Wolfgang Pietsch Agglomeration Processes Phenomena, Technologies, Equipment
Wolfgang Pietsch
Agglomeration Processes Phe no mena, Tech no Iogi e s, Eq ui p me nt
8WILEY-VCH
Dr.-lng. Wolkang Pietsch, EUR INC COMPACTCONSULT, INC. 2614 N. Tamiami Trail, #520 Naples, Florida 34103-4409 USA
In Europe: Holzweg 127 67098 Bad Durkheim, Germany Cover Illustration Like an agglomerate, the picture on the cover is composed of many disparate components, all of which relate to the topics discussed in this book. The panels on the left and right are microphotographs of naturally agglomerated nano-particles. The top and the bottom panels depict different products from spray drying and fluid bed agglomeration. The four sectors (between the panels and the circle) represent Scanning Electron Micrographs (SEMs) of agglomerate structures as well as photographs of coated agglomerates and of granules. The top half of the circle shows products from tumble/growth agglomeration and the lower half are briquettes from roller presses as well as product from compaction/granulation. The center square includes tablets from punch and in die presses. The originals of the individual pictures from which sections are reproduced were supplied by (in alphabetical order): Albemarle Corp., Baton Rouge, LA, USA; Cabot Corp., Tuscola, IL, USA: Eirich, Hardheim, Germany: Euragglo, Qievrechain, France; Niro A/S, Soeborg, Denmark Norchem Concrete Products, Inc., Fort Pierce, FL, USA; Koppern GmbH & Co, KG, Hattingen, Germany. Their support is appreciated and acknowledged.
This book was carefully produced. Nevertheless, author and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: Applied for. British Library Cataloguingin-Publication Data: A catalogue record for this book is available from the British Library.
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Die Deutsche Bibliothek CIP Cataloguingin.Pub. lication Data: A catalogue record for this publication is available from Die Deutsche Bibliothek.
Wiley-VCH Verlag GmbH, Weinheim, 2002 Printed on acid-free paper. All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. In this publication, even without specific indication, use of registered names, trademarks, etc., and reference to patents or utility models does not imply that such names or any such information are exempt from the relevant protective laws and r e g ulations and, therefore, free for general use, nor does mention of suppliers or of palticular commercial products constitute endorsement or recommendation for use. Mittenveger & Partner Kommunikationsgesellschaft mbH, Plankstadt Printing betz.druck GmbH, Darmstadt Bookbinding GroBbuchbinderei J. Schaffer GmbH & Co. KG, Griinstadt
Composition
Printed in the Federal Republic of Germany. ISBN 3-527-30369.3
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Contents Dedication, Acknowledgements and References
VII
1
Introduction
2
A Short History o f Agglomeration
3
Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
4
Glossary of Agglomeration Terms
5
Agglomeration Theories
5.1 5.1.1 5.1.2 5.2 5.2.1 5.2.2 5.3 5.3.1 5.3.2 5.4 5.5
The Development of Strength of Agglomerates 32 Binding Mechanisms 35 Binders, Lubricants, and Other Additives 42 Estimation of Agglomerate Strength 55 Theoretical Considerations 55 Laboratory and Industrial Evaluations 62 Structure of Agglomerates 76 General Considerations 78 Porosity and Techniques That Influence Porosity 89 Other Characteristics of Agglomerates 100 Undesired and Desired Agglomeration 109
6
Agglomeration Technologies
7
Tu mble/G rowth Agglomeration
7.1 7.2 7.3 7.4 7.4.1 7.4.2
Mechanisms of TumblejGrowth Agglomeration 140 Kinetics of Tumble/Growth Agglomeration 144 Post-treatment Methods 150 Tumble/Growth Agglomeration Technologies 151 Disc and Drum Agglomerators 153 Mixer Agglomerators 164
1
3
11
29
133 13 9
5
VI
I
Contents
7.4.3 7.4.4 7.4.5 7.4.6
Spray Dryers 187 Fluidized Bed Agglomerators 196 Other Low Density Tumble/Growth Agglomerators Agglomeration in Liquid Suspensions 221
8 8.1 8.2 8.3 8.4 8.4.1 8.4.2 8.4.3 8.4.4
Mechanisms of Pressure Agglomeration 23 1 Structure of Pressure Agglomerates 236 Post-treatment Methods 241 Pressure Agglomeration Technologies 252 Low-Pressure Agglomeration 253 Medium-Pressure Agglomeration/Pelleting 266 High-pressure Agglomeration 300 Isostatic Pressing 373
9
Agglomeration by HeatlSintering 385
9.1 9.2 9.2.1 9.2.2
Mechanisms of Sintering 385 Sintering Technologies 389 Batch Sintering 390 Continuous Sintering 397
10
Special Technologies Using the Binding Mechanisms of Agglomeration
10.1 10.2 10.2.1 10.2.2 10.3
Coating 415 Separation Technologies 440 Gas/Solid Separation 440 Liquid/Solid Separation 442 Fiber Technologies 447
11
11.2 11.3
Engineering Criteria, Development, and Plant Design 453 Preselection of the Most Suitable Agglomeration Process for a Specific Task 462 Laboratory Equipment, Testing, and Scale-Up 468 Peripheral Equipment 492
12
Outlook
13
Bibliography 525 List of Books or Major Chapters on Agglomeration and Related Subject References 530 Author’s Biography, Patents, and Publications 53 1 Tables of Contents of Related Books by the Author 541
11.1
13.1 13.2 13.3 13.4 14
14.1 14.2 14.3
Pressure Agglomeration
212
229
409
507
Indexes 543 List of Vendors 543 Wordfinder Index 580 Subject Index 591
526
Dedication, Acknowledgements and References When this book was first planned, the idea was to combine in one volume concise descriptions of the agglomeration phenomena, technologies, equipment, and systems as well as a compilation of the applications of agglomeration techniques in industry. The latter was intended to demonstrate the widespread natural, mostly undesired occurrences of the phenomena, including possibilities to avoid them, and discuss the varied old, conventional, and new beneficial uses of the technologies. However, it soon became obvious that, in its entirety, this project became too voluminous and required much more time than anticipated. Therefore, it was decided to split the subject’s presentation into two volumes whereby both books will be “stand alone” publications that are also complementary. The first volume, available here, covers the fundamental phenomena that define agglomeration as well as the industrial technologies and equipment for the size enlargement by agglomeration. Applications are mentioned in a general way throughout the text of this presentation but without going into details. These applications will be presented in a separate book entitled “Agglomeration Technologies - Industrial Applications” that is scheduled for publication in 2003. A preliminary table of contents is given in Section 13.4. Many persons, institutions, and companies have contributed to the two volumes of this book. First and foremost, I wish to thank my wife Hannelore for her support and understanding while, thomghout my professional career, I was compiling various papers and books (see Section 13.3). All are dedicated to her. Without my wife’s active participation in preparing almost all publications, in elaborating the textbook entitled “Size Enlargement by Agglomeration” [B.42],which is a major reference for this publication (see also below), and her, if sometimes reluctant, acceptance that I was not available for long hours on many days during almost four decades, the books, in particular, could not have been completed. It is impossible to acknowledge all the help, extensive and small, that was provided by a large number of individuals and companies. In Section 14.1,a list ofvendors and other organizations is compiled which mentions those who have, in one way or another, contributed as well as some others who may be of interest as potential contacts for the readers of this book. While I have decided not to clutter the text with references, sources have been acknowledged if figures or tables were provided by or are based
Vlll
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Dedication, Acknowledgements and References
on information from particular companies. The Discialmer at the beginning of this book (see page IV) should be referred to when using such cross-references. Regarding references to literature, Chapter 13 should be consulted. The earlier textbook “Size Enlargement by Agglomeration” [B.42]contains treatments as well as many references relation to the developing science of the unit operation and covers in some detail the sizing of and scale-up methods for agglomeration equipment. Since the emphasis of the new book is on practical considerations and industrial applications, not theory, the earlier book “Size Enlargements by Agglomeration” (Wiley, 1991) should always also be referred to. Information on the availability of reprints is available at the beginning of Section 13.1 and as a footnote later in the same Section. Since Size Enlargement by Agglomeration is one of the unit operations of Mechanical Process Technology (see Chapter 1) and, for the design and construction of agglomeration systems of any kind, many or all of the other unit operations are required, together with the associated transport and storage technologies, often even in multiplicity, and the analytical methods are applied for process evaluation and control, the reader who is interested in the topic of this book should also learn about or have access to information on the other fields of Mechanical Process Technology. This is also emphasized in Chapter 13. At this point I wish to acknowledge two books of general importance to which I have contributed chapters on agglomeration and ofwhich major portions were included in this book. They are: “Handbook of Powder Science and Technology” M. E. Fayed, L. Otten (eds,), 1st ed., Van Nostrand Reinhold Co., New York, NY (1983) and 2nd ed., Chapman & Hall, New York, NY (1997). Source references can be found in [B.21] and [B.56], Section 13.1. Finally, I like to mention with gratitude the following individuals who, as professionals and experts in their own fields, are or have been colleagues and/or partners in several continuing education courses over many years in the USA as well as in Europe. They have agreed that statements during their presentations and the elaborations for their course notes can be used directly, adopted, or modified for this book. They are, in alphabetical order: T. van Doorslaer, W. E. Engelleitner, M. E. Fayed, M. Gursch, D. C. Hicks, S. Jagnow, R. H. Leaver, R. Lobe, K. Masters, S. Mortensen, H. B. Ries, F. V. Shaw, J. Storm, R. Wicke, and R. Zisselmar. For additional references and acknowledgements please refer to Sections 13.1 and 13.2. Naples, November 2001
Wolfgang Pietsch
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
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1
Introduction In 1957, under the leadership of Professor Dr.-Ing. Hans Rumpf at the Technical University (TH) of Karlsruhe, Germany, Mechanical Process Technology or Particle Technology IB.111 was first introduced as a field of science in its own right. It comprises the interdisciplinary treatment of all activities for the investigation, processing, and handling of solid particles as well as the interactions of such particulate solids. Four unit operations and associated techniques were defined (Fig. 1.1).Other common English names for this field of science, which was quickly adopted around the world, are Mechanical Process Engineering, Powder Technology, or Powder & Bulk Solids Technology. Size enlargement by agglomeration is the generic term for that unit operation of mechanical process engineering which is characterized by “combination with change in particle size” (Fig. 1.1).The author of this book had the privilege to become one of the first assistants of Professor Rumpf. For several years he was responsible for the research and development of size enlargement by agglomeration at the Institute of
Fig. 1.1
Unit operations and associated techniques
of Mechanical Process Technology
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1 Introduction
Mechanical Process Engineering and earned his PhD with a doctoral thesis on specific aspects of a binding mechanism [1.1]of agglomeration. Webster’s Unabridged Third New International Dictionary [1.2] defines the verb agglomerate as: “to gather into a mass or cluster; to collect or come together in a mass; to collect into a ball, heap, or mass, specifically: clustered or growing together but not coherent”, and the noun agglomerate as: “a cluster of disparate elements; an indiscriminately (= randomly) formed mass”. A technical dictionary [ 1.31 defines agglomeration as: “sticking or balling of (often very fine) powder particles due to short range physical forces. Therefore these forces become active only if the individual particles (forming the agglomerate) are brought closely together by external effects”. These definitions distinguish the term size enlargement by agglomeration from the more general size enlargement such that particle growth occurring, for example, during crystallization or the production of particulate solids by melt solidification are not part of this unit operation of Mechanical Process Technology.
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
2
A Short History of Agglomeration As a basic physical efect, agglomeration has existed since particulate solids were first formed on Earth. Binding mechanisms between solid particles cause the stability of wet and dry soil and (often under the influence ofheat and pressure) participate in the development of rock formations. Sandstone is the most easily recognized “agglomerate”. Agglomeration as a phenomenon, e.g. the natural caking and build-up of particulate solids, must have been observed and has been used by higher developed organisms and later by humans since prehistoric times. Sea creatures covered themselves with protective coats, birds as well as other animals built nests, and humanoids formed artificial stones, all from various solids, sand, clay and different binders that were often secreted by the creature itself. As a “tool” to improve powder characteristics, agglomeration was used by ancient “doctors” in producing pills from medicinal powders and a binder (e.g.honey) or by food preparers during the making of bread from flour whereby the inherent starchy components act as binder. In spite of this long “history”,agglomeration as a technology is only about 150 years old today (excluding small scale pharmaceutical and some little-known ancient, mostly Chinese applications as well as brick and bread making). Agglomeration as a unit operation, defined within solids processing, started around the middle of the nineteenth century as a method to recover and use coal fines. Agglomeration as a science is very young. It began in the 1950s with the formal definition of the binding mechanisms of agglomeration, interdisciplinary collection of knowledge relating to all aspects of agglomeration, and fundamental research which was no longer application oriented [B.42].At approximately that time, the first recurring series of professional meetings were organized which were exclusively devoted to the science and technology of agglomeration (International Briquetting Association (IBA),- today Institute for Briquetting and Agglomeration (IBA) -, beginning in 1949 with biennial meetings and proceedings: International Symposia on Agglomeration, initiated in 1962 with proceedings, (see also Section 13.1)). Since that time, agglomeration science, technology, and use have experienced rapid growth but still without finding a corresponding awareness at institutions of higher learning and in the technical or process engineering communities. This book is the second by the author on the general subject of size enlargement by agglomeration. While frequently referring to fundamentals and specifics which are
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2 A Short History ofAgglomeration
covered in more detail in the first book [B.42], this new text tries to provide an updated, comprehensive summary of the state-of-the-art of agglomeration, its basics, technologies, and applications, at the beginning of the 21st century.
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
3
Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science As mentioned in the previous chapter, size enlargement of particulate solids by agglomeration is as old as the existence of solids themselves. Originally, agglomeration happened naturally during the development of soil, stone, and rock formations. Later, unwanted agglomeration occurred during handling and storage of particulate matter particularly when hygroscopic and/or soluble materials (such as salt) “set-up’’ into lumps or large, more or less solidified masses. In the animal world agglomeration was used to develop protective coatings (e.g. many marine worms, Fig. 3.1), to build nests (e.g. swallows, termites, Fig. 3.2), and to provide a nourishing and protective environment for the offsprings (e.g. dung beetles, Fig. 3.3). Humans most probably first used agglomeration during the making of bread by taking flour (= particulate solids including an inherent binder, starch) and liquid additives (= additional binder for plasticity and “green”bonding), mixing and forming the mass, and, finally, “curing”the product, the removal of much of the moisture that was added during the mixing and agglomeration steps, to obtain structure and permanent bonding during baking. The technology of bread making combines all com-
Fig. 3.1 Protective agglomerated coating of a Rhizopod, a creping marine Protozoan (Difflugia urceolata)
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3 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
Fig. 3.2a Nest of swallows made by agglomeration from mud, the bird's saliva as a binder, and organic fibers for strengthening
Fig. 3.2b Nest o f termites made from earth as well as the animal's excrements andlor secretions as binder
3 Agglomeration as a Generic, Independent, and lnterdisciplinary Field of5cience 17
Fig. 3.3
Dung beetle, Scarabaeus Sacer, “pelletizing” dung
ponents of a complex agglomeration process including preparation of solid feed particles by milling (= adjustment of particle size and activation of the inherent binder, starch),mixing of particulate solids with additional binder@),forming the mass into a “green”agglomerate, and a “post-treatment” (curing =baking = heating and cooling) to provide strength and texture. Very early it was also found that the porosity of the final product could be modified (= increased) by making use of gases that are produced during fermentation (initiated by sour dough or yeast) and result in bubbles in the green mass. These voids are stabilized by “strengthening”the bread during post-treatment (baking). For the construction of permanent shelter, humans may have observed the activities of animals that formed nests and protective “walls” from wet clay which hardened during drying (Fig. 3.2). By copying this behavior, wet clay, which was soon reinforced and made more water resistant by mixingin straw or other fibrous material, was filled into a framework of wood branches and let harden during natural drying. To make building activities independent of the location of clay “mines”, during prehistoric times bricks were already produced from clay and sand and, after hardening, transported to building sites. Since fire was known for providing heat, the accidental “firing” of a piece of clay most probably resulted in the adaptation of an improved posttreatment that yielded waterproof bricks for areas where rock was not easily available, thus allowing the development of villages and, during the 4th millenium B.C. in Mesopotamia, cities with large brick structures. By experience, humans learned that certain natural materials helped cure specific illnesses. Minerals as well as dried animal and plant matter were ground to powder and “formulated”to yield medicines. Since powders cannot be easily consumed orally, natural binders, such as honey which, incidentally, also masked the unpleasant taste of
8
I some of the medicinal components, were mixed with the powder and the resulting 3 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
plastic mass was rolled by hand into little balls (= pills). The sticky binder(s) caused pills to adhere to each other; therefore, it was soon found that coating the pills by rolling them in flour or pollen solved this problem (see also Section 10.1). These three, well known ancient agglomeration techniques were used with little change through the ages of human history. Many other, lesser known and somewhat more recent processes could be added. However, it is not the objective here to produce a history book. Rather, these examples relating to three major modern “industries”, food, building materials, and health products, were selected to show that humans always lived with and used agglomeration. As a result, agglomeration technologies as all the other unit operations and associated techniques of Mechanical Process Technology (see Fig. 1.1)were considered to be “normal activities” which, with the beginning ofindustrialization in the 18th and 19th centuries A.D., were merely mechanized by simulating what was done manually before. During these early modernization efforts it was not considered necessary to question the fundamentals of the processes and “improvements” were based on empirical developments. Until very recently, agglomeration technologies had been developed independently in the particular industries in which they were applied. Because the process requirements are fundamentally different in such unlike industries handling, for example, coal and ores on one hand or food and pharmaceuticals on the other, no interdisciplinary contact and exchange of information took place. In fact, although agglomeration techniques developed along similar lines, application related “theories” were defined which were derived from investigations of specific requirements and their solutions together with a terminology that was often incomprehensible and, therefore, not useable by the “agglomeration expert” of another industry (see Chapter 4). Agglomeration as a science began when an effort was made to interdisciplinarily combine the extensive knowledge that had been accumulated during sometimes hundreds of years in specific fields of human activities. This approach showed that (in alphabetical order, not indicating importance): Baking: A thermal post-treatment process, does not only induce the development of final bonding, structure, and consistency in bread but produces similar characteristics during the heat curing of any “green” agglomerate. Briquetting: Is not predominantly a technique for the enlargement of coal fines for beneficial use but equipment which was specifically developed for that application is also suitable for such diverse uses as, for example, the briquetting of salt for the regeneration of water softeners, the briquetting of flaked DMT to decrease the bulk volume and improve handling and shipping, the briquetting of frozen vegetable pulps to be used as rations for field kitchens, the hot briquetting of sponge iron to reduce this commodity’s reactivity and allow open handling and storage, or the production of fertilizer spikes and the manufacturing of artificial fireplace logs. Coating: Is not only suitable for the modification of surface characteristics or the control of dispersion and dissolution of medicinal specialties but also to achieve similar properties in agrochemicals as well as human and animal foods, among others.
3 Agglomeration as a Generic, Independent, and Interdisciplinaty Field of Science 19
Compacting: Is not only applicable for the production of “green” bricks or other ceramic bodies prior to firing but finds many uses in powder metallurgy or for the production of battery cathodes, etc. Granulating: Is not primarily a method to improve flowability of powders and formulations in the pharmaceutical industry but also in the fertilizer and bulk chemical industries as well as for carbon black, silica fume, and many other materials. Instantizing: As an example of a relatively new agglomertion process, is not limited to applications in the food industry for easily dissolvable drink and soup mixtures but is equally important for pigments, insectizides, fungizides, and many more. Pelleting: Originally developed for the shaping of animal feed formulations by extrusion, is also applicable for the production of catalyst carriers and other materials requiring uniform size and shape together with relatively high porosity. Sinteuing: When going back to the fundamentals of this process, was found to be not only a high temperature process for the agglomeration of ores but, at much lower temperatures, also for plastics and other man made powders with low melting points or softening ranges, and, quite obviously, for powder metallurgy, mechanical alloying, or the like at many different temperature levels including extremely high ones for refractory metals. The above is only a small selection of the many diverse applications of particular agglomeration methods which, in all the different environments, follow the same fundamentals, apply the same rules, and use essentially the same equipment and systems if looked at from an interdisciplinary point of view. Although these facts become more and more known, there is still the understandable preconceived notion of, for example, somebody working in an ultraclean environment, such as the pharmaceutical, food, or electronic industries, that developments, expertise, and know-how gained in the “dirty”plants of, for example, minerals or metals production and processing, can not be considered as valid information that may be applied for the solution of a “clean”problem - and vice versa. In the case of “dirty”industries, a typical concern is that the often more deeply and completely investigated technologies originating in “clean” industries can not be applied because the production capacities are too small, the process may be batch, the equipment too complex, the execution and the materials of construction too expensive, etc., etc. However, as will be shown among other topics in this book, methods for the selection of the most suitable agglomeration process for a specific application (see Section 11.1)are the same for all projects. While some requirements, for example in regard to equipment or system capacity, or on the shape, size, and special properties of the products, may result in the definition of “cleaner” or “more heavy-duty, rugged” processes already in the preselection phase, the normal approach is to determine the preferred method and/or technique by considering the fundamentals as well as an interdisciplinary pool of expertise and know-how first. Conditions of the particular application such as, for example, “hot and dusty large volume processing”, or the opposite, “clean, small capacity operation with cGMP (= current Good Manufacturing Practice) and CIP (= Cleaning In Place) capabilities” are special design criteria that can be added to most of the systems later during the engineering phase.
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3 Agglomeration as a Generic, Independent, and interdisciplinary Field of Science
Nevertheless, for manufacturing reasons and sometimes also because of special requirements on the company’s test facilities (see Section 114, some vendors specialize in equipment for one or the other industry. This is a decision of convenience by the individual supplier and does not indicate the existence of a fundamentally different technology. In fact, techniques or apparatus that were developed for a specific industry can be adopted for use in areas with different environment and requirements while still maintaining the fundamental underlying principle as well as the general machinery and process. Examples are flaking (see Section 8.4.3), instantizing (see Section 5.4), spheronizing (see Section 8.3), and spray dryer agglomerators (see Sections 7.4.3 and 7.4.4).
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
4
Glossary o f Agglomeration Terms Newly developing fields of science are organized according to universally recognized classifications using well-defined terms to describe the fundamentals, correlations, equipment, procedures, and processes. This is not the case for those technologies that were known for centuries and have been developed empirically and independently for different applications (see also Chapters 2 and 3). In such cases the same process, procedure, activity or piece of equipment may have different names in different industries or the same term may have different meanings in different fields of application. The earlier book “Size Enlargement by Agglomeration” [ B.421 contained already a glossary of agglomeration terms. In the following this glossary is repeated and updated. Although the author and many others that are active in the promotion of “agglomeration” are trying to use scientific and technical terms consistently in an interdisciplinary manner (terms shown bold), it is still helpful to also explain some of the more common names and expressions including a few historical ones. In the following, crossreferences are indicated by italic letters. The same and many more “agglomeration terms”, the latter mostly descriptive and/or trade names, are mentioned and used in the text of the book. Sections 14.2 and 14.3 help locate these words and expressions. Abrasion [n.]
Abrasion resistance Accretion [n.]
Accumulate [vb.] Accumulation [n.] Adhesion [n.]
Removal of solid matter from the surface or edges of an agglomerate. The matter removed is much smaller than the agglomerate itself. (See also attrition, erosion.) Measure for the ability of a body, for example an agglomerate, to withstand abrasion. The process of growth or enlargement by a gradual buildup, such as: increase by external addition or accumulation, for example by adhesion of external parts or particles. (See also agglomeration, aggregation, build-up.) To heap up into a mass; pile up. The action or process of accumulating; an accumulated mass, quantity, or number. A sticking together of solids. The molecular attraction exerted between the surfaces of solids. Distinguished from cohesion.
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I Agglomerate [vb.]
4 Glossary of Agglomeration Terms
Agglomerate [n.]
Agglomeration [n.] Agglomerator [n.] Aggregate b.1
Aggregate [vb.] Aggregation [n.]
Agitation [n.] Agitator [n.] Ammoniation [n.] Angle of repose Angle o f compaction Anticaking agent
Apparent density Atomizer [n.] Atomizing [vb.] Attrition [n.] Auger [n.] Axial extruder
To gather (particulate solids) into a ball, mass, or cluster. (See also aggregate.) An assemblage of particles which is either loosely or rigidly joint together. Particles adhering to each other. (See also conglomerate.) The action or process o f gathering particulate solids into a conglomerate. Specific equipment in which agglomeration is accomplished. Any o f several hard, inert materials (as sand, gravel, rock, slag) used for mixing with a binding material to form concrete, mortar, plaster or, for example, road surfacing products. Also: A mass or body o f units or parts somewhat associated with one another. To collect or gather into a mass. (See also agglomerate.) A group, body, or mass composed o f many distinct parts or individuals; the collection of units or parts into a mass or whole; the condition o f so collected. (See also agglomerate, aggregate, cluster, agglomeration, accretion, build-up.) Changes in characteristics ofparticulate solids or agglomerates that occur naturally with time. (See also post-treatment.) A state of movement of particulate solids and/or fluids induced by external effects or forces. See mixing tool, intensijer bar. The formation o f fertilizer granulates using ammonia to obtain chemical modification and bonding. The basal angle of a pile o f powder that has been freely poured onto a horizontal surface. See nip angle. Liquid or solid matter applied to the surface of, for example, agglomerates that prohibits sticking or growing together. (See also caking.) The weight of the unit volume of a porous mass, for example, an agglomerate. See nozzle. Finely dispersing liquids. The unwanted break-down of agglomerates. (See also abrasion, erosion.) See screw. Low, medium, or high pressure extruder with a flat die plate at the end of a barrel; the material is extruded in the same direction as it is transported by the screw(s).
4 Clossarj of Agglomeration Terms
Backmixing [n.]
Bag set
Ball [n.] Ballability [n.]
Balling [n.]
Barrel [n.] Basket extruder Beading [n.]
Bin [n.] Binder [n.] Binding mechanism
Biomass [n.] Blade [n.] Blunger [n.] Boiling Bed Bonding [n.] Bowl [n.]
Bridging [n.]
Briquette [n.] Briquetter [n.]
During the flow of particulate solids, reverse movement of some particles due to their stochastic motion caused by turbulence or special equipment design. Typical in the fertilizer industry; unwanted agglomeration of particulate solids in a closed bag during storage. Mostly caused by recrystallization of dissolved materials. Synonymous with spherical agglomerate. (See also pellet.) Typical in the iron ore industry; the capability of particulate solids to form more or less spherical agglomerates during growth agglomeration. Originally in the iron ore industry; any method producing spherical agglomerates by tumble or growth agglomeration. (See also pelletizing.) Cylindrical (or sometimes tapered) housing for screws, e.g. offeeders or extruders. Low pressure extruder in which the die plate resembles a basket, using rotating or oszillating extrusion blades. Formation of bead-like particles; typical in solidification of melt droplets. (See also prilling, pastillation, melt solidijcation.) A container, box, frame, crib, or enclosed volume used for storage. (See also hopper, silo.) An inherent component of or an additive to particulate matter providing bonding between the disparate particles. Physical and chemical effects that cause adhesion and bonding between solid surfaces. See Section 5.1, Tab. 5.1 and Fig. 5.8. Organic plant and animal residuals. Often organic waste material that is especially used as a source for fuel. See extrusion blade. Typical in the ceramic and fertilizer industries; double shafted pug mill. SeeJuid bed. The process of binding particles together by the action of binding mechanisms. A vertical or inclined, cylindrical, conical or convex vessel enclosing and defining the operating volume of some coaters, mixers, spheronizers, etc. Unwanted arching of solid matter in a converging discharge chute or cone. Prohibits discharge of particulate solids from containers or chutes. Also briquet; agglomerate produced and shaped by highpressure agglomeration. (See also compact, tablette.) Also briquetting machine; equipment that produces briquettes.
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4 Glossary of Agglomeration Terms
Briquetting [n.] Brittleness [n.] Build-up [n.]
Bulk density
Capillary [adj.] Capping [n.]
Cake [n.] Caking [n.] Cement [n.]
Cement [vb.] Cementitious [adj.] Channel [n.]
Chelate [adj.]
Chelate [n.] Chopper [n.] Clam shelling
Closed pore Cluster [n.] Clustering [n.] Coalesce [vb.]
The process of forming briquettes or compacts. The tendency of particles or agglomerates to break down in size easily. (See also friability.) The unwanted coating of surfaces with particles which adhere naturally due to their fineness and/or inherent binding mechanisms. The weight of the unit volume of a particulate mass under non-specific condition, e.g. in storage or in a shipping container. (See also density.) Describing full liquid saturation. Separation of a thin layer from the face(s) of compacts during decompression. Defect in tablettes caused by the recovery of elastic deformation and/or expansion of compressed air. See sheet; typical in fertilizer applications. Unwanted agglomeration during storage mostly by recrystallization of dissolved materials. (See also bag set.) A powder of alumina, silica, lime, iron oxide, and magnesium oxide burned together in a kiln, finely pulverized, and used as an ingredient of mortar and concrete. Also any mixture used for a similar purpose. (See also pozzolan.) To unite or make firm by or as if by cement. Having the properties of cement. Open ended compacting tool set for high pressure extrusion in a ram press; also any elongated opening through which material is extruded. (See also pressway.) Relating to, producing, or characterized by a cyclic structure usually containing five or six atoms in a ring in which a central metallic ion is held in a coordination complex by one or more groups each of which can attach itself to the central ion by at least two bonds. To combine with a metal to form a chelate ring or rings. See knive head. Opening of the leading or trailing edge of briquettes discharging from roller presses; one-sided splitting along the web. Also duck billing, oyster mouthing. A pore not communicating with or connected to the surface of a porous body. A number of similar individual entities that occur together. (See also accretion, agglomerate, aggregation.) The growing together of primary agglomerates to form larger entities. (See also satellites formation.) To unite by growth.
4 Glossary of Agglomeration Terms 115
Coalescence [n.] Cohesion [n.]
Coating [n.]
Coating pan
Cold bonding
Compact [n.]
Compact disperse Compactibility [n.] Compacting [n] Compacting tool set Compaction/granulation
Composite [adj.] Compressibility [n.]
Compression ratio
Conditioning [n.]
Cone agglomerator
A growing together or union in one body, form, or group. (See also growth agglomeration.) Molecular attraction by which the particles of a body (e.g. agglomerate) are united throughout the mass whether like or unlike. Distinguished from adhesion. Applying a layer of material, a film, or a finish to a substrate; in agglomeration, application of a layer of solids to a particulate unit. Specially shaped p a n in which a material layer is applied on agglomerates (such as tablettes) usually in the presence of liquid, heat, or both. Typical in the pharmaceutical and food industries. A binding process that occurs at ambient or low temperatures and uses the cementitous or pozzolanic reactions of many hydroxides; often assisted by pressure. An object of specific size and shape produced by the compression of particulate matter. Synonymous with briquette. A state of particulate solids in which individual particles are closely packed. Distinguished from discrete dispers. See compressibility. Also compaction. The method of producing sheet. The part or parts making up the confining form in which a powder is pressed. Synonymous with die. The normally dry methods of obtaining granular products by crushing and screening compacts and/or sheet into granulate. Consisting of two or more separate materials whereby each retains its own identity. The capacity of a particulate matter to be compacted. Compressibility may be expressed as the pressure or force to reach a required density or, alternately, the density at a given pressure or force. Synonymous with compactibility. The ratio of the volume of loose particulate matter in a die to the volume of the compact made from it. Synonymous with fill ratio. In low and medium pressure extruders, the total thickness of material that is under compression in a die (including any inlet chamfer) divided by the nominal hole diameter. Development of special characteristics of particulate solids by, for example, treatment with steam, kneading, heating, etc., or surface treatment by, for example, anticaking agents. Pan with relatively high conical rim.
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4 Glossary ofAgglomeration Terms
Contact point Coordination number
Core rod Couffinhal press
CUP Ln.1 Curing [n.] Cut size Decrepitation [n.] Densification [n.] Density [n.] Deposit [n.] Die [n.]
Die plate Disc [n.] Discrete dispers
Dispers [adj.] Dispersibility [n.]
Distribution plate
Doctor blades Dome extruder Double action pressing
An adhering mass of particles made up of parts from different sources or of various kinds. (See also agglomerate.) Area at which two particles touch each other. Sum of all near and contact points of a particle with surrounding particles in a structure made up of particulate solids, for example an agglomerate. Member of the compacting tool set that forms a through hole in the compact. (See also mandrel.) Punch-and-die press with multiple die sets on an indexed table for making large (e.g. coal) briquettes. (No longer used.) See pocket. h d u r a t i o n of green agglomerates by any method. (See also post-treatment.) The actual value at which separation of a particle size distribution into “coarse” and “fines” has taken place. Breakdown in the size of particles or agglomerates due to internal forces, generally induced by heat. The act or process of making dense. Mass per unit volume of matter at specific conditions. For example: apparent, bulk, or true densities. A (natural) accumulation of particles. Member of the compacting tool set that forms the periphery of the part being produced. Also open ended channels for extrusion. Plates, rings, or other machine parts with perforations for extrusion. (See also die.) See pan. A state ofparticulate solids in which individual elements can be clearly distinguished. Distinguished from compact dispers. See particulate. Measure for the ease with which, under specific conditions (e.g. in liquids), an agglomerate breaks down into primary particles. Perforated plate at the bottom of a j u i d bed through which fluidizing gas enters from the plenum. (See also gil plate.) See scraper. Axial, low pressure extruder, most often with two screws, in which the die plates resemble domes. A method by which particulate solids are pressed between opposing punches which are both moving relative to the die.
4 Glossary of Agglomeration Tirms
Downdraft [n.] Drum agglomerator Dry granulation Duck billing Dwell time
Encapsulation [n.] Erosion [n.]
Equivalent diameter
Expansion [n.]
Exter press Extrudate [n.] Extruder [n.] Extrusion [n.]
Extrusion blade
Feeder [n.] Feed screw Fill ratio
Flake [n.]
Flake breaker Flashing [n.] Flight [n.]
Downward flow of gas, for example through a particle bed. Slowly rotating, slightly inclined drum for growth a g glomeration. See compaction/granulation. See clam shelling, oyster mouthing. In compacting, time during which certain process conditions, for example pressure, persist or are held constant. Typically used as microencapsulation. The gradual wearing away of an agglomerate by the progressive removal of small pieces of material. (See also abrasion.) Diameter of immaginary monosized spherical particles which feature the same property as the particulate mass to be characterized. For example: surface equivalent diameter. Increase in volume of, for example, an agglomerate after production or during post-treatment. Converse of shrinkage. See ram extruder. Product of extrusion. (See also pellet.) Machinery for the production of extrudates. (See also screw and ram extruder.) The formation of (often cylindrical) agglomerates by forcing a “plastic” mass through open ended channels or holes in (perforated) dies. In low pressure extruders, the flat, curved, or engineered blade that pushes material through the openings of a die plate; it is the part closest to the die plate. Device to deliver feed material to a processing unit. (see also force feeder.) Element(s) providing forces onto particulate solids in a feeder. (See also screw.) Typically used in tabletting or other confined volume compression equipment. Synonymous with compression ratio. See sheet. Also: 1. Grains or other malleable particles flattened between smooth rollers. 2. Material solidified from a melt on a rotating, cooled drum (flaker) and removed by scrapers. A primary crusher (often two rollers with teeth) used to reduce the size of sheet. See web. A continuous or semi-continuous spiral flat plate that is attached to the shaft of a screw.
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4 Glossmy ofAgglomeration Terms
Floc [n.] Flocculant [n.] Flocculate [vb.] Flocculation [n.] Flocculent [adj.] Fluid bed
Fluid bed agglomeration Force feeder Fraction [n.]
Fragmentation [n.]
Friability [n.] Friction plate
Funicular [adj.] Gap w.1
Gear pelleter
Gil plate Globulation [n.] Granular [adj.] Granulate [n.]
Aflocculent mass formed by the aggregation of a number of fine suspended particles. A flocculating agent. To aggregate or coalesce into small lumps or loose clusters or into aflocculent mass or deposit. The act or process offlocculating. Containing, consisting of, or occurring in the form of loosely aggregated particles or soft flocs. Also fluidized bed. A bed of particles in which the particulate solids are kept in suspension by forces caused by an upward flowing fluid. Growth agglomeration in a fluid bed. A feeder that provides forces onto particulate matter by, for example, the action offeed screws. That portion of a sample of particulate solids which is between two particle sizes (see cut) or in a stated range (e.g. fine, coarse, etc.). The process whereby aparticle (or agglomerate) splits into usually a large number of smaller parts with a range of sizes. The tendency of particles to break down in size during storage and handling. (See also brittleness.) In spheronizers, a circular flat disc with a rough surface or uniformly spaced grooves which rotates inside a cylindrical bowl. Describing the transitional liquid saturation. In pressure agglomeration,the distance between the surfaces of compacting tool sets; specifically: in extrusion, the distance between the pressure generating device and the die plate, in roller presses, the closest distance between the rollers. Double-rollpellet mill in which the rollers are in the shape of coarse, intermeshing gears with bores at the root sections between the gear teeth. (Also gear pelletizer.) Distribution plate in which the perforations are manufactured such that they produce a directional flow of gas. See melt solidijcation. Present as particles in “grain” shape and size. Coarsely particulate. Also Granule. From Latin granula = grain, particle. Any kind of relatively coarse particulate matter. In size enlargement, synonymous with agglomeration to a size range of between approx. 0.1 and 10 mm. In size reduction, synonymous with crushing into approx. the same size range. Granulate is normally considered dustfree, free flowing, and non-segregating.
4 Glossary of Agglomeration Terms
Granulate [vb.]
Granulation [n.] Green [adj.] Grid [n.] Growth [n.] Growth agglomeration Heat bonding Heel [n.]
Hopper [n.] Hot melt agglomeration Hot pressing Immiscible binder agglomeration Induration [n.] Inkbottle pore
Instant [adj.] Instantizing [n.]
Intensifier bar
Interconnected porosity
Producing a granular solid matter; possible by size enlargement (agglomeration, melt solidijhtion [pastillation, prilling], and crystallization) or by size reduction (crushing). (See also compaction/granulation.) A general term for the production of solids in granular form by either size reduction or size enlargement. As in “green agglomerate”, “green pellet”, etc., means fresh, moist, uncured, etc. In spheronizers, the design (size and shape) of the grooves on the friction plate surface. An increase in dimension by for example agglomeration or crystallization. (See also coalescence.) See coalescence, tumble agglomeration. See sintering. In batch processing, for example agglomeration, a percentage of the previous batch retained in or returned to the processing vessel. The funnel or chute that stores material and/or directs it into equipment. (See also bin, silo.) Granulation of a hot melt of e.g. urea or ammonium nitrate in a pan. The simultaneous heating and molding of a compact or briquetting of hot material. Selective agglomeration of particles suspended in a liquid by adding an immiscible binder during agitation. (See also oil agglomeration.) Strengthening of green agglomerates, mostly by heat. Non-cylindrical pore with varying diameter; particularly a pore with narrow entrance followed by a large, internal volume. Quickly soluble. Characteristic as, for example, in “instant coffee”. Producing agglomerated products with instant characteristics, i.e. material exhibiting, as compared with the untreated powder, particularly high solubility, even in cold liquids. In high shear mixers and agglomerators, an independently driven bar, rotating with high speed, usually carrying mixing tools and, sometimes means for atomizing liquid binder, that extends into the particulate mass to be mixed and causes an additional turbulent motion of the particles. (See also knive heads.) A network of contiguous pores in and extending to the surface of a porous body, e.g. agglomerate.
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4 Glossary of Agglomeration Terms
A plane or other surface forming a common boundary of two bodies or spaces. The densijcation of a particulate mass by subjecting it to Isostatic pressing nominally equal pressure from every direction. In high shear mixers and agglomerators, independently h i v e head driven high speed rotating tools which extend into the particulate mass and cause additional turbulent motion of the particles as well as desagglomeration in mixing and controlled destruction of agglomerates in agglomeration. (See also intensijer bar.) The area surrounding briquette pockets on the roller surLand area face of briquetters. (See also flashing, web.) During briquetting in roller presses the forward edge of a Leading edge discharging briquette. A member of the compacting tool set that determines the Lower punch powder fill level and forms the bottom of the part in a punch-and-die press. Extruder in which the die plates consist of screens or thin, Low pressure extruder perforated sheets and exert small frictional resistance during extrusion. An agent mixed with or incorporated into particulate Lubricant [n.] matter or applied to the tooling to facilitate pressing and ejection of a compact, tablette, or extrudate. See second meaning of aggregate. Lump [n.] Also mandril. A metal bar that serves as a core around Mandrel [n.] which material may be bent, cast, forged, molded, or otherwise shaped. (See also core rod.) Sometimes used to describe a particle which has been Marum [n.] spheronized. See spheronizer. Original (Japanese)name. Marumerizer [n.] A technology of powder metallurgy by which powders of Mechanical alloying metals, that cannot be combined in molten stage, are mixed and compacted to form the alloy. Medium pressure extruder See pellet mill. A method by which molten substances are converted Melt solidification into particulate solids by cooling droplets of the melt. (See also beading, pastillation, prilling.) A method by which small portions of liquids, particulate Microencapsulation [n.] solids, or gases are enclosed by a shell (membrane, capsule) to form a dry, free flowing product often with spherical particle shape. The capsule shell may provide specific product characteristics (e.g. dispersibility, solubility). Trle formation of small agglomerates, usually not larger Micropelletizing in.] than 3 mm, by growth agglomeration. (See also pelletizing.)
4 C/ossay of Agglomeration Terms
Mixer agglomeration Mixing tool
Mix-Muller [n.] Mold [n.] Muller [n.]
Multiple pressing Near point
Nip [n.]
Nip angle
Nodulizing [n.]
Nozzle [n.] Nucleus [n.], Nuclei [pl.]
Oil agglomeration
One pot processor
Open pore Orifice [n.]
&tation and growth agglomeration in powder mixers. Any of a large number of differently shaped tools that are attached to a rotating shaft and cause irregular movement in a particle bed. See Muller. See die. Also Mix-Muller. Originally, a device that used a heavy stone roller to grind and/or mix particulate solids. Today, a blender with one, two or four large metal rollers that mix and knead (densify) material. Often used prior to pressure agglomeration. (See also pan grinder.) A method of pressing whereby two or more compacts are produced simultaneously in separate die cavities. Area at which two particles approach each other closely enough for a binding mechanism to become effective. (See also coordination number.) In roller presses and pellet mills, converging space (volume) between two counter-rotating rollers and, respectively, the pressure generating device and the extrusion surface. (See also nip angle.) In rollerpresses, radial angle defining the line on the roller surface at which the speed of the particulate mass is identical with that of the roller; in extruders, the angle between the extrusion surface (e.g. dieplate) and the pressure generating device (e.g. extrusion blade, screw, roller). Formation of nearly spherical lumps (agglomerates)from a wet mixture of particulate solids by either drying or chemical reaction during tumbling; typically accomplished in dryers or rotary kilns. Also atomizer. Means for atomizing liquids. Primary agglomerate(s) consisting of only a few particles on which further growth occurs. (See also seed.) Also spherical agglomeration. Selective agglomeration of suspended particles in water by adding a bonding oil during agitation; typical in coal preparation. (See also immiscible binder agglomeration.) A batch processing vessel in which several process steps, for example mixing, agglomeration, post-treatment, and finishing, are carried out without opening the vessel during the entire processing sequence. A pore communicating with or connected to the surface of a porous body. (See also inkbottle pore.) The mouth or opening of something, for example an extrusion channel, that forms material into defined shape.
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4 Clossaty of Agglomeration Terms
Pan [n.] Pan grinder Particle [n.] Particle size Particulate [adj.] Pastillation [n.]
Pastille [n.] Pellet [n.]
Pellet mill Pelleting [n.]
Pelletizing [n.]
Pelletization [n.]
Pelletizer [n.] Pendular [adj.] Penetrating pore Pin mixer Piston press Plenum [n.]
Plow [n.] Plug flow
See clam shelling, duck billing. Also disc. An inclined rotating circular plate with low cylindrical rim for growth agglomeration. See Muller. A piece of solid material that is an entity in itself. The controlling dimension of an individual particle as determined by analysis. Of or relating to separate particles. A method of melt solidijcation by which droplets of a molten material are solidified on a cooled, moving stainless steel belt. Product of pastillation. Name for many different types of agglomerates. Most commonly used in the iron ore industry for nearly spherical agglomerates formed by growth agglomeration in pans, cones, or drums and in the animal feed industry for extrudates produced by pelleting. Often synonymous with agglomerate. Equipment for extrusion through perforated dies. Agglomeration by extrusion of plastic material or of particulate matter containing binders through bores of dies in “pelleting machines” or pellet mills. Originally, production of pellets by growth agglomeration. Today typically agglomeration by balling. Often also used as synonym for agglomeration. Typical in the (iron) ore industry; any agglomeration method involving growth agglomeration with subsequent heat induration. (See also sintering.) Usually rotating pan, drum, cone, or the like for growth agglomeration. (See also “gear pelletizer”.) Describing the liquid bridge model. A pore that connects opposite sides of a porous body, for example, an agglomerate. (See also through pore.) A stationary, cylindrical mixer using a single shaft agitator with pins. See punch-and-die press. Specially designed chamber at the bottom of afluid bed from which fluidizing gas enters the apparatus through the openings of a distribution plate. Plow shaped mixing tool. Forward movement of particulate solids to the discharge end of tumbling drums orfluid beds, caused by a continuous particle feed and optionally assisted by downsloping the drum or the application of gil plates influid beds.
4 Glossary of Agglomeration Terms
Pocket [n] Pore [n.] Pore volume Porosity [n.] Porous [adj.] Post-treatment
Pozzolan [n.]
Pozzolanic [adj.] Powder [n.] Powder metallurgy
Powder rolling Pressway [n.]
Pressure agglomeration
Prill [n.] Prilling [n.] Pug mill Punch [n.] Punch-and-die press
Radial extruder
Indentation on the surface of rollers, normally forming one half of a briquette shape. (See also cup.) An inherent or induced cavity in a particle or void space between particles within an object e.g. agglomerate. Void space (volume) in porous objects. (See also porosity.) The amount of pores (voids) in an object expressed as percentage of the object’s total volume. Possessing or “full of” pores. Any treatment of green agglomerates to modify moisture content, strength, structure, etc., by, for example, aging, drying, heating, sintering, etc. (See also curing.) Also Pozzolana. Finely divided siliceous or siliceous and aluminous material that reacts chemically with slaked lime at ordinary temperature and in the presence of moisture to form a strong, slow hardening cement. Having the properties of pozzolan. Particles of dry matter typically with a maximum dimension of less than approx. 1,000 pm. The art of producing metal powders and of their utilization for the production of massive materials and shaped objects as well as for mechanical alloying. See roll compacting. Also used in powder metallurgy for direct rolling of sheet from metal powders. In extruders, the (length of the) channel in which frictional resistance causes the extrusion pressure; the total distance material is compressed inside a die. Also press agglomeration. Agglomeration technique during which agglomerates are formed by pressure. Distinguished from tumble agglomeration. Product of prilling. In the fertilizer industry often (incorrectly!!) synonymous with agglomerate. The formation of spherical particles by solidification of melt droplets. (See also melt solidijcation, shot forming.) A paddle type mixer usually with open top, single or double shafts, and trough shaped chamber. Part of a compacting tool set which transmits pressure to the particulate matter in the die cavity. A mechanically or hydraulically actuated press in which a reciprocating piston compacts particulate matter in a die. Low pressure extruder in which part ofthe barrel consists of a screen or perforated thin sheet through which moist, plastic material is passed by extrusion blades to form extrudates; the material is extruded radially to the direction in which it is transported.
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4 Glossary ofAgglomeration Terms
Ram [n.] Ram extruder
Ram press Rim [n.] Ring die
Ring die extruder Ring roll press Roller [n.]
Roll(er) compacting
Roll(er) press Roll(er) pressing
Rope [n.] Rotary press Satellites formation
Saturation [n.]
Schugi flexowall
Scraper [n.]
Synonymous with punch. Press in which a fly-wheel powered reciprocating ram densifies and extrudes particulate solids through a long extrusion channel. Particularly suitable for elastic materials (such as peat, lignite, biomass, etc.). Also ram press, exter press. See ram extruder. Cylindrical or conical wall surrounding the circular plate of pan, disc, or cone agglomerators. A usually narrow hollow cylinder that is equipped with perforations for extrusion. See pellet mill. Special roller press with one press roller within a large ring-shaped die. (No longer used.) Also Roll. Cylindrical rotating body that is: 1.Paired with an identical, counter-rotating one in a suitable frame. This arrangement is used for briquetting, compacting, pelleting, densijication, Jaking, and granulating particulate solids. 2. Rolling close to a die plate and forces material to flow through openings, for example, in flat die pellet mills. 3. Mixing and kneading material in a cylindrical or “figure eight”-shapedbowl. (See also Muller.) Also powder rolling. The (progressive) compacting of (metal) powders in roller presses (often called “rolling mills”). (See also roll pressing.) Equipment for pressure agglomeration between two rollers. Densification between two counter-rotating rollers. (See also compacting.) In spheronization, referring to the rotating particulate material. Tabletting machine in which compacting tool sets are arranged on a rotating table (= turret). In agglomeration, the attachment of smaller solid entities, often agglomerates, to other agglomerates by binding mechanisms. (See also clustering.) Relative amount of pores in an agglomerate filled with a liquid or solid substance, as in “liquid saturation”, “binder saturation”. High speed, high shear mixer and/or agglomerator with vertical axis, adjustable mixing tools, flexible shell, flexing roller cage, and short residence time. A tool for removing build-up in agglomeration equipment. Also doctor blades.
4 Glossary of Agglomeration Terms
Screen [n.]
Screw [n.]
Screw extruder Seed [n.] Segregation [n.]
Selective agglomeration
Sheet [n.]
Shot forming Shrinkage [n.] Silo [n.]
Single action pressing Sinter [n.] Sintering [n.]
Slug [n.]
Slugging [adj.]
Slugging press
A (usually mounted) perforated thin plate or cylinder or a meshed wire or cloth fabric used to: 1. Separate coarser from finer particles or 2. Form extrudates. A mechanical device spiral in form or appearance; a conveyor working on the principle of a screw; a conveying tool in afeeder, mixer, or extruder. Also auger, worm. Extruder in which screw(s) produce the extrusion pressure. See nucleus. The desirable or undesirable separation (according to mass, shape, size, etc.) of one or more components of a particulate mass. Agglomerationof only one component of a powder mixture controlled by, for example, binding mechanism,binder, particle size. (See also immiscible binder agglomeration.) A more or less continuous band of compacted material produced in roller presses featuring smooth or shallowly profiled rollers and a gap between those rollers. Also, anything that is thin in comparison to its length and/or breadth. The solidification of a melt into little spheres in a tall form tower. (See also prilling.) A decrease in dimension. In agglomeration, usually of a compact during sintering. Converse of expansion. A trench, pit, or especially a tall cylinder (as of wood, metal, or concrete) often sealed and used for storing particulate solids. (See also hopper, bin.) A method by which a particulate mass is pressed in a stationary die between one moving and one fixed punch. Agglomerated product of sintering. Technique involving induration of green agglomerates by heat. Generally, bonding at a temperature below the melting or softening points of the main constituent of a mixture by the application of heat. (See also heat bonding.) Large, flat faced compressed disk prepared for the purpose of stabilizing the mixture of ingredients in the pharmaceutical industry. 1. Producing slugs in a sluggingpress. 2. Influid bed technology, the slow, upward movement of large, somewhat cohesive masses of particulate solids. Punch-and-die press for the production of large tablettes or slugs which are crushed to obtain granulate. Mostly in the pharmaceutical industry. (See also tabletting machine.)
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4 Glossary of Agglomeration Terms
Spherical agglomeration Spheronizing [n.]
See oil agglomeration. Rounding of soft, plastic (usually green) agglomerates (usually extrudates) in a spheronizer. Vertical drum with rotating bottom for spheronizing. Spheronizer [n.] (See also Marumerizer.) Characteristic parameter for roller presses; defined as Specific force pressing forcelactive roller width. The formation of granular solids or small spherical agSpray drying glomerates by dispersing a liquid or slurry in droplet form at the top of a tower and evaporating the liquid in the presence of drying gases. The formation of small, spheroidal agglomerates in a Spray granulation &id, circulating, or spouted bed by spraying a solution, slurry or melt onto the particles; often combined with drying. Stabilize [vb.] Avoid segregation by agglomerating a powder mixture. An elongated body; synonymous with uncut extrudate. Strand [n.] Strip [n.] See sheet. Surface equivalent diameter The diameter of immaginary monosized spherical particles, calculated from the mass related specific surface area, in m2/g,of a particle size distribution, that produce the same specific surface area as the powder. The state ofparticulate solids which are uniformly mixed Suspension [n.] with but undissolved in a fluid. Also Tablet. A compressed agglomerate made of particuTablette [n.] late solids, specifically, in pharmacy, a small compact of a medicated particulate formulation usually in the shape of a disc or a flat polyhedral body. (See also briquette.) The process of forming tablettes. Tabletting [n.] Tabletting machine Compaction press for the manufacture of tablettes. A tall tower with enlarged conical bottom. Tall form tower To drive in or down by a succession of light or medium Tamp [vb.] blows; predensify. See tamp. Tamper [n.] A receptacle for holding, storing, or transporting liTank [n.] quids. Using heat to fuse particulate solids into agglomerates. Thermal agglomeration (See also heat bonding, sintering.) The property of various materials to become fluid when Thixotropy [n.] disturbed (as by shaking, vibration, pressure, etc.). Materials tending to exhibit Thixotropy. Thixotropic [adj.] See penetrating pore. Through pore Parts making up the compacting tool set of a tabletting Tooling [n.] machine.
4 Glossary of Agglomeration Terms
Tower [n.]
Trailing edge True density Tumble agglomeration
Turret [n.] Updraft [n.] Upper punch Wear [n.] WDG Web [n.] Wet agglomeration Withdrawal process
Worm [n.] WSG
In spray drying or prilling, a cylindrical structure in which liquid droplets that were formed at the top solidify during their descend in a gas atmosphere with suitable temperature. During briquetting in roller presses the back edge of a discharging briquette. The mass of the unit volume of a solid material that is free of pores. Agglomeration technique during which agglomerates are formed by growth during tumbling; synonymous with growth agglomeration. (See also coalescence.) Rotating table carrying the compacting tool set of some tabletting machines. Upward flow of gas, for example through a particle bed. Member of the compacting tool set that closes the die and forms the top of the part being produced. Similar to erosion, but usually refers to the surface of a solid body such as a part of machinery. (Easily) Water Dispersible Granulate. Thin Jlashing surrounding briquettes made in roller presses; caused by the land area. Tumble and growth agglomeration in which the major binder is a liquid. Operation of some tablettingpresses by which the die descends over a fixed lower punch to reduce density variation in the tablette and facilitate removal of the compact. See screw. (Easily) Water Soluble Granulate. (See also instant.)
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
5
Agglomeration Theories The distinguishing characteristic of size enlargement by agglomeration is the formation of larger entities from particulate solids by sticking particles together by short range physical forces between the particles themselves or through binders, substances that adhere chemically or physically to the solid surfaces and form a material bridge between the particles. The components of an agglomerate are often widely disparate and, except if matrix binders are applied (see Section 5.1.2) or after shrinkage during sintering (see Sections 5.3.2 and 9.1), void spaces are present between the particles forming an agglomerate. The above definition of size enlargement by agglomeration sets this unit operation of Mechanical Process Technology apart from other grain growth techniques, particularly crystallization whereby a uniform solid body grows from a mother liquor by forming a structure in which the same atoms and/or molecules have a regularly repeating internal arrangement. As will be shown later (see Section 7.4.6), agglomeration may also play a role during crystallization if nuclei or crystallites adhere to each other in the mother liquor and form macroscopically amorphous, porous structures. Size enlargement by agglomeration is also distinguished from another particle forming technique, melt solidification. In this process a molten material is divided into droplets or extruded through die plates and cut into cylindrical pellets. The product is then solidified by cooling. The melt may be directly synthesized, as in the case of urea prilling, or obtained by heating the solid. In the latter case, similar to the meaning of the term granulation, melt solidification can be a particle size reduction, if large chunks of a solid are melted and then divided into small droplets or extrudates that are solidified, or a particle size enlargement, if a powder is melted, divided into relatively larger droplets or extrudates, and solidified. Droplet formation can be by spraying through a number of differently designed nozzles (see also Section 7.4.3)or by dividing a liquid stream either naturally, by mechanical means, or by gas or liquid impingement. Solidification is accomplished during the free fall in a cooling tower (Fig. 5.la) which results in spherical “prills” (Fig. 5.2), on a cooled stainless steel belt (Fig. 5.lb) yielding flattened “pastilles” (Fig. 5.3), or in water (Fig. 5 . 1 ~ producing ) cylindrical extrudates (underwater granulation/peIleting).
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Fig. 5.la
Fig. 5.1 b
Fig. 5.1 Schematic representations o f the three most c o m m o n melt solidification processes. (a) Prilling [B.42], (b) pastillation (courtesy Sandvik, Totowa, NJ, USA), (c) underwater granulation/ pelletizing (courtesy Gala, Eagle Rock, VA, USA).
Agglomeration Theories
Fig. 5.2 Photograph o f urea prills (courtesy KaltenbachThuring, Beauvais, France).
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Fig. 5.3 Photograph of the discharge end o f a pastillator also showing pastillated product (courtesy Berndorf Band, Berndorf, Austria).
Since most of the commercially produced urea for fertilizer applications is prilled by the tower melt solidification process and urea is one of the most important nitrogen providing fertilizers, farmers and suppliers often wrongly name all spheroidal agrochemicals “prills” even if they were produced by true agglomeration processes, for instance on discs or in drums (see also Section 7.4.1). In the following only size enlargement by agglomeration will be covered.
5.1 The Development of Strength of Agglomerates
Fig. 5.4 is the random cut through part of an agglomerate. Obviously, in reality, the structure is three dimensional. In such a body strength can be caused in several ways. In Fig. 5.4a the entire pore space is filled with a matrix binder. Typical examples of agglomerates held together in this manner are concrete, where the matrix between the aggregate particles consists of hardened cement (Fig. 5.5), or road surfaces, in which bitumen occupies the volume between crushed stone (Fig. 5.6). Fig. 5.413 generally looks very similar to 5.4a but shows an agglomerate structure in which the entire void volume is filled with a liquid that wets the solid particles. If concave menisci form at the pore ends on the surface of the agglomerate, a (negative) capillary pressure develops within the pores which affords strength to the body. As explained in Fig. 5.7, depicting a series of situations representing different liquid saturations in particulate bulk solids or of agglomerates, distinct distribution models exist which depend on the amount ofliquid in the structure. The term liquid saturation is defined as the percentage of total void space that is filled with the liquid. A precondition for cohesiveness of particulate solids due to the presence ofliquid is that the liquid wets the solids. Although, depending on the application, other liquids may be used to totally or partially fill the voids between particulate solids, in agglomeration water is most commonly used. Referring to Fig. 5.7, absolutely dry particulate bulk solids (Fig. 5.7a) are non existent under normal atmospheric conditions. The water molecules of adsorption layers (Fig.
5.7 The Development of Strength of Agglomerates
Fig. 5.4 Random cut through part o f an agglomerate or a particulate bulk solid mass and explanations o f how strength may be caused. (a) Pore volume filled with a matrix binder. (b) Pore volume filled with a wetting liquid. (c) Liquid bridges at the coordination points. (d) Adhesion forces at the coordination points.
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Fig. 5.7 Schematic representations o f different liquid saturations in particulate bulk solids o r o f agglomerates. (a) Dry, (b) adsorption layers, (c) liquid bridges (“pendular” state), (d) transitional (“funicular” state), (e) fully saturated (“capillary” state), (f) droplet.
5.7b) that quickly form on the solid surfaces are bonded so strongly that they are not mobile and, therefore, do not cause “liquid saturation” or moisture content which can be measured with “normal”laboratory equipment. However, as will be shown later (see Section 5.2.1), adsorption layers can participate in the development of strength by enhancing molecular (van-der-Waals) forces. With small amounts of “free” water, i.e. producing moisture contents of little more than a few tenth of a percent and, correspondingly, very small “saturation”, liquid bridges begin to form at the contact points between particles. With increasing moisture content or saturation liquid bridges form at all coordination points (see below) in the structure (Figs 5 . 4 ~and 5 . 7 ~ )Further . increase in liquid saturation produces a transitional situation in which liquid bridges and void spaces that are filled with liquid coexist (Fig. 5.7d). The theoretically highest saturation (100 %) is reached, when all voids within a bulk mass or an agglomerate are filled (Fig. 5.4b) and concave menisci are formed at the pore ends (Fig. 5.7e). Beyond complete saturation, liquid droplets shaped by the surface tension may enclose solid particles (Fig. 5.7f). Slurries, bulk particulate solids containing an excess amount of water, are shapeless. All above models exist in wet agglomeration, methods that are based on the processing of slurries, suspensions, or solutions (see Sections 7.4.3 and 7.4.6) or the presence of liquids as binders (see Section 7.4).
5.1 The Development of5trength of Agglomerates
Fig. 5.4d depicts the action of solid bridges or forces at the coordination points of a particle with other particles surrounding it in the agglomerate structure. Coordination points are points of contact with other particles and near points, areas of the particle surface which are so close to a neighboring particle surface that significant adhesion forces act or bridges can form. The coordination number is the average of the sum of all contact and near points of each particle with others surrounding it in a particular agglomerate structure (see also Section 5.3.1). Typical examples of agglomerates bonded in this manner are “natural” aggregates of very fine particles which are held together by molecular forces or agglomerates with solid bridges at the coordination points which have formed during drying of originally wet agglomerates by recrystallizing materials which had been dissolved in the liquid. 5.1.1
Binding Mechanisms
The binding mechanisms of agglomeration were first defined and classified by H. Rumpf and his co-workers (see Chapter 1).According to Tab. 5.1 they are divided into five major groups, I to V, and several subgroups (see also Fig. 5.8).
Tab. 5.1
Binding mechanisms of agglomeration
I. Solid bridges 1. Sintering 2. Partial melting 3. Chemical reaction 4. Hardening binders 5. Recrystallization G. During drying: a) Recrystallization (dissolved substances) b) Deposition (colloidal particles) II. Adhesion and cohesion forces 1. Highly viscous binders 2. Adsorption layers ( < 3 nm thickness) I I I . S u r f c e tension and capillary pressure 1. Liquid bridges 2. Capillary pressure IV. Attraction forces between solids 1. Molecular forces
a) Van-der-Waals forces b) Free chemical bonds (Valence forces) c) Associations (nonvalence);hydrogen bridges 2. Electric forces (electrostatic, electrical double layers, excess charges) 3. Magnetic forces V. Interlocking bonds
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(4 Molecular Forces (E.1) Electrostatic Forces ( E . 2 ) Magnetic Forces (E.3)
(e) Interlocking(Y)
(f) Matrix Binder (1.3,1.4,E.l) Capillary Forces (Conglomerates Saturated with Liquid ) (El2 )
Fig. 5.8 Pictorial representation of the binding mechanisms o f agglomeration.
I. Solid Bridges If the temperature in a disperse system rises above approximately two- thirds of 1. the melting temperature or softening range of the solids, diffusion of atoms or molecules from one particle to an other one occurs at the points of contact. The solid bridges that develop with time are called sinter bridges. The velocity of diffusion depends on temperature, size of contact area, and contact pressure. It increases with rising temperature, larger contact area, and higher pressure. Heat can be introduced from an external source or created during agglomeration by friction and/or energy conversion (see also Section 9.1). 2. At the contact points of particles, roughness peaks may melt due to heat caused by friction and/or pressure. In such cases, liquid bridges develop which solidify quickly due to the large heat sink provided by the solids themselves. This mechanism, called partial melting, is often responsible for unwanted agglomeration and caking of substances with low melting point or softening temperature. 3./4. The formation of solid bridges by chemical reaction or hardening binders depends only on the participating materials, their reactivity, and their tendency to harden. Elevated temperature and/or pressure may improve the reaction and result in a modified, potentially stronger bridge structure. These binding mechanisms are often activated by moisture.
5.I The Development of Strength of Agglomerates
5. Temperature fluctuations can result in recrystallization and bridge formation within otherwise stable or sealed bulk particulate solids. The temperature induced physical recrystallization of some substances may extend through the interface at contact points causing solid particles to grow together. Salts or mixtures of salts that contain some free moisture may cake when exposed to varying temperatures, even if the amount of moisture is very small and the material is packed in airtight enclosures. This is because, often, more salt dissolves at higher temperature which recrystallizes if the temperature drops, forming crystal bridges between the solid particles in the bulk. During temperature fluctuations caused, for example, by day and night or seasonal differences, this is a continuing process that will, with time, result in more and stronger caking (see also Section 5.5). 6. The more common method of forming solid bridges by recrystallization of dissolved substances or deposition of suspended colloidal particles is to evaporate the liquid. The strength of crystal bridges depends not only on the amount of the dissolved and recrystallizing material but also on the speed of crystallization. At higher crystallization rates a finer bridge structure is formed which results in higher strength (see also Section 5.2.2). Colloidal particles form solid bridges if the liquid between the macroscopic particles of a disperse system consists of a colloidal suspension. During drying the colloidal particles concentrate in diminishing liquid bridges and the pressure caused by the liquid’s surface tension compacts the colloidal particles. After complete evaporation of the liquid, solid bridges remain which are made up of colloidal particles. Adhesion in the bridges is mostly caused by molecular forces which may be enhanced by electrical and magnetic effects (see Group IV below). 11. Adhesion and Cohesion Forces 1. If highly viscous binders, such as bitumen, honey, pitch, tar, etc., are applied, adhesion forces at the solid-binder interface and cohesion forces within the viscous material can be fully exploited for agglomerate strength until the weaker of the two fails. Highly viscous binders are often used as matrix binders (see also Sections 5.1 and 5.2.1). 2. Most finely divided solids easily attract free atoms or molecules from the surrounding atmosphere. The thin adsorption layers thus formed are not mobile. However, they can contact and penetrate each other. It can be assumed that molecular forces can be fully transmitted if the adsorption layer is thinner than 3 nm. Such forces are often high enough to cause deformation of solid particles at the contact points (Fig. 5.9) thus increasing the contact area and, therefore, strength of the bond between adhering partners. The application of external forces and/or elevated temperatures may increase the contact area and strength further [B.14, pp 97-1291.
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Fig. 5.9 Viscoelastic deformation at the contact point between two glass spheres due to molecular attraction.
Adsorption layers may also increase adhesion forces if the layers do not contact or touch each other (see Section 5.2.1). 111. Suface Tension and Capillary Forces One of the most common binding mechanism of wet agglomeration is liquid bridges at the coordination points between the particles forming the agglomerate. Liquid bridges can develop from free water or by capillary condensation. They are often the precondition for the formation of solid bridges (see above, 1.G). If the entire pore volume between the particles of a disperse system is filled with a liquid and concave menisci form at the pore ends on the surface of the system, a negative capillary pressure exists in the interior causing strength. Wet agglomerates are very often bonded by a combination of the above two mechanisms. In that case partial volumes exist which are completely filled with the liquid while in others liquid bridges prevail. Technically it is almost impossible to attain 100 % saturation because there is a high probability that during the agglomeration process air is trapped in some pores. IV. Attraction Forces Between Solid Particles Attraction forces between solid particles are often the cause for unwanted agglomeration: bridging, caking, coating, and build-up. The most important binding mechanisms in this category are molecular, electric, and magnetic forces (Fig. 5.10). At extremely small distances between the adhesion partners these forces can be very high but, due to their short range effect, they diminish quickly with increasing distance at the coordination points. Since particles approach each other with roughness peaks (Fig. 5.11) and the absolute roughness of smaller particles is less than that of larger ones, the adhesion probability, i.e. the chance of such particles moving closer together, increases as powders become finer. High adhesion forces are obtained if fine and ultrafine or nano-sized particles are involved.
5.1 The Development of Strength of Agglomerates
Molecular Forces van-der-Waals Forces
1.a.
/
\ Valence Forces at newly created surfaces (Recombination Bonding)
1. b.
Nonvalence Association e.g.. Hydrogen bridges betweenoxygenand hydroxyl radicals a: Association - H interacts with nonbinding electron pair of oxygen b: Water molecules intensify association c: Bridging by nonvalence association of bipolar (water) molecules
Electrostatic Forces
(-I---
3. I S
N y - - - [ N ] ,
Fig. 5.10
Magnetic Forces
-_-
I
Attraction forces between solid surfaces o r particles.
1.a) Van-der-Waals forces are naturally occurring forces at the surfaces of all solid materials. The molecules, atoms, or ions in the interior of a solid interact with each other such that they retain their relative, equilibrium positions. At the surface of, for example, a particle, the molecular forces that are directed to the outside are not satisfied and produce a force field that interacts with that of other particles.
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Fig. 5.11 Model depicting the true situation at a coordination point between two particles. Roughness exists o n all real particles. 09; is the representative distance between the particles.
Then, van-der-Waals forces arise because of the electric polarization induced in each of the particles by the presence of the other ones. Forces are in the order of 0.1 eV and decrease with the sixth power of the distance between the partners. The maximum range of the van-der-Waalsinteraction is in the order of 100 nm which, compared to chemical bonds (valence forces), is large. 1.b) During size reduction (comminution) bonds between the atoms and molecules of a solid are stressed and ultimately part creating new surfaces. Immediately after separation, unsatisfied valences exist on these newly created surfaces. Normally, the free radicals quickly combine with atoms and molecules from the surrounding atmosphere, thereby becoming neutralized. However, conditions exist where either the newly created surface area is so large at any given moment that the number of atoms and molecules that are available in the immediate vicinity is too small to satisfy all the available valences or mobile, reactive atoms and molecules that could neutralize the free radicals are not present. In those cases, the valences themselves may recombine if newly created surfaces come close to each other. Such recombination bonding occurs during fine grinding due to the first mechanism, eventually resulting in an equilibrium between size reduction by comminution and size enlargement by agglomeration (“grindinglimit”) (see also Section 5.5). Recombination bonding also occurs during high-pressure agglomeration (see Section 8.1). If brittle particles break in the compact under the influence of high forces, new surfaces are created within a densifying mass of particulate solids where the possibilities are limited to satisfy the exposed free valences with gaseous atoms or molecules. At the same time, high compaction forces cause particle surfaces, including the newly created, reactive ones, to approach each other so closely that, after some lateral movement of the fractured pieces, free valences recombine, forming strong, permanent bonds. 1.c) Nonvalence associations of certain molecular groups can also cause bonding and provide strength to a particulate bulk solid. One important phenomenon, hydrogen bridges, is, for example, the prevailing, naturally occurring binding mechanisms between organic macromolecules in coal. Hydrogen bridges form if a hydrogen atom is bonded to a strongly electronegative atom, such as oxygen in a typical OH group, and the hydrogen atom interacts with the non-binding electron pair of another electronegative atom, e.g. oxygen of a COOH group. Water
5.1 The Development ofstrength of Agglomerates
(H - 0 - H) intensifies this association and the bipolar molecules can also form nonvalence association bridges which participate in the development of strength (see also Fig. 5.10, l.c, b and c). 2. At their surfaces, ionic solids possess an unsatisfied electrostatic field which is superimposed on that produced by the van-der-Waals forces. The strength of this field diminishes rapidly with distance from the surface and is soon negligible. However, this external electrical field can induce a dipole or a higher order moment in the charge distribution of the molecules in an adsorbed layer thus participating in adhesion. When two solid surfaces come in contact with each other, electrostatic forces of attraction arise as a result of the contact potential, forming electrical double layers. The physical cause for the transfer of electrons when two solid bodies come into contact is the difference between their electron work functions. Electrons migrate from the body with the smaller work function to the one with the larger one until equilibrium is reached (double layer). The action of this mechanism is permanent. Particles also can be charged by providing electrons from external sources (e.g. spray electrodes). Such excess charges can also cause attraction (or repulsion). Because of the field character of this binding mechanism, strength is independent of particle size. Also, the strength due to excess charges is very small and the charges tend to equalize (disappear) with time. Therefore, this mechanism is, in most cases, only significant for initial, temporary bonding (typical application: electrostatic precipitators/filters). As mentioned before, it is also possible that bonding between two oppositely charged solid surfaces is caused by the nonvalence association of bi- or multipolar molecules or radicals. Hydrogen bonding is a well known example. 3. The attraction mechanism caused by magnetic forces is similar to that of electrostatic forces. The presence of magnetic forces is limited to ferromagnetic particles although, recently, based on the understanding of the nature of magnetism, it was reported that it is now possible to engineer completely man-made plastic materials with magnetic properties. The latter may enlarge the applicability of this mechanism in the future. V. Interlocking Bonds Normally, interlocking bonds occur if the particulate solids have the shape of, for example, fibers, threads, or lamellae that twist, weave, and bend about each other or entangle during agglomeration. Sometimes interlocking bonds of elongated, fibrous additives are used to strengthen agglomerates which are otherwise too weak (see also Section 5.3.1). In high-pressure agglomeration, another interlocking mechanism may occur if a mixture of rigid and plastic materials is compacted. In this situation, the plastic component flows into recesses and, more generally, envelopes the exterior structure of harder particles, thus producing a strong structural bond that resembles the effect of a matrix binder (see also Section 8.1).
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Fig.s 5.8 and 5.10 describe pictorially the binding mechanisms that were reviewed above. It should be pointed out that only the two-dimensional situation at one coordination point between two particles or solid surfaces is shown. In reality, each particle has many interaction sites (coordination points) with other particles in the three-dimensional structure. It should be further understood that in typical particulate bulk solids and agglomerates large numbers of particles are present per unit volume (see also Section 5.3.1) and participate in bonding due to the binding mechanisms presented above. With exception of capillary and matrix bonded structures of particulate solids, it is unlikely that only one binding mechanism acts on all the coordination points within a mass. If molecular and electric forces as well as liquid bridges and the solid bridges, resulting from the latter by one or the other of the mechanisms that were discussed above, are considered, it must be assumed that the effect of each binding mechanism is different at essentially every coordination point due to varying microscopic surface structures and distances at each interaction point (see also Section 5.2.1). 5.1.2
Binders, Lubricants, and Other Additives If size enlargement by agglomeration is desired and the correct agglomeration technique is selected, many of the binding mechanisms described in the previous Section 5.1.1 are inherently available or can be activated. Under certain conditions, some binding mechanisms also act naturally to produce undesirable agglomeration phenomena. Generally speaking, if agglomeration is wanted, means to enhance the available binding mechanisms must be developed and applied, while the effect of binding mechanisms must be eliminated or reduced to avoid unwanted agglomeration. Both aspects will be covered in much more detail in Section 5.5. As will be shown in Sections 5.2 and 5.3, particle size of the particulate solids plays an important role in agglomeration. While the surface area of particles, the interface at which all binding mechanisms act, decreases with the second power of particle size, volume and, therefore also, mass, the most important particle properties which result in forces that challenge adhesion and cause separation of bonds, diminish with the third power of the particle size. If the particle size reaches a few pm or is in the n m range, the natural adhesion forces dominate and particles which contact each other or come into close proximity adhere to one another. This phenomenon can not be economically eliminated so that very fine particles always adhere and form loose agglomerates which may be desirable or undesirable (see Section 5.5). Naturally available adhesion tendencies can be considerably increased if moisture is added during the agglomeration process. Application of external forces can contribute to the enhancement of inherently present binding mechanisms. Depending on the magnitude and nature of these forces, improved structure (by shear and low to medium compression) or plastic deformation and brittle breakage (due to high external forces) can occur. Plasticity, an often preferred response to external forces that results in high agglomerate strength (see Section 8.1), increases with many solids if the temperature of the material rises. Therefore, hot densification is often a desirable agglomeration technology, particularly for minerals and metal bearing materials.
5.I The Development of Strength ofAgglomerates
Since all binding mechanisms rely on molecular interactions on and between surfaces or interfaces, the structure and distance at these points is of great importance for the ability of powders to agglomerate. Often, the presence of ultra fine particles facilitates size enlargement of coarser particulate matter. Fines that are suspended in a liquid accumulate during drying at coordination points and form solid bridges which are bonded by molecular forces. Dry fines may fill areas with high surface energy, such as holes and depressions, thus reducing the effective distance between larger particles and increasing the attraction force (similar to the influence of adsorption layers; see Section 5.2.1). With other materials, e.g. certain coals and chemicals with low softening or melting points or containing such components, mechanical energy, introduced by dynamic forces, compression, or shear and converted into thermal energy, activates the inherently available binding properties. Under this influence, momentary softening and melting can occur upon contact at minute roughness peaks which, after almost instantaneous solidification, produce a small solid bridge between the powder particles. Similar mechanisms are responsible for the bonding of soluble materials in the presence of moisture. Mechanical energy converted into heat or the direct external supply of thermal energy result first in dissolution and then in recrystallization at the coordination points. The larger the number of coordination points in a unit volume (increasing with decreasing size of the agglomerate forming particles), the higher will be the strength of the agglomerated part. In spite of the availability of all these “natural” binding mechanisms and the various possibilities to enhance them for the desirable production of agglomerates, sometimes no economic method can be found to process a specific material and form a product with sufficient strength. Grinding the particulate solid to a sufficient fineness for strong molecular bonding and/or heating it to high enough temperatures that result in either sufficient dissolution for recrystallization, plasticity for large area contact and bonding, or sintering and melt solidification, would be too expensive and, therefore, prohibit economic processing. In those cases where no bonding can be achieved, particle size is relatively large, or specific product characteristics must be obtained, binders, mostly for higher strength, lubricants, mostly for improved density and structure, and other additives, which produce special properties, can or must be used. Binders are components which are added prior or during agglomeration to increase the strength of the agglomerated product at otherwise unchanged processing conditions. They can affect strength directly or after a curing step. Binder selection depends on many considerations which are specific for the particular application. They must be compatible with the materials to be agglomerated and the proposed uses of the product. For example, for pharmaceutical and food applications only officially approved materials may be used and for the agglomeration of metal bearing dusts which are intended for recirculation into steel mills, sulfur containing binders are normally prohibited. Many such limitations can be defined for specific materials and applications.
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For those reasons, binder development can not be generally treated. Rather, each individual case must be evaluated separately. However, a few common characteristics can be considered before starting a specific development program. Binders can be divided into inorganic or organic components and their distribution in the agglomerate structure may be in the form of films and bridges or a matrix. Film or bridge type additives are normally fluids which coat particles or are drawn to the coordination points where they form bridges. If applicable, only relatively small additions are required; porosity of the agglomerates as well as their freely accessible surface area (including internal surfaces, see also Section 5 . 3 . 2 ) are only insignificantly changed. Water is the most well known film and bridge forming binder. Matrix forming binder components, on the other hand, more or less fill the entire pore space and, therefore drastically reduce porosity and accessible surface area. Cement is a typical matrix forming additive. Water or other liquids may act as matrix binder in fully saturated wet agglomerates (capillarystate, see Section 5.1). However, this is only a temporary binding mechanism and the liquid will disappear naturally or during a posttreatment step (see Section 7 . 3 ) so that pores open up and surfaces become accessible again. Still other binders will react chemically with different components of the additive mixture or with some or all of the materials to be processed. Such reactions can result in high strength products with, for example, waterproof bonds. Tab. 5 . 2 lists examples of organic and inorganic binders that were previously employed in agglomeration. It shows that many different substances and materials have already been used. Commonly available and applied binders are printed in italic letters. Investigation of by-products or wastes as binders may result in the discovery of cheap and very acceptable additives. For example, molasses, a by-product of sugar making, is an excellent and nutritionally beneficial binder for animal feed and organic wastes can be incorporated in fertilizers as nutrient and binder. Binder development must take into consideration the availability of the substance at the point of ultimate use and over time. Normally, evaluations begin at a vendor facility with traditional and/or new materials that are available at that time and location. Often such developments become unacceptable when during the final cost analysis the binder turns out to be excessively expensive due to the need for its transportation to the location of the planned industrial agglomeration facility. A recent example for the “drying out” of a binder source with time is Brewex, the somewhat modified by-product of a specific beer brewing technology. The material, a liquid starch material, which was available at reasonable cost in the USA and quickly enjoyed a relatively widespread use, had to be taken off the market when the beer brewing technology changed and the by-product source disappeared. In such a situation, the operator of an already established agglomeration process has to search for a replacement binder with similar properties, acceptable price, and good availability to be able to remain in business and continue to be profitable. Therefore, unless there is a safe and unlimited binder supply for a particular application, it is prudent to continuously observe the market, evaluate new developments, and be ready for change.
5.1 The Development of Strength of Agglomerates Examples o f organic and inorganic binders that were previously employed in agglomeration (in alphabetical order).
Tab. 5.2
Organic binders
Inorganic binders
Albumates (Albuminates) Alcohols Alcotac" Alginates AsphaltlAsphalt EmulsionslReJned Asphalts Brewex Carnauba Wax Caseins CAFA (Chemically Activated FlyAsh) Cellulose Compounds Chicken Manure C M C (Carbo-Methyl Cellulose) Coal Tar, Pitch, and Creosote Coke Oven Tar Covol Crude Oil Dextnne Drying Oils Elveron" Fir Tar (Pine Wood Tar) Fish Waste Gelatine Gilsonite@ (Natural Asphalt) Glues Gums (e.g. Arabic) Humates (Humic Acid) Lignins (Liquor and Powder) Lignite Lignite Tar Lignosulfonates Maltose Molasses Orimulsion" Paper Pulp (from secondary paper making) Paraffin Peat Petroleum Pitch Peridura Pittsburgh Flux Polyvinyl Alcohol ( P V A ) Resins (Natural and Synthetic) Rosin Sawdust Seaweed Slaughterhouse Refuse Starches, pregelatinited (e.g. Corn, Potato, Tapioca, Wheat) Straw (Ground or Pulped) Sucrose
Alkali Silicates (e.g. Sodium, Potassium) Alum Alumina (see Colloidal ..) Attapulgite (Clay) Bentonite (Montmorillonite Clay) Caustic Soda Cements (e.g. Portland, Slag) Clays Colloidal Alumina, Silica, etc. Dolomite Fuller's Earth Gypsum Lime Lime Hydrate (often as hardener) MagnesialMagnesium Oxide Magnesium Chloride Metal Swarf Metal Fibers Plaster of Paris Salts Silica (see Colloidal ..) Silicates (see Alkali Silicates) Sodium Borate Sulfates (e.g. Copper) Water
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Sugars Tanning Liquors (Tannic Acid) Terravest (Liquid Polybutadiene Emulsion) Thermoplastic Powders Tree Sap Vegetable Pulp Waxes and Wax Tailings Wood Pulp
Lubricants may be either liquid or solid additives (Tab. 5 . 3 ) . They reduce the coefficient of friction between the particles of a bulk mass and, therefore, result in a somewhat higher agglomerate density or lower porosity, E . According to the relationship k E = j~ (see Section 5.2.1) additional adhesion sites (characterized by the coordination number, k ) are activated by which increased agglomerate strength is expected. Tab. 5.3
Examples of some typical liquid and solid lubricants
Liquids
Solids
Glycerine Oil/Water Emulsions Water Dry Starch Molybdenum Disulfide Stearates (Metallic, e.g. Magnesium Stearate) Talc Etylene Glycol Oils Silicones
Graphite Paraffin Stearic Acid Waxes
In pressure agglomeration, lubricants also reduce the coefficient of friction between the material to be compacted and the tooling. This results in a more uniform structure of the compact and in less density variation (see also Section 8.2). During ejection from a die or release from a mold lower forces are required for separation and, therefore, higher survival rates are obtained. Development and selection of lubricants must apply the same considerations as discussed for binders above. While, in some cases, binders may be valuable ingredients of the final product or disappear during post-treatment, lubricants are almost always contaminants. For this reason and to keep costs down, the most acceptable lubricants are those that are effective in very small amounts. In former times lubricants were mixed into the formulation prior to, for example, tabletting, even if the lubricant was only meant to reduce the friction between the solids and the tooling. Newer developments came-up with applicators that deposit the lubricant on the surfaces of the tooling thus decreasing amount, cost, and product contamination considerably (see also Section 8.4.3).
5. I The Development of Strength of Agglomerates
With the growing importance of size enlargement by agglomeration for the manufacturing of engineered products (see also Chapter 12), many other additives are used as “functional” components. Particularly in the food industry (Fig. 5.12), but also in other, by the public less well known applications, materials with specific, predetermined, and controlled properties are formulated from particulate ingredients and then agglomerated to yield consumer products that feature desirable characteristics. For example, convenience foods can be easily and quickly used such as “instant” soups, sauces, and drinks or products that were recombined from fine, ground food stuffs, contain already the correct amount of spices as well as other aromas, and, after preparation, feature a texture and taste that pleases the palate. Functional foods, also called designer foods on the other hand, have been treated to eliminate unhealthy ingredients, such as fat. They are then recombined with additives that replace the removed components without sacrificing the “mouthfeel” that is expected from the untreated food. Functional foods may also contain dietary additives that make a product particularly acceptable for a special group of often chronically sick people, such as, for example, diabetics. For those reasons, the market for food additives is growing overproportionately, largely due to the increasing production of more nutritious and better balanced designer foods whereby calorie reduction agents are the largest segment. Fun foods are the wide range of modern sweets and snacks where mostly sugar and fat based binders are applied to obtain agglomerates or, for example, bar shaped products from a multitude of ingredients for the consumer martket. A more complete coverage of these fast growing technologies is far beyond the scope of this book. They are mentioned to demonstrate the wide range of applications of agglomeration in areas that are not immediately recognized as common uses of the unit operation. Still other additives are more generally introduced to overcome problems caused by the need to obtain sufficient strength for packaging, handling, and storage. Special components may have to be added to the formulation which assist in the break-up of the agglomerated product when it comes into contact with water or other liquids. Such materials are commonly starches or their derivatives and other compounds that swell when absorbing liquids. Fibers may be added for a number of reasons, for example, as a dry binder, a structural component, a moisture absorbent, and a conduit for liquid. Mixtures of carbonates will produce carbon dioxide with water and result in the well known effect of effervescence. Fig. 5.13 shows schematically the influence of wicking by fibers or swelling of suitable components on the dispersion of agglomerates in liquids. Both effects may be also used together. Produced from renewable resources, organic fibers and their derivatives have a wide range of functional applications. In the pharmaceutical and food industries, the presently best known cellulosic additive is microcrystalline cellulose (MCC). It is obtained from wood cellulose by acidic hydrolysis. The product does no longer contain lignins, hemicelluloses, or other impurities and is bleached to produce a high degree of brightness. In a cellulose molecule, approx. 15,000 D-glucose units are connected in a 1.4-pglucosidic linear arrangement to form a filamentary molecule. Individual molecules of Other Additives
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Fig. 5.12
A few examples o f modern food products that were manufactured using agglomeration technologies. (a.1-a.4) Cereals and cereal bars (courtesy Kellog Co., Battle Creek, MI, USA); (b) cubed and granulated beef bouillon, both with "instant" (see Section 5.4) characteristics (courtesy Borden Foods/Wyler's, Columbus, OH/Chicago, IL, USA); (c.1-c.3) various snack bars from cereals, whole grains, nuts, dried fruit, and processed food materials (courtesy Hosokawa Bepex/Hutt, Leingarten, Germany); (c.4) various dumplings (courtesy Hosokawa Bepex/Hutt, Leingarten, Germany).
5. I The Development of Strength of Agglomerates
WlCKlNG
4
4 8
SWELLING
0
Fig. 5.13 Schematic representation o f the influence o f wicking and swelling on the dispersion o f agglomerates in liquids.
cellulose are bonded together by hydrogen bridges yielding pseudo crystalline stmctures. Although the hydrogen bonding is not destroyed by acidic hydrolysis, the cellulose chains are depolymerized and form “microcrystallites”. Fig. 5.14 depicts the structural and molecular formulas. n is 500 and 1,000, respectively. MCC is insoluble, physiologically inert, has high microbial purity, and is no substrate for microorganisms.
Structural formula:
Molecular formula: (C,H,,O,), Fig. 5.14
Chemical composition o f cellulose.
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5 Agglomeration Theories
Fig. 5.15 Examples of PC cellulosic fibers (Vivapur 101, courtesy J. Rettenmaier & Sohne, Rosenberg, Germany).
Powdered cellulose (PC) is also prepared from plant material by chemical digestion and purification processes. Further mechanical processing, without the use of chemical additives, yields high purity fibers. They are chemically inert and insoluble in water, organic solvents, and dilute acids or alkaline liquors. Depending on the specific technical requirement of a customer, different qualities of PC fibers can be developed and manufactured (Fig. 5.15). Today, both MCC and PC fiber grades are widely used in tabletting. Depending on the composition of the formulation, one or the other cellulose product results in better hardness, friability, and disintegration values. However, the quantity of MCC required to yield comparable tablet properties is normally at least one-third higher than that of PC fibers. Since, because of a more economical production process, the cost of PC fibers is also lower than that of MCC, monetary advantages can be derived from using powdered cellulose. Other organic fiber products which are mostly used in foods as “dietary” ballast additives are made from wheat, oats, tomato, apples, and citrus. Such dietary fibers are “non-starch” polysaccharides obtained from cell walls only, which can not be broken down by the digestive enzymes of the human organism and, therefore, constitute inert ballast materials. Color, taste, and odor relate to the fiber source. Unlike cereal brans or dietary fibers derived from, for example, sugar beets, which are often rejected by consumers because of their specific taste, wheat, oat, tomato, apple, and citrus fibers offer physiological properties that are much more readily accepted. Although these fibers are primarily used in foods, there are also applications in other industries. Functional characteristics of the fibers include high water binding and retention capacity (as a rule: the longer the fibre, the more water it retains),no synergy effect with thickening agents in the normal dosage range (for example, up to 10 % of wheat fiber can be added to absorb and bind liquids and oils before a thickening effect can be detected), improvement of the rheological properties of various thickening agents (e.g., improvement of the thixotropic qualities of carbomethyl cellulose (CMC)),and free flowlanticaking agent (with very low dust content). Starches and compounds derived from starches have long been known as additives in many industry. These materials improve flowability and act as binders as well as disintegrants.
5. I The Development of Strength of Agglomerates
A particularly interesting newer starch derivative is sodium starch glycolate (SSG). It is the sodium salt of the carboxymethylether of potato starch or other starches (e.g. wheat, maize (corn), rice, etc.) and is a fine, almost white, odorless and tasteless, free flowing powder. Because of the low degree of substitution (see Fig. 5.16), the form and particle size of the original starch remains almost unchanged. SSG is practically insoluble in organic solvents and forms translucent suspensions or clear gels with water. Until recently, sodium starch glycolate has been used exclusively as a disintegrant in pharmaceutical solid dosage forms. Since it was found that the manufacturing process can be modified, specific SSG grades are produced for different new applications (Tab. 5.4). Finally, as further examples in the context ofthis chapter, the beneficial use of totally different fibers than discussed above shall be mentioned and reviewed. Metal swarf, fine, elongated grindings and turnings which are fibers in a generic sense, may be applied to “mechanically reinforce” briquettes made from metal bearing dusts for recycling into metal making processes. For this application, it is important to produce high strength of which at least a certain part is retained at high temperatures, until melting occurs, so that secondary contamination due to premature release ofdust is avoided. The influence of these fibers on briquette strength is demonstrated in Fig. 5.17 which depicts that the strength of briquettes increases with growing addition of swarf while the necessary amount of chemical binder, constituting contamination and non temperature resistant bonding, decreases [B.42]. Fig. 5.17 also shows a broken cylindrical compact (a)that was manufactured with a laboratory piston press during process development (see also Section 11.2) and actual
g... Fig. 5.16
Structural formula o f sodium starch glycolate (SSC).
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5 Agglomeration Theories Tab. 5.4
New applications of sodium starch glycolate (SSC), according to J. Rettenmaier & Sohne (JRS). Application as
Disintegrant
Quasi soluble disintegrant
Wet granulation Taste masking binder
Thickening agent Gel former
Grade P ")
Grade P5000
Grades PlSOO, PSOOO
Grade P5000
Grades P1000, P3500, P5000
Guarantees excellent disintegration times of tablets, (film-)coated tablets, capsules, and granules.
Swells very much in water and forms translucent gels. These are particularly suitable to improve disintegration of tablets, effetvescent tablets, soluble tablets, granules, etc.
Have very good adhesion properties and are sui. table as binding agents with disintegration properties in wet granulation. They can be added in powder form and granulated with water.
Is used because of its gel forming properties to mask the taste in lozenges and chewable tablets.
Are used as Forms clear gels thickening and which are stable stabilizing agents within a wide in juices, suspen- range of sions, emulsions, temperatures. ointments, creams, etc.
Grade PO100
Typical dosage
1-5 %
2-20 %
up to 5 %
5-20 %
2-5 % in special 5 - 2 0 % cases also higher
")"P" in the grade designation refers to potato starch as origin. All grades can also be made from other starches ("M" = maize (corn), "R" = rice, " W =wheat, etc.). For formulations that are incompatible with alcohol the grade designation "SF" guarantees an alcohol content below 1 %.
briquettes (b) obtained in the industrial plant. Concerns that the swarf "fibers" would prohibit separation of briquettes that are produced with a roller press did not turn out to be a problem. Similar to concrete, refractory linings and components are agglomerates in which highly temperature resistant aggregates and mortars represent a system which is shaped and fired to yield bricks or other components that are then set into mortar for mounting, or is applied by casting or gunning. Today's high temperature processing industries demand high performance and predictable service life from the refractory. The latest generation of low cement, ultra-low cement, and self-flow castables, which are resistant to high temperatures, continue to be weak in tension and offer minimal resistance to damage from sudden changes in stress. Thermal cycling or shock as well as mechanical impact or vibration can all cause cracking, which, in turn, may lead to premature failure and substantial costs. Because the development of cracks can not be avoided in the rough environments of the typical applications of refractories, the probability must be reduced that such cracks result in failure. This is possible by reinforcement with fibers. Some materials that have been added to accomplish this are stiff, needle-like chopped wire or slit sheet fibers which are sometimes even supplied with, for example, hooks on their ends to increase anchorage. As schematically shown in Fig. 5.18 steel fibers in the refractory structure arrest the cracks and prohibit their propagation. Newer reinforcement
5.1 The Development of Strength of Agglomerates
2001
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1
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Z
o 180-
V
O 0-,
I
2 c
% content of ?:inforcing swarf and 3 p a r t s sulfite-waste powder as a binder
160-
01
I)
(J L
140-
a
0-0-
'ii 120-
reinforcing swarf and 5 parts sulf I t e -waste powder as a binder
E
7, 80 0,
i
-
4 - . 0
0-O
vt
5
40-
z! 0
O-
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2
-
waste powder as a binder I
3 L Hardening duration (days)
Fig. 5.17 Cold crushing strength o f briquettes from metal bearing dust, containing different amounts o f dry lignosulfonate binder (called "sulfite waste"), with and without swarf reinforcement, as a function o f (natural) curing time. Photographs o f "reinforced" metal bearing dust briquettes (scales not identical). (a) Cylindrical test briquette and fracture surface, (b) commercial pillow-shaped briquettes.
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5 Agglomeration Theon‘es
materials use direct spun stainless fibers that are rapidly cooled. The resulting products are fully annealed and, therefore, more pliable and ductile, feature better flow characteristics and an improved aspect ratio that results in optimum dispersion due to easy disentanglement of the fibers during mixing with the wet refractory system, and offer exceptional resistance to high temperature corrosion since a beneficial metallurgical structure is “frozen” during the ultra-rapid cooling process. The photograph in Fig. 5.18 shows a representative selection of some of these stainless steel fibers for the refractory industry.
Fig. 5.18 Schematic depiction o f the crack stopping mechanism of steel fibers in a refractory. Photographs o f some typical stainless steel fibers for the reinforcement o f refractories (courtesy RIBTEC, Gahanna, OH, USA).
5.2 Estimation of Agglomerate Strength
5.2 Estimation of Agglomerate Strength
The most important property of all agglomerates, desired or undesired, is their strength. For the practical and industrial investigation of agglomerate strength, stresses that occur in reality during storage and handling are experimentally simulated (see Section 5.2.2). In addition to the frequently used crushing, drop, and abrasion tests, methods for the determination of impact, bending, cutting or shear strength are employed. All values obtained by these methods are strictly empirical and cannot be predicted by theory because it is not known which component of the applied stresses causes the agglomerate to fail. For the same reason, the experimental results from different methods can not be compared with each other. Therefore, Rumpf (see Chapter 1) proposed to determine the tensile strength of agglomerates. It is defined by the tensile force at failure divided by the cross section or, if the test body has no uniform shape, the area of the failure plane(s) of the agglomerate@) (see Section 5.2.2). Because failure occurs in all stressing situations with great probability under the influence of the highest tensile force, this proposal is justified. Moreover, tensile force and strength can be approximated by models and theoretical calculations. 5.2.1 Theoretical Considerations
All binding mechanisms of agglomeration (see Section 5.1.1) can be described by one of three models (see Section 5.1, Fig. 5.4): 1. The entire pore volume of the agglomerate is filled with a substance that can 2. 3.
transmit forces and, thereby, causes strength (matrix binder, Fig. 5.4a). The pore volume of the agglomerate is entirely filled with a liquid (Fig. 5.4b). Binding forces are transmitted at the coordination points of the primary particles forming the agglomerate (Fig. 5.4d).
Liquid bridges at the coordination points (Fig.s 5 . 4 ~and 5 . 7 ~are ) described by model ( 3 ) while the transitional state (Fig. 5.7d) is connected with model (2) through the liquid saturation, S (see Section 5.1).
ad 1 ) Maximum tensile strength the pore volume is filled with a strength-transmitting substance If the pore volume of the agglomerate is completely filled with a stress transmitting substance, e.g. a hardened binder, three strength components can define agglomerate strength: (a) ota(pore volume strength) = tensile strength of the binder substance, (b) ota(grain boundary strength) = tensile strength caused by the adhesion between binder and particulate solids forming the agglomerate,
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Fig. 5.19 Two-dimensional schematic representation o f the failure lines derived from the three models describing strength o f agglomerates with a matrix binder.
(c) o ~ ( ~ = strength -~) of the particulate solids forming the agglomerate. The relatively lowest component determines the agglomerate strength. Fig. 5.19 depicts schematically the expected failure lines in a two dimensional schematic representation.
ad 2 ) Maximum tensile strength ifthe pore volume i s filled with a liquid If a liquid that wets the solid(s)fills the entire pore volume of an agglomerate to such a degree that concave menisci are formed at the pore ends on the surface, a negative capillary pressure pc develops in the interior of the agglomerate. Because the membrane forces at the surface are negligibly small in relation to the capillary pressure, the tensile strength otcof agglomerates that are completely filled with a liquid can be approximated by the capillary pressure: Otc
Pc
(Eq. 5.1)
Assuming that the pore diameter is characterized by the mean half hydraulic radius of the pore system, further assuming perfect wetting and spherical monosized particles, the following formula is obtained: Otc N
pc = a’ ( 1 - & ) / & ax
(Eq. 5.2)
The maximum tensile strength of agglomerates that are completely filled with a perfectly wetting liquid depends on the porosity of the agglomerate, characterized by the strong term (1- & ) / e ,the surface tension a of the liquid, and the size 3c of the particles forming the agglomerate. The empirical correction factor a’ has values between G and 8. An approximation of the agglomerate strength 5‘ , in the transitional (“funicular”) state, in which a certain percentage S (= saturation, see Section 5.1) of the pore volume is filled with liquid, is possible by multiplying the maximum strength otcwith the appropriate saturation S: Ott
s (Jtc
(Eq. 5 . 3 )
5.2 Estimation of Agglomerate Strength
ad 3 ) Maximum tensile strength ifforces are transmitted at the coordination points ofthe particles forming the agglomerate Estimation of the strength of agglomerates which is caused by solid bridges at the coordination points assumes that the entire solid binder material is uniformly distributed at all coordination points and forms bridges with constant strength oB.If, in addition, failure only occurs through solid bridges, the relative cross section of that material defines the agglomerate strength: (3tB
A,
(MB
p ~ / PB) ~ p -
OB =
VB
oB
(Eq. 5.4)
MB is the mass of the bridge building material and Mp the mass of the agglomerate building particulate solids, pBand pp are the densities of the respective solid materials, 1 - E is the relative volume of the particulate solids building the agglomerate, E is the specific void volume (porosity) of the agglomerate, and vBis the fraction of voids in the agglomerate that is filled with the bridge building material. Strength may be also caused by adhesion forces A acting at the coordination points of the particles forming the agglomerate. Based on statistical considerations and a simple model, Rumpf [5.1] developed a general formula that is often used to describe agglomerate strength: ot = (1-
E)/E
k A/d
(Eq. 5.5)
E is the specific void volume (porosity) of the agglomerate and (1- E) the respective volume of the particulate solids, TI = 3.14...., k the average coordination number, and x the representative size of the particulate solids forming the agglomerate. For k an empirical approximation exists:
kE
N
(Eq. 5.6)
TC
with which Equation 5.5 is simplified to: O~ =
(1-
E)/E
A/x'
(Eq. 5.7)
Theoretical Approximation o f Adhesion Forces The still unknown term in Equation 5.7 is the adhesion force A. Firstly, it must be recognized that, normally, more than one
binding mechanism participates in the production of agglomerate strength. Secondly, due to differences in micro conditions, it must be expected that the adhesion force A, at each coordination point is different. Therefore, Equation 5.7 becomes in its most general form: ot = (1-
E)/E
CAi/x2
(Eq. 5.8)
Work of many researchers concentrates on modelling and calculating adhesion forces that are caused by the different binding mechanisms [B.42].So far, all models are based on simplified conditions at the coordination points. For example, modelling of the adhesion force of a liquid bridge is based on two monosized spherical particles with a distance a from each other (Fig. 5.20). Adding the two adhesion force components, one caused by the negative capillary pressure in the bridge and the other by the boundary force at the solid/liquid/gas
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Fig. 5.20 I
Liquid bridge between two monosized spherical
particles
contact line, a general formula for the adhesion force of a liquid bridge AiLcan be derived: Ai,
=
u x f(P,G,a/x)
(Eq. 5.9)
The adhesion force of a liquid bridge between two monosized spherical particles depends on the surface tension of the liquid a, the particle diameter x, and a function of the angle P which defines the size of the bridge, the angle of contact or “wetting angle” 6, and a dimensionless term a / x which represents the distance at the coordination point. Obviously, a large number of different partner shapes, other than sphere to sphere, are possible and, normally, the size of the partners will be different and can vary infinitely. As already mentioned in Section 5.1.1 and shown schematically in Fig. 5.11 all particles also feature rough surfaces. Proving roughness, even on the macroscopically smoothest surfaces, depends only on the magnification. Therefore, when modelling surface interactions, this can be done macroscopically,disregarding surface roughness (for example the liquid bridge model above), or microscopically. In the case of liquid bridges the latter means that the distance a is an average as depicted in Fig. 5.11 and the angle of contact depends on the microscopic topography and, therefore, results in very complicated bridge geometries which can not be modelled. Generally, the description of the true shape of a particle, including surface roughness can not yet be described unequivocally. New techniques, such as fractal dimensions [B.37], may be applied in the future to solve this problem. As another example of modelling efforts, the estimation of the van-der-Waals adhesion force will be discussed. Three different situations at the coordination point, two flat surfaces, a spherical particle opposing a flat surface, and two spherical particles, are presented. Because van-der-Waals forces are field forces, models take into consideration the atomic and molecular interactions between the two entities. A microscopic
5.2 Estimation of Agglomerate Strength
theorie (Hamaker [5.2])assumes that all interactions may be added up and obtains the van-der-Waals adhesion force AivdWby integrating over all pairs of atoms and molecules. The characteristic term His the “Hamakerconstant” with a value ofapprox. lo-” to J. The macroscopic theorie (Lifshitz [5.3], Krupp [5.4]) calculates the interaction force from the energy dissipation of the electromagnetic fields that emanate from the bodies and obtains a similar van-der-Waals adhesion force. In this case hw is the Lifshitz-van-der-Waals constant with a value of approx. 1.6.10-20 to 1.6.10-18 J. Fig. 5.21 summarizes the model conditions and the results. 0 is the respective unit area on the opposing flat surfaces. The equations in Fig. 5.21 are only valid for distances a that are less than 150 nm. However, because already at much smaller distances at the coordination points the contribution of van-der-Waals adhesion to the strength of agglomerates becomes insignificant, this limitation is of no concern. It should be also noted that for very small distances, the “Born repulsion” is predominant as shown in Fig. 5.22. In addition to the already discussed influence of the actual micro topography at the coordination point, other conditions may influence the true adhesion forces that act between the solid partners. In the case of van-der-Waals forces the average distance a as shown in Section 5.1.1, Fig. 5.11, may be changed by the presence of adsorption layers (Fig. 5.23). From an adhesion physics point of view, adsorption layers with a thickness of less than 3 nm are so strongly bonded that they are immobile and can be considered as part of the solid. Because adsorption occurs primarily at energetically favorable locations, such as in depressions or valleys, it tends to smooth-out the surface roughness resulting in a reduction of the actual distance between the particles at the coordination point
Hamaker microscopic
Lifshitz macroscopic
Andw/O= H/6na3
A,,d,,/O
Fig. 5.21 Three model conditions for the estimation ofthe van-derWaals adhesion force and the results o f two theories.
=
hw/8n2a3
(Eq. 5.10)
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5 Agglomeration Theories ARepulsion
A
A vdW. man
van- der-Waals Adhesion
AAdhesion Fig. 5.22 Relationship of Born repulsion and van-der-Waals adhesion as a function o f the distance a at the coordination point
(Fig. 5.23) and an increased adhesion force. At ambient conditions the adsorption of atoms and/or molecules from the atmosphere is a natural phenomenon. Therefore, it can also happen during storage in bulk solids and even within agglomerates. While in the latter case agglomerate strength is enhanced which, in most cases, is not detrimental, the development of adsorption layers in bulk solids can lead to difficulties during discharge, feeding, and metering.
Fig. 5.23 Model explaining the increase in strength due to adsorption layers during van-der-Waals bonding [5.1].
5.2 Estimation ofAgglomerate Strength
5.2.2 Laboratory and Industrial Evaluations
Major parameters determining the properties of agglomerates are: The primary particle size, x , distribution,&), surface area, s(x), and shape. The agglomerate size, d, distribution, Ad), and shape. The apparent and bulk densities as well as the porosity E (= voids between the primary particles), also the pore sizes and their distribution in the agglomerate. The strength of the agglomerate. Primary particle size, distribution, surface area as well as micro (= surface structure) and macro shape, define the agglomerative behavior of a given type of particulate solids. The agglomerate (used as a generic term) size, distribution, and shape together with the characteristics discussed in Section 5 . 3 determine most of the advantages of agglomerated materials. The apparent density describes the mass of the agglomerates themselves, and the bulk density delineates the space filling behavior (e.g.the packing volume) of an agglomerated product. The porosity of agglomerates (see Section 5.3.2) is another method of describing their apparent density; it is the void volume between the primary particles forming the agglomerate and defines the accessibility of the internal surface area while the pore sizes and their distribution regulate the capillary suction which is responsible for “takingup” liquids (as in absorbents). The strength of agglomerates is one of their most important properties and may have many different meanings. In most cases the attribute “strength” defines a survival characteristic and may be defined as crushing, bending, cutting, shear, or tensile strength, as tolerance to one or several drops from a specific height, thereby reproducing stresses experienced at transfer points, or as resistance to attrition and the formation of dust [B.42]. For special applications still other measures of “strength” may be elaborated that simulate the real handling or processing conditions. Scientifically the only unequivocally defined and reproducible strength, that is ultimately and with a high degree of probability responsible for all failure modes and can be also approximated by theoretical calculations, is the tensile strength. A general formula describing the tensile strength ot of agglomerates, which are held together by binding mechanisms acting at the coordination points, was given in Section 5.2.1, Equation 5.8. The equation shows, that the porosity of agglomerates plays the most important role for their strength. The lower the porosity or, in other words, the higher the apparent density of the agglomerate, the stronger is the agglomerate. Since many of the desirable characteristics of agglomerated products require high porosity, sufficient strength is obtained in such cases by selecting a suitable binding mechanism featuring high adhesion or binding forces, using a powder with a small representative particle size, applying suitable curing techniques that produce permanent bonds with high strength (e.g. by sintering), and/or incorporating temporary additives in the feed. During or after the curing step such components are removed by melting, evaporation, or combustion (see Section 5.3.2).
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Results of experimental determinations of agglomerate strength have been published in many scientific works and were summarized in numerous specific books on agglomeration or in major chapters of more general handbooks (see Section 13.1). In the following, a few examples will be presented to describe generally important trends. For a more detailed coverage, the literature, particularly also the proceedings of the International Symposia on Agglomeration [B.4, B.14, B.18, B.23, B.35, B.48, B.701 should be consulted. Because strength depends critically on porosity, this property should be always measured first. To allow a comparison of individual strength values which were determined on different agglomerates they must be adjusted to fit a representative porosity. Then a larger number of results should be averaged and presented together with the statistical standard deviation or the minimum and maximum deviation of single values. If the density (specific mass p,) of of the solid particles forming the agglomerate (composite density if more than one material participates) is known and the volume of the agglomerate can be accurately determined, the porosity can be calculated as:
Some Results of Laboratory Determinations o f Agglomerate Strength
Agglomerates often contain moisture. If this is the case, they must be dried prior to the determination of the solid mass, M,.Also, with the exception of flat, cylindrical tablettes (see Section 8.4.3) and similarly well defined shapes, agglomerate volume can not be easily calculated. In those cases, the buoyancy of the agglomerate in a liquid is often measured. Since, according to the principle of Archimedes, the buoyancy is equal to the mass of the displaced liquid (under the assumption that the liquid does not penetrate into the agglomerate) the volume can be calculated as: Vagglomerate =
ML/PL
(Eq. 5.14)
MLis the mass of the liquid which is displaced by the agglomerate during the buoyancy test and pL is the liquid’s specific mass. The requirement that the liquid must not penetrate into the liquid can be met by using of a non wetting liquid (mercury was applied widely, also because of its high specific mass, by coating the surface of the agglomerate with a liquid repellant (e.g. oil)),or by painting a thin film of lacquer onto the agglomerate. The error caused by any of the protective measures is insignificant. If the binding mechanism between the agglomerate forming particles is not destroyed by the liquid, it is also possible to totally saturate the porous body and then reimmerse it to determine the buoyancy. By inserting Equation 5.14 into 5.13 another formula for determining porosity is obtained: E =
1-
(PL/Ps)
(M,/ML)
(Eq. 5.15)
Porosity can be also measured by pressure permeation methods [B.GO] ifthe agglomerate can not be treated and submerged in a liquid without losing its integrity. Fig. 5.24 depicts some laboratory methods for the determination of the strength of agglomerates and cohesive powders. Often, even in a scientific environment, the transversal crushing force is measured (Fig. 5.24a). This method is quite acceptable for
5.2 Estimation of Agglomerate Strength
I bl
la1
)”
IP Agglomerate
Adhesive
If I
R
6
Fig. 5.24 Laboratory methods for the determination o f t h e strength o f agglomerates or cohesive powders [B.42].
1
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perfect cylinders, such as tablettes and some extrudates. However, any spheroidal agglomerate is so irregular that a perfect diametral loading is impossible. This results in undefined stressing with compression, shear, and tensile forces acting in unknown ratios so that a wide scatter of data is obtained from undefined sources. Also, the definition of a compression “strength”by dividing the force at failure by the projection area ofthe agglomerate is, from a scientific point ofview, not acceptable. Normally, the statistical mean force at failure of testing a large number of agglomerates is reported. Crushing a sheet or cylindrical agglomerate by loading parallel opposite flat surfaces between plates is even more problematic because in very few cases the faces of the agglomerate are truly parallel resulting in uneven loading or the lateral expansion is blocked by friction between the agglomerate and the plates so that uncontrolled stress concentrations build up which may be the true cause for failure. As a consequence, data obtained from transversal crushing tests are seldom comparable. More reproducible results are obtained if the shear strength of a well defined, often specially prepared agglomerate is measured (Fig. 5.24b). This method was adapted from the well known shear cell for the evaluation of cohesive particulate solids (Jenike shear cell and derivatives [5.5]). During fundamental work on the binding mechanisms and strength of model agglomerates or cohesive powders, most of the laboratory evaluations determine tensile strength (Fig.s 5 . 2 4 ~- g). Machinable agglomerates are converted into cylinders that are glued between two adapters (Fig. 5.24~)and torn apart in a standard tensile test machine (e.g. Frank, Instron, see also Section 11.2). Other methods use “split” dies, with or without mandrils, for the manufacturing of a compacted agglomerate which is then pulled apart at a cross section which is defined by the split mold. Low strength caused by various binding mechanisms with or without prior densification is measured in flat split containers of which one part is fued and the other part is movable with insignificant frictional resistance. The load can be applied by slowly lifting up the support table (Fig. 5.24e) or providing an incrementally increasing horizontal force (Fig. 5.24f). Often, it is desirable or necessary to measure the strength of agglomerates which are partially or completely filled with a liquid. Particularly, in the high range of saturation the correlation of strength with the capillary pressure and their change during wetting or drying of the bed (hysteresis effect) is of interest. For this purpose, the simple method shown in Fig. 5.24fwas modified as shown in Fig. 5.248 [B.42]. The most reliable results of tests determining agglomerate or cohesive powder strength that can be also interpreted best are those in which the binding mechanism is caused by the surface tension of liquids and/or the resulting capillary forces (see Section 5.1.1, I11 in Tab. 5.1). With a high degree of probability the influence of other binding mechanisms can be excluded in agglomerates or powders that are bonded by a liquid. As shown in Equations 5.2, 5.3, and 5.7 together with 5.9 (see Section 5.2.1), this binding mechanism in defined by the surface tension a as well as other characteristics of the liquid and the solid, such as the wetting angle 6, the porosity E , and the representative size of the particles forming the agglomerate. Fig. 5.25 depicts the tensile strength, determined in the laboratory according to the method shown in Fig. 5.24c, of nearly saturated agglomerates made from narrowly distributed quartz and limestone powders as a function of the size 3c of the particles
5.2 Estimation of Agglomerate Strength
d
5 m C
E
c
m
Fig. 5.25 Tensile strength crt o f nearly saturated agglomerates as a function o f the size x o f the particles forming the agglomerate. Porosity adjusted to E =
0.0 0 6 1 o,ooLI
I
2
0.35. otcaccording t o Eq. 5.2.
1 1
1
1
1
I
I
L 6 8 10 20 LO P a r t i c l e s i z e x Ipm)
I
60
/
100
forming the agglomerate. The porosities of the individual agglomerates were adjusted arithmetically to E = 35 %. The diagonal lines represent the theoretical tensile strengths according to Equation 5.2 with a’ = 6 and a’ = 8, respectively. The diagram shows that the relationship o, l / x is fulfilled. The actual values are lower than theoretically predicted because the agglomerates which were produced in a pan (see Section 7.4.1) are not fully saturated with water and the structure of technically manufactured agglomerates is not perfect. Although not unequivocally visible in Fig. 5.25, regression analyses of these and many more sets of data revealed that the representative particle size for agglomeration processes is the surface equivalent diameter, x,. The importance of this representative diameter for the unit operation is not surprising as structure and bonding of the products critically depend on the surfaces of the particles forming an agglomerate as well as on the surfaces’ microscopic and macroscopic conditions. Of course, only the exterior particle surface is responsible for the effects; potentially internal surface area of the agglomerate forming particles must not be included when calculating the surface equivalent diameter of a particulate mass. Therefore, experimentally, surface area should be determined by permeametry, for example the well known and in the cement industry universally applied Blaine method [B.60]. The data in Fig. 5.26 confirm that the relationship between tensile strength o,,agglomerate forming particle size x , and surface tension of the binder liquid a and the porosity function (1- E)/E as per Equation 5.2 is correct and Fig. 5.27 proves that the (compression) strength of agglomerates increases linearly with the surface tension of the binder liquid as indicated by Eq. 5.2. Finally, Fig. 5.28 presents the tensile strength o,of moist and wet agglomerates as a function of liquid saturation S. At the two extremes S = 0 % and S = 100 % the strength
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3.0
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T
1 0.L
LT
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1
/ Y
I
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0.6
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1.2 0.8 1.0 Porosity function ( 1 - E ) / E
l.L
Fig. 5.26 Relative tensile strength % / a o f agglomerates made from sperical glass powder related to the porosity function (1 - E)/E and compared with the theory (Eq. 5.2) [B.42].
N -
1.0
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E E
-2 0.8b
0.6 5 Gl C
-
c
m
710 (Nlcm)
$0 0 0
Surface tension a
x
lo5
8b
Fig. 5.27 Compression strength 0 ofspherical wet agglomerates as a function o f the surface tension a o f the liquid [8.42]. Surface tensions are that of pure alcohol and water and o f 30/70 and 10/90 vol.% mixtures o f alcohol and water.
is close to zero. “Bone dry” powders feature very low tensile strength unless they are compacted or the representative size of the agglomerate forming particles is <1 pm. If it is assumed that at S = 100 % concave menisci are no longer formed at the pore ends on the agglomerate’s surface, capillary pressure is zero which, according to Equation 5.1 also lets strength go to zero. Between those extremes the conditions of Fig. 5.7 (c,d, and e) (Section 5.1) exist. On the left, Fig. 5.28 shows three curves for different dimensionless values a / x that were calculated using Equations 5.7 and 5.9. It can be assumed that up to a saturation of S = 30-40 % discrete liquid bridges prevail at the coordination points between the particles forming the agglomerate. In the right part of Fig. 5.28 the experimentally determined capillary pressure is plotted (solid dots). This curve is obtained when
5.2 Estimation of Agglomerate Strength
E E
EE
“0
20
LO 60 L i q u i d saturation
80
100
J I L (YO)
Fig. 5.28 Tensile strength cq and capillary pressure pc as functions of liquid saturation 5 [B.42].
the liquid from a totally saturated agglomerate is drained, for example, in an arrangement as shown in Fig. 5.248. As mentioned before, at S = 100 % the capillary pressure is zero. If, starting at this point, the agglomerate is drained, the capillary pressure rises steeply (development of concave menisci at the pore ends on the surface of the agglomerate) and then turns to a much slower rate of increase. The point at which the tangents to both curves intersect is defined as the entry suction pressure p , at which liquid begins to recede into the agglomerate. It is generally located at S>90 %. At approximately that point the maximum tensile strength of wet agglomerates exists. The circular and square open symbols represent average values of experimental results of measurements of tensile strength with their standard deviation. In the bridge model (“pendular”)state the results seem to fit the curve for a / x = 0.02 best and for high saturations (“capillary” state) they approach pe. In the transition (“funicular”)range between 30 % < S < 95 % both binding mechanisms, liquid bridges and saturated pores, contribute to the development of strength. The fact that in the transition range a difference exists between the strength of agglomerates to which liquid was added (e.g. during agglomerate growth, circular open symbols) and agglomerates from which liquid was drained (square open symbols) confirms that liquid can only be drained from saturated pores and liquid bridges are not influenced. The mechanism of capillary flow in wet agglomerates is an important factor if the liquid is a solution or becomes one (because all or some of the agglomerate forming particles are soluble) and the dissolved material recrystallizes during drying [1.1,B.421. If the agglomerate is highly saturated, drying takes place only on the surface. Liquid moves by capillary flow to the surface where evaporation occurs and recrystallizing substances deposit. The formation of a crust may influence further drying of the porous body considerably. The developing crust reduces the drying rate and may, after
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forming a dense crust, stop drying altogether. Since the crystal structure is influenced by the drying rate, the strength of recrystallizing substances in agglomerates during drying will be controlled by either the drying temperature, the crust, or both. Fig. 5.29 presents the tensile strength q of the core of dry agglomerates with recrystallized salt bridges which was obtained after removing a surface layer (including a crust, if applicable). The diagram shows that for agglomerates with very low initial moisture contents (curves 1 and 2) the strength increases as expected, almost linearly with an increasing amount of available salt (rising saturation) and with the drying temperature. At higher drying rate, finer and stronger crystallites grow at the coordination points in the agglomerate and, because the liquid (solution) was concentrated in discrete, immobile bridges, no crust had developed. At an initial saturation of 20 % (curve 3) the formation of a crust begins to influence strength at high drying temperatures while for the highest liquid saturations (S = 45 % and 60 %) the dense crust, formed at all temperatures, is the deciding factor for drying and development of strength (curves 5 and 6). Above 175- 200 'C the temperature within the porous body rises so quickly that the vapor pressure building up below the dense crust causes the agglomerate to burst (Fig. 5.30). The unexpectedly high tensile strength obtained at a liquid saturation of 30 % and a drying temperature of 350 "C is due to the formation of a network of small cracks in the crust that did not cause the agglomerate to fracture but increased the drying rate and, thus, the tensile strength of the dry agglomerate core.
Drying temperature t d ('C) Fig. 5.29 Tensile strength ct o f the core o f agglomerates with salt bridges as a function o f t h e drying temperature td for different liquid saturations 5 prior to drying [1.1, 8.421.
5.2 Estimation of Agglomerate Strength
Fig. 5.30 Photographs ofcylindrical agglomerates which contained a high amount o f a nearly saturated salt solution and burst during drying.
The above mentioned incrustation may be positive or negative. On the positive side, the phenomenon can be used as a method to achieve encapsulation of agglomerates if a film forming, easily soluble polymer is dissolved in the liquid phase. On the other hand, if a dryer is controlled by sensing the moisture content in the off-gas,the process instrumentation may mistakenly identify a heavily encrusted product as being dry when, underneath of the crust, moisture still remains. Such a product can, of course, cause a whole host of problems, such as caking during storage when the liquid slowly redistributes and problems during a secondary process, for example tabletting of a still partially moist granulated pharmaceutical formulation, as well as many more difficulties. During the initial phase of drying, when all evaporation occurs on the surface of the porous bodies, the temperature of the material to be dried stays at or below 100 “C. For highly temperature sensitive materials this temperature can be lowered by the application of vacuum. However, if incrustation occurs, the temperature of the mass to be dried increases to the temperature of the drying gas and can cause damage to the material. A considerable amount of fundamental research is going on in many places of the world trying to increase knowledge of all binding mechanisms and develop numerical methods to calculate or at least estimate binding forces as well as agglomerate strength. In addition to the “classic” standard methods discussed above many novel technologies, such as, for example, application of the atomic force microscope (AFM) (also called lateral force microscope (LFM) or scanning probe microscope (SPM)) for the measurement of adhesion in the micron and submicron particle range, and new theories, for instance, Fractals [B.37] and the Chaos Theory, are applied to agglomeration research. However, as mentioned earlier (see Section 5.1.1), it is unlikely that only
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one binding mechanism acts on all coordination points within even a single agglomerate. Moreover, the microscopic conditions at each coordination point are so diverse that bonding at virtually each individual coordination point is different. Therefore, although big advances are being made, the science of agglomeration is still far away from formulating a useful general theory. Furthermore, this book is devoted to a more practical coverage of agglomeration. Therefore, the reader is encouraged to search for and study the increasing number of publications that report on the advances in this area (see Sections 13.1 and 13.2). Industrial Evalutations of Agglomerate Strength Determination of agglomerate strength in industry is much more pragmatic [B.42].Although knowledge and understanding of the fundamentals of agglomeration, particularly the nature and effect of the binding mechanisms and how they can be influenced, become more and more important during the development of new and for the optimization of existing agglomeration processes, agglomeration as a unit operation is still more an art than a science. While an increasing number of criteria are known for the preselection of the most suitable agglomeration process for a specific application (see Chapter 11), it is still necessary to test the selected equipment in the laboratories of vendors or development organizations (see Section 11.2). Often, if the process is a new one, it is even desirable to operate a smaller pilot plant or to involve a “toller”,an outside processor for hire, prior to an investment decision for a large scale plant (see Section 11.2). Agglomerate strength in industry is defined as a commercial or process characteristic of the particular intermediate or final product. For example, if the agglomerated material is a final product, strength may be defined as resistance to breakage, chipping, or abrasion. The definition of this property and of other strength related requirements will differ whether it is an industrial bulk material or a consumer product. While the former may break down to a certain extent, as long as it remains free flowing and dust free, a consumer product must have perfect and pleasing appearance where even minimal chipping or breakage into large chunks must be avoided. Intermediate products must have characteristics that are suitable for the intended further processing. For example, a material may have to be strong enough and abrasion resistant for storage and handling to avoid bridging, flow problems, dusting and segregation of components. If it is a feed material for tabletting or other pressure agglomeration methods, the agglomerates must break down totally under pressure and produce a uniform final product structure. Other agglomerated intermediates may have to feature the opposite property, i.e. to yield a filter with bimodal pore size distribution it must retain its shape and structure during pressing (see also Section 5.3.2). For those reasons, “strength”means many different things in industry. Typically, measurement of strength is based on a simulation of the stresses which a particular agglomerated product must withstand. Very few industrial methods for the determination of this property are standardized or even known. In a competitive environment it is of less interest to compare quality between rivals than to make sure internally that the product properties that are expected by the industrial or public consumer are maintained. Therefore, most measurements of strength are undertaken as quality assurance. A few will be described below as examples.
5.2 Estimation of Agglomerate Strength
A general problem associated with the determination of product properties in industry is sampling [B.24, B.271. Particularly the measurement of strength is in most cases based on totally or partially destructive methods. If taken during production, these “lost” samples are extracted from the product stream in a random but representative manner and either tested directly “in-line”or, sometimes after again sampling the sample, in a quality assurance laboratory which is associated with production. Afterwards they are discarded. During initial and occasionally repeated process optimization, the influence of different process parameters on agglomerate strength is determined. Results of the measurement of strength are often used to adjust process parameters as required. If there is a difference between “green”and “cured”or final strength, both strength values may have to be evaluated to allow adjustment of the respective process steps. Even bigger problems exist iflarge bulk masses (e.g.stock piles, silos, ship loads, rail cars, trucks, etc.) must be sampled. This is done to guarantee product quality prior to or after shipment and at the point of consumption. Results of those tests are only of commercial value because, typically, they can no longer be corrected but may influence acceptability or price of the commodity. Often, if quality is below standard but does not meet the guarantee, the price will have to be adjusted by offering discounts or rebates. Among the few standardized methods for determining agglomerate strength are the compression strength ( I S 0 TC 102/Sc 3 DP 4700 and ASTM E 382-97) and tumble ( I S 0 3271 1975 E and ASTM E 279-97)tests for iron ore pellets as well as the “tumbler test” for coke (ASTM D 294-72). In this case, a group of consumers (steel companies) forced a growing number of independent suppliers to test and guarantee agglomerated bulk commodities by formulating the standards. For iron ore the tests are on finished pellets, either prior to shipment or at the consumer’s facility and, therefore, are not intended or even suitable for process control. It will be shown later that iron ore pellets are first produced as “green” agglomerates and then indurated by sintering. For the determination of compression strength, a bulk sample is first screened and at least GOO pellets are taken from the size range in which the maximum is found. In a “Riffle” splitter [B.24, B.271 four samples, containing at least 100 pellets each are prepared. From two of the samples, individual pellets are placed between the parallel, surface-hardened platens of a compression testing machine, loaded with a constant speed, and crushed. The maximum force at which each pellet breaks is determined and recorded. After testing 100 pellets of each of the two samples, the arithmetic averages for the batches are calculated. If they deviate by more than a predetermined amount, another 100 pellets are tested to confirm one or the other value. In the tumble test the abrasion resistance of the pellets is measured. The “ASTM drum” is a cylindrical container with specific dimensions which is rotated around its horizontal axis at a predetermined speed and for a defined number of revolutions. A given mass containing a representative sample of clean pellets is filled into the drum, tumbled for so many revolutions at the constant speed, removed and screened at GOO m. The “strength”of the pellets is defined by the amount of “fines”smaller than GOO m that was abraded during the test. For coke, the “tumbler test” is carried out correspondingly in a similar drum (Fig. 5.31).
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SIOL LLLVATIOY. M U f I N S L E W ON
Fig. 5.31
*I
LIm C L C V A I I W
,W
f I*
Itnlol4
om
A&
Sketch o f the ASTM “tumbler test apparatus” for coke
Both compression and tumble tests have been widely used and modified for other applications. The crushing test can be utilized for any agglomerate that is large enough for individual testing and, sometimes, a single layer of many narrowly sized granules is crushed by this method. However, as discussed above, in most cases, due to a more or less irregular macroscopic and microscopic shape of the agglomerates, stressing is not uniform or reproducible and, therefore, the results can not be used for scientific or general purposes. If large enough numbers are crushed and the results are statistically treated and evaluated, the average values are good enough for quality control in a specific plant. It has been repeatedly shown, however, that a comparison of data between different plants, laboratories, and even between technicians in the same laboratory (often referred to as the “human effect”) is not possible. Many publications also report on the fact, that most agglomerates do not break under the influence of a single, well defined force. Rather, because agglomerates are porous bodies, which are made up from particulate solids with binding mechanisms acting between them, and often feature irregularities in their structure, they will disintegrate in steps. It is possible, that several small pieces break from the agglomerate before it finally fails catastrophically. Other products, particularly wet agglomerates, deform plastically before failure occurs. Some binding mechanism, for example those caused by highly viscous binders or capillary forces (see Section 5.1.1), can also produce a “selfhealing” effect after a first, smaller crack has developed. Therefore, even an unequivocal definition of the crushing strength is problematic. For the testing of other agglomerates by tumbling, the drum shape and execution is often modified. To avoid a sliding motion and produce cascading during the test, square drums have been designed or varying numbers of differently designed lifters have been built into cylindrical drums. The composition and mass of the sample to be tested, the rotational speed, the duration of the test, and the screen size defining “fines” are varied to fit particular needs.
5.2 Estimation of Agglomerate Strength
If the abrasion resistance of smaller granules, for example of fertilizers, agrochemicals, intermediate products, etc., must be tested, specific, often smaller drums can be used as described above. Recently, based on this technique, again influenced by pressure from consumers and the desire to develop a quality assurance plan, the Saskatchewan Potash Producers Association has defined a standard procedure for the determination of degradation characteristics (= “strength”)of this granulated bulk fertilizer [5.6]. More often however, a representative sample is placed on the particular test screen that defines the “fines” and vibrated or shaken in a laboratory screening machine (e.g. Rotap, Fritsch, etc.) for a predetermined time [5.7].To produce a sufficiently significant amount of abrasion for quality control, “grinding media”, such as a specific number of steel bearing balls of a particular size or other pieces with the same purpose, are added to the granular sample. If the separation size defining “fines”is very small and, therefore, the screen is delicate, the test can be carried out in the pan. The amount of fines is then determined in a separate screening step. Because of the well defined shape of tablets, crushing tests are regularly and with great success used in the pharmaceutical industry in-line or off-line and often automatically, in combination with an automatic sampler, for monitoring tablet strength. Other modern, fully automated equipment measures tablet weight, thickness, diameter, and hardness for quality control and validation (Fig. 5.32). For some agglomerated products it is important to make sure that they meet certain strength related characteristics. For example, many animal feeds are pelleted by extruding mixtures of conditioned components through cylindrical bores in flat or cylindrical dies (see also Section 8.4.2).While pelleted food for fowl or fish is swallowed whole, products for feeding mammals need to be chewable. A compromise must be found between high strength and abrasion resistance, which allows storage, transportation, and handling without breakdown andlor the production of fines, and the requirement that pellets must be safely crushed between the teeth of the animal. A crushing test to measure this type of crushing strength was developed (Fig. 5.33) and is used for quality control in feed mills.
Fig. 5.32 Photograph o f the “Schleuniger Autotest 4” tablet testing system for the quick and automatic measurement of tablet weight, thickness, diameter, and hardness (courtesy Dr. Schleuniger Pharmatron, Manchester, NH,USA).
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5 Agglomeration Theories
Fig. 5.33 Handheld (a) and automated (b) crushingtest equipment for the determination o f strength o f pelleted animal feed (courtesy Amandus Kahl, Reinbek, Germany).
In many industries, the requirements on product quality are not very stringent. The agglomerates must withstand handling, including the loading of silos and transport vessels, as well as transfers. Generally speaking, they must survive several drops with only limited breakage and the creation of a minimum of fines. For the measurement of this characteristic “drop tests” are carried out. The actual equipment and procedure may vary widely and is normally a simulation of the expected “abuse” which the material will encounter on its way from production to consumption or use. Fig. 5.34 is the sketch of a typical arrangement. A drop test arrangement can be easily built and applied in the field. The “equipment” consists of a heavy plate (1)made from steel with at least 20 m m thickness, a concrete slab, or - in the most simple case - a stone laboratory floor on which a
5.2 Estimation of Agglomerate Strength
Fig. 5.34
Sketch o f a drop test arrangement (explanations see text).
tube or some kind of collar (2) is placed to contain the sample after the drop and avoid material losses. Diameter, height, and material of construction of the retaining wall must be such that, after impact, pieces can dissipate without experiencing secondary breakage. A vertical pipe (3), the upper end of which is at a distance h from the impact plate on the floor, extends into the retaining container. Length and diameter of the pipe depend on the size of the agglomerates to be tested. The diameter should be at least 5times or, even better, lotimes greater than the largest agglomerate dimension. The length is simulating the expected drops during further handling of the product. The pipe must end at a sufficient distance from the impact plate to allow free lateral movement of the mass upon impacting the plate. The test itself can be carried out in different ways. One method is to drop batches, each, for example, consisting of five large agglomerates (in most cases briquettes), one after the other, from different, increasing heights. “Strength” is defined as that height from which all five agglomerates still survive the drops without damage. This test determines the maximum drop height that can be tolerated in a plant which must produce whole agglomerates and handle them without breakage. Such a requirement may exist if products are manufactured that must have a certain appeal such as charcoal briquettes for barbecueing, salt briquettes for the regeneration of home water softeners, or, generally, consumer products. The test as described before is carriedout during system development, prior to plant design: later, for quality control during operation, representative samples are extracted in regular intervals from the product stream and dropped from the predetermined height to recheck and confirm their survival. Often it is not necessary to produce industrial agglomerates that must survive all handling completely intact. In this case, a relatively great drop height is selected
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and individual agglomerates are tested. The particle size distribution of the broken pieces is determined by screening and the result of the drop test is judged based on the amount of “fines”that is produced during impact. The definition of what constitutes fines and their permissible amount depend on the application. In still other cases, the collective behavior of a large number of agglomerates is of interest. Then, a sample, sometimes weighing several kilograms, is dropped, all at once, from a bucket through the pipe. This setup simulates the “cushioning” effect of a bed of material at the impact point. The evaluation is again carried out by screening and determining the amount of “fines”.During an alternative procedure the entire sample or only pieces larger than “fines”are dropped again or repeatedly for a specific number of times. The evaluation of the test is done in the same way as before whereby data are determined either after each drop or after a certain number of drops. These few examples of industrial methods for the determination of agglomerate strength and descriptions of how these test may be carried out shall suffice. The technical literature ofthe past century is full ofreports that cover industrial plants producing agglomerates and evaluations oftheir “strength”,whatever that characteristic may mean in a particular case. Readers who wish to know more are encouraged to seek out these publications whereby it should be recognized that, in the past, most papers have appeared in application oriented journals and proceedings of conferences. The above descriptions of alternatives that are possible in the execution of the same test to obtain specific information for agglomerate handling or use should also help to understand how existing methods, which may have been used in a completely different context for other agglomerates, can be adapted interdisciplinarily to fit new requirements.
5.3 Structure of Agglomerates
Agglomerates are bodies that are, often artificially and with purpose, produced from individual “small” particles. The term “small” is to be understood in relation to the agglomerate. Although there are agglomerates, for example in the food industry (see Section 5.1.2 and Fig. 5.12), or natural, often undesired agglomerates (see Section 5.5), which consist of only a few particles, typical agglomerates contain very large numbers of particles (see Section 5.3.1, Table 5.6) with sizes that are orders of magnitude smaller than that of the agglomerate. Binding mechanisms (see Section 5.1.1) cause these particles to temporarily or permanently stick together and form a lose or porous entity (see Section 5.3.2). Since binding mechanisms act in different ways (see Section 5.1), the structure of agglomerates is of great importance for all properties of agglomerates. The sketch in Fig. 5.35 depicts a random cut through an agglomerate. The area within the heavy solid lines is arbitrarily defined as “one”. Fig. 5.35 seems to show particles and their distribution. In reality, what is visible are cross sections through particles at a random level. If another random cut through the same agglomerate is made, a totally different picture is obtained. Moreover, particles that seem to float in space are in contact with other particles at some level. For example, the shaded cross section maybe the result of cutting the particle, shown in elevation on
5.3 Structure of Agglomerates
Examples of:
@
Contact points
0 Nearpoints
Elevation (Side View)
Fig. 5.35 Sketch o f a random cut through an agglomerate.
the side of Fig. 5.35, at the indicated line. Obviously this particle will have a completely different outline at another level. The same observation is true for the void spaces (= porosity) that are visible between the particle cross sections. If the heavily bordered square in Fig. 5.35, which represents the area “one”,is large enough and contains a great number of the two significant structural characteristics, i.e. outlines of cross sections through particles and of pores between the particles, a statistical evaluation of any random cut will produce generally valid results with an accuracy that can be described by the standard deviation which is associated with that statistical treatment. Therefore, for example, scanning the picture of the cut will produce information on particle size and distribution, porosity, E, solids content, 1 - E, and, with the appropriate software, a shape factor and the specific surface area of the particles [B.GO]. Accuracy can be increased by investigating multiple cuts through the same agglomerate and determining the statistical averages for all of them. A visual evaluation of the enlarged picture of the cut through an agglomerate also reveals certain other features, although the observations can only be used to explain phenomena and do not serve any scientific purpose. The shaded circles in Fig. 5.35 indicate, for example, some of the contact points between particles in this particular cross section and the open circles depict some of the “near points” at which a binding mechanism, such as liquid bridges or one ofthe field forces (see Section 5.1.1), could develop. The average of the sum of both types of interaction points for one particle defines the coordination number k. Taking into consideration the statements made above in regard to random cuts, it is of course possible that “near points” in a particular cut are actually contact points in a level slightly above or below and it is impossible to determine all the interaction points which are distributed three-dimensionally around a particle.
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5 Agglomeration Theories 5.3.1
General Considerations
The structure of agglomerates depends on many different parameters. Generally, they are parameters related to the particles building and all the processes involved in forming the green and final agglomerate. Particularly during high-pressure agglomeration (see Section 8.2) and various post-treatment (curing)processes (see Sections 5.3.2,7.3, and 8.3) parameters that relate to the original feed particles may change. Parameters Related to Particles Building an Agglomerate. The most important particle related parameters that influence agglomerate structure are:
Particle size, Particle size distribution, Macroscopic particle shape, Microscopic particle shape (surface configuration, e.g. roughness). With the exception of geometrically well defined particles, particularly spheres and cubes, it is difficult to describe with one dimension and measure particle size (Fig. 5.36). During particle size analysis [B.60],the response of each particle to a physical effect is determined; for example, whether a particle will pass a defined opening (screening),how fast a particle will settle in a stationary fluid under the influence of gravity or centrifugal force (sedimentation),at what speed of a gas flow a particle will be entrained (sifting), how much extinction will be caused by a particle passing through a sensing zone (sensor output),what is the outline of a picture or projection of a particle (scanning),how much energy is reflected from a particle at a particular angle (scattering), etc., etc. Only absolutely spherical, microscopically smooth particles will produce results with which particle size (diameter of the sphere) is determined unequivocally and by modifying the effect which determines size, the distribution of the sizes of spherical particles can be determined, if dilute samples are analyzed where the particles do not influence each other during the test. For all other situations, particle shape has an overwhelming effect on how they behave during a test or in any process. Shape is characterized by form and proportions. Form refers to the degree to which a particle approaches a definite form, such as a sphere, cube, tetrahedron (Fig. 5.36), or higher order polyhedron. The relative proportions distinguish one spheroid, cuboid, tetrahedron, or polyhedron from another of the same class. Macroscopically, shape may be described rather subjectively by comparison with “standard shapes” or defined by coefficients (Fig. 5.37). The major problem of characterizing the three-dimensional shape of a particle by its size is that size is one-dimensional and coefficients of “standard shapes” are two-dimensional. To overcome this problem, particles may be described by polar coordinates, for example, radius vectors from the center of gravity extending to any point of the surface. By using the radius and the two polar coordinate angles, the shape of the particle surface can be described to any desired degree of accuracy. Obviously, for the time being, this technique is limited to scientific work.
5.3 Structure of Agglomerates
Microscopically, particle shape, particularly surface texture, may be defined by fractals or Fourier functions. It must be realized that in nature no absolutely smooth surfaces exist. With increasing magnification macroscopically smooth, e.g. polished, surfaces first reveal scratches, caused by the polishing media, and later “natural”roughness with peaks and valleys. Since the size of small particles which are interacting with other, larger particles extends into the nano range and, therefore, such particles are themselves similar to or potentially smaller than many surface features on other particles, it is understandable that knowledge of the microscopic particle texture is of great importance for agglomeration. However, even with the quickly growing technological advances in the nano scale it is still impossible in practice to apply the information for general theories with which agglomerate characteristics can be predicted. As will be shown below, the extremely large number of particles that are involved and their variability (it can be assumed that no two particles are exactly alike) is another reason for today’s inability to generally and unequivocally describe the interactions between particles in an agglomerate.
0 97
00000
0 95
00000
0 93
00000 00000
0 91
0 89 1 .
0 87
+.
p L oi p Ln
Fig. 5.37 “Standard set o f shapes” for the determination of particle sphericity according to Rittenhouse [8.42].
~
00000 00000
085
00000
083
00000
0 81
oQDO0
0 79
00000
0 77
00000
075
00000
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All that is normally known about a particle is its silhouette, projection, or profile or, in those cases where size is derived from other physical effects, such as, for example, the settling velocity, a dimension related to volume, mass, or surface texture. Therefore, methods must be found that interpret information from cuts through the particle, scans of portions of the surface area, or information from particle behavior in, for example, fluids and connect it with overall shape. Unless the measured outline of the particle misses a unique, dominant feature of the particle shape, the result will be representative of the particle. The methods are still very complicated and require a large number of discrete items of information to describe a particle signature reasonably well. Shape influences particle behavior in powder packings and the representative (particle) equivalent diameter changes with the physical situation. In agglomeration, in addition to the surface equivalent diameter of an entire particle size distribution, which is the representative value for estimating the strength of agglomerates (see Section 5.2.2), for the packing structure, the diameter of an inscribed average circle representing the particle projection is less significant than that of the circle enveloping all peaks and protrusions (Fig. 5.38). This is particularly true for loose packings (Fig. 5.38, top). A different equivalent diameter would be representative for closely packed particles (Fig. 5.38, bottom). It appears possible that, in the future, equivalent particle diameters can be computed for loose and dense packings. Furthermore, it should become feasible to calculate the work that is required to go from one packing structure to another, the resistance of a powder to penetration, and its angle of repose. It is already possible to characterize the pore structure of a particle system by using automatic scans of a cross section and employing fractals to analyze the data. Nevertheless, the characterization of particulate matter and the structure of particle systems is still at the beginning of becoming an exact and widely used science.
P a r t i c l e size versus packing r a d i u s Size Packing radius-
Close packed p a r t i c l e s
Fig. 5.38
Effect ofparticle shape on its packing behavior.
5.3 Structure of Agglomerates
Much of the industrial research of packing structures is still based on spherical particles. The most fundamental information is obtained if the regular packings of monosized spheres are evaluated. Fig. 5.39 shows the six regular packing structures of monosized spheres. For the packings depicted in Fig. 5.39 porosity E , the void volume between the monosized spherical particles, and the coordination number k, the number of interaction points of a sphere in the structure with neighboring spheres, can be exactly determined (Tab. 5.5). All coordination points in these structures are contact points.
Fig. 5.39 Systematic arrangements o f spherical particles ("regular packings"). For explanations see text and Table 5 . 5 .
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5 Agglomeration Theories Porosities, coordination numbers, and approximations according to Eq. 5.6 for the “regular packings” shown in Figure 5.39.
Tab. 5.5
-
Coordination number, k
k
E
Cubic (Figure 5.39a)
0.476
6
6.599
Orthorhombic, two alternatives (Figure 5.3%)
0.395
8
7.953
(Figure 5.3%)
0.395
8
7.953
Tetragonal-spheroidal [Figure 5.39d)
0.302
10
10.40
Rhombohedral (pyramidal) [Fig. 5.39e)
0.260
12
12.08
Rhombohedral (hexagonal) (Fig. 5.3%)
0.260
12
12.08
Geometric arrangement ~~~
Porosity
n/c
(Eq. 5.6)
~
Tab. 5.5 shows that the porosity of regular packings ranges from 47.6 % for the most open structure to 26 % for the densest packing. Real packings of monosized spheres are called irregular packings. Unlike the specified location of each sphere in a regular packing (deterministic system),the position of any sphere in a randomly packed bed can only be described by a probability distribution (stochastic system). Moreover, the density (or porosity) of a randomly packed bed depends on the mode of packing. Normally, in freely developing, infinite beds two structures are distinguished: a very loose random packing with a typical porosity of 40-43 % and a lose random packing with 39 -41 % porosity. If packings are produced in a container they are influenced by the “wall effect” (Fig. 5.40). On and near rigid walls the positioning of the spherical particles can not occur freely and this disturbance is continuing into the packing, creating voids and other irregularities. Nevertheless, poured random packings in containers may attain 37 - 39 % porosity, depending on the dimensions of the container in relation to the size of the spheres and, if the container is vibrated or vigorously shaken, a porosity of approx. 36 % may be obtained. Tab. 5.5 also shows that the coordination numbers for regular packings of monosized spheres are 6, 8, 10, and 12 and that even for these unique conditions the approximation of Equation 5.6 is rather good. Therefore, it can be assumed that Equation 5.6 results in a close approximation of k which indicates that, based on a purely mathematical estimation, high densities of <10 %, which are, for example, obtained during high-pressure agglomeration (see Section 8.2), are associated with coordination numbers of >3O, explaining, among other reasons, the immediate high strength of agglomerates produced by these methods. Even though many studies have been and are being carried out to characterize packings derived from two or more sphere sizes, there is still no theory that satisfactorily describes the structure and allows an universally valid prediction of density or porosity, pore sizes and distribution as well as the coordination number of specific packings. For particles with irregular shape and a particle size distribution, the typical case in industry, a general understanding of structure and its characteristics is still remote. If
5.3 Structure of Agglomerates
Fig. 5.40
Examples o f packings demonstrating the "wall effect"
[5.8].
packing parameters need to be known they must be determined experimentally. Nevertheless, some interesting information has evolved from the many tests that were carried out over time. It relates to so called optimum packings. The most important optimum packing is the densest regular packing of spherical particles. It can be most easily derived from the two loosest regular packings (a and c in Fig. 5.39) of the largest spherical particles in a mixture (Fig. 5.41). It is obtained by inscribing the largest possible spherical particle into the void between the four or three larger spheres (depending on the model used) and adding the appropriate amount of these smaller spherical particles to fill all the voids between the larger ones. This method is then continued as shown in Fig. 5.41.
I
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5 Agglomeration Theories
--r-- I-----
I
Fig. 5.41 Sketches o f the systematic arrangements of differently sized spherical particles to obtain the densest possible packing.
Obviously, the resulting particle sizes and their relative masses do not fit a continuous distribution. The mixture consists of discrete classes of particles. In reality, where such densest packings are desired for a number of reasons, for example in the production of high quality concrete to obtain high strength and water impermeability by minimizing porosity and the presence of interconnected pores (see Section 5.3.2), narrow size ranges of aggregate particles are mixed in the appropriate amounts as
5.3 Structure of Agglomerates
prescribed by the model. Although neither particle size nor shape of the components correspond with the assumptions of the model (spherical monosized particles in different classes) the real random packing produces the desired high density and strength. To obtain the best possible impermeability in concrete, the smallest particles that are added today feature particle sizes <500 nm (e.g. silica fume). In those cases where it is not practical to mix discrete classes of narrowly sized particles, an approximation yields a continuous particle size distribution. Mathematically, it is described by the “Fuller distribution” [B.42],an exponential distribution in which the exponent must be between 1/3 and 213. For agglomerates, strength depends primarily on porosity and particle size (see Section 5.2.1). The smaller they are, the higher is the expected strength. This is due to the fact that, with reduced particle size, the number of particles per unit volume increases and with decreasing porosity the coordination number increases. Both effects multiply the binding forces to sometimes very high levels resulting in considerable agglomerate strength. For example, iron ore pellets which, for other reasons (see Section 11.8),are produced from concentrate particles <44 pm, are so strong after sintering that they will not be crushed by truck or front end loader tires. The effect of porosity on the coordination number has been discussed above. Tab. 5.6 shows that if a 1 g sphere, made of a material with a specific mass of 1 g/cm3, is converted into spherical particles (for example by melting, spraying, and consolidation) with 1 pm diameter, 1.9 billion (1.9 trillion according to the American system) particles are produced. Dividing the same mass into spherical particles with 0.5 m m (500 pm) diameter still yields 15,000 particles. These relations have to be kept in mind when considering the effects of binding mechanisms on agglomerates. Since the structure of agglomerates is so important for many properties of agglomerates and scientific or theoretical predictions or descriptions are not yet possible, empirical approaches must be taken to understand and evaluate agglomerate structure. This is particularly true when real particles, i.e. those with irregular shape and an often wide, sometimes multimodal size distribution are involved. As will be obvious in this book, scanning electron microscopy (SEM) is a very powerful tool for the observation of surface and/or interior structures of most solids, including agglomerates. The main advantage of the scanning electron microscope is, that it can not only enlarge the field of vision such that even submicron features are clearly visible but that it also provides depth of focus which allows a three dimensional evaluation. Tab. 5.6 Some properties o f spherical particles (density 1 g/cm3) and o f agglomerates made from these particles.
Mass Diameters
Volume
Porosity
Igl
Lmml
w31
[%I
1
p-12.4 a: -
p
1
p
=
1
p
=
a-14.75 0.5 a-14.34
P = particle(s), a
=
Spec. Surface
[m’lgl
p
=
0; a: -
p=l
~ - 4 . 8 , 1 0 - ~a:; -
p : - ; a-l.G7
p
=
0; a
=
40
p-1.9.10’2
p : - ; a-5.97-6
p : - ; a-1.54
p
=
0; a
=
35
~-1.5.10~
p : - ; a-1.2.10-~
=
agglomerate
1; a: -
Numbers
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Since they were first invented the size and cost of scanning electron microscopes has come down considerably, the time for sample preparation has been shortened, and other complications of the procedure have been reduced. Today, relatively low priced desk top microscopes are available and many scientific or corporate laboratories are equipped with larger units so that this technology is accessible to almost everyone. Parameters Related to Processes Forming Agglomerates. Processes applied for the manufacturing of green or final (after post-treatment) agglomerates strongly influence agglomerate structure. As will be shown in more detail later (see Chapter G), agglomeration processes generally belong to three disparate technological fields:
Tumble/Growth Agglomeration Pressure Agglomeration Agglomeration by Heat/Sintering According to the mechanisms that prevail in each of these technologies, the structure of agglomerates is different. Tumble/Crowth Agglomeration. As the name implies, in tumble/growth agglomera-
tion small particles adhere to each other after colliding during their irregular, stochastic motion in a particle bed and form a new entity that is held together by binding forces. After passing the difficult and critical stage of seed formation (see Section 7.1) the particle assembly grows by the attachment of additional particles to its surface. The structure of agglomerates resulting from growth during tumbling depends on the density of the particle bed, the energy imparted the tumbling mass, the acting binding mechanisms, and the time, among others. Agglomerates formed in a low density cloud of particles, for example in a fluidized bed or other low density tumble/growth agglomerators (see Sections 7.4.4 and 7.4.5), feature a very loose structure, high porosity, far beyond that of even the most open regular packing (Fig. 5.39 and Tab. 5.5), and contain few particles. In high density tumbling beds of particles, as realized, for example, in disc, drum, or cone agglomerators (see Section 7.4.1),and even more so if mixing tools provide additional energy and shear, such as in mixer agglomerators (see Section 7.4.2), particles that have attached themselves to the surface are either torn off again or moved to an energetically more favorable location as shown in Fig. 5.42. This results in dense structures with porosities between that of the cubic and orthorhombic regular packings (Fig. 5.39a - c and Tab. 5.5). It must be realized, however, that once particles are incorporated in the structure, a change of that arrangement is only possible during secondary processing or post-treatment.
5.3 Structure of Agglomerates 187
I
Fig. 5.42 Conceptual model depicting how a small particle is incorporated into the surface o f a (wet) agglomerate in a high density tumbling bed o f particles during tumble/growth agglomeration.
Pressure Agglomeration During pressure agglomeration external forces act on an at least partially confined mass of particles. As shown in Fig. 5.43 two different densification phases are conceivable (see Section 8.1),influencing the structure of agglomerates obtained by pressure. First, requiring relatively little force, particle adhesion and interparticle friction are overcome and some densification of the particle mass is realized. The amount of densification depends on the bulk density of the original feed particle mass. Sometimes the bulk density is very low, corresponding to high bulk volume, because natural adhesion forces are high, for example due to the presence of very small particles, and/or because considerable interparticle friction occurs which is caused, for example, by irregular shape and/or significant surface roughness of the powder particles. During this initial densification phase, the sizes and shapes of the feed particles are not altered or only very little, for example by breaking off some roughness peaks. Some of the pressure agglomeration techniques which make use of binders or inherent binding mechanisms do not apply forces that densify to or far beyond this point (see Sections 8.4.1 and 8.4.2). Once the densest packing of the unaltered feed particles in a confined space is obtained, - which, by the way, does not correspond to the absolutely highest density because solid particles can not flow freely into still available voids as would the molecules of liquids or gases -, once this density is reached and the external forces are
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cp
Fig. 5.43
Mechanisms of densification of particulate solids.
continuing to increase, particles deform and/or break. During this phase of compaction, structure is changed drastically and void space is reduced until the “hydrostatic” state is reached at which no further densification occurs and the compact responds like an incompressible solid (see Section 8.1).Porosity of agglomerates that are produced at high pressure is in the 10 to 20 % range or may be as low as 5 % and many closed pores have developed. Therefore, for some agglomerates resulting from high-pressure agglomeration, it may be necessary to introduce porosity that may be required or desired during post-treatment processes (see Sections 5.3.2 and 8.3).
5.3 Structure of Agglomerates
During sintering, atoms and molecules move at elevated temperatures across the interface at the contact points between two solid particles and form a bridge (see Sections 5.3.2 and 9.1). This process is influenced by temperature, contact area, and pressure as well as time. As a primary technology, agglomeration by heat occurs in bulk masses which are deposited onto a stationary or moving (“travelling”)grate. Heat is provided by hot flue gases or burning solid fuel which has been mixed into the bulk mass. As a result, the structure of “sinter”is relatively open, particularly if the heat was caused by solid fuel which disappears to a large extent (only leaving ashes) during burning. Often, sintering is used as a post-treatment process to provide strength to agglomerates (see Sections 7.3 and 8.3) or modify the porosity. Depending on how the sintering process is controlled, the agglomerate shrinks and densifies (up to almost 100 % density or 0 % porosity) or porosity is maintained and, sometimes, even increased (see Section 5.3.2). Agglomeration by Heat/Sintering
5.3.2
Porosity and Techniques That Influence Porosity
The properties of materials that are produced from fine (FP) and ultrafine (UFP) or “nano”particles by agglomeration are critically influenced by the void volume between the particles forming the agglomerate. This product attribute is called porosity and is defined by the presence, size, shape, and distribution of pores. To a considerable extent, the following text is based on two recently published books [B.Gl, B.621 which are recommended for further reading. Generally, there are two types ofpores (Fig. 5.44). Open pores are connected with the body’s surface while closed pores are isolated within and may be filled with a fluid. Penetrating pores are a special type of open pores; they feature at least two ends and connect opposing surfaces of the porous body. Pore models are typically based on cylindrical tubes (Fig. 5.45a). In reality, pores are produced by voids between particles and, therefore, feature narrow necks and wider parts (Fig. 5.45b) and form a complicated network which is strongly dependent on the
Open pores
Fig. 5.44 Schematic representation of different pore types [B.61].
Closed pores
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Fig. 5.45
Pore models. (a) Cylindrical, (b) realistic.
size, shape, and distribution of the particles forming the agglomerate (Fig. 5.46). Another special type of open pores are “inkbottle” pores (see Fig. 5.44) which have a narrow opening and expand below the surface. Porous materials are solids which contain void spaces. Agglomerates are special in that the particles forming the porous body must not be uniform. In fact, it is more common that the primary particles which are loosely or rigidly joint together by binding mechanisms are different in size, shape, structure, and composition. For most industrial applications, for example those that make use ofthe accessibility of the large specific surface area of the primary FPs and/or UFPs in agglomerates, open pores are required. Penetrating pores are necessary if fluids must flow through agglomerates in, for example, filters, fluid distributors, or catalyst carriers. Agglomerates with closed pores are primarily used for sonic and thermal insulators and for light weight building materials. Many of the statements and explanations in the following subchapters are similar to treatments already covered in the previous section (Section 5.3.1). For reasons of clarity and to better connect the items, a certain amount of redundancy will be found. Structure and porosity of agglomerates are very closely related. Porosity and Agglomerate Strength One of the equations that is most commonly used to describe the strength of agglomerates (see Section 5.2.1) connects the porosity, E , with the coordination number, k, the sum of all adhesion forces at each coordination point, A,, and the particle size, x, (Eq. 5.8). The term (1 - E ) / E indicates that the strength of agglomerates which are held together by binding mechanisms acting at the coordination points within the aggglomerates is strongly dependent on porosity. Unless special efforts are made, high porosity, a feature of agglomerates that is often desired, results in low agglomerate strength, which is equally as often not acceptable in agglomerated industrial products such as, for example, catalyst carriers. The same influence of porosity on strength is obtained for agglomerates that are completely saturated with a wetting liquid (for example water). In this case, the sum of all adhesion forces, A,, is replaced by the liquid’s surface tension, a (Eq. 5.2).
5.3 Structure of Agglomerates
Fig. 5.46 Schematic representation o f pore configurations (adapted from [8.61]). (a) Most agglomerates have a geometry o f openings between particles. Fundamentally the pore shape is angular. (b) Agglomerates produced from or containing plate-like particles have a geometry o f openings between plates. When the platelets pile up, porosity becomes less. (c) Agglomerates consisting o f or containing elongated particles usually feature high porosity which becomes less when fibers pile up. (d) An interconnected network o f
homogeneous pores is observed if, for example, in glassy agglomerates a leaching technique decomposes spinodal components. (e) In this pore geometry, large pores are connected by small ones. The removal o f pore forming agents during a posttreatment o f agglomerates results in this structure. (f) Agglomerates that are produced from porous particles have a pore structure with networks o f both large and small pores.
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If in matrix bonded agglomerates (see Section 5.1), such as, for example, concrete (matrix: cement) and road surfaces (matrix: asphalt), a single, narrowly sized aggregate would be used, large pores result requiring much binder to fill them and yielding low quality products. In addition to the possibility of many faults in the matrix (incomplete filling, gas bubbles, etc.) which act as stress raisers and cause failure, the binder becomes the quality determining part of the system and causes low strength in concrete and insufficient hardness at elevated temperatures in road surfaces. The densest packing of aggregate (= particles forming the agglomerate structure) in such materials can be determined by considering models that are based on regular packings of monosized spheres (see Section 5.3.1, Fig. 5.41). Although, in reality, neither monosized spherical particles nor regular packings are obtained, the gradation (= size fraction) of the aggregate and the respective amounts of each to achieve dense packing or low porosity and, with it, high product quality can be determined. In addition to obtaining good strength and other mechanical quality specifications, introduction of UFPs (as, for example, nano-sized silica fume) in the above mentioned models and during formulation, results in high impermeability for water in ultra high quality concrete for e.g. prestressed structures and airport runways. Typical Agglomerate Porosities Different agglomeration methods yield different por-
osities of the resulting agglomerates. In this respect, three methods with several subgroups can be distinguished: I. Growth or tumble agglomeration (Chapter 7) 1. High density tumbling bed (Section 7.4.1) 2. High shear tumbling bed (Section 7.4.2) 3. High densitylhigh shear with abrasion or crushing transfer (Section 7.4.2) 4. Low density fluidized bed (Section 7.4.4) 5. Low density particle clouds (Section 7.4.5) 6. Agglomeration in stirred suspensions (Section 7.4.6) 7. Immiscible liquid agglomeration (Section 7.4.6) 11. Pressure agglomeration (Chapter 8) 1. Low-pressure agglomeration: Extrusion through screens (Section 8.4.1) 2. Medium-pressure agglomeration: Pelleting, extrusion through perforates die plates (Section 8.4.2) 3. High pressure extrusion: Ram presses (Section 8.4.3) 4. High-pressure agglomeration a) In confined spaces: Punch-and-die pressing, tabletting (Section 8.4.3) b) In confined spaces: Isostatic pressing (Section 8.4.4) c) In semi-confined spaces: Roller presses (Section 8.4.3) 111. Agglomeration by heatlsintering (Chapter 9) I. Growth or Tumble Agglomeration (see Chapter 7) The mechanism of growth (see Section 7.1) ofthis method, the addition of single or aggregated particles to the outside of an agglomerate that increases in size, results in a typical structure with rather constant porosity. Porosity depends mostly on the size, distribution, shape, and surface
5.3 Structure of Agglomerates
morphology of the particles forming the agglomerate and very little on the growth mechanism itself. The only difference in agglomerate structure results from whether dense particle beds and suspensions (1.1, 6, and 7 ) or low density fluidized particle beds, clouds, and suspensions (1.4- 7) are present during agglomeration. In dense environments, where many particles are tumbling in close proximity, the separating forces are more pronounced, particularly if shear is induced by mixing tools (1.2). In this case, weakly attached particles or loosely associated particle assemblies are removed by attrition and have a chance to become reconnected in a more favorable, normally closer position resulting in higher density or lower porosity. In low density fluidized beds, clouds, or suspensions, on the other hand, even weak, loosely bonded agglomerates with very high porosity survive. Once definite positioning of particles in the structure of a growing agglomerate is obtained, it is virtually impossible to change the size and network of pores unless very high external forces are applied (see, for example, 11.3 and 4). Another possibility to increase density and decrease porosity during growth is to introduce high shear through mixing tools (1.2) or to install high speed choppers that are integrally mounted but individually powered and rotate at several thousand RPM to achieve abrasion or crushing transfer (1.3, see also Section 7.1). Even if forced abrasion and crushing transfer are applied, the feed particle size distribution is chosen to form dense structures, and selective agglomeration of the fine components is avoided, growth or tumble agglomeration very infrequently produces agglomerates with porosities of less than 40 % but may yield very loose structures with porosities as high as 95 % if UFPs agglomerate in low density particle clouds. II. Pressure Agglomeration (see Chapter 8) Pressure agglomeration applies external forces to shape and densify particle masses, with or without a binder, and to produce strength. Low-pressure agglomeration (see Section 8.4.1) is carried out by passing (extruding) plastic and sticky particle masses through screens or contoured, thin, perforated metal sheets ( I L l ) , whereby mostly shaping with very little densification occurs. It results in agglomerates which, after drying, typically have a porosity of between 40 and GO %. No change of this porosity is possible after extrusion. Some reshaping and surface densification is achieved during spheronizing, a rounding process that may be applied to still plastic extrudates (see Section 8.3). In medium-pressure agglomeration (see Section 8.4.2), the extrusion through perforated die plates (11.2),higher densification can be obtained by two effects. First, the feed, that must have binding characteristics and be somewhat plastic, is predensified prior to extrusion by the screw(s) or between the press roller(s) and the die. Secondly, the material is passed through holes in the die where pressure builds up due to wall friction. The length over diameter ratio of the hole (= extrusion channel) and its shape as well as the properties of the feed determine final densification. Because of the wall friction, the surface of the extrudate is always denser than the center (see also Section 8.2). After production, no change in porosity other than potentially caused by some shrinking during drying is possible. Porosity is typically in the range of 30 to 50 %.
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During high-pressure agglomeration (see Section 8.4.3) the forces may be so high that plastic deformation and flow or breakage of brittle particles followed by rearrangement can occur (see also Sections 5.3.1 and 8.2). Correspondingly, porosities can be as low as 5 % or even less, approaching theoretical density. On the other hand, it is relatively difficult to produce compacts with porosities of more than 30 % without resorting to some post-treatment method. Such possibilities (see below) are rather important because punch-and-die and isostatic pressing methods (II.4.a and b) are often used to produce green bodies, particularly in ceramics and powder metallurgy, which may require a controlled network of pores in the final product with larger porosity than originally obtained in the green part. A particular characteristic of all pressure agglomeration methods is that the density of a compacted body made from particulate solids is not uniform (see Section 8.2). This is due to the presence of adhesion and frictional forces between the particulate mass and the pressing tools, the existence of interparticle friction, and the division of force at the points of contact between the particles. This phenomenon is most pronounced in products from high-pressure agglomeration in confined spaces (II.4.a) which is the method of choice for “near-net-shape” green parts in the ceramics and powder metal industries. Because non uniform density leads to deformation during the post-treatment process that is required to obtain final strength and properties, isostatic pressing (11.4.b),both cold and hot, has been developed in which the pressure acts from all sides through a fluid in an autoclave thus producing green bodies with uniform density distribution. This does not mean that density and porosity are constant throughout the body; density is still higher at the surface and less in the center due to the dissipation of forces and interparticle friction. However their distribution is uniform and, therefore, does not cause uncontrolled deformation during post-treatment. Elastic and/or fibrous materials such as many organic materials (peat, lignite, straw, hay, wood, vegetable, and other plant materials as well as their wastes, etc.) pose a particular problem in agglomeration. Unless powdered in some way, which is expensive, such materials can not be agglomerated by growth and tumble agglomeration. It is also difficult to successfully use pressure agglomeration because after the typically very short application of force, deformation and densification are not permanent and elastic spring back occurs after pressure release (see Section 8.1). For such materials ram presses (11.3) are used. They feature a long extrusion channel in which many compacts are held under pressure by the influence of elastic forces and wall friction. These compacts undergo repeated new densification and deformation phases when moving forward with each ram stroke, thus producing stable briquettes even from highly elastic materials (see Section 8.4.3). Because of the particular shaping and densification process, porosity is very low on all surfaces but relatively high inside. Products are ideally suited as primary or secondary fuels. Ill. Agglomeration by Heat (see Chapter 9) At elevated temperatures, close to the melting point or softening range of a material, the atoms and molecules at the surface of a solid particle become so mobile that they can move across the interface at a contact point between two particles; solid bridges, so called sinter bridges, develop.
5.3 Structure of Agglomerates
Although this phenomenon may occur with any material at the appropriate temperature (including relatively low ones for e.g. some man-made plastic and other organic materials), sintering was originally invented as an agglomeration method for metal ores. In this process, ore fines are mixed with a solid fuel (e.g. coal); the mixture is then loosely deposited as a fixed bed on a stationary or moving grate, ignited, and kept burning by passing air through the bed. As a result, sinter bridges develop while the fuel is consumed and leaves large voids. After cooling, the agglomerate is broken into suitable pieces and used as feed for blast furnaces. During the reduction process, the large voids and a relatively open pore network provide good access of reducing gases to the ore particles in the sinter structure. Sintering is also used as a post-treatment process to obtain final and permanent strength and structure of green bodies which were formed by agglomeration (see Sections 7 . 3 and 8.3). Methods for Influencing the Porosity o f Agglomerates It is possible to produce, for example, quickly disolvable, “instant”,or easily dispersible granules (so called water soluble or dispersible granules, WSGs and WDGs) with sufficient strength for storage, handling, and application by tumble or growth agglomeration, particularly in low density fluidized bed agglomerators. It is also feasible to manufacture agglomerates with pores of atomic scale for gas separation or catalysis from zeolites, silica gel and other similar materials featuring such small pores by sticking the nanoporous primary particles together and connecting them with the relatively large interparticle voids of the agglomerate (Fig. 5.469. In one application, hardening binders, that ultimately result in high strength and also seal the large voids between the particles, are used to avoid a partially penetrating open network of coarse pores, and pressure agglomeration methods are selected to obtain well formed product shapes as designed for a particular application. In another case, a bimodal pore structure, in which a penetrating network of large pores is maintained by selecting a suitable binder that acts only at the coordination points between the particles, allows selective filtration. Such structures are, for instance, used in bioreactors. Enzymes or bacteria are immobilized in the small pores and the large pores are used as channels for transporting reactants and products [B.61]. The large group of agglomerated materials with high permanent strength, large porosity, and mostly open or even penetrating pores require post-treatment methods to achieve the theoretically impossible: high strength and a large amount of voids. In the following, some such methods will be discussed. For more information, the book “Porous Materials” [B.61] is recommended as additional reading material. An important mechanism for the production of porous products during a post-treatment process is sintering of green agglomerated bodies. The driving force for sintering is the reduction of surface area that is associated with pores. The free surface of the particles in the agglomerate has a specific surface energy which is caused by the atoms in the surface and the lack of opposing atoms. A decrease of the surface results in a reduction of surface energy. Therefore, the total free energy of an agglomerate decreases during sintering. At the same time the body often becomes denser. This phenomenon is shown in Fig. 5.47 in a very basic configuration of three spheres.
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Particles in contact
Particles adhere and form necks
Necks grow and open porosity decreases
Open pores disappear and closed Necks become large and pores change shape to spheroidal pores appear Grain boundary migration occurs leaving spherical closed pores isolated from grain boundary diffusion routes Fig. 5.47 The phases of sintering that are associated with shrinkage as shown in a simple model involving three spherical particles; adapted from [B.61].
Fig. 5.48 depicts schematically the sintering processes that can occur between two spherical particles. The numbers correspond to those in Tab. 5.7. It can be concluded that several mechanisms causing material transport (=diffusion paths) do not produce densification. While sintering which results in densification is well researched (see Section 9.1), diffusion phenomena which do not cause shrinkage and are, therefore, important for the production of porous products are less known. For the realisation of porous products during sintering two important preconditions must be fulfilled. One is the manufacturing of green agglomerates with homogeneous packing structure and low density. The other is the necessity to influence the sintering process such that no significant densification occurs but strong, well shaped bridges
Fig. 5.48 Model o f the sintering processes that can occur between two particles [B.61]. (a) Depicts schematically bonding. X, Y, a, and p are: bridge radius, penetration depth, particle radius, and neck radius, respectively. (b) Is the bonding area in more detail showing the diffusion paths (Tab. 5.7).
5.3 Structure of Agglomerates Tab. 5.7
Description o f path routes of diffusion during sintering.
No.
Diffusion path routes
Diffusion source
Diffusion sink
Densification
1
Surface diffusion
Surface
Neck
No
2
Volume diffusion
Surface
Neck
No
3
Evaporation/condensation
Surface
Neck
No
4
Grain boundary diffusion
Grain boundary
Neck
Yes
5
Volume diffusion
Grain boundary
Neck
Yes
G
Volume diffusion
Dislocation
Neck
No
develop. The first precondition is obtained by adjusting and controlling the powder characteristics and by CIPing (cold isostatic pressing; see II.4.b, above, and also Section 8.4.4)with low pressure. The second is accomplished by using sintering conditions during which surface diffusion and evaporation/ condensation prevail. These requirements differ considerably from those of “normal”sintering for the production of dense products. Porous structures in green agglomerates can be achieved if the feed particles are preagglomerated (Fig. 5.49) or consist of bi- or multi-modal particle size fractions. Another method is the production of loose packings by sedimentation. Then, to retain the open porosity, the sintering process must be carried out at a low temperature and for a short time or, to obtain more open pores, pore forming additives are mixed with the powder. Pore forming additives can be either liquids or solids. Liquid additives are used if the green agglomerate is produced by either extrusion, injection molding, or casting. In those cases, the porosity of the green body is controlled by the amount of liquid in the viscous mix. Because liquids are not compressible they occupy volume elements which
Fig. 5.49
Pores in a structure made up o f pre-agglomerated particles [8.61].
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remain as pores after drying. If the liquid is a solution, strength is produced during drying by recrystallization. Depending on the final application and strength requirements, the dry agglomerate can be further strengthened by sintering. Solid pore forming additives (Tab. 5.8) are mixed with the primary particles and the blend is then agglomerated with any of the different available methods. As discussed above, agglomeration by heat (111, Sintering) produces directly a porous agglomerate when the solid fuel burns and, with exception of its ashes, disappears. Green agglomerates obtained by other methods are subjected to one or more post-treatments during which final strength develops and the pore forming agent burns, evaporates, or is removed from the structure as a liquid leaving pores behind (Fig. 5.50). It is difficult to produce small pores with this technique because very fine pore forming solids tend to selectively agglomerate during mixing so that they do not distribute uniformly. The low cost method is, however, very good for the production of porous materials with large, open pores. Tab. 5.8
Examples from literature of solid pore forming additives (for refs. see [B.61])
______~_____
~
Ammonium tetrachloride Carbonyl Coal Iodine fluoride Paraffin Petroleum coke Spherical polymer (PMMA) Wood dust
Carbon black Charcoal Dextrin Melamine Peridur" Salicylic acid Starch
Another possibility for the manufacturing of solid and permanently porous agglomerates is the utilization of binders which melt during post-treatment at high temperatures and solidify during cooling to solid bridges (so called vitrification). Fig. 5.51 explains the process schematically. Reaction sintering is another technology that can be used for the production of solid, porous materials. By this means, chemicals in the powder mixture react under specific conditions with the atmosphere and/or other particles whereby solid bridges are formed. Porous ceramics may be produced by reaction sintering which is then called reaction bonding.
Fig. 5.50 Schematic representation of pore forming with solid additives [B.61].
5.3 Structure of Agglomerates
Fig. 5.51 Schematic representation of pore forming by vitrification [B.61].
It is often difficult to produce a highly porous body, which, in the green state, has little strength, and to sinter it without destroying its shape during handling and losing porosity. One possibility to overcome these problems is to sinter the green compact in the pressing tool by passing electric current through it. A new technology which is particularly suitable for the production of porous products is pulsed electric current sintering (PECS) which is sometimes also called spark plasma sintering (SPS). Hot isostatic pressing (HIP) is often applied to densify sintered materials and to correct casting defects. Recently a new HIP process for the production of porous materials was developed. Partially densified compacts are sintered in a high pressure atmosphere. The pressurized gas in the open pores delays densification and porosity remains largely intact. Finally, the so called Sol-gelprocesses should be mentioned in which porosity, pore size, and polarity of products manufactured by this method can be controlled. Processing begins with a solution (= sol) which becomes a gel. The solution can be prepared from either inorganic salts or organic compounds. It is then hydrolized to a sol or condensed to form a gel. The process can be terminated in the sol-phase, where a dispersion of colloidal particles in a liquid exists, or continued to the gel-phase, the development of a three-dimensional, linked solid structure in which the pores are filled with a liquid. In the wet gel-phase, the pores are interconnected. The process of gel forming or gelation involves first the formation of particle clusters which are held together by hydrogen bonds with silanol groups (Fig. 5 . 5 2 ) followed by the development of a three-dimensional network which is strengthened by bridge growth between the particles. The size of the primary particles and their coordination number determine the porosity and the average pore diameter. In addition to a large number of parameters that control “Sol-Gel”processing, the drying conditions are most important for the production of porous materials. During “normal” drying, considerable shrinkage occurs which is attributed to the negative capillary pressure which depends on the liquid’s surface tension. To minimize this effect, the original liquid which may have high surface tension can be exchanged by one with lower surface tension prior to drying. As mentioned previously, for additional information the book “Porous Materials” [B.Gl] should be consulted.
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5 Agglomeration Theories ~ i - nI/
Fig. 5.52 Schematic representation of changes during the gelation o f particulate sols. Dots represent hydrogen bonds; curved lines are micelle surfaces.
5.4
Other Characteristics o f Agglomerates
As the particle size of solids decreases, many characteristics of individual units change. Particle behavior in bulk masses also changes. Tab. 5.9 illustrates the influence of particle size, listing some important examples. In this context it is immaterial how size is defined: the table considers the relative decrease of a linear particle dimension which is randomly called “particle size”. The upper part of Tab. 5.9 presents Influence of particle size on some important characteristics of tine particulate solids <0.5 mm.
Tab. 5.9
Characteristics o f Single Particles
With Decreasing Particle Size
Homogeneity Elastic/plastic behavior Probability of breakage Strength Resistance to attrition Vapor pressure, solubility, reactivity, etc. Color perception and intensity
Increasing Increasing ductility Decreasing Increasing Increasing Increasing Changing
Characteristics o f Bulk Masses
With Decreasing Particle Size
Bulk density (space-filling behavior) Flow characteristics, Flowability (of particles) Mixing efficiency Separating efficiency Ease of fluidization Occurrence of undesired agglomeration Dusting - Losses - Ignition Behavior/Explosiveness
Decreasing Decreasing Decreasing Decreasing Decreasing Increasing Increasing Increasing
5.4 Other Characteristics ofAgglomerates
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characteristics of single (individual) particles, while the lower describes the behavior of bulk masses. The most important single particle characteristics are homogeneity and specific surface area. Both increase with decreasing particle size. Homogeneity improves because, during size reduction, breakage begins preferably at faults (pores, microcracks, imperfections, contaminants, and others) leaving the resulting smaller particle more and more without flaws until a perfect structure is obtained. Therefore, as size reduction progresses, materials that first exhibit brittle behavior become increasing ductile, the probability of breakage decreases, and strength or resistance to attrition increase. And, if fine or ultrafine particles are produced directly, for example by precipitation, the probability that imperfections develop decreases with smaller particle size. Since volume and mass decrease with the third power of the characteristic linear particle dimension and surface area is only proportional to the square of that particle property, the specific surface area, expressed in m2/cm3or m2/g, increases with decreasing particle size. This results in higher vapor pressure, improved solubility, and increased reactivity as well as related changes of other, more complex properties. Also related to size and surface characteristics are the changes of color perception and intensity. As knowledge of these and other particle characteristics became more readily available by applying new and powerful microscopes, miniaturized tools, and novel methods of investigation, interest increased in the production and use of ultra-fine particles (UFPs) with physically predictable and sometimes new properties (so-called Nano Technology). Many particles that require extreme cleanliness in, for example, the food, pharmaceutical, fine chemicals, and metals industries are obtained by precipitation from gases or liquids yielding UFPs [B.55]. Fig. 5.53 depicts the size ranges of particulate matter in process technologies together with some reference points (i.e. radii of atoms, sizes of viruses, wave lengths of visible light). The particle size range of mechanical processes and of all mechanical unit operations (see Chapter 1, Fig. 1.1)now extends down to nano meters. Although, in comparison with the atomic scale, ultra-fine particles <0.1 pm or
Processes with changes in atomic structure
Processes of colloid physics and biochemistry
-----
I
Viruses
--Mechanical processes
_ 1 1 _ 1 1 -
Thermal processes
--- Wave lengths of visible light
------Macromolecules + --
I
Radii of atoms
Including electrical + magnetic separation
--------
Molecular disperse
Disperse systems
------.---.+.------.----------------+------------.
Colloid ,o-13
10-12
1fm
Fig. 5.53
104 10-8
10-l~ 1Prn
IA
10.7
10.6 10.5
Inm
Particle sizes o f various disperse systems.
Fine
lo4 1I.lm
Coarse
1 0 . ~ lo-* 10.’ l o o 10’ l o 2 [cm] Irnm I c m I d m I m
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bodies, their size is less than optical wave lengths and, therefore, require electron microscopy to make them visible, i.e. individual nano-particles can not be observed optically. Since the smallest UFPs only contain approx. 100 atoms, new particle qualities may be defined by answers to questions such as: “How many atoms are required for a particle to exhibit characteristics that are common to a metal?” or “When does a particle begin to show biological interactions and functions?” [B.55]. UFPs of a particular size are the smallest units of solids, just as the smallest unit in the world of microorganisms today is thought to be the virus, a “particle” about the same size as that of UFPs. Therefore, further to the changes of properties with decreasing size of single fine particles as shown in Tab. 5.9, additional, new, and desirable characteristics are attributed to nano-particles. They also include, for example, biochemical interactions with microbes [B.55]. The disadvantages of fine and ultra-fine particles in bulk relate mostly to the storage, handling, and processing of such disperse systems. Interactions of extremely small particles with each other or various components in bulk masses cause a number of problems. Many result from the changing competition between volume and surface related forces. For example, since the weight or mass of particles decreases faster (volume related) than the adhesion force between particles (surface related), very fine particles adhere naturally to each other or to larger entities. This phenomenon causes decreased bulk density or increased bulk volume, reduced flowability,lower mixing or separation efficiencies, reduced ease of fluidization, increased tendency for undesired agglomeration (such as bridging, caking, build-up, etc.). Material losses due to dusting result from small size and mass while increased selfignition behavior and explosiveness (dust explosions) or, generally, high reactivity relate to the large specific surface area of fine powders. As shown in Tab. 5.10, size enlargement by agglomeration can improve the bulk properties of particulate solids in many ways. Agglomeration is characterized by an increase of the apparent size so that, for example, dusting or material losses are reduced and flowability is improved. At the same time the characteristics of the single particles, such as surface area and, with this, solubility, reactivity, and other Tab. 5.10
Some of the most important advantages of agglomerated products.
No or low content of dust: therefore, increased safety during handling of, for example toxic or explosive materials, and, generally, fewer losses which may cause primary or secondary pollution. Freely flowing. Improved storage and handling characteristics. Improved metering and dosing capabilities. No segregation of co-agglomerated materials. Increased bulk density and lower bulk volume. Defined size and shape. Sometimes, defined weight of each agglomerate. Within limits, porosity or density can be controlled; thus, dispersibility, solubility, reactivity, heat conductivity, and other properties can be influenced. Improved product appeal. Increased sales value.
5.4 Other Characteristics of Agglomerates
related properties, are maintained. This is due to the fact that agglomerates are porous bodies, in which the primary particles, held together by binding mechanisms, are still identifiable and retain most of their individual qualities. Depending on the specific use of agglomerates,the typical advantages listed in Tab. 5.10 may have different importance. For example, in waste treatment, recycling, or pollution control, the low content of dust and, respectively, reduced dustiness are generally important, also to prevent secondary pollution. Flowability, on the other hand, often has little priority in these applications.The same is true for improved metering and dosing capabilities as well as for defined shape and weight. Quite significant are, however, improved handling and storage characteristics, often due to increased bulk density and lower bulk volume. Co-agglomerated materials, from mixtures containing many different components with often considerable differences in characteristics and amount, do not segregate. This plays a role in the pharmaceutical industry where the active drug component and various excipients are co-agglomerated to form a free flowing, dust free, non segregating feed material for high speed tabletting presses. It is also important when hazardous materials are incorporated into concrete blocks. In such cases it is often also necessary that the hazardous components will not leach and that the binder does not age and disintegrate. If wastes become secondary raw materials, improved characteristics resulting in enhanced appeal and increased value are often desirable. These are just a few examples with many more in existence. In the next decades, particulate solids processing will focus on a shift away from just attaining size and distribution of products toward the more fundamental area of microstructure and morphology. The emphasis of new products and processes will be on better control of primary particle physical properties, highly specific product size and composition, as well as the creation of desirable characteristics. This will lead to an increased demand for “engineered”particulate materials with high value that are produced in low tonnages. In the future, for many products special consideration will be on high value and special effects rather than “simple”bulk commodities. In general, the industry will have to cater increasingly to the needs of the end user. Agglomeration science and technology will play a vital role in the search for such novel, differentiated particulate products and the means for making them. A major requirement in these new areas is for a much greater degree of flexibility in regard to agglomerate structure and morphology. This will not only require a deeper understanding of the fundamentals of agglomeration but also an interdisciplinary effort of mechanical, chemical, and process engineers with physical chemists, colloid and biochemical scientists as well as other researchers in various industries. Among the many “engineered”products that exhibit specific properties after size enlargement by agglomeration are the following: Easily soluble (“instant”)products Easily dispersible products Easily degradable products Highly absorbent, stable products Highly porous, stable products
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Products with controlled reactivity - Controlled high reactivity - Precisely controlled reactivity - Reduced reactivity
This list is by no means exhaustive and new “engineered”,agglomerated materials with specific characteristics are being developed at an accelerating pace. Many of the new products are results of or influenced by advances in nano-technologies. Easily Soluble (“Instant”) Products The manufacturing of easily soluble, “instant” granules is one of the oldest and most thoroughly researched applications of “engineered” products. The term “instant” is normally used in the food industry and related fields, such as pharmaceuticals and animal feeds, to describe characteristics of drink powders (including coffee, tea milk, milk replacers, soft drinks, vitamins, medicated powders, etc.), soups, sauces and the like. Instant agglomerates are also desirable for pigments and other chemicals that are ultimately dissolved in a diluent. All instant products must quickly dissolve in a specified solvent or, more generally, in liquids of any kind and temperature (particularly also at ambient or even cold conditions) without residue or sediment. Each industry has a more or less well defined procedure to determine the maximum allowable time. Typically, complete dissolution should be accomplished within a few seconds in warm liquid and approx. 30-GO s in cold liquid. Special drink powders will have to meet the shorter times, even in cold liquids. Such instant granules may contain certain substances that assist in the breakup of the granules during the dispersion phase (see Section 5.1.2). Easily Dispersible Products Easily dispersible products are very similar to “instant” products. The only difference is in the fact that the primary particles are not soluble. Typical examples are pigments, carbon black, silica fume, etc. There are two different applications for easily dispersible agglomerated products: 1. Products that are dispersible in liquid phase. 2. Products that are dispersible in solid phase.
In addition, transitional applications exist which require that agglomerated products must be dispersible in wet bulk solids or slurries.
ad 1) Dispersion in liquid phase must achieve complete separation of the primary particles to obtain statistically uniform mixing with other solid particles that are present in the liquid phase, or, if the particles are small enough, to produce a suspension. Suspensions should be stable indefinitely or for a long time and not form sediments. To further reduce the formation of a sediment, the viscosity of the liquid can be increased by incorporating so called stabilizers into the agglomerate which are liberated during dispersion and “thicken” the liquid. Such materials may be pregelatinized starch, pectines, alginates, and the like. Because the primary particles are not soluble, the break-up of agglomerates must occur only by a weakening or destruction of the binding mechan-
5.4 Other Characteristics of Agglomerates
ism(s).For that reason, it is much more common in dispersion that high shear forces are applied by liquid mixing techniques. The effect of shearing is often enhanced by a high viscosity of the liquid or the developing suspension. ad 2) Other easily dispersible agglomerated materials are manufactured to improve the storage, transportation, and metering characteristics of an intermediate product. During the mixing with bulk solids that are dry or contain various amounts of moisture (e.g. slurry) such agglomerates must disintegrate into the primary, finely divided or nano particles. In addition to considerable shear forces, mass related forces are created which help to overcome the binding mechanisms in the agglomerates. If moisture is present (for example during the production of concrete) the influence of liquid on the strength and survival of binding mechanisms should be also considered during product design. Easily Degradable Products Typical examples are carrier materials for fertilizers, insectizides, fungizides, and many other chemicals. In liquid phase the active substance is highly concentrated and, in most cases, toxic. By adsorbing these liquids on the mostly inner surface of granules that are manufactured from fine particles by agglomeration, the toxin is diluted and the products are rendered safe for handling and application. For example, spreading of granular agro-chemicals by conventional equipment is possible. Newer applications of the technology also include special micronutrients. Easily degradable carrier materials must feature a large accessible (inner) surface area, i.e. high porosity, and sufficient strength to withstand processing, storage, and handling. The binding mechanism must survive the impregnation process with the active substance. For example, if molecular or electric forces were used for dry agglomeration, it is possible that, after impregnation, the active substance replaces (e.g.by recrystallisation during a drying step) or enhances (e.g. by viscous liquid bonding and/or chemical reaction) the original binding mechanism. On the other hand, the granules must break down easily and quickly under the influence of moisture either from the soil or the atmosphere (rain or dew). Therefore, the product must easily wet and even small amounts of moisture should reduce the strength. Surfactants improve wetting and components that swell in the presence of moisture may assist breakdown. Sometimes, particularly in the case of fertilizers and micronutrients, interactions with bacteria participate in the breakdown. Highly Absorbent, Stable Products The basic requirements for these materials are the same as for the previous ones. However, as compared with “easily degradable” products, after absorbing liquids the granules must not disintegrate. For easy application and disposal, they should not be as hard and stable as the “highly porous, stable” carrier materials (see below). Typical examples are cat litter and absorbents for nuisance liquids (e.g. spilled oil).
Most absorbents are consumer products. For that reason, a new requirement is customer appeal. For absorbents this means that the material must be uniform, easily spreadable, and completely dust free with high abrasion resistance. In both areas
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of main interest, cat litter and absorbents for spilled liquids, considerable research and development have been carried out in recent years to “engineer”the product for specific uses. One major requirement is particle size; an optimal size exists for perfect wetting. Another is particle shape. For cat litter and for some liquid absorbents spherical shape is undesirable. It has been found that cats tend to play with spherical particles, and round liquid absorbents scatter too widely during application thus, in both cases, causing widespread contamination. On the other hand, some absorbents are round to allow easier distribution into far corners and to produce a more uniform layer at the spill site. While, nowadays, cat litter is supposed to lump for easy removal of the used portion from the litter box, granules applied for the clean-up of liquid spills are supposed to remain separate for effective gathering with mechanical cleaning equipment. Although all absorbents must retain some strength and not convert to slush, mud, or slurry they must disintegrate when being stepped on to avoid accidents by slipping. Particularly cat litter must also absorb odors and/or provide a pleasent smell. Therefore, antibacterial and odor-suppressing substances or perfumes are added whereby it is more important that the smell pleases the cat than the human owner. Highly Porous, Stable Products Another group of agglomerated materials is used, for example, as carriers for catalysts. These products must feature high, easily accessible surface area before and after impregnation with the catalyst. In contrast to the “easily degradable” or “highly absorbent” products discussed above, these carrier materials must be permanently strong and stable. They should not break down during the chemical process which is often accompanied by harsh conditions, particularly high temperatures and aggressive chemicals. The primary particles forming the carrier itself need to be inert in respect to the chemicals used for impregnation and during reaction. In addition, the shape and structure of the carrier must be defined and uniform and the strength high to guarantee good permeability of the activated carrier (catalyst)bed. Little or no mechanical breakdown must occur due to the overburden pressure in the column. Products With Controlled Reactivity This group of agglomerated materials with adjustable, specific characteristics includes the widest range of choices. Products may have controlled high reactivity, for example explosives as well as specialty products, such as airbag chemicals, precisely predetermined reactivity, e.g. chemical oxygen generators and pyrotechnic components, or drastically reduced reactivity as, for instance, densified iron and titanium sponges (the products resulting from direct reduction of the oxides of these metals) or hazardous (including toxic and radioactive) wastes. In contrast to the previously discussed products, no common requirements can be defined for materials with controlled reactivity.Therefore, a few examples in the areas of:
Products with controlled high reactivity Products with precisely predetermined reactivity Products with drastically reduced reactivity will be discussed in the following.
5.4 Other Characteristics of Agglomerates
Products With Controlled High Reactivity High reactivity of particulate solids is con-
nected with large specific surface area and, respectively, small primary particle size. Because reactions take place on the surface of solids, agglomerated products with high reactivity must feature voids which are interconnected with a continuous network of pores that is accessible from the outside. In this respect, their structure is similar to those that are easily wettable. The main difference is, that, in most cases, the chemicals reacting with the solid are gases. The most common reactions are oxidation and reduction. In the case of explosives or, for example, chemicals that activate airbags, large amounts of gas (products of combustion) are produced during a very fast reaction which, in addition, expand because of high system temperatures thereby creating the destructive forces or the pressure in an airbag. After activating a primary detonation, the chemical reaction of these explosives is a rapid decomposition by oxidation whereby the necessary oxygen may be part of the material itself or made available from oxygen-rich molecules such as chlorates. The effect of an explosive depends on its density, the energy produced during reaction, and the speed of reaction. Particularly if oxygen is made available from separate oxygen carriers in a mixture, it is necessary to provide good contact between the components. Product characteristics can be adjusted by modifying the degree of densification. At higher density the stored energy concentration increases but, because of lower surface area, the unassisted reactivity decreases. The reaction of such explosives must be initiated with primers. A primer is a highly reactive material that is easily ignited by friction, percussion, or electricity and will, in turn, set off a less reactive explosive. High reactivity is also desirable of other agglomerated products. One of the most important large scale application of size enlargement by agglomeration is the pelletization of iron oxides (see also Section 11.8).The technology was first developed for iron ores the concentration or purity of which are too low for economic processing in the blast furnace. Typical examples for such ores are the Taconites with large reserves in North America or the Itabirites. To make these ores economically useful, they must be upgraded. Prior to any of several concentration processes the ore must be ground to below separation fineness which is defined by the ore structure and the requirement that each particle contains only iron oxide or gangue. With the before mentioned ores this particle size is <45 km (<425 mesh). Such concentrates can not be fed to and reduced in the blast furnace. To overcome this problem, iron ore pelletizing was developed. Because the size range of the resulting pellets is very narrow, new important advantages in the blast furnace were obtained. One results from the more uniform bed structure which increases permeability and thereby process speed (production capacity). The other is due to the fact that, in comparison with the performance of dense, natural lump ore, the porous agglomerates, which consist of small primary particles, feature a significantly better reducibility. The latter is so important that, in the meantime, iron ores which do not need upgrading are also pelletized. In that case, the ore concentrate must be ground to the particle size that is required for tumble and growth agglomeration.
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Products With Precisely Predetermined Reactivity To this group belong, for example, pyrotechnic articles or components and chemical oxygen generators. These materials must feature precisely predetermined, reproducible, and often variable speed(s) of reaction. Pyrotechnic articles contain components that transmit ignition and produce light, sound, fire, and explosion effects such as coal, sulfur, milk sugar, resins, etc. as easily combustible materials, nitrates and chlorates as oxygen sources, and various chemicals for special effects. Among others, antimony sulfide and aluminum or magnesium powders emit white light while colored light is produced by sodium (yellow), strontium nitrate (red),barium nitrate (green),and caustic copper carbonate (blue);aluminum, magnesium, and iron filings exhibit sparks and mixtures of potassium nitrate and potassium picrate result in hissing sounds. In the pyrotechnic article these components are packed with materials that provide ignition, fire, explosion and, in the case of rockets, propulsion. The other product featured in this group is the chemical oxygen generator. These “oxygen candles”, which are, for example, installed in aircraft to provide each passenger with oxygen if an emergency arises, must produce a variable amount of oxygen during a period of time in which the plane descends to lower altitudes. The production of oxygen occurs during the thermal dissociation of chlorates and perchlorates. Therefore, the oxygen candle is an agglomerate containing chlorate, fuel, and catalyst and is ignited by a primer charge that is activated when the oxygen mask is pulled down. It is a requirement that immediately after decompression a large amount of oxygen is made available as it is assumed that the emergency begins at great altitude. Because it is assumed that the aircraft quickly descends to a denser atmosphere, during the “candle’s’’ 12 to 25 min of burning time, the production of oxygen can diminish. Products With Drastically Reduced Reactivity There are many products which, immediately after manufacturing, are highly reactive mainly because of their small particle size and/or large specific surface area. The greatest danger with such materials exists in potential self ignition and/or spontaneous combustion. Since many such reactions are exothermic, the reaction may accelerate and result in a catastrophic development of heat. In other cases, if, for example, water provides oxygen for the reaction, hydrogen is liberated which may produce explosive mixtures with air. In recent years an important development in the field of iron and steel production is the direct reduction of iron ore. In this technology iron oxide is reduced to virgin iron in the presence of suitable reductants (often CO and/or H,) and at elevated temperatures (but still in solid state). After the removal of oxygen a highly porous, sponge-like iron product remains that features a very high specific surface area. Depending on its microstructure, DRI (direct reduced iron) or “spongeiron”has a self ignition temperature in the order of 150 to 230‘C [5.9, 5.101. Even at ambient temperaures a slow reoxidation occurs. With water as oxygen carrier, sponge iron reacts quickly thereby producing heat and hydrogen. Sea water, an electrolyte, accelerates this process. Because all reactions proceed exothermically and due to its structure, direct reduced iron is a heat insulator, in the core of a bulk mass (e.g. storage piles or ship holds) a catastrophic heating can occur that stops only ifthe source of oxygen (i.e. water and air) is
5.5 Undesired and Desired Agglomeration
removed or replaced. Without passivation, fine sponge iron from fluidized bed reduction processes can not be stored and handled openly. To overcome this problem, the spongy structure is destroyed and the specific surface area, particularly on the exterior of the product, is reduced so that it becomes inert at all technically relevant transport, storage, and handling conditions. In other examples of this group, toxic, radioactive, or hazardous particulate solids of any size, from large pieces with dimensions of several centimeters to dusts and nanosized particles, are agglomerated to render them safe for disposal. In those cases it is necessary to produce an “agglomerate” which is stable for long periods of time, does not emit radiation or gaseous components and is not leached by liquids.
5.5
Undesired and Desired Agglomeration
Agglomeration is a natural phenomenon. Therefore, under certain conditions, it happens, whether it is desired or not [for specific ref. see B.561. Tab. 5.11 lists the operations of Mechanical Process Technologies (see Chapter 1, Fig. 1.1)and indicates if agglomeration is desired or undesired or, sometimes, both. Separation During separation unwanted agglomeration may occur and must be
avoided if a particle collective is to be divided into well defined classes. The separation curve is a measure for separation quality. In a diagram the degree of separation (the percentage amounts of particles above and below the desired separation size) is plotted versus the particle size. Fig. 5.54 is a qualitative presentation of several separation curves. Line (a) in Fig. 5.54 represents the ideal or perfect separation of a particle size distribution x,,<x<x,, at cut size xtl which is possible only in theory. In industrial separation equipment, curves of the type (b) are obtained. The cut size is that particle
Ideal separation at x,, Technical separation at Ideal separation Technical separation; separation limit xrZ Technical separation with agglomeration separation limit xIy. I
Fig. 5.54 Examples of separation curves.
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5 Agglomeration Theories The occurrence of undesired and desired agglomeration in mechanical and related process technologies.
Tab. 5.11
Unit Operation
Process
Agglomeration Undesirable
Separation
Mixing
Comminution Particle size enlargement
Conveying
Storage Batching, metering Drying
Screening, sieving Classifying, sorting Flotation Dust precipitation Clarification, thickening Particle size analysis Dry mixing Wet mixing Stirring Suspending, dispersing Fluidized bed Dry grinding Wet grinding Agglomeration Briquetting Granulating Pelletizing Pelleting Sintering Tabletting Mechanical conveying Vibratory conveying Pneumatic conveying Silos, hoppers, stockpiles
Explanations: (+) sometimes yes,
+ yes, ++ decisively yes,
Desirable
+ +
+ + + +
+
++ ++ ++ ++ *+ ++
++ i+) i+) -
(-) sometimes no, - no, - - decisively no
size of which half end up in the coarse fraction and half in the fine. The sharpness of separation increases with the slope of the curve. If the abscissa uses a logarithmic scale, separation curves representing similar separation efficiencies at different cut sizes are parallel to each other. The influence of agglomeration must be judged differently. If the separation task is to remove all particles x,,,<x<x,, from a fluid the desired cut size is x,,,. Line (c) in Fig. 5.54 describes the ideal, only theoretically possible separation curve. In reality, a certain amount of smaller particles remains suspended in the fluid and the actual cut size is xt,>xmi, (curve d). If agglomeration occurs, the finest particles may form larger entities or attach themselves to larger particles thereby changing the separation curve in Fig. 5.54 to (e).The new cut size x,,.is still somewhat larger than the desired xmi,, where all particles would have been removed from the fluid, but agglomeration definitely helps to move the actual cut size closer to the ideal one. In general, as described above, whenever the task is to remove all particulate solids from a fluid, agglomeration will be advantageous. Since particularly the smallest, low
5.5 Undesired and Desired Agglomeration
mass particles are, on one hand, the most difficult to remove and, on the other hand, feature the highest natural adhesion tendency, the chance agglomeration of these particles improves separation efficiency. As will be shown later (see Sections 7.4.5 and 7.4.6) techniques for enhancing the natural agglomeration tendency of very fine particles are often applied during gas and liquid cleaning. Even relatively loose conglomerates of particles behave according to their combined weight during, for example, settling and particularly in the centrifugal fields of cyclone separators. For all those separation cases, however, which attempt to separate a particle collective according to certain properties of the particulate solids, agglomeration is most often undesired. Techniques for which this statement is true include screening, sifting, classification, sorting, flotation, and, as a general analytical method, particle size characterization. It should also be realized that the respective separation property is not only size; it could be density, shape, color, chemical composition, and others. During screening, unwanted agglomeration is often facilitated by the motion of the material on the screen; spherical agglomerates are frequently formed from material containing fines or featuring other binding characteristics, for example if it is moist. Among others, binding mechanisms are: For finely divided solids, molecular and electrical forces and/or adsorption layers, for plastics, electrostatic forces, for ores, magnetic forces, for moist powders, liquid bridges and capillary forces, for fibers, interlocking, and for materials with low melting points, partial melting and solidification.
In some substances several bonding mechanisms may occur simultaneously. In all cases, the result of screening is distorted because agglomerated fines are classified as coarse. The immediate and complete removal by dedusting or “scalping” of the finest fraction prior to screening into the desired property classes is one practical method to avoid selective agglomeration of the fines or adhesion of fines to larger particles. During screening itself, the effect of adhesion is reduced by mechanical destruction of agglomerates with, for example, rubber cubes or balls placed on or under the screen decks, the application of brushes, air jets, or ultrasound, or the modification of screen amplitude or, respectively, frequency (e.g. ultrasonic screen excitation). During the screening of moist bulk materials, difficulties increase with moisture content, but agglomeration tendencies are almost completely eliminated during wet screening when the particles are suspended in a liquid. Since, in moist screening, particles are often held in the mesh openings by liquid bridges, by direct resistance heating, inductive heating, or by modifying the wetting angle and/or the surface tension with surfactants the separation of such materials is facilitated and blinding of the screen is avoided. In air classijcation, products from, for example, dry fine grinding are separated. Particular problems arise if the material to be separated contains agglomerates that were formed during comminution. Conglomerates would be recirculated into the mill and “overgrinding”occurs. Therefore, attempts are made to destroy them by spe-
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speed or direction of flow and by installing air jet mills in front of the classifier. When classifying cement, it was determined that grinding aids, used during comminution to avoid agglomeration, also improve separation in the classifier. In the classifier itself, agglomerates are formed by molecular forces that may be reinforced by adsorption layers if separation is carried out with ambient air, by liquid bridges if moist materials are processed, and by electrostatic forces in a dry environment. As an example, Fig. 5.55 depicts separation curves of various air classifiers. With decreasing particle size, the amount found in the coarse fraction increases, which is due to agglomeration, whereby fine particles adhere to larger ones and conglomerates of fines behave as if they were coarser particles. Both effects reduce the separation efficiency and can be avoided only if the causes for adhesion are removed, that is, mostly by eliminating moisture in the material and humidity in the air. Sorting processes that separate materials according to particle characteristics other than size are often carried out in liquids. During flotation, one of the technologies, the relative capacity of light components to float is enhanced by the addition of chemicals which form bubbles that attach selectively to one component. Agglomeration can again reduce the separation efficiency when smaller particles of other components stick to larger ones of the floating type or to bubbles. By using modified chemicals, processing very dilute suspensions, or applying multiple separation steps efficiency can be improved. On the other hand, agglomeration can be also beneficial in dense media sink/ swim separation, centrifugal separation, or during jigging if particles of a particular ingredient can be made to selectively adhere to each other and form larger, heavier agglomerates. During particle size analysis, in addition to screening, sifting, and counting, sedimentation techniques are often used which produce unequivocal results only if the individual particles can move without influencing each other. For that reason, very dilute suspensions are used. Nevertheless, it is possible that agglomerates form or already
Particle size x Fig. 5.55
Separation curves of different air classifiers
5.5 Undesired and Desired Agglomeration
present conglomerates do not disperse completely. Therefore, dispersion aids are often added which reduce particle affinity (see also Section 7.4.6). The molecules of dispersion aids attach to the particles, eliminating polarities and/or reducing interfacial tensions. Separation forces, such as ultrasonic vibrations, can be also introduced for improved desagglomeration and dispersion. In connection with particle size analysis, the importance of correct sample preparation must be stressed. Because natural, unintentionally formed agglomerates always incorporate a larger than average number of the finest particles, the result of particle size analysis will be incorrect if preexisting agglomerates are not destroyed or conditions prevail during testing that promote agglomeration. Mixing Many of the previously mentioned considerations apply to the formation and prevention of undesired agglomerates during mixing. Little needs to be added concerning mixing in liquids by stirring or methods for the production of suspensions and dispersions. The addition of dispersion agents is always recommended if the tendency of the solid particles to agglomerate is high. Agglomerates or flocs that are already intentionally, for example to improve handling characteristics of fine powders, or unintentionally present prior to mixing can be destroyed by shear forces in the liquid. Consequently, the generation of the highest possible shear gradient is often considered advantageous when selecting agitators. During extended storage, the particles in (pharmaceutical) suspensions often form agglomerates that can be no longer destroyed by shaking the preparation. This is of particular concern in, for example, eye drops. The problem can be avoided by controlled flocculation of the solids. After the addition of an electrolyte,the fine particles aggregate to loose flocs that can be easily redispersed by shaking the dispenser prior to application. When mixing dry or moist bulk solids, agglomerates may form which originate from the finest components of the mixture. They are held together by molecular and electrostatic as well as capillary forces. These undesired agglomerates should be broken up by shear or frictional stresses, generated by the motion of the bulk mass, or by special disintegration devices that are built into the blender (see Section 7.4.2). Comminution Duringfine grinding in roller crushers and tube mills containing grinding media, with all materials certain problems begin to develop at a certain fineness of the solids to be milled. Two types of phenomena can be distinguished. In the first case, the finest particles start to adhere to walls and the grinding media in the mill. On this first coating, even coarser particles find excellent conditions for adhesion and massive deposits form rapidly. Experiments, during which the particle size distribution across thick layers of build-up were investigated, showed that the finest particles are indeed found in the lowest layers. Fig. 5.56 is the photograph of clean grinding balls and of those which, after a short period of operation, are already covered with a light primary deposit upon which additional layers will build during extended grinding. Fig. 5.57 are the photographs of the interior side of a manhole cover of a ball mill before and after use illustrating the extent of such deposits. These layers adhering to the inside walls and the grinding media produce a cushioning effect which lowers the intensity of stressing and, therefore, increase the duration of grinding and decrease efficiency.
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Fig. 5.56
Grinding balls before (right) and after (left) brief milling.
The second phenomenon during dry fine grinding is the occurrence of agglomerates in the freely moving charge itself. Formation of such agglomerates is associated with the so called limit of grinding. For each material a fineness exists at which, in spite of continued consumption of energy, the finest particles in the charge do not seem to become finer. Agglomeration and adhesion in mills can be attributed to various binding mechanisms (see Section 5.1.1). Since the mill housing may become highly charged by the friction between its contents and the walls, electrostatic forces are often the cause for initial build-up. This effect can be eliminated quite easily by grounding the mill. In
Fig. 5.57 Manhole cover o f a ball mill before (top) and after (bottom) grinding.
5.5 Undesired and Desired Agglomeration
other cases, wall deposits will begin with particles of the size that generally corresponds to that of the wall roughness. The strength of the layer depends on the contact pressure which is magnified by the mill charge consisting of grinding media and material to be crushed. Adhesion is largely affected by molecular forces; however, partial melting and sintering are also possible. Agglomerates are formed in the freely moving charge of a tube mill by the compaction of fine particles between the grinding media and by recombination bonding (see also Section 5.1.1). Adhesion is affected by van-der-Waals forces between the particles that have been compressed very tightly and by the recombination of free valence forces at newly created surfaces. Since these agglomerates are very strong, a grinding equilibrium exists which has been observed and described by many. It means, that in fine grinding, after a certain time, a state of equilibrium between size reduction and size enlargement by agglomeration occurs. From that point on, agglomerates are formed, crushed, and re-formed so that the apparent particle size does not change. However, since destruction of particles continues to occur, a growing amorphization of the material can be observed which also results in increasing specific surface area and is often called mechanical activation as, in many cases, higher reactivity is obtained. Every form of agglomeration during size reduction reduces the efficiency of grinding and the fineness obtained at the limit of grinding is often insufficient for many tasks, even though the agglomerates actually contain much smaller particles. Therefore, it is desirable to prevent or, at least, reduce these effects. In milling, one possibility to achieve less unwanted agglomeration is to add surface-active substances, so called surfactants or grinding aids. It has long been known that small amounts of such additives may reduce the grinding time required to reach a particular fineness by 20 30 %. Molecules ofthese substances, which are present in a gas or vapor phase, quickly saturate free valences at the newly created surfaces and avoid recombination bonding. The effect of some of these grinding aids on the fineness of cement after a specific grinding time is depicted in Fig. 5.58. It can be seen that with the exception of soot, the desired effect is produced only if the amount used is very small. At higher concentrations the agglomeration tendency increases due to the formation of adsorption layers and liquid bridges. In the case of soot a greater quantity is required because it is a solid which, as compared with molecules in the form of gas or vapor, is not very mobile. As a rule, grinding aids also reduce caking. Fig. 5.59 demonstrates the effect of 0.1 % sodium stearate during the grinding of cement clinker. Other surface-active substances can delay build-up for longer periods or prevent them entirely up to a certain fineness (for cement clinker 0.1 % triethanol-amine, for example, Fig. 5.60). Fig. 5.59 also shows that the specific surface, which is a measure of the fineness of cement, increases when 0.1 % sodium stearate is added and the build-up consists of finer particles.
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cm*/g 3000 a,
3 ‘fIn
2500
2 0
a,
cna 2000
1500
1000
e a: soot 0 b: Acetone 0 c : Sodium stearate @ d : Water 0 e: Naphthenic acids 0.5
Addition of auxiliary grinding agent
%I Fig. 5.58 Effect o f some “grinding aids” on the fineness o f cement after grinding clinker for the same time in a rod mill.
The formation oflamellar flakes or flat agglomerates in tube mills has been attributed to compaction between the grinding media during impact. The same mechanism occurs in all comminution processes in which the material to be crushed is subjected to stresses between two surfaces, for example in roller mills. Since the second condition
OWithout additions *+0.1% Na-stearate
Fig. 5.59 Specific surface o f the build-up (= accretion) and o f the free charge as well as amount o f the build-up (= accretion quantity) Grinding duration l o 3 (mill revolutions)
with and without the use o f a grinding aid during size reduction o f cement clinker in a rod mill.
5.5 Undesired and Desired Agglomeration
Fig. 5.60 Changes in the amount o f freely moving charge during the grinding o f cement in a laboratory ball mill with and without the addition o f triethanolamine.
Lim e s t on e
='lYm
s = 122, pm Aw9,5
Cement clinker x = 9BOcJm s = 122,Spm A- 8 De ree of re uktion
B
A = -X
s
Fig. 5.61 Agglomerates produced during the grinding o f limestone and cement clinker in a roller mill with a high reduction ratio.
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volved, the occurrence of strong, flat flakes is mostly observed during fine grinding. One measure for the fineness, the intensity of stressing, and the unintentional formation of agglomerates is the so called reduction ratio, that is, the quotient of maximum feed particle size and gap between the rollers. Fig. 5.61 depicts typical agglomerates produced in a roller mill with a high reduction ratio. Since the fine material is immediately compacted, almost all free valences at the newly created surfaces participate in recombination bonding. Consequently, the formation of agglomerates in roller mills can be only avoided if a smaller reduction ratio is chosen or by applying friction between the rollers. More recently it was found that, as compared with the stochastic crushing process in a tube mill with grinding media, the combination of the well defined stressing in a high pressure roller mill with large reduction ratio and the ensueing desagglomeration of the conglomerated flakes that were produced in the mill result in a significantly lower overall energy consumption during fine grinding of brittle materials, such as cement clinker and many ores; in those cases the unintentional and unavoidable agglomeration of the fine particles is not only tolerable but also results in a more economical fine grinding method. Agglomerates can be also formed during impact grinding. Fig. 5.62a shows schematically the fracture lines that develop during impact stressing of a glass sphere. A cone of fine material is created at the impact point and is compacted by the pressure resulting from the kinetic energy of the system into an agglomerated mass (Fig. 5.62b and c). Here too, the effect of free valence forces on newly created surfaces is used to its almost complete extent, yielding a quite strong agglomerate. It is very difficult to avoid this type of agglomeration; it can be affected only by reducing the impact velocity
a: Resfkege/ b: Seilensplifter c : Feingutkegel
Fig. 5.62 (a) Schematic representation of the fracture lines caused in a glass sphere by impact stressing. (b) Agglomerated cone o f fines created during the impact stressing of a glass sphere (sphere dia.: 8 m m , impact velocity approx. 150 m/s). (c) Agglomerated cone o f fines created during impact stressing o f a sugar crystal (shown o n the left).
5.5 Undesired and Desired Agglomeration
which, in turn, results in a lower degree of comminution. For glass spheres, for example, the formation of agglomerates was observed only at impact speeds exceeding 80 m/s. During the impact crushing of thermoplastic materials or inorganic substances with low melting points, solidified bridges of partially melted material may further increase adhesion and the strength of agglomerated fines. Since the rise in temperature depends on the energy input and is constant for a given impact velocity, cryogenic milling, whereby the particles to be crushed are cooled prior to feeding into the mill, not only results in an increased brittleness but may also avoid partial melting and unwanted agglomeration. In wet grinding, as a rule, agglomeration is avoided by suspending the particles in liquid. Sometimes, the product of dry fine grinding is subjected to a brief final wet grinding to destroy the previously formed agglomerates. Nevertheless, some materials also tend to flocculate in wet grinding. Since the adhesion forces causing flocs are mostly electrical in nature, the addition of a small amount of electrolyte to the suspending liquid nearly always suffices to prevent flocculation. By definition, the unit operation size enlargement by agglomeration makes use of all binding mechanisms and often enhances them in suitable environments and equipment. All agglomerates that are produced are made intentionally and are desired. Nevertheless, there are instances where adhesion and agglomeration are unwanted and undesired. Because, particularly in tumble/growth agglomeration, binders are added, the effect of these binders is still present on the surface of green agglomerates and during post-treatment new binding mechanisms between the agglomerates may develop which result in the formation of clusters of agglomerates. Of course, because agglomerates are larger bodies and only a few interaction points ( = coordination points) are present in a unit volume, even relatively strong solid bridges which may have developed by recrystallisation, chemical reaction, or sintering during post-treatment can be broken relatively easily. Nevertheless, such clusters could be detrimental during storage, feeding, or metering and, therefore, should be avoided or broken up prior to a following process step. More information on potential problems is given in the subchapter on Storage below. Agglomeration
During the conveying ojparticulate solids, especially of finely dispersed powders, the unintentional formation of agglomerates and (sometimes thick) coatings on walls is often observed. Whereas agglomerates occur mostly on vibrating or shaking conveyors and inclined conveyors or chutes, wall build-up is more common in pneumatic conveyors. The main causes of agglomeration during the conveying of fine particulate solids are molecular and electric forces as well as binding mechanisms related to moisture and, as a result of mechanical or thermal energy input, binding mechanisms such as partial melting and solidification can be activated, too. Although it is very difficult to avoid agglomeration on vibrating or shaking conveyors, several possibilities exist for the prevention of wall build-up and deposits during pneumatic transport. Since the adhesion of the finest particles always begins in the roughness depresTransportation
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sions, one ofthe most important conditions to avoid a common reason for initial build-up is to provide smooth inner wall surfaces of pneumatic conveying lines. Because high drag forces tend to remove particles that have already adhered to the walls, high transport velocities also reduce the danger of build-up. Deposits preferably start in dead or calm flow zones; therefore, when designing such systems, low speed areas must be avoided. On the other hand, sudden changes in the direction of flow will cause high energy impacts of particles with the wall, causing build-up. Friction between particulate solids and pneumatic conveyor walls may result in high electrostaticcharges on both partners. They depend to a large extent on whether the particles and/or the walls are electrically conductive and the lines are grounded or not. System design must take these conditions into consideration. To further explain some of the phenomena that occur during pneumatic conveying, the results of some pilot investigations will be summarized and presented in the following. Pneumatic transportation tests were carried out in a 58.51 m long horizontal pipe with a diameter of0.7 m. The pressure within the system was determined at seven locations which were distributed along the measured length of the pipe. Ap = p1 - p7 is the total pressure drop in the conveying system. In Fig. 5.63 the pressures at three different location are plotted over time. Since the fan located behind the dust collector at the end of the conveyor generates a slight negative pressure in the filter housing this also can be measured in the pipe as long as it is clean. After a few seconds, however, pressure p1 rises and the other locations follow after a short delay. Part of the pressure increase is caused by loading the air with particles, but a major portion is due to the build-up of deposits in the pipe. If the pipe was inspected after runs of 20 s and 50 s, respectively, no deposit was found after the short duration but after the longer run deposits had build up in the feed end portion while the part closer to the end still remained clean.
Tube diameter: D = 0.7m hP= 1440 kg/hr mp/m,= 40
I
Fig. 5.63 Pressure changes a t three locations o f a pilot pneumatic conveying system during the first 150 s of a test run.
5.5 Undesired and Desired Agglomeration
E
1
~
Qt
7200
B
2 150
Measured tube length: AL =58.51m Tube diameter :D = 0.7 m rn,/rnf
mn=516kg/hr
..-
=158
ci:l865rn/s
r
rn,, = 325ikg/hr
- .
8 'D
100.
I
20
40
60 Time tlininl
,o 0 rn ~ 1 4 4 0 kg/hr P 36 kg/hr
rn / r n f = 40
P
u'=21lrn/s
rn,=
-
50
0
_--
5
10 15 Time t h i n 1
Fig. 5.64 Pressure drop along the measured pipe length o f a pilot pneumatic conveying system as a function o f time.
Fig. 5.64 depicts the total pressure drop between both ends of the pipe. The lower diagram represents results of the same test as shown in Fig. 5.63.After about 2.5 min, the system is in equilibrium and the total pressure drop in the systems remains almost constant. This indicates that, at least macroscopically, no further build-up takes place after this time. The upper diagram in Fig. 5.64 represents a different behavior. The total pressure drop increases more slowly. This is mostly due to a lower solidslfluid ratio and a higher transport velocity than used during the experiment depicted in the lower part of Fig. 5.64. In rather regular time intervals, however, a high pressure peak developed which was first measured at the feed end and propagated quickly to the discharge end. This behavior, together with some other observations, indicated that deposits fall off and are pushed along the pipe, thus momentarily increasing the pressure drop. Ifthe system is opened immediately after such a pressure wave, the inner walls are found almost completely clean.
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At high conveying velocities or in vertical pipes, deposits build up uniformly and, in the following, shall be called “crusts”.In horizontal pipes, bonds between particles and the wall are stressed by the weight of the build-up in the upper part while they are strengthened in the lower part by the gravitational forces. Therefore, especially during conveying at low velocities and high solidslfluid ratios, a second type of build-up is observed in horizontal pipes that shall be called “massy” deposits. The lower part of Fig. 5.65 describes schematically the different types of build-ups in horizontal pneumatic transport pipes. They are affected by gravity, grow in the direction of mass flow, and are composed of finer particles because these particles exhibit a higher adhesion tendency and a “sieving” or classification effect takes place in the charge by which finer components move to the bottom layer of the moving mass. The upper part of Fig. 5.65 is a photograph taken during a model experiment. With exception of the formation of a crust, all other stages, including a “dune” of freely mobile particles moving over the deposits, can be distinguished. Fig. 5.66 is the view into a pipe after pneumatically conveying a slightly moist, finely divided quartz powder showing the heavy build-up in crust and massy deposit as well as the remainder of a dune. Massy deposits can be also formed by the action of other influences, such as centrifugal and inertial forces at elbows. Another important agglomeration phenomenon is that deposits may be shaped such that they yield a more effective flow channel. This can be explained by the fact that separation and drag forces influence adhesion. Particles preferably build up in zones without flow or where the direction of flow is changed by, for example, eddies. A typical example is shown in Fig. 5.67. On the bottom the partial cross section of a Moller pump is presented. Such pumps are used for feeding powders into a pneumatic conveying system. Powder and air enter a mixing chamber through a screw and a nozzle, respectively, and are then forced into the piping. The photograph in the upper part of
,>
, , ,,x
jl
,,,x , ,x
x
0
, I/
/
x
,,
I
x x
‘\CfUSt
k -
Growing “mussy” dsposits
‘Massy“ deposit Crust
Fig. 5.65 Sketch and photograph o f a model experiment explaining the different types of build-up in a horizontal pneumatic conveying PlPe.
5.5 Undesired and Desired Agglomeration I 1 2 3
Fig. 5.66 View into the pipe of a horizontal pneumatic conveyor after conveying a slightly moist, finely divided quartz powder at low velocity.
Fig. 5.67 is a view (in direction A-A)of such a mixing chamber which was opened after conveying fine zinc oxide. Opposite the nozzle a deposit in a Venturi-like shape has built up which defines the most effective flow channel at this point. Similar deposits can be found along the line in pneumatic systems that were not optimally designed and/or arranged. Adhesion phenomena are involved in, for example, the bridging ofparticulate solids in hoppers. In the case of relatively coarse materials, bridge formation is caused by dome-like structures which are supported on the inclined walls in the lower conical part of bins. With decreasing particle size, the participation of true adhesion forces in bridging and agglomeration increases. Binding mechanisms are molecular forces and adsorption layers or liquid bridges. The latter often play an important role whereby liquid bridges form by capillary condensation at the coordination points. For example, feeding warm and moist material into silos must be avoided, even if its moisture content is very low. Evaporating moisture condenses on the cooler silo walls and drips into the charge forming wet agglomerates and causes strong capillary adhesion bonding of particles on the walls. Insulation of the silos and/or forced large volume venting can be employed to avoid condensation and agglomeration problems. Bridging can totally block the discharge from silos, thus causing severe operating problems. Because adhesion even of finely dispersed DRY solids can not be avoided, agglomerates and bridges must be destroyed by special devices. For this purpose, inflatable cushions are mounted to the inside walls of silos or the material is momentarily fluidized by the (pulsed) injection of air jets. In the case or coarser solids, which tend to form domes, it is often sufficient to select a cone with steeper sides (= “mass flow” design). Small remaining flow problems due to adhesion can be overcome by installing vibrators or “hammers” on the outside silo walls. Unwanted agglomeration is often observed if the particulate materials are soluble or if chemical reactions can occur, particularly in the presence of moisture. These phenomena are very common in the fertilizer industry and are called caking if they occur in bulk masses or bagset if material solidifies in bags. Caking of fertilizers and of other soluble materials has long been and still is a great problem. To get an idea about its importance and scale, three examples shall be preStorage
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Moller pump (schematically) Mixture solids/air
Fig. 5.67
Photograph and sketch of a "Moller pump". The photograph shows Venturi-like deposits after conveying finely divided zinc oxide.
5.5 Undesired and Desired Agglomeration
Fig. 5.68
Manually unloading a shiplead o f caked sylvite
sented. Fig. 5.68 depicts the unloading of a shipload of Sylvite that was expected to arrive at its destination as a free-flowing particulate mass, similar to the state it was in when it was loaded. This historical photograph shows that, instead, it had badly caked so that, owing to the limited room in the shiphold, the very expensive and time consuming method of manual unloading had to be chosen. Fig. 5.69 is another historical photograph. It was taken by TVA (Tennessee Valley Authority, Muscle Shoals, AL, USA) in 1947 and recalls the recovery of nongranulated triple superphosphate from a curing pile which had to be blasted first to break the pileset. Although today, agglomeration methods are used to produce a granular product which does no longer cake to such an extend that it can not be broken up by, for example, front-end loaders, some fertilizers, particularly those with high nitrogen content, are so hygroscopic that, additionally, they must still be stored in bulk storage facilities with controlled low humidity to prevent excessive caking. Breaking a potential pile-set by high energy input, such as blasting, is not possible for nitrate fertilizers.
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Fig. 5.69 Recovery o f nongranulated triple superphosphate from a curing pile after blasting t o break the "pile-set".
The third photograph (Fig. 5.70) demonstrates potential difficulties that may be experienced by the end user. The granular fertilizer in the left bag displays the desired free flowing behavior while, in the other bag, the same product, that was not treated with an anticaking agent, has set up (bagset). Even after mechanically breaking up such a caked mass it may no longer exhibit the same uniform distribution characteristics as the product that did not experience secondary agglomeration. Different materials become caked during various storage and handling procedures but caking itself is almost exclusivelyby solid bridges or, more specifically, by chemical reaction and crystallization ofdissolved substances (see also Section 5.1.1). Other binding mechanisms contribute only slightly to caking. The rate and extent to which caking takes place depends on the moisture content, the particle size or specific surface area, the pressure under which the material is stored (e.g. top or bottom ofthe pile), the temperature and its variation during storage, as well as the time. The effect of these parameters changes with different materials. Fig. 5.71
Fig. 5.70 Granular fertilizer treated with an anticaking agent (left) and untreated control (right) showing severe "bag-set".
5.5 Undesired and Desired Agglomeration
Fig. 5.71 Variation of crushing strength with caking pressure (left) and time of storage (right). (a) NaNO,, (b) (NH,),S04, (c) Urea, (d) KCI (potash), (e) (NH,)H,PO,, (f) superphosphate.The numbers in brackets indicate the respective moisture contents.
depicts results obtained in a “caking bomb”. It can be seen that the crushing strength of caked masses rises with increasing moisture content (numbers in brackets, curves (a)),caking pressure and time of storage. The influence of temperature and temperature variations depends on the solubility of the solids. Fig. 5.72 shows four different temperature-solubility curves. Whereas the solubility of sodium chloride changes little with temperature, this is not true for potassium chloride (or potash) and potassium nitrate, for example. Especially the latter features a very steep curve. Some salts, such as sodium sulfate, exhibit various temperature dependent solubility ranges. Salts or mixtures of different salts, such as fertilizers, for example, containing a small amount of moisture, may cake during storage and/or transport if exposed to changing temperatures even if the moisture content is very small and the material is packed in airtight containers. In many cases (see Fig. 5.72) more salt will be dissolved at higher temperatures which recrystallizes and forms solid bridges between the particles when the temperature drops again. Repeated cycling, for instance due to climatic changes or differences in day and night temperatures, reinforces this bonding and causes bag-set. The crushing strength of caked materials depends on the number of bridges formed per unit volume and, therefore, decreases with increasing particle size. As mentioned earlier, mixtures of powdered soluble materials that were granulated by agglomeration may still set up somewhat due to the mechanism described above, however, normally the granulated material can be broken and desagglomerated easily. In conclusion, it can be stated that the tendency for caking of a fertilizer mixture, for example, will vary with the physical and chemical properties of the components and their proportions in the mixture. It also depends on the method of mixing, the particle size after processing, which often includes size enlargement by agglomeration, and the storage conditions to which the finished products are exposed.
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Fig. 5.72
Solubility curves o f four different salts.
The answer to what can be done to avoid or, at least, lessen caking is complex but generally the same as in all other cases where unwanted adhesion or agglomeration occurs: Detect the binding mechanism that is responsible and the parameters that influence the process and then try to reduce their effect. In the following some examples will be discussed briefly. If (unobjectional) chemical reactions between components of a mixture do occur, these components should be mixed separately until the reaction is completed. The resulting intermediate product can then be blended with the other components and no longer induces caking. An example for this is any mixture that contains both ammonium sulfate and superphosphate. An almost trivial precaution is very often to lower the moisture content. However, this is not always necessary. Different maximum moisture levels exist that depend on the material. Fig. 5.71 shows that the crushing strength of caked superphosphate containing 1.1% moisture is very low while the strength of some other
5.5 Undesired and Desired Agglomeration
Fig. 5.73 Granules o f 12-12-12 NPK fertilizer showing typical crystalline hulls o f an urea-ammonium chloride complex after storage for 3 months in bags. Uncured (left) and cured for 7 days prior to bagging (right).
caked fertilizers is much higher although they contain considerably less water. It was found during microscopic studies of several types of high-analysis fertilizers that caking usually resulted from bonding by the crystals of soluble salts. These crystals often covered the entire granule surface in the form of a “veneer” or hull. Fig. 5.73 shows typical 12-12-12 NPK fertilizer granules that were produced with an ammonia-urea solution after 3 months of storage. They were illuminated from below and photographed at a higher magnification to reveal details of the crystalline hull. Bondingphase salts identified during the study were potassium nitrate, ammonium chloride, monoammonium phosphate, ammonium nitrate, and an urea-ammonium chloride complex; all, particularly the last one, are highly soluble. Those salts migrated to the surface of the granule, leaving numerous small cavities within. This mechanism requires water and drying should, therefore, reduce caking. Fig. 5.74 is a photograph taken with crossed Nicol prisms. It shows the difference in hull thickness between undried and predried 12-12-12 grade NPK fertilizer granules. The crushing strength of caked material after storage decreases correspondingly. (c) Curve b (ammonium sulfate) in the right hand side diagram of Fig. 5.71 shows the typical behavior of materials that respond favorably to several days of bin or pile curing prior to bagging. Such products cake in a few days to their final strength but the resulting lumps are broken up before the cured materials are bagged and put into long term storage. Curing can even accelerate hull formation owing to the heat and moisture retention in the bin or pile. In products that respond well to curing, additional development of crystals on the granule surfaces during subsequent storage is not sufficient to cause significant caking.
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Fig. 5.74
Difference in the hull thickness o f undried (top) and predried (bottom) granular 12-12-12 NPK fertilizer made with Ammonia-Urea solution.
However, many products do not improve during this type of curing. Fig. 5.73 depicts the comparison of uncured (left) and cured (right; for 7 days prior to bagging) 12-12-12 N P K fertilizer granules that were made from ammonium sulfate, potassium chloride, and superphosphate with ammonium-urea solution and sulfuric acid. Although ammonium sulfate is present, the caking behavior of the other components dominates and both the cured and uncured granules exhibit continued growth of the hulls and caking during storage. Another curing method will be described under (e) below. (d) The oldest method of conditioning fertilizers is the coating with a parting agent. Storage properties are improved after the addition of up to 3 % of an extremely fine particulate solid such as diatomaceous earth, kaolin, vermiculite, pulverized limestone, magnesium oxide, and a variety of other inexpensive, very fine powders. Again, microscopic studies revealed the fundamental properties of a “conditioner” which are threefold: 1.The powder coating acts as a separator between the individual fertilizer granules and prevents intergrowth of crystals during and after drying. 2. The hulls from beneath the coating crystals rarely project beyond the layer of conditioner. 3. The moisture is distributed uniformly over the surface of the granules due to the high sorptive capacity of the finely porous coating. Thus the localized growth of crystals at the coordination points is prevented and the surface hulls are much finer grained, more intergrown, and more densely packed than those covering unconditioned products. Such anticaking conditioning agents are usually applied by mixing them with the granular fertilizer in a rotary tumbler (typically a drum) prior to bagging. (e) A modern variation of the above mentioned conditioning process is the coating with surface-activeorganic chemicals. it was found, however, that not all surfactants improve the physical conditions of mixed fertilizers. It was reported that the caking ten-
5.5 Undesired and Desired Agglomeration
(4
dencies can be reduced by as much as 45 % if non-ionic chemicals were used but increased by as much as 37 % with the use of anionic materials. Where in the process the surface-activeagents were applied was also found to be of decisive importance. Typical cationic anticaking agents are fatty amines with a general formula R - NH, with R representing C16 and C18 chains. They are believed to attach directly to the fertilizer particles with their surface-active amine group. The fatty, hydrophobic part of the molecule extends outward, thus preventing hygroscopic products from attracting moisture. Of course, this is only true, if a monomolecular layer covers the fertilizer granules and all amine molecules extend their hydrophobic portion outward. Therefore, too much conditioner may cause rather than prevent caking. Multiple layers are alternately hydrophobic and hydrophilic. The above makes an alternative curing process advantageous. The molecules of a second molecular layer, if attached, would position themselves with the amine group extending outward. These amine groups are free to interact with other fertilizer particles, especially the phosphate portion of incompletely coated granules, to form an amine-phosphate salt. Pressure intensifies this effect. The chemical “bridge”is not as strong as a recrystallized salt bridge and the “set”can be broken easily. Since, on the other hand, the amine- phosphate bond is stronger than the RR bond, a more uniformly covered product results from a short bin cure (1 - 2 days) which is unlikely to set again (see Fig. 5.70, left side). Sometimes a combination of the two types of conditioners is used. An example for this approach is finely divided kaolin treated with surfactant. A last but not least method is granulation. Today, this technology is almost obligatory, particularly for mixed fertilizers. Size-enlarged, granular fertilizers offer fewer coordination points per unit volume where solid bridges can develop. If the strength of the bridges is low anyway, as in the case of superphosphate with 1.1 % moisture or monoammonium phosphate with 0.06 % moisture (see Fig. 5.71) granulating alone is sufficient to prevent severe caking.
The above examples were selected to demonstrate how unwanted agglomeration problems can be studied and possible remediation techniques be determined. They date back to a point in time when the fundamentals ofunwanted agglomeration in different industries were first investigated and means were developed to avoid some of these phenomena. While this part of size enlargement by agglomeration, unwanted agglomeration, is often very important, because its effects may result in considerable losses of production and profit, most of the past and present major publications deal only with the methods and equipment for the production of agglomerates with beneficial properties. Therefore, it is an important achievement that recently a book entitled “Cake Formation in Particulate Systems” [B.44] was published that does cover the unwanted adhesion and agglomeration phenomena. The author distinguishes four major classes of caking in particulate systems: Mechanical caking Plastic-flow caking Chemical caking Electrical caking.
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In addition, several sub-classes are defined whereby certain properties of components, either pure substances or part(s) of a formulation, can be expected to cause caking under certain conditions. After describing the above, considerable emphasis is given in the book to laboratory techniques and test procedures that need to be considered by those engaged in solving caking problems. Therefore, this publication is recommended for further reading.
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
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Agglomeration Technologies In two previous chapters (Sections 5.3 and 5.3.2) reference was already made to the three technologies that are available for the desired size enlargement of small particulate solids by agglomeration, i.e. Tumblelgrowth agglomeration Pressure agglomeration Agglomeration by heat/sintering and the division into the following sub-groups: I. Tumble/growth agglomeration (Chapter 7) 1. High density tumbling bed (Section 7.4.1) 2. High shear tumbling bed (Section 7.4.2) 3. High density/high shear with abrasion or crushing transfer (Section 7.4.2) 4. Low density fluidized bed (Section 7.4.4) 5. Low density particle clouds (Section 7.4.5) 6. Agglomeration in stirred suspensions (Section 7.4.6) 7. Immiscible liquid agglomeration (Section 7.4.6) 11. Pressure agglomeration (Chapter 8) 1. Low-pressure agglomeration: Extrusion through screens (Section 8.4.1) 2. Medium-pressure agglomeration: Pelleting, extrusion through perforates die plates (Section 8.4.2) 3. High pressure extrusion: Ram presses (Section 8.4.3) 4. High-pressure agglomeration a) In confined spaces: Punch-and-Die pressing, tabletting (Section 8.4.3) b) In confined spaces: Isostatic pressing (Section 8.4.4) c) In semi-confined spaces: Roller presses (Section 8.4.3) 111. Agglomeration by heatlsintering (Chapter 9).
Additionally, most technologies can be subdivided into two techniques, those utilizing no binder, and those requiring a binder. It should be pointed out that the binding mechanism in binderless agglomeration often resembles that of bonding with a binder. This is due to the fact that binders
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are sometimes inherently available and act during agglomeration and/or post-treatment. A typical example for this process is the wet agglomeration of materials that are easily soluble in the liquid. The modified surface tension of the liquid solution may already influence the strength of the green agglomerate and during drying, the necessary post-treatment to convert the intermediate wet agglomerate into the dry, final product, solid bridges develop by recrystallization of the dissolved substance(s) (see also Sections 5.1.1, Chapter 7, and Section 7.3). Other inherently available binders have to be activated by a so called conditioning process prior to agglomeration. A typical example for this technique is in the pelleting of animal feed where the starchy component of feed grains becomes plastic and sticky during moistening and heating with steam while mixing the formulation in a kneader. After conditioning the starch provides plasticity that is required for extrusion through bores in the dies of pelleters (see Section 8.4.2) as well as green and dry product strengths. The basic mechanism of tumble/growth agglomeration is shown in Fig. 6.1. Adhesion of individual particles to each other or to solid surfaces is controlled by the competition between volume and surface related forces (see also Section 5.4). To cause permanent adhesion, certain criteria must be fulfilled. The most important of all is that any system force (e.g. caused by gravity, inertia, drag, etc.) must be smaller than the attraction forces between the adhering partners. According to Fig. 6.2 and Equation 6.1, the ratio between the binding forces Bi(x)and the sum of the active components of all ambient forces F,,(x) is a measure for the adhesion tendency T,:
(Eq. 6.1)
T,= X B,(x)/XF,,(x)>l
Both the attraction and the ambient forces are mainly dependent on the size x of the powder particle(s).To cause adhesion, T,must be larger than “one”.In most cases, to keep the particle(s) adhering, the sum of all moments Q ( x ) must be zero, too: Q ( x ) = x/2 ZFjx(x)= 0
(Eq. 6.2)
It has been discussed (Section 5.1.1) that most of the attraction forces have only a short range; their magnitude and strength decreases quickly with increasing distance. Therefore, because the surfaces of all particulate matter are, at least microscopically, rough (see Section 5.1.1, Fig. 5.11), and the mass of the particles decreases with the third power of the particle size, the adhesion tendency increases with decreasing particle dimensions.
Growth
Nucleation Coalescence
Layering
Fig. 6.1: Basic m e c h a n i s m o f tumble/growth agglomeration.
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Fig. 6.2: Schematic representation o f the adhesion tendency o f a spherical particle on a flat wall.
The mechanism as depicted in Fig. 6.1 occurs naturally if the agglomerate forming particles are nano-sized. In the case of larger, micron-sized particles the adhesion forces must be produced by the addition of binders (mostly water and other liquids) or enhanced by conditioning and the probability of collision must be increased by providing a high concentration of particles. Such conditions are obtained (Fig. 6.3)
Ti/-
% u
i
Fig. 6.3: Schematic representation o f typical equipment for size enlargement by tumble/ growth agglomeration (left) and the post-treatment steps required to obtain a final product (right). Left, from the top: inclined disc (pan), drum, continuous mixer, batch mixer, fluidized bed. (1) Liquid binder (spray), (2) fresh feed, (3) recirculating fines, (4) dryer, (5) cooler, (6) double deck screen, (7) mill, (8) conditioning drum
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in inclined discs (pans),rotating drums, and any kind of powder mixer (see also Sections 7.4.1 and 7.4.2). Relatively lose agglomerates are obtained in fluidized beds which realize an irregularly moving particle bed with lower concentrations (see also Section 7.4.4). Sometimes, simple rolling and tumbling motions, for example on inclined stationary or moving surfaces, are sufficient for the cheap formation of crude agglomerates (see Section 7.1 and Chapter 10). In most instances, tumblelgrowth agglomeration processes yield first so called green agglomerates after growing nuclei into larger, nearly spherical aggregates by coalescense and/or layering (Fig. 6.1).These wet agglomerates are temporarily bonded by the effects of surface tension and capillary forces of the liquid binder. While, occasionally, components within the green agglomerate naturally produce permanent bonding by, for example, cementitious reactions, in most cases post-treatments consisting of all or some of the following processes are required to obtain permanent and final strength (see right hand side of Fig. 6.3): heating, potentially chemically reacting, drying, and, sometimes, sintering or partial melting, cooling, screening, adjustment of product properties by crushing and conditioning as well as recirculating undersized material. Since it is difficult, if not impossible to screen green (wet) agglomerates without blinding the screen cloths, separation of undersize material for recycle occurs normally after drying, reacting (if applicable), and cooling. Although, the mostly pre-agglomerated recirculating particles often play an important role in tumble/growth agglomeration because they provide most of the nuclei that are necessary for an accelerated growth of product-size agglomerates (see also Section 7.2), the sometimes very large percentage of recycle (often > 300 %) must be again activated for agglomeration by rewetting and needs to pass once more through the entire process, including heating, drying and cooling, which, in final analysis, may render this technology uneconomical. Relatively uniformly shaped and sized agglomerates can be obtained with low- and medium-pressureagglomeration (see also Sections 8.4.1 and 8.4.2). For these processes, the feed mixture must still be made up of relatively small particles and inherently available, activated, or externally added binders (see above). The moist, often sticky mass of particulate solids as well as plastic and liquid binders is extruded through holes in differently shaped screens or perforated dies (Fig. 6.4). Agglomeration and shaping are caused by the pressure forcing the mass through the holes and by the frictional forces developing during the material’s passage. Depending on the plasticity of the feed mix and the dimensions of the holes, short “crumbly”,elongated “spaghetti-like”,or cylindrical green extrudates are produced. Particularly the thin, string shaped agglomerates that are obtained from low-pressure agglomeration (Fig. 6.4, a.1 -a.5) are often spheronized, i.e. rolled into small spherical particles while the product is still plastic. In most cases a post-treatment (typicallydrying and cooling) is required to yield final, permanent strength. As far as applicability is concerned, high-pressure agglomeration (Fig. 6.5) is the most versatile technique for size enlargement of particulate solids by agglomeration (see also Section 8.4.3). If certain characteristics of the feed materials and conditions occurring during densification (see Section 8.1)are considered during equipment selection as well as plant design and operation (see also Section ll.l),particulate solids of
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Fig. 6.4: Schematic representation o f equipment for (a) l o w and (b) medium. pressure agglomeration. (a.1) Screen, (a.2) basket, (a.3) radial, (a.4) dome, (a.5) axial. (b.l) screw, (b.2) flat die, (b.3 - b.5) different designs o f cylindrical dies, (b.6) gear.
b.3
b.5
b.4 I
b.6
any kind and size, from nanometers to centimeters, and at any condition, for example with temperatures from below freezing to 1,000 “C, can be successfully processed. Typically, the products from high-pressure agglomeration feature high strength immediately after discharge from the equipment. Nevertheless, to further increase strength, addition of a small amount of binder and/or application of post-treatment methods are possible. The mechanism of densification of particulate solids (Fig. 6.6) includes, as a first step, a forced rearrangement of particles requiring little pressure followed by a steep pressure rise causing brittle particles to break and malleable ones to deform plastically. During the entire process, porosity decreases so that fluids which originally occupied the pore space of the bulk feed must be able to escape and the initial elastic deformation must have sufficient time to either cause breakage or convert into plastic deformation (see also Section 8.1).These requirements limit the speed ofdensification and, therefore, the production capacity. The agglomeration by heat or sintering has been developed and is mostly applied in industries processing minerals and ores for the size enlargement of fines prior to further use (see also Chapter 9). Because the technology requires large amounts of thermal energy, special efforts are made to recover heat or use sources of waste heat. The resulting agglomerates are crude but meet the requirements of the industry.
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Fig. 6.5:
Schematic representation o f equipment for high-pressure agglomeration. Ram press (upper left), punch and die press (upper right), roller presses for compaction (lower left) and briquetting (lower right).
Another large field of application of sintering in agglomeration is in post-treatment where the phenomenon (see also Section 9.1) is used to produce strong permanent bonds in many parts that may have been produced by virtually any one of the other agglomeration techniques (see also Sections 5.3.2,7.3, and 8.3).Particularly in powder metallurgy, sintering is the most important finishing process for the achievement of final strength and stucture.
Deformation d
-.
Fig. 6 . 6 The mechanisms occurring during the densification o f particulate solids.
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
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Tumble/Crowth Agglomeration AS mentioned several times before (see Sections 5.3, 5.3.2, 5.4, and Chapter 6) and indicated in Chapters 2 and 3, the “natural”adhesion of small particles is the most basic agglomeration phenomenon. Therefore, it is that which is most often responsible for unwanted agglomeration (see Section 5.5) and the conglomeration of fine particles that is frequently observed in nature. To arrive at methods which achieve size enlargement by agglomeration in a desired and controlled manner, both a movement of particles and binding mechanisms must be created and enhanced. As the solids move in relation to each other, for example in the relatively dense bed of a rotating or otherwise actuated containment of some sort or in a low density suspension, particles of any size and kind, will collide from time to time and, if the attraction force at the collision site is high enough, coalesce. Theoretically, for this phenomenon to occur, no specific piece of equipment is necessary. As long as the solid particles are kept in irregular, stochastic motion, the probability for collision and coalescence exists. If, additionally, the binding force that has developed upon impact is strong enough to withstand the separating effects of all system forces (see Chapter 6) and does not disappear with time without being replaced by some other binding mechanism, the “seed agglomerate” will survive and eventually collide with other particles or agglomerates. At each instance of collision the bonding criterium as defined in Chapter 6, Fig. 6.2, Equations 6.1 and 6.2, will be tested leading to either growth, indifference, that is, the colliding partners will separate again and remain single, or the destruction of weaker agglomerates. To achieve growth, the individual mass of adhering particles must be small and their surface large. This is equivalent to the requirement that the size of agglomerating particles must be small. Typically, the surface equivalent diameter (see Section 5.2.2) should be in a range below approx. 100-200 pm. Micron and submicron or nano-sized solids (approx. < 10 pm), even if they are dry, will adhere naturally and form agglomerates. Larger particles necessitate the addition of binder for successful growth agglomeration. The limitation to small dimensions of the particles forming the agglomerate and the fact that, in most cases, only temporary bonds are formed constitute major drawbacks of all tumble/growth agglomeration methods. If particles are larger than required, crushing to achieve the necessary fineness is normally uneconomical.
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Immediately after growth agglomeration, in the green (moist or wet) stage, the main binding mechanisms are caused by bridges of freely movable liquids, capillary pressure at the surface of particle conglomerates that are filled with a freely movable liquid, or adhesion caused by viscous binders and slurries. To a lesser degree, other binding mechanisms, such as van-der-Waals, electric, and magnetic forces, may also participate. After curing, which often results also in a considerably strengthening of the agglomerates, bonding is achieved by solid bridges resulting from sintering, chemical reactions, partial melting and solidification, or recrystallization of dissolved substances. Some tumblelgrowth agglomeration equipment can handle large volumes effectively if the above requirements (small primary particle size and instantaneous bonding with high strength) are fulfilled. The apparatus is simple and the design is unsophisticated (see Section 7.4.1) but control depends largely on operator experience. Curing is normally the expensive part of plant investment and also contributes to a large extent to operating costs, both of which may render an otherwise perfect technology uneconomical. However, if very large amounts of solids must be agglomerated and the finely divided particulate form of the primary particles is required for other reasons, for example, the concentration of valuable components of ores (see Section 5.4),tumble agglomeration is the preferred technology. In those cases the main binder is water. At production capacities exceeding 1 million t/y, the curing facilities become cheaper and more economical and methods for, for example, the recuperation of heat to make the process more efficient and reduce operating costs become feasible. Other reasons for the application of tumblelgrowth agglomeration, even at small capacities, may be the high porosity of the agglomerates with other attendant beneficial product characteristics (see also Section 5.4, such as high surface area (e.g. for catalyst carriers) and easy solubility (e.g. for food {drink} and pharmaceutical products). These advantages may be so valuable that additional costs for grinding to obtain the necessary small particle size for agglomeration will be acceptable and high operating costs can be absorbed. In these cases, even the agglomeration liquids (binders for the formation of green agglomerates) may be so costly that they are condensed from the dryer off-gas and recirculated.
7.1
Mechanisms of Tumble/Crowth Agglomeration
With the exception of very few applications where particles are so small that they naturally agglomerate in the dry state, tumble/growth agglomeration methods utilize binders. Even if materials contain binder components inherently, this constituent is so obvious in the bulk mass that the process can not be classified as binderless. In this general section, only those tumble/growth agglomeration methods will be discussed in which discrete solid particles, (seed) agglomerates, and fragments of agglomerates attach themselves to each other. Other technologies, such as spray drying, use almost identical equipment as, for example, fluid bed agglomerators; however, since they are utilizing different growth mechanisms, their fundamentals will be covered in Section 7.4.3.
7.1 Mechanisms of TumblelGrowth Agglomeration
In tumble/growth agglomeration distinct process steps can be defined in which (see also Chapter 6, Fig. 6 . 3 ) : 1. Green agglomerates are formed from solid particles and binder. 2. Green agglomerates are cured. 3. If necessary, the cured agglomerates are sized (undersized material is recirculated and oversized agglomerates are crushed and rescreened or recirculated). 4. If desired, post-treatment takes place, for example, the application of anticaking agents, coating, etc.
Steps 3 and 4 may sometimes move in front of step 2 to avoid the expense of energy that is required for repeated drying and rewetting of large circulating streams of material. However, since sticking and other unwanted agglomeration problems (see also Section 5.5) may be encountered during sizing and oversize crushing, application of this alternative may not always be possible. In a broad sense, process equipment for tumble/growth agglomeration itself, may be divided into: Apparatus producing movement of a densely dispersed mass of particulate solids - dense phase tumblelgrowth agglomeration (Sections 7.4.1 and 7.4.2). 11. Apparatus producing movement while keeping solid particulate matter suspended or loosely dispersed in a suitable fluid - suspended solids agglomeration (Sections 7.4.4 and 7.4.5).
I.
In both cases, finely divided binder is added in a suitable manner to the turbulently agitated mass of particles. If solid particles are suspended in a liquid, agglomerates may be formed after adding a second, immiscible bridging (binder) liquid - immiscible binder agglomeration (see also Section 7.4.6). In the widest sense, this technology belongs to the Type 11-processes. It was previously mentioned (see Chapter 6, Fig. 6.1 and 6.2) that the basic adhesion criterion of tumble/growth agglomeration is that two solid entities colliding with one another coalesce and the resulting bond is stronger than the combined effects of all system forces which try to separate it again. This principal process continues, causing size enlargement by agglomerate growth. However, as it proceeds, somewhat more complicated mechanisms evolve. Fig. 7.1 and 7.2 [B.42]present almost identical explanations of what is happening. While Fig. 7.1 is the more easily understandable series of sketches defining nucleation, random coalescense, abrasion transfer, as well as crushing and layering (preferential coalescense), Fig. 7.2 distinguishes between size enlargement and size reduction phenomena, both of which take place simultaneously. Nucleation, the production of primary agglomerates or, in general technical terms, of “seeds”,occurs when several individual particles adhere to each other. Nucleation is the most difficult and time consuming part of any tumblelgrowth agglomeration process. The reason for this is, that most seed agglomerates are very weak because the kinetic energy of the impact during particle collision is low which translates into the
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o+o+o
w+
- 6b -@
0 0 +o +
Nucleation
Random coalescence
+o +o
c
+O
Abrasion t r a n s f e r
Fig. 7.1: Sketches explaining the different processes taking place during tumble/growth agglomeration 18.421.
Crushing a n d layering ( p r e f e r e n t i a l coalescence)
Size enlarqement
1
Coalescence f,
t
5 --P/+/
O+O-Q
Size reduction
1
Breakage
P, -P,
+ f,
O--a+O
I
La ye ring
Working u n i t P,
0
Fig. 7.2: Schematic representation o f the mechanisms involved in size changes during tumble/ growth agglomeration [B.42].
7.1 Mechanisms of Tumb/e/Crowth Agglomeration
development of only small adhesion forces, only a few interactive adhesion sites exist, and individual primary particles are not yet embedded in a structure where forces at several coordination points participate in the bonding. As a result, under the effects of system forces, nuclei tend to disintegrate again into individual particles. Since only a small number of nuclei survives at any given time, this initial part of the growth process is time consuming. As long as individual particles are available they tend to adhere, trying to form nuclei or attach themselves to larger agglomerates. The latter becomes the preferential process because the larger entities with more mass and higher kinetic energy easily “pick up” individual particles and incorporate them into their surface structure (see also Section 5.3 and, for example, Fig. 5.42). Therefore, to accellerate the tumblelgrowth agglomeration process specific operating strategies that influence the nucleation stage are commonly applied. If the agglomeration is carried out batch, at the end of the process only to 3/4 of the agglomerated mass is removed. The rest, often called a “heel”remains in the apparatus which is then filled to the predetermined level with fresh powder mix. As soon as tumbling and, if applicable, the addition of binder fluid start, the preagglomerated material, that remained from the previous batch, participates actively in agglomerate growth by random coalescense, abrasion transfer, as well as crushing and layering (see Fig. 7.1) and, at the same time, but not influencing the agglomeration rate, nuclei are being formed. In some cases, where low force natural bonding of submicron- or nanoparticles is used and, therefore, nuclei are particularly weak, the time for completion of the agglomeration process has been reduced from days to hours by this measure, for example, during the densification of silica fume in a batch fluidized bed (see also Section 7.4.5). In continuously operating tumblelgrowth agglomeration processes, the recirculating fines take the place of seeds. Although the recirculating particles are smaller than the lower product size limit, most of them are preagglomerated entities which become rewetted, if they had passed the drying stage, and then easily pick-up single feed particles while “regular” nuclei are also developing in the tumbling charge. Therefore, recirculating fines are not only a burden on productivity and operating cost, in many tumble/growth agglomeration systems they also play a vital part in agglomeration efficiency. Particularly in the pharmaceutical industry and other applications requiring high cleanliness, modern batch agglomeration equipment is now often operated as a “one pot processor”. This means that all agglomeration steps, including dry powder mixing, is done in the same vessel without opening the containment between steps and transferring intermediate materials. A “one pot process” blends the components, agglomerates (stabilizes) the mixture by adding binder liquid, potentially adjusts agglomerate size by crushing, further agglomerates while introducing additional binder liquid, and finally dries and cools the agglomerated mass. In such processes, fines may be removed by screening from the one pot processor discharge and returned into the processor with the appropriate amount of fresh feed components to act as seeds. Retaining a heel or returning fines which act as seed material may also effectively prevent the potential of selective agglomeration of the finest component(s)of the for-
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mulation. Because the adhesion tendency increases with decreasing particle size (see also Sections 5.1.1 and 5.4) very small particles adhere easily to larger ones but also preferentially to each other. Such selective agglomeration may change the uniformity of the mix and/or composition of individual agglomerates. Because of the larger mass of the preagglomerated returning fines they participate in the destruction of selectively growing agglomerates and, thus, help in maintaining a uniform distribution of all components of a formulation in an agglomerated, mix stabilized product. As the mass of the growing agglomerates increases they may break apart at structurally weaker areas or as a result of the force of impact. Abrasion will also take place resulting in newly liberated primary particles or small conglomerates which then try to attach themselves to entities offering better binding conditions. Particularly in batch operations, both mechanisms help to prevent the growth of a few agglomerates to excessively large sizes. To make sure that the production of oversized agglomerates is prevented or, at least, reduced, individually controlled cutting or shredding devices are often installed which will continuously or intermittently operate and mechanically assist the breakdown of agglomerates (see also Section 7.4.2). Such operational methods may lead to a relatively uniform and narrow agglomerate size distribution if their application has been developed for a particular process in the laboratory. However, these procedures need to be redefined in the large scale commercial unit because scale-up is extremely difficult, if not impossible. Depending on the density of the tumbling material, the (changing) mass of the individual agglomerates, and the type of equipment causing agitation, the growth phenomena and, herewith, the agglomerate properties will differ. One reason for change is the varying extent of the previously mentioned naturally occurring or mechanically induced abrasion, break-down, and reagglomeration. Another is how new particles are attached and incorporated into the structure (see also Section 5.3 and, for example Fig. 5.42). It is obvious that particle beds, tumbling in rotating equipment or agitated by mixing tools, will produce denser agglomerates than obtained in the low density particle clouds of fluidized beds. These effects will be described in more detail in the appropriate chapters of this book.
7.2
Kinetics of Tumble/Crowth Agglomeration
For all methods of tumble/growth agglomeration, during which size enlargement occurs in an irregularly moving mass of particulate solids by the adhesion of single particles, nuclei, conglomerates, and pieces of agglomerates to each other, growth is a function of time. Investigation of the kinetics of tumblelgrowth agglomeration, i.e. the change of sizes and their distribution with time, is of direct and practical importance, particularly in regard to the determination of the end-point of the process at which the desired product properties are attained. Kinetic studies deal with theoretical and experimental investigations of the motion in agglomeration equipment, the growth mechanisms caused by these movements, and the operating and equipment parameters influencing the process. To be able
7.2 Kinetics of TumblelGrowth Agglomeration
to explain the phenomena, knowledge of the binding mechanisms acting in the charge and between the particles is necessary. It is the task of these studies to correlate the equipment parameters and the material characteristics, both ofthe feed and the product as well as of the binder(s),ifapplicable, such that the conditions in the apparatus and the agglomerated product quality can be predicted, And it is the ultimate goal of these studies to provide fundamental input for the automatic control of the processes. At the beginning, very much influenced by the emerging large scale iron ore pelletization processes in the early 19GOs, batch operating drums were employed to investigate the growth of agglomerates during “balling”. Typical results of such tests are shown in Fig. 7.3 and 7.4 [B.42]. In Fig. 7.3 agglomerate growth is plotted over time. The latter is characterized by the number of revolutions ofthe drum. Four tests were carried out with moist (42.5 vol.% water) limestone powder and the curve confirms good reproducibility. In such a batch process, at first small nuclei are formed which grow into larger agglomerates as time progresses. The absolute number of agglomerates diminishes during the process as
Fig. 7.3: Average pellet diameter as a function of agglomeration time (measured in drum revolutions). (A) Region of seed (nuclei) formation; (B) transition region; (C) agglomerate growth region.
QI
m
e >
a
Revolutions of the d r u m n
100 80 OI
.C_
$ 60 0
Q c
$ LO 2
a 20 Fig. 7.4: Agglomerate size distribution after different times, n = number o f drum rev0I uti on s.
0
Pellet d i a m e t e r d Irnm)
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7 Tumb/e/Crowth Agglomeration
small and weak conglomerates are crushed and, then, the pieces adhere to larger a g glomerates. This is depicted in Fig, 7.4 where the cumulative mass distribution curves of agglomerate sizes are plotted after different processing times which, again, are characterized by the total number of drum revolutions. In continuously operating equipment, these growth regions take place simultaneously. As mentioned previously (see Section 7.1) the critical phase of the process is the formation of nuclei. This must always take place at a sufficient rate to guarantee the transformation of the often highly adhesive feed material into a freely flowing granular mass. Investigations of the kinetics of growth agglomeration continues using more modern laboratory equipment and modeling techniques [e.g. 7.1 -7.31. Population and mass balance equations are widely used to describe the conditions with rather mixed success. For example, Sastry states [7.1]:“Formal derivations of these model equations can be found in the literahre, including details on deriving rate terms for the individual mechanisms. Such equations can be adapted appropriately to specific applications, for example, in describing plug flow drum agglomeration, fluidized bed particle coating, high speed mixer agglomeration, or fully mixed flocculation systems. Relevant initial and boundary conditions must be introduced. Then, of course, we need analytical expressions or correlations for the nucleation, coalescense, and layering process parameters (among others) as a function of all material, machine, and operating conditions. After all this, one finds that analytical solutions for the model equations are most unlikely because of the complex nature of the system equations. Consequently, one resorts to numerical solutions”. The problems encountered in mathematical modeling of tumble/growth agglomeration do not relate to the theories, formulas, and possib es to solve the ever more complicated equations. With modern computing possibilities, a whole series of assumptions can be introduced into the model equations and responses to certain imaginary process conditions can be predicted. However, the real system often produces unexpected results intermittently or even consistently without offering a clear indication of why such deviations occur. Introduction of new mathematical methods, such as, for example, fuzzy logic or chaos theory, produce more complicated model equations and “closer to life” results but still are not able to serve as unequivocal bases for control schemes. The real problem is, of course, the determination of and correlations between process data as input for the model and its solutions. Such expressions are different for each situation, i.e. they depend on feed material and binder characteristics, equipment design and operation, process variations, final product properties, and many more. Data that can serve as input for the model equations must be obtained experimentally. Since access to commercial, often restricted or large scale operations is not available or possible, typically the determination of data and their correlation is based on model experiments. In addition to the difference in size and operation between the laboratory model and the real system, the gathering of data is interrupting and critically changing the process. For example, referring to Fig. 7.3 and 7.4, to obtain each data point in Fig. 7.3 and curve in Fig. 7.4 the batch laboratory drum agglomerator was stopped after the indi-
7.2 Kinetics of Tumble/Crowth Agglomeration
cated number of revolutions, the charge was removed and screened. After that, the material was placed back into the drum which then rotated for the additional number of revolutions until the next set of data was determined. It is obvious that the interruptions of the process and the handling will have some, but indeterminable influence on how agglomeration as a whole proceeds. Of even more concern is that the laboratory model experiment is carried out in much smaller scale and almost exclusively in batch mode. Even small continuous tests require so much material to reach equilibrium during the multiple test runs that are necessary to evaluate different process parameters that their use is very infrequent. To overcome the need for large amounts of material and the associated handling problems, agglomerates are sometimes crushed and reused as “fresh feed” which introduces a completely new set of problems. If, on the other hand, a continuous process is simulated in a batch operation, the influence of recycle and the question of process equilibrium remain unsolved problems. Scale-up, even from laboratory batch to commercial batch operations, is very difficult and can not be totally predicted by tendencies or “rules” that are obtained as results of systematic variations of assumptions in model equations. As a result, vendors maintain an often extensive test facility, potentially with differently sized equipment (see Section 11.2) to more accurately predict the behavior of the commercial system. Particularly in the more regulated industries, which process materials such as food and pharmaceuticals with high profit margins, a new trend is towards tolling operations (see Sections 11.2 and 14.1) either during the development phase or for the manufacturing of an intermediate or a particular final product during its entire life. Equally difficult is the definition of suitable, easily measurable process variables that can be used to control the performance of a commercial installation. In continuous operations it is often sufficient to measure the inputs, such as the mass flows of fresh and recycling material and the amount of binder as well as its location and means of application at defined operating conditions of all system components and keep them constant. If, after reaching equilibrium, product yield and quality are not acceptable, all process variables must be evaluated and changed until optimal conditions are reached. Sampling plays an important role during this endeavor. For that reason it is most important to plan for and provide sampling points at crucial points of the process when first designing the system (see also Section 11.3). Determination of the conditions in and the need for modifications of process variables of batch tumble/growth agglomerators are normally not possible. It is not feasible to sample the contents of batch operating equipment during a process run to determine the progress and state of agglomeration. Therefore, methods have been proposed to control the performance of such equipment by measuring, for example, the sound of the tumbling charge and its change or the momentary energy consumption of the drive motor. Measurement and analysis of the power consumption during tumblelgrowth agglomeration in a drum have been carried out by many researchers [e.g. 7.4, 7.51. Fig. 7.5 is a typical power consumption curve obtained during the agglomeration of lactose with water in a batch high energy mixer/granulator as a function of time
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S in [mL liquid/100 g of powder]. Although the actual shape of the curve does change with material, binder, process, and product characteristics, the overall result can be characterized by the schematic representation of Fig. 7.6. The tumblelgrowth agglomeration process is carried out by placing the dry components into the apparatus. As described previously the equipment could be operated as “one pot processor” (see Section 7.1). Then the goal of interpreting the power consumption curve would be to determine the endpoint of agglomeration, that is the time at which agglomeration is completed and drying begins. During “standard”processing, where curing (drying, cooling and, if applicable, other post-treatment procedures) are carried out externally, the “endpoint of agglomeration” defines the time when the agglomerator should be emptied. Referring to Fig. 7.5 and 7.6, in phase I, powder mixing takes place. During this process S1 is “zero” (or very low) and tumbling conditions are not altered; therefore, the power consumption remains constant. After mixing has been completed, which is defined by the previously, experimentally determined time that is required to uniformly blend the powder components, addition of binder liquid begins. Typically, binder is added as a fine spray and at constant rate. For a short time (see Fig. 7.6), the power consumption curve will not be influenced until, at Sz,it begins to increase. In phase 11, liquid bridges develop between the powder particles and nuclei as well as smaller agglomerates are formed. The increase in power consumption is, on one hand, due to the added mass of binder liquid and, on the other hand, caused by the increasing cohesiveness ofthe moist powder. In phases 111 and IV, from saturations S3
A 300Phose V I
3
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Time Imin)
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1L 21 28 Total liquid addition Iml liquid1100g powder)
Fig. 7.5: Power consumption o f a batch high energy mixer/granulator during the agglomeration o f lactose with water as binder liquid, plotted vs. time and, respectively, the amount o f liquid added
-
7.2 Kinetics of Tumble/Crowth Agglomeration I149
POWER OR ENERGY CONSUMPTfO
T
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PHASE I1 I I I I I I I I I I I I
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r
/ I
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PHASE 111 I I
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AMOUNT OF GRANULATING LIQUID OR TIME
Fig. 7.6: Schematic representation o f the general shape o f power consumption curves as a function o f time or, respectively, liquid saturation, 5,.
to S5,the interparticle void volume in the agglomerates fills up with liquid until, at Ss, complete saturation is achieved. From a control point of view, for practical purposes, phase 111 is the most important one as only in this range of saturations (between S,and S,) agglomerates are obtained that feature acceptable size distribution and quality. If a batch granulator is equipped with both mixing tools and cutter heads (see Section 7.4.2) it is in this phase that particle size adjustment is accomplished by intermittently activating the high speed cutter heads. In phase IV, particularly towards its end which is characterized by Ss, local ovenvetting takes place which may be corrected to some extent by operating the cutter heads and redistributing the moisture more uniformly (approaching the straight line depicted in Fig. 7.6),however it is often recommended not to exceed saturation S4 if a well controllable operation of the process is desired. Phase V should be avoided altogether. Operation becomes erratic due to excessive sticking and the build-up of large, wet conglomerates. While, with identical feed materials, the overall shape of curves obtained with various types of mixer/agglomerators is the same, as shown in a very generalized manner in Fig. 7.6, different amounts of water are required to reach the specific saturations that define phases I to V. For example, Fig. 7.7 depicts results obtained with three equipment designs [B.42]. Referring to saturations S2 and S5, the absolute variation in liquid requirement is very small but for S3, the minimum amount of liquid that is required to achieve an agglomerated state of the powder, the difference is approx. 100 % between two of the granulators chosen for the experiment. Equally as large is the difference between two of the granulators at S4, the safe maximum amount
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Fig. 7.7: Liquid requirements for obtaining the specific saturation levels 5, as defined by the energy consumption curve in Fig. 7.6 for different mixer designs [B.42, 7.41. o Intensive, high energy mixer with vertical axis of rotation of the mixing tools (Mfg. DIOSNA). # Intensive, high energy mixer with horizontal axis of rotation ofthe mixing tools (Mfg. LODICE). $r Low shear planetary mixer (Mfg. LOEPTHIEN).
ofbinder liquid addition, but with a change in relation. This means, in reference to this particular example, that the very turbulent environment in the intensive, high energy mixer with vertical axis of rotation of the mixing tools produces good agglomerates with a relatively small amount of liquid in a narrow range, while the intensive, high energy mixer with horizontal axis of rotation of the mixing tools requires somewhat more liquid before larger, more uniformly tumbling agglomerates are formed but also tolerates a larger amount of binder before entering the increasingly unstable phase IV. For the low shear planetary mixer it is particularly important to point out that the difference between S, and S5 is the smallest which means that the charge may become easily ovenvetted resulting in erratic and uncontrollable process conditions. In the end, when considering the kinetics of tumblelgrowth agglomeration, which is really the art of controlling an agglomeration system such that high quality products with the desired properties are obtained, external consultants who use fundamental and/or interdisciplinary knowledge and know-how are often the only ones that can help to bring non-performing plants into compliance or optimize under-performing systems. For further information on the kinetics of agglomeration in pans a study by W. Dotsch [B.33] should be referred to also.
7.3
Post-treatment Methods
Several times before it had been mentioned that green agglomerates that grew during tumbling in the presence of a liquid binder are bonded by temporary binding mechanisms and must be cured, using post-treatment methods to achieve permanent bonding (see, for example, Chapter G and Fig. 6.3). Tab. 7.1 is a general summary of the effects of post-treatment methods that are commonly applied during size enlargement by agglomeration. In addition to the
7.4 Tumb/e/Growth Agglomeration Technologies
achievement of the final binding mechanism after the removal of liquid binder components, which is necessary in almost all applications of tumble/growth agglomeration, other accomplishments are feasible and may be used in the design of any agglomeration system. Tab. 7.1: Effects of post-treatments (curing) in size enlargement by agglomeration.
Achievement
offinal binding mechanisms (by recrystallization, sintering, chem. reaction,..)
Development
offinal agglomerate characteristics (by drying, hardening, disinfection, impregnation,..)
Improvement
ofjnagl agglomerate characteristicts (by coating, polishing, conditioning,..)
Modijcation Change
of agglomerate characteristicts (by removal of temporary ingredients [porosity],..) of the applicabiiity of agglomerates (by secondary agglomeration [tabletting, spheronizing, instantizing,..], encapsulation,..)
Many of the curing methods and their effects were already mentioned in previous chapters (see, for example, Sections 5.3, with subchapters, and 5.4) while others will be covered later in the appropriate chapters. Therefore, the summary in Tab. 7.1 shall suffice at this location to highlight the topic.
7.4
Tumble/Crowth Agglomeration Technologies
In the following six subchapters the technologies and the equipment for beneficial agglomeration by growth during tumbling will be described. In the context of these chapters tumbling means the irregular, turbulent (stochastic) movement of all participating particulate matter in a suitable environment. As mentioned before (see Chapter 6) no specific equipment is necessary for this movement to occur. Any means that will produce the stochastic movement of the solids in any environment can be utilized and adapted to become an “agglomerator”. For practical reasons, four different types of agglomeration technologies will be distinguished and covered in this part of the book:
High density tumbling particle beds (Sections 7.4.1 and 7.4.2), drying of solutions and suspensions (Section 7.4.3), low density particle clouds (Sections 7.4.4 and 7.4.5),and agglomeration in liquid suspensions (Section 7.4.6).
page page page page
153-187 187-196 196-221 221-227
Drying of solutions and suspensions in so called spray dryers (Section 7.4.3)make use of the binding mechanisms of agglomeration which develop during drying, but secondary agglomeration, that is the adhesion of semi-dry and/or dry particles during collisions in the drying tower, is not the primary objective of the process. Nevertheless, the technology will be covered in these chapters because it is sometimes combined with fluidized bed agglomeration (Section 7.4.4).
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High Density Tumbling Particle Beds (Sections 7.4.1 and 7.4.2) These tumblelgrowth agglomerators are characterized by the presence of an agitated particle bed in which, during the addition of a binder, collisions and coalescence occur. Typical equipment for realizing this technology include rotating inclined pans, cones, and drums as well as any mechanical powder mixer that has been modified to allow the addition ofbinder into the live blend. Particularly the latter group of equipment, powder mixers, is applied to an ever increasing extent because particle movement that is caused by specific container shapes as well as movements or by mixing tools together with separately driven cutter bars or heads allows to exert considerable control over the process. According to the type of agitation, mixers can be classified into several design groups. For agglomeration, the most important distinguishing characteristic is the amount of shear that is created in the particle bed. Low shear particle mixers (Section 7.4.1) typically employ rotating containers featuring shapes that produce irregular particle flow which, sometimes, is enhanced by baffles or similar structures that are built into the machine. For use as agglomerators, liquid spray arrangements and often cutter bars or heads are installed. High shear particle mixers (Section 7.4.2) are usually equipped with stationary shells and rotating mixing tools inside. The axis ofthese mixing tools may be either vertical or horizontal. For processing reasons some shells may be mounted at an angle to the horizontal with a corresponding inclination of the axis of the mixing tools. Again, for agglomeration spray systems are added and additional shear may be introduced by shear plates and/or cutter heads. Drying o f Solutions and Suspensions (Section 7.4.3) Although binding mechanisms of agglomeration are developing during the process, these technologies are primarily dryers that are often used to produce the primary particles for agglomeration by some other method. Depending on the feed to the dryer, either solutions or suspensions, including slurries of many consistencies and filter cakes, the structure of the solid particles after drying is quite different. Low Density Particle Clouds (Sections 7.4.4 and 7.4.5) If agitation of particulate solids occurs by gas flows or jets, the speed of the suspending fluid is normally so high that the so called fluidization point or incipient bubbling velocity is exceeded and an expanded, low density particle bed develops. In this condition the stochastic movement of the solids is attained and coalescense may occur during particle collisions. Because the separating forces in the low density particle cloud are relatively little and, at a given gas velocity, only a narrow distribution of particle sizes can be retained in the fluidized state, small, very porous agglomerates are obtained.
Solid particles that are suspended in liquids may agglomerate as a result of two basically different phenomena: Flocculation is the aggregation of solid particles into relatively loose conglomerates (flocs) after collision and coalescense have occurred. Adhesion may be enhanced by the addition of polymers (so called flocculants). Agglomeration in Liquid Suspensions (Section 7.4.6)
7.4 TumblelCrowth Agglomeration Technologies
Immiscible liquid agglomeration is using the affinity to certain particulate solids of a binder liquid that is dispersed in the form of tiny droplets in and is immiscible with the suspending liquid. Particles are selectively bonded with the immiscible binder liquid, form agglomerates, and can be separated as enlarged entities from the suspension. 7.4.1
Disc and Drum Agglomerators Disc, Dish, or Pan Agglomerators The basic disc agglomerator is a simple, inclined, flat-bottomed, shallow pan that, owing to the particular pattern of particle motion, features a distinctive classification effect whereby only the largest agglomerates discharge over the rim (Fig. 7.8). To achieve special effects, modified pan designs are available (see below).
A typical shallow disc "pelletizer" is shown in Fig. 7.9. It consists of the the pan (A) which is often equipped with an expanded metal liner (B) to reduce wear. The height h of the rim forming the pan is small compared with the diameter D of the disc ( h / D 0.10-0.20). Normally, the pan angle can be adjusted between 40 and GO' to the horizontal, in this case with a handwheel operated jacking screw (C).The disc with angle adjustment and drive is mounted on a heavy base structure (D). Also attached to the base is a frame (E) which carries the plows or scrapers (F), the spray nozzles (G), the positioning of which can, in this case, be adjusted by flexible metal hoses, and, if applicable, the dust cover. The particulate feed is delivered to the pan by suitable metering equipment and dropped onto the wetted moving mass. As shown in Fig. 7.10, shallow inclined discs are manufactured with diameters of less than 0.5 m to more than 7.5 m or, in special cases, exceeding 10 m in diameter. The design of shallow inclined pans which, if left uncovered, allows to observe the movement of the charge during operation and its response to changes, such as rotational speed and pan angle as well as method(s),position(s), and amount(s) of fresh N
Fig. 7.8:
Photograph o f a disc "pelletizer" with narrowly sized finished agglomerates discharging over the rim.
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Fig. 7.9: Photographs of a typical shallow disc "pelletizer" depicting the structural components (courtesy FEECO International, Green Bay, WI, USA).
feed and binder additions. This has resulted in extensive and successful scientific evaluation of that type of growth/tumble agglomerator. The most striking feature of the shallow inclined pan is a very defined pattern of particle motion and a size classification both over the pan area and in the tumbling particle bed. As sketched in Fig. 7.11 and discernible in Fig. 7.8, in a clockwise rotating disc the material is lifted up from around the G o'clock position through 9 o'clock to near the top from where it fans out and cascades back down to the lower portion; here, the cycle begins again.
Fig. 7 . 1 0 Photograph showing a range o f differently sized shallow disc "pelletizers" (courtesy FEECO international, Green Bay, Wi, USA).
7.4 Tumble/Crowth Agglomeration Technologies
12
3
Surface: Development of different layers
Movement of feed in various stages
Fig. 7.11: Sketch o f the pattern o f particle motion in a shallow inclined disc agglomerator (adapted from IB.421).
Observation and/or sampling of the contents of the operating pan reveals that in the left half of the clockwise rotating disc, where material is lifted up, a particle bed exists that extends to the edge of the rim. Due to a natural segregation effect, the largest particles move on the top of the bed and discharge over the rim as more material is added to the pan. The smallest particles, unagglomerated feed and seed agglomerates, are concentrated on the bottom of the bed. Under the weight of the bed, the lowest layer of moist feed and small agglomerates tends to stick to the bottom and is cleaned off by a series of staggered scrapers. As mentioned above, to minimize wear, in many cases an expanded metal liner is installed in the pan to encourage the build-up of a layer of material, the thickness of which is controlled by the plows; at the same time, the frictional characteristics which are required for uniform movement of the charge are optimized. In the right half of the clockwise rotating pan, directed and partially produced by the scraper plows, a curtain of small and seed agglomerates moves down over the disc bottom. Considering the growth mechanism of tumble agglomeration (see Chapter 6, Fig. 6.1, and Section 7.1, Fig. 7.1 and 7.2), control of agglomerate growth in an inclined, shallow pan can be affected by the relative positions of binder liquid and fresh feed addition. For example, if the liquid sprays impinge the moving courtain of small agglomerates roughly along the 3 o’clock radius and fresh, relatively dry powder feed is added on the 5 o’clock radius, powder particles will adhere to the wetted seeds and make them grow. When entering the bed at roughly the 6 o’clock position the newly adhering powder particles will be either “compacted” onto the growing agglomerate or removed by attrition and transferred to the surface of other agglomerates, thus causing growth. Adding liquid and fresh feed according to this pattern results in the production of many relatively small agglomerates. Nevertheless, the largest ones
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will move on the top of the oval shaped “tumbling center” close to the rim in the left half of the pan and discharge by overflow at the 7 to 8 o’clock position. On the other hand, if liquid is added to the top of the bed at, say, the 11o’clock radius and fresh feed is dumped into the turbulently moving foot of the bed at approximately 6 o’clock, large agglomerates will be grown. Angle of tilt, rim height, and rotational speed (influencing the pattern of motion) together with adjustments of method and location of binder and fresh feed addition, can further modify the growth behavior and may be used for control. It is no surprise that, after the inclined shallow pan agglomerator was invented around the middle of the first half of the 20th century for the agglomeration of fertilizers, cement raw meal, and other mineral powders, it quickly attracted the interest of many researchers, including the author of this book, because it lent itself to easy scientific scrutiny and evaluation. Rather extensive coverages of the subject are included in earlier books [e.g. B.9, B.16, B.331to which the interested reader should refer. Unfortunately, the tumblelgrowth agglomerator that is best described and understood when operated in a laboratory environment, is very difficult to control when used in large scale and an industrial environment. Small fluctuations in binder liquid and fresh feed addition as well as in frictional conditions in the charge and between the charge and the equipment components, most of which are caused by temporary overor under-wetting, render the operation of inclined shallow pan agglomerators an art. There are a few large scale applications, for example the pelletization of iron ores, where the highly uniform and fine feed (particle size <40 pm) and the desired pellet size (12.5 mm average) produce an optimum system response and require little operator interference. The benefit of such an operation is the manufacturing of narrowly sized, spherical “balls” which do not need extensive sizing and handling of large amounts of recycle. Most other applications, particularly those attempting the production of often desired “micro agglomerates” (approx. < 5 mm), require the continuous attendance
Fig. 7.12: Schematic representation o f scraper arrangements in inclined shallow pan agglomerators. Left: Stationary wall scraper and rotating bottom scraper. Right: Stationary wall and bottom scrapers.
7.4 TumblelCrowth Agglomeration Technologies
of an experienced operator who can remedy problems by identifying, understanding, and correcting excessive and/or ongoing fluctuations. Because fluctuations are often beginning at the scrapers where moisture may accumulate and build-up can occur which, from time to time breaks off as large slabs, manufacturers of inclined shallow pan agglomerators sometimes propose to use motor-powered rotating scrapers. As shown schematically in Fig. 7.12 they not only scrape the bottom but also somewhat extend into the particle bed to serve a similar purpose as the cutter heads in mixer agglomerators (see Section 7.4.2). The rotating scrapers can be operated co- or counter-currently. For some applications a more uniform, less operator intensive production is achieved. Modified Disk or Pan Agglomerators Inclined shallow disk agglomerators lend themselves easily to modifications. By employing innovative pan designs (Fig. 7.13 and 7.14), the particle motion in parts of the disk may be changed to achieve special effects. Unfortunately the already mentioned sensitivity to operational upsets is even more pronounced with these tumble/growth agglomerator designs and, therefore, these interesting process variations are seldomly used. Nevertheless, they are mentioned here as examples of ideas that could be beneficially applied for the manufacturing of special products. Fig. 7.13 shows schematically four pans with collars or other re-roll designs. The purpose of the addition of collars (Fig. 7.13a and b) is to provide a separate re-roll space for agglomerates discharging from the main operating area of the pan. The re-roll can serve to smooth the surface of the agglomerates, densify their outer layer, or, generally, produce a more spherical shape. In certain applications, the re-roll space is also utilized for additional processing. This may include the coating with components for subsequent use, such as coke breeze on green iron ore “balls” (as a fuel or a solid reductant), limestone powder on coal pellets (for desulfurization of the combustion gas), or anticaking agents (to improve the storage characteristics of, e.g., fertilizers). Fig. 7.15 is the photograph
Fig. 7.13: Sketches o f inclined shallow pan agglomerators that are equipped with collars or partitions to achieve special effects. (a) Basic collar design, (b) collar with rim baffle, (c) concentric vertical partitioning rings, (d) peripheral rings on the rim.
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Fig. 7 . 1 4 Sketches of inclined shallow pan agglomerators with stepped side wall or bottom. (a) Basic onestepped design, (b) multistepped design, (c) partially raised bottom.
of uncoated (white) and coated (gray) as well as of cut pellets (to show the coating) which were produced by this technology. The collar or re-roll ring can be fastened flush (Fig. 7.13a) or recessed to provide a baffle (Fig. 7.13b) over which the agglomerates must discharge. Similar effects can be obtained by installing concentric vertical partitioning rings or peripheral rings on the rim (Fig. 7 . 1 3 ~ and d). To enhance the segregation of fines and of smaller or larger agglomerates, stepped side wall designs have been suggested (Fig. 7.14). Proponents of the multistep sidewall (Fig. 7.14b) claim that stronger, more uniform agglomerates are produced because the larger pellets impact and roll on the stepped sidewall rather than on a “soft bed” of agglomerates and fines. The inventor of an inclined shallow pan with a concentric raised central bottom part (Fig. 7 . 1 4 ~reports ) that a more stable operation is obtained because larger lumps, which are normally breaking off the scrapers in a flat bottom design, are not present. This is due to the fact that liquid binder and fresh feed are sprayed and, respectively, fed onto the raised bottom part which is held
Fig. 7.15: Photograph o f uncoated (white) and coated (gray) a s well as cut pellets (to show the coating). The coating was applied on a re-roll ring (courtesy EIRICH, Hardheim, Germany).
7.4 JumblelCrowth Agglomeration Technologies
clean by a stationary scraper. The “seeds”thus produced are directed into the annular space between the raised bottom and the rim where uniform growth occurs without surging. Deep Disk or Pan Agglomerators Some manufacturers offer inclined pans with a diameter to rim height ratio exceeding 0.25 and claim that the increased mass (hold-up) in the pan results in additional strengthening of the agglomerates due to overburden pressure and longer residence time. One design in particular (EIRICH, see Section 7.4.2) also uses an eccentrically mounted, rotating scraper/mixer which causes partial disintegration of already formed agglomerates, thus forcing crushing transfer/layering and producing more uniform structure and sizes of pellets. W ,a etr
connection
Ma t e r i a l operating level
ts
Fig. 7.16: M M C Mars Mineral drum pelletizer. (a) Schematic representation o f the design principle, (b) photograph o f a large industrial M M C d r u m pelletizer (courtesy Mars Mineral, Mars, PA, USA).
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Fig. 7.17: Sketch o f a “cone pelletizer”.
Of particular interest is the deep pan agglomerator with integral rear auger feeder (Fig. 7.16). By transporting the material to be agglomerated directly into the “seeding area” at the bottom of the pan, this design promotes the production of new seeds and eliminates the free fall of fine feed from the top which may cause dusting. In top-fed inclined pan agglomerators the installation of covers and dust collection is necessary to avoid particulate pollution. This impairs physical and visual access to the pan and is avoided by bottom feeding. Since it is difficult to reach the smaller seeds with the binder spray, most of the liquid impinges on the larger agglomerates, and the material has a longer residence time in the deep pan, larger pellets are typically being produced. As shown in Fig. 7.16b, which depicts an “MMC drum pelletizer”, the deep pan begins to resemble a drum agglomerator (see below). The main difference is a still rather steep tilt angle with a 30” adjustment range. The charge moves upward from the feed to the discharge end, which results in a relatively short “drum”,requiring little space, and still features the production of narrowly sized pellets due to the natural segregation that is typical for inclined pan agglomerators. The “MMC drum pelletizer” is also available in a multiple-depth design that provides three different drum depths in one unit. Another, no longer available inclined pan agglomerator, which was approaching the design of a drum but still featured size classification in the charge, used a truncated cone rotating around its axis (Fig. 7.17). Feed and binder were added from the top but the higher peripheral speed, which was necessary to retain the charge in the cone, resulted in additional surface “compaction” of the larger agglomerates as they traveled to the base of the cone prior to discharge. Drum Agglomerators They represent the most simple type of equipment for growth agglomeration by tumbling. They are used in industries for the processing of large amounts of bulk solids where in the relatively crude and rough environment unsophisticated machinery performs best. Drum agglomerators consist normally of a cylindrical steel tube with a slight (typically up to 10” from the horizontal) slope a toward the discharge end (see Fig. 7.18). Retaining rings are often fitted to the feed and discharge ends of the drum to avoid spill-back and, respectively, to increase the bed depth of material and/or its residence time.
7.4 TumblelCrowth Agglomeration Technologies
Liquid
/ Sc:aper
Fig. 7.18: Sketch o f a drum agglomerator.
P
Fig. 7.19 is the photograph of a typical drum agglomerator for the wet granulation of fertilizers. The tubular shell (1)is fitted with steel tires (2). The drum rests on forged trunnions ( 3 ) with antifriction bearings which are mounted on a heavy structural frame (4).Sturdy, adjustable thrust rolls ( 5 ) keep the inclined drum in place. A roller chain girth drive (G) guarantees a “soft” start and smooth running of the equipment. It consists of a hardened drive sprocket, sectionalized girth sprocket, and rugged roller chain. Gear drives are also available as an alternative. Retaining rings are fitted on both ends of the drum. Material enters by way of an inclined chute (not shown) and exits through an enclosed manifold. A liquid feed assembly (8)serves to introduce the binder into the tumbling mass in the drum. Similar to the practice used for inclined discs (see above),the interior of drums may be covered with cement or expanded metal to encourage build-up of material as an
Photograph o f a typical drum agglomerator for fertilizers (courtesy A.J. SACKETT & Sons, Baltimore, MD, USA). For explanations see text.
Fig. 7.19
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“autogenous” wear liner. To control the thickness of this coating, different designs of scrapers are employed [B.42]. As in the case of the inclined pan agglomerator, depending on the tube slope and diameter, the material, and the moisture content of the charge, the rotational speed of the drum must be adjusted such that the bed begins to separate from the wall at approximately the 10 to 11 o’clockposition or, respectively,the 1to 2 o’clock position in a counter-clockwise rotating drum (see Fig. 7.18) so that the particles tumble down the inclined bed. Binder liquid is sprayed onto the bed surface and fresh material is added to the turbulent zone at the foot of the tumbling mass. More extensive coverages of the theoretical treatment, including calculations and scale-up procedures, are published in earlier books [e.g. B.42, B.651 to which the interested reader should refer. Although, within the kidney shaped tumbling mass of solids, production of seeds as well as growth of agglomerates takes place and some segregation by size occurs, the major difference between drum and pan is, that the entire bed moves forward in a plug flow fashion. To avoid discharge of ovenvetted material, binder addition is limited to the first 114 (minimum)to 3/4 (maximum)length of the tube. In the remaining portion of the tube, moisture distribution is equalized by abrasion and crushing of conglomerates as well as reagglomeration ofthe fines (see Section 7.1, Fig. 7.1 and 7.2). Nevertheless, many fines and pieces remain in the drum discharge which consists of a wide
y 7
Fresh feed
Additive(s)
1 (Optional)
$.
Alternative
Mixer (and Binder (prewetter) liquid
I Vibrating screen
Alternative LI Roller screen
Green pellets to curing Fig. 7 . 2 0 Sketch o f the flow sheet o f a drum agglomerator that operates in closed circuit prior t o curing the green pellets.
7.4 TumbIelCrowth Agglomeration Technologies
distribution of sizes including over-, product-, and undersized agglomerates as well as some still unagglomerated material. Since the wide distribution of sizes in the discharge from drum agglomerators is not acceptable for most applications, they typically work in closed circuit (Fig. 7.20). Green agglomerates exiting the drum are sized on vibrating or roller screens. Large pieces are shredded and returned to the drum agglomerator together with the screen fines. The correctly sized product is sent to curing. Because green agglomerates are somewhat sticky and tend to blind vibrating screens (alternative I in Fig. 7.20), most of the fines are first “scalped off’ whereby the mass of the larger pieces helps to pass the fines and keeps the screen cloth open. Product is separated from the oversized material after all the fines are eliminated. More reliable are self-cleaning roller screens (see Section 11.3) with individually driven rollers and steadily increasing gap. In most cases the rotation of the screen rollers is against the gravitational flow of solids on the downward sloping machine which induces a rolling movement of the material on the “deck” and causes an additional rounding of the spherical agglomerates. Inspite of these screening “tricks” it is often not possible to separate the moist discharge from a drum agglomerator and prevent blinding of the screen cloth for an acceptable time. In those cases, curing, which always includes the removal of water, must be carried out first, as shown in Chapter 6, Fig. 5.3, so that classification occurs in a dry state. The disadvantage is, of course, that the recycle, often amounting to several hundred percent of the system’s production capacity, must be rewetted and again cured which results in increased operating costs. To enhance the natural segregation behavior in drums and ultimately avoid the need for agglomerate sizing, a number of alternative drum designs have been proposed, all of which mimic the inclined pan or cone. For example, Fig. 7.21 shows a drum agglomerator that consists of a cylindrical tube with a slight (typicallyup to 10“ from the horizontal) rise toward the discharge end [B.42,B.65l.A retaining ring at the feed end avoids spill-back.The operating principle is that of the deep pan agglomerator (see above). The difference is in the type and arrangement of accessories. Referring to Fig. 7.21, powder to be agglomerated is metered into the lower feed end of the drum by, for example, a screw conveyor (1)and liquid binder is sprayed onto the tumbling charge by suitable means (2). As usual, a scraper (3) controls the build-up on the wall. Finished agglomerates discharge at point (4) which is the highest point of the drum. Therefore, only closely sized agglomerates are produced. The rise of the drum can be adjusted/changed at point (5) and the drive is connected near the drum’s thrust support (6).
Fig. 7.21: Sketch of a segregating drum agglomerator.
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Other similar proposals include multi-partitioned drums with weirs that let only closely sized agglomerates discharge into the next chamber for controlled growth, or a series of conical inserts that serve the same purpose. Scale-up of these modified drums more closely resembles that of inclined pan agglomerators. 7.4.2
Mixer Agglomerators
The irregular, stochastic movement that is required for mixing particulate solids also produces ideal conditions for growth agglomeration by coalescense. Therefore, unwanted agglomeration is often observed in powder mixers, especially if the particle size of the powder is small and/or some moisture is present. Considerable problems can arise if components of the bulk mass have different characteristics and/or sizes because, in that case, particles that have higher adhesion tendency and/or the smaller size fraction(s) may selectively agglomerate, thus making it impossible to obtain an ideal mixture. Such selective agglomeration is of particular concern in the pharmaceutical industry where often an extremely small amount of finely divided active substance (the drug) must be mixed uniformly and reliably with a relatively large amount of inert filler material (excipient). On the other hand, if the conditions in the mixer can be adjusted such that a statistically uniform blend is obtained, segregation can be avoided through “stabilizing” the relative distribution by agglomeration. Often, this can be achieved in the same apparatus during or after the mixing phase by a controlled addition of binder. Since, normally, the binder is a liquid, green agglomerates are formed which require curing before they can be safely stored and handled. As mentioned before (see Section 7. I),in batch, ultraclean applications, particularly in the pharmaceutical industry, all of these process steps can be carried out in the same vessel (“onepot processing”) thus avoiding contamination of the product and/or the environment. Literally all powder mixers can be modified to operate as agglomerators. The relative movement of the particles that is required to obtain uniform mixing causes particles to collide with each other. Such particle to particle contact may result in coalescense and agglomeration if the adhesion criterion (see Chapter 6, Fig. 6.2) is fulfilled. In some cases, this occurs due to van-der-Waals or other attraction forces ifthe particles are very small (typically, particles must be nano-sized).In all other instances, binders (mostly liquids) must be added to achieve the growth of agglomerates. Therefore, the major and often only modification that is required to convert a powder mixer into an agglomerator is the installation of suitable means for binder addition. Introducing a liquid uniformly into the tumbling powder mass to entice agglomeration is not as easy as it sounds. For certain simple, batch operating mixing tasks where, at the end, the blend should be agglomerated, it is feasible to dump the entire predetermined amount of binder liquid into the mixer and cause it to distribute uniformly by the application of shear. Eventually, agglomerates will form. More commonly, however, liquid is slowly metered into the tumbling mass whereby it is important that no ovenvetting of certain volume elements occurs and agglomerate growth is obtained as the amount of binder liquid increases.
7.4 TumblelGrowth Agglomeration Technologies
Fig. 7.22: Cross sections through two phase (pressurized gas assisted) spray nozzles. (a) External and (b) internal mix set-up (courtesy BETE Fog Nozzle, Greenfield, MA, USA); (c) photograph of an operating nozzle, cross section through the nozzle, and spray patterns that are obtainable with such nozzles (courtesy Spraying Systems Co., Wheaton, IL, USA).
To avoid overwetting, liquid binder is introduced by means of more or less complicated and sophisticated spray systems which employ nozzles to atomize the liquid. Particularly in batch operating mixers it is important that the nozzles do not drip during the blending phase and are not clogged when atomizing begins. In many cases, tightly closing external valves and the application of two phase nozzles, in which atomization is caused or assisted by compressed gas (Fig. 7.22), is sufficient. In other cases, nozzles are required that seal themselves mechanically while they are not in use. Binder liquid should wet the solid particles and not the interior walls or the mixing tools of the equipment. If parts of the mixer are wetted, build-up occurs which is difficult to remove. Different spray patterns are available with commercially available single or two phase nozzles (Fig. 7 . 2 2 ~and 7.23) and a suitable pattern should be selected to assure that liquid impinges only on the moving powder. Since the spray pattern depends critically on the cleanliness of the orifice area, as mentioned before, nozzles must be drip free and installed such that they remain clean or are blown free by the action of atomizing gas. This is particularly important for low capacity nozzles which are often preferred because the binder liquid should be always completely ab-
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Fig. 7.23: Spray patterns ofsingle phase (liquid only) spray nozzles (courtesy Spraying Systems Co., Wheaton, IL, USA).
sorbed by the growing agglomerates to generate a “dry” tumbling mass and avoid build-up. For the purpose of agglomeration, mixers can be classified into: Low shear mixers, high shear mixers, low or high shear mixers with intensifiers. Low Shear Mixers The most simple low shear mixers use horizontally oriented cylindrical drums that rotate about their axis. Many of these mixers are operating in a batch mode and feature differently shaped lifters or internal baffles or the vessels are shaped such that the otherwise regular flow of material is modified to a stochastic movement. Continuously operating cylindrical drum blenders resemble the previously described drum agglomerators (see Section 7.4.1).
One of the earliest drum mixers that was applied in the fertilizer industry for agglomeration (granulation) produced a curtain of solid materials as sketched in Fig. 7.24. If acid and ammonia are sprayed onto the curtain and/or added by sparger tubes granulation occurs during ammoniation. The co-processing (ammoniation and granulation) makes this technique a most beneficial one.
7.4 Turnble/Cro wth Agglomeration Technologies
Fig. 7.23:
(continued)
Other low shear mixers use double or slanted cone and V-shaped vessels. Fig. 7.25 depicts schematically these always batch operating blenders which rotate about a horizontal axis. The lines and arrows in the sketches try to explain the particle paths that cause mixing. In Fig. 7.26 photographs of typical equipment are presented. The modification to a mixer agglomerator involves the installation of spray nozzles in the axis of rotation and often a so called intensifierbar, a separately driven shaft with paddles or other dispersion means that rotates at high speed and serves to produce additional turbulence. It also helps to disintegrate large, loose, ovenvetted agglomerates. The intensifier bar may also carry the spray nozzles or other distribution means
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Fig. 7 . 2 4 Sketch o f a drum mixer with internal baffles which produce curtains of material during rotation as well as spray and Sparger tubes that modify the mixer to a co-processing agglomerator for the granulation o f fertilizers.
for liquid binder (Fig. 7.27). With the intensifier bar design as shown in Fig. 7.27 the liquid spray issues through an adjustable tiny slot around the entire periphery of the dispersion blades. Although it can not be avoided that the walls are wetted to some extent, the periodic covering and uncovering with material together with the sliding action of the mass tend to keep the interior clean. The baffle inserts of low shear drum mixers can also have other designs than the simple radial lifters shown in Fig. 7.24. Fig. 7.28 is a cut-away view of a rotary batch blender with internal mixing flights. Liquid manifolds with nozzles can be installed along the imaginary central axis to convert the blender to a mixer agglomerator. As depicted in Fig. 7.29, this mixer agglomerator can be also operated continuously whereby cleaning and accessibility are guaranteed by the quick and easy removal
Fig. 7.25:
Schematic representation o f different batch operating low shear mixers with indication o f basic particle movements. (a) Double cone, (b) slanted cone, (c) V-shape (adapted from CEMCO, Middlesex, NJ, USA).
7.4 TumblelCrowth Agglomeration Technologies
Fig. 7.26 Photographs oftypical batch operating low shear mixers. (a) Double cone (courtesy Abbe, Little Falls, N), USA.), (b) slanted cone (courtesy CEMCO, Middlesex, NJ,USA.), (c) V-shape (courtesy Abbe, Little Falls, NJ, USA.).
or exchange ofthe internals (Fig. 7.29b). The internals can be specifically designed for the changing process requirements of each application. As shown in Fig. 7.30, agglomerates from low shear mixers are loosely assembled and bonded entities of irregular shape (Fig. 7.30a) which become somewhat denser and more rounded as they grow to larger sizes (Fig. 7.30b). The combination of a high shear blender and a low shear agglomerator is the P-K Zigzag continuous blender/agglornerator (Fig. 7.31). It consists of a slowly turning eccentric drum with a dispersion head inside that rotates at high speed (Fig. 7.32) and a V-shaped tumbling shell that is attached to the drum and causes multiple internal material recyclings to produce uniform blends. If the equipment is operated as an agglomerator, liquid is introduced by slots (similar to what is shown in Fig. 7.27) and atomized; the powder that is aerated by the action of the high speed dispersion head is uniformly wetted and seeds form in the mass which grow to loosely bonded agglomerates in the low shear Zig-Zag portion of the machine. Fig. 7.33 is the photograph of such a machine.
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ded
wide spray band
controls spray fineness, from a mist to droplets
Spray issues through tiny slot around entire periphery. Width of sproy bond
Liquid inlet to centi'01 tube
Dipersion blades to aerote ond suspend solids in area of sproy bond.
Liquid addition01 detoil Fig. 7.27:
Sketches o f t h e design o f a specific intensifier bar and the liquid addition detail (courtesy Patterson-Kelley, East Stroudsburg, PA, USA).
7.4 JumblelCrowth Agglomeration Technologies
Fig. 7.28: Cut-away view of a batch rotary blender with internal mixing flights (courtesy Munson Machinery Co., Utica, NY, USA).
High Shear Mixers High shear mixers are characterized by a normally stationary vessel with mixing tools inside that cause a stochastic movement of the particulate charge. The most simple blenders with mixing tools are single or twin shafted pug mills. They consist of a bottom portion with semi-cylindrical trough(s), connecting to vertical side, feed and end walls, and feature a flat cover (Fig. 7.34). The latter is often kept at least partially open for observation and the mounting of liquid manifolds. The shaft(s)carries(y)a series of paddles and rotate(s) relatively slowly to cause the mixing action. Because of the slow movement of the paddles and the charge, pug mills constitute a transition between low and high shear mixers. They are often used as part of a transportation system for bulk solids and, if liquid is added to the charge, form crude
Fig. 7.29 Artists rendering of a continuous rotary blender with internal mixing flights. (a) Cut-awayview, (b) different internal mixing flight designs (courtesy Munson Machinery Co., Utica, NY, USA).
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Fig. 7.30 Photographs o f two typical agglomerates that were produced in a batch low shear agglomerator with V-shaped shell Left: Agglomerate size approx. 750 p n , Right: agglomerate size approx. 4 m m (courtesy Patterson-Kelley, East Stroudsburg, PA, USA) .
agglomerates which are suitable for dustfree deposition of particulate waste in a land fill or for granulated products with low quality requirements. The first true high shear mixer that got patented in Germany in 1934 to carry out agglomeration was the Eirich granulating mixer. The patent describes the mixing and rolling of particulate solids on the flat bottom of a rotating pan by means of eccentrically arranged, counter-currently moving mixing tools. The mixing elements of these tools are either blades or bars which extend into the material to be agglomerated (Fig. 7.35). The patent claims that, due to the intensive shearing, practically all
inlet
Drum and
chute
accessdoor
Dispersion head on liquid-solids model (excluded on solid-solids model)
/
Tire
vent
Shd harQe
I
Discharge shroud
-
f Drive chain and sprocket
Drive motor andassemblv reducer
Normal product working volume is50%01totaldrum volume.
Fig. 7.31: Schematic representation of a P-K Zig-Zag continuous blender/agglomerator (courtesy Patterson-Kelley, East Stroudsburg, PA, USA).
Note: OSnA.aD?ro.ea G.arcs p'?lecI res oe 9s an2 c n a ns on i m a e s
.
7.4 Turnble/Crowth Agglomeration Technologies I 1 7 3
Fig. 7 . 3 2 (a) Sketch depicting how liquid is dispersed into aerated solids in the eccentric drum of a P-K Zig-Zag blender/agglomerator. (b) The close-up photograph shows a dispersion head inside the eccentric drum o f a P-K Zig-Zag blender agglomerator featuring multiple dispersion blades and liquid spray apertures between discs (courtesy Patterson-Kelley, East Stroudsburg, PA, USA).
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7
-
P\
k 7
~
Fig. 7.33: Pnorograpn of a P-X 2 g-Zag D enoer agg omeraroc (co,cres) Patrersoi-Xe e) East Srro,asD,rg PA J S A )
Fig. 7.34: Photograph o f a double trough (twin shafted) pug mill (courtesy FEECO International, Green Bay, WI, USA).
t
Fig. 7.35: (a) High shear mixing and agglomeration tools o f an Eirich counter-current granulating mixer. (b) Schematic representation o f the pattern o f movement in the Eirich counter-current mixer (courtesy EIRICH, Hardheim, Germany).
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7.4 TumblelCrowth Agglomeration Technologies
Fig. 7.36 (a) Photograph o f a n Eirich inclined, deep pan granulator with counter-currently operating high shear mixing and agglomeration tools. (b) Schematic cut-away view of the equipment shown in (a) describing its function (courtesy EIRICH, Hardheim, Germany).
solid powders can be agglomerated into relatively uniform granules without, if the material features sufficient natural binding characteristics, or with the addition of a binder. While, originally, the Eirich granulating mixer used a deep, flat pan with vertical axis and operated in a batch mode, it was later modified by tilting the axis, thus yielding a continuously operating deep pan agglomerator but maintaining the high shear, counter-currently rotating mixing tools. Fig. 7.36 is the photograph of such a piece of equipment and a cut-away view describing its function. In 1949, also in Germany, Lodige invented another important new tool for high shear mixing, the plow (Fig. 7.37). This element is fastened, in multiplicity, to a horizontal shaft as shown in Fig. 7.38 and rotates in a stationary drum (Fig. 7.38a). The rotational speed depends on the material, the diameter of the drum as well as the shape of the plows and is empirically determined to produce an intensive, three-dimensional particle motion as shown schematically in Fig. 7.38b. Often, the condition of the turbulently moving, aerated particle mass is described by the term “mechanically fluidized”.
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Fig. 7.37: The plow-shaped mixing tool as invented by Lodige at the middle of the 20th century (courtesy LODICE, Paderborn, Germany).
Fig. 7.38:
(a) Shaft with plow-shaped mixing tools. The half plows at each end serve to keep the mixer end walls clean. (b) Cut-awayview of plow-shaped mixing tools installed in a drum. (c) Cut-awayview of a batch mixer indicating the particle movement caused by the plowshaped mixing tools (courtesy LODICE, Paderborn, Germany).
After expiration of the patent protection for Lodige, the plow-shaped element has become one ofthe most common mixing tools for high intensity, high shear batch and continuous mixers and mixer/agglomerators with stationary cylindrical shell and horizontal axis. Other high shear mixers and mixer/agglomerators include pin mixers (Fig. 7.39) employing pins and paddles of many different shapes (Fig. 7.39b), whereby the elements can be arranged in straight lines (Fig. 7.39a) or resembling a spiral (Fig. 7.39c), and ribbon blenders with a large number of varying mixing tool designs (Fig. 7.40) as well as single or double shaft execution. Particularly in batch mixers and when cohesive powders had to be blended or if the mixer was modified to operate, at least during certain phases, as an agglomerator, the problem always existed to avoid unwanted agglomeration or the formation of oversized conglomerates. Although the application of mixing tools and the resulting shear in the
7.4 JurnblelCrowth Agglomeration Technologies
Fig. 7.39: (a) Pin mixer with open cover showing the built-in screw feeder (foreground) and detail o f a the pin mixer shaft (courtesy FEECO, Green Bay, WI, USA) (b) Detail o f different mixing elements (courtesy LODIGE, Paderborn, Germany) (c) Detail o f a pin mixer shaft with elements arranged in a staggered, overlapping, double helical pattern (courtesy Mars Mineral, Mars, PA, USA).
tumbling mass caused some desagglomeration or destruction of oversized agglomerates, both undesired phenomena still persisted. To overcome this problem, in 1957 in Germany, Lodige, the same company that had earlier invented the plow-shaped mixing tools, invented the independently driven, high speed “knife heads” or choppers (Fig. 7.41). As shown in Fig. 7.41b the knife head extends into the vessel such that it does not
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(4 Fig. 7.40: Five different element designs for the ribbon mixers o f one supplier. (a) Double ribbon, (b) interrupted outer ribbon, (c) double ribbon with flanged shaft, (d) split ribbon, (e) "mullers" can be added to any ribbon design t o yield a spatula-like action (courtesy Abbe, Little Falls, NJ, USA).
interfere with the mixing tools. Again, after expiration ofthe patent protection for Lodige essentially all manufacturers of high intensity mixers and their modifications for agglomeration offer and use knife heads. They are called accelerators, intensifiers, turbines, mills, choppers, and many similar names. Fig. 7.42 depicts various single and multiple action elements of another mixer manufacturer. As long as the mixer is used for blending powders, the knife heads are used as accelerators and intensifiers for the mixing action. They may be operating continuously at a speed of 1,800 rpm and more to destruct undesired agglomerates which hamper mixing. If the process enters the agglomeration mode or if a mixer is used primarily for agglomeration, the operating parameters of the knife heads must be modified. In that case the choppers are applied from time to time to mechanically destruct agglomerates, thus arriving at a more uniform agglomerate structure and size distribution by crushing and layering (see Section 7.1, Fig. 7.1).
Fig. 7.41: (a) Multiple knife head. (b) Schematic representation o f how a knife head is installed in an intensive mixer with plow-shaped mixing tools (courtesy LODICE, Paderborn, Germany).
7.4 TumblelCrowth Agglomeration Technologies
Fig. 7.42: Various modern single and multible action chopper elements o f a specific mixer manufacturer (courtesy DRAIS, Mannheim, Germany).
The purpose of the knife heads is to reduce the size of agglomerates in a controlled fashion whereupon the fragments reagglomerate with still available and/or freshly produced fines. The method also serves to distribute the binder liquid more uniformly. The mode of operation to control agglomeration by this manner is characterized by applying the following consecutive steps: Filling/metering of components. Mixing, potentially with knife heads operating. 3. Spraying of some binder and agglomeration. 4. Tumbling without binder addition (optional). 5. “Chopping” with knife heads. 6. Spraying of some additional binder and agglomeration. 7 . Repeat(s) of steps 4-6 until the final amount of binder has been added and uniform agglomeration has been obtained. 8. Begin of the next mode of operation, for example dryinglcooling or discharge of green agglomerates. 1. 2.
Agglomeration occurs by alternating growth and disintegration while slowly adding binder. Within limits, the final size and density of the agglomerates can be controlled by the number and relative duration of the individual steps. It should be realized that scale-up of successful sequencing, that was determined in a small laboratory mixer, can not occur linearly. Rather, the purpose of the different steps and the amounts of powder and liquid involved in every step must be considered. It is often necessary to again carry out optimization trials with the full scale equipment. The above mentioned mechanisms for achieving agglomerate size and density control have only limited applicability for continuous mixer/agglomerators.In most cases their mixing chamber is subdivided into imaginary zones in which the processes mixing, binder addition and agglomeration as well as “finishing” take place. Choppers may be applied in the agglomeration and early finishing sections to accomplish similar modifications as described above for batch mixers.
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a
Fig. 7.43: Schematic representation ofthevortexflow pattern in a Henschel high intensity mixer (courtesy HENSCHEL Mixers
The mixer/agglomerators which were discussed so far employ tools that rotate around a horizontal axis. A large, important group of mixers, which are also used as agglomerators, is equipped with vertical tool shafts. They always operate in a batch mode and are equipped with mixing blades that are located close to the bottom. One of the first of these types of machines, the “Henschel” mixer, was introduced around 1955. It provides a (mechanically) fluidized vortex pattern of movement (Fig. 7.43) in which all particles move freely, essentially independent of size, density, or coefficient of friction. Therefore, originally, the equipment was applied for fast and optimal desagglomeration of powders and dispersion of additives. Later, it was found that the highly turbulent particle movement is also causing agglomeration if liquid binders are added to the charge. To obtain one or the other effect, not only the correct additives must be introduced into the batch but also the mixing tools must be adapted. For that purpose a variety of mixing tools is available, including special tools of different design. However, even the so called “standard tools” are very versatile (Fig.
Fig. 7.44: Design o f a “standard mixing tool” (courtesy HENSCHEL Mixers America, Houston, TX).
7.4 TurnblelCrowth Agglomeration Technologies
r
I
-
Fig. 7.45: Sketches oftypical designs of bowl mixer/agglomerators. (a) Basic design, (b) equipment for “one pot” processing (courtesy DIOSNA, Osnabruck, Germany).
7.44); they are of modular design thus allowing adjustment of the energy input into the batch by the installation of different spacers. Fig. 7.45 depicts schematically typical designs of bowl rnixevs which, with the exception of the bowl shape, feature similar designs as the “original” Henschel equipment. The shape of the bowl promotes formation of a vortex flow and the mixing tool has minimum clearances to the inner equipment walls for maximum product yield. Often, as shown in the sketches of Fig. 7.45a and b, the impeller can be lifted, sometimes even hydraulically, for improved cleaning. A chopper (or multiple ones) is located such that it extends into the zone of greatest material velocity to perform the same functions as described previously. Particularly in the pharmaceutical industry, cleaning requirements may necessitate installation of the chopper(s)through a removable roof as shown in Fig. 7.45b. This apparatus is designed for efficient mixing, granulating, gas stripping, and vacuum drying in a one-pot manner. To avoid condensation the so called “auto-lift lid” is also heated in this case. Fig. 7.46 depicts the design of a modern one-pot mixing, granulating, and drying system in which the particular advantages of microwave drying are utilized. In this drying method, the internationally standardized microwave energy of 2,450 MHz
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Fig. 7.46 Schematic o f a “one-pot” mixing, granulating, and drying system featuring microwave drying (courtesy FUKAE Powtec Corp., Kobe City, japan).
causes the water molecules in the moist agglomerates to vibrate at high speed. Heat that results in evaporation is generated by friction between the water molecules throughout the mass to be dried. Therefore, drying does no longer occur from the outside by transfer and conduction of heat; it now proceeds at a faster rate, particularly if the solids exhibit poor heat conductivity. Although bowl mixers and agglomerators always operate in a batch mode, new equipment has sometimes a rather large volume (up to 2,000 L bowl volume) of which, depending on the material and application, between 30 and 80 % are useable per batch thus allowing large production rates because, typically, processing times are short. If, in addition, the drying process is carried out externally, as shown in Fig. 7.47 in which an external fluidized bed dryer is applied, a quasi-continuous process is obtained. In such an arrangement, a closed system, from loading the raw materials to the discharge of dry granular product, is also achieved. Other batch or continuous mixers with an almost vertical axis of the mixing tool are the orbiting type screw blenders (Fig. 7.48) often also called “Nauta” mixers. The total contents of the conical silo is mixed by the orbiting screw which rotates such that it transports material from the depth of the bin to the surface. Although not often
7.4 JumblelGrowth Agglomeration Technologies
Fig. 7.47: Quasi-continuous system for mixing, granulating, and drying featuring a bowl mixer/granulator with 600 L bowl volume and external fluidized bed dryer. A schematic o f the entire system is shown in the inset (courtesy DIOSNA, Osnabruck, Germany).
used as granulator, agglomerate growth can be accomplished in the turbulently moving center of the material surface if the screw speed is high. Under those circumstances, material exiting the screw at the bed surface is thrown towards the whirling center were it begins to descend. If binder liquid is introduced at this location, agglomeration occurs. This example is mentioned not because of the importance of this blender type for agglomeration but to demonstrate that any mixer that causes irregular and
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Fig. 7 . 5 0 (a) Schematic representation o f a cross section through the operating parts o f a "Schugi Flexomix". (b) Photograph of the opened-up roller cage o f a "Schugi Flexomix" also showing the vertical shaft with the mixing blades (after removal o f the flexible sleeve that defines the mixing chamber) (courtesy HOSOKAWA SCHUGI, Lelystad, The Netherlands).
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r/min
L
.-
v) L
L
2
’
10
8
L
6 L
01
a
// I--
3000 rlmin
Granule diameter I p m )
Fig. 7.51: Effect o f “Schugi” rotor speed on granule size distribution.
equipped with adjustable blades, rotates at high speed (1,000to 3,000 rpm). The number and position of the knife blades and their angle of attack are selected to suit the particular process needs. The shaft is suspended from heavy duty bearings in the drive system above the vertical cylindrical mixing chamber, resulting in a continuous, completely unobstructed discharge of the moist, agglomerated product. The mixing chamber of the “Schugi” consists of a flexible sleeve that is continually deformed from the outside by rollers which are moving up and down, thus preventing build-up on the interior (Fig. 7.50). The rollers are mounted in a cage which is pneumatically operated. Roller cage and mixing chamber are easily accessible for cleaning and servicing (Fig. 7.50b). Powders are dropped into the upper end of the mixing chamber such that a low concentration of solids in the mixing chamber develops. Hold-up or retention time, which is only approx. one second or less, can be, to a certain extent, influenced by the angle of the knife blades; they may increase or decrease the free fall speed component of the rotating charge. Agglomeration of the solid particles occurs either by desagglomeration of a wet feed and reagglomeration using the liquid that is available in the feed or by wetting dry powder with binder liquid. For liquid addition, a wide range of spray or atomizing nozzles is available; selection and installation of these wetting arrangements depends on the liquid and the desired product characteristics. Agglomerates from the “Schugi” normally feature a small but somewhat adjustable particle size in the range from 0.2 to 2 m m and narrow distribution. With increasing rotor speed the width of the particle size distribution tends to become smaller [B.42] (Fig. 7.51). Fig. 7.52 is the flow sheet of a continuous granulation system using a Schugi Flexomix as the agglomerator. Although this is only a process schematic it shows that the
7.4 TumblelCrowth Agglomeration Technologies
Flow sheet of a continuous granulation system using a “Schugi Flexomix” as the agglomerator (courtesy HOSOKAWA SCHUCI, Lelystad, The Netherlands).
Fig. 7.52:
heart of the plant, the “Schugi”, is almost inconsequential in size. This figure, in addition to partially depicting what is necessary to obtain a complete granulation process using a “Schugi”,demonstrates again, that in most wet agglomeration plants, the peripheral equipment and installations, such as raw material receiving and storage, powder preparation - which often comprises metering and premixing of several components -, liquid receiving, storage, preparation, metering, and addition, green agglomerate drying, product sizing - often including crushing of oversized agglomerates in a closed milling loop -, dust collection, and fines recirculation, are much larger and more expensive, both in regard to investment and operation, than the agglomerator itself (see also Chapter 11). 7.4.3 Spray Dryers
Spray drying is the drying of a spray [B.43]. As the name implies, spray dryers are primarily used to obtain dry products from liquid or wet feed stocks. In all of these methods, feed in a liquid or semiliquid form is dispersed in a gas stream to produce granular solids through heat and/or mass transfer. Features that are common to these techniques are: 1. The feed must be pumpable and dispersible into droplets. 2. Product size is limited to particles with approx. 1m m diameter and is often much
smaller.
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Uncontrolled adhesion resulting in oversized clusters and fines carryover are often a problem but the systems are designed to recover and/or recycle them. 4. The processes are normally amenable to continuous, automated large scale operation. Spray drying is one of the most important continuous drying methods for the conversion ofsolutions, emulsions, or slurries into powders. The pumpable feed is dispersed into droplets which are dried in a controlled flow of hot gas. Particles are formed as moisture evaporates from each droplet. Particularly in regard to agglomeration, there is a fundamental difference between producing dry particles from solutions or emulsions and from suspensions or slurries. If droplets of solutions or emulsions are dried, a solid hull if formed in the supersaturated layer on the surface. This solid casing soon gains structural integrity and defines the size of the final product particle which is only slightly smaller than that of the original droplet. During further drying, solids deposit onto this hull mostly from the inside so that, finally, hollow spherical particles are formed (Fig. 7.53a).With some materials it is possible that the casing becomes impermeable to steam which results in cracked hollow spheres (7.53b).In other cases the particle hulls may collapse forming doughnut-like shapes (7.53~). In any case the bulk density of these particles is extremely low. Also, the formation of the solid is not characterized by an agglomeration process but by crystallization and/or deposition.
Fig. 7.53:
Photographs o f typical particles (explanations see text) that are obtained during the spray drying o f solutions or emulsions (courtesy CEA/NIRO, Soeborg, Denmark) .
7.4 TumblelCrowth Agglomeration Technologies
Fig. 7.54:
Model explaining (see text) the drying o f droplets o f suspensions or slurries (courtesy GEA/NIRO, Soeborg, Denmark).
The solids formation process is much different if suspensions or slurries are dried. In this case, the droplets already contain the solid in the form of small particles. As shown in the model depicted in Fig. 7.54 drying proceeds as follows: Due to the surface tension of the liquid, suspensions first form a spherical droplet while slurries, depending on their solids content, obtain a more or less (less in the case of high solids loading) spheroidal shape. At the beginning, liquid evaporates only on the surface. If particles in the droplet are still somewhat movable, i.e. if particles are suspended in relatively low concentration, some densification occurs when the droplet becomes smaller due to evaporation of the liquid. This is normally not the case if the dryer feed is a slurry. Since agglomerates of the particles that are contained in the droplet develop during drying, some binding mechanism must develop between the particles when the capillary forces disappear. This binding mechanism is typically the recrystallization of dissolved substances. Most commonly it is the solid material itself or, in the case of a mixture of solids, at least one of the components which is soluble in the liquid and recrystallizes during drying. Since drying only occurs at the surface, supersaturated solution develops there and some crystals form near the outer pore ends between the particles. b) When the excess liquid has evaporated, the capillary bonding state is obtained. The solid particles are now packed as tightly as they will ever be and are held together by the negative capillary pressure of the liquid that completely fills all the void spaces (pores) between the particles (see also Sections 5.1 and 5.1.1). a)
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Evaporation continues to take place on the surface of the agglomerate, where dissolved material recrystallizes, and solution is made available from the interior by capillary flow. c) At a certain stage of the drying process, some pores become empty and the residual liquid concentrates in liquid bridges at the coordination points between the particles, while some other pores are still filled with liquid. Capillary flow can only occur in completely filled pores. Therefore, as drying progresses, the drying zone moves into the interior of the agglomerate and recrystallization now also occurs inside the structure. d) Just before reaching the dry state, all remaining moisture is in the form of liquid bridges between the particles which form crystal bridges as drying is completed. Therefore, while during the spray drying of solutions and emulsions, mostly hollow spherical particles are produced by a process which is not directly defined as agglomeration, the production of dry particles from suspensions and slurries uses the binding mechanisms of agglomeration to yield true agglomerates. Besides the gas handling, heating, and cleaning facilities, the two most important system components are the equipment for dispersion of the wet feed and the containment in which hot gas contacts the dispersed wet material and drying takes place. Dispersion of the wet feed is typically accomplished by specially designed nozzles. Although different designs of nozzles (see also Section 7.4.2) are offered and used by competing manufacturers of spray dryers, below, as examples, only two often applied nozzle designs will be described. The most widely applied type of spray dryers by one supplier is provided with a rotary atomizer (Fig. 7.55 and 7.56). The system disperses fluids by centrifugal force. It has a high degree of flexibility, with capacities from small (a few kg/h) to very large (over 200 t/h). Various wheel designs (Fig. 7.55) have been developed for the handling of different types of liquid feed such as solutions, emulsions, suspensions, slurries, pastes, and melts. Fig. 7.56 is the photograph of a rotary atomizer in operation. The main operating parameter affecting droplet size is the peripheral speed of the atomizer wheel.
Fig. 7.55: Schematic representation and partial cut through one design o f a rotary atomizer wheel. (1) (2) (3) (4) (courtesy CEA/NIRO, Soeborg, Den m ark).
7.4 TumblelCrowth Agglomeration Technologies
Fig. 7.56:
Photograph o f a rotary atomizer in operation (courtesy GEA/NIRO, Soeborg, Denmark).
ADS
=
C x FRQ/rpmbx diac x nd
(Eq. 7.1)
with ADS = average droplet size, C = a constant, FR = feed rate, rpm = speed of the atomizer wheel, dia = wheel diameter, n = number of vanes, and a, b, c, and d = empirical exponents. Often, the droplet size is inversely proportionate to the peripheral wheel speed to the power of 0.8. The size of the dry particles depends on the primary droplet size as well as the above mentioned factors that affect size during drying, such as shrinkage, expansion, rupture, and agglomeration, whereby the latter may include some densification. The normal way to obtain dry powders or granules that are not dusty is to produce larger droplets. However, as droplet size is increased, the diameter of the drying chamber must be enlarged, too, to avoid the formation of deposits on the wall. Especially for smaller plant capacities, this requirement makes it impractible to use only spray drying to obtain this product characteristic. In those cases a combination between spray drying and fluidized bed agglomeration is being used (see Section 7.4.4). Spray dryers with nozzle atomizers use two different types of dispersers: singlephase, hydraulic pressure nozzles and two-phase, pressurized gas assisted nozzles (see also Section 7.4.2). The latter has a relatively limited application in spray drying because of the relatively large flow of atomizing gas, which influences the flow pattern of the drying gas in the tower, and the broader particle size distribution that is produced by this type of nozzle.
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Fig. 7.57: Photograph o f a single phase, hydraulic pressure nozzle in operation (courtesy GEA/NIRO, Soeborg, Denmark).
The feeds for spray dyers with nozzle atomizers are normally solutions, emulsions, and dilute suspensions. As with rotary atomizers the product particle size of a “nozzle plant” depends on primary droplet size. For a given feed and nozzle type and size, the droplet diameter is inversely proportionate to the liquid pressure to the power of e.g. 0.3 and directly proportionate to the square root of the orifice diameter. Typical liquid pressures are 5 to 50 bar and the orifice diameter is between 1 and 4 mm. With these
c
0 0
v Fig. 7 . 5 8
Different drying chamber designs. (a) Co-current, (b) counter-current, (c) mixed flow patterns (courtesy GEA/NIRO, Soeborg, Denmark).
7.4 Jumb/e/Growth Agglomeration Technologies
parameters, average particle sizes range from 50 to 350 pm. As droplet size is enlarged the height of the cylindrical section of the drying chamber must be increased to avoid the formation of deposits. The capacity of a single pressure nozzle is only up to 1 t/h; therefore, high tonnage plants use multiple nozzles. Since the spray pattern from these nozzles can be relatively narrow (see Fig. 7.57) many such nozzles can be installed in the roof of the drying chamber with minimal interference between the sprays. Drying chambers are designed to realize co-current, counter-current, and mixed flow patterns of drying gas and droplets or wet particles (Fig. 7.58). Especially the counter-current and, to some extent, the mixed flow patterns increase the residence time of the descending solids in the drying chamber, create a certain amount of turbulence, and, therefore, result in collisions between partially dried solid units. These conditions lead to coalescense and to the formation of agglomerates. Fig. 7.59 is the simplified flow sheet of a co-current spray drying system with recirculation of fines that is another possibility to accomplish agglomeration in a spray dryer. Often, the fines are returned to the liquid feed bin and redissolved or redispersed (see, for example Fig. 7.6813 and c). However, if fine particles are returned to the spray zone, as shown in Fig. 7.59, they are captured by the droplets (similar to a wet scrubbing effect) and incorporated in the product. In regard to gas handling and flow, the most involved and costly part of any spray drying system, the flow sheet of Fig. 7.59 shows an open system. However, for economical or other reasons, closed, semi-closed or even aseptic plant gas flow schemes may be applied (Fig. 7.60). In Fig. 7.61 a photo of some spray dried powders is presented. Successful drying of sticky, hygroscopic, thermoplastic, or slowly crystallizing products into free flowing agglomerated powders requires powder temperatures at much lower levels than possible in conventional spray dryers. Under those conditions, such materials would tend to adhere to the discharge cone of the tower and potentially produce massive build-up which results in process upsets. Also, completion of drying
Fig. 7.59 Simplified flow sheet of a co-current spray drying system with rotary atomizer and recirculation of fines. (1) Feed container, (2) feed pump, (3) rotary atomizer, (4) drying chamber (tower), (5) dust collector, (6) blower, (7) dry product (courtesy CEA/NIRO, Soeborg, Denmark).
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Fig. 7.62: Artist's conception and flow sheet of a "filtermat" spray dryer together with the photograph o f an agglomerated powder product sample (courtesy CEA/NIRO, Soeborg, Denmark).
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requires that the material is held at the (lower) drying temperature for a much longer time. This is accomplished in the integrated belt spray dryer termed ‘tfiltermat” spray d y e r (FMD). In Fig. 7.62 the flow sheet and an artist’s conception of this system is shown. To avoid build-up, the drying chamber is wider towards the lower end. In this tower, agglomeration and the first drying phase are accomplished. The semi-dried material accumulates as an agglomerated, porous layer on a moving belt that is permeable to gas. The still warm gas from the tower is sucked through the material and the belt so that drying continues (1)while the material is transported into three other hooded chambers that are divided from each other by baffles. Further processing occurs in the efficient downdraft gas flow pattern. In chamber (2) hot gas is provided for high rate drying, a mixture of hot and cold gas completes drying in chamber (3), and in chamber (4) the product is cooled with dehumidified gas prior to its discharge. The agglomerated powder sample in Fig. 7.62 reveals that the dry product consists of relatively large, loose (porous) agglomerates. For further information on spray drying and associated matters it is recommended to consult the book authored by K. Masters [B.43]. 7.4.4
Fluidized Bed Agglomerators
As a result of the mechanical action of mixing tools in high intensity mixers (see Section 7.4.2) an aerated, turbulent particulate matter system with stochastic particle movement develops. Similar conditions exist if the particles are suspended in a fluidized bed. The main difference between the two methods is that in the mixers particle movement is caused by mechanical forces while in fluidized beds drag forces, that are induced by a flow of gas, are the main reason for the movement of the particulate matter. Therefore, fluidized beds are not only used as excellent environments in which gas efficiently and intimately contacts particles but also for dry mixing of particulate solids and coalescence of particles which, in the presence of binding mechanisms, causes agglomeration. Originally, the fluidized bed technology was developed during pioneering work in the mid 1920s with the “Winkler generator” for the gasification of bituminous coal in Germany and of Standard Oil and Kellogg in the United States. The latter improved the catalytic break-up of heavy oil by replacing the less efficient fixed bed crackers [B.42]. Almost immediately it was also recognized that cooling and drying of particulate solids was easily accomplished in fluidized beds if the carrier gas was dry and cool or heated. As an alternative to fluidization, the spouted bed was developed for the drying ofwheat where the particles are too coarse and uniform for the development of a good, regular fluid bed. Fluidization begins when the upward flow of gas through the voids between the particles in the bed attains a frictional resistance equal to the weight of the bed. This condition is called incipient buoyancy. However, at that flow rate, the particles are still so closely together that they do not have any appreciable mobility. The desired bed homogeneity and particle mobility is achieved if the gas velocity is increased
7.4 TumblelCrowth Agglomeration Technologies
further until the particle mass seems to behave like a boiling liquid (therefore, sometimes, the term boiling bed is used to describe this condition).To obtain agglomeration, the bed movement must be more vigorous which is accomplished by the formation of bubbles [B.42]. The gas velocity at which bubbles first appear is referred to as incipient bubbling velocity. From the description of conditions that are required to obtain a fluidized bed it is obvious that the size distribution of the particles in the fluidized bed must be narrow. Larger particles will not be sufficiently lifted by the gas and sink down. This effect can be and often is used to accomplish discharge of agglomerates that have grown to the proper size. At the other extreme, if the particles are too small, they are entrained in the gas and carried out of the chamber. Such solids must be removed from the off-gas in dust collectors and are normally recirculated, either to the liquid feed stock for redispersion and drying or directly into the fluidized bed for reagglomeration. Inspite of the obvious, good conditions for agglomeration, a high probability of coalescence between particles, it took several decades and the beginning of interdisciplinary evaluation of processes before, by design, fluidized and spouted beds were utilized for agglomeration [B.42, B.571. Particularly if dry powder is produced in a spray dryer plant from suspensions or slurries, agglomeration can be accomplished if the partially solidified but still wet particles are tumbled in an associated fluidized bed where, in most cases, final drying also takes place. Fig. 7.63 is the schematic flow sheet of a fluidized spray dryer (FSD). As compared with Fig. 7.59 (Section 7.4.3), the conventional spray dryer, a somewhat modified gas handling system is the most obvious new feature ofthe FSD. Drying gas (9) not only enters the top of the tower for co-current drying but also a so called “plenum”, a specially designed chamber at the bottom of the tower, from which the hot gas enters the tower through a distribution plate (for further details see below). The amount of drying gas entering with the dispersed feed (3) is controlled such that, while the droplets descend in the tower, only partial drying is accomplished. The still wet, slightly sticky particles are captured in a fluidized bed (5) where they collide and form larger agglomerates. Fines (7)that are removed from the off-gas (10) in a dust collector (6) are recirculated to the fluidized bed where they are attached
Fig. 7.63: Schematic o f a fluidized spray dryer (FSD) with open plant gas flow (courtesy GEA/NIRO, Soeborg, Denmark). Explanations see text.
f 8
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Fig. 7.64 Two-stage back mixinglplug flow fluidized bed according to European patent EP 0 749 560 61.
Fig. 7.65:
Comparison o f FSD-dried products with those obtained in other systems (courtesy CEA/NIRO, Soeborg, Denmark).
7.4 TumblelCrowth Agglomeration Technologies
to the growing agglomerates. While the solids tumble in the fluidized bed and grow by agglomeration they are also dried with the hot fluidizing gas. Dry, agglomerated product (8)is removed from the fluidized bed in a suitable manner. Fig. 7.64 depicts a patented possible discharge system of a fluidized bed spray dryer. It realizes back mixing and plug flow (for details on these characteristics see below) and avoids short circuiting of wet particles from the primary fluidized bed (13) into the product discharge (17). Further control of the discharge rate is accomplished by, for example, a conventional rotary vane valve. The photographs in Fig. 7.65 demonstrate the larger powder particle size that can be obtained with the FSD in comparison with other equipment and Fig. 7.66 depicts a typical particle size distribution from a fluidized spray dryer (FSD) as compared with those from a conventional spray dryer with rotary atomizer (SD) and a “tall form spray dryer” (TFD) (see Fig. 7.68c, below). In Fig. 7.67 a patent drawing shows the integration of the special fluidized bed design of Fig. 7.64 in an FSD. Other special features of this FSD are that final drying occurs in the central fluid bed portion with hot gas (6a),the product is cooled in the annular fluid bed portion with cold air (Gb),product discharge is by overflow (6c). and the spent air exits the chamber through highly efficient filters (11)which are mounted within the tower so that fine particles that are dislodged during the cleaning cycle of the filters are directly recirculated. Originally, the gas distributors in fluidized beds were made of perforated steel plates (see Fig. 7.70, top). The size of the holes, in most cases the diameter of circular bores, the percent open area, defined by the sum of all hole areas, sometimes the distribution pattern of the holes in the plate, and the gas pressure in the plenum below the distribution plate, which, together with the other dimensions, defined the gas flow rate, were major design parameters. To obtain a good, stable fluidized bed, the gas velocity has to be uniform across the entire area ofthe bed and must be adjusted such that, as a result, the solid particles are in a suspended state. Perforated plate gas distributors, although relatively cheap and easy to manufacture, have a number of disadvantages (Tab. 7.2). Characteristics of perforated plate gas distributors for fluidized beds.
Tab. 7.2
+ -
Simple construction, therefore, relatively cheap Particles remain static between holes. Tendency to form deposits. - Dead volumes. -
-
Vibration needed to prevent air channeling. Particle sifting through holes.
-
Poor directional flow control.
For structural reasons, holes must have a certain distance from each other causing static particles in dead volumes on the “land areas” and potential deposits. Also, particles tend to sift through the holes necessitating periodic cleaning of the plenum.
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99.9
Fig. 7.66 Typical particle size distribution of a dry powder produced in the FSD and comparison with powders from SD and TFD (courtesy CEA/NIRO, Soeborg, Denmark).
66
6c
60
Fig. 7.67: Schematic o f a fluidized spray dryer with a modified two stage fluidized bed (Fig. 7.64) and integrated filters according to WO 97/14288 (courtesy GEA/NIRO, Soeborg, Denmark).
Furthermore, the fluidized bed can not be shut down while material is still in the chamber. The desire for directional flow control will be discussed below. Agglomeration and/or drying can be more effectivelycontrolled, if the fluidized bed is arranged externally. Fig. 7.68 shows three different spray dryer systems with external fluidized bed agglomerator/dryer/cooler. Although the designs were originally introduced only for final drying and product cooling, additional agglomeration can be accomplished if the tower discharges still moist and slightly sticky material. In all three examples fines are entrained in the off-gasesfrom the spray tower and the fluidized bed, removedinafines separator (dustcollector)priorto exhaust, andrecirculatedeithertothe
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7.4 TumblelCrowth Agglomeration Technologies 201
sprayzoneorintothewetfeed. Thesketchofthespraytowerin Fig. 7.68crepresentstheso calledtallformspraydryer (TFD)inwhich powder separationfromthe off-gasoccursvery efficiently in the enlarged cone section of the tower. Continuous fluidized beds are normally characterized by the residence time distribution of individual particles in the unit. A wide residence time distribution is obtained in a fluidized bed with a relatively small length to width ratio. A circular fluidized bed at the bottom of a spray tower (see, for example, Fig. 7.63) is a perfect example of this concept. Such an arrangement is called back-mixed fluidized bed. In
Fig. 7.68: Schematic representation o f three different spray dryer systems with external fluidized bed agglomerator/dryer/ cooler (courtesy CEA/ NIRO, Soeborg, Denmark). (a) "Standard" spray dryer (SD) with fines recycling to the spray zone, (b) "standard" spray dryer (SD) with fines recycling into the wet feed, (c) "tall form spray dryer" (TFD) with fines recycling into the wet feed. (1) Feed container, (2) feed pump, (3) spray nozzle, (4) drying chamber, (5) dust collector, (6) external fluidized bed agglomerator/dryer/cooler, (7) blower, (8) re-dissolution or dispersion tank.
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of Chemical Engineering it can be modeled by an agitated tank with an overflow (Fig. 7.69a). The residence time distribution is wide because individual units (powder particles or agglomerates) can “short circuit” to the discharge with extremely low residence time, while others may be retained in the bed for a very long time. A narrow residence time distribution is obtained if the length to width ratio of a fluidized bed is large. Such a design corresponds to the external fluidized beds that are schematically shown in Fig. 7.68. The residence time distribution is narrow because all particles are pushed forward towards the discharge by the material that enters the feed end. This arrangement is called plug flow fluidized bed and in terms of Chemical Engineering it can be modeled by a large number of agitated tanks in series (Fig. 7.69b) whereby every single agitated tank represents a “mini” back-mixed fluidized bed. As mentioned above, even and stable distribution of the fluidizing gas is a prerequisite for sustained operation. Distribution plates ensure optimal fluidization and powder movement. The original distribution plate design, the perforated plate (top of Fig. 7.70),has a number of disadvantages (see Tab. 7.2). It was also discovered that it could be advantageous if the direction of particle flow in fluidized beds can be influenced. This, and the desire to overcome some of the other disadvantages
t
Fig. 7.69 Explanation o f t h e conditions in back-mixed (a) and plug flow (b) fluidized beds.
7.4 Tumble/Crowth Agglomeration Technologies
Perforated plate
4-M-
Gill plate
Brigde plate
Flex plate
Non-sifting plate Fig. 7 . 7 0 Sketches of different gas distribution plates for fluidized beds (courtesy CEA/NIRO, Sceborg, Denmark).
of the simple perforated plate design, led to the development of modified distribution plates. Although all companies that design and/or offer fluidized bed particle processing units have developed such new distribution plate designs that differ in details from each other, the development of one vendor of fluidized bed equipment will be discussed in the following as an example. Fig. 7.70 are sketches of the basic perforated plate design and of new and improved distribution plates. Tab. 7.3 lists the most important characteristics of some new plate designs. The most important features are that, because air enters at an angle (see Fig. 7.70), no deposits form at the land areas between the holes and the fluidized particles are moving in the direction of the airflow which can be used for lateral powder transport. Depending on the application, the latter can be also employed to develop circular or linear flow patterns which are particularly advantageous for the design of fluidized beds that require plug flow. The non-sifting plate designs feature hole covers which are larger than the holes themselves so that, if the gas flow stops and the powder settles onto the plate, a powder surface develops near the holes which depends on the angle of repose. If the geometry of the hole covers and the respective size of the holes are selected correctly, powder will not sift through the holes. Fig. 7.71 is a photograph showing three of the new gas distribution plates for NIRO fluidized bed processors. Particularly if used for size enlargement by agglomeration, totally back-mixed fluidized beds are often not suitable because the discharging product is still sornewhat moist since it is near equilibrium with the entire humid exhaust air. Furthermore, because the particles from a back-mixed fluidized bed will have had different residence times (see above), a large portion of the material is either over- or under-dried.
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7 TumblelGrowth Agglomeration Characteristics of NIRO Distribution Plates for Fluidized Bed Processors (courtesy CEA/NIRO, Soeborg, Denmark).
Table 7.3:
Gill PlateT'
+ +
+ +
+
Less tendency for deposit formation. Improved directional air flow. Improved powder transport control. Good bed emptying properties. Vibration normally not necessary. Particles are still sifting through holes.
Bridge Plate-"
+ +
Essentially the same performance as Gill PlateTN,but: Creates slower fonvard particle movement. Particle sifting prevented when bridges overlap the holes. Expensive fabrication procedure, particularly if bridges overlap.
Flex Plate'"
+ +
Similar features as Gill PlateT",but: Further improvement in powder transport and movement control as gill openings can be located to face in any direction resulting in high flexibility. Good bed emptying properties.
Non-S$ing Gill Plate'" and Non-Sifting Flex PlateT" Essentially the same as Gill and Flex PlatesT", but: + Because "Gills" are overlapping with the plate, particles are no longer sifting through
Fig. 7.71:
Photograph of three advanced gas distribution plates for fluidized beds (courtesy CEA/ NIRO, Soeborg, Denmark).
To overcome these problems, plug flow fluidized beds with controlled particle residence time are selected. By using directional airflow, through the application of specific gas distribution plates, optimally operating fluidized beds are obtained. They are either circular or rectangular. In circular fluidized beds with plug flow (Fig. 7.72), the feed, which must be directly fluidizable, is introduced into the center of the fluid bed and the fluidized particles are forced to follow a long narrow path (between the spiralshaped baffle in Fig. 7.72) to the periphery of the fluid bed where they discharge over a weir. In this apparatus, good control of the residence time of each particle is achieved and, on discharge, the product is near equilibrium with the dry hot gas so that very low
7.4 TumblelCrowth Agglomeration Technologies
Fig. 7.72: Schematic representations of a circular fluidized bed with plug flow (courtesy CEA/NIRO, Soeborg, Denmark).
residual moisture can be obtained without overheating the material. In rectangularly shaped fluidized beds, the plug flow of the fluidized particles is achieved with transversely arranged baffles (Fig. 7.73). Since, as mentioned before, the feed to such fluidized bed processors must be directly fluidizable, they are charged with particulate solids. Ideally suited materials may be formed in independent processing steps, such as precipitation, crystallization, coagulation, and polymerization, followed by drying, or by grinding which may be followed by upgrading, again potentially drying, and/or mixing. Particles may also be formed by spray drying. In those cases where solutions are spray-dried, which results in the production of hollow particles (see Section 7.4.3),it may be desirable to destroy the hollow structures prior to feeding them into an agglomerator to obtain denser products. Size enlargement in fluidized beds that are charged with dry particles occurs after the addition of atomized binder liquid in the turbulently moving charge by particle collisions and coalescense. Therefore, this process is called rewet agglomeration in fluidized beds and, as far as the growth mechanism is concerned, is equivalent to the mechanisms of tumble/growth agglomeration (see Section 7.1, Fig. 7.1 and 7.2). As discussed earlier, a particular characteristic of fluidized beds is that, because of the necessity that lifting forces, resulting from the gas flow, and the weight of the particles must be in equilibrium, only a narrow size distribution can be kept in the fluidized state. Therefore, only an incremental growth can be obtained in a given fluidized bed. It is also difficult to obtain agglomeration in a fluidized bed featuring plug
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Fig. 7.73: Sketch o f a rectangular fluidized bed with plug flow (courtesy GEA/NIRO, Soeborg, Denmark).
flow. The relatively uniformly advancing flow of the tumbling mass is not as amenable to particle collisions, which constitute the fundamental mechanism of tumblelgrowth agglomeration, as the turbulently agitated back-mixed fluidized bed. Therefore, the plug flow designs are primarily used for the final drying of narrowly sized green agglomerates that were formed in a previous agglomeration step. A combination that can be used for agglomeration and drying applies a two-stage circular fluidized bed apparatus (Fig. 7.74). By arranging a totally back-mixed fluid bed on top of a plug flow fluid bed, agglomeration and predrying can be achieved in the first stage if moist powders (e.g. filter cakes) or fine particles, small agglomerates, and fines as well as atomized binder liquid are fed to this back-mixed fluidized bed. In the case of feeding moist powders, a rotary distributor (see Fig. 7.74) disperses small chunks of the material evenly over the back-mixed section. In the fluidized bed, attrition, abrasion transfer, crushing, and layering (see Section 7.1, Fig. 7.1 and 7.2) as well as predrying take place. Fig. 7.75 is the photograph of the turbulently moving surface of a fluidized bed above which the atomizing nozzle for binder liquid is located. Other designs use atomization nozzles that are submerged in the bed to achieve more intimate contact between the liquid and the solids. Solids, fresh feed and recycle, are typically fed into the bed below its surface to allow collisions with other particles, adhesion, and agglomeration before they may be entrained in the off-gas leaving the apparatus. The fines are then removed from the gas, collected, and once more recirculated to the back-mixed fluidized bed.
7.4 Tumb/e/Crowth Agglomeration Technologies
Fig. 7.74 Schematic drawings of two 2-stage fluidized bed processors. (a) Plug flow (bottom) with radial baffles and alternating transfer ports. (b) Contact heating panels in the back-mixed (top) section (courtesy CEA/NIRO, Soeborg, Denmark).
In both cases, the feeding of moist powders or the wetting of dry particles, moist agglomerates are collecting near the bottom of the back-mixed bed and discharge into the beginning of the plug flow fluidized bed where final drying occurs. Since the backmixed fluidized bed is always on top of the plug flow fluidized bed, the solids flow counter-currently to the drying gas; thus, space requirement, installation cost, and energy consumption are minimized. Fluidized beds of this design may also feature more than two fluid beds which are stacked upon each other. Generally, all equipment with two or more stacked fluidized beds are called multi-tier fluidized beds. Another multistage fluidized bed processor is depicted in Fig. 7.76. It is a rectangular apparatus in which the different chambers are separated by walls with transversely arranged connections or (as shown in Fig. 7.76) openings at the bottom, close to the gas distribution plates, both providing plug flow. Although in the sketch a common
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Fig. 7.75: Photograph o f the surface o f a fluidized bed and an atomizing nozzle supplying liquid binder (courtesy CEA/ NIRO, Soeborg, Denmark).
Fig. 7.76 Rectangular plug flow fluidized bed apparatus with spray nozzles and internal dust filters (courtesy CEA/NIRO, Soeborg, Denmark),
7.4 Tumbleltrowth Agglomeration Technologies
plenum for the fluidization gas is shown, more typically the plenum is subdivided to supply gas of different pressure (or speed) and temperature to the individual chambers. As a result several different operating modes are possible. For example, solutions or suspensions may be spray-dried whereby the product size increases in consecutive chambers. The feed port on the left may supply recycle as nuclei and for incorporation into the product, although the schematic in Fig. 7.76 indicates optionally that in each chamber internal dust filters are installed which directly recirculate any fines. Through the feed port dry powder (plus, potentially, recycle) may be also introduced. In this case, binder liquid is sprayed onto the fluidized bed to entice agglomeration and, again, consecutive chambers produce ever larger agglomerates. Finally, it is possible
Fig. 7.77: Two sketches o f a rectangular two-stage "contact fluidizer". The numbers in (b) indicate: (1) Feed inlet, (2)/(3) process gas inlet (2) back-mixed section, (3) plug flow section], (4) product discharge, ( 5 ) process gas outlet, (6) heating panels, (7) wet feed rotary distributor (courtesy CEA/NIRO, Soeborg, Denmark).
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to cool the material and/or provide some product treatment (e.g. coating, application of anticaking agent, impregnation, etc.) in one or more of the final chambers. Fig. 7.77 shows two schematic representations of a rectangular contact fluidizer which, in this case, also incorporates back-mixed as well as plug flow sections. Therefore, it can be utilized in much the same way as discussed before. Plug flow is achieved with baffles arranged transversely (see also Fig. 7.73). As shown, a rotary distributor disperses the wet feed evenly over the back-mixed section (dry feed and atomized binder liquid could be also used) which, in addition, is equipped with contact heating surfaces that are immersed in the fluidized bed (see also Fig. 7.74b). As shown in Fig. 7.77b, the heating panels can be easily removed for cleaning and maintenance. The supply of thermal energy is selected such that a substantial portion of the required heat is provided by the panels. Therefore, it is possible to reduce both the temperature and the amount of gas through the system significantly which is particularly important if the material to be treated is heat sensitive. In those cases where the solids to be processed can not be easily fluidized due to a broad particle size distribution, highly irregular particle shape, or low abrasion resistance (requiring relatively low fluidization gas velocity), a shallow vibrated fluidized bed apparatus (also called “vibro fluidizer”) is applied (Fig. 7.78). It is normally designed as a long rectangular trough with (natural) plug flow and is vibrated with a frequency of 5 -25 Hz and a half amplitude of a few millimeters. The vibration vector is applied at an angle to the vertical (typically <45’) so that the material is easily transported along the trough by the combined effects of fluidization and vibration. If the feed is moist and has self-agglomerating tendency or by providing atomized binder liquid near the feed end of the vibrated fluidized bed agglomeration can be achieved which is followed by drying in the same apparatus. While all of the examples that were discussed so far featured continuously operating equipment, it should be pointed out, that in some industries, for example the pharmaceutical industry, fluidized bed processors are used in batch modes. Fig. 7.79 is the
Fig. 7.78: Schematic representations o f a vibrated fluidized bed (“vibro fluidizer”) (courtesy CEA/ NIRO, Soeborg, Denmark).
7.4 TumblelCrowth Agglomeration Technologies
simplified flow sheet of a batch fluid bed granulator system. Continuous processes that could include premixing, wetting, granulation (agglomeration),classifying, collection and recirculation of fines, drying, cooling, and post-treatment (e.g. coating) may carry out each step or a few combined steps in separate pieces of equipment requiring transport, potentially intermediate storage, and handling in between. A batch fluidized bed apparatus, as shown in Fig. 7.79, can realize all these steps in one unit providing one-pot processing (see also Section 7.4.2). During a processing cycle, the powder constituents are weighed-out and dispensed into the chamber which is then closed. First, a fluidized powder bed is established and the different components of the formulation are uniformly mixed. When this condition is reached, binder liquid is sprayed onto the turbulently moving bed and agglomeration begins. As particle size increases, the fluidizing gas flow must be adjusted (increased) to compensate for the larger particle mass. All along, fines are entrained in the off-gas and collected in the bag filters that are mounted within the chamber. During the automatic cleaning cycle of the filters, fines are recirculated into the bed were they get another chance to adhere to the growing agglomerates. After reaching the optimal particle size, drying begins which is followed by cooling and, potentially, a post-treatment (e.g. coating, see Section 10.1). The apparatus is only opened after the last processing step is completed so that a true one-pot operation has been realized.
5
Fig. 7 . 7 9 Batch fluid bed spray granulator system (1) Air filter and heater, (2) powder batch/fluidized bed, (3) spray nozzle, (4)/(5) liquid feed tank and pump, (6) bag filter, (7) exhaust fan (courtesy GEA/NIRO, Soeborg, Denmark)
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Further system improvements may be obtained by recirculating the fluidizing gas and, after condensing the evaporated fluid during the drying cycle, also returning the liquid. The first allows an aseptic operation and recovers energy during the drying cycle and the second is particularly advantageous if the agglomeration liquid is expensive (for example, alcohol). Without going into more detail, Tab. 7.4 lists parameters in fluidized bed agglomerationlgranulation which are grouped into material variables and process variables. In summary, Fig. 7.80 provides an overview over different fluidized bed processes, the product structure obtained with these processes, and the average particle size (50 % point of the distribution in mm) of the product. The first five methods (entries 1- 5) relate to processes for the conversion of liquid feed (solutions, emulsions, suspensions, thin slurries) into dry powders and the second six methods (entries G - 11) make use of moist or dry powders and liquids as binders. The last two of the second group (entries 10, 11) apply powder mixers (see Section 7.4.2) for agglomeration and fluidized beds as dryer/cooler/post-treatment equipment. Rewet agglomeration (the 3rd last in Fig. 7.80) may use mixers or fluid bed processors for agglomeration.
'
I
I
Spray DryeriRotary Atomizer Spray DryeriNozzle Atomizer
Liquid
TFD-N
'
~
FSD ~-N,R
Fluidized Spray Dryer
FILTERMAT Dryer
FMD
~
Fluid Bed Agglameratar
*
*
~
SFD
Fluid Bed Granulator
Powde
Spray Fluidizer
Liquid Rewet Agglomeratar
I
1
,
1
' ~
FsA
FSG SFD RWA
~
01-03
0 1-0.4
Spray Fluidizer ~
0 02-0 2
*
1
4
0.15-3 0
0
a 1a a '
0.3-3.0
0.2-0.6 0.5-3.0 0.5-3.0
0 3-2 0
Tumbler Agglomerator Tumbler Agglomerator
Fig. 7.80: Overview over different fluid bed processes (courtesy CEA/NIRO, Sceborg, Denmark).
7.4 JumbIelCrowth Agglomeration Technologies 7.4.5
Other Low Density Tumble/Growth Agglomerators
There are further, less well known but often important techniques that make use of one or the other or all of the previously discussed agglomeration methods in low density turbulently moving particle beds and clouds. Tab. 7.4: Parameters in fluid bed agglomeration/granulation (based on information from CEA/NIRO, Soeborg, Denmark). Material Variables Feed materials
Binder
Product
Process Variables Particle size and distribution Surface area Chemistry, wetting characteristics Type, chemistry Quantity If solution, concentration and quantity Viscosity Particle size and distribution/dust Strength/abrasion resistance Bulk densityiporosity
Dispersibility/solubility/reactivity
Fig. 7.81: Schematic representation o f the PCS rotary-valved pulse combustor/atomizer. Explanations see text (courtesy Pulse Combustion Systems, San Rafael, CA, USA).
Fluidized bed Binder distribution
Fluidizing gas Processing time
Load Moisture content Nozzle position/spray shape/droplet size Liquid flow rate Velocity/pressure Inlet/outlet temperatures
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An alternative to spray drying of solids that are suspended in a liquid using high pressure single and two phase or rotary nozzles (see Section 7.4.3) is the break-up of a low pressure stream of slurry in gas dynamic atomization. In this process, the fluid is pumped to an orifice where it is released into a pulsating flow ofhot gas (Fig. 7.81) and atomized. The heart of this drying system is a rotary-valved pulse combustor. Referring to Fig. 7.81, combustion air (1)is pumped at low pressure into the pulse combustor’s outer shell and flows through an unidirectional air valve (2) into a tuned combustion chamber [“HelmholtzResonator” (3)]where fuel (4)is added. The air valve closes. The fuelair mixture is ignited by a pilot (5) and explodes, creating hot air which is pressurized to approx. 0.2 bar above combustion air fan pressure. The hot gas exits the chamber through a pipe (6)towards the atomizer area (7).Just above the atomizer, quench air (8) is blended in to achieve the desired process temperature. The orifice releases the liquid (9) into the carefully balanced gas flow which dynamically controls the break-up of the liquid stream (atomization),drying, and particle trajectories. The pulse cycle can be up to 100 per second. After break-up, the particles enter a conventional tall-form drying chamber (10) and the downstream equipment is also identical to that used with spray drying as well as fluidized bed agglomeration (see Sections 7.4.3 and 7.4.4). In gas dynamic atomization, a liquid stream is broken-up into small entities which are dried very quickly. Because the formation of droplets occurs in aerodynamic suspension, the material experiences no shear and the liquid temperature does not rise above the local dew point, despite high gas temperatures. Since drying and subsequent cooling are rapid, organic materials do not have time to oxidize, degrade, or experience any other damage. Food powders often exhibit better flavor, texture, and instant characteristics than comparable powders from other spray dryers. Because a low pressure stream of slurry is pumped and dispensed, the system can also handle corrosive and abrasive products easily. Control over particle size is normally better. Fig. 7.82 depicts SEM photographs of some typical products. In designing pulse combustor/atomizer drying systems, the pulse intensity as well as the temperature and velocity of the gas at the point of atomization are optimized for each product. A particular advantage of the technology is, that the plant’s control system can modify the process conditions such that a variety of dry powder characteristics are met without physically changing the equipment. These characteristics primarily include particle size, flowability, texture, temperature history, residual moisture content, flavor, and ease of reconstitution. An alternative to wetting by atomized liquid for rewet agglomeration in low density turbulently moving particle clouds is steam jet agglomeration [7.6] (Fig. 7.83). In this process a dry powder, consisting of single particles or weak aggregates of primary particles that are bound by van-der-Waals or electrical forces (see Section 5.1.1), is fed into the apparatus. Since such powders do not usually flow freely, the method of conveying and dispensing is of great importance to the result of agglomeration as, at this stage, the number, size, and morphology of the dry feed conglomerates are determined. As shown in Fig. 7.83a, for more easily handleable powders a vibratory feeder could be used while for cohesive powders a powder dispenser with two rotary cylinders (Fig. 7.83b) is proposed.
7.4 TumblelCrowth Agglomeration Technologies
Fig. 7.82: SEM photographs o f some typical product from dryers with PCS pulse combustion atomizers (courtesy Pulse Combustion Systems, San Rafael, CA, USA). (a) non-fat dry milk, 200x; (b) non-fat dry milk, 2,000~;(c) sodium alkene sulfonate, 350x; (d) sodium alkene sulfonate, 2,000~;(e) soy sauce, 2,000~;(f) silica alumina, 2,ooox.
In the chamber, steam jets are directed parallel to or slightly impinging the flow of feed. This serves two purposes: to wet the solids and to cause particle movement resulting in collisions and coalescense. Contact of the steam with cold particle surfaces results in condensation and thermal energy transfer; also, droplets can form in the vapor phase. Therefore, two different mechanisms contribute to the wetting processes in the agglomeration zone:
1. Condensation of steam on cold particle surfaces and 2. collision of particles with liquid droplets. Calculations and measurements have shown that condensation is the dominant effect. As condensation only occurs until the temperature of the solids reaches the dew point temperature that corresponds to the relative humidity of the gas atmosphere, particle inlet temperature has an important influence on the amount of water that can condense. Steam jet agglomeration is normally applied for the size enlargement of water soluble materials in the food industry to obtain products with instant characteristics (see also Section 5.4). Since during the condensation of steam not only a thin, uniform coating of water is formed on the solids but also thermal energy is transmitted, a maximum of the water soluble material is dissolved which produces recrystallization bonds during drying (see Section 5.1.1)
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Solids
-
0
)f
($
Powder stirrer
Feeder
Solids supply
Steam jet agglomerator
Exhaust gas + fines I
I
Agglomerated product
Fig. 7.83: (a) Schematic representation o f a steam jet agglomerator. (b) Sketch of a dispenser with rotating cylinders for cohesive powders [7.6].
If the solid particles are very small, typically in the submicron or nanometer range, the natural molecular attraction forces are large in comparison with the particle mass so that particles that collide with each other in a fluidized bed will adhere to each other and form weak agglomerates. As discussed earlier (Section 7.1), formation of nuclei is the most difficult and time consuming step. Therefore, development of the earliest applications of binderless agglomeration in fluidized beds occurred by chance until, quite some time later, interdisciplinary application of the fundamentals of agglomeration explained the phenomena and allowed the design of simple, reliably functioning fluidized bed agglomerators. To avoid the time consuming nucleation step, a heel of agglomerated product is now retained in batch operating equipment or “seeding” with recirculating, undersized but pre-agglomerated material is carried out. In either case, growth of the already available nuclei occurs by the mechanisms described in Section 7.1 while new nuclei are also formed in the bed. Typical applications of this technology are for the agglomeration of carbon black and silica fume. Silicafitme, in contrast tofitmed silica which is produced by purposefully volatilizing silica, is a by-product from the manufacturing of ferrosilicon and silicon metal. From the violently turbulent bath of the submerged-arc furnaces, that are used in these production processes, tiny droplets of Silica (Si02)emerge which, under the influence of surface tension, become spherical, solidify, and are removed from the furnace offgas in baghouse filters. Because of the very fast cooling effect, the spherical particles consist of amorphous silica, have extremely small particle size (Fig. 7.84), and feature large specific surface area. These characteristics of silica fume make it an excellent
.:a
7.4 TumblelCrowth Agglomeration Technologies
s
2 40
Q
20 Fig. 7.84: Typical silica fume particle size distribution (courtesy Norchem Concrete Products, Fort Pierce, FL, USA).
0 005 0 1
0 2 05prnl
admixture to, for example, high strength concrete as well as high performance grouts and mortars. The amorphous structure and high surface area render the particles highly reactive causing pozzolanic effects and the small particle size also results in highly impermeable structures of building materials (see also Section 5.3.1, Fig. 5.41). In prestressed concrete structures, the latter protects the rebars from attack by water.
Fig. 7.85: Schematic o f a fluidized bed agglomerator for dry silica fume (courtesy Norchem Concrete Products, Fort Pierce, FL, USA).
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Ilu idisation
compaction
flu id isat ion
Fig. 7.86: The principle o f pressure swing gra n uI atio n.
As produced, directly from the bag house, silica fume is very dusty, difficult to handle, self agglomerating, - causing bridging, build-up, and lumping -, and can not be transported and handled economically. Because of its very low bulk density (160- 240 kg/m3),bulk tanker trucks only hold 8- 10 t and require long pump-off times and if bags are used they are large and bulky. A simple dry fluid bed agglomeration process (Fig. 7.85) converts silica fume by dry agglomeration into a product which is less dusty, flows well, and can be handled pneumatically. Product density may be such that the tanker truck now holds approx. 25 t and can be adjusted to fit different handling and end use applications. At the same time, agglomerate bonding is so weak that the product disperses easily, for example in cement mixers. Similar considerations, which, through observation, investigation, and understanding of the processes that are involved, led to the development of the dry agglomeration
Fig. 7.87: Photograph of a pressure swing granulator, Model DQ-350 (courtesy Fuji Paudal, Osaka, Japan).
7.4 Tumble/Crowth Agglomeration Technologies
technique for ultrafine particles that was described above, yielded another novel dry agglomeration method for fine solid particles: the pressure swing granulator (PSG) 17.7: B.70, pp. 15-24].The principle of this intensive, dry, fluidized bed agglomeration process is shown in Fig. 7.86. It consists of a cylindrical chamber with a perforated gas distribution plate at the bottom and operates in batch mode using cyclic fluidization and compaction of the material. During the fluidization and upward motion, agglomerates are formed, mostly due to molecular (e.g. van-der-Waals) forces (see Section 5.1.1). These agglomerates have a rather wide particle size distribution. During the downward flow, the bed of material is compacted with compressed air: weak agglomerates collapse and larger agglomerates break due to compaction forces or they loose small particles due to attrition. The overall result is that the larger agglomerates will become smaller and more spherical while the smaller agglomerates pick-up fines and become larger and more spherical during each compaction step. After a number of compaction and fluidization cycles granules with relatively uniform size and spherical shape are obtained. Fig. 7.87 is the photograph of a Fuji Paudal pressure swing granulator, model DQ-350, for a batch size of 25 kg WCco-wax. Fig. 7.88 depicts different states of particle beds that develop above a gas distribution plate. In the first two on the left, particles are immobile. At the incipient fluidization point, the forces caused by the flowing gas and the particle mass are in equilibrium and the bed volume has reached its maximum before, at somewhat higher gas velocities, particles begin to move freely and randomly in the fluidized beds that are shown on the right.
Fixed bed
4
Incipient fluidization
Particulate or smooth flu id ization
Aggregate or bubbl i ng fluidization
I
1 Fluidization air or gas
Fig. 7.88: Sketches depicting the different states o f particle beds above a gas distribution plate.
SI ugg i ng
Spouted bed
220
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4
pouting
Fig. 7.89 (a) Schematic representation o f an industrial spouted bed. (b) Flow sheet o f a continuously operating spouted bed agglomeration system.
For natural, dry agglomeration of very fine particles as described above, destructive forces, which are, for example, caused by bubbling or slugging, must be avoided. Therefore, the gas distribution plate is a finely pored diaphragm such as a sintered glass frit or a cloth which is supported by screens (see Fig. 7.85). Rewet agglomeration in fluidized beds, on the other hand, often requires the turbulent movement and wakes caused by the rising gas bubbles [B.42]. The sketch on the far right of Fig. 7.88 identifies a spouted bed [B.56]. Typically, to obtain the spouted bed condition, gas is introduced in a relatively narrow area in the bottom center of the particle bed (Fig. 7.89a). Movement of the particles in such an arrangement is caused by a vertical, steady axial jet and is rather regular. The bed particles circulate much like a water fountain; they are carried-up in the central spout as a dilute phase until they loose their momentum and fall back onto the top of the bed towards the outer periphery. The particles then recirculate back down as a dense moving bed, are directed back into the gas stream by the normally conical base of the apparatus, and begin again their upward flow. Liquid, injected as a spray into the base of the chamber together with hot spouting gas, deposits a thin layer of liquid onto the recirculating seeds and particles so that fresh feed particles adhere causing agglomerate growth. Permanent bonding oc-
7.4 TumblelCrowth Agglomeration Technologies
curs by recrystallization during drying or solidification (see Section 5.1.1) in the spout. The gas-solids contacting efficiency of fluidized systems becomes impaired at particle sizes larger than, say, 1m m because more and more gas bypasses the solids in the form of large bubbles. Spouted beds avoid that problem and are, therefore, suited for the formation of larger agglomerates. Other advantages of the spouted bed over regular fluidized beds are: Materials with caking tendencies can be processed. Higher gas inlet temperatures are permissible. Layer-by-layergrowth favors well rounded and uniform granules. Due to the absence of a gas distribution plate none can get scaled or plugged. A classification effect at the top of the bed allows selective removal of the largest particles through the outlet pipe, yielding a relatively narrow product particle size distribution. Inspite of the last statement, continuous operation of spouted beds normally does require the separation of product from over- and undersized agglomerates; both off-scale material streams, the oversized after crushing to below maximum product size, are recirculated (Fig. 7.89b). If back-flow of particles into the spout is avoided by enclosing the spout with a concentric pipe and the spray liquid is a hot melt or a saturated solution, an excellent coater (“Wurster”coater, see Section 10.1) is obtained. 7.4.6
Agglomeration in Liquid Suspensions
Finely divided solids that are suspended in liquids may be difficult to deal with. The sizes of the individual particles are often so small that conventional methods to capture and remove them, such as different types of filters, are not effective unless some sort of size enlargement is applied first. The traditional methods for the agglomeration of fine particles in liquids, such as flocculation (see Section 10.2.2),rely on relatively small interparticle bonding forces to form rather weak agglomerates. The objective of these size enlargement procedures is simply to remove the fine particles from the liquid, thereby cleaning a contaminated effluent. In contrast, the present discussion deals with those techniques in which stronger bonding and specialized equipment are used to form generally larger and more permanent agglomerates in liquid suspensions [B.17, B.21, B.42, B.56, B.581. In addition to the capture and removal of particles from suspensions, these methods have other broad objectives as shown in Tab. 7.5. They include the production of granular, often spherical material, displacement of as much suspending liquid as possible from the product, direct agglomeration of crystals during crystallization, and selective agglomeration of one or more components of a wet multiparticle mixture. During crystallization, solid particles are formed in the solution (the so called “mother liquor”) by reducing the concentration of the solvent (evaporation), solvent
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7 TumblelCrowth Agglomeration Agglomeration processes that are carried-out in liquid systems (according to Capes and Darcovich [8.58]).
Tab. 7.5:
Process Objective
Material treated and Process used
Formation of spheres and production of coarse granular product
Nuclear fuel and metal powder production by sol-gel processes. Manufacturing of small spheres from refractory and high melting point solids (e.g. tungsten carbide) by immiscible liquid wetting. Spherical crystallization: direct agglomeration of crystals during crystallization for drug delivery systems.
Removal and recovery of fine solids from liquids
Removal of soot from refinery waters by wetting with oil. Recovery of fine coal from coal preparation plant effluents to allow recycling of water.
Displacement of the suspending liquid
Dewatering of various sludges by flocculation followed by mechanical drainage on belt filters, in rotary drums, etc. Displacement of moisture from fine coal by wetting with oil.
Selective agglomeration of certain components from mixtures of particles in liquids
Removal of ash-forming impurities from coal and tar-sands. Coal-gold agglomeration to recover very low valuable concentrations in gold ore. Solvent extraction and simultaneous soil agglomeration to remediate oil-contaminated soil.
(ex-)change,neutralization, or temperature decreasing methods. It is often difficult to control crystal growth and avoid undesired clustering by the growing together of numerous crystallites into an amorphous, irregular particle aggregate. As a result, sizing or even sorting of the product of crystallization may become necessary to arrive at suitable crystal sizes, compositions, and/or structures for specific uses. A new technology, spherical crystallization [7.8],has been developed to design the properties of (pharmaceutical) crystals and improve their yield as well as powder handling. Because agglomeration occurs simultaneously with the crystallization process and does not require binders, processing steps, such as separation of crystallites and drying prior to external agglomeration with binders, are avoided. The product particles resulting from this process are uniform, spherical agglomerates of fine crystals. Fine particles in liquid suspension can be transformed into often large and dense agglomerates of considerable integrity by adding a second binder liquid while suitably agitating the system. The second liquid must be immiscible with the suspending liquid and must wet the particles to be agglomerated. Therefore, this technology is often called immiscible liquid agglomeration. An example is the addition of oil to the aqueous suspension of fine coal. The oil is immiscible with the water and adsorbs preferentially on carbon particles; it forms oil bonds between these particles when they
7.4 TumblelGrowth Agglomeration Technologies
collide in the agitated liquid system and coalesce. Inorganic impurities, i.e. those particles that would form ash, are not wetted by the oil and remain in suspension. A similar result would be obtained if silicious particles suspended in oil are agglomerated with water as the immiscible binder. In immiscible liquid agglomeration, the same structural and bonding conditions exist as discussed for tumble/growth agglomeration (see Section 5.1, Fig. 5.7, and Section 7.1).At low levels of binder liquid, only bridges can form between the particles (“pendular”state) which results in an unconsolidated floc structure. If these agglomerates were allowed to settle, a loose mass with a volume that is larger than that of the un-flocculated particles would be obtained. As more oil is added, a transitional (“fuar”) state is reached and the flocs consolidate somewhat. Soon, agglomerates appear and increase in number until, about midway in the transitional region, all the agglomerating particles have been transformed into “micro-agglomerates”.As the amount of binding liquid is further increased, the agglomerates grow in size, using a mechanism that is again similar to that of tumblelgrowth agglomeration (see Section 7.1, Fig. 7.1 and 7.2),and reach a peak of strength and sphericity near the “capillary”region (compare Section 5.2.2, Fig. 5.28). Beyond this point, additional binder liquid addition results in the formation of pasty lumps in which the solids are dispersed in the binder liquid. A most useful feature of immiscible liquid agglomeration is that the size of extremely fine, suspended solid particles, including those in the nanometer range, can be enlarged. This allows wet, ultrafine grinding of minerals in which valuable components or impurities are very finely distributed and the recovery of agglomerates with much higher purity or, respectively, the removal of gangue components as agglomerates. A number of such applications have reached either commercial or semicommercial operation in the minerals industry. The general relationships described above are not specific to a certain system. However, given the need for optimal separation of valuable particles from the associated gangue, the colloid and surface chemistries that are involved may be quite complex. As in theflotation process, selective agglomeration by immiscible liquids depends strongly on the relative wettability of surfaces, and the same fundamentals of surface chemistry apply to the conditioning of particles to yield the required affinity for the wetting liquid. For a variety of applications, spherical particles are required. Many of these are associated with the field of powder metallurgy. While it is relatively easy to produce spherical particles from low melting materials by conventional techniques, such as shot or prilling towers (see Chapter 5 ) , refractory solids in general and, specifically, high melting point metals can not be converted by these techniques. However, if the solid is available in powder form, various methods are available to produce spherical particles by agglomeration. One such spherical agglomeration process uses an immiscible binder liquid to form spheroidal products from particles that are suspended in a second liquid. These highly specialized materials are required in small amounts and, therefore are carried-out in small, high energy, batch shaking devices as shown in Fig. 7.90. In this apparatus tungsten carbide spheres are manufactured which, after sintering, yield ball pen
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tips [B.56, B.581. The closely sized particles with 1 m m diameter are prepared by agitating tungsten carbide and cobalt powders in a closed teflon container with hemispherical ends on a high-speed reciprocating shaker. Halogenated solvents are used as the suspending liquid and water is the binder. The addition of approx. 6 % cobalt to the tungsten powder is required to lower the sintering temperature to more acceptable levels. In the batch process, compaction and rounding occurs during many collisions between the agglomerates and with the container walls. The advantage of products from this process is that finishing operations, such as lapping and grinding after a preliminary sintering step, are greatly reduced if compared to those necessary for, for example, spherical compacts from press molding. When comparatively high (but still relatively small) production rates are required, continuous processes are better suited. Fig. 7.91, for example, depicts a drum agglomerator featuring an internal screen classifier for the formation of uniform spheres by immiscible liquid agglomeration [B.56]. In these tumbling agglomerators, the pre-
TEFLON VESSEL
Fig. 7.90: Teflon cylinder with hemispherical ends, mounted in a reciprocating shaker, and used to form small spheres by the "spherical agglomeration process" [ B. 561.
7.4 TumblelCrowth Agglomeration Technologies
sence of a liquid slurry is useful to reduce dusting, especially if toxic powders are processed. The liquid environment also avoids avalanging because particles and immicible binder liquid are uniformly distributed throughout the agglomerating mass thus allowing conglomerates to grow into larger entities in a much more controlled manner. The liquid charge also helps in the development of a desirable tumbling and cascading motion in the equipment because it is more voluminous and better interparticle lubrication occurs than would be the case if no suspending liquid were present. Furthermore, the solids are carried with the liquid which makes internal classification possible. As shown in Fig. 7.91 a spiral screen that rotates at a slower speed than the drum passes through the charge and picks-up agglomerates. Undersized particles fall through the screen openings and return to the agglomerating liquid mass. Larger material moves along the spiral until it reaches a tube at the axis of the drum which directs the finished agglomerates to a discharge point. In immiscible liquid agglomeration, particles with a small amount of adsorbed binding liquid on their surfaces collide and coalesce to form larger entities. In the sol-gel process (see also Section 5.3.2, Fig. 5 . 5 2 ) ,another agglomeration technique that occurs in liquid phase, fine particles are initially suspended in a binder liquid. The suspension is then formed into spherical droplets and the excess binder is removed to solidify the droplets into a particulate product.
Particles
t Fig. 7.91: Drum agglomerator with internal spiral screen classifier for the formation o f uniform spheres by immiscible liquid agglomeration [ B. 561.
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The sol-gel process has been developed for the production of spherical oxide fuels with a particle size of up to 1 m m for use in nuclear reactors [B.56]. The following operations are involved in converting the initially aqueous sol of colloidal particles into calcined microspheres: 1. Dispersion of sol into droplets. 2. Suspension of sol droplets in an immiscible fluid that will extract water to cause
3. 4.
5.
gelation. Separation of gel microspheres. Recovery of the immiscible liquid for reuse. Drying, calcining, and sintering of the microspheres.
Equipment used to accomplish steps 1 - 4 in a continuous operation is shown in Fig. 7.92. In this process, the aqueous sol of colloidal particles is dispersed into drops at the top of a tapered vessel. The droplets are fluidized by the upward flow of the waterextracting fluid, such as 2-ethyl-1-hexanol. Interfacial tension holds the droplets in a spherical shape, but there is a maximum size since larger drops tend to more distortion. A surfactant is added to the immiscible liquid to prevent coalescense of the sol droplets with each other or sticking to the walls of the vessel and lumping of partially dehydrated drops. As water is removed and the sol is converted to a gel, the particles become denser and their settling velocity increases. Vessel design and flow rates are controlled such that the densified gel particles continuously drop-out to the product
COOLlNO WAlIR
Fig. 7.92: Flow diagram of a sol-gel process for the formation ofgel microspheres and the cleaning o f the water-extracting fluidizing liquid [B.56].
7.4 TurnblelCrowth Agglomeration Technologies
receiver while fresh sol droplets are added at the top. The extracting liquid is separated from the product and a portion of it is sent to the distillation system for purification to maintain a sufficiently low water content in the fluidizing liquid. Agitation in baffled vessels can also be used to disperse and suspend sol droplets in an extracting fluid. Compared with the system of Fig. 7.92, the more vigorous agitation produces smaller microspheres of less than 100 pm in diameter.
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Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
8 Pressure Agglomeration During the agglomeration by growth in tumbling particle beds (see Chapter 7 and subchapters), “natural” adhesion forces, which are either totally inherent, enhanced by suitable methods, introduced by binders, or acting as a combination of two or all of these effects, cause particles to stick together when they collide in a stochastically moving mass of particulate solids. With exception of forces that are exerted during the interaction between the particles, the surrounding atmosphere and equipment walls as well as, in some cases, various mixing tools, no externally induced directional forces or pressures act on the growing agglomerates and no shaping, other than caused by attrition, occurs. As a result, depending on the level of interaction, which is largely influenced by the tumbling bed density, more or less spherical agglomerates are grown. Because of the relatively small forces caused by interactions in and with the tumbling charge, porosity of the agglomerates is high and increases as bed density decreases. Also, since the adhesion forces are small, too, and separation forces which try to destruct the growing agglomerates are mass related, the particles forming the agglomerate must be little (see Section 5.2.2). Typically, with a few exceptions, temporarily bonded “green” agglomerates are produced which require post-treatment to achieve permanent, final strength. In pressure agglomeration, new, enlarged entities are formed by applying external forces to particulate solids in more or less closed dies. In contrast to tumblelgrowth agglomeration, pressure agglomeration is used to achieve one or more and sometimes all process conditions and product characteristics that are summarized in Tab. 8.1. Of course, as will be shown in the following chapters, certain conditions and characteristics are better obtained with one or the other pressure agglomeration process and, sometimes, one or more of the parameters can not be met with a specific technique and/or equipment, system, or plant. Pressure or press agglomeration, using a large number of different extrusion machines, punch-and-die presses, isostatic pressing equipment, and roller presses as well as some lesser known machinery, represents a major share among commercial applications of size enlargement by agglomeration. This technology is largely independent of feed particle size and the forces acting upon the particulate solids can be small with some or may be very high with other equipment. Therefore, it constitutes the most versatile field of size enlargement by agglomeration.
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Tab. 8.1:
Process conditions
Larger feed particle size High initial strength Dry processing No or little binder Hot processing No post treatment Processing of elastic materials Automatic operation Easy clean-out Quick turn.over Product characteristics
Specific shape Large pieces Specific mass (or weight) High density Low porosity High final strength Long shelf life Amenable to the production of near net-shape parts
The equipment in which pressure agglomeration occurs is a machine that operates
with well defined mechanical parameters that are independent of the performance and characteristics of the particulate solids to be processed. Therefore, pressure agglomeration techniques lend themselves readily to automation and remote control and are essentially independent of operator presence and/or skill. Because the equipment is relatively complex and the throughput per unit is often limited, this technology finds its largest usage in low to medium-sized applications (approx. 1 to 50 t/h). Of course, this statement is relative. Specialty products, such as those in the pharmaceutical industry, may be processed in very small and sophisticated machinery, handling only a few kilograms per hour, while certain high-tonnage materials, for example some fertilizer, refractory, and mineral materials, are briquetted or compacted in large facilities employing multiple units. An advantage of high-pressure agglomeration is that, in most cases, essentially dry solids are processed which do not tend to set-up, so that the process can be stopped at almost any time and re-started easily; also, the amount of material in the system is relatively small. Therefore, pressure agglomeration methods, specifically those applying high pressure, lend themselves particularly well to batch or shift operation and to applications in which several products must be manufactured from different feed mixtures in the same unit. At the end of a campaign, the system can be easily and completely emptied in a relatively short time. If the danger for cross- contamination is unimportant, for example in the fertilizer industry, a new campaign with a different feed can be started immediately (see also Chapter 12). Another possibility for fast and quick change-over is to install different feed and product discharge/handling systems
8.7 Mechanisms of Pressure Agglomeration
as, in most cases, the expensive pressure densification/shaping equipment itself can be easily cleaned or adapted to the manufacturing of a new product. In general, if it is the task to agglomerate several million tons per year of always the same feed composition, - as is often the case in ores or minerals mining, upgrading, and processing -, pressure agglomeration will not normally be the preferred choice. In all other cases, one of the different methods of pressure agglomeration should be considered. All pressure agglomeration processes have in common that externally provided forces act on particulate solids and that some sort of a tool or die defines the shape of the agglomerated product. All other process conditions and product characteristics that are listed in Tab. 8.1 are more or less well fulfilled or, sometimes, do not apply at all. The level of force that is applied during densification and shaping is the most distinguishing factor in pressure agglomeration. Therefore, the technology is subdivided into low (Section 8.4.1),medium (Section 8.4.2),and high (Section 8.4.3)pressure techniques. As will be shown in Sections 8.1 and 8.2 it is another important distinguishing characteristic whether the material to be pressed is subjected to forces in open ended (extrusion presses), totally confined (punch-and-die and hydrostatic presses), or semi-confined (roller presses) compacting tool sets.
8.1 Mechanisms o f Pressure Agglomeration
There is a great variety of pressure agglomeration methods, each corresponding to one or more ofthe binding mechanisms of agglomeration (see Section 5.1.1, Tab. 5.1, Fig. 5.8 and 5.10). While in low and medium-pressure agglomeration all binding mechanisms are equally possible, in high-pressure agglomeration attraction forces (Fig. 5.10) provide the most common bonding. According to the mechanisms involved, the processes can be further categorized as those using binders and those without binders (see also Chapter 6 ) . Binderless pressure agglomeration comprises all size enlargement processes that apply high compaction forces and use one or more of the binding mechanisms: Solid bridges caused by chemical reaction, partial melting, or sintering; adsorption layers; molecular, electrical, and magnetic forces; recombination bonds; non-valence associations; and interlocking. Several materials contain binders naturally, e.g. the bituminous components in most coals or starch in many foods. Since this inherent binder was not added specifically, pressure agglomeration methods processing such materials are considered binderless. Some particulate solids, especially those that are relatively coarse, do not exhibit inherent binding tendencies; therefore, a binder must be added to secure adhesion of the particles. The main binding mechanisms for such cases are: Bridges of highly viscous media and the negative capillary pressure in liquid bridges caused by the surface tension of freely movable liquids that wet the solids. Processes that use added viscous and liquid binders apply low to medium-pressure agglomeration (see Chapter
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6, Fig. 6.4). Products from such methods are initially of low or medium strength. To obtain stronger bonds, subsequent aging, drying, firing, or curing is necessary. Then, the following binding mechanisms apply: Solid bridges caused by crystallization of dissolved substances, chemical reaction, hardening, or sintering. The most versatile techniques of pressure agglomeration use high forces to compact essentially dry particulate matter into tablets and briquettes of specific size and shape or into compacts. Under the acting pressure, the solid particles approach each other closely which results in strong bonds due to molecular adhesion forces; fusion may even take place at the grain boundaries and many of the free valence forces on newly created surfaces will recombine. Normally, high-pressure agglomeration (see Chapter G, Fig. 6.5) yields products that exhibit high strength instantaneously. Binders are not necessary in most cases. Other pressure agglomeration techniques use lower forces; this medium-pressure agglomeration technology is often called pelleting (see Chapter 6, Fig. 6.413.2 - b.6). In these processes, feed materials with sufficient natural binding tendency, often after an appropriate conditioning step, or those containing binders are forced through open die channels or through holes in dies of different shapes. Typically, the briquettes or pellets formed in this way are cylindrical with a predetermined diameter but variable length. Such products may still require curing to gain final strength. In low-pressureagglomeration (see Chapter 6, Fig. 6.4a.l - a.5), moist masses or particulate solids are processed which are plastic or have been plasticized by various conditioning methods (see also Section 8.4.2). Similar requirements exist in regard to size of the feed particles and binding characteristics as for tumblelgrowth agglomeration (see also Chapter 7). Since very low forces act on the particle mass while it passes the openings in screens or perforated dies that are made of thin sheets, a small degree of densification and little strengthening, other than that caused by the binder, occur. The difference to tumble/growth agglomeration is, that the shape of the green agglomerates does not result from growth and attrition in a tumbling mass of particles but is defined by the openings through which the material is passed. Because a considerable “stickiness”is one of the preconditions for successful agglomeration by this technique, often vermicelli-like extrudates are first produced (see Section 8.4.1), which break or are deformed during post-treatment (see also Section 8.3) into the final product shape. Corresponding to the previous discussion, pressure agglomeration can be carried out by a number of techniques. Each method results in the manufacture of different types of products with respect to size, shape, and physical properties. However, all have in common a basic compaction mechanism. Fig. 8.1 is another presentation of what has been discussed and illustrated before (see Section 5.3 and Chapter 6, Fig. 5.43 and 6.6).The upper part of Fig. 8.1 shows with four model sketches the structural change of a bulk mass of particulate matter during densification in a die, the attendant change in volume, and an indication of the modifications of particle shape and size that occur at high pressure. The lower part of Fig. 8.1 depicts the build-up of pressing force with time under the assumption that the forward movement of the punch occurs with a constant rate until pmaxis reached at which point the direction of movement reverses and the punch now retracts with the same constant (or a higher) speed.
8.7 Mechanisms of Pressure Agglomeration
f
(Brittle)
(Plastic)
I I Densified
Bulk
Compacted
Expansion of compressed air and/or Elastic springback
Low
// Time Fig. 8.1: Sketches explaining the mechanism o f pressure agglomeration.
Referring to Fig. 8.1, as a first step, pressure agglomeration achieves a rearrangement of particles which requires little force and does not change particle shape and size. This is followed by a steep rise of pressing force during which brittle particles break and malleable particles deform. Sketches 3 (brittle) and 4 (plastic) occur either/ or and often simultaneously if both brittle and malleable particles are present in the mix. Two important phenomena limit the speed of compaction and, therefore, the capacity of any pressure agglomeration equipment: compressed residual gas (air) in the pores and elastic springback. Both cause different, equipment specific cracking and a weakening or, sometimes, total destruction of the products (see also [B.12b; B.42; BSG] and Sections 8.4.1 through 8.4.3). Low and medium-pressure agglomeration apply small forces (up to approx. the beginning of the steep slope of the curve in Fig. 8.1)but, nevertheless, the removal of a relatively large amount of air must be guaranteed (the time-axis is directly proportional to the volume change since the punch advances with constant speed). Development of compressed air pockets within the densifying product can be avoided if compaction occurs slowly enough so that all gas is able to escape from the diminishing pore space. High-pressure agglomeration extends into the steep increase of the pressing force. In this range, particle size and shape change by breakage and or deformation and porosity is further reduced. The maximum pressing force is normally defined by an overload feature of the equipment. Since a predetermined final strength and structure (see Section 8.2) must be reached, equipment selection must take into consideration that a sufficiently high maximum pressing force can be attained.
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After arriving at the maximum pressing force, pressure is released. If, as shown in Fig. 8.1, compaction is performed by a punch in a die, the direction of travel of the piston reverses and, when no expansion of the densified body occurs, the pressing force should drop to zero immediately (vertical line). In reality, there is always a more or less pronounced “spring-back which is caused by the expansion of compressed gas and the relaxation of elastic deformation. As mentioned before, this effect becomes more pronounced with increasing speed of densification until, at a certain compression rate, the compacted body disintegrates partially or totally upon depressurization. Therefore, it is often necessary to find an optimal compromise between densification speed (= capacity) and product integrity (= quality). The problem becomes greater with finer particle size because such materials are naturally more cohesive and, therefore, in the feed state, feature lower bulk density or, respectively,higher bulk volume. In these cases, cohesive arches will collapse at low pressure whereby large amounts of gas are driven out. At the same time, pores between fine particles are small which results in low diffusivity so that it takes a relatively long time for the large amount of displaced gas to escape. To help overcome problems associated with degassing or deaeration, special design features, such as force feeders and/or various provisions for venting, are applied with all pressure agglomeration methods, particularly if fine powders must be processed. If the mechanism of densification is considered (refer to the sketches in the upper part of Fig. 8.1),it becomes clear that the pores in the feed for a pressure agglomeration process of any kind must not be filled completely (saturated) with a liquid. An example of such a material would be a normal filter cake, i.e. one that has not been blown dry or otherwise further dewatered. Since liquids are incompressible, the pressing force would increase quickly and mechanical dewatering would have to occur, which further reduces the speed of densification. It would also require an effective separation of solids and liquid during the densification process; this is a task which, so far, has not been solved satisfactorily. Therefore, with increasing pressure applied to the particulate solids, which typically results in higher densification or lower porosity, the moisture content of the feed must diminish. In high-pressure agglomeration the feed must be essentially dry! The destructive effects of expanding compressed air and relaxation of elastic deformation can be also reduced if the maximum pressure is held for some time, called dwell time, before it is released. Fig. 8.2 shows, that, without special technical provisions, this is only achieved in ram extruders (Fig. 8.2b, see also Section 8.4.3).In such equipment, a number of briquettes is retained in the long pressing channel and is redensified during each stroke. After the wall friction is overcome and the entire line of briquettes moves forward, the pressing force remains almost constant. A similar, but much smaller effect is obtained in pellet presses (see also Section 8.4.2). Since, as mentioned before, a dwell time and, particularly, the application of several densification cycles also helps to convert temporary elastic deformation into permanent plastic deformation, these techniques are especially suitable for the densification of elastic materials such as, for example, biomass. If required or desired, in the case of punch-and-die presses (Fig. 8.2a) special drive systems must be used to accomplish a dwell time (see also Section 8.4.3).It is obvious
8.7 Mechanisms of Pressure Agglomeration
from Fig. 8.2c, that no such possibility exists in roller presses (see also Section 8.4.3) where a continuous rolling action densifies the material between approaching surfaces until, immediately after passing the point of closest approximation, the relative motion is reversed, the surfaces retract, and the pressing force drops, ideally to zero (see also Section 8.4.3).
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8.2 Structure of Pressure Agglomerates
As compared with the structure of tumble/growth agglomerates (see Chapter 6, Fig. 6.1) where particles adhere to each other and form a porous body by natural coalescense and in which the agglomerate forming particles largely retain their individuality, size, shape, and characteristics, during pressure agglomeration, depending on the level of densification and the applied forces, this structure may change dramatically. Referring again to the upper part of Fig. 8.1 (Section 8.1) it can be seen that, as densification progresses (represented by the x-axis) and the pressing force increases (represented by the y-axis), the volume of the particulate matter becomes smaller which indicates a decrease in the void volume between the individual particles. In low-pressure agglomeration (see Chapter 6 , Fig. 6.4a), only the porosity of the mass is changed while the original particle sizes and shapes are retained. Typically, wet particle mixtures or those exhibiting a sufficient amount of natural plasticity and lubricity are passed through the openings of a screen or thin, perforated sheet by the wiping action of a suitably designed tool (see Section 8.4.1). The product is a crumbly mass or small extrudates, one dimension of which is defined by the screen or perforated sheet openings. Porosity is still high and the fresh (“green”)strength is normally obtained by liquid or “sticky” binders. Because, as experienced in tumblelgrowth agglomeration as a result of the competition between adhesive and destructive forces, preferential coalescense does not take place in low-pressure agglomeration, a system that densifies feed by passing it through openings and relying on inherent or added binders for green strength, porosity of the product particles is higher than in agglomerates obtained in high density tumbling beds and is comparable with that of granules from low density fluidized beds. The advantage of low-pressure agglomerators over fluidized bed agglomerators is that the size of the product granules is better controllable and that larger agglomerates can be made directly from a moist powder mixture. The green granules from low-pressure agglomeration are also well suited for spheronizing (see Section 8.3).Final strength is achieved by curing which normally involves drying. Medium-pressure agglomeration or “pelleting” comprises processes in which sufficiently plastic and “lubricated” particle mixtures are extruded through perforated dies (see Chapter 6, Fig. 6.4b). In contrast to the “dies”used in low-pressure agglomeration, the openings feature significant length and the densification pressure is caused by the frictional resistance in the orifice during extrusion (see Section 8.4.2). As a result, lower porosity, higher green strength, and a better defined product shape are obtained. As shown in Fig. 8.1 (Section 8.1),often some deformation of the agglomerate forming particles is obtained if they are sufficiently plasticized during a conditioning step (often by “steaming”to activate starchy components, see also Section 8.3) and a high enough pressing force results from the dimensions of the extrusion channels. Brittle breakage of agglomerate forming particles does not normally take place because plasticity is a precondition for successful pelleting. The fresh, green strength of products from medium-pressure agglomeration is caused by capillary, adhesion, cohesion, and attraction forces as well as by interlocking due to plastic
8.2 Structure of Pressure Agglomerates
flow. In most cases a drying step follows during which additional bonding, mostly by recrystallization of dissolved substances, occurs. High-pressure agglomeration (see Chapter 6, Fig. 6.5) extends densification into the reduction of void spaces by changing particle size and shape. As product density comes near to the true density of the solids and porosity approximates zero, the pressing force curve asymptotically approaches a vertical line. At this state, very large increases in force result from small changes in volume which endanger the structural integrity of the equipment. For that reason, densification is only carried out to a save level (pma in Fig. 8.1) and overload protection is included in the equipment design. In size enlargement by pressure agglomeration, the aim of densification is to bring the primary particles into sufficiently close contact so that the forces acting between them become large enough to yield adequate strength for the agglomerate’s intended use. This may be achieved directly and/or after a post-treatment. In dry, high-pressure agglomeration, it is often necessary to carry compaction into the bulk compression stage in which stressing is hydrostatic in character. Then, broken or deformed particles are no longer able to change position because only few, small voids remain and a certain degree of particle conformity has been achieved. The rate at which the apparent density approaches theoretical density depends on the yield point of the solids. It is more difficult to compact brittle materials to high density by pressure alone because fragmentation decreases due to the development of hydrostatic pressure conditions and smaller particles exhibit higher strength (see also Section 5.4). When voids become fully disconnected, a considerable internal gas pressure may develop in the isolated pores which, together with stored energy from residual elastic deformation, contribute to the potential weakening or destruction of compacts when the pressure is released (see also Section 8.4.3). Compact density and its distribution is also strongly influenced by interactions between the solid particles and of the particulate mass with equipment features (e.g. die walls, punch surfaces, roller press pockets, etc.). If a perfectly lubricated particulate solid (i.e. featuring no interparticle friction) were compacted in a cylindrical die with frictionless walls, it could be expected that the force exerted by the smooth, flat punch is transmitted through the entire volume of material resulting in uniform pressure and, therefore, uniform density throughout the compact. In reality, the presence of frictional and shear forces leads to a non-uniform pressure distribution and irregular particle movement (displacement) causing variations in compact density. Density variations are present in products from most pressure agglomeration techniques and may lead to a weakening of the compact (see Sections 8.4.2 and 8.4.3). If post-treatment includes crushing to yield a granular product, particles with different hardness and strength are obtained, and if sintering is applied, distortion of the final product is caused by the density variations (see Section 8.3). To demonstrate some of the possible effects of real particulate masses and equipment features on the outcome of densification, results of tests are shown in Fig. 8.3 which depict lines of constant solids content in a cylindrical compact after uniaxial compaction at different pressures [B.42].Since, as explained in Section 5.2.1, solids content in an agglomerate can be related with the term (1- E ) to the porosity E, a solids content of, for example, 39.6 % corresponds to a porosity of 60.4 %. Therefore, the
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2.8 MN /m2
6.1 M N / m Z
39 MN/m2
66 MN/m2
200 MN/mL
Fig. 8.3: Density distribution in cylindrical compacts at progressive stages of densification IB.421.
feed, even though not shown, had a solids content of approx. 30 % or a porosity of 70 % which is indicative of and rather typical for a fine bulk solid. After subjecting identical bulk volumes ofthe same powder to the indicated compaction forces, as a first result, the volumes of the densified cylindrical bodies were reduced to the sizes that are represented by the corresponding rectangles. For structural evaluation, the residual pores were filled with a highly fluid, perfectly wetting polymer which hardened without shrinkage by chemical reaction. Upon solidification, rings were machined from the cylindrical body and, after leaching out the polymer, the solids content in each ring was determined by weighing the solids and relating their mass to the volume of the ring. Points of identical solids content were connected to obtain the lines shown in Fig. 8.3. The differences in density (or porosity, respectively) can be explained by interparticle friction, wall friction, force dissipation from particle to particle, and sliding under shear. Referring to the three highest densified samples in Fig. 8.3 (the lower row of compacts) in which the effects are most distinctly expressed, the high density in the upper corners is caused by the downward movement of the punch and the frictional resistance of the powder mass on the die wall while the low density in the lower
8.2 Structure of Pressure Agglomerates
corners is the result of lower pressure due to force dissipation, interparticle friction, and friction on the wall. The low density in the top center indicates a lateral frictional arrest of the powder mass because of its intimate contact with the punch. Finally, the high density in the lower center is obtained where the shear plane faults, which typically occur at an angle of approx. 45” in pressurized particulate masses, intersect. Similar explanations of density variations are possible for all other products of pressure agglomeration. In pelleting, for example, the extrusion through longer, mostly cylindrical openings in dies, the pressing force develops as a result of the friction between the extruding mass and the extrusion channel walls. Therefore, in most cases, the extrudate features a distinct, highly densified “skin” of defined thickness on the outside while the center is much less densified. As already mentioned in Section 5.2.2 this characteristic is advantageous for animal feed, the main field of application of this technology, because the highly densified surface provides good abrasion resistance for transport, handling, and application while, at the same time, the transverse crushing strength is relatively low yielding a feed that is easily chewable by the animals for which it is intended. On the other hand, crumbling or granulating an extruded material by crushing, destroys the strengthening effect of the skin and often results in a dusty product with low abrasion resistance (see also Section 8.3). In general, density variations during pressure agglomeration increase with higher pressing force and with greater height or thickness of the compact, decrease for cylindrical compacts with increasing diameter, even if the height to diameter ratio is constant, are slightly reduced by the addition of a lubricant to the powder, and are considerably reduced by lubricating the die walls and/or pressing tools. To avoid uneven densification, originally to alleviate distortion of pressed parts during sintering, isostatic pressing was developed. In this process, particulate material is shaped and compressed in a flexible mold by a pressurized fluid. By this method, the pressing force acts uniformly (isostatically) from all sides on the powder to be densified. Of course, prior to the application of pressure, the powder in the mold must be well evacuated to avoid the build-up of compressed gas in the densifying material. The compacts resulting from isostatic pressing still feature a different density, for example on the surface and in the center of the parts, but, because the density gradient is uniform, no distortion occurs during sintering (see Sections 8.4.4 and 9.1). Segregation during feeding and/or the filling of die cavities also results in density variations in the compacted product owing to localized changes in particle size and/or distribution and, in the case of mixtures, due to different distributions of plastic and friable components. There is ample evidence in Mechanical Process Technology that macroscopic flow of solid particles within powder masses is negligible. Particulate systems do not behave like gases or liquids in which molecules are mobile and can freely move. Therefore, it must be expected that variations in overall or localized density of the particulate feed before compaction will have a definite and significant effect on the uniformity and quality of the compacts.
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Fig. 8.4: Structures ofcompacts from high-pressure agglomeration after processing equal feed materials (recrystallized KCI, Potash) at low (left, brittle disintegration) and elevated (right, plastic deformation) feed temperatures in identical roller presses with the same operating parameters. (For more information see text.).
In the foregoing, it was mentioned several times that the structure (and the characteristics) of products from high-pressure agglomeration depends on whether the particles are brittle and break or deform plastically during compaction. The structure of composit materials can be influenced not only by their mechanical and morphological features but also by shape and size. Since most materials become more malleable with increasing temperature and may be brittle at low temperature, heating or refrigerating particulate solids prior to feeding them to high-pressure agglomeration equipment can be also used to influence product structure. For example, Fig. 8.4illustrates the influence of feed temperature on the structure of specific compacts. These scanning electron microscope (SEM) photographs depict microstructures of recrystallized (upgraded/concentrated) fertilizer grade potassium chloride (potash) after high-pressure agglomeration in a roller press (see also Section 8.4.3).The pictures on the left demonstrate brittle behavior, while the structure on the right results from plastic deformation. All parameters of the feed, the process, and the operation, with exception of the temperature, were kept constant. The feed temperature was at ambient on the left, a temperature at which potash crystals respond brittle to high speed loading, and approx. 130 "C on the right, indicating that at this feed temperature plastic deformation occurs. To appreciate the effects fully, it should be pointed out, that the feed particle size is in a range from 10 to several hundred pm, with most of the particles larger than 100 pm and only a very small percentage of fines < 10 pm. Thus, the pictures on the left (the one above at higher magnification for better scrutiny) prove that all potash feed particles have disintegrated into small, cubic crystallites. The photograph on the right (same magnification as the picture on the lower left) indicates that at the higher feed temperature the original crystals have survived but were deformed such that they now contact each other with large surface areas while fines are also plastically deformed and packed tightly into the pore space between the larger particles.
8.3 Post-treatment Methods
Although, the structure obtained from either brittle or plastic behavior of feed materials during high-pressure agglomeration is quite different, as demonstrated in Fig. 8.4, compression strength of the compacts is often equal. In the case of brittle response, a much larger number of considerably smaller particles is produced, which results in increased strength (see Section 5.2.1) and recombination bonding (see Section 5.1.1) participates importantly in the development of strength. If plastic deformation occurs, high strength is obtained inspite of retaining the larger particles because large surfaces contact each other intimately and interlocking bonds can occur when plastic components partially or completely flow around harder solids. In regard to residual porosity, it is often observed that brittle materials still retain a considerable amount of narrow but open porosity, which can be useful if the material must easily disintegrate and disperse in a liquid, while most of the porosity in compacts from plastically deformed feed particles is isolated, rendering a product with low dispersibility. As shown in Section 5.2.1, the tensile strength of agglomerates, in which the bonding occurs by binding mechanisms acting at the coordination points of the agglomerate forming particles, is directly proportionate to the solids content (1- E ) as well as the sum of all adhesion forces A, caused by these bonds, and inversely proportionate to the porosity E as well as a representative equivalent particle size. If in tumble/growth agglomeration, where the size of the agglomerate forming particles does not change during the process, all acting adhesion forces are known, as, for example, in the case of capillary bonding, strength can be estimated. This is not possible for high-pressure agglomeration during which, as indicated in Fig. 8.4, particle sizes within the compact change ifbrittle solids are involved or unknown plastic deformation and approaches of surfaces take place; the extent of these modifications can not be estimated. Therefore, much more work needs to be done to learn about the structure of compacts from pressure agglomeration, particularly of those from high-pressure agglomeration. To that end it is necessary to understand the development of structure and to determine the parameters that control the structure as well as all product characteristics that are influenced by structure. 8.3 Post-treatment Methods
Referring to Section 7.3, where post-treatment methods were first covered in connection with tumblelgrowth agglomeration techniques, Tab. 7.1 summarized the effects of post-treatments on the final characteristics of agglomerates. As mentioned there, the methods and their effects “are feasible and can be used in the design of any agglomeration system”,which includes those based on pressure agglomeration technologies. In tumblelgrowth agglomeration, most frequently green (moist)agglomerates are produced which require drying as a post-treatment method to gain final, permanent strength. Only a few “natural” tumblelgrowth agglomeration technologies, involving ultrafine (nano-sized)particles, yield dry agglomerates which do not need further processing for strength. Additional post-treatment of aggregates from any tumble/growth
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agglomeration method is dictated by the desired or required final product characteristics. In pressure agglomeration, the properties of freshly produced compacts depend on the level of forces that have been exerted during densification and shaping. Low-pressureagglomeration requires feeds that are moist and plastic to allow passing the material through the structurally weak die plates (Section 8.4.1). As explained in Section 8.2, the structure of the resulting agglomerates resembles that of products obtained in low density tumbling particle beds. That means, they feature high porosity as well as low strength and are initially held together by bonds based on the surface tension of liquids and/or the adhesion of viscous substances. Therefore, they always need to be dried whereby final strength is obtained. The feed to medium-pressure agglomeration or pelleting equipment contains much less moisture. Plasticity and lubricity are normally obtained by the activation of feed components during a conditioning step. Such conditioning may be the mixing and kneading with small amounts of solid or liquid lubricants, or the contacting with steam to moisten and heat as well as gelatinize starchy materials, or may simply mean heating to soften certain components. An additional rise in temperature results from working the feed with the extrusion tools and by friction in the extrusion channels. In many cases, the products of pelleting need only cooling to remove the heat and gain final strength. The small amount of moisture that is present typically evaporates naturally prior to cooling. In some cases where for various reasons more moisture had to be added prior to pelleting, drying may be also necessary. As mentioned in Section 8.1,high-pressureagglomeration requires essentially dry feed materials due to the high densification to low residual porosities and the incompressibility of liquids. Typically products from high-pressure agglomeration feature high strength immediately and very seldom need post-treatment for strengthening. Excluded from this statement are applications in ceramics and powder metallurgy where final structure and strength are obtained during sintering (see also Section 5.3.2 and Chapter 9). Therefore, while in tumble/growth agglomeration at least one post-treatment step is almost always a necessary part of any system, the same is true for all low-pressure a g glorneration but only for some medium-pressure agglomeration and very seldom for highpressure agglomeration processes. Increasing with the applied force during the process, post-treatment methods in pressure agglomeration are directed towards modifications of product size, shape, and/or structure. At first it seems that one of the most distinguishing features of pressure agglomeration, the production of a particular shape by densifying and forming particulate solids in various dies, is very desirable. However, for physical and technical reasons (see also Sections 8.4.1 through 8.4.4) it is only feasible to produce relatively large compacts. For a number of considerations it may be preferred to apply one of the pressure agglomeration techniques but still seek a granular product with particle sizes of only a few millimeters in diameter or even below that dimension. With the exception of low-pressure agglomeration, where extrudates with diameters of as little as 0.8 m m can be produced from selected feed materials, in medium or high-pressure agglomeration such sizes can not be obtained directly.
8.3 Post-treatment Methods
Most ofthe methods for the reduction and adjustment of size are based on crushing the compacts and screening the crusher discharge to yield a particle size distribution of predetermined width and accuracy. The narrower the distribution and the greater the desired accuracy of the sizing cuts, the lower is the yield of acceptable product and the greater are the amounts of over- and undersized particles. The rejected material streams (the one containing the larger particles after milling) are normally recirculated to the agglomerator. However, to increase or optimize product yield, a closed loop recrushing of the oversized particles is often preferred (see below and Section 11.3). This technology is called compaction/granulation. Agglomerates are assemblages of solid particles. Compared with the solid itself, in which atoms and molecules are held together by valence forces and form regular arrangements with an organization that depends on the types of the atoms and/or molecules, agglomerates are made up of smaller particles with different size and shape that are arranged in irregular structures and joined together by binding mechanisms. Because small particles contain only a few irregularities and flaws, their strength is relatively high (see also Section 5.4) and always exceeds that of the binding forces acting between the particles (see also Section 5.1). Agglomerates also feature void spaces between the particles, so called porosity (see Section 5.3.2). As a result of these characteristic properties of all agglomerates, in most cases it is easy to convert them back into the primary particles from which the agglomerate was originally made. Therefore, breaking larger compacts into smaller, but still agglomerated granules, which is the task of this particular post-treatment method, and avoiding the formation of excessive amounts of fines requires a different set of crushing parameters than those used for the “normal” size reduction of solids. For optimal granulation of agglomerates by crushing, it is most important to control the input of crushing energy, minimize the interaction between particles during crushing, and immediately remove fines from the crushing chamber. Taking into consideration the well known fracture mechanics [8.1]and interdisciplinarily applying crushing know-how, the following applies for compaction/granulation: 1. A low energy input must be maintained during crushing to avoid total destruction of agglomerates into or beyond the original powder particles. 2. Fines from any source, either leaking through the pressure agglomerator or produced during handling, should be removed to avoid overloading the crusher and, thereby, overgrinding the agglomerated material. 3. Crushing must be carried out gently to produce only few fines, a certain amount of product, and a considerable amount of oversized material; after screening, the latter is being recrushed in the same mill (closed circuit) or in another one (second crushing step). 4. Even in a separate second crushing step, energy input should be such that still some oversized material is produced, removed during screening, and recrushed to further optimize the yield of granular product.
The shape of granular material from compaction/granulation is irregular and angular (Fig. 8.5).This is in stark contrast to the spherical or spheroidal shapes of agglomerates
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Fig. 8.5: Photographs o f granular products from compaction/granulation showing the irregular, angular shape o f the particles.
from tumble/growth agglomeration. On an industrial scale, the first agglomerated granular materials were produced by rolling and growth. Since some of the major requirements on the characteristics of granular products are good flowability and distribution (as,for example, in mechanical spreaders for granulated fertilizers) as well as superior metering capabilities and high abrasion resistance, to avoid the production of nuisance dust, i.e. fine, airborne particles produced by erosion, it was thought that spherical or spheroidal agglomerates fulfill these requirements better than angular ones. Therefore, initially, granular products obtained by compaction/granulation were judged to be inferior to micro-agglomerates from tumblelgrowth agglomeration. This perception changed when it was shown in the pharmaceutical and the fertilizer industries that dry, high-pressure agglomeration and granulation by crushing and screening offered definite advantages (see also Chapter 11).Processing of dry particulate materials proved to be more economical because of the absence of a liquid binder that is costly and can also cause unwanted chemical reactions, the immediate obtaining of strength, the much lower energy requirement because drying of large amounts of material is not necessary, the easier house keeping, cleaning, and changeover possibilities with dry powders, and many other reasons. It was also shown that, for example, the spreading of granular fertilizers from compactionjgranulation is accomplished as well and uniformly as that of “conventional” granulated products. A valid concern is, however, that the edges, corners, and peaks on the irregular broken granules tend to rub-off during handling, producing the dreaded nuisance dust. Because such dust manifests itself only at the distributors or users it leads to complaints as it is widely visible and does not settle quickly. The manufacturer faces claims even though the actual amount of airborne material is minimal. On the other hand, if this dust is produced by rounding the material during a post-treatment process in the plant prior to packing or intermediate storage, loading, and shipping, it can be separated and recirculated to the agglomerator for reuse. Therefore, in those cases
8.3 Post-treatment Methods
where granules are very irregular in shape and feature a strength that “favors” the production of dust, abrasion drums (see Chapter 11.3) or other suitable equipment are sometimes used to erode and/or round the particles. Rounding can be achieved by spraying a liquid onto the surface of a tumbling bed of granules after which the peaks on the now softened surface are flattened and/or dust particles are re-attached. While after erosion a dedusting step and fines collection as well as recirculation are necessary, no fines are produced during rounding but some drying may be required. A further possibility to round irregularly shaped granules from a crushing step is coating (Section 10.1). Particularly in the fertilizer industry, melt coated particles, in which the coating material constitutes at least one of the nutrients of the final multi component product, are an excellent solution of the problem with additional beneficial improvements. As always, interdisciplinary application of such methods can result in similar benefits for other granular products in different industries. Another rounding technology in agglomeration with quickly increasing, varied application is spheronization. It is the technique of converting plastic extrudates or particles that were formed otherwise into a rounded spherical or spheroidal shape. Approx. 40 years ago, the Japanese inventors coined the name marumerizer for this device which means translated “round maker”. In many industries, the technique is still called “marumerizing”although equipment by other vendors may be in use. The original apparatus looked very similar to the machines that are in use today; the changes since then have centered around auxiliaries and improvements to the internal structure which now allows a wider range of applications and offers the availability of competitive suppliers. In the beginning, spheronization had been primarily used in the pharmaceutical industry as a final forming method for formulations with high active loading. A spherical particle was needed for coating with a thin polymeric material for controlled release (see also Section 10.1).Today, spheronization is also applied for animal medicines, herbicides, enzymes, specialty fertilizers, advanced ceramics, and many more. The ability of an agglomerated material to be spheronized depends mainly on its rheology. Extrudates must break into shorter pieces and those as well as other agglomerates must have the right amount of plasticity to deform by impact and during rolling. The rheology can be adjusted by the addition of binders and lubricants or more wetting agent (usually water). With this feed characteristic and the frequent desire to produce small (i.e. in the millimeter range) spherical or spheroidal products, low-pressure a g glornerution is the predominant technology preceding spheronizing. In fact, many wet mixtures and thixotropic materials that, during handling, processing, or compaction, become too pasty for use in any other agglomeration equipment, can be successfully densified in extruders, shaped into discrete agglomerates, and further treated to yield uniform rounded particles. In modern applications, perfectly spherical particles are often not required. Spheronization is then being used if extrudates do not break into short enough lengths or can not be cut in uniform pieces when exiting the die plate. In these cases the function of the spheronizer equipment is to reduce the size of long extrudates (Fig. 8.6a) into short cylinders with rounded edges (Fig. 8.6b,c). After processing the same material for several minutes (Fig. 8.6d), almost perfectly rounded particles are obtained. In
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Fig. 8.6:
Extrudates (a) and products from spheronization treatments after 5 s (b), 30 s (c), and several min (d) (courtesy LCI Corp., Charlotte, NC, USA).
regard to Fig. 8.6, it should be pointed out that, with more plastic materials, the spherical shape is often reached after much shorter times. A very round particle is produced if the end use dictates the need for this shape, for example for uniform coating in the pharmaceutical industry or well defined fixed bed packing of catalyst carriers. Spheronization begins in most cases with wet extrudates obtained from low (Section 8.4.1) or medium (Section 8.4.2) pressure agglomeration. To retain a maximum of plasticity, the elongated, often spaghetti-like extrudates are immediately charged into the spheronizer (Fig. 8.7) which consists of a vertical hollow cylinder (called “bowl”)with a horizontal rotating disc (called “friction plate”) located inside. The friction plate is the most important component of a spheronizer. It features a variety of different textures designed for specific purposes [B.42].Upon contacting the friction plate, which rotates with several hundred (up to approx. 1,600)revolutions per minute, the extrudates break almost instantly into short pieces of uniform length. As shown in Fig. 8.8, the segments are flung outwards by the centrifugal force that is exerted by the friction plate and form a rotating mass that contacts the wall of the bowl. The proper motion of the moving mass of particles should resemble a twisting rope (Fig. 8.9)
8.3 Post-treatment Methods
Charoe " chute
Side wall
-
discharge Fig. 8.7: Sectional representation of a marumerizer-type spheronizer (courtesy LCI Corp., Charlotte, NC, USA).
which turns at a significantly slower speed than that of the spinning friction plate. Mechanical energy is transformed into kinetic energy and the still plastic particles in the mass are being worked by contact with the friction plate as well as by collisions between particles and of particles with the wall. Continued processing causes a gradual deformation into a more and more spherical shape (see Fig. 8.6). Several auxiliary devices have been developed to improve and/or accelerate the rounding process (Fig. 8.10). During deformation and densification, excess moisture may migrate to the surface or the mass can exhibit thixotropic behavior. In such cases, a slight dusting by means of a suitable powder feeder reduces the likelihood of particles sticking together. Warm or cool dry air can be also introduced under the plate to remove some of the surface water from the particles. Other special features may in-
Fig. 8.8: Photograph o f a marumerizer, Type QJ-400, in operation (courtesy LCI Corp., Charlotte, NC, USA).
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Fig. 8.9: Schematic presentations o f the flow o f particles that are being spheronized (courtesy LCI Corp., Charlotte, NC, USA).
Fig. 8.10: Auxiliary devices for a marumerizer-type spheronizer (courtesy LCI Corp., Charlotte, NC, USA).
8.3 Post-treatment Methods
clude cooling or heating of the bowl through a jacket or cleaning of the friction plate with brushes. A moving baffle, consisting of several arms with pitched blades that are placed close to the wall and to the friction plate, serves to increase the agitation by wiping the inside wall and directing product into better contact with the friction plate. Variable speed drives are standard options since process conditions vary widely between applications (see below, Fig. 8.14).Also, formulation changes or different production rates will require modified rotational speeds of the friction plate for best performance. Spheronization equipment is designed for batch operation (see Fig. 8.7, 8.10, and 8.11). Continuous operation is possible by employing multiple units or cascade flow [B.42]. Both methods use two or more spheronizers. Multiple batch operation, for example with two spheronizers (Fig. 8.12),is sequenced such that one unit discharges
r-Mixer
Extruder
Surge hopper Load cell activated cycling
I Fig. 8.11: Schematic representation of a batch spheronizing system including mixer, extruder, and spheronizer (courtesy LCI Corp., Charlotte, NC, USA).
Mar ume r i z e r
Product t o dryer
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Fig. 8.12: Schematic flow diagram o f a complete (quasi-) continuous spheronization syst e m employing multiple (two) spheronizers (courtesy LCI Corp., Charlotte, NC, USA).
and subsequently begins with loading while the other one is in the second half of its spheronization cycle. A reversing belt or, as shown in Fig. 8.12, a diverter gate can be applied to alternately feed each machine. In cascade operation, two or more spheronizers are linked in series to extend the total processing (residence) time while, overall, realizing continuous flow through the system. Feed is continuously charged into the first unit resulting in partially spheronized material being displaced and overflowing into the next one(s). Scale-up of spheronizers depends on the mode of operation. For batch units, it is volumetric. Each machine has a typical operating volume as shown in Fig. 8.13 as a function of bowl diameter. These relationships also apply for each batch spheronizer in a multiple batch, (quasi-)continuous system.
Fig. 8.13: Working volume of marumerizer-type spheronizers as a function o f bowl diameter (courtesy LCI Corp., Charlotte, NC, USA)
8.3 Post-treatment Methods
For continuous cascade operation, the friction plate is lowered in the bowl so that a volume of material always remains inside while excess overflows. The residual volume can be either measured experimentally or calculated, assuming that the cross-section of the rope may be approximated by a fourth of a circle (quarter torus). To obtain a particular spheronization effect, an overall residence time must be maintained. The processing time in each machine can be calculated as the ratio of residual volume divided by the volumetric feed rate (= volumetric throughput). Since bowl diameters are predetermined and fixed by the design, the position of the friction plate in the bowl is the only variable which can be modified to match a certain feed rate or system capacity to the desired or necessary residence time. The friction plate speed is scaled-up by maintaining the tangential (or circumferential) accelleration constant. The formula for scale-up is: (rotational ~peed,)~/(rotational speed,),
=
(Eq. 8.1)
bowl radius,/bowl radius,
Results of Equation 8.1 are plotted in Fig. 8.14. In many cases it is an advantage that products of pressure agglomeration have uniform shape and often also feature the same size. For example, in the pharmaceutical industry it is a requirement that all tablettes made from the same formulation are of the same shape and size because they also represent the dosage form. Since, during therapy, individual tablettes are to be taken, packing of such products is easy and selection as well as retrieval are errorfree. Many agglomerated consumer products, such as foods, snacks and sweets, some flavoring mixtures, certain detergents, etc., have similar requirements. In the metallurgical industry, sometimes alloying elements are agglomerated such, that the large briquettes represent the quantity to be added to a certain amount of liquid metal, and for home heating, briquettes have long been easily chargeable solid fuels.
1800 1600 1400 1200 1000
800 600
400 Fig. 8.14 Relationship between plate diameter o f a spheronizer and r p m to maintain constant tangential accelleration (courtesy LCI Corp., Charlotte, NC, USA).
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Monosized pieces in bulk, on the other hand, particularly if they are not spherical, do not reach high density or mass. For example, run-of-mine,coarsely crushed coal that is loaded into trucks, railroad cars, or ship holds results in relatively high bulk mass, as smaller particles fill the voids between larger ones. If that same coal is briquetted, the identical transport device holds much less and, therefore, in comparison, renders shipping less economical. Crushing briquettes and sizing the broken pieces to be within the same limits as run-of-mine coal results in identical bulk mass and meets standard loading requirements. If agglomerates that were produced by any of the pressure agglomeration methods are broken, it must be realized, however, that individual pieces may exhibit different density and strength (see Section 8.2, Fig. 8.3 and corresponding text). In contrast to what has been said before (see above), sometimes it may, therefore, be inappropriate to crush agglomerates too gently. Weak particles may survive the stressing and then disintegrate during handling, producing excessive amounts of fines. Such breakdown of the granulated product does not normally result in airborne “nuisance dust” as defined above, but may still not be accepted by a particular user because the amount of “fines”,whatever that term means in a particular case, is greater than specified and again results in claims to the supplier. An important post-treatment method for many parts obtained from powders by high-pressure agglomeration is sintering (see Chapter 9). This technology produces final strength and structure in most ceramic and powdermetallurgical parts. Finally, because during high-pressure agglomeration porosity is often reduced to very low levels, post-treatment methods may be required to regain a more open pore structure and larger voids (see Section 5 . 3 . 2 ) .
8.4
Pressure Agglomeration Technologies
In the following four subchapters the technologies and the equipment for the beneficial agglomeration by pressure will be described. As already mentioned in Chapter 8, in pressure agglomeration, new, enlarged entities are formed by applying external forces to particulate solids in differently shaped and operating dies. There are two major distinguishing characteristics which define different pressure agglomeration techniques: Pressure and Die or Tool ConJguration. The first three sections are organized according to the level of pressure that is applied, while the fourth one describes a method that is set apart from the others by how the pressure is applied. Although it has been decided not to break the sections into further subsections, the different die configurations are so important that they will be described and collected in specific paragraphs with appropriate group headlines. The following is a summary of the methods and how they will be identified:
8.4 Pressure Agglomeration Technologies
Pages 8.4.1 Low-pressure agglomeration (Extrusion) 253-266 Gravity feed and extrusion blade(s) (screen and basket extruders) 253-256 Screw feeder(s) and extrusion blades (radial, axial, and dome extruders) 257-262 262 - 266 Gravity feed and roller(s) (flat die extruders) 8.4.2 Medium-pressure agglomeration (Pelleting) Hollow, perforated cylinder(s), feed from the outside Hollow, perforated cylinder, feed from the inside Flat, perforated die plate with press roller(s) Gear-shaped press rollers, feed from the outside Medium pressure axial screw extruders
2GG - 299 273 -276 277 - 283 284 - 289 290 - 294 294 - 299
8.4.3 High-pressure agglomeration High pressure axial screw and ram extruders Punch-and-die presses Roller presses
300-373 300-315 315-335 335-373
8.4.4 Isostatic Pressing
373 - 383
Another criterion that might be used to distinguish between the different pressure agglomeration techniques could be whether or not they operate continuously. However, although systems, overall and in practical terms, can be almost always designed to discharge product continuously, with the exception of screw extruders and roller presses, they do not actually perform continuously. From a fundamental point of view, in relation to the process and the formation of each individual agglomerate, most pressure agglomeration equipment operates discontinuously. For example, in low and medium-pressure agglomeration extrusion takes place only as long as pressure is exerted on a particular row of orifices and in ram extrusion as well as punchand-die presses the pressing tool reciprocates, forming each compact in a separate densification cycle, etc.
8.4.1
Low-Pressure Agglomeration Gravity Feed and Extrusion Blade(s) The techniques of low-pressure agglomeration may be the oldest method for the production of granular material by agglomeration. Originally, a moist mass was passed through a sieve by the eminence of the hand, a spatula, specially designed hand tools, or a sturdy brush. The “crumbled” mass was dried and yielded a granular product that was used directly, mostly in the food and pharmaceutical industries, or further processed, for example by pressing it into cubes or tablettes. This method, the use of which dates back several hundred years, was carried out to obtain free flowing and non segregating powder mixtures, which still is the major task for most granulation methods. In more recent times, the procedure was mechanized and what had been done by hand is now accomplished by motorized rotating or oscillating extrusion blades (Fig.
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6, Fig. 6.4a.l and a.2). The oldest and most basic equipment is the screen extruder (Fig. 8.15a). A system for the production of dry agglomerates is shown in Fig. 8.16. Wet feed (1)is fed by gravity onto a circular flat screen and wiped through the mesh openings with a rotating blade. The agglomerates with an approx. size of the screen openings are dried in a suitable piece of equipment. Since
Fig. 8.15: Schematic representation o f low-pressure agglomerators using gravity feed and screens or thin perforated sheets. (a) Screen extruder, (b) trough extruder, (c) basket extruder.
(Optional I mill
2
1
Fig. 8.16 Sketch o f a low-pressure agglomeration system with screen extruder, dryer, and (optional) mill. (1) Wet feed, (2) granular product and fines.
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8.4 Pressure Agglomeration Technologies 255
Fig. 8.17: Photograph o f a small trough-type granulator with horizontal rotor axis (courtesy Erweka, Heusenstamm, Germany).
the feed must be very plastic, it is possible that agglomerates stick together in the dryer and form large lumps. If this is the case an optional mill is used to break the dry material into the final granular form (2) which contains a certain amount of fines that, depending on the application, may have to be separated and recirculated (not shown). Fig. 8.17 is the photograph of a small trough granulator (see also Fig. 8.15b). Inside the screen trough is a rotating or oscillating cage with wiper bars that passes the material through the screen. Most modern low-pressure agglomerators using gravity feed and screens are basket extruders (Fig. 8.1%). Fig. 8.18 depicts more detailed sketches of this machine and its
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1. I
Fig. 8.19 Photograph o f a basket extruder, model BR-450, and detail of extrudates being formed (courtesy LCI Corp , Charlotte, NC, USA)
function. In basket extruders, the meshed or perforated cylinder sits upright. Feed material falls into the chamber and in front of the extrusion blades (Fig. 8.18a). The material is compressed in the nip between the rotating blades and the cylinder and forced through the screen openings, forming extrudates (Fig. 8.18a). The extrudates break-off naturally or are cut-off by a rotating knive on the outside of the cylinder and fall into an inclined collection chute or, as depicted in Fig. 8.18a, onto a rotating plate. In both cases, the green agglomerates leave the equipment through a discharge chute. Because pressure is only exerted by the extrusion blade, basket extruders feature the least compaction of the various low pressure extrusion devices and are especially suited for easily extruding materials yielding products with high porosity. Fig. 8.19 and 8.20 are photographs of industrial basket extruders and of extrudates being formed during operation. Fig. 8.20 also depicts the major components of a basket extruder showing feed hopper, extrusion blades, and perforated basket die. To achieve higher densification, additional compressive forces must act on the feed material. This can be attained by means of integrated screw feeders or by heavy and/or pressurized rollers. At the same time, the perforated die must become structurally
Fig. 8.20:
Photograph of the major components (feed hopper, extrusion blades, and perforated die) of a small basket extruder (Bextruder BX 150, courtesy Hosokawa Bepex CmbH, Leingarten, Germany).
8.4 Pressure Agglomeration Technologies
stronger to withstand these forces which results in somewhat longer extrusion channels and higher frictional resistance that also yield higher densification. Screw Feeder(s) and Extrusion Blades A modern machine that may, alternatively, apply low or medium pressure to a wet or moist particulate mass of solids is the screw extruder. In these machines the phenomenon of movement caused by the flights of rotating screws in more or less tightly fitting barrel-shaped housings is used to produce the necessary pressure to overcome the friction in open-ended channels. Screw extruders may feature single or twin (= two) screws. In low pressure screw extruders extrusion blades are used to create a wiping effect at the die plate. Two fundamentally different arrangements are possible: extruders with peripheral (or radial) and axial discharge (see also Chapter 6, Fig. 6.4a.3 and a.5). Pressures in extruders with radial discharge are typically less than 1 MPa. The extrudates exit radially from a screen cage located at the end of the screw(s) (Fig. 8.21). The extrusion blades are either tapered cylinders with vanes in single screw extruders or intermeshing blades iftwin screws are applied (Fig. 8.22). While the screw(s) transport(s) the particulate mass through the barrel and provide(s) pressure, the extrusion blades push the material through the screen in a similar way as shown in Fig. 8.18b. In addition to the blade forces and the frictional resistance to slip, the force exerted by the screw(s) acts on the mass. If the feed mix to be extruded features unfavorable flow characteristics, some of the material may collect in front of the extrusion blades and rotate across the screen. Then the screw(s) must impart more driving force (work), increasing the internal pressure which participates in pushing the mass through the openings. For that purpose different screw designs are available (see be-
\
/+
Feed hopper
'Screw Fig. 8.21:
Schematic cross section through a peripheral or radial low pressure screw extruder.
Fig. 8.22: Conical, vaned and intermeshing extrusion blades for radial low pressure extruders (courtesy LCI Corp., Charlotte, NC, USA).
Screen
Extrusion blade
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Fig. 8.23: Photograph of the extrusion section o f a low pressure extruder with radial discharge (courtesy LCI Corp., Charlotte, NC, USA).
low, Fig. 8.26).The direction of extrudate flow is perpendicular to the axis of the equipment as shown in Fig. 8.23. In an axial extruder, pressure is again developed by a screw or screws which forces the particulate mass through uniform openings in a die plate that seals the end of the barrel (Fig. 8.24).According to the principal mechanical operations that are performed along the screw(s),three major zones are defined in an axial extruder: Feed zone, transport and compression zone, extrusion zone. The feed zone is the area where the moistened formulation is first introduced into the extrusion device. It includes a hopper to channel and distribute the flow of material into the chamber containing the screw(s).Most screw extruders will be operated with only a slight excess of feed or even in a somewhat starved state. Because, for extrusion, the material must be plastic, too much feed tends to build up over the screws and bridging is likely to occur. The screw(s)move the mass from the feed zone into the compression zone. In some machines, liquid can be introduced in this zone and the material is kneaded to form a Feed hopper
/
\
/
Gear box and drive mechanism
Screw
t
/
Coolingiheating jacket
Die plate
5 5 5 2 $ Feed $ Compression 5 *Extrusion zone $ zone $ 5
8 Fig. 8.24
/
/ d
Schematic cross section through an axial extruder.
zone
8.4 Pressure Agglomeration Technologies
Feed
4 Vacuum
moist homogeneous mass. Some mixing of different powders can be also accomplished. Most manufacturers of screw extruders offer both single and twin screw designs. The twin screw extruder has the advantages of less bridging in the feed zone due to the larger open area and better transport into the compression and extrusion zones. Also, a greater throughput is achieved. On the other hand, a single screw extruder is capable of delivering more power per unit mass of the material to be processed which may help to produce a harder and/or denser extrudate. In the compression zone, as a result of the specific design of the screw(s)in this part of the equipment, the void volume between the particles is reduced as particles are forced to approach each other more closely and gas (in most cases wet air) is expelled from the loose mass (see also Sections 8.1 and 8.2). As shown in Fig. 8.25, some extruders have vents, which may be open or connected to a vacuum and exhaust treatment, to remove the displaced gas. Screw design varies in accordance to how much compression is needed. A very low pressure extruder may feature screws with regularly spaced flights and a straight shaft. They will provide some compression but the main function of such screws is just to transport the material along the barrel of the extruder to the extrusion zone. Other designs use progressively closer screw flights, i.e. variable pitch, and/or tapered screw shafts (Fig. 8.26).
Fig. 8 . 2 6 screws.
Sketches o f various types o f extrusion
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Fig. 8.27: Schematic representation o f a low pressure axial extruder with extrusion blades, also showing a typical drive arrangement [B.42].
Sometimes, a space is left between the end of the screw(s) and the die plate. If rheological properties of the material are such that further densification takes place in this transition space, a denser extrudate is produced. In most cases, a large gap is used in high pressure screw extruders (see Section 8.4.3)where the necessary forces to obtain extrusion are solely developed by the screw(s) and high hydrostatic pressure is required to induce hydraulic flow through the extrusion channel(s). In low pressure axial extruders, extrusion blades are commonly attached to the end of the screw shaft (Fig. 8.27). In those cases, the gap is small and the plastic mass is compressed in the
Fig. 8.28:
Photograph ofstrands dischargingfrom a twin screw low pressure axial extruder without cutters (courtesy LCI Corp., Charlotte, NC, USA)
8.4 Pressure Agglomeration Technologies
Material inlet
Fig. 8.29: Schematic cross section through a low pressure dome extruder.
nip between the blades and the die face, forcing the material to flow through the openings utilizing a localized “drag flow” pressure. With axial extruders, it is difficult to cut the extrudates into uniform lengths because the material extrudes faster on the outside of the die plate than nearer to the center (Fig. 8.28).This phenomenon is a result ofthe complex flow and pressure patterns that are developed in the extrusion zone. A rotating knife or an oscillating wire can be used to separate the extruded strands into pellets but, nevertheless, their length will usually vary considerably. The dome extruder, also called extended die extruder, is a hybrid between the axial and radial low pressure extruders (Fig. 8.29, see also Chapter 6, Fig. 6.4a.4). This design was developed while trying to find a way to make the flat die plates of axial extruders last longer. The latter tend to bend out when they are overloaded if, for example, difficult to extrude materials are processed. Geometrically, the best balance of forces happens on a sphere and the extrusion stresses are spread over a three-dimensional area. An additional advantage is the increase in production capacity even if compared to radial extruders. Although the extrusion area is smaller than that of
Fig. 8.30: Dome type low pressure extruder in operation showing extrudates emerge f r o m the semi.spherical die plates. Also visible is a double shafted pug mill for mixing and conditioning the feed prior to the extruder (courtesy LCI Corp., Charlotte, NC, USA).
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the radial extruder, a steeper nip angle at the extrusion blade can be used which results in a more efficient wiping motion that overcomes the increase in pressure due to the smaller screen area. Typically, dome type low pressure extruders are equipped with two screws and, consequently, with two domes. The extrudates from all low pressure extruders, but particularly those from low pressure dome extruders, are slightly curved since they bend under their own weigth after leaving the orifice (Fig. 8.30). Gravity, acting on the overhanging mass of extrudates, causes strands to break irregularly wherever and for whatever reason a cross section is weaker. Accordingly, extrudates feature different lengths. Gravity Feed and Roller(s) Flat die extruders (see Chapter 6, Fig. 6.413.2) are commonly designed for medium pressure extrusion or pelleting. They consist of a horizontally arranged flat die plate on which normally two (or more) rollers move along (Fig. 8.31). Feed material is charged by gravity from the top into a chamber surrounding the circular die plate, pressed by the rotating rollers, and squeezed through the die openings. Typically, a rotating knife cuts off the extrudates below the die plate. Because of the high forces that are normally exerted by the rollers and involved in densifying and extruding the plastic particulate feed, the die plate must be thick to offer sufficient structural integrity and, consequently, the extrusion channels (in most cases cylindrical bores) are long, requiring medium pressure for extrusion (see Section 8.4.2). Since the principle is of interest for the agglomeration of some materials by low pressure extrusion, a method was found to support a die which is made from perforated thin sheet and, thus, obtain a low pressure flat die extruder. This machine resembles a low pressure screen extruder (see above, Fig. 8.15a and 8.16) in which the extrusion blade(s) has (have) been replaced by rollers. Fig. 8.32 is the photograph of a small, low pressure flat die extruder in operation. In extrusion, material flow through the openings of a die is very complex and related not only to the physical characteristics of the particulate mass but also to the die plate or screen configuration. As an example, Fig. 8.33 shows how the extrusion rate varies in relation to the dimension of the orifices (defined by the die thicknesses and hole diameters) as well as the percentage of free area (defined as the sum of all hole cross
Fig. 8.31: Schematic representation and working principle of a flat die extruder.
8.4 Pressure Agglomeration Technologies
Fig. 8.32: Photograph o f a small, low pressure flat die extruder in operation. The front half o f the extrusion chamber is transparent t o make the press rollers visible (courtesy LCI Corp., Charlotte, NC, USA).
sections divided by the total area of the die plate 100). In low pressure extrusion, a radial discharge extruder with screens will typically have more than six times the extrusion area than an axial extruder with the same barrel diameter. If a given material can be processed on either type of machine, this relationship translates into a higher capacity and cost advantages for the extruder with radial discharge. The differences between a screen and a perforated sheet or plate are quite substantial. Up to 1 mm, screens are usually of the same thickness as the hole diameter; they are rarely thicker than 1.5 m m due to physical limitations of mounting. Perforated sheets and plates are >1 m m (see also Section 8.4.2). The moisture level of the feed directly influences extrusion capacity and material flow (Fig. 8.34). In low pressure extrusion, most materials can only be extruded within a relatively narrow moisture range, typically 2 to 5 %. Even then, the extrusion characteristics may vary dramatically. At the lower end, the extrudate may have a rough surface, cause high power
500 450
400 n
L
c a Y
350
\
300
U
150 100
50 1
2
3
4
5
6
7
Die thickness (mm) Fig. 8.33: Extrusion rate as a function o f die thickness, hole diameter, and percent free area (courtesy Fuji Paudal, Osaka, Japan).
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1200 1100 1000
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C \
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z v
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cn
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0
Fig. 8.34
Extrusion rate o f a rubber chemical as a function o f the moisture content and hole diameter (courtesy LCI Corp., Charlotte, NC, USA).
consumption, exhibit high temperature rise, and may become excessively dense (see also Section 8.4.2). At the same time, the machine responds with a high demand of power and the screen or die plate may become overloaded. At moisture levels approaching the upper limit, moisture may be squeezed from the material, the extrudates may stick together, and the material may adhere to the feed screw thus reducing its transport efficiency (see below). Scale-up of low pressure extruders usually begins in the laboratory with testing on smaller equipment. After extensive experimentation with the formulation and equipment, an optimal set of parameters is defined which includes information on the material’s bulk density (before and after extrusion), the extrusion rate, the power consumption during extrusion, and the product’s temperature rise. An efficiency factor is then determined by ratioing the actual extrusion rate obtained on the small equipment to the calculated theoretical maximum extrusion rate. Efficiency factors are in the range of 5 - 35 % for axial, 15 - 55 % for radial, and 35 - 85 % for dome extruders. This efficiency factor is then applied to the theoretical extrusion rates of the industrial extruder. Many manufacturers of extruders will also include an application related “experience factor” for the determination of a safe but reasonable expected extrusion rate. Fig. 8.35 depicts relative levels of extrusion pressure and shear that are applied by the various low pressure extrusion equipment. For all machines that rely on a screw or screws for material movement and the development of pressure, it is important to understand how a conveyor screw works. In general, the mass flow rate drn/dt of, for example, a screw extruder is de-
8 4 Pressure Agglomeration Technologies
4
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Axial Roll
7
Extrusion pressure Fig. 8.35: Relative levels o f extrusion pressure and shear applied in different low pressure extruders (courtesy LCI Corp., Charlotte, NC, USA).
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Shear applied per weight of product
~
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i
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i
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LBasket
termined by the combined influences of screw transport and die resistance. The operating point, defining pressure and capacity, is obtained in a mass flow/pressure diagram (Fig. 8.36) as the intersection point between the lines characterizing the screw and, respectively, the die performances. Because of the influence of both, the theory of screw extruders is rather complex (see also Section 8.4.3). The actual operating point results from the superposition of two extremes: of screw conveying with no back pressure and pumping/mixing against a completely closed end [B.42]. The difficulty to theoretically describe the conditions in a screw extruder becomes even more complicated if, as described above, special kneading, densification, and deaeration sections are included in the design. To further understand the function of a screw, whether used for feeding (see also Section 8.4.3) or to build up pressure, it should be always realized that a “perfect” performance of a pressure creating transport screw requires that, along its length, the screw and barrel diameters as well as the pitch and flight thickness are constant, no build-up occurs on either the screw or the housing, the volume between the flights is completely filled, and the particulate solids do not rotate and/or densify by deaeration. In reality, all these conditions are not fulfilled. Shaft and barrel (or housing) dimensions often change for process reasons, build-ups may appear with time, the space between flights may partially empty due to rearrangements of particles, the solids may begin to rotate with the screw, mostly because of surface blemishes and build-up on the screw flights, and uncontrolled separation of solids and gas may occur. In screw design, a correction factor is trying to correct these inefficiencies.
cNozzle
Fig. 8.36 Extrusion rate dm/dt o f a screw extruder as a function o f the (back-)pressure of the mass t o be extruded [B.42].
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Most critical is the fact, that the particulate solids often begin to rotate with the screw. In an ideal case the screw flights are so smooth that no friction occurs with the particulate solids and, in contrast, the barrel walls exhibit so much frictional resistance that the particulate mass remains stationary in relation to the housing and is “screwed”forward efficiently. Sometimes this condition is also lost if the screw turns too fast. Then the solids may begin to exhibit turbulent movement and lose contact with the barrel wall, thus initiating rotation. Once this situation has occurred, the screw or screws may have to be stopped, emptied, and restarted with lower speed. Occasional cleaning of the screw flights, the frequency of which depends on the characteristics of the material and the condition of the screw, may also be necessary. Coating the screw flights with a “non stick surface may improve efficiency. 8.4.2 Medium-Pressure Agglomeration/ Pelleting
Referring back to Section 8.1, Fig. 8.1, and the accompanying text, medium-pressure agglomeration or pelleting is characterized by forces that are in the transition range. Structurally, most or all of the rearrangement of particles has taken place in the compact and, at the high end of medium-pressure agglomeration, some particle deformation may take place. Since pelleting is carried out by extrusion, the materials to be agglomerated need to be plastic or deformable. Therefore, the occurrence of brittle disintegration, the other mechanism that causes changes in particle shape (and size) in pressure agglomeration, is very unlikely. As in low-pressure agglomeration, differently shaped extrusion dies are used in which agglomerates with cross sections that are defined by the orifices are formed. The forces for densification, extrusion, and shaping are provided by the movement of the dies themselves and/or by press rollers and by the frictional resistance in the die channels. To accomplish higher densification, the ratio “lengthldiameter” or, respectively, “length/cross sectional area of the extrusion channels” must become greater, resulting in a higher frictional resistance. Since the extrusion force must be larger than the frictional resistance, considerable pressures develop in front of the extrusion channels. Therefore, the die body thickness must be selected such that it will not break during operation even if, occasionally, overloading takes place. Also, the open area as defined in Section 8.4.1, Fig. 8.33 (“% free area”) is normally relatively small, also to achieve structural integrity; that means that the “land” between the holes is large. These conditions influence the design of the extrusion channels. The cheapest execution of an orifice, the straight cylindrical bore, is very seldom applied. Fig. 8.37 depicts six commonly used designs of extrusion channels for medium-pressure agglomeration; in all cases the direction of extrusion is from top to bottom. As shown in Fig. 8.37a, in the simplest case there is at least a small inlet chamfer to compensate for the large land area that is necessary between the orifices; it serves to guide the material into the extrusion bore. Sketches 8.37b and 8 . 3 7 ~indicate that this feature may be more pronounced and varied to fit specific applications. Because the die must be thick, an extrusion channel through the entire body may be too long, the channel
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8.4 Pressure Agglomeration Technologies 267
Fig. 8.37: Sketches ofsix typical extrusion channel designs for medium-pressure agglomeration.
gqgg (dl
(fl
ratio maybe too large, and, as a result, the frictional resistance maybe too high. In such a case, the orifice length is reduced by increasing (relieving) the size on the inlet (Fig. 8 . 3 7 ~or ) the exit (Fig. 8.37d). In pelleting, organic materials are often processed which feature a certain amount of elasticity. It is possible that, when such products reach the end of the orifice, some of the initial elastic deformation has not yet transformed into permanent plastic deformation. If this is the case, the green densified pellet strands expand when they leave the extrusion channel (see also Section 8.1, “elastic springback). If the channel exit is designed with a sharp edge, the sudden expansion results in cracking and the pellets exhibit a surface structure that is called “Christmas tree shape”; the cracks caused by the expansion upon exiting the bores impart a very ragged pellet surface which resembles the shape of a pine (Christmas) tree. This “defect” can be avoided if, instead of ending with a sharp edge, the exit is tapered (Fig. 8.37e), thus allowing a controlled expansion with no cracking. Sketch 8.37f is a channel with inlet chamfer, tapered exit, and relieve bore. Fig. 8.38 is used for a more detailed description of all die hole characteristics. The diagram assumes that the die body is a cylindrical ring with thickness T and that the direction of material flow is from the inside of the ring die to its outside (for more details on this design, see below). As mentioned before and as will be further discussed in this section, dies may be also flat or machined into a gear shape and, in cylindrical or gear shaped ring dies, the flow of material may be also from the outside to the inside. Referring now again to Fig. 8.38, with exception of the elastic recovery or “springback”, d represents the pellet diameter and L is the effective length in which work is performed on the material during extrusion. T is the total, overall thickness of the die body which has been selected to withstand all the stresses that may develop within the equipment. Xis the counter bore depth; it reduces the die thickness T to the effective channel length L. The counter bore may feature a tapered part with angle B to obtain a gradual elastic expansion of the exiting strand and avoid structural defects on the surface of the pellets. Other counter bores have only a cylindrical bottom; this design, or straight bores with no counter bore, can be used for plastic materials with no or negligible elastic expansion. The tapered inlet (or chamfer) from diameter D to channel diameter d with angle @ is required to either increase the open area without sacrificing the strength of the die body and/or to obtain additional compression according to D2/ dZ. The first effect is particularly important for fibrous material which may produce matting and ultimately cause clogging of the die if too large land areas exist between
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Fig. 8.38:
Die hole characteris-
tics [B.42].
the holes. The second reason for a tapered inlet may be dictated by the need for a high overall densification of feeds with low bulk density. Dies deteriorate by “washing out” the inlet and discharge areas as well as the cross section of the extrusion channels. This is due to the high friction between the extruding mass and the walls of the orifices. Die life is determined by the increase of channel cross sections to such dimensions that either the product size is no longer acceptable or the back pressure due to decreasing frictional resistance becomes too low. In the latter case, compression is reduced and, therefore inadequate product density and/or strength are obtained. If neither pellet size, density, or strength are critical, the limiting die life is defined by the decreasing structural integrity of the die body. Particularly if extrudates with small cross section and high density and/or strength must be produced, the necessary thickness of the die body and the effective length of the extrusion channel may be rather incompatible. In this case, replaceable insert plates with the required short length of the bores may be used (Fig. 8.39a). Other inserts may be utilized (Fig. 8.39b) if extrusion channels have worn out to salvage the massive, expensive die body which, for strength reasons, is often made from forged high quality steel. On the left side of Fig. 8.39, in both cases the inserts are pictured as being installed in gear type dies. However, it is obvious to those skilled in the art that the principle is universally applicable in medium-pressure agglomeration by extrusion. If thin inserts are used in thick die bodies to obtain short bores for extrudates with small cross section (Fig. 8.39a), there is sometimes a problem in discharging the pellets from the recessed die plates if the product is too sticky and does not easily break off. The formulation may have to be adapted for successful operation.
8.4 Pressure Agglomeration Technologies
(b)
Fig. 8.39 Replaceable inserts for medium-pressure agglomeration by extrusion with (a) short and (b) long bores (courtesy HOSOKAWA BEPEX/Hutt, Leingarten, Germany).
Of course, in the case of inserts for wear replacement, any bore including a single channel, for example as depicted in Fig. 8.38, can be applied. Fig. 8.40 depicts the basic principle of medium-pressure agglomeration by extrusion or “pelleting”. Although sketch (a) shows the situation obtained between an internal press roller and a perforated ring die and (b) represents a press roller moving on a flat extrusion die, the following discussions are valid, with corresponding adjustments, for all arrangements of medium pressure extrusion. In Fig. 8.40,in both cases a cylindrical pressing tool rolls over a layer of material that was deposited by some feeding and distribution means on a perforated (only a few holes with simplified design are shown) support (= die). In the wedge-shaped nip, material is first compressed and then extruded through the holes. Fig. 8.40b includes some additional information. First, it should be recognized that the nip geometry between the roller (1)and the flat die (4,characterized by (3), (4),and (S), is different from that depicted in Fig. 8.40a where the roller is within a ring-shaped die. Because of nip geometry, more volume is densified in the case of a flat die and, if two perforated hollow rollers contact each other and form a nip (see below, Fig. 8.42c), this volume and its change during rotation is even greater. These are the types of adjustments that have to be made when applying the principles to different arrangements and designs. With the changing nip volume and compression rate, the pressure in the material as well as on the roller and the die, characterized by the curve 3-m-6which is equivalent to the curve that was first presented in Section 8.1, Fig. 8.1, also becomes different. The material layer is first densified between (3) and (4). In this regime, the increasing pressure is still smaller than the frictional resistance in the bores and no extrusion takes place. At point (4)the static frictional force is overcome and the maximum pressing force is reached (m).Between (4) and (5) extrusion occurs through the die holes and the pressure remains almost unchanged (see Section 8.1, Fig. 8.2b [although representing the conditions in a ram extruder (see Section 8.4.3),the conditions in the channels of medium pressure extruders are similar]).At the point of closest approach
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Roller
ossemblv
Fig. 8.40
Sketches explaining the basic principle o f medium-pressure agglomeration by extrusion [8.42]. For explanations see text.
(5) a gap remains and, therefore, a predensified layer of feed covers the extrusion die which helps to avoid damage by metal to metal contact. Later, this coating serves to obtain optimal drag of and improved bonding with the new feed and to reach better densification. As already discussed in Section 8.1, if materials to be pelleted exhibit a certain elasticity, the residual layer expands between ( 5 ) and (6).The curve 3-m-6 represents a typical profile of all the forces that act in the compression, extrusion, and expansion zones. Because of product and equipment design considerations, the forces that can be exerted on the mass to be pelleted are higher than those in low-pressure agglomeration but are still relatively small. Therefore, binders and lubricants (see Section 5.1.2) play an important role for the technology and the product is normally not highly densified.
8.4 Pressure Agglomeration Technologies
4-1
S l i p force
1
Fig. 8.41: Sketches depicting the nip area o f a medium pressure extruder with concave die and press roller explaining the forces at work [B.42]. For explanations see text.
Overfeeding is a common problem of medium-pressure agglomeration equipment. If, as shown in Fig. 8.41a for a concave die, the thickness of the layer of fresh material in front of the pressing tool increases from TFto 2TF as the feed rate is doubled, the force component which is directed forward and tends to push material away increases and the downward (compression and extrusion) force decreases. As a result of these conditions feed material may build up in front of the roller to a point where the pressing tool can no longer entrain it. The entire die cavity may then fill with material and the equipment will plug up. The same can occur in a fluctuating feed situation even if the machine is only temporarily overfed. This effect decreases for the flat die (Fig. 8.42a),is even less pronounced in machines with convex rings (Fig. 8.42c), and almost never occurs with gear type dies (see Fig. 8.66).
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Fig. 8.42: Schematic representations of the three major designs of medium-pressure agglomerators also called “pelleting machines” or “pellet mills”. (a) Flat, (b) concave, and (c) convex die configuration.
Nevertheless, machines with concave die rings and internal press rollers do have advantages. For example, if the feed material exhibits a certain elastic behavior, because the forces in the relatively long and slender nip increase slowly, a more complete conversion of temporary elastic into permanent plastic deformation takes place. Fig. 8.41b is another presentation of the forces at work. Feed, ideally deposited in a uniform layer on the die, is pulled into the space (nip) between roller and die and compressed. Friction between roller, die, and material as well as interparticle friction in the mass are responsible for the “pull”of the feed into the nip and for densification. Smooth surfaces of roller and/or die may result in slip. Axial grooves in the roller, which may also favor build-up of a thin layer of material, and the above mentioned residual layer of densified feed on the die effectively reduce slip. Low interparticle resistance to flow or a distinct plasticity result in a more or less pronounced tendency of the mass to “avoid the squeeze” (back-flow),thus reducing densification and potentially choking the machine (see above). In medium-pressure agglomeration equipment (Fig. 8.42), the perforated support (die) can be either flat (a), concave (b),or convex (c). For all three designs, intermeshing, toothed executions have been proposed in the patent literature [B.42] to avoid slippage and improve extrusion as well as extrudate quality, but only the so called “gear pelleter”, in which the convex dies (Fig. 8 . 4 2 ~are ) hollow gears with extrusion channels between the teeth, has reached commercial importance (see below). In the following the different, commercially available equipment and some of their characteristics are described in more detail. Often, these machines are called pellet
mills.
8.4 Pressure Agglomeration Technologies I273
Machines With Hollow, Perforated Cylinder(s) and Feed From the Outside The principle of these machines is shown in Fig. 6.4b.3 and b.4 (Chapter 6). Although, in the historic literature, equipment featuring the design of Fig. 6.413.4 is mentioned, today’s commercial offerings are limited to the execution depicted in Fig. 6.4b.3. Fig. 8.43 is a current schematic sketch as published by the manufacturer. Contrary to the drawing of Fig. 6.4b.3 the press roll is neither solid nor of the same size as the perforated die cylinder. As shown in a photograph of the two operating parts of such a machine (Fig. 8.44), the hollow press roller is a cylinder with roughened surface and features a diameter that, in most cases, is somewhat smaller (between approx. 81 and 87 %, see Tab. 8.2) than that of the perforated die. It is claimed that the difference in circumferential speed, that causes a certain amount of shear in the nip, improves the extrusion characteristics and, thereby the quality of the granulated product. The working tools (Fig. 8.44) ofthe Alexanderwerk “moist granulator” are two counter-rotating cylinders. One is perforated and acts as a die while the press cylinder is solid. Because the press roll is normally hollow, it can be equipped for cooling or heating. Feed is fed from above, mostly by gravity, and pressure is build up in the nip. When the pressure is high enough to overcome the static frictional resistance in the bores, the densified moist, plastic material passes through the appropriate perforations and extrudates are formed within the die cylinder. A scraper plate, located inside the perforated cylinder, cuts the cylindrical ropes into granules. Fig. 8.45 is the front view of such a machine, also showing the scraper plate support extending into the open die. A common problem of all ring die extruders in which material passes from the outside to the inside of a cylinder was and is that the extruded material must reliably discharge from the inside of the die without becoming entangled and/or stuck. To allow successful extrusion, the particulate mass must be moist and, to produce
Fig. 8.43: Schematic o f the Alexanderwerk “moist granulator” (courtesy Alexanderwerk, Remscheid, Germany).
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8 Pressure Agglomeration Tab. 8.2: Technical data of 'standard' "moist granulators"
(according to Alexanderwerk, Remscheid, Germany). Parameter
Working length Die diameter Orifice diameter Press roll dia. Throughput Drive power Approx. weight
Model
Unit
mm mm mm mm kg/h kW kg
CA 65
C1/100/160 S C1/148
80 70 1-5 GO 30-50 1 90
1GO 110 1-8 90 100-500 4 420
160
110 1-8 186 100-500
4 800
C1/168
C1/244
250 180 2-10 15G 500-1,000 10 1,100
240 270 2-10 218 1,000-3,000 15 1,300
strength, it must exhibit good binding characteristics. Although cut into short lengths, sticking of the product granules to each other, to the scraper blade support, and to the inside of the die cylinder is a definite possibility, particularly if the die cylinder is small in diameter. As shown in Fig. 8.46, to facilitate discharge, the machines are typically mounted on a slanted support such that gravity assists in product removal from the die interior. Because the nip between the rollers is fed by gravity, a too steep mounting platform will preferably feed orifices near the front of the die cylinder which causes an increase in the variation of extrudate length. Therefore, a compromise must be found between good feed distribution and acceptable discharge when designing the sloping support structure. A further problem of all ring die extruders in which the material is fed from the outside and passes to the inside is that at the closest line of approach between the
8.4 Pressure Agglomeration Technologies I 2 7 5
Fig. 8.45: Front view o f an Alexanderwerk “moist granulator”, also showing a view into the die cylinder and the scraper blade (courtesy Alexanderwerk, Remscheid, Germany).
roller and the die, theoretically, there should be no gap to avoid that material extrudes through this space or is compacted without entering the extrusion channels or simply leaks through. However, in reality, a gap can not be avoided. First, from a design point ofview, metallic contact between the two tool parts must be avoided. Second, clearance in the support bearings opens up a small gap when the operating pressure is acting on the two cylinders. Third, even though the operating pressure is low, because both the roller and the die are hollow cylinders, it is conceivable that some small deformation occurs in the area of highest force which coincides with the line of closest approach. Furthermore, over time, both the roller and die will wear which increases the gap that exists by necessity due to the three previously mentioned reasons. As a result, as sketched in Fig. 8.47, some material will always pass in between the two cylinders and collect below. This leakage or production of “fines” will increase with time due to wear. Provisions must be made to clean out the housing in regular intervals or to recirculate the leakage to the feed mixer/conditioner for reprocessing. In some cases, where the presence of fines in the product is not a problem, the small amount (typically <10 %) of this off-grade material can also be combined with the product. Because of the need to keep the clearance between the two cylinders to a mini-
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Fig. 8.46 Three “moist granulators” on a c o m m o n base (courtesy Alexanderwerk, Remscheid, Germany).
mum, they are mounted in fixed relative positions. If a larger piece of tramp material enters the nip between the cylinders (which should be avoided at all cost, for example by sizing the feed just before it enters the equipment), overload protection can be only achieved by stalling the drive. Nevertheless, if this should happen, the perforated cylinder is often damaged. Although this part is easily replaced, the spare extrusion die is costly. In spite of the problems that are associated with this design, it is often applied, particularly in clean environments and for easily deformable materials which require small extrusion pressure. Tab. 8.2 lists the “standard” models offered by one manufacturer and summarizes their most important technical details. Among the mediumpressure agglomerators or pellet mills, the Alexandenverk “moist granulator” represents equipment that operates with the lowest forces. In Fig. 8.48 three examples of products are shown. As can be easily seen, granules or pellets can be well formed (a and c) or somewhat crumbly (b). If the latter is not acceptable, the extrusion characteristics of the feed may be adjusted by changing (in this case increasing) the moisture, binder, and/or lubricant contents.
Flow of material Ring die
ction of Fig. 8.47: Sketch explaining the operation tation o f a medium pressure extruder in which
duct
a feed material is densitied on the outside o f a cylindrical extrusion die and passes into the interior. For further explanations see text.
8.4 Pressure Agglomeration Technologies
Fig. 8.48: Three examples of products that were manufactured with the Alexanderwerk "moist granulator", (a) 3 rnm dia., (b) 4 m m dia., (c) 5 m m dia (courtesy Alexanderwerk, Remscheid, Germany).
"Pellet-Mills" With Hollow Perforated Cylinder and Feed From the Inside This design represents the most commonly installed medium-pressure agglomeration equipment (see Chapter 6, Fig. 6.4b.5). It is primarily used in feed mills for the granulation of animal feed and associated products. A large number of vendors in many countries manufacture and offer these machines. Fig. 8.49 is another schematic representation, published by one of the manufacturers (CPM), showing the typical design of a pellet mill with cylindrical die and two internal press rollers. The execution suggested by Fig. 6.4b.5 (Chapter 6), 8.40, and 8.41 with one press roller is only used in laboratory and small production machines. For structural and process reasons, the perforated concave die can not be very wide (Fig. 8.50). Therefore, to increase the capacity of a given press and more uniformly distribute the forces acting on the ring, up to three rollers (see below, Fig. 8.52) are installed. With two rollers (Fig. 8.49) the capacity of a machine doubles as compared with a single roll pellet mill and triples with three rollers; from a ring loading point of view, the latter also results in the best distribution of the pressing forces.
Fig. 8.49 Schematic representation by one of the manufacturers (CPM) o f the operating principle of medium pressure extrusion in a "pellet mill" with ring die and internal press rollers.
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Fig. 8 . 5 0 A selection o f typical concave die rings for pellet mills with internal press roller(s) (courtesy Sprout-Matador, Muncy, PA, USA).
It is clear from Fig. 8.49, that pellet mills with ring dies and internal press roller(s) have the great advantage over the previous design, in which the material passed from the outside to the inside of the cylindrical die, that the pellets exit on the outside periphery of the ring. Therefore, they can be easily cut and discharged and no leakage of feed material occurs. However, to obtain uniform pellet quality, minimize pellet length variations, avoid uneven die wear, and ascertain constant power demand, it is necessary to distribute the feed evenly across the entire working width (= perforated area) of the die. Since the particulate feed can only enter the operational area of the machine from the open front of the die ring and, additionally, the interior is to a large extent occupied by the press rollers, this requirement is not easily met. Fig. 8.51 is a partial cut through a pellet mill showing the most important internal parts as designed and offered by this particular manufacturer. The machine is a directly gear driven model. For pellet mills that are used in the animal feed and similar industries, it is rather common that the feeding arrangement includes a conditioner (top), in which the different components are mixed and the feed characteristics are adjusted to exhibit optimum plasticity and extrusion properties by adding moisture (binder and/or lubricant) as well as steam (for example, for the activation of starchy ingredients by heating and moistening during condensation). The conditioned mass is then fed to the operating area ofthe pellet mill. In Fig. 8.51, a special feeder is used to overcome most of the previously mentioned distribution problems. The feed enters this “centri feeder” after, as a safety provision, a magnet in the feed chute catches any tramp metals. A transport device with a screw and paddles is used to advance an annular flow of material uniformly to the entire conical cover area of the die (Fig. 8.52). As shown in the schematic and in the photograph, adjustable plows, that are located between the (stationary) press rollers, divide the advancing feed stock into equal portions and direct it evenly in front of each press roller. For easy maintenance and cleaning, all modern pellet mills are equipped with a hinged door in the front of the machine which can be opened to access the internal machine parts (Fig. 8.53).To also meet the demands of short production runs, a “quick change pelleting cartridge” (Fig. 8.54), which contains the die housing, the die, the
8.4 Pressure Agglomeration Technologies
Fig. 8.52: Schematic and photograph o f a "Centri-Feeder" distributor (courtesy Sprout-Matador, Muncy, PA, USA). See text for explanations.
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conical die covers, the roller assemblies with the feed plows and the main shaft, can be removed as a whole in directly gear driven machines (Fig. 8.51) and replaced. This is important if, when changing products, no cross-contamination is tolerated (for example, in the case of medicated animal feed) or different pellet sizes and/or shapes are required. Fig. 8.55 shows a ring die and three press rollers with different surface configurations. The surfaces of the rollers are roughened to improve the drag and make the pressing tools roll on the material. If the material is too slippery and/or the rollers are too smooth the material to be pelleted will not be entrained and the equipment may stall. If this tends to happen with a particular material or application, it is preferable to select a pellet mill with V-belt drive rather than a directly gear driven machine, as slippage of the belts may act as additional safety precaution. Often the rollers are also furnished with an abrasion resistant coating to extend their life. Pellet dies should resist wear, corrosion, and breakage. Because the extrusion bores must be machined economically, selection of the material of construction to meet all require-
8.4 Pressure Agglomeration Technologies
Fig. 8.55: Ring die and three press rollers with different surface configuration (courtesy CPMRoskamp Champion, Waterloo, IA, USA).
ments is limited. Nevertheless, the use of modern alloyed steels, which are selected with a particular application in mind, has increased the life expectancy of dies considerably, particularly if also good equipment maintenance is provided. Tab. 8.3 lists, as examples, the technical data of one manufacturer of pellet mills. Rather typical equipment characteristics are: die inner diameters (ID) from 419 to 1,143 mm, working widths from 79 to 360 mm, pelleting surface areas from 0.1 to 1.29 m2, number of press rollers: (l),2, or 3 , roller outer diameters (OD) from 206 to 457 mm, and power requirements from 160 to 620 kW.
Fig. 8.56 Typical cylindrical animal feed pellets.
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8 Pressure Agglomeration Examples of technical data of pellet mills offered by one manufacturer (according to Sprout-Matador, Muncy, PA, USA)
Tab, 8.3:
Parameter
Unit
Characteristic
Comment
149 Ultra-V All grease 419 or 533 79-152 0.1-0.26 2 or 3 254(2) or 206(3) Gravity Centrifeeder
Max.allowable
186 Direct gearing Cartridge Tapered die fit Gearbox Main shaft 533 108 or 152 0.18 or 0.26 2 or 3 254(2) or 206(3) Gravity Centrifeeder
Max.allowable
Model V-200 Series - V-Belt Drive Pellet Mills Main drive mot01 Drive type Lubrication method Die sizes
Number of rollers Roller size Die feeding
kW
ID Operating width Working area
mm mm m2
OD 2 roll design 2/3 roll design
Single reduction V-belt (No messy oil changes)
3 minimize stress
Model 21 -250 Series - Gear driven Pellet Mills kW
Main drive motor Drive type Quick die change Lubrication method Die sizes
Number of rollers Roller size Die feeding
ID Operating width Working area
mm mm m2
OD 2 roll design 2/3 roll design
mm
Single reduction helical Optimum wear Inherent piloting effect Circulating oil cooled/filtered Grease
3 minimize stress
Model V-500 Series - V-Belt Drive Pellet Mills ~
~~
Main drive motor Drive type Jackshaft design
Quick die change Lubrication method Die sizes
Number of rollers Roller size Die feeding Mechanical design Maintenance
kW
ID Operating width Working area
mm mm mz
375 V-Belt
Tapered die fit All grease 660 or 812 102 - 305 0.21-0.78
Max. allowable Single reduction Eliminates belt pull on motor shaft Permits use of std. short shaft motors Inherent piloting effect (No messy oil changes)
3
OD
254 or 305 Centrifeeder Heavy duty Electric hoist
For severe stress operation Optional
8.4 Pressure Agglomeration Technologies Tab. 8.3: continued Parameter
Unit
Characteristic
Comment
375 Direct gearing Cartridge Tapered die fit Gearbox Main shaft GGO or 812 108-305 0.22 -0.78 2 3 254 or 304 Centrifeeder Cast Electric hoist
Max. allowable Single reduction helical For 660 m m ID dies For 660 or 812 mm ID dies Circulating oil cooled/filtered Grease
597 Direct gearing Tapered die fit Gearbox Main shaft 812 or 1,143 203 - 360 0.52-1.29 3 305 or 457 Centrifeeder Cast Electric hoist
Max. allowable Single reduction helical Inherent piloting effect Circulating oil cooled/filtered Grease
Model 500 Series - Gear driven Pellet Mills kW
Main drive motor Drive type Quick die change Lubrication method Die sizes
ID Operating width Working area
mm mm m2
Number of rollers Roller size Die feeding Mechanical design Maintenance
OD
Light duty Heavy duty
Internals: heat treated steel Optional
Model 800 Series - Gear driven Pellet Mills
kW
Main drive motor Drive type Quick die change Lubrication method Die sizes
Number of rollers Roller size Die feeding Mechanical design Maintenance
ID Operating width Working area OD
mm mm m2
Internals: heat treated steel Optional
All features depend on the application and the feed properties. Capacities, although strongly influenced by the material as well as the pellet size and shape, may be as low as a few hundred kg/h and, on the high end, exceed 80 t/h. As visible from the dies shown in Fig. 8.50, most of the extrusion channels in pellet mills are cylindrical bores; however, as depicted in the center of this Fig. (&SO), square openings (and others) are provided for specific applications (for example, the manufacturing of catalyst carriers). As a typical product of pellet mills, Fig. 8.56 presents some cylindrical animal feed pellets.
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Equipment Using Flat, Perforated Die Plates and Press Rollers The above general designs of medium-pressure agglomerators (extruders) “suffered” from a potential lack of structural integrity if larger and/or denser and stronger pellets had to be produced, from feeding difficulties when trying to distribute the mass to be agglomerated evenly, and, sometimes, from leakage of “fines”as well as a limited possibility to control the densification of the feed and influence the extrusion process. Equipment that uses flat, perforated dies and press rollers can overcome most of these shortcomings. The basic principle of flat die pelleting machines was already shown in Chapter 6, Fig. 6.4b.2 and above, Fig. 8.40b as well as Fig. 8.42a. Fig. 8.57 is the photograph of some flat dies and of several press roller arrangements. The drawing in Fig. 8.58 demonstrates how the individual items fit together. As can be deduced from both figures, the machine may be constructed in a very heavy duty execution which allows the build-up of high pressures for the production of highly densified pellets, even from difficult to extrude materials. The perforated flat dies can be quite massive with good structural support. The roller arrangements consist of a minimum of two rollers. With larger machines, this number can increase to a max-
8.4 Pressure Agglomeration Technologies Range of technical characteristics of flat die pelleting ma. chines [not including low pressure applications (Section 6.4.1) with perforated thin sheet dies] (according to Amandus Kahl, Hamburg, Germany).
Tab. 8.4:
Parameter
Unit
Characteristic
Outer diameter of the flat die Medium roller track diameter Track width = Roller width Roller diameter Number of rollers Open track area = C hole area Specific open track area Extrusion channel (= hole) dia. Flat die plate thickness Circumferential roller speed Speed of roller assemblies Motor power (press only) Specific press motor power Throughput (dep. on product)
mm mm mm mm
175-1,250 125 - 1,026 22 - 200 130 - 450 2-6 90-6,220 20-30 2-40 20 - 200 0.6 - 2.7 50-160 2 - 400 5-80 50-> 30,000
cm2 cm2/kW mm mm mis rpm of main shaft kW kWjt kgih
-
imum of six. As the number of rollers is directly proportionate to the capacity (throughput)of the machine, depending on the material and the pellet diameter, production rates per machine can reach more than 30 t/h. Including a laboratory press, the drive motor power may be between 2 and 400 kW. Tab. 8.4 summarizes the range of technical characteristics of flat die pelleting machines. Fig. 8.59 is the partial cut through a Kahl flat die pelleting machine with explanations. Because the principle of rollers running on the bottom of a flat pan has been known for grinding long before the flat die pelleting machine was invented and, in milling, this device was called “pan grinder” or “muller” the press rollers are sometimes identified as “pan grinder rollers” or “muller wheels”. Referring to Fig. 8.59, feed (called product) enters the machine at the top and falls down by gravity. The flat top of the hydraulic roller adjustment device rotates with the speed of the roller assembly and diverts the material into the annular space that is bordered by the cylindrical guide plate. In some cases a cone is mounted on the top to avoid build-up of material. The feed, thus diverted, falls rather evenly onto the rollers and the track of the die, i.e. that portion ofthe plate that is perforated and over which the pressing tool rolls (see also Tab. 8.4), so that uniform densification and extrusion occur. A scraper removes feed that may have been pushed to the housing wall and directs it back to the track. Below the die plate, knives (called cutting devices) rotate with the roller assembly and shear the protruding strands into pellets. The product falls on a plate, is collected by a wiper arm and deposited into a discharge chute through which it leaves the machine. Fig. 8.59 shows the drive situation that is most often used and is applicable for “normal” materials which form discrete pellets with little stickiness. The main shaft that carries and rotates the wiper arm, the cutting devices, the roller assemblies, the scraper, and the hydraulic roller adjustment is driven by a worm gear from below.
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If the material to be extruded is very moist and/or sticky the cutting device would lump the strands into globs of wet material and the discharge plate with wiper bar would further aggravate this situation. A special design is available in which the main shaft is driven from the top. Then, the wet strands fall directly into a dryer or on a belt, thus minimizing aggregation (Fig. 8.60). As in all medium-pressure agglomerators that use rollers for the densification and extrusion of moist and/or plastic materials, the primary cause for roller rotation is friction between the roller surface and the material to be processed. The roller is mounted on a shaft with sealed antifriction bearings and is brought to close proximity
8.4 Pressure Agglomeration Technologies
with the die plate. Feed is wedged between the roller and the die (see Fig. 8.40 and 8.41) and the roller rotates due to the friction between its surface and the material. Roller rotation is necessary for proper operation (see above). It has been discussed previously, that, for several reasons, a gap is provided between the roller and the die. An important process feature is the layer remaining after extrusion which binds easily with fresh feed and is predensified thus increasing pellet density and, with it, quality.
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Fig. 8.61: Sketch o f the Kahl hydraulic roller adjustment device (“hydraulic nut”). On the left, the roller has been lifted to maximum clearance and, on the right, hydraulic pressure has reduced the gap against the force o f the springs (courtesy Amandus Kahl, Hamburg, Germany).
In Kahl flat die pellet presses, a unique feature is the hydraulic roller adjustment device. As depicted in Fig. 8.61, the roller assembly can slide up and down the main shaft and is supported by strong springs. A so called “hydraulic nut”, a hydraulic cylinder arrangement, with which the rollers can be pushed down or lifted, is installed at the end of the shaft. This allows to adjust the clearance or gap between the rollers and the die to optimize densification and extrusion characteristics for different materials and to modify the thickness of the predensified layer after the extrusion step. Since adjustments can be made anytime, even during operation, it is possible to optimize press performance as needed. To avoid slip ofthe rollers, the surface is axially grooved as shown in Fig. 8.62. Other surface configurations are also available; all serve to increase the drag and avoid slippage. For some difficult to process, very slippery materials rotation of the rollers can not be reliably guaranteed. In those cases, the rollers can be driven by a direct bevel gear drive (Fig. 8.63). Obviously, a vertical roller adjustment is not possible with this drive arrangement and, therefore, is not shown in Fig. 8.63. If a cylindrical pressing tool rolls over a circular track, shear is caused in the material that is wedged between the roller and the die because the circumferential speed of the main shaft rotation only matches that of the roller(s) at one point of the roller surface. The inside edge turns faster and the outside edge slower than the overall rotation of the tool. This additional shear may be of advantage for some materials to be processed, but for others, for example those exhibiting thixotropic properties, this speed gradient must be avoided. The uneven speeds also cause more wear than would be experienced with a uniform movement. To overcome this problem, conical press rollers are used when necessary (Fig. 8.64).As shown in the schematic these rollers as well as cylindrical ones can be equipped with channels for cooling with a liquid. Another, rather unique process feature of flat die pellet presses is that excessive amounts of liquid can be mechanically removed from the feed during the predensification step. By providing a space where liquid can collect within the housing at the periphery of the flat die and installing drain lines, some liquid can be squeezed out. Although this is not a major application for this type of equipment it is worth mentioning.
8.4 Pressure Agglomeration Technologies
Fig. 8.62: Photograph of a Kahl flat die with the hopper housing removed showing a roller arrangement with four grooved rollers, the die plate, the scrapers, and the hydraulic nut (courtesy Amandus Kahl, Hamburg, Germany).
Tab. 8.5 lists the technical data of machines that are offered by a manufacturer of flat die pellet presses. For ranges of typical equipment characteristics, reference should be made to Tab. 8.3 above. While this type of equipment is also widely used for the production of animal feed, the ability to exert higher forces and the availability of special machine features (not all of which have been mentioned in this section) make these pellet presses amenable to applications that can not normally be handled with the ring die models which were discussed earlier. Of particular interest in this respect are many difficult to handle waste materials that need to be transformed into a relatively large particulate shapes for reuse as secondary raw materials. Fig. 8.65 shows a few examples.
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Fig. 8.63: Photograph and schematic o f a positively driven roller arrangement for the processing of slippery materials (courtesy Amandus Kahl, Hamburg, Germany).
Pellet Presses With Gear-Shaped Press Rollers and Feed From the Outside So far, all medium pressure extruders or pellet mills had in common, that the feed material is able to “avoid the squeeze” and, as a consequence, will not always extrude, resulting in clogging and stalling of the machine. Different surface configurations of the pressing tools are used to increase the drag into the nip and changes in the feed formulation may be made to render the material less slippery. However, in any case there is a more or less pronounced shear and back-flow in the nip that causes frictional heat which may be objectionable and normally can not be controlled by cooling the operational parts, which is possible with some machines. In the patent literature, several toothed, intermeshing pressing tool and die arrangements have been proposed [B.42]. However, to the knowledge of the author, only one has gained commercial importance and is offered as standard equipment. Fig. 8.66 shows as a schematic, published by the manufacturer, what had already been presented in Chapter 6, Fig. 6.4b.6 and mentioned several times before. This pellet mill, the so called gear pelletizer, features two intermeshing hollow gears with large modulus and extrusion channels at their roots between the teeth. Feed enters by gravity from the top, is caught by the teeth, entrapped between the teeth, densified and then extruded into the gear’s interior by the punch-like action of the opposing tooth. Inside,
8.4 Pressure Agglomeration Technologies
Fig. 8.64 Photograph and schematic o f a roller arrangement with conical pressing tools (courtesy Amandus Kahl, Hamburg, Germany).
protruding strands are often cut into pellets by scraper blades or they break under their own weight. Because of the positive feeding and displacement that is caused by the teeth, the material to be processed experiences very little shear due to back flow. Therefore, the gear pelletizer is ideally applied for temperature sensitive and thixotropic materials as well as for low melting point and waxy products. Widely used for the pelleting of rubber chemicals, foodstuffs, and pharmaceuticals, it is also particularly effective in applications requiring medium pressure and small diameters (see also Fig. 8.39 and corresponding text). Smaller extrudates may be spheronized to obtain final agglomerate shape. As shown in Tab. 8.6, for manufacturing and process reasons, the gear diameters are relatively small and the working width is limited due to potential difficulties in discharging extruded products from the depths of wider hollow gears. Throughput capacity is also relatively low because only one extrusion occurs per revolution of the gear. In this respect the HUTT gear pelletizer is similar to the Alexandenverk moist granulator (see above). With some feeds it can also not be excluded that, after extrusion, some material drops from the space between the teeth, thus requiring periodic removal of densified fines from under the machine. As depicted in Fig. 8.67 the gears are connected to a double output-shaft gear reducer which is normally powered
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8 Pressure Agglomeration Technical data of pellet presses with rollers and a flat die (according to Amandus Kahl, Hamburg, Germany).
Tab. 8.5:
Parameter
Unit
Model
17-250
24-390
25 - 500
Laboratory
Small Prod.
Overhead
175 130127
390 200175 2 37 2.11 617 1,800
0v erh ead 500
3 0.5-1.3 106 260
250 160/35 2 7.5 1.27 203 450
200/75 4 37 2.7 844 1,800
with motor
with motor
wjo support
w/o support
33-390 or -500 34-GOO
38 -GOO
38 - 780
Standard 390 or 500
14-175
Designation Die diameter Roller diameterlwidth Number of rollers Power of drive motor Roller speed Perforated die area Approx. machine weight
mm mm
L
kW mls cm2 kg
Designation Die diameter Roller diameterlwidth
mm mm
Number of rollers Power of drive motor
kW
Roller speed Perforated die area Approx. machine weight
Designation Die diameter Roller diameterlwidth Number of rollers Power of drive motor Roller speed Perforated die area Approx. machine weight :;'
mi s cm2 kg
mm mm kW m/s cm2 kg
Standard
Standard
Standard
780
230177
600 2801102
2 or 3 or 4 1 5 - 30>k
3 or 4 45-55wr
3 or 4 55 -75"
780 280 or 3501 102 4 or 5
2.2 617 or 840 990 or 1,300
2.7 1,382
2.6 1,382
2.6
with motor
max. 2,430 with motor
max. 2,430 with motor
3,400 wjo motor
37-850
39-1000
45-1250
GO - 1250
Standard 850
Standard 1,000
Standard
Double drive
3501130 3-5 132:': 2.5 2,695 4,600 w/o motor
4501156 3-5 160 - 200" 2.6 5,400 5,400
1,250 4501192 or 1156 4 or 5 200- 250" 2.7 5,900 9,000
1,250 4501192 4 or 5 2.160-200" 2.6 5,900 9,370
w/o motor
w/o motor
w/o motors
2801 102
90 - 100" 1,916
Motor rpm 1,500, other speeds possible; -I;;? Motor rpm 750
by a mechanical (shown) or electrical variable speed drive. The gears are mounted to the cantilevering shafts (CS models) which, for larger extrusion forces, are held parallel by additional support bearings in front (as shown in Fig. 8.67), further obscuring the discharge area. The largest gear pelletizer is built as a mill shaft design (MS) machine (for details on such designs see Section 8.4.3, roller presses).
8.4 Pressure Agglomeration Technologies
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8 Pressure Agglomeration Tab. 8.6 Some technical data of standard “Gear Pelletizers” (according to Hosokawa Bepex/HUTT, Leingarten, Germany). Parameter
Gear diameter Gear width Drive power Weight
Unit
mm mm
kW kg
Model CCS200/40
CCSZOO/80
GCS300/80
GCS300/120 CMS300/200
200 40 4
200 80
300
7.5
300 80 11
1s
300 200 22
800
1,000
3,000
3,200
3,800
120
Fig. 8.67: Photograph o f a typical CS model “gear pelletizer” (courtesy Hosokawa Bepex/ H U T , Leingarten, G e r m any) .
Medium Pressure Axial Screw Extruders Axial screw extruders normally operate with low (see Section 8.4.1) or high (see Section 8.4.3) pressure. The basic principle of an axial screw extruder was shown in Fig. 6.4b.l (Chapter 6) and Fig. 8.24 (Section 8.4.1). The pressure that is developed by the screw(s) depends on the power of the drive and the frictional resistance in the extrusion channel or other discharge device. Axial screw extruders that rely solely on the pressure developed by the rotating screw(s) employ hydrostatic pressure as the driving mechanism for extrusion. Such machines generally use high pressure. However, under certain conditions and for specific applications, some can be classified as medium-pressure agglomerators. Three different medium pressure axial screw extruders will be discussed in the following as examples. Fig. 8.68 is a schematic representation of an “Extrud-0-Mix”.It is designed to process plastic masses or generate its own suitable conditions by mixing and working solids and additives (binders and/or lubricants, see Section 5.1.2) prior to extrusion. The “Extrud-0-Mix”features a single horizontal shaft which carries rows of paddles that are arranged in a spiral pattern and move the material(s) to be processed through a cylindrical housing. Stationary blocks (called anvils) attached to the inside wall of the housing are inserted between the rotating paddles. As the blades pass the anvils, a portion of the material is moved forward and the remainder lags behind. The
8.4 Pressure Agglomeration Technologies
blocks also prevent the material from rotating with the shaft so that a continuous kneading and mixing action occurs. Furthermore, as indicated in Fig. 8.68, various orifice plates may be installed within the barrel to increase the uniformity of the feed mixture. Fig. 8.69 is a view into the open housing of an “Extrud-0-Mix”showing the different parts. At the end of the barrel a final die plate is located through which the completely processed material is extruded into pellets with various shapes and sizes. A cutting device may be used to control the length of the product particles. Normally, extrudates are cylindrical with diameters between 0.5 and 6 mm. Several equipment sizes are offered with capacities ranging from approx. 350 kg/h to 4.5 t/h and drive power ratings from 7.5 to 100 ltw. Other medium-pressure agglomerators that are offered by several manufacturers and are mostly used in the food and animal feed industries are pressure cooker extruders.They apply medium pressure because the mostly grain and/or vegetable based starchy, organic feeds are conditioned by pressure and heat into easily deformable and extrudable masses. Fig. 8.70 is a drawing depicting the functional components. At the inlet on top ofthe equipment, the slightly premixed feed components enter first a mixing and predensification screw. Pressure is build up by the changes in shaft diameter, sometimes variable pitch of the screw flights, and by a collar at the end of the screw shaft (see inset in Fig. 8.73, below) or a “pressure piece”, both forming an annular space with reduced area through which the material must pass. Such a screw conveyor, mixer, and processor is often called expander. The so processed material drops into the cooker in which pressurized steam is injected and paddles or screws move the material to accomplish optimal contacting. The
Fig. 8.68 Schematic representation of an “Extrud-0-Mix” medium pressure axial extruder (courtesy Hosokawa Bepex, Minneapolis, MN, USA).
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Fig. 8.69: Open top view of an "Extrud-0-Mix" showing the working parts exposed (courtesy Hosokawa Bepex, Minneapolis, M N , USA).
previously mentioned plug at the end of the mixing and predensification screw acts as a dynamic seal so that the cooker is kept under pressure. Fig. 8.70 depicts a single cooker vessel with the indicated dimensions and paddles. However, depending on the capacity and cooking time required for a specific application, the cooker vessel dimensions and/or number (multiple ones are mounted on top of each other, see Fig. 8.73 below) as well as the type of agitators may be changed. By varying the speed of the agitator(s) and the operating pressure in the cooker, almost infinite time/temperature combinations can be obtained. Pressurized steam cooking decreases work and power use, cuts production costs, and increases production capacity as much as 50 % of other extruders that accomplish heating by the conversion of mechanical into thermal energy through friction. The pressure (steam)cooker accomplishes much
Fig. 8.70
Schematic drawing o f a typical pressure cooker extruder (model TME 2000) with main dimensions in feet (') and inches (") (courtesy Sprout-Matador, Muncy, PA, USA) 1' = 0.3048 rn, 1" = 25.4 mm.
8.4 Pressure Agglomeration Technologies Examples o f technical data o f pressure cooker extruders (according to Sprout.Matador, Muncy, PA, USA).
Tab. 8.7:
Parameter
Model
Unit
TME2000
Dia. of mixing screw Mixing screw drive Cooker assembly Cooker drive Water injection system Steam addition system Electric control panel Extruder drive assembly Extruder diameter Segmented extruder barrel Insert type extrusion die Adjustable knive cutter assbly Cutter drive Construction Total electrical req. Steam requirements Water requirements
mm kW mm kW
kW mm
kW kW
TME1500
MDL450
200 (240 optional) 200 160 7.5 3.5 11 (7.5) 760 dia. x 3,020 760 dia. x 2,100 530 dia. x 2,130 5.5 5.5 2 Standard Standard Standard Standard Standard Standard Standard Standard Standard 150 110 55 200 110 200 Standard Standard Standard Standard Standard Standard Standard Standard Standard 5.5 5.5 3.5 Stainless Steel in Product Zone 167 (170) 130 67 10 kg steam/1,000 kg material, dry basis 8 to 12 % of dry feed, weight basis
(Dimensions were converted to the metric system from nonmetric figures.)
of the work that is conventionally done by the extruder; therefore, the life span of the extruder screw, inserts, barrel, die plate, and bearings is significantly increased. The cooker is easy to maintain and operate and, because the agitators are of simple design, these may be rebuilt numerous times to regain critical clearances.
Fig. 8.71: Photograph o f the pressure cooker extruder depicted in Fig. 8.70 (courtesy Sprout-Matador, Muncy, PA, USA).
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8.4 Pressure Agglomeration Technologies
The processed mass is transferred into the screw extruder where hydrostatic pressure is developed which causes axial extrusion through the openings of the die plate. The orifices produce extrudates which often feature different cross sections (for example [see also Fig. 8.741, for processed cereals tubes yielding rings and for dog food a bone shape may be used). The ropes are cut with an adjustable and, for cleaning purposes, replaceable device (lower right in Fig. 8.70) from which they discharge for further post-treatment (e.g. drying and cooling). Fig. 8.71 is a photograph ofthe equipment presented in Fig. 8.70. Tab. 8.7 summarizes, as examples, the technical data of three pressure cooker extruders. Fig. 8.72, the front view of a similar machine with the cutter device removed, shows the end ofthe extruder and Fig. 8.73 is the schematic of a three barrel pressure cooker with screws as agitators. Fig. 8.74 depicts some products obtained with pressure cooker extruders. Sometimes, the lumpy shape ofthe processed (often called “expanded”)mass can be directly used in a post-treatment facility (for example, “puffing” snack pieces during drying); in that case rotating cell wheels, that also act as pressure seals, are applied to discharge the processed mass (lower right end in Fig. 8.73).This idea has been recently modified by another manufacturer of expanders. Fig. 8.75a is an artist’s conception of the annular gap extruder, another medium-pressure agglomeration device, showing the steam manifold and feed lines, indicating the turbulent movement of the particles inside the barrel as well as their expansion and mass densification, and demonstrating the extrusion of material through the annular space at the discharge end. The cross section of the annular gap can be changed hydraulically, even during operation, by moving the conical piece that creates the back pressure in or out of a beveled seat. Fig. 8.75b depicts an optional execution of the discharge end whereby two different, hinged discharge configurations can be attached alternatively to the extruder.
Fig. 8.75: (a) Artist’s conception o f the operation o f an annular gap extruder. (b) Optional configuration o f the discharge end: (1) beveled seat and perforated die plate (retracted) with cutter, (2) “standard” conical resistance piece and seat, defining the annular space and backpressure (courtesy Amandus Kahl, Hamburg, Germany).
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High-pressure Agglomeration
Once again referring back to Section 8.1, Fig. 8.1, and the related text, high-pressure agglomeration is characterized by densification that is accompanied by particle deformation and destruction, often requiring very high specific forces. Therefore, particulate solids of any kind and size can be processed, making this technology the most versatile of all the various agglomeration techniques. As long as feed particles can be fed into the high pressure zone of the agglomeration equipment and high enough forces can be exerted without destroying the machine, there is no maximum size limit. Large pieces will either be deformed or broken down and incorporated in the structure of the agglomerate. However, if the feed only consists of large particles and no or not enough fines are present to fill the large pores, deaeration may require a long time if brittle pieces suddenly collapse or may be not at all possible if the deformation of large plastic particles results in closed pores in which gas becomes trapped. On the other hand, if very fine particles are densified into the bulk compression stage (see Section 8.2), gases can not be easily expelled from the diminishing pore space because of high resistance in pores with extremely small diameters and, since small particles feature high strength (see Section 5.4), a considerable elastic deformation occurs. Therefore, generally, when pressure release begins after compaction, gas must have been completely removed from the structure to avoid destructive expansion of compressed gas pockets and the energy of elastic deformation must have been converted into other forms of energy as much as possible to avoid detrimental elastic spring-back. As mentioned several times before, the speed of densification or the rate of pressure rise is the most important parameter for successful high-pressure agglomeration. In some cases, repeated densification prior to final discharge or an extended period during which the maximum pressure is applied will reduce the effects of high speed densification while such corrective measures are not possible with other high-pressure agglomeration methods (see also Section 8.1). Since speed of densification is directly proportionate to the production capacity, its limitation, determination, modification, and optimization is of the greatest importance for all high-pressure agglomeration techniques. High Pressure Axial Screw and Ram Extruders Extrusion is possible by low, medium, and high pressure. In all three cases the same underlying basic principle is responsible for the agglomeration process. Tab. 8.8 lists the requirements. The major difference between low and medium pressure extruders is the execution of the machine. To exert and contain the forces that are necessary for high-pressure agglomeration of masses which consist to a large extent of particulate solids, the drives must be capable of delivering high torque, the processing chambers as well as mixing and densification tools must be heavy duty, and the orifice(s) must produce a high back pressure. To avoid the development of closed pores that are filled with residual pressurized gas, the particulate mass must be kneaded, densified, and degassed, often by applying vacuum, prior to entering the hydrostatic zone in which the pressure in the mass is equalized.
8.4 Pressure Agglomeration Technologies Tab. 8.8:
Requirements for the agglomeration of particulate solids by
extrusion. Material
Particulate solids must be premixed to match formulation. Size of solid particles should be smaller than cross section of orifice Particles to be plastic or becoming deformable during conditioning. Interparticle friction must be small or decreased by lubricants. Conditioned mixture to be de-gassed and plastic (deformable). Binding characteristics must be inherent or caused by binders. Equipment
Mixing, conditioning, and degassing must be accomplished by suitable tools. Pressure must be built up in the conditioned and degassed mass. Special extrusion tools may be necessary to feed the mass to the orifice(s.) Orifice dimensions define resistive force and cross section of extrudate. Orifice design must enable flow and assist in pressure release at discharge. Cutting devices may be necessary or desirable to divide strand into pellets. Drive must be capable of sustaining mixing, conditioning and extrusion. Individual process steps may be carried out in-line in separate equipment.
Fig. 8.7Ga is a schematic cross section through an extruder with vacuum degassing. It shows first the end of the mixing/conditioning equipment (in this case an open pug mill but more commonly a closed paddle or screw mixer/ conditioner [see, for example, Fig. 8.70, Section 8.4.21) in which the feed is prepared for extrusion. The mass is then densified in a pug sealer in which a compacted plug of material is formed that provides a dynamic seal against the vacuum that is created in the feed chamber of the extruder. While dropping into the extruder screw by gravity, gas is removed from the particulate mass. Alternatively, there is a vertical feeder/sealer (Fig. 8.7Gb) for smaller capacities which mounts directly to the vacuum chamber of the extruder and accomplishes a similar degassing effect. The characteristic pressure distribution within the axial high pressure screw extruder is depicted in Fig. 8.77. The pressure zones that are identified in this figure can be described as follows [8.2]. Zone 1: Up to line A, this is the feeding area. In this conveying zone, the material is still relatively loose and moves along the barrel essentially without any densification. There is little pressure build-up and the bulk density remains largely unchanged. Zone 2: From lines A to B is the densification region of the screw and the loose material becomes compacted. Zone 3: From line B to the end of the continuous screw flights metering takes place. This results from the number of partial flights or “wings” on the screw shaft tip. The double or “split” wing, as shown in Fig. 8.77, is by far the most common execution. Split wing tips are used to attenuate the unbalanced flow which is coming off the screw.
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Fig. 8.76 Schematic cross section through an extruder with vacuum degassing 18.21. (a) Horizontal pug sealer, (b) vertical pug sealer.
8.4 Pressure Agglomeration Technologies
Rf = Resistive force due to sliding friction of material on barrel, augers, and die surfaces. Rs = Resistive force due to shear of material. Fig. 8.77: Depiction o f the characteristic pressure distribution within an axial high pressure screw extruder [8.2].
Zone 4: This zone, from lines C to D, is a space in which the pressure in the material that is delivered by the metering device is more evenly distributed. In this volume, the previously mentioned hydrostatic pressure is achieved. Zone 5: The resistance to flow in the die, from D to E, results in a pressure drop to atmospheric pressure at its front (discharge) face. This pressure drop depends on the frictional resistance of the die, the volumetric rate of the material flowing through the die, and the rheological properties of the mass.
Fig. 8.78: Sketches o f extrusion plates. (a) Single, machined die plate, (b) sandwich die plate [8.2].
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Fig. 8.79: Two methods of dividing single strands with large cross section into smaller ropes [8.2].
In high pressure axial screw extruders, dies and their orifice(s) can have different shapes and sizes. Their selection depends on the plasticity and flow properties of the materials to be extruded and, obviously, on the desired product shape and size. Two basic types of dies are used in extrusion. For the formation of small agglomerates, such as strands or pellets, extrusion plates are used. Such plates can be simple or more complex, the latter, so called sandwich types, consist of multiple plates that are made of different materials. Fig. 8.78 presents sketches oftwo extrusion plate designs. Parameters are based on manufacturer know-how as well as vendor experience from different applications and can not be discussed here.
Fig. 8.80 Schematic presentation o f the lubri. cation o f single stream tapered dies 18.21.
8.4 Pressure Agglomeration Technologies
Single stream tapered dies, as shown in Fig. 8.76 and 8.77, are used for shapes such as bricks and, using mandrels, for the production of tubes or cored blocks. However, this type of die may be also acceptable for the production of smaller agglomerates. As shown in Fig. 8.79, some simple methods exist by which an extruded strand with large cross section can be divided into several ropes. Cutters can then be employed to yield smaller extrudates. If the material to be processed exhibits properties that make it suitable for successful extrusion with a single large orifice it is an advantage that less wear occurs because relatively little material contacts the orifice wall and it is further possible to lubricate the die (Fig. 8.80) resulting in still lower power requirement, still less wear, and a better surface quality of the extruded strand. Plasticizers or “extrusion aids” are commonly used in preparing materials for extrusion which are otherwise not sufficiently flowable and deformable. Many of these also act as binders and lubricants (see Section 5.1.2) and, quite often, water may be a cheap choice. The photographs in Fig. 8.81 show, as examples, two different executions of J.C. Steele’s (see Section 14.1) Model 90AD extruder. Fig. 8.81a includes a horizontal mixer/pug sealer and dual hinged dies while Fig. 8.81b shows a hydraulic die changer in which the die plates are not installed in the holders. Die changers are used to allow the exchange of plates with worn orifices with a minimum of downtime or to switch over from one cross section to another. Tab. 8.9 presents technical information on some
Fig. 8.81: Photographs o f two high pressure axial screw extruders showing different die changers. (a) Extruder (model 90AD with horizontal mixer/pug sealer and dual hinged mouthpieces (= dies), (b) extruder (model 90AD with hydraulic horizontal die changer; dies are not installed in the die holders (courtesy J.C. Steele, Statesville, NC, USA).
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8 Pressure Agglomeration Tab. 8.9: Technical information on some typical high pressure axial extruders and pug sealers (according to J.C. Steele, Statesville, NC, USA). Parameter
Unit
Model
(Model: rpm)
25A Extr.
75ADExtr.
90AD Extr.
(Hor./Vert.Pug sealer)
Screw shaJ
rpm rPm rPm
10-37
18-38
22-41
(25A 10-37/25A: 10-37) (75AD: 21 -41) (90BD: 25-44)
Motor power
kW kW kW
22-75
110-260
225 - 375
(25A 11-45/25A: 7.5 - 30) (75AD: 75-85) (90BD: 150-300)
Std. US bricks
/h
3, - 9,000
12, - 24,500
22, - 41,000
Capacity
tlh
25 - 54
48-90
10,000-20,000
11,000-21,000
Machine weight kg
5,000-6,000
typical high pressure axial extruders. Because one of the main uses of the equipment is the production of bricks, the number of “standard US brickslh” is included as a capacity figure. Other high pressure axial extruders are mostly applied for the processing of pastes and of plastic materials, particularly thermoplastic polymers. Because much specific work is done during the mixing, kneading, and plasticizing of these materials, instead of conventional continuously flighted screws, they often apply specially configured tools that are attached to single or twin shafts. They accomplish processing tasks as well as transportation and the development of extrusion pressure (Fig. 8.82). If the materials are sticky and/or plastic, the tools are intermeshing with stationary parts in the barrel of single shafted machines or with each other if twin shafts are applied. Close tolerances prevent build-up and result in a self-wiping cleaning action. With the use of highly wear resistant internal parts and dies, modern applications include those for the processing of masses consisting of or containing large amounts of particulate solids. Such machines can be classified as equipment for high-pressure agglomeration.
Fig. 8.82: Photograph o f the open barrel o f a double shafted high pressure extruder showing the processing elements for mixing and conveying (courtesy Readco, York, PA, USA).
8.4 Pressure Agglomeration Technologies
Extrusion, particularly if high pressures are applied, is particularly well amenable to the processing of elastic materials. As mentioned before (Section 8.1, Fig. 8.2b), the densification process builds-up pressure in the mass itself. With increasing length of the extrusion channel, which is synonymous with the application of higher force, it becomes more and more likely that extrudates remain in the die for an extended period before they discharge at the mouth. During this time additional degassing and conversion of elastic into plastic deformation can and, in most cases, will occur. The use of a “pressure chamber” between the end of the tool that provides forward transportation and the extrusion channel and the application of a long extrusion channel will result in the production of extrudates with minimum spring-back and/or expansion even if the feed was loose (i.e. containing much gas or, in other words, requiring a large percentage of densification) and had elastic properties. The above phenomenon is further enhanced if, instead of utilizing continuously operating screws or screw-like devices, the reciprocating movement of a punch is applied. Consequently, the so called Exter press, a horizontal ram extrusion press, was developed for the briquetting of peat. This highly elastic organic material which, after appropriate drying, becomes also very loose, was greatly desired as a cheap fuel when such material was thought after in large amounts for the quickly expanding use of the steam engine in industry and for locomotion [8.3]. The principle of the ram extrusion press (Exter press) is depicted schematically in Fig. 8.83. A typical feature is the horizontal extrusion channel which first converges somewhat to allow the development of sufficient pressure in the mass for initial bonding. The reciprocating punch presses feed against briquettes which were formed during previous strokes and remain wedged in the channel. During each stroke, fresh feed as well as all the other briquettes in the channel are compressed until the axial force becomes high enough to overcome the wall friction and a potential back pressure acting at the mouth of the channel. When this happens, shortly before the end of each stroke, the entire column of briquetted material moves forward and a briquette emerges at the discharge end of the machine. Fig. 8.84 shows the sequence of events during a briquetting cycle. The reciprocating motion is, for example, produced by an eccentric drive which is symbolized by the circular representation on the left. The diagram on the right indicates the development of force that is exerted by the ram onto the material to be briquetted. The figure is self-explanatory.Only a few important operating stages will be mentioned below.
Fig. 8.83: Schematic representation o f the Exter, reciprocating ram, or ram extrusion press [B.42].
‘Briquettes
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i Fig. 8.84: Sequence ofevents during a briquetting cycle in the ram press [8.3].
At position (3), lower left, the force produced by the ram has reached the level that is required to overcome the friction of all briquettes in the channel as well as, if applicable, the back pressure caused, for example, by the briquettes in a cooling channel. The entire line of briquettes moves forward with the force remaining approx. constant (position 4, upper right). During the back stroke, the energy of the drive is stored in a flywheel and made available later to overcome the deceleration/acceleration at the return points of the reciprocating motion and also to help during compaction.
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Bulk 'eed
Fig. 8.85:
Schematic representation of the decrease in elastic recovery and increase o f density o f a briquette during consecutive press cycles in a ram press [8.3].
8.4 Pressure Agglomeration Technologies
At the beginning of the back stroke (when the eccentric drive has passed position 4) and if elastic materials, such as peat or, generally, biomass are being processed, at first the ram face does not separate from the newly created briquette because of its considerable elastic recovery and expansion. At a typical rotational speed of the eccentric drive of 90 rpm, the duration of the compression phase is only 0.04 s. This time is too short to achieve total conversion of elastic into plastic deformation. Therefore, the elastic recovery during the back stroke is high. Without the characteristic of ram extrusion presses that, during each compression stroke, many briquettes that remain wedged in the extrusion channel are again loaded and compacted, whereby more and more permanent plastic deformation is obtained, successful briquetting of elastic materials would not be economically feasible. For example, in a punch-and-die press (see below) densification would have to occur very slowly and pressure would have to be held at maximum for a long time (dwelltime) to make sure that the conversion takes place during the single stroke and be completed prior to pressure release and removal of the compact from the die. As a result, only few compacts could be made per unit time in an expensive machine resulting in uneconomical operation. Fig. 8.85 is the schematic representation of the increasing density and the decreasing elastic recovery of a particular briquette during repeated pressing as it moves forward in the extrusion channel. It is important to note that, in contrast to the conditions in medium pressure extrusion presses where in longer extrusion bores of pellet mills material may similarly pass during more than one pressing event (see Section 8.4.2) but bind into a continuous extrudate structure, even after the first stroke the surface produced by the ram
Detail
Z
Fig. 8.86 Cross section through a modern ram extrusion press (courtesy ZEMAG, Zeitz, Germany).
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face is so highly densified that, during the next stroke and for phases 2 , 3 , and 4 in Fig. 8.84, it acts as the solid bottom of a confined volume densification chamber. During the entire production process, the surfaces of adjacent briquettes do not develop s i g nificant bonding. Therefore, upon discharge from the press mouth or, if applicable, the cooling channel, the product will separate into single briquettes. There are physical limits to the design of such machines because friction and drive power as well as overall stressing of the equipment increase quickly with channel length. In a technically feasible channel reaching the conditions of Fig. 8.85 may not be possible and, consequently, the briquettes will retain a certain elastic deformation. If suddenly released, the elastic recovery may be large enough to damage or even destroy the integrity of the product. Therefore, for most applications, a gradual release is provided by slowly increasing the cross section of the channel prior to product discharge (see also Section 8.4.2, discussion of Fig. 8.37 and 8.38). Fig. 8.86 is the cross section through a modern ram extrusion press. The upper channel wall is adjustable such that different release angles can be obtained. In addition, a flexible support system at this point serves as a safety device to avoid overloading due to tramp material in the feed or “overcompaction”. As compared with a closed mold (punch-and-diepresses, see below) in which a predetermined pressure is reached with no difficulty, in high pressure ram extrusion presses the situation is complex (see Fig. 8.85). The peak pressure that is developed at each stroke depends not only on the force exerted by the ram but also on the resistance to the forward movement of the briquettes in the extrusion channel as well as a potential back pressure. The two latter ones are influenced by the shape and length of the channel, the changes in cross section in relation to length, the smoothness of the channel walls, the nature of the material to be processed, including parameters such as temperature, structure, plasticity, etc., and, if applicable, the type and length of the curing (cooling) channel. The rate of pressure increase is also important. It depends on the stroke frequency and length as well as, again, the rather complicated relationship between the movement of the ram and the magnitude of the resisting frictional force between briquette and die wall and the force caused by the column of already compressed material that is
Fig. 8.87 Photograph showing different traditional (coal) briquette shapes that were produced with ram extrusion presses.
8.4 Pressure Agglomeration Technologies
Fig. 8.88: Partial view o f a row of twin (two channel) ram extrusion presses in a lignite briquetting plant (courtesy KRUPP Fordertechnik, Essen, Germany).
being pushed forward. These forces change with both the state of compaction and the rate of movement. As with all reciprocating equipment and after consideration of the above conditions related to forces, densification, permanent plastic deformation, and development of strength, capacity of ram extrusion presses is restricted. To overcome this limitation, multiple extrusion channels are used in a single machine and relatively large briquettes are made. Fig. 8.87 is the photograph of several traditional (coal) briquettes demonstrating the size and shape of such products. The approx. dimension of the one on the top left is 153 m m long x 67 m m wide x 45 m m thick and, if made binderless from German brown coal (lignite),it weighs approx. 500 g. Inspite of using multiple channels and producing large briquettes, industrial plants employ numerous presses (Fig. 8.88). The partial view of the “press house” of this lignite briquetting plant shows the discharge ends of eight double channel (twin) ram extruders. Fig. 8.89 demonstrates the crank shaft drive mechanism of a three channel ram extrusion press after removal of the crank case cover.
Fig. 8.89 Photograph of a triple (three channels) ram extrusion press with the crank case opened to show the three crank shafts (courtesy KRUPP Fordertechnik, Essen, Germany).
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Tab. 8.10 summarizes some technical information for high pressure ram extrusion presses. Tab. 8.10a presents machine details and Tab. 8.10b indicates the approximate briquette output per channel of the shapes shown at the top. In Tab. 8.10b “impact area” means the face area of the ram that is contacting (impacting) the material to be briquetted; it is approximately equivalent to the face area of the briquettes as represented by the shapes specified in the first three lines of Tab. 8.10b. As mentioned before, at typical ram speeds the contact time is so short for each cycle (0.04 s was mentioned, see above) that compacting is often referred to as being carried out by a blow. From its invention in 1857 to approx. the 19GOs, the Exter press was almost exclusively applied for the binderless briquetting of processed (partiallydried) peat and soft coal (mostly lignites). For several reasons, during the second part of the 20th century use of such briquettes became no longer acceptable and many of the classic manufacturers of ram presses, who were mostly located in Europe, folded or gave up Tab. 8.10: Technical information on some typical high pressure extrusion presses. (a) machine data, (b) briquette output (according to ZEMAG, Zeitz, Germany). (a) Model
PSA 200-1 PSB 200-1 PSC 200-1 PSA 400-1 PSB 400-1 PSC 400-1 PZA 300
Max. load [MNI
Fly wheel Largest speed width [rpml [mml
No. o f rams
2.5 2.5 2.5 3.8 3.8 3.8 3.0
120 120 120 120 120 120 120
4 2 2 4 2 2
(b) Briquet output
M..
Salon briquette
shape
Semi briquette
shape
Industrial briquette
shape
Impact area
211 211 211 315 315 315 273
cm2
used with presses of rated size
No. o f flywheels [approx. tons]
Press weight
2 1 1
102 51 51 132 67 67 147
2 1 1 2
4 0
.
Req. motor
[kW] 450 250 250 630 280-355 280-355 630
aooo......
Gljl82 H2/182
H2j210
)4/209
H3/273
H3/315
)5/261
125
115
120
136
158
170
200
200
200
200
300 400
300 400
300 400
400
Largest permissible contact pressureg
MPa
200
217,s
208
279
240
223,s
190
in contin. operation
MPa
140
140
140
195
168
155
133
Briquette output at 100/min and 45 mm thickness
t/h
7,45
($86
7.2
8.1
9.4
10
12
Briquette output at 75/min and 40 mm thickness
t/h
5
4.55
4.8
5.4
6.25
6.65
8
8.4 Pressure Agglomeration Technologies
this sector of their businesses [8.3].However, since the technology is available, mature, and has continued to be developed until a few decades ago, some companies found new niche markets, particularly in the field of biomass based solid fuels. The particular characteristic of this press type, is the possibility to successfully and permanently densify and shape materials that feature high elasticity or require the removal of large amounts of interstitial gas. Therefore, the reciprocating ram extrusion press and, after some modifications which particularly provide a longer extrusion channel, screw extrusion presses are also amenable for the briquetting of very fine powders that feature a certain amount of “lubricity” and of inert elastic materials which inherently contain binders or to which binders have been added. Typical examples for the latter are lignin in or for wood based products, either as a natural ingredient or as an added waste product (lignosulfonates from paper making), and natural sugars or the byproduct of sugar making (molasses)for bagasse or spent slices of sugar beets. For these applications new, typically less heavy duty machines were developed [B.22, B.411. Fig. 8.90 depicts the cross section through a ram extrusion press that has been modified for the briquetting of organic waste materials. In all major major components it resembles the machines that were described above and were based on the Exter press. The most obvious difference is the application of a vertical feed screw which is required to stuff the typically loose, voluminous feed material into the press chamber. Providing predensified material allows to produce briquettes with sufficient size (thickness) without excessive stroke length of the ram.
Fig. 8.90: Cross section through a ram extrusion press for the briquetting o f biomass [B.41].
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8 Pressure Agglomeration
In contrast to the intermittent build-up of pressure in reciprocating ram presses, screw extrusion presses operate by continuously forcing material into and through a long die channel. As a result, the material is experiencing pressure, that is caused by wall friction, for a long time while being pushed through the extrusion channel. During this period, densification, deaeration, and conversion of elastic into plastic deformation continue. The product is a continuous strand of compacted material that is normally cut into cylindrical pieces. For the briquetting of biomass or similar material, different designs of screw extrusion presses are on the market. They are: machines with conical or cylindrical screw(s) with or without externally heated dies. The conical screw press provides predensification before the material to be processed enters the extrusion channel where final density is obtained. The main disadvantage of this design is the severe wear in the screw if the material consists of or contains abrasive components. Fig. 8.91 is the cross section through a screw extrusion press with externally heated die. Most commonly, heating is accomplished by an electric resistance heater which is wired around the die. Heat is either used to activate the binder or a lubricant, to achieve more plasticity of the material during densification, or, at least, to begin drying off moisture. If heating is applied for the latter, a system of vents allows the steam that is generated to escape from the material. In this case, the process can accept raw materials with a free moisture content of up to 35 % without mechanically squeezing water from the structure or limiting densification by the development of hydrostatic pressure in the pores.
-D I E
i i7 SCREW
rE L E C T R I C
I
I
U
COIL
\
\
r
"OPPER
T
3 - Q MOTOR
1 B
U
1 biomass with heated die [B.41].
8.4 Pressure Aggiomeration Technologies I 3 1 5
Punch-and-Die Presses Punch-and-die presses for the compaction of particulate solids are the oldest (high) pressure agglomeration machines [B.42].The densification of powders in a totally confined volume is a well defined process (see also Section 8.1, Fig. 8.2a) and the products resulting from such compaction can and most often do feature excellent uniformity in size, shape, and even mass. Punch-and-die presses are used by numerous industries for a wide variety of purposes [B.42]. Today, the largest application, in terms of numbers of machines and compacts produced, is most probably in the pharmaceutical industry for the production of solid dosage forms. However, punch-and-die presses are also widely used by the ceramic, powder metal, confectionary, catalyst, and, to an increasing extent, the general chemical industries. Principally, the equipment can be divided into two main groups: Vertical and horizontal presses and the vertical equipment can be categorized into:
Reciprocating or single-stroke machines and Rotary machines. Vertical Punch-and-DiePresses
Reciprocating Machines Reciprocating punch-and-die presses operate with one upper and one lower punch in a single die (see Chapter 6, Fig. 6.5, upper right). They are mainly used for the production of large compacts or complex shapes where high pressure and/or low output are required (typically less than 100 compressions per minute). Machines using the reciprocating punch-and-die arrangement can be subdivided into two types: ejection and withdrawal presses. Ejection presses are among the most versatile machines. They are built as simple hand-operated equipment, with pressure capabilities of <20 MN/m*, and as highly complex units which may exert pressures of more than 1,000 MN/m2 and produce compacts with a very high degree of density and accuracy. Even hand-operated machines (Fig. 8.92) incorporate the features that are common to all ejection presses. Contrary to the situation shown in Fig. 8.92 where the pressing force is acting from below, Fig. 8.93 depicts a die (d), containing the material to be pressed (e),with an upper punch (c) as well as lower closing (4and bottom plates (g) that are placed between the hydraulic cylinder (a),operating from above, and a fixed press table (i) below. When the upper punch moves downward, compaction is carried out in the die cavity until a predetermined pressure is reached. After releasing the hydraulic pressure, the bottom plate (g) is removed and the compact is extracted from the die through an exit port (h)in the press table by again activating the hydraulic cylinder. For more specific, always recurring applications the die is mounted in the fixed press table and upper and lower punches are attached to moving rams. Fig. 8.94 demonstrates schematically the sequence of events in an eccenter driven ejection press. In such machines, the lower punch descends in the die to allow its filling with powder from a fill shoe (A). Often, all the compression is accomplished by the upper punch which is moving toward the stationary lower one (B+C).Later the lower punch ejects
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8 Pressure Agglomeration
Fig. 8.92: Hand-operated hydraulic platen press with the pressing force acting from below (courtesy Carver, Wabash, IN, USA).
n
Fig. 8.93: Schematic representation o f a simple ejection press [B.32]. For explanations see text.
8.4 Pressure Agglomeration Technologies
the compact upward from the die, the fill shoe moves it to a discharge chute (D+A),and the cycle begins again. As discussed in Section 8.2 (Fig. 8.3), interparticle and wall friction, force dissipation from particle to particle, and sliding under shear cause differences in density distribution in a compact that is produced in a die with one-sided compression by one of the punches. To obtain a somewhat more uniform structure, compaction can be carried out by both punches. If both move at the same rate and for the same stroke length, thus exerting identical forces, and assuming uniform filling of the die, a mirror image of the density distributions that were shown in Fig. 8.3 (Section 8.2) develops along a neutral plane which, under those conditions, is located in the middle of the compact. Machines that operate in this manner may be identified as presses with “double pressure” (see also below). A large variety of ejection presses has been developed for different applications. They vary in the size and complexity of products that can be made and in the amount of pressure that may be exerted during the formation of compacts. Another differentiation is the type of drive that is used to move the punches. Small machines are often hand-operated hydraulic presses (see Fig. 8.92) or the platen is actuated by moving it up and down with a ball screw drive. Larger machines use mechanical or hydraulic drives. Fig. 8.95 shows schematically the principles of the most common drive arrangements [B.25, B.421. In practical terms, apart from the output, the effectiveness of mechanical and hydraulic systems is equal. The cycle time of hydraulic presses varies with the stroke. The low pressure portion of each stroke can be made quite fast by using a multistage pump but, as the higher pressure cuts in, the remainder of the stroke becomes progressively slower. The length of the high pressure stroke depends directly on the thickness of the piece which is being pressed. In addition, when it is used near maximum pressure, the pumping system of the hydraulic press can rarely achieve a cycle time that is comparable with that of a mechanical press.
Fig. 8.94: Diagram depicting the operational stages o f an ejection press [B.32].
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,-Main
bearings
(a.1)
‘Crosshead
I
Eccentric shaft
Cronkshoft
Crosshea
Accumulator
?\
oll I
.
(4
Safety valve /&Hydraulic
cylinder
T ,-
c , m1 ~ , I
I
-
P
I
*Crosshead
Control valve
Return lines O i l pump a n d reservoir
Fig. 8.95: Schematic represen. tations o f the most c o m m o n drive systems for reciprocating powder presses [B.25, 8.421. (a) Mechanical drives, (a.1) eccentric or crank drives, (a.2) toggle or knuckle drive, (b) hydraulic drive.
8.4 Pressure Agglomeration Technologies
Therefore, mostly because of their low output, the use of hydraulic presses is restricted to the ceramic and powder metal industries where high compacting pressures are required. The disadvantage of all mechanical punch drives that are shown in Fig. 8.95 (a.l and a.2) is that, while the compression speed becomes smaller as the eccentric connection of the rotating drive member approaches dead center, overall compaction takes place very quickly and is associated with a sudden release of force after reaching the maximum. This is a particular problem if the material to be processed is very fine and aerated or features elastic properties. Such products reach sufficient deaeration or permanent plastic deformation and strength only after comparatively slow densification and/or remaining under pressure for some time (see also Section 8.1).Fast compaction and/or premature pressure release result in excessive expansion of the product which may destroy its structural integrity and result in well known failure modes (cracking, lamination, etc.) indicating “overpressing”. The only reliable means to overcome these problems in punch-and-diepresses is to employ hydraulic actuation of the punch(es) (Fig. 8.95b). The timing of the punch strokes as well as the rate of increasing or decreasing pressure and the “dwell time” can be easily adjusted. In addition, hydraulic presses typically feature overload protection by gas filled accumulators and, because there is no physical limit to the length of the stroke, densification ratios can be very high, thus allowing successful compaction of large amounts of feed even if its initial bulk density is low. New activities in the development of press drives are directed towards hybrid punchand-die presses. This equipment combines mechanical and hydraulic components with electronics to yield machines that incorporate the advantages of both drive systems together with easy process control and data logging. The concept is particularly advantageous for withdrawal presses (see below) in which, for example, the die is moved hydraulically while the fill shoe and top punch are mechanically actuated. As a result, by utilizing the high cycle numbers of mechanical presses and the freely programmable characteristics of the hydraulic drive, a very flexible, reliable, and reproducible operation for the manufacturing of parts with unsurpassed, high quality is obtained. Smaller machines are employed in many fields, including the pharmaceutical, confectionary, and fine chemicals industries, if only a limited output is required and, to a certain extent, for development work in all areas of high-pressure agglomeration (see Section 11.2). Larger ejection type presses are mainly used in the powder metal and ceramic industries. However, even there, the applications are in most cases limited to compacts that feature no or little change in cross section. Withdrawal presses always operate with two (independent)drives. One controls the movement of the upper punch and the other moves the die (Fig. 8.96). Whereas the majority of ejection presses is mechanically operated, for the withdrawal type both the mechanical and hydraulic drives are commonly used. In a withdrawal press, compaction and ejection take place with a continuous downward movement of the upper punch and the die. As shown in Fig. 8.96, at the beginning of the press cycle, the die is positioned on top of the lower punch which remains stationary at all times. Material is filled into the die cavity and compressed while both
1
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8 Pressure Agglomeration Movement of upper punch
I
t
t
Movement of d i e
Ready
Filled
Compaction
I
I
t
& t
Eject ion
4 Return
Fig. 8.96 Diagram indicating the operating phases o f a withdrawal press [6.42].
the upper punch and the die travel downward. The simultaneous movement of the die with the punch minimizes the influence of wall friction on compact structure and results in highly uniform product density. At the end of the compression stroke, the upper punch is lifted while the die continues to move down until it has been completely separated from the compact. During this ejection procedure, the compact is supported by the stationary lower punch. After removal of the densified product, the die moves up and the cycle begins again. Tooling for withdrawal presses is much more expensive and complex than that required for ejection presses. It consists of a die set which is removable from the machine as a complete unit. This has the advantage that the tooling is exchangeable between presses. Further advantages lie mainly in its adaptability to the production of complex components. It is also possible to obtain greater accuracy. Compacts can be made on this type of tooling with dimensional tolerances of less than 4 x m m and uniform density. This makes withdrawal presses particularly well suited for powder metallurgical and ceramic applications where uneven shrinkage during sintering must be avoided to make the production of “near net shape” parts possible. Some of the most common shapes of products from reciprocating presses are solid and perforated cylinders (i.e. bushings), rectangular (i.e. bricks) or cubic pieces, and structured shapes (Fig. 8.97).In terms of variety of applications, complexity of shapes, and accuracy of parts, punch-and-die pressing is the most versatile agglomeration method. All other agglomeration techniques offer either only one more or less defined shape with different sizes and little accuracy (for example all tumble/growth agglomeration methods, see Chapter 7 ) or a relatively small number of shapes with some accuracy (for example roller presses and pelleting, see Sections 8.4.2 and 8.4.3,below).To achieve this versatility,the basic principle of punch-and-diepressing is often modified [B.25, B.421. On the other hand, capacity of reciprocating punch-and-die presses becomes quite low if they are used for the manufacturing of, for example, high definition ceramic and powder metal parts. Since these products are made from expensive materials and almost always undergo post-treatment by sintering, it is of utmost importance that they feature uniform structure to avoid uneven shrinkage and rejects.
8.4 Pressure Agglomeration Technologies
Fig. 8.97: (a) Some parts made from metal powders, metal oxides, ferrites, ceramic materials abrasives, and other particulate solids in punch-and-die presses (courtesy Komage, Kell a m See, Germany); (b) A selection o f different products made with hydraulic presses [B.42]
One possibility to improve the structure of compacts is to reduce friction (see also Section 8.2, Fig. 8.3). The addition of lubricants (see also Section 5.1.2) was first introduced in the pharmaceutical industry for the improvement of tablette quality from high speed punch-and-diepresses (see below). Even today, lubricants are often a considerable part of the formulation of solid drug dosage forms. Mixed into the entire powder mass such lubricants must be included as inert excipients but always constitute additives that need to be accounted for. Because interparticle friction is normally very little affected by mass lubrication, the amount of lubricants, which, in reality, is meant mostly to reduce wall friction, must be much higher than is commensurate with its effect. Therefore, developments were directed toward the lubrication of only the tool surfaces. For this task, nozzles and solenoid valves which operate reliably in millisecond intervals were used and, later, the lubricant was also electrostatically charged to become attracted by and adhere to the tooling walls [B.42]. This technology was also adapted for other applications. Particularly the pressing of metal powders is much improved by lubricating the die cavity. Parts become more uniform, die wear is reduced, and the lubricant, which is often a contaminant, is eliminated from the metal powder mixture. Fig. 8.98 shows schematically and as a photograph a die wall lubrication system which uses an electrostatically charged powder for
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y--J Discharge
Fig. 8.98: Schematic and photograph of a die wall lubrication system using an electrostatically charged powder (courtesy Casbarre, DuBois, PA, USA).
8.4 Pressure Agglomeration Technologies WI t hout 'DreDr . . e ss'
High
Correct amount of 'prepress'
Mid
Too m u c h 'prepress'
Low
Fig. 8.99 Sketches describing possibilities to influence the location o f the "neutral plane" in upper punch pressing and controlled withdrawal die [8.25, 8.421.
lubricating the die cavity of larger presses in powder metallurgy. A timed pneumatic function conveys the powder lubricant from the hopper to the charge gun where an electrostatic charge is induced. A second timed pneumatic function transfers the charged powder through a nozzle, discharge hose, and discharge block into the die where it adheres to the cavity walls. After powder pressing and discharge of the compacted part, the cycle begins again. Particularly in complex parts, it is also necessary to control the position ofthe neutral plane, the low density zone that is approximately perpendicular to the direction of pressing. This is achieved by the relative motions of the tooling members (Fig. 8.99). It is also important to understand that, particularly under pressure, particles will not move from one level or position in the developing structure of a part to another one. As a consequence, if parts are pressed that feature more than one level, separate pressing forces must be applied simultaneously for each level. As a result, neutral planes will exist for each part level (Fig. 8.100). Fig. 8.101 is the photograph of a typical large vertical hydraulic press for the manufacturing of refractory brick and the three presses in Fig. 8.102 depict examples of a mechanical (a), a hydraulic (b), and a hybrid press that represent special powder presses of one German manufacturer. As an example of the wide variability, Tab. 8.11 summarizes technical information indicating the ranges of design data from that same manufacturer. Obviously a large number of other suppliers will offer a wide range of other presses.
tral planes'' in single and multilevel parts p . 2 5 , 8.421.
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One
Two
Three
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Fig. 8.102:
Examples of different special powder presses. (a) Hydraulic, (b) mechanic, and (c) hybrid drives (courtesy Komage, Kell a m See, Germany).
8.4 Pressure Agglomeration Technologies
Summary of technical information on the design ranges offive different special powder presses (according to Komage, Kell am See, Germany).
Tab. 8.11:
Parameter
Type Pressing force Stroke (top punch) Die hold. force Strokes;’: Die fill depth Std. height Space req. Width Depth Height Weight Power req.
Models
Unit K
KHA
0-50/0-500”
20- 1200
S 20-1200
KFMA 20-250
30
hydraulic 200-12,000 225 -425
hydraulic 200-12,000 100-300
hybrid 200-2,500 140-218
hybrid 300 140
mechanic kN may. 50-500 mm 110-200
KMA
kN min-’ mm mm
20-80 (2509r’k)140-8,400 10-80/6-15 14/2 0-50/0-120” 100-300 650-920 1,100-2,700
140-8,400 15/3 100-300 1,100-2,700
140-1,350 170 7-50/3-15 10-40 140-250 1,045-1,895
mm mm mm
1,400-2,100 1,600-2,300 2,000-2,700 GOO -4,200 4.0-7.5
2,800-6,300 3,000-4,500 2,600-6,800 5,000- 46,000 25-130
2,700-3,500 3,500-6,000 2,600-5,500 3,450- 18,800 15-100
kg kW
Small machinesilarge machines:
9r;’r
2,800-6,300 3,000-4,500 2,600-6,800 5,000- 46,000 25-130
special machines.
Rotary machines Rotary punch-and-die presses were developed to meet the ever increasing demand for higher outputs of relatively small tablettes, primarily in the pharmaceutical industry. Their basic principle of operation is similar to that of simple reciprocating machines. The difference lies in the fact that a series of dies is mounted into a circular steel table (the so called turret) near its periphery (Fig. 8.103) and that two punches (one upper and one lower) are associated with each die. The punches are moved by stationary cams while the turret with the dies and punches is rotating. An evoluted presentation of one pressing cycle is shown in Fig. 8.104. Feed is supplied to the table by an open frame, often called “feed shoe”, which is connected to a hopper above. While the feed frame momentarily covers a particular die, the bottom punch that is associated with that die is pulled down to the lowest position by its cam thus allowing the die to fill with powder. It then rises up on adjustable ramp to eject excessive powder from the die. The powder surplus is scraped off flush with the top of the turret at the highest point of the “weight adjustment ramp”. Assuming uniform fill density, this always leaves the same volume of powder to be compacted in the die. It is common practice to let the lower punch drop down slightly after the surplus material has been scraped off. This is done to prevent uncontrolled displacement or “blow-out” of powder from the die when the upper punch enters. Both punches are then moved together by their respective cams to achieve densification and compaction. If, optionally, the ramps moving the punches remain parallel for some distance after reaching
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T 7 HOPPER
FEED Fig. 8.103: Sketch ofthe layout of a rotary punch-and-die press [ 8.561.
maximum densification, a so called “dwell time” is introduced. During this time, the compact remains under pressure so that additional deaeration and conversion of elastic deformation into permanent plastic deformation can occur and expansion upon pressure relief is minimized. The overall opposite movement of both punches during densification and compaction produces the effect of double pressure and, therefore results in a relatively uniform structure of the tabletted product. Finally, the upper punch is lifted from the die and the lower punch travels up to eject the finished compact. As shown in Fig. 8.105, another evoluted presentation of a typical high speed, high pressure rotary tabletting machine, quite often, the maximum pressure is produced by two press rollers that oppose each other. One or both are supported by springs to provide overload protection. In such machines, the final compaction takes place very quickly and is followed by a sudden pressure relief. This is similar to what happens in roller presses (see below) but, because in tabletting machines the roller diameter is very small and the table speed is high, this process takes place extremely fast. Therefore, capping (see below) is a commonly observed problem if high speed tabletting is desired and the preparation of specially prepared particulate feeds by pre-granulation (see below) is frequently a necessity to overcome this defect.
Feed shoe
Upper punch cam
1
Scraper
Rotation of table tand punches
Mat Rotatinr table
t
tower punch cam Filling
Adjustment Ready
Compaction
Election Dwell
Fig. 8.104 Evoluted (straightened) schematic o f a rotary punchand-die press.
Ready Return
8.4 Pressure Agglomeration Technologies
Paths of the punches in a rotary punch-and-die (tabletting) press in evoluted presentation [B.56].
Fig. 8.105:
The simplest type of rotary machine is “single sided” with one feed location and a certain number (as few as four) of “stations” (= dies) on the table. One rotation of the turret produces as many compacts as there are dies (and punch sets) on the machine. Therefore, the output of single sided rotary machines depends on the maximum allowable speed of and the number of stations on the table. It is normally in the range of 300-800 tablettes per minute and can be doubled by installing two feed locations. In this case, the stations are filled twice on opposite sides of the rotating table and two compressions are carried out in each die per revolution of the turret. Obviously, to maintain the rate of densification and compaction the number of stations on the correspondingly larger table would have to be doubled, too. Outputs of more than 3,000 tablettes per minute can be obtained from well compacting material with double sided machines. Although the above production numbers also seem to indicate large volumetric capacities this is not the case because the individual compacts often weigh less than 1 g each. For example, at a tablet weight of one gram an output of 3,000 tablettes per minute translates into a capacity of 180 kg/h. A further increase in numbers of (typicallysmall) compacts produced per minute in rotary punch-and-die presses can be achieved by dual or multiple tooling (two or more die sets) per station (see below). In many reciprocating and most rotary presses for the pharmaceutical and similar industries, the original and still most common “standard”shape of compacts is a more or less cylindrical tablet (also tablette). As depicted in Fig. 8.106, this description in-
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cludes flat, faceted, and crowned products. For these shapes, simple die and punch configurations are applicable. Since these agglomerates are consumer products aesthetics, requirements that are dictated by the medical application (i.e. an easy identification of a particular formulation by the user), and the marketing driven desire to distinguish between manufacturers have more recently resulted in the development of special shapes, some ofwhich are shown in Fig. 8.107. Additionally, the punches may be engraved as demonstrated, for example, in Fig. 8.108. Finally, as already mentioned above, the tooling for smaller tablettes can be designed such that in a single pressing station two or more die cavities can be associated with correspondingly shaped punches to produce several compacts at once (Fig. 8.109). Of course, such punchand-die designs are very delicate and require high precision press designs as well as excellent maintenance. Expulsion of entrapped gas (air) from granulated or (particularly) powder feeds is very important because it reduces lamination and capping of the tablettes. As repeatedly mentioned (see, for example, Section 8.1),if gas is entrapped in compacts where it becomes compressed in the residual pore spaces and/or elastic deformation is still present when the compaction pressure is released, products from pressure agglomeration methods are partially or totally destroyed during ejection. In the high speed rotary tabletting presses, capping, the separation of a thin layer of material from the main body of the tablette on one or both faces (Fig. 8.110),is a particular problem. In regard to processing it is caused by particulate solid feeds that are not suitable for quick, high pressure densification or, in other words, by too high compaction forces and/or excessive speed of densification. Raw particulate solids for tabletting may be described by three types: 1. Noncompressible powders, 2. compressible powders possessing poor flow characteristics, and 3. compressible powders featuring good flow properties. Noncompressible powders are either pregranulated wet, which adds a binder component that also renders the granulate
'-4
Flat
d
Faceted
Fig. 8.106: "Standard" tablette shapes.
b-
-m Crowned
8.4 Pressure Agglomeration Technologies
I
Fig. 8.107: Designs and photograph of some special tablette shapes.
compressible, or, if the dosage level is sufficiently low, they are mixed with a powder excipient of type 3 so that the blend becomes compressible and free flowing. The same methods are used to improve the characteristics of type 2 whereby, if pregranulation is selected, application of the dry compaction/granulation methods (see below) may be advantageous. Type 3 powders are called directly compactible. While for punch-and-die presses with relatively few strokes per minute (see, for example, Tab. 8.11)accurate and reproducible filling of the die is normally not a problem and the rate of densification in those machines can be adjusted to match the compactibility of the particulate feed, the very high speed of rotary presses often
Fig. 8.108: Photograph o f an assortment of engraved punches (courtesy Kilian, Koln, Germany).
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Approximate seoaration lines
-__---U
_----_. m
Fig. 8.110 Photograph of tablettes with "capping" and sketch explaining the capping phenomenon [B.42].
8.4 Pressure Agglomeration Technologies
Fig. 8.111: Schematic representation o f a force feeder for rotary tabletting machines (courtesy Kilian, Koln, Germany).
causes problems. If powders are pregranulated, owing to their now larger apparent (agglomerate) size, flow characteristics are usually superior to those of naturally free flowing powders, compactibility can be adjusted, and a good granule size and distribution can be selected that yields an optimal feed bulk density. Nevertheless, rotary presses often require more than the equivalent of a simple shuttle feeder or fill shoe [B.25, B.421. Fig. 8.111 shows schematic representations of a force feeder for the accurate high speed feeding of rotary tabletting presses and Fig. 8.112 is the photograph of a modern machine on which such a feeder as well as a tablette
Fig. 8.112: Photograph of a modern rotary tabletting machine with force feeder and tablette discharge unit (courtesy Kilian, Koln, Germany).
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discharge unit are installed. Depending on the equipment to which they are connected, the latter feature good tablette unloading, bad tablette channel, and sample extractor. Electromagnetic gates may also separate tablettes that are off-specification during machine start-ups and shut-downs. Validation and cleanliness requirements considerably burden the designs of equipment for the pharmaceutical industry. To avoid cross contamination it is necessary to include CIP (cleaning in place) or at least WIP (washing in place) features on modern machines. It is easily understandable that such techniques are difficult, at least, when considering the complicated mechanical design of multistation (up to 79 per turret, see Tab. 8.12) rotary tabletting presses. Nevertheless, WIP is one of the latest features of such machines (Fig. 8.113) and often, to meet the stringent requirements of the regulatory authorities, from machines that were designed during the last decade, the entire turret assembly, complete with die table, upper and lower punches as well as upper and lower cam tracks (Fig. 8.114a) can be removed for cleaning, exchange, or maintenance. Smaller machines are equipped with integrated handling and mounting devices (Fig. 8.114b) while the assemblies of larger machines require remote handling systems (Fig. 8.114c,d). Tab. 8.12 summarizes some technical specifications of rotary punch-and-dietabletting presses of one manufacturer to demonstrate the range of sizes and capabilities.
Fig. 8.113: Glove box design o f the processing part of a rotary punch-and-die tabletting machine demonstrating (WIP) "washing in place" (courtesy Fette, Schwarzenbek, Germany).
8.4 Pressure Agglomeration Technologies
Horizontal Punch-and-Die Presses Some punch-and-die presses are arranged horizontally. Normally, such machines use a hydraulic drive. Because they are typically used for the briquetting of voluminous materials, such as metal turnings and borings or biomass, hay, straw, wood shavings, bark, saw dust, etc., shredded plastic, cardboard, paper, etc., and fine dusts with low bulk density and, therefore, require a large Some technical specifications by one manufacturer of three families of rotary punch.and-die tabletting presses for the pharmaceutical industry (according to Fette, Schwarzenbek, Germany) Tab. 8.12
Parameter
Model family
Unit
Punch types Tablet output Max. compr. Max. precomp. Max. tablet 0 Table speed Die 0 Die height Punch shank 0 :';
T
=
h 'min. maw. kN kN mm min mm mm mm
'
P 1200
PT 2090
EU 19/1"/1" - 441 IPT 19/1" 30-48 T* 120-230.4 T+ 80 50 11/13/16/25 25 - 100 22/24/30.16/38.1 22.22123.8 19/25.35
EU 19/1"/1" IPT 19/1"
PT 3090 -
441135
19.8-42.3 T* 105.6-338.4 Tq 100 100 11/13/16/25/34 15-lZOjl5 -80 22/24/30.16/38.1/52 22.22123.8130 19/25.35/35
thousands, for example: 30 T = 30,000; 1,004.88 T
=
1,004.880
EU 19/1"/1" - 441135 IPT 1911" 355.2-1,004.88 T" 100 100 11/13/16/25/34 30- lOOj15 -80 22/24/30.16/38.1/52 22.22/23.8/30 19/25.35/35
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degree of densification, they feature long, slow speed strokes, high forces, and almost always include a certain dwell time at maximum pressure. Hydraulic drives are ideally suited for these tasks. Fig. 8.115a depicts the principle design of a large hydraulic die press with horizontal ram movement for the briquetting of metal scrap. In this machine both the “chip box” and the punch move. At the beginning of the cycle the ram is retracted and the chip box with the integrated die is pressed forward against the anvil plate, closing the bottom of the die which is part of the chip box. Next, the punch moves forward and pushes feed material into the die. If the material is very loose, a “tamping device” can be added which predensifies the chips and holds them down. When the punch enters the die the pressure stroke begins during which the briquette is made. The finished briquette is held against the anvil while the chip box retracts. As soon as the ram begins to retract the briquette falls into a discharge chute. The cycle begins again when the ram is fully retracted and the chip box is fully moved forward and pressed against the anvil plate. Fig. 8.115b is the photograph of a typical press and Fig. 8 . 1 1 5 ~shows some actual briquettes from this type of press.
Fig. 8.115: (a) Drawings showing the principle design o f large hydraulic ram presses with horizontal punch movement; (b) photograph of a typical press for the briquetting o f metal scrap; (c) briquettes from such a press (courtesy Svedala Lindemann, Dusseldorf, Germany).
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8.4 Pressure Agglomeration Technologies 335
Fig. 8.116: (a) Schematic o f a horizontal hydraulic punch-and-die press for the briquetting o f loose stripper dust; (b) photograph o f the press showing the horizontal ram and the vertical predensification channel; (c) photograph o f loose feed and the highlydensified briquette (courtesy Pneumafil, Charlotte, NC, USA).
While machines for the briquetting of metals and many other recyclables require high forces, special machines have been developed for lower force processing, particularly ofbiomass. In many such applications, the material to be briquetted is difficult, because large amounts of densification and air removal are necessary, they have, however, good binding characteristics. For example, the grinding dust from smoothing the surfaces ofwood chip boards contains the chip board binder as well as a small amount of water from dust suppression. The biggest problem for the processing of such dusts is its fineness and looseness. Fig. 8.116adepicts the schematic of a press for the production of cylindrical briquettes from stripper dust (Fig. 8.116~). As can be seen, the loose dust is transported with a screw from the feed hopper into a vertical rectangular channel in which it is predensified by a hydraulically operated “tamper”.The material, thus densified, is briquetted by a horizontal, hydraulically actuated punch in a die with a force of, in this particular case, up to 42 tons. The product can be picked up with the dumpster of a trash service for regular disposal or it can be burned as a man-made solid fuel. Fig. 8.1161, shows the hydraulic press part with the vertical tamping channel. Roller Presses Traditionally, roll pressing is of greatest interest for industries in which large amounts of finely divided solids, both valuable and worthless (wastes), must be converted into larger, agglomerated pieces. The most widely used machines feature two rolls of identical size which rotate countercurrently and achieve compaction by squeezing the feed in the nip area (Fig. 8.117),much in the same manner as in rolling mills [B.8].Around the middle of the 19th century, roller presses were originally developed as an economic method to agglomerate coal fines [B.12b, B.421. More recently this method of size enlargement by high-pressure agglomeration is applied for a large number of materials in the chemical, pharmaceutical, food processing, mining, minerals, and metallurgical industries. The versatile technology lends itself to such different uses as compaction/granulation of highly heat and pressure sensitive pharmaceutical materials as, for example pancreatin or penicillin; the briquetting of
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r-13 I. s :
,
Fig. 8.1 17:
The basic principle o f roll pressing.
extremely corrosive and hazardous materials as, for example, chlorinated or brominated biocides and sodium cyanide; or the briquetting of crude, hot materials as, for example, metal chips and turnings, ores, and “sponge iron” with temperatures of up to 1,000 ‘C. An important new application is in the vast field of environmental control where often micron- or submicron-sized particulate solids must be economically enlarged for recycling or disposal. The rollers themselves or pockets and indentations that are machined into the working surfaces of the rolls form compacts or briquettes. Between smooth, fluted, corrugated, or waffled rollers, material is compacted into dense sheets (Fig. 8.118, see also Chapter 6, Fig. 6.5, lower left). Normally, these sheets are crushed and screened to yield a granular product. This process is called compaction/granulation. If the two rollers carry rows of identical pockets or moulds and the rolls are timed such that the pockets, representing roughly one half of the final product shape, match exactly (Fig. 8.119),so calledbriquettes are produced (see also Chapter 6, Fig. 6.5, lower right). Roller presses do not produce compacts with the same fine detail and uniformity as tabletting machines or other punch-and-die presses. The “web” or “flashing” that is caused by the land area around each pocket is usually found on the outer edges of all briquettes from roller presses. Even if they are thin and brittle, they can not be totally removed, for example on a screen, and, if they do break off, a clean edge is seldom obtained. In most cases some webbing remains on the product which may be objectionable in itself and because it may produce fines during handling. Because of these characteristics, briquetting roller presses find their natural field of application where relatively large scale production with low investment and operating costs is more important than the absolute uniformity of the product. The other important application is for the essentially dry compaction of powders into sheets followed by crushing and screening into a granular product of almost limitless average particle size and distribution. Granule size and distribution depend critically on crushing and screening which must be optimized for maximum yield and economical operation (see also below as well as Sections 8.3 and 11.1). Originally, roller presses were not conceived to exert high pressure and use high forces for briquetting. As mentioned before, they were invented for the economical conversion of fine coal into briquettes that could be applied as solid fuel for the quickly expanding use of the steam engine [B.l, B.21. For that purpose, coal fines were mixed with milled coal tar pitch and heated in a vertical pug mill by direct impingement with
8.4 Pressure Agglomeration Technologies
(b)
(d)
Fig. 8.118 Sketches showing different roller surface configurations for compaction and representations o f the corresponding products. (a) Smooth, (b) corrugated, (c) fluted offset, (d) fluted peak-to-peak, (e) waffled.
)(,&t
(e) - b -
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Fig. 8.120 Drawing of an early double roll briquetting press designed by Schuchterrnann and Kremer (a company no longer existing). w, w, = Roller shafts; f, f, = Rollers with briquette pockets; z = internal coupling gears; v = Feeder, pan with rotating distributor arms.
steam. The resulting blend was warm, moist, and sticky but still reasonably free flowing. As shown in Fig. 8.120, early roller presses consisted of two large, hollow drums with briquette pockets machined or cast into sleeves or segments that were shrunk or bolted onto the drum body. They used gravity feeding. The conditioned blend was transferred into a feeder pan with rotating distributor arms which moves and passes the blend through rectangular slots above the nip area between the rolls (Fig. 8.121~). Feed control was accomplished by “tongues”(Fig. 8.121) which were moved, mostly by hand, to increase or restrict the flow of material into the nip. Briquetting was more a forming than a compacting process. Final product strength was obtained during cooling by solidification of the coal tar pitch. For this duty, the machines were powered with flat belts and speed adjustment was accomplished by means of pulleys and crude open gear reduction. Only one roller was driven and timing occurred by open (“naturally”lubricated by coal dust) coupling gears
8.4 Pressure Agglomeration Technologies
Fig. 8.121: Drawings o f different gravity feed controls. (a) Standard tongue, (b) tongue with parallel movement, (c) mechanical distribution (see also Fig. 8.120) with standard tongue.
that were fastened to the drum bodies between two pocketed rings. No pressurizing system existed; the two rollers were fixed in the frame and supported by sleeve bearings. Sometimes, one bearing housing was located in the frame by shear pins to provide overload protection in case tramp material entered the nip. From the beginning, the coal tar pitch which made briquetting so easy was also a constant source of concern and, eventually, the down fall of this technology. Burning of briquettes resulted in the production of excessive amounts of sooty, acrid smoke and, therefore, efforts were made to, at least, reduce the amount of coal tar pitch that was necessary for the production of good briquettes. Correspondingly, more and more pressure was required for briquetting to overcome the lack of binder which slowly changed the mechanical design of the machines. Nevertheless, although the drives were now by electric motors through enclosed gear boxes, the original coupling gears were still used (Fig. 8.122a) which were later moved to a location outside the frame, enclosed in a sheet metal housing and lubricated by dipping into an oil bath (Fig. 8.122b), and springs were added to support one set of bearing housings to result in a floating roller for pressure control and overload protection. Since more and more machines were used for compaction where, during operation, a larger gap opens up which defines the sheet thickness, coupling gears, although manufactured with very large modulus, tended to disengage and frequently teeth were damaged or broke. Therefore, practically all modern machines are equipped with a synchronized double output-shaft gear reducer and misalignment (e.g. gear tooth) couplings between the fixed gearbox shafts and the adjustable or moving roller shafts (Fig. 8.122~). As will be shown below, other drive arrangements and hydraulic pressurizing systems are being used today. Since roller presses were originally gravity fed (see Fig. 8.121),the rolls had been always arranged side-by-side. Later, when a few roller press manufacturers tried to
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(cl
Fig. 8.122: Schematic representations depicting three different roll drives (explanation see text).
overcome the loss of business, after coal briquetting was phased out, and some new companies entered the market, mostly to supply small, ultraclean equipment to the pharmaceutical industry, force feeders became necessary to supply the often very fine and aerated powders to the nip between the rollers and higher pressures were required. Fig. 8.123 describes the compaction of a particulate solid in the nip between two gravity fed counter rotating rolls. For clarity, the roller diameter D and the distance between the rolls h, are not to scale. In reality the roll gap is much smaller as compared with the roller diameter (e.g. D/h, -100/2 to 100/5).Compaction between two smooth rolls may be explained by dividing the nip area into three zones: The feed zone, the compaction zone, and the extrusion zone.
8.4 Pressure Agglomeration Technologies
- -
Fig. 8.123: Conditions in the nip between two smooth, counter rotating rollers during the compaction o f particulate solids. Definitions o f geometry, angles, and roll force.
The feed zone is defined by the two angles uE’and uE.In the feed zone, the material is pulled into the roller nip by friction on the roller surface and between the feed particles. Densification is solely due to rearrangement of particles (see Section 8.1, Fig. 8.1).The density of the feed is characterized by the bulk density yo and reaches the tap density yt at aE.The peripheral speed of the rolls is higher in this zone than the downward velocity of the material to be compacted. u, is the so called “angle of delivery” which is defined by the width h, of the rectangular feed opening above the rollers. The angle that is enclosed by the two tangents on the rollers at u, is called “angle of entry” af (not shown). The compaction zone follows after the heavy solid line (Fig. 8.123), defined by the angle uE which is known as the “angle of rolling”, the “gripping angle”, the “angle of nip”, or the “angle of compaction”. In the compaction zone the full pressing force becomes quickly effective and the feed particles deform plastically and/or break if they are brittle (see Section 8.1, Fig. 8.1). ag is the “neutral angle” where the sign (direction) of the frictional force changes. At this point, the pressure in the material and the density reach their highest values and, for many materials, the velocity of the densified solids increases towards the centerline; therefore, this zone is called extrusion zone and this phenomenon assists in the release of the compacted material from the rollers.
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a, (assumed zero in Fig. 8.123) is the “angle of elastic compression of the rolls” which determines the thickness h, of the compacted sheet. a, becomes zero and the sheet thickness is equal to h, the theoretical gap width, if the elastic deformation of the rollers is insignificant. However, in most cases, the sheet is even thicker than h, due to the elastic recovery of the compacted material and the expansion of entrapped compressed gas (see Section 8.1). The actual sheet thickness is h, and the angle aR corresponding to the release plane is called “angle of release”. In the case of briquetting, the conditions in the nip of roller presses become much more complex. The distance between the outer contour of the rollers approaches zero to produce briquettes with only thin webs that easily break into singles. Nevertheless, in many cases, briquette separators are required to accomplish the task of producing individual briquettes (see Section 11.1).Fig. 8.124 depicts the mechanism of briquetting in roller presses. Only the final compaction phase is of particular interest. It begins when the leading (lower in Fig. 8.124) axial land area between successive pockets passes through the line connecting the centers of the two rollers. At this point, the pocket forming the briquette is practically closed at the leading (lower in Fig. 8.124) edge while the trailing (upper in Fig. 8.124) edge is still open and connected with the material in the nip. Immediately following this condition, during the continuous rolling action of the briquetting rolls, the formerly closed leading edge of the pocket opens while, now, the trailing edge closes and completes the compaction of the briquette. Above the final compaction phase, depicted in Fig. 8.124, similar conditions exist as shown in Fig. 8.123 which are modified by the fact that the roller surfaces are not smooth and, therefore, an “interlocking effect” assists in pulling material into the nip. The feed and compaction zones are less clearly defined, only determined by interparticle friction, and no longer depend on the friction between material and roller surfaces. However, it has been determined in a roll press simulator [B.12b, B.421 that, as a result of insufficient interparticle friction, with certain particulate solids, large portions of material, that was initially contained between one pair of pockets, are squeezed out and move back into the following space. Feed
1
1 Discharge
Fig. 8.124 Five successive momentary conditions o f briquetting between two counter currently rotating rollers with matching pockets.
8.4 Pressure Agglomeration Technologies .B.
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__ ----------.
-.-_ - - -.... - - _- - - _ ________
,
Fig. 8.125: Sketch o f an unsatisfactory pillow shaped briquette from a roller press.
The specific compaction process, described in Fig. 8.124, may result in beneficial or in negative effects. As the leading edge ofthe pocket opens, the force acting vertically to the line connecting the roller centers tries to “extrude” the briquette from the pocket, thus assisting in the release of the briquette, provided the pocket shape is correctly designed [B.42].On the other hand, since this affects mostly that part which is already completely densified, it may also cause a number of product imperfections (e.g. cracks, soft trailing edge, etc., Fig. 8.125) which also depend mostly on pocket shape and size. Another common defect ofbriquettes produced with roller presses is that they open up at the plane of pocket contact. In the vast majority of cases, this opening is at the trailing (last compacted) edge, but, occasionally, opening at the leading (first compacted) edge of the briquettes has been described and explained by overcompaction as well as volume recovery upon pressure release. Independent of their positioning, these latter faults are known as “clam-shelling”,“oyster-mouthing”,“duck-billing”,or similarly descriptive terms. In order to influence the conditions in the nip area of any roller press, provide a certain amount of control to the process, and allow higher pressures and densification rates, even in machines with relatively small roller diameter and if the feed consists of fine, aerated powder mixtures, in more recent times, gravity feeding has been replaced by force feeders, which are mostly based on the application of feed screws (Fig. 8.126, see also discussion below). With this modification, it is no longer necessary to position the rollers side by side. As will be shown later, vertical and even diagonal roller arrangements are offered today, particularly with small machines for specialty applications, such as in the pharamceutical industry. However, it should be mentioned at this point, that, in the opinion of the author, vertical feeding, which translates into horizontal roller positioning, is always preferable, as gravity, the natural force acting always and everywhere, assists in uniform vertical feeding while it may negatively influence the flow of solids in horizontal feed arrangements. Similarly to what is true for any of the other technologies of size enlargement by agglomeration, much of the knowledge about roller press operation is phenomenological in nature. Because of the change of particle sizes and shapes during high pressure densification and compaction of particulate solids (see Section 8.1, Fig. 8.1), a comprehensive theory is not even available for the “simple” punch-and-die process. The complex conditions in the nip between two counter rotating rollers (Fig. 8.123 and 8.124) makes theoretical predictions even more difficult, although certain similarities exist with the much better defined, investigated, and understood deforma-
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Fig. 8.126: Schematic representation o f some typical force (screw) feeders [B.lZb]. (a) Vertical straight or slightly tapered, (b) inclined straight, (c) vertical tapered (conical), (d) horizontal straight.
tion of metal (a continuum) in rolling mills [B.8]. Details of the more scientifically oriented treatment can be gleaned from two earlier books by the author [B.12b, B.421. In keeping with the emphasis of this book “industrial applications, not theory” readers who are interested in those aspects are referred to the previously mentioned literature (see also Chapter 2). In addition to what has been already covered and can be extracted from several publications that are listed in Section 13.1, the following discusses, from a practical point of view, the parameters which are required and used to size and scale up roller presses. Tab. 8.13 is a summary of these items: Roller presses were invented for the economical size enlargement of coal fines and built as large machines with gravity feed and rolls that often exceeded 1 m and sornetimes were as large as 2 m in diameter, typically carrying two sets of rings with a set of coupling gears in the middle (see above). Modern roller presses are descendants of these machines. Therefore, descriptions as well as design and scale-up considerations will be first based on equipment which, as far as size is concerned, has been directly
8.4 Pressure Agglomeration Technologies
developed from those earlier presses (as examples of early machine designs see Fig. 8.120 and 8.127). Although, almost everything which will be said for large machines also applies for the smaller ones, specific executions of the new generation of small roller presses meet the requirements of particular industries. Such details will be covered later. Fig. 8.128 and 8.129 present a series of photographs and artist’s renderings of different roller presses. These pictures, together with the contents of Tab. 8.13, will be used to explain important items. Fig. 8.128a.1 and a.2 show photographs of recent roller presses for the briquetting of coal fines with binder. Both still feature gravity feeders. The smaller one (Fig. 8.128a.1) uses a simple movable plate for the control of feed volume (Fig. 8.130),a single pair of wide, pocketed rollers, hydraulic pressurization of the floating roller, and a drive system featuring variable speed (frequency modulated, SCR) electric motor, double output-shaft gear reducer, and misalignment couplings. Since in a gravity feeder, the downward flow of material is retarded near the walls of the chute by friction, the edges ofwider rollers tend to be underfed, producing partially soft briquettes or softer bands on the sides of sheets, if roller compaction is employed. To compensate for this, the tongue(s) or movable plate(s) may be curved such that the feed opening in the middle of the roller is restricted (Fig. 8.131a). It is also possible to modify the volume of the briquette near the edge of the roller as shown in Fig. 8.132. The larger press (Fig. 8.128a.2),equipped with 1.4 m diameter rollers, has two gravity feed chutes, with manual and, after switching to automatic, electrically actuated tongue controls inside, feeding two pairs of pocketed rollers (not shown in Fig. 8.128a.2). Actually, when installed, the rollers will be connected to a double output-shaft gear reducer via misalignment couplings with a timing feature for matching the pockets. Therefore, the split of each roller into two separate rings is not done to accommodate the coupling gears ofthe old designs (see,for example, Fig. 8.120) but to accomplish a more uniform feed across the now narrower working faces and to provide additional deaeration possibilities. The first (uniform feed), requires a constant level of the material column in the feed chute which, in this case, is guaranteed by an overflow chute. In this chute a particulate solids flow meter (see Section 11.1)is installed and the feed streams to the presses are adjusted such that always a trickle overflow is measured, thus keeping the level in the feeder constant. Regarding the requirement to reliably and completely remove all gas from the particulate solids during densification (see Section 8.1)in roller presses, several routes for the escape of gas exist. Fig. 8.133 shows the different deaeration paths in roller presses. The two left representations depict side views of a narrow and a wide roller and the right sketch is a front view showing the rollers, the nip, the product sheet (which may also be a string of briquettes), and the feed hopper. When gas (air)is squeezed from the densifying material in the nip, it can leave between the feeder base and the top of the rollers (a),between the rollers and the cheek plates sealing the nip on the sides (b),and against the flow of feed through the loose bulk material (c). No escape is possible through the roller gap other than as compressed trapped gas; but this must be avoided. The sketch suggests several important considerations for the design and operation of roller presses. 1. The open space between the feeder base and the the top of the
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8 Pressure Agglomeration Tab. 8.13:
Summary o f design features and parameters o f roller
presses. Design feature or parameter
Various levels o f details or subparameters
Feeder
Gravity
Design
Unrestricted Manual adjustment Screw/other Force Screw Vertical/horizontal Drive Gear reducer w. motor Direct E-motor variable Parameters Gravity Standpipe/overburden Force Screw Speed Design Fixed cheekplates Other features Antifriction (ball/roller) Design Bearings Conical/selfaligning Tapered/withdr. sleeve Solid, forged Shafts Heated/cooled Single pair Rollers/general Solid Heated/cooled Side-by-side Compacting Smooth Side-by-side Briquetting Number of pockets Pocket shape Diameter/width Parameters Speed Gap Fixed rollers Design None No overload protection One roller Elastic/general Helical Spring Handpump Hydraulic W. accumulator Specific force [kN/cm] Parameters Force characteristic Accumulator pressure Cantilever Design Cast Fixed/bolted Enclosed (dust-/airtight) Parameters Strength (cold) Open Design Gearing Gear box (singl. outp.) Roll synchr No or coupling gears Uneven no. of teeth Regular Couplings Misalignm. couplings W. or wjo. roll timing Tongue
Feeder base Roll arrangement
Pressurizing
Frame
Drive
Flow stimulators Automatic control Manual/automatic Single/multiple Fixed/variable Direct hydraulic Flight - and pitch Adjustable cheekplates Sleeve Other Cylindrical Composite Multiple pairs Tires/segments Other Profiled (specif.) Other Size of pockets Shoulder (seal) (Force) Gap control Shear pin or similar Both rollers Other Automatic Wjo. accumulator Max. force Accum. volume Mill shaft Fabricated Hinged Open Strength (hot) G.b. (doubl. outp.) By gear box Spec. synchr. device Universal joint
8.4 Pressure Agglomeration Technologies Tab. 8.13 cont’d:
Summary of design features and parameters
of roller presses. Design feature or parameter
Various levels o f details or subparameters
Power supply Transmission Parameters Execution
General
Electric motor Hydraulic drive Belt (flat or V) Power [kW] Speed reduction ratio Heavy duty Clean (CIP, WIP) Rough environment Ambient
Variable speed Other Direct Speed Torque Light duty Stainless steel Special mat. constr. Hot
rollers is a major deaeration feature in roller presses ((a)in Fig. 8.133). However, ifthe material is very fine and aerated and this space is too big, large amounts of material may flush over the rollers and end-up in the discharge of the press as fines, thus reducing efficiency. It is possible to instal baffles in this area which allow the escape of gas and retain solids by depositing them onto the roller surfaces for transport back into the nip. 2. The escape of gas between the rollers and the cheek plates ((b)in Fig. 8.133) is the most important and often also the most misunderstood deaeration path. Cheek plates (for design and more detailed descriptions see below), the heart-shaped pieces sealing the side of the nip between the rollers, can not be in rubbing contact with the rollers as excessive wear would take place and the constant friction, which is aggravated by the presence of fine powder particles, causes the rollers and the cheek plates to quickly become red hot. Rather, well adjusted cheek plates have a finite clearance which is selected such that the leakage of fine material is minimized. In this
Fig. 8.127: Early roller press for the briquetting of caol fines, built by Zimmermann & Hanrez (a company no longer existing).
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Fig. 8.128: Photographs and artist's renderings o f different roller presses. (a.1, courtesy Lewis, Pocatello, IA, USA) and (a.2, courtesy Koppern, Hattingen, Germany): modern presses for the briquetting o f coal; (b, courtesy Koppern, Hattingen, Germany): roller press with external gear reducer and coupling gears; (c, courtesy Otsuka, Tochigi-City, Japan): roller press with pressurization by helical springs; (d, courtesy Sahut-Conreur, Raismes, France): narrow faced large roller press with screw feeder; (e.1, courtesy Koppern, Hattingen, Germany) and (e.2, courtesy Hosokawa BEPEX, Minneapolis, M N , USA): artist's conceptions o f roller presses exposing important internal parts; (f, courtesy Koppern, Hattingen, Germany): large roller compactor o f latest design for mineral fertilizers: (g.1, 8.2, courtesy Alexanderwerk, Remscheid, Germany): roller presses with horizontal feed and vertical roller arrangement
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Fig. 8.128: cont’d
Fig. 8.129: Hinged frame. (a) Drawing depicting the principle, (b) photograph o f a roller press with hinged frame, opened on one side for maintenance (courtesy Ko ppern, H attingen, Germany).
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Fig. 8.130 Sketch of movable plate volume control in the gravity feeder o f a roller press for briquetting. One or two plates may be used; actuation is manual (shown) or automatic.
position, large volumes of gases that are squeezed out during densification can escape. Since this gas flow entraps fine particles, leakage of material seemingly becomes excessively large and, since some pressure is lost in the material to be compacted at the edge of the nip, the already present effect of underfeeding at this point (see above) increases, causing soft briquettes or edges or compacted sheet. However, as became obvious when, due to improved designs and materials of construction, the rollers could become increasingly wider, the center portion of the nip between these rolls lost its ability to let squeezed out gas flow to and escape at the cheek plates. If this happens, more and more of the gas tries to flow against the flow of material ((c) in Fig. 8.133), particularly in the central portion of wide rollers. In a gravity feed situation, this results in a cycling of the flow of feed and the performance of the press. Gas escaping upwards, against the flow of feed aerates the still loose material above the nip so that the particulate solid’s condition becomes similar to that of a fluidized bed (see Section 7.4.4). The bulk density offeed to the nip diminishes to such an extent that full densification is no longer possible and aerated, little compacted material passes through the rolls. At the same time, the roll pressure and the drive torque drop to near no load conditions. Since in this state, most of the gas passes the rollers with the less compacted material, the upward flow of air ceases and the “fluidized bed” collapses. Immediately afterwards, material with “normal”,high bulk density enters the nip and good compaction takes place. At the same time pressure and torque peak. However, because gas is now again squeezed out and flows upwards, the cycle begins again. This operating condition is not acceptable because not only the yield of good compacts drops considerably, thus reducing process economics, but, equally important, the chattering, caused by the large fluctuations in pressure and torque, may result in serious damage to the roller assemblies, bearings, couplings, gear reducers, and drives, often to the point of destruction. To overcome the above mentioned problems, wide rollers are subdivided into two or more rings with one or more gap in between where cheek plates are arranged and deaeration can take place. After leaving the original and more recent designs of roller presses for the briquetting of coal (see also several papers by the author, listed in Section 13.3) and before going on to more modern roller presses that were conceived for the processing of a
8.4 Pressure Agglomeration Technologies
I ! \ Section 8-8 I
I!\ ’
i Section A-A
I’
I
Sheet
I’
Sheet
Fig. 8.131: Some feeder designs for wide faced roller presses. (a) Curved tongue (or plate) o f gravity feeders, (b) independent and (c) overlapping multiple screws [B.12b].
variety of new materials, Fig. 8.12% shows that, in certain cases and for a number of reasons, the application of older design principles may still be preferred. The machine depicted in this photograph makes use of a simple, cheap, fabricated frame and directly attached coupling gears. The press produces briquettes so that fluctuations in gap width, the main problem for machines with coupling gears, are minimal. Because the material to be processed is not abrasive, there is practically no need to exchange rollers or do other maintenance which makes a bolted frame acceptable. In spite of a large speed reduction, the arrangement is compact and requires little floor space (compare with a machine of similar frame design, roller diameter, and capacity but with double output-shaft gear reducer and misalignment coupling that is shown in Fig. 8.128e.1. On the other hand, the press features such modern details as force (screw) feeders and hydraulic pressurization of the floating roller.
Fig. 8.132: Cross section through a roll sleeve with reduced volume o f the border pockets to improve quality o f these briquettes.
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b Fig. 8.133: Schematic repre. sentation o f the different deaeration paths in roller presses. Also describing the difference between narrow and wide rolls.
Fig. 9 . 1 2 8 ~is the photograph of a modern machine with a spring loaded pressurization system that avoids the use of hydraulic fluid in and potential contamination of the processing area. Also, combinations of springs, for example nesting helical springs with different spring characteristics or pakets of springs of other designs (e.g. disc springs), create forces in response to gap changes that are not achievable with hydraulic systems and may be of particular interest for certain processing reasons. Fig. 8.128d is the photograph of a hydraulically pressurized roller press, driven through a double output-shaft gear reducer and featuring a (force) screw feeder with variable speed (SCR) electric motor. Force feeders are used to provide a controlled amount of particulate solids with a defined bulk density to the nip area between the rollers to, according to the conditions explained in Fig. 8.123, accomplish the desired densification and compaction. Force feeders are also applied to overcome the previously mentioned problem of fluidization of fine feed particles during deaeration. They provide a downward pressure, thus prohibiting development of the fluidized bed condition, but this action can also hinder proper deaeration and may cause failure of the compacted product due to the expansion of compressed gas. It is important to consider the relationship between screw diameter and roller width. As shown in Fig. 8.134, top views, there is a fundamental problem in feeding the nip between two rollers, which is rectangular in cross section, with a rotating screw that features circular projection and material delivery areas. Such an arrangement tends to overfeed the center portion of the nip while the edges, which are lacking feed anyway due to wall friction and deaeration as well as leakage at the cheek plates, become severely starved. Since additionally, particles in a bulk mass, particularly if it is under pressure and interparticle contact has been established, do not have the ability to move from one area to another as, for example, the molecules of gases or liquids, screw feeders can provide the necessary mass flow, bulk density, and feed pressure but they may also cause uneven feeding. A simple partial remedy is to use a screw diameter which is slightly larger than the roller diameter and push excess material into the nip by suitably shaped cheek plates.
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8.4 Pressure Agglomeration Technologies 355
A further characteristic of screw feeders, which results in uneven compaction, is the fact that, in single-flighted screws, the end of the blade extends farthest into the nip and exerts a rotating point-forceonto the particulate mass. While, in briquetting machines, the consequence of this moving pressure point can be shown only indirectly by the varying density and strength of individual briquettes within a product batch, it can be easily demonstrated with roll compactors, particularly if smooth rollers are used for compaction. As shown in Fig. 8.134, a sine-wave pattern of somewhat more densified material is visible on the compacted sheet. The frequency of this sine-wave depends on the screw and roller speeds. If such sheet is crushed and screened into a granular product, density and strength variations are commonly obtained. Double-flighted screws with blade ends on opposite sides and two rotating pressure points produce offset, overlapping sine-waves and, as a whole, a more uniformly briquetted or compacted product. Finally, again referring to Fig. 8.134, as the width of the rollers gets bigger the diameter of the screw must be increased correspondingly. As shown in the front views (lower part of Fig. 8.134), a screw feeder diameter which approaches the diameter of the rollers, which is synonymous with rollers that feature the same diameter and width, the screw more and more “applies breakes” to the rollers. A considerable
Fig. 8.134: Sketches showing the relationship between the diameter of a single-screw feeder and the width o f the rollers [B.lZ(b)].
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amount of the screw force acts on the roller surface and not on the particulate material in the nip. Such a situation results in wasted energy (of the main and screw drives), exaggerated uneven feeding, and insufficient predensification of the particulate mass in the nip. As a rule of thumb, the diameter of the screw should not exceed GO - 70 % of the roller diameter and, preferably be in the 50 % range. As shown in Fig. 8.128e.1, e.2, and f, wider rollers that, for process reasons, require screw feeders are equipped with multiple screws. In Fig. 8.1311, and c, the reasoning behind this choice is explained. To provide material to the nip between rollers with growing width, the longer rectangular feed area is approximated by a series of small diameter screws. Each of these screws, whether single- or double-flighted,will produce a sine-wave pressure pattern (Fig. 8.131b) which can be further modified by the use of overlapping screws (Fig. 8.131~). As depicted in Fig. 8.128e.1, e.2, and f, multiple screws are often arranged under an angle to allow easier loading of the feed hopper (see also Fig. 8.12Gb). Although coming from different directions, the screws in the artist’s conception of Fig. 8.128e.2 are working in a common hopper extension and, therefore, feature the characteristics of overlapping screw flights, while each screw in Fig. 8.128e.1 is housed in separate pipes. Sometimes, all screws are angled from the same side, particularly if the overlapping feature is used. Then, the screw axes will be directed towards the fixed roller to avoid excessive movement of the floating roller as a result of always possible bulk density variations. As visible through the open doors in the photograph of Fig. 8.128fthe need for optimal deaeration may also direct the use of multiple rings as discussed above. The main difference between the artist’s conceptions in Fig. 8.128e.1 and e.2 are the choices of bearings for supporting the rollers. Fig. 8.128e.1 represents what the majority of manufacturers of large roller presses use today. As shown in more detail in Fig. 8.135a the bearings are selfaligning roller bearings which are mounted onto cylindrical shaft journals with conical withdrawal sleeves. The advantages of this design are that bearings can be mounted and removed quickly without the danger of doing any damage to the seats, cylindrical (straight) shaft journals are easily manufactured accurately and maintained, even after repeated removal seating is well reproducible, and, if the floating roller momentarily moves out of parallel, due to the spherical (selfaligning) design no forces act on the bearings and no bending of the shaft is experienced. Another bearing arrangement, which was suggested by metal rolling applications where the rolls, by design, do not cock, uses heavy duty conical (Timken) roller bearings. These bearings are typically directly seated on conical shaft journals (Fig. 8.135b shows the removal procedure), therefore require reshimming during every renewed installation, and do cause bending if cocking of the floating roller occurs. Fig. 8.135a also suggests the design and shows the simple labyrinth seal of a sheet metal housing surrounding the rollers (see photograph in Fig. 8.128f), mostly for dust containment. Other details will not be discussed, as they exceed the scope of this book. Reference should be made to [B.l2(b)]and [B.42]as well as to other literature in Sections 13.3 and 14.1. Another feature of most modern roller presses, that merits a specific mentioning, is the hydraulic pressurization system. It is used on all presses shown in Fig. 8.128 with
8.4 Pressure Agglomeration Technologies
Fig. 8.135: Bearing design: (a) Cross-sectional drawing depicting a roller assembly with selfaligning roller bearings and other modern features (see Section 13.3 [102]); (b) conical (Timken) roller bearing with removal device.
the spring loaded machine, pictured in Fig. 8.128c, being the only exception. The hydraulic pressurization of the floating roller selves to provide a controllable operating pressure and an overload feature if and when tramp material enters the rollers. A typical advanced schematic is shown in Fig. 8.136. The hydraulic pressure is produced by a pump which, in this system, is motorized and often submerged in the oil reservoir. In the most simple case, the pressurized fluid actuates hydraulic cylinders that push against the bearing blocks of the floating roller. In a no load situation, the bearing blocks are held in place by shimmed stops, avoiding metallic contact of the rollers and defining the “no load gap”. Since hydraulic fluid is incompressible, an accumulator, a partially gas-filled pressure vessel, is associated with each bearing block. Before pumping up the hydraulic system, the accumulator is filled with gas, typically nitrogen, to a pressure that is lower than the expected operating pressure. As shown in Fig. 8.137, if an empty hydraulic system begins to be filled up at time marker (0),the hydraulic pressure in the system begins to increase until it reaches the gas pressure in the accumulator (A) at time marker (1).Before the system pressure increases further the volume of the compressible gas in the accumulator is reduced while pumping continues and, for some time, no pressure change is observed. (This is an easy
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8 Pressure Agglomeration
A
Floatina roller
A
I
Nonryturn valve
I
1 Contact Dressure
Motor pump
\
Safety valve
Fig. 8.136: Schematic diagram o f a hydraulic pressurization system for the floating roller of a high pressure roller press (see Section 13.3 [102]).
on-site check to determine the functioning and pressure of the accumulator(s) if no pressure gauge for the gas filling is available). After the equilibrium gas volume is reached at time marker (2), the pressure in the system rises again until it reaches a predetermined pressure (B) at time marker ( 3 ) and pumping is discontinued. The no-load pressure (B) has been determined during tests such that, if operation of the roller press begins at time marker (4)and material opens the “no load gap” to the operating gap, the corresponding movement of the hydraulic piston in the closed hydraulic system increases its pressure to the operating pressure (C). While material is being densified and compacted, the pressure will fluctuate around the operating pressure depending on momentary changes in feed bulk density. Normally, the hydraulic system is then switched to “automatic” whereby the signal from the contact pressure gauge (Fig. 8.136) turns the pump on if the minimum pressure is reached and off when the maximum is obtained. If the pressure increases beyond the maximum, the safety valve dumps hydraulic fluid back into the reservoir. While many machines use only one hydraulic accumulator (as shown, for example, in Fig. 8.1288.1)and the two hydraulic cylinders are connected with common hydraulic tubing, the more sophisticated (andbetter) system uses at least one accumulator per bearing block and pressure lines that are separated from each other by non-return valves. In this design (as shown in Fig. 8.136), ifthe floating roller cocks, the hydraulic pressure in that part of the system that moves more also increases more and tries to push the bearing block back to regain a parallel position. Systems that are always in
8.4 Pressure Agglomeration Technologies
(0) - (4) (A)
: Time markers : Accumulator pressure
(B) (C)
: no load ( pre) pressure : Operating pressure
(A)
(0)
(1)
(2)
(3)
(4)
-b
Time
Fig. 8.137: Operational diagram o f a hydraulic pressurization system for the floating roller of a high pressure roller press.
equilibrium, do not offer this possibility so that a non parallel position of the floating roller can persist forever or until the situation is manually remedied and may reoccur again. The gas accumulator(s) in the hydraulic system of a roller press is (are) often considered not very important by the operators. This is far from true. First and foremost, the hydraulic accumulator provides flexibility to the system and avoids overload situations. Peak loads due to variations of the bulk density of the feed can not be avoided and are compensated by the accumulator(s).If an accumulator is damaged and does not contain a gas cushion, the system is rigid, because hydraulic fluid is incompressible, and very short high peak loads, which are not picked up and displayed by standard instrumentation, reduce the life of many critical machine components (e.g. bearings, shafts, couplings, gearing, etc.). The hydraulic accumulators also influence compaction by defining the response to overload situations. If the volume of the gas cushion is large, the floating roller moves easily in response to load changes. Vice versa, if the volume of gas at the operating pressure is small, the response is more rigid. This performance can be influenced by the size of the accumulator and the gas pressure (determined at the no-load condition).The larger the volume and/or the closer the (noload) gas pressure to the operating pressure the softer the response. A “standard” starting accumulator pressure is normally set at GO - 75 % of the operating pressure but if, for example, the material to be processed easily overcompacts, i.e. showing signs of lamination, clam-shelling, cracking, etc., accumulator volume and/or pressure should be changed which may result in improved product quality.
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Modern roller presses often use very high forces (up to and beyond 1 MN) which, through the hydraulic system, act on the bearing blocks ofthe floating roller. Although the hydraulic fluid is under pressure (presented, for example, in kN/cm2)the piston or pistons exert a force onto the bearing block which is defined by the area of the piston. These high forces may deform the bearing block and damage the bearing or, at least, reduce bearing life. Therefore, high pressure roller presses feature two punches per bearing block which act above and below the centerline of the rollers and the bearing block is designed asymmetrically (i.e. more steel on the side where the force is acting). Fig. 8.138 is the photograph of a hydraulic block featuring to cylinders and an accumulator between a frame member and the bearing block. At this point it should be mentioned that, for roller presses, a densification and/or compaction pressure can not be calculated. Referring to Fig. 8.123 it is easily understood that a volume element on which the maximum pressure acts can not be defined. Fig. 8.139 translates the curve of Fig. 8.1 (Section 8.1)to a smooth roll, gravity fed high pressure roller press. Because there are no defined, unequivocal volume elements in which the pressure can be determined, the curve represents force. For comparison of press performance, the specific force, a physically meaningless designation, which is, however, useful for, for example, scale-up, has been developed. It is defined as the (normally operating) force divided by the operating width of the roller and measured in kN/cm. If, for example, the roller width between the cheek plates is 0.75 m and the average total force exerted by all hydraulic cylinders onto the bearing blocks of the floating roller is 5,000 kN, the press operates with a specific force of approx. 66.7
Photograph o f the hydraulic block in the frame o f a modern high pressure roller press (courtesy Koppern, Hattingen, Germany)
Fig. 8.138:
8.4 Pressure Agglomeration Technologies
kN/cm. The same specific force characteristic is used to identify the capabilities of a particular press. Fig. 8.140 shows typical specific forces which are required by some different materials for successful compaction in roller presses. At and above approx. 150 kN/cm the local yield pressure of even the best materials for the manufacturing of roller surfaces (at that level almost exclusively segments [B.42])is overcome; therefore, for technical reasons, the limit for roller pressing is reached at that specific force. With increasing pressure, wear of the roller surfaces becomes more and more of a concern, particularly if the material to be processed is also abrasive, such as many metals, minerals, and ores. In such cases, the rollers must be frequently removed from the frame for maintenance. Because the rollers must be always larger in diameter than the bearings and those become increasingly larger with higher specific forcerequirements, in early machines, such roller maintenance meant opening the frame after removing most everything above the press and lifting out the roller sets. For fabricated, bolted frames (see for examples Fig. 8.128a.2, b, and e.1) this is quite time consuming. A big improvement was made when the hinged frame was invented (Fig. 1.829).As can be seen, the frame can be opened easily and quickly and the rollers can be removed to the sides of the machine where they are picked-up by a lifting device (Fig. 8.129a). If the installation is planned properly, nothing above, below, or around the press must be removed other than items that are directly related to the rollers (e.g. cooling, lubrication), the frame (e.g. hydraulic pressurizing sys-
Fig. 8.139:
Pressure conditions in the nip between two counter rotating smooth rollers during the passing o f particulate solids.
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kN/cm
Pressure agglomeration
150
Minerals Ores, Bunt l h Metallurglcal dusts. Sponge iron hot) Caustic MgO
100
"I 60
20 10
H i g h pressure comminution
Ammonium sulphate NPK- mixed f er tillzer8 Coal without binders Rock salt
Ores
Sodo
Cement clinker
FGD gypsum NPK-mixed f erlilirers with urea Briquetliig with binders Potosh, Oil shole Ceromic raw materials Haleic anhydride. DMT Cool with binders Sholes. Clay Earthy ores
Cool
Fig. 8.140 Specific forces required for the successful compaction o f some different materials with roller presses (courtesy Koppern, Hattingen, Germany).
tem), and the press (e.g. roller housing).A real maintenance situation is shown in Fig. 8.129b. So far, only presses with rollers arranged side-by-side, were presented. Although most of the presses with horizontal feed and rollers on top of each other are small, manufactured by relative newcomers to the field, and applied in the pharamaceutical industry for ultraclean processing, Fig. 8.1288.1 and 8.2 are examples of large machines of this design. Everything that has been said about roller presses so far and will still be discussed below is valid for presses of any design and size. Selection and design details are mostly influenced by the application. As mentioned repeatedly, roller presses that were originally developed for the economic size enlargement of coal fines by briquetting have, after losing their own field of application, found many new uses and, accordingly, were modified to fit the particular requirements. As shown in Fig. 8.141 modern roller presses are currently used in three different fields. Particularly for the new high pressure comminution, where efficient brittle breakage during compaction in the nip is the primary reason for passing minerals through the rollers and the agglomerates that are formed are later dispersed into individual small particles by total destruction of the bonds (in mills), large throughput capacities were desired. This led to the construction of very large roller presses (Fig. 8.142) which incorporate the newest developments of roller press design plus some specific ones which were suggested by the size of this equipment. If, given the extremely torque requirements, the typical mo-
8.4 Pressure Agglomeration Technologies
tor-gear reducer-misalignment coupling drive system would have been chosen, the “foot print” of such installations would have become excessively large. Therefore, as shown in Fig. 8.142 direct hydraulic drives are often being employed. Just for comparison, Fig. 8.143 is the photograph of a complete compaction/ granulation system that is used in the pharmaceutical industry for the conversion of dry powder mixtures into dust free, free flowing, non-segregating granular intermediate products for subsequent tabletting. The roller press is shown in the upper center. So far, all machines that were presented used the mill shaft design in which the roller shafts are supported on both sides by bearings that are mounted in a frame. Another, relatively new design of roller presses uses cantilevered shafts and rolls. With these machines, the shafts are supported in a frame and the rollers are mounted on the front of that frame. Fig. 8.144 shows four examples. Because the feeder, the rollers, the cheek plates, and the nip are relatively freely accessible from the outside, these machines are used for frequently changing small applications, for development work, and in the pharmaceutical industry.
(b)
(a)
(C)
Feed
Briquettes
Feed
Compacted Sheet
I 8
(brittle solids1
Agglomerated
Schematic representation o f the three current fields of application o f modern roller presses. (a) Pressure agglomeration/ briquetting, (b) pressure agglomeration/compacting, (c) high-pressure com mi nut ion.
Fig. 8.141:
Fig. 8.142:
Large roller press
for high pressure comminution (courtesy K o ppern, H attingen, Germany).
Crushed Solids
1
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Fig. 8.143: Dry, high pressure compaction/ granulation system employing a small roller press (courtesy Fitzpatrick, Elmhurst, IL, USA)
Selection, sizing, and scale-up of roller presses are based on experience and testing. In addition, some very basic formulae support these efforts. As has been discussed above (see also Fig. 8.140), the specific force, which is required for the production of an acceptable product, obviously represents one of the most important results of testing. For its determination, a representative sample of the material to be processed is compacted in a roller press. The roll diameter of this test press should be as close to that which foreseen for the large scale application. The roller diameter can not be easily scaled up. This is due to the conditions in the nip which become unpredictably complicated if the roller surface is not smooth and vary with the changes in structure, size, and shape of the particulate solids during densification and compaction. As shown in Fig. 8.145, the nip shape and size are quite different between rollers with different diameters. In addition to the modifications caused by the changes in diameter, the gap would have to become narrower as the rollers are getting smaller. It can be easily recognized that, if, for example, the circumferential speed v, is kept constant and, therefore, the angle of nip, a, remains approximately the same, the densification and compaction behavior must be quite different. Between larger rollers, the process occurs much less suddenly and such important processes as deaeration and conversion into plastic deformation are achieved more completely. Even though circumferential speeds are normally only in the range of 0.5 -0.9 m/s (for easily compactible materials, such as common salt, vc maybe as high as 1.5 - 2.0 m/s and in high pressure comminution it can be in excess of 3.0 m/s) it should be realized that the entire process in the nip happens in fractions of a second.
8.4 Pressure Agglomeration Technologies
Fig. 8.144: Four examples o f roller presses featuring cantilevered shafts and rolls. (a) Side view drawing (a.1) and photograph (a.2) o f a model CS machine with vertical feeder (courtesy Hosokawa BEPEX, Minneapolis, M N , USA); (b) schematic drawing (b.1) and photograph (b.2) of a model CS machine with horizontal feeder (courtesy Komarek, Elk Grove Village, IL, USA); (c) outline drawing (c.1) and photograph (c.2) o f a small cantilevered roller press with horizontal feeder and integrated flake breaker; the photograph also includes an integrated granulator (mill) (courtesy Alexanderwerk, Remscheid, Germany); (d) overall photograph (d.1) and detail (d.2) of a cantilevered roller press with vertical feeder and integrated granulator (mill) (courtesy Sahut Conreur, Raismes, France).
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8.4 Pressure Agglomeration Technologies
Fig. 8.144 cont'd
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Fig. 8.144 cont’d
Fig. 8.145: diameters.
N i p shape and size between rollers with different
8.4 Pressure Agglomeration Technologies
The following are useful practical equations for sizing and understanding roller presses: Circumferential speed:
vc = 7c 2r rprn 1/60 [m/s]
(Eq. 8.2)
Throughput (compaction)
Q = n 2r s 1 rprn GO y [kg/h]
(Eq. 8.3)
(briquetting)
=
z V rprn GO y [kg/h]
(Eq. 8.4)
(Eq. 8.5) (Eq. 8.6) (Eq. 8.7) With:
2r=
Ypm S
1 Y z
V
P
D
Roller diameter [cml Roller speed [~/min] Sheet thickness (avg.) [cm] Roller width [cml Apparent density” [kg/cm31 Number of briquette pockets per roller Briquette volume (avg.) [cm3] Specific force [kN/cmI
(” average of the compacted sheet or, respectively, the briquettes)
More equations can be found in the literature [e.g. B.8, B.12b, B.42, B.47, B.561. If the specific force, the average density of the product, and the desired throughput capacity of the machine are known, with the sheet thickness or the briquette size a roller size can be calculated and a machine can be selected based on its maximum specific force capability. The drive power is determined from the torque requirement during testing and safety factors are added to all machine and process parameters. After installation it is normally necessary to readjust and optimize all conditions in-line. In Tab. 8.14 some parameters of roller presses from different suppliers are summarized and Fig. 8.146 shows photographs of typical products that are obtained with roller presses.
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8 Pressure Agglomeration Tab. 8.14:
Some typical parameters of roller presses from different
suppliers. Vendor (see Section 14.1)
Type Roller
Frame design Feeder design
Accessories Execution Drive power Throughput Weight
Orientation Arrangement Diameter Width Surface Design Support Pressurizing system Specific press. force Drive Motor Speed Standard Multiple Special Standard Special Standard Rollers Approx. typii Approx.
Vendor (see Section 14.1)
m e Roller
Orientation Arrangement Diameter Width Surface Design Support Pressurizing system Specific press. force Drive Motor Speed
Frame design Feeder design
Accessories
Standard Multiple Special Standard Special
Unit
mm mm
Alexandetwerk
Lab/Pharm H C 120-300 25-120
Fitzpatrick
Lab/Pharm V C/M 200-400 200-400 C Rs/Si/wc AFB/cyl H/wAcc
Large V M
Rs/Si/wc AFB/cyl H/wAcc
Large H M 250- 1,000 120 - 1,000 C Rs/Si/wc AFB/cyl H/wAcc
G/do EI/vS
G/do EI/vS
G/do EI/vS
Gdo El
CF sc Yes
HF sc Yes
HFjCF sc No
hg sc No
md/hd/De
Id/ss/De
md
to 60,000
to 2,000
C/B Rs AFB HAc
kN/cm
m/s
md/ss kW kg/h to 400 1,000kg to 4.0 Unit
mm mm
Hosokawa BEPEX
m/s
Large V M 300- 1,400 100-1,600 SIC SIC Si/Rs/Sg/wc Rs/Si/Sg/wc AFB,R AFB,R con slv/sph H/wAcc H/wAcc 125 150 (160) G/do G/do,Dd El/ss,Hy/vs El/ss, vs 0.1-1.2 0.1-1.5 HFjhg HF Sc/co,Gr/ag Sc/cy,Gr/ag Yes Yes
Lab/Pharm Large V V M M/C 200-400 300 - 1,100 50-350 75-1,100 SIC Rs/wc AFB,B,R
kN/cm
K6ppern
CYl No 55 G/do El/ss, vs 0.1-1.5 HF/tb(hg) Sc/co,(Gr) Yes ‘b/T HYd
P/v/rb/vP Lub
P/V/CO Lub Hot exec./ HYd
Commin. V M 500-2,400 200-1,700 C Si/Sg/wc AFB,R slv/sph H/wAcc 150 G/do,Dd Hy/vs,El <3.0 HFjhg Gr/ag No
Lub
8.4 Pressure Agglomeration Technologies Tab. 8.14:
continued. Hosokawa BEPEX
Vendor (see Section 14.1)
Unit
Execution Drive power Throughput Weight
ld/md/ss/Sa md/hd/De/Sa md/hd/De kW 3-45 7.5-1,100 to 1,000 kg/h to 160,000 1,000kg to 8.0 2.0-150.0 to 100.0
Standard Rollers Approx. typical Approx.
Vendor (see Section 14.1)
Type Roller
Orientation Arrangement Diameter Width Surface Design Support
Unit
KR Kornarek
mm mm
Accessories Execution Drive power Throughput Weight
Multiple Special Standard Special Standard Rollers Approx. typical Approx.
Sahut Conreur
Labpharm Large V V C M 150-400 400-1,400 30-100 150-1,600
H/wAcc 20- 168 G/do El/vs 0.08-0.89 CF sc
SIC Rs/Sg/wc AFB,R con H/wAcc 40-130 G/do El/vs 0.09 - 1.1 OF/HF SciGr
No
Yes
SIC SIC Rs/wc Rs/Si/Sg/wc AFB,R AFB,R CYl slv/sph H/wAcc H/wAcc 40-90 90-170 G/tg,G/do G/do El/vs El/ss,vs 0.02-0.42 0.04-1.40 CF OF/HF sc/co/vp, Gr/ag No Yes vacuum deaeration No Lub
CYl
Standard
md/hd/De 150-3,000 to 2 x lo6 to 400.0
Large V M 330-1,371 50-685
Lab H C 130-457 10-150 SIC Rs/wc AFB,R
Pressurizing system Specific press. force kN/cm Drive Motor Speed mls Frame design Feeder design
Koppern
No No HYd Lub ld/md/ss/De md/hd/ss/D)e kW 2.2-60 30-220 kg/h to 6,000 to 80,000 1,000kg 0.8-12.0 5.0-28.0
ld/ss/De 3-37 to 1,500 0.5-4.0
hd/ss/De 37 - 1,000 to 120,000 5.0-120.0
~~
Vendor (see Section 14.1)
Type Roller
Orientation Arrangement Diam et eI Width Surface Design Support Pressurizing system Specific press. force Drive
Unit
mm mm
kN/cm
Turbo Kogyo (Lic. Alexandewerk)
Lab H C/M 90-230 30-170 C Si/Rs/wc AFB H/wAcc to 36 G/do
Large H M 265-662 170-660 C Si/Rs/wc AFB H/wAcc to 45 G/do
Zemag
Large V M 250-1,000 150-1,250 SIC Rs/ Sg/wc AFB,R H/wAcc 24-150 G/do, Dd
I
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372
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8 Pressure Agglomeration Tab. 8.14:
continued.
Vendor (see Section 14.1)
Unit
Motor Speed Frame design Feeder design
Execution Drive power Throughput Weight
Rollers Approx. typical Approx.
Explanations Roller .... Orientation Arrangement Diam et er Width Surface
Design
H V C M [mml [mml B C Rs Si Sg
Support
Pressurizing system
wc AFB CYl con slv SPh H S
No
Specific press. force Drive
Motor
wAcc [kN/cmI G Dd tg do El HY VS
ss
Speed Frame design
[mlsl HF OF CF
kW kg/h 1000kg
Zernag
El/vs
El/vs
CF/HF sc Yes
HF sc Yes
El/vs, Hy/vs 0.04-1.0 OF/HF Sc/cyl, Gr/ag,v Yes
ld/md/ss/De/ Sa 0.2-7.5 12-600 to 2.8
md/hd/ss/De
hd/ss/De
5.5-200
14-630 200-55,000 to 86.0
4 s
Standard Multiple Special Standard Special Standard
Accessories
Turbo Kogyo (Lic. Alexandetwerk)
800 - 20,000 to 42.0
Horizontal feed: rolls on top of each other Vertical feed: rolls horizontal to each other Cantilevered rolls outside frame Rolls between two frame members (= mill design) If applicable, range Range Briquetting Compacting Ring (sleeve) Solid (integral shaftiroll) Segmented Watercooled Antifriction bearings (B = ball; R = roll: N = needle) Cylindrical bore and shaft journal Conical bore and shaft journal Sleeve and cylindrical shaft journal Self aligning (spherical) Hydraulic Spring No pressurizing system (fixed rollers) With hydraulic accumulator Max., range Gear reducer Direct drive (each roller directly driven) Timing gear Double outputshaft (synchronized) Electric Hydraulic Variable speed Single speed Circumferential speed of rollers (range) H - frame 0 - frame Cantilever frame
8.4 Pressure Agglomeration Technologies Tab. 8.14
continued.
Explanations (cont‘d)
Feeder design
hg tb Gr ag sc CY
co P v
Accessories
Execution
Throughput
rb “P Lub HYd Id md hd ss De Sa [kg/hl
Hinged Tie bars Gravity Adjustable gate Screw Cylindrical screw Conical screw Parallel screw shafts V - arrangement, inclined Ribbon flights Variable pitch screw Automatic lubrication system Hydraulic support functions (feederiframe dismantling) Low duty Medium duty Heavy duty Stainless steel Dust enclosure Sanitary Typical, range
8.4.4 Isostatic Pressing
One of the potential problems of all pressure agglomeration methods is, that friction between the material and tooling walls, interparticle friction, and the often one sided or, generally, directional introduction of the compression force result in density variations in the compacted product (see Section 8.2, Fig. 8.3). For many applications this is not a problem. In fact, the highly densified “skin”of pelleted or briquetted products acts almost like a packaging of the compacted material. The dense surface provides higher strength and, particularly, good abrasion resistance while the interior is somewhat softer. This may be of advantage for, for example, certain pelleted or briquetted animal feeds or for metal briquettes which must feature an inert surface to avoid reoxidation. Particularly ceramic and metal powders are formed and densified by pressure agglomeration techniques to yield intermediate products which are heat treated (sintered) to reach final shape, density, and strength. Since many of these materials can not be easily machined after sintering due to their abrasiveness and/or hardness, “near net shape” articles are often desired. This means, that, after taking into consideration the dimensional changes caused by, during, and after the sintering process, the part should posses dimensions which need no or minimal adjustment for its intended use. In isostatic pressing, the consolidation pressure is applied by a fluid that is pressurized and acts uniformly from all sides onto particulate solids that are enclosed in a
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Fig. 8.146 Photographs of different products from roller presses. (a) Briquettes, (b) compacts and granules, (c) feed and product of comminution.
flexible container (“mold”)which is immersed in the fluid [B.12a].This results in as uniform a consolidation as is technically possible. Of course, there is still a density gradient across the part, the center is always less compacted than the surface (see also Section 8.3), but the gradient is uniform and, typically, does not cause distortion during sintering. Although isostatic pressing was already mentioned and patented around 1910, only approx. 30 years later the technology found a general and large scale application for the direct isostatic pressing of spark plug insulator preforms [B.42].Technically it is quite immaterial whether or not sintered ceramic spark plug insulators are totally straight. However, as a consumer product which, at that time, needed replacement often, the part, in addition to functioning well, had to look good. After introducing isostatic
8.4 Pressure Agglomeration Technologies I 3 7 5
pressing into the manufacturing process, the number of rejects became very low. Later, the technology was used for the production of many other ceramic parts, particularly of the high performance variety, and for many applications in the metal powder industry, including mechanical alloying, for composite parts, consisting of ceramic, metallic, and plastic components, and for plastics (especially PTFE), explosives, chemicals as well as pharmaceutical specialties. Isostatic pressing is carried out cold or hot. The most common application is still cold isostatic pressing (CIP) which is performed at ambient temperatures. In hot isostatic pressing (HIP) the forming and densification process is achieved uniformly by heated high-pressure gas in an autoclave. The material to be processed itself is often also brought to elevated temperature prior to loading. Contrary to CIP, in which powders are always containerized, HIP may be applied for containerized powders and also for preformed metal, ceramic or plastic components. Manufacturing of the preforms may be by any method, including, for example, casting. In the latter case, porosity and structural flaws can be virtually eliminated and the properties as well as the service life of the parts may be markedly improved. Related functions are near net shape forming and diffusion bonding of dissimilar materials. The term hydrostatic pressing is often used as a synonym of isostatic pressing. Isostatic pressing is the generic term covering liquids and gases as the pressure transmitting medium whereas hydrostatic pressing is best reserved for liquids. However, the two are used interchangeably to cover both aspects. If flexible containers are used, their arrangement may be such that they contract or dilate by the application of pressure. Whether the tooling is an integral part of the press or loaded and removed during each compaction cycle determines if it is a “dry bag” or “wet bag” process. The difference between the two methods is illustrated in Fig. 8.147.
Fig. 8.147: Schematic representation o f the differences between dry bag and wet bag pressing [B.l2a, 5.42).
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Compaction ( a ) F i l l e d c a v i t y is clamped ( b l Apply pressure ( c l Compaction and dwell
Upstroke
Ejection and filling la1 Decompress ( b l E j e c t i o n f r o m l s o l a s t i c mold l c l Dispense measured q u a n t i t y of powder, t r a n s f e r p a r t
Fig. 8.148: Operational sequence ofa dry bag isostatic press for the manufacturing of spark plug blanks [B.lZa, B.421.
In the dry bag process, the flexible container is fixed in the pressure vessel and the powder is loaded directly. The tool forms a membrane between the fluid and the powder. Optionally, the flexible container may be placed inside of a primary diaphragm so that the powder never comes in contact with the fluid, even if the flexible mold is damaged or breaks. Therefore, dry bag tooling also has the advantage that the fluid is not contaminated with the powder. However, because the container must stand up to many pressing cycles and since changing it is time consuming, it has to be made of a very durable material. The wet bag process, in which the container, that has been filled externally with the powder, is entirely submerged in the fluid inside of a pressure vessel, utilizes the simplest type of equipment. This process is commonly applied in the laboratory or for pilot plants and is commercially used for the production of single large components or a large number of small parts. While dry bag tooling can be fitted with means to remove the gas that is being displaced during densification (for example, through internal rigid formers and the breech plug, see Fig. 8.148), the material in containers for wet bag pressing must be consolidated and evacuated prior to closing and loading into the autoclave to avoid compressed air pockets which, upon pressure release, may damage the structure of the compacted parts (see Section 8.1).
8.4 Pressure Agglomeration Technologies
The basic principles of isostatic powder pressing are: The wet bag pressing oflarge and/or complex shapes in which the flexible container is filled and prepared outside the pressure vessel and then immersed in the fluid and compacted. 2. The dry bag pressing of smaller, regular shapes in which the tooling forms an integral part of the pressure vessel. 3. The use of internal or external rigid formers to produce accurate surfaces. 4. Pressurization by systems using pumps or by direct compression with pistons in a die. 5. Handling systems for the filling, preparation, loading and unloading of powders and parts as well as for the tooling and pressing equipment. 1.
Therefore, isostatic powder compaction equipment consists of powder storage and dispensing facilities, at least one pressure vessel with means for loading and unloading the tooling or parts, pressure generator(s),and related items that enable effective and safe operation of the process. Particularly dry bag pressing is used for the production of small components at a high rate. As shown in Fig. 8.148, which demonstrates the compaction, ejection, and filling of a dry bag press during the manufacturing of a spark plug insulator, it is relatively easy to automate the operation of this process. The permanent location of the tool and small fluid volume surrounding it contribute to a fast operation. Production rates in the neighborhood of 100 parts per minute are common. Where mass production of simple, small compacts from powder is the task (e.g.
F Illina z ( a ) Apply vacuum to i s o l a s t i c mol d t o e n s u r e accurate c a v i t y ( b ) Dispense measured q u a n t i t y of powder i n t o the mold
'
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Ejection ( a ) Remove t h e compacted p a r.t f r o m the isolastic mold ( b ) Transfer the p a r t from t h e p r e s s
W
Compact ion ( a ) Clamp t h e integrated t o o l set a g a i n s t press frame ( b ) Apply i s o l a s t i c p r e s s u r e ( c ) Decompress Fig. 8.149: Operational sequence of an automatic isostatic press with round, timed tooling table [B.12a, B.421.
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c-
Fig. 8.150 The principle o f intensifiers [B.l2a, B.421.
Secondary
Pressure
Fig. 8.151: Photograph and schematic (inset) of a modern high pressure intensifier p u m p (courtesy EPSI, Haverhill, MA, USA).
spark plug insulator blanks, other electrical insulators, grinding media, carbide tool bits, etc.), the equipment usually takes the form of a multi cavity press or a series of small machines which operate similarly to conventional hydraulic presses. The actual production rates depend on powder properties, size of the part, maximum pressure and potential dwell time requirements, the number of tool cavities as well as preparation and handling needs. Large automatic units have been developed. Some such installations include a round, sequenced pressure chamber system (Fig. 8.149). Generally, the time needed to reach the required pressure is of great concern in isostatic pressing. It also depends on a number of factors, including the volume of the autoclave vessel, the compaction ratio of the powder, the compressibility of the fluid, and, most importantly, the delivery rate of the pumping system. To speed up pumping, it is possible to apply a number of pumps in parallel. Alternatively,
8.4 Pressure Agglomeration Technologies
Fig. 8.152 Photograph o f an automated wet bag cold isostatic press (pressure chamber dimension: 430 m m dia. 1,000 m m high, 200 MPa max. pressure) with mold washing and handling system (courtesy EPSI, Haverhill, MA, USA).
a pump system using different types of pumps to reach different pressure levels may be designed. Intensifiers (Fig. 8.150) increase the primary pressure according to the relationship of the piston areas. Fig. 8.151 is the photograph of a modern high pressure pump system with intensifier and double action design (see inset). The pump provides a constant flow of pressurized liquid from two opposed cylinders as the central hydraulic piston cycles back and forth. Fig. 8.152 is the photograph of an automated wet bag cold isostatic press. As mentioned before, one of the disadvantages of the wet bag technology is the danger of contaminating the pressure transmitting fluid with powder that either adheres to the outside of the bag, as a result of the filling, processing, and handling procedures, or arises from a failure of the container. Up to approx. 400 MN/m2 (= MPa),
Fig. 8.153: View into the cavity of a large wet bag production CIP (pressure chamber dimension: 1,220 mm dia. . 2,500 mm high, 70 MPa max. pressure) (courtesy EPSI, Haverhill, MA, USA, and Seagoe Adv. Ceramics, N.-Ireland).
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Fig. 8.154:
Pressure levels [in MPa] required during cold isostatic powder pressing for the manufacturing o f certain product groups (courtesy EPSI, Haverhill, MA, USA).
most cold isostatic presses operate satisfactorily on an oil/water emulsion or hydraulic oil which can be easily cleaned with conventional means or discarded after contamination. At higher pressures, special liquids may have to be used for which compatible tooling materials are required and with which problems may develop when it becomes necessary to dispose of the often toxic fluids. To avoid or delay contamination, mold washing may become part ofthe handling system (Fig. 8.152). Fig. 8.153 is a view into the open pressure chamber of a cold isostatic press and also shows the breech plug handling frame. As illustrated in Fig. 8.154, different pressure levels are required for the consolidation ofvarious powders. Pressures below 200 MPa are sufficient for the manufacturing
Fig. 8.155: Some typical parts (see also Fig. 8.1 54) that were manufactured by cold isostatic powder pressing (courtesy EPSI, Haverhill, MA, USA).
8.4 Pressure Agglomeration Technologies
Fig. 8.156:
Photograph ofan open production scale (pressure chamber dimension: 850 mm dia. 1,700 mm high, 200 MPa rnax. pressure, 1,400'C) hot isostatic pressing (HIP) unit (courtesy EPSI, Haverhill, MA, USA).
of parts from carbon/graphite, refractories, certain ceramics and, particularly, plastics (PTFE). Cemented carbides and powder metals, on the other hand, may necessitate pressures ofup to GOO MPa. Fig. 8.155 depicts some typical examples of parts that were produced by cold isostatic powder pressing. During hot isostatic pressing, parts to be HIPed are loaded into a pressure vessel which contains a modular electric furnace. A thermal barrier is placed around the furnace to direct the heat toward the parts and away from the water cooled vessel walls. Argon or other (inert) gases are used as the pressure transmitting medium. Depending on the temperature requirement and the atmosphere, interchangeable plug-in furnaces are available. Iron-chromium-aluminum (FeCrAl) furnaces create temperatures of up to 1,200'C and are capable of operating with a concentration of up to 20 % oxygen in a balance of argon: molybdenum furnaces, with temperatures
Fig. 8.157: Schematic lay-out o f a complete industrial HIP installation (courtesy EPSI, Haverhill, MA, USA).
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Fig. 8.159: Schematic representation of three different types of hot isostatic pressing [B.61]. (a)/(a'): Capsule HIP/sintering; (b)/(b'): sintered preform/HIP; (c)/(c'): HIP/sintering for making porous products.
8.4 Pressure Agglomeration Technologies
to 1,450 “C, are suitable for applications requiring a clean environment; and graphite furnaces are designed for use in argon and nitrogen up to 2,000 ’C. Graphite furnaces have a high resistivity and are, therefore, very well suited for operation in vacuum where the use of low voltage is necessary. As with all modern technologies, a computer control system is a standard feature. It is particularly desirable in hot isostatic pressing for the flexible programming of cycles, for process supervision, and data logging. Typically, all current system conditions are displayed and each cycle is stored under a specific name for review at a later time and quality assurance. Fig. 8.156 is the photograph of an open hot isostatic press and Fig. 8.157 is the schematic layout of a complete installation. Ancillary equipment may include load preparation stations, electrohydraulic compressor(s),vacuum pump(s),high pressure valve system, and closed loop vessel cooling system. Cooling of the parts and the interior of the furnace after finishing the consolidation cycle is particularly important in hot isostatic pressing. Cooling gas must be circulated uniformly through the entire work zone so that thermal distortion and grain growth are minimized. Fig. 8.158 shows three typical HIPed parts. While, originally, hot isostatic pressing was developed and used to remove defects and/or produce parts with minimum porosity and, consequently, ultimate density from powders and preforms, the study and understanding of the mechanisms of pressure agglomeration also led to a modification of the process which is then actually used to produce parts with a controlled high porosity [B.Gl] (see also Section 5.3.2). Fig. 8.159 illustrates schematically the different types of hot isostatic pressing. (a)/ (a’) and (b)/(b’)represent the conventional applications of hot isostatic pressing which ultimately result in dense products. HIPing which, because of the temperature of the process is associated with sintering (see Section 9.1), is applied to powders that are encapsulated in containers made from metal or glass (a) or to presintered bodies with closed pores (see also Section 5.3.2, Fig. 5.47). During the new HIP for making porous products, open, non-containerized powder compacts or loosely sintered bodies are HIPed directly at high temperatures in a high pressure gas atmosphere. In this situation, densification is avoided or delayed by the high pressure gas that fills the open pores. High open porosity of sintered materials, which experience considerable densification during conventional sintering alone, can thus be obtained by the combination of HIP and final sintering.
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Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
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9
Agglomeration by Heat/Sintering At a certain elevated temperature, which is different for various materials, while still in solid state, atoms and molecules begin to migrate across the interface where particles touch each other. Depending on temperature, time, and intensity of contact (caused by pressure during the manufacturing of the preform or the sintering process itself) diffusion of matter forms bridge-like structures between the surfaces which solidify upon cooling. The process may also result in a densification of a compact which is associated with a densification by the elimination of pores and shrinkage. This entire group of phenomena is called sintering. Sintering is a binding mechanism (see Section 5.1.1), a size enlargement process by agglomeration (see Sections 9.2, 9.2.1, and 9.2.2), and a method of post-treatment to create final characteristics of agglomerated products (see Sections 7.3 and 8.3). In the following, after a short introduction into the sintering mechanisms, sintering as a size enlargement process by agglomeration will be covered. Different parts of the book should be consulted for information on the other meanings of the word. 9.1 Mechanisms o f Sintering
As already mentioned earlier, particles in a powder mass can be bonded in solid state at elevated temperatures below the melting or softening temperature of the material(s) (see Section 5.1.1). This process is called (solid state) sintering. If the powders are predensified in a compact, sintering is typically accompanied by further densification of the body until only few and isolated pores remain, but other processes, which retain some open porosity can occur, too [B.Gl] (see also Section 5.3.2). The geometrical arrangement of the particles largely determines what can happen during sintering although it does not predict whether or how fast particular changes occur [B.12c]. The driving force for sintering is the diminution of surface area of the assembly of original particles. The accessible internal surface of the particles or surface ofthe pores between the particles features a specific surface energy. This specific surface energy is due to the fact that surface atoms have no neighbors. The reduction of free surface also leads to a reduction in surface energy. Therefore, sintering occurs with a reduction in total surface energy and, accordingly, the total free energy of the powder decreases with sintering.
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Most attention has been focused on the kinetics and mechanisms of sintering and less on the geometry of the process [B.l2c]. In general, a particulate multiphase body consists of a series of cells (pores) and grains which meet, in stable configurations, three to an edge and four to a point. These three-grain edges and four-grain corners are the fundamental units of structure and their shapes are determined by the equilibrium between the various surface energies involved. When there are two phases, one of which is porosity, it is energetically favorable for the second phase (porosity) to occupy, in order of preference, four-grain corners, threegrain edges, two-grain faces, and, lastly, sites in the interior of grains, because the total amount of interface or of interfacial energy is thereby reduced. Basically, sintering occurs in stages. During the first stage of sintering,the points of contact in a collection of particles grow into necks by mass transport (see also Section 5.3.2, Fig. 5.48).The difference in free energy or chemical potential between the surface of the neck area and the surface of the particle provides the driving force [B.Gl]. After a time and at a point where porosity becomes approx. 15 %, the grain boundary energy begins to be a significant contributor to the total energy of the system and the grain boundaries begin to rearrange themselves to minimize their total area. The geometry in this second stage of sintering becomes that of an assembly of polyhedral grains with pores along the three-grain edges (Fig. 9.1) and the tendency to minimize the grain boundary area results in grain growth. The remaining pores continue to shrink as sintering proceeds and, in the third stage of sintering, become unstable as approximate cylinders along the three-grain edges and pinch off to become isolated pores at four-grain corners. This third stage commences at approx. 5 % total porosity and may continue until all porosity is eliminated. Fig. 9.2 depicts typical curves of the change of open and closed porosity during the second and third stages of sintering as functions of relative density [B.l2c]. A new phenomenon may occur during the third stage of sintering. If gas is trapped in the closed pores its pressure will rise as the pore size decreases until further shrinkage stops when an equilibrium is reached. It is, however, energetically favorable from the point of energy stored in the compressed gas and neutral from the point of surface energy if gas transfers from small pores with high pressure to large pores with low pressure. If this is possible, the result is an increase in the volume of the part, a phenomenon called bloating.
,Pore
\ Particle
Fig. 9.1: Schematic drawing of the pore structure developing in the second stage of sintering.
9. I Mechanisms of Sintering 15-
-c
1
a
c
B 5-
Fig. 9.2: Example o f the change of open and closed porosities during the second and third stages of sintering as a function o f relative density [B.lZc].
Relotive density (%)
The adjustments that are needed during sintering to minimize the surface energy in a body necessarily involve the movement of matter. This has been previously discussed in some detail in Section 5.3.2 (Fig. 5.48 and Tab. 5.7) but shall be repeated here in a somewhat different presentation. In the case of a single solid phase, matter can be moved under the influence of surface energy from the neighborhood of convex surfaces to that of concave surfaces by several mechanisms. These are: 1. Evaporation and subsequent recondensation, influenced by vapor pressure and surface curvature. 2 . Diffusion over the surface, atom by atom. 3. Plastic flow by dislocation movement in a crystalline material. 4. Viscous flow in an amorphous material, requiring a liquid phase. 5. Bulk diffusion through the solid, down to the vacancy gradient produced by the pressure gradient resulting from variations of the surface curvature. Of these, the first two are not capable of moving the centers of particles closer together and, therefore, do not cause densification. They are, however, causing neck growth and strengthening of the initial assembly or compact of particles. The other processes can cause both neck growth and shrinkage. In addition to solid state sintering, liquid phase sintering is possible. From a binding mechanism point ofview, this process could be described as one using partial melting (see Section 5.1.1). In connection with sintering, an appreciable volume of a liquid phase must be present, the solid must be soluble in the liquid, and the liquid phase must completely wet the solid. These are criteria that are also required for growth agglomeration (see Section 7.1), although the liquid source and the structural consequences are different in sintering. Liquid phase sintering is, for example, used for hard metal alloys from powders [B.12c].Because of the great hardness of the carbide particles, which constitute the
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bulk of hard metal powders, it is impossible to press the powder to a density >GO % of theoretical. Yet, by liquid phase sintering a perfect pore-free compact can be obtained. The detailed metallurgical description of the sintering operation is highly complex because of the complicated nature of the phase equilibria that are involved. Very simply explained, sintering begins by solid diffusion, frequently aided by a carburizing atmosphere, and results in the formation of a tungsten-cobalt-carbon eutectic with a relatively low melting point at suitable points of contact between particles. Once small amounts of liquid have formed, further liquefaction proceeds rapidly and surface tension forces cause rapid flow and wetting of all the other solid surfaces. This is accompanied by considerable contraction (shrinkage), often 40 % by volume. The whole process occurs with great speed at only a few degrees above the eutectic temperature. Very small quantities of liquid phase, approx. 1 % by volume, which is much less than one might assume necessary, are sufficient to result in rapid and near complete densification. The high packing density must be caused by a considerable rearrangement of the positions of carbide particles. However, because of the small volume of liquid that is involved it is not possible that this densification is obtained by particle rearrangement alone. Even with the best packing of the solid particles, the liquid phase would be insufficient to completely fill the interstices. Therefore, some change in particle shape must also occur and there must be considerable material transfer at the contact points to bring about this shape change. For this, the only possible phenomenon is the solution-reprecipitation process. Ceramic parts are often made from finely powdered components which are shaped by a pressure agglomeration technique and then sintered by the application of heat. In some cases this simple technique is not applicable because the powders may not sinter together, the sintering temperature or atmosphere requirements may be impractical or uneconomic, or a base material may not be readily available or may decompose under normal sintering compositions. In some of these cases, reaction sintering may offer the possibility of making sound ceramic bodies, often also with high density, which are impossible or at least difficult to produce by other methods [B.l2c]. The term reaction sintering is not very well defined. In the simplest case, the powder that decomposes during heating is substituted although, in the true sense of the word, this is not actually reaction sintering. In true reaction sintering processes, two (or more) components of the desired ceramic compound are selected such that they react with each other during sintering. In a first method, the powder components are mechanically mixed, shaped, and reaction-sintered. It is particularly suited for those materials which are solids at room temperature and feature relatively high melting points. Another process is applicable when one of the constituents is gaseous at room temperature (e.g. oxides, nitrides) or oflow melting point relative to the other components (e.g. phosphides, sulphides and some silicides or aluminides). Then the more refractory powder components are shaped and afterwards reacted hot with the other constituent in gaseous or liquid form. The preshaped body must be porous to allow for entry of the reactant(s) and for extra volume (if any) of the product of reaction. As mentioned before, sintering may be beneficially carried out under pressure (see also Section 8.4.4, HIP). During pressure sintering of a powder, an external pressure is applied and the initial stage of compaction, probably up to a relative density of approx.
9.2 Sintering Technologies I 3 8 9
85 %, includes the complex mechanisms of pressure agglomeration, i.e. particle packing, sliding, fragmentation, and deformation (see Section 8.1).The subsequent intermediate (featuring connected porosity) and final (with closed porosity) stages both involve a solid matrix and a definite pore system. A theory of pressure sintering [B.l2c]is valuable because it allows to extrapolate experimental data for a given material, for predicting performance under changed conditions, and also allows the calculation of viscosity or diffusion data, thus affording a means of assessing possible results when changing the composition of the material. In pressure assisted sintering, the more accurate name for pressure sintering, pressure and heat are applied simultaneously to a powder that is enclosed in a die. Generally, it permits the use of lower temperatures and pressures and shorter processing times than those required for cold pressing and subsequent sintering. It can also assist in the production of parts with finer grain size, lower porosity, and higher purity. As mentioned in Chapter 9, sintering is a binding mechanism and the different technologies may be used for size enlargement by agglomeration and as methods for the creation of final characteristics of various products. Depending on their applications, the required properties of pieces or parts after the sintering process may vary widely. Correspondingly, different methods for powder preparation, the manufacturing of preforms, and the application of heat must be chosen. For example, iron ore pellets, which must feature high strength to guarantee the excellent transportation and handling characteristics required for a bulk commodity, uniform size and shape for good and reproducible packing in shaft and blast furnaces, and a large percentage of open porosity for optimum reducibility, are made by tumblelgrowth agglomeration in discs and drums (see Section 7.4.1), dried and sintered to produce necks between the ore particles but retain the high porosity of the agglomerates. Powder metal or most ceramic parts, on the other hand are formed into dense compacts by pressure agglomeration (see Sections 8.4.1 through 8.4.4) and then sintered, possibly with the assistance of pressure (HIP, see Section 8.4.4) to yield well (sometimes near net) shaped parts with nearly theoretical density and high strength. Other powder metal or ceramic parts may have to become filters or catalyst carriers requiring large numbers of penetrating pores (see Section 5.3.2), uniform structure, and high strength. In those cases sintering of preforms is carried out such that no densification occurs and porosity remains unchanged.
9.2
Sintering Technologies
To remain within the scope of this book, the description of industrial agglomeration processes, only those sintering technologies will be reviewed which are used either directly for the size enlargement of particulate solids or for the post-treatment of agglomerates to gain final product properties. The many highly sophisticated sintering techniques that have been developed during the past few decades for the production of new, engineered, often composite materials with novel characteristics, will be covered in a future book [B.71].
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9.2.1
Batch Sintering
Batch sintering in stationary furnaces of different kinds is used primarily for the posttreatment of agglomerated products to gain final strength and properties but also for laboratory work in connection with the development of continuous sintering technologies and for pilot plants [B.12c, B.16, B.251. The two most important applications of batch sintering are in the ceramic and powder metal industries. Muffle, bell, elevator, or pit type furnaces are used. As shown in Tab. 9.1, the processes occurring during sintering are somewhat different for ceramic and powder metal parts. During sintering, ceramic ware is heated to a temperature between 700 and 2,00OCC.Because raw ceramic parts are almost always “green” (= moist), removal of water is the first post-treatment step. Following or simultaneously and immediately preceeding firing, binders and plasticizers, which have provided the properties needed for forming, are also removed. The amount of residual moisture and/or additives that can be tolerated in the part when firing begins depends on its shape and structure as well as on the heating rate of the furnace. If the part is made by dry pressing, removal of additives can be incorporated into the heat-up stage of the sintering furnace if the time for this process is not too long. However, since additives are often cellulose, wax, or starch type products, they can be conveniently decomposed by oxidation in air at low temperature prior to loading the parts into the furnace. In the furnace, clay minerals usually dehydroxylize between 500 and 600 ”Cwhereby steam is produced. The loss of strength that occurs at this stage may result in cracking. Often, carbon and sulphur compounds are present in unfired ceramics. They must be oxidized before sintering densification has advanced too far to avoid black cores. Oxidation can be accomplished by holding the temperature at a certain level which varies with the manufacturing method that was used for and the type of the body but is often in the range of 300°C. A decomposition of carbonates and sulphates may produce bloating in vitrified parts. Silica, which exists in many different crystalline forms, is an important constituent of most ceramics. The conversion from one form into another is accompanied by sometimes large volume changes. Because this occurs during heating and cooling, the rate of temperature change must be considered and may have to be adjusted to avoid deformation and/or cracking. Processes occurring during the sintering of ceramic and powder metal parts [B.l2c].
Tab. 9.1:
Ceramic Parts
Powder Metal Parts
Removal of water Removal of binder and organic media Dehydroxylation Oxidation Decomposition Phase transformation Cooling
nia Burn-off of pressing lubricant n/a Heating to sintering temperature Soaking nia Cooling
9.2 Sintering Technologies I 3 9 1
Fig. 9.3: Photographs ofsimple atmospheric muffle furnaces (courtesy Gasbarre, Sinterite Furnace Div., St. Marys, PA, USA).
Most ceramic products are fired in air, i.e. under oxidizing conditions. The ideal kiln for the firing of ceramics is capable of heating and cooling the parts uniformly at the maximum rate of temperature change for each of the stages mentioned in Tab. 9.1. Simple muffle furnaces are typically used for batch sintering in the ceramics industry (Fig. 9.3). For high quality wares, temperature control is very important to avoid the previously mentioned potential problems in different processing stages. It can be accomplished easiest and most accurately in batch furnaces although many bulk ceramic products must be of such low cost that continuous furnaces are used which operate more economically (see Section 9.2.2). Some materials must be, at least during certain stages, fired in reducing atmosphere which can be also easily provided in batch kilns. In powder metallurgy, sintering requirements are different. Although the volatilization of pressing lubricant from the compact prior to sintering is sometimes carried out separately, it is more typically an integral part of the process. During sintering itself, temperature is held constant so that no distortion takes place and full bonding is obtained. Therefore, temperature control and soaking periods are most important. Additionally, it is necessary to retain the composition of the atmosphere to ensure reproducibility of strength, carbon content, dimensional stability, etc. of the final part. Therefore, ingress of air into the furnace during sintering must be avoided. This is achieved by using either a gas tight furnace shell or a muffle or retort which is usually manufactured from a nickel-chromium alloy. Batch sintering furnaces are employed for: 1. Low output production, 2. special duties, (because there are no moving parts, batch furnaces can be designed for higher temperatures; furthermore, since it is possible to seal the interior more effectively, purer atmospheres can be realized and maintained) and 3. experimental work.
For (1)and/or (2), a small manual pusher furnace can be applied in which parts on a tray are moved through a furnace, one tray at a time (Fig. 9.4). If it features gas tight
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UNLOAD
COOL
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TRAVEL
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CURTAIN
Fig. 9.4 Sketch o f a longitudinal section-box or manual pusher furnace [B.25].
interlocks and/or doors for charging and discharging, it can also be used for sintering processes in which the atmosphere must be well controlled. The bell type furnace (Fig. 9.5) is widely used for P/M parts requiring long sintering cycles. Typical equipment consists of one or more supporting bases with removable sealed retorts, to cover the loads and to retain the protective atmosphere around them throughout the entire heating and cooling cycles, a portable heating bell, and a standby (idling) base, a hoist, and an optional (not shown) cooling bell. The elevator type furnace (Fig. 9.6) is useful for sintering heavy and/or bulky loads. It has an elevated heating chamber with open bottom in a fixed position, a mechanism for raising and lowering load supporting cars into and out of the furnace, a stand-by car to plug the kiln opening during idling periods, and optional cooling chambers. It is also applicable if protective atmospheres of exceptionally high purity are required. Flexible hoses carry atmosphere gas and cooling water to and from the cars.
IDLING BASE
HEAT ING BELL ON RETORT
RETORT ON LOAD
Fig. 9.5: Schematic representation of a bell furnace [8.25]
LOAD ON BASE
9.2 Sintering Technologies
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+
Fig. 9.6: Schematic representation o f an elevator furnace [B.25].
Batch kilns can be operated under vacuum and as direct-resistance furnaces for the sintering of refractory metals (e.g. for tungsten at 3,000 "C). For hardmetal processing, lower temperatures are used. Fig. 9.7 shows typical pressure and temperature profiles. A complete sintering cycle may take from G to 14 h or longer for thick parts. Therefore, such furnaces are often connected in pairs to a common vacuum pumping system and power supply as well as a single set of controls because approx. 50 % of the cycle is required for heating under vacuum and the balance for cooling in inert gas. During various phases of the heating cycle, inert or active gases may be injected into the vacuum system, thereby changing the partial pressure in the sintering chamber, to
4-
!
700 i-
500
T i m e (Hours) Fig. 9.7: Typical pressure and temperature profiles of a vacuum sintering cycle in a batch furnace [B.25].
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Pot grate Insulating material : fired pellets
3
Pot grate Insulating material : retroctory lining
/-
1.) Green pellets
1.) Green pellets
2 . ) hsulating side walls : pellet fragments 3.) Grate bars :cast alloys 4.) Insulating hearth layer
2 . ) hsulating side w o k : bricks or rammed mass
3.) Grate bars: usually silicium carbide
Fig. 9.8: Two-pot grate furnaces for the heat treatment of (iron ore) pellets [B.l6].
achieve special effects which influence the structure and properties of the sintered part. Since batch furnaces are normally relatively small and can be controlled easily, they are also commonly used for development work. For the sintering of ceramic or powder metal parts the results from small scale testing can be directly transferred to larger or continuously operating kilns. As will be shown in Section 9.2.3, large continuous sintering facilities are used in the minerals industry for the size enlargement of fine ores and the induration of “green balls”, spherical agglomerates made by tumble/growth agglomeration from fine ore concentrates (see Section 7.4.1).Since, in the development phase, the heating and gas flow conditions in industrial plants must be simulated, batch pot grate sintering equipment (Fig. 9.8) is being used [B.lG]. Representative tests for determining the performance of travelling grate, grate-kiln, and shaft furnace processes (see Section 9.2.3) can be carried out in these laboratory and pilot facilities. According to the actual conditions in industry, pot grates are operated either with insulating side walls of fired pellet fragments and a hearth layer (Fig. 9.8, left) or are equipped with a corrugated refractory brick lining (Fig. 9.8, right). In the first case, the pot grate itself is a metal container with a bottom of metallic grate bars. By placing indurated pellets between the hot wall and, respectively,the grate and the green pellets which are to be sintered, the charge is protected by a refractory envelope and overheating of the metallic parts is avoided. Other pot grates feature a refractory lining and a high temperature resistant grate (Fig. 9.8, right). To eliminate the wall effect
9.2 Sintering Technologies
in these relatively small furnaces and to be able to test samples of different ores during a single test run, stainless steel baskets, which are filled with the appropriate ores or pellets, are embedded in the charge. After the test, they are retrieved and the characteristics of the fired materials are determined. During a pot grate test the following parameters are determined and may be varied: Direction of the air flow, gas volume, suction, and pressure in the wind box, preheating rate and temperature profile, fuel type (gas or oil), gas atmosphere (using additional oxygen, if necessary). Fig. 9.9 shows the operation of a pot grate and the locations of thermoelements. In this case, drying is carried out first with updraft warm air (flowing up through the pellet bed), followed by downdraft sintering with hot air from the burner above the bed, and, finally, cooling is accomplished by an updraft flow of ambient air. It is also possible to design the system such that during sintering separate up- and downdraft stages can be used. During the entire process, the temperatures are continuously monitored as they are decisive for the quality of the fired product. In a pot grate as shown in Fig. 9.9, the different process stages occur intermittently, one after the other. When the flow of gas is reversed, the temperature of the peripheral
Burnet hood mvable
TZ M i l e ot pellet bed
16 In wirdbox Wasle gas M Drying gas or Cwling oir
Fig. 9.9 Operation and temperature control of a pot grate [B.16].
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V - 1
Fig. 9.10:
iv
U
n
I
Pol grote positions _______(
Movable pot grate system for sintering tests lB.16)
parts, which are associated with the pot grate itself, must first change to the new condition which can critically modify and impair the test conditions. To avoid this, a movable pot grate system as shown in Fig. 9.10 was developed [B.lG] in which all equipment for the different steps is kept at process conditions and the pot grate is moved as necessary into the various positions. It should be mentioned that pot grate sintering machines may be also used for the industrial sintering of small amounts of metal ores. Fig. 9.11 shows the flow diagram of a pan sintering plant [B.23]. In such sintering systems, heat is provided by the burning of carbonaceous solid fuels that were mixed with and are uniformly distributed in the charge. After ignition of the bed surface, for example by depositing red-hot CYCLONE SEPARATOR
RETURN S I N T E R C O K E ORE
WET SCRU 8 B € A
SlNTERlNG
Fig. 9.11:
Flow diagram of a pan sintering plant p . 2 3 , p. 2121.
9.2 Sintering Technologies
charcoal or coke, oxygen from the air, which is pulled through by a suction fan, produces the intense heat that is necessary for sintering. During sintering, the entire charge becomes one large cake which, after cooling, is broken and screened into the desired sinter size. Fines are recirculated and blended with fresh fine ore and fuel. Such plants typically produce 30-50 t/day of sized sinter for use in reduction furnaces. 9.2.2 Continuous Sintering
As mentioned in Section 9.2.1, sintering of ceramic wares normally occurs in oxidizing atmosphere and without a special gas environment. Therefore, continuous sintering furnaces are often directly flame heated. Fig. 9.12 is the side elevation of a tunnel furnace for the firing of ceramic parts, indicating the direction of material (car)and gas movement as well as the process zones. The diagram below depicts the temperature profile over the length of the furnace and shows that temperature control is normally quite unpretentious. Most tunnel kilns for ceramics are of the car type. Cars are more rugged and reliable than belts and other continuous methods of movement. Fig. 9.13 is a schematic cross section through the sintering zone of a directly fired, atmospheric tunnel furnace. The tunnel is enclosed by refractory walls and a simple sand seal prohibits the exit of hot combustion gases at the car base and wheels. Operation of modern furnaces is computer controlled and continuous movement is accomplished with automatic loading and unloading systems. Fig. 9.14 is the partial view of a state-of-the-art tunnel kiln for the firing of table ware and also shows an automated car handling system. Direction of cars
aL
Direction of goses *
W Reheat zone--
f,
h
h6t:*Y+
*
Y
Firing z m e
0. 4
A
--
4
I200 r
3L
Air blower
I
s-0
300
200 I00
Fig. 9.12: Side elevation (schematic) o f a directly flame heated tunnel kiln for the firing o f ceramic parts and typical temperature profile [B.lZc].
W
Cooling z m -
ia
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wheel
base
Fig. 9.13: Cross section through the sintering zone of a tunnel kiln [B.l2c].
In comparison, continuous furnaces for the sintering of P/M parts are more complicated because of the needs for a more differentiated temperature profile (Fig. 9.15) as well as for controlled gas environments (see Section 9.2.1). The latter requires some sort of separation of the atmospheres in different sections along the kiln, either by oscillating doors or by gas curtains.
Fig. 9.14
Photograph o f a modern tunnel kiln for the tiring o f table ware with a fully automated car handling system (courtesy Eisenmann, Boblingen, Germany).
9.2 Sintering Technologies
+ BURNOFF -+-SINTER -+-? SLOW 1 HEAT 1 SOAK I HEAT I SOAK ]COOL(
TI ME Fig. 9.15: Typical temperature profile o f a continuous furnace for the sintering o f powder metallurgical parts [B.25].
Fig. 9.17: Schematic and photograph o f a horizontal mesh-belt sintering furnace including an optional "accelerated delube system'' (ADS) (courtesy Gasbarre, Sinterite Furnace Div., St. Marys, PA, USA).
COOL
-- 7
!
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The most commonly used conveyor in continuous furnaces for the sintering of small, light parts is the mesh-belt. Fig. 9.16 is a schematic longitudinal section through a mesh-belt sintering furnace and includes an indication of the different zones. The doors, which can be manually operated, are usually left open and the atmospheric conditions within the furnace are created by and between the gas inlets. This puts a certain strain on the atmosphere generating equipment as ample capacity must be available. Fig. 9.17 shows both the schematic and a photograph of a horizontal mesh-belt sintering furnace which includes an optional "accelerated delube system" (ADS) for the rapid removal of powder metal lubricants. A variation ofthe straight mesh-belt furnace is the humpback kiln (Fig. 9.18),which is used where high purity of the atmosphere in the sintering zone is required. The belt in a long, inclined, gas tight purge chamber carries the work from the charge area up to the elevated hot zone. This design is particularly advantageous iflight gases are used in the sintering zone because these gases tend to naturally rise to the highest point of the furnace. Fig. 9.19 is a schematic representation of a roller hearth furnace. Loaded trays are conveyed through the kiln by riding on individually driven rolls (Fig. 9.20). Depending on roll spacing, a section is capable of holding a substantially greater load than an equivalent length of mesh-belt. The grade of alloy used for the rolls limits furnace temperature to between 1,150 and 1,260 "C. The charge and discharge doors are automatically opened or closed and are interlocked with the tray handling system. Because they are only opened when a tray is charged or discharged, the amount of atmosphere gas is optimized and heat losses are minimized. HEATING CHAMBER
COOLING CHAMBER EXIT INCLINE
EN1. R A K E INCLINE
MUFFLE ~. .............
WIRE MESH BELT
Fig. 9.18:
Fig. 9.19
Schematic representation o f a hump-back furnace [B.25]
Schematic longitudinal section through a continuous roller hearth sintering furnace [B.25].
IDLING
PULLEY-J
9.2 Sintering Technologies
Fig. 9.20: Roller drive system using cogged belts which offer durability, low maintenance, and quiet operation Roller hearth sintering furnaces with temperatures of up to 1,450 ‘ C can be equipped with this design (courtesy Eisenmann, Boblingen, Germany)
MECHANICAL PUSHER
PURGE 8 PRCHEAT CMPIYDER
k
HIGH H E A T CHAMBER
WATERCOOLEOCHbYBER
UNLOADING P L A T F O R Y
m a w ) . . r i ~ e sY I O Y H e i r
Fig. 9.21: Longitudinal section through a continuous pusher furnace lB.251.
The pusher type furnace that was shown schematically as manually operated equipment in Fig. 9.4 (Section 9.2.1) can be mechanized and then becomes a continuous kiln. Fig. 9.21 is the longitudinal section through a continuous mechanical pusher furnace. It is suitable for sintering metal parts which are too heavy for the meshbelt and production rates do not warrant the roller hearth. It can also be used for sintering temperatures of up to 1,GSO”C which are too high for the mesh-belt and the roller hearth furnaces. Mechanically or hydraulically operated, intermittent or continuous, stoker type pushers are available. Fig. 9.22 is the photograph of a pusher furnace for the high temperature sintering of powder metal compacts. The final typical design of continuous sintering furnaces for powder metallurgy and similar applications is the walking beam furnace (Fig. 9.23). With this furnace the weight of the work that can be conveyed safely is practically unlimited and the m a imum continuous operating temperature is only limited by the refractory material used to line the hot zone chamber and by the compatibility of the atmosphere with the heating element. The sintering temperature may be as high as 1,800”C. Fig. 9.24 is a schematic cross section through the hot zone of a walking beam furnace. A comparison with Fig. 9.13, above, shows the hermetically closed furnace housing which is typical for all metal sintering furnaces and the vertical (left) or horizontal (right) noncontaminating electrical resistance heating elements. Both are required to
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Fig. 9.22: Photograph o f a high temperature pusher furnace (courtesy Casbarre, Sinterite Furnace Div., St. Marys, PA, USA).
I
W A L K I N G BEAM'
HORIZONTAL MOTION CYLINDER
8 Fig. 9.23: Longitudinal section through a walking beam sintering furnace [B.25].
maintain a particular composition of the atmosphere in the kiln. Movement of the charge in a walking beam furnace is accomplished by a mechanism that lifts and pushes forward, by only a few centimeters each, a bottom tray with the beams below (see hydraulic cylinders (A) and (B) in Fig. 9.23). The two, timed displacements produce a rectangular motion which conveys the work through the furnace at the required speed. The use of sintering for the induration of ceramic products (bricks, pots, vases, etc., see also Chapters 2 and 3) is quite old but through the centuries, even though empirically improved, was exclusively carried out in batch kilns. Continuous heat treatment of ceramic, powder metal, and other pre-agglomerated parts is less than 150 years old. Another original, also quite old application of sintering is found in the minerals industry that is associated with technologies for the production of metals. There, agglomeration by heat was introduced many centuries ago for the size enlargement of fine ores. Primitive versions of the pan sintering process (see Section 9.2.1, Fig. 9.11) were used to produce sinter from fine ores which had been mixed with a particulate solid fuel. The necessary heat was produced by blowing air through a bed of ore particles with bellows and burning charcoal that was uniformly distributed within (Fig. 9.25). Continuous sinter plants for ores were developed at the beginning of the 21st century for the size enlargement of fine ores, flue dust, mill scale and other fine metal bearing materials [B.42].At the beginning, metal cars with perforated or slotted bottom
9.2 Sintering Technologies I 4 0 3
IGHT S H E L L
-
Y
Fig. 9.24: Sketch showing a cross section through the hot zone o f a walking beam furnace (left: vertical heating element; right: horizontal heating element) [B.25].
were pushed through a directly fired tunnel furnace. As discussed in Section 9.2.1, Fig. 9.8, left, screened, fired fines were placed between the metal walls and bottom and the feed containing the solid fuel to avoid overheating of the mechanical parts. Later, the carts were connected to form a continuous grate belt which was moved continuously through a tunnel furnace by a motorized drive. Fig. 9.26 shows schematically the operating principle of such a travelling grate sintering machine. First, recirculated, fired (= sintered) fines from the sinter screens are deposited on the grate as a hearth layer. Then, feed, consisting of a blend of fine ore
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Fig. 9.25: Georgius Agricola's (De Re Metallica, 1556) representation o f a small ore sintering furnace.
and solid fuel, is placed onto the insulating hearth layer with a swinging conveyor (for uniform bed depth across the wide grate carts) and leveled with a roll feeder. Next, the charge thus built passes through an ignition furnace which is a short hood with burners inside. The flames impinge the surface of the bed and ignite solid fuel that is close to the hot interface. Beginning at this point and after leaving the hood, air is pulled through the completely open bed in a downdraft fashion; combustion of the solid fuel is sustained and enhanced by providing excess oxygen and the burning hot zone travels downward through the bed. The amount of air which is pulled through the entire length of the bed and the speed of the grate are adjusted such that, at the end of the machine, the fuel has disappeared and the entire charge has sintered together. The porous solidified mass is broken in a (hot) sinter breaker before the pieces are screened into the desired particle size distribution and externally cooled. Fines are recirculated to provide the hearth layer and potentially oversized pieces are recrushed in closed loop with a screen. Solid particles that are entrained in the combustion air, settle in bins which are part of the main suction duct and fine dust is removed in a dust collector. These solids are recirculated to the burden preparation plant and ultimately fed back to the sintering machine.
Feed i f r a m burden preparot!onl
Swinging conveyor
Roll f e e d e r sin:^: b r e a k e r
c o i l e c t o r ond
Counterweighted dust valves
Fig. 9.26
Schematic of an early travelling grate sintering machine.
'
9.2 Sintering Technologies
Although, after the development of pelletizing (see below), sinter has lost some ofits importance as a sized feed for reduction furnaces, the technology is still used, particularly in the iron and steel industry, and occasionally new sinter plants are installed. However, overall, since the 19GOs, worldwide production of sinter is decreasing in favor of pelletizing (Fig. 9.27).Nevertheless, even today reports dealing with improved sinter machine designs are published [B.48].While the basic process is still the same, new developments are directed towards better burden preparation, particularly in connection with unusual ore compositions, improved temperature control, minimization of pollution, general process optimization, and reduced energy consumption (Fig. 9.28). During the middle of the 20th century in several locations, particularly in the USA, development work started to render the large reserves of Taconite and Itabirite, low grade iron ores, useable for iron and steel making. In the early 1960s iron ore pelletizing was developed. The iron ore concentrates which, after upgrading, have a particle size <44 pm (see also Section 5.4) are agglomerated by tumblelgrowth methods (pan, drum, cone, Section 7.4.1) into nearly spherical “green” agglomerates with a narrow particle size distribution around 12 mm. These so-called iron ore pellets are held together by surface tension and capillary forces. During a post-treatment, which consists of drying, induration (sintering),and cooling, final strength and structural properties (e.g. porosity) are obtained. Fig. 9.29 shows schematically the three induration furnaces and the corresponding plant layouts that have been developed [B.42].The furnace types are l. the shaft furnace, 2. the straight (or sometimes circular) travelling grate (or strand), and 3. the Grate-Kiln.
Fig. 9.27: Development o f blast furnace charge compositions in Germany [B.48, p.1611.
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Fig. 9.28:
Flow sheet o f a modern sintering plant with improvements and cost saving features [8.42].
G r e e n agglomerates B u r n e r chambers
Shaft Pellets
D Drying B Firing C Cooling
Fig. 9.29
Schematic representation o f the three most c o m m o n heat induration (sintering) furnaces for iron ore pellets and sketches depicting the corresponding plants. (a) Shaft furnace, (b) straight (travelling) grate, (c) Grate-Kiln [B.42].
Balling d r u m clrcuits
9.2 Sintering Technologies
Fig. 9.29 cont’d
During post-treatment of the green iron ore pellets, they must be first dried and preheated before induration by sintering occurs. As in many similar curing processes that are used for the strengthening of moist agglomerates, the problem of this sequence of events is that after drying the original binding mechanisms (capillaryforces
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and surface tension) have disappeared while sintering, which requires approx. twothirds of the melting or softening temperature, has not yet begun. Therefore, there is a time interval during which the dry agglomerates exhibit almost no strength. Theoretically, only the travelling grate exerts low enough stresses in the essentially stationary bed that the weak pellets can survive. In reality, even these machines, because of their relatively crude design, which must be suitable for hot operation, introduce vibrations and other dynamic forces which endanger the survival of the dry agglomerates. To overcome this problem, additives are used during tumblelgrowth agglomeration (see Section 5.1.2) which retain some bonding characteristics in the dry state and improve the chance of survival until sintering begins. In iron ore pelletizing, the additive is traditionally bentonite, a natural montmorillonite clay. Unfortunately, bentonite not only increases the cost of the process but also introduces impurities (slag components in iron making). Therefore, efforts to optimize the process have yielded additives which do retain strength in the dry stage but burn out or otherwise disappear at high temperature, thus avoiding unwanted contamination. Considering that the first commercial plants for iron ore pelletizing were put into operation in the early 1950s [B.42],this technology has experienced quick acceptance and growth. Fig. 9.30 shows the worldwide development of installed capacity. If operating capacity is considered, the trend after 1990 fell short of expectations because some of the older plants were phased out, the associated mines have reached the end of their economical life, and cheaper high quality lump ore became available together with other iron sources, notably direct reduced iron (DRI).While, in the past, approx. two-thirds of the iron ore pelletizing plants were located at the mine sites, approx. one-quarter at the shipping or receiving ports, and the remaining approx. 10 % at integrated steel works, new systems are often built in connection with direct reduction plants, which are located were energy and reductant are cheaply and abundantly available. I
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1955 1960
1965 1970
1975 1980
1985
1990
Fig. 9.30 Graph showing the world-wide growth o f iron ore pel letizing [B.42].
1995
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
I
10
Special Technologies Using the Binding Mechanisms of Agglomeration As mentioned several times before, agglomeration is a natural phenomenon and methods for the size enlargement of solids by agglomeration date back to the beginning ofhuman life on earth (see particularly Chapters 2 and 3). A new era began when, during the past century, the Mechanical Process Technologies and, among the others (see Chapter 1, Fig. l,l),the unit operation “Size Enlargement by Agglomeration” were recognized as fields of science in their own right and applied interdisciplinarily. It was then found that a common body of concepts and techniques exists which applies to any of many different processes and situations in all industries that handle and process particulate solids. The basic phenomenon agglomeration applies to all situations where at least two solid particles are joined together under the action of a binding mechanism (see Section 5.1.1). Thereby, one type of the particulate solid may be very small and the other extremely large. Such a condition exists, for example, if a single particle or, in other cases, a small, well controlled number of particles, which typically are or may be microor nanometer-sized, adhere(s)to a surface. The result of this manner of agglomeration is either detrimental if the particles are contaminants on, for example, electronic circuit boards or chips [B.34], or individual particle deposition is highly desirable if it is accomplished in a controlled fashion to achieve special material properties (see Section 10.1 and Chapter 12). If hundreds, thousands, millions or more solid particles are involved and stick together due to binding mechanisms (see Section 5.1.1) to form agglomerates, the product characteristics are defined by the final size, shape, and strength as well as the structure and porosity of the parts thus produced. In such cases, it is not necessary that the entire product features the same structure throughout. As will be shown in Section 10.1, agglomeration may be used on the surface and is then responsible for only a small part of a final product. With the above in mind, agglomeration phenomena exist always and in all cases were solid particles are bonded to each other or to another surface and, no matter how the product looks like and what it is used for, its characteristics are defined and controlled by the same fundamentals that can be collected and applied on an interdisciplinary basis. It should be also recognized that a solid “particle” can be many different things (see Sections 5.3.1 and 10.3),from a sphere or cube to an elongated filament or fiber. All feature surface configurations which may be anything from
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10 Special Technologies Using the Binding Mechanisms of Agglomeration
v-
I 3
-
1
Fig. 10.1: Droplet granulation system. (1) Powder, (2) granular 2 ’ product, (3) powder recycle, (4) drop former, (5) screen, (6) dryer [B.42].
microscopically smooth to macroscopically rough with all kinds of protrusions which may enhance or hinder adhesion. In the following as well as in Sections 10.1- 10.3 a few special technologies will be discussed as examples. The presentation is by no means complete and, as is true for the entire contents of this book, should serve as “food for thoughts” to trigger new ideas and/or developments. Similar to the agglomeration ofparticles by heat (see Section 9.1),where aggregation often occurs by the action of the binding mechanism alone, there is a possibility to obtain agglomerates by direct capillary action. Fig. 10.1 shows a process that uses this principle. Although it was used by the inventors (IG Farben, Germany [B.42])for the production of spherical, easily dispersible granules, so far, the method did not gain major industrial importance. Agglomerates are formed by droplets of a suitable, wetting liquid with high surface tension (for example water) which fall into a bed of powder that is transported on a belt conveyor. A spherical wetted area is formed within the powder bed and the powder particles in that volume are held together by the negative capillary pressure (see also Section 7.1). For successful operation, the frequency of droplet production and their spacing as well as the conveyor speed must be controlled such that the volume elements which are wetted by the droplets remain separated in the powder bed. The green agglomerates are separated from the excess amount of powder by screening and are further processed by suitable post-treatment methods (see Section 7.3). Unagglomerated powder is recirculated and redeposited, together with fresh feed, on the belt conveyor. Even though the technique never found large scale applications it is a good example of an interesting alternative approach to size enlargement by agglomeration, particularly if loosely bonded, easily dispersible granules must be produced from corrosive or highly abrasive powders and a minimal equipment cost for these conditions is desired. Since this process was invented and first used, reliable droplet formers have been developed which make it easily applicable if a desire for such products arises (see “melt solidification”, Chapter 5). Macroscopically, the direct effect of molecular forces can be observed in a layer of dust on surfaces. Small, light particles that settle on, for example, furniture are held to the substrate and to each other by the always present molecular forces and removal requires wiping (e.g. with a dust cloth) or suction (e.g. with a vacuum cleaner). Microscopic natural adherance of submicron particles due to molecular forces will be discused in Sections 10.2.1 and 10.2.2.
10 Special Technologies Using the Binding Mechanisms of Agglomeration
Other special technologies use tumble/growth agglomeration for the realization of simple, low cost particle size enlargement processes, in which shape and quality are of minor importance. Such applications are, for example, in the fields of recovery of small amounts of valuable constituents by leaching from low grade ores or waste [B.42]and the agglomeration of refuse during processing for disposal. Since, in both cases, large amounts of material with low or no value must be processed, the cheapest possible method that fulfills the process requirements must be selected. Some of such low cost solutions will be described in the following as examples of how the binding mechanisms of agglomeration can be used in innovative ways [B.42]. Fig. 10.2 shows stockpile agglomeration. In this process, an inclined conveyor discharges particulate solids that were mixed with appropriate amounts of CaO and cement (= dry binder) from several meters above onto a stockpile. The stream of material which is falling from the end of the conveyor is wetted with water (and/or other liquid) sprays (= liquid binder). Below the spray area, several heavy (dispersion) bars are suspended in the falling curtain and act as simple, stationary mixer. The wetted mass than tumbles down the slopes of the pile and agglomerates into lumps. Strengthening occurs by natural curing, begins immediately, and continues until the cementitious reactions are completed. Around the foot of the pile, a front-end loader picks up the agglomerates and transfers them into a dump truck or directly onto the leaching heap of a metal (e.g. gold) recovery plant [B.42].
Fig. 10.2: Sketch o f stockpile agglomeration [ B 421 NaCN s o l u t i o n or H,O
Fig. 10.3: Sketch of belt conveyor agglomeration [B 421.
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Another simple agglomeration method is belt conveyor agglomeration (Fig. 10.3). It is a modified stockpile agglomeration system with additional sprays and mixing at each transfer point. The inclination and speed of the belt can be chosen such, that some agglomerates tend to roll down (backwards) from time to time until held by a mass of material. During such rolling, additional fines may be picked up and agglomerate growth can occur (see also below: reversed belt agglomeration). The number of transfer points depends on the amount of fines that must be bonded onto larger particles or agglomerates. Fig. 10.4 depicts shaking trough or vibrating deck agglomeration. (a) Shows the principle of the shaking trough agglomerator. In the wave or surf like motion, particles collide and coalesce if binding properties are favorable. Agglomeration occurs if the powder particles are very fine and/or if the mass is moist. (b) Is a sketch of the vibrating deck agglomerator. Dry binder, if applicable, is added prior to the vibrating conveyor and liquid is sprayed onto the particle bed at the beginning of the downward inclined vibrating deck. The conveyor deck is executed with a number of steps over which the material tumbles whereby mixing and agglomeration take place. A further low cost method is the reversed belt agglomerator (Fig. 10.5). It uses a steeply inclined conveyor belt to which the material, possibly including dry binder, is fed near the upper end. Liquid binder sprays are located at the upper one third of the steeply inclined belt. Belt movement is such that it attempts to convey the mass to the top of the equipment but, due to the steep inclination, material rolls downward. Depending on the angle and the speed of the conveyor the material can be retained on the belt long enough to provide for adequate mixing and agglomeration.
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Fig. 10.4: Schematic (a) o f shaking trough and sketch (b) of vibrating deck agglomeration [B.42].
70 Special Technologies Using the Binding Mechanisms of Agglomeration I 4 1 3 Deflectors
4 x 2 5 f t belt, 4
Fig. 10.5:
v a r i a b l e speed
Reversed belt agglomeration [B.42]
The basic principle of tumble/growth agglomeration is that solid particles move irregularly and independently resulting in collisions (see Section 7.1). They coalesce upon impact if the binding forces which are present or activated at that moment are higher than the separating forces acting on the newly created entity. Therefore, in addition to what has been discussed previously (see Chapter 7 and subchapters) and above, many alternative methods of particle excitation can be used. Of particular interest is sonic (or acoustic) agglomeration [B.42].This technique is being developed for the agglomeration of submicron particles in flue gases and process off-gases which otherwise remain airborne as respirable solid contaminants. Acoustic pressure and velocity are superimposed on the natural Brownian movement causing collisions between even the smallest particles as well as agglomeration. The larger entities can then be removed with conventional methods. Since all of the methods of pressure agglomeration (Chapter 8 and subchapters) require specific equipment in which external forces are exerted onto particles to densify and shape the mass into an agglomerated product, absolutely new special techniques of pressure agglomeration are difficult to envisage. Most probably, any novel pressure agglomeration technology will be a modification of the already known ones. However, an alternative, unusual binding mechanism which is applied in pressure agglomeration for a specific task shall be mentioned in this context to demonstrate the great variety of ideas and phenomena that are part of the science of agglomeration. If supercooled ice or other frozen particles are compressed, the conversion of mechanical energy into heat causes roughness peaks and a thin layer of the particles to melt momentarily and produce liquid bridges or, generally, a liquid phase. However, because the bulk of the material remains at temperatures that are substantially below freezing, immediately after pressure release the liquid solidifies and the resulting shaped body becomes solid and features high density or a certain amount of porosity. Before continuous ice makers for the quick production of “cubed” ice became available, this binding mechanism was used in large commercial ice production plants to recover undersized ice or shape fines into small pieces by ice briquetting. More re-
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Fig. 10.6
Photographs of (a) briquetted frozen vegetable pulp and (b) an example of reconstituted food from such briquettes (courtesy Sahut Conreur, Raismes, France).
70.1 Coating
cently, roller presses were applied to shape frozen, pulped vegetables into ration-sized briquettes for deep freeze storage and use in field kitchens (Fig. 10.6).
10.1 Coating
The binding mechanisms of agglomeration (Section 5.1.1) can be also applied for coating. In most cases the method of choice for the application of powder coatings is tumblelgrowth agglomeration (see Chapter 7 and subchapters). As discussed during the review of the mechanisms of tumblelgrowth (Section 7.1), layering or preferential coalescense occurs often during size enlargement by agglomeration. If layered agglomerates are produced, this growth mechanism takes place during the entire process from nucleation until the final agglomerate is obtained. In coating, nuclei or cores are provided from elsewhere and layering occurs in irregularly, often turbulently moving beds of relatively large particles and coating powder. In most cases, a liquid binder is added to assist in the formation of a layer. Furthermore, coating materials can be brought in by means of suspensions. Although not directly identifiable as an agglomeration method, coatings can be also applied by spraying a solution or a melt onto a bed of stochastically moving particles and simultaneously drying (from a solution) or cooling (from a melt) the mass. Since the latter technologies are frequently used to coat agglomerates they will be also covered in the following. The first coatings used by humans were manually applied layers of wet minerals onto shaped clay items (e.g. bricks or containers such as vases or pots). During firing these minerals reacted chemically and glazed thereby producing color and a dense surface. The coatings were applied to achieve surface hardness, for water resistance and tightness as well as for decorative purposes. Pill making, another ancient agglomeration technology (see also Chapter 3), used viscous binders, such as honey, which rendered the pills themselves sticky so that they tended to adhere to each other, forming clumps during storage. To remove the stickiness such pills were coated with adsorptive fine powders, such as talcum or pollen. The coating was applied by shaking the freshly made pills with the powder in a bag and, potentially, rerolling them by hand to increase the bond and smoothen the surface. Beginning in the Middle Ages, sweets were coated with, often colored sugar layers to make them look neat, polished, and shiny. These coatings were applied by either spraying sugar solutions onto heated cores or dipping the sweets into molten sugar. The first mechanized machines were rotating pan coaters. This equipment is still used almost unchanged today (Fig. 10.7).The generally round or pear shaped rotating vessels are operating in a batch mode. Cores (e.g. tablettes but also nuts, raisins, or similar food and other products) are placed into the rotating bowl, sprayed with a solution or suspension, and dried with hot air, or coated with a melt and solidified with cold air. Hot or cold air is blown into the pan and circulates over the surface of the tumbling bed. The products are well polished, normally sugar coated pieces (Fig. 10.8).Today, larger units are also available,for example as shown in Fig. 10.9, which, depending on the material to be coated, can process between 50 and 500 kg per batch.
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Fig. 10.7: Simple revolving coating and polishing pans with open style base and round or pear shaped bowls (courtesy LMC Int’l, Elmhurst, IL, USA).
Fig. 10.8 Sugar/cocoa coated raisins and almonds (courtesy LMC Int’l, Elmhurst, IL, USA).
Fig. 10.9: Artist’s conception of a large modern bowl coater (courtesy Trybuhl, Dassei-Markoldendorf, Germany).
70.7 Coating
More recently it was found, that coating is not only suitable for taste masking or the enhancement of flavor, for the improvement of surface conditions, yielding a smooth, polished exterior, and for better oral administration of, for example, solid dosage forms because they can be swallowed easier, but that coating can also provide important functional properties. Functional coatings may be soluble or insoluble in water, soluble only in liquids with specific characteristics and/or composition, permeable, impermeable, or partially permeable, permanently plastic or elastic, elastic featuring a well defined burst pressure, insulating or conductive, etc., etc. While many of the functional properties are used in medicine, other applications are feasible; many of them are already used or are being envisaged for a multitude of products and a virtually unlimited number may be developed in the future (see also Chapter 12). The dimensions, structure, and uniformity of functional coatings must be much better controlled than those of the “classic” enrobing of particles. Sugar and other coatings of particulate foods often disguise the irregular shape of the core particles (see, for example, Fig. 10.8). To accomplish functionality, it is important to cover the surface with a uniform coating (= film coating),which is often only a few molecular or powder particle layers thick and must not have holes, due, for example, a shadow effect, or “blobs” of coating material which make the coating ineffective at these points. Therefore, it is most important to apply strict process control and modern drum coating equipment features at least four support areas as shown in Fig. 10.10. To obtain uniform coverage, the cores (typically tablettes or spheronized agglomerates) must tumble in the apparatus, the liquid sprays must cover the entire length of the particle bed, and the flow of warm or hot air must be directed such that each particle is instantaneously dried to guarantee the production of a smooth surface. Correct movement of the core particles is achieved by installing baffles or lifters or by using polygonally shaped drums. Spray systems have become very sophisticated whereby the stainless steel spray arms with nozzles are often telescopic and can be extracted through the front door for cleaning (Fig. 10.11).If slurries are used, spraying is air assisted to unplug the nozzles and keep them clean (Fig. 10.12).
1
D Fig. 10.10 Diagram depicting schematically the flow sheet o f a typical (film) coating facility [B.42]. (a) Programmable Logic Controller (PLC), (b) storage tanks for spray liquid(s) and metering/ pumping system, (c) equipment for supplying and processing air, (d) air cleaning and exhaust system.
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Depending on the application, the flow of air may be directed in different ways to obtain specific effects. Fig. 10.13 shows three alternatives which are possible in the same polygonal coating drum of one manufacturer by simply switching some valves. In such drum coaters some or all of the panels are double walled and perforated to allow air inlet and exhaust in a controlled manner. Another design utilizes stationary, hollow, perforated paddles (Fig. 10.14) which are immersed in the product and create an unidirectional, constant, and homogeneous flow of air in the tumbling particle bed. Similar to what has been shown in Fig. 10.13, in drum coaters using paddles air can be directed in different ways, too (Fig. 10.15). Most of the drum coaters are used in ultraclean industries (for foods and pharmaceuticals). Therefore, modern coaters are made in sanitary, seamless design from stainless steel and feature smooth exterior housings (see, for example, Fig. 10.16). Additionally, in accordance with the rules of cGMP (current good manufacturing practice), the equipment parts which are contacting product must be capable of CIP (cleaning in place). Fig. 10.17 shows schematically the automatic cleaning of a drum coater according to these requirements. The drum and the cleaning tub that is built into the housing are separated from the "technical part" by water tight seals. After wet cleaning in four steps, the machine's own air system is used for drying.
10.1 Coating I 4 1 9
Product
Product
-4 -
TOD view Manual shut-off.
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\
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Standard system equipped with four stations
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I 375 Air- f low
-12 0
1 '
!
II
1,8018 0180
Tumble drum \e;gth h r j s , standard 6 f t shown
-li -----
Tumble drum length + 2L in S l u r r y return t o reservoir Side view
Dimensions in inches
Fig. 10.12: Top and side view o f a slurry spray system [B.42]
As everywhere else in modern industrial technology, continuous operation of drum coating is desired. Fig. 10.18 depicts schematically a continuous drum coater. Although the long, perforated drum is divided into three independent zones for spraying, distribution/polishing, and drying, the results of coating are not as uniform and reproducible as in batch equipment. Fig. 10.19 is the photograph of the machine shown schematically in Fig. 10.18. To enhance flow of solids through the drum, its axis is inclined and the slope is adjustable. CIP is also available. Continuous coating is also possible in balling pans (see Section 7.4.1) if they are equipped with re-roll collars (see, for example, Fig. 7.13). Coating is much cruder and applied, for example, for fertilizer products (see e.g. Fig. 7.15). It should be pointed out in this context, that coating is by no means limited to the clean and ultraclean industries and products which were mentioned previously. Among the many and varied recent developments, a large, relatively new field of use is in the agricultural industry for the coating of plant seeds with fertilizer, fungizide, and insectizide. These materials may be applied in combination or individually; they feed and/or pro-
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Fig. 10.13: Three alternative air flows which are possible in the polygonal d r u m o f one supplier (courtesy Driam, Eriskirch/ Bodensee, Germany). (a) Reverse air flow from the bottom through the particle bed with air exhaust at the top; (b) direct airflow from the t o p through the particle bed with air exhaust at the bottom; (c) axial reverse flow from the bottom through the particle bed with air exhausting through the hollow shaft.
tect plant seedlings during sprouting and initial growth. The continuous drum coater as shown in Fig. 10.19 is an example of typical machinery that is being used for this purpose. While during most of the previous discussions much emphasize was given to the development of a relatively thin, uniform coating, other reasons for enrobing particles are also possible. Fig. 10.20 shows cross sections through melt coated fertilizer granules. In this case, the cores of conventionally tumble/growth granulated TSP (= triple super phosphate) were melt coated with sulfur, to provide an additional nutrient for sulfur deficient soils and/or obtain a slow release fertilizer. A flow sheet of the process which may, for example, be used to achieve this result is shown in Fig. 10.21. In the fluid drum granulator (FDG)TSP granules are sprayed with liquid sulfur and coated in a fluidized pan and the tumbling bed (Fig. 10.22). A similar flow sheet can be also used
10. I Coating
Fig. 10.14: Photographs o f two types o f air-blowing paddles (courtesy CS Coating Systems, Osteria Crande (Bologna), Italy).
to round crystals (Fig. 10.23) or the irregularly shaped granules from compaction/ granulation (see Section 8.3) as shown in Fig. 10.24 or to “fatten” smaller granules (Fig. 10.25). Therefore, the purpose of such coating processes is to provide functional layers (e.g. for controlled release of nutrients), to add a component (e.g. sulfur), to round irregular granules (e.g. from compaction/granulation), or to enlarge small particles (fattening of urea prills). Another melt coating method, the rotocoat process, uses a turbine in which a finely divided molten coating material contacts solid particles which are at room temperature (Fig. 10.2Ga).Due to the surface tension of the melt, the individual particles are enrobed with the liquid which cools to form the coating. Depending on the amount of solids passing radially through the turbine, secondary agglomeration may also occur if particles come into contact with each other while the coating material is still sticky. Fig. 10.2Gb is a simplified flow sheet of the process. The above method is suitable for the coating of smaller particles, down to 0.1 mm. However, more commonly, small particles, either powders, crystals, or agglomerates, the shape ofwhich may be irregular, spheroidal, tabletted, or spheronized, are typically coated in specially designed fluid bed equipment (see also Section 7.4.4). As with all other coaters, the heart of fluid bed processes is the type and location of the delivery
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I2
i4
i5
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Fig. 10.15: Schematic flow sheets of two different air flow regimes using air-blowing paddles (courtesy CS Coating Systems, Osteria Crande (Bologna), Italy) (a) Hot air through the paddles into the particle bed with air exhaust through the hollow shaft, (b) hot air through the hollow shaft onto the particle bed with air exhaust through the paddles (1) Inlet air handling unit, (2) control panel, (3) solution tank, (4) dosing system for liquid to be sprayed, (5) sliding support arm for spray nozzles, (6) coating pan, (7) air-exhaust or -blowing paddle device (8) dust collector, (9) outlet air fan, (10) powder dosing device
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Fig. 10.18: Schematic representation o f a continuous d r u m coater (courtesy Driam, Eriskirch/Bodensee, Germany).
Fig. 10.19: Photograph o f the continuous d r u m coater shown schematically in Fig. 10.18 (courtesy Driam, Eriskirch/ Bodensee, Germany).
Fig. 10.20 Broken (cross sections through) melt coated fertilizer granules. Cores: triple superphosphate (TSP) granules; coating: sulfur (courtesy Kaltenbach-Thuring, Beauvais, France).
70.7 Coating I 4 2 5
SCRUBBING
GRANULATION
SCREENING
Fig. 10.21: Flow sheet of a fluid drum granulating (FDC) process for the coating or fattening of granules with sulfur and/or purge solution (courtesy Kaltenbach-Thuring, Beauvais, France). seeds and recycled product
fluidization air
Fig. 10.22: Sketch describing the principle ofthe fluid granulation drum (FCD) for coating particles (courtesy Kaltenbach-Thuring, Beauvais, France).
10.1 Coating
Fig. 10.26 Principle (a) and simplified flow sheet (b) o f the rotocoat process (courtesy Sandvik, Totowa, NJ, USA).
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Nozzle [hydraulic or pneumatic)
(C)
Fig. 10.27: Sketches o f the material processing sections o f three fluidized bed coaters [B.42]. (a) Top spray, (b) tangential spray (rotating disc fluidized bed coater), (c) bottom spray (Wurster coating system).
10.7 Coating I429
Fig. 10.28: Artist's conception of a top spray fluidized bed coating system (courtesy Vector, Marion, IA, USA).
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system for the liquid coating material. For this, three methods are available: top, tangential, and bottom spraying (Fig. 10.27). The nozzles are often binary, i.e. liquid is supplied at low pressure to an orifice and is atomized by pressurized gas. Such pneumatic nozzles produce finer droplets, an advantage when coating smaller particles. However, it is also an important requirement of coating that the liquid, solution, or suspension droplets impact the core particles and uniformly distribute on the surface before the liquid is dried off. Since very fine droplets evaporate quickly as they travel from the nozzle to the particle bed, solids concentration and viscosity of solutions and suspensions increase. Therefore, droplets may fail to spread satisfactorily when they contact the substrate surface, resulting in an imperfect coating. This drying of the coating spray can be severe in top-spray coaters (Fig. 10.27a) in which the most random particle movement exists and liquid is sprayed against the flow of drying air. Nevertheless a substantial share of coating is performed in this type of equipment because larger amounts can be processed per batch and the design is simple. Fig. 10.28 is an artist’s conception of a top-spray coater showing the fully integrated processing systems and matching accessories. The rotating disc fluidized bed coater (Fig. 10.27b) combines centrifugal, high intensity mixing with the efficiency of fluid bed drying. A major advantage of this method is its ability to layer large amounts of coating materials onto cores consisting either of robust granules, crystals, or nonpareil nuclei. Because of the unit’s high drying rate, relative large gains in product weight can be achieved in a short period of time. In this respect it is similar to the fluid drum granulator (Fig. 10.22) if this is used for “fattening” cores. Another advantage ofthe rotating disc fluidized bed coater is the possibility to layer dry powders that are dispersed in the fluid bed onto nuclei which have been wetted with a liquid. Because the spray nozzle(s) is (are)located below the bed surface, the above mentioned problems with early drying are not experienced. The same is true of the Wurster coating process (Fig. 10.27~). This is the only bottom-spray fluid bed coating method which is applicable for tablettes, pellets, and coarse granules as well as fine particles. The Wurster coating chamber is cylindrical and the basic model contains a concentric inner tube with approximately half the diameter of the outer chamber. At the base of the apparatus is a perforated plate which features larger holes underneath the inner tube. The liquid spray nozzle is located in the center of the orifice plate and the tube is positioned at a certain distance above the plate to allow the movement of material from the outside annular space to the higher velocity airstream inside the tube. This design creates a very organized flow of material which is similar to that of the spouted bed (see Section 7.4.5, Fig. 7.89). Solids move upwards in the center where coating and highly efficient drying occur. Contrary to what happens in the spouted bed, where some mass exchange occurs between the solids moving upwards in the center and those in the outside downward flow, the high speed upward flow regime in a Wurster coater is contained in the center tube, so that no backmixing occurs. At the top of the tube, the material discharges into an expansion area and then flows down, as a near-weightless gaslsolids suspension, in the annular space outside the tube. Design variations include different chamber configurations for use in coating tablettes, coarse granules, or fine particles (Fig. 10.29). For scale-up, the outer vessel diam-
10.1 Coating I 4 3 1
w
Product retention screen
Fig. 10.29: Sketches o f the different chamber configurations of single tube Wurster coaters as used for (a) tablettes, (b) coarse granules, and (c) fine particles
Filter housing
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eter and the number - rather than the size - of inner tubes increase. Fig. 10.30 is the simplified explanation of how the capacity of a (laboratory or pilot) Wurster coater (Fig. 10.29) is scaled up 3, 5 , 8, and lotimes and Fig. 10.31 is a view into the coating section of a Wurster coater with ten tubes; each tube features its own gas distributor and spray nozzle and is essentially identical with that in the test equipment which was used during development work. During studies of particle and gas flow patterns in traditional Wurster coaters (Fig. 10.32a) it was found that flow paterns were dominated by the particles rather than the gas. This explains why sometimes, even in the well defined traditional Wurster coaters, uniform coating is difficult to control. To overcome this problem, the precision coater (Fig. 10.32b) was developed. An essential feature of this new Wurster coater design is the highly controlled gas flow pattern in the coating zone. This is achieved by application of the so-called Swirl Accelerator, a guiding system in which the gas is accelerated, stabilized, and given a precise amount of swirl which eliminates slugging, often seen in traditional coaters, and stabilizes multi-tube systems. Particles are entrained into the swirling air on an individual basis. This results in an optimized probability of impact with the droplets of atomized liquid and to an even application of coating material.
Tra d i t i o na I C o a t e r
Coating column
Down flow bed
Air distributor
plate Two-fluid spray nozzle
Air inlet Fig. 10.32: Operating principles o f a traditional Wurster coater (a) and (b) a precision coater (courtesy Aeromatic-Fielder, Bubendorf, Switzerland).
PRECf5lOhl COATERTM
Two-fluid spray nozzle
Fig. 10.32:
cont’d
Another coating technique is encapsulation. Although this is a relatively new technology, many different processes have been developed, a large number of applications has evolved, and many new uses are conceivable and found, literally on a daily base (see also Chapter 12). In Section 5.2.2, the mechanism of capillary flow in wet agglomerates or, more generally, in porous bodies that are filled with a liquid (= continuous phase) was described. During drying, in a first drying phase, evaporation takes place on the surface and the liquid is replenished by capillary flow of the continuous phase from the interior of the porous body. If the liquid is a solution or suspension, solids are deposited at the pore ends on the surface and causes more or less severe incrustation (see Section 5.2.2). If a film forming, easily soluble polymer is dissolved in the continuous phase as emulsion or dispersion, encapsulation occurs during drying. These encapsulation processes can be carried-out with agglomerates of any size and shape and result in a large number of special effects which, depending on the type and composition of the coating or incrustation, modify final product properties (see also Section 7 . 3 ) . More commonly used and widely researched is microencapsulation. In this context, the partial word “micro”refers to the dimension of the encapsulated product which is typically <1- 2 m m in size or, increasingly, in the 10 to a few 100 pm range. If a slurry containing a solution, emulsion, or suspension of polymer is dispersed into small particles and dried in a spray dryer (see Section 7.4.3) microencapsulated particles
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Coherent. Homoaeneous element, SimDle miCrOCaDSUI? Coating can be porous to various levels of molecular weight chemicals or impermeable, of various solubility, and rigid or soft.
Simale multilaver microcatxuI?
+-- Early season herbicide Fertilizer
‘Late season herbicide Simole Heteroaeneous rnicrocaosule [Carrier, cohesion phase can be continuous or discontinuous]
(c)
Heterogeneous structure can be multiplexed with capsules within capsules ad nauseam
d) Heteroaeneous coating [Sparse or Dense: Tenacious or Temporary
-0
Capsule
Encapsulation
Blending
Ordered Mixture Fixing (em bedding) Composite
Fig. 10.33: Schematic description of possible structures if microcapsules (courtesy Brian Kaye Associates, Sudbury, Ont. Canada).
of the type described above are formed. Such a process yields a dry, free flowing powder which, in most cases, satisfies the criteria defined for instant products (see also Section 5.4). However, microencapsulation becomes more and more a sophisticated packaging method in which the “packing material” (= coating) features a specific, well defined functionality. With this technology small agglomerates or tiny portions of powders, liquids, and even gases are individually wrapped into a shell (= capsule) to form
10.1 Coating I 4 3 5
Fluid wall deposition
Wall solidifies
Fig. 10.34 Schematic representation o f four stages o f the coacervation process (courtesy Brian Kaye Associates, Sudbury, Ont. Canada).
free flowing particles which are often spheroidal. Fig. 10.33 describes schematically possible structures of microcapsules. Originally, drugs, chemicals, toner materials, pigments, and the like were encapsulated to facilitate or improve handling and to bring about special product characteristics. In most cases, the capsules according to (a), (b),and (c) are produced by sol-gel processes (see also Section 5.3.2) or coacervation, an electrostatically assisted coating process. As shown in Fig. 10.34, the core material to be encapsulated (often a liquid) is placed in a (immiscible) carrier liquid (to form small, individual droplets). The coating material is also suspended or is present in a dissolved state in this carrier liquid. To induce the process that is known as coacervation, the temperature, pH, or other conditions of the system are changed in such a way that the wall material comes out of solution, the resulting particles or those which were originally suspended as solids aggregate around the cores, and continuous encapsulating walls are formed. In a final stage of the process the capsules are hardened. While many other microencapsulation techniques also use surface and in situ polymerization methods or, generally, interfacial reactions to produce soluble or insoluble and impermeable or permeable capsule walls, in addition to the coating and spray drying methods that were discussed previously, a growing number of processes de-
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10 Special Technologies Using the Binding Mechanisms of Agglomeration Core
2
(b)
Encapsulation System developed by
IITRI
Fig. 10.35: (a) Principle of microencapsulation by electrostatic, aerosol based coating [ ( l ) coating particles, ( 2 ) core particles] [B.42] and (b) sketch ofa possible equipment configuration (courtesy Brian Kaye Associates, Sudbury, Ont., Canada).
posit particles onto cores or solid surfaces whereby the binding mechanisms of agglomeration are utilized. Three such methods will be mentioned below as examples. Fig. 10.35 are sketches of the principle (a) and the equipment (b) of electrostatic, aerosol based microencapsulation. The two components to be turned into a microencapsulated product, the coating and, respectively, the core particles, are given an ionic charge of the appropriate sign using a sub-corona discharge system. To achieve sufficient encapsulation, the apparatus must be designed such that a high rate of collisions between the two components occurs in a turbulent supportive gas system. In this process, the coating materials must be selected so that they will uniformly and completely cover the core particles. They include softened wax particles which solidify upon cooling or polymers which form a skin by interfacial action between a component in the core and another in the coating material or upon exposure to a suitable gas phase, etc. The coating (capsule) can be also finished by heating (= sintering). Another method of manipulating coating particles uses magnetic forces. In the m a g netically assisted impaction coating process (MAIC) the coating material is applied onto the cores by the actions of the coating material (i.e. impacts due to turbulent particle movement) and by magnetic attraction if either coating or core materials or both are magnetic in character or by the action of magnetic elements in a bipolar
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Mater i a1
Effect
Spherical Ia rge l f i ne particles (inorganic/ plastic)
Surface fusion
.. . .
Surface f u s i o n and
I r regu l a r large/ f i ne partic I e s
Nearly same size particles (hard lsof t 1
0.
sp heroniz in! effect
o**
*:0
0
0
With
D is persion I surface fusion
agglomerated particles
I r regu I ar Iy shaped plastics
Grinding l surface fusion
00
Spheronizing effect
0
Fine powders (dyelpigment 1
Fig. 10.39: Pictorial presentation o f the differerent possible effects o f mechanofusion [B.42].
Precision mixing
I
10. I Coating
oscillating magnetic field which fluidizes the coating material, the cores, and the m a g netic elements (Fig. 10.36). Particle to particle impacts cause peening of the coating material onto the cores. If neither the coating particles nor the cores are magnetic, the bipolar oscillating magnetic field causes impingement of the magnetic elements into the coating particles which forces them onto the core material with a peening action (Fig. 10.37). With this method coatings can be developed which do not require a separate binder. Adhesion is accomplished by molecular forces which are enhanced by drastically reducing the distance at the contact point and by partially embedding the coating particles in the surface of the core. In the end, the mechanism that causes strong adhesion of the coating particles to the cores is due to mechanical forces acting in the system. It works best, if the coating particles are somewhat harder than the core material. Because the coating particles are often so small, that no dislocations are present in their structure, they behave as very hard entities (see Section 5.4). Therefore, it is for example possible to partially embed submicron-sized Ti particles in the surface of glass. A similar method uses mechanical forces outright. The process was developed in the early 1970s by Hosokawa in Japan and is called rnechanofusion [B.42] or hybridization. In the high energy environment of a special mill, particles are held by centrifugal force against the inner wall of a fast rotating cylinder and wedged into the space between a
Fig. 10.40 Microphotographs o f four products from hybridization (courtesy Nara, Tokyo, Japan),
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stationary, curved inner piece (Fig. 10.38a). During densification in the converging clearance, the particles experience complex forces such as compression, attrition, shearing, and rolling (Fig. 10.38b). Depending on the characteristics of the materials that are involved and on the operating conditions of the equipment one or more of the following effects can occur: Solid-solid mechanochemical reaction (mechanofusion), microspheronizing, intensive dispersion (precision mixing), and granulation (agglomeration). Fig. 10.39 illustrates these effects pictorially [B.42]. Coating processes, particularly MAIC, mechanofusion, and hybridization are often used to produce materials with new, engineered physical properties (see Chapter 12). First and foremost flowability is typically improved but other characteristics such as abrasion resistance, electrical conductivity or insulation, magnetic properties, and many others may be attained in a controlled fashion. Fig. 10.40 shows photographs of four products from hybridization.
10.2
Separation Technologies
As discussed in Section 5.5, separation technologies may suffer from unwanted agglomeration when particles, that should be separated according to a certain property (including size, shape, density, composition, color, etc.),stick together and can not be dispersed sufficiently well that a good separation efficiency is obtained. On the other hand, many separation technologies depend on agglomeration for the effective removal of particulate solids, especially those featuring micron and submicron size, from gases and liquids in environmental control. Without going into details, in the following two subsections a few remarks are made and some examples will be presented which are intended to show the importance of agglomeration phenomena in connection with separation technologies. As is generally true for this book, no claim is made for completeness. Rather, ideas should be instilled in those readers who are faced with developing, using, and/or optimizing separation technologies. 10.2.1
Cas/Solid Separation
In the field of dust collection or the separation of ultrafine particles (UFPs) from gas streams the mechanisms of tumble/growth agglomeration in low density fluidized beds and particle clouds are frequently applied. As particle size decreases, i.e. if it is in the micron or submicron (nano) range, particle mass becomes very small. As a result, for example, centrifugal forces, which are the underlying effect for particle separation in cyclone separators, or inertia, which controls uniform particle movement in a gas stream and determines the collision probability with e.g. filter media, are neglectable. Therefore, such particles follow the stream lines of the flowing gas and exit cyclones or packed bed filters with the “clean” gas (see also Section 10.3). Since, at the same time, these particulate solids are respirable and, owing to their
10.2 Separation Technologies
very large surface area, exhibit high reactivity, they represent the most dangerous particulate contamination from the human health point of view. If ultrafine particles can be agglomerated, the mass of the new entity is equal to the sum of all particles in the structure and mass related forces as well as inertia increase proportionately. After agglomeration, ultrafine particles, in their new form, can be removed in standard dust collection devices such as cyclones and packed bed filters. As mentioned several times before, the natural adhesion forces (see Section 5.1.1), caused, for example, by molecular (e.g. van-der-Waals) or electrical forces (e.g. due to asymmetric molecular structures), may become much larger than the separation forces which are mass and shape related. Therefore, if collisions or, generally speaking, contact between ultrafine particles occur, a rather strong bond will develop. This phenomenon is also responsible for the fact that most nano-sized particles do not exist as individuals but as assemblies of many particles (Fig. 10.41);this might be a problem in those applications where ultrafine particles must be deposited individually or in monolayers (see Chapter 12). For the effective separation of such particles from gases, however, agglomeration is desired and must be promoted. Ultrafine particles that are suspended in a fluid exhibit Brownian motion, a random movement resulting from the impact with molecules of the fluid surrounding the particles. Inspite of the randomness of the motion, it is very unlikely that particle-
Fig. 10.41:
Microphotographs
of naturally agglomerated nano particles (courtesy CABOT, Tuscola, IL, USA).
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to-particle impacts occur because the amplitudes of the movement and the particle sizes are very small. Therefore, other methods must be applied to entice particle collisions which will result in coalescence and agglomerate growth. Such techniques use artificially induced turbulences, for example in front of filters and cyclones, uni- or bipolar charging in so-called electrostatic precipitators, magnetism, and ultrasound [B.42].Although, from a process point of view, it is very advantageous that with these methods particle accretion occurs in the free flowing fluid, a major draw-back is that the collision probability changes with the square of the particle concentration. Therefore, as the number of particles becomes smaller, also during the cleaning process itself by the incorporation of dust particles into the growing agglomerates, the collision probability decreases and the desired low concentrations of ultrafine particles in the off-gas may not be reached. A different, new strategy for the effective removal of aerosols from gases uses a principle which is already well known from the capture of ultrafine particles by comparatively large liquid droplets in wet scrubbers. In contrast to using liquid droplets, which combine in a sump together with the collected solids and transfer at least part of the separation problem to a secondary cleaning process, i.e. the removal of fine particulate solids from a liquid (see Section 10.2.2),it is proposed to pass the contaminated gas through a fluidized bed of solids [10.1, 10.2, 10.31. In such a fluidized bed dust collector, the aerosol particles adhere to the large surface area of the fluidized collector medium and form a coating which is densified when coated collector particles collide with each other. It has been determined that, later, attrition of the coating may result in the formation of secondary dust particles. However, these secondary particles are normally agglomerates and substantially larger than the original aerosol so that they can be easily separated in conventional dust collectors. 10.2.2
Liquid/Solid Separation
Aggregation of fine particulate solids also takes place in liquids. Some of the phenomena have already been described and were discussed in Section 7.4.6. In environmental control, the removal of particulate solids from liquid process effluents is of great importance. As in the case of gaslsolid separations (see Section 10.2.1),when the size of the solids diminishes and reaches the micron or submicron (nano) range, mass of the individual particles becomes so small that they remain in suspension and can not be removed by settling. Because of the fineness, membranes would be required to retain particles on or in a diaphragm which is uneconomical for the cleaning of large volumes of contaminated liquids from industrial plants or waste water treatment facilities. However, remembering the mechanisms of growth agglomeration (Section 7. l ) ,if particles can be forced to impact with each other, it is possible that they adhere to one another. Therefore, when water, that is contaminated with suspended fine solids, is stirred, flocs may form naturally. If this happens, the size and shape of these aggregates depend on the circumferential speed of the stirrer and the processing time. Fig. 10.42 shows that flocs are larger if the shear forces are low and the processing time is
70.2 Separation Technologies Circurn'erentiol speed of p-opel'er
Original sample
1 m/s
0 6 !r/s
0 27
R/S
0 18 rnis
30 min
60 min
90 min
S t a t i o w r y sample affer 1 5 h mixed
Pro less1 ng time
Fig. 10.42 Natural flocculation o f solid contaminants in river water [8.42]. Parameters are the circumferential speed ofthe stirrer and the processing time.
short. But further investigation revealed that higher speed of the stirrer and/or longer duration of mixing ultimately result in denser and more stable agglomerates. This is due to the previously discussed mechanism of growth agglomeration (see Section 5.3, Fig. 5.42) whereby loosely attached particles are removed under the influence of ambient forces (in this case, for example, shear) and later have the chance to become reattached in energetically more favorable positions thus yielding denser and stronger products. It depends on the process that will be used for solids removal, which of the two agglomerate structures is required, the loose flocs resulting from relatively gentle movement or stronger agglomerates from a more vigorous stirring for a longer time. In the large diameter circular thickenerlclarifier (Fig. 10.43), which is commonly used in municipal water treatment plants and in many industrial applications, water, flowing slowly from the feedwell in the center to the overflow around the periphery of the circular tank, is gently moved by a slowly rotating arm. Loose flocs are formed which settle by gravity to the slightly conical bottom. Differently shaped scrapers (rakes)are used to move the sludge to the discharge cone at the lowest point from where it is transported to conventional liquid filters. The more vigorous stirring which produces stronger agglomerates is used when the resulting agglomerates are moved with the water to an off-site filtering system and, therefore, must survive transport. In many cases, even if collisions between particles do take place, the naturally available binding mechanisms, mostly molecular forces which are considerably lower in a
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Superstructure
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Fig. 10.43: Partial t o p view and elevation (a) (according to EIMCO, Div. Baker Hughes, South Walpole, MA, USA) and photograph (b) of a circular thickener/clarifier.
70.2 Separation Technologies
liquid environment than in a gas atmosphere, do not create bonds with sufficient strength to withstand the various separating effects and sufficient flocculation does not occur. For quite some time it has been known that polymers, added to liquid based particulate systems, have a dramatic influence on particle interaction. Molecules may attach themselves to solid surfaces and, depending on the characteristics of the exposed radicals, can cause particle attraction [B.26]or dispersion [B.51]. The second, dispersion, is applied to avoid agglomeration or enhance disintegration of aggregates. There are two ways in which polymers can promote aggregation: 1. By making particles more susceptible to salts or 2. by flocculating the system without the aid of electrolytes. These processes are known as sensitization and adsorption flocculation, respectively. The second is more common. To create aggregates or flocs, the polymer adsorbs on different particles simultaneously which is best accomplished by using substances with high molecular weight and a strong affinity to the particles to be agglomerated. Fig. 10.44 explains the principle. In nearly all applications of polymeric flocculants, the polymer addition and the subsequent flocculation process are carried out under conditions where the suspension is agitated in some way, for example by stirring. This way, the polymer molecules are distributed uniformly throughout the system and adsorb onto the particles which are then encouraged to collide and form aggregates. As described earlier in other contexts, bridging may be followed by break-up if the bond is not strong enough and, later, re-attachment during another impact. Fig. 10.45 is the sketch of a flocculate. Care must be taken not to oversaturate the suspension with polymer. If too much polymer is adsorbed, the particles may become restabilized (= deactivated) because of surface saturation or by steric stabilization [B.26].Fig. 10.46 demonstrates schematically bridging, which results in the desired flocculation, and restabilization. Commercial flocculants are used extensively in practice, for instance in water purification. By influencing the affinity ofthe polymer, it is also possible to obtain selective agglomeration. This method is used in the upgrading of certain minerals and ores, for example during flotation. Less well known is the fact that, more often than not, solids and immiscible droplets dispersed in aqueous solution are electrically charged due to preferential adsorption of certain ion species, charged organics, and/or dissociation of surface groups [B.42]. Depending on such variables as nature of the material, its pretreatment, pH, and composition of the solution, these charges can be either positive or negative. Since the
W Fig. 10.44
Principle o f polymer adsorption and flocculation [B. 261. (a) Adsorption of polymer molecule on the particle, (b) rearrangement o f adsorbed chain, (c) collisions between destabilized particles and bridging to form aggregates (flocs), (d) break-up o f flocs.
/
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Fig. 10.45: Structure o f a flocculate (floc) bonded by a polymer [8.42].
Fig. 10.46: Schematic illustration o f (a) polymer bridging between particles and (b) restabilized
(b) particles [8.26]. +
+
Fig. 10.47: Schematic representation o f two +
+
- +
-
- +
+
-
+
particles with electrical double layers in a liquid [B.42].
surface charges on particles are compensated by an equal but opposite countercharge surrounding them (Fig. 10.47) an electrical double layer develops (see also Section 5.1.1). Even though, as a whole, the system is electrically neutral, repulsion between the particles occurs. Upon addition of indifferent (= non adsorbing) electrolyte (e.g. a salt), the double layers become less active and, as a consequence, the particles can now approach each other more closely before repulsion sets in. If enough salt is added, the particles may eventually come so close that van-der-Waals attraction binds them together. This is, in principle, the explanation of the sensitivity of colloids and suspensions to salts and may, in other environments, be used to destroy stable colloids or suspensions and cause flocculation. For technical applications, electrocoagulators are used [B.42] to charge the solids in contaminated effluents. Metal hydroxides are produced by a system of soluble electrodes (anodes) which, in suitable electrolytes, cause coagulation of particles into larger flocs.
70.3 Fiber Technologies 10.3
Fiber Technologies
The influence of fibers on the strength, structure, and characteristics of agglomerates was discussed in Section 5.1.2. The binding mechanism “interlocking bonds” (see Section 5.1.1), the intertwining of fibers and threads, is also used directly to produce “agglomerates” by producing non-wovens, felts, filters, webs, paper, etc. Each of these applications, which produce agglomerates from fibers featuring interlocking bonds, are technological fields that are covered in the literature, in books and scientific as well as commercially oriented papers. Therefore, they will not be covered here. The purpose of this short chapter is, to remind the reader that many products are in reality some sort of agglomerates and their characteristics are based on and controlled by the fundamentals of agglomeration, particularly the binding mechanisms of agglomeration (Section 5.1.1). In the following, one specific technology will be discussed in more detail, because it describes first how the “agglomerate” is made and then strengthened by post-treatment and, secondly, indicates that, during application, agglomeration plays an important role again. This technology is the manufacturing and use of fiber based filter media. For pleated bag and cartridge filters for example [10.4],there are wet-laid filter media, made from cellulose or synthetic fibers, needled felt or spunbonded, metallized or carbon impregnated polyester media, or acrylic, Nomex, Ryton, and P-84 (designations by TDC Filter Manufacturing, see Section 14.2) needled felt (Fig. 10.48). Wet-laid media (Fig. 10.48a) is made by mixing a slurry of cellulose or synthetic fibers (= particles), or of both, with a chemical resin (= binder) that holds the fibers together and protects them from picking up excessive moisture. The mixture is pressed flat, dried, and cured in a high temperature oven (= post-treatment). Wetlaid cellulose fiber, typically made from hardwood, softwood, or grass, produces filter media which is suitable for moderate filtering efficiency, but is not applicable in high temperature, high moisture, or oily environments. Filters from wet-laid synthetic fiber, however, offers excellent filtering efficiency in moist and/or abrasive conditions up to 135 ”C. Needled felt media (Fig. 10.4%) consists of intertwined short fibers, pressed together, and mechanically fixed with a needle punch machine. The efficiency of such filter media varies with its density, composition, and relative thickness. Needled felt media is strong and durable, but, for maximum filtering efficiency requires the build-up of a dust layer (see below). Spunbonded media (Fig. 10.48~) is made from continuous polyester filaments. During manufacturing, polyester is melted, extruded, spun, and drawn to form laminated webs. The webs are rolled, thermally bonded, and embossed to form sheets. Spunbonded media feature a hard, slick surface that is ideal for increasing the filtering efficiency and, at the same time, retains little dust during cleaning. Also, spunbonded media provides 30 to 50 % more surface area than other filter media. Such filter media can be either used to capture dust by building a layer on their surface or to collect particles on the fibers in the interior. The build-up of a dust layer is controlled by the relative open (pore) area on the surface. Cleaning is accomplished
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Fig. 10.48: Photomicrographs of different fiber based filter media (courtesy TDC, Cicero, IL, USA).
10.3 Fiber Technologies
Fig. 10.49: Enlarged view of the interior o f a filter mat with particles sticking t o the fiber s u rfaces.
by back blowing or mechanical shaking. It is advantageous if strong enough bonds (which may be enhanced by adding a suitable “binder”to the gas stream) have developed in the dust layer so that during cleaning some agglomeration remains for ease of transportation to disposal, avoiding re-entrainment of dust particles (secondary pollution). Media that collects dust on the fibers in the interior consist mostly of empty space. Typically,the fiber volume represents only a few percent of that of the filter mat. Therefore the voids are orders of magnitude larger than the dust particles. Fig. 10.49 is the enlarged view of the interior of a filter medium with small particles sticking to the fiber surfaces. The fiber diameter in this picture is approx. 50 pm and the size of the particles ranges from 3 to 10 pm. There are a number of problems associated with this type of dust collection. First, the particles to be attached to the surface (Fig. 10.50) may not stick immediately because they bounce back due to the elastic energy that develops during the impact. Secondly, if particles adhere, they may be torn off again by the drag force of the flowing gas; this effect is often increased by the development of a momentum if the particles are not closely attached to the fiber surface and extend into the gas stream (Fig. 10.50).
Fig. 10.50: Detail o f a dust laden fiber from Fig. 10.49 showing how particles extend into the gas stream.
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A further problem is that differently sized particles follow stream lines in a different way. It must be a goal during filtering that the particles collide with the fiber and adhere upon impact. As shown by the results of model calculations (Fig. 10.51) various particle sizes not only move differently but modified system conditions also influence the behavior of the particulate solids [10.5]. Fig. 10.51a depicts the lines of motion for 1, 2, 3, 5, and 10 pm particles. Originating at the points on the left, all do not impact the fiber and show well the influence of particle size. Even if the paths ofthe three smallest particle sizes begin almost at the center line, they still pass around the fiber without colliding. Often, particles or fibers carry an electrical surface charge which causes electrostatic forces (see also Section 10.2.1). Fig. 10.51b shows the flow lines and origination points of the previously defined particles for the case that the product of particle charge q and fiber charge per unit length Q is equal to Coul*/cm, a typical value which has been determined by measurements. In this case the smallest particles move farthest away from the fiber. For a similar case, i.e. the existence of electrical charges, Fig. 1 0 . 5 1 depicts ~ the paths that result in collisions with the fiber. As can be seen, small particles may even collide on the back of the fiber if electrical charges are present. Fig. 10.52 shows that the natural agglomeration of very fine particles (see also Chapter 10 and Section 10.2.1) and the resulting larger mass of the conglomerate also contribute to the separation mechanism on filter fibers. Filter media which collects particles on fibers within the media are either discarded after saturation with dust or washed with suitable liquids. Particles may be also dissolved by the action of solvents which do not attack the fibers and/or modify their surface characteristics.
Fig. 10.51: Results o f model calculations showing the paths of differently sized particles around a fiber at different conditions [10.5]. Explanations see text.
10.3 Fiber Technologies
Fig. 10.52: Naturally formed agglomerates of small (8 pm) glass spheres adhering to a filter fiber (10.5, B.421.
Finally, in closing this chapter, a relatively new fiber based agglomeration technology shall be briefly introduced. It was already mentioned at the beginning of this chapter that paper making is one of the techniques that uses predominantly fibers, together with fillers, if applicable, and sometimes binders to yield products with widely differing qualities, from newsprint to papers for applications in the arts, the building industry, for decorative purposes, and many more. The common base of all these products is that fibers, mostly cellulosic from plant material, are used which intertwine and interlock in a thin slurry layer. The wet individual sheets or continuous bands are pressed, dewatered, and dried to yield the final product. Strength, mostly defined as the resistance against tearing, depends directly on the type and, particularly, the length of the fibers. The least stringent requirements on fiber length are for the manufacturing of newsprint. Triggered by growing environmental concerns, the already well established recycling of paper offers several advantages. It reduces the requirement for fresh fiber, saving forests, and makes cheaper secondary raw materials available. It also eliminates the need for large new disposal sites or the construction of expensive waste incineration plants. During recycling, waste paper is deinked and redissolved to form a slurry from which most of the fillers and binders are removed. However, to obtain secondary paper (mostly newsprint, tissue, and packing paper) with acceptable quality, the slurry must be separated by suitable means into a stream containing long fibers, suitable for recycling, and one in which fiber length is below specification. Originally, the stream with the short fibers had been dewatered for disposal in landfills or burning in incinerators. It was found that, after filtering, the waste fibers can be easily agglomerated by tumble/growth methods (see Section 7.4 and subsections). After drying, a granular product is obtained which has excellent absorbtion characteristics for liquids and, due to a structure of interlocking fibers, retains strength after wetting. This material can be used, for example, as absorbent to bind spilled liquids or as cat litter (see also Section 5.4).Additives can produce special characteristics such as extra strength, odor control, avoidance of bacterial growth, and others.
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Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
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Engineering Criteria, Development, and Plant Design Size enlargement by agglomeration is one of the unit operations of Mechanical Process Technology (see Chapter I),the technical field that deals with the processing and handling of solids. When developing, designing, constructing, installing, and operating an industrial plant that includes size enlargement by agglomeration, many or all of the other unit operations of Mechanical Process Technology, each sometimes more than once, as well as the associated techniques and the analytical support functions (see Fig. 1.1, Chapter 1) are required and used. Since Mechanical Process Technology encompasses the oldest techniques serving mankind and because these methods are all based on natural phenomena, they were applied by various users in parallel so that similar but separate techniques evolved in different fields. For centuries, development was purely empirical until recently, beginning less than 150 years ago, one after the other, the unit operations were recognized and treated as generic fields of engineering science and approx. 50 years ago they were evaluated and used interdisciplinarily (see also Chapter 3). At that time, efforts began to apply experience and know-how that was available in one field to solving problems in another, potentially with totally different requirements on, for example, plant size, process cleanliness, and product characteristics. While in the newer fields of industrial technologies, for example chemistry, electronics, communications, etc., process research and plant development started from first principles and many of the equipment and system designs had to be newly elaborated for a particular purpose using modern thought processes as well as manufacturing and industrial methods, even the new Mechanical Process Technologies mostly still rely on fundamentals that are rooted in the purely empirical past enhanced by the know-how of expert persons and/or companies. In addition, since mechanical processes and installations are widely considered simple and the techniques are viewed as being dirty, - because particulate solids in different size ranges, including dusts and slurries, are always involved and present -, having little technical and scientific appeal, and requiring only the combination of known conventional equipment for successful operation, very often designs are crude, old fashioned, and far from optimal. Furthermore, the different disciplines that are today involved in the elaboration of new industrial plants often lack a common background and an understanding of the availability of new, improved technologies so that the resulting designs commonly include stunning misconceptions.
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Fig. 11.1: Cartoon satirizing the frequently experienced development, design, manufacturing, and installation o f a new industrial plant.
The above is demonstrated in a cartoon-like fashion in Fig. 11.1.The idea and execution of this series of sketches was not conceived by the author. It was given to him many years ago during one of his numerous industrial contacts and consultations. The originator is unknown. However, whoever dreamed up and drafted this sequence understood well what is often found in today’s industrial practice. His knowledge and draftsmanship deserve credit. Referring to Fig. 11.1,at the beginning there is a site and an idea which is discussed between the customer, owning or having control of the site, and a supplier of engineering, equipment, and services. (1) Is the supplier’s engineering; (2) is the specification of the purchasing department; (3) is what was actually built; (4)are the instructions for installation; (5) shows how the installation was executed; (6) is the modification which was made in the field prior to start-up; (7) depicts what the customer really wanted; and (8) is what the supplier shows as reference in his advertisements. Although this picture story is exaggerated, it indicates that finally, in the field, even under adverse circumstances, modifications attempt and typically succeed in making the installation work. The result is, however, far from optimal and from what was really intended and/or desired. A fix of all problems, for which outside experts and consultants are typically called in, which should, after the fact, result in a well designed system and an economical operation, is normally not feasible. Some improvements can always be made but an optimal solution is only possible with a completely new installation and expert project management.
7 7 Engineering Criteria, Development, and Plant Design
To improve the development, design, and selection process, some guidelines will be developed below and in the two subsections that follow this introduction. As will be seen and is easily understandable, selection of the proper agglomeration equipment, that which is best suited for a specific task, as well as procurement of the optimal peripheral equipment and system layout depend greatly on the application and the industry for which the process and plant are destined. Tab. 11.1 summarizes the most important parameters in determining the best suited approach for a particular project. The characteristics to be considered fall into four main categories: Particulate feed, agglomerated product, method options, and site.
Tab. 11.1: Considerations during the selection o f a suitable agglomeration
process for a particular project (see Section 13.3, e.g. [128, 141, 143, 1521).
Parameters $the particulate feed Feed particle size and shape (dimensions and distribution, surface area, shape factor, fractals, etc.) Moisture content (free, encapsulated, crystal water) Material characteristics (chemistry, density, porosity, plasticity, brittleness, elasticity, wettabiiity, abrasivity, etc.) Special material characteristics (heat and/or pressure sensitivity, toxicity, reactivity, etc.) Bulk characteristics (temperature, density, flowability, etc.) Binding characteristics
Parameters of the agglomerated product Agglomerate size and shape (dimension(s),distribution, volume, weight, tolerances, etc.) Strength - Green strength (if applicable) - Cured (final) strength Stmcture and other characteristics (porosity, specific surface area, dispersibility, solubility, reactivity, abrasion resistance, etc.)
Parameters of the agglomeration method Batch or continuous operation (interruptions or downtimes tolerated or not) Capacity per hour and per year or per campaign Wet or dry operation Simultaneous processing Space and energy requirements Investment and operating costs
Site, supply, and environmental conditions, infrastructure Relative location to suppliers and consumers (raw materials, additives and binders, energy, users, etc.) Site accessibility and transportation facilities Climatic conditions Availability of skilled and other labor Availability of support functions Regulations (e.g. EPA, OSHA, etc.)
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Particulate feed Particle size and distribution should be determined and considered first. A limit in the range of a few hundred micrometers defines the applicability of methods using growth mechanisms based on coalescence in moving beds of particles (tumblelgrowth agglomeration). Larger particles, which may also constitute seed agglomerates or recycle, can only be incorporated if an adequate amount of binder or sufficiently small particles are present. Small particles tend to embed larger ones (see Sections 5.3 and 7.1). Agglomerate strength is defined by the matrix of fine powder in this case. Generally speaking, it is difficult to agglomerate narrow particle-size distributions or monosized particles. Adding binder can cause relatively large particles to agglomerate. However, it may be more economical to crush larger particles to render material suitable for tumblelgrowth agglomeration. This is particularly true if the product must feature high porosity. Pressure agglomeration can be applied for larger feed sizes, e.g., sandlike material and particles of up to 20 - 30 mm. Since the external forces acting upon the mass result in particle disintegration or deformation, the upper limit of feed particle size is determined more by geometrical restrictions of the feeder than the ability of the material to agglomerate. Inmost cases, consolidation takes place in a short time. To obtain sound agglomerates, considerable amounts ofair must be removed during compaction. Because ofincreasing resistance to flow with decreasing particle size (dueto smaller pore radii),very fine bulk solids (below about 150 pm) can be agglomerated by pressure methods only if certain preconditions, particularly low speed and dwell time, are fulfilled (see Section 8.1). Moisture content, especially, free moisture, can play an important role in growth agglomeration by coalescence. Here, moisture provides the binder or prevailing binding mechanism. The maximum volume of liquid must not be more than about 95 % of anticipated agglomerate porosity. Wet (tumblelgrowth) agglomeration is sensitive to this limit because a small excess of moisture beyond 100 % saturation causes the entire charge to turn to mud (see Section 5.22, Fig. 5.28). Further, since the addition of moisture in the agglomerator is an important tool to control growth (see Section 7.2) and other agglomeration parameters, the feed moisture should be several percentage points below the critical maximum moisture content, as defined above. The moisture content is less critical in fluidized-bed agglomerators which also act as dryers. Here, the moisture must often be high enough to make the feed pumpable (see Section 7.4.4). In pressure agglomeration, moisture must be kept low. In most cases, dry feed is a precondition for high-pressure agglomeration. The reasons are that, due to the high compaction forces, crushing, rearrangement, and deformation of the solid take place which result in a considerable reduction of porosity. Excess water either is squeezed out or remains in the mass as an incompressable component (see Section 8.1). Both result in low strength. Filter cakes turn into mud upon discharge from filter presses unless moisture is reduced by applying, for example, vacuum or compressed air. Material characteristics, such as chemistry, particle density or porosity, brittleness, elasticity, plasticity, wettability, and abrasivity, etc., play important roles in the choice of an agglomeration method. A particular chemistry may be necessary to bring about
1 1 Engineering Criterja, Development, and Plant Design
the required chemical bonding or may be incompatible with certain conditions of a method (such as the necessary addition of water or other liquids in most tumble/ growth agglomeration techniques). Density (or porosity) of the feed particles, through particle weight, determines gravitational and other field forces which may be counteractive to adhesion by coalescence. Brittleness, elasticity, plasticity, and abrasivity are most important for pressure agglomeration but are of less concern for tumble/growth methods. Wettability, on the other hand, is the single most significant parameter for all agglomeration methods using surface tension and capillary forces in the growth regime. Wetting is required for green strength. In some industries, special material characteristics, such as heat or pressure sensitivity, toxicity, and reactivity must be considered. These parameters are particularly important in the pharmaceutical industry. Pressure agglomeration has only limited applicability for heat- or pressure-sensitive materials. Toxicity may limit the appeal of tumble agglomeration because of the difficulty to contain dust.
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Bulk characteristics, such as temperature, bulk density, and flowability, may be adjusted in a preparatory step to improve size enlargement. Prior to briquetting into ration-sized agglomerates, vegetables, food pulps, and fruit juices may be frozen. Metal dusts and powders, as well as certain minerals, are often heated and briquetted hot to make use of their increased malleability at elevated temperatures. High bulk volume and unacceptable flowability are sometimes corrected by using two agglomeration methods in series. For example, fine feed powders are pre-agglomerated (Fig. 11.2) to reduce the compaction stroke and improve the flow of feed into a die. This increases, for example, the speed of the turret of rotary tabletting machines. At the same time, this technique avoids segregation of the feed mix by stabilizing the blend in a granular form (see Section 8.4.3). Roller presses are particularly sensitive to the problems associated with high bulk volume and the need for sufficient deaeration. As shown in Fig. 11.3,bulk density can be increased by either using two presses in line (Fig. 11.3a) or by producing a sufficient amount of predensified recycle (Fig. 11.3b). Finally, the possibility of obtaining bonding with a candidate method must be determined to decide whether agglomeration can be carried out binderless (potentially, by making use of an inherent binder in the material and/or high pressure or elevated temperature) or requires the addition of binders. Agglomerated product The desired shape, dimensions, and size distribution of the ag-
glomerated product also influence the selection of a suitable method. Results of size enlargement may be, for example, free flowing, dustfree, granular products with strict requirements on the limits of size distribution, accurately shaped compacts with extreme demands on tolerance, or large, highly densified and strong briquettes. Many other requirements, including predetermined volume and/or weight, are conceivable. Granular, feelflowing, dustfee products can be manufactured using almost all methods of size enlargement. The (often necessary) task of narrowing the size distribution of the discharge from tumble/growth agglomeration is done by screening whereby under- and oversized components are produced. While the fines are recirculated to the agglomerator, oversized particles are crushed and either rescreened (to recover additional product) or directly recirculated with the fines (see Chapter 7). Granular products can be also obtained by crushing and screening large agglomerates such as tablettes, cylindrical pellets, compacted sheets from smooth roll compactors, and briquettes or tablettes (slugs) from various equipment (Fig. 11.2). In this application, selection criteria are often defined by other parameters, such as product porosity or density, solubility, reactivity, or inertness. Yield of the end product may be very small (less than 25 % for narrow size distributions) or relatively high (75 -80 % for wide distributions) but, in most cases, sizable amounts of fines must be recirculated to the agglomerator (alternatives 1 and 2 in Fig. 11.2) unless granulating (crushing or milling) yields a product that can be used directly (granular product, normal, in Fig. 11.2). Only the confined die pressing in punch-and-die machines is suitable for producing accurately shaped compacts with extreme demands on tolerance. Such requirements exist, for example, for dry dosage forms in the pharmaceutical industry and for near net-shape preforms in powder metallurgy.
I I Engineering Criteria, Development, and Plant Design I
Fig. 11.3: Two flow sheets o f precompaction arrangements with roller presses.
The reciprocating movement of the pistons and the often small volume of the die cavity restrict the capacity of these machines, even if modern, multistation, rotary tabletting presses are considered. With the production of larger pieces in, for example, hydraulic presses or roller briquetting machines, accuracy in weight, shape, and dimensions sometimes is no longer obtained. Shape is often also important. In many cases, spherical products of size enlargement are desired. The approximate shape can be obtained with all tumblelgrowth agglomeration methods. On the other hand, unless extremely accurate feed control is established in some punch-and-die machines or by using wet bag isostatic pressing, spherical products cannot be produced with pressure agglomeration. The nearest approximation would be pillow-, lens-, or almond-shaped compacts. Strength is significant for the final product, but also plays a role during size enlargement itself. Particularly in growth agglomeration, green agglomerates are first formed which then must be cured to obtain permanent bonding. In most cases, a weak intermediate state exists when the binding mechanism of the green agglomerate disappears and before the permanent, cured bond sets in. Unless large amounts of matrix binders are used, or agglomerates are cured at extremely high temperatures (e.g. sintering, partial melting) or by some chemical reactions, growth agglomeration will result in weaker products than most pressure agglomeration methods. Porosity plays a major role, too. Different strength levels develop primarily because agglomerates growing by coalescence feature higher porosity than those from pressure agglomeration. However, strength may not be the only determining characteristic. In fact, materials which must be easily dispersible and are only agglomerated to improve handling of the intermediate product should have just enough strength to survive their short existence. In other cases, a large spec& surface (e.g. catalyst carriers) is more important than high density and strength. Generally, with increasing external forces acting upon the particulate matter during size enlargement, porosity and characteristics related to this parameter decrease, while density and strength increase.
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Method options Depending on the requirements of the application, agglomeration may be batch or continuous. Batch processes are normally low capacity, but feature a high degree of control. Most large-volume applications operate continuously with corresponding variations in quality. Maintenance and cleaning reduce the annual capacity. In growth agglomeration, uncontrolled build-up must be removed, and in pressure agglomeration, worn parts require replacement. In plant design, the annual production is often calculated on the basis of 330 days per year and 20-22 hours per day to allow for scheduled and unscheduled downtime. Sometimes other processes that are closely related with the agglomeration system operate in campaigns which may last for a few months or more than a year. In those cases, it is necessary to select an agglomeration method or instal suitable redundancies to meet this requirement. Most of the growth methods are wet processes using liquids as binders for forming green agglomerates. In contrast, most high-pressure techniques are dry processes. These operating conditions play an important role, if, for whatever reason, a particulate solid must not be wetted during processing. On the other hand, addition of liquids may bring about desirable effects such as specific binding mechanisms or chemical reactions and, therefore, require tumble/grow-th techniques. The latter qualify as simultaneous processing if agglomeration and chemical reactions occur concurrently. Some classic fertilizer agglomeration methods (ammoniators) operate in this fashion. More often, simultaneous processing happens in mixer-granulators, granulator-dryers, or even mixer-granulator-dryers. Mixers are often also granulators in which both processes occur but in different zones. However, in fluid-bed granulators, agglomeration and drying take place simultaneously. Space and energy requirements as well as investment and operating costs frequently render an otherwise perfectly feasible process uneconomical. Incorrectly, these factors also sometimes direct interest toward methods, which after superficial investigation seem to offer cheaper alternatives because indirect or hidden costs are not recognized. The entire process must be considered. For example, in the granulation of fertilizers a granulation drum may seem cheaper than a roller compactor. If only investment costs are considered, this may still be true. However, if space requirements of the complete system, which includes dryer(s)and cooler(s),as well as the energy and operating costs for entire processes are compared, in many cases a different conclusion may be drawn. New applications for size enlargement by agglomeration include the recycling wastes which contain valuable ingredients, and the disposal of particulate wastes without value in an environmentally safe and acceptable way. Particularly in the latter case, an economical solution seldom can be found. However, because legislation forbids dumping or landfilling in a quickly increasing number of countries, agglomeration must be applied. Sometimes, economic justification can not be obtained and the process is used solely in compliance with laws. In other cases, the application of bonuses (e.g. incentive payments by communities) or credits (e.g hazardous materials become nonhazardous due to size enlargement) may result in an economical or even profitable operation.
1 1 Engineering Criteria, Development, and Plant Design
Site, supply, and environmental conditions, infrastructure The relative location of a processing plant and its influence on economics is of great importance. Many pitfalls can endanger the success of a project. Since, in most cases, one of the major reasons for size enlargement is the improvement of material handling characteristics, plants should be built at the source of the particulate solid. Therefore, a suitable method must consider the availability and cost of utilities and auxiliary materials, such as binders. The availability of waste steam or byproduct gas for heating may render wet granulation economical. On the other hand, where energy must be purchased, the same task may best be accomplished using roller presses for dry compaction, and granulation by crushing and screening. If binders are required for strength, the binder, often developed during tests in an equipment vendor’s laboratory, must be available in sufficient quantity and at an acceptable cost where the plant will be located. Feasibility of many well thought out projects disappeared when it became clear that the necessary binder was not available at the proposed plant site and existing processes had to close when the binder source “dried up” (see also Section 5.1.2). When wastes containing valuable components are processed for recycling, it is necessary to not only identify the need for and method of use of the agglomerated secondary raw material, but also an actual user and its relative location. The cost for transportation from the source to the potential consumer often becomes prohibitive. Since higher strength of the product is normally associated with higher production cost (due to more binder, higher curing temperature, higher compaction force, and higher wear), unless there are special requirements, agglomerate strength should be just high enough to survive handling and transportation. Therefore, products for a more distant user may need higher strength, costing more, and resulting in reduced economics. The accessibility of the site, including infrastructure, availability of a labor force and of maintenance as well as support fac es, availability or lack of already existing transport facilities, and climatic conditions must be considered early during project development. Regarding climate for example, long periods of freezing may require excessively high costs for winterization and hot and humid summer months may make air conditioning necessary, not for the comfort of the workers but to avoid condensation and/or adsorption of moisture onto powders with large surface area and attendant changes in flowability and binding characteristics that result from this condition. If not considered and resolved during project implementation, agglomeration plants may show marked differences in performance and product quality during winter and summer which, in the worst case, prohibit a successful year-round operation. Environmentalregulations can influence the selection of agglomeration equipment in several ways. A first concern is always the finely divided particulate nature of the feed materials. Often, these are precipitated dusts or solids removed from fluids in pollution control devices. Recontamination of the environment is an obvious concern and is typically regulated. The equipment must provide dust control and be completely enclosed, particularly if the material is hazardous or toxic. Many of the growth agglomeration methods, for example, drums or discs, do not easily fulfill these requirements. In the USA, for example, all installations handling, treating, and/or processing fine
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particulate solids must obtain permits (e.g. from OSHA and EPA as well as state and local authorities) prior to planning an installation.
11.1
Preselection of the Most Suitable Agglomeration Process for a Specific Task
Since a large variety of techniques and equipment is available to carry out size enlargement by agglomeration, one must first determine which method is the most likely choice for a specific task. Fig. 11.4 is a simplified preselection guide for picking a suitable method. The arrow tips indicate the direction of an increasing feed or product property and point to the technology that is most suitable for treatment of that material or obtaining a specific product characteristic. For example: With increasing feed particle size, tumble/growth agglomeration becomes less applicable until only high-pressure agglomeration remains. Or: Products from high-pressure agglomeration have low residual porosity, while agglomerates from tumble/growth methods typically feature porosities of approx. 40 % and more. Although there are many exceptions to these rules, Fig. 11.4 provides a simple and often valuable guide to preselection. Particularly post-treatments (see Sections 7 . 3 and 8.3) or the use of matrix binders (see also Section 5.1) may change final product strength and reactivity such that they become independent of the agglomeration method.
Fig. 11.4 method.
Simplified selection guide for choosing an agglomeration
7 7 . 7 Preselection ofthe Most Suitable Agglomeration Process for a Specific Task Technolo Process
“umble/Growth High density L o w density : Pan D r u m Mixer Fluid with Bed mech. agit
.
TOLEXANCE TO FEED MATERIALS: €hes H H H A D coarse partic. L L L VL wide distr. A A A L low bulk dens. €I H H VB low flowabil. L/A L/A A H elast. partic. VH VH VB H moisture cont. H H H VH high temp. VL V L VL A
A
BINDER REQUIR.:H
H
A D
Pressure low Screen with extrud. spheron.
A/H
H
VL
A A/H
L
VH
H A
H H
A
VH
VH
medium pellet./ die extrud.
H VL
A/H H
H A H H
high ram tabexlet. trud.
r o l l . press briq. come/ gran.
A(H) H
A(H) A(H) B V H
L(A) A V
V H V H V H V H
VL
VH VL
H H H L
A/H
H
A/H
L
L
L L L
L L L
A L L/A
L A/L A
V L V L V L A/L A/L A/L A A/H A/H
H
H
H H
A
H
H
L L
H L
L
A
L/A L/A vr. - vr. ._ V L L L L L/A L/A
L L L L L
VE
H
L/A
A
L/A L L/A
L/A
H A L/A L L
H A
L
L
DANGER OF:
segregation H dust (pollut.) H temp. rise VL PRODUCT : capacity max. size density strength (after curing) solub./disp. uniform size uniform shape abrasion res. (after pOSt treatment)
VL Fi
H A
L L H H A
A
V
L L H
H A
r. L L B H A
A L/A L/A A L/A L/A A/H L/A L/A
A
A L/A
L L L/A L A A/H H
A/H A A
A
A
H L
H
H
A/H
H
L VE
H
H
H
VE
A A/H
A A
A/H A/H
H H
V
H
V H
H H H
V
L
H
L A/H A/H A/H
H
Fig. 11.5: Comparison of some o f the most important considerations for different agglomeration methods. Notations: A = average, H = high, L = low, V = very (as, for example, in VH =very high).
Fig. 11.5 is another attempt to compare some of the most important selection considerations for different agglomeration methods. It must be realized that general, always valid statements for all processes and all conditions can not be made. The notations in Fig. 11.5 represent the most likely result or observation in each category. Also, it should be pointed out that all indications are relative to the typical application of each process. When using an interdisciplinary approach to process selection, where certain knowhow and experience from one industry is applied to solving problems in another field, somewhat different results may be obtained than are commonly known from the traditional application of a particular method. For example, product dispersibility (and solubility) is normally considered to be low if high-pressure agglomeration is used for size enlargement. It has been found, however, that powder formulations that contain dry binders and/or dispersants can be compacted with roller presses, which normally apply high pressure (see Section 8.4.3, Fig. 8.140),now using a linear force which may be considerably less than 1kN/cm (e.g. 0.2 kN/cm). Inspite of the low force, sheets are formed that can be crushed and screened to produce a narrowly sized granular material with good yield, sufficient strength, and excellent dispersibility both in dry and wet environments (see also Section 5.4). The notations in Fig. 11.5 can only be used to “get a feeling” for what a particular agglomeration method needs and is able to produce. For the actual preselection of a process, relative ratings must be expressed by numbers which can be added up to allow overall comparisons. Fig. 11.6 represents such a numerical ranking format. It is only
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feed ihorocterirtic (method's toleionce of): h e
10'
10
10
I0
(OUrsi
2
1
2
I?,uiit Hold Chrcsive
8
8 9 1
8
1 9
5
5
9 10 10
3 8 9 9
5 8 10 10
8
8 8
5 5
3
4
2
10 4
tlom
Ploslr 8iinle low hulk density Poor!! lowing High lernpeiotuie
9 7 9 10 10
a
8 3
Method's typical ogglornerate choroderirtic Srnoll rile Lcrge rile Tioht i r e toleionce
Equipmenl: [oil: Investment Ogeroting Required rnoinlermrce (opociw lhroughput PldUaiGn
[Ole 01 OpefOfiGfl tore 01 outornotion Requiied eneigy
5
a
10
10
3 3 3
2
3 3 7
1
7
7
10 I0
5
9
9
10 lo/?' 4 5 5
10 !0/3' 4 5 5
5
5
5
5
6
9 9/3' 1
7 5
5
5
5
4 5 5/3' 5 2 3
5 10 10
5/8' 5 8 5 9 10 10 7 2 2 2 s
2 2
1
7!
8
?I
5
5
5
5 4 8 8
I' '
5 5 3 3
i0 2
3 9
8
1 2 9 1 0 4 1 0 3 1 0 9 9 10 9 10 9 9 N d 9 N d 1 0 N d
1/9' 7
5
9
8
2
2
3
2
5
6
6 9
9/31
8 8 5
5
1
5 5 5
5 7
5
5 5
7 4
5 1 :
10 10
1
9
10
10 3 9 10
9 8 5 9
9 8 9
10 1 10 5 1
5
0
2 2 3
sf?
3 1
1
8
5
a
8
I 2 2 2
10 B 9 8
9
I0
5
5 10 10
KB KA A!,
1 5 Nk
NA NA
2 3 4 8 9
9 1
3 4 1
NA NA 9
8
5 5 10 10
8 10 10
Fig. 11.6 Ranking o f some agglomeration methods by c o m m o n feed, agglomerate, and equipment factors. Notes: (b) rankings: 1 = low, 5 = average, 10 = high; (c) N A = not applicable; (d) these (second) rankings are valid for the production of granular material. When manufacturing granular products, a fairly large amount o f undersized material is recycled which can limit production capacity; the lower numbers apply for those projects that require a narrow size distribution (= large amount o f recycle). (e) ? = these characteristics are unknown.
an example and rates some of the most common factors (similar to what has been presented in Tab. 11.1, Chapter 11) for a selection of agglomeration methods. Also, as mentioned before, the ranking is based on traditional applications. If a specific project needs to be evaluated, after determining own prior experience, related published work, and vendor input as well as potentially employing the services of an unbiased consultant in the field, a knowledgeable person within the company must review the list of factors, adding or subtracting specific items, and revise the individual ranking numbers. By using a scale of 1 to 10, with 1 = low, 5 = average, and 10 = high, a more subtle distinction of responses to the different factors by individual agglomeration methods can be obtained. It should be pointed out, however, that the numbers always mean what they indicate, literally. Therefore, they may sometimes imply different conclusions. For example, if the tolerance to fines is 10, this
1 1 . 1 Preselection of the Most Suitable Agglomeration Process for a Specific Task
indicates excellent performance of the method if the feed is a fine powder. But if the energy requirement is rated 10, this means that much energy is consumed which, normally, represents a negative process performance. To demonstrate the preselection procedure that is based on the aforementioned selection guides, two examples will be presented in Tables 11.2 and 11.3. The evaluations are based on the rankings in Fig. 11.6, and for the elaboration of these examples, the numbers reflecting certain important factors are taken without any adjustment to particular conditions of the two projects. As mentioned before, additional factors could be defined and ranked which take into consideration the special projects and, based on published information or experience, the numbers, indicating responses of different agglomeration methods to these factors, could be modified. Contrary to what has been done in the following, referring to Tab. 11.1, in most cases, the influences of site, supply, and environmental conditions as well as infrastructure considerations should be included. Example 1 (Tab. 11.2) refers to the production of an easily dispersible granular material from dry powders (see Section 5.4). The following factors were selected for consideration during the preselection exercise: Feed characteristics: fine, brittle, low bulk density (3 x). Agglomerate characteristics: small size, high porosity ( 3 x). Equipment specifications: low operating cost, low maintenance, ease of operation, ease of automation (3 x ) . The individual response numbers of Fig. 11.6 are added up for each category and then totaled with and without the opposite influence of costs. The highest respective number indicates the most promising choice for carrying out this task by agglomeration. In comparing the results with the agglomeration methods in Fig. 11.6, without the influence of costs (operating and maintenance), for the first 6 choices the following ranking is obtained: 1. fluidized bed, 2. mixer, 3. disc or cone, 4. deep disc or drum, 5. spray drying, and 6. vibrating trough. As expected in Example 1, all suitable methods belong to the tumblelgrowth group of techniques. If costs are included in the evaluation, the disc or cone and spray drying are tied for third place and the vibrating trough moves to the last of the six choices. Sometimes, a better preselection can be obtained if, in each category, the most important characteristic is weighted by multiplying it with a factor (for example 3 x ) . After doing this, not much change in ranking occurs in this example. The only remarkable modification is, that spray drying moves up in rank and looks now as good as or better than the mixer. Also, pelleting, a medium-pressure agglomeration technique, becomes a feasible alternative. While, generally, the results of this exercise agree with what is commonly used in existing industrial applications, modifying the basis, by adding new techniques and/or introducing new factors and determining how they fare with the individual agglomeration methods, enhances the preselection process. For example, low pressure extrusion with and without spheronizing could be included as a viable alternative method and, under certain conditions (e.g. the use of low specific force),compaction/granulation could become feasible for the project.
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I I Engineering Criteria, Development, and Plant Design Tab. 11.2: Preselection Exercise: Example 1. Production o f easily dispersible granular material from dry powders. Feed Characteristics: Fine, brittle, low bulk density (3 x ) , poorly flowing Agglomerate Characteristics: Small size, high porosity (3 x ) Equipment: Low operating and maintenance, ease ofoperation, ease o f automation (3 x )
Feed Characteristics 36 36 35 Product 18 17 17 Characteristics Easeofoperation/ 9 9 14 Automation (Cost + 14 14 11 Maintenance [C&M]
30 20
37 20
17 17
31 7
26 7
28 7
5
34
34 5
33 9
25 10
32 1G
34 9
34 9
7
12
8
16
13
16
12
12
11
18
13
10
10
9
13
-
11
16
12
12
12
12
10
12
10
10)
54
46
51
51
51
53
53
61
53
53
Total(exc1. C&M) Ranking
63 3
62 4
66 2
57 6
69 1
42
Total (incl. C&MJ Ranking
49 3
48 4
55
46 5
56 1
-
43
30
39
39
39
41
43
49 3
43
43
2
Weighted Total (excI. C&M) Ranking
105 102 110 91
123
-
89
84
87
89
89
91
89
111 83
83
4
5
3
6
1
91
88
99
82
110 -
3
4
2
5
1
Weighted Total (incl.C&M) Ranking
5
6 78
68
75
77
77
2
79
79
99
G
(6) 2
73
73
Note: During weighting the most important responses in the three categories are multiplied with three (3 x )
Example 2 (Tab. 11.3) evaluates the production ofhighly densified, inert (passivated) briquettes from hot sponge iron (see Section 5.4). The following factors were selected for consideration during this preselection exercise: Feed characteristics: coarse, abrasive, plastic, high temperature ( 3 x ) . Agglomerate characteristics: large size, high density = 10 - high porosity ( 3 x). Equipment specifications: low operating, maintenance, and energy ( 3 x ) costs as well as ease of operation and automation. The individual response numbers from Fig. 11.6 are again added up for each category and then totaled with and without the opposite influence of costs. The totals without the influence of costs (operating, maintenance, and energy) indicate that the ranking for the first 6 choices is: 1. sintering, 2. roller press, 3. punch-and-diepressing, 4. ram extrusion, 5 . screw extrusion, and 6. pelleting tied with plastification. As expected, all methods belong to the heat and pressure agglomeration groups of techniques. If costs are included in the evaluation there is only a small change in that pelleting drops out. If the weighting procedure is performed, plastics granulation as well as plastification become highly rated apparent choices.
1 1 . 1 Preselection ofthe Most Suitable Agglomeration Processfor a Specific Task Tab. 11.3: Preselection Exercise: Example 2. Production o f highly densified, inert (passivated) briquettes from hot sponge iron. Feed Characteristics: Coarse, abrasive, plastic, high temperature (3 x ) ; Agglomerate Characteristics: Large size, high density = 10 - high porosity (3 x); Equipment: Low operating, maintenance, and energy (3 x ) costs, ease o f operation and automation.
Feed Characteristics Product Characteristics EaseofOperation/ Automation (Cost + Maintenance [C&M]
22 6
22 7
18 6
18 2
24 3
3
24 16
20 13
35 15
25 13
23 13
23 11
39 15
25 4
22 12
22 13
9
9
14
7
12
8
16
13
16
12
12
11
18
13
10
10
19
19
16
12
21
-
16
21
17
17
17
17
20
22
15
15)
Total (excl. C&M) Ranking
37
38
38
27
39
-
56 3
46
66 2
50 4
48
45 6
72 1
42
44
45
Total (incl. C&M) Ranking
18
40 3
25
49 2
33 4
31 5
28
52 1
20
29
Weighted Total (excl. C&M) Ranking
47
50
52
31
59
-
76
64
102 76
74
67
102 64
80
81
1
4
5
1
2
3
Weighted Total (incl. C&M) Ranking
18
21
26
12
22
-
43
75
49
47
40
62
55
56
1
G
4
3
19
22
15
18 -
4
50 5
5
2
G
30 G
22
Note: During weighting the most important responses in the three categories are multiplied with three (3 x )
Example 2 demonstrates some of the potential problems and pitfalls of this preselection method. As mentioned before, the response numbers of Fig. 11.6 have been collected without a specific project in mind and, therefore, represent the most common applications of the individual agglomeration methods. The hot densification of directly reduced iron (DRI) to achieve passivation of this initially highly reactive material (see Section 5.4) features and requires very specific responses to the different factors so that, prior to carrying out the preselection process, the response numbers should have been reviewed and adjusted. In the industry, hot densification with roller presses has emerged as the exclusive agglomeration method for this task. As can be seen from Tab. 11.3, even without an adjustment of the response numbers, the roller press is selected in the #2 spot and moves to the #1 selection if the most important factors are weighted and the requirement for low costs is included. However, the rankings seem to indicate that sintering is another desirable method for the task. This is due to the fact, that this technology can not only handle but requires high temperatures, is a very rugged technique, and, in some sintering applications which were actually considered during the determination of the response factors in Fig. 11.6 (for example HIP, see Section 8.4.4), high density is achieved.
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Nevertheless, for several reasons, a discussion of which exceeds the scope of this book, sintering does not become the preferred method. The emergence of plastics granulation and plastification in the top rankings if the most important factors are weighted is due to the fact that they represent easy responses of plastics processing. Since hot DRI is by no means comparable with plastics, these results of the preselection process will be disregarded by anybody who is somewhat skilled in the art.
11.2 Laboratory Equipment, Testing, and Scale-Up
As has been shown in Section 11.1,knowledge of the binding mechanisms of agglomeration, the parameters controlling the processes of agglomeration, the characteristics of the equipment that is available for size enlargement by agglomeration, and the requirements on product quality as well as plant design, together with the availability of interdisciplinary research and operational know-how and experience, does allow a certain preselection of the most suitable method(s)of size enlargement by agglomeration. Nevertheless, development of all agglomeration techniques is still more an art than a science. Having done the preselection, which, after collecting all the information that is summarized in Tab. 11.1 (Chapter 11), is a desk job, it becomes normally necessary to carry out further investigations and do testing, particularly for example, to determine if and potentially what kind of a binder must be added and how much of it is required. After that, tests with actual equipment must be conducted to find limitations regarding capacity as well as product size, shape, and characteristics and needs for peripheral equipment and/or post-treatment. Closed loop processing and recirculation must be evaluated, too. Unfortunately, in many cases the availability of the actual material that needs to be processed in a new installation is limited at the time of plant design. The testing of similar materials from different sources, even if they are chemically identical and seem to be physically comparable, is not recommended because traces of impurities and minuscule changes of surface structure, for example, can decisively change many or all aspects of a material’s agglomerative behavior. Therefore, a common desire is to use small laboratory, often desk top equipment in an effort to develop the sizing and parameters of a large scale industrial plant. Referring to Tab. 11.1 (Chapter l l ) ,to be able to evaluate a particulate solid or the mixtures of particles and/or powders, the chemical and, particularly, the physical properties of this material must be known. A description of all the methods and procedures that are available today to completely characterize particulate solids and determine all the quality attributes that are necessary for a meaningful determination of their behavior under different process conditions is a book project in itself. Therefore, in the context of this book, only a few will be mentioned. Those selected represent relatively new laboratory equipment that combines the analysis of different powder characteristics in one piece of equipment and/or improve the gathering of data, that is of particular interest for agglomeration.
11.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.7: Photograph o f the micron powder tester (courtesy Hosokawa Micron Powder Systems, Summit, NJ, USA).
Fig. 11.7 is the photograph ofthe Micron powder tester by Hosokawa Micron Powder Systems. It is a multi purpose instrument that provides seven mechanical and three supporting measurements in a single unit. It has been developed to improve the handling efficiency and accuracy in testing the bulk properties of dry, fine particulate solids. The mechanical measurements determine the angle of repose, the compressibility, the angle of spatula, the cohesiveness, the angle of fall, the dispersibility, and the angle of difference. Supporting measurements provide the aerated bulk density, the packed bulk density, and the uniformity of the powder. The instrument employs a microprocessor, an electronic balance, and a built-in dust collection system. The results can be printed or, with a data communication port which is provided, they may be sent directly to a computer file. With this feature, the apparatus can be also used later for quality control (QC) and quality assurance (QA) in the industrial plant. Fig. 11.8 are photographs of two instruments offered by Amherst Process Instruments (API).Fig. 11.8a is the API aerosizer, a high resolution particle size analyzer for fine powders (range 0.2 - 700 pn),which is based on aerodynamics. A gas containing the entrained particles expands through a nozzle at supersonic velocities into a partial vacuum which is contained within a barrel shock envelope. The exit velocity of a particle depends on its density and size. Two laser beams, separated by a defined distance, and two photomultipliers form the measurement zone. From the velocity and the known density of the particulate solid, the aerosizer determines size, one by one, with a speed of up to 100,000 particles per second and an accuracy of better than 1 % which make this instrument rather unique. The system uses interchangeable dispersers for different types of particles and, in total (refer to Fig. 11.8a), consists of particle dispersers (A), sensor unit (B), vacuum system (not shown), and computer (C).
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Fig. 11.8: Photographs of (a) the API aerosizer and (b) the API aero-flow. (a) also includes a schematic presentation o f the meas. uring principle (courtesy API, Amherst, MA, USA).
11.2 Laboratory Equipment, Testing, and Scale-Up
Many other particle size analyzers are available for a variety ofparticulate solids in all particle size ranges. They are based on a large number of different physical principles. For an up-to-date coverage of this topic the literature should be consulted [B.GO]. API also offers an automated powder flowability analyzer (Fig. 11.8b)which features real time, low pressure, non intrusive powder flow analysis. It is being used both in the laboratory and for process control. The Aero-Flow utilizes the deterministic chaos theory [B.59] to measure the time intervals of a series of catastrophic avalanches. The particular advantage of this system is that it eliminates the need for any operator subjective measurement. Other, already conventional methods to measure flowability and adhesion tendencies of particulate solids are based on the shear cell developed by Jenike and Johanson [B.11] and adapted or modified by many other researchers. Particle shape is another characteristic that is of particular importance for agglomeration (see Section 5.3.1)and, for a long time, could not be determined easily. Fractals [B.37]have become one possibility of describing macroscopic and microscopic particle shape. The company Particle Characterization Measurements is now offering the Powder WorkBench 32 (Fig. 11.9), a particle size and shape analyzer, that identifies, differentiates, and categorizes powders and particles based on advanced morphological characteristics, such as shape, size, roughness, microroughness, and partial symmetry; it also performs a complete Fourier analysis. In all, the Powder WorkBench 32 offers over 50 specific morphological particle characterization features. It recognizes that not all particles are spheroidal. By applying advanced mathematical solutions it uniquely identifies and characterizes each particle whereby their orientation will not effect the results. This orientation independence is achieved by two simple operations. First the center of gravity of each particle is located and then each profile is rotated coincident with its principal axis. This is called “standard orientation”. With it, results are reproducible since identical particles which are only oriented differently produce the same data. Finally, another new laboratory technique, that is particularly important for the evaluation of agglomerates, measures porosity. Fig. 11.10 is the photograph of a Poremaster automatic pore size analyzer which is based on mercury porosimetry. While this technology is not new, the automatic features and computer control, including
Fig. 11.9 Photograph of the Powder WorkBench 32 (courtesy Particle Characterization Measurements, Iowa City, IA, USA).
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Photograph o f a Poremaster automatic pore size analyzer (courtesy Quantachrome, Boynton Beach, FL, USA). Fig. 11.10:
data transmittal and reduction software, make the instrument particularly easy to use. Representative for many other modern analyzers in different fields of applications, Tab. 11.4 demonstrates what data reduction software can do with a basic set of results. Such conversion features are included in most modern computer assisted laboratory instrumentation and allow, with a mouse click, to evaluate many related properties of what has been investigated in the first place. While mercury porosimetry still ranks among the most commonly used methods for the determination of pore structure, volume, size, and related data, concerns associated with the presence of elemental mercury, although well contained, triggered the development of alternative equipment. For example, Fig. 11.11 shows different
Poremaster data reduction software (according to Quantachrome, Boynton Beach, FL, USA). Tab. 11.4:
Cummulative pore volume vs. pressure or pore diameter Cummulative surface area vs. pressure or pore diameter Differential pore volume vs. pressure or pore diameter Differential pore area vs. pressure or pore diameter Pore number fraction vs. pressure or pore diameter Particle size distribution (Mayer-Stowe and Smith-Stermer Theories) Pore tortuosity Permeability Throat/pore ratios Fractal dimension Statistics Sample compressibility
77.2 Laboratory Equipment, Jesting, and Scale-Up
Fig. 11.11: Photographs showing non-mercury instruments for the determination o f porosity and related data. (a) Capillary flow porometer for gas as well as liquid permeability and the testing o f filter integrity; (b) gas pycnometer; (c) bulk/absolute density analyzer; (d) BET sorptometer (courtesy PMI, Ithaca, NY, USA).
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instruments in non-mercury technology for the determination of gas and liquid permeability in a capillary flow porometer (a),which can be also used for the testing of filter integrity; for the measurement of the absolute (true) density of particulate solids with a gas pycnometer (b), applying helium or other non-reacting gas; for the measurement of bulk density (c), using wetting fluids; and for performing physisorption and chemisorption in a BET sorptometer (d)with an extremely low range of adsorption pressures to obtain surface area, micropore and mesopore information, isotherms, and density. In addition many traditional and other new laboratory technologies are available for the determination of those parameters that were mentioned in Tab. 11.1 (Chapter 11) and which are necessary to evaluate feed materials and agglomerated products during the first development phase.
71.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.12: Photograph o f an all purpose drive stand for the attachment o f different work modules (courtesy Erweka, Heusenstamm, Germany).
After having determined all or at least the most important characteristics of the feed material, the preselected agglomeration method must be used to produce agglomerated products. In the laboratory, such experimental work is relatively easy and meaningful for all tumble/growth agglomeration methods. Small discs, drums, mixers, and fluidized bed processors are available that can simulate the growth process satisfactorily. Sometimes, small scale equipment is available in a modular design. For example, to the all-purpose drive stand, shown in Fig. 11.12, a multitude of attachments can be fitted that allow the testing of most tumblelgrowth agglomeration methods in the laboratory. Fig. 11.13 depicts the most important work modules; they are a disc or pan agglomerator (a),a coating pan (b), a bowl blender (c), a planetary bowl blender (d),which is similar to the no longer available Loepthien planetary mixer that was used for one set of data points in Fig. 7.7 (Section 7.2), a double cone blender (e), a cube mixer (f), a high shear mixer (g),and a pug milllkneader (h).The corresponding large scale equipment was discussed in Sections 7.4.1 (a),7.4.2 (c through h), and 10.1 (b). The fluidized bed technology (Section 7.4.4) can be easily scaled down. Fig. 11.14 is a picture and the dimensional layout of an MP-micro fluidized bed apparatus. It features variable process control of airflow, temperature, and liquid addition as well as interchangeable product containers and is equipped with a blowback filter system. It can be easily stripped down for cleaning and may be applied for dry mixing and liquid addition with a spray nozzle, for particle size enhancement by spray granulation/agglomeration, for spray coating, and for fluid bed drying. All processes can be carried out in sequence to obtain one-pot operation or simulate a continuous system. Another table top laboratory fluid bed is depicted in Fig. 11.15. In three schematic representations the different main uses of this equipment are also shown which are granulating, coating, and drying. Finally, a further flexible modular system is presented in Fig. 11.16 together with a table which indicates the different process modules that can be realized with this equipment. The terms granulating and pelletizing are alternative names for size enlargement processes by agglomeration. In the context of this presentation, granulating may also include powder mixing with a variable speed
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Fig. 11.13: Photograph of different tumble/growth agglomeration work modules that are attached to the all purpose drive stand o f Fig. 11.12. (a) disc or pan agglomerator; (b) coating pan; (c) bowl blender; (d) planetary bowl blender; (e) double cone blender; (f) cube mixer; (g) high shear mixer; (h) pug mill/kneader (courtesy Erweka, Heusenstamm, Germany).
11.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.15: Tab. top laboratory fluid bed and schematic representations of the various possible applications (courtesy AeromaticFielder, Columbia, MD, USA).
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PROCESS MODULES PLANT TYPE
i--GRANULATINC
MELT PELLETIZING
WET PELLETIZING
Fig. 11.16: Flexible modular system for RSLD powder processing projects (courtesy Aeromatic-Fielder, Columbia, MD, USA).
VACUUM RANULATINC
MICROWAVE DRYING
7 1.2 Laboratory Equipment, Testing, and Scale-Up
impeller. The parameters can be registered on optional chart recorders or data display and recording systems. Power, water, and air are simply plugged in and, after finishing a test, the whole system can be easily cleaned. Of course, if considering the mechanisms (Section 7.1) and kinetics (Section 7.2) of tumble/growth agglomeration it becomes obvious, that smaller containments and masses of tumbling particulate solids translate into lower forces acting during impact and coalescense as well as in the system as separating forces. This results in weaker bonds and, because the forces of the moving environment are small, in less destruction and, therefore, more porous structures (see also Sections 7.1 and 7.2). These conditions, in turn, may and generally will result in higher binder requirements, lower strength, quicker dispersibility, and differences in a whole host of other agglomerate characteristics if they are later compared with products from larger scale industrial operations. While no easy solution can be offered, this problem must be mentioned at this point to alert researchers and project developers to the differences that will exist between the products from small scale laboratory and large scale industrial operations (also see below). It is much more difficult to carry out small laboratory tests for most of the pressure agglomeration methods and obtain results which are meaningful and can be used for process development. The technique that lends itself best to small scale laboratory development and evaluation is low-pressure agglomeration (Section 8.4.1) which may be followed by spheronization (see Section 8.3). Since in this method of pressure agglomeration a wet mixture is passed through the openings of a screen or a thin perforated sheet, very little pressure is exerted and it is essentially a shaping process. Therefore, even if tests are performed on a small perforated die, in regard to product characteristics, the results are also representative for larger units. The same is true for the spheronizing process. As an example for laboratory size equipment, Fig. 11.17 shows the photograph and a dimensional drawing of one system which uses a low pressure axial extruder and a small spheronizer. The machines are mounted on a common base cabinet which includes the controls and the display of process parameters (i.e. power consumption and screw speed of the extruder, extrusion pressure, temperatures of the product and of cooling or heating mediums, and speed of the spheronizer). Small radial, flat screen, and basket extruders are available as well. Medium-pressure agglomeration in pellet mills can be easily simulated because even a single bore with the correct diameter to length ratio and featuring all other details of the orifice (e.g. inlet chamfer, discharge cone, relief bore, etc., see Section 8.4.2) can be used to determine the extrusion characteristics of a particular feed material and the properties of the extrudate. A laboratory set-up can be easily deviced and used for process development. A standard motor powered test stand (Fig. 11.18) can be selected and modified for this purpose. Fig. 11.19 is the photograph of another all purpose drive stand together with the five modules that can be installed for laboratory evaluations of different processes. The attachments provide planetary mixing (a); rotary granulation (b) in which a bar-cage rotor gently passes dry (compacted) or moist materials through a screen to produce a consistent particle size distribution; grating, shredding, milling, and
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Fig. 11.17: Photograph and dimensional drawing o f a laboratory low-pressure agglomeration and spheronizing system utilizing an axial extruder (courtesy WLS Gabler, Ettlingen, Germany).
granulating (c) of dry and moist materials by a rotating wiper blade which pushes the products through a perforated plate; moist granulating (d) in which moist feed is mechani- cally extruded through a perforated cylinder to produce uniform pellets; and mincinglextruding (e) in a simple screw extruder to form strands of compacted moist material.
77.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.18: Motorized single (a) and two (b) column test stands for exerting tensile or compression forces (courtesy Chattillon, Largo, FL, USA).
Samples of punch-and-die pressing can be produced in a variety of home made or purchased small machines. The previously mentioned force/pressure test stands (Fig. 11.18),which may also use hydraulic actuation with hand or motor pumps, can be applied in connection with home made punch-and-die arrangements. Many laboratories are equipped with automatically or hand operated hydraulic laboratory presses, for example as shown in Fig. 11.20 (see also Fig. 8.92, Section 8.4.3). From the suppliers of such machines a large number of simple or sometimes highly sophisticated and automated presses are available. They are used for the determination of a variety of strength and force or pressure related product characteristics and, although the densification and compaction mechanisms are quite different from those of roller presses and can not be correlated, punch-and-die compacts are often made and evaluated to preliminarily investigate the compactibility of different feed materials or powder mixtures and to determine the type and amount of potential binders. Tabletting research and development can be carried out in single station punch-anddie presses which use different drive mechanisms. One such possibility is the application of an attachment to the drive stand that was depicted in Fig. 11.12 (Fig. 11.21).
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Fig. 11.19: All purpose drive stand with five laboratory modules that can be installed. (a) Planetary mixer; (b) rotary fine granulator; (c) grater/shredder; (d) moist granulator; (e) mincer/extruder (courtesy Alexanderwerk, Remscheid, Germany).
77.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.20:
Photograph o f an automatically operated hydraulic "Carver" laboratory press with explanations of the components (courtesy Carver, Wabash, IN, USA).
Fig. 11.21: Tablet press attachment to the universal drive stand o f Fig. 11.12 (courtesy Erweka, Heusenstamm, Germany).
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Fig. 11.22: Small single station eccentric tabletting press. (a) press with stand; (b) selection o f different die assemblies; (c) data measurement, display, and storage system; (d) example o f a pressing force vs. displacement (densification) diagram (courtesy Korsch, Berlin, Germany).
11.2 Laboratory Equipment, Jesting, and Scale-Up
Fig. 11.23: Photograph of a laboratory electro-hydraulic four column press in which a cold isostatic press modul has been installed (courtesy Weber, Remshalden, Germany).
Most manufacturers of tabletting machines also offer a small machine that is suitable for laboratory and development work. Fig. 11.22 is an example of such a machine in which any kind of die assemblies can be installed, including those with unusual shapes and multiple punches (Fig. 11.22b). If used for high precision tabletting and/or R&D work, instrumentation is added (Fig. 1 1 . 2 2 ~with ) which data can be recorded, stored, and processed. For tabletting research, determination of the pressing force over displacement (densification) diagram (Fig. 11.22d) is of great value (see also Section 8.4.2). For evaluating isostatic pressing in the laboratory a specially designed press chamber can be inserted between the platens of a suitable press. Fig. 11.23 shows a powerful laboratory electro-hydraulic four column press in which a cold isostatic press module has been installed. The most difficult laboratory evaluation is that of high pressure roller presses. As discussed in Section 8.4.3 the conditions in the nip between the two counter rotating, converging roller surfaces depends on so many parameters, that it is practically impossible to accurately predict press performance. To simulate the process and get an insight into the macroscopic and microscopic events that occur during densification in the nip, a roller press simulator (Fig. 11.24) was designed and extensively used. For details, an earlier book by the author should be consulted [B.12b]. One of the disadvantages of this roller press simulator was that it still needed a relatively large amount of material and movement was slow and limited. There is a special need in the pharmaceutical industry to accurately predict the compaction behavior of dry powder formulations in roller presses during the development of
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Fig. 11.24 Photograph (a) and diagrammatic representationofthe NCBjCRE (National Coal Board/Coal Research Establishment, UK) roller press simulator; (b) shows the link and drive mechanism [ B. 12 b].
71.2 Laboratory Equipment, Testing, a n d Scale-Up
Fig. 11.25: Photograph of the Polygran Micropactor and close-ups o f the arrangement (left) and the nip (right) during a test (courtesy Certeis, Jona, Switzerland).
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new drugs. Since roller presses that are used in the pharmaceutical industry are typically small, testing could be carried-out in actual machines. However, because, particularly in the development stage, very little active substance is available which, in addition, is extremely expensive, even the smallest machines require too much material especially if the limiting roller speed (= capacity) is to be determined. The Polygran Micropactor (see Section 14.1, Gerteis) tries to solve this problem; it is claimed that with only a few grams of material, meaningful results are obtained. Fig. 11.25 is a photograph of the equipment and also shows close-ups of the arrangement (left) and the nip during a test (right); the front cheek plate, which seals the nip, has been removed to show the material. A narrow roller faces a straight metal strip which, during the test moves downward with a speed that is identical to the circumferential speed of the roller. This way one half of the nip produces ' / 2 compacted strip which can be tested for strength and other characteristics (e.g. porosity) as well as granulation and tabletting behavior. During the test, gap, force, and torque are measured which can be used for designing an industrial machine. Another modular system is often called laboratory equipment; it can be used for small scale production and for the laboratory evaluation of small samples. Fig. 11.26 depicts the design and some of the accessories. In the most simple execution a hopper feeds a pair of rollers which are driven by a hand crank. The rollers can be solid and may be equipped with compacting or briquetting surfaces (see Section 8.4.3) or two perforated, geared, intermeshing pelleting rolls (see Section 8.4.2) are installed to accomplish medium pressure extrusion. In a modular fashion the rollers can be motorized, screw feeders can be added, and the rolls may be oriented vertical or horizontal or in any other direction. As shown in the photographs of Fig. 11.27 the roller frame can be totally enclosed for dust control if toxic or hazardous materials are processed. A panel includes controls and instrumentation for data display and collection. As has been demonstrated, test equipment is available in all areas of interest for the determination of feed and product characteristics, including new techniques that have been developed in response to advancements in modern Mechanical Process Technol-
Fig. 11.26: Schematic representation o f t h e design and some o f the accessories o f a modular laboratory roller press (courtesy Bepex/HUTT, Leingarten, Cermany) ([38]in Section 13.3).
17.2 Labarataty Equipment, Testing, and Scale-Up
Fig. 11.27: Two photographs o f totally enclosed laboratory roller presses (courtesy B e p e x / H U T , Leingarten, Germany).
ogy and to new applications for the manufacturing of novel, for example, engineered
products (see Section 5.4). However, testing is only as good and predicts industrial performance of the projected plants as correctly as test conditions reflect what will be found later in the actual installation. A common problem during the development of any new process of Mechanical Process Technology, particularly also including size enlargement by agglomeration is the requirement to investigate a representative sample of the future feed material. As mentioned before, in many cases the actual feed material which must be agglomerated in the course of a new plant flow sheet is not yet available in large quantities. Often, it is manufactured within the same or another new project, either by processing a natural resource, changing the characteristics of already available solids, or synthesizing from various raw materials. In those cases, the feed for laboratory tests is itself the results of tests or of a small scale pilot plant. The properties of such materials may change considerably when they will be produced in the industrial scale and/or in-line. The results of tests with such material are questionable, at best, which must be considered when evaluating the data and designing the plant. In other circumstances the feed material is already available from an industrial installation, either, for example, mines or chemical and mechanical processing plants. It may also be a waste stream or by-product from an unrelated manufacturing facility. In those cases, it has become necessary at a later time to modify material characteristics and improve their properties (see Section 5.4, Tab. 5.10). For testing, this material must be sampled so that it accurately represents what needs to be continuously pro-
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cessed in the planned agglomeration system. Such sampling must take into consideration potential segregation as well as normal, unavoidable fluctuations in material consistency and properties. Sampling of particulate solids is a complex problem in itself [B.24, B.271. Its coverage is beyond the scope of this book. Another, often overlooked influence on the results of testing and their applicability is that, in practically all cases, the test facility is located at a vendor or research facility,often hundreds or thousands of kilometers away from the source of the material to be evaluated. Assuming correct sampling, i.e. excluding this problem, the material is bagged in a suitable way, which, today, often includes “big (bulk)bags”, so called FIBCs (flexible intermediate bulk containers). During handling and transshipment, particulate solids, in addition to potential chemical changes, may segregate, break, and/or cake and generally will change their bulk characteristics. This is particularly true if the modern FIBC is used because one of the characteristics of this packing method is its flexibility. However, the “old fashioned” packing in drums or packages may cause at least some of the same problems. To obtain the best possible feed for testing, the original bulk properties must be reinstated which, as is easily recognizable, is difficult or even impossible. Segregation and settling may be reversed by tumbling and mixing, but changes in particle size and shape, either during transshipment and handling or the breaking of lumps, and other particle modifications are irreversible. Furthermore, aging of materials is a common, but little recognized problem. This term refers in most cases to a modification of the surfaces of the particulate solids by adsorption of moisture and other atoms or molecules and/or oxidation and other chemical reactions. Sometimes, new (often whisker-like) crystal growth is also observed (see Section 5.5). The product(s) of aging have a marked effect on the results of testing, because, in agglomeration, binding mechanisms rely on chemical and physical interactions at and between surfaces of the particles to be agglomerated and, if applicable with the binder component(s). In conclusion, even if a representative sample is provided, a material which is several days, weeks, or months old and may have had to be reheated, dried, rewetted, delumped, mixed, fluffed, etc. to bring it back to conditions that are corresponding to or comparable with those found or expected in the real plant environment may yield completely different results than obtained later “in-line”.Although, as described in the previous chapters of this book, as reviewed in other publications (see Section 13.1),and as made available by vendors (see Section 14.1)in their brochures and newsletters, certain characteristic relationships have been developed for most agglomeration methods and performance factors can be collected in charts for the preselection of methods which are most likely suitable for a particular application (for example, Fig. 11.5 and 11.6, Section 11.1),determination ofthe actual design parameters remains a serious problem. This means that, as a general rule, tests must be carried out with representative samples of the specific, unaltered particulate solids which need to be processed by an agglomeration method and installation of a pilot plant on-site and/or in-line should be considered if the risks, which are always connected with new installations, are to be minimized. Furthermore, even if plants are already successfully operating in other places and “the same material” from new or existing sources or particulate solids with essentially
17.2 Laboratory Equipment, Testing, and Scale-Up
identical chemical composition must be agglomerated in a new location, experience teaches that it can not be safely assumed that the new installation can use the same design and operating parameters to obtain a product with comparable quality. Minute differences in feed characteristics, such as particle shape, size distribution, surface roughness, wettability, porosity, physical contamination with nanometer dust or adsorbed layers, chemical modification with trace elements, etc. may result in significantly different process and operating conditions. A plant which is comfortably sized in a “reference location” may, at another site, handling “the same material”, be grossly underperforming if for the design and execution of the new project only data from the “reference plant” were utilized. While the pilot plant approach during project development must be investigated and decided upon on an individual basis, testing of equipment is always necessary. For that reason, nearly all manufacturers and/or vendors maintain sometimes rather elaborate facilities [ B.421 which normally include machines of different sizes, including large scale equipment, to avoid scale-up problems. These test facilities must also include peripheral equipment such as mixers, heaters or coolers, conveyors, crushers, dryers, screens, etc. although the variations that are available in these special areas can not be offered. Therefore, additional tests for the evaluation of the best peripheral equipment are often necessary at different facilities. All of these tests have the same problems as mentioned above. Furthermore, for cost reasons, even if only a limited selection of process equipment would be used, the continuous operation of an entire production line is normally not possible during testing. In those cases where in the actual plant recycle will be produced and recirculated in one way or another, product is first made from the fresh feed, the expected type(s) ofrecycle is (are) produced, and for further testing the anticipated amount(s) is (are) mixed with the fresh feed or other material streams, such as, for example, the crusher or screen feeds, to simulate the conditions in a continuously operating system. All data from tests and evaluations at often many different locations and with various equipment, sometimes also including information from pilot plant operations, is collected, compared with related know-how and experience, if available, and used for designing the new plant and the selection of process equipment. In spite of all the efforts that normally go into the determination of the design data, it is prudent to include safety factors in the design stage which will allow to optimize the system if and when it comes on stream. Such optimization is a normal requirement for all installations of Mechanical Process Technology. Selecting equipment for the lowest expected material flow rates and/or product qualities, which often seems necessary to meet project cost limitations, later leaves no room for modifications and often results in underperforming plants. Remediation of this situation may not be possible or becomes much more costly than a small project cost overrun would have been. A relatively new development is the emergence of tolling companies (see Section 14.1). These are installations which maintain production lines for contract manufacturing, co-manufacturing, and back-up manufacturing. Originally, most of the “tollers” were formed when the sometimes very large and extensive test and development es of diversified companies became profit centers as the head office’s management philosophy was changed. The growing desire for out-sourcing instead of new
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cifically formed to accept waste materials and by-products from the industry for conversion into secondary raw materials. And, especially in the pharmaceutical industry, intermediate products are produced by specialized tollers and co-production or backup production is established to meet often seasonal peak demands. At certain times, these companies also offer their processing capabilities during project development for the testing of new materials and the evaluation of products. Although, in most cases, a major percentage of the manufacturing capabilities are dedicated to supporting a limited number of customers with particular needs and to the production of specific products, tolling companies typically also offer their services on the open market and can, for example, do the pilot plant stage in the development of a large project. Another possibility is that new materials are made for exploratory marketing purposes and/or to bridge the gap between product demand and supply during a new facilities’ start-up phase.
11.3
Peripheral Equipment
If a complete installation, which includes size enlargement by agglomeration, is considered, the entire range of equipment and technologies that is related to the unit operations and associated fields of Mechanical Process Engineering (see Chapter 1, Fig. 1.1) may be utilized in different locations. Which of the various alternatives are optimal solutions for specific tasks depends on the application and the requirements that are defined by specific needs or regulations of a particular industrial field. It would go beyond the scope of this book to discuss details, pros and cons as well as selection criteria of all of the machines and techniques of Mechanical Process Technology. For the purpose of this publication, only some typical equipment, which is closely related to the agglomeration process or constitutes a necessary part of the agglomeration system itself, will be discussed. Particular emphasis will be on characteristics and operating behavior that is or needs to be different if the equipment is applied as part of an agglomeration method. More detailed information will be provided in a future book by the author on the industrial applications of agglomeration [ B.711. Excluding agglomeration by heat (sintering),which, as discussed in Chapter 9 and subsections, has conditions and requirements of its own, the industrial agglomeration technologies can be roughly divided into tumblelgrowth and pressure agglomeration techniques. The various agglomeration methods work best with different feed characteristics and product parameters. Thus, agglomerators and their components must be designed with these conditions in mind. Fig. 11.28 shows generic tumblelgrowth (a) and pressure (b)agglomeration systems. The flow sheets are simplified but include the areas in which special considerations must be observed. During a “walk through these block diagrams, different requirements will be pointed out.
71.3 Peripheral Equipment
Final Product
Crusher
Screen
a. TumblelGrowth Agglomeration
Metered Feed Materials 1-n from Day and Surge Bins
c
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Final (Under and (Milled) O v e r - S i z e d l
Screen
I b. Pressure Agglomeration
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Fig. 11.28: Simplified block diagrams of tumble/growth (a) and pressure (b) agglomeration systems.
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Crusher7
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(a) Tumble/growth agglomeration
If more than one feed powder is agglomerated, the components must be metered and premixed. Homogenization may be necessary since there is a pronounced danger of selective agglomeration, e.g., if the sizes and distributions of the particulate solids are very different. This could involve separate or joint milling of the components. During mixing, some of the liquid or dry binders could be added. It is also possible to feed all or part of the recycle into the mixer. The premixed material should still be loose and aerated such that, after feeding it into the agglomerator, the solid particles are able to move individually and randomly, pick up more binder, and agglomerate upon impact. A metered addition of recycle to the agglomerator improves and accelerates agglomerate growth by seeding the charge. This is because recycle, in spite of its representing undersized product, consists largely of somewhat preagglomerated material. Control of the growth mechanism also requires addition of at least some of the liquid or dry binders in the agglomerator. Tumble/growth units discharge green agglomerates that are preferentially bonded by liquids. These units include drums, inclined pans, all kinds of mixers, and fluidized beds (see Chapter 6, Fig. 6.3). With the exception of products from inclined pans (see Section 7.4.1), the agglomerate sizes and shapes vary within wide limits. Green agglomerates are often weak and sticky, tending to blind conventional screens. Therefore, separation of over- or undersized material at this point must be frequently bypassed. In some cases, it is possible to screen the green agglomerates and feed only a narrow distribution to the post-treatment step. For this process step, special screens are required. Fig. 11.29 shows schematic representations and the photograph of a roller (conveyor) screen in operation. Originally, this type of equipment was used for the rounding of spherical “pellets” and the removal of fines in iron ore pelletizing plants prior to feeding clean agglomerates to the drying and sintering machine (see Section 9.2.2). Each of the rollers, which together form a downward slanted surface, is individually driven and, as shown in the upper left, the spherical agglomerates are rounded when moving down the slope while fines fall through the gaps between the rollers. The moist fines (recycle) are sent directly back to the agglomerator. Modern roller screens feature a horizontal deck. Fig. 11.30a depicts the principle of such a machine. Each of the rollers is still individually driven, as shown in the photograph (Fig. 11.30b), but the movement of the material is caused by intermeshing triangular lobes (discs)which, at the same time, loosen up the bed to free the fines which drop through the gaps. Since the materials are often tacky, scrapers, which in their entirety resemble combs, clean the screen openings. During post-treatment, the temporary liquid bonding is transformed into a permanent bond. In this step, liquid is removed and permanent bonding is obtained by recrystallization of dissolved substances, sintering, partial melting, or chemical reactions. The discharge from the post-treatment should be screened or rescreened because fines are still present or may have formed by abrasion and breakage, and oversized agglomerates may have developed by secondary agglomeration of the still moist and sticky green agglomerates. Oversized agglomerates must be crushed. This recycle is
77.3 Peripherai Equipment Movemenl 01 geen pellels pellei
Pellel dischory
lo lrowlling grole
Fig. 11.29: Schematic presentation o f the principle (a) o f a roller screen in iron ore pelletization and photographs o f such screens (b) showing the design (left) and operation (right) [B.16, 6.421.
dry and, therefore, should be returned to the mixer. A preferred feature of this flow sheet is a surge bin in which undersized material and dust from the dedusting system are collected. This allows the metered addition of recycle, thus contributing to a better defined process performance in the agglomerator and a more uniform product. (b) Pressure agglomeration On first sight, the block diagram for pressure agglomeration looks almost identical to that of tumble/growth agglomeration, particularly when post-treatment is required. Typically, this is the case if low- and medium-pressure agglomeration methods are used. These methods require liquid binders to guarantee easy formability (see Sections 8.4.1 and 8.4.2). A typical high-pressure agglomeration system does not include posttreatment and in most applications, the addition of binders is limited to dry additives.
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wldth of screen bed
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roller packages
Fig. 11.30 Principle (a, side and top views) and photograph (b) of a modern horizontal roller screen for the separation o f sticky fines (courtesy ZEMAC, Zeitz, Germany).
11.3 Peripheral Equipment
Fig. 1 1 . 3 0 cont'd
(b)
In contrast to tumblelgrowth agglomeration, which requires a feed with a surface equivalent diameter of less than a few hundred micrometers (see Section 7.1), as well as excellent dispersion and aeration, pressure agglomeration tolerates a wide particle size distribution. The maximum allowable particle size increases with increasing pressure, and aeration of the feed prior to agglomeration must be avoided by all means. As mentioned repeatedly (see for example Section 8.1)and also discussed below, air in the feed must be removed from the compaction area during densification. Large particles are easily incorporated during the forming of an agglomerate under pressure. If high forces are applied, brittle disintegration and plastic deformation occur (see Fig. 8.1, Section 8.1).In any case, a considerable volume reduction takes place which is largest for high-pressure agglomeration. Densification ratios of 1 : 2 and 1: 3 are common and may be as high as 1 : 5. Since gases (air) in the bulk feed must be totally removed to avoid compressed air pockets, blending prior to agglomeration should not be carried out in high speed powder mixers. A good piece of equipment for this task is the batch or continuous mixmuller (Fig. 11.31). In pressure agglomeration, it is preferable to meter recycle into the mixer. However, when product quality does not need to be tightly controlled, recycle can be added to the fresh feed in the agglomerator. The ratio of fresh feed to recycle should be kept constant, however, because variations in feed composition change the agglomerate quality which, in turn, influences the recycle rate. Therefore, a surge bin for the recycle must be provided from which the fines are metered back into the system. (Note: This statement is valid for all agglomeration methods.)
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lnnerand outer ploughs
chute
Crib
Fig. 11.31:
Sketches o f a batch (a) and continuous mix-muller (courtesy National Engineering, Aurora, IL, USA).
While surging is normal for most tumble/growth agglomeration techniques, in pressure agglomeration, particularly at high pressures, uncontrolled surging, caused by varying recirculation rates, may totally disrupt the process. The feed/recycle ratio must be changed if there is a change in the amount of recycle produced in pressure agglomeration as determined by level probes in the surge bin. This often requires changing operating parameters of the agglomerator and of downstream equipment. Agglomerates leaving pressure agglomerators first hang together then break into pieces due to their own weight. Agglomerate strength increases with higher pressures during densification and forming. Knives must be used to cut extrudates, and various separators are used to break a string of briquettes into singles (Fig. 11.32). Pressure agglomeration can also be applied to produce a granular product (Fig. 11.33). In such use, the separator, if required, is often a prebreaker. Granular product is obtained between the two decks of double-deck screens. Oversized material is crushed in a suitable mill and rescreened; undersize is recirculated. Multistep crushing and screening operations are used to improve the yield and obtain a cleaner granular product (see also Section 8.3).
11.3 Peripheral Equipment
+
roller presses [for details refer to 6.421.
(d)
(dl
Yield can be also increased by installing mills using a gentler crushing mechanism. However, because compacted materials are not uniformly dense and strong, the product may be softer and produce more fines during storage and handling. Granular materials which must be shipped in bulk, such as fertilizers, should be produced with mills using high energy input, thus giving a somewhat lower yield but high product strength. Nevertheless, as discussed in Section 8.3, the particles should be stressed only once or multiple times individually during unrestricted movement,
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Fig. 11.33: Flow sheet o f an optimized compaction/granulation system for the manufacturing o f granular product (for example mixed [NPK] fertilizer) ((1471 Section 13.3).
i.e. without being retained in the crushing chamber by, for example, exit screens. The discharge from the size reduction equipment is then screened and the oversize is recrushed to produce more product while minimizing fines production (see Fig. 11.33). An excellent crusher design with almost unlimited adjustability of the crushing process is the cage mill (Fig. 11.34).As shown in the artist's conception (a) and the representation of the operating principle (b),in this design two independently driven bar-cages, each consisting of two cylindrical rows or bars which interlink with the opposite ones, operate in a common housing. The material to be crushed enters through a chute in the center and, driven by centrifugal force, travels through the bar cages to the periphery and drops by gravity out of the housing which is open below. There are literally unlimited possibilities for varying the degree of stressing by selecting co- or counter-rotation and varying the absolute and relative speeds of the cages. The number of cages can be also changed with different models and only one single or multiple cage may intermesh with stationary opposing bars.
71.3 Peripheral Equipment
Fig. 11.34 Artist's conception (a) and principle (b) o f a cage mill (courtesy Cundlach, Belleville, IL, USA).
The irregular particle shape resulting from crushed agglomerates may be rounded in post-treatment steps. Since screening is always a part of the process, the particular material movement on the extremely flat screen decks of gyratory screens (Fig. 11.35) is sometimes providing some such rounding. In general, screens of this design are often preferred for agglomeration and, particularly, compaction/granulation plants because residence time on the deck is relatively long and movement is gentle without the impacts between solid particles and the screen that are typical for vibrating or mechanically excited machines and may cause fines through secondary breakage or attrition. On the other hand, loosely bonded particles and corners or edges on the irregularly shaped granules from crushed compacts, are abraded during the rolling action on the screen. As discussed in several parts of this book, agglomerates, particularly immediately after their production, are weaker than solid particles with comparable size. Therefore, transportation equipment should be selected to handle agglomerates gently. Normally
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Fig. 11.35: Schematic presentation ofthe particle path (a) on a flat, gyratory screening machine and photograph (b) o f such equipment (courtesy Rotex, Cincinnati, OH, USA) In both pictures, feeding is on the right and discharge on the left.
mechanical belt conveyors are used which, to avoid sliding and tumbling if the direction of transport is up or down, are preferably pocketed (Fig. 11.36).Smaller agglomerates can be transported pneumatically if alternatives to the high velocity, low density system, which causes destruction by impact and results in fines and/or build-up, are selected (Fig. 11.37). In Section 11.2 it was pointed out that, with any new agglomeration system, after installation and start-up it is normally necessary to adapt its operation to the in-line and site conditions and to perform optimization. Unless batch or small, for example pharmaceutical, installations are considered, the often large continuous plants feature considerable internal mass flows (see, for example, Fig. 6.3, Chapter 6, and Fig. 11.33, above) in closed or recirculation loops. Since in installations that handle and process particulate solids, dusting and particulate contamination is a common and objectionable problem, the equipment of more recently designed plants is enclosed and aspiration points are connected to dust collection systems. As a result, it is surprising that personnel, operating systems for quite some time, has no idea what happens inside the plant and how mass flows are influenced when processing parameters are changed. This is particularly true for the closed loop screening and crushing cycles of granulation plants where the final product size and distribution are adjusted. Optimization requires a good knowledge of the mass flow rates at critical points of the plant and of the characteristics of the particulate solids at these points. Until relatively recently, flow rates of solids had to be determined by opening a system, bypassing and collecting the entire amount of material for a measured time, and weighing it. Today, many different mass flow meters for solids are available on the market. Fig. 11.38 shows the principle (a) and two executions (b) of such an instrument. All measuring principles are based on determining the force that results from the mass of particulate solids impacting onto or flowing over a plate which momentarily supports a fraction of the material. Installation of such solids flow meters at as many points as
11.3 Peripheral Equipment
Fig. 11.36 Photograph o f an open, pocketed mechanical conveyor (a), detail of the pocketed belt (b), and some typical configurations (c) (courtesy Unitrac, Port Hope, Ont., Canada).
possible indicates the flow patterns in the system and allows to influence system performance by changing process parameters and evaluating their effect. In addition, sampling equipment or, at least, access points must be provided to draw material samples for evaluating feed, agglomerate, intermediate, and product characteristics [B.24, B.271. Although, the above represents only a very limited discussion of the peripheral equipment in agglomeration plants, these comments shall suffice for this publica-
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Fig. 11.37: Schematic representations o f the operation of different pneumatic conveyors. (a), (b), and (c) show high-velocity conveying systems. (d), (e), and (f) depict low-velocity systems for gentle conveying (courtesy Buhler, Uzwil, Switzerland).
T 1.3 Peripheral Equipment
Fig. 11.38: Principle (a) and two alternative configurations (b) o f a solids mass flow meter (courtesy El, Wilmington, NC, USA).
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tion. Special equipment considerations and the selection of the best suited machinery for a particular flow sheet depend largely on the application and the characteristics of feed and product as well as on the agglomeration method. Coverage of the applications of agglomeration in industry is planned in an additional book [B.71]. A more detailed evaluation of the peripheral equipment for various applications will be presented there.
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
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12
Outlook Although agglomeration is a natural phenomenon (see Chapters 2 and 3) and was used by animals and humans for a number of purposes for millions and, respectively, thousands of years (see Chapter 3), it has been recognized as a technology only at the beginning of industrialization, approximately two hundred years ago, and has been defined as a unit operation of Mechanical Process Technology and a field of science in its own right during the 20th century. The understanding of the fundamentals of agglomeration (see Chapter 5 and subchapters) has quickly led to a large number of new and improved processes, some of which, such as the briquetting of coal, the pelletizing of iron ore, the pelleting of animal feed, the development of a large number of solid dosage forms in the pharmaceutical industry, the shaping of new food products, the granulation of fertilizers, agrochemicals, and, generally, of many chemicals for the most diverse uses, the compaction of wastes for recycling, the production of ceramic and metallic materials for high strength and high temperature applications, and many more, have been produced in large bulk quantities and have helped revolutionize particulate solids technologies. More recently, requirements for the vast novel field of life sciences and for many other modern applications have started a new trend in the manufacturing and/or manipulation of solids. As already discussed in Section 5.4, particulate solids processing is shifting away from just attaining a particular shape, size, and distribution of products and moves toward the realization of improved composition, microstructure, morphology, and characteristics. In other words, the emphasis of new products and processes will be on better control of primary particle physical properties, highly specific product size and composition, as well as the creation of desirable properties. Many products have high value, feature special effects, and are being produced in small quantities. There is a shift away from the production of simple bulk commodities with average, widely useable, but not always optimized quality to specifically engineered materials that respond directly to the particular needs of the end user. A few examples of such processes which use the fundamentals of agglomeration and/or conventional or modified agglomeration processes will be discussed to illustrate what future developments may entail.
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Granulation of Fertilizers
For many readers, fertilizer granulation may be a rather unlikely topic responding to the above description of new trends. Shortly after postulating the need for the feeding of commercially grown plants with the basic nutrients - nitrogen (N), phosphorus (P), and potassium (K) - by Justus von Liebig during the middle of the 19th century, large scale production of fertilizers and their application by farmers everywhere began. While up to this time, empirically developed fertilization with organic wastes was practiced in agriculture, mineral sources of nutrients were defined, mined, and processed in large amounts, and shipped in bulk to the growers. Since availability of the elements depends to a large extend on their solubility, fertilizers must be soluble and, therefore, if transported or stored for longer periods of time they tend to develop unwanted agglomeration (see also Section 5.5) resulting in lumping, pile set, or, more generally, an uncontrolled size enlargement which hinders or prohibits uniform distribution on the fields. To overcome this problem and to be able to serve the quickly increasing needs of growers around the world, during the first half of the 20th century granulation of fertilizers was introduced. As described in Section 5.4, Tab. 5.10, size enlargement by agglomeration produces freely flowing products with improved handling and storage characteristics and low content of dust, with defined size and shape, featuring no segregation of different components, if applicable, and often with increased bulk density. At the beginning, granulation was accomplished by tumble/growth agglomeration methods, mostly in drums (see Section 7.4.1) with water and post-treatment, strengthening the particles by recrystallization during drying (see Section 7.3). Later, particularly in the potash industry, dry compaction/granulation was used (see Sections 8.3 and 8.4.3). In both cases bulk volumes were produced in facilities with increasingly large capacities. As agrochemical research and the understanding of the needs of growing plants in various environments expanded, it became clear that plant species in different soils and climates have very distinct requirements. Not only does a particular planting perform best if it is fed with a very defined NPK relationship but a large number of so-called trace elements, such as copper, iron, or sulfur, to name a few, are required for optimum results. In addition, the fertilizer granules may be coated with insectizide and/or fungizide to protect the new plant growth or availability may be varied by coatings which provide for delayed or slowed dissolution. Fertilizers will also feature distinctly different composition if they are applied during various growth stages. To avoid overfertilization or the application of only partially suitable fertilizers and achieve all the other desirable features, an agrochemical evaluation of the soil is combined with climatological data and the needs of the particular plant species. Then, a special multicomponent fertilizer is defined, mixed from a multitude of ingredients, granulated, and potentially subjected to specific post-treatment methods, for example coating, to arrive at the best possible plant food. For this modern method of fertilization, which is particularly desirable for tropical, subtropical, and other high performance farming areas, even if the farming area is relatively large, due to varying soil qualities which necessitate adjustment in fertilizer quality, only small amounts
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of the fertilizer are desired. This material must be produced on demand and by economical means. The classic method of fertilizer agglomeration using tumble/growth methods is not suitable for this complex task of particulate solids processing. To produce designer plant foods as described above, which are often no longer called fertilizers but are identified as agrochemicals, compaction/granulation is being applied (for a typical flow sheet refer to Section 11.3, Fig. 11.33 and for representative references see Section 13.3 [29, 35, 40, 41, 95, 96, 101, 104, 106-108, 110, 112, and, particularly, 1441). The most important advantage of fertilizer granulation by compaction is its versatility as demonstrated by the following list: 1. With the exception of a few materials (such as urea or TSP (triple superphosphate) for which maximum amounts exist that can be used in a formulation) literally all solid particulate plant nutrients can be processed. This includes, for example, dry digested sludge from municipal waste treatment plants and also the addition of small amounts (typically <10 %) of liquid additives. 2. To minimize the cost of the product, raw materials can be purchased on the world markets without specific requirements on particle size, shape and mass. Fines which are off-specification for, for example, fertilizer bulk blending can be used and are often even preferred. 3. Compaction/granulation plants can be designed for economic operation at any feed rate. Production capacities per line are feasible between 0.1 and 50 t/h. 4. Larger plants are preferably equipped with two or more lines fed by only one large compounding (= batching or formulation) system. The rest of the plant is designed with separate lines to improve availability; only one line is shut down during maintenance and emergency shut-downs. 5. If a plant is equipped with multiple lines and features separate day bins for fresh material, recirculating fines, and granulated product, each line can process different formulations. 6. Economical production of small batches is feasible. Depending on the extent of cleaning that is necessary during change-over (determined by how much crosscontamination can be tolerated), up to three different formulations (batches) can be produced in an 8 hour shift. 7 . Fertilizer granulation plants using compaction can be combined with either custom designed batching systems or standardized formulation or bulk blending units. Particularly the latter allows easy expansion of bulk blending to include mixed fertilizer granulation. 8. Any fertilizer compaction/granulation system can be also utilized as a regional production facility for the manufacturing of bulk blending grade material from off-specification fines. This capability also includes special formulations which may be required by the local market such as indigenous fillers, with or without major nutrients or carriers incorporating micronutrients. Such products can then be used together with imported bulk blending grade materials in bulk blending.
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9.
To demonstrate the ultimate in flexibility it should be mentioned that plants using roller presses for compaction can be easily modified to also produce unusual fertilizer materials. For example, urea supergranules for deep placement in wetland rice production may be manufactured by changing the rollers to a pocketed (= briquetting) configuration, bypassing the flake breaker (9 in Fig. 11.33) and obtaining the product as oversize discharge from the scalping screen (10 in Figure 11.33) which is also used for separating the briquettes.
The previously mentioned coating of the granulated fertilizer to achieve the other product characteristics may be accomplished with almost any of the methods described in Section 10.1. Production of Agglomerated Materials with Instant Characteristics As briefly discussed in Section 5.4, the term “instant” is normally used in the food industry, for drink powders, soups, sauces, and the like, as well as in related fields, for example for pharmaceuticals and animal feeds, and describes the easy solubility of these products. Instant agglomerates are also desirable for pigments and other finely divided chemicals which are ultimately applied with and in a diluent. Many of these materials are insoluble and, therefore, only require complete dispersion. All instant products must be able to quickly disperse and, if applicable, dissolve in a specific liquid at any temperature, particularly also at ambient or even cold conditions, without residue and sediment. Instant characteristics of an agglomerate are defined by the three (dispersion) or four (dissolution) mechanisms (see Section 13.3 [147, 149-151, 153, 1581) listed in Tab. 12.1. All three or four phases of dispersion or dissolution are proceeding individually whereby some overlapping may occur, depending on the amount of material involved. Instant properties are a function of time. Each industry has a more or less well defined procedure to determine the maximum allowable time. Typically,complete dispersion or dissolution should be accomplished within a few seconds in warm liquid and in approx. 30-GO s in cold liquid. Particularly in the food and pharmaceutical industries, instant agglomerates may contain certain substances that assist in the break-up during the dispersion phase (see Section 5.1.2, Fig. 5.13). In the pigment and chemical industries dispersion is often assisted by some sort of agitation [B.51]. It is interesting to note, that, for many of these modern and very likely for most of the future engineered products, quality control procedures must be newly established. The characteristics which control their performance have previously been unknown, were
Dispersion and, respectively, dissolution mechanisms of “instant” agglomerates.
Tab. 12.1:
1. 2. 3. (4.
Penetration of liquids into the pores of the agglomerates (also called wetting) Submergence of the agglomerates in the liquid (also called sinking behavior) Break-up of the agglomerates into the primary particles (also called dispersibility) If the solid is soluble, dissolution of the primary particles (also called solubility)).
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unimportant, and/or could not yet be determined because of scientific or technical limitations at that time. For example, to evaluate the instant properties of agglomerates their size, which influences the wetting or suction (= take-up of liquid) behavior, and the dispersion characteristics must be measured. Fig. 12.1 depicts a technique for the measurement ofthe dynamic wetting behavior. The force measured by the weigh cell is proportional to the liquid volume that has entered the particle bed by suction and wetting. With the method shown in Fig. 12.2 the speed of penetration of a liquid into a particle mass can be determined. After forming a layer of material with thickness h on the bottom diaphragm and adding a plexiglass cylinder to avoid wetting from above, the test cell is submerged in the liquid. The time is measured until the entire bed is wetted and liquid appears on the surface. If for each measurement differently fractionated (= sized) agglomerates are used to build the bed, the optimal granule size for quick wetting and liquid penetration can be determined. The results of this and other tests are influenced by various potential particle behaviors as shown in Fig. 12.3. Fig. 12.4 represents a method by which both wetting and dispersion behavior can be determined. The apparatus consists of a 250 mL, temperature controlled beaker with flip top in which wetting and dispersion occur and a photometric device with a flowthrough cell (= cuvette) for the measurement of the degree of dispersion. Fig. 12.5, in which transmission is plotted vs. time is a typical result of this test.
Fig. 12.1: Measurement o f the dynamic wetting behavior or particulate materials [B.20]. Left: ready position, right: measuring position.
Powder Diaphragm
Plexiglass
Powder Fig. 12.2: Determination o f the speed of penetration o f a liquid through a powder bed [12.1, 12.21,
Screen
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72 Outlook
\
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1
'
.
5
6
7
Fig. 12.4: Apparatus for measuring the wettability and dispersibility o f "instant products" [B.42]. ( 1 ) Instant product, (2) cuvette, (3) amplifier with lens, (4) thermostat, (5) magnetic stirrer, (6) beaker, (7) light source, (8) light conductor, (9) photodiode; 1/1, = Transmission, H = 4 0 mm.
8
1.c -m
0.6 \ L,
4
g 0.6 ..u
5
0.L
I
-
I
z
I
c
0.2 -Wetting
0
--+ 0.5
I
,
Fig. 12.5:
Dispersion I
1
1
5
Time l r n i n l
j 10
Transmission vs.
time of an agglomerated instant 13 powder evaluated using the apparatus o f Fig. 12.4 [B.42].
First, the sample of material to be investigated is uniformly deposited onto the surface of the liquid by the flip top. During the entire test, a small amount of liquid, which does not change the stationary behavior of the liquid mass in the beaker, is transported by a pulse-free peristaltic pump through the cuvette. At the beginning, the magnetic stirrer is not operated. If, after wetting the sample, particles sink in the liquid and enter the suction port of the pump, they are transferred to the photometric test cell where a reduction of light intensity is observed. The measurement determines the transmission, i.e. the actual light intensity I referred to the light intensity I , of the clear liquid, as
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a function of time. Materials that wet more easily show a decrease in transmission earlier than less readily wettable solids. Later, the magnetic stirrer is operated in a defined manner and additional dispersion is obtained. The graph plotting transmission vs. time (Fig. 12.5) provides a measure of dispersion velocity. A plateau of the transmission cuwe indicates the best (stationary) dispersion. By applying ultrasound, the degree of dispersion can be further improved in most cases. The test results may be used to determine several characteristic dispersion conditions of a particulate solid. The results define, for example, the speed of dispersion, dispersion without and with different levels of stirring, and final dispersion after the application of ultrasound. The level of ultrasound that is necessary to disperse aggregated particles in a liquid can also be used as a measure of agglomerate strength of, for example, granulated silica fume in a somewhat modified test arrangement [12.3]. This discussion shall suffice at this point to demonstrate the need for the development of novel technologies and methods for new generation, agglomerated, engineered materials. While the techniques that were discussed above use relatively simple fundamentals and equipment, many others are based on highly sophisticated electronic and physical methodology. Products with instant characteristics, for example from powdered food materials, can be obtained from a variety of different, rather conventional processes (Fig. 12.6); most of these use agglomeration techniques (A in Fig. 12.6). Because granule size should be small and porosity must be high, instant food products are most commonly manufactured by rewet agglomeration in mechanically agitated beds (see Section 7.4.2) or fluidized beds in which turbulent movement is induced by flowing gas (see Section 7.4.4). Spray drying (see Section 7.4.3) combined with mixer agglomeration (see Section 7.4.2) or fluidized bed agglomeration (see Section 7.4.4) are also often used. A more recent development in instantizing utilizes compaction/granulation. One of the most important binding mechanisms of high-pressure agglomeration (see Section 8.1) is caused by van-der-Waals forces. This short range molecular attraction does not develop solid bridges between the agglomerate forming particles. Since in wet environments van-der-Waals forces are lower by a factor of approx. 10, agglomerates thus bonded disperse easily in liquids. This knowledge led to the adaptation of compaction/
A: Aaalomeration Techniaues A.l
A.2 A.3 A.4
8 : Techniques utilizing other processes
Rewetting of Powders in Fluid B1 Beds A 1.a mechanically induced 8.2 gas induced A. 1.b: 8.3 Spray Drying and Agglomeration Combinations of (A.l) and (A.2) Press Agglomeration A.4.a: CompactionlGranulation A 4.b: ExtrusionlCrumbling
Improvement of Wetting with Additives (Surfactants) Improvement of Wetting by Extraction (e.g. of Fat) Improvement of Solubility (e.9. amorphous structure)
Fig. 12.6: Principles that are most commonly used to manufacture instant products from powdered food materials [12.2]
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granulation for the manufacturing of instant materials. Originally, this method was considered an unlikely candidate technology for the production of easily dispersible agglomerates because the product particles are dense and feature low porosity. However, it turns out that the higher density of the material, which translates into larger mass of each product particle, coupled with the fact that agglomeration occurs in the dry state, make this technique uniquely applicable for, for example, the granulation of detergents. The heavier products can be fitted into smaller packages and the smaller dispensing cup suggests a more economical use as well as less environmental damage. Since submergence (= sinking behavior, Tab. 12.1) is no problem with these heavy particles, quick wetting must be guaranteed by, for example, the addition of surfactants (see Section 5.1.2) prior or after compactionjgranulation. Although, as mentioned above, the binding forces are lower in a liquid environment, additional dispersion may be realized by employing aids such as using amorphous components or adding materials such as micro crystalline cellulose (MCC, see Section 5.1.2), or by producing effervescent granules. In some cases fluidized bed agglomeration is not possible, because high enough strength can not be produced or the resulting agglomerates are too loose and friable, and compaction/granulation is also not feasible, because material components are pressure sensitive or become too dense. In such instances, low pressure extrusion (see Section 8.4.1) followed by drying, cooling, “crumbling”, and screening can be used. Undersized fines are recirculated. The resulting instant products are somewhat denser and less friable than those obtained from fluidized beds but are typically easier dispersible without employing disintegration aids and are lighter than compacted particles. Coating, Microencapsulation, Mechanofusion, and Hybridization
These technologies were already covered in some detail in Section 10.1. All are used to modify the surface characteristics of pieces, particles, granules, or agglomerates of any kind and shape. Coating is the covering of relatively large pieces with macroscopic layers of solids originating from either powders, suspensions, solutions, melts, or vapors. In plasma vapor deposition (PVD) solids pass directly into the vapor phase and deposit by sublimation as nano particles onto surfaces where they sinter together to form layers. Because these ultrafine particles feature no dislocations they are extremely hard, a characteristic which is retained in the coating. Hard coatings have significantly improved the performance of tools and other parts that are exposed to friction and wear. New developments are directed at producing protective coatings with a combination of advantageous material characteristics, such as enhanced hardness or wear resistance, while other properties, e.g. friction and high temperature behavior, are at least kept at their established levels. Such demands can be met by new combinations of materials in a single layer where phases with the desired characteristics are combined (multiphase coatings) or by depositing multiple layers which, in combination, produce the desired effect [12.4, 12.51. Traditionally, many powder color coatings are applied as suspensions in organic liquids. Anything from small parts, such as toys, to large items, such as automobile
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bodies, were painted in this way. Recently, clean air legislation in many countries has required to sharply reduce the emission of VOCs (volatile organic compounds). As a result, to avoid using solvent based paint systems, most manufacturers have turned to the application of dry powders [12.6].Todays powder coatings come in many colors, textures, and finishes, but only in two basic types - thermosetting and thermoplastic. Thermoset powders consist of lower molecular weight resins, such as polyesters and acrylics, that chemically crosslink (cure) to form a coating which, in its chemical structure, is quite different from the initial powder. Once cured, a thermoset coating will not resoften. In contrast, thermoplastic powders, which do not crosslink, can remelt. Until recently, thermoset powder coatings have been used primarily for metals. The challenge is to lower the temperature needed to fuse powders into a smooth, continuous layer which then crosslinks so that powder coatings can be used on woods and plastics. Alternative curing methods using, for example, ultraviolet (UV) radiation are being developed also. Encapsulation and, particularly, microencapsulation produces a thin coating typically on smaller particles, granules, and agglomerates but is also used to enclose powders, liquids, and even gases. The particular characteristic of these capsules is normally that they feature properties which are enhancing the application of the encap sulated material. For example in pharmacy, microencapsulation often creates a drug delivery system. By being soluble only in specific bodily liquids and/or under specific ambient conditions, drugs can be transported to the location of need where they will be released. Other delivery systems use the slow release performance of a semipermeable capsule. In carbonless copying papers microcapsules which contain the dry or liquid ink are uniformly distributed in the paper structure which consists mostly of fibers (see Section 10.3). The individual capsules must be small and arranged individually but so closely together that by applying pressure with the type of a typewriter or the tip of a writing tool individual capsules will burst and release the ink. To obtain a clear copy, the capsules must have a diameter of between approx. 0.5 to 10 pm. Quality of the paper is determined by the uniformity of size and distribution of the microcapsules as well as their strength. The capsules must be so strong that the paper does not smudge by mere contact during storage and handling but must reliably burst when contacted by a type or the tip of a pen. The challenge is to produce the microcapsules according to this specification with a very narrow bursting pressure which is in the range of 200 pN and, for quality control, to be able to measure the bursting force [12.7]. Another exciting new development is the manufacturing of hollow capsules in the submicrometer (= nano) to micrometer size range [ 12.81. Such hollow capsules constitute an important class of new particulate materials that are employed in very diverse technological applications. Uses include the encapsulation and controlled release of different substances (e.g. drugs, cosmetics, dyes, inks, etc.), application in catalysis and in acoustic insulation as lightweight composite materials as well as the development of piezoelectric transducers, of materials with low dielectric constant as fillers in electronic components, and of the manufacturing of “advanced materials”. Today, hollow capsules comprising composites of polymers, glasses, metals, and ceramics are routinely produced by using various chemical and physicochemical methods.
516
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A novel, versatile technique for the synthesizing of uniform hollow capsules from a broad range of materials is based on a combination of colloidal templating and selfassembly processes [12.8]. Fig. 12.7 describes schematically the concept. Colloidal templates of different composition, size, and geometry (although spheroidal shape is preferred) can be employed. Materials range from spherical polymer particles to nonspherical biocolloids, all with diameters in the nano- to micrometer range. The first step, (1) in Fig. 12.7, involves the deposition of a charged polymer layer (+) onto the colloidal particles. In a next step, oppositely charged (-) polymer, (2) in Fig. 12.7, or nanoparticles, (3) in Fig. 12.7, resulting in another layer, are added. Additional layers can be produced as shown in Fig. 12.7 by repeated deposition which
Fig. 12.7: Schematic diagram describing hollow capsule production by exploiting colloidal templating and self-assembly methods [12.8]. Explanation see text.
12 Outlook I 5 1 7
makes use of the surface charge reversal occurring every time a layer is adsorbed. Colloidal core/multilayer-shell particles are manufactured. After the desired thickness of the layer is obtained, excess unadsorbed polyelectrolyte or nano particles are removed by repeated centrifuging or filtering and wash cycles. Finally, hollow capsules are produced by the removal of the core from the composite colloids. This is achieved either by chemical or thermal means. If a solvent is used, only the core is dissolved which results in hollow polymer, (4)in Fig. 12.7, or composite, (6) in Fig. 12.7, capsules. Heat treatment (= calcination) of the coated particles, (5) in Fig. 12.7, removes both the colloidal core and the bridging polymer, thereby producing hollow inorganic spheres. By combining colloidal templating with self-assembly, the manufacturing of a broad range of coated colloids and, ultimately, of hollow capsules in the nano- and micrometer range is possible, featuring various and defined composition. Capsule geometry, size, and wall thickness can be controlled with nanometer precision by the use of colloids with given shape and dimensions and by varying the number of coating cycles.
Fig. 12.8: Diagram demonstrating the process o f hybridization and showing photomicrographs o f intermediate as well as final particles (courtesy Nara, Tokyo, Japan). Explanation see text.
518
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Fig. 12.9: Flow sheet and photograph o f a hybridization system and dimensioned outline 3 f the NHS-1 (see Tab. 12.2) la>oratory system (courtesy Nara, Tokyo, Japan).
Uniformity in the size of the hollow capsules is defined by the monodispersity of the colloidal templates. Mechanofusion and hybridization modify the surface structure and the characteristics of fine particles by embedding nano-sized particles into or coating such particles onto the core substrates. Both technologies were described earlier in this book (see Section 10.1) and mechanofusion was already covered in an earlier book by the author [B .42]. Fig. 12.8 demonstrates again how hybridization works and Fig. 12.9 is a schematic flow sheet. Core particles with a size between 0.1 and 500 pm are mixed with nano particles that, depending on the size of the cores, may feature sizes between 0.01 and
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I I
'I I
0 m 0
-
1,115
Fig. 12.9
*
cont'd
Tab. 12.2: Power requirements and approximate production capacities o f standard Hybridization systems (according to Nara, Tokyo, Japan). Model
NHS-I (Laboratory) NHS-2 NHS-3 NHS-4 NHS-5
Power Requirement
Production Capacity
[kwl
Ikglhl
3.7-5.5 7.5-11 15-22 30-45 55-90
approx. approx. approx. approx. approx.
3.5 G.O 15.0 35.0 50.0
800 x ssa
..
I
5J9
520
12 Outlook
50 pm, (1)in Fig. 12.9. After mixing a coating has developed on the cores (see left photomicrographs in Fig. 12.8). These prepared particles are metered, (2) in Fig. 12.9, into the hybridizer, (3) in Fig. 12.9. During a short time (1-5 min), the hybridizer introduces so much mechanical and thermal energy into the product, that the fine particles are imbedded in or are permanently bonded to the surface of the core material (see right photomicrograph in Fig. 12.8). The whole process is controlled from a panel, (5) in Fig. 12.9, and the finished product is transferred to a product collection container (4) in Fig. 12.9. As shown in the center right of Fig. 12.8,multiple surface layers can be produced. If the core material of such products is removed with solvents or during calcination, hollow capsules as described above may be produced. As shown in Tab. 12.2, the production capacity ofhybridization systems is measured in kg/h and is quite small. However, since many of these particles with modified characteristics of their surfaces are used for very specialized applications, where numbers rather than mass count, at this point in time, larger production units are not necessary. Deposition and Bonding of Individual Particles on Surfaces
A very new group of methods for the deposition and bonding of particulate solids onto surfaces assembles nano- to micrometer-sized particles in a predetermined and orderly fashion onto a substrate [12.9]. Such techniques produce an organized structure, made up of particles, on solid surfaces which can bring about many interesting properties; for example, it is possible to create microdevices and microstructures with multiple functions.
Fig. 12.10 Overview schematically describing the manipulation techniques for small solid particles [12.9].
7 2 Outlook
So far, only a few methods can produce such materials. Fig. 12.10 is an overview of the two groups of techniques that are available for the manipulation of nano- to micrometer particles. With the methods of one group, single particles are deposited on the substrate, one by one. The scanning probe microscope, laser, and microneedles are used for particle manipulation (upper part of Fig. 12.10).Particles can be deposited at specific positions with high accuracy but only at a very low rate. In contrast, the techniques of the other group can deposit a great number of particles by using particle jets (lower part of Fig. 12.10).The disadvantage is a low accuracy for the positioning of each particle. A new method aims to overcome these limitations. Fig. 12.11 shows a concept that can be used to assist the deposition of fine particles by an electron beam drawing [12.9]. The technique, electrification assisted controlled particle deposition, is based on the fundamentals of electrophotography. As shown in Fig. 12.11, at first, the electron beam produces an electrified pattern on the substrate surface. Next, positively electrified particles are attracted by electrostatic forces to the electrified pattern and adhere there. By repeating the electron beam drawing and the adhesion steps, using different types of particles, composite deposits can be created. Fig. 12.12 shows the steps that are required for the electrically assisted deposition of small particles. The oppositely electrically charged particles are made available in a suspension into which the substrate, carrying the electron beam drawing, is dipped. After remaining in the suspension for a predetermined time, the substrate
Electr fication by electron beam drawing
Electron beam drawing
Powder particle arrangement Fig. 12.11: Concept of a process for the assembly of small powder particles on a substrate that is assisted by electron beam drawing [12.9].
\
3-D structure
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521
522
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Drawing an electrified Dattern on a substrate
Dipping the substrate into a suspension
Rinsing the and then drying it
Fig. 12.12: Schematic representation o f the steps that are required for the electrically assisted deposition o f small particles by electron beam drawing 112.91.
is removed, rinsed, and dried. Stronger bonds between particles and substrate can be achieved with a suitable post-treatment (for example sintering). The micrographs in Fig. 12.13 show how silica spheres with a diameter of approx. 5 pm were arranged on two negatively electrified lines (substrate is CaTiO,). The upper photograph (a) depicts the two lines which are 1,600 pm long and 800 pm apart and are composed of silica spheres. The lower photograph (b) is an enlargement of one of the lines in Fig. 12.13a and shows how the silica spheres are arrayed along the electrified line on the substrate.
(b)
Fig. 12.13: SEM photographs of silica spheres arranged along negatively electrified lines [12.9].
l2 out'ook
Many persons, scientists, and experts in Mechanical Process Technology, may not agree that some of the examples, particularly the latter ones, are new applications of agglomeration. However, the adhesion, bonding, and final structure are all controlled by the fundamentals of agglomeration, especially the binding mechanisms. Therefore, ideas for new products consisting of more or less defined particle assemblies of various size, structure, and properties can be derived from an in depth knowledge of agglomeration mechanisms.
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Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
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13
Bibliography Contrary to the format of the author’s earlier book [B.42],in which numerous individual bibliographical references were listed, many ofwhich referred to the fundamentals and the scientific treatment of the unit operation “Size Enlargement by Agglomeration”, the present work is trying to offer a complete, up-to-date compilation of the various agglomeration techniques and their applications. To that end, in addition to introducing the properties of agglomerates and the specific characteristics of the different technologies, descriptions of equipment and their special features for particular uses are the main topic of the book. Emphasis is on industrial applications, not theory. The explanations of details of equipment, processes, systems, plants, and applications as well as the descriptions of products and of their uses are largely based on information from vendors, the experience of the author as well as input from many of his colleagues that are active in this field. Therefore, it was decided that it is not necessary to collect the numerous individual publications that, in one way or another, report on technical and practical developments and review specific industrial features, applications, and products. Rather, with the exception of a few annotations (Section 13.2), reference is made to books or major chapters dealing with all facets of agglomeration and related subjects (Section 13.1) and to the vendors (Section 14.1) who, either by direct communication or through their technical sales literature and/or brochures, supplied the information that has been processed by the author to yield an unbiased presentation. Size enlargement by agglomeration is a unit operation of Mechanical Process Technology, the science which is concerned with all activities that are related to the processing and handling of particulate solids (see also Chapter 1).As has been repeatedly shown in the book, all unit operations of Mechanical Process Technology as well as the peripheral techniques (see Chapter 1, Fig. 1)are being used, sometimes several times, in the design and execution of agglomeration systems and plants. Therefore, in addition to what has been presented in Section 13.1 it should be pointed out, that some of the books in which major chapters deal with Agglomeration are also valuable sources of information on other topics of Mechanical Process Technology. Specifically,those references are (in numerical order): Winnacker-Kiichler “Chemical Technology” [B.11], “Handbook of Powder Technology” [B.17],“Handbook of Powder Science and Technology” [B.21, B.561, “Series on Bulk Materials Handling” [B.24, B.291, “Developments in Mineral Processing” jB.3 11, “Ullmann’s Encyclopedia of Industrial Chemistry”
525
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[B.32], “Fortschr. Ber. VDI” (Reihe 3, Verfahrenstechnik) [B.33, B.38 - B.40, B.571, “Drugs and Pharmaceutical Sciences” [B53, B.541, Kirk-Othmer “Encyclopediaof Chemical Technology” [B.58].Obviously, there are numerous others, for example, Perry’s Chemical Engineer’s Handbook, Dubbel, and Hiitte as well as many more, particularly those published in different parts of the world and in other languages.
13.1
List o f Books or Major Chapters on Agglomeration and Related Subject (With exception of the more recent ones, most of the following references are “out-of-print”.However, today, there is a growing number of suppliers and/or publishers of out-of-printbooks available on the Internet. They either offer antiquarian publications for sale or on loan or will try to obtain books for “on d e m a n d reproduction. One such service is, for example, the reprinting service for out-of-print books of Bell & Howell Information and Learning Company, Ann Arbor, MI, USA, at http://~.bellhowell,infoleaming.com (see also footnote on page 528). Other sources are listings of antiquarian book such as http://www,bibliofind.comor http:// w.zvab.com The Library of the US Congress (http://www.loc.gov), listing almost all books which were published at any time, anywhere in the world, and the German Library ( h t t p : / / w . d d b . d e ) make books available at their reading rooms.) G. Franke, Handbuch der Brikettbereitung (Handbook of [Coal] Briquetting), Verlag Ferdinand Enke, Stuttgart, Germany (1909). B.2 K. Kegel, Aufiereitung und Brikettiemng (Processing and Briquetting [of Coal]), Wilhelm Knapp Verlag, Halle/Saale, Germany (1948). B.3 Proceedings of the Biennial Conferences of the Institute for Briquetting and Agglomeration (IBA), V O ~ .1-27 (1949, 1951, 1953, ....., 2001). B.4 W.A. Knepper (ed.),Agglomeration, Proc. 1st International Symp. Agglomeration, Philadelphia, PA, USA, John Wiley & Sons, New York, NY, USA, and London, UK (1962). B.5 W.A. Ritschel, Die Tablette. Gmndlagen und Praxis des Tablettierens, Granulierens und Dragierens (The tablette. Fundamentals and applications of tabletting, granulating and coating), Editio Cantor KG, Aulendorf, FR Germany (19G6). B.6 W. Pietsch, KornvergroiSemng (Agglomerieren), (Size enlargement by agglomeration), In: “Fortschritte der Verfahrenstechnik, VDI-Verlag GmbH, Dusseldorf, FR Germany, vol. 9 (1971), 831-872; V O ~ .10 (1972), 223-235: V O ~ . 11 (1973), 162-172; V O ~ .1 2 (1974), 133146; V O ~ .13 (1975), 143-163; v01. 14 (1976), 149-160; V O ~ .16 (1978), 73-89. B.7 A.S. Goldberg (ed.),Compaction ‘73, Proc. 1st International Conf. on Compaction and Consolidation of Particulate Matter, Powder Technology Publ. Series No. 4, Powder Advisory Centre, London, UK (1972). B.8 W. Herrmann, Das Verdichten von Pulvern zwischen zwei Walzen (The densification of powders between two rollers), Verlag Chemie GmbH, Weinheim, FR Germany (1973). B.9 D.F. Ball, I. Dartnell, J. Davison, A. Grieve, and R. Wild, Agglomeration of Iron Ores, American Elsevier Publishing Co., New York, NY, USA (1973). B.10 S.K. Nikol (ed.),Pellets and Granules, Proc. Symp. Pellets and Granules, The Australian Inst. of Mining and Metallurgy, Newcastle, NSW, Australia (1974). B.ll H. Rumpf, Mechanische Verfahrenstechnik (English ed.: Particle Technology), Monograph in Winnacker-Kuchler, Chem. Technology, vol. 7, 3rd ed., Carl Hanser Verlag, Munchen, FR Germany/Wien, Austria (1975);Engl. ed.: F.A. Ball (Translator), Particle Technology, Chapman & Hall, London, UK (1990). B.12 A.S. Goldberg (series ed.), Monographs in Powder Science and Technology, Heyden & Son Ltd., London, UK/Rheine, FR Germany/New York, NY, USA: a) P. Popper, Isostatic Pressing B.l
13.1 List of Books or Major Chapters on Agglomeration and Related Subject
B.13
B.14 B.15 B.16 B.17
B.18
B.19
B.20 B.21
B.22 B.23 B.24 B.25 B.26 B.27 B.28 B.29
B.30
B.31
B.32
(1976);b) W. Pietsch, Roll Pressing (1976),2nd ed. (1987);c) M.B. Waldron and B.L. Daniell, Sintering (1978);d) J.K. Beddows, The production ofmetal powders by atomization (1978);e) P.J. Sherrington and R. Oliver, Granulation (1981). H.C. Messmann and T.E. Tibbets (eds.), Elements of Briquetting and Agglomeration vol. 1, The Institute for Briquetting and Agglomeration (IBA),Hudson, WI, USA (1977);R.N. Koerner and J.A. McDougall (eds.), Elements of Briquetting and Agglomeration vol. 2, The Institute for Briquetting and Agglomeration (IBA), Hudson, WI, USA (1983). K.V.S. Sastry (ed.),Agglomeration 77, vols. 1 and 2, Proc. 2nd International Symp. Agglomeration, Atlanta, GA, USA, AIME, New York, NY, USA (1977). H. Schubert et al., Mechanische Verfahrenstechnik (Mechanical Process Technology), Deutscher Verlag fur Grundstoffindustrie, Leipzig, DR Germany (1977). K. Meyer, Pelletizing of Iron Ores, Springer-Verlag, Berlin/Heidelberg, FR Germany/New York, NY, USA - Verlag Stahleisen mbH, Diisseldorf, FR Germany (1980). C.E. Capes, Particle Size Enlargement. In: J.C. Williams and T. Allen (series eds.) “Handbook of Powder Technology” vol. 1, Elsevier Scientific Publishing Co., Amsterdam, The Netherlands/Oxford, UK/New York, NY, USA (1980). 0. Molerus and W. Hufnagel (eds.),Agglomeration 81, vols. 1 and 2, Proc. 3rd International Symp. Agglomeration, Nurnberg, FR Germany, Nurnberger Messe- und Ausstellungsgesellschaft, Niirnberg, FR Germany (1981). W. Pietsch, Agglomeration. In: “Fortschritte der Verfahrenstechnik (in English), VDI-Verlag GmbH, Dusseldorf, FR Germany, vol. 19 (1981),133-149; vol. 21 (1983), 121-139; vol. 23 (1985), 125-139. H. Schubert, Kapillaritat in porosen Feststoffsystemen (Capillarity in porous solid systems), Springer-Verlag, Berlin/Heidelberg, FR Germany/New York, NY, USA (1982). C.E. Capes, W. Pietsch, et al., Size Enlargement Methods and Equipment. In: M.E. Fayed and L. Otten (eds.)“Handbook of Powder Science and Technology”, ch. 7, Van Nostrand Reinhold Co., New York, NY/Cincinnati, OH, USA/Toronto, Canada/London, UK/Melboume, Australia (1983). U. Bossel (ed.),Brikettieren und Pelletieren von Biomasse (Briquetting and pelleting of biomass), SOLENTEC Fachbuchvertrieb, Adelebsen, FR Germany (1983). C.E. Capes (ed.),Agglomeration 85, Proc. 4th International Symp. Agglomeration, Toronto, Ont., Canada, The Iron & Steel Society, Inc. (ISS), Warrendale, PA, USA (1985). J.W. Merks, Sampling and Weighing of Bulk Solids. Series on Bulk Materials Handling, vol. 4, Trans Tech Publications, Clausthal-Zellerfeld, FR Germany (1985). S. Bradbury (ed.),Powder Metallurgy Equipment Manual, 3rd ed., Metal Powder Industries Federation, Princeton, NJ, USA (1986). B.M. Moudgil and P. Somasundaran (eds.), Flocculation, Sedimentation & Consolidation, Proc. Engineering Foundation Conference, United Engineering Trustees, Inc., USA (1986). K. Sommer, Sampling of Powders and Bulk Materials, Springer-Verlag, Berlin, Heidelberg, FR Germany (1986). Committee on Raw Materials, Sinter and Pellets. Production and Use Capacities (State: 1987), International Iron and Steel Institute (IISI), Brussels, Belgium (1987). British Materials Handling Board, Particle Attrition - State-of-the-ArtReview, Series on Bulk Materials Handling, vol. 5, Trans Tech Publications, Clausthal-Zellerfeld, FR Germany (1987). P. Ridgeway-Watt, Tablet Machine Instrumentation in Pharmaceuticals - Principles and Practice, Ellis Hanvood Series in Pharm. Technology, John Wiley & Sons, New York, NY, USA (1988). J. Srb and 2. Ruzickova, Pelletization of Fines (Minerals, Ores, Coal). In: D.W. Fuerstenau (advisory ed.) “Developments in Mineral Processing”,vol. 7, Elsevier Science Publishers B.V., Amsterdam, The Netherlands (1988). K. Sommer, Size Enlargement. In: “Ullmann’s Encyclopedia of Industrial Chemistry”, 5th ed., vol. B.2, ch. 7, Verlag Chemie GmbH, Weinheim FR Germany (1988), 1-37.
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528
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B.33 W. Dotsch, Agglomerationskinetik zur Simulation von Agglomerationsprozessen i m Agglomerierteller (Kinetics of agglomeration for the simulation of agglomeration processes in the pan granulator), Fortschr.-Ber. VDI, Reihe 3, Nr. 157, VDI-Verlag GmbH, Diisseldorf, FR Germany (1988). B.34 K.L. Mittal (ed.),Particles on Surfaces 1,2,and 3: Detection, Adhesion, and Removal, Plenum Publishing Corp., New York, NY, USA (1988, 1989, 1990). B.35 Agglomeration 89, Proc. 5th International Symp. Agglomeration, Brighton, UK, The Institution of Chemical Engineers (IChemE), Rugby, UK (1989). B.36 I. Ghebre-Sellassie (ed.), Pharmaceutical Pelletization Technology, The Pharmaceutical Sciences Series, No. 37, Marcel Dekker, New York, NY, USA (1989). B.37 B.H. Kaye, A Random Walk Through Fractal Dimensions, VCH Verlagsgesellschaft mbH, Weinheim, FR Germany (1989). B.38 B. Schetter and J. Funcke, Agglomeration der dispersen Phase von Aerosolen durch starlte Schallfelder (Agglomeration of the disperse phase of aerosols by strong sound fields), Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 196, VDI-Verlag GmbH, Diisseldorf, Germany (1990). B.39 P.Schultz, Trocltnung kapillarporoser Korper bei Anwesenheit auskristallisierender Stoffe in der Gutsfeuchte/Trocknung mit Krustenbildung (Drying ofwet porous bodies which contain dissolved substances/Drying with incrustation), Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 201, VDI-Verlag GmbH, Diisseldorf, Germany (1990). B.40 C:J. Klasen, Die Agglomeration partikelformiger Feststoffe in Matrizenpressen (The agglomeration of particulate solids in pellet presses), Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 220, VDI-Verlag GmbH, Diisseldorf, Germany (1990). B.41 S. Eriksson and M. Prior, The briquetting of agricultural wastes for fuel, FA0 Environment and Energy Paper 11, Food and Agriculture, Organization ofthe United Nations, Rome, Italy (1990). B.42 W. Pietsch, Size Enlargement by Agglomeration, John Wiley & Sons Ltd., Chichester, UK/ New York, NY, USA/Brisbane, Australia/Toronto, Canada/Singapore - Otto Salk Verlag GmbH & Co., Frankfurt/M, Germany - Verlag Sauerlander AG, Aarau, Switzerland (1991).This book is out-of-print and the copyright is now held by the author.+< B.43 K. Masters, Spray Drying Handbook, 5th ed., Longman, London, UK/John Wiley & Sons, New York, NY, USA (1991). B.44 E.J. Griffith, Cake Formation in Particulate Systems, VCH Publishers, Inc., New York, NY, USA (1991). B.45 Agglomerations- und Schiittgut-Technik (Agglomeration and Bulk Solids Technologies), Preprints, GVC, VDI Gesellschaft Verfahrenstechnik und Chemieingenieunvesen, Dusseldorf, Germany (1991). B.46 F. Loffler and J. Raasch, Grundlagen der Mechanischen Verfahrenstechnik (Fundamentals of Mechanical Process Engineering), Friedr. Viehweg & Sohn Verlagsgesellschaft mbH, Braunschweig/Wiesbaden, Germany (1992). B.47 2. Drzymala, Industrial Briquetting. Fundamentals and Methods, Studies in Mechanical Engineering, vol. 13, Elsevier Science Publishers, Amsterdam, The Netherlands/London, UK/New York, NY, USA/Tokyo, Japan - PWN Polish Scientific Publishers, Warzawa, Poland (1993). B.48 AGGLOS, Proc. 6th International Symp. Agglomeration, Nagoya, Japan, The Society of Powder Technology, Japan - The Iron and Steel Institute of Japan - The Society of Chemical Engineers, Japan (1993).
Arragements have been made with “Books on Demand”, the reprinting service for out-of-print books of Bell & Howell Information and Learning Company, to makethe publication availabletothose who are interested in it (Order # 2067035). Addi-
4
tional information, also on the availabilityof another book bythe author (“Roll Pressing” [B. 12b], Order # AU00526) and further out-of.print books that may be listed in Section 13.1, can be obtained through the web site www.bellhowell.infolearning.com.
13.1 List of Books or Major Chapters on Agglomeration and Related Subject
B.49 First International Particle Technology Forum (1st IPTF), Denver, CO, Proceedings, PTF of AIChE, New York, NY, USA (1994). B.50 W. Herman de Groot, I. Adami, and G.F. Moretti, The Manufacture of modern Detergent Powders, Herman de Groot Academic Publishers, Wassenaar, The Netherlands (1995). B.51 B.M. Moudgil and P. Somasundaran (eds.),Practical Dispersion. A Guide to Understanding and Formulating Slurries, VCH Publishers, Inc., New York, NY,USA (1996). B.52 The 5th World Congress of Chemical Engineering and 2nd IPTF (Int’l Particle Technology Forum), San Diego, CA, Proceedings, AIChE and PTF ofAIChE, New York, NY, USA (199G). B.53 G. Alderborn and Ch. Nystrom (eds.), Pharmaceutical Powder Compaction Technology, Drugs and Pharmaceutical Sciences, vol. 71, Marcel Dekker, Inc., New York, NY, USA (1996). B.54 D.M. Parikh (ed.),Handbook of Pharmaceutical Granulation Technology, Drugs and Pharmaceutical Sciences, vol. 81, Marcel Dekker, Inc., New York, NY, USA (1997). B.55 Ch. Hayashi, R. Uyeda, and A. Tasalci (eds.), Ultra-Fine Particles. Exploratory Science and Technology, Noyes Publications, Westwood, NJ, USA (1997). B.56 W. Pietsch, Size Enlargement by Agglomeration, In: “Handbook of Powder Science and Technology”, M.E. Fayed and L. Otten (eds.),2nd ed., ch. 6, Chapman & Hall, New York, NY, USA (1997), 202-377. B.57 R:D. Becher, Untersuchung der Agglomeration von Partikeln bei der Wirbelschicht-Spriihgranulation (Investigation of the agglomeration of particles during fluidized bed spray granulation), Fortschr.-Ber.VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 500, VDIVerlag GmbH, Diisseldorf, Germany (1997). B.58 C. Edward Capes and K. Darcovich, Size Enlargement, In: Kirk-Othmer, “Encyclopedia of Chemical Technology”, 4th ed., vol. 22, John Wiley & Sons, Inc., New York, NY, USA (1997), 222-255. B.59 B.H. Kaye, Powder Mixing, Chapman & Hall, London, UK (1997). B.60 T. Allen, Particle Size Measurement, 5th ed., vol. 1: Powder Sampling and Particle Size Measurement, vol. 2: Surface Area and Pore Size Determination, Chapman & Hall, London, UK (1997). B.61 K. Ishizaki, S. Komarneni, and M. Nanko, Porous Materials. Process Technology and Applications, Kluwer Academic Publishers, Dordrecht, NL,Boston, USA, London, UK (1998). B.62 R.W. Rice, Porosity of Ceramics, Marcel Dekker, Inc., New York, USA, Basel, Switzerland, Hong Kong (1998). B.63 World Congress on Particle Technology 3, Brighton, UK, including 3rd IPTF (“Emerging Particle Technologies: A Vision to the Future”), IChemE, Rugby, UK (1998). B.G4 J. Scheirs, Polymer Recycling: Science, Technology, and Applications, John Wiley & Sons, Ltd., Chichester, West Sussex, England (1998). B.65 G. Heinze, Handbuch der Agglomerationstechnik (Handbook of Agglomeration Technology), Wiley-VCH Verlag, Weinheim, Germany (2000). B.66 J.F. Scamehorn and J.H. Hanvell (eds.), Surfactant-based Separations. Science and Technology, American Chemical Society, Washington, DC, USA (2000). B.67 D. Ramkrishna, Population Balances. Theory and Applications to Particulate Systems in Engineering, Academic Press, San Diego, CA, USA (2000). B.68 E.M. Petrie, Handbook ofAdhesives and Sealants, McGraw.Hi11, New York, NY,USA (2000). B.69 H. Uhlemann and L. Morl, Wirbelschicht-Sprtihgranulation(Fluidized bed spray granulation), Springer Verlag, Heidelberg, Germany (2000). B.70 Preprints 7th Int’l Symposium Agglomeration, Albi, France, Progep, 18 chemin de la loge, F-31078 Toulouse, Cedex 4, France (2001). B.71 W. Pietsch, Agglomeration Technology - Industrial Applications. Wiley-VCH Verlag, Weinheim, Germany (2003).
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References W. Pietsch, “Festigkeit und Trocknungsverhalten von Granulaten, deren Zusammenhalt durch bei der Trockung auskristallisierende Stoffe bewirkt wird.” (Strength and drying behavior of agglomerates, the induration of which is caused by the recrystallization of dissolved solids during drying.), Diss. (Ph.D.thesis) Universitat (T.H.) Karlsruhe, FR Germany (1965). 1.2 “Webster’sThird New International Dictionary of the English Language” (unabridged),Merriam-Webster, Inc. Springfield, MA, USA (1986). 1.3 K. Schiefer (ed.), “RoRoRo Techniklexikon, Verfahrenstechnik, Bd. 3” (RoRoRo Technical Lexicon, Process Technology, vol. 3), Rowohlt Taschenbuch Verlag GmbH, Hamburg, FR Germany (1972). H. Rumpf, “The strength of granules and agglomerates”, in [B.4], 379-418. 5.1 H.C. Hamaker, “The London - van der Waals attraction between spherical particles”, Physica 5.2 4 (1937) 10, 1058-1072. E.M. Lifshitz, ”The theory of molecular attractive forces between solids”, Soviet Phys. JETP 2 5.3 (1956)1,73-83. 5.4 H. Krupp, “Particle adhesion - Theory and experiment”, Adv. Coll. & Interf. Sci. (1967)1, 112-239. F.M. Thomson, “Storage and flow ofparticulate solids”, In: Handbook of Powder Science and 5.5 Technology, M.E. Fayed and L. Otten (eds.), 2nd ed., ch. 8, Chapman & Hall, New York, NY, USA (1997), 389-486. 5.6 B. Wist, “Ball-mill degradation test for quality control of granular potash products”, (revised), PCS Potash, Saskatoon, Sask., Canada (1997). H. Rieschel, K. Zech, “Comparison of various test methods for the quality control of Potash 5.7 granulate”, Phosphorous & Potassium, Brit. Sulphur Corp., Sept./Oct. (1981). 5.8 S. Debbas, “Uber die Zufallsstruktur von Packungen aus kugeligen oder unregelmassig geformten Kornern” (The random structure of packings of spherical and irregular particles), Diss. (Ph.D. thesis) Universitat (T.H.) Karlsruhe, FR Germany (1965). 5.9 W. Pietsch, “Storage, shipping, and handling of direct reduced iron”, AIMEjSME Transactions 262 (1977)3,225-234. 5.10 W. Pietsch and W. Schiitze, “HBI - A safe DRI-based source of iron units”, Paper at World Iron Ore 96, Orlando, FL (1996), Skillings’ Mining Review 86 (1997)18, 4-9. 7.1 K.V.S. Sastry, “Process Engineering of Agglomeration Systems”, in [B.46], 37 -44. 7.2 A.A. Adetayo et al., “Drum granulation of fertilizers: Modelling and circuit simulation”, in [B.46],105-110. 7.3 J.R. Wynnyckyj, “Microstructure and growth mechanisms in pelletizing - A critical re.assessment”, in [B.46],143 - 159. 7.4 H. Leuenberger, “Design and optimization approaches in the field of granulation, drying, and coating”, in “Topics in Pharmaceutical Sciences 1993”, Proc. 53rd Int’l. Congr. of Pharmaceutical Sciences of F.I.P., Tokyo, Japan, D.J.A. Crommelin, K.K. Midha, and T. Nagai (eds.), Scientific Publishers, Stuttgart, Germany (1994), 493 -510. T.S. Chirkot, “Characterization of a pharmaceutical wet granulation process in a V-type low 7.5 shear granulator”, Ph.D. thesis, The Union Institute, Cincinnati, OH, publ. by UMI Co., Ann Arbor, MI, USA (1998). S. Hogekamp, H. Schubert, and S. Wolf, “Steam jet agglomeration ofwater soluble material”, 7.6 Powder Technology 86 (1996), 49-57. K. Nishii, Y. Itoh, and N. Kawakami, “The characteristics of pressure swing granulation”, in 7.7 [B.46], 111-116. 7.8 Y. Kawashima, “Spherical crystallization technique: A new tool for micromeritic design of crystals and preparation of drug delivery systems”, in [B.46],487-492. 8.1 S.P. Shah, S.E. Swartz, Ch. Ouyang, “Fracture Mechanics of Concrete: Applications of Fracture mechanics to concrete, rock, and other quasi-brittle materials”. John Wiley & Sons, Inc., New York, NY, USA, and Toronto, Ont., Canada (1995). 1.1
73.3 Author’s Biography, Patents, and Publications
8.2 8.3
10.1
10.2
10.3
10.4 10.5
12.1
12.2 12.3
12.4 12.5
12.6 12.7
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R.B. Steele, “Agglomeration of Steel Mill By-productsvia Auger Extrusion”. Proc. 231d Biennial Conf. IBA (see also [B.3]), Seattle, WA, USA (1993) 205-217. W. Pietsch, “Ram Pressing - An almost extinct technology with interesting new applications in coal and other solid fuel processing”. Proc. 25thInt’l.Technical Conf On Coal Utilization & Fuel Systems, Clearwater, FL (ZOOO), 37-48 (see also Section 13.3). Y. Doganoglu, V. Jog, K.V. Thambimuthu, and R. Clift, “Removal of fine particulates from gases in fluidised beds”, Trans IChemE 56 (1978),239-248. R. Clift, M. Ghadiri, and K.V. Thambimuthu, “Filtration of gases in fluidised beds”, In: Progress in Filtration and Separation, vol. 2, R.J. Wakeman (ed.),Elsevier, Amsterdam, The Netherlands (1981),75-123. S. Fleck and U. Riebel, “Einfluss der Fluidisiemngsbedingungenauf Abscheidung und Agglomeration von Aerosolen beim Durchgang durch Wirbelschichten” (Influence of the fluidization conditions on removal and agglomeration of aerosols during the passage through fluidized beds), Chemie Ingenieur Technik 71 (1999)4,361 -364. V.A. Bielobradek, “Selectinga better media for your pleated bag and cartridge filter”, Powder and Bulk Engineering 14 (2000)10, 77-81. K. Schonert, “Mechanische Verfahrenstechnik - Insbesondere Umgang mit feinen Partikeln” (Mechanical Process Technology - Particularly processing of fine particles), Fridericiana, 2.der Univ. Karlsruhe, Verlag C.F. Miiller, Karlsmhe, FR Germany (1977)21,12-33. H. Schubert, “Eine Schnellmethode zur Messung der Instanteigenschaften pulverformiger Stoffe” (A fast method for measuring the instant characteristics of products from powder materials), 2. Lebensmitteltechnologie und -verfahrenstechnik 36 (1985)5, 149 - 152. H. Schubert, “Instantisieren pulverformiger Lebensmittel” (Instantizing of powdered food materials), Chem.-1ng.-Tech.62 (1990)11, 892-906. J. Wolsiefer, Sr., “The measurement and analysis of silica fume particle size distribution and dispersion”,Norchem Concrete Products, Inc. (see Section 14.1),Paper at 5th CANMET/ACI Int’l Conf. on Durability of Concrete, Barcelona, Spain, June 4, 2000. Th. Zehnder and J. Patscheider, “Nanocomposite TiC/a-C:H hard coatings deposited by reactive PVD”, Surface and Coatings Technology 133-134 (2000), 138-144. P.H. Mayrhofer and C. Mitterer, “High-temperatureproperties of nanocomposite Ti,BNy and TiB,C, coatings”, Surface and Coatings Technology 133- 134(2000),131- 137. Ch. Crabb, “Powder coatings find cures”, Chemical Engineering (2000)2, 54, 56, 57. 2.Zhang, R. Saunders, and C.R. Thomas, “Mechanical strength of single microcapsules determined by a novel micromanipulation technique”, J. Microencapsulation 16 (1999)1, 117-124. F. Caruso, “Hollow capsule processing through colloidal templating and self-assembly”, Chem. Eur. J. G (2000)3,413-419. H. Fudouzi, M. Kobayashi, and N. Shinya, “Arrangement of microscale particles by electrification”, Kona (1999)17, 55-63.
13.3 Author’s Biography, Patents, and Publications
Dr. Pietsch is a Senior Consultant in the general fields of Mechanical Process Engineering (Powder & Bulk Solids Technologies) and, particularly, Size Enlargement by Agglomeration for COMPACTCONSULT, Inc. of Naples, FL, USA. He received the equivalents of B.S. and M.S. degrees in Mechanical and, respectively, Chemical Engineering from the Technical University (TH) of Karlsruhe, West Germany, in 1959 and 1962 (Dip1.-Ing.).In 1965 he earned his Ph.D. (Dr.Ing.) at the same University with work on the Fundamentals of Binding Mechanisms of Agglomeration. Prior to his industrial career, Dr. Pietsch did further research in the
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general field of Size Enlargement by Agglomeration at the Institute of Mechanical Process Engineering at the Technical University (TH) of Karlsruhe until 1967. Later, while in industry, he taught the “Unit Operations of Mechanical Engineering” at the University of Stuttgart, Heilbronn Branch, Heilbronn, West Germany. Beginning in 1967, his industrial positions included: Research Scientist, Allis-Chalmers, Milwaukee, WI, USA; Staff Consultant, Komarek Greaves (today HOSOKAWA BEPEX),Rosemont, IL, USA; Technical Director, HUTT GmbH (today HOSOKAWA BEPEX), Leingarten, West Germany; Managing Director, Technical, Lemforder Kunststoff GmbH, Lemforde, West Germany; Director Agglomeration Systems and Product Technology, MIDREX Corp., Charlotte, NC, USA; General Manager Large Metallurgical Systems, Leybold-Heraeus GmbH, Hanau, West Germany; Senior Technical Manager, Maschinenfabrik KOPPERN GmbH & Co KG, Hattingen/ Ruhr, Germany; Executive Vice President and, later, President, KOPPERN Equipment, Inc., Charlotte, NC, and Pittsburgh, PA, USA, until 1995 when he retired from industry. As of the publication date of this book, Dr. Pietsch is the author of more than 150 papers, 3 books, including the textbook “Size Enlargement by Agglomeration” published by Wiley & Sons in cooperation with Salle + Sauerlander in 1991 (now outof-print but available from “Books on Demand, see Sections 13.1 and 13.2), and holds 9 patents. He is a member of six professional organizations in the USA and Germany and is active in a number of technical committees. He is a frequent lecturer of workshops, short courses, and continuing education events in the fields of Mechanical Process Technology and Agglomeration in the USA and Europe. COMPACTCONSULT, Inc., was founded in 1983 and incorporated in the state of North Carolina, USA, in early 1984. It is 100 % owned and operated by Mrs. Hannelore Pietsch and, therefore, qualifies as a woman-ownedsmall business concern. The primary purpose of COMPACTCONSULT, Inc. is, to make international experts available to industries as well as private and government agencies. The fields of expertise of consultants are the Unit Operations of Mechanical Process Technology (see Chapter 1, Fig. 1.1)in all areas producing, handling, and processing particulate solids (particles, powders, bulk masses, etc.) as well as hot and cold metal bearing particulate matter, including direct reduced iron (DRI). Specific expertise exists in Size Enlargement by Agglomeration. Other important activities are in the fields of processing and recirculating particulate wastes as secondary raw materials. In 1991 COMPACTCONSULT, Inc. moved temporarily to the State of Pennsylvania, USA, where it operated as “foreign” enterprise while still incorporated in North Carolina. After relocating to Naples, Florida, USA, the company was reincorporated in the State of Florida on August 17, 1995. Ownership has remained unchanged. Dr.-Ing. Wolfgang Pietsch (Ph.D.),EUR ING, joined COMPACTCONSULT,Inc. in 1983 as Senior Consultant and, after leaving KOPPERN Equipment, Inc. of Pittsburgh, PA, in 1995, continues working in this position as an unaffiliated consultant and independent contractor. During his entire professional career, from becoming a student helper to Prof. Dr.Ing. Hans Rumpf at the Institute of Mechanical Process Engineering of the Technical University (TH)of Karlsruhe, West Germany, in 1960 to now exclusively working as a
13.3 Author’s Biography, Patents, and Publications
consultant, Dr. Pietsch was always involved in Mechanical Process Technology, particularly the unit operation of Size Enlargement by Agglomeration and fields related to any aspect of agglomeration, as a researcher, teacher, process developer, designer, and user on two continents. While in industry (from 1967 to 1995) as a vendor representative, he has travelled to almost all countries on our planet to evaluate customer’s needs, develop suitable solutions, offer equipment and systems, and, if successful, help with the implementation, process optimization, and maintenance. Therefore, this book is based on that long and varied experience to which innumerable professionals have contributed. Even though they remain anonymous, these persons deserve credit. Also acknowledged should be the countless “students”that partook in the seminars and continuous education programs which Dr. Pietsch has either personally conducted or in and to which he has actively participated and contributed as a faculty member. At these sessions as well as during hundreds of consulting assignments, discussions with those faced with technical problems and with the development of solutions, to which Dr. Pietsch often contributed, have played a significant role in collecting the know-how that has been partially presented in this book. Much of the experience and know-how that was gathered by Dr. Pietsch during forty years of professional work has also been published in books, papers, and patents as well as in semipublic course notes. Although many of the more important statements and conclusions are reproduced in various parts of this book, it may be of interest to refer to the complete listing of these publications. The titles, summarized below, are always clear indications of the contents and, therefore, may complement what is submitted briefly in this book by directing the reader to a more detailed coverage. Patents
Priority patents only: related patents filed or issued in many foreign coun-
tries.) 1.
2.
3.
4. 5. 6. 7. 8.
9.
C. Buchholz and W. Pietsch, Verfahren zur Aufiereitung von feuchten Metallspanen zum Wiedereinschmelzen (Process treating moist metal chips for melting), German patent DP 2 151 819, filed Oct. 18, 1971, issued Oct. 24, 1974. H.-J. Pitzer and W. Pietsch, Verfahren und Vorrichtung zur Ausnutzung von bei der Spanplattenherstellung anfallenden Sagespane- und Schleifstaubteilchen (Process and equipment for the recovery of wood dust and chips produced during chipboard manufacturing), Swiss patent SP 530 262, filed Oct. 22, 1971, issued Nov. 15, 1972. W. Pietsch, Verfahren zur Pressgranulation von in Entstaubungsanlagen abgeschiedenen Industriestauben (Briquetting process for industrial dusts), German patent DP 2 314 637, filed March 23, 1973, issued March 6, 1975. W. Pietsch, Binder Composition, US patent No. 4,032,352, filed May 3, 1976, issued June 28, 1977. W. Pietsch, Apparatus for continuous passivation of sponge iron material, US patent No. 4,033,559, filed April 5, 1976, issued July 5, 1977. W. Pietsch, Method for continuous passivation of sponge iron material, US patent No. 4,076,520, filed April 5, 1976, issued Feb. 28, 1978. W. Pietsch, Compacted, passivated metallized iron product, US patent No. 4,093,455, filed December 22, 1976, issued June 6, 1978. W. Pietsch and Ch.A. Schroer, Briquet and method of making same, US patent No. 4,105,457, filed June 22, 1977, issued August 8, 1978. W. Pietsch, Metallized iron briquet, US patent No. 4,116,679, filed March 24, 1977, issued, Sept. 26, 1978.
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Publications 1.
2.
3.
4.
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6.
7. 8.
9.
10. 11.
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14. 15. 16. 17. 18. 19. 20.
W. Pietsch, Festigkeit und Trocknungsverhalten von Granulaten, deren Zusammenhalt durch bei der Trocknung auskristallisierende Stoffe bewirkt wird. (Strength and drying behavior of agglomerates, the induration of which is caused by solids crystallizing during the drying operation.) Diss. (Ph.D. thesis) Universitat (TH) Karlsmhe (1965). W. Pietsch, Die Beeinflussungsmoglichkeiten des Granuliertellerbetriebes und ihre Auswirkungen auf die Granulateigenschaften. (The possibilities of influencing the pelletizing pan operation and their effects on the properties of the pelletized material.) Aufiereitungs-Technik 7 (1966)4, 177-191. H. Rumpf and W. Pietsch, Festigkeit und Trocknungsverhalten von Granulaten mit Salzbriickenbildung. (Strength and drying behavior of granules with salt bridges.) Chemie-Ingenieur-Technik 38 (1966)3, 371 -372. W. Pietsch, Neue Entwicklungen auf dem Gebiet der Granuliertechnik und die Festigkeitseigenschaften von Granulaten. (New agglomeration procedures and physical properties of the agglomerates produced.) Revue Technique Luxembourgeoise (1966)2, 68 -90. W. Pietsch and H. Rumpf, Transport phenomena, crystallization and development of tensile strength during the drying of moist agglomerates containing NaC1-Solutions. Comptes-Rendues, Coll. Int. CNRS Nr. 169, Paris (1966), 213-235. W. Pietsch, Zweites Europaisches Symposium “Zerkleinern”. (2nd European Symposium on Comminution.) a) Aufiereitungs-Technik 7 (1966)11, 655 -665; b) Chemie-Ingenieur-Technilt 38 (1966)12, 1307-1309; c) Staub-Reinhalt. Luft 27 (1967)1, 52-55. W. Pietsch, Das Agglomerationsverhalten feiner Teilchen. (The agglomeration tendencies of fine particles.) Staub-Reinhalt. Luft 27 (1967)2, 64-65; English ed.: 27 (1967)1, 24-41. W. Pietsch, Einfluss der Verkmstung auf die Trocknung kapillar-poroser Korper. (Influence of the incrustation on the drying of porous bodies.) Staub-Reinhalt. Luft 27 (1967)2, 64-65; English ed.: 27 (1967)2, 10-11. W. Pietsch, Die Festigkeit von Granulaten mit Salzbriickenbindung und ihre Beeinflussung durch das Trocknungsverhalten. (The strength of granules with salt bridges and its change due to their drying behavior.) Aufiereitungs-Technik 8 (1967)6, 297 - 307. W. Pietsch, Die Festigkeit von Agglomeraten. (The strength of agglomerates.) Chemie Technik 19 (1967)5, 259-266. W. Pietsch, Die Grundlagen der Kornvergro&erung, ihre wissenschaftliche Untersuchung und technische Anwendung. (The fundamentals of size enlargement, its scientific investigation and technical application.) Revue Technique Luxembourgeoise 59 (1967)2, 53 - 65. H. Rumpf and W. Pietsch (Hrsg./eds.), Zerkleinern. (Comminution.) Dechema-Monographien Nr. 993- 1026 (2 Bde.), Verlag Chemie, GmbH, WeinheimlBergstrasse (1967). W. Pietsch and H. Rumpf, Haftkraft, Kapillardmck, Flussigkeitsvolumen und Grenzwinltel einer Fliissigkeitsbriicke zwischen zwei Kugeln. (Binding force, capillary pressure, liquid vol39 ume and critical angle of a liquid bridge between two spheres.) Chemie-Ingenieur-Technik (1967), 885-893. J.E. Moore and W. Pietsch, Briquetting and compacting of lime and lime-bearing materials. Proc. 10th Biennial Conf. of IBA, Albuquerque, N M (1967), 38-50. W. Pietsch, Tensile strength of granular materials. Nature, 217 (1968)130, 736-739. W. Pietsch, Stand der Eisenerzpelletierung. (Pelletizing of iron ore, worldwide.) Aufiereitungs-Technik 9 (1968)5, 201-214. W. Pietsch, An evaluation of techniques for particle size analysis, Part I and 11. Minerals Processing 11 (1968), 6-11; 12 (1968), 12-14, 24. W. Pietsch, Adhesion and agglomeration of solids during storage, flow and handling-A survey. Journal of Engineering for Industry (Trans. ofthe ASME), Series B, 91 (1969)2,435-449. W. Pietsch, E. Hoffman, and H. Rumpf, Tensile strength of moist agglomerates. I & EC Product Research and Development 8 (1969), 58-62. W. Pietsch, The strength of agglomerates bound by salt bridges. The Canadian Journal of Chemical Engineering 47 (1969), 403-409.
73.3 Author’s Biography, Patents, and Publications 21. 22.
23.
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25. 26. 27. 28. 29.
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W. Pietsch, Improving powders by agglomeration. Chem. Engng. Progress 66 (1970)1,31- 35. W. Pietsch, Die Bedeutung der Walzenkonstruktion von Brikettier-, Kompaktier- und Pelletiermaschinen fur ihre technische Anwendung. (The importance of roll design for roll-type briquetting, compacting, and pelleting machines as defined by their technical application.) Aufbereitungs-Technik 11 (1970j3, 128- 138. W. Pietsch, Brikettieren, Kompaktieren und Kompaktieren/Granulieren von Kalk und kalkhaltigen Stoffen. (Briquetting, compacting and compacting/granulating of lime and lime bearing materials.) Zement-Kalk-Gips 59 (1970)5, 210-215. W. Pietsch, a) Granuliererfahren fur die pharmazeutische Industrie. Die Pharmazeutische Industrie 32 (1970)5, 383 - 389; b) Granulation techniques for pharmaceutical applications. Drugs made in Germany 13 (1970)2, 58-66. W. Pietsch, Erwiinschte Agglomeration mit Granulatformmaschinen. (Wanted agglomeration with pelleting machines.) Maschinenmarkt MM - Industriejournal 77 (1971)10, 193 - 196. W. Pietsch, Size enlargement of solids. Particulate Matter (Bulletin of the Powder Advisory Center) 2 (1971)1, 15-22. W. Pietsch, Roll designs for Briquetting-Compacting Machines. Proc. 11th Biennial Conf. of IBA, Sun Valley, Idaho (1969), 145-163. W. Pietsch, Kornvergrofierung. (Size enlargement.) Abschnitt 26, “Fortschritte der Verfahrenstechnik”, Bd. 9, VDI-Verlag GmbH, Dusseldorf (1971), 831 -872. W. Pietsch, Das Kornen von Dungemitkeln mit dem Kompaktier-Granuliererfahren. (Granulating of fertilizers by means of the compacting/granulating procedure.) Aufbereitungs-Technik 12 (1971)11, 684-690. W. Pietsch, Possibilita di miglioramento delle qualita fisiche delle polveri tramite di agglomerazione. (Possibilities to improve the physical characteristics of powders by agglomeration methods.) Ing. Chim. Ital. 7 (1971)11, 161-166. W. Pietsch, Granulieren durch Kornvergrofierung. (Granulation by size enlargement.) CZChemie Technik 1 (1972)3, 116-119. W. Pietsch, Anwendungen und Vorteile von Walzdruck-Brikettiermaschinen bei der Aufbereitung mineralischer Rohstoffe. (Applications and advantages of roll type briquetting machines for mineral processing.) Proc. IXth Int. Min. Processing Congr., Prag (1970)3, 255-259, Ustav Pro Vyzkum rud, Praha (1972). W. Pietsch, Torque mill studies. A new approach in grinding research. “Particle Technology”, Proc. of Seminar, Indian Institute of Technology (IITj, Madras (1971), 203-232. W. Pietsch, Size enlargement. Lit. (33), 276-290. W. Pietsch, Granulation of fertilizers using compacting/granulation methods. Lit. (33), 335 348. W. Pietsch, Agglomerieren problemlos - Kompaktierorgang in Walzdruckbrikettier- und Kompaktiermaschinen. (Agglomeration without problems - The process of compaction in roll-type briquetting and compacting machines.) Maschinenmarkt MM - Industriejournal 78 (1972)88, 2036-2040. W. Pietsch, Wet grinding experiments in a torque ball mill. In “Zerkleinern” Symposion in Cannes 1971, Dechema-Monographien, Band 69, Nr. 1292- 1326, 751 -779, Verlag Chemie GmbH, Weinheim/Bergstrasse (1972). W. Pietsch and H. Liebert, Design and application of a laboratory machine for briquetting, compacting and pelleting research. Proc. 12th Biennial Conf. of IBA, Vancouver, Brit. Columbia (1971), 19-30. W. Pietsch, KornvergroISerung. (Size enlargement.) In: “Fortschritte der Verfahrenstechnik” (1970/71), Bd. 19, Abt. B: Mechanische Verfahrenstechnik I, 221-235, Hrsg. VTG i m VDI, VDI-Verlag GmbH, Dusseldorf. W. Pietsch, Das Kornen von Dungemitteln mit dem Kompaktier-Granuliererfahren. (Granulation of fertilizers using compaction/granulation methods.) Proc. 2. Wissenschaftlich-Technische Konferenz “Mineraldunger”, Varna, Bulgarien, Hrsg.: Wissenschaftlich-Techn. Verband fur chem. Industrie, Sofia (1972), 359-379.
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41. 42.
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57. 58. 59. 60.
W. Pietsch, A granulcao de adubos pel0 sistema de granulacao-compactacao. (Granulation of fertilizers using compaction/granulation methods.) Productos Quimicos 12 (1972)9/12,3- 10. W. Pietsch, Agglomerieren problemlos - Kompaktiervorgang in Walzdruck-Brikettier. und Kompaktiermaschinen. (Agglomeration made easy - The process of compaction in roll type briquetting and compacting machines.) Europa Industrie Revue (1973)1, 28-31. W. Pietsch, Der Kompaktiervorgang in Walzdruck-Brikettier- und Kompaktiermaschinen. (The process of compaction in roll type briquetting and compacting machines.) Proc. Symposium Pracovnku Binskkho Prumyslu, Hornickd Prhram ve Vede a Technice, Prhram, CSSR (1972), 683-710. C. Buchholz and W. Pietsch, Neue Anwendungen der Kompaktiertechnik - Auhereitung von Abfallmaterialien zu hochwertigen Rohstoffen. (New applications of compacting - Production of secondary raw materials from waste materials). CZ-Chemie-Technik 2 (1973)8, 319- 321. W. Pietsch, Mechanische Verfahrenstechnik im Dienst der Umwelttechnik. (Mechanical Process Engineering in Pollution Control.) CZ-Chemie-Technik 2 (1973)9, 351 -354. W. Pietsch, The many versatile applications of size enlargement in pollution control. Proceedings ofthe First Int’l Conf. on the Compaction and Consolidation of Particulate Matter, Brighton, England, The Powder Advisory Centre, London (1973), 227-235. W. Pietsch, Granulieren, Agglomerieren und Kornvergrogerung in der Pharmazeutischen Industrie. (Granulation, agglomeration and size enlargement in the Pharmaceutical Indus. try.) APV-Informationsdienst, Mainz 19 (1973)2/3, 147- 182. W. Pietsch, Kornvergrogerung. (Size enlargement). In: “Fortschritte der Verfahrenstechnik”, Bd. 11, 1972, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1973),162172. W. Pietsch, Anwendung der Brikettierung i m Umweltschutz am Beispiel der Riickfiihrung von Filter- und Erzstauben in metallurgischen Anlagen. (Application of briquetting in pollution control as demonstrated by recycling of filter- and ore-dusts from metallurgical plants.) Aufbereitungs-Technik 14 (1973)12, 818-821. W. Pietsch, Application of briquetting in pollution control-recycling of filter and ore dusts in metallurgical plants. Proc. 13th Biennial Conf. of IBA, Colorado Springs, CO (1973), 1-12. W. Pietsch, The new HUTT laboratory Kompaktor and the Pharmapalctor. Proc. 13th Biennial Conf. of IBA, Colorado Springs, CO (1973), 113-117. W. Pietsch, Kornvergrogemng. (Size enlargement.) In: “Fortschritte der Verfahrenstechnik”, Bd.12, 1973, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1974), 131146. R.H. Snow, B.H. Kaye, C.E. Capes, R.F. Conley, J. Sheehan, F. Schwarzkopf, and W. Pietsch, Size reduction and size enlargement. Section 8. In: R.H. Perry, C.H. Chilton “Chemical Engineer’s Handbook, 5th ed., McGraw-Hill (1973), 1-65. W. Pietsch, Kornvergrogemng. (Size enlargement.) In: “Fortschritte der Verfahrenstechnik”, Bd. 13, 1974. Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1975),143163. W. Pietsch, Kornvergrofierung mit Walzenpressen - Eine alte Technologie mit neuen Anwendungen. (Roll pressing - An old technology with new applications.) Aufiereitungs-Technik 17 (1976)3, 120-127. W. Pietsch, Kornvergrogerung. (Size enlargement.) In: “Fortschritte der Verfahrenstechnik”, Bd. 14, 1975, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1976),149160. W. Pietsch, Roll pressing. Heyden & Son Ltd., London/New York/Rheine (1976). W. Pietsch, Use of sponge iron in foundries. AFS Int’l Cast Metals Journal 1 (1976)2, 43- 50. W. Pietsch, Storage, shipping, and handling of MIDREX iron. Preprint Nr. 76-B-317, SMEAIME Meeting & Exhibit, Denver, CO (1976). W. Pietsch and G.A. Mott, Face to face interview: Direct reduced iron.....past and present. Modern Casting 66 (1976)9, 50-52.
13.3 Author’s Biography, Patents, and Publications
61. W. Pietsch, The use of sponge iron in foundries. Modern casting 66 (1976)9, 53-55 (condensed form of [SS]). 62. W. Maschlanka and W. Pietsch, Aplicacion del hierro esponja como material de carga en fundiciones. (Application of sponge iron as charge material in foundries.) Proc. Congreso Fundicion, ILAFA (1976), 75-85. 63. W. Pietsch, Charging with direct-reduced iron may reduce costs, improve chemistry. Foundry Operation Planbook (McGraw-Hill Inc.) (1977)4, 45-48. 64. D.L. Keaton and W. Pietsch, An update of MIDREX Direct Reduction techniques and innovations. Proc. 50th Annual Meeting Minnesota Section AIME, Duluth, MN (1977), 5-23. 65. W. Pietsch and R. Kreimendahl, Us0 de hierro esponja en la elaboracion de hierro. (Use of sponge iron in iron making.) In: “Us0 y comercializaci6n del hierro esponja”. Proc. Congreso ILAFA - Reduccion Directa, Macuto, Venezuela (1977), 233-239. 66. W. Pietsch, MIDREX - Direlctreduktion - Stand der Technik. (MIDREX direct reduction - The state of the art.) Aufbereitungs.Technil< 18 (1977)8, 410-416. 67. J.E. Bonestell and W. Pietsch, MIDREX direct reduction - State of the art. Proc. SEAS1 Direct Reduction Conf., Bangkok, Thailand (1977). 68. W. Pietsch, Technical development of a merchant direct reduced iron facility. Annual Convention and Iron and Steel Exposition, Cleveland, OH (1977). 69. W. Pietsch, Storage, shipping, and handling of direct reduced iron. AIMEjSME Transactions 262 (1977)3,225-234. 70. W. Pietsch, Pressure agglomeration - State ofthe art. In: K.V.S. Sastry (ed.)Agglomeration 77, AIME New York (1977), 649-677. 71. W. Pietsch, The MIDREX cold briquetting system: An economic answer to direct reduced iron fines recovery. Iron and Steel Int’l 51 (1978)2, 119, 121-123. 72. W. Pietsch, Direct reduced iron: A new charge material for iron and steel foundries. The British Foundrymen 71 (1978)4, 89-93. 73. W. Pietsch, Agglomerieren. (Agglomeration.)In: “Fortschritte der Verfahrenstechnik”, Bd. 16, 1978, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1978), 73 -89. 74. W. Pietsch, Agglomeration and direct reduction: A technical symbiosis. Mining Magazine 139 (1978)4, 414 -42 1. 75. J.E. Bonestell and W. Pietsch, The floating direct reduction plant - A feasible future reality. a) Preprint, 3rd Int’l Iron and Steel Congress, Chicago, IL, USA (1978);b) Continental Iron & Steel Trade Reports 18 (1978),707-709; c) Proc. 3rd Int’l Iron & Steel Congress, Chicago, IL, USA (1978), 186-194. 76. W. Pietsch, The availability o f direct reduced iron - An assessment of the technology and production capabilities through 1985. 82nd AFS Casting Congress and Exposition, Detroit, MN, USA (1978). 77. W. Pietsch, Direct reduced iron - A new charge material for iron and steel foundries. SEAISI 1978 Singapore Seminar “Modern Foundry Practice” (1978). 78. W. Pietsch, Development, installation, and operation o f a briquetting system for direct reduced iron fines. Proc. 15th Biennial Conf. of IBA, Montreal, Canada (l977), 83-96. 79. W. Pietsch, The influence of raw material and reduction temperature on the structure and characteristics of direct reduced iron. SME of AIME Transactions 264 (1978), 1784- 1789. 80. W. Pietsch, The role of vacuum metallurgy in the production and processing of non-iron metals. Proc. VII. Ritkaf6m Konferencia, Budapest, Hungary (1979), 75 -99. 81. W. Burgmann and W. Pietsch, Modern technologies i n steel degassing and ladle metallurgy. Proc. Int’l Symposium Modern Developments in Steelmaking, Jamshedpur, India (1981), 7.8.1-7.8.26. 82. W. Pietsch, Vakuumverfahren in der Metallurgie. (Vacuum process technology in metallurgy.) a) Vortrag Messe “Pulvermetallurgie”,Minsk, BSSR (1981) (in Russian): b) Fachberichte, Hiittenpraxis, Metallverarbeitung 19 (1981)10, 808-817 (in German); c) World Steel and Metalworking Export Manual (1981), 93- 101 (in English).
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83. W. Pietsch, Agglomeration. In: “Fortschritte der Verfahrenstechnik, Bd. 19, 1981, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1981), 133- 149. 84. W. Pietsch, Agglomeration. 3rd Int’l Symposium. Aubereitungs-Technik 22 (1981)9, 488494. 85. W. Pietsch, New production technologies for metal and alloy powders. 1981 Int’l Industrial Seminar on Pilot Plant Experiences, Amelia Island, FL, USA (1981). 86. W. Pietsch, Agglomeration. 17th Biennial Conf. of IBA, Reno, Nevada, USA (1981).Aufbereitungs-Technik 23 (1982)2, 92-99. 87. W. Pietsch, New production technologies for metal and alloy powders. In: “Competing i n the World Market - New technology for the Metals Industry”, Proc. 35th Annual Conf., Sydney, Australia (1982), 17-24. 88. H. Stephan, W. Pietsch, H. Ettl, and H. Aichert, Degassing of metal powders and the filling of degassed powders into capsules for the manufacturing of ingots and discs. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982), 179- 191. 89. W. Pietsch, H. Stephan, A. Feuerstein, J. Heimerl, and R. Ruthardt, Some new results of the atomization of reactive and refractory metals with the EBRD Process. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982),481 -499. 90. W. Pietsch, New production technologies for metal and alloy powders. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982), 739-754. 91. W. Pietsch, Titanium - From sponge to powder. VII Yugoslav Conf. on Contemporary Materials, Subotica, Yugoslavia (1982). 92. W. Pietsch, Die KornvergroBemng in der Verfahrenstechnik und ihre industrielle Anwendung am Beispiel der Direktreduktion von Eisenerzen. (Size enlargement in process engineering and its industrial application as exemplified by the direct reduction of iron ores.) Aufbereitungs-Technik (part 1) 23 (1982)4, 193-200; (part 2) 23 (1982)5, 248-257. 93. R. Ruthardt, W. Pietsch, and H. Stephan, Atomization techniques for high quality metal powder production. Unpublished manuscript (1982). 94. W. Pietsch, H. Stephan, A. Feuerstein, J. Heimerl, and R. Ruthardt, Atomization of reactive and refractory metals by the electron beam rotating disc process. Powder Metallurgy Int’l 15 (1983j2, 77-83. 95. W. Pietsch, Energy conservation in the fertilizer industry - The compaction/granulation process for mixed (NPK) fertilizers. a) Proc. 18th Biennial Conf. of IBA, Colorado Springs, CO, USA (1983), 243-265; b) Proc. Int’l Conf. Fertilizer ’83, London, UK 2 (1983), 467-479. 96. W. Pietsch, Modern equipment and plants for potash granulation. Proc. 1st Int’l Potash Technology Conf. Potash ’83, Saskatoon, Sask., Canada (1983), 661-669. 97. W. Pietsch, Einsatz grosser Walzenbrikettiermaschinen in der Koksherstellung. (Large roller briquetting machines in coke production.) Aufbereitungs-Technik 25 (1984)1, 29 - 38. 98. W. Pietsch, Agglomerate bonding and strength. Section 7.2 in M.E. Fayed and L. Otten (eds.) “Handbook of Powder Science and Technology”, Van Nostrand Reinhold Co., Inc., New York (1984), 231-252. 99. W. Pietsch, Roll pressing, isostatic pressing and extrusion. Section 7.4 in M.E. Fayed and L. Otten (eds.)“Handbook of Powder Science and Technology”,Van Nostrand Reinhold Co., Inc., New York (1984), 269-285. 100 W. Pietsch, Agglomeration. In: “Fortschritte der Verfahrenstechnik, Bd. 21, 1983, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1983), 121- 139. 101. W. Pietsch, Granulation of mixed fertilizers i y compaction. Proc. 34th Annual Meeting, Fertilizer Round Table, Baltimore, MD (1984),48-58. 102. H.-G. Bergendahl and W. Pietsch, Hot briquetting with roller presses. Proc. 4th Int’l Symposium on Agglomeration, Toronto, Canada, Iron and Steel Society, Inc. (1985), 543-550. 103. W. Pietsch, Agglomeration - Key to reycling of metal bearing fines. Proc. Int’l Symposium on Recycle and Secondary Metals, Fort Lauderdale, FL (1985), 683-699. 104. W. Pietsch, Compaction/granulation of dry, digested sludge from municipal waste treatment plants. Proc. 19th Biennial Conf. of IBA, Baltimore, MD, USA (1985), 179-194.
13.3 Author’s Biography, Patents, and Publications
105. W. Pietsch, Agglomeration. In: “Fortschritte der Verfahrenstechnik”, Bd. 23,l Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1985), 125- 139. 106. W. Pietsch and C. Rodriguez, Granulation of fertilizers by compaction. Proc. 20th Biennial Conf. of IBA, Orlando, FL (1987), 113-126. 107. P. Schafer, Ph. Wolstenholme, W. Pietsch, and R. Holland, Compaction and granulation of dried sludge at Ocean County, NJ. 60th Annual Conf. WPCF (Water Pollution Control Federation), Philadelphia, PA (1987). 108. W. Pietsch, Mixed fertilizer granulation by compaction, History, application, and present status of mixed fertilizer granulation by compaction, Proc. Int’l Conf. Fertilizer South America, Caracas, Venezuela. The British Sulfur Corp. Ltd., London, England (1989), 153- 173. 109. W. Pietsch and R. Zisselmar, Pressure agglomeration with roller presses for waste processing and recycling. Proc. 5th Int’l Symposium Agglomeration, Brighton, UK, IChemE, Rugby, UK (1989), 117-130. 110. W. Pietsch, Granulation offertilizers by compaction. Proc. IFDC Workshop “Supplying quality multinutritional fertilizers in the Latin American and Caribbean Region - Emphasizing bulk blending and the complementary role of agglomeration”, Guatemala City, Guatemala (1989), 183-196. 111. W. Pietsch, Briquetting of coal (Can an ancient technology be modified for the production of environmentally safer smokeless fuels?). Proc. 2lst Biennial Conf. of IBA, New Orleans, LA (1989), 303-320. 112. W. Pietsch, Granulation of fertilizers by compaction. Proc. IFDC Workshop “Urea-based NPK plant design and operating alternatives”, Muscle Shoals, AL (1990), 89-98. 113. W. Pietsch, Briquetting of non-ferrous waste for economic recycle. Proc. 2nd Int’l Symposium “Recycling of metals and engineered materials” (J.H.L. van Linden, D.L. Stewart, Y. Sahai, eds.), TMS, Warrendale, PA (1990), 667-670. 114. W. Pietsch, Size enlargement by agglomeration. John Wiley & Sons Ltd. - Salle + Sauerlander, Chichester, UK/New York, NY, USA/Brisbane, Australia/Toronto, Canada/Singapore - Aarau, Switzerland/Frankfurt/M., Germany/Salzburg, Austria (1991). 115. M.E. Fayed and W. Pietsch, Particulate solids characterization and agglomeration. Course notes, AIChE Continuing Education, Miami Beach, FL (1992) (revised). 116. W. Pietsch and H. Ries, Agglomerieren - Granulieren. (Agglomeration - Granulation.) Course notes, Technische Akademie Wuppertal e.V., Wuppertal-Elberfeld, Germany (1992) (revised). 117. W. Pietsch, Fundamentals of agglomeration. Course notes, Workshop at Powdex NJ, Somerset, NJ (1992) (revised). 118. W. Pietsch, Pressure agglomeration: Fundamentals and applications. Course notes, Workshop at Powdex N J , Somerset, NJ (1992). 119. H.O. Kono and W. Pietsch, Tumble/growth agglomeration of fine powders: Present state and new developments. Course notes, Industrial awareness seminar at Powdex NJ, Somerset NJ (1992) (revised). 120. W. Pietsch, Briquetting of aluminum swarf for recycling. Light Metals ’93, TMS, Warrendale, PA (1993), 1045-1051. 121. W. Pietsch, Size enlargement by agglomeration in the pharmaceutical industry with special emphasis on pressure agglomeration. Course notes, Workshop at Interphex - USA New York, NY (1993). 122. W. Pietsch and H. Giinter, New applications of roller presses in coal-related technologies. Proc. 18th Int’l Techn. Conf. on Coal Utilization & Fuel Systems, Clearwater, FL (1993), 837-852. 123. W. Pietsch, Agglomeration technologies for environmental protection and recycling. Proc. 6th Int’l Symposium on Agglomeration, Nagoya, Japan (1993), 837-847. 124. W. Pietsch, Size enlargement by agglomeration for solid waste treatment or minimization and for hazardous waste stabilization. Preprints: 4th Pollution Prevention Topical Conference, Seattle, WA, USA, AIChE, New York, NY, USA (1993), 202-208.
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125. H.-G. Bergendahl and W. Pietsch, Roller presses, their applications, sizes, and capacities as well as their limitations. Proc. 23rd Biennial Conf. “The Institute for Briquetting and Agglomeration’’, Seattle, WA (1993), 185-204. 126. W. Pietsch and H. Giinter, Briquetting as an upgrading process for different types of coals. Proc. 19th Int’l Techn. Conf. on Coal Utilization & Fuel Systems, Clearwater, FL (1994), 181 195. 127. W. Pietsch, Granulation by agglomeration in the pharmaceutical industry. Course notes, Workshop at Interphex - USA, New York, NY (1994) (revised). 128. W. Pietsch, Parameters to be considered during the selection, design, and operation ofagglomeration systems. Proc. 1st Int’l Particle Technology Forum, AIChE, Denver, CO (1994),Part I, 248 - 257. 129. W. Pietsch, Economical and innovative methods for the agglomeration of dusts and other wastes from metallurgical plants for recycling. In: H.Y. Sohn (ed.) “Metallurgical Processes for the Early Twenty-First Century”, vol. 11, Technology and Practice, TMS, Warrendale, PA (1994),487-495. 130. W. Pietsch, A review of agglomeration fundamentals and industrial techniques to enhance productivity. Course notes, Workshop at Powdex ’94, Houston, TX (1994). 131. W. Pietsch, Aglomeracion en plantas para reciclado: Metodos economicos e innovativos para la aglomeracion de polvos y otros desechos. Siderurgia Latinoamericana 10 (1994) 414, 27- 34. 132. F:H. Grandin and W. Pietsch, Compaction of aluminum chips and turnings and of other particulate aluminum scrap with roller presses for secondary aluminum melting without losses. Light Metals ’95, TMS, Warrendale, PA (1995), 799-802. 133. W. Pietsch, Agglomeration: Controlling pollution and permitting waste recycling. Powder & Bulk Engng. 9 (1995)2,53, 54, 56, 57, 59-62. 134. W. Pietsch, Briquetting of coal with roller presses. An important technology for the production of coal-based compliance fuel. Proc. 20th Int’l Techn. Conf. on Coal Utilization & Fuel Systems, Clearwater, FL (1995), 87-95; also in: S.D. Serkin (ed.) “Coal Fines: The Unclaimed Fuel”, Coal & Slurry Technology Assoc., Washington, DC (1995), 91-99. 135. W. Pietsch, Agglomeration technologies for environmental protection and recycling. Course notes, Workshop at Powder & Bulk Solids ’95, Rosemont, I L (1995). 136. W. Pietsch, Review of particle formation by compaction processes. 16th IFPRI Annual Meeting, Urbana, I L (1995). 137. F.-H. Grandin, W. Pietsch, and G. Medina y Espafia, Compaction of aluminum scrap on high pressure roller presses. Proc. 4th Australian, Asian, Pacific Course & Conference on Aluminum Cast House Technology, Sidney, Australia (1995). 138. W. Pietsch, J. Jagnow, and R. Lobe, Schiittguter pelletieren, extrudieren, granulieren, briltettieren, kompaktieren. Problemlosungen fur industrielle Anwendungen. (Bulk solids pelletizing, extruding, granulating, briquetting, compacting. Solutions for problems of industrial applications.) Course notes. Seminar of the Technical Akademie Wuppertal, Altdorf, Germany (1995). 139. W. Pietsch, Roller presses -Versatile equipment for mineral processing. Proc. XIX IMPC, San Francisco, CA (1995),vol. 1, 59-66. 140. W. Pietsch, Roller presses for secondary metal recycling. Proc. 3rd Int’l Symp. Recycling of Metals and Engineered Materials. P.B. Queneau and R.D. Peterson (eds.), Point Clear, AL (1995),233-241. 141. W. Pietsch, Evaluation of parameters for the selection, design, and operation of agglomeration systems. Proc. 24th Biennial Conf. “The Institute for Briquetting and Agglomeration”, Philadelphia, PA (1995), 175-189. 142. W. Pietsch, The briquetting of coal in Europe. Proc. COAL PREP 96, 13th Int’l Coal Preparation Conf., Lexington, KY (1996), 167-183. 143. W. Pietsch, Successfully use agglomeration for size enlargement. Chem. Engng. Progr. 92 (1996)4, 30-45.
73.4 Tables ofContents of Related Books by the Author
144. W. Pietsch, Recent developments in dry granulation of fertilizers by compaction. Proc. 5th World Congress of Chemical Engineering, San Diego, CA (1996),vol. V, 552-558. 145. W. Schiitze and W. Pietsch, HBI - Survey ofthe significance and development of sponge iron hot briquetting and the application of this technology in various plants and reduction processes. Proc. Conf. Pre Reduced Products and Europe, Milan, Italy (1996). 146. W. Pietsch and W. Schutze, HBI - A safe DRI-based source of iron units. Paper at World Iron Ore 96, Orlando, FL (1996), Skillings Mining Review 86 (1997)18, 4-9. 147. W. Pietsch, Granulate dry particulate solids by compaction and retain key powder particle properties. Chem. Engng. Progr. 93 (1997)4, 24-46. 148. W. Pietsch, Size enlargement by agglomeration. ch. 6 (175 pages) In: M.E. Fayed and L. Otten (eds.) ”Handbook of Powder Science & Technology”, 2nd ed., Chapman & Hall, New York, NY (1997). 149. W. Pietsch, Agglomeration techniques for the manufacturing of “instant” granules from fine powder mixtures. In: R. Hogg, C.C. Huang, and R.G. Cornwall (eds.) “Fine Powder Processing Technology”, The Pennsylvania State University (1998), 233 -242. 150. W. Pietsch, Agglomeration techniques for the manufacturing of granular materials with specific product characteristics. Proc. 25th Biennial Conf. “The Institute for Briquetting and Agglomeration”, Charleston, SC (1997), 149- 164. 151. W. Pietsch, Particle engineering by agglomeration in the pharmaceutical industry. Course notes, Workshop at INTERPHEX ‘99, New York, NY (1999). 152. W. Pietsch, How to select an agglomeration method. (part I) Powder and Bulk Engineering 13 (1999)2,60-65; (part 11) Powder and Bulk Engineering 13 (1999)3, 21-31. 153. W. Pietsch, Readily engineer agglomerates with special properties from micro- and nanosized particles. Chem. Engng. Progr. 95 (1999)8, 67-81. 154. W. Pietsch, The porosity of agglomerates. Proc. 26th Biennial Conf. “The Institute for Briquetting and Agglomeration”, San Diego, CA (1999), 1- 14. 155. W. Pietsch, Granulation of pharmaceutical formulations to improve handling, processing, and use. Course notes. Workshop at INTERPHEX 2000, New York, NY (2000). 156. W. Pietsch, Compaction methods for granulation and the manufacturing of dry dosage forms. Course notes. Workshop at INTERPHEX 2000, New York, NY (2000). 157. W. Pietsch, Ram pressing -An almost extinct technology with interesting new applications in coal and other solid fuel processing. Proc. 25th Int’l Technical Conf. on Coal Utilization & Fuel Systems, Cleanvater, FL (ZOOO), 37-48. 158. W. Pietsch, Agglomeration methods in particle engineering. Proc. XXI Int’l Mineral Processing Congress, Rome, Italy (2000),A4: 87-96. 159. W. Pietsch, An interdisciplinary approach to size enlargement by agglomeration. Proc. 7th International Symposium on Agglomeration, Albi, France (2001). 160. W. Pietsch, Agglomeration -An old and key technology serving mankind. Proc. 3rd European Congress of Chemical Engineering (3rd ECCE), Niirnberg, Germany (2001). 13.4 Tables o f Contents of Related Books by the Author
To help the reader decide whether related books by the author may contain additional information, below the tables of contents of three related books are reproduced. W. Pietsch, Roll Pressing. 1st ed., Heyden & Son Ltd., London/New York/Rheine (1976);2nd ed., Powder Advisory Centre, London (1987). Introduction/Fundamentals of Roll Pressing/Phenomenology of Roll Briquetting/Phenomenology of Roll Compacting/Design Principles/Machine Designs/Roll Designs/Feeder Designs/Design of the Discharge System/Instmmentation and Control of Roll Presses/Product Design/Assessment of Compact Quality/Auxiliaries/ Binders and Lubricants/Applications/TheCost of Roll Pressing/Application of Solids Flow Properties to Roll Presses/Recommendations/Equipment.
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W. Pietsch: Size Enlargement by Agglomeration. John Wiley & Sons Ltd. - Salle + Sauerlander, Chichester, UK/New York, NY, USA/Brisbane, Australia/Toronto, Canada/Singapore - Aarau, Switzerland/Frankfurt/M., Germany/Salzburg, Austria. (1991). Introduction/Fundamentals of Agglomeration/Experimental Investigations/IndustrialSize Enlargement Equipment and Processes/Industrial Applications of Agglomeration/Past, Present, and Future of Size Enlargement by Agglomeration. W. Pietsch, Agglomeration Technologies - Industrial Applications. VCH-Wiley, Weinheim (2003). Introduction/Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science/Glossary of Application-related Agglomeration Terms/Undesired Agglomeration and Methods to avoid or lessen it/Industrial Applications of Size Enlargement by Agglomeration/Powder Metallurgy/Applicationsin Evironmental Control/Development of Industrial Applications/Optimization and Troubleshooting of Agglomeration Systems and Plants/Applications of Agglomeration Phenomena for Single Particles and in Nano-Technologies/Engineering Criteria, Development, and Plant Design/Outlook/Bibliography/Indexes.
Agglomeration Processes Wolfgang Pietsch Copyright 0Wiley-VCH Verlag GmbH, Weinheim, 2002
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Indexes As discussed in Chapter 4, one of the problems encountered in connection with the presentation of technologies that were known for centuries and henceforth have been developed empirically and independently for different applications and industries (see also Chapter 2) is that the reader of books and the student of the methods and processes find an often confusing terminology. Even the suppliers of equipment may present themselves in a manner that does not unequivocally define their activities. To help locate information, three different indexes are offered in the following: Section 14.1 is a list of addresses of vendors of equipment for size enlargement by agglomeration and of peripheral techniques as well as of their telefone and telefax numbers. It is subdivided into fields of activities and, if a vendor is active in different areas, its listing is repeated again under the appropriate heading. Of course, no claim for completeness is made and mentioning a specific vendor does not constitute an endorsement by the author of this book. Also included is a listing of some tollers with a description of their activities. Section 14.2 is a “Wordfinder Index”. It is provided to give the reader a ready reference to the way in which words and terminologies are used in this book. Throughout this publication, historical, modern, and application oriented terms of agglomeration can be found (including certain important trade names, refer to “Disclaimer” at the beginning of the book). In the “Wordfinder” approach, the location is indicated were a word or term is explained and in the text it is highlighted by bold print. Section 14.3 contains references to the locations of words and terms in a “classic” Index of Subjects. This index often provides many page numbers for the same topic. When a word is encountered without specific explanations in the text, the reader can go to the Wordfinder Index, which gives direction to the definition location, to learn about it or refresh his or her memory.
14.1 List o f Vendors
When planning the book, the author intended to prepare a worldwide, comprehensive list of vendors of agglomeration equipment as well as of associated resources and services. To that end, he collected technical and process information, particularly
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in Europe, North America, and Japan. Also, the author’s extensive personal files and library, many of which were already reviewed for and incorporated in his earlier books [B.12b,B.421 and articles (see Section 13.3) were referred to. Furthermore, participants of the author’s many international continuing education courses were interviewed and work of several ofhis colleagues, who covered specific topics at these courses, was used as basis for some of the chapters and/or sections of this book (see Acknowledgements). At some point of his work, all of the vendors that he and others knew were contacted and asked for input to be used in this publication. Unfortunately, only very few provided information that fitted formats which the author had prepared so that an unbiased comparison of data and technical details, publication of which does have the supplier’s approval, can be provided. Therefore, the latter idea was given up and those vendors that were or became known to the author while preparing the book are included in the following listing. Companies and individuals that have provided support beyond the normal are identified in the Acknowledgementsand are referred to with gratitude in figure and table captions. To include also vendors, resources, and services in countries that are less well known in the western hemisphere, particularly Russia and the former Eastern Block countries, China and the countries of the Far East, India, the Near East, Africa, and South America, technical and trade organizations were contacted and asked to provide lists of vendors and specialists working in the field. Unfortunately, again, most did not even respond so that information was exclusively gleaned from a few freely available brochures and publications. It was most disappointing to the author that, with his limited possibilities and the language as well as accessibility problems in other continents, it was virtually impossible to adequately research and cover the undoubtedly vast resources of the “Eastern” countries (particularly Russia, China, and the Far East, excluding Japan). Since the topic of this book deals particularly with “agglomeration” it was found, however, that in Australia, India, the Near East, South America as well as Africa and in many smaller countries, sources of agglomeration equipment are primarily local subsidiaries or foreign and home office representatives of those that are mentioned in the list below. Other sources are international engineering companies, their local subsidiaries and representatives which are specifying and using European, North American, and Japanese equipment. The following listing is subdivided according to methods, technologies, resources and technologies. It was decided to list vendors that are active in different field in each of the classifications to facilitate the search if a particular method has been preselected, for example, by methods that were described in Section 11.1. (The author of this book is working on another, complementary publication which is tentatively entitled: ‘Agglomeration Technologies - Industrial Applications”, WilqVCH (2003) jB.711; see also Section 13.4. Suppliers of equipment, technologies, and services in the field are invited to submit information to the author for possible inclusion in the forthcoming book.)
74.7 List of Vendors
The entries are organized according to the following “Tab. of Contents” pages a
Growth/Turnble Agglomeration - Disc (Pan)/Drum/Mixer - Fluid Bed
Spray Nozzles and Systems - Agglomeration in Suspensions a
Pressure Agglomeration - Low Pressure Extrusion
Spheronizing Medium Pressure Extrusion (Pelleting) High Pressure Extrusion (Ram Presses, Extruders) High Pressure Agglomeration (Punch-and-Die,Tabletting, Isostatic) High Pressure Agglomeration (Roll) a Sintering a Coating a Melt Solidijcation 0 Applications a Binders a Test Equipment and Peripherals a Organizations a Tollers -
Growth/Tumble Agglomeration Disc (Pan)/Drum/Mixer
Paul 0. Abbe, Inc. 139 Center Avenue Little Falls, NJ 07424, USA
Tel.: +1- 973- 256- 4242 Fax.: +1- 973- 256- 0041
Aeromatic-Fielder Div. Niro Inc. 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1-410- 997- 7070 Fax.: +1-410- 997- 5021
Allgaier-Werke GmbH & Co. KG Bereich Sieb- und Aufiereitungstechnik Ulmerstr. 75 D-73066 Uhingen, Germany
Tel.: +49- (0)7161- 301. 313 Fax.: +49- (0)7161- 301- 440
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, M N 55413, USA
Tel.: +1- 612- 331- 4370 Fax.: +1-612- 627- 1444
545 545 549 551 552 552 552 553 554 554 556 559 561 562 563 564 565 567 5 74 5 75
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I Hermann Berstorff Maschinenbau GmbH 14 fndexes
An der breiten Wiese 3-5 D-30625 Hannover, Germany
Tel.: +49- (0)511- 5702- 0 Fax.: +49- (0)511- 561916
L.B. Bohle, Inc. 1504 Grundy Lane Bristol, PA 19007, USA
Tel.: +1-215- 785- 1121 Fax.: +1-215- 785- 1221
H.C. Davis Sons Mfg. Co., Inc. Box 395 Bonner Springs, KS 66012, USA
Tel.: +1- 913- 422- 3000 Fax.: +1-913- 422-7220
Dierks & Sohne, GmbH & Co, KG. (DIOSNA) Sandbachstr. 1 D-49074 Osnabriick, Germany
Tel.: +49- (0)541- 3310- 0 Fax.: +49- (0)541- 3310- 410
Draiswerke GmbH Speckweg 43 - 5 1 D-68305 Mannheim, Germany
Tel.: +49- (0)621- 7504- 00 Fax.: +49- (0)621- 7504- 233
Maschinenfabrik Gustav Eirich Walldurner Str. 50, Postfach 1160 D-74736 Hardheim, Germany
Tel.: +49- (0)6283- 51- 310 Fax.: +49- (0)6283- 51- 304
Eirich Machines, Inc. American Process Systems Div. Delany Business Center 4033 Ryan Rd. Gurnee, IL 60031, USA
Tel.: +1-847- 336- 2444 Fax.: +1-847- 336- 0914
FEECO International 3913 Algoma Rd. Green Bay, WI 54311, USA
Tel.: +1-920- 468- 1000 Fax.: +1-920- 469- 5110
Palex Corp. (Fukae Powtec Corp) P.O. Box 65 Tajimi, Gifu-Pref. 507-0033, Japan
Tel.: +81- (0)572- 229152 Fax.: +81- (0)572- 242722
GEMCO The General Machine Company of New Jersey 301 Smalley Avenue Middlesex, NJ 08846, USA
Tel.: +1-908- 752- 7900 Fax.: +1-908- 752- 5857
14.1 List of Vendors
Hayes & Stolz Ind. Mfg. Co., Inc. 3521 Hemphill Street Fort Worth, TX 76110, USA
Tel.: +1-817- 926- 3391 Fax.: +1- 817- 926- 4133
Henschel Mixers America, Inc. P.O. Box 800607 Houston, TX 77280-0607, USA
Tel.: +1-713- 690- 3333 Fax.: +1-713- 690- 3353
Hosokawa Micron Powder Systems 10 Chatham Road Summit, NJ 07901, USA
Tel.: +1-908- 273- 6360 Fax.: +1-908- 273- 7432
Italvacuum Srl. (Criox) Via Stroppiana 3 1-10071 Borgaro (Turin), Italy
Tel.: +39- 11-470- 4651 Fax.: +39- 11-470- 1010
Jaygo, Inc. 675 Rahway Ave. Union, NJ 07083, USA
Tel.: +1- 908- 688- 3600 Fax.: +1- 908- 688- GO60
Brian Kaye Associates Ltd. 30 Courtney Hill Sudbury, Ont. P3E 5W5, Canada
Tel.: +1-705- 688- 1432 F a . : +1- 705- 688- 1432
Key International, Inc. 480 Route 9
Englishtown, NJ 07726, USA
Tel.: +1-908- 536- 1500 Fax.: +1-908- 972- 2630
Littleford Day, Inc. 7451 Empire Drive Florence, KY 41042-2985, USA
Tel.: +1- 606- 525-7600 Fax.: +1- 606- 525- 1446
Gebrtider Lodige Maschinenbau GmbH Elsener Str. 7-9 D-33102 Paderborn, Germany
Tel.: +49- (0)5251- 309- 0 Fax.: +49- (0)5251- 309- 123
MAP S.R.L Via Cavour, 388/B 1-41030 Ponte Motta, Cavezzo (MO), Italy
Tel.: +39- 535- 49911 Fax.: +39- 535- 49900
Mars Mineral P.O. Box 719 Mars, PA 16046, USA
Tel.: +1- 724- 538- 3000 Fax.: +1-724- 538- 5078
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C.G. Mozer GmbH & Co. KG Postfach 943 D-73009 Goppingen, Germany
Tel.: +49- (0)7161- 6735- 0 Fax.: +49- (0)7161-6735- 35
MTI, Mixing Technology, Inc. 3303 FM 1960 West, Suite 490 Houston, TX 77068, USA
Tel.: +1-281- 583- 8610 Fax.: +l-281- 583- 0190
Munson Machinery Co., Inc. P.O. Box 855, 210 Seward Ave. Utica, NY 13503-08555, USA
Tel.: +1-315- 797- 0090 Fax.: +1-315- 797- 5582
Nara Machinery Co., Ltd. 5-7, 2-chome, Jonan-Jima Ohta-ku, Tokyo 143, JAPAN
Tel.: +81-(0)3- 3799- 5011 Fax.: +81- (0)3- 3790- 8055
Nara Zweigniederlassung Europa Europaallee 46 D-50226 Frechen, Germany
Tel.: +49- (0)2234-23063 Fax.: +49- (0)2234-23067
GEA/NIRO A/S Gladsaxevej 305, P.O. Box 45 DK-2860 Soborg, Denmark
Tel.: +45- 3954- 5454 Fax.: +45- 3954- 5800
NIRO, Inc. 9165 Rumsey Road Columbia, MD 21045, USA
Tel.: +1-410- 997- 8700 Fax.: +1-410- 997- 5021
Patterson-Kelley Co. 100 Burson Street P.O. Box 458 East Stroudsburg, PA 18301, USA
Tel.: +1-570- 421- 7500 Fax.: +1- 570- 421- 8735
Phlauer, A&J Mixing International, Inc. 8-2345 Wyecroft Road Oakville, Ont. L6L 6L4, Canada
Tel.: +1-905- 827- 7288 Fax.: +1-905- 827- 5045
Processall, Inc. 10596 Springfield Pike Cincinnati, OH 45215, USA
Tel.: +1-513- 771- 2266 Fax.: +1-513- 771- 6767
Robot Coupe USA, Inc. Scientific-Industrial Division P.O. Box 16627 Jackson, MS 39236-6627, USA
Tel.: +1-601- 956- 3216 Fax.: +1-601- 956- 5758
14. I List of Vendors
ROMACO, Inc. 104 American Road Morris Plains, N J 07950, USA
Tel.: +1-973- 605- 5370 Fax.: +1-973- 605- 1360
Charles Ross and Son Co. 710 Old Willets Path Hauppauge, NY 11788, USA
Tel.: +1-631- 234- 0500 Fax.: +1-631- 234- 0691
The A.J. Sackett & Sons Co. 1701 South Highland Ave. Baltimore, MD 21224, USA
Tel.: +1-301- 276- 4466 Fax.: +1-301- 276- 0241
Hosokawa Schugi B.V. Chroomstraat 29 NL-8211 AS Lelystad, Netherlands
Tel.: +31- (0)320- 28 66 66 Fax.: +31- (0)320- 24 47 94
Sejong Machinery Co., Ltd. #159-11 Dodangdong Wonmi-ku 420-130 Puchon-city Kyunggi-do, Korea
Tel.: +82- 32- 672- 7811/2 Fax.: +82- 32- 672- 7813
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax.: +1- 319- 377- 5574
Zanchetta & C. S.R.L (see also Key/ROMACO) Via della Contea, 24 1-55010 S. Salvatore (Lucca), Italy
Tel.: +39- (0)583- 934626 Fax.: +39- (0)583- 217317
ZETTL GmbH & Co. KG Oldenbourgstr. 11 D-81247 Miinchen, Germany
Tel.: +49- (0)89- 81809- 0 Fax.: +49- (0)89- 81809- 55
Fluid Bed
Aeromatic Ltd. (member GEA/NIRO) Hauptstr. 145 CH- 4416 Bubendorf, Switzerland
Tel.: +41- (0)61- 931- 2575 Fax.: +41- (0)61- 931- 2678
Aeromatic-Fielder Div. Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1-410- 997- 7070 Fax.: +1-410- 997- 5021
Applied Chemical Technology, Inc. (ACT) 4350 Helton Drive Florence, AL 35630, USA
Tel.: +l-256- 760- 9600 Fax.: +1- 256- 760- 9638
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Allgaier Werke GmbH & Co. KG Ulmerstr. 75 D-73066 Uhingen, Germany
Tel.: +49- (0)7161- 301- 0 Fax.: +49- (0)7161- 34268
Allgaier Verfahrenstechnik GmbH A-4492 Hofkirchen 93. Austria
Tel.: +43- 45 67 22 59 01 25 Fax.: +43- 72 25 64 23
AMMAG Dahlienstr. 11 A-4623 Gunskirchen, Austria
Tel.: +43- 7246- 6408- 0 Fax.: +43- 7246- 6408- 39
APV Anhydro AS Ostmarken 7 DK-2860 Soborg, Copenhagen, Denmark
Tel.: +45- 3969- 2811 Fax.: +45- 3969- 3880
APV Anhydro 182 Wales Avenue Tonawanda, NY 14150, USA
Tel.: +1- 716- 692- 3000 Fax.: +1- 716- 692- 6416
Babcock-BSH GmbH August Gottlieb Str. 5 D-36222 Bad Hersfeld, Germany
Tel.: +49- (0)6621-81449 Fax.: +49- (0)6621- 81393
DMR Prozesstechnologie Rinaustr. 380 CH-4303 Kaiseraugust, Switzerland
Tel.: +41- 61- 813- 10- 60 Fax.: +41- 61- 813- 10- 62
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81-06- 933- 1511 Fax.: +81- 06- 933- 1531
Glatt GmbH Process Technologie Biihlmuhle D-79589 Binzen, Germany
Tel.: +49- (0)7621-664- 0 Fax.: +49- (0)7621-647- 23
Glatt Air Technique, Inc. 20 Spear Road Ramsey, NJ 07446, USA
Tel.: +1- 201- 825- 8700 Fax.: +1- 201- 825- 0389
A. Heinen AG Anlagenbau Achternstr. 1-17 D-26316 Varel, Germany
Tel.: +49- (0)4451- 122- 0 Fax.: +49- (0)4451- 122- 159
14. I List of Vendors
BWI Hiittlin Daimlerstr. 7 D-79585 Steinen, Germany
Tel.: +49- (0)7627- 9117- 0 Fax.: +49- (0)7627-8851
GEA/NIRO A/S Gladsaxevej 305, P.O. Box 45 DK-2860 Soborg, Denmark
Tel.: +45- 3954- 5454 Fax.: +45- 3954- 5800
NIRO, Inc. 9165 Rumsey Road Columbia, MD 21045, USA
Tel.: +1-410- 997- 8700 Fax.: +1-410- 997- 5021
NIRO, Inc. (Food & Dairy Industries) 1600 O’Keefe Road Hudson, WI 54016, USA
Tel.: +1-715- 386- 9371 Fax..: +1- 715- 386- 9376
Pulse Combustion Systems 135 Eye Street, Suite B San Rafael, CA 94901, USA
Tel.: +1-415- 457- 6500 Fax.: +1-415- 723- 3727
SprayDryConsult Int’l. ApS Krathusparken 2 DK-2920 Charlottenlund, Denmark
Tel.: +45- (0)3964- 5030 Fax.: +45- (0)3964-6050
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax.: +1- 319- 377- 5574
Spray Nozzles and Systems
Bete Fog Nozzle, Inc. P.O. Box 1438, 50 Greenfield Street Greenfield, MA 01302-1438, USA
Tel.: +1-413- 772- 2166 +1-413- 772- 0846 Fax.: +1- 413- 772- 6729
BEX, Inc., Spray Nozzles 37709 Schoolcraft Rd. Livonia, MI 48150-1009, USA
Tel.: +1- 734- 464- 8282 Fax.: +1- 734- 464- 1988
Lechler GmbH & Co. KG P.O. Box 1323 D-72544 Metzingen, Germany
Tel.: +49- (0)7123-962- 0 Fax.: +49- (0)7123-962- 333
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Lechler, Inc. 445 Kautz Road St. Charles, IL 60174, USA
Tel.: +1- 630- 377- 6611 Fax.: +1-630- 377- 6657
Processall, Inc. 10596 Springfield Pike Cincinnati, OH 45215, USA
Tel.: +1- 513- 771- 2266 Fax.: +1- 513- 771- 6767
Spray Dynamics 108 Bolte Lane St. Claire, MO 63077, USA
Tel.: +1- 314- 629- 7366 Fax.: +1- 314- 629- 7455
Spraying Systems Co. P.O. Box 7900 Wheaton, IL 60189-7900, USA
Tel.: +1- 630- 665- 5000 Fax.: +1- 630- 260- 0842
Agglomeration in Suspensions
Dr. Ed Capes/ Dr. Ken Darcovich National Research Council/NRC-CNRC Chemical Division/ I C P ET M 12-15, Montreal Road Ottawa, Ontario K1A OR6, Canada
Tel.: +1- 613- 993- 6848 Fax.: +1- 613- 941- 2529
EIMCO, Div. of Baker Hughes Dillenburger Str. 100 D-51105 Koln, Germany
Tel.: +49- (0)221- 9856- 0 Fax.: +49- (0)221- 9856- 102
Baker Hughes Co. 100 Neponset Street South Walpole, MA 02071, USA
Tel.: +1- 508- 668- 0400 Fax.: +1- 508- 668- 6855
Pressure Agglomeration Low Pressure Extrusion
Aeromatic-Fielder Div. Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax.: +1- 410- 997- 5021
Alexandenverk AG Kippdorfstr. 6-24 D-42857 Remscheid, Germany
Tel.: +49- (0)2191- 795- 216 Fax.: +49- (0)2191- 795- 350
74.7 List of Vendors
Alexandenverk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1-215- 442- 0270 Fax.: +1- 215- 442- 0271
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, M N 55413, USA
Tel.: +1-612- 627- 1412 Fax.: +1-612- 627- 1444
Hosokawa BEPEX GmbH (HUTT) Postfach 1152 / Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131-907-0 Fax.: +49- (0)7131-907-301
Caleva Process Solutions Ltd. Butts Pond Industrial Estate Sturminster Newton, Dorset DTlO lAZ, England
Tel.: +44- (0)1258-471122 Fax.: +44- (0)1258-471133
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81-06- 933- 1511 Fax.: +81-06- 933- 1531
WLS GABLER Maschinenbau KG Nobelstr. 16 a D-76275 Ettlingen, Germany
Tel.: +49- (0)7243- 5431- 0 Fax.: +49- (0)7243- 5431-54
LCI Corporation P.O. Box 16348 Charlotte, NC 28297-8804, USA
Tel.: +1-704- 394- 9474 Fax.: +1-704- 392- 8507
Spheronizing
Aeromatic-Fielder Div. Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1-410- 997- 7070 Fax.: +1-410- 997- 5021
Caleva Process Solutions Ltd. Butts Pond Industrial Estate Sturminster Newton, Dorset DTlO l A Z , England
Tel.: +44- (0)1258-471122 Fax.: +44- (0)1258-471133
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81- 06- 933- 1511 Fax.: +81-06- 933- 1531
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WLS GABLER Maschinenbau KG Nobelstr. 16 a D-76275 Ettlingen, Germany
Tel.: +49- (0)7243- 5431- 0 Fax.: +49- (0)7243- 5431-54
LCI Corporation P.O. Box 16348 Charlotte, NC 28297-8804, USA
Tel.: +1- 704- 394- 9474 Fax.: +1-704- 392- 8507
Medium Pressure Extrusion (Pelleting)
Andritz, Inc. (Sprout-Waldron-Bauer-Matador) 35 Sherman Street Muncy, PA 17756, USA
Tel.: +1-570- 546- 8211 Fax.: +1-570- 546- 1306
Biihler AG CH-9240 Uzwil, Switzerland
Tel.: +41- (0)71- 955- 1111 Fax.: +41- (0)71-955- 3379
Buhler Inc. Box 9497 Minneapolis, M N 55440, USA
Tel.: +1-612- 545-1401 Fax.: +1-612- 540- 9296
California Pellet Mill Co. 1114 E. Wabash Avenue Crawfordsville, IN 47933, USA
Tel.: +1- 765- 362- 2600 Fax.: +1- 765- 362- 7551
CPM (California Pellet Mill) Roskamp Champion 2975 Airline Circle Waterloo, IA 50703, USA
Tel.: +1-319- 232- 8444 Fax.: +1-319- 236- 0481
Amandus Kahl Nachf. Dieselstr. 5 / P.O. Box 1246 D-21465 Reinbek b. Hamburg, Germany In USA see: LCI, Corp.
Tel.: +49- (0)40- 72771- 0 Fax.: +49- (0)40- 72771- 100
High Pressure Extrusion (Ram Presses, Extruders)
Biihler AG CH-9240 Uzwil, Switzerland
Tel.: +41- (0)71-955- 1111 Fax.: +41- (0)71-955- 3379
Buhler Inc. Box 9497 Minneapolis, M N 55440, USA
Tel.: +1- 612- 545- 1401 Fax.: +1-612- 540- 9296
14.1 List of Vendors
The Bonnot Co. 1520 Corporate Woods Pkwy. Uniontown, OH 44685, USA
Tel.: +1- 330- 896- 6544 Fax.: +1- 330- 896- 0822
ENTEX Fust & Mitschke GmbH Heinrichstr. 67 D-44805 Bochum, Germany
Tel.: +49- (0)234-85636 Fax.: +49- (0)234-85638
Handle GmbH Industriestr. 47 D-75417 Muhlacker, Germany
Tel.: +49- (0)7041- 891- 1 Fax.: +49- (0)7041-821- 232
Krupp Werner Sr Pfleiderer 663 East Crescent Avenue Ramsey, NJ 07446, USA
Tel.: +1-201- 372- 6300 Fax.: +1-201- 825- 6494
Krupp Fordertechnik GmbH Altendorfer Str. 120 D-45143 Essen, Germany
Tel.: +49- (0)201-828- 04 Fax.: +49- (0)201-828- 2566
Svedala Lindemann GmbH Erkrather Str. 401 D-40231 Dusseldorf, Germany
Tel.: +49- (0)211- 2105- 0 Fax.: +49- (0)211- 2105- 376
List AG CH-4422 Arisdorf, Switzerland
Tel.: +41- (0)61- 811- 3000 Fax.: +41- (0)61- 811- 3555
List, Inc. 42 Nagog Park Action, MA 01720, USA
Tel.: +1-978- 635- 9521 Fax.: +1-978- 263- 0570
Readco Manufacturing, Inc. 901 S. Richland Avenue York, PA 17405-0552, USA
Tel.: +1- 717- 848- 2801 Fax.: +1- 717- 848- 2811
Spanex BHSU Luft- und Umwelttechnik GmbH Otto-Brenner-Str. 6 D-37170 Uslar, Germany
Tel.: +49- (0)5571-304- 0 Fax.: +49- (0)5571- 304- 111
J.C. Steele & Sons, Inc. 715 S. Mulberry Street, Box 951 Statesville, NC 28677, USA
Tel.: +1-704- 872- 3681 Fax.: +1- 704- 878- 0789
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Theysohn Maschinenbau GmbH J.-F. Kennedy Str. 48 D-38228 Salzgitter, Germany
Tel.: +49- (0)5341-551- 110 Fax.: +49- (0)5341-551- 177
ZEMAG GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441-880- 205 Fax.: +49- (0)3441- 212993
High-pressure Agglomeration (Punch-and-Die, Tabletting, Isostatic)
AIP American Isostatic Presses, Inc. 1205 South Columbus Airport Road Columbus, OH 43207, USA
Tel.: +1-614- 497- 3148 Fax.: +1-614- 497- 3407
Best Press Corp. 102 Crowatan Road, Northside Industrial Park Castle Hayne, NC 28429, USA
Tel.: +1-910- 675- 2429 Fax.: +1- 910- 675- 1395
CAPPlus Technologies, Inc 21622 N. 7th Avenue #7 Phoenix, AZ 85027, USA
Tel.: +1-623- 582- 2800 Fax.: +1- 623- 582- 4099
Carver Inc. 1569 Morris Street Wabash, IN 46992-0544, USA
Tel.: +1- 219- 563- 7577 Fax.: +1- 219- 563- 7625
GEI Courtoy N.V. Bergensesteenweg 186 B-1500 Halle, Belgium
Tel.: +32- (0)2- 3638300 Fax.: +32- (0)2-3560516
Dorst Maschinen & Anlagenbau GmbH & Co., KG Tel.: +49- (0)8851- 188- 0 Mittenwalder Str. 61 Fax.: +49- (0)8851- 188- 310 D-82431 Kochel a. See, Germany Elizabeth Carbide Die Co., Inc. 601 Linden Street, PO Box 95 McKeesport, PA 15135, USA
Tel.: +1-412- 751- 3000 Fax.: +1-412- 754- 0755
Elizabeth Carbide Europe NV Av. du roi Albert 134 B-1082 Bruxelles, Belgium
Tel.: +32- (0)2- 46900- 30 Fax.: +32- (0)2-46900- 15
74.1 List of Vendors
Elizabeth - Hata International, Inc. 14559 Route 30, 101 Peterson Drive North Huntingdon, PA 15642, USA
Tel.: +1-412- 829- 7700 Fax.: +1-412- 829- 9345
EPSI, Engineered Pressure Systems, Inc. 165 Ferry Road Haverhill, MA 01835, USA
Tel.: +1- 978- 469- 8280 Fax.: +1-978- 373- 5628
EPSI, Engineered Pressure Systems International NV Tel.: +32- (0)3-711- 2464 Walgoed Straat 19 B-9140 Temse, Belgium Fax.: +32- (0)3-711- 1870 Wilhelm Fette GmbH Grabauer Str. 24 D-21484 Schwarzenbek, Germany
Tel.: +49- (0)4151- 12-0 Fax.: +49- (0)4151- 3797
Fette America, Inc. 400 Forge Way Rockaway, NJ 07866, USA
Tel.: +1-973- 586- 8722 Fax.: +1-973- 586- 0450
Flow Pressure Systems AB SE-721 66 Vasteris, Sweden
Tel.: +46- 21- 32- 7000 Fax.: +46- 21- 14- 1817
Flow Autoclave Systems, Inc. 3721 Corporate Drive Columbus, OH 43231, USA
Tel.: +1-614- 891- 2732 Fax.: +1- 614- 891- 4568
Gasbarre Products, Inc. 590 Division Street Dubois, PA 15801, USA
Tel.: +1- 814- 371- 3015 Fax.: +1-814- 371- 6387
Horn & Noack, Pharmatechnik ROMACO GmbH Am Heegwald 11 D-76229 Karlsruhe, Germany
Tel.: +49- (0)721-4804- 0 Fax.: +49- (0)721-4804- 225
I.M.A. SPA Via Emilia 428-442 1-40064 Ozzano Emilia (BO), Italy
Tel.: +39- (0)51- 651- 4111 Fax.: +39- (0)51- 651- 4666
Key International, Inc. 480 Route 9 Englishtown, NJ 07726, USA
Tel.: +1- 201- 536- 1500 Fax.: +1- 201- 972- 2630
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14 fndexes
Kilian & Co., GmbH Emdener Str. 10 D-50735 Koln, Germany
Tel.: +49- (0)221- 7174- 02 Fax.: +49- (0)221- 7174- 110
Kilian & Co, Inc. 415 Sargon Way, Unit 1 Horsham, PA 19044, USA
Tel.: +1- 215- 957- 1871 Fax.: +1-215- 957- 1874
Kikusui Seisakusho Ltd. 104, Minamikamiai-cho Nishinokyo, Nakagyo-ku Kyoto, 604, Japan
Tel.: +81-(0)75- 841- 6326 Fax.: +81- (0)75- 803- 2077
KOMAGE Gellner GmbH & Co. Maschinenfabrik KG Dr. Hermann-Gellner Str. 1 D-54427 Kell am See, Germany
Tel.: +49- (0)6589-9142- 0 Fax.: +49- (0)6589- 9142- 19
Korsch Pressen GmbH Breitenbachstr. 1 D-13509 Berlin, Germany
Tel.: +49- (0)30-43576- 0 Fax.: +49- (0)30-43576- 350
Krupp Fordertechnik GmbH Altendorfer Str. 120 D-45143 Essen, Germany
Tel.: +49- (0)201-828- 04 Fax.: +49- (0)201-828- 2566
Laeis Bucher GmbH Schiffstr. 3 D-54293 Trier, Germany
Tel.: +49- (0)651- 9492- 0 Fax.: +49- (0)651- 9492- 200
BWI Manesty Evans Road, Speke Liverpool L24 9LQ, Great Britain
Tel.: +44- (0)151-486- 1972 Fax.: +44- (0)151-486- 5639
Pentronix, Inc. (PTX) 1737 Cicotte Lincoln Park, MI 48146, USA
Tel.: +1- 313- 388- 3100 Fax.: +1- 313- 388- 9171
Pneumafill P.O. Box 16348 Charlotte, NC 28297-8804, USA
Tel.: +1- 704- 399- 7441 Fax.: +1- 704- 393- 2758
14.1 List of Vendon
Riva S.A. Libertador San Martin 431 1702 Ciudadela, Pcia. Buenos Aires, Argentina in USA: SMI P.0. Box 219 Whitehouse, NJ 08888, USA
Tel.: +1-908- 534- 1500 Fax.: +1-908- 543- 1546
Ruf GmbH & Co KG Tussenhausener Str. 6 D-86874 Zaisertshofen, Germany
Tel.: +49- (0)8268-9090- 0 Fax.: +49- (0)8268-9090- 90
SAMA Machinenbau GmbH Schillerstr. 21 D-95136 Weissenstadt, Germany
Tel.: +49- (0)9253-8890 Fax.: +49- (0)9253- 1079
Sejong Machinery Co., Ltd. 159-11 Dodang Dong, Wonmi-Gu Buchun-City, Kyunggi-Do, Korea 421-130
Tel.: +82- 32- 672- 781112 Fax.: +82- 32- 672- 7813
DT Industries, Stokes Division 1500 Grundy’s Lane Bristol, PA 19007, USA
Tel.: +1-215- 788- 3500 Fax.: +1-215- 781- 1122
Tel.: +54- 1- 653- 870518392 y 488- 5181 Fax.: +54- 1- 441- 7142
Paul-Otto Weber, Maschinen-Apparatebau GmbH Fuhrbachstr. 4 - 6 Tel.: +49- (0)7151- 72600 D-73630 Remshalden, Germany Fax.: +49- (0)7151-72509 High-pressure Agglomeration (Roll)
Alexandewerk AG Kippdorfstr. 6 - 24 D-42857 Remscheid, Germany
Tel.: +49- (0)2191- 795- 216 Fax.: +49- (0)2191-795- 350
Alexandewerk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1-215- 442- 0270 Fax.: +1-215- 442- 0271
Hosokawa BEPEX GmbH ( H U T ) Postfach 1152 / Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131-907- 0 Fax.: +49- (0)7131-907- 301
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, MN 55413, USA
Tel.: +1-612- 627- 1412 Fax.: +1- 612- 627- 1444
I
559
560
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14 Indexes
The Fitzpatrick Company 832 Industrial Drive Elmhurst, I L 60126, USA
Tel.: +1- 630- 530- 3333 Fax.: +1-630- 530- 0832
Gerteis Maschinen- + Processengineering AG Stampfstr. 74 CH-8645 Jona, Switzerland
Tel.: +41- (0)55-212- 1121 Fax.: +41- (0)55-212- 1140
K.R. Komarek, Inc. 1825 Estes Avenue Elk Grove Village, IL 60007, USA
Tel.: +1-847- 956- 0060 Fax.:+1-847- 956- 0157
Maschinenfabrik KOPPERN GmbH & Co. KG Konigsteinerstr. 2 - 12 D-45529 Hattingen/Ruhr, Germany
Tel.: +49- (0)2324- 297- 0 Fax.: +49- (0)2324-207- 207
Koppern Equipment, Inc. 2201 Water Ridge Parkway Charlotte, NC 28217, USA
Ludman Machine Co., LLC. S. 82 W. 18664 Gemini Dr. Muskego, WI 53150, USA Lewis Corporation 15134 West Hunziker Pocatello, ID 83202, USA
Tel.: +1-704- 357- 3322
Fax.:+1- 704- 357- 3350
Tel.: +1-414- 679- 3120 Fax.: +1-414- 679- 9272
Tel.: +1-208- 237- 1314 Fax.: +1- 208- 238- 1834
Matsubo Co., Ltd. 8-21 Toranomon 3-chome Minato-Ku, Tokyo, 105-0001, Japan
Tel.: +81-3- 5472- 1733 Fax.: +81- 3- 5472- 1730
Powtec Maschinen und Engineering GmbH Berghauserstr. 62 D-42859 Remscheid, Germany
Tel.: +49- (0)2191- 389- 194 Fax.: +49- (0)2191- 389- 196
Prater Industries, Inc. 1515 South 55th Court Cicero, I L 60804, USA
Tel.: +1-708- 656- 8500 Fax.: +1-708- 656- 8576
74.7 List of Vendors
Riva S.A. Libertador San Martin 431 1702 Ciudadela, Pcia. Buenos Aires, Argentina in USA: SMI P.O. Box 219 Whitehouse, N J 08888, USA
Tel.: +54- 1- 653- 870518392 y 488- 5181 Fax.: +54- 1- 441-7142 Tel.: +1-908- 534- 1500 Fax.: +1-908- 543- 1546
Sahut Conreur S.A. 700 Rue Corbeau, BP 49 F-59590 Raismes, France
Tel.: +33- (0)3-27- 46 90 44 Fax.: +33- (0)3-27- 29 97 65
Turbo Kogyo Co., Ltd. 2-10, Uchikawa 1-chome Yokosuka-Shi, Kanagawa, 239-0836, Japan
Tel.: +81- (0)468- 36- 4900 Fax.: +81- (0)468- 35- 6516
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax.: +1-319- 377- 5574
ZEMAG GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441-880- 205 Fax.: +49- (0)3441-212993
Sintering
Deltech, Inc. 750 W. 39th Ave. Denver, CO 80216, USA
Tel.: +1- 303- 433- 5939 Fax.: +1- 303- 433- 2809
Eisenmann Maschinenbau KG Postfach 1280 D-71002 Boblingen, Germany
Tel.: +49- (0)7031- 78- 0 Fax.: t49- (0)7031-78- 1000
Eisenmann Corp. USA 150 East Dartmoor Drive Crystal Lake, IL 60014, USA
Tel.: +1- 815- 455- 4100 Fax.: +1- 815- 455- 1018
Fuller Company Member of the F.L. Smidth-Fuller Engineering Group Tel.: +1- 610- 264- G O 1 1 2040 Avenue C Tel.: +1- 610- 264- 6170 Bethlehem, PA 18017-2188, USA
I
561
562
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14 Indexes
Gasbarre Products, Inc. 590 Division Street Dubois, PA 15801, USA
Tel.: +1-814- 371- 3015 Fax.: +1- 814- 371- 6387
Gasbarre Sinterite Furnace Div. 310 State Road St. Marys, PA 15857, USA
Tel.: +1- 814- 834- 2200 Fax.: +1-814- 834- 9335
LsrL Special Furnace Co., Inc. 20 Kent Road Aston, PA 19014-1494, USA
Tel.: +1-610- 459- 9216 Fax.: +1-610- 459- 3689
Lurgi Metallurgie GmbH Ludwig-Erhard-Str. 21 D-61408 Oberursel, Germany
Tel.: +49- (0)69693-0 Fax.: +49- (0)69693- 1234
HED International, Unique/Pereny 449 Route 31 Ringoes, N J 08551, USA
Tel.: +1- 609- 466- 1900 Fax.: +1- 609- 466- 3608
Coating
Aeromatic-Fielder Div. Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, M D 21045, USA
Tel.: +1- 410- 997- 7070 Fax.: +1- 410- 997- 5021
AVEKA, Inc. 2045 Wooddale Drive, Bldg. 553-C Woodbury, MN 55125, USA
Tel.: +1- 651- 730- 1729 Fax.: +1- 651- 730- 1826
Dinnissen bv Horstenveg 66 NL-5975 N B Sevenum, Netherlands
Tel.: +31- 77- 467- 3555 Fax.: +31- 77- 467- 3785
DRIAM Metallprodukt GmbH & Co, KG Aspenweg 19-21 D-88097 Eriskirch a. Bodensee, Germany
Tel.: +49- (0)7541- 9703- 0 Fax.: +49- (0)7541- 9703- 10
Fluid Air, Inc. 2550 White Oak Circle Aurora, I L 60504-9678, USA
Tel.: +1- 630- 851- 1200 Fax.: +1- 630- 851- 1244
74.7 List of Vendors
GS Coating Systems Via Friuli 38/40 1-40060 Osteria Grande (Bologna), Italy
Tel.: +39- (0)51- 94- 6608 Fax.: +39- (0)51- 94- 5624
BWI Hiittlin Daimlerstr. 7 D-79585 Steinen, Germany
Tel.: +49- (0)7627- 9117- 0 Fax.: +49- (0)7627- 8851
IMA Solid Dose Div., Kilian & Co., Inc. 415 Sargon Way, Unit 1 Horsham, PA 19044, USA
Tel.: +1-215- 957- 1871 Fax.: +1-215- 957- 1874
Kaltenbach-Thiiring 9, rue de 1’Industrie F-60000 Beauvais. France
Tel.: +33. 44- 02- 8900 Fax.: +33- 44- 02- 8910
LMC (Latini) International 893 Industrial Drive Elmhurst, IL 60126, USA
Tel.: +1- 630- 834- 7789 Fax.: +1-630- 834- 9473
O’Hara Technologies 65 Skagway Ave. Toronto, Ont. M1M 3T9, Canada
Tel.: +1-416- 265- 1800 Fax.: +1-416- 265- 6658
Sandvik Process Systems, Inc. 21 Campus Road Totowa, NJ 07512, USA
Tel.: +1- 201- 812- 1066 Fax.: +1- 201- 812- 0733
Thomas Engineering, Inc. 575 West Central Road Hoffmann Estates, IL 60195-0198, USA
Tel.: +1- 847- 358- 5800 Fax.: +l-847- 358- 5817
Trybuhl Dragiertechnik GmbH Obere Torstr. 20 D-37586 Dassel-Markoldendorf, Germany
Tel.: +49- (0)5562-91101 Fax.: +49- (0)5562- 91127
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax.: +1- 319- 377- 5574
Melt Solidification
Berndorf Band GesmbH A-2560 Berndorf, Austria
Tel.: +43- (0)2672-2930 Fax.: +43- (0)2672-4176
I
563
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I
14 Indexes
Berndorf ICB, Inc. 820 Estes Ave. Schaumburg, IL 60193, USA
Tel.: +1-847- 891- 8650 Fax.: +1-847- 891- 7563
Gala Industries, Inc. 181 Pauley Street Eagle Rock, VA 24085, USA
Tel.: +1-540- 884- 3160 Fax.: +I- 540- 884- 2310
Goudsche Machinefabriek B.V. Coenecoop 88 NL-2740 AJ Waddinxveen, Netherlands
Tel.: +31- 182- 623723 Fax.: +31- 182- 619217
Gebr. Kaiser, Chem Verfahrenstechnik Magdeburger Str. 17 D-47800 Krefeld, Germany
Tel.: +49- (0)2151-474051 Fax.: +49- (0)2151- 474053
Kaltenbach-Thiiring 9, rue de 1’Industrie F-60000 Beauvais, France
Tel.: +33- 44- 02- 8900 Fax.: +33- 44- 02- 8910
Sandvik Process Systems, Inc. 21 Campus Road Totowa, NJ 07512, USA
Tel.: +1-201- 812- 1066 Fax.: +1-201- 812- 0733
Applications
Babcock-BSH GmbH August Gottlieb Str. 5 D-36222 Bad Hersfeld, Germany
Tel.: +49- (0)6621-81449 Fax.: +49- (0)6621- 81393
Dynamit Nobel GmbH Explosivstoff- und Systemtechnik Forschung und Entwicklung Kronacher Str. 63 D-90765 Fiirth, Germany
Tel.: +49- (0)911- 7930- 0 Fax.: +49- (0)911- 7930- 655
Fleissner GmbH & Co Wolfsgartenstr. 6 D-63329 Egelsbach, Germany
Tel.: +49- (0)6103-401- 0 Fax.: +49- (0)6103-401- 440
Kellogg Co./W.K. Kellogg Institute for Food and Nutrition Research 2 Hamblin Ave. East Tel.: +I- 616- 961- 2000 Battle Creek, MI 49016-3232, USA Fax.: +1-616- 660 6557
14.1 List of Vendors
Norchem Concrete Products, Inc. 985 Seaway Drive Fort Pierce, FL 34949, USA
Tel.: +1- 561- 468- 6110 Fax.: +1-561- 468- 9702
NRS, National Recovery Systems 5222 Indianapolis Boulevard East Chicago, IN 46312, USA
Tel.: +1-219- 397- 0200 Fax.: +1-219- 392- 1419
Puritan Bennet Aero Systems 10800 Pflumm Road Lenexa, KS 66215, USA
Tel.: +1-913- 469- 5400 Fax.: +1-913- 469- 8419
Sintec Keramik GmbH Romantische Str. 18 D-87642 Buching, Germany
Tel.: +49- (0)8368- 9101- 0 Fax.: +49- (0)8368- 9101- 30
TDC Filter Manufacturing, Inc. 1331 S. 55th Court Cicero, IL 60804, USA
Tel.: +1- 708- 863- 4400 Fax.: +1- 708- 863- 4472
Binders
Allied Colloids Cleckheaton Road Low Moor, Bradford West Yorkshire BD12 OJZ, UK
Tel.: +44- (0)124- 41700 Fax.:+44- (0)124-606499
Borregaard Lignotech P.O. Box 31 NL-7213 ZG Gorssel, Netherlands P.O. Box 162 N-1701 Sarpsborg, Norway
Tel.: +31- (0)5759-3488 Fax.: +31- (0)5759-4575 Tel.: +47- (0)6911- 8000 Fax.: +47- (0)6911- 8790
Lignotech USA 100 Highway 51 South Rothschild, WI 54474-1198, USA
Tel.: +1-715- 359- 6544 Fax.: +1-715- 355- 3648
CABOT Corp., Cab-0-Sil Division 700 E. US Highway 36 Tuscola, IL 61953-9643, USA
Tel.: +1- 217- 253- 9643 Fax.: +l-217- 253- 4334
DuPont (UK) Ltd. (ElveronB) P.O. Box 401 Wilton, Middlesbrough TS6 8JJ,England
Tel.: +44- (0)1642-445521 Fax.: +44- (0)1642-445510
I
565
566
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14 Indexes
FMC Corp., Pharmaceutical Division 1735 Market Street Philadelphia, PA 19103, USA
Tel.: +1-215- 299- 6534 Fax.: +1-215- 299- 6821
GPC,Grain Processing Corp. 1600 Oregon Street Muscatine, IA 52761, USA
Tel.: +1- 319- 264- 4265 Fax.: +1-319- 264- 4289
Green Wood Canada, Inc. 239 Russel Street, P.O. Box 2559 Sturgeon Falls, Ontario POH 2G0, Canada
Tel.: +1- 705- 753- 2822
Hoogovens Technical Services Wenkebachstraat 1 1951 J Z Velsen Noord, P.O. Box 10.000 NL-1970 CA Ijmuiden, Netherlands
Fax.:+1-705- 753- 1270
Tel.: +31- (0)2514- 97847
Fax.:+31- (0)2514- 70030
Koch Minerals Company P.O. Box 2219 Wichita, KS 67201-2219, USA
Tel.: +1-316- 832- 6662 Fax.: +1-316- 832- 8028
Penwest Pharmaceuticals, Mendell 2981 Rt. 22 Patterson, NY 12563-9970, USA
Tel.: +1-914- 878- 3414 Fax.:+1-914- 878- 3484
J. Rettenmaier & Sohne GmbH & Co. Faserstoff-Werke Holzmuhle 1 D-73494 Rosenberg, Germany
Tel.: +49- (0)7967- 152- 0 Fax.: +49- (0)7967- 152- 222
J. Rettenmaier USA LP Manufacturers of Fibers 16369 US Hwy. 131 Schoolcraft, MI 49087, USA
Tel.: +1- 616- 679- 2340 Fax.: +1-616- 679- 2364
Reed Lignin, Inc. 100 Highway 51 South Rothschild, WI 54474-1198, USA
Tel.: +1-715- 359- 6544 Fax.: +1- 715- 355- 3648
RDE, Inc. 101 N. Virginia St. Crystal Lake, IL 60014, USA
Tel.: +1-815- 459- 0470 Fax.: +1-815- 439- 8043
14.1 List of Vendors
RIBTEC Ribbon Technology Corp. P.O. Box 30758 Gahanna, OH 43230, USA Schuurmans & van Ginneken Keizersgracht 534 NL-1017 EK Amsterdam, Netherlands Wyo Ben, Inc. 3044 Hesper Road, P.O.Box 1979 Billings, Montana 59103, USA
Tel.: +1-614- 864- 5444 F a . : +1-614- 864- 5305
Tel.: +31- 20- (0)626-0711
Tel.: +1-406- 652- 6351 Fax.: +1-406- 656- 0748
Test Equipment and Peripherals
AC Compacting LLC 1577 Livingston Ave. North Brunswick, NJ 08902-7266, USA
Tel.: +1- 732- 249- 6900 Fax.: +1-732- 249- 6909
Schenck AccuRate Corp. 746 E. Milwaukee St. Whitewater, WI 53190, USA
Tel.: +1-262- 473- 2441 Fax.: +1- 262- 473- 4384
Aeromatic-Fielder Div. Niro Inc. 9165 Rurnsey Rd. Columbia, M D 21045, USA
Tel.: +1-410- 997- 7070 Fax.: +1-410- 997- 5021
Alexanderwerk AG Kippdorfstr. 6 - 24 D-42857 Rernscheid, Germany
Tel.: +49- (0)2191- 795- 216 FLU.: +49- (0)2191- 795- 350
Alexanderwerk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1- 215- 442- 0270 Fax.: +1-215- 442- 0271
API Amherst Instruments, Inc. Mountain Farms Technology Park Hadley, MA 01035-9547, USA
Tel.: +1-413- 586- 2744 Fax.: +1-413- 585- 0536
Babcock-BSH GmbH August-Gottlieb-Str. 5 D-36222 Bad Hersfeld, Germany
Tel.: +49- (0)6621- 81449 F a . : +49- (0)6621- 81393
1
567
568
I
14 Indexes
Hosokawa BEPEX GmbH (HUTT) Postfach 1152 / Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131- 907- 0 Fax.: +49- (0)7131- 907- 301
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, MN 55413, USA
Tel.: +1-612- 627- 1412 Fax.: +1- 612- 627- 1444
Brabender Technology 6500 Kestrel Road Mississauga, Ont. L5T 1Z6, Canada
Tel.: +1- 905- 670- 2933 Fax.: +1- 905- 670- 2557
Bristol Equipment Co. 210 Beaver Street Yorkville, IL 60560-0696, USA
Tel.: +1- 630- 553- 7161 Fax.: +1-630- 553- 5981
Biihler AG CH-9240 Uzwil, Switzerland
Tel.: +41- (0)71-955- 1111 Fax.: +41- (0)71-955- 3379
Buhler Inc. Box 9497 Minneapolis, M N 55440, USA
Tel.: +1-612- 545- 1401 Fax.: +1-612- 540- 9296
Carrier Vibrating Equipment, Inc. Box 37070 Louisville, KY 40233, USA
Tel.: +1- 502- 969- 3171 Fax.: +1- 502- 969- 3172
Chatillon Products, Ametek, Inc. 8600 Somerset Drive Largo, FL 33773, USA
Tel.: +1- 813- 536- 7831 Fax.: +1-813- 539- 6882
Carver, Inc. 1569 Morris Street Wabash, IN 46992-0544, USA
Tel.: +1- 219- 563- 7577 Fax.: +1- 219-563- 7625
Derrick Manufacturing Corp. 590 Duke Road Buffalo, NY 14225, USA
Tel.: +1- 716- 683- 9010 Fax.: +1-716- 683- 4991
Despatch Industries P.O. Box 1320 Minneapolis, M N 55440-1320, USA
Tel.: +l-612- 781- 5363 Fax.: +1-612- 781- 5353
14.1 List of Vendors
Dings Magnetic Group 4740 W. Electric Avenue Milwaukee, WI 53219-9990, USA
Tel.: +1-414- 672- 7830 Fax.: +1-414- 672- 5354
EI, Eastern Instruments 416 Landmark Drive Wilmington, NC 28412, USA
Tel.: +1- 910- 392- 2490 Fax.: +1-910- 392- 2123
Eriez Magnetics 2200 Asbury Road Erie, PA 16508, USA
Tel.: +1-814- 835- 6000 Fax.: +1- 814- 838- 4960
Erweka GmbH Ottostr. 20- 22 D-63150 Heusenstamm, Germany
Tel.: +49- (0)6104- 6903- 0 Fax.: +49- (0)6104- 6903- 40
Erweka Instrument, Inc. 56 Quirk Rd. Milford, CT 06460, USA
Tel.: +1- 203- 877- 8477 Fax.: +1-203- 874- 1179
Flexicon Corp. 1375 Stryker's Road Phillipsburg, NJ 08865-5269, USA
Tel.: +1-908- 859- 4700 Fax.: +1-908- 859- 4826
Flexicon Europe Ltd 89 Lower Herne Road
Herne, Herne Bay, Kent CT6 7PH, England
Tel.: +44- (0)1227- 374710 Fax.: +44- (0)1227- 365821
Flottweg GmbH (Member of the Krauss-Maffei Group) Industriestr. 6-8 D-84137 Vilsbiburg, Germany
Tel.: +49- (0)8741- 301- 0 Fax.: +49- (0)8741- 301- 300
Gerteis Maschinen- + Processengineering AG Stampfstr. 74 CH-8645 Jona, Switzerland
Tel.: +41- (0)55- 212- 1121 Fax.: +41- (0)55- 212- 1140
T.J. Gundlach Machine Co. One Freedom Drive Belleville, IL 62226, USA
Tel.: +1-618- 233- 7208 Fax.: +1-618- 233- 6154
Gustafson Sampling Systems, Inc. 7290 Golden Triangle Drive Eden Prairie, M N 55344, USA
Tel.: +1- 612- 941- 1630 Fax.: +1-612- 941- 9371
I
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74 Indexes
Hagglunds Drives, Inc. 2275 International Street Columbus, OH 43228, USA
Tel.: +1- 614- 527- 7400 Fax.: +1-614- 527- 7401
Hardy Instruments 3860 Calle Fortunada San Diego, CA 92123-1825, USA
Tel.: +1-619- 278- 2900 Fax.: +l- 619- 278- 6700
Hi Roller Enclosed Belt Conveyors 5100 W. 12th Street Sioux Falls, SD 57107-0514, USA
Tel.: +1-605- 332- 3200 Fax.: +1-605- 332- 1107
IMI Industrial Magnetics, Inc. 1240 M-75 South Boyne City, MI 49712-0080, USA
Tel.: +1- 231- 582- 3100 Fax.: +1-231- 582- 2704
Intersystems Sampling Systems 17330 Preston Road, Suite 105D Dallas, TX 75252, USA
Tel.: +1- 972- 380- 0791 Fax.: +1- 972- 250- 4135
Kason Corp. 67-71 E. Willow St. Millburn, NJ 07041-1416, USA
Tel.: +1- 973- 467- 8140 Fax.: +l- 973- 258- 9533
Korsch Pressen GmbH Breitenbachstr. 1 D-13509 Berlin, Germany
Tel.: +49- (0)30-43576- 0 Fax.: +49- (0)30-43576- 350
K-Tron America Routes 55 & 553 Pitman, N J 08071-0888, USA
Tel.: +1- 609- 589- 0500 Fax.: +1- 609- 598- 8113
K-Tron Switzerland Industrie Lenzhard CH-7202 Niederlenz. Switzerland
Tel.: +41- 62- 885- 7171 Fax.: +41- 62- 891- 6663
Krauss-Maffei Verfahrenstechnik GmbH Krauss-Maffei-Str. 2 D-80997 Miinchen, Germany
Tel.: +49- (0)89-8899- 0 Fax.: +49- (0)89- 8899- 3299
Krauss-Maffei Corp. 7095 Industrial Road Florence, KY 41022-6270, USA
Tel.: +1- 859- 283- 0200 Fax.: +1- 859- 283- 1878
14.1 List of Vendors
MP Machine and Process Design, Inc. 820 McKinley Street Anoka, MN 55303, USA
Tel.: +1-763- 427- 9991 Fax.: +1- 763- 427- 8777
Mark-10 Corp. 458 West John Street Hicksville, NY 11801, USA
Tel.: +1- 516- 822- 5300 Fax.: +1-516- 822- 5301
Hosokawa Micron Powder Systems 10 Chatham Rd. Summit, NJ 07901, USA
Tel.: +1-908- 273- 6360 Fax.: +1-908- 273- 7432
Minox Siebtechnik GmbH Interpark D-76877 OffenbachlQueich, Germany
Tel.: +49- (0)6348-9828- 0 Fax.: +49- (0)6348-4086
Minox/Elcan Industries, Inc. 59 Plain Avenue New Rochelle, NY 10801, USA
Tel.: +1-914- 235- 0161 Fax.: +1-914- 654- 9835
Modern Process Equipment, Inc. 3125 South Kolin Ave. Chicago, IL 60623, USA
Tel.: +1- 312- 254- 3929 Fax.: +1-312- 254- 3935
Monitor Manufacturing, Inc. 44W320 Keslinger Road Elburn, IL 60119-8048, USA
Tel.: +1-630- 365- 9403 Fax.: +1-630- 365- 5646
Natoli Engineering Co., Inc., Tableting Accessories 28 Research Park Circle Tel.: +1- 314- 926- 8900 St. Charles, MO 63304, USA Fax.: +l-314- 926- 8910 Nerak Systems, LP 6 Debbie Lane Cross River, NY 10518, USA
Tel.: +1-914- 763- 8259 Fax.: +1-914- 763- 9570
Nicolet Instrument Corp. 5225 Verona Road Madison, WI 53711-4495, USA
Tel.: +1-608- 276- 6100 Fax.: +1-608- 273- 5046
Nordberg Group P.O.Box 307 33101 Tampere, Finland
Tel.: +358- 20- 484- 140 Fax.: +358- 20- 484- 141
I
571
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74 Indexes
Nordberg Americas 3073 South Chase Avenue Milwaukee, WI 53207, USA
Tel.: +1-414- 769- 4300 Fax.: +1-414- 769- 4730
Particle Characterization Measurements Div. of Business Systems International, Inc. 453 Highway 1 West Iowa City, IA 52246, USA
Tel.: +1- 319- 354- 5889 Fax.: +1-319- 354- 0526
Pennsylvania Crusher Co. GOO Abbott Drive Broomall, PA 19008-0100, USA
Tel.: +1-610- 544- 7200 Fax.: +1-610- 543- 0190
PMI Porous Materials, Inc. Cornell Business & Technology Park 83 Brown Rd. Ithaca, NY 14850, USA
Tel.: +1-607- 257- 5544 Fax.: +1-607- 257- 5639
Quadro, Inc. 55 Bleeker Street Millburn, NJ 07041-1414, USA
Tel.: +1-973- 376- 1266 Fax.: +1-973- 376- 3363
Quantachrome Corp 1900 Corporate Drive Boynton Beach, FL 33426, USA
Tel.: +1-561- 731- 4999 Fax.: +1- 561- 732- 9888
Quantachrome GmbH Rudolf-Diesel Str. 12 D-85235 Odelzhausen, Germany
Tel.: +49- (0)8134- 9324- 0 Fax.: +49- (0)8134- 9324-25
The Rapat Corp. 919 O’Donnell Street Hawley, M N 56549-4310, USA
Tel.: +1-218- 483- 3344 Fax.: +1-218- 483- 3535
Rhewum GmbH Rosentalstr. 24 D-42899 Remscheid, Germany
Tel.: +49- (0)2191- 98306- 0 Fax.: +49- (0)2191- 51840
Rotex, Inc. 1230 Knowlton Street Cincinnati, OH 45223, USA
Tel.: +1- 513- 541- 1236 Fax.: +1- 513- 541- 4888
Russel Finex Ltd. Russel House, Browells Lane Feltham, Middlesex TW13 7EW, England
Tel.: +44- (0)181-818- 2000 Fax.: +44- (0)181-818- 2060
14.7 List of Vendon I 5 7 3
Russel Finex, Inc. 10709-AGranite Street Charlotte, NC 28273,USA
Tel.: +1- 704-588-9808 Fax.: +1- 704-588-0738
Carl Schenck AG D-64273Darmstadt, Germany
Tel.: +49-(0)615132-0 Fax.: +49-(0)6151-32-1100
Dr. Schleuniger Pharmatron AG Schongriinstrasse 27 C H -450 1 Solothurn, Switzerland
Tel.: +41-(0)32624-4080 Fax.: +41-(0)32624-4088
Dr. Schleuniger Pharmatron, Inc. One Sundial Avenue, Suite 214 Manchester, NH 03103,USA
Tel.: +1- 603-645-6766 Fax.: +1- 603-645-6726
Sepor, Inc. P.O. Box 578 Wilmington, CA 90748,USA
Tel.: +1-310-830-GGOl Fax.: +1- 310-830-9336
Shimadzu Scientific Instruments, Inc. 7102 Riverwood Drive Columbia, M D 21046,USA
Tel.: +1- 410-381-1227 Fax.: +1-410-381-1222
Simpson Technologies Corp.
751 Shoreline Drive Aurora, IL 60504-6194, USA
Tel.: +1- 630-978-0044 Fax.: +1-630-978-0068
Simpson Technologies Baarerstrasse 77 CH-6300 Zug, Switzerland
Tel.: +41-(0)41711-1555 Fax.: +41-(0)41711-1387
GR Sprenger Engineering, Inc. 736 West Hemlock Circle Louisville, CO 80027,USA
Tel.: +1- 303-665-7069 Fax.: +1- 303-665-5346
S.S.T. Schiittguttechnik Lechwiesenstr. 21 D-86899Landsberg am Lech, Germany
Tel.: +49-(0)8191-335951 Fax.: +49-(0)8191-335955
Stedman Machine Co. P.O. Box 299 Aurora, I N 47001,USA
Tel.: +1-812-926-0038 Fax.: +1- 812-926-3482
574
I
14 Indexes
SVS Sauk Valley Systems, Inc. P.O. Box. 1013 Sterling, IL 61081, USA
Tel.: +1- 815- 625- 5573 Fax.: +1- 815- 625- 5593
SWECO 8029 US Hwy 25 Florence, KY 41022-1509, USA
Tel.: +1-606- 283- 8400 Fax.:+1-606- 283- 8469
Tecnetics Industries, Inc. 1811 Buerkle Road St. Paul, MN 55110, USA
Tel.: +1- 612- 777- 4780 Fax.: +1- 612- 777- 5582
W.S. Tyler 8570 Tyler Boulevard Mentor, OH 44060, USA
Tel.: +1-440- 974- 1047 Fax.:+1-440- 974- 0921
W.S. Tyler Germany Ennigerloher Str. 64 D-59302 Oelde, Germany
Tel.: +49- (0)2522- 30- 0 Fax.: -1-49-(0)2522-30- 404
Unitrac Corp. Ltd. Box 330, 299 Ward Street Port Hope, Ontario L1A 3W4, Canada
Tel.: +1-905- 885- 8168 Fax.: +1-905- 885- 2614
Paul-Otto Weber, Maschinen-Apparatebau GmbH Tel.: +49- (0)7151-72600 Fuhrbachstr. 4 - 6 Fax.: +49- (0)7151-72509 D-73630 Remshalden, Germany ZEMAG GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441-880- 205 Fax.: +49- (0)3441-212993
Organizations
Institute for Briquetting and Agglomeration (IBA) 721 Indian Springs Lane Tel.: +1- 847- 229- 6126 Buffalo Grove, IL 60089-1403, USA Fax.: +1- 847- 541- 8947 Institute of Coal Preparation (IOTT) Panki, District of Lubertzi Moscow oblast 14004, Russia
14.1 List of Vendors
Institute “Mechanoz Engineering” 2/linia, dom 8-a St. Peterburg B-26, Russia Metal Powder Industries Federation (MPIF) 106 College Road East Princeton, N] 08540-6692, USA
Tel.: +1-609- 452- 7700 Fax.: +1-609- 987- 8523
PennState, The Pennsylvania State University, College of Engineering Materials Characterization, P/M Lab. 118 Research West Tel.: +1- 814- 863- 6809 University Park, PA, 16802-6809, USA Fax.: +1-814- 863- 8211 Particle Technology Forum (PTF) American Institute of Chemical Engineers (AIChE) 3 Park Avenue Tel.: +1-212- 591- 8100 Fax.: +1-212- 591- 8888 New York, NY 10016, USA Society for Mining, Metallurgy, and Exploration, Inc. (SME), (Member American Institute of Mining, Metallurgical, and Petroleum Engineers [AIME]) P.O. Box 625002 Tel.: +1- 303- 973- 9550 Littleton, CO 80162-5002, USA Fax.: +1-303- 973- 3845 University of Florida Engineering Research Center for Particle Science and Technology 418 Weil Hall, Box 116135 Tel.: +1- 352- 846- 6135 Gainesville, FL 32611-6135, USA Fax.: +1-352- 846- 1196 (A National Science Foundation Engineering Research Center for Particle Science & Technology depending on active industry support through its Industrial Partners Program [IPP]). Verfahrenstechnische Gesellschaft (VTG) Verein Deutscher Ingenieure (VDI) Graf Recke Str. 84 D-40239 Diisseldorf, Germany
Tel.: +49- (0)211- 6214- 600 Fax.: +49- (0)211- 6214- 169
Tollers
(Out-sourcing by contract manufacturing, co-manufacturing, and “backup” manufacturing.)
(Note: In addition to the companies listed below which specifically ofer at least some contract manufacturing sewices that are related to Size Enlargement by Agglomeration, essentially all manufacturers and suppliers/distributors of equipmentfor the unit operations of Mechanical
I
575
576
I Process Technology and related industrial and analytical techniques 14 Indexes
(see Chapter 1, Fig. 1.1)maintain a more or less extensivefacility and laborutoryfor testing materials, developing
applications, and determining process parameters). Abbott Laboratories, Contract Manufacturing Services Tel.: +1- 847- 937- 1009 1401 Sheridan Road Fax.: +1-847- 938- 2875 North Chicago, IL 60064-6321, USA (Pharmaceuticals: powders and granules, tablettes, etc.)
The ASC Group Tel.: +1- 217- 834- 3301 309 E. Yates St., Box 200 Allertown, IL 61810, USA Fax.: +1- 217- 834- 3655 (Custom (pan) pelletizing, full-scale testing, process design and engineering) AVEKA, Inc., Headquarters, R&D Tel.: +1-612- 730- 1729 2045 Wooddale Dr. (Building 553-C) Fax.: +1- 612- 730- 1826 Woodbury, M N 55125, USA AVEKA Manufacturing, Large-Scale Manufacturing Tel.: +1- 319- 237- 5010 279 Woodward Avenue Fredericksburg, IA 50630, USA Fax.: +1- 319- 237- 5056 AVEKA Foods, Food Processing 106 Bremer Avenue Tel.: +1- 715- 962- 9106 Colfax, WI 54730, USA Fax.: +1- 715- 962- 3129 (Originating from 3M’s Fine Particle Pilot Plant and after adding large-scale manufacturing as well as food processing, AVEKA has become an R&D, service, and smallor large-scale particle processing group. Capabilities include: Agglomeration, blending, classification, compounding, dispersion preparation, granulation, grinding, microencapsulation, particle characterization, particle coating, particle surface modification, prilling, screening, and spray drying) Catalytica Pharmaceuticals P.O. Box 1887 Tel.: +1- 252- 707- 2330 Greenville, NC 27835-1887, USA Fax.: +1- 252- 707- 2450 (Development, scale-up, and large scale manufacturing of complete packaged pharmaceutical products, dosage form includes tablettes and granules.) Coating Place, Inc. 200 Paoli St., P.O. Box 930310 Verona, WI 53593, USA (Coating, encapsulation, fluid bed processing.)
Tel.: +1- 608- 845- 9521 Fax.: +1-608- 845- 9526
14. I List of Vendors
Custom Granular, Inc. 4846 Hwy. 26 North Tel.: +1- 608- 868- 3838 Janesville, WI 53546, USA Fax.: +1-608- 868- 5448 (Roll compaction, briquetting, milling, particle classification, blending) Custom Powders Ltd. Gateway, Crewe Tel.: +44- 1270- 530020 Cheshire, CW1 G Y T , England Fax.: +44- 1270- 500250 Custom Powders BV Grasbeemd 10 Tel.: +31- 492- 598598 NL-5705 DG Helmond, Netherlands Fax.: +31- 492- 598591 (Size enlargement, size reduction, particle separation, dry blending, liquid addition, drying (water), hot melt processes.)
The Dow Chemical Co., Contract Manufacturing Services 100 Larkin Center Tel.: +1- 517- 636- 1000 Fax.: +1- 517- 832- 1465 Midland, MI 48674, USA (Development, scale-up, and manufacturing of products from agricultural, pharmaceutical, and intermediates to specialty chemicals.) Erie Foods International, Inc. 401 Seventh Ave. Erie, IL 61250, USA
Tel.: +1- 309- 659- 2223 Fax.: +1- 309- 659- 2822 (Developing and manufacturing specialty milk proteins for use in the food and pharmaceutical industries; agglomeration at Rochelle, IL, facility.)
Fuller Company, Research & Development Member of the F.L. Smidth-Fuller Engineering Group Tel.: +1-610- 266- 5035 2040 Avenue C Bethlehem, PA 18017-2188, USA F a . : +1-610- 266- 5109 (Crushing/classification, material preparation [including drum conditioners/pelletizers, pans, extruders, compaction/granulation], pyroprocessing [including calcination, high temperature processing, mineral roasting, drying and cooling, reduction], rotary kilns, flash calciners/dryers, physical and chemical laboratories, bulk material handling, pneumatic conveying, etc.) GEA NIRO, Inc. (Food & Dairy Industries) Tel.: +1-715- 386- 9371 1600 O’Keefe Road F a . . : +1- 715- 386- 9376 Hudson, WI 54016, USA (Testing facility and pilot plants for liquid processing, spray drying and agglomeration, evaporation and concentration, product handling and packaging.) Howard Industries, Inc. 1840 Progress Avenue Columbus, OH 43207, USA
Tel.: +1-614- 444- 9900 Fa.: +1- 614- 444- 4571
I
577
578
I
14 Indexes
(Custom processing includes blending, milling, classification, agglomeration [with pans, pin mixers, tablet presses, roller presses, extruders], calcining, centrifuging, double arm mixing, compounding, flaking, drying, packaging.) IFP, Inc. Tel.: +1- 507- 334- 2730 2125 Airport Drive, Hwy. 21 & 1-35 Fax.: +1-507- 334- 7969 Faribault, MN 55021-7798, USA (Contract food processing and packaging, including spray drying, agglomeration, particle coating/encapsulation, instantizing, milling, blending.)
International Processing Corp., Member of the GLATT Group Tel.: +1-859- 745- 2200 1100 Enterprise Drive Fax.: +1-859- 745- 6636 Winchester, KY 40393-9888, USA Glatt Air Techniques, Ramsey, N J , USA IPC Processing Center, Dresden, Germany (Blending and granulating, tabletting, extrusion and spheronizing, coating.) L. Robert Kimball & Associates, Bituminous Coal Research Facility Tel.: +1-814- 472- 7700 615 W. Highland Ave. Fax.: +1-814- 472- 7712 Ebensburg, PA 15931, USA (Processing and briquetting of coal.) K.R. Komarek Briquetting Research, Inc. Tel.: +1- 256- 831- 5741 20 Wm. F. Andrews Drive Fax.: +1-256- 831- 1331 Anniston, AL 36207, USA (Roller press briquetting and compaction/granulation.)
Materials Processing Technology, Inc. Tel.: +1-973- 279- 4132 95 Prince Street Fax.: +1-973- 279- 4435 Paterson, NJ 07501, USA (Agglomeration, Coating, Encapsulation, Granulation, Mixing/Blending, Screening/ Classifying). M.I.E. (Marietta Industrial Enterprises, Inc.) Tel.: +1- 740- 373- 2252 Rt. 4, BOX 179-1A Fax.: +1- 740- 373- 6369 Marrietta, OH 45750, USA (Custom crushing, grinding, milling, screening, and roll briquetting.) Metrics, Inc. Tel.: +1-252- 752- 3800 1240 Sugg Parkway Tel.: +1- 252- 757- 2573 Greenville, NC 27834, USA (Contract and manufacturing services for the pharmaceutical industry, including encapsulation, fluid bed processing, granulation, mixing/blending, etc.)
14.1 List of Vendors
Quintiles Tel.: +1-818- 767- 3900 10245 Hickman Mills Drive Kansas City, MO 64137, USA Fax.: +I-818- 767- 3950 Research Avenue South Tel.: +44- 131- 451- 2074 Riccarton, Edinburgh EH1 4AP, UK Fax.: +44- 131- 451- 2063 (Contract and manufacturing services for the pharmaceutical industry, includes wet granulation, direct compression, fluid bed processing, film coating, encapsulation, bead manufacture, mixing/blending, etc.) R.Tech (Results Technology) 4001 Lexington Ave. N. Arden Hills, MN 55126, USA
Tel.: +1-612- 481- 2207 Fax.: +1-612- 486- 0837 (Technical, analytical, development, and manufacturing services for the food industry, including, among many others, spray drying, agglomeration, instantizing, extrusion, dry blending, etc.)
J. Rettenmaier & Sohne GmbH & Co. Contract Service Holzmuhle 1 Tel.: +49- (0)7967- 152- 0 D-73494 Rosenberg, Germany Fax.: +49- (0)7967- 152- 222 (Mixinglhomogenizing, sifting/air classification, grinding/cryogenic grindinglpulverizing, dryinglconditioning, encapsulating/coating, agglomerating/compacting/ granulating/pelleting, filling/refilling.) ScheringPlough Third Party Business Tel.: +1-908- 629- 3200 1095 Morris Avenue Union, NJ 07083-7137, USA Fax.: +1- 908- 629- 3164 (Pharmaceutical tabletted products at Kenilworth, N J: granulation and blending, compression and coating, in-process testing.) Stellar Manufacturing Co. Tel.: +1-618- 337- 4747 1647 Sauget Business Blvd. Sauget, IL 62206, USA Fax.: +1-618- 337- 0003 (Blending, compacting, granulating, milling, sizing, tabletting, pouching, bagging, packaging.) Svedala Industries, Inc., Process Research and Test Center (PRTC) Tel.: +1-414- 762- 1190 9180 Fifth Avenue Fax.: +1-414- 764- 3443 Oak Creek, WI 53154, USA
(Fully equipped facility with the capabilities to perform complex material and process testing and evaluations as well as simulating complete flowsheets that can be assembled to represent a commercial plant with many different unit operations. The test center is designed to perform comminution, agglomeration, and thermal processing studies.)
I
579
580
I
14 Indexes
Toll Compaction Service, Inc. 14 Memorial Drive Tel.: +1- 732- 776- 8225 Fax.: +1-732- 776- 8306 Neptune, NJ 07753, USA (Roll compaction, pan agglomerating, screening, blending, and grinding of pharmaceuticals and chemicals.) Vector Corporation Tel.: +1- 319- 377- 8263 675 44th Street Fax.:+1- 319- 377- 5574 Marion, IA 52302, USA (Agglomeration, coating, encapsulation, fluid bed processing, granulation including roller press compaction/granulation, mixing/blending.) Welch Laboratories, Inc. 4270 Sunnyside Drive Tel.: +1-616- 399- 2711 Holland, MI 49424, USA Fax.: +1-616- 399- 6889 (Compaction services for the pharmaceutical, food, and chemical industries.)
14.2 Wordfinder Index
ChapterlSection
Page
8.3 5.4 10.2.2 5.1.1 5.4 11.2 11.2 11.2 1 7.2 5.2 5.2.2 5.2.2 1 12 7.4.1 8.4.2 5.5 5.2.2 5.1.1 8.4.1 8.4.3
245 105 445 37 102 471 469 490 2 145 55 62 70 2 509 163 299 130 69 38 258 301
A
Abrasion drums Absorbents Adsorption flocculation Adsorption layers Advantages of agglomerated products Aero-Flow Aerosizer Aging Agglomerate Agglomerate growth Agglomerate strength (Definition) Agglomerate strength (Determination) Agglomerate strength in industry Agglomeration Agrochemicals Alternative drum designs Annular gap extruder Anticaking conditioning agents Atomic Force Microscope (AFM) Attraction forces between solid particles Axial extruder Axial high pressure screw extruder
14.2 Wordfinder Index
B
Back-mixed fluidized bed Baffle inserts Bag-set Basic compaction mechanism Basic mechanism of tumble/growth agglomeration Basket extruder Batch sintering Bell type furnace Belt conveyor agglomeration Binder development Binderless agglomeration Binderless pressure agglomeration Binders Binder selection Bridge type additives Briquette Bulk compression stage By-products as binders C Caking Cantilevered shafts and rolls Capillary flow in wet agglomerates Capillary pressure Capping Carbonless copying paper Carriers for catalysts Cat litter Characteristics of single particles Chattering Cheek plates Chemical oxygen generator Chemical reaction Co-agglomerated materials Coal tar pitch Cocking of the floating roller Cold isostatic pressing (CIP) Compaction/granulation
Compressed residual gas Conditioner Conditioning
7.4.4 7.4.2 5.5 8.1 6 8.4.1 9.2.1 9.2.1 10 5.1.2 6 8.1 5 5.1.2 5.1.2 5.1.2 8.4.3 8.2 5.1.2
201 168 123 232 134 255 390 392 412 44 133 231 29 43 43 44 336 237 44
5.5 8.4.3 5.2.2 5.1.1 8.4.3 12 5.4 5.4 5.4 8.4.3 8.4.3 5.4 5.1.1 5.4 8.4.3 8.4.3 8.4.4 8.3 8.4.3 8.1 8.4.2 5.5 8.3
123 363 67 38 328 515 106 106 101 352 347 108 36 103 338 356 375 243 336 233 278 130 242
I
581
582
I
14 Indexes
Contact fluidizer Continuous drum coater Continuous fluidized beds Continuous mechanical pusher furnace Continuous mixer/agglomerators Control of feed volume Convenience foods Conveyor screw Coordination number Coordination points Coupling gears Crystal bridges Crystallization Cut size Cyclone separators
7.4.4 10.1 7.4.4 9.2.2 7.4.2 8.4.3 5.1.2 8.4.1 5.1 5.1 8.4.3 5.1.1 5 5.5 10.2.1
210 419 201 40 1 179 345 47 264 35 35 337 37 29 109 440
8.4.3 5.5 5.1.1 5.1.2 12 5.2.2 5.2.2 5.1.2 5.1.2 8.4.3 5.3.2
345 109 37 47 509 70 71 46 50 321 96 410 410 108 153 50 197 201 338 160 418 417 376 515 234
D
Deaeration paths in roller presses Degree of separation Deposition of suspended colloidal particles Designer foods Designer plant foods Determination of agglomerate strength in industry Determination of product properties in industry Development of lubricants Dietary ballast additives Die wall lubrication system Diffusion path Direct capillary action Direct effect of molecular forces Direct Reduced Iron (DRI) Disc agglomerator Disintegrants Distribution plate Dome extruder Double output-shaft gear reducer Drum agglomerator Drum coaters using paddles Drum coating equipment Dry bag process Drug delivery system Dwell time
10 10
5.4 7.4.1 5.1.2 7.4.4 8.4.1 8.4.3 7.4.1 10.1 10.1
8.4.4 12 8.1
74.2 Wordfinder lndex
E
Easily degradable carrier materials Easily dispersible products Effervescence Efficiency of grinding Ejection press Elastic springback Electrical double layers Electrification assisted controlled particle deposition Electrocoagulators Electrostatically assisted coating process Electrostatic field Electrostatic forces Electrostatic precipitators Elevator type furnace Encapsulation Encapsulation of agglomerates Endpoint of agglomeration Engineered particulate materials Engineered products Entry suction pressure Excess charges Expander Expansion of compressed gas Experimental determination of agglomerate strength Explosives Exter press Extruder with radial discharge
5.4 5.4 5.1.2 5.5 8.4.3 8.1 5.1.1 12 10.2.2 10.1 5.1.1 5.1.1 10.2.1 9.2.1 10.1 5.2.2 7.2 5.4 5.1.2 5.2.2 5.1.1 8.4.2 8.1 5.2.2 5.4 8.4.3 8.4.1
105 104 47 115 315 233 41 521 446 435 41 41 442 392 433 69 148 103 47 67 41 295 234 62 107 307 257
8.4.3 8.4.3 12 10.3 5.1.2 8.4.3 10.1 5.1.2 5.1.2 9.1 8.4.2 11.2 8.4.3 7.4 10.2.2
337 337 508 447 47 315 417 44 42 386 284 490 338 152
F
Feed control by tongues Feeder pan Fertilizer granulation Fiber based filter media Fibers Fill shoe Film coating Film type additives Fine particles First stage of sintering Flat die pelleting machine Flexible intermediate bulk container Floating roller Flocculation
###
I
583
584
I Fluid drum granulator 14 Indexes
Fluidized bed dust collector Fluidized bed technology Fluidized spray dryer (FSD) Food additives Force feeder Formation of a crust Functional components Functional coatings Functional foods Fun foods
10.1 10.2.1 7.4.4 7.4.4 5.1.2 8.4.3 5.2.2 5.1.2 10.1 5.1.2 5.1.2
420 442 196 197 47 338 67 47 41 7 47 47
G Gas dynamic atomization Gear pelletizer Granulation of detergents Grate Kiln Gravity feeding Grinding aids Grinding equilibrium Growth of agglomerates Growth phenomena
7.4.5 8.4.2 12 9.2.2 8.4.3 5.5 5.5 7.2 7.1
214 290 514 405 337 112 115 145 144
5.1.1 5.1.1 8.4.3 8.4.3 5.5 7.4 7.4.2 8.4.4 12 8.4.3 5.1.2 5.3.2 8.4.4 9.2.2 10.1 8.4.3 8.4.3 8.4.3 8.4.3 5.1.1 8.4.4
36 37 300 362 118 152 171 383 515 333 42 99 375 400 439 319 359 319 356 40 375
H
Hardening binders Highly viscous binders High-pressure agglomeration High pressure comminution High pressure roller mill High shear (particle) mixers HIP for making porous products Hollow capsules Horizontal punch-and-die presses Hot densification Hot isostatic pressing (HIP) Hump-back kiln Hybridization Hybrid punch-and-die presses Hydraulic accumulator Hydraulic presses Hydraulic pressurization system Hydrogen bridges Hydrostatic pressing
74.2 Wordfinder Index I585
I
Ice briquetting Immiscible binder agglomeration Immiscible liquid agglomeration Incipient bubbling velocity Incipient buoyancy Influencing the nucleation stage Inherently available binding properties Innovative pan designs Instant products Integrated belt spray dryer Intensifier bar Interdisciplinary approach to process selection interlocking bonds Iron ore pellets Isostatic pressing
10 7.4 7.4 7.4.6 7.4.4 7.4.4 7.1 5.1.2 7.4.1 5.4 7.4.3 7.4.2 11.1 5.1.1 9.2.2 8.4.4
413 153 153 222 197 196 143 43 157 104 196 167 463 41 405 373
7.2
144
5.5 5.1.1 9.1 5.1 8.4.1 8.4.1 8.4.1 7.4 7.4.2 5.1.2
114 38 387 32 253 262 257 152 166 46
8.2 10.1 5.1.1 9.2.1 8.3 5.5 5.4
239 436 41 391 245 123 106
7.2 5.1 5.1.2
146 32 44
K
Kinetics of tumblelgrowth agglomeration L
Limit of grinding Liquid bridges Liquid phase sintering Liquid saturation Low-pressure agglomeration Low pressure flat die extruder Low pressure screw extruders Low shear particle mixers Lubricants M
Macroscopic flow of solid particles Magnetically assisted impaction coating Magnetic forces Manual pusher furnace Mammerizer Mass flow design Materials with controlled reactivity Mathematical modelling of tumblelgrowth agglomeration Matrix binder Matrix forming binder components
586
I
14 Indexes
Maximum pressing force Mechanical activation Mechanical dewatering Mechanical Process Engineering Mechanical Process Technology Mechanical punch drives Mechanism of briquetting in roller presses Mechanofusion Medium-pressure agglomeration Medium pressure axial screw extruder Melt solidification Mesh-belt sintering furnace Metal swarf Microcrystalline cellulose (MCC) Microencapsulation Microwave drying Mill shaft design Misalignment coupling Muffle furnace Multi-tier fluidized beds
8.1
5.5 8.1 1 1 8.4.3 8.4.3 10.1 8.4.2 8.4.2
5 9.2.2 5.1.2 5.1.2 10.1 7.4.2 8.4.3 8.4.3 9.2.1 7.4.4
233 115 234 1 1 319 341 439 266 294 29 400 51 47 43 3 181 363 338 391 207
N
Nan0 technology Natural adhesion of small particles Natural binding mechanisms Near net shape articles Neutral plane New generation of small roller presses Nip area Nonvalence associations Nozzle atomizer Nucleation Nuisance dust 0 Optimum packings Organic fibers Overcompaction
5.1.2 8.4.4 8.4.3 8.4.3 8.4.3 5.1.1 7.4.3 7.1 8.3
101 139 43 373 317 345 338 40 191 141 244
5.3.1 5.1.2 8.4.3
83 47 343
10.2.1 5.3.1 9.2.1 10.3 7.4.1
440 81 396 45 1 157
5.4
7
P
Packed bed filters Packing structure Pan sintering plant Paper making Pans with collars
14.2 Wordfinder Index I587
Parameters determining the properties of agglomerates Partial melting Particle shape Particle signature Particles in bulk Particle size analysis Particle technology Pelleting Pellet mills Pelletization of iron oxides Pile-set Plasma vapor deposition (PVD) Plasticity Plug flow fluidized bed Polygonal coating drum Pore forming additives Pore size analyzer Pores of atomic scale Porosity Porosity of agglomerates Pot grate Powder & Bulk Solids Technology Powder coatings Powder color coatings Powdered Cellulose (PC) Powder flowability analyzer Powder Technology Powder tester Powder Workbench 32 Precision coater Pressure agglomeration Pressure cooker extruders Pressure sintering Pressure swing granulator (PSG) hilling Production of primary agglomerates Product properties in industry Pulsed electric current sintering (PECS) Pyrotechnic articles
5.2.2 5.1.1 5.3.1 5.3.1 5.4 5.3.1 1 8.4.2 8.4.2 5.4 5.5 12 5.1.2 7.4.4 10.1 5.3.2 11.2 5.3.2 5.3.2 5.2.2 9.2.1 10.1 12 5.1.2 11.2 1 11.2 11.2 10.1 8 8.4.2 9.1 7.4.5 5 7.1 5.2.2 5.3.2 5.4
61 36 78 80 102 78 1 266 272 107 125 514 42 202 418 97 471 95 89 61 394 1 415 5 14 50 471 1 469 471 432 229 295 388 219 29 141 71 99 108
8.4.3 5.3.2 5.3.2 9.1
307 98 98 388
1
R
Ram extrusion press Reaction bonding Reaction sintering
588
I
14 Indexes
Reciprocating punch-and-die presses Recombination bonding Recrystallization Recrystallization at the coordination points Recycling of paper Reduction ratio Refractory linings and components Regular packings Relaxation of elastic deformation Release of briquette Representative (particle) equivalent diameter Representative sample Reversed belt agglomerator Rewet agglomeration in fluidized beds Roller hearth furnace Roller presses Roller screen Roll press simulator Rotary atomizer Rotary punch-and-die presses Rotating disc fluidized bed coater Rotating pan coaters Rotating point-force Rotocoat process
8.4.3 5.1.1 5.1.1 5.1.2 10.3 5.5 5.1.2 5.3.1 8.1 8.4.3 5.1.3 11.2 10 7.4.4 9.2.2 8.4.3 7.4.1 11.3 8.4.3 11.2 7.4.3 8.4.3 10.1 10.1 8.4.3 10.1
315 40 37 43 45 1 118 52 81 234 343 80 489 412 205 400 335 163 494 342 485 190 325 430 415 355 421
5.5 11.2 11.2 5.3.1 8.4.3 8.4.3 9.1 11.1
113 489 49 1 85 354 354 386 463 46 455 143 356 108 445 109 405 412
S Sample preparation Sampling Scale-up Scanning Electron Microscopy (SEM) Screw diameter Screw feeder Second stage of sintering Selection considerations Selection of lubricants Selection process Selective agglomeration Self aligning roller bearings Self ignition Sensitization Separation curve Shaft furnace Shaking trough agglomerator
5.1.2 11 7.1 8.4.3 5.4 10.2.2 5.5 9.2.2 10
14.2 Wordfinder Index
Sharpness of separation Sheet thickness Simplified preselection guide Sintering Size enlargement (general) Size enlargement by agglomeration Sinter bridges Sol-gel processes Solid bridges Solids flow meter Solid state sintering Sonic agglomeration Spark plasma sintering (SPS) Specific force Specific surface energy Spherical agglomeration process Spherical crystallization Spheronization Split of each roller Sponge iron Spontaneous combustion Spouted bed Spray dryers Spray pattern Spray systems Spring loaded pressurization system Steam jet agglomeration Stockpile agglomeration Strength of agglomerates Structure of agglomerates Structure of agglomerates obtained by pressure Struct. of agglomerates resulting from growth during tumbling Structure of sinter Surface-active substances Surface texture
5.5 8.4.3 11.1 9 5 1 5.1.1 5.3.2 7.4.6 5.1.1 11.3 9.1 10 5.3.2 8.4.3 9.1 7.4.6 7.4.6 8.3 8.4.3 5.4 5.4 7.4.5 7.4.3 7.4.2 7.4.2 8.4.3 7.4.5 10 5.2.2 5.3 5.3.1 5.3.1 5.3.1
110 340 462 385 29 1 36 99 225 36 502 385 413 99 360 385 223 222 245 345 108 108 220 187 165 165 354 214 41 1 61 76 87
5.3.1
85 89 115 79
7.4.4 3 5.2 12 9.1 8.3
201 5 55 515 386 245
5.5
T
Tall form spray dryer (TFD) Technology of bread making Tensile strength of agglomerates Thermoset coating Third stage of sintering Thixotropic materials
I
589
590
I
14 lndexes
Tolling companies Top spray coaters Transport screw Transversal crushing force Travelling grate sinter machine Tunnel kilns for ceramics Turret
11.2 10.1 8.4.1 5.2.2 9.2.2 9.2.2 8.4.3
49 1 430 265 62 403 397 325
5.4 5
101 29
5.1.1 5.1.1 8.2 8.4.3 8.4.3 7.4.4 5.3.2
40 39 237 336 315 210 412 98
9.2.2 5.1.2 5.1.2 5.1.1 8.4.4 10.2.1 5.1.2 8.4.3 10.1
40 1 44 50 38 376 442 47 319 430
U
Ultra-fine particles (UFPs) Underwater granulation/pelleting V
Valences Van-der-Waals forces Variations in compact density Vertical pug mill Vertical punch-and-die presses Vibrated fluidized bed Vibrating deck agglomerator Vitrification
10
w Walking beam furnace Wastes as binders Water binding and retention capacity of fibers Wet agglomerates Wet bag process Wet scrubbers Wicking by fibers Withdrawal press Wurster coating process
74.3 Subject Index 1591 14.3 Subject Index
a abrasion - resistance 73 - transfer 141 absorbent 105, 451 accellerators 178 accumulator - hydraulic 359 - pressure 358 - - standard 360 action, of the coating material 436 additives 43 - dietary 47, 50 - disappear at high temperature 408 -food 47 - functional 47 - impurities 408 - organic fiber 50 - pore forming 97 - solid pore forming 98 - starches 50 - swelling 47 adhesion - physics GO - permanent 134 adhesion forces 57 - mechanism 439 - natural 42, 229 - van-der-Waals 34, 40, 58, 59, 513 adsorption - charged organic 445 - ion 445 -layer 32, GO - preferential 445 advanced material 515 agglomerate - angular 244 - breakdown 144 -broken 252 -buoyancy 62 - characteristics 70, 243, 409, 479 - completely filled with a liquid 56 - components 29 - density 179 - dispersion 47 - failure 55 - final size 179 - freely accessible surface area 44 - green 136, 140, 150, 241 - growth 183, 442, 494
- growth or tumble, porosity 92 - higher purity 223 - instant properties 511 - level of ultrasound 513 - liquid saturation 32 -mass of 144 - matrix bonded 92 - micro 156 - more stable 443 - optimal granulation 243 - oversized 144, 495 - particle size adjustment 149 - plasticity 245 - porosity 44, 61, 62, 92, 140, 229, 242 - properties 61, 525 - quality 497 - random cut 32, 76 - ration-sized 458 - recrystallizing substance 68 - roller mills 118 - seed 141 - solid bridges 57 - spherical 229, 244 - strength 32, 55, 61, 62, 90, 242, 461 - - determination, industrial method 76 - - in industry 70 - - standardized methods 71 - structure 76 - 100, 300 -volume 62 - wet - - development of strength 67 --drying 68 agglomerated products, characteristics 61 agglomeration -acoustic 413 - ancient technique 8 - applications 9 - - n e w 523 -art 70 - basic phenomen 139 - basic physical effect 3 - batch 143, 460 - beneficial - - pressure 252 - - growth, technology / equipment 151 - binding mechanisms 35, 523 - -models 55 - concept 409 - continuous 143, 460 - controlled 139
592
I
14 Indexes
- during crystallization 222 - efficiency 143 - equipment - - motion 144 - - procurement 455 - - selection 455 - expert 8 - field of science 507 - fundamentals 70, 507, 523 - fundamentals, interdisciplinary application 216 - by heat 94, 95, 137 - history 3, 4 - immiscible binder 141 - immiscible liquid 223 - in the dry state 514 - industrial 389, 492 - innovative 411 - interdisciplinary 8 - liquid system 222 - mechanism 523 - method, preselected 475 - most versatile technique 136, 300, 320 - naturally 5, 109, 507 - on the surface 409 - phenomenom 3, 409 - plant, peripheral equipment / wet 187 - pressure agglomeration (see there) - principle, fundamental 10 - processes 86 - representative particle size 65 - rewet agglomeration 214, 215, 513 - science 3 - secondary 126, 421 - selective 111, 144, 222, 445, 494 - simplified selection guide 462 - single particle 409 - size enlargement 119 - - alternative approach 410 - - methods 468 - - most versatile 229 - - preselection 468 - steam jet 216 - symposia 3 - system - - art of controlling 150 --design 151 - - installation 502 - - necessary part 492 - - start-up 502 - - new 502 - technique - - development 468 - - different 525
- - instant characteristics 513 - - preselection 468 - - special features 525 - - specific characteristics 525 - technology 3, 409, 507 -terms, glossary 11-27 -theories 29-131 - tool configuration 252 - tool to improve powder characteristics 3 - tumble / growth (see there) - undesirable 42 - unit operation 507 - unwanted 5, 36, 176, 508 - - remediation technique 131 agglomerator 151 - drum - - retaining rings 160 - low-pressure 254 - D-K Zig-Zag continuous 169 - p a n 155 - - additional processing 157 - - bottom feeding 160 - - collar 158 - -deep 160 - - growth behavior 156 - - industrial 156 - - large scale 156 - - modifications 157 - - operations 156 - - process variations 157 - - re-roll designs 157 - - rear auger feeder 160 - - rim, / classification effect 153 - - scraper arrangement 156 - - scraper plows 155 - - stepped side wall designs 158 agitator, type 296 agricultural industry 419 agrochemicals 105, 507 - research 508 air conditioning 461 air pockets, compressed 233, 497 air removal 334 airbag chemicals 106 airflow - direction 203 - powder transport 203 alloying, mechanical 375 alloying elements, agglomerated 251 ambient forces 134 ammoniation 167 amorphization 115 angle - of compaction 341
74.3 Subject lndex 1593
- of difference 469 - of fall 469 - ofnip 341 - of release 342 - of repose 469 - of rolling 341 - of spatula 469 animal feed 73, 104, 134,239,277, 278, 283, 289, 295, 373, 510 - medicated 280 - pelleting 507 annual production 460 anticaking agent, cationic 131 anular gap 299 anvil 294 - plate 334 application - development work 364 -small 364 aspiration point 502 ASTMdrum 71 atomization 214 atomizer wheel, peripheral speed 191 autoclave 376 -volume 378 auxiliary materials 461
b baffles 205, 417 bag filter 218 bag-set 126 bagasse 313 baking 8 ball pen tips 224 balling 145 bark 333 basket test 395 batch operations 144 - equipment 147 -vacuum 393 batching system 509 bearing - blocks 358 - life 360 - reshimming 357 - with conical withdrawal sleeves 356 bentonite 408 BET sorptometer 474 binder 43-46, 133, 139, 458, 461, 468 - addition 494 - availability 44 - bridges 44 - cost 44 - development 44
- evaluation 44 -extrusion 305 -films 44 - immiscible bridging liquid 141 - inherent 231 - inherently available 134 - inorganic 44 - liquid 136, 140, 205 - - atomizing 165 - matrix 44 - organic 44 - replacement 44 - supply 44 -viscous 415 - wet granulation 52 binding - characteristics 334 - mechanisms 35, 55 - - adhesion forces (see there) - - destruction 105 - - effects 85 - - field forces 58 --final 151 - - inherently available 42 - - liquid bridge 57 - - means to enhance 42 biomass 234, 309, 333, 334 bitumen 32, 37 bituminous components 231 black core 390 Blaine method 65 blender - cylindrical drum 166 - orbitting type screw 182 - P-K zig-zag continuous 169 -ribbon 176 bloating 386, 390 boiling bed 197 bonding - chemical 457 - criterium 139 - recombination 115 Born repulsion 59 bread, making of 5 breech plug 376 brick 7, 305 bridging 123 briquette 335 - for deep frozen storage 415 - highly densified 466 - inert 466 - large 311 - metal 373 - ration-sized 415
594
I
74 lndexes
- separators 342, 498 - single 310 -thin web 342 briquetting 8 - coal 338, 507 - fine powders 313 - highly elastic organic material 307 -hot 8 - inert elastic material 313 - metal 334 -peat 307 brominated biocide 335 Brownian movement 413, 441 build-up 113, 122, 165, 196 bulk -bags 490 - characteristics 458 - commodity 389, 507 - compression stage 300 - density 87, 188, 234 - - aerated 469 - - feed 331, 353, 358 - - measurement 474 --packed 469 - masses, behavior 101 - original properties 490 - - changes 490 -volume 234 burden preparation plant 404 by-product 492 C
cage mill 500 caking 36, 37, 69, 115 - avoid 128 - fertilizers 123, 127 - laboratory technique 132 - particulate system 131 - temperature variations 127 - test procedure 132 campaigns 460 cams 325 capacity 277, 285, 320, 460 capillary - condensation 38 - flow 67, 190, 433 - forces 38, 136 - pressure 32, 56, 67, 190 - region 223 - state 34 capping 326, 328 capsule 434 carbide tool bits 378 carbides, cemented 381
carbon 380 carbon black 104, 217 card-board 333 carrier liquid, immiscible 435 carrier material 105, 106 cat litter 105, 451 catalyst 106 - carrier 246, 283, 389 cereals, processed 299 cellulose - chemical composition 49 - molecules 49 -powdered 50 - tabletting 50 -wood 47 cement 32, 156 - clinker 118 cementitious reaction 136 centrifugal forces 440 ceramics 94, 242, 319, 380, 388, 390, 402, 507 - applications 320 - high performance 375 -industry 391 -porous 98 cGMP 9, 418 channel (see also extrusion channel) 257, 266, 267 - cooling -flow 122 - length 267, 307 chaos theory 146 characteristics - differences 492 -product 507 - relationship 490 cheek plates 352 chemical 435 chemical reaction 128 chemistry, incompatible 457 chip box 334 chlorinated biocide 335 choppers 177 - mode of operation 179 CIP (cleaning in place) 9, 418, 423 circular thickener / clarifier 443, 444 circumferential speed 364 clam-shelling 343 cleaning 278, 460 - CIP (cleaning in place) 9, 332, 418, 423 cleanliness 332 climatic conditions 461 closed circuit 163 clusters 119
coacervation 435 coal tar pitch 336 coal 231 - briquetting 340, 507 - bulk mass 252 -fines 347 - organic macromolecules 40 - run-of-mine 252 - Soft 312 coalescence 442 - preferential 236 - random / preferential 141 coating 8, 119, 157, 245, 415, 514 - agglomerates 415 - blobs of material 417 - by magnetic attraction 436 - cores 415 - delivery system, material 421 - drum --batch 419 - - continuous 419, 424 - fluidized bed 429 - - bottom-spray 430 - functional 421 - functional properties 417 - hard 514 - harder particle 439 - imperfect 430 - liquid binder 415 - manually applied 415 - mechanism 439 - mechanized 415 - melt 420 - multiphase 514 - multiple layer 514 - nuclei 415 - peening of the material 439 - plant seeds 419 - powder materials 415 - protective 5, 514 - separator 130 - single layer 514 - sorptive capacity 130 - spray 430 - surface-activeorganic chemicals 130 - thin 420 - uniform 417, 420 - Wurster 221, 430 cobalt 224 cohesiveness 469 collision - probability 442 - high rate 436 colloid, sensitivity 446
colloidal templating 516 compact 335 - accuracy 315 - accurately shaped 458 -density 315 - complex shape 315 -crushed 501 - densification 385 - large 315 - of pressure agglomeration 242 -structure 320 compacted sheet 356 compactibdity 329 compaction / compacting 9, 115, 329 - brittle breakage 363 - behavior, predict 485 - compaction / granulation 513 - - economic operation 509 - - multiple lines 509 - - small batches 509 - shape of granular material 243 - zone 340 components - bituminous 231 -porosity 375 - preformed 375 - pyrotechnic 106, 108 - structural flaw 375 composite material 240 composite parts 375 composition, product 507 compounding 509 compressed - air, expanding 234 -gas 355 compressibility 469 compression - force 373 - one-sided 317 - phase, duration 309 - rate 234 - strength 64, 241 - testing 71 concrete 32 - high quality 85 -high strength 218 conditioning 134, 232, 236 conditions in-line -optimize 369 - readjust 369 consolidation, uniform 374 consultants 454 consumer product 47, 106, 251, 328, 375 contact potential 41
596
I
74 Indexes
contamination, particulate 502 continuous system, simulation 475 conveyor - belt 502 - mesh-belt 400 - pneumatic 119, 504 - pocket mechanical 503 cooker 295 - pressure 296 cooking, pressurized steam 296 cooling 196, 214 -rapid 214 coordination -number 77 - point 34 co-processing 167 cored block 305 cosmetics 515 costs - investment 140 - operation 140, 163 coupling gear 339 - fluctuation in gap width 354 -problem 354 cross contamination 280, 332, 509 crushing - chamber, exit screen 499 - know-how 243 - mechanism 499 - test, transversal 64 cryogenic milling 119 crystal bridges - strength 37 -structure 37 crystalline hull 129 crystallization 2, 29 - crystal growth 222 - undesired clustering 222 cubedice 413 curing 129, 140 curing method 151 - ultraviolet radiation 515 customer appeal 106 cyclones 441 cylinder, perforated 276
d data -logging 383 - reduction software 472 deaeration 352, 355, 364 dedusting 111, 245 degassing 301 dense sheet 336
densest packings 84, 87 densification - cycles 234 - mechanism 234 -outcome 237 - ratio 497 - - very high 319 - speed of 137, 234, 300 density - apparent 61 - differences 238, 317 - distribution 237 - gradient, uniform 374 - localized 239 - overall 239 - pressure agglomeration 239 - theoretical 237 - uniform 237 - variations 237, 239, 373 deposit 114, 119, 122, 191, 199, 203 desagglomeration 118, 186 descriptive names 11 design data, determination 491 design parameter 490 - new location 491 detergent 251, 514 development 410 - empirical 453 - phase, first 474 -work 394 diaphragm -cloth 220 - finely pored 220 - sintered glass 220 die 325 - assemblies 485 - changer 305 - channels 232 - counter bore 267 - cylinder - - discharge from the inside 273 - - internal press rollers 277 - - no leakage 278 - - small in diameter 274 - cylindrical ring 267 -extrusion 267 - - flat 269, 284 - - perforated ring 269 - gear shaped 267 -holes 232 - - land area 267 -insert 2689 - life 268 - life expectancy 281
14.3 Subject lndex
- lubrication 305 - openings 262 - perforated 136 - plate 258 - - circular 262 --domed 261 - - flat 261, 262 - ring, open front 278 - screens 136 -track 285 diffusion 387 - ofmatter 385 digested sludge 509 direct reduction plant 408 discharge system, sub-corona 436 disintegrant 51, 52 disintegration aid 514 dispersibility 469, 510 dispersion 510, 511 -aid 113, 514 displaced gas 234 disposal sites 451 distribution plates 202 -modified 203 - non-sifting 203 diversified companies, test and development facilities 492 dog food 299 dosage form 251 -dry 459 - solid 315, 417 downdraft 196, 395, 404 downtime 460 drag 288 -flow 261 - forces 120 DRI (direct reduced iron) 408 drink powder 510 drive, variable speed 292 drop test 74 droplet formation 29 drugs 435, 515 drums, polygonally shaped 417 dry bag - pressing 377 - tooling 376 dry powders 214 - characteristics 215 - flavor 215 dryer 69 - external fluidized bed 182 - spray dryer (see there) drying 242 - continuous method 188
emulsions 188 - fluidbed 430 - high rate 430 -rapid 214 - solutions 188 - slurries 189 - spray drying (see there) - suspensions 189 drying chamber 191 - Filtermat 196 - flow patterns 193 - height 193 - roof 193 - turbulence 193 drying zone 190 duck-billing 343 dung beetle 7 dust 495, 502 - collecting system 201, 440, 449, SO2 - control 462 - layer 449 dwell time 309, 326, 378 dyes 515 dynamic seal 296, 301 -
e eccentric drive 309 economic justification 460 effective distance 43 efficiency factor 264 EIRICH granulating mixer 172 elastic - deformation 137 - - relaxation 234 - expansion 267 - recovery 310 - - characteristic, ram 309 - spring-back 267, 300 electric resistance heater 314 electrical double layer 446 electrolyte 119 electron beam drawing 521 electron work functions 41 electronic circuit board 409 electrostatic precipitators 41 emergency shut-downs 509 encapsulation, during drying 433 energy 386 - requirements 460 engineered products 103 - agglomerated 513 - new generation 513 engineering science, generic fields 453 entire process 460
I
597
598
I
14 Indexes
environmental - conditions 461 - control 335, 440, 442 - regulations 461 EPA 462 equation, population / mass balance 146 equipment - description 525 -desk top 468 -features 237 - maintenance 281 - new laboratory 468 - overload features 233 - overload protection 237 - parameters 145 - small laboratory 468 - small scale, modular design 475 - special 506 -testing 491 - transportation 502 eutectic 388 - temperature 388 evaporation 387 - porous bodies 433 excipients 321 expansion - compressed gas pockets 300 -elastic 267 experience 453, 463 - factor 264 explosives 106, 375 extrudate 93, 232, 255 - converting plastic 245 - different cross section 299 - discharging 268 -length 261 - size reduction 245 -skin 239 - spaghetti-like 246 - sticky 268 - with minimum spring-back 307 - with small cross section 268 extruder - anular gap 299 - axial 257 -basket 255 - cost advantage 263 -dome 262 - extrusion area 263 - gravity feed 255 - high pressure 304 - - configured tools 306 - - pastes 306 - - plastic materials 306
- - thermoplastic polymers 306 - life span 297 - optional execution 299 - peripheral 257 - pressure cooker, technical data 299 - r a m 234 - ring die 274 - screen 254, 294 - screw 257, 299 --axial 304 - - characteristic pressure distribution 301 - - conditions 265 - - conveying zone 301 - - densification region 301 - -design 265 - single screw 259 -trough 255 - twin screw 259 Extrud-0-Mix 294 extrusion zone 338 extrusion - aid 305 - blade 253, 255 - channel 93, 256, 257, 262 - - cylindrical bore 283 - -design 266 - - frictional resistance 294 - - length 267, 307 - - multiple 311 - - release 310 - - relieving 267 - - square opening 283 - - straight cylindrical bore 266 - - tapered inlet 268 - - with inlet chamfer 267 - dies - - differently shaped 266 - - % free area 266 - forces to obtain 260 - high pressure 294 - hydrostatic pressure 294 - material flow 262 - medium-pressure 294 - plate, design 304 - pelleting 266 - pressure 276 - rate 264 -zone 340
f farming area - high performance 508 - large 508 fatty amines 131
14.3 Subject Index
feeder 278 feeder base, baffles 352 feeding - high speed 331 - horizontal 344 - positive 291 - rotary tabletting presses 331 - vertical 344 feed screws 343 feed zone 340 felt 447 ferromagnetic particles 41 ferrosilicon 218 fertilizer 156, 161, 230, 419, 499 - bulk blending 509 - caking 123, 127 - coating 508 - defined NPK relationship 508 - granulation 126, 127, 131, 167, 244, 420, 460, 507, 508 - - coating 510 - - compaction / granulation 509 - - on demand 509 - high-analysis 129 -industry 245 - multicomponent 508 - nitrate 1 2 5 - partially suitable 508 - slow release 420 - trace elements 508 fiber 41, 409, 447 - briquette strength 51 - cellulose 447 - direct spun stainless 52 - metal 51 - reinforcement 52 - synthetic 447 - wire 52 filament 409 fill shoe 317, 331 filter / filtering 389, 447 - cake 234 -cartridge 447 - efficiency 447 - manufacturing 447 - media 440, 447, 450 - off-site system 443 - packed bed 441 - pleated bag 447 - u s e 447 fine - coal 223 -dust 333 - metal
fines 252 - excessive amounts 243 - recirculating 143 firing 390 flashing 336 flavor, enhancement 417 flexible containers 375 - fixed 376 - for wet bag pressing 376 floating roller - hydraulic pressurization 354 - springs 354 floc 223, 442 flocculant, commercial 445 flocculation 113, 221, 445 flotation 223, 445 flow -channel 122 - characteristics 331 - meter, solids 347, 502, 505 - of air 418 - patterns in the system 503 - rates, mass - zones, calm 120 flowability 458 flue dust 402 fluid - contamination 379 - pressure transmitting 379 - toxic 380 fluidized bed 86 - agglomeration 196, 211, 219 - - size enlargement 204 - arranged externally 200 - back mixed 202 - bag filters 212 -batch 211 - binder liquid 210, 214 - circular 201, 204 - combination 206 - condensing, evaporated fluid 2 12 - contact heating surface 210 - continuously operating 211 - conversion, liquid feed 214 - cooling 196, 214 - discharge, agglomerates 197 - dry mixing 196 - dry powders 214 - drying 196, 214 - energy consumption 207 - heat sensitive material 210 - heating panels 210 - incremental growth 206 - installation cost 207
I
599
Wolfgang Pietsch Agglomeration in Industry Occurrence and Applications
Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
Also of Interest: Pietsch, W.
Smulders, E.
Agglomeration Processes
Laundry Detergents
2002
2002
ISBN 3-527-30369-3
ISBN 3-527-30520-3
Kaye, B. H.
Smith, H. M. (Ed.)
Characterization of Powders and Aerosols
High Performance Pigments
1999
2002
ISBN 3-527-28853-8
ISBN 3-527-30204-2
Zlokarnik, M.
Albrecht, W., Fuchs, H., Kittelmann, W. (Eds.)
Scale-up in Chemical Engineering 2002
Nonwoven Fabrics 2003
ISBN 3-527-30406-1 ISBN 3-527-30266-2
Ko¨hler, M., Fritzsche, W.
Nanotechnology 2004
ISBN 3-527-30750-8
Wolfgang Pietsch
Agglomeration in Industry Occurrence and Applications
Dr.-Ing. Wolfgang Pietsch, EUR ING COMPACTCONSULT, INC. 2614 N. Tamiami Trail, #520 Naples, Florida 34103-4409 USA In Europe: Holzweg 127 67098 Bad Du¨rkheim Germany
All books published by Wiley-VCH are carefully produced. Nevertheless, author and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No. applied for. British Library Cataloguing-in-Publication Data: A catalogue record for this book is available from the British Library. Bibliographic information published by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data is available in the Internet at
. ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printed in the Federal Republic of Germany Printed on acid-free paper Typesetting Mitterweger & Partner, Kommunikationsgesellschaft mbH, Plankstadt Printing Strauss GmbH, Mo¨rlenbach Bookbinding Litges & Dopf Buchbinderei GmbH, Heppenheim ISBN 3-527-30582-3
V
Contents Preface IX Volume 1 1
Introduction
2
Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science 3
1
3
Agglomeration Fundamentals
3.1 3.2 3.3 3.4 3.5 3.6 3.7
Structure and Bonding 7 Binding Mechanisms 11 Particle Size of the Particulate Solid Binders and Additives 14 Strength 15 Determination of Strength 18 Porosity 21
7
14
4
Undesired Agglomeration: Methods of Avoiding or Lessening its Effect
4.1 4.2 4.3 4.4 4.5 4.6
Separation 23 Mixing 27 Comminution 27 Agglomeration 30 Transportation 30 Storage 31
23
5
Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods
5.1 5.2 5.3 5.4
Tumble or Growth Agglomeration 38 Pressure Agglomeration 47 Agglomeration by Heat/Sintering 53 Technologies Using the Phenomena and Fundamentals of Agglomeration for Purposes other than Size Enlargement 54
Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
37
VI
Contents
6
Industrial Applications of Size Enlargement by Agglomeration
6.1
General Applications
6.1.1 6.1.2 6.1.3
Tumble/Growth Technologies 77 Pressure Agglomeration Technologies Other Technologies 83
61 80
6.2
Pharmaceutical Applications
6.2.1 6.2.2 6.2.3
Tumble/Growth Agglomeration Technologies Pressure Agglomeration Technologies 114 Other Technologies 147
85
6.3
Applications in the Chemical Industry
6.3.1 6.3.2 6.3.3
Tumble/Growth Technologies 169 Pressure Agglomeration Technologies Other Technologies 202
90
167
6.4
Applications in the Food Industry
6.4.1 6.4.2 6.4.3
Tumble/Growth Technologies 215 Pressure Agglomeration Technologies Other Technologies 244
181
206
6.5
Applications for Animal Feeds
6.5.1 6.5.2 6.5.3
Tumble/Growth Technologies 248 Pressure Agglomeration Technologies Other Technologies 266
229
247
6.6
Fertilizers and Agrochemicals
6.6.1 6.6.2 6.6.3
Tumble/Growth Technologies 270 Pressure Agglomeration Technologies Other Technologies 297
252
266
6.7
Building Materials and Ceramics
6.7.1 6.7.2 6.7.3
Tumble/Growth Technologies 304 Pressure Agglomeration Techniques Other Technologies 333
281
303 313
6.8
Applications in the Mining Industry (Minerals and Ores)
6.8.1 6.8.2 6.8.3
Tumble/Growth Technologies 349 Pressure Agglomeration Technologies Other Technologies 381
374
6.9
Applications in the Metallurgical Industry 385
6.9.1 6.9.2 6.9.3
Tumble/Growth Technologies 386 Pressure Agglomeration Technologies Other Technologies 412
386
347
59
Contents
6.10
Applications for Solid Fuels
6.10.1 6.10.2 6.10.3
Tumble/Growth Technologies 417 Pressure Agglomeration Technologies Other Technologies 455
415
6.11
Special Applications
6.11.1 6.11.2 6.11.3
Tumble/Growth Technologies 459 Pressure Agglomeration Techniques Other Technologies 469 Indexes
419
459 464
I1
List of Vendors I 1 Subject Index I 53 List of Figures I 80 List of Tables I 103 Volume 2 7
Powder Metallurgy
479
8
Applications in Environmental Control
8.1 8.1.1
Collection, Stabilization, and Deposition of Particulate Solids 489 Size Enlargement by Agglomeration during the Collection of Particulate Solids 489 Size Enlargement by Agglomeration for the Stabilization and Disposal of Particulate Solid Wastes 499 Recycling/Secondary Raw Materials 502 Historical Review of Waste Production 502 Agglomeration Technologies for the Size Enlargement of Wastes 504 Applications in the Mineral, Metallurgical, and Energy Related Industries 505 Applications in Regional and Municipal Material Recycling Plants 518 Other Applications 533 Recycling of Polymers 537
8.1.2 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.5 8.2.6
485
9
Development of Industrial Applications
9.1 9.2 9.3
Test Facilities 545 Tolling Operations: Contract Manufacturing Scale-Up 573
541 566
10
Optimization and Troubleshooting of Agglomeration Systems
10.1 10.1.1 10.1.2 10.1.3 10.1.4
Tumble/Growth Technologies 596 Particle Size 597 Particle Size Distribution 597 Particle Shape 599 Chemical and Physical Surface Properties
601
589
VII
VIII
Contents
10.1.5 10.1.6 10.1.7 10.1.8 10.2 10.2.1 10.2.2 10.3
Binder Interaction 601 Equipment Type and/or Size 601 Optimization 602 Troubleshooting 604 Pressure Agglomeration Technologies 616 Low- and Medium-Pressure Methods 618 High-Pressure Agglomeration Methods 621 Other Technologies 630
11
Applications of Agglomeration Phenomena for Single Particles and in Nanotechnologies 633
11.1
11.2 11.3
Occurrence and Applications of Agglomeration Phenomena for the Attachment and Bonding of Single Particles to Surfaces and Substrates 639 Some Examples of Nanoparticles with Special Characteristics 642 Applications of Agglomeration in Nanotechnologies 644
12
Outlook
655
13
Bibliography
13.1 13.2 13.3
List of Books or Major Chapters on Agglomeration and Related Subjects 662 References 669 Author’s Biography, Patents, and Publications 676
14
Glossary of Application-Related Terms Associated with Agglomeration
661
15
Indexes
15.1 15.2 15.3 15.4
List of Vendors 721 Subject Index 773 List of Figures 800 List of Tables 823
721
693
IX
Preface When this book was first planned, the idea was to combine in one volume concise descriptions of agglomeration phenomena, technologies, equipment, and systems as well as a compilation of the applications of agglomeration techniques in industry. The latter was intended to demonstrate the widespread natural, mostly undesired occurrences of the phenomena and possible ways of avoiding them as well as the old, conventional, and new, varied beneficial uses of the technologies. However, it soon became obvious that, in its entirety, this project was too extensive and required much more time than anticipated. Therefore, it was decided to split the subject into two complementary books. The first book, Agglomeration Processes – Phenomena, Technologies, Equipment (ISBN 3-527-30369-3) was published by Wiley-VCH, Weinheim, Germany, in 2002. It covers the fundamental phenomena that define agglomeration and industrial technologies and equipment for size enlargement by agglomeration. Applications are mentioned in a general way throughout this text but without going into details. This second book is an up-to-date overview dealing with the occurrence and key applications of agglomeration, including size enlargement in pharmaceutical, food and animal feed, chemical, fertilizer and agrochemical, mineral, building material and ceramic, metal, solid fuel, and other industries. Furthermore, the book emphasizes recent developments at the level of single particles and applications of agglomeration phenomena in nanotechnologies. Many people, institutions, and companies have contributed to the two books. First and foremost, I wish to thank my wife Hannelore for her support and understanding, particularly during the years when I was compiling these books. They are both dedicated to her. Without my wife’s active participation in preparing almost all my publications, including the first textbook entitled Size Enlargement by Agglomeration, which is a major reference for the current two books, and her acceptance that I was not available for many hours almost every day during much of two decades, these publications could not have been completed. It is impossible to acknowledge all the help that was provided by a large number of individuals and companies. Chapter 15.1 is a list of vendors and other organizations, which mentions those who have, in one way or another, contributed as well as some others that might be of interest as potential contacts for the readers of these books. While I have decided not to clutter the text with references, sources have been acknowl-
X
Preface
edged if figures or tables were provided by or are based on input from particular companies. The Disclaimer at the beginning of this book should be referred to when using such information. Chapter 13 lists literature references. The earlier textbook, Size Enlargement by Agglomeration, contains treatments and many references relating to the developing science of the unit operation and covers the sizing of agglomeration equipment in some detail. Since the emphasis of the new books is on industrial applications, rather than theory, the earlier book should be referred to for the theoretical background. Information on the availability of reprinted copies of Size Enlargement by Agglomeration (Wiley, 1991) is available in Chapter 13.1 as a footnote. I have also contributed major chapters on agglomeration to two other books, portions of which are used in this book. The other books are: Handbook of Powder Science and Technology (Eds: M. E. Fayed, L. Otten), 1st edn, Van Nostrand Reinhold, New York (1983) and 2nd edn, Chapman & Hall, New York (1997). Full references can be found in Chapter 13.1. Since size enlargement by agglomeration is one of the four unit operations of Mechanical Process Technology (see Chapters 1 and 2) and, for the design and construction of agglomeration systems and plants of any kind, many or all of the other unit operations are required together with the associated transport and storage technologies, often even in multiplicity, and the analytical methods are applied for process evaluation and control, the reader who is interested in the topic of this book should also learn about or have access to information on the other fields of Mechanical Process Technology (references in Chapter 13). Finally, I wish to thank the following individuals who, as professionals and experts in their own fields, are or have been colleagues and/or partners in several continuing education courses over many years in the USA and in Europe and have agreed that their presentations and course notes can be used directly, adopted, or modified for this book. They are, in alphabetical order: T. van Doorslaer, W. E. Engelleitner, B. J. Ennis, M. E. Fayed, M. Gursch, D. C. Hicks, S. Jagnow, R. H. Leaver, R. Lo¨be, K. Masters, S. Mortensen, H. B. Ries, F. V. Shaw, N. Stanley-Wood, J. Storm, R. Wicke, and the late R. Zisselmar. Other major contributors were M. Karel, Y. Kawashima, the late B. Kaye, and H. Schubert.
Wolfgang Pietsch, Naples, FL, USA, September 2004
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Introduction This book is complementary to the author’s earlier books, Roll Pressing (1976/1987) [B.13b], Size Enlargement by Agglomeration (1991) [B.48], and Agglomeration Processes – Phenomena, Technologies, Equipment (2001) [B.97], as well as some major contributions to professional handbooks, such as Size Enlargement by Agglomeration (1997) [B. 71]. While Roll Pressing [B.13b] dealt exclusively with aspects of pressure agglomeration in roller presses, Size Enlargement by Agglomeration [B.48] covered the entire operation and some related fields from a fundamental view point. It described in much detail the newly evolving science of the natural phenomenon “Agglomeration”, which has been used by living creatures, including humans and modern mankind, for thousands of years, and the technologies that were derived from it. In contrast, Agglomeration Processes – Phenomena, Technologies, Equipment [B.97] was trying to offer a complete, up-todate compilation of the various agglomeration techniques and, in general terms only, their applications. To that end, in addition to introducing the properties of agglomerates and the specific characteristics of the different technologies, descriptions of equipment and their special features for particular uses as well as engineering know-how and information on specific peripheral equipment were the main topic of the book. Emphasis was on up-to-date practical knowledge, not theory. The present book again does not claim to be a scientific publication. Agglomeration, both as a phenomenon and as the beneficial size enlargement of particulate solids, has become part of the newly defined interdisciplinary technical field of Mechanical Process Engineering and a science in its own right, as described earlier [B.97], (Chapter 2). Nevertheless, much of the research and development are phenomenological in nature and most of the designs for equipment and industrial plant still rely heavily on the experience and know-how of vendors as well as experts in the field. Therefore, after again reviewing the fundamentals of agglomeration in an even more abbreviated form, this book attempts to summarize and describe the occurrence of agglomeration in industry, the industrial applications of size enlargement by agglomeration, and other beneficial uses of particle adhesion. The latter, in particular, is found increasingly in new fields such as nanotechnology, life sciences, and even the communication industry (nanoelectronics). Obviously, it is not possible to describe all the many occurrences of agglomeration and to cover every application of size enlargement by agglomeration in industry. ParAgglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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1 Introduction
ticularly during the past 50 years, because of an increase in the production of fine and ultra-fine primary, intermediate, and final solid materials as well as the collection of ever finer particulate wastes, circumstances of undesired build-up, lumping, and other troublesome aggregations have increased tremendously. During that same period of time, a better understanding of the agglomeration phenomenon has led to many beneficial applications of desired and controlled size enlargement to obtain an increasing number of benefits (Chapter 6, Tab. 6.1). Therefore, in planning the contents of this book, in spite of its growing importance, just one chapter was allocated to deal with the undesired occurrences of agglomeration and the most common methods to avoid or at least lessen its effect. For the presentation of desirable size enlargement processes in industry, the entire field was sub-divided into a number of segments. In each, the most important application(s) is (are) being presented in considerable detail as examples, describing the history, development, and present state of the technique(s). From this, knowledge and guidance may be gleaned for interdisciplinary use during the evaluation of similar applications in other industries and/or for different materials. In some areas, the author has decided to describe or indicate in varying detail some less well known, normally more partial methods for size enlargement by agglomeration. These were selected to broaden understanding of the possibilities for beneficially applying the phenomenon for obtaining specific product characteristics and may suggest the use of similar approaches for other cases.
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Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science Agglomeration, the sticking together of particulate matter, is a natural phenomenon and is as old as the existence of solids. Originally, it happened during the development of soil, stone, and rock formations. Later, unwanted agglomeration occurred during handling and storage of particulate matter by humans, especially when hygroscopic and/or soluble materials (such as salt) set into lumps or large more-or-less solidified masses. In the animal world agglomeration was and is used to develop protective coatings, to build nests, and to provide a nourishing and protective environment for the offspring (e.g., by the legendary Scarabaeus Sacer, the dung beetle, Fig. 2.1, the mascot of this agglomeration expert). “Size enlargement by agglomeration” is the generic term for the operation in Mechanical Process Engineering that is characterized by the descriptions “combination with change in particle size” (Fig. 2.2). It is distinguished from the more general “size enlargement” such that particle growth occurring, for example, during crystallization
Fig. 2.1 Beetle, Scarabaeus Sacer, “pelletizing” dung and an artist’s impression of this animal’s procedure Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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2 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
Separation
Combination
Without Change in Particle Size
Mechanical Separation (Filters, Classifiers, Screens, Sifters)
Powder Mixing and Blending
With Change in Particle Size
Size Reduction (Crushing and Grinding)
Size Enlargement by Agglomeration
Particle and Bulk Material Characterization (Size, Distribution Shape, Volume, Surface, Density, Mass, Porosity, Moisture Content, etc.)
Transport and Storage of Bulk Materials
Fig. 2.2 Operations of Mechanical Process Technology and associated techniques
or the production of particulate solids by melt solidification are not part of this operation [B.97]. Most probably, humans first used agglomeration during the making of bread. The technology of bread making combines all components of a complex agglomeration process including preparation of solid feed particles by milling (adjustment of particle size and activation of the inherent binder, starch), mixing of particulate solids with additional binder(s), forming the mass into a green agglomerate, and a post-treatment (curing = baking = heating and cooling) to provide strength and texture. Very early it was also found that the porosity and, thus, the ease and enjoyment of consumption of the final product could be modified (increased) by making use of gases that are produced during fermentation (initiated by sourdough or yeast) and result in bubbles in the green mass. These voids are stabilized by strengthening the bread during posttreatment (baking). For the construction of permanent shelter, humans may have observed the activities of animals that formed nests and protective “walls” from wet clay, which hardened during drying. By copying this behavior, wet clay, which was soon reinforced and made more water resistant by mixing-in straw or other fibrous material, was filled into a framework of wood branches and let harden during natural drying. To make building activities independent of the location of stone quarries, during prehistoric times bricks were already produced from clay and sand when rock was not easily available, thus allowing the development of villages and, during the 4th millennium BC in Mesopotamia, cities (e.g., Uruk) with large brick structures. By experience, humans learned that certain natural materials helped cure specific illnesses. Minerals as well as dried animal and plant matter were ground to powder and formulated to yield medicines. Since powders cannot be easily consumed orally, natural binders, such as honey which, incidentally, also masked the unpleasant taste of some of the medicinal components, were mixed with the powder and the resulting plastic mass was rolled by hand into little balls (pills). The sticky binder(s) caused pills to adhere to each other; therefore, it was soon found that coating the pills by rolling them in flour or pollen solved this problem.
2 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
These three well-known ancient agglomeration techniques were used with little change through the ages of human history. Many other, lesser known or more recent processes could be added and some will be mentioned later in the appropriate sections of this book. At this point, the short review and the examples relating to three major modern industries (food, building materials, and health products) were selected to show that humans always lived with and used agglomeration. As a result, technologies for the mechanical processing of solids were considered normal activities, which, with the beginning of industrialization in the 18th and 19th centuries, were merely mechanized by simulating what was done manually before. During these early modernization efforts it was not considered necessary to question the fundamentals of the processes and improvements were based on empirical developments. Until very recently, agglomeration technologies, as all the other unit operations and associated techniques of Mechanical Process Technology (Fig. 2.2), had been developed independently by the particular industries in which they were applied. Because the process requirements are very different in such unlike industries handling, for example, coal and ores on one hand or food and pharmaceuticals on the other, no interdisciplinary contact and exchange of information took place. In fact, although agglomeration techniques developed along similar lines, application related theories were defined, which were derived from investigations of specific requirements. Solutions of such work were not universally useable and the terminology was often incomprehensible to the agglomeration expert of another industry. Agglomeration as a science began when an effort was made to interdisciplinarily combine the extensive knowledge that had been accumulated during sometimes hundreds of years in specific fields of human activities. It was found that, in all the different environments, agglomeration methods follow the same fundamentals, apply the same rules, and use essentially the same equipment and systems if looked at from a basic interdisciplinary point of view. Although these facts become more and more known, there is still the understandable preconceived notion of, for example, somebody working in an ultra-clean environment, such as pharmaceutical, food, or electronic industries, that developments, expertise, and know-how gained in the dirty plants of, for example, minerals or metals production and processing, can not be considered as generally valid information and, therefore, may not be applied for the solution of a clean problem, and vice versa. In dirty industries, a typical concern is that the often much more deeply and completely investigated technologies that originate in clean industries can not be applied because the production capacities are too small, the process may be batch, the equipment too complex, the execution and the materials of construction too expensive and so on. However, as has been shown among other topics in the author’s previous book [B.97], methods for the selection of the most suitable agglomeration process for a specific application are the same for all projects. While some requirements, for example in regard to equipment or system capacity or on the shape, size, and special properties of the products, may already in the pre-selection phase result in the definition of cleaner or more heavy-duty, rugged processes, the normal approach is to determine the preferred method and/or technique by considering the fundamentals as well as an
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2 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
interdisciplinary pool of expertise and know-how first. Special conditions of the particular application, such as, for example, hot and dusty large volume processing, or the opposite, clean, small capacity operation with cGMP (current good manufacturing practice) and CIP (cleaning in place) capabilities, are special design criteria that can be added to most of the systems later during the engineering phase. Nevertheless, for manufacturing reasons and sometimes also because of special requirements on the company’s test facilities (Chapter 9.1), some vendors specialize in equipment for one or the other industry. This is a decision of convenience by the individual supplier and does not indicate the existence of a fundamentally different technology. In fact, techniques or apparatus that were developed for a specific industry can be adopted for use in areas with different environment and requirements while still maintaining the fundamental underlying principle as well as the general machinery and process. Examples are crushing with high pressure roller presses, flaking, instantizing, spheronizing, and agglomeration with spray dryers.
Further Reading
For further reading the following books are recommended: B.3, B.4, B.6, B.7, B.8, B.12, B.13a through e, B.15, B.16, B.17, B.19, B.21, B.22, B.24, B.26, B.35, B.36, B.40, B.41, B.48, B.49, B.51, B.52, B.55, B.56, B.58, B.63, B.66, B.67, B.68, B.70, B.72, B.73, B.82, B.83, B.89, B.93, B.94, B.97, B.99, B.106 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold.
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Agglomeration Fundamentals The major distinguishing characteristic of agglomeration is the formation of larger entities from particulate materials. The particles stick together by short-range physical forces between the solids themselves or through binders: substances that adhere chemically or physically to the surfaces and form a material bridge between the particles. The components of an agglomerate are often widely disparate and, except if matrix binders are applied or after shrinkage during sintering, voids between the particles forming an agglomerate are present in the product. As will be shown later (Chapter 11), an increasing number of occurrences of agglomeration relate to single, often nanosized particles, and new techniques make use of the adhesion of individual particles to substrates for the manufacturing of materials with unique properties. All apply the same agglomeration phenomena, fundamentals, relationships, and rules.
3.1
Structure and Bonding
Fig. 3.1 shows random sections through a part of an agglomerate; the structure is really three dimensional. In such a body, strength can arise in several ways. In Fig. 3.1a the entire pore space is filled with a matrix binder. Typical examples of agglomerates held together in this manner are concrete, where the matrix between the aggregate particles consists of hardened cement, or road surfaces, in which bitumen occupies the volume between crushed stone or other aggregate. Fig. 3.1b looks very similar to Fig. 3.1a but represents an agglomerate structure in which the entire void volume is filled with a liquid that wets the solid particles. If concave menisci form at the pore ends on the surface of the agglomerate a (negative) capillary pressure develops within the pores, which gives strength to the body. As explained in Fig. 3.2, depicting a series of situations representing different liquid saturations in particulate bulk solids or of agglomerates, distinct distribution models exist that depend on the amount of liquid in the structure. The term “liquid saturation” is defined as the percentage of total void space that is filled with the liquid. A precondition for cohesiveness of particulate solids due to the presence of liquid is that the liquid wets the solids. Although, depending on the application, other liquids may be used to Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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3 Agglomeration Fundamentals
Fig. 3.1 Random sections through part of an agglomerate or a particulate bulk solid mass: a) pore volume filled with a matrix binder; b) pore volume filled with a wetting liquid; c) liquid bridges at the coordination points; d) adhesion forces at the coordination points
completely or partially fill the voids between the particles, in agglomeration water is used most commonly. Referring to Fig. 3.2, absolutely dry particulate bulk solids (Fig. 3.2a) are non-existent under normal atmospheric conditions. The water molecules of adsorption layers (Fig. 3.2b), which, under ambient atmospheric conditions, form quickly on solid surfaces, are bonded so strongly that they are not mobile, and, therefore, do not cause “liquid saturation” or moisture content that can be measured with “normal” laboratory equipment. However, adsorption layers can participate in the development of strength by enhancing molecular (van der Waals) forces [B.48, B.97]. With small amounts of “free” water (i.e., producing moisture contents of little more than a few tenth of a percent) and, correspondingly, very small “saturation”, liquid bridges begin to form at the contact points between particles. With increasing moisture content or saturation, liquid bridges become present at all coordination points (see below) in the structure (Fig. 3.1c and 3.2c). A further increase in liquid saturation produces a transitional situation in which liquid bridges and void spaces that are filled with liquid coexist (Fig. 3.2d). The theoretically highest saturation (100 %) is reached when all voids within a bulk mass or an agglomerate are filled (Fig. 3.1b) and concave menisci are formed at the pore ends (Fig. 3.2e). Beyond complete saturation, liquid droplets, shaped by the surface tension, may enclose solid particles (Fig. 3.2f). Slurries, defined as bulk particulate solids containing an excess of water, are shapeless. All the above
3.1 Structure and Bonding Fig. 3.2 Diagrams of different liquid saturations in particulate bulk solids or agglomerates: a) dry; b) adsorption layers; c) liquid bridges (“pendular” state); d) transitional (“funicular” state); e) fully saturated (“capillary” state); f) droplet
models exist in “wet agglomeration”, methods that are based on the processing of slurries, suspensions, solutions, or the presence of liquids as binders. Fig. 3.1d depicts the action of solid bridges or forces at the coordination points of a particle with all other particles surrounding it. Coordination points are points of contact with other particles and “near points”, areas of the particle surface that are so close to a neighboring particle surface that significant adhesion forces act or bridges can form. The coordination number is the average of the sum of all contact and near points of each particle with others surrounding it in a particular agglomerate structure. Typical examples of agglomerates bonded in this manner are “natural” aggregates of very fine particles that are held together by molecular forces, agglomerates with solid bridges at the coordination points that have formed during drying of originally wet agglomerates by recrystallizing dissolved materials, or products of pressure agglomeration in which the distance at the coordination points has been reduced by the effect of external forces.
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3 Agglomeration Fundamentals
Agglomerates are bodies that are, often artificially and with purpose, produced from individual “small” particles. The term “small” is to be understood in relation to the agglomerate. Although there are agglomerates, for example in the food industry (Section 6.4), or natural, often undesired agglomerates (Chapter 4), which consist of only a few particles, typical agglomerates contain very large numbers of particles with sizes that are orders of magnitude smaller than that of the agglomerate. Binding mechanisms (see below) cause these particles to temporarily or permanently stick together and form a lose or porous entity. Since binding mechanisms act in different ways, the structure of agglomerates is of great importance for all their properties. The sketch in Fig. 3.3 depicts a random section through an agglomerate. The area within the heavy solid lines is arbitrarily defined as “one”. Fig. 3.3 seems to show particles and their distribution. In reality, what is visible are cross sections through particles at a specific level. If another random section through the same agglomerate is made, a totally different image is obtained. Moreover, particles that seem to float in space are in contact with other particles at some level. For example, the shaded cross section may be the result of cutting the particle shown in elevation on the side of Fig. 3.3, at the indicated line. Obviously this particle will have a completely different outline at another level. The same observation is true for the void spaces (porosity) that are visible between the particle cross sections. If the heavily bordered square in Fig. 3.3 that represents the area “one”, is large enough and contains a great number of the two significant structural characteristics, i.e. outlines of cross sections through particles and of empty space (pores) between the particles, a statistical evaluation of any random section will produce generally valid results with an accuracy that can be described by the standard deviation that is associated with that statistical treatment. Therefore, for example, scanning the
Fig. 3.3
Sketch of a random section through an agglomerate
3.2 Binding Mechanisms
image of the section will produce information on particle size and distribution, porosity (e), solid content (1 – e), and, with the appropriate software, a shape factor and the specific surface area of the particles [B.75]. Accuracy can be increased by investigating multiple sections through the same agglomerate and determining the statistical average for all of them. A visual evaluation of the enlarged image of the section through an agglomerate also reveals certain other features, although the observations can only be used to explain phenomena and do not serve any scientific purpose. The circles in Fig. 3.3 indicate, for example, some of the contact points between particles in this particular cross section and the rectangular boxes depict some of the “near points” at which a binding mechanism, such as liquid bridges or one of the field forces (see below), could develop. The average of the sum of both types of interaction points for one particle defines the coordination number k. Taking into consideration the statements made above in regard to random sections, it is of course possible that “near points” in a particular section become actually contact points in a level slightly above or below and it is impossible to determine all the interaction points that are distributed three-dimensionally around a particle in such a cross section.
3.2
Binding Mechanisms
The binding mechanisms of agglomeration (Tab. 3.1) are divided into five major groups (Roman numerals) and several subgroups (Arabic numerals). They have been described and discussed in detail elsewhere [B.48, B.71, B.97]. Fig. 3.4 and 3.5 describe pictorially the binding mechanisms. It should be pointed out that only the two-dimensional situation at one coordination point between two particles or two solid surfaces is shown. In reality, each particle has many interaction sites (coordination points) with other particles in the three-dimensional structure. It should be further understood that in typical particulate bulk solids and agglomerates large numbers of particles that participate in bonding are present per unit volume. With the exception of capillary and matrix bonded structures of particulate solids (Fig. 3.1a and b), it is unlikely that only one binding mechanism acts on all the coordination points within a mass. If molecular and electric forces as well as liquid bridges and the solid bridges, resulting from the latter by one or the other of the mechanisms that were indicated above and discussed elsewhere [B.48, B.71, B.97], are considered, it must be also assumed that the effect of each binding mechanism is different at every coordination point because of varying microscopic surface structures and distances at each interaction point. If size enlargement by agglomeration is desired and the correct agglomeration technique is selected, many of the binding mechanisms described above are inherently available or can be activated. Under certain conditions, some binding mechanisms also act naturally to produce undesirable agglomeration phenomena (Chapter 4). Generally speaking, if agglomeration is wanted, means to enhance the available binding
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3 Agglomeration Fundamentals Tab. 3.1
Binding mechanisms of agglomeration
I
Solid bridges, caused by 1 Sintering 2 Partial melting 3 Chemical reaction 4 Hardening binders 5 Recrystallization 6 Drying a Recrystallization (dissolved substances) b Deposition (colloidal particles)
II
Adhesion and cohesion forces 1 Highly viscous binders 2 Adsorption layers (< 3 nm thickness)
III Surface tension and capillary pressure 1 Liquid bridges 2 Capillary pressure IV Attraction forces between solids 1 Molecular forces a van-der-Waals forces b Free chemical bonds (valence forces) c Associations (non-valence); hydrogen bridges 2 Electric forces (electrostatic, electrical double layers, excess charges) 3 Magnetic forces V
Interlocking bonds
Fig. 3.4
Pictorial representation of the binding mechanisms of agglomeration
3.2 Binding Mechanisms Fig. 3.5 Attraction forces between solid surfaces or particles
mechanisms must be developed and applied, while the effect of binding mechanisms must be eliminated or reduced to avoid unwanted agglomeration. Adhesion of individual particles to each other or to solid surfaces is controlled by the competition between volume and surface related forces. To cause permanent adhesion, certain criteria must be fulfilled. The most important is that any system force (e.g., caused by gravity, inertia, drag, etc.) must be smaller than the attraction forces between the adhering partners According to Fig. 3.6, the ratio between the binding forces Bi(x) and the sum of the active components of all ambient forces Fjy(x) is a measure of the adhesion tendency Ta (Chapter 5 [B.48, B.97]).
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Fig. 3.6
Diagram of the adhesion tendency of a spherical particle on a flat surface
3.3
Particle Size of the Particulate Solid
Both the attraction and the ambient forces are mainly dependent on the size x of the powder particle(s). While the surface area of individual particles, the interface at which all binding mechanisms act, decreases with the second power of particle size, volume and consequently also mass, the most important particle properties that result in forces that challenge adhesion and may cause separation of bonds, diminish with the third power of the particle size. Therefore, if the particle size reaches a few micrometers or is in the nanometer range, the natural adhesion forces dominate and particles that contact each other or come into close proximity adhere to one another. This phenomenon can not be economically eliminated, so very fine particles always adhere and form loose agglomerates, which may be desirable (Chapter 5) or undesirable (Chapter 4). Since all binding mechanisms rely on molecular interactions on and between surfaces or interfaces, the structure and distance at these points is of great importance for the ability of powders to agglomerate. Often, the presence of ultra fine particles facilitates size enlargement of coarser particulate matter. Fine particles that are suspended in a binder liquid accumulate during drying in liquid bridges at coordination points and eventually form solid bridges, which are bonded by molecular forces. Dry fine particles may fill areas with high surface energy, such as holes and depressions, thus reducing the effective distance between larger particles and increasing the attraction force (similar to the influence of adsorption layers, [B.48]).
3.4
Binders and Additives
Naturally available adhesion tendencies can be considerably increased if moisture is added during the agglomeration process. Application of external forces can contribute to the enhancement of inherently present binding mechanisms. Depending on the
3.5 Strength
magnitude and nature of these forces, improved structure (by shear and low-to-medium compression) or plastic deformation and brittle breakage (due to high external forces) can occur. Plasticity, an often-preferred response to external forces that results in high agglomerate strength (Chapter 5), increases in many solids if the temperature of the material rises. Therefore, hot densification is often a desirable agglomeration technology, particularly for metals, minerals, and metal-bearing materials. With other materials, featuring low softening or melting points or containing such components, mechanical energy, introduced by dynamic forces, compression, or shear and converted into thermal energy, activates the inherently available binding properties. Under this influence, momentary softening or melting can occur upon contact at minute roughness peaks which, after almost instantaneous solidification, produce a small solid bridge between the powder particles. Similar mechanisms are responsible for the bonding of soluble materials in the presence of moisture. There, mechanical energy converted into heat or the direct external supply of thermal energy result first in additional dissolution and then in recrystallization at the coordination points, even in airtight containers and without changing the moisture content. In both cases, the higher the number of coordination points in a unit volume (increasing with decreasing size of the agglomerate forming particles), the higher will be the strength of the resulting agglomerated part. In spite of the availability of these “natural” binding mechanisms and the various possibilities to enhance them for the desirable production of agglomerates, sometimes no economic method can be found to process a specific material and form a product with sufficient strength. Grinding the particulate solid to a sufficient fineness for strong molecular bonding and/or heating it to high enough temperatures that result in either sufficient dissolution for recrystallization, plasticity for large area contact and bonding, partial melting followed by solidification, or solid state sintering would be too expensive and, therefore, prohibit economic processing. In those cases where no bonding can be achieved by the above-mentioned measures, particle size is relatively large, or specific product characteristics must be obtained, binders, mostly for higher strength, lubricants, mostly for improved density and structure, and other additives that produce special properties, can or must be used [B.48, B.97].
3.5
Strength
The most important property of all agglomerates, desired or undesired, is their strength. To obtain a physically meaningful value of strength it was proposed to determine the tensile strength of agglomerates [B.12]. It is defined by the tensile force at failure divided by the cross section or, if the test body has no uniform shape, the area of the failure plane(s) of the agglomerate(s). Because in all stressing situations failure occurs with great probability under the influence of the highest tensile force, this proposal is justified. Moreover, tensile force and strength can be approximated by models and theoretical calculations.
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All binding mechanisms of agglomeration (Tab. 3.1, and Fig. 3.4 and 3.5) can be described by one of three models. 1. The entire pore volume of the agglomerate is filled with a substance that can transmit forces and, thereby, causes strength (matrix binder, Fig. 3.1a). 2. The pore volume of the agglomerate is entirely filled with a liquid (Fig. 3.1b). 3. Binding forces are transmitted at the coordination points of the primary particles forming the agglomerate (Fig. 3.1d). Liquid bridges at the coordination points (Fig. 3.1c and 3.2c) are described by Model 3 while the transitional state (Fig. 3.2d) is connected with Model 2 through the liquid saturation, S. If the pore volume of the agglomerate is completely filled with a stress-transmitting substance (Fig. 3.1a), such as a hardened binder, three strength components can define agglomerate strength. (a) rte (pore volume strength) = tensile strength of the binder substance (b) rta (grain boundary strength) = tensile strength caused by the adhesion between binder and particulate solids forming the agglomerate (c) rt(1 – e) = strength of the particulate solids forming the agglomerate. The relatively lowest component determines the agglomerate strength. Fig. 3.7 depicts schematically the expected failure lines in a two-dimensional schematic representation [B.97]. If a liquid that wets the solid(s) fills the entire pore volume of an agglomerate to such a degree that concave menisci are formed at the pore ends on the surface (Fig. 3.1b), a negative capillary pressure pc develops in the interior of the agglomerate. Because the membrane forces at the surface are negligibly small in relation to the capillary pressure, the tensile strength rtc of agglomerates that are completely filled with a liquid can be approximated by the capillary pressure [B.97] rtc pc = a’ (1 – e)/e a/x
(3.1)
Fig. 3.7 Two-dimensional diagram of the failure lines derived from the three models describing strength of agglomerates with a matrix binder (Fig. 3.1a)
3.5 Strength
The maximum tensile strength of agglomerates that are completely filled with a perfectly wetting liquid depends on the porosity of the agglomerate, characterized by the strong term (1 – e)/e, the surface tension a of the liquid, and the size x of the particles forming the agglomerate. The empirical correction factor a’ has values between 6 and 8. An approximation of the agglomerate strength rtt in the transitional (“funicular”) state (Fig. 3.2d), in which a certain percentage S (saturation) of the pore volume is filled with liquid, is possible by multiplying the maximum strength rtc by the appropriate saturation S rtt Srtc
(3.2)
Strength may be also caused by adhesive forces A acting at the coordination points of the particles forming the agglomerate (Fig. 3.1d). Based on statistical considerations and a simple model, a general formula was developed that is often used to describe agglomerate strength [B.48] rt = (1 – e)/p kA/x2 (1 – e)/e A/x2
(3.3)
where e is the specific void volume (porosity) of the agglomerate and (1 – e) the respective volume of the particulate solids, k the average coordination number (ke p), and x the representative size of the particulate solids forming the agglomerate The still unknown term in Equation 3.3 is the adhesion force A. As already mentioned above, it must be recognized that, normally, more than one binding mechanism participates in the production of agglomerate strength and that, due to differences in distance and micro conditions, the adhesion force Ai at each coordination point is different. Therefore, Equation 3.3 becomes in its most general form rt = (1 – e)/eRAi/x2
(3.4)
The work of many researchers has concentrated on modeling and calculating adhesion forces that are caused by the different binding mechanisms [B.48, B.97]. So far, all models are based on simplified conditions at the coordination points. A large number of different partner shapes is possible, other than, for example, sphere to sphere, the simplest model that is often used, and, normally, the size of the partners will be different and can vary infinitely. Particles also feature rough surfaces. Therefore, when modeling surface interactions, this can be done macroscopically, disregarding surface roughness, or microscopically. Generally, the true shape of a particle, including surface roughness, can not yet be described unequivocally. New techniques, such as fractal dimensions [B.42], may be applied in the future to solve this problem. Estimation of the strength of agglomerates, which is caused by solid bridges at the coordination points (similar to Figures 3.1c and 3.2c) assumes that the entire solid binder material is uniformly distributed at all coordination points and forms bridges with constant strength, rB. If, in addition, failure only occurs through solid bridges, meaning that the strengths of the particle and of the adhesion bond at the interface are
17
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3 Agglomeration Fundamentals
higher than that of the binder (solid bridge), the relative cross section of that material defines the agglomerate strength rtB (MBqp/MpqB)(1 – e)rB = wBerB
(3.5)
where MB is the mass of the bridge building material and Mp the mass of the agglomerate building particulate solids, qB and qp are the densities of the respective solid materials, 1 – e is the relative volume of the particulate solids building the agglomerate, e is the specific void volume (porosity) of the agglomerate, and wB is the fraction of voids in the agglomerate that is filled with the bridge building material.
3.6
Determination of Strength
“Strength of agglomerates” may have many different meanings. In most cases the attribute “strength” defines a survival characteristic and may be defined as crushing, bending, cutting, shear, or tensile strength, as tolerance to one or several drops from a specific height, thereby reproducing stresses experienced at transfer points, or as resistance to attrition and the formation of dust [B.48, B.97]. For special applications still other measures of “strength” may be elaborated that simulate the real handling or processing conditions. Scientifically the only unequivocally defined and reproducible strength, which is ultimately and with a high degree of probability responsible for all failure modes and can be also approximated by theoretical calculations, is the tensile strength (see above). Results of experimental determinations of agglomerate strength have been published in many scientific works and were summarized in numerous specific books on agglomeration or in major chapters of more general handbooks (Section 13.1). In particular, the proceedings of the International Symposia on Agglomeration [B.4, B.16, B.21, B.26, B.40, B.56, B.94] should be consulted. A considerable amount of fundamental research is going on in many places of the world trying to increase knowledge of all binding mechanisms and develop numerical methods to calculate or at least estimate binding forces as well as agglomerate strength. In addition to the “classic” methods mentioned above, many novel technologies, such as, for example, application of the atomic force microscope [AFM] (also called lateral force microscope [LFM] or scanning probe microscope [SPM]) for the measurement of adhesion in the micron and submicron particle range, and new theories, for instance, fractals [B.42] and the chaos theory, are applied to agglomeration research. However, as mentioned earlier, it is unlikely that only one binding mechanism acts on all coordination points within even a single agglomerate. Moreover, the microscopic conditions at each coordination point are so diverse that bonding at virtually each individual coordination point is different. Therefore, although big advances have been and are being made, the science of agglomeration is still far away from formulating a useful general theory. Furthermore, this book is devoted to a more practical coverage of agglomeration. Therefore, the reader is encouraged to search for and
3.6 Determination of Strength
study the increasing number of publications that report on the advances in this area (Section 13.1). Determination of agglomerate strength in industry is rather pragmatic [B.48]. For the practical and industrial investigation of agglomerate strength, stresses that occur during storage and handling are experimentally simulated. In addition to the frequently used crushing, drop, and abrasion tests, methods for the determination of impact, bending, cutting, or shear strength are employed [B.97]. All values obtained by these methods are strictly empirical and cannot be predicted by theory because it is not known which component of the applied stresses causes the agglomerate to fail. For the same reason, the experimental results from different methods can not be compared with each other. It is even possible that results obtained with the same method but carried out by different laboratories and/or different technicians are not compatible because of characteristic procedures that are inadvertently used by different people. A further problem that is associated with any determination of agglomerate strength stems from the fact that agglomerates are not uniform bodies. Many have no welldefined size or shape and, even if this is the case (such as with, for example, cylindrical compacts or spherical pellets) or if a test specimen is made to specific directions, they are all made from a large number of particulate solids, held together by widely varying binding mechanisms, have void spaces (pores) between the particles, and often feature many structural faults, including cracks, density variations, and others. As a result, there is no well-defined circumstance of failure. The curve in Fig. 3.8 shows a possible results from a compression test with a spherical pellet (assumed to be a perfect sphere). The heavy solid line represents the theoretical increase of force as provided by the test equipment (constant rate). At the beginning, the compression force increases steadily and then begins to deviate slightly from the theoretical curve due to some deformation, mostly at the contact points. At (1) a bulk densification be-
Fig. 3.8 Diagram of force against time curves that may be obtained during the compression strength test of spherical agglomerates
19
20
3 Agglomeration Fundamentals
gins, which is arrested at (2) and the compression strength continues to increase without causing an appreciable densification of the agglomerate. At point (3) different conditions may arise. Pieces of a brittle agglomerate may break off, indicating a first failure. However, it is possible that most of the agglomerate remains intact and, after some densification between (3a) and (4), can be compressed further until final catastrophic disintegration occurs at (5). Plastic agglomerates, on the other hand, may never come apart. After considerable densification (between (3) and (6)) the compression force may increase more or less steeply, indicating formation of a less compressible cake. With this type of agglomerates, “strength” can be only determined by defining a maximum degree of deformation and associating the measured force at this point. For this evaluation a force/displacement curve must be determined (which is similar to the force/time graph shown in Fig. 3.8). While for some applications it is important to know the force causing final catastrophic failure, particularly in such cases where the production of fines from brittle agglomerates must be avoided during handling, the first indication of partial breakage could be selected to determine the strength. This opens the reporting of test results to a wide range of operator-dependent interpretations. Although knowledge and understanding of the fundamentals of agglomeration, particularly the nature and effect of the binding mechanisms and how they can be influenced, become more and more important during the development of new and for the optimization of existing agglomeration processes, agglomeration as a unit operation is still more an art than a science. While an increasing number of criteria are known for the preselection of the most suitable agglomeration process for a specific application [B.97], it is still necessary to test the selected equipment in the laboratories of vendors or development organizations (Section 9.1). Often, if the process is a new one, it is even desirable to operate a smaller pilot plant or to involve a “toller”, an outside processor for hire (Section 9.2), prior to an investment decision for a large scale plant. Agglomerate strength in industry is also defined as a commercial or process characteristic of the particular intermediate or final product. For example, if the agglomerated material is a final product, strength may be defined as resistance to breakage, chipping, or abrasion. The definition of this property and of other strength-related requirements will differ whether it is an industrial bulk material or a consumer product. While the former may break down to a certain extent, it is acceptable as long as it remains free-flowing and dust-free. A consumer product, on the other hand, must have perfect and pleasing appearance where even minimal chipping or breakage into large chunks must be avoided. Intermediate products must have characteristics that are suitable for the intended further processing. For example, a material may have to be strong enough and abrasion resistant for storage and handling to avoid bridging, flow problems, dusting, and segregation of components. However, if it is a feed material for tabletting or other pressure agglomeration methods (Chapter 5), the agglomerates must break down totally under pressure and produce a uniform final product structure. Other agglomerated intermediates may have to feature the opposite property, i.e. they must retain their shape and structure during pressing, for example to yield a filter with bimodal pore size distribution (Chapter 7).
3.7 Porosity
For those reasons, “strength” means many different things in industry. Typically, measurement of strength is based on a simulation of the stresses that a particular agglomerated product must withstand. Very few industrial methods for the determination of this property are standardized or even publicly known. In a competitive environment it is of less interest to compare quality between rivals than to make sure internally that the product properties that are expected by the industrial or public consumer are maintained. Therefore, most measurements of strength are undertaken as quality assurance.
3.7
Porosity
All previously presented equations for strength show that the porosity of agglomerates plays a very important role for their strength. The lower the porosity or, in other words, the higher the apparent density of the agglomerate, the stronger it is. Since many of the desirable characteristics of agglomerated products require high porosity, sufficient strength is obtained in such cases by selecting a suitable binding mechanism featuring high adhesion or binding forces, using a powder with a small representative particle size, applying suitable curing techniques that produce permanent bonds with high strength (sintering), and/or incorporating temporary additives in the feed. During or after the curing step the latter are removed by melting, evaporation, or combustion [B.97].
Further Reading
For further reading the following books are recommended: B.1, B.3, B.4, B.6, B.7, B.8, B.9, B.12, B.13a through e, B.15, B.16, B.17, B.19, B.21, B.22, B.23, B.24, B.26, B.29, B.35, B.36, B.40, B.41, B.42, B.44, B.48, B.49, B.51, B.52, B.55, B.56, B.58, B.64, B.66, B.67, B.68, B.70, B.71, B.72, B.73, B.78, B.82, B.83, B.86, B.88, B.89, B.90, B.92, B.93, B.94, B.97, B.99, B.106, B.107 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold.
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4
Undesired Agglomeration: Methods of Avoiding or Lessening its Effect Agglomeration is a natural phenomenon. Under certain conditions, it happens, whether it is desired or not [B.71, B.97]. The adhesion of small particles is the most basic mechanism of agglomeration, which is most often responsible for unwanted build-up and the conglomeration of fine particles that is frequently observed when handling particulate solids. Tab. 4.1 lists the operations of mechanical process technologies (Chapter 2, Fig. 2.2) and indicates if agglomeration is desired or undesired or, sometimes, both.
4.1
Separation
During separation unwanted agglomeration may occur and must be avoided if a particle collective is to be divided into well-defined classes. The separation curve is a measure of separation quality. In a diagram the degree of separation the percentage amounts of particles above and below the desired separation size) is plotted against the particle size. Fig. 4.1 is a qualitative presentation of several separation curves. Line (a) in Fig. 4.1 represents the ideal or perfect separation of a particle size distribution xmin < x < xmax at cut size xt1 (possible only in theory). In industrial separation equipment, curves of type (b) are obtained. Then, the cut size is that particle size of which half end up in the coarse fraction and half in the fine. The sharpness of separation increases with the slope of the curve. If the abscissa uses a logarithmic scale, separation curves representing similar separation efficiencies at different cut sizes are parallel to each other. The influence of agglomeration must be judged differently. If the separation task is to remove all particles x < xmin from a fluid the desired cut size is xmin. Line (c) in Fig. 2.1 describes the ideal (only theoretically possible) separation curve. In reality, some small particles remain suspended in the fluid and the actual cut size is xt2 > xmin, curve (d). If agglomeration occurs, the finest particles may form larger entities or attach themselves to larger particles, thereby changing the separation curve in Fig. 2.1 to (e). The new cut size xt2’ is still somewhat larger than the desired xmin, at which all particles would have been removed from the fluid, but agglomeration definitely helps to move the actual cut size closer to the ideal one. Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
24
4 Undesired Agglomeration: Methods of Avoiding or Lessening its Effect Tab. 4.1 The occurrence of undesired and desired agglomeration in mechanical and related process technologies Unit operation
Process
Separation
Mixing
Comminution Particle size enlargement
Conveying
Storage Batching, metering Drying Explanations
Agglomeration
Screening, Sieving Classifying, Sorting Flotation Dust Precipitation Clarification, Thickening Particle Size Analysis Dry Mixing Wet Mixing Stirring Suspending, Dispersing Fluidized Bed Dry Grinding Wet Grinding Agglomeration } Briquetting } Granulating } Pelletizing } Pelleting } Sintering Tabletting Mechanical Conveying Vibratory Conveying Pneumatic Conveying Silos, Hoppers, Stockpiles
(+) sometimes YES ( ) sometimes NO
+ YES NO
Undesirable
Desirable
+ + + (-) (-) ++ + + + + + + +
(+) (+) + + -(+) + (+) + -
(+)
++
+ + + + + +
++ ++ (+) (+) --+
++ decisively YES - - decisively NO
In general, whenever the task is to remove all particulate solids from a fluid, agglomeration will be advantageous. Since the smallest, low-mass particles are, on one hand, the most difficult to remove and, on the other hand, have the highest natural adhesion tendency, the chance agglomeration of these particles improves separation efficiency. Therefore, techniques for enhancing the natural agglomeration tendency of very fine particles are often applied during gas and liquid cleaning. Even relatively loose conglomerates behave according to the combined weight of all adhering particles during, for example, settling or in the centrifugal fields of cyclone separators. For all those separation cases that attempt to separate a particle collective according to certain properties of the particulate solids, agglomeration is most often undesired. Techniques for which this statement is true include screening, sifting, classification, sorting, flotation, and, as a general analytical method, particle size characterization. It should also be realized that the respective separation property does not have to be size; it could be density, shape, color, chemical composition, and others.
4.1 Separation Fig. 4.1 curves
Examples of separation
During screening, unwanted agglomeration is often facilitated by the motion of the material on the screen; spheroidal agglomerates are frequently formed from material containing fines or featuring other binding characteristics, for example if it is moist. Among others, binding mechanisms are: for finely divided solids, molecular and electrical forces and/or adsorption layers; for plastics, electrostatic forces; for ores, magnetic forces; for moist powders, liquid bridges and capillary forces; for fibers, interlocking; and for materials with low melting points, partial melting and solidification. With some substances several bonding mechanisms may occur simultaneously. In all cases, the result of screening is distorted because agglomerated fines are classified as coarse. The immediate and complete removal by dedusting or “scalping” of the finest fraction prior to screening into the desired property classes is one practical method of avoiding selective agglomeration of the fines or adhesion of fines to larger particles. During screening itself, the effect of adhesion is reduced by mechanical destruction of agglomerates with, for example, rubber cubes or balls placed on or under the screen decks, the application of brushes, air jets, or ultrasound, and the modification of screen amplitude or frequency (e.g., ultrasonic screen excitation). During the screening of moist bulk materials, difficulties increase with moisture content, but agglomeration tendencies are almost completely eliminated during wet screening when the particles are suspended in a liquid. Since, in moist screening, particles are often held in the mesh openings by liquid bridges, the separation of such materials is facilitated by direct resistance heating, inductive heating, or by modifying the wetting angle and/or the surface tension with surfactants, and blinding of the screen is avoided. In air classification, products from, for example, dry fine grinding are separated. Particular problems arise if the material to be separated contains agglomerates that were formed during comminution. Such conglomerates would be recirculated into the mill and “overgrinding” occurs. Therefore, attempts are made to destroy them by special feeder designs. Destructive forces are caused by, for example, sudden
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4 Undesired Agglomeration: Methods of Avoiding or Lessening its Effect
changes in speed or direction of flow and by installing air jet mills in front of the classifier. In the classifier itself, agglomerates are formed by molecular forces that may be reinforced by adsorption layers if separation is carried out with ambient air, by liquid bridges if moist materials are processed, and by electrostatic forces in a dry environment. As an example, Fig. 4.2 depicts separation curves of various air classifiers. With decreasing particle size, due to agglomeration, the amount found in the coarse fraction increases, because fine particles adhere to larger ones and conglomerates of fines behave as if they were coarser particles. Both effects reduce the separation efficiency and can be avoided only if the causes for adhesion are removed, mostly by eliminating moisture in the material and humidity in the air. Sorting processes that separate materials according to particle characteristics other than size are often carried out in liquids. Agglomeration can again reduce the separation efficiency when smaller particles of other components stick to larger ones. On the other hand, agglomeration can be also beneficial in dense media sink/swim separation, centrifugal separation, or during jigging if particles of a particular ingredient can be made to selectively adhere to each other and form larger, heavier agglomerates. During particle size analysis, in addition to screening, air sifting, and counting, sedimentation in liquids is often applied, which produces unequivocal results only if the individual particles can move without influencing each other. For that reason, very dilute suspensions are used. Nevertheless, it is possible that agglomerates form or conglomerates that are already present do not disintegrate completely. Therefore, dispersion aids are often added, which reduce particle affinity. The molecules of dispersion aids attach to the particles, eliminating polarities and/or reducing interfacial tensions. Separation forces, such as ultrasonic vibrations, can be also introduced for improved disagglomeration and dispersion. In connection with particle size analysis, the importance of correct sample preparation must be stressed. Because natural, unintentionally formed agglomerates always incorporate a larger than average number of the finest particles, the result of particle size analysis will be incorrect if pre-existing agglomerates are not destroyed or conditions prevail during testing that promote agglomeration.
Fig. 4.2 Separation curves of different air classifiers [B.71, B.97]
4.3 Comminution
4.2
Mixing
Many of the previously mentioned considerations apply to the formation and prevention of undesired agglomerates during mixing. Little needs to be added concerning mixing in liquids by stirring or methods for the production of suspensions and dispersions. The addition of dispersion agents is always recommended if the tendency of the solid particles to agglomerate is high. Agglomerates or flocs that are already intentionally, for example to improve handling characteristics of fine powders, or unintentionally present prior to mixing can be destroyed by shear forces in the liquid. Consequently, the generation of the highest possible shear gradient is often considered advantageous when selecting agitators. During extended storage, the particles in pharmaceutical suspensions often form agglomerates that can no longer be destroyed by shaking the preparation. This is of particular concern in, for example, eye drops. The problem can be avoided by controlled flocculation of the solids. After the addition of an electrolyte, the fine particles aggregate to loose flocs that can be easily redispersed by shaking the dispenser prior to application. When mixing dry or moist bulk solids, agglomerates may form, which originate from the finest components of the mixture. They are held together by molecular and electrostatic as well as capillary forces. These undesired agglomerates should be broken up by shear or frictional stresses, generated by the motion of the bulk mass, or by special disintegration devices that are built into the blender [B.97].
4.3
Comminution
During fine grinding in roller crushers and tube mills containing grinding media, with all materials problems begin to develop at a certain fineness of the solids to be milled. Two types of phenomena can be distinguished. In the first case, the finest particles start to adhere to walls and the grinding media in the mill. On this first coating, even coarser particles find excellent conditions for adhesion and massive deposits form rapidly. These layers adhering to the inside walls and the grinding media produce a cushioning effect, which lowers the intensity of stressing and, therefore, increase the duration of grinding and decrease efficiency. The second phenomenon during dry fine grinding is the occurrence of agglomerates in the freely moving charge itself. Formation of such agglomerates is associated with the so called “limit of grinding”. For each material a fineness exists at which, in spite of continued consumption of energy, the finest particles in the charge do not seem to become finer. Agglomeration and adhesion in mills can be attributed to various binding mechanisms. Since the mill housing may become highly charged by the friction between its contents and the walls, electric forces are often the cause for the initial build-up. This effect can be eliminated quite easily by grounding the mill. In other cases, wall deposits
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4 Undesired Agglomeration: Methods of Avoiding or Lessening its Effect
will begin with particles of a size that generally corresponds to that of the wall roughness. The strength of the layer depends on the contact pressure, which is magnified by the mill charge consisting of grinding media and material to be crushed. Adhesion is largely affected by molecular forces; however, partial melting and sintering are also possible. Agglomerates are formed in the freely moving charge of a tube mill by the compaction of fine particles between the grinding media and by recombination bonding (Chapter 5). Adhesion is affected by van-der-Waals forces between the particles that have been compressed very tightly and by the recombination of free valence forces at newly created surfaces. Since these agglomerates are very strong, a “grinding equilibrium” exists, which has been observed and described by many. It means that in fine grinding, after a certain time, a state of equilibrium between size reduction and size enlargement by agglomeration occurs. From that point on, agglomerates are formed, crushed, and re-formed so that the apparent particle size does not change. However, since destruction of particles continues to occur, a growing amorphization of the material can be observed, which also results in increasing specific surface area and is often called “mechanical activation” as, in many cases, higher reactivity is obtained. Every form of agglomeration during size reduction reduces the efficiency of grinding and the fineness obtained at the limit of grinding is often insufficient for many
Fig. 4.3 Agglomerates produced during grinding in a roller mill with a high reduction ratio
4.3 Comminution
tasks, even though the agglomerates actually contain much smaller particles. Therefore, it is desirable to prevent or, at least, reduce these effects. In milling, one possibility to achieve less unwanted agglomeration is to add surface-active substances, so called surfactants or “grinding aids”. It has long been known that small amounts of such additives may reduce the grinding time required to reach a particular fineness by 20–30 %. Molecules of these substances, which are present in a gas or vapor phase, quickly saturate free valences at the newly created surfaces and avoid recombination bonding. As a rule, grinding aids also reduce caking. The formation of lamellar flakes or flat agglomerates in tube mills has been attributed to compaction between the grinding media during impact. The same mechanism occurs in all comminution processes in which the material to be crushed is subjected to stresses between two surfaces, for example in roller mills. Since the second condition for the formation of agglomerates is a sufficient fineness of the particles that are involved, the occurrence of flat flakes is mostly observed during fine grinding. One measure for the fineness, the intensity of stressing, and the unintentional formation of agglomerates is the so called “reduction ratio” that is, the quotient of maximum feed particle size and gap between the rollers. Fig. 4.3 depicts typical agglomerates produced in a roller mill with a high reduction ratio. Since the fine material is immediately compacted, almost all free valences at the newly created surfaces participate in recombination bonding. Consequently, the formation of agglomerates in roller mills can be avoided only if a smaller reduction ratio is chosen or by applying friction between the rollers. Agglomerates can be also formed during impact grinding. Fig. 4.4a shows the fracture lines that develop during impact stressing of a glass sphere. A cone of fine material is created at the impact point and is compacted into an agglomerated mass by the pressure resulting from the kinetic energy of the system (Fig. 4.4b and c). Here too, the effect of free valence forces on newly created surfaces is used to its almost complete extent yielding a quite strong agglomerate. It is very difficult to avoid this type of agglomeration; it can be affected only by reducing the impact velocity which, in turn, results in a lower degree of comminution. For glass spheres, for example, the formation of agglomerates was observed only at impact speeds exceeding 80 m/s.
Fig. 4.4 a) Schematic representation of the fracture lines caused in a glass sphere by impact stress. b) Agglomerated cone of fines created during the impact stressing of a glass sphere (sphere diameter 8 mm, impact velocity 150 m/s). c) Agglomerated cone of fines created during impact stressing of a sugar crystal (shown on the left)
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During the impact crushing of thermoplastic materials or inorganic substances with low melting points, solidified bridges of partially melted material may further increase adhesion and the strength of agglomerated fines. Since the increase in temperature depends on the energy input and is constant for a given impact velocity, cryogenic milling, whereby the particles to be crushed are cooled prior to feeding into the mill, not only results in an increased brittleness but may also avoid partial melting and unwanted agglomeration. In wet grinding, as a rule, agglomeration is avoided by suspending the particles in liquid. Sometimes, the product of dry fine grinding is subjected to a brief final wet grinding to destroy the previously formed agglomerates. Nevertheless, some materials also tend to flocculate in wet grinding. Since the adhesion forces causing flocs are mostly electrical in nature, the addition of a small amount of electrolyte to the suspending liquid nearly always suffices to prevent flocculation.
4.4
Agglomeration
By definition, the size enlargement by agglomeration operation makes use of all binding mechanisms and often enhances them in suitable environments and equipment. All agglomerates that are manufactured are made intentionally and are desired. Nevertheless, there are instances where adhesion and agglomeration are unwanted and undesired. Because, particularly in tumble/growth agglomeration, binders are added, the effect of these binders is still present on the surface of green agglomerates and during post-treatment new binding mechanisms between the agglomerates may develop, which result in the formation of clusters. Of course, because agglomerates are larger bodies and only a few interaction points (coordination points) are present in a unit volume of the cluster, even relatively strong solid bridges, which may have developed by recrystallization, chemical reaction, or sintering during post-treatment can be broken relatively easily. Nevertheless, such clusters could be detrimental during storage, feeding, or metering and, therefore, should be avoided or broken up prior to a following process step. More information on potential problems is given in the section 4.6 on storage.
4.5
Transportation
During the conveying of particulate solids, especially of finely dispersed powders, the unintentional formation of agglomerates and (sometimes thick) coatings on walls is often observed. Whereas agglomerates occur mostly on vibrating or shaking conveyors and inclined conveyors or chutes, build-up on walls is more common in pneumatic conveyors. The main causes of agglomeration during the conveying of fine particulate solids are molecular and electric forces as well as binding mechanisms related
4.6 Storage
to moisture and, as a result of mechanical or thermal energy input, binding mechanisms such as partial melting and solidification can occur, too. Although it is very difficult to avoid agglomeration on vibrating or shaking conveyors, several possibilities exist for the prevention of wall build-up and deposits during pneumatic transport. Since the adhesion of the finest particles always begins in the roughness depressions, one of the most important conditions for avoiding a common reason for initial build-up is to provide smooth inner wall surfaces of pneumatic conveying lines. Because high drag forces tend to remove particles that have already adhered to the walls, high transport velocities also reduce the danger of build-up. Deposits preferably start in dead or calm flow zones; therefore, when designing such systems, low speed areas must be avoided. On the other hand, sudden changes in the direction of flow will cause high energy impacts of particles with the wall, causing build-up. Friction between particulate solids and pneumatic conveyor walls may result in high electrostatic charges on both partners. They depend to a large extent on whether the particles and/or the walls are electrically conductive and if the lines are grounded or not. System design must take these conditions into consideration.
4.6
Storage
Adhesion phenomena are involved in, for example, the bridging of particulate solids in hoppers. In the case of relatively coarse materials, bridge formation is caused by domelike structures, which are supported on the inclined walls in the lower conical part of bins. With decreasing particle size, the participation of adhesion forces in bridging and agglomeration increases. Binding mechanisms are molecular forces and adsorption layers or liquid bridges. The latter often play an important role whereby liquid bridges form by capillary condensation at the coordination points. Feeding warm and moist material into silos must be avoided, even if the moisture content is very low. Evaporating moisture may condense on the cooler silo walls and drip into the charge, forming wet agglomerates and causing strong capillary adhesion bonding of particles on the walls. Insulation of the silo and/or forced large volume venting can be employed to avoid condensation and agglomeration problems. Bridging can totally block the discharge from silos, thus causing severe operating problems. Because adhesion even of finely dispersed dry solids cannot be avoided, agglomerates and bridges must be destroyed by special devices. For this purpose, inflatable cushions are mounted to the inside walls of silos or the material is momentarily fluidized by the (pulsed) injection of air. In the case of coarser solids, which tend to form domes, it is often sufficient to select a cone with steeper sides (“mass flow” design). Small remaining flow problems due to adhesion can be overcome by installing vibrators or “hammers” on the outside silo walls. Unwanted agglomeration is often observed if the particulate materials are soluble or if chemical reactions can occur, particularly in the presence of moisture. These phenomena are very common and are called caking if they occur in bulk masses or bag-set if material solidifies in bags.
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Different materials become caked during various storage and handling procedures but caking itself is almost exclusively by solid bridges or, more specifically, by chemical reaction and crystallization of dissolved substances. Other binding mechanisms contribute only slightly to caking. The rate and extent to which caking takes place depends on the moisture content, the particle size or specific surface area, the pressure under which the material is stored (e.g., top or bottom of the pile), the temperature and its variation during storage, as well as the time. The influence of temperature and temperature variations depends on the solubility of the solids. Fig. 4.5 shows four different temperature–solubility curves. Whereas the solubility of sodium chloride changes little with temperature, this is not true for potassium chloride (or potash) and potassium nitrate, for example. The latter features a very steep curve. Some salts, such as sodium sulfate, exhibit various temperature-dependent solubility ranges. Salts or mixtures of different salts containing a small amount of moisture, may cake during storage and/or transport if exposed to changing temperatures even if the moisture content is very small and the material is packed in airtight containers. In many cases (Fig. 4.5), more salt will be dissolved at higher temperatures, which recrystallizes and forms solid bridges between the particles when the temperature drops again. Repeated cycling, for instance due to climatic changes or differences in day and night temperatures, reinforces this bonding and causes bag-set.
Fig. 4.5 Solubility curves of four different salts
4.6 Storage
The crushing strength of caked materials depends on the number of bridges formed per unit volume and, therefore, decreases with increasing particle size. As mentioned earlier, mixtures of powdered soluble materials that were granulated by agglomeration may still set up somewhat due to the mechanism described above. However normally, the granulated material can be broken and disagglomerated easily. The answer to what can be done to avoid or, at least, lessen caking is complex but generally the same as in all other cases where unwanted adhesion or agglomeration occurs: Detect the binding mechanism that is responsible and the parameters that influence the process. Then try to reduce their effect. In the following some examples will be discussed briefly. If (unobjectional) chemical reactions between components of a mixture do occur, these components should be mixed separately until the reaction is completed. The resulting intermediate product can then be blended with the other components and no longer induces caking. An almost trivial precaution is very often to lower the moisture content. However, this is not always necessary. Different maximum moisture levels exist that depend on the material. It was found during microscopic studies that caking usually resulted from bonding by the crystals of soluble salts. These crystals often covered the entire granule surface in the form of a veneer or hull. Those salts migrated to the surface of the granule as a solution, leaving numerous small cavities within. Since this mechanism requires water, drying should reduce caking. Some materials respond favorably to several days of curing prior to bagging. Such products cake in a few days to their final strength and the resulting lumps are broken up before the cured materials are bagged and put into long term storage. Curing can even accelerate hull formation owing to heat and moisture retention. In products that respond well to curing, additional development of crystals on the granule surfaces during subsequent storage is not sufficient to cause significant caking. However, many products do not improve during this type of curing. Another curing method will be described below. The oldest method of conditioning granular materials is coating with a parting agent. Storage properties are improved after the addition of up to 3 % of an extremely fine particulate solid such as pollen, diatomaceous earth, kaolin, vermiculite, pulverized limestone, magnesium oxide, and a variety of other inexpensive, very fine powders. Again, microscopic studies revealed the fundamental properties of a conditioner, which are threefold. 1. The powder coating acts as a separator and prevents crystal bridge growth between the individual granules during and after drying. 2. The hull crystals from beneath the coating rarely project beyond the layer of conditioner. 3. The moisture is distributed uniformly over the surface of the granules due to the high sorptive capacity of the finely porous coating. Thus the localized growth of crystals at the coordination points is prevented and the surface hulls are much finer grained, more intergrown, and more densely packed than those covering un-
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4 Undesired Agglomeration: Methods of Avoiding or Lessening its Effect
conditioned products. Such anticaking conditioning agents are usually applied by mixing them with the granular material in a rotary tumbler (typically a drum) prior to bagging. A modern variation of the above mentioned conditioning process is the coating with surface-active organic chemicals. It was found, however, that not all surfactants improve the physical conditions of granules. It was reported that the caking tendencies can be reduced by as much as 45 % if non-ionic chemicals were used but increased by as much as 37 % with the use of anionic materials. Where in the process the surfaceactive agents were applied was also found to be of decisive importance. Typical cationic anticaking agents are fatty amines with a general formula R-NH2 with R representing C16 and C18 chains. they are believed to attach directly to the particles with their surface-active amine group. The fatty, hydrophobic part of the molecule extends outward, thus preventing hygroscopic products from attracting moisture. Of course, this is only true if a monomolecular layer covers the granules and all amine molecules extend their hydrophobic portion outward. Therefore, too much conditioner may cause rather than prevent caking. Multiple layers are alternately hydrophobic and hydrophilic. The above makes an alternative curing process advantageous: The molecules of a second molecular layer, if attached, would position themselves with the amine group extending outward. These amine groups are free to interact with other particles. Pressure intensifies this effect. The resulting chemical “bridge” is not as strong as a recrystallized salt bridge and the “set” can be broken easily. Sometimes a combination of the two types of conditioners is used. An example of this approach is finely divided kaolin or talcum powder treated with surfactant. A last but not least method is granulation by agglomeration. Today, for very fine solids, this technology is almost obligatory, particularly for powder mixtures. Size-enlarged, granular products offer fewer coordination points per unit volume where solid bridges can develop. If the strength of the bridges is low anyway, granulation alone is sufficient to prevent severe caking. Agglomeration also prevents segregation during storage. The above examples were selected to demonstrate how unwanted agglomeration problems can be studied and how possible remediation methods and techniques
Tab. 4.2
Some dvantages of uncontrolled, natural agglomeration
Separation
Mixing Comminution Agglomeration Transportation Storage
After agglomeration, size enlarged ultrafine particles can be removed by conventional separation equipment. Selective agglomeration of specific particles in liquid suspensions (immiscible binder agglomeration. Selective agglomeration (powder coating). Mechanical activation (amorphization by crushing and recombination bonding). See Table 6.1 Larger, heavier particles. Smaller number of coordination points per unit volume.
4.6 Storage
can be determined. They date back to a point in time when the fundamentals of unwanted agglomeration in different industries were first investigated and means were developed to avoid some of these phenomena. Tab. 4.2 and 4.3 summarize what has been discussed. Tab. 4.2 shows that some of the uncontrolled, natural agglomeration phenomena are not necessarily detrimental. Tab. 4.3 lists a few of the possibilities to avoid or at least lessen the effect of unwanted agglomeration. While undesired agglomeration is often very important, because its effects may result in considerable losses of production and profit, most of the past and present Tab. 4.3 Summary of some of the possibilities to avoid or at least lessen the effect of unwanted agglomeration Separation
Mixing
Dry/moist Wet
Comminution
Agglomeration Transportation
Storage
Remove or “scalp-off” fines (immediately) at the source. Destroy agglomerates mechanically (rubber cubes/balls, shear, impact, brushes, gas (air) jets, ultrasound, etc.) Modify amplitude and/or frequency. Remove moisture from material and/or gas (air) environment. Heat (direct resistance/inductive). Modify wetting angle and/or surface tension of liquid (surfactants). Disperse particulate solids in suitable liquid (wet separation). Ground equipment (removal of electric charges). Use dispersion aids (chemicals, mechanical {e.g. stirring, ultrasound}). Apply shear or frictional stresses (tumbling mass, mixing tools, special accessories (e.g. shredders, baffles) Use shear (stirring) or dispersion agents. Utilize controlled flocculation (addition of electrolyte) that allows repeatable redispersion by shaking. Remove fines and/or moisture. Ground equipment (removal of electric charges). Coating of the inner walls of mills and crushers (elastic or non-stick). Add grinding aids (steam, vapors, surface-active substances). Lower reduction ratio (potentially multiple grinding steps). Use cryogenic milling (making use of material brittleness at low temperatures). Apply wet grinding (possibly with the addition of a small amount of electrolyte). Use drying and other post treatment measures. Modify surfaces by coating or encapsulation. Remove fines and/or moisture. Ground all lines (removal of electric charges). Make equipment, chutes, and pipes from (grounded) electrically conducting materials (avoid plastics). Finish inner walls smooth or with coating (e.g. Teflon). Remove fines and/or moisture. Cool materials to ambient conditions prior to storage. Insulate and/or heat walls. Ground (electrically conductive) hoppers, silos, and storage containers. Smooth and/or coat (e.g. with Teflon) all inner walls. Use steep (as close to vertical) walls, especially towards discharge openings. Employ mechanical bridge breakers (vibration, shock, {pulsed} air jets, inflatable inserts, etc.). Apply load (overburden) relieving means (e.g. “chinese hats”, baffles, etc.). Carry-out conditioning/curing prior to storage.
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4 Undesired Agglomeration: Methods of Avoiding or Lessening its Effect
major publications deal only with methods, equipment, and systems for the production of agglomerates with beneficial properties. Therefore, it is an important achievement that a book authored by Griffith and entitled Cake Formation in Particulate Systems [B.50] was published that discusses especially the unwanted adhesion and agglomeration phenomena. The author distinguishes four major classes of caking in particulate systems: * * * *
mechanical caking, plastic-flow caking, chemical caking, and electrical caking.
In addition, several sub-classes are defined whereby specific properties of components, either pure substances or part(s) of a formulation, can be expected to cause caking under certain conditions. After describing the above, considerable emphasis is given in the book to laboratory techniques and test procedures that need to be considered by those engaged in solving caking problems. Griffith [B.50] states that one of the greatest difficulties in classifying caking (undesired agglomeration) problems comes from the very large number of variables that can contribute to its occurrence. They include material type and composition, including modifications by trace elements, which may be desired ingredients or contaminants, moisture, temperature, pressure, motion, and others. Therefore, in the section that deals with the experimental design of laboratory studies a few of the statistical methods of identifying and ranking variables are also discussed along with an evaluation of possible interactions.
Further Reading
The book [B.50] is recommended for further reading.
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5
Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods Three technologies are available for the desired size enlargement of small particulate solids by agglomeration: * * *
tumble/growth agglomeration, pressure agglomeration, and agglomeration by heat/sintering.
They can be divided into the following sub-groups [B.97]. A 1. 2. 3. 4. 5. 6. 7.
Tumble or growth agglomeration: high-density tumbling bed, high-shear tumbling bed, high-density/high-shear with abrasion or crushing transfer, low-density fluidized bed, low-density particle clouds, agglomeration in stirred suspensions, immiscible liquid agglomeration.
B Pressure agglomeration: 1. low-pressure agglomeration, extrusion through screens, 2. medium-pressure agglomeration, pelleting, extrusion through perforates die plates, 3. high-pressure extrusion, ram presses, 4. high-pressure agglomeration, – in confined spaces, punch-and-die pressing, tabletting, – in confined spaces, isostatic pressing, – in semi-confined spaces, roller presses. C Agglomeration by heat/sintering: 1. agglomeration of stationary particle beds by sintering, 2. bonding of pre-agglomerated bodies or parts during post-treatment to obtain final product properties, 3. agglomeration and bonding during special pressure agglomeration processes (i.e., hot isostatic pressing). Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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5 Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods
In addition, existing and new innovative technologies use the phenomena and fundamentals of agglomeration for purposes other than size enlargement. Specifically, they produce changes or improvements of the properties of particulate solids or achieve modifications of the surfaces of solids. Others manipulate individual particles or deposit ultrafine particles onto substrates in a controlled manner and subsequently bond them with the base material and/or with each other. D Technologies using the phenomena and fundamentals of agglomeration for purposes other than size enlargement: 1. coating, 2. hybridization, mechanofusion, 3. deposition and/or manipulation of individual particles. Most technologies in categories (A) to (D) can be subdivided into two techniques: * *
those utilizing no binder, and those requiring a binder.
It should be pointed out that the binding mechanism in binderless agglomeration often resembles that of bonding with a binder. This is due to the fact that binders are sometimes inherently available and act during agglomeration and/or post-treatment. A typical example of this process is the wet agglomeration (Fig. 5.2, left) of materials that are easily soluble in the liquid. The modified surface tension of the solution may already influence the strength of the green agglomerate and during drying (the necessary post-treatment to convert the intermediate wet agglomerate into the dry, final product) solid bridges develop by recrystallization of the dissolved substance(s). Other inherently available binders have to be activated by a so-called conditioning process prior to agglomeration. A typical example of this technique is in the pelleting (Fig. 5.10, b1–b6) of animal feed where the starchy component of feed grains becomes plastic and sticky during moistening and heating with steam while mixing the formulation in a kneader. After this conditioning, the starch provides plasticity that is required for extrusion through bores in the dies of pelleters as well as green and dry product strengths (see Section 6.5.2).
5.1
Tumble or Growth Agglomeration
The basic mechanism of tumble/growth agglomeration is shown in Fig. 5.1. Adhesion of individual particles to each other or to solid surfaces is controlled by the competition between volume-related separation and surface-related adhesion forces [B.48, B.97]. To cause permanent adhesion, certain criteria must be fulfilled. The most important is that the sum of all separation forces in the system (e.g., caused by gravity, inertia, drag, etc.) must be smaller than the attraction forces that act between the adhering
5.1 Tumble or Growth Agglomeration
partners. According to Fig. 3.6 and Equation 5.1, the ratio between the binding forces Bi(x) and the sum of the active components of all ambient forces Fjy(x) is a measure of the adhesion tendency Ta Ta = RBi(x)/RFjy(x) > 1
(5.1)
Both the attraction and the ambient forces are mainly dependent on the size x of the powder particle(s). To cause adhesion, Ta must be larger than one. In most cases, to keep the particle(s) adhering, the sum of all moments Qj(x) must be zero, too: Qj(x) = x/2 RFjx(x) = 0
(5.2)
Most of the attraction forces (Tab. 3.1) have only a short range; their magnitude and strength decreases quickly with increasing distance. Therefore, because the surfaces of all particulates are, at least microscopically, rough, and the mass of the particles decreases with the third power of the particle size, the adhesion tendency increases with decreasing particle dimensions. The mechanism as depicted in Fig. 5.1 occurs naturally if the agglomerate forming particles are micron and submicron or nanosized solids (beginning at about <10 lm), even if they are dry. In the case of larger particles, the adhesion forces must be produced by the addition of binders (mostly water and other liquids) or enhanced by conditioning and the probability of collision must be increased by providing a high concentration of particles. Such conditions are obtained (Fig. 5.2) in inclined discs (pans), rotating drums, and any kind of powder mixer. Relatively lose agglomerates are obtained in fluidized beds, which realize an irregularly moving particle bed with lower concentrations. Sometimes, simple rolling and tumbling motions, for example on inclined stationary or moving surfaces, are sufficient for the cheap formation of crude agglomerates [B.48, B.97]. In most instances, tumble/growth agglomeration processes yield first so-called green agglomerates (see below) after growing nuclei into larger, nearly spherical aggregates by coalescence and/or layering (Fig. 5.1). These wet agglomerates are temporarily bonded by the effects of surface tension and capillary forces of the liquid binder. While, occasionally, components within the green agglomerate naturally produce permanent bonds by, for example, cementitious reactions, in most cases post-treatments consisting of all or some of the following processes are required to obtain permanent and final strength (right hand side of Fig. 5.2): heating, potentially chemically reacting,
Fig. 5.1 Basic mechanism of tumble/growth agglomeration
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5 Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods Fig. 5.2 Diagram of typical equipment for size enlargement by tumble/growth agglomeration (left) and the post-treatment steps required to obtain a final product (right). Left, from the top: inclined disc (pan), drum, continuous mixer, batch mixer, fluidized bed. 1) liquid binder (spray), 2) fresh feed, 3) recirculating fines, 4) dryer, 5) cooler, 6) double deck screen, 7) mill, 8) conditioning drum
drying, and, sometimes, sintering or partial melting, cooling, screening, adjustment of product properties by crushing and conditioning as well as recirculating of undersized material. Since it is difficult, if not impossible, to screen green (wet) agglomerates without blinding the screen cloths, separation of undersize material for recycling occurs normally after drying, reacting (if applicable), and cooling. Although the mostly pre-agglomerated recirculating particles often play an important role in tumble/ growth agglomeration, because they provide most of the nuclei that are necessary for an accelerated growth of product-size agglomerates (B.97], the sometimes very large percentage recycled (often > 300 %) must be again activated for agglomeration by rewetting and needs to pass once more through the entire process, including heating, drying, and cooling, which, in the final analysis, may render the technology uneconomical [B.97]. To arrive at the methods that achieve size enlargement by agglomeration in a desired and controlled manner, both a movement of particles and binding mechanisms must be created and enhanced. As the solids move in relation to each other, for example in the relatively dense bed of a rotating or otherwise actuated containment of some sort or in a low-density suspension, particles of any size and kind will collide from time to time and, if the attraction force at the collision site is high enough, coalesce.
5.1 Tumble or Growth Agglomeration
Theoretically, for this phenomenon to occur, no specific piece of equipment is necessary. As long as the solid particles are kept in irregular, stochastic motion, the probability of collision and coalescence exists. If, additionally, the binding force that has developed upon impact is strong enough to withstand the separating effects of all system forces (above and Fig. 3.6, Chapter 3) and does not disappear with time without being replaced by some other binding mechanism, the “seed agglomerate” will survive and eventually collide with other particles or agglomerates (Fig. 5.1). At each instance of collision the bonding criterion as defined in Equations 5.1 and 5.2 will be tested, leading to either growth, indifference, that is, the colliding partners will separate again and remain single, or the destruction of weaker agglomerates. To achieve growth, the individual mass of adhering particles must be small and their surface large. This is equivalent to the requirement that the size of agglomerating particles must be small. Typically, the surface equivalent diameter [B.48, B.75, B.97] should be in a range below about 100–200 lm. The limitation to small dimensions of the particles forming the agglomerate and the fact that, in most cases, only temporary bonds are formed constitute major drawbacks of all tumble/growth agglomeration methods. If particles are larger than required, crushing to achieve the necessary fineness is normally uneconomical. Immediately after growth agglomeration, in the green (moist or wet) stage, the main binding mechanisms (Chapter 3) are caused by bridges of freely movable liquids, capillary pressure at the surface of particle conglomerates that are filled with a freely movable liquid, or adhesion caused by viscous binders and slurries. To a lesser degree, other binding mechanisms, such as van-der-Waals, electric, and magnetic forces, may also participate. After curing, which often results also in a considerable strengthening of the agglomerates, bonding is achieved by solid bridges resulting form sintering, chemical reactions, partial melting and solidification, or recrystallization of dissolved substances. Some tumble/growth agglomeration equipment can handle large volumes effectively if the above requirements (small primary particle size and instantaneous bonding with high strength) are fulfilled. The apparatus is simple and the design is unsophisticated but control depends largely on operator experience. Curing is normally the expensive part of plant investment and also contributes to a large extent to operating costs, both of which may render an otherwise perfect technology uneconomical. However, if very large amounts of solids must be agglomerated and the finely divided particulate form of the primary particles is required for other reasons, for example, the concentration of valuable components of ores (Section 6.8), tumble agglomeration is the preferred technology. In those cases the main binder is water. At production capacities exceeding 1 million t per year, the curing facilities become cheaper and more economical and methods for, for example, the recuperation of heat to make the process more efficient and reduce operating costs become feasible. Other reasons for the application of tumble/growth agglomeration, even at small capacities, may be the high porosity of the agglomerates with other attendant beneficial product characteristics, such as high surface area, e.g., for catalyst carriers, and easy solubility, e.g., for food (drink) and pharmaceutical products (Sections 6.2.1, 6.3.1, and 6.4.1). These advantages may be so valuable that additional costs for grinding to
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5 Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods
obtain the necessary small particle size for agglomeration will be acceptable and high operating costs can be absorbed. In these cases, even the agglomeration liquids (binders for the formation of green agglomerates) may be so costly that they are condensed from the dryer off-gas and recirculated. In tumble/growth agglomeration distinct process steps can be defined in which (Fig. 5.2): 1. green agglomerates are formed from solid particles and binder, 2. green agglomerates are cured, 3. if necessary, the cured agglomerates are sized (undersized material is recirculated and oversized agglomerates are crushed and rescreened or recirculated), 4. if desired, post-treatment takes place, for example, the application of anticaking agents, coating, etc. Steps 3 and 4 may sometimes move ahead of step 2 to avoid the energy cost of repeated drying and rewetting of large circulating streams of material. However, since sticking and other unwanted agglomeration problems may be encountered during sizing and oversize crushing, application of this alternative may not always be feasible. In a broad sense, equipment for the tumble/growth agglomeration process itself may be divided into the following. I. Dense Phase Tumble/Growth Agglomeration: apparatus producing movement of a densely dispersed mass of particulate solids. II. Suspended Solids Agglomeration: apparatus producing movement while keeping solid particulate matter suspended or dispersed in a fluid. In both cases, finely divided binder is added in a suitable manner to the turbulently agitated mass of particles. If solid particles are suspended in a liquid, agglomerates may be formed after adding a second, immiscible bridging (binder) liquid. In the widest sense, this technology (called Immiscible Binder Agglomeration) belongs to the type II processes. Generally, the basic process, described in Fig. 5.1, continues, causing size enlargement by agglomerate growth. However, as it proceeds, somewhat more complicated mechanisms evolve. Fig. 5.3 and 5.4 [B.48, B.97] present almost identical explanations of what is happening. While Fig. 5.3 is the more easily understandable series of sketches defining nucleation, random coalescence, abrasion transfer, as well as crushing and layering (preferential coalescence), Fig. 5.4 distinguishes between size enlargement and size reduction phenomena, both of which take place simultaneously. Nucleation, the production of primary agglomerates or “seeds”, occurs when several individual particles adhere to each other. Nucleation is the most difficult and time consuming part of any tumble/growth agglomeration process. Since only a small number of nuclei survives at any given time, this initial part of the growth process is time consuming. As long as individual particles are available they tend to adhere, trying to form nuclei or attach themselves to larger agglomerates. The latter becomes the preferential pro-
5.1 Tumble or Growth Agglomeration Fig. 5.3 Sketches explaining the different processes taking place during tumble/ growth agglomeration [B.48, B.97]
Fig. 5.4 Diagram of the mechanisms involved in size changes during tumble/growth agglomeration [B.48, B.97]
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5 Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods
cess because the larger entities with more mass and higher kinetic energy easily “pickup” individual particles and incorporate them into their surface structure. Therefore, to accelerate the tumble/growth agglomeration process specific operating strategies that influence the nucleation stage are commonly applied [B.97]. Most of the industrial tumble/growth agglomeration methods require liquid binders to achieve satisfactory bonding for nucleation and continuing size enlargement. Although these liquids are suitably dispersed, for example by atomization [B.97], the size of the droplets produced by even the finest liquid spray nozzles is often much larger than that of the agglomerate forming primary particles. Therefore, as described by Ennis [5.1], nucleus formation in a dense tumbling particle bed (type I process) occurs as depicted in Fig. 5.5. As a droplet contacts fine particulate solids, the initial distribution of binding liquid, which depends on its wetting characteristics, influences the sizes of the resulting nuclei. Perfect wetting results in relatively strong large primary agglomerates that are saturated with liquid and are consolidated and held together by capillary forces, while imperfect wetting produces moistened particles, which may coalesce later and produce much weaker nuclei with different sizes which, in addition, are prone to attrition. If, on the other hand, a droplet makes contact with a larger entity, i.e., an agglomerate, wetting either results in spreading of the liquid on the surface or, at least partially, penetration into empty pore spaces below (Fig. 5.6). Both result in a moistening of the surface, which then attracts and captures other units, single particles or agglomerates, causing growth. Fig. 5.7 explains schematically the processes [5.1]. As the mass of the growing agglomerates increases they may break apart at structurally weaker areas or as a result of the force of impact. Abrasion will also take place resulting in newly liberated primary particles or small conglomerates, which then try
Fig. 5.5 Nucleus formation in a dense tumbling bed of fine particles [5.1]
5.1 Tumble or Growth Agglomeration Fig. 5.6 Spreading or penetration of a droplet into a powder bed [5.1]
to attach themselves to entities offering better binding conditions (Fig. 5.3 and 5.4). Particularly in batch operations, both mechanisms help to prevent the growth of a few agglomerates to excessively large sizes. To make sure that the production of oversized agglomerates is avoided or, at least, reduced, individually controlled cutting or shredding devices are often installed, which will continuously or intermittently operate and mechanically assist the breakdown of agglomerates (high-density/high-shear methods with abrasion or crushing transfer [B.97]). Depending on the density of the tumbling material, the (changing) mass of the individual agglomerates, and the type of equipment causing agitation, the growth phenomena and, herewith, the agglomerate properties will differ. One reason for change is the varying extent of the previously mentioned naturally occurring or mechanically induced abrasion, break-down, and reagglomeration. Another is how new particles are attached and incorporated into the structure. It is obvious that particle beds, tumbling in rotating equipment or agitated by mixing tools, will produce denser agglomerates than obtained in the low-density particle clouds of fluidized beds.
Fig. 5.7 Processes occurring between particles of a tumbling powder bed after wetting with droplets of a binder liquid [5.1]
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5 Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods Fig. 5.8 Diagram of particle and droplet distribution in a low-density fluidized bed and of particle penetration into a liquid [5.1]
Referring to Fig. 5.8 and comparing it with Fig. 5.6, in low-density fluidized particle beds (type II process), particles and droplets coexist and, in most cases, agglomerates form by particles penetrating into the liquid. This is the prevailing growth phenomenon in re-wet fluidized bed agglomerators, were growth is limited by the requirements of particle fluidization. A more basic growth mechanism in fluidized beds, which resembles that of “natural” agglomeration is obtained if the particle surfaces are moistened by condensing steam in “steam jet agglomeration” [B.97] or by ultrafine liquid droplets (including solutions and suspensions) produced with the “RESS process” [5.2].
Tab. 5.1 Some conditions, reasons, and requirements for the selection of tumble/growth agglomeration. Process conditions Small feed particle size (xo < 100–200 lm) Wet processing Normally, (liquid) binder(s) is (are) required Green (wet) agglomerates with low initial strength Post-treatment for final strength and properties Simple equipment design Control depends largely on operator experience Difficult to clean (danger of cross-contamination) Not amenable to quick turn-over. Product characteristics Irregular (overall spheroidal) shape Final strength depends on binder(s) and post-treatment Wide distribution of agglomerate sizes and weights High porosity High solubility, dispersibility, reactivity, etc. Shelf-life often limited
Cold processing
System “footprint” is large
Small pieces
Low density
5.2 Pressure Agglomeration
During the agglomeration by growth in tumbling particle beds, “natural” adhesion forces, which are either totally inherent, enhanced by suitable methods, introduced by binders, or acting as a combination of two or all of these effects, cause particles to stick together when they collide in a stochastically moving mass of particulate solids. With the exception of forces that are exerted during the interaction between the particles, the environment and equipment walls as well as, in some cases, various mixing and/or shredding tools, no externally induced directional forces or pressures act on the growing agglomerates and no shaping, other than caused by attrition, occurs. As a result, depending on the level of interaction, which is largely influenced by the tumbling bed density, more or less spherical agglomerates are grown featuring a size distribution that depends on the system properties. Because of the relatively small forces caused by interactions in and with the tumbling charge, porosity of the agglomerates is high and increases as bed density decreases. Also, since the adhesion forces are small, too, and separation forces, which try to destroy the growing agglomerates, are mass related, the primary particles forming the agglomerate must be small. Typically, with a few exceptions, temporarily bonded “green” agglomerates are produced, which require posttreatment to achieve permanent, final strength. Narrowly sized products are obtained by additional processing such as sizing, crushing, and shaping or attrition. In summary, Tab. 5.1 compiles some of the conditions, reasons, and requirements that are typical of tumble/growth agglomeration. Of course, this table does not claim to be complete but is meant to provide some guidelines that can be also gleaned in more detail from the discussion above and from other sources (Section 13.1).
5.2
Pressure Agglomeration
In pressure agglomeration, new, enlarged entities are formed by applying external forces to particulate solids in more or less closed dies that define the shape of the agglomerated product. In contrast to tumble/growth agglomeration, pressure agglomeration is used to achieve one or more and sometimes all process conditions and product characteristics summarized in Tab. 5.2. The level of force that is applied during densification and shaping is the most distinguishing factor in pressure agglomeration. Therefore, the technology is subdivided into low-, medium-, and high-pressure techniques. Of course, as will be shown later, certain conditions and characteristics are better obtained with one or the other pressure agglomeration process and, sometimes, one or more of the parameters of Tab. 5.2 can not be met with a specific technique and/or equipment, system, or plant. Each of the great variety of pressure agglomeration methods corresponds to one or more of the binding mechanisms of agglomeration (Chapter 3, Tab. 3.1, Fig. 3.4 and 3.5). While in low- and medium-pressure agglomeration all binding mechanisms are equally possible, in high-pressure agglomeration attraction forces (Fig. 3.5) provide the most common bonding. According to the mechanisms involved, the processes can be again further categorized as those using binders and those without binders. However, all have in common a basic compaction mechanism.
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5 Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods Tab. 5.2 Some conditions, reasons, and requirements for the selection of pressure agglomeration. Process conditions Larger feed particle size Dry processing Hot processing Processing of elastic materials Easy clean-out Product characteristics Specific shape Specific mass (or weight) Low porosity Long shelf-life Amenable to the production of near-net-shape parts
High initial strength No or little binder No post-treatment Automatic operation Quick turn-over Large pieces High density High final strength
The upper part of Fig. 5.9 shows with four model sketches the structural change of a bulk mass of particulate matter during densification in a die, the attendant change in volume, and an indication of the modifications of particle shape and size that occur at high pressure. The lower part of Fig. 5.9 depicts the build-up of pressing force with time under the assumption that the forward movement of the punch occurs with a constant rate until pmax is reached, at which point the direction of movement reverses and the punch retracts with the same or a higher speed. Referring to Fig. 5.9, as a first step, pressure agglomeration achieves a rearrangement of particles that requires little force and does not change particle shape and size. This is followed by a steep rise of pressing force during which brittle particles break and malleable particles deform. Sketches 3 (brittle) and 4 (plastic) occur either/or and often simultaneously if both brittle and malleable particles are present in the mix.
Fig. 5.9
Sketches explaining the mechanism of pressure agglomeration
5.2 Pressure Agglomeration
Two important phenomena limit the speed of compaction and, therefore, the capacity of any pressure agglomeration equipment: compressed residual gas (in most cases air) in the pores and elastic springback. Both cause different, equipment specific cracking and a weakening or, sometimes, total destruction of the products [B.13(b), B.48, B.97]. Low- and medium-pressure agglomeration apply small forces (up to about the beginning of the steep slope of the curve in Fig. 5.9) but, nevertheless, the removal of a relatively large amount of gas must be guaranteed (the time axis is directly proportional to the volume change since the punch advances with constant speed). Development of compressed gas pockets within the densifying product can be avoided if compaction occurs slowly enough for all gas to escape from the diminishing pore space. High-pressure agglomeration extends into the steep increase of the pressing force. In this range, particle size and shape change by breakage and or deformation and porosity is further reduced. The maximum pressing force is normally defined by an overload feature of the equipment. Since a predetermined final strength and structure must be reached, equipment selection must take into consideration that a sufficiently high maximum pressing force can be attained. After arriving at the maximum pressing force, pressure is released. If, as shown in Fig. 5.9, compaction is performed by a punch in a die, the direction of travel of the piston reverses and, when no expansion of the densified body occurs, the pressing force should drop to zero immediately. In reality, there is always a more or less pronounced rebound, which is caused by the expansion of compressed gas and the relaxation of elastic deformation. This effect becomes more pronounced with increasing speed of densification until, at a certain compression rate, the compacted body disintegrates partially or totally upon de-pressurization. Therefore, it is often necessary to find an optimal compromise between densification speed (capacity) and product integrity (quality). The problem becomes greater with finer particles because such materials are naturally more cohesive and, therefore, in the feed state, feature lower bulk density or higher bulk volume. In these cases, cohesive arches will collapse at low pressure whereby large amounts of gas are driven out. At the same time, pores between fine particles are small, which results in low diffusivity so it takes a relatively long time for the large amount of displaced gas to escape. To help overcome problems associated with degassing or deaeration, special design features, such as force feeders and/or various provisions for venting, are applied with all pressure agglomeration methods, particularly if fine powders must be processed [B.97]. If the mechanism of densification is considered (refer to the sketches in the upper part of Fig. 5.9), it becomes clear that the pores in the feed to a pressure agglomeration process of any kind must not be filled completely (saturated) with a liquid. An example of such a material would be a normal filter cake: one that has not been blown dry or otherwise further de-watered. Since liquids are incompressible, the pressing force would increase quickly and mechanical de-watering would have to occur, which further reduces the speed of densification. It would also require an effective separation of solids and liquid during the densification process; this is a problem that has not yet been solved. Therefore, with increasing pressure applied to the particulate solids, which typically results in higher densification or lower porosity, the moisture content
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5 Beneficial Uses of Agglomeration: Agglomeration Technologies and Methods Fig. 5.10 Diagram of equipment for: a) low-; b) medium-pressure agglomeration: a1, screen; a2, basket; a3, radial; a4, dome; a5, axial; b1, screw; b2, flat die; b3–b5, different designs of cylindrical dies; b6, gear
of the feed must diminish. In high-pressure agglomeration the feed must be essentially dry. The equipment in which pressure agglomeration occurs is a machine that operates with well-defined mechanical parameters that are independent of the performance and characteristics of the particulate solids to be processed. Therefore, pressure agglomeration techniques lend themselves readily to automation and remote control and are essentially independent of operator presence and/or skill. Because the equipment is relatively complex and the throughput per unit is often limited, this technology finds its greatest usage in low- to medium-sized applications (about 0.1–50 t/h). Of course, this statement is relative. Specialty products, such as those in the pharmaceutical industry (Section 6.2.2), may be processed in very small and sophisticated machinery, handling only a few kilograms per hour, while certain high-tonnage materials, for example some fertilizer, refractory, and mineral materials, are briquetted or compacted in large facilities employing multiple units (Sections 6.6.2 to 6.10.2). Relatively uniformly shaped and sized agglomerates can be obtained with low- and medium-pressure agglomeration. For these processes, the feed mixture must still be made-up of relatively small particles and inherently available, activated, or externally added binders (above). The moist, often sticky mass of particulate solids as well as plastic and liquid binders is extruded through holes in differently shaped screens or perforated dies (Fig. 5.10). Agglomeration and shaping are caused by the pressure
5.2 Pressure Agglomeration
forcing the mass through the holes and by the frictional forces developing during the material’s passage. Depending on the plasticity of the feed mix and the dimensions of the holes, short “crumbly”, elongated “spaghetti-like”, or cylindrical green extrudates are produced. Particularly the thin, string shaped agglomerates that are obtained from low-pressure agglomeration (Fig. 5.10, a1–a5) are often spheronized, that is, rolled into small spherical particles while the product is still plastic [B.48, B.97]. In most cases a post-treatment (typically drying and cooling) is required to yield final, permanent strength. As far as applicability is concerned, high-pressure agglomeration (Fig. 5.11) is the most versatile technique for the size enlargement of particulate solids by agglomeration. If certain characteristics of the feed materials and conditions occurring during densification are considered during equipment selection as well as plant design and operation, dry particulate solids of any kind and size, from nanometers to centimeters, and at any condition, for example with temperatures from below freezing to 1000 8C, can be successfully processed. Typically, the products from high-pressure agglomeration feature high strength immediately after discharge from the equipment. Nevertheless, to further increase strength, addition of a small amount of binder and/or post-treatment methods are possible. An advantage of high-pressure agglomeration is that, as discussed above, in most cases, essentially dry solids are processed, which do not tend to set, so that the process can be stopped at almost any time and re-started easily; also, the amount of material in the system is relatively small. Therefore, pressure agglomeration methods, specifically those applying high pressure, lend themselves particularly well to batch or shift operation and to applications in which several products must be manufactured from different feed mixtures in the same unit. At the end of a campaign, the system can be easily and completely emptied in a relatively short time. If the danger for crosscontamination is unimportant, for example in the fertilizer industry, a new campaign
Fig. 5.11 Diagram of equipment for high-pressure agglomeration. Ram press (upper left), punch and die press (upper right), roller presses for compaction (lower left) and briquetting (lower right)
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Fig. 5.12 Cycles of force build-up in the three different high-pressure agglomeration techniques
with a different feed can be started immediately (Section 6.6.2). Another possibility for fast and quick change-over is to install different feed and product discharge/handling systems as, in most cases (for example the pharmaceutical industry, Section 6.2.2), the expensive pressure densification/shaping equipment itself can be easily cleaned or adapted to the manufacturing of a new product. The above-mentioned destructive effects of expanding compressed air and relaxation of elastic deformation can be reduced if the maximum pressure is held for some time, called dwell time, before it is released. Fig. 5.12 shows that, without special
5.3 Agglomeration by Heat/Sintering
technical provisions, this is only achieved in ram extruders (Fig. 5.11, upper left). In such equipment, a number of briquettes are retained in the long pressing channel and are redensified during each stroke. After the wall friction is overcome and the entire line of briquettes moves forward, the pressing force remains almost constant (Fig. 5.12b). A similar, but much smaller effect is obtained in pellet presses (Fig. 5.10, b1–b6). Since a dwell time and, particularly, the application of several densification cycles also helps to convert temporary elastic deformation into permanent plastic deformation, these techniques are especially suitable for the densification of elastic materials such as, for example, biomass. If a dwell time is required or desired in punch-and-die presses (Fig. 5.11, upper right), special drive systems must be used [B.48, B.97]. It is obvious from Fig. 5.11, lower left and right, that no such possibility exists in roller presses where a continuous rolling action densifies the material between approaching surfaces until, immediately after passing the point of closest approximation, the relative motion is reversed, the surfaces retract, and the pressing force drops, ideally to zero if no expansion due to compressed gas and/or stored elastic energy takes place.
5.3
Agglomeration by Heat/Sintering
At a certain elevated temperature, which is different for various materials and can be quite low for organic products or very high for minerals, atoms and molecules begin to migrate across the interface where particles touch each other. While still in solid state, depending on temperature, time, and intensity of contact (caused by pressure during the manufacturing of a pre-form or the sintering process itself), diffusion of matter forms bridge-like structures between the surfaces, which solidify upon cooling. The process may also result in a densification of the compact, which is due to an elimination of pores and associated shrinkage. The entire group of phenomena is called sintering [B.13(c), B.97]. Agglomeration by heat or sintering has been developed and is applied mostly in industries processing minerals and ores for the size enlargement of fines before further use (Section 6.8.3). Because the technology requires large amounts of thermal energy, special efforts are made to recover heat or use sources of waste heat. The resulting agglomerates are crude but meet the requirements of the industry. Another large application of sintering in agglomeration is in post-treatment where the phenomenon is used to produce strong permanent bonds and/or specific final properties in many parts that may have been manufactured by virtually any one of the other agglomeration techniques. Particularly in powder metallurgy, sintering is the most important finishing process for the achievement of final strength and structure (Chapter 7). More recently, in this field and for other post-treatment processes, it has been found that several mechanisms exist that cause material transport at elevated temperatures for bridging and, thus, strengthening of the preform or the agglomerate, but do not result in shrinkage. Therefore, sintering can also yield strong final products with high porosity [B.78, B.97].
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5.4
Technologies Using the Phenomena and Fundamentals of Agglomeration for Purposes other than Size Enlargement
Among the technologies that use the phenomena and fundamentals of agglomeration for purposes other than size enlargement the oldest is coating, which was first used to avoid the sticking together of pills and/or to mask the taste of the often unpleasantly tasting medicinal remedies. The coating material was either applied as a fine naturally occurring powder (such as pollen) together with some moisture or as a natural (e.g., nectar) or man-made sugar solution. The coating was made permanent by drying (originally by the heat of the sun). Today this technology has evolved into an important field of solids processing [B.48, B.97]. It is used in almost all industries that are specifically covered in Chapter 6 and has further potentials in many other applications. The most common modern methods employ equipment similar to that utilized in tumble agglomeration [B.97]. Inclined pans, often pear shaped, are applied for simple coatings (for example in the food industry for sugar or chocolate covers, Section 6.4.3) and sophisticated drum designs are used widely if coatings with uniform distribution and well-controlled thickness are required (for example in the pharmaceutical industry for functional layers, Section 6.2.3). Fine solid particles and a liquid binder, solutions, or suspensions are sprayed onto or into the tumbling bed of solids that are to be coated and form a temporary wet layer on their surface. Gas (normally dry hot air) is simultaneously passed through the bed, evaporating the liquid and leaving a solid coating,
Fig. 5.13 Sketch of the material processing section of a bottom spray fluidized bed coater (Wurster coating system) [B.97]
5.4 Technologies Using the Phenomena and Fundamentals of Agglomeration for Purposes
Fig. 5.14
Diagram of possible structures of microcapsules [B.97]
the thickness of which grows as the process continues. Unique design features have been developed with regard to the shape of the container used for tumbling, the feeding of solids (if any), the spraying of the liquid, and the input as well as exhaust of the drying gas [B.97]. Another technique sprays melts onto the moving bed of particles, which form a coat that solidifies upon cooling with a flow of ambient air (Section 6.6.3). The stochastic movement in low-density fluidized beds is used for coating relatively small particles with narrow particle size distribution. Many of the newer designs use some modification of the Wurster coater [B.48, B.97] (Fig. 5.13).
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Fig. 5.15 Diagram depicting the process of hybridization and microphotographs showing particles with intermediate as well as final coating [B.97]
A further coating method is encapsulation, most commonly called microencapsulation [B.48, B.97]. It is a packaging technique that enrobes powders, particles, liquids, or even gases and forms free flowing, dustfree particulate products. The most basic method makes use of the capillary flow of liquid and the recrystallization of dissolved (or deposition of colloidal) solids on the surface of saturated wet agglomerates during drying. More recently, technologies are being developed in which the capsule features a specific, well-defined functionality. The capsule material may offer controlled-release characteristics (e.g., pharmaceutical products, Section 6.2.3), insolubility and well-defined bursting strength (e.g., “dry” inks and toners, Section 6.11.3), delayed availability (e.g., agrochemicals, Section 6.6.3), and other properties. Fig. 5.14 describes schematically the possible structures of microcapsules. As shown in Fig. 5.14d, bottom, heterogeneous coatings on a core can be also fixed by embedding. The processes that accomplish this are called mechanofusion or hybridization [B.97]. Mechanical forces act either on a previously deposited layer or on systems consisting of larger core and ultrafine coating particles. Often the core particles are relatively soft and inert with the coating providing functionality (for example
5.4 Technologies Using the Phenomena and Fundamentals of Agglomeration for Purposes
Fig. 5.16 Overview diagram describing manipulation techniques for small solid particles [5.3]
Fig. 5.17 Precise arrangement of SiO2 spheres on pinpointed locations: a) voltage contrast image of a dotted line; b) silica spheres arranged on the dotted line [5.3]
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electrical conductivity, Section 6.11.3) by embedding in the surface (Fig. 5.15). Because the coating particles are often so small that no dislocations are present in their structure, they behave as very hard entities. Therefore, it is, for example, possible to embed ultrafine titanium particles in the surface of glass spheres. A very new group of methods for the deposition and bonding of particulate solids onto surfaces assembles nano- to micrometer sized particles in a predetermined and orderly fashion onto substrates [5.3]. Fig. 5.16 gives an overview of the two groups of manipulation techniques that are available. While with the upper methods particles can be deposited with great accuracy but low rate, with the lower ones larger numbers of particles can be delivered with less accuracy. The limitations of both can be overcome if the intended locations of the particles are predetermined by electrification with a focused ion beam (FIB). Fig. 5.17a is the voltage contrast image of a line in which the dots where formed by the impingement of a Ga+-FIB. The spacing of the dots is 5 lm. As shown in Fig. 5.17b, 5 lm silica spheres are attracted to the dotted electrified line and form an almost perfect array of particles. Microdevices or microstructures with multi-functions (Chapter 11) can be obtained after permanently fastening and interconnecting the particles with the substrate and with each other, for example by sintering.
Further Reading
With exception of the following: B.14, B.20, B.23, B.27, B.30, B.42, B.75, B.81, B.91, B.103, B.104, all publications listed in Chapter 13.1 are recommended for further reading.
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6
Industrial Applications of Size Enlargement by Agglomeration All techniques of beneficial size enlargement by agglomeration yield products that feature some or all of the advantages listed in Tab. 6.1. In certain cases many of the characteristics are desired, but sometimes one specific requirement may be the sole reason for adopting a suitable agglomeration method in a manufacturing line. Agglomerated materials may constitute final products, such as tabletted pharmaceutical specialties (Section 6.2), pelleted animal feed (Section 6.5), instant drink concentrates (Section 6.4), briquetted solid fuels (Section 6.10) and so on, or intermediate products, such as granulated pharmaceutical formulations as feed for tabletting
Tab. 6.1
General advantages of agglomerated products
Enlarged apparent size. No or low content of dust. Increased safety during the handling of, for example, toxic or explosive (highly reactive) materials. Fewer losses (by dusting or elutriation). Less primary and/or secondary pollution. Freely flowing. Improved storage and handling characteristics. Better metering and dosing capabilities. Reduced tendency of unwanted agglomeration. Increased bulk density and reduced bulk volume. Smaller size of packages and storage or transportation volume. No segregation of co-agglomerated materials. More uniform supply of multi-component materials to users. Defined size and shape. Sometimes defined weight of each agglomerate (dosage form). Distribution may be narrowed by post treatment (sizing). Shape may be modified by post treatment (e.g. spheronizing). Within limits, porosity or density can be controlled. Possibility to influence dispersibility, solubility, reactivity, heat conductivity, or other related properties. Improved product appeal Material properties can be adjusted to meet government, local, or industry standards or requirements. Higher sales value and increased profit potential. Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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presses (Section 6.2), briquetted DMT (di-methyl therephtalate) flakes for safe and economic shipment to the fiber production plants (Section 6.3), agglomerated silica fume to reduce its volume and improve the handling characteristics during transportation (Section 6.7), and so on. The most common reason for size enlargement by agglomeration is, as the term implies, the desire or need to obtain a product with larger particle dimensions. As described in Chapter 3, the resulting entity is only apparently a new unit. The original solid particles are still present in the structure [B.97], often with a completely unaltered shape and size, and are held together by binding mechanisms. Furthermore, the new entity features porosity: voids between the agglomerate-forming particles (Chapter 3). This is beneficial in some cases, for example, for easily dispersible food (Section 6.4) and pigment (Section 6.3) granules, for “designer” foods (Section 6.4), and for catalyst carriers (Section 6.11), or undesirable in others, as in hot densification of sponge iron (DRI) to reduce reactivity (Section 6.9) and high-density ceramic (Section 6.7) and powder metallurgical (Chapter 7) parts. As a further result of this modification of the original particulate solids, many bulk characteristics of the new product are changed too, mostly for the better. For example, larger particles have less dust, exhibit improved flow behavior, and feature much-reduced sticking tendencies. Therefore, storage, handling, and feeding is less risky, even for “difficult” materials. After production, many modern particulate solid materials are very aerated and remain loose, largely due to their small particle size and high surface area, which cause instant, natural adhesion and prohibit consolidation by settling. If packaged, loaded, or stored, such products occupy large volumes and require excessively large containers for packaging or big silos. During agglomeration the natural adhesive bonds are broken and the individual particles are bonded more closely together, resulting in higher density, so that the aforementioned problems are eliminated. If mixtures are agglomerated after blending, the distribution of the different components is fixed in the agglomerates and thereafter segregation is avoided. Agglomerated products are better sized and shaped and often can be used as solid dosage forms. Their properties may be adjusted to meet specific requirements by influencing the agglomerate structure, either directly or by post-treatments. Generally, many agglomerated materials have better appeal and command higher prices and profits. After agglomeration some particulate solids exhibit properties that are undesirable for their use or application. In most cases, these negative characteristics relate to the new porosity of the agglomerate, which may be, for example, too low for easy dispersibility, or too high, resulting in excessive reactivity. Bad product properties can also be caused by unsuitable binding mechanisms, for example absorbents for liquids may disintegrate when wetted or, vice versa, a binder in water-dispersible granules may not dissolve when the product is submerged in the liquid. More recently, agglomeration has entered new fields in which individual particles, mostly in the nanometer size range, are attached to each other or to substrates in a controlled fashion (Chapter 11). Areas of application include the manufacturing of new composite materials with novel characteristics and of microscopic structures for electrical circuits or material associations on a molecular level. The latter result
6.1 General Applications
in the evolution of unique drug-delivery systems or life-science products, while the former is increasingly used for electronics and in the communication industries. In the following sections examples of specific applications of beneficial agglomeration by size enlargement in different industries are presented. According to the commonly used classification of the available methods (Chapter 5) they are subdivided into tumble/growth, pressure, and other agglomeration technologies. The latter include agglomeration by heat/sintering and technologies using the phenomena and fundamentals of agglomeration for purposes other than size enlargement, such as coating and some of the already industrially applied technologies involving nanoparticles for the building of new engineered materials and novel particle-modification technologies.
6.1
General Applications The most general application of beneficial size enlargement by agglomeration is the granulation of powders. Basically, the terms “granules” or “granulate” mean relatively coarse (0.1–10 mm) particulate solids (Chapter 14). Contrary to the often performed granulation of solids by crushing (the size reduction of larger solid pieces), granulation of powders uses the technologies of size enlargement to achieve the same goal, the production of a “granular” product. Although with regard to the overall granulometry both products are similar or even identical, physically they are different. Granular material obtained by the crushing of solids consists of particles that have similar structure to the solid from which they were produced. Because failure during crushing begins preferentially at imperfections (dislocations, faults, cracks, inclusions, and other defects), the smaller particles become increasingly more homogeneous and, with decreasing size, approach the structure of the ideal solid. In contrast, the granulation of powders involves binding mechanisms and suitable processing of fine and/or ultrafine particulate solids. The individual particles are brought closely together during impacts or by applying external forces. At that instant, binding forces that are larger than the separating forces caused by the ambient conditions (Fig. 3.6 and [B.48, B.97]), develop between the solid surfaces, resulting in particles sticking together. By these mechanisms, larger units develop by growth or as defined by a die in or by which they are formed. However, the new entities are only seemingly solid. In reality, unless modified by a post-treatment procedure (Chapter 7), the resulting agglomerates are made-up from the largely unaltered powder particles, retaining most of these particle characteristics, and feature porosity, that is interconnected voids, which may represent a volume percentage as high as 95 % or more and as low as 5 % or less. Granular products from powders may be produced by tumble/growth agglomeration (Section 6.1.1); while some are immediately in the correct size range, others re-
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quire post-treatment and/or sizing to remove over- and under-sized materials. In the latter case, it is necessary to consider what to do with the rejects. In many cases, large agglomerates are produced first, to make use of economical tumble/growth and pressure agglomeration or sintering methods (Sections 6.1.1, 6.1.2 and 6.1.3); the pieces resulting from these procedures are then crushed to form the granular material. Effective size reduction into a defined size range (granulation), whereby over- and undersized particles must be kept within certain narrow limits, is a fundamental problem of fracture mechanics. Moreover the failure mode of solids is different from that of agglomerates. Therefore, these phenomena and how they influence granulation will be described in more detail below. Granulation of solids by crushing, results in a product distribution that typically includes oversized and undersized particles. The size distribution of the discharge from a crusher depends (among many other parameters) on the stressing mechanism (Figs. 4.3 and 4.4, Chapter 4). Other important influences are the feed size and shape, the energy input, the crushing tool and chamber configuration, and the reduction ratio, which is a characteristic derived from the previous variables. The larger the reduction ratio (the difference between feed and product size, Fig. 4.3), the higher the energy input must be and the more fines are produced. When a solid body is stressed, which requires the addition of energy from the outside, the tensions that are produced within stretch all bonds between the molecules. Theoretically, if the solid features an ideal structure with no imperfections and only tensile forces are applied, the bonds separate in a structural plane in which the load exceeds the bond strength and the solid splits cleanly into two parts. Then all other areas return to their equilibrium structure, and the unused energy is freed and converted into other forms: kinetic and thermal energy and/or sound. However, real solids contain many flaws, which become stress raisers and initiate cracks at loads much below those required for the separation of an ideal body, even if only tensile forces are applied. The stressed system is unstable and cracks expand rapidly, accelerating to high propagation velocities: only the bonds at the crack tips are breaking at any instant. Under combined stresses, the normal condition in an industrial crushing situation, many cracks will be initiated and propagate in a direction perpendicular to the local tensile stress but, sooner or later, they run into a region of compression, which stops further crack growth [B.12]. Therefore, the highest number of cracks will be initiated and cause failure near the point(s) of energy input. Also, if primary fractures relieve the
Fig. 6.1-1 Diagram of failure lines in single spherical and irregular particles during: left) compression. right) impact crushing [B.12]
6.1 General Applications
Fig. 6.1-2 Photographs of: a) the fines cone; b) coarse residual pieces obtained during impact crushing of a single glass sphere [6.1.1]
stress sufficiently rapidly, vibrations are induced that result in new tensile stresses and may cause secondary fractures. As shown in Fig. 6.1-1, mainly fine material is produced when a high energy density is released at or near the point(s) of energy input. The volume of the fines cones and, therefore, the mass of fines grow with increasing energy input or, in other words, with larger reduction ratio. Fig. 6.1-2 shows the fines cone and coarse residual pieces obtained during the impact-crushing of a single glass sphere [6.1.1]. Since the main objective of the crushing of solids is to achieve a certain fineness and avoid oversized particles, many impact crushers and mills (e.g., hammer mills) feature some sort of a restriction (bar cage, screen, or perforated plate) at the discharge from the chamber in which the material is stressed. This exit grate, screen, or perforated plate retains material in the chamber until it is small enough to pass through the openings. Although this eliminates the presence of oversized particles in the product, a number of problems can or will arise. *
*
*
*
Such a measure will increase the amount of fines, as each stressing event will produce fines as discussed above. A finer than desired product may be produced because particles that are just below the size of the openings have a low probability of exiting it and, therefore, are retained, stressed again, and broken to finer sizes. To reduce this effect, the exit grid may be increased which, of course, increases the danger of oversized particles in the product. Depending on the requirements on size distribution this measure may not be acceptable. If crushers operate near their upper capacity limit, temporary or permanent choking of the grinding chamber may occur, requiring downtimes for clean-out.
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6 Industrial Applications of Size Enlargement by Agglomeration *
*
If the solid feed contains hard inclusions, selective crushing may occur. Hard, difficult-to-crush particles may remain larger than the openings of the exit grid and accumulate within the chamber, eventually also causing choking. Finally, but not least important, the exit restriction is a highly stressed part. It needs frequent maintenance (exchange or rebuilding) and, in some cases, for example in the ultraclean food and pharmaceutical industries but also in other applications where minute contaminations with iron or alloying components cause problems (for example, discoloration of glass when small amounts of iron are present), are not acceptable.
Large amounts of fines are produced by different mechanisms and increase with energy input or larger reduction ratio. They are often undesirable in the final product, so most of them must be removed by sizing, for example by screening or air classification. Although these fines normally have the same composition as the solid and, therefore are potentially valuable, they were traditionally discarded as waste. Size enlargement by agglomeration makes it possible to produce materials from fines for recycling or for use as undesirableecondary raw materials (Section 8.2). Nevertheless, since the conversion of fines into products for beneficial use is costly, it is more economical to minimize the production of fines during crushing. When the mechanisms that occur during size reduction of solids were first described by Rittinger [6.1.2] and later modified by Griffith [6.1.3] it became clear that to minimize fines production, a low energy input must be used. While this measure reduces fines, a substantial amount of oversized material is also obtained, which must be restressed. As shown in Fig. 6.1-3aa the simplest solution to this problem is to separate the larger pieces from the discharge and recirculate them to the mill, which must be oversized to be capable of handling the considerably greater throughput. As mentioned before, particles (of any size) in the milled product tend to be stronger because imperfections have been removed. Therefore, the optimal energy input for crushing oversized particles that resulted from a primary size-reduction event is increased. The arrangement shown in Fig. 6.1-3ab, whereby the second mill operates at a higher energy input than the first one, would seem to be appropriate. Although a somewhat improved result (narrower product size distribution with less fines) is obtained from both mills, they do not represent an optimized solution, because system ab still produces some oversized particles, which may have to be removed and otherwise used or (Fig. 6.1-3ac), possibly recirculated to the second mill (“closed loop” handling of larger pieces). Today it is commonly understood [6.1.4] that, from an energetic point of view, sizereduction processes are considered optimal when: *
* *
the particles to be crushed obtain a specific stressing energy directly from the milling tools (which may include grinding media and specially designed chamber walls) an undefined interaction between particles does not occur energy is not wasted on particles that are already fine enough – such materials are removed from the chamber
6.1 General Applications
Fig. 6.1-3 Diagrams of a crusher with oversize recycling and various two- or three-stage crushing circuits [6.1.4]. Circles represent crushers or mills; horizontal lines flanked by + and – represent classifiers (screens) that split the crushed material into over- and undersized particles. “A” is an air-classifier that removes fines by entrainment
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6 Industrial Applications of Size Enlargement by Agglomeration *
fines that tend to interfere with the controlled energy input are eliminated, both prior to and during size reduction.
With this in mind, Fig. 6.1-3ba depicts a much better system, which can be further improved (if all particles above a certain size must be eliminated) by recirculating the secondary crusher discharge into the classifier (Fig. 6.1-3bb). Alternatively, a second classifier separates oversized particles from the secondary crusher and only recirculates this portion to the secondary crusher (Fig. 6.1-3bc). If the emphasis is on reducing fines in the product and optimizing performance of the secondary crusher, an air classifier to remove fines by entrainment could be installed between the primary and secondary crushers (Fig. 6.1-3bd). Figs. 6.1-3ca, cb, and cc show the next step of optimization, which is three-stage crushing. Depending on the quality requirements on the final product, more grinding steps and/or additional closed loops can be added, on the understanding that crushing circuits become more expensive with each step. Therefore, a compromise between best result and cost must be found. In the sketches of Fig. 6.1-3 it has been assumed that by optimizing crushing the amount of fines has been reduced to such an extent that they can remain in the product, but fines elimination at the end of the circuit, possibly by installing a screen or similar separator for fines, may be necessary as a final quality adjustment step. As defined by the method of energy input, excluding shear, there are two main crushing mechanisms: compression and impact (Fig. 6.1-1). Although the above comments are valid for both mechanisms, the explanations referred more directly to impact stressing. As shown in Fig. 4.3 (Chapter 4), the importance of the reduction ratio for the outcome of milling is even more important when compression is applied. If the reduction ratio is too high, flat agglomerates are formed in which coarser pieces that are in at least one dimension smaller than the narrowest final distance or gap, are embedded in fines. Since the fine particles are immediately compacted after formation, almost all free valences at the newly created surfaces participate in recombination bonding [Chapter 3, B.48]. Recently it was found that compared with the stochastic milling process in a tube mill with grinding media, the combination of well-defined compression stressing in a high-pressure roller mill with large reduction ratio (Fig. 4.3) and the ensuing disagglomeration of the compacted flakes that were produced result in a significantly lower overall energy consumption during the fine grinding of brittle materials, such as cement clinker and many ores. Nevertheless, during “normal” crushing of solids by compression, either between two approaching flat (jaw crusher) or curved (roller mill) surfaces, the formation of compacted flakes or similar agglomerates must be avoided. This can be only achieved if relatively small reduction ratios are applied. In Fig. 6.1-3 it can be seen that a certain amount of oversized material is produced after a particular crushing step and the corresponding system sketches of Fig. 6.1-3 can be applied whereby the second, third, or potentially higher-stage compression crushers feature decreasing gap sizes (as compared with higher energy input, mostly defined by speed, in the case of impact crushers).
6.1 General Applications
Fig. 6.1-4 Diagram of multi-stage crushing between three sets of rollers and photographs of two-stage and three-stage Gran-U-Lizer roller mills (courtesy Modern Process Equipment, Inc. (MPE), Chicago, IL, USA)
If rollers are used for crushing, several sets with decreasing gaps can be installed in the same housing (Fig. 6.1-4). Because the material flows through the machine under the influence of gravity, and if the throughput is such that the nip area is not overfed (no pile-up), fines produced in the first stage that are smaller than the gap width of the second stage pass it without additional stressing while larger particles are crushed. This is demonstrated in Fig. 6.1-5, which shows the comparison of typical cumulative particle size distributions after crushing solids in a single stage roller mill and an MPE
Fig. 6.1-5 Comparison of typical cumulative particle size distributions after crushing solids in a singlestage roller mill and an MPE Gran-U-Lizer with three sets of rollers. Note the much reduced amount of oversize (shaded area) and the almost unchanged amount of fines (courtesy Modern Process Equipment, Inc. (MPE), Chicago, IL, USA)
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Fig. 6.1-6 Diagrams of various arrangements of profiled rolls for roller mills: a) peak-to-valley arrangement showing the “cracking” of a particle; b) parameters determining control of the particle size in a peak-to-valley arrangement; c) peak-to-peak arrangement; d) example of a peak-to-valley arrangement with modified pitch and gap (courtesy Modern Process Equipment, Inc. (MPE), Chicago, IL, USA)
Gran-U-Lizer with three sets of rollers. The shaded area, upper right, depicts the much-reduced amount of oversize material after three-stage roller mill crushing, while the amount of fines (lower left) remains almost unchanged. For certain materials and applications, the result of crushing in roller mills can be improved by using profiled roller surfaces. Fig. 6.1-6a shows a circumferentially grooved pair of rollers with a “peak-to-valley” arrangement and how a particle is crushed between such rollers. The mating peaks and valleys on the rollers act as fulcrum points that actually “crack” a particle that is bridged as shown. Variables that influence the result of crushing are: the pitch and gap adjustment (Fig. 6.1-6b) and the positioning of the matching rollers. Fig. 6.1-6a and b describe the staggered arrangement (peak-to-valley); another possibility is a directly opposed arrangement (peak-to-peak, Fig. 6.1-6c. Fig. 6.1-6d shows, as an example, another pitch and gap in a peak-to-valley arrangement. The granulation of powders by size enlargement, whereby agglomerates are first formed, then crushed into the desired particle size range, and finally classified to obtain the desired product size distribution, is even more difficult to analyze than the complex size reduction of solids. The reason for this is twofold. 1. During the crushing of solids the production of fines is expected, often with a certain amount desired, and in other cases the production of fine particles is the sole objective of the operation, but the optimal result of the granulation of powders by agglomeration and crushing would be to produce no fines at all. This is understandable as powders are, by definition, fines and granulation is performed to produce larger particles. However, since the formation of fines can not be totally avoided, it is most important to select methods that minimize this effect.
6.1 General Applications
2. Agglomerates are assemblages of powder particles that are joined together by binding mechanisms. If compared with the (ideal) structure of the solid itself, where atoms and molecules are held together by intermolecular attraction forces (valence forces) and form regular arrangements, which depend on the type of the atoms and molecules and on the formation mechanism, the structure of agglomerates is made up of irregularly arranged small particles with different size and shape and features void spaces (porosity). Because small particles contain few structural irregularities and flaws, their strength is high and always exceeds that of the binding mechanism acting between the solids. As a result of this characteristic property of all agglomerates, it is easy to break them into the powder particles from which they were originally made. Therefore, breaking down larger agglomerates into smaller, but still agglomerated granules (featuring sizes that are greater than those of the agglomerate-forming primary particles) must be done with gentle, low-energy methods to avoid the formation of excessive amounts of fines. Harsh methods of stressing and high energy input may
Fig. 6.1-7 Diagram of the structure of different agglomerates: a) tumble/growth agglomeration; b) low- and medium-pressure agglomeration (extrusion); c) high-pressure agglomeration
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easily produce solids that are, at least partially, finer than the particles from which the agglomerates were originally made. Generally, the same measures must be applied for agglomerates as described above for solids to obtain a good result from granulation without the production of excessive amounts of fines. This means that the energy input should be selected so that the agglomerate to be granulated breaks into as many large fragments as possible with minimum fines production at the points of stress initiation. Because of the porous structure and the method of creating strength (bonding at coordination points, Chapter 3), application of shear is particularly detrimental to avoiding fines. Powder particles can be easily removed from agglomerates by abrasion, especially from the surfaces of freshly produced fragments. Therefore, the use of exit grates or screens in crushers to retain larger material in the milling chamber and avoid oversized particles in the product is a sure way of creating additional fines when granulating agglomerates. Referring back to Fig. 6.1-3, single-stage impact milling of agglomerates should employ the system depicted in aa. In single-stage compression crushing, recirculation of oversized particles is ineffective. Both types of crushing can be optimized by applying two or more crushing stages. Systems bc and cd are particularly well-suited if impact milling is used, while compression stressing is best carried out in roller mills with multiple sets of rolls (Fig. 6.1-4) and profiled roll surface (Fig. 6.1-6). An additional peculiarity of the granulation of powders by agglomeration and crushing is due to the fact that most agglomerates to be reduced in size feature different structure, strength, and/or density over the cross section of the body. Large agglomerates resulting from tumble/growth agglomeration are produced in dense moving particle beds (in drum or mixer agglomerators [B.48, B.97]). According to the agglomeration mechanism in such equipment (Chapter 5), powder particles adhere to the outside surface of nuclei or intermediate aggregates and, if the bond is stronger than the sum of all separating system forces (Fig. 3.6) become an integral part of the growing entity. As the mass of the agglomerate becomes larger, under the influence of kinetic and reaction forces from the surrounding material and equipment walls, the newly attached particles may be more tightly embedded in the current surface structure of the agglomerate. Therefore, it is possible that the density and strength are increasing towards the surface of large agglomerates that have been produced by tumble/growth mechanisms in dense particle beds, particularly if these bodies feature onion-skin structure (Fig. 6.1-7a1) [B.48]. As described in Chapter 5, tumble/growth agglomeration occurs almost always in the presence of a liquid binder, which wets the solid particles and derives strength from surface tension and capillary forces. Such bonding is only temporary because if the liquid evaporates and no other binding mechanism takes over, the agglomerate looses its entire strength. The most common secondary binding mechanisms that develop during drying of wet or moist agglomerates are the recrystallization of dissolved substances or the deposition of colloidal particles at coordination points within the agglomerate (Chapter 3, [B.48, B.71]). If the pores within an agglomerate are completely or partly filled with liquid (Fig. 3-1b and Fig. 3-2d and e, Chapter 3), drying
6.1 General Applications
begins on the surface of the body and the drying zone remains there as long as liquid is replenished from the interior by capillary flow. Only when pores cease to be filled continuously, will the drying zone move into the agglomerate. During the first phase of drying, which takes place only on the entity surface, dissolved substances or suspended particles will deposit on or near the surface, resulting in a strong and dense crust [B.48, B.71]. Since large agglomerates made by tumble/growth methods require a high moisture content to reach bigger sizes and crushing for the production of a granular product always occurs after drying, when the permanent binding mechanism has developed, it is common that such agglomerates feature a strong surface layer and a looser core (Fig. 6.1-7a2). In extrusion (low- and medium-pressure agglomeration, Fig. 5-10, Chapter 5) the force causing densification results from friction between the material and the die channel. In addition to a somewhat higher density close to the die wall, as compared with the center of the extrudate, due to force dissipation, shear from the sliding motion of the material in the channel produces a “skin” on the surface (Fig. 6.1-7b). The extent of this effect depends on the forces applied, which are a function of the equipment and the material characteristics, and on the length of the extrusion mold. It is more pronounced in products from medium-pressure agglomeration and almost not noticeable if wet powder masses are passed through screens (low-pressure agglomeration). In high-pressure agglomeration, particulate solids are compressed and densified in confined-volume, open-ended, or converging dies (Fig. 5.12, Chapter 5). When forces (or pressures) are applied onto particulate solids they dissipate at the contact points of one particle with other particles surrounding it. Because each particle contacts several others and the number of contact points increases during densification, the forces decrease quickly towards the center of the compact. This dissipation becomes larger with smaller particles, and if forces are introduced from opposite sides, a neutral plane exists where the directional sign of the acting forces changes. The location of the neutral plane depends on the size and direction of the opposing forces [B.28, B.48]. As a result of this dissipation of forces, particulate solids are more densified near the points or planes where the external forces are exerted onto the particulate mass and less dense in more distant locations. While, normally, there is a rather well-defined highly densified surface layer, overall a density gradient exists towards the center or the agglomerate. Because strength is strongly dependent on porosity (the opposite of density) and decreases with increasing porosity (or lower density), a strength gradient also exists across the agglomerated body. Fig. 6.1-7 shows the situations; the shaded areas feature high density and strength: c1, briquette from high-pressure ram extrusion or cylindrical compact from punch-and-die pressing; c2, briquettes from roller pressing; c3, flat sheet from roller pressing; c4, corrugated sheet from roller pressing. With this understanding of how agglomerates are structured and considering the laws of fracture mechanics it is now possible to define phenomenologically how the granulation of powders by crushing agglomerates should be best carried out. By now it should be obvious that a direct, single-stage reduction of the particle size from the agglomerated feed to the granulated product will normally result in an excessive amount of fines. Therefore, in a first step, agglomerates from any process source must be cracked, taking into consideration the surface layer with higher
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strength (Fig. 6.1-7), to form as many particles as possible, including oversized pieces. During this crushing step the energy input must be selected to minimize the amount of fines. The discharge must be separated to yield fines, correctly sized product, and oversized particles. Most of the oversized pieces are the strongest parts of the agglomerate. They must be recirculated to the crushing equipment or fed to another sizereduction process. If, depending on the system used, oversized particles are again obtained after restressing, they will be even stronger and require additional size reduction. This can be accomplished with various systems using different methods of disintegration. Beginning with compression stressing, the aforementioned multiple-pass roller mills (Fig. 6.1-4) promise best results. These mills are designed so that the gap between the rollers can be easily adjusted (Fig. 6.1-8), even during operation, to allow optimization of the product particle size distribution. The gap of the last set of rollers defines the maximum particle size and is normally adjusted such that no oversized material is obtained. Depending on requirements on the product particle size distribution, fines may have to be removed and discarded or otherwise used, including recirculation into the agglomeration process. Differently profiled rollers (Fig. 6.1-6) may be advantageous in certain cases. Today, by far the largest number of granulation systems that crush agglomerates into a specific particle size range use some kind of impact mill. Hammer mills are most commonly applied but other types, notably cage mills with one static bar cage and one or more rotating one(s), may be selected. As mentioned before, to avoid the production of excessive amounts of fines, if this is a requirement, agglomerated solids should be stressed once, thereafter immediately removed from the grinding chamber, and separated into at least three fractions containing undersized (fines), correctly sized (product), and oversized particles. Because larger than desired pieces are always unacceptable, this fraction is either recirculated or fed to a secondary crusher. Fig. 6.1-9 is a simplified comparison of the results of granulation by crushing agglomerates, using three different milling circuits. In all cases hammermills are utilized. All diagrams depict the agglomerated feed with 100 % in a relatively narrow, large particle size range and the desired (shaded) particle size of the granular product. For the result shown in Fig. 6.1-9a, a hammermill with discharge restriction in the form of a bar cage or a screen was used (Fig. 6.1-10). The restriction is meant to avoid oversized pieces in the discharge of the mill and the opening size of this feature is selected such that nothing leaves the milling chamber with a size larger than the
Fig. 6.1-8 Automatic adjustment with pneumatic servomotors and micrometer or digital readout in multi-stage roller mills (courtesy Modern Process Equipment, Inc. (MPE), Chicago, IL, USA)
6.1 General Applications
Fig. 6.1-9 Comparison of the results of granulation by crushing agglomerates: a) a mill with exit grate or screen, b) an impact (hammer) mill with unobstructed discharge and recirculation of oversized particles (Fig. 6.1-3aa), and c) two-stage crushing (Fig. 6.1-3bb) applying two individually controlled impact mills with unobstructed discharge (for explanations see text)
upper limit of the granular range. Oversized material, including already sufficiently reduced pieces and fines, which can not immediately clear the discharge openings, remain in the milling zone and experience turbulent movement and a multitude of stressing events, including shear. In addition to clean impacts from the hammers, largely undefined energy input occurs, mostly between the hammers, the chamber walls, and the discharge restriction that leads to the production of fines, particularly since the material to be crushed consists of agglomerates. The example in Fig. 6.19a assumes that the acceptable granular size range is relatively narrow. Therefore, only 40 % product and 60 % fines (undersized particles) are obtained. This represents an ineffective method of granulation.
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.1-10 Diagram of an impact mill with hinged or fixed hammers on a horizontal rotor and discharge restriction (bar cage or screen) to avoid oversized particles in the product: a) grate bars, b) cage with axially elongated holes, c) cage with round holes
It should be mentioned, however, that in some applications, especially those in which the granular material represents an intermediate product, manufactured as a freely flowing, essentially dustfree, non-segregating, directly compressible feed for punch-and-die presses (for example in the pharmaceutical industry, Section 6.2), the entire output from such a mill, including all of the fines, may be adequate for further use. Also if, in other cases, the granular size range can be wider, the yield of acceptable product becomes larger so that the correspondingly smaller amount of fines may be recirculated or otherwise used without sacrificing the economics of the process. In Fig. 6.1-9b it may be assumed that the same mill as used in example A is employed but without discharge restriction. The tip speed of the hammers (defining the energy input) is selected such that during crushing a wide size range of broken pieces, including oversized ones, is produced. After separation into undersized (fines, 25 %), correctly sized (product, 40 %), and oversized (35 %) particles, the large pieces are recirculated to the mill (Fig. 6.1-3aa) where it is assumed that they will disintegrate into the same particle size distribution as experienced before with the whole agglomerates. From the discussion above, which stated that oversized particles after the crushing of agglomerates represent the stronger parts of the structure, this assumption is not totally correct. If the oversized particles are recirculated into the same mill, exerting an identical energy input as during the first crushing event, the stronger pieces should produce a somewhat different size distribution. For the sake of this presentation the difference is disregarded. Since the oversized fraction, which, in a closed loop recirculation layout, results from a multitude of cycles, produces material fitting the three size fractions during every stressing event, the final yield of product and fines, the only two streams discharging from the system, comes from an infinite accumulation of iterative amounts whereby all the coarse particles are ultimately converted such that they fall into the two size ranges. As compared with Fig. 6.1-9a, 21.5 % more product (61.5 %) and, correspondingly, 21.5 % less fines (38.5 %) are produced with the granulation method shown in Fig. 6.1-9b. As discussed before, this result can be further improved if the granular size range is wider and, depending on the requirements on the granulated material, the now much-smaller percentage of fines may actually be an acceptable component of the product.
6.1 General Applications
Because after the first crushing step the oversized particles represent the strongest portions of the agglomerates, a further improvement of granular yield and reduction of fines can be obtained if, instead of returning this size fraction back to the same mill (Fig. 6.1-3aa) another similar or identical mill, the latter employing modified energy input, is installed to crush the oversized portion. This second-stage mill could be in a straight through arrangement (Fig. 6.1-3ba), employ an internal closed loop (Fig. 6.1-3bb), or feature a separate closed loop for the still-remaining oversized pieces (Fig.s 6.1-3 bc and bd). By being able to control the energy input of the second mill, which now receives only the oversized particles from previous crushing events, it can be assumed that the particle size distribution of the discharge is improved in regard to product and fines (for example: undersized (fines, 25 %, unchanged), correctly sized (product, 50 % instead of 40 %), and oversized (25 % instead of 35 %) particles). With this and using crushing systems according to Fig. 6.1-3bb or bc, Fig. 6.1-9c shows that a further small increase in product yield of 1.9 % to 63.4 % and the corresponding reduction in fines (to 36.6 %) can be achieved. Instead of recirculating the coarse fraction from the second mill to that same one (Fig. 6.1-3bb, bc, or bd), a third crushing stage could be added (Fig. 6.1-3ca, cb, and cc) and even higher-order multi-stage systems are conceivable. Based on the small improvement of granular yield by adding a second mill (comparison between Fig. 6.19b and Fig. 6.1-9c) it may seem uneconomical to use two or higher order granulating circuits. However, with different agglomerates, employing other types of crushers, and by optimizing the results of crushing in each milling stage through modified energy input, it is possible to achieve dramatic improvements of the yield from the granulation of powders via agglomeration and controlled size reduction. It should be pointed out that the maximization of granular product yield may not always be the preferred solution. As will be described later (Section 6.6.2), granular yield can be maximized by employing gentle crushing methods, low energy input, and multi-stage milling but, at the same time, weaker parts of the original agglomerates may survive and become product. Such granules feature lower abrasion resistance and produce dust during bulk storage, transshipment, and distribution, which is objectionable for some materials, leading to complaints from the customers. Although specific applications of granulation by agglomeration and crushing will be described in many of the sections covering different industries, in Sections 6.1.1, 6.1.2, and 6.1.3 some generic flowcharts will be introduced and discussed. They are mostly applied to generally improve the handling characteristics of finely divided particulate solids in mechanical process technologies without trying to modify other properties of the materials or obtain special, beneficial product qualities. In most cases, the reason for increasing the size of fine or ultrafine particles by agglomeration is to avoid undesired adhesion or agglomeration (Chapter 4). To achieve this, the particle size of the material is commonly adjusted to 0.1–1.0 mm. In this size range solids, whether naturally compact or agglomerated, have insufficient mass for the always-present adhesion forces to cause permanent bonding between the granular particles or with surfaces of any kind. If bonding occurs due to the presence of binders (moisture) or extraneous binding mechanisms (e.g., electrostatic charges), the strength of the agglomerate is low and the aggregation can be easily destroyed or build-ups and coatings can be removed without difficulties.
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To explain this in the context of common incidents occurring in the process industry, two examples are discussed in more detail. The first example refers to the adhesion of fine particles to walls of storage, transfer, or processing contrivances. Although ultrafine particles may permanently stick to technically smooth surfaces due to natural adhesion forces, an even stronger bond is obtained if the size of the solids is in the range of or smaller than the roughness of the substrate. Because all technical surfaces feature a microscopic texture that results from the machining or polishing technology used, powders will always contain some particles, which, in addition to the action of molecular forces, will interact mechanically with the surface topography. On such a first layer of adhering particles, more of the very small particles will attach themselves. Because a large number of bonding sites exist per unit volume and weakly bonded particles are removed by external forces so that only strong bonds survive, such a build-up will continue to grow and will be difficult to remove by methods other than mechanical cleaning and, potentially, washing.
Fig. 6.1-11 Glass cylinders filled with the same gravimetric amount of a powder having different particle sizes (a) from left to right: 4 mm, 1.5 mm, 100 lm, 12 lm, 5 lm, 28 nm, 18 nm, 16 nm, 12 nm and (b) left 4 mm, right 12 nm
6.1 General Applications
On the other hand, when the powder particles have been agglomerated into granules with somewhat larger size, these new entities may still adhere to walls if the conditions are favorable. However, because the number of bonds between the particles is smaller and since the agglomerates have greater mass, the cohesion of the build-up is so small that it will break-off under its own weight when it reaches a certain thickness. The second example relates to agglomerates that form in a particulate mass. Apart from the fact that the previously discussed build-up grows on a substrate (wall), similar conditions apply. If the particles are very small, a large number of bonds exist per unit volume rendering the resulting agglomerate strong and difficult to destroy. Secondary agglomerates from larger sized granulated powders are weaker and can be easily dispersed by the forces prevailing, for example, in a moving mass of particulate solids. At the same time, it is a precondition that the agglomerated granules, making-up the easily dispersible secondary agglomerate, are so strong that they withstand those same forces without breaking-up. A further problem that is associated with fine and ultrafine solids and can be remedied by size enlargement is that the entire mass of powder particles responds to the presence of the naturally existing molecular adhesion. This means that, in bulk, they adhere to each other and do not settle under the influence of gravity. This effect increases with diminishing particle size and is particularly pronounced when the solids are in the nanometer range. Primarily this results in a large, often excessive bulk volume (low bulk density) (Fig. 6.1-11) but also in bad flowability and in discharge difficulties. As shown in Tab. 6.1, size enlargement by agglomeration can overcome these handling and process problems.
6.1.1
Tumble/Growth Technologies
As discussed before, size enlargement by agglomeration for general applications is typically used to improve the handling characteristics of finely divided particulate solids in mechanical process technologies without trying to modify other properties of the materials or obtain special, beneficial product qualities. In most cases, the reason for increasing the size of fine or ultrafine particles by agglomeration is to avoid undesired adhesion or agglomeration (Chapter 4). To achieve this, the particle size of the material is commonly adjusted to 0.1–1.0 mm. In Chapter 5 the following methods of tumble/growth agglomeration were listed. 1. 2. 3. 4. 5. 6. 7.
high-density tumbling bed high-shear tumbling bed high density/high shear with abrasion or crushing transfer low-density fluidized bed low-density particle clouds agglomeration in stirred suspensions immiscible liquid agglomeration
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A detailed discussion of these methods and of the equipment used to accomplish them can be found in the author’s earlier books on the subject [B.48, B.71, B.97]. Although in Section 6.1 it was indicated that, theoretically, granular products can be made by crushing cured (dried) large agglomerates from tumble/growth technologies, the problems inherent with these procedures were also mentioned which, in most cases, prohibit the use of such a technique. Therefore, the granulation of powders by tumble/growth agglomeration for the improvement of general handling properties in the process industry normally tries to manufacture directly the desired particle size. Owing to this requirement some of the above seven methods are preferred for this task while others are normally not used. At “normal” operating conditions, the high density tumbling bed obtained in discs, cones, and drums best produces agglomerates in the range 10–15 mm. In addition, particularly from drums, a wide agglomerate size distribution is obtained. Therefore drums are not used for the granulation of powders by agglomeration. Under certain conditions and with continuous, extensive observation and control by experienced operators, discs with low rim height [B.48] can be applied for “micro-agglomeration” (often called “micropelletizing”) yielding particles in the range 1–3 mm. However, because of the costly operator involvement, this technique is only used for materials with high value, for example the granulation of certain agrochemicals (Section 6.6.1), or if a worker is required at the pan for other reasons (for example, packing the discharge into drying trays, see Section 6.11.1) who will observe and control the operation. High shear tumbling beds are produced in mixer agglomerators with rotating tools [B.48, B.71, B.97]. The small size and relatively narrow distribution requested for the granulation of powders can be produced best if agglomeration occurs in high density/ high shear particle beds with abrasion or crushing transfer (Figs. 5-3 and 5-4, Chapter 5). This procedure applies shear plates between the mixing tools or independently driven, high speed knife heads (also called shredders or chopper elements), which are periodically operated [B.48, B.71, B.97]. These provisions are used to destroy oversized agglomerates and make the fragments available for new growth within the desired size range. Although, with good preparation and control, batch or continuous mixer agglomerators can produce rather well-defined granular materials from powders they are infrequently used for general applications. Since in this rather unpretentious field the emphasis is on large throughput capacity and low cost processing, mixer agglomerators do not normally meet these conditions. Because of the mechanisms defining agglomeration in low-density fluidized beds or particle clouds [B.48, B.71, B.97], where only relatively small, narrowly sized granules can be manufactured and kept suspended without either settling or becoming entrained, this group of processes seems to be well suited for converting fine or ultrafine particles into granular products. However the motions, forces, and growth mechanisms that are at work result also in relatively weak agglomerates with low density, which are not suitable to withstand the stresses encountered during regular handling in mechanical processing plants. The structure of agglomerates obtained in low density fluidized beds or particle clouds yield other beneficial product qualities, which make them more suitable for specific applications, for example in the food (Section 6.4.1)
6.1 General Applications
Fig. 6.1-12 Block diagram of a powder granulation system by tumble/growth agglomeration for general applications also showing various optional features
or pharmaceutical industries (Section 6.2.1). Nevertheless, certain solids, which are prepared by spray drying of solutions, suspensions, or slurries are granulated by simultaneously agglomerating them in a fluidized bed that is associated with the spray dryer (Section 6.7.1) to make them more easily handleable. Agglomeration in stirred suspensions and immiscible liquid agglomeration are always used to produce special, beneficial material qualities and, therefore are not of interest for general applications. With exception of the dry granulation of carbon black (Section 6.11.1) and silica fume (Section 6.7.1) all tumble/growth processes that are employed for granulation just to obtain improved handling properties of powders make use of wet agglomeration. Therefore, they include drying as a post-treatment process because the temporary bonding provided by liquids must be converted into a permanent strength producing mechanism. As a result, a block diagram for these general applications is as depicted in Fig. 6.1-12. If different powders are to be granulated they are proportioned (metered) from day bins into a blender, which may operate in batches or continuously. It is also possible to install a mill instead, especially if the various raw feeds have different particle size distributions that might lead to segregation and selective agglomeration in the size enlargement step. Particularly if a blender is applied, it is feasible to introduce additives and/or some of the liquid binder(s) at this point (preconditioning). However, care must be taken that not all of the binder liquid is added, which would eliminate an important control factor for size adjustment in the tumble/growth agglomerator.
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A single component or homogeneous material feed is directly metered into the unit accomplishing size enlargement by tumble/growth agglomeration. The remaining binder liquid, normally atomized by suitable means (spray nozzles), is added and causes the tumbling particle mass to coalesce and form the desired granules. In dense beds of particulate solids a wide distribution of agglomerates, including oversized ones, is developing. The application of high shear or the operation of intensifier bars and intermittent use of knife heads [B.48, B.97] specifically results in the destruction of large agglomerates and of new growth from the fragments. In size enlargement by tumble/growth in any of the processes, green (moist or wet) agglomerates are formed, which are held together by temporary binding mechanisms. If natural curing, for example the hydration of cementitious components, does not take place, some post-treatment is required, almost always encompassing the elimination of the binder liquid and, at the same time, the development of a new, permanent binding mechanism (e.g., the recrystallization of a dissolved solid). Particularly if the granules are highly saturated, which is typical if size enlargement has occurred in a dense tumbling bed, liquid travels to the surface of the agglomerates during drying by capillary flow. Since, on the other hand, the green granules are weak and often become weaker during drying before a new bond takes over, curing takes often place in a stationary bed in which, caused by the previously described mechanisms, the aggregates may stick together and form oversized lumps. This is normally not observed with granules that were obtained in fluidized particle beds or clouds as drying takes place simultaneously or in another fluidized bed that is closely associated with the agglomerator. In such an arrangement, the particles are in constant motion and do not stick together. While for general applications the presence of some over and under sized particulates is often not objectionable, as an option the discharge from post-treatment may be separated into two or three size fractions. Fines (undersized particles) are discarded, used elsewhere, or recirculated to the agglomerator and coarse pieces (oversized particles and lumps) may be crushed in a closed loop whereby the fragments, which typically include again all three fractions, are recirculated to the sizing operation. Selection criteria for the crusher have been discussed in Section 6.1.
6.1.2
Pressure Agglomeration Technologies
Pressure agglomeration is also frequently used to produce granular materials for general applications. The same improvements of the powder properties as discussed in Section 6.1.1 are sought, which means that the particle size of the material is typically adjusted to 0.1–1.0 mm by granulation. In Chapter 5 the following methods of pressure agglomeration were listed. 1. low-pressure agglomeration: extrusion through screens 2. medium-pressure agglomeration: pelleting, extrusion through perforates die plates 3. high-pressure extrusion: ram presses
6.1 General Applications
4. high-pressure agglomeration – in confined spaces: punch-and-die pressing, tabletting – in confined spaces: isostatic pressing – in semi-confined spaces: roller presses A detailed discussion of these methods and of the equipment used to accomplish them can be found in the author’s earlier books on the subject [B.13b, B.48, B.71, B.97]. Only with low-pressure agglomeration can small extrudates with diameters < 1 mm be manufactured directly by passing formable (normally moist) particulate masses through screens. Because these machines are delicate and feature low capacity they are not used for general applications. All other pressure-agglomeration equipment produces relatively large, more-or-less densified pieces called briquettes, compacts, extrudates, pellets, sheets, slabs, and similar names. To meet the requirement that most of the granular materials, manufactured solely to improve the handling behavior of finely divided particular solids (general applications), should be in a particle size range of < 1 mm, the regular products from medium- and high-pressure agglomeration must be crushed. A plant embodying such a process, irrespective of which type of pressure agglomeration equipment is used, is called a compaction/granulation system. Fig. 6.1-13 is the block diagram of a typical compaction/granulation system. Right away it should be mentioned that the advantage of this technique of granulating powders is that any product size and distribution can be obtained. The final quality only depends on the crusher(s) and the sizing method(s) used. In this connection the discussion of Section 6.1 must be considered. A disadvantage is that always fines are also
Fig. 6.1-13 Block diagram of a powder granulation system by pressure agglomeration (compaction/granulation) for general applications also showing various optional features
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produced, the amount of which depends on the cut size (Chapter 4) that is associated with this property and whether or not undersized particles are detrimental and must be removed. If the granular material must be free of fines, the rejected percentage increases with narrower product size distribution. As discussed in Section 6.1, the granulation of agglomerates by crushing is a difficult undertaking. It is, of course, always the primary requirement of a compaction/ granulation plant to produce the highest possible yield of high-quality granules in the desired particle size range. To accomplish this it is necessary to define and obtain the granule quality and size range. Both properties may differ widely. The quality of granules that are produced solely to improve the handling characteristics of fine particulate solids may be defined such that they just survive the stresses, for example at transfer points or in feeders, but easily disperse when finally processed or, in the other extreme, they must be hard and abrasion resistant if they are, for example, produced to facilitate safe disposal. The size range of the granulated solids may be wide and consist of the entire output from the granulator, even including the unavoidable fine particles, or narrowed down to a small fraction by sizing. This can be accomplished by merely removing the undersized portion from a granulator discharge that contains no coarse particles because, for example, a screen or bar cage are installed in the crusher exit. As discussed in Section 6.1, when agglomerates are stressed in such a mill, excessive amounts of fines must be expected, which have to be used or discarded elsewhere or recirculated. The latter increases the equipment sizes in the system considerably. Therefore, if a narrow size range of the granular product is required, in keeping with the explanations of Section 6.1, the optional closed crushing loop for oversized particles featuring a secondary optimized crusher and twin separation steps (with a double deck screen) should be considered. It has been mentioned in Section 6.1 that the products of medium- and high-pressure agglomeration feature a well defined density and, therefore, strength gradient. Both density and strength decrease from the surface of the compact to its center. The effect depends on the size (thickness) and the shape of the pressure agglomerate (Fig. 6.1-7). As will be shown in more detail in Section 6.6.2, where it has been a common problem, crushing the compacts too gently results in relatively soft parts of the agglomerate becoming part of the final product. This may not be objectionable if granulation is solely performed to improve, often only temporarily, the storage and handling problems of fine particulate solids. However, the fact that efforts to maximize the yield of granular material may lead to the presence of soft, low-strength granules should be acknowledged and remedied if the production of (additional) fines during handling becomes a problem. Even though general applications of size enlargement by agglomeration normally do not require special or high granule quality and are often only produced as an intermediate product with temporary life, sometimes the irregular shape featuring corners and edges (Fig. 6.1-14) is undesirable, mostly because these parts rub-off easily and create particles that are so small that they become airborne. In such cases conditioning may be added. Conditioning typically happens in rotating drums where the granules are rounded during tumbling and the abraded fines are separated by entrainment in an
6.1 General Applications
Fig. 6.1-14 Typical granules from compaction/granulation showing the irregular shape of the unconditioned particles
air stream or afterwards mechanically on a screen; alternatively, a small amount of mostly liquid coating may be added to provide either adhesion properties for fines, which are then picked-up by the moistened agglomerates, or to smoothen and/or strengthen the surface of the granules.
6.1.3
Other Technologies
“Other technologies” comprise agglomeration by heat, also called sintering, and methods that manipulate fine particulate solids such that specific structures are produced. They are not normally used to produce granular materials for general applications
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because the first, requiring a large amount of thermal energy, is too expensive and the second does not manufacture agglomerates solely to improve the handling behavior; in fact, particle manipulation procedures and the use of binding mechanisms of agglomeration are always applied in specific industries to engineer products with defined new properties. One technique that is being described in more detail in Section 6.9.2 as a preparatory step for the hot briquetting of steel mill dusts for recycling shall be mentioned here, also to introduce the idea for future use with a variety of materials. The process could be conceived simply for the conversion of fine, reactive (e.g., metal-bearing) dusts, which are not valuable enough for reuse as secondary raw material (Chapter 8), into dustfree, easily handleable granular aggregate for safe disposal. In this respect it meets the requirement of “general applications” because no specific characteristics in regard to size, structure, or properties are necessary other than the need to be dust free, of sufficient strength, and easily handleable. During the production of, for example, metals by the liquid route, particularly in gasblown converters, ultrafine particles are produced that are so small that they are difficult to capture with conventional methods, become airborne, and, for example, were responsible for the brownish atmosphere in and around steel mills during the late 19th and the early part of the 20th centuries. Although most of these pollutants are oxidized impurities, tiny droplets of metal are also produced, which quickly solidify and become part of the dust. Because these particles are very small their specific surface area is so large that the smallest ones feature self-ignition characteristics. This means that, in the presence of oxygen, they are liable to oxidize, even at ambient temperatures. This chemical reaction is exothermic (producing heat). In an effort to clean the air in and around metal producing centers, improved dry dust collection systems were and are being developed, which remove the particulate contaminants but, at the same time, result in large volumes of difficult to handle dust. It is possible to use the auto-oxidation behavior of such materials beneficially for the production of agglomerates by sintering. For this purpose, the dust is tumbled in an inclined steel drum and air (or oxygen) is introduced into the bed by a suitable sparger pipe. The reoxidation reaction produces heat, the level of which is controlled by the supply of oxygen, and sintering into agglomerates of various sizes occurs. While the binding mechanism is that of sintering, the agglomeration mechanism is that of tumble/growth. Typically, this process operates continuously and fines discharging with the agglomerates may be separated and reintroduced into the feed end of the drum. Because more and more of the particulate solids that are removed from plant effluents are or contain ultrafine particles and legislation requires the safe disposal of these materials to avoid recontamination of air and/or water (Chapter 8), the high reactivity of these particles and the exothermic production of heat during oxidation can be generally used to achieve low cost granulation by heat. In this context it should be remembered that, while for the sintering of metal and mineral particles very high temperatures (typically exceeding 1200 8C) are required, the sintering mechanism begins at about two thirds of the melting or softening temperature of a particular solid. Therefore, it is feasible to apply the technology for the granulation of other reactive ultrafine particles at much lower temperatures.
6.2 Pharmaceutical Applications
6.2
Pharmaceutical Applications
By trial and error, humans found that certain herbs and other organic materials and some minerals can be used to alleviate or cure ailments. Early physicians dried the materials, ground them to yield fine powders, proportioned and blended different ingredients, and finally administered the remedy in a traditional way. Since some of these medications were given orally, “solid dosage forms” were already prepared early in human history by the tribal medicine people, because fine powders can not be swallowed easily and the concoctions often had a bad taste. To that end, the powdered drugs were mixed with binders, for example starch (flour) and water or honey, and rolled into spherical pills. Honey was often also used to mask the taste. Because the pills thus produced were still sticky after rolling, they were sometimes coated, for example with pollen, to render them dry and easily storable. Even today, in many rural dispensing pharmacies, in developed countries for homeopathic remedies, and in lesser developed parts on Earth for many, often natural drugs, the ancient pill making is still performed by producing a formable mixture from the ingredients, proportioning the mass into the dosage size, and manually rolling these pieces into spherical pills (Fig. 6.2-1) [6.2.1]. During the 10th century AD, Arabs used a simple press to form a moist medicinal powder into shapes [6.2.2]. The mixture was “compacted” by hand between the two halves of a tongue-shaped tool, which was made from bone, ivory, wood, or stone. For many centuries, pills and manually produced compacts were well accepted by the patients as solid medicines for oral application. Nevertheless, the invention of the tabletting press during the middle of the 19th century in the UK and the USA initiated a veritable revolution in the fledgling pharmaceutical industry. Aside from the production of previously unheard of numbers of tablets, the major difference between today’s manufacturing of oral dosage forms and the technology that was successfully performed for centuries is that many powders are agglomerated dry. Tab. 6.2-1 summarizes the reasons for and/or results of size enlargement by agglomeration in pharmaceutical applications. Since larger amounts and numbers of different powders, which for reasons of uniform product composition are very fine [B.48], had to be tabletted in machines operating with ever increasing speed, flowability, determining the ease of filling the die, and compactibility, controlling the production of high-quality tablets during very short densification cycles, must be often improved. This is accomplished by pre-agglomeration into a free-flowing granular feed. It was originally carried-out by wet agglomeration, another centuries-old method, and is now increasingly done by dry compaction/ granulation. Under certain conditions, a single modern rotating table tabletting machine is now capable of producing more than 1 million tablets per hour [B.97]. The two most important aspects of agglomerated pharmaceutical products are the following.
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Fig. 6.2-1 Sketches of tools for manual pill making [6.2.1]. 1, “Pill machine”: a) rolling plate, b) rope cutter, lower part, c) rope cutter, upper part, d) rolling board, e) area for ropes and finished pills. 2, Mortar (with pestle). 3, Pill rolling (disc) tool *
*
They are intermediate or final dosage forms, which must carry reliably and reproducibly the required amount of active substance. Final dosage forms are consumer products; therefore, consumer appeal is of great concern, which means for agglomerated specialties that they must have uniform, aesthetically pleasing, and reproducible shape and weight.
Tab. 6.2-1 Reasons for and/or results of size enlargement by agglomeration in pharmaceutical applications Agglomerated particulate solids contain no or low amounts of dust; therefore, they provide increased safety during the handling and processing of toxic or medicinally active materials, cause no workplace pollution or operator health risks, and, generally, result in fewer losses. Agglomerated mixtures of different particulate solids do not segregate. Agglomerated finely dispersed solids are freely flowing, therefore: Storage and handling characteristics are improved, Metering and dosing properties are better, Bulk density is increased and bulk volume is smaller. Within limits, the strength and porosity or density of agglomerates can be controlled; thus: disintegration, dispersibility, solubility, reactivity, and other properties of agglomerated intermediate or final products can be influenced. By coating or encapsulating, the agglomerate characteristics can be modified further; for example, solubility may be delayed or drug components activated by or in certain environments. Agglomerates may feature a specific shape, e.g. tablettes. Agglomerates have defined sizes and their distribution can be controlled. Monosized products or a defined amount of smaller agglomerates, the latter often enclosed in, for example, gelatine capsules, represent the dosage size. The final product has high consumer appeal and increased sales value.
6.2 Pharmaceutical Applications
The above are essential requirements because agglomerated products are supposed to facilitate measurement of or, in the case of tablets, define the dosage unit. Particularly for tablets and capsules, accurate weight is of utmost importance. Since, in most cases, the medicinally active component represents only a small fraction of the commercial drug form, uniform mixing and stabilization of the mixture (by pre-agglomeration) are necessary to avoid segregation. During manufacturing of the agglomerated specialties, a number of potential problems may have to be considered and overcome. These result from the specific properties of the highly sophisticated and often very sensitive active drug component. For example, the natural or chemically derived materials may not tolerate the presence of water or other liquids and may be sensitive to heat and/or pressure; for some substances shear forces must be avoided while still other materials are unstable or may discolor if they are brought in contact with certain chemical elements such as iron or various metal compounds. If intermediate or final pharmaceutical agglomerates are produced by tumbling, accretion, and growth, involving binder liquids, drying is required to obtain final and permanent bonding and strength (Section 6.2.1). Because many of the components of commercial drug systems are to a certain extent soluble in the liquid, not only the special case of drying of a porous body must be considered but also crust formation. Particularly if materials are temperature sensitive, which is often the case in pharmaceutical agglomeration, the phenomenon of incrustation during drying is of special importance. Agglomerates are porous bodies. The accretion and growth processes are normally controlled such that highly saturated agglomerates are produced; this means that a large portion (80–90 %) of the pore space is filled with the liquid. During drying, evaporation occurs at first only on the surface and liquid moves from the interior by capillary flow. This mechanism continues until, at about 15–25 % saturation, only liquid bridges remain at the coordination points between the particles forming the agglomerate. If the liquid flowing to the surface and evaporating there contains dissolved substances, these substances will crystallize at the pore ends and form a crust. As long as evaporation takes place, the temperature of the agglomerate remains low due to the heat of evaporation that is consumed during drying. Vacuum drying enhances this effect. However, it is possible that the crust becomes so dense that no additional liquid can reach the surface of the agglomerate from within while the core is still wet. If this happens, two alternative processes may occur. *
Because many dryers are controlled by the partial vapor pressure in the drying chamber or the dryer off-gas and will cease heating if this parameter falls below a certain value, the granules may be dry on the surface but still contain appreciable amounts of liquid inside. The same will be the case in vacuum dryers where low temperatures prevail. With time, for instance during storage of such insufficiently dried material, the residual moisture will migrate and cause lumping or, if the granules are destroyed, for example during tabletting, liquid will be liberated and cause a number of different problems.
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If drying is carried-out at relatively high temperatures and, for temperature sensitive materials, the cooling effect caused by the removal of the heat of evaporation is used to keep the temperature of the granular mass at acceptable levels, this benefit is lost when formation of a dense crust occurs. As evaporation at the surface ceases the temperature in the agglomerate rises until the build-up of internal vapor pressure results in cracking of the crust and, at least temporarily, a continuation of drying, or the temperature will reach that of the dryer atmosphere. Both can be high enough to damage heat sensitive materials.
To avoid these problems care must be taken that crust formation is light. This can be accomplished by either limiting the contents of soluble materials in the formulation or producing agglomerates that contain only small percentages of liquid, thus realizing the bridge model. Incrustation can be also avoided if agglomeration and drying are taking place at the same time as, for example, in fluidized bed agglomeration when hot gas is used for fluidization. In this case drying and recrystallization of the dissolved substance(s) begin directly after liquid bridge bonding so that agglomerates never reach high liquid saturation, which is the precondition for capillary flow and crust formation. All concerns with drying and incrustation are eliminated if agglomeration is carried out dry by compaction/granulation or tabletting (Section 6.2.3). Other problems encountered in pharmaceutical applications are associated with the need to process small batches and to avoid cross-contamination when switching from one formulation to another. Therefore, equipment is typically small, designed for quick, easy, and complete cleaning, and frequently made from stainless steel or special materials with and without protective coatings (e.g., plating with chromium, nickel, or, in extreme cases, noble metals). Increasing concerns about worker protection lead to one-pot-technologies, in which several process steps, for example mixing, stabilization by agglomeration, drying, cooling, and de-dusting, are carried-out without the necessity to open the equipment to transfer the charge, and complete containment during operation is realized. “Through-the-wall designs”, whereby parts handling and processing the drug are totally separated from the technical components by an impermeable wall until a final, safe dosage form is produced, become increasingly common. This development requires new cleaning methods such as WIP (washing in place) and CIP (cleaning in place). For some applications where successful CIP is not possible and cross-contamination must be absolutely avoided, the use of specific equipment must be limited to only one formulation. Agglomeration is a technique by which particulate solids, comprising one or multiple components, are bonded together to form larger entities (granules, extrudates, tablets). According to the binding mechanisms of agglomeration, such permanent bonding can be accomplished by solid bridges or physical adhesion and chemical or molecular forces. Because of their short range, the latter require densification of the particulate mass to produce sufficient strength. Although, today, from dosage and/or marketing (appeal, acceptance) points of view, agglomeration of solid drugs is a necessity in most cases, compromises in regard to product characteristics must be often accepted. For instance, high strength (for stability during handling, packa-
6.2 Pharmaceutical Applications
ging, and storage) requires high density obtained, for example, by compaction or matrix binders which, in turn, results in reduced solubility. Therefore, special components are sometimes added, which assist in the break-up of the agglomerated product when it comes in contact with water or other liquids (e.g., effervescent or swelling components, fibers) [B.97]. On the other hand, agglomerated pharmaceutical formulations are ideally suited for the application of coatings to accomplish a multitude of tasks (Section 6.2.3). Such coatings may simply furnish a more pleasing shape and color, or enhance or mask the taste (e.g., by applying sugar coatings), and, generally, facilitate swallowing. Coatings can also provide characteristics that are required to render the drug medicinally more effective by, for example, retarding dissolution in the digestive system. These coatings are also applied by agglomeration methods (Section 6.2.3). Another “coating” technique is microencapsulation. It coats liquid droplets or solid particles with a skin and forms “microcapsules” with dimensions in the range 1– 5000 lm. The skin consists of natural or synthetic polymers and may be dense, permeable, or semi-permeable. Therefore, this technology allows the production of microcapsules containing a reactive substance, which can be liberated in a controlled fashion by destruction of the skin or by permeation. It is also possible to carry-out reactions within the capsules after permeation of reaction partners from the outside. The outer shape of the microcapsules depends on the type of material in the core and the method of depositing the wall material. Microcapsules may be smooth, spherical particles and grape-like conglomerates or irregular particulates with smooth or rough surfaces. Since all methods of size enlargement by agglomeration still need to be fine tuned by experimentation in a vendor’s testing facility (Section 9.1) or corresponding installations at an engineering company or the final user [B.97], the new, very strict requirements for process validation, which is part of the permitting and ongoing quality control in the pharmaceutical industry, put a severe burden on the development of new pharmaceutical products and the processes that include one or more agglomeration steps for manufacturing them. Because validation also includes the need to select and define equipment sizes and operating parameters of the final commercial systems already during the permitting phase (development) and scale-up is often not easy, definition of all the requirements that must be satisfied by the mechanical processes is challenging, to say the least. Also, difficulties with scale-up do normally not allow to build a larger plant if a new drug experiences unexpected success in the market. For these reasons, the trend in the pharmaceutical industry for the production of solid dosage forms is towards relatively small, standardized systems and processes, which can be already used and optimized during the development phase. Larger capacities that may become necessary during commercialization are obtained by installing identical additional systems at a central location or in regional facilities. Contract manufacturing (Section 9.2), where the burden of equipment validation is shifted to external manufacturers (Section 15.1), is also becoming more and more attractive and important. The manufacturing of primary components, both drugs and excipients, and intermediate feed materials, such as pre-agglomerated powders and powder mixtures, may still be produced in large central locations for a multitude of finishing plants.
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As will be shown in the following sections 6.2.1, 6.2.2, and 6.2.3 all the main types of processes and techniques of size enlargement by agglomeration (Chapter 5) are being used in the pharmaceutical industry. Characteristic design features are based on what has been discussed above. The equipment is typically small, built from stainless steel, and can be cleaned easily inside and out to avoid cross-contamination. In addition, more and more installations are executed with a physical separation between the (dirty) drives and other mechanical parts that require technical maintenance and the containments that process the charge. Sophisticated control and recording devices are a common part of all systems, in part to satisfy the requirements of process validation. In accordance with the importance in the pharmaceutical industry of size enlargement by agglomeration and the use of agglomeration phenomena for other purposes, an increasing number of publications becomes available, particularly also in book form. Section 13.2 lists many of the more important books although it is not exhaustive and does not include works that where undoubtedly published in other languages.
Further Reading
For further reading the following books are recommended: B.3, B.6, B.7, B.8, B.16, B.19, B.21, B.22, B.24, B.26, B.33, B.34, B.36, B.40, B.41, B.48, B.49, B.51, B.56, B.58, B.64, B.66, B.67, B.68, B.70, B.71, B.72, B.73, B.78, B.82, B.89, B.93, B.94, B.95, B.97, B.99 (Chapter 13.1). Books mostly devoted to the subject matter are printed bold.
6.2.1
Tumble/Growth Agglomeration Technologies
As discussed in Chapter 2 and Section 6.2, the very first use of agglomeration in medicine was the making of pills from ground dry animal and plant matter, minerals, and other solid remedies to define the dosage and improve ingestion. Pills are spheroidal bodies that are produced from portioned moist, formable material by rolling with the flat hand or between two flat boards (Fig. 6.2-1). While wetting the medicinal mixture with water is often sufficient to render the material plastic and suitable for pill making, more sticky binders, such as honey, were frequently used. The sweet and relatively strong flavor of honey was also used to mask the often unpleasant taste of the concoctions. Although, in some countries and/or pharmacies, pill forming was and still is carried out by manual pressure during rolling, this technique can be considered the first application of wet agglomeration in pharmacy because the binding mechanism is caused by a liquid or viscous binder and, at least directly after manufacturing, a “green” agglomerate is obtained, which may be cured by drying or coated with a dry powder to reduce the product’s stickiness. Apprentice pharmacists still learn to manually make pills with the “pill machine” (Fig. 6.2-1). Since the first step of forming the mixture, which has been moistened in a mortar, is the production of a rope, which is then
6.2 Pharmaceutical Applications
divided into the portions to be rolled, today, industrial manufacturing of spherical dosage forms is performed by extrusion and spheronizing (Section 6.2.2). Granulation, to yield a free-flowing, dust-free, and non-segregating product, was also carried out by wet agglomeration for a long time. The formulation is blended, moistened, and passed through a screen by hand, and then dried to yield the granular product. Today, this is accomplished by low-pressure agglomeration with modern mechanized and typically motorized equipment (Section 6.2.2). Other wet agglomeration techniques, which were developed during recent decades for different applications, have been modified for use in the pharmaceutical industry and become important, as they can be particularly well designed to meet the special new requirements of validation, control, and documentation. They employ tumble/ growth agglomeration and are almost exclusively used for granulation [B.48, B.68]. In most cases, contemporary pharmaceutical wet agglomeration processes yield granules from pharmaceutical blends that are directly compressible and are, therefore, produced as free-flowing, dust-free, and non-segregating feeds for the modern high-speed tabletting machines (Section 6.2.2). Other applications manufacture a limited number of granular products that are either used directly, that is, for dosage by spoon or packaged in small envelopes, or as fill for gelatin capsules. Most other pharmaceutical specialties resulting from wet agglomeration feature instant characteristics (Section 6.4.1), for example mineral supplements and vitamin drink formulations. Shangraw [6.2.1.1] stated that the process of (pre-)granulation is historically embedded in the pharmaceutical industry. It produces in a single process (although many steps may be involved, see Fig. 6.1-13 and 6.1-14, and below) the two primary requisites for reproducibly making a high-quality compact/tablet (Section 6.2.2), that is, good flowability and compressibility. In more detail, the advantages of granulating a pharmaceutical press feed are summarized in Tab. 6.2-2. When all particles of a mixture are proportionally incorporated in agglomerates, thereby effectively stabilizing the blend, each granule has the ideal composition and any segregation of the granulated product has no effect on the ingredients in a tablet (1). By increasing the apparent particle size and sometimes also the sphericity of the granulated feed, the material becomes free-flowing, thus guaranteeing a conTab. 6.2-2 Advantages of (pre-) granulation in the pharmaceutical industry (adapted from Shangraw [6.2.1.1]) 1. No segregation: Permits handling and feeding without loss of mix quality. 2. Freely flowing: Improves flow of powders by increasing the particle size. 3. No dust: Reduces the level of workplace dust and cross-contamination. 4. Good bulk density: Increases and improves uniformity of feed density. 5. Improved degassing: Reduces the amount of air entrapment. And for wet granulation processes only 6. Better bonding: Improves cohesion during and after compaction. 7. Liquid addition: Allows for (small!) addition of liquid phase to powders. 8. Surface modification: Makes hydrophobic surfaces hydrophilic.
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stant fill of a container (for example gelatin capsule) or the die cavity(ies) of a tabletting machine (Section 6.2.2) even at very high fill speeds (2). Since, by definition, dust particles are bonded within the agglomerate and attrition is minimized, the level of dust in the manufacturing area is low. Health risks to the workers and cross-contamination between production lines is essentially eliminated (3). Although components of the formulation may have different densities, all are incorporated in the granules, which themselves feature a uniform, average density. Within limits, the bulk density of the granulated feed can be adjusted by controlling the agglomerate size range through post-treatments (e.g., screening) or directly in the process (4). Because the agglomerates are porous and feature a certain strength, densification first takes place by rearrangement of the relatively big granules (Fig. 5-9 and Fig. 10-15) whereby the large amount of gas (air), expelled in this phase, can easily escape from the diminishing interparticle volume. Only when the pressing force approaches the high-pressure region (Fig. 5-9 and Fig. 10-15), are the granules destroyed and the original powder particles incorporated into the uniform tablet structure. Porosity and pore sizes are drastically reduced resulting in a considerable increase in diffusion resistance. However, because in this final densification phase only a relatively small amount of air remains to be displaced, the volume of trapped, compressed gas in isolated pores, if any, is small and does not negatively influence tablet quality (5). The above are valid for all granulation processes, wet or dry (below and Section 6.2.2). If wet granulation is selected, additional advantages may apply. In many cases, a thin layer of binder (added as a liquid, in suspension, or in solution) forms around substances that have poor bonding characteristics. This effect, often called functionalizing (Fig. 10-14), improves cohesion during and after tabletting (6). Although, typically, granulated products are dried prior to their use in compaction, liquids are added during mixing and/or agglomeration. This allows for the addition of drugs or other ingredients (e.g., dyes) in solution thereby achieving a more homogeneous distribution (7). By chemical or physical surface modification, initiated by special liquid phases added during wet granulation, hydrophobic substances can be made more hydrophilic (Section 10.1), which improves the dissolution rate of the tabletted products (8). One technology is wet granulation by mixer agglomeration [B.48, B.68, B.97] which, after adaptation from other industries, has evolved from simple drums with mixing elements and integrated spray-nozzles for liquid additives and/or binders to highly sophisticated, specially designed, equipped, and instrumented “one pot” granulating and processing systems. Another utilizes spray drying and/or fluid-bed agglomeration [B.48, B.49, B.68, B.93, B.97] One of the first high-shear mixers, suitable for pharmaceutical applications, was the L€odige drum with plows as mixing tools on a horizontal shaft and high-speed choppers in the drum shell. After expiration of the basic patents, in addition to the original manufacturer, this design is now offered by many vendors worldwide. Fig. 6.2-2 shows this equipment. The size, number, positioning, geometric shape, and peripheral speed of the mixing elements are selected to achieve a three-dimensional movement of the particulate solids within the mixing drum [6.2.1.2]. The resultant turbulence with total particle mobility prevents the formation of dead zones and results in gentle precision mixing and agglomeration within the shortest possible time. During
6.2 Pharmaceutical Applications
wet granulation, particularly if liquid is added, the separately driven high-speed choppers are operated to destroy larger lumps that may have formed and control agglomerate size [B.48, B.97]. For pharmaceutical applications small to medium sized, versatile, and often modular equipment is desired that must meet extreme cleanliness requirements (below). Fig. 6.2-3 shows a laboratory or small production dust-tight plow mixer/agglomerator with cantilevered shaft, interchangeable drum, easy and quick opening front door for good accessibility, chopper, and liquid addition. Larger cantilevered units are often mounted on the wall to separate the potentially contaminating technical parts, such as drives, from the clean room in which the pharmaceutical formulations are handled and processed (Fig. 6.2-4). In Fig. 6.2-4, right, the front door is opened, allowing a view into the interior of the polished stainless steel drum with shaft, plows, and chopper element. To further facilitate cleaning, the shaft with the mixing elements may be fitted with a pull-out mechanism (Fig. 6.2-5). Furthermore, particularly for larger mixer/granulators, WIP (washing-in-place) can be added: Fig. 6.2-6 is a P&I (process and instrumentation) diagram of such a machine. Horizontal, batch and continuously operating drum mixer/agglomerators with plows and many other mixing tools and chopper elements [B.48, B.68, B.97] are widely used in the pharmaceutical industry, mostly for the pre-granulation of formulations prior to tabletting. The addition of steam jackets or other heat sources and/or the application of vacuum allows single-pot processing (below), which is a fast growing technology in the pharmaceutical industry. Because the equipment must not be opened between different process steps, it avoids or minimizes contamination of both the charge and the workplace. A newer, but widely applied design is the vertical high-shear system, often called “bowl” mixer/agglomerator (Fig. 6.2-7) The shape of the container promotes formation of a vortex flow and the mixing tool has minimum clearances to the inner equipment walls for maximum product yield. This mode of operation assures rapid, inten-
Fig. 6.2-2 Diagram of the operating principle of a horizontal plow mixer/agglomerator with chopper, and two alternative means of liquid addition (courtesy L€ odige, Paderborn, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-3 Laboratory or small production dust-tight plow mixer/ agglomerator with cantilevered shaft, interchangeable drum, front opening door, chopper, and liquid addition (courtesy L€ odige, Paderborn, Germany)
Fig. 6.2-4 Wall mounting of larger cantilevered mixer/agglomerators in a pharmaceutical manufacturing facility (courtesy L€ odige, Paderborn, Germany)
6.2 Pharmaceutical Applications
Fig. 6.2-5 The activated pull-out mechanism, exposing the shaft and plow mixing elements of a mixer/agglomerator for easy cleaning (courtesy L€ odige, Paderborn, Germany)
sive movement of the entire charge even if components have diverse bulk densities and particles feature widely different shapes and sizes. Often, as shown in Fig. 6.2-7a, the impeller can be lifted, sometimes hydraulically, for improved cleaning. A chopper (or multiple ones) is located such that it extends into the zone of greatest material velocity to perform the same functions as described previously.
Fig. 6.2-6 P&ID of a horizontal, batch operating plow mixer/agglomerator, equipped for WIP (courtesy L€ odige, Paderborn, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.2-7 Typical designs of vertical high-shear (bowl) mixer/agglomerators: a) basic design, b) equipment for “one-pot processing”, c) process diagram (courtesy Diosna, Osnabr€ uck, Germany)
Cleaning requirements in the pharmaceutical industry may necessitate installation of the chopper(s) through a removable roof as shown in Fig. 6.2-7c. This apparatus is also designed for efficient mixing, granulating, gas stripping, and vacuum drying in a “one pot” manner. In this case, to avoid condensation, the bowl and lid are doublewalled and heated. Fig. 6.2-8 depicts the complete design of a modern one-pot mixing, granulating, and drying system in which the particular advantages of microwave drying are utilized. In this drying method, the internationally standardized microwave energy of 2450 MHz causes the water molecules in the moist agglomerates to vibrate at high speed. Heat that results in evaporation is generated by friction between the water molecules throughout the mass to be dried. Therefore, drying does no longer occur from the outside by transfer and conduction of heat; it now proceeds at a faster rate, particularly if the solids exhibit poor heat conductivity. For pharmaceutical applications, cleanliness and avoiding cross-contamination are important requirements. Fig. 6.2-9 shows that even the smallest vertical high-shear mixers can be executed such that the stainless steel vessels are exchangeable to accommodate different materials by dedicating a specific bowl to a particular formulation or to allow easy external cleaning. Larger units may feature a similar design. Although
6.2 Pharmaceutical Applications
Fig. 6.2-8 Diagram of one-pot mixing, granulating, and drying system featuring microwave drying (courtesy FUKAE Powtec Corp., Kobe City, Japan)
Fig. 6.2-9 Small vertical highshear mixer/agglomerator with exchangeable stainless steel vessels of 1, 2, 4, and 8 L volumes (courtesy Diosna, Osnabr€ uck, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-10 Typical through-thewall installation of a mid-sized vertical high-shear mixer/agglomerator (600 L bowl) with container feeding through the ceiling in a pharmaceutical processing plant (courtesy Diosna, Osnabr€ uck, Germany)
vertical high-shear mixer/agglomerators always operate in batch mode, new equipment sometimes has a rather large volume (up to 2000 L) of which, depending on the material and application, 30–80 % is useable per batch. This allows large production rates because, typically, processing times are short (Fig. 6.2-10). In addition, if the drying process is carried-out externally, as shown in Fig. 6.2-11, in which a fluidized bed dryer is applied in line, a quasi-continuous process is obtained. In such an arrangement, a closed system from loading the raw materials to the discharge of dry granular product is also achieved. It has been already mentioned that in order to maintain a clean room environment in the processing department, “through-the-wall installation” is increasingly applied. The execution of all equipment for the granulation of pharmaceutical formulations, whether employing wet tumble/growth or any of the pressure agglomeration methods, can be either free-standing or through-the-wall, depending on the specific cleanliness requirements. Fig. 6.2-12 depicts the difference between the two system designs, using a simple vertical high-shear mixer/agglomerator as example.
6.2 Pharmaceutical Applications
Fig. 6.2-11 A quasi-continuous system for mixing, granulating, and drying in a pharmaceutical setting featuring a bowl mixer/granulator of 600 L volume
and an external fluidized bed dryer (courtesy Diosna, Osnabr€ uck, Germany)
Another commonly desired equipment feature in the pharmaceutical industry is a modular design. As already shown (Fig. 6.2-9) one of the reasons for this is the easy exchange of machine parts for cleaning or the commitment of, for example, a vessel or tooling to only one material to avoid any cross-contamination. The latter is particularly necessary if the machine parts are complicated and not easily washed and/or disinfected. Modular attachments (Fig. 6.2-13) also allow modifications to include, for
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.2-12 Diagram of through-the-wall and free-standing installations of a vertical high-shear mixer/agglomerator (courtesy Diosna, Osnabr€ uck, Germany)
example, vacuum processing with and without solvent recovery, CIP, different lids with or without filter attachments, various internal tools, instrumentation and so on. Although the above discussions were limited to two types of high-shear mixer/agglomerator, the irregular, stochastic movement that is required for high-performance blending of particulate solids in all types of blenders also produces ideal conditions for growth agglomeration by coalescence. Agglomeration (granulation) is achieved in the apparatus during or after the mixing phase by a controlled addition of binder. In the pharmaceutical industry, maintaining a uniform distribution of all components in the granular product is of particular interest since an often extremely small amount of very finely divided active substance (the drug) must be mixed uniformly and reliably with a relatively large amount of inert filler material (excipient) and segregation avoided by stabilization through agglomeration.
Fig. 6.2-13 Diagram of modular design features offered with a vertical high-shear mixer/agglomerator (courtesy H€ uttlin, Steinen, Germany)
6.2 Pharmaceutical Applications
Fig. 6.2-14 Diagrams, including indication of particle movements, and photographs of different batch-operating low-shear mixer/ agglomerators: a) double-cone (courtesy Abbe´, Little Falls, NJ, USA), b) slanted-cone (courtesy Gemco, Middlesex, NJ, USA), c) V-shape (courtesy Abbe´, Little Falls, NJ, USA) [B.97]
For some applications, low-shear, batch-operating mixer/agglomerators, such as double-cone, slanted-cone, or V-shape blenders (Fig. 6.2-14) are successfully applied in the pharmaceutical industry if the particles are soft or brittle and degradation during blending should be avoided. However, considerable problems can arise if components of the mixing and agglomerating tumbling mass have different characteristics and/or sizes. In such cases, particles that, for any reason whatsoever, feature higher adhesion tendency and/or the smaller size fraction(s) of the formulation may selectively agglomerate, thus making it impossible to achieve the ideal mixture in the granulates. The gentle tumbling in low-shear mixers does not produce sufficiently high forces to destroy these agglomerate and make the particles available for renewed, more uniform attachment to granules. The incorporation of choppers, which in mixers with stationary vessels and moving tools help in the destruction and rebuilding of agglomerates, is difficult but can be accomplished if absolutely necessary (Fig. 10-11). Aside from the gentler processing, the low-shear mixer/agglomerators produce loosely assembled granules with low density and high bulk volume. The first increases somewhat as larger agglomerates acquire more mass (Fig. 6.2-15). Furthermore, it is more difficult to achieve uniform distribution of the binder liquid. Upon impact with the powder mass, droplets wet larger areas (Fig. 5-5) and, because of the low shear, the
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liquid remains trapped in this volume. Although, during tabletting, the low-density intermediate products require longer punch strokes and higher amounts of gas need to be removed, necessitating some dwell time (Section 10.2), the compacted solid dosage forms obtained from such granules often feature improved structure, higher porosity, and better dispersibility. Combinations of high-shear blender and low-shear agglomerator, the P-K blender agglomerator, for example, unite continuous operation with a more uniform binder distribution. As shown in Fig. 6.2-16, this design consists of a slowly turning eccentric drum to which a V-shaped (zigzag) shell is mounted. Inside the drum is a dispersion head that rotates at high speed, aerates the powder, and supplies finely atomized liquid via special slots [B.97]. The charge is uniformly wetted and seeds are formed, which through repeated internal recycle from the zigzag portion back into the drum, grow uniformly into loosely bonded agglomerates. During the forward flow in the final part of the V-tube, granules are rounded and grow somewhat more. Because near the discharge end of the machine essentially no shear occurs, some segregation and selective agglomeration may still be experienced. If granulated pharmaceutical formulations with uniform composition and low density are required, application of the fluidized bed [B.41, B.48, B.49, B.68, B.93, B.97] has become increasingly prevalent. The technology, in this industry originally mostly used for the gentle and uniform drying of green agglomerates (Fig. 6.2-11), has lately emerged as an important method for the production of granulated formulations. Because the agglomerated particles are relatively small and have low density, most of the instant pharmaceutical specialties are now produced by fluidized bed processes. Particularly if dry powder is produced in a spray dryer plant from solutions, suspensions, or slurries, agglomeration can be accomplished if the partially solidified but still moist particles are tumbled in an associated fluidized bed where, in most cases, final drying also takes place. Fig. 6.2-17 is the schematic flow diagram of a continuous fluidized spray dryer (FSD). As compared with the conventional spray dryer [B.48, B.49, B.71, B.93], a somewhat modified gas handling system is the most obvious new feature of the FSD. Drying gas (9) not only enters the top of the tower for cocurrent drying but also a so-called “plenum”, a specially designed chamber at the bot-
Fig. 6.2-15 Typical agglomerates that were produced in a batch low-shear mixer/agglomerator with V-shaped shell. Agglomerate sizes: left, about
0.750 mm; right, about 4 mm (courtesy PattersonKelly, East Stroudsburg, PA, USA)
6.2 Pharmaceutical Applications
Fig. 6.2-16 A P-K zigzag continuous blender/agglomerator (courtesy Patterson-Kelly, East Stroudsburg, PA, USA)
tom of the tower, from which the hot gas is introduced through a distribution plate. The amount of drying gas entering with the dispersed feed (3) is controlled so that, while the droplets descend in the tower, only partial drying is accomplished. The still partially wet, slightly sticky particles are captured in a fluidized bed (5) where they
Fig. 6.2-17 Diagram of a continuous fluidized spray dryer (FSD) with open plant gas flow (courtesy GEA/NIRO, S€ oborg, Denmark) [B.97]
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6 Industrial Applications of Size Enlargement by Agglomeration
collide and form larger agglomerates. Fines (7) that are removed from the off-gas (10) in a dust collector (6) are recirculated to the fluidized bed where they are attached to the growing agglomerates. While the solids tumble in the fluidized bed and grow by agglomeration they are also dried with the hot fluidizing gas. Dry, agglomerated product (8) is removed from the fluidized bed in a suitable manner [B.97]. In the pharmaceutical industry, fluidized bed technology is also commonly used for the granulation of powders or mixed powder formulations by re-wet agglomeration. For reasons of containment and cleanliness, batch operating is the preferred process execution. Fig. 6.2-18 shows the principle of the process (right) and the outline of an apparatus (left) depicting, from bottom to top, a container for collecting the product, the plenum with gas distribution plate, the tower in which the fluidized bed develops and liquid binder is sprayed onto the charge and integral bag filters for capturing
Fig. 6.2-18 left) The principle of batch fluidbed granulation; right) the outline of an apparatus (courtesy Glatt, Binzen, Germany)
Fig. 6.2-19 P&ID of a typical batch fluid-bed granulation system (courtesy Glatt, Binzen, Germany)
6.2 Pharmaceutical Applications
Fig. 6.2-20 Germany)
Two batch fluid-bed granulation units (courtesy Glatt, Binzen,
entrained fines. Fig. 6.2-19 is the process and instrumentation diagram of a typical installation showing the supply of hot air into the plenum at the bottom and the discharge of gas through filters at the top. The flow is adjusted and balanced by means of a number of automatically controlled dampers (called “control flaps” in the figure). The top-spray arrangement for binder liquid is clearly visible. Fig. 6.2-20 shows actual equipment and Fig. 6.2-21 presents a typical product. Another advantage of the fluidized bed technology is the relatively simple shape of the processing chamber which, in general terms, is a vertical tower consisting of several easily removable and exchangeable sections. For example, as seen in Fig. 6.2-22, two mid-sections may be installed on the frame of the apparatus, designed for quick exchange. While one is in operation the other can be cleaned or modified. It is also easy to perform CIP, a fully automatic, reproducible cleaning process without the need to open the apparatus (Fig. 6.2-23a). Of course, problem zones such as the filters, distribution plate, sealing joints, and bulls eye windows must be specially adapted.
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-21 Acetaminophen (paracetamol) granules produced in a top-spray fluid-bed processor: a) surface, b) cross section, revealing internal voids (compare with Fig. 6.2-32). Magnification 60 (courtesy Glatt, Binzen, Germany)
Fig. 6.2-22 Batch fluid-bed granulation unit with two mid-sections installed on the frame for easy and quick exchange (courtesy Glatt, Binzen, Germany)
6.2 Pharmaceutical Applications Fig. 6.2-23 CIP of fluid-bed granulation units: a) principle of automatic cleaning, b) principle of washable pulse blow-back filters and view into the filter area of a unit, c) metal cartridge filter, d) hydraulically extendable washing nozzles (courtesy Glatt, Binzen, Germany)
The filters are a particular concern. Fig. 6.2-23b shows schematically and as a view into the filter area the washable pulse blow-back filters of one manufacturer. The shape of the metal filter cartridge (Fig. 6.2-23c), which is entirely made from stainless steel, combines optimum filtration with outstanding cleaning properties and maximum durability. The spray nozzles for washing are installed in the shell of the apparatus but, because they would disturb the material and gas flows if they were protruding into the chamber during operation of the fluidized bed, they are retracted or flush with the wall inside when not in use and extend automatically if connected with the line providing pressurized cleaning liquid (Fig. 6.2-23d).
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-24 SC (“SuperClean”) fluidized bed granulator equipped for CIP and total containment (courtesy Glatt, Binzen, Germany)
The ability to perform WIP or CIP is part of the process validation in the pharmaceutical industry. While WIP means a thorough automatic pre-cleaning but, to achieve the required results, still needs opening of the unit for a manual final cleaning, CIP is a fully automated, reproducible cleaning process with a defined result. Both are applied depending on machine design and material to be processed. Equipment for the latter is specially executed (Fig. 6.2-24) and must not be opened between campaigns or when changing from one formulation to the other. Because there may be several units in a production facility that periodically undergo WIP or CIP, the equipment supplying washing liquid with the required quantity, pressure, and temperature is often mounted on a skid (Fig. 6.2-25), which is moved and connected to a particular apparatus as required. The unit may also include the metering of the correct amount of cleaning agent(s) into the liquid.
6.2 Pharmaceutical Applications
Fig. 6.2-25
WIP/CIP skid (courtesy Glatt, Binzen, Germany)
Fig. 6.2-26 Diagram of the principle of continuous fluid-bed drying and granulation (courtesy Glatt, Binzen, Germany)
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For certain applications, the fluid bed can be operated continuously. Fig. 6.2-26 depicts the principle of this design. In most units a liquid raw material is dried while particle size is building up. Entrained fines, collected with external filters, are recirculated. It is also possible to continuously add other particulate solids. Both act as seed material for the granulation process. The fluid bed guarantees that all raw materials are homogeneously mixed in the agglomerates, which are discharged continuously through a centrally arranged pipe in the circular distribution plate. The final product size is determined by the velocity of air flowing upwards in the discharge pipe. Fluidized bed equipment lends itself particularly well to vacuum processing. Fig. 6.227 is the process and instrumentation diagram of a batch vacuum top-spray fluid-bed granulator with liquid (solvent, binder) recovery. The advantages of vacuum processing are: * *
* *
*
constant conditions, independent of atmospheric influences, a reduction of the vaporization temperature and shortening, up to 90 %, of the drying time (Fig. 6.2-28), the production of granules with higher porosity (Fig. 6.2-29), an expensive system to provide an inert gas atmosphere, if necessary in “open” systems, is not required, and up to 99 % of the liquid used in the process can be recovered and, in most cases, reused.
Fig. 6.2-30 shows photographs of two batch vacuum fluid-bed granulators with liquid recovery, indicating the extent of the vacuum and recovery systems. In a relatively new development, the distribution plate in a batch fluid-bed granulator is replaced by a solid rotating plate fitted into a conical bottom section [B.97]. Process air enters the apparatus through the annular space between the plate and the container walls. Its amount can be varied by changing the annular gap width
Fig. 6.2-27 P&ID of a batch vacuum top-spray fluid-bed granulator with solvent recovery (courtesy Glatt, Binzen, Germany)
6.2 Pharmaceutical Applications Fig. 6.2-28 Comparison of the drying temperatures and times achieved in contact and vacuum fluid-bed dryers (courtesy Glatt, Binzen, Germany)
by moving the plate up or down. Fig. 6.2-31 depicts the principle and the outline of an apparatus. The rotation of the plate produces a torus-like movement, which rounds and densifies agglomerates much in the same manner as in a spheronizer (Fig. 6.2-65). Fig. 6.2-32 shows microphotographs of a pharmaceutical product which, in spite of some densification, still features a considerable amount of internal void spaces. A similar rounding effect but without the same high densification can be obtained in a continuous fluid-bed drying/granulation process in which the bottom plate features rings of different perforations to modify airflow (Fig. 6.2-33). The airflow pattern induces a rotating ring of material in which the liquid droplets impinge solids and are dried. Granules are growing, rounded, and classified before they dis-
Fig. 6.2-29 SEM image of a granule produced in a vacuum fluid-bed granulator (courtesy Glatt, Binzen, Germany)
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Fig. 6.2-30 Two vacuum batch fluid-bed units with closed loop and solvent recovery systems (courtesy Glatt, Binzen, Germany)
charge through the central exit pipe (Fig. 6.2-26) and are cooled in an external fluid-bed cooler. Fig. 6.2-34 depicts the process and instrumentation diagram of such a process and Fig. 6.2-35 shows photographs of an apparatus and a typical product. Worldwide, there is a growing number of manufacturers that offer specialized equipment for the pharmaceutical industry using tumble/growth agglomeration/ granulation, particularly in mixers and fluid beds. Special process and equipment execution of all types in this industry (see also Sections 6.2.2 and 6.2.3) is driven by an extreme need for cleanliness and containment and by government-imposed registration and validation [6.2.1.3]. Stainless steel and other, sometimes exotic materials of construction, smooth external and internal surfaces, special seals, valves, probes and instrumentation, modular design, and one-pot processing, during which multiple steps are performed in one container or a sealed production line without the
Fig. 6.2-31 Diagrams of the principle and outline of a fluid-bed granulator with solid rotating bottom plate (courtesy Glatt, Binzen, Germany)
6.2 Pharmaceutical Applications
Fig. 6.2-32 Acetaminophen (paracetamol) granules produced in a rotary fluid-bed processor: a) surface, b) cross section, still revealing internal voids (compare with Fig. 6.2-21), magnification 60 (courtesy Glatt, Binzen, Germany)
Fig. 6.2-33 Principle of a special rounding continuous fluid-bed drying/granulation process (courtesy Glatt, Binzen, Germany)
Fig. 6.2-34 P&ID of a rounding continuous fluid-bed drying/ granulation process (courtesy Glatt, Binzen, Germany)
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Fig. 6.2-35 A rounding continuous fluid-bed drying/granulation process apparatus and a typical product at two different magnifications (courtesy Glatt, Binzen, Germany)
need to open, handle, and transfer intermediate product forms, are the most distinguishing characteristics of pharmaceutical processing and equipment. While the techniques of only a few vendors are being introduced and discussed above and in Sections 6.2.2 and 6.2.3, many additional suppliers are available, some of which are mentioned in Section 15.1.
6.2.2
Pressure Agglomeration Technologies
As already discussed, the rolling of pills from moist and sticky medicinal blends is most probably the oldest application of agglomeration in pharmacy although, to the author’s knowledge, there is no historical document available that proves its exact origin. These older forms of solid dosage are the direct precursors of today’s primary solid pharmaceutical product, the tablet, in all its forms and presentations. While references in the Egyptian “Papyrus Ebers” already seem to confirm the use of some sort of agglomerated preparation, the manufacturing of “pills” was first reported by the Greeks and Romans well before the beginning of our calendar and in Persia and Arabia in the 9th and 10th centuries AD (Tab. 6.2-3). Such agglomerates were also often coated to improve their taste and/or appeal. Originally pills were, and sometimes still today are, rolled by hand whereby manual pressure is applied directly or with rolling boards and discs (Fig. 6.2-1). During the
6.2 Pharmaceutical Applications Tab. 6.2-3 [B.48]
Early history of tabletting for pharmaceutical specialties
Time
Source/Inventor(s)
Shape or manufacturing method
BC BC 850 – 923 980 – 1037 Tenth century 1448 1606 1837 1837
Greeks Romans Rhazes al-Razi (Persia) Avicenna (Perisa) Al-Zahrawie (Arabia) Apothecary, Florence (Italy) Jean de Renou (France) M. Labelonie (France) A. Fortin (France)
1838 1840
M. Deschamps (France) E. Mayer, F. Roman (France)
1843 1848 1874
W. Brockedon (England) US Dispensatory, 12th edition (USA) J.A. McFerran (USA)
1874
Th.J. Young (USA)
1875 1876
J.P. Remington (USA) J. Dunton (USA)
1877 1877
G. Gercke, Jr (Germany) Th.J. Young (USA)
1878
Ch. Charter (USA)
1881
A. Edler von Hofmann (Germany)
1882
J.T. Jones (USA) Ch.T. Jones
1885 1889
J. Lusby (USA) O. Smith (USA) H.K. Mulford
1891 1894 1895 1896 – 7
W. Kilian P.M. Justice (England) F. Kilian (Germany) P.J. Noyes (USA)
1897 1898 1898
P.E.M. Jamain (France) W. Kra¨mer (Germany) W.R. Dodd (USA)
1900
F. Kilian (Germany)
1901
Allen and Hanburys Ltd (England)
1903
Henning and Martin (Germany)
’Katapotia’ ’Pilulae’ Pills with Psyllium seed Silver- and gold-coated pills Pastille shapes Silver- and gold-coated pills in Europe ’Tabellae’ Sugar-coated polls (dragees) Patent 5116 for the manufacture of sugar coatings of pills Honey and Acacia powder Patents 6222 and 6449 for the manufacture of pill coatings of sugar and Acacia powder English Patens 9977 for a tabletting press Manufacturing of dragees US Patent 152666, tabletting press (improvement in pill machines) US Patent 156398, improvement in machines for making pills, lizenges, etc. Tabletting press US Patent 174790, tabletting press (improvement in pill machines) German Patent 5006, rotating press US Patent 189005, improvement in machines for marking pills US Patent 207013, improvement in coated, compressed pills, lozenges, etc. German Patent 15535, machine for pressing of flowery, powdery, or grainy materials US Patent 256573, machine for making pills, lozenges, etc. US Patent 323349, pill-making machine German Patent 54817, machine for the production of pills German Patent 63185, ’pastille’ press German Patent 81470 pastille and pill press German Patent 88514, ’pastille’ press US Patents 568488 and 582794, press for sugar-coating pills, pill machine German Patent 99282, pastille press German Patent 112286, tablette press German Patent 113018, press for the manufacturing of pills, pastilles, tabletts, etc. German Patents 120903 and 126493, rotating table press German Patent 146340, press for the manufacture of medicinal tablettes German Patent 158023, press with rotating table and punches
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6 Industrial Applications of Size Enlargement by Agglomeration Tab. 6.2-3
(continued)
Time
Source/Inventor(s)
Shape or manufacturing method
1904
F. Kilian (Germany)
1906
F.F.W. Stieler (Germany)
1907
Th. O. Kend (England)
1908
F. Kilian (Germany)
1914 1917
Tietz and Co. (Germany) F.J. Stokes (USA)
German Patents 160550 and 165401, press with rotating table German Patent 190355, method for making pastilles in presses with movable upper and lower punches German Patent 202270, manufacture of pharmacuetical tablettes with a movable upper and lower punch German Patent 204824, press with rotating table and upper punches which are held coaxially in relation to the respective lower punch German Patent 287776, coating press US Patent 1248571, press for the manufacture of coated tablets
second part of the 10th century AD, Arabian “pharmacists” used a simple manually operated press to form moist powder [6.2.2]. Material was compacted between the two halves of a tongue-shaped tool made of bone, ivory, or wood. For many centuries, these forms of solid medicines for oral application had been well accepted by the patients and were produced in relatively large numbers. Nevertheless, the invention of the tabletting die press in 1843 by William Brockedon (Tab. 6.2-3) initiated a revolution in the manufacturing of pharmaceutical solid dosage forms. Although very simple and manually operated, his patent (British Patent 9977, 1843) entitled Shaping pills, lozenges, and black lead by pressure described the still-valid basic principle of punch-and-die presses, Fig. 6.2-36 [6.2.2.1]. It took some time until this method was accepted but the pace accelerated in 1874 when two additional patents for “improved pill machines” were granted in the USA. Both new machines were suitable for use with a belt drive. Thomas J. Young’s patent (US Patent 156 398, Fig. 6.2-37 [6.2.2.1]) used an eccentric movement that was disengaged when the upper punch reached its highest position. After manually filling the cavity with powder, the operator connected the eccentric drive with the fly-wheel causing the upper punch to descend, compact the powder, and then withdraw to the highest position where the drive was automatically disengaged. After manually pushing out the tablet, the cycle could begin once more. Joseph A. McFerran’s machine (US Patent 152 666, Fig. 6.2-38 [6.2.2.1]) already used a round, indexed table carrying several dies with their associated lower punches. A spindle drive, automatically turning left and right, moved the upper punch up and down and the press table was moved ahead by one step when the upper punch reached its highest point. Both filling the die and removal of the finished tablet were also already accomplished automatically. After that time, further improvements were invented around the world in rapid succession (Tab. 6.2-3). They were aimed at more accurate tablet shape and weight and increased the capacity of each unit, mostly by additional state-of-the-art automa-
6.2 Pharmaceutical Applications Fig. 6.2-36 Patent drawing of Brockedon’s hand tool for tabletting. British Patent 9977, 1843
Fig. 6.2-37 Patent drawing of Young’s eccentric drive tabletting machine. US Patent 156 398, 1874
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-38 Patent drawing of McFerran’s indexed table tabletting machine. US Patent 152 666, 1874
tion. While the basic principles, defined in the early inventions, remained largely unchanged, improvements which, during the past decades, have been strongly influenced by Government imposed cGMP (current good manufacturing practice), validation, and cleanliness requirements, result in the design and offering of quickly and perpetually improved compaction equipment for the pharmaceutical industry [B.34, B.41, B.48, B.66, B.71, B.97, B.99, B.105]. Today, high-pressure agglomeration is still the most widely used technology for obtaining dry pharmaceutical dryosage forms, so-called tablets. Because individual units are small, with diameters usually in the range 5–17 mm and weights of 0.1– 1.0 g, and must be of uniform, highly reproducible shape, only machines operating with punches and dies are applicable. Although other punch-and-die presses, such as simple ejection and withdrawal machines [B.97], are used for special applications and for research and development, to meet the throughput requirements of modern pharmaceutical production, measured by the number of tablets made per unit time, special equipment, so-called rotary tabletting machines [B.6, B.48, B.66, B.97, B.99, B.105], has been introduced, which is capable of producing up to 1 million compacts per hour.
6.2 Pharmaceutical Applications Fig. 6.2-39 The layout of a rotary punch-and-die press [B.71, B.97]
The basic operating principle of rotary punch-and-die presses is similar to that of all reciprocating ejection machines. The difference lies in the fact that a series of dies is mounted into a circular steel table (the so-called “turret”) near its periphery (Fig. 6.239) and that two punches (one upper and one lower, Fig. 6.2-40) are associated with each die. The punches are moved by stationary cams while the turret with the dies and punches is rotating. An evoluted presentation of one pressing cycle is shown in Fig. 6.2-40. Feed is supplied to the table through a frame, often called the “feed shoe”, which may include a mechanical, in most cases rotating flow stimulator. The feeder is connected to a hopper above. When a particular die moves under the feed shoe, the bottom punch that is associated with that die is pulled down to its lowest position by its cam thus allowing the die to fill with powder. The punch then rises up an adjustable ramp to eject excessive powder from the die. The powder surplus is scraped off flush with the top of the turret at the highest point of the “weight adjustment ramp”. Assuming uniform fill density, this always leaves the same volume of powder to be compacted in the die. It is common practice to let the lower punch drop down slightly after the surplus material has been scraped off. This is done to prevent uncontrolled displacement or “blow-out” of powder from the die when the upper punch enters. Both punches are then moved together by their cams to achieve densification and compaction. If, optionally, the ramps moving the punches remain parallel for some distance after reaching maximum densification, a so-called ’dwell time” is introduced. During this time, the compact remains under pressure so that additional deaeration and conversion of elastic deformation into permanent plastic deformation can occur and expansion upon pressure relief is minimized (below and Section 10.2). The overall opposing movements of both punches during densification and compaction produce the effect of double pressure and, therefore result in a relatively uniform structure of the tabletted product (Fig. 6.7-23). Finally, the upper punch is lifted from the die and the lower punch travels up to eject the finished compact. As shown in Fig. 6.2-40b, another but more technically detailed evoluted presentation of a typical high speed, high-pressure rotary tabletting machine, quite often, the maximum pressure is produced by one (or two) set(s) of press rollers that oppose each other. One or both are supported by springs to provide overload protection. In such machines, the final compaction takes place very quickly and is followed by a sudden
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Fig. 6.2-40 a) Evoluted (straightened) diagram of a rotary punch-and-die press. b) Paths of the punches in rotary punch-and-die (tabletting)
presses with one or two sets of press rollers in evoluted presentation [B.71, B.97]
6.2 Pharmaceutical Applications
pressure relief. This is similar to what happens in roller presses [B.13b, B.48, B.97] but, because in tabletting machines the roller diameter is very small and the table speed is high, this process takes place extremely fast. Therefore, capping (below) is a commonly observed problem in high-speed tabletting. To avoid this and still maintain high-production capacity, the preparation of special particulate feeds by pre-granulation (below) is frequently a necessity to overcome this defect. The simplest type of rotary machine is “single sided” with one feed location and a certain number (as few as four) of “stations” (dies) on the table. One rotation of the turret produces as many compacts as there are dies (and punch sets) on the machine. Therefore, the output of single sided rotary machines depends on the maximum allowable speed of and the number of stations on the table. It is normally in the range 300–800 tablets per minute. It can be doubled by installing two feed locations. In this case, the stations are filled twice on opposite sides of the rotating table and two compressions are carried out in each die per revolution of the turret. Obviously, to maintain the rate of densification and compaction the number of stations on the correspondingly larger table would have to be doubled, too. Outputs of more than 3000 tablets per minute can be obtained from well compacting material with double sided machines. Although the above production numbers also seem to indicate large volumetric capacities this is not the case because the individual compacts often weigh less than 1 g each. For example, with a tablet weight of one gram an output of 3000 tablets per minute translates to a capacity of 180 kg/h. A further increase in numbers of (typically small) compacts produced per minute in rotary punch-anddie presses can be achieved by dual or multiple tooling (two or more die sets) per station (below and, for example, Section 6.3.2). In many reciprocating and most rotary presses for the pharmaceutical and similar industries, the original and still most common “standard” shape of compacts is a more or less cylindrical tablet. As depicted in Fig. 6.2-41, this description includes flat, faceted, and crowned products. For these shapes, simple die and punch configurations are applicable. Since tabletted dry dosage forms in the pharmaceutical industry are consumer products, aesthetics and requirements that are dictated by the medical application (easy identification of a particular formulation by the user) and the marketing-driven desire to distinguish between manufacturers have more recently resulted in the development of special shapes, some of which are shown in Fig. 6.2-42. Additionally, the punches may be engraved as demonstrated, for example, in Fig. 6.2-43. Finally, as already mentioned above, the tooling for smaller tablets can be designed such that in a single pressing station two or more die cavities are associated with correspondingly shaped punches to produce several compacts at once (Section 6.3.2, Fig. 6.3-14). Of course, such punch and die designs are very delicate and require high-precision press designs and excellent maintenance. Expulsion of entrapped gas (air) from granulated or (particularly) powder feeds is very important because it reduces lamination and capping of the tablets. As repeatedly mentioned (also, for example, Section 10.2 and [B.48, B.97]), if gas is entrapped in compacts where it becomes compressed in the residual pore spaces and/or elastic deformation is still present when the compaction pressure is released, products from pressure agglomeration methods are partially or totally destroyed during ejec-
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Fig. 6.2-41
“Standard” tablet shapes [B.48, B.97]
tion. During the manufacturing of pharmaceutical dry dosage forms with high-speed rotary tabletting presses, capping, the separation of a thin layer of material from the main body of the tablet on one or both faces (Fig. 6.2-44), is a particular problem. It is caused by particulate solid feeds that are not suitable for quick, high-pressure densification or, in other words, by too high compaction forces and/or excessive speed of densification (Section 10.2).
Fig. 6.2-42 Some special tablet shapes [B.97]
6.2 Pharmaceutical Applications Fig. 6.2-43 An assortment of engraved punches (courtesy Kilian, K€ oln, Germany)
Raw particulate solids for tabletting may be of three types: (1) non-compressible powders, (2) compresssible powders possessing poor flow characteristics, and (3) compressible powders featuring good flow properties. Non-compressible powders are either pre-granulated wet, which, in some cases, may add a binder component that also renders the granulate better compressible (Section 6.2.1, Tab. 6.2-2), or, if the dosage level is sufficiently low, they are mixed with a powder excipient of type (3) so that the blend becomes compressible and free-flowing. The same methods are used to improve the characteristics of type (2) whereby, if pre-granulation is selected, application of the dry compaction/granulation methods (below) may be advantageous. Type (3) powders are called directly compressible. Validation and cleanliness requirements considerably burden the designs of all equipment for the pharmaceutical industry. To avoid cross-contamination it is necessary to include on modern machines CIP or at least WIP features. It is easily understandable that such techniques are difficult, to say the least, when considering the com-
Fig. 6.2-44 Tablets with “capping” and sketch explaining the capping phenomenon [B.48]
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-45 Glove box design of the processing part of a rotary punch-and-die tabletting machine demonstrating WIP (courtesy Fette, Schwarzenbek, Germany)
Fig. 6.2-46 Photographs showing: a) complete turret assembly, b) handling of the turret assembly in a smaller machine, c) turret assembly removal from a larger press, d) special handling system (courtesy Fette, Schwarzenbek, Germany)
6.2 Pharmaceutical Applications
plicated mechanical design of multi-station (up to more than 75 per turret [B.66, B.97, B.99, B.105]) rotary tabletting presses. Nevertheless, WIP is one of the latest features of such machines (Fig. 6.2-45). To meet the stringent requirements of the regulatory authorities and reduce downtimes, from newer machines (that were designed during the last decade or so) the entire turret assembly, complete with die table, upper and lower punches and upper and lower cam tracks (Fig. 6.2-46a) can be removed for cleaning, exchange, or maintenance. Smaller machines are equipped with integrated handling and mounting devices (Fig. 6.2-46b) while the assemblies of larger machines require separate handling systems (Fig. 6.2-46c and d).
Fig. 6.2-47 a) Modular system concept; b) modern automated standard tabletting system with rotary tabletting machine and tablet discharge units (courtesy Kilian, K€ oln, Germany)
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The available space for and requirements of tabletting lines vary greatly. This is why many manufacturers have adopted a modular concept consisting of presses with various feed systems, tablet discharge, de-dusting, de-burring, metalcheck, and collection devices, automatic sampling and analysis, and in-process control and documentation (Fig. 6.2-47a). Fig. 6.2-47b shows a modern automated standard tabletting system. Depending on the equipment used, such lines feature good tablet unloading, bad tablet channel, and sample extractor. Electromagnetic gates may also separate tablets that are off-specification during machine start-ups and shut-down. Since punch-and-die presses of various designs are widely used in the pharmaceutical industry and represent the most important equipment for the manufacturing of solid dosage forms, many books have been published on the subject in essentially all languages around the world. It exceeds the context of this book to go into more detail. Rather, the specialized literature should be sought and reviewed, which can be obtained from the pharmaceutical associations existing in all major countries, from international sellers of academic, scientific, and technical books that can be found on the Internet, and from vendors (Section 15.1). The books cited by the author [B.6, B.34, B.41, B.48, B.66, B.97, B.99, B.105] are those that are in his personal library. Originally, when tabletting was first introduced, the powder feeds to the machines were almost always somewhat moist. While, normally, for punch-and-die presses with relatively few strokes per minute accurate and reproducible filling of the die is not a problem and the rate of densification in those machines can be adjusted to match the compressibility of the particulate feed, the very high speed of rotary presses often causes problems. As finer, dry mixtures had to be tabletted and an ever increasing pressing speed was used to optimize the machines’ production rates, it was found that some excipients which, up to that point, behaved very well during densification did not yield products with acceptable quality. At that time (the early 20th century), without knowing all the reasons for this behavior, the term “direct compression” was coined and later (after 1950 [6.2.1.1]) used to identify a process by which tablets are pressed directly from powder blends of the active ingredient and suitable excipients without any pretreatment. With time, the composition of solid drug forms became more complex and feed mixtures for direct compression now also include fillers, disintegrants, and lubricants. It is now increasingly difficult to formulate suitable blends even though directly compressible tablet vehicles (excipients), such as spray-dried lactose, microcrystalline cellulose (MCC), modified starches, and others, are commercially available, increasingly and cheaply. Although the simplicity of the direct compression process is obvious, if pharmaceutical blends are pre-treated in one way or another prior to tabletting, flow characteristics can be improved, compressibility adjusted, and a good particle size and distribution can be selected that yields an optimal feed bulk density. However, the pretreatment of dry dosage formulations becomes more and more complicated to render them suitable for use with the sophisticated high-speed tabletting machines [B.34, B.41, B.48, B.66, B.71, B.97, B.99, B.105]. In simple terms, for those presses it is a requirement that the feed flows quickly and uniformly into the die cavities where it can be converted into well-shaped and firm compacts in a very short time. Tablet quality is a result of the blend’s “compressibility”. This characteristic not only includes an excel-
6.2 Pharmaceutical Applications
lent densification behavior, obtained by low interparticle friction, possibly assisted by the addition of lubricants, and suitable deformation properties, often defined by the nature of the main excipient, but also the quick and complete escape of air that is displaced during the fast densification stroke. For that, the absence of elasticity, which may be associated with large crystallites, and a uniformly diminishing porosity are required so that no elastic spring-back occurs upon pressure release and no isolated pores are formed in which air could be trapped and end-up as compressed gas pockets. As the active components in solid dosage forms become more potent and represent a decreasing percentage of the overall composition, an extremely uniform distribution is required to guarantee the correct content of the drug in each individual tablet [B.48]. Therefore, an increasingly important reason for pre-treatment is the necessity of avoiding segregation after mixing during handling and while feeding the press. (Pre-) Granulation of the powder feed blends for tabletting machines solves all above mentioned potential problems (Tab. 6.2-2). Flow characteristics of pre-granulated formulations, owing to the larger apparent (agglomerate) size, are usually superior, even to those of naturally free-flowing powders. While there are indications that some dry granulation for tabletting was already performed by “slugging” (below) prior to 1900 in the pharmaceutical industry, the “classic” process is carried-out by wet granulation (Section 6.2.1). Alternatively, the formulation is blended, moistened, and originally was passed through a screen by the eminence of the hand; drying then yields the granular product. Technically however, this is a low-pressure agglomeration method; modern mechanized and typically motorized equipment is shown schematically in Fig. 6.2-48. Since it is possible that lumps are produced during drying, an optional mill may be used to obtain a uniform, granular product (Fig. 6.2-49). Fig. 6.2-50 and 6.2-51 show of equipment according to 6.2-48 (b) and (c) [B.48, B.68, B.97]. While, as shown in Tab. 6.2-3, wet granulation may offer certain advantages, the addition of liquid(s), which is required to initiate agglomeration, is often objectionable. With liquid addition care must be taken: *
*
*
that the pharmaceutical components are not modified, for example by chemical reaction(s), (partial) dissolution and/or recrystallization, or physical changes, the liquid must be removed after granulation by drying and, if clean and/or expensive liquids are used, should or must be recovered and recirculated, which adds to the cost of the process, and control of particle size of the granular product is difficult; if sizing is required prior to further use, the rejected portion(s) must be recirculated to avoid excessive losses due to the often very high cost of the pharmaceutical formulation and to eliminate contamination or disposal problems caused by toxic drugs.
The modernized equipment depicted in Fig. 6.2-48 is still used for the granulation of certain powder mixtures in the pharmaceutical industry. However, other agglomeration methods, which were developed during the 19th and 20th centuries for different applications, have been modified for use as (pre-)granulation techniques in the pharmaceutical industry and have become more important, as they can be particularly well
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Fig. 6.2-48 Diagram of low-pressure agglomeration equipment using gravity feed and screens or thin perforated sheets: a) screen, b) trough, c) basket [B.97]
Fig. 6.2-49 Sketch of a low-pressure agglomeration system with screen granulator, dryer, and (optional) mill: 1) wet feed mixture, 2) granular product including fines [B.97]
6.2 Pharmaceutical Applications Fig. 6.2-50 Small trough-type granulator with horizontal rotor axis (courtesy Erweka, Heusenstamm, Germany)
designed to meet the special new requirements of validation, control, and documentation. In view of the above mentioned concerns that are associated with wet granulation, improved granulation processes make use of dry powder compaction. Historically, “slugging” was the first such technique. For this, as shown in Fig. 6.2-52, left, large (in diameter and thickness) tablets were made with often old punch-and-die machines, so-called slugging presses, that are no longer suitable for the manufacturing of highquality pharmaceutical dry dosage forms and, therefore, operated relatively cheaply. The compacts were broken to yield a granular feed forgranular feed for tabletting machines. At the beginning, this was mostly done to pre-densify the powder, thereby reducing the press stroke during tabletting, and speed-up the process. Later, the additional advantages described in Tab. 6.2-3 led to a quickly increasing use of the technology. To meet the industry’s capacity requirements, slugging presses
Fig. 6.2-51 The major components (feed hopper, extrusion blades, and perforated die) of a small basket extruder (Bextruder BX 150, Courtesy Hosokawa Bepex GmbH, Leingarten, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-52 Simplified flow diagram of dry granulation in the pharmaceutical industry using slugging or roller presses with optional circuit alternatives [Section 13.3, ref. 147]
must produce large compacts. While, at the beginning, the tablets were crushed by hand with mortar and pestle, eventually, specially designed motorized screen granulators with oscillating or rotating rod cages (Fig. 6.2-53) as breaking tools were developed to arrive at a semi-continuous operation. The final tablets made from the crushed “slugs” (dry granulated feed) often showed a “checkered” appearance, that is, the surface featured alternating dull and shiny areas. Since pharmaceutical specialties are health remedies and consumer products that are closely scrutinized by the patient, the customers frequently rejected such tablets, resulting in call-backs by and financial losses to the manufacturer. Although other industries (e.g., ceramics) had already realized that density variations exist in pressure compacted powder masses (Section 6.7.2) which, for example, cause the distortion of parts during sintering and, eventually, triggered the development of isostatic pressing in the 1920/30s [B.13a, B.48, B.97], the pharmaceutical industry was unaware of this fact. In 1957 the English pharmacologist Train [6.2.2.2] determined and published lines of constant density in compacts after applying various pressures from the top in a cylindrical die (Fig. 6.2-54). He found that, even in such
6.2 Pharmaceutical Applications Fig. 6.2-53 a) Granulator for pharmaceutical applications with screen and rod cage, b) designed for easy cleaning by disassembly (courtesy Alexanderwerk, Remscheid, Germany)
Fig. 6.2-54 Density distribution in cylindrical compacts [6.2.2.2]. Progressive stages of compaction from the top after applying the indicated
pressures. The curves in the compact are lines of constant density (1 – e) in % (e = porosity)
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simple shapes, due to the presence of frictional forces in the mass and on the tooling surfaces, considerable density differences and gradients develop. Because the largest density variations occur if a great volume of particulate solids is densified with the application of high forces, this behavior applies particularly to the products of slugging presses. It was then realized that, during granulation (i.e., the break-up of large tablets or slugs), hard pieces were obtained from the “press-skin” and other highly densified parts of the compact while weak granules resulted from the less densified interior. During final tabletting, hard granules on the surface were compacted again and created the shiny spots in a matrix of less, but still sufficiently compacted material, explaining the “checkered” appearance. A number of other disadvantages of using slugging presses also existed. For example, they generate large amounts of dust during compaction, resulting in workplace contamination and high material losses. Since the machines are mechanically complex, they require extensive cleaning when changing from one formulation to another and cross-contamination must be avoided. The operating costs are high because of piston and die wear, and lubricants are often necessary to obtain the required compaction ratio. However the greatest disadvantage of using slugging presses is that, in addition to the above mentioned effects of friction, density and hardness also vary from compact to compact because of feeding problems associated with fine powders. This further amplifies the variations in quality of both the granulate and the final tablets. Remedying these problems is very difficult. Today, dry granulation has become a well-accepted method for the processing of powder blends prior to tabletting but slugging has been replaced in most locations by compacting roller presses (Fig. 6.2-52, right). Up to the 1960s, roller presses were almost exclusively used for “dirty” applications in the coal (Section 6.10.2), mineral (Section 6.8.2), and metallurgical (Section 6.9.2) industries. They were typically large, heavy-duty machines and it was thought that they are not particularly well suited for the compaction of very fine powders since the need to operate the rollers at low speed for successful deaeration results in small unit capacities [B.97]. The latter, however, is of no concern for the pharmaceutical industry. Individual batches for the production of a single tabletted formulation are small, often only a few hundred kilograms and sometimes even less. In addition, the value of the material to be processed and the profit margins are much higher than in most other industries. Therefore, roller speeds can be low and capacities small while still maintaining profitability. All roller compacting presses in the pharmaceutical industry use one or multiple force feeder(s) to deliver the blend to the nip between the rollers and to keep the powder firmly pressed into the nip against the flow of air expelled during compaction. The roller surfaces are smooth or slightly profiled to improve the pull of material into the nip and are set at a narrow gap, producing a thin sheet. Feeder and rollers are driven independently with variable speed drives. The feeder/roller speed ratio determines the degree of densification and the hardness of the compacted powder. Product quality can be kept constant by employing an automatic control device by which the feeder motor is slaved to the current drawn by the roll drive motor. The roller presses for pharmaceutical applications are relatively small (Fig. 6.2-55), made of stainless steel, and designed for easy cleaning (Fig. 6.2-56) to avoid cross-
6.2 Pharmaceutical Applications
Fig. 6.2-55 Four small roller presses with: a) one, b) two integrated in-line granulators for the manufacturing of tabletting feed in the pharmaceutical industry (courtesy: a1) Powtec, Remscheid, Germany; a2) Vector, Marion, IA, USA; b1) Alexanderwerk, Remscheid, Germany; b2) Riva, Buenos Aires, Argentina)
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Fig. 6.2-56 Design features for easy cleaning of roller presses for pharmaceutical applications (courtesy: a) Hosokawa Bepex, Leingarten, Germany; b) Powtec, Remscheid, Germany; c) Bonals, Barcelona, Spain)
contamination when switching from one formulation to another. To eliminate contact with oil or grease, the rollers are often fixed in the frame. This design is acceptable in the pharmaceutical industry because the blends are extremely fine, clean, and free of tramp materials. Compared with the large-scale, heavy-duty applications in other industries, the compaction forces are relatively low so that torque limiting safety features are sufficient to protect the machines mechanically. The rollers are completely enclosed in a dust-tight stainless steel housing that is kept at a slight negative pressure by an aspiration system. Therefore, workplace contamination is excluded and the loss of material is only about one tenth of that encountered with slugging presses. Compacted sheets or flakes result from the continuous rolling action between the two counter-rotating rollers. Therefore, densification is very uniform and wear is low. The thickness of the compacted material is only in the range of a few millimeters either as flat or corrugated sheets/flakes (Fig. 6.2-57). Even though, microscopically, there are small density variations in the sheet or flakes, they are so small, compared with those found in the large tablets from slugging presses, that they can be disregarded. Produc-
6.2 Pharmaceutical Applications
Fig. 6.2-57 Flat and corrugated sheets/flakes and granulated product from pharmaceutical formulations
tion of checkered tablets from granular feeds manufactured by roller press-based compaction/granulation is eliminated. Referring again to Fig. 6.2-52, for many applications in the pharmaceutical industry, the compacted sheet or flakes are simply crushed and the resulting “normal” granular material is directly used as feed for tabletting machines. Although this product features a wide particle size distribution, the top size is limited because in the granulator (mill) the entire throughput is passed through a screen. In spite of the presence of fine agglomerated particles, the granulated product is free-flowing, does not segregate, and features a relatively high bulk density, which reduces the stroke length during tabletting and allows high press speeds. However, if necessary or desired, either fines can be removed by screening the granulator discharge (alternative 1 in Fig. 6.2-52) or larger particles can be produced during crushing followed by double deck screening and recirculation of the oversized material to the mill (alternative 2 in Fig. 6.2-52). The latter reduces the amount of fines produced during crushing (Section 6.1). In both cases, fines are returned to the roller press for recompaction.
Fig. 6.2-58 Diagram of the three design alternatives for positioning the rollers in roller presses for pharmaceutical applications [6.2.2.3]
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Originally and in most contemporary large-scale, heavy-duty roller presses the press rollers are arranged side-by-side and the particulate solids move vertically by gravity supported force feeding or by gravity alone into and through the machines [B.13b, B.48, B.97], but the smaller presses for pharmaceutical applications may feature side-by-side, one-above-the-other, and slanted roller positions (Fig. 6.2-58). Obviously, if a dry powder formulation flows very easily, it is possible that it passes through the gap between essentially smooth rollers without being compacted and it is difficult to start and/or maintain compaction. The use of corrugated rollers may solve this problem. Fig. 6.2-59 shows a small, instrumented compaction/granulation system. The unit features a stainless steel bucket elevator to fill a feed hopper with fresh formulation blend and/or recyclate, a horizontal discharge screw from the hopper into a vertical force feeder, a roller press with side-by-side rollers, an in-line granulator, attached to the press discharge chute, and a double-deck circular screen for separating over- and undersized particles and a narrowly sized granular product. Fig. 6.2-60 shows two views of an older, larger pharmaceutical granulation system, also employing a roller press with side-by-side rollers and vertical screw feeder. With an adjustable (through variable speed drives) production capacity of several hundred kg/h it represents one of the largest facilities used in the pharmaceutical industry. In this plant, the wellblended formulation is transferred from the mixer in special transport containers and deposited into a glove-box hopper resting directly on top of the screw feeder or the roller press. The compacted sheets are crushed in a (screen) granulator, which
Fig. 6.2-59 Small, instrumented compaction/ granulation system (courtesy Fitzpatrick, Elmhurst, IL, USA)
6.2 Pharmaceutical Applications
Fig. 6.2-60 Two views of a plant for dry, high-pressure granulation of pharmaceutical products (courtesy H€ ochst AG, Frankfurt/M.-H€ ochst, Germany)
is clamped dust-tight to the discharge chute. The granulator is mounted on wheels and can be easily removed from the system for cleaning. The feed hopper, complete with screw and feeder drive, or the screw alone can be lifted hydraulically after opening some quick release fasteners and swiveled out (Fig. 6.2-56b and c) for easy cleaning of the feed hopper, the screw, the compaction chamber and the nip between the rollers. In the case shown, the granulated product discharging from the granulator is transported by a sanitary screw conveyor to a diverter gate that allows the alternative filling of two transport containers, which are then stored or directly transferred to the tabletting department.
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-61 Diagram of two typical designs and arrangements of roller presses for pharmaceutical applications [Section 13.3, ref. 147]
Fig. 6.2-61 depicts two typical designs and arrangements of roller presses for pharmaceutical applications. As mentioned above, small presses in particular are sometimes equipped with rollers one-above-the-other (alternative a2). In this case, the force feeding screw is installed horizontally. It is often said that this design allows for better de-aeration as shown in Fig. 6.2-62. In this diagram, (1) is the raw blend, (2) is a special hopper equipped with a rotating flow stimulator, a separate de-aeration chamber, which is also used for feeding recycle (3) and additives (if applicable), and air removal port (4), (5) is a vacuum pad option for further direct de-aeration from the horizontal feed screw, and (6) is the recirculation of powder leakage [B.13b, B.48, B.97]. Fig. 6.2-61 also shows two step granulation (b). In those cases where the entire output from a single granulator (Fig. 6.2-55a, 6.2-59, 6.2-60, and 6.2-64) is directly used as tabletting feed, the presence of fines is often beneficial (above). However, excess amounts of fines may again result in reduced flowability, segregation, and dusting. The amount of fines can be lowered by two-step milling (Fig. 6.2-55b and 6.2-63) whereby the first mill is equipped with a screen featuring larger openings and only the second granulator, mounted in-line, determines the largest particle size of the final product.
6.2 Pharmaceutical Applications
Fig. 6.2-62 Diagram of a new arrangement for improved de-aeration in roller presses with vertical roller arrangement (courtesy Alexanderwerk, Remscheid, Germany)
Fig. 6.2-63 is the photograph of a pharmaceutical compaction/granulation system with vertical roller arrangement and horizontal feed screw, essentially as shown schematically in Fig. 6.2-62 but without the special de-aeration hopper. Granulation is accomplished by two-stage milling. The granulator assembly can be easily detached and rolled out for cleaning. The inset in Fig. 6.2-63 is a photograph of the partially disassembled granulator assembly. Some pharmaceutical formulations contain components that become somewhat plastic during compaction due to the conversion of mechanical energy into heat. While the rise in temperature is small and very short lived and does not normally damage even sensitive materials, sticking on the roller surfaces may occur as these parts heat up during continuous operation. To avoid this, roller cooling is available (Fig. 6.2-56c and 6.2-63). Scrapers may be also installed to keep the rollers clean (Fig. 6.2-64, item 12). When the material becomes temporarily somewhat plastic, continuous sheets are produced, which do not enter the granulators (mills) easily. In those cases, a simple flake breaker is installed (indicated at the discharge in Fig. 6.2-62), which breaks the strip into short pieces (flakes). There are good arguments by the vendors (Section 15.1) why each of the basic arrangements of rollers, horizontal, side-by-side or vertical, one-above-the-other, are more advantageous. Another manufacturer proposes, however, that the rollers should be slanted. The potential problem with horizontally mounted rollers is well understood
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Fig. 6.2-63 Pharmaceutical compaction/granulation system with vertical roller arrangement, horizontal feed screw, and granulation by two-stage milling (courtesy Alexanderwerk, Remscheid, Germany)
(above) and with vertically mounted rollers some powder particles may stay considerably longer in the compaction area, particularly in front of the lower roller, possibly becoming physically or chemically damaged. To avoid this, as shown in Fig. 6.2-64, the roller position in a newer design is between the two extremes [6.2.2.3]. In Section 6.2.1 it was stated that, today, the industrial manufacturing of spherical dosage forms is performed by extrusion and spheronization. The process is based on low to medium pressure extrusion and rounding (spheronizing) the moist, still plastic
6.2 Pharmaceutical Applications Fig. 6.2-64 Diagram of a roller press with slanted roller arrangement, with screw feeders and integrated one-stage granulation (courtesy Gerteis, Jona, Switzerland) [6.2.2.3]
extrudates in a cylindrical vessel with vertical wall and rotating bottom plate (spheronizer) (Fig. 6.2-65). According to a flow diagram (Fig. 6.2-66), the process includes the steps: feeding (1), (2), and (3), mixing and wetting of the powder components followed by kneading the wet formulation to obtain plasticity (4), and extruding damp cylindrical agglomerates (5). The extrudates are either simply separated from fines on a de-dusting sieve (6), yielding clean extrudates or converted into spherical particles in the “marumerizer” (spheronizer) (7). In both cases post-treatment (drying and potentially cooling, not shown) is required to remove the moisture and obtain dry strength of the final product. In the pharmaceutical industry, extrudates (Fig. 6.2-67a) are used, for example, in the field of herbal remedies, including instant teas. Spheronized particles (Fig. 6.267b), the diameter of which can be quite small (down to about 0.5 mm), but more commonly is in the range 1.0–1.5 mm, may be coated if desirable or necessary to obtain specific functionality (Section 6.2.3). Since the product, with or without coating, has excellent flow and metering characteristics it is suitable for filling into soft or hard gelatine capsules (“gel-caps”, Fig. 6.2-68). After ingestion and dissolution of the capsule, the particles are released and, according to the type of coating, may make the drug(s) available at different times in various parts of the digestive system. For these reasons, spheronized products have been quickly accepted by the pharmaceutical in-
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dustry and a number of competing equipment vendors are on the market (Section 15.1). While the smaller spheres with diameters of 0.5–1.2 mm are normally made from extrudates that are produced with low-pressure screen extruders (basket, radial, and dome extruders [B.48, B.97], Fig. 6.2-69), larger ones are manufactured with axial extruders (single or twin screw [B.48, B.97], Fig. 6.2-70). As shown in Fig. 6.2-69 and 6.2-70 the green agglomerates discharge from the extruders as spaghetti-like ropes. Adjustment of their length, which, for spheronizing, should be about
Fig. 6.2-65 Diagram of a marumerizer-type spheronizer (courtesy LCI, Charlotte, NC, USA)
Fig. 6.2-66 Flow diagram of a continuous granulation system featuring a low-pressure extruder with or without spheronizer (marumerizer) [B.48]
6.2 Pharmaceutical Applications
Fig. 6.2-67 a) Extrudates with small diameter (1.0 mm) obtained after de-dusting (Fig. 6.2-64, item 6); b) spheronized particles (Fig. 6.2-64, item 7)
Fig. 6.2-68 Photographs of open and closed gel-caps filled with spheronized, coated pharmaceutical granules and of tabletted dry dosage forms (for comparison)
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Fig. 6.2-69 The discharge areas of basket, radial, and dome extruders (courtesy LCI, Charlotte, NC, USA)
1.2 times its diameter, depends on the size and properties of the extrudates. Thinner ones (in the +/- 1 mm range) are charged directly, as produced onto the spheronizer’s rotating plate, which is often provided with a pattern of grooves [B.48], where they break almost instantaneously into the required short segments with rather uniform length. Products with larger diameter, normally made with axial single or twin screw extruders, are brought to length by cutting means, knifes or wires rotating in front of the extrusion plate. If the length/diameter ratio is in the desired 1.2 range, the diameter of the extrudate determines the size of the spheres after spheronization. In the spheronizer, the still plastic extrudate segments are put into rotation by the rotating friction plate (bottom of the bowl) and thrown against the vertical wall by centrifugal forces. As shown in Fig. 6.2-71, a view into an operating machine, a rotating, twisting torus-shaped rope is formed in the edge of the bowl. There, the agglom-
Fig. 6.2-70 Two low-pressure axial extruders and of extrudates/spheronized particles (courtesy LCI, Charlotte, NC, USA, and WLS-Gabler, Ettlingen, Germany)
6.2 Pharmaceutical Applications Fig. 6.2-71 View into an operating spheronizer and schematic presentation of its operation (courtesy LCI, Charlotte, NC, USA)
erates collide and rub with each other and the wall. Mechanical energy is transformed into kinetic energy, which causes a gradual deformation of the extrudate segments into spherical particles (Fig. 6.2-72). During deformation some densification can occur also and excess moisture may migrate to the surface of the rounding agglomerates. Small amounts of moisture will contribute to the pick-up and incorporation of fines by larger particles so that the product is essentially free of fines. If larger amounts of moisture are set free, a slight dusting of the mass by a suitable powder dispenser reduces the likelihood of the entire charge sticking together. Other special features include cooling or heating of the bowl through a jacket and cleaning of the friction plate between charges with brushes. Although the flow diagram of Fig. 6.2-67 seems to indicate that the equipment is interconnected, for the normally small production capacities in the pharmaceutical industry, modular components are used in most cases and transfers from one machine to the next may be accomplished manually. Since the spheronizer operates batchwise, handling typically 5 kg in 1–3 min intervals, the charging of a dosing
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-72 a) Extrudates, b) products from different spheronization treatments after 5 s, c) after 30 s, e) after several minutes (courtesy LCI, Charlotte, NC, USA)
unit serving the extruder and the transfer of extrudates that were collected in trays are commonly manual procedures. Larger systems include a batch proportioner in front of the spheronizer, which renders the process quasi continuous [B.48]. Nevertheless, they are still assembled from modular units, which are either dedicated to the processing of one formulation and are easily exchanged or can be wheeled into cleaning bays for decontamination if they are used for different formulations. Continuous feed preparation and post-treatment, even including coating after drying, together with appropriate methods of transfer are used in some large scale operations. Because of their particle shape and nature, cellulose and other plant based or organic drugs and pharmaceutical excipients often exhibit elastic properties. For that reason, as mentioned several times in this book (see, for example, Chapter 5 and Section 10.2), after fast compaction densified products may experience spring-back and loose structural integrity, strength, and/or “quality” as defined by a multitude of descriptions. To overcome possible problems, the rate of densification must be lowered to potentially unacceptable (technically and/or monetary) levels to allow conversion of temporary elastic alteration in shape and volume into permanent plastic deformation. Another possibility to achieve good densification of elastic materials is to use extrusion, for example in flat die pelleting machines [B.48, B.97]. In these presses the material is first predensified between rollers and a flat perforated die and then forced through the die bores, often during several press intervals (Chapter 5, Fig. 5-12b, for explanation) during which additional displaced air can escape and elastic deformation can become permanently plastic. Carboxymethylcellulose (CMC), a compound used as thickening, emulsifying, and stabilizing agent and a bulk laxative in medicine [B.97], is such a product. To become a directly compactable component of dry dosage formulations, granulation is required. Fig. 6.2-73 depicts the flow diagram of a gran-
6.2 Pharmaceutical Applications Fig. 6.2-73 Flow diagram of a flat die pelleting/granulation plant for carboxymethylcellulose (courtesy Amandus Kahl, Reinbek, Germany)
ulation plant for CMC that is based on flat die pelleting. Although the material needs some wetting to make it extrudable and more plastic, the water evaporates during pellet cooling. While elsewhere in this book (Section 6.1, Fig. 6.1-8b) it is stated that a highly densified skin is produced on the surface of extrudates due to shear and wall friction and that the center is somewhat less densified, which leads to the production of granules with different hardness (above), the advantage of overcoming the elastic properties by using pelleting for the densification of CMC overrides the potential disadvantages during tabletting (mostly also because granulated CMC is only a partial component of the dry dosage formulation).
6.2.3
Other Technologies
It was also already discussed in Chapter 2 and Section 6.2 that the first use of coating in medicine was for enrobing pills that were made from ground dry animal and plant matter, minerals, and other solid remedies. While wetting the medicinal mixture with water is often sufficient to render the materials suitable for pill making, more sticky binders, such as honey, were frequently used to mask the often unpleasant taste of the
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concoctions. A disadvantage of moist and sticky pills is that they tend to adhere to each other and feature unacceptable short shelf life by quickly forming lumps. Therefore, most probably the earliest form of coating was to tumble the pills immediately after manufacturing with very fine organic materials, such as pollen, or mineral powders, such as talcum or dried natural clays, to eliminate their stickiness. The surface of the pills is deactivated by the addition of a layer of dry fine particles. At the beginning this was done in bowls, shaking the pill/powder mixture by hand, and then removing the excess fines. Later, especially when powders were compressed into tablets, sugar coatings were applied to make the product look nicer, extend its shelf life, particularly if components of the formulation are hygroscopic and/or swell when moisture is absorbed, make it easier to swallow, and also, in some cases, to mask the drug’s taste. For this, the batch coating pan was developed [B.48, B.97]. These machines are still widely used in the pharmaceutical industry today (Fig. 6.2-74) for sugar coating and glazing. The tabletted cores are often lentil shaped or crowned (Fig. 6.2-41) and the resulting product is called drage´e. Coating is accomplished by spraying (often colored) sugar solutions onto the tumbling bed of cores and simultaneously evaporating the solvent with warm air. The shapes of the pan and the tablets have been selected to achieve stochastic movement and a uniform coating by exposing all surfaces of the cores to the liquid spray. Fig. 6.275 shows two photographs of coating pans in manufacturing plants. Special executions include, among others, double walled pans for heating or cooling (Fig. 6.2-76).
Fig. 6.2-74 Outlines of different modern coating pans (courtesy Stechel, Alfeld, Germany)
Fig. 6.2-75 Coating pans in two manufacturing plants (courtesy Stechel, Alfeld, Germany)
6.2 Pharmaceutical Applications Fig. 6.2-76 Specially equipped coating pans (courtesy Stechel, Alfeld, Germany)
More recently it was discovered that tablets can be functionalized by applying special coatings. Functional coatings may be soluble or insoluble in water, soluble only in liquids with specific characteristics and/or composition, permeable, impermeable, or partly permeable, permanent plastic or elastic, elastic featuring a defined burst pressure. [B.97]. Some are based on recrystallizing dissolved substances, others on the development of polymeric surface layers, and still others on powder coatings. All, however, are produced or assisted by spraying a liquid onto the cores and evaporating the solvent. Because for these new applications, the characteristics of the final coating, particularly its thickness, uniformity, and structure, are determining the performance of the final product, much more stringent requirements on the deposition and buildup of the layer(s) are imposed. Among the first modifications of the behavior of tablets were those that delay the availability of drugs until they reach the intestinal system by providing a coating that is not dissolved by the juices in the stomach. Later, granules were coated with materials featuring different dissolution behavior and either incorporated in varying proportions in tablets or filled into gelatine capsules. The latter are now commonly based on spheronized particles (Section 6.2.2) and the coatings are distinguished by color (Fig. 6.2-68). Such pharmaceutical specialties provide a long-term (controlled release, or retard) effect by liberating the drug(s) at different times after ingestion. Today, sophisticated drug delivery systems are being developed that target specific parts of the body and avoid the indiscriminate broad-band supply of drugs to the entire system. Some of these controlled release reactions can be accomplished by suitably coating particulate active pharmaceutical components prior to formulating the composition of the dry dosage form, for example, tablets or granules. The micron sized coated particles enter the blood stream and liberate the drug when they come to, for example, inflamed body parts and experience higher temperatures. It is understandable that these new coating tasks require more defined deposition methods. Particularly the achievement of very thin, uniform, and completely closed layers (film coating) is a major problem. The entire surface of cores, which may be well-formed compacts or spheronized extrudates, but also irregularly shaped particles or agglomerates, and sometimes very small particles, must be exposed to sprays to
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Fig. 6.2-77 The flow diagram of a typical film coating facility: a) programmable logic controller (PLC), b) storage tanks for spray liquid(s) and metering/pumping system, c) equipment for supplying and processing air, d) air cleaning and exhaust system [B.48, B.97]
form layers that are often only a few molecules or powder particles thick and must have no holes. Areas of coatings become ineffective, for example, if “blobs” of coating material attach to the core or a “shadow effect” causes incomplete coverage (holes) and the desired functionalism is compromised at those points. Therefore, it is also most important to apply strict process control. Modern batch drum film coating equipment [B.48, B.97] features at least four support areas as shown in Fig. 6.2-77. To obtain uniform coverage, while the cores tumble in the apparatus, the liquid sprays must spread over the entire length of the particle bed and the flow of warm or hot air must be directed such that each particle is instantaneously dried to guarantee the production of a smooth surface. Correct movement of the core particles is achieved by installing baffles or lifters or by using polygonally shaped drums. Such high-definition film coaters always operate in batch. Spray systems have become very sophisticated whereby the stainless steel spray arms with nozzles are often telescopic and can be extracted through the front door for cleaning (Fig. 6.2-78). If slurries are used, spraying is air assisted to unplug the nozzles and keep them clean. Depending on the application, the flow of air may be directed in different ways to obtain specific effects. In such drum film coaters some or all of the panels are double walled and perforated to allow air inlet and exhaust in a controlled manner [B.48, B.97]. Another design utilizes stationary, hollow, perforated paddles that are immersed in the product and create an unidirectional, constant, and homogeneous flow of air in the tumbling particle bed. Similar to what has been achieved with double walled, perforated and segmented panels in polygonal drums, in cylindrical drums using paddles, air can be directed in different ways, too (Fig. 6.2-79). Drum film coaters, used in the ultra-clean pharmaceutical industry (Fig. 6.2-80), are made in sanitary, seamless design from stainless steel and feature smooth exterior housings. Additionally, in accordance with the rules of cGMP, the equipment parts, which are contacting product must be capable of CIP. Fig. 6.2-81 shows the automatic cleaning of a drum film coater according to these requirements. The drum and the cleaning tub that is built into the housing are separated from the “technical part” by water tight seals. After wet cleaning in four steps, the machine’s own air system is used for drying.
6.2 Pharmaceutical Applications Fig. 6.2-78 Typical telescopic spray nozzle mounting assembly for film coating drums and detail of an individual spray nozzle attached to this assembly (courtesy O’Hara, Richmond Hill, Ontario, Canada)
Small particles, either powders, crystals, or agglomerates, the shape of which may be irregular, or spheronized, are typically coated in specially designed fluid-bed equipment (Section 6.2.2). As with all other coaters, the heart of fluid-bed processes is the type and location of the delivery system for the liquid coating material. For this, three methods are available: top, tangential and bottom spraying [B.48, B.97]. The nozzles are often binary, that is, liquid is supplied at low pressure to an orifice and is atomized by pressurized gas. Such pneumatic nozzles produce finer droplets, an advantage when coating smaller particles. However, it is also an important requirement of coating that the liquid, solution, or suspension droplets impact the core particles and uniformly distribute on the surface before the liquid is dried off. Since very fine droplets evaporate quickly as they travel from the nozzle to the particle bed, solids concentration and viscosity of solutions and suspensions increase. Therefore, droplets may fail to spread satisfactorily when they contact the substrate surface, resulting in an imperfect coating. This drying-up of the coating spray can be severe in top-spray coaters (similar to Fig. 6.2-18) in which the most random particle movement exists and liquid is sprayed against the flow of drying air. Nevertheless a substantial share of coating is performed in this type of equipment because larger amounts can be processed per batch and the design is simple. Fig. 6.2-82 is an artist’s impression of a topspray coater in a pharmaceutical manufacturing facility showing the fully integrated
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Fig. 6.2-79 Flow diagrams of two different air flow regimes using air-blowing paddles in a cylindrical film coating drum (courtesy GS Coating Systems, Osteria Grande (Bologna), Italy): a) hot air through the paddles into the particle bed with air exhaust through the hollow shaft; b) hot air through the hollow shaft onto the particle bed with air exhaust through the paddles; 1) inlet air handling unit, 2) control panel, 3) solution tank, 4) dosing system for liquid to be sprayed, 5) sliding support arm for spray nozzles, 6) coating pan, 7) air-exhaust or blowing paddle device, 8) dust collector, 9) outlet air fan, 10) powder dosing device
6.2 Pharmaceutical Applications Fig. 6.2-80 Two drum film coaters installed in the ultraclean environment of pharmaceutical processing (courtesy Driam, Eriskirch/Bodensee, Germany)
processing systems and matching accessories with through the ceiling separation of the technical support functions and product handling below the floor of the clean room. The rotating disc fluidized bed coater (similar to Fig. 6.2-31) combines centrifugal, high-intensity mixing with the efficiency of fluid-bed drying. A major advantage of this method is its ability to layer larger amounts of coating materials onto cores consisting either of robust granules, crystals, or nonpareil nuclei. Because of the unit’s high dry-
Fig. 6.2-81 Sketch showing the CIP of a polygonal drum film coater (courtesy Driam, Eriskirch/Bodensee, Germany). Cleaning phases: 1) drum inside by cleaning spray bar, 2) drum outside and air distributor, 3) air channels inside, 4) rinsing of tub
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Fig. 6.2-82 Artist’s impression of a top-spray fluid-bed coating system (courtesy Vector, Marion, IA, USA)
ing rate, relative large gains in product weight can be achieved in a short period of time. Another advantage of the rotating disc fluidized bed coater is the possibility to layer dry powders that are dispersed in the fluid bed onto nuclei, which have been wetted with a liquid. Because the spray nozzle is located below the bed surface, the above mentioned problems with early drying are not experienced.
6.2 Pharmaceutical Applications
The same is true of the Wurster coating process (Fig. 6.2-83). This is the only bottom-spray fluid bed coating method, which is not only applicable for fine particles and coarse granules but for tablets and pellets as well. The Wurster coating chamber is cylindrical and the basic model contains a concentric inner tube with approximately half the diameter of the outer chamber. At the base of the apparatus is a perforated plate, which features larger holes underneath the inner tube. The liquid spray nozzle is located in the center of the orifice plate and the tube is positioned at a certain distance above the plate to allow the movement of material from the outside annular space to the higher velocity airstream inside the tube. This design creates a very organized flow of material, similar to that of a spouted bed [B.48, B.97]. Solids move upwards in the center where coating and highly efficient drying occur. Contrary to what happens in the spouted bed, where some mass exchange occurs between the solids moving upwards in the center and those in the outside downward flow, the high-speed upward flow regime in a Wurster coater is contained in the center tube, so that no backmixing occurs. At the top of the tube, the material discharges into an expansion area and then flows down, as a near-weightless gas/solids suspension, in the annular space outside the tube. Fig. 6.2-84 shows a Wurster coater for pharmaceutical applications. Design variations include different chamber configurations for use in coating tablets, coarse granules, or fine particles (Fig. 6.2-85). For scale-up, the outer vessel diameter and the number (rather than the size) of inner tubes increase. Each tube features its own gas distributor and spray nozzle and is essentially identical with that in the test equipment that was used during development work [B.97]. During studies of particle and gas flow patterns in traditional Wurster coaters it was found that flow patterns were dominated by the particles rather than the gas. This explains why sometimes, in spite of the well defined flow, uniform coating is difficult to control. To overcome this problem, a precision Wurster coater was developed [B.97]. An essential feature of this new design is a better controlled gas flow pattern in the coating zone. This is achieved by application of a guiding system in which the gas flow is accelerated, stabilized, and given a precise amount of swirl, which eliminates slugging, often seen in traditional coaters, and stabilizes multi-tube systems. Particles are
Fig. 6.2-83 Principle of the Wurster coater (right) and outline of an apparatus (left) (courtesy Glatt, Binzen, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-84 Wurster coater for pharmaceutical applications (courtesy Glatt, Binzen, Germany)
entrained into the swirling air on an individual basis. This results in an optimized probability of impact with the droplets of atomized liquid and an even application of coating material. Fig. 6.2-86 shows scanning electron micrographs of various particles that were coated with different fluidized bed coaters. Although, as shown in Fig. 6.2-85a, short Wurster coaters may be used for the processing of tablets, it is more common to use drum coaters for this task. Since spray systems projecting into the tumbling bed are subject to relatively high mechanical forces, are damaged easily, and become clogged quickly, usually the top-spray method is employed. As mentioned before, the disadvantage of this arrangement is that many of the fine spray droplets are deflected by the strong counterflow of hot air and dried before they ever reach the particles to be coated. This results in the loss of film forming material and an irregular surface coverage. Furthermore, the coating is non-uniform because neutral or dead zones occur in the bed so that it can not be assured that all tablets travel to and are exposed at the bed’s surface for the same time to the spray.
6.2 Pharmaceutical Applications Fig. 6.2-85 Sketches of the different chamber configurations of single tube Wurster coaters as used for: a) tablets, b) coarse granules, c) fine particles [B.48]
To overcome these problems, a vertical centrifugal coater was developed [6.2.3.1]. As depicted in Fig. 6.2-87 it consists of a perforated bowl-shaped container that rotates around a vertical axis, and a static return flow cone located therein. Centrifugal force is utilized to produce the movement of the tablets to be coated. The particles initially rise up the wall of the rotating bowl and, on reaching the top rim, are led into the return flow cone. They then descend and, in the center, drop back to the bottom of the bowl where the cycle starts again. When the tablets fall down from the return cone they are sprayed with the coating material by a radial spray nozzle with a narrow spray angle of 308. The major advantages of this new machine are:
Fig. 6.2-86 SEM photographs of particles that were coated with fluidized bed coaters: a) coated with ethylcellulose in a top-spray fluid-bed processor, b) coated with ethylcellulose in a Wurster apparatus, c) vacuum top-spray coated (retard) pellet, cut open, d) Wurster coated (retard) pellet, cut open (courtesy Glatt, Binzen, Germany)
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Fig. 6.2-87 Diagram of the operating principle of a vertical centrifugal coater (courtesy Diosna, Osnabr€ uck, Germany) * * * * * *
reduction of the processing time by 50 % or more, uniform coating, intense air circulation, and little loss of coating material, high-quality through reproducible thickness and uniformity of the coating, possibility to build and use (due to shorter processing time) smaller units, complete, automatic discharge, and ease of adding WIP.
Fig. 6.2-88 is a view into the open top of an operating vertical centrifugal coater. Another coating technique is encapsulation. Although this is a relatively new technology, many different processes have been developed, a large number of applications has evolved, and many new uses are conceivable and found, literally on a daily base (Chapters 5, 11, and 12). Some encapsulation can be achieved during the drying of wet (green) agglomerates, including spheronized extrudates, the pores of which are filled with a liquid (continuous phase) [B.48, B.97]. In a first drying phase, evaporation takes place only on the surface and the liquid is replenished by capillary flow of the continuous phase from the interior of the porous body. If the liquid is a solution or suspension, solids are deposited at the pore ends on the surface and cause encrustation. If a film-forming, easily soluble polymer is dissolved in the continuous phase as emulsion or dispersion, en-
6.2 Pharmaceutical Applications
Fig. 6.2-88 View into the open top of an operating vertical centrifugal coater (courtesy Diosna, Osnabr€ uck, Germany)
capsulation occurs during drying. These encapsulation processes can be carried-out with agglomerates of any size and shape and result in a large number of special effects which, depending on the type and composition of the coating or incrustation, modify final product properties. More commonly used and widely researched is microencapsulation. In this context, the prefix “micro” refers to the dimension of the encapsulated product, which is typically 1–2 mm in size or, increasingly, 10–100 lm range. If a slurry containing a solution, emulsion, or suspension of polymer is dispersed into small particles and dried in a spray dryer microencapsulated particles of the type described above are formed. Such a process yields a dry, free-flowing powder which, in most cases, satisfies the criteria defined for instant products. However, microencapsulation becomes a more and more a sophisticated packaging method in which the “packing material” (coating) features a specific, well-defined functionality. With this technology small agglomerates or tiny portions of powders, liquids, and even gases are individually wrapped into a shell (capsule) to form free-flowing particles, which are most often spheroidal [B.97]. Fig. 6.2-89 illustrates possible structures of microcapsules. In the pharmaceutical industry, drugs are encapsulated to facilitate or improve handling and, very importantly, to bring about special product characteristics (functionalizing). Microencapsulation techniques make use of sol-gel processes, coacervation, surface and in situ polymerization methods or, generally, interfacial reactions to produce soluble or insoluble and impermeable or permeable capsule walls. In addition to the coating and spray drying methods that were discussed previously, a growing number of other processes deposit particles onto cores or solid surfaces whereby the binding mechanisms of agglomeration are utilized [B.97] (Chapter 11).
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Fig. 6.2-89
Diagram of possible structures of microcapsules [B.97]
As examples, Fig. 6.2-90 depicts sketches of the principle and the equipment of electrostatic, aerosol based microencapsulation. The two components to be turned into a microencapsulated product, the coating and the core particles, are given an ionic charge of the appropriate sign using a sub-corona discharge system. To achieve sufficient encapsulation, the apparatus must be designed such that a high rate of collisions between the two components occurs in a turbulent supportive gas system. In this process, the coating materials must be selected so that they will uniformly and completely cover the core particles. They include softened wax particles, which solidify
6.2 Pharmaceutical Applications
Fig. 6.2-90 a) Principle of microencapsulation by electrostatic, aerosol based coating (1, coating particles; 2, core particles); b) sketch of a possible equipment configuration [B.48, B.97]
upon cooling or polymers, which form a skin by interfacial action between a component in the core and another in the coating material or upon exposure to a suitable gas phase. The coating (capsule) can be also finished by heating (sintering). Although, technically, agglomeration in liquid suspensions is a wet agglomeration method, for the purposes of this book it qualifies under “other technologies” in the pharmaceutical industry as a new growth process for the manufacturing of welldefined, engineered particles. Crystallized active pharmaceutical ingredients which, because of their particle size and shape, do not exhibit good handling and/or compression behavior must be modified to render them suitable for direct use in the modern high-speed rotary tabletting machines. A promising new technique is spherical crystallization [6.2.3.2]. This agglomeration method greatly improves the micrometric (size, distribution, shape, and structure) and compaction properties of crystals. Crystal agglomeration in liquid suspensions is possible according to two different mechanisms, which depend on the amount of drug solution (for example ascorbic acid dissolved in a good solvent, for example, water) added to a poor solvent system (e.g., ethyl acetate). Fig. 6.2-91 shows the two alternative methods: emulsion solvent diffusion (ESD) and spherical agglomeration (SA).
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.2-91
Mechanisms of crystal agglomeration [6.2.3.2]
When, in the first case, the two liquids, kept at different temperatures, are mixed with a small water-to-ethyl acetate volumetric ratio (1:100), using a suitable stirrer and no baffles, a W/O emulsion forms first. Without baffles, the emulsion droplets do not break down. Then, as they cool and water/ethyl acetate diffuse, solubility in the emulsion droplets decreases and crystals precipitate on the droplet surfaces. When crystallization is complete, agglomerates have formed. This mechanism is called emulsion solvent diffusion or ESD. On the other hand, if the water-to-ethyl acetate volumetric ratio is large (4:150) and baffles are built into the agitator tank to promote turbulence, crystals precipitate in the same way after an emulsion has formed. Then, a small amount of water that was liberated from the ethyl acetate phase acts as a liquid bridging agent between the crystals, causing particles to randomly agglomerate (immiscible liquid agglomeration [B.48, B.71, B.73, B.97], see also Chapter 5). This mechanism is called spherical agglomeration or SA. The main difference between the two agglomerated crystal forms is that, as depicted in Fig. 6.2-92, in ESD particles, crystals grow towards the center (a2), while with the SA method, primary crystals were randomly assembled and formed into agglomerates (b2). Fig. 6.2-93a is another SEM image of a spherically crystallized drug showing again the random arrangement of primary crystals and Fig. 6.2-93b indicates the exceptional uniformity of the product, which is responsible for its excellent flowability and handling properties, both allowing quick feeding into the dies of high-speed tabletting machines.
6.2 Pharmaceutical Applications
Fig. 6.2-92 Scanning electron micrographs of: 1) external appearance, 2) cross section of a) ESM, b) SA agglomerated crystals of ascorbic acid [6.2.3.2]
As described above, in SA the crystallization system is in reality a ternary mixture composed of a good solvent dissolving the drug, a poor solvent for precipitating the drug, and a third liquid, immiscible with the system, termed bridging agent. The latter preferentially wets and agglomerates the crystals produced. In the two solvent approach (mixing poor and good solvents), a small amount of the binding liquid is liberated from the system and becomes available for bridging. As shown in the three step sketch of Fig. 6.2-94, a three-solvent system (containing good and poor solvents and an added bridging liquid) allows even better quality control of the agglomerates [6.2.3.3]. With decreasing amount of bridging liquid the agglomerates become more uniformly
Fig. 6.2-93 Scanning electron micrographs of: a) spherically agglomerated crystals of a drug; b) sample of the same product, showing its uniformity (courtesy AstraZeneca R&D, M€ olndal, Sweden)
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.2-94
Mechanisms of spherical crystallization in a three-solvent system [6.2.3.3]
shaped and feature a narrower particle size distribution (Fig. 6.2-95). Also, as shown in Fig. 6.2-96, the greater interfacial tension of the added aqueous bridging liquid in the ternary system, as compared with the ethanol–aqueous bridging liquid, causes stronger bonds, producing agglomerates with a thick shell and highly densified surface, made up of needle-like crystals. Ultimately, however, the interest of the pharmaceutical industry in a new, engineered material for tabletting is not only in its improved handling and die filling behavior but also in an enhanced compactibility. Direct tabletting of a granulated formulation is only feasible if all of these combined characteristics are available. In a series of experiments acebutolol hydrochloride, an anti-arrhythmic agent, was used for tabletting trials [6.2.3.4]. The material was chosen because it is easily water soluble and in its dry crystalline form exhibits strong cohesion and high static electricity properties. From an aqueous solution of the drug a W/O emulsion with isopropyl acetate
Fig. 6.2-95 Size distribution of SA crystals produced in a three-solvent system [6.2.3.3]. Ethanol 5 mL, bridging liquid (water): *) 0.5 mL, ~) 0.8 mL, &) 1.2 mL, ^) 1.5 mL
6.2 Pharmaceutical Applications
Fig. 6.2-96 Scanning electron micrographs of SA crystals prepared with two- and three-solvent systems [6.2.3.3]
was prepared in a stirred vessel containing baffles. A small amount of seed crystals (drug powder) was then added to promote crystallization and agglomeration. After solidification was complete, the dispersing solid was decanted and the agglomerated crystals were filtered and dried. This process conforms to spherical agglomeration (SA) with a two solvent system. Fig. 6.2-97 shows scanning electron micrographs of drug crystals (5–20 lm) and of spherically agglomerated product (208–600 lm). As shown in Fig. 6.2-98, the fill volume (called relative volume, crossed symbols) in the die of the spherical agglomerates is almost twice as high as that of the crystals. This is due to the fact that crystals are dense and fines fill the voids between the particles
Fig. 6.2-97 Scanning electron micrographs of single dry crystals of acebutolol hydrochloride and of spherically agglomerated crystals [6.2.3.4]
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.2-98 Relationship between relative volume and compression pressure during the compaction of single dry (*) and spherically agglomerated () acebutolol hydrochloride crystals [6.2.3.4]
while the micro-spheres pack less densely, contain no fines, and are porous. With increasing pressure the crystals compact uniformly while the spherical agglomerates exhibit the typical densification behavior of larger particles (Section 5.2 Fig. 5.9). The latter includes rearrangement of the spheres without change in size and shape (a–b in Fig. 6.2-98), production of some fines that fill voids between the spheres (b–c), breakage and deformation of the spheres and building of a new, dense structure (c–d), and, at the end, breakage and densification of the needle-like crystals themselves (d–e). In the final phase (c–d), a somewhat denser compact structure is formed with the agglomerates as compared with the single crystals (Fig. 6.2-99).
Fig. 6.2-99 Scanning electron micrographs of cross-sections of tablets prepared with various compression pressures (1, 10 MPa; 2, 50 MPa, 3,
200 MPa) [6.2.3.4]: a) tablets from single crystals, a1–a3 1500; b) tablets from spherically agglomerated crystals, b1 750, b2 and b3 1500
6.3 Applications in the Chemical Industry
The overall evaluation must take into account the extreme handling problems and die filling difficulties and a pronounced capping tendency (Section 6.2.2, Fig. 6.2-44) of single crystals. In contrast, the very good flowability of the spherically agglomerated drug allows a higher speed of compression whereby, because freshly created surfaces resulting from agglomerate fracture enhance the plastic interparticle bonding [6.2.3.4], capping does not occur. Therefore, utilizing spherically agglomerated drugs result in directly tablettable formulations.
6.3
Applications in the Chemical Industry The chemical industry shows the most diverse applications of size enlargement by agglomeration. They are found in both inorganic and organic chemistry and are typically used to modify the characteristics of intermediate products by eliminating dusting, improving storage, flow, and metering properties, increasing bulk density, providing good dispersibility or, generally, desirable characteristics for further use, giving high surface area in stable particulate masses, and causing many more features (Tab. 6.1). There are very few chemicals or compounds that are produced in quantities similar to, for example, agrochemicals, animal feeds, ceramics, fertilizers, foods, minerals, pharmaceuticals, or solid fuels (mentioning, in alphabetical order, other major sections of this book). Each of the many different chemicals is treated by the method that is required to obtain the desired characteristics. Processes may include wet or dry granulation, extrusion, spheronization, briquetting, compacting, or sintering and apply any of the methods of size enlargement by agglomeration (Chapter 5). Tab. 6.3-1 lists, in alphabetical order, chemicals that are cited in the sales literature of different vendors of equipment for size enlargement by agglomeration. They were compiled by the author, not to paint a concise picture of the use of the technology in the chemical industry but to suggest the great variety of potential applications. Several of the chemicals are synthesized materials that are also found in nature and may also be mentioned in connection with agrochemical (including fertilizer), feed, food, pharmaceutical, and special applications. Tab. 6.3-1 Chemicals (in alphabetical order) that are cited in the sales literature of different vendors of equipment for size enlargement by agglomeration Acrylonitrile butadiene resins Acetylsalicylic acid Adipic acid Alkyl-aryl sulfonates Aluminum chloride Aluminum formiate Aluminum hydroxide Aluminum phosphate
Chromium sulfate Cobalt sulfide Citric acid CMC (Carboxy-methyl-cellulose) Codeine phosphate Copper hydroxide Copper oxide Copper oxichloride
Nickel hydroxide Nickel sulfide Nitrates Nitro-penta-erythritol Organic pigments Oxalic acid Paracetamol Pectin
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6 Industrial Applications of Size Enlargement by Agglomeration Tab. 6.3-1 Chemicals (in alphabetical order) that are cited in the sales literature of different vendors of equipment for size enlargement by agglomeration (continued) Aluminum silicate Aluminum stearate Amino acid Ammonium chloride Ammonium nitrate Ammonium phosphate Ammonium sulfate Ampicillin Ampicillin trihydrate Amylase Antrachinon Aspartame Aspirin Atrazine Barium carbonate Barium chloride Barium hydroxide Barium sulfate Bismuth carbonate Borax Boric acid Cadmium carbonate Cadmium chloride Cadmium sulfide Calcium acetate Calcium bromide Calcium caseinate Calcium gluconate Calcium hypochloride Calcium lactate Calcium oxide Calcium saccharate Calcium silicate Calcium stearate Calcium sulfate Catalysts (bismuth, iron, nickel, platinum, silica, vanadium, zinc) Cellulose acetate Chrome tannin Chrome-yellow Chromium dioxide Sodium bichromate Sodium bromide Sodium carbonate Sodium chloride Sodium chromate Sodium citrate Sodium cyanide Sodium ethylxanthate
Copper sulfate Cryolite Cupric oxide DBDMH (Dibromo-dimethyl hydentoin) Dextrose monohydrate Dicalcium citrate Di-potassium and -sodium orthophosphate Dispersing agents DMT (Di-methyl-terephthalate) Ethylene-vinyl acetate Fatty alcohol sulfate Glycerides Graphite Hypochloride Ibuprofen Indigo Iron oxide Iron chelate Lead oxide Lead stearate Lead sulfate Lithium chloride Lithopone Magnesium aluminum silicate Magnesium carbonate Magnesium chloride Magnesium hydroxide Magnesium oxide Manganese carbonate Manganese oxides Maleic anhydrate Maltodextrins MCC (Micro crystalline cellulose) Melamin formaldehyde resin Monosodium glutamate Mono-sodium and -potassium orthophosphate Niacinamide Nickel carbonate Sodium phosphate Sodium silicate Sodium silico aluminate Sodium silico fluoride Sodium sorbite Sodium sulfate Sodium thiosulfate Sorbitol
Penicillin Phenoles Phenol formaldehyde resin Phtalocyanides Phosphates Polyacrylate Polyacrylonitrile Polyamide Polycarbonate Polyethylene Polyethylene terephthalate Polyformaldehyde Polypropylene Polystyrene Polyvinyl acetate Polyvinyl pyrrolidone Potassium bicarbonate Potassium carbonate Potassium chloride Potassium monopersulfate Potassium nitrate Potassium permanganate Potassium peroxymonosulfate Potassium persulfate Potassium phosphate Potassium sorbate Potassium sulfate PVA (Polyvinyl alcohol) PVC (Polyvinyl chlorides) Rubber accelerants Saccharin Saccharose Salicylic acid Selenium sulfonate Silicic acid Soda ash Sodium acetate Sodium aluminum silicate Sodium ampicillin Sodium antimonate Sodium benzoate Thorium carbonate Titanium oxide Uranium dioxide Urea Urea formaldehyde resin Xanthates Xanthene Zeolite
6.3 Applications in the Chemical Industry Tab. 6.3-1 Chemicals (in alphabetical order) that are cited in the sales literature of different vendors of equipment for size enlargement by agglomeration (continued) Sodium Sodium Sodium Sodium Sodium Sodium
gluconic acid hydrogen sulfate hypochlorite lauryl sulfate orthophosphate perborate
Streptomycine sulfate Styrene resins Sulfates Sulfonates Synthetic tannin Tetra potassium polyphosphate
Zinc Zinc Zinc Zinc Zinc
carbonate chromates oxide potassium chromate sulfate.
In addition, a few groups of chemical products were selected by the author, which are discussed in more detail in this section as examples of what can be accomplished by modifying chemicals with agglomeration techniques. They are: aspartame (artificial sweeteners) biocides (water treatment) catalysts (laundry) detergents DMT (dimethylterephthalate) pigments plastics (master batch) sodium cyanide
improved packing, dosing and dispersion eliminate dust, increase stability improved packing and increased activity improved density, packing and dissolution increased bulk density, improved handling improved handling, metering and dispersion avoiding segregation, better metering reduce toxicity, improve handling
Further Reading
For further reading the following books are recommended: B.3, B.7, B.13.e, B.16, B.21, B.22, B.26, B.29, B.33, B.40, B.48, B.49, B.50, B.56, B.58, B.60, B.64, B.67, B.70, B.82, B.89, B.90, B.92, B.93, B.94, B.97, B.98, B.102, B.103 (Chapter 13.1). Books mostly devoted to the subject matter are printed bold.
6.3.1
Tumble/Growth Technologies
As indicated above, rather than discussing a multitude of applications, a few well-established uses of size enlargement by tumble/growth agglomeration in the chemical industry will be reviewed and described in some detail as examples. Aspartame (Artificial Sweeteners) Artificial sweeteners are obtained by either hydrolysis of starch (e.g., maltodextrin) or chemically as crystalline compounds that are unrelated to carbohydrates (low calorie) and several hundred times sweeter than cane sugar (e.g., saccharin and aspartame). As produced, all products are very fine, do not wet easily, feature bad flow properties, and 6.3.1.1
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can not be easily handled. They are also not stable, adhere to walls, and lump during storage. Size enlargement by agglomeration is used to overcome these problems (see also Sections 6.3.2 and 6.3.3). As an example of growth agglomeration, the production of aspartame (a-L-aspartylL-phenylalanine methyl ester or a-APM) as a stabilized ingredient in low-calorie chewing gum is presented [6.3.1.1]. Many efforts have been made to stabilize APM, including encapsulating or otherwise coating the material. Most techniques use fluidized bed technology, for example, by co-drying a solution containing APM and an encapsulation agent, such as gum arabic, or by coating dry particles in a Wurster-type fluidized bed coater (Section 6.3.3). A recent patent [6.3.1.1] describes the mixing of non-coated APM with a dry binder, such as modified cellulose (e.g., hydroxypropyl methylcellulose), and a liquid, such as water. The mixer is a planetary or other such machine that applies compressive forces between the components. The resulting moist blend, characteristically dust-free, nonflowing, and crumbly, is dried and crushed to produce fine agglomerated APM particles (< about 0.4 mm). They can be added to a chewing gum formulation without a further coating. However, since these agglomerates release their sweetness instantly, it is still preferable to use them in combination with APM particles resulting from encapsulation or other treatments (Section 6.3.3) that produce a slower release during chewing. By blending APM materials with different release rates into the gum, it is possible to obtain stable products, featuring a strong initial taste and a milder longlasting sweetness. Agglomerated products, including aspartame and other artificial sweeteners, can also be made directly in high-shear mixers with knife heads or similar high-speed, high-energy tools. Such machines include drums with horizontal axes (e.g., pin or plow mixers), bowl types with vertical axes (Diosna, Fukae, or Henschel mixers) and vertical tube arrangements (e.g., Schugi) [B.97] (Section 15.1). The first two normally operate on batches and may apply one-pot processing, includings drying, but the latter works continuously, discharging moist agglomerates, with external drying, most commonly in a fluidized bed. Because of the use of high shear, which both builds and destroys agglomerates (Fig. 5-3 and 5-4), all produce the desired particle size directly and yield products with “instant properties”. As mentioned above, these agglomerated particles can be further engineered and processed, for example by adding suitable coating(s) (Section 6.3.3), to improve their storage, handling, and application characteristics.
Laundry Detergents Washing clothes is one of the oldest human activities. Originally, it was carried out by immersing the material to be washed in water and beating, treading, and/or rubbing it to dislodge the dirt. Later, in relatively recent times, it was found that water not only flushes out the soiling substances but also reduces by a factor of ten the natural van-der-Waals adhesion forces that participate in holding the dirt. In this respect it was discovered that soft water (rain water) has a better effect. Soda ash was known in ancient Egypt as a wash additive and charcoal was used in medieval times. 6.3.1.2
6.3 Applications in the Chemical Industry
Soap has been known for more than 3000 years: it is the oldest of the surfactants. For a long time it was applied as a cosmetic and a remedy. About 1000 years ago it came to be used as a general purpose washing and laundering agent [B.102]. “Modern” detergents were invented in Germany by Henkel. Synthetic soda ash and sodium silicate formed the basis for “Bleichsoda”, the first commercial detergent, introduced in 1878. With the beginning of the 20th century and the introduction in Germany of the first self-acting laundry detergent (Henkel’s “Persil”, 1907), soap, now synthetically produced by the saponification of fats with soda ash, became an ingredient in multi-component systems for the washing of textiles. Soap was combined with so called builders, usually sodium carbonate, sodium silicate, and sodium perborate, and bleach. The next important development was the transition from manual laundry to machine washing, which required appropriate changes in the formulation of detergents. Soap, which is sensitive to water hardness, was gradually replaced by other synthetic surfactants; the first neutral detergent for delicate fabrics was introduced in Germany in 1932 (Fewa) and in the USA in 1933 (by Dreft). The general acceptance worldwide of the new synthetic surfactants (mostly products from the petrochemical industry) is a development of the 1940s; Procter & Gamble introduced in 1946 the synthetic detergent Tide in the USA. Because of the increasing pollution of rivers and lakes, the biodegradability of detergent products, its testing, and corresponding environmental legislation became important tasks in the 1950s and 1960s. Subsequently fillers, such as sodium carbonate, were replaced by complexing agents (sodium di- and tri-phosphates) and more recently by zeolites, particularly in those countries where phosphate legislation was enacted. Tab. 6.3-2 summarizes the main components of laundry detergents and their functions [B.60, B.102]. To produce conventional laundry detergent powder, spray drying is the established method (the tower process). Fig. 6.3-1 shows the block diagram of such a system [B.60]. All raw materials are procured either in liquid or powder form. While ensuring a certain quality, the selection is mostly based on price. The physical characteristics of the components is not important since they will be dispersed or dissolved and mixed with other liquids to form a solution or slurry. This slurry is pumped by low- or medium-pressure pumps to the spray dryer (Fig. 6.3-2). An air vessel (pressure accumulator) is used to even-out pressure peaks. The spray towers can work in con- or counter-current fashion [B.97]. Concurrent contact yields light powders (mostly made-up of hollow particles) with a bulk density of about 100–150 g/L and a moisture content of 3–10 %. Also, the hollow beads tend to break-up and form dust. Countercurrent drying (Fig. 6.3-2) produces powder with a bulk density of 300–500 g/L and a moisture content of 7–15 % (commonly 10 %). Most plants use this method because the higher bulk density is almost always desired. The tower process is limited in its ability to produce powders with bulk densities greater than 500 g/L and the inclusion of non-ionics in the formulation. The latter, which can not be spray dried due to its volatility at the prevailing drying temperatures in the tower, must be post-added in either a rotary drum or other simple machines. The final detergent could then have a maximum content of 4 % and still feature a bulk
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6 Industrial Applications of Size Enlargement by Agglomeration Tab. 6.3-2 The main components of laundry detergents and their functions [B.60, B.102] Function
Components
Surfactants (anionics)
Soap Alkylbenczene-sulphonate Fatty alcohol sulfate Alpha-olefinsulphonate Alpha-sulpho-methylester Ethoxylated alkyl-phenols Ethoxylated fatty alcohols Sodium carbonate Sodium di- and tri-(poly)phosphate Poly-carboxylates (NTA) Citrates Zeolite A (ion exchangers) Sodium perborate Sodium percarbonate (Tetra-acetyl-ethylene-diamine) Sodium sulfate Water Corrosion inhibitors (sodium silicate) Foam boosters (alkylolamides) Foam inhibitors (hydr. soap/silic. oils) Stabilizers/sequestrants Optical brighteners/fluorescent whitening agents Soil antiredepositioning agents (CMC, Bentonite) Enzymes (alcalase, protease) Minors (dyestuffs, perfume)
Surfactants (nonionics) Builders
Bleaches (and activators)
Fillers and processing aids Specific additives and minors
density of < 500 g/L. Several producers have used the post-treatment system not only to add the non-ionics but to also supply and include heavy powders in an effort to produce a somewhat higher bulk density of the laundry detergent (Fig. 6.3-3) [6.3.1.2]. The ZigZag blender depicted in Fig. 6.3-3 (and in Fig. 6.3-4 and 6.3-9) is a unique combination of high-speed, high-shear mixing in the first part (drum with eccentrically arranged mixing tool) and gentle agglomeration and rounding in the second (zigzag) [B.97]. To further overcome the limitations of spray drying and extend the applicability of the post-treatment shown in Fig. 6.3-3, fluid-bed dryers were introduced for final drying (Fig. 6.3-4). Now the mixer, which can have designs other than shown, is operated as an agglomerator, producing moist granules, which are dried in the fluidized bed that follows. Such dryers can handle larger, denser particles and have high thermal efficiency, thus allowing the removal of large amounts of moisture. Therefore, the combination of a spray dryer with an agglomerator and a fluid-bed dryer results in a very flexible plant, capable of producing different powder qualities. Fines entrained during drying may be captured in a bag filter and recycled into the agglomerator.
6.3 Applications in the Chemical Industry
Fig. 6.3-1 Diagram of a spray-drying system for the production of conventional laundry detergent powder [B.60]
Non-tower methods are also used for the production of complex detergent powders. Such processes are carried out in horizontal, continuous drum, or other high-shear mixers [B.97] that are equipped with various blending tools, often assisted by knife heads. As shown in Fig. 6.3-5, in the reactor/mixer dry powder raw material, which may include spray dried components, zeolites, and soda ash, is used to bind surplus moisture that is brought in with the other ingredients. Owing to the high-energy input by the blending tools, agglomerates are formed. At the same time neutralization with sodium carbonate or sodium hydroxide takes place. Since the granules may be sticky, mainly due to the non-ionics in the composition, they may be powdered (coated) with zeolite in a second mixer. If the water binding capacity of the dry components is still not sufficient, a further step, such as a fluidized bed dryer, may have to be added (Fig. 6.3-6). In the 1980s, beginning in Japan, a strong demand developed for solid laundry detergents with a density of more than 700 g/L and a content of active matter (anionic and non-ionic) of up to 50 %. Wet agglomeration (Fig. 6.3-7), performed by adding water, polymer solutions, anionic surfactant pastes, or surfactant gels at highshear rates, is a common process for densifying. It consists of the following steps [B.102].
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Fig. 6.3-2 Flow diagram of a traditional spray drying process for the manufacture of laundry detergent powder [B.102]
Fig. 6.3-3 Flow diagram of a tower process with post-addition of powders and non-ionics and agglomeration [6.3.1.2]
6.3 Applications in the Chemical Industry
Fig. 6.3-4 Flow diagram of a tower process with post-addition, wet agglomeration, fluid-bed drying, and dust recycling [6.3.1.2]
1. Grinding in a high-speed mixer, for example a L€ odige CB type (Fig. 6.3-8); addition of polymer solution (7–12 %), binder (water), and other ingredients via spray nozzles; pre-granulation. 2. Granulation (potentially adding more binder liquid) and conditioning in a high- or medium-shear mixer (optional but preferred). 3. Evaporation of excess water in a fluidized bed and/or cooling down. 4. Removal of coarse particles from the product by sieving (normally, fine particles remain in the product), milling the oversize, and recirculating the powder to the agglomerator. 5. Addition of minor ingredients in a low-energy mixer. Using several mixers of different type in a cascade increases the process flexibility. Energy input, granulation temperature and time, location and method of dosage of
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.3-5 Block diagram of a non-tower agglomeration process for the production of laundry detergents [B.102]
liquids, powdering of the particles to reduce stickiness, and selection of machine types represent the know-how of the detergent’s manufacturers. A similar system, that is an extension of the tower process, is shown in Fig. 6.3-9 [6.3.1.2]. In this case, if an increase in the amount of non-ionics in the product is desired, a portion is introduced in the first blender and the remainder, after drying of the agglomerates, is added in the second mixer, together with the minors. Fig. 6.3-6 Flow diagram of a non-tower process for the manufacture of laundry detergents with two mixers and a fluidized bed dryer [B.102]
6.3 Applications in the Chemical Industry
Fig. 6.3-7 Flow diagram of a typical wet granulation process for the manufacture of heavy laundry detergent [B.102]
While the spray dryer, sometimes including size enlargement by agglomeration in the tower or in a secondary fluid-bed agglomerator/dryer/cooler [B.97], is the most commonly used method for the production of basic detergent powders, the manufacture of high-density laundry detergents or of defined product forms for special applications also uses pressure agglomeration techniques (Section 6.3.2). Pigments Pigments are particulate colorants that are not soluble in a solvent or binder liquid. Chemically, they may be inorganic or organic. Since their coloring effect is caused by the reflection and the partial absorption of light, particle size and dispersion are important characteristics. Not only does a smaller particle size produce more brilliance, in some cases, changing the particle size modifies the reflection and absorption properties of the coating, resulting in a different perception of color by the human eye. The oldest known pigments are natural inorganic compounds such as chalk, carbon black, graphite, ocher, sienna, or umber. Originally, the earthy minerals were finely ground between stones or by mortar and pestle, dispersed in water and applied as paint. Carbon black, obtained as soot, a residue from the burning of carbonaceous 6.3.1.3
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Fig. 6.3-8 Type CB high-speed, high-shear mixer that is often used for the densification and pre-granulation of laundry detergent components. The inset shows details of the mixing tools (courtesy L€ odige, Paderborn, Germany)
materials, was often mixed with oil and applied as a cosmetic. Today special mills (e.g., impact (hammer or pin) mills) are used and carbon black of sub-micron particle size, is produced in large quantities by special methods (e.g., the complete combustion of oil, the thermal (often catalytic/Ni) dissociation of CO, the burning of natural gas, Chapter 11). Natural organic pigments include indigo and sepia. Artificial inorganic pigments, produced by chemical or physical modification of inorganic materials, are, for example, barium sulfate, chromium yellow, cobalt blue, iron oxides, red lead, or titanium dioxide. Particularly with the beginning of cyclic organic chemistry, many artificial organic pigments were synthesized, many reproducing and/or modifying naturally occurring compounds, such as indigo, and, more recently, particulate inorganic and organic so called high-performance pigments (HPP) are being developed, which are defined as colored, black, white, pearlescent, luminescent, or fluorescent products for a specific use, with well-defined quality, and an optimized cost [B.103]. All modern pigments must feature excellence of performance, application permanence, and compatibility with health, safety, and environmental issues. The latter are imposed and controlled by local and/or national legislation.
6.3 Applications in the Chemical Industry
Fig. 6.3-9 Tower process followed by wet agglomeration, fluid-bed drying, and post-treatment for the manufacture of dense laundry detergents [6.3.1.2]
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6 Industrial Applications of Size Enlargement by Agglomeration Tab. 6.3-3
Examples of some inorganic and organic pigments
Inorganic
Examples
Organic
Examples
Elements
Carbon black Al-flakes, Zn-dust Fe2O3, Fe3O4 TiO2 Cr3O3, Pb3O4 ZnFe2O4 (Co,Ni,Zn)2TiO4 Ti(Cr,Nb)O2 ZnS, CdS CeS2 Pb(Cr,S)O4 Pb(Cr,Mo,S)O4 BiVO3 Na3Al6Si6O24S3 KFe[Fe(CN)6]
Azo-compounds Anthraquinones Benzene derivatives Carbazoles
Bisazomethines
Oxides
Mixed metal oxides
Sulfides Chromates Vanadates Silicates Cyanides
Benzimidazolones Carbazol violet
Diketopyrrolopyrroles (DPP) HP Naphtols Perylenes Phtalocyanides Polycyclics
Anilines Isoindolinones Quinophthalone
The characteristic performance (perception of a particular color) of any pigment is determined by its chemical composition and particle size, the latter being typically in a range from several hundred nano- to a few tens of micrometers, and its ability to uniformly disperse, resulting in an even coloration of the material to which is has been applied. Pigments are available as suspensions or pastes in various solvents or as dry powders. Although the liquid forms have many advantages, packing, storage, and transportation of these products are expensive, as a large percentage is represented by the solvent. Therefore, especially those pigments that are produced and applied in bulk quantities, such as carbon black for use in the manufacture of tires and black, yellow and red iron oxides or manganese oxides for dyeing concrete, are dry powders. Because of their small particle size they tend be dusty, causing dirty workplaces and endangering the health and cleanliness of workers, adhere to walls and tools, bridge in silos, settle during prolonged storage, and, generally, exhibit unfavorable flow and bad metering characteristics. Automatic handling and application systems do not work reliably and the use of such powders becomes very expensive. To overcome these difficulties, pigments are now often micro-agglomerated. Such products are dust free, easily flowing, withstand handling and shipping without degradation, can be metered easily, and will adequately re-disperse back into their fine particulate form during process application. These characteristics are those of “instant” agglomerates (Tab. 6.4-3, Section 6.4.1). Suitable granules are commonly made by tumble/growth agglomeration. Methods include spray drying and the re-wetting of powders in mechanically and gas induced fluid beds (Tab. 6.4-4, A1–A3). Mechanical high-shear mixers, employing pins, plows, and other high-speed tools, often equipped with additional knife heads, are used for the manufacture of smaller batches of specifically color formulated products while bulk masses are processed in drums, on pans, or with gas fluidized beds.
6.3 Applications in the Chemical Industry
As an example, the widely used method for the production of agglomerated pigments for the coloring of concrete will be described [6.3.1.3]. The granules from this process consist of one or more pigments, one or more binders to aid dispersal of the pigments in the concrete during mixing, and optional other additives. As compared with concrete surfaces that have been decorated by painting and need periodic repair, exposed surfaces made from dyed concrete mix will keep their color for many years without further maintenance. Therefore, typical products are colored concrete blocks and slabs, concrete roofing tiles, landscape bricks and stones, composite blocks and mortars and grouting materials. Agglomerates are made from dry powder mixes by rewet agglomeration in drums and pans or from slurries in spray dryers, often followed by secondary agglomeration, drying, and cooling in suitable equipment, such as fluidized beds [B.97]. To obtain quick and uniform dispersion in the concrete mixer, the pigment is blended prior to agglomeration with a material that acts as a binder, which is specifically selected to also promote dispersion during application. A large number of materials fulfills these requirements [6.3.1.3] but a common, cheap, and effective binder/dispersion agent is lignin sulfonate, available as a by-product from spent sulfite liquor in the paper industry. The granules typically contain up to 15 % binder, feature a particle size distribution with at least 90 % > 20 lm, and have a moisture content of not more than 4.2 %, preferably less [6.3.1.3]. To determine whether the agglomerated pigment disperses well in the concrete mixer and does not produce color spots in or uneven dyeing of the final product, a comparison is made with samples obtained using powdered pigment (control) of the same composition. The best agglomerated products will mix readily with the concrete, requiring the same time, moisture content, and mineral composition, and result in the same or better uniformity of color. The dispersion agent that acts as binder for achieving the required handling, storage, and shipping properties of the micro-granules, plays an important role during final application. As mentioned above, the need for ease of re-dispersion during final application requires that many agglomerated pigments are “instant” products. As briefly discussed in Section 6.4.1, under certain conditions, press agglomeration may yield intermediates with such characteristics (Tab. 6.4-4, A4). On the other hand, pigment granules are also widely used to color plastic master batches. Since very high-shear forces are present during the manufacture of these premixed products, somewhat lower requirements for easy re-dispersion exist in this case (Section 6.3.2.6).
6.3.2
Pressure Agglomeration Technologies
The technologies of pressure agglomeration (Chapter 5) include the most versatile methods of size enlargement because, when certain preconditions are observed, very few restrictions exist that would prohibit the application of one or the other method of this group of techniques. In particular, if the right technique is used, there is
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virtually no limitation to feed size both small and large, unlike the requirement for tumble/growth agglomeration that the feed particle size must be small and generally below a certain dimension (Tab. 5-1, Chapter 5). In high-pressure agglomeration, the feed size must be smaller than the feeder dimensions because the high forces will both crush or deform and then agglomerate the material. For these reasons, pressure agglomeration is also widely used for the size enlargement of chemicals. However, only a few well-established uses of size enlargement by pressure agglomeration in the chemical industry will be reviewed and described in some detail as examples.
Aspartame (Artificial Sweeteners) There are two major applications for pressure agglomeration for artificial sweeteners, such as aspartame and saccharin: granulation and tabletting. Since both product forms must easily disperse and dissolve in cold or hot liquids, they should and in most cases do exhibit “instant” properties. Dry granulation of, for example, aspartame, is carried out by compaction/granulation (Fig. 6.1-13, Section 6.1.2) mainly to reduce dusting and improve flowability and metering. Aspartame, as produced, like many other chemicals, features long, needlelike crystals that tend to clump together in storage and during handling. Compaction in a dry granulation system is normally accomplished with relatively small roller presses that are specially designed and executed for applications in the food and pharmaceutical industries (Sections 6.2 and 6.4). Because production capacities are relatively low, the entire system is integrated in a single machine (Fig. 6.3-10). Crystallized aspartame, possibly pre-mixed with additives such as fillers and disintegrants, is deposited in a feed hopper (1), kept fluid but settling (partial deaeration) with a slowly rotating discharge aid (2), and enters a horizontal screw (4). The feed assembly is mounted on a support (3) that can be moved horizontally and swiveled for easy access and cleaning. The screw, passing through a deaeration box (5), forces the powder feed into the nip between two vertically arranged rollers (6) that produce a more or less continuous compacted strip that is guided to a sheet breaker (7) for disintegration into small pieces. Final size adjustment is accomplished by a granulator (mill) (8) featuring a discharge screen. Because all parts that are in contact with the material to be processed are cantilevered, enclosed, and separated from the drives and controls, which are located in the machine housing, automatic CIP (cleaning in place) features can be added and operated between batches. A major disadvantage of the system shown in Fig. 6.3-10 is that, while totally enclosed and sanitary, a large amount of fines is produced and, at least initially, is part of the discharge. Although the powder feed has been granulated, the fine portion, resulting from leakage at the rollers and the crushing and milling processes, gives rise to dusting, reduced flowability, and potentially lumping. Improved compaction/granulation processes employ multiple crushing, milling, and classification steps (Section 6.1.2) shown in Fig. 6.3-11. This flow diagram, which is the reproduction of a patent drawing [6.3.2.1], first removes and recycles leakage and then applies three breaking/milling 6.3.2.1
6.3 Applications in the Chemical Industry Fig. 6.3-10 Small roller press compaction/ granulation system for the processing of materials such as artificial sweeteners. A product sample is shown in the inset (courtesy Turbo Kogyo, Kanagawa, Japan)
and two classification steps, producing final granules as sieve overs and recycling the fines from crushing as sieve unders. Hydrolyzed starch products, such as maltodextrins, are produced by the partial hydrolysis of cereal (e.g., corn) or root (such as potato) base starches and are commercially available in spray dried, particulate form. As manufactured it has a relatively low sweetness level and, if used alone as a sweetener, the food product can not be characterized as 100 % artificially sweetened, a characterization that is often desired from a marketing standpoint. However, maltodextrins can be used as a bulking agent or carrier for synthetic sweeteners, such as aspartame, and then, the resulting product can be characterized as 100 % artificially sweetened. However, the commercially available spray dried maltodextrin exhibits the same drawbacks as other similar powders: due to its small particle size it tends to dusting, features low bulk density, has unfavorable flow properties, and does not readily dissolve in liquids. Most of these disadvantages could be overcome by rewet agglomeration (Section 6.3.1). During such a process, maltodextrin particles stick together
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.3-11 Flow diagram of a compaction/granulation system with multiple crushing/milling and classification steps and fines recirculation for the reduction of undersized material in the product [6.3.2.1]
and form larger clusters, which produce less dust during handling and feature improved flow and dissolution properties. But because these agglomerates contain many large voids, their bulk density is even lower than the original material; therefore, this product is not well suited for use in automatic packaging machines. The compaction/granulation process (above and Section 6.1.2), preferably applying roller presses, produces agglomerates by breaking and screening densified strips. However, if this is used for maltodextrin, the material quickly begins to stick to the rollers, which, in spite of cooling [B.48, B.97], become warm, forcing frequent shut-downs and resulting in an uneconomic operation. Adding a lubricant to the feed allows operation for a longer time but the highly densified granules exhibit very poor dissolution properties and the lubricant leaves an objectionable scum in or film on the liquid. All drawbacks can be overcome by mixing the maltodextrin with a volatile solvent (e.g., ethyl alcohol) and forming a moist blend prior to feeding the roller press (Fig. 6.3-12) [6.3.2.2]. While, normally, the feed to high-pressure agglomeration equipment should be essentially dry (Chapter 5 and [B.13b, B.48, B.97]), according to the invention, the moist blend contains sufficient moisture that liquid filled pores remain in the compacted strip and, during passage through the nip, a small amount of the liquid is squeezed to the exterior of the sheet to lubricate and cool the rollers thus preventing the maltodextrin from sticking. The sheet is then broken in a closed loop with con-
6.3 Applications in the Chemical Industry
Fig. 6.3-12 Patent drawing of a compaction/granulation system using a roller press for the densification of an artificial sweetener, mixed with a volatile liquid [6.3.2.2]
ventional equipment, whereby a substantial portion of the trapped liquid is liberated, and then classified. During the final drying step, most of the remaining liquid that occupies the spaces between the compacted particles is driven off. The end result is a mass of densified particles of maltodextrin with acceptable bulk density. Although having a crystalline appearance, the surface topography features cracks, crevices, and fissures, which causes a relatively rapid solution rate in liquids, but the flow and dusting problems of the material are eliminated. Compaction/granulation is frequently used prior to tabletting to improve the flow properties and increase the bulk density, also, if applicable, to reduce dusting and avoid the segregation of powder mixtures. As mentioned several times elsewhere (for example in Section 6.2.2), this pre-agglomeration step is applied to fill the cavities of tabletting machines faster and more reliably. Artificial sweeteners are often converted into small tablets (Fig. 6.3-13) that represent a comparative dosage size. For example, a label on the dispenser shown in Fig. 6.3-13 specifies that “one tablet (1.4 kJ) is equivalent in sweetness to one level teaspoon of sugar (70 kJ)”. Although, in the example, the sweetness of aspartame is reduced by the addition of lactose, L-leucine, and croscarmellose sodium (20 % aspartame and 80 % additives), the tablets, producing the equivalent sweetness, must be
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Fig. 6.3-13 Dosage forms of pressure agglomerated artificial sweetener (aspartame): left) tablets and tablet dispenser, right) granules and portion packages. (Equal is a registered trademark of Merisant Co.)
very small. For manufacture, rotary punch-and-die machines are used featuring multiple dies per station. Fig. 6.3-14 is the photograph of a tool set featuring nine dies. It requires very good flowability to quickly (which is typical of rotary presses) and reproducibly fill such small dies. Since the feed also consists of a mixture of ingredients (above), segregation must be avoided. These requirements are fulfilled when supplying a pre-agglomerated, stabilized, granular (small sized) intermediate product to the tabletting press. Such material is best made by dry compaction/granulation. Fig. 6.3-15 depicts a tabletting line that might be used for the manufacture of artificial sweetener tablets. From right to left it shows a control panel with data recorder, the rotary tabletting machine, in-line de-dusting and transportation, and bulk packaging. The final consumer items (e.g., tablet dispensers) are produced and filled by the distributor. 6.3.2.2 Biocides
Biocides are chemical substances that are destructive to many different organisms. They are used as pesticides in agriculture (Section 6.6) and as water treatment chemicals, particularly also for swimming pools.
6.3 Applications in the Chemical Industry Fig. 6.3-14 Left) upper and right) lower punches and the die insert for the multi-die station of a rotary tabletting press (courtesy Kilian, K€ oln, Germany)
Halogenated (mostly Cl and Br) biocides (such as hypochloride or DBDMH, see Tab. 6.3-1) are the most commonly used chemicals for sanitizing water. They are strong oxidizers and, as dust and products of decomposition (below), irritate respiratory tracts. Therefore, airborne, inhalable fine particles and all dust must be avoided. This is also true for non-chlorine (or bromine) oxidizers that are used for the same
Fig. 6.3-15 Tabletting line for, for example, the manufacture of artificial sweetener tablets. From right to left: Control panel with data recorder, rotary
tabletting machine, in-line de-dusting and transportation, bulk packaging (courtesy Kilian, K€ oln, Germany)
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purpose. An example of such compounds is potassium peroxymonosulfate (often simply called potassium monopersulfate). Stability of oxidizers is also a concern. Halogenated products generate chlorine and bromine. Both are irritating, poisonous gases. Other materials decompose with the production of less objectionable elements or compounds. For example, the relatively stable potassium monopersulfate (Oxone), emits oxygen when it decomposes but at elevated temperatures, may also generate sulfuric acid, sulfur dioxide, or sulfur trioxide [6.3.2.3]. Small amounts of moisture (and, depending on the product, many other chemicals) reduce the stability of all oxidizers. Since these products are often manually handled during, for instance, swimming pool cleaning and maintenance, they must be converted into shapes with properties that avoid nuisance dust and enhance stability. The pure chemicals, which, as produced, may be very fine (e.g., sometimes precipitated), are often mixed with other components for specific performance. For example, Oxone can be blended with many additives, including sodium-sulfate, -carbonate (especially the dense version), -bicarbonate, -perborate, -tripolyphosphate, -metasilicate, tetrasodium pyrophosphate, citric-, malic-, and tartaric-acids and surfactants and fragrances. To maintain stability, all ingredients must be anhydrous or hold hydrated water tightly. Formulated Oxone is used as a shocking agent (auxiliary oxidant) in swimming pools and spas for the purpose of reducing the organic content of the water. It improves the efficiency of the regular sanitizing chemicals such as chlorine and bromine. It can also be used as one part of a two-part disinfecting system for spas and hot tubs with sodium bromide. In such a system, Oxone oxidizes or activates the bromide ion to bromine, which rapidly forms the active sanitizer hydrobromous acid. Upon reaction with bacteria or other water contaminants, hydrobromous acid is reduced back to bromide ion, which can be activated over and over again, thus recycling the active bromine reactant. To reduce dust and increase stability, halogenated biocides or other products, such as Oxone, are often pressed into tablets (Fig. 6.3-16) of different sizes and shapes
Fig. 6.3-16 Tabletting of swimming pool oxidizer compound with a rotary punch-and-die press (courtesy Stellar, Sauget, IL, USA)
6.3 Applications in the Chemical Industry
(Fig. 6.3-17). These tablets are used in the skimmer or in, often floating, “chlorinators” where they slowly dissolve. Tablets must be formulated and pressed to retain their shape in water without falling apart. They should be so dense that they dissolve on the surface only. For shocking, a granular material that dissolves quickly is produced by compaction/granulation (Section 6.1.2). To produce tablets of sufficient strength and density, pre-granulated material may be used to feed the punch-and-die presses. This is a common practice in all applications where the quick and reliable filling of a die is required and segregation of multiple components that are incorporated in the granulated feed must be avoided (Section 6.2.2). Because of their high activity and relatively low stability, all oxidizers have a limited shelf-life. They must be stored cool and dry. Packaging must be water tight in drums with sealed lids (granular or tabletted, Fig. 6.318a), bags with water-resistant liners (granular, Fig. 6.3-18b), or individually wrapped and welded into plastic (tabletted, Fig. 6.3-18c). 6.3.2.3 Catalysts
A catalyst is a substance that enables a chemical reaction to proceed under more favorable conditions, for example with higher speed or at lower temperature, than otherwise possible. Normally, except for potentially picking-up contaminants from the usually liquid or gaseous components that participate in the reaction(s), the material inducing the catalytic reaction (catalyst) remains chemically unchanged at the end of the process. It may, however, participate in intermediate steps, forming a temporary compound which, in turn, reacts to yield the desired product(s) and regenerates the catalyst. The rate of reaction induced by a solid catalyst is generally proportional to its surface area and the concentration of the so-called active centers or catalyst sites. The latter are locations of high chemical activity on the surface. Since the specific surface of particulate solids increases with decreasing particle size, the effectiveness of solid catalysts
Fig. 6.3-17 Some different shapes of tablets for swimming pool sanitizing (courtesy Stellar, Sauget, IL, USA)
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Fig. 6.3-18 Packaging of agglomerated swimming pool sanitizing products: a) water-tight in drums with sealed lids (granular or tabletted), b) bags with water-resistant liners (granular), c) individually wrapped and welded into plastic (tabletted) (courtesy Stellar, Sauget, IL, USA)
also increases with decreasing particle size. However such powders exhibit typical drawbacks, which include dustiness, low bulk density, and, generally, unfavorable handling, storage, flow, feeding, and metering characteristics. In addition, because the reactants have to be in close contact with the solid catalyst, the vessel in which the processes take place often contains a (stationary) catalyst bed through which liquids or gases or both flow and produce the new compound(s). With time, fine powders pack densely in certain areas, caused by the capture of small suspended particles in pores, thus partially obstructing the flow (Section 8.2), resulting in a non-uniform flow pattern, and leading to an ineffective usage of the catalyst. Therefore, size enlargement by agglomeration is used to convert catalyst powder into shaped, porous bodies with sufficient strength to withstand the mechanical stresses during handling and loading and to survive the prevailing process conditions (including elevated temperatures, if applicable). A packed bed reactor in which solid-catalyzed fluid-phase reactions take place must be filled easily with an optimum amount of catalyst (just enough) to produce a permanent, non-deteriorating bed with high permeability. This has led to the production of porous but strong solid carriers, which are later impregnated with the catalyst. For impregnation the active material is dissolved or dispersed in a liquid and during a post-
6.3 Applications in the Chemical Industry
treatment (for example drying) it is deposited as a thin coating throughout the pore space of the carrier. The structural integrity of the bed is provided by the carrier particles while the catalyst is made available to the reactants on the very large internal surface area of the carrier. Catalysts used in industrial processes, whether made from the active substance directly or deposited on carriers, are often porous cylindrical pellets produced by medium pressure extrusion (Fig. 5-10b1–b6, Chapter 5). While almost all methods of size enlargement by agglomeration can be used to produce porous bodies from solid powders and post-treatment processes can yield strong pieces with a high accessible internal porosity [B.23, B.78, B.97] that fulfill the previously discussed requirements, pelleting offers a number of advantages. In extrusion, final densification and shaping is accomplished in the die holes by forces that are exerted by differently designed and shaped extrusion tools (rollers, screws, wipers) and the frictional resistance opposing the flow of extruding material [B.97]. Depending on the material’s consistency and the geometry of the die openings, considerable pressures develop in front of and within the die holes whereby wall friction plays a decisive role. The materials to be agglomerated by extrusion need to be formable and exhibit some lubricity. These characteristics are most often achieved by the addition of a sufficient amount of suitable liquid, particularly if the solid itself is hard and brittle. Liquid in a particulate mass produces plasticity, creates a lubrication film, and fills pores during densification. The latter are retained in the product after drying and potentially further treatment, for example by calcination, are accessible from the outside, and provide an internal network of surfaces consisting of catalyst or, if it is a carrier, are available for coating with active material. Extruders that are most often used for the production of catalysts or catalyst carriers are screw extruders and flat die pellet presses (Fig. 5-10b1 and b2, Chapter 5). Both machines can exert high pressures and are sufficiently rugged for the application. Because of design limitations, flat die pellet presses (Fig. 6.3-19) can only produce cylindrical shapes with diameter to lengths ratios of between about 1:1 and 1:3. After drying and calcination, these carrier materials are impregnated with the catalyst and serve as products that can be easily poured into containers to form a stable bed with satisfactory permeability. If the “natural” porosity, which is partially created by pores that are filled with liquid, opened during drying, and stabilized in the calcination process, is not high enough, additional porosity may be produced by adding temporary particulate solids to the feed mix. Such pore building materials are removed from the pellet structure during drying and/or calcination, leaving behind large open voids [B.78]. Nevertheless, in some packed bed applications, the permeability created by the porous, cylindrical extrudates is not high enough. For these, in addition to rods with larger aspect ratio and hollow rods, rings or supported rings (Fig. 6.3-20) and many other, often proprietary shapes (not shown) are made as carriers with specially designed screw extruders (Fig. 6.3-21). Since many of the materials, such as high performance aluminas, kaolins, or molecular sieves, require considerable pressure for extrusion and are quite abrasive, extra heavy duty components, made of special abrasion and corrosion resistant steels, are used in equipment design.
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Fig. 6.3-19 Installation of a flat die pellet press for the manufacture of cylindrical catalyst or carrier pellets (courtesy Amandus Kahl, Reinbek, Germany). For a schematic, partial cut through a flat die pellet press see Fig. 6.5-4a (Section 6.5.2).
Fig. 6.3-20 Various shapes of extruded catalyst carriers (courtesy Bonnot, Uniontown, OH, USA)
6.3 Applications in the Chemical Industry
Fig. 6.3-21 Special extruder for the manufacture of catalysts or catalyst carriers. A hinged die holder allows the quick exchange of extrusion plates (courtesy Bonnot, Uniontown, OH, USA)
6.3.2.4 Laundry Detergents
As already mentioned in Section 6.3.1, in the 1980s, beginning in Japan, a strong demand developed for solid laundry detergents with a density of more than 700 g/L and a contents of active matter (anionic and non-ionic) of up to 50 %. It was also indicated that, as a general rule, agglomeration was added to the manufacture of laundry detergents to improve the dispersibility of the chemicals in water. Such materials are often described as having “instant” properties. The use of medium-pressure agglomeration (extrusion) is relatively new. The complete system (Fig. 6.3-22 [B.102]) uses mixing of solid components with a lubricant, plasticizing and forming the moist mass with a double screw extruder into small diameter, spaghetti-like strands, spheronizing [B.48, B.97] the product into uniform spherical particles, drying/cooling in a fluidized bed, screening (overs, after crushing (not shown) and unders are recirculated), and post-addition of minors in a final mixer. Extruded particles exhibit some of the highest densities (about 1400 g/L) achieved in detergent manufacture. Whereas other processes deliver small, irregular granulates, the extruded and spheronized particles, called Megaperls, are spherical, uniform, and quite large (about 1.5 mm diameter) (Fig. 6.3-23). Apart from being an extraordinary basis for the manufacture of super-compact laundry detergents, they feature additional advantages, including total absence of dust, very high homogeneity, no segregation of particles due to their uniformity, and excellent flow characteristics. Extruded detergents allow anionic surfactant contents of more than 20 % along with the very high density [B.102].
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Fig. 6.3-22 Flow diagram of an extrusion process for the manufacture of Megaperls [B.102]
Fig. 6.3-23 Appearance of commercial heavy laundry detergent products: a) granular, b) extruded and sheronized (Megaperls) [B.102] (courtesy Henkel, D€ usseldorf, Germany)
6.3 Applications in the Chemical Industry
Granulation by high-pressure agglomeration, although a logical choice for the densification of particulate solids, was first ruled out because it was assumed that the dense particles would feature low dispersibility. In fact, in the industry, salt (NaCl) is commonly briquetted as an agent for the regeneration of ion-exchange water softeners whereby the resulting product features mechanical stability in water and a slow dissolution rate (Section 6.8.2), clearly a performance not acceptable for laundry detergents. However, as discussed in Section 6.4.1 (Tab. 6.4-4 A4), if the binding mechanisms that develop during high-pressure agglomeration are mainly caused by molecular adhesion (van-der-Waals forces), granules from compaction/granulation (Section 6.1.2) are easily dispersible because these binding forces are reduced to about 1/10th of their strength in liquids. To achieve this condition, only solid compounds should be used for formulating the laundry detergent. The density of the resulting, irregularly shaped granules is high (> 700 g/L). A concern, that the edges of agglomerates, that were produced by crushing a consolidated sheet, break off easily during handling, packing, storage, and application, causing dust, can be overcome by tumbling the granules in a polishing drum with or without the addition of small amounts of additives (e.g., perfumes). During this procedure sharp corners are removed and/or rounded. A final de-dusting step yields a mechanically stable, dust-free product (Section 6.1.2). Tablets, produced from powder mixtures with punch-and-die presses, are always an easy and precise dosage form. They are also convenient for laundry detergents since no dosing and dispensing aids are required. Other advantages include smaller packages as compared with powder or granular products due to their highly concentrated form, extra portability, and a more accurate sense of how many wash portions remain in the detergent box. They were first introduced in Europe in 1997 and in 2000 already held a share of 10 % in the heavy-duty detergent category. In the year 2000, marketing also began in the Japanese and North-American markets. Tablets may consist of one or two (differently colored) layers (Fig. 6.3-24), each providing a specific performance. They are the most compact delivery form of non-liquid
Fig. 6.3-24 Two-layer laundry detergent tablets (courtesy Henkel, D€ usseldorf, Germany)
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detergents. To achieve the best efficiency, all ingredients must be in a dissolved state from the very beginning of the wash cycle. Another specific demand made on heavyduty detergent tablets is sufficient hardness to enable handling, packaging, transportation, and use. Several problems must be resolved [B.102]. *
*
*
The low surface-to-volume ration of detergent tablets adversely affects the dissolution rate. In the presence of surfactants, especially non-ionics, gel phases may form on first contact with water. Gel formation impedes fast dissolution. Hardness or strength of tablets is synonymous with high densification and low porosity, which lowers dispersibility and dissolution.
To overcome these problems, laundry detergent tablets contain special disintegrants [B.97]. These can be classified into four groups or combinations thereof [B.102]. a Effervescents such as carbonate/hydrogencarbonate/citric acid b Swelling agents such as cellulose, carboxymethyl cellulose (CMC), or cross-linked poly(N-vinylpyrrolidone) c Quickly dissolving materials, such as halogen (Na, K) acetate or citrate d Rapidly dissolving water-soluble coatings such as dicarboxylic acids. The composition of heavy-duty detergent tablets differs from other forms of supercompact detergents mostly in their contents of disintegrants. Two-phase (two-layer) tablets allow the separation of certain detergent ingredients that might otherwise adversely affect each other during storage, for example enzymes and activated bleach. All manufacturers use rotary punch-and-die machines for tablet production, whereby twolayer presses feature two feeding stations on the turret [B.48, B.97, B.99]. Fig. 6.3-25 is the evoluted (straightened) diagram of such equipment. From left to right: the die is filled with the first component while the lower punch moves down. After scraping-off the excess amount of feed, compression begins by moving the upper punch down. Pre-compaction is achieved between the small press wheels and, after a dwell time, final compression takes place between the large wheels. The upper punch is
Fig. 6.3-25 Evoluted (straightened) diagram of a rotary double-layer punch-and-die press (courtesy Fette, Schwarzenbek, Germany)
6.3 Applications in the Chemical Industry
then retracted to make room for filling and volume adjustment of the second component. Compaction follows. At the end of the cycle (one revolution of the turret) the upper and the lower punches move up to expel the finished tablet. In the design depicted in Fig. 6.3-25, most of the travel is done by the upper punch. The lower one only moves during the filling and adjustment steps and for tablet extraction. In other machines both the upper and the lower punches participate during compression, performing double sided compaction [B.48, B.97]. Other laundry washing additives are also produced and applied as tablets. Fig. 6.3-26 depicts a water-softening product for European automatic washing machines. The
Fig. 6.3-26 Two-phase (two-layer) water softening product for European automatic washing machines (Calgon is a registered trade mark of Benckiser, Ludwigshafen/Rh., Germany)
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machines include a heating coil that gets coated with calcareous deposits if hard water is used. The tablets have two phases but contain no active detergent components, brighteners, or perfumes, are added with the wash load, and remove the hardness when washing begins. Therefore, they must also feature quick disintegration, which is again achieved by the inclusion of disintegrants (above). By using this additive regularly, the machine drum and the heating coil are kept clean and continue to perform efficiently for a long time. As shown in Fig. 6.3-26, to increase shelf-life, laundry detergent and additive tablets are often individually wrapped. 6.3.2.5 DMT (dimethylterephthalate)
Chemicals that are produced as a melt are cooled and solidified with drum coolers. Such machines feature a polished, internally cooled, slowly rotating metal drum that dips into a bath of molten material (Fig. 6.3-27). A thin layer of material is picked-up by
Fig. 6.3-27
Rotary drum cooler (dip feed flaker) (courtesy GMF- Gouda, Waddinxveen, The Netherlands)
6.3 Applications in the Chemical Industry
and sticks to the surface of the drum as it emerges from the liquid. After the coating is cooled and solidified it is scraped from the drum and forms thin flakes. DMT, a major starting material for polyester fibers and coatings, is processed by this method. It is synthesized in chemical plants as an intermediary product and must be shipped to the manufacturers of fibers, which are normally not part of the same chemical complex. In fact, DMT is made in a few locations only in large quantities and shipped for remelting and final processing to many users throughout the world. The flakes have bad flow properties, feature extremely low bulk density, and, if packed, for example into bulk bags, initially require a large volume. On the other hand, they tend to settle in the flexible containers, forming lumps that are difficult to discharge and are not suitable for easy feeding to remelt equipment at the fiber manufacturing plant. These problems are resolved by briquetting. The DMT flakes are readily densified and bonded under moderate pressure in a roller press that is equipped with a screw feeder and pillow shaped pockets. The briquettes have high density and are well formed. During a screening operation they are separated into singles and leakage and land areas that surround the briquettes and are rubbed-off on the screen, are recirculated to the press. The briquettes have a volume of about 4 cm3, are about 20 mm square and 14 mm thick, pack, store, and handle well, and can be easily metered and remelted. Other flaky chemicals obtained from drum coolers or dryers that have similar properties as those of the fresh DMT can be similarly converted into dense free flowing intermediate products by pelleting, briquetting, or compaction/granulation. Drum dryers feature heated drums that either dip into a bath of suspended particles or are coated by means of axial dispensers with the suspension. As in the case of drum coolers, dry solids are removed from the drum with scrapers and are typically bulky and dusty before suitable agglomeration has taken place. 6.3.2.6 Pigments
Agglomerated pigments must feature instant properties. As in many other industrial applications requiring similar characteristics, such as laundry detergents (above), and food (Section 6.4.2) or animal feed products (Section 6.5.2), granulation by high-pressure agglomeration was first ruled out because it was assumed that the dense particles would feature low dispersibility. However, as discussed in Section 6.4.1 (Tab. 6.4-4 A4), if the binding mechanisms that develop during high-pressure agglomeration are mainly caused by molecular adhesion (van-der-Waals forces), granules from compaction/granulation (Section 6.1.2) are easily dispersible because these binding forces are reduced to about 1/10th of their strength in liquids. The widely used inorganic pigments for the coloring of concrete have already been used as examples in Section 6.3.1 to demonstrate the reasons, importance, and results of agglomeration. A major disadvantage of tumble/growth agglomeration is the need for a binder that must be mixed with the powder, forms bonds between the pigment particles throughout the mass to be processed, and ultimately provides the strength of the granular product. The “vehicle” for introducing the binder material(s) and facilitating granule formation is a liquid, so that initially “green” (moist) agglomerates are
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formed. A post-treatment, at least involving drying for the removal of the liquid and formation of permanent bonding, yields dust-free, dry, free flowing, and easily handleable granular product. Because the binder is distributed throughout the granules, the shear forces in a concrete mixer are not high enough to fully and reliably disperse the pigment. Color spots and streaks are the result of such ineffective distribution. To overcome this problem, the binders have been selected from a group of materials that promote the particle dispersion in the presence of moisture (Section 6.3.1). Nevertheless, tumble/ growth agglomeration requires the additional cost of the binder and the post-treatment, which reduce the economics and the profit potential. A more effective way to produce pigment granules, which, if applied for concrete, have a coloring effect that is at least equal to that of the unagglomerated powder, uses compaction/granulation (Section 6.1.2) [6.3.2.4]. Although, as mentioned above, the natural adhesion forces (van-der-Waals) between dry, compacted, inorganic powder (pigment) particles are reduced to 1/10th of their strength in a liquid environment, the addition of a dry additive that promotes dispersion in a concrete mixer improves the ease and uniformity of coloration. The major advantage of the new method is the dry processing, which does not require a post-treatment to dry out liquid and achieve permanent granule strength. The process uses a flow diagram that is fundamentally in agreement with Fig. 6.1-14 (Section 6.1.2). Pigment and a small amount of dispersing agent are thoroughly mixed and then compacted with a roller press. The strip emerging from the rollers is milled and classified to yield product granules sized 0.2–0.6 mm. Oversized material is returned to the mill and re-granulated while all fines (undersized particles) are recirculated and mixed into the compactor feed. Because the highest possible yield of on-size granules, the utilization of the entire pigment feed through internal recirculation, and the absence of dust in the finished product are major objectives, an optimized flow diagram uses multiple (at least two) milling (crushers) and classifying (screens) steps (Fig. 6.3-28).
Fig. 6.3-28 Flow diagram of an optimized granulation process, based on roller press compaction, for (for example) an inorganic pigment
6.3 Applications in the Chemical Industry
Instead of a roller press a pellet mill may be used for densification. The rest of the system is essentially the same. However, for successful extrusion (pelleting) some lubricant (liquid) may have to be added. Although the temperature rise caused by friction in the die holes may often be sufficient to naturally dry-out the small amount of liquid, the resulting granules do still not exhibit the same uniform quality as those originating from sheets made with roller presses. As exemplified by the application of pressure compaction/granulation for pigments that are applied for the coloring of cementitious materials, this technology may be also used for other inorganic and organic materials. In many cases the reduction in strength of the molecular adhesion forces when they are in contact with liquids may suffice to render the granules sufficiently dispersible without the need for an addition of dispersing agents. 6.3.2.6 Plastics (Master Batch)
A master batch is an intermediate material that has been formulated for the manufacture of thermoplastic parts by pressing, extrusion, injection molding, and other forming and/or finishing processes. It contains all ingredients that are necessary for achieving the structure, color, strength, and other properties of the final plastic piece. Specifically it may hold pigments, fibers, fillers, chemical reactants, and many others. Most master batches are formulated from solid powders by mixing the different ingredients in a conventional (normally batch) mixer. Since considerable differences in size and shape of the individual components exist, for example between pigments and fibers and thermoplastic powders, segregation of the dry blend is a major concern. Also, the end users prefer to only re-melt the fully formulated feed and process it into parts for industrial and consumer applications. To solve the segregation problem and guarantee a uniform distribution of all the components in the master batch, the thermoplastic components of the blends are partially or completely softened or melted in double shafted screw or other high-pressure extruders [B.97]. Heat is created by the conversion of mechanical into thermal energy in the high-shear mixing and homogenizing section of the extruder or provided by external heating. The plasticized mass is passed through holes in a die plate and the extruded strands are cut into short, normally cylindrical pieces with a diameter to length ratio of between 1:1 and 1:2. If the compound is warm or hot and very plastic, the mouth piece (carrying the die plate) of the machine is under water wherein the strands are cooled prior to cutting (“under water granulation”). 6.3.2.7 Sodium Cyanide
As produced, sodium cyanide (NaCN) is a very poisonous powdery salt. The material is manufactured and applied in relatively large quantities for the treatment of steel, electroplating, and fumigating. Dust, particularly airborne particles, must be avoided to protect workers and allow safe transportation and use. While for the formulation of fumigating agents, agglomerates must be milled again to yield an aerosol powder, applications in the metals industries can accept larger pieces.
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The most common agglomeration technology for the conversion of sodium cyanide into a safe product is briquetting with roller presses. Almond- or pillow-shaped compacts are made. The systems are completely enclosed, equipped with highly effective dust collection systems to safeguard the operators, and executed in stainless steel to keep corrosion in check. The discharge from the roller press is screened within the enclosed system, fines are recirculated internally to the briquetter feed bin, and clean product is immediately packed into sealed containers. This is a typical example of the use of pressure agglomeration for the transformation of a dusty, hazardous chemical into an easily handleable compacted product.
6.3.3
Other Technologies
Other technologies of size enlargement by agglomeration (Chapter 5, groups C and D) are not as frequently used in the chemical industry as the other two major groups (tumble/growth and pressure agglomeration). The applications are more specific and not as well known to the general or even the knowledgeable public. Nevertheless, some of the materials defined as examples in Section 6.3 and discussed in Sections 6.3.1 and 6.3.2 are being modified by other agglomeration technologies as demonstrated below. Aspartame (Artificial Sweeteners) Aspartame is very sensitive to high temperatures and an alkaline pH. Therefore, without modification, it is not suitable for baking applications such as pre-mixes for cakes, sweet (Danish) rolls, and cookies. Suitably coated, it would be capable of resisting these adverse effects. However, aspartame, like other products with similar characteristics, for example niacinamide, a nutritional food supplement (Section 6.4), or ibuprofen, an anti-inflammatory drug (Section 6.2), has a long, needle like crystalline structure, exhibits very bad flow properties, and can not be encapsulated easily. Encapsulation coating is typically carried out in fluid beds (for example the Wurster coater [B.48, B.97]) where the elongated crystals tend to intertwine and form unacceptable soft, fibrous aggregate balls. A patent [6.3.3.1] proposes and industrial applications later use compaction/granulation (12, 14, 16 in Fig. 6.3-29) prior to encapsulation. During the pressure agglomeration process, the needle-like crystals break easily and are incorporated into the structure of the compacted sheet that is crushed and screened to yield granules in the size range 40–840 lm, which readily form a fluid bed in a top spray coater (18 in Fig. 6.3-29). Fines removed with a sieve (16) are recirculated to the compactor feed. Preferred encapsulation materials for the process are molten hydrogenated lipids and waxes. In the case of aspartame, it could be hydrogenated soybean oil stearine. An advantage of this fat coating is believed to be that it effectively protects the artificial sweetener from the moist heat developed during baking [6.3.3.1]. 6.3.3.1
6.3 Applications in the Chemical Industry
Fig. 6.3-29 Modified patent drawing of a fluidized bed encapsulation process incorporating compaction/granulation as a preparatory step (according to [6.3.3.1])
6.3.3.2 Laundry Detergents
In Section 6.3.2 it was mentioned that several problems must be resolved if laundry detergents are tabletted and three specific areas were listed [B.102]. Two have to do with the fact that the dissolution rate of compacted material is low and that, to arrive at acceptable performance, disintegrants have to be added that assist in the break-up during application. Another approach is the manufacture of a friable, unstable detergent tablet using low forces during compaction. Such a product already breaks-up just by handling it so that it can not be packed or stored. To stabilize these tablets they have to be coated with materials that form a hard, easily melting or dissolving layer or shell. Suitable materials are C12–14 dicarboxylic acids, which are combined with a disintegrant [B.102]. Suitable equipment for such coating are enrobers, more typically used in the food industry (Section 6.4.2). Fig. 6.3-30 explains the operation of such equipment. Pieces to be enrobed are transported on a wire mesh belt and, after entering the machine from the left, are kept in place by a holding down device. Coating is accomplished with a bottoming roller and by pouring mass onto the pieces from above.
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Fig. 6.3-30 The operating principle of an enrobing machine (courtesy Hosokawa Kreuter, Hamburg, Germany)
6.3.3.3 Pigments
Since the coloring effect of pigments is caused by reflection and the partial absorption of light, particle size is one of the most important characteristics. Not only does a smaller particle size produce more brilliance, in some cases changing the particle size modifies the reflection and absorption properties of the coating, resulting in a different perception of color by the human eye. To be independent of unintentional changes, more and more pigments are precipitated or synthesized as sub-micron (nano) particles, which initially exist in suspension. To be able to filter, wash, and dry the solid product without loosing the pigment properties, an agglomeration process is necessary. Later, the binder and residual water are evaporated and the dry granulated pigment can be easily redispersed for use in the coloring of surfaces. In the example presented here, an organic pigment (di-methyl-quinacridone) is produced in suspension during the drowning-out (dilution) of highly concentrated sulfuric acid in which the synthesized pigment is dissolved [6.3.3.2]. Because the precipitated particles are in suspension, an immiscible liquid binder (di-N-butylamine) is used for agglomeration. During the “classic” (immiscible binder agglomeration) method it is first necessary to disperse the binder phase into small droplets. This occurs in a stirred vessel with baffles (Fig. 6.3-31). The pigment particles coalesce with the droplets and form agglomerates [B.97], which can then be subjected to the previously mentioned processing. As in all growth agglomeration processes, binder dispersion and formation of nuclei are time consuming (Fig. 6.3-32). A new process reduces this time by adding a binder emulsion, which was prepared in a separate step [6.3.3.2]. Fig. 6.3-33 is the comparison of agglomerate growth with and without binding liquid pre-dispersion by a rotor-stator mixing device. Disadvantages are a complicated installation scheme, high-power consumption, and a modification of the physicochemical properties of the system due to the temperature rise caused by the conversion of energy.
6.3 Applications in the Chemical Industry Fig. 6.3-31 Stirred vessel with baffles for the “classic” immiscible binder agglomeration [6.3.3.2]
In the second new process, binder nano-droplets are produced chemically in-situ by using the acid–basic properties of the binding agent [6.3.3.2]. The di-N-butylamine is soluble in sulfuric acid solution resulting in an ionized compound, the sulfate of di-Nbutylamine. If the neutralization of the suspension is carried out after injection of the
Fig. 6.3-32 Comparison of growth curves obtained during different operating conditions of the “classic” immiscible binder agglomeration showing long time intervals for the formation of nuclei [6.3.3.2]
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Fig. 6.3-33 Comparison of agglomerate growth with and without binding liquid pre-dispersion by a rotor-stator mixing device [6.3.3.2]
amine, very fine droplets of the binding liquid are directly and homogeneously generated within the suspension (reaction between di-N-butylammonium and hydroxide ions). This results in an optimal contacting of binder droplets with solid particles and a fast and uniform increase in agglomerate size with narrow distribution. In addition, as shown in Fig. 6.3-34, very fine particles disappear with time, which makes further filtration and washing steps easier. An already classic packaging method for pigments is the microencapsulation of toner particles. Such products are dust-free, free flowing, and feature a long shelflife. They can be used directly in copying machines where, after electrostatically assisted deposition on paper, pigment is liberated between pressure rollers by rupturing the capsules. Another application is the embedding of uniformly distributed
Fig. 6.3-34 Fast and uniform increase in agglomerate size with narrow distribution obtained during optimal contacting of in-situ formed binder droplets with solid particles [6.3.3.2]
6.4 Applications in the Food Industry
microcapsules between the fibers during paper making, yielding self-copying paper. In this case the capsules must be small enough to fit into the thickness of the paper structure and strong enough to avoid smudging during normal paper handling. They must be positioned in close proximity so that, after destruction of the membrane by the force of the writing tool, a clear and continuous line is produced.
6.4
Applications in the Food Industry Besides meats and greens, humans first consumed kernels of cereal grasses by eating them raw, green or naturally dried. Later, grains were liberated through manual threshing and separating the chaff by “wind sifting”, early applications of unit operations of mechanical process technology. Maybe before learning to grind dry grains, green kernels were flattened to yield a flaked product. As human development continued it can be assumed that agglomeration was first used during the making of bread. “Whole grain” bread or flat cakes, made with flaked kernels, were most probably the first manufactured foods. As already mentioned in Chapter 2, for the making of “advanced breads” dry grains were ground between rough (mill) stones. The resulting flour (particulate solids including an inherent binder, starch) and liquid additives (additional binder for plasticity and “green” bonding) were mixed and kneaded; this mass was then formed (agglomerated) into loaves and finally “cured”, the removal of much of the moisture that was added during the mixing, kneading, and agglomeration steps, to obtain structure and permanent bonding, first by drying in the sun and later by baking in an oven. The “technology of bread making” combines all unit operations of mechanical process technology (Fig. 2-2, Chapter 2) and the components of a complex agglomeration process, including separation of grain kernels from the chaff, preparation of solid feed particles by milling (adjustment of particle size and activation of the inherent binder, starch), mixing and kneading of particulate solids with additional binder(s), agglomerating the mass into a “green” form, and a “post-treatment” (curing, drying or baking, heating) to provide strength and texture. Very early in history it was also found that the porosity of the final product could be modified (increased) by making use of gases that are produced during fermentation (initiated by sourdough or yeast) and result in bubbles in the moist, rising mass. These voids (pores) are stabilized by “strengthening” during post-treatment (baking). Although the availability of bread is documented in some of the earliest human writings, pictorial descriptions of the process only date back to the Egyptians (Fig. 6.4-1) and the Romans (Fig. 6.4-2) [6.4.1]. Fig. 6.4-1 describes the mixing, forming and baking of bread and includes special, most probably devotional shapes (top center). It is reproduced from a painting in a tomb from the time of Pharaoh Rameses III (about 1175 BC) [6.4.1, 6.4.2].
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Fig. 6.4-1
The bakery of Rameses III, about 1175 BC [6.4.1, 6.4.2]
Fig. 6.4-2 is a lithograph of a stone relief from 50–25 BC on the tomb of a well-to-do baker named Marcus Vergilius Eurysaces, which now stands at Porta Maggiore in Rome, Italy. It shows in the lower two strips the (mule powered) grinding of flour (center) and mixing of dough (lower right), the forming and baking of the loaves, and the weighing and transportation of the bread in the upper strip.
Fig. 6.4-2 Lithograph of stone relief from 50–25 BC on the tomb of a well-to-do baker named Marcus Vergilius Eurysaces in Rome [6.4.1]
6.4 Applications in the Food Industry
Bread is still a major basic food for most people on earth and its manufacture has not changed in principle. Fig. 6.4-3 shows a modern bakery [6.4.1]. Other than obtaining pre-processed components (flour, yeast, salt, and potentially other ingredients(3)) and using mechanized kitchen tools and modern ovens (1) or other “curing devices”, even in advanced societies individuals still mix, knead, and form bread in much the same way as people did millions of years ago. Bakeries where large amounts of bread are produced for sale, have evolved into medium to large industries. In particular, big bakeries serving customers in national regions or selling to international markets, have become bread-making factories: thousands of loaves of bread are made on continuous production lines, which use extruders (17) for the agglomeration step and in which extensive instrumentation is employed for automated handling, processing, storage, and packing. During the past 100 years, many more methods of food processing have developed into sizable industries, many of which use size enlargement by agglomeration for the beneficial modification of intermediate components (Section 6.4.1) and final products (Sections 6.4.2 and 6.4.3). A major influence on the development of large food manufacturing facilities has been the availability of reliable long-distance transport by railroads, steam ships, and, more recently, trucks and refrigeration. The latter played a major role in the evolution of food processing. After Linde invented the refrigerator using ammonia compression/condensation in 1874, within a few years large cooling plants were built around the world, which allowed the long-term storage of food for transportation to centralized processing facilities where the raw components are modified and combined to yield new food products. As mentioned above, many employ agglomeration at some point of manufacturing.
Fig. 6.4-3
Diagram of a modern bakery [6.4.1]
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The latest development in food technology is the manufacture of engineered products. Additives are often used as functional components [B.71] to obtain materials with specific, predetermined, and controlled properties. They are formulated from particulate ingredients and then agglomerated to yield consumer products that feature desirable characteristics. For the new food groups, descriptive names have been coined during the past years [B.97]. For example, “convenience foods” can be easily and quickly used, such as “instant” soups, sauces, and drinks, or products that were recombined from fine, ground food stuffs, contain already the correct amount of spices and other aromas, and, after quick and easy preparation, feature a texture and taste that pleases the palate. “Functional foods”, also called “designer foods” on the other hand, have been treated to eliminate unhealthy ingredients, such as fat. They are then recombined with additives that replace the removed components without sacrificing the “mouthfeel” that is expected from the untreated product. Functional foods may also contain dietary additives that make a product particularly acceptable for a special group of often chronically sick people, such as diabetics. The market for food additives is growing over-proportionately, largely due to the increasing production of more nutritious and better balanced convenience and designer foods. Calorie reduction agents represent the largest segment. “Fun foods” comprise the wide range of modern sweets and snacks in which mostly sugar and fat-based binders are applied to obtain agglomerates, for example bar-shaped products (Section 6.4.2), from a multitude of ingredients for the consumer market. As new food products are developed and their production is industrialized, problems arise and have to be solved because of the need for mechanized and automated processing. For example, for centuries the baker realized that different flours require different amounts of additives and, by experience, modified the recipe to obtain the desired consistency of the dough. It was also known that, for cakes and cookies, different results were obtained after baking if the sugar had not been fully dissolved or the fat was not finely divided. The beating of a mixture of water or milk with sugar and fat, often in a hot water bath, yielding an emulsified, foamy mixture as an ingredient of dough, was an important part of baking and was modified as needed by changing the composition and the preparation methods or times. Such individual visual and tactile evaluation by a human is, of course, no longer possible with an automated production line, producing high-quality products reliably and reproducibly, ready to be packed, shipped, stored, sold, and consumed. It became necessary to analyze and define food characteristics and their modifications and process parameters, knowledge of which were previously transferred from generation to generation and applied by experience. It also became necessary to measure, apply, and control all variables to be able to guarantee the desired product quality. While, in the past, some modifications in the presentation of individually prepared foods was accepted, merely reflecting on the skill of the baker or the cook, a new understanding of quality by the consumer requires absolutely consistent appearance, composition, texture, taste, and many other attributes, which may be specially defined for a particular product. Questions of shelf life and the need for specific methods of packing also became important.
6.4 Applications in the Food Industry
As in many other areas, but most easily described in connection with food products, industrialization converted the manufacturing of foods from a craft to a mechanized process technology, which is controlled by chemistry and physics and their interrelationships (Chapter 12). Since the 1980s, the polymer science approach to the study of the glassy state, glass transitions, and their importance for structures, properties and water relationships in food materials, products, and processes was recognized by a growing number of food scientists and technologists [6.4.3]. As a result, the following questions arose (Chapter 3 in [B.53]): “What is a glass? What is a glass transition? Why is the temperature at which a glass transition occurs (Tg) so important to the processing and storage stability of so many foods? Why is the effect of water, used as a plasticizer, on Tg of such widespread relevance to food products and processes? Why are considerations of non-equilibrium glassy solid and rubbery liquid states in foods more germane to issues of food quality and safety rather than equilibrium phases? Why are the kinetics of heat/moisture processes for foods and of deterioration in food systems during storage more often appropriately interpreted in terms of the Williams–Landel–Ferry (WLF) rather than the Arrhenius equation? What is the “food polymer science” approach with its central concepts of “glass dynamics” and “water dynamics”, and why has this research been so useful to the study of glasses and glass transition in foods?” Answers to some of these questions are as follows. A glass is an amorphous (noncrystalline) solid; it is actually a supercooled liquid of such high viscosity that it exists in a metastable solid state in which it is capable of supporting its own weight against gravitational flow. In a food agglomerate it represents the solid-bridge binding mechanism. A glass transition in amorphous systems is a temperature-, time-, and composition-dependent, material-specific change in the physical state, from a rigid glassy solid to a rubbery viscous liquid or vice versa. In the polymer science approach, the importance of the glassy state, the glass-transition temperature, Tg, together with its location relative to the temperature of storage, and the critical role of water as a plasticizer of food glasses (depressing Tg with increasing moisture content) were recognized as controlling the quality, safety, and storage stability of a wide range of food systems. An increasing awareness of the inherent non-equilibrium nature of most food products and processes has developed. This is exemplified by the development of intermediate moisture foods (IMFs) in which amorphous carbohydrates (polymeric and/or monomeric) and proteins are major functional components. Glass dynamics deals with the time- and temperature-dependent relationships between composition, structure, thermodynamic properties, and functional behavior of foods. It focuses on the glass-forming solids in a food system containing water, the resulting glass that can be produced by (often drying and) cooling to T < Tg, and the effect of the glass transition and its Tg on processing and process control. Temperatures during individual processing steps, for example during agglomeration, may be deliberately chosen to be first above and then below Tg (providing the binding mechanism). Some selected examples of food systems whose behavior is governed by dynamics far from equilibrium and of practical problems of food science and technology posed by their non-equilibrium nature, are presented in Tab. 6.4-1.
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6 Industrial Applications of Size Enlargement by Agglomeration Tab. 6.4-1 Some selected examples of food systems with non-equilibrium behavior (adopted from Chapter 3 in [B.53]) * *
* * * * * *
Water-vapor sorption/desorption hysteresis in concentrated food polymer systems. Graininess and iciness in ice cream; reduced survival of frozen enzymes and living cells; reduced activity and shelf-stability of freeze dried proteins. Caking of dry powders; sugar bloom on chocolate. Recipe requirements for gelatine deserts. Cooking of cereals and grains Expansion of bread or collapse of cake during baking; effects of flour and sugar on cookie baking. Baked goods become stale Effects of sugar-water glasses and rubbers on texture and storage stability.
Cookies and crackers have been used as examples for research on how the food polymer science approach expands the quantitative and practical knowledge of commercial processes [6.4.3]. A sucrose phase diagram (Fig. 6.4-4) was developed as a tool for the understanding of cookie and cracker baking. In many different food products and processes, the glass-forming and crystallizing behaviors of sucrose constitute key functional attributes. In Fig. 6.4-4, center left, at typical temperatures prior to baking, the locations of lean (low sugar/fat ratio) and rich (high sugar/fat ratio) cracker doughs are sufficiently above the glass curve, so that it is easy to dry these products during baking without structural modifications. When the temperature is raised to the vaporization curve (for example, vertical line from the location labeled “rich”), water in the dough begins to evaporate. As baking continues, more water is removed and the concentration of dissolved sucrose increases. Depending on how much flour, sugar, and water are added to the mixture for highsugar cookies, and on how much crystalline sugar dissolves prior to the beginning of baking, the final state of this dough may be on either side of the solidus curve (A or B in Fig. 6.4-4). Then, when baking begins, either water evaporates first and additional
Fig. 6.4-4 Phase diagram for the sucrose-water system, illustrating the locations of the glass, solidus, liquidus, and vaporous curves and the points Tg’ and Te (eutectic melting temperature) corresponding to the intersections of the liquidus/glass and liquidus/solidus curves, respectively [6.4.3]
6.4 Applications in the Food Industry
sugar dissolves later (from A) or additional sugar dissolves first and water evaporates later (from B). Once all of these doughs are baked and cooled, their state falls into a box of final product conditions (D) that spans the glass curve. For typical cracker doughs containing sucrose (“lean” or “rich” in Fig. 6.4-4), the relatively small amount of sugar is so far below its saturation limit in water that it can be completely dissolved during dough mixing, remain in solution during baking, and then, without likelihood of recrystallization, convert to the glassy solid state in the finished product. In contrast, in the dough for a cookie the sucrose content may be already higher than the saturation concentration (67.5 % at 25 8C); then (B in Fig. 6.4-4), part of the remaining crystalline sucrose will dissolve during baking (along Tsolidus in Fig. 6.4-4), depending on the time/ temperature/moisture-loss profile of the oven. Nevertheless, sufficient moisture loss during baking of any (high sucrose) cookie dough (either A or B in Fig. 6.4-4) can create a supersaturated sucrose solution (located within the metastable rubbery region, C in Fig. 6.4-4), from which some sugar may recrystallize during final baking, cooling, or product storage (Fig. 6.4-5), conditions that are not acceptable from a quality point of view. Other food systems can be characterized and investigated with similar phase diagrams. As explained earlier (Fig. 6.4-4), amorphous foods may enter the rubbery state, in which contacting or colliding particles adhere to each other, either at constant moisture or due to an increase in temperature. Very recently the following definitions were formulated (Chapter 5 in [B.108]). “The physical state and physicochemical properties of food components affect food behavior in processing and storage. Many of the food components can exist in the amorphous state, especially at low temperatures and/or low moisture contents. Amorphous materials may exist in a solid-like, “glassy” or in a viscous, “rubbery” state. The
Fig. 6.4-5 Diagram of the transitions of food materials, which can be present as crystalline solids, glasses, rubbers, liquids, or in solution [B.108]
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transitions of materials, which can be present as crystalline solids, as glasses and rubbers, as molten liquids, or in solution, are shown schematically in Fig. 6.4-5. The glassy state is not an equilibrium state but is determined by kinetic considerations, such as rates of cooling or dehydration. However, once established, the glasses are stable until the temperature exceeds the glass-transition temperature Tg.” Because water plasticizes hydrophilic food components, their glass transition is strongly dependent on water content. The effect of water on the glass-transition temperature of several amorphous carbohydrates, calculated with the Gordon Taylor equation [B.79], is depicted in Fig. 6.4-6. Within the range of the materials shown, Tg decreases with lower average molecular weight and/or increased concentration of plasticizer (water). The rubbery and glassy states of amorphous food products play important roles for agglomeration and caking. As the product temperature exceeds Tg, amorphous materials enter the rubbery state and the decreasing viscosity induces flow, deformation, and bonding. The latter may be desired and is stabilized when the glassy state of the final product is obtained at or below Tg by drying and/or cooling, or it may initiate caking, undesired build-up during food processing [6.4.4]. For example, the solids in dehydrated fruit juices contain mostly fructose, glucose, and sucrose. In combination, their Tg is estimated to be below room temperature (Fig. 6.4-6), which makes them very sticky even at typical ambient temperatures; to avoid build up during dehydration in spray dryers it is recommended to cool the walls. In another example, as the amount of high molecular weight carbohydrates, such as maltodextrins, added to infant formula grows, Tg is raised, which increases the stability against caking if the product is kept dry. Therefore, while the transition into the glassy state stabilizes the bonding of agglomerated products, a strict control of moisture content and storage at low temperature is typically required to avoid undesired caking and deterioration of many food products.
Fig. 6.4-6 Effect of water on the glass-transition temperature of several carbohydrates, calculated with the Gordon-Taylor equation [B.108]
6.4 Applications in the Food Industry
Since in many cases, particularly if the food powders are hygroscopic, adsorption of moisture from the air during processing and packing and later during storage, due to liquid migration through package walls, is already sufficient to substantially change Tg, anticaking agents may have to be added to improve stability (Chapter 4).
Further reading
For further reading the following books are recommended: B.21, B.26, B.33, B.40, B.44, B.49, B.50, B.53, B.54, B.56, B.59, B.67, B.72, B.77, B.86, B.89, B.93, B.94, B.108 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold.
6.4.1
Tumble/Growth Technologies
In mammals, milk is nature’s food for the offspring and, therefore, represents one of the best and richest nutriments for this animal group, including people. Reduced-fat milk from which only cream has been skimmed and, to a lesser degree, whey from which fat and coagulated milk solids have been removed (mostly for the manufacture of cheeses) are still highly nutritional foods. They are processed for a number of reasons. The evaporation of milk has been known for many years, even as early as 1200 when Marco Polo described the production of a paste-like milk concentrate in Mongolia [6.4.1.1]. Approximately 600 years later the concentration of milk and of other liquid food products, for example extracts such as coffee, was taken-up as an industrial technology, eventually ending in the production of a dry powder. During water removal, pronounced changes in physical structure and appearance take place. Since the process starts with a thin, water-like liquid and ends with a dry powder, it was found that one method of liquid removal is not optimal for all conditions In the food and dairy industry, the methods listed in Tab. 6.4-2 have been adopted for liquid removal. Tab. 6.4-2 indicates that sometimes liquids containing small amounts of suspended and/or dissolved solid matter may be concentrated first by evaporation. This was oriTab. 6.4-2 Methods that have been adopted in the food and dairy industry for liquid removal [6.4.1.1, B.97] Evaporation Spray Drying Vibrating Fluid Bed Integrated Fluid Bed Integrated Belt Drying
Changing from a water like liquid to a high viscosity concentrate Transforming the concentrate into droplets and further evaporating water to get a dry powder Introduced for additional drying and cooling to improve drying efficiency and powder quality Further improvement of drying economy and possibility of drying difficult materials Adding a moving belt dryer at the bottom of the drying chamber; applied for materials that are extremely difficult to dry by spray drying.
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ginally carried-out in an open pan, later in more economical forced recirculation evaporators, and now mostly in highly efficient falling film evaporators [6.4.1.1]. In any case, spray drying has become the heart of all liquid-removal facilities for the production of a dry powder. The associated fluid bed or belt dryers improve drying efficiency and powder quality and allow the processing of difficult materials from which, for example, the final moisture content can not be removed easily or which require cooling. The reason for liquid removal and manufacturing of a powder is to reduce the volume and mass of the liquid product and to stabilize the dry concentrate. Later the dry powder is reconstituted in water or another suitable liquid. During spray drying, particularly if the solid content of the liquid feed is low, the spray droplets must be small to achieve the necessary removal of liquid. This, in turn, results in the production of tiny solid particles which, because of the ensuing drying mechanism, are hollow (Fig. 6.4-7). The reconstitution of a powder consisting of small, light particles in liquid is difficult because they exhibit bad wetting characteristics, which results in powder floating on the liquid surface. Intensive mixing is required to accomplish reconstitution, which is still ineffective and time consuming. Tab. 6.4-3 defines the mechanisms that must occur if a powder is dispersed (nonsoluble particles) or dissolved (soluble particles) in a liquid. All three or four phases of dispersion or dissolution proceed individually whereby some overlapping may occur, depending on the amount of material involved. In the food industry good reconstitution of powders in a liquid, which is a function of time, is associated with the term “instant product” and is often synonymous with agglomerated materials [B.97]. In addition to the inter-particle porosity of the powder, pores in the agglomerates assist in the desired quick liquid penetration. Agglomerates are also larger (heavier) thus improving submergence. The binding mechanisms holding agglomerates together must be such that they easily and quickly break-up in the liquid.
Fig. 6.4-7 Micrograph of an overheated spraydried particle that has burst, showing that it is hollow (courtesy Niro A.S., Soeborg, Denmark)
6.4 Applications in the Food Industry Tab. 6.4-3 Mechanisms occurring during the dispersion and, respectively, dissolution of powder in a liquid 1. Penetration of liquids into the dry powder (also called wetting) 2. Submergence of the powder in the liquid (also called sinking behavior) 3. Break-up of the powder mass into the primary particles (also called dispersibility) (4. If the solid is soluble, dissolution of the primary particles (also called solubility)).
Manufacturers and/or users have a more or less well-defined procedure to determine the maximum allowable time. Typically, complete dispersion or dissolution should be accomplished within a few seconds in warm liquid and in about 30–60 s in cold liquid. Instant agglomerates may contain certain substances, such as fibers and other disintegrants, which swell on contact with liquid and assist in break-up during the dispersion phase [B.63, B.97]. Products from powdered food materials with instant characteristics can be obtained with a variety of different, rather conventional processes (Tab. 6.4-4); most of these use agglomeration techniques (A in Tab. 6.4-4). Because granule size should be small and porosity must be high, instant food products are most commonly manufactured by rewet agglomeration in mechanically agitated beds or fluidized beds. Spray drying combined with mixer agglomeration or fluidized bed agglomeration, in which turbulent particle movement is induced by flowing gas, are also often used. The term “instant” is normally used in the food industry for drink powders, including milk, coffee, tea, soups, sauces, and the like. All instant products must be able to disperse quickly and, if applicable, dissolve in a specific liquid at any temperature, particularly also at ambient or even cold conditions, without residue and sediment. Spray drying has been found to be the most suitable process for converting milk into a dry powder while still keeping the nutritional properties. In the modern dairy industry the drying of milk dates back to around 1800 with larger systems introduced around 1850, but early methods required the addition of sugar, sulfuric acid or alkali and, therefore, the product could not be considered pure. Breakthroughs in the production of high-quality milk powder occurred at the beginning of the 20th century. Tab. 6.4-4 Principles that are most commonly used to manufacture instant products from powdered food materials [B.97] A
Agglomeration techniques
B
Techniques utilizing other processes
A1
Rewetting of powders in fluid beds A1a mechanically induced A1b gas induced Spray drying and agglomeration Combinations of A1 and A2 Press agglomeration A4a Compaction/granulation A4b Extrusion/crumbling
B1
Improvement of wetting with additives (surfactants) Improvement of wetting by extraction (e.g. of fat) Improvement of solubility (e.g., amorphous structure)
A2 A3 A4
B2 B3
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Stauf in Germany in 1901, and Grey and Jensen in the USA in 1913, applied for patents on the use of spray nozzles. Rotary atomizers were developed in 1912 in Germany by Kraus and in 1933 in Denmark by Nyrop [6.4.1.1]. Because of the danger of bacterial destruction, as for milk, other foodstuffs that are in liquid solution or in the form of a slurry or a paste have limited shelf life. Similar to refrigeration or the addition of preservatives, the removal of liquid reduces this bacterial activity. Foods that are free (or relatively free) of liquid can be stored for an almost unlimited time if kept dry and cool. Such products may contain proteins, carbohydrates (including the most important one, starch), fat, and other ingredients such as vitamins, flavorings, including salts and sugars, emulsifiers, stabilizers, colors, and chemicals. If the dry foodstuffs are powders resulting from spray drying they all exhibit difficulties during reconstitution. As mentioned before, size enlargement by agglomeration improves this situation (Discussion of Tab. 6.4-3). During spray drying, agglomeration may occur spontaneously or in a controlled manner. Spontaneous agglomeration is mostly caused by coalescence of partially dried particles with dissimilar diameters. Such particles collide with one another due to their different deceleration paths and adhere to each other. As shown in Fig. 6.4-8, the resulting agglomerates are small and normally do not markedly improve reconstitution. Controlled, forced agglomeration provides conditions that are favorable for particle coalescence and/or size enlargement by growth. These methods yield instant products since most powders achieve that characteristic by mere agglomerate growth. Favorable conditions for collisions exist in multi-nozzle spray dryers where two or more atomization clouds penetrate each other (Fig. 6.4-9) resulting in the attachment of larger, often equal-sized particles and the production of larger agglomerates. This is in contrast to what has been shown in Fig. 6.4-8, in which only smaller particles are captured by larger ones.
Fig. 6.4-8 Typical particle from single stage spray drying with small “satellites” attached [6.4.1.1]
6.4 Applications in the Food Industry Fig. 6.4-9 Forced agglomeration by the interaction of two atomized clouds of droplets from opposing nozzles [6.4.1.1]
Fig. 6.4-10
Sketches of fines return methods for nozzle atomizers [6.4.1.1]
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Fig. 6.4-11
Sketches of fines return methods for rotary atomizers [6.4.1.1]
More common, however, is the spraying of concentrate onto particles in a fluidized bed of a down stream dryer. Of course, this is only possible if two or multi-stage dryers are used [6.4.1.1, B.97]. Growth agglomeration (Chapter 5) occurs after re-wetting the surfaces of the partially dried particles. A further technology for the production of instant (agglomerated) materials in a spray dryer uses the recirculation of dry, already pre-agglomerated particles into the spraying zone of the dryer. Recirculating fines are particles captured in cyclones of the dust collection system and product screen undersize. Figs. 6.4-10 and 6.4-11
6.4 Applications in the Food Industry
demonstrate how the recycling dry particulates are introduced into the dryer near the atomization devices (nozzles or rotary atomizers) where they meet and collide with droplets (called primary particles in Figs. 6.4-10 and 6.4-11) that wet their surfaces. Subsequently, collisions form agglomerates that consist of several particles stuck together. Depending on the parameters selected, the final agglomerates discharging from the system can have a size of 100–200 lm. Fig. 6.4-12 is a schematic flowchart of a complete plant for the production of agglomerated dry solids from a liquid, such as milk [6.4.1.1]. On the left, raw liquid receiving and storage is followed by concentration (partial evaporation) in a falling film, vapor recompression system. On the right the concentrate and recycling cyclone fines are fed to a compact spray dryer which is followed by a vibrating fluidized bed dryer/agglomerator/instantizer and a vibrating fluidized bed cooler. Fig. 6.4-13 is the photograph of a fluidized bed machine in a food processing plant. Fines entrained in the effluent gases from the spray dryer and the vibrating fluidized beds are directed to the cyclones from where they are recirculated into the spray dryer. Re-wet agglomeration of dry, fine food material (e.g., from spray drying) to obtain instant products is carried out in special tower structures or in mixer agglomerators. The first instant non-fat dry milk product was marketed in 1954 based on pioneering research by Peebles [6.4.1.1]. Since steam condensing on solids wets surfaces directly and uniformly and, at the same time, transfers heat very effectively, which helps to partially dissolve solids that are then acting as binders (discussion of Fig. 4.5, Chapter 4), steam is used directly [B.97] or added for improved results (Fig. 6.4-14).
Fig. 6.4-12 Flowchart of a complete plant for the production of agglomerated dry solids from a liquid [6.4.1.1]
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Fig. 6.4-13 Photograph of a fluidized bed machine (dryer/agglomerator/instantizer) in a food processing plant (courtesy Niro A.S., Soeborg, Denmark)
Another example of re-wet agglomeration was developed by Nestle´ for the instantizing of milk powders, chocolate flavored beverages, and soups, as shown in Fig. 6.4-15 [6.4.1.1]. It uses a wetting/agglomeration tower (5) in which an about 10 % solution (1) of the material to be agglomerated (4) is sprayed onto the dry feed with a high-velocity flat fan type nozzle (3). The now sticky particles adhere to each other and are dried in a vibrating (7) fluid bed (6) with warm air (8) to a residual moisture content of < 3 % (9) and bonded by solidification of the dissolved material prior to packing. So far, in all examples the method for agglomeration was based on the use of lowdensity gas fluidized beds. As discussed in Chapter 5 and in much more detail in earlier publications [B.48, B.93, B.97], adhesion by coalescence of irregularly moving particles in low-density fluidized beds yields relatively small, structurally loose, and low-strength agglomerates, which (if reconstitution in liquids is desired) exhibit instant characteristics. In most cases these processes operate continuously, but batch operations for very sensitive materials (vitamins) are also possible. Fig. 6.4.1-16 shows as examples the photomicrographs of two typical products (dry milk and coffee extract). Mechanical agitation can also be used for this purpose. A low-density mechanically activated cloud of particles is obtained in the “Schugi Flexomix”. This machine features a vertical open-ended cylindrical mixing chamber in which a shaft, equipped with
6.4 Applications in the Food Industry
Fig. 6.4-14 Diagram of the Peebles instantizer [6.4.1.1]
Fig. 6.4-15
Schematic flowchart of the Nestle´ re-wet instantization plant [6.4.1.1]
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Fig. 6.4-16 Photomicrographs of two typical products: a) coffee extract; b) milk (courtesy Niro A.S., Soeborg, Denmark)
adjustable blades, rotates at high speed (1000–3000 rpm). The number and position of the knife blades and their angle of attack are selected to suit the particular process needs. The shaft is suspended from heavy duty bearings in the drive system above the vertical cylindrical mixing chamber, resulting in a continuous, completely unobstructed discharge of the moist, agglomerated product [B.48, B.97]. The mixing chamber of the “Schugi” consists of a flexible sleeve that is continually deformed from the outside by rollers that are moving up and down, thus preventing build-up on the interior (Fig. 6.4-17). The rollers are mounted in a cage that is pneu-
6.4 Applications in the Food Industry
Fig. 6.4-17 a) Diagram of a cross section through the operating parts of a “Schugi Flexomix”. b) Photograph of the opened-up roller cage of a “Schugi Flexomix”, showing the vertical shaft with the mixing blades after removal of the flexible sleeve that defines the mixing chamber. c) Artist’s conception of the “Schugi” (courtesy Hosokawa Schugi, Lelystad, The Netherlands)
matically operated. Roller cage and mixing chamber are easily accessible for cleaning and servicing (Fig. 6.4-17b). Wet or dry powders are dropped into the upper end of the mixing chamber so that a low concentration of solids in the mixing chamber develops. Hold-up or retention time, which is only about 1 s or less, can be to a certain extent influenced by the angle of the knife blades: they may increase or decrease the free fall speed component of the rotating charge. Agglomeration of the solid particles occurs either by disagglomeration of a wet feed and reagglomeration using the liquid that is available in the feed or by wetting dry powder with binder liquid. For liquid addition, a wide range of spray or atomizing nozzles is available; selection and installation of these wetting arrangements depends on the liquid and the desired product characteristics. Agglomerates from the “Schugi” normally feature a small but somewhat adjustable particle size in the range 0.2–2 mm and narrow distribution. With increasing rotor speed, the width of the particle size distribution tends to become smaller [B.48, B.97]. Because agglomerates have formed by accretion after impacting in the low-density particle cloud, they typically feature instant characteristics. The four photographs
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shown in Fig. 6.4-18 show the quick and complete (instant) dispersion of an agglomerated product in water. Although, theoretically and sometimes practically, instant products can be also manufactured in high-shear mixer agglomerators (Section 6.3.1), in the food industry spray drying and agglomeration (A2 in Tab. 6.4-4) are used as the original, still very common
Fig. 6.4-18 Sequence of photographs showing the quick and complete dispersion of an agglomerated material in water (courtesy Hosokawa Schugi, Lelystad, The Netherlands)
6.4 Applications in the Food Industry
method and, more recently, re-wetting of powders in gas or mechanically agitated particle bed and clouds (A1 in Tab. 6.4-4) and several combinations of the two (A3 in Tab. 6.4-4) are applied as the dominant techniques. Tab. 6.4-5 lists food materials (in alphabetical order) that are processed by the methods described above and Fig. 6.4-19 is a summary in which, in various categories, specific food materials are mentioned and one each (indicated by an asterisk *) is presented microscopically and macroscopically. As usual, the list is not complete and exhaustive but is presented to demonstrate the large variety and wide acceptance of growth agglomeration in this industry. Without going into detail, some low- and high-shear mixer and pan agglomerators are being also used in the food industry (for example, for grated cheese, dry yeast, cake mixes, fat/flour mixtures, food additives, including so-called nutraceuticals). As in the case of pharmaceutical applications (Section 6.2.1), the equipment is executed in food grade stainless steel or suitably coated on those surfaces that come into contact with the product and are designed for easy, thorough cleaning. Such methods are used to increase the bulk density of fine powders, reduce their dustiness, and improve the flowability to ease the packaging and the metering during further processing or final use. Obtaining products with instant characteristics is not a primary objective in such cases. In most applications small granules with particle sizes around 1 mm (0.5– 3.0 mm), narrow distribution, and the above-mentioned desirable physical characteristics are manufactured. For the fast growing market of cereals and snacks and the dietary and health foods, agglomeration plays an increasing role (Section 6.4.2). Agglomerates consisting of
Tab. 6.4-5 List of some dry and dried food materials as well as mixed food formulations that are converted by growth agglomeration into free flowing, granulated, often instant, and dust free products for easy packaging, metering, and reconstitution Artificial sweeteners Baby formulas Cake mixes Cereal dust Cocoa mixes (w&w/o sugar) Coffee extracts, powders, and substitutes Cream Dairy product blends Decaffeinated coffee extract Dextrose Drink mixes Flavorings Flours and flour mixes Food colors Fruit juices (w&w/o fillers/additives) Gravy mixes Herb extracts Malt extract
Milk powdered whole skim low fat non-fat mixed fat filled permeate protein Molasses Pudding mixes Sauce mixes Soup mixes Soy milk and protein Spices, powders and extracts Starches and derivatives Teas, powders and extracts Yeasts
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Fig. 6.4-19 Various categories of and specific food materials that are dried and agglomerated to yield powders; * indicates which material is shown in the pictures (courtesy Niro A.S., Soeborg, Denmark)
6.4 Applications in the Food Industry
Fig. 6.4-20 a) Growth agglomerated cereal mixtures; b) “Rice Krispies” (courtesy Kellogg Co., Battle Creek, MI, USA)
only a few large particles are produced to avoid segregation (Fig. 6.4-20a) or to yield a product that can be served with liquids, especially milk, without loosing its crispiness (Fig. 6.4-20b). Growth occurs in blenders with mixing tools after adding (often warm) binders, such as honey, syrup, caramel, fats, sugar. The resulting mass is dried and cooled, then screened to remove dust, which is used elsewhere, and chunks that are broken and rescreened. It is also possible to extrude a thick sheet with roller extruders (Section 6.4.2), cure it, and break and screen the solidified mass as described before.
6.4.2
Pressure Agglomeration Technologies
Many modern food products are processed and/or finished by methods of pressure agglomeration, particularly extrusion. But other techniques, such as punch-and-die pressing and compaction/granulation, are also used for particular applications.
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Bread making has already been mentioned and described as one of the oldest applications of agglomeration by people. Fig. 6.4-3, items 10, 11, 12, and 17, show roller presses and extruders for the shaping of dough into flat sheets or ropes from which raw bakery goods (bread loaves and rolls) are made by cutting the extrudates. Machines featuring two counter-rotating rollers are the most common equipment for the shaping of modern food products by extrusion. Fig. 6.4-21 shows a toothed roller press. The feed to be processed, solid, viscous, or, generally, plastic masses to which coarse components can be admixed, is drawn in by the teeth on the rollers, transported to the pressure zone below, and extruded through interchangeable dies as strands (ropes) or slabs (sheets). The rollers are timed such that the oscillating die, which extends over the entire working width of up to 1000 mm, wipes the surface of the rollers clean. With this press, considerable pressures can be exerted. The strands or slabs emerge with constant speed that can be varied infinitely. Fig. 6.4-22 shows the Fig. 6.4-21 Diagram of a toothed roller press for the processing of solid, viscous, or, generally, plastic masses to which coarse components can be admixed (courtesy Hosokawa Bepex, Leingarten, Germany)
Fig. 6.4-22 Photograph of the top side of a toothed roller form-press, featuring a clear feed hopper that allows a view into the nip of the toothed rollers (courtesy Hosokawa Bepex, Leingarten, Germany)
6.4 Applications in the Food Industry
top side of such a form-press, featuring a clear feed hopper that allows a view into the nip of the toothed rollers. For some materials and applications, the oscillating motion of the extrusion die is a disadvantage, as it may make the deposition of the extrudates, that require a certain strength to avoid breakage, onto the following equipment difficult. To overcome this problem, the rotary bar roller press is now often preferred. Fig. 6.4-23 shows the principle of this design. Specially shaped bars are inserted in semi-circular grooves in the rollers and held in position with a lip exposed by spring action. In this situation, similar to the action of toothed rollers, the feed material, which is again solid and viscous (plastic) and may also contain coarse components, is pulled into the nip, transported and pressurized, and extruded through interchangeable, differently shaped die plates. Fixed scrapers clean the body of the rollers and bars by momentarily turning the latter ones. After cleaning, the bars return immediately to their basic position. Very high pressures can be produced with this press. The compression ratio and the extrusion speed are adjustable and can be varied infinitely. The two rollers must not necessarily interact with each other. This is depicted in Fig. 6.4-24. Each rotary bar roller operates in a partially closed barrel and is physically separated from the other one. On the feed side a grooved roll assists in feeding, creating a nip from which the material is transported along the barrel wall into a pressure chamber below; as usually, the rotary bar roller is cleaned by a fixed scraper. With this design principle, more than two press rollers and associated feeders can be combined
Fig. 6.4-23 Design principle of a rotary bar roller press (courtesy Hosokawa Bepex, Leingarten, Germany)
Fig. 6.4-24 Diagram of the design of a double rotary bar roller press (courtesy Hosokawa Bepex, Leingarten, Germany)
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(below). Extrusion is accomplished in sheets or ropes. Particularly in the latter case, filled or multi-layer products can be made. Fig. 6.4-25 shows two possible nozzle types for the manufacturing of double layered and hollow strands. Fig. 6.4-26 is a collection of diagrams of cross sections through ropes from single, double, and triple rotary bar roller presses. All rollers can be up to 1300 mm wide and accommodate a multitude of nozzles for strand forming (Fig. 6.4-27). Wide sheets can be multi-layered by extrusion through slots in multiple rotary bar roller presses or by laying several slabs on top of each other (Fig. 6.4-28, Fig. 6.4-31a). Fig. 6.4-29 shows single, double, and triple rotary bar roller presses with “standard” (cantilevered) drive.
Fig. 6.4-25 Two possible nozzle types for the manufacturing of: a) double layered; b) hollow strands in a double rotary bar roller press (courtesy Hosokawa Bepex, Leingarten, Germany)
Fig. 6.4-26 Diagrams of the designs of single, double, and triple rotary bar roller presses and collection of cross sections through ropes that may be manufactured (courtesy Hosokawa Bepex, Leingarten, Germany)
6.4 Applications in the Food Industry
Fig. 6.4-27 Nozzles on a double rotary bar roller press for the forming of: a) filled ropes, b) flat-single, c) double-layered strands (courtesy Hosokawa Bepex, Leingarten, Germany)
Fig. 6.4-28 Two situations in which a second sheet is deposited on top of a previously made slab (courtesy Hosokawa Bepex, Leingarten, Germany)
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Fig. 6.4-29 Photographs of three cantilevered rotary bar roller presses: single (DP 200-800), double (DDP 200-1000), and triple (DP/3 250-300) formers (courtesy Hosokawa Bepex, Leingarten, Germany)
6.4 Applications in the Food Industry
The model numbers represent roller diameter and width in millimeters. All machines are executed in stainless steel and feed hopper, housing, side walls, rollers and die plates or nozzles are double walled and can be tempered (heated or cooled) independently of each other. Individual snack bars can be made either by cutting ropes (Fig. 6.4-30a) of by slitting slabs and cutting the strips (Fig. 6.4-30b). During slitting and cutting, the resulting strips and bars are automatically separated to avoid sticking together. Fig. 6.4-31a is a close-up photograph of a triple-layered food bar and Fig. 6.4-31b and c show other snack bars, including some that are coated with chocolate or very coarse food particles. Since the equipment following the extrusion is associated with belts (Fig. 6.4-30b), the roller presses are often executed in a bridge type fashion (Fig. 6.4-32), called mill shaft design in high-pressure roller presses [B.48, B.97]. An actual combination of formers, one bridge type and the other cantilevered, and a cutter are depicted in Fig. 6.4-33. Other equipment is available that can be added to complete lines, including: *
*
*
Smooth roll formers for the extrusion of pressure-sensitive or aerated masses that must be processed in viscous, pasty, kneadable, or sticky consistency; the forming system works without pressure according to the dragging principle (Fig. 6.4-34). Shape formers that are installed after, for example, rotary bar roller presses; they are two-roll formers (lower part of Fig. 6.4-35a) in which the roll surface carries molds that correspond to the product shape (Fig. 6.4-35b). This procedure is applied for cookies and articles that are difficult to produce with other systems. The shaped products are taken off with a special belt and transferred to a subsequent transport belt. Coolers (Fig. 6.4-30b), coaters (Fig. 6.4-30b), tunnel ovens for baking, enrobers and decorators (Fig. 6.4-36), and packing units.
Fig. 6.4-37 is a summary of some of the many different foods that can be shaped by or based on low-, medium-, and high-pressure agglomeration (extrusion). Fig. 6.4-30 Production of individual bars: a) by cutting ropes, b) by slitting slabs and cutting the strips (courtesy Hosokawa Bepex, Leingarten, Germany). In (b) are shown: (5) mixer/conditioner, (6) smooth roller former, (7) cooling drum, (8) rotary bar roller press, (9) cooling tunnel, (10) strand slitter, (11) fanning (separation) belt, (12) cutter, (13) tempering unit, (14) coater, (15) cooling tunnel
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Fig. 6.4-31 Close-up of a triple layered food bar and other snack bars, some coated with very coarse food particles (courtesy Hosokawa Bepex, Leingarten, Germany)
Originally, the equipment described above was developed for sweets, such as marzipan (egg-, roll- or bar-shaped), but with the advent of convenience, designer, fun, and functional foods, which are often offered in the form of bars or other shapes, its application has widened and is now a major manufacturing tool of small and large, multinational manufacturers of foods. Tab. 6.4-6 is a listing of some of the materials that are processed with roller extrusion presses and associated equipment in the food industry. It shows that, today, not only confectionery products (sweets) but many others that were not extruded, are industrially handled and converted by these methods, mostly into convenience foods (ready to prepare or eat). Of course, many of the foods listed in Tab. 6.4-6 are not made-up or do not contain solid particles in large quantities and, therefore, even in the widest sense, can not be considered agglomerates. They are mentioned to show the versatility of the technology and tendencies in modern food processing and preparation. Other low-, medium-, and high-pressure extruders are also sometimes applied for the agglomeration of food products. Many of the materials mentioned in Tab. 6.4-6 are processed in such equipment; however the relationship between machine vendor and food processor is often a close one and governed by secrecy agreements.
6.4 Applications in the Food Industry Tab. 6.4-6 Listing of some materials that are processed with roller extrusion presses and associated equipment in the food industry (in alphabetical order) Confectionary Aerated nougat Aerated sugar masses Candy creme Chewing masses Chocolate masses Coconut masses Cream masses Fat fondant Fruit caramel Fudge Hard croquant Honey nougat Liquorice masses Marzipan
Others Noisette masses Nougat Nougat Monte´limar Nut pastes Peanut brittle Peppermint toffees Persipan* Praline´ mixture Soft caramel Soft croquant Sugar pastes Nougat Truffle masses
Biscuit masses Cereal mixtures Cheeses Chewing gum Corn starch masses Crisp rice (w.binder) Diet food masses Doughs (all kinds) Fats Fish masses Fruit pastes Honey cakes Jelly masses
Liver pastes Meat masses Muesli Oat mixtures Peanut butter Popcorn (w. binder) Potato masses Protein masses Puffed rice (w. binder) Ricemix masses Semolina masses Vegetable masses Yeast
..... masses contain viscous components to produce plasticity Binders may be fat, chocolate caramel toffee, sugar, starches, etc. * Persipan is a marzipan substitute made with apricot kernels
Fig. 6.4-32 Roller extrusion press executed in bridge type design (courtesy Hosokawa Bepex, Leingarten, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.4-33 Combination of roller press formers, one bridge type and the other cantilevered, and a cutter (courtesy Hosokawa Bepex, Leingarten, Germany)
Fig. 6.4-34 Sketch of the principle of smooth roll formers for the extrusion of pressure sensitive or aerated masses (courtesy Hosokawa Bepex, Leingarten, Germany)
Fig. 6.4-35 Shape forming: a) two-roll formers installed after, for example, a rotary bar roller press; b) the roll surface carries molds that correspond to the product shape (courtesy Hosokawa Bepex, Leingarten, Germany)
6.4 Applications in the Food Industry Fig. 6.4-36 Enrobed and decorated food products (courtesy Hosokawa Bepex, Leingarten, Germany)
Fig. 6.4-37 Summary photograph of the some of the many different foods that can be shaped by or based on low-, medium-, and high-pressure agglomeration (extrusion) (courtesy Hosokawa Bepex, Leingarten, Germany)
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Several manufacturers offer pressure-cooker extruders [B.97] for the food industry. These machines apply medium pressure because the mostly grain and/or vegetable based starchy organic feeds are conditioned by pressure and heat into easily deformable and extrudable masses. Fig. 6.4-38 shows the functional components. At the inlet on top of the equipment, the slightly pre-mixed feed components enter first a mixing and predensification screw. Pressure builds-up by the changes in shaft diameter, sometimes variable pitch of the screw flights, and by a collar at the end of the screw shaft or a “pressure piece”, both forming an annular space with reduced area through which the material must pass. Such a screw conveyor, mixer, and processor is often called expander. The processed material drops into the cooker in which pressurized steam is injected and paddles or screws move the material to accomplish optimal contact. The plug at the end of the mixing and predensification screw acts as a dynamic seal so that the cooker is kept under pressure. Depending on the capacity and cooking time required for a specific application, the cooker vessel dimensions and/or number (multiple ones are mounted on top of each other) and the type of agitators may be changed. By varying the speed of the agitator(s) and the operating pressure in the cooker, almost infinite time/temperature combinations can be obtained. Pressurized steam cooking decreases work and power use, cuts production costs, and increases production capacity as much as 50 % compared with other extruders that accomplish heating by the conversion of mechanical into thermal energy through friction. The pressure (steam) cooker accomplishes much of the work that is conventionally done by the extruder; therefore, the life span of the extruder screw, inserts, barrel, die plate, and bearings is significantly increased. The cooker is easy to maintain and operate and, because the agitators are of simple design, these may be rebuilt numerous times to regain critical clearances. The processed mass is transferred into the screw extruder where hydrostatic pressure is developed, which causes axial extrusion through the openings of the die plate. The orifices produce extrudates, which often feature different cross sections, for processed cereals, for example, tubes. The ropes, featuring various cross sectional shapes, or, for example, tubes are cut with an adjustable and, for cleaning purposes, replace-
Fig. 6.4-38 Diagram of a typical pressure-cooker extruder with main dimensions in feet and inches: 1’ = 0.3048 m, 1” = 25.4 mm (courtesy Sprout-Matador, Muncy, PA, USA)
6.4 Applications in the Food Industry
Fig. 6.4-39 Some examples of food products obtained with pressure-cooker extruders (courtesy Sprout-Matador, Muncy, PA, USA)
able device yielding short pieces or rings (Fig. 6.4-39). The lumpy shape of the processed (often called “expanded”) mass can often be directly used in a post-treatment facility (for example, “puffing” snack pieces during drying). Pellet mills (Fig. 5-10b2–b6, Chapter 5) are used in the food industry for the size enlargement of coffee and tea meal, herb powders, yeast, products with instant characteristics (for example mixtures of cocoa and powdered sugar, effervescent drink powders), and others. The cylindrical extrudates are bagged and used mostly by institutional customers (large kitchens). The most common application of punch-and-die presses in the food industry is for the production of bouillon cubes (Fig. 6.4-40) with dimensions typically in the range 13–15 mm. To obtain efficiency, rotary presses, so-called “cubers”, are used. The multi-component mass that results from mixing salt, sweeteners, binders (such as hydrolized corn gluten, modified corn starch, yeast), emulsified cooked meats, flavors, preservatives, various amounts of fat, and water (moisture content: 45–55 %) in a blender, is dried to < 2–3 % moisture with a belt dryer. The dry porous cake is broken and screened into three fractions. “Overs” are recirculated to the beginning of the process, the fraction between 2–5 mm is sold as granular product with “instant” properties (due to the particle’s high porosity, Fig. 6.4-40, bottom right), and fines are cubed. Since the small feed particles to the cuber contain substantially fewer pores and are deformed under the high pressing force to yield dense cubes, dissolution requires hot liquid, stirring, and a long time (several minutes). With modern cooking requirements such dissolution behavior is no longer acceptable. Therefore, bouillon cubes may now
Fig. 6.4-40 Cubed and granulated beef bouillon, both with “instant” characteristics (courtesy Borden Foods/Wyler’s, Columbus, OH/Chicago, IL, USA)
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contain organic fibers, which act as conduits for liquid, swell, and assist in the product disintegration, or starches and/or their derivatives as disintegrants [B.97]. Other, mostly rectangular, compacted cooking ingredients, such as concentrated gravy, sauces, and soups are formulated and produced in similar ways. Fines separated from crushed salt, sugar, and other crystalline food products tend to stick together and cake. For that reason, for a long time until recently, they were redissolved and reprocessed in crystallizers. When high-pressure roller press manufacturers began to look for alternative applications for their equipment and started building clean, pharmaceutical and food grade machines (totally enclosed, stainless steel execution, easy cleaning), compaction/granulation (Fig. 6.1-14, Section 6.1) became a feasible alternative for size enlargement. Because the granules, the size of which can be adjusted as desired, are made from fine particles, they are not as hard as crystals of the same size and are, therefore, particularly suitable as sprinkling sugar for cookies or pretzel salt. Very large granules are made as fishery salt for use on fishing vessels. The same relative softness of agglomerated granules from fines as compared with crystals of the same material and size, even if made by high-pressure compaction/ granulation, make them ideal feed materials for secondary compression in punchand-die presses. The material is dust-free, easily flowing, and, therefore, can be metered well by and into all equipment, including high-speed rotary machines. During compression, the granules disintegrate and result in a uniform structure of the final compact. To avoid segregation, additives, such as flavors, colorants, or vitamins, can be incorporated in the first granulation step. As already indicated in Section 6.4.1 (Tab. 6.4-4, A4), contrary to common belief, granules obtained from press agglomeration may have instant characteristics. One of the most important binding mechanism of high-pressure agglomeration is caused by van-der-Waals forces (Chapter 3). This short-range molecular attraction does not develop solid bridges between the agglomerate forming particles and is lower in liquid environments by a factor of about 10, causing such granules to disperse easily and quickly. The addition of organic fibers, which, in foods, may at the same time constitute dietary ballast components, and/or of disintegrants [B.97] further improves the product’s instant properties. To further demonstrate the varied uses of size enlargement in the food industry, a relatively recent development shall be described that has led to the recovery of dust, which was previously considered a waste and discarded. After grinding roasted coffee beans, the product must be dedusted to meet customer expectations. Fines at the bottom of bags with ground coffee are not accepted. On the other hand, the large specific surface area of this dust results in almost instant brewing results, if it could be offered in an acceptable form. The solution of the problem is to produce a thin (< 1 mm) sheet by pressure agglomeration between smooth rollers in a modified flaking mill and crush it to yield flakes. With these, filter pockets are filled and bags are produced, called “coffee singles” by the manufacturer, which are similar to what was already known and available for (loose) teas for a long time. To engineer the product further, freeze concentrated coffee and flavors can be added and marketed as “gourmet coffee singles” as shown in Fig. 6.4-41.
6.4 Applications in the Food Industry
Fig. 6.4-41 Examples of “gourmet coffee singles”, single portion filter bags filled with flaked and flavored roast coffee fines and freeze-concentrated coffee (Folgers Coffee/Procter & Gamble, Cincinnati, OH, USA)
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6.4.3
Other Technologies
Agglomeration by heat or, more generally, thermal agglomeration, is sometimes used for special applications in food or food related industries. The oldest thermal agglomeration method was used in large communal “ice houses” that were built for the long-term storage of perishable foods such a meats and vegetables. Since large amounts of energy had to be used for freezing water, it was economical not to let small chips and fines, which were produced during a number of processing steps (crushing), melt. One of the reasons for crushing ice was to produce smaller pieces for the packing of foods in preparation for shipping in insulated containers. It was found that if supercooled (< 0 8C) ice pieces are compressed, the conversion of mechanical energy into heat causes roughness peaks and a thin surface layer to melt momentarily and to produce a liquid phase resulting in liquid bridges between the ice particles. However, because the amount of liquid produced is very small, typically less than 1 %, and the bulk of the mass remains at a temperature that is substantially below freezing, immediately after pressure release (when the energy supply ceases) the liquid solidifies, bonding the shaped body into an ice piece with high density. Since relatively small, briquetted, almond or pillow-shaped ice can be easily poured and metered, it became a superior cold-packing material. Roller presses were preferred for this operation because the thermal contraction of machine parts does not cause operational problems. Dry ice, compressed and shaped solid CO2, is an even better material for cold packing because it evaporates rather than melts and its cooling capacity is almost three-times higher than that of water ice. Dry ice compacts are typically made with hydraulic presses and are still used today. Today, many vegetable and fruit coulis are available in frozen blocks. After the food blocks (dimensions: 500 mm 500 mm 100 mm, temperature: –20 8C) have been milled to < 10 mm, compacts are made with a roller press (Fig. 6.4-43), as depicted in the inset sketch. Briquettes, still at –20 8C, are good looking and stable. They can be identified with patterns, machined into the form pockets on the rollers by electrochemical milling [B.48], and offer a presentation apart from the usual packaging and a product that is easy to meter, quick to defrost, and useable at home, by caterers, in restaurants, and in field kitchens (Fig. 6.4-42). The roller presses are very simple (Fig. 6.4-43) and have been supplied for capacities of 2.5–15 t/h. Because the feed is very clean and does, by definition (food), not contain any foreign material, contrary to most other designs, these roller presses do not have a floating roller so that the area that is in contact with the food can be totally sealed against oil or grease and synchronization is accomplished with timing gears. During production, the machine head is enclosed with food grade sheeting and the gears are completely covered. Feeding is by gravity. Agglomeration of foods by heat is mostly limited to the production of bonding between often large food particles (such as nuts). Sugar mixed with such materials is partially melted and caramelized, producing the desired bonds and forming clusters. At the same time the process enhances the taste. This is a very old method
6.4 Applications in the Food Industry
Fig. 6.4-42 Reproduction of a leaflet showing frozen food pulp briquettes and the rendition of a reconstituted meal (courtesy Sahut-Conreur, Raismes, France)
of making candy, originating in the Arab world, widely practiced by street vendors and at fairs. Nuts and sugar are stirred by hand in a metal bowl over charcoal fire or burners until the caramelization has proceeded to the point where clusters are formed. After cooling, so that sticking between the product pieces is minimized, the candy is packed and sold.
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Fig. 6.4-43 Photograph of a special roller press for the briquetting of food coulis (capacity 2.5 tonne/h) and sketch of the briquetting process (inset) (courtesy Sahut-Conreur, Raismes, France)
Coating of food products is performed most frequently with enrobers whereby food bars of the fun food category receive a layer of, for example, a chocolate mass (Section 6.4.2, Fig. 6.4-30b, items 13 and 14). This process is, however, not an agglomeration technique. Liquid coating mass is applied to the surfaces of the food article and solidified in a cooling tunnel. True coating by agglomeration, for example with powdered cocoa or sugar, may be carried-out in a coating drum or a Wurster type coater [B.97]. This technique is used for the finishing of small compacts (tablets), called cores, which are tumbled or fluidized while the powdered solids are added, binder liquid is atomized to cause adhesion to the surfaces of the cores, and warm air flows through the mass to evaporate the liquid and strengthen the powder layer. Rubbing against and collisions with each other densify and smooth the coating (Section 6.2.3).
6.5 Applications for Animal Feeds
6.5
Applications for Animal Feeds
When humans began to keep certain animals as pets and later raised a growing number of different animal species for work or transportation and as a source of food, clothing, and many other uses, the necessity of feeding independently of natural availability and the seasons arose. In addition to the production of hay, many animals shared human food scraps with their keepers. However, certain species, for examples the trained elephants in Southeast Asia, where so much removed from their natural food sources and foraging habits that the mahouts produced large balls of plant material and minerals, sort of giant hand-made agglomerates, which are fed directly into the elephant’s mouth, a method, which can still be observed today. Until the middle of the 19th century natural products, mostly plants and plant seeds, prevailed in animal feeding. The raising and keeping of animals was decentralized and concentrated near large human settlements. Only a few products, such as cheese, dried or cured meats, and processed eggs, were suitable for longer time storage and large distance transportation. The same reasons (i.e., improved long distance transportation and refrigeration) as described for food production (Section 6.4) triggered a change in livestock farming and feeding. The animals were now raised in large numbers at geographically favorable locations and their products transported to, stored near, and distributed to a large number of consumers. In addition, at approximately the same time, more knowledge about the digestion of feedstuffs by animals and the need for minerals and other ingredients for animal health and development became known; later the addition of vitamins, and, more recently, the desire to include medications and the controversial inclusion of hormones, became common practice. All that led to the formulation of today’s concentrated or compound feeds. Also at approximately the same time, large food processing and packing facilities were developed, which produced huge amounts of byproducts from plants, vegetables, and animals. In addition to milled feed grains, the major source of starch, these materials became excellent sources of nutritional, highly digestible components for livestock feeds. However, because of their physical consistency these very desirable feed components are not accepted by many animals in their finely particulate, suspended, or dissolved state. Suspended and dissolved byproducts had to be dried and, together with other fine solid components, size enlargement became necessary or desirable for many applications. Concentrated feed is formulated with the specific needs of each animal group and the requirements of the particular industry in mind, whereby growth (fat and/or muscle development), endurance, subsistence, and special performances (e.g., high rate of egg production) are encouraged and supported. Since, in comparison to the basic nutritional feed components carbohydrates, proteins, fats, amides, fibers, and water, the relative gravimetric and volumetric amounts of additives, such as minerals and vitamins, are very small, uniform distribution in the bulk feed became a challenge (Section 6.2.1) and segregation a problem. It was also found that certain animals did
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not take-up and eat feed additives if they where supplied as separate entities, even if they were processed for easy consumption. Smell and/or taste causes the animal to avoid such compounds. All these reasons led to the evolution of a large, highly efficient industry in which socalled feed mills formulate and process special nutriments for different groups of animals. While, at the beginning, this development was centered around the traditional domestic livestock, mammals and fowl, today’s products include feeds for new food production technologies, for example fish farming, where fish or shrimp are raised and fed for optimum results. Another important new market is pet food. Tab. 6.5-1 is a summary of the most important reasons for size enlargement in the animal feed industry.
6.5.1
Tumble/Growth Technologies
The first applications of agglomeration in animal feeding were in veterinary medicine for the treatment of sick animals. Remedies were mixed with a feed base from which pills and later tablets were made (Section 6.2) that were dispensed to the keeper (later farmers) and fed to the animals. This technology is almost as old as the manufacturing of agglomerated medicines for humans. For mammals, milk is nature’s food for the offspring and, therefore, represents one of the best and richest nutriments for this animal group, including humans. Reduced fat milk from which only cream has been skimmed and, to a lesser degree, whey from Tab. 6.5-1 Summary of the most important reasons for size enlargement by agglomeration in the animal feed industry *
* * * * * * * * * * *
* *
* * *
Conversion of mixtures of feed compounds and additives with different particle sizes and shapes, specific weights, hygroscopicities, and physical states into a stable mixed formulation. No losses and no dust annoyance (compliance with emission control laws). Improved flow, storage, transportation, metering, and feeding behavior. Avoidance of the segregation, separation, or selection (by the animals) of feed components. Better acceptance of the feed by animals. Possibility to prepare species specific forms of feed (e.g. for chickens, fish, etc.). Prevention of lumping, build-up, and bag-set. Reduction of losses by oxidation and other reactions during storage. Improved pickup of liquids added during feeding. In some cases, improved dispersion and solution in liquids. Possibility to include a wide variety of waste materials, including liquids, with nutritional value. Particularly after conditioning in the so-called expander and pelleting, improved availability of nutrients for digestion. Increased starch-value of the feedstuff. Lower moisture requirement for (pressure) agglomeration (pelleting) after preconditioning; therefore, reduced energy requirement for drying. Specifically for pet food, pleasing appearance of the product for the buying consumer. Increased economy of the manufacturing process. Higher profit margin.
6.5 Applications for Animal Feeds
which fat and coagulated milk solids have been removed (mostly for the manufacturing of cheese) are still highly nutritional feeds. Nevertheless, they are processed for a number of reasons (Section 6.4.1). Animal feeds contain mostly carbohydrates, proteins, fat, starch, and organic fibers, all of which can spoil with time. Particularly all types of milk and whey degenerate due to the activity of bacteria destroying the nutritive value unless it is kept at low temperatures or preservatives are added. Removal of water from the product will also reduce the bacterial activity and, thus, ensure an almost infinite shelf life if the product is dry enough and kept in a sheltered, cool place. A common animal feed that is processed by growth agglomeration is milk replacer. This product is a replacement for whole milk in which the butter fat, removed for mostly human consumption, is replaced by a cheaper animal or vegetable fat. It is typically used to feed calves, chickens, and pigs. Calf milk replacer is primarily used for breeding, as, at birth, calves have underdeveloped stomachs, lack the ability to digest fibrous feed, and must receive a liquid diet. Weaning newborn calves off cows milk frees it up for other diary based operations, such as processing milk for human consumption or conversion to more valuable products, for example butter and cheeses. Furthermore, all replacers are for fattening of animals, including calves. It is possible to make modifications of components in the mix and supply unique nutrients that are normally not present in whole milk. Therefore, however, the finished product has no fixed composition, as shown in Tab. 6.5-2 [6.4.1.1]. Milk replacers may be in liquid or dry powder form. The latter offers various benefits when compared with liquids; they cost less to transport, which helps improve the overall efficiency of diary production, and, as mentioned above, microbial growth during storage is much reduced. The farmers desire powdered milk replacer that are stable until reconstitution (rehydration, dispersion and dissolution in water), undergo little or no fat separation prior to, during, or after hydration, are easily and quickly processed in cold or hot water, feature an optimum fat particle size distribution for easy digestion by the animal, and exhibit no or very little lumping during storage prior to its application [6.5.1.1]. To fulfill the above requirements, manufacturers of milk replacers seek to obtain the following: high wettability, high dispersability, low sedimentation tendencies (little residue or sediment after reconstitution), and stability of the fat-in-water emulsion. Referring back to Section 6.4.1, Tab. 6.4-3, the first three are characteristics of instant products. In addition, the stable fat-in-water emulsion is maintained with the help of emulsifiers. Tab. 6.5-2
Typical ingredients and range of compositions of milk replacers [6.4.1.1] %
Skim milk solids Fat Dextrose, lactose, possibly whey solids Emulsifying agents (lecithin, mono-glycerides, sucro-glycerides) Flour Minerals, vitamins, antibiotics (Ca, Na, Mg, Cu, vitamins A, D, E)
65–80 15–20 0–10 0–2 0–7 0–0.5
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The two major components (Tab. 6.5-2) must be brought together and agglomerated. Originally, the loading of skimmed milk powder with fat was accomplished by spraying fat with a nozzle onto powder passing on a belt conveyor. However, this product had fat exposed on the surface, did not have good shelf life, and could not be reconstituted easily. Later fat was sprayed into an airstream that carried skim milk powder. In this process, milk particles adhere to and cover the fat droplets. Although this milk replacer behaves better during storage and reconstitution, the fat particles are much too big, causing poor storage properties and indigestion for the animals. Other processes used contacting a pre-made fat compound and skim milk powder in a tower with steam [B.97], secondary agglomeration of the warm and moist mixture with water in a tumble agglomeration unit, cooling, and sizing. This method suffers from inconsistency and inefficiency and does not accommodate flexible incorporation of additives. Until recently, the best product was produced by spray drying an emulsion of skim milk concentrate and fat, followed by agglomerating and/or cooling in a vibrating fluidized bed. In many ways, a spray/dryer/agglomerator/cooler system for, for example, the manufacturing of calf milk replacer (Fig. 6.5-1) resembles a plant for dry, instant milk (Section 6.4.1). Mixing (homogenizing) of liquid fat (about 65 8C) and concentrated (40–45 % solids, 65 8C) skimmed milk, producing the emulsion, is either accomplished discontinuously in mixing tanks or continuously in a “combinator” [6.4.1.1]. In both cases, the purpose of homogenizing is to reduce the size of the fat droplets (above) with an aim of < 3 lm and a minimum of 90 % less than 1 lm.
Fig. 6.5-1 Spray dryer/agglomerator/cooler system for the manufacturing of calf milk replacer (courtesy Niro AS, Soeborg, Denmark)
6.5 Applications for Animal Feeds
As documented in the patent literature [6.5.1.1], there is a constant effort among manufacturers of milk replacer to produce a material that better meets the aforementioned requirements. In a new process [6.5.1.1], a powdered nutritional composition, with components corresponding to the range suggested in Tab. 6.5-2, is introduced into a mixer. This machine is a modified Schugi Flexomix (Section 6.4.1, Fig. 6.4-17, and [B.48, B.97]). One or more agglomerating and emulsifying agents are also added to the mixer. The feed powder blend is agglomerated whereby its particle size is increased and forms a moist intermediate product, which is dried and cooled. Care is taken to maintain the shape and size distribution of the agglomerates during post-treatment. The milk replacer is then classified to obtain the final product for packaging. The mixer has been modified to deliberately produce agglomerates with a relatively large particle size of 200 lm and larger. Contrary to competitive milk replacers, which may initially feature even larger but friable agglomerates, the particle size in the new product is substantially or fully retained until the time of reconstitution and helps (due to large size and no fines) to enhance the hydration characteristics. This is caused by a rather continuous coating that is formed on and between the particulate components and on the exterior of the agglomerates. Fig. 6.5-2 shows SEM micrographs at two
Fig. 6.5-2 SEM photographs of milk replacer produced by: 1) conventional method, 2) new process [6.5.1.1] at two different magnifications (a and b) (courtesy Land O’Lakes, Inc., Arden Hills, MN, USA)
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different magnifications (a and b), of milk replacer powder produced by a conventional method and the new process [6.5.1.1]. The non-uniform and discontinuous nature of the bonding and coating of an agglomerate made by the old method is visible at high magnification. At lower magnification, showing a larger area, the loose attachment of particles and the many voids, gaps, and crevices can be distinguished even better. Components are rubbed-off easily during packing and handling, causing disintegration of the agglomerates and the formation of dust. In contrast, product from the new process is completely enrobed by a continuous coating whereby the coating material is selected to allow quick and complete dispersion of wetted particles in lukewarm (about 45 8C) tap water. A number of animal feed compositions, often containing fat, proteins, vitamins, antibiotics, and special nutrients, normally not found in natural products, are granulated for grain-eating animals, particularly chicken and other farm or pet birds. Since, during agglomeration, the moist mass often becomes sticky, special blenders with various self-cleaning mixing tools [B.97] are applied. Birds of different size pick-up only narrowly sized grains that are typically related to the bird’s proportions: small ones require fine and large ones coarse agglomerates. Accordingly, even though the feed composition may be identical, different fractions may be produced and sold.
6.5.2
Pressure Agglomeration Technologies
At the beginning of the 20th century, the fast growing world population and the concentration of humans in industrialized centers of developed and developing countries required a new, highly efficient production and distribution of food. Animals in the food chain for milk, meat, and, later, also eggs, were no longer raised and processed on relatively small farms but in ever larger breeding and rearing facilities. Depending on the type, hundreds (cattle and swine) to several tens of thousands (chicken) of animals are kept in one location, needing to be efficiently fed. Also, in countries with extensive grass lands (e.g., USA and Argentina), cattle are raised in large herds and transferred to “feed lots” for fattening just prior to slaughter. In such facilities, specially formulated, highly concentrated feeds are provided to the animals for fast growth within only a few weeks. More recently, to counteract the overfishing of streams, lakes, and oceans and to satisfy growing demand, an increasing number of fish and shellfish species are raised in fish farms and fed with dry feed. As mentioned in Section 6.5, all these modern food production plants are no longer relying on naturally available animal feed sources. Research into the needs of the various animal groups for healthy, fast, and controlled (e.g., low fat or large eggs) development has led to the definition of preferred feed components and the development of complex rations, which do not only provide food but also guarantee animal health and the desired growth rate. The multi-component concentrated feed compositions consist of a few main ingredients and an increasing number of minor elements, which often also include antibiotics. As in pharmaceutical (Section 6.2) and similar products, it is important that
6.5 Applications for Animal Feeds
each morsel of food contains the same amount of all ingredients. To accomplish this, the basic components are ground to yield feed meal into which the lesser constituents are blended. Since only a few animals take up powdery feed and those who do tend to leave bad tasting ingredients behind, the blend must be stabilized by agglomeration to make it acceptable and the taste masked by suitable means. While wet granulation (Section 6.5.1) is possible in some cases, pressure agglomeration by pelleting (Chapter 5, Fig. 5.10 b1–b6) soon became the production method of choice [B.48, B.97]. The reasons for this preference are the easy preconditioning of the components, whereby the starchy ingredients are activated and the plasticity of organic materials is enhanced, the possibility to process materials that, because of their origin, still feature a certain elasticity, the reliable shaping of the blend and the production of feed pellets with adjustable physical, particularly strength and structure related properties. A recent study [6.5.2.1] estimates that the annual “world compound feed production is presently at about 600 million tons and is expected to reach 630 million tons by 2006”. Much of this is agglomerated, typically pelleted, which means that almost every feed mill on earth is equipped with one or more pelleting systems. It also means that by volume the agglomeration (pelleting) of animal feed is the largest application of the unit operation “size enlargement by agglomeration” followed by the pelletization of iron ores (Section 6.8.1) which, with an available capacity of just over 300 million tons in 1999/2001 [6.8.1.8], is a distant second. Pelleting of animal feed started at the beginning of the 20th century with the use of screw extruders, which had just been invented for the shaping of clays (Section 6.7.2). Fig. 6.5-3 is the reproduction of a brochure from the 1920s describing screw extruders (Chapter 5, Fig. 5.10b1) and showing a flowchart for the pelleting of animal feed meal blends. Shortly thereafter, responding to the growing demand, flat die (Chapter 5,
Fig. 6.5-3 Reproduction of a brochure from the 1920s describing screw extruders and a flowchart for the pelleting of animal feed meal blends (courtesy Amandus Kahl, Reinbek, Germany)
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Fig. 5.10b2) and cylindrical die (Chapter 5, Fig. 5.10b5) pellet mills and their designs were proposed by many inventors in short succession [B.48]. Although other designs were and, to a limited extend, still are offered for specific applications, the flat die design with muller-like press rollers (Fig. 6.5-4a) and ring dies with interior press rollers (Fig. 6.5-4b) are the dominant machines for the processing of animal feeds. Typically, both machines are equipped with integral feeder/conditioners (lower part of Fig. 6.5-4b). The largest parts of a feed mill (Fig. 6.5-5) are raw material receiving, storage, preparation, proportioning, and blending (left side of Figure 6.5-5) and the packing and shipping departments (right side of Figure 6.5-5). In the first the components are received, stored, and processed as required. Grains are milled to produce a meal and grasses in the form of hay or alfalfa and other dry plant materials are chopped to become suitable for blending with the major and/or minor ingredients. All components are then conventionally formulated by metering and mixing to form the feed to the pellet mill(s). The flowchart in Fig. 6.5-5 is a versatile plant for the production of 40 t/h of pellets. It is designed for the storage of a total of 400 ts of up to 24 dry and several liquid components. Compounding and mixing is carried out in 10 batches per hour of 4000 kg each. Two independently operating conditioning and pelleting lines, each equipped with one flat die pellet mill (Fig. 6.5-4a), allow the simultaneous manufacturing of two different animal feed formulations. Correspondingly, the 20 product silos are arranged in two groups, each with two loadout points for bulk carriers. Alternatively, bagging systems can be added if desired.
Fig. 6.5-4 Diagrams of the principles and designs of the two dominant types of pellet mills for the processing of animal feeds: a) flat die pellet press (courtesy Amandus Kahl, Reinbek, Germany); b) ring die pellet press, also showing integral feeder/conditioners in the lower part (courtesy CPM, Waterloo, IA, USA)
6.5 Applications for Animal Feeds
Fig. 6.5-5 Flow sheet of a typical feed mill (Courtesy Amandus Kahl, Reinbek, Germany): 1) receiving, 2) silos, 3) proportioning and weighing, 4) premixing, 5) grinding and mixing, 6) conditioning and pelleting, 7) liquids storage and metering, 8) coating (Rotospray), 9) product storage and loading, 10) miscellaneous support functions, 11) electrical and control equipment
Fig. 6.5-6 is the flow diagram of a pelleting system using a ring die pellet mill (Fig. 6.5-4b and 6.5-7) and a vertical cooler (Fig. 6.5-8) [B.3, 1971]. The pellet mill (Fig. 6.5-7) is fitted with a variable speed screw conveyor, which is necessary to provide an even feed to the conditioner. This machine is a continuous, flow-through mixer that is equipped with fixed or adjustable pins or paddles (lower part of Fig. 6.5-4b). The purpose of the conditioner, which is fitted with steam and liquid injection manifolds, is to prepare the materials properly for optimum pelleting results. Conditioning is almost always accomplished by the addition of controlled amounts of constant-quality process steam, which supplies moisture for lubrication and softening of plant material, liberates natural oils, and causes the partial gelatinization of starches. Viscous liquids, such as molasses, may also be added as nutrients and binders just prior to extrusion. In some cases the lubricating, plasticity, and binding characteristics of the dry blend are adequate for successful pelleting. In such few cases, the conditioner (Figure 6.5-7) may be omitted. The conditioned, warm, moist, plastic, and sticky feed is transferred to the extrusion section of the pellet mill. For good operation and product quality it is imperative to distribute the feed uniformly over (flat) or in (ring) the die; this is one of the major difficulties, which is addressed and solved by various distributor designs [B.48, B.97]. Since densification and shaping of the feed is due to the frictional resistance during
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Fig. 6.5-6 Flow diagram of a pelleting system using a ring die pellet mill and a vertical cooler [B.3, 1971]
6.5 Applications for Animal Feeds Fig. 6.5-7 Components of a ring die pellet mill [B.3, 1971]: 1) variable speed screw conveyor, 2) conditioner, 3) ring die and roller (extrusion) section, 4) speed reducer, 5) main motor, 6) machine base
extrusion and a certain amount of moisture is required for lubrication, the pellets as produced are warm (70–95 8C) and feature a moisture content of up to 18 %. Both are too high. Therefore, in a pellet cooler (horizontal [B.48, B.97]) or vertical (Fig. 6.5-6 and 6.5-8) design) ambient air is pushed through a bed of pellets affecting at the same time cooling and drying. In some cases, particularly with the more complex and sensitive mixtures of modern animal feeds (below) a hot air dryer is installed first.
Fig. 6.5-8 Schematic of a vertical pellet cooler [B.3, 1971]: 1) feed hopper with level sensing device, 2) cooling columns, 3) plenum or air chamber, 4) discharge gate drive motor, 5) discharge star gates, 6, 7) air fan with drive motor
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Fig. 6.5-9 Pellet mill floor of a conventional animal feed mill employing ring die machines with conditioners (courtesy CPM, Waterloo, IA, USA)
Many feed pellets are sold in bulk or packed and offered to the animals as cylindrical extrudates. However, it is also possible to install so called crumblers, often mills with fluted of corrugated rollers, to produce granules, mostly for fowls. Depending on the specification, such feed may be screened to remove fines. The latter are recirculated to the mixing area of the feed mill for reuse. Although the need for animal feeds is constantly growing, the conventional pelleting as described above and shown in Fig. 6.5-5, 6.5-9, and 6.5-10 is overbuilt and new capacities are not as frequently built as in the past. The feeds processed in these mills are the basic staples, which include [B.3, 1971] the following. *
*
*
*
High-grain complete feeds (50–80 % grain, 12–25 % protein). These are high-starch formulas requiring conditioning to reach high moisture content and temperature. Heat-sensitive feed with sugar, dried milk, or dried whey (5–25 %). Since sugar and milk products begin to caramelize around 60 8C, the heat of friction must be kept low by employing thin dies, low speed, and adding fat and/or water as lubricant/ coolant. High natural protein (25–45 %) supplements and concentrates. Commonly these feeds also contain 5–30 % molasses. Similar to high-grain complete feeds they require high heat but less moisture addition. Low-protein (12–16 %) complete feeds. They contain little grain and high amounts of bulky fibers. Since these feeds can accept only a small amount of moisture, addition of steam must be limited resulting in low moisture and temperature of the conditioned feed. Consequently, the manufacturing of high quality pellets is difficult.
6.5 Applications for Animal Feeds Fig. 6.5-10a Flat die pellet mill in one of the production lines shown in Fig. 6.5-5 (courtesy Amandus Kahl, Reinbek, Germany)
Fig. 6.5-10b Multiple flat die pellet mills in a conventional animal feed mill (courtesy Amandus Kahl, Reinbek, Germany)
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High-urea (6–30 %), high-molasses (5–20 %) feeds. This type, particularly with higher concentrations, is very difficult to extrude, requiring little or no steam, thin dies, low speed, dusting of the pellets to remove stickiness, and drying prior to cooling.
The above indicates that, while a feed mill may be equipped with one or more pellet mills that are selected to handle the production capacity with the most common feed type, conditioning, die design, extrusion speed, and, potentially, post-treatment must be variable. This is accomplished by controls, variable speed drives, the availability of various dies, and an adaptable flowchart with diverters (circles in Fig. 6.5-6) for different process path selections. Installation of redundant equipment helps to chose the most effective system and maintaining production during machine modifications and maintenance. As already mentioned above, research into the needs of various animal groups for healthy, fast, and controlled development has led to the definition of preferred feed components and the development of complex rations, which do not only provide food but also guarantee animal health and growth. This is particularly true for feeds for specific animal farming technologies, such as the already traditional feed lot formulations for cattle or the more recently formulated nutrients for fish and shrimp farming, and for pet foods. Regarding pet food developments, pelleted dry dog food is an example [6.5.2.2]. Fig. 6.5-11 shows that the research into the nutritional needs of dogs results not
Fig. 6.5-11
Analysis of nutrients in extruded dry dog food [6.5.2.2]
6.5 Applications for Animal Feeds Tab. 6.5-3 Typical generic recipe of a high quality dry dog food suitable for pelleting [6.5.2.2] 48 % 12 % 5% 8% 8% 6% 4% 2% 2% 3% 2%
wheat or flour fish meal meat meal soy flour corn (maize) wheat bran (beer) yeast dry extracted sugar beet slices potato flour lime and minerals minor ingredients (e.g. drugs, colors, flavors, etc.)
only in a best composition but also reveals that the percentages of the main ingredients and the amount of energy that is made available should be modified according to dog age and size. Dogs are also carnivores and in contrast to, for example, swine have a digestive system that is about 2/3 shorter. Although, theoretically, dogs may be nourished with food components that originate from materials other than meat and bones, the formulation must be energy rich and easily digestible. Tab. 6.5-3 is a typical generic recipe of a high-quality dry dog food suitable for pelleting. To make this mixture easily digestible, all particles, particularly the plant-based ones must be ground to < 0.6 mm. The large surface area of finer particles facilitates the pre-gelatinization of the starchy components (Fig. 6.5-12) which, in turn, increases digestibility. Pre-gelatinization itself is accomplished in the conditioner (called expander [B.97]) by heating (with steam) and moistening. Originally, the warm and moist dog food formulation was pelleted in one of the traditional pellet mills as described above for the conventional types of animal feeds. More recently it was found that the expander itself can be modified to produce pellets by installing a hydraulically adjustable die plate with knife head (detail in Fig. 6.5-13), thereby effectively becoming an extruder [B.97]. As in the case of dog food, research into the nutritional needs of other pets and of farm-raised animals, also taking into account animal sizes and ages, led to the requirement to produce a large number of different formulations for optimal care (pets) and
Fig. 6.5-12 Starch gelatinization of pet food as a function of particle size [6.5.2.2] characterized by the diameters of the openings in the discharge screen of a hammermill (Section 6.1). “Special” refers to a screen producing particles < 0.6 mm
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Fig. 6.5-13 Flow diagram of a modern animal feed extrusion system (courtesy Amandus Kahl, Reinbek, Germany)
growth (farming) in relatively small quantities. To accomplish this in an efficient, profitable way, the bulk processing conventional feed mill is no longer suitable. New systems must be small, versatile, and quickly adaptable to new formulations. Fig. 6.5-13 shows the flowchart of such a manufacturing plant. Production begins with the exact proportioning of solids that have been ground to the optimal particle size (above) and are made available from silos with feeding screws, and of liquids, steam, and water into a blender. After mixing, the material is transferred to a holding tank (conditioner) with bottom agitator/discharge feeder. There, during a residence time of about 10 min, the liquids penetrate into the solids. A screw conveyor feeds the tubular expander–extruder [B.97] that is equipped with double walls for heating or cooling and a hydraulically adjustable die. With the latter, blockages are avoided or can be removed quickly. Some designs are fitted with multiple dies for quick change [B.97]. However, in any case, even without this provision, die change of expander–extruders is fast and can be carried out without tools. Following the extrusion, several alternative flow paths are possible. In most cases, because the pellets from expander–extruders are relatively wet and have low green strength, the discharge is carefully positioned onto a belt dryer, which dries-off excess moisture at minimum mechanical stress while the final binding mechanism is developed (e.g., by recrystallization of dissolved substances, increased viscosity of molasses,
6.5 Applications for Animal Feeds
or chemical reaction between, for example, molasses and lime). Screening of the dryer discharge to remove fines is optional. If used, the fines together with dust from dust collection are recirculated to the blender. Coating of screened pellets is used when specific feed characteristics are desired (below). If the product is still too warm, cooling (with a belt dryer) may be employed. Feed for chicken, other fowl, and fish or shrimp must be granulated (crushed) by a crumbler followed by fractionating into different sizes (alternative on the lower right of Fig. 6.5-13). According to the manufacturer (Amandus Kahl, Reinbek, Germany, see Section 15.1), the expander–extruder operation allows starch modification (pre-gelatinization) of 80– 90 % for good digestibility, fat contents of 20–30 % for high energy feeds, and considerable water binding (200–300 %) prior to extrusion resulting in high porosity after drying. The latter is of importance if, to increase the water uptake of animals, the pellets must be soaked prior to feeding but retain sufficient stability. Depending on die design, the cross section of the pellets may be round, oval, cloverleaf, bone shaped and so on (Fig. 6.5-14). Final feeds may be blends of different products (Fig. 6.5-14, last). New applications include the manufacturing of feed for use in water farms. For young, small fish, granules with sizes in the range 0.1–2 mm are required (system alternative at the lower right of Fig. 6.5-13 and Fig. 6.5-15a). Coating allows the production of floating or slowly sinking pellets for tilapia, carp and catfish, of slowly sinking pellets with high fat content (up to 30 %) for trout, salmon, and perch, and of sinking, water stable pellets for shrimp and other crustaceans. Such pellets have diameters in the range 2–12 mm (Fig. 6.5-15b–e).
Fig. 6.5-14 Different extruded dry dog and cat foods (courtesy Amandus Kahl, Reinbek, Germany)
Fig. 6.5-15 Extruded dry fish and shrimp feed: a) crumbled feed for young fish, b) pellets 2–12 mm, c) floating pellets, d) slowly sinking pellets, e) water
stable (2–8 h) pellets (courtesy Amandus Kahl, Reinbek, Germany)
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Dry plant material that is used as animal feed, including cereal, hay, and alfalfa, is very voluminous, even after baling. Therefore, it is not easily transported and stored. Baled material needs to be torn apart prior to feeding and milled before it can be incorporated into the formulation of complete animal feed rations, which are then pelleted as described above. Also bales take-up water if they are not protected from rain. The early development of stationary and mobile baling presses (Fig. 6.5-16) [6.5.2.3] and the successful application of high pressure for the production of relatively small, highly densified compacts (Fig. 6.5-17 and 6.5-18), which can be collected and handled in bulk, have suggested the use of punch-and-die or roller presses for the production of cylindrical and almond or pillow shaped briquettes, mainly consisting of shredded hay and alfalfa enriched with small amounts of minerals and other animal feed components. While after conditioning (heating and moistening with steam) briquetting is feasible and economical, field trials showed that such material can not be used as Fig. 6.5-16 Sketches demonstrating the development of baling presses [6.5.2.3]: a) old American box frame press, b) first stationary high-pressure press (USA, about 1870), c) first German mobile straw press (1896), d) swing-piston press (Raussendorf, Singwitz/Saxonia, 1938)
6.5 Applications for Animal Feeds
Fig. 6.5-17 Mobile hay collection equipment with high pressure ram press [6.5.2.3]. The cylindrical compacts are transferred into an open car by pushing them through the transport pipe on top of the machine
Fig. 6.5-18 Comparison of volume of compacts (left) produced with the equipment shown in Fig. 6.5-17 and a conventional bale (right) from the same material [6.5.2.3]
feed. The highly densified pieces swell to a multiple of their volume in the stomachs of, for example, cows and endanger their wellbeing. Therefore, this development was given up. High-pressure briquetting, mostly with ram or punch-and-die presses, is used, however, for the manufacturing of so called range cubes or licking stones. These large briquettes are used on farms for free-range cattle and provide salt, minerals, and minor ingredients (drugs, hormones) to the animals that lick the pieces. It is important that such briquettes have high density and often contain a binder that makes them rain resistant.
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6.5.3
Other Technologies
Applications of coating and nanotechnologies are new developments in the design, formulation, and production of animal feed. Coating, as discussed in Section 6.5.2 and shown as a possible process step in Fig. 6.5-13, is being used to obtain special effects, such as resistance to moisture pickup during storage, and for the production of water stable pellets, for example for shrimp feed. Coating, often as micro-encapsulation (Chapters 5 and 11), may be also used as a preparatory step for feed components to influence the availability of nutrients or drugs in the animal’s digestive system. Such processing takes place prior to creating the formulation in the feed mill and is often carried out by the supplier of the ingredient off-site. Nanotechnologies (Chapter 11) are of increasing interest for the manufacturing of functional feed components, mostly in veterinary medicine (Section 6.2.3). Functionalized ingredients that influence animal growth and health are being developed for incorporation in complete feed formulations.
6.6
Fertilizers and Agrochemicals For centuries, through ancient and medieval times, man has been interested in improving crop yields. Various mineral and organic substances have been used to improve productivity by exploiting effects discovered by accident or empirically by trial and error. Such materials include: manure, ground bones, wood and other ashes, saltpeter, and gypsum. However, the results were not predictable and a treatment that benefited one field could have no or even an adverse effect on another. The foundation of modern fertilizer technology was laid by Justus von Liebig in 1840. He postulated that the mineral elements nitrogen, phosphorus, and potassium (N, P, and K) in the soil are responsible for plant nutrition and stressed the necessity of replacing those elements to maintain soil fertility. The availability of nutrient elements for plant life depends to a large extent on solubility. In most cases a high solubility is desired, requiring a large specific surface, which is synonymous with small particle size. Additional micronutrients are necessary, which must be added as fine powders because of the small amounts of these trace elements in a fertilizer formulation. Such powder systems exhibit a number of problems, as shown in Tab. 6.6-1. To overcome these difficulties, it is not surprising that size enlargement by agglomeration of powdered plant nutrients has been investigated almost since the beginning of their use.
6.6 Fertilizers and Agrochemicals Tab. 6.6-1 Potential problems associated with the processing and handling of powdered fertilizer formulations * * * * *
Uniform mixing is difficult and time consuming. Dusting is excessive during handling. Segregation of components occurs due to differences in particle size and/or density. Danger of caking exists during storage and transportation. Difficulties prevail during application (dusting, which may result in health hazards, run-off with water, scattering by wind, etc.).
After definition of the main plant nutrients, naturally occurring materials containing those elements were systematically used for fertilization. Ground bones constituted the first phosphate fertilizer, and wood ashes, sugar beet wastes, and saltpeter were early sources of potassium. For many years, the need to supply nitrogen was considered to be of secondary importance, as natural supplies in rain water and from the air with a system of crop rotation were deemed adequate [6.6.1]. By about 1840, treatment of phosphate rock with sulfuric acid was found to yield an effective phosphate fertilizer, called superphosphate. The first successful commercial production started in England in 1842, and by 1870 there were 80 factories operating in the UK [6.6.1]. Mining of potassium chloride salt deposits began in Germany in 1860 and dominated the world market for 75 years. The first products were low-grade unrefined ores such as manure salts (20–25 % K2O) and Kainite (19 % K2O). The development of refining methods gradually increased the grade and, today, potassium chloride (60– 62 % K2O) is the main product. Toward the end of the 20th century it became evident that the food needs of a growing world population could be met only by an increased supply of fixed nitrogen in fertilizers and three processes were developed. In 1903 the arc process was commercially introduced in Norway which, after several process steps, produces calcium nitrate. At about the same time the calcium cyanamide process was perfected and in 1913 direct synthesis of ammonia from nitrogen and hydrogen was first carried out successfully on a commercial scale in Germany [6.6.1]. More recently, urea production has grown rapidly and urea is now the leading form of nitrogen in fertilizers. First reports of fertilizer granulation appeared at the beginning of the 20th century [B.48]. Original research by the US Department of Agriculture and the Tennessee Valley Authority (TVA) started in 1922 and aroused worldwide interest. In 1927 a branch of the German IG Farbenindustrie introduced for the first time a “grained” fertilizer material (Nitrophoska) [6.6.2]. Until 1933 the process involved granulating a slurry of di-ammonium phosphate and ammonium nitrate in pug mills with the addition of potash salts. After 1933 the formulation was changed several times, but the production of Nitrophoska continued (later at BASF). One of the first commercial fertilizer granulation processes in North America was carried-out by COMINCO (Consolidated Mining and Smelting Co.) in Trail, BC, Canada [6.6.3]. This also employed a double-shaft pug mill, called a “blunger”. Sulfuric acid, a by-product from the smelting operation, was used to make ammonium sulfate and phosphoric acid with phosphate rock; ammonia was also produced from electrolytic hydrogen.
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Three granular fertilizers were made: triple superphosphate, monoammonium phosphate (11-47-0), and ammonium phosphate-sulfate (16-20-0). This technology, with various modifications, was later used in numerous fertilizer plants around the world and was one of the more important granulation processes for many years. The first application of a drum for fertilizer granulation was in the early 1930s in Baltimore, MD, USA, in the Oberphos process [6.6.4], a batch method for producing superphosphate from phosphate rock and sulfuric acid. In the closed drum a chemical reaction and granulation took place simultaneously. In the 1930s the first granular fertilizers were introduced in the UK by ICI (Imperial Chemical Industries). Six grades of granular “concentrated complete fertilizers” were produced, based on ammonium phosphate with additions of ammonium sulfate and potash salts [6.6.5]. ICI probably used a slurry process, similar to the one introduced by COMINCO, except that much of the feed was solid ammonium sulfate and potash with only ammonium phosphate in slurry form. Relatively large agglomerates were formed and the final granules were produced by crushing and screening the dried product. Prilling is the spraying and solidification of molten urea, which produces spherical granules but is not really an agglomeration technology [B.48, B.97]. There are also chemical reaction methods in which granulation is a side effect (e.g., the TVA “continuous ammoniator”, in which both a chemical finishing process and granulation occur [6.6.6]). Apart from these, further large scale commercial adoption of fertilizer granulation by any agglomeration method did not take place until the 1950s. The production of mixed (NPK) granulated fertilizers from dry components began then, using mixers, drums, pans, and suspended-solids agglomerators (spouted and fluidized beds). By 1934 a patent had been issued to the machine manufacturer Eirich in Germany (DRP 647 651) for a granulating mixer intended for fertilizers. The patent describes the mixing and rolling of particulate solids on the flat bottom of a pan mixer by means of eccentrically arranged mixing tools. The mixing elements are designed as either blades or bars, which rotate and extend into the material to be agglomerated. The patent stated that, due to the intense movement of the material, practically all solid powders can be agglomerated into relatively small, uniform granules with or without the addition of binder(s). Utilization of the inclined-pan granulator in the fertilizer industry was first reported in Germany in 1953 for the granulation of ordinary superphosphate and the first documented application of suspended-solids agglomerators was in the early 1960s using a spouted bed [B.48]. Although the tabletting (punch-and-die) press was invented in 1843, its use for small-volume fertilizers did not occur until much later. Today this technology is still not very important in the fertilizer industry; it is mostly used for special garden and plant-nursery applications. In 1950, another pressure-agglomeration technology emerged as a method for the granulation of potassium chloride (mineral potash) after concentration by flotation or selective crystallization. This method uses roller presses for the production of a highly densified sheet, which is then crushed and screened (compaction/granulation) to yield granular and coarse potash fertilizer grades. The technique quickly found general acceptance around the world [Section 13.3, ref. 108]. Because roller presses can be
6.6 Fertilizers and Agrochemicals
easily adapted to a wide range of capacities and feed materials, ten years later compaction/granulation was also introduced and is now applied for the production of granulated mixed NPK fertilizers. Much more recently, during the past 20 years or so, agglomeration methods began to make inroads into other agrochemical technologies. Coating is now used to modify the availability of the nutrient by controlling the release time or to improve seeds. Agglomerates are also employed as carriers and diluents for toxic chemicals, such as herbicides, insecticides, and pesticides. These agglomerates must feature specific properties, particularly that they are easily degradable. Easily degradable carrier materials must offer a large accessible (inner) surface area (high porosity), so that they can be easily loaded (impregnated) with liquid components, and have sufficient strength to withstand processing, storage, and handling. The binding mechanism must survive the impregnation process with the active substance. For example, if molecular or electric forces were used for dry agglomeration, it is possible that, after impregnation, the active substance replaces (e.g., by recrystallization during a drying step) or enhances (e.g., by viscous liquid bonding and/or chemical reaction) the original binding mechanism. On the other hand, the granules must break down easily and quickly under the influence of moisture either from the soil or the atmosphere (rain or dew). Therefore, the product must wet easily and even small amounts of moisture should reduce the strength. Surfactants improve wetting and components that swell in the presence of moisture may assist in breakdown. Sometimes, particularly in the case of fertilizers and micronutrients, interactions with bacteria also participate in degradation. With the large-scale industrial production of straight fertilizers each containing only one primary nutrient, and the merger of land into large operations using mechanical equipment for farming including fertilization, over-fertilization and pollution by runoff became a major concern. Also, farmers striving for higher yields and greater productivity requested more and more complex fertilizers, which now also contain secondary elements and micronutrients (Tab. 6.6-2). Formulations are determined by Tab. 6.6-2 Primary-, secondary-, and micro-nutrients for plant fertilization, each in alphabetical order [6.6.2]
Primary nutrients
Secondary elements
Micronutrients
Element
Symbol
Nitrogen Phosphorus Potassium Calcium Magnesium Sulfur Boron Chlorine Copper Iron Manganese Molybdenum Zinc
N P K Ca Mg S B Cl Cu Fe Mn Mo Zn
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agronomists on the basis of soil analyses and knowledge of the requirements of specific plants. Since secondary and micronutrients are required in only tiny amounts, they must be added as fine powders and uniform distribution becomes a challenge. At the same time, increased caking problems result from the production of fertilizer components with higher analyses while the common use of mechanical application equipment calls for dependably free-flowing, dust-free materials. This is why the granulation of fertilizers, either as components for bulk blending or as mixed fertilizers for direct use became popular during the second part of the 20th century. Today, even small farms, particularly in subtropical and tropical climate zones, obtain specially formulated multi-component fertilizers, which are compounded and granulated in special fertilizer agglomeration plants using compaction/granulation for size enlargement (Section 6.6.2).
Further Reading
For further reading the following books are recommended: B.3, B.7, B.16, B.21, B.22, B.26, B.40, B.48, B.56, B.58, B.64, B.67, B.81, B.82, B.89, B.93, B.94, B.97, B.98 (Chapter 13.1).
6.6.1
Tumble/Growth Technologies
The first fertilizers that were industrially produced were the result of reacting a solid (such as phosphate rock), with a liquid (such as sulfuric acid), in a pug mill. The reaction yielded the fertilizer material, in this case superphosphate. Other early fertilizers used similar production methods. Granules formed naturally during mixing and reaction. Sometimes the agglomerates were too large, so they were subsequently crushed and screened into the desired size distribution. An improvement was the accretion process in which a recirculating load of granules received a thin coating of slurry in the mixer prior to drying. The dry product was screened to recover product-sized particles and the rest was recirculated. Beginning in the 1950s, for reasons already discussed, more complex mixed fertilizers were developed and produced. This trend required either compatible granules (in size and/or mass) of single fertilizers and additives for bulk blending or the production of multi-component granules from dry, mixed fertilizer formulations. To accomplish this, size enlargement by agglomeration of the particulate solids was introduced. Since some earlier processes had yielded agglomerates from solids after wetting them with liquids, the first techniques that quickly became the standard for fertilizer granulation followed these precedents. Such plants (Fig. 6.6-1) include an appropriate number of silos for the raw components (1) from which the correct amounts are metered (2). Recycling fines are added at a fixed rate. These feed materials can be alternatively mixed (3) for added uniformity or fed directly into the tumble agglomerator
6.6 Fertilizers and Agrochemicals
Fig. 6.6-1
Flow diagram of a typical wet granulation plant for dry fertilizers
(e.g., rotating drum, (4), [B.48, B.97]) where a liquid binder (normally water or a fertilizer solution) is added and the growth of agglomerates takes place in the moving particle bed. Alternative tumble agglomerators can be inclined pans or many types of mixers (including pug mills, the Eirich mixer, ribbon blenders, and others [B.97]). The discharge from the tumble/growth agglomerator consists of a wide distribution of green (moist) agglomerates, which must be dried (5) and cooled (7) to achieve permanent final product strength. Since the formation of oversized lumps in the agglomerator and the dryer can not be avoided, the cooled material is screened (9) to yield the finished product. Oversized and undersized particles pass through a mill (10) and are returned, together with the dust from the cyclones (11) that clean the effluent air, via a surge bin and a metering device to the process. It should be mentioned that the recirculating, pre-agglomerated particles that are removed on the product screens and adjusted in size by the mill play an important role in tumble/growth agglomeration. Since nucleation, the formation of seed agglomerates from powder, is the most difficult and time-consuming step in growth agglomeration [B.97], recycling introduces nuclei, which help agglomerate formation and growth. Fig. 6.6-2 shows the discharge end of a granulation drum and the chute feeding the green agglomerates into the rotary dryer. The dryer is a co-current design with the burner (6 in Fig. 6.6-1) visible under the support structure. Fig. 6.6-3 shows the discharge of well-sized green agglomerates from two pan agglomerators. Fig. 6.6-4 is the overall view of a system specified in Fig. 6.6-1. One of the major problems associated with the wet granulation of fertilizers is the very reason why the wetted particulate mass agglomerates and forms granules. The moistened solid particles not only adhere to each other but also to equipment walls and parts. This build-up can not be avoided and must be controlled. In mixers employing agitators, such as pug mills, ribbon blenders, or Eirich-type pan mixers, the tools them-
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.6-2 The discharge end of the granulation drum and the feed end of the rotary dryer of a wet fertilizer granulation plant (courtesy Krupp Polysius, Beckum, Germany)
Fig. 6.6-3 Discharge of granules from two pan agglomerators in a wet fertilizer granulation plant (courtesy Krupp Polysius, Beckum, Germany)
6.6 Fertilizers and Agrochemicals
Fig. 6.6-4 Overall view of a system for the wet granulation of dry fertilizers according to Fig. 6.6-1 (courtesy Krupp Polysius, Beckum, Germany)
selves limit the thickness of the build-up to the distance between the equipment walls and the agitators. In addition, scraper blades may be installed to clean those areas that are not reached by the mixing tools. The drum and pan agglomerators, which are the most commonly used for wet granulation of particulate fertilizer components, do not employ mixing tools. Therefore, special scrapers must be installed to limit build-up. In inclined pans, the bottom scrapers (Fig. 6.6-5, left) are also used to enhance the separation of granule sizes in the downward moving part of the pan (Fig. 6.6-5, right) and, for that reason, their position is adjustable. Fig. 6.6-6 shows a pan of 4.6 m diameter with unadjusted scrapers. In agglomeration drums, stationary (Fig. 6.6-7a and b) or movable scrapers (Fig. 6.6-7c and d) are used. Fig. 6.6-7 depicts only four examples of many different designs.
Fig. 6.6-5 left) Diagram of the wall and bottom scrapers in a pan granulator; right) sketch of the particle motion that is assisted by the position of the bottom scrapers [B.97]
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.6-6 Photograph of a 4.6 m diameter pan showing the still-unadjusted vane-type plow scrapers (E) that are individually mounted on a support structure (D), which moves with the tilt of the pan (courtesy Feeco, Green Bay, WI, USA)
Fig. 6.6-8 is a sketch of the operating principle of a drum granulator depicting the spray bar for liquid addition, the movement of the particulate charge, and the scraper. In spite of these control measures, build-up takes place. If the coating has a controlled thickness and grooves from scraper tips, it can act as wear and/or corrosion protection. It may also improve the tumbling action by modifying the friction between the coating and the charge. However, with time the coatings become very hard and increase the drive
Fig. 6.6-7 Four typical designs of internal scrapers of drum granulators [B.48]: a) single-stage, sectional adjustable straight edge; b) two-stage scraper consisting of tungsten carbide cutters and a secondary adjustable straight edge; c) hydraulically powered reciprocating; d) rotary spiral
6.6 Fertilizers and Agrochemicals Fig. 6.6-8 Sketch of the operating principle of a drum granulator depicting the spray bar for liquid addition, the movement of the particulate charge, and the scraper (B.48)
power required which, in turn, is converted into frictional heat within the apparatus. Therefore, it is necessary from time to time to stop the equipment and remove the buildup, a time consuming, dirty, manual operation. Often such cleaning must also take place before production can be switched to another formulation. Because many of the components are relatively fine high-analysis fertilizers and moisture is part of the process, many other parts of a fertilizer granulation system are also prone to build-up which, owing to chemical reactions or after natural evaporation of contained liquids, also harden. This affects dust-collection ducts, collectors, and chutes and transport devices, dryers, and screens. Generally speaking, wet agglomeration plants in the fertilizer industry must be cleaned often and good housekeeping is an important part of reliable operation. Fig. 6.6-9 is another flow diagram of a plant for the granulation of dry fertilizer materials [6.6.6]. Compared with the system presented in Fig. 6.6-1, there are the fol-
Fig. 6.6-9 Typical flow diagram of a plant for steam granulation of dry fertilizer materials [6.6.6]
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lowing differences. Particularly if mixed fertilizers are to be formulated and granulated, it is important that segregation of the components is avoided as much as possible prior to and during agglomeration. Since the raw materials tend to cake or lump and, as supplied, may feature different particle size distributions they are screened prior to the intermediate storage and oversized material is crushed in a disintegrator (mill). Another, even better way to homogenize the feed to an agglomerator (granulator) is to mill the mixed formulation as shown in Fig. 6.6-25. Since the materials all have similar size distribution and are metered continuously and simultaneously onto a collection belt, mixing in the chutes, the bucket elevator, the feed hopper, and the drum granulator is considered sufficient to obtain satisfactory homogeneity. Also, recycling of fines from the product screen, the oversize crusher, and the dust collectors is added prior to the bucket elevator without going through a surge bin and metering. This is a questionable practice. It is not unusual that surging occurs in the granulator and dryer, which results in large fluctuations of the amount of recycling. For uniform plant operation it would be better to level this out by means of a recycle surge hopper. In the drum granulator, water is added as a binder and steam is also discharged under the bed of material at the feed end of the drum. Any of the tumble/growth agglomerators mentioned earlier can be installed at this position. If steam is injected, the process is often called “steam granulation”. Size enlargement is controlled by the amounts of steam and water added. For each mixture a “percentage liquid phase” exists at which granulation efficiency (agglomerate growth rate) is optimal. The liquid phase consists of the amounts of moisture and dissolved salt. Since the solubility of fertilizer salts increases with temperature (Fig. 4-5, Chapter 4), the higher the temperature the less spray water is required. For any given fertilizer formulation an optimum moisture content exists at each temperature; this may be described by an experimentally determined curve such as shown in Fig. 6.6-10. Since the condensation
Fig. 6.6-10 Influence of the relationship between temperature and water content on the agglomeration behavior of fertilizers during steam granulation [6.6.6]
6.6 Fertilizers and Agrochemicals
of steam in a particle bed is one of the most efficient means of transferring heat to particulate solids, the overall moisture content is reduced and dryer operation becomes cheaper. At the same time a stronger and often denser agglomerate is formed in the dryer when the larger amount of dissolved salt recrystallizes to form the permanent bonding. In the granulator, conditions of mixtures that are below the range shown in Fig. 6.6-10 result in insufficient agglomeration and those above the range produce excessive growth, build-up, and even the formation of a slurry, requiring shut down of the system and extensive cleaning of the granulator and equipment downstream. As depicted in Fig. 6.6-9, oversized material is removed after the dryer on a doubledeck screen, crushed, and recirculated as fines. As considerable amounts of oversized lumps can form in the granulator and the dryer, returning the discharge of the crusher that would now operate at lower levels of stressing (Section 6.1, Fig. 6.1.9), to the screen may increase the product yield. The screen is installed after the dryer to unburden the cooler because the recirculating load in wet granulation systems is often high (up to 500 %). The flow diagram also indicates the use of a coating drum after the cooler. Although granulated fertilizers are much less prone to setting during storage (Chapter 4), the inherent hygroscopic nature of many plant nutrient materials may cause problems. Coating with anti-caking materials (Chapter 4) may prevent difficulties, and coatings with functional layers can add new characteristics (i.e., slow release, herbicide, fungicide, insecticide: Section 6.6.3). Wet granulation is mostly used for size enlargement of multi-nutrient fertilizers formulated from dry components, but processes to form a granular fertilizer product by chemical reaction accompanied by agglomeration are still carried-out. In the USA, for example, the introduction by TVA of a continuous ammoniator-granulator (US patent 2 729 554) had a significant effect [6.6.6] on the development of fertilizer granulation. The method was originally developed for a more efficient ammoniation of superphosphate; however, it was found that agglomeration occurred during this process and could be controlled by the addition of water or steam or by adjusting the formulation to provide the required temperature of 80–100 8C. When the heat of reaction of the ammoniation of superphosphate was insufficient, sulfuric or phosphoric acid was added along with more ammonia through sparger tubes embedded in the tumbling mass (Fig. 6.6-11). Granulation of two NPK grades (6-12-12 and 10-2020) was demonstrated in a pilot plant in 1953 and by 1962, 164 installations used the process in the USA, about two-thirds of all US granulation plants [6.6.6]. Later the process was adapted to receive a pre-reacted slurry (Fig. 6.6-11b) for use with formulations in which the heat of reaction is too great for release in the ammoniator– granulator. Fig. 6.6-12 is a typical flow diagram of a TVA-type ammoniation-granulation plant for the production of granulated NPK fertilizers [6.6.6]. The liquid phase that initiates granulation in a tumble/growth agglomerator can also be a melt. For example, using the TVA pipe-reactor [6.6.6] phosphoric acid reacts with ammonia to produce a melt of ammonium polyphosphate (APP), which is sprayed onto the moving bed of solids and causes agglomeration. Fig. 6.6-13 is the flow diagram of a plant producing granulated NP fertilizers by spraying a highly concentrated
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Fig. 6.6-11 a) Sketch of the TVA pilot ammoniator-granulator; b) cut-away view of a large scale ammoniator-granulator designed to accept pre-reacted slurry [6.6.6]
urea solution, an APP melt, and scrubber solution onto recirculating material from the process in a pug mill, and Fig. 6.6-14 is a system for the production of granulated NPK fertilizers from solid ammonium nitrate or urea and potash with an APP melt in a special drum. Fig. 6.6-15 is a cut-away drawing of the granulating drum in Fig. 6.6-14. Other processes using melts are applied to round irregular fertilizer granules by coating or to add additional nutrients (Section 6.6.3).
6.6 Fertilizers and Agrochemicals
Fig. 6.6-12 Flow diagram of a TVA type ammoniation-granulation plant for the production of granulated NPK fertilizers [6.6.6]
Fig. 6.6-13 Flow diagram of a TVA pipe reactor-pugmill process producing granular NP fertilizer (urea, ammonium polyphosphate) [6.6.6]
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Fig. 6.6-14 Flow diagram of a granulation plant using the TVA pipe reactor for the production of NPK fertilizers [6.6.6]
Fig. 6.6-15
Cut-away drawing of the drum used in Fig. 6.6-14 [6.6.6]
6.6 Fertilizers and Agrochemicals
6.6.2
Pressure Agglomeration Technologies
Since 1950 granulation by compaction has been an accepted method for the manufacture of high-quality granular potash [Section 13.3, ref. 96]. Today, this technology is used by the majority of potash producers worldwide. About 10 years later, in the early 1960s, pressure agglomeration emerged as an alternative to the conventional tumble agglomeration methods in mixers, drums, pans, and suspended solids granulators for mixed NPK fertilizer granulation [Section 13.3, ref. 108]. Because, in contrast to conventional wet granulation methods, high-pressure compaction is processing dry feed materials from an almost unlimited number of sources, without special requirements on particle size or distribution, this technique is gaining increasing importance. Compaction/granulation with the most commonly used equipment, roller presses, which can be easily adapted to a wide range of capacities and feed materials, is even better suited for multi-component fertilizers then for the single potash product. Roller presses that are used for the compaction of fertilizers (Fig. 6.6-16) feature selfaligning roller bearings, optimally sized steel bearing blocks, and a hydraulic pressurizing system with proprietary functions and hydraulic accumulators. The latter allow adjustment of the pressure-response characteristic and provide for overload protection. While in some cases simple gravity feeders with flow control baffles are provided, most applications require one or more screw feeder(s) with variable (e.g., hydraulic) speed drives to force the material to be compacted into the nip between the rollers [B.48, B.97]. To maximize availability and minimize potential problems that could be caused by insufficient routine maintenance, the machines are equipped with water cooling and automatic grease lubrication. Double output-shaft gear reducers provide for completely enclosed, dust-tight drives connected with the rollers by gear-tooth couplings and spacers, which allow transmittal of full torque even at relatively high misalignment. In the case of machines with high torque requirement, the oil of the gear reducer is circulated, filtered, and cooled and the gear-tooth couplings may be optimally equipped with continuous greasing to guarantee long life and availability. Particularly if fine nutrient powders or carriers for agrochemicals are processed, deaeration, which is the removal of air from the densifying mass, requires special design and operational considerations. In machines with large production capacities this includes the “split roller” design, which is characterized by two separate compaction rolls on each shaft with a gap in the middle that provides additional venting of air at the center cheek plates. Many fertilizer materials are hard minerals that cause wear. Other nutrients or solid agrochemicals are salts or compounds which, especially in the presence of moisture, may cause corrosion. For these reasons it is inevitable that the “pressing tools”, rings or segments, which are fastened to the roller core by suitable means, must be exchanged at regular intervals for remachining or replacement [B.48]. The “hinged frame”, which is available from some manufacturers (Fig. 6.6-17) facilitates this work and minimizes downtime due to maintenance.
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Fig. 6.6-16 Two views of a modern roller press for the compaction of fertilizers (courtesy K€ oppern, Hattingen/Ruhr, Germany)
6.6 Fertilizers and Agrochemicals
Fig. 6.6-17 Schematic and photograph of a “hinged frame” for roller presses (courtesy K€ oppern, Hattingen/Ruhr, Germany)
Fig. 6.6-18 depicts the most versatile flow diagram of a fertilizer granulation plant using a roller press for compaction. Pre-mixed powder formulation (1) is fed into a day bin (2). Recycled fines (17) from the process (below) and dust from the pollution control system (20) are transported to bin (18). The latter should be sized so that in an emergency or unscheduled changeover the entire hold-up of the plant can be accepted. Prior to running a new formulation the contents of the recycling bin (18) must be dumped via diverter gate (19). During normal operation fresh pre-mixed (if applicable) feed (2) and recyclate (18) are proportioned by rotary gates and weigh belts (3). The ratio of fresh feed to recyclate should be kept constant; it is only adjusted if the level controls in bins (2) and (18) require modification of compactor feed composition. Typically a changed relationship “fresh feed to recyclate” necessitates readjustment of the compactor (8) and, sometimes, the oversize crusher (16) parameters. Fresh feed and recyclate are homogenized in a low-intensity mixer (e.g.,. pug mill, mix muller) (4) and transported by buck-
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Fig. 6.6-18 Flow diagram of a fertilizer granulation plant using a roller press for compaction [B.48, B.97]
et elevator (5), metal detector (6), and drag chain conveyor (7) to the roller press (8). Independent of the type of feeder used to transport material into the roller nip, it is imperative to avoid “starved” feeding conditions at all times. Therefore, a small overflow, measured by a solids flow meter (11), is maintained. The signal from the flowmeter (11) may be used to adjust the system feed rate by controlling the rotary gates and weigh belts (3). The compacted sheets exiting the roller press (8) are pre-crushed in the flake breaker (9) and screened (10) to remove fines. Coarse material is transported to screen (13) by bucket elevator (12). On the screen, product is separated and transferred to storage silo (14) while oversized material is crushed in granulator (16) and again separated into three fractions on the double-deck screen. All undersized fines, including dust from the pollution control system, are recirculated to recycle bin (18). Potential modifications to optimize this basic flow diagram are as follows (compare Figs. 6.6-18 and 6.6-19).
6.6 Fertilizers and Agrochemicals
Fig. 6.6-19 Flow diagram of an optimized fertilizer compaction/ granulation system incorporating changes [B.48, B.97]
a Elimination of flake breaker (9) Sometimes the sheet produced in compactor (8) breaks up easily and, therefore, flake breaker (9) is not required. b Elimination of screen (10) Since fines separated at this point amount to only 10 % (mostly “leakage” from the compactor cheek plates) screen (10) may be eliminated. However, crusher screen (13) and/or primary granulator (22), if applicable, may be less efficient. c Flake curing (21) Some materials yield a relatively soft sheet immediately after compaction (e.g., due to liquid phases resulting from energy input during compaction, for examples see below) but quickly “cure” (i.e., solidify) when cooling and reach higher strength. In such cases it is desirable to install a “time delay” of between a few seconds and several minutes (curing belt) between compactor and flake breaker.
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d Addition of primary granulator (22) The yield of granular product can be optimized by lowering the reduction ratio of crushing (Section 6.1, Fig. 6.1.9). This can be accomplished by the installation of a primary granulator (22) or, more generally, multi-stage crushing. e Improved product characteristics Particularly if primary granulator (22) is installed, the loading of double deck screen (13) with finer material is so high that its separation efficiency deteriorates. On the other hand, product users often impose limitations on the amounts of oversize and fines; also, the presence of product-grade material in the oversize and/or undersize streams reduces yield; therefore, screen decks with larger and smaller openings may be selected for screen (13). Then, final separation is achieved on secondary screen (23). f Product particle rounding (24) Studies have shown [6.6.2.1] that the irregular (angular) shape of granular fertilizer obtained by compaction, crushing, and screening (Fig. 6.6-20) does not have a negative influence on the efficiency of and uniformity of distribution by modern mechanical rotary spreaders. Nevertheless it may be preferable to remove sharp edges and corners in an “abrasion drum” (24) to avoid excessive production of dust during handling and transportation. Fines produced during tumbling are separated from the product on screen (25) and recirculated. g Product conditioning (26) In some cases it is desirable to condition or treat the product with anticaking reagents, insecticides, or fungicides. Such treatment can be accomplished in a conditioning drum (26) (Section 6.6.3). Modern roller presses for the fertilizer and agrochemical industries feature special designs (Figs. 6.6-16 and 6.6-17). Operating parameters are determined during tests with a representative sample of the particular formulation. As for all applications, they include: specific pressing force, response characteristic of the floating roller (i.e., in-
Fig. 6.6-20 Compacted sheet and granular fertilizer obtained by crushing and screening
6.6 Fertilizers and Agrochemicals
fluence of the hydraulic accumulator pressure), roller diameter, sheet thickness, and roller speed, which is limited by deaeration and potential elastic properties of feed components. As a result of testing and experience the surface configuration of the rollers (e.g., smooth, corrugated, waffled, welded), the type (gravity or force) and number of feeder(s), and the drives, size (kW) and method (single or variable speed, electric or hydraulic), are chosen. As always, the most important parameter for roller press selection is the specific pressing force necessary to obtain a highly densified sheet that (after crushing and screening) produces strong enough granules with acceptable yield in the required particle size range. The specific pressing force in kN/cm is defined as total force exerted by the pressurizing system of the machine divided by the active width of the rollers. It is different for each fertilizer or formulation and varies in the range about 30–120 kN/cm if materials are processed in presses featuring rollers with 1000 mm diameter and operating at 12–14 rpm producing sheets 12 mm thick (Tab. 6.6-3). Simple mathematical relationships make it possible to convert these figures to the conditions that are valid for different roller diameters and speeds or sheet thicknesses [B.97]. In mixed fertilizers the presence of substantial amounts of material requiring low specific force (e.g., urea) reduce the necessary force and act as a binder while the admixture of hard components (e.g., raw phosphate or Thomas slag) result in the need for higher forces. Selection of peripheral system components, such as mixers, crushers, screens, material handling, and storage, depends on the material(s) to be processed and, to a cer-
Tab. 6.6-3 Specific pressing force, water content, and feed particle size that were determined for the compaction of some fertilizer materials in roller presses [B.48] Fertilizer
Specific pressing forcea (kN/cm)b
Water content (%)
Feed particle size (mm)
Ammonium sulfate Potash 60% K2O Feed temperature > 1208C Feed temperature 208C Potash 40% K2O Feed temperature 908C Potassium sulfate Feed temperature > 708C Potassium nitrate Calcium nitrate Calcium cyanamide Urea Mixed fertilizer containing – No raw phosphate or Thomas slag – Raw phosphate or Thomas slag – Urea
100 – 120
0.5 – 1.0
< 1.0
45 – 50 70
dry dry
< 1.0, with max of 3% < 0.06
60
dry
< 1.0
1.0 0.5 – 1.0 dry dry dry
< 0.5 < 1.0 < 1.0 < 0.4 2 – 3 to < 1.0
< 1.0 < 1.0 < 1.0
< 1.0 < 1.0 < 1.0
70 100 60 60 30 – 40 30 – 80 > 80 30 – 40
a) Indicated pressing force is for machine having 1.0 m diameter rollers. b) 1 kN/cm = approximately 0.1 t/cm.
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tain extent, on whether the plant is dedicated to the production of only one fertilizer or to a variety of formulations. Materials that reach final strength only after some curing time (such as formulations containing urea, partially acidulated phosphates, or superphosphates) and fertilizers obtaining strength from recrystallization, such as ammonium and potassium sulfates, must be handled, crushed, and screened gently. Other applications, such as the granulation of potash, require high energy input during crushing and screening so that a strong, abrasion-resistant product, suitable for bulk shipment, is obtained [Section 13.3, ref. 96]. At the beginning, compaction was applied to produce a strong, abrasion resistant granular fertilizer from KCl fines because dry material could be processed directly. Later, this process was used in the mixed fertilizer industry [Section 13.3, refs 108 and 110] primarily to save energy. Today the specific location and local costs of energy determine whether this feature is still a deciding factor. Other advantages often play a more important role, the most valuable being its versatility as demonstrated by the following list.
1. With the exception of a few materials, such as urea or triple superphosphate, for which maximum amounts exist that can be used in a formulation, literally all particulate solids can be processed by compaction. This includes also, for example, dry digested sludge from municipal waste treatment plants (Section 8.2). 2. To minimize cost, raw materials can be purchased on the world markets without specific requirements on particle size. Off-specification fines can be used and often are even preferred. 3. Compaction/granulation plants can be designed for economic operation at any feed rate. Production capacities of 0.1–50 t/h per line are feasible. 4. Larger plants are preferably equipped with two or more lines fed by one large compounding (batching or formulation) system. Otherwise the lines are kept separate to improve availability because only one line is down during maintenance and emergency shut-downs. 5. If a plant equipped with multiple lines features separate day bins for fresh feed, recirculating fines, and granulated product, each line can be operated on different formulations. 6. Production of small batches is feasible. Depending on the amount of cleaning necessary during change-over (determined by how much cross-contamination can be tolerated), several different formulations (batches) per 8 h shift can be produced. 7. Fertilizer granulation plants utilizing compaction can be combined with either custom designed batching systems or with standardized formulation or bulk-blending units. The latter allows easy expansion of bulk-blending to mixed fertilizer granulation. 8. Any fertilizer compaction/granulation system can be utilized as a regional production facility for the manufacturing of bulk-blend grade material from off-specification feeds. This capability also includes special formulations that are required by the local market such as indigenous fillers, with or without major nutrients, or carriers
6.6 Fertilizers and Agrochemicals
for micronutrients. Such product can then be used together with imported bulkblend grade materials in bulk-blending. 9. It should also be mentioned that plants with roller presses can be easily adapted to the manufacturing of urea supergranules for deep placement in wetland rice production. In this case, the roller surface must be modified, the flake breaker (9 in Fig. 6.6-18) bypassed and the granulator (16 in Fig. 6.6-18) blocked-off. The freedom to select raw materials from a large number of different sources on the free market allows one to take advantage of special offers and, thus, optimize the facility’s cost structure. It is also possible to use otherwise marginally or not suitable raw materials offering special agronomic characteristics, including the incorporation of micro-nutrients. Furthermore, it is feasible to work closely with individual farmers and formulate fertilizers for their particular crops, soils, and climatic conditions. Actual operating data confirm that production runs of as short as 1 h duration can be economical and that during a typical day, three or more different formulations can be manufactured for specific customers. This exceptional versatility is most interesting for tropical and sub-tropical agricultural zones where many different crops are planted on relatively small plots. Fig. 6.6-21 shows block diagrams of different installations using compaction for the granulation of finely divided particulate solids (including materials such as fertilizers
Fig. 6.6-21 Block diagrams of different installations using compaction for the granulation of finely divided particulate solids (including materials such as fertilizers or agrochemicals) [Section 13.3, ref. 101]. F, fresh feed (possibly premixed); P, product(s); A, mixer; B, compactor; C, flake-breaker; D, screen(s); E, crusher(s); G, wet granulator; H, dryer; I, cooler
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or agrochemicals). Fig. 6.6-21a depicts the simplest, basic flow diagram of a compaction/granulation plant. Feed (F), potentially premixed, coming from an outside source, is mixed in a blender (A) with recycle (R) from the product screen (D). The blend is compacted in the roller press (B) and sheets are produced, which are crushed in the flake breaker (C). The double deck screen (D) separates the discharge from the flake breaker into product (P1) and oversized and undersized material. Oversized particles are stressed again in a granulator (E) and returned to the screen (D) for sizing. Fines (recycle R) are returned to the blender and compaction. Fig. 6.6-22 is the photograph of such an installation, which is placed alongside the raw material storage facility (left). On the right are a day bin for premixed feed material and a recycle surge hopper. Below is the roller press, equipped with two feed screws (Fig. 6.6-23) and the structure behind the compactor contains the crushers, screens, and material handling equipment. Fig. 6.6-24 is the flow diagram of this particular system. When compared with the conventional wet granulation of fertilizers, the advantages of compaction/granulation became more widely known and accepted, dry systems according to Fig. 6.6-21a were installed at several sites that already operated a wet granulation plant. In fact, the system presented in Fig. 6.6-22 and 6.6-24 is in such a facility. The block diagram depicting both plants and how they are interconnected is shown in Fig. 6.6-21b. The rather extensive formulation system for the fresh feed and the mixer are common to both the wet and dry granulation plants. After this point, a diverter valve feeds either one of the two plants. A decision as to which of the two systems should be used depends on the feed characteristics and economical consid-
Fig. 6.6-22 Mixed fertilizer compaction/granulation plant (courtesy S€ udchemie AG, M€ unchen/Kehlheim, Germany)
6.6 Fertilizers and Agrochemicals
Fig. 6.6-23 Roller press similar to the machine used shown in Fig. 6.6-22 (courtesy K€ oppern, Hattingen/Ruhr, Germany)
Fig. 6.6-24
Flow diagram of the plant shown in Fig. 6.6-22
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erations. As indicated by the dotted connection, some or all of the fines (recycle) form the wet granulation system can be treated in the dry compaction/granulation plant, thus avoiding repeated re-wetting, re-drying, and re-cooling of a substantial amount of the fertilizer. A further energetically advantageous application of compaction/granulation, making use of the fact that systems with small capacities are economical, installs a compactor at the tail end of a conventional wet granulation plant (Fig. 6.6-21c). All of the off-grade (under- and oversized) dried and cooled material is compacted. The oversize crusher of the wet granulation plant may have to be upgraded in this case to also handle the compacted sheet but the screening capacity is normally sufficient. The product (P1 + P2) is a mixture of nearly spherical particles from wet granulation and irregularly formed granules from crushing a compacted sheet. Compacting off-grade dry material instead of recirculating it to wet granulation increases the capacity of the original system and, at the same time, considerably reduces energy consumption per ton of granular product. However, in a wet granulator, recyclate ( undersized but already pre-agglomerated material) often plays an important role for the kinetics and the ease of agglomeration [B.48, B.97]. Therefore, it is often preferred not to treat 100 % of the recyclate by compaction/granulation but to return 10– 20 % to the wet agglomeration system. Referring to (7) above, the flow diagram presented in Fig. 6.6-25 depicts what is also shown in Fig. 6.6-21d: the combination of a standard bulk blending system with a versatile compaction/granulation system (with two lines). On one hand, bulk blending (Fig. 6.6-25, items 1–7) can be used to accomplish the formulation and mixing of the fresh feed for the dry, multi-component fertilizer granulation plant (beginning with item 8, bucket elevator, in Fig. 6.6-25) employing one or more (two in the case on hand) roller presses for compaction. On the other hand, according to (8) above, the compaction/granulation system can be used as production facility for the manufacturing of bulk-blend grade material from off-specification feeds. Off-specification fertilizer components can be fines or, for example, micronutrients, which need to be attached to, often inert, carriers for uniform incorporation into a large mass of major nutrient materials. To avoid segregation of the bulk blended mixed fertilizer product, all components must meet special particle size and mass requirements. After adjusting these properties by compaction/granulation the products can be successfully used in the shown or any other bulk blending plant. Fig. 6.6-26 is a photograph of the actual plant according to the flow diagram of Fig. 6.6-25 showing bulk blending on the left and compaction/granulation on the right. Fig. 6.6-27 is a view of one of the compactors with compacted material on the discharge/cooling belt conveyor. The technologies, equipment, and plants described above are mostly used for single or multi-component fertilizers and for fillers (such as limestone) and mixtures of inert carriers loaded with small amounts of micronutrients. After screening-out a granular particle distribution (typical size about 1–6 mm), this product is either directly suitable for bulk shipment or bagging and use or, as discussed before, it is a component for bulk blending. As mentioned earlier, it is also possible to produce pillow- or almondshaped briquettes (about 20 cm3) as urea supergranules for deep placement in wetland
6.6 Fertilizers and Agrochemicals
Fig. 6.6-25 Flow diagram of a plant combining fertilizer bulk blending (2–7) and compaction/ granulation (8–27). 1, front-end loader input of components; 2, 8 ,18, bucket elevators; 3, distributor; 4, silos for components; 5, compounding scale; 6, blender; 7, bulk blend receiving hopper (courtesy Sackett, Baltimore, USA). 9, mill for feed homogenization; 10, feed bin; 11, 27, screw conveyors; 12, belt mixer; 13, metal separators; 14, 25,
drag-chain conveyors; 15, roller presses compactors; 16, solids flow meters; 17, curing belt conveyors; 19, double-deck screens; 20, mills (granulators); 21, product belt conveyor; 22, belt scale; 23, 24, dust collection system equipment and building; 26, recycle surge bin (courtesy Sackett, Baltimore, USA, and K€ oppern, Hattingen/Ruhr, Germany)
rice production. Machines for this purpose are often very small, homemade roller presses, sometimes even manually driven with simple hand cranks, to make the product by the farmer himself on site. A further application of briquetting roller presses in the plant nutrient industry is the manufacturing of fertilizer spikes (Fig. 6.6-28) for the direct, long-term feeding of shrubs and trees. To produce these highly densified, strong, and hard shapes, elongated, pointed matching pockets are machined axially across the face of the rollers and the spikes are produced in normal briquetting fashion (Section 6.3.2) from the mixed fertilizer feed. During application, the pointed end of the spikes is punched (hammered) into the soil, for example near the trunk of a tree, where it slowly dissolves, thus releasing the nutrients in a controlled manner. Other high-pressure agglomeration techniques for the production of special fertilizer spikesertilizer products use punch-and-die presses with reciprocating punches (eccentric or rotary designs; Chapter 5). They are limited in capacity because of the acceleration and deceleration forces at the upper and lower dead centers. Even though rotary tablet presses with multiple dies and punches are capable of producing large numbers of typically flat, cylindrical (tabletted) compacts [B.97], the mass processed
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Fig. 6.6-26 Plant described in Fig. 6.6-25. Bulk blending is on the left and compaction/granulation on the right (courtesy Ferquigua, Guatemala)
per unit time is low. Applications in the fertilizer industry are in the area of specially formulated products for house or nursery plants, where each tablet provides the ration for one plant, one container, or, after dissolution in water, for a certain amount of liquid plant food. Although, in the agrochemical industry, easily degradable products (Section 6.6.1) are more commonly made by growth and tumble agglomeration (Section 6.6.1), some are also made by pressure agglomeration. Typical examples are carrier materials for liquid fertilizers, insecticides, fungicides, and many other chemicals. In the liquid phase the active substance is highly concentrated and, in most cases, toxic. By adsorbing these liquids on the mostly inner surface of granules that are manufactured from
6.6 Fertilizers and Agrochemicals
Fig. 6.6-27 One of the two roller presses in the plant according to Figures 6.6-25 and 6.6-26 also showing compacted material on the discharge/curing belt conveyor (courtesy Ferquigua, Guatemala)
fine particles by compaction/granulation, the toxin is diluted and the products are rendered safe for handling and application. For example, spreading of granular agrochemicals by conventional equipment is possible. Newer applications of the technology also include special micronutrients.
Fig. 6.6-28 Photograph of fertilizer spikes
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In contrast to common belief, compaction/granulation is well suited for the manufacturing of easily degradable carrier particles since one of the main binding mechanism, molecular forces (van-der-Waals), diminishes by a factor of 10 in the presence of water. Therefore, if rain or dew wet the granules they lose strength, readily degrade, and liberate the active substance. A possible drawback is that high-pressure compaction results in a material with little residual porosity, which limits the amount of active components that can be incorporated. To overcome this problem, some carriers, with or without the active substance included, are extruded at medium pressure with pellet mills (Fig. 5-10b1–b6, Chapter 5). The product consists of cylindrical extrudates and can be used either directly or after impregnation with a chemical. Snail and slug control pellets are a typical application of this technique (Fig. 6.6-29).
Fig. 6.6-29
Pelleted agrochemical product (snail and slug control pellets)
6.6 Fertilizers and Agrochemicals
6.6.3
Other Technologies
Coating (Chapter 5) is the most commonly used other technology in the fertilizer and agrochemicals fields. Agglomerated fertilizers (granulated or shaped by applying pressure) are coated in drums, to achieve rounding with an additional fertilizer component (melt coating), to supply a beneficial component (such as non-fertilizer agrochemicals), to reduce sticking tendencies (coating with anti-caking materials), or to provide functional properties (e.g., slow release). Non-fertilizer agrochemicals (e.g., herbicides, insecticides, pesticides) are typically impregnated on porous (often agglomerated) carriers and the product may then be also coated to obtain a certain functionality (e.g., release control). In the latter case the method of choice is now sometimes microencapsulation. In Section 6.6.1 the use of a melt as the liquid phase that initiates granulation in a tumble/growth agglomerator was described. Flow diagrams and a special drum design to accomplish this were shown (Fig. 6.6-13–15). A drum that accomplishes coating or rounding of irregularly shaped fertilizer granules, mostly from compaction/granulation (Section 6.6.2), by applying a melt is now introduced. Fig. 6.6-30 shows cross sections of broken melt-coated fertilizer granules [B.97]. In this case, the cores of conventionally granulated (by tumble/growth agglomeration, Section 6.6.1) TSP (triple super phosphate) were melt-coated with sulfur to provide an additional nutrient for sulfur-deficient soils and/or obtain a slow-release fertilizer. Fig. 6.6-31 is the flow diagram that is used for this process. The centerpiece of the system is the so-called fluid drum granulator (FDG). The fluid drum granulator (Fig. 6.6-32) is one vendor’s design to most efficiently achieve a uniform coating. Other coaters can be also used for the task. The fluid drum coater is a cylindrical horizontal drum, rotating around its axis, and fitted with special
Fig. 6.6-30 Cross sections through broken melt-coated fertilizer granules: cores, triple superphosphate (TSP) granules; coating, sulfur (courtesy Kaltenbach-Thuring, Beauvais, France)
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Fig. 6.6-31 Flow diagram that is used for the manufacturing of products as shown in Fig. 6.6-1. The centerpiece of the system is the fluid drum
granulator (FDG) (courtesy Kaltenbach- Thuring, Beauvais, France)
anti-clogging lifters. A fluidized bed develops in a tray that is mounted inside the drum and supplied with atmospheric (cool) air. Seed solids fed to the drum can be either granules, produced by a separate agglomeration system, prills from a melt solidification process, or recyclate. The lifters transport the seed material to the upper part of the
Fig. 6.6-32 Sketch depicting the principle of the fluid granulation drum (FGD) for granulation or the coating of granular materials (seeds) (courtesy Kaltenbach-Thuring, Beauvais, France)
6.6 Fertilizers and Agrochemicals
drum, from where it falls onto the surface of the fluidized bed and is cooled by the fluidizing air. Because the tray is sloped, the granules move across the bed and, at the edge, drop down into the lower part of the drum (falling curtain). During its fall, the material is sprayed with a melt or a solution, coating the particles, which are then lifted up and the cycle repeats itself. The drum can operate batch but more typically works continuously with finished product discharging over a weir. The process can also be used outright for the granulation of fertilizer components whereby a melt and/or solution, sprayed onto the falling curtain, provide the liquid binder component for growth agglomeration. Although, as shown in Section 6.6.2, the irregular shape of fertilizer granules obtained by, for example, compaction/granulation (Fig. 6.6-20) is not detrimental to the uniform distribution with mechanical spreaders in the field, there are various reasons for the desire to produce a rounded granule by coating, including a more pleasing appearance of those products destined for sales to small volume consumers (home and garden market) or the fattening of small (undersized) particles. Fig. 6.6-33a shows again some granules that were produced by compaction/granulation and Fig. 6.6-33b is the rounded product after coating in a fluid drum granulator. The FDG can be also used for the agglomeration of special agrochemical materials. Examples are the production of granulated degradable sulfur and of ammonium nitrate fertilizers. To obtain degradable sulfur granules a swelling agent is added, which assists in the break-up when moisture (from irrigation, rain, or dew) becomes present. Fig. 6.6-34 is the flow diagram of a manufacturing process that was designed for this purpose. Approximately 10 % of a bentonite clay (swelling agent) is added to the molten sulfur. The slurry together with make-up water is sprayed into the FDG (for startup, stored recycle from earlier production runs is introduced first) which, for this purpose is slightly modified as shown in Fig. 6.6-35. The discharge is screened and the undersized particles are returned to the granulator for fattening. In contrast to explosive low-density ammonium nitrate (LDAN, Section 6.11.3), fertilizer grade AN must have higher density (high-density ammonium nitrate = HDAN).
Fig. 6.6-33 a) Irregularly shaped fertilizer granules from compaction/ granulation (phosphate); b) rounded fertilizer granules obtained by coating by the FGD process (courtesy Kaltenbach-Thuring, Beauvais, France)
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Fig. 6.6-34 Flow diagram of a system using the fluid drum granulator (FDG) for the production of granulated degradable sulfur for agricultural use (courtesy Kaltenbach-Thuring, Beauvais, France)
Fig. 6.6-35 Sketch of a modified fluid granulation drum (FGD) for the production of granulated degradable sulfur for agricultural application (courtesy Kaltenbach-Thuring, Beauvais, France)
6.6 Fertilizers and Agrochemicals
Calcium AN (CAN) is an ammonium nitrate incorporating a filler (e.g., Dolomite) and, therefore, with reduced nitrogen content (26–34.5 %) and increased safety. The fluid drum granulator process is one of the alternatives for the manufacturing of HDAN and CAN. Fig. 6.6-36 is the flow diagram for the production of these granulated fertilizer grade ammonium nitrate materials from a 98.5 % AN solution; again, for startup, the FGD must be filled with recycle material from earlier production. The granulator discharge is screened on a double deck screen. The oversized particles are redissolved and undersize is returned for further size enlargement in the FGD. Sometimes it may be necessary to crush a part of the product to balance the closed loop recirculation system. Correctly sized particles are cooled in a fluidized bed cooler with dry air (from air conditioner). To avoid sticking of the hygroscopic product the granules are coated in a rotary drum with anti-caking agents (Chapter 4). Another interesting application of coating in this field is the enrobing of seeds with fertilizers and agrochemicals such as fungicides, herbicides, or insecticides. The technology is used particularly for light seeds, such as grass, and is often applied for reseeding golf courses. The coating with fertilizer makes the individual seeds larger and heavier thus allowing easier spreading and the addition of agrochemicals keeps birds, fungi, and insects in check. For other seeds control of weeds and grasses with suitable agrochemicals is a more important task. Microencapsulation (Chapter 5, [B.33, B.67, B.97]) is another one of the methods to obtain products with controlled release properties. It allows to isolate an active component from external media by forming a polymeric network or differently structured
Fig. 6.6-36 Flow diagram of a plant for the production of fertilizer grade granulated ammonium nitrate (courtesy Kaltenbach-Thuring, Beauvais, France)
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wall in which a solid or a liquid substance are enclosed. Objectives for modern agrochemical products are to limit pollution by the uncontrolled release of active substance into the environment, to reduce the quantity of active substance required for a given application, and to improve the material’s efficiency and duration of action in the field. As an example of the use of microencapsulation in agrochemistry, a relatively recent development will be presented [6.6.3.1], although it deals with the “packaging” of a liquid and, therefore, is only connected with agglomeration through encapsulation. It is reviewed to show the principle. The goal of that work is to enclose an insecticide and to mix the microcapsules with disinfectant liquid and yield a two-effect product with long term release properties. In a first step, a liquid-liquid dispersion is formed in a static mixer. The insecticide and a monomer are in the immiscible phase and the continuous phase is an aqueous solution of polyvinyl alcohol. A second monomer is added to the liquid-liquid dispersion to achieve interfacial polymerization, a process in which monomers polymerize at the interface of two immiscible substances. In this case, the resulting microcapsules consist of a liquid core and a permeable wall. The encapsulated insecticide is stored in a concentrated disinfectant. While non-encapsulated insecticide and disinfectants are chemically incompatible and the active substance (insecticide) is destroyed after a relatively short time, the microencapsulated material remains essentially unchanged during long periods of storage. For final agricultural application, the concentrated product must be diluted with water to make a 2 % solution, which is applied with compressed air by hand held or power spray equipment. In the field, as shown in Fig. 6.6-37, the disinfectant solution surrounding the microcapsules evaporates, providing the disinfectant action. Then, the insecticide diffuses through the capsule wall and becomes available on the outside surfaces in small, highly effective amounts of the active substance. As the insecticide dissipates it is replenished by more chemical from the inside. Thus, the product is still working after most other insecticides have lost their effect.
Fig. 6.6-37 Description of the action of a microencapsulated two-effect agrochemical product [6.6.3.1]: a) microcapsule surrounded by a
disinfectant film, b) disinfectant evaporation, c) diffusion of the insecticide through the wall and dissipation of the active substance
6.7 Building Materials and Ceramics
6.7
Building Materials and Ceramics As already mentioned in Chapter 2, the manufacture of artificial building blocks (bricks) for the construction of shelters (protective walls and houses) and of ceramic materials as containers or for decorative purposes were among the earliest applications of agglomeration. Particulate solids (sand) and naturally occurring binders (clays) were mixed and formed into rectangular shapes, dried, and later fired to obtain a readily useable building material. Finer clays (later called china clay) were easily formed into hollow bodies (vases, containers) or artistic shapes that were also heat treated to yield permanent strength and were often decorated by mineral glazes, which fuse during firing into colorful and waterproof coatings. With time the manufacture of kitchen and tableware (earthenware, porcelain), tiles, and numerous industrial articles was added and synthetic raw materials (e.g., cement) and additives (e.g., silica fume, inorganic pigments) were introduced. Since heat treatment of the agglomerated solid particle mixture is a necessary and integral part of any application, size enlargement in this field is always a two-stage procedure, although, sometimes, both operations are carried-out in the same equipment. Both procedures, the production of the green pre-shape (agglomerate) and the heat treatment (sintering), determine the properties of the final product, which include porosity, cold and hot strength, and permanent changes on re-heating [B.13c]. To optimize the characteristics of the final product, particulate solid (feed, powder) treatment has become an important step in the manufacture of high-quality building materials and ceramics (Fig. 6.7-1). As shown in Tab. 6.7-1, agglomeration increasingly plays a major role in the preparation of raw materials and additives. It is used to improve the storage and handling properties of building and ceramic raw materials and additives and to enhance the flow properties and packing efficiency of fine powders prior to forming. As shown in Fig. 6.7-1, particularly for the manufacture of industrial ceramic parts, hot forming (called hot pressing or pressure sintering) or, more precisely, pressure assisted sintering (PAS), the simultaneous application of pressure and heat to a powder mass that is enclosed in a die, gains importance. In general, this technique allows the use of lower temperatures and pressures
Fig. 6.7-1 Block diagram of fabrication routes for building materials and ceramics
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6 Industrial Applications of Size Enlargement by Agglomeration Tab. 6.7-1 Alphabetical listing of raw materials and additives for building materials and ceramics that have been and/or are being agglomerated and the products using or representing agglomerated parts Raw materials and additives Aggregate, alumina (aluminum oxide), cement raw mix, clay, dolomite, fluorspar, kaolin, light weight aggregate, lime, limestone, magnesia (magnesium oxide), pigments, silica (silicon dioxide), silica fume Products Bricks, building blocks, cement, decorative parts, earthenware, industrial ceramics, insulators, light weight aggregate, porcelain, pottery, manufactured concrete parts, refractory materials and parts, tiles
and shorter processing times than required for the two-stage process while, at the same time, producing finer grain sizes, lower porosity, higher purity, and precise dimensional control, resulting in near-net-shape parts. Tab. 6.7-1, without claiming completeness, lists some of the most important materials that are related to building materials and ceramics and have been agglomerated for a multitude of purposes. Further Reading
For further reading the following books are recommended: B.3, B.4, B.6, B.7, B.13a+c, B.15, B.16, B.21, B.22, B.26, B.40, B.48, B.51, B.55, B.56, B.61, B.64, B.70, B.72, B.76, B.80, B.81, B.82, B.89, B.93, B.94, B.97, B.98, B.103 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold.
6.7.1
Tumble/Growth Technologies
Most of the agglomeration techniques used for the manufacture of building materials andceramicsapplypressureforthedensificationandshapingofminerals(Section6.7.2). However, for better handling, metering, and feeding it is often desirable to produce a granular, free-flowing intermediate material in a preparatory step, which may use agglomeration in drums, pans, mixers, and, originating from suspensions, in spray dryers. For the manufacture of cement, two major technologies have been developed, using either dry or wet processing (Section 6.7.3). Before the preheating of the raw materials in high-efficiency cyclones became a standard method to achieve the most economic production of cement, pre-agglomeration of cement raw meal in balling drums was an accepted method of improving feeding to, environmental control of, and clinker production by the traveling grate used for sintering (Section 6.7.3). However, today this technology has all but disappeared. Finely divided clay, often after drying and grinding to destroy natural agglomerates, is a common binder in tumble/growth agglomeration. One of the more common and best known is bentonite (Section 15.1), a montmorillonite (Al[Si2O5]OH). The most
6.7 Building Materials and Ceramics
Fig. 6.7-2 Planetary intensive mixer with a flat bowl (courtesy Eirich, Hardheim, Germany)
valuable clay, used for fine ceramics, such as tableware (porcelain), is kaolinite (Al2[Si2O5](OH)4), which is cleaner but has similar binding characteristics. Therefore, it is easy to agglomerate such materials in drums or on pans with water as binder. However, production of small granules is not possible with this equipment. To achieve the manufacture of small granules, high shear forces must be created in the tumbling mass, preferably in a batch mixer. Typical equipment for this application includes the Eirich planetary mixer as a versatile rugged machine, which was originally designed for the mineral industries (especially building materials and ceramics) [B.48, B.97]. Later the same operating principle that had been carried out in flat bowls (Fig. 6.7-2 and Fig. 9.5, left, Section 9.1) was modified to accomplish better granula-
Fig. 6.7-3 Planetary intensive mixer/granulator with an inclined bowl (courtesy Eirich, Hardheim, Germany)
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tion by using an inclined bowl design (Fig. 6.7-3 and Fig. 9.6, Section 9.1). Although most of the moisture must be removed prior to further use, the spheroidal shape of the dense granules results in excellent flowability, which is often of great interest in the fine ceramics industry and warrants the additional cost of wet agglomeration over dry compaction/granulation (Section 6.7.2). Today’s intermediate ceramic products, particularly for industrial applications, such as silicate molding materials for floor tiles or compounds for high-performance ceramics, must satisfy stringent quality standards. Therefore, conventional processes have been superseded by new technologies. For example, similar to one-pot-processing in the pharmaceutical industry (Section 6.2.1), mixing, homogenization, agglomeration, and drying, all in a vacuum, can be performed in a specially equipped and instrumented planetary mixer (Fig. 6.7-4). Dry prepared raw components, in the case of molding
Fig. 6.7-4 Flow diagram of an Evactherm preparation plant for ceramic molding materials (courtesy Eirich, Hardheim, Germany)
6.7 Building Materials and Ceramics
compound for ceramic floor tiles consisting of clay, feldspar, and grog, pre-ground to < 63 lm, are combined in a batching scale and transferred to the Evactherm mixer. Special (binder) additives are not required. After blending, using steam and water, the homogeneous mix is wetted to 10–11 % and agglomerated (granulated). Finally, the finished product is gently dried by the combined action of steam and vacuum in the same unit to about 6 %, the moisture desired for the next (forming) process. To meet the granule size specification (0.1–1.2 mm, Fig. 6.7-5), oversized particles are removed by screening and recirculated to the feed end of the process where, after milling, it is used as a component of the batch. The process has minimum impact on the environment as exemplified by Fig. 6.7-6 showing the extreme cleanliness in two Evactherm preparation plants. Through the use of vacuum technology, product quality and yield can be raised considerably. In response to the needs of the industry, similar processes have been developed by other vendors using different mixer designs (Section 15.1). Spray drying [B.48, B.97] is used in the manufacture of granulated press powders for tiles and electronic parts. It also plays an important role in the industrial production and development of high-performance (advanced) ceramics because of the ability of this process to meet size distribution requirements and give granules with a smooth surface and spherical shape, all resulting in excellent flowability, which is important for the subsequent forming step. Another reason for the widespread use of spray dryers in the ceramic industry is the fact that they can handle abrasive feedstocks without problems.
Fig. 6.7-5 Scanning electron micrograph of silicate ceramic granules produced in an Evactherm preparation plant (courtesy Eirich, Hardheim, Germany)
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Fig. 6.7-6 Two preparation plants with Evactherm mixers demonstrating the extreme cleanliness of this process technology (courtesy Eirich, Hardheim, Germany)
6.7 Building Materials and Ceramics
In spray drying of ceramic materials, suspensions of particulate solids or pumpable slurries are divided into small droplets at the top of a tower by special wear resistant nozzle(s) [B.97] and dried with hot air. Since drying begins at the surface of the droplets, it is possible that voids are formed within the granules, which reduce the density of the product. To obtain the required density of the green (before sintering, Section 6.7.2) ceramic part, a longer stroke of the press is necessary which, sometimes, may be objectionable. Although the high cost of the energy to evaporate the suspending liquid burdens the process increasingly, the low expenditure for personnel, only a few persons are required for operation and maintenance, together with other advantages, related to granule shape and size, make the technology still attractive for the ceramic industry. In the building materials industry, particularly for the do-it-yourself markets, it becomes an increasing trend to pre-mix all components. Such blends may contain sand, lime, cement, pigments, plasticizers and others. Owing to different particle sizes and densities of the ingredients and the often limited mixing capabilities of the user, they tend to segregate in the bag or container and during application, causing quality and color variations in the finished product. Separation during packing, storage, and handling of the homogenized blend is avoided by encouraging some agglomeration during mixing, combining the components uniformly into loose, non-segregating, easily dispersible granules. Manufacturers of high-quality building materials require machines and plants that must be rugged and tough to withstand the harsh conditions caused by the abrasive raw materials. Particularly the mixer/agglomerators must be of relatively simple but efficient design to allow wear protection by hard metal liners and the application of overlay coatings on the blending tools. Fig. 6.7-7 shows the building with storage silos of a plant for the production of dry mortars and Fig. 6.7-8 is a view into a facility with high-intensity mixers for the processing of sand-lime brick aggregates. Silica fume, a by-product of the manufacture of ferrosilicon and silicon metal that is captured in the plants’ dust collection systems (Section 8.1), consists of amorphous silica particles that are spherical, have very small dimensions (0.05–0.5 lm [B.97]), and feature a large specific surface area. These characteristics make it an excellent admixture to, for example, high-strength concrete and high-performance grouts and mortars. The amorphous structure and high surface area render the particles very reactive, causing pozzolanic effects, and the small particle size results in highly impermeable structures of building materials. In pre-stressed concrete, the latter protects the reinforcement bars from attack by water. As collected, silica fume is very dusty, difficult to handle, self-agglomerating (causing bridging, build-up, and lumping), and can not be transported and handled economically. Because of its very low bulk density (160–240 kg/m3), bulk tanker trucks only hold 8–10 ts and require long pump-off times and if bags are used they are large, light, and bulky. A simple dry fluid bed agglomeration process (Fig. 6.7-9 [B.97]) converts silica fume by dry agglomeration into a product, which is less dusty, flows well, and can be handled pneumatically. Product density may be such that the tanker truck now holds about 25 ts and can be adjusted to fit different handling and end use applications. At the same time, agglomerate bonding is so weak that the product disperses easily, for example in cement mixers (compare Section 6.3.1, pigments).
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.7-7 Plant for the production of dry mortars with storage silos
A final application of tumble/growth agglomeration for building materials that shall be mentioned is the manufacture of expanded clay, often also called ESCS (expanded shale, clay, and slate). The product is a lightweight ceramic aggregate. After mining the clay, the raw materials are cleaned (removal of coarse and foreign components), homogenized by kneading, and divided into small pieces (Section 6.7.2). At the cold end of a long rotary kiln the clay pieces are first agglomerated and rounded. Later, at temperatures of about 1200 8C, the clay expands forming a porous interior while the outer surface fuses into a strong ceramic skin (Fig. 6.7-10). For different applications, various sizes in the range 0–25 mm are produced by screening. The manufacture and raw material selection processes are strictly controlled to ensure a uniform, high-quality product that is structurally strong, stable, durable and inert, yet also lightweight and heat and sound insulating. Therefore, ESCS gives building designers greater flexibility in creating solutions to meet the challenges of wall and
6.7 Building Materials and Ceramics
Fig. 6.7-8 View into a facility with high-intensity mixers for the processing of sand-lime brick aggregates (courtesy Eirich, Hardheim, Germany)
Fig. 6.7-9 Diagram of a fluidized bed agglomerator for dry silica fume (courtesy Norchem Concrete Products, Fort Pierce, FL, USA)
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Fig. 6.7-10 Expanded clays, showing the porous interior and the fused ceramic skin (courtesy Fibo ExClay, Lamstedt, Germany)
construction specifications, dead load, terrain, seismic conditions, and budgets. According to the Expanded Shale, Clay, and Slate Institute (Section 15.1), for nearly a century (originally developed in 1908) expanded clay products have been used successfully around the world in more than 50 different types of applications. The most notable among these are lightweight concrete masonry, high-rise buildings, concrete bridge decks, high-performance concrete (HPC), pre-cast and pre-stressed concrete elements, asphalt road surfaces (chip seal, asphalt surface treatment), soil conditioners, and lightweight geotechnical fills. Today ESCS aggregate, as manufactured by the rotary kiln process, is available throughout the world. ESCS ceramic lightweight aggregate improves concrete in several ways. Tab. 6.7-2 summarizes the advantages of this product in the building industry. Tab. 6.7-2 Advantages of ESCS in the concrete building industry (adapted from information provided by The Expanded Shale, Clay, and Slate Institute, Salt Lake City, UT, USA, and Fibo ExClay, Lamstedt, Germany) * *
*
*
*
Ability to design concrete with specified density, especially low density concrete. Concrete with ESCS lightweight aggregate features * better thermal properties * better fire resistance * reduced shrinkage * reduced chloride ion permeability * improved freezing and thawing durability * improved contact zone between aggregate and cement matrix * less micro-cracking as a result of better elastic compatibility * better blast resistance * better shock and sound absorption * no attack by rodents and vermin * no decay and decomposition. ESCS carrying absorbed water provides internal curing (often referred to as self-curing or water entrainment) that improves the hydration of the cement. High-performance lightweight aggregate concrete also has less cracking, improves skid resistance, and is readily placed by the concrete pumping method. Concrete with ESCS lightweight aggregate can be recycled (typically after crushing).
6.7 Building Materials and Ceramics
6.7.2
Pressure Agglomeration Techniques
In most cases, the shaping of building materials and ceramic parts (collectively called “ceramics”) is accompanied with a densification of raw mineral ingredients with or without additives (such as binders, lubricants, plasticizers, pore formers) and mixtures thereof. Also, many final parts, particularly high-performance ceramics, must in the end have net shape, meaning that they exhibit ultimate dimensions and tolerances. In this respect, the manufacturing process is similar to that encountered in powder metallurgy (P/M, Chapter 7). While for P/M parts, a near-net-shape that requires some final machining before the intended use is often satisfactory, additional machining of ceramics is normally not feasible or possible because of their low cost (e.g., building materials) and, generally, their hardness, abrasiveness, and brittleness. Therefore, during the manufacture of ceramic pre-shapes for subsequent sintering, it is most important to take into consideration possible shrinkage and/or distortion during firing of the green parts. The extent of densification during the initial forming of ceramic powders into green parts defines the amount and orientation of shrinkage during post-treatment (sintering). Within certain limits, the density of a final part sintered from a low-density green ceramic pre-form will be the same as that obtained from a green part with high density, but the higher shrinkage during firing makes it more difficult to anticipate and control the shrinkage to produce a well-dimensioned and shaped final part. Therefore, for the manufacture of true-to-size ceramic components the density of the green part should be as high as possible. Component size, particularly the relation between vertical and horizontal dimensions of a green part, also plays an important role in how the outline of a body changes due to shrinkage during sintering. For example (Fig. 6.7-11, [B.21, pp. C39–C52]), if the same mass of a ceramic raw material (steatite) is compressed by a punch in a die with square cross section using varying compaction pressures, different green densities, characterized by the height Hu (upper part of Fig. 6.7-11) of the resulting body, are obtained. With increasing compaction pressure, the density of the part increases (decreasing volume). Of course, the length of the edges, Lu, is always the same and is equal to the sides of the die. During firing, these bodies shrink and, as mentioned above, all parts attain approximately the same density (in the case reported here, 2.75 g/cm3). However, as shown by Hg, the heights, and Lg, the edge lengths of the sintered parts, the shape of each body is different. Because with increasing pressure a more pronounced orientation of non-isometric particles in the raw materials occurs, the deformation of the fired parts as a function of compaction pressure is further intensified (lower part of Fig. 6.7-11). Sh (shrinkage of the height) and Sq (shrinkage of the edges) do not change at the same rate. Many raw materials for ceramic parts contain clays. During densification, the lamellar shape of clay minerals results in a more-or-less defined texture because the foliated silicates tend to orient themselves vertically to the direction of force. Since lamellar particles shrink non-isotropically, such textures may result in high stresses in the sintered part, causing cracking during cooling. The orientation effect also plays a role in
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.7-11 Above) dimensions of fired (sintered) steatite parts obtained from green bodies with identical mass that were densified with different pressures, Lu = unsintered length, Lg = sintered length, Hu = unsintered height, Hg = sintered height; below) vertical and horizontal shrinkage that occurred during sintering, Sh = height shrinkage, Sq = length shrinkage [B.21]
Fig. 6.7-12 Vertical and horizontal shrinkage occurring during the firing (sintering) of parts from raw materials with different densities, which were compressed to the same green density and the relation of both [B.21]
6.7 Building Materials and Ceramics
defining the final dimensions since it may influence the horizontal and vertical shrinkage in different ways. For example, tests using steatite raw materials with different densities revealed that, if green parts are produced with the same density by the application of different compression forces (high- for low-density particles and low- for high-density particles), the shapes of the sintered bodies are not the same [B.21, pp. C39–C52]. According to Fig. 6.7-12, with increasing density of the raw materials the vertical shrinkage decreases while the horizontal contraction increases somewhat. As a result, the relationship between vertical and horizontal shrinkage declines steeply. Therefore, if higher compaction pressures are used during the shaping of green bodies, the resulting sintered parts are shorter and wider. This was explained by the more pronounced flow of material during increased densification when higher forces are applied, which results in a distinctly intensified orientation of the lamellar particles perpendicularly to the direction of the force. It has been shown earlier (Section 6.2.2, Fig. 6.2-14) that during the densification and compaction of particulate solids, density variations occur as a result of the effects of internal and external friction. Together with the associated changes in primary particle orientation, which are further modified by variations in the direction of the acting forces, rather unpredictable shrinkage behavior during sintering is obtained if these effects are not considered during manufacture of the green bodies. This is particularly important if parts with different cross sections are to be made and show the importance of the agglomeration step for the entire manufacturing process and the shape and the dimensions of the sintered component. More generally, a thorough understanding of the microstructural dependence of the properties of green and fired ceramics is necessary [B.80]. It is the critical link between forming, processing, and properties and/or performance of the finished part. Forming (always an agglomeration method) is the manufacture of specific shapes by punchand-die pressing, isostatic pressing, injection molding, extrusion, and slip casting and includes as crucial parameters also the effects of binders, mixing, and consolidation for producing the actual size and shape of uniform green bodies. Processing refers to the post-treatment by the application of heat and includes the influences of particle character, green density, and sintering parameters on the microstructure of the finished components. Forming and processing are often referred to in one term, fabrication, because the microstructural features of the final part are a direct consequence of the starting material’s character and the parameters of forming and processing. Fabrication of ceramics typically begins with the consolidation of powders. After that, two alternative paths are commonly followed [B.80]. One is to obtain a desired porosity to achieve key application characteristics. The requirements may include a particular specific surface area, which increases with porosity but must be balanced against other physical properties that decrease with increasing porosity, particularly strength (Section 6.7.3, Tab. 6.7-5). The other path is used when high or maximum levels of physical properties, such as strength, optical transmission, thermal conductivity, are sought, which necessitate low to zero porosity. With increasing firing conditions (temperature and time), many characteristics of the final component, such as thermal and electrical conductivity and elastic properties, increase at different rates for
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a given material to varying plateaus. Similarly, other properties, especially most mechanical attributes, initially increase at varying rates but pass through variable maxima and then decrease (Fig. 6.7-13). All these changes are related to the starting particle size and shape, their orientation and packing (density) in the green body and other parameters, which result from the different agglomeration methods. As mentioned at the beginning of this section, in most cases, the shaping of building materials and ceramic parts is accompanied by a densification of raw mineral ingredients with or without additives (such as binders, lubricants, plasticizers, pore formers) and mixtures thereof to yield green bodies, which must be post-treated (Section 6.7.3) to achieve the ceramic part’s final properties. Although a number of different agglomeration methods are used to a varying degree for the manufacture of green ceramic bodies, three pressure-based technologies are the most common ones: * * *
punch-and-die pressing extrusion, and isostatic pressing.
The oldest ceramic product is bricks, a building material made from naturally occurring moist or wetted clay, originally shaped by hand in wooden forms, sometimes reinforced with fibers (straw), and air (sun) dried to obtain a building block. In later times, the green body was fired to yield a strong and waterproof product. While today in some undeveloped areas, this basic (manual shaping and sun drying) manufacturing principle is still used and applied for the construction of adobe buildings, large quantities of bricks with many different qualities and for a large number of applications are produced by punch-and-die presses and extruders. At the beginning of industrialization, various designs of mechanical punch-and-die presses with single or multiple cavities, including “stone presses” with indexed die tables, were used, particularly for the manufacture of high-quality bricks that required dimensional precision for their application (e.g., refractory bricks). Among the earliest machines of this type was the Couffinhal press (Fig. 6.10-14, Section 6.10.2). Later, hydraulic presses were also used.
Fig. 6.7-13 Diagram of maxima of properties, such as strength, that are dependent on both porosity decreases and grain size increasing as a function of firing temperature or time [B.80]. The dashed line suggests that maxima may change with different material and fabrication parameters
6.7 Building Materials and Ceramics
Fig. 6.7-14 Typical robust double-screw extruder with integrally mounted pug sealer for the processing of stiff materials (courtesy J.C. Steele, Statesville, NC, USA)
For mass production, however, extrusion has become the technology of choice [B.97]. After first applying ram extrusion (Exter press), although this machine was originally invented for the briquetting of solid fuels (Fig. 6.10-3, Section 6.10.2), the continuous single- or double-shaft screw extruder was introduced and, today, is the prevalent equipment for the manufacture of clay-based building materials, including bricks [B.97]. Fig. 6.7-14 shows a typical, albeit smaller, robust double-screw extruder with integrally mounted pug sealer for the processing of stiff materials, such as clays. Pug sealers are conditioners that combine an open pug mill for intensive high-shear mixing with an enclosed (sealing) screw for degassing and a shredder for easy feeding into the vacuum chamber of the extruder with two counter-rotating screws. The tough, plastic and/or sticky clay is transported into a wide throat area, which ends in a flanged front barrel that accepts a wide variety of dies and die holders. A variety of dies is available for the extrusion of solid and hollow bricks and specially shaped building blocks, roofing tiles, and other hollowware for construction purposes. Fig. 6.7-15 shows some of the finished extruded products. On the extruder mouth, the dies can be mounted fixed or in single- and double-hinged holders or a hydraulic chan-
Fig. 6.7-15 Some typical extruded clay-based building products (courtesy J.C. Steele, Statesville, NC, USA)
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ger, the latter to allow quick change-over between different shapes [B.97]. The continuous strands emerging from the extruder are cut to size with different cutter designs. Individual large extruders can produce more than 40 000 standard bricks per hour representing a capacity of about 80 t/h. More complicated shapes, required in much smaller numbers, are made on versatile, die-changer-equipped smaller machines as shown, for example, in Fig. 6.7-14. Lower duty, simpler machines are used during the manufacture of expanded clay (Section 6.7.1). As depicted in Fig. 6.7-16, the product still shows that it originated from cut extruded ropes. A similar expanded clay material is made using pellet mills, particularly the heavy-duty flat die machines (Chapter 5, Fig. 5.10, b.2), for the manufacture of green clay extrudates. Extruders are also used in the production of sintered industrial parts. Of special importance is their application for the manufacture of catalyst carriers (Section 6.3.2, Figs. 6.3-20 and 6.3-21). The use of wear-resistant alloys for all parts that come in contact with the material to be extruded make the extruders suitable for the processing of highly abrasive catalysts, other chemicals, and minerals, such as molecular sieves, high-purity aluminas, and kaolin carriers. Interchangeable die plates allow the extrusion of an almost unlimited variety of sizes and shapes on the same basic machine that is then equipped with a variable-speed screw-drive to adjust retention time, pressure, and production rate. For more structured industrial, sanitary, and household ceramic parts mechanical and hydraulically operated punch-and die presses are used. Fig. 6.7-17 depicts a modern mechanical automatic press showing the complexity of such equipment. Such machines are available with pressing forces of 60–4500 kN, filling height of up to 180 mm, pressing travel (densification) of up to 90 mm, and stroke rates of 4–60 per minute (decreasing with increasing press size). Hydraulic presses (Fig. 6.7-18) are stronger (up to 26 000 kN) but all feature lower stroke rates (maximum 30 per minute and typically less). With both types of presses the platens can be subdivided into smaller sections (Fig. 6.7-19) to accommodate multiple die holders for the manufacture of smaller parts and to increase the production rate for a more economical operation. Fig. 6.7-20 shows several industrial ceramic parts demonstrating the detail that can be achieved with this technology.
Fig. 6.7-16 Expanded clay produced from green extrudates (courtesy J.C. Steele, Statesville, NC, USA)
6.7 Building Materials and Ceramics
Fig. 6.7-17 Modern automatic mechanical press for the production of green ceramic bodies (courtesy Dorst, Kochel am See, Germany)
During densification, particularly of powders, density variations are obtained as a result of the effects of internal and external frictional forces. Fig. 6.7-21 depicts the simplified density distribution obtained after the unidirectional compaction of particulate solids in a cylindrical mold (Section 6.2.2, Fig. 6.2-54). In all applications that require post-treatment by heat to obtain final properties (Chapter 7), density variations or gradients in a green body must be of concern as they will cause uneven shrinkage and may result in distorted or warped final parts. During the unidirectional compaction of a part with variable cross sections, the density is higher just above an internal corner because of the elevated shear stress at this point in the mold (Fig. 6.7-22). Under the corner, the density is lower since the high-density region above prevents the powder from flowing downward. In addition to all the previously mentioned shape related problems due to this density distribution, a crack is likely to open up in the corner, which may already be present undetected in the green body [B.61].
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Fig. 6.7-18 Modern automatic hydraulic press for the production of green ceramic bodies (courtesy Laeis Bucher, Trier, Germany)
Fig. 6.7-19 Example (ref. Fig. 6.7-18) of a mold use and performance table (courtesy Laeis Bucher, Trier, Germany)
6.7 Building Materials and Ceramics
Fig. 6.7-20 Industrial ceramic parts demonstrating the detail that can be achieved with this technology (courtesy Komage, Kell an See, Germany, and Dorst, Kochel am See, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.7-21 The density distribution obtained after the unidirectional compaction of particulate solids in a cylindrical mold
Fig. 6.7-22 Areas of major density variation obtained during the unidirectional compaction of a part with variable cross sections [B.61]
The various potential defects resulting from the changing density in a compressed powder part are of great concern. Net-shape articles are often desired because of the abrasiveness and/or hardness of many of the components preventing machining after sintering. This means, that, after taking into consideration the unavoidable dimensional changes during and after the sintering process, the finished item should posses dimensions that need no or minimal adjustment for its intended use. One way of reducing the development of uneven densities in green parts during compaction is to lower friction by the addition or application of lubricants [B.48, B.97] (Section 6.2.2). A secondary effect of lubrication is that wear of the tooling is also reduced. “Double pressing”, that is, the densification of particulate solids by the movement of both the top and bottom punches, is used to overcome at least part of the uneven density distribution caused by unidirectional pressing. If both punches move towards each other with the same speed, exerting the same force, a mirror image of the density distributions across a “neutral axis”, which, in this case, is in the middle of the body, will be created (Fig. 6.7-23). If, in addition, the die walls move with or to some extent in regard to the travel of the punches (withdrawal die), the location of the neutral plane
6.7 Building Materials and Ceramics Fig. 6.7-23 Sketch of the density distribution and the location of the “neutral axis” in a simple (cylindrical) green part obtained by symmetrical “double pressing”
(axis) can be further influenced (B.28, B.48, B.97]. It is necessary to control the position of the neutral planes (low-density zones that are perpendicular to the direction of pressing) particularly in complex parts. This is achieved by the relative motions of the tools (Fig. 6.7-24). It is also important to understand that within the developing structure of a compact, especially under pressure, particles will not move from one level or position to another. As a consequence, if parts are pressed that have more than one shape feature, separate forces and movements must be applied simultaneously, and neutral planes will exist in each level (Fig. 6.7-25).
Fig. 6.7-24 Drawings describing ways of influencing the location of the “neutral plane” in unidirectional (upper punch) pressing by controlled die withdrawal [B.28, B.48, B.97]
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Fig. 6.7-25 Different “neutral planes” in single- and multi-level parts [B.28, B.48, B.97]
Today, the best method of overcoming the problem of uneven densities in a green body is isostatic pressing. In this compaction process, the pressure is applied by a fluid that is pressurized and acts uniformly from all sides on dry particulate solids that are enclosed in a flexible container (“mold”) immersed in the fluid [B.13a]. This results in the most uniform consolidation possible. Of course, there is still a density gradient across the part, the center is always less compacted than the surface, but the gradient is uniform and, typically, does not cause distortion during sintering. Isostatic pressing was patented around 1910, and about 30 years later the technology found a general and large scale application for the direct isostatic pressing of spark plug insulator pre-forms [B.48]. Technically it is quite immaterial whether or not sintered ceramic spark plug insulators are totally straight. However, as a consumer product which, at that time, needed replacement often, the part, in addition to functioning well, had to look good. After introducing isostatic pressing into the manufacturing process, the number of rejects became very low. Later, the technology was used for the production of many other ceramic parts, particularly of the high-performance variety, and for many applications in powder metallurgy (Chapter 7). Isostatic pressing is carried-out cold or hot using dry powders as feed material. The most common application is still cold isostatic pressing (CIP), which is performed at ambient temperatures. In hot isostatic pressing (HIP) the forming and densification process is achieved uniformly by heated high-pressure gas in an autoclave. Especially in powder metallurgy (Chapter 7), the material to be processed is itself often also brought to elevated temperature prior to loading. Unlike CIP, in which powders are always containerized, HIP may be applied for containerized powders and also for preformed non-containerized components manufactured by any method. Hydrostatic pressing is a term often used as a synonym for isostatic pressing. Isostatic pressing is the generic term covering liquids and gases as the pressure transmitting medium whereas hydrostatic pressing is best reserved for liquids. However, the two are used interchangeably to cover both aspects. If flexible containers are used, their arrangement may be such that they contract or dilate under the application of pressure. Whether the tooling is an integral part of the
6.7 Building Materials and Ceramics
press or loaded and removed during each compaction cycle determines if it is a “drybag” or “wet-bag” process [B.48, B.97]. The difference between the two methods is illustrated in Fig. 6.7-26. In the dry-bag process, the flexible container is fixed in the pressure vessel and the powder is loaded directly. The tool forms a membrane between the fluid and the powder. Optionally, the flexible container may be placed inside a primary diaphragm so that the powder never comes in contact with the fluid, even if the flexible mold is damaged or breaks. Therefore, dry-bag pressing also has the advantage that the fluid is not contaminated by the powder. However, because the container must stand up to many pressing cycles and since changing is time consuming, it has to be made of a very durable material. The wet-bag process, in which the container that has been filled externally with the powder, is entirely submerged in the fluid inside a pressure vessel, uses the simplest type of equipment. This process is commonly used for the production of single large components or a large number of small parts. While dry-bag tooling can be fitted with means to remove the gas displaced during densification, the material in containers for wet-bag pressing must be consolidated and evacuated prior to closing and loading into the autoclave to avoid compressed air pockets which, upon pressure release, may damage the structure of the compacted parts. The basic principles of isostatic powder pressing are summarized in Tab. 6.7-3. Pressure equipment consists of powder storage and dispensing facilities, at least one pressure vessel with means for loading and unloading the tooling or parts, pressure generator(s), and related items that enable effective and safe operation of the process. Dry-bag pressing is used for the production of small components at a
Fig. 6.7-26 Diagram of the differences between dry- and wet-bag pressing [B.13a, B.48, B.97]
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6 Industrial Applications of Size Enlargement by Agglomeration Tab. 6.7-3
Summary of the basic principles of isostatic powder pressing [B.97]
1. The wet bag pressing of large and/or complex shapes in which the flexible container is filled and prepared outside the pressure vessel and then immersed in the fluid and compacted. 2. The dry bag pressing of smaller, regular shapes in which the tooling forms an integral part of the pressure vessel. 3. The use of internal or external rigid formers to produce accurate surfaces. 4. Pressurization by systems using pumps or by direct compression with pistons in a die. 5. Handling systems for the filling, preparation, loading and unloading of powders and parts as well as for the tooling and pressing equipment.
high rate. Fig. 6.7-27 demonstrates the compaction, ejection, and filling of a dry-bag press during the manufacture of spark plug insulators. It is relatively easy to automate the operation of this process. The permanent location of the tool and small fluid volume surrounding it contribute to a fast operation. Production rates in the neighborhood of 100 parts per minute are common. The actual production speed depends on powder properties, size of the part, maximum pressure and potential dwell time requirements, the number of tool cavities and pressure and handling needs. Some automated installations feature a round, sequenced pressure-chamber system (Fig. 6.7-28) while others hold multiple dies in a
Fig. 6.7-27 Operational sequence of a dry-bag isostatic press for the manufacture of spark-plug blanks [B.13a, B.48, B.97]
6.7 Building Materials and Ceramics
common tool holder (Fig. 6.7-29). The cold isostatic press shown in Fig. 6.7-29 is designed for the production of balls (5–90 mm diameter), tubes (4–150 mm diameter, up to 250 mm long), rods (3–114 mm diameter, up to 250 mm long), filter inserts, and valve bodies. The three main, of several possible, demolding (extraction) methods are demonstrated in Fig. 6.7-30, and Fig. 6.7-31 shows some typical finished parts. While hot isostatic pressing was originally developed and used to remove defects and/or produce parts with minimum porosity and, consequently, ultimate density from powders and preforms, the study and understanding of the mechanisms of pressure agglomeration also led to a modification of the process, which is actually used to produce parts with a controlled high porosity [B.78, B.97]. During this new HIP process for making porous products, open, non-containerized powder compacts or loosely sintered bodies are HIPed at high temperatures in a high-pressure gas atmosphere. In this situation, overall densification of the part is avoided by the high-pressure gas that fills the open pores. Thus, high open porosity of sintered components can be obtained by the combination of open HIPing and simultaneous sintering from materials that normally experience considerable densification if conventional HIP or CIP followed by sintering are applied. The porous products are, for example, suitable as filters. Today, the efforts to avoid distortion of ceramic products during firing by the use of isostatic pressing for the production of preforms has even led to the design and application of this technology for tableware. Fig. 6.7-32 shows the open tooling section in a “free fall” horizontal isostatic press, the two parts of the die system, and the depic-
Fig. 6.7-28 Operational sequence of an automatic isostatic press with round, timed (sequenced) tooling table [B.13a, B.48, B.97]
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Fig. 6.7-29 Photograph and diagram of a dry-bag CIP system (courtesy Dorst, Kochel am See, Germany). 1, protective enclosure; 2, control cabinet; 3, hydraulic unit; 4, high-pressure intensifier pump; 5, loading device; 6, hopper; 7, dosing device;
8, closing lever with preload cylinder at top; 9, upper tool; 10, device for parts removal from above (Fig. 6.7-30); 11, lower tool; 12, conveyor belt; 13, tool component for ejection from below (Fig. 6.7-30); 4, closing lever with preload cylinder at bottom
6.7 Building Materials and Ceramics
Fig. 6.7-30 The three main demolding (extraction) methods (courtesy Dorst, Kochel am See, Germany)
Fig. 6.7-31 Some typical parts manufactured by cold isostatic powder pressing (courtesy Dorst, Kochel am See, Germany)
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tions of a raw ceramic bowl and the corresponding tooling design. In the latter, the cross-hatched parts represent polyurethane as a (protective) coating (top) and a membrane (bottom) through which the isostatic pressure for consolidation is applied. Fig. 6.7-33 presents some shapes and actual flat ware products that can be made Fig. 6.7-32 Tooling section of a horizontal isostatic press for the manufacture of tableware preforms: a) open section in a “free fall” horizontal isostatic press, b) the two parts of the die system, c) parts of a raw ceramic bowl and the corresponding tooling design (courtesy Sama, Weissenstadt, Germany)
6.7 Building Materials and Ceramics
Fig. 6.7-33 Some shapes and actual flat ware products that can be made with a “free fall” horizontal isostatic press (courtesy Sama, Weissenstadt, Germany)
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Fig. 6.7-34 A “free fall” horizontal isostatic press for the manufacture of tableware preforms (courtesy Sama, Weissenstadt, Germany)
by this method and Fig. 6.7-34 shows a machine that transforms dry ceramic spray granulate into evenly compacted dry-pressed articles that do not deform or lose shape and size during firing. The method can be applied for the manufacture of articles that are round, smooth or festooned, square or multi-cornered, with reliefs or not round, small or large (from saucers of 10 cm diameter to oval plates 40 cm long) and flat or deep (including 9–10 cm deep salad bowls) from porcelain, stoneware, earthenware, vitreous china, and bone china.
6.7 Building Materials and Ceramics
6.7.3
Other Technologies
Agglomeration by the application of heat (sintering) is a technology that is used during the manufacture of one of the most important building materials, cement, and of others and for the post-treatment (finishing) of essentially all ceramics. Chapter 2 described how the production of materials for the building of shelters was among humankind’s first activities. The search for materials to bond stones together also has a long history, which extends back to Neolithic times [B.69]. Carbon dating of materials found at a site in southern Galilee indicates that a lime-based concrete, used for the construction of polished floors, was made around 7000 BC. The extensive excavated floor area reveals that considerable quantities of lime were necessary to produce such a large amount of concrete. From this it was deduced that the technology of burning (calcining) limestone to form calcium oxide (CaO, lime), slaking the lime with water to form Ca(OH)2, and then mixing the slaked lime with limestone aggregate to obtain concrete was well known to the Neolithic builders. Lime-based concrete hardens slowly by reaction with carbon dioxide forming calcium carbonate. Later, in the construction of the Pyramid of Cheops (2613–2494 BC), another inorganic cementing material was made from gypsum (CaSO4 2H2O). The calcium sulfate was first dehydrated, then mixed with sand, and, when water was added, rehydration occurred, forming interlocking gypsum needles, which give the mortar its strength. Other historians have somewhat different opinions on the composition of early cements but all confirm the long history of using inorganic cementing materials by humans. Hydraulically setting cements were first developed by the Greeks and Romans [B.69]. It was found that, after the addition of volcanic ash (pozzolana) to the slaked lime and sand, a mortar was obtained that possessed superior strength. This was discovered by the Greeks (700–600 BC) and later passed on to the Romans (150 BC) who called it Caementum. It was used, for example, for the construction of the Colosseum in Rome. Hydraulically setting concrete, a mixture of aggregate, sand, and Caementum, has been used since Roman times. Throughout the centuries, a number of modifications and improvements were made but without changing the basic composition and the use of natural pozzolanic materials. In 1818 in France, Vicat prepared an “artificial Roman cement” by calcining a mixture of limestone and clay, which was distinctly different from naturally occurring clay/lime admixtures [B.69]. It was the forerunner of Portland cement, although Portland cement was only patented by Aspdin in 1824. The modern high-temperature process was discovered in 1844 by Johnson who heated the ingredients to a temperature at which they partially melted. It was recognized that the amounts of clay and lime had to be carefully proportioned but it took until 1887 (Le Chatelier) before upper and lower limits were defined. The production of Portland cement normally involves the firing of a mixture of finely ground calcareous and argillaceous materials in a kiln at about 1500 8C and the formation of a clinker, which consists of a number of compounds that set (harden) when the clinker is ground to a fine powder (cement) and then mixed with water. Today it is known that the quality of cement is defined by the fineness and the exactly
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Fig. 6.7-35
Diagram of a rotary cement kiln [B.69]
controlled composition of the mixture of raw materials, the temperature and duration of the calcination, the method of cooling the clinker, and the specific surface area (fineness) of the final product. In ancient Roman and later times vertical (now called shaft) kilns were used. Originally, they were manually charged and controlled. The irregular operation often resulted in clinker with unpredictable and inferior properties. More recently, the raw materials were finely ground, uniformly mixed, agglomerated in drums or discs, and fired employing tight process control (Section 6.8.1, Fig. 6.8-17a). The disadvantages of this kiln design are the large overburden pressure under which pellets may break, disturbing the gas flow and resulting in uneven clinker quality, and a relatively low production capacity. The development of the rotary kiln, now predominantly used for the production of Portland cement, started around the late 1870s but the technology was not patented until 1885 (Ransome [B.69]). A rotary kiln (Fig. 6.7-35) is a long refractory-lined steel cylinder that is inclined at about 3–68 to the horizontal. At the lower end is a burner (coal, oil, or gas fired) and the material to be calcined enters on the other end. While passing down the kiln, chemical and physical reactions take place in the oxidizing atmosphere. Hot clinker emerges at the burner end and must be suitably cooled before being milled to yield cement. The rotary kiln process may be carried out with wet (slurry), semi-dry, or dry feed. Two agglomeration processes occur successively in the rotary kiln using wet feed. The schematic diagram of a coal (dust) fired wet process rotary kiln, also showing ancillary equipment and components, is depicted in Fig. 6.7-36 [6.7.3.1]. The raw materials are mixed and ground before water is added to form a slurry (37–39 % moisture), which is fed into the kiln (right side of Fig. 6.7-36). In the kiln, water is driven off and, as the moisture evaporates, the flow properties of the slurry change, tumble/ growth agglomeration occurs, and granules are formed. In Fig. 6.7-37 typical temperature profiles of gas and solids along the length of the kiln are shown [B.69]. As the granules dry in the early part of the preheating zone (material temperature < 100 8C) the colloidal clay provides a permanent bond, which is enhanced by partial melting and sintering in the calcining and clinkering zones. Air (gas) is preheated during clinker cooling and used in the burners and the rotary kiln to boost the temperature. The wet process, starting with a large amount of water in the feed (slurry), requires costly pre-drying to initiate agglomeration within the rotary kiln. Therefore, as soon as balling (wet tumble/growth agglomeration) in drums and pans became a reality
6.7 Building Materials and Ceramics
Fig. 6.7-36 Diagram of a coal (dust) fired wet process rotary kiln, also showing ancillary equipment and components [6.7.3.1]
around the middle of the 20th century and was applied for the size enlargement of fertilizers (Section 6.6.1) and iron ores (Section 6.8.1, Fig. 6.8-17b and c), in an effort to develop a more economical method for the manufacture of Portland cement, cement raw meal mixtures were also converted into spherical pellets, now containing only 10– 12 % moisture. With this greatly improved feed (in addition to the lower moisture content pellets also feature more uniform composition and structure than the nodules formed in the rotary kiln), grate and grate-kiln machines were introduced to carry out the highly efficient “semi-dry” cement making processes.
Fig. 6.7-37 Typical temperature profiles of gas and solids along the length of a rotary cement kiln [B.69]
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Fig. 6.7-38 Traveling grate for the drying and partial calcination of pellets (agglomerates, nodules) prior to entry into the rotary kiln [B.69]
Because the feed mixture to the cement clinker manufacturing furnace is already agglomerated, a more efficient method of heat exchange can be used. In the grate-kiln type furnaces (Section 6.8.1, Fig. 6.8-11c and 6.8-16) pellet drying and preheating are carried out on a traveling grate in downdraft fashion with the hot gases coming from the rotary kiln (Section 6.8.1 and Fig. 6.7-38). Fig. 6.7-39 shows the use of only a traveling grate for the production of cement clinker. Calcination and clinkering are accomplished by the burning of fuel (coal) that has been incorporated into the feed pellets during tumble/growth agglomeration in a drum (Section 6.8.1, Fig. 6.8-13) and/or added to the agglomerated material bed on the grate. Because the temperature must reach the clinkering temperature, the structure of the traveling grate must be protected. This is accomplished by depositing suitably sized clinker from the screen (11 in Fig. 6.7-39) into a hearth layer prior to building the bed of raw meal/fuel pellets (Section 6.8.1, Fig. 6.8-15). All kilns using grates, partially or totally, suffer from high installation and maintenance costs. Therefore, although the grate-kiln and straight grate machines have been
Fig. 6.7-39 Simplified flow diagram of a grate kiln cement clinker manufacturing facility (courtesy, Lurgi, Frankfurt/M., Germany). 1, 2, raw material receiving and pre-crushing; 3, 4, 5, raw material drying and milling; 6, 7, 8, 14, raw meal (6) and fuel
(coal, 8) proportioning and mixing; 9, 10, drum agglomeration and pellets sizing (roller screen); 11, clinker screen; 12, sinter grate; 13, 15, dust collection
6.7 Building Materials and Ceramics
and are used in a great number of cement plants and are particularly suitable if systems must be shut down or started-up quickly and without problems (e.g., rotary kilns must be turned with emergency power and drives as long as they are hot to avoid major damage due to bending), the rotary kiln process is still considered simpler and features larger production capability. A major problem associated with the wet rotary kiln is the very bad heat transfer between the hot gases from the kiln and the slurry that essentially coats the furnace walls until granulation occurs and the charge begins to tumble. To obtain satisfactory economy, a number of methods, all designed to improve the transfer of heat at the feed end of the rotary kiln, have been invented and used. The oldest and among the most effective is the installation of slack metal chains in the furnace tube (Fig. 6.7-40). They absorb heat from the hot gas and, as the kiln rotates and the hot chains mix with the feed, transmit it to the slurry. However, there are problems with slurry adhering to the chains and other heat exchanger designs have their own difficulties. When it was found that dry cement raw meal mixtures produce clinker from lumps formed in the furnace at elevated temperatures due to partial melting and sintering if the materials can be contained and the particulate solids discharging with hot gas can be controlled by collection and recirculation, the basic principle of the dry cement manufacturing process was born. The method is only feasible with the rotary kiln because fine, dry raw feed can not be retained on grates. The most important requirement is to remove entrained solids form the hot off-gas with highly efficient dust collection equipment (cyclones) and recirculate the particulates into the feed end of the kiln. Since drying of slurry (wet process) or partially pre-dried feed (semidry process) is no longer necessary, the length of the dry process rotary kiln can be reduced (Fig. 6.7-41a, b, and c) while, at the same time, the throughput capacity increases (Tab. 6.7-4). It was also found that the cyclones can be used to efficiently dry and preheat the raw feed (Fig. 6.7-42) further decreasing the length of the kiln (Fig. 6.7-41d). The latest development is to also transfer calcination of the raw materials into the external preheating section by installing a combustion chamber that boosts the temperature of hot
Fig. 6.7-40 Sketch of a chain curtain for the heat transfer from the kiln gases to the slurry in a wet process rotary cement kiln [B.69]
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Fig. 6.7-41 Sketches of the relative sizes of different rotary cement kilns [B.69]
air coming from the clinker cooler, which is then injected into the system. This reduces the length of the rotary kiln further (Fig. 6.7-41e). Nevertheless, a cement manufacturing plant, including all the auxiliary equipment and processes is still a very large facility (Fig. 6.7-43). Generally, rotary kilns and sinter strands are suitable machines for the heat treatment and/or calcination of a great variety of building materials. Lime, although often produced in shaft furnaces, is calcined on sinter strands or with rotary kilns. Another example is the firing of ESCS (Section 6.7.1) for the production of lightweight aggregate. Still other applications are in the manufacture of artificial aggregate, both dense and light weight, from mineral wastes, crushed debris, and ashes, particularly fly ash
6.7 Building Materials and Ceramics Tab. 6.7-4 Comparison of diameter, length, and clinker production (throughput) of wet and dry process cement kilns [B.69]
Kiln diameter Kiln length Clinker
[m] [m] [tonnes/day]
Wet process
Dry process
5.0 165 1.050
4.0 70 2.300
(Section 8.2). The advantage of using a sinter strand for the agglomeration by heat is that the machine can be easily used for the manufacture of additives to building materials with different compositions and qualities (Fig. 6.7-44 [6.3.7.1]). For most shaped building materials, notably all types of bricks and preshaped structural parts, and for all ceramic articles, post-treatment, which almost always includes the application of heat, is an important final manufacturing step. During such treatment, the final properties and physical characteristics are produced. Tab. 6.7-5 lists some important characteristics of finished ceramic or building materials. Particularly in regard to modern high-performance ceramics, this list is not exhaustive. Also, some properties are more important for specific applications, for example for building materials, dimensions and strength, for technical ceramics, dimensions and density/por-
Fig. 6.7-42 Diagram of a dry-process rotary cement kiln employing cyclones for the collection of entrained solids and the drying and pre-heating of the raw materials [6.7.3.2]
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.7-43 Artist’s impression of a complete dry-process rotary kiln cement manufacturing plant [6.7.3.2]. 1, primary crushing; 2, raw material storage; 3, raw meal grinding; 4, raw meal silos; 5, heat exchangers; 6, rotary kiln; 7, dust collection; 8, clinker cooler; 9, clinker storage; 10, coal
grinding; 11, coal storage; 12, main air blower; 13, cement grinding; 14, gypsum storage; 15, gypsum grinding; 16, cement silos; 17, discharge facility; 18, bagging; 19, office and laboratory; 20, power distribution and plant controls
Fig. 6.7-44 Sinter strand operating on the production of light-weight building material from fly ash [6.7.3.1]
6.7 Building Materials and Ceramics Tab. 6.7-5 *
Some important characteristics of finished ceramic or building materials
Dimensions Shape * Fit (free of distortion) Density / Porosity * Shock absorption * Sound and/or heat conductivity * Permeability Strength * Crushing / Tensile / Shear * Abrasion / Hardness / Wear * Impact resistance * Elasticity / Toughness * Freeze – Thaw * Thermal stress and/or shock resistance Quality * Surface * Color * Translucency * Electrical conductivity and other non mechanical properties *
*
*
*
osity, and for household ceramics (tableware) dimensions and certain quality indicators (e.g., surface finish, color, and translucency). Shaped building materials and ceramic parts are most often post-treated (hardened, calcined, sintered) in batch or continuously operated furnaces [B.97]. For certain building blocks and ceramics, such equipment may also include the use of special (inert or reactive) atmospheres. Fig. 6.7-45 shows the manufacturing process of (hollow) building blocks made from lime and sand. Milled burnt lime and sand are mixed and hydrated in a batch drum (10) with steam (for about 1 h). This reaction can be also carried out continuously in a silo with appropriate residence time. Blocks are formed from the moist mass in a “stone press” (punch-and-die with indexed rotating table, (14 in Fig. 6.7-45), stacked on carts, and hardened in closed chambers (17) with steam for 8–14 h at a pressure of 8–15 bar. During post-treatment, the final, permanent bonding is obtained and the building blocks are ready for use. More sophisticated and higher temperature batch furnaces include muffle, bell, elevator, or pit designs [B.97]. Tab. 6.7-6 lists the processes occurring during the sinTab. 6.7-6 * * * * * * *
Processes occurring during the sintering of most ceramic parts [B.13c]
Removal of water Removal of binder and organic media Dehydroxylation Oxidation Decomposition Phase transformation Cooling
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.7-45 Diagram of the manufacturing process of (hollow) building blocks made from lime and sand. 1, lime ball mill; 2, elevator; 3, wind sifter; 4, 9, 11, silos; 5, metering bucket; 6, screw conveyor; 7, sand feed; 8, screen; 10, batch mixing and
hydration drum; 12, proportioning; 13, elevator; 14, stone press; 15, block cart; 16, transfer platform; 17, post-treatment (hardening) chambers; 18, steam production
tering of most ceramic parts. Ceramic ware is heated to a temperature between 700 and 2000 8C. Because raw ceramic parts are often “green” (moist), removal of water is the first post-treatment step. Following or simultaneously and immediately preceding firing, binders and plasticizers, which have provided the properties needed for forming, are also removed. The amount of residual moisture and/or additives that can be tolerated in the part when firing begins depends on its shape and structure and on the heating rate of the furnace. If the part is made by dry pressing, removal of additives can be incorporated into the heat-up stage of the sintering furnace if the time for this process is not too long. However, since additives are often cellulose, wax, or starch type products, they can be more conveniently decomposed by oxidation in air at low temperature prior to loading the parts. In the furnace, clay minerals usually dehydroxylize at 500–600 8C whereby steam is produced. The loss of strength that occurs at this stage may result in cracking. The next step, oxidation, can be accomplished by holding the temperature at a certain level, which varies with the manufacturing method that was used for and the type of the body but is often about 900 8C where a decomposition of carbonates and sulfates may produce bloating in vitrified parts. Silica, which exists in many different crystalline forms, is an important constituent of most ceramics. The conversion from one form into another is accompanied by sometimes large volume changes. Because this occurs during heating and cooling, the rate of temperature change must be considered and may have to be adjusted to avoid deformation and/or cracking.
6.7 Building Materials and Ceramics
Most ceramic products are fired in air: under oxidizing conditions. The ideal kiln for the firing of ceramics is capable of heating and cooling the parts uniformly at the maximum rate of temperature change for each of the stages mentioned in Tab. 6.7-6. In the ceramics industry, muffle furnaces are typically used for batch sintering [B.97]. For high-quality wares, temperature control is very important to avoid the above mentioned problems in different processing stages. This can be accomplished easiest and most accurately in batch furnaces. Some materials must be, at least during certain stages, fired in reducing or other special atmosphere that is also best provided in batch kilns. However, many bulk ceramic products need to be of such low cost that continuous furnaces must be used, which operate more economically. In conclusion, batch sintering furnaces are used for: * *
*
low-output production, special duties (because there are no moving parts, batch furnaces can be designed for higher temperatures; furthermore, since it is possible to seal the interior more effectively, purer atmospheres can be realized and maintained), and experimental work.
For the first two of these, manual pusher furnaces (Fig. 6.7-46) can be applied in which parts on a tray are moved through a furnace, one tray at a time [B.97]. If it features gas tight interlocks and/or doors for charging and discharging, it can be also used for processes in which the atmosphere must be well controlled. The bell furnace is widely used for parts requiring long sintering cycles [B.97]. Typical equipment consists of one or more supporting bases with removable sealed retorts, used to cover the loads and retain the protective atmosphere around them throughout the entire heating and cooling cycles, a portable heating bell, a stand-by base, a hoist, and an optional cooling bell. The elevator furnace [B.97] is useful for sintering heavy and/or bulky loads. It has an elevated heating chamber with open bottom in a fixed position, a mechanism for raising and lowering load supporting cars into and out of the furnace, a stand-by car to
Fig. 6.7-46
Sketch of a manual pusher furnace [B.28, B.97]
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6 Industrial Applications of Size Enlargement by Agglomeration
plug the kiln opening during idling periods, and optional cooling chambers. It is also applicable if protective atmospheres of exceptionally high purity are required. Flexible hoses carry atmosphere gas and cooling water to and from the cars. Since batch furnaces can be small and controlled easily, they are also commonly used for development work. The results from small-scale testing can be directly transferred to larger or even continuously operating kilns. As mentioned above, sintering of ceramic wares normally occurs in oxidizing atmosphere and without a special gas environment. Therefore, continuous sintering furnaces are often directly flame heated. Fig. 6.7-47 is the side elevation of a tunnel furnace for the firing of ceramic parts, indicating the direction of movement of material (car or mesh belt) and gas and the process zones. The diagram below depicts the temperature profile over the length of the furnace and shows that temperature control is normally quite unpretentious. Most tunnel kilns for ceramics are of the car type. Cars can carry more weight, are more rugged, and are more reliable than belts and other continuous methods of movement. Operation of modern furnaces is computer controlled and continuous movement is accomplished with automatic loading and unloading systems (Fig. 6.7-48). The pusher furnace that was shown schematically as manually operated equipment in Fig. 6.7-46 can be mechanized and then becomes a continuous kiln. The most commonly used continuous furnace for the sintering of small, light parts is the mesh-belt sintering furnace. A variation of the straight belt arrangement is the hump-back kiln, which is used where high purity of the atmosphere in the sintering zone is required [B.97]. Another sintering furnace used in the ceramic industry is the roller hearth furnace in which loaded trays are conveyed through the kiln by riding on individually driven rolls [B.97].
Fig. 6.7-47 Side elevation (diagram) of a directly flame-heated tunnel kiln for the firing of ceramic parts and typical temperature profile [B.13c]
6.7 Building Materials and Ceramics
Fig. 6.7-48 Modern tunnel kilns for the firing of sanitary parts, tableware, and fine china, all with fully automated car systems, and of a completely
automated handling area for loading and unloading (courtesy Eisenmann, B€ oblingen, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration
As mentioned above, the use of sintering for building and ceramic products (bricks, structural parts, pots, vases) is quite old. Through the centuries, even though empirically improved, it was exclusively carried out in batch (periodic) kilns. Even today, large pieces and all other types of ceramics are still frequently fired in modern periodic hearth, shuttle, elevator, or bell kilns (Fig. 6.7-49). Continuous heat treatment of pre-agglomerated parts is less than 150 years old. Fig. 6.7-50 shows a continuous kiln for drying and firing load-bearing structural bricks. The most recent development is hot isostatic pressing (Section 6.7.2) in which forming of the part(s) from dry particulate raw materials and heat treatment are combined in one piece of equipment [B.13a, B.97]. This technology is especially used for the manufacture of high-performance ceramics and of composites containing ceramic and other (e.g., metal) components.
Fig. 6.7-49 A large periodic shuttle kiln for the firing of chimney flues (courtesy Eisenmann, B€ oblingen, Germany)
Fig. 6.7-50 A continuous kiln for the drying and firing of load-bearing structural bricks (courtesy Eisenmann, B€ oblingen, Germany)
6.8 Applications in the Mining Industry (Minerals and Ores)
6.8
Applications in the Mining Industry (Minerals and Ores)
With exception of the extraction of mud and sand from the ground for building materials and clays for the same purpose as well as for ceramics (Section 6.7), in human history mining began with the search for and recovery of pure metals and minerals. Among the first minerals that were mined was coal (Section 6.10), the first metals that were found and used were gold and copper, the first decorative materials that were used together with the metals for making jewelry were precious and semi-precious stones, and the first nutrition-related material was salt (Sections 6.3.2 and 6.4.2). Metallic (telluric) iron was found much less frequently. It was either naturally reduced iron of volcanic origin or meteoric iron; however, man recognized that, as compared with bronze tools and weapons, it provided superior quality when forged into shape. After learning that iron can be produced from ores by relatively simple heating with charcoal in shallow cavities that were dug into the earth, early industrialization centers developed quickly. About 1400 BC, particularly in the Near East and in China, monopolies developed from which iron was distributed and later its manufacture was spread to other cultural areas. Until very recently, while the production of iron developed away from the primitive forms and casting of iron and the blast furnace were introduced after approximately the 14th century, the raw material was still high-quality lump ore. The sintering (Section 6.8.3) of high-quality ore fines into lumps was invented several hundred years ago and first applied mostly to enlarge the size of non-iron ores prior to smelting. Later this technology was also used for high-grade iron ore fines. Beginning during the first part of the 20th century, as a response to a higher demand for iron and steel, the capacity of blast furnaces was increased by improving the burden composition with sized and fluxed sinter. After World War II a great backlog of industrial demand, including the production of steel, triggered the interest in abundantly available low-grade ores, particularly the Taconites of North America. Although the concentration of ores, based on density differences between heavy and light mineral components, had been known for some time, the separation size of these lower grade ores, that is the particle size to which the raw material must be ground to effectively accomplish separation into concentrate and tailings, is so small (typically < 325 mesh or < 44 lm) that the enriched product can not be used directly and even sintering is not applicable (Section 6.8.3). To overcome this problem, size enlargement by agglomeration must be used. During the earlier development of an optimal burden for the blast furnace, consisting of lump ore, crushed sinter, coke, and fluxing additives, it was found that optimal particle size ranges should be 5–50 or, better, 5–30 mm [B.18]. Particularly for the former size range, which is typical when high-grade lump ores are used and, at that time, was the more common requirement, briquetting was the available and accepted agglomeration method (Section 6.8.2). Although the metallurgical behavior of briquettes in smelting furnaces is very good, iron ore briquetting was too expensive (mostly because of high
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6 Industrial Applications of Size Enlargement by Agglomeration
wear) and the production capacity of briquetting units too small when compared with the anticipated large quantities of concentrates to be agglomerated. Therefore, a new method had to be developed. Since the particle size of the concentrate is small enough for growth agglomeration, discs, drums, and, originally also cones were used for the production of green (wet) spherical balls, which were hardened by heat in special sintering furnaces (Section 6.8.1). It was found that the large tumble/growth (called “balling”) equipment was best suited to producing spherical agglomerates (called “balls”) with a size range of 9– 15 mm and, according to specification, does not include any particles below 5 or above 25 mm. Even after sintering, this product contains pores that are accessible from the pellet surface (open porosity), so it is easier to reduce than lump ore. Furthermore, additives can be mixed with the fine ore feed to yield a self-fluxing product. These characteristics, together with the development of an excellent burden structure, resulting in increased capacity and better control of blast furnaces, made iron ore pellets the preferred feed material for the blast furnace and, later, direct reduction plants (Section 6.8.1). Therefore it became an accepted practice to grind even high-grade ores to the required fineness and produce pellets, with or without the addition of flux. Also during the first part of the 20th century, other finely divided ores and minerals, often recovered during the mining operation, were first enriched and then upgraded by agglomeration (granulation or briquetting, Section 6.8.2) for many different uses. A major application is the treatment of by-product sodium chloride from the processing of potassium chloride (Potash) for fertilizer and chemical applications (Sections 6.3.2, 6.6.2, and 6.8.2). Fines from rock salt mining and sea salt manufacture are also agglomerated for many applications (Section 6.8.2). Tab. 6.8-1 lists ores and minerals that have been and/or are being agglomerated for a multitude of purposes. Further Reading
For further reading the following books are recommended: B.3, B.4, B.7, B.8, B.10, B.11, B.15, B.16, B.18, B.21, B.22, B.26, B.31, B.35, B.40, B.48, B.55, B.56, B.89, B.94, B.98 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold. Tab. 6.8-1 Alphabetical listing of minerals and ores that have been and/or are being agglomerated Alumina Cement Raw Mix Clays Dolomite Graphite Lead Ore Magnesia Lateritic Nickel Ore Potassium Sulfate Salts Zinc Ore
Ammonium Sulfate Chalk Cobalt Ore Fluorspar Gypsum Lime Manganese Ore Phosphate Rock Rare Earth Oxides Soda Ash (heavy and light) Zirconium Ore
Bauxite Chromium Ore Copper Ore Glass Batch Iron Ores Limestone Niobium Ore Potassium Chloride Rock Salt Sodium Chloride Zeolite
6.8 Applications in the Mining Industry (Minerals and Ores)
6.8.1
Tumble/Growth Technologies Pelletization of Iron Ores By far the largest application of tumble/growth agglomeration in the mining industry is the pelletization of iron ores. Iron ore pelletization was developed around the middle of the 20th century in North America (particularly by the US Bureau of Mines) to make very fine (< 325 mesh or < 44 lm) Taconite concentrates suitable for use in the blast furnace. In Sweden the process was developed to agglomerate magnetite fines for the same purpose [B.8, paper 36]. Strongly metamorphic, originally sedimentary iron ores, similar to the Taconites, are found elsewhere on earth: for example, Itabirites in Brazil and similar ones in other major mining districts. Therefore, after early successes of the newly developed technology in the 1950s in the USA, many large plants were built worldwide during the following 40 years. In the USA, a production capacity of 15 million t/y was reached in 1960. Worldwide, as shown in Fig. 6.8-1 [B.48], a capacity of 50 million t/y was available in about 1963, and 200 million t/y in about 1977. After a maximum of over 300 million t/y, according to the statistics of UNCTAD [6.8.1.1], 232 million t/y of iron ore pellets were produced in 2001. This is 11.7 % (30.7 million t/y) down from 2000. The pellets are manufactured in 20 countries where the available total production capability is still about 300 million t/y. The lower actual capacity reflects the reduced volume of world steel making and the growing importance of alternative iron units (Sections 6.9 and 8.2). Excluding product that is consumed in the producer’s own semi- or fully integrated steel plants (particularly in Japan) or by domestic users, iron ore pellets are exported to foreign steel making 6.8.1.1
Fig. 6.8-1 Graph showing the development of worldwide iron ore pelletizing capacity during the first 40 years (1950–1990) and projections at that time for further growth [B.48]
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6 Industrial Applications of Size Enlargement by Agglomeration
facilities by 10 countries (Brazil, Sweden, Canada, USA, Chile, Peru, Venezuela, Australia, India, and Bahrain). Much of the above mentioned decline has happened in the USA, although Minntac, which began operation in 1967, has become the single largest processor in the world with an annual production capability of 16.4 million tons of pellets after several expansions and recent modernization efforts, lasting for 6 years [6.8.1.2]. In October 2002 the cumulative pellet output from Minntac had reached 2 billion tons. Because of the remarkable uniformity of size, shape (Fig. 6.8-2), and (adjustable) composition of iron ore pellets, major improvements in blast furnace operation were achieved. They include better handling characteristics of the ore feed and increased production capacity per blast furnace, which, to a large extent, is due to a more uniform burden structure, the higher reducibility of the furnace charge, and, more recently, the “engineering” of the pellets’ composition, mostly to decrease the contents of gangue and obtain self-fluxing characteristics. This has led to modified purchasing specifications of blast furnace operators who increasingly require the supply of pellets over direct shipping lump ore, in spite of their somewhat higher price. As a result, pelletizing plants have been built in several areas, processing high-grade ores that would not normally necessitate concentration, and converting them, after fine grinding and formulating a preferred metallurgical composition by mixing, into superior quality pellets. When direct reduction (DR) was developed as an alternative to the blast furnace route to steel, most of the commercially successful technologies were and stillarebasedontheuseofpelletsasorefeed.Sincedirectreducediron(DRI,Section6.9.2) is a charge material for electric arc furnaces, DR grade pellets are now produced, which are specially formulated to have a high total iron content (low gangue). An increase in blast furnace capacity and a reduction in coke requirements resulted from the first agglomerated iron ore charges used by Gr€ ondal in Finland in 1899. For that process, briquetting was used, the only large-scale agglomeration technique available at the time (Section 6.8.2). As the benefits of a sized, agglomerated feed became recognized, many other agglomeration techniques were patented in the early 20th century, of which only the one that Andersson originated in Sweden was of importance. He had the idea of agglomerating fine iron ore in a drum, even the use of a binder to strengthen the green balls, and subsequently drying and firing the product. However, Andersson’s patent was never used commercially and his work was soon forgotten [B.8, paper 36]. Independently, in 1930 a pilot plant was constructed in Rheinhausen, Germany, to use the patent by Brackelsberg in which so-
Fig. 6.8-2
High-quality iron ore pellets (courtesy CVRD, Vitoria, Espirito Santo, Brazil)
6.8 Applications in the Mining Industry (Minerals and Ores)
dium silicate was employed as a binder and the green balls were hardened at low temperature. The plant was unable to produce an acceptable product, was closed, and also forgotten. The methods that finally were to develop into the universally accepted iron ore pelletizing technology at the US Bureau of Mines, the University of Minnesota and in Sweden were first described by Firth (USA) and Tigerschiold/Ilmoni (Sweden) in the 1940s. Much additional information about the early research can be gleaned from a paper by Goldstick [B.8, paper 36]. The agglomeration of iron ore uses a two-stage process. Such processes are characterized by the production of discrete agglomerates by size enlargement from the particulate feed with or without binders in a first stage and the development of final strength in a second stage. For the first (size enlargement) stage, many of the known tumble/growth and pressure agglomeration techniques can be used and for the second (hardening) stage, new methods had to be developed. Although at least three major low temperature or “cold” pellet hardening processes were invented (Fig. 6.8-3) [B.3, B.18], the application of heat (sintering) became the dominant technology. Depending on the most efficient method of heating for the specific purpose, different types of furnaces are applied. In modern iron ore pelletizing, nearly spherical pellets are produced by tumble/ growth agglomeration in the first stage. Since high throughput capacities must be handled, large drums and discs (pans) are used (Fig. 6.8-4). Although the agglomeration pan, due to a natural segregation effect, has the advantage of directly producing narrowly sized agglomerates [B.48, B.97], for most installations drums are applied. In spite of the fact that the agglomeration circuit (Fig. 6.8-5) must be able to handle several hundred percent (typically 300–500 %) of the production capacity and, therefore,
Fig. 6.8-3
Pellet hardening methods without firing [B.18]
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.8-4 Tumble/growth agglomeration equipment for the balling of iron ores. (Left) Diagram of: a) the disc (pan), b) the cone, c) the drum (including recirculation circuit). (Right) Actual equipment (c: drum only) (courtesy Feeco, Green Bay, WI, USA (a and c), file photo, Kennedy Van Saun, New York, NY, USA (b))
6.8 Applications in the Mining Industry (Minerals and Ores)
Fig. 6.8-5
Typical balling drum circuit for the agglomeration of fine iron ores [B.48]
needs to be much oversized, the considerably simpler operation and control of drum agglomerators is responsible for this preference. In some early installations balling cones were used (Fig. 6.8-4b). It was said that these “deep pans” produced the same segregation pattern as inclined shallow discs but, because of the larger “hold-up” in the machine, additional strengthening of the green pellets occurs due to overburden pressure and a longer retention time. However, in reality some of the defined pattern of charge movement, which is typical of the shallow discs with cylindrical rim (Fig. 6.8-4a), is sacrificed and operation of balling cones is not easily controlled. Therefore, this design is no longer used and is, for all practical purposes, forgotten. Balling drums (Fig. 6.8-4c) offer the advantages that very large throughput capacities can be handled in a single unit and operation is simple. They consist of a cylindrical tube, normally made of steel with or without a variety of liners, and are installed with a slight slope towards the discharge end (typically 108 from the horizontal). A retaining ring is often fitted to the feed end of the drum to avoid spill back. Depending on the slope and diameter of the drum and the physical characteristics and moisture content of the charge, the rotational speed of the equipment must be adjusted such that the particle bed begins to separate from the wall at approximately the 3–4 o’clock position in a counter clockwise rotating drum (Fig. 6.8-6). The mass then cascades down the inclined bed and liquid binder, if applicable, is sprayed onto the surface with nozzles from a manifold extending into the first part of the drum.
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.8-6 Diagram of the optimum charge movement in a balling drum [B.18]
Within the kidney shaped tumbling mass of solids, production of seeds and growth of agglomerates takes place. Although some natural segregation by size occurs in the bed and even if a retaining ring would be provided at the outlet of the drum, the product consists of a wide distribution of agglomerate sizes because the entire charge moves forward in a plug flow fashion. Normally, in iron ore pelletizing no retaining ring is provided downstream and the discharge end features a scroll (spiral extension, Fig. 6.8-7) that serves to distribute the pellets over the width of a screen. As shown in the sketch of an agglomeration circuit (Fig. 6.8-5), fine ore (concentrate), dry binder (see below), and (fluxing) additives are mixed (fluffer) with recycled undersized material and deposited into the feed end of the drum. Although the charge material is normally moist and may contain enough binder (water and dry binder) to accomplish balling, a manifold with spray nozzles is installed in the first about 1/3 of the drum to initiate and control agglomerate growth if the tumbling mass is too dry. A sufficient number of nuclei, some of which are also supplied with the recyclate, must
Fig. 6.8-7 The spiral discharge of a large balling drum for iron ore (file photo, McKee [Section 13.3, Ref. 16])
6.8 Applications in the Mining Industry (Minerals and Ores)
be produced in the drum at all times to replace the pellets that are removed from the circuit, and the growth rate per pass must be such that the required production rate of green balls is consistently maintained. To avoid surging, the rate of pellet production must be stabilized and the balance between material in the drum and recycling rate must be kept constant. This may necessitate installation of a surge bin in the closed recycle loop from which a metered amount of undersized returns is fed into the drum. Originally, many drums were coated on the inside with cement or expanded metal to encourage build-up of material as an autogenous wear liner. Different designs of scrapers [B.48], called cutter bar in Fig. 6.8-5, are then installed to control its thickness. More modern installations use a special rubber coating, which prevents buildup and still protects the drum from excessive wear. The green (moist) balls produced in iron ore pelletizing are rather weak and require gentle handling before they reach final strength during sintering in the second stage. Because of this, and since vibrating screens tend to clog, roller screens (Fig. 6.8-8) were
Fig. 6.8-8 a) Diagram of the principle of a roller screen in iron ore pelletizing; b) the design, and c) operation of such screens [B.97]
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6 Industrial Applications of Size Enlargement by Agglomeration
developed [B.97]. In most cases, the rotation of the often individually driven rollers (Fig. 6.8-8b) is against the gravitational flow of material on the downward sloping machine deck. This induces a rolling movement of the pellets (Fig. 6.8-8a, upper left) causing additional cleaning and rounding. The final, typically very high strength of the iron ore pellets is obtained in the second process stage by the development of sinter bridges between the ore particles at high temperatures, the so-called sintering temperature [B.48, B.97]. During the post-treatment, the green iron ore pellets must be first dried and preheated before hardening by sintering occurs. The problem of this sequence of events is that the original binding mechanism, caused by capillary forces and the surface tension of the liquid, has disappeared after drying and sintering, which requires about two-thirds of the ore’s softening temperature, has not yet begun. Therefore, there is a time interval during which the dry pellets have almost no strength. To partially overcome this problem and increase the chance of survival, binders that retain some bonding characteristics in the dry state, are added during tumble/growth agglomeration. The traditional additive for this purpose is bentonite, a natural montmorillonite clay (Section 6.7.1). Originally, when the binder was mixed with the feed to the tumble/growth agglomerator rather inefficiently with a simple fluffer (a rotating mixing tool crudely turning-over the material on a belt conveyor, see Fig. 6.8-5), 2 % (and sometimes more) of bentonite were added to obtain an acceptable effect. However, from a metallurgical point of view, this additive is a detrimental constituent as it increases the acid gangue portion in the product. Fig. 6.8-9 depicts this effect of bentonite. Particularly in highly concentrated ores (lowest line), which have been upgraded at high cost, the increase in acid gangue components may be 100 % thus canceling some of the ore improvements. Today, bentonite is often blended into the ore concentrate with a highly efficient mixer. Fig. 6.8-10 shows the influence of a good binder mix on green and dry pellet compression strengths [B.18]. It shows that the green strength is almost unaffected. Nevertheless, the material has a beneficial effect in the wet stage as the colloidal nature of the clay imparts a certain plasticity to the pellets, which makes them less friable and better formable (for example during rounding on a roller screen, see above and Fig. 6.8-8a). During the final stage of drying, the suspended ultrafine clay moves with the retreating liquid to the coordination points between the ore particles and develops solid bridges (Chapter 3, Tab. 3.1, item I-6b). Depending on the amount of bentonite added, a considerable increase in dry strength is obtained. Since a compressive dry strength per pellet of 20 N is normally considered sufficient and because the contamination level of the product should be kept as low as possible, in most plants using high-efficiency mixing, the bentonite rate is kept at < 0.7 % and often below 0.5 %. Even with these low amounts of binder addition a certain percentage of the ore concentration effort is reversed. Therefore, numerous alternative materials were proposed and tried to produce a higher quality pellet. The most effective additives in this respect are organic materials (for iron ore concentrates, for example Peridur [B.97]), which do retain strength in the dry and preheating stages but burn-out (or otherwise disappear) at high (sintering) temperatures and do not leave binder-related contaminants behind. In the end, their application is defined by an economical evaluation [B.97].
6.8 Applications in the Mining Industry (Minerals and Ores) Fig. 6.8-9 Influence of bentonite addition on acid gangue components in iron-ore pellets [B.18]
Fig. 6.8-10 The influence of the amount of bentonite in a well-mixed iron ore concentrate feed on green and dry pellet compression strengths [B.18]
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.8-11 Diagrams of the three major furnace types used for the hardening of iron ore pellets: a) shaft, b) traveling grate, c) grate-kiln; D, Drying; F, firing (sintering); C, cooling
Based on development work in the 19th century, in the early 20th century, modern thermal treatment units had been developed for the drying, heating, and cooling of minerals, particularly for the manufacture of cement (Section 6.7.3) and the sintering of metal ores (Section 6.8.3). These were converted for application as the second stage of the newly developed iron ore pelletization process. Three furnace types evolved as the dominant thermal treatment methods (Fig. 6.8-11): the shaft furnace, the traveling grate, and the grate-kiln [Section 13.3, Ref. 16]. Shaft furnaces had been used for several centuries for the smelting of ores and the burning of lime. Because of their high thermal efficiency and ease of operation they were the first, both in Sweden and the USA, to be adapted for iron ore pelletizing. In North America the first pilot plants were installed in Aurora and Babbitt, which started-up between 1948 and 1952. The first commercial plant was built for Erie Mining Co., Hoyt Lake, Minnesota, beginning in 1954 and began operation in 1955. Green pellets are fed into a narrow, slender shaft, which was modified to a rectangular (from the originally round) shape and is heated by burning oil or gas in chambers that are arranged on its sides. Since the sintering zone is located only about 50 cm below the top of the pellet bed and hot gas and solids move countercurrently, the green agglomerates are quickly dried and preheated. Therefore, they must be resistant to thermal shock. Also, because ore sintering and softening temperatures are close
6.8 Applications in the Mining Industry (Minerals and Ores)
together, the charge tends to form lumps, which disrupt the uniform flow of solids and gas in the furnace and must be broken at the bottom of the hot zone (Fig. 6.8-12). In the first shaft furnaces for the hardening of iron ore pellets, a solid fuel was added to the charge to supply additional thermal energy. Later, shaft furnaces were almost exclusively used for the treatment of pellets that were produced from Magnetite, as the exothermic oxidation reduces the heat requirements from the outside and simplifies furnace control. Additional improvements were obtained after the long shaft furnace with internal cooling (Fig. 6.8-12a) was converted to a medium shaft height with external coolers and recovery of sensible heat (Fig. 6.8-12b). Nevertheless, the shaft furnace has lost its importance. Tab. 6.8-3 summarizes the advantages and disadvantages. While at the beginning and still to date for some facilities it is an advantage that smaller units, particularly if they process Magnetite pellets, are economically feasible, in today’s environment, where iron ore pelletizing plants produce several million tons per year in one unit (below), this is considered a disadvantage. Because pellets are prone to spalling by thermal shock and easily abrade in the constantly downward moving charge they must be of the highest quality, which normally means a high amount of binder addition (particularly bentonite). The biggest disadvantage of shaft furnaces is, however, that because every process component takes place in close proximity in one unit, there is little chance of influencing the process stages, which translates into low flexibility. Even small disturbances in charge consistency (e.g., due to lumping), which are unavoidable, results in disrupted gas and charge flow patterns and inconsistent product quality. At the time when iron ore pelletizing was first developed, simple traveling grates had already been used for ore sintering. They are the oldest machines for the production of agglomerates from fine grained ores (Section 6.8.3). As shown in Fig. 6.8-13, the early traveling grate machines combined all process steps and little control was possible. The feed, consisting of a blend of fine ore and solid fuel (coal), is first placed as uniformly as possible onto an endless perforated belt, made-up of connected and hinged cast iron or steel sections, and then passes through an ignition furnace, which is a short hood with burners inside. The flames impinge on the surface of the bed and ignite solid fuel that is close to the hot interface. From that point on, air is pulled through the completely open bed, in a downdraft fashion; combustion of the solid Tab. 6.8-3 Advantages and disadvantages of shaft furnaces for the induration of iron ore pellets Advantages
Disadvantages
Simple design Small number of moving parts Refractory lining of entire furnace Intensive heat exchange Feasible for low production capacity Best for Magnetite pellets
Little chance of influencing process stages Low flexibility Charge is in constant movement Pellets must be of highest quality Limited production capacity High fuel consumption Commonly disrupted flow pattern (lumping) Inconsistent product quality
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Fig. 6.8-12 Details of two shaft furnace designs: a) long shaft furnace with internal cooling, b) medium shaft furnace with external cooling and two alternative methods of sensible heat recovery [B.18]
6.8 Applications in the Mining Industry (Minerals and Ores)
Fig. 6.8-13
Diagram of an early traveling grate sintering machine [B.97]
fuel is sustained and enhanced by providing oxygen from the air and the burning hot zone moves downward through the bed. The amount of air that is pulled through the entire length of the bed and the speed of the grate are adjusted such that, at the end of the machine, the fuel has disappeared and the entire charge has sintered together. The porous, solidified, still hot mass is broken in a sinter breaker, the resulting pieces are screened into the desired particle size distribution, and the product is cooled externally. Solid particles that are entrained in the combustion air, settle in bins, which are part of the main suction duct and fine dust is removed in a dust collector. These solids are transported to the burden preparation plant and ultimately fed back to the sintering machine. Fig. 6.8-13 already includes two improvements. First, oversized pieces from the sinter crusher are recrushed in closed loop with a screen (not shown) and all screen fines are recirculated to provide a hearth layer, which protects the grate from excessive temperatures. Second, to achieve uniform bed depth across the grate, feed is placed onto the insulating hearth layer with a swinging conveyor and leveled with a roll feeder. This traveling grate machine was adapted for use in iron ore pelletizing plants in US test facilities at Carrollville and Babbitt between 1952 and 1954 [Section 13.3, Ref. 16]. The first commercial plants were started-up at Reserve Mining Co., Silver Bay, Minnesota, USA, in October 1955, at International Nickel Co. of Canada Ltd., Copper Cliffs, Ontario, Canada, in February 1956, and at Cleveland-Cliffs Iron Co., Ishpeming, Michigan, USA, in October 1956 [B.18]. The major modifications, which were later further optimized, comprised the separation of the process stages of drying, pre-heating, firing, and cooling, including recuperation of sensible heat. This resulted in much better control and improved the economics of the process. As shown in Fig. 6.8-14, the traveling grate is now completely enclosed and the housing is subdivided to produce the different process conditions. The belt consists of pallet cars with grate bottoms and side walls, connected with each other by hinges to form a continuous unit (Fig. 6.8-15a). To protect the pallet walls, recirculating fired pellets are deposited in the hearth and side layers in which the green pellet bed rests [B.18].
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Fig. 6.8-14 Principles of two traveling grate hardening systems for iron ore pellets [B.18]: a) McKee design; b) Lurgi–Dravo design
The hoods and windboxes depicted in Fig. 6.8-14 are easily and effectively isolated from each other. While the original traveling grate was applying only a downdraft flow of process gas through the particle bed, the new, improved design employs both upand downdraft sections to optimize the efficiency of thermal energy transfer and use. Also, the originally necessary addition of solid fuel was abandoned in favor of heating with burners. Different ores require distinct heating patterns. Their behavior is determined in laboratory pot grate furnaces [B.97] and pilot plant installations. While the process sections of a traveling grate installation can be designed for a particular ore, an advantage of this machine is that by merely changing the speed of the pallet belt an existing hardening facility can be adjusted to meet the requirements of changing ore compositions. As shown in Tab. 6.8-4 the total duration of the process may change between about 33 min for magnetitic ores and 41 min if the raw material contains limonite and hematite.
6.8 Applications in the Mining Industry (Minerals and Ores) Tab. 6.8-4 Ranges of thermal treatment zones for different ore compositions (Figure 6.8-14b) [B.18] Thermal treatment
Time [min] (range)
Updraft drying Downdraft drying Preheating (including dehydration, calcination and oxidation) Firing (sintering) After firing Cooling Total duration arithmetric
4–7 2–5 7–9
% of overall process (range) 12 – 17 6 – 12 17 – 22
7 – 11 17 – 28 3–5 8 – 12 12 – 14 30 – 35 35 – 51 (actual 33 – 41)
Fig. 6.8-15 a) Side and hearth layer as well as green pellet feeding of a Lurgi-Dravo traveling grate machine. b) Sketch and thermal insulating effect of the hearth and side layers [B.18]
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The traveling grate, which may be executed straight or circular [Section 13.3, Ref. 16, B.48, B.97], is the most commonly used hardening machine for iron ore pellets throughout the world. In most cases the straight grate design is employed. Advantages are its easy adaptation to operation with practically all ore types, the fact that the pellets remain stationary throughout the process, thus allowing the processing of agglomerates with lower strength (requiring less binder), the production of pellets with uniform quality (because of the processing in a relatively thin layer), the possibility of obtaining high energy efficiency (including effective heat recuperation), the potential use of many different fuels for firing, and, last but not least, its applicability for large capacities, which further increases the economy of the system (below). Still better process control can be achieved if the three components (drying/preheating, firing, and cooling) can not only be influenced separately and individually but are physically detached equipment units. This is the case in the so called grate-kiln that was developed and optimized by Allis-Chalmers Mfg. Co., Milwaukee, Wisconsin, USA, for the production of cement clinker (Section 6.7.3). Work to test the feasibility of the grate-kiln system for the hardening of iron ore pellets began at the Allis-Chalmers pilot plant at Carrollville near Milwaukee, Wisconsin, USA, in the 1950s. The first commercial plant started-up in the mid 1960s at the Humboldt Mining Co., in Michigan, USA [Section 13.3, Ref. 16]. Fig. 6.8-16 is a diagram of a grate-kiln system for the hardening of iron ore pellets [B.18]. The traveling grate, consisting of an endless chain of grate plates, is used for drying, preheating, and oxidation of magnetite. The green balls are charged directly onto the grate plates without hearth layer and remain in a relative position of rest. For drying and pretreatment, hot waste gases from the rotary kiln and sometimes also from the cooler pass through them in different directions and zones. Use of the tra-
Fig. 6.8-16 Sketch of the grate-kiln hardening systems for iron ore pellets [B.18]
6.8 Applications in the Mining Industry (Minerals and Ores)
veling grate with its various functions is imperative for the successful operation of the system. For thermal, operational, and quality reasons, the rotary kiln alone is not sufficient to do the task. During transfer from the preheating grate to the rotary kiln, special care must be taken to avoid breakage of the still weak pellets by installing special (often spiral) chutes. In the rotary kiln, which is equipped with refractory lining and fired by oil, gas, or, to a certain extent, pulverized coal, thermal energy to achieve uniform heating and sintering of the pellets is mainly provided by radiation. To reach the required hardening temperature of 1320–1340 8C, the wall temperature must be higher, which may lead to reactions between oxide dust and the hot lining, causing patchy or ringshaped accretions. This is one of the main problems that can occur in the rotary kiln and must be avoided. The pellets roll spirally in a very thin layer towards the discharge end where they leave the kiln as hardened hot product. Subsequently, they pass into the third unit, a circular or annular cooler. Having three different process-specific units allows excellent adaptation of the system to a specific ore or mixture of ores. Residence times and heat supply in each part can be adjusted to achieve optimum results in regard to power consumption, thermal efficiency, and pellet quality. Particularly in regard to the latter, it is often said that the roundness and surface quality are higher after rolling the pellets in the kiln at high temperature. This translates into higher abrasion resistance and less production of nuisance dust during shipping and handling. As in the case of the traveling grate, large capacity grate-kiln systems are most economical. Fig. 6.8-17 shows isometric drawings of the three major iron ore pelletizing processes. The three small shaft furnaces in Fig. 6.8-17a are each fed from a balling drum circuit, while the straight traveling grate in Fig. 6.8-17b, which always handles a much larger throughput, receives its feed from three balling drum circuits. To give an impression of the size of such systems, Fig. 6.8-18 is a partial view into the drum agglomeration section of an iron ore pelletizing plant showing five drums and associated process equipment. In Fig. 6.8-17c, depicting a plant using the grate-kiln hardening process, it is indicated that either balling pans or closed loop balling drums may be used for the production of green pellets. Of course, complete iron ore pelletization plants also include the feed preparation. As discussed in Section 6.8 and at the beginning of this Section, iron ore pelletization was originally developed for the size enlargement of ore concentrates which, after beneficiation (upgrading by removal of gangue), were too fine for direct use in the blast furnace or other smelting equipment. After crushing and fine grinding to liberate the high-grade ore particles in the raw, as mined material, separating the iron oxide from the gangue by mechanical (jigging, spiral) or chemical (flotation) sink/float processes, and dewatering, the resulting ore concentrate is fine enough for growth agglomeration by one of the balling processes (Fig. 6.8-4). The simplified flow diagram of a complete plant, which includes crushing to the required fineness for growth agglomeration of concentrate produced at the mine site and feed preparation, is shown in Fig. 6.8-19. Again to give an impression of the size of such plants, Fig. 6.8-20 shows views into the fine grinding bays of multi-million ton per year iron ore pelletizing plants using semi-autogenous mills (a) or rod and ball mills (b) for size reduction.
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.8-17 Isometric drawings of the three major iron ore pelletizing processes [B.48, B.97]. a) Balling drum circuits with shaft furnaces. b) Balling drum circuits with straight travelling grate machine. c) Balling drum circuit(s) or balling pan(s) with grate-kiln hardening system
6.8 Applications in the Mining Industry (Minerals and Ores)
Fig. 6.8-18 Partial view into the drum agglomeration section of on iron ore pelletizing plant showing five drums and associated process equipment
Depending on the operating philosophy, both iron ore concentration and pelletizing can be combined in a single facility or in two separate plants. For example, Fig. 6.8-21a is the aerial photograph of National Steel Pellet Company’s Keewatin, Minnesota, USA, installation on the Mesabi (Taconite) iron range [6.8.1.3]. In this plant with a rated production capacity of 5.5 million t/y, iron ore is concentrated and 1 % limestone pellets are produced for exclusive consumption by the US National Steel Corporation. Similar plants in different parts of the world produce pellets from iron ore, mined and upgraded on site, for export to the free market. Other mines concentrate the iron ore on site and ship so called pellet fines to pelletizing plants that are owned by the same group or by others, Fig. 6.8-21b is the aerial photograph of such an installation [6.8.1.3]. The picture shows the pelletizing facility of Quebec Cartier Iron Co. at Port Cartier, Quebec, Canada. The ore is mined and beneficiated at the Mt. Wright mining area in Fermont, Quebec, Canada. Similar installations are located at import harbors, particularly in Japan, where pellet fines from, for example Australia, are received, potentially reground (below), and pelletized for use in nearby steel mills. The improvements in blast furnace operation from the use of pelletized and sometimes additionally engineered (i.e., self-fluxing) iron ore burdens has been so great that naturally high-grade iron ores are being ground to balling fineness and converted into
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Fig. 6.8-19 Simplified flow diagram of a complete iron ore pelletizing plant (courtesy CVRD, Vitoria, Espirito Santo, Brazil)
high-quality pellets. Fig. 6.8-22 is the simplified flow diagram of a natural ore regrind and pelletization process [B.10]. Of course, in reality the feed preparation section consists of several ball mills with the associated equipment and the single straight traveling grate machine is fed from a number of balling drum circuits. The difference in the processing of natural ores is that the clay minerals have not been removed by either concentration or de-sliming processes and, therefore, play a part in the operation. Generally, the fineness of feed materials is not only important for successful growth agglomeration but must also be compatible with subsequent operations. For example, pellets made from very fine material, for example due to the presence of clay, can cause problems in the drying stage of the induration section. Also, the filtering of slurry produced in wet tumble mills is very difficult if clay is still included. Therefore, as shown in Fig. 6.8-22, closed circuit dry ball milling and the removal of clay in a thickener is the preferred method of feed preparation for a natural ore pelletizing plant although other flow diagrams are also used.
6.8 Applications in the Mining Industry (Minerals and Ores) Fig. 6.8-20 Views into the fine grinding bays of multi-million ton per year iron ore pelletizing plants using: a) semi-autogenous mills, b) rod and ball mills for size reduction
Fig. 6.8-21 a) aerial view of US National Steel Pellet Company’s Keewatin, Minnesota, concentration and pelletizing plant on the Mesabi (Taconite) iron range. b) aerial photograph of the pelletizing (only) facility of Quebec Cartier iron Co. at Port Cartier, Quebec, Canada. [6.8.1.3]
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Fig. 6.8-22 Simplified flow diagram of a regrind and pelletization process for natural ore [B.10]
Tumble/Growth Technologies for Non-Ferrous Minerals and Ores As in the case of iron ore concentrates that are too fine for direct use in reduction and smelting processes, size enlargement by tumble/growth agglomeration is used for non-ferrous minerals and ores that feature the same characteristics and limitations, that is they are fine because they have been upgraded or they are produced as finely divided particulate solids. Among the first group are copper, nickel, platinum, zinc, and other ore concentrates [B.35] and many precipitated products and by- or waste-materials from processing plants belong to the second group. While the former need modification for their proper use, the latter are treated for recirculation (Section 8.2). Preparation of pelletized charges for hydrometallurgical, melting, reduction, or roasting processes follows the same two-stage agglomeration that was described for fine iron ores and uses similar equipment (Fig. 6.8-23) or employs a binder for the development of sufficient strength for the process that follows [B.35]. In some cases, layered pellets are produced, for example as feed to the lead-making shaft fur6.8.1.2
6.8 Applications in the Mining Industry (Minerals and Ores)
filter cake
dust collection oil air
drying drum
to thickener Surge bin
to dump balling pan
wring belts
green balls
traveling grate machine
roller screen
to dust collection
product screen
fines pile overs pile to storage bins Fig. 6.8-23 Simplified flow diagram of a two-stage manganese ore agglomeration plant. Above) concentrate drying and balling stage; below) pellet hardening stage [B.35]
nace (Fig. 6.8-24). For such applications, the collared or stepped pan agglomerator [B.48, B.97] is often preferred as it is better suited for the balling of multi-component mixes, enables the controlled take-up of individual materials for the layered structure, and produces pellets in a narrow size range. If binders are used, they may, at the same time, be part of the reaction system that is used for the extraction of non-ferrous components from, for example, roasted pyrite residues (Fig. 6.8-25) [B.35]. Another reason for size enlargement by tumble/growth agglomeration is that, for better reactivity or solubility, a large specific surface area of the solids (i.e., small particle size) is desirable, which is retained in the agglomerated product. After size en-
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.8-24 Diagram of a layered pellet [B.35]: 1, core (recycled undersize); 2, flue dust; 3, limestone; 4, lead containing concentrate + SiO2 + iron ore
largement, the bigger porous entities can be easily handled and fed into reactors without experiencing excessive losses by dusting or oxidation. Examples for such applications are finely ground cement raw meal, and glass batch. Many of the fundamental studies of the pan agglomerator were carried out in connection with the pelletizing of cement raw meal [Section 13.3, Ref. 2], which was widely investigated in the 1950s and 60s. Some experimental plants were built, but later this approach was abandoned in favor of preheating fine raw materials in fluidized bed heat exchangers.
Fig. 6.8-25 Processing plant for the firing of roasted pyrite residues using CaCl2 as a binder and reactant for the extraction of non-ferrous components [B.35]. 1, balling pan; 2, belt dryer (max. 250 8C); 3, shaft furnace
6.8 Applications in the Mining Industry (Minerals and Ores)
Fig. 6.8-26 Flow diagram of a glass batch agglomeration system and photograph of the control panel (courtesy Philips Lighting BV, Winschoten, The Netherlands)
Glass batch agglomeration was also extensively evaluated during the second half of the 20th century [6.8.1.4–7]. For the making of good quality glass, a high-standard of batch preparation (feed to the melting furnace) is essential. Not only is the selection of the type and size of raw material (mineral or synthetic, wet or dry, fine or coarse) important, but after compounding and mixing it is necessary to ensure that either caking or segregation and/or cross-contamination do not occur. The advantages of agglomerated glass batch (pellets) include [6.8.1.7]: pellets are free-flowing, therefore, they are easily transported by many systems and are simply and reliably charged into the furnace, they do not cake, their components do not segregate, and very little dust is produced, they can be stored easily, can be packed for prolonged storage, and several recipes can be kept in stock, suitably separated and ready for use. The grains of the raw materials for growth agglomeration, particularly the sand, must be finer than those of regular batch. This is due to the fact that the agglomerate forming particles must have a certain minimum fineness to create strong enough bonds to accomplish growth and green strength (Chapter 5 and [B.48, B.97]). Green pellets may contain up to 15 % moisture. This allows the use of various liquid components, such as sodium hydroxide, arsenic acid, and soluble colorants. The result is a nearly ideal distribution of refining agents and colorants. After drying at 400 8C the
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Fig. 6.8-27 Green pellets discharging from a pan agglomerator Fig. 6.8-26 and of dry pelletized glass batch (courtesy Philips Lighting BV, Winschoten, The Netherlands)
pellets are strong, their water content is below 2 %, and the bulk density of the agglomerated batch has been increased substantially [6.8.1.7]. Fine-grained components can not be used in regular glass batch because they feature a higher gas release rate and bonding of the melting batch takes place, forming a floating insulating layer on the melt. Pellets, on the other hand that by necessity incorporate finer components, are more reactive but the feed layer remains porous during meltdown and preserves the ability to freely release gas. Melting without cullet is possible. As a result of all this and in addition to the advantages described above, there is ample evidence that the use of agglomerated glass batch results in an increased melt-
6.8 Applications in the Mining Industry (Minerals and Ores)
ing rate and a decreased power consumption. The use of pellets allows a reduction in the temperature of the hot spot (up to 30 8C [6.8.1.7]) without sacrificing glass quality and furnace output. This and the lower amounts of dust and carryover increase the life of the furnace, the recuperators, and the regenerators and reduce pollution of the environment. In spite of these advantages, little industrial use of glass batch agglomeration takes place. However, when the Special Glass Factory of Philips Lighting BV at Winschoten, The Netherlands, was built (start-up in 1980), it was decided to operate it with 100 % pellets as batch. Fig. 6.8-26 depicts the flow diagram of the agglomeration system and a photograph of the control panel. Fig. 6.8-27, shows photographs of green pellets discharging from the pan agglomerator and of the dry product. The diameter of the pellets is 12 mm on average, each featuring the required chemical composition and physical properties. The merits of using agglomerated batch was found so great in practice that the company began to offer this high-quality material to other glass manufacturers [6.9.1.7]. As of this writing the plant is still in operation. Over eighty different raw components are used to produce agglomerated batch compositions with a high demand on homogeneity and quality for various special glass types.
6.8.2
Pressure Agglomeration Technologies
Towards the end of the 19th century several briquetting machines for the size enlargement of clay for the production of building materials and of coal fines as shaped solid fuel had been invented and many plants for these purposes had been built and were operating successfully (Sections 6.10 and 6.10.2). Therefore, it is not surprising that, when the first commercially acceptable process for the agglomeration of fine iron ores was developed in 1899 by Gr€ondal, it was based on briquetting [B.8, paper 36]. The Gr€ondal process originated in Finland, using equipment similar to that applied for the shaping and hardening of clay bricks (Section 8.7.2). Briquettes were made by pressing fine iron ore, mixed with water as a binder, into rectangular shapes about the size of building bricks. These green agglomerates were then loaded onto cars and passed through a gas fired tunnel with a temperature of about 1400 8C in the combustion zone. It was said that the strength of the fired product resulted from the heat evolved by rapid oxidation in the combustion zone. The porous briquettes produced in this manner were hard, featured extremely low sulfur, and had a ferric iron content of more than 90 %, regardless of the originating ore type. In spite of their unfavorable size and shape, mostly because of the fineness of the original ore and the porosity of the briquettes, they made an excellent blast furnace feed and it was established that Gr€ondal briquettes increased blast furnace capacity and reduced coke requirements. The success of the first Gr€ondal plant at the Pitkaranta Iron Works in Finland led to the installation of a similar one in Sweden in 1902. By 1913, there was a total of 38 plants throughout the industrially developed world. Of these, 16 were in Sweden, 12 in the UK, and 6 in the USA.
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Many other agglomeration techniques for ores and minerals were patented early in the 20th century, but, excluding sintering (Section 6.8.3), the Gr€ ondal process was the only success until pelletization was developed and found worldwide application with an unprecedented pace (Section 6.8.1). As for the size enlargement of iron ores, briquetting with roller presses, which was a widely accepted technology in the coal industry since the middle of the 19th century (Section 6.10.2), was also evaluated for the treatment of many ores and minerals. However, because the machines were not yet designed to exert high forces and withstand elevated temperatures (below), binders were required to produce sufficient product strength. These binders included asphalt, bitumen, coal tar pitch, cement, lime, silicates, sulfite liquor, and many others [B.48, B.97]. Most binders were not desirable constituents because they contained impurities, were costly, and, during final use, lost strength prematurely. In addition most briquettes were relatively weak and the economics of the process suffered from excessive wear. During the mid 1950s when coal briquetting started to decline (Section 6.10.2) and some equipment manufacturers searched for new applications of their machines, stronger presses were built, improved roller designs developed, and novel materials of construction selected [B.13b, B.48], which individually or together made applications feasible, which heretofore had been uneconomical. In particular, the segmented roller design, which had been proposed much earlier but failed due to the lack of suitable wear- and high-temperature resistant steel grades [B.13b, B.48], together with extensive cooling of machine components, allowed the processing of hot materials. Since metals, ores, and minerals become quite malleable at temperatures in the range 500–1000 8C hot briquetting was evaluated for several applications. The first hot briquetting facilities were used in pilot plants of the fledgling technologies of DR iron ores and for the de-oiling, densification, and shaping of cast iron chips (Section 6.9.2). Since it quickly became obvious that iron ore pelletization for the size enlargement of fine iron ores by balling and heat hardening (Section 6.8.1) is only economical in plants processing more than 1 million t/y and hot briquetting with roller presses promised efficient and profitable operation at much lower capacities, several pilot and demonstration systems were built and operated in the 1960s. Fig. 6.8-28 is a schematic elevation of the largest iron ore hot briquetting plant that was installed at the Steel Company of Wales in Margam, UK, with participation by the engineering companies Dravo and Davy-Ashmore [Section 13.3, Ref. 32]. Cost comparisons between sintering, pelletizing, and hot briquetting, which were based on the operation of the plant during the years 1964/65, showed that the investment for briquetting is 50 % less than for the two others and that overall production costs are 27 and, respectively, 23 % lower. A large part of the financial advantages is due to the fact that no binders are required, which eliminates its cost and the need for binder-related installations and equipment and directly influences the product value by not adding a contaminant. In spite of the use of high-quality steel grades for the segments and other machine parts, wear is considerable and, while, when compared with other methods, the total costs associated with this item are not excessive, the process interruptions, which can be only avoided if redundant stand-by equipment is installed, are not tolerable. In
6.8 Applications in the Mining Industry (Minerals and Ores)
Fig. 6.8-28 Large scale pilot plant (10 tonne/h) at the Steel Company of Wales, Margam, UK, for the hot briquetting of iron ore [Section 13.3, ref. 32]
addition, it was found in another pilot plant, which was built and operated at Arbed, Luxembourg, in the late 1960s for the hot briquetting of Minette ore, that the considerable amount of limestone contained in this material is converted to calcium and magnesium oxide during heating of the briquetter feed and renders the product unstable. The oxides hydrate with ambient moisture; this unavoidable reaction is associated with an increase in volume, which causes even the strongest briquettes to weaken or completely fall apart. It was found that, in a stockpile, the product deteriorates in 2–3 days so much that it can no longer be reclaimed and fed into the blast furnace without causing excessive dusting and losses. An attempt to use logistics (first in – first out) in producing, depositing, and reclaiming the product was unrealistic in the blast furnace environment, and the project was abandoned. Another relatively early application of size enlargement by pressure agglomeration uses roller presses for the compaction/granulation of phosphate rock prior to calcination in a system for the production of elemental phosphorous. The previously described grate-kiln process (Section 6.8.1) was modified by Allis-Chalmers for the calcination, but run-of-mine rock produced an arc furnace feed of varying quality due to widely different sizes and structure of the raw material. In addition, fines had to be removed and discarded because they became entrained in the off-gases from the heat treatment (calcination) facility. Allis-Chalmers modified a state-of-the-art roller mill into a “compactor mill” (Chapter 4, Fig. 4.3) to crush and densify phosphate rock into a compacted sheet that is broken and screened into a granular product with narrow size distribution. This feed to the calcining system resulted in optimal drying, heating (firing), and cooling and produced an excellent charge material for the electric arc furnaces (Fig. 6.8-29). The advantages of using roller presses for such processes over tumble/growth agglomeration are that there are essentially no requirements on the fineness of the material to be agglomerated and that normally no binders are necessary. The compaction/
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Fig. 6.8-29 Flow diagram of a plant for the production of elemental phosphorus [Section 13.3, ref. 32]
granulation process also allows to adjust the product size and distribution to the optimal feed properties. Nevertheless, some other plants use roller briquetting, which always produces the same, typically larger and almond or pillow sized shape, requiring longer calcination time but resulting in an even better electric arc furnace feed. The higher cost of the pocketed rollers and more noticeable wear are offset by the elimination of the granulator (breaker) and a much smaller amount of fines to be recirculated. In addition to size enlargement, densification is often a major requirement when pressure agglomeration is used. One such application is the briquetting of magnesium oxide (MgO) prior to high-temperature sintering (dead-burning) in rotary kilns for use in refractories (Fig. 6.8-30). Magnesite, the raw material, is either mined and, after milling, concentrated or precipitated from sea water. The first calcination step, the conversion into MgO, is often carried-out in rotary hearth furnaces and the resulting product would be fine enough for successful tumble/growth agglomeration. However, the most important requirement for the sintered end-product (dead-burned magnesia) is a tolerance for extremely high temperatures in the refractory material. This is only achieved if the magnesium oxide reaches theoretical density during deadburning which, in turn, requires a high density of the feed to the sintering furnace. Therefore, although the MgO powder is very fine and aerated, a condition, which is not very desirable for briquetting [B.97], densification in roller presses must be applied.
6.8 Applications in the Mining Industry (Minerals and Ores) Fig. 6.8-30 Flow diagram of a sea-water magnesium oxide (magnesia) plant: 1, precipitating thickener; 2, pump; 3, washing thickener; 4, vacuum filter; 5, rotary hearth furnace; 6, screw conveyor/cooler; 7, 15, bucket elevator; 8, feed bin; 9, roller briquetting press with screw feeder; 10, screw conveyor; 11, screen; 12, 13, chip (undersized fines) recycling; 14, chip surge bin; 16, magnesia (sintering, dead-burning) kiln; 17, magnesia cooler
Because large amounts of air are squeezed from the powder during briquetting, a vertical screw feeder must be applied to force the material into the nip between the rollers [B.48, B.97]. Even then, particularly if the magnesium oxide originated from precipitated material and, therefore, was extremely fine, the natural densification ratio of a medium sized roller press (typical machines featured a roller diameter of about 520 mm) was not high enough to produce a high yield of well-densified briquettes. In such cases, separate equipment is used for predensification (for example, Fig. 6.8-31a) or already partially densified material (“chips” in Fig. 6.8-30) is recirculated in a closedloop system and added to the fresh feed (Fig. 6.8-31b). The systems shown in Fig. 6.8-31 have been often used to render smaller roller presses suitable for the briquetting of very fine aerated materials, such as magnesium oxide from seawater precipitation. If presses with larger roller diameters (e.g.,> 1000 mm) are used and the material is fed hot, directly from the rotary hearth calciner, a high yield of briquettes with excellent density is obtained and only a very small
Fig. 6.8-31 Two flow diagrams of precompaction arrangements for roller presses [B.48]
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amount of undersized material must be recirculated. Modern plants have been equipped with large hot briquetting roller presses. As a final example of the use of pressure agglomeration for minerals, various applications of roller presses in the salt (sodium chloride) industry shall be discussed. The technology is used for the size enlargement of rock salt fines and of crystallized byproduct salt from the concentration of potassium chloride and for potassium chloride itself and for other salts. All salts deform easily under pressure and, while becoming corrosive in the presence of water, do not cause much mechanical wear unless hard solid impurities or contaminants are present, which happens rather infrequently. Most of the size enlargement of salt by pressure agglomeration is accomplished by compaction/granulation (Section 6.6.2). The application of the granulated product determines its size and distribution, which are easily adjusted by changing the openings of the screen decks and, possibly, modifying the crusher (granulator) type or operating characteristics. Sodium chloride as a household and industrial seasoning (Section 6.4.2) is the finest (about < 2 mm) while fishery salt used on fishing vessels is coarse (> 5 mm). Granular salt for industrial applications (for example potassium chloride) is also designated and produced as coarse. Sodium chloride is also briquetted into a well-densified pillow or almond shaped product that is sold directly to the consumer (Fig. 6.8-32). These briquettes are used to maintain a concentrated salt brine for the automatic regeneration of ionic water softeners. For the proper functioning of the regeneration cycle, the briquettes should not fall apart when immersed in water; rather they must dissolve, much as if they were large salt crystals. That means, they must feature high density and bonds that are waterproof. Such a binding mechanism is obtained when during briquetting the salt particles are plastically deformed, come into close contact, and a natural recrystallization (healing of the crystal structure) takes place. The quality of such briquettes is determined with a so-called mush test, which determines whether or not submerged briquettes fall apart and form a sludge. The latter is unacceptable.
Fig. 6.8-32 Salt briquettes that are used for the regeneration of ion-exchange water softeners
6.8 Applications in the Mining Industry (Minerals and Ores)
In Section 6.8.1.2 the advantages of glass batch agglomeration were discussed. Taking into consideration that, in addition to the common properties of agglomerated materials, that is no segregation of components, good flow and storage characteristics, and low content and production of dust, some of the specific reasons for improvements that were experienced with pelletized glass batch are higher apparent density (better penetration and sinking behavior) and better solubility (because of smaller particles), it was an obvious conclusion that briquetted (compacted) glass batch should feature even more pronounced advantages. Therefore, developments employing roller briquetting machines were piloted in several companies, particularly in the USA (Corning Glass, FMC, Ball, US Gypsum, and others). From these tests, which are carried out with essentially dry batches, thus avoiding expensive drying, it was initially concluded that the pressure consolidation of glass batch leads to significantly shorter melting and refining rates and better glass homogeneity (as compared with raw and growth agglomerated feed) and, therefore, warrants further investigation. However, it was later found that, because of the hardness and abrasiveness of most of the glass batch components (particularly silica sand), considerable wear takes place during briquetting. Although this by itself is not necessarily detrimental, as new, hard roll materials and novel roller designs (segments) have become available, even small contaminations with iron and other (alloying) metals result in a discoloration of the glass. Since batch agglomeration is particularly interesting for high-quality glass, pressure consolidation has been ruled out and testing was discontinued.
6.8.3
Other Technologies
Sintering, size enlargement by heat, has been known for a long time [B.97]. Originally, it was mostly applied on a small scale by medieval alchemists for the fusing of different metal-bearing materials and in the laboratory by assayers for the preparation of samples. It was used on a larger scale for the lumping of ore fines when larger and more numerous shaft and blast furnaces were introduced, particularly to satisfy the great need for iron at the beginning of the industrial era. In the beginning, industrial sintering was carried-out batch-wise in pan-like structures on grates that were heated and supplied with air from below (updraft) with additional heat produced by the burning of solid fuel that had been mixed with the raw ore. Later, the bed was ignited from above (see below) and air, containing the oxygen that is necessary to sustain burning of the solid fuel, was pulled in a downdraft fashion through the material layer by a fan. Even today, modern pan sintering facilities are still used where small amounts of metal ores must be agglomerated (Fig. 6.8-33). As usual, heat is provided in such systems by carbonaceous solid fuels that have been uniformly distributed in the ore charge. After ignition, for example by depositing red-hot charcoal or coke onto the bed surface, air is pulled through the machine by a suction fan. When the burning fuel, which produced the intense heat necessary for sintering, is depleted, the material is cooled. Since the entire charge has become one large cake it must be removed by
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Fig. 6.8-33
Flow diagram of a contemporary pan sintering plant [B.97]
suitable means, broken, and screened into the desired sinter size. Oversize particles are re-crushed in the closed breaker loop and fines are recirculated to be mixed with fresh ore and fuel. Such plants may produce 30–50 ts/day of sized sinter [B.97]. In most cases, however, economic and processing reasons require the treatment of larger amounts of several hundred tons per day or, in the case of iron ore, of over 1000 ts/day to satisfy the needs of modern base metal smelting facilities. Such quantities also necessitate the use of a continuous system. For this, introduced by several companies at almost the same time at the beginning of the 20th century, the traveling grate, a slowly moving endless belt, also called a strand, made up of hinged steel plates with grate bars (Section 6.8.1), was used. The early machines (Fig. 6.8-13) used open belts with only a short ignition furnace located at the beginning after the bed is deposited and a de-dusting hood at the end where the sintered cake is broken off. More modern installations with essentially the same design include a number of traveling gratemprovements and cost-saving features (Fig. 6.8-34) [B.48]. They are related to better feed preparation and more controlled deposition of the hearth layer and of a deeper bed through surge bins and charging devices, energy savings by the use of hot air from a separate sinter cooler for bed preheating and in the ignition furnace, additional heat recovery in boilers, improved cooling efficiency by employing a pre-breaker in front of an optimized cooler, desulfurization and denitrification of the exhaust gas, and electric power savings from the use of more efficient fans. Although, in general, the process has not changed during further development, experience that was gained during the modification of the system for the hardening of pellets (Section 6.8.1) was applied to sintering. In particular this includes the complete enclosure of the traveling grate to accommodate specific combustion and off-gas handling procedures. While, during the induration of pellets, changes between downand updraft airflow patterns produces advantages, in sintering the downdraft method is used throughout.
6.8 Applications in the Mining Industry (Minerals and Ores)
Fig. 6.8-34 Flow diagram of a modern sintering plant with improvements and cost saving features highlighted [B.48]
Fig. 6.8-35 shows a traveling grate sintering plant featuring enclosures [B.40, pp.8393]. Waste gas minimization by recycling is used together with energy recovery. Only a relatively small part of the gas in the system is exhausted and, possibly after desulfurization, released to the environment. In three of the gas recycling loops the temperatures are high enough for reasonable heat utilization for electric power generation.
Fig. 6.8-35 Diagram of a traveling grate sintering plant featuring enclosures and waste gas minimization by recycling [B.40, pp.83–93]
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Advantages are also obtained from two-stage cooling. Sinter is first cooled from 500–600 8C to an average temperature of 250 8C on the strand, broken to < 200 mm during discharge, and separated at 50 mm on a scalping screen. The oversize is crushed to < 50 mm by a double roll crusher arranged in closed loop with the scalping screen. The material is then again separated at 5 mm; fines are recycled “hot”, thus providing some heat to the feed mix (supplemental energy recovery), and the coarse fraction is transferred to a conventional secondary cooler. As the sinter has obtained its product size before final cooling, this process stage becomes more efficient, also resulting in a more uniform and lower discharge temperature that poses less danger if belt conveyors are used for transport. The structure of and the solid fuel distribution in the material bed on the traveling grate critically determine how well sintering proceeds. Since ore, fuel, and sometimes flux (such as limestone and/or lime) need to be blended prior to the mixture’s deposition on the strand and, if moisture is present or after the addition of moisture, the motion of solid particles in a mixer will initiate agglomeration by growth, it has become a rather common practice to granulate the feed mix, thereby improving the bed structure on the traveling grate, and causing fuel and ore to get into closer contact (Fig. 6.8-36). The use of modern, sophisticated control features is the latest development in ore sintering. Fig. 6.8-36 shows the instrumentation installed at a sinter plant in China [B.56, pp. 450–454]. The multi-variable process control is adaptive in nature, that is it keeps track of variations and automatically adjusts the operation to the most optimal conditions. To overcome the time delays that are inherently experienced in a process of long duration, a prediction algorithm has been included. However, since random, unpredictable disturbances are often experienced, a proportioning expert system is necessary to yield rational and uniform results.
Fig. 6.8-36 Sensor and control systems at the no.3 sintering plant at Anshan, China [B.56, pp. 450–454]
6.9 Applications in the Metallurgical Industry
Batch and straight or circular, and occasionally differently shaped continuous sintering plants are being used not only for iron ores but also for non-ferrous ores and lately are applied also for the treatment of metal bearing fines, such as flue dusts, mill scale, and ground slags, for recirculation (Section 8.2).
6.9
Applications in the Metallurgical Industry Applications of size enlargement by agglomeration in the metallurgical industry are in three major fields: * * *
raw materials and additives for metal production; modification of metal products; and processing of metal or metal-bearing wastes for recycling.
The agglomeration methods used for the raw materials for metal production have been covered in Section 6.8 (mining industry: minerals and ores). Applications for additives and other materials required in different steps of metal production are discussed in the following section. A major modification of metal products is carried-out in powder metallurgy (Chapter 7) while others are reviewed in Section 6.9.2. The processing of metal wastes for recycling is also included in Section 6.9.2, while the recovery of metal-bearing wastes, mostly dusts from metal making, is covered in Section 8.2. Tab. 6.9-1, without supposing completeness, lists some of the most important materials that have been and/or are being agglomerated for a multitude of purposes in the metallurgical industry. Tab. 6.9-1 Alphabetical listing of raw materials, additives, metal products, and metal bearing wastes that have been and/or are being agglomerated to obtain various benefits Raw materials and additives or materials for metal production Alloying elements, electrode mass, minerals, ores, oxidizers, refractories, fluxes, Modification of metal products Direct reduced iron (DRI, sponge iron), metal powders, titanium sponge Processing of metal or metal-bearing wastes for recycling Aluminum chips and turnings, blast furnace dust, brass turnings and swarf, cast iron turnings, converter dust, copper turnings and swarf, copper wires, electric arc furnace dust, ground metal bearing slags, metals sludges, metal swarf, mill scale, zinc and lead enriched dust
Further Reading
For further reading the following books are recommended: B.3, B.4, B.7, B.8, B.10, B.11, B.13,c,d, B.14, B.15, B.16, B.18, B.20, B.21, B.22, B.26, B.31, B.35, B.40, B.48, B.55, B.56, B.64, B.82, B.87, B.89, B.94, B.97, B.98 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold.
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6.9.1
Tumble/Growth Technologies
One of the requirements for the successful application of tumble/growth technologies for size enlargement of particulate solids by agglomeration is a sufficient fineness of the particles (Chapter 5). Others are the need for binder liquids and post-treatment. Since size reduction by comminution of metallic materials is difficult and, if possible, expensive and the production of metal powders by atomization of melts, followed by solidification, normally produces ultraclean particles for special applications (Chapter 7), only those metals that are directly available in a finely divided form could be candidates for the use of growth agglomeration. Also, suitable binders that do not introduce objectionable impurities or become lost during post-treatment while giving sufficient strength, are difficult to find at acceptable cost. A few applications exist that combine metal or metal-bearing powders by dry mixing to yield formulated alloying components that are stabilized by wet agglomeration and post-treatment to become a dust free and easily handleable granular product, but most processes in the metallurgical industry use methods of pressure agglomeration (Section 6.9.2). Typically, this technology is also used for the recovery of metal-bearing dusts from metallurgical manufacturing and processing plants (Section 8.2). Certain special metallic parts, such as hard metal inserts for cutting tools and permanent magnets, are pressed into shape and hardened by powder metallurgy (Chapter 7). For good structure and performance the primary particles, metal carbides (tungsten or titanium carbide) and ferrites, must be small but the final parts should be dense. To achieve the required structural density, the powder (produced by chemical reaction and precipitation) has to be filled reliably, quickly, and tightly into the molds with a minimum of air space between the particles. As for all pressing applications (Section 6.2), this necessitates free-flowing granules with excellent metering characteristics and high bulk density. Such intermediate, agglomerated products may be produced by fluidized-bed spray granulation from particle suspensions [B.93]. Similar applications of tumble/growth agglomeration are conceivable if powders or suspensions of metallic solids (obtained by precipitation or other ultrafine particle formation methods, for example in nanotechnology (Chapter 11)) must be converted into a dry, freely flowing granular material for further processing.
6.9.2
Pressure Agglomeration Technologies
During past centuries, the blast furnace was developed to become the dominant producer of raw iron in the metallurgical industry. To be competitive, gigantic installations were built with unit capacities of several million t per year, operating in integrated steel mills that produce semi-finished products from ore and coal in a single industrial complex. Modifications of the ore and coal by agglomeration methods (Sections 6.8 and 6.10) helped to improve blast-furnace operation and further established the dominance of the process. Until recently, the blast furnace route to steel was the accepted technology everywhere in the world.
6.9 Applications in the Metallurgical Industry
In the 1960s the idea of “mini-mills” was introduced. These are regional steel manufacturing plants based on electric arc furnace (EAF) technology, melting scrap and pig iron and making a growing number of steel grades for local markets. Because, at any given time, these facilities produce and process relatively small amounts of liquid metal they are very flexible and can supply local markets economically with custommade steels, Therefore, they quickly found widespread acceptance. Pig iron, being a product of the blast furnace, is a clean but expensive material. Its availability is controlledby thelarge primarymills.Thecompositionandpriceof scrap, on the other hand, vary widely. Only the most expensive and scarce scrap (home or machining scrap and #1 bundles), which contains restricted amounts of contamination and highly alloyed or coated steel, is suitable for melting in EAF without extensive and costly adjustment of the composition. Therefore, the price of this commodity varies widely depending on its production, availability, and trader-controlled market forces. As a consequence, several companies became interested in solid-state reduction of high-grade or upgraded (pelletized, Section 6.8.2) iron ores by the direct reduction (DR) processes. They produce virgin iron with little gangue (depending on the total iron content of the ore) by utilizing gaseous or solid reductants [B.14, B.20, B.87]. After a burst of development in many parts of the world, yielding many ideas and patents, as well as pilot and demonstration plants, only a few were industrially and economically feasible and prevailed. Today DR facilities are based on gas and coal as reductants and use shaft furnaces, fluidized beds, rotary kilns, and circular grates. Processes using solid reductant (coal) are used where natural gas or other sources of gaseous reductant are not available or too expensive. The product often contains some ash and residual solid fuel. The capacity per unit is relatively small, 10 000–500 000 t/y, and this method is increasingly being used for the processing of metal-bearing waste materials for recycling (Section 8.2). Processes utilizing gaseous reductants (reformed natural gas, hydrogen, suitable process off-gases, or products from oil or coal gasification) yield cleaner products and are carried-out in shaft furnaces or fluidized bed reactors. The highest quality is obtained if the reductant is a cleaned (desulfurized) reformed gas or hydrogen. Such installations may have a capacity of up to 1.5 million t/y per unit in the case of shaft furnaces or up to 2.5 million t/y by employing multiple modules (Section 9.3). While most of the earlier large-scale DR processes were built in conjunction with EAF shops as integrated mini-mills, in which the directly reduced iron (DRI) was used on site (Fig. 6.9-1), more and more so called merchant DR plants have been installed. The latter produce DRI as premium iron units for export and sale to EAF steel-making facilities around the world. Particular advantages of merchant DRI are its cleanliness, reliable chemistry, and predictability of feeding characteristics (size and shape). To be competitive with high-quality scrap during times when this commodity is offered cheaply, merchant DRI facilities are installed where high-grade iron ore or pellets are easily available and/or the gaseous reductant (natural gas) is in ample supply. Today, such places are north-eastern Venezuela, north-western Australia, western Iran and India, and parts of Russia and China, where iron ore and natural gas are available and the added value of the merchant DRI is of great interest to the local economy.
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Fig. 6.9-1 a) Schematic flow diagram of a mini-steel-mill using DR and EAF (courtesy Hylsa, San Nicolas de los Garza, NL, Mexico); b) Hadeed, Al Jubail, Saudi Arabia; foreground left: ore storage, center: two DR plants, upper right: steel mill (courtesy Midrex, Charlotte, NC, USA)
Other sites are, for example, the south central USA, Malaysia, or Trinidad where the ore feed on its way from the source to the consumer can be easily landed and value can be added by utilizing local sources of natural gas. Except for some thermal and mechanical cracking and degradation, the external macroscopic physical form of DRI is the same as that of the feed material (Fig. 6.9-2). Chemically, oxygen that was associated with the iron (iron oxides) has been removed in the solid state. This leaves a very porous (sponge) iron structure. Fig. 6.9-3a shows an SEM image of the internal surface of a DRI pellet and Fig. 6.9-3b is a polished and etched cross section through the same pellet showing
6.9 Applications in the Metallurgical Industry
Fig. 6.9-2 a) Mixture of iron ore lumps and pellets, b) top: direct reduced iron pellets, bottom: direct reduced iron lumps. Observe the slight cracking
the amount of intraparticle porosity. As a result, the apparent density of DRI is low (about 2 g/cm3) and the specific surface is extremely high (around 1 m2/g). To a certain extent, both density and specific surface area depend on the raw material, the type of solid state DR technology, and the operating conditions during direct reduction (Section 13.3, ref. 79). Nevertheless, because of its nature and structure, DRI always behaves differently in many ways if compared with solid (pig) iron. An important characteristic, which is common to all products, is caused by the large specific surface area, and is most critical for the shipment of merchant DRI, is the material’s tendency to reoxidize at ambient temperatures (Section 13.3, ref. 69). Most of the possible chemical reactions taking place during reoxidation are exothermic in nature, that is they produce heat. Since both the thermal conductivity (DRI is an insulator)
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Fig. 6.9-3 a) SEM image of the internal surface of a DRI pellet; b) micrograph of a polished and etched cross section through the same pellet
and the temperature at which spontaneous reoxidation begins are low, it is possible that DRI in bulk masses (storage piles), in silos, or in ship holds will heat due to such reactions and reach the self-ignition temperature from which oxidation proceeds, selfsustained and accelerating, as long as oxygen is available. If water is present, reoxidation is more easily initiated and hydrogen is produced during the reaction, which can lead to the formation of explosive oxyhydrogen gas. Seawater accelerates this process. To avoid catastrophic situations, such as documented in Fig. 6.9-4, the International Maritime Organization (IMO) has, as part of its code of safe practice for solid bulk cargoes, declared DRI as “material hazardous in bulk” (MHB), which requires certain precautions to allow shipment and obtain insurance. Untreated DRI can be transported if the ship is equipped with safety measures as shown in Fig. 6.9-5 [6.9.2.1]. Before loading the cargo, the floor of each hold is wired with thermocouples at specific points for temperature monitoring during the voyage (Fig. 6.9-5a). Steel pipes are also installed in each hold (Fig. 6.9-5b) to allow a purging of the cargo with nitrogen after loading. If the hatch covers are tightly sealed, safe shipment of DRI is possible for short distances (coastal, river and lake) if strict rules are observed [6.9.2.1]. The above is not acceptable for long haul and intercontinental ocean shipments where (salt) water can enter the holds during storms or other unusual conditions (accidents). It also does not solve the reaction and self-ignition problem during transshipment and storage on the receiving side. While the latter can be avoided by keeping the material cool and dry from the time DRI is unloaded until it is used in the steel mill, other precautionary measures must be taken for ocean shipping. It is also desirable for merchant DRI to be inert so that it can be safely handled without losses in conventional scrap yards. Therefore, all companies that are active in the field of DR have put considerable effort into the development of suitable passivation methods. At the same time, test methods have been developed to determine the reactivity of DRI products, simulate the conditions during storage, understand the factors controlling
6.9 Applications in the Metallurgical Industry
Fig. 6.9-4 US national press reacting to a DRI shipping accident (Section 13.3, ref. 69)
reactivity, and ascertain techniques to reduce or inhibit reoxidation (Section 13.3, ref. 69). All DR facilities processing pellets and/or lump ores can be designed to produce cold or hot DRI. Cooling is used before discharge if the material requires only limited handling and storage before its use. This is the case if a meltshop is associated or nearby (Fig. 6.9-1b). The option of cold discharge does not exist for plants using fluidized bed reactors as the fine-grained product features an even higher exposed specific surface and, therefore, lower self-ignition temperature. To overcome this problem, based on the success and experience with the hot briquetting of cast-iron borings (below), early pilot plants (ESSO FIOR, Purofer, US Steel) were equipped with roller presses to shape and densify the hot, fine DRI into pillow-shaped briquettes (Fig. 6.9-6). It was found that, if the briquettes attained a density of at least 5.0 g/cm3, the compacted product had become essentially inert. This is caused by a marked reduction of porosity (Fig. 6.9-7) and a highly densified outer “skin”, which makes the remaining internal surface area largely
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Fig. 6.9-5 Cutaway sketches of a ship’s hold equipped for the transportation of DRI [6.9.2.1]: a) locations of thermocouples and oxygen and hydrogen monitoring, b) installation of inerting pipes
inaccessible. As a result, IMO has amended the code to exclude “DRI, hot molded, pressed at temperatures equal to or higher than 650 8C and attaining an apparent density of at least 5.0 g/cm3” from the MHB classification. The reasons why this rather recent development was described in so much detail are: the author’s participation in its evolution from the first pilot plant stages to today and it is an example of how the analysis of technical problems and the evaluation of their roots results in the definition of remedies and industrial solutions. All true (producing exclusively for export) merchant DR plants use roller presses to produce hot briquetted iron (HBI) although other methods of densification and shaping are also feasible (Section 13.3, Patents 5–9). Profitable merchant plants use ores that are more likely to produce fines or avoid the additional cost of procuring highquality lump ore or pellets altogether by feeding natural ore fines or concentrates directly into fluid-bed reactors. As mentioned before, such facilities require hot den-
6.9 Applications in the Metallurgical Industry
Fig. 6.9-6
Briquettes made from hot fine DRI (courtesy FIOR, Puerto Ordaz, Venezuela)
sification to render the product handleable and suitable as a source of iron for steelmaking. The particular advantage of briquetting is not only the morphological change from an iron sponge to a high-density compact structure (Fig. 6.9-7) but also that the densified skin incorporates most of the fines that are discharged from the process. Such fines may result from thermal decrepitation and abrasion (lump ore and/or pellet feed) or represent the entire output (fine ore feed). Fig. 6.9-8 depicts three typical hot briquetting systems for DRI. Hot feed in the form of reduced pellets and/or lumps and/or fines is forced by a vertical screw feeder into the nip between two counter-rotating rollers with matched pockets [B.48, B.97]. The briquettes are pillow shaped, 90–140 mm long, 50–60 mm wide, about 30 mm thick, and weigh 450–800 g each (Fig. 6.9-9). Although the rollers are set closely together, a distance of 2–3 mm is maintained between their surfaces to make sure that there is
Fig. 6.9-7 Micrographs comparing the structures of direct reduced iron (DRI, left) and hot briquetted iron (HBI, right) (Section 13.3, ref. 146)
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Fig. 6.9-8 Schematic flow diagrams of three typical hot briquetting systems for DRI: a) left: hot pellet and/or lump feed from shaft furnaces; right: hot fines feed from fluidized bed reactors; both including hot fines recycling; b) hot pellet and/or lump feed without hot fines recycling: 1, roller press; 2, separator; 3, hot screen; 4, briquette cooler; 5, hot bucket elevator, fines recycling (Section 13.3, ref. 146)
6.9 Applications in the Metallurgical Industry
Fig. 6.9-9
Photograph showing differently sized HBI
never any metal-to-metal contact. This is particularly important because a high specific pressing force of 150 kN/cm is required to attain the necessary density. During operation, this gap increases to 3–6 mm. Since material in the area between the briquette pockets experiences the highest forces and the resulting material bridge is 3–6 mm thick, a continuous string of briquettes is produced rather than individual ones. A separator, producing mostly single briquettes by breaking the continuous string, is an important part of all hot DRI briquetting plants. Briquettes made from reduced pellets and/or lump ore, typically produced in a shaft furnace, feature stronger connections than those made from fine DRI, typically produced in fluid bed reactors. Therefore, the separator consists of a high energy rotating wheel in the first case (Fig. 6.9-8a, left) and a tumble drum with lifters in the second (Fig. 6.9-8a, right). In the preferred embodiment, the briquettes are cooled by immersing them in water (quench cooling) on a conveyor (belt or vibrating) but other methods of cooling (e.g., evaporative) are also available. Fig. 6.9-8a also shows a hot fines separation (double deck screen) and recirculation (bucket elevator) system. Fines and chips are produced by leakage in the roller press [B.13b, B.48, B.97] and during breakage in the separator. More recently, plants are installed without fines recycling (Fig. 6.9-8b). In such cases, the product is screened after cooling and again prior to loading. The metallized fines and chips can be recirculated to the DR plant as “Remet” or, generally, the material is sold for special applications. In many publications, photographs of HBI show nicely formed briquettes. While briquettes from fine DRI obtained in fluidized bed reactors are more frequently well shaped (Fig. 6.9-6), product made from pellets and/or lump ore and after high energy separation includes not only single briquettes but also some multiples and broken ones (Fig. 6.9-10). Metallurgically, such mixtures behave the same as single, well-formed briquettes. Nevertheless, there is often a concern because broken
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.9-10 Commercial HBI from a mixture of reduced pellets and lump ore (Section 13.3, ref. 146)
pieces expose the less densified interior. One of the commonly observed “defects” is caused by the rolling action of a roller press (Fig. 6.9-11), which results in that pockets are never completely closed (Fig. 6.9-11, left) and that edges are missing (Fig. 6.9-11, right) or open-up (so-called clam-shelling, oyster-mouthing, duck-billing). Nevertheless, the briquettes are inert and can be shipped and handled without problems by truck (Fig. 6.9-12, top left) or ship (Fig. 6.9-12, top right) and stored outside (Fig. 6.9-12, bottom).
6.9 Applications in the Metallurgical Industry
Fig. 6.9-11 Left: five successive momentary conditions of briquetting between two counter currently rotating rollers with matching pockets showing that molds are never completely closed. Right: sketch of a defective pillow-shaped briquette from a roller press [B.48, B.97]
Fig. 6.9-12 Top left, HBI being shipped by truck; top right, HBI being unloaded by magnet from a ship hold; bottom, outside storage of HBI in Venezuela before loading it on ships (courtesy Midrex, Charlotte, NC, USA, and OPCO, Puerto Ordaz, Venezuela)
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To obtain the required density, according to IMO the temperature of the feed must be > 650 8C (better > 700 8C). Therefore, all parts of the system that come in contact with the hot material, especially the roller presses and separators must be made of temperature-resistant steel, vigorously cooled, and, at least partially inert (Section 13.3, refs. 102, 125). Fig. 6.9-13 shows views into the hot briquetting bays of different merchant DR plants, depicting the specialized roller presses, and Fig. 6.9-14 shows photographs the facilities. Until hot densification of DRI was introduced for merchant plants the dominant shaft furnace technologies (Midrex and HYL) were designed to discharge DRI cold, even if the facilities were next to and the material was mostly used in an associated EAF steel mill, as shown in Fig. 6.9-1. The disadvantage of this arrangement is that to a large extent (only some heat recovery is carried-out in the DR process itself) the heat from direct reduction is wasted and the EAF charge must be preheated, requiring either special equipment or electric power. The recent development of methods for hot, inertized transport has made it possible to use hot discharging DR furnaces, heat recovery, and consequent substantial savings. Installation of roller presses for the hot densification or external cooling of excess DRI or during upsets in the steel mill (Fig. 6.9-15) results in superior layouts of DRI/EAF steel mills. With many of the blast furnaces at integrated steel mills outdated or due for costly major overhaul during the first decades of the 21st century, steel companies are trying to switch to alternative iron and steel making technologies, also using new iron units as feed [6.9.2.2]. Because of the technically limited size of EAF (maximum capacity about 150 t of liquid metal and several hours tap-to-tap time), resulting in relatively small steel mills with production rates of a few 100 000 t/y using multiple EAFs, the DRI/ EAF route is not an alternative for the large integrated steel companies for the production of mass steels for construction of every kind (automobiles, ships, industrial and domestic structures). One new development that is directed towards the replacement of the multi-million t/y blast furnaces for the production of liquid steel is carried out at POSCO, Pohang, South-Korea. In this new process, fine ore in sinter-feed quality, typically 23 % cheaper than lump ore and even more when compared with high-quality iron ore pellets, is reduced in fluidized bed reactors (process named FINEX) with off-gas from the downstream melter gasifier. The resulting DRI, which also contains a suitable amount of lime for desulfurization, is hot compacted, broken into smaller pieces (hot compacted iron: HCI), transported hot to the smelting facility, mixed with briquetted common bituminous coals (typically 24 % cheaper than metallurgical coal) and charged to the melter gasifier (modified Corex vessel) in which liquid steel and reduction gas for the FINEX DR process are produced. It is anticipated that with this technology million t/y installations, producing liquid steel at considerably lower cost than blast furnaces, can be built and operated [6.9.2.2]. Size enlargement by agglomeration with roller presses is used for the briquetting of coal (Section 6.10.3) and the hot densification of DRI. Still other processes, all using agglomeration at some point, are being developed at different locations [6.9.2.2]. As mentioned in Section 6.9, methods for the recovery and use of metal-bearing (waste) materials are covered in Section 8.2.
6.9 Applications in the Metallurgical Industry
Fig. 6.9-13 Views into the hot briquetting bays of different merchant DR plants showing some of the roller presses (courtesy Orinoco Iron, Puerto Ordaz, Venezuela)
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Fig. 6.9-14 Panoramas of two merchant DR plants (courtesy Orinoco Iron, Puerto Ordaz, Venezuela (FINMET), and Midrex, Charlotte, NC, USA (COMSIGUA HBI, Matanzas, Venezuela))
6.9 Applications in the Metallurgical Industry
Fig. 6.9-15 Schematic flow diagrams of steel mini-mills using DR with hot discharge: a) hot (pneumatic) transport to the EAF, hot briquetting of excess hot material (courtesy HYLSA, San Ni-
colas de los Garza, NL, Mexico); b) typical hot link with transfer located under the DR furnace and external cooling of excess hot material (courtesy Midrex, Charlotte, NC, USA)
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Fig. 6.9-16 Flow diagram of a hot briquetting plant for cast-iron borings [B.3, Vol. 10 (1965), 16–22]
At the beginning of this chapter it was stated that the use of hot briquetting for the densification of DRI was influenced by the earlier successful testing and use of roller presses for the briquetting of oily cast-iron borings to yield a product resembling pig iron for remelting. This development was initiated by General Motors in the USA, where in two central foundry divisions (Saginaw, MI, and Tonawanda, NY) large numbers of cast-iron motors were produced and machined leaving mountains of borings which, because they were heavily contaminated with boring oil, could not be easily reused. Disposal of the material was out of the question because it was very valuable, well-defined home scrap. The process that was developed [B.35, Vol. 10 (1965), 16–22] uses burning (removal) of the boring oil in a special multiple-hearth furnace to evaporate the liquid and heat the dry metal particles to about 675 8C. At this temperature the cast-iron borings have become malleable enough to produce a high-density briquette while requiring relatively low force and resulting in acceptable (minimized) roll wear. After the roller press, the material is cooled before further handling (Fig. 6.9-16) The briquettes feature a number of advantages as listed in Tab. 6.9-2 and are an ideal melt charge for the foundries’ cupolas. Based on this and other success stories and the need to conserve raw materials and avoid waste (Chapter 8), other, particularly non-ferrous, materials are collected and converted into high-density shaped products (briquettes) by pressure agglomeration techniques. The aim of the briquetting process is to obtain similar characteristics to those listed in Tab. 6.9-2. The aluminum industry has one of the most advanced and complete recycling programs in the metals market (Section 13.3, refs. 120, 132, 137). Among the reasons for this are that while aluminum has become one of the major metals used for construc-
6.9 Applications in the Metallurgical Industry Tab. 6.9-2 Advantages of briquettes made from hot cast iron borings as a melt charge for foundries [B.3, Vol. 10 (1965), 16-22] 1. 2. 3. 4. 5. 6. 7.
The briquettes will take several severe handling abuses without breakage. The briquettes can be stored without deterioration for great lengths of time. The briquettes can be easily transported and weighed (metered). The briquettes are a clean foundry charge material, free of moisture, oil, and other contaminants. The briquettes do not disintegrate in a hot gas stream, no fines blowing away. The briquettes’ high density results in a high heat transfer rate regardless of the type of melting. The briquettes’ curved outer surface (pillow shape) insures sufficient voids in the furnace charge and does not prevent the free flow of hot gases during melting. 8. The briquettes’ chemical composition closely resembles that of the iron being made; little adjustment of the chemistry is required and the slag volume is low.
tion and packing, its production from Bauxite requires large amounts of electrical energy so that, today, most of the aluminum producers are located in areas where cheap (hydroelectric) power is available. Transportation from these locations to the finishing mills adds substantially more to the cost of the virgin material. A large segment of aluminum recycling is based on the collection and processing of used beverage cans (UBC), other containers, and automotive scrap: “old scrap” resulting from obsolescence (Section 8.2). In the context of this chapter, the following will exclusively deal with “home scrap” and “prompt industrial scrap”. Home scrap is recycled within the mill, processing (alloying) and shaping (rolling, extruding) metal into intermediate products, while prompt industrial scrap is new scrap from fabricators who do not choose or are not equipped to remelt. The latter is collected by and marketed through secondary scrap dealers and should be segregated according to alloy, shape, and quality. Aluminum home scrap in the form of swarf is produced in considerable amounts during the trimming of ingots and blocks by milling for sheet and foil manufacturing, in large machining centers, for example in automobile wheel and motor production, and in the aircraft industry. If the turnings and borings form loose, tangled masses, they must first be crushed. This processing step reduces the length of the swarf components, which then become relatively free-flowing and pack more densely. If swarf is also wet and/or oily, an additional cleaning step by washing and drying or thermal treatment is recommended or required. Even after such processing, melting of the now dry, crushed swarf still causes problems, mostly because of difficulties in feeding the low-density bulk material (about 0.25 ts/m3) into remelting furnaces and excessive oxidation due to its large surface area. Fig. 6.9-17 is the block diagram of an improved secondary aluminum smelting operation (Section 13.3, ref. 132). Compared with earlier installations, the “compacting” process step has been added. As described above, to render the material suitable for feeding into the compaction equipment, the swarf must be clean and reasonably free flowing. The flowability and, therefore, its metering characteristics depend to a great extent on particle shape, which is influenced by material hardness. Swarf from softer alloys tends to be thicker and spirally wound; such material is bulky, interlocks, and
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.9-17 Block diagram of a secondary aluminum smelting operation that includes compacting the processed swarf (Section 13.3, ref. 132)
entangles and may have to be crushed. Harder alloys produce thinner, straight chips, which can be used directly. For the compaction of clean processed metal swarf, two methods are available. The non-continuous confined volume punch-and-die process (Fig. 6.9-18 a) and the continuous compaction in the nip of two counter-rotating rollers (Fig. 6.9-18 b). The advantage of the non-continuous punch-and-die process is that highly entangled, very loose swarf can be compacted to yield a relatively high apparent density. Preconditions are that the material can be fed to the die and that the stroke of the punch (densification ratio) is long and/or multiple feeding and compression cycles are possible to form a single compact. In the case of aluminum, the product shape is normally cylindrical with diameters of 80–190 mm, heights of 30–120 mm, and weights of 0.4–7.3 kg. The technology also has a number of disadvantages of which the most important is the relatively slow movement of the hydraulic rams, which must be further reduced to overcome elastic behavior of the swarf, and a dwell time is necessary [B.97]. To improve the unfavorable production/investment cost ratio, it is better to produce large compacts. Owing to interparticle and wall friction during densification, such briquettes feature lower and less uniform density, particularly if the simpler singlesided machines (one punch pressing against a fixed anvil) are used. Double-sided
Fig. 6.9-18 Diagrams of the two methods used to compact aluminum swarf: a) punch-and-die, b) roller press. 1, anvil (removable for ejection of compact; 2, die; 3, punch (reciprocating); 4, feed; 5, compact; 6, rollers; 7, force (screw) feeder; 8, compacted sheet (product).
6.9 Applications in the Metallurgical Industry
machines, in which compaction is accomplished between two hydraulically operated punches (Fig. 6.9-19), produce higher density and good transfer stability of the briquettes. With these and other characteristics in mind ([B.97], Section 13.3, ref. 132) punchand-die presses are used when relatively small amounts of aluminum swarf and/or alloys, producing turnings and borings with high elasticity, must be compacted. The flow diagram of a complete processing system is shown in Fig. 6.9-20. The vibro-screen feeders in front of the centrifuges and the press eliminate and by-pass large pieces that might damage the machines. Fig. 6.9-21 shows processed aluminum swarf and briquettes produced with a punch-and-die press.
Fig. 6.9-19 a) Diagram of a hydraulic punch-and-die press for metal swarf briquetting featuring two-sided pressing; b) sketches explaining the operating principle of the press depicted in (a) (courtesy Metso Lindemann, D€ usseldorf, Germany)
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Fig. 6.9-20 Flow diagram of a complete metal swarf processing and compacting system using a two-sided punch-and-die press for briquetting (courtesy Metso Lindemann, D€ usseldorf, Germany)
Fig. 6.9-21 Photograph of processed aluminum swarf and of briquettes produced with a punch-and-die press
The continuous compaction between two rollers offers the advantages of large capacity and high apparent density. However, since an endless thin sheet is formed from elongated and often tortuous pieces, there is no hope of making individual compacts that can be easily charged into the remelting furnaces, similar to the cylindrical briquettes from the punch-and-die process. Use of the continuous roller press was developed for compacting relatively large amounts of home scrap that has been segregated by alloy to realize the highest economical advantage. Fig. 6.9-22 is the flow diagram of such a plant. Not shown in the flow diagram is that the swarf may have to be crushed to yield chips with dimensions of about 0.8 mm 12 mm 20–60 (max. 100) mm. Washing and subsequent drying are required if the oil content (from milling) is > 2 g oil/kg chips. The clean dry aluminum is delivered to the plant pneumatically (1), collected in a cyclone (2), and distributed by a shuttle conveyor (3) into silos (I to VI). From the silos the material is metered by vibrating feeders (4) onto a belt (5) and an elevator (6) into a feed bin (7). An amount, larger than required for the roller press, is removed from the bin by vibratory conveyor (8) and passes a magnetic separator. The integral force (screw) feeder (9) of the roller press (10) supplies the material into the nip between the rollers for continuous densification. The excess material that was made available to reliably avoid a starved feed condition, overflows (11) and is recirculated to bin (7). The compacted strip discharging from the roller press is cut into slabs of approx. uniform length in a double roll trimming shear (12). Fig. 6.9-23 shows three different processed aluminum home scrap samples (top) and sheared compacted strips. On a vibro-screen
6.9 Applications in the Metallurgical Industry
Fig. 6.9-22 Flow diagram of a processed metal (aluminum) swarf briquetting system using a roller press for compaction (explanations see text)
feeder (13) potentially uncompacted chips are screened out and recycled to elevator (6) by a vibrating feeder (14) and conveyor (15). The compacted slabs are alternatively deposited by a diverter gate into one of two carts (16) which may be positioned in a sound-proof chamber that is connected to dust collection. Conveyor (4) can be reversed to clean out individual bins or the entire chip storage facility into transport containers (17). During start-up, optimization trials, or towards the end of a run, unsatisfactory compacts may be produced. In this case, the vibroscreen feeder (13) can be turned to remove the off-spec material from the system via vibro-feeder (14) and conveyor (15), which is reversed to transport the material into a dumpster.
Fig. 6.9-23 Three different processed aluminum home scrap samples (top) and sheared compacted strips produced by the continuous roller press process (Fig. 6.9-22)
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Fig. 6.9-24 shows the flow diagram and a photograph of another plant for the briquetting of 8 t/h processed aluminum home scrap (swarf) in two roller press compaction lines of 4 t/h each. Although the capability is larger, after the plant was put into operation total annual production was about 23 000 ts. The slabs produced in such facilities have high density (> 2.3 g/cm3) and are particularly well-suited as feed for reverberatory furnaces. Typically an aluminum loss of 15–20 % is experienced if uncompacted particulate scrap is melted. This is reduced to 2–5 % if compacted slabs are charged, resulting in a sizable commercial advantage. Other high-quality metal refuse that is suitable for remelting is not produced or collected in such large amounts as cast-iron borings or aluminum swarf, discussed above. Such materials are densified and shaped in newly designed small hydraulic punch-and-die presses. Fig. 6.9-25 shows a collection of briquettes from different
Fig. 6.9-24 a) Flow diagram; b) Plant for the briquetting of 8 tonne/h processed aluminum home scrap (swarf) in two roller press compaction lines of 4 tonne/h each (Rhe´nalu, Neuf Brisach,
France): 1, pneumatic transport system; 2, storage silos; 3, reversible horizontal belt conveyor; 4, dumpster; 5, elevators; 6, day bins; 7, roller presses; 8, rotating shears; 9, product collection carts
6.9 Applications in the Metallurgical Industry
Fig. 6.9-25 Collection of briquettes made from different metals with small, hydraulically operated punch-and-die presses (courtesy Ruf, Zaisertshofen, Germany)
materials and Fig. 6.9-26 depicts the principle and a photograph of such a press. A predetermined volume of the loose, clean metal dust, chips, turnings, borings, wire clips, foil, wool, etc. are fed from an agitated hopper (to avoid bridging) by a screw conveyor into the press (Fig. 6.9-26a). A tamper (vertical, hydraulically operated piston) predensifies the charge, which is then compacted by the horizontal hydraulic ram. The press features two oscillating pressing chambers and, in addition to the press ram, two rods, one of which expels the finished briquette while the other one is densified (Fig. 6.9-26a, top). Such machines have production capacities of 50– 3000 kg/h (depending on machine size and material to be briquetted) and may exert pressures of up to 5000 kg/cm2. There are many other fine materials in the metallurgical industry that are converted with methods of pressure agglomeration. A major group comprises additives that are used in the liquid metal bath to achieve a number of processes, most commonly either alloying or chemical reactions, such as desoxydation, denitrification, desulfurization (Section 6.9.1). For good dispersibility and/or solubility these components should feature small particle size, but that makes them unsuitable for feeding into metallurgical equipment. In the thermally turbulent gas atmosphere above the melt and the normally strong flow induced by the dust collection fans, small particles are entrained and end-up in the off-gas filters of the systems. Introducing the fine solids into the bath with lances is often not feasible and/or economical. Therefore, the materials should be agglomerated into larger pieces or granules, which are dense and dust free but still retain the large surface area of the originating particles. These requirement can be obtained with briquetted or compacted/granulated products. Since the manufacturing of pressure agglomerated products for the metallurgical industry requires high forces, in some cases, additional benefits can be experienced. The following describes what may occur as an example. During high pressure agglom-
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Fig. 6.9-26 a) Principle; b) photograph of a small, hydraulically operated punch-and-die press (courtesy Ruf, Zaisertshofen, Germany)
6.9 Applications in the Metallurgical Industry Fig. 6.9-27 SEM image of the internal structure of a mineral briquette showing particle disintegration and cracking (Section 13.3, ref. 23)
eration plastic particles are deformed and brittle solids break (Chapter 5, Fig. 5-9). In the latter case, due to the fact that the structure of the compact becomes rather dense, in addition to producing fine particles, cracks are formed within the solids (Fig. 6.9-27). This increases the specific surface area of the material. Among the common steel making additives that are briquetted are limestone and lime. Fig. 6.9-28 shows roller presses in operation for this application. The almond shaped briquettes have better storage characteristics, particularly if quick lime has been processed, and exhibit better solubility in the melt. Before its application in the steel mill, the very porous burnt lime absorbs water and hydrates, forming the more stable calcium hydrate. The higher density of the briquetted material slows this process. Even if briquetted lime is stored outside, only a surface layer will hydrate causing those briquettes to swell, disintegrate, and form a protective coat on the pile, effectively sealing the material below. In the steel mill, the heavier briquettes easily penetrate the slag layer and are quickly submerged in the metal bath. Lime produced from natural lump still features mineralogical bridges, which require time for dissolution. Briquettes are made from small particles, which, in addition, feature microcracks
Fig. 6.9-28 left) Three lime briquetting machines; right) close-up of briquettes on the discharge conveyor
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after they have been subjected to the high forces during briquetting (Fig. 6.9-27). Furthermore, they are held together by a binding mechanism (molecular forces). When the liquid melt penetrates into the pores of briquettes, the binding forces are readily destroyed whereupon the small, cracked particles are set free and dissolve quickly. The mechanisms discussed above are universally present advantages of products obtained from high-pressure agglomeration. They may be applied for all materials destined for similar uses and requiring these characteristics. The most important properties of briquettes or granules produced with high pressure agglomeration methods are: * * * * * * * *
higher density and weight, larger surface area, increased reactivity due to the presence of microcracks, binding mechanisms that are more readily destroyed, reduced dusting, improved flowability, feeding, and metering, easy storage and reclamation, and a whole host of other, more specific beneficial modifications that depend on the particular material and processing details.
6.9.3
Other Technologies
For a variety of applications, spherical particles are required. Many of these are associated with the field of powder metallurgy (Chapter 7). While it is relatively easy to produce spherical particles from low-melting materials by conventional techniques, in which melt droplets are produced and solidified in spray (prilling) towers [B.97], refractory solids in general and, specifically, high-melting metals can not be converted by this simple technique. However, if the solid is available in powder form (Section 13.3, refs. 82, 85, 89, 90, 94), various methods are available to produce spherical particles by agglomeration. One such “spherical agglomeration process” uses an immiscible binder liquid to form spheroidal products from particles that are suspended in a second liquid. These highly specialized materials are required in small amounts and, therefore are carried out in small, high-energy, batch-shaking devices as shown in Figure 6.9-29. In this apparatus, tungsten carbide spheres are manufactured, which after sintering yield ball-pen tips [B.73]. The closely sized particles of 1 mm diameter are prepared by agitating tungsten carbide and cobalt powders in a closed Teflon container with hemispherical ends on a high-speed reciprocating shaker. Halogenated solvents are used as the suspending liquid and water is the binder. The addition of about 6 % cobalt to the tungsten powder is required to lower the sintering temperature to more acceptable levels. In the batch process, compaction and rounding occurs during many collisions between the agglomerates and with the container walls. The advantage of products
6.9 Applications in the Metallurgical Industry Fig. 6.9-29 Teflon cylinder with hemispherical ends, mounted in a reciprocating shaker, and used to form small spheres by the spherical agglomeration process [B.73, B.97]
from this process is that finishing operations, such as lapping and grinding after a preliminary sintering step, are greatly reduced compared with those necessary for spherical compacts from press molding. When comparatively high (but still relatively small) production rates are required, continuous processes are better suited. Figure 6.9-30 depicts a drum agglomerator featuring an internal screen classifier for the formation of uniform spheres by immiscible liquid agglomeration [B.73]. In these tumbling agglomerators, the presence of a liquid slurry is useful to reduce dusting, especially if toxic powders are processed. The liquid environment also avoids avalanching because particles and immiscible binder liquid are uniformly distributed throughout the agglomerating mass thus allowing conglomerates to grow into larger entities in a much more controlled manner. The liquid charge also helps in the development of a desirable tumbling and cascading motion in the equipment because it is more voluminous and better interparticle lubrication occurs than would be the case if no suspending liquid were present. Further-
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Fig. 6.9-30 Drum agglomerator with internal spiral screen classifier for the formation of uniform spheres by immiscible liquid agglomeration [B.73, B.97]
more, the solids are carried with the liquid, which makes internal classification possible. As shown in Figure 6.9-30 a spiral screen that rotates at a slower speed than the drum passes through the charge and picks up agglomerates. Undersized particles fall through the screen openings and return to the agglomerating liquid mass. Larger material moves along the spiral until it reaches a tube at the axis of the drum that directs the finished agglomerates to a discharge point. In immiscible liquid agglomeration, particles with a small amount of adsorbed binding liquid on their surfaces collide and coalesce to form larger entities by growth agglomeration. In the sol-gel process, another agglomeration technique that occurs in the liquid phase, fine particles are initially suspended in a binder liquid. The suspension is then formed into spherical droplets and the excess binder is removed to solidify the droplets into a particulate product (Section 6.10.3). Agglomeration by heat or sintering is also very common in the metallurgical industry. However it is mostly applied for the size enlargement of feed materials (minerals, Section 6.8.3), for the preparation of secondary raw materials from metal bearing byand waste-products (Section 8.2) and for the development of final properties of powder metallurgical parts (Chapter 7). These chapters should be consulted for further information.
6.10 Applications for Solid Fuels
Finally, the modification of metals, for example to prepare improved catalysts, and the production of novel metal or metallic compounds by product engineering, all using nanostructural assembly, is a fast growing modern technological field that often includes desired and undesired size enlargement by agglomeration. Some references to these applications can be found in Chapter 11.
6.10
Applications for Solid Fuels Humans have used solid fuels from the time fire was harnessed to provide warmth and light. In the beginning, solid fuel consisted exclusively of dry plant material, mostly wood, but nomadic tribes living with their grazing mammals in steppes and on the brink of deserts, also learned to burn dry animal excrement. Other vegetation-based solid fuel was dry peat in certain geographical areas. Later, wax, resin, tar, tallow, and fat were found to sustain fire. These materials can be liquefied by melting and torches, originally dry (porous) pieces of wood that were impregnated with such liquids, were the first manufactured solid fuels and, because the aforementioned materials are also moldable, candles incorporating a wick were formed as “advanced” sources of light. Relatively early charcoal, the residue of anaerobic burning (distillation) of wood in an earth-covered pile, was produced to enhance the heating value of the wood and natural outcroppings of mineral coal were already mined in prehistoric times for use as solid fuel. When human culture entered the metal ages (Bronze and Iron) more high-quality solid fuels were required for smelting. This increased the charring and mining activities but, because the metallurgical and other applications of these or, more generally, of all solid fuels necessitates relatively large pieces for optimal burning and heat production, fines, which are the unavoidable by-product of solid fuel preparation, had to be removed and were discarded. The development of advanced blast furnaces in the 16th century, the invention of the steam engine by Watt in 1765, the introduction of modern iron- and steel-making technologies in the 19th century, and the beginning of large scale production of electricity for lighting and power in 1882 (New York, Edison) further increased the demand for solid fuels (particularly coal) and their use grew exponentially during the 19th and 20th centuries. Coal mining and processing became a major industry and, although the share of coal in world energy production is declining, has reached annual production rates of more than 2 milliard (US: billion) tons per year. Together with this growing capacity and the ever-increasing requirements on coal quality, which includes strict product size specifications, the amounts of rejected coal fines have also increased disproportionately. In the industrialized, densely populated countries of Europe, where coal mining had been going on for centuries and mining conditions were becoming more difficult, resulting in high solid fuel cost, technologies were developed to collect, clean, and convert coal fines into useful products for a
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number of industrial and domestic applications (Section 6.10.2). This increased the yield of the valuable raw material and reduced the need for scarce disposal sites. In other parts of the world, where good coal was still available abundantly and could be mined relatively cheaply and where plenty of disposal areas existed, the practice of removing the fines and dumping them into lagoons or landfills prevails to this day. For example, in the mid-1990s in the USA the US Department of Energy determined that “more than 5 US billion tons of coal fines may be locked in disposal sites across the country and that, each year, approximately 40–100 million tons of waste coal are added to these impoundments” [6.10.1]. It was stated that, in a very real sense, coal fines are an “unclaimed fuel”. Landfill areas can be reclaimed if old deposits are recovered and the need for new ones can be avoided if these and newly produced fine “waste coal” are turned into a viable fuel for power producers. While many proposals for the utilization of coal fines in the USA are based on turning fines into coal slurries for direct burning, size enlargement by agglomeration is also being considered. The oldest and most common application of size enlargement by agglomeration for solid fuels, which now also include biomass [B.25, B.48] (Section 8.2), utilizes different forms of pressure agglomeration. More modern uses of the technology try to convert abundantly available cheap sub-bituminous coal from, for example, the Powder River Basin in Wyoming, USA, into an improved product with increased heating value [6.10.2] (Section 6.10.2). Solid fuels other than carbon-based materials can also be agglomerated to obtain specific properties. For example, in recent developments spherical agglomerates are produced from enriched uranium powder as a fuel for specific nuclear reactors (Section 6.10.3). Tab. 6.10-1 lists some of those solid fuels that have been or are being processed most commonly with agglomeration technologies to improve their properties for various applications. Tab. 6.10-1 List of some materials that can be used as or converted to solid fuels and have been or are being processed most commonly with agglomeration technologies to improve their properties (see also Tab. 6.10-3) Municipal refuse Vegetable refuse Wood
Charcoal
Peat Coal
Mineral oil Uranium
Separated (paper, packing materials, organic wastes, plastic) Dried digested sludge (see Table 6.10-3) Bark Chips Saw dust Wood Nutshells Bones Bituminous (soft, brown coal) Subbituminous Anthracite (hard, black coal) From disposal sites Shale Sludge Enriched powder
6.10 Applications for Solid Fuels
Further Reading
For further reading the following books are recommended: B.1, B.2, B.3, B.7, B.13b, B.16, B.21, B.22, B.25, B.26, B.35, B.37, B.40, B.46, B.48, B.55, B.56, B.58, B.64, B.82, B.89, B.93, B.94, B.97, B.98 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold. 6.10.1
Tumble/Growth Technologies
The briquetting of fine coal was used extensively after the second part of the 19th century (Section 6.10.2). When, about 100 years later, tumble/growth agglomeration methods became more commonly known and were applied for many different materials, numerous studies were initiated and efforts began to use this seemingly simple technology for the size enlargement of fine coal [6.10.1.1]. The big success in the production of spherical iron ore pellets from fine concentrates (Sections 6.8.1 and 6.9.1) and the great improvements in the blast furnace process resulting from their use, triggered the interest in many quarters to also apply agglomeration pans or discs, drums, and cones (Chapter 5) for other applications. As reported during the 3rd International Symposium on Agglomeration [B.21, H36–H51], in 1977, based on pilot plant studies at the University of California, Berkeley, CA, and financed by the Electrical Power Research Institute (EPRI), Palo Alto, CA, a conceptual design of a coal pelletizing circuit was established by Kaiser Engineers in the USA (Fig. 6.10-1). Selection of the proposed equipment and the system layout was much influenced by experience from the large investments in the quickly growing application of iron ore pelletizing in North America and around the world (Section 6.8). To be economical, a plant using the circuit shown in Fig. 6.10-1 had to treat 100 t/h of filter cake consisting of < 0.6 mm coal fines with 10 % ash and 25 % moisture. The filter cake is an intermediate product from either a wet upgrading process or the recovery of high-quality coal fines from slurry disposal ponds. Corn starch was chosen as binder but the use of other additives, including liquids, was anticipated. It was assumed that 35 % moisture is necessary for the formation of good-quality green pellets, thus allowing the controlled addition of water in the agglomeration device. Six pelletizing drums, each 3 m diameter by 10 m long, or six discs of 8 m diameter were considered necessary for the proposed plant. Although the size distribution of agglomerates discharging from a pan is relatively narrow, installation of a roller screen (Section 6.8.1 and [B.97]) with closed loop recirculation of undersized material was provided in any case to improve operating flexibility. Assuming two shifts per day for 5 days per week, an annual capacity of 375 000 ts was projected. The dryer in which moisture is removed and the final, permanent binding mechanism develops, represents a major cost item, both in regard to investment and operation. Although it represents 25 % of the investment and 35 % of the operating costs, in the economical analysis it was argued that a dryer is always necessary to reduce the moisture content if a coal filter cake is to be processed. However, nevertheless, in the
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Fig. 6.10-1
Flow diagram of a proposed coal pelletizing circuit [B.21]
final evaluation the conversion cost was considered too high and no large scale commercial system was built then or yet. Compared with the prevailing coal briquetting (Section 6.10.2), which also necessitates an essentially dry feed (a dryer if wet coal is to be agglomerated), the wet granulation system also requires a binder (equal to about 25 % of the operating cost) and a guaranteed fineness of the feed < 0.6 mm (or, more accurately, a surface-equivalent diameter of < 0.2 mm, see Chapter 5). The requirement that, to become suitable for tumble/growth agglomeration, coal fines (as all other materials, too) must have a certain fineness, seems to make this technology most applicable for the recovery efforts that are centered around the slurry ponds that are found in all coal mining areas [6.10.1.2]. Vast quantities of undersized coal are impounded in these disposal sites. For example, several estimates indicate that in the eastern states of North America alone, more than 2 US billion t of recoverable coal fines are already contained and, each year, coal preparation plants in this area continue to send 30–50 million t of fines to slurry ponds. About 10 years after the proposed coal pelletizing circuit (shown in Fig. 6.10-1 and described above) was developed, additional efforts in the field were summarized [6.10.1.1]. It was concluded then that several technically feasible methods are available but commercial application will depend on the particular location, economic considerations (which may change with time), and the costs of the binder and pre- and post-treatments that will be necessary to render the coal suitable for processing and/or the market. Crushing coarser fractions to meet the size requirements for wet agglomeration will always make the material prohibitively expensive.
6.10 Applications for Solid Fuels
With the development of coal–water slurry fuels and their successful use in power plants, direct combustion of fines in circulating fluidized beds and the injection of dry coal fines into a number of thermal and metallurgical processes, the interest in tumble/growth agglomeration for the size enlargement of coal fines from any source has diminished to almost zero.
6.10.2
Pressure Agglomeration Technologies
Among the oldest known and mined coal deposits are the ones around Fushun in Northern China. Even today, in and around this location, an ancient coal agglomeration technique can be observed. After adding a binder, coal dust and fines, distributed by the mines to their workers for personal use, are manually formed into brick-shaped agglomerates in wooden frames. Binders are wet clay, flour (starch) pastes, and, more recently, oils and tars. Air-curing results in sufficient strength for handling and use as domestic fuel. In Europe, as recently as the beginning of the 20th century, similar “production facilities” were still in operation (Fig. 6.10-2) in some locations (Section 13.3, ref. 122). However, these agglomerates were of poor quality. They had low heating value and strength. In response to the large quantity of unmarketable coal fines produced as a result of the much increased need for solid fuel during the 19th century and the high cost of mining coal in Europe, efforts were undertaken to develop new products from coal fines with superior properties. Such coal products, featuring many or all the characteristics in Tab. 6.1 (Chapter 6), are made by pressure agglomeration and are commonly called “briquettes”. The first coal briquetting trials were carried out in the 1840s. In Belgium, France, and the UK bituminous coal fines were briquetted with “sticky” binders while in the USA and Germany peat, lignite, and other carbonaceous fines were dried and shaped
Fig. 6.10-2 Manual agglomeration of coal fines in Germany in about 1900 (Gewerkschaft Susanna, [B.1])
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without a binder to obtain a product with higher heating value. During that time, independent of each other, two totally different briquetting technologies were developed: binderless briquetting and briquetting with binders. The “double roll type briquetting machine” or roller press, the most commonly used equipment for the briquetting of coal with binders, was invented around the middle of the 19th century. The objective of its invention was the economic conversion of coal fines into a coarsely particulate product, suitable for burning in stoves, in furnaces, and on the firing grates of industrial ovens. In the beginning, the machine was also called ‘Belgian’ press because the first successfully operating one was constructed by the Belgian Louiseau and installed not in Europe but in a coal briquetting plant at Port Richmond, USA, in the late 1870s [B.1]. Mashek, whose company merged later with the Traylor Engineering Company in New York, built an ‘American’ roller press and Komarek and Greaves, Co. (today Hosokawa BEPEX, see Section 15.1) was formed in Chicago, among others, at the end of the 19th century. However, predictably, the largest application was in Europe where companies such as Fouquemberg, Humboldt, K€oppern, Sahut-Conreur, Sch€ uchtermann & Kremer, Wedag, Zimmermann & Hanrez, and others supplied great numbers of roller presses [B.13b, B.48]. As shown earlier (Section 6.2.2), around the middle of the 19th century the basic principles of punch-and-die presses were also developed and widely patented. Particularly the indexed table arrangement of McFerran (1874, Section 6.2.2, Fig. 6.2-38) seems to have been influenced by a similar but larger press for the briquetting of coal, the so-called Couffinhal press (invented 1859). Contrary to the success of the rotating table press in the pharmaceutical industry, for coal, this machine was in industrial use for only a short period of time (below) during the same period when the first roller presses were installed. Both the Couffinhal and the roller presses produced briquettes from mixtures of fine hard coal (e.g., Anthracite) and a binder (mostly coal tar pitch). This blend was preconditioned by steam to soften the binder and moisten the mix; therefore, relatively little force was required to form the briquettes and the process entailed more shaping than densification. However, in the search for additional, easily available, and cheap solid fuels, other carbonaceous materials also became of interest for size enlargement by briquetting, notably bituminous or “brown” coal (lignite) and peat. Both were known for a long time as natural resources; they were “mined” in open pits, dried to remove the often very high content of water (particularly in peat), and then burned. For use in modern domestic and industrial furnaces it was desirable to densify and shape these materials and improve the handling, feeding, and burning behavior. Carbonization of the organic matter making up peat and lignite has not proceeded far and very little natural densification has occurred, because their deposits are on or near the surface of the Earth, are water logged, and covered with no or little overburden. Therefore, they still contain large amounts of loose fibers and fossil plant matter, which render the product elastic. Economically, such materials can not be briquetted with either punch-and-die or roller presses, even if a binder is used. After the relatively quick densification in such machines, because of high residual elastic deformation, the products expand during pressure release and weaken or
6.10 Applications for Solid Fuels
fall apart completely. To overcome this problem, in 1857 Exter invented what later became known as the ram extrusion press (Prussian Patent no. 6015). In this machine, due to repeated densification in the extrusion channel (below), elastic dry peat or lignite is converted in steps into strong, well formed briquettes without using a binder. The first plant performing binderless briquetting of lignite with an “Exter Press” was installed in 1858 at Ammendorf in the eastern part of Germany (Section 13.3, ref. 157) [B.13b, B.48]. As will be shown later, within a short period of time, beginning during the second half of the 19th century, many coal briquetting plants were built, originally using all three types of machines. The technology reached its peak during the first third of the 20th century with a short but high peak after World War II before, for a number of reasons, coal briquetting technology went into a steep decline. Although several new uses, mostly involving Biomass (below), have been and are being proposed, at the beginning of the 21st century the briquetting of solid fuels is at a historical low. Binderless Briquetting of Peat and Lignin (Soft Coal) Fines As mentioned before, the briquetting machine used for this application is the ram extrusion press, also called the Exter press, or reciprocating ram press. The principle of this equipment is shown in Fig. 6.10-3. A typical feature is the horizontal extrusion channel, which first converges somewhat to allow build-up of sufficient pressure. During continuous operation, the reciprocating punch feeds against briquettes that were formed during previous strokes and compresses the material and all the briquettes in the channel until the wall friction and a potential back pressure at the mouth of the press are overcome shortly before the end of each stroke thus incrementally moving forward the entire column of briquetted material [B.48]. Fig. 6.10-4 depicts the sequence of events during a briquetting cycle [B.97]. The reciprocating motion is produced by, for example, an eccentric drive, symbolized by the circular representation on the left. The diagram on the right indicates the progress of force that is exerted on the material to be briquetted. The figure is self-explanatory. Only a few important operating stages will be discussed. At position 3 the force produced by the ram has reached a level that is sufficient to overcome the friction of all briquettes in the pressing channel and, if applicable, the back pressure caused by the column of briquettes in the cooling channel [B.48]. The 6.10.2.1
Fig. 6.10-3 Diagram of the ram extrusion, Exter, or reciprocating ram press [B.48]
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.10-4 Sequence of events during a briquetting cycle in a ram extrusion press [B.97]
entire line of briquettes moves forward, with the force remaining approximately constant, and a new briquette emerges from the “mouth” of the press (position 4). During the back stroke, the energy of the drive is stored in a flywheel and made available again to overcome the deceleration/acceleration at the return points and to help during compaction. At the beginning of the back stroke (when the eccentric drive has passed position 4) and if elastic materials, such as peat, lignite, or biomass are processed, at first the ram face does not separate from the newly created briquette because of considerable elastic recovery. At a typical rotational speed of the eccentric drive of 90 rpm, the duration of the compression phase is only about 0.04 s. This time is too short to achieve total conversion of elastic into plastic volume change; therefore, the elastic recovery during the back stroke is high and directed toward the feed position because the briquettes near the discharge end of the press are firmly wedged in the extrusion channel. Without the characteristic of ram presses that, during each compression stroke, all briquettes in the pressing (extrusion) channel are again loaded and compacted, whereby more and more permanent plastic deformation is achieved, successful briquetting of material with high elasticity would not be possible. Fig. 6.10-5 is a diagram of the increasing density and decreasing elastic recovery of a particular briquette during repeated pressing and forward movement in the extrusion channel. This performance is a significant difference from the densification process in, for example, roller presses [B.48]. It is important to note, however, that even after the first stroke, the surface produced by the ram face is so highly densified that, during the next stroke and for phases 2, 3 and 4 in Fig. 6.10-4, it acts as the bottom of a confined volume densification chamber until friction is overcome and the product column moves forward. During the entire production process the surfaces of adjacent briquettes do not develop significant bonding; therefore, upon discharge from the “press mouth” or the cooling channel, if applicable, the product will readily separate into single briquettes.
6.10 Applications for Solid Fuels
Fig. 6.10-5 Diagram of the decrease of elastic recovery and increase of density during consecutive press cycles in a ram extrusion press [B.97]
To accomplish this, the design of a ram press must provide a relatively long extrusion channel. However, there are physical limits to this parameter because friction and drive power and overall stressing of the equipment increase with channel length. In a technically feasible channel, reaching the conditions of Fig. 6.10-5 may not be possible. Then briquettes can retain a certain elastic deformation which, if suddenly released, could be large enough to damage or even destroy product integrity. Therefore, in most applications, a gradual release is provided by slowly increasing the cross section of the channel prior to product discharge. Fig. 6.10-6 is a cross section through a modern ram extrusion press. The upper channel wall is adjustable such that different release angles can be obtained. In addition, a flexible support system at this point serves as a safety device to avoid overloading due to tramp material in the feed or “overcompaction”. As compared with a closed mold (punch-and-die), in which a predetermined pressure is reached with no difficulty, in extrusion presses the situation is complicated (Fig. 6.10-5). The peak pressures developing at each stroke depend not only on the force exerted by the ram but also on the resistance to the forward movement of the briquettes in the extrusion channel. The latter is influenced by many things: the shape and length of the channel, the changes in cross section in relation to length, the smoothness of the channel walls, the nature of the material to be processed, including parameters such as temperature, structure, plasticity, and, if applicable, the type and length of a curing (cooling) channel.
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.10-6 Cross section through a modern ram extrusion press (courtesy Zemag, Zeitz, Germany)
The rate of pressure increase is also important; it depends on the stroke frequency and length and the rather complicated relationship between movement of the ram and magnitude of the resisting frictional force between extrudate and die and the force caused by the column of already compressed product being pushed forward. These forces change with both the state of compaction and the rate of movement. Therefore, to control the briquetting pressure, channel cross section, configuration and length and back pressure, resulting from pushing-ahead long lines of finished briquettes in the cooling channels, are varied [6.10.2.1]. Because of its well-known, large, and highly exploited lignite (called “brown coal”) deposits, Germany and countries with similar mines surrounding it have been the leaders in binderless briquetting of this fuel with ram extrusion presses. Fig. 6.10-7 is the flow diagram of one of the latest lignite (“brown coal”) drying and briquetting plants in Germany, which was commissioned in 1956 [6.10.2.2] but has long since been shut down and is dismantled. Run-of-mine coal, containing about 60 % moisture, was precrushed in the mine (open pit) to < 150 mm. In the plant it was first dried in four indirectly heated rotary tubular dryers (6), crushed and screened to < 3 mm in a chain conveyor (10), cooled in another chain conveyor (13), and then briquetted without binder in seven three-channel ram extrusion presses (16, 17). Each press feeds briquettes into three cooling channels (a total of twenty one), which also provide back-pressure for coal densification in the press. The capacity, initially 1300 t/day (as depicted in Fig. 6.10-7) of “union-type” briquettes, was later doubled to 2600 t/day (about 0.8 million t/y). Fig. 6.10-8 shows a diagram of an indirectly heated rotary tubular dryer (drum fitted with heating tubes inside, (6) in Fig. 6.10-7). Fig. 6.10-9 and 6.10-10 are two different views of the extrusion presses and Fig. 6.10-11 shows briquettes being loaded into a rail car after cooling. Fig. 6.10-12 depicts different briquette shapes that can be obtained with ram extrusion presses. The briquette in the upper left features the most common, so called “union” shape.
6.10 Applications for Solid Fuels
Fig. 6.10-7 Flow diagram of one of the last lignite (brown coal) briquetting plants built in Germany [6.10.2.2]
Fig. 6.10-8 Diagram of a rotating tube dryer for the reduction of moisture content of lignite or “brown coal” (courtesy Zemag, Zeitz, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.10-9 Partial view of the briquetting bay showing the drive side of the ram extrusion presses [6.10.2.2]
Fig. 6.10-10 Partial view of the briquetting building showing the briquetter heads and the beginning of the cooling channels [6.10.2.2]
6.10 Applications for Solid Fuels Fig. 6.10-11 “Union-type” briquettes being loaded into a rail car [6.10.2.2]
Fig. 6.10-12 Different briquette shapes that can be obtained with ram extrusion presses
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6.10.2.2 Heydays and Downfall of Binderless Briquetting of Bituminous Coal Fines
Fig. 6.10-13 (Section 13.3, ref. 126 and [6.10.2.3]) indicates that in Germany, historically the largest manufacturer of binderless lignite briquettes using Exter ram extrusion presses, production began to take-off in 1890 and grew until it reached about 60 million t per year shortly before the end of World War II. Particularly in East Germany (GDR) it quickly regained a dominant role in the post-war energy picture of this country. Between 1955 and 1960 production of “brown coal” briquettes in Germany (FRG + GDR) peaked at almost 80 million t per year. Although representing the larger and more populous part, the Federal Republic (FRG) accounted for less than a quarter of that total, indicating the growing environmental concerns in the west, which curbed the use of this “dirty” energy source. As shown in Fig. 6.10-13, the decline after the post-World War II peak until the early 1970s was caused by the phasing-out of “brown coal” briquette usage in the west, which was driven by clean-air legislation. In the mid-eighties some reduction of lignite briquette production and use also began to take hold in the GDR as a result of agreements between the two Germanys and after reunification many plants were closed immediately causing a precipitous drop in production and a very bleak forecast for the future of this technology. (It should be noted that “brown coal” is still a major energy source in Germany where it is used in large, modern power plants with strict environmental control for the production of electricity (Section 8.2). Similar developments are planned for other parts of the world where “brown coal” is abundant.) 6.10.2.3 Briquetting of Hard Coal Fines with Binders (Punch-and-Die Presses)
With the binderless briquetting technique, only relatively soft coals can be briquetted. The saturation or bed moisture of the coal is a good measure of whether a particular coal is soft enough. The transition is at about 40–45 % free moisture content. Coals with higher bed moisture are normally briquettable without binder, while at lower saturation this is only possible in exceptional cases and/or if special techniques are applied. Lower moisture is due to higher coal density, which results in significantly harder particles. Such coals must be briquetted with a binder. After adding a thermoplastic
Fig. 6.10-13 Development of “brown coal” (bituminous coal) briquetting in Germany [6.10.2.3]
6.10 Applications for Solid Fuels
binder material, such as coal tar pitch, for a short period beginning at approximately the middle of the 19th century, large, mostly brick-shaped briquettes were produced in punch-and-die presses. Among different designs the so called “Couffinhal” press (Fig. 6.10-14) enjoyed the widest application. For example, at the briquetting plant
Fig. 6.10-14 a) Model and photograph of a Couffinhal press. b) Vintage general arrangement drawing of a Couffinhal press (courtesy K€ oppern, Hattingen/ Ruhr, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.10-15 Some typical coal briquettes (bricks) produced with punch-and-die presses
“Wiesche” in the German Ruhr District, where originally a Belgian “Mazeline” press had been installed producing twenty-four 9.5 kg bricks per minute (about 13 t/h), the first of several Couffinhal presses was introduced in 1887 for the production of 1, 3 and 7 kg briquettes that were used in the firing holes of locomotives [6.10.2.4]. Although the Couffinhal press was semi-automatic, employing an indexed table with several molds and separate feed, pressing and discharge stations, the operation was only economical if large briquettes, each weighing several kilograms (up to as much as 10 kg) were made, requiring handling, stacking, and feeding by hand, mostly for use in locomotives and older (smaller) stationary steam engines. Fig. 6.10-15 shows some typical coal briquettes (bricks) produced with punch-and-die presses (Section 13.3, Ref. 126). 6.10.2.4 Briquetting of Hard Coal Fines with Binders (Roller Presses)
The quickly developing need for large amounts of briquetted carbonaceous solid fuels, allowing bulk transportation, storage and handling, necessitated the production of much smaller, egg or pillow shaped briquettes (Fig. 6.10-16). An economical method for this task uses “roller presses” (Section 13.3, refs 111 and 134) and [B.13b, B.48, B.97]. Fig. 6.10-17 represents schematically the operating principle of roller briquetting presses. These machines achieve compaction of particulate feed by squeezing the material between two synchronously counter-rotating drums. Matching pockets repre-
Fig. 6.10-16 High-quality pillowand egg-shaped coal briquettes
6.10 Applications for Solid Fuels Fig. 6.10-17 The operating principle of roller briquetting presses
senting briquette halves are cut into the working surface of the drums (rollers) and form the briquettes. During any briquetting or compacting process (Chapter 5, Fig. 5.9), densification begins by a rearrangement of feed particles resulting in the filling of larger voids in the bulk mass. The pressure rise during this phase is relatively small. This is followed by a steep increase in pressure during which further densification results in plastic deformation and/or brittle destruction of the particulate feed particles [B.13b, B.48, B.97]. Since a considerable reduction in volume and pore space is associated with the densification process, gas that previously occupied the pores between and within the particles to be compacted must be totally removed during densification to avoid compressed air pockets. This process of dissipating air is time consuming, particularly if the permeability of the mass is low (i.e., if fine particles cause small pore diameters), and, therefore, limits the speed of compaction. The single densification step in roller presses is completed in a very short period of time, typically a fraction of a second when the material passes the narrowest part of the nip between the rollers. If air is not totally expelled, pockets of compressed air, together with remaining elastic deformation will result in expansion when the pressure is suddenly released [B.13b, B.48, B.97]. In roller presses where this is the case and can not be avoided, damage to or destruction of the structure of the densified material may occur. In contrast, the ram extrusion press, discussed above, provides “dwell times” under pressure: the column of previously made briquettes is held in the press channel and then advances during the next press cycle. The briquettes (or compacts) are repeatedly redensified; thus, in addition to the conversion of temporary elastic densification into permanent plastic deformation, both effects (repressurization and redensification) assist also in the complete removal of air. Because the maximal pressure attainable with the extrusion principle is limited and sometimes not sufficient for the successful briquetting of harder, but still elastic lignites and to overcome the above mentioned problems of roller presses with such materials, the ring roller press was developed in the 1930s (Fig. 6.10-18). Significantly higher briquetting pressures could be obtained with this equipment and, due to the large diameters of the ring and the internal press roller and the long and slender nip, the speed of densification is considerably lower, thus allowing better deaeration and achieving less residual elastic energy (reduced spring-back). Nevertheless, technical and economic reasons limited its application. Only a few machines were built and since the 1950s this design principle is no longer in use.
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Fig. 6.10-18
The “ring roller press” [6.7.3.1]
The shaping of briquettes in roller presses is performed during a rolling motion of the two synchronously rotating press cylinders. As depicted in Fig. 6.9-11 (Section 6.9.2) the two halves of the pockets are never completely closed. Particularly if high pressure is necessary to obtain good densification and strength, a number of shape inadequacies, characterized by such descriptive terms as “clam-shelling”, “duck-billing”, “oyster-mouthing” (Chapter 14), can be observed on the final briquettes (Sections 6.3.2 and 6.9.2). Since, during their peak acceptance for domestic (heating and cooking) applications, coal briquettes had become consumer products, which must not only perform well but also have a pleasing appearance, such defects would not have been accepted. This was another reason for applying a binder. The conditioned (below) coal and binder mix became so plastic and sticky that, after some densification and under moderate pressure, well-shaped briquettes were formed, which hardened during cooling. In fact, coal briquetting with binders in roller presses was more a shaping than a compaction process, which was so easy that it became feasible to in-
6.10 Applications for Solid Fuels
clude distinguishing marks in the pocket design to determine the source of the briquetted product for marketing reasons (Fig. 6.10-16). Fig. 6.10-19 is the flow diagram of an anthracite (hard) coal mixing, drying, conditioning, briquetting, and cooling plant demonstrating the extent and complexity of the more modern (about 1950) hard-coal briquetting systems [6.10.2.2]. To guarantee specific quality requirements, which included strength (handling properties), heating value, and burning characteristics, coals from different sources were often supplied, crushed, mixed, and dried in the feed preparation plant (top and upper right part of Fig. 6.10-19) preceding briquetting. In the specific plant shown here, the briquetting section encompasses four lines, each with three roller presses and all the other associated equipment. Three trains are sufficient to produce the 200 t/h of briquettes for which the plant was designed., Thus, one line is always available as a stand-by and for scheduled maintenance, the latter mostly for the replacement of wear parts. The starting materials are different coals with particle sizes in the range 0–80 mm. After mixing, the coal is dried to < 5 % moisture content, crushed to < 3 mm particle size, and briquetted using, in this case, pitch, clay, and sulfite liquor as binders. Fig. 6.10-20 is a major elevation of the plant showing from left to right the raw coal bins with coarse crushers above and metering below, the drying section, featuring indirectly heated drum dryers, the dry coal storage, fine crushing, and binder mixing section, the
Fig. 6.10-19 Flow diagram of a modern anthracite (hard) coal mixing, drying, conditioning, briquetting, and cooling plant [6.10.2.2]
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Fig. 6.10-20 Major elevation of the plant depicted in Fig. 6.10-19 [6.10.2.2]
coal conditioning and briquetting lines, and, finally, the product drying and cooling conveyors on which excessive moisture evaporates and is removed to the atmosphere while the binder system hardens. One of the most important process steps in a conventional coal briquetting system is the conditioning of the coal/binder mixture prior to feeding it into the briquetting machine. This is accomplished in a vertical pug mill (Fig. 6.10-21) where the blend of coal and binder(s) is converted into a plastic mass by heating with saturated or superheated steam. In addition, the steam acts as a “wetting agent”, which assists in the uniform coating of all coal particles with binder.
Fig. 6.10-21 Sketch (partially cut, interior view right of the center line) of a vertical pug mill for steam conditioning mixtures of fine coal before briquetting [Section 13.3, ref. 111]
6.10 Applications for Solid Fuels
The residence time in or the throughput of a vertical pug mill are determined by the moisture content of the material entering the apparatus and the temperature to which the particular coal blend must be heated to obtain optimum plasticity. If these parameters and the desired production rate of the plant are known, the pug mill can be designed to match the application. Through a system of nozzles, distributed around and along the entire shell, steam at 250–300 8C and a pressure of 50–250 kPa is injected into the mass. Since the transfer of thermal energy to solids during the condensation of steam is highly efficient, heating of the coal mixture occurs quickly and uniformly. Although the necessary quantity of steam per metric ton of material varies, it is normally in the range 30–60 t/h. Because different amounts of steam are required in various zones of the pug mill, each nozzle is provided with a regulating valve. Excess steam collects in a dome and is vented to the atmosphere. Stirring arms (mixing elements) on a central, vertical shaft agitate the material and avoid build-up on the walls. The design, number, and distribution of the mixing elements depends on the particular material to be processed. Each pug mill is normally driven individually. At the bottom, a rotating discharge device removes the plasticized coal blend through remotely adjustable gates. Signals from level indicators are used to maintain the correct level by adjusting the feed accordingly. To control wear, exchangeable sleeves are mounted to the inner walls of the shell. External insulation and jackets heated with steam or hot water may be used to minimize heat losses. The principle, the design, and special considerations of roller briquetting presses (Fig. 6.10-17) have been covered in much detail in three earlier books by the author [B.13b, B.48, B.97] and in many of his papers (Section 13.3). Machines for the briquetting of hard coal fines with binder(s) are relatively simple because the process entails more a moderate densification and forming of the plasticized mass into defined shapes than a high-pressure compaction of solid particles. Although featuring modern design, recent equipment for this application is still very similar to the presses that were built shortly after the invention of this technology and throughout its common use. Fig. 6.10-22 depicts the general arrangement drawing of a press built in 1913 by one of the German manufacturers. The machine is equipped with a rotating distribution device to accomplish the uniform feeding of two sets of molded rings, which are synchronized by wide, rugged spur gears that are mounted in the center of the shafts, thus separating each briquetting roller into two halves of identical dimensions (1000 mm diameter 150 mm wide). Such a press, operating at a rotational speed of the rollers of 6 rpm, was capable of producing 6 t/h of relatively small (15–50 g, Fig. 6.10-16) hard coal briquettes that were suitable for home heating purposes. The required specific force, that is the applied pressing force divided by the active roller width, described in kN/cm, is low for hard coal fines briquetting with binder(s), typically 10–20 kN/cm. Therefore, to lower the mass of the rollers, originally and even in more recent designs (Fig. 6.10-23), they were made of hollow cylinders equipped with a ring that carries the pockets and is exchangeable as a wear part. Typically, the vertical pug mill conditioner (Fig. 6.10-21) was mounted close to the roller press and the plasticized coal/binder blend was metered into the rotating feed
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.10-22 General arrangement drawing of a vintage (1913) roller press for the briquetting of hard coal with binder(s) (courtesy K€ oppern, Hattingen/Ruhr, Germany)
distributor by a horizontal screw from which excess steam was vented. Well-formed briquettes discharged from the rollers onto a perforated chute where fines were separated (Fig. 6.10-24a). Fig. 6.10-24b is a photograph of the briquetting floor of a relatively recent plant (installed in 1952, now dismantled) that had, with multiple presses, an annual capacity of 1.5 million ts. The equipment in the upper center is the vertical pug mill feeding the roller press in the left foreground. As compared with the machine depicted in Fig. 6.10-24a, the major difference in the design of this later installation is the use of electrical motors (instead of transmission belts), gear boxes, and the extensive use of vent lines to collect and remove vapors. Feed distribution is still achieved with a rotating distribution device to two pairs of briquetting rings, coupled and synchronized with open gears located between the rings, the press frame is open, and adjustment of the tongues (gates) for feed control [B.13b, B.48] is manual. 6.10.2.5 Heydays and Downfall of Hard Coal Briquetting with Binders
Shortly after introduction of the roller press technology in the late 19th and early 20th centuries, hard coal briquetting with binder(s) applying roller presses found extensive
6.10 Applications for Solid Fuels
Fig. 6.10-23 a) Elevation of an early roll type briquetting press designed by Sch€ uchtermann and Kremer [B.13b] and b) detail of the rollers of a relatively modern machine (about 1980, courtesy K€ oppern, Hattingen/ Ruhr, Germany), both featuring hollow roller cores
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.10-24 Vertical pug mill conditioner and roller press arrangements: a) general arrangement drawing dating from 1918; b) Photograph of the
briquetting floor at the Rosenblumendelle mine in Germany (courtesy K€ oppern, Hattingen/Ruhr, Germany)
6.10 Applications for Solid Fuels Tab. 6.10-2 Development of hard coal briquetting with binder(s) using roller presses in some of the coal mining areas of Germany (Ruhr, Aachen, Lower Saxonia) Year
Number of plants
Number of presses
Production [Mt/y]
Average/press
1890 1900 1910 1921 1924 1956 1976
18 32 53 48 55 25 4
19 100 243 205 201 90 19
0.361 1.612 3.847 4.547 4.984 7.196 1.347
0.019 16.12 15.83 22.18 24.80 79.96 70.89
use in the traditional European industrial zones, located in the UK and the central part of Europe, particularly Belgium, Northern France, and Germany, and in other industrialized countries, such as USA and Japan. Before the worldwide depression in 1929, the European production of bituminous coal briquettes with binders reached a peak of tens of millions of t per year and then declined sharply. In the 1930s and during World War II a recovery occurred in Europe without reaching the pre-depression peak. The end of World War II caused another downturn followed by a recovery period. In 1962/ 63 the Cuban missile crisis resulted in a last peak of bituminous coal briquetting in the western world. At that time, Japan and western Europe together produced about 25 million ts per year while the capacity of the USA, whose production had peaked in 1947/48, was already far below 1 million ts/year (Section 13.3, ref. 111, 122, 126, 134, 142, 157). During this history of roller press briquetting, in addition to becoming more modern in design, the machines also became larger. This is shown in Tab. 6.10-2 which lists for several years the number of plants, presses, and their capacity in one of Europe’s largest coal mining districts. Today, only a few plants remain that produce “smokeless” briquettes from carefully selected coals and additives for special applications such as burning in home fireplaces. These briquettes are consumer products requiring a “pleasing” appearance and are sold shrink wrapped in small quantities in specialty stores. Most of the large industrial plants were dismantled and scrapped. 6.10.2.6 Present Status and New Developments in Coal Briquetting
In the “classic” coal mining areas of the western industrialized countries briquetting of coal has decreased to marginal levels. The reasons for this are three-fold. *
*
Briquettes were originally produced as cheap fuel for domestic use and the rapidly growing power and transportation industries. Between about 1890 and 1920 most of the briquettes were consumed by the railroads and stoker fired power stations. With increasing demand for briquetted fuel, briquetting plants became larger, more sophisticated, and more expensive to operate. These developments eventually resulted in the replacement of briquettes by newly emerging cheaper fuels such as gas and oil.
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6 Industrial Applications of Size Enlargement by Agglomeration *
Raw materials for “cheap” fuel briquettes are bituminous coal fines with coal tar pitch, bitumen, or asphalt as binders or lignite (brown coal). These briquettes produce considerable smoke and air pollution. “Clean air legislation”, now enacted in most industrial countries, bans such fuels.
In spite of or, more likely, because of the decline in coal briquetting, new developments were achieved during the past decades. They were either directed towards a more economical production of briquettes, rendering this fuel more competitive, or the manufacturing of “compliance fuel”, briquettes or compacts that can be burnt without violating anti-pollution laws. More recent developments are using so called “opportunity fuels”, waste materials with calorific value that need to be disposed of without landfilling. To achieve more economical production, the remaining two types of equipment for the briquetting of solid fuels, the ram extrusion press and the roller press, have been redesigned extensively. In ram extrusion, the press drives were modernized for more power input and the shape of the extrusion channels was optimized to allow the application of higher pressures. Since the capacity of these machines, even if producing large briquettes, is limited to a maximum of somewhere around 20 t/h, less with smaller product sizes, higher capacities are achieved by using several channels per press (Fig. 6.10-25) and ultimately multiple lines (Figs. 6.10-7, 6.10-9 and 6.10-10). Roller presses are now available for throughputs of as high as 100 t/h per machine (Fig. 6.10-26, and Fig. 6.10-33 below) [B.97]. However, in those areas where many of the future solid-fuel related applications are envisaged, that is low sulfur sub-bituminous coals, biomass, and other opportunity fuels, the roller press has limited applicability. Unless extensive and costly preparation or conditioning steps are used, which include the addition of new, effective binders, the sometimes very elastic organic materials cannot be processed successfully in roller presses. The single, quick densification, which is followed by immediate, complete pressure release does not allow the manufacturing of permanently bonded briquettes or compacts from such solids in one pass [B.13b]. Another potential problem of roller presses is related to briquette size which, particularly if relatively small briquettes are desired (20–30 cm3), influences selection of equipment size and roller width that can still be fed uniformly and, therefore may limit machine capacity. These considerations must be taken into account when developing new solid fuel related applications for roller presses. The application of ram extrusion presses overcomes the problems of roller presses. Machines with multiple (up to four; for example Fig. 6.10-25, bottom) extrusion channels can produce high capacities per unit and, if this is desired or required, comparatively small amounts of binders will produce, for example, water repellant compacts. Since it is preferable to produce a crushed product (below), initial briquette shape is of no concern and can be optimized for capacity. 6.10.2.7 New Applications of Ram Extrusion Presses: Elastic Materials, Coal Logs
Although the production of the “traditional” brown coal briquettes has come to very low levels, the technology, based on the ram extrusion principle, is available, mature, and has been further developed until recently. While many classic manufacturers of
6.10 Applications for Solid Fuels
Fig. 6.10-25 Photographs of a triple (top) and a quadruple (bottom) channel extrusion press (courtesy Krupp F€ ordertechnik GmbH, Essen, Germany)
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.10-26 a) Elevations and plan view of a modern large capacity roller press for the production of about 100 t/h of coal-based compliance fuel with a binder [Section 13.3, ref. 134] (courtesy
K€ oppern, Hattingen/Ruhr, Germany); b) photograph of one of the latest large roller presses that was designed for the briquetting of coal (courtesy Sahut-Conreur, Raismes, France)
6.10 Applications for Solid Fuels
ram presses in western Europe folded or gave-up this section of their business, some found new niche markets. The particular characteristic of this press type, the possibility to successfully and permanently densify and shape materials that are typically organic in nature and feature high elasticity (Fig. 6.10-5), can also be used for the briquetting of materials other than peat and lignite that have similar properties [6.10.2.5, 6.10.2.6]. Tab. 6.10-3 lists some of the materials for which ram extrusion presses are already being used. New non-coal applications also include the pressing of very fine powders and of materials that either inherently contain binders or to which binders are added. For example, typical binders in or for wood-based products are lignins, either as a natural ingredient or a waste product (lignosulfonates), and for Bagasse or spent sugar beet slices the binder is unrefined sugar or the by-product molasses. For some of the new applications, less heavy duty machines have been developed [B.25, B.46, B.97]. Many of these use screws for the development of force and, therefore, accomplish continuous extrusion. The rope emerging from the die is cut into pieces, often by a rotating knife. In a much smaller scale, the conditions in the die holes during medium pressure agglomeration or pelleting (Chapter 5, Fig. 5-10b1–b6) realize the same densification process as developed in the (long) channel of ram or screw extrusion presses. Therefore, medium pressure agglomerators can be also used for the production of “fuel pellets” from, for example, shredded paper, saw dust, plastic foil, and similar waste materials [B.97]. Fig. 6.10-27 shows two examples of lose raw materials and the resulting pelleted product. Another potential application of Exter ram presses is in the field of transportation. It has been proposed to produce cylindrical, water repellent “coal logs”, which are transported over long distances in so called “freight pipelines” [6.10.2.8]. The coal log pipeline (CLP) technology is being developed at the Capsule Pipeline Research Center (CPRC) of the University of Missouri–Columbia. One of the key requirements for the economic application of this technology is the production in a very short time of large numbers of good quality cylindrical logs with a diameter of more than 125 mm (5”) and an aspect ratio (length over diameter) of about 1.8. Although most of the laboratory and semi-industrial pilot work is still being carried out with punch and die presses, ram extrusion has also been tested with good preliminary results [6.10.2.9, 6.10.2.10]. Ultimately, the application of this technology for the com-
Tab. 6.10-3 Examples of some new materials, including biomass and opportunity fuels, for which Ram Presses have been successfully applied [6.10.2.7] Bagasse Bark Biomass Cannery wastes Grape wkins Hay
Mineral powders (various, fine) Municipal waste (processed) Paper Polymers Rubber (from shredded tires) Saw dust
Scrap leather Seed husks Straw Sugar beet slices Vegetable wastes Wood chips
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.10-27 Two waste materials and the resulting products after medium pressure agglomeration in a flat die pelleting machine (courtesy Amandus Kahl, Reinbek/Hamburg, Germany)
mercial production of logs from coal and other materials, will require the design, manufacturing, and testing of new ram extrusion presses. 6.10.2.8 New Applications of Roller Presses: Charcoal, Smokeless Fuel, Formed Coke,
Coal-Based Solid Compliance Fuels A relatively new but already mature and presently overbuilt (excess capacity) application of solid fuel agglomeration is charcoal briquetting. While the production of charcoal from wood belongs to the oldest commercially and industrially used technologies of mankind, application of the material as a solid fuel was restricted to chunky pieces which, during their use in the furnace, developed the necessary openness (voids) of the particle bed to achieve an effective flow of air and, as a result, optimal burning. Fines were removed prior to charcoal use as a fuel and used elsewhere (ground finely as a black coloring agent for many applications, including paints and cosmetics, and later in gun powder and as additive in animal feeds). Charcoal briquetting evolved when backyard barbecuing became popular. The additional need for good quality chunks of charcoal which, in the meantime, had become a valuable material for many industrial applications (e.g., filtering, catalysts, chemical synthesis) could not be satisfied. Also, because the material became a consumer product, it was expected by the buyer that pieces looked perfect and contained virtually no fines in the bag (Fig. 6.10-28). Briquettes from charcoal fines resulted in a product with better ignition, particularly if a liquid lighter is employed, and superior heating behavior. Because charcoal briquettes are used in food preparation, government authorities soon regulated their composition. Therefore, the process, which requires a food grade binder (pregelatinized starch) to yield a well-formed, good quality consumer product, became standardized as shown in Fig. 6.10-29.
6.10 Applications for Solid Fuels
Fig. 6.10-28
Typical barbecue charcoal briquettes
Most plants use hardwood charcoal (F), which is crushed to < 3 mm (1), stored in a small surge bin (2), and then mixed with the binder. Often premixing occurs in a simple paddle mixer (3) and conditioning in a double shaft vertical pug mill ((4) called vertical fluxer), similar to the one shown in Fig. 6.10-21 but without steam injection. A typical mix formulation consists of 76 % charcoal, 4 % pregelatinized starch, and 20 %
Fig. 6.10-29 “Standard” barbecue charcoal briquetting system. Explanation see text
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6 Industrial Applications of Size Enlargement by Agglomeration
water. If soft wood charcoal or a mixture of different charcoals is used, the amount of starch may be somewhat greater and the water content can be as high as 45 %. The well-conditioned mixture is briquetted in a low-pressure double roll press (6) with a paddle feeder ((5) to stimulate uniform flow into the nip between the rollers). The green (moist) briquettes are uniformly deposited by a special conveyor (7) onto the mesh belt of a tunnel dryer (8) and hardened at about 120 8C for 2.5–3 h. The finished briquettes are screened (9) and fines are returned (10) to the paddle mixer for reprocessing. After natural cooling the product (P) is packed or shipped in bulk. To overcome the main problem of conventional coal briquettes, excessive pollution, beginning in about the 1960s, research started in Europe, particularly the UK and in Japan to develop “smokeless” coal briquettes. As a result of fundamental studies into and evaluations of the briquetting of coal at the British National Coal Board’s Coal Research Establishment (NCB/CRE) a book on roll pressing was published in 1976 [B.13b]. Smokeless fuel was expected to become again a consumer product with all its new requirements, especially excellent physical quality and pleasing appearance. The latter motivated one producer in the UK to dip the briquettes into a special gold bronze die during curing to eliminate the “dirty black coal look”. Although a number of processes were developed, which were based on the partial removal of volatile matter and/or either cold cure binders or hot briquetting, the meanwhile abundant availability of cheap, clean energy did not let this technology grow as was originally anticipated. The few plants built, mostly in the UK and Japan, were small and used low-capacity roller presses for briquetting. Most of these enterprises became quickly unprofitable and closed. In the 1950s and 1960s, a number of companies and organizations worked on socalled formed coke processes. The interest on such technologies developed in the wake of an unprecedented growth in iron and steel production and forecasts predicting a continuation of this trend for quite some time. At the same time, fears came up that coking coal reserves might soon be insufficient to satisfy the demand and a growing awareness of the need for a better protection of the environment became a topic of much concern. Therefore, the goals of all formed coke developments were as summarized in Tab. 6.10-4 [Section 13.3, ref. 97]. While some of the processes combined known technologies, for example hard coal briquetting with pitch binder and conventional coke making in indirectly heated slot type ovens, others used a number of novel partial steps which, for the complex overall
Tab. 6.10-4 * *
* *
Goals of formed coke developments
A coke product that would perform as well as or better than conventional coke A process that permits use of a wide range of either coking or non-coking coals and produces a consistently uniform coke A process that meets present and future environmental laws A process that produces coke at essentially the same cost as that from conventional coke making while meeting the above requirements.
6.10 Applications for Solid Fuels
system, presented the inherent difficulty of arriving at a continuous operation. Further problems in process development resulted from the large number of goals, particularly the intended increase in the range of applicable coals. New technologies included the process steps volatilizing or semi-coking by low-temperature carbonizing, adapted mixing and agglomeration methods and final carbonizing and/or post-treatment. While during development most of the researchers adopted roller briquetting for agglomeration, balling in drums or discs (Section 6.10.1) and extrusion were also tried. For technical and economic reasons, in those processes reaching commercial acceptability, briquetting is ultimately favored. Fig. 6.10-30 is a generally valid schematic into which all formed coke technologies can be fitted. A fundamental distinction was made into those using hot or cold agglomeration. Parallel to the efforts to develop a process in which formed coke is produced as a new commodity, the partial briquetting of coke oven charges was introduced (Fig. 6.10-31). Only part of the coke oven feed is densified and shaped by briquetting, subsequently mixed with loose coal (Fig. 6.10-32), and charged into conventional slot type coke ovens. This increases the bulk density of the coal feed and, thereby, improves coke quality while, at the same time, allowing the utilization of lower grade or non-coking coal with a high percentage of volatile matter and unfavorable swelling characteristics.
Fig. 6.10-30 Block diagram depicting the different processes for the manufacturing of formed coke [Section 13.3, ref. 97]
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.10-31 Block diagram of the partial briquetting techniques for coke making [Section 13.3, ref. 97]
As in the case of formed coke, the economical and environmentally safe production of briquettes requires large agglomeration equipment, with roller presses as the machines of choice. Fig. 6.10-33 is a collection of photographs showing details of machines, which are used in several SUMICOAL partial coal briquetting plants in Japan and South Africa. Each press is capable of producing between 90 and 120 t/h of coal briquettes. Similar to the machines built at the beginning of the century (Figs. 6.10-22, 6.10-23a, and 6.10-24a) each roller is equipped with two pocketed rings (1400 mm diameter, 900 mm wide). Each set of rings is fed by a gravity feed chute with overflow and tongue control [B.48]. Although a binder system had been added, the mixture to be briquetted is still free flowing; therefore, this simple type of feeder and feed control could be selected. In the early 1970s, when most processes had developed into large pilot plant or early production stages, the initial enthusiasm of the project sponsors disappeared. This was due to first indications of the approach of what since has been termed the 1970s steel crisis and the beginning of a fundamental technological change in the steel industry involving a move away from the blast furnace, the major consumer of (metallurgical) coke. Some plants experienced premature closure, others never reached full production capacity, and plans for big installations were canceled. Nevertheless, interest in the development of alternative coking processes continues, particularly in regard to economy (i.e., use of lower quality coals) and better environmental control. Fig. 6.10-34 shows the flow diagram of the 6 t/h pilot plant of one of the latest attempts [6.10.2.11]. The process is designed for a feed mix including fines that contains 50 % coking coal and 50 % lower-cost non-coking coal (as compared with 25 % in conventional coke making). The fines are hot briquetted before feeding the coke
6.10 Applications for Solid Fuels
Fig. 6.10-32 Mixture of briquettes and loose coal on its way to the coke ovens in one of the plants for partial briquetting (courtesy Iscor, New Castle Works, South Africa)
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.10-33 a) Large briquetting machine for the briquetting of coal (courtesy K€ oppern, Hattingen/Ruhr, Germany); b) partially assembled roller press showing the two sets of rings (courtesy K€ oppern, Hattingen/Ruhr, Germany); c) roller press installed in a plant for the partial briquetting of coke oven charges (courtesy Iscor, New Castle Works, South Africa)
6.10 Applications for Solid Fuels
Fig. 6.10-33c
oven together with preheated coarse coal. Called SCOPE 21 for “Super Coke Oven for Productivity and Environmental enhancement toward the 21st century”, the project is a joint development of the Center for Coal Utilization, Japan (CCUJ, Tokyo) and 12 Japanese steel and chemical companies. Coking is accomplished at lower temperatures (750 8C vs. 1000 8C) and in a shorter time (10 h against 30 h). The entire system is enclosed and quenching is done by nitrogen. Both reduce dust generation and the latter provides better heat transfer for energy recovery.
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6 Industrial Applications of Size Enlargement by Agglomeration
Fig. 6.10-34 Flow diagram of the 6 t/h pilot plant of the proposed SCOPE 21 coal coking process [6.10.2.11]
The low-sulfur sub-bituminous coals from, for example, the Wyoming Powder River Basin (PRB), which are natural compliance fuels, typically contain high amounts of moisture (30–50 %, sometimes more) and, therefore, must be dried prior to briquetting, also to improve their heating value [6.10.2.12]. During drying these coals disintegrate into a finely divided particulate mass and then exhibit a pronounced tendency of auto-ignition. Therefore, the dry coal must be processed with a suitable technology that either directly or by the application of binders (which must not introduce undesirable, polluting components and/or reduce the heating value and must be low in cost) forms highly densified, inert briquettes or compacted pieces. The production of coal-based compliance fuel must yield a bulk commodity at high production rate and with low conversion cost to be competitive with alternative measures to achieve compliance (for example desulfurization processes). While in other briquetting applications well-shaped, uniform briquettes are required or preferred, compliance fuel must meet the size criteria of coal for power plants, which is, for example, 0–50 mm with only a small amount of “dust”, a characteristic that is not defined but represents a subjective requirement for “safe” loading and handling. Coal for power plants is transported in bulk in large trucks, railroad hopper cars, unit trains, barges, and seagoing vessels. Because the volume of these transportation means is predetermined by their existing designs, the specific mass of a new, briquetted product in t per m3 must be close to that of raw coal to guarantee delivery of the same amount. Monosized pieces, such as briquettes of any size or shape, do not pack closely enough. Therefore, it is desirable to produce broken briquettes or compacts by passing them through special “calibration” equipment. As shown in Fig. 6.10-35 (Section 13.3, ref. 134) the screened product after calibration is similar to regular screened coal and, due to some break-down of larger pieces, will also contain a certain amount of finer particles, which fill the voids and increases the bulk density but do not produce airborne “nuisance” dust.
6.10 Applications for Solid Fuels
Fig. 6.10-35 Screened coal-based compliance fuel after the sizing (calibration) of briquettes (Section 13.3, ref. 134)
So far, numerous processes were proposed and evaluated. Many laboratory tests and pilot plants and several larger scale demonstration plants were conceived and operated. All were based on roller presses and the application of binders. The developments, each of which by itself represents an important technological advance, did not arrive at an economical large-scale concept, suitable for the conversion of hundreds of millions of tons of PRB coal annually. To overcome the deaeration and elastic spring-back problems, the roller presses had to be run at low speeds requiring multiple briquetting lines to produce the necessary amount of high-quality compacts. Alternatively, extra high percentages of binders could be used to enhance the binding characteristics and produce compacts, which will survive the combined spring-back caused by compressed air and stored elastic energy. However, binder cost, its on site storage and handling and the need for complicated systems for mixing them into the roller press feed render the technology uneconomical. In spite of the fact that such briquettes retain their integrity, microcracks in the structure contribute to the breakdown during bulk handling and shipping or excessive fines. As a result, to date, no system is successfully operating in the multi-million t per year range that is required for electrical energy production. Instead, in the USA, for example, in 1999 about 400 million tons of PRB coal were shipped in run-of-mine condition, mostly by 10 000 t unit trains (100 cars at 100 t each), to power plants throughout the country. Because, as an average, run-of-mine PRB coal contains 35 % water, each unit train transports 3500 t of water, a total of about 14 million tons annually, and, as a consequence, coal with low heating value. This should be a strong incentive to continue the search for an economical briquetting system. Another new application of coal briquetting that is already being used, is the densification and shaping of solid reductant for the direct reduction of iron ores. Such briquettes are mixed with iron ore and fed into the reactor where controlled combustion provides heat and reducing gas for the production of direct reduced iron (DRI).
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The application of pressure agglomeration by extrusion has been also proposed for the beneficial recovery of impounded coal fines from slurry ponds (Section 6.10.1 [6.10.1.2]). One process produces coal-fiber pellets from a 50:50 mixture of fine coal and waste paper fiber. Since the latter contains little bound sulfur and nitrogen, burning the coal-fiber product can yield lower emissions of sulfur dioxide, nitrogen oxide, carbon dioxide, and possibly particulates (including trace metals) than the coal alone. Fig. 6.10-36 is a simplified block diagram of the manufacturing process. The “pelletizer” may be a (flat die) pellet mill (Fig. 6.10-27) or a screw extruder (Fig. 6.10-37). Another application is the BioBinder process. In this method, small quantities of digested municipal sewage or similar sludges are mixed with the coal and allow the formation of pellets. Since the plasticizer/binder is a waste product that is costly to dispose of, its price is negative (assuming the disposal fee is applied as a credit) resulting in low pelleted fuel cost. Fig. 6.10-37 is a flow diagram of the proposed process. To arrive at a weather-resistant pellet an additive is added to the sludge and coal mixture. Finally, another extrusion process for the manufacturing of a fuel from waste materials will be mentioned. After mixing saw dust and other finely divided wood waste with waxy materials the blend is extruded and cut into artificial coal logs for burning in
Fig. 6.10-36 Simplified block diagram of the coal-fiber pellet manufacturing process [6.10.1.2]
6.10 Applications for Solid Fuels
Fig. 6.10-37
Flow diagram of the proposed BioBinder extrusion process [6.10.1.2]
domestic fire places. The product ignites easily, burns cleanly, has high output of heat, and, after the addition of traces of suitable chemicals, emits colorful flames for added enjoyment.
6.10.3
Other Technologies
Oil-agglomeration is increasingly important for the treatment of suspension of fine coal in water [6.10.3.1]. Although, referring to the beginning of Chapter 5, the technology is part of group A, sub-groups 6 and 7, the methods applied for the cleaning of fine suspended coal are sufficiently different to warrant their coverage in this chapter. As with froth flotation, the separation effect of oil-agglomeration relies on differences in the surface properties of coal and impurities. Coal fines are preferentially wetted and agglomerated by oil, which is mixed with the suspension and is immiscible with water. Ash forming impurities remain in suspension and are rejected when the agglomerates are recovered, for example by screening. The major factors affecting oil-agglomeration of coal are amount and type of oil, degree and type of agitation, density of the suspension, particle size distribution, and wetting properties of the coal. The most significant is the oil concentration. As progressively larger amounts of bridging liquid (oil) are added, a variety of agglomerates form [6.10.3.1]. In the capillary region (Chapter 5, [B.48, B.97]), requiring 10 %
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or more of the bridging liquid, oil filled coal pellets (spheroidal agglomerates) are formed. In spite of the ability to recover an excellent coal product from sludges that are normally discarded in conventional coal cleaning operations, the high cost of the oil used was an impediment to the commercial adoption of the technology. Therefore, during recent years, decreasing oil levels were used for the production of micro-agglomerates. This approach requires a highly efficient mixer to disperse the oil and centrifugal drying to reach the desired product moisture. Oil-agglomeration can be tailored to form the type of product that is necessary for a given application. Micro-agglomerates may be used in coal dust burners of power plants and larger agglomerates are easily handled and transported. The properties of the hydrocarbons (oils) selected as collectors are important as they afford the wetting of the coal and, hence, the selectivity and yield of the process. Since different oils can be used and their price represents the major operating cost it is necessary to choose the most economical oil for the required ash rejection and coal recovery levels. It is recommended [6.10.3.1] that lighter (< 0.9 g/cm3 density), more refined oils of higher paraffin content be used when optimum rejection of ash is an important consideration. The denser, more viscous, and, generally, more aromatic oils are preferred if recovery and dewatering of coal fines are a major and ash rejection a secondary objective [6.10.3.1]. Because highly variable natural materials are involved, laboratory testing of the wetting behavior is advised for each specific application. Oxidized coals or those of lower rank are relatively more hydrophilic than bituminous coals and, therefore, respond less well to selective wetting by hydrocarbons. Improvements may be obtained by the use of chemical additives that influence surface wetting [6.10.3.1]. Agglomeration at low oil levels produces a coal product with fewer impurities. This is due to the fact that only particles with lower ash content are wetted sufficiently to produce aggregates. On the other hand, the absorption of oil on coal fines during agglomeration displaces water. The moisture content of the recovered product is primarily made up of surface water which, in turn, depends inversely on agglomerate size and oil level. It may be reduced to less than 10 % by agglomeration that is only followed by simple mechanical dewatering, provided the diameter of the agglomerates is larger than a certain minimum [6.10.3.1]. Smaller aggregates with less oil content require centrifuging to obtain the same low moisture level. Fig. 6.10-38 is the basic flow diagram of a commercial oil-agglomeration process for the recovery of fine coal that has been installed in a coal cleaning plant in Pennsylvania, USA [6.10.3.1, B.73]. It uses a low oil level for the production of micro-agglomerates with few impurities. This approach requires both an efficient, high shear mixer to finely disperse the oil and centrifugal drying to reach the product moisture requirements. A skimmer tank was also added to recover very small agglomerates that were lost in the screening operation. After the screen bowl centrifuges, the agglomerate size of the dewatered product is enlarged by rolling the mass in a balling disc pelletizer. The plant is designed to recover 20–30 t/h of clean coal agglomerates from the underflow of a wet coal cleaning plant thickener containing particles with 50 % < 40 lm; this sludge was previously discarded as waste.
6.10 Applications for Solid Fuels
Fig. 6.10-38 Flow diagram of a commercial oil agglomeration process for the recovery of fine coal [B.73]
A technique related to immiscible liquid agglomeration in suspensions is using the sol-gel process for the formation of agglomerated spherical oxide fuel particles with a diameter of up to 1 mm for nuclear reactors [B.73]. Fine particles are initially contained in an excess of a bridging phase (sol), which is then dispersed by spraying into an immiscible liquid, forming a droplet suspension. The immiscible liquid is selected such that it extracts bridging phase and causes gelation. The gel particles are removed from the suspension, dried, calcined, and sintered to yield the final product. The immiscible fluid is recovered for reuse.
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Fig. 6.10-39 is the flow diagram of a sol-gel process for the formation of gel microspheres from a water based sol in a fluidized extraction column and the cleaning of the water extracting fluidizing liquid [B.97]. The post-treatment steps (drying, calcining, and sintering) are not shown. In the process the aqueous sol of colloidal particles is dispersed into drops at the top of a tapered vessel. The drops are fluidized by the upward flow of a water-extracting fluid, such as 2-ethyl-1-hexanol. A surfactant is added to the immiscible liquid to prevent coalescence of the droplets, sticking to the walls, or lumping together. As water is removed and the sol is converted to a gel, the particles become denser and settle toward the product collection port. Vessel design and flow rates are controlled such that densified gel particles drop continuously into the product receiver while fresh droplets are added at the top. Extracting liquid is separated from the system and for cleaning. At least part of it is sent to distillation for the removal of excess water. It is possible to further disperse the droplets in stirred and baffled vessels. As compared with the fluidization method depicted in Fig. 6.10-39, smaller gel particles are obtained in such systems.
Fig. 6.10-39 Flow diagram of a sol-gel process for the formation of gel microspheres from a water based sol in a fluidized extraction column and the cleaning of the water extracting fluidizing liquid [B.97]
6.11 Special Applications
6.11
Special Applications Agglomeration is the action or process of gathering particulate solids into a conglomerate, and an agglomerate (another word for particles adhering to each other) is an assemblage of particles, which is either loosely or rigidly joined together with or without featuring a specific shape (Chapter 14). These occurrences or procedures are caused by a natural phenomenon, which may be enhanced by a number of means, such as use of wet and/or dry binders, pressure, and/or temperature. Therefore, literally millions of applications, some happening without intent but still producing beneficial results, are known, carried-out, or occur in all fields that handle and/or process particulate solids. Many applications can be associated with specific industries and, therefore, have been covered in some detail in this book. It should be recognized, however, that all are based on the same fundamentals and could be used for other materials in different industries, often after only minimal application-related modifications (e.g., materials of construction, equipment size, duty). In the following three sections a few examples of special industrial applications of the technology will be described to round-off this book’s compilation. It will be pointed out that agglomeration methods and techniques are often used repeatedly during the preliminary, intermediate, and final manufacturing processes. Although not specifically mentioned in other chapters, this repeated application of agglomeration is a general trend when, in one way or another, fine particulate solids are involved. A further large new special application of controlled size enlargement of small solid items uses the fifth binding mechanisms of agglomeration (Chapter 3, Tab. 3.1 V: interlocking bonds), involving fibers and various additives to achieve specific product characteristics. This technology yields “non-woven” engineered fabrics for a multitude of industrial and consumer applications. 6.11.1
Tumble/Growth Technologies
As discussed in several sections of this book (Sections 6.1, 6.2, 6.3 and Chapter 11), many physical characteristics of individual particles improve with decreasing size (Tab. 11.1, Chapter 11). This tendency is often maximized when the solid entities Tab. 6.11-1 Some areas that are cleaned with gas-phase filters (adapted from Purafil, Doraville, GA, USA) Airports Archives Casinos Clean room manufacturing Hospitals Hotels
Laboratories Museums Office buildings Prisons Restaurants
Retail Stores Schools Zoos
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reach nanoscale dimensions where sometimes even chemical properties are changed, producing or allowing new and beneficial reactions. Ultrafine particles (UFP), defined as being in a size range from a few micrometers down to nanometers, feature natural adhesion tendencies, which strongly increase with decreasing linear dimensions or increasing specific surface area. On the one hand, this may be a disadvantage, because nanosized particles always exist as agglomerates (Fig. 11.1, Chapter 11) and, if individual nanoscale particles are required, special
Fig. 6.11-1 Photograph of a pan agglomeration system for gas cleaning media (courtesy Purafil, Doraville, GA, USA)
6.11 Special Applications
environments must be provided to produce and keep them separate or costly disagglomeration steps must be added. In bulk, UFP form accretions and adhere to surfaces, and have generally bad handling properties due to low flowability, inaccurate metering characteristics, and lumping. On the other hand, such particles easily form agglomerates which, if sufficient accessible internal void space is present, retain to a large extent the desirable properties of the agglomerate-forming UFP. Such granular products are best made by tumble/growth agglomeration since only small forces are exerted and minimal structural densification occurs. One of the main qualities of these granules is their large specific surface area that is available through open porosity for surface related reactions (as catalysts and catalyst carriers, Section 6.3) while the product is dust-free, free-flowing, and, generally, can be handled effortlessly. Also, such mainly spheroidal agglomerates can be packed easily and reproducibly into static beds in columns or containers of various sizes and shapes. The conditioning and control of indoor environments is an ever increasing problem. Lately this does not only include the traditional control of temperature and moisture but also the elimination of contaminants and odors from the atmosphere. Particulates, including organic matter, such as mold, germs, and viruses, can be captured and retained by sometimes electrically assisted ultrafiltration. Cartridges are either discarded or cleaned/reactivated. For the elimination of chemicals and odors, absorption, adsorption, and chemical reactions with air purification media are required. While diluting the indoor environment with outside air is a simple method to reduce the levels of contamination, gas-phase filtration has been shown to be more efficient and cost effective. Indoor sources of contamination include people, cleaning compounds, disinfectants, emissions from new carpeting and furniture, office equipment, such as copiers and printers, and so on. Nearby exhausts from, for example, industrial manufacturing, furnaces, and incinerators and gasoline or diesel engines can also participate when the fumes are brought inside through building HVAC systems. As microelectronics and microcircuitry continue to develop and businesses are more and more depending on computers for data processing, the control of operations, and the management of manufacturing processes and facilities, indoor environments must be kept very clean to avoid interference of gaseous contaminants with computer controls and the corrosion of electronic components. Tab. 6.11-1 lists some of the applications of these decontamination technologies. While fiber-based filters or other porous media can be impregnated with customized decontamination compounds, these air cleaning units are only suitable for relatively low contamination levels and small air flows. For larger cleaning requirements, powdered carrier and/or active materials (e.g., sodium bicarbonate, activated alumina, activated carbon) are impregnated with chemically active substances (for example potassium permanganate, potassium hydroxide, sodium thiosulfate) and agglomerated by pelleting or pan agglomeration. Fig. 6.11-1 shows a pan agglomeration system, which makes use of the classification effect of this equipment. The quite uniform spherical wet agglomerates are collected in trays, transferred into and passed through an oven with a trolley system, and dried. The cured agglomerates are screened to remove fines and are then ready for use (Fig. 6.11-2).
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6 Industrial Applications of Size Enlargement by Agglomeration Fig. 6.11-2 Vials with different pan agglomerated gas cleaning media (courtesy Purafil, Doraville, GA, USA)
Fig. 6.11-3 depicts a deep bed filtering and decontamination system and a so-called deep bed scrubber, which contains a blower section, one or more deep beds of media in series, and particulate pre- and high-efficiency final filters. Such units sit outside and provide pressurized recirculating air, maximum contaminant removal, and long service life. Fig. 6.11-4 shows the principle of an extended surface system and the artist’s conception of a partially open unit. The large surface of the V-shaped gas filter media module banks increases the contact area in spite of relatively small bed thickness (< 75 mm) and, therefore, features a low pressure drop. The powered or nonpowered unit is designed to filter low to moderate levels of gaseous contaminants in less polluted areas of the process industry. Other units are for cleaning the air in instrumentation rooms or for computer racks, for compressor intake filtration, and many more. One manufacturer lists over 1000 specific gases and 100 general gas categories as a guide for selecting the most appropriate cleaning media. Fig. 6.11-5 is an example of media solutions for contaminant gases, indicating some of the more important pollutants that can be removed.
Fig. 6.11-3 a) Diagram of a deep bed filtering and decontamination system; b) photograph of a so-called deep bed scrubber (courtesy Purafil, Doraville, GA, USA)
6.11 Special Applications
Fig. 6.11-4 a) Principle of an extended surface gas cleaning system, b) artist’s impression of a partially open unit (courtesy Purafil, Doraville, GA, USA)
To guarantee the efficient and reproducible manufacturing of the final product, a number of other particulate solids must be granulated, too. Because of their fineness, these materials are difficult to handle but do feature properties that are important for their ultimate application and, therefore, must be retained (Section 6.1). The beneficial characteristics of particulate matter formulations, consisting of several, often widely disparate ingredients, may be even greater after mixing the components in one or more preparatory steps. For these blends, not only the handling problems but also the danger of segregation are a great concern. For example, brake linings contain finely divided, hard, abrasion resistant particles (Tab. 11-1, Chapter 11) in a complex matrix that provides the bonding in the lining itself and with the metallic supports. The formulation, containing all components, may also include such materials as metallic, mineral and, synthetic fibers. During production of the break drums, discs, or shoes, the blend is pressed onto the structural supports with specially designed presses to produce uniform, homogeneous wear pads.
Fig. 6.11-5 Examples of media solutions for some of the more important contaminant gases (courtesy Purafil, Doraville, GA, USA)
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First, however, the mixture must quickly and evenly flow into the die, whereby no segregation can be tolerated. This is only possible if the mass has been pre-agglomerated, which is often accomplished in specially executed batch high shear mixer/agglomerators. Another example is found in the manufacturing of dry-alkaline batteries. The active ingredients of the cathode (positive electrode) in primary (non-rechargeable) batteries are MnO2, either as chemical (CMD) or finely divided ground electrolytic manganese dioxide (EMD), graphite, and a KOH electrolyte solution and a small amount of binder. The ultimate task is to pack a maximum of uniformly distributed active substance into the battery container. Again, to avoid segregation, pre-agglomeration is required. Although, to achieve the desired performance, particularly long life and good cell-discharge efficiency, high density is necessary in the final product and, for that reason, granulation by pressure is in many cases preferred (Section 6.11.2), agglomeration with high shear mixer/agglomerators is also feasible.
6.11.2
Pressure Agglomeration Techniques
The particular advantage of pressure agglomeration over tumble/growth agglomeration is that external forces are applied and act upon particulate solids, causing particles to approach each other more closely, even break or deform, and, depending on the pressure level, result in sometimes considerable densification of the powder mass. The latter may be used to reduce the reactivity of materials, for example of directly reduced, “sponge” metals, such as iron or titanium (Section 6.9.2). Other special applications of pressure agglomeration take advantage of the fact that the solid components of a compacted mixture are in intimate contact. This does, for example, allow the fast progress of chemical reactions from particle to particle or the immediate availability of elements (e.g., oxygen) for the sustenance of interactions between chemical entities (e.g., by oxidation or reduction). In general terms, special pressure agglomeration techniques can yield products with controlled reactivity [B.97]. In the case of explosives or, for example, chemicals that activate airbags, large amounts of gas (products of combustion) are produced during a very fast reaction which, in addition, expand because of high system temperatures thereby creating the destructive forces or the pressure in an airbag. After activating a primary detonation, the chemical reaction of these materials is a rapid decomposition by oxidation whereby the necessary oxygen may be part of the solid system itself or made available from oxygen-rich ingredients, such as chlorates. The effect of an explosive depends on its energy density (the relative stored energy concentration), the amount of energy released during the reaction, and the speed of the reaction. Particularly if oxygen is made available from separate oxygen carriers in a mixture, such as airbag chemicals, it is necessary to provide good contact between the components. Product characteristics can be adjusted by modifying the degree of densification resulting from pressure agglomeration, for example compaction/ granulation. With higher density the stored energy concentration increases but, because the available surface area is
6.11 Special Applications
lower, the unassisted reactivity decreases. The reaction of such explosives or chemicals must be initiated with primers. A primer is a highly reactive material that is easily ignited by friction, percussion, or electricity that will detonate the somewhat less-reactive explosive. Another product with precisely predetermined reactivity is the chemical oxygen generator [6.11.2.1]. These “oxygen candles”, which are, for example, installed in aircraft to provide each passenger with oxygen if an emergency arises, must produce oxygen while the plane descends to lower altitudes. The production of oxygen occurs during the thermal dissociation of chlorates and perchlorates. The chemical core in which the reaction proceeds is an agglomerate containing chlorate, fuel, and catalyst that control the decomposition of the “candle” (core) and is ignited by a primer charge that is activated when the oxygen mask is pulled down. Fig. 6.11-6a depicts a cross-sectional view of a typical chemical oxygen generator and Fig. 6.11-6b is the photograph of actual equipment for aircraft that is housed behind the panels above each seat. The heart of such an oxygen generator is the chemical core. By varying the physical shape and the amount of chemicals that make up the core, a wide range of oxygen outputs may be achieved. Generally, the larger the core diameter, the more oxygen is produced, and the greater its length, the longer the duration of the oxygen production. If in a high-flying aircraft, an aneroid, operated by the effect of ambient air pressure on a diaphragm, senses that decompression has occurred, a signal is supplied that
Fig. 6.11-6 a) Cross-sectional view of a typical chemical oxygen generator, b) actual equipment for aircraft (courtesy Puritan Bennet, Lenexa, KS, USA)
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releases the overhead compartment door and the masks are presented within the reach of the passengers. The flow starting mechanism (primer), which is activated by releasing the spring loaded pin while pulling down the mask, initiates the chemical decomposition. As the chemical reaction zone travels along the core from one end to the other (right to left in Fig. 6.11-6a), the oxygen produced flows through the insulation until it reaches the core locator, a metal plate separating the candle from the filter media. The gas passes through holes around the perimeter of the plate into the filter where particulate matter and traces of contaminating gases, resulting from core decomposition (Section 6.11.1), are removed. Clean oxygen is supplied through a manifold and tubing to the mask. The chemical oxygen generators for airline application are designed to produce oxygen at a rate varying with time that coincides with the physiological needs of humans during descent. The requirements are stated in the US FAA (Federal Aviation Administration) regulation FAR 25.1443. Fig. 6.11-7a shows a typical altitude profile (cabin altitude against time after decompression). It is a requirement that immediately after decompression a large amount of oxygen is made available as, typically, the emergency begins at great altitude. Because it is assumed that, after a short period, during which the crew reacts to the situation, the aircraft quickly descends to a denser atmosphere, the production of oxygen can diminish during the “candle’s” 12–25 min burning time. In Fig. 6.11-7b a trace of the actual output of an aviation chemical oxygen generator is indicated on the flow requirement profile. Once the aircraft reaches an altitude of 10 000 feet (about 3000 m), oxygen is normally no longer required and production ceases. The variable production of oxygen is determined by the dimensions, shape, and chemical composition of the core. To make sure that the reaction proceeds uniformly, the core is made by pressure agglomeration. For that purpose, different mixtures of chemicals, which may be pre-agglomerated to avoid segregation and improve metering, are filled in layers into the die of a mechanically or hydraulically operated punch-anddie-press. After compaction, the chemical components of the blend are in very close contact so that the influences of the different layers are only visible as little defined steps in the curve representing the actual production of oxygen in Fig. 6.11-7b. Since a major
Fig. 6.11-7 a) Typical altitude profile (cabin altitude against time after decompression), b) trace of the actual output of an aviation chemical oxygen
generator as compared with the flow requirement profile (courtesy Puritan Bennet, Lenexa, KS, USA)
6.11 Special Applications
concern when working with oxygen is fire susceptibility, high-pressure oxygen systems are dangerously sensitive to nearby fires or heat, particularly in aircraft. Chemical oxygen generators will not explode, are a low fire contributors, and even gunfire fragmentation tests resulted in no signs of combustion within the chlorate core. Referring to some of the discussions in Section 6.11.1, manganese dioxide based cathode blends for the manufacturing of dry, alkaline batteries may be preferably agglomerated by compaction/granulation to produce higher density of the dust free, easily handleable, and non-segregating particles. This can be of particular interest if CMD (chemical magnesium dioxide) is used which, contrary to crushed solid EMD (electrolytic magnesium dioxide), is made of porous spherical particles. The interaction of certain types of synthetic graphite, which perform better in battery blends, is also improved by pressure granulation. Ultimately, it is necessary to pack the cathode tightly into the battery container. This is especially difficult for the small (diameter) cells. Therefore, the cathodes of many modern high performance batteries are made from pre-compacted rings. It is necessary to make these rings with high precision, particularly in respect to thickness, since several rings are filled into the container and must fit its defined (standardized) height with only very little adjustment possible during final pressing. Even these rings are made from pre-granulated cathode mass to improve metering and avoid segregation of the components. The application of rings, which feature small density variation, if any, avoid the otherwise marked density gradient that would result from the considerable volume reduction required to compact the pre-granulated mass into the final cathode in one step. In the latter case, the friction on the container wall and on the mandrel that is necessary for the production of a central hole for the anode, does cause the density gradient that is unacceptable in high quality batteries. In Section 6.11.1, the manufacturing of brake linings from pre-agglomerated masses with special presses has already been mentioned. This additional processing is a further application of pressure agglomeration, yielding highly densified brake pads in which all components are well bonded. A post-treatment may be necessary to obtain final properties (Section 6.11.3). Another special application of agglomeration is in the field of polyimides. In this group of polymers thermoplastic, thermosetting, and non-melt processable characteristics exist. The resins of the latter type, in conjunction with fillers (e.g., graphite, MoS2, PTFE) or discrete fibers (chopped or spun glass, carbon), comprise typical molding compounds [6.11.2.2]. Fabrication of these formulations into engineered products occurs in many ways and, for non-melt processable compounds, includes methods of powder metallurgy for the manufacturing of near-net or net shaped parts (Chapter 7.0). Other techniques are conventional injection, transfer, and compression molding and standard plastic extrusion. Stock shapes, produced by these processes, are available as sheets, rods, tubes, plaques, rings, discs, and bars (Fig. 6.11-8) and can be machined (much the same as brass) on standard metalworking equipment [6.11.2.2]. Many products, particularly the often very complex near-net or net shape parts (Fig. 6.11-9), which also include simpler gears, self-lubricating or high temperature bearings, bushings, retainer hubs, roller guides, thrust washers and discs, wire guides with molded-in holes, wear strips, and many more, are made on punch-and-die
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Fig. 6.11-8 Stock shapes (sheets, rods, tubes, plaques, rings discs, and bars) and simple parts (bushings) of Vespel polyimide non-melt processable compound (courtesy, DuPont, Newark, DE, USA)
presses with special tooling. Taking into consideration the superior quality of the engineered items, beginning in some cases with batch sizes of 500 pieces per design, development and manufacturing of new parts become feasible end economical. The process begins with the formulation of the molding compound. Particulate resin is uniformly mixed with specific, application related additives and then agglom-
Fig. 6.11-9 a) Complex near-net or net shape parts for wafer handling and processing, b) IC (integrated circuits) handling and testing and other semiconductor manufacturing made of Vespel polyimide non-melt processable compound (courtesy, DuPont, Newark, DE, USA)
6.11 Special Applications
erated by compaction/granulation, mostly, as always, to avoid segregation and improve handling, particularly the metering and packing into the dies of the densification and shaping equipment. Especially if complex parts of the types shown in Fig. 6.11-9 are produced, not only the bulk density and flowability, but also the particle size distribution, the macro- and microsurface roughness and the strength and structure of the granules must be tightly controlled and maintained. Regarding particle size distribution, a sufficiently large amount of “fines” that fill the voids between larger particles, produce a denser packing in the die, reduce the necessary stroke length of the punch, and increase the structural uniformity of the compact should be present. The granule shape and macro- and microsurface roughness will influence interparticle friction and, therefore, the fill density and ease of densification during the manufacturing process. Removing edges and roughness by abrasion in a tumbling drum (Section 6.6.2), with or without de-dusting, may improve the packing characteristics. Lastly, because the structure of the final part must feature uniform and high density, it is necessary that the pre-agglomerated molding compound is totally dispersible (i.e., destructible) under the prevailing pressure in the die during compaction. Therefore, the strength of the granules must meet this requirement. The objective of pre-agglomeration will be to stabilize the uniform mixture of all components and produce not the highest possible abrasion resistance but the best strength for obtaining the optimal structure of the final parts (i.e., a compromise between “high enough for good handling” and “low enough for easy disintegration under pressure”). Compaction/granulation is the most versatile pre-agglomeration technique for this task. By modifying the pressing force the density and strength can be controlled, granule size and distribution is adjustable, using two- or multi-step crushing and screening, and, within limits, shape and surface roughness can be changed. Parts produced by compaction and shaping with the various, previously mentioned techniques are cured by heating in specially designed furnaces. While “stock shapes” (Fig. 6.11-8) can and will be machined to final tolerances, net or near-net shape parts (Fig. 6.11-9) must have so small density variations after pressing that they do not distort during heat treatment. Well granulated molding compounds, taking into consideration the above discussions, and the use of optimally designed dies will yield products that meet this requirement.
6.11.3
Other Technologies
Most of the “other agglomeration technologies” in the field of special applications are related to the use of nanoparticles and to coating, without but, increasingly, with nanoparticles and techniques. Agglomeration by heat, mostly as a post-treatment, is also commonly present. Another large new special application of controlled size enlargement of small solid items uses the fifth binding mechanisms of agglomeration (Chapter 3, Tab. 3.1 V: interlocking bonds), involving fibers and various additives to achieve specific product characteristics.
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It has been repeatedly mentioned in the previous chapters that one objective of agglomeration, particularly during early preparatory manufacturing steps, is the stabilization of a uniform mixture of different dry solids to avoid segregation during storage, handling, and further processing. In addition to the conventional granulation methods, in which binders are added or inherent binding mechanisms are activated to achieve bonding between the particles, a few years ago the term “ordered mixture” was created to describe blending of particulate solids in the presence of interparticular attraction with and without forces causing organization [6.11.3.1]. Nanoscale particles easily agglomerate naturally and, therefore, make mixing of, for example, two nanosized solids extremely difficult (Chapter 11). If the particles of one component, suspended in a suitable fluid, are electrostatically charged with the same polarity, for example tribo-electrically by using high shear forces [6.11.3.1], they become optimally dispersed due to repulsion between the entities. A second component of equal size and similarly prepared but with opposite polarity, can then be mixed uniformly with the first material whereby the two types of particles attract each other. This is best achieved in a liquid environment because van-der-Waals forces are lower in liquid than in air and the electrostatic charges prevail. Because the electrostatic charges dissipate after mixing, natural re-agglomeration occurs, which stabilizes the blend. As shown in Fig. 6.11-10, re-agglomeration by aggregation also occurs with time after only one type of nanosized particulate solids (in this case SiO2, Aerosil OX50, manufacturer Degussa, Chapter 11 and Section 15.1) was first electrostatically dispersed. Fig. 6.11-11 shows another result of electrostatically assisted mixing. The coating of relatively large spherical lactose with nanosized SiO2 particles (Aerosil R972, manufacturer Degussa) occurs if the lactose is charged positively and the Aerosil negatively [6.11.3.1]. The result is an interactive, homogeneous particle blend. Small particles adhering to the surface of larger solid entities, which, prior to this modification, featured unfavorable flow properties, do improve flowability because they impose a distance between the large particles and avoid sticking.
Fig. 6.11-10 Re-agglomeration (aggregation) of electrostatically dispersed SiO2, Aerosil OX50 (manufacturer Degussa): left) original sample;
center) at the end of dispersion/charging; right) 10 min after dispersion/mixing [6.11.3.1]
6.11 Special Applications
Fig. 6.11-11 SEM photograph of relatively large spherical lactose with immobilized (adhering) nanosized SiO2 particles [6.11.3.1]
A purely mechanical modification of surface structure and characteristics of particles is accomplished by mechanofusion and hybridization [B.48, B.97] (Chapters 5.0 and 11.0). With these technologies nanosized particles are embedded into or coated onto core substrates. As discussed above, flowability of the solid cores may be improved but, depending on the properties of the adhering substances, many other characteristics, such as wettability, electrical conductivity, magnetic properties, color perception, dispersibility, solubility, etc. can be changed, too. Generally, as shown in Fig. 6.11-12 the technologies are used to alter particle shape and structure, allowing particle engineering. Recently, another method was developed that can be used for particle modification by allowing dispersion and processing of powders of 0.1–50 lm by fluidization [6.11.3.2]. As depicted in Fig. 6.11-13, left, conventional fluid bed systems are unable to disperse powders < 50 lm because the attraction forces between the particles are greater than the fluidizing forces exerted by the gas (drag and buoyancy). A new rotating fluid bed processor, termed Omnitex by the manufacturer (Fig. 6.11-13, right), consists of a plenum chamber, a cylindrical porous drum as gas distributor, and a filter in the center of the drum (Fig. 6.11-14). The porous drum is made of sintered stainless steel with a pore size between 1 and 20 lm, depending on the application, and rotates within the chamber to create centrifugal forces of up to 50g. Fine powder is placed into the drum. During operation, due to centrifugal forces, the particles move to the inside wall where they form a layer and (hot) fluidizing gas flows radially inward through the porous drum. By proper selection of the rotational speed of the drum and the gas flow rate, sufficiently high drag forces are created to overcome the centrifugal and cohesive forces, thus dispersing the particles and creating a turbulent movement. A spray nozzle may be installed in the drum for the addition of coating or binding agents as required. Product is collected on the internal filter. From time to time during operation, particles and fines are reintroduced into the process by a compressed air blow-back action and, at the end of the cycle, product is removed from the filter using
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Fig. 6.11-12
Different possible effects of mechanofusion [B.48, B.97]
the same mechanism. In addition to the conventional powder modifications to improve flowability, produce direct tabletting formulations (Sections 6.2.1 and 6.2.2), coat particles or agglomerates to produce drug delivery systems or mask taste (Section 6.2.3), other surface alterations, the manufacturing of synthetic composite materials, and many more fine powder processing techniques are feasible with this method.
6.11 Special Applications
Fig. 6.11-13 Comparison between the fluidization mechanisms in a conventional fluid bed and the rotating Omnitex FB processor (courtesy Nara, Tokyo, Japan)
In Section 6.11.1, the manufacturing of brake linings from pre-agglomerated masses with special presses has already been mentioned. After application of the pads by pressing, a thermal post-treatment is required in most cases to obtain final bonding and properties. A similar thermal post-treatment is also necessary to obtain the uniform, high product quality of polyimide stock and engineered parts (Section 6.11.2). So far in this book, the binding mechanism V: interlocking bonds (Tab. 3.1, Chapter 3) has been mentioned in connection with the sometimes heat-induced plastic deformation during press agglomeration (Section 6.6.2) and the application of fibers for the reinforcement of agglomerates (Section 8.2) and for assisting disintegration of
Fig. 6.11-14 Diagram of the principle of the Omnitex FB processor (courtesy Nara, Tokyo, Japan)
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compacts in liquids (Section 6.2.2) ([B.97]). With the exponential increase in the production of both natural and artificial industrial fibers during the 20th century, in the second half of the century, inspired by military developments for and during World War II, a number of fibrous materials were no longer woven but consolidated and bonded in different ways. This technology yields “non-woven” engineered fabrics for a multitude of industrial and consumer applications [B.107]. Although one type of fiber-based agglomerate, paper with all its derivatives, which is, technically, a non-woven material, dates back to the beginning of the 2nd century in China and the methods of paper making were further developed and improved during all the following centuries, real advances were only made during the past 100 years. The origins of other non-wovens resulted from the recycling of fibrous wastes and low quality fibers left from industrial processes such as wood processing for the manufacturing of cellulose, weaving, or leather processing and also from raw material restrictions during and after World War II. Therefore, non-wovens go beyond the limits of paper products or textiles; the fibrous web may be also made of plastics and foils and their humble beginning is responsible for two misconceptions that are still lingering today: they are often assumed to be (cheap) substitutes and/or associated with disposable items, which are cheap and of low quality. Non-wovens neither depend on the interlacing of yarn for internal cohesion nor do they have an organized geometrical structure. Non-wovens are the result of relationships between individual fibers and feature new properties. Therefore, they are by no means cheap or substitutes but high quality materials in their own right offering special characteristics and performances. Mostly due to their irregular, random but relatively permeable structure, non-wovens are applied in filters, hygiene, health care, cleaning, household, and automotive articles, agriculture, civil engineering, food wrap and packaging, and clothing to just name a few areas of end-use. Tab. 6.11-2 is a more detailed compilation. For the manufacturing of non-wovens, there are three main routes to web forming. *
* *
The dry-lay processes with carding or airlaying of natural, man-made, and inorganic fibers. The wet-lay methods using short fibers and cellulose. The polymer-based web bonding originating with particles from which fibers or film are formed.
Fig. 6.11-15 is a diagram of these routes indicating the applicable raw materials and their preparations, the web forming and bonding, the post-treatments (processing of non-wovens) and the type of products [B.107]. Since their early development about 50 years ago, many different non-woven products have been developed for use in a wide variety of applications (Tab. 6.11-2). In spite of this relatively short history, so much information is already available that it goes beyond the scope of this book to describe in detail the various manufacturing methods and the special properties of the products. Reference is made to a recent book that is a coherent presentation of the present state of the art. It is available in both German and English [B.107].
6.11 Special Applications Tab. 6.11-2 [B.107])
Some current application of non-wovens (adapted from
General area
Specific applications
Filtration
Dry air and gas, HVAC and process air technologies, particle filters, gas sorption, liquid particle filters, surface filters for > 5 mg/m3, deep filters for low (< 1 lg/m3) and average (10–500 lg/m3) concentration. Baby diapers, feminine hygiene products, adult incontinence items, dry and wet pads, nursing pads, nasal strips, etc. Surgical drapes, gowns, and packs, face masks, dressings and swabs, bag liners, etc. Bed and table linens, furnishings, fabric softeners, filters, food wraps, tea bags, scouring media, wipes, dusters, etc. Trunk liners, molded hood liners, heat shields, airbags, tapes, trim, decorative fabrics, oil and air filters, etc. Rooting pads and pots, soil stabilization, etc. Asphalt overlay, drainage pads and pieces, sedimentation and erosion control, etc. Roofing and tile underlay, thermal and noise insulation, floor pads, house wrap, etc. Cable insulation, abrasive pads, reinforced plastics, battery separators, satellite dishes, artificial leather, coating, etc. Leisure and travel, school and office, interlinings, insulation and protective clothing, industrial work wear, chemical defense suits, shoe components, etc.
Personal care and hygiene Health care Home, household, cleaning Automotive Agriculture Geotextiles Construction Industrial Clothing
Nevertheless, a few statements are in order to show how non-wovens are truly a part of agglomeration. The main characteristic of all non-woven products is that fibers, with a wide range of properties and originating from a multitude of sources, are at first loosely and, most importantly, irregularly deposited to form a 3D web. For many applications layers, made-up of different fibers with specific characteristics and forming distinct structures, are laid down. For example, Fig. 6.11-16 is a SEM micrograph of the cross section through a laminated non-woven product that is composed of synthetic leather and microfiber non-wovens. This household cleaning material combines high cleaning performance on glossy surfaces with smear free drying due to the presence of an absorbent core [B.107]. Many other high volume consumer products make use of one or more core layers with absorbent properties. Among the most quickly and highly developed applications are baby diapers, which use super absorbent fibers. Super absorbent polymer (SAP) molecules can trap and hold hundreds to thousands of times their own weight in fluid, ultimately forming a gel. The super absorbent core layer in a diaper is between a nonwoven cover stock, a one or two layer non-woven fluff/pulp sheet that takes up, distributes, and draws liquid into the core, and a microporous back sheet. In addition, elastomeric materials and waterproof elements are incorporated. The super absorbent core not only stores liquid but actively pulls moisture out of the damp or even wet fluff/ pulp, thus leaving the contact areas soft and dry.
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Fig. 6.11-15
Diagram of the manufacturing routes to non-wovens [B.107]
While laying down (dry or wet) one or more fiber layer(s), a weak bonding occurs by entanglement between the fibers (Tab. 3.1, Chapter 3), which can be enhanced by crimping or looping the fiber strands. Particulate components and/or other solid or liquid additives that provide special properties (e.g., odor absorbents or perfumes, anti-bacterial substances) or later act as binders during a post-treatment are also incorporated during web forming. To obtain final and permanent bonding, structure, and strength, finishing processes are required prior to shaping, cutting, packing, and distributing the product. Post-treatments include shrinking, compacting and creping, glazing and calandering, embossing and goffering, molding and stamping, and a
Fig. 6.11-16 SEM micrograph of the cross section through a laminated non-woven household product that is composed of synthetic leather and microfiber non-wovens. Layer sequence (from top to bottom): macroporous synthetic leather surface, non-woven absorbent core, microfiber non-woven [B.107]
6.11 Special Applications
Fig. 6.11-17 Summary of the different bonding processes as described in an ISO standard (ISO/DIS 11224 [B.107])
multitude of chemical finishing methods and web bonding by needling, stitch bonding, knitting, and drying, heat setting, ultrasound and/or chemical methods to activate binders. Bonding is very important for the performance and life of non-wovens [B.107]. Fig. 6.11-17 is a summary of the different bonding processes as described in an ISO standard. While needling, such as stitch bonding, knitting, and other, similar techniques, uses special tools to reorient some of the fibers or introduces continuous filaments or threads in a sewing fashion, other bonding processes are based on friction, cohesion, and adhesion forces between fiber surfaces. A cohesive bond occurs between two identical web fibers and, therefore, no binders are present while in adhesively bonded non-wovens, cross-linked or coagulated binder fluid, solidified droplets, originating from binder fibers or powders, attach the matrix fibers to one another. Fig. 6.11-18 shows schematically some possible bonding sites. Non-wovens that contain binder fibers“,4› have both cohesive and adhesive bonds. The latter form after melting or softening and are predominantly bonds at fiber intersection points (Fig. 6.11-18c). In Fig. 6.11-19 two microphotographs of non-wovens depict fiber bonding.
Fig. 6.11-18 Diagram of some possible non- woven bonding sites: a) large area, enveloping fiber intersection points, b) small area and punctiform bonding, c) bonding of fiber intersection points [B.107]
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Fig. 6.11-19
Two micrographs of non-wovens depicting fiber bonding [B.107]
Further Reading
Once again, for further reading the book “Nonwoven Fabrics – Raw Materials, Manufacture, Applications, Characteristics, Testing Processes” [B.107] is recommended.
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Powder Metallurgy Powder metallurgy (PM) is a method for manufacturing ferrous and non-ferrous industrial parts. Although relatively young, the technology is highly developed and very reliable. In accordance with its great importance for modern high-tech metal products and components, many publications are available on the subject (see Further Reading) and everywhere in the developed world scientific groups, industrial and trade organizations, and other institutions exist that promote the knowledge and applications of powder metallurgy. Many are easily accessible, for example: www.mpif.org; www.ifam.fraunhofer.de; www.PowderMetalWeb.com; www.ipmd.net. Tab. 7.1 lists the advantages of the PM process and products. The technology is cost effective because it produces parts, simple or complex, at or very close to final dimensions. This is often called “near-net-shape” production (Section 6.7.2). As a result, only minor, if any, machining is required making it a “chipless” metal working process. Typically more than 97 % of the starting material (metal powder) is in the finished part. Because of this, powder metallurgy is a process that saves energy and raw materials. Most PM parts are small and even larger ones weigh typically less than 2 kg, although pieces of up to 15 kg can be fabricated with conventional PM equipment.
Tab. 7.1 *
Advantages of the P/M process and of parts made by it (adapted from MPIF, Princeton, NJ, USA)
Eliminates or minimizes machining (near-net-shape) * Eliminates or minimizes scrap losses * Maintains close dimensional tolerances * Produces good surface finishes
*
Permits wide variety of alloy systems * Facilitates manufacture of complex or unique shapes which would be impractical or impossible with other metal working processes * Provides materials which may be heat treated for increased strength or increased wear resistance * Metallurgically immiscible materials can be combined to produce uniform structures and “exotic alloys” that are not attainable by pyrometallurgy
*
Provides controlled porosity for, for example, self-lubrication or filtration
*
Offers long term performance reliability
*
Suited to moderate and high volume production requirements
*
Cost effective
Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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Fig. 7.1 Some complex parts produced by powder metallurgy (courtesy of Komage-Gellner, Kell am See, Germany, and Dorst, Kochel am See, Germany)
7 Powder Metallurgy
Production rates range from a few hundred or less to several thousand parts per hour. While many early powder metallurgical items, such as cutting tool tips, bushes, or bearings, had very simple shapes (cubes, rectangular blocks, discs, rings, cylinders), parts with complex contours and multiple levels are produced economically today (Fig. 7.1) [B.28, B.48, B.97]. Many gears, cams, and intricately shaped parts that would require expensive machining when produced from cast, forged, or wrought stock can be made from metal powders. Counterbores, flanges, hubs, and holes as well as keyways, keys, D-shaped bores, and other fastening devices can be an integral feature of the part and two or more parts may be combined into a single unit if product design permits. Compared with other metallurgical manufacturing processes, PM offers the additional advantage of precise control. Powder metallurgists are able to exercise their influence over the entire production process from the pure metal to the equally pure powder, the (agglomerated) pre-form, and the finished part. By mixing metal powders, which may be of different elemental or alloy origin, size, and/or shape, various material compositions and finished products can be created. Powder metallurgical manufacturing eliminates impurities and inclusions, avoids uneven internal stresses, and excludes poor finishes, unworkable tolerances, and many other factors that might affect the rate of production and the quality of the finished part. PM assures uniformity and optimum performance characteristics of products and offers long-term performance reliability in critical applications. As shown in Fig. 7.2, after the formulation and blending step, shaping (forming) of the part and the development of strength are accomplished with methods of agglomeration by pressure and heat followed by a choice of post-treatment steps, if required. While powder production is accomplished by the atomization of pure molten metals or alloys [B.13d and Section 13.3, Refs. 82, 85, 87, 89, 90, 91, 93, 94], the manufacturing process for parts occurs in the solid state. Therefore, metallurgically immiscible materials can be combined to produce uniform structures and “exotic alloys” that are not attainable by melt methods. Dissimilar metals, non-metallics, and other components of widely different characteristics can be mixed, compacted, sintered, and further processed, if required, into components that exhibit unique properties. For example: ceramics can be blended with metals to make cermets (Section 6.7.2). Carbon and copper form electrical brushes with high electrical conductivity and wear resistance. Tungsten and silver are combined in the manufacture of electrical contacts and switch gear. Copper, tin, iron, lead, and graphite are compacted into heavy-duty friction material. The combination of materials through the use of PM techniques is essentially unlimited. Application for a new task only requires research and experimentation. Since powder metallurgy is a technology in its own right, this book is not going into more detail. As far as agglomeration tools for the densification and shaping of the powders and of powder blends are concerned, Fig. 7.2 classifies the methods under hot and cold compaction. The equipment available and used for these tasks has been described in detail in an earlier book by the author [B.97]. In many respects the shaping and densification of metal powders is similar to that of high-performance ceramics and many of the presses can be used for both applications (Section 6.7.2). The same is true for sintering. Here too, some related material is included in Section 6.7.3
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Fig. 7.2 Process diagram of powder metallurgy (courtesy of MPIF, Princeton, NJ, USA)
7 Powder Metallurgy
and the author’s earlier book [B.97]. Additional information can be gleaned from the numerous websites associated with the search word “powder metallurgy” and its foreign language forms as well as from the books listed at the end of this chapter. As can be seen from the process diagram in Fig. 7.2, hot compaction combines the steps of densifying, shaping, and sintering in one piece of equipment. For many applications it is the modern way of accomplishing the task. In most cases, after hot compaction only additional post-treatment (optional operations) is necessary if required. Isostatic pressing, either hot or cold [B.13a, B.97], results in the most uniform structure, which minimizes distortion during heat treatment (sintering) that is used either for the development of strength or the modification of density (reduction of porosity) during re-sintering (Section 6.7.2). Punch-and-die pressing is used for the mass production of simple small parts but also for complex symmetrical designs (Fig. 7.1) [B.28]. Rolling with roller presses is a relatively new process for the manufacture of sheet with special properties from powders that are often mechanically alloyed and could not be produced by traditional methods.
Further Reading
For further reading the following books are recommended: B.4, B.13a, c, and d, B.28, B.47, B.57, B.65, B.76, B.78, B.79, B.84, B.85, B.86, B.88, B.95, B.96, B.100, B.101 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold.
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Applications in Environmental Control Minimization of solid wastes, pollution control, and conservation of natural resources have become important and popular topics in all countries of the world. We live in an age of increasing government intervention, resulting in environmental laws, regulations, and enforceable penalties for offenders. Legal federal, state, and private citizen actions are taken increasingly often. They often result in considerable fines and sometimes even jail for those found responsible. Every year billions are spent to capture, contain, and remedy solid wastes after their generation, and costs for “hazardous waste” treatment and disposal have multiplied during the past few years because of the limited availability of treatment and disposal sites and facilities.
Tab. 8.1 Origins of particulate wastes and sources of pollution (presented in alphabetical order) Primary wastes * Agricultural wastes * Excrements * Mining wastes Processing wastes Chemical processing wastes * Metallurgical processing wastes * Mineral processing wastes * Vegetable and other food processing wastes * Waste by-products * Wood processing waste *
Production wastes Energy production wastes * Food production wastes * Waste by-products * Wastes from the forming, shaping, and finishing of solid products *
Secondary wastes Wastes created during the disposal of wastes * Wastes created during the handling of wastes * Wastes created during the processing of wastes *
Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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8 Applications in Environmental Control
Most solid wastes are particulates with an increasing portion in the fine particle size range. As shown in Tab. 8.1, they are produced during primary activities, the processing of raw and advanced materials, and the handling, processing, and disposal of wastes from previous sources. Agricultural wastes include, for example, straw. Formerly straw was burnt but this practice has been outlawed in many regions. Excrements used to be applied as a natural fertilizer. Today, concerns about public health forbid the direct application of raw feces, manure, and sewage on farmland or gardens in most developed countries. People have mined the earth for thousands of years. Originally only high-grade ores and minerals were removed for further processing and use. Many of the oldest mine sites can be identified by the associated dumps of low-grade or undesirable material. Even today, tailings, below-grade slurries, and slimes that remain after high-efficiency upgrading and may now also be contaminated with chemicals, are still frequently disposed of in ponds or other deposits from which ground water is threatened by seepage. During all processing steps, wastes develop featuring very diverse characteristics. In chemical processing, the solids are often very fine (dispersions), toxic, or hazardous. Metallurgical processing produces large amounts of fine (often metal-bearing) dusts. In mineral processing, size reduction is an important step that yields fines with particle sizes that are too small for direct further use and particulate, often contaminated, tailings are discarded from concentration plants. Vegetable, food, and wood-proces-
Tab. 8.2
Origin, forms, and characteristics of particulate solid wastes
Origin * Single-component or composite materials * Primary or secondary (Tab. 8.1) Forms (organized by size) Large and/or bulky parts * Lumps and pieces * Granular materials (0.1–10 mm) * Fibrous materials (turnings, borings, organic fibers, etc.) * Dusts (0.1–100 lm) * Ultrafine (nano) particles (0.1–100 nm) * Cakes, sludges, slurries, and slimes * Dispersions: suspensions (solids in liquid), smokes (solids in gas) * Ultrafine particulate systems (colloids and aerosols) *
Characteristics (arranged alphabetically) Hazardous (by composition, definition, nature, or size) * Inert * Obsolete * Organic * Radioactive * Reactive * Toxic * Useless * Valuable *
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sing facilities require the disposal of large amounts of organic matter, which is today summarily called “biomass”. By-products, such as molasses from sugar making, lignins from paper mills, oils and tars from the coking of hydrocarbons, or slags from metallurgical processing may be considered wastes because either their amounts are too large, contaminations prohibit their direct use, or, more recently, legislation requires mandatory processing, often to reduce secondary pollution and/or health hazards. In many of these by-products, the particulate solids are dissolved or suspended in a liquid phase. Similarly, various particulate wastes develop during the production of industrial or consumer products. In particular, the shaping, forming, and finishing of articles from solid material yields turnings, borings, chips, grindings, and dust. By-products, for example gypsum or ammonium sulfate obtained during flue-gas desulfurization as required by “clean air acts”, are increasingly obtained in response to anti-pollution legislation. The production of food, such as canning and fish or meat packing, again results in large amounts of biomass, which must be dealt with in an environmentally safe manner. The broad category “energy production” comprises wastes from all fuelpowered motors and generators of any kind and size. Secondary pollution is a cause for great concern during the handling, processing, and disposal of solid refuse. Since primary waste materials are often fine, they tend to
Tab. 8.3 Possible effects and applications of size enlargement by agglomeration in environmental control (A) Collection of contaminants * Improvement of the collection efficiency after agglomeration by – Particle contacts in turbulent flow regimes – Sonic agitation – Coalescence in the presence of moisture – Flocculation by stirring with or without polymer addition. (B) Handling of particulate solid wastes * Elimination of dust by size enlargement * Increased bulk density or decreased bulk volume * Improved flow, including derived characteristics of bulk materials (metering) * Optimized or modified product shape. * Reduced reactivity (C) Processing of particulate solid wastes * Same as (B) * Additionally – Modification of properties (strength, porosity, dispersibility, solubility, reactivity) – Adjustment or engineering of composition. (D) Disposal of particulate solid wastes * Same as (B) * Additionally: – Production of permanently strong and/or dense pieces – Encapsulation of toxic or radioactive materials – Achievement of leach-prove bonds
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dust if they become dry, may be washed away with liquids, or seep into the ground where they may contaminate soil and aquifers. Tab. 8.2 lists typical origins, forms, and characteristics of particulate solid wastes. They must be considered during handling, processing, and disposal. Solid wastes may consist of materials containing one single component or of composites comprising two or more substances. They may be primary discards, such as domestic refuse and solids contained in sewage, or secondary wastes such as ashes from refuse incinerators and digested filter cakes from municipal waste-water treatment plants. Secondary pollution can also arise from the emanation of dust or ultrafine particles from primary wastes. The dimensions of solid wastes span the range from meters (for example automobile scrap and obsolete structures and equipment) to nanometers (in particulate residues from combustion and products of sublimation). Originally, the removal of particulate pollutants from liquids and gases was accomplished by settling or with relatively simple filters and collectors. Ultrafine particles are so tiny that they can not be observed with the eye or by light microscopy [B.70]. Also, because of their small mass they remain suspended in gases (especially in air) and dispersed in liquids. Furthermore, they follow the streamlines of flowing fluids and so can not be captured by conventional means. Electron microscopy made such particles visible and relatively recent research revealed that very fine particles enter the respiratory tract during breathing or are resorbed from drinks, causing previously unrecognized health risks (for example silicosis and asbestosis or intestinal infections). For these reasons, the characterization of hazardous solid materials has changed and, with it, the requirements for pollution control. In addition to its nature, described by certain properties (e.g., radioactive) and compositions (e.g., containing toxic components), legislation often defines the term “hazardous material” by “below a certain particle size”. With this definition, totally inert materials, consisting of or containing ultrafine particles, become hazardous by law and require processing. Since in most cases the dimensions of particulate solid wastes, particularly those of contaminants, are small and, therefore, cause problems during collection and handling and with secondary pollution, size enlargement by agglomeration offers technologies that can assist in solid waste handling, processing, and disposal in many different ways. Tab. 8.3 summarizes the possibilities. Increasingly, wastes are no longer just disposed of but processed for direct recycling or as secondary raw materials. With these technologies environmental protection laws are fulfilled, raw material sources are conserved, problem-causing waste deposits or extensive land fills are avoided, and, over all, energy is saved. This results in considerable reductions in costs and increases in profit potentials in all industries.
8.1 Collection, Stabilization, and Deposition of Particulate Solids
Further Reading
For further reading the following books are recommended: B.3, B.7, B.8, B.15, B.16, B.18, B.19, B.20, B.21, B.22, B.24, B.25, B.26, B.35, B.36, B.37, B.40, B.43, B.46, B.48, B.49, B.51, B.55, B.56, B.69, B.70, B.83, B.87, B.89, B.93, B.94, B.97 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold.
8.1
Collection, Stabilization, and Deposition of Particulate Solids 8.1.1
Size Enlargement by Agglomeration during the Collection of Particulate Solids
Most effluents from processing plants of any kind are contaminated in one way or another. Contaminants may be gaseous, liquid, or solid. Gaseous pollutants are often removed by chemical reaction, which may result in a solid by-product that needs to be processed, for example the removal of sulfur from the flue gas of power plants, or they are burned, which may produce particulates (soot) that become an increasing environmental concern. Liquid contaminants may be precipitated and then also form finely dispersed, suspended particles. While for quite some time particles with sizes down to a few hundred micrometers have been successfully removed by conventional dust collection (mostly using gravitational and centrifugal or other field forces) and dry and wet filtration, air borne (Fig. 8.1) and suspended solid pollutants continued to be a great problem. Because of the small mass of fine and ultrafine particles they do not settle, even if high-field forces are produced, and they follow the flow lines in filter media so that impacts, which are necessary for collection (Fig. 8.2), do not take place. For a long time, natural agglomeration has helped to improve the collection efficiency of, for example, filters and cyclones. Knees, bends, restrictions (e.g., valve seats), and even build-up in the pipes and channels of the cleaning device caused turbulence, particle collisions, and the formation of aggregates that, as a whole, have the combined mass of all the particles adhering to each other and a correspondingly larger cross section. Both effects allow the removal of particles that, individually, would be too small (Fig. 8.3). Originally, the introduction of such turbulence was unintentional and led to sometimes surprising results: low efficiency if short straight connection lines are used between the source of particulate pollution and the collection device or unexpectedly high efficiency if, often for design purposes, tortuous, twisting lines had to be installed. More recently, process designers have applied this knowledge in the routing of effluent lines to assist dust collection. Ultrafine particles that are suspended in a fluid exhibit Brownian motion, a random movement resulting from the impact with molecules of the fluid surrounding the particles. In spite of the randomness of the motion, it is very unlikely that particleto-particle impacts will occur because the amplitudes of the movement and the par-
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Fig. 8.1 Example of an industrial plant (pyroprocessing of minerals) heavily polluted with airborne particulate solids
ticle sizes are very small. Therefore, other methods must be used to cause particle collisions that will result in coalescence and agglomerate growth. In addition to artificially induced turbulence, such techniques use uni- or bipolar charging in so-called electrostatic precipitators, magnetism, and ultrasound [B.48]. Although, from a process point of view, it is very advantageous that with these methods particle accretion occurs in the free-flowing fluid, a major drawback is that the collision probability changes with the square of the particle concentration. Therefore, as the number of particles becomes smaller, during the cleaning process itself by the incorporation of dust particles into the growing agglomerates, the collision probability decreases and the desired low concentrations of ultrafine particles in the off-gas may not be reached.
8.1 Collection, Stabilization, and Deposition of Particulate Solids Fig. 8.2 a) Enlarged view of the interior of a filter mat with particles sticking to the fiber surfaces. b) Detail of a dust laden fiber from (a) showing how particles extend into the gas stream [B.71]
Fig. 8.3 Naturally formed agglomerate of small (8 lm) glass spheres adhering to a filter fiber photographed during a laboratory experiment [B.48, B.97]
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Fig. 8.4 String-like agglomerates of “brown smoke” particles produced by natural magnetic coagulation [B.48]
For a long time, images similar to the one shown in Fig. 8.1 could have been seen at any steel-making complex in the world. The air was heavily polluted with “brown smoke”, consisting mostly of submicron c-Fe2O3 particles that are individually too small for collection by the early, rather basic gas-cleaning systems. After the invention of high-efficiency cyclones and multi-clones which, for physical reasons (low particle mass), can not collect submicron particles from contaminated gas, and their use in the steel and other metals or mineral-processing industries, it was found that, nevertheless, some of the brown smoke and similar airborne dust was removed anyway. In the meantime, it had become possible to observe ultrafine particles with the newly developed transmission electron microscope (TEM). It was determined that, because they are ferromagnetic dipoles, c-Fe2O3 particles naturally attach to each other, resulting in string-like agglomerates (Fig. 8.4). The larger of these aggregates were removed but an objectionable reddish plume was still discharged. The ultimate clean-up became possible after the naturally produced agglomerates (Fig. 8.4) were charged in an electrical field or by spray electrodes causing them to grow into larger dendritic structures (Fig. 8.5) that can be captured and collected [B.48]. It must be a goal during the collection of particles in filters that they collide with the medium and adhere upon impact. As shown by the results of model calculations,
Fig. 8.5 Dendritic growth of “brown smoke” agglomerates (Fig. 8.4) in an electrostatic field [B.48]
8.1 Collection, Stabilization, and Deposition of Particulate Solids
various particle sizes not only move differently but modified system conditions also influence the behavior of the particulate solids, for example if particles and/or fibers carry a natural or acquired electrical surface charge, which causes electrostatic forces [B.97]. Another new strategy for the removal of ultrafine contaminants from a gas uses a particulate collector medium that is fluidized in a suitable vessel by the gas to be cleaned. Aerosol particles adhere to the large surface area of the fluidized medium and form a coating that is densified when the collector particles collide with each other. As the coating becomes thicker, attrition results in the formation of secondary particles. These are agglomerates and substantially larger than the original aerosol so that they can be easily separated in conventional dust collectors [B.97]. Like many original methods of mechanical process technology, an old technique for the successful removal of airborne dust was first observed in nature. The capacity of rain to “clear the air” has been used since ancient times to remove suspended dust particles by passing the contaminated gas through a water spray (wet scrubbers). The liquid droplets capture the solid (and some of the gaseous) pollutants and collect them in a sump. While this reduces air pollution it transfers part of the separation problem to a secondary cleaning process: the removal of fine particulate solids from a liquid. Although these pollution control devices are cheap and efficient, it becomes more and more difficult to satisfy the increasingly stringent legislation of environmental control that is introduced almost everywhere. Traditional sprinkler sprays that require pumps and high-pressure water systems with substantial water usage, tend to exacerbate the problem by producing contaminated water runoff, potential spillage, and more high-cost clean-up. This is of particular concern if the locations of the sources of pollution and, with it, the collection points are not stationary. For example, dockside mobile loaders (DML, Fig. 8.6) are used during the unloading and transfer of dusty materials [8.1.1]. To avoid the aforementioned disadvantages, a new development causes dust suppression by wet agglomeration and accomplishes collection of the enlarged particles with fabric filters in the same unit. For the wet agglomeration step, an almost dry fog is produced by an air or (inert) gas-driven oscillator. This device atomizes liquids by passing them through high-frequency sound waves. The air or gas expands in a convergent section (nozzle) into a resonator cap where it is reflected back to complement and amplify the primary shock wave. The result is an intense field of sonic energy focused between the nozzle body and the resonator cap. Any liquid that can be pumped into the shock wave is sheared and forms fine droplets. Gas bypassing the resonator carries the atomized droplets away and forms a soft plume. The droplets have low mass, a low velocity, and ensure uniform distribution of liquid with minimum overspray and waste. It was also found that large, relatively fast-moving droplets (from sprinklers) displace so much air that the flow around it prevents dust particles from contacting the droplets (Fig. 8.7, left) while the fine dust particles easily impact small droplets, triggering agglomeration (Fig. 8.7, right; see also Chapter 5, discussion of the importance of droplet size for wet agglomeration). Sonic energy can be also used to initiate and/or accelerate dry agglomeration of ultrafine particles that are suspended, for example, in flue gases [B.43, B.48]. An acoustic field imposes sound pressure and energy. For a typical pressure of 160 dB the acoustic velocity is about 5 m/s and a typical frequency of 2000 Hz causes a fully en-
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Fig. 8.6 Loading of a truck by heavy duty clam shell grab via dockside mobile loader (DML) to avoid pollution of the environment [8.1.1]
trained particle to move back and forth 2000 times a second over a distance of about 600 lm. Particle entrainment is explained by a factor that is a function of acoustic frequency, particle diameter and density, and the dynamic viscosity of the gas [B.48]. If the factor is 1, full entrainment is achieved, while with a value of 0, no entrainment occurs. The latter means that such particles are not affected by the acoustic field and remain still. For each acoustic frequency a particle size exists below which particles are so much entrained (factor > 0.5) that they are sufficiently moved. For example, for a sound frequency of 2000 Hz, this “cut size” is about 4.5 lm. The flight paths of larger particles remain essentially unchanged, while smaller particles move with large displacements, colliding with the large particles, and adhering because of high van-der-Waals forces.
Fig. 8.7 Demonstration of the importance of droplet size for particle agglomeration
8.1 Collection, Stabilization, and Deposition of Particulate Solids
In the hot gas cleaning system of a coal-burning power plant, acoustic agglomeration could be installed between the first cyclones, capturing coarser particles, and highefficiency (multi) cyclones, removing the agglomerated fines. The power requirement to operate the acoustic agglomerator would be about 0.02–0.5 % of the power plant output [B.48]. This means that for a 250 MW power plant several hundred kilowatts of acoustic power are needed, which is very high (compared with about 36 kW, the acoustic power output of a four-engine jet aircraft on take-off). To harness such large acoustic powers, considerable additional research and development are required. Aggregation of fine particulate solids also takes place in liquids. In environmental control, the removal of particulate solids from liquid process effluents is of great importance. As for gas/solid separations, when the size of the solids diminishes and reaches the micron or submicron (nano) range, the mass of individual particles becomes so small that they remain in suspension and cannot be removed by settling. Because of the fineness, membranes would be required to retain particles on a diaphragm, which is uneconomical for the cleaning of large volumes of contaminated liquids from industrial plants or waste-water treatment facilities. However, remembering the mechanisms of growth agglomeration (Chapter 5), if particles can be made to impact with each other, it is possible that they will adhere to one another in liquids, too. Therefore, when water that is contaminated with suspended fine solids is stirred, flocs may form naturally. If this happens, the size and shape of these aggregates depend on the circumferential speed of the stirrer and the processing time. Fig. 8.8 shows that flocs are larger if the shear forces are low and the
Fig. 8.8 Natural flocculation of solid contaminants in river water [B.48]. Parameters are the circumferential speed of the stirrer and the processing time
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processing time is short. But further investigation revealed that higher speed of the stirrer and/or longer duration of mixing ultimately result in denser and more stable agglomerates. This is because of the previously discussed mechanism of growth agglomeration (Chapter 5) whereby loosely attached particles are removed under the influence of ambient forces (in this case, shear) and later have the chance to become re-attached in energetically more favorable positions thus yielding denser and stronger products. It depends on the process that will be used for solids removal, which of the two agglomerate structures is required, the loose flocs resulting from relatively gentle movement or stronger agglomerates from a more vigorous stirring for a longer time. In the large diameter circular thickener/clarifier (Fig. 8.9) that is commonly used in municipal water-treatment plants and in many industrial applications, water, flowing slowly from the feedwell in the center to the overflow around the periphery of the circular tank, is gently moved by a slowly rotating arm. Loose flocs are formed, which settle by gravity to the slightly conical bottom. Differently shaped scrapers (rakes) are used to move the sludge to the discharge cone at the lowest point from where it is transported to conventional liquid filters. The more vigorous stirring that produces stronger agglomerates is used when the resulting agglomerates are moved with some of the water to an off-site filtering system and, therefore, must survive transport. In many cases, even if collisions between particles do take place, the naturally available binding mechanisms, mostly molecular forces, which are considerably lower in a liquid environment than in a gas atmosphere, do not create bonds with sufficient strength to withstand the various separating effects and satisfactory flocculation does not occur. For quite some time it has been known that polymers, added to liquid-based particulate systems, have a dramatic influence on particle interaction. Molecules may attach themselves to solid surfaces and, depending on the characteristics of the exposed radicals, can cause particle attraction [B.29] or dispersion [B.63]. The second, dispersion, is applied to avoid agglomeration (Chapter 4) or enhance disintegration of aggregates. There are two ways in which polymers can promote aggregation: either by making particles more susceptible to salts or by flocculating the system without the aid of electrolytes. These processes are known as sensitization and adsorption flocculation, respectively. The second is more common. To create aggregates or flocs, the polymer adsorbs on different particles simultaneously, which is best accomplished by using substances with high molecular weight and a strong affinity to the particles to be agglomerated. Fig. 8.10 explains the principle. In nearly all applications of polymeric flocculants, the polymer addition and the subsequent flocculation process are carried out under conditions in which the suspension is agitated in some way, for example by stirring. In this way, the polymer molecules are distributed uniformly throughout the system and adsorb onto the particles, which are then encouraged to collide and form aggregates. As described earlier in other contexts, bridging may be followed by break-up if the bond is not strong enough and, later, re-attachment during another impact. Fig. 8.11 is a sketch of a flocculate. Care must be taken not to oversaturate the suspension with polymer. If too much polymer is adsorbed, the par-
8.1 Collection, Stabilization, and Deposition of Particulate Solids
(a) (b)
Fig. 8.9 a) Diagram; b) photograph of a circular thickener/clarifier (according to EIMCO, Div. Baker Hughes, South Walpole, MA, USA)
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8 Applications in Environmental Control Fig. 8.10 Principle of polymer adsorption and flocculation [B.29]: a) adsorption of polymer molecule on the particle; b) rearrangement of adsorbed chain; c) collisions between destabilized particles and bridging to form aggregates (flocs); d) break-up of flocs
Fig. 8.11 Structure of a flocculate (floc) bonded by a polymer [B.48]
ticles may become restabilized (deactivated) because of surface saturation or by steric stabilization [B.29]. Fig. 8.12 demonstrates diagrammatically bridging, which results in the desired flocculation, and restabilization. Commercial flocculants are used extensively, for instance in water purification. By influencing the affinity of the polymer, it is also possible to obtain selective agglomeration. Less well known is the fact that, more often than not, solids and immiscible droplets dispersed in aqueous solution are electrically charged because of preferential adsorption of certain ion species, charged organics, and/or dissociation of surface groups [B.48]. Depending on such variables as the nature of the material, its pretreatment, pH, and composition of the solution, these charges can be either positive or negative. Since the surface charges on particles are compensated by an equal but opposite
Fig. 8.12 a) Diagram of polymer bridging between particles; b) restabilized particles [B.29]
8.1 Collection, Stabilization, and Deposition of Particulate Solids Fig. 8.13 Diagram representation of two particles with electrical double layers in a liquid [B.48]
countercharge surrounding them (Fig. 8.13) an electrical double layer develops (Chapter 5). Even though, as a whole, the system is electrically neutral, repulsion between the particles occurs. Upon addition of an indifferent (non-adsorbing) electrolyte (a salt), the double layers become less active and, as a consequence, the particles can now approach each other more closely before repulsion sets in. If enough salt is added, the particles may eventually come so close that van-der-Waals attraction binds them together. This is, in principle, the explanation of the sensitivity of colloids and suspensions to salts and why flocculation may be caused by the addition of salts. For certain technical applications, electrocoagulators may be used to charge the solids in contaminated effluents [B.48]. Metal hydroxides are produced by a system of soluble electrodes (anodes) which, in suitable electrolytes, cause coagulation of suspended solid particles into larger flocs.
8.1.2
Size Enlargement by Agglomeration for the Stabilization and Disposal of Particulate Solid Wastes
Current legislation in most developed countries classifies as hazardous most fine and all ultrafine particulate solids separated and collected from fluids with environmental control devices of any kind. In many cases, the mere fact that solids are micron or submicron sized, constitutes a reason for this classification. Owing to their fineness, dusts and slurries or moist residues from pollution abatement that are or become dry particulate matter easily become airborne causing a renewed threat to the environment and to human and animal or plant life (secondary pollution). Also, the large surface area of fine particulate solids results in high solubility so that toxic or otherwise undesirable substances may leach out and spill into surface water or end-up in aquifers if such fines are stored outside or deposited in unprotected landfills. Therefore, a common task is to use agglomeration for the size enlargement of fine materials that result from pollution control measures. Emphasis in this case is on the production of large and heavy aggregates to avoid scattering by wind or water and of permanent bonds that are waterproof, survive freeze–thaw cycles, and, preferably, immobilize leachable compounds. Because in this section recycling and the manufacturing of secondary raw materials are not a topic (for that, see Section 8.2), the only purpose of size enlargement is to render fine particulate solids suitable for safe handling, storage, and disposal. The cost
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for carrying out the necessary processes is an extra burden and, consequently must be kept low. In Section 8.2, in connection with the size enlargement of low-grade ores or recovered fines for heap leaching, stockpile agglomeration is discussed as a cheap method. For the final disposal of terminal wastes, the same principle of agglomeration can be applied. Agglomeration is achieved at multiple transfer points from belts to belts (Fig. 8.14), on shaking or vibrating conveyor decks (Fig. 8.15), or on steeply inclined belt conveyors where material tumbles downward against the upward motion of the belt (Fig. 8.16) [B.48, B.97]. To obtain permanent, strong, and waterproof bonding, cement is often added as a low-cost, easily applicable binder. Agglomeration is initiated and bonding is activated by water sprays. Post-treatment for the production of final strength and long-term aggregate properties occurs in most cases naturally in curing piles prior to loading the material for transfer to the final disposal site, which does not need to be environmentally secured. Other low-cost processing of terminal waste may simply use mixing of the material with a suitable matrix binder (cement is again an option but hot mixing with bitumen Fig. 8.14 Sketch of belt conveyor agglomeration [B.48, B.97]
Fig. 8.15 a) Diagram of shaking trough; b) sketch of vibrating deck agglomeration [B.48, B.97]
8.1 Collection, Stabilization, and Deposition of Particulate Solids
Fig. 8.16
Reversed belt agglomeration [B.48, B.97]
is a common alternative) and forming the blend into crude, brick-like bodies that harden into inert disposable pieces. Depending on the hazard level, more sophisticated (and expensive) methods may have to be chosen that can finally include some sort of additional sealing by surface coating or encapsulation. In this context, encapsulation could mean, for example, the pressing of the prepared mass into drums or barrels (Fig. 8.17) that are closed after hardening of the contents has been completed.
Fig. 8.17 Heavy-duty drum compactor (courtesy S&G Enterprises, Germantown, WI, USA)
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In a growing number of cases, particularly if radioactive wastes are involved, glass is used as the matrix binder. Since, in this case, achieving safety requirements is more important than cost, rather elaborate processes are developed and used. They include the impregnation of evacuated porous agglomerates and, sometimes, also coating with liquid glass or the high-temperature post-treatment of agglomerates that contain ground waste glass. Although recycling, the production of secondary raw materials, waste minimization, and resource conservation (Section 8.2) are the declared goals of all industrial societies, useless, terminal waste is also produced in growing amounts, which needs to be processed to avoid the costly disposal on secured sites or in managed landfills. Size enlargement by agglomeration is a new and quickly growing field of activities for this application. Knowledge of the fundamentals of this unit operation [B.48] and of the methods and equipment used to accomplish a multitude of tasks [B.97] and their interdisciplinary evaluation allows the development of suitable processes for safeguarding the environment, nature, enjoyment of life, and the future of the Earth.
8.2
Recycling/Secondary Raw Materials
Since industrial wastes are commonly particulate solids, of which a large portion becomes increasingly finer, size enlargement is often required to render these materials suitable for beneficial use. Agglomeration, the sticking together of particles with the help of various binding mechanisms, results in products with controllable composition, strength, size, shape, density or porosity, and other desirable properties. The latter include lowered or increased reactivity, improved solubility or dispersibility, increased abrasion resistance, reduced dustiness, better flow and metering, or, generally, greatly enhanced handling characteristics.
8.2.1
Historical Review of Waste Production
People have mined the earth for thousands of years. Initially these activities were limited to the gathering of metal, mostly gold nuggets and gem stones from the surface, but soon digging and tunneling into the earth began and materials other than the desired ones were also excavated. At the same time crushing was introduced to liberate valuable ingredients from unwanted solids. Fines, produced during crushing, and tailings (residue) were discarded in waste piles or dumped into holes. The location of many of the earliest mines is revealed by the waste heaps found nearby. Between 4000 and 3000 BC the Bronze Age began: humans learned to extract copper and tin from ores by heat and produced the alloy bronze. Accordingly, mining activities increased and wastes were still simply discarded into heaps. Somewhat before 1000 BC, the Iron Age started in western Asia and Egypt as man learned to smelt
8.2 Recycling/Secondary Raw Materials
iron from its ores. Large amounts of smoke and fumes were emitted from the increasing number of small furnaces, but no one felt a need to capture or control it and slag was dumped. Relatively early during the history of mankind carbonaceous materials, such as peat, lignite, and other coals, were collected and later mined to be burnt for various purposes. Like other mineral mine waste, coal fines were discarded, often in ponds after washing the coal lumps. There are indications that even in ancient times poor people retrieved the fines, mixed them with fat or oil, and manually formed them into bricks that hardened during open-air drying, a method that can still be observed today in coal mining districts of less-developed countries and in China. Such manually recovered agglomerated coal fines were used for home cooking and heating. Nevertheless, and in spite of these isolated recycling efforts, until the early part of the 20th century and sometimes even today, wastes from mining, including fine coal, ores, and many other minerals, from the processing of organic materials, such as wood, plants of all kinds and types, and fruits, and from the production of industrial goods were mostly discarded. During the past 100 years this situation has drastically changed. Industrial processing and production, yielding wastes of all kinds, skyrocketed. The desire and later the requirement to effectively clean gaseous and liquid plant effluents created large amounts of dusts and other fine or ultrafine particulate solids. Today, many of these materials are classified as hazardous because they may again pollute the environment and can be inhaled or ingested and resorbed, causing health risks. At the same time, industrial production and output are no longer concentrated in a few developed areas but distributed worldwide. Therefore, solid waste collection, treatment, processing, recycling, and disposal have become very important (often mandatory) industrial activities whereby handling of the finest materials poses the biggest challenge and costliest efforts. In addition to the wastes produced during the collection, extraction, upgrading, and processing of natural raw materials, the quickly and still increasing production capacities in all industries result in large quantities of rejects and by-products, which inherently represent great value. Furthermore, the useful life of many industrial products is relatively limited and tends to become ever shorter. The obsolete items: equipment, such as machinery, cars, electronic components; packing materials, including paper, cardboard, foil, plastics, bottles, cans; wastes, for example newsprint, glass, wood, organic residue; and many more, contain a multitude of potentially valuable ingredients that must be separated and/or cleaned for further use. Tab. 8.4 summarizes the most important advantages of agglomerated solid mineral and metallurgical wastes. Bullet points 1 to 3 are self-explanatory since the particle size of most solid wastes is too small for direct processing or is ultrafine after they were removed from gaseous or liquid plant effluents as solid pollutants (Section 8.1). Because of their size-related large specific surface area, fine and ultrafine solid wastes, especially metallic fines or those containing metals or cellulosic dusts, are very reactive. Even at ambient conditions they combine easily with oxygen in an exothermic reaction that may cause accelerated, catastrophic heating or “dust explosions”. Size enlargement by agglomeration, accompanied by densification, sometimes
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Advantages of agglomerated solid wastes
*
Agglomerated particulate solid wastes contain no or low amounts of dust; therefore, they provide increased safety during handling, even of hazardous materials, no secondary pollution, and, generally, fewer losses.
*
Agglomerated solids are freely flowing, featuring: – Improved storage and handling characteristics, – Better metering and dosing properties, – Increased bulk density and lower bulk volume.
*
Agglomerates have larger sizes and their size distribution can be controlled.
*
Within certain limits, depending on the process used, product porosity or density can be influenced; leachability and reactivity as well as other properties can be controlled.
*
Secondary raw materials can be crafted by: – Enlarging the size of wastes containing valuable ingredients, – Combining different solid waste components to obtain improved feed materials with controlled composition, – Including additives to bring about desirable processing characteristics.
hot, renders such dusts sufficiently inactive for safe handling and use. Terminal solid wastes, destined for final storage, sometimes contain toxic or other hazardous components which, if untreated, may become airborne or leach into groundwater, requiring lined and/or covered disposal sites. Size enlargement by agglomeration using permanent binders, often in the form of a matrix, filling all interparticle voids, eliminates the need for special conditions for the deposition of such materials. 8.2.2
Agglomeration Technologies for the Size Enlargement of Wastes
A common classification of methods for the size enlargement of particulate solids distinguishes between two types of process (Chapter 5), * *
natural and growth or tumble agglomeration (no external forces) and pressure agglomeration (low, medium, or high external forces)
and two techniques, binderless agglomeration and * agglomeration with the addition of binders. *
For fine metal ores and, more recently, for iron-bearing waste materials, agglomeration by heat (sintering), a less frequently used size-enlargement technology, is used. All agglomeration methods can be used to manufacture products for recycling and secondary raw materials. Growth/tumble agglomeration happens in a similar fashion to natural agglomeration. Because the particles to be agglomerated are larger (surface equivalent diameter 10 < xo < 200 lm [B.97]), the particle-to-particle adhesion must be enhanced by binders and the collision probability must be increased by providing a high particle con-
8.2 Recycling/Secondary Raw Materials
centration. In most cases, growth/tumble agglomeration first yields “green” agglomerates, which are only held together temporarily by liquid binders. Final strength and potentially other product characteristics are obtained during post-treatment. Because of the growth mechanism, the agglomerated bodies are more or less spherical. If a narrowly sized product is desired, screening is used. Oversize (after milling) and undersize are recirculated to the agglomeration equipment. Pressure agglomeration processes use external forces to enhance bonding by densification and to shape the product. As far as applicability is concerned, high-pressure agglomeration is the most versatile technology for the size enlargement of particulate wastes. If certain characteristics of the feed materials and conditions occurring during densification are considered and controlled during equipment selection, design, and operation, any kind and size (from nano- to millimeters) of particulate material can be successfully processed. Since high-pressure agglomeration is essentially a dry process, a limitation exists with regard to the highest tolerable moisture content of the feed. Filter cakes, for example, a common initial form of waste, can not be agglomerated by high pressure without first reducing the moisture to very low levels by additional drying.
8.2.3
Applications in the Mineral, Metallurgical, and Energy Related Industries
In the mineral and metallurgical industries, resource conservation by the recovery of valuable components from waste and the recycling of materials containing recoverable ingredients has already reached a high level of acceptance. New recovery technologies make the secondary processing of old mine waste deposits, particularly of ores and coal, economical and deposition of new wastes is minimized by the application of methods that convert a host of different, formerly obsolete materials into “secondary raw materials”, which are recycled, replacing primary raw feed sources. The latter also include iron and non-iron scrap, turnings, borings, grindings, dusts, etc. Mixing different raw materials, additives, and binders, agglomerating the blend, and subjecting the product to different post-treatment methods to achieve special properties is known as material engineering. Industrial wastes can be included in such raw materials and additives can produce, for example, a fluxed feed for metallurgical operations, secondary raw materials with predetermined alloying ingredients, or smokeless fuels (Sections 6.8, 6.9, and 6.10). Tab. 8.5 lists the areas in which size enlargement by agglomeration can benefit solid waste management in mineral and metal processing. Waste minimization is a preventive measure by which particulate solids are agglomerated to avoid pollution in the first place. Examples are in the bulk handling and transportation of naturally dusty minerals. In pollution control agglomeration has many applications. In most cases, collection of fine solids is improved or the removal of ultrafine particles is made possible (Section 8.1). Examples are the agglomeration of smoke particles or the flocculation of solid contaminants in effluent water. By agglomeration, by-products can be converted into materials that are suitable for beneficial use. Examples are fertilizers containing FGD
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8 Applications in Environmental Control Tab. 8.5 Areas in which size enlargement by agglomeration benefits solid waste management *
Waste minimization
*
Pollution control
*
Beneficial use of waste by-products
*
Recycling/reprocessing
*
Production of secondary raw materials
*
Terminal wastes disposal
*
Prevention of secondary pollution
(flue gas desulfurization) ammonium sulfate or pulverized Thomas slag, lump gypsum from FGD gypsum for use in cement production, and building materials from ashes. Recycling identifies the re-utilization of off-size solids after agglomeration in the same process in which this material was originally rejected. Examples are metal-bearing dusts from smelters, metal turnings, borings, grindings, etc., and coal, ore, and mineral fines from crushing and/or beneficiation. With modern processing technologies, many fine or low-grade raw materials which for decades and sometimes centuries were discarded at the mine or former processing sites can be used after recovery and agglomeration for reprocessing. Examples are fines deposits of rejected bauxite, coal, and ores (copper, gold, iron). With the exception of waste minimization all aforementioned applications are part of “resource conservation” by increased emphasis on secondary raw materials, process input obtained from wastes, rejects, and obsolete products, in most instances involving agglomeration. In some cases, the nature of terminal solid wastes, those which can no longer be processed for beneficial use, require agglomeration for disposal (Section 8.1). One reason for this can be to avoid leaching of toxic or radioactive components into the groundwater by the application of matrix binders such as cement or glass for stabilization. Another is to avoid secondary pollution because, after separation from plant effluents, many terminal wastes contain very fine particles in dusts, slurries, sludges, or slimes. Size enlargement by agglomeration is increasingly required to avoid costly disposal in scarce, small, specially designed hazardous waste deposition sites. Some examples will be presented in more detail. They are selected such that, with the exception of natural agglomeration (used, for example, for the densification of silica fume, Section 6.7.3), at least one type of the available agglomeration processes is demonstrated. Concerns regarding acid rain led to legislation in all industrialized countries aimed at limiting the sulfur content of flue gases, typically from power plants (Section 6.10.2, compliance fuel) and metallurgical processes. One of the most widely used desulfurization processes uses limestone as absorbent. The resulting calcium hydrogen sulfite is converted by oxidation into (synthetic) gypsum, called FGD gypsum (calcium sulfate dihydrate). Fig. 8.18 shows an absorber and associated processing facilities in a coalfired power plant.
8.2 Recycling/Secondary Raw Materials
Fig. 8.18 Sulfur absorption in the flue system of a coal-fired power plant (EVS, Heilbronn, Germany): left) absorber; right) limestone processing and gypsum recovery (briquetting) systems
The product from such a desulfurization system is a slurry and, after mechanical dewatering, a filter cake, containing finely divided solids. Disposal of the wet FGD gypsum is expensive and can result in secondary pollution if it becomes dry and is not stabilized. It has been found that, after agglomeration, the material can be used in cement kilns (replacing natural lump gypsum), economically transported to and used in plants producing gypsum board for the building industry, or safely discarded. Principally three methods are used to convert the finely divided moist gypsum into the required lumpy form, all requiring no additional binder. 1. Tumble agglomeration of the moist gypsum in discs, drums, or mixers (Chapter 5, Fig. 5.2) and hardening the green, spherical pellets by thermal curing. 2. Densification of gypsum with reduced moisture content (8–10 %) in pellet mills (Chapter 5, Fig. 5.10b1–b6) and hardening the green cylindrical pellets by thermal curing. 3. Drying the gypsum to < 1 % moisture content and pressing and shaping it into finished, pillow-shaped briquettes with roller presses (Chapter 5, Fig. 5.11, lower right).
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Fine particulate solids, dry or moist and from any source can be converted into larger products by growth/tumble agglomeration. The example shown in Fig. 8.19 depicts a simple system using an inclined disc for agglomeration, a rotary kiln for post-treatment, and screening and crushing for finishing. Moist particulate solid wastes, such as predried FGD gypsum filter cake, can be extruded (Fig. 8.20) in, for example, flat die pellet presses (4 in Fig. 8.20) yielding cylindrical agglomerates, which are dried and screened prior to load-out. Since in high-pressure agglomeration with roller presses densification is so high that the residual porosity can only hold very small amounts of moisture (Chapter 5),
Fig. 8.19 Size enlargement of particulate solid waste by growth/ tumble agglomeration (courtesy Eirich, Hardheim, Germany)
Fig. 8.20 Pelleting of wet FGD gypsum by medium-pressure agglomeration (flat die pelleting, courtesy Amandus Kahl, Reinbek, Germany)
8.2 Recycling/Secondary Raw Materials
if this technology is used, the FGD gypsum must be thermally dried to < 1 % moisture content prior to briquetting. The latter is very simple as shown in Fig. 8.21. To avoid starved feeding of the roller press and guarantee the production of high-quality briquettes, a small stream of excess material overflows at the end of the horizontal conveyor, is measured by a solids flow meter [B.97], and controls the discharge from the day bin, that is, the amount of press feed. Fig. 8.22 depicts photographs of a roller press in a FGD gypsum briquetting plant and of product samples. Unless only fine dusts must be treated, the more versatile processes for size enlargement in recycling and particularly the secondary raw materials industry use pressure agglomeration technologies. Especially with the high-pressure machines (ram extrusion, hydraulic punch-and-die, and roller presses, Chapter 5, Fig. 5.11) a wide variety of feed materials and particle sizes can be processed. The sometimes very large energy input results in extensive densification, yielding high molecular forces for bonding, disintegration of brittle particles followed by recombination bonding, momentary melting of roughness peaks at the contact points, resulting in solid bridges, and plastic deformation of malleable components. The latter can be enhanced if hot feed materials are used (Sections 6.6.2 and 6.9.2). As early as 1885, briquetting of iron-bearing fine residues from cupriferrous pyrites after roasting and the extraction of copper was carried out commercially for recycling in the UK [Section 13.3, ref.103]. The processes used punch-and-die presses and a binder, producing relatively large, brick-like pieces that were subsequently fired to obtain final strength. Around the same time, great success was reported in the bri-
Fig. 8.21 Flow diagram of a FGD gypsum roller press briquetting plant. Thermal drying of the feed is not shown
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Fig. 8.22 Roller press for the briquetting of (synthetic, FGD) gypsum in a flue gas desulfurization plant: inset, synthetic gypsum briquettes (courtesy K€ oppern, Hattingen/Ruhr, Germany)
quetting of blast furnace dust since this by-product contains lime, alumina, and soluble silica and, therefore, like cement, already possesses hydraulic binding properties. However, although they had been considered as quite important at the time, these early technologies were soon abandoned because, due to increasing costs of labor, energy, and, where applicable, binder, it became more economical to simply dump waste materials into industrial landfills. The desire for environmental protection and the growing need to conserve raw materials, such as, fuels, ores, or strategic alloying components, and natural resources, such as land and water, have revitalized these efforts, leading to new processes utilizing modern equipment and technologies. Therefore, today’s briquetting systems have little in common with early brick-making procedures. Fig. 8.23 is the flow diagram of a briquetting plant for metallized, metal-bearing, or other recyclable fines. The possibility of adding solid or liquid binders, either alone or in combination, render this simple system very versatile. As shown, the feed material(s) is (are) dry and cold and the binders are self-curing, that is, they gain final strength at ambient temperatures in a ventilated (to remove heat, moisture, and gaseous reaction products) briquette storage bin. Incorporation of certain materials, such as oxide fines or certain mill scales, and/or restrictions on binder amount or composition may require the installation of a curing oven. Typically the briquetted product is pillow shaped, strong, free of dust, and can be easily stored, handled, and metered during recycling (Fig. 8.24).
8.2 Recycling/Secondary Raw Materials
Fig. 8.23 Flow diagram of a generic briquetting plant for metallized, metal-bearing, or other recyclable fines
For recycling and the manufacture of secondary raw materials, successful size enlargement by agglomeration often hinges on finding a binder that is chemically (no contamination) and economically (low cost) acceptable and physically effective (adequate strength, high abrasion resistance, good flow and metering characteristics). The addition of fibers or other elongated particles is an innovative method for reinforcing agglomerates [B.97].
Fig. 8.24 Close-up view of a pile of typical briquettes produced by roller presses for recycling
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Fig. 8.25 Flow diagram of a plant for the production of paper fluff from old newsprint, as a binder for use in a roller-press briquetting plant [8.2.1]
A simple, low-cost fibrous binder component that is available everywhere can be obtained by grinding waste paper. Fig. 8.25 is the flow diagram of a plant for the production of paper fluff from, for example, old newsprint as a binder for use in a roller press briquetting plant. This cellulosic material mixes quickly and uniformly with dry powders in any kind of blender. Even small percentages (2 %) result in a marked increase in (crushing) strength if compared with two other common binders (Fig. 8.26) [8.2.1].
Fig. 8.26 Effect of binder type on the crushing strength of briquettes made from metallurgical dust [8.2.1]. Amounts of binder added: waste paper, 2 %; molasses, 6 %; lime, 6 %; starch, 6 %
8.2 Recycling/Secondary Raw Materials
Another application of “fibers” as a reinforcing binder component is the addition of grinding swarf during the briquetting of metal-bearing filter dust [B.48, B.97]. Up to 50 % of high-grade steel grinding swarf < 30 mm and a small amount of the conventional binder lignosulfonate powder (sulfite waste from paper making) were added to residue (filter dust and sludge) from specialty (alloyed) steel production. Since the product is destined for recirculation into the metal-making process, it is important to produce high strength of which at least a certain part is retained at high temperature, until the surface of the liquid metal bath is penetrated and melting occurs. Thereby, secondary contamination because of premature release of dust is avoided. This is achieved by the presence of reinforcing fibers in the structure. Fig. 8.27 indicates that the briquette strength increases with addition of swarf while the necessary amount of chemical binder, constituting contamination and non-temperature resistant bonding, decreases. During simple atmospheric curing a considerable increase in strength occurs because of hardening of the chemical binder component, which improves the product’s handling and storage properties. Fig. 8.28 shows a broken cylindrical compact that was manufactured during process development with a laboratory punch-and-die press and actual briquettes obtained in an industrial plant. Concerns that, when produced with a roller press, the addition of “fibers” would prohibit separation into mostly single, handleable pieces (see also Section 6.9.2) were unfounded. Most dusts from pyroprocessing plants in the minerals and metals industries contain calcium and/or magnesium oxides. They hydrate easily, especially also during storage at high or even ambient humidity, only more slowly in the latter case. Since this reaction is coupled with a large increase in volume, when natural hydration occurs, with time even small amounts (> 0.5 %) of alkaline earth oxides tend to lower the strength of any agglomerate, including high-quality briquettes, and, in many cases, eventually result in complete destruction. Therefore, if such components are present, an appropriate amount of water is mixed with the dust in a preparatory step
Fig. 8.27 Cold crushing strength of briquettes from metal-bearing dust and sludge containing different amounts of dry lignosulfonate binder (called “sulfite waste powder”) with and without reinforcement by the addition of swarf [B.48, B.97]
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8 Applications in Environmental Control
Fig. 8.28 a) Broken cylindrical compact that was manufactured during process development with a laboratory punch-and-die press; b) actual briquettes obtained in an industrial plant [B.48, B.97]
and the oxides are allowed to hydrate prior to agglomeration. If afterwards additional moisture is required alone or as part of a binder system, a second blender must be installed in the agglomeration plant. Because the design of agglomeration plants, especially of those using briquetting, can be very compact, they are often suitable for installation at or near the dust-collection system(s) (Fig. 8.29). They are fully automatic, needing practically no operator attention, and can be monitored from a nearby control room. In the case of dust recirculation from specialty steel production, the briquettes that were reinforced with alloyed steel grinding swarf were an excellent feed addition for electric arc fur-
Fig. 8.29 Compact briquetting system for metallurgical filter dust. Capacity 11–20 t/h, depending on feed characteristics (courtesy: Thyssen Stahl AG, Krefeld, Germany)
8.2 Recycling/Secondary Raw Materials
naces. Some 98 % of the chromium and 99 % of the nickel contained in the residue can be recovered by this practice [Section 13.3, ref. 103]. Fig. 8.30 shows how, by employing size enlargement by agglomeration, almost all solid wastes within, for example, the steel industry can be recycled (furnace and converter dusts) or processed into secondary raw materials (Waelz oxide, sinter), resulting in very little terminal residue going to disposal or to outside use (road building material). Hot and cold briquetting with roller presses are the predominant methods for size enlargement in this case because it can handle materials of varied composition, sizes, and properties. The rotary kilns shown in front of the hot briquetting machines are used for heating the feed to the required temperature. Alternative methods of heating can be chosen, too. Other agglomeration processes shown in Fig. 8.30 apply hot nodulizing (a tumble/growth method) in the Waelz kiln and sintering (agglomeration by heat). Sometimes, when wastes are recycled, technological, physical, and/or chemical, limitations must be considered. For example, it is not possible to continue re-using steel mill dust (as shown in the right part of Fig. 8.30) indefinitely. Because of decrepitation and attrition, this would result in the production of an ever increasing amount of new dust and briquettes. Therefore, a maximum amount of briquettes in the feed per charge must be determined and the rest must be sold to other users. Also, an
Fig. 8.30 Size enlargement by agglomeration in iron and steel making for the minimization of terminal residue through recycling and the production of secondary raw materials [Section 13.3, refs 129, 166]
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enrichment of contaminants, particularly zinc, takes place in the dust with each cycle. To determine its level, dust must be sampled and analyzed regularly. Once a certain concentration of zinc is reached, the entire dust that is in the system at this time is passed to the Waelz plant. The Waelz oxide is briquetted hot and treated in an Imperial Smelting (IS) shaft furnace to recover zinc and lead. As an example [8.2.2], in one plant, 3–4 t of briquettes are charged per heat and enrichment from about 5 % to more than 17 % of zinc occurs in 3–4 weeks. At that time, 350–400 t of briquettes are removed from the cycle for reprocessing in the Waelz kiln. Certain processes shown in Fig. 8.30, such as the manufacturing of Waelz oxide and the recovery of zinc and lead, can not be supported by and operated economically in a single steel mill. Therefore, independently owned plants are often processing the waste from several producers and are, preferably, located in the geographical center of an industrialized area where suppliers of the metal-bearing dusts, users of the recovered metals, and customers for the by-products (road building materials) are close by. Also, with a trend away from the giant integrated steel mills towards smaller regional producers and the application of alternative iron units for steel making (direct reduced iron, Section 6.9), the treatment and recovery of metallurgical wastes is no longer economically feasible in these new facilities, although, in plants that are also using rolling mills and wire drawing operations for the manufacturing of intermediate products such as slabs, sheets, rods, and coils, they may additionally include mill scale and metal grindings, sludges, and slimes. This situation is generally found in all industries generating recoverable and recyclable waste. In response, maximum amountolling processors (Section 9.2) and specialized service companies are being created to take wastes and convert them into beneficially useable products or treat them for safe disposal. These companies fulfil the desires, requirements, and/or charter of the industry and of governments to conserve resources and to reduce the need for increasingly expensive disposal and scarce, often specially designed and protected deposition sites. While many are serving a small number of customers and are strictly of regional importance (see below), some operators are becoming large, even multi-national, globally active corporations. An example of a large, multi-national service provider is Heckett MultiServ (Section 15.1). In their own words, this company, “delivers specialist services to more than a quarter of the world’s steel production, operating at over 160 sites in 33 countries and employing more than 9000 people. (Currently) every year (they) process 40 million tons of slag and debris, recover 9 million tons of steel, handle 11 million tons of scrap, and transport 10 million tons of liquid slag and 6 million tons of liquid steel. (Heckett MultiServ) design, build, and operate facilities for materials handling and preparation, waste processing, and environmental services ... (They work) within customer’s own premises and plants, providing tailor-made services using specialist technologies and equipment operated by their own highly-trained people”. The above indicates that the services offered by this group include much more than processes that are related to size enlargement by agglomeration. In regard to the latter the corporation states that they have “extensive experience with a variety of steel industry by-products and that all processes offer the recovery of iron units and valuable metal-bearing components as well as reduced disposal costs”. Using binder systems,
8.2 Recycling/Secondary Raw Materials
both pellets (Fig. 8.31) and briquettes (Fig. 8.32) are made from the most appropriate by-product blends, meeting the demands of the iron and steel making process and the local environment. Tab. 8.6 lists materials that are successfully pelletized or briquetted by Heckett MultiServ around the world. Fig. 8.33 depicts stacks of pellets (left) and briquettes (right) that are ready for recycling. Pelletizing (Fig. 8.31) is applied to convert waste and by-products from a steel mill that also operates a sinter plant into agglomerates with improved handling characteristics. The pellet composition is site specific. It depends on the number of components to be incorporated and their physical and chemical characteristics, including moisture content. By preparing the materials in this way, not only is recycling achieved but the productivity of the sinter plant is also enhanced. The inputs and ratios depend on the mill’s specifications for so-called control elements, such as zinc, alkali, and oil. To date, Heckett MultiServ operates two pelletizing plants, both in Europe, with annual capacities of 100 000 and 200 000 t/year.
Fig. 8.31 Disc pelletizer for the recycling of waste dusts, slurries, and plant fines (courtesy Heckett MultiServ, Butler, PA, USA)
Fig. 8.32 Briquetting plant for and typical briquette from steel mill by-products (courtesy Heckett MultiServ, Butler, PA, USA)
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8 Applications in Environmental Control Tab. 8.6 Materials that are successfully pelletised or briquetted by Heckett MultiServ (Section 15.1) around the world *
Sinter plant dusts
*
Blast furnace dusts
*
BOS grits and sludges
*
Magnetic fines from desulfurization and steel slags
*
Mill scale and oily mill scale sludges – EAF dusts – AOD/CLU/VOD (converter) dusts – Grindings and swarf
Fig. 8.33 Stacks of pellets (left) and briquettes (right) that are ready for recycling (courtesy Heckett MultiServ, Butler, PA, USA)
Briquetting (Fig. 8.32) is used for the recycling of particulate wastes and by-products in steel mills if a sinter plant does not exist or the inputs are unsuitable for the sinter plant/blast furnace route. In addition to the accurate proportioning and mixing of the materials with a suitable binder, control of the blend’s moisture content is critical. At the present time Heckett MultiServ is operating seven briquetting plants with tonnages ranging from 7500 to 150 000 t/year in Canada and Europe and is in the process of installing another one in Australia.
8.2.4
Applications in Regional and Municipal Material Recycling Plants
As a further example of major recycling efforts that use agglomeration methods to convert sorted wastes into recyclable secondary raw materials, regional material recycling facilities (MRF) [8.2.3] and municipal waste processing plants shall be discussed. Beginning in the 1980s, MRF were developed in several countries (USA – MRF; Germany – DSD, Duales System Deutschland) to separate, through process design, tech-
8.2 Recycling/Secondary Raw Materials Tab. 8.7 Commonly considered advantages of material recycling facilities [8.2.3] *
Since MRFs permit co-mingling, public participation is made easier as less sorting is required by the waste generator. This leads to higher recycled volumes and unburdens the conventional municipal solid waste collection and processing.
*
Collection costs can be reduced since less expensive equipment can be employed and time consuming sorting at the curb is eliminated or reduced.
*
Higher volumes of recyclable materials offer greater market influence and acceptance since industry quality standards can be better met by producing superior forms of secondary raw materials.
nology, and human inspection, co-mingled recyclable materials and convert them into marketable commodities. These facilities are in addition to the conventional municipal solid waste processing plants. While for the latter wastes are picked up indiscriminately from households or collection points, the feed for MRF, although still more or less co-mingled, has been pre-sorted by the originators and is collected from curb sides or special collection containers. Tab. 8.7 lists the three principal characteristics of MRF that are commonly considered as advantages and Tab. 8.8 provides a breakdown of MRF separated recyclables in a specific plant, typical processing costs, and expected market prices of the resulting secondary raw material [8.2.3].
Tab. 8.8 Breakdown of MRF separated recyclables in a specific plant as well as typical processing costs and expected market prices (US Midwest, end of 1992) of the resulting secondary raw material (adopted and modified from [8.2.3]) Material
% of total
Paper Newsprint Cardboard Mixed
50
Cans Steel Aluminum (UBCs)
10.50
Glass Clear (flint) Brown (amber) Green Mixed
29.00
Plastic HDPE PET Polystyrene Foil
7.75
Waste Residue
2.75
Processing cost
Market price
US$/t
US$/t
33.59 42.99 36.76
0–20 10–30 5–25
(6.4) (4.1)
67.53 143.51
49–78 500–900
(11.8) (3.9) (4.7) (8.6)
72.76 111.52 87.38 50.02
50 25–40 5–15 0–30
(5.1) (2.5) (0.08) (0.07)
187.95 183.84 – –
40–160 40–200 – –
–
–
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Tab. 8.8 shows that the most valuable material in MRF recycling streams are used (aluminum) beverage cans (UBC) which, with the exception of paper (see below), represent the only material that, at the assumed market conditions, can be sold for a profit. More recently polymer recycling has become feasible too (see below). As discussed in Section 6.9.2, the aluminum industry has one of the most advanced and complete recycling programs in the metals market [Section 13.3, refs 120, 132, 137]. The major reason for this is that the production of aluminum from Bauxite requires large amounts of electrical energy so that, today, most of the producers are located in areas where cheap (hydroelectric) power is available. Transportation from these locations to the finishing mills adds substantially more to the cost of the virgin material. Aluminum is becoming one of the major metals used for packing, replacing steel and, in many cases, glass. As a result, a large segment of aluminum recycling is based on the collection and processing of used beverage cans (UBC) and other aluminum (food) containers [Section 13.3, ref. 132]. Competitively priced high-quality can stock can be made from re-melting aluminum containers after they have been decoated (delaquered) by solvent or, more commonly, thermal processes. The objective of decoating is to remove paint, lacquer, plastic, paper, and other contaminants (food residues) with minimal modification of the metal. The nature and amount of the coatings vary widely. They include organic chemical compounds, which are predominantly volatile (VOC, volatile organic compound), inorganic components, which are added for coloring and mass, plastic or paper laminates, and oil and water from machining and forming operations. After proper decoating, it is also possible to recycle, in addition to UBC, such materials as clean foils, printed foils, painted packaging, paper- and plastic-laminated foil, food containers, litho plate, and contaminated extrusions. The raw aluminum scrap separated, for example in an MRF, from the other materials is rather voluminous and, if decoating is not available on site, which is the most common situation, are baled, an agglomeration process typically using hydraulic presses (Section 6.9.2, Fig. 6.9.19) for transportation to the thermal treatment facility. There, the key to efficiently producing a quality decoated aluminum scrap product is feed preparation, which includes shredding during which bales are broken-up and larger parts are reduced in size to less than about 50 mm. This material is then subjected to screening to remove fines < 3 mm, which are discarded, and to magnetic separation. Decoated aluminum scrap leaves the thermal treatment unit in particulate form with a temperature of about 500 8C. Since, in most cases, the remelting furnaces are also at a different location, the product is again baled (hot) and sold as premium “old” aluminum scrap to secondary aluminum smelters. A disadvantage of using baling at this point is that, prior to feeding the scrap into remelting furnaces, these bales must be torn apart again because: * * *
the bales are too large for direct charging, operators want to make sure that the scrap quality is consistent throughout, and bales may have picked up excessive amounts of moisture, which must be removed.
8.2 Recycling/Secondary Raw Materials
A disadvantage of using the resulting loose scrap, although found of acceptable quality, is that considerable oxidation losses (dross) occur during melting. As an alternative to baling, particulate decoated aluminum scrap can be converted into highly densified (>2.0–2.3 g/cm3) slabs using roller presses (Section 6.9.2) [Section 13.3, ref. 140]. Fig. 8.34 shows in the upper part different processed MRF aluminum wastes. They are from left to right: light foil, dense foil, and UBC. Densifying these materials in roller presses as described in Section 6.9.2 yields products as depicted in the lower part of Fig. 8.34. Although UBC do not reach a density of more than 2.0 g/cm3, even if they are hammer-milled or mixed with other, heavier aluminum wastes before compaction, a considerable improvement of recovery is obtained with a product as shown in the right lower part of Fig. 8.34. The slabs are particularly advantageous as feed for reverberatory furnaces. They can be easily charged and the metal losses experienced during the melting of loose, particulate aluminum scrap, which typically amount to 15–20 %, are reduced to 2–4 %. In general, for the compaction of clean processed metal swarf, two methods are available. The non-continuous confined volume punch-and-die process and the continuous compaction in the nip of two counter-rotating rollers (Section 6.9.2). With the punch-and-die process clean, delaquered, and crushed UBC can be compacted to yield a relatively high apparent density. The product shape is normally cylindrical with diameters between 80 and 190 mm, heights between 30 and 120 mm, and weights between 0.4 and 7.3 kg (Section 6.9.2). Although the punch-and-die technology is by far the most widely applied choice for the compaction and shaping of UBC-based aluminum scrap it also has a number of disadvantages of which the most important is the relatively slow movement of the (hydraulic) rams. Therefore, it is used when relatively small amounts of processed aluminum cans and containers must be compacted. Influenced by developments in connection with the briquetting of aluminum home scrap (Section 6.9.2), the continuous compaction between two rollers is now a feasible
Fig. 8.34 Three different MRF aluminum wastes as collected and processed (top) and after roller press compaction [Section 13.3, ref 132]. From left
to right: loose foil, densified (granulated) foil, delaquered and hammer-milled used beverage cans (UBC)
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alternative. It offers the advantages of large capacity and high apparent density. However, since an endless sheet is formed with these machines, there is no hope of producing directly individual compacts, such as cylindrical briquettes from the punch-anddie process that can be easily charged into the remelting furnaces. Similar to the solution presented in Section 6.9.2 a suitable cutting device (shear) must be employed to divide the strip into slabs, pieces with acceptable dimensions. Other examples of regional processors and recyclers are modern modifications of the conventional municipal waste disposal plants. Until recently there were two alternatives for the removal, handling, and elimination of municipal wastes. Originally, after pick-up from the curb or from specific collection points, the entire mass was disposed of in landfills, either filling natural or excavated holes or building mountains, which, after completion, were covered with dirt and topsoil and left to rot. Aside from the scarcity of suitable land near population centers to accept the exponentially growing masses of waste, problems arose with time from the leaching of harmful chemicals into aquifers and the production of increasing amounts of methane within the waste. These required the rehabilitation of many of the older, sometimes already abandoned landfills and triggered environmental laws, severely restricting the use of existing and opening of new landfills and/or requiring special design considerations (such as leakproof liners, collection and removal or use of methane gas, etc.) for new disposal sites thus making this method expensive and unattractive. Efforts to entice consumers (households) to segregate wastes into recyclable components, such as paper, metals, and plastics (see above), and rejects, reduced the amount of materials to be disposed of but created mostly organic discards, which become smelly and produce gases and other environmental contaminants if not properly handled and stored. The logical solution to this problem, the municipal waste incinerator, often faces public opposition. Therefore, communities and waste processors, supported by new laws that require the conversion of solids with a certain heating value into (industrial) fuels, are forced to develop new methods for further segregation of wastes and the processing of the resulting components. Since not all regions have adopted the separate collection of recyclables and not all waste generators accept the desire to segregate, the first processing step in a modern municipal solid waste treatment plant is to segregate the waste by scalping (screening, size), manual and visual selection (paper and paper products, wood, plastic containers), shredding of large pieces, re-screening (size), magnetic separation (metals), jigging (density, plastic), etc. Metals, divided into magnetic and non-magnetic (aluminum) varieties, are sold as iron-bearing (magnetic) and non-iron, heavy scrap or transferred to aluminum scrap processing (see above). Increasingly, natural and man-made organic materials that can be burnt are converted into a secondary solid fuel product. The latter is exemplified with the description of a recently installed (1999) large plant in Schwarze Pumpe, Germany, for the production of pellets from municipal (household) solid waste, the light fraction from a shredder plant, DSD (see above) plastics, and waste wood (chips, saw dust, crushed, or reduced to fibers). Typically, the mixture to be pelleted consists of 50 % household waste, 10 % light shredder fraction, 25 % DSD plastics (with up to 75 % foil), and 15 % wood. Depending on the composition of these ingredients (Tab. 8.9), the feed can be coarse and light (20–60 mm long,
8.2 Recycling/Secondary Raw Materials Tab. 8.9 Approximate compositions of two common feed mixtures to the pelleting machines of the secondary solid fuel manufacturing plant at Schwarze Pumpe, Germany (Courtesy Amandus Kahl, Reinbek, Germany) Description/Component
Light mixture
Fine mixture
Size range Thickness Moisture content Bulk density
[mm] [mm] [mass %] [t/m3]
20–60 <5 approx. 10 0.06–0.25
0–20 <5 approx. 10 0.06–0.46
Paper, cardboard Organic waste Composit (packing) materials Wood Foils Hygiene articles Plastics Glass, stone, sand, ash Textiles Foams, rubber, felt
[%] [%] [%] [%] [%] [%] [%] [%] [%] [%]
22–50 8–15 7–16 1 5–15 <3 5–15 <1 1–7 approx. 1
5–10 50–70 0.5–2 { { { 20–25 { <1 –
< 5 mm thick, bulk density < 0.25 t/m3) or fine and somewhat heavier (0–20 mm long, < 5 mm thick, bulk density < 0.46 t/m3).
Fig. 8.35 Flow diagram of the pelleting section of the secondary solid fuel manufacturing plant at Schwarze Pumpe, Germany (courtesy Amandus Kahl, Reinbek, Germany)
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Fig. 8.35 is the flow diagram of the pelleting section of the plant that is equipped with two identical lines. Each line (Fig. 8.36) features four flat die pellet presses [B.48, B.97], which are fed volumetrically from a common chain conveyor and individually with screw feeders, in which, if applicable, a liquid binder (molasses) can be added. Although a liquid binder system is installed, pellets with good quality can often be produced without a liquid additive if up to 15 % wood fiber is present in or added to the feed mix. Each press produces up to 2.5 t/h of green (< 10 % moisture content) pellets with a diameter of 16 mm and 50–70 mm in length, which are collected and transferred to a belt cooler. During pelleting heat is produced by the conversion of mechanical into thermal energy and friction, which is removed in the coolers. At the same time, some of the moisture is evaporated, increasing the heating value of the product. Problems can arise from the presence of metals, stones, long items (video tapes, stockings, foils, etc.), non-compactible materials (glass, hard rubber, and similar materials with a thickness > 5 mm), tropical hardwood, and broken plywood. Tab. 8.10 describes some of these limitations in more detail. For the safety of the pelleting system it is imperative to limit the tramp material contents in the feed accordingly. With the increasing availability of materials that should, preferably, no longer be deposited in landfills and are recyclable, entrepreneurs are developing new regional processing plants that serve particularly the building industry. The manufacturing of artificial aggregate, dense and lightweight, is currently the most common application. Fig. 8.37 is the generic block diagram of a versatile plant using extrusion [B.48, B.97] for the agglomeration step in the production of lightweight aggregate from waste materials [8.2.4]. To achieve bloating during firing of the green extrudates, chemistry within certain ranges is required. Materials that meet all or most of these criteria are clay and clay-like wastes, including fly ash, bottom ash, contaminated river, lake, and harbor bottom solids, and fines resulting from igneous rock washing. In addition to these primary wastes, secondary materials can be added, such as various waste sludges. Sludge may make up as much as 25–30 % of the dry weight of an aggregate mix. However because drying is not generally economical, the sludge can carry at most the amountof waterthat is requiredfor the extruder feed. Solid fuel (coal) fines add heating value that will lower the process energy requirement during the firing stage. Other new developments aim to enhance the value of the materials to be recycled while producing the secondary raw material. In particular, iron-bearing solids from Tab. 8.10 Limitations on the contents of tramp material in the feed to the pellet presses for the production of secondary solid fuels from waste (Courtesy Amandus Kahl, Reinbek, Germany) *
Stones and glass with diameters of up to 2 mm are acceptable without influencing operation.
*
Stones and glass with diameters < 10 mm are acceptable but may reduce die life.
*
Removal of ferrous, particularly also stainless steel, and non-ferrous metal parts from the feed must be guaranteed.
*
The presence of up to 3 % of oversized material, other than mentioned above, does not reduce the plant’s performance. Higher percentages and long items (see text) may cause a number of problems resulting in unscheduled downtime.
8.2 Recycling/Secondary Raw Materials Fig. 8.36 Four flat die pellet presses of the secondary solid fuel manufacturing plant at Schwarze Pumpe, Germany (courtesy Amandus Kahl, Reinbek, Germany)
Fig. 8.37 Generic block diagram of a versatile plant using extrusion in the production of lightweight aggregate from waste materials [8.2.4]
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different sources are reduced to yield a value added material containing a certain percentage of metallic iron. The mostly gas based direct reduction processes for iron ores (Section 6.9.2) are only economical if operated at large scale (>500 000 t/y). Regional recycling plants must be profitable at much lower capacities. One possibility to achieve this is using a rotary hearth furnace (FASTMET) for the processing of pellets from blast furnace and steel making dusts (Fig. 8.38). Iron-bearing oxide fines, reductant (coal), and binder are mixed, agglomerated on an inclined pan, and dried (Section 6.8.1) prior to feeding the rotary hearth furnace. The carbon in the pellets acts as a reducing agent (reductant), reacting with the iron oxide and leaving a direct reduced iron (DRI) with a high metallic content. The hot product may be directly transferred to an electric arc furnace for melting or briquetted (Section 6.9.2) for external marketing. Zinc recovered from the off-gas can be also recycled. A further new technology uses agglomerated dusts, coal, and air in a multiple hearth furnace (PRIMUS) to recover highpurity lead and zinc and produce highly metallized (90–95 %) iron concentrate (Fig. 8.39). While many cellulosic wastes, particularly wood, can be and are being converted to secondary solid fuels (see above and Section 6.10.2) efforts are also undertaken to produce agglomerated, value-added secondary raw material that can be used in the manufacturing of engineered wood products. One such method is described in Swiss patent SP 530 261 [Section 13.3]. It shows that saw dust, which is produced in large
Fig. 8.38 Flow diagram and artist’s impression of a recycling plant for the production of secondary raw material (DRI) from iron-bearing oxide fines,
employing a rotary hearth furnace (FASTMET) for the reduction of pellets with coal as reductant (courtesy Midrex, Charlotte, NC, USA)
8.2 Recycling/Secondary Raw Materials
Fig. 8.39 Flow diagram of a recycling plant for the manufacturing of secondary raw material (DRI) from iron-bearing dusts, employing a multiple hearth furnace (PRIMUS) for the production of highly metallized iron concentrate with coal as reductant and the recovery of high-purity lead and zinc (courtesy Paul Wurth, Luxembourg)
amounts during the cutting to size of plywood and pressboards in their manufacturing plants, can be converted into small, thin, chip-like briquettes with roller presses and used, together with virgin wood chips, in the center layer of plywood pressboards. The adhesive (binder) that is contained in the saw dust and causes environmental concerns when burnt in, for example, boilers, is activated by the heat produced during briquetting (conversion of mechanical into thermal energy) and participates in the bonding of the artificial secondary wood chips. Using this technology conserves natural resources and avoids environmental pollution. As in other industries, within recycling facilities, agglomeration equipment is increasingly used for other applications than originally anticipated (Section 6.11.2, high-pressure roller mills). An example, utilizing flat die pellet mills, will be described in the following. A German recycler of wood and building materials feeds bulky wastes, carpets, parts of man-made plastics, textiles, cardboard, and wood that have all been pre-crushed to 90 % <40 mm, contain 10–20 % moisture, and have a bulk density of 50–110 kg/m3, to flat die pellet presses, featuring 22 mm diameter die holes. Fig. 8.40 depicts the arrangement and flow diagram of the plant, showing the potential to ultimately install six pellet mills, and Fig. 8.41 is a photograph of the system in its current four-mill execution. Each machine has a throughput capacity of about 5 t/h. As the feed materials are compressed by the press rollers and forced through the die holes they experience a further reduction in size and leave the press(es) as a partially pelleted, easily handleable, and free flowing “fluff” that can be readily
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Fig. 8.40 Arrangement drawing and flow diagram of the plant (future execution) of a German recycler of wood and building materials, ordinarily producing “fluff” (courtesy Amandus Kahl, Reinbek, Germany)
metered for use, for example, as solid fuel addition to gas burners in the cement industry. With a somewhat different die configuration a completely pelleted product can be obtained (Fig. 8.42, Section 6.10.2) that can be applied as a partial secondary solid fuel in stoker fired incinerators or boilers.
Fig. 8.41 Photograph of the system depicted in Fig. 8.40 in its current four mill execution (courtesy Amandus Kahl, Reinbek, Germany)
8.2 Recycling/Secondary Raw Materials
Fig. 8.42 Pelleted secondary fuel, right, from combustible domestic and industrial waste, left (courtesy Amandus Kahl, Reinbek, Germany)
As mentioned several times before, in most developed countries massive disposal of organic municipal refuse into landfills is prohibited. Sludge from waste-water processing and cleaning facilities poses a particular problem that requires new solutions. Biological digestion, dewatering, and drying is often used to arrive at a material that, with a residual water content of 1 % or a solids content of 99 %, can be burnt if suitable boilers are available. However, as shown in Tab. 8.11 the dried digested sludge powder, for example from OCUA (see below), is very fine and light [Section 13.3, ref. 107]. Because the incinerator is normally not nearby, considerable storage and transportation problems are experienced (Chapter 4). Some years ago, Ocean County Utility Authority (OCUA), NJ, USA [Section 13.3, ref. 107], with the help of the US Environmental Protection Agency (EPA), searched for an innovative approach to their sludge processing. This summer vacation and resort area has the additional problem of seasonally changing amounts of waste water; the actual daily quantities of dry sludge solids in the 1990s were 31 t/d in summer while the yearly average was only 25 t/d. It was forecast at the time that this relation will change to 39 t/d (summer) and 35 t/d (yearly average). The four major components of a sludge management system, processing, storage, transportation, and disposal or utilization were first investigated. It was decided to develop a site-specific method of Tab. 8.11 Characteristics of OCUA (Ocean County Utility Authority, NJ, USA) dried sludge powder [Chapter 13.3, Lit. 107] Parameter
Unit
Value
Moisture content Particle size range Temperature (cooled) Organic solids content Bulk density Angle of repose
[%] [mm] [8C] [%] [t/m3] [8]
1 90 % >0.1 and<0.5 <65 60 0.56 37
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“sludge drying and beneficial use”. To obtain the aforementioned amounts of waste solids that render advanced processing feasible and economical, even in winter, the sludge from three water pollution control facilities (WPCF) had to be combined. Each WPCF features similar treatment systems, including primary clarifiers (Section 8.1), activated sludge secondary treatment and thickening, and anaerobic digestion of mixed primary and activated sludge. The digested sludge is dewatered (by filter presses) and, heretofore, has been disposed in local sanitary landfills. A diagram of the new overall sludge management is shown in Fig. 8.43. It was decided to install an innovative new sludge drying system using mechanical vapor recompression (MVR, Carver–Greenfield process). The resulting powder (Tab. 8.11) is biologically inactive but tends to dusting, poses fire and explosion hazards, easily picks-up static electricity, features an explosion limit of 0.2 g/L and tends to self-ignition above 150 8C. Because of the high organic solids content and a considerable amount of agronomically available nitrate it was assumed that, after a suitable size enlargement by agglomeration, the material could be used as a soil conditioner component in fertilizer bulk blending plants (Section 6.6.2). For many years already, the Milwaukee (WI, USA) Metropolitan Sewerage District used pelleting with cylindrical dies (Chapter 5, Fig. 5.10b5) to create “Milorganite”, a soil conditioner/fertilizer product. In this application, fines, produced in rotary sludge dryers, are converted into cylindrical pellets, which are crumbled (crushed) and screened into a granular product featuring a particle size range of 0.25–1.4 mm. A number of reasons precluded the use of this technology for the OCUA project. To be suitable as a component in bulk blending fertilizer plants, the particle size range must be between 1 and 3.3 (<4) mm. When pellets are broken, only smaller particles can be made because the interior of such extrudates remains relatively weak and friable (Section 6.1, Fig. 6.1-7). The low moisture content of powder from the Carver-Greenfield MVR plant, resulting from an EPA requirement of “further reducing pathogens”,
Fig. 8.43 Diagram of the new overall sludge management of OCUA [Section 13.3, ref. 107]
8.2 Recycling/Secondary Raw Materials
and the high content of sand (mostly in summer from vacationer’s beach activities) threatened excessive wear (already a major problem in Milwaukee) of the extrusion dies and uneconomically high costs. For the above reasons, OCUA decided to install a compaction/granulation system utilizing a roller compactor for the manufacturing of a highly densified sheet, which is crushed and screened to yield the desired product (Section 6.6.2). Fig. 8.44 is the flow diagram of the plant. Some specific considerations for this project were the following [Section 13.3, ref. 104]. Influence of Fibers, Strings, etc. During the preliminary testing, fibrous ingredients in the sludge powder were identified as a potential problem during granulation (crushing) of the compacted sheet. However, this was not confirmed in large scale operation; although short (2–5 mm since earlier processing steps did grind all sludge material) fibers, hair, pieces of cord, and other stringy material are present and visible in the powder, the recycled material, and the product, there is no evidence that they had a negative effect on the process. Nevertheless, caution must be exercised since over longer periods of operation fibrous materials may accumulate and cause problems; they may blind heat exchangers (in the drying section), screens, fans and filters requiring unscheduled shut-downs and maintenance. 8.2.4.1
Fig. 8.44 Flow diagram of the compaction/granulation system at OCUA [Section 13.3, ref. 104, 107]
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8.2.4.2 Influence of Temperature
Prior to entering the compaction/granulation system, the dried powder is cooled to <65 8C. For proper compaction, a small amount of water must be added to the mixer in which fresh powder and recycled material are combined, adjusting the feed moisture content to about 8 %. During blending the temperature of the material increases by about 25 8C because of exothermal chemical reactions and the input of mechanical energy. For the latter reason, a further temperature rise of at least 20 8C occurs during compaction and a small increase in temperature must be expected during milling. Because, as a result of this, the material in the plant may reach temperatures around or exceeding the boiling temperature of water, steam must be expected in all parts of the system. Although this may be advantageous from the point of view of suppressing the explosion hazard it has to be considered during plant design. All equipment must be well vented. Particularly the product silos, which are installed outside (Fig. 8.45), must also be insulated to avoid condensation. Final dust removal and odor control must be carried out wet. Finally, the recirculating material must be cooled to keep the temperatures in check.
Fig. 8.45 Photograph of the outside product storage silos at OCUA (courtesy Ocean County Utility Authority, Toms River, NJ, USA)
8.2 Recycling/Secondary Raw Materials
8.2.4.3 Alternative Product Use
Chemical analyses of the powder revealed that its actual composition changes with the seasons and as a result of many other factors. In view of its intended use as an agriculturally beneficial soil sweetener, a significant amount of heavy and other metals raised concerns. Based on legislation, valid at the time of project execution and during the immediate future, the contaminant levels were considered acceptable for partial use in Florida’s citrus groves. However, to provide a process alternative that could be used if the granular material is no longer accepted by farmers and/or brokers as a fertilizer conditioner, diverter gates are installed after the product screen to alternatively discharge the large pieces from the top as product and recycle the two other, finer streams, bypassing the secondary hammer mill. In this case, the larger pieces (<50 mm) are used as secondary solid fuel in industrial boilers, utilizing the heating value of the mostly organic solids. If the lower size limit of this product must be larger than 4 mm the top deck of the double deck screen should be changed and, to avoid the recirculation of exceedingly large particles to the feed mixer and roller compactor, a small change of the flow diagram will direct the middle fraction to the secondary hammer mill (grate bar distance and hammer tip speed may have to be readjusted). The discharge of the mill goes directly to the recycling loop.
8.2.4.4 Energy Efficiency
Among other plant components, the OCUA solids management project also includes facilities for waste energy usage. Firstly, an energy recovery system is associated with the sludge drying process. A fluidized bed reactor is installed to burn waste sewage oil and scum and a waste heat boiler produces steam. Secondly, a co-generation system, consisting of six turbo-charged, lean burn, digester gas driven engine/generators with a total electrical output of 1800 kW became part of the project. With this system, a significant portion of the electrical requirements for the northern and central WPCFs (Fig. 8.43) is produced. When producing a partially dried (about 90 % solids content) digested sludge that does not contain large amounts of abrasive contaminants (sand), it can be also converted into a secondary solid fuel by pelleting and drying. Fig. 8.46 is the simplified flow diagram of a facility using a flat die pellet press and a belt dryer/cooler and Fig. 8.47 shows photographs of the pellet press and the dryer/cooler in an actual plant (City of N€ urnberg, Germany). The pellets can be easily stored, transported, and, for example, metered as complementary solid fuel into the stoker of coal-fired power plants or boilers.
8.2.5
Other Applications
Sometimes, simple tumbling motions, for example on the slope of stock piles or on other inclined surfaces, are sufficient for the formation of crude agglomerates [B.48, B.97]. These low-cost particle size enlargement processes are primarily used in a spe-
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8 Applications in Environmental Control Fig. 8.46 Simplified flow diagram of a facility using a flat die pellet press and a belt dryer/cooler for the production of secondary solid fuel from digested sewage sludge (courtesy Amandus Kahl, Reinbek, Germany)
cial segment of secondary raw material processing and for the production of agglomerates during the processing of refuse for disposal (Section 8.1). Applications in the first area are, for example, in the recovery of small amounts of valuable, mostly metallic ingredients, such and gold and silver from previously discarded tailings of older mines. Originally, these rejects could not be economically processed. In the meantime, a technology was developed for the extraction of valuable components in very low-grade ores, called “heap leaching” [B.48]. It can be also used for the retreatment of tailings, which then become secondary raw materials. The main problem during these applications is the segregation of coarse and fine particles in the heap during its construction and/or the movement of fine particles with the leach solution. This primary and secondary segregation creates areas with significantly lower permeability as shown in a simulation of the liquid flow (Fig. 8.48, [8.2.5]). Consequently, the leach solution follows paths of least resistance through “open” areas and bypasses or only barely wets those containing large amounts of fines. This results in lower extraction efficiency, longer leaching time, and higher reagent (chemical or bacterial) consumption. The percolation problems can be minimized if fines are attached to each other or to coarser particles by agglomeration. When building the heap, fines are now uniformly distributed and, if the bonds within the agglomerate are strong enough and do not deteriorate during leaching, remain immobile. A more uniform wetting (Fig. 8.49, [8.2.5]) and efficient operation is obtained. Because the heap leaching technology is a low-cost process and the amount of recoverable ingredients is small, high expenses for agglomeration are not warranted. The basic principle of “cheap” size enlargement is demonstrated and used in stockpile agglomeration (Fig. 8.50, [B.48, B.97]). An inclined conveyor transports tailings that
8.2 Recycling/Secondary Raw Materials Fig. 8.47 a) Pellet press and b) dryer/cooler in an actual plant according to Fig. 8.46 (courtesy Amandus Kahl, Reinbek, Germany)
were mixed with appropriate amounts of CaO, cement, and/or other suitable dry binders to a point several meters above ground. The stream of material falling from the end of the conveyor is wetted with water (and/or other liquid) sprays. Below the spray area, several heavy (dispersion) bars extend into the falling curtain and act as a simple, stationary mixer. The wetted mass then tumbles down the slope of a stockpile and agglomerates into lumps that collect at the foot of the pile. Strengthening occurs by natural curing, begins immediately, and continues until the cementitious reactions
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Fig. 8.48 Simulation of liquid flow in a heap constructed of non-agglomerated ore [8.2.5]
are completed. A front-end loader picks-up the agglomerates from the periphery of the pile and transfers them into a dump truck or directly onto the leaching pad of a reclamation plant. The same principle of agglomeration can be applied at multiple transfer points from belts to belts, on shaking or vibrating conveyor decks, or on steeply inclined belt conveyors where material tumbles downward against the upward motion of the belt [B.48, B.97] (Section 8.1, Figs. 8.14, 8.15, and 8.16).
Further Reading
For further reading on the above subjects the following publications by the author, some of which were referenced above, are recommended (Section 13.3): 32, 44, 45, 46, 49, 50, 55, 58, 61, 62, 63, 71, 74, 78, 92, 103, 104, 107, 109, 113, 120, 122, 123, 124, 126, 129, 131, 132, 133, 134, 135, 137, 140, 142, 157, 166.
8.2 Recycling/Secondary Raw Materials
Fig. 8.49 Simulation of liquid flow in a heap constructed of narrowly sized (agglomerated) ore [8.2.5]
Fig. 8.50
Diagram of stockpile agglomeration [B.48, B.97]
8.2.6
Recycling of Polymers
Plastic man-made materials, mostly in the form of polymers, have become a major component of industrial and municipal (Tab. 8.8) wastes. While some modern materials are biodegradable and, therefore, can be disposed of in unsecured landfills, others are permanent contaminants and, if incinerated, may produce harmful compounds, such as dioxins. Growing environmental awareness has prompted plastics recycling programs in most developed countries. In spite of this, however, currently only some 5–25 % of plastic waste is being recycled but, as worldwide consumption of plastics is set to grow, this will have to expand [B.83]. As shown in Tab. 8.8, plastics constitute about 8 % by weight of all MRF materials. By volume this represents about 18 % of which
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40 % comprise packaging materials that are of particular concern because of their short use life. If these plastics go to a landfill, their low density, which make them prone to being picked up by wind, high visibility, and voluminous nature cause considerable public resentment. Commercial incentives for the recycling of polymers include ecological reasons, consumer demand, recycling legislation, and the lowering of costs by reusing the waste’s chemical components and inherent energy content by producing a secondary raw material. Until recently, these incentives had to be weighed against variable material composition, possibility of contamination, loss of mechanical properties because of degradation, lack of standards, and variations in supply. Now many of these problems have been solved through such measures as the use of sophisticated automated sorting, restabilization, implementation of recycled material quality standards, and integrated collection networks [B.83]. The three essential elements of polymer recycling are: * * *
a stable supply source, which involves reliable collection and sorting, an economical, established, and environmentally sound recycling process, and end-use applications for the recycled polymer that yield economic market values and capture consumer interest and confidence.
A recent book entitled Polymer Recycling [B.83] describes in detail the science involved, the technologies used, and the applications of recycled plastics. Its content covers: * * * * * * * * * * * * * * *
sorting and separation techniques, size reduction (and associated agglomeration) of recycled plastics, melt filtration of contamination in recycled polymers, recycling of PET (poly(ethyleneterephthalate)), recycling of polyolefines, recycling of PVC (poly(vinylchloride)), polystyrene recycling, nylon recycling, recycling of engineering thermoplastics, recycling of polyurethanes, recycling of polymer composites, rubber tire recycling, feedstock recycling: pyrolysis, hydrogenation, and gasification, incineration of plastic waste with energy recovery, plastic lumber based on recycled polymers.
Although some recycling methods are based on chemical (recycling of PET, nylon, and polyurethanes) and thermal treatments (melt filtration), most apply mechanical processes for the separation, cleaning, and granulation of the wastes prior to the manufacturing of parts from the secondary plastic raw materials. It is beyond the scope of this book to go into the details of polymer recycling. The aforementioned publication [B.83] is recommended for further reading.
8.2 Recycling/Secondary Raw Materials
It shall suffice in the context of the present review to point out that, while some products with high quality and value are being made from recycled plastics, most of the applications are low-cost manufactured parts. These include [B.83] the following. *
*
*
* * *
PET: staple fiber, filaments, non-wovens (Section 6.11.3), insulating fiber fills, carpets, strapping, sheet, films, non-food contact containers, injection moldings, large moldings, and engineering resins. Polyolefins: large molded containers (e.g., trash cans), blow molded bottles (e.g., detergent containers), bottle crates, pallets and large injection molded parts, drainage pipe, grocery sacks, garbage bags, structural applications (e.g., decking, fence posts, road posts, plastic lumber, etc.), battery cases, automobile bumpers. PVC: pipes, cladding, guttering, window frames, plastic wood, conduit for cables, pipe fittings, floor coverings, fibers, non-food bottles, surface coatings, sound protection panels, automotive acoustic insulation, floor mats. Polystyrene: packaging foam, loose fill, wood substitute, sorbent polymers. Nylon: engineered parts Rubber tires: filler, paviors, mats (skid resistant), athletic tracks, dock systems, playground cover, slope stabilization, road fill, rubberized asphalt and concrete.
Since most of these applications involve some sort of forming, first of the recycled material into a granular feed and then during the manufacturing of the final parts, agglomeration processes are widely used. After sorting, cleaning, and size reduction, pellets are formed in the mill itself by partial melting or by pressure agglomeration (pelleting, extrusion), which are then fed to the final shaping process. The latter mostly uses modified punch-and-die pressing and extrusion techniques, which are again methods of size enlargement by agglomeration.
Further Reading
Once again, for further reading the book “Polymer Recycling: Science, Technology and Applications” [B.83] is recommended.
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Development of Industrial Applications Size enlargement by agglomeration is one of the unit operations of mechanical process technology (Chapter 2), the technical field that deals with the processing and handling of solids. Whendeveloping,designing, constructing, installing, andoperating an industrial plant that includes size enlargement by agglomeration, many or all of the other unit operations of mechanical process technology (each sometimes more than once), associated techniques, and analytical support functions (Fig. 2.2, Chapter 2) are required and used. Since mechanical process technology encompasses the oldest techniques serving mankind, these methods are all based on natural phenomena. They have been applied by various users in different fields so that similar but separate techniques have evolved. For centuries, development was purely empirical until (less than 150 years ago) one after the other, the unit operations were recognized and treated as generic fields of engineering science. During the second half of the 20th century the engineering community began to evaluate and use them interdisciplinarily (Chapter 2). At that time, efforts began to apply experience and know-how that is available in one field to solve the problems of another, often involving totally different feed properties and requirements for plant size, process cleanliness, and product characteristics. In the newer fields of industrial technologies, such as advanced chemistry, electronics, and communications, process research and plant development started from first principles and much of the equipment and system designs had to be newly elaborated for a particular purpose, using modern ideas, manufacturing, and industrial methods. In contrast, most mechanical process technologies rely on fundamentals that are rooted in the purely empirical past, enhanced by the expertise of individuals and companies. Also, mechanical processes and installations are often considered simple and dirty, because particulate solids in different size ranges, including dusts and slurries, are involved. To students and many practitioners they have little technical or scientific appeal; they are thought to require only conventional equipment for successful operation. As a result, designs are very often crude, old-fashioned, and far from optimal. Furthermore, the different disciplines that are involved in construction of industrial plants often lack a common background and understanding of improved technologies so that the resulting designs can include stunning misconceptions. This is demonstrated in a cartoon (Fig. 9.1). This series of sketches was not conceived or drawn by the author. It was given to him many years ago by an industrial contact and Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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Fig. 9.1 Cartoon allegorizing the frequently experienced development, design, manufacture, and installation of a new industrial plant
its origin is unknown. However, whoever drew this sequence understood well what is often found in industrial practice and his knowledge and draftsmanship deserve credit. In Fig. 9.1, at the beginning there is a site and an idea that is discussed between the customer and a supplier of services: (1) is the supplier’s engineering; (2) is the specification of its purchasing department; (3) is what was actually built; (4) are the instructions for installation; (5) shows how the installation was executed; (6) is the modification that was made in the field prior to start-up; (7) depicts what the customer really wanted; and (8) is what the supplier shows in his advertisements. Although this picture story is exaggerated, it indicates that in the field, even under adverse circumstances, modifications eventually succeed in making the project work. The result is, however, far from what was intended and/or desired. Solutions to all the problems that consultants are typically asked to resolve are not feasible. Some improvements can be made but an optimal execution would require a new installation and expert project management. To facilitate the development, design, and selection of a new plant, guidelines have been developed [B.97]. Of course, proper agglomeration equipment (that is best suited for a specific task) and the procurement of optimal peripheral equipment and system layout depend on the application for which the process and plant are destined. Tab. 9.1 [B.97] summarizes the most important parameters that need to be considered when evaluating the best possible approach for a particular project. The characteristics fall into four categories:
9 Development of Industrial Applications Tab. 9.1 Considerations during the selection of a suitable agglomeration process for a particulate project [B.71] Parameters of the particulate feed * Feed particle size and shape (dimensions and distribution, surface area, shape factor, fractals, etc.) * Moisture content (free, encapsulated, crystal water) * Material characteristics (chemistry, density, porosity, plasticity, brittleness, elasticity, wettability, abrasivity, etc.) * Special material characteristics (heat and/or pressure sensitivity, toxicity, reactivity, etc.) * Bulk characteristics (temperature, density, flowability, etc.) * Binding characteristics Parameters of the agglomerated product Agglomerate size and shape (dimension(s), distribution, volume, weight, tolerances, etc.) * Strength – Green strength (if applicable) – Final (cured) strength * Structure and other characteristics (porosity, specific surface area, dispersibility, solubility, reactivity, abrasion resistance, etc.) *
Parameters of the agglomeration method Batch or continuous operation (interruptions or downtimes tolerated or not) * Capacity per hour and per year or per campaign * Wet or dry operation * Simultaneous processing * Space and energy requirements * Investment and operating costs *
Site, supply, and environmental conditions, infrastructure o Relative location to suppliers and consumers (raw materials, additives and binders, energy, users, etc) * Site accessibility and transportation facilities * Climatic conditions * Availability of skilled and other labor * Availability of support functions * Regulations (e.g. EPA, OSHA, etc.)
* * * *
particulate feed, agglomerated product, method options, site, supply and manpower situation, environmental conditions, and infrastructure.
Methods for the selection of the best agglomeration process for a specific application are the same for all projects. Some requirements for equipment or system capacity, or for the shape, size, and special properties of the products, may result in the definition of “cleaner” or “more heavy-duty, rugged” processes. However, the normal approach is to determine the preferred method and/or technique by first considering the fundamentals. The conditions, such as “hot and dusty large volume processing”, or “clean, small capacity operation with cGMP (current good manufacturing practice) and CIP (cleaning in place) capabilities” are special design criteria that can be added to most of the systems later during the engineering phase.
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When evaluating the best agglomeration method for a new application, references in the literature or in industry should be researched first. Even if a seemingly suitable reference plant is operating successfully elsewhere, the new application should be freshly evaluated because small variations in, for example, feed characteristics, may require a different approach. At the very least, operating parameters, productivity, and product quality must be newly determined. Next, the most important characteristics in the four categories of Tab. 9.1 must be selected and evaluated [B.48, B.97]. For example, if the feed particle size is large, tumble/growth agglomeration methods and low-pressure agglomeration are less suitable than high-pressure agglomeration. However, different techniques can be more or less acceptable depending on, for example, whether binders are inherently present or may be added. Wet feed materials can not normally be processed with high-pressure agglomeration equipment. Large product sizes can not be made with tumble/growth agglomeration methods, and fluidized bed technologies fare worse than agglomeration by tumbling in a pan or drum. With certain tumble/growth techniques, the agglomeration of moisture or heat sensitive materials becomes questionable because liquid binders are often required and drying is necessary for the development of final product characteristics. A more detailed discussion of this selection process is beyond the scope of this book and chapter. Readers who are interested in the topic should refer to other publications by the author [B.48, B.71, B.97, and, particularly, Section 13.3, refs. 128, 141, 152, 159, 165, and 168]. The valuation of the feed, product, method, and site-related parameters is totally independent and not connected with any application. Nevertheless, determination and sometimes weighted use of the most relevant parameters within the four groups in Tab. 9.1 for a particular task and summation of the values for each agglomeration technology will result in a ranking from “most suitable” to “least suitable”. Final selection of the best agglomeration technology for the specific task, procurement of the optimal peripheral equipment, and design of the most productive system ultimately depend on the application for which the process and plant are destined. It is also influenced by the owner, the user, the engineering group involved, the vendors, and, possibly, one or more consultants. All agglomeration methods, although designed to achieve various goals and operate in different environments, are based on the same fundamentals, apply the same rules, and use essentially the same equipment and systems if looked at from an interdisciplinary point of view. These facts are becoming better known. Nevertheless, there is still the understandable prejudice of, for example, somebody working in an ultra-clean environment, such as the pharmaceutical, food, or electronic components industries that expertise gained in the “dirty” plants of mineral or metals production and processing can not be considered relevant to the solution of a “clean” problem: and vice versa. In the case of “dirty” industries, a typical concern is that the technologies originating in “clean” industries can not be applied because the production capacities are too small, the process may be batch, the equipment too complex, the execution and the materials of construction too expensive, and so on. In spite of the realization that a common body of concepts and techniques applies to the very different processes and situations of size enlargement by agglomeration, for
9.1 Test Facilities
manufacturing reasons and sometimes also because of special requirements on test facilities (e.g., cleanliness, Section 9.1), some vendors specialize in equipment for one or a particular group of industries (for example, food, pharmaceutical, and fine chemicals or mining, metals production, minerals, and wastes). This is a decision of convenience by the individual supplier and does not indicate the existence of fundamentally different technologies. In fact, techniques or apparatus that were developed for a specific industry can be adopted for use in applications with another environment and different requirements while still maintaining the basic underlying principle and the general machinery and process. Following the collection of information and the data summarized in Tab. 9.1 and after completing the pre-selection process, which is a desk job, it normally becomes necessary to carry out laboratory tests, for example, to determine what type of binder must be added and how much of it is required. After that, trials with the actual equipment must be conducted to find process and capacity limitations and optimize product size, shape, and characteristics. Needs for peripheral equipment, post-treatment, closed-loop processing, and recirculation must be also evaluated.
Further Reading
For further reading the following books are recommended: B.1, B.2, B.3, B.4, B.5, B.6, B.7, B.8, B.9, B.10, B.11, B.12, B.13 a-e, B.15, B.16, B.17, B.18, B.19, B.21, B.22, B.24, B.25, B.26, B.35, B.36, B.37, B.40, B.41, B.45, B.46, B.48, B.49, B.51, B.52, B.55, B.56, B.60, B.61, B.66, B.67, B.68, B.71, B.73, B.74, B.83, B.89, B.93, B.95, B.97, B.99, B.104, B.106, B.107, B.109 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold.
9.1
Test Facilities
Knowledge of the binding mechanisms of agglomeration, the parameters controlling the processes of agglomeration, the characteristics of the equipment available for size enlargement by agglomeration, and the requirements for product quality and plant design, together with the availability of interdisciplinary research, operational know-how, and experience allows pre-selection of the most suitable method(s) of size enlargement by agglomeration for a particular task [B.97]. Nevertheless, the development of all agglomeration techniques is still more an art than a science. Often the material to be agglomerated is produced in the project itself and, therefore, samples can not be supplied at the time of plant design. If the actual material that needs to be processed in a new installation is available, its amount may be limited. The testing of similar materials from different sources, even if they are chemically identical and seem to be physically comparable, is not recommended because traces of impurities and miniscule changes of surface structure, for example, can de-
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cisively change many aspects of a material’s agglomerative behavior. Therefore, at the beginning of testing, a common desire is to use small laboratory, often desk-top equipment in an effort to define the basis for sizing and develop the parameters for a largescale industrial plant. Tab. 9.1 shows that to be able to evaluate a particulate solid or the mixtures of particles and/or powders, the chemical and, especially, the physical properties of this material must be known. Descriptions of the methods and procedures that are available today to completely characterize particulate solids and determine all the quality attributes that are necessary for a meaningful evaluation of their behavior under different process conditions can be found in the literature. Some reference books are mentioned in Section 13.1 [e.g., B.12, B.17, B.23, B.27, B.30, B.42, B.52, B.70, B.75] and other books listed in the same Section 13.1 do contain sections that deal with laboratory data collection methods and equipment for general or specific size enlargement technologies [e.g., B.1, B.2, B.6, B.10, B.18, B.32, B.35, B.41, B.48, B.49, B.50, B.55, B. 66, B.67, B.68, B.88, B.89, B.93, B.95, B.97, B.99, B.102]. After having determined all or at least the most important characteristics of the feed material, the pre-selected agglomeration method must be used to produce agglomerated products. In the laboratory, such experimental work is relatively easy and meaningful for all tumble/growth agglomeration methods. Small discs, drums, mixers, and fluidized bed processors are available that can simulate the growth process satisfactorily. Sometimes, such small scale equipment is available in a modular design [B.97]. Of course, when considering the mechanisms and kinetics of tumble/growth agglomeration [B.48, B.97] it becomes obvious that smaller containments and masses of tumbling particulate solids translate into lower forces that act during impact and coalescence or as separating forces in the system. This results in weaker bonds and more porous structures but, because the forces of the moving environment are small, also in less destruction. These conditions, in turn, may and generally will result in higher binder requirements, lower strength, quicker dispersibility, and differences in a whole host of other agglomerate characteristics if they are later compared with products from larger scale industrial operations. While no easy solution can be offered, this problem must be mentioned at this point to alert researchers and project developers to the differences that will exist between the products from small scale laboratory and large scale industrial operations (Section 9.3). It is much more difficult to carry out small laboratory tests for most of the pressure agglomeration methods and obtain results that are meaningful and can be used for process development. In this field, the technique that lends itself best to small scale laboratory development and evaluation is low pressure agglomeration (Section 5.0, Fig. 5.10, and Section 6.2.2), which may be followed by spheronization. Since in this method of pressure agglomeration a wet mixture is passed through the openings of a screen or a thin perforated sheet, very little pressure is exerted and it is essentially a shaping process [B.97]. Therefore, even if tests are performed on a small perforated die, in regard to product characteristics, the results are also representative for larger units. The same is true for the spheronizing process.
9.1 Test Facilities
Medium pressure agglomeration in pellet mills can be easily simulated because even a single bore with the correct diameter to length ratio and featuring all other details of the orifice (e.g., inlet chamfer, discharge cone, relief bore, etc. [B.97]) can be used to determine the extrusion characteristics of a particular feed material and the properties of the extrudate. Samples of punch-and-die pressing can be produced in a variety of simple homemade or purchased small machines using mechanical actuation or hydraulic pressurization with hand or motor pumps. A large number of sometimes highly sophisticated and automated presses is also available [B.97]. They are used for the determination of a variety of strength and force or pressure related product characteristics and, although the densification and compaction mechanisms are quite different from those of roller presses and can not be correlated (Section 9.3), punch-and-die compacts are often made and evaluated to preliminarily investigate the compactibility and strengthening of different feed materials or powder mixtures and to determine the type and amount of potentially necessary binders. Tabletting research and development can be carried out in single station punch-anddie presses, which use different drive mechanisms. If used for high precision tabletting or development work, instrumentation is added with which data can be recorded, stored, and processed. For tabletting research, determination of the pressing force over displacement (densification) diagram is of great value [B.97]. For evaluating isostatic pressing in the laboratory a specially designed press chamber can be inserted between the platens of a suitable press. The most difficult laboratory evaluation is that of high-pressure roller presses because the conditions in the nip between the two counter rotating, converging roller surfaces depends on so many parameters that it is practically impossible to accurately predict the performance of commercial presses with small machines. In conclusion, test equipment is available in all areas of interest for the determination of feed and product characteristics, including new techniques that have been developed in response to advancements in modern mechanical process technology and to new applications for the manufacturing of novel, for example, engineered products. However, testing is only as good and predicts industrial performance of the projected plants as correctly as test conditions reflect what will be found later in the actual installation. A common problem during the development of any new process of mechanical process technology, particularly also including size enlargement by agglomeration is the requirement to investigate a representative sample of the future feed material. As mentioned before, in many cases the actual feed material that must be agglomerated in the course of a new plant flow diagram, is not yet available in large quantities. Often, it is manufactured within the same or another new project, either by processing a natural resource, changing the characteristics of already available solids, or synthesizing from various raw materials. In those cases, the feed for laboratory tests is itself the result of tests or of a small scale pilot plant. The properties of such materials may change considerably when they are later produced on the industrial scale and/or inline. The results of tests with such material are questionable, at best, which must be considered when evaluating the data and designing the plant.
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In other circumstances the feed material is already available from an industrial installation, either, for example, mines or chemical and mechanical processing plants. It can also be a waste stream or a by-product from such plants or from an unrelated manufacturing facility. At a later time, it may become necessary to modify material characteristics and improve their properties in the first case or to allow recycling in the second case. For testing, this material must be sampled so that it accurately represents what needs to be continuously processed in the planned agglomeration system. Such sampling must take into consideration potential segregation and normal, unavoidable fluctuations in material consistency and properties. Sampling of particulate solids is a complex problem in itself [B.27, B.30]. Another, often overlooked influence on the results of testing and their applicability is that, in practically all cases, the test facility is located at a vendor or research facility, often hundreds or thousands of kilometers away from the source of the material to be evaluated. Assuming correct sampling, i.e., excluding this problem, the material is packed in a suitable way, which, today, often includes “big (bulk) bags”, so-called FIBC (flexible intermediate bulk containers). During handling and transshipment, particulate solids, in addition to potential chemical changes, may segregate, break, and/or cake and generally will change their bulk characteristics. This is particularly true if the modern FIBC is used because one of the properties of this packing method is its flexibility. However, the “old fashioned” packing in drums or bags may also cause at least some of the same problems. To obtain the best possible feed for testing, the original bulk properties must be reinstated which, as is easily understandable, is difficult or even impossible. Segregation and settling may be reversed by tumbling and mixing, but changes in particle size and shape, either during transshipment and handling or the breaking of lumps, and other particle modifications are irreversible. Furthermore, “aging” of materials is a common, but often little recognized problem. This term refers in most cases to a modification of the surfaces of the particulate solids by adsorption of moisture and other atoms or molecules and/or oxidation and other chemical reactions. Sometimes, new (often whisker-like) crystal growth is also observed, particularly if the materials are moist [B.48, B.71, B.97]. The product(s) of aging have a marked effect on the results of testing, because, particularly during agglomeration, binding mechanisms rely on chemical and physical interactions at and between surfaces of the particles to be agglomerated and, if applicable with the binder component(s). Therefore, although a representative sample may have been provided, a material that is several days, weeks, or months old and may have had to be reheated, dried, rewetted, delumped, mixed, fluffed, etc. to bring it back to conditions that are corresponding to or comparable with those found or expected in the real plant environment may yield completely different results from that obtained later “in-line”. Even if plants are already successfully operating in other places and “the same material” from new or existing sources or particulate solids with essentially identical chemical composition must be agglomerated in a new location, experience teaches that it can not be safely assumed that the new installation can use the same design and operating parameters to obtain a product with comparable quality. Minute differ-
9.1 Test Facilities
ences in feed characteristics, such as particle shape, size distribution, surface roughness, wettability, porosity, physical contamination with nanometer dust or adsorbed layers, chemical modification with trace elements, etc. may result in significantly different process and operating conditions. A plant that is comfortably sized in a “reference location” may, at another site, handling “the same material”, be grossly underperforming if for the design and execution of the new project only data from the “reference plant” were utilized. Although, as reviewed in other publications (Section 13.1) and made available by vendors (Section 15.1) in their brochures and newsletters, certain characteristic relationships have been developed for most agglomeration methods and performance factors can be collected in charts for the pre-selection of methods, which are most probably suitable for a particular application (Chapter 9 and [B.48, B.97]), determination of the actual design parameters remains a serious problem. This means that, as a general rule, tests must be carried out with representative samples of the specific, unaltered particulate solids that need to be processed by an agglomeration method. For cost reasons, even if only a limited selection of process equipment is used, simulating the continuous operation of an entire production line is normally not possible during testing. In those cases where in the actual plant recycle will be produced and returned in one way or another, product is first made from the fresh feed during its evaluation, the expected type of recyclate is produced, and, for further testing, the anticipated amount is mixed with the fresh feed or other material streams, such as, for example, the crusher or screen feeds, to simulate the conditions in a continuously operating system. Installation of a pilot plant on-site and/or in-line should be considered if the risks that are always connected with new installations, are to be minimized. While the pilot plant approach during project development must be investigated and decided upon on an individual basis, testing of the process equipment is always necessary. For that reason, essentially all manufacturers and/or vendors maintain sometimes rather elaborate facilities [B.48], normally with machines of different sizes, including, in some cases, large scale equipment to avoid scale-up problems (Section 9.3). Test facilities for agglomeration methods must also include some peripheral equipment, such as mixers, heaters or coolers, conveyors, crushers, dryers, screens, etc., although the variations that are available in these special areas from outside sources can not be offered. Therefore, additional tests for the evaluation and selection of the best peripheral equipment are often necessary at different facilities. All tests, in-house or externally, have the same problems as mentioned above. These difficulties can be summarized as the results of “aging”. Many agglomerates also require some sort of post-treatment. Nearly all products of growth agglomeration are at the beginning “green” (wet or moist); they must be dried and sometimes hardened (with chemical or thermal processing) [B.48, B.97]. For small production units, for example in the pharmaceutical industry, all process steps may now be carried out in one container (one-pot processing, [B.97, B.99]) and tested in the supplier’s laboratory. But for larger installations and the selection of optimized peripheral equipment it is necessary to make use of the facilities of specialized vendors. In those cases, it is a challenge to avoid loss of moist-
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ure and sticking (secondary agglomeration) during the transfer of samples. Some production of fines, due to surface drying and crushing, and a potential change of the internal distribution of moisture, which may influence the kinetics of the following process, is commonly observed and must be considered during the evaluation of test data and system layout. Compacted products are often granulated by crushing and screening. While, as mentioned above, some preliminary size reduction and particle separation must be carried-out at the facilities of the pressure agglomeration equipment manufacturer (to determine the estimated amount of and produce recycle), the most suitable large scale equipment is tested at and selected by specialized vendors. Owing to the conversion of mechanical into thermal energy, especially the products of high-pressure agglomeration (e.g., roller presses) exit the machine warm. As a result, some components of the densified mixture may have become soft, resulting in a very different crushing behavior from that of cold, aged compacts, which may now be brittle. Generally speaking, freshly produced high-pressure agglomerated particulate solids will require some time until final binding characteristics are obtained. Additionally, residual elastic deformation, causing internal stresses, will disappear with time. Material that is packed and shipped to a distant testing facility will have reached final (aged) properties when it arrives there and perform accordingly. In-line, in the actual production plant, these conversions have normally not fully taken place (unless intermediate curing is a part of the process, Section 10.2) and it is impossible to reintroduce the same conditions prior to secondary testing. Another example is the external development of separation equipment for the individualization or cutting of hot briquetted or compacted, metal-bearing materials (Section 6.9.2). Reheating the material after cooling and transportation does not normally bring the material back to its in-line condition. From all this it should now be well understood why and that all manufacturers of mechanical process equipment need extensive test centers, which include a laboratory for the determination of feed and product properties and facilities for the evaluation and selection of design and process data. The personnel of this department are also the link to the customers, both new and old (existing), from whom new ideas and feedback (know-how) are obtained that are an important part of machine design, process lay-out, and equipment performance, including predicted operating parameters. The availability of test centers is not only a necessity but has become an important part of the competitive presentation of vendors of mechanical process equipment in the market. As a result, special brochures describing the capabilities are commonly offered (including material and process questionnaires and material safety data sheets (MSDS) that need to be filled out, often to comply with government regulations) and several journals publish short annual reports on company test centers. Since every vendor maintains some sort of facility and in this book it is not intended to singleout individual suppliers in one way or another, it is recommended to refer to the (admittedly incomplete) vendor list (Section 15.1) and request information directly from the companies, to buyer’s guides, which are published regularly (often annually) by journals and trade organizations around the world, and to the catalogues of trade shows.
9.1 Test Facilities
Nevertheless, it was felt by the author that at least an idea of what can be expected and what is commonly available should be given but that this coverage should be anonymous, only pointing out certain features. For that purpose, the author has searched his files and selected photographs, some of which may be old and, if recognized by the owner, at this point in time may be outdated and replaced by a more modern installation. In the following these pictures are presented and discussed, whereby certain general features, procedures, and limitations are explained. The series of photographs shown in Fig. 9.2 depicts various views of support laboratories. Here, particle size distributions and, sometimes, specific surface area, moisture content, powder flowability, product strength (defined, for example, by crushing, drop, or abrasion) and density or porosity and other, more specific information, such as dispersibility, solubility, reactivity, are determined [B.97]. The results define the feed characteristics and check the product properties. Often data acquisition and computerized evaluation systems are part of these supporting laboratory facilities. Fig. 9.3 represents a rather typical equipment test set-up. In the foreground drums are visible in which the raw material has arrived and into some of which product samples are loaded for shipment to the customer and for their evaluation. In the back-
Fig. 9.2
Various views of support laboratories
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Fig. 9.3 Typical test facility for the evaluation of the performance of particulate solids during mechanical processing
cont’d
9.1 Test Facilities
ground, individual pieces of equipment that were taken from the vendor’s stores have been assembled and are partially connected to provide a temporary system. In most cases, the machine types, sizes, and performances do not relate to each other in an optimal way; they are selected from what happens to be available and simulate the process steps that are expected in a proposed plant. On the right are visible a pellet mill into which feed materials are metered from different sources and discharge and transport devices; a mill and cyclone dust collector are in the center and several types of separation equipment (screens) on the left. During the next campaign with other feed materials and for a new customer, the same area will most probably look quite different but equipment will be assembled in a similar fashion to accomplish the desired task. Fig. 9.4 is the photograph of another general purpose test facility. The major equipment to be operated and investigated is a bowl-type mixer agglomerator (right foreground, [B.97]) which, in the picture, is connected to a control unit (far right) and features a bottom agitator, a shredder/knife head (driven from the left on the bowl), and metering and de-dusting attachments (on the opened lid). In this small facility the support laboratory section (Fig. 9.2) is integrated into the room (left background). The design and operating principle of the mixer agglomerator allows the performance of all process steps (metering, mixing, moistening, size enlargement, agglomerate size control, drying, cooling, and sometimes even post-treatment by, for example, coating or conditioning) in one pot [B.97]. Therefore, although the room is not specially equipped for sanitary operation, in this case, clean testing as required for the food and pharmaceutical industries is possible (below). A rather common test set-up for the development of growth agglomeration in a pan agglomerator is depicted in Fig. 9.5. It shows a small planetary mixer on the left in which, after hand feeding proportioned amounts of dry particulate solid components and possibly some liquid or viscous binder(s), the blend to be agglomerated is pre-
Fig. 9.4 General purpose test facility with a bowl-type mixer agglomerator
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Fig. 9.5 Common test set-up for the development of growth agglomeration in a pan agglomerator
pared. This feed is then transferred to the balling pan (rotating clock-wise) where liquid binder is sprayed onto the tumbling mass, growth agglomeration takes place, size segregation occurs, and finished nearly spherical pellets (visible in the now stopped pan) discharge over the rim (lower left quadrant, [B.97]). Fig. 9.6 is the partial view of another test center in which two deep pan mixer agglomerators are shown. Note that this equipment is located in a room where clean and sensitive materials can be evaluated (floor and wall tiles), however, for reasons of possible cross-contamination with traces of materials that were processed earlier, products are not suitable for, for example, human evaluation by consumption.
Fig. 9.6 Partial view of a test center in which two deep pan mixer agglomerators are shown
9.1 Test Facilities
The green (moist) agglomerates are dried, in small batches in a laboratory drying oven elsewhere for the evaluation of drying behavior and the selection of a suitable large scale dryer. The blend of ingredients could also be transferred into a balling drum. In this case the discharge is not segregated according to size. Therefore, the green agglomerates must be screened. Product is further treated as described before and the undersized particles are returned, coarsely mixed with fresh feed, and reintroduced into the drum for further growth agglomeration. In some cases, if the green pellets are too sticky and tend to blind the screens, separation of undersized material is only possible after drying. In this case, even if large drying equipment is not available, a sufficient amount of dry recyclate must be produced since the conditions in the drum (wetting and liquid binder requirement) is considerably changed particularly if one takes into account that the recycling rate is often several hundred percent (300– 600 %) of fresh feed. Even more complicated than the drum agglomerator is the testing of fluidized bed processes. In those cases it is not possible to separate the process steps sufficiently to carry them out individually. As a result, small laboratory equipment may be used as described elsewhere [B.97] or systems that resemble pilot plants are applied (Fig. 9.7). The latter also reduce scale-up problems if larger capacities will be required in the proposed new installation.
Fig. 9.7 Test facility for the evaluation of fluidized bed processes
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As mentioned above and discussed in Section 9.3, the use and transfer of data obtained with (“laboratory”) roller presses featuring small roller diameter to largescale production machines is questionable and generally very difficult. For that reason, roller press manufacturers that cater for customers with big capacity applications, for example for fertilizers (Section 6.6.2), the mining (Section 6.8.3) and metallurgical (Section 6.9.2) industries, or for solid fuels (Section 6.10.2), maintain test presses with large roller diameters to minimize scale-up problems. Fig. 9.8 is an older photograph of such a facility. The press on the left features rollers of 1000 mm diameter and the one on the right is somewhat smaller, but still a relatively large unit (700 mm diameter). Depending on the characteristics of the mass to be densified, different gravity or force feeders can be installed on the roller press frames (a screw feeder with hydraulic motor is mounted on the larger (left) press). To minimize the sample size per test, the rollers are narrow. With this, a highly instrumented machine, and the computerized recording of all data, meaningful results can be obtained with a feed volume of about 50 L, although the duration of each test is often less than 1 min.
Fig. 9.8 Test facility of a roller press manufacturer featuring presses with large roller diameters
9.1 Test Facilities
As in the case of drum agglomeration, discussed above, particularly when compacted sheets are produced, which are subsequently crushed and screened to yield a granular product but also during the investigation of briquetting where only a small amount of fines and chips are present, the influence of recycling on the process parameters must be simulated by manufacturing suitable amounts of undersized material and adding it to the feed of each test in proportion. In different parts of the test center the analytical support laboratory is located (Fig. 9.2) and strength testers (Fig. 9.9) and other equipment, such as mixers (Fig. 9.10a), sample splitters and screens (Fig. 9.10b), feeders and mills (Fig. 9.10c), ovens (Fig. 9.10d) and so on, are available for material preparation, product evaluation or treatment, and recycling (Fig. 9.11). Especially in pressure agglomeration, testing of the influence of recycling fines in the feed on the operating parameters is very important. While in growth agglomeration recycling particles always act as nuclei and improve the kinetics of size enlargement, in pressure agglomeration recycling may be beneficial, detrimental, or neither. Common advantages of returned fines, which are mostly made up of pre-densified but undersized granules, are a reduced requirement for deaeration and an increased compact strength due to the higher density of the feed. A disadvantage may be that the sometimes hard undersized particles are not well integrated into the structure of the agglomerates, resulting in non-uniform bonding and breaking upon discharge. Such inclusions may also produce stress peaks during granulation, thus negatively influencing the crushing behavior and the product yield. For these reasons, the above described simulation of the effects of recycling by introducing undersize particles into the feed during batch testing may not be sufficient. Therefore and for a number of other purposes, it has become rather common that major suppliers of medium and high-pressure agglomeration equipment also maintain well-sized pilot plants in which continuous processing of materials is possible. Fig. 9.12 depicts such a system, which is designed to mostly process animal feed (Section 6.5.2). The elevator on the right transports the materials to be agglomerated, possibly including recycling, to various alternative mixers on the uppermost platform. Metering equipment for solids and liquids allows the correct adjustment of the composition. The blend may be further processed in a conditioner where, for example, steam can be added to activate binding characteristics. Agglomeration can be carriedout in an extruder, a so called expander, or a flat die pellet mill (the latter is located near the person in the picture). The system also includes a belt dryer/cooler and the possibilities to granulate the agglomerates by crushing and screening and to pack the product, for example into big bags. Fig. 9.13 shows a similar plant with a roller press for the high-pressure agglomeration of dry particulate solids and Fig. 9.14 demonstrates how many different pieces of equipment may be combined if a system for the testing of iron ore processing, pelletizing, firing (sintering), and cooling must be temporarily assembled in a specialized vendor’s facility. It has already been mentioned that some applications, especially in the food and pharmaceutical industries, require a clean environment. It was also pointed out that there is still the understandable preconceived notion of somebody working in those industries that developments, expertise, and know-how gained in the “dirty” plants of, for example, minerals or metals production and processing, can not be con-
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Fig. 9.9 Agglomerate strength testers: top) hydraulic four-column press for the determination of compression strength; bottom) from left, rotating tube for measuring degradation at transfer points, drop test arrangement, and drum abrasion tester
9.1 Test Facilities
Fig. 9.10
Laboratory test equipment: a) mixers, b) sample splitters and screen, c) feeder and mills, d) ovens
sidered as valid information that may be applied for the solution of a “clean” problem. With exception of fluid-bed agglomeration, coating, and tabletting all size enlargement technologies using agglomeration were originally applied for ceramics, coal, fertilizer, and minerals, which definitely quality as “dirty” industries. The mentality of developers in these fields, accepted test facilities as shown, for example, in Fig. 9.3 well. However, even if it is acknowledged that product samples from testing can no longer be used for the material’s intended application and, for the testing of compositions the (expensive and active) food or medicinal part has been replaced by a (cheap and inactive) filler component, a pharmacist or food technologist, for example, can not be convinced that equipment tested in a non-sanitary environment will be suitable for his or her application.
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9 Development of Industrial Applications Fig. 9.10c
From the beginning, test facilities of tabletting (Fig. 9.15, see also Section 6.2.2) or coating (Fig. 9.16, see also Sections 6.2.3 and 6.4.3) equipment manufacturers catered to the pharmaceutical and food (Fig. 9.17, see also Section 6.4.2) industries. As a consequence, the rooms are designed for easy cleaning (tiled floors with drains and tiled or washable walls) and a dust free atmosphere (room de-dusting and, more recently, air conditioning). Laboratory personnel are required to wear white coats and hats. Although such clothing and other “clean looking measures” are only superficial aspects and have nothing to do with the quality of the equipment or the product, the customer representative witnessing the tests feels “at home”, which is translated by him or her into a more acceptable service and supply. It is interesting to observe how changes in the scope of supply of vendors require the adoption of new philosophies. The desire or even need of laboratory cleanliness, when dealing with food, pharmaceutical, and other high quality materials, became of special
9.1 Test Facilities
Fig. 9.10d
Fig. 9.11 Product evaluation and recycle processing
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Fig. 9.12 Pilot system in which continuous processing by extrusion of particulate solids is carried out
concern when roller press manufacturers, seeking alternative applications to the traditional briquetting of coal and minerals (Sections 6.10 and 6.10.2), tried to enter the market of dry granulation of tabletting formulations by compacting, crushing, and, potentially, screening (Section 6.2.2). At the beginning, it was very difficult to convince the pharmaceutical industry that small appropriately designed (e.g., for oiland greaseless processing and easy cleaning) and executed (e.g., in stainless steel) machines that were seen as “cousins” and next to large and crude presses for minerals, are suitable for sanitary applications. The solution of the problem is depicted in Fig. 9.18. It shows the “clean” room of the test center of a roller press manufacturer with, from the right, a gear type pelleting machine [B.48, B.97], a small roller press, opened for cleaning, and a medium sized machine with special features for sanitary applications. All three machines are equipped with force (screw) feeders. On the far wall two of the ancillary pieces of equipment (mill and screen) are visible.
9.1 Test Facilities
Fig. 9.13 Manufacturer’s pilot plant with a roller press for high-pressure agglomeration of dry particulate solids
All results from tests and evaluations at often many different locations and with various equipment, sometimes also including information from pilot plant operations, are collected, compared with related know-how and experience, if available, and used for the engineering of the new plant and the selection of process equipment. In spite of all the efforts that normally go into the determination of the data, it is prudent to include safety factors during the design stage that will allow optimization of the system if and when it comes on stream. Such initial modification, in the best case only involving process and operating parameters, is a normal requirement for all installations of mechanical process technology. Selecting equipment for the lowest expected material flow rates and/or product qualities, which often seems necessary to meet project cost limitations, later leaves no room for these changes and often results in underperforming plants. Remediation of this situation may not be possible or becomes much more costly than a small project cost overrun in the project phase would have been.
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Fig. 9.14 Temporarily assembled system for the testing of iron ore processing, pelletizing, firing (sintering), and cooling in a specialized vendor’s facility
Fig. 9.15
Test facility of a tabletting machine manufacturer
9.1 Test Facilities
Fig. 9.16
Testing facility of a coating equipment manufacturer
Fig. 9.17
Food processing (forming, cooling, and cutting) test center
Fig. 9.18
“Clean” room of the test center of a roller press manufacturer
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9.2
Tolling Operations, Contract Manufacturing
A relatively new development is the emergence of tolling companies (Section 15.1). These are installations that maintain production lines for contract manufacturing, comanufacturing, and back-up manufacturing. Many of the “tollers” were formed when the sometimes very large and extensive test and development centers of big, diversified companies became profit centers as the head office’s management philosophy was changed. Rather than maintaining costly R&D facilities and financing them with annual budgets, these department(s) were spun-off and made independent. Testing of new applications and materials, which before was often offered free as a customer service, now has to be paid either by the parent (often an engineering and/or manufacturing company) as part of their proposal cost or directly by the potential customer. Since the new entity is now required to finance itself, services are offered to all parties seeking testing, development, or manufacturing support, sometimes even independently of the interests of a remaining original ownership in whole or in part, as long as there is no conflict of interest. Other companies were founded for the specific purpose of contract manufacturing. Fig. 9.19 is the partial reproduction of a brochure (Fluid Air, Inc., Aurora, IL, USA, Section 15.1) showing, as an example, the separate product development and manufacturing facility of a vendor. This is a crossover from the basic supplier test centers (Section 9.1) to a more elaborate customer service department. It is still owned by the equipment manufacturer, is under the direction of a well-educated and experienced veteran expert in the field, and offers not only feasibility testing but also product development and process optimization through small scale manufacturing. Fig. 9.19 presents an image of the plant’s layout and describes the available machinery (the dark areas house central utilities). The office wing, also providing personnel and social support functions, is not shown. Other R&D departments, originally owned by a company, have become completely independent. A typical example is Aveka, Inc. of Woodbury, MN, USA (Section 15.1), a “particle processing and custom research group”. It was founded in 1994 with the divestiture of 3M Corporation’s fine particle pilot plant. The facility had been set up for the development and testing of toner before being converted to general particle processing in 1986. In 1994 it was purchased by two former employees. After adding large scale manufacturing in 1996 and food processing in 1997, Aveka has become an R&D, service, and small- or large-scale particle processing group. Their philosophy is stated as follows (www.aveka.com): “We believe that particle processing for our customers should be approached with the idea of providing the best product at the lowest price by using the correct technology or combination of technologies, rather than force-fitting a method to produce a marginal product. This means that we must have a wide range of flexible processing equipment for both small- and large-scale processing. We also have an extensive commitment to developing new proprietary processing for internal use and for licensing to our customers. We welcome the opportunity to define, refine, and implement traditional and novel process solutions. Finally, we recognize the need to be able to characterize our
9.2 Tolling Operations, Contract Manufacturing
Fig. 9.19 Partial reproduction of a brochure showing, as an example, the separate product development and manufacturing facility of a vendor (courtesy Fluid Air, Inc., Aurora, IL, USA)
processes and the resulting materials. Therefore, we have invested heavily in analysis equipment for use in the group’s process development, patent work, and process qualification.” This philosophy is in contrast to that of vendor controlled laboratories and test facilities (Section 9.1 and above) who do the work primarily to evaluate, size, and sell the company’s range of equipment and/or technologies. As a result, in addition to largescale manufacturing, Aveka group service, processing, and research capabilities include (in alphabetical order): agglomeration, blending, classification, compounding, dispersion preparation, granulation, grinding, microencapsulation, particle characterization, particle coating, particle surface modification, prilling, screening, spray drying, and more, as required.
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As another example, the contract service department of J. Rettenmaier & S€ ohne (JRS) GmbH & Co., Rosenberg, Germany (Section 15.1), although a profit center, is still part of the parent company. However, JRS is not an equipment supplier but one of the world’s leading manufacturers of cellulose based fibers and their derivatives, which are used in a variety of applications, such as tabletting, mostly in the pharmaceutical industry to improve hardness, friability and/or disintegration values, or as multi-functional additives for food and a variety of technical products [B.97]. To be able to enter new fields and promote the use of various fibers in novel solid compounds, it became necessary to offer test and small production facilities, the latter originally to allow the evaluation of market acceptance by the potential customer. In the meantime, as shown in Fig. 9.20, a new building housing the contract service department, was designed and constructed at the company’s headquarters complex. In addition to the basic laboratory and test functions, the establishment consists mostly of storage areas for raw and intermediate materials, equipment and products, processing bays, in which machinery is installed and/or systems are erected for specific jobs, and receiving and shipping facilities. Fig. 9.21 is a collage of diagrams from the company’s catalogue describing the capabilities. The sketch in the lower right shows, as an example, a system that was put together at the request of a customer combining mixing, compacting, granulating, screening (i.e., dry granulation by compaction/granulation), packaging and palletizing.
Fig. 9.20 Building housing the JRS contract service department at the company’s headquarters complex (courtesy J. Rettenmaier & S€ ohne (JRS), Rosenberg, Germany)
9.2 Tolling Operations, Contract Manufacturing
In this service facility an increasing amount of contract manufacturing is performed, work for the production of intermediate components, which contain fibers and disintegrants. The resulting materials include the functionality as provided by the company’s products and are used for the formulation of the customer’s final specialty, for example easily dispersible pharmaceutical tablets or detergent compacts. Another independently owned company is Stellar Manufacturing Co., Sauget, IL, USA (Section 15.1). From the beginning, this plant (Fig. 9.22) was designed as a back-up and co-manufacturing facility for a nearby supplier of chemicals. Owing to the seasonal demand for an agglomerated (granulated, briquetted, or tabletted) chemical product, additional external processing capacity was desired by this chemical manufacturer without tying down money for systems that would be only used part of the time. As required by the original client, Stellar’s experience is in the processing of particulate solid chemicals. Extensive capabilities (Fig. 9.23) for particle enlargement and reduction are available and manufacturing is accomplished in integrated systems, which meet the client’s needs in quality, cost, and timeliness of supply. For size enlargement, mechanical and rotary punch-and-die presses produce tablet sizes from 250 mg to 1200 g, exerting 5–200 t of force and compaction/granulation systems, equipped with 50, 75, and 150 t roller presses, yield granular materials that are packaged for a variety of industrial markets. As in all other tolling plants, the facilities of co- and back-up manufacturers need large warehousing areas for receiving and storing the raw materials, often in controlled environments, and for the collection of packaged products for shipment (Fig. 9.24). Although protecting and respecting their client’s confidential information, to be profitable, off-site co-manufacturers, such as Stellar Mfg. Co., must also attract other customers in an effort to guarantee constant employment and avoid idle capacity. As a result, the emphasis of the owners of such facilities is in finding long-term partner-
Fig. 9.21 Collage of diagrams from the JRS contract service department catalogue describing the capabilities (courtesy J. Rettenmaier & S€ ohne (JRS), Rosenberg, Germany)
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9 Development of Industrial Applications
Fig. 9.22 Artist’s impression and photographs of the Stellar back-up and co-manufacturing facility (courtesy Stellar Manufacturing Co., Sauget, IL, USA)
ships and collaborations by providing on-time, on-budget, well-planned quality manufacturing services. They also strive to enhance relationships by focusing on improvements in product form and quality and cost reduction. The growing desire for out-sourcing instead of new investments support the developments described above. Especially in the pharmaceutical industry, intermediate or final products are produced by specialized tollers and co- or back-up production is established to meet peak demands. On the other hand, the large research departments and sometimes even the production plants of globally active pharmaceutical companies increasingly offer their facilities to outside customers for the development of new drugs, pre-production services, involving precise pilot-scale programs that can circum-
9.2 Tolling Operations, Contract Manufacturing Fig. 9.23 Some of the major capabilities of Stellar Mfg. Co. for particle enlargement, reduction, and sizing, product handling and packing and support facilities (courtesy Stellar Manufacturing Co., Sauget, IL, USA)
vent processing problems, delays, and unexpected costs, and trial manufacturing for marketing purposes and initial supply before full-scale production is set in motion (Section 15.1). More recently, tolling companies have been specifically formed to accept waste materials and industrial by-products for conversion into secondary raw materials. Waste processing facilities are set up in many countries in central or otherwise strategic locations at which, by combining the materials from different sources, large installations can be fed and maintained, thus lowering the conversion costs per unit mass. Municipal refuse combustion plants are typical examples in which agglomeration may
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9 Development of Industrial Applications
Fig. 9.24 Sketch of the layout of a tolling plant showing its extensive warehousing areas (courtesy Stellar Manufacturing Co., Sauget, IL, USA) and distribution facilities (courtesy IFP, Inc., Faribault, MN, USA)
9.3 Scale-Up
be performed to compact the metallic remains or shape the often toxic ashes into inert non-leachable pieces for safe disposal (Section 8.1). In several locations special collection and processing plants have been erected for the treatment and agglomeration of metal bearing particulate wastes into secondary raw materials for use in various metallurgical facilities (Section 8.2). As already mentioned, at certain times tolling companies also offer their processing capabilities during project development for the testing of new materials and the evaluation of products. Although, in most cases, a major percentage of the manufacturing facilities are dedicated to supporting a limited number of customers with particular needs and to the production of specific products, tolling companies may sell their services on the open market and can, for example, do the pilot plant stage in the development of a large project. Another possibility is that, as mentioned for pharmaceutical applications, new materials are made for exploratory (marketing) purposes and/or to bridge the gap between product demand and supply during a new facilities’ start-up phase or, later, to perform co- and back-up-manufacturing, thus smoothing widely varying market requirements or satisfying an unexpectedly large sales volume.
9.3
Scale-Up
It has been stated several times before that the development of all agglomeration techniques is still more an art than a science. After concluding a pre-selection, which is a desk job but is based on laboratory evaluations of the feed solid’s properties, it becomes necessary to carry-out additional investigations, particularly, for example, to determine if and potentially what kind of a binder must be added and how much of it is required. Then, tests with actual equipment must be conducted to find limitations in regard to capacity and product size, shape, and characteristics. Needs of peripheral equipment, for post-treatment and of closed loop processing and recirculation must be also evaluated. All results from tests and evaluations at often many different locations and with various equipment, sometimes also including information from in-line testing and from pilot plant operations, are collected, compared with related know-how and experience, if available, and used for the engineering of the new plant and the selection of process equipment. In spite of all the efforts that normally go into the determination of these data, it is prudent to include safety factors during the design stage, which will allow optimization of the system if and when it comes on stream. This engineering exercise is called the scale-up process, during which the limited information, often obtained in small scale, is translated into the sizing of equipment, the design of system(s), and the expected operating performance and the typically guaranteed production capacity and product quality. Scale-up from laboratory or small equipment faces different problems for the various agglomeration methods. Scale-up is also more or less of concern in certain industries. For example, for practical reasons, new registration, approval, and validation requirements in the pharmaceutical industry do no longer make it feasible to produce
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9 Development of Industrial Applications
a final solid dosage form with larger equipment than used during the development phase. The entire design information and detailed process and operating parameters are part of the ongoing validation process, which is defined in the approval documents for a particular drug. Therefore, in-house or external laboratories are now often equipped with the same machines that are later intended for manufacturing and actually used during production. As sales grow, larger capacities are achieved by a duplication of entire systems, which are sometimes provided by external co- or back-upmanufacturers (Section 9.2). This applies only for the final formulation (Section 6.2). Large quantity production of components, both pharmaceutically active drugs and excipients, and intermediate feed materials, such as pre-agglomerated powders and powder mixtures, may still be produced in large central locations for a multitude of finishing plants. While the composition and cleanliness of these materials is also controlled by the appropriate authorities, the manufacturing facilities are not scrutinized to the same extent as those producing the final specialty and dosage form. Other difficult scale-up considerations may be encountered and caused by the size of the investment that ultimately hinges on successful agglomeration at the end of the process. Such projects may comprise a multitude of components. For example, the BHP hot briquetted iron facility in Port Hedland, Western Australia, included, in addition to the 2.5 million t/y Finmet direct reduction plant (Section 6.9.2), the construction of an entire infrastructure with ore receiving through an upgraded rail line from a distant mine, the energy supply (gas and electricity), an ore concentrator, and a new deep harbor for product shipment with large vessels. Excluding the mine, railroad and harbor but including all other infrastructure the project was budgeted to cost $1.5 milliard (US: billion). Later this amount was considerably exceeded. Since the Finmet direct reduction plant processes fine ore in a fluidized bed, the product (DRI), even after cooling, would be so reactive that it could not be stored safely and transportation would be prohibited (Section 6.9.2). Therefore, to make the investment a success, a passivation process must convert the material from this merchant plant into an inert product that is suitable for road, rail, and worldwide ocean shipment. The solution of the problem is to discharge the DRI hot and shape and densify it with special equipment (Section 6.9.2) to yield HBI (hot briquetted iron). The 12 large roller presses that are required to process the 2.5 million t/y of hot, fine DRI represented a value of about $20 million. The total value of the passivation system, including briquette separation and cooling, product handling, associated installations such as dust collection, water cooling and cleaning, utilities, controls, and the structure (building) to house this part of the plant had a value of about $ 100 million. Compared with the entire budgeted cost, this represents less than 7 % and the exposure of the briquette machine manufacturer alone is just a little over 1 %. On the other hand the entire project risk hinges on this small portion of the entire investment. If the hot briquetting system does not perform as predicted and the product is not sufficiently passivated, product can not be sold to the prospective overseas customers and the plant is a failure, at least until another, better method is found, developed, built, and installed, which would idle the facility for a long time, require extensive modifications, and cost substantial amounts of money.
9.3 Scale-Up
In the case in question, a single, old, and much smaller fluidized bed direct reduction plant with hot briquetting was operating in Venezuela and a shipload of the Australian ore could be processed during a test run. Nevertheless, innovations throughout the new facility, including the roller presses, and the always present changes of physical and chemical material properties at the actual site, still resulted in considerable project risks. If the possibility of a large-scale test in a similar plant would not have been possible and/or the commercial benefits of the new installation, which was based on already known successes elsewhere, would have been less convincing, construction of a much smaller, less costly pilot plant and scale-up in small steps would have been necessary to limit the technical and financial risks. In addition to these industry and project related concerns, scale-up of the agglomeration methods themselves are not easy. In general it can be stated that the problems encountered during the scale-up of equipment for the processing of particulate solids, including all agglomeration technologies, are so complicated that they can not be solved with numerical mathematics. If at all, they can be only overcome with the aid of partial similarity. In addition, apart from the geometrical and process-related similarity, the material similarity must be considered too (Section 9.1). Therefore, aside from using the results of tests and factors derived from experience, the mathematical method to approach scale-up in all areas of mechanical process technology is dimensional analysis [B.104]. When considering the mechanisms and kinetics of tumble/growth agglomeration [B.48, B.97, Section 5.0] it becomes obvious that smaller containments and masses of tumbling particulate solids translate into lower forces that act during impact and coalescence and as separating forces in the system. This results in weaker bonds and more porous structures but, because the forces of the moving environment are small, also in less destruction. These conditions, in turn, may and generally will result in higher binder requirements, lower strength, quicker dispersibility, and differences in a whole host of other agglomerate characteristics if they are later compared with products from larger scale industrial operations. For the two best-understood and most easily defined tumble/growth agglomeration methods, the drum and the pan, the similarity approach that is based on dimensional analysis has been used before [B.48] to determine scale-up factors. For drum agglomerators, the following variables are considered (to reduce the complexity of the problem their number has been limited to the most important ones). D drum diameter [L] n drum speed [T–1] g gravitational constant [LT–2] C capacity [MT–1] l drum length [L] i inclination (pitch) of drum [dimensionless] N power input to drum [MLT–1] tr residence time in the drum [T] q density of feed [ML–3] R recirculating load [MT–1]
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9 Development of Industrial Applications
Dimensions are: L, length; T, time; M, mass. The following dimensionless equation was derived from the above variables (N/Cl) = k1 (Dn2/g)c1 (qnD3/C)c2 (qD4/Ntr)c3 (C/R)c4 (D/l)c5 ic6
(9.1)
where k1 is a conversion factor. The exponents c1 to c6 are unknown and, for similarity considerations, need not be known. The significance of each dimensionless group is as follows. (N/Cl) This group shows that the power consumed is not a linear function of the drum capacity. (Dn2/g) This group is characteristic of rotating machinery. It is the ratio of gravitational to centrifugal acceleration (Froude number). This group characterizes the ratio of the volume of material being (qnD3/C) processed to the available equipment volume. (qD4/Ntr) this group represents the physical work required (D[qD3]) divided by the work input. (C/R) This is the ratio of capacity to circulating load. (D/l) This is the familiar diameter to length ratio that is typical of characterizing rotating cylindrical drums. From similarity considerations, the following scale-up factors can be derived n 1/D0.5
(9.2)
C D2.5
(9.3)
N D3.5
(9.4)
tr D0.5
(9.5)
The above assumes that, along the length of the drum, first growth occurs and, then, in the latter part, densification of the agglomerates takes place. This means that the power consumed by balls being densified increases faster than that due to increased throughput. Therefore, drums cannot be scaled-up geometrically. In scaling-up a drum, either the l/D ratio must be made smaller or the pitch of the drum must be decreased to lower the residence time. A similar dimensional analysis can be made for the balling pan. Again, to reduce the complexity of the problem the number of variables has been limited to the most important ones [B.48]. D pan diameter [L] H rim height [L] n pan speed [T–1] d pellet diameter at discharge [L] C capacity [MT–1] M mass in the pan [M] g gravitational constant [LT–2] q density of feed [ML–3]
9.3 Scale-Up
c b
angle of repose (of the material) [dimensionless] tilt angle to the horizontal of pan [dimensionless] The following dimensionless equation was derived from these variables (Mg/qn2D4) = k2 (Dn2/g)c7 (Mn/C)c8 (H/D)c9 (d/D)c10 cc11 bc12
(9.6)
where k2 is a conversion factor. The exponents c7 to c12 are unknown and, for similarity considerations, need not be known. The significance of each dimensionless group is (Mg/qn2D4) This group is a special form of Newton’s law of inertia. By substituting M from N nMD, the group transforms into from which the power requirement for the operation of balling pans (Ng/qn3D5) can be derived. (Dn2/g) This group is characteristic of rotating machinery. It is the ratio of gravitational to centrifugal acceleration (Froude number). (Mn/C) This group characterizes the residence time, M/C, in the balling pan. (H/D) This is the relationship between rim height and pan diameter. It must be geometrically analogous to (d/D) for scale-up. (d/D) This is the relationship between pellet diameter at discharge and pan diameter. If the diameter of the discharging agglomerates should be kept constant, when increasing (scaling-up) the pan diameter, the rim height in (H/D) must be reduced. In addition to the angle of inclination b (tilt angle of the bottom of the pan to the horizontal), the angle of repose of the material c plays an important role in the development of the pattern of charge motion in the balling pan [B.48]. In both cases, particularly in the balling pan, the influence of externally introduced forces by scrapers disturbs the pattern and the scale-up considerations sufficiently that the introduction of factors that are derived from experience, is required. In addition to guidelines obtained from dimensional analysis, the results of experiments and the already repeatedly mentioned experience factors must be applied. Mixer agglomeration, which features a multitude of mixing elements and often only temporarily operating knife heads [B.97], eludes the successful use of only the dimensional analysis. In growth/tumble agglomeration, after using similitude to scale-up the equipment and its operating parameters, stochastic effects causing collisions and coalescence but also abrasion and natural or forced crushing followed by renewed coalescence and growth (Chapter 5, Figs. 5.3 and 5.4) take place, which can not be predicted mathematically and modeling is only possible when a number of limiting assumptions are introduced. Therefore, it is normally necessary to adjust and optimize the large scale operation in the field. This optimization is easiest for the pan agglomerator [B.48]. Due to segregation in the depth of the bed, with the smallest (powder and seed) particles travelling near the bottom and the largest agglomerates (ready to discharge over the rim) concentrated on the bed surface, the competition between gravitational and centrifugal forces results in
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9 Development of Industrial Applications
a fanning-out of the charge as shown in Fig. 9.25. While the largest pellets remain in a kidney-shaped area on top of the bulk of the charge (rising quadrant of the pan), there are distinct paths of differently sized agglomerates whereby the smallest particles are found closest to the pan bottom and the opposing wall (rim). By selecting areas in which binder spray and powder feed are added, the operator can determine the size of the balls that will discharge from the top of the bed. In regard to the individual trajectories, binder liquid is always added ahead (i.e., in the top half of the pan) of the powder. The latter than adheres to the wetted particles. Size control is accomplished by which size fraction is fed. For example, if the finest particles (in a counter clock-wise rotating disc, Fig. 9.25, farthest to the left) are wetted and powdered, these particles grow fastest, the bed is lifted-up, and smaller pellets discharge. In contrast, if binder and powder feed are added closer to the vertical centerline, the already bigger balls grow, a lower amount of seeds is produced and larger agglomerates are formed and discharge over the rim. The area and extent of separation of the trajectories shown in Fig. 9.25 can be modified by changing the rotational speed and/or the inclination of the pan. In addition to the procedure described above, pellet size can be also influenced by the material
Fig. 9.25 Diagram of the paths of differently sized agglomerates in a balling pan during operation
9.3 Scale-Up
throughput. Higher material flow rate, that is, lower residence time, result in smaller agglomerate sizes. Since even the largest discs are or can be at least temporarily open and, therefore, the motion of the charge can be observed, an optimization in regard to pellet size and properties can be easily accomplished by changing speed, inclination, throughput, and locations of liquid and powder addition. The effects on pan performance can be visually observed and confirmed. In all other tumble/growth agglomeration apparatus, the charge movement that results in collisions, coalescence, disintegration, and reattachment of particles and pieces of agglomerates occurs in a closed containment and can not be observed. Such equipment may be operated batch or continuous. What has been produced can only be judged when the batch or the discharge (in continuous operation) are available for evaluation. Continuously operating methods can be scaled-up and controlled easiest. Already during testing in smaller scale equipment, a wide range of agglomerate sizes is produced, which is classified by suitable means into product with a desired or acceptable size range and over- and under-sized material. Both off-grade material (the overs after size reduction) are returned to the agglomerator and play an important role in growth kinetics [B.48, B.97]. The yield of product or the recirculation rate of the off-grade material, the latter often amounting to several hundred percent, is determined during testing and scaled-up with experience factors. During equipment scale-up the additional recirculating load must be considered in sizing to obtain the desired production capacity. Particularly in the pharmaceutical industry, the need for extreme cleanliness and the requirement to limit worker exposure to the drugs has led to the development of onepot growth agglomeration methods (Section 6.2.1). All processing steps, including mixing, binder addition, agglomeration, size control (sometimes by disagglomeration with knife heads and re-agglomeration), drying, potentially coating, and cooling, are carried-out in one apparatus, which is only opened at the end of the entire procedure. At that point, the major portion or preferably all of the product should feature the desired characteristics, that is, the granules must have the correct size, distribution, strength, and structure, be free of dust and flow easily. The major problem on which the success of such processes and of batch growth agglomeration methods in other industries hinges is the need to understand what happens in the closed, inaccessible apparatus and control the final batch properties. The first can be investigated during testing by periodically stopping the process, analyzing the charge, and determining the progress of agglomeration [B.48], also including the effects of operating knife heads, if applicable, and transferring this knowledge to the commercial installation. Control of final batch properties, particularly in regard to size distribution, is difficult since it is not feasible to sample the contents of batch operating equipment during a production run and determine the progress of agglomeration and the state of the charge. Therefore, methods have been proposed to monitor the performance of such machines by, for example, recording the sound of the tumbling charge and its change or measuring the momentary energy consumption of the drive motor and using this information to determine the endpoint of agglomeration [B.48, B.97].
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Measurement of the power consumption during growth agglomeration in a batch operating drum or mixer results in a curve as characterized by the schematic representation depicted in Fig. 9.26 [B.97]. In phase I, powder mixing takes place. Towards the end of this phase, the duration of which has been determined beforehand during laboratory blending tests, the addition of binder begins. Shortly afterwards, in phase II, liquid bridges develop between the powder particles and nuclei and smaller agglomerates are formed. The increase in power consumption is due to the added mass of binder and the increased cohesiveness of the charge. In phases III and IV, the interparticle volume in the growing agglomerates fills with liquid until, at the end of phase IV, complete saturation is reached. In phase V, uncontrolled growth and sticking to walls and tooling occurs. From a control point of view and for practical purposes, phase III is the most important one as only in this range of liquid saturations agglomerates are obtained that feature acceptable size distribution and quality. If a granulating drum is not only equipped with mixing tools but also with knife heads [B.97], it is in this phase that agglomerate size adjustment is accomplished by intermittently activating the high speed cutter heads. The above refers to the high density tumble/growth agglomeration methods with and without shear (for definitions and explanations see Chapter 5 and [B.48, B.97]). Although, in most cases, the entire process must include a post-treatment, during which the final strength and other properties of the agglomerates are obtained, in the context of this book, scale-up is first and foremost concerned with the tumbling and growth procedures that result in the discharge of green agglomerates. Materials, binders, and products must be tested, performance of small-scale equipment estab-
Fig. 9.26 Diagram depicting the result of the measurement of power consumption during growth agglomeration in a batch operating drum or mixer [B.97]
9.3 Scale-Up
lished, and the results scaled-up as described above. The sizing of commercial posttreatment systems, - in most cases dryers or other thermal equipment, sometimes size reduction, classification, transportation, storage, metering, and other associated techniques, such as, for example, environmental control methods -, is performed by external suppliers. In the end, the group that has responsibility for the entire system, either in-house or external, must apply safety factors and common sense to come up with what needs to be procured and built. Agglomeration in low density fluidized beds is different in that at least part of the drying takes already place during growth and in the equipment itself. The stochastic movements of gases, droplets, and particles, the occurrence of coalescence and bonding and the mechanisms of drying can be modeled and mass, particle size, heat and moisture balances can be established [B.49, B.72, B.93]. Nevertheless, it is an art, performed by experienced personnel of specialized vendors, to successfully scale-up from the performance of test units to commercial equipment and systems. Capacity increases are often obtained by installation of multiple feed points (gas and powder inlets, liquid spray nozzles), which are housed in suitably sized and shaped containments (e.g., towers). So far, the sizing of machines for the agglomeration in stirred suspensions and immiscible liquid agglomeration is totally based on common sense approaches, experience with similar applications and trial and error. Thickeners are primarily designed to handle the amount of contaminated liquid at low flow conditions, which do not disturb the accretion (assisted by flocculation agents [B.48, B.97]) and the settling behavior of the suspended agglomerating solids. It is much more difficult to carry out small-scale tests for most of the pressure agglomeration methods and obtain results that are meaningful and can be used for process development and scale-up. The technique that lends itself best to small-scale evaluation is low pressure agglomeration (Chapter 5, Fig. 5.10, and Section 6.2.2), which may be followed by spheronization. Since in this method of pressure agglomeration a wet mixture is passed through the openings of a screen or a thin perforated sheet, very little pressure is exerted and it is essentially a shaping process [B.97]. Therefore, even if tests are performed on a small perforated die, in regard to product characteristics, the results are also representative for larger units. Many of the low pressure agglomeration methods use a screw to transport the extrudable material into a chamber that is enclosed by a screen or perforated thin sheet, where it is pressurized and passed through the openings, often assisted by some sort of a wiper blade [B.48, B.97]. As in all other cases, scale-up of these extruders begins with testing on smaller scale machines. After extensive evaluation of formulations, equipment, and process arrangements, an optimal set of parameters is defined that includes information on a particular products extrusion rate, bulk density (before and after extrusion), power consumption, and, if applicable, temperature rise (below). An efficiency factor is then calculated by dividing the actual extrusion rate, as obtained during testing, by the calculated theoretical (maximum) extrusion rate Q = F (q2D2nh sinf cosf)
(9.7)
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where Q = volumetric output [MT–1], F = frictional coefficient [dimensionless], q = density of feed [ML–3], D = screw shaft diameter [L], n = screw speed [T–1], h = height of screw flight [L], f = (angle of ) screw pitch [dimensionless]. Efficiency factors, which are not only influenced by partial filling and friction but also by material sticking to the barrel, the screws, and the extrusion blades, are in the range 5–35 % for axial, 15–55 % for radial, and 35–85 % for dome extruders (Chapter 5, Fig. 5.10a3–a5). They, together with additional manufacturers’ experience factors, are applied to the theoretical extrusion rate of the commercial extruder. Scale-up of spheronizers depends on the mode of operation [B.48, B.97]. For batch units it is purely volumetric. Each machine has a typical operating (working) volume, which is a function of bowl diameter (Fig. 9.27). These relationships also apply for each batch spheronizer in a quasi-continuous (using multiple batches) system. For continuous (cascade) operation, the plate is lowered in the bowl so that a certain volume always remains inside. This residual volume can be either measured experimentally or calculated, assuming that the cross section of the rope resembles a fourth of a circle (quarter torus). If material is fed to the unit at a constant rate, the average residence time, tr, a most important parameter that had been previously determined during testing, can be calculated tr [T] = residual volume [L3] / volumetric feed rate [L3T–1]
(9.8)
Since bowl diameters are fixed by the manufacturer, the plate height is the only variable that is changed to match a certain feed rate to the required residence time. The plate speed is scaled-up from small (test) units to large (commercial) machines by maintaining the tangential acceleration. From this results the scale-up formula n22/n12 = R1/R2
(9.9) –1
with n = rotational speed [T ] and R = bowl radius [L]. Medium pressure agglomeration in pellet mills can be also easily simulated because even a single bore with the correct diameter to length ratio and featuring all other details of the orifice (e.g., inlet chamfer, discharge cone, relief bore [B.97]) can be used to determine the extrusion characteristics of a particular feed material and
Fig. 9.27 Operating (working) volume of batch spheronizers [B.97]
9.3 Scale-Up
the properties of the extrudate. Because, in extrusion, the densification pressure results from wall friction, a considerable increase in the temperature of the die and potentially damaging temperatures of the product may be experienced during continuous operation. This is especially possible if long bores (high pressure) are necessary to achieve pellet strength and if the ratio l/d (bore length/bore diameter) is large. Tests must be run sufficiently long to determine whether such danger exists. To avoid these problems, binders and/or lubricants may have to be added to the feed to allow shorter bores and/or reduce wall friction. The capacity of a commercial machine can be calculated by multiplying the throughput of a single bore with the number of all bores in the large scale die-ring or -plate [B.48, B.97] and applying factors (efficiency and safety) that are derived from experience. Machine sizes that result from this calculation feature drive designs and sizes that are sufficient for the purpose. Samples of punch-and-die pressing can be produced in a variety of simple home made or purchased small machines using mechanical actuation or hydraulic pressurization with hand or motor pumps. A large number of sometimes highly sophisticated and automated laboratory presses is also available [B.97]. They are used for the determination of a variety of strength and force or pressure related product and machine characteristics and, although the densification and compaction mechanisms in a punch-and-die press are quite different from those of roller presses and can not be correlated, punch-and-die compacts are often made and evaluated to preliminarily investigate the compactibility and strengthening of different feed materials or powder mixtures and to determine the type and amount of potentially necessary binders. Tabletting research and development can be carried out in single station punch-anddie presses, which use different drive mechanisms. If used for high precision tabletting and/or R&D work, instrumentation is added with which data can be recorded, stored, and processed. For tabletting research, determination of the pressing force over displacement (densification) diagram is of great value [B.97]. Fig. 9.28 shown an example. Scale-up is very simple. In essence, after determining the compression speed and force, also including a potentially necessary dwell time (Section 6.2.2 and [B.97]) that are necessary to obtain the desired compact quality, the commercial machines that are available in “standard” sizes from the manufacturers, are selected according to capacity. For eccentric machines [B.97] the number of strokes per unit time (e.g., per hour) define the production. If multiple dies and related punches are installed, this result must be multiplied by the number of cavities to arrive at the total capacity. For rotary presses [B.97], the production per unit time is defined by the number of dies per table (one or more per station (individual location on the rotating die holder table or turret)) multiplied by the rotational speed. In simple terms: A rotary press produces as many compacts per revolution as there are dies on the table. This capacity can be doubled by installing two feed locations and associating a complete pressing cycle with one half of the turret. In this case, the stations are filled twice on opposite sides of the rotating table and two compressions are carried-out in each die per revolution of the turret. For the production of very small tablets, some manufacturers offer more than two feed stations per table, which increases the output per revolution accordingly.
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9 Development of Industrial Applications
Fig. 9.28 Example of a pressing force against displacement (densification) diagram [B.97]
Although all important factors, including the compressibility of the feed material and its formulation for successful compaction and the mechanical parameters, such as gravity or force feeding, compression force and speed, residence time under pressure (dwell), ejection and discharge mechanisms and sometimes more specific others (particularly if complicated shapes are produced [B.48, B.97], Chapter 7), must be and have been determined during the initial test phase, optimization by performing final adjustments must be carried-out in the field, sometimes at the beginning of each production run. These modifications are performed manually and compacts produced during this period of time are discarded. In general, during start-up, whether operating parameters are changed or not, product is preferably bypassed until equilibrium conditions are reached. For evaluating isostatic pressing in the laboratory a specially designed press chamber can be inserted between the platens of a suitable press [B.97]. Scale-up is volumetrically, maintaining the gas or liquid environments, pressures, and temperatures that were found necessary for achieving the necessary densification and desirable product quality during testing. The most difficult laboratory evaluation is that of high-pressure roller presses because the conditions in the nip between the two counter rotating, converging roller surfaces depends on so many parameters that it is practically impossible to accurately
9.3 Scale-Up
predict the performance of commercial presses with small machines. Low pressure roller presses that accomplish the shaping of plastic materials, for example of coal fines with binder (Section 6.10.2), are scaled-up volumetrically (Eqs. 9.12 and 9.13, below). As mentioned above, the compactibility of a particulate solid, the development of strength, and the potential need for modifications of the feed (e.g., particle size and distribution, composition, temperature, binder) can be evaluated with a laboratory punch-and-die press. However, this result only provides an answer to the question whether or not a material can be compacted and under which conditions. The pressure used during such an experiment, for example, can in no way be converted to the force that a roller press will have to exert for achieving the same good quality compacts. Although the typical compaction curve (Fig. 5.9, Chapter 5, and [B.97]) can be measured, in the converging nip between the two counter-rotating rolls of a roller press (shaded area in Fig. 9.29), a specific volume element, on which the maximum, mechanically or hydraulically produced force acts, can not be defined. Therefore, for the comparison of press performance, the specific force, a physically meaningless designation, has been defined. It is the operating force divided by the active width of the roller and measured in kN/cm. If, for example, the roller width between the cheek plates is 0.75 m and the average total force exerted by the pressurization system onto the floating roller is 5000 kN, the press operates with a specific force of 66.7 kN/cm. This parameter and whether or not force feeding [B.13b, B.48, B.97]
Fig. 9.29 Conditions in the nip between two counter-rotating smooth rollers during the passing of a particulate solid
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is required must be experimentally determined in a roller press. Low pressure roller presses require a specific force of about < 20 kN/cm and the specific force exerted by high-pressure roller presses may be as high as 150 kN/cm. As the roller diameter can not be easily scaled-up, in the test press it should be as close to what is foreseen for the commercial application. This is due to the fact that the conditions in the nip vary with changing relationships between nip geometry and particle as well as compact size and/ or thickness. They become totally unpredictable when the roller surface is not smooth and, as is normally the case in high-pressure roller presses, the structure, size and shape of the particulate solids change during densification and compaction. The nip shape and size are quite different between rollers with different diameters (Fig. 9.30). It can be easily recognized that, if the circumferential speed vc is kept constant and, therefore, the angle of nip a remains approximately constant, the throughput is the same but the densification and compaction behavior are quite different. Between larger rolls, densification occurs much less suddenly and such important processes as deaeration and conversion of elastic into plastic deformation [B.97] are achieved more completely. Even though circumferential speeds are normally in the range 0.5–0.9 m/s (only for easily compactable materials, such as common salt, vc may be as high as 1.5–2.0 m/s), it should be recognized that the entire compaction process in the nip happens in fractions of a second.
Fig. 9.30
Nip shape and size between rollers with different diameters
9.3 Scale-Up
The following are useful practical equations for sizing presses Circumferential speed: vc = p 2r rpm 1/60 Throughput (compaction) Qc = p 2r s l rpm 60 c (briquetting) Qb = z V rpm 60 c Relationships: D – p p2 (D2/D1)1/2 p1 D–s s2 (D2/D1)1/2 s1 s–p p2 (s2/s1)1/2 p1 with
and understanding roller [m/s] [kg/h] [kg/h]
(9.10) (9.11) (9.12) (9.13) (9.14) (9.15)
2r = D roller diameter [cm] rpm roller speed [1/min] s average sheet thickness [cm] l roller width [cm] c apparent density* [kg/cm3] z number of briquette pockets per roller V average briquette volume [cm3] p specific force [kN/cm] (* average of the compacted sheet or the briquettes) More equations can be found in the literature [B.9, B.13b, B.48, B.55, B.71]. If the specific force, the average density of the product, and the desired throughput capacity of the machine are known, with the sheet thickness or the briquette size a roller size can be calculated and a machine can be selected based on its maximum specific force capability. The drive power is determined from the torque requirement during testing and safety factors are added to all machine and process parameters. As always, after installation it is normally necessary to readjust and optimize all conditions in-line. In conclusion, test equipment is available in all areas of interest for the determination of feed and product characteristics, including new techniques that have been developed in response to advancements in modern mechanical process technology and to new applications for the manufacturing of novel, for example, engineered products. However, testing is only as good and predicts industrial performance of the projected plants as correctly as test conditions reflect what will be found later in the actual installation. This means that, as a general rule, tests must be carried out with representative samples of the specific, unaltered particulate solids that need to be processed by an agglomeration method. Although, as reviewed in other publications (Section 13.1) and made available by vendors (Section 15.1) in their brochures and newsletters, certain characteristic relationships have been developed for most agglomeration methods and performance factors can be collected in charts for the pre-selection of methods that are most probably suitable for a particular application [B.48, B.97], determination of the actual design parameters remains a serious problem. For cost reasons, even if only a limited selection of process equipment is used, simulation of the continuous operation of an entire production line is normally not possible during testing. In those cases where, in the actual plant, recycle will be produced and recirculated in one way or another, product is first made from
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the fresh feed, the expected type(s) of recycle is (are) produced, and, for further testing, the anticipated amount(s) is (are) mixed with the fresh feed or other material streams, such as, for example, the crusher or screen feeds, to simulate the conditions in a continuously operating system. Installation of a pilot plant on-site and/or in-line should be considered if the risks, which are always connected with new installations, are to be minimized. While the pilot plant approach during project development must be investigated and decided upon on an individual basis, testing of equipment is always necessary. For that reason, essentially all manufacturers and/or vendors maintain sometimes rather elaborate facilities (Section 9.1 and [B.48, B.97]), which normally feature machines of different sizes, including, in some cases, large scale equipment, to avoid scale-up problems. These test facilities must also include some peripheral equipment, such as mixers, heaters or coolers, conveyors, crushers, dryers, screens, etc., although the variations that are available in these special areas from outside sources can not be offered. Therefore, additional tests for the evaluation and selection of the best peripheral equipment are often necessary at different facilities.
Further Reading
For further reading, particularly on the use of dimensional analysis, the book “Scale-up in Chemical Engineering” by M. Zlokarnik [B.104] is recommended.
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Optimization and Troubleshooting of Agglomeration Systems “Engineers know failure is an option” is the title of a short editorial by Henry Petroski [10.1], Professor of Engineering and History at Duke University, Durham, NC, USA. In his article he quotes Theodore von Ka´rma´n as saying: “Scientists seek to understand what is, while engineers seek to create what never was”. For mechanical process technology as a whole, including all agglomeration systems, this is true as an engineer designing a plant for the processing of particulate solids, even if it is, on the surface, an exact copy of another, earlier facility, must consider it as a totally new endeavor and expect new responses, experiences, and, possibly, failures (Chapter 9). In this context and with this in mind, here are a few more quotes from Professor Petroski’s piece. “A common misconception about how (engineered devices, machines, or systems) come to be is that engineers simply apply the theories and equations of science.” As discussed earlier (Chapter 2), the unit operations of mechanical process engineering, although representing the oldest technologies known to and used by mankind, began as a science only rather recently. At that time, an effort was made to combine the extensive, mostly empirical knowledge that had been accumulated during sometimes hundreds of years in specific fields of human activities and to search for their common roots in an interdisciplinary way. Although, today, some theories and equations are available, the above statement still holds true as a multitude of material properties play decisive roles in how a particular process will perform. Since such important characteristics of the participating particulate solids as size, distribution, macroscopic and microscopic shape, internal structure and related properties (e.g., porosity, density, strength, thermal and electrical conductivity), surface topology and chemistry, and many more will define the instantaneous and the long-term interactions between the particles, a large number of variations is possible. Even after extensive material testing and the evaluation and consideration of past experience (Chapter 9), process design, scale-up (Section 9.3) as well as hardware and software selection include so many uncertainties that a particular performance can often only be guaranteed within rather broad limits. “The design of any device, machine, or system is fraught with failure. Indeed, the way engineers achieve success in their designs is by imagining how they might fail. Much of the design is thus defensive engineering. Obviously, total success can only come if every possible mode of failure is identified and defended against.” In this Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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10 Optimization and Troubleshooting of Agglomeration Systems Tab. 10-1 Examples of guide words used in a HAZOP study (adapted from an ICI HAZOP seminar) Guide Word
Meaning
No or Not More of Less of As well as Part of Reverse Other than Sooner (later) than
An activity or a response ceases or does not take place A quantitative increase in an activity or a response A quantitative decrease in an activity or a response A further activity or response occurs in addition The incomplete performance of an activity or response The inversion of an activity or response A substitution of one activity or response for another An activity or response occurring at the wrong time
respect, testing of all system components, preferably with the actual material (Sections 9.1 and 9.2) and consideration of past direct and similar experience with the materials in question, the process, and the design engineering are indispensable. A method for carrying out process evaluation during project execution and/or immediately prior to entering start-up procedures was formalized and pioneered in the early 1960s by ICI (Imperial Chemical Industries, UK) [10.2]. The technique, called Hazard and Operability study (HAZOP), is designed to minimize sources and occurrences of failure. Basically, it is a deviation analysis. For the HAZOP study, a definition of the future purpose of the system or plant is required, which is then examined in detail to determine all possible variations from that purpose. This definition could be a simple description or a list of the events and of their sequence that are intended and must be carried out to achieve a certain goal. It is then examined by a team of people with varying backgrounds and expertise in a systematic way [10.2], looking at all process stages and plant components individually (steps) and using guide words to identify potential deviations from what was intended. The guide words should be appropriate for the particular situation being studied and are employed to assist the study team in imagining what deviations are possible. Each step (stage and component) of the process is examined in turn using all the guide words. A general set of guide words, originally developed from method study techniques, is listed in Tab. 10.1. Such guide words are utilized by asking questions such as: “What happens if ... does not occur?”, or “What happens if ... does occur?”, or “What happens if more of ... occurs?”, or “What happens if less of ... occurs?”, etc. The questions are addressed to any piece of hardware and its components, any software, or any parameter that can be imagined when evaluating the intent. If such events can occur then the question is asked “Does it matter?”, will it cause a problem that is of concern and if the answer is “Yes”, methods to remedy the situation must be available or introduced. Often, the installation of redundant equipment or controls may be necessary. There are two very important elements in this approach. Firstly, the study is systematic, it is carried out in exactly the same manner every time, and secondly, without
10 Optimization and Troubleshooting of Agglomeration Systems Tab. 10-2 Helpful phrases for a study leader to expose reality (adapted from an ICI HAZOP seminar) Sensitizing
Comforting
Good grief The deviation will only be seconds It will cost a fortune The news services will love it The whole site will shut down for days We will need two more operators We can’t start-up We can’t operate without it All the fish will die People will smell (see) it miles away
So what, so what ... ? The deviation will last for hours or days We are talking pennies here Who’s to notice? We shut down for an hour at most So he has to walk a little farther So you have to open two valves So he has to climb some stairs without it It’s only a drop in the ocean People won’t notice a thing
exception, all the guide words are used for all the steps. Any situation may be examined in this way, provided there is an adequate description of the future intent. In the process industry, a HAZOP study is most commonly applied to process line diagrams. A standard P&ID (Process and Instrumentation Diagram) is a description of the future intent, typically the steady state of a continuous process. Batch systems can also be studied but in those cases a description of the batch process steps is required in addition to the P&ID. In systems of mechanical process technology (powder and bulk solids), at various points, the characteristics of the solids and, if applicable, their interactions with each other and with process fluids and their possible deviations must be included in the study. When examining a system using guide words, people new to the method tend to confine their thinking to the particular component under study and, especially, to the one they know best. This is a mistake. All participants should allow their thoughts to indiscriminately investigate every item and go both up- and down-stream to identify potential causes of deviations and consequences or other problems of concern. Generally, causes and consequences must be explored in depth as the two are closely linked. A deviation may lead to a certain consequence that, in itself, may be the cause of other consequences elsewhere in the plant. Remedial actions must be taken if problems are identified. It is tempting to consider only hardware changes. HAZOP studies have been blamed for an unnecessary proliferation of instrumentation, particularly alarms, which can be very expensive and counterproductive for the continuity of process operation. It should be remembered that software solutions, such as changes of operating procedures or methods of operation, can also eliminate problems. The level of concern about problems identified by a HAZOP study will vary widely. Some participants may wonder what the fuss is all about while others are very excited with a high level of concern. It is useful to consider the sequence of events from the time the deviation occurs to the time the hazard or problem sets in. Sometimes the concern arises immediately and must be eliminated, but if the sequence of events takes time, then the team can consider how the problem’s progress can be interrupted
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or modified to avoid the hazard. In any case, it is helpful for the study leader to have a number of phrases available to prompt the team members in an attempt to obtain consistency in the level of concern, either by sensitizing or comforting (Tab. 10.2). Such phrases are designed to point out reality and get agreement from the team. In this respect, Prof. Petroski states [10.1] that while “engineers are pessimists, managers are optimists about technologies.” Managers, especially those responsible for the budget, think that “successful, albeit flawed (projects) indicate ... not a weak but a robust” system and “because (engineers) are humbled by their creations, (they) are naturally conservative in their expectations of technology. They (the engineers) know that the perfect system is the stuff of science fiction, not of engineering fact, and (therefore) everything must be treated with respect.” As a result of these differences in opinion and approach to a new project between project management and engineering, a common problem exists, which often culminates in, on the one hand, the pressure of management to conserve money, go with the cheapest vendor, offer, or subcontractor, and avoid redundancies and, on the other hand, the fight of engineering to be overcautious, select only the most well-known vendors, and install a large number of alarms, enunciators, electronic safeguards, and redundant process paths. Suppliers, faced with the increasing possibility of costly law suits after accidents involving their equipment or because of performance below specification, will design a multitude of safety features into their scope of supply. Machine manufacturers, in particular, tend to include all components and insist on all operating procedures that have, in the past, been developed to avoid dangerous situations and/or have successfully averted operating problems. This inflates the price, reduces competitiveness, and often causes unnecessary downtime that result from the installation of excessive safety features. To remedy such costly complications, a reality check at the beginning of order execution, using the HAZOP analysis and evaluating its concerns, as indicated in Tab. 10.2, should determine the level of necessary design and safety features that are required for a particular job. Tab. 10-3 [10.4]) *
* * *
* * *
*
*
Draft principles of “Green Engineering” (adapted from
Processes and products should be engineered holistically using systems analysis and integrating environmental impact assessment tools. Natural ecosystems should be conserved and improved while protecting human health and well-being. Life-cycle thinking should be used in all engineering activities. It should be ensured that all material and energy in- and outputs are as much as possible inherently safe and benign. Depletion of natural resources should be minimized. Waste prevention should be endeavored. Engineering solutions should be developed that are cognizant of local geography, aspirations, and culture. Engineering solutions beyond current or dominant technologies should be created. * Improve, innovate, and/or invent technologies that achieve sustainability. Communities and stakeholders in the development of engineering solutions should be actively engaged in new projects.
10 Optimization and Troubleshooting of Agglomeration Systems
As discussed in Chapter 8, the design of modern plants and systems must take into consideration pollution prevention and waste minimization, particularly by employing recirculation of valuable by- and waste-products. Raw material conservation and clean production as a result of “green engineering” can greatly reduce waste generation with up to 200 % internal rate of return [10.3]. The proposed principles of green engineering are summarized in Tab. 10.3 [10.4]. In view of the now often required validation and documentation, especially in the pharmaceutical, medicinal, and food but increasingly also in other industries (e.g., specialty and fine chemicals, high-performance powder metallurgy, and nanotechnology), conventional and multi-product or -purpose plant designs must be weighed against the use of modular systems [10.5, 10.6]. For conventional plants, engineering, procurement, and construction are carried out in a logical fashion, with some parts done in parallel and some in series. The goal is to complete the project as expediently and cost-effectively as possible. Equipment and materials are procured from worldwide suppliers and delivered to the site where roads are paved, foundations poured, structural steel is erected, equipment is set, and piping, electrical wiring, and controls are completed, all in accordance with drawings, specifications, and standards developed during the detailed engineering phase of the project [10.5]. In a multi-product plant, some or all of the process equipment is intended for the manufacturing of a number of different products. Occasionally the same term is used for a multi-purpose plant. Generally, the difference between the two depends on whether the processes and products are known from the outset (multi-product) or are likely to change (multi-purpose, also called general-purpose). The pharmaceutical and specialty and fine chemical process industries typically require multi-purpose plants, although they may be designed as multi-product facilities. In fact, it is not uncommon for a multi-purpose plant to be initially sanctioned on the basis of specific multi-product requirements that, due to new, often unforeseen circumstances, change once the plant is erected and operating [10.6]. For many of the newer projects, the conventional approach may not be the most costeffective method for their execution. Factors that may favor one design and construction approach over another, thus directly or indirectly affecting project feasibility and execution, include infrastructure, labor availability, weather conditions, and logistics of transporting materials and equipment to and products from the site, schedule considerations, and permission issues [10.5] (Chapter 9). Prefabrication, pre-assembly, modularization, and off-site fabrication may be warranted for a given project. The decision for modularization (complete or partial) and any of the other construction or execution methods should be made early during the conceptual phase of the project. Although, in many cases, adoption of these procedures has resulted in cost savings, such advantages are not assured and a decision should not be made lightly. A final consideration for a new project is the implementation of a suitable, workable, and sustainable process safety management (PSM) [10.7]. This incorporates a management systems approach in such areas as operating procedures, hot working, incident investigations, management of changes, and employee and contractor safety. PSM has been successful in reducing incidents since formalized and effective management
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systems reduce the variability in the ways people do their work and help to ensure that important tasks are not omitted. In this respect, PSM must be as simple and efficient as possible and must not include unnecessary requirements. During the design of new industrial plants, their installation, and operation, people from different disciplines are involved who often lack a common background and an understanding of the availability of new, improved technologies and methodologies. Therefore, the resulting installations commonly include stunning misconceptions (Chapter 9). It is still typical that a new employee in any of the participating disciplines, who has completed a formal education at school, college, or university, is trained by an experienced person who mentors the new employee and transfers his or her accumulated knowledge. This involves extensive discussions of the process, its equipment, performance, and past problems. Circumstances that occur at the fringes, such as the evolvement of wastes or by-products, are commonly treated almost as an afterthought, although consideration of these materials and effluents are becoming most important (Chapter 8 and Tab. 10-3). A new proposal suggests [10.3] to mentally step outside the box, defined by the plant design, study the product(s), waste(s), by-product(s), and effluent(s), and ask how the input(s) to and the operation in the box can be modified to optimize product yield and minimize or beneficially use all other streams (Tab. 10.4). When carrying out the analysis according to Tab. 10.4, it should be realized that the only materials that are truly valuable to the business are the raw material(s), any intermediate(s) and additive(s) that are required for the process, and the final product(s).
Tab. 10-4 *
*
*
*
Effluent stream analysis (adapted from [10.3]
Other than the product(s) list all components of streams exiting the process along with their key parameters. For example, for solids these could be the physical (particle size and distribution, bulk density, angle of repose, moisture content, etc.) and chemical (main and contaminants, separate or intermingled, etc.) characteristics and conditions; for a waste liquid stream they could be content of water as well as organic and inorganic compounds (both dissolved and suspended), pH, surface tension, viscosity, etc.). Identify those components triggering concern, e.g. Hazardous Air Pollutants (HAPs), carcinogenic compounds, materials regulated under the Resource Conservation and Recovery Act (RCRA), etc. o Determine the sources of these components in the process (box). o Develop process improvement options to reduce or eliminate generation of these components. o Develop and use procedures for the recirculation of valuable components (secondary raw materials) or those covered by the RCRA. Identify the highest volume effluent component. This material, together with others (see below), controls the investment costs (equipment and processes must be sized to also handle these components) and significantly impacts on the manufacturing cost. o Determine the source of this highest volume component in the process. o Apply process improvements to reduce or eliminate generation of this component, recirculate it to the feed end of the process, or manufacture secondary raw material(s). If the highest volume effluent component is successfully minimized, eliminated, or beneficially used, identify the next (set of) components that have a large impact on investment and operating cost (or both) and repeat the above procedure.
10 Optimization and Troubleshooting of Agglomeration Systems
Although this approach must start with the effluent streams and move backwards through the system(s), it must be done during the conceptual engineering to be fully effective. In contrast to chemical processes, where the feed materials and process aids (catalysts) are in most cases well defined, clean, and consistent, the particulate solids in mechanical process technologies, especially if they are or were derived from naturally occurring raw materials (minerals, concentrates), may vary widely in composition and/ or physical characteristics. As mentioned before (Chapter 3), when agglomeration processes are involved, the surface properties of the particles (macroscopic and microscopic shape, roughness, contamination by adsorption and absorption) are of particular importance and modifications that are sometimes difficult to detect, may require a new set of operating parameters or the introduction of, for example, binders, surfactants, or other additives. During a typical project execution, after testing the materials (Section 9.1), possibly piloting the process (Section 9.2), scale-up (Section 9.3), and the evaluation of the most desirable plant execution and lay-out (above), management often insists on minimum investment, minimum operating cost, and maximum yield or throughput. Finally the design is further optimized to maximize profit. Once the plant is operating, the management requires to run the process continuously to generate revenue; typically there is no opportunity for modifications or optimization because the business needs the cash. For installations of mechanical process technology, whether new or adapted to different process conditions, feed materials, and/or products, these limiting preconditions on design, selection, execution, and initial operational availability should be avoided. Particularly if agglomeration processes are involved (above, Chapter 3, [B.48, B.97]), the material characteristics and equipment performances can change dramatically from those found during testing (Section 9.1). Therefore, plant design should always include some extra capacity, sometimes even redundancy, and sufficient time during start-up and initial operation to allow adjustment of hard- and software settings to the actual conditions and optimize the process to realize the best possible product yield and quality. In the following sections, deviations that are often experienced and common methods of optimization and troubleshooting the different agglomeration systems will be described. In some cases, design considerations will be mentioned also. However, it should be recognized that the treatments can not be exhaustive. Rather, the author, with the help of several colleagues, has tried to compile ideas that will be helpful in recognizing the potential problems and developing suitable remedies. The project engineer(s), with the help of experienced vendors, mentors, and consultants and by applying formalized methods for project evaluation (above), must try to understand the potential influences and deviations. The type and extent of remedies and precautions that become part of the plant design must then be evaluated by a team that includes all disciplines, the future owners, and/or the operator of the facility.
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10.1
Tumble/Growth Technologies
Most deviations from the anticipated system efficiencies, design expectations, and product properties of size enlargement by tumble/growth agglomeration are rooted in the mechanisms of bonding and aggregate structure. Reference should be made to Fig. 3.6 (Chapter 3) and Figs. 5.1 and 5.3–5.8 (Chapter 5). Size enlargement by growth agglomeration is caused by the natural adhesion of particles to each other, which may be enhanced by the addition of binders. The tumbling motion, during which collisions between particles occur and coalescence is initiated, is produced mechanically by the equipment used for the process (Fig. 5.2, Chapter 5). Adhesion of particles and growth of agglomerates is only possible if, at the time of collision, bonds between the newly impacting particle and the substrate are created instantaneously that are greater than the opposing components of all destructive forces (Fig. 3.6, Chapter 3). This mechanism also includes attachment to the relatively rough surface of a particle assembly that was formed earlier and, due to its topography, allows the particle to move to an energetically preferred position for permanent bonding (Fig. 10.1). The growth agglomeration process is greatly influenced, on the one hand, by the size and distribution of the primary particles, their shape, their macroscopic and microscopic structure, their chemical and physical surface properties, the presence and characteristics of binders and their interaction with the agglomerate forming solids, and
Fig. 10.1 Conceptual model describing how a small particle is incorporated into the surface of a (wet) agglomerate during tumble/growth agglomeration [B.48, B.97]
10.1 Tumble/Growth Technologies
many more. On the other hand, the separating forces, trying to prevent adhesion and growth, are influenced by the type and size of the equipment used to produce the tumbling action. The aggregates that are typically formed by these methods are first “green”, that is, moist, and held together primarily by the capillary attraction caused by the surface tension of liquids; since capillary attraction is only a temporary binding mechanism, the agglomerates reach final bonding and strength only after some post-treatment (normally involving heat or chemical reactions). Although covered in various parts of earlier chapters and the author’s prior books [B.48, B.97], in the following the main factors are summarized, their influence on potential deviations presented, and methods for optimization or troubleshooting discussed.
10.1.1
Particle Size
The size of the particulate solid feed to an agglomeration system is a most important property. For the successful application of any of the tumble/growth agglomeration methods, the particle size must be small. This is because almost all separation mechanisms, challenging the adhesion force during agglomerate growth, are related to size. Primarily this is the mass of the particles, which (together with the gravitational force) defines their weight and, in moving systems, results in, for example, inertial or centrifugal forces. Especially in fluidized beds, it is also the aspect ratio or the particle projection, which result in drag forces. Since for the evaluation of the influence of particle size, only a single parameter can be considered, the surface equivalent diameter, xs, of a particulate feed is used (Chapter 5, [B.48, B.75, B.97]). Typically, this diameter should be in a range below 100– 200 lm.
10.1.2
Particle Size Distribution
Fig. 10.1 shows a model with uniformly sized, spherical particles to explain the permanent attachment of a new particle into the surface of a growing wet agglomerate, yet narrowly sized particles of similar shape are not well-suited for growth agglomeration. In fact, only submicron (nano) monosized particles do naturally adhere to each other and form loose aggregates (Chapter 11). To be applicable for growth agglomeration, technical powders should feature a particle size distribution: it is especially desirable that a sufficient amount of fines is present. As shown by the two-dimensional models in Fig. 10.2, the best effect of fines in the structure of agglomerates is obtained if they fill the voids between larger particles (densest packing). They do not only reduce the void fraction but, because of the increased number of coordination points per unit volume (Chapter 3), the strength of the agglomerates is higher.
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10 Optimization and Troubleshooting of Agglomeration Systems Fig. 10.2 Sketches of the systematic arrangement of differently sized particles to obtain dense packing [B.97]
The previously mentioned representative surface equivalent diameter, xs, is calculated from the specific surface area (m2/g) of a bulk powder. Therefore, it takes into account the respective surfaces of all particle sizes. Since the mass decreases with the third power of diameter but the surface area only with its square, the specific surface area of smaller particles has a relatively larger influence on the surface equivalent diameter. This means that the requirement of xs < 200 lm can be reached while some of the particles in the feed are much larger, for example up to 1 mm in diameter. The model in Fig. 10.3 shows that, if a sufficient quantity of fine particles is available, a matrix-like structure forms around larger pieces, which may also consist
10.1 Tumble/Growth Technologies Fig. 10.3 Model explaining how fine particles embed larger pieces in the structure of an agglomerate [B.48]
of recirculated, undersized agglomerated material. As defined by the representative surface equivalent diameter, the overall strength of such an agglomerate is largely determined by the fines. Therefore, agglomerates incorporating a few large pieces can and do survive.
10.1.3
Particle Shape
Because all binding mechanisms interact with the surface of the participating particles and in tumble/growth agglomeration this interaction occurs to a large extent at the coordination (i.e., contact and near) points, macroscopic and microscopic shape play an important role for the two properties defining the methods’ success: growth and strength. Macroscopically, particles may be represented rather subjectively by comparison with “standard shapes” (Fig. 10.4). The most easily recognized one is the sphere which, for that reason, is often used for modeling. Alternatively, a factor describing the deviation of the real particle from a sphere is also commonly applied for shape
Fig. 10.4 Some geometrical approximations of the form and proportions of particles [B.97]
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10 Optimization and Troubleshooting of Agglomeration Systems Fig. 10.5 “Standard set of shapes” for the determination of particle sphericity according to Rittenhouse [B.48]
characterization (Fig. 10.5). The major problem of determining and describing shape is that “particle size” is presented as a one-dimensional length (“diameter”); outlines for comparison, according to Fig. 10.5 for example, are two-dimensional, and real particles are three-dimensional. Therefore, this property of particulate solids, which is so critical for the structure and bonding of agglomerates, is presently unsatisfactorily defined [B.97] and results in many, potentially rather detrimental deviations from design and process expectations, especially of tumble/growth agglomeration systems (below). Microscopically, it must be realized that in nature and technically there is no such thing as an absolutely smooth surface: it requires only a large enough magnification to make the roughness visible. Since wetting properties and the ability of particles to approach each other closely depend on the microscopic surface topography (Fig. 10.6), this characteristic is very important for the success of growth agglomeration and small variations, which are often difficult to detect, may result in major process deviations.
Fig. 10.6 Model sketches depicting the influence of surface roughness on the approach of particles to each other (left) and wetting (liquid bridge formation, right). The crosshatched area is the actual
bridge. a* represents the mean distance between the particles, the outlines of the ideal particles (averaging out roughness), and the theoretical bridge contours assuming perfect wetting
10.1 Tumble/Growth Technologies
10.1.4
Chemical and Physical Surface Properties
The interactions between solids (and, if applicable, binders) that cause agglomerate bonding and strength, are strongly dependent on the chemical and physical surface properties. The latter are mostly influenced by the surface topography (above and Fig. 10.6). Most of the tumble/growth agglomeration processes are wet methods; this means that to enhance adhesion, liquid binders are added, which must themselves adhere to the solids. This is only possible if the liquid wets the solid. Many chemical surface modifications can influence wetting. They may involve contamination with oily components or other hydrophobic coatings and chemical reactions, such as oxidation, all of which may change the wetting characteristics and agglomerate growth and strength.
10.1.5
Binder Interaction
As already repeatedly mentioned, tumble/growth agglomeration requires in most cases the activation of an inherent binder component or the addition of a suitable liquid to enhance adhesion and prevent the destruction of bonds by ambient forces. The molecular forces between dry solids are only strong enough to withstand separating system forces if the particles are ultrafine (submicron- or nanosized) (Section 6.7.1 and Chapter 11). To achieve good interaction between the solid(s) and liquid binder(s) it is not only necessary to provide a wetting surface as discussed above but also to distribute the liquid uniformly on the surface and in the structure of the growing agglomerates. This requirement is influenced by the manner in which the liquid is added to the tumbling mass of particles and to what extent shear is created for secondary liquid distribution by destroying overwetted masses.
10.1.6
Equipment Type and/or Size
The tumbling action that is necessary to produce particle collisions and coalescence may be achieved in many containments and by various methods of agitation [B.48, B.97]. While substantial driving forces, causing the stochastic movement of the solids for agglomerate growth are required, that same mechanism is responsible for stressing the newly created bonds, thus hindering agglomeration. In many cases, a delicate balance between these two influences must be found and maintained, which is easily lost during scale-up and/or process modifications (Section 9.3).
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10.1.7
Optimization
Optimization seeks to obtain the best product quality with the highest reliability and the lowest effort. In tumble/growth agglomeration this means that the process should be carried out with minimal requirement for hands-on operator involvement and realize high yield (i.e., low recirculation rate due to over- and under-sized output). While recycling, particularly in continuous but also in many batch systems, plays an important role for the kinetics of growth agglomeration [B.97], excessive amounts of recirculating material increase costs due to the need for oversized equipment within the system as defined by the recycle loop. Optimization involves the search for the best possible feed particle size distribution, which must include the percentage and composition of the recycle, the position(s) and method of binder addition, sometimes the type of build-up control (e.g., scrapers), and the operating parameters of the equipment (speeds, residence time, etc.). During the selection of the best feed particle size, the surface area of the fresh feed should be measured and the resulting surface equivalent diameter calculated. To be safely below the experimentally determined particle size limit for the specific application, the fines content may have to be adjusted. To avoid surging caused by variable recycling rates, an average percentage of recovery must be determined and metered into the feed from a surge hopper. Uncontrolled reintroduction of recycling material should be avoided. Although particle shape plays an important role during growth agglomeration, especially by influencing structure, porosity, and, thereby, green strength as well as final quality of the new entities, it is seldom determined or even modified during the development, scale-up, and optimization of a new size-enlargement process. Normally it is assumed that this particle parameter remains unchanged from the material as tested and of the one used later in the industrial plant. As will be shown below under “troubleshooting”, this assumption is not always true and may be the cause of problems. The need for and application of binder(s) is determined early in the development phase, typically during the first bench-scale testing of material(s) to be agglomerated (Chapter 9 and Section 9.1). When planning the addition of binder(s) in pilot and, especially, industrial-scale systems, the growth mechanisms must be envisaged by the designer of the agglomeration unit (e.g., pan, drum, mixer, fluidized bed). While during the test and development stages manual binder addition and distribution is commonly utilized, both must be automatic and optimized in the semi-industrial or full-scale equipment. Based on the mental modeling of what happens in the tumbling batch or continuous charge (compare Figs. 5.3 and 5.4, Chapter 5) it must be decided whether the experimentally determined amount of binder(s) is added continuously or intermittently, at one point or several locations, and by what means. As discussed in Chapter 5 the method of application, finely or coarsely atomized or as intermittent or continuous stream(s), influences nucleation, growth, structure, and size distribution of agglomerates but also potential build-up in the apparatus and, if applicable, on tools. In the case of batch operation, binders are typically added intermittently, potentially with knife head operation in between to help distribute moisture, destroy
10.1 Tumble/Growth Technologies
oversized agglomerates, and allow the build-up of new, more favorably structured agglomerates [B.97]. In continuously operating equipment, the introduction of binder(s) is normally distributed along the agglomerator’s first section (up to 1/2 to 2/3 of the total length) and liquids are atomized with nozzles [B.97]. Equipment (Agglomerator) Size
Agglomeration and the separating forces that are trying to prevent adhesion and growth, are influenced by the type and size of the equipment and the energy used to produce the tumbling action. When transferring the experience gained during laboratory or pilot testing, scale-up considerations (Section 9.3) must again include a good understanding and mental visualization of the mechanisms of growth agglomeration and of the destructive forces that are caused by the equipment’s operation (Figs. 5.3 and 5.4, Chapter 5). Since industrial plants process large amounts of solids, the forces required to transport these masses and to cause the turbulent, stochastic movement of individual particles, nuclei, and agglomerates are much bigger than those experienced in the smaller developmental systems. Therefore, it is not unusual that additional and/or more effective binder(s) is (are) required or the feed size must be adjusted to feature a smaller surface equivalent diameter of the fresh input, possibly combined with modified recycle properties, such as smaller size obtained by a suitable mill to be installed in the return loop. The application and installation of turbulizers, knife heads, and/or other tooling causing shear that was found desirable or necessary in small scale testing must be carefully evaluated during scale-up. Typically, their location in an industrial unit does not correspond with that in the laboratory equipment as the relative filling with material (as a percentage of the container volume) and the resulting distribution of the charge may be changed dramatically. In most cases, the time(s) and the frequency and duration of operation (e.g., continuous or intermittent) will have to be modified and optimized. Continuously operating industrial installations differ from laboratory equipment and systems by the need for long time availability. Because it is almost impossible to avoid undesired build-up in and coating of the interior of large-scale growth agglomeration units by non-mechanical means, scrapers or flexible wall designs are used to remove these unwanted deposits (Chapter 4). Lumps or slabs of often very wet and sometimes strongly bonded material can, at least temporarily, upset the movement of the agglomerating charge and cause variations in the output properties (e.g., surging, resulting in changing recycle rates). Therefore, the influence of such disturbances must be also considered during scale-up and optimization. Because, in most cases, binder added for growth agglomeration is liquid, the discharge from equipment in which size enlargement occurs is moist (green) and only temporarily bonded by the effects of surface tension and capillary forces. With few exceptions (e.g., pan and some fluid bed agglomerators) a wide size distribution is obtained which, without removal of over- and under-sized material, is not acceptable as product. Since separation techniques that do not blind when processing wet solids,
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for example roller screens [B.48, B.97], are only suitable for rather specific applications and particle ranges, final product sizing is typically accomplished after drying. During the optimization of system design, the actual condition of the recycling, either wet or dry and possibly containing some residual binder, must be carefully evaluated as it may influence the kinetics of agglomerate growth and the addition (amount and locations) of new binder. 10.1.8
Troubleshooting
Even if the considerations discussed above are carefully followed during scale-up and the design of industrial units and systems, problems, typically associated with unacceptable capacity, process yield, and various changes of product properties, may be experienced during the early phases of running a new plant or after a long successful period of operation. The first of these inconsistencies (i.e., during initial operation) can be solved by optimizing the design parameters. In most cases this can be accomplished by modifying the locations and respective amounts of binder addition, the residence time in the equipment (for instance by changing the machine set-up (e.g., inclination) and/or the speeds of rotational equipment and accessories or of the fluidizing gas), the operating schedule of, for example, knife heads, if applicable, or, in some cases, the composition and or condition of the feed, including recycle (e.g., adjusting particle size distributions, premixing of components and additives, prewetting). It is more difficult to understand and remedy problems that occur, often without premonition, after the long and successful operation of a system or plant. In such cases it is necessary to go back to the fundamentals, try to envision what happens mechanically, physically, and sometimes even chemically during agglomeration, understand for the specific case the mechanisms of particle interaction, determine what, how, and why something has changed, develop corrective measures, and implement suitable modifications. Adsorption Layers
To explain a common problem, its reason, and its possible elimination, the very basic agglomeration of UFPs in a fluid bed shall be described. This process uses the natural, binderless adhesion of submicron (nano) particles and, therefore, can be easily evaluated. Nevertheless, it is surprising that, with the increasing availability and use of UFPs, the problem often arises in industry and still causes puzzlement. A typical description of the problem is as follows: the agglomeration process works as designed and without problems during most of the year but in winter it is sometimes impossible to obtain the desired size enlargement and bulk characteristics. Or, less frequently: during certain periods, agglomerates are produced but the desired densification (bulk density) is not reached. Such deviations from the expected performance occur, for example, during the agglomeration and densification of silica fume (Section 6.7.1, Fig. 6.7-9).
10.1 Tumble/Growth Technologies
When evaluating such problems, it can be assumed and also confirmed by electron microscopy that the particle sizes and shapes have not changed. Since the chemical composition and physical properties are also unchanged and the only applicable binding mechanism is caused by the naturally occurring molecular adhesion force (vander-Waals) between solids, the different agglomerative behavior must be explained by the presence, absence, or variability of adsorption layers. At ambient conditions the adsorption of atoms and/or molecules (mostly water) from the atmosphere is a natural phenomenon. Its extent is mostly controlled by the relative humidity of the surrounding gas (air). From an adhesion physics point of view, adsorption layers with a thickness of less than 3 nm are so strongly bonded to the solid that they are immobile and must be considered as an extension of the particle diameter. Therefore, as shown in Fig. 10.7, the average distance between two particles “a*” (compare Fig. 10.6) is reduced by the presence of adsorption layers to the smaller equivalent distance “a”. Because the van-der-Waals attraction force is inversely proportional to the square or third power (depending on the model situation at the coordination point [B.48, B.71, B.97]) of the equivalent distance, UFPs stick together more easily and strongly if this dimension decreases. Although the amount of moisture in adsorption layers is very small and the adsorbate is so tightly bonded that it can not be determined with regular equipment for the measuring of moisture [B.97], the following explanations for the behavior as described above are proposed. *
*
In many geographical areas the humidity is very low during winter. Since the gas used for the fluidization of silica fume (Section 6.7.1, Fig. 6.7-9) is ambient air, little, if any, adsorption of moisture takes place. This, in turn, reduces the enhancement of bonding due to adsorption layers, which takes place during other seasons of the year and results in little or no agglomeration. In the other case, the particle size and distribution of the UFPs is such that agglomeration proceeds well without the enhancing effect of adsorption layers. However, during hot and humid summer months adsorption of moisture does take place which,
Fig. 10.7 Model explaining the effect of adsorption layers on van der Waals bonding [B.48, B.97]
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because of increased van-der-Waals forces between the particles, results in the formation of loose aggregates. These entities are no longer destroyed by the acting field forces (gravity, drag) and, therefore, can not be rebuilt into dense agglomerates. If such undesirable performance occurs often and disrupts the operation of the granulation and densification system excessively, the air used for fluidization must be processed to achieve constant moisture content and, possibly, also constant temperature. The best conditions should be determined in an optimization step and then kept at all times. In most other cases that require troubleshooting due to unacceptable variations, many more parameters are involved in the building of undesirablegglomerates, in their growth and their green and/or final quality. A few possible causes will be mentioned below but it should be understood that any characteristic of the solids, binders, and/or additives or of the equipment involved in the process may have unintentionally changed and, therefore must be investigated and evaluated. After one or more changes have been identified, efforts must be undertaken to return to the original conditions or to develop a modified procedure that will result in the desirable system performance and product quality. Particle Size Distribution, and Shape; Physical Surface Properties
The importance of particle size distribution, and shape for the efficiency of growth agglomeration has been discussed in detail above. Normally it is assumed that these properties remain unchanged during the plant’s entire life. Because, today, owners and operators have understood the controlling effect of feed composition for the performance and results of tumble/growth agglomeration, the feed attributes of raw materials, including their particulate conditions, are in most cases specified when ordering from sub-suppliers and controlled upon receipt. They may be also adjusted during feed preparation on site, either alternatively or additionally. Typically, particulate solids are defined as being entirely between two size limits (xmin < x < xmax) or featuring certain percentages below or above specified sizes (for example, 90 % < x1; or max. 10 % > x1 and 5 % < x2). Still other specifications may define an average particle size (e.g., x50 = 120 lm, where x50 represents the 50 % point of the cumulative size distribution curve) or several representative sizes (e.g., x5 = 10 lm, x50 = 120 lm, and x98 = 300 lm); in these cases an acceptable interval may be defined, too (for example: x50 = 120 lm +/– 5 lm or: x5 = 10 lm +/– 2 lm, x50 = 120 lm +/– 5 lm, and x98 = 300 lm–20 lm). Predominantly, a single representative dimensional size (using particle length (diameters) as defined by a specific particle size analyzer [B.12, B.48, B.97, 10.1.1]), sometimes with an acceptance interval, is used, which allows infinite variability of the actual distribution above and below. It is even possible that, in reality, bi- or multimodal distributions are represented by such a specification (Fig. 10.8). Remembering the discussion above in regard to particle size and distribution it is easily understood that, in spite of maintaining specifications, the actual particle distributions may be so much different that the performance of growth agglomeration processes is
10.1 Tumble/Growth Technologies Fig. 10.8 Several different particle size distributions responding to the same specifications (x50 = 120 lm, x5 = 10 lm, x98 = 300 lm)
changed, which does result in variations of kinetics (i.e., production of different agglomerate sizes and distributions, influencing product yield) and/or agglomerate qualities. Such problems are rather common after a longer period of operation. When investigating the particulate composition of the feed and determining that the actual size distribution has changed, the variation can be often explained by the effect(s) caused by the replacement of process equipment (normally crushers or milling circuits) down the line (e.g., at sub-suppliers or during feed preparation, upstream). If an old crusher or a milling circuit breaks or becomes obsolete and is replaced by a similar but new machine, the operating parameters can be adjusted to produce a material that meets the aforementioned specification(s) but, as per Fig. 10.8, the product may have a different distribution. Especially variations in fines content may cause great changes in the agglomerative behavior of a particulate solid. A better characterization of a particulate feed for tumble/growth agglomeration is by the surface equivalent diameter, xs,, which would directly indicate changes in the fines content (e.g., shift to a smaller size if more fines are present). In addition, it is recom-
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10 Optimization and Troubleshooting of Agglomeration Systems Fig. 10.9 Circles (defining particle diameter) with the same area as the projections of a needle shaped particle. The situations on the right represent various positions in space (viewed in directions x and y) of the same particle
mended to determine and file the entire size distribution, used at the beginning of successful plant operation, for reference and, from time to time, carry-out checks. Deviations form the curves on file must be investigated and corrected. Particle sizes are sometimes misrepresented by the particle size analyzers due to shape. In most cases the instruments determine or calculate the diameter of the circular outline of particles passing an opening (e.g., sieving) or of circles with the same area as projections of real particles dispersed in space (e.g., photosedimentometer [B.12, B.48]. Discrepancies between the measured “particle sizes” and reality are of particular concern if the shape of the particulate solid is elongated (e.g., rods or needles, Fig. 10.9). For example, the particle size analysis of a needle-shaped material (e.g., aspartame, niacinamide, or ibuprofen, Sections 6.2.1, 6.2.3, and 6.3.3) could at first indicate that all crystallites are smaller than 20 lm and the average size x50 is around 10 lm. Therefore, it should be well-suited for tumble/growth agglomeration. Because of the fineness of the powder, even the possibility to accomplish natural, binderless agglomeration could exist. In reality, however, the material that, judged by its size characteristics, should be an ideal candidate for growth agglomeration, does not perform as anticipated. Under the microscope it is revealed that the actual particles are thin, long needles with an aspect ratio of 3 to 5 or more (Fig. 10.10) which, during particle size analysis, are represented by the diameters of circles featuring the same area as the
Fig. 10.10 Micrographs of needle-shaped particles (crystallites) featuring large aspect ratios
10.1 Tumble/Growth Technologies
projections of the true particles. In addition, different orientations in space will greatly change the projection of elongated particles, thus further misrepresenting their size (Fig. 10.9). If dense, well-structured granules are to be manufactured economically from such materials by tumble/growth agglomeration, the needle-shaped particles must be broken to aspect ratios between 1 and 2 before using them in the feed mixture (i.e., with fillers and bulking additives) for agglomeration. The direct application of elongated particles with high aspect ratios requires excessive amounts of binder(s) and other additives, thus reducing profitability. With particles that deviate much from sphericity (i.e., elongated shape, Fig. 10.5) and feature high aspect ratios, the transfer of experience gained in small or medium-sized equipment to large scale industrial machines may be a problem, too. In one case, the growth agglomeration in batch laboratory and pilot mixer agglomerators worked well and, based on the results, an industrial process was designed using large double cone tumble mixers. In the small equipment, mixing tools (plows and intermittently operating knife heads) were used to agitate a fraction of or, during piloting, a few kilograms of premixed (in a ribbon blender) material because it was assumed that tumbling alone in the laboratory agglomerator did not create high enough kinetic and shear forces to obtain good granule density. Liquid binder was added in intervals by pouring small amounts on the stationary bed of solids after stopping and opening the agglomeratormixer. The required agglomerate quality was obtained during this development and the slightly larger pilot work. Since, similar to what has been discussed above, the results of feed particle size analyses indicated that the solids should agglomerate well with binder addition in a tumbling bed and the laboratory and pilot work seemed to confirm this assumption, a large, expensive (because of extreme cleanliness requirements in a pharmaceutical manufacturing plant) granulation system, using a double cone blender for the processing of 1.5–2.0 t per batch, was designed and built. The supplier argued that the large size of the equipment, resulting in high cascading and vigorous tumbling action, combined with the atomization of binder liquid with two-phase (pressurized gas (nitrogen) assisted [B.97]) spray nozzles would not necessarily duplicate the laboratory and pilot work but produce the same granule size distribution and quality. After start-up, the time to produce agglomerates was much longer and granule size and quality were not acceptable. At that point it was recommended to investigate the shape of the solids with the Scanning Electron Microscope (SEM). Although it was well documented in the scientific literature that the crystallites of a major feed component are needle shaped, featuring large aspect ratios, this was the first time the developers saw the particles and realized their implications. Additional investigations revealed that during the small scale development work, these needles were unintentionally broken in the feed mixer and the agglomerator, a fact that had remained hidden to the researchers and developers, while in the large production scale system the crystallites remained unchanged. As a result, loosely structured agglomerates were formed, most of which held together during tumbling, and grew into large granules with low density. Either premixing the
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Fig. 10.11 Diagram and photograph of the interior of a vacuum double cone mixer with knife heads (courtesy Italvacuum, Borgaro, Italy)
feed in a high shear mixer and/or the installation of knife heads in the double cone mixer/agglomerator (Fig. 10.11, a less well-known but available design) duplicated the break-up of the needles, as achieved unknowingly during development in the laboratory, and resulted in the manufacturing of a well sized and densified granular product, suitable for further processing as specified. Although, in the above case, the particles were so small that electron microscopy was required for observation, the shape that was revealed is still the macroscopic outline. In
10.1 Tumble/Growth Technologies
Fig. 10.12 Diagram of the conditions at the coordination point between two particles [B.48]
addition, the microscopic surface roughness, as shown schematically in Fig. 10.12, plays an important role for bonding at the coordination points within the agglomerate structure. Similarly to what has been discussed before above, the surface roughness can change when upstream equipment is modified or replaced. Fig. 10.12 depicts on the left the conditions at the coordination point of two similarly sized particles (diameter x1). The representative average distance is “a”. Due to the roughness of the surface two other situations exist: Highlighted by the circles named “A” roughness peaks touch each other and circle “B” is a near-point. While at “A” high molecular forces persist and solid bridges can form, at “B” the adhesion force is much lower but a liquid bridge may form. In reality, a multitude of different interaction points are present at each coordination point between particles, which are defined by the microroughness and, in their entirety, determine bonding and strength. This surface topography is, on one hand, caused by the raw material structure and composition and the size reduction mechanism used for pulverization of the solid, or, on the other hand, by the type of particle formation (e.g., precipitation, crystallization, chemical reaction). Any modification in the feed particle preparation can and most probably will change the microscopic surface configuration and, thereby, agglomeration. This influence is particularly relevant during tumble/growth agglomeration because, in contrast to pressure agglomeration (Section 10.1.2), only relatively small external forces are acting during impact and coalescence in the turbulently moving mass. On the right of Fig. 10.12 is also shown that a much smaller particle can attach directly to a roughness peak. Since molecular forces are typically inversely proportionate to the particle size [B.48, B.97], the adhesion of such a pairing is strong as the small
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particle diameter x2 and the dimensions of the peak, not the diameter x1 of the large piece, are applicable. While, the absolute roughness declines with diminishing particle size, the influence of changing fines in a particulate mass on agglomeration is enhanced by the existence of larger particles with rough surfaces and their potential modification by variations in upstream processing. As mentioned elsewhere [B.42], although the topography of particles can be described to any, even the smallest detail by fractals, the practical application of this technique is still elusive. To nevertheless be able to evaluate the macroscopic and microscopic shape of particles and how they might change, electron microscopy should be used. The scanning electron microscope (SEM) is an excellent tool that helps to visualize and understand particle shape and roughness (Fig. 10.13). A single view or, even better, a series of pictures, preferably at different magnifications, can explain the behavior of a particular material during agglomeration. Collecting, filing, and comparing SEM photographs routinely as part of the quality assurance (QA) program of a manufacturing plant, will point to potential problems if changes are detected. To avoid unacceptable variations in plant performance, the physical par-
Fig. 10.13
SEM images depicting particles with different shapes and roughnesses
10.1 Tumble/Growth Technologies
ticle properties (including micro surface topography) must be kept within narrow limits. Corrective measures should be initiated as soon as modifications become detectable. Chemical Surface Properties: Binder Interaction
Surface properties can also change due to chemical modifications. The most common chemical reaction is oxidation by which a more or less continuous (often with “normal” methods undetectable) oxide coating or film are formed. Often this modification is called “aging” because it takes time to develop and, in most cases, occurs naturally during open-air storage, consuming oxygen from the atmosphere. Under certain conditions, some materials also react with other elements or compounds that may be present in the environment (including liquids) surrounding the solids. Modifications in chemical surface properties may influence the interaction with binders, particularly if the wetting behavior of the surface is changed (above and Chapter 9). The potential problems that arise from testing aged materials during the development phase of a new project while the future plant processes freshly produced material can also become a concern during the ongoing operation of an existing plant. If a facility consists of several different manufacturing units, it is common practice to provide intermediate, often called “emergency” storage. If upstream components of a plant combining several process systems continue operating, while, for example, the downstream agglomeration undergoes planned or unscheduled maintenance or repair, the feed material is deposited in this storage facility (often an open or covered stockpile) also to provide a buffer if the availability of plant components changes in other ways. When the downstream system comes back on line it is often directly reconnected and begins to consume freshly produced or processed material from the upstream facilities. The intermediate storage remains untapped. If the material in storage ages and is used at a much later time, when the supply situation has changed, problems may arise due to the modified surface properties and, in most cases, the variation in binder interaction. If the causes of this behavior are not recognized by the plant operators, a major process problem could be suspected and emergency activities, including the involvement of a consultant, may be initiated. Often, the problem can be overcome by using additional amounts or types of binders, which may, however, be detrimental to product quality and will, in any case, reduce profitability. In addition, if the true reasons for the variations are not determined and corrective measures have been taken, a new problem arises if the system’s feed is reverted to fresh material, either after consuming the intermediate stock or when the supply situation goes back to normal. To defeat such operational problems, the intermediate, “emergency” storage should not be an off-line facility but be included in the overall flow diagram as a surge capacity. This means that, to maintain constant operation of separate but dependent systems even during downtimes of one or the other, adequate intermediate storage capacity should be installed through which material flows, at times accumulates, and later empties. To avoid or reduce the effects of aging, the first-in-first-out principle must be maintained.
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Fig. 10.14 Diagram of the attachment of functional molecules to the surfaces of solids [B.95]. Left: a) adsorption, molecules are mobile, b) contact between neighboring molecules, c) several molecules create stable islands, d) an ordered
molecular film develops. Right: Above a critical concentration amphiphilic molecules aggregate spontaneously and form microscopic micelles in the liquid phase
Sometimes the surface of solids is or becomes hydrophobic, thus featuring bad wetting characteristics. Such materials do not perform well during wet tumble/ growth agglomeration. It is recommended to process these solids in a suitable manner by washing or chemical treatment. If possible, the hydrophobic surface layer should be removed or coated with functional molecules. As shown in Fig. 10.14, functional molecules (e.g., surfactants or emulsifiers) feature on one end a lipophilic and on the other a hydrophilic group (amphiphilic molecules, Chapter 11), attach to the surface in single layers if added in a correct amount, and render the particles hydrophilic. Sometimes the simple addition of a small amount of detergent to the binder liquid improves the wetting characteristics of slightly hydrophobic solid sufficiently for successful growth agglomeration. Equipment (Agglomerator) Type, Size, and Execution
The correct type, size, and execution of agglomeration equipment is selected during scale-up (Section 9.3) and modified during the start-up phase of a new plant (above). An example of how troubleshooting may be applied during operation for fluid bed spray granulators [B.48, B.49, B.93, B.97] is given in Tab. 10.5 Following the guidelines of this representation, similar explanations and corresponding procedures can be defined for any of the other tumble/growth agglomeration methods and equipment.
10.1 Tumble/Growth Technologies Tab. 10-5 Troubleshooting fluid bed spray granulators (adapted from an unpublished guideline by Niro, Inc., Columbia, MD, USA) Problem
Cause
Capacity
1. Leaks Keep the system tight and operate the Leaks of air into the system can steal a chamber at only a slight vacuum. significant amount of the heat input. They can also distort temperature readings and cause condensation resulting in wet deposits (see below) 2. Airflow Evaporative capacity is directly proportional to the product of mass flow of air and temperature difference inlet-outlet. A loss in capacity is often caused by low airflow which can result from several sources.
3. Incorrect Temperatures If a dryer has correct air flow but evaporative capacity is low, the temperatures must be wrong. Poor mixing of an air stream after it is heated causes a thermal gradient (determined by temperature measurements near the dryer inlet). Deposits
Corrective Measures
Check for: a: Clogged/dirty inlet filters b: Loose/incorrectly positioned dampers c: Fan operating backwards d: Worn/loose fan belts e: Obstructions in ducts or plenums f: Blinding of bag collectors g: Clogged air distribution plate. Check for: a: Broken or wrong thermocouples (TCs) b: Leaks near TCs c: Deposits on TCs d: Thermal gradient in the inlet duct work
1. Outlet Temperatures An equilibrium exists between air humidity, powder temperature, and product moisture. At a given outlet temperature, the powder will be higher in moisture if the air humidity at the outlet is higher.
The outlet temperature must be high enough to prevent wet powder which will stick to the dryer/granulator. Installation of a relative humidity meter will often prove useful.
2. Atomization Nozzles must produce a uniform spray pattern of droplets sufficiently fine to dry in the granulator without creating wet lumps in the bed or deposits on the walls.
The feed should feature constant viscosity and be free of foam and grit which can clog nozzles. Duplex strainers just before the feed pump are often used to accomplish the latter.
3. Insulation The humid air leaving the dryer is often very close to its dew point causing the danger of condensation.
Outlet ducts, cyclones, and dust collectors must be well insulated and leak free.
Particle Size 1. Conveying Control During conveying, recycle may break down more than desired for the process. 2. Screening Screens are sometimes blinded by (wet) material.
Check potential size reduction of pneumatic systems, other conveyors, rotary valves, screens, etc. and, if necessary, modify. Use cleaning devices and produce dry particles.
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10 Optimization and Troubleshooting of Agglomeration Systems Tab. 10-5 Problem
continued Cause
Corrective Measures
3. Predrying Adjust spray position. If the spray point is located too high, droplets dry completely before contacting the bed (excessive fines/no agglomerates). 4. Atomizing Nozzles The spray pattern must be uniform at all times.
Select correct or improved nozzles/ keep them clean.
10.2
Pressure Agglomeration Technologies
In contrast to tumble/growth agglomeration, pressure agglomeration technologies apply external forces, different dies, and/or various molding techniques to enhance the binding mechanisms and shape the products, (Chapter 5 [B.48, B.97]). Therefore, the feed particle size, distribution, and shape, both macroscopic and microscopic, do not have the same importance for the agglomeration process and the product characteristics and quality. The influence of shaping pressure and method on the results increases with the applied force. In high-pressure agglomeration, product structure, strength and most other properties are largely independent of feed particle size, distribution, and shape. Because of the dependence on force or pressure, the entire technology is typically subdivided into low-, medium-, and high-pressure agglomeration techniques (Chapter 5). Since low-, medium-, and high-pressure methods, their equipment, and product characteristics are totally different, in the context of optimization and troubleshooting they will be covered in two separate sections below. First, however, some characteristics that are common to all pressure agglomeration techniques will be reviewed. As first discussed in this book in Chapter 5 (Fig. 5.9) and later reiterated in one way or another in all sections dealing with pressure agglomeration technologies (particularly Sections 6.n.2) two main areas of concern must be always considered when pressure agglomeration techniques are involved: *
*
the removal of fluids from the pores during densification of the particulate mass, and the elastic behavior of solids during and after the compaction process.
Contrary to tumble/growth agglomeration, where individual particles or clusters of particles coalesce with each other and successively build a structure by material addition on the surface of a growing mass, in pressure agglomeration, a defined volume of loose, sometimes pre-processed feed material is introduced into a more-or-less closed or closing die and mechanically compressed by external forces that are caused by the
10.2 Pressure Agglomeration Technologies
movement of die members. Therefore, in the first case (growth), the pores are formed during the attachment of particles and remain essentially unchanged during further growth of the green agglomerate, while in the second case (pressure), the size of interparticle voids in the loose feed is reduced, sometimes during successive densification steps, to an often very low pore volume that finally remains in the product. Also, during tumble/growth agglomeration, the adhesion mechanism, responsible for the attachment of new matter to the outside of the new entity, is largely independent on whether the solids are brittle, elastic, or plastic. However, as shown in Fig. 5.9 (Chapter 5), after densification by the rearrangement of particles has proceeded to its maximum degree, further compaction is associated with the breakage of brittle and deformation of malleable (elastic and plastic) particles. Therefore, these physical characteristics of each and all participating solids are of utmost importance in pressure agglomeration. Removal of Fluids from Masses of Particulate Solids during Densification
As discussed elsewhere in detail (Chapter 5, [B.13b, B.48, B.97]), while particulate solids are densified, the fluids that occupy the inter-particle pore space must be removed instantly and completely as this volume diminishes. When very high compaction forces are applied, towards the end of pressing, fluids in pores within breaking and/or deforming particles must be considered, too. During this removal process, which is controlled by fluid flow and diffusion, gases and liquids behave quite differently. Driven by relatively small pressure differences, gases flow easily and quickly even through the smallest openings. Diffusion assists in this process. If the flow of gas is temporarily stopped, the medium gas is compressed and its pressure rises, which often results in the opening of new channels and, subsequently, decompression. These effects are influenced by the speed of densification. If the compaction and shaping of the particulate mass occurs very quickly, pockets of compressed gas may form inside, which need time and/or opportunity for equalization. Because the resistance to flow and diffusion is a function of pore diameter, length, tortuosity, and surface roughness, it increases with decreasing particle size, either in the feed or caused by the break-up of particles at high compaction force. Plastic deformation of particles does also reduce the size and shape of open pores or may even close them altogether. Resistance to the flow of liquids depends on the same pore parameters but is, by nature, much higher. Therefore, during typical processing speeds, it is impossible to remove liquids from the pore space. In addition, liquids are incompressible. Therefore, if the pores are completely filled with liquid from the outset or as soon as the diminishing pore volume becomes saturated, densification ceases and the applied force increases suddenly endangering the equipment if no safety features are provided. The characteristic of the pressure rise depends on the type of drive and whether it is mechanical or hydraulic.
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Influence of Particulate Solid Elasticity during and after Densification
If all or some of the particles to be agglomerated by pressure are elastic, under certain conditions, the temporary elastic deformation can be converted into permanent plastic change of shape. For this to occur, the most important parameters are time and temperature. If elastically deformed solids are kept under pressure for some time, certain structural features, such as lattices, dislocations, etc., move into stable new positions by creep. At elevated temperatures, but still well below the softening point, most solids, even those that are brittle or tough at ambient temperatures, become more malleable and deform readily under pressure, To obtain and keep the required product quality, agglomerates that are formed by external pressure must maintain their shape and density after release of the pressing force and removal from the die. If residual elastic deformation is present within the compact, elastic recovery, accompanied by an increase in volume, will occur when the pressure is released and the part is ejected from the die or leaves the shaping tools. This causes structural defects such a microcracks, which lower the strength or separation of pieces by capping (Section 6.2.2) or lamination (Section 6.9.2). To overcome the problems associated with compressed gases and their uncontrolled expansion, densification must occur slowly and/or dwell times must be designed into the pressing cycle before the compact is released. The same process conditions, potentially enhanced by heating the particulate solids to above ambient temperatures, will also assist in a sufficient conversion of elastic deformation to yield a permanent shape.
10.2.1
Low- and Medium-Pressure Methods
Low- and medium-pressure agglomeration methods apply extrusion through holes in die plates with different thicknesses. Low pressure is required for passing particulate solids through the openings of screens or thin perforated sheets (Chapter 5, Fig. 5.10a1–a5, and Section 6.2.2) [B.48, B.97]. Since only minimal densification occurs during this process, strong binding mechanisms must act between the agglomerate forming particles to maintain the shape of the extrudates. Also, the openings and the extrudate must be small in cross section (< about 2 mm diameter). While screens produce irregularly shaped, “crumbly” materials, typically round perforations in sheets result in thin, vermicelli-like ropes or portions thereof. Therefore, preconditions for the successful productions of agglomerates by lowpressure agglomeration are a small enough particle size to safely pass the openings, and the presence of a (liquid) binder. The first requirement means that the top size of any particle in the particulate mass must be smaller than the diameter of the opening through which extrusion occurs. Normally, the maximum particle size should be smaller than 50 % of the opening’s smallest dimension, in the case of mesh screens, or 60–70 % of the diameter of circular bores. In many cases, mechanical damage to the equipment is caused by non-adherence to this requirement either in
10.2 Pressure Agglomeration Technologies
the raw material(s) or when recycle is used or re-work is attempted. It is recommendable to crush recycle or re-work prior to blending it with fresh material and binder and to scalp any larger particles from the raw material components. The second precondition means that binder is required to produce the necessary plasticity and stickiness of the feed mixture to allow extrusion and provide sufficient strength. Because, normally, at least one binder is a liquid, green agglomerates, crumbs or vermicelli, are first produced, which need post-treatment. If well-defined particles are desired, the still moist extrudates may be spheronized (Section 6.2.2). Final product characteristics are always obtained after drying. Since agglomerate forming is accomplished with low pressure, non-spheronized, dry materials typically exhibit instant characteristics (Sections 6.3.1 and 6.4.1). Optimization and troubleshooting of low-pressure agglomerators/extruders involves almost always the development, preparation, and maintenance of a good feed blend by high shear mixing solid components with optimal particle sizes and distributions and the addition of plasticizers, if necessary, lubricants, if applicable, and binders; at least one of them is a liquid. The throughput (production rate) depends on the open area of the screen or die plate and the speed of the extrusion blade(s) either attached to a rotating shaft (screen and basket extruders) or the end of a single or double screw (radial, axial, or dome extruders). Operating problems, other than the presence of too large, hard particles or aggregates, are caused by incorrect plasticity of the feed and/or material build-up and plugged die openings. High moisture content, although sometimes desirable for easy extrusion, may result in a lumping together of the discharge. As with other pressure agglomeration methods (below), it is often assumed that the production capacity can be adjusted by merely changing the speed of the extrusion blade(s). While a downturn is possible over a wide range, a highest speed and, therefore, a maximum capacity exists beyond which the material begins to rotate with the blade(s) and the production rate becomes erratic or drops or operation ceases altogether. When this happens, the feed area becomes overloaded and often plugs-up, requiring stopping and cleaning the machine. If the feed becomes (too) dry, the resistance to the mass passing the openings increases and the delicate die plates of low-pressure extruders may be damaged, deform or break. If the die plates become thicker and the extrusion channels longer, the technology of pelleting is performed (Chapter 5, Fig. 5.10b1–b6) [B.48, B.97]. The perforated dies may be flat (Fig. 5.10b1 and b2) and in the shape of rings (Fig. 5.10b2–b5) or hollow gears (Fig. 5.10b6). While, according to Fig. 10.15, the pressure increase with time during extrusion is very low in low-pressure agglomeration (lower left branch of the curve in Fig. 10.15) and densification of the feed occurs only by re-arrangement of the particulate solids, the resistance to flow in the longer bores (extrusion channels) increases with their length or, more correctly, with the active length/diameter (or length/cross section) ratio [B.48, B.97]. Therefore, the applied forces are higher, generally in the range defined by the second vertical line and the upward bend in the curve in Fig. 10.15. This means that with the strongest pellet presses, it is possible to exceed the densification that is caused solely by particle rearrangement and without change in particle shape. Since the feed mix for pelleting must be suitable for extrusion and,
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therefore, feature a certain plasticity and sufficient flow properties, which are often achieved by (steam) conditioning [B.48, B.97], some components will be soft enough to deform under the still relatively low applied pressure. This can give rise to the formation of isolated pores or the plugging of pores and the possibility of compressed gas pockets. In pelleting the forces caused as a result of the friction in the bores during extrusion are higher (i.e., medium-pressure agglomeration) than during the passing of moist masses through screens or perforated sheets. Therefore, the limitation to fine particles is not so critical, also because the diameters of the extrusion channels can be much larger (> 2 mm up to 40 mm [B.97]). However, because of the greater pressing forces and friction in the bores, the outside of the extrudates exhibits a distinct, highly densified skin (Section 6.1, Fig. 6.1.8b) while the center is much less compact. This may be important for optimization and troubleshooting. As in the case of low-pressure extruders, operating problems with pellet presses are typically caused by incorrect plasticity of the feed and/or material build-up and plugged die openings. In addition, because the machines are normally much larger, the uniform distribution of the feed is a major concern [B.48, B.97]. While in machines with flat dies this is a minor problem, with ring dies it is very difficult to spread the often sticky feed material consistently across the entire die face and in front of the press rollers. Only if a constant amount of feed is pulled into the nip between press rollers and die, uniform densification and extrusion will occur. Unequal distribution not only influences product quality (i.e., pellet length, density, and strength) but the perforated dies, which due to their design exhibit relatively low structural integrity, are endangered and uneven wear takes place. The assumption that the production capacity can be adjusted by merely changing the speed of the press rollers and/or the dies (whichever are driven in a particular design) is even less correct in pelleting. In addition to the failure of pulling excessive amounts of material into the nip and under the press rollers for extrusion when the throughput is pushed-up too much, the necessary uniform feed distribution can be no longer
Fig. 10.15 Sketches explaining the mechanisms of pressure agglomeration (see also Fig. 5.9, Chapter 5)
10.2 Pressure Agglomeration Technologies
guaranteed. In that case, the interior of the pellet mill fills-up and eventually chokes the machine, requiring its shut-down. After such an unscheduled stop, the cleaning of pellet mills is very difficult because the many long bores are filled with material which, depending on the binder system used, may harden. Then, in many cases, all extrusion channels must be individually bored-out; since, in a larger die, there may be several thousand bores, this is quite a task. To avoid this complication, the bores can be cleaned with a special, non-hardening feed mixture prior to a scheduled shut-down; this allows the renewed start-up with regular feed. A diverter gate in the discharge chute of the pellet mill must be installed, however, to collect contaminated (mixed) material during cleaning and start-up.
10.2.2
High-Pressure Agglomeration Methods
The three high-pressure agglomeration technologies and the four basic methods are shown in Fig. 5.11 (Chapter 5). The machines include all the modifications of ram extrusion, punch-and-die, and roller presses. Referring to Fig. 10.15, in high-pressure agglomeration, the forces applied to particulate solids carry densification of the mass beyond the stage of mere particle rearrangement, resulting in brittle breakage and/or elastic/plastic deformation. As a consequence, sometimes very low residual porosities can be obtained. This is associated with a great possibility of forming isolated or very narrow pores and compressed air pockets, which expand upon pressure release and ejection of the product. Because of the often pronounced degradation and deformation of the feed particles in response to high pressures in the mass during compaction, the final particle size in and the structure of the product are largely independent of the feed’s particulate composition. This effect increases with the force that a particular machine can exert. As long as pieces are smaller than the feed opening and can enter the equipment, the pressing force will increase to a level at which the solid breaks or deforms and fills the die. In the interest of obtaining high capacity, which is required to arrive at an acceptable return and profit margin of the expensive machinery, densification should occur quickly. Therefore, the extensive breakage and deformation during the final stages of shaping often cause the build-up of a considerable residual elastic energy at the end of compaction which results in spring-back upon pressure release and ejection of the product. Fig. 5.9 (Chapter 5) depicts in its entirety the force or pressure diagram of a typical cycle of high-pressure agglomeration as a function of time. The effect of the two phenomena that are responsible for most problems associated with this technology, expansion of compressed air and/or elastic spring back, are shown to occur after having reached Pmax and take some time. The more important changes caused by these phenomena, however, are an increase in volume and porosity (Fig. 5.9) and the potential lowering of the product’s structural integrity. In the best case, they will (only) lead to reduced strength, which may be corrected by a suitable post-treatment (e.g., coating,
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encapsulation, impregnation, sintering) or, in more severe cases, to lamination (Section 6.9.2) or capping (Section 6.2.2) that render the product unfit for the intended use. Fig. 10.16 shows some fictional pressure/densification plots of high-pressure agglomeration, which illustrate the above and some additional characteristics of this technology, particularly the influence of particle size and distribution on degassing during and spring-back after compaction. It was assumed that all materials reach a final density of 90 % (residual porosity 10 %) at maximum pressing force. Densification curve (1) in Fig. 10.16a represents conditions if a monodisperse mass of particles is compacted. The particle size in this example is in the range of a few hundred micrometers. Real particles are irregular in shape and feature surface roughness (Section 10.1). The term monodisperse in the present context means a very narrow distribution with no over- and undersized particles. The bulk density of such a feed is high, e = 60 % porosity or solids content 1 – e = 40 %. When densification begins, interparticle friction causes an immediate increase in pressing force, giving rise to some deformation or breakage of roughness peaks and some weaker particles. Massive deformation and breakage of the solids, which is associated with quickly increasing pressing forces begins early in the cycle. In many cases, conditions during early densification can be improved by adding a lubricant (curve (2) in Fig. 10.16a). Such a lubricant can be a specially formulated additive that reduces the interparticle friction or, sometimes, a small percentage of very fine particles of the same material, which assists in the formation of a more uniform structure. As shown by curve (2), the rearrangement of particles then proceeds to higher densities at lower pressure. Although it can be expected that using a lubricated mass results in higher densification at Pmax, this effect was disregarded for the general presentation in Fig. 10.16. During the entire compaction cycle, air must be able to escape from the diminishing pore volume and elastic deformation must convert into permanent changes of particle size by breakage and/or plastic modification of shape. This becomes particularly difficult during the final densification phase when Pmax is approached, pores become very small, and compact density is very high. The vertical dotted line represents the theoretical situation if no springback occurs after pressure release and ejection of the part. In reality, a volume enlargement, represented by an increase “s” in porosity (De = 5 %), must be expected because, to obtain an acceptable production capacity, the cycle time must be as short as possible and a certain amount of compressed gas in the pores and of residual elastic deformation can not be avoided. The aim of optimization of highpressure agglomeration equipment is to find the shortest cycle time that will yield a good or, at least, as a compromise, a still satisfactory product quality. As shown in Figs. 10.16b and c, different particle sizes and distributions also affect the springback, particularly due to the expansion of compressed gas in the structure. Fig. 10.16b represents the densification cycle obtained if the feed contains solids with a wide particle size distribution. In this case, even in the loose bulk stage, void spaces between larger particles are filled with smaller ones, resulting in high feed density (Fig. 10.2). A starting porosity of 48 % and, as before, the same residual porosity of 10 % under maximum pressure were arbitrarily chosen. In real applications somewhat higher densification (lower final porosity) can be obtained with wide feed particle
10.2 Pressure Agglomeration Technologies
Fig. 10.16 Typical, fictitious pressure/densification plots developing during high-pressure agglomeration [Chapter 13.3, ref. 147]
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size distributions. However, deaeration during compaction is more difficult because, from the start, the resistance to flow and diffusion is greater. On the other hand, less volume reduction means that not as much gas must be removed. Fig. 10.16c represents the densification cycle if the feed consists of very fine solids with a particle size distribution ranging from a few hundred micrometers to micron or sub-micron dimensions. In such masses, natural adhesion forces are so high that large voids exist between bridges and arches of adhering particles. This results in a very low feed bulk density of, say, 20 % or even less, corresponding to an 80 % void (pore) volume. If compaction begins at 80 % pore space and proceeds with uniform speed to the arbitrarily chosen residual porosity of 10 % at maximum pressing force, much densification must occur. Because the flow of air from a mass of very fine particles is time consuming, large amounts of gas are entrapped, resulting in a considerable expansion of the compact after pressure release and at least a partial destruction of the product (curve (1) in Fig. 10.16c and expansion s). This problem, which is commonly experienced in the pharmaceutical industry when fine medicinal formulations are to be directly compressed into, for example, tablets (Section 6.2.2), can be overcome by utilizing either a very low densification speed, which may be unprofitable because it reduces the equipment capacity (above), or by keeping the compact for some time (dwell time, Section 6.2.2) at maximum pressure. The latter (curve (2) in Fig. 10.16c) is more often used because it allows fast densification and, overall, a short pressing cycle. When holding the compact for some time at the maximum pressure, additional de-aeration and conversion of elastic deformation occur, resulting in less expansion (s* in Fig. 10.16c). Dwell time and re-expansion are interrelated and the optimum must be determined experimentally. Even very short dwell times of a fraction of a second produce surprising improvements. The ability to control and modify the compaction cycle in high-pressure agglomeration depends on the machine design (Figs. 5.11 and 5.12, Chapter 5). Although the ram extruder and the punch-and-die press are depicted with mechanical eccentric drives, different methods are possible. For example, hydraulic means of moving the piston(s) are commonly applied and rotary tableting uses cams to actuate the punches (Section 6.2.2). For roller presses, however, only the continuous rolling action is possible. Considering the conclusions of Fig. 10.16, the following statements regarding applicability of high-pressure agglomeration equipment for specific particulate feed compositions can be made. Materials characterized by curve (1) in Fig. 10.16a require high energy input during the entire densification cycle. This can be readily accomplished with hydraulically driven ram and punch-and-die presses. It is difficult to process such materials with roller presses. As coarse monosized materials are not easily pulled into the nip between the rollers, large diameter rollers and high force feeder pressures would be needed. The densification ratio in roller presses is limited to about 3:1. Therefore, material requiring the densifications assumed in Figs. 10.16a and c can not be processed with these machines in one pass. Possible remedies are to compact in two stages or to add
10.2 Pressure Agglomeration Technologies
predensified recyclate to increase the bulk density of the feed [B.13b, B.48, B.97]. In all roller press briquetting and compacting systems some off-size material is produced and added to the feed for this purpose. Wide particle size distributions, associated naturally with higher bulk densities, are best for roller presses. Reciprocating presses, such as the ram extruder and all punch-and-die presses, also benefit from feeds with high bulk density. This is because the densification stroke is shorter, simplifying the drive, and less air is expelled. Generally speaking, feeds with high bulk density, either resulting from wide particle size distributions or the addition of predensified material (e.g., recycle), are preferable for all high-pressure agglomeration methods. The greatest difficulties are experienced with very fine feed materials (Fig. 10.16c). Owing to the low starting bulk density, high densification ratios are necessary and large volumes of gas must be removed. Because of the inherent densification range limit of roller presses, multiple passes or high recirculation ratios are necessary (Section 6.8.2, Fig. 6.8-31). Also, the compaction process in the nip of rotating rolls does not allow dwelling at maximum pressure. In fact, immediately after reaching the highest compaction pressure, the force is suddenly released (Fig. 5.12c, Chapter 5) resulting in often explosive expansion of entrapped gas and massive destruction of the compact, particularly at high roller speeds (i.e., attempting to reach unrealistically large throughput capacities). For that reason, roller presses are the least suitable high-pressure agglomeration equipment for the compaction of fine materials In reciprocating ram presses, the need of long strokes for the compaction of very fine powders makes hydraulic drives most suitable [B.97]. Hydraulic actuation also permits adjustment of the punch speed, such as initially fast; slow during high-pressure densification with a short dwell time at the end; and a very quick return travel to minimize the overall cycle time. The other advantage of ram extrusion presses for very fine (and/or highly elastic, Section 6.10.2) feed materials is that, depending on the design, a variable, often large number of pre-compressed compacts remains in the pressing channel. Before, during a new stroke, the static friction is overcome and the entire contents of the channel moves forward, all already pre-densified bodies experience pressure again. This occurs during each cycle as long as a particular compact remains in the channel. To a certain extend this phenomenon may even happen in the bores of pelleting machines (above). With all machines it contributes to additional deaeration and conversion of elastic into plastic deformation. In the case of ram presses, back pressure applied at the discharge end of the pressing channel (the so called press mouth) will enhance this effect further [B.97]. By using hydraulic cylinders for compaction in punch-and-die presses, which densify particulate materials in a totally enclosed, diminishing volume, in addition to being able to modify punch speed, it is also easy to maintain the highest pressure during an (adjustable) dwell time. Similar speed and dwell time adjustments are also possible with adjustable cam drives in rotary tabletting machines (Section 6.2.2 [B.48, B.97]). In spite of all these provisions and the development and maintenance of conditions for optimal deaeration and avoiding elastic spring back, the small remaining expansion may still damage the compact when it is ejected from the die. It is always a good
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idea to provide a slight taper on the ejection side of the die to guarantee a gradual expansion. In addition to the potentially grave problems caused by insufficient deaeration and conversion of elastic into plastic deformation, roller presses exhibit a behavior that is often overlooked during optimization and/or troubleshooting. With these machines, excessive spring-back, resulting in below standard products or complete failure to produce a desired quality, is not only a result of too high speed of compaction (i.e., diameter related roller speed) but may be also influenced by the flexibility of the pressurization system of the floating roller. The earliest roller (briquetting) presses featured two counter rotating, pocketed, often hollow drums, which were rigidly mounted into a steel frame and supported by sleeve bearings. With the exception of a few special machines, especially for the pharmaceutical industry (Section 6.2.2), today’s equipment design includes a pressurizing and overload system. One of the rollers is installed in the frame such that it can float; it is pressed into position by springs or, now more commonly, hydraulic cylinders. Both types of flexible support are adjusted to produce a specific spring characteristic. When hydraulic pressurization is used, this characteristic is adjustable and produced by one or more hydraulic accumulator(s) [B.13b, B.48, B.97]. Fig. 10.17 is the typical operational diagram of the hydraulic pressurizing system for the floating roller of a high-pressure roller press (Fig. 10.18). When an empty hydraulic system is being filled with hydraulic fluid beginning at time marker (0), the pressure in the system increases until it reaches the gas pressure in the accumulator(s) (A) at time marker (1). The accumulator(s) is (are) normally pressurized prior to system start-up by filling the accumulator(s) with an inert gas, typically nitrogen. Since,
Fig. 10.17 Typical operational diagram of the hydraulic pressurizing system for the floating roller of a high-pressure roller press [B.97]
10.2 Pressure Agglomeration Technologies Fig. 10.18 Diagram of a hydraulic pressurizing system for the floating roller of a high-pressure roller press [B.97]
as will be discussed below, the relationship between accumulator and operating pressure may have to be changed during plant optimization or troubleshooting sessions, it is advisable to install isolation and pressure release valves (not shown in Fig. 10.18) between the hydraulic lines and the accumulator(s). Coming back to the pressurization diagram (Fig. 10.17), before the system pressure increases beyond (A), the volume of the compressible gas in the accumulator(s) is reduced while pumping continues and, for some time, no pressure change is observed. This is an easy on-site examination to determine the functioning and pressure of the accumulator(s) if no separate pressure gauge for checking the gas pressure is available. After the equilibrium gas volume is reached at time marker (2), the pressure in the system rises again until it reaches a predetermined level (B) at time marker (3) and pumping is stopped. The no-load pressure (B) has been determined during previous testing such that, if operation of the roller press begins at time marker (4) and feed material enters the nip, opening the “no load gap” to the “operating gap” [B.97], the corresponding movement of the hydraulic piston(s) increases the pressure in the closed hydraulic system to the operating pressure (C). Afterwards, when particulate solids are being continuously densified and compacted, the pressure of the hydraulic system will fluctuate around the operating pressure due to momentary changes in feed bulk density and the condition (leakage?) of the hydraulic system. While the start-up phase is customarily performed by hand, the system may now be switched to “automatic” whereby the signal from the contact pressure gauge (Fig. 10.18) turns on the
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pump if the minimum pressure is reached, for example due to small leaks or after the safety valve(s) has (have) been actuated, and off when the maximum pressure is obtained. The pressure accumulator(s) in the hydraulic system of a roller press is (are) often considered not very important by the operators. This is far from correct! First and foremost the hydraulic accumulator(s) provide(s) flexibility to the floating roller and avoid(s) overload situations. Peak loads due to variations of feed bulk density can not be avoided and are compensated by the accumulator(s). However, because hydraulic fluid is incompressible, the positioning of a particular bearing block becomes rigid if the associated accumulator is damaged and does no longer contain a gas cushion. The very short but high peak loads, which are often not picked up and displayed by standard, slowly responding instrumentation, may reduce the life of many critical machine components (e.g., bearings, shafts, couplings, gearing, etc.). The pressure and condition of the hydraulic accumulator(s) also influence compaction and product quality by defining the machine’s response to the fluctuating loads in the nip. If the volume of the gas cushion at operating pressure is large, which is the result of high gas pressure in an accumulator (as determined at no-load or while separated from the hydraulic system), the floating roller moves easily in response to load changes. Vice versa, a small volume of gas at the operating pressure (i.e., low gas pressure in the accumulator) results in a more rigid performance. These reactions can be also influenced by the size of the accumulator. In general: The larger the volume of the accumulator and/or the closer the (no-load) gas pressure to the operating pressure in the accumulator, the softer the response. Fig. 10.19 attempts to pictorially explain these effects in the partial operational hydraulic diagram, which is a modification of Fig. 10.17. The solid curve, already shown in Fig. 10.17 after time marker (4), depicts an optimal machine performance and setting. The system pressure remains within the max/min limits in spite of the small gap changes caused by feed density variations. The dotted curve represents the response if the accumulator(s) had been highly pressurized and, therefore, exhibit a soft response to gap changes. In this case, the pressure fluctuates less although the actual gap changes may be larger. The accumulator(s) with a small gas cushion (i.e., low initial gas pressure) behave as shown by the dashed curve. Since already very small variations in gap width do result in pronounced pressure peaks, it is possible that spikes occur that exceed the max/min limits but are so short and quick that they do not actuate the safety valve(s) or the pump, respectively ((a) in Fig. 10.19). Other peaks ((b) in Fig. 10.19) are relieved by the safety valve or cause the pump to start. Overall, the machine performs very erratically and, as already indicated above, the large pressure changes endanger the structural integrity of the machine. A hard response to the unavoidable gap changes often also results in excessive elastic recovery as a high-pressure deformation in the gap area may occur in the material just before it passes the center line and, therefore, expands immediately afterwards. This causes decrepitation or lamination of the compact or, generally, reduced product quality.
10.2 Pressure Agglomeration Technologies
Fig. 10.19 Partial operational hydraulic diagram showing the effects of different compressed gas volumes in the accumulator(s) of a high-pressure roller press during operation (compare Fig. 10.17)
Determining the correct accumulator gas pressure for the actual operating system pressure used during continuous compaction may be of great importance. The sensitivity of the machine performance to this parameter depends on the material to be processed. It is more pronounced for elastic and fine feeds and may be of little or no concern if the particulate solids are coarser and/or plastic. At the beginning of plant operation, a “standard” gas pressure in the accumulator(s) could be set at 50– 75 % of the expected operating pressure. But if later the solids overcompact, that is show signs of lamination, clam-shelling, cracking, etc., or lower than anticipated quality, the volume of and/or gas pressure in the accumulator(s) could be modified up or down. Normally, if done correctly, this does lead to improved product quality and should be tried during optimization or troubleshooting to arrive at the best possible result. Of course, it must be realized that, while reducing the risk of excessive elastic deformation and spring-back, a soft response of the floating roller to changing pressures in the nip (easily moving) results in a greater fluctuation of gap width. This is of little or no consequence for compacting roller presses producing a flat or somewhat profiled sheet or slabs but may, from time to time, produce too thick land areas between briquettes, which can not be broken easily. In many cases, the reliable separation of briquettes into singles is a process requirement (Section 6.9.2). For this reason, and some other, less important ones, adjustment of the gas pressure in the accumulator(s) and the behavior of the floating roller must be optimized by finding a com-
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promise between maintaining an acceptable variation of gap width and minimizing compact expansion due to the relaxation caused by elastic recovery.
10.3
Other Technologies
In this book, other technologies making use of the fundamentals of agglomeration encompass (Chapter 5): * * * *
agglomeration by heat or sintering, coating, hybridization/mechanofusion, and deposition and/or manipulation of individual UFPs.
In sintering, two generally different procedures must be distinguished: * *
agglomeration of particles by the application of heat, and use of heat for the post-treatment of agglomerates to produce final properties, particularly strength and structure.
Sintering for the agglomeration of mostly mineral particles is an old technology but stillwidely usedinthe mining(Section 6.8.3)andmetallurgicalindustries(Section6.9.3). Today, optimization and troubleshooting of the various old, conventional and some new methods is mostly concentrated on solving environmental concerns. The production of dioxin, caused by the interaction of mineral contaminants and fuel components is a major problem, which can be avoided by closely controlling the combustion and by specialized gas cleaning. Optimization also includes heat recovery and the recycling of fine particulate solids. In some cases, vaporized ingredients, such as lead and zinc, are precipitated and utilized in secondary metal production plants (Section 6.9.2). Troubleshooting often entails cleaning or modifying housings and piping to eliminate gas leaks into or from the process. When heat is used during and/or for post-treatment of pre-agglomerated parts (Section 6.7.3 and Chapter 7), optimization involves in most cases the establishment or improvement of the ambient conditions (i.e., oxidizing, reducing, or inert atmosphere) in the apparatus and control and maintenance of temperatures and their gradients and of heating, soaking, and cooling rates [B.13c, B.28, B.47]. Aside from potential corrections of the aforementioned conditions, in very few instances can troubleshooting correct the most important difficulties, that is those that result in unacceptable final products with structural defects. Normally, the sources of these manufacturing problems (e.g., insufficient strength, distortion or warping, too high or too low porosity, uneven surface) are unsuitable structural properties of the “green” agglomerate, such as density variations and gradients. Therefore, they must be corrected during pre-agglomeration by, for example, employing isostatic pressing [B.13a, B.48, B.97] (Chapter 7).
10.3 Other Technologies
Coating has become a highly sophisticated technology, which does no longer only enrobe particulate solids but is used to functionalize the product by applying thin films that are either hard and/or abrasion resistant or lubricating, light reflecting or absorbing, producing special color effects, electrically conductive or insulating, soluble or insoluble, permeable, semi-permeable, or impermeable, permanently plastic or elastic, elastic, featuring a well-defined burst pressure, etc., etc. While, originally, coatings were used to make a product look or taste better, whereby the thickness of the layer was immaterial and, in fact, often had to cover-up irregularities, the new functional coatings must be thin, uniform in thickness, and especially have no holes. As a consequence of the above, optimization and troubleshooting often involves adjustments of the coating material properties (e.g., particle size and distribution of powders, viscosity of liquids, strength of solutions), the cores (size and shape), and the equipment (e.g., location of nozzles, atomization droplet size and pattern, tumbling behavior of the cores and/or powders, the latter by changing the speed of the container movement or of the gas used for fluidization or transport or by adding or modifying baffles). Since an important part of film coating is the simultaneous drying of the solvent or binder liquid, temperatures may have to be adjusted and the direction of flow reversed. The latter can be accomplish with drum coaters, many of which can be operated with two or more alternative flow patterns [B.97] (Section 6.2.3, Fig. 6.2-79). Hybridization or mechanofusion [B.48, B.97] (Chapter 5 and Section 6.11) and deposition and/or manipulation of individual UFPs (Chapters 5 and 11) are emerging new technologies. Much of the equipment is still in the development stage and the few manufacturing units are for experimental or small scale production. As a result, general experience and rules for optimization and troubleshooting are not yet available.
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Applications of Agglomeration Phenomena for Single Particles and in Nanotechnologies Fine and ultrafine (nanosized) particles exhibit characteristics that are often very different from thosse of larger units of the same material. Tab. 11.1 lists some of the traditional, well-known, and more recently determined property changes that occur with decreasing particle size. The most important characteristics of a single particle are size and homogeneity. They are inversely proportional; homogeneity increases with decreasing particle size. This is due to the fact that, during mechanical size reduction, breakage begins preferably at faults (pores, microcracks, imperfections, contaminants), which act as stress raisers. The smaller particles, resulting from repeated stressing and breakage, become less and less flawed until, at a very small size, a perfect structure is obtained. Therefore, as size reduction progresses, dependent properties also change; particles that first exhibit brittle behavior, largely due to structural imperfections, be-
Tab. 11.1 Some major effects of decreasing size on properties of fine and ultrafine (nano) particles [B.97, 11.1] Characteristics of single particles (general)
With decreasing particle size
Homogeneity Probability of breakage Strength Resistance to attrition / hardness Elastic/plastic behavior Vapor pressure, solubility, reactivity, etc. Color perception and intensity
Increasing Decreasing Increasing Increasing Increasing ductility Increasing Changing and increasing
Characteristics of nanopartlicles and nanoassemblies
With decreasing particle size
Catalytic activity (e.g. Pt@Al2O3) Mechanical reinforcement (e.g. carbon black in rubber) Electrical conductivity of ceramics (e.g. CeO2) Electrical conductivity of metals (Cu and alloys, Ni, Fe, Co) Magnetic coercivity (Fe2O3)
Increasing Increasing Increasing Decreasing Changing towards superparamagnetic behavior Increasing Increasing
Blue shift of optical spectra of quantum dots Luminescence of semiconductors
Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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come increasingly ductile, the probability of breakage decreases, and strength or resistance to attrition (hardness) both increase. Since the volume (or mass) decreases with the third power of the representative linear particle dimension and surface area is proportional to the square of that characteristic, the specific (volume or mass related) surface area increases with decreasing particle size. This, together with the changes in overall curvature and surface structure, results in higher vapor pressure, solubility, and reactivity and modifications of other dependent, more complex properties. Also related to surface characteristics of small particles are, for example, a possible change in the perception and an increase in the intensity of colors. Some of the characteristics of nanoparticles and nanoassemblies (including nanophase materials and nanocomposites) will be discussed below. Immediately above the surfaces of all solids, molecular forces are present. Since they are weak and only acting over a short range, they are becoming quickly irrelevant with increasing distance. Therefore, molecular forces are of no consequence for larger units with sizes exceeding 5–10 lm or so. However, particularly if individual particles are to be deposited onto substrates, electrical charges or magnetization can be applied to enhance adhesion, often in a directionally controlled manner, opening the opportunity to attach somewhat larger particles (below and Chapter 5). Furthermore, the volume and mass of particles that are smaller than 1 lm, that is ultrafine and nanosized entities, are so diminutive, that the ambient separation force components are always smaller than the natural molecular adhesion between the solids (Fig. 3.6, Chapter 3). As a result, if such particles collide, they will adhere to each other. As presented earlier (Chapter 4), this phenomenon also results in the so-called limits of grinding, a point at which during mechanical stressing of bulk solids size reduction due to breakage is in equilibrium with size enlargement by agglomeration. Although energy is consumed, no further change in size distribution is observed. Since breakage is immediately followed by aggregation, an increasing amorphization of the solids occurs during this process. While Tab. 11.1 and the discussion above suggest that it may be desirable to reduce the particle size by comminution until a perfect structure and physically predictable properties are obtained, this is normally not possible by mechanical means. Application of grinding aids and of special mills (Chapter 4) allows the production of smaller particles (in the range of several hundred nanometers), which may be used for some nanotechnological applications. However, today the more common approach to the manufacturing of nanoparticles is to synthesize the solids from precursors in gaseous or liquid environments. The oldest and best known techniques use gas-phase synthesis for the production of nanoparticles [11.1]. Even before the term “nanotechnology” was coined, a number of manufacturers used flame reactors for the generation of pyrogenic silica, titania, alumina, and industrial carbon black. For example, “fumed silica” (not to be confused with “silica fume”, a by-product of silicon metal and alloy melting, used in concrete, Section 6.7.1) is mostly applied as a reinforcing filler in silicone rubber and to control the rheology of coatings and colorants, while industrial carbon black is particularly used in tires but also as a pigment in printing inks, coatings, plastics, and building materials (Sections 6.3 and 6.7).
11 Applications of Agglomeration Phenomena for Single Particles and in Nanotechnologies
The tendency of nanoparticles to instantaneously adhere to each other is only a function of size and increases with decreasing particle dimensions. In addition, non-valence associations, for example hydrogen bridge linkages (Chapter 3, Fig. 3.5), may participate. Consequently, nanosized solid materials are almost always in a more or less agglomerated state. Because these aggregates tend to be very loose and extended (Fig. 11.1), they occupy a large volume and their bulk density is very low (Section 6.1, Fig. 6.1-11). Product quality and application characteristics of nanoscaled solids depend strongly on size distribution and morphology, so that the degree of aggregation defined by the size and number of primary particles. In flame reactors, within a few milliseconds during the early stages of the synthesis process, chemical reaction of the precursor
Fig. 11.1
TEM images of different nanoscaled products (courtesy Degussa AG, Hanau, Germany)
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leads to the production of monomers by nucleation or direct inception. Thereafter, as shown in Fig. 11.2, three different growth mechanisms dominate final particle formation [11.1]. *
*
*
Precursor molecules may react on the surface of newly formed particles resulting in “surface growth”. Dispersed particles move randomly due to the Brownian motion and, particularly at high concentrations, collide with each other, resulting in coagulation, an intrinsic mechanism that inevitably occurs in all aerosol processes. Coalescence and fusion are often sufficiently fast in the high-temperature zones of the flame reactor to effect a reduction of aggregation or sometimes even the formation of larger spherical particles, all due to several potential levels of melting.
By selecting process parameters, such as nature of the precursor, temperature, time, reactant state, and/or reactor geometry, product quality can be influenced. However, measurements in gas-phase reactors are a problem as times are extremely short, temperatures very high, and atmospheres often aggressive. Therefore, numerous models for process simulation, all based on particle population balances [B.91, 11.1], have been developed as useful tools to better understand particle formation and support product and process optimization. Fig. 11.3 depicts schematically a sampling system fitted to a flame reactor with which newly created particles can be captured on a transmission electron microscope (TEM) grid, enabling direct measurement of sizes and morphology. Comparison of average aggregate shapes obtained from simulation with actual TEM pictures confirms that modeling is successful [11.1]. If nanoparticles are produced in flame reactors, where temperatures in the formation zone are very high, some materials form hard agglomerates by partial melting and
Fig. 11.2 Formation mechanisms that are relevant in gas-phase synthesis of particles [11.1]
11 Applications of Agglomeration Phenomena for Single Particles and in Nanotechnologies
Fig. 11.3 Diagram of a TEM grid sampling system mounted to a flame reactor, TEM images of sampled product, and results of simulations [11.1]
sintering. Since the resulting solid bridges can not be easily broken and nanoparticles should be in a readily dispersible state prior to and for the manufacturing of the finished engineered nanophase products (below), special efforts must be made to avoid such solid bridging (called coalescence in Fig. 11.2). While, theoretically, the formation of agglomerates can be reduced by producing a very dilute aerosol in which particle collisions are unlikely, this drastically lowers the capacity of such reactors. It is much more efficient to cool the particles on inception to well below the melting or sintering temperatures (lower part of Fig. 11.4) [11.2]. The most successful rapid cooling technique uses a “cold finger” (a surface cooled from the inside with liquid nitrogen) on which particles condense and are continuously scraped off (top of Fig. 11.4). Exposure to air and oxidation and uncontrolled secondary agglom-
Fig. 11.4 Diagram of two methods applied to avoid agglomeration during the production of nanoparticles [11.2]
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eration can be avoided by locating a compaction device (below) within the inert gas atmosphere of the reactor housing. It has been reported that with a scaled-up industrial version of this method 40 t/month of compacts derived from nanoparticles are being produced [11.2]. The simplest, albeit low-capacity, method to obtain mostly individual nanoparticles is a rapid dilution with cold, condensable gas by fast expansion (bottom of Fig. 11.4). Special nozzle designs can produce a one-dimensional temperature gradient, a highly uniform quench rate, and nanoparticles with a narrow size distribution. Owing to their small mass, nanoparticles (whether present as individual or coagulated entities) will continue to adhere to each other and form larger clusters. Owing to their large surface area they are also very reactive. For successful handling and storage, particularly in the gas phase, nanoparticles may have to be treated by various methods to avoid uncontrolled aggregation or reaction, which could reduce or, in some cases, totally inhibit their functionality. Among the most common modifications are physical or chemical transformations, protection by encapsulation (Section 6.3.3), embedding, or mixing with host particles at a time when the ultrafine material is still largely unaggregated (Fig. 11.5) [11.2]. For the manufacturing of nanocomposites, uniform mixing of nanosized components is required. This is often no longer possible if the different particles are produced and collected separately because large aggregates are present. By electrical bipolar mixing, in which the chemically different aerosols are separately charged with opposite polarity during formation and prior to blending, the process is improved. While the aggregation rate of particles of the same composition is reduced, it is increased for different ones [11.2]. To make coagulated bulk nanomaterials (Fig. 11.2), for example fumed silica or carbon black, more suitable for industrial applications, a controlled aggregation Fig. 11.5 Possible methods for the modification of nanoparticles for improved handling and processing [11.2]
11.1 Occurrence and Applications of Agglomeration Phenomena
step may have to be applied during which the natural bonds are first broken (e.g., by mechanical dispersion) and the individual particles are then agglomerated to yield entities with a more tightly packed but easily dispersible structure (below). Similar to the agglomeration processes for pigments (Section 6.3), which are also often ultrafine particles, dustless, free flowing, easily handleable granules are produced that feature “instant” characteristics. Other methods for synthesizing nanoparticles use chemical reactions in liquid environments and precipitation of the solid product. Although the natural molecular attraction forces are by an order of magnitude smaller in liquids than in gas, some clustering also occurs, particularly during and after extraction from the liquid. Under certain circumstances and/or in specific environments, self-assembly or chemical modification (e.g., doping) of nanoparticles may occur, yielding materials with new properties or defined shape and structure (below). Nanosized particles touching any kind of substrate will also strongly adhere to the surface of the solid and can not be easily removed, which may be desirable or detrimental (below). Since nanoparticles are smaller than the wavelengths of light, they are not visible and can not be observed with optical techniques. New tools had to be developed for the imaging and manipulation of these particles, the smallest of which are only little larger than the basic building blocks of matter: atoms and molecules. The new techniques are centered around scanning probe microscopes, the scanning tunneling microscope, and the atomic force microscope, all developed after 1980. Particularly the latter is not only capable of creating pictures of atom- and molecule-sized solids but it can also mechanically move such particles from place to place. This opens the possibility to craft new solid entities and/or structures from and with individual, nanosized building blocks (below and Fig. 5.16, Chapter 5).
11.1
Occurrence and Applications of Agglomeration Phenomena for the Attachment and Bonding of Single Particles to Surfaces and Substrates
Beginning in the 1980s, with the increasing importance of microelectronics and the manufacturing of very small physical structures, the accidental adhesion of individual nanoparticles on surfaces, which occurs in spite of the consistent application of clean room manufacturing facilities, became recognized as an important topic. In response, in 1988, a series of symposia was inaugurated, discussing means for the detection, the fundamentals of adhesion, and (if feasible) methods for the removal of ultrafine particles from surfaces [B.34]. Of particular concern are particles on electronic boards, processors, or chips and in micromachines. They can cause electrical shorts on the first and blockages of mechanical parts in the second. Because of their extremely small size they are strongly bonded to the contaminated surfaces by molecular forces (Fig. 11.6), can not be detected easily (above), and are difficult to remove [B.39]. The same phenomenon can be applied beneficially for the deposition, assembly, and bonding of nano- to micrometer-sized particles on a substrate in a predetermined and
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Fig. 11.6 SEM images of submicron poly(vinylidene fluoride) (PVF2) particles on a polished silicon substrate (silicon wafer, left) and a polyester copolymer (right). The (van-der-Waals) attraction force between the particles and the substrates is so great that particles embed into the soft polymeric material (right) but not into the silicon (left), although in the latter case they are flattened at the contact points [B.39, Vol. 2, p. 51]
orderly fashion (Chapter 5.0, [5.3]). An organized structure, made up of individual particles, is produced on solid surfaces, which can bring about many interesting properties; it is possible to create microdevices and microstructures with multiple functions. So far, to accomplish the above, only a few methods exist. Fig. 5.16 (Chapter 5) is an overview of the two groups of techniques that are available for the manipulation of small particles. With the methods of one group, single particles are deposited on the substrate, one by one. The scanning probe microscope, laser, or microneedles are used for particle manipulation (Fig. 5.16, upper part). Particles can be deposited at specific positions with high accuracy but only at a very low rate. In contrast, the techniques of the other group can deposit a great number of particles by using particle jets (Fig. 5.16, lower part). The disadvantage is a lower accuracy for the positioning of each particle. Fig. 11.7 shows a concept that can be used to assist the deposition of fine particles by an electron beam drawing ([5.3], Chapter 5). The technique, called “charge-assisted controlled particle deposition”, is based on the fundamentals of electrophotography. As shown in Fig. 11.7, at first, the electron beam produces a charge pattern on the substrate surface. Next, positively charged particles are attracted by electrostatic forces to the charge pattern and adhere there. By repeating the electron beam drawing and the adhesion steps, using different types of particles, composite deposits can be created. Fig. 11.8 shows the steps that are required for the charge-assisted deposition of small particles. The oppositely electrically charged particles are made available in a suspension into which the substrate, carrying the electron beam drawing, is dipped. After
11.1 Occurrence and Applications of Agglomeration Phenomena Fig. 11.7 Concept of a process for the assembly of small powder particles on a substrate that is assisted by electron beam drawing [5.3]
Fig. 11.8 Diagram of the steps that are required for the charge-assisted deposition of small particles by electron beam drawing [5.3]
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11 Applications of Agglomeration Phenomena for Single Particles and in Nanotechnologies Fig. 11.9 SEM images of silica spheres arranged along negatively charged lines [5.3]
remaining in the suspension for a predetermined time, the substrate is removed, rinsed, and dried. Stronger bonds between particles and substrate can be achieved with a suitable post-treatment (e.g., sintering). The micrographs in Fig. 11.9 show how silica spheres with a diameter of about 5 lm were arranged on two negatively charged lines (substrate is CaTiO3). The upper photograph (a) depicts the two lines, which are 1600 lm long and 800 lm apart and are composed of silica spheres. The lower photograph (b) is an enlargement of one of the lines in Fig. 11.9a and shows how the silica spheres are arrayed along the charged line on the substrate. The adhesion, bonding, and final structure of these particle arrangements are controlled by the fundamentals of agglomeration, especially the binding mechanisms. Therefore, ideas for new products consisting of more or less defined particle assemblies of various size, structure, and properties can be derived from an in depth knowledge of agglomeration mechanisms.
11.2
Some Examples of Nanoparticles with Special Characteristics
As mentioned before, the small size of nanoparticles and nanostructured materials in respect to their macroscopic forms is responsible for the different mechanical, chemical, electrical, electronic, optical, magnetic, and/or biological properties that make them suitable for applications in new products.
11.2 Some Examples of Nanoparticles with Special Characteristics
Experimentally and theoretically, nanoarchitectured metal oxides show fundamentally new material properties, which are influenced by the size of the crystallites and the nanoscale topological features. In particular, the electrical conductivity increases by several orders of magnitude. For example, upon exposure of titania nanotubes to hydrogen gas, unprecedented changes in electrical conductivity were found, which are over an order of magnitude greater than the hydrogen sensitivity of other materials [11.3]. This opens the opportunity to manufacture extremely sensitive hydrogen detectors and use them, for instance, for the control of industrial processes, for clinical applications where hydrogen is an indicator of bacterial infections, and to maintain the economy of hydrogen based fuels. Nanoarchitectured metal oxides also exhibit enhanced photocatalytic and photovoltaic performances. For example, in a system called “quantum dot”, electrons are confined to a small domain (e.g., nanoparticle) and their resulting new energy levels are defined by quantum confinement effects [11.2]. This can be observed as a shorter wavelength optical absorption edge, indicated by a spectral “blue shift” (Tab. 11.1). By using particle size effects, quantum dots can be also used to produce light emitters of various colors. As an example, LEDs (light emitting diodes) were produced, which have a voltage-controlled tunable output color. Vanadium pentoxide is often used as catalyst for oxidation reactions in gas sensors and in lithium and magnesium batteries. Specific chemical and electronic properties develop when the oxide is doped with other elements. Doping may result in changes of morphology and electronic parameters and affect strongly the reactivity of the oxide. Because the way by which the dopants are introduced is of great influence, new technologies, such as nanostructural assembly, allow the production of materials that can not be synthesized with conventional preparation methods. For example, titanium doped nanostructured V2O5 is prepared by spray drying a mixture of vanadyl oxalate with TiO2 and calcination of the resulting powder for 2 h at 500 8C. Both methods are agglomeration techniques. The V2O5 particles feature a rod-like shape and the titanium atoms are located non-uniformly in a thin layer on the surface, which seems to stabilize the V2O5 structure against electron beam irradiation [11.4]. In addition to these few, recently reported examples, a large number of other nanoparticles with special characteristics are being developed in rapid succession by the scientific community. While many of the new species exhibit often surprising and highly desirable properties, which stimulate largely theoretical, futuristic ideas of building new or improved nanotechnological materials, potential industrial applications suffer from still extremely small production quantities and time consuming and complicated manufacturing procedures that are technologically very demanding. As simpler, low cost, and high capacity processes for the creation of functional nanoparticles are found, methods using the principles of (size enlargement by) agglomeration will result in the building of nanostructures, nanophases, or nanocomposites with improved or new levels of performance. Fig. 11.10 shows the current (2002) distribution of nanotechnology start-ups and spin-offs in the USA [6.2.3.1]. It shows that the greatest activities are in materials production techniques, medical and pharmaceutical products (Section 6.2.3) and electronics as well as in research. Exploitation of the other areas of interest is still relatively small following the advise of experts in the field to
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Fig. 11.10 Present (2002) distribution of nanotechnology start-ups and spin-offs in the USA [6.2.3.1]
focus on market-driven applications and not spend time developing great solutions for problems nobody has (www.nanobusiness.org).
11.3
Applications of Agglomeration in Nanotechnologies
Nanotechnologies deal with the border between the realm of individual atoms and molecules, where quantum mechanics rule, and the macroworld, where bulk properties of materials emerge from the collective behavior of a large number of atoms and molecules [11.5, B.97]. Two different approaches to the fabricating of nanostructures have been developed, so called “top-down” and “bottom-up” methods [11.5]. The basic principle of agglomeration, the attachment of small entities to similar ones or to other solid surfaces by binding mechanisms (Chapter 3) to form larger units, is used in bottom-up manufacturing. Nanoparticles, having at least one dimension of between 1 and 100 nm, are produced by processes that allow fundamental control over the physical and chemical attributes of molecular-scale structures and are then combined to form larger units with superior chemical, mechanical, electrical, or optical properties. Nanophase materials feature a three-dimensional structure and a domain size of less than 100 nm. They are usually produced by compaction of a nanoscale powder and are characterized by a large number of grain boundary interfaces in which the local atomic arrangements are different from those of the crystal lattice [11.2]. Nanocomposites, in contrast, consist of nanoparticles that are dispersed in a continuous matrix, creating a compositional heterogeneity of the final structure. The matrix is usually either ceramic or polymeric. Only the manufacturing of ceramic nanocomposites applies the principles of agglomeration (Section 6.7).
11.3 Applications of Agglomeration in Nanotechnologies
Nanoparticles, exhibiting special properties (above), have found new applications in nanophase resistors and varistors. Granular films, consisting of small conductive particles, embedded in an insulating matrix of ceramic or glass, exhibit a great variability of electrical conductivity that can be influenced by a proper control of the volume fractions [11.2]. They can be used as thick film resistors. It was also shown that, owing to their large number of grain boundaries, they increase the breakdown voltage of varistors. Dielectric elements that are based on nanostructures are of recent interest for the scaling-down of DRAMs (dynamic random access memories) [11.2]. The need to reduce capacitance requires materials with larger dielectric permittivity. One method to achieve this is to disperse conductive particles in a dielectric matrix; by using nanoparticles, the dissipation factor is kept low. As mentioned above, quantum dots can be used to produce light emitters of various colors. Although, this application, like many others in the nanoworld, seems attractive, it is technologically very demanding. Among other characteristics, the quantum dots must be stable and their surface must be passivated. To circumvent the inherent reactivity and instability (of all nanoparticles) they should be embedded (above, Fig. 11.5). Nanocomposite structures, consisting of quantum dots embedded in glass or a semiconductor material, can be used in current optical and electro-optical devices. Structural modifications of engineered materials are caused by the incorporation of nanoparticles as passive basic building blocks and lead, for example, to superplastic ceramics or extremely hard metals. Functional applications, on the other hand, rely on the transformation of external signals, such as the filtering of light, the change of electrical resistance in different environments, or the occurrence of luminescence when electrically activated (Tab. 11.1). Other bottom-up manufacturing methods use self-assembly processes to produce larger structures. If the ambient conditions are right, atoms or molecules spontaneously form ordered arrangements of, for example, fullerenes or nanotubes and wires and a quickly increasing array of others, often called nanoparticle ensembles [11.6]. Although, under certain conditions, quite large units (e.g., tubes or wires) may be grown by self-assembly, they are still considered individual building components for nanophases or nanocomposites. For example, in spite of their often relatively great length (high aspect ratio), semiconductor nanowires (e.g., luminous CdTe wires) can be assumed as one-dimensional and are, therefore, key structural blocks for new generations of electronics, sensors, and photonics materials [11.7]. Their diameter corresponds to the size of the primary nanoparticles, from which the wires were formed by self-assembly, and, correspondingly, emit light in different parts of the spectrum. Several methods are available to produce even larger structures from nanoparticles. For example, a novel, versatile technique for the synthesizing of uniform hollow capsules from a broad range of materials is based on a combination of colloidal templating and self-assembly processes [11.8]. Fig. 11.11 describes schematically the concept. Colloidal templates of different composition, size, and geometry (although spheroidal shape is preferred) can be employed. Materials range from spherical polymer particles to non-spherical biocolloids, all with diameters in the nano to micrometer range. The
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Fig. 11.11 Diagram describing hollow capsule production by exploiting colloidal templating and self-assembly methods [11.8]
first step, (1) in Fig. 11.11, involves the deposition of a charged polymer layer (+) onto the colloidal particles. In a next step, oppositely charged (–) polymer, (2) in Fig. 11.11, or nanoparticles, (3) in Fig. 11.11, are added resulting in another layer. Additional layers can be produced as shown in Fig. 11.11 by repeated deposition, which makes use of the surface charge reversal occurring every time a layer is adsorbed. Colloidal core/multilayer-shell particles are manufactured. After the desired thickness of the layer is obtained, excess unadsorbed polyelectrolyte or nanoparticles are removed by repeated centrifuging or filtering and wash cycles. Finally, hollow capsules are produced by the removal of the core from the composite colloids. This is achieved either
11.3 Applications of Agglomeration in Nanotechnologies
by chemical or thermal means. If a solvent is used, only the core is dissolved, which results in hollow polymer, (4) in Fig. 11.11, or composite, (6) in Fig. 11.11, capsules. Heat treatment (calcination) of the coated particles, (5) in Fig. 11.11, removes both the colloidal core and the bridging polymer, thereby producing hollow inorganic spheres. By combining colloidal templating with self-assembly, the manufacturing of a broad range of coated colloids and, ultimately, of hollow capsules in the nano- and micrometer range is possible, featuring various and defined composition. Capsule geometry, size, and wall thickness can be controlled with nanometer precision by the use of colloids with given shape and dimensions and by varying the number of coating cycles. Uniformity in the size of the hollow capsules is defined by the monodispersity of the colloidal templates. Rather conventional means for the manufacturing of hollow microspheres with diameters between 1 and 1000 lm have been developed [11.9]. Methods include spray drying and dripping as well as emulsion or suspension techniques. The microspheres feature low effective and bulk densities coupled with high specific surfaces. Typical wall thicknesses are in the range 1–10 % of the diameter. Potential wall materials include glass, ceramic and mixed oxides, silicates and aluminosilicates, polymers and polycondensates, and metals. Surface phenomena, which may be modified by chemical reactions, additives, and/or post-treatments, play an important role for microsphere formation, properties, and stability. Fig. 11.12 is the photomicrograph of a calcined hollow microsphere [11.9].
Fig. 11.12 Photomicrograph of a hollow TiO2 microsphere after calcination at 650 8C. The wall thickness of the 20 lm diameter sphere is about 200 nm [11.9]
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Similar to the macroscopic use of lightweight aggregate (Sections 6.7.1 and 6.7.2), hollow microspheres can be applied in microscopic structures to achieve weight reduction or improved heat and sound insulation of special building materials or plastic compounds and composites. Other applications are for the immobilization of active substances or catalytic materials. In the health and pharmaceutical industries, hollow microspheres are used for the synthesis of artificial cell structures with functionalized surfaces and the manufacturing of drug-delivery systems (Section 6.2.3). The results of encapsulation and coating processes are defined by the particle size and structure (degree of agglomeration) of the participating solids. Fig. 11.13 depicts the formation of either a dense or a porous capsule [11.9]. Even in the world of nanoparticles, in addition to the always present natural adhesion forces (e.g., van-der-Waals), additives, for example ligand or linker molecules, are sometimes required, which work on a completely different level from the previously discussed classic binders (Chapter 3) for macroscopic agglomeration processes. The possibility of generating ordered structures from nanoparticles by self-assembly is often governed by the size and local concentration ratios of the constituent particles. Furthermore, the interparticle spacing of nanoparticle ensembles can be controlled by the choice of the ligand or linker molecules [11.6]. Generally speaking, such nanostructures (ensembles) form by self-assembly of suitably functionalized nanoparticles. Fig. 11.14 shows the building of nanoaggregates after complementary functionalization of nanobeads. (a) shows the linking of two classes of beads and (b) the attachment of smaller beads to a larger one [B.95]. The mechanical properties of plasma sprayed nanostructured coatings, on the other hand, depend on the presence of molten and non-molten portions of the feedstock, a classic binding mechanism of agglomeration, whereby the molten component acts as a binder by enveloping and anchoring the non-molten particles in the microstructure of the coating [11.10].
Fig. 11.13 Depiction of the formation of either a dense or porous capsule during encapsulation and coating processes [11.9]
11.3 Applications of Agglomeration in Nanotechnologies
Fig. 11.14 Diagram of the formation of nanostructures by self-assembly of complementarily functionalized nanobeads: a) linking of two classes of beads, b) attachment of smaller beads to a larger one [B.95]
Many nanoparticle preparations lack sufficient stability (above, Fig. 11.5) to allow the ordered assembly of two-dimensional or three-dimensional materials and structures, in which the particles are closely packed, without the onset of uncontrolled aggregation (agglomeration). To overcome this problem, the particles must be rendered chemically stable, for example by ligand stabilization, also to avoid degradation processes such as partial oxidation or undesired sintering of particles [11.6].
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11 Applications of Agglomeration Phenomena for Single Particles and in Nanotechnologies
Mechanofusion and hybridization (Chapter 5) are borderline methods of nanotechnology. They modify the surface structure and the characteristics of fine particles by embedding fine or ultrafine particles (UFP) into or coating such particles onto the core substrates [B.48, B.97]. Fig. 5.15 (Chapter 5) demonstrates how hybridization works and Fig. 11.15 is a flow diagram. Core particles with a size between 0.1 and 500 lm are mixed with UFP that, depending on the dimensions of the cores, may feature sizes between 0.01 and 50 lm, (1) in Fig. 11.15. After mixing a coating has developed on the cores (left photomicrographs in Fig. 5.15). These prepared particles are metered, (2) in Fig. 11.15, into the hybridizer, (3) in Fig. 11.15. During a short time (1–5 min), the hybridizer introduces so much mechanical and thermal energy into the product, that the fine particles are imbedded in or permanently bonded to the surface of the core material (right photomicrographs in Fig. 5.15). The whole process is controlled from a panel, (5) in Fig. 11.15, and the finished product is transferred to a product collection container (4) in Fig. 11.15. As shown in the center right of Fig. 5.15, multiple surface layers can be produced. If the core material of such products is removed with solvents or during calcination, hollow capsules as described above may be produced, too. The application of a classic method of size enlargement for the processing of nanoparticles, that is already in use since a time when nanotechnology was not even defined yet, shall be mentioned as a final example. It has been described above that nanosized solids feature an extremely low bulk density caused by their almost instantaneous adhesion and formation of loose flocs. Such materials also tend to dusting and exhibit many other unfavorable characteristics. This is also true for industrial carbon black that has been produced in flame reactors for more than half a century [11.11]. Large volumes of this material are being used primarily to reinforce tires and other rubber products and as a pigment (Sections 6.3.1 and 6.3.2). The latter has a
Fig. 11.15 Flow diagram and photograph of a hybridization system (courtesy Nara, Tokyo, Japan)
11.3 Applications of Agglomeration in Nanotechnologies
number of advantages when compared with black organic dies, including color stability, solvent, acid and alkaline resistance, thermal stability, and high covering power. Many carbon blacks also fulfil the governmental standards for additives used in articles that come into contact with food, drinking water, and toys. Fig. 11.16 shows the layout of a modern furnace black manufacturing facility and a cross section through the furnace reactor [11.11]. It is a continuous process, using liquid and gaseous hydrocarbons as feed stock (oil, 1) and heat source (gas, 2), respectively. After the carbon black is formed at a very high temperature in the refractory lined furnace, the process mixture is quenched with water to prevent unwanted secondary reactions (3 and inset). The carbon black laden gas then passes through a heat exchanger (4) for further cooling. At the same time process air is preheated before entering the furnace (5). The carbon black particles are separated from the gas stream in a bag filter system (6). In most cases, the clean, combustible gas is burnt in a boiler to generate steam (7). As depicted in Fig. 11.17, the primary furnace black particle size is about 10–80 nm. To meet the requirements of environmental, health, and workplace safety and protection and improve handling and transportation properties, a large percentage of industrial carbon black is granulated. Depending on product application, one of three growth agglomeration methods is used. The wet granulation process employs water and a binding agent in a specially designed pin mixer (8 in Fig. 11.16 [B.48, B.97]). The green (wet), spherical agglomerates are dehydrated in a rotary dryer (9). The binding agent ensures that the granular product can be easily stored (10) and transported with-
Fig. 11.16 Layout of a modern furnace black manufacturing facility and cross section through the reactor (5) [11.11]
651
652
11 Applications of Agglomeration Phenomena for Single Particles and in Nanotechnologies
Fig. 11.17
Primary particle size distributions of different carbon black varieties [11.11]
out excessive attrition. Such material is typically used in the rubber industry where high shear forces of the mixers break-up the agglomerates and uniformly disperse the carbon black particles in the polymer matrix. Pigment black is transformed into easily dispersible granules by dry agglomeration in rotating drums [11.12]. As shown in Fig. 11.18, the spherical agglomerates, produced by natural agglomeration, grow in size along the length of the drum. As a further alternative, for oil-agglomerated carbon black, also used primarily in the pigment industry, mineral oils are applied in the granulation process. Because of a light
Fig. 11.18 Diagram of a dry agglomeration system for the granulation of furnace black in a rotating drum [11.12]
11.3 Applications of Agglomeration in Nanotechnologies Fig. 11.19 Three different carbon black products: 1) powder, 2) dry granulated, 3) wet granulated [11.11]
oil coating, this product allows essentially dustfree handling and is even easier dispersible during manufacturing of the mineral oil based printing inks [11.11]. Fig. 11.19 is a photograph depicting three different carbon black products: powder (1) and dry (2) as well as wet (3) granulated.
Further Reading
For further reading the following books are recommended: B.29, B.39, B.33, B.42, B.43, B.49, B.50, B.58, B.67, B.70, B.72, B.76, B.86, B.88, B.90, B.91, B.95, B.103, B.109 (Chapter 13.1). Books mainly devoted to the subject matter are printed bold. Also, a recent issue (November 2003) of Chemical Engineering Progress (CEP), the official journal of the American Institution of Chemical Engineers (AIChE, see Chapter 15.1), was dedicated to the topic. It is mentioned here because it contains a listing of international companies applying nanotechnology to the chemicals and advanced materials markets.
653
655
12
Outlook During the past 100 years, based on the classic, often centuries-old methods of size enlargement by agglomeration, many new fields of application for the modernized processes have been introduced in industry. In addition, with the emerging scientific knowledge of the underlying facts and theories of agglomeration, adhesion, and bonding, together with a better understanding of aggregate structures and properties, new technologies were developed with which fine and ultrafine particulate solids are converted into products with novel characteristics. A selection of the more important, traditional and contemporary technologies has been presented in Sections 6.1–6.11 as well as Chapters 7, 8, and 11. In conclusion, Tab. 12.1 lists industrial materials that are being modified with size enlargement by agglomeration and indicates the methods that are most commonly applied. The table is not an exhaustive summary of all industrial applications but is presented to demonstrate the wide distribution of the unit operation in industry and the versatility of its use. Tab. 12.1 Examples of industrial materials that are produced and/or modified with size enlargement by agglomeration and listing of the methods that are most commonly applied Products
Agglomeration methods General
Specific
Aggregate (+ 3 mm) Agrochemicals
All All
Animal feed Biomass Charcoal Manure (fertilizer)
Extrusion Press aggl. Press aggl. Extrusion Press aggl. Growth aggl. All Press aggl. All All All Press aggl. All
Drum, compaction/granulation, sintering Pan, mixer, pelleting, compaction/granulation, fluid bed, spheronizing Pelleting, extruder Pelleting, ram or screw presses Briquetting (roller press) Pelleting Compaction/granulation Mixer Pan, pelleting, compaction/granulation Pelleting, compaction/granulation Pan, mixer, compaction/granulation Pan, compaction/granulation Mixer, pelleting, compaction/granulation Pelleting, spheronizing, roller press
Carbon black Carrier materials Catalysts Cat litter Cement raw material Ceramic materials Chemicals Organic Inorganic
Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
656 Tab. 12.1
continued
Products
Agglomeration methods General
Specific
Clays
Growth aggl Press aggl.
Mixer Pelleting, roller presses
Colorants/pigments Detergents Dusts (< 0.5 mm) Explosives
All All All Press aggl.
Mixer, fluid bed, compaction/granulation Mixer, fluid bed, compaction/granulation
Fertilizers Fillers Filter cakes Food
All All Growth aggl. All
Fruit kernels and shells Fume Functional products Hay/straw Ice (crushed) Inorganic chemicals Instant products Metal-bearing dusts Metallic wastes (scrap)
Press aggl. Growth aggl. Nanotech. Extrusion Press aggl. All Growth aggl. Press aggl. Press aggl. Press aggl.
Metal powder
Press aggl.
Metal sponges (e.g. Fe, Ti) Minerals Mud Ores
Press aggl. All Growth aggl. All
Pharmaceuticals
Polymers Powders Salts
All Press aggl. All Press aggl. Nanotech. see Colorants Growth aggl. Extrusion Press Aggl. All Press aggl.
Sewage sludge/dry(digested) Slurry Straw/hay Sugar
Press aggl. Growth aggl. see Hay Press aggl.
Vegetables (frozen)
Press aggl.
Raw material Dosing form Gran./direct Gran./caps Functional Pigments/colorants Plastics
Punch-and-die or roller presses Prilling and coating (fattening) Pan, drum, compaction/granulation, fluid bed Compaction/granulation Drum, pan, fluid bed, pelleting Mixer, fluid bed, extruder, punch-and-die or roller presses Punch-and-die or roller presses Sonic agglomeration, fluid bed Coating, encapsulation, hybridization Pelleting, ram or screw presses Punch-and-die or roller presses Mixer, fluid bed Compaction/granulation Briquetting (roller press), sintering Punch-and-die presses, briquetting (roller press) Punch-and-die or roller presses, isostatic presses, sintering Punch-and-die or roller presses Pan, drum, roller presses, sintering Pan, drum, mixer, fluid bed Growth agglomeration + sintering Mixer, fluid bed, compaction/granulation Punch-and-die presses (tabletting) Mixer, fluid bed, compaction/granulation Low pressure extrusion + spheronizing Coating, encapsulation, hybridization Hot mixer, shear milling Pelleting, extruder Pelleting, extruder, punch-and-die Punch-and-die or roller presses, compaction/granulation Pelleting, compaction/granulation Pan, drum, mixer, fluid bed Punch-and-die or roller presses, compaction/granulation Briquetting (roller press)
657 Tab. 12.1
continued
Products
Vegetable waste Wastes Organic Inorganic
Agglomeration methods General
Specific
Extrusion Extrusion All
Pelleting Pelleting, ram pressing, spray drying Pan, drum, roller press, fluid bed
Referring once again to Tab. 12.1, if one notes the listings under “general”, considers that the statement “all” includes pressure agglomeration, and realizes that “extrusion” is a part of press agglomeration, size enlargement technologies using some sort of external force for the shaping and densification of particulate solids are the most widely used and versatile. Still discussing present-day uses of size enlargement by agglomeration, Tab. 12.2 summarizes some of the more common benefits of the larger entities that are produced by this technology and mentions typical industrial applications. Combined, Tabs. 12.1 and 12.2 represent in the most condensed form the current importance of size enlargement by agglomeration in industry. Tab. 12.2 Some of the more common benefits of the larger entities that are produced with size enlargement by agglomeration and typical current industrial applications Benefit
Examples of current industrial applications
Production of useful structural forms and shapes Preparation of definite quantity units
Pressing of intricate shapes in powder metallurgy Manufacturing of spheres by rolling (spheronization) Metering, dispensing, and administering of drugs in pharmaceutical agglomerated products Definition of dosage Agglomeration of fine raw materials Size enlargement of waste fines Tabletting of detergents Briquetting of solid fuels for domestic use Granulation of fertilizers Cubing of sugar Pelleting of animal feeds Granulation of pharmaceutical formulations for tabletting Granulation of ceramic clay for pressing Agglomeration of fine solids prior to packaging Briquetting of flaked products (e.g., DMT) Densification of fumes (e.g., silica fume) Agglomeration of carbon black Briquetting of (reactive) metal sponges Compaction of caustic materials Granulation of bleach Agglomeration of manure (as fertilizer)
Reduced dusting losses Better product appearance Prevention of caking and lump formation Improvement of flow properties
Greater bulk density to improve storage and shipping Reduction of handling hazards of reactive, irritating, and obnoxious materials
658
12 Outlook Tab. 12.2
continued
Benefit
Examples of current industrial applications
Control of solubility
Production of instant food products Granulation of pigments Incorporation of disintegrants in pharmaceuticals Coating of agglomerates to delay dissolution Pelleting of catalyst carriers Granulation of molecular sieves Agglomeration of filtering media for odor control Pelletization of (e.g. iron) ores Granulation/briquetting of glass batch as furnace feed (Acoustic) agglomeration of fine particles in flue gases and other gaseous effluents Flocculation of solid contaminants in waste water and other liquid effluents Selective agglomeration (e.g., of coal) in fines slurries by immiscible liquid (e.g., oil) bonding
Control of porosity and surface-to-volume ratio Increased heat transfer rates Removal of particles from gases
Fractionation of particle mixtures in suspensions
In the near future, particulate solids processing will focus on a shift away from just attaining size and distribution of products towards the more fundamental area of microstructure and morphology. The emphasis of new products and processes will be on better control of the primary particle physical properties, highly specific product size, shape, and composition, and the creation of desirable characteristics of the final manufactured materials. This will lead to an increased demand for “engineered” particulate solids with high value that are produced in small amounts. Particular consideration will be on high quality and special effects rather than simple bulk commodities with broad applications. The industry will also have to cater increasingly to the needs of the end user. For these reasons, the profile of professionals and, especially, engineers in the process industries has changed profoundly [12.1]. The development of improved, preferably unique products often moves the consumer’s needs and desires into the forefront. In addition to all facets of performance, the design, involving user-friendliness or convenience of application and appearance through shape, color, and packaging, all resulting in brand recognition and the all-important price, are becoming aspects that must be taken into consideration during product development. To accomplish these new requirements, marketing must be involved during the entire engineering process. This involvement begins with the idea, continues until the product launch, and lasts through the ongoing customer service. Feedback from the field and market research are very important tools for brand maintenance, product improvement, and the evolution of new or follow-up goods. Agglomeration science and technology play a vital role in the search for these novel differentiated particulate products and the means for making them. A major requirement is a much greater degree of flexibility regarding agglomerate composition, struc-
12 Outlook
ture, performance, and appearance. This not only requires a deeper understanding of the fundamentals of agglomeration but also an interdisciplinary effort from mechanical, chemical, and process engineers, chemists, physicists, colloid and biochemical scientists, and other researchers in industries that are related to all aspects of the product, and the attention of marketing experts as mentioned above. In the future, basic and intermediate materials will still be manufactured and agglomerated in large quantities with high-capacity production plants. However, even there, new technologies are being developed, which do not only emphasize bulk product size, shape, and strength but also define and modify particle structure, yielding new functional material properties. Final agglomerated products are then assembled in specialized, small facilities, using pre-functionalized components and ultraclean, often nanosized particles and methods, that result in predetermined and controlled compositions and specific characteristics of the conglomerates. As mentioned before (Section 6.4), among the oldest technologies for the manufacturing of “engineered” particulate solids with such improved properties was the production of “instant” granules, agglomerates that disperse and, if applicable, dissolve easily and quickly. Newer processes use the phenomenon agglomeration for the production of solid chemical, pharmaceutical, food, and other specialties or, generally, materials for life-science applications. These substances are especially designed and composed to respond to specific requirements. In addition to the controlled bonding of particles, resulting in desirable structures, post-treatments, involving coating, encapsulation, or hybridization (Chapter 5) and others, are beneficially modifying the product properties. Although the classic methods of size enlargement by agglomeration, discussed briefly in Chapter 5 and covered in more detail in the literature (Chapter 13), will still dominate industrial applications, the “bottom-up” approach of particle building and material assembly (Chapter 11) will be used more and more for high-value products with specific characteristics. Today, the manufacturing of drug delivery vehicles and systems in the pharmaceutical industry (Section 6.2.3) and of miniaturized electronic components (Chapters 5 and 11) are most commonly using these new nanotechnologies. The author hopes that this book, together with an earlier publication [B.97], will give the reader sufficient basic knowledge to successfully tackle new developments with which they will be confronted in the industry and involve agglomeration phenomena of any level.
659
661
13
Bibliography
After a short review of the various agglomeration phenomena, the underlying fundamentals, the undesired occurrence, and the techniques used for their beneficial application for the size enlargement of particulate solids, the bulk of this book is trying to present a complete, up-to-date compilation of how these are found in past, present, and future industries. To that end, in addition to introducing the properties of agglomerates and the specific characteristics of the different technologies, descriptions of applications in various industries and of special features for particular uses are the main topic of the book. Emphasis is on industrial applications, not theory. The explanations of details of processes, systems, plants, and applications as well as the descriptions of products and of their uses are largely based on information from vendors, the experience of the author as well as input from many of his colleagues that are active in this field. Therefore, as in his earlier book [B.97], it was decided that it is NOT necessary to collect the numerous individual publications that, in one way or another, report on technical and practical developments and review specific industrial features, applications, and products. Rather, with the exception of a few annotations (Section 13.2), reference is made to books or major chapters dealing with all facets of agglomeration and related subjects (Section 13.1) and to the vendors (Section 14.1 as well as specific acknowledgements in the text and figure or table captions) who, either by direct communication or through their technical sales literature and/or brochures, supplied the information that has been processed by the author to yield an unbiased presentation. Size enlargement by agglomeration is a unit operation of mechanical process technology, the science of which is concerned with all activities that are related to the processing and handling of particulate solids (Chapter 2.0). As has been repeatedly shown in the book, all unit operations of mechanical process technology as well as the peripheral techniques (Section 2.0, Figure 2.2) are being used, sometimes several times, in the design and execution of agglomeration systems and plants. Therefore, in addition to what has been presented in Chapter 13.1 it should be pointed out, that some of the books or series in which major chapters deal with Agglomeration are also valuable sources of information on other topics of Mechanical Process Technology. Specifically, those references are (in numerical order): Winnacker-Ku¨chler, CheAgglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
662
mical Technology [B.12], Handbook of Powder Technology [B.19], Handbook of Powder Science and Technology [B.24, B.71], Series on Bulk Materials Handling [B.27, B.32], Developments in Mineral Processing [B.35], Ullmann’s Encyclopedia of Industrial Chemistry [B.36], Fortschrittsberichte VDI (Reihe 3, Verfahrenstechnik) [B.38, B.43, B.44, B.45, B.72], Drugs and Pharmaceutical Sciences [B.66, B.68], Kirk-Othmer, Encyclopedia of Chemical Technology [B.73]. Obviously, there are numerous others, for example, Perry’s Chemical Engineer’s Handbook, Dubbel, and Hu¨tte as well as many more, particularly those published in different parts of the world and in other languages.
13.1
List of Books or Major Chapters on Agglomeration and Related Subjects
B.1
B.2
B.3
B.4
B.5
B.6
B.7
B.8
B.9
B.10
Handbuch der Brikettbereitung (Handbook of [Coal] Briquetting) G. Franke Verlag Ferdinand Enke, Stuttgart, Germany (1909) Aufbereitung und Brikettierung (Processing and Briquetting [of Coal]) K. Kegel Wilhelm Knapp Verlag, Halle/Saale, Germany (1948) Proceedings of the Biennial Conferences of the Institute for Briquetting and Agglomeration (IBA), USA Volumes 1-28 (1949, 1951, 1953, ....., 2003) Agglomeration W.A. Knepper, Editor Proc. 1st International Symp. Agglomeration, Philadelphia, PA, USA John Wiley & Sons, New York, NY, USA, and London, UK (1962) Briquetting D.C. Rhys Jones In: Chemistry of Coal Utilization, H.H. Lowry (Editor), Supplementary Volume, Chapter 16, J. Wiley & Sons, New York, NY, USA (1963) Die Tablette. Grundlagen und Praxis des Tablettierens, Granulierens und Dragierens (The tablet. Fundamentals and applications of tabletting, granulating and coating) W.A. Ritschel Editio Cantor KG, Aulendorf, FR Germany (1966) Kornvergr€osserung (Agglomerieren) (Size enlargement by agglomeration) W. Pietsch In: Fortschritte der Verfahrenstechnik VDI-Verlag Gmbh, D€ usseldorf, FR Germany, Vol. 9 (1971), 831-872, Vol. 10 (1972), 223-235, Vol. 11 (1973), 162-172, Vol. 12 (1974), 133-146, Vol. 13 (1975), 143-163, Vol. 14 (1976), 149-160, Vol. 16 (1978), 73-89 Compaction ’73 A.S. Goldberg, Editor Proc. 1st Int. Conf. on Compaction and Consolidation of Particulate Matter Powder Technology Publ. Series No. 4, Powder Advisory Centre, London, UK (1972) Das Verdichten von Pulvern zwischen zwei Walzen (The densification of powders between two rollers) W. Herrmann Verlag Chemie GmbH, Weinheim, FR Germany (1973) Agglomeration of Iron Ores D.F. Ball, J. Dartnell, J. Davison, A. Grieve, R. Wild American Elsevier Publishing Co., New York, NY, USA (1973)
13.1 List of Books or Major Chapters on Agglomeration and Related Subjects B.11
B.12
B.13
B.14
B.15
B.16
B.17
B.18
B.19
B.20
B.21
B.22
Pellets and Granules S.K. Nikol, Editor Proc. Symp. Pellets and Granules, The Australian Inst. of Mining and Metallurgy, Newcastle, NSW, Australia (1974) Mechanische Verfahrenstechnik (English Ed.: Particle Technology) H. Rumpf Monograph in Winnacker-K€ uchler, Chem. Technology, Vol. 7, 3rd Edition, Carl Hanser Verlag, M€ unchen, FR Germany/Wien, Austria (1975) Engl. Ed.: Particle Technology, F.A. Ball (Translator) Chapman & Hall, London, UK (1990) Monographs in Powder Science and Technology A.S. Goldberg, Series Editor Heyden & Son Ltd., London, UK/Rheine, FR Germany/New York, NY, USA a: P. Popper: Isostatic Pressing (1976) b: W. Pietsch: Roll Pressing (1976) 2nd Edition (1987) c: M.B. Waldron, B.L. Daniell: Sintering (1978) d: J.K. Beddows:The production of metal powders by atomization (1978) e: P.J. Sherrington, R. Oliver: Granulation (1981) f: J.C. Williams, J.J. Benbow: Die Compaction and Extrusion (unpublished manuscript, 1974) Direktreduktion von Eisenerz. Eine bibliographische Studie. (Direct Reduction of Iron Ore. A bibliographic study) Commission of the European Community, Verlag Stahleisen m.b.H, D€ usseldorf, FR Germany (1976) Elements of Briquetting and Agglomeration The Institute for Briquetting and Agglomeration (IBA), Hudson, WI, USA Vol. 1: H.C. Messmann, T.E. Tibbets, Editors (1977) Vol. 2: R.N. Koerner, J.A. McDougall, Editors (1983) Agglomeration 77, Vols. 1 and 2 K.V.S. Sastry, Editor Proc. 2nd International Symp. Agglomeration, Atlanta, GA, USA AIME, New York, NY, USA (1977) Mechanische Verfahrenstechnik (Mechanical Process Technology) H. Schubert et al. Deutscher Verlag f€ ur Grundstoffindustrie, Leipzig, GR Germany (1977) Pelletizing of Iron Ores K. Meyer Springer-Verlag, Berlin/Heidelberg, FR Germany/New York, NY, USA - Verlag Stahleisen mbH, D€ usseldorf, FR Germany (1980) Particle Size Enlargement C.E. Capes Handbook of Powder Technology, J.C. Williams, T. Allen, Series Editors, Vol. 1 Elsevier Scientific Publishing Co., Amsterdam, The Netherlands/ Oxford, UK/New York, NY, USA (1980) Direct Reduced Iron. Technology and Economics of Production and Use. R.L. Stephenson, R.M. Smailer The Iron and Steel Society of AIME, Warrendale, PA, USA (1980) Agglomeration 81, Vols. 1 and 2 O. Molerus, W. Hufnagel, Editors Proc. 3rd International Symp. Agglomeration, N€ urnberg, FR Germany N€ urnberger Messe- und Ausstellungsgesellschaft, N€ urnberg, FR Germany (1981) Agglomeration W. Pietsch In Fortschritte der Verfahrenstechnik (in English) VDI-Verlag GmbH, D€ usseldorf, FR Germany Vol. 19 (1981), 133-149 Vol. 21 (1983), 121-139 Vol. 23 (1985), 125-139
663
664
13 Bibliography B.23
B.24
B.25
B.26
B.27
B.28
B.29
B.30
B.31
B.32
B.33
B.34
B.35
B.36
B.37
Kapillarit€at in por€osen Feststoffsystemen (Capillarity in porous solid systems) H. Schubert Springer-Verlag, Berlin/Heidelberg, FR Germany/New York, NY, USA (1982) Size Enlargement Methods and Equipment C.E. Capes, W. Pietsch, et al. In Handbook of Powder Science and Technology, M.E. Fayed, L. Otten, Eds. Chapter 7 Van Nostrand Reinhold Co., New York, NY/Cincinnati, OH, USA/Toronto, Canada/London, UK/Melbourne, Australia (1983). Brikettieren und Pelletieren von Biomasse (Briquetting and pelleting of biomass) U. Bossel, Editor SOLENTEC Fachbuchvertrieb, Adelebsen, FR Germany (1983). Agglomeration 85 C.E. Capes, Editor Proc. 4th International Symp. Agglomeration, Toronto, Ont., Canada The Iron & Steel Society, Inc. (ISS), Warrendale, PA, USA (1985) Sampling and Weighing of Bulk Solids J.W. Merks Series on Bulk Materials Handling, Vol. 4, Trans Tech Publications, Clausthal-Zellerfeld, FR Germany (1985) Powder Metallurgy Equipment Manual, 3rd Edition S. Bradbury, Editor Metal Powder Industries Federation, Princeton, NJ, USA (1986) Flocculation, Sedimentation & Consolidation B. M. Moudgil, P. Somasundaran, Editors Proc. Engineering Foundation Conference United Engineering Trustees, Inc., USA (1986) Sampling of Powders and Bulk Materials K. Sommer Springer-Verlag, Berlin, Heidelberg, FR Germany (1986) Sinter and Pellets. Production and Use Capacities (State: 1987) Committee on Raw Materials International Iron and Steel Institute (IISI), Brussels, Belgium (1987) Particle Attrition - State-of-the-Art Review British Materials Handling Board Series on Bulk Materials Handling, Vol. 5, Trans Tech Publications, Clausthal-Zellerfeld, FR Germany (1987) Microcapsule Processing and Technology A. Kondo Marcel Dekker, Inc., New York, NY, USA (1987) Tablet Machine Instrumentation in Pharmaceuticals - Principles and Practice P. Ridgeway-Watt Ellis Harwood Series in Pharm. Technology, John Wiley & Sons, New York, NY, USA (1988) Pelletization of Fines (Minerals, Ores, Coal) J. Srb, Z. Ruzickova Developments in Mineral Processing, D.W. Fuerstenau, Advisory Editor, Vol 7 Elsevier Science Publishers B.V., Amsterdam, The Netherlands (1988) Size Enlargement K. Sommer In Ullmann’s Encyclopedia of Industrial Chemistry, 5th Edition Vol B.2, Chapter 7 Verlag Chemie GmbH, Weinheim FR Germany (1988), pp. 1-37 An International Workshop on Biomass Fuel Briquetting in Developing Countries. The National Energy Administration, Sudan Proceedings, Khartoum, Sudan, 23-26 October (1988).
13.1 List of Books or Major Chapters on Agglomeration and Related Subjects B.38
B.39
B.40
B.41
B.42
B.43
B.44
B.45
B.46
B.47
B.48
B.49
Agglomerationskinetik zur Simulation von Agglomerationsprozessen im Agglomerierteller (Kinetics of agglomeration for the simulation of agglomeration processes in the pan granulator) W. D€otsch Fortschr.-Ber. VDI, Reihe 3, Nr. 157, VDI-Verlag GmbH, D€ usseldorf, FR Germany (1988) Particles on Surfaces 1, 2, and 3: Detection, Adhesion, and Removal K.L. Mittal, Editor Plenum Publishing Corp., New York, NY, USA (1988, 1989, 1990) Agglomeration 89 Proc. 5th International Symp. Agglomeration, Brighton, UK The Institution of Chemical Engineers (IChemE), Rugby, UK (1989) Pharmaceutical Pelletization Technology I. Ghebre-Sellassie, Editor The Pharmaceutical Sciences Series, No. 37 Marcel Dekker, New York, NY, USA (1989) A Random Walk through Fractal Dimensions B.H. Kaye VCH Verlagsgesellschaft mbH, Weinheim, FR Germany (1989) Agglomeration der dispersen Phase von Aerosolen durch starke Schallfelder (Agglomeration of the disperse phase of aerosols by strong sound fields) B. Schetter and J. Funcke Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 196, VDI-Verlag GmbH, D€ usseldorf, Germany (1990) Trocknung kapillarpor€oser K€orper bei Anwesenheit auskristallisierender Stoffe in der Gutsfeuchte / Trocknung mit Krustenbildung (Drying of wet porous bodies which contain dissolved substances / Drying with incrustation) P. Schultz Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 201, VDI-Verlag GmbH, D€ usseldorf, Germany (1990) Die Agglomeration partikelf€ormiger Feststoffe in Matrizenpressen (The agglomeration of particulate solids in pellet presses) C.-J. Klasen Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 220, VDI-Verlag GmbH, D€ usseldorf, Germany (1990) The briquetting of agricultural wastes for fuel S. Eriksson and M. Prior FAO Environment and Energy Paper 11, Food and Agriculture Organization of the United Nations, Rome, Italy (1990). High Temperature Sintering H.I. Sanderow MPIF Publication, Princeton, NJ, USA (1990). Size Enlargement by Agglomeration W. Pietsch John Wiley & Sons Ltd., Chichester, UK/New York, NY, USA/Brisbane, Australia/Toronto, Canada/Singapore - Otto Salle Verlag GmbH & Co., Frankfurt/M, Germany - Verlag Sauerl€ander AG, Aarau, Switzerland (1991). This book is out of print and the copyright is now held by the author* Spray Drying Handbook K. Masters 5th Edition, Longman, London, UK/John Wiley & Sons, New York, NY (1991)
* Arrangements have been made with “Books on Demand”, the reprinting service of ProQuest Information and Learning (www.lib.umi.com/bod or www.Alibris.com), to make the publication available under order number WBI-2067035-009).
665
666
13 Bibliography B.50
B.51
B.52
B.53
B.54
B.55
B.56
B.57
B.58 B.59
B.60
B.61
B.62
B.63
B.64
B.65
B.66
Cake Formation in Particulate Systems E.J. Griffith VCH Publishers, Inc., New York, NY, USA (1991) Agglomerations-/Sch€ uttgut-Technik (Agglomeration and Bulk Solids Techn.) Preprints, GVC, VDI Gesellschaft Verfahrenstechnik und Chemieingenieurwesen, D€ usseldorf, Germany (1991) Grundlagen der Mechanischen Verfahrenstechnik (Fundamentals of Mechanical Process Engineering) F. L€offler and J. Raasch Friedr. Viehweg & Sohn Verlagsgesellschaft mbH, Braunschweig/Wiesbaden, Germany (1992) Physical Chemistry of Food H.G. Schwartzberg (Editor) Marcel Dekker, Inc., New York, NY, USA (1992) The glassy State in Foods J.M.V. Blanshard and P.J. Lillford Nottingham University Press, Loughborough, Leicestershire, UK (1993) Industrial Briquetting. Fundamentals and Methods. Z. Drzymala Studies in Mechanical Engineering, Vol. 13 Elsevier Science Publishers, Amsterdam, The Netherlands/London, UK/New York, NY, USA/ Tokyo, Japan - PWN Polish Scientific Publishers, Warzawa, Poland (1993) AGGLOS Proc. 6th International Symp. Agglomeration, Nagoya, Japan The Society of Powder Technology, Japan - The Iron and Steel Institute of Japan - The Society of Chemical Engineers, Japan (1993) Powder Metallurgy Science, 2nd Edition R.M. German MPIF Publication, Princeton, NJ, USA (1994) First International Particle Technology Forum (1st IPTF), Denver, CO Proceedings, PTF of AIChE, New York, NY, USA (1994) Phase Transitions in Foods Y.H. Roos Academic Press, London, UK (1995) The Manufacture of modern Detergent Powders W. Herman de Groot, I. Adami, and G.F. Moretti Herman de Groot Academic Publishers, Wassenaar, The Netherlands (1995) Ceramic Processing R.A. Terpstra, P.P.A.C. Pex, and A.H. De Vries (Editors) Chapman & Hall, London, UK (1995) Fracture Mechanics of Concrete: Applications of fracture mechanics to concrete, rock, and other quasi-brittle materials. S.P. Shah, S.E. Swartz, Chengsheng Ouyang John Wiley & Sons, Inc., New York, NY, USA (1995) Practical Dispersion. A Guide to Understanding and Formulating Slurries R.F. Conley VCH Publishers, Inc., New York, NY, USA (1996) The 5th World Congress of Chemical Engineering and 2nd IPTF (Int’l Particle Technology Forum), San Diego, CA Proceedings, AIChE and PTF of AIChE, New York, NY, USA (1996) Sintering Theory and Practice R.M. German John Wiley Publications, New York, NY, USA (1996). Pharmaceutical Powder Compaction Technology G. Alderborn, Ch. Nystr€om, Editors Drugs and Pharmaceutical Sciences, Vol. 71 Marcel Dekker, Inc., New York, NY, USA (1996)
13.1 List of Books or Major Chapters on Agglomeration and Related Subjects B.67
B.68
B.69
B.70
B.71
B.72
B.73
B.74
B.75
B.76
B.77
B.78
B.79
B.80
B.81
B.82
Microencapsulation – Methods and Industrial Applications S. Benita Marcel Dekker, Inc., New York, NY, USA (1996) Handbook of Pharmaceutical Granulation Technology D.M. Parikh, Editor Drugs and Pharmaceutical Sciences, Vol. 81 Marcel Dekker, Inc., New York, NY, USA (1997) Cement and Concrete M.S.J. Gani Chapman & Hall, London, UK (1997) Ultra-Fine Particles. Exploratory Science and Technology Ch. Hayashi, R. Uyeda, A. Tasaki, Editors Noyes Publications, Westwood, NJ, USA (1997) Size Enlargement by Agglomeration (Monograph) W. Pietsch In: Handbook of Powder Science and Technology, M.E. Fayed, L. Otten, Eds. 2nd Edition, Chapter 6 Chapman & Hall, New York, NY (1997), 202-377 Untersuchung der Agglomeration von Partikeln bei der Wirbelschicht-Spr€ uhgranulation (Investigation of the agglomeration of particles during fluidized bed spray granulation) R.-D. Becher Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 500, VDI-Verlag GmbH, D€ usseldorf, Germany (1997) Size Enlargement C. E. Capes and K. Darcovich In: Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Vol. 22 (1997), 222-255 Powder Mixing B. H. Kaye Chapman & Hall, London, UK (1997) Particle Size Measurement, Fifth Edition T. Allen Vol. 1: Powder Sampling and Particle Size Measurement Vol. 2: Surface Area and Pore Size Determination Chapman & Hall, London, UK (1997) An Introduction to Composite Products. Design, Development, and Manufacture. K. Potter Chapman & Hall, London, UK (1997) Principles of Food Processing D.R. Heldmann, R.W. Hartel Chapmann & Hall, (ITP), New York, NY, USA (1997) Porous Materials. Process Technology and Applications K. Ishizaki, S. Komarneni, and M. Nanko Kluwer Academic Publishers, Dordrecht, NL, Boston, USA, London, UK (1998) Powder Metal Technologies and Applications, ASM Handbook Vol. 7 W.B. Eisen et al., Editors ASM International Publication, Materials Park, OH, USA (1998). Porosity of Ceramics R. W. Rice Marcel Dekker, Inc., New York, USA, Basel, Switzerland, Hong Kong (1998) Lime and Limestone J.A.H. Oates Wiley-VCH Verlag GmbH, Weinheim, Germany (1998) World Congress on Particle Technology 3 (WCPT3), Brighton, UK including 3rd IPTF (“Emerging Particle Technologies: A Vision to the Future”), IChemE, Rugby, Warwickshire, UK (1998)
667
668
13 Bibliography B.83
B.84
B.85
B.86
B.87
B.88
B.89
B.90
B.91
B.92
B.93
B.94 B.95
B.96
B.97
B.98 B.99
B.100
B.101
Polymer Recycling: Science, Technology, and Applications J. Scheirs John Wiley & Sons, Ltd., Chichester, West Sussex, England (1998) Powder Metallurgy Design Manual, 3rd Edition Anonymous (MPIF) MPIF Publication, Princeton, NJ, USA (1998). Powder Metallurgy of Iron and Steel R.M. German ASM International Publication, Materials Park, OH, USA (1998). Aggregation Phenomena in Complex Systems J. Schmelzer. G. R€opke, R. Mahnke Wiley-VCH Verlag GmbH, Weinheim, Germany (1999) Direct Reduced Iron – Technology and Economics of Production and Use J. Feinman, D.R. Mac Rae (Editors) The Iron and Steel Society, Warrendale, PA, USA (1999) Disperse Systems Makoto Takeo Wiley-VCH Verlag GmbH, Weinheim, Germany (1999) Handbuch der Agglomerationstechnik (Handbook of Agglomeration Technology) G. Heinze Wiley-VCH Verlag GmbH, Weinheim, Germany (2000) Surfactant-based Separations. Science and Technology J. F. Scamehorn, J. H. Harwell, Editors American Chemical Society, Washington, DC, USA (2000) Population Balances. Theory and Applications to Particulate Systems in Engineering D. Ramkrishna Academic Press, San Diego, CA, USA (2000) Handbook of Adhesives and Sealants E. M. Petrie McGraw-Hill, New York, NY, USA (2000) Wirbelschicht-Spr€ uhgranulation (Fluidized bed spray granulation) H. Uhlemann, L. M€orl Springer Verlag, Heidelberg, Germany (2000) 7th Int’l Symposium Agglomeration, Albi, France PROGEP, 18 chemin de la loge, 31078 Toulouse Cedex 4, France (2001) Nanotechnologie (Nanotechnology) M. K€ohler Wiley-VCH Verlag GmbH, Weinheim, Germany (2001) Mechanical Alloying for Fabrication of Advanced Engineering Materials S. El-Eskandarany William Andrew Publishing, Norwich, NY, USA (2001). Agglomeration Processes – Phenomena, Technologies, Equipment W. Pietsch Wiley-VCH Verlag GmbH, Weinheim, Germany (2002) World Congress on Particle Technology 4 (WCPT4), Sydney, Australia (2002) Die Tablette – Handbuch der Entwicklung, Herstellung und Qualit€atssicherung (The Tablet – Handbook for the development, manufacturing and quality assurance), 2nd updated edition (see also B.5) W.A. Ritschel, A. Bauer-Brandl ECV – Editio Cantor Verlag, Aulendorf, Germany (2002) Fundamentals of Powder Metallurgy L.F. Pease, W.G. West MPIF Publication, Princeton, NJ, USA (2002). International Atlas of Powder Metallurgical Microstructures P. Beiss, K. Dalal, R. Peters MPIF Publication, Princeton, NJ, USA (2002).
13.2 References B.102
B.103
B.104
B.105
B.106
B.107
B.108
B.109
Laundry Detergents E. Smulders Wiley-VCH Verlag GmbH, Weinheim, Germany (2002) High Performance Pigments H.M. Smith (Editor) Wiley-VCH Verlag GmbH, Weinheim, Germany (2002) Scale-up in Chemical Engineering M. Zlokarnik Wiley-VCH Verlag GmbH, Weinheim, Germany (2002) Encyclopedia of Pharmaceutical Technology (2nd Edition) J. Swarbrick and J.C. Boylan (Editors) Marcel Dekker, Inc., New York, NY, USA (2002) Handbuch der Mechanischen Verfahrenstechnik, 2 B€ande (Handbook of Mechanical Process Technology, 2 Volumes) H. Schubert (Editor) Wiley-VCH Verlag GmbH, Weinheim, Germany (2003) Nonwoven Fabrics W. Albrecht, H. Fuchs, W. Kittelmann (Editors) Wiley-VCH Verlag GmbH, Weinheim, Germany (2003) Physical Principles of Food Preservation (2nd Edition) M. Karel and D. Lund Marcel Dekker, Inc., New York, NY, USA (2003) Nanotechnology and Nanoprocesses – Introduction, Valuation, W. Fahrner (Editor), Springer Verlag, Heidelberg, Germany (2003).
13.2
References 5.1
5.2
5.3
6.1.1
6.1.2
6.1.3
6.1.4
B.J. Ennis, J.D. Litster “Size Enlargement” Section 20, Perry’s Chemical Engineers’ Handbook, 7th Edition, R. Perry, D. Green (Editors) McGraw-Hill, Inc., New York, NY, USA (1999). T.-J. Wang, A. Tsutsumi, H. Hasegawa, T. Mineo “Mechanism of particle coating granulation with RESS process in a fluidized bed” Powder Technology 118 (2001), 229-235. H. Fudouzi, M. Kobayashi, N. Shinya “Arrangement of microscale particles by electrification” Kona (1999) 17, 55-63. J. Priemer “Untersuchungen zur Prallzerkleinerung von Einzelteilchen” (Investigations of the impact crushing of single particles) PhD Thesis, University (TH) Karlsruhe, Germany (1964). R.P. von Rittinger “Lehrbuch der Aufbereitungskunde” (Textbook Process Technology) Ernst v. Korn, Berlin, Germany (1857). A.A. Griffith “Phenomena of Rupture and Flow in Solids” Phil. Trans. Roy. Soc. (London, UK) 221A, 163 (1920). “The Theory of Rupture” Proc. First Int. Congr. Applied Mechanics, Delft, The Netherlands (1924). H. Schubert “Aufbereitung fester mineralischer Rohstoffe (Mechanical Process Technology), Vol 1, 4th rev. Edition VEB Verlag f€ ur Grundstoffindustrie, Leipzig, GDR (1989).
669
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13 Bibliography 6.2.1
6.2.2
6.2.1.1
6.2.1.2
6.2.1.3
6.2.2.1
6.2.2.2
6.2.2.3
6.2.3.1
6.2.3.2
6.2.3.3
6.2.3.4
6.3.1.1
6.3.1.2
6.3.1.3
H. Kaiser (Editor) “Der Apothekerpraktikant” (The apprentice pharmacist) Wissenschaftliche Verlagsges. mbH, Stuttgart, Germany (1957). S.K. Hamarneh “Early Arabic pharmaceutical instruments” J. Am. Pharm. Ass., Practical Pharmacy Edition 21 (1960) 2, 90-92. R.F. Shangraw (Course Director) “Modern Granulation, Tabletting and Capsule Technology” Course Notes, Center for Professional Advancement (1988). R. Holzm€ uller “Untersuchungen zur Sch€ uttgutbewegung beim kontinuierlichen Feststoffmischen” (Investigation of the movement of particulate solids during continuous mixing). PhD Thesis, University of Stuttgart, Germany (1984). R.A. Nash “Streamlining Validation” Chemical Engineering Nov. (2003), 47-50. K. Zentis “Untersuchungen zur Entwicklung der Tablettenherstellung unter pharmazie- und technikgeschichtlichen Gesichtspunkten” (Historical investigations of the development of tablet making in pharmacy) PhD Thesis, Ludwig-Maximilian University, M€ unchen, Germany (1985). D. Train “Transmission of forces through a powder mass during the process of pelleting” Trans. Inst. Chem. Engrs. 35 (1957) 4, 258-266. G. Shlieout, R.F. Lammens, P. Kleinebudde “Dry granulation with a roller compactor. Part I: The functional units and operation modes” Pharmaceutical Technology Europe Nov. (2000), 6 pages. Anonymous (Diosna) “Farewell to irregularities – New coater assures uniform layer thickness” Pharma + Food (H€ uthig) 4 (2001) May, 2 pages. Y. Kawashima, M. Imai, H. Takeuchi, H. Yamamoto, K. Kamiya “Development of agglomerated crystals of ascorbic acid by the spherical crystallization technique for direct tabletting and evaluation of their compactibilities” Kona 20 (2002), 251-262. Y. Kawashima, F. Cui, H. Takeuchi, T. Niwa, T. Hino, K. Kiuchi “Parameters determining the agglomeration behavior and the micromeritic properties of spherically agglomerated crystals prepared by the spherical crystallization technique with miscible solvent systems” International Journal of Pharmaceutics 119 (1995), 139-147. Y. Kawashima, F. Cui, H. Takeuchi, T. Niwa, T. Hino, K. Kiuchi “Improvements in flowability and compressibility of pharmaceutical crystals for direct tabletting by spherical crystallization with a two-solvent system” Powder Technology 78(1994), 151-157. S.F. Zibell, M.M. Patel, J.C. Dave, R.A. Payne “Method of making a fast release stabilized aspartame ingredient for chewing gum” US Patent No. 5,221,543, dated June 22, 1993. O. Koppel “Compact Detergent Powder” (Various presentations on the technology for manufacturing compact detergent powders). GEA Niro A.S., Soeborg, Danmark, 3rd issue (1993). A.E. Jungk “Verfahren zum Einf€arben von Beton (Process for dyeing concrete)” European Patent EP 0 268 645 B1, 07.11.90 United States Patent # 4,946,505, 08.07.90
13.2 References 6.3.2.1
6.3.2.2
6.3.2.3
6.3.2.4
6.3.3.1
6.3.3.2
6.4.1
6.4.2
6.4.3
6.4.4
6.4.1.1
6.5.1.1
6.5.2.1
6.5.2.2
6.5.3
6.6.1
T. Kataoka et al. “Method of preparing granules of Dipeptide” European Patent EP 0 585 880 B1, 31.08.93 United States Patent # 5,358,186, 25.10.94 D.W. Caton “Starch hydrolyzate product and method of producing same” United States Patent # 4,810,307, 07.03.89 DuPont “OXONE monopersulfate compound” Technical Information TC 2.40, DuPont Specialty Chemicals, Wilmington, DE, USA (1997). J.T. Will “Process for coloring concrete using compacted inorganic granules” United States Patent # 5,853,476, 29.12.98 D. W. Perkins, A.V. Petricca “Pretreatment in encapsulation process” United States Patent # 4,588,612, 13.04.86 L. Madec, H. Muhr, E. Plasari “Development of new methods to accelerate and improve the agglomeration of submicron particles by binding liquids” Powder Technology 128 (2002) 236-241. H. Eiselen (Publisher) “Brotkultur” (The culture of bread) DuMont Buchverlag, K€oln, Germany (1995). H.E. Jacob “Six Thousand Years of Bread. Its Holy and Unholy History” Lyons & Burford Publishers, New York, NY, USA (1997) (1st Ed. 1944). L. Slade, H. Levine, J. Ievolella, M. Wang “The glassy state phenomenon in applications for the food industry: Application of the food polymer science approach to structure-function relationships of sucrose in cookie and cracker systems”. J. Sci. Food Agric. 63 (1993), 133-176. J.M. Aguilera, J.M. del Valle, M. Karel “Caking phenomena in amorphous food powders” Trends in Food Science & Technology 61 (1995) 5, 149-155. V. Westergaard “Milk Powder Technology. Evaporation and Spray Drying”, 4th Edition Niro A/S, Copenhagen (1994). B.L. Miller, M.R. Higgins, P.Casey “Method and process for producing an improved milk replacer” United States Patent # 6,406,729 B1, filed April 14.2000, patented June 18, 2002. Anonymous “Markets update” Powder & Bulk Engineering (2002) 9 Sept., 16. A. Buschhart, H.W. Lucht “Herstellung von Hundefutter mit dem Expander OE-E-15.2” (Manufacturing of dog food with the Expander OE-E-15.2) Unpublished report Amandus Kahl, Reinbek, Germany (1999). H.-J. Matthies “Entwicklung und Forschung auf dem Gebiet des Verdichtens von Halmgut” (Development and research in the field of densification of dry plant material) Landtechische Forschung 13 (1963) 6, 157-163. Anonymous (T.P. Hignett, IFDC) “Fertilizer Manual” United Nations Industrial Development Organization (UNIDO), International Fertilizer Development Center (IFDC), Muscle Shoals, AL, USA (1979).
671
672
13 Bibliography 6.6.2
6.6.3
6.6.4
6.6.5
6.6.6
6.6.2.1
6.6.3.1
6.7.3.1
6.7.3.2
6.8.1.1
6.8.1.2
6.8.1.3
6.8.1.4
6.8.1.5
6.8.1.6
6.8.1.7
G. Lueth “40 years of Nitrophoska” Proc. 17th Annual Meeting of the Fertilizer Round Table (1967), 114-117. J.J. Schultz, T.P. Hignett “Granulation of Fertilizers: History, current practices, and future trends” In “Agglomeration ’85” (C.E. Capes, Editor), Proceedings of the 4th Int. Symposium on Agglomeration, Toronto, Ont., Canada, the Iron and Steel Society, Inc., Warrendale, PA, USA (1985), 339-355. J.O. Hardesty “ Granulation” in “Superphosphate – Its history, chemistry, and manufacture” US DOA and TVA, US Government Printing Office, Washington, DC, USA (1964). W.G.T. Packard “Superphosphate, its history and manufacture” Trans. Inst. Chem. Engrs. (UK), 15 (1937), 21-44. L..D. Yates, F.T. Nielsson, G.C. Hicks “TVA’s continuous ammoniator” Farm Chemicals 117 (1954) 7, 38-48 and 8, 34-41. M. Br€ ubach “The influence of particle size, strength, and friction on the distribution of granular fertilizers and insecticides using rotary spreaders”. PhD Thesis, Techn. University Berlin, FRG (1973). K. Hirech, S. Payan, G. Carnelle, L. Brujes, J. Legrand “Microencapsulation of an insecticide by interfacial polymerisation” Proceedings of the 7th Int. Symp. Agglomeration, Albi, France, PROGEP, 18 chemin de la loge, 31078 Toulouse Cedex 4, France (2001), 167-176. Anonymous “Lurgi Handbuch” (Lurgi Handbook) Lurgi Gesellschaften, Frankfurt/M., Germany (1960). E. Gr€asle, G. Goldbeck, W. Vogeno “100 Jahre Humboldt 1856-1956” (100 Years of Humboldt) Werk Humboldt, K€oln-Kalk, Germany, 2nd Edition (1958). Anonymous “Operating pellet plants in the world – (1)” The TEX Report Ltd., Tokyo, Japan, Vol.34, No. 8123, Sept.18 (2002). Anonymous “Operating pellet plants of the world – (3)” The TEX Report Ltd., Tokyo, Japan, Vol.34, No. 8125, Sept.20 (2002). P.J. Kakela “Dramatic times for North American iron ore” Skillings Mining Review, July 20 (2002), 17-30. W.H. Engelleitner “Pellets cut cost improve quality” The Glass Industry, March (1972), 4 pages. W.H. Engelleitner “Agglomeration in the Glass Industry – An energy and environmental tool” isa (Instrument Society of America) N.F. 77-85 preprint, Niagara Falls, NY, October (1977) 229249. A.J.C.M. Sparidaens “Batch pelletization – the key to glass quality improvement” Society of Glass Technology, October symposium, Kidderminster, England (1990). “Pelletized Batch to Order” A.J.C.M. Sparidaens “Batch pelletisation – the key to glass quality improvement” Glass Technology Vol. 32, No.5, October (1991) 149-152.
13.2 References 6.8.1.8
6.9.2.1
6.9.2.2
6.10.1
6.10.2
6.10.1.1
6.10.1.2
6.10.2.1
6.10.2.2
6.10.2.3
6.10.2.4
6.10.2.5
6.10.2.6
6.10.2.7
6.10.2.8
6.10.2.9
Anonymous “World Iron Ore Pellet Statistics by UNCTAD” Skillings Mining Review July 15 (2000), 3. V.C. Vora “Shipping and Handling of DRI” Skillings Mining Review, Feb. 28 (1998), 4-8. S. Moore - D. Hairston (Editor) “Steelmakers’ iron resolve” Chemical Engineering (2002) 10, 27S. D. Serkin, Editor “Coal Fines: The Unclaimed Fuel” Coal & Slurry Technology Association, Washington, DC, USA (1995). P. W. Woessner, J. T. Wilbur “AMAX-HICAL Briquetting Project” Proc. 18th International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, FL, USA, Coal & Slurry Technology Association, Washington, DC, USA (1993), 83-94. W.H. Engelleitner “Developments in the agglomeration of fine coal” Proc. 3rd USA-Korea Joint Workshop on Coal Utilization Technology, Pittsburgh Energy Technology Center (PETC), Pittsburgh, PA, USA (1986), 49-64. S.D. Serkin (Editor) “Coal Fines: The unclaimed Fuel” Coal & Slurry Technology Association, Washington, DC, USA (1995). E. Rammler € ber die Theorien der Braunkohlenbrikettentstehung” (On the theories of mechanisms in“U volved in the production of brown coal briquettes). Sitzungsber. S€achs. Akad. Wissenschaften (Leipzig), Vol. 109 (1), Akademie Verlag, Berlin, GDR (1970). Anonymous “100 Jahre (years) Humboldt” Anniversary Publication, 2nd Edition, Werk Humboldt of Kl€ ockner-Humboldt-Deutz AG, K€ oln-Kalk, FRG (1958). F. Knauth “Brown Coal Briquetting in Germany and the former State-Trading Countries of East Europe” Braunkohle (1992)11, 16-19. Anonymous “Die Steinkohlenbrikettierung auf den Zechen des M€ uhlheimer Bergwerks-Vereins” (Hard coal briquetting at the mines of the M€ uhlheim Bergwerks-Verein) M€ uhlheimer Bergwerks-Verein AG, M€ uhlheim/Ruhr, FRG (1954). U. Bossel (Editor) “Brikettieren und Pelletieren von Biomasse” (Briquetting and pelleting of biomass). SOLENTEC Fachbuchvertrieb, Adelebsen, Germany (1983) S. Eriksson, M. Prior “The briquetting of agricultural wastes for fuel”. FAO Environment and Energy Paper 11 (book), United Nations, Rome (1990). W. Pietsch “Ram Pressing - An almost extinct technology with interesting new applications in coal and other solid fuel processing”. Proc. 25th Int. Technical Conf. on Coal Utilization & Fuel Systems, Clearwater, FL., USA, Coal & Slurry Technology Association, Washington, DC, USA (2000), 37-48. H. Liu, R.L. Gandhi, M.R. Carstens,G. Klinzing (contributing authors) “Freight Pipelines: Current Status and anticipated Future Use”. J. Transportation Engng. 124(1998)4, 300-310. T.R. Marrero, H. Liu, W.J. Burkett “Coal Log Fabrication: State of the Art for Pipeline Transportation”. Proc. 11th Annual Int. Pittsburgh Coal Conference, Pittsburgh, PA (1994), 841-847.
673
674
13 Bibliography 6.10.2.10 Q. Deng, H. Liu “Analysis of Coal Log Ram Extrusion”. Powder Technology 91(1997), 31-41. 6.10.2.11 Anonymous (Nippon Steel, Tokyo) “Coal-coking process cuts costs and emissions” Chemical Engineering (2002)8, 21. 6.10.2.12 P.W. Woessner, J.T. Wilbur “AMAX - HICAL Briquetting Project”. Proc. 18th Int’l Techn. Conf. on Coal Utilization and Fuel Systems, Clearwater, FL (1993), 83-94. 6.10.3.1 C.E. Capes “Oil agglomeration process principles and commercial application for fine coal cleaning” In: Coal Preparation, 5th Edition, J.W. Leonard, III, B.C. Hardinge (Editors) Topics of special interest, Part 4, pp 1020-1041. SME, Littleton, CO, USA (1991) 6.11.2.1 Anonymous “Chemical Oxygen Generators for Aviation” PB Puritan Bennett Aero Systems, Lenexa, KS, USA (1972). 6.11.2.2 R. van Ryper “Bearing Design with polyimides” DuPont Engineering Polymers, Newark, DE, USA (1986) 6.11.3.1 K.-E. Wirth, M. Linsenb€ uhler “Elektrostatisch unterst€ utztes Mischen feink€ orniger Partikel” (Electrostatically assisted mixing of finely dispersed particles) Chemie Ingenieur Technik 75(2003)6, 701-704. 6.11.3.2 M. Nanu “Neue Materialien kreieren – Nanoskalige Beschichtung mit dem Omnitex” (Creating new materials – Nano-scale coating with the Omnitex) CITplus (2003)4, 42-43. 8.1.1 Anonymous “New developments in DML’s for unloading dusty cargoes” Bulk Solids Handling 23(2003)1, 44-45. 8.2.1 H. G€ unter “The application of waste paper as a binder in pressure Agglomeration” In [B.3], Vol. 23 (1993), 25-34. 8.2.2 J. Heiss et al. “Hot briquetting of LD dust in the steel plant of VA-Linz” Proc. 1st European Oxygen Steelmaking Congress, D€ usseldorf, Germany (1993), 194-197. 8.2.3 W.B. Steverson “MRFs and UBCs: A concern yet an opportunity” Light Metals 1995, J. Evans (editor), The Minerals, Metals & Materials Society (TMS), Warrendale, PA, USA (1995), 1,303-1,307. 8.2.4 M.H. Weinecke, B.P. Faulkner “Production of lightweight aggregate from waste materials” Mining Engineering (2002)11, 39-43. 8.2.5 S. Orr “Enhanced heap leaching - Part 1: Insights” Mining Engineering (2002)9, 49-56. S. Orr, V. Vesselinov “Enhanced heap leaching - Part 2: Applications” Mining Engineering (2002)10, 33-38. 10.1 H. Petroski “Engineers know failure is an option” Naples Daily News (New York Times News Service), Naples, FL, USA, September 7 (2003), 3B. 10.2 C.D. Swann, M.L. Preston “Twenty-five years of HAZOPs” Journal of Loss Prevention in the Power Industry 8 (1995) 6, 349-353.
13.2 References 10.3
10.4
10.5
10.6
10.7
10.1.1
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
11.10
11.11
K.L. Mulholland “Think outside the box to reduce wastes” CEP (Chemical Engineering Progress (2003) 6, 46-49. Anonymous “Defining Green Engineering” CEP (Chemical Engineering Progress (2003) 9, 78. R.E. Meissner III “Modular vs. conventional construction – Choosing the right path” Chemical Engineering (2003) 9, 46-52. D. Ainsworth, M. Brocklebank “Multiproduct plant design” Chemical Engineering (2003) 7, 42-49. M.C. Welch, R.B. Hartman “How sustainable are your PSM processes?” CEP (Chemical Engineering Progress (2003) 6, 50-53. M. Rhodes “Introduction to Particle Technology” John Wiley & Sons Ltd., Chichester, UK (1998). A. Gutsch, M. Kr€amer, G. Michael, H. M€ uhlenweg, M. Prid€ ohl, G. Zimmermann “Gas-phase production of Nanoparticles” Kona 20 (2002), 24-37. F.E. Kruis, H. Fissan “Nano-process technology for synthesis and handling of nanoparticles” Kona 17 (1999), 130-139. O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, E.C. Dickey, C.A. Grimes “Extreme Changes in the electrical resistance of titania nanotubes with hydrogen exposure” Advanced Materials (Weinheim, Germany) 15 (2003) 7-8, 624-627. G.N. Kryukova, G.A. Zenkovets, N. Pf€ander, D.-S. Su, R. Schl€ ogl “Synthesis and characterization of the titanium doped nanostructural V2O5” Materials Science and Engineering A343 (2003), 8-12. G. Stix “Big Little Science” Scientific American, Sept. (2001), 32-37. M. Brust, Ch.J. Kiely “Some recent advances in nanostructure preparation from gold and silver particles: a short topical review” Colloids and Surfaces A: Physicochemical and Engineering Aspects 202 (2002) 175-186. Z. Tang, N.A. Kotov, M. Giersig. “Spontaneous organization of single CdTe nanoparticles into luminescent nanowires” Science, 297 (2002) 5579, 237-240. F. Caruso “Hollow capsule processing through colloidal templating and self-assembly” Chem. Eur. J. 6 (2000) 3, 413-419. J. Bertling, J. Bl€omer, R. K€ ummel “Mikrohohlkugeln” (Hollow Micro Spheres) Chemie Ingenieur Technik 75 (2003) 6, 669-678. R.S. Lima, A. Kucuk, C.C. Berndt “Bimodal distribution of mechanical properties on plasma sprayed nanostructured partially stabilized zirconia” Materials Science and Engineering A327 (2002) 224-232. Anonymous (Degussa) “What is carbon black?” Publication of Degussa AG, Business Unit Advanced Fillers and Pigments, Frankfurt/M., Germany (2003).
675
676
13 Bibliography 11.12
12.1
M. Katzer, S. Pirl, S. Esser, J. Kopietz, Th. Rieckmann, J. Behnisch, J. Klasen “Verweilzeitverteilung in Granulationstrommeln am Beispiel von Industrieruß” (Residence time distribution in granulation drums for industrial carbon black). Chemie Ingenieur Technik 75 (2003) 4, 354-358. W. R€ahse, S. Hoffmann “Produkt-Design – Zusammenwirken von Chemie, Technik und Marketing im Dienste des Kunden” (Product design – Cooperation of production, engineering, and marketing for customer satisfaction) Chemie Ingenieur Technik (CIT) 74 (2002) 9, 1220-1229.
13.3
Author’s Biography, Patents, and Publications
Dr. Pietsch is a Senior Consultant in the general fields of Mechanical Process Engineering (powder and bulk solids technologies) and, particularly, size enlargement by agglomeration for Compactconsult Inc. of Naples, FL, USA. He received the equivalents of a BSc in Mechanical Engineering and an MS in Chemical Engineering from the Technical University (TH) of Karlsruhe, West Germany, in 1959 and 1962 (Dipl-Ing). In 1965 he earned his PhD (Dr-Ing) at the same university with work on the fundamentals of binding mechanisms of agglomeration. Prior to his industrial career, Dr. Pietsch did further research in the general field of size enlargement by agglomeration at the Institute of Mechanical Process Engineering at the Technical University (TH) of Karlsruhe until 1967. Later, while in industry, he taught the unit operations of mechanical engineering at the University of Stuttgart, Heilbronn Branch, Heilbronn, West Germany. Beginning in 1967, his industrial positions were: Research Scientist, Allis-Chalmers, Milwaukee, WI, USA; Staff Consultant, Komarek Greaves (today Hosokawa Bepex), Rosemont, IL, USA; Technical Director, HUTT GmbH (today Hosokawa Bepex), Leingarten, West Germany; Managing Director, Technical, Lemf€ order Kunststoff GmbH, Lemf€orde, West Germany; Director Agglomeration Systems and Product Technology, Midrex Corp., Charlotte, NC, USA; General Manager Large Metallurgical Systems, Leybold-Heraeus GmbH, Hanau, West Germany; Senior Technical Manager, Maschinenfabrik GmbH & Co KG, Hattingen/Ruhr, Germany; Executive Vice President and, later, President, K€ oppern Equipment, Inc., Charlotte, NC, and Pittsburgh, PA, USA, until 1995 when he retired from industry. Dr. Pietsch is the author of more than 170 papers, four books, including the textbooks Size Enlargement by Agglomeration, published by Wiley & Sons in cooperation with Salle + Sauerl€ander in 1991 (see note on availability in Section 13.1) and Agglomeration Processes – Phenomena, Technologies, Equipment, Wiley-VCH, 2002, and holds nine patents. He is a member of six professional organizations in the USA and Germany and is active in a number of technical committees. He is a frequent lecturer of workshops, short courses, and continuing education events in the fields of Mechanical Process Technology and Agglomeration in the USA and Europe. Compactconsult Inc. was founded in 1983 and incorporated in the state of North Carolina, USA, in early 1984. It is 100 % owned and operated by Mrs. Hannelore Pietsch and, therefore, qualifies as a Woman-Owned Small Business Concern.
13.3 Author’s Biography, Patents, and Publications
The primary purpose of Compactconsult Inc. is to make international experts available to industries as well as private and government agencies. The fields of expertise of consultants are the unit operations of mechanical process technology in all areas producing, handling, and processing particulate solids (particles, powders, and bulk masses) as well as hot and cold metal-bearing particulate matter, including direct reduced iron (DRI). Specific expertise exists in size enlargement by agglomeration. Other important activities are in the fields of processing and recirculating particulate wastes as secondary raw materials. In 1991 Compactconsult Inc. moved temporarily to the State of Pennsylvania, USA, where it operated as a “foreign” enterprise while still incorporated in North Carolina. After relocating to Naples, Florida, USA, the company was reincorporated in the State of Florida on 17 August 1995. Ownership has remained unchanged. Dr-Ing Wolfgang Pietsch (PhD), Eur Ing, joined Compactconsult Inc. in 1983 as Senior Consultant and, after leaving K€oppern Equipment Inc. of Pittsburgh, PA, in 1995, continues working in this position as an unaffiliated consultant and independent contractor. During his entire professional career, from becoming a student helper to Prof. DrIng Hans Rumpf at the Institute of Mechanical Process Engineering of the Technical University (TH) of Karlsruhe, West Germany, in 1960 to now exclusively working as a consultant, Dr. Pietsch has always been involved in mechanical process technology, particularly the unit operation of size enlargement by agglomeration and fields related to any aspect of agglomeration, as a researcher, teacher, process developer, designer, and user on two continents. While in industry (1967–1995) as a vendor representative, he has traveled to almost all countries to evaluate customers’ needs, develop suitable solutions, offer equipment and systems, and, if successful, help with the implementation, process optimization, and maintenance. This book and the earlier one Agglomeration Processes – Phenomena, Technologies, Equipment, Wiley-VCH (2002), see Section 13.1, are based on Dr. Pietsch’s long and varied experience to which innumerable professionals have contributed. Even though they remain anonymous, these people deserve credit. Also acknowledged should be the countless students who took part in the seminars and continuous education programs that Dr. Pietsch has either conducted or in which he has actively participated. At these sessions, as well as during hundreds of consulting assignments, discussions with those faced with technical problems and with the development of solutions have played a significant role in collecting the know-how that has been partially presented in these books. Much of the experience and know-how gathered by Dr. Pietsch during 40 years of professional work has also been published in papers and patents as well as in course notes. Although many of the more important statements and conclusions are reproduced in various parts of this book, it may be of interest to refer to the complete listing of these publications. The titles summarized below are clear indications of the contents and, therefore, may complement what is submitted briefly in this book by directing the reader to a more detailed coverage.
677
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13 Bibliography
Patents (Priority patents only: related patents filed or issued in many foreign countries) 1.
2.
3.
4.
5.
6.
7.
8.
9.
C. Buchholz, W. Pietsch Verfahren zur Aufbereitung von feuchten Metallsp€anen zum Wiedereinschmelzen. (Process treating moist metal chips for melting.) German patent DP 2 151 819, filed Oct. 18, 1971, issued Oct. 24, 1974. H.-J. Pitzer, W. Pietsch Verfahren und Vorrichtung zur Ausnutzung von bei der Spanplattenherstellung anfallenden S€agesp€ane- und Schleifstaubteilchen. (Process and equipment for the recovery of wood dust and chips produced during chipboard manufacturing.) Swiss patent SP 530 262, filed Oct. 22, 1971, issued Nov. 15, 1972. W. Pietsch Verfahren zur Pressgranulation von in Entstaubungsanlagen abgeschiedenen Industriest€auben. (Briquetting process for industrial dusts.) German patent DP 2 314 637, filed March 23, 1973, issued March 6, 1975. W. Pietsch Binder Composition. US patent No. 4,032,352, filed May 3, 1976, issued June 28, 1977. W. Pietsch Apparatus for continuous passivation of sponge iron material. US patent No. 4,033,559, filed April 5, 1976, issued July 5, 1977. W. Pietsch Method for continuous passivation of sponge iron material. US patent No. 4,076,520, filed April 5, 1976, issued Feb. 28, 1978. W. Pietsch Compacted, passivated metallized iron product. US patent No. 4,093,455, filed December 22, 1976, issued June 6, 1978. W. Pietsch, Ch.A. Schroer Briquet and method of making same. US patent No. 4,105,457, filed June 22, 1977, issued August 8, 1978. W. Pietsch Metallized iron briquet. US patent No. 4,116,679, filed March 24, 1977, issued, Sept. 26, 1978.
Publications 1.
2.
3.
W. Pietsch Festigkeit und Trocknungsverhalten von Granulaten, deren Zusammenhalt durch bei der Trocknung auskristallisierende Stoffe bewirkt wird. (Strength and drying behaviour of agglomerates, the induration of which is caused by solids crystallizing during the drying operation.) Diss. (Ph.D. thesis) Universit€at (TH) Karlsruhe, 1965. W. Pietsch Die Beeinflussungsm€oglichkeiten des Granuliertellerbetriebes und ihre Auswirkungen auf die Granulateigenschaften. (The possibilities of influencing the pelletizing pan operation and their effects on the properties of the pelletized material.) Aufbereitungs-Technik 7(1966)4, 177-191. H. Rumpf, W. Pietsch Festigkeit und Trocknungsverhalten von Granulaten mit Salzbr€ uckenbildung. (Strength and drying behaviour of granules with salt bridges.) Chemie-Ingenieur-Technik 38(1966)3, 371-372.
13.3 Author’s Biography, Patents, and Publications 4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
W. Pietsch Neue Entwicklungen auf dem Gebiet der Granuliertechnik und die Festigkeitseigenschaften von Granulaten. (New agglomeration procedures and physical properties of the agglomerates produced.) Revue Technique Luxembourgeoise (1966)2, 68-90. W. Pietsch, H. Rumpf Transport phenomena, crystallization and development of tensile strength during the drying of moist agglomerates containing Na-Cl-Solutions. Comptes-Rendues, Coll. Int. CNRS Nr. 169, Paris (1966), 213-235. W. Pietsch Zweites Europ€aisches Symposium “Zerkleinern”. (2nd European Symposium on Comminution.) a) Aufbereitungs-Technik 7(1966)11, 655-665. b) Chemie-Ingenieur-Technik 38(1966)12, 1307-1309. c) Staub-Reinhalt. Luft 27(1967)1, 52-55. W. Pietsch Das Agglomerationsverhalten feiner Teilchen. (The agglomeration tendencies of fine particles.) Staub-Reinhalt. Luft 27(1967)2, 64-65; English Edition: 27(1967)1, 24-41. W. Pietsch Einfluss der Verkrustung auf die Trocknung kapillar-por€ oser K€ orper. (Influence of the incrustation on the drying of capillaryporous bodies.) Staub-Reinhalt. Luft 27(1967)2, 64-65, English Edition: 27(1967)2, 10-11. W. Pietsch Die Festigkeit von Granulaten mit Salzbr€ uckenbindung und ihre Beeinflussung durch das Trocknungsverhalten. (The strength of granules with salt bridges and its change due to their drying behaviour.) Aufbereitungs-Technik 8(1967)6, 297-307. W. Pietsch Die Festigkeit von Agglomeraten. (The strength of agglomerates.) Chemie Technik 19(1967)5, 259-266. W. Pietsch Die Grundlagen der Kornvergr€osserung, ihre wissenschaftliche Untersuchung und technische Anwendung. (The fundamentals of size enlargement, its scientific investigation and technical application.) Revue Technique Luxembourgeoise 59(1967)2, 53-65. H. Rumpf, W. Pietsch (Herausgeber/Editors) Zerkleinern. (Comminution.) Dechema-Monographien Nr. 993-1026 (2 Bde), Verlag Chemie, GmbH, Weinheim/Bergstrasse (1967). W. Pietsch, H. Rumpf Haftkraft, Kapillardruck, Fl€ ussigkeitsvolumen und Grenzwinkel einer Fl€ ussigkeitsbr€ ucke zwischen zwei Kugeln. (Binding force, capillary pressure, liquid volume and critical angle of a liquid bridge between two spheres.) Chemie-Ingenieur-Technik 39(1967), 885-893. J.E. Moore, W. Pietsch Briquetting and compacting of lime and lime-bearing materials. Proc. 10th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Albuquerque, NM (1967), 38-50. W. Pietsch Tensile strength of granular materials. Nature, 217(1968)130, 736-739.
679
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13 Bibliography 16.
17.
18.
19.
20.
21.
21a
22.
23.
24.
25.
26.
27.
28.
W. Pietsch Stand der Eisenerzpelletierung. (Pelletizing of iron ore, worldwide.) Aufbereitungs-Technik 9(1968)5, 201-214. W. Pietsch An evaluation of techniques for particle size analysis, Part I and II. Minerals Processing 11(1968), 6-11, 12(1968), 12-14, 24. W. Pietsch Adhesion and agglomeration of solids during storage, flow and handling A survey. Journal of Engineering for Industry (Trans. of the ASME), Series B, 91(1969)2, 435-449. W. Pietsch, E. Hoffman, H. Rumpf Tensile strength of moist agglomerates. I & EC Product Research and Development 8(1969), 58-62. W. Pietsch The strength of agglomerates bound by salt bridges. The Canadian Journal of Chemical Engineering 47(1969), 403-409. W. Pietsch Roll designs for Briquetting-Compacting machines. Proc. 11th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Sun Valley, ID (1969), 145-163. W. Pietsch Die Bedeutung der Walzenkonstruktion von Brikettier-, Kompaktier- und Pelletiermaschinen f€ ur ihre technische Anwendung. (The importance of roll design for roll-type briquetting, compacting, and pelleting machines as defined by their technical application.) Aufbereitungs-Technik 11(1970)3, 128-138. W. Pietsch Improving powders by agglomeration. Chem. Engng. Progress 66(1970)1, 31-35. W. Pietsch Brikettieren, Kompaktieren und Kompaktieren/Granulieren von Kalk und kalkhaltigen Stoffen. (Briquetting, compacting and compacting/granulating of lime and lime bearing materials.) Zement-Kalk-Gips 59(1970)5, 210-215. W. Pietsch a) Granulierverfahren f€ ur die pharmazeutische Industrie. Die Pharmazeutische Industrie 32(1970)5, 383-389. b) Granulation techniques for pharmaceutical applications. Drugs made in Germany 13(1970)2, 58-66. W. Pietsch Erw€ unschte Agglomeration mit Granulatformmaschinen. (Wanted agglomeration with pelleting machines.) Maschinenmarkt MM - Industriejournal 77(1971)10, 193-196. W. Pietsch Size enlargement of solids. Particulate Matter. (Bulletin of the Powder Advisory Center) 2(1971)1, 15-22. W. Pietsch Roll designs for Briquetting-Compacting Machines. Proc. XIIIth Biennial Conf. of IBA, Sun Valley, Idaho (1969), 145-163. W. Pietsch Kornvergr€osserung. (Size enlargement.) Abschnitt 26, Fortschritte der Verfahrenstechnik, Bd. 9, VDI-Verlag GmbH, D€ usseldorf (1971), 831-872.
13.3 Author’s Biography, Patents, and Publications 29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
W. Pietsch Das K€ ornen von D€ ungemitteln mit dem Kompaktier-Granulierverfahren. (Granulating of fertilizers by means of the compacting/granulating procedure.) Aufbereitungs-Technik 12(1971)11, 684-690. W. Pietsch Possibilita` di miglioramento delle qualita` fisiche delle polveri tramite di agglomerazione. (Possibilities to improve the physical characteristics of powders by agglomeration methods). Ing. Chim. Ital. 7(1971)11, 161-166. W. Pietsch Granulieren durch Kornvergr€osserung. (Granulation by size enlargement.) CZ-Chemie Technik 1(1972)3, 116-119. W. Pietsch Anwendungen und Vorteile von W€alzdruck-Brikettiermaschinen bei der Aufbereitung mineralischer Rohstoffe. (Applications and advantages of roll type briquetting machines for mineral processing.) Proc. IXth Int. Min. Processing Congr., Prag (1970)3, 255-259, Ustav Pro Vzkum rud, Praha (1972). W. Pietsch Torque mill studies. A new approach in grinding research. “Particle Technology”, Proc. of Seminar, Indian Institute of Technology (IIT), Madras (1971), 203-232. W. Pietsch Size enlargement. Lit. (33), 276-290. W. Pietsch Granulation of fertilizers using compacting/granulation methods. Lit. (33), 335-348. W. Pietsch Agglomerieren problemlos - Kompaktiervorgang in W€alzdruckbrikettier- und Kompaktiermaschinen. (Agglomeration without problems - The process of compaction in roll-type briquetting and compacting machines.) Maschinenmarkt MM - Industriejournal 78(1972)88, 2036-2040. W. Pietsch Wet grinding experiments in a torque ball mill. In “Zerkleinern”Symposion in Cannes 1971, Dechema-Monographien, Band 69, Nr. 1292-1326, 751-779, Verlag Chemie GmbH, Weinheim/Bergstrasse (1972). W. Pietsch, H. Liebert Design and application of a laboratory machine for briquetting, compacting and pelleting research. Proc. 12th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Vancouver, Brit. Columbia (1971), 19-30. W. Pietsch Kornvergr€osserung. (Size enlargement.) In: Fortschritte der Verfahrenstechnik (1970/71), Bd. 19, Abt. B, Mechanische Verfahrenstechnik I, 221-235, Hrsg. VTG im VDI, VDI-Verlag GmbH, D€ usseldorf. W. Pietsch Das K€ ornen von D€ ungemitteln mit dem Kompaktier-Granulierverfahren. (Granulation of fertilizers using compaction/granulation methods.) Proc. 2. Wissenschaftlich-Technische Konferenz “Minerald€ unger”, Varna, Bulgarien, Hersg.: Wissenschaftlich-Techn. Verband f€ ur chem. Industrie, Sofia (1972), 359-379. W. Pietsch A granulcao de adubos pelo sistema de granulacao-compactacao. (Granulation of fertilizers using compaction/granulation methods.) Productos Quimicos 12(1972)9/12, 3-10.
681
682
13 Bibliography 42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
W. Pietsch Agglomerieren problemlos - Kompaktiervorgang in W€alzdruck-Brikettier- und Kompaktiermaschinen. (Agglomeration made easy - The process of compaction in roll type briquetting and compacting machines.) Europa Industrie Revue (1973)1, 28-31. W. Pietsch Der Kompaktiervorgang in W€alzdruck-Brikettier- und Kompaktiermaschinen. (The process of compaction in roll type briquetting and compacting machines,) Proc. Symposium Pracovnku´ Ba´nske´ho Pru´myslu, Hornicka´ Prbram ve Vede a Technice, Prbram, CSSR (1972), 683-710. C. Buchholz, W. Pietsch Neue Anwendungen der Kompaktiertechnik - Aufbereitung von Abfallmaterialien zu hochwertigen Rohstoffen. (New applications of compacting - Production of secondary raw materials from waste materials). CZ-Chemie-Technik 2(1973)8, 319-321. W. Pietsch Mechanische Verfahrenstechnik im Dienst der Umwelttechnik. (Mechanical Process Engineering in Pollution Control.) CZ-Chemie-Technik 2(1973)9, 351-354. W. Pietsch The many versatile applications of size enlargement in pollution control. Proceedings of the First Int’l Conf. on the Compaction and Consolidation of Particulate Matter, Brighton, England, The Powder Advisory Centre, London (1973), 227-235. W. Pietsch Granulieren, Agglomerieren und Kornvergr€ osserung in der Pharmazeutischen Industrie. (Granulation, agglomeration and size enlargement in the Pharmaceutical Industry.) APV-Informationsdienst, Mainz 19(1973)2/3, 147-182. W. Pietsch Kornvergr€osserung. (Size enlargement). In: Fortschritte der Verfahrenstechnik, Bd. 11, 1972, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, D€ usseldorf (1973), 162-172. W. Pietsch Anwendung der Brikettierung im Umweltschutz am Beispiel der R€ uckf€ uhrung von Filter- und Erzst€auben in metallurgischen Anlagen. (Application of briquetting in pollution control as demonstrated by recycling of filter- and ore-dusts from metallurgical plants.) Aufbereitungs-Technik 14(1973)12, 818-821. W. Pietsch Application of briquetting in pollution control-recycling of filter and ore dusts in metallurgical plants. Proc. 13th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Colorado Springs, CO (1973), 1-12. W. Pietsch The new HUTT laboratory Kompaktor and the Pharmapaktor. Proc. 13th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Colorado Springs, CO (1973), 113-117. W. Pietsch Kornvergr€osserung. (Size enlargement.) In: Fortschritte der Verfahrenstechnik, Bd.12, 1973, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, D€ usseldorf (1974), 131-146. R.H. Snow, B.H. Kaye, C.E. Capes, R.F. Conley, J. Sheehan, F. Schwarzkopf, W. Pietsch Size reduction and size enlargement. Section 8. In: R.H. Perry, C.H. Chilton “Chemical Engineer’s Handbook”, 5th edition McGraw-Hill (1973), 1-65.
13.3 Author’s Biography, Patents, and Publications 54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
W. Pietsch Kornvergr€osserung. (Size enlargement.) In: Fortschritte der Verfahrenstechnik, Bd. 13, 1974. Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, D€ usseldorf (1975), 143-163. W. Pietsch Kornvergr€osserung mit Walzenpressen - Eine alte Technologie mit neuen Anwendungen. (Roll pressing - An old technology with new applications.) Aufbereitungs-Technik 17(1976)3, 120-127. W. Pietsch Kornvergr€osserung. (Size enlargement.) In: Fortschritte der Verfahrenstechnik, Bd. 14, 1975, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, D€ usseldorf (1976), 149-160. W. Pietsch Roll pressing. Heyden & Son Ltd., London/New York/Rheine (1976). W. Pietsch Use of sponge iron in foundries. AFS Int’l Cast Metals Journal 1(1976)2, 43-50. W. Pietsch Storage, shipping, and handling of MIDREX iron. Preprint Nr. 76-B-317, SME-AIME Meeting & Exhibit, Denver, CO (1976). W. Pietsch, G.A. Mott Face to face interview: Direct reduced iron.....past and present. Modern Casting 66(1976)9, 50-52. W. Pietsch The use of sponge iron in foundries. Modern casting 66(1976)9, 53-55 (condensed form of [58]). W. Maschlanka, W. Pietsch Aplicacion del hierro esponja como material de carga en fundiciones. (Application of sponge iron as charge material in foundries.) Proc. Congreso Fundicion, ILAFA (1976), 75-85. W. Pietsch Charging with direct-reduced iron may reduce costs, improve chemistry. Foundry Operation Planbook (McGraw-Hill Inc.) (1977)4, 45-48. D.L. Keaton, W. Pietsch An update of MIDREX Direct Reduction techniques and innovations. Proc. 50th Annual Meeting Minnesota Section AIME, Duluth, MN (1977), 5-23. W. Pietsch, R. Kreimendahl Uso de hierro esponja en la elaboracion de hierro. (Use of sponge iron in iron making.) In “Uso y comercializacio´n del hierro esponja”. Proc. Congreso ILAFA - Reduccion Directa, Macuto, Venezuela (1977), 233-239. W. Pietsch MIDREX - Direktreduktion - Stand der Technik. (MIDREX direct reduction - The state of the art.) Aufbereitungs-Technik 18(1977)8, 410-416. J.E. Bonestell, W. Pietsch MIDREX direct reduction - State of the art. Proc. SEASI Direct Reduction Conf., Bangkok, Thailand (1977). W. Pietsch Technical development of a merchant direct reduced iron facility. Annual Convention and Iron and Steel Exposition, Cleveland, OH (1977).
683
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13 Bibliography 69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
W. Pietsch Storage, shipping, and handling of direct reduced iron. AIME/SME Transactions 262(1977)3, 225-234. W. Pietsch Pressure agglomeration - State of the art. In K.V.S. Sastry, editor, Agglomeration 77, AIME New York (1977), 649-677. W. Pietsch The MIDREX cold briquetting system: An economic answer to direct reduced iron fines recovery. Iron and Steel Int’l 51 (1978)2, 119, 121-123. W. Pietsch Direct reduced iron: A new charge material for iron and steel foundries. The British Foundrymen 71(1978)4, 89-93. W. Pietsch Agglomerieren. (Agglomeration.) In: Fortschritte der Verfahrenstechnik, Bd. 16, 1978, Abtlg. B, Mechanische Verfahrenstechnik, VDI-Verlag, D€ usseldorf (1978), 73-89. W. Pietsch Agglomeration and direct reduction: A technical symbiosis. Mining Magazine 139(1978)4, 414-421. J.E. Bonestell, W. Pietsch The floating direct redution plant - A feasible future reality. a) Preprint, 3rd Int’l Iron and Steel Congress, Chicago, IL, USA (1978) b) Continental Iron & Steel Trade Reports 18(1978), 707-709. c) Proc. 3rd Int’l Iron & Steel Congress, Chicago, IL, USA (1978), 186-194. W. Pietsch The availability of direct reduced iron - An assessment of the technology and production capabilities through 1985. 82nd AFS Casting Congress and Exposition, Detroit, MN, USA (1978). W. Pietsch Direct reduced iron - A new charge material for iron and steel foundries. SEAISI 1978 Singapore Seminar “Modern Foundry Practice” (1978). W. Pietsch Development, installation, and operation of a briquetting system for direct reduced iron fines. Proc. 15th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Montreal, Canada (1977), 83-96. W. Pietsch The influence of raw material and reduction temperature on the structure and characteristics of direct reduced iron. SME of AIME Transactions 264(1978), 1784-1789. W. Pietsch The role of vacuum metallurgy in the production and processing of non-iron metals. Proc. VII. Ritkafe´m Konferencia, Budapest, Hungary (1979), 75-99. W. Burgmann, W. Pietsch Modern technologies in steel degassing and ladle metallurgy. Proc. Int’l Symposium Modern Developments in Steelmaking, Jamshedpur, India (1981), 7.8.1-7.8.26. W. Pietsch Vakuumverfahren in der Metallurgie. (Vacuum process technology in metallurgy.) a) Vortrag Messe “Pulvermetallurgie”, Minsk, BSSR (1981) (in Russian). b) Fachberichte, H€ uttenpraxis, Metallverarbeitung 19(1981)10, 808-817 (in German). c) World Steel and Metalworking Export Manual (1981), 93-101 (in English).
13.3 Author’s Biography, Patents, and Publications 83.
84.
85.
86.
87.
88.
89.
90.
91.
92.
93.
94.
95.
96.
W. Pietsch Agglomeration. In: Fortschritte der Verfahrenstechnik, Bd. 19, 1981, Abtlg. B Mechanische Verfahrenstechnik, VDI-Verlag, D€ usseldorf (1981), 133-149. W. Pietsch Agglomeration. 3rd Int’l Symposium. Aufbereitungs-Technik 22(1981)9, 488-494. W. Pietsch New production technologies for metal and alloy powders. 1981 Int’l Industrial Seminar on Pilot Plant experiences, Amelia Island, FL, USA (1981). W. Pietsch. Agglomeration. 17th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Reno, Nevada, USA (1981). Aufbereitungs-Technik 23(1982)2, 92-99. W. Pietsch New production technologies for metal and alloy powders. In: Competing in the World Market - New technology for the Metals Industry, Proc. 35th Annual Conf., Sydney, Australia (1982), 17-24. H. Stephan, W. Pietsch, H. Ettl, H. Aichert Degassing of metal powders and the filling of degassed powders into capsules for the manufacturing of ingots and discs. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982), 179-191. W. Pietsch, H. Stephan, A. Feuerstein, J. Heimerl, R. Ruthardt Some new results of the atomization of reactive and refractory metals with the EBRD Process. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982), 481-499. W. Pietsch New production technologies for metal and alloy powders. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982), 739-754. W. Pietsch Titanium - From sponge to powder. VII Yugoslav Conf. on Contemporary Materials, Subotica, Yugoslavia (1982). W. Pietsch Die Kornvergr€osserung in der Verfahrenstechnik und ihre industrielle Anwendung am Beispiel der Direktreduktion von Eisenerzen. (Size enlargement in process engineering and its industrial application as exemplified by the direct reduction of iron ores.) Aufbereitungs-Technik (part 1) 23(1982)4, 193-200, (part 2) 23(1982)5, 248-257. R. Ruthardt, W. Pietsch, H. Stephan Atomization techniques for high quality metal powder production. Unpublished Manuscript. W. Pietsch, H. Stephan, A. Feuerstein, J. Heimerl, R. Ruthardt Atomization of reactive and refractory metals by the electron beam rotating disc process. Powder Metallurgy Int’l 15(1983)2, 77-83. W. Pietsch Energy conservation in the fertilizer industry - The compaction/granulation process for mixed (NPK) fertilizers. a) Proc. 18th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Colorado Springs, CO, USA (1983), 243-265. b) Proc. Int’l Conf. Fertilizer ’83, London, UK 2(1983), 467-479. W. Pietsch Modern equipment and plants for potash granulation. Proc. 1st Int’l Potash Technology Conf. Potash ’83, Saskatoon, Sask., Canada (1983), 661-669.
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13 Bibliography 97.
98.
99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
109.
110.
W. Pietsch Einsatz grosser Walzenbrikettiermaschinen in der Koksherstellung. (Large roller briquetting machines in coke production.) Aufbereitungs-Technik 25(1984)1, 29-38. W. Pietsch Agglomerate bonding and strength. Chapter 7.2 in “Handbook of Powder Science and Technology”, M.E. Fayed, L. Otten, editors, Van Nostrand Reinhold Co., Inc., New York (1984), 231-252. W. Pietsch Roll pressing, isostatic pressing and extrusion. Chapter 7.4 in “Handbook of Powder Science and Technology”, M.E. Fayed, L. Otten, editors, Van Nostrand Reinhold Co., Inc., New York (1984), 269-285. W. Pietsch Agglomeration. In: Fortschritte der Verfahrenstechnik, Bd. 21, 1983, Abtlg. B Mechanische Verfahrenstechnik, VDI-Verlag, D€ usseldorf (1983), 121-139. W. Pietsch Granulation of mixed fertilizers by compaction. Proc. 34th Annual Meeting, Fertilizer Round Table, Baltimore, MD (1984), 48-58. H.-G. Bergendahl, W. Pietsch Hot briquetting with roller presses. Proc. 4th Int’l Symposium on Agglomeration, Toronto, Canada, Iron and Steel Society, Inc. (1985), 543-550. W. Pietsch Agglomeration - Key to reycling of metal bearing fines. Proc. Int’l Symposium on Recycle and Secondary Metals, Fort Lauderdale, FL (1985), 683-699. W. Pietsch Compaction/granulation of dry, digested sludge from municipal waste treatment plants. Proc. 19th Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), Baltimore, MD, USA (1985), 179-194. W. Pietsch Agglomeration. (engl.) In: Fortschritte der Verfahrenstechnik, Bd. 23,1 Abtlg. B Mechanische Verfahrenstechnik, VDI-Verlag, D€ usseldorf (1985), 125-139. W. Pietsch, C. Rodriguez Granulation of fertilizers by compaction. Proc. 20th Biennial Conf. “The Institute of Briquetting and Agglomeration” (IBA), Orlando, FL (1987), 113-126. P. Schafer, Ph. Wolstenholme, W. Pietsch, R. Holland Compaction and granulation of dried sludge at Ocean County, NJ. 60th Annual Conf. WPCF (Water Pollution Control Federation), Philadelphia, PA (1987). W. Pietsch Mixed fertilizer granulation by compaction. History, application, and present status of mixed fertizlier granulation by compaction. Proc. Int’l Conf. Fertilizer South America, Caracas, Venezuela. The British Sulfur Corp. Ltd., London, England (1989), 153-173. W. Pietsch, R. Zisselmar Pressure agglomeration with roller presses for waste processing and recycling. Proc. 5th Int’l Symposium Agglomeration, Brighton, UK, IChemE, Rugby, UK (1989), 117-130. W. Pietsch Granulation of fertilizers by compaction. Proc. IFDC Workshop “Supplying quality multinutritional fertilizers in the Latin American and Caribbean Region - Emphasizing bulk blending and the complementary role of agglomeration”, Guatemala City, Guatemala (1989), 183-196.
13.3 Author’s Biography, Patents, and Publications 111. W. Pietsch Briquetting of coal (Can an ancient technology be modified for the production of environmentally safer smokeless fuels?). Proc. 21st Biennial Conf. “The Institute for Briquetting and Agglomeration (IBA), New Orleans, LA (1989), 303-320. 112. W. Pietsch Granulation of fertilziers by compaction. Proc, IFDC Workshop “Urea-based NPK Plant design and operating alternatives”, Muscle Shoals, AL (1990), 89-98. 113. W. Pietsch Briquetting of non-ferrous waste for economic recycle. Proc. 2nd Int’l Symposium “Recycling of metals and engineered materials” (J.H.L. van Linden, D.L. Stewart, Y. Sahai, editors), TMS, Warrendale, PA (1990).\, 667-670. 114. W. Pietsch Size enlargement by agglomeration. John Wiley & Sons Ltd.//Salle + Sauerl€ander, Chichester, UK / New York, NY, USA / Brisbane, Australia / Toronto, Canada / Singapore // Aarau, Switzerland / Frankfurt/M., Germany / Salzburg, Austria. (1991). 115. M.E. Fayed, W. Pietsch Particulate solids characterization and agglomeration. Course notes, AIChE Continuing Education, Miami Beach, FL (1992) (revised). 116. W. Pietsch, H. Ries Agglomerieren - Granulieren. (Agglomeration - Granulation.) Course notes, Technische Akademie Wuppertal e.V., Wuppertal-Elberfeld, Germany (1992) (revised). 117. W. Pietsch Fundamentals of agglomeration. Course notes, Workshop at Powdex NJ, Somerset, NJ (1992) (revised). 118. W. Pietsch Pressure agglomeration: Fundamentals and applications. Course notes, Workshop at Powdex NJ, Somerset, NJ (1992). 119. H.O. Kono, W. Pietsch Tumble/Growth agglomeration of fine powders: Present state and new developments. Course notes, Industrial awareness seminar at Powdex NJ, Somerset NJ (1992) (revised). 120. W. Pietsch Briquetting of Aluminum swarf for recycling. Light Metals ’93, TMS, Warrendale, PA (1993), 1045-1051. 121. W. Pietsch Size enlargement by agglomeration in the pharmaceutical industry with special emphasis on pressure agglomeration. Course notes, Workshop at Interphex - USA New York, NY (1993). 122. W. Pietsch, H. G€ unter New applications of roller presses in coal-related technologies. Proc. 18th Int’l Techn. Conf. on Coal Utilization & Fuel Systems, Clearwater, FL (1993) 837-852. 123. W. Pietsch Agglomeration technologies for environmental protection and recycling. Proc. 6th Int’l Symposium on Agglomeration, Nagoya, Japan (1993), 837-847. 124. W. Pietsch Size enlargement by agglomeration for solid waste treatment or minimization and for hazardous waste stabilization. Preprints: 4th Pollution Prevention Topical Conference, Seattle, WA, USA, AIChE, New York, NY, USA (1993), 202-208.
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13 Bibliography 125. H.-G. Bergendahl, W. Pietsch Roller presses, their applications, sizes, and capacities as well as their limitations. Proc. 23rd Biennial Conf. “The Institute for Briquetting and Agglomeration” (IBA), Seattle, WA (1993), 185-204. 126. W. Pietsch, H. G€ unter Briquetting as an upgrading process for different types of coals. Proc. 19th Int’l Techn. Conf. on Coal Utilization & Fuel Systems, Clearwater, FL (1994), 181-195. 127. W. Pietsch Granulation by agglomeration in the pharmaceutical industry. Course notes, Workshop at Interphex - USA, New York, NY (1994) (revised)). 128. W. Pietsch Parameters to be considered during the selection, design, and operation of agglomeration systems. Proc. 1st Int’l Particle Technology Forum, AIChE, Denver, CO (1994), Part I, 248-257. 129. W. Pietsch Economical and innovative methods for the agglomeration of dusts and other wastes from metallurgical plants for recycling. In: Metallurgical Processes for the Early Twenty-First Century, H.Y. Sohn, editor, Vol. II, Technology and Practice, TMS, Warrendale, PA (1994), 487-495. 130. W. Pietsch A review of agglomeration fundamentals and industrial techniques to enhance productivity. Course notes, Workshop at Powdex ’94, Houston, TX (1994). 131. W. Pietsch Aglomeracio´n en plantas para reciclado: Metodos economicos e innovativos para la aglomeracio´n de polvos y otros desechos. Siderurgia Latinoamericana 10 (1994), No. 414, 27-34. 132. W. Pietsch Compaction of Aluminum chips and turnings and of other particulate Aluminum scrap with roller presses for secondary Aluminum melting without losses. Light Metals ’95, TMS, Warrendale, PA (1995), 799-802. 133. W. Pietsch Agglomeration: Controlling pollution and permitting waste recycling. Powder & Bulk Engng. 9(1995)2, 53, 54, 56, 57, 59-62. 134. W. Pietsch Briquetting of coal with roller presses. An important technology for the production of coal-based compliance fuel. Proc. 20th Int’l Techn. Conf. on Coal Utilization & Fuel Systems, Clearwater, FL (1995), 87-95. Also: In: “Coal Fines: The Unclaimed Fuel”, S.D. Serkin, Editor, Coal & Slurry Technology Assoc., Washington, DC (1995), 91-99. 135. W. Pietsch Agglomeration technologies for environmental protection and recycling. Course notes, Workshop at Powder & Bulk Solids ’95, Rosemont, IL (1995). 136. W. Pietsch Review of particle formation by compaction processes. 16th IFPRI Annual Meeting, Urbana, IL (1995), June 11-14. 137. F.-H. Grandin, W. Pietsch, G. Medina y Espana Compaction of Aluminum scrap on high pressure roller presses. Proc. 4th Australian, Asian, Pacific Course & Conference on Aluminum Cast House Technology, Sidney, Australia (1995)
13.3 Author’s Biography, Patents, and Publications 138. W. Pietsch, J. Jagnow, R. L€obe Sch€ uttg€ uter pelletieren, extrudieren, granulieren, brikettieren, kompaktieren. Probleml€osungen f€ ur industrielle Anwendungen. (Bulk solids pelletizing, extruding, granulating, briquetting, compacting. Solutions for problems of industrial applications.) Course notes, Seminar of the Akademie Wuppertal, N€ urnberg, Germany (1995). 139. W. Pietsch Roller presses - Versatile equipment for mineral processing. Proc. XIX IMPC, San Francisco, CA (1995), Vol.1, 59-66. 140. W. Pietsch Roller presses for secondary metal recycling. Proc. 3rd Int’l Symp. Recycling of Metals and Engineered Materials (P.B. Queneau and R.D. Peterson, editors), Point Clear, AL (1995), 233-241. 141. W. Pietsch Evaluation of parameters for the selection, design, and operation of agglomeration systems. Proc. 24th Biennial Conf. “The Institute for Briquetting and Agglomeration” (IBA), Philadelphia, PA (1995), 175-189. 142. W. Pietsch The briquetting of coal in Europe. Proc. COAL PREP 96, 13th Int’l Coal Preparation Conf., Lexington, KY (1996), 167-183. 143. W. Pietsch Successfully use agglomeration for size enlargement. Chem. Engng. Progr. 92(1996)4, 30-45. 144. W.Pietsch Recent developments in dry granulation of fertilizers by compaction. Proc. 5th World Congress of Chemical Engineering, San Diego, CA (1996), Vol. V, 552-558. 145. W.R. Sch€ utze, W. Pietsch HBI - Survey of the significance and development of sponge iron hot briquetting and the application of this technology in various plants and reduction processes. Proc. Conf. Pre Reduced Products and Europe, Milan, Italy (1996) 146. W. Pietsch, W. Sch€ utze HBI - A safe DRI-based source of iron units. Paper at World Iron Ore 96, Orlando, FL (1996) Skillings Mining Review 86(1997)18, 4-9. 147. W. Pietsch Granulate dry particulate solids by compaction and retain key powder particle properties. Chem. Engng. Progr. 93(1997)4, 24-46. 148. W. Pietsch Size Enlargement by Agglomeration. Chapter 6 (175 pages) in “Handbook of Powder Science & Technology”, 2nd edition (M.E. Fayed & L. Otten, editors), Chapman & Hall, New York, NY (1997) 149. W. Pietsch Agglomeration Techniques for the manufacturing of “instant” granules from fine powder mixtures. in “Fine Powder Processing Technology” (R. Hogg, C.C. Huang, and R.G. Cornwall, editors), The Pennsylvania State University (1998), 233-242. 150. W. Pietsch Agglomeration Techniques for the manufacturing of granular materials with specific product characteristics. Proc. 25th Biennial Conf. “The Institute for Briquetting and Agglomeration” (IBA), Charleston, SC (1997), 149-164. 151. W. Pietsch Particle Engineering by Agglomeration in the Pharmaceutical Industry. Course notes, Workshop at INTERPHEX ’99, New York, NY, April 20-22 (1999) 152. W. Pietsch How to select an agglomeration method Part I: Powder and Bulk Engineering 13(1999)2, 60-65 Part II: Powder and Bulk Engineering 13(1999)3, 21-31
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13 Bibliography 153. W. Pietsch Readily Engineer Agglomerates with Special Properties from Micro- and Nanosized Particles. Chem. Engng. Progr. 95(1999)8, 67-81 154. W. Pietsch The Porosity of Agglomerates Proc. 26th Biennial Conf. “The Institute for Briquetting and Agglomeration” (IBA), San Diego, CA (1999), 1-14. 155. W. Pietsch Granulation of Pharmaceutical Formulations to Improve Handling, Processing, and Use Course notes, Workshop at INTERPHEX 2000, New York, NY, March 21-23 (2000) 156. W. Pietsch Compaction Methods for Granulation and the Manufacturing of Dry Dosage Forms Course notes, Workshop at INTERPHEX 2000, New York, NY, March 21-23 (2000) 157. W. Pietsch Ram Pressing - An almost extinct technology with interesting new applications in coal and other solid fuel processing. Proc. 25th Int’l Technical Conf. on Coal Utilization & Fuel Systems, Clearwater, FL., March 6-9 (2000), 37-48. 158. W. Pietsch Agglomeration Methods in Particle Engineering Proc. XXI Int’l Mineral Processing Congress, Rome, Italy, July 23-28 (2000), A4 - 87-96. 159. W. Pietsch An Interdisciplinary Approach to Size Enlargement by Agglomeration Proc. (Preprints) 7th International Symposium on Agglomeration, Albi, France, May 29-31(2001), Vol 1, 25-35 and Powder Technology 130(2003)1-3, 8-13. 160. W. Pietsch AGGLOMERATION - An old and key technology serving mankind Proc. 3rd European Congress on Chemical Engineering, N€ urnberg, Germany, June 25-28 (2001). 161. W. Pietsch AGGLOMERATION - The neglected unit operation of process technology Proc. 27th Biennial Conf. “The Institute for Briquetting and Agglomeration” (IBA), Providence, R.I. (2001), 9-17. 162. W. Pietsch Agglomeration Processes – Phenomena, Technologies, Equipment WILEY-VCH Verlag GmbH, Weinheim, Germany (2002). 163. M.E. Fayed, W. Pietsch Pharmaceutical Powder Characterization and Agglomeration – An Overview. Course notes, Workshop at Interphex – USA, New York, NY (2002) 164. B. Ennis, W. Pietsch, N. Stanley-Wood. Agglomeration Phenomena and Technologies. Course notes, Tutorial at Powder & Bulk Solids ’02, Rosemont, IL (2002). 165. W.Pietsch Methods to pre-select the most suitable Agglomeration Process for a specific Task. Course notes, Workshop at Powder & Bulk Solids ’02, Rosemont, IL (2002). 166. W. Pietsch Agglomeration Technologies for the Recycling and Waste Treatment in Mineral and Metal Processing Proc. TMS Fall 2002 Extraction and Processing Div. Meeting on Recycling and Metal Processing in Mineral and Metal Processing: Technical and Economic Aspects, Lulea, Sweden, June 16-20 (2002), Vol. 1, 119-128. 167. W. Pietsch The Past, Present, and Future of Agglomeration in Particle Technology. Proc. World Congress on Particle Technology 4 (WCPT4), July 21-25 (2002) Sydney, Australia 168. W. Pietsch Systematische Entwicklung von Verfahren zur Kornvergr€ osserung durch Agglomeration. (Systematic development of processes for the size enlargement by agglomeration). Chemie Ingenieur Technik (CIT) 74(2002)11, 1517-1530.
13.3 Author’s Biography, Patents, and Publications 169. W. Pietsch Mixers for growth and tumble agglomeration. Powder & Bulk Engineering 17(2003)2,19-25. 170. W. Pietsch Agglomeration Phenomena in Conveying and Handling of Particulate Solids. Proc. 4th International Conference for Conveying and Handling of Particulate Solids (ChoPS4), May 27-30 (2003), Budapest, Hungary, Vol.1, 3.51-3.56. 171. W. Pietsch Agglomeration during Conveying and Handling of Particulate Solids. Proc. 28th Biennial Conf. “The Institute for Briquetting and Agglomeration” (IBA), Santa Fe´, NM, USA (2003).
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Glossary of Application-Related Terms Associated with Agglomeration Newly developing fields of science are organized according to universally recognized criteria using well-defined terms to describe the phenomena, fundamentals, correlations, equipment, procedures, and processes. This is not the case for those technologies that have been known for centuries and have been developed empirically and independently for different applications (Chapter 2). In such cases the same process, procedure, activity or piece of equipment may have different names in different industries or the same term may have different meanings in different fields of application. Earlier books by the author “Size Enlargement by Agglomeration” [B.42] and “Agglomeration Processes – Phenomena, Technologies, Equipment” [B.71] already contain glossaries of agglomeration terms. In the following glossary these definitions are repeated and updated. Although the author and many others that are active in the promotion of agglomeration are trying to use scientific and technical terms consistently in an interdisciplinary manner, it is helpful also to explain some of the common names and expressions that are used in industry (both by vendors and users), including a few historical ones and a few trade names. Therefore, several such descriptions have been included. Many of the definitions have been adapted from Merriam-Webster’s Collegiate Dictionary, 10th Edn (1993–1998). If a word has several explanations, such as common language and technical meanings, the technical reference is being presented which, in some cases, has been amended by the author to more closely describe its use in mechanical process technology and/or agglomeration. In the following, cross-references are indicated by CAPITAL LETTERS. More agglomeration terms are mentioned and used in the text of the book, where they are self-explanatory. Absorb [vb.] To suck up or take up. To take in and make part of an existent whole. Absorption [n.] The process of absorbing or being absorbed. Distinguished from ADSORPTION. Abrasion [n.] Removal of solid matter from the surface or edges of an AGGLOMERATE. The matter removed is much smaller than the AGGLOMERATE itself. (See also ATTRITION, EROSION). Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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14 Glossary of Application-Related Terms Associated with Agglomeration
Abrasion resistance Accretion [n.]
Accumulate [vb.] Accumulation [n.] Adhesion [n.]
Admixture [n.] Adobe [n.] Adsorption [n.]
Agglomerate [vb.] Agglomerate [n.]
Agglomeration [n.] Agglomerator [n.]
Aggregate [n.]
Aggregate [vb.] Aggregation [n.]
Aging [n.]
Measure for the ability of a body, for example an AGGLOMERATE to withstand ABRASION. The process of growth or enlargement by a gradual BUILDUP, such as: increase by external addition or accumulation, for example by ADHESION of external parts or PARTICLES. (See also AGGLOMERATION, AGGREGATION, BUILD-UP) To heap up into a mass; pile up. The action or process of ACCUMULATING; an ACCUMULATED mass, quantity, or number. A sticking together of solids. The molecular attraction exerted between the surfaces of solids. Distinguished from COHESION The fact of being MIXED. A BRICK or building material of sun dried CLAY or earth and straw. A structure made of adobe BRICKS. The adhesion in an extremely thin layer of molecules (as of gases, solutes, or liquids) to the surfaces of solid bodies. Distinguished from ABSORPTION. To gather (particulate solids) into a ball, mass, or cluster. (See also AGGREGATE) An assemblage of PARTICLES that is either loosely or rigidly joint together with or without a specific shape. PARTICLES adhering to each other. (See also CONGLOMERATE) The action or process of gathering particulate solids into a CONGLOMERATE. Specific equipment in which AGGLOMERATION is accomplished. One or something performing AGGLOMERATION Any of several hard, inert materials (as sand, gravel, rock, slag) used for mixing with a binding material to form concrete, mortar, plaster or, for example, road surfacing products. Also: A mass or body of units or parts somewhat associated with one another. To collect or gather into a mass. (See also AGGLOMERATE) A group, body, or mass composed of many distinct parts or individuals; the collection of units or parts into a mass or whole; the condition of so collected. (See also AGGLOMERATE, AGGREGATE, CLUSTER, AGGLOMERATION, ACCRETION, BUILD-UP) Changes in characteristics of PARTICULATE solids or AGGLOMERATES that occur naturally with time. (See also POST-TREATMENT)
14 Glossary of Application-Related Terms Associated with Agglomeration
Agitation [n.] Agitator [n.] Algorithm [n.] Ammoniation [n.] Amphiphilic [adj.] Amphiphobic [adj.] Ancillary [adj.] Angle of repose Angle of compaction Annunciator [n.] Anticaking agent
Apparent density Argillaceous [adj.] Aspartame [n.] Aspect ratio Aspirator [n.]
Assay [vb.] Assayer [n.] Atomize [vb.] Atomizer [n.] Atomizing [vb.] Attrition [n.] Auger [n.] Autogenous mill Axial extruder
A state of movement of PARTICULATE solids and/or fluids induced by external effects or forces. See MIXING TOOL, INTESIFIER BAR. A step-by-step procedure for solving a problem or accomplishing some end, esp. by computer. The formation of fertilizer GRANULATES using Ammonia to obtain chemical modification and BONDING. Attracting both polar and non-polar solvents or lipids and water (compare AMPHIPHOBIC). Repelling both polar and non-polar solvents or lipids and water compare AMPHIPHILIC). Subordinate, subsidiary, AUXILIARTY, supplementary. The basal angle of a pile of POWDER that has been freely poured onto a horizontal surface. See NIP ANGLE. A usually electrically controlled board or indicator. Liquid or solid matter applied to the surface of, for example, AGGLOMERATES that prohibits sticking or growing together. (See also CAKING). The weight of the unit volume of a POROUS mass, for example, an AGGLOMERATE. Of, relating to, or containing CLAY or CLAY-like materials. A crystalline compound that is used as a low calorie sweetener (see also SACCHARIN). A ratio of one dimension to another, for example length to diameter. Also: Aspiration system. An apparatus for producing suction or moving or collecting materials by suction. Dust collection systems. To put to a test. Commonly used in the pharmaceutical industry for (material) testing. Person that ASSAYS. To reduce to a fine SPRAY. See NOZZLE. Finely dispersing liquids. The unwanted break-down of AGGLOMERATES (See also ABRASION, EROSION). See SCREW. Tumbling mill in which the coarse lump feed (.e.g. ore) will serve as the grinding medium while it is itself being ground. Low, medium, or high pressure EXTRUDER with a flat DIE PLATE at the end of a BARREL; the material is EXTRUDED in the same direction as it is transported by the SCREW(S).
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14 Glossary of Application-Related Terms Associated with Agglomeration
Backmixing [n.]
Baffle (n.) Bag set
Ball [n.] Ballability [n.]
Balling [n.]
Barrel [n.] Basket extruder
Batch [n.] Beading [n.]
Beneficiate [vb.] Bin [n.] Binder [n.]
Binding mechanism Biocide [n.] Biomass [n.] Blade [n.] Blend [vb.] Blender [n.] Blunger [n.] Boiling bed
During the flow of PARTICULATE solids, reverse movement of some particles due to their STOCHASTIC motion caused by turbulence or special equipment design. A device (as a plate, wall, or insert) to deflect, check, or regulate flow and/or movement of particulate solids and fluids. Typical in the fertilizer industry; unwanted AGGLOMERATION of PARTICULATE solids in a closed bag during storage. Mostly caused by recrystallization of dissolved materials. Synonymous with spherical AGGLOMERATE. (See also PELLET) Typical in the iron ore industry; the capability of PARTICULATE solids to form more or less spherical AGGLOMERATES during GROWTH AGGLOMERATION. Originally in the iron ore industry; any method producing spherical AGGLOMERATES by TUMBLE or GROWTH AGGLOMERATION. (See also PELLETIZING) Cylindrical (or sometimes tapered) housing for SCREWS, e.g. of FEEDERS or EXTRUDERS. Low pressure EXTRUDER in which the DIE PLATE resembles a basket, using rotating or oscillating EXTRUSION BLADES. The quantity of material prepared or required for one operation. Formation of bead-like PARTICLES; typical in solidification of melt droplets. (See also PRILLING, PASTLLATION, MELT SOLIDIFICATION) To concentrate or otherwise prepare for SMELTING. A container, box, frame, crib, or enclosed volume used for storage. (See also HOPPER, SILO) An inherent component of or an additive to PARTICULATE matter providing BONDING between the disparate PARTICLES. Physical and chemical effects that cause ADHESION and BONDING between solid surfaces Substance that is destructive to many different organisms. Organic plant and animal residuals. Often organic waste material that is especially used as a source for fuel. See EXTRUSION BLADE. To combine or associate so that the separate constituents are no longer distinguishable. (See also MIX) See MIXER. Typical in the ceramic and fertilizer industries; double shafted PUG MILL. See FLUID BED.
14 Glossary of Application-Related Terms Associated with Agglomeration
Bonding [n.] Bowl [n.]
Branch [vb.] Brick [n.]
Bridging [n.]
Briquette [n.]
Briquetter [n.] Briquetting [n.] Brittleness [n.] Build-up [n.]
Bulk density
Calcareous [adj.] Calcine [vb]
Campaign [n.] Cantilevered [adj.] Capillary [adj.] Capping [n.]
Cake [n.] Caking [n.] Cement [n.]
The process of binding PARTICLES together by the action of BINDING MECHANISMS. A vertical or inclined, cylindrical, conical or convex vessel enclosing and defining the operating volume of some coaters, mixers, or spheronizers. To put forth branches or outgrowth. A handy-sized unit of building or paving material, typically rectangular and made from moist clay hardened by heat; an agglomerate resembling a BRICK. Unwanted arching of solid matter in a converging discharge chute or cone. Prohibits discharge of PARTICULATE solids from containers or chutes. Also Briquet; AGGLOMERATE produced and shaped by HIGH PRESSURE AGGLOMERATION. (See also COMPACT, TABLET) Also Briquetting Machine; equipment that produces BRIQUETTES. The process of forming BRIQUETTES or COMPACTS. The tendency of PARTICLES or AGGLOMERATES to break down in size easily. (See also FRIABILITY) The unwanted coating of surfaces with PARTICLES that ADHERE naturally due to their fineness and/or inherent BINDING MECHANISMS. The weight of the unit volume of a PARTICULATE mass under non-specific condition, e.g. in storage or in a shipping container. (See also DENSITY) Consisting of or containing calcium carbonate. To heat inorganic materials to a high temperature in order to drive off volatile matter or to effect changes (as oxidation) but without FUSING. A series of operations designed to bring about a particular result. Supported at only one end. Describing full liquid SATURATION. Separation of a thin layer from the faces of COMPACTS during decompression. Defect in TABLETS caused by the recovery of elastic deformation and/or expansion of compressed air. See SHEET; typical in fertilizer applications. Unwanted AGGLOMERATION during storage mostly by recrystallization of dissolved materials. (See also BAG SET) A POWDER of alumina, silica, lime, iron oxide, and magnesium oxide burned together in a kiln, finely pulverized, and used as an ingredient of MORTAR and CONCRETE. Also any mixture used for a similar purpose. (See also POZZOLAN)
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14 Glossary of Application-Related Terms Associated with Agglomeration
Cement [vb.] Cementitious [adj.] Ceramic [adj.]
Ceramic [n.] cGMP Channel [n.]
Chelate [adj.]
Chelate [n.] Chopper [n.] Clinker [n.] Clam shelling
Classification [n.] Classifier [n.]
Classify [vb.] Clay [n.]
Closed pore Cluster [n.]
Clustering [n.] Coacervate (n.) Coacervate (vb.)
To unite or make firm by or as if by CEMENT. Having the properties of CEMENT. Of or relating to the manufacture of any product (as EARTHENWARE, PORCELAIN, or BRICK) made essentially from a non-metallic mineral (as clay) by firing at elevated temperature. The art or process of making CERAMIC articles; the product of CERAMIC manufacture Current Good Manufacturing Practice: In the pharmaceutical Industry, generally imposed manufacturing rules. Open ended COMPACTING TOOL SET for high pressure EXTRUSION in a RAM PRESS; also any elongated opening through which material is extruded. (See also PRESSWAY) Relating to, producing, or characterized by a cyclic structure usu. containing five or six atoms in a ring in which a central metallic ion is held in a coordination complex by one or more groups each of which can attach itself to the central ion by at least two bonds. To combine with a metal to form a CHELATE ring or rings. See KNIFE HEAD. Stony (mineral) matter fused together. Opening of the LEADING or TRAILING EDGE of BRIQUETTES discharging from ROLLER PRESSES; one-sided splitting along the WEB. (See also DUCK BILLING, OYSTER MOUTHING) The act or process of classifying. One that classifies; a machine for sorting out the constituents of a material according to specific properties (e.g. size, shape, color, density, composition) To arrange in classes; to assign to a category (e.g. of sizes, shapes, colors, densities, compositions). An earthy material that is plastic when moist but hard when FIRED, mainly composed of fine PARTICLES, less than a specified size, of hydrous aluminum silicates and other materials, used for BRICK, tile and pottery. A PORE not communicating with or connected to the surface of a POROUS body A number of similar individual entities that occur together. See also ACCRETION, AGGLOMERATE, AGGREGATION) The growing together of primary AGGLOMERATES to form larger entities. (See also SATELLITES FORMATION) An aggregate of COLLOIDAL droplets held together by electrostatic attraction forces. To form a COACERVATE.
14 Glossary of Application-Related Terms Associated with Agglomeration
Coacervation (n.)
The process of producing COLLOIDAL PARTICLES from solutions or suspensions by changing the system parameters (e.g. temperature, pH) during the formation of a COACERVATE. Coalesce [vb.] To unite by GROWTH. Coalescence [n.] A growing together or union in one body, form, or group. (See also GROWTH AGGLOMERATION) Coating [n.] Applying a layer of material, a film, or a finish to a substrate; in AGGLOMERATION, application of a layer of solids to a PARTICULATE unit. Coating pan Specially shaped PAN in which a material layer is applied on agglomerates (such as TABLETS) usu. in the presence of liquid, heat, or both. Typical in the pharmaceutical and food industries. Cogeneration [n.] The production of electricity using waste heat (as in steam) from an industrial process. Cohesion [n.] Molecular attraction by which the PARTICLES of a body (e.g. AGGLOMERATE) are united throughout the mass whether like or unlike. Distinguished from ADHESION Cold bonding A binding process that occurs at ambient or low temperatures and uses the CEMENTITIOUS or POZZOLANIC reactions of many hydroxides; often assisted by pressure. Colloid [n.] A substance that consists of PARTICLES DISPERSED throughout another substance; the PARTICLES are too small for resolution with an ordinary light microscope but are incapable of passing through a semi-permeable membrane. Colorant [n.] A substance used for coloring a material; see PIGMENT, PAINT. Comminute [vb.] To reduce to minute PARTICLES (PULVERIZE; see also CRUSH) Compact [n.] An object of specific size and shape produced by the compression of PARTICULATE matter. Synonymous with BRIQUETTE Compact disperse A state of PARTICULATE solids in which individual PARTICLES are closely packed. Distinguished from DISCRETE DISPERS Compactibility [n.] See COMPRESSIBILITY Compacting [n] Also Compaction. The method of producing SHEET. Compacting tool set The part or parts making up the confining form in which a powder is pressed. Synonymous with DIE Compaction/granulation The normally dry methods of obtaining GRANULAR products by crushing and screening COMPACTS and/or SHEET into GRANULATE. Compliance [n.] Conformity to official or legal norms.
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14 Glossary of Application-Related Terms Associated with Agglomeration
Component [n.] Composite [n.]
See CONSTITUENT. A solid material composed of two or more substances having different physical characteristics and in which each substance retains its identity while contributing desirable properties to the whole. Composite [adj.] Made up of distinct parts. Consisting of two or more separate materials whereby each retains its own identity. Compressibility [n.] The capacity of a PARTICULATE matter to be compacted. Compressibility may be expressed as the pressure or force to reach a required DENSITY or, alternately, the DENSITY at a given pressure or force. Synonymous with COMPACTIBILITY Compression ratio The ratio of the volume of loose PARTICULATE matter in a DIE to the volume of the COMPACT made from it. Synonymous with FILL RATIO. In LOW and MEDIUM PRESSURE EXTRUDERS, the total thickness of material that is under COMPRESSION in a DIE (including any inlet chamfer) divided by the nominal hole diameter. Conditioning [n.] Development of special characteristics of PARTICULATE solids by, for example, treatment with steam, kneading, heating, or surface treatment by, for example, ANTICAKING AGENTS. Conditioning [vb.] To adapt or modify PARTICULATE SOLIDS to obtain properties essential to the occurrence of something else; e.g. break-up and hydrolize starch to become a BINDER or heat a BINDER contained in a MIX to form a plastic mass. Cone agglomerator PAN with relatively high conical RIM. Confect [vb] To put together from varied material. Confection [n.] Something CONFECTED, particularly a sweet food or a medicinal preparation made with sugar, syrup, or honey. Conglomerate [n.] An adhering mass of PARTICLES made up of parts from different sources or of various kinds. (See also AGGLOMERATE) Constituent [n.] Serving to form, compose, or make-up a unit or whole. COMPONENT. Contact point Area at which two PARTICLES touch each other. Contaminant [n.] Something that CONTAMINATES. Contaminate [vb.] To make inferior or impure by admixture or introduction of undesirable elements of compounds. Contract manufacturing The paid manufacturing of goods in a non-owned, external facility. (see also TOLLING OPERATION). Convolution [n.] A form or shape that is executed in curved windings. Coordination number Sum of all NEAR and CONTACT POINTS of a PARTICLE with surrounding PARTICLES in a structure made up of PARTICULATE solids, for example, an AGGLOMERATE
14 Glossary of Application-Related Terms Associated with Agglomeration
Core rod Corrugated [adj.] Couffinhal press
Coulis [n.] Crush [vb.] Cullet [n.] Cup [n.] Cure [vb.] Curing [n.] Cut size Damper [n.] Day bin
Debris [n.] Decrepitate [vb] Decrepitation [n.] Dendrite [n.]
Dendritic [n.] Densification [n.] Density [n.] Deposit [n.] Die [n.]
Die plate Digest [vb.] Direct reduction Disc [n.]
Member of the COMPACTING TOOL SET that forms a through hole in the COMPACT. (See also MANDREL) Made with alternating ridges and grooves. PUNCH-AND-DIE press with multiple DIE sets on an indexed table for making large (e.g. coal) BRIQUETTES. (No longer used) A thick sauce made with pureed vegetable or fruit. To squeeze or force by pressure so as to alter or destroy structure. To reduce to PARTICLES by GRINDING. In glass making: broken or refuse glass added to raw BATCH material to facilitate melting. See POCKET. To prepare or alter, esp. by chemical of physical processing for keeping or use. INDURATION of GREEN AGGLOMERATES by any method. (See also POST-TREATMENT) The actual value at which separation of a PARTICLE size distribution into “coarse” and “fines” has taken place. A valve or plate for regulating the flow of gas. A BIN in which the daily requirement of a FEED MATERIAL is stored. The BIN may also hold more or less than a day’s requirement; often contains sufficient feed for a CAMPAIGN. The remains of something broken down or destroyed. To break-up upon heating. Breakdown in the size of PARTICLES or AGGLOMERATES due to internal forces, generally induced by heat. A BRANCHING figure produced on or in a mineral by a foreign substance; a crystallized BRANCHING PARTICLE; any BRANCHING shape. Resembling or having DENDRITES. The act or process of making dense. Mass per unit volume of matter at specific conditions. For example: APPARENT, BULK, or TRUE DENSITIES A (natural) ACCUMULATION of PARTICLES. Member of the COMPACTING TOOL SET that forms the periphery of the part being produced. Also open ended CHANNELS for EXTRUSION. Plates, rings, or other machine parts with perforations for EXTRUSION. (See also DIE) To convert (food) into absorbable form. To soften, decompose, or break down by heat and moisture or chemicals. Solid state reduction of (metal, typ. iron) oxides into the metallic state. See PAN.
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14 Glossary of Application-Related Terms Associated with Agglomeration
Discrete disperse
A state of PARTICULATE solids in which individual elements can be clearly distinguished. Distinguished from COMPACT DISPERSE Disperse [adj.] See PARTICULATE. Disperse [vb.] To distribute (as fine PARTICLES) more or less evenly throughout a medium. Dispersibility [n.] Measure for the ease with which, under specific conditions (e.g. in liquids), an AGGLOMERATE breaks down into primary PARTICLES. Distribution plate Perforated plate at the bottom of a FLUID BED through which fluidizing gas enters from the PLENUM. (See also GIL PLATE) Doctor blades See SCRAPER. Dome extruder Axial, low pressure EXTRUDER, most often with two SCREWS, in which the DIE PLATES resemble domes. Double action pressing A method by which PARTICULATE solids are pressed between opposing PUNCHES that are both moving relative to the DIE. Downdraft [n.] Downward flow of gas, for example, through a PARTICLE bed. Downstream [adv.] In the direction of flow. Drage´e [n.] A sugar COATED TABLET, nut, or fruit. DR Direct reduction. DRI Direct reduced iron. Drum agglomerator Slowly rotating, slightly inclined drum for GROWTH AGGLOMERATION Dry granulation See COMPACTION/GRANULATION Duck billing See CLAM SHELLING, OYSTER MOUTHING. Dwell time In COMPACTING, time during which certain process conditions, for example pressure, persist or are held constant. EAF Electric arc furnace. Earthenware [n.] CERAMIC ware made of slightly POROUS opaque clay fired at low temperature. Effluent [n.] Waste material (as smoke, liquid industrial refuse, or sewage) discharged into the environment, esp. as a POLLUTANT. Encapsulation [n.] Typically used as MICROENCAPSULATION. Engineered materials Modified or newly produced materials from (often NANO scale) building blocks whereby the new materials exhibit predetermined, special properties. (see also NANOTECHNOLOGY) Entity [n.] Something that has separate and distinct existence. Equivalent [adj.] Equal in value. Equivalent diameter Diameter of monosized spherical PARTICLES that feature the same property as the PARTICULATE mass to be char-
14 Glossary of Application-Related Terms Associated with Agglomeration
Erosion [n.]
Excipient [n.] Expansion [n.]
Exter press Extrudate [n.] Extruder [n.] Extrusion [n.]
Extrusion blade
Feeder [n.] Feed material Feed screw Festoon [n.] Filler Fill ratio
Fines [n., pl.] Fire [vb.] Flake [n.]
Flake breaker Flashing [n.] Flight [n.]
acterized. For example: SURFACE EQUIVALENT DIAMETER The gradual wearing away of an AGGLOMERATE by the progressive removal of small pieces of material. (See also ABRASION). A usually inert (FILLER) substance (as lactose, starch) that forms a vehicle (as for a drug). Increase in volume of, for example, an AGGLOMERATE after production or during POST-TREATMENT. Converse of SHRINKAGE See RAM PRESS. Product of EXTRUSION. (See also PELLET) Machinery for the production of EXTRUDATES. (See also SCREW and RAM EXTRUDER) The formation of (often cylindrical) AGGLOMERATES by forcing “plastic” mass through open ended CHANNELS or holes in (perforated) DIES. In LOW PRESSURE EXTRUDERS, the flat, curved, or engineered blade that pushes material through the openings of a DIE PLATE; it is the part closest to the DIE PLATE. Device to deliver feed material to a processing unit. (see also FORCE FEEDER) Raw material entering equipment or systems for processing. Element(s) providing forces onto PARTICULATE solids in a FEEDER. (See also SCREW) A molded ornament (e.g. on CERAMICS) representing a decorative chain. See EXCIPIENT Typically used in TABLETTING or other confined volume compression equipment. Synonymous with COMPRESSION RATIO Always used as plural. UNDERSIZED REJECTS from a SIZING operation To process by applying heat. See SHEET. Also: 1. Grains or other malleable particles flattened between smooth ROLLERS. 2. Material solidified from a melt on a rotating, cooled drum (flaker) and removed by SCRAPERS. A crusher (often two ROLLERS with teeth) used to reduce the size of SHEET. See WEB. A continuous or semi-continuous spiral flat plate that is attached to the shaft of a SCREW.
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14 Glossary of Application-Related Terms Associated with Agglomeration
Floc [n.]
A FLOCCULENT mass formed by the AGGREGATION of a number of fine SUSPENDED PARTICLES. Flocculant [n.] A FLOCCULATING agent. Flocculate [vb.] To AGGREGATE or COALESCE into small LUMPS or loose CLUSTERS or into a FLOCCULENT mass or DEPOSIT. Flocculation [n.] The act or process of FLOCCULATING. Flocculent [adj.] Containing, consisting of, or occurring in the form of loosely AGGREGATED PARTICLES or soft FLOCS. Fluid Bed Also Fluidized Bed. A bed of PARTICLES in which the PARTICULATE solids are kept in SUSPENSION by forces caused by an upward flowing fluid. Fluid bed agglomeration GROWTH AGGLOMERATION in a FLUID BED. Flux [n.] A substance used to promote FUSION as of metals and minerals. Flux [vb.] To cause to become fluid. Force feeder A FEEDER that provides forces onto PARTICULATE matter by, for example, the action of FEED SCREWS. Fractal(s) [n., pl.] Often used as plural. Any of various extremely irregular curves or shapes for which any suitably chosen part is similar in shape to a given larger or smaller part when magnified or reduced to the same size. Applied for the approximation and description of complicated outlines of (PARTICLE) shapes. Fraction [n.] That portion of a sample of PARTICULATE solids that is between two PARTICLE sizes (see CUT) or in a stated range (e.g. fine, coarse). Fragmentation [n.] The process whereby a PARTICLE (or AGGLOMERATE) splits into usually a large number of smaller parts with a range of sizes. Friability [n.] The tendency of PARTICLES to break down in size during storage and handling. (see also BRITTLENESS) Friction plate In SPHERONIZERS, a circular flat disc with a rough surface or uniformly spaced grooves that rotates inside a cylindrical BOWL. Fulcrum [n.] The support about which a LEVER turns. Functional [adj.] Existing or used to contribute to the development or maintenance of a larger whole: having a useful function. Designed or developed chiefly from the point of view of use. Funicular [adj.] Describing the transitional liquid SATURATION. Fuse [vb.] To become fluid with heat; to become joined by or as if by melting together. Gap [n.] In PRESSURE AGGLOMERATION, the distance between the surfaces of COMPACTING TOOL SETS; specifically: in EXTRUSION, the distance between the pressure gener-
14 Glossary of Application-Related Terms Associated with Agglomeration
Gear pelleter
Gel [n.] Gelatinization [n.] Gelatinize [vb.] Gelatinous [adj.] Gelation [n.] Gil plate Globulation [n.] GMP Granivorous [adj.] Granular [adj.] Granulate [n.]
Granulate (vb.)
Granulation [n.] Grate [n] Green [adj.] Grid [n.] Grind [vb.] Grog [n.]
ating device and the DIE PLATE, in ROLLER PRESSES the closest distance between the ROLLERS. Double-roll PELLET MILL in which the ROLLERS are in the shape of coarse, intermeshing gears with bores at the root sections between the gear teeth. (Also gear PELLETIZER) A COLLOID in a more solid form than a SOL. The process of converting (e.g. starch) into a GELATINOUS form. Converting into a gelatinous form or into a jelly. Resembling gelatin or jelly. The formation of a GEL from a SOL. DISTRIBUTION PLATE in which the perforations are manufactured such that they produce a directional flow of gas. See MELT SOLIDIFICATION. In the pharmaceutical industry: Good Manufacturing Practice (see also cGMP) Feeding on seeds or grains, e.g. birds. Present as PARTICLES in “grain” shape and size. Coarsely PARTICULATE Also Granule. From Latin granula = grain, PARTICLE. Any kind of relatively coarse PARTICULATE matter. In Size Enlargement synonymous with AGGLOMERATION to a size range of between approx. 0.1 and 10 mm. In Size Reduction synonymous with crushing into approx. the same size range. GRANULATE is normally considered dustfree, free flowing, and non-segregating. Producing a GRANULAR solid matter; possible by size enlargement (AGGLOMERATION, MELT SOLIDIFICATION [PASTILLATION, PRILLING], and crystallization) or by size reduction (crushing). (See also COMPACTION/GRANULATION) A general term for the production of solids in GRANULAR form by either size reduction or size enlargement. A barred frame; also an enclosure made-up of bars. As in “green AGGLOMERATE”, “green PELLET” means fresh, moist, uncured. In SPHERONIZERS, the design (size and shape) of the grooves on the FRICTION PLATE surface. To reduce to POWDER or small FRAGMENTS. Refractory material, such as crushed pottery and FIREBRICKS used in the manufacturing of industrial CERAMICS to reduce shrinkage during drying and FIRING.
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14 Glossary of Application-Related Terms Associated with Agglomeration
Growth [n.]
An increase in dimension by, for example, AGGLOMERATION or crystallization. (See also COALESCENCE) Growth agglomeration See COALESCENCE, TUMBLE AGGLOMERATION. HBI Hot Briquetted (direct reduced) Iron. HCI Hot Compacted (direct reduced) Iron. Heat bonding See SINTERING. Heel [n.] In batch processing, for example AGGLOMERATION, a percentage of the previous batch retained in or returned to the processing vessel. Hold-up [n.] Mass of material held at any given time in a continuously operating piece of equipment (for example drum or mixer). Holistic [adj.] Emphasizing the organic or functional relation between parts and wholes. Hopper [n.] The funnel or chute that stores material and/or directs it into equipment. (See also BIN, SILO) Hot melt agglomeration GRANULATION of a hot melt of e.g. urea or ammonium nitrate in a PAN. Hot pressing The simultaneous heating and molding of a COMPACT or BRIQUETTING of hot material. HVAC Heating, Ventilation, and Air Conditioning Hydrate [n.] A COMPOUND formed by the union of water with some other substance. Hydrate [vb.] To cause to take-up or combine with water or the elements of water. Igneous [adj.] Relating to or resulting from volcanic activity. Immiscible binder Selective AGGLOMERATION of PARTICLES suspended in agglomeration a liquid by adding an immiscible BINDER during AGITATION. (See also OIL AGGLOMERATION) IMO International Maritime Organization Induration [n.] The process of or condition produced during or by growing hard. Especially the strengthening of GREEN AGGLOMERATES, by the effect of binders or heat. Inkbottle pore Non-cylindrical PORE with varying diameter; particularly a PORE with narrow entrance followed by a large, internal volume. Instant [adj.] Quickly soluble. Characteristic as, for example, in “INSTANT coffee” Instantizing [n.] Producing AGGLOMERATED products with INSTANT characteristics, that is material with, as compared with the untreated POWDER, particularly high solubility, even in cold liquids. Intensifier bar In high shear mixers and AGGLOMERATORS, an independently driven bar, rotating with high speed, usu. carrying MIXING TOOLS and, sometimes means for atomizing liquid binder, that extends into the PARTICULATE mass
14 Glossary of Application-Related Terms Associated with Agglomeration
to be mixed and causes an additional turbulent motion of the PARTICLES. (See also CHOPPER, SHREDDER) Interconnected porosity A network of contiguous PORES in and extending to the surface of a POROUS body, such as an AGGLOMERATE. Interface [n.] A plane or other surface forming a common boundary of two bodies or spaces. Isostatic pressing The DENSIFICATION of a PARTICULATE mass by subjecting it to nominally equal pressure from every direction. Jacket [n.] An outer covering or casing; a covering that encloses an intermediate space through which a temperature-controlling fluid may be circulated. Knife head In high shear mixers and AGGLOMERATORS, independently driven high speed rotating tools that extend into the PARTICULATE mass and cause additional turbulent motion of the PARTICLES as well as desagglomeration in mixing and controlled destruction of AGGLOMERATES in AGGLOMERATION. (See also CHOPPER, INTENSIFIER BAR, SHREDDER) Land area The area surrounding BRIQUETTE POCKETS on the ROLLER surface of BRIQUETTERS. (See also FLASHING, WEB) Leading edge During BRIQUETTING in ROLLER PRESSES the forward edge of a discharging BRIQUETTE. Lever [n.] A bar used for prying or dislodging something. A rigid piece that transmits and modifies force or motion when forces are applied at two points and it turns about a third on a FULCRUM. Lower punch A member of the COMPACTING TOOL SET that determines the powder fill level and forms the bottom of the part in a PUNCH AND DIE PRESS. Low pressure extruder EXTRUDER in which the DIE PLATES consist of SCREENS or thin, perforated SHEETS and exert small frictional resistance during EXTRUSION. Lubricant [n.] An agent mixed with or incorporated into PARTICULATE matter or applied to the TOOLING to facilitate pressing and ejection of a COMPACT, TABLET, or EXTRUDATE. Lump [n.] See second meaning of AGGREGATE. Malleable [adj.] Capable of being altered or formed by outside forces; capable of being shaped or extended by beating or rolling. Mandrel [n.] Also mandril. A metal bar that serves as a core around which material may be bent, cast, forged, molded, or otherwise shaped. (See also CORE ROD) Marum [n.] Sometimes used to describe a particle that has been SPHERONIZED. Marumerizer [n.] See SPHERONIZER. Original (Japanese) name.
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14 Glossary of Application-Related Terms Associated with Agglomeration
Mechanical alloying
Medium[n.] Medium pressure extruder Melt solidification
Membrane [n.] Meter [vb.] Micelle [n.]
Micro-encapsulation
Micropelletizing [n.]
Mill [n.]
Mill [vb.] Mix [vb.] Mixer [n.] Mixer agglomerator Mixing tool
Mix-Muller [n.] Mold [n.]
A technology of POWDER METALLURGY by which POWDERS of metals, that cannot be combined in molten stage, are mixed and compacted to form the alloy. A surrounding or enveloping substance. See PELLET MILL. A method by which molten substances are converted into PARTICULATE solids by cooling droplets of the melt. (See also BEADING, PASTILLATION, PRILLING). A thin sheet or layer, often soft and pliable and of animal and plant origin. To supply in a measured or regulated amount. A unit of structure built up from polymeric molecules or ions. An ordered region in a fiber (as of cellulose or rayon). A molecular aggregate that constitutes a colloidal particle, obtained by association of dissolved molecules (e.g., soap in water) if a certain concentration is exceeded. Because micelles can be filled with substances that are ordinarily insoluble they play a great role in emulsifiers and detergents. A method by which small portions of liquids, PARTICULATE solids, or gases are enclosed by a shell (membrane, capsule) to form a dry, free flowing product often with spherical PARTICLE shape. The capsule shell may provide specific product characteristics (e.g., DISPERSIBILITY, SOLUBILITY). The formation of small AGGLOMERATES, usu. not larger than 3 mm, by GROWTH AGGLOMERATION. (See also PELLETIZING) A machine for CRUSHING or COMMINUTION. Also: A building provided with machinery or process equipment for manufacturing; for example Steel Mill. To grind into flour, meal or POWDER. To combine or BLEND onto one mass. To combine with one another. To bring into close ASSOCIATION. A container, device, or machine for MIXING or BLENDING. POWDER mixers modified for AGITATION and GROWTH AGGLOMERATION. Any of a large number of differently shaped tools that are attached to a rotating shaft and cause irregular movement in a PARTICLE bed. See MULLER. See DIE.
14 Glossary of Application-Related Terms Associated with Agglomeration
Muller [n.]
Multiple pressing NanoNanotechnology
Near point
Nip [n.]
Nip angle
Nodulizing [n.]
Non-woven(s) [n.]
Nozzle [n.]
Nucleus [n.] Nuclei [pl.] Ocher [n.]
Also MIX-MULLER. Originally, a device that used a heavy stone ROLLER to grind and/or mix particulate solids. Today, a blender with one, two or four large metal ROLLERS that mix and knead (densify) material. Often used prior to PRESSURE AGGLOMERATION. (See also PAN GRINDER) A method of pressing whereby two or more COMPACTS are produced simultaneously in separate DIE cavities. One billionth (10 –9) part: nanometer [nm], nanosecond [ns]. The art of manipulating materials on an atomic or molecular scale especially to build (new) engineered materials and microdevices. Area at which two PARTICLES approach each other closely enough for a BINDING MECHANISM to become effective. (See also COORDINATION NUMBER) In ROLLER PRESSES and PELLET MILLS, converging space (volume) between two counter-rotating ROLLERS and, respectively, the pressure generating device and the EXTRUSION surface. (See also NIP ANGLE) In ROLLER PRESSES, radial angle defining the line on the ROLLER surface at which the speed of the PARTICULATE mass is identical with that of the ROLLER; in EXTRUDERS, the angle between the EXTRUSION surface (e.g. DIE PLATE) and the pressure generating device (e.g. EXTRUSION BLADE, SCREW, ROLLER). Formation of nearly spherical LUMPS (AGGLOMERATES) from a wet mixture of PARTICULATE solids by either drying or chemical reaction during TUMBLING; typically accomplished in dryers or rotary kilns. Product(s) made from fibers; it (they) does (do) not depend on the interlacing of yarn for internal cohesion nor does (do) it (they) feature an organized structure. Means for ATOMIZING liquids. (See also ATOMIZER). Also orifices for injecting steam Primary AGGLOMERATE(S) consisting of only a few PARTICLES on which further GROWTH occurs. (See also SEED) See UMBER
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14 Glossary of Application-Related Terms Associated with Agglomeration
Oil agglomeration
One-pot processor
Open pore Opportunity fuels Orifice [n.]
Oversize [n.] Oyster mouthing Oxidize [vb.] Oxidizer [n.] Paint [n.]
Pan [n.] Pan grinder Particle [n.] Particle size Particulate [adj.] Pastillation [n.]
Pastille [n.] Pellet [n.]
Pellet mill Pelleting [n.]
Also SPHERICAL AGGLOMERATION. Selective AGGLOMERATION of suspended PARTICLES in water by adding a BONDING oil during AGITATION; typical in coal preparation. (See also IMMISCIBLE BINDER AGGLOMERATION) A batch processing vessel in which several process steps, for example mixing, AGGLOMERATION, POST-TREATMENT, and finishing, are carried out without opening the vessel during the entire processing sequence. A PORE communicating with or connected to the surface of a POROUS body. (See also INKBOTTLE PORE) Waste materials with calorific value that can be processed to become solid fuels. The mouth or opening of something, for example a NOZZLE for injecting or an EXTRUSION CHANNEL that forms material into defined shape. Particulate solids that are larger than a specific size. See CLAM SHELLING, DUCK BILLING. To combine with oxygen. To dehydrogenate esp. by the action of oxygen. An OXIDIZING agent. A mixture of a PIGMENT and a suitable liquid to form a closely adherent coating when spread on a surface in a thin layer. Also DISC. An inclined rotating circular plate with low cylindrical RIM for GROWTH AGGLOMERATION. See MULLER. A piece of solid material that is an entity in itself. The controlling dimension of an individual PARTICLE as determined by analysis. Of or relating to separate PARTICLES. A method of MELT SOLIDIFICATION by which droplets of a molten material are solidified on a cooled, moving stainless steel belt. Product of PASTILLATION. Name for many different types of AGGLOMERATES. Most commonly used in the iron ore industry for nearly spherical AGGLOMERATES formed by GROWTH AGGLOMERATION in PANS, CONES, or DRUMS and in the animal feed industry for EXTRUDATES produced by PELLETING. Often synonymous with AGGLOMERATE. Equipment for EXTRUSION through perforated DIES. AGGLOMERATION by EXTRUSION of plastic material or of PARTICULATE matter containing BINDERS through bores of DIES in “Pelleting Machines” or PELLET MILLS.
14 Glossary of Application-Related Terms Associated with Agglomeration
Pelletizing [n.]
Pelletization [n.]
Pelletizer [n.] Pendular [adj.] Penetrating pore
Percolation [n.] Permeable [adj.] Permeate [vb.] Pharmacologist [n.] Pharmacology [n.] Pigment [n.]
Pin mixer Piston press Plasticizer [n.]
Plenum [n.]
Plow [n.] Plug flow
Plywood [n.]
Pocket [n] Pollutant [n.]
Originally, production of PELLETS by GROWTH AGGLOMERATION. Today typically AGGLOMERATION by BALLING. Often also used as synonym for AGGLOMERATION. Typical in the (iron) ore industry; any AGGLOMERATION method involving GROWTH AGGLOMERATION with subsequent heat INDURATION. (See also SINTERING) Usually rotating PAN, DRUM, CONE, or the like for GROWTH AGGLOMERATION. Also “Gear Pelletizer”. Describing the liquid bridge model. A PORE that connects opposite sides of a POROUS body, for example, an AGGLOMERATE. (See also THROUGH PORE) The slow passage of liquid through a POROUS MEDIUM. Capable of being PERMEATED; also having PORES or openings that permit liquids or gases to pass through. To pass through the PORES or interstices of something. A person working in pharmacology. The science of drugs including their manufacture. A powdered substance that is mixed with a liquid in which it is (relatively) insoluble and used esp. to impart color to coating materials (as PAINT) or to inks, plastics, rubber. A stationary, cylindrical mixer using a single shaft AGITATOR with pins. See PUNCH-AND-DIE PRESS. A component (such as a chemical or CLAY) added to PARTICLE MIXTURES to impart flexibility, workability, and/or stretchability. Specially designed chamber at the bottom of a FLUID BED from which fluidizing gas enters the apparatus through the openings of a DISTRIBUTION PLATE. Plow shaped MIXING TOOL. Forward movement of PARTICULATE solids to the discharge end of tumbling drums or FLUID BEDS, caused by a continuous PARTICLE feed and optionally assisted by downsloping the drum or the application of GIL PLATES in FLUID BEDS. A structural material consisting of sheets of wood glued together with the grains of adjacent layers arranged at right or wide angles. In cheaper versions, the layer(s) in the center may be made up of wood chips. Indentation on the surface of ROLLERS, normally forming one half of a BRIQUETTE shape. (See also CUP) Something that POLLUTES.
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14 Glossary of Application-Related Terms Associated with Agglomeration
Pollute [vb.] Pollution [n] Porcelain [n.] Pore [n.] Pore volume Porosity [n.] Porous [adj.] Post-treatment
Pozzolan [n.]
Pozzolanic [adj.] Powder [n.] Powder metallurgy
Powder rolling Precursor [n.] Pregelatinize [vb.]
Pressway [n.]
Pressure agglomeration Prill [n.] Prilling [n.]
To make physically impure or unclean; to CONTAMINATE (an environment) esp. with man-made waste. The action of POLLUTING especially by environmental CONTAMINATION with man-made waste. A hard, fine-grained, non-POROUS, white CERAMIC ware that is fired at high temperature. An inherent or induced cavity in a PARTICLE or void space between PARTICLES within an object. Void space (volume) in POROUS objects. (See also POROSITY) The amount of PORES (voids) in an object expressed as percentage of the object’s total volume. Possessing or “full of” PORES. Any treatment of GREEN AGGLOMERATES to modify moisture content, strength, structure, by, for example, AGING, drying, heating, SINTERING. (See also CURING) Also Pozzolana. Finely divided siliceous or siliceous and aluminous material that reacts chemically with slaked lime at ordinary temperature and in the presence of moisture to form a strong, slow hardening CEMENT. Having the properties of POZZOLAN. PARTICLES of dry matter typically with a maximum dimension of less than approx. 1,000 lm. The art of producing metal POWDERS and of their utilization for the production of massive materials and shaped objects as well as for MECHANICAL ALLOYING. See ROLL COMPACTING. Also used in POWDER METALLURGY for direct rolling of SHEET from metal POWDERS. RAW MATERIAL from which another is SYNTHESIZED. See GELATINIZE. In the case of, for example, starch: Activate (by cooking) the starchy component for easy solubility when used as a BINDER. In EXTRUDERS, the (length of the) CHANNEL in which frictional resistance causes the EXTRUSION pressure; the total distance material is compressed inside a DIE. Also Press Agglomeration. AGGLOMERATION technique during which AGGLOMERATES are formed by pressure. Distinguished from TUMBLE AGGLOMERATION. Product of PRILLING. In the fertilizer industry often (incorrectly) synonymous with AGGLOMERATE. The formation of spherical PARTICLES by solidification of melt droplets. (See also MELT SOLIDIFICATION, SHOT FORMING)
14 Glossary of Application-Related Terms Associated with Agglomeration
Product [n.] Pug mill
Pulverize [vb.] Punch [n.] Punch-and-die press
Radial extruder
Ram [n.] Ram extruder
Ram press Random [adj.] Raw material
Recirculate [vb.] Recycle [n.] Redundant [adj.] Reject [vb.] Rejects [n., pl.]
Remet [n.]
Resistor [n.] RESS process
The amount, quantity, or total produced. Something resulting from or necessarily following from a set of conditions. A paddle type mixer usually with open top, single or double shafts, and trough shaped chamber. (see also VERTICAL PUG MILL) To reduce, as by CRUSHING or GRINDING, to very small PARTICLES. Part of a COMPACTING TOOL SET that transmits pressure to the particulate matter in the DIE cavity. A mechanically or hydraulically actuated press in which a reciprocating piston compacts PARTICULATE matter in a DIE. Low pressure EXTRUDER in which part of the BARREL consists of a SCREEN or perforated thin SHEET through which moist, plastic material is passed by EXTRUSION BLADES to form EXTRUDATES; the material is EXTRUDED radially to the direction in which it is transported. Synonymous with PUNCH. Press in which a fly-wheel powered reciprocating RAM densifies and extrudes PARTICULATE solids through a long EXTRUSION CHANNEL. Particularly suitable for elastic materials (such as peat, lignite, BIOMASS). (See also RAM PRESS) See RAM EXTRUDER. Lacking or seeming to lack a regular plan, purpose of pattern (STOCHASTIC). Crude or processed material that can be converted by manufacture, processing, or combination into a new and useful PRODUCT. To return REJECTS to a process for reuse. Something RECIRCULATED. Serving as a duplicate to prevent failure of an entire system if a single component fails. To refuse to accept, consider, take for some purpose, or use. In mechanical process technology particles that are too large (OVERSIZE) or too fine (UNDERSIZE, FINES) or exhibit other undesirable properties (for example in regard to shape, color, composition). In DIRECT REDUCTION: Partially or highly metallized undersized chips or fines that are RECIRCULATED to the DIRECT REDUCTION process. A device possessing electrical resistance used in an electric circuit for protection, operation, or current control. Formation of superfine particles by Rapid Expansion of a Supercritical fluid Solution at the end of a NOZZLE.
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14 Glossary of Application-Related Terms Associated with Agglomeration
Reverberatory furnace Re-work [n.] Rim [n.] Ring die Ring die extruder Ring roll press Roller [n.]
Roll(er) compacting
Roll(er) press Roll(er) pressing Rope [n.]
Rotary press Saccharin [n.]
Satellites formation
Saturation [n.]
Sanitary [adj.]
A furnace in which heat is radiated from the roof onto the material treated. To reprocess (as used or below specification material) for further use. Cylindrical or conical wall surrounding the circular plate of PAN, DISC, or CONE AGGLOMERATORS. A usu. narrow hollow cylinder that is equipped with perforations for EXTRUSION. See PELLET MILL. Special ROLLER PRESS with one press ROLLER within a large ring-shaped DIE. (no longer used) Also Roll. Cylindrical rotating body that is: 1.- paired with an identical, counter-rotating one in a suitable frame. This arrangement is used for BRIQUETTING, COMPACTING, PELLETING, DENSIFICATION, FLAKING, and GRANULATING PARTICULATE solids. 2.- rolling close to a DIE PLATE and forces material to flow through openings, for example, in flat DIE PELLET MILLS. 3.- mixing and kneading material in a cylindrical or “figure eight’-shaped bowl. (See also MULLER) Also POWDER ROLLING. The (progressive) COMPACTING of (metal) POWDERS in ROLLER PRESSES (often called “rolling mills”). (See also ROLL PRESSING) Equipment for PRESSURE AGGLOMERATION between two ROLLERS. Densification between two counter-rotating ROLLERS.(see also COMPACTING) In food extrusion, an elongated body; synonymous with uncut EXTRUDATE. See also STRAND. In SPHERONIZATION, referring to the rotating PARTICULATE material. TABLETTING MACHINE in which COMPACTING TOOL SETS are arranged on a rotating table (= TURRET). A crystalline compound that is unrelated to carbohydrates, is several hundred times sweeter than cane sugar and is used as a calorie-free sweetener (see also ASPARTAME). In AGGLOMERATION, the attachment of smaller solid entities. Often AGGLOMERATES, to other AGGLOMERATES by BINDING MECHANISMS. (See also CLUSTERING) Relative amount of PORES in an AGGLOMERATE filled with a liquid or solid substance, as in “liquid saturation”, “BINDER saturation”. Characterized by or readily kept in cleanliness.
14 Glossary of Application-Related Terms Associated with Agglomeration
Scalp [vb.]
To remove an upper part from, esp. removing large pieces from, for example, PARTICULATE SOLIDS; e.g. SCALPING SCREEN. Schugi flexowall High speed, high shear mixer and/or AGGLOMERATOR with vertical axis, adjustable MIXING TOOLS, flexible shell, flexing roller cage, and short residence time. Scraper [n.] A tool for removing BUILD-UP in AGGLOMERATION equipment. (See also DOCTOR BLADES) Screen [n.] A (usu. mounted) perforated thin plate or cylinder or a meshed wire or cloth fabric used to: 1.- separate coarser from finer PARTICLES or 2.- form EXTRUDATES. Screw [n.] A mechanical device spiral in form or appearance; a conveyor working on the principle of a screw; a conveying tool in a FEEDER, mixer, or EXTRUDER. (See also AUGER, WORM) Screw extruder EXTRUDER in which screw(s) produce the EXTRUSION pressure. Scroll [n.] A spiral or CONVOLUTED form derived from the curves of a rolled cylinder. Scum [n.] Extraneous matter or impurities risen to or formed on the surface of a liquid (often as a foul, filmy covering). Sedimentation [n.] The movement (in any direction) of solid PARTICLES through a fluid as a result of gravitational or other forces. Seed [n.] See NUCLEUS. Segregation [n.] The desirable or undesirable separation (according to mass, shape, size) of one or more components of a PARTICULATE mass. Selective agglomeration AGGLOMERATION of only one component of a POWDER mixture controlled by, for example, BINDING MECHANISM, BINDER, PARTICLE size. (See also IMMISCIBLE BINDER AGGLOMERATION). Self-fluxing Containing FLUX to become fluid more easily. Semi-autogenous mill AUTOGENOUS MILL in which some grinding media (e.g. steel balls) is added to assist in grinding. Semi- permeable [adj.] Partly but not freely or wholly PERMEABLE; PERMEABLE to some but not other molecules. Separator [n.] Equipment that removes solid PARTICLES from fluids or classifies them according to size. Sheet [n.] A more or less continuous band of COMPACTED material produced in ROLLER PRESSES featuring smooth or shallowly profiled ROLLERS and a gap between those ROLLERS. Also, anything that is thin in comparison to its length and/or breadth. See also SLAB
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14 Glossary of Application-Related Terms Associated with Agglomeration
Shot forming Shredder [n.] Shrinkage [n.]
Sienna [n.] Silo [n.]
Similarity [n.] Similitude [n.] Single-action pressing Sinter [n.] Sintering [n.]
Sizing [n.] Slab [n.] Slime [n.]
Sludge [n.] Slug [n.]
Slugging [adj.]
Slugging press
Slurry [n.] Smelt [vb.] Spall [vb.]
The solidification of a melt into little spheres in a TALL FORM TOWER. (See also PRILLING) See CHOPPER, INTENSIFIER BAR, KNIFE HEAD. A decrease in dimension. In AGGLOMERATION, usually of a compact during SINTERING. Converse of EXPANSION See UMBER. A trench, pit, or esp. a tall cylinder (as of wood, metal, or concrete) often sealed and used for storing PARTICULATE solids. (See also HOPPER, BIN) The quality or state of being similar. A comparable aspect. Correspondence in kind or quality. See also SIMILARITY. A method by which a PARTICULATE mass is pressed in a stationary DIE between one moving and one fixed PUNCH. AGGLOMERATED product of SINTERING. Technique involving INDURATION of GREEN AGGLOMERATES by heat. Generally, BONDING at a temperature below the melting or softening points of the main constituent of a mixture by the application of heat. (See also HEAT BONDING) Bring to proper or suitable size. (See also CLASSIFY, CLASSIFICATION) In food extrusion, an elongated wide body or uncut EXTRUDATE. See also SHEET A product of wet crushing consisting of ore ground so fine that it passes separators and remains SUSPENDED until THICKENED. A muddy or slushy mass, deposit, or sediment. Large, flat faced COMPRESSED disk prepared for the purpose of STABILIZING the mixture of ingredients in the pharmaceutical industry. 1.- Producing SLUGS in a SLUGGING PRESS. 2.- In FLUID BED technology, the slow, upward movement of large, somewhat COHESIVE masses of PARTICULATE solids. PUNCH-AND-DIE press for the production of large TABLETS or SLUGS that are crushed to obtain GRANULATE. Mostly in the pharmaceutical industry. (See also TABLETTING MACHINE). A water mixture or SUSPENSION of insoluble matter. To melt or fuse (as ore) often with an accompanying chemical change, usually to separate metal. To break up or reduce by chipping; to break-off chips, scales, or slabs.
14 Glossary of Application-Related Terms Associated with Agglomeration
Sparger pipe
From sparge = spray. A submerged, often perforated pipe through which a fluid is introduced into a liquid for (gas) agitation or into a moving bed of PARTCULATE SOLIDS for various processing purposes (e.g. liquid binder; reaction, drying, or cooling gas). Spherical agglomaration See OIL AGGLOMERATION. Spheronizing [n.] Rounding of soft, plastic (usu. GREEN) AGGLOMERATES (usu. EXTRUDATES) in a SPHERONIZER. Spheronizer [n.] Vertical drum with rotating bottom for SPHERONIZING. (See also MARUMERIZER) Specific force Characteristic parameter for ROLLER PRESSES; defined as ‘pressing force/active ROLLER width’. Specific surface The relative surface area of a POWDER measured in m2/g or area m2/cm3. Spray drying The formation of GRANULAR solids or small spherical AGGLOMERATES by dispersing a liquid or slurry in droplet form at the top of a TOWER and evaporating the liquid in the presence of drying gases. Spray granulation The formation of small, spheroidal AGGLOMERATES in a FLUID, circulating, or SPOUTED BED by spraying a solution, slurry or melt onto the PARTICLES; often combined with drying. Stabilize [vb.] Avoid SEGREGATION by AGGLOMERATING a POWDER mixture. Stochastic [adj.] RANDOM (e.g. processes). Stoker [n.] A machine for feeding a fire. Strand [n.] An elongated body; synonymous with uncut EXTRUDATE. Also: an elongated body resembling a belt – See TRAVELING GRATE Strip [n.] See SHEET. Substrate [n.] A base on which material builds-up or an organism lives. Surface equivalent The diameter of monosized spherical PARTICLES, calculadiameter ted from the mass related specific surface area, in m2/g, of a PARTICLE size distribution, that produce the same specific surface area as the POWDER. Suspension [n.] The state of PARTICULATE solids that are uniformly mixed with but undissolved in a fluid. Swarf [n] Fine particles removed by a cutting or grinding tool. Metal swarf means turnings, borings, chips, wires, clippings, and foil. Synthesis n.] The composition or combination of parts or elements so as to form a whole. The production of a substance by the union of elements, groups or simple compounds or by the degradation of a complex compound. Synthesize [vb.] To combine or produce by SYNTHESIS.
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14 Glossary of Application-Related Terms Associated with Agglomeration
Tablet [n.]
A compressed AGGLOMERATE made of PARTICULATE solids, specifically, in pharmacy, a small COMPACT of a medicated PARTICULATE formulation usu. in the shape of a disc or a flat polyhedral body. (See also BRIQUETTE) Tabletting [n.] The process of forming TABLETS. Tabletting machine Compaction press for the manufacture of TABLETS. Tailings [n., pl.] Inferior or refuse material separated as residue in processing. Talc (n.) A very soft mineral that is a basic silicate of magnesium, has a soapy feel and is used esp. in making Talcum powder. (See also ANTICAKING AGENT) Tall form tower A tall TOWER with enlarged conical bottom. Tamp [vb.] To drive in or down by a succession of light or medium blows; predensify. Tamper [n.] See TAMP. Tank [n.] A receptacle for holding, storing, or transporting liquids. Template [n.] A body used as a guide to form a piece being made. Template (vb.] To form a piece by employing a model (TEMPLATE). Thermal agglomeration Using heat to fuse PARTICULATE solids into AGGLOMERATES. (See also HEAT BONDING, SINTERING). Thicken [vb.] To make thick, dense, or viscous in consistency. Thickener [n.] An apparatus for the SEDIMENTATION and collection of SUSPENDED solids in industrial liquids. Thixotropy [n.] The property of various materials to become fluid when disturbed (as by shaking, vibration, pressure) Thixotropic [adj.] Materials tending to exhibit THIXOTROPY. Through pore See PENETRATING PORE. Toll [n.] A fee paid or compensation taken for services rendered, as a charge for the use of facilities. Toller [n.] Independent industrial facility offering CONTRACT MANUFACTURING. Tolling operation See CONTRACT MANUFACTURING. Tooling [n.] Parts making up the COMPACTING TOOL SET of a TABLETTING MACHINE. Tortuosity [n.] Something winding or twisted. Tortuous [adj.] Winding. Marked by repeated twists, bends, or turns. Tower [n.] In SPRAY DRYING or PRILLING, a cylindrical structure in which liquid droplets that were formed at the top solidify during their descend in a gas atmosphere with suitable temperature. Trailing edge During BRIQUETTING in ROLLER PRESSES the back edge of a discharging BRIQUETTE. Traveling grate Endless belt consisting of hinged plates with GRATE bars on which solids are processed by heat (SINTERED, CALCINED). Also called STRAND.
14 Glossary of Application-Related Terms Associated with Agglomeration
True density Tumble agglomeration
Turret [n.] UFP Umber [n.]
Undersize [n.] Updraft [n.] Upper punch Upstream [adv.] Varistor [n.] Vertical pug mill
Wear [n.] WDG Web [n.] Wet agglomeration Wick [n.] Withdrawal process
Worm [n.] WSG Yield [n.]
The mass of the unit volume of a solid material that is free of PORES. AGGLOMERATION technique during which AGGLOMERATES are formed by GROWTH during tumbling; synonymous with GROWTH AGGLOMERATION. (See also COALESCENCE) Rotating table of some TABLETTING MACHINES carrying the COMPACTING TOOL SET. Ultra Fine PARTICLE. A brown earth that is darker in color than OCHER and SIENNA because of its content of manganese and iron oxides and is highly valued as a permanent PIGMENT either in the raw or CALCINED state. Particulate solids that are smaller than a specific size. (see also FINES) Upward flow of gas, for example, through a PARTICLE bed. Member of the COMPACTING TOOL SET that closes the DIE and forms the top of the part being produced. Nearer to the source. An electrical RESISTOR whose resistance depends on the applied voltage. Mostly in the BRIQUETTING of coal with BINDERS. An upright cylindrical MIXER with vertical rotating shaft, carrying MIXING ELEMENTS, and NOZZLES for steam injection. Used to convert (CONDITIONING) coal binder mixtures into a plastic mass suitable for BRIQUETTING. (see also PUG MILL) Similar to EROSION, but usu. refers to the surface of a solid body such as a part of machinery. (Easily) water dispersible GRANULATE. Thin FLASHING surrounding BRIQUETTES made in ROLLER PRESSES; caused by the LAND AREA. TUMBLE and GROWTH AGGLOMERATION in which the major BINDER is a liquid. A bundle of fibers that by CAPILLARY action draw-up a steady supply of liquid (often of melted fuel to be burned). Operation of some TABLETTING PRESSES by which the DIE descends over a fixed lower punch to reduce density variation in the TABLET and facilitate removal of the COMPACT. See SCREW. (Easily) water soluble GRANULATE. (See also INSTANT) The amount or quantity produced (PRODUCT) or returned.
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I1
Indexes As discussed in Chapters 2 and 14, one of the problems encountered in connection with the discussion of technologies that have been known for centuries, and have been developed empirically and independently for different applications and industries, is that the reader of books and the student of the methods and processes find an often confusing terminology. Even the suppliers of equipment may present themselves in a manner that does not unequivocally define their activities. To help locate information, two different indexes are offered. The first section is a list of addresses, telephone, and fax numbers as well as web sites of vendors of equipment for size enlargement by agglomeration and of some peripheral techniques. It is subdivided into fields of activities and, if a vendor is active in different areas, its listing is repeated again under the appropriate heading. Of course, no claim for completeness is made and mentioning a specific vendor does not constitute an endorsement by the author of this book or its publishers. Also included is a listing of some tollers or contract manufacturers (Section 9.2) with a description of their activities. The second section is a subject index, which often provides many page numbers for the same topic.
List of Vendors
When planning the book “Agglomeration Processes – Phenomena, Technologies, Equipment” [B.97], the author intended to prepare a worldwide, comprehensive list of vendors of agglomeration equipment and of associated resources and services. To that end, he collected technical and process information, particularly in Europe, North America, and Japan. It was found that in Australia, India, the Near East, South America as well as Africa and in many smaller countries, sources of agglomeration equipment are primarily local subsidiaries of foreign companies or foreign and home office representatives. Other sources are international engineering companies and their local subsidiaries and representatives which are specifying and using European, North American, and Japanese equipment.
Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
I2
Indexes
At some point in preparing the present book, many of the vendors that the author and others knew were again contacted and asked for input that could be used in this publication. As experienced before, only very few provided information beyond submitting their sales literature. Companies and individuals that have exceeded the normal in supporting the project are identified at the beginning of the book (Acknowledgments) and are referred to with gratitude in figure and table captions. Also, the author’s extensive personal files, many of which were already reviewed for and incorporated in his earlier books [B.13b, B.48] as well as his library and articles were referred to. The vendor list resulting from these efforts, updated and extended as compared with what was presented earlier [B.97], is reproduced below. The following listing is subdivided according to methods, technologies, resources and technologies. It was decided to list vendors that are active in different fields in each of the classifications to facilitate the search if a particular method is desired for a specific application. The entries are organized according to the following headings: *
*
* * * * * * * *
Growth/Tumble Agglomeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . – Disc (Pan)/Drum/Mixer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – Fluid Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spray Nozzles and Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – Agglomeration in Suspensions . . . . . . . . . . . . . . . . . . . . . . . . . . . Pressure Agglomeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – Low Pressure Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spheronizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – Medium Pressure Extrusion (Pelleting) . . . . . . . . . . . . . . . . . . . . . – High Pressure Extrusion (Ram Presses, Extruders) . . . . . . . . . . . . . – High Pressure Agglomeration (Punch-and-Die, Tabletting, Isostatic, Tooling). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – High Pressure Agglomeration (Roll) . . . . . . . . . . . . . . . . . . . . . . . Sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Melt Solidification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Binders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Test Equipment and Peripherals . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tollers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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List of Vendors
Growth/Tumble Agglomeration Disc (Pan)/Drum/Mixer
Paul O. Abbe´, Inc. 139 Center Avenue Little Falls, NJ 07424, USA
Tel.: +1- 973- 256- 4242 Fax:+1- 973- 256- 0041 www.pauloabbe.com
Aeromatic-Fielder AG, Div. Niro Inc. Hauptstr. 145 CH-4416 Bubendorf, Switzerland
Tel.: +41- (0)61- 93636- 36 Fax: +41- (0)61- 93636- 00 www.niropharmasystems.com
Aeromatic-Fielder Ltd. School Lane Eastleigh, Hants SO53 4ZD
Tel.: +44- (0)23- 80267131 Fax: +44- (0)23- 80253381 www.niropharmasystems.com
Great Britain Aeromatic-Fielder, Inc. 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax: +1- 410- 997- 5021 www.niropharmasystems.com
Allgaier-Werke GmbH & Co. KG Bereich Sieb- und Aufbereitungstechnik Ulmerstr. 75 D-73066 Uhingen, Germany
Tel.: +49- (0)7161- 301- 0 Fax: +49- (0)7161- 34268 www.allgaier.de
ACT Applied Chemical Technology 4350 Helton Drive Florence, AL 35630, USA
Tel.: +1- 256- 760-9600 Fax: +1- 256-760-9638 www.appliedchemical.com
AVA-Huep GmbH & Co. KG Arzbergerstrasse 10 D-82211 Herrsching, Germany
Tel.: +49- (0)8152- 9392- 0 Fax: +49- (0)8152- 93492- 91 www.ava-huep.de
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, MN 55413, USA
Tel.: +1- 612- 331- 4370 Fax: +1- 612- 627- 1444 www.hosokawamicron.com
Hermann Berstorff Maschinenbau GmbH An der breiten Wiese 3-5 D-30625 Hannover, Germany
Tel.: +49- (0)511- 5702- 0 Fax: +49- (0)511- 561916
L.B. Bohle Maschinen + Verfahren GmbH Industriestrasse 18 D-59320 Ennigerloh, Germany
Tel.: +49- (0)2524- 9323- 0 Fax: +49- (0)2524- 9323- 29 www.lbbohle.de
I3
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Indexes
L.B. Bohle, Inc. 1504 Grundy Lane Bristol, PA 19007, USA
Tel.: +1- 215- 785- 1121 Fax: +1- 215- 785- 1221 www.lbbohle.com
Bolz-Summix International Oostzeestraat 6 NL-7202 CM Zutphen The Netherlands
Tel.: +31- (0)575- 593155 Fax: +31- (0)575- 593166 www.mpe.nl
CEMTEC Cement & Mining Technology GmbH Stadlgasse 8a A-4470 Enns, Austria
Tel.: +43- (0)7223- 83620 Fax: +43- (0)7223- 85083 www.cemtec.at
Continental Products Corp. P.O. Box 762 Milwaukee, WI 53201, USA
Tel.: +1- 414- 964- 0640 www.continentalrollomixer.com
H.C. Davis Sons Mfg. Co, Inc. Box 395 Bonner Springs, KS 66012, USA
Tel.: +1- 913- 422- 3000 Fax: +1- 913- 422- 7220 www.hcdavis.com
Dierks & So¨hne, GmbH & Co, KG. (DIOSNA) Sandbachstr. 1 D-49074 Osnabru¨ck, Germany
Tel.: +49- (0)541- 3310- 0 Fax: +49- (0)541- 3310- 410 www.diosna.de
DIOSNA USA 270 US Highway 46 Rockaway, NJ 07866, USA
Tel.: +1- 973- 586- 3708 Fax: +1- 973- 586- 3731
Dinnissen b.v., Process Technology Horsterweg 66 5975 NB Sevenum, Holland
Tel.: +31- 77- 467- 3555 Fax: +31- 77- 467- 3785 www.dinnissen.nl
Draiswerke GmbH Speckweg 43-51 D-68305 Mannheim, Germany
Tel.: +49- (0)621-7504- 00 Fax: +49- (0)621- 7504- 233 www.draiswerke.com
Draiswerke, Inc. 40 Whitney Road Mahwah, NJ 07430, USA
Tel.: +1- 201- 847- 0600 Fax: +1- 201- 847- 0606 www.draiswerke-inc.com
Maschinenfabrik Gustav Eirich Walldu¨rner Str. 50, Postfach 1160 D-74736 Hardheim, Germany
Tel.: +49- (0)6283- 51- 0 Fax: +49- (0)6283- 51- 325 www.eirich.com
List of Vendors
Eirich Machines, Inc. American Process Systems Div. Delany Business Center 4033 Ryan Rd. Gurnee, IL 60031, USA
Tel.: +1- 847- 336- 2444 Fax: +1- 847- 336- 0914 www.eirichusa.com
Fabtech, Inc. 23250 Pinewood Warren, MI 48091, USA
Tel.: +1- 810- 756- 2335 Fax: +1- 810- 756- 2831 www.fabtech-usa.com
FEECO International 3913 Algoma Rd. Green Bay, WI 54311, USA
Tel.: +1- 920- 468-1000 Fax: +1- 920- 469- 5110 www.feeco.com
Fluid Air, Inc. 2550 White Oak Circle Aurora, IL 60504-9678, USA
Tel.: +1- 630- 851-1200 Fax: +1- 630- 851- 1244 www.fluidairinc.com
Palex Corp. (Fukae Powtec Corp) P.O. Box 65 Tajimi, Gifu-Pref. 507-0033, Japan
Tel.: +81- (0)572- 229152 Fax: +81- (0)572- 242722 www.palex.jp
GEMCO The General Machine Co of New Jersey 301 Smalley Avenue Middlesex, NJ 08846
Tel.: +1- 908- 752- 7900 Fax: +1- 908- 752- 5857 www.okgemco.com
Gericke AG Althardstr. 120 CH-8105 Regensdorf-Zu¨rich Switzerland
Tel.: +41- 1- 871- 3636 Fax: +41- 1- 871- 3600 www.gericke.net
Glass GmbH & Co. KG Frankfurter Weg 28 D-33106 Paderborn, Germany
Tel.: +49- (0)5251- 77991- 0 Fax: +49- (0)5251- 77991- 77 www.Glass-Maschinen.de
Hayes & Stolz Ind. Mfg. Co., Inc. 3521 Hemphill Street Fort Worth, TX 76110, USA
Tel.: +1- 817- 926- 3391 Fax: +1- 817- 926- 4133 www.hayes-stolz.com
Thyssen Henschel Industrietechnik GmbH Henschel-Mixers & Systems Henschelplatz 1 D-34112 Kassel, Germany
Tel.: +49- (0)561- 801- 5890 Fax: +49- (0)561- 801- 6943 www.henschel-mixers.de
I5
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Indexes
Thyssen Henschel America, Inc. Green Bay, WI 54304, USA
Tel.: +1- 920- 336-4000 Fax: +1- 920- 336- 3131 www.thamerica.com
Hosokawa Micron Powder Systems 10 Chatham Road Summit, NJ 07901, USA
Tel.: +1- 908- 273- 6360 Fax: +1- 908- 273- 7432 www.hosokawamicron.com
Hu¨ttlin GmbH An IWK Company Daimlerstr. 7 D-79585 Steinen, Germany
Tel.: +49- (0)7627- 9117- 0 Fax: +49- (0)7627- 8851 www.huettlin.de
I.M.A. SPA Via Emilia 428-442 I-40064 Ozzano Emilia (BO), Italy
Tel.: +39- (0)51- 651- 4111 Fax: +39- (0)51- 651- 4666 www.ima.it
Italvacuum Srl. (Criox) Via Stroppiana I-10071 Borgaro (Turin), Italy
Tel.: +39- (0)11- 470- 4651 Fax: +39- (0)11- 470- 1010 www.italvacuum.com
Jaygo, Inc. 675 Rahway Ave. Union, NJ 07083, USA
Tel.: +1- 908- 688- 3600 Fax: +1- 908- 688- 6060 www.jaygoinc.com
Kaltenbach-Thu¨ring 9. rue de l’Industrie F-60000 Beauvais, FRANCE
Tel.: +33- 44- 02- 8900 Fax: +33- 44- 02- 8910 www.afa.com.eg/Kaltenbach
Kemutec Group, Powder Technology Springwood Way, Macclesfield Cheshire SK10 2ND, UK
Tel.: +44- (0)1625- 412000 Fax: +44- (0)1625- 412001 www.kemutec.com
Key International, Inc. 480 Route 9 Englishtown, NJ 07726, USA
Tel.: +1- 732- 536- 9700 Fax: +1- 732- 972- 2630 www.keyinternational.com
Littleford Day, Inc. 7451 Empire Drive Florence, KY 41042-2985, USA
Tel.: +1- 606- 525- 7600 Fax: +1- 606- 525- 1446 www.littleford.com
Gebr. Lo¨dige Maschinenbau GmbH Elsener Str. 7-9 D-33102 Paderborn, Germany
Tel.: +49- (0)5251- 309- 0 Fax: +49- (0)5251- 309- 123 www.loedige.de
List of Vendors
Lodige Process Technology, Inc. One Greentree Centre, Suite 201 Marlton, NY 08053-3105, USA
Tel.: +1- 856- 988- 5579 Fax: +1- 856- 596- 1324 www.loedige.de
MAP S.R.L Via Cavour, 388/B I-41030 Ponte Motta, Cavezzo (MO), ITALY
Tel.: +39- 535- 49911 Fax: +39- 535- 49900
Mars Mineral P.O. Box 719 Mars, PA 16046, USA
Tel.: +1- 724- 538- 3000 Fax: +1- 724- 538- 5078 www.marsmineral.com
mhs Verfahrenstechnik GmbH PO Box 1144 D-48330 Sassenberg, Germany
Tel.: +49- (0)2583- 9340- 00 Fax: +49- (0)2583- 9340- 18 www.mhsverfahren.de
C.G. Mozer GmbH & Co. KG Adolf-Safft-Str. 10 D-73037 Go¨ppingen, GERMANY
Tel.: +49- (0)7161- 6735- 0 Fax: +49- (0)7161- 6735- 35 www.allgaier.de/mozer
m-tec Mathis Technik GmbH Otto-Hahn-Strasse 6 D-79395 Neuenburg, Germany
Tel.: +49- (0)7631- 709- 0 Fax: +49- (0)7631- 709- 120 www.m-tec-gmbh.de
MTI, Mixing Technology, Inc. 3303 FM 1960 West, Suite 490 Houston, TX 77068, USA
Tel.: +1- 281- 583- 8610 Fax: +1- 281- 583- 0190 www.mti-america.com
Munson Machinery Co., Inc P.O. Box 855, 210 Seward Ave. Utica, NY 13503-08555, USA
Tel.: +1- 315- 797- 0090 Fax: +1- 315- 797- 5582 www.munsonmachinery.com
Nara Machinery Co., Ltd. 5-7, 2-chome, Jonan-Jima Ohta-ku, Tokyo 143, JAPAN
Tel.: +81- (0)3- 3799- 5011 Fax: +81- (0)3- 3790- 8027 www.nara-m.co.jp/english/index.html
Nara Machinery Co., Ltd. Zweigniederlassung Europa Europaallee 46 D-50226 Frechen, GERMANY
Tel.: +49- (0)2234- 23063 Fax: +49- (0)2234- 23067 www.nara-e.de
GEA / NIRO A/S Gladsaxevej 305, P.O. Box 45 DK-2860 Soeborg, Denmark
Tel.: +45- 3954- 5454 Fax: +45- 3954- 5800 www.niro.com
I7
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Indexes
NIRO, Inc. 9165 Rumsey Road Columbia, MD 21045, USA
Tel.: +1- 410- 997- 8700 Fax: +1- 410- 997- 5021 www.niroinc.com
O’Hara Technologies, Inc. 20 Kinnear Ct. Richmond Hill, L4B 1K8, Ont., Canada
Tel.: +1- 905- 707- 3286 Fax: +1- 905- 763- 6749 www.oharatech.com
Patterson-Kelley Co. 100 Burson Street P.O. Box 458 East Stroudsburg, PA 18301, USA
Tel.: +1- 570- 421- 7500 Fax: +1- 570- 421- 8735 www.patkelco.com
Pemat Mischtechnik GmbH Hauptstrasse 29 D-67361 Freisbach, Germany
Tel.: +49- (0)6344- 9449- 20 Fax: +49- (0)6344- 9449- 510 www.pemat.de
Phlauer, A&J Mixing Intermational, Inc. 8-2345 Wyecroft Road Oakville, Ont. L6L 6L4, CANADA
Tel.: +1- 905- 827- 7288 Fax: +1- 905- 827- 5045 www.ajmixing.com
Processall, Inc. 10596 Springfield Pike Cincinnati, OH 45215, USA
Tel.: +1- 513- 771- 2266 Fax: +1- 513- 771- 6767 www.processall.com
Robot Coupe USA, Inc., Scientific-Industrial Division P.O. Box 16627 Jackson, MS 39236-6627, USA
Tel.: +1- 601- 956- 3216 Fax: +1- 601- 956- 5758 www.robocpe-si.com
Romaco Group Am Heegwald 11 D-76216 Karlsruhe, GERMANY
Tel.: +49- (0)721- 4804- 0 Fax: +49- (0)721- 4804-258 www.romaco.com
Romaco, Inc. 104 American Road Morris Plains, NJ 07950, USA
Tel.: +1- 973- 605- 5370 Fax: +1- 973- 605- 1360 www.romaco.com
Charles Ross and Son Co. 710 Old Willets Path Hauppauge, NY 11788, USA
Tel.: +1- 631- 234- 0500 Fax: +1- 631- 234- 0691 www.rossmixing.com
Gebr. Ruberg GmbH & Co. KG Friedrich-Wilhelm-Weber-Str. 31 D-33039 Nieheim, Germany
Tel.: +49- (0)5274- 98510- 0 Fax: +49- (0)5274- 98510- 50 www.gebr-ruberg.de
List of Vendors
Ruberg-Mischtechnik KG Halbersta¨dter Str. 55 D-33106 Paderborn, Germany
Tel.: +49- (0)5251- 1736- 0 Fax: +49- (0)5251- 1736- 999 www.ruberg.de www.clever-cut.de
The A.J. Sackett & Sons Co. 1701 South Highland Ave. Baltimore, MD 21224, USA
Tel.: +1- 301- 276- 4466 Fax: +1- 301- 276- 0241 www.ajsackett.com
Hosokawa Schugi B.V. Chroomstraat 29 NL-8211 AS Lelystad THE NETHERLANDS
Tel.: +31- (0)320- 28 66 66 Fax: +31- (0)320- 24 47 94 www.hosokawamicron.com
Sejong Machinery Co., Ltd. #159-11 Dodang-dong Wonmi-ku ROK-420-130 Puchon-city Kyunggi-do KOREA
Tel.: +82- 32- 672- 7811/2 Fax: +82- 32- 672-7813 www.sejong.com
Sejong Pharmatech Co., Ltd. 409-3, Chungchun-Dong Bupyong-Gu, Incheon City Korea 403-030
Tel.: +82- 32- 508- 128- 0 Fax: +82- 32- 508- 128- 9 www.sejong-trading.com
Uhde GmbH Friedrich-Uhde-Strasse 15 D-44141 Dortmund, Germany
Tel.: +49- (0)231- 547- 0 Fax: +49- (0)231- 547- 3032 www.thyssenkrupp.com/uhde
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax: +1- 319- 377- 5574 www.vectorcorporation.com
Zanchetta &C. S.R.L (see also Key/ROMACO) Via della Contea, 24 I-55010 S. Salvatore-Montecarlo (Lucca), ITALY
Tel.: +39- (0)583- 2171 Fax: +39- (0)583- 217317 www.romaco.com
ZETTL GmbH & Co. KG Oldenbourgstr. 11 D-81247 Mu¨nchen, Germany
Tel.: +49- (0)89- 81809- 0 Fax: +49- (0)89- 81809- 33 www.zettl-munich.de
I9
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Indexes
Fluid Bed
Allgaier Werke GmbH & Co KG Ulmerstrasse 75 D-73066 Uhingen, GERMANY
Tel.: +49- (0)7161- 301- 0 Fax: +49- (0)7161- 34268 www.allgaier.de
Allgaier Verfahrenstechnik GmbH A-4492 Hofkirchen 93, AUSTRIA
Tel.: +43- 45 67 22 59 01 25 Fax: +43- 72 25 64 23
ACT, Applied Chemical Techn., Inc. 4350 Helton Drive Florence. AL 35630, USA
Tel.: +1- 256- 760- 9600 Fax: +1- 256- 760- 9638 www.appliedchemical.com
Aeromatic Ltd. (member GEA/NIRO) Hauptstrasse 145 CH-4416 Bubendorf, Switzerland
Tel.: +41- (0)61- 936- 3660 Fax: +41- (0)61- 936- 3600 www.niro.com
Aeromatic-Fielder (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax: +1- 410- 997- 5021 www.niroinc.co
AMMAG Dahlienstrasse 11 A-4623 Gunskirchen, Austria
Tel.: +43- 7246- 6408- 0 Fax: +43- 7246- 6408- 39 www.ammag.com
APV Anhydro AS Østmarken 7 DK-2860 So¨borg, Copenhagen Denmark
Tel.: +45- 3969- 2811 Fax: +45- 3969- 3880 www.anhydro.com
APV Anhydro 182 Wales Avenue Tonawanda, NY 14150, USA
Tel.: +1- 716- 692- 3000 Fax: +1- 716- 692- 6416
Babcock-BSH GmbH Parkstrasse 10 D-47829, Krefeld, Germany
Tel.: +49- (0)2151- 448-0 Fax: +49- (0)2151- 448-592 www.babcock.com
DeSpain/Babcock 935-D East Mountain St. Kernersville, NC 27285, USA
Tel.: +1- 336- 996- 0692 Fax: +1- 336- 996- 8573 www.despaineng.com
DMR Prozesstechnologie Rinaustrasse 380 CH-4303 Kaiseraugust Switzerland
Tel.: +41- 61- 813- 10- 60 Fax: +41- 61- 813- 10- 62 www.dmr-prozess.com
List of Vendors
Fluid Air, Inc. 2550 White Oak Circle Aurora, IL 60504-9678, USA
Tel.: +1- 630- 851-1200 Fax: +1- 630- 851- 1244 www.fluidairinc.com
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81- 06- 933- 1511 Fax: +81- 06- 933-1531 www.fujipaudal.co.jp
Glatt GmbH Process Technologie Bu¨hlmu¨hle D-79589 Binzen, Germany
Tel.: +49- (0)7621- 664- 0 Fax: +49- (0)7621- 647- 23 www.glatt.de
Glatt Air Techniques, Inc. 20 Spear Road Ramsey, NJ 07446, USA
Tel.: +1- 201- 825- 8700 Fax: +1- 201- 825- 0389 www.glattair.com
A. Heinen AG Anlagenbau Achternstrasse 1-17 D-26316 Varel, Germany
Tel.: +49- (0)4451- 122- 0 Fax: +49- (0)4451- 122- 159 www.heinen.biz
Hu¨ttlin GmbH An IWK Company Daimlerstr. 7 D-79585 Steinen, Germany
Tel.: +49- (0)7627- 9117- 0 Fax: +49- (0)7627- 8851 www.huettlin.de
GEA / NIRO A/S Gladsaxevej 305, P.O. Box 45 DK-2860 So¨borg, Denmark
Tel.: +45- 3954- 5454 Fax: +45- 3954- 5800 www.niro.dk
NIRO, Inc. 9165 Rumsey Road Columbia, MD 21045, USA
Tel.: +1- 410- 997- 8700 Fax: +1- 410- 997- 5021 www.niroinc.com
NIRO, Inc. (Food & Dairy Industries) 1600 O’Keefe Road Hudson, WI 54016, USA
Tel.: +1- 715- 386- 9371 Fax..:+1- 715- 386- 9376 www.niroinc.com
Pulse Combustion Systems 135 Eye Street, Suite B San Rafael, CA 94901, USA
Tel.: +1- 415- 435- 4225 Fax: +1- 508- 437- 7726 www.pulsedry.com
VA TECH WABAG Fliessbett Systeme GmbH Escher-Wyss-Strasse 25 D-88212 Ravensburg, Germany
Tel.: +49- (0)751- 83- 03 Fax: +49- (0)751- 83- 3033 www.vatech.ch
I 11
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Indexes
VA TECH USA 2901 Wilcrest Suite 345 Houston, TX 77042, USA
Tel.: +1- 713- 780- 4200 Fax: +1- 713- 780- 2848 www.VATECHfluidbedUS.com
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax: +1- 319- 377- 5574 www.vectorcorporation.com
Spray Nozzles and Systems
Acosystem Prozesstechnologie GmbH Klus 5 D-31073 Delligsen, Germany
Tel.: +49- (0)5187- 9440- 0 Fax: +49- (0)5187- 9565-30 www.acosystem.de
Bete Fog Nozzle, Inc. P.O. Box 1438, 50 Greenfield Street Greenfield, MA 01302-1438, USA
Tel.: +1- 413- 772- 2166 Fax: +1- 413- 772- 6729 www.bete.com
BEX, Inc., Spray Nozzles 37709 Schoolcraft Rd. Livonia, MI 48150-1009, USA
Tel.: +1- 734- 464- 8282 Fax: +1- 734- 464- 1988 www.bex.com
HIROX Co., Ltd. 2-15-17 Koenij-Minami Suginami-ku Tokyo 166-0003, Japan
Tel.: +81- 3- 3311- 9911 Fax: +81- 3- 3311- 7722 www.hirox.com
Lechler GmbH & Co KG P.O. Box 1323 D-72544 Metzingen, Germany
Tel.: +49- (0)7123- 962- 0 Fax: +49- (0)7123- 962- 333 www.lechler.com
Lechler, Inc. 445 Kautz Road St. Charles, IL 60174, USA
Tel.: +1- 708- 377- 6611 Fax: +1- 708- 377- 6657 www.lechler.com
Processall, Inc. 10596 Springfield Pike Cincinnati, OH 45215, USA
Tel.: +1- 513- 771- 2266 Fax: +1- 513- 771- 6767 www.processall.com
Schlick-Du¨sen GmbH Hutstrasse 4 D-96253 Untersiemau, Germany
Tel.: +49- (0)9565- 9481- 0 Fax: +49- (0)9565- 2870 www.duesen-schlick.de
List of Vendors
Orthos Liquid Systems, Inc. (Schlick) P.O. Box 1267 Bluffton, SC 29910, USA
Tel.: +1- 843- 987- 7200 Fax: +1- 843- 987- 7203 www.orthosnozzles.com
Spray Dynamics 108 Bolte Lane St. Claire, MO 63077, USA
Tel.: +1- 636 - 629- 7366 Fax: +1- 636- 629- 7455 www.spraydynamics.com
Spraying Systems Co. P.O. Box 7900 Wheaton, IL 60189-7900, USA
Tel.: +1- 630- 665- 5000 Fax: +1- 630- 665- 0432 www.spray.com
Toftejorg GmbH Lademannsbogen 136 D-22339 Hamburg, Germany
Tel.: +49- (0)40- 538- 1012 Fax: +49- (0)40- 538- 1112 www.toftejorg.com
Agglomeration in Suspensions
EIMCO, Div. of Baker Hughes Dillenburger Str. 100 D-51105 Ko¨ln, Germany
Tel.: +49- (0)221- 9856- 0 Fax: +49- (0)221- 9856- 102 www.bakerhughes.com/ baker-process
Baker Hughes Co. (EIMCO) 100 Neponset Street South Walpole, MA 02071, USA
Tel.: +1- 508- 668- 0400 Fax: +1- 508- 668- 6855 www.bakerhughes.com
Pressure Agglomeration Low Pressure Extrusion
Aeromatic-Fielder Div, Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax: +1- 410- 997- 5021 www.niroinc.com
Alexanderwerk AG Kippdorfstr. 6-24 D-42857 Remscheid, Germany
Tel.: +49- (0)2191- 795- 0 Fax: +49- (0)2191- 795- 350 www.alexanderwerk.com
Alexanderwerk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1- 215- 442-0270 Fax: +1- 215- 442- 0271 www.alexanderwerkinc.com
I 13
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Indexes
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, MN 55413, USA
Tel.: +1- 612- 627- 1412 Fax: +1- 612- 627- 1444 www.hosokawamicron.com
Hosokawa BEPEX GmbH (HUTT) Postfach 1152 / Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131- 907-0 Fax: +49- (0)7131- 907-301 www.hosokawamicron.com
Caleva Process Solutions Ltd. Butts Pond Industrial Estate Sturminster Newton, Dorset DT10 1AZ ENGLAND
Tel.: +44- (0)1258- 471122 Fax: +44- (0)1258- 471133 www.caleva.co.uk
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81- 06- 933- 1511 Fax: +81- 06- 933-1531 www.fujipaudal.co.jp
WLS GABLER Maschinenbau KG Nobelstrasse 16 a D- 76275 Ettlingen, Germany
Tel.: +49- (0)7243- 5431- 0 Fax: +49- (0)7243- 5431-54 www.wls-gabler.de
LCI Corporation P.O. Box 16348 Charlotte, NC 28297-8804, USA
Tel.: +1- 704- 394- 9474 Fax: +1- 704- 392- 8507 www.lcicorp.com
Spheronizing
Aeromatic-Fielder Div, Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax: +1- 410- 997- 5021 www.niroinc.com
Caleva Process Solutions Ltd. Butts Pond Industrial Estate Sturminster Newton, Dorset DT10 1AZ ENGLAND
Tel.: +44- (0)1258- 471122 Fax: +44- (0)1258- 471133 www.caleva.co.uk
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81- 06- 933- 1511 Fax: +81- 06- 933-1531 www.fujipaudal.co.jp
WLS GABLER Maschinenbau KG Nobelstrasse 16 a D- 76275 Ettlingen, Germany
Tel.: +49- (0)7243- 5431- 0 Fax: +49- (0)7243- 5431-54 www.wls-gabler.de
List of Vendors
LCI Corporation P.O. Box 16348 Charlotte, NC 28297-8804, USA
Tel.: +1- 704- 394- 9474 Fax: +1- 704- 392- 8507 www.lcicorp.com
Medium Pressure Extrusion (Pelleting)
Andritz, Inc. (Sprout-Matador[-Waldron-Bauer]) 35 Sherman Street Muncy, PA 17756, USA
Tel.: +1- 717- 546- 8211 Fax: +1- 717- 546- 8211 www.andritz-na.com
Bu¨hler AG CH-9240 Uzwil SWITZERLAND
Tel.: +41- (0)71- 955- 1111 Fax: +41- (0)71- 955- 3379 www.buhler-group.com
Buhler Inc. 1100 Xenium Lane, Box 9497 Minneapolis, MN 55440, USA
Tel.: +1- 612- 545-1401 Fax: +1- 612- 540- 9296 www.buhler-group.com
California Pellet Mill Co. 1114 E. Wabash Avenue Crawfordsville, IN 47933, USA
Tel.: +1- 765- 362- 2600 Fax: +1- 765- 362- 7551 www.cpmroskamp.com
CPM (California Pellet Mill) Roskamp Champion 2975 Airline Circle Waterloo, IA 50703, USA
Tel.: +1- 319- 232- 8444 Fax: +1- 319- 236- 0481 www.cpmroskamp.com
Amandus Kahl Nachf. Dieselstr. 5 / P.O. Box 1246 D-21465 Reinbek b. Hamburg, Germany In USA see: LCI, Corp. UMT (Paladin), Universal Milling Technology See also Andritz Industriestrasse 15a D-40822 Mettmann, Germany
Tel.: +49- (0)40- 72771- 0 Fax: +49- (0)40- 72771- 100 www.amandus-kahl-group.de
Tel.: +49- (0)2104- 9197- 0 Fax: +49- (0)2104- 12054 www.umt-group.com
High Pressure Extrusion (Ram Presses, Extruders)
Best Press Corp. Northside Industrial Park 102 Crowatan Road Castle Hayne, NC 28429, USA
Tel.: +1- 910- 675- 2429 Fax: +1- 910- 675- 1395 www.best-press.com
I 15
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Indexes
Bu¨hler AG CH-9240 Uzwil Switzerland
Tel.: +41- (0)71- 955- 1111 Fax: +41- (0)71- 955- 3379 www.buhler-group.com
Buhler Inc. 1100 Xenium Lane, Box 9497 Minneapolis, MN 55440, USA
Tel.: +1- 612- 545-1401 Fax: +1- 612- 540- 9296 www.buhler-group.com
The Bonnot Co. 1520 corporate Woods Pkwy. Uniontown, OH 44685, USA
Tel.: +1- 330- 896- 6544 Fax: +1- 330- 896- 0822 www.bonnot.cc
Coperion Corp. (see also Werner & Pfleiderer) 663 East Crescent Avenue Ramsey, NJ 07446 USA
Tel.: +1- 201- 372- 6300 Fax: +1- 201- 825- 6494 www.coperion.com
Entex Rust & Mitschke GmbH Heinrichstrasse 67 D-44805 Bochum, Germany
Tel.: +49- (0)234- 89122- 0 Fax: +49- (0)234- 89122- 99 www.entex-bochum.de
Ha¨ndle GmbH (see also J.C. Steele) Industriestrasse 47 D-75417 Mu¨hlacker, Germany
Tel.: +49- (0)7041- 891- 1 Fax: +49- (0)7041- 891- 232 www.haendle.com
Holzmag Recycling Technology Mu¨hlenmattstrasse 22 CH-4104 Oberwil, Switzerland
Tel.: +41- (0)61- 406- 99- 00 Fax: +41- (0)61- 406- 99-10 www.holzmag.com
Holzmag Briquetting Systems Suite 101-1001 West Broadway Vancouver, BC Canada V6H 4E4
Tel.: +1- 604- 818- 0287 Fax: +1- 604- 874- 7103 www.biquettingsystems.com
ThyssenKrupp Fo¨rdertechnik GmbH Altendorfer Strasse 120 D-45143 Essen, Germany
Tel.: +49- (0)201- 828- 04 Fax: +49- (0)201- 828- 2566 www. Krupp-foerdertechnik.com
Lihotzky Extrusion Finsinger Str. 1 D-94526 Metten, Germany
Tel.: +49- (0)991- 9107- 0 Fax: +49- (0)991- 9107- 153 www.lihotzky.de
Metso Lindemann GmbH Erkrather Strasse 401 D-40231 Du¨sseldorf, Germany
Tel.: +49- (0)211- 2105- 0 Fax: +49- (0)211- 2105- 376 www.metsominerals.com
List of Vendors
List AG CH-4422 Arisdorf SWITZERLAND
Tel.: +41- (0)61- 815- 3000 Fax: +41- (0)61- 815- 3001 www.listgrp.com
List, Inc. 42 Nagog Park Action, MA 01720, USA
Tel.: +1- 978- 635- 9521 Fax: +1- 978- 263- 0570 www.listgrp.com
Readco Manufacturing, Inc. 901 S. Richland Avenue York, PA 17405-0552, USA
Tel.: +1- 717- 848- 2801 Fax: +1- 717- 848- 2811 www.readco.com
Rieter Automatik GmbH Ostring 19 D-63762 Grossostheim, Germany
Tel.: +49- (0)6026- 503- 0 Fax: +49- (0)6026- 503- 109 www.rieter-automatik.de
S&G Enterprises, Inc. N115 W19000 Edison Drive Germantown, WI 53022-3024, USA
Tel.: +1- 414- 251- 8300 Fax: +1- 414- 251- 1616 www.ramflat.com
Sela Maschinen GmbH Am Glu¨sig 3 D-39365 Harbke, Germany
Tel.: +49- (0)39- 406- 669- 0 Fax: +49- (0)39- 406- 669- 10 www.sela-gmbh.com
Spa¨nex BHSU Luft- und Umwelttechnik GmbH Otto-Brenner-Strasse 6 D-37170 Uslar, Germany
Tel.: +49- (0)5571- 304- 0 Fax: +49- (0)5571- 304- 111 [email protected]
J.C. Steele & Sons, Inc. (see also Ha¨ndle) 715 S. Mulberry Street, Box 951 Statesville, NC 28677, USA
Tel.: +1- 704- 872- 3681 Fax: +1- 704- 878- 0789 www.jcsteele.com
Coperion Werner & Pfleiderer GmbH & Co. KG Theodorstrasse 10 Tel.: +49- (0)711- 897- 0 D-70469 Stuttgart Fax: +49- (0)711- 897- 3999 Germany www.coperion.com ZEMAG GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441- 880- 0 Fax: +49- (0)3441- 212993 www.zemag.com
I 17
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Indexes
High Pressure Agglomeration (Punch-and-Die, Tabletting, Isostatic, Tooling)
AIP American Isostatic Presses, Inc. 1205 South Columbus Airport Road Columbus, OH 43207
Tel.: +1- 614- 497- 3148 Fax: +1- 614- 497- 3407 www.aiphip.com
Best Press Corp. 102 Crowatan Road, Northside Industrial Park Castle Hayne, NC 28429, USA
Tel.: +1- 910- 675- 2429 Fax: +1- 910- 675- 1395 www.best-press.com
J. Bonals S/A Oliverar 6 E-08940 Cornella de Llobregat Barcelona, Spain
Tel.: +34- 93- 471- 4580 Fax: +34- 93- 376- 9712 www.jbonals.es
CAPPlus Technologies, Inc 21622 N. 7th Avenue #7 Phoenix, AZ 85027, USA
Tel.: +1- 623- 582- 2800 Fax: +1- 623- 582- 4099 www.capplustech.com
Carver Inc. 1569 Morris Street Wabash, IN 46992-0544, USA
Tel.: +1- 219- 563- 7577 Fax: +1- 219- 563- 7625 www,carver-inc.com
GEI Courtoy N.V. Bergensesteenweg 186 B-1500 Halle, Belgium
Tel.: +32- (0)2- 3638300 Fax: +32- (0)2- 3560516 www.courtoy.be www.niro-pharma-systems.com
Dorst Maschinen & Anlagenbau GmbH & Co, KG Mittenwalder Strasse 61 D-82431 Kochel a. See, GERMANY
Tel.: +49- (0)8851- 188- 0 Fax: +49- (0)8851- 188- 310 www.dorst.de
Durit Hartmetall Saarbru¨ckerstr. 16 D-42289 Wuppertal, Germany
Tel.: +49- (0)202- 55109- 0 Fax: +49- (0)202- 55109- 25 www.durit.de
Elizabeth Carbide Die Co., Inc. 601 Linden Street, PO Box 95 McKeesport, PA 15135, USA
Tel.: +1- 412- 751- 3000 Fax: +1- 412- 754- 0755 www.eliz.com
Elizabeth Carbide Europe NV Av. du roi Albert 134 B-1082 Bruxelles, Belgium
Tel.: +32- (0)2- 46900- 30 Fax: +32- (0)2- 46900- 15 www.eliz.com
List of Vendors
Elizabeth - Hata International, Inc. 14559 Route 30, 101 Peterson Drive North Huntingdon, PA 15642
Tel.: +1- 412- 829- 7700 Fax: +1- 412- 829- 9345 www.eliz.com
EPSI Engineered Pressure Systems, Inc. 165 Ferry Road Haverhill, MA01835, USA
Tel.: +1- 978- 469- 8280 Fax: +1- 978- 373- 5628 www.epsi-highpressure.com
EPSI Engineered Pressure Systems International NV Walgoed Straat 19 B-9140 Temse Belgium
Tel.: +32- 3- 711- 2464 Fax: +32- 3- 711- 1870 www.epsi.be
Fette GmbH LMT (Leitz Metalworking Techn. Group) Grabauer Strasse 24 D-21484 Schwarzenbek, Germany
Tel.: +49- (0)4151- 12- 0 Fax: +49- (0)4151- 833371 www.fette.com
Fette America, Inc. 400 Forge Way Rockaway, NJ 07866, USA
Tel.: +1- 973- 586- 8722 Fax: +1- 973- 586- 0450 www.fetteamerica.com
Flow Pressure Systems AB SE-721 66 Va¨steras Sweden
Tel.: +46- 21- 32- 7000 Fax: +46- 21- 14- 1817 www.flowae.com
Flow Autoclave Systems, Inc. 3721 Corporate Drive Columbus, OH 43231, USA
Tel.: +1- 614- 891- 2732 Fax: +1- 614- 891- 4568 www.flowae.com
Gasbarre Products, Inc. 590 Division Street Dubois, PA 15801, USA
Tel.: +1- 814- 371- 3015 Fax: +1- 814- 371- 6387 www.gasbarre.com
I. Holland Ltd. Meadow Lane Long Eaton, Nottingham NG10 2GD England
Tel.: +44- (0)115- 972- 6153 Fax: +44- (0)115- 973- 1789 www.iholland.co.uk
Horn & Noack, Pharmatechnik ROMACO GmbH Am Heegwald 11 D-76229 Karlsruhe, Germany
Tel.: +49- (0)721- 4804- 0 Fax: +49- (0)721- 4804- 211 www.Romaconet.com
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Indexes
I.M.A. SPA Via Emilia 428-442 I-40064 Ozzano Emilia (BO), Italy
Tel.: +39- (0)51- 651- 4111 Fax: +39- (0)51- 651- 4666 www.ima.it
Key International, Inc. 480 Route 9 Englishtown, NJ 07726, USA
Tel.: +1- 732- 536- 9700 Fax: +1- 732- 972- 2630 www.keyinternational.com
Kilian & Co., GmbH Emdener Str. 10 D-50735 Ko¨ln, Germany
Tel.: +49- (0)221- 7174- 02 Fax: +49- (0)221- 7174- 110 www.kilian.lion.de
Kilian & Co, Inc. 211 Sinclair Street Bristol, PA 19007, USA
Tel.: +1- 215- 826- 8500 Fax: +1- 215- 826- 0400 www.ima.it
Kikusui Seisakusho Ltd. 104, Minamikamiai-cho Nishinokyo, Nakagyo-ku Kyoto, 604, Japan
Tel.: +81- (0)75- 841- 6326 Fax: +81- (0)75- 803- 2077 www.kikusui.com
KOMAGE Gellner GmbH & Co. Maschinenfabrik KG Dr. Hermann-Gellner Strasse 1 D-54427 Kell am See, Germany
Tel.: +49- (0)6589- 9142- 0 Fax: +49- (0)6589- 9142- 19 www.komage.de
Korsch AG Breitenbachstrasse 1 D-13509 Berlin, Germany
Tel.: +49- (0)30- 43576- 0 Fax: +49- (0)30- 43576- 350 www.korsch.de
Korsch America, Inc. 18 Bristol Drive South Easton, MA 02375, USA
Tel.: +1- 508- 238- 9080 Fax: +1- 508- 238- 9487 www.korschamerica.com
ThyssenKrupp Fo¨rdertechnik GmbH Altendorfer Strasse 120 D-45143 Essen, Germany
Tel.: +49- (0)201- 828- 04 Fax: +49- (0)201- 828- 2566 www.krupp-foerdertechnik.com
Laeis Bucher GmbH Schiffstrasse 3 D-54293 Trier, GERMANY
Tel.: +49- (0)651- 9492-0 Fax: +49- (0)651- 9492- 200 www.laeis-bucher.com
Metso Lindemann GmbH Erkrather Strasse 401 D-40231 Du¨sseldorf, GERMANY
Tel.: +49- (0)211- 2105- 0 Fax: +49- (0)211- 2105- 376 www.metsominerals.com
List of Vendors
LuxPharmTec Bischoff & Munneke GmbH Frauenthal 8 D-20149 Hamburg, Germany
Tel.: +49- (0)40- 471100- 72 Fax: +49- (0)40- 471100- 99 www.bmg-hh.de www.luxpharmtec.de
Manesty An IWKA Company Kitling Road, Knowsley Merseyside, England L34 9JS
Tel.: +44- (0)151- 547- 8000 Fax: +44- (0)151- 547- 8001 www.manesty.com
Pentronix, Inc. (PTX) 1737 Cicotte Lincoln Park, MI 48146, USA
Tel.: +1- 313- 388- 3100 Fax: +1- 313- 388- 9171 www.ptx.com
PMC (Pharma Maschinen-Center) Buchenweg 3 D-89284 Pfaffenhofen, Germany
Tel.: +49- (0)7302- 92177- 0 Fax: +49- (0)7302- 92177- 1 www.pmc-service.de
Pneumafill P.O. Box 16348 Charlotte, NC 28297-8804
Tel.: +1- 704- 399- 7441 Fax: +1- 704- 393- 2758 www.pneumafil.com
Riva S.A. Libertador San Martin 431 1702 Ciudadela, Pcia Buenos Aires, ARGENTINA
Tel.: +54- 114- 653- 2000 Fax: +54- 114- 653- 3100 www.rivasa.com
Ruf GmbH & Co KG Tussenhausener Str. 6 D-86874 Zaisertshofen, Germany
Tel.: +49- (0)8268- 9090- 0 Fax: +49- (0)8268- 9090-90 www.brikettieren.de
S&G Enterprises, Inc. N115 W19000 Edison Dr. Germantown, WI 53022, USA
Tel.: +1- 262- 251- 8300 Fax: +1- 262- 251- 1616 www.ramflat.com
SAMA Machinenbau GmbH Schillerstrasse 21 D-95136 Weissenstadt GERMANY
Tel.: +49- (0)9253- 8890 Fax: +49- (0)9253- 1079 www.sama-online.com
Sejong Machinery Co., Ltd. 159-11 Dodang Dong, Wonmi-Gu Buchun-City, Kyunggi-Do Korea 421-130
Tel.: +82- 32- 672- 781- 1/2 Fax: +82- 32- 672- 781- 3 www.sejong.com
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Indexes
Sejong Pharmatech Co., Ltd. 409-3, Chungchun-Dong Bupyong-Gu, Incheon City Korea 403-030
Tel.: +82- 32- 508- 128- 0 Fax: +82- 32- 508- 128- 9 www.sejong-trading.com
SFOB Pharma 23,27 Rue Branly F-77465 Lagny-sur-Marne Cedex
Tel.: +33- 1- 641267- 00 Fax: +33- 1- 641267- 01 [email protected]
DT Industries, Stokes Division 1500 Grundy’s Lane Bristol, PA 19007, USA
Tel.: +1- 215- 788- 3500 Fax: +1- 215- 781- 1122 www.stokesdti.com
Paul-Otto Weber, Maschinen-Apparatebau GmbH Fuhrbachstr. 4-6 Tel.: +49- (0)7151- 75033- 0 D-73630 Remshalden Fax: +49- (0)7151- 75033- 22 GERMANY www.p-o-weber.de
High Pressure Agglomeration (Roll)
Alexanderwerk AG Kippdorfstr. 6-24 D-42857 Remscheid, Germany
Tel.: +49- (0)2191- 795- 0 Fax: +49- (0)2191- 795- 350 www.alexanderwerk.com
Alexanderwerk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1- 215- 442-0270 Fax: +1- 215- 442- 0271 www.alexanderwerk.com
Hosokawa BEPEX GmbH (HUTT) Postfach 1152 / Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131- 907-0 Fax: +49- (0)7131- 907-301 www.hosokawamicron.com
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, MN 55413, USA
Tel.: +1- 612- 627- 1412 Fax: +1- 612- 627- 1444 www.hosokawamicron.com
J. Bonals S/A Oliverar 6 E-08940 Cornella de Llobregat Barcelona, Spain
Tel.: +34- 93- 471- 4580 Fax: +34- 93- 376- 9712 www.jbonals.es
The Fitzpatrick Company 832 Industrial Drive Elmhurst, IL 60126, USA
Tel.: +1- 630- 530- 3333 Fax: +1- 630- 530- 0832 www.fitzmill.com
List of Vendors
Fitzpatrick Company Europe N.V. Entrepotstraat 8 B-9100 Sint Niklaas, Belgium
Tel.: +32- 3- 777- 7208 Fax: +32- 3- 766- 1084 www.fitzpatrick.be
Gerteis Maschinen- + Processengineering AG Stampfstrasse 74 CH-8645 Jona, Switzerland
Tel.: +41- (0)55- 212- 1121 Fax: +41- (0)55- 212- 1140 [email protected]
K.R. Komarek, Inc 1825 Estes Avenue Elk Grove Village, IL 60007, USA
Tel.: +1- 847- 956- 0060 Fax: +1- 847- 956- 0157 www.komarek.com
Maschinenfabrik KPPERN GmbH & Co. KG Ko¨nigsteinerstr. 2-12 D-45529 Hattingen/Ruhr GERMANY
Tel.: +49- (0)2324- 297-0 Fax: +49- (0)2324- 207- 207 www.koeppern.com
Koppern Equipment, Inc. 2201 Water Ridge Parkway Charlotte, NC 28217, USA
Tel.: +1- 704- 357- 3322 Fax: +1- 704- 357- 3350 www.koeppern.com
Ludman Machine Co., LLC. S. 82 W. 18664 Gemini Dr. Muskego, WI 53150, USA
Tel.: +1- 262- 679- 3120 Fax: +1- 262- 679- 9272 www.ludman.net
Matsubo Co., Ltd. 8-21 Toranomon 3-chome Minato-Ku, Tokyo, 105-0001 JAPAN
Tel.: +81- 3- 5472- 1733 Fax: +81- 3- 5472- 1730 www.matsubo.co.jp
Powtec Maschinen und Engineering GmbH Berghauserstr. 62 D-42859 Remscheid, Germany
Tel.: +49- (0)2191- 389- 194 Fax: +49- (0)2191- 389- 196 www.powtec.de
Prater Industries, Inc. 2 Sammons Court Bolingbrook, IL 60440, USA
Tel.: +1- 630- 759- 9595 Fax: +1- 630- 759- 6099 www.praterindustries.com
Riva S.A. Libertador San Martin 431 1702 Ciudadela, Pcia. Buenos Aires, Argentina
Tel.: +54- 114- 653- 2000 Fax: +54- 114- 653- 3100 www.rivasa.com
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Indexes
Sahut Conreur S.A. 700 Rue Corbeau, BP 49 F- 59590 RAISMES - France
Tel.: +33- 3- 27- 46 90 44 Fax: +33- 3- 27- 29 97 65 www.sahutconreur.com
Turbo Kogyo Co., Ltd. 2-10, Uchikawa 1-chome Yokosuka-Shi, Kanagawa, 239-0836 Japan
Tel.: +81- (0)468- 36- 4900 Fax: +81- (0)468- 35- 6516 www.matsubo.co.jp
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax: +1- 319- 377- 5574 www.vectorcorporation.com
ZEMAG-01 GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441- 880- 0 Fax: +49- (0)3441- 212993 www.zemag.com
Sintering
Deltech, Inc. 750 W. 39th Ave. Denver, CO 80216, USA
Tel.: +1- 303- 433- 5939 Fax: +1- 303- 433- 2809 [email protected]
Eisenmann Maschinenbau KG Postfach 1280 D-71002 Bo¨blingen, GERMANY
Tel.: +49- (0)7031- 78- 0 Fax: +49- (0)7031- 78- 1000 www.eisenmann.com
Eisenmann Corp. USA 150 East Dartmoor Drive Crystal Lake, IL 60014, USA
Tel.: +1- 815- 455- 4100 Fax: +1- 815- 455- 1018 www.eisenmann.com
Fuller Company Member of the F.L. Smidth-Fuller Engineering Group 2040 Avenue C Bethlehem, PA 18017-2188, USA
Tel.: +1- 610- 264- 6011 Fax: +1- 610- 264- 6170 www.fullerco.com
Gasbarre Products, Inc. 590 Division Street St. Marys, PA 15857, USA
Tel.: +1- 814- 371- 3015 Fax: +1- 814- 371- 6387 www.gasbarre.com
Gasbarre Sinterite Furnace Div. 310 State Road St. Marys, PA 15857, USA
Tel.: +1- 814- 834- 2200 Fax: +1- 814- 834- 9335 www.gasbarre.com
List of Vendors
L&L Special Furnace Co., Inc. 20 Kent Road Aston, PA 19014-1494
Tel.: +1- 610- 459- 9216 Fax: +1- 610- 459- 3689 www.hotkilns.com
Lurgi Metallurgie GmbH An Outokumpu Technology Comp. Ludwig-Erhard-Str. 21 D-61408 Oberursel, Germany
Tel.: +49- (0)6171- 9693- 0 Fax: +49- (0)6171- 9693- 123 www.outokumpu.com
Nabertherm Bahnhofstr. 20 D-28865 Lilienthal/Bremen, Germany
Tel.: +49- (0)4298- 922- 0 Fax: +49- (0)4298- 922- 129 www.nabertherm.de
SACMI Forni Via dell’Artigianato, 10 I-42010 Salvaterra di Casalgrande (RE)
Tel.: +39- (0)522- 997011 Fax: +39- (0) 522- 840875 www.sacmi.it
HED International, Unique/Pereny 449 Route 31 Ringoes, NJ 08551, USA
Tel.: +1- 609- 466- 1900 Fax: +1- 609- 466- 3608 www.hed.com
Coating
Acosystem Prozesstechnologie GmbH Klus 5 D-31073 Delligsen, Germany
Tel.: +49- (0)5187- 9440- 0 Fax: +49- (0)5187- 9565-30 www.acosystem.de
Aeromatic-Fielder Div, Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax: +1- 410- 997- 5021 www.niroinc.com
AVEKA, Inc. 2045 Wooddale Drive, Bldg.553-C Woodbury, MN 55125, USA
Tel.: +1- 651- 730- 1729 Fax: +1- 651- 730- 1826 www.aveka.com
BRACE GmbH Taunusring 50 D-63755 Alzenau, Germany
Tel.: +49- (0)6023- 32317 Fax: +49- (0)6023- 4973 www.brace.de
Dinnissen bv Horsterweg 66 NL-5975 NB Sevenum The Netherlands
Tel.: +31- 77- 467- 3555 Fax: +31- 77- 467- 3785 www.dinnissen.nl
I 25
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Indexes
DRIAM Anlagenbau GmbH Aspenweg 19-21 D-88097 Eriskirch/Bodensee, Germany
Tel.: +49- (0)7541- 9703- 0 Fax: +49- (0)7541- 9703- 10 www.driam.com
Fluid Air, Inc. 2550 White Oak Circle Aurora, IL 60504-9678, USA
Tel.: +1- 630- 851- 1200 Fax: +1- 630- 851- 1244 www.fluidairinc.com
GS Coating Systems Via Friuli 38/40 I-40060 Osteria Grande (Bolognia) Italy
Tel.: +39- (0)51- 94- 6608 Fax: +39- (0)51- 94- 5624
Hu¨ttlin GmbH An IWK Company Daimlerstr. 7 D-79585 Steinen, Germany
Tel.: +49- (0)7627- 9117- 0 Fax: +49- (0)7627- 8851 www.huettlin.de
I.M.A. SPA Tel.: +39- (0)51- 651- 4111 Via Emilia 428-442 I-40064 Ozzano Emilia (BO), Italy
Fax: +39- (0)51- 651- 4666 www.ima.it
Kilian I.M.A. Verpackungssysteme Emdener Str. 10 D-50735 Ko¨ln, Germany
Tel.: +49- (0)221- 7174- 500 Fax: +49- (0)221- 7174- 501 www.kilian-tabletpresses.com
Kilian & Co., Inc. IMA Solid Dose Div. 415 Sargon Way, Unit 1 Horsham, PA 19044, USA
Tel.: +1- 215- 957- 1871 Fax: +1- 215- 957- 1874 www.kilian-tabletpresses.com
Kaltenbach-Thu¨ring 9. rue de l’Industrie F-60000 Beauvais, FRANCE
Tel.: +33- 44- 02- 8900 Fax: +33- 44- 02- 8910 www.afa.com.eg/Kaltenbach
LMC (Latini) International 893 Industrial Drive Elmhurst, IL 60126, USA
Tel.: +1- 630- 834- 7789 Fax: +1- 630- 834- 9473
Manesty An IWKA Company Kitling Road, Knowsley Merseyside, England L34 9JS
Tel.: +44- (0)151- 547- 8000 Fax: +44- (0)151- 547- 8001 www.manesty.com
List of Vendors
O’Hara Technologies, Inc. 20 Kinnear Ct. Richmond Hill, L4B 1K8, Ont., Canada
Tel.: +1- 905- 707- 3286 Fax: +1- 905- 763- 6749 www.oharatech.com
Sandvik Process Systems, Inc. 21 Campus Road Totowa, NJ 07512, USA
Tel.: +1- 201- 812- 1066 Fax: +1- 201- 812- 0733 www.processsytems.sandvik.com
Sandvik Process Systems GmbH Salierstr. 35 D-70736 Fellbach, Germany
Tel.: +49- (0)711- 5105- 0 Fax: +49- (0)711-5105- 196 www.sandvik.com
Sejong Pharmatech Co., Ltd. 409-3, Chungchun-Dong Bupyong-Gu, Incheon City Korea 403-030
Tel.: +82- 32- 508- 128- 0 Fax: +82- 32- 508- 128- 9 www.sejong-trading.com
SFOB Pharma 23,27 rue Branly, BP 506 F-77465 Lagny-sur-Marne Cedex France
Tel.: +33- 1- 6412- 6700 Fax: +33- 1- 6412- 6701 [email protected]
Friedhelm Stechel GmbH Am Ku¨lf 27 D-31061 Alfeld/Leine (OT Dehnsen) Germany
Tel.: +49- (0)5181- 6465 Fax: +49- (0)5181- 5980 www.stechel-coatingsystems.de
Thomas Engineering, Inc. 575 West Central Road Hoffmann Estates, IL 60195-0198, USA
Tel.: +1- 847- 358- 5800 Fax: +1- 847- 358- 5817
Trybuhl Dragiertechnik GmbH Obere Torstrasse 20 D-37586 Dassel-Markoldendorf Germany
Tel.: +49- (0)5562- 91101 Fax: +49- (0)5562- 91127
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax: +1- 319- 377- 5574 www.vectorcorporation.com
Melt Solidification
Berndorf Band Gesm Leobersdorferstrasse 26 A-2560 Berndorf, Austria
Tel.: +43- (0)2672- 800- 0 Fax: +43- (0)2672- 84176 www.berndorf-band.at
I 27
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Indexes
Berndorf ICB, Inc. 820 Estes Ave. Schaumburg, IL 60193, USA
Tel.: +1- 847- 891- 8650 Fax: +1- 847- 891- 7563
BRACE GmbH Taunusring 50 D-63755 Alzenau, Germany
Tel.: +49- (0)6023- 32317 Fax: +49- (0)6023- 4973 www.brace.de
Gala Undustries, Inc. 181 Pauley Street Eagle Rock, VA 24085, USA
Tel.: +1- 540- 884- 3160 Fax: +1- 540- 884- 2310
Goudsche Machinefabriek B.V. Coenecoop 88 NL-2740 AJ Waddinxveen The Netherlands
Tel.: +31- 182- 623723 Fax: +31- 182- 619217 www.gmfgouda.nl
Gebr. Kaiser, Chem Verfahrenstechnik Magdeburger Strasse 17 D-47800 Krefeld, Germany
Tel.: +49- (0)2151- 474051 Fax: +49- (0)2151- 474053
Kaltenbach-Thu¨ring 9. rue de l’Industrie F-60000 Beauvais, France
Tel.: +33- 44- 02- 8900 Fax: +33- 44- 02- 8910 www.afa.com.eg/Kaltenbach
Sandvik Process Systems, Inc. 21 Campus Road Totowa, NJ 07512, USA
Tel.: +1- 201- 812- 1066 Fax: +1- 201- 812- 0733 www.processsystems. sandvik.com
Sandvik Process Systems GmbH Salierstr. 35 D-70736 Fellbach, Germany
Tel.: +49- (0)711- 5105- 0 Fax: +49- (0)711-5105- 196 www.sandvik.com
Binders
Allied Colloids Cleckheaton Road Low Moor, Bradford West Yorkshire BD12 0JZ, UK
Tel.: +44- (0)124- 41700 Fax: +44- (0)124- 606499
Black Hills Bentonite, LLC Box 9 Mills, WY 82644, USA
Tel.: +1- 307- 265- 3740 Fax: +1- 307- 265- 8511 www.bhbentonite com
List of Vendors
Borregaard LignoTech P.O. Box 162 N-1701 Sarpsborg, Norway
Tel.: +47- (0)6911- 8000 Fax: +47- (0)6911- 8790 www.borregaard.com
Borregaard LignoTech USA 100 Grand Avenue Rothschild, WI 54474-1198, USA
Tel.: +1- 908- 429- 6660 Fax: +1- 908- 429- 1112 www.ltus.com
CABOT Corp., Cab-O-Sil Division 700 E. US Highway 36 Tuscola, IL 61953-9643, USA
Tel.: +1- 217- 253- 9643 Fax: +1- 217- 253- 4334 www.cabot-corp.com
Invista (Elveron) P.O. Box 401 Wilton, Middlesborough TS6 8JJ England
Tel.: +44- (0)1642- 445- 400 Fax: +44- (0)1642- 445- 440 www.Invista.com
FMC Corp., Pharmaceutical Division 1735 Market Street Philadelphia, PA 19103, USA
Tel.: +1- 215- 299- 6534 Fax: +1- 215- 299- 6821 www.fmc.com
GPC, Grain Processing Corp. 1600 Oregon Street Muscatine, IA 52761, USA
Tel.: +1- 319- 264- 4265 Fax: +1- 319- 264- 4289
Green Wood Canada, Inc. 239 Russel Street, P.O. Box 2559 Sturgeon Falls, Ontario P0H 2G0 CANADA
Tel.: +1- 705- 753- 2822 Fax: +1- 705- 753- 1270
Hoogovens Technical Services Wenkebachstraat 1 1951 JZ Velsen Noord, P.O. Box 10.000 1970 CA Ijmuiden, THE NETHERLANDS
Tel.: +31- (0)2514- 97847 Fax: +31- (0)2514- 70030
Koch Minerals Company P.O. Box 2219 Wichita, KS 67201-2219, USA
Tel.: +1- 316- 832- 6662 Fax: +1- 316- 832- 8028
Penwest Pharmaceuticals, Mendell 2981 Rt. 22 Patterson, NY 12563-9970, USA
Tel.: +1- 914- 878- 3414 Fax: +1- 914- 878- 3484
I 29
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Indexes
J. Rettenmaier & So¨hne GmbH & Co. Faserstoff-Werke Holzmu¨hle 1 D-73494 Rosenberg, Germany
Tel.: +49- (0)7967- 152- 0 Fax: +49- (0)7967- 152- 222 www.jrs.de
J. Rettenmaier USA LP Manufacturers of Fibers 16369 US Hwy. 131 Schoolcraft, MI 49087, USA
Tel.: +1- 269 (or 616)- 679- 2340 Fax: +1- 269 (or 616)- 679- 2364 www.jrsusa.com
Reed Lignin, Inc. Highway 51 South Rothschild, WI 54474-1198, USA RDE, Inc. 101 N. Virginia St. Crystal Lake, IL 60014, USA RIBTEC Ribbon Technology Corp. P.O. Box 30758 Gahanna, OH 43230, USA
Tel.: +1- 815- 459- 0470 Fax: +1- 815- 439- 8043
Tel.: +1- 614- 864- 5444 Fax: +1- 614- 864- 5305
Schuurmans & van Ginneken Keizersgracht 534 NL - 1017 EK Amsterdam, The Netherlands
Tel.: +31- 20- (0)626- 0711
Wyo Ben, Inc. 3044 Hesper Road, P.O. Box 1979 Billings, Montana 59103, USA
Tel.: +1- 406- 652- 6351 Fax: +1- 406- 656- 0748 www.wyoben.com
Test Equipment and Peripherals
AC Compacting LLC 1577 Livingston Ave. North Brunswick, NJ 08902-7266, USA
Tel.: +1- 732- 249- 6900 Fax: +1- 732- 249- 6909
Schenck Accurate Corp. 746 E. Milwaukee St. Whitewater, WI 53190, USA
Tel.: +1- 800- 558- 0184 Fax: +1- 414- 473- 2489
Aeromatic-Fielder Div. Niro Inc. 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax: +1- 410- 997- 5021
List of Vendors
Alexanderwerk AG Kippdorfstr. 6-24 D-42857 Remscheid, Germany
Tel.: +49- (0)2191- 795- 216 Fax: +49- (0)2191- 795- 350
Alexanderwerk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1- 215- 442- 0270 Fax: +1- 215- 442- 0271
API Amherst Instruments, Inc. Mountain Farms Technology Park Hadley, MA 01035-9547, USA
Tel.: +1- 413- 586- 2744 Fax: +1- 413- 585- 0536
Babcock-BSH GmbH August-Gottlieb-Strasse 5 D-36222 Bad Hersfeld, Germany
Tel.: +49- (0)6621- 81449 Fax: +49- (0)6621- 81393
Hosokawa BEPEX GmbH (HUTT) Postfach 1152 / Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131- 907-0 Fax: +49- (0)7131- 907-301
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, MN 55413, USA
Tel.: +1- 612- 627- 1412 Fax: +1- 612- 627- 1444
Brabender Technology 6500 Kestrel Road Mississauga, Ont. L5T 1Z6, Canada
Tel.: +1- 905- 670- 2933 Fax: +1- 905- 670- 2557
Bristol Equipment Co. 210 Beaver Street Yorkville, IL 60560-0696, USA
Tel.: +1- 630- 553- 7161 Fax: +1- 630- 553- 5981
Bu¨hler AG CH-9240 Uzwil Switzerland
Tel.: +41- (0)71- 955- 1111 Fax: +41- (0)71- 955- 3379
Buhler Inc. Box 9497 Minneapolis, MN 55440, USA
Tel.: +1- 612- 545-1401 Fax: +1- 612- 540- 9296
Carier Vibrating Equipment, Inc. Box 37070 Louisville, KY 40233, USA
Tel.: +1- 502- 969- 3171 Fax: +1- 502- 969- 3172
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Chatillon Products, Ametek, Inc. 8600 Somerset Drive Largo, FL 33773, USA
Tel.: +1- 813- 536- 7831 Fax: +1- 813- 539- 6882
Carver, Inc. 1569 Morris Street Wabash, IN 46992-0544, USA
Tel.: +1- 219- 563- 7577 Fax: +1- 219-563- 7625
Derrick Manufacturing Corp. 590 Duke Road Buffalo, NY 14225, USA
Tel.: +1- 716- 683- 4991 Fax: +1- 716- 683- 4991
Despatch Industries P.O. Box 1320 Minneapolis, MN 55440-1320, USA
Tel.: +1- 612- 781- 5363 Fax: +1- 612- 781- 5353
Dings Magnetic Group 4740 W. Electric Avenue Milwaukee, WI 53219-9990, USA
Tel.: +1- 414- 672- 7830 Fax: +1- 414- 672- 5354
EI, Eastern Instruments 416 Landmark Drive Wilmington, NC 28412, USA
Tel.: +1- 910- 392- 2490 Fax: +1- 910- 392- 2123
Eriez Magnetics 2200 Asbury Road Erie, PA 16508, USA
Tel.: +1- 814- 835- 6000 Fax: +1- 814- 838- 4960
Erweka GmbH Ottostr. 20-22 D-63150 Heusenstamm, Germany
Tel.: +49- (0)6104- 6903- 0 Fax: +49- (0)6104- 6903-40
Erweka Instrument, Inc. 56 Quirk Rd. Milford, CT 06460, USA Flexicon Corp. 1375 Stryker’s Road Phillipsburg, NJ 08865-5269, USA Flexicon Europe Ltd 89 Lower Herne Road Herne, Herne Bay, Kent CT6 7PH, UK
Tel.: +1- 203- 877- 8477 Fax: +1- 203- 874- 1179 Tel.: +1- 908- 859- 4700 Fax: +1- 908- 859- 4826
Tel.: +44- (0)1227- 374710 Fax: +44- (0)1227- 365821
List of Vendors
Flottweg GmbH (Member of the KRAUSSMAFFEI Group) Industriestr. 6-8 D-84137 Vilsbiburg, Germany
Tel.: +49- (0)8741- 301- 0 Fax: +49- (0)8741- 301- 300
Gerteis Maschinen- + Processengineering AG Stampfstrasse 74 CH-8645 Jona, Switzerland
Tel.: +41- (0)55- 212- 1121 Fax: +41- (0)55- 212- 1140
T.J. Gundlach Machine Co. One Freedom Drive Belleville, IL 62226, USA
Tel.: +1- 618- 233- 7208 Fax: +1- 233- 6154
Gustafson Sampling Systems, Inc. 7290 Golden Triangle Drive Eden Prarie, MN 55344, USA
Tel.: +1- 612- 941- 1630 Fax: +1- 612- 941- 9371
Ha¨gglunds Drives, Inc. 2275 International Street Columbus, OH 43228, USA
Tel.: +1- 614- 527- 7400 Fax: +1- 614- 527- 7401
Hardy Instruments 3860 Calle Fortunada San Diego, CA 92123-1825, USA Herbold Zerkleinerungstechnik GmbH Petersbergstrasse 9 D-74909 Meckesheim, Germany
Tel.: +1- 619- 278- 2900 Fax: +1- 619- 278- 6700 Tel.: +1- (0)6226- 923- 0 Fax: +1- (0)6226- 60455
HiRollerR Enclosed Belt Conveyors 5100 W. 12th Street Sioux Falls, SD 57107-0514, USA
Tel.: +1- 605- 332- 3200 Fax: +1- 605- 332- 1107
IMI Industrial Magnetics, Inc. 1240 M-75 South Boyne City, MI 49712-0080, USA
Tel.: +1- 231- 582- 3100 Fax: +1- 231- 582- 2704
InterSystems Sampling Systems 17330 Preston Road, Suite 105D Dallas, TX 75252, USA
Tel.: +1- 972- 380- 0791 Fax: +1- 972- 250- 4135
Kason Corp. 67-71 E. Willow St. Millburn, NJ 07041-1416, USA
Tel.: +1- 973- 467- 8140 Fax: +1- 973- 258- 9533
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Korsch Pressen GmbH Breitenbachstrasse 1 D-13509 Berlin, Germany
Tel.: +49- (0)30- 43576- 0 Fax: +49- (0)30- 43576- 350
K-Tron America Routes 55 & 553 Pitman, NJ 08071-0888, USA
Tel.: +1- 609- 589- 0500 Fax: +1- 609- 598- 8113
K-Tron Switzerland Industrie Lenzhard CH-7202 Niederlenz. Switzerland
Tel.: +41- 62- 885- 7171 Fax: +41- 62- 891- 6661
Krauss-Maffei Verfahrenstechnik GmbH Krauss-Maffei-Str. 2 D-80997 Mu¨nchen, Germany
Tel.: +49- (0)89- 8899- 0 Fax: +49- (0)89- 8899- 3299
Krauss-Maffei Corp. 7095 Industrial Road Florence, KY 41022-6270, USA
Tel.: +1- 606- 283- 0200
MP Machine and Process Design, Inc. 820 McKinley Street Anoka, MN 55303, USA
Tel.: +1- 763- 427- 9991 Fax: +1- 763- 427- 8777
Mark-10 Corp. 458 West John Street Hicksville, NY 11801, USA
Tel.: +1- 516- 822- 5300 Fax: +1- 516- 822- 5301
Hosokawa Micron Powder Systems 10 Chatham Rd. Tel.: +1- 908- 273- 6360 Summit, NJ 07901, USA
Fax: +1- 908- 273- 7432
Minox Siebtechnik GmbH Interpark D-76877 Offenbach/Queich, Germany
Tel.: +49- (0)6348- 9828- 0 Fax: +49- (0)6348- 4086
Minox/Elcan Industries, Inc. 59 Plain Avenue New Rochelle, NY 10801, USA
Tel.: +1- 914- 235- 0161 Fax: +1- 914- 654- 9835
Modern Process Equipment, Inc. 3125 South Kolin Ave. Chicago, IL 60623, USA
Tel.: +1- 773- 254- 3929 Fax: +1- 773- 254- 3935
List of Vendors
Monitor Manufacturing, Inc. 44W320 Keslinger Road Elburn, IL 60119-8048, USA
Tel.: +1- 630- 365- 9403 Fax: +1- 630- 365- 5646
Natoli Engineering Co., Inc., Tableting Accessories 28 Research Park Circle Tel.: +1- 314- 926- 8900 St. Charles, MO 63304, USA Fax: +1- 314- 926- 8910 Nerak Systems, LP 6 Debbie Lane Cross River, NY 10518, USA
Tel.: +1- 914- 763- 8259 Fax: +1- 914- 763- 9570
Nicolet Instrument Corp. 5225 Verona Road Madison, WI 53711-4495, USA
Tel.: +1- 608- 276- 6100 Fax: +1- 608- 273- 5046
Nordberg Group P.O. Box 307 33101 Tampere, Finland
Tel.: +358- 20- 484- 140 Fax: +358- 20- 484- 141
Nordberg Americas 3073 South Chase Avenue Milwaukee, WI 53207, USA
Tel.: +1- 414- 769- 4300 Fax: +1- 414- 769- 4730
Particle Characterization Measurements Div. of Bus. Sys. International, Inc. 453 Highway 1 West Iowa City, IA 52246, USA Pennsylvania Crusher Co. 600 Abbott Drive Broomall, PA 19008-0100, USA PMI Porous Materials, Inc. Cornell Business & Technology Park 83 Brown Rd. Ithaca, NY 14850, USA Prater Industries, Inc. 2 Sammons Court Bolingbrook, IL 60440, USA Quadro, Inc. 55 Bleeker Street Millburn, NJ 07041-1414, USA
Tel.: +1- 319- 354- 5889 Fax: +1- 319- 354- 0526
Tel.: +1- 610- 544- 7200 Fax: +1- 610- 543- 0190
Tel.: +1- 607- 257- 5544 Fax: +1- 607- 257- 5639
Tel.: +1- 630- 759- 9595 Fax: +1- 630- 759- 6099
Tel.: +1- 973- 376- 1266 Fax: +1- 973- 376- 3363
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Indexes
Quantachrome Corp 1900 Corporate Drive Boynton Beach, FL 33426, USA
Tel.: +1- 561- 731- 4999 Fax: +1- 561- 732- 9888
Quantachrome GmbH Rudolf-Diesel Str. 12 D-85235 Odelzhausen, Germany
Tel.: +49- (0)8- 134- 9324- 0 Fax: +49- (0)8- 134- 9324-25
The Rapat Corp. 919 O’Donnell Street Hawley, MN 56549-4310, USA
Tel.: +1- 218- 483- 3344 Fax: +1- 218- 483- 3535
Rhewum GmbH Rosentalstr. 24 D-42899 Remscheid, Germany
Tel.: +49- (0)2191- 98306- 0 Fax: +49- (0)2191- 51840
Rotex, Inc. 1230 Knowlton Street Cincinnati, OH 45223, USA
Tel.: +1- 513- 541- 1236 Fax: +1- 513- 541- 4888
Russel Finex Ltd. Russel House, Browells Lane Feltham, Middlesex TW13 7EW, UK
Tel.: +44- (0)181- 818- 2000 Fax: +44- (0)181- 818- 2060
Russel Finex, Inc. 10709-A Granite Street Charlotte, NC 28273, USA
Tel.: +1- 704- 588- 9808 Fax: +1- 704- 588- 0738
Carl Schenck AG D-64273 Darmstadt, Germany
Tel.: +49- (0)6151- 32- 0 Fax: +49- (0)6151- 32- 1100
Dr. Schleuniger Pharmatron AG Scho¨ngru¨nstrasse 27 CH-4501 Solothurn, Switzerland
Tel.: +41- (0)32- 624- 4080 Fax: +41- (0)32- 624- 4088
Dr. Schleuniger Pharmatron, Inc. One Sundial Avenue, Suite 214 Manchester, NH 03103, USA
Tel.: +1- 603- 645- 6766 Fax: +1- 603- 645- 6726
Sepor, Inc. P.O. Box 578 Wilmington, CA 90748, USA
Tel.: +1- 310- 830- 6601 Fax: +1- 310- 830- 9336
Shimadzu Scientific Instruments, Inc. 7102 Riverwood Drive Columbia, MD 21046, USA
Tel.: +1- 410- 381- 1227 Fax: +1- 410- 381- 1222
List of Vendors
Simpson Technologies Corp. 751 Shoreline Drive Aurora, IL 60504-6194, USA
Tel.: +1- 630- 978- 0044 Fax: +1- 630- 978- 0068
Simpson Technologies Baarerstrasse 77 CH-6300 Zug, Switzerland
Tel.: +41- (0)41- 711- 1555 Fax: +41- (0)41- 711- 1387
GR Sprenger Engineering, Inc. 736 West Hemlock Circle Louisville, CO 80027, USA
Tel.: +1- 303- 665- 7069 Fax: +1- 303- 665- 5346
S.S.T Schu¨ttguttechnik Lechwiesenstrasse 21 D-86899 Landsberg am Lech, Germany Stedman Machine Co. P.O. Box Aurora, IN 47001, USA Stela Laxhuber KG Maschinenbau, Trocknungstechnik D-84323 Massing/Bayern, Germany
Tel.: +49- (0)8191- 335951 Fax: +49- (0)8191- 335955
Tel.: +1- 812- 926- 0038 Fax: +1- 812- 926- 3482 Tel.: +49- (0)8724- 899- 0 Fax: +49- (0)8724- 899- 80
SVS Sauk Valley Systems, Inc. P.O. Box. 1013 Sterling, IL 61081, USA
Tel.: +1- 815- 625- 5573 Fax: +1- 815- 625- 5593
SWECO 8029 US Hwy 25 Florence, KY 41022-1509, USA
Tel.: +1- 606- 283- 8400 Fax: +1- 606- 283- 8469
Tecnetics Industries, Inc. 1811 Buerkle Road St. Paul, MN 55110, USA
Tel.: +1- 612- 777- 4780 Fax: +1- 612- 777- 5582
W.S. Tyler 8570 Tyler Boulevard Mentor, OH 44060, USA
Tel.: +1- 440- 974- 1047 Fax: +1- 440- 974- 0921
W.S. Tyler Germany Ennigerloher Str. 64 D-59302 Oelde, Germany
Tel.: +49- (0)2522- 30- 0 Fax: +49- (0)2522- 30- 404
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Unitrac Corp. Ltd. Box 330, 299 Ward Street Port Hope, Ontario L1A 3W4, Canada
Tel.: +1- 905- 885- 8168 Fax: +1- 905- 885- 2614
Paul-Otto Weber, Maschinen-Apparatebau GmbH Fuhrbachstr. 4-6 Tel.: +49- (0)7151- 72600 D-73630 Remshalden, Germany Fax: +49- (0)7151- 72509 ZEMAG GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441- 880- 205 Fax: +49- (0)3441- 212993
Applications
Albemarle Corporation 451 Florida Street Baton Rouge, LA 70801-1765 USA
Tel.: +1- 225- 388- 7402 Fax: +1- 225- 388- 7848 www.Albemarle.com
AstraZeneca LP, R&D Box 14 Mo¨lndal SE-431 21 Sweden
Tel.: +46- (0)31- 776- 30- 00 Fax: +46- (0)31- 776- 30- 10 www.astratech.com www.astazeneca.com
Babcock-BSH GmbH August Gottlieb Strasse 5 D-36222 Bad Hersfeld, Germany
Tel.: +49- (0)6621- 81449 Fax: +49- (0)6621- 81393 www.babcock-bsh.de
Reckitt Benckiser Detergents GmbH Ludwig-Bertram-Str. 8+10 D-67059 Ludwigshafen/Rh. Theodor-Heuss-Anlage 12 D-68165 Mannheim Germany
www.reckittbenckiser.de Tel.: +49- (0)621- 3246- 0 Fax: +49- (0)621- 3246- 500 www. reckittbenckiser.com
BHP Billiton Iron Ore Pty. Ltd. (Boodarie Iron Plant, Port Hedland, WA) 200 Georges Terrace Perth, WA 6000, Australia
Tel.: +61- 8- 9320- 4444 Fax: +61- 8- 9320- 4178 www.bhpbilliton.com
Borden Foods Corporation 180 East Broad Street Columbus, OH, USA
Tel.: +1- 614- 225- 4000 www.bordeninc.com
List of Vendors
C.A.R.M.E.N. (Centrales Agrar-Rohstoff-Marketingund Entwicklungs Netzwerk Schulgasse 18 D-94315 Straubing Germany CVRD (Companhia Vale do Rio Doce) Pelletizing and Metallics Unit Av. Dante Micheline, 5.500 Jardim Camburi - Ponta de Tubara~o 29090-900 Vito´ria, Espirito Santo, Brasil
Tel.: +49- (0)9421- 960- 300 Fax: +49- (0)9421- 960- 333 www.carmen-ev.de
Tel.: +55- 27- 335- 5685 Fax: +55- 27- 335- 4802 www.cvrd.com.br
Degussa AG Verfahrenstechnik & Engineering / Project House Nanomaterials / Creavis GmbH Advanced Nanomaterials (ANM) Rodenbacher Chaussee 4 D-63457 Hanau-Wolfgang, Germany
Tel.: +49- (0)6181- 59Fax: +49- (0)6181- 59www.degussa.com, www.creavis.com
DuPont Engineering Polymers Pencader Site – Vespel Newark, DE 19714-6100, USA
Tel.: +1- (800)- 972-7252 Fax: +1- (800)- 477-5790 www.dupont.com/engpolymers/
Dynamit Nobel GmbH Explosivstoff- und Systemtechnik Forschung und Entwicklung Kronacher Strasse 63 D-90765 Fu¨rth, Germany
Tel.: +49- (0)911- 7930- 0 Fax: +49- (0)911- 7930- 655
Fleissner Faserstrassen GmbH & Co Wolfsgartenstr. 6 D-63329 Egelsbach, Germany
Tel.: +49- (0)6103- 401- 0 Fax: +49- (0)6103- 401- 440 www.fleissner.de
Grupo Disagro Anillo Perife´rico 17-36 Zona 11 Guatemala City Guatemala 01011 Ferquigua km 124 Ruta Atlantico Teculutan, Guatemala
Tel.: +502- 473-1453/9 Fax: +502- 473- 2611 www.disagro.com
Fibo ExClay Deutschland GmbH Rahdener Strasse 1 D-21769 Lamstedt, Germany
Tel.: +49- (0)4773- 896- 0 Fax: +49- (0)4774- 896- 133 www.fiboexclay.de
Tel.: +502- 934- 7137; 7007/9; 7216 www.disagro.com/ infraestructura/ferquigua.htm
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The Folger Coffee Co. A subsidiary of Procter & Gamble P.O. Box 5549 Cincinnati, OH 45202-5549, USA
www.folgers.com www.pg.com
Heckett MultiServ International Harsco House, Regent Park 299 Kingston Road Leatherhead, Surrey KT22 7SG, UK
Tel.: +44- (0)1372- 381- 400 Fax: +44- (0)1372- 381- 499 www.heckettmultiserv.com
Heckett MultiServ North America 612 North Main Street PO Box 1071 Butler, PA 16003-1071, USA
Tel.: +1- 724- 283- 5741 Fax: +1- 724- 283- 2410 www.heckettmultiserv.com
HENKEL KGAA Henkelstr. 67 D-40589 Du¨sseldorf Germany
Tel.: +49- (0)211- 797- 0 Fax: +49- (0)211- 798- 2352 www.Henkel.de
HYL Ave. Munich 101 San Nicolas de los Garza, NL 66452 Mexico
Tel.: +52- 81- 8865- 2801 Fax: +52- 81- 8865- 2810 www.hylsamex.com/hyl
Interstar Materials, Inc. 4255 Portland Boulevard Sherbrooke (Quebec) Canada J1L 3AS
Tel.: +1- 819- 563- 9975 Fax: +1- 819- 563- 1317 www.interstar.ca
ISCOR Ltd. PO Box 2 2940 Newcastle, South Africa
Tel.: +27- 34- 314- 7094 Fax: +27- 34- 314- 7325 www.iscor.com
Kellogg Co. W.K. Kellogg Institute for Food and Nutrition Research 2 Hamblin Ave. East Battle Creek, MI 49016-3232, USA
Tel.: +1- 616- 961- 2000 Fax: +1- 616- 660 6557 www.kelloggs.com
Hosokawa Kreuter GmbH Essener Strasse 104-108 D-22419 Hamburg, Germany
Tel.: +49- (0)40- 527- 209- 0 Fax: +49- (0)40- 527- 209- 99 www.hosokawamicron.com
List of Vendors
Merisant US, Inc. 10 South Riverside Plaza, Suite 850 Chicago, IL 60606
Tel.: +1- 312- 840- 6000 www.merisant.com
ThyssenKrupp Fo¨rdertechnik GmbH Altendorfer Strasse 120 D-45143 Essen Germany
Tel.: +49- (0)201- 828- 04 Fax: +49- (0)201- 828- 2566 www.krupp-foerdertechnik.com www.thyssenkrupp.com
Land O’ Lakes, Inc. 1080 Country Road F West Shoreview, MN 55164-0281 USA
Tel.: +1- 800- 618- 6455 Fax: +1- 651- 494- 5040 www.landolakesinc.com
Midrex Technologies, Inc. 2725 Water ridge Parkway, Suite 100 Charlotte, NC 28217, USA
Tel.: +1- 704- 373- 1600 Fax: +1- 704- 373- 1611 www.midrex.com
Norchem Concrete Products, Inc. 985 Seaway Drive Fort Pierce, FL 34949, USA
Tel.: +1- 772- 468- 6110 Fax: +1- 772- 468- 9702 www.norchem.com
NRS, National Recovery Systems 5222 Indianapolis Boulevard East Chicago, IN 46312, USA
Tel.: +1- 219- 397- 0200 Fax: +1- 219- 392- 1419 www.nrs.com
OCUA Ocean County Utilities Authority 501 Hickory Lane, PO Box P Bayville, NJ 08721 USA
Tel.: +1- 732- 269- 4500 Fax: +1- 732- 269- 4173 www.ocua.com
OPCO, Operaciones al sur del Orinoco, C.A. Apartado 497 Zona Postal 8015 Puerto Ordaz, Edo. Bolivar VENEZUELA Orinoco Iron (RDI plant Fior and Iron Plant Finmet) Zona Ind. Matanzas Av. Fuerzas Armandas P.O. Box 120, 56, 120 Puerto Ordaz, Edo. Bolivar, VENEZUELA
Tel.: +58- 286- 303- 511/810 Fax: +58- 286- 225- 722 www.kobelco.co.jp
Tel.: +58- 286- 994- 0520 Fax: +58- 286- 994- 0071 www.orinoco-iron.com
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Philips Lighting B.V. P.O. Box 150 NL-9670 AD Winschoten The Netherlands
Tel.: +31- 5970- 51911 Fax: +31- 5970- 51265 www.lighting.philips.com
POLYSIUS Corp. 180 Interstate North Parkway Suite 300 Atlanta, GA 30339-2194, USA
Tel.: +1- 770- 955- 3660 Fax: +1- 770- 955- 8789 www.polysiusUSA.com
Purafil 2654 Weaver Way Doraville, GA 30340, USA
Tel.: +1- 770- 662- 8545 Fax: +1- 770- 263- 6922 www.purafil.com
International Purafil Europa BV Kerkbuurt 27-29 NL-1511 BB Oostzaan, The Netherlands
Tel.: +31- (0)75- 684- 2486 Fax: +31- (0)75- 684- 2495
Puritan-Bennet Aero Systems Co B/E Aerospace, Inc 10800 Pflumm Road Lenexa, KS 66215, USA
Tel.: +1- 913- 469- 5400 Fax: +1- 913- 469- 8419 www.beaerospace.com
Sintec Keramik GmbH Romantische Strasse 18 D-87642 Buching, Germany
Tel.: +49- (0)8368- 9101- 0 Fax: +49- (0)8368- 9101- 30 www.sintec-keramik.com
Su¨d-Chemie AG Werk Kelheim Su¨d-Chemie-Str. 3 D-93309 Kehlheim, Germany
Tel.: +49- (0)9441- 29507- 0 Fax: +49- (0)9441- 29507- 52 www.sud-chemie.com
TDC Filter Manufacturing, Inc. 1331 S. 55th Court Cicero, IL 60804, USA
Tel.: +1- 708- 863- 4400 Fax: +1- 708- 863- 4472 www.tdcfilter.com
Paul Wurth S.A. 32, rue d’Alsace, B.P. 2233 L-1022 Luxembourg
Tel.: +352- 4970- 1 Fax: +352- 4970- 709 www.paulwurth.com
List of Vendors
Organizations
American Institute of Chemical Engineers (AIChE), see also PTF Three Park Avenue New York, NY 10016-5991, USA
Tel.: +1- 800- 242- 4363 www.aiche.org
Association of Powder Process Industry and Engineering (APPIE) The Society of Powder Technology, Japan (SPTJ) No. 5 Kyoto Building, 181 Kitamachi Tel.: +81- 75- 354- 3581 Rokujo-agaru, Karasuma-dori Fax: +81- 75- 352- 8530 Shimogyo-ku, Kyoto 606-8176 www.iijnet.or.jp/APPIE Japan www.iijnet.or.jp/SPTJ Expanded Shale, Clay & Slate Institute (ESCSI) 2225 Murray Holloday Road Suite102 Salt Lake City, UT 84117, USA
Tel.: +1- 801- 272- 7070 Fax: +1- 801- 272- 3377 www.escsi.org
Institute for Briquetting and Agglomeration (IBA) P.O. Box 297 Tel.: +1- 715- 543- 2750 Manitowish Waters, WI 54545 Fax: +1- 847- 541- 8947 USA www.agglomeration.org Fraunhofer Institut fu¨r Fertigungstechnik und angewandte Materialforschung (IFAM) Wiener Strasse 12 D-28359 Bremen Germany
Tel.: +49- (0)421- 2246- 0 Fax: +49- (0)421- 2246- 300 www.ifam.fraunhofer.de
Fraunhofer Institut fu¨r Umwelt-, Sicherheits-, Energietechnik (UMSICHT) Osterfelder Strasse 3 D-46047 Oberhausen Germany
Tel.: +49- (0)208- 8598- 0 Fax: +49- (0)208- 8598- 1290 www.umsicht.fhg.de
Museum der Brotkultur Ulm (ehemals/formerly Deutsches Brotmuseum) Salzstadelgasse 10 D-89073 Ulm GERMANY
Tel.: +1- 49- (0)731- 69955 Fax: +1- 49- (0)731- 602- 1161 www.museum-brotkultur.de
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National Mining Research Center (NMRC) – Stochinsky Institute of Mining Institute of Solid Fossil Fuels Preparation (IOTT) 411 Oktyabrsky prospekt, Lyubertsy Tel.: +7- (0)95- 554- 8513 4, Moscow oblast 140004 Fax: +7- (0)95- 554- 5247 Russia www.igd.ru National Research Council/NRC-CNRC Dr. Ed Capes/ Dr. Ken Darcovich, Chemical Division/ICPET M 12-15, Montreal Road Ottawa, Ontario K1A 0R6, CANADA
Tel.: +1- 613- 993- 6848 Fax: +1- 613- 941- 2529 www.nrc.ca
Metal Powder Industries Federation (MPIF) 106 College Road East Princeton, NJ 08540-6692 USA
Tel.: +1- 609- 452- 7700 Fax: +1- 609- 987- 8523 www.mpif.org
PennState, The Pennsylvania State University, College of Engineering Materials Characterization, P/M Lab. 118 Research West University Park, PA, 16802-6809 USA
Tel.: +1- 814- 863- 6809 Fax.: +1- 814- 863- 8211 www.psu.edu
Particle Technology Forum (PTF) American Institute of Chemical Engineers (AIChE) 3 Park Avenue Tel.: +1- 212- 591- 8100 New York, NY 10016-5991 Fax: +1- 212- 591- 8888 USA www.aiche.org Society for Mining, Metallurgy, and Exploration, Inc. (SME), (Member American Institute of Mining, Metallurgical, and Petroleum Engineers [AIME]) P.O. Box 277002 Tel.: +1- 303- 973- 9550 Littleton, CO 80127-7002 Fax: +1- 303- 973- 3845 USA www.smenet.org University of Florida (UF-PERC) NSF Engineering Research Center for Particle Science and Technology 205 particle Sci. & Tech. Gainesville, FL 32611 USA
Tel.: +1- 352- 846- 1194 Fax: +1- 352- 846- 1196 www.erc.ufl.edu
List of Vendors
Verfahrenstechnische Gesellschaft (VTG - GVC) Verein Deutscher Ingenieure (VDI) Graf Recke Strasse 84 D-40239 Du¨sseldorf Germany
Tel.: +49- (0)211- 6214- 600 Fax: +49- (0)211- 6214- 169 www.vdi.de
Tollers
(Out-sourcing by Contract Manufacturing, Co-Manufacturing, and “backup” Manufacturing) (Note: In addition to the companies listed below which specifically offer at least some contract manufacturing services that are related to Size Enlargement by Agglomeration, essentially all manufacturers and suppliers/distributors of equipment for the unit operations of Mechanical Process Technology and related industrial and analytical techniques (see Chapter 2, Figure 2.2) maintain a more or less extensive facility and laboratory for testing materials, developing applications, and determining process parameters). Abbott Laboratories Contract Manufacturing Services Tel.: +1- 847- 937- 1009 1401 Sheridan Road Fax: +1- 847- 938- 2875 North Chicago, IL 60064-6321, USA (Pharmaceuticals: Powders and granules, Tablettes, etc) ACD, American Custom Drying, Co. Tel.: +1- 609- 387- 3933 109 Elbow Lane Fax: +1- 609- 387- 7204 Burlington, NJ 08016-4123, USA www.americancustomdrying.com (Spray drying services for the food, fine chemical, and chemical industry, liquid storage ISO 9002 certified, wet mixing and blending, large and small production, pilot test facilities). allphamed PHARBIL, site Go¨ttingen Arzneimittel GmbH Tel.: +49- (0)551- 382-0 Hildebrandstrasse 12 Fax: +49- (0)551- 382- 470 D-37081 Go¨ttingen, Germany (Pharmaceutical contract manufacturing: Powders, wet and dry granules, tablets (film coated, dispersible, effervescent) and associated equipment). The ASC Group Tel.: +1- 217- 834- 3301 309 E. Yates St., Box 200 Fax: +1- 217- 834- 3655 Allertown, IL 61810, USA (Custom (pan) pelletizing, full-scale testing, process design and engineering)
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Indexes
AVEKA, Inc., Headquarters, R&D 2045 Wooddale Dr. (Building 553-C) Woodbury, MN 55125, USA
Tel.: +1- 612- 730- 1729 Fax: +1- 612- 730- 1826 www.aveka.com
AVEKA Manufacturing Large-Scale Manufacturing 279 Woodward Avenue Fredericksburg, IA 50630, USA
Tel.: +1- 319- 237- 5010 Fax: +1- 319- 237- 5056
AVEKA Foods, Food Processing Tel.: +1- 715- 962- 9106 106 Bremer Avenue Fax: +1- 715- 962- 3129 Colfax, WI 54730, USA (R&D, service and small or large scale particle processing. Capabilities include: Agglomeration, blending, classification, compounding, dispersion preparation, granulation, grinding, microencapsulation, particle characterization, particle coating, particle surface modification, prilling, screening, and spray drying) Carolina Pelleting & Extrusion, Inc. Tel.: +1- 828- 695- 8505 1694 Fisher Court Fax: +1- 828- 695- 8508 Newton, NC 28658 www.Carolinapelleting.com (Powder blending/liquid addition, flat die and extruder pelleting, drying and cooling, product classification, raw material and product handling) Catalytica Pharmaceuticals Tel.: +1- 252- 707- 2330 P.O. Box 1887 Fax: +1- 252- 707- 2450 Greenville, NC 27835-1887, USA (Development, scale-up, and large scale manufacturing of complete packaged pharmaceutical products, dosage form includes tablettes and granules). Coating Place, Inc. 200 Paoli St., P.O. Box 930310 Verona, WI 53593, USA (Coating, Encapsulation, Fluid bed processing)
Tel.: +1- 608- 845- 9521 Fax: +1- 608- 845- 9526
Custom Granular, Inc. Tel.: +1- 608- 868- 3838 4846 Hwy. 26 North Fax: +1- 608- 868- 5448 Janesville, WI 53546, USA (Roll compaction, Briquetting, Milling, Particle Classification, Blending) Custom Powders Ltd. Gateway, Crewe Cheshire, CW1 6YT, England
Tel.: +44- 1270- 530020 Fax: +44- 1270- 500250
List of Vendors
Custom Powders BV Tel.: +31- 492- 598598 Grasbeemd 10 Fax: +31- 492- 598591 5705 DG Helmond, The Netherlands (Size enlargement, Size reduction, Particle separation, Dry blending, Liquid addition. Drying (water), Hot melt processes) The Dow Chemical Co. Contract Manufacturing Services Tel.: +1- 800- 447- 4369 100 Larkin Center Fax: +1- 517- 832- 1465 Midland, MI 48674, USA (Development, scale-up, and manufacturing of products from agricultural, pharmaceutical, and intermediates to specialty chemicals) Erie Foods International, Inc. Tel.: +1- 309- 659- 2223 401 Seventh Ave. Fax: +1- 309- 659- 2822 Erie, IL 61250, USA (Developing and manufacturing specialty milk proteins for use in the food and pharmaceutical industries / agglomeration at Rochelle, IL, facility) Fluid Air, Inc. Product Development and Manufacturing Tel.: +1- 630- 851-1200 2550 White Oak Circle Fax: +1- 630- 851- 1244 Aurora, IL 60504-9678, USA www.fluidairinc.com (Fluid bed processors, high shear mixers, size reduction systems, screeners, and analytical methods and instruments). Fuller Company, Research & Development Member F.L. Smidth-Fuller Engng Group Tel.: +1- 610- 266- 5035 2040 Avenue C Fax: +1- 610- 266- 5109 Bethlehem, PA 18017-2188, USA (Crushing/Classification, Material preparation [including drum conditioners/pelletizers, pans, extruders, compaction/granulation], Pyroprocessing [including calcination, high temperature processing, mineral roasting, drying and cooling, reduction], Rotary kilns, Flash calciners/ dryers, physical and chemical laboratories, Bulk material handling, Pneumatic conveying, etc.). GEA Atlantic Pharmaceutical Services, Inc. Tel.: +1- 410- 413- 1000 11200 Gundry Lane Fax: +1- 410- 413- 2000 Owing Mills, MD 21117, USA www.apsoutsource.com (Oral solid dosage forms, from development to manufacturing. Spray drying, spray congealing, high shear granulation, hot melt granulation, particle/pellet coating, extrusion/spheronization, drug layering, tabletting – beads/particles in tablettes, double – triple layer tablettes - immediate release, controlled release, coated, matrix).
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Indexes
GEA NIRO, Inc. (Food & Dairy Industries) Tel.: +1- 715- 386- 9371 1600 O’Keefe Road Fax..:+1- 715- 386- 9376 Hudson, WI 54016, USA (Testing facility and pilot plants for liquid processing, spray drying and agglomeration, evaporation and concentration, product handling and packaging). Howard Industries, Inc. Tel.: +1- 614- 444- 9900 1840 Progress Avenue Fax: +1- 614- 444- 4571 Columbus, OH 43207, USA www.chemical-processing.com (Custom processing includes Blending, Milling, Classification, Agglomeration [with pans, pin mixers, tablet presses, roller presses, extruders] Calcining, Centrifuging, Double arm mixing, Compounding, Flaking, Drying, Packaging) IFP, Inc Tel.: +1- 507- 334- 2730 2125 Airport Drive, Hwy. 21 & I-35 Fax: +1- 507- 334- 7969 Faribault, MN 55021-7798, USA (Contract food processing and packaging, including Spray drying, Agglomeration, Particle coating/encapsulation, Instantizing, Milling, Blending). Inprotec AG Tel.: +49- (0)7634- 5099- 0 Neuer Weg 1 Fax: +49- (0)7634-5099- 29 D-79423 Heitersheim, Germany www.inprotec-ag.de (Innovative production technologies, 3D micro granulate, process development, particle laboratory, large volume production capacity [> 10,000 t]) International Processing Corp. Member of the GLATT Group 1100 Enterprise Drive Tel.: +1- 859- 745- 2200 Winchester, KY 40391-9888, USA Fax: +1- 859- 745- 6636 Glatt Air Techniques, Ramsey, NJ, USA (Blending and granulating, Tabletting, Extrusion and spheronizing, Coating) IPC Process Center GmbH Tel.: +49- (0)351- 2584- 0 Grunaer Weg 26 Fax: +49- (0)351- 2584- 328 D-01277 Dresden, Germany [email protected] (Blending and granulating, Tabletting, Extrusion and spheronizing, Coating) L. Robert Kimball & Associates, Bituminous Coal Research Facility 615 W. Highland Ave. Tel.: +1- 814- 472- 7700 Ebensburg, PA 15931, USA Fax: +1- 814- 472- 7712 (Processing and briquetting of coal) www.lrkimball.com
List of Vendors
K.R. Komarek Briquetting Research, Inc. Tel.: +1- 256- 831- 5741 20 Wm. F. Andrews Drive Fax: +1- 256- 831- 1331 Anniston, AL 36207, USA (Roller press briquetting and compaction/granulation). Materials Processing Technology, Inc. Tel.: +1- 973- 279- 4132 95 Prince Street Fax: +1- 973- 279- 4435 Paterson, NJ 07501, USA (Agglomeration, Coating, Encapsulation, Granulation, Mixing/Blending, Screening/ Classifying). M.I.E. (Marietta Industrial Enterprises, Inc.) Tel.: +1- 740- 373- 2252 Rt. 4, Box 179-1A Fax: +1- 740- 373- 6369 Marrietta, OH 45750, USA (Custom crushing, grinding, milling, screening, and roll briquetting) Metrics, Inc. Tel.: +1- 252- 752- 3800 1240 Sugg Parkway Fax: +1- 252- 757- 2573 Greenville, NC 27834, USA (contract and manufacturing services for the pharmaceutical industry, including encapsulation, fluid bed processing, granulation, mixing/blending, etc.) Boehringer Ingelheim Promeco, S.A. de C.V. Maiz 49, Xochimilco 16090 Me´xico D.F. Me´xico
Tel.: +52- 55- 5629-8300 Fax: +52- 55- 5676- 0905 www.boehringer-ingelheim. com.mx www.maquilasbi.com.mx (Development and manufacturing of oral dosage forms: tablettes, capsules, granulates) Quintiles Tel.: +1- 818- 767- 3900 10245 Hickman Mills Drive Fax: +1- 818- 767- 3950 Kansas City, MO 64137, USA Research Avenue South Tel.: +44- 131- 451- 2074 Riccarton, Edinburgh EH1 4AP, UK Fax: +44- 131- 451- 2063 (Contract and manufacturing services for the pharmaceutical industry, includes wet granulation, direct compression, fluid bed processing, film coating, encapsulation, bead manufacture, mixing/blending, etc.) R’Tech (Results Technology) Tel.: +1- 612- 481- 2207 4001-Lexington Ave. N. Fax: +1- 612- 486- 0837 Arden Hills, MN 55126, USA (Technical, analytical, development, and manufacturing services for the food industry, including, among many others, Spray drying, Agglomeration, Instantizing, Extrusion, dry blending, etc.).
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Indexes
J. Rettenmaier & So¨hne GmbH & Co. Tel.: +49- (0)7967- 152- 0 Contract Service Fax: +49- (0)7967- 152- 222 Holzmu¨hle 1 D-73494 Rosenberg, Germany (Mixing / homogenizing, Sifting / air classification, Grinding / cryogenic grinding / pulverizing, Drying / conditioning, Encapsulating / coating, Agglomerating / compacting / granulating / pelleting, Filling / refilling) Rottendorf Pharma GmbH Tel.: +49- (0)2524- 268- 0 Ostenfelder Str. 51-61 Fax: +49- (0)2524- 268- 100 D-59320 Ennigerloh, Germany www.rottendorf.de (Pharmaceutical solid dosage forms: Fluid bed and mixer (vacuum) granulation, dry (compaction) granulation, tabletting, coating, drying, mixing, screening, analytical). Schering-Plough Third Party Business Tel.: +1- 908- 629- 3200 1095 Morris Avenue Fax: +1- 908- 629- 3164 Union, NJ 07083-7137, USA (Pharmaceutical tabletted products at Kenilworth, NJ: Granulation and blending, Compression and coating, In-process testing) Stellar Manufacturing Co. Tel.: +1- 618- 337- 4747 1647 Sauget Business Blvd. Fax: +1- 618- 337- 0003 Sauget, IL 62206, USA (Blending, Compacting, Granulating, Milling, Sizing, Tabletting, Pouching, Bagging, Packaging) Svedala Industries, Inc. (a member of Metso minerals) Process Research & Test Center (PRTC) Tel.: +1- 262- 762- 1190 9180 Fifth Avenue Fax: +1- 262- 764- 3443 Oak Creek, WI 53154, USA www.metsominerals.com Pyro products Tel.: +1- 570- 275- 3050 Danville, PA, USA Fax: +1- 570- 275- 6789 (Fully equipped facility with the capabilities to perform complex material and process testing and evaluations as well as simulating complete flowsheets that can be assembled to represent a commercial plant with many different unit operations. The test center is designed to perform comminution, agglomeration, and thermal processing studies). Toll Compaction Service, Inc. Tel.: +1- 732- 776- 8225 14 Memorial Drive Fax: +1- 732- 776- 8306 Neptune, NJ 07753, USA (Roll compaction, Pan agglomerating, Screening, Blending, and Grinding of pharmaceuticals and chemicals).
List of Vendors
Tropon GmbH Neurather Ring 1 D-51063 Ko¨ln, GERMANY (Pharmaceuticals: Mixing, granulating, tabletting, support functions).
Tel.: +49- (0)221- 6472- 343 Fax: +49- (0)221- 6472- 507 www.tropon.de coating, encapsulation as well as
TTC (Technology Training Center) Bu¨hlmu¨hle D-79589 Binzen, Germany (Blending and granulating, Tabletting, Extrusion Fluid bed agglomeration, Coating).
Tel.: +49- (0)7621- 664- 308 Fax: +49- (0)7621- 664- 623 [email protected] and spheronizing, Spray drying,
Vector Corporation Tel.: +1- 319- 377- 8263 675 44th Street Fax: +1- 319- 377- 5574 Marion, IA 52302, USA www.vectorcorporation.com (Agglomeration, Coating, Encapsulation, Fluid bed processing, Granulation including roller press compaction/granulation, Mixing/Blending). Welch Laboratories, Inc Tel.: +1- 616- 399- 2711 4270 Sunnyside Drive Fax: +1- 616- 399- 6889 Holland, MI 49424, USA (Compaction services for the pharmaceutical, food, and chemical industries)
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Subject Index
Subject Index
a abrasion drum 286 abrasion transfer 42 accumulator – gas pressure 629 – volume 629 acid gangue 356 additives 15 adhesion 13 – criteria 38 – naturally existing 77 adhesion forces – calculating 17 – modeling 17 adhesion tendency 39 adobe 316 adsorption layers 8, 605 – immobile 605 – natural phenomenon 605 – strongly bonded 605 agglomerates 10 – abrasion resistance 75 – breakdown 45 – crust 71 – determination of strength 18 – estimation of the strength 17 – failure mode 62 – for disposal 534 – from fine grained ores 359 – granulation 70 – green 39, 80, 271 – growth 596 – hard 636 – high porosity 41 – in pharmaceutical suspensions 27 – instant 180, 217 – mechanical destruction of 25 – open porosity 348 – oversized 45 – plastic 20 – porosity of 21 – saturations 7 – seed 271 – solid bridging 636 – spherical 145 – spheroidal 461 – strength 15 – structure of 69 – tensile strength 15 – typical examples 7
– uniformly shaped 163 – weaker parts 75 – chlorate 465 – components 7 – destruction 41 – fiber-based 474 – growth 41, 490 – pore volume 16 – porosity 17 – properties 45 – random sections 7, 10 – secondary 77 – seed 41 – size adjustment 580 – strength 7 – structure 9, 60 – temperature 88 agglomerate strength – adhesive forces 17 – compression test 19 – determination 19 – empirical 19 – experimental results 19 – in industry 19 – maximum tensile strength 17 – quality assurance 21 – solid bridge 18 – state 17 – transitional 17 agglomerated feed – benefits of 350 agglomerated glass batch 373 agglomeration – acoustic 495 – additives 303 – ancient 5 – avoid 33 – beneficial uses 37 – binderless 38 – binding mechanisms 11, 642 – building of nanostructures 643 – by heat 53, 83, 244 – crystal 161 – dirty industries 559 – discs 78 – distinguishing characteristic 7 – drums 78 – dry 85, 493 – endpoint 580 – external forces 14
Agglomeration in Industry. Wolfgang Pietsch Copyright ª 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN 3-527-30582-3
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Subject Index – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
fibers 459 fluidized beds 78 fundamentals 459, 642 general theory 18 glass batch 373 high-pressure 49, 71 hot densification 15 hydrate prior to 514 ice 244 immiscible liquid 162, 457, 581 in liquid suspensions 161 in stirred suspensions 581 industrial applications 59 investment decision 20 low density fluidized beds 581 low- and medium-pressure 49, 71, 618 low-cost 534 low-pressure 71 mechanism 70 medium-pressure 71 metal-bearing powders 386 metallurgical industry 385 mixer 78 natural 489 natural phenomenon 23, 459 natural, binderless 604 new fields 60 occurrences 7 of UFPs 604 of ultrafine particles 493 performance factors 549 pharmaceutical applications 85 pressure 464 raw materials 303 re-wet 104 science of 18 secondary 638 selective 498 selectively 101 size enlargement 30, 59 special industrial applications 459 spherical 161 spontaneous 218 stockpile 534 techniques 5 technologies 61 test equipment 587 thermal 244 tumble/growth 38, 348, 596 two-stage 370 two-stage process 351 uncontrolled 649 undesired 75 unit operation 20
– unwanted 23 – wet 9, 85, 386, 493 – with a binder 38 agglomeration in pharmacy – oldest application 114 agglomeration method – best 544 agglomeration methods – characteristic relationships 587 – interdisciplinary 544 – performance factors 587 – scale-up 575 agglomeration pan – drums 351 agglomeration plants – design of 514 – using briquetting 514 agglomeration techniques – development of 573 agglomeration technology 37 – best 544 – ranking 544 agglomerative behavior – different 605 – variations 607 agglomerator – drum 413 – size 603 – tumble 270 aggregate – mix 524 aggregation – in liquids 495 aging 613 – effects of 613 agitation – methods of 601 agrochemical materials – special 299 agrochemicals – non-fertilizer 297 air – complete removal 431 – decontamination 461 air cleaning 461 air purification media – sources of contamination 461 airbag chemicals 464 alternative iron units 349 aluminum – baled 520 – compacted slaps 408 – for packing 520 – loss of 408
Subject Index – re-melting 520 – recovery 521 – recycling 402 – scrap 520 aluminum industry 402 aluminum scrap – punch-and-die process 521 – roller press compaction 521 ammoniator-granulator 277 ammonium nitrate 299 ammonium polyphosphate (APP) 277 amorphous food products 214 angle of nip 586 animal feed – baling 264 – basic staples 258 – coating 266 – formulations 254 – highly densified compacts 264 – ingredients 254 – nanotechnologies 266 anticaking agents 34 applications – metallurgical 505 – mineral 505 – for agrochemicals 266 – for animal feeds 247 – for building materials 303 – for ceramics 303 – for fertilizers 266 – for solid fuels 415 – in environmental control 485 – in the chemical industry 167 – in the food industry 207 – in the metallurgical industry 385 – in the mining industry 347 – energy related 505 approach to a new project – of engineering 592 – of management 592 artificial aggregate 524 artificial sweeteners 169 – granulation 182 – pressure agglomeration 182 – small tablets 185 – tabletting 182 aspartame 169, 182 – agglomerated 170 – coated 202 – coating 170 – crystalline structure 202 – dry granulation 182 – encapsulated 202 – non-coated 170
aspect ratio – needle-shaped material 608 aspiration system 134 atomic force microscope 18, 639 atomization 44 atomizers – rotary 218 attachments – modular 99 attraction forces – short range 39 auto-oxidation behavior 84 automatic packaging 184
b baby diapers 475 back-up manufacturing 565 backmixing 155 bag-set 31 bakeries 209 baking – cookies and crackers 212 baling – disadvantage 520 baling presses 264 ball mills 365 balling 348 balling cones 353 balling disk 456 balling drums 353 balling pan 353 – pattern of charge motion 577 – performance 578 – segregation 577 – variables 576 batteries – cathode 467 – dry, alkaline 464, 467 Bauxite 403 bed – permeability 190 bentonite 304, 356 binder 7, 15, 618 – acceptable 511 – addition 601 – application of 602 – bitumen 500 – cement 500 – cold cure 446 – corn starch 417 – costs 418, 453 – disappear 356 – fibrous 512 – glass 502
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Subject Index – immiscible 42, 412 – inherent 601 – interactions 601 – liquid 80 – matrix 7, 500 – need for 602 – paper fluff 512 – permanent 504 – pitch 446 – produced chemically in-situ 205 – recirculated 42 – thermoplastic 429 – top-spray arrangement 105 binder addition – automatic 602 – manual 602 binding liquid – pre-dispersion 204 binding mechanisms 11 – enhancement 14 – models 16 – natural 15 BioBinder process 454 biocides 186 – granular 188 – oxidizers 186 – tablets 188 biomass 53, 421, 487 blast furnace 347, 386 – alternative 398 – burden structure 350 – increased capacity 348 – operation 350 – optimal burden 347 – outdated 398 Bleichsoda 171 blend – stabilizing 91 blender – all types 100 – combinations 102 bonds – permanent 499 bouillon cubes 241 – contain organic fibers 241 bowl mixer/agglomerator 93 brake linings 463, 467, 473 brand maintenance 658 brand recognition 658 bread – making of 4, 207 bricks 316 – high-quality 316 – mass production 316
– refractory bricks 316 bridging 31 briquettes – chip-like 527 – continuous string 395 – cooling 395 – defects 395 – egg or pillow shaped 430 – for home heating 435 – Gro¨ndal 375 – hard coal 435 – high density 380 – inert 452 – large 430 – maximum amount 515 – properties of 412 – separation 629 – single 395 – size 587 – smokeless 439 – union-type 424 – well-densified 379 briquetting – anthracite 433 – cast iron chips 376 – cast-iron borings 402 – charcoal 444 – excessive wear 376 – glass batch 381 – hot 376, 379 – iron ore 347 – material with high elasticity 422 – of coal 398 – of fine coal 417 – of fine iron ores 374 – of iron-bearing fine residues 509 – roller presses 376 – solid fuels 440 briquetting or compacting process 431 briquetting plants – lignite 424 brown smoke 492 brownian motion 489 build-up 76, 271 – undesired 603 building blocks (bricks) – artificial 303 building materials – abrasive 309 – great variety 338 – pre-mix 309 – shaped 339 building products – clay-based 317
Subject Index bulk blending 270, 288 bulk characteristics 548 bulk volume 77 bulking agent 183
c cage mills 72 caking 31, 270 – avoid 33 – during food processing 214 caking problems 36 capillary flow 80, 87 capillary pressure 7, 16 capping 122, 618, 622 capsule – functionality 56 – material 56 carbon black 177, 634, 650 – oil-agglomerated 652 carboxymethylcellulose (CMC) 146 carrier materials 269 – easily degradable 294 cast-iron borings 402 catalyst 189 – bed 190 – carriers 190, 318 – chemically unchanged 189 – cylindrical pellets 190 – porous bodies 190 – strength 190 – surface area 189 – vanadium pentoxide 643 cathode mass – pre-granulated 467 cement 303, 333 – fineness 333 cement manufacturing – dry-process 337 – semi-dry process 337 – wet process 337 cement raw meal 372 – pre-agglomeration of 304 cementing materials 333 ceramic floor tiles 307 ceramic materials 303 ceramic parts – industrial 318 – punch-and die presses 318 – sanitary, and household 318 ceramic pre-form 313 ceramic products – intermediate 306 ceramics – additives 343
– composites 346 – consolidation of powders 315 – forming 315 – high-performance 313, 346 – incorporation of 645 – post-treatment 333 – properties 339 cereal 207 cermets 481 cGMP (current good manufacturing practice) 118, 150 chaos theory 18 charcoal 415 – barbecuing 444 – industrial 444 charcoal briquettes – binder pregelatinized 444 – consumer product 444 – in food preparation 444 charcoal briquetting 444 – mix formulation 445 chemical industry – diverse applications 167 chemical oxygen generator 465 – for airline application 466 chemical products 169 chemical reactions – exothermal 532 – exothermic 84, 389 chemical surface properties – modifications 613 chemicals 167 – flakes 198 – molten 198 – water treatment 186 chlorates 464 chutes – spiral 365 – transfer 365 CIP 88, 108, 123, 150, 182 classification 24 clay – expanded 318 clay minerals 313 clean air legislation 428, 440 clean production 593 clean room 93 cleaning – CIP 108 – external 96 – mechanical 76 – pharmaceutical applications 93 – requirements in the pharmaceutical industry 96
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Subject Index – washing 76 – wet 150 – WIP 108 clinker 334 closed system 98 CMC – directly compactable 147 co-generation 533 co-manufacturing 565 co-manufacturing facility 569 coal – agglomerates 456 – applicable 447 – auto-ignition 452 – binderless briquetting 428 – binders 452 – briquettes 419 – brown 420 – coking 446 – density 428 – deposits 419 – disposal sites 418 – extrusion 454 – fine suspended coal 455 – fines 415 – for power plants 452 – hard 420 – in water 455 – logs 444, 454 – lower rank 456 – micro-agglomerates 456 – oxidized 456 – pelletizing 417 – quality 415 – recovery 456 – sludges 456 – sub-bituminous 452 – wetting 456 coal briquettes – consumer products 432 – conventional 446 – domestic 432 – smokeless 446 coal briquetting – binderless 419 – history 436 – larger 436 – new developments 439 – present status 439 – quality requirements 434 – with binders 419, 432 coal fines – briquetting 430 – extrusion 454
– hard 430 – new products from 419 – unmarketable 419 coal logs 443 coal tar pitch 429 coalescence 41 coater – fluid-bed 151 – vertical centrifugal 157 coating 54, 89, 297 – drum 54 – fluidized beds 55 – fully integrated 151 – functionalize the product 631 – functional 149 – nanostructured 648 – melt 297 – process control 149 – shadow effect 149 – undesired 603 – with sulfur 297 – Wurster 155 coating pan 148 coating pans – special executions 148 coffee – powder 215 coke – conventional 446 – metallurgical 448 coke oven charges – partial briquetting 447 coking – alternative 448 – lower quality coals 448 cold isostatic pressing (CIP) 324 collector medium – particulate 493 collision probability 490 colloidal templating 645 compacted pieces – inert 452 compaction – continuous 406 – double sided 197 – low densification speed 624 – metal swarf 404 – non-continuous 404 – of very fine particles 624 – roller press 406 – unidirectional 319 – versatility 288 compaction curve 585 compaction cycle
Subject Index – control 624 – cycle time 622 – machine design 624 – short 622 compaction mechanism 47 compaction/granulation 242, 281 – advantages 290 – block diagram 81 – dry granulation 569 – for pharmaceutical applications 136 – for pigments 201 – improved 182 – prior to encapsulation 202 – prior to tabletting 185 – roller compactor 531 – salt by 380 compaction/granulation plants – economic operation 288 compactor mill 377 compacts 71 – density differences 130 – dry ice 244 – spherical 413 complex rations 260 compliance fuel 440 – bulk commodity 452 – coal-based 452 – natural 452 composite materials 60 composites 488 compressed air 52 compressed air pockets 431 compressed gas – expansion of 49 – pockets 617 compressed residual gas 48 compression stroke 422 concave menisci 16 concentrate 347 concern – environmental 489 concrete – history 333 – hydraulically setting 333 conditioner 255 conditioning – steam 255 conditioning drum 286 confectionery products 236 conflict of interest 566 consultants 542 consumer appeal 86 consumer product 324 – pleasing appearance 446
contaminants 380 – from processing 489 contaminations 64 continuous operation – simulating 549 contract manufacturing 89, 565 contract service 568 convenience foods 210 convenience of application 658 cooker 240 cooling – efficient 384 coordination number 17 coordination point 9, 70, 597 – conditions at 611 – sites 11 core – removal of 646 Corex 398 costs – investment 41 – operating 41 couffinhal press 420, 429 creep 618 cross-contamination 92 crushing 61 – compacts 82 – industrial 62 – multi-stage 75 – particle size distributions 67 – rollers 67 – selective 64 crushing and layering 42 crushing behavior 550 crushing mechanisms – compression 66 – impact 66 crust formation 88 crystallization 3 crystals – agglomerated 165 – needle-like 182 cullet 373 curing 41, 500 – natural 80, 535 customer service 566 customer service department 566 cut size 82 – ideal 23 cutting ropes 235 cyclones 337, 489 – high-efficiency 492
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Subject Index
d data – computerized recording 556 decontamination 146 defects 322 defensive engineering 589 dehydrated fruit juices 214 densest packing 597 densification – minimal 618 – process 422 – speed 49 density – gradient 71 – variations 315, 319 deposition – charge reversal 646 – repeated 646 design – misconceptions 594 – success 589 – safety factors 563 design features – characteristic 90 design parameters 587 – optimizing 604 designer foods 210 detergent – fillers 171 – first commercial 171 – high-density 177 – multi-component 171 – phosphate 171 – process flexibility 177 – supercompact 196 – synthetic 171 – washing additives 198 – water-softening 198 detergent powders – non-tower methods 173 detergent tablet 196 – coated 203 – composition 196 development – empirical 5, 541 diaper 475 die – extrusion 231 – oscillating 230 – plates 231 dielectric elements 645 diffusion 617 dimensionless group 576 dioxin 630
direct compression 126 direct reduced iron (DRI) 350, 387, 526 – briquettes 391 – fluidized bed 391 – hot briquetting 393 – hot densification 393 – material hazardous in bulk (MHB) 390 – passivation 390 – physical form 388 – porosity 388 – reactivity 390 – safe shipment 390 – shaft furnace 395 – specific surface 388 direct reduction (DR) 348, 350, 387, 526 – of iron ores 453 – solid reductant 453 dispersion – by fluidization 471 dispersion aids 26 disposal – final 500 – particulate solid wastes 499 DMT (dimethylterephthalate) 198 – briquettes 199 – flakes 199 dockside mobile loaders (DML) 493 documentation 593 doping 643 dosage forms – dry 118 double pressing 322 double-screw extruder 317 drage´e 148 droplets – electrically charged 498 – fine 493 drug – approval documents 574 drug delivery systems 149, 648 drum – porous 471 – wear liner 355 drum agglomerators – variables 575 drum coolers 198 drum dryers 199 drum granulator 274 dry – pharmaceutical 118 dry granulation – powder blends 132 – roller presses 132 dry-bag process 325
Subject Index drying – capillary flow 70 – drying zone 70 – externally 98 – incrustation 87 – microwave 96 – partial 103 DSD, Duales System Deutschland 518 dust – formation of 252 – nuisance 452 dust collection 84, 489 dust explosions 503 dwell time 52, 119, 404, 624 dyed concrete 181 dynamic seal 240
e effluent stream analysis 594 Eirich planetary mixer 305 elastic deformation 52, 618 – relaxation of 49 – residual 420, 618 – temporary 618 elastic recovery 422, 618 – excessive 628 electric arc furnace (EAF) 387 electric power generation 383 electrical double layer 499 electrification – focused ion beam 58 electrocoagulators 499 electron beam – charge pattern 640 – drawing 640 electron microscopy 488 emulsifiers 249, 614 emulsion solvent diffusion (ESD) 161 encapsulation 56 – encrustation 158 – soluble polymer 159 – special effects 159 energy – conversion 244 – forms 62 – input 62 – recovery 533 engineered materials 61, 645 engineered particulate solids – high quality 658 – high value 658 – special effects 658 engineering – fundamentals 543
– requirements 543 – special design criteria 543 enrobers 203 enrobing of seeds 301 entrapped gas 121 – explosive expansion 625 environment – protection of 446 environmental control 493 equipment – causing agitation 45 – of the charge 603 – operation 603 – output 603 – redundant 260 – relative filling 603 – size 601 – type 601 – upset 603 ESCS (expanded shale, clay, and slate) 310 evactherm 306 excipient 100, 126 – elastic properties 146 expanded clay 310 expander 240 expander–extruder 262 expansion of compressed gas 622 experience – past 590 explosives 464 Exter Press 421 extrudates – densified skin 620 – quality 620 – shape of 618 extruders – axial 142 – interchangeable die plates 318 – pressure-cooker 240 – screen 142 extrusion 71, 140, 618 – aerated masses 235 – axial 240 – characteristics 583 – continuous 443 extrusion channel 421 – adjustable 423 – briquettes in 423 – gradual release 423
f fabrics – engineered 474 – non-woven 459
I 61
I 62
Subject Index falling curtain 299 farming – cattle or 260 – fish and shrimp 260 FASTMET 526 feed – compound 247 – concentrated 247 – water farms 263 – with high bulk density 625 feed characteristics 551 feed components – preferred 260 feed composition – controlling effect 606 feed for tabletting machines – granular material 135 feed material – characteristics 546 – compactibility 547 feed mill 248 – conventional 262 – new systems 262 – pelleting 253 feed mix – agglomeration 384 feed mixture – plasticity 619 feed pellets 258 feed shoe 119 feedback 550 fertilization 267 fertilizer – accretion process 270 – bulk blending 530 – bulk-blend grade 288 – complex 269 – granular 268 – granulation 267 – granulation plant 283 – mixed (NPK) granulated 268 – multi-component granules 270 – natural 486 – nitrogen 267 – potassium 267 – slow-release 297 – specially formulated 270 – wet granulation 273 fertilizer granulation – drum 268 – granulating mixer 268 – NPK 281 – potash 281 – roller presses 268
fertilizer salts – solubility of 276 fertilizer spikes 293 – special 293 fertilizer technology 266 FGD gypsum 507 FIBC (flexible intermediate bulk containers) 548 fiber 475 – accumulate 531 – artificial 474 – bonding processes 477 – binder 477 – entanglement 476 – influence of 531 – manufacturers of 568 – natural 474 – super absorbent 475 – wood 523 film coaters – drum 150 – high-definition 150 film coating 149, 631 filter cakes 505 filtering – deep bed 462 – high-efficiency 462 filters 489 filtration – gas-phase 461 – gaseous contaminants 462 final binding mechanism 262 final part – distorted 319 – structure of 469 fine feed materials – reciprocating ram presses 625 – roller presses 625 fine grinding 27 fines 182, 597 – charcoal 444 – conversion 64 – formation of 68 – minimize the production of 64 – recycling 183 – undesirable 64 FINEX DR process – fluidized bed 398 Finmet – innovations 574 – scale-up considerations 574 firing of ceramics – ideal kiln 343 fishery salt 380
Subject Index flake breaker 139, 284 flame reactors 634 flat die pelleting – waste materials 444 floating roller – response 629 flocculation 496 – addition of salts 499 flocculation agents – suspended 581 flotation 24 flowability 471 flue gases – desulfurization 506 fluid bed – bottom-spray 155 – coater 154 – coating 155 – operated continuously 110 – rotating disc 154 – rounding effect 111 fluid drum granulator (FDG) 297 fluid flow – gases 617 – liquids 617 fluid-bed granulation – filters 107 – spray nozzles 107 fluid-bed granulator – rotating plate 110 – vacuum 110 fluidization 88 fluidized bed – drying 102 – granulated formulations 102 – processing chamber 105 – vacuum processing 110 fluidizing forces 471 flywheel 422 food 213 – additives 210 – amorphous 213 – bars 236 – convenience 236 – dietary ballast components 242 – engineered 210 – functional 210 – functional components 211 – glasses 211 – intermediate moisture 211 – manufactured 207 – organic fibers 242 – processing and preparation 236 – re-wet agglomeration 221
food agglomerate 211 food coulis – briquetting of 246 food industry – high-shear mixer 227 – pan agglomerators 227 food materials – list of 227 food polymer science 211 food processing 209 food products – coating of 246 – extrusion 229 – glass transitions 211 – glassy state 211 – instant 217 food system – glass dynamics 211 forces – binding 61 – centrifugal 471 – physical 7 – separating 61 – short-range 7 formed coke 446 – developments 446 – roller briquetting 447 foundry – briquettes 402 fractal dimensions 17 fractals 612 fracture mechanics 62 freight pipelines 443 frictional force 50, 424 Froude number 576–577 fuel – agglomerated spherical oxide 457 fumed silica 634 Fun foods 210 functional coatings 631 Functional food 210 functional molecules 614 furnace – bell 343 – elevator 343 – manual pusher 343 – mesh-belt 344 – multiple hearth 526 – roller hearth 344 – rotary hearth 526 future solid-fuel related applications 440
I 63
I 64
Subject Index
g gap – changes 628 gas – flow patterns 155 gelatine capsules 141 general applications 61 generic flowcharts 75 glass – contaminations 381 – discoloration 64 glass batch 373 – abrasiveness 381 – briquetted 381 glass transition 211 – water content 214 glass-forming – of sucrose 212 good binder mix 356 good housekeeping 275 Gro¨ndal process 374 gradient – porosity 71 – strength 71 gradual expansion – ejection 626 granular fertilizer 288 granular material – for further processing 386 – pressure agglomeration 80 granular product – for the electric arc furnaces 377 – from tumble/growth technologies 78 – yield of 286 granulate – ceramic 332 granulated powders 77 granulation 34, 62 – advantages of 91 – by crushing 61 – by crushing agglomerates 72 – by mixer agglomeration 92 – dry 79, 129 – efficiency 276 – energy input 73 – fluid-bed agglomeration 92 – low-pressure agglomeration 91 – of powders 61, 68, 71, 75 – under water 201 – wet 92, 129 – yield 75, 82 granulation of powders – by agglomeration and crushing 68 – optimal result 68
granule – agglomerated 77 – average density 92 – crumblers 258 – directly compressible 91 – fill for gelatin capsules 91 – formed naturally 270 – from press agglomeration 242 – improved structure 102 – instant 91 – irregular shape 82 – loosely assembled 101 – molecular adhesion 195 – properties of 412 – quality 82 – small 305 graphite – synthetic 467 grate-kiln 336, 364 – calcination 377 gravity feed chute 448 green engineering 593 green part 313 – dimensions of 313 – firing of 313 green pellets – resistant to thermal shock 358 grinding – between two surfaces 29 – dry 27 – efficiency 27 – impact 29 – limit of 27 – wet 30 grinding aids 29 grinding chamber – choking 63 grounding 27 growth agglomeration – carbon black 651 – formation of nuclei 204 – kinetics 602 – mechanisms of 495 – one-pot 579 – transfer of experience 609 growth agglomeration process 596 growth phenomena 45 growth/tumble agglomeration – stochastic effects 577 gypsum 507
h hammer mills 72 – exit grate 63
Subject Index Hazard and Operability study (HAZOP) 590 HAZOP – deviation 591 – guide words 590 – investigate every item 591 – level of concern 591 – phrases 592 – questions 590 – reality check 592 – remedial actions 591 – sequence of events 591 health risks 92, 488 heap – permeability 534 heap leaching 534 hearth layer 361 heat of evaporation 88 high-pressure agglomeration 51 – brittle breakage 621 – compressed air pockets 621 – degassing 622 – dispersibility 195 – expensive machinery 621 – low residual porosities 621 – plastic deformation 621 – pressure/densification plots 622 – spring-back 622 high-pressure roller mill 66 high-shear mixers – lo¨dige 92 – mixing tools 92 high-speed choppers 93 high-speed tabletting – capping 121 hot briquetted iron (HBI) 392 hot briquetting 376, 446 – Minette ore 377 – no binders 376 hot compacted iron: HCI 398 hot compaction 483 hot gas 88 – downdraft 336 hot isostatic pressing (HIP) 324 – to remove defects 327 hump-back kiln 344 hybridization 56, 471, 650 hybridizer 650 hydration – increase in volume 513 – natural 513 hydraulic accumulator 287 hydraulic pressurizing system – accumulator(s) 626 – operational diagram 626
– roller press 626 hydrostatic pressing 325
i ice briquettes 244 immiscible binder agglomeration 204 immiscible liquid agglomeration – immiscible binder 413 impact mill 72 imperfections 61 incinerators – stoker fired 528 incomprehensible – terminology 5 individual particles – organized structure 640 industrial applications – development of 541 inert filler material 100 infant formula 214 innovative technologies 38 instant pharmaceutical specialties 102 instant products 159 instrumentation 583 integrated steel mills 386 interdisciplinary effort 659 interfacial polymerization 302 interlocking bonds 473 International Maritime Organization (IMO) 390 investment – size of 574 iron 347 iron ore – beneficiation 365 – pelletization 349 – pellets 348 iron ore pelletization plants – complete 365 iron ore pelletizing – green (moist) balls 355 iron ore pellets – composition 350 – post-treatment 356 – self-fluxing 350 – uniformity 350 isostatic pressing 323, 584, 630 – ceramic parts 324 – for making porous products 327 – for tableware 327 – laboratory 547 – uniform consolidation 324 itabirites 349
I 65
I 66
Subject Index
j jet mills 26 jigging 26 Justus von Liebig 266
k kaolinite 305 kiln – shut down or started-up 337 knife heads 78 knowledge – empirical 589 – interdisciplinary 589
l laboratory cleanliness 560 laboratory tests 545 lamination 618, 622 landfills – ehabilitation 522 – managed 502 – special design considerations 522 – unprotected 499 lateral force microscope 18 laundry – machine washing 171 – manual 171 laundry detergent 170, 193, 203 – higher bulk density 172 – high density 193 – shelf-life 198 – spheronizing 193 – solid compounds 195 – tablets 195, 203 laundry detergent powder – spray drying 171 law suits – after accidents 592 legislation – anti-pollution 487 – disposal 84 licking stones 265 life-science applications 659 ligand stabilization 649 lightweight ceramic aggregate 310 lignin sulfonate 181 lignite 420 – harder 431 lignite briquette – environmental concerns 428 lignosulfonate 443, 513 lime 411 – briquetted 411 – slaked 333
limestone 411 liquid – distribution 601 – incompressible 617 liquid addition – objectionable 127 liquid bridges 8 liquid removal – method of 215 – reason for 216 liquid saturation 7–8 livestock farming 247 livestock feeds 247 logistics – first in – first out 377 long sintering cycles 343 losses – dusting 372 – oxidation 372 low and medium pressure agglomeration 50 low cost – granulation 84 low-pressure agglomeration – small extrudates 81 low-pressure agglomerators/extruders – operating problems 619 – throughput 619 lubricant 15, 322, 622 luminescence – electrically activated 645
m magnesium oxide (MgO) 378 – briquetting 378 – calcination 378 – from seawater 379 – precipitated 379 – refractories 378 magnetite 359 maintenance 64 making of bread – agglomeration 4 maltodextrin 169, 183 mandrel 467 manganese dioxide 464 manufacturing – furnace black 651 – small scale 566 manufacturing facilities – clean room 639 market – success 89 market research 658 marketing 88
Subject Index marumerizer 142 marzipan 236 master batch 201 – ingredients 201 material – agglomerative behavior 545 – aging of 548 – amorphization2 28 – caked 33 – circulating streams 42 – coating 54 – comparable 548 – engineering 505 – functionality 569 – granular 61 – hazardous 488 – hygroscopic 3 – manufacturing of 7 – nanophase 644 – nanostructured 642 – nanotechnological 643 – oversized 64 – organic 443 – properties 546 – sampling 548 – soluble 3 – surface properties 595 – temperature sensitive 87 – truly valuable 594 – with unique properties 7 maximum amount 516 maximum pressing force 49 Mazeline press 430 mechanical process engineering 3 mechanical process technology – based on natural phenomena 541 – installations of 595 – plant design 595 – unit operations 207 mechanical processes – designs 541 – misconceptions 541 mechanical vapor recompression 530 mechanism of densification 49 mechanofusion 56, 471, 650 medicines 4 – for oral application 85 Megaperls 193 melt solidification 4 melts – coating 55 merchant DR plants 387 merchant DRI 387 – inert 390
– inhibit reoxidation 390 – shipment of 389 metal oxides – nanoarchitectured 643 metal powders 481 – shaping and densification of 481 – sintering 481 metallized fines and chips 395 metallurgical industry – additives 409 metals 347 – extremely hard 645 – heavy 533 method(s) of size enlargement by agglomeration – pre-selection of 545 micro-agglomeration 78 microcapsules – long term release 302 microcracks 453 microencapsulation 56, 89, 297 – electrostatic 160 – functionalizing 159 – in agrochemistry 302 – packaging method 159 micronutrients 266, 270 microroughness 611 microscopic surface structures 11 microspheres – hollow 647 microwave drying 96 milk 215, 248 – evaporation of 215 milk replacer 249 – agglomerated 250 – coating 251 – dry powder 249 – large size 251 Milorganite 530 mineral coal 415 mineral ingredients – densification 313 minerals 347 mini-mills 387 Minntac 350 mixed fertilizer – segregation 292 mixer agglomeration 577 mixer/agglomerators – low-shear 101 mixing 27 – dry 27 – electrical bipolar 638 – electrostatically assisted 470
I 67
I 68
Subject Index – high-efficiency 356 – moist bulk solids 27 mixing tools 78 mixture – segregation 60 – sludge and coal 454 – stabilization of 470 – stabilizing 91 modifications – application-related 459 modular design 99 modularization 593 moisture 14 molasses 255, 443 molding compound 467 – formulation 468 – granules 468 – particle size distribution 468 – pre-agglomerated 469 molecular forces 634 molecules – amphiphilic 614 – functional 614 multi-clones 492 municipal refuse – organic 529 municipal sewage 454 municipal waste – incinerator 522 – landfills 522 – pellets 522 municipal waste processing 518 mush test 380
n nanoaggregates 648 nanocomposites 638, 644 nanoparticle – agglomerated 635 – comminution 634 – controlled aggregation 638 – functional 643 – functionalized 648 – gas-phase synthesis 634 – incorporation of 645 – individual 638 – larger structures 645 – low bulk density 650 – modifications 638 – new products 642 – not visible 639 – ordered structures from 648 – precipitation 639 – processing 650
– preparations 649 – self-assembly 639 – small mass 638 – uncontrolled aggregation 638 – with special characteristics 643 nanoparticle ensembles 645 nanophase resistors 645 nanoscale powder – compaction of 644 nanostructural assembly 643 nanostructures – bottom-up manufacturing 644 – fabricating of 644 nanotechnology 633, 659 nanotube 643 nanowire 645 natural materials 4 natural resources – conservation of 485 near-net-shape 313, 479 needle-shaped particles – broken 609 – direct application 609 net-shape articles 322 neutral axis 322 neutral plane 71, 323 new plant – engineering 573 Nitrophoska 267 no load – gap 627 nodulizing 515 non-wovens – agglomeration 475 – applications 474 – bonding 477 – end-use 474 – fibrous web 474 – finishing 477 – high quality materials 474 – manufacturing methods 475 – misconceptions 474 – origins of 474 – post-treatments 477 – products 474 – properties 475 – web forming 474 nonpareil nuclei 153 nozzle – flat fan 222 – imperfect 151 – liquid spray 44 – spray 218 – steam 435
Subject Index nucleation 42, 271 nuclei 40 nuisance dust 365 nutraceuticals 227 nutrient – availability 266 – elements 266 – plant 267 nutritional needs – animals 261
o Ocean County Utility Authority (OCUA) 529 oil-agglomeration 455 old mine waste deposits – secondary processing of 505 Omnitex 471 one-pot processing 549 one-pot-technologies 88 operating – gap 627 operating parameters 550 operating pressure – fluctuate 627 operation – optimal conditions 384 operational hydraulic diagram – machine performance 628 operator involvement – minimal 602 opportunity fuels 440 optimization 578, 584, 602 – compromise 66 ordered mixture 470 ore fines 347 – sintering 347 ores 347 – concentration of 347 – low-grade 347 organic feeds – conditioned 240 organic matter 420 out-sourcing 571 outlook 655 oxidizers 188 – shocking 188 – stability 188 oxone 188 oxygen – variable production of 466 oxygen candles 465
p P&ID (Process and Instrumentation Diagram) 591 packed bed reactor 190 pallet cars 361 pan agglomeration 461 pan agglomerator – stepped 371 Papyrus Ebers 114 parameters – evaluation of 544 part – with variable cross sections 319 particle – adhering 76 – affinity 26 – arrangements 642 – aerosol 493 – array of 58 – brittle 48, 617 – coating 56 – collisions 596 – colloidal 70 – collisions 489 – concentration 490 – core 56 – deposition of 640 – elastic 617 – electrostatically charged 470 – engineered 161 – flow patterns 155 – individual 459 – interaction 11 – malleable 48 – manipulation 640 – mass 598 – monodisperse 622 – motion 41 – nanoscale 460, 470 – narrowly sized 597 – natural adhesion 596 – original 60 – oversized 74 – physical characteristics 617 – plastic 617 – plastic deformation 617 – population balances 636 – pre-agglomerated 40, 271 – rearrangement 48 – real 599 – recirculating 40 – sampling 636 – segregation of 534 – shape 599, 602
I 69
I 70
Subject Index – size 17, 41, 459 – size 14, 597, 618 – size distribution 597 – small 10 – spherical 145, 597 – stronger 74 – surface area 14, 598 – surface properties 595 – suspended 412 – ultrafine 76, 84, 489 – with high aspect ratios 609 particle engineering 471 particle formation – growth mechanisms 636 particle size 600 – acceptance interval 606 – furnace black 651 – misrepresented 608 – needle-shaped material 608 – representative 606 – specifications 606 particle size analysis 26 particle size analyzers 608 particle size distribution – bi- or multimodal 606 – infinite variability 606 – specification 606 particles in – manipulation techniques 58 particulate bulk solids – dry 8 – saturations 7 particulate solid – collection 489 – directly compressible 123 – for tabletting 123 – hazardous 499 – particle size 14 – pre-granulated 123 particulate systems – cake formation 36 parts – complex 467 – net shape 467 peat 420 pellet – coal-fiber 454 – free-flowing 373 – hardening processes 351 – higher quality 356 – liquid components 373 – layered 372 – magnetite 359 – multi-component 371
– quality 365 – reactive 373 – self-fluxing 367 pellet cooler 257 pellet fines 367 pellet mill 241, 296 – conditioner 255 – cylindrical die 254 – design 260 – extrusion 255 – flat die 254, 454, 527 – integral feeder/conditioners 254 pelleting 253, 443, 619 – advantages 191 – compressed gas pockets 620 – feed distribution 621 – flat die 146 – friction in the bores 620 – operating problems 620 – production capacity 621 pelleting of animal feed – screw extruders 253 pelletizing facility – pellet fines 367 percolation 534 perforated dies – cleaning of 621 – low structural integrity 620 performance – guaranteed 589 – undesirable 606 peridur 356 peripheral system components – selection of 287 pet food – developments 260 pharmaceutical – pre-treated 126 pharmaceutical applications 85 – roller presses for 132 pharmaceutical compaction/granulation system – single granulator 139 – two-stage milling 139 pharmaceutical industry 624 – needs 112 – profit margins 132 – roller compacting presses 132 – ultra-clean 150 – validation requirements 574 – value of the material 132 – wet agglomeration techniques 91 pharmaceutical specialties 149 phenomenon
Subject Index – natural 3 phosphate rock 377 pig iron 387 pigment 177, 199, 204, 303 – artificial inorganic 178 – artificial organic 178 – characteristic performance 180 – coloring of concrete 181 – dry processing 200 – granules 200 – high-performance 178 – ineffective distribution 200 – instant properties 199 – micro-agglomerated 180 – natural inorganic compounds 177 – particle size 204 – particulate colorants 177 – perception of color 177, 204 – synthesized 204 pigment black – dry agglomeration 652 pill making 85 – binders 90 pills 4 – coating the 4 pilot plant 547, 588 – continuous processing 557 plant – conventional 593 – designing 589 – general-purpose 593 – modular 593 – multi-product 593 – multi-purpose 593 – underperforming 563 plastic master batches 181 plasticity 15 plenum 102 polishing drum 195 pollutants – particulate 488 pollution – secondary 487, 499 pollution control 485 pollution prevention 593 polyimides 467 polymer – affinity 498 – flocculation 496 – recycling of 537 – super absorbent 475 polymer recycling – elements of 538 polymer science
– glass transitions 211 – glassy state 211 polymeric flocculants 496 polymers pore size – distribution 20 pores 617 porosity 10, 61 – high 21 – temporary additives 21 portland cement 333 post-treatment 42, 79, 172, 458, 597, 619, 659 – agglomeration by heat 469 – application of heat 339 – atmosphere 630 – external suppliers 581 – problems 630 – puffing 241 – sintering 53 – thermal 473 potash – granular 281 potassium chloride 267, 348 powder – composition 533 – drink 217 – floating 216 – in a liquid 217 – milk 217 – pre-agglomerated 89 – reconstitution of 216 – toxic 413 powder metallurgical manufacturing 481 powder metallurgy 53 – advantages 479 – agglomeration tools 481 – hot and cold compaction 481 powder metallurgy (PM) 479 power plant – coal-fired 506 pozzolana 333 PRB coal – PRB coal 453 pre-agglomerated particles – recirculation of 220 pre-agglomeration 85 – compaction/granulation 469 – objective of 469 pre-forms – spark plug insulator 324 pre-granulated formulations 127 pre-granulation – prior to tabletting 93
I 71
I 72
Subject Index preconditioning 79 precursor 636 predensification 379 preferential coalescence 42 press agglomeration – plastic deformation 473 presses – punch-and-die 53 – roller 53 pressing channel – friction 421 pressure – no-load 627 – operating 627 pressure accumulator – damaged 628 – overload 628 – response 628 – size 628 pressure agglomeration 47 – advantage of 464 – batch or shift operation 51 – campaign 51 – defined volume 616 – degree of densification 464 – equipment 50 – high-pressure 51 – laboratory tests 546 – level of force 47 – low- and medium-pressure 50 – mechanical parameters 50 – methods 47 – most widely used and versatile 657 – selection of 48 – throughput per unit 50 – versatile processes 509 pressure agglomeration technologies 616 pressure assisted sintering (PAS) 303 pressure increase – rate of 424 prilling 268 primer 465 PRIMUS 527 process – aerosol 636 – capacity 604 – economics 74 – fundamentals 543 – inconsistencies 604 – modifications 601 – optimization 636 – pre-selection 545 – requirements 543 – selection 543
– special design criteria 543 – yield 604 process design – misconception 589 process equipment – replacement of 607 – selection 573 – sintering 341 process path 260 process research – empirical 541 process safety management (PSM) 593 process validation 89–90 processing – of natural ores 368 – of powders 471 processing systems 153 product – with controlled reactivity 464 – best 567 – bulk characteristics 60 – characteristics 88, 579 – compacted 202 – conditioning 82 – consumer 20 – density 60 – engineered 467 – granular 61 – granular size range 74 – for house or nursery plants 294 – instant 216 – industrial bulk material 20 – intermediate 82 – properties 60 – quality 636 – quality requirements 66 – yield of 579 product development 658 product integrity 49 product properties 551 – changes of 604 product quality 574 product silos – vented 532 product yield – maximization 75 production capacity 574 – of low-pressure extruders 619 project – parameters 542 project execution – typical 595 project management 542 project risks 575
Subject Index pug milly 267 – blunger 267 pug sealer 317 punch – slow movement 404 – stroke 404 punch-and-die presses – coal briquettes 430 – cubers 241 – dwell time 625 – ejection 118 – feed for 74 – hydraulic cylinders 625 – in the food industry 241 – laboratory 585 – principle of 116 – rotary 118 – small hydraulic 408 – withdrawal 118 punch-and-die pressing – small machines 547
q quality assurance (QA) 612 quantum dot 643, 645
r ram extruders 52 ram extrusion press 421 – briquetting cycle 421 – conversion of elastic into plastic deformation 625 – deaeration 625 – hydraulic drives 625 – new applications 440 – non-coal applications 443 – redesigned 440 random coalescence 42 range cubes 265 raw material conservation 593 raw materials – secondary 488 re-agglomeration 470 re-work 619 reciprocating ram press 421 recirculating load 579 recirculation – closed loop 74 recirculation rate 579 recombination bonding 28 recoverable coal fines 418 recovery – binder 110 recovery of dust
– coffee 242 recrystallization 70, 213 recuperation of sensible heat 361 recycle 549, 619 – surge hopper 276 – uncontrolled 602 recycled material – quality standards 538 recycled plastics – applications 539 – low-cost manufactured parts 539 recycling 488, 602 – agglomeration equipment 527 – aluminum 520 – facilities 519 – hot 383 – influence of 557 – legislation 538 – major 518 – plastics 537 – polymers 538 – waste gas 383 recycling fines – influence of 557 reduction ratio – importance of 66 reference plant 544, 549 regional material recycling 518 rejects 62 reoxidation – exothermic 84 representative samples 587 residence time 579, 582 resistance to flow 617 resource conservation 505 RESS process 46 reverberatory furnaces 408, 521 ring roller press 431 risks – technical and financial 575 roasted pyrite residues 371 roller briquetting presses 435 roller crushers 27 roller mill 67 – multiple-pass 72 – profiled roller surfaces 68 roller press 132, 244, 420 – briquetting 377, 430 – cantilevered 232 – circumferential speed 586 – compaction/granulation 377 – de-aeration 138 – densification ratio 625 – floating roller 626
I 73
I 74
Subject Index – for pharmaceutical applications 138 – high-pressure 242, 585 – hinged frame 283 – hot briquetting 398 – hot densification of DRI 398 – laboratory evaluation 547 – large 448 – large hot 380 – low pressure 585 – new applications 444 – new solid fuel related applications 440 – nip 586 – operating parameters 286 – optimization 626 – performance 585 – phosphate rock 377 – pressurization system 626 – redesigned 440 – rotary bar 231 – screw feeder(s) 281 – specific force 586 – speed of compaction 626 – toothed 230 – troubleshooting 626 roller screen 355, 604 rotary kiln 334 – agglomeration 334 – problems 365 – process 334 – wet process 334 rotary press – multiple dies per station 186 rotary punch-and-die presses – feeder 119 – pressing cycle 119 – punches 119 rotary tabletting machine – adjustable cam drives 625 – capacities 121 – double sided 121 – multiple tooling 121 – overload protection 119 – single sided 121 rotary tabletting presses – multi-station 125 rotating fluid bed processor 471 roughness peaks – melting 15 rounded granule – by coating 299
s saccharin 169 safety factors 573
safety features 592 – excessive 592 – required 592 salt 380 salt (sodium chloride) 32, 242 – briquettes 380 – crystallized by-product 380 – pretzel 242 – rock 380 – water softeners 380 sample – representative 547 sanitary applications 562 satellites 218 scale-up 89, 549, 573, 589, 601, 603 – art 581 – begins with testing 581 – dimensional analysis 575 – experience factors 579 – geometrically analogous 577 – low pressure agglomeration 581 – pellet mills 582 – problems 556 – punch-and-die 582 – rotary presses 583 – spheronizers 582 – tabletting 583 scaled-up – common sense 581 – tumbling and growth procedures 581 scalping screen 384 scanning probe microscope 18, 639 scanning probe microscopes 639 Schugi 222, 251 science – agglomeration 5 SCOPE 21 451 scrap 387, 505 – bales 520 – home 403 scrapers 355 – to limit build-up 273 screen – spiral 414 screening 24 – of moist bulk materials 25 – wet 25 screw extrusion presses 443 scrubber – deep bed 462 – wet 493 secondary fractures 63 secondary plastic raw materials 538 – extrusion techniques 539
Subject Index – pellets 539 – punch-and-die pressing 539 secondary pollution 507 secondary raw material 502, 505, 534, 538 – metallic iron 524 secondary solid fuel – pelleting 533 seeds – coating 301 segregation 463, 470, 534 – avoiding 127 – by size 354 – during agglomeration 276 self-assembly 645 self-ignition 84 semi-autogenous mills 365 semi-coking 447 separation 23 – after drying 603 – curve 23 – cut size 23 – degree of 23 – influence of agglomeration 23 – property 24 – quality 23 – sharpness 23 service provider 516 services – to all parties 566 settling behavior 581 shaft furnaces 358 – hardening of iron ore pellets 359 shaft kilns 334 shape – macroscopic outline 610 – microscopic surface roughness 610 shape characterization 600 shape formers 235 shaping pressure – dependence on 616 – influence of 616 sheet thickness 587 shelter 4 sifting 24 silica fume 303, 309 silo – mass flow 31 similarity 575 – considerations 576 – geometrical 575 – material 575 – partial 575 – process-related 575 simulation
– effects of recycling 557 – entire production line 587 single particle – characteristics 633 – homogeneity 633 single-pot processing 93 sink/float processes 365 sinter – two-stage cooling 384 sinter breaker 361 sintering 53, 84, 333, 381, 414 – batch 381 – continuous 381 – controlling the combustion 630 – downdraft 382 – environmental concerns 630 – final strength 355 – for recirculation 385 – gas cleaning 630 – heat recovery 630 – high porosity 53 – in solid state 53 – ore fines 381 – pan 381 – shrinkage 7, 53 – solid fuels 381 – sophisticated control features 384 sintering furnaces – batch 343 – continuous 344 sintering plant 383 size enlargement – cost 499 – in the animal feed industry 248 – safe handling, storage, and disposal 499 size enlargement by agglomeration – advantages 59 – beneficial 59 – current importance 657 – for general applications 77 – future 657 – general application 61 – new fields of application 655 – reason for 60 size-reduction processes – optimal 64 slitting slabs 235 sludge – drying 530 – management 530 – processing 529 slugging 129 slugging presses – disadvantages 132
I 75
I 76
Subject Index slurry 8 small particles – adhesion of 23 soap 171 sodium chloride 348 sodium cyanide 201 – briquetting 202 soil conditioner 530 sol-gel process 414, 457 solid dosage forms 85 – in the pharmaceutical industry 89 solid fuel 415 – coal 415 – complementary 533 – fines 415 – fluff 528 – secondary 523, 526, 533 – for smelting 415 solid materials – characterization of 488 solid waste 486 – minimization 485 – particulate 486 – pre-sorted 519 – steel industry 515 solid waste management 505 solids – adhesion between 634 – amorphization 634 – characteristic of 606 – electrically charged 498 – failure mode 62 – interactions 601 – nanoscaled 635 solution – of a “clean” problem 559 solvent recovery 110 sonic energy 493 sorting 24 sparger tubes 277 special applications – other agglomeration technologies 469 special design criteria 6 specialized vendors 550 specific force 585 specific pressing 287 spheres – hollow 647 spherical agglomeration 412 spherical crystallization 161 spherical dosage forms 140 spherical particles 412 – melt droplets 412 – powder 412
spheronization 140 spheronizer – batch 582 – continuous (cascade) operation 582 – flow diagram 141 – friction plate 144 – modular components 145 – special features 145 sponge iron 388 spouted bed 155 spray dryer – fluidized 102 – in the ceramic industry 307 – multi-nozzle 218 spray drying 79, 216 – different powder qualities 172 – forced agglomeration 218 – limitations of 172 spray drying and agglomeration – food industry 226 spray nozzles 80 – manifold 354 spray systems – coaters 156 spray towers 171 stabilization – particulate solid wastes 499 standard shapes 599 starch – binder pregelatinized 444 – gelatinization 255, 261 starch modification 263 starved feed 406 starved feeding 284 steam – condensation of 277, 435 steam granulation 276 steel manufacturing – regional 387 stochastic movement 601 stock piles – slope of 533 stockpile agglomeration 500 storage – intermediate 613 storage bin – ventilated 510 strength – bursting 56 stress raisers 62 structure – brick 4 – ideal solid 61 – improved 15
Subject Index sugar – sprinkling 242 SUMICOAL 448 superphosphate 270 support laboratories 551 surface – alterations 472 – chemical modifications 613 – hydrophobic 614 – mechanical modification 471 – modifications 601 – oxidation 613 – properties 601, 613 – roughness 600, 611 – smooth 600 – technical 76 – texture 76 – topography 611 – wetting 601 surface configuration – microscopic 611 surface equivalent diameter 41, 597 – limit 602 surface tension 17 surface topography – variations 600 surfactant 458, 614 – soap 171 – synthetic 171 surging 276, 355, 602 swarf 403 system – commercialization 89 – duplication 574 – future purpose 590 – modification 563 – optimization 563 – variations 590
t tablet – functionalized 149 – number of 118 – off-specification 126 – pressed directly from powder blends 126 – quality 126 – rejected 130 – shape of 121 – two-layer 196 – variations in quality 132 tabletting – early history 115 – research 547 tabletting die press
– invention 116 tabletting lines – automated 126 – modular concept 126 tabletting machines 129 tabletting press 85 taconite 347 – concentrates 349 tailings 347, 486 tamper 409 technical problems – analysis of 392 – industrial solutions 392 technologies – particle-modification 61 technology of bread making 207 temperature – monitoring 390 – rise 532 – self-ignition 390 test conditions 547, 587 test facilities 545, 588 – general purpose 553 – peripheral equipment 549 – requirements 545 test set-up – equipment 551 – growth agglomeration in a pan 553 testing 89 – a balling drum 555 – aged materials 613 – clean 553 – clean environment 557 – dust free atmosphere 559 – easy cleaning 559 – facilities 549 – in a non-sanitary environment 559 – of fluidized bed 555 – of similar materials 545 – original bulk properties 548 – roller presses 555 tests – evaluations 563 – many different locations 573 – peripheral equipment 588 – results 563, 573 – various equipment 573 thermal shock 359 thickener/clarifier 496 through-the-wall designs 88 through-the-wall installation 98 tolling 565 tolling companies – development of a large project 573
I 77
I 78
Subject Index – secondary raw materials 573 toner particles – microencapsulation of 206 tooling – dry-bag 325 – special 468 – wear 322 – wet-bag 325 tools – relative motions 323 tower process 171 toxic drugs 127 tramp material 524 transmission electron microscope (TEM) 492 transport – hot 398 traveling grate 336, 359 – changing ore compositions 362 – circular 364 – heating 362 – modifications 361 – straight 364 – traveling grate 382 – windboxes 362 trimming shear 406 troubleshooting 604 – fluid bed spray granulators 614 – tumble/growth agglomeration 614 tube mills 27 tumble/growth agglomeration – basic mechanism 39 – binding mechanisms 41 – drawbacks of 41 – in the chemical industry 169 – mining industry 349 – non-ferrous minerals 370 – of fine coal 417 – optimization 602 – scale-up factors 575 – selection 46 – similarity approach 575 – small scale equipment 546 – UFP 461 tumble/growth processes – block diagram 79 – granulation 79 tumble/growth technologies 596 tunnel kilns 344 turnings and borings 403 turret 119, 583
u ultrafiltration 461 ultrafine particles (UFP) 460 undesirable 64 – parameters 606 unwanted agglomeration – agglomeration 30 – air classification 25 – avoid 35 – comminution 27 – during screening 25 – mixing 27 – particle size analysis 26 – pneumatic conveyors 30 – sorting 26 – storage 31, 34 – transportation 30 urea 267 US Bureau of Mines 349 used beverage cans (UBC) 403, 520 user-friendliness 658
v vacuum technology 307 validation 89, 593 validation process 574 vertical pug mill 434 – mixing elements 435 veterinary medicine 248 void volume – filled with a liquid 7
w Waelz kiln 515 warehousing 570 wash cycle 196 waste – bulky 527 – cellulosic 526 – historical review 502 – industrial 502 – plastic man-made 537 – segregation 522 – terminal 500 waste energy 533 waste minimization 593 waste paper 512 waste processing facilities 573 water-treatment – municipal 496 wet granulation 127 wet granulation systems – recirculating load 277 wet-bag process 325
Subject Index wetting 44, 601 – chemical additives 456 – modifications 613 WIP (washing-in-place) 88, 93, 108, 123 wood chips – artificial secondary 527 wood products – engineered 526 worker protection 88
workplace contamination 134 Wyoming Powder River Basin (PRB) 452
y yield 602
z zeolites 171
I 79
I 80
List of Figures
List of Figures
Fig. 2.1 Beetle, Scarabaeus Sacer, „pelletizing“ dung and an artist’s impression of this animal’s procedure Fig. 2.2 Operations of Mechanical Process Technology and associated techniques Fig. 3.1 Random sections through part of an agglomerate or a particulate bulk solid mass: a) pore volume filled with a matrix binder; b) pore volume filled with a wetting liquid; c) liquid bridges at the coordination points; d) adhesion forces at the coordination points Fig. 3.2 Diagrams of different liquid saturations in particulate bulk solids or agglomerates: a) dry; b) adsorption layers; c) liquid bridges („pendular“ state); d) transitional („funicular“ state); e) fully saturated („capillary“ state); f) droplet Fig. 3.3 Sketch of a random section through an agglomerate Fig. 3.4 Pictorial representation of the binding mechanisms of agglomeration Fig. 3.5 Attraction forces between solid surfaces or particles Fig. 3.6 Diagram of the adhesion tendency of a spherical particle on a flat surface Fig. 3.7 Two-dimensional diagram of the failure lines derived from the three models describing strength of agglomerates with a matrix binder (Fig. 3.1a) Fig. 3.8 Diagram of force against time curves that may be obtained during the compression strength test of spherical agglomerates Fig. 4.1 Examples of separation curves Fig. 4.2 Separation curves of different air classifiers [B.71, B.97] Fig. 4.3 Agglomerates produced during grinding in a roller mill with a high reduction ratio Fig. 4.4 a) Schematic representation of the fracture lines caused in a glass sphere by impact stress. b) Agglomerated cone of fines created during the impact stressing of a glass sphere (sphere diameter 8 mm, impact velocity 150 m/s). c) Agglomerated cone of fines created during impact stressing of a sugar crystal (shown on the left) Fig. 4.5 Solubility curves of four different salts
Fig. 5.1 Basic mechanism of tumble/ growth agglomeration Fig. 5.2 Diagram of typical equipment for size enlargement by tumble/growth agglomeration (left) and the post-treatment steps required to obtain a final product (right). Left, from the top: inclined disc (pan), drum, continuous mixer, batch mixer, fluidized bed. 1) liquid binder (spray), 2) fresh feed, 3) recirculating fines, 4) dryer, 5) cooler, 6) double deck screen, 7) mill, 8) conditioning drum Fig. 5.3 Sketches explaining the different processes taking place during tumble/growth agglomeration [B.48, B.97] Fig. 5.4 Diagram of the mechanisms involved in size changes during tumble/ growth agglomeration [B.48, B.97] Fig. 5.5 Nucleus formation in a dense tumbling bed of fine particles [5.1] Fig. 5.6 Spreading or penetration of a droplet into a powder bed [5.1] Fig. 5.7 Processes occurring between particles of a tumbling powder bed after wetting with droplets of a binder liquid [5.1] Fig. 5.8 Diagram of particle and droplet distribution in a low-density fluidized bed and of particle penetration into a liquid [5.1] Fig. 5.9 Sketches explaining the mechanism of pressure agglomeration Fig. 5.10 Diagram of equipment for: a) low-; b) medium-pressure agglomeration: a1, screen; a2, basket; a3, radial; a4, dome; a5, axial; b1, screw; b2, flat die; b3–b5, different designs of cylindrical dies; b6, gear Fig. 5.11 Diagram of equipment for highpressure agglomeration. Ram press (upper left), punch and die press (upper right), roller presses for compaction (lower left) and briquetting (lower right) Fig. 5.12 Cycles of force build-up in the three different high-pressure agglomeration techniques Fig. 5.13 Sketch of the material processing section of a bottom spray fluidized bed coater (Wurster coating system) [B.71] Fig. 5.14 Diagram of possible structures of microcapsules [B.97]
List of Figures Fig. 5.15 Diagram depicting the process of hybridization and microphotographs showing particles with intermediate as well as final coating [B.71] Fig. 5.16 Overview diagram describing manipulation techniques for small solid particles [5.3] Fig. 5.17 Precise arrangement of SiO2 spheres on pinpointed locations: a) voltage contrast image of a dotted line; b) silica spheres arranged on the dotted line [5.3] Fig. 6.1-1 Diagram of failure lines in single spherical and irregular particles during: left) compression. right) impact crushing [B.12] Fig. 6.1-2 Photographs of: a) the fines cone; b) coarse residual pieces obtained during impact crushing of a single glass sphere [6.1.1] Fig. 6.1-3 Diagrams of a crusher with oversize recycling and various two- or threestage crushing circuits [6.1.4]. Circles represent crushers or mills; horizontal lines flanked by + and – represent classifiers (screens) that split the crushed material into over- and undersized particles. „A“ is an airclassifier that removes fines by entrainment Fig. 6.1-4 Diagram of multi-stage crushing between three sets of rollers and photographs of two-stage and three-stage Gran-U-Lizer roller mills (courtesy Modern Process Equipment, Inc. (MPE), Chicago, IL, USA) Fig. 6.1-5 Comparison of typical cumulative particle size distributions after crushing solids in a single-stage roller mill and an MPE Gran-U-Lizer with three sets of rollers. Note the much reduced amount of oversize (shaded area) and the almost unchanged amount of fines (courtesy Modern Process Equipment, Inc. (MPE), Chicago, IL, USA) Fig. 6.1-6 Diagrams of various arrangements of profiled rolls for roller mills: a) peak-to-valley arrangement showing the „cracking“ of a particle; b) parameters determining control of the particle size in a peak-to-valley arrangement; c) peak-to-peak arrangement; d) example of a peak-to-valley arrangement with modified pitch and gap (courtesy Modern Process Equipment, Inc. (MPE), Chicago, IL, USA) Fig. 6.1-7 Diagram of the structure of different agglomerates: a) tumble/growth agglomeration; b) low- and medium-pressure agglomeration (extrusion); c) high-pressure agglomeration
Fig. 6.1-8 Automatic adjustment with pneumatic servomotors and micrometer or digital readout in multi-stage roller mills (courtesy Modern Process Equipment, Inc. (MPE), Chicago, IL, USA) Fig. 6.1-9 Comparison of the results of granulation by crushing agglomerates: a) a mill with exit grate or screen, b) an impact (hammer) mill with unobstructed discharge and recirculation of oversized particles (Fig. 6.1-3aa), and c) two-stage crushing (Fig. 6.1-3bb) applying two individually controlled impact mills with unobstructed discharge Fig. 6.1-10 Diagram of an impact mill with hinged or fixed hammers on a horizontal rotor and discharge restriction (bar cage or screen) to avoid oversized particles in the product: a) grate bars, b) cage with axially elongated holes, c) cage with round holes Fig. 6.1-11 Glass cylinders filled with the same gravimetric amount of a powder having different particle sizes (a) from left to right: 4 mm, 1.5 mm, 100 lm, 12 lm, 5 lm, 28 nm, 18 nm, 16 nm, 12 nm and (b) left 4 mm, right 12 nm Fig. 6.1-12 Block diagram of a powder granulation system by tumble/growth agglomeration for general applications also showing various optional features Fig. 6.1-13 Block diagram of a powder granulation system by pressure agglomeration (compaction/granulation) for general applications also showing various optional features Fig. 6.1-14 Typical granules from compaction/granulation showing the irregular shape of the unconditioned particles Fig. 6.2-1 Sketches of tools for manual pill making [6.2.1]. 1, „Pill machine“: a) rolling plate, b) rope cutter, lower part, c) rope cutter, upper part, d) rolling board, e) area for ropes and finished pills. 2, Mortar (with pestle). 3, Pill rolling (disc) tool Fig. 6.2-2 Diagram of the operating principle of a horizontal plow mixer/agglomerator with chopper, and two alternative means of liquid addition (courtesy Lo¨dige, Paderborn, Germany)
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List of Figures Fig. 6.2-3 Laboratory or small production dust-tight plow mixer/agglomerator with cantilevered shaft, interchangeable drum, front opening door, chopper, and liquid addition (courtesy Lo¨dige, Paderborn, Germany) Fig. 6.2-4 Wall mounting of larger cantilevered mixer/agglomerators in a pharmaceutical manufacturing facility (courtesy Lo¨dige, Paderborn, Germany) Fig. 6.2-5 The activated pull-out mechanism, exposing the shaft and plow mixing elements of a mixer/agglomerator for easy cleaning (courtesy Lo¨dige, Paderborn, Germany) Fig. 6.2-6 P&ID of a horizontal, batch operating plow mixer/agglomerator, equipped for WIP (courtesy Lo¨dige, Paderborn, Germany) Fig. 6.2-7 Typical designs of vertical highshear (bowl) mixer/agglomerators: a) basic design, b) equipment for „one-pot processing“, c) process diagram (courtesy Diosna, Osnabru¨ck, Germany) Fig. 6.2-8 Diagram of one-pot mixing, granulating, and drying system featuring microwave drying (courtesy FUKAE Powtec Corp., Kobe City, Japan) Fig. 6.2-9 Small vertical high-shear mixer/ agglomerator with exchangeable stainless steel vessels of 1, 2, 4, and 8 L volumes (courtesy Diosna, Osnabru¨ck, Germany) Fig. 6.2-10 Typical through-the-wall installation of a mid-sized vertical high-shear mixer/agglomerator (600 L bowl) with container feeding through the ceiling in a pharmaceutical processing plant (courtesy Diosna, Osnabru¨ck, Germany) Fig. 6.2-11 A quasi-continuous system for mixing, granulating, and drying in a pharmaceutical setting featuring a bowl mixer/ granulator of 600 L volume and an external fluidized bed dryer (courtesy Diosna, Osnabru¨ck, Germany) Fig. 6.2-12 Diagram of through-the-wall and free-standing installations of a vertical high-shear mixer/agglomerator (courtesy Diosna, Osnabru¨ck, Germany) Fig. 6.2-13 Diagram of modular design features offered with a vertical high-shear mixer/agglomerator (courtesy Hu¨ttlin, Steinen, Germany)
Fig. 6.2-14 Diagrams, including indication of particle movements, and photographs of different batch-operating low-shear mixer/ agglomerators: a) double-cone (courtesy Abbe´, Little Falls, NJ, USA), b) slanted-cone (courtesy Gemco, Middlesex, NJ, USA), c) V-shape (courtesy Abbe´, Little Falls, NJ, USA) [B.97] Fig. 6.2-15 Typical agglomerates that were produced in a batch low-shear mixer/ agglomerator with V-shaped shell. Agglomerate sizes: left, about 0.750 mm; right, about 4 mm (courtesy Patterson-Kelly, East Stroudsburg, PA, USA) Fig. 6.2-16 A P-K zigzag continuous blender/agglomerator (courtesy PattersonKelly, East Stroudsburg, PA, USA) Fig. 6.2-17 Diagram of a continuous fluidized spray dryer (FSD) with open plant gas flow (courtesy GEA/NIRO, So¨borg, Denmark) [B.97] Fig. 6.2-18 left) The principle of batch fluid-bed granulation; right) the outline of an apparatus (courtesy Glatt, Binzen, Germany) Fig. 6.2-19 P&ID of a typical batch fluid-bed granulation system (courtesy Glatt, Binzen, Germany) Fig. 6.2-20 Two batch fluid-bed granulation units (courtesy Glatt, Binzen, Germany) Fig. 6.2-21 Acetaminophen (paracetamol) granules produced in a top-spray fluid-bed processor: a) surface, b) cross section, revealing internal voids (compare with Fig. 6.2-32). Magnification 60 (courtesy Glatt, Binzen, Germany) Fig. 6.2-22 Batch fluid-bed granulation unit with two mid-sections installed on the frame for easy and quick exchange (courtesy Glatt, Binzen, Germany) Fig. 6.2-23 CIP of fluid-bed granulation units: a) principle of automatic cleaning, b) principle of washable pulse blow-back filters and view into the filter area of a unit, c) metal cartridge filter, d) hydraulically extendable washing nozzles (courtesy Glatt, Binzen, Germany) Fig. 6.2-24 SC („SuperClean“) fluidized bed granulator equipped for CIP and total containment (courtesy Glatt, Binzen, Germany) Fig. 6.2-25 WIP/CIP skid (courtesy Glatt, Binzen, Germany)
List of Figures Fig. 6.2-26 Diagram of the principle of continuous fluid-bed drying and granulation (courtesy Glatt, Binzen, Germany) Fig. 6.2-27 P&ID of a batch vacuum top-spray fluid-bed granulator with solvent recovery (courtesy Glatt, Binzen, Germany) Fig. 6.2-28 Comparison of the drying temperatures and times achieved in contact and vacuum fluid-bed dryers (courtesy Glatt, Binzen, Germany) Fig. 6.2-29 SEM image of a granule produced in a vacuum fluid-bed granulator (courtesy Glatt, Binzen, Germany) Fig. 6.2-30 Two vacuum batch fluid-bed units with closed loop and solvent recovery systems (courtesy Glatt, Binzen, Germany) Fig. 6.2-31 Diagrams of the principle and outline of a fluid-bed granulator with solid rotating bottom plate (courtesy Glatt, Binzen, Germany) Fig. 6.2-32 Acetaminophen (paracetamol) granules produced in a rotary fluid-bed processor: a) surface, b) cross section, still revealing internal voids (compare with Fig. 6.2-21), magnification 60 (courtesy Glatt, Binzen, Germany) Fig. 6.2-33 Principle of a special rounding continuous fluid-bed drying/granulation process (courtesy Glatt, Binzen, Germany) Fig. 6.2-34 P&ID of a rounding continuous fluid-bed drying/granulation process (courtesy Glatt, Binzen, Germany) Fig. 6.2-35 A rounding continuous fluidbed drying/granulation process apparatus and a typical product at two different magnifications (courtesy Glatt, Binzen, Germany) Fig. 6.2-36 Patent drawing of Brockedon’s hand tool for tabletting. British Patent 9977, 1843 Fig. 6.2-37 Patent drawing of Young’s eccentric drive tabletting machine. US Patent 156 398, 1874 Fig. 6.2-38 Patent drawing of McFerran’s indexed table tabletting machine. US Patent 152 666, 1874 Fig. 6.2-39 The layout of a rotary punchand-die press [B.71, B.97] Fig. 6.2-40 a) Evoluted (straightened) diagram of a rotary punch-and-die press. b) Paths of the punches in rotary punch-anddie (tabletting) presses with one or two sets of press rollers in evoluted presentation [B.71, B.97]
Fig. 6.2-41 „Standard“ tablet shapes [B.48, B.97] Fig. 6.2-42 Some special tablet shapes [B.97] Fig. 6.2-43 An assortment of engraved punches (courtesy Kilian, Ko¨ln, Germany) Fig. 6.2-44 Tablets with „capping“ and sketch explaining the capping phenomenon [B.48] Fig. 6.2-45 Glove box design of the processing part of a rotary punch-and-die tabletting machine demonstrating WIP (courtesy Fette, Schwarzenbek, Germany) Fig. 6.2-46 Photographs showing: a) complete turret assembly, b) handling of the turret assembly in a smaller machine, c) turret assembly removal from a larger press, d) special handling system (courtesy Fette, Schwarzenbek, Germany) Fig. 6.2-47 a) Modular system concept; b) modern automated standard tabletting system with rotary tabletting machine and tablet discharge units (courtesy Kilian, Ko¨ln, Germany) Fig. 6.2-48 Diagram of low-pressure agglomeration equipment using gravity feed and screens or thin perforated sheets: a) screen, b) trough, c) basket [B.97] Fig. 6.2-49 Sketch of a low-pressure agglomeration system with screen granulator, dryer, and (optional) mill: 1) wet feed mixture, 2) granular product including fines [B.97] Fig. 6.2-50 Small trough-type granulator with horizontal rotor axis (courtesy Erweka, Heusenstamm, Germany) Fig. 6.2-51 The major components (feed hopper, extrusion blades, and perforated die) of a small basket extruder (Bextruder BX 150, Courtesy Hosokawa Bepex GmbH, Leingarten, Germany) Fig. 6.2-52 Simplified flow diagram of dry granulation in the pharmaceutical industry using slugging or roller presses with optional circuit alternatives [Section 13.3, ref. 147] Fig. 6.2-53 a) Granulator for pharmaceutical applications with screen and rod cage, b) designed for easy cleaning by disassembly (courtesy Alexanderwerk, Remscheid, Germany)
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List of Figures Fig. 6.2-54 Density distribution in cylindrical compacts [6.2.2.2]. Progressive stages of compaction from the top after applying the indicated pressures. The curves in the compact are lines of constant density (1 – e) in % (e = porosity) Fig. 6.2-55 Four small roller presses with: a) one, b) two integrated in-line granulators for the manufacturing of tabletting feed in the pharmaceutical industry (courtesy: a1) Powtec, Remscheid, Germany; a2) Vector, Marion, IA, USA; b1) Alexanderwerk, Remscheid, Germany; b2) Riva, Buenos Aires, Argentina) Fig. 6.2-56 Design features for easy cleaning of roller presses for pharmaceutical applications (courtesy: a) Hosokawa Bepex, Leingarten, Germany; b) Powtec, Remscheid, Germany; c) Bonals, Barcelona, Spain) Fig. 6.2-57 Flat and corrugated sheets/ flakes and granulated product from pharmaceutical formulations Fig. 6.2-58 Diagram of the three design alternatives for positioning the rollers in roller presses for pharmaceutical applications [6.2.2.3] Fig. 6.2-59 Small, instrumented compaction/granulation system (courtesy Fitzpatrick, Elmhurst, IL, USA) Fig. 6.2-60 Two views of a plant for dry, high-pressure granulation of pharmaceutical products (courtesy Ho¨chst AG, Frankfurt/ M.-Ho¨chst, Germany) Fig. 6.2-61 Diagram of two typical designs and arrangements of roller presses for pharmaceutical applications [Section 13.3, ref. 147] Fig. 6.2-62 Diagram of a new arrangement for improved de-aeration in roller presses with vertical roller arrangement (courtesy Alexanderwerk, Remscheid, Germany) Fig. 6.2-63 Pharmaceutical compaction/ granulation system with vertical roller arrangement, horizontal feed screw, and granulation by two-stage milling (courtesy Alexanderwerk, Remscheid, Germany) Fig. 6.2-64 Diagram of a roller press with slanted roller arrangement, with screw feeders and integrated one-stage granulation (courtesy Gerteis, Jona, Switzerland) [6.2.2.3] Fig. 6.2-65 Diagram of a marumerizer-type spheronizer (courtesy LCI, Charlotte, NC, USA)
Fig. 6.2-66 Flow diagram of a continuous granulation system featuring a low-pressure extruder with or without spheronizer (marumerizer) [B.48] Fig. 6.2-67 a) Extrudates with small diameter (1.0 mm) obtained after de-dusting (Fig. 6.2-64, item 6); b) spheronized particles (Fig. 6.2-64, item 7) Fig. 6.2-68 Photographs of open and closed gel-caps filled with spheronized, coated pharmaceutical granules and of tabletted dry dosage forms (for comparison) Fig. 6.2-69 The discharge areas of basket, radial, and dome extruders (courtesy LCI, Charlotte, NC, USA) Fig. 6.2-70 Two low-pressure axial extruders and of extrudates/spheronized particles (courtesy LCI, Charlotte, NC, USA, and WLSGabler, Ettlingen, Germany) Fig. 6.2-71 View into an operating spheronizer and schematic presentation of its operation (courtesy LCI, Charlotte, NC, USA) Fig. 6.2-72 a) Extrudates, b) products from different spheronization treatments after 5 s, c) after 30 s, e) after several minutes (courtesy LCI, Charlotte, NC, USA) Fig. 6.2-73 Flow diagram of a flat die pelleting/granulation plant for carboxymethylcellulose (courtesy Amandus Kahl, Reinbek, Germany) Fig. 6.2-74 Outlines of different modern coating pans (courtesy Stechel, Alfeld, Germany) Fig. 6.2-75 Coating pans in two manufacturing plants (courtesy Stechel, Alfeld, Germany) Fig. 6.2-76 Specially equipped coating pans (courtesy Stechel, Alfeld, Germany) Fig. 6.2-77 The flow diagram of a typical film coating facility: a) programmable logic controller (PLC), b) storage tanks for spray liquid(s) and metering/pumping system, c) equipment for supplying and processing air, d) air cleaning and exhaust system [B.48, B.97] Fig. 6.2-78 Typical telescopic spray nozzle mounting assembly for film coating drums and detail of an individual spray nozzle attached to this assembly (courtesy O’Hara, Richmond Hill, Ontario, Canada)
List of Figures Fig. 6.2-79 Flow diagrams of two different air flow regimes using air-blowing paddles in a cylindrical film coating drum (courtesy GS Coating Systems, Osteria Grande (Bologna), Italy): a) hot air through the paddles into the particle bed with air exhaust through the hollow shaft; b) hot air through the hollow shaft onto the particle bed with air exhaust through the paddles; 1) inlet air handling unit, 2) control panel, 3) solution tank, 4) dosing system for liquid to be sprayed, 5) sliding support arm for spray nozzles, 6) coating pan, 7) air-exhaust or blowing paddle device, 8) dust collector, 9) outlet air fan, 10) powder dosing device Fig. 6.2-80 Two drum film coaters installed in the ultraclean environment of pharmaceutical processing (courtesy Driam, Eriskirch/Bodensee, Germany) Fig. 6.2-81 Sketch showing the CIP of a polygonal drum film coater (courtesy Driam, Eriskirch/Bodensee, Germany). Cleaning phases: 1) drum inside by cleaning spray bar, 2) drum outside and air distributor, 3) air channels inside, 4) rinsing of tub Fig. 6.2-82 Artist’s impression of a topspray fluid-bed coating system (courtesy Vector, Marion, IA, USA) Fig. 6.2-83 Principle of the Wurster coater (right) and outline of an apparatus (left) (courtesy Glatt, Binzen, Germany) Fig. 6.2-84 Wurster coater for pharmaceutical applications (courtesy Glatt, Binzen, Germany) Fig. 6.2-85 Sketches of the different chamber configurations of single tube Wurster coaters as used for: a) tablets, b) coarse granules, c) fine particles [B.48] Fig. 6.2-86 SEM photographs of particles that were coated with fluidized bed coaters: a) coated with ethylcellulose in a top-spray fluid-bed processor, b) coated with ethylcellulose in a Wurster apparatus, c) vacuum top-spray coated (retard) pellet, cut open, d) Wurster coated (retard) pellet, cut open (courtesy Glatt, Binzen, Germany) Fig. 6.2-87 Diagram of the operating principle of a vertical centrifugal coater (courtesy Diosna, Osnabru¨ck, Germany) Fig. 6.2-88 View into the open top of an operating vertical centrifugal coater (courtesy Diosna, Osnabru¨ck, Germany) Fig. 6.2-89 Diagram of possible structures of microcapsules [B.97]
Fig. 6.2-90 a) Principle of microencapsulation by electrostatic, aerosol based coating (1, coating particles; 2, core particles); b) sketch of a possible equipment configuration [B.48, B.97] Fig. 6.2-91 Mechanisms of crystal agglomeration [6.2.3.2] Fig. 6.2-92 Scanning electron micrographs of: 1) external appearance, 2) cross section of a) ESM, b) SA agglomerated crystals of ascorbic acid [6.2.3.2] Fig. 6.2-93 Scanning electron micrographs of: a) spherically agglomerated crystals of a drug; b) sample of the same product, showing its uniformity (courtesy AstraZeneca R&D, Mo¨lndal, Sweden) Fig. 6.2-94 Mechanisms of spherical crystallization in a three-solvent system [6.2.3.3] Fig. 6.2-95 Size distribution of SA crystals produced in a three-solvent system [6.2.3.3]. Ethanol 5 mL, bridging liquid (water): *) 0.5 mL, ) 0.8 mL, &) 1.2 mL, ^) 1.5 mL Fig. 6.2-96 Scanning electron micrographs of SA crystals prepared with two- and threesolvent systems [6.2.3.3] Fig. 6.2-97 Scanning electron micrographs of single dry crystals of acebutolol hydrochloride and of spherically agglomerated crystals [6.2.3.4] Fig. 6.2-98 Relationship between relative volume and compression pressure during the compaction of single dry () and spherically agglomerated () acebutolol hydrochloride crystals [6.2.3.4] Fig. 6.2-99 Scanning electron micrographs of cross-sections of tablets prepared with various compression pressures (1, 10 MPa; 2, 50 MPa, 3, 200 MPa) [6.2.3.4]: a) tablets from single crystals, a1–a3 1500; b) tablets from spherically agglomerated crystals, b1 750, b2 and b3 1500 Fig. 6.3-1 Diagram of a spray-drying system for the production of conventional laundry detergent powder [B.60] Fig. 6.3-2 Flow diagram of a traditional spray drying process for the manufacture of laundry detergent powder [B.102] Fig. 6.3-3 Flow diagram of a tower process with post-addition of powders and non-ionics and agglomeration [6.3.1.2] Fig. 6.3-4 Flow diagram of a tower process with post-addition, wet agglomeration, fluidbed drying, and dust recycling [6.3.1.2]
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List of Figures Fig. 6.3-5 Block diagram of a non-tower agglomeration process for the production of laundry detergents [B.102] Fig. 6.3-6 Flow diagram of a non-tower process for the manufacture of laundry detergents with two mixers and a fluidized bed dryer [B.102] Fig. 6.3-7 Flow diagram of a typical wet granulation process for the manufacture of heavy laundry detergent [B.102] Fig. 6.3-8 Type CB high-speed, high-shear mixer that is often used for the densification and pre-granulation of laundry detergent components. The inset shows details of the mixing tools (courtesy Lo¨dige, Paderborn, Germany) Fig. 6.3-9 Tower process followed by wet agglomeration, fluid-bed drying, and posttreatment for the manufacture of dense laundry detergents [6.3.1.2] Fig. 6.3-10 Small roller press compaction/ granulation system for the processing of materials such as artificial sweeteners. A product sample is shown in the inset (courtesy Turbo Kogyo, Kanagawa, Japan) Fig. 6.3-11 Flow diagram of a compaction/ granulation system with multiple crushing/ milling and classification steps and fines recirculation for the reduction of undersized material in the product [6.3.2.1] Fig. 6.3-12 Patent drawing of a compaction/granulation system using a roller press for the densification of an artificial sweetener, mixed with a volatile liquid [6.3.2.2] Fig. 6.3-13 Dosage forms of pressure agglomerated artificial sweetener (aspartame): left) tablets and tablet dispenser, right) granules and portion packages. (Equal is a registered trademark of Merisant Co.) Fig. 6.3-14 Left) upper and right) lower punches and the die insert for the multi-die station of a rotary tabletting press (courtesy Kilian, Ko¨ln, Germany) Fig. 6.3-15 Tabletting line for, for example, the manufacture of artificial sweetener tablets. From right to left: Control panel with data recorder, rotary tabletting machine, in-line de-dusting and transportation, bulk packaging (courtesy Kilian, Ko¨ln, Germany)
Fig. 6.3-16 Tabletting of swimming pool oxidizer compound with a rotary punch-anddie press (courtesy Stellar, Sauget, IL, USA) Fig. 6.3-17 Some different shapes of tablets for swimming pool sanitizing (courtesy Stellar, Sauget, IL, USA) Fig. 6.3-18 Packaging of agglomerated swimming pool sanitizing products: a) watertight in drums with sealed lids (granular or tabletted), b) bags with water-resistant liners (granular), c) individually wrapped and welded into plastic (tabletted) (courtesy Stellar, Sauget, IL, USA) Fig. 6.3-19 Installation of a flat die pellet press for the manufacture of cylindrical catalyst or carrier pellets (courtesy Amandus Kahl, Reinbek, Germany). For a schematic, partial cut through a flat die pellet press see Fig. 6.5-4a (Section 6.5.2). Fig. 6.3-20 Various shapes of extruded catalyst carriers (courtesy Bonnot, Uniontown, OH, USA) Fig. 6.3-21 Special extruder for the manufacture of catalysts or catalyst carriers. A hinged die holder allows the quick exchange of extrusion plates (courtesy Bonnot, Uniontown, OH, USA) Fig. 6.3-22 Flow diagram of an extrusion process for the manufacture of Megaperls [B.102] Fig. 6.3-23 Appearance of commercial heavy laundry detergent products: a) granular, b) extruded and sheronized (Megaperls) [B.102] (Courtesy Henkel, Du¨sseldorf, Germany) Fig. 6.3-24 Two-layer laundry detergent tablets (courtesy Henkel, Du¨sseldorf, Germany) Fig. 6.3-25 Evoluted (straightened) diagram of a rotary double-layer punch-and-die press (courtesy Fette, Schwarzenbek, Germany) Fig. 6.3-26 Two-phase (two-layer) water softening product for European automatic washing machines (Calgon is a registered trade mark of Benckiser, Ludwigshafen/Rh., Germany) Fig. 6.3-27 Rotary drum cooler (dip feed flaker) (courtesy GMF- Gouda, Waddinxveen, The Netherlands) Fig. 6.3-28 Flow diagram of an optimized granulation process, based on roller press compaction, for (for example) an inorganic pigment
List of Figures Fig. 6.3-29 Modified patent drawing of a fluidized bed encapsulation process incorporating compaction/granulation as a preparatory step (according to [6.3.3.1]) Fig. 6.3-30 The operating principle of an enrobing machine (courtesy Hosokawa Kreuter, Hamburg, Germany) Fig. 6.3-31 Stirred vessel with baffles for the „classic“ immiscible binder agglomeration [6.3.3.2] Fig. 6.3-32 Comparison of growth curves obtained during different operating conditions of the „classic“ immiscible binder agglomeration showing long time intervals for the formation of nuclei [6.3.3.2] Fig. 6.3-33 Comparison of agglomerate growth with and without binding liquid pre-dispersion by a rotor-stator mixing device [6.3.3.2] Fig. 6.3-34 Fast and uniform increase in agglomerate size with narrow distribution obtained during optimal contacting of in-situ formed binder droplets with solid particles [6.3.3.2] Fig. 6.4-1 The bakery of Rameses III, about 1175 BC [6.4.1, 6.4.2] Fig. 6.4-2 Lithograph of stone relief from 50–25 BC on the tomb of a well-to-do baker named Marcus Vergilius Eurysaces in Rome [6.4.1] Fig. 6.4-3 Diagram of a modern bakery [6.4.1] Fig. 6.4-4 Phase diagram for the sucrosewater system, illustrating the locations of the glass, solidus, liquidus, and vaporous curves and the points Tg and Te (eutectic melting temperature) corresponding to the intersections of the liquidus/glass and liquidus/ solidus curves, respectively [6.4.3] Fig. 6.4-5 Diagram of the transitions of food materials, which can be present as crystalline solids, glasses, rubbers, liquids, or in solution [B.108] Fig. 6.4-6 Effect of water on the glass-transition temperature of several carbohydrates, calculated with the Gordon-Taylor equation [B.108] Fig. 6.4-7 Micrograph of an overheated spray-dried particle that has burst, showing that it is hollow (courtesy Niro A.S., Soeborg, Denmark) Fig. 6.4-8 Typical particle from single stage spray drying with small „satellites“ attached [6.4.1.1]
Fig. 6.4-9 Forced agglomeration by the interaction of two atomized clouds of droplets from opposing nozzles [6.4.1.1] Fig. 6.4-10 Sketches of fines return methods for nozzle atomizers [6.4.1.1] Fig. 6.4-11 Sketches of fines return methods for rotary atomizers [6.4.1.1] Fig. 6.4-12 Flowchart of a complete plant for the production of agglomerated dry solids from a liquid [6.4.1.1] Fig. 6.4-13 Photograph of a fluidized bed machine (dryer/agglomerator/instantizer) in a food processing plant (courtesy Niro A.S., Soeborg, Denmark) Fig. 6.4-14 Diagram of the Peebles instantizer [6.4.1.1] Fig. 6.4-15 Schematic flowchart of the Nestle´ re-wet instantization plant [6.4.1.1] Fig. 6.4-16 Photomicrographs of two typical products: a) coffee extract; b) milk (courtesy Niro A.S., Soeborg, Denmark) Fig. 6.4-17 a) Diagram of a cross section through the operating parts of a „Schugi Flexomix“. b) Photograph of the opened-up roller cage of a „Schugi Flexomix“, showing the vertical shaft with the mixing blades after removal of the flexible sleeve that defines the mixing chamber. c) Artist’s conception of the „Schugi“ (courtesy Hosokawa Schugi, Lelystad, The Netherlands) Fig. 6.4-18 Sequence of photographs showing the quick and complete dispersion of an agglomerated material in water (courtesy Hosokawa Schugi, Lelystad, The Netherlands) Fig. 6.4-19 Various categories of and specific food materials that are dried and agglomerated to yield powders; * indicates which material is shown in the pictures (courtesy Niro A.S., Soeborg, Denmark) Fig. 6.4-20 a) Growth agglomerated cereal mixtures; b) „Rice Krispies“ (courtesy Kellogg Co., Battle Creek, MI, USA) Fig. 6.4-21 Diagram of a toothed roller press for the processing of solid, viscous, or, generally, plastic masses to which coarse components can be admixed (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-22 Photograph of the top side of a toothed roller form-press, featuring a clear feed hopper that allows a view into the nip of the toothed rollers (courtesy Hosokawa Bepex, Leingarten, Germany)
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List of Figures Fig. 6.4-23 Design principle of a rotary bar roller press (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-24 Diagram of the design of a double rotary bar roller press (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-25 Two possible nozzle types for the manufacturing of: a) double layered; b) hollow strands in a double rotary bar roller press (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-26 Diagrams of the designs of single, double, and triple rotary bar roller presses and collection of cross sections through ropes that may be manufactured (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-27 Nozzles on a double rotary bar roller press for the forming of: a) filled ropes, b) flat-single, c) double-layered strands (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-28 Two situations in which a second sheet is deposited on top of a previously made slab (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-29 Photographs of three cantilevered rotary bar roller presses: single (DP 200800), double (DDP 200-1000), and triple (DP/3 250-300) formers (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-30 Production of individual bars: a) by cutting ropes, b) by slitting slabs and cutting the strips (courtesy Hosokawa Bepex, Leingarten, Germany). In (b) are shown: (5) mixer/conditioner, (6) smooth roller former, (7) cooling drum, (8) rotary bar roller press, (9) cooling tunnel, (10) strand slitter, (11) fanning (separation) belt, (12) cutter, (13) tempering unit, (14) coater, (15) cooling tunnel Fig. 6.4-31 Close-up of a triple layered food bar and other snack bars, some coated with chocolate or very coarse food particles (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-32 Roller extrusion press executed in bridge type design (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-33 Combination of roller press formers, one bridge type and the other cantilevered, and a cutter (courtesy Hosokawa Bepex, Leingarten, Germany)
Fig. 6.4-34 Sketch of the principle of smooth roll formers for the extrusion of pressure sensitive or aerated masses (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-35 Shape forming: a) two-roll formers installed after, for example, a rotary bar roller press; b) the roll surface carries molds that correspond to the product shape (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-36 Enrobed and decorated food products (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-37 Summary photograph of the some of the many different foods that can be shaped by or based on low-, medium-, and high-pressure agglomeration (extrusion) (courtesy Hosokawa Bepex, Leingarten, Germany) Fig. 6.4-38 Diagram of a typical pressurecooker extruder with main dimensions in feet and inches: 1’ = 0.3048 m, 1“ = 25.4 mm (courtesy Sprout-Matador, Muncy, PA, USA) Fig. 6.4-39 Some examples of food products obtained with pressure-cooker extruders (courtesy Sprout-Matador, Muncy, PA, USA) Fig. 6.4-40 Cubed and granulated beef bouillon, both with „instant“ characteristics (courtesy Borden Foods/Wyler’s, Columbus, OH/Chicago, IL, USA) Fig. 6.4-41 Examples of „gourmet coffee singles“, single portion filter bags filled with flaked and flavored roast coffee fines and freeze-concentrated coffee (Folgers Coffee/ Procter & Gamble, Cincinnati, OH, USA) Fig. 6.4-42 Reproduction of a leaflet showing frozen food pulp briquettes and the rendition of a reconstituted meal (courtesy Sahut-Conreur, Raismes, France) Fig. 6.4-43 Photograph of a special roller press for the briquetting of food coulis (capacity 2.5 tonne/h) and sketch of the briquetting process (inset) (courtesy Sahut-Conreur, Raismes, France) Fig. 6.5-1 Spray dryer/agglomerator/cooler system for the manufacturing of calf milk replacer (courtesy Niro AS, Soeborg, Denmark) Fig. 6.5-2 SEM photographs of milk replacer produced by: 1) conventional method, 2) new process [6.5.1.1] at two different magnifications (a and b) (courtesy Land O’Lakes, Inc., Arden Hills, MN, USA)
List of Figures Fig. 6.5-3 Reproduction of a brochure from the 1920s describing screw extruders and a flowchart for the pelleting of animal feed meal blends (courtesy Amandus Kahl, Reinbek, Germany) Fig. 6.5-4 Diagrams of the principles and designs of the two dominant types of pellet mills for the processing of animal feeds: a) flat die pellet press (courtesy Amandus Kahl, Reinbek, Germany); b) ring die pellet press, also showing integral feeder/conditioners in the lower part (courtesy CPM, Waterloo, IA, USA) Fig. 6.5-5 Flow sheet of a typical feed mill (Courtesy Amandus Kahl, Reinbek, Germany): 1) receiving, 2) silos, 3) proportioning and weighing, 4) premixing, 5) grinding and mixing, 6) conditioning and pelleting, 7) liquids storage and metering, 8) coating (Rotospray), 9) product storage and loading, 10) miscellaneous support functions, 11) electrical and control equipment Fig. 6.5-6 Flow diagram of a pelleting system using a ring die pellet mill and a vertical cooler [B.3, 1971] Fig. 6.5-7 Components of a ring die pellet mill [B.3, 1971]: 1) variable speed screw conveyor, 2) conditioner, 3) ring die and roller (extrusion) section, 4) speed reducer, 5) main motor, 6) machine base Fig. 6.5-8 Schematic of a vertical pellet cooler [B.3, 1971]: 1) feed hopper with level sensing device, 2) cooling columns, 3) plenum or air chamber, 4) discharge gate drive motor, 5) discharge star gates, 6, 7) air fan with drive motor Fig. 6.5-9 Pellet mill floor of a conventional animal feed mill employing ring die machines with conditioners (courtesy CPM, Waterloo, IA, USA) Fig. 6.5-10a Flat die pellet mill in one of the production lines shown in Fig. 6.5-5 (courtesy Amandus Kahl, Reinbek, Germany) Fig. 6.5-10b Multiple flat die pellet mills in a conventional animal feed mill (courtesy Amandus Kahl, Reinbek, Germany) Fig. 6.5-11 Analysis of nutrients in extruded dry dog food [6.5.2.2] Fig. 6.5-12 Starch gelatinization of pet food as a function of particle size [6.5.2.2] characterized by the diameters of the openings in the discharge screen of a hammermill (Section 6.1). „Special“ refers to a screen producing particles < 0.6 mm
Fig. 6.5-13 Flow diagram of a modern animal feed extrusion system (courtesy Amandus Kahl, Reinbek, Germany) Fig. 6.5-14 Different extruded dry dog and cat foods (courtesy Amandus Kahl, Reinbek, Germany) Fig. 6.5-15 Extruded dry fish and shrimp feed: a) crumbled feed for young fish, b) pellets 2–12 mm, c) floating pellets, d) slowly sinking pellets, e) water stable (2–8 h) pellets (courtesy Amandus Kahl, Reinbek, Germany) Fig. 6.5-16 Sketches demonstrating the development of baling presses [6.5.2.3]: a) old American box frame press, b) first stationary high-pressure press (USA, about 1870), c) first German mobile straw press (1896), d) swing-piston press (Raussendorf, Singwitz/ Saxonia, 1938) Fig. 6.5-17 Mobile hay collection equipment with high pressure ram press [6.5.2.3]. The cylindrical compacts are transferred into an open car by pushing them through the transport pipe on top of the machine Fig. 6.5-18 Comparison of volume of compacts (left) produced with the equipment shown in Fig. 6.5-17 and a conventional bale (right) from the same material [6.5.2.3] Fig. 6.6-1 Flow diagram of a typical wet granulation plant for dry fertilizers Fig. 6.6-2 The discharge end of the granulation drum and the feed end of the rotary dryer of a wet fertilizer granulation plant (courtesy Krupp Polysius, Beckum, Germany) Fig. 6.6-3 Discharge of granules from two pan agglomerators in a wet fertilizer granulation plant (courtesy Krupp Polysius, Beckum, Germany) Fig. 6.6-4 Overall view of a system for the wet granulation of dry fertilizers according to Fig. 6.6-1 (courtesy Krupp Polysius, Beckum, Germany) Fig. 6.6-5 left) Diagram of the wall and bottom scrapers in a pan granulator; right) sketch of the particle motion that is assisted by the position of the bottom scrapers [B.97] Fig. 6.6-6 Photograph of a 4.6 m diameter pan showing the still-unadjusted vane-type plow scrapers (E) that are individually mounted on a support structure (D), which moves with the tilt of the pan (courtesy Feeco, Green Bay, WI, USA)
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List of Figures Fig. 6.6-7 Four typical designs of internal scrapers of drum granulators [B.48]: a) singlestage, sectional adjustable straight edge; b) two-stage scraper consisting of tungsten carbide cutters and a secondary adjustable straight edge; c) hydraulically powered reciprocating; d) rotary spiral Fig. 6.6-8 Sketch of the operating principle of a drum granulator depicting the spray bar for liquid addition, the movement of the particulate charge, and the scraper (B.48) Fig. 6.6-9 Typical flow diagram of a plant for steam granulation of dry fertilizer materials [6.6.6] Fig. 6.6-10 Influence of the relationship between temperature and water content on the agglomeration behavior of fertilizers during steam granulation [6.6.6] Fig. 6.6-11 a) Sketch of the TVA pilot ammoniator-granulator; b) cut-away view of a large scale ammoniator-granulator designed to accept pre-reacted slurry [6.6.6] Fig. 6.6-12 Flow diagram of a TVA type ammoniation-granulation plant for the production of granulated NPK fertilizers [6.6.6] Fig. 6.6-13 Flow diagram of a TVA pipe reactor-pugmill process producing granular NP fertilizer (urea, ammonium polyphosphate) [6.6.6] Fig. 6.6-14 Flow diagram of a granulation plant using the TVA pipe reactor for the production of NPK fertilizers [6.6.6] Fig. 6.6-15 Cut-away drawing of the drum used in Fig. 6.6-14 [6.6.6] Fig. 6.6-16 Two views of a modern roller press for the compaction of fertilizers (courtesy Ko¨ppern, Hattingen/Ruhr, Germany) Fig. 6.6-17 Schematic and photograph of a „hinged frame“ for roller presses (courtesy Ko¨ppern, Hattingen/Ruhr, Germany) Fig. 6.6-18 Flow diagram of a fertilizer granulation plant using a roller press for compaction [B.48, B.97] Fig. 6.6-19 Flow diagram of an optimized fertilizer compaction/granulation system incorporating changes [B.48, B.97] Fig. 6.6-20 Compacted sheet and granular fertilizer obtained by crushing and screening
Fig. 6.6-21 Block diagrams of different installations using compaction for the granulation of finely divided particulate solids (including materials such as fertilizers or agrochemicals) [Section 13.3, ref. 101]. F, fresh feed (possibly premixed); P, product(s); A, mixer; B, compactor; C, flake-breaker; D, screen(s); E, crusher(s); G, wet granulator; H, dryer; I, cooler Fig. 6.6-22 Mixed fertilizer compaction/ granulation plant (courtesy Su¨dchemie AG, Mu¨nchen/Kehlheim, Germany) Fig. 6.6-23 Roller press similar to the machine used shown in Fig. 6.6-22 (courtesy Ko¨ppern, Hattingen/Ruhr, Germany) Fig. 6.6-24 Flow diagram of the plant shown in Fig. 6.6-22 Fig. 6.6-25 Flow diagram of a plant combining fertilizer bulk blending (2–7) and compaction/granulation (8–27). 1, front-end loader input of components; 2, 8 ,18, bucket elevators; 3, distributor; 4, silos for components; 5, compounding scale; 6, blender; 7, bulk blend receiving hopper (courtesy Sackett, Baltimore, USA). 9, mill for feed homogenization; 10, feed bin; 11, 27, screw conveyors; 12, belt mixer; 13, metal separators; 14, 25, drag-chain conveyors; 15, roller presses compactors; 16, solids flow meters; 17, curing belt conveyors; 19, double-deck screens; 20, mills (granulators); 21, product belt conveyor; 22, belt scale; 23, 24, dust collection system equipment and building; 26, recycle surge bin (courtesy Sackett, Baltimore, USA, and Ko¨ppern, Hattingen/Ruhr, Germany) Fig. 6.6-26 Plant described in Fig. 6.6-25. Bulk blending is on the left and compaction/ granulation on the right (courtesy Ferquigua, Guatemala) Fig. 6.6-27 One of the two roller presses in the plant according to Figures 6.6-25 and 6.626 also showing compacted material on the discharge/curing belt conveyor (courtesy Ferquigua, Guatemala) Fig. 6.6-28 Photograph of fertilizer spikes Fig. 6.6-29 Pelleted agrochemical product (snail and slug control pellets) Fig. 6.6-30 Cross sections through broken melt-coated fertilizer granules: cores, triple superphosphate (TSP) granules; coating, sulfur (courtesy Kaltenbach-Thuring, Beauvais, France)
List of Figures Fig. 6.6-31 Flow diagram that is used for the manufacturing of products as shown in Fig. 6.6-1. The centerpiece of the system is the fluid drum granulator (FDG) (courtesy KaltenbachThuring, Beauvais, France) Fig. 6.6-32 Sketch depicting the principle of the fluid granulation drum (FGD) for granulation or the coating of granular materials (seeds) (courtesy Kaltenbach-Thuring, Beauvais, France) Fig. 6.6-33 a) Irregularly shaped fertilizer granules from compaction/granulation (phosphate); b) rounded fertilizer granules obtained by coating by the FGD process (courtesy Kaltenbach-Thuring, Beauvais, France) Fig. 6.6-34 Flow diagram of a system using the fluid drum granulator (FDG) for the production of granulated degradable sulfur for agricultural use (courtesy KaltenbachThuring, Beauvais, France) Fig. 6.6-35 Sketch of a modified fluid granulation drum (FGD) for the production of granulated degradable sulfur for agricultural application (courtesy Kaltenbach-Thuring, Beauvais, France) Fig. 6.6-36 Flow diagram of a plant for the production of fertilizer grade granulated ammonium nitrate (courtesy KaltenbachThuring, Beauvais, France) Fig. 6.6-37 Description of the action of a microencapsulated two-effect agrochemical product [6.6.3.1]: a) microcapsule surrounded by a disinfectant film, b) disinfectant evaporation, c) diffusion of the insecticide through the wall and dissipation of the active substance Fig. 6.7-1 Block diagram of fabrication routes for building materials and ceramics Fig. 6.7-2 Planetary intensive mixer with a flat bowl (courtesy Eirich, Hardheim, Germany) Fig. 6.7-3 Planetary intensive mixer/granulator with an inclined bowl (courtesy Eirich, Hardheim, Germany) Fig. 6.7-4 Flow diagram of an Evactherm preparation plant for ceramic molding materials (courtesy Eirich, Hardheim, Germany) Fig. 6.7-5 Scanning electron micrograph of silicate ceramic granules produced in an Evactherm preparation plant (courtesy Eirich, Hardheim, Germany)
Fig. 6.7-6 Two preparation plants with Evactherm mixers demonstrating the extreme cleanliness of this process technology (courtesy Eirich, Hardheim, Germany) Fig. 6.7-7 Plant for the production of dry mortars with storage silos Fig. 6.7-8 View into a facility with high-intensity mixers for the processing of sand-lime brick aggregates (courtesy Eirich, Hardheim, Germany) Fig. 6.7-9 Diagram of a fluidized bed agglomerator for dry silica fume (courtesy Norchem Concrete Products, Fort Pierce, FL, USA) Fig. 6.7-10 Expanded clays, showing the porous interior and the fused ceramic skin (courtesy Fibo ExClay, Lamstedt, Germany) Fig. 6.7-11 Above) dimensions of fired (sintered) steatite parts obtained from green bodies with identical mass that were densified with different pressures, Lu = unsintered length, Lg = sintered length, Hu = unsintered height, Hg = sintered height; below) vertical and horizontal shrinkage that occurred during sintering, Sh = height shrinkage, Sq = length shrinkage [B.21] Fig. 6.7-12 Vertical and horizontal shrinkage occurring during the firing (sintering) of parts from raw materials with different densities, which were compressed to the same green density and the relation of both [B.21] Fig. 6.7-13 Diagram of maxima of properties, such as strength, that are dependent on both porosity decreases and grain size increasing as a function of firing temperature or time [B.80]. The dashed line suggests that maxima may change with different material and fabrication parameters Fig. 6.7-14 Typical robust double-screw extruder with integrally mounted pug sealer for the processing of stiff materials (courtesy J.C. Steele, Statesville, NC, USA) Fig. 6.7-15 Some typical extruded claybased building products (courtesy J.C. Steele, Statesville, NC, USA) Fig. 6.7-16 Expanded clay produced from green extrudates (courtesy J.C. Steele, Statesville, NC, USA) Fig. 6.7-17 Modern automatic mechanical press for the production of green ceramic bodies (courtesy Dorst, Kochel am See, Germany)
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List of Figures Fig. 6.7-18 Modern automatic hydraulic press for the production of green ceramic bodies (courtesy Laeis Bucher, Trier, Germany) Fig. 6.7-19 Example (ref. Fig. 6.7-18) of a mold use and performance table (courtesy Laeis Bucher, Trier, Germany) Fig. 6.7-20 Industrial ceramic parts demonstrating the detail that can be achieved with this technology (courtesy Komage, Kell an See, Germany, and Dorst, Kochel am See, Germany) Fig. 6.7-21 The density distribution obtained after the unidirectional compaction of particulate solids in a cylindrical mold Fig. 6.7-22 Areas of major density variation obtained during the unidirectional compaction of a part with variable cross sections [B.61] Fig. 6.7-23 Sketch of the density distribution and the location of the „neutral axis“ in a simple (cylindrical) green part obtained by symmetrical „double pressing“ Fig. 6.7-24 Drawings describing ways of influencing the location of the „neutral plane“ in unidirectional (upper punch) pressing by controlled die withdrawal [B.28, B.48, B.97] Fig. 6.7-25 Different „neutral planes“ in single- and multi-level parts [B.28, B.48, B.97] Fig. 6.7-26 Diagram of the differences between dry- and wet-bag pressing [B.13a, B.48, B.97] Fig. 6.7-27 Operational sequence of a dry-bag isostatic press for the manufacture of spark-plug blanks [B.13a, B.48, B.97] Fig. 6.7-28 Operational sequence of an automatic isostatic press with round, timed (sequenced) tooling table [B.13a, B.48, B.97] Fig. 6.7-29 Photograph and diagram of a dry-bag CIP system (courtesy Dorst, Kochel am See, Germany). 1, protective enclosure; 2, control cabinet; 3, hydraulic unit; 4, highpressure intensifier pump; 5, loading device; 6, hopper; 7, dosing device; 8, closing lever with preload cylinder at top; 9, upper tool; 10, device for parts removal from above (Fig. 6.7-30); 11, lower tool; 12, conveyor belt; 13, tool component for ejection from below (Fig. 6.7-30); 4, closing lever with preload cylinder at bottom Fig. 6.7-30 The three main demolding (extraction) methods (courtesy Dorst, Kochel am See, Germany)
Fig. 6.7-31 Some typical parts manufactured by cold isostatic powder pressing (courtesy Dorst, Kochel am See, Germany) Fig. 6.7-32 Tooling section of a horizontal isostatic press for the manufacture of tableware preforms: a) open section in a „free fall“ horizontal isostatic press, b) the two parts of the die system, c) parts of a raw ceramic bowl and the corresponding tooling design (courtesy Sama, Weissenstadt, Germany) Fig. 6.7-33 Some shapes and actual flat ware products that can be made with a „free fall“ horizontal isostatic press (courtesy Sama, Weissenstadt, Germany) Fig. 6.7-34 A „free fall“ horizontal isostatic press for the manufacture of tableware preforms (courtesy Sama, Weissenstadt, Germany) Fig. 6.7-35 Diagram of a rotary cement kiln [B.69] Fig. 6.7-36 Diagram of a coal (dust) fired wet process rotary kiln, also showing ancillary equipment and components [6.7.3.1] Fig. 6.7-37 Typical temperature profiles of gas and solids along the length of a rotary cement kiln [B.69] Fig. 6.7-38 Traveling grate for the drying and partial calcination of pellets (agglomerates, nodules) prior to entry into the rotary kiln [B.69] Fig. 6.7-39 Simplified flow diagram of a grate kiln cement clinker manufacturing facility (courtesy, Lurgi, Frankfurt/M., Germany). 1, 2, raw material receiving and precrushing; 3, 4, 5, raw material drying and milling; 6, 7, 8, 14, raw meal (6) and fuel (coal, 8) proportioning and mixing; 9, 10, drum agglomeration and pellets sizing (roller screen); 11, clinker screen; 12, sinter grate; 13, 15, dust collection Fig. 6.7-40 Sketch of a chain curtain for the heat transfer from the kiln gases to the slurry in a wet process rotary cement kiln [B.69] Fig. 6.7-41 Sketches of the relative sizes of different rotary cement kilns [B.69] Fig. 6.7-42 Diagram of a dry-process rotary cement kiln employing cyclones for the collection of entrained solids and the drying and pre-heating of the raw materials [6.7.3.2]
List of Figures Fig. 6.7-43 Artists impression of a complete dry-process rotary kiln cement manufacturing plant [6.7.3.2]. 1, primary crushing; 2, raw material storage; 3, raw meal grinding; 4, raw meal silos; 5, heat exchangers; 6, rotary kiln; 7, dust collection; 8, clinker cooler; 9, clinker storage; 10, coal grinding; 11, coal storage; 12, main air blower; 13, cement grinding; 14, gypsum storage; 15, gypsum grinding; 16, cement silos; 17, discharge facility; 18, bagging; 19, office and laboratory; 20, power distribution and plant controls Fig. 6.7-44 Sinter strand operating on the production of light-weight building material from fly ash [6.7.3.1] Fig. 6.7-45 Diagram of the manufacturing process of (hollow) building blocks made from lime and sand. 1, lime ball mill; 2, elevator; 3, wind sifter; 4, 9, 11, silos; 5, metering bucket; 6, screw conveyor; 7, sand feed; 8, screen; 10, batch mixing and hydration drum; 12, proportioning; 13, elevator; 14, stone press; 15, block cart; 16, transfer platform; 17, post-treatment (hardening) chambers; 18, steam production Fig. 6.7-46 Sketch of a manual pusher furnace [B.28, B.97] Fig. 6.7-47 Side elevation (diagram) of a directly flame-heated tunnel kiln for the firing of ceramic parts and typical temperature profile [B.13c] Fig. 6.7-48 Modern tunnel kilns for the firing of sanitary parts, tableware, and fine china, all with fully automated car systems, and of a completely automated handling area for loading and unloading (courtesy Eisenmann, Bo¨blingen, Germany) Fig. 6.7-49 A large periodic shuttle kiln for the firing of chimney flues (courtesy Eisenmann, Bo¨blingen, Germany) Fig. 6.7-50 A continuous kiln for the drying and firing of load-bearing structural bricks (courtesy Eisenmann, Bo¨blingen, Germany) Fig. 6.8-1 Graph showing the development of worldwide iron ore pelletizing capacity during the first 40 years (1950–1990) and projections at that time for further growth [B.48] Fig. 6.8-2 High-quality iron ore pellets (courtesy CVRD, Vitoria, Espirito Santo, Brazil) Fig. 6.8-3 Pellet hardening methods without firing [B.18]
Fig. 6.8-4 Tumble/growth agglomeration equipment for the balling of iron ores. (Left) Diagram of: a) the disc (pan), b) the cone, c) the drum (including recirculation circuit). (Right) Actual equipment (c: drum only) (courtesy Feeco, Green Bay, WI, USA (a and c), file photo, Kennedy Van Saun, New York, NY, USA (b)) Fig. 6.8-5 Typical balling drum circuit for the agglomeration of fine iron ores [B.48] Fig. 6.8-6 Diagram of the optimum charge movement in a balling drum [B.18] Fig. 6.8-7 The spiral discharge of a large balling drum for iron ore (file photo, McKee [Section 13.3, Ref. 16]) Fig. 6.8-8 a) Diagram of the principle of a roller screen in iron ore pelletizing; b) the design, and c) operation of such screens [B.97] Fig. 6.8-9 Influence of bentonite addition on acid gangue components in iron-ore pellets [B.18] Fig. 6.8-10 The influence of the amount of bentonite in a well-mixed iron ore concentrate feed on green and dry pellet compression strengths [B.18] Fig. 6.8-11 Diagrams of the three major furnace types used for the hardening of iron ore pellets: a) shaft, b) traveling grate, c) gratekiln; D, Drying; F, firing (sintering); C, cooling Fig. 6.8-12 Details of two shaft furnace designs: a) long shaft furnace with internal cooling, b) medium shaft furnace with external cooling and two alternative methods of sensible heat recovery [B.18] Fig. 6.8-13 Diagram of an early traveling grate sintering machine [B.97] Fig. 6.8-14 Principles of two traveling grate hardening systems for iron ore pellets [B.18]: a) McKee design; b) Lurgi–Dravo design Fig. 6.8-15 a) Side and hearth layer as well as green pellet feeding of a Lurgi-Dravo traveling grate machine. b) Sketch and thermal insulating effect of the hearth and side layers [B.18] Fig. 6.8-16 Sketch of the grate-kiln hardening systems for iron ore pellets [B.18]
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List of Figures Fig. 6.8-17 Isometric drawings of the three major iron ore pelletizing processes [B.48, B.97]. a) Balling drum circuits with shaft furnaces. b) Balling drum circuits with straight travelling grate machine. c) Balling drum circuit(s) or balling pan(s) with gratekiln hardening system Fig. 6.8-18 Partial view into the drum agglomeration section of on iron ore pelletizing plant showing five drums and associated process equipment Fig. 6.8-19 Simplified flow diagram of a complete iron ore pelletizing plant (courtesy CVRD, Vitoria, Espirito Santo, Brazil) Fig. 6.8-20 Views into the fine grinding bays of multi-million ton per year iron ore pelletizing plants using: a) semi-autogenous mills, b) rod and ball mills for size reduction Fig. 6.8-21 a) aerial view of US National Steel Pellet Companys Keewatin, Minnesota, concentration and pelletizing plant on the Mesabi (Taconite) iron range. b) aerial photograph of the pelletizing (only) facility of Quebec Cartier iron Co. at Port Cartier, Quebec, Canada. [6.8.1.3] Fig. 6.8-22 Simplified flow diagram of a regrind and pelletization process for natural ore [B.10] Fig. 6.8-23 Simplified flow diagram of a two-stage manganese ore agglomeration plant. Above) concentrate drying and balling stage; below) pellet hardening stage [B.35] Fig. 6.8-24 Diagram of a layered pellet [B.35]: 1, core (recycled undersize); 2, flue dust; 3, limestone; 4, lead containing concentrate + SiO2 + iron ore Fig. 6.8-25 Processing plant for the firing of roasted pyrite residues using CaCl2 as a binder and reactant for the extraction of nonferrous components [B.35]. 1, balling pan; 2, belt dryer (max. 250 8C); 3, shaft furnace Fig. 6.8-26 Flow diagram of a glass batch agglomeration system and photograph of the control panel (courtesy Philips Lighting BV, Winschoten, The Netherlands) Fig. 6.8-27 Green pellets discharging from a pan agglomerator Fig. 6.8-26 and of dry pelletized glass batch (courtesy Philips Lighting BV, Winschoten, The Netherlands) Fig. 6.8-28 Large scale pilot plant (10 tonne/ h) at the Steel Company of Wales, Margam, UK, for the hot briquetting of iron ore [Section 13.3, ref. 32]
Fig. 6.8-29 Flow diagram of a plant for the production of elemental phosphorus [Section 13.3, ref. 32] Fig. 6.8-30 Flow diagram of a sea-water magnesium oxide (magnesia) plant: 1, precipitating thickener; 2, pump; 3, washing thickener; 4, vacuum filter; 5, rotary hearth furnace; 6, screw conveyor/cooler; 7, 15, bucket elevator; 8, feed bin; 9, roller briquetting press with screw feeder; 10, screw conveyor; 11, screen; 12, 13, chip (undersized fines) recycling; 14, chip surge bin; 16, magnesia (sintering, dead-burning) kiln; 17, magnesia cooler Fig. 6.8-31 Two flow diagrams of precompaction arrangements for roller presses [B.48] Fig. 6.8-32 Salt briquettes that are used for the regeneration of ion-exchange water softeners Fig. 6.8-33 Flow diagram of a contemporary pan sintering plant [B.97] Fig. 6.8-34 Flow diagram of a modern sintering plant with improvements and cost saving features highlighted [B.48] Fig. 6.8-35 Diagram of a traveling grate sintering plant featuring enclosures and waste gas minimization by recycling [B.56, pp.83–93] Fig. 6.8-36 Sensor and control systems at the no.3 sintering plant at Anshan, China [B.56, pp. 450–454] Fig. 6.9-1 a) Schematic flow diagram of a mini-steel-mill using DR and EAF (courtesy Hylsa, San Nicolas de los Garza, NL, Mexico); b) Hadeed, Al Jubail, Saudi Arabia; foreground left: ore storage, center: two DR plants, upper right: steel mill (courtesy Midrex, Charlotte, NC, USA) Fig. 6.9-2 a) Mixture of iron ore lumps and pellets, b) top: direct reduced iron pellets, bottom: direct reduced iron lumps. Observe the slight cracking Fig. 6.9-3 a) SEM image of the internal surface of a DRI pellet; b) micrograph of a polished and etched cross section through the same pellet Fig. 6.9-4 US national press reacting to a DRI shipping accident (Section 13.3, ref. 69) Fig. 6.9-5 Cutaway sketches of a ship’s hold equipped for the transportation of DRI [6.9.2.1]: a) locations of thermocouples and oxygen and hydrogen monitoring, b) installation of inerting pipes
List of Figures Fig. 6.9-6 Briquettes made from hot fine DRI (courtesy FIOR, Puerto Ordaz, Venezuela) Fig. 6.9-7 Micrographs comparing the structures of direct reduced iron (DRI, left) and hot briquetted iron (HBI, right) (Section 13.3, ref. 146) Fig. 6.9-8 Schematic flow diagrams of three typical hot briquetting systems for DRI: a) left: hot pellet and/or lump feed from shaft furnaces; right: hot fines feed from fluidized bed reactors; both including hot fines recycling; b) hot pellet and/or lump feed without hot fines recycling: 1, roller press; 2, separator; 3, hot screen; 4, briquette cooler; 5, hot bucket elevator, fines recycling (Section 13.3, ref. 146) Fig. 6.9-9 Photograph showing differently sized HBI Fig. 6.9-10 Commercial HBI from a mixture of reduced pellets and lump ore (Section 13.3, ref. 146) Fig. 6.9-11 Left: five successive momentary conditions of briquetting between two counter currently rotating rollers with matching pockets showing that molds are never completely closed. Right: sketch of a defective pillow-shaped briquette from a roller press [B.48, B.97] Fig. 6.9-12 Top left, HBI being shipped by truck; top right, HBI being unloaded by magnet from a ship hold; bottom, outside storage of HBI in Venezuela before loading it on ships (courtesy Midrex, Charlotte, NC, USA, and OPCO, Puerto Ordaz, Venezuela) Fig. 6.9-13 Views into the hot briquetting bays of different merchant DR plants showing some of the roller presses (courtesy Orinoco Iron, Puerto Ordaz, Venezuela) Fig. 6.9-14 Panoramas of two merchant DR plants (courtesy Orinoco Iron, Puerto Ordaz, Venezuela (FINMET), and Midrex, Charlotte, NC, USA (COMSIGUA HBI, Matanzas, Venezuela)) Fig. 6.9-15 Schematic flow diagrams of steel mini-mills using DR with hot discharge: a) hot (pneumatic) transport to the EAF, hot briquetting of excess hot material (courtesy HYLSA, San Ni-colas de los Garza, NL, Mexico); b) typical hot link with transfer located under the DR furnace and external cooling of excess hot material (courtesy Midrex, Charlotte, NC, USA)
Fig. 6.9-16 Flow diagram of a hot briquetting plant for cast-iron borings [B.3, Vol. 10 (1965), 16–22] Fig. 6.9-17 Block diagram of a secondary aluminum smelting operation that includes compacting the processed swarf (Section 13.3, ref. 132) Fig. 6.9-18 Diagrams of the two methods used to compact aluminum swarf: a) punchand-die, b) roller press. 1, anvil (removable for ejection of compact; 2, die; 3, punch (reinprocating); 4, feed; 5, Compact; 6, rollers; 7, force (screw) feeder; 8, Compacted sheet (product). Fig. 6.9-19 a) Diagram of a hydraulic punch-and-die press for metal swarf briquetting featuring two-sided pressing; b) sketches explaining the operating principle of the press depicted in (a) (courtesy Metso Lindemann, Du¨sseldorf, Germany) Fig. 6.9-20 Flow diagram of a complete metal swarf processing and compacting system using a two-sided punch-and-die press for briquetting (courtesy Metso Lindemann, Du¨sseldorf, Germany) Fig. 6.9-21 Photograph of processed aluminum swarf and of briquettes produced with a punch-and-die press Fig. 6.9-22 Flow diagram of a processed metal (aluminum) swarf briquetting system using a roller press for compaction (explanations see text Fig. 6.9-23 Three different processed aluminum home scrap samples (top) and sheared compacted strips produced by the continuous roller press process (Fig. 6.9-22) Fig. 6.9-24 a) Flow diagram; b) Plant for the briquetting of 8 tonne/h processed aluminum home scrap (swarf) in two roller press compaction lines of 4 tonne/h each (Rhe´nalu, Neuf Brisach, France): 1, pneumatic transport system; 2, storage silos; 3, reversible horizontal belt conveyor; 4, dumpster; 5, elevators; 6, day bins; 7, roller presses; 8, rotating shears; 9, product collection carts Fig. 6.9-25 Collection of briquettes made from different metals with small, hydraulically operated punch-and-die presses (courtesy Ruf, Zaisertshofen, Germany) Fig. 6.9-26 a) Principle; b) photograph of a small, hydraulically operated punch-and-die press (courtesy Ruf, Zaisertshofen, Germany)
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List of Figures Fig. 6.9-27 SEM image of the internal structure of a mineral briquette showing particle disintegration and cracking (Section 13.3, ref. 23) Fig. 6.9-28 left) Three lime briquetting machines; right) close-up of briquettes on the discharge conveyor Fig. 6.9-29 Teflon cylinder with hemispherical ends, mounted in a reciprocating shaker, and used to form small spheres by the spherical agglomeration process [B.73, B.97] Fig. 6.9-30 Drum agglomerator with internal spiral screen classifier for the formation of uniform spheres by immiscible liquid agglomeration [B.73, B.97] Fig. 6.10-1 Flow diagram of a proposed coal pelletizing circuit [B.21] Fig. 6.10-2 Manual agglomeration of coal fines in Germany in about 1900 (Gewerkschaft Susanna, [B.1]) Fig. 6.10-3 Diagram of the ram extrusion, Exter, or reciprocating ram press [B.48] Fig. 6.10-4 Sequence of events during a briquetting cycle in a ram extrusion press [B.97] Fig. 6.10-5 Diagram of the decrease of elastic recovery and increase of density during consecutive press cycles in a ram extrusion press [B.97] Fig. 6.10-6 Cross section through a modern ram extrusion press (courtesy Zemag, Zeitz, Germany) Fig. 6.10-7 Flow diagram of one of the last lignite (brown coal) briquetting plants built in Germany [6.10.2.2] Fig. 6.10-8 Diagram of a rotating tube dryer for the reduction of moisture content of lignite or „brown coal“ (courtesy Zemag, Zeitz, Germany) Fig. 6.10-9 Partial view of the briquetting bay showing the drive side of the ram extrusion presses [6.10.2.2] Fig. 6.10-10 Partial view of the briquetting building showing the briquetter heads and the beginning of the cooling channels [6.10.2.2] Fig. 6.10-11 „Union-type“ briquettes being loaded into a rail car [6.10.2.2] Fig. 6.10-12 Different briquette shapes that can be obtained with ram extrusion presses Fig. 6.10-13 Development of „brown coal“ (bituminous coal) briquetting in Germany [6.10.2.3]
Fig. 6.10-14 a) Model and photograph of a Couffinhal press. b) Vintage general arrangement drawing of a Couffinhal press (courtesy Ko¨ppern, Hattingen/Ruhr, Germany) Fig. 6.10-15 Some typical coal briquettes (bricks) produced with punch-and-die presses Fig. 6.10-16 High-quality pillow- and eggshaped coal briquettes Fig. 6.10-17 The operating principle of roller briquetting presses Fig. 6.10-18 The „ring roller press“ [6.7.3.1] Fig. 6.10-19 Flow diagram of a modern anthracite (hard) coal mixing, drying, conditioning, briquetting, and cooling plant [6.10.2.2] Fig. 6.10-20 Major elevation of the plant depicted in Fig. 6.10-19 [6.10.2.2] Fig. 6.10-21 Sketch (partially cut, interior view right of the center line) of a vertical pug mill for steam conditioning mixtures of fine coal before briquetting [Section 13.3, ref. 111] Fig. 6.10-22 General arrangement drawing of a vintage (1913) roller press for the briquetting of hard coal with binder(s) (courtesy Ko¨ppern, Hattingen/Ruhr, Germany) Fig. 6.10-23 a) Elevation of an early roll type briquetting press designed by Schu¨chtermann and Kremer [B.13b] and b) detail of the rollers of a relatively modern machine (about 1980, courtesy Ko¨ppern, Hattingen/Ruhr, Germany), both featuring hollow roller cores Fig. 6.10-24 Vertical pug mill conditioner and roller press arrangements: a) general arrangement drawing dating from 1918; b) Photograph of the briquetting floor at the Rosenblumendelle mine in Germany (courtesy Ko¨ppern, Hattingen/Ruhr, Germany) Fig. 6.10-25 Photographs of a triple (top) and a quadruple (bottom) channel extrusion press (courtesy Krupp Fo¨rdertechnik GmbH, Essen, Germany) Fig. 6.10-26 a) Elevations and plan view of a modern large capacity roller press for the production of about 100 tonnes/h of coalbased compliance fuel with a binder [Section 13.3, ref. 134] (courtesy Ko¨ppern, Hattingen/ Ruhr, Germany); b) photograph of one of the latest large roller presses that was designed for the briquetting of coal (courtesy SahutConreur, Raismes, France)
List of Figures Fig. 6.10-27 Two waste materials and the resulting products after medium pressure agglomeration in a flat die pelleting machine (courtesy Amandus Kahl, Reinbek/Hamburg, Germany) Fig. 6.10-28 Typical barbecue charcoal briquettes Fig. 6.10-29 „Standard“ barbecue charcoal briquetting system Fig. 6.10-30 Block diagram depicting the different processes for the manufacturing of formed coke [Section 13.3, ref. 97] Fig. 6.10-31 Block diagram of the partial briquetting techniques for coke making [Section 13.3, ref. 97] Fig. 6.10-32 Mixture of briquettes and loose coal on its way to the coke ovens in one of the plants for partial briquetting (courtesy Iscor, New Castle Works, South Africa) Fig. 6.10-33 a) Large briquetting machine for the briquetting of coal (courtesy Ko¨ppern, Hattingen/Ruhr, Germany); b) partially assembled roller press showing the two sets of rings (courtesy Ko¨ppern, Hattingen/Ruhr, Germany); c) roller press installed in a plant for the partial briquetting of coke oven charges (courtesy Iscor, New Castle Works, South Africa) Fig. 6.10-33c Fig. 6.10-34 Flow diagram of the 6 tonne/h pilot plant of the proposed SCOPE 21 coal coking process [6.10.2.11] Fig. 6.10-35 Screened coal-based compliance fuel after the sizing (calibration) of briquettes (Section 13.3, ref. 134) Fig. 6.10-36 Simplified block diagram of the coal-fiber pellet manufacturing process [6.10.1.2] Fig. 6.10-37 Flow diagram of the proposed BioBinder extrusion process [6.10.1.2] Fig. 6.10-38 Flow diagram of a commercial oil agglomeration process for the recovery of fine coal [B.73] Fig. 6.10-39 Flow diagram of a sol-gel process for the formation of gel microspheres from a water based sol in a fluidized extraction column and the cleaning of the water extracting fluidizing liquid [B.97] Fig. 6.11-1 Photograph of a pan agglomeration system for gas cleaning media (courtesy Purafil, Doraville, GA, USA) Fig. 6.11-2 Vials with different pan agglomerated gas cleaning media (courtesy Purafil, Doraville, GA, USA)
Fig. 6.11-3 a) Diagram of a deep bed filtering and decontamination system; b) photograph of a so-called deep bed scrubber (courtesy Purafil, Doraville, GA, USA) Fig. 6.11-4 a) Principle of an extended surface gas cleaning system, b) artists impression of a partially open unit (courtesy Purafil, Doraville, GA, USA) Fig. 6.11-5 Examples of media solutions for some of the more important contaminant gases (courtesy Purafil, Doraville, GA, USA) Fig. 6.11-6 a) Cross-sectional view of a typical chemical oxygen generator, b) actual equipment for aircraft (courtesy Puritan Bennet, Lenexa, KS, USA) Fig. 6.11-7 a) Typical altitude profile (cabin altitude against time after decompression), b) trace of the actual output of an aviation chemical oxygen generator as compared with the flow requirement profile (courtesy Puritan Bennet, Lenexa, KS, USA) Fig. 6.11-8 Stock shapes (sheets, rods, tubes, plaques, rings discs, and bars) and simple parts (bushings) of Vespel polyimide non-melt processable compound (courtesy, DuPont, Newark, DE, USA) Fig. 6.11-9 a) Complex near-net or net shape parts for wafer handling and processing, b) IC (integrated circuits) handling and testing and other semiconductor manufacturing made of Vespel polyimide non-melt processable compound (courtesy, DuPont, Newark, DE, USA) Fig. 6.11-10 Re-agglomeration (aggregation) of electrostatically dispersed SiO2, Aerosil OX50 (manufacturer Degussa): left) original sample; center) at the end of dispersion/charging; right) 10 min after dispersion/mixing [6.11.3.1] Fig. 6.11-11 SEM photograph of relatively large spherical lactose with immobilized (adhering) nanosized SiO2 particles [6.11.3.1] Fig. 6.11-12 Different possible effects of mechanofusion [B.48, B.97] Fig. 6.11-13 Comparison between the fluidization mechanisms in a conventional fluid bed and the rotating Omnitex FB processor (courtesy Nara, Tokyo, Japan) Fig. 6.11-14 Diagram of the principle of the Omnitex FB processor (courtesy Nara, Tokyo, Japan) Fig. 6.11-15 Diagram of the manufacturing routes to non-wovens [B.107]
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List of Figures Fig. 6.11-16 SEM micrograph of the cross section through a laminated non-woven household product that is composed of synthetic leather and microfiber non-wovens. Layer sequence (from top to bottom): macroporous synthetic leather surface, nonwoven absorbent core, microfiber non-woven [B.107] Fig. 6.11-17 Summary of the different bonding processes as described in an ISO standard (ISO/DIS 11224 [B.107]) Fig. 6.11-18 Diagram of some possible nonwoven bonding sites: a) large area, enveloping fiber intersection points, b) small area and punctiform bonding, c) bonding of fiber intersection points [B.107] Fig. 6.11-19 Two micrographs of non-wovens depicting fiber bonding [B.107] Fig. 7.1 Some complex parts produced by powder metallurgy (courtesy of KomageGellner, Kell am See, Germany, and Dorst, Kochel am See, Germany) Fig. 7.2 Process diagram of powder metallurgy (courtesy of MPIF, Princeton, NJ, USA) Fig. 8.1 Example of an industrial plant (pyroprocessing of minerals) heavily polluted with airborne particulate solids Fig. 8.2 a) Enlarged view of the interior of a filter mat with particles sticking to the fiber surfaces. b) Detail of a dust laden fiber from (a) showing how particles extend into the gas stream [B.71] Fig. 8.3 Naturally formed agglomerate of small (8 lm) glass spheres adhering to a filter fiber photographed during a laboratory experiment [B.48, B.97] Fig. 8.4 String-like agglomerates of „brown smoke“ particles produced by natural magnetic coagulation [B.48] Fig. 8.5 Dendritic growth of „brown smoke“ agglomerates (Fig. 8.4) in an electrostatic field [B.48] Fig. 8.6 Loading of a truck by heavy duty clam shell grab via dockside mobile loader (DML) to avoid pollution of the environment [8.1.1] Fig. 8.7 Demonstration of the importance of droplet size for particle agglomeration Fig. 8.8 Natural flocculation of solid contaminants in river water [B.48]. Parameters are the circumferential speed of the stirrer and the processing time
Fig. 8.9 a) Diagram; b) photograph of a circular thickener/clarifier (according to EIMCO, Div. Baker Hughes, South Walpole, MA, USA) Fig. 8.10 Principle of polymer adsorption and flocculation [B.29]: a) adsorption of polymer molecule on the particle; b) rearrangement of adsorbed chain; c) collisions between destabilized particles and bridging to form aggregates (flocs); d) break-up of flocs Fig. 8.11 Structure of a flocculate (floc) bonded by a polymer [B.48] Fig. 8.12 a) Diagram of polymer bridging between particles; b) restabilized particles [B.29] Fig. 8.13 Diagram representation of two particles with electrical double layers in a liquid [B.48] Fig. 8.14 Sketch of belt conveyor agglomeration [B.48, B.97] Fig. 8.15 a) Diagram of shaking trough; b) sketch of vibrating deck agglomeration [B.48, B.97] Fig. 8.16 Reversed belt agglomeration [B.48, B.97] Fig. 8.17 Heavy-duty drum compactor (courtesy S&G Enterprises, Germantown, WI, USA) Fig. 8.18 Sulfur absorption in the flue system of a coal-fired power plant (EVS, Heilbronn, Germany): left) absorber; right) limestone processing and gypsum recovery (briquetting) systems Fig. 8.19 Size enlargement of particulate solid waste by growth/tumble agglomeration (courtesy Eirich, Hardheim, Germany) Fig. 8.20 Pelleting of wet FGD gypsum by medium-pressure agglomeration (flat die pelleting, courtesy Amandus Kahl, Reinbek, Germany) Fig. 8.21 Flow diagram of a FGD gypsum roller press briquetting plant. Thermal drying of the feed is not shown Fig. 8.22 Roller press for the briquetting of (synthetic, FGD) gypsum in a flue gas desulfurization plant: inset, synthetic gypsum briquettes (courtesy Ko¨ppern, Hattingen/ Ruhr, Germany) Fig. 8.23 Flow diagram of a generic briquetting plant for metallized, metal-bearing, or other recyclable fines Fig. 8.24 Close-up view of a pile of typical briquettes produced by roller presses for recycling
List of Figures Fig. 8.25 Flow diagram of a plant for the production of paper fluff from old newsprint, as a binder for use in a roller-press briquetting plant [8.2.1] Fig. 8.26 Effect of binder type on the crushing strength of briquettes made from metallurgical dust [8.2.1]. Amounts of binder added: waste paper, 2 %; molasses, 6 %; lime, 6 %; starch, 6 % Fig. 8.27 Cold crushing strength of briquettes from metal-bearing dust and sludge containing different amounts of dry lignosulfonate binder (called „sulfite waste powder“) with and without reinforcement by the addition of swarf [B.48, B.97] Fig. 8.28 a) Broken cylindrical compact that was manufactured during process development with a laboratory punch-and-die press; b) actual briquettes obtained in an industrial plant [B.48, B.97] Fig. 8.29 Compact briquetting system for metallurgical filter dust. Capacity 11–20 tonne/h, depending on feed characteristics (courtesy: Thyssen Stahl AG, Krefeld, Germany) Fig. 8.30 Size enlargement by agglomeration in iron and steel making for the minimization of terminal residue through recycling and the production of secondary raw materials [Section 13.3, refs 129, 166] Fig. 8.31 Disc pelletizer for the recycling of waste dusts, slurries, and plant fines (courtesy Heckett MultiServ, Butler, PA, USA) Fig. 8.32 Briquetting plant for and typical briquette from steel mill by-products (courtesy Heckett MultiServ, Butler, PA, USA) Fig. 8.33 Stacks of pellets (left) and briquettes (right) that are ready for recycling (courtesy Heckett MultiServ, Butler, PA, USA) Fig. 8.34 Three different MRF aluminum wastes as collected and processed (top) and after roller press compaction [Section 13.3, ref 132]. From left to right: loose foil, densified (granulated) foil, delaquered and hammer-milled used beverage cans (UBC) Fig. 8.35 Flow diagram of the pelleting section of the secondary solid fuel manufacturing plant at Schwarze Pumpe, Germany (courtesy Amandus Kahl, Reinbek, Germany) Fig. 8.36 Four flat die pellet presses of the secondary solid fuel manufacturing plant at Schwarze Pumpe, Germany (courtesy Amandus Kahl, Reinbek, Germany)
Fig. 8.37 Generic block diagram of a versatile plant using extrusion in the production of lightweight aggregate from waste materials [8.2.4] Fig. 8.38 Flow diagram and artist’s impression of a recycling plant for the production of secondary raw material (DRI) from iron-bearing oxide fines, employing a rotary hearth furnace (FASTMET) for the reduction of pellets with coal as reductant (courtesy Midrex, Charlotte, NC, USA) Fig. 8.39 Flow diagram of a recycling plant for the manufacturing of secondary raw material (DRI) from iron-bearing dusts, employing a multiple hearth furnace (PRIMUS) for the production of highly metallized iron concentrate with coal as reductant and the recovery of high-purity lead and zinc (courtesy Paul Wurth, Luxembourg) Fig. 8.40 Arrangement drawing and flow diagram of the plant (future execution) of a German recycler of wood and building materials, ordinarily producing „fluff“ (courtesy Amandus Kahl, Reinbek, Germany) Fig. 8.41 Photograph of the system depicted in Fig. 8.40 in its current four mill execution (courtesy Amandus Kahl, Reinbek, Germany) Fig. 8.42 Pelleted secondary fuel, right, from combustible domestic and industrial waste, left (courtesy Amandus Kahl, Reinbek, Germany) Fig. 8.43 Diagram of the new overall sludge management of OCUA [Section 13.3, ref. 107] Fig. 8.44 Flow diagram of the compaction/ granulation system at OCUA [Section 13.3, ref. 104, 107] Fig. 8.45 Photograph of the outside product storage silos at OCUA (courtesy Ocean County Utility Authority, Toms River, NJ, USA) Fig. 8.46 Simplified flow diagram of a facility using a flat die pellet press and a belt dryer/cooler for the production of secondary solid fuel from digested sewage sludge (courtesy Amandus Kahl, Reinbek, Germany) Fig. 8.47 a) Pellet press and b) dryer/cooler in an actual plant according to Fig. 8.46 (courtesy Amandus Kahl, Reinbek, Germany) Fig. 8.48 Simulation of liquid flow in a heap constructed of non-agglomerated ore [8.2.5]
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List of Figures Fig. 8.49 Simulation of liquid flow in a heap constructed of narrowly sized (agglomerated) ore [8.2.5] Fig. 8.50 Diagram of stockpile agglomeration [B.48, B.97] Fig. 9.1 Cartoon allegorizing the frequently experienced development, design, manufacture, and installation of a new industrial plant Fig. 9.2-1 Various views of support laboratories Fig. 9.2-2 Fig. 9.3 Typical test facility for the evaluation of the performance of particulate solids during mechanical processing Fig. 9.4 General purpose test facility with a bowl-type mixer agglomerator Fig. 9.5 Common test set-up for the development of growth agglomeration in a pan agglomerator Fig. 9.6 Partial view of a test center in which two deep pan mixer agglomerators are shown Fig. 9.7 Test facility for the evaluation of fluidized bed processes Fig. 9.8 Test facility of a roller press manufacturer featuring presses with large roller diameters Fig. 9.9 Agglomerate strength testers: top) hydraulic four-column press for the determination of compression strength; bottom) from left, rotating tube for measuring degradation at transfer points, drop test arrangement, and drum abrasion tester Fig. 9.10 Laboratory test equipment: a) mixers, b) sample splitters and screen, c) feeder and mills, d) ovens Fig. 9.10c Fig. 9.10d Fig. 9.11 Product evaluation and recycle processing Fig. 9.12 Pilot system in which continuous processing by extrusion of particulate solids is carried out Fig. 9.13 Manufacturer’s pilot plant with a roller press for high-pressure agglomeration of dry particulate solids Fig. 9.14 Temporarily assembled system for the testing of iron ore processing, pelletizing, firing (sintering), and cooling in a specialized vendor’s facility Fig. 9.15 Test facility of a tabletting machine manufacturer Fig. 9.16 Testing facility of a coating equipment manufacturer
Fig. 9.17 Food processing (forming, cooling, and cutting) test center Fig. 9.18 „Clean“ room of the test center of a roller press manufacturer Fig. 9.19 Partial reproduction of a brochure showing, as an example, the separate product development and manufacturing facility of a vendor (courtesy Fluid Air, Inc., Aurora, IL, USA) Fig. 9.20 Building housing the JRS contract service department at the company’s headquarters complex (courtesy J. Rettenmaier & So¨hne (JRS), Rosenberg, Germany) Fig. 9.21 Collage of diagrams from the JRS contract service department catalogue describing the capabilities (courtesy J. Rettenmaier & So¨hne (JRS), Rosenberg, Germany) Fig. 9.22 Artist’s impression and photographs of the Stellar back-up and co-manufacturing facility (courtesy Stellar Manufacturing Co., Sauget, IL, USA) Fig. 9.23 Some of the major capabilities of Stellar Mfg. Co. for particle enlargement, reduction, and sizing, product handling and packing and support facilities (courtesy Stellar Manufacturing Co., Sauget, IL, USA) Fig. 9.24 Sketch of the layout of a tolling plant showing its extensive warehousing areas (courtesy Stellar Manufacturing Co., Sauget, IL, USA) and distribution facilities (courtesy IFP, Inc., Faribault, MN, USA) Fig. 9.25 Diagram of the paths of differently sized agglomerates in a balling pan during operation Fig. 9.26 Diagram depicting the result of the measurement of power consumption during growth agglomeration in a batch operating drum or mixer [B.97] Fig. 9.27 Operating (working) volume of batch spheronizers [B.97] Fig. 9.28 Example of a pressing force against displacement (densification) diagram [B.97] Fig. 9.29 Conditions in the nip between two counter-rotating smooth rollers during the passing of a particulate solid Fig. 9.30 Nip shape and size between rollers with different diameters Fig. 10.1 Conceptual model describing how a small particle is incorporated into the surface of a (wet) agglomerate during tumble/ growth agglomeration [B.48, B.97]
List of Figures Fig. 10.2 Sketches of the systematic arrangement of differently sized particles to obtain dense packing [B.97] Fig. 10.3 Model explaining how fine particles embed larger pieces in the structure of an agglomerate [B.48] Fig. 10.4 Some geometrical approximations of the form and proportions of particles [B.97] Fig. 10.5 „Standard set of shapes“ for the determination of particle sphericity according to Rittenhouse [B.48] Fig. 10.6 Model sketches depicting the influence of surface roughness on the approach of particles to each other (left) and wetting (liquid bridge formation, right). The crosshatched area is the actual bridge. a* represents the mean distance between the particles, the outlines of the ideal particles (averaging out roughness), and the theoretical bridge contours assuming perfect wetting Fig. 10.7 Model explaining the effect of adsorption layers on van der Waals bonding [B.48, B.97] Fig. 10.8 Several different particle size distributions responding to the same specifications (x50 = 120 lm, x5 = 10 lm, x98 = 300 lm) Fig. 10.9 Circles (defining particle diameter) with the same area as the projections of a needle shaped particle. The situations on the right represent various positions in space (viewed in directions x and y) of the same particle Fig. 10.10 Micrographs of needle-shaped particles (crystallites) featuring large aspect ratios Fig. 10.11 Diagram and photograph of the interior of a vacuum double cone mixer with knife heads (courtesy Italvacuum, Borgaro, Italy) Fig. 10.12 Diagram of the conditions at the coordination point between two particles [B.48] Fig. 10.13 SEM images depicting particles with different shapes and roughnesses
Fig. 10.14 Diagram of the attachment of functional molecules to the surfaces of solids [B.95]. Left: a) adsorption, molecules are mobile, b) contact between neighboring molecules, c) several molecules create stable islands, d) an ordered molecular film develops. Right: Above a critical concentration amphiphilic molecules aggregate spontaneously and form microscopic micelles in the liquid phase Fig. 10.15 Sketches explaining the mechanisms of pressure agglomeration (see also Fig. 5.9) Fig. 10.16 Typical, fictitious pressure/densification plots developing during high-pressure agglomeration [Chapter 13.3, ref. 147] Fig. 10.17 Typical operational diagram of the hydraulic pressurizing system for the floating roller of a high-pressure roller press [B.97] Fig. 10.18 Diagram of a hydraulic pressurizing system for the floating roller of a highpressure roller press [B.97] Fig. 10.19 Partial operational hydraulic diagram showing the effects of different compressed gas volumes in the accumulator(s) of a high-pressure roller press during operation (Fig. 10.17) Fig. 11.1 TEM images of different nanoscaled products (courtesy Degussa AG, Hanau, Germany) Fig. 11.2 Formation mechanisms that are relevant in gas-phase synthesis of particles [11.1] Fig. 11.3 Diagram of a TEM grid sampling system mounted to a flame reactor, TEM images of sampled product, and results of simulations [11.1] Fig. 11.4 Diagram of two methods applied to avoid agglomeration during the production of nanoparticles [11.2] Fig. 11.5 Possible methods for the modification of nanoparticles for improved handling and processing [11.2] Fig. 11.6 SEM images of submicron poly(vinylidene fluoride) (PVF2) particles on a polished silicon substrate (silicon wafer, left) and a polyester copolymer (right). The (vander-Waals) attraction force between the particles and the substrates is so great that particles embed into the soft polymeric material (right) but not into the silicon (left), although in the latter case they are flattened at the contact points [B.39, Vol. 2, p. 51]
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List of Figures Fig. 11.7 Concept of a process for the assembly of small powder particles on a substrate that is assisted by electron beam drawing [5.3] Fig. 11.8 Diagram of the steps that are required for the charge-assisted deposition of small particles by electron beam drawing [5.3] Fig. 11.9 SEM images of silica spheres arranged along negatively charged lines [5.3] Fig. 11.10 Present (2002) distribution of nanotechnology start-ups and spin-offs in the USA [6.2.3.1] Fig. 11.11 Diagram describing hollow capsule production by exploiting colloidal templating and self-assembly methods [11.8] Fig. 11.12 Photomicrograph of a hollow TiO2 microsphere after calcination at 650 8C. The wall thickness of the 20 lm diameter sphere is about 200 nm [11.9] Fig. 11.13 Depiction of the formation of either a dense or porous capsule during encapsulation and coating processes [11.9]
Fig. 11.14 Diagram of the formation of nanostructures by self-assembly of complementarily functionalized nanobeads: a) linking of two classes of beads, b) attachment of smaller beads to a larger one [B.95] Fig. 11.15 Flow diagram and photograph of a hybridization system (courtesy Nara, Tokyo, Japan) Fig. 11.16 Layout of a modern furnace black manufacturing facility and cross section through the reactor (5) [11.11] Fig. 11.17 Primary particle size distributions of different carbon black varieties [11.11] Fig. 11.18 Diagram of a dry agglomeration system for the granulation of furnace black in a rotating drum [11.12] Fig. 11.19 Three different carbon black products: 1) powder, 2) dry granulated, 3) wet granulated [11.11]
List of Tables
List of Tables
Tab. 3.1 Binding mechanisms of agglomeration Tab. 4.1: The occurrence of undesired and desired agglomeration in mechanical and related process technologies. Tab. 4.2: Some advantages of uncontrolled, natural agglomeration. Tab. 4.3: Summary of some of the possibilities to avoid or at least lessen the effect of unwanted agglomeration Tab. 5.1: Some conditions, reasons, and requirements for the selection of tumble/ growth agglomeration. Tab. 5.2: Some conditions, reasons, and requirements for the selection of pressure agglomeration. Tab. 6.1 General advantages of agglomerated products Tab. 6.2-1 Reasons for and/or results of size enlargement by agglomeration in pharmaceutical applications Tab. 6.2-2 Advantages of (pre-) granulation in the pharmaceutical industry (adapted from Shangraw [6.2.1.1]) Tab. 6.2-3 Early history of tabletting for pharmaceutical specialties [B.48] Tab. 6.3-1 Chemicals (in alphabetical order) that are cited in the sales literature of different vendors of equipment for size enlargement by agglomeration Tab. 6.3-2 The main components of laundry detergents and their functions [B.60, B.102] Tab. 6.3-3 Examples of some inorganic and organic pigments Tab. 6.4-1 Some selected examples of food systems with non-equilibrium behavior (adopted from Chapter 3 in [B.53]) Tab. 6.4-2 Methods that have been adopted in the food and dairy industry for liquid removal [6.4.1.1, B.97] Tab. 6.4-3 Mechanisms occurring during the dispersion and, respectively, dissolution of powder in a liquid Tab. 6.4-4 Principles that are most commonly used to manufacture instant products from powdered food materials [B.97]
Tab. 6.4-5 List of some dry and dried food materials as well as mixed food formulations that are converted by growth agglomeration into free flowing, granulated, often instant, and dust free products for easy packaging, metering, and reconstitution Tab. 6.4-6 Listing of some materials that are processed with roller extrusion presses and associated equipment in the food industry (in alphabetical order) Tab. 6.5-1 Summary of the most important reasons for size enlargement by agglomeration in the animal feed industry Tab. 6.5-2 Typical ingredients and range of compositions of milk replacers [6.4.1.1] Tab. 6.5-3 Typical generic recipe of a high quality dry dog food suitable for pelleting [6.5.2.2] Tab. 6.6-1 Potential problems associated with the processing and handling of powdered fertilizer formulations Tab. 6.6-2 Primary-, secondary-, and micronutrients for plant fertilization, each in alphabetical order [6.6.2] Tab. 6.6-3 Specific pressing force, water content, and feed particle size that were determined for the compaction of some fertilizer materials in roller presses [B.48] Tab. 6.7-1 Alphabetical listing of raw materials and additives for building materials and ceramics that have been and/or are being agglomerated and the products using or representing agglomerated parts Tab. 6.7-2 Advantages of ESCS in the concrete building industry (adapted from information provided by The Expanded Shale, Clay, and Slate Institute, Salt Lake City, UT, USA, and Fibo ExClay, Lamstedt, Germany) Tab. 6.7-3 Summary of the basic principles of isostatic powder pressing [B.97] Tab. 6.7-4 Comparison of diameter, length, and clinker production (throughput) of wet and dry process cement kilns [B.69] Tab. 6.7-5 Some important characteristics of finished ceramic or building materials Tab. 6.7-6 Processes occurring during the sintering of most ceramic parts [B.13c]
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List of Tables Tab. 6.8-1 Alphabetical listing of minerals and ores that have been and/or are being agglomerated Tab. 6.8-3 Advantages and disadvantages of shaft furnaces for the induration of iron ore pellets Tab. 6.8-4 Ranges of thermal treatment zones for different ore compositions (Figure 6.8-14b) [B.18] Tab. 6.9-1 Alphabetical listing of raw materials, additives, metal products, and metal bearing wastes that have been and/or are being agglomerated to obtain various benefits Tab. 6.9-2 Advantages of briquettes made from hot cast iron borings as a melt charge for foundries [B.3, Vol. 10 (1965), 16-22] Tab. 6.10-1 List of some materials that can be used as or converted to solid fuels and have been or are being processed most commonly with agglomeration technologies to improve their properties (see also Tab. 6.10-3) Tab. 6.10-2 Development of hard coal briquetting with binder(s) using roller presses in some of the coal mining areas of Germany (Ruhr, Aachen, Lower Saxonia) Tab. 6.10-3 Examples of some new materials, including biomass and opportunity fuels, for which Ram Presses have been successfully applied [6.10.2.7] Tab. 6.10-4 Goals of formed coke developments Tab. 6.11-1 Some areas that are cleaned with gas-phase filters (adapted from Purafil, Doraville, GA, USA) Tab. 6.11-2 Some current application of non-wovens (adapted from [B.107]) Tab. 7.1 Advantages of the P/M process and of parts made by it (adapted from MPIF, Princeton, NJ, USA) Tab. 8.1 Origins of particulate wastes and sources of pollution (presented in alphabetical order) Tab. 8.2 Origin, forms, and characteristics of particulate solid wastes Tab. 8.3 Possible effects and applications of size enlargement by agglomeration in environmental control Tab. 8.5 Areas in which size enlargement by agglomeration benefits solid waste management Tab. 8.6 Materials that are successfully pelletised or briquetted by Heckett MultiServ (Section 15.1) around the world
Tab. 8.7 Commonly considered advantages of material recycling facilities [8.2.3] Tab. 8.8 Breakdown of MRF separated recyclables in a specific plant as well as typical processing costs and expected market prices (US Midwest, end of 1992) of the resulting secondary raw material (adopted and modified from [8.2.3]) Tab. 8.9 Approximate compositions of two common feed mixtures to the pelleting machines of the secondary solid fuel manufacturing plant at Schwarze Pumpe, Germany (Courtesy Amandus Kahl, Reinbek, Germany) Tab. 8.10 Limitations on the contents of tramp material in the feed to the pellet presses for the production of secondary solid fuels from waste (Courtesy Amandus Kahl, Reinbek, Germany) Tab. 8.11 Characteristics of OCUA (Ocean County Utility Authority, NJ, USA) dried sludge powder [Chapter 13.3, Lit. 107] Tab. 9.1 Considerations during the selection of a suitable agglomeration process for a particulate project [B.71] Tab. 10-1 Examples of guide words used in a HAZOP study (adapted from an ICI HAZOP seminar) Tab. 10-2 Helpful phrases for a study leader to expose reality (adapted from an ICI HAZOP seminar) Tab. 10-3 Draft principles of “Green Engineering” (adapted from [10.4]) Tab. 10-4 Effluent stream analysis (adapted from [10.3] Tab. 10-5 Troubleshooting fluid bed spray granulators (adapted from an unpublished guideline by Niro, Inc., Columbia, MD, USA) Tab. 11.1 Some major effects of decreasing size on properties of fine and ultrafine (nano) particles [B.97, 11.1] Tab. 12.1 Examples of industrial materials that are produced and/or modified with size enlargement by agglomeration and listing of the methods that are most commonly applied Tab. 12.2 Some of the more common benefits of the larger entities that are produced with size enlargement by agglomeration and typical current industrial applications
