Decanter Centrifuge Handbook

Decanter Centrifuge Handbook

1st Edition

This Page Intentionally Left Blank

Decanter Centrifuge Handbook 1st Edition

Alan Records Ken Sutherland

ELSEVIER ADVANCED TECHNOLOGY

UK

USA JAPAN

Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB.UK Elsevier Science Inc. 665 Avenue of the Americas, New York, NY 10010,USA Elsevier Science Japan, Tsunashima Building Annex, 3-20-12 Yushima, Bunkyo-ku, Tokyo 113, Japan Copyright Q 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. First edition 2001 Library of Congress Cataloging-in-Publication Data Records, Alan Decanter centrifuge handbook / Alan Records, Ken Suther1and.-1st ed. p. cm. Includes index. ISBN 1-8 5 6 17-369-0 (hardcover) 1.Centrifuges-Handbooks, manuals, etc. 2. CentrifugationHandbooks, manuals, etc. I. Sutherland, Ken. 11. Title. QD54.C4 R43 2000 660'.2842-d~21 00-049 524 British Library Cataloguing in Publication Data A catalogue record for this title is available from the British Library.

ISBN 1 8 5 6 1 7 369 0

No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Published by Elsevier Advanced Technology, The Boulevard, Langford Lane, Kidlington, Oxford OX5 l G B , UK Tel: +44(0) 1865 843000 Fax: +44(0) 1865 843971 Typeset by Variorum Publishing Ltd, Rugby Transferred to digital printing 2005 Printed and bound by Antony Rowe Ltd, Eastboume

CONTENTS Preface and Acknowledgements Chapter 1

Introduction

1.1

1.2

1. 3 1. 4 1.5 Chapter 2

xiii

The Decanter Centrifuge 1.1.1 The basic decanter 1.1.2 Separation principle 1.1.3 Decanter applications The History of the Decanter 1.2.1 Origins 1.2.2 Machine and application development Decanter Manufacturers Present Trends References

2 2 3 5 6 6 8 10 13 14

Decanter Design

2.1 2.2

Basic Construction Basic Components 2.2.1 Orientation 2.2.2 Flow 2.2.3 Materials of construction 2.2.4 Bowl 2.2.4.1 Front hub 2.2.4.2 Centrate weirs 2.2.4.3 Liner 2.2.4.4 Front hub bearing 2.2.5 Beach 2.2.5.1 Rear hub and bearings 2.2.5.2 Cake discharge 2.2.5.3 Liner 2.2.6 Conveyor 2.2.6.1 Conveyorhub 2.2.6.2 Flights 2.2.6.3 Feedzone

17 19 19 19 21 21 22 22 23 24 25 26 28 28 29 29 31 31

vi

2.3

2.2.6.4 Floc/rinse zone 33 2.2.6.5 Wear protection 33 2.2.6.6 Conveyor bearings and seals 34 2.2.7 Gearbox 36 Frame 2.2.8 37 2.2.8.1 Bearing supports 38 2.2.8.2 Feed tube 38 2.2.8.3 Vibration isolators 39 2.2.9 Casing 40 2.2.9.1 Casing baffles 41 2.2.9.2 Cake discharge 41 2.2.9.3 Centrate discharge 42 2.2.9.4 Casing seals 42 2.2.9.5 Vents 42 2.2.10 Sub-frame 43 2.2.11 Main drive 43 2.2.12 Back-drive 45 Variations to Main Components 47 2.3.1 Orientation 47 2.3.1.1 Vertical vs. horizontal 47 2.3.1.2 Vertical decanter seals and bearings 4 9 2.3.1.3 Vertical decanter casing seal 51 Flow 51 2.3.2 Materials of construction 52 2.3.3 54 Bowl variants 2.3.4 2.3.4.1 Front hub 54 2.3.4.2 Centrate weirs 55 2.3.4.3 Liner 56 2.3.4.4 Main bearing 58 Beach 59 2.3.5 2.3.5.1 Rear hub 61 2.3.5.2 Cake discharge 61 2.3.5.3 Beach liner 64 Conveyor 64 2.3.6 2.3.6.1 Conveyor hub 66 2.3.6.2 Flights 66 2.3.6.3 Feedzone 67 2.3.6.4 Floc/rinse zone 69 2.3.6,s Wear protection 71 2.3.6.6 Bearings and seals 73 Gearbox 73 2.3.7 76 Frame 2.3.8 2.3.8.1 Bearing supports 76 2.3.8.2 Feed tube 76

vii

2.4

2.3.8.3 Vibration isolators Casing 2.3.9 2.3.9.1 Baffles 2.3.9.2 Cake discharge 2.3.9.3 Centrate discharge 2.3.9.4 Casing seals 2.3.9.5 Vents 2.3.10 Sub-frame 2.3.11 Main drive 2.3.12 Back-drive Special Features Basic construction 2.4.1 2.4.1.1 Screen-bowl decanter 2.4.1.2 Three-phase decanter 2.4.1.3 The countercurrent extractor decanter 2.4.1.4 Decanters for temperature and pressure extremes 2.4.1.5 The cantilevered bowl 2.4.1.6 The “hubless” conveyor 2.4.1.7 Thickening decanter 2.4.1.8 The dual beach decanter Centripetal pump 2.4.2 Skimmer pipe 2.4.3 Centrate weir design 2.4.4 2.4.4.1 Cup dam 2.4.4.2 Notcheddam 2.4.4.3 Inflatable dam 2.4.5 Noise suppression Bowl baffles 2.4.6 2.4.6.1 Cake baffledisc 2.4.6.2 Bafflecone 2.4.6.3 Floater disc 2.4.6.4 Conveying baffle 2.4.6.5 Longitudinal baffle 2.4.7 Clarification enhancement 2.4.7.1 Quasi-axial flow 2.4.7.2 Fully axial flow 2.4.7.3 Vanes 2.4.7.4 Discs 2.4.8 Conveyor rake 2.4.9 Conveyor tiles 2.4.10 Conveyor pitch 2.4.10.1 Variable pitch

77 77 77 78 79 79 80 80 80 82 86 86 86 86 89 90 90 90 90 92 93 95 96 96 96 97 97 99 99 100 101 102 103 104 104 105 105 106 107 108 109 109

viii

2.5 Chapter 3

2.4.10.2 Reverse pitch 2.4.1 1 Counterbalance and scraper flights 2.4.12 Feedzone 2.4.13 The reslurry collector 2.4.14 CIP 2.4.1 5 The Rotodiff 2.4.16 Power regeneration 2.4.1 7 Dual main drive motor 2.4.18 Floating conveyor 2.4.19 Decanter controls References

Applications

3.1 3.2 3.3

Application Classes Application Analysis Waste Sludge Processing 3.3.1 Industrial wastes 3.3.2 Water treatment sludges 3.3.3 Municipal sewage treatment 3.4 Energy Materials Production 3.5 Processed Fuels 3.6 Minerals Extraction and Processing 3.7 Food and Food By-products Meat and meat products processing 3.7.1 3.7.2 Fish processing 3.7.3 Fruit andvegetable products 3.7.4 Other food processing 3.8 Beverages 3.9 The Chemicals Industry 3.9.1 Bulk inorganic chemicals 3.9.2 Bulk organic chemicals 3.9.3 Fine and household chemicals 3.9.4 Pharmaceutical and medicinal chemicals 3.10 Other Applications

Chapter 4

110 110 112 113 114 114 115 116 116 116 118

122 125 127 127 129 129 132 134 135 136 136 137 138 140 141 142 143 143 144 144 146

Decanter Theory

4.1

4.2 4.3

Basic Theories 4.1.1 Acceleration force 4.1.2 Differential 4.1.3 Conveyor torque 4.1.4 Process performance calculations Particle Size Distribution Clarification 4.3.1 Sigma theory

149 149 150 151 151 154 159 159

IX

4.4 4.5 4.6 4.7

4.8 4.9

4.10 4.1 1

4.12

4.13

4. 14 4.15 Chapter 5

4.3.1.1 Usingsigma 4.3.2 Sigma enhancement 4.3.3 Flocculant requirement Classification Three-Phase Separation Thickening Conveying 4.7.1 TheBeta theory 4.7.2 Conveying on the beach 4.7.3 Dry solids conveying Conveyor Torque Dewatering and Washing 4.9.1 Solids dewatering 4.9.2 Washing 4.9.3 Solids compaction Dry Solids Operation Fluid Dynamics 4.1 1.1 Reynolds number 4.11.2 Moving layer 4.1 1.3 Cresting 4.1 1.4 Feed zone acceleration Power Consumption 4.12.1 Main motor sizing 4.12.2 Main motor acceleration Mechanical Design 4.1 3.1 Maximum bowl speed 4.1 3.2 Critical speeds 4.13.3 Liquid instability problems 4.13.4 Length/diameter ratio 4.13.5 Bearing life 4.13.6 Gearboxlife 4.13.7 Feedtube Nomenclature References

165 166 167 168 170 173 175 175 176 177 179 180 180 181 185 186 192 192 194 194 195 196 197 198 200

200 202 203 204 204 206 206 208 213

Flocculation

5.1 5.2

5.3 5.4 5.5

The Principle of Flocculation Polymer Solution Make-up 5.2.1 Dissolving solid polymers 5.2.2 Diluting dispersions 5.2.3 Final flocculant solution characteristics Polymer Choice Pretreatment Admitting Flocculant to the Decanter

217 220 220 221 222 225 229 230

X

5.6 Flocculant Suppliers 5.7 Low-Toxicity Polymers 5.8 Applications 5.9 Performance 5.10 References Chapter 6

Test Work and Data

6. 1 6.2 6.3 6.4

Chapter 7

Test Equipment Test Procedures TestLog SomeTest Data 6.4.1 Spent grain 6.4.2 Agricultural products 6.4.3 Lime sludge classification 6.4.4 Clay classification 6.4.5 Waste activated sludge thickening 6.4.6 Digested sludge thickening 6.4.7 Lactose washing 6.4.8 Coal tailings dewatering 6.4.9 Dry solids (DS)dewatering

245 248 2 52 255 255 258 259 261 263 265 267 269 269

Calculations and Scaling

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 Chapter 8

233 235 236 237 241

Basic Calculations Three-Phase Calculations Classification Calculations Washing The Probability Scale Scale-Up of Centrate Clarity Limiting Applications Simple Dewatering and Torque Scale-Up Main Motor Sizing DS Scaling

284 288 291 294 298 300 302 306 308

Instrumentation and Control

8.1 8.2

Decanter Plant Modules Instrumentation 8.2.1 Flow meters 8.2.2 Solids concentration meters 8.2.3 Level probes 8.2.4 Speed probes 8.2.5 Temperature probes 8.2.6 Torque measurement 8.2.7 Timers 8.2.8 Counters 8.2.9 Electrical meters

317 319 319 319 320 321 32 1 321 321 322 322

XI

8.3

8.4 8.5 8.6 8.7 Chapter 9

8.2.10 Bearing monitors Controlled Equipment 8.3.1 On/off devices 8.3.2 Variable output devices Controllers Integrated Controller CIP References

322 323 323 324 325 328 3 30 331

The Decanter Market

9.1

9.2 9.3

Market Characteristics Market Trends Market Size Estimates 9.3.1 Overall decanter market size 9.3.2 Regional market estimates 9.3.3 Application market estimates 9.3.4 Suppliers' market shares

3 34 335 336 336 337 337 338

Chapter 10 Suppliers' Data

339

Chapter 11 Glossary of Terms

363

Appendix Index

37 9 413

This Page Intentionally Left Blank

Preface and Acknowledgements By virtue of its title, which involves the word "handbook", this book is intended, above all else, to be useful. Its aims include the explanation of the nature and methods of operation of the decanter centrifuge, and a description of the kind of performance that might be expected from a decanter. The decanter centrifuge is a device for continuously separating particulate solids from a suspending liquid or liquids by sedimentation and decanting. As such, it is part of the general range of sedimenting, filtering and other mechanical equipment used for separation processes. A distinguished range of books exists that describes this complete spectrum of equipment, and the processes by which they operate. A previous book covers the whole range of centrifuges, both sedimenting (like the decanter) and filtering, but this is the first book to deal solely with the solid-bowl, scroll-discharge centrifuge, which is the decanter. The book is aimed at all those for whom the decanter may be part of their studies, of their research, or of their working life. It is intended to be of value in undergraduate courses on filtration and separation, but it will also offer the practising engineer in end-user companies m u c h that is of direct value to the daily job of designing, specifying or operating this sophisticatedly engineered, but very useful, piece of processing equipment. This handbook will find use in research establishments and equipment manufacturers' engineering departments, as it gives guidance on basic design and operating features, some in regular use and some only recently introduced to the market. This essentially practical text nevertheless covers the underlying theory of centrifugal sedimentation separations in some detail, which further extends its usefulness to the research or design engineer looking for new ideas. The a r r a n g e m e n t of the handbook follows a logical pattern: a general introduction, followed by technical descriptions of equipment features and the industrial uses of the decanter. Then comes the theory of the decanter's design, and detailed descriptions of operational and test procedures. The book finishes with some marketing data, and descriptions of the equipment ranges of the main manufacturers.

xiv

Prefaceand Acknowledgements

The authors (both Chartered Chemical Engineers) have a wealth of experience in the decanter business: 9 Alan Records retired from a senior equipment application and development role with Alfa Laval, after almost a full lifetime's job involved with decanters, covering research, design, commissioning, operation and service, in a wide range of industrial applications; and 9 Ken Sutherland, for a time Technical Manager for Sharpies, has later been heavily involved with the marketing aspects of separation equipment, including centrifuges. The putting together of a book of this nature requires the help and cooperation of many individuals and organisations. The contributions, help, advice, work and kind permissions of those mentioned below are most gratefully acknowledged. Lenny Shapiro and Jan Cederqvist contributed to the mechanical information, while Bert Guille assisted with the electrical content. The process data were obtained as a result of painstaking work in the field, often in far less than a salubrious environment, by numerous field engineers, our former colleagues, and in particular John Joyce, Betina Pedersen, and Keith Smith. Apologies are extended to all those not mentioned. Denis Locke contributed to the work on many of the illustrations, professionally executed by Mike Nicklinson. Graham Dawson, with the help of some of his former colleagues, advised on the section on flocculant technology. Keith Kernahan advised on the details of the Viscotherm equipment. The Triton Electronic Company co-operated in providing photographs and details of their CST equipment. The decanter centrifuge market is a highly competitive one, and thus manufacturers are, understandably, reticent in providing specific data and information on their products. Without such data and information, however, this book would be reduced in value. The authors are therefore especially grateful for the data supplied by the companies Alfa Laval. Baker Process (Bird Machine and Bird Humboldt), Broadbent. Centriquip, Centrisys, Flottweg. Gennaretti, Guinard, Hiller, Hutchison-Hayes, Noxon, Pennwalt India, Pieralisi, Siebtechnik, and Westfalia/Niro. Permission to reproduce sketches and drawings has been obtained from Alfa Laval, Bird Machine, Bird Humboldt, Broadbent, Centriquip, Centrisys, Cyclo, Flottweg, Noxon, Siebtechnik, Tomal, Viscotherm and Westfalia Separator. Finally, gratitude is expressed to Bent Madsen and his colleagues for checking the early manuscripts. The book owes its origin to Nick CornerWalker, then Director of Engineering with Alfa Laval, to whom the authors are indebted for the inspiration, for his personal support, and for putting the resources of a major manufacturer of decanters behind the venture. The

Preface and Acknowledgements

xv

authors are very happy to acknowledge that debt here, but also to acknowledge the input from the other companies whose ideas and illustrations have been used at the appropriate parts of the text. To these, and all of the other workers involved with the decanter for the 60 years of its effective operating history, the authors express their thanks. Alan Records Ken Sutherland

This Page Intentionally Left Blank

CHAPTER 1 Introduction The decanter centrifuge has become a major processing tool in a wide range of liquid/solid separation applications. This handbook aims to be a thorough introduction to the design, performance and application of the decanter. It aims also to be a useful guide for the centrifuge engineer, both in equipment manufacturing companies and in the end-user companies, and their associated contractors and consultancies. The handbook's first chapter introduces the reader to the decanter, to its history and to the manufacturing sector within which it is made. The contents of this chapter are intentionally brief, with major expansion of the topics covered in later chapters of the book.

1.1 The Decanter Centrifuge The solid-bowl scroll-discharge centrifuge w now almost universally known as the decanter centrifuge has, indeed, become the workhorse of a wide range of liquid/solid separation activities. Its application to the dewatering of waste sludges has made it a most valuable tool in combating environmental pollution. This has made the decanter a well-known and widely appreciated piece of equipment. 1.1.1 The basic decanter

Although a complicated piece of machinery, the decanter centrifuge embodies a simple principle, that of the screw conveyor. In basic terms, the decanter comprises a solid cylindrical bowl, rotating at high speed. Inside the bowl is a scroll (screw conveyor) rotating at a slightly different speed. The differential speed between bowl and scroll provides the conveying motion to collect and remove the solids, which accumulate at the bowl wall. A slurry of liquid and suspended solids is fed along the centre line. to some fixed position within the bowl, and is accelerated outwards to join the pond of liquid held on the bowl wall by the centrifugal force. This same force then causes the suspended solids to settle, and accumulate at the bowl wall. The clarified liquid then flows along the bowl, to leave at one end of it, over some kind of weir design, which sets the level of the liquid surface in the bowl. The other end of the bowl is sloped inwards, towards the centre, thus providing a beach, up which the solids are conveyed, to be discharged from the bowl, at the top of the beach. Whilst the solids are conveyed up the beach, some, hopefully most, of the entrained liquid drains back into the pond, to join the liquid flow towards the far end. The scroll usually is carried on a hollow axial hub, t h r o u g h which the slurry feed tube passes to the feed zone. The diameter, the number, and the pitch of the conveyor flights are chosen to match the needs of the slurry being treated as are the depth of the pond, the length of the bowl, the conveyor differential speed, and the angle of slope of the beach. Most decanters operate with their axis horizontal, in which case they usually are mounted in substantial bearings at each end of the bowl. Vertical

Introduction

Liquids Dtsctmrl~

C~,m8 .

C~veyor t

Bowl

Fli#t

Feed

Solids

Feed

Zone

Dischtr~

Tube

3

Figure 1.1. The main operating parts of a decanter centrifuge.

operation is possible, in which case the bowl is carried only on one set of bearings, at the top. If the decanter is short, then cantilevered horizontal operation is also possible, with bearings at one end only. The rotating bowl is enclosed in a casing, which is divided to ensure that the discharged liquid (the "centrate") and solids cannot remix after separation. The basic decanter is completed with a drive motor, usually electrical, and a gearbox, which controls the differential speed of the conveyor. Aspects of the physical forms of the decanter in its different versions are described in Chapter 2.

1.1.2 Separation principle The decanter operates mainly by sedimentation, a process causing the separation of suspended solids by virtue of their higher density than the liquid in which they are suspended. If the density difference is high, then gravity may provide sufficient driving force for the separation to occur in a reasonable time as is the case with large-tank clarifiers and clariflocculators, or with lamella and inclined-plate separators. If the difference in density is small, or the particle size is very small, then gravity separation would take too long, and the separation force must be augmented by the imposition of centrifugal forces many times that of gravity alone. The centrifugal force may be imposed by virtue of the flow of the slurry, as in a hydrocyclone, or by means of mechanically driven rotation, as in the sedimenting centrifuge.

4

TheDecanterCentrifuge

There are several types of solid bowl sedimenting centrifuge, including: 9 the tubular bowl centrifuge, mainly used for liquid/liquid separation, for which use any suspended solids would require cessation of operation for their removal (the tubular bowl centrifuge is also used for very difficult solid/liquid separations, where there is a low concentration of solids, which cannot be flocculated): 9 the imperforate basket centrifuge, which is operated batch-wise for the removal of collected solids; 9 the disc-stack centrifuge, originally developed for liquid/liquid separation (cream from milk), but which has been improved to achieve continual solids removal (although, in most cases, not fully continuous), by a variety of devices at the outer periphery of the bowl; and 9 the decanter. The prime beneficial characteristic of the decanter in this spectrum of sedimentation equipment is its ability to remove separated solids from the separation zone on a fully continuous basis. It can operate, unattended, for weeks, if not months, at a time. By comparison, therefore, with: 9 gravity sedimentation the decanter can achieve separations that would be impossibly lengthy (or just impossible) in a clarifier or lamella separator, and it produces drier solids; 9 h y d r o c y c l o n e s - the decanter has a much higher liquid capacity, can handle much higher slurry concentrations, and produces much drier solids: 9 tubular bowl centrifuges the decanter offers higher capacities, the ability to handle concentrated slurries, and continuous operation: 9 imperforate basket centrifuges the decanter operates continuously, can handle much higher solids concentrations, and produces much drier solids: and 9 disc-stack c e n t r i f u g e s - the decanter is truly continuous in operation, can handle much higher solid concentrations in the feed slurry (although it cannot usually match the high centrifugal forces of the disc-stack designs, and so does not have the same clarification performance), and produces drier solids. In addition to these other types of sedimentation centrifuge, the decanter competes effectively with several types of solids recovery f i l t e r - such as the plate-and-flame filter press, and the various types of band press, without requiring the use of filter aids. The theory of the separation and dewatering behaviour of the decanter is described in Chapter 4.

(It must be rcmcmbercd that there are many other types of industrial c:enIril'iigr, hul: t.tiese achieve sepratiori by riieans of filtration rather than sedimenl:;ii.ion -- :Ill.hoiigh the srreen-howl dwariler r:omhiries t.he two sepa r ii t.ion rn ~h a n isms .) 1.1.3 Decanter applications

Thc dccantcr cciitrifugc can be used for most types of liquidjstrlid separ;ition, and its ability to handle a wide rarige of feed slurry L.r,rir:entrol.iorrsodds to its general versatility. 11 C ~ I be I 11sed Tor t . 1 clussificntian ~ of' solids in Liquid suspc.nslon. where a single CUI. is required tielween I W C )sixes ofsolid particle (or, less often. bctweeii solids of differing density). I1 i s ii very gcind device for this purpose, and its early history inclnded developme,nt for thc kaoliii (china clay) iiidustry. The drt:ant.er can be used for the clnrjficntioriof a liquid. it can be operated so as lo give a high degree of clarification, although it is not usually used to clocify o slurry that contains only a small amnunt.nf d i d s in suspensiun, It (::in also be iwd in thc recovrr~jd a valuable scilid irom i t s suspcnsioii in a liquid, :ind li!Llowing such rcrovery it is capable o f \.wr.shing t.he recovcrcd solid free of the original mother liquor, and of drliqimrin[j ( d w n t e r i i i g ) the wrls1it.d solids to a high dcgree of dryri Whcrc thc slurry is a waste needing treatment prior to sale dispos;iI, t h c decanter again can dewat.er such slurries to a high luvcl of ilryriess. Finally the decariler c:in be opcratcd so as to a c t as il i h i r k m v - , producing a clcar liquid and i-l m o r e concentratcd slurry either in a manufacturing proccss, or i t 1 wsstc treatment. This wide mnge of'potcntial uses. coupled with its continuous operalion, its ability tn accept a wide range of feed concentrations. and its arailahility in a wide raiigc o l fred capacities, t.
1.2 The History of the Decanter . ..

The effective history of the decanter centrifiige occupies the second half of' the twentieth century, although it originated at the st;irt of that century. 'I'he tubular bowl and the disc centrifuges had. by then. been in usc for some time. and thcrc was. clcarly, soiiic inccntivc to producc a scntrifugc which would enablc the continuous rcmoval of solids which accumulated during t,he proc:essiriy o f ij liquitl/liquid mixture. The fdlc.iwirig discussion descrihes how that devel(Ipmerit. was achiwed, arid h o w h e modern decnnt,erevolved. 1.2.1 Origins

The first. patent rlrscrihirrg a d c v i w like t h e decanter was grantcd to Liedheck. a Swedish inventor (wilh r i o corporate assignee). in 1902 [ I 1, This was il vcrtically mounted device, wit,h ils tlrivc r~iolorsituated below it. and with the hrach section at thc top, discharging into ;in open s p x c in t.tit. casing. which also contained ths feed tank. Its rrsemblant:e t.o ;i veriiual basket oentrifugc is sLrong. but it is still astonishing to see Ihat 1,iedbeck's design. which was intended t o separate solids from two distinct liquids, contains all ill the main features of ij modern three-yhasc dccantcr. (An e;irlier p a i e n l . by Stewart. working in ihtr sugar ir'idustry, looks similar to a decanter. in that it had a corivcyor iii two parts, but is actually a filtering ccntril'uxe. wilh t h conveyor ~ scrolling solids across it screen.) 'I'his first dccanter patent iIppt!;ired a1 a time whcn Gustaf de 1 , ~ i w It.he ~ inverilor or the continuous crc'mi separator, w a s active in dcvcloping this system. De 1,aval's first patent for a continuous ceriirifuuge. i n thc form of ;I roughly spherical bowl. was granted in 18 7 X [ 21. The key patent covering the disc stack was griirlted t.o vnn Btchtolshcim in 1888 3 ] +and was immediately bought by dc Laval's c:ompai.iy, which he joined. Quite soon i n this developrriciit i t was realised that a build u p of solids in the howl wuuld be a serious disadvantage. and a number of ideas wcre developed to prrrilit thcir rcmoval. One such idea featured a slowly moving helical scraper (slowly. Lhat is, in relation 1.0the bowl). mounted outside the disc stack -- but ihis design u7as dropped, because. rrf high mcchanical wear.

Introduction

7

Similar problems affected any solid bowl centrifuge (imperforate basket or tubular bowl), and it would seem that the decanter centrifuge designs available at the turn of the century would have been the answer to this problem. That these designs were not developed with commercial advantage may have been due to cost, and to the fact that the n u m b e r of commercial applications for such a device was still quite small cream separation dominated the applications for continuous centrifuges, and this could manage without solid removal, except at a shut-down for cleaning. Liedbeck's design was, admittedly, quite complex for the early 1900s. It featured a vertically mounted rotating assembly, with a cylindrical lower section, and a conical top part, the solids discharging over the edge of the beach, into a collector, and the two liquids leaving separately from the bottom. The drive mechanism was mounted directly below the rotating assembly, and was directly coupled to it, with a gearbox giving the differential speed to the conveyor screw. The drive shaft for the conveyor ran through that for the bowl, and the feed slurry entered the bowl at its bottom end. The drawing strongly resembles a vertical basket centrifuge, with the conveyor and beach added. There was also, at that time, quite a lot of interest in the use of centrifuges capable of handling sludge for the dewatering of starch, and the German companies Uhland and Jahn made centrifuges that embodied the decanter principle for this purpose. Apart, however, from a slight resurgence of interest in starch processing in the 1920s, the decanter effectively vanished for 40 years. It re-emerged at the end of the 193()s in the patent literature: Pecker (application February 1938, granted May 1942) [4], showing a conical decanter, with feed at the base of the cone. Then not again until the mid1940s: Ritsch, assigned to the Process Development Company (application September 1945, granted November 1950) [ 5 ], showing a conical bowl and a more sharply conical beach section, and intended to separate two solids, one settling and one floating, from the suspending liquid. Meanwhile, the Sharpies Corporation was patenting basket and tubular bowl centrifuge developments, and the Bird Machine Company had not yet been formed. So the relatively sudden appearance on the market of conical bowl decanters by both companies in 1 9 4 5 / 1 9 4 6 , is, in retrospect, quite surprising. The Sharpies P-IO00 decanter had quite a rapid impact on the whale and fish oil market, so much so that AB Separator (not yet called Alfa Laval) was forced to copy the Sharpies machine, or lose a good market in the Norwegian fishing industry. Bird Machine applied for a patent early in 1946 on kaolin production [ 6]. in which decanters were included merely as items in a flowsheet, as processing tools not otherwise described, while a patent granted in 194 7, but which had been filed in 1940 [7], showed a similar use in the cement industry, for classification by size. A December 1949 application by Milliken and Topping (also for Bird Machine) shows a three-section bowl, ready for solid washing on

8

TheHistoryoftheDecanter

a screen section [8]. The combination of the slow but sure economic recovery after World War II, and the evidence of successful operations by Bird and Sharpies, brought a rush of competitors to the market. As well as Alfa Laval in Sweden, these included Kl6ckner, Krupp and Rahmesohl & Schmidt (Westfalia) in Germany, International Combustion in the UK, and Dorr Oliver in the USA.

1.2.2 Machine and application development In the ensuing 55 years, since that first really commercial entry into the marketplace by Bird and Sharpies, rapid developments in both machine design and application technology have occurred. These developments were largely market-led, with demands from end users quickly converted into machine improvements. The main manufacturers had strong technical development and engineering groups, responsible for keeping their products both in line with market needs, and at least level with, if not ahead of, their competitors. In relative terms, the greater changes occurred in the first 2 5 years of this period, up to 1970. This period saw the decanter expand into more than 100 applications within food and by-products processing, mining, energy materials and systems, chemical, petrochemical and pharmaceutical industries, and environmental engineering. The major manufacturers produced decanters suitable for a wide range of these applications, but there also grew smaller companies, specialising in just one industry, such as olive oil or starch. Basic decanter performance (in terms of solids recovery, centrate quality and solids dryness) remained relatively little changed in this period, but improving engineering and materials of construction enabled the use of longer bowls to give greater feed capacity. Of the major design variants (all described in Chapter 2), the screen-bowl design was developed by Bird in the mid-1940s. Sharpies produced the first vertical, pressurisable decanter in 1958, and three-phase operation came along in the early 1960s. The main gearbox choices were all in place before 1970, while the potentially wearing surfaces of the conveyor could be protected by a range of treatments or materials, including the range of tungsten carbide tiles developed by Sharpies (tiles being small flat pieces of hard material fixed to the scrolling face of the conveyor flight). As a separating device, dependent largely upon a combination of particle size and density to achieve good separation of solids from liquids, the decanter can show much better performance if the particles can be agglomerated before or during separation. The appearance onto the market of polymeric coagulating agents at the end of the 1960s quite revolutionised the use of the decanter for waste sludge dewatering, and offered great improvements in performance in other applications as well.

Introduction

9

In the subsequent 30 years, relatively few "new" applications have been developed, with the difficulty of achieving fully sanitary operation keeping the decanter from some of the food and biochemical operations. However, major changes have occurred in the refinement of machine design, and, therefore, performance. These changes have been most apparent in the way in which the decanter handles the discharging solids, and especially in the achievement of the maximum dryness (least moisture content) in those solids. Thus there are restrictions available at the base of the conical section (such as Sharpies' "Centri-Seal" patent by Lee), operations with deep ponds, and improved control over conveyor speed and torque, leading to the "Dry Solids" decanter. The availability of stronger stainless steels has enabled the production of very long bowls, as well as quite large diameter bowls. Very high rotational speeds are now available, giving separational forces of up to 10 000 times the force of gravity on smaller models. Mechanical improvements include the ability to profile the protective tiles on the conveyor face, to use better bearings, and to use three-stage planetary gearboxes. Improved machine driving mechanisms include inverter drives, and the use of back-drives with power regeneration. Alternative means for liquid discharge have been developed, largely from other centrifuge types, such as centripetal and skimmer pumps. Decanters have been fitted with discs to improve clarification performance, and sanitary performance has been improved by the development of clean-in-place (CIP) methods. A very important system development has been the improvement in control methods, enabling the decanter to react automatically and quickly to changing feed conditions.

1.3 Decanter Manufacturers By the mid-1940s, only two companies were working on decanters Bird Machine and Sharpies. In the ensuing 55 years, the number of manufacturers has increased many-fold, through a peak in numbers, with a decreasing number at the turn of another century. The 1990s trend in mergers and acquisitions, extending into the new century, has meant that, of the eight leading manufacturers mentioned at the end of Section 1.2.1, not one remains as an independent company, even if they exist at all. The decanter centrifuge is simple in concept, but complicated in practice. It is therefore expensive to make, and has relatively low profit margins. It follows that a flesh entry into the marketplace needs strong corporate support, coupled with good engineering, a willingness to invest in a strong process engineering department, and possibly a niche market to target. One way to enter the business has been to establish working relationships with an existing manufacturer, as, for example, Broadbent did with Bird, and, later, with Tanabe, or as Tomoe did with Sharpies and then Alfa Laval. That so many companies have entered the decanter business is a sign of the importance of the decanter to the process industries. It has not proved to be an easy manufacturing sector to stay in: at the time of writing three of the world's leading decanter manufacturers are for sale. The list of significant former manufacturers of decanters is quite long, including: 9 Comi Condor, Italy, recently stopped making decanters (but still makes other types of centrifuges). 9 Dorr Oliver, USA, acquired first by Krauss Maffei, then by a Canadian investment company who sold the centrifuge interests to Alfa Laval, but the decanter range had already been dropped. 9 International Combustion, UK, acquired by Rolls Royce and stopped making centrifuges altogether. 9 Robatel, France, acquired by Rousselet, who decided not to continue the decanter range (but still make other types of centrifuges). 9 Kl6ckner, Germany, K16ckner's KHD subsidiary, including Humboldt decanters, acquired by Baker Process/Bird Machine.

Introduction

11

Krupp, Germany, stopped making decanters many years ago. Sharpies, USA, acquired by Pennsalt/Pennwalt, then sold to Alfa Laval, and the decanter ranges merged. In the late 1960s a small ripple in the decanter world was caused by a Danish engineer, Kruger, who developed the Total decanter, with good performance in waste sludge treatment. This was acquired by Niro, a spin-off from the Danish sugar company, but the Niro decanter has disappeared, following the company's acquisition by GEA, which merged the decanter business with Westfalia. Other well known companies have been acquired over the years, but have kept their identity, as illustrated in the list below. Despite these changes, the decanter manufacturing sector is still a large one. The following list gives the manufacturers believed to be operating in the year 2000, by country of ownership, with parent companies where these are known.

Denmark FFG Separation France Guinard Centrifugation, owned by Andritz group (Austrian) Germany Flottweg, owned by Krauss Maffei, which has been sold as Atecs by Mannesmann to a consortium of Siemens and Bosch Hiller MAF SAMAG (Sangerhausen) Siebtechnik Westfalia, owned by GEA, itself owned by Metallgesellschaft Greece Centrifugal Environment India Pennwalt India Italy Amenduni Cornello Gennaretti Officine Mecaniche Toscane (Athena) Pieralisi Group Rapanelli lapan IHI Ishikawa-Harima Heavy Industries Kokusan Seiko Kotobuki Techrex

12

Decanter Manufacturers

Mitsubishi Kakoki Kaisha Sumitomo Tanabe Tomoe Sweden Alfa Laval Separation, owned by Industri Kapital (Swedish) and Tetra Laval (Swiss) Noxon, owned by Waterlink (USA) Switzerland Chematec UK Broadbent Centriquip USA Bird (including Humboldt). owned by Baker Process Centrisys DMI Decanter Machine lnc Hutchison-Hayes Of these 30 or so manufacturers, over 90% of the market is covered by just eight: Alfa Laval, Bird/Humboldt, Flottweg, Guinard, IHI. Pieralisi, Tomoe and Westfalia. Each of these market leaders has been in the business long enough to have developed a full range of decanters, making them capable of selling to the whole market. It is only among the smaller companies that any degree of market specialisation is seen, and that is mostly for olive oil processing. The decanter manufacturers are described, together with the key features of their range of decanters, in Chapter 10.

1.4 Present Trends The key features of decanter development described as occurring during the last 30 years are by no means fully developed. In capacity and size terms, there will still be larger, longer and faster machines to come. There is much still to be done in the development of control systems for the decanter, leading to complete a u t o m a t i o n of decanter processes, with associated telemetering. As new production processes develop, in parallel with the derivation of new products, then the decanter will be adapted to keep pace with such changes. Oil production and refining will continue to be a challenge to the decanter manufacturer, especially as production moves into less hospitable zones. There is a wealth of food, protein, biochemical and pharmaceutical applications awaiting the efficient clean-in-place process for decanters, while the increasing demands on municipal and industrial waste treatment will also add to the application range. If nuclear power returns to favour, then here also the decanter will have a part to play, especially once it is fully automated. The need to be able to process low-grade metal ores will also need help from the decanter.

1.5 References 1. A Liedbeck. Centrifugal apparatus. US Patent 750668, 27 October 1903 (patented in Sweden in 1902) 2. G de Laval. (AB Separator) Centrifugal creamer. US Patent 247804, 4 October 1881 (patented in Sweden in 1878) 3. C yon Bechtolsheim. (AB Separator) Centrifugal liquid separator. US Patent 432719, 12 May 1890 (patented in France in 1888) 4. J S Pecker. Centrifugal machine. US Patent 2283457, 19 February 1938 5. H P Ritsch. (Process Development Co) Centrifugal separator. US Patent 2528974, 19 September 1945 6. S C Lyons. (Bird Machine) Improving kaolin and products thereof. US Patent 2524816~ 21 February 1946 7. F A Downes. (Bird Machine) Cement manufacture. US Patent 2424746, 25 September 1940 8. G A Milliken, K E Topping. (Bird Machine) Centrifugal separator. US Patent 2600372, 16 December 1949

CHAPTER 2 Decanter Design The decanter centrifuge is, in principle, a relatively simple device, t h o u g h far from simple to manufacture, being a rotating d r u m with a screw conveyor in it; clarified liquid decants out of one end while dewatered solids are scrolled out of the other. The prime virtue of the decanter is its ability to remove quite high levels of suspended solids from a liquid, with a reasonably low level of retained liquids in the separated solids. The decanter can handle slurries containing solids occupying 100% of the volume of the slurry. So long as the slurry is pumpable, the decanter will handle it. The moisture content of m a n y of the dewatered cakes from decanters is such t h a t the cakes can be stacked without m u c h seepage of moisture from the stack. Some cakes are free flowing and friable, while a few are pasty or clay-like. This apparent simplicity of the decanter is, however, complicated by a very wide range of design variants. It is the aim of this chapter to describe both the basic operating elements of the decanter, and the range of design variants and special features (concentrating on those variations that affect operating performance).

16

Decanter Design

Gearbox Guard

\

Conveyor Assembly Bowl Assembly \

Upper Casing

Belt

Guard (Upperl

Rear Mare Bearing Assembly Gearbox .~... Assembly

Feed Tube

Front ~ " Main Bearing Assembly

~..~

Torque Arm / (in place of Brake m upper illustration)

Drive Belts

Belt Guard (Lower)

Lower Casing

Sub Frame Torque Control

Centrifuge Frame

Main Drive Assembly

Figure 2.1. A basic decanter centrifuge ( By courtesy of Alfa Laval ).

Fluid Coupling (when fitted)

2.1 Basic Construction The construction of a basic centrifuge with all its main components is s h o w n in Figure 2.1. The heart of the decanter is the rotating assembly, w h i c h comprises a mainly cylindrical bowl housing an Archimedian screw conveyor, with a small clearance between it and the bowl. One end of the bowl is conical in shape, providing the means whereby solids can be removed from it. Affixed to one end of the bowl is usually a gearbox to effect a small differential speed between the conveyor and bowl. The rotating assembly, usually horizontal, is supported by a bearing in a pillow block at either end of the assembly. Surrounding the bowl is a casing to collect at one end the clarified liquor and dewatered cake at the other. The bearing pillow blocks are m o u n t e d accurately on a rigid frame together with the casing. Sometimes the frame is m o u n t e d on a sub-frame together with the drive motor, and where necessary a back-drive system, to control the gearbox pinion shaft, which will in turn control the conveyor-to-bowl differential speed. The back-drive system will be described later, but for the present it suffices to say that it is essentially a braking motor or similar device coupled to the gearbox pinion shaft. The main motor is offset from the bowl and drives the bowl by m e a n s of a set of V-belts. The back-drive can also be offset, in which case it would be connected with a timing belt. The timing belt is to facilitate more accurate speed control. However the back-drive system can also be m o u n t e d direct in line with the pinion of the gearbox. The sub-frame assembly, or the main frame if there is not a sub-frame, usually is supported by vibration isolators. The feed enters the bowl t h r o u g h ports in the conveyor, having entered the conveyor hub through a stationary coaxial feed tube projecting into the conveyor from a support, m o u n t e d on the main frame. Feed slurry is metered t h r o u g h the feed tube into the rotating bowl. Suspended solids sediment to the bowl wall, where they are picked up by the conveyor and scrolled as a saturated cake to the conical end of the bowl, over the heel of cake which builds up in the small clearance between the bowl and conveyor. The resulting clarified liquor flows to the opposite end of the bowl and decants over weirs into the casing for discharge. The cake scrolls up the

18

Basic Construction

conical section of the bowl, the beach, before it falls into the casing for discharge. The heel, the thin layer of process solids which builds up between bowl and conveyor, can progressively consolidate with coarser particles bedding themselves into it. This, while providing an aid to scrolling efficiency, can be an u n w a n t e d source of abrasion for the conveyor. However, generally, there is a tendency for the heel to move, albeit at a m u c h lower rate t h a n the cake itself. Thus there is a tendency for the heel slowly, but continuously, to regenerate itself. Materials of construction are i m p o r t a n t considerations in the basic design. Most decanters are constructed with the parts in contact with the process in some form of stainless steel. Although some m a n u f a c t u r e r s successfully use carbon steel, others have not been as fortunate, due to severe corrosion and associated problems.

2.2 Basic Components Many of the basic components of the decanter have been introduced in describing the construction of the decanter. These need to be described in more detail. The four major c o m p o n e n t assemblies are the rotating assembly, the flame and casing together, and the drive and back-drive assemblies. The rotating assembly includes the bowl, beach, conveyor and gearbox. It is the most important (and expensive) part of the decanter, where all the work is done, and which contains the most sophisticated technology, both process and mechanical. For such a heavy component, weighing up to several tons and producing a force field of several t h o u s a n d g, a high level of precision engineering is required, followed by precise balancing. Bearings and seals used in the rotating assembly and gearbox are an important part of the decanter. Bearings in general have to be lubricated to work properly. To do this, seals separate the lubricated bearings from the process environment, both to protect the bearings and to avoid c o n t a m i n a t i o n of the product or environment, by the lubricant. Seals are also needed to contain process liquids and vapours within the centrifuge casing. Seals are especially important where the process requires a positive pressure or v a c u u m , and where vapours are flammable or toxic. 2.2.1 Orientation

The rotational axis of the decanter can be horizontal or vertical. The vertical designs are most frequently used for special applications and are described in Section 2.3.1. Thus the horizontal design will be taken from here on as the basic design. 2.2.2 Flow

The flow of clarified liquor and cake in the decanter can be either co-current or countercurrent. In the co-current design, both solids and liquid travel in the same direction, axially, in the separating zone, with the clarified liquid diverting to the opposite end to the solids discharge t h r o u g h off-take channels. With the c o u n t e r c u r r e n t design, solids and liquid travel in opposite directions,

20

Basic Components

axially, in the separating zone, and discharge at opposite ends. Both designs have strong proponents and arguments. For the moment, countercurrent flow is assumed, and co-current flow is discussed further in Section 2.3.2. Conventionally the front end of the decanter is the liquid discharge end and the solids end is referred to as the rear. The solids discharge is more usually referred to as the cake. While defining flow and positional conventions, it is worth mentioning that later in the book when discussing the interior of the bowl, terms such as "up", "over", and "bottom" for instance will be used. These terms relate to the centrifugal field, and thus "bottom" refers to the bowl wall, "up" and "over" mean towards the bowl axis.

-

-

-,~-aa6etoo60eooeeoqPoq~eo00oooooo0o0oo0oeoe0ooo

|

~

.~a.aLI.IimAjp.OOOOOOOOOOOOlPtuu~

_ _

i |

~

-

"~Lt

_.

e _ o o e e e ~ _ "~,au O O O 0 O O O-g'ql,~

9, ~ . e e o e o o

"'

In

/

.....;

...... ~ "

.

.

.

.

.

.

.

.

_

.

.

.

.

.

~

.

.

.

...,1

gurto

oo

.

.

.

.

.

oo.,A~-.--,

.

.

oo

m

.

m

.

9 9 ooo

.

Centrate

.

.

.

.

.

.

m

,,,u,I

9 9 ..-

m __....-----.,,-,,,,,,,,,,,,,.,,,.,,,--,-,,,--,,---.

9

o

m

m

m

m

m

m

Boo 9 9 Jhw-

. . . . . . . .

Figure 2.2. Countercurrent flow.

..........

el~e,j,~

=,~ 9 o o o e e o ' l P ~ " ~pooooo4

it

Feed

i m m

Cake

: : : ; ~ . : : : : : : : :::..~... ----'-.---4. dm~ o

oooooo "~oooo "-4Jo

Feed n

-

!

Cake

Centrate

Figure 2.3. Co-current flow.

DecanterDesign

21

2.2.3 Materials of construction

Materials of construction of the decanter are m a n y and varied. It is more usual to make the contact parts, particularly in the rotating assembly, of some form of stainless steel. This is to avoid assembly problems and misalignment due to corrosion on mating surfaces. This has to be avoided with high speed rotating equipment. Nevertheless, it must be said that there are m a n y decanters in operation with bowls of carbon steel, where their m a n u f a c t u r e r claims to be able to o v e r c o m e , or avoid, corrosion. For stationary contact components there is no need for a high grade of stainless steel. When the process used is non-arduous, simple neoprene seals and gaskets will suffice. Supporting framework will be in ordinary or even cast steel. Materials of construction for the decanter are discussed in more detail in Section 2.3.3. 2.2.4 Bowl

The bowl in a modern d e c a n t e r is a cylindrical tube with a flange at either end, on which are bolted at one end the liquid discharge bowl hub. and, on the other end. the cake discharge hub, or the beach followed by the cake discharge hub. The first cylindrical bowls used a filler piece in the end of the bowl to form the beach. On modern bowls, particularly the larger ones, the beach is bolted to a flange at one end of the cylindrical section, although with some overlap to provide mechanical location. The thickness of the bowl wall is dictated by the material of construction used, the m a x i m u m speed at which the bowl will be rotated, and the m a x i m u m weight of process material, feed, centrate or cake, likely to be held in the bowl. Thus the density of the process materials in use can have a major effect on the safe working speed of the bowl.

I .

i

/

Front Hub

Bowl Shell

Fignre 2.4. Basic bowl assembly.

Beach

Rear Hub

22

Basic Components

The inside surface of the bowl can be plain machined. However, some effort is often m a d e to encourage cake to stick to the bowl, to aid scrolling instead of slipping r o u n d with the conveyor. The means of doing this could be by knurling the inside of the bowl for instance. This can w e a r smooth relatively quickly. More often longitudinal ribs are welded, or a liner with similar ribs is fitted (see Section 2.2.4.3). At each end of the bowl the outside bowl diameter can be increased to provide, if necessary, excess metal for removal during balancing. In particular, it can provide a position for m a c h i n i n g grooves, which will mate with corresponding baffles in the casing. Together, the grooves and baffles form labyrinths to c o u n t e r a c t cross-contamination of the products discharging at either end of the casing. 2.2.4.1 Front hub

The front h u b (the liquid discharge hub) bolts to one end of the bowl. It has an inner spindle to locate the conveyor, its bearing and seals, and an outer spindle for the fitting of the front main bearing and pillow block. Seals will also be fitted to the outer spindle as required. The discharging liquid is commonly known as the centrate.

Centrate Discharge Ports Inner Spindle

Outer Spindle Figure 2.5. A decanter fl'ont hub.

2.2.4.2 Centrate weirs

In a basic decanter the centrate discharges from the front hub over weirs. These weirs, sometimes called dam plates, cause a pond to form in the bowl. The level of liquid in the bowl, the distance from the bowl wall to the inner

Decanter Design

23

edge of the weir, is known as the pond height. The simplest form of weir is a r e c t a n g u l a r plate with slotted holes bolted to the outside face of the front hub. The pond height is adjusted by loosening the bolts, repositioning the plate and then re-securing the bolts (see Figure 2.6). Accurate location of the weir plates is necessary to enable a d j u s t m e n t of the pond level to within, say, 1 m m or better. This has necessitated the development of better designs (see Section 2.3.4.2). For best process control, the weir width needs to be maximised to reduce the level of cresting over the weir. The crest is the extra level of liquid above the weir inner edge, necessary to effect flow, as seen over weirs in rivers. This cresting varies with feed rate, but will be an inverse function of the weir length. Thus the larger this is, the smaller is the variation due to feed rate, or more properly, centrate flow changes.

Front Hub

Weir Plate

l

Adjustment Slot

s #

I

!

!

Fixing Bolts

/ Figure 2.6. A simple centrate weir.

2.2.4.3 Liner

A liner is a metal sheet rolled to spring into the shape of the inside of the bowl. On the inside surface of the sheet will be welded longitudinal strips. The liner is to combat erosion, but more particularly to form a key for the settled cake to improve scrolling efficiency. The liner will be held in position in the bowl by tack, or spot, welds. On the smaller sizes of centrifuge the liner will be full length. On larger machines it can be full length, but sometimes it will cover only a partial length of the bowl from the beach junction forward to a little way past the feed zone. The diameter of the conveyor and the profile of the larger end of the beach need to be adjusted to accommodate the liner. Thus the use of a liner should be decided before the centrifuge is built. Fitting a bowl liner is not an easy thing to do as an afterthought.

24

Basic Components Bowl Shell

Bowl Liner

Figure 2.7. A bo~vl shell with liner.

2.2.4.4 Front hub bearing

One of the two pillow block bearings is fitted to the front hub. It is supported in a housing and sealed with a non-contacting flinger, wind back and labyrinth parts on each side. The housing is accurately mounted to the main flame and aligned with the bearing housing at the opposite end of the rotor. The bearing shown in Figure 2.9 is chosen to result in a long Lloh life (above 1 0 0 0 0 0 hours) at its speed and load conditions (see Section 4.13.4 for definition of Linch). Lubrication is usually by oil, static, circulating or mist. Circulating oil, while usually the most expensive, is the best and most reliable. Most actual bearing failures are due to lubrication failure or foreign contamination, not load. A circulating system flushes out contaminants and introduces only cooled, filtered oil to the bearing. The oil drains from each housing must be large enough to discharge the oil quickly, after it passes through the bearing.

Grubscrew ScrewScrew ,-.,. -, \ End Plate t-unger L;over ~~1i ]

Co

Outer Flinger

Roller Bearing ~ - /

glinger- End PlateBearing Bearing

ScrewSeal Holder

PillowBlock

~

HolderSeal \

End PlateBearing

Seal O-ringDisc Casing Seal ] ~ End Cover

SpacerSeal

Figure 2.8. Components of a main bearing assembl!t.

Seal Disc

ScrewEnd Cover

Decanter Design

25

Housing Fliger

Figure 2.9. An alternative main bearing assembly.

Smaller decanters are often grease lubricated to reduce cost. The bearings of smaller decanters are often cylindrical roller, or ball type. Bearing housing seals must have sufficient axial clearance to permit t h e r m a l expansion of the rotor, and at least one bearing must float axially. The seal between the casing and front hub is usually a close clearance bushing. The space between the bearing housing and the casing is best vented to ensure separation of the oil and the process liquid. Most leakage is from outside air entering the casing, due to the slight v a c u u m produced by the rotating front hub. If a positive seal is required, both axial mechanical and radial mechanical seals are used. A radial seal, which uses two split, floating, carbon rings with a gas buffer riding on a tungsten carbide coated runner, is an example of an advanced design, permitting axial movement and both high- and lowt e m p e r a t u r e operation. 2.2.5 Beach

The beach is the conical section at the end of the bowl, and is considered a part of the bowl assembly. The front hub and the beach together enable a pool of liquor to be held in the bowl. Being a component in contact with the process liquor, the beach will be fabricated in the same material as the bowl. The beach will be flanged and bolted to the end of the bowl or inserted into the end of the bowl as a filler piece. To the rearmost end of the beach is fitted the bowl's rear hub. There are a n u m b e r of possible configurations involving these two components, to

facilitate the oaku discharge. 'l'lie discharge holes car1 be rourid, slotted or spccially shaped. 'I'lwse holes. i r ports. are genurally in the heach. but car1 hp i i l thc rear h u b , or t J O t h . The hiilf'inrluded iinglr ol' t.he cone shape o l ' t h r heach i s commonly known ;is tht: hwch m g l e . A different beach iingle, o r ;i combination o"Igles in a compound beach, cvuld be selected lo t'acilitate better dryness, better washing. or pcrhaps casicr scrulling. dcpcnding upon thc prnrcss application. A brach angle of 8 to 10 dcgrccs is a common valuc choscn for many prnccsscs. 'I'he hcach is usually ribbcd or grooved to assist in cnnvcying thc solids. Alternative designs are dr:st:rihed i r i Sectiorl 2 . 3 . 5 . 2.2..5:1 R r a r huh and hearings

Thc rear hub bolts to the beach. The rear h u h m;iin hearings arid seals ;1rr similar to t h a t ofthc front h u b (SCC Section 2.2.4 .)I),The rear h u h si1pport.sone end ot t he cnnveyor wllh ball. cylindrical rollcr or sphcrical roller bearings. A hall or roller hearing also resists the axial thrust ofthc convcyor. away from the beach. due 111 the solids conveying torque. All o f the conveynr hearings are grease lubrici~tedand sealed. usually with elastomeric lip scals. This is possible sincc theseal rubbing velocily ir; low. due to thc low diffcrciitial spccd bctwccn the convcyor and howl, Care is required to ensure that centrifugal forcc docs

Decanter Design

27

Rear Bowl Hub

-

-1

_Q, ,..a,

J I

i

i

____............3

Figure 2.11. A rear bowl hub.

Conveyor

Conveyor Thrust Beanng

Rear Hub

Thrust Bearing Seals

21

i

11

i

"-q Ill It Ill

,. ,. ~,,-, -, ,-:,4;2; ,; .

.

.

.

.

.

.

';~,

.

. . . .

........

Illll---"-""-J Rear

Bear

i

Rear Seals i,,,,

Figure 2.12. An alternative rear bo~,l hub and bearing assembly.

not separate the rubbing seal member from the contact surface. Larger decanters usually use a tension bar, to transfer the axial load to a bearing located in the driven pulley. This prevents the axial load produced by scrolling torque from being imposed on the bowl shell bolts. Where the axial load is resisted by bearings in the opposite end of the rotor, this load

28

Basic Components

must be added to the internal liquid and solids pressure load contained by the bowl shell bolts. All well-designed decanters permit the re-greasing of the conveyor bearings without the requirement to disassemble the casing. 2.2.5.2 Cake discharge The cake discharges at the rear of the beach, between the beach proper and the rear hub. In its simplest form, the cake discharge will be a series of radial holes around the beach end. These holes will usually be lined with some form of erosion protection, quite often in the form of a sintered tungsten carbide cylinder in a steel holder. For process reasons it is important to have a defined cake discharge diameter. This is the diameter of the inner edge of the beach (radially, outer axially), over which the solids decant into the casing. Thus, prior to the discharge ports will be a ring or ledge providing a definite discharge level. 2.2.5.3 Liner

The beach surface is the most difficult section over which the cake has to be scrolled, being an incline in a field of a few thousand g. It is therefore c o m m o n to provide a scrolling aid in the form of grooves or ribs. orientated axially. The grooves would be machined into the beach surface, whereas the ribs would be welded on or form a part of an inserted liner.

Figure 2.1 _3. Beach ribs and cake discharge apertures.

2.2.6 Cunveyor

T h e c(lIiveyor (or scroll) is in the form of'iivArchirriedian screw. fitting insidc i,he beach and bowl bctwcen the two end hubs. with a small clearance of less than 2 m m radially. It has a iiurnbcr of functions. Not only does il.c:rlnvfiy I.he sol.ids, a f k r they form a cake, along thc cyliudrical bowl section and up t h e beitch, if. also accepts the feed and ac.celeratcs it up to bowl speed. In its simplest form, t.he conveyor has a cyliiidrical central h u b with a sct of flights welded onto it., t,u I'orrn one r:cmtiriuous helix. l'he conveyor bearings and associated seals are housed in bot.h ends ol'i1.scenlral hub. Somewhere in between the bearings will be a charnbcr c,allcd the feed zone. sealed and isolated frotii both bearings, In some applications, whcrc tlic solid partidcs are too fine to separate on l.heir o w n , it is rwcessary to use a flocculating aid. The flocculant can be added upslrearn of the decanter, but there are inany circumstances where, for best efficiency. it is adiiiitted in the bowl. On these occasions there will be an extra c h a m h w , t.he "flnc znne", huilt into the huh of the conveyor. M!herr nect.ssary this 11oc c h a m b e r car1 he used ;is i j rinse chiimber insl.e;id, t.o admit rinse liquor onto the scrolling cake. 2.2.6.1 Uorrveyor hub

This p x t cil'the conveyor is a substantial tubular st.ccl constructinn. 1 1 nay bc tapered at thc bcach cnd. It could. if necessary, he erilargerl in diameter a t ei1l-h end to take thc coiivtcyor bearings. In each end of thc cotlvcynr h u l ~w i l l he the conveyor bearings with their assnciatcd scals. A d j a m i t hi o n e olthe bearings (in thc basic dcsigii it will hc

30

Basic Components

Feed Port

Rear Conveyor E,earlng Housing

Figure 2.1 5. A conveyor hub.

the front bearing) will be some form ofbushing. This bushing could be splined, keyed or specially shaped, e.g. lobed, to mate with the gearbox shaft, and so provide the conveyor drive. The feed zone will be built into the hub to discharge at the start of the cylindrical section of the bowl adjacent to the beach. Next to the feed zone, a second chamber for flocculant or rinse may be fabricated within the hub. A buffer chamber between the feed and additive chamber will sometimes be built, with simple exit ports into the pond. By putting distance between the feed and the additive chamber bv use of the buffer chamber, there is less chance of the additive chamber being contaminated by feed material. The natural vibrational frequency of the conveyor can be a limiting feature controlling the maximum speed of the centrifuge. This becomes especially critical when the L/D (length to diameter) ratio reaches 4.0 and more, and modern decanters are getting longer in order to give higher separating capacity. If the hub diameter gets smaller, the conveyor flexibility increases, thus lowering the natural frequency. Increasing the hub diameter will solve this problem, but with modern decanters using deeper ponds in m a n y applications, the hub becomes immersed in the pond. Immersed hubs can result in more hydraulic turbulence, and thus lower separation due to friction on the liquor surface, and possible build-up on the hub due to a sticky floating phase. Surface non-concentricity results in mechanical vibration due to nonsymmetrical buoyancy effects, so high precision is needed in geometry. Air flow and degassing of the feed stream become more complicated with submerged hubs. Some new designs avoid these problems by permitting small hubs designed with high stiffness and high natural frequency [1]. However, within the last decade, immersed hubs have been designed to float on the pond, considerably reducing potential vibration and enabling higher speeds [ 2 ].

Decanter Design

31

2.2.6.2 Flights

The conveyor flights are fabricated from segments of a n n u l a r discs suitably stretched and welded end to end, to form a regular helix. Naturally the helix profile has to be tapered to suit the beach section. Each section is welded in turn to the conveyor hub and then welded to the adjacent section. Double welding (both sides) with grinding afterwards is essential where hygiene is of importance. However double welding is c o m m o n practice, even w h e n hygiene is not required. The flights will be normally perpendicular to the decanter axis or bowl wall. On the beach the flights will either remain at 90 ~ to the axis or will be at 90 ~ to the beach surface, depending upon the decanter m a n u f a c t u r e r ' s choice. It is not always appreciated w h a t a complex shape the surface of a flight is. The usual pitch angle for a decanter is a little over 5 ~ . The pitch angle is the angle the tip of the flight subtends to a right circle of the bowl. To m a i n t a i n the flight at a constant angle to the axis, the pitch angle of 5 ~ nearly doubles at the root of the flight. If the flights are not to be protected from wear, then their tips will be ground smooth and perhaps chamfered, to provide a m i n i m u m of area in contact with the heel, to minimise torque. 2.2.6.3 Feed zone

There are a large n u m b e r of designs, both complex and simple, for the feed zone. The feed enters the feed zone c h a m b e r from the feed tube. Once in the

II

Figure 2.16. A conveyor flight section before welding to the conveyor hub.

32

Basic Components

Accelerator Blades

Feed Po~s

\

s! s

Feecl Pipe

Figure 2.17. A typical feed zone.

feed zone, it has to be accelerated up to the bowl speed before spilling into the pond via the exit ports. To assist the feed up to speed, a c c e l e r a t o r v a n e s will sometimes be found on the " t a r g e t " , the plate opposite the feed tube end. These vanes could be radial, at an angle to the radii, or curved. In extreme cases of wear, parts inside the feed zone are hard surfaced or specific erosion resistant c o m p o n e n t s are used. Hard s u r f a c i n g is often used on the accelerator plate, p a r t i c u l a r l y on leading edges and the tips of the vanes. w h e r e most w e a r takes place. Some shaped accelerators have been m a d e completely of u r e t h a n e rubber. W h e n the feed leaves the feed tube, in most cases it is at a high axial velocity. W h e n it hits the r o t a t i n g target, some splashing inevitably occurs. In fact a dense aerosol mist is often produced. At the back of the feed zone a tube is sometimes built in, to s u r r o u n d the end of the feed tube. On the outside of this tube, small accelerator v a n e s are welded to accelerate and condense the mist and also to accelerate liquor up to speed, should the feed zone become flooded. Ideally, air is allowed to enter the feed zone from a r o u n d the feed tube. It will be d r a w n in by the fan effect of the feed zone and t h r o w n out of the feed zone exit ports. The air would t h e n pass along the bowl to exit over the centrate. This d r a u g h t helps to prevent splash back of feed from the feed zone. The exit ports from the feed zone are themselves subject to a considerable range of designs and i n n o v a t i o n s . It is not usual to h a v e just one exit port. For s y m m e t r y and balance an even n u m b e r of ports is usual, two, four, six or eight. The basic design has these ports fitted with t u b u l a r nozzles, often lined with a ceramic w e a r protection. New feed zones have been introduced recently to reduce feed particle attrition, by slowing and e x t e n d i n g the acceleration time to bring the feed up to speed, and to reduce the inlet t u r b u l e n c e in the s e p a r a t i o n zone.

Decanter Design

33

2.2.6.4 Floc/rinse z o n e

T h e floc or rinse zone is a c h a m b e r in the c o n v e y o r h u b b e h i n d the feed zone, s o m e t i m e s s e p a r a t e d by a buffer c h a m b e r to minimise cross c o n t a m i n a t i o n . T h e required t h r o u g h p u t of this c h a m b e r is a n order of m a g n i t u d e less t h a n t h a t of the feed zone. Therefore, it does n o t r e q u i r e the s a m e sophistication as the feed zone, n o r does it r e q u i r e erosion p r o t e c t i o n . F r o m the floc zone, c h a n n e l s or tubes are provided to lead the flocculant into the area of feed discharge to e n s u r e a n i n t i m a t e a n d e c o n o m i c mix at the a p p r o p r i a t e point. Flocculant c a n be relatively expensive, and on an effluent application c o n t r i b u t e s a large p e r c e n t a g e to the total t r e a t m e n t costs. It is t h e r e f o r e very i m p o r t a n t to e n s u r e t h a t just the right a m o u n t of flocculant is used, and that no e x t r a is r e q u i r e d due to b y p a s s i n g . As a rinse c h a m b e r , the exit p a t h s are quite different. Generally these will be o n t o the b e a c h section, and even onto t h e dry b e a c h section, if not at the j u n c t i o n of wet a n d dry zones. 2.2.6.5 Wear protection

This is a very i m p o r t a n t topic and will be covered a g a i n in Section 2 . 3 . 6 . 5 , a n d m o r e fully in Section 2.4.9. Various levels a n d grades of w e a r p r o t e c t i o n m a y be applied to the c o n v e y o r d e p e n d i n g u p o n the application. The m a i n areas on the c o n v e y o r r e q u i r i n g p r o t e c t i o n are the feed zone, flight scrolling surfaces a n d the flight tips. M e c h a n i c a l l y i n t e r c h a n g e a b l e w e a r inserts are m o r e e c o n o m i c a l to replace t h a n welded or b o n d e d w e a r protection.

Pt //

t~

II

ii ii

II

t I

II II ii

I

IIi

;,, r~

Feed Zone

Figure 2.18. An additive chamber.

, ~,/

1

Additive Chamber

34

Basic Components

Figure 2.19. Floc Addition.

Feed Tube

~/

-

-

,.

~ 1 -

U

,

J;

U

U~

--

.-,.-:,,, :..-:,-:::-..... ' / 1

/

,"

,/

Figure 2.20. Rinsing.

2.2.6.6 Conveyor bearings and seals Bearings are fitted into the ends of the conveyor and are generally grease packed. A seal will be used to retain the grease and a second outer seal will be used to prevent ingress of process fluids and solids into the bearing. Thus two seals are fitted back to back at each end. Grease channels have to be

Decanter Design Areas of Protection

,--.

p-

-~-~-,,

r ~~1 7 6 ~ ' ' ~ ~ -9

o,

,o

e ~.,

-

..........~

_

~

,

o ~

Figure 2.21. Areas of wear protection.

Figure 2.22. A typical conveyor bearing assembly with seals.

,,

[~

35

36

Basic Components

incorporated into the design of the conveyor hub and both bowl hubs, to facilitate the greasing of the conveyor bearings from the outside. The differential rotation between the two races of each bearing is low and thus the life of these bearings should be good when adequately greased and sealed. 2.2.7 Gearbox

The gearbox is a major component of the rotating assembly, which creates the differential speed between the bowl and the conveyor. There are two main types of gearboxes used on decanters. These are the epicyclic gearbox and the Cyclo gearbox, made by Sumitomo of Japan. However there are a number of decanters which have eliminated the gearbox by using a hydraulic system called a Rotodiff manufactured by the Swiss company Viscotherm. The Rotodiff and the Cyclo gearbox will be covered in more detail in Sections 2.3.7 and 2.4.15, respectively. The epicyclic system consists of a pinion shaft and gear, which engages three planetary gears (mounted on carrier plates) which in turn engage a ring gear fixed to the gearbox casing. For the decanter the epicyclic gearbox involves two stages, although recently three stages have been in use. The carrier plate of the first stage holds a second pinion shaft carrying the sun gear for the second stage. The ratio of the gearbox is the product of the ratios for each stage. The maximum practical ratio for any stage is just over 1 3, giving a 2nd Stage Ring Gear (Annulus)

1st Stage Ring Gear (Annulus)

100

~L

~

I

~

99

Gearbox in operation

~-

1 Revolution Output Spindle Shaft "2nd Stage Sun Gear

I st Stage Sun Gear

m

Figure 2.2 3. A two stage epic!lclic,qearbox ( 1 O0 Ratio).

Decanter Design 37

m a x i m u m ratio for a two-stage epicyclic gearbox of 170 to 180. Three stage epicyclic gearboxes with ratios over 500 have been used on decanters. If the central pinion shaft is held stationary, the differential speed between conveyor and bowl will be the bowl speed divided by the gearbox ratio. If the pinion shaft is allowed to rotate at some speed below the bowl speed, then the differential between bowl and conveyor will be the difference between bowl speed and pinion speed, divided by gearbox ratio. If the pinion speed is controlled by using a brake, or a variable speed motor, differential speed m a y be varied from close to m a x i m u m , w h e n the brake is at its slowest speed, to nearly zero, w h e n the brake is almost at bowl speed. Reducing the pinion speed below zero, i.e. by reversing, enables higher differential speeds to be obtained. Using an epicyclic gearbox causes the conveyor to rotate slower than the bowl, whereas it is normally faster w h e n using a Cyclo gearbox. Generally the conveyor flight helix is "left h a n d e d " with an epicyclic gearbox and right handed with a Cyclo gearbox. 2.2.8 Frame

On smaller decanters, the frame has often been made from cast iron. More usually it is fabricated from steel channel or box sections. The flame needs to be a rigid support for the rotating assembly. The surfaces for the main bearing pillow blocks are accurately machined in the same plane, and in line, to ensure no end-to-end misalignment of the rotating assembly, which would cause p r e m a t u r e bearing failure. Some m a n u f a c t u r e r s fill part of the main flames of their larger machines with concrete effectively to form an inertia

J

Figure 2.24. Decantern~ainJrame.

38

Basic Components

block, and for noise reduction. Some flames have been used as a reservoir for the lubricating oil for the main bearings. The flame and casing (see Section 2.2.9) act as the link between the high g field of the rotating assembly and the stationary area a r o u n d it. 2.2.8.1 Bearing supports The m a i n bearings with seals are m o u n t e d onto the m a i n flame in pillow blocks. When oil lubricated, the pillow blocks will be piped to an oil system, which will include a circulating pump, an oil reservoir and cooler with associated pressure, flow, and t e m p e r a t u r e i n s t r u m e n t a t i o n (see Section 2.2.4.4). 2.2.8.2 Feed tube In its simplest form the feed tube is a plain cylindrical tube. A clamp or flange holds it on a support extension from the main flame. It extends to the feed zone and within a few centimetres of the accelerator in the feed zone.

Casing Seal

Pillow Block

3~x

[[ii-Iti!:ii| I'lllll'llllt i I!

Feed Tube

'j

'~

i iI

Pillow Block

Figure 2.2 5. A main hearing and pillow block assemblH.

Decanter Design

39

Figure 2.26. Feed tube.

The geometry of m a n y decanters is such t h a t there is a risk of r e s o n a n t vibration of the feed tube at frequencies a r o u n d the bowl speed. To c o u n t e r this, feed tubes have been made slightly tapered, and made of lighter materials such as glass fibre and even carbon fibre, and sometimes composite material. Entering the conveyor with the feed stream is a flow of leakage air, w h i c h passes through the clearance between the conveyor and the feed tube. This air flow must eventually be vented, and in those applications where odour or toxicity is an issue, minimising this air in-flow is important. A simple, lightly contacting lip seal is often used. On critical sealing processes, m e c h a n i c a l seals are required. 2.2.8.3 Vibration isolators

Out-of-balance forces in the rotating assembly are isolated from the ground by interposing vibration isolators between the flame and ground. When a subflame is used the mounts are placed under the sub-flame. Even with a well balanced m a c h i n e , out-of-balance can occur when solids build up unevenly, when there is uneven wear, or when some u n p l a n n e d mechanical movement occurs within the rotating assembly. Considerable oscillations of the rotating assembly can occur during run up and shut down, w h e n the speed of the bowl passes t h r o u g h critical speeds. Owing to the presence of these isolators, all process connections to the decanter must be flexible. Likewise, oil lubrication connections m u s t also be flexible.

Spring

Carbon Seal Rings

Spring

Figure 2.27. A feed tube seal.

O-ring

Housing

40

Basic Components

Fn

Screw

Su .3hloroprene Elastomer

];i

;;J

,,,. -,, ~-.-..-~/?..-.-.L ~ ; ~.; .-,..._~:..~::: :..~,. :, '. . .,.,-,-~-~ ..-:,_.-.4,.. ....... .-.--.~;-~;:;." .. ". ....

9 "" F l o o r

9

Figure 2.28. A decanter vibration isolator.

2.2.9 Casing T h e c a s i n g a c t s as t h e c o l l e c t o r for t h e p r o d u c t s d i s c h a r g e d f r o m t h e r o t a t i n g a s s e m b l y , a n d c h a n n e l s t h e m to r e c e i v e r s for o n w a r d h a n d l i n g . T h e c a s i n g must, obviously, keep the separated products apart.

Upper Casing

a . . . . . . . . . .

~

Lower Casing

Figure 2.2 9. Decanter casing.

ii

Decanter Design

41

Simply stated, this casing is a stationary collector for cake at one end, and centrate at the other. There are m a n y design variants and each m a n u f a c t u r e r has its own recognisable design w h e t h e r it be just its finish or its shape and functionality. In its more usual format, a lid is hinged onto a bottom half and bolted with a flat rubber gasket between the two halves.

2.2.9.'1 Casing baffles Inside, the casing is compartmenta]ised by several baffles, which are welded to the inside surface of the casing and fit very close to the rotating assembly. This close fit may be to the plain surface of the bow], or to a shoulder on the bowl, or into a labyrinth groove machined into the bowl.

2.2.9.2 Cake discharge To cope with the wide range of rheologies of cakes experienced, the cake chute of the casing needs to be as open and large as possible without ledges, or any n a r r o w i n g of the opening. The cake discharges from the bowl at high velocity t h r o u g h 360 ~. It is necessary to ensure that this cake does not stick to the casing, and is directed down into the receiver.

\:,',, \ Casing Baffles

Figure 2.30. Casing baffles.

42 BasicComponents 2.2.9.3 Centrate discharge

It is usual to cone down the centrate end of the casing in order to mate w i t h an off take pipe. The pipe size needs to be sufficient to allow free flow of the copious quantities of air at this end. Alternatively a separate vent pipe will be introduced. The centrate tends to continue its circular motion on leaving the bowl and swirls around in the casing. It is therefore sometimes seen t h a t the centrate discharge is offset from the centre line to take a d v a n t a g e of this tangential flow. 2.2.9.4 Casing seals

A flat gasket seals the two halves of the casing. This will be of, say, neoprene, Viton or silicone rubbers, depending upon the application. Sometimes these gaskets are moulded with various cross-sectional shapes to provide good location and secure positioning. The clearance between the casing and the outer spindles of the bowl hubs can be open, or have fitted some form of seal depending upon the degree of sealing required. For atmospheric operation a simple surface contact of a PTFE ring often suffices (see Section 2.2.4.4). Horizontal decanters, operating at a slight positive or negative pressure. require careful sealing w h e n the casing is split, especially at the corners and the end. Some higher pressure designs avoid this problem by h a v i n g the casing cylindrical, with disassembly in the axial direction, thus using simple O-ring seals. However, m a c h i n e disassembly is then complicated and added floor space is required for m a i n t e n a n c e . Sealing process discharge vents, as well as feed lines, requires the use of commercial and special flexible connectors. These connectors m u s t be carefully designed to limit the forces imposed on the centrifuge and m o d u l a t e the forces transmitted to the plant piping and structure. The material of these flexible connectors must be chosen to resist the process temperature, pressure and corrosive characteristics. 2.2.9.5 Vents

The bowl, rotating at high speed, acts like a fan, and drags large quantities of air around with it in a t u r b u l e n t motion. This turbulent motion of air, called windage, tends to move outwards due to centrifugal action, and in doing so drags air towards the centre line to replace it. Windage needs to be channelled and vented rather t h a n suppressed. Suppressing windage can cause cross c o n t a m i n a t i o n rather t h a n prevent it. It can be advantageous to allow air into the centre c o m p a r t m e n t to satisfy the windage created at either end of the bowl. In so doing the air travels from the centre outwards and thus helps to prevent escape of products from their designated discharge c o m p a r t m e n t s .

Decanter Design

43

If air is allowed in, then it has to be allowed out, and thus it is n e c e s s a r y to e n s u r e that the product lines or receivers are vented a n d the lines are a d e q u a t e in size to carry both p r o d u c t and the air flow. Air flow e n t r a i n s a lot of the discharged c e n t r a t e . This e n t r a i n e d liquor has a propensity to migrate a r o u n d baffles. T h u s the bottom of the casing is generally slightly sloped to drain any spilled liquor back to the c e n t r a t e . In thickening applications, the slope will be in the opposite direction, as a little extra dirty liquor in a fluid cake is preferable to dirty liquor in a clean c e n t r a t e . 2.2.10 Sub-frame

W h e n the m a i n f l a m e a d e q u a t e l y fulfils its function, a s u b - f l a m e is not used. The sub-flame forms a base for the m a i n f l a m e c a r r y i n g the r o t a t i n g assembly plus the main drive motor and back-drive system w h e n used. It e n s u r e s t h a t there is no u n w a n t e d differential m o v e m e n t w i t h either of the drives and the r o t a t i n g assembly. Vibration isolators are strategically placed u n d e r the subf l a m e to share the load properly The use of a s u b - f l a m e subjects the drive m o t o r to more vibration, but at the same time it m a k e s installation easier. 2.2.11 Main drive

The m a i n centrifuge drive is usually an electrical m o t o r m o u n t e d on slide rails on the frame or sub-frame and connected to the centrifuge drive pulley with V-belts. A purpose built belt g u a r d will cover the two pulleys and belts. Motors on the larger d e c a n t e r s can c o n s u m e a few h u n d r e d kilowatts of power. Such large motors are more often directly m o u n t e d on the floor, in w h i c h case a special belt-tensioning device is incorporated to allow for the differential m o v e m e n t of the rotating assembly.

Figure 2.31. Sub-frame.

The mijin motor IS t o i\CC
Decanter Design

45

Motors can be two-, four- or six-pole giving s y n c h r o n o u s speeds of 3 0 0 0 , 1500 or 1000 rpm at 50 Hz, respectively. The most commonly used motor is the four-pole, which is a more usual standard in motors and is capable of being better balanced t h a n the two-pole. Because of the low speed the six-pole m o t o r would be an u n u s u a l choice. The torque available from the main motor varies according to the method of start up, w h e t h e r it is star-delta or direct-on-line. A m o t o r connected in star produces a starting torque one third of that provided w h e n starting direct-on-line. Direct-on-line starting torque can be two and a half times the motor's full load torque, with a starting current of six times full load current. A standard motor has a very steep direct-on-line torque/speed curve characteristic, rising from a m i n i m u m of 1 8 0 - 2 5 0 % full load torque, to a m a x i m u m of 3 0 0 - 5 0 0 % full load torque. This m a x i m u m , called the pull out torque, occurs at about 85 % of full speed. Main motors need starter overload and short circuit protection. High rupture fuses (HRC) will protect the motor against short circuit conditions, and will interrupt the electrical supply in milliseconds of the fault occurring. It is essential that fuses of this type are always fitted. Conventional overload protection, thermal or magnetic, can offer no protection to a motor with an extended acceleration time. Thermistor overload protection is the only true protection for a motor under these conditions. A thermistor is embedded in each of the motor's three windings and connected in series. The resistance of these thermistors is designed to increase rapidly at a set temperature, depending upon the insulation class of the motor. The thermistors are connected to an electronic amplifier control unit in the starter enclosure, and will trip the starter contacts w h e n required. The device will not reset until the motor has sufficiently cooled. In Europe the main motor is usually an AC motor, using a star-delta starter. An inverter for the main motor is becoming more common, particularly with the smaller decanters. The inverter enables a soft start, and allows speed adjustment for different process requirements. Inverter motors can cause u n w a n t e d electrical interference, and harmonic wave forms, on the main electrical supply lines. These problems can be minimised by using electrical filters and the latest advanced electrical technology. 2.2.12 Back-drive

The back-drive system is a means of controlling the speed of the gearbox pinion shaft (and thereby the conveyor differential speed) using, for instance, a motor or a brake. This could be offset from the gearbox shaft, in the same m a n n e r as the main drive, and connected by a belt. This belt would be a timing belt because of the accurate control required. Normally the back-drive is connected directly and in line with the gearbox pinion.

Basic Components

46

. ~

!

.-~--;'

'1~1 '~I1~

~~'~!

I

|i ~''~..

~'~-'"

~

"~

~" 2-;~*';---'"

S~ . . . .

-~--~'"

.,~oO~i

i

Figure 2.3 3. A decanter back-drive system.

The main component of the back-drive assembly can be an eddy c u r r e n t brake, inverter motor or a DC motor. The Viscotherm Rotodiff hydraulic conveyor drive is a variable speed device, powered by a fixed speed hydraulic motor.

2.3 Variations to Main Components The basic components of the decanter have been described. It is now possible to discuss the alternative designs of these components. 2.3.1 Orientation

The orientation of the axis of rotation (horizontal or vertical) does not affect the process performance of a decanter. The g force is at right angles to the axis in both cases, so rotors that are equivalent in diameter, speed, geometry and design will give comparable separation. There is one main alternative to the basic horizontal orientation and that is vertical. However, the Flottweg Company does have an inclined decanter, inclined at the angle of the beach, which facilitates full emptying of the bowl for clean-in-place processes. The availability of inexpensive vertical decanters is very limited, such that in some circumstances one has to select a horizontal model even though a vertical one might have been preferred. The vertical decanter is designed and built primarily for high-temperature and/or pressurised operation. The vertical orientation lends itself to better and more reliable pressurisation, with only one end to be provided with rotational seals. The amount of engineering required for pressurisation and sealing makes the vertical decanter more expensive than the non-pressurised and open horizontal decanter. 2.3.1.7 Vertical vs. horizontal

Figure 2.35 compares the seal and structural differences of these types of decanters. While each type requires three slow speed seals for the conveyor and one slow speed seal for the gearbox, these seals, due to their very low surface rubbing speeds, do not cause problems if designed properly. The horizontal type requires two large high-speed seals to be mounted on the rotor, and one smaller high speed feed tube seal. The vertical type, in which the rotor and gearbox are pendulum suspended from a flexibly mounted spindle, requires only one high speed seal between the bearings in the spindle and the casing. There are no high speed bearings or seals at the bottom of the

48

Variations to Main Components

Rotary Joint for Gearbox Lube Oil Feed and Take-off Drive Pulley Flexible Suspension System

Spindle

Rubber / I s o l a t i o n Mounts

Spindle Seal Flexible Casing Seal Gearbox Upper Casing Wear Ring (used when fine abrasive solids present in effluent) Effluent Discharge

Conveyor ~.~

Upper Casing ~

J/

&

Casing Vent

"~-Frame

Bowl Feed Accelerator Conveyor Wear Sleeve

Feed Tube Solids Discharge Spray Ring

Abrasive Solids Wear Sleeve Conveyor Sleeve Bearing

Rinse Connection

Conveyor Support Washer Bowl Bumper Lower Casing Feed Connection

Figure 2.34. A vertical decanter (By courtesy of Alfa Laval).

Decanter Design

49

Horizontal

High Speq Low Speed Seals Flush Ve~n,t 1Flush Lo y II ~ T High Speed Seals

High Speed Seal

\

Spe Se~

Rinse

, 9 ,,,,

;~

,-,-

"

~

, ' ' I

:

'~ Low

Centrate Seals Stationary

Rigid ~

~

..

"ll'a .q

~

N~ _o~.;c~._;

~

Feed

Centr

Vibration Isolators

Solids

Stationary Vibrating

T

Solids

Figure 2.3 5. Comparison of vertical and horizontal decanters.

rotor, and no high speed feed tube seal needed. In addition, with the casing and flame of the horizontal decanter flexibly m o u n t e d and vibrating, all process and electric connections must be flexible, thus needing periodic attention. The vertical decanter flame and casing are rigidly mounted. Therefore there is no need for a vibration isolator between it and the external connections. With no second bearing and seal on the vertical decanter, thermal expansion and alignment issues are almost eliminated. Almost all of the vertical decanters installed have been designed to meet the Unfired Pressure Vessel, Explosion Proof Code requirements and chemical industry piping, vessel lubrication and i n s t r u m e n t a t i o n codes. Due to this, the cost of this design is higher t h a n horizontal m a c h i n e s of equal process capacity. For those horizontal m a c h i n e s that meet the same e n v i r o n m e n t and code requirements, the cost is comparable. In general, the emitted noise level of vertical m a c h i n e s is m u c h lower t h a n horizontal machines due to less vibrating surface exposed to the work area. 2.3.7.2

Vertical

decanter

seals and

bearings

The main high speed seal assembly used in vertical machines is usually a mechanical seal. This is m o u n t e d on the spindle assembly cartridge, which is separable from the gearbox by m e a n s of a tapered joint, thus allowing seal and bearing m a i n t e n a n c e w i t h o u t removal and disassembly of the main rotor. Maintenance can then be done in a clean e n v i r o n m e n t . Most seals installed are titanium bellows with carbon nose, rubbing on a hard coated stainless steel or solid carbide or ceramic m a t i n g ring. The materials used are chosen to meet the most difficult of corrosive and t e m p e r a t u r e and pressure environments. Seal assemblies having two seals and one m a t i n g ring, and a buffer liquid between, are available.

50

Variations to Main Components

Figure 2.36. A vertical decanter seal and bearing assembly (By courtesy of Alfa Laval)

Decanter Design

51

It should be noted t h a t the horizontal design will impose an axial load on one of the m a i n bearings, equal to the process pressure times the feed tube seal area. The axial load (upward) on a vertical design reduces the axial bearing load due to the rotor weight. This reduction in bearing load is equal to the process pressure times the high-speed spindle seal area. In fact, with a high enough process pressure, the axial load can be near zero on the m a i n bearing and can even push up on it. This load reversal must be considered in the design. 2.3.1.3 Vertical decanter casing seal

Since this design permits m o v e m e n t between the spindle housing and casing, a flexible static seal is required which is not needed in a horizontal machine. The device most used is a single reinforced elastomeric bellow, sufficient to resist the internal vapour pressure in the casing, and lined with PTFE to resist the corrosion of the process. 2.3.2 Flow

The main alternative to the c o u n t e r c u r r e n t flow inside the bowl already described is co-current. In this design, the feed enters a feed c h a m b e r situated close to the bowl front hub, from a feed tube t h r o u g h the front main bearing. Both the cake and the clarifying liquor travel together towards the beach end. Prior to the beach, the clarified liquor decants into channels built onto the conveyor hub, dipping into the pond, which direct the centrate back to the front hub for discharge over adjustable weirs in the normal way, already described. In this design the gearbox is fitted to the rear of the bowl. The drive is usually at this end also. The co-current design allows the shortest feed tube. It also means that the finer solids, settled the furthest distance from the feed zone, do not have to return u n d e r the turbulent area of the feed zone, and so risk being resuspended. The co-current design requires horizontal channels built onto the conveyor hub, dipping into the pond, to direct clarified liquor back to the front hub. These can suffer from fines settlement. However, in the c o u n t e r c u r r e n t design the majority of the solids are removed from the clarification zone early, allowing more room for clarification. Separation problems, which result in more solids in the centrate than usual, can necessitate the decanter to be shut down for rodding out of plugged return tubes. Turbulence at the entrance to the r e t u r n tubes, near the solids discharge, can cause solids re-suspension. Despite these differences in behaviour, there are m a n y t h o u s a n d s of decanters of each design successfully operating in the field, and both are currently sold. It thus can be concluded that performance differences are marginal. There are only a few applications where the supplier might claim the physical difference of his preferred design offers an advantage.

52

Variations to Main Components

Reinforced Elastomer~ PmFE Liner-~

~

___

~~"~'~.~~ ~ ' ~ . ~ ~ ~ ~ t i Casing

pindle

Figure 2.3 7. A vertical decanter casing seal (By courtesy of Alfa Laval).

2.3.3 Materials of construction

Each decanter m a n u f a c t u r e r can choose from a range of construction materials, and will have its own preferred materials. Choice m a y be due to the m a n u f a c t u r e r ' s own particular design, the process materials, the speed at

Decanter Design

53

I

Figure 2.3 8. A co-current decanter( BI! courtesy of KHD ).

w h i c h the bowl is to be rotated, or s o m e t i m e s e v e n the a r r a n g e m e n t s with t h e p a r t i c u l a r steel supplier. The most critical parts, the c o n t a c t parts of the r o t a t i n g assembly, are m o s t frequently m a d e in one of the m a n y stainless steels. N e v e r t h e l e s s some m a n u f a c t u r e r s c o n t i n u e to supply bowls in c a r b o n steel. The 3 1 6 - t y p e stainless steel is a c o m m o n material for the smaller decanters, but for the larger m a c h i n e s at h i g h e r g-forces, m a t e r i a l s with h i g h e r s t r e n g t h , such as the duplex stainless steels, must be used. W h e r e high t e m p e r a t u r e s , or e x t r a high speeds, or corrosive materials are in use, special c o r r o s i o n - r e s i s t a n t or t e m p e r a t u r e - r e s i s t a n t materials m a y be used. A c o m m o n r e a s o n for corrosion on d e c a n t e r s is the presence of chlorides in the process material. The chlorides c a n c a u s e pitting corrosion, crevice corrosion a n d stress corrosion of the d e c a n t e r parts in c o n t a c t with t h e process. To avoid pitting and crevice c o r r o s i o n in severe e n v i r o n m e n t s , m o r e corrosion resistant materials, n o r m a l l y special stainless steels with a h i g h e r c o n t e n t of alloying elements, are used. In e x t r e m e l y corrosive e n v i r o n m e n t s , m o r e exotic materials, such as the nickel-based Hastelloy or e v e n t i t a n i u m , m a y be used. Stress c o r r o s i o n cracking (SCC) is a special c o r r o s i o n form, w h i c h is seen as f i n e - b r a n c h e d t r a n s g r a n u l a r cracks in the material. SCC is c a u s e d by a c o m b i n a t i o n of m e c h a n i c a l stress, the p r e s e n c e of chlorides a n d elevated t e m p e r a t u r e . At h i g h t e m p e r a t u r e a n d h i g h m e c h a n i c a l stress, SCC of 3 1 6 type stainless steel can occur at chloride levels even below 1 0 0 0 ppm. T h e a n s w e r to this is to use a more c o r r o s i o n - r e s i s t a n t stainless steel or a duplex stainless steel. Duplex stainless steels n o r m a l l y do not s h o w SCC at t e m p e r a t u r e s below 100~ The fabrication m e t h o d s for the bowl are i m p o r t a n t w h e n c o n s i d e r i n g its safe w o r k i n g speed. Some bowls are m a c h i n e d from a simple cast ingot a n d others from a special centrifugal casting. Some are even fabricated by rolling a

54

Variations to Main Components

sheet and welding. Quality control of casting, and especially rolled and welded parts, is critical. In the case of contact parts that are not rotated, such as the casing, the specification of the steel need not be so stringent. Bowls in stainless steel and casings in a non-stainless steel are not u n k n o w n . Materials for erosion protection are described in Section 2.3.6.5. Elastomers used for seals and gaskets again depend upon the e n v i r o n m e n t . Neoprene is a standard material, but Viton and m a n y other materials, even PTFE, are used when necessary. In some parts of the decanter, such as bowl and beach liners and feed zone liners, they can be made of a similar material to the bowl, w h e n they just act as a preferential sacrificial wear component, to protect a more expensive component. In the cake discharge c o m p a r t m e n t of the casing, linings of rubber, PTFE, PVC, steel and even Stellite have all been used variously to combat wear or to overcome cake sticking. Flexing of some materials here is encouraged to aid the desticking process. 2.3.4 Bowl variants

The bowl is a simple cylinder and therefore the variations possible are limited, although one of the first bowls made was fully conical and thus constituted both bowl and beach together. Some of the longer bowls are made in sections, which are flanged and bolted together to give a set of standard bowl lengths. This allows c o m m o n components with the shorter designs. The m a c h i n i n g of the inside diameter will depend upon w h e t h e r a liner is to be fitted or not. The outside is machined to mate with baffles in the casing and with grooves when required to form a labyrinth with the casing baffles. 2.3.4.'1 Front hub

The design of the front (centrate discharge) hub varies considerably from machine to machine. However these variations are generally for mechanical considerations, having little effect, by themselves, on process performance. The thickness of the hub is dictated by the mechanical strength and stiffness requirements. When centrate discharge ports are drilled radially into it, as for some three-phase designs, extra thickness is needed. With m a n y different designs for centrate discharge, which are incorporated on the front hub, the mechanical design is made to suit. The precise bearings specified for the inner and outer spindles of the hub will need their own m a t i n g diameters. Devices added to the front hub, such as centripetal pumps, noise a b a t e m e n t rings if used, and the gearbox, all require their own special designs and fixings.

Decntiter Design

55

When simple weirs are used, then the hub will be machined with recesses to locate the weirs and their support plates. On the smaller diameter bowls the number of weir plates will probably be four, while Iarger models may take six or even eight. 2.3.4.2 Centrate weirs

An advance on the plain design (see Section 2.2.4.2) is to recess the weir plates into holes machined into the hub. A thicker cover plate would secure each weir plate. The outer edge of the weir and cover plate would be circular to locate in the recess. Different. pond depths are then obtained with different weir plates. In the “moon” design the inside edge of the weir plate would be radiused to match the pond surface. The longer this edge is the better, as this minimises cresting, which facilitates easier process control. The cover plate would have its inner radius larger than the shallowest weir plate usable. The deeper the weir plate the more important it is to secure it with a support plate. This is because the dam, being relatively thin because of the range of sizes necessary, can easily distort with the pressure of the pond at speed. The better cover plates will have a protruding lip fabricated on their inner edge. This is to ensure that the discharging centrate separates from the bowl at. the smallest possible radius. This minimises power consumption. One alternative to the multi-plate weir is to have a complete circle split in two, t n facilitate fitting, and bolted over the ports. These hairrings have t o be very thick to avoid distortion and loss of seal. Another alternative is to have a complete circular plate n7itb slots cut at a specitic radius to cover the hub holes. Other sets of slots. maybe as many as threc. are cut at. alternativc radii but offset. circumferentially. To change the pond setting the plate is unboltcd and rotated. to engage another slot radius Over the hub holes. and then re-secured. These last two variants tend to bc uneconomic unless it is known that only a small number or pond settings will be requircd. A further type of pond adjustment is provided by a set of close-fitting circular inserts in the bowl hub apertures. An eccentric hole is turned in each casting insert. By rotating the circular casting in its seat, an almost infinite range of pond settings can be obtained. The shortcoming of this design is that the overflow edge is locally circular, not matching the pond surface, such that a large crest forms over the weir. the height ofwhich varies considerably with the process rate. This does not allow good process control when needed. When the weir height is set above the cake discharge height a considerable amount of feed can flow from the cake discharge at start up. until a cake seal is created. There are various ways of combating this. such as stopping and starting with a full bowl of solids, setting a minimal differential a t start up, and feeding at a low rate at the start. A better method is to use notched weir plates

56

Variations to Main Components

(see Section 2.4.4.2). This method of operating the decanter (liquid discharge radius smaller than the cake discharge radius) is sometimes k n o w n as "negative pond operation". It is used to improve scrolling efficiency, and to enhance the control of scrolling, by making use of the hydraulic head difference between the discharges. 2.3.4.3 Liner

Liners can sit inward of the bowl surface. Alternatively a recess can be machined into the bowl to take the thickness of the liner with its ribs. In the latter case, the option of w h e t h e r to fit a liner or not can be taken after m a n u f a c t u r e . However, in this case and operating without a liner, a m u c h bigger heel results, which could give out of balance problems, should part of the heel dry out and break away after the bowl remains stationary for any length of time. An alternative to the ribbed liner, recently introduced, is an expanded metal sheet ("Expamet") fitted in a recessed bowl. This is now used frequently with success. Instead of a liner, m a n y bowls have longitudinal ribs spot welded to the inside of the bowl. A n o t h e r alternative to a liner is knurling or r o u g h e n i n g of the bowl, which is a simple procedure, but would normally be done only as a temporary measure as such surfaces will be quickly worn smooth. Some applications do not require the assistance of a bowl liner as the process material has sufficient friction, even with a smooth m a c h i n e d bowl surface, to provide e n o u g h keying to allow an adequate scrolling efficiency. Using a liner on these applications could raise the scrolling torque to an unacceptable level.

Centrate Discharge

Figure 2.39. A centrate

"moon"dam with

cover plate.

Decanter Design

_SplitDams

_..

\

Figure 2.,10. A split dam.

DamP l a t e ~ D ~

Figure 2.41. A single piece dam.

amHolder

57

2.3.4.4 M d i r i b r a r i r i g

Several different bearing types ;Jnd hwring combinnlions arc used for (lwarilers. A (:omrriort solution i s to hitvr ii deep groove balI-bcaring in onc end. usually the drive end, which fixes the dccantcr in thc axial dircction a n d a cylindrical rrillcr bcarinE i n thc other cnd. which allows for therrrial expansion of thc bowl. Sornc manufacturers. i n particular on large dccanters. usc doublc spherical roller bearings. which havc high load capacity. [f t h e decanter has deep groove ball bearings. or other bearings which d o not allow axial displacement, in both ends, the decariter tiiust be desiprtrd to acctmimodate the thermal expansion of the rotor. When selecting milin bearings lor [Iec;jnters sevltral p;ir;jmet.rrs must he considered. The rn;iin par;imeters ;ire l h e rotat.ioria1 s ~ L ' cthe ' ~ ,radial 10;id. thc operating temperature anti the luhricatiori riiethod. 'l'he expected bcarlng lifc can bc calculatcd bascd on bcarinR data and thc dynamic arid static loads on thc bcarinE. F o r a giver1 decanter. size a large convcyor h u b shaft dianictcr is oftcn dcsirahle both to increase t h e stiffness or the rotor and to give space for feed pipe nnd splinr shafl.s. A s t.he allowahle speed for a given bearing type dccrcases with increasing size, the hearing selettioil will he a compromise. An oil-lubricated bearing will have a higher allow able^ speed arid t.!ierr,fwr, o i l luhricatinri is often uscd for larger bearings and bearings with ;i high speed or rotitlion.

Decanter Design

59

Angular Contact Ball Bearing

Deep Groove Ball Bearing

Cylindrical Roller Bearing

Cylindrical Roller Bearing (Type NUP)

Spherical Roller Bearing (with Cylindrical Bore)

Figure 2.43. Some bearings usedfor supporting the rotating assembl!j.

The design of main bearings, their sealing, and their lubrication is a complicated task, which requires a high degree of expertise in bearings and decanter design. See also Section 2.2.4.4. 2.3.5 Beach

There are m a n y variants for the beach. A major beach parameter is the beach angle. Typically this is 8 - 1 0 ~ semi-included cone angle. However, an angle as low as 5 ~ is not u n c o m m o n , nor are angles up to 20 ~ A few particular special designs have been built with beach angles up to 45 ~ These special designs are used for thickening applications. Here the bulk of the solids are discharged t h r o u g h special nozzles in the bowl wall at the foot of the beach and only the coarsest solids discharge conventionally. In some designs the pitch of the

60

Variations to Main Components

10 ~

12"

11 ~

6"

I

10"

10 ~

Figure 2.44. Some alternative beach designs.

Decanter Design

61

conveyor on the beach is reversed to convey any solids which pass the nozzles back to them. Another variant of the beach angle is to have a compound angle. In this design, some distance from the foot of the beach the angle is increased or decreased resulting in a concave or convex design. The concave design is an attempt to make a more gradual transition from the horizontal bowl section to the final beach angle. The convex design is used to maximise the dry beach length where this is a limiting factor, and to facilitate conveying. Rarely a completely, or near completely, curved concave contour is used. This is used for very difficult scrolling problems. The large increase in cost of fabrication of this beach and corresponding conveyor section inhibits the wide use of this design. In some old designs a beach is not used at all. A simple flat rear hub is used with cake discharge holes. The conveyor is tapered as if there were a beach, and the cake then forms its own beach. This design can work reasonably well with stiff cohesive cakes, but very high conveyor torques can result. Scrolling improvement and abrasion protection add to the beach variations. These include ribbing, grooving, tiling and liners. The cake discharge is generally a part of the beach fabrication but this will be covered in Section 2.3.5.2. 2.3.5.'I Real hub

The precise design of the rear hub will vary from decanter to decanter and from m a n u f a c t u r e r to m a n u f a c t u r e r . There will be variations according to the method of lubrication of the rear conveyor bearing. The bowl design speed and the type of bearings used will affect the detailed design. These details will not affect process performance. The main parameter, which will give rise to the major hub variations, is the design of the cake discharge. If the cake discharge is wholly in the beach wall then there is a simple a t t a c h m e n t of the hub to the beach. Sometimes, however, the cake discharge is extended into the hub or is completely in the hub. The cake discharge variants are more fully described in the next section.

2.3.5.2 Cake discharge The cake discharge is an i m p o r t a n t part of the decanter process design, especially because the ability to separate solids continuously is the prime reason for the existence of the decanter. An i n a d e q u a t e design will reduce process performance. In the case of very high solids loading, the design t h a t has a row of radial holes towards the end of the beach m a y be improved by the introduction of a

2.3.4.4 M d i r i k a r i i i g

Several differen1 bearing lypes ynrl h w r i n g cornbimabms arc uscd for h a v e ii deep groove ball-bcaring in onc end. usuaily the drive end, which fixes the decanter in thc axial diruction and a ~yliiidrical rollcr bcaring iii thc other cnd. which allows for therrrial expansion of thc bowl. Sornc nianufacturcrs. i n particular on large dccanters. usc dnublc spherical roller benrings. which havc high Inad capacity. If t h e decanter has deep groove ball bearings. or other bearings which do not allow axial displacemeot. i n both ends, the decarikr must be desigrlt:d 1.0 a ~ ~ o ~ ~ i n i othe d a thermal te expansion of the rotor. When selecting milin bearings lor [lec;jnlers several p;ir;jmelers must bP considered. The m;iin p;ir;imeters ;ire I tie rolai.iorlal sped, Ihe radial load, thrr operating temprrotrlre and he luhricatiori riielhod. 'Ihc expected bcarlng life can bc calculatcd bascd on bearing data and thc dynamic arid static loads on thc bearing. For a giver1 decanter size a large convcyor h u b shaft diamctcr is oftcn dcsirablt. both t o increase t h e stiffness or the rotor and to give space for feed pipe and spline shaf1.s. As t.he allowahle speed for a given bearing type decreases with increasing size,lhe hearing selection will he a compromise. A n nil-lubricated bcaring will have a higher iIllowilhle speed a r i d I.herefore oil li~hricatioriis often uwd for larger bearings iind bearings with ;i high speed or

c k r m l e r s . A cornrriori solution i s to

rot ;I I'. l o l l .

r-

L.

q~

....~

L

64

Variations to Main Components

cut-outs m a c h i n e d from a r o u n d the outer end of the beach. An advance on this is to provide matching castellations on the rear hub to improve discharge area. As designs have developed there has been a tendency to enlarge and shape holes and i n t r o d u c e castellations. On very large decanters the thickness of the beach wall in the area of the cake discharge is such as to allow the m a c h i n i n g of exit ports of a helix (snail shell) shape to minimise contact of the cake with the surface of the discharge ports. This reduces wear in the area. Some m o d e r n designs adopt a 360 ~ discharge method. Cake decants over the end of the beach t h r o u g h o u t 3 6 0 ~ passing between support pillars fabricated axially on the outboard edge of the beach. These pillars enable the a t t a c h m e n t of the rear hub. The pillars are restricted in length by mechanical strength considerations, but provide sufficient axial gap for free solids discharge. In some applications it is found necessary to surround the cake discharge area with a stationary collector, fixed to the casing, to prevent cake sticking to the main casing. The function of the collector is assisted by impellers, w h i c h are bolted to the outer surface of the beach and/or the rear hub. The impellers sweep clean the inside of the collector, although they also add to the noise level of the decanter. The type and design of hard surfacing add to the variants of cake discharge. Cast or sintered pieces of various ceramics and tungsten carbides can be fabricated to fit regular shapes such as circular holes and the face of the 360 ~ discharge. For irregular shapes, flame applied hard surfacing is used on the discharge area or on a steel casting made to fit the discharge. Alternatively a plain steel sacrificial c o m p o n e n t is used. On the pillars of the 3 6 0 ~ discharge, half cylinders of sintered tungsten carbide are used. The latest development of this component, by Alfa Laval, is to use a saddle shape, w h i c h improves the wear life of the component. 2.3.5.3 Beach liner

While grooves or ribs are used on most beaches it is unusual to use a liner. However in extreme cases of erosion, a beach liner of say Stellite with its own Stellite ribs is used. Alternatively the beach can be covered with small carbide or alumina tiles like a R o m a n mosaic. Tiles of two different thicknesses allow the formation of in-built ribs or grooves as desired.

2.3.6 Conveyor The conveyor is a c o m p o n e n t that has a major influence on the process performance of the decanter. It thus has a large number of variations in design. In general the shape of the conveyor has to match the inside profile of the bowl and beach assembly, with from 0.5 to 2 mm radial clearance. The

DecanterDesign

i

i

65

I Figzire 2.47. A beach liner.

exception is the beachless bowl where the taper of the conveyor is chosen for the particular application and the process material itself forms its own "beach". The large n u m b e r of conveyor variants arise from the p e r m u t a t i o n of the different hubs, flight design, feed and floc zone types, and the different types and degrees of hard surfacing. Devices added to the conveyor, and the type of flow, two-phase, three-phase (see Section 2.4.1.2), co-current or c o u n t e r c u r r e n t , add to the permutations. The co-current flow conveyor will have its feed zone at the front (large diameter) end and some return channels will be built into the conveyor hub to lead the clarified centrate from the foot of the beach back to the front hub, between the exit ports of the feed zone. The c o u n t e r c u r r e n t conveyor has its feed zone in the conveyor but abreast of the foot of the beach. Clarified liquor is free to flow a r o u n d or t h r o u g h the conveyor flights to the front hub while the cake is scrolled to the rear, and up the beach. The orientation of the bowl (vertical or horizontal) does not usually give rise to a variation in conveyor design. The conveying action can induce a large axial thrust towards the end of the bowl opposite to the beach. On the larger horizontal decanters, Alfa Laval employs a tension bar on the conveyor to c o u n t e r b a l a n c e this thrust and take off the load t h a t could be placed on the front bowl hub. The tension bar consists of a substantial hollow bar bolted to the rear face of the conveyor. It projects t h r o u g h the spindle of the rear hub and is locked in position with a large nut o u t b o a r d of the spindle. The hollow centre affords access for the feed tube.

66

Variations to Main Components

2.3.6.'1

Conveyor hub

The conveyor hub will vary according to the type and position of the feed and floc zones. Also the size and type of c o n v e y o r bearings and seals will make some differences to the e n g i n e e r i n g design of the ends of the hub. However, these last differences will not affect process performance. The diameter of the h u b is usually minimised w i t h i n limits of, for example, mechanical s t r e n g t h and the size of the feed c h a m b e r to maximise pond volume. However, in some applications it is necessary to increase the h u b size to shorten the flight height and so e n h a n c e the scrolling torque capacity of the conveyor. To the hub will be a t t a c h e d a n y baffle discs or similar devices w h i c h the process needs. These are discussed in Section 2.4.6. 2.3.6.2

Flights

An alternative to the solid helical flight is one with " w i n d o w s " to allow axial flow of the clarified liquor from the feed zone area to the centrate discharge. The w a y these w i n d o w s are m a n u f a c t u r e d can vary. The simplest design has holes cut out of the flights before they are welded onto the hub. A more sophisticated design (see also Section 2.4.7.1 ) has a ribbon flight welded to the extremities ofpillars h a n g i n g from the c o n v e y o r hub. The rake or cant of the flight is varied for some applications, as is the c o n v e y o r pitch or pitch angle (see Sections 2.4.8 and 2 . 4 . 1 0 ). The vast majority of d e c a n t e r c o n v e y o r s have a single flight, or lead. There are some, however, w h i c h have two, three or more leads. A multi-lead c o n v e y o r would have the flights equispaced intertwined on the c o n v e y o r hub. W h e n the multi-lead conveyors have solid flights, which is more usual, care has to be taken t h a t the feed is distributed evenly a m o n g all the flights. Multilead conveyors can reduce the out-of-balance problems, which may result from the a s y m m e t r y of the single lead designs ( see also Section 2.4.1 1 ).

h

r I

Figure 2.48. A co-current conveyor.

t,,

2.3.6.3 Fred m r w

Simplc variations in thc dcsign of thc fccd zonc iiicltldc thc number of exit ports. the number arid shape of the accelerator vanes and lie size 0 1 the chamber .

'l'he exit ports of the feed zone can be simple holes cut in the conveyor h u h , possibly hard surfaced. or with specially I'abriuakd cylinders hohed into position. More sophisticated exit ports will be specially shaped t o rriiriiniise contact with the process material. Sometirries, exit ~.ioulesare used 1 00irec.t

68

Variations to Main Components

Second Flight

First Flight

oeconu

First Flight

r~lgnl

Figure 2.52. A double lead conveyor.

feed onto the back of the conveyor flight (directional feed nozzles), to minimise disturbance in the pond. The smaller the feed zone is relative to the feed rate, then the more sophisticated has to be its design, to ensure that the feed is accelerated to speed without flooding and consequent spillage back t h r o u g h the feed e n t r a n c e . To this end, sometimes there will be seen accelerator blades to the rear of the feed zone. Also the entrance to the feed zone will sometimes have a tube fitted to surround the feed tube. This ensures a high velocity for the air sucked in around the feed tube to prevent escape of liquor from the feed c h a m b e r . Directional Feed Nozzle

Directional Feed Nozzle Figure 2.53. A directional feed nozzle.

Decanter Design

69

Round Feed Ports

Oval Feed Ports

Long Elongated Feed Ports

Figure 2.54. Some alternative designs offeed zones.

If large t r a m p material e n t e r s the feed zone, from the feed tube, it m a y lodge in the feed zone if this zone c o n t a i n s s u b s t a n t i a l dead areas. Thus, the walls a n d shape of the feed zone m u s t be designed, not only to accelerate the feed adequately, but also so as to avoid plugging. 2.3.6.4 Floe~rinse z o n e

The variations of floc and rinse zones are m a i n l y associated with size a n d h o w the rinse or flocculant discharges into t h e m a i n process stream. Simple accelerators on the c h a m b e r wall are s o m e t i m e s fitted. The m o r e efficient floc c h a m b e r s will discharge the flocculant evenly a d j a c e n t to the feed d i s c h a r g e into the pond.

70

Variations to Main Components

Flocculant

Zone

Feed

I

Pipe

)

Additive

Figure 2.55. A floc zone.

' " "

.9 .

.

.

.

,

~

:~l'"'

-"

:': ~

,~ ,..~..ee~r .

H " ~ .

~

.

~

'

Feed Tube

l

" " ~

,--~,

"

",."

'

~':i.""

.

-~-," .:-'-:"

"

Rinse

: _~

~

~u,~r.,Ch~~~.~/_/

~'_.~..-t~.. "

.Po.d.,. "

.

~-

.

-::

.

",~ ...... ": ~:.~-"~"'" " " "\ " ""

, -

"

Nozzles

~,.

\'\

~~,~ ~ ~ '.... ": Rinse

~

'..... Rinse

Rinse

"

'

r

,i~"

;

_n

-

-I

I~~

~

~

~.

~

19

~

R in s e

Z o n e

.

~---

9"

Figure 2.56. A rinse zone.

On the slower r o t a t i n g bowls t h e r e is less need to mix the flocculant inside the bowl and flocculant is t h e n a d m i t t e d into the feed zone t h r o u g h a flexible pipe inside the feed tube, or directly into the feed line. The discharge from the floc zone is either simply t h r o u g h holes in the c o n v e y o r h u b or via tubes or c h a n n e l s fabricated in it.

Decanter Design

71

As a rinse zone, discharge is at a different point from flocculant, often at the junction between the wet and the dry beach or spread over a finite length of wet and/or dry beach. Special spray nozzles are often used to spread and direct the spray, to improve rinse efficiency. 2.3.6.5 Wear protection

W e a r protection is required on various parts of the rotating assembly w h e n abrasive sludges are processed. It can also be required in the casing, where the discharged cake impacts. Materials for hard surfacing where required are m a n y and varied, and will depend upon where they are used and the degree of erosion expected. In the bowl, wear protection, w h e n required, will be applied to the conveyor flight tips, the flight conveying surface, in the feed zone, and at the cake outlet. The flight tips can be coated with a flame-applied material, such as a metal alloy (nickel- or cobalt-based) c o n t a i n i n g tungsten carbide particles of varying size and shape, followed by a fusion process. Sometimes liners are fitted, for wear protection, to the inside of the bowl and occasionally to the beach surface. Fitted to the bowl or beach, the liner would double as a scrolling aid. Colmonoy or Stellite, which are cobolt-based alloys, are alternative materials that are applied by flame spraying. While these can have higher

Figure 2.5 7. Some flight tile designs.

72

Variations to Main Components

Hard Surfacing on Flights

Flights Figure 2.58. Flame applied hard surfacing to flight tips.

corrosion resistance they tend to be porous, and have less wear resistance than tungsten carbide. This is even t h o u g h flame applied tungsten carbide is more porous than the cobalt alloys. Alternatively coatings can be applied by arc welding using special welding rods. Other application methods exist, such as plasma spraying and HVOF (high-velocity oxy fuel), but in general they will not be able to produce the same layer thickness as flame spraying. The wear resistance can, however, be higher. Instead of coatings, special tiles can be affixed. These tiles can be specially shaped and profiled stainless steel, or even Hastelloy, backing plates brazed, silver soldered, or epoxy bonded with, say, plates of sintered tungsten carbide, alumina or even more exotic material. The fixing method, ceramic to backing plate, depends upon the e n v i r o n m e n t in which they are to be used. The tile assemblies are carefully welded to the flight tip. Some tiles are even riveted in place. Where welding is involved care must be taken in using the correct welding rods to prevent electrolytic corrosion, or i n t e r g r a n u l a r corrosion, in use. Tungsten carbide is a c o m m o n material used for tiles, but its exact m a n u f a c t u r i n g method can give at least one order of m a g n i t u d e difference in its abrasion resistance. Care therefore has to be taken w h e n trading quality for costs. Tungsten carbide, w h e n used as a tile, is a matrix of tungsten carbide particles, sintered with an alloy binder. Care must be taken with the materials for the matrix in which they are sintered as these m a y corrode, leaving a porous honeycomb of the actual abrasion-resistant material, which readily collapses. The quality of the sinter depends upon the processing conditions, the binding alloy, and the composition and size distribution of the carbide. Many other ceramics are used for hard surfacing, particularly alumina. Silicon carbide is another i m p o r t a n t ceramic used for w e a r protection, which is stronger and more wear resistant t h a n alumina.

Decanter Desiqn

73

Besides ceramics (the most frequently used material), some types of rubber, such as urethane, can be used, mainly in the feed zone. One m a n u f a c t u r e r has even used rubber on the conveyor flights. Thus, the variations in hard surfacing follow from the type of material used, its extent and how it is applied. Separate wear components are to be preferred wherever possible, as repair is more simple and inexpensive when high wear rates, higher t h a n expected, are experienced.

2.3.6.6 Bearings and seals The type of bearings used in the conveyor varies from m a n u f a c t u r e r to manufacturer, and varies with the duty. For example, elevated-temperature operation could require a different bearing clearance rating. Similarly the materials used for the construction of the seals will vary with the application and the environment. Note t h a t the conveyor seals are not only required to contain the lubricating m e d i u m but also to exclude process material. Some bearings used will be grease packed while others require forced oil lubrication. 2.3.7 Gearbox

The epicyclic gearbox is described in Section 2.2.7. The main alternative is the Cyclo type gearbox. The German Cyclo Company was founded in 19 31 w h e n Lorenz Braren invented the Cyclo gearbox. Shortly after this date Sumitomo of Japan licensed the device and eventually absorbed the Cyclo Company into its own group. Another German company, Maun, m a n u f a c t u r e s gearboxes that work on similar principles. These use interlocking toothed gears, whereas the Cyclo gearboxes employ cycloid discs. The outer edges of the cycloid discs engage a series of rollers, situated on the inside of the gearbox casing. Because of the mechanical complexity of cycloid discs, it is easier to describe the Maun gearbox first. Consider a gearwheel with external teeth rotating clockwise inside a slightly larger gearwheel with internal teeth. As the inner wheel processes around the inside of the outer wheel, it revolves anticlockwise on its own axis. An input shaft, on the axis of the outer gearwheel, with an eccentric disc central with the inner gearwheel, is fitted such that rotation of the shaft causes the inner gearwheel to travel around the outer gear. A bearing is fitted on the eccentric disc to carry the inner gearwheel. A series of equispaced holes perforate the inner gearwheel at a fixed radius. An output shaft is fitted on the same axis as the input shaft, with a series of pins which engage in the aforementioned holes of the inner gearwheel. The holes in the inner gearwheel are such a size as to allow this gearwheel to engage the outer gearwheel at any position on the outer gearwheel's teeth

74

Variations to Main Components

Cycloid Disc 2

Centrifuclal Force from

G Torque from Disc 2 Torque from Disc 1

Cycloid Disc Centrifugal F Disc 1 Balance of Internal Forces

Figure 2.5 9. A C!lclo gearbox (By courtesy of C!lclo ).

without distorting the output shaft. Whatever are the relative positions of the two gearwheels, the holes and pins remain in contact. Note that Figure 2.59 depicts a Cyclo gearbox, where the teethed gear wheels of the Maun-type box are replaced by cycloid shaped discs.

Decanter Design

75

Conveyor Drl//- "'-'"'- q [

Lobed Excentnc Wheels

" Tracks

Figure 2.5 9.-(continued).

W h e n used as a decanter gearbox, the input shaft, called the pinion shaft in decanter terminology, is held stationary or is braked to a speed below bowl speed. The body of the gearbox, holding the outer gearwheel, rotates at bowl speed. In consequence, the output shaft, which engages the conveyor, rotates at a slightly faster speed than the bowl. In practice, the device as described so far would cause balancing problems. To overcome this situation, the input shaft has a double eccentric fitted with two inner gearwheels 180 ~ apart. The pins of the output shaft protrude t h r o u g h the holes of both inner gearwheels, so that contact of the hole of one gearwheel with a pin is 180 ~ around that pin from the contact of the hole of the other gearwheel (see Figure 2.59 ). The Maun Company m a n u f a c t u r e s a similar second design of gearbox with pins fixed to an end face of the gearbox casing, and the gearwheel with internal teeth as a part of the output shaft. This second design is such that the conveyor rotates slower t h a n the bowl. The Cyclo gearbox, as stated, uses cycloid discs (see Figure 2.59) in place of the inner gearwheels of the Maun gearbox. The outer gearwheel is replaced by a series of rollers on pins. There is one less roller than the n u m b e r of lobes on the cycloid disc. The drive pins of the output shaft are also fitted with rollers. The ratio of a Cyclo gearbox is from about 6 to a m a x i m u m of about 120. Therefore, for the decanter, m a n y Cyclo gearboxes need only be single stage, which substantially reduces m a n u f a c t u r i n g costs. It remains to be described how the output shaft of the Cyclo gearbox is attached to the two cycloid plates. The output shaft has another ring of pins and rollers on its inner end. These rollers engage in corresponding circular holes in the two cycloid plates. The holes in the cycloid plates are of such a diameter that each hole wall is always in contact with its corresponding roller in spite of the eccentric throw of the cycloid plate. To calculate conveyor differential, the same relationships as described for the epicyclic gearbox are used, except, as already stated, the Cyclo differential is faster than the bowl speed whereas the epicyclic is slower.

76

Variations to Main Components

The Cyclo gearbox torque characteristics are different from those of the epicyclic. Its torque capability varies with differential speed while the maximum torque transmission of the epicyclic remains essentially constant for all differentials used. According to the manufacturer's performance data. the torque capacity of the Cyclo gearbox approximately halves when differential is increased from minimum to maximum. For both designs the conveyor torque equals the pinion torque times the gearbox ratio whatever the differential. For a given torque rating the Cyclo gearbox tends to be slightly larger in diameter but shorter than the epicyclic gearbox. The Rotodiff (see Section 2.4.15) will be less heavy than either, which gives it an advantage when extra high torques are required. This is because rotodynamic and mechanical stress constraints limit the weight that can be cantilevered from the rotating assembly. When choosing a gearbox its weight for the required torque capacity is an important consideration. 2.3.8 Frame

The basic variations in flame design have already been discussed in Section 2.2.8. One further variant is one where the frame and lower half of the casing are fabricated together. The casing then becomes a part of the flame. 2.3.8.7 Bearing supports

The pillow blocks housing the main bearings are generally bolted and dowelled to the flame. Sometimes the blocks form an integral part of the flame. The bearings will have an interference fit in the pillow block. Alternatively the pillow block could be split allowing easy separation of the rotating assembly from the frame. Frame and bearing designs should permit interchangeability between rotating assemblies to permit reduced spare parts cost and lower maintenance time with multiple machine installations. 2.3.8.2 Feed tube

The main variant of the feed tube is one with a floc feature. In this design two tubes are held together concentrically. The inner tube takes the feed while the outer shorter tube is used for the flocculant or rinse. The flocculant or rinse leaves the outer tube via holes drilled radially at the sealed front end. As discussed in Section 2.2.8.2, care has to be taken in avoiding resonance of the feed tube. The concentric floc tube helps with the rigidity. Tapering the feed tube and making it of lighter materials also help. The resonance frequency of the feed tube is reduced when feed is admitted to it. It is not unusual for the feed tube's resonance frequency to be higher than bowl speed when empty, and lower than bowl speed after feed starts to flow.

Decanter Design

Flocculant

77

Flocculent Machined Surface

lj

O |

~

,

/,

~Feed

Locating Shoulder Flocculant Figure 2.60. A feed tube with floc feature.

2.3.8.3 Vibration isolators

There are several types of vibration isolator in use. Some rely on the elasticity of rubber components, while others use coiled springs. It is important that each mount is rated for the share of the load it has to take. The rubber mounts would tend to be used for smaller machines, while the springs would be used for larger models. Damping features of vibration isolators limit the magnitude of vibration during the starting and stopping of the decanter.

2.3.9 Casing As well as the standard casing, split and gasketted along the centre line, there is the single-piece casing that is fitted over a cantilevered rotating assembly (see Section 2.4.1.5). A second variant is where the casing is made of two separate split casings, one for the liquor discharge and the other for the cake discharge. This type of casing has the advantage that cross contamination is eliminated completely. (The variant where the casing is integral with the flame has already been mentioned in Section 2.3.8.) Cylindrical casings, with no gasket required, are also used, but these require axial disassembly from the ends. Special tools and greater floor space are then needed to remove the rotating assembly. 2.3.9.1 Baffles

The number of baffles used in the casing depends upon the manufacturer and the potential for cross-contamination of the two products relative to what is permissible. It would be unusual for there to be more than two baffles each for either product end. The simpler and slower the decanter, the less is the baffling needed. Some baffles are fitted with a large clearance from the bowl while others penetrate grooves in the bowl to form labyrinths.

78

Variations to AIain Components

The method of sealing the two halves of each baffle also varies. In some there is no attempt to seal, leaving a gap the thickness of the main casing gasket. Alternatively the two halves of the baffles are overlapped and slightly sprung. A gasket at each baffle joint is u n u s u a l , but not u n k n o w n . W h e n the centrate discharges from the bowl a lot of turbulence occurs with splashing back onto the bowl. Some of this splashing m a y get into the baffle grooves causing an unacceptable level of cross c o n t a m i n a t i o n . To prevent this, a gutter is sometimes fitted to the baffle adjacent to the centrate discharge.

2.3.9.2 Cake discharge The variations in the casing cake discharge are mainly associated with aids introduced to maintain a smooth cake discharge without sticking. Liners fitted to the inside of the casing are not unusual, either in the form of HDPE, PTFE or urethane rubber to prevent sticking, or u r e t h a n e rubber or, say, Stellite to counteract erosion. Sometimes an inner casing is closely fitted outside the bowl discharge ports to form something akin to a racetrack (or " R e n n b a h n " ) . This can be aided by impellers, bolted on the outside of the bowl adjacent to the bowl discharge ports. The outlet of this inner casing or collector is strategically sized and

,~,~

Baffle Gutter

iI Figure 2.61. A casing baffle with guttering.

Decanter Design

79

positioned to e n s u r e that the discharge is directed at the m a i n c a s i n g outlet. R a c e t r a c k s with impellers add greatly to a m b i e n t noise. On a very old design a similar device w a s used but, instead of impellers on the bowl, an e x t e r n a l screw or conveyor was used. As an a l t e r n a t i v e to the collector, a vibrator or a reciprocating scraper inside the m a i n casing has been used.

2.3.9.3 Centrate discharge The alternative to free discharge of centrate into and from the casing is to use a pump or skimmer inside the bow]. These devices are discussed in Sections 2.4.2 and 2.4.3.

2.3.9.4 Casing seals There are several possible variations in the type of gasketting used, the types of material employed and the shape of the gasketting. From the simple flat gasket, there are then those with a circular section, sitting in a groove, to those of L or U s h a p e located over the casing flange. Care has to be taken with choice of materials, especially with n o n - a q u e o u s liquors, to avoid swelling and deterioration.

Bowl

Cake Discharge Figure 2.62. A cake collector.

80

Variations to Main Components

There are variations as to how the rotating assembly hubs are sealed t h r o u g h the casing ends. A plain bearing seal such as PTFE can be used. In more sophisticated designs, a rotating seal is used particularly w h e r e it is necessary to have a contained process. Mechanical seals are used w h e n the casing needs to be pressurised. See also earlier sections, particularly Section 2.2.4.4. 2.3.9.5 Vents

To control windage associated with the decanter, the centre c o m p a r t m e n t of the casing is sometimes connected to a ventilation system. It is necessary to ensure that air is allowed into the centre c o m p a r t m e n t and allowed free exit at either end. This helps to reduce the likelihood of cross c o n t a m i n a t i o n of the two products. 2.3.10 Sub-frame

The simple function of the sub-frame does not allow m u c h variation apart from w h e t h e r or not to use one at all. The sub-frame on smaller designs provides a platform for m o u n t i n g main flame, main motor, and back-drive, enabling rigid connections before m o u n t i n g onto vibration isolators. Modern engineering designs allow easy m o u n t i n g of both motors, with flange mountings, onto the main frame. 2.3.11 Main drive

The variations in the main drive are in the choice of motor design. With modern electrical developments the n u m b e r of choices has expanded. These include: 9

9 9 9 9 9 9

three-phase motor with fluid coupling and direct on-line start; specially w o u n d three-phase motor with star-delta starter; DC motor; inverter motor: high voltage t h r e e - p h a s e m o t o r ; soft start motor: and hydraulic motor.

In processes using solvents, flameproof drives will be demanded. The fluid coupling, a slipping clutch type of design, is a device sometimes mounted on the motor shaft allowing it to be started at full speed with minimal torque. Frictional drag from the fluid within the coupling slowly brings the rotating assembly up to speed with the release of heat within the fluid. It allows the use of a standard motor in spite of the high inertial load. Small

Decanter Design

81

Main Motor

Fluid Coupling Figure 2.63. A main motor with fluid coupling.

Gearbox

.

.

fT .

.

. .. .. .. . . .. .. .

I"I iI iI li III I l"~-.l.i . . . . . . I~

Guard

TT

. . . .

iI

i

--I--~" --T-r

Gearbox

I _ ,J(~'Im--II I--!1"11 I'11 r I

I

I

Hi.4 i~ki i I ~LJ, P - II I

z

Assembly

Ill III __

...........

iI iiii,.,,, I1, IL~LI

. ,

II I u_ i- ----

Upper iaslng

, i~G~ii

I II L JJ

I1

iP'qt 11"~ I,~1

.....

' , I

I I I I

I I

I

Flanged

Main

tX I1

J rll I II , III

,m. I IIII11 I IP,~ll I

ImlllI lllllllI

ilt1111 IP':P, I I ~ II'k----ImMII I IIill IRitU lilT - "-I~,ttll ! I II I li+~lll I

I III

~ I I I

I I II I

I lip I T PII I

u

ll~ll I I~tll

I~:'PIII

i

n Frame

Vibration Mount

Motor

(Frame mounted)

Figure 2.64. A.flanged main motor mounted on main fl'ame.

decanters often use standard motors, but larger decanters need special motors or starters, since the inertia seen by the motor increases more rapidly than the power needed, m a k i n g starting more difficult. Some of the ways of m o u n t i n g the motor have already been discussed. These include m o u n t i n g oil slide rails, on a sub-flame, m o u n t i n g on the f l a m e

82

Variations to Main Components

"'~""q

L~

///i..--.---,~..A.~~

-~,

/ f ..........

!I

F" ~

~

i J

~".. I

Figure 2.6 5. A floor mounted main motor with belt tensioning device.

direct, and bolting to the ground direct with a special tensioning device to allow for movement of the centrifuge assembly on its vibration isolators. Flange mounting the drive motor onto the main flame is also an option. 2.3.12 Back-drive

There are many methods for controlling the conveyor differential. When variation of differential is not required the system could simply be a balanced arm on the gearbox pinion shaft, held stationary by resting against a sprungloaded stop. With a differential requirement of something less than the maximum then the gearbox pinion shaft has to be rotated at a fixed speed. This can be done using a fixed speed three-phase motor with belt and pulleys. Alternatively a countershaft system is used whereby an idler shaft is rotated by a pulley and belt from the gearbox casing. Different belt and pulleys then drive the gearbox pinion shaft from the idler shaft. Both these last two systems need a rotating clutch somewhere in the system to cater for overload situations. They also benefit from the ability to permit some change in the choice of differential by appropriate change of pulley sizes. An advance on the fixed speed differential is the variable speed back-drive. This design uses variable speed motors. They may be variable mechanically

Decanter Design

83

with, say, split pulleys or special mechanical gearboxes. Otherwise they will use variable speed electrical motors, DC or inverter type or even a hydraulic motor. A further alternative, which has been widely used, is the eddy current brake. This offers simplicity and cost effectiveness, but its disadvantage is that it cannot drive, nor can it regenerate power. The eddy current brake comprises a copper torque tube, rotating in a magnet field induced by an electric current through copper coils. Eddy currents are generated in the torque tube producing a braking torque. The extent of the braking torque is controlled by the amount of current which is allowed to flow through the coils. Hydraulic conveyor drives are inherently variable (see Section 2.2.12). Direction of Rotation e Arm

~ Microswitch

Compression Link

Torque Control Lever

Figure 2.66. A torque overload arm S!lstem.

84

Variations to 2vlain Components

\

\

?L-L;;

. . . . . :;,,, ,,,.,.

9 ,.,..!

J...- ,.

",'.": .z,; >,;' ",..

' " "'"

Figz~re 2 . 6 7 . A coz~ntershaft b a c k - d r i v e .

- - --

....

Decanter Design

Figure 2.68. Aneddy current brake back-drive system.

83

2.4 Special Features It could be argued t h a t m a n y special features have been covered already and indeed at least some have been mentioned. It is considered that the variants already discussed are alternatives to do the same job as the basic component, but in a different way. The special features, now to be described, are designs that enable a unique task, or designs used by only a minority of manufacturers, or t h a t are used for specific applications, or have e n h a n c e d efficiency in some respect. 2.4.1 Basic construction

There are a n u m b e r of special v a r i a n t s of the basic horizontal or vertical decanter for simple clarification and dewatering. Special designs are available for three-phase separation, for thickening applications, with a filtration section, for leaching, for washing, and those for operating at extra high temperature and pressure. 2.4.1.1 Screen-bowl decanter

The screen-bowl decanter is a basic decanter w i t h an extra cylindrical section attached to the end of the beach. This extra cylindrical section is perforated with a bar screen fitted on the inside wall. The cake is scrolled from the beach and over the screen to undergo further dewatering by filtration. Some rinsing may be added to the screen. The conveyor itself will be extended with a cylindrical section to perform the scrolling over the screen. This decanter can be built in co-current and countercurrent format, but has a narrow range of applications. It has been used in the low-temperature separation of paraxy]ene, and in coal washeries. On abrasive applications the bar screen must be resistant to erosion. Hard surfaced and carbide variants of bar screen have been used for such applications. 2.4.1.2 Three-phase decanter

The three-phase decanter is a common variant for the separation of oil, water and solids. In these designs the cake is separated and removed as in the

Decanter Design

87

FRONT (Centrate Discharge) Screen Section

I

Decanting Section

I

Beach Section

I

REAR (Cake Discharge)

Figure 2.69. A screen-bowl decanter.

two-phase design. Extra devices or baffles are required inside the bowl to separate and discharge separately the two liquids. The lighter liquid will discharge over the weirs already described in the normal way. The heavier liquid needs to decant under a reversed weir into a chamber in the hub and then can flow over and through nozzles of adjustable height. Depending upon the relative quantities of the two liquids, it may be desired to decant the light phase from the nozzles and the heavy over the weir plates. This can be done by adjusting the relative heights of the weirs and nozzles, and altering the liquid/liquid baffle. Sometimes the weir ports in the front hub are alternately light and heavy, making a compact design. Half the weir ports only will be as for the two-phase design. The other half will have an inverted weir on the inside, to preferentially receive the heavy phase, with a dam plate on the outside to control the hydraulic head. The two separated liquids, with this latter design, have thus to be kept separate by appropriate baffling in the casing, allowing for the ever-present problem of windage. To ensure separate collection of the two liquid phases, the discharge positions of the two phases need to be separated by a finite axial distance. To do this one of the discharges can be taken radially through the hub via drilled channels. If necessary a shroud is fitted to the bowl end to take the discharge further from the front. Naturally, in three-phase designs there are many possibilities for variations with the fitting of special baffles, chambers and channels into the front end of the bowl but the principle is the same. However the use of a skimmer (see Section 2.4.3) in three-phase designs is very useful in controlling the equilibrium line between the two liquid phases while the bowl is at speed. The casing design is, of course, different.

88

Special Features

..

.

.

.

r..

,'--

,

Water

Figure 2.70. A three-phase decanter.

Oil

Decanter Design

89

2.4.1.3 The c o u n t e r c u r r e n t extractor d e c a n t e r

This type of decanter is patented [3] and built by Westfalia, and is used for countercurrent extraction. It is similar to a three-phase decanter, except that the second liquid phase is purposely added separately to pass, in the bowl, through the first liquid phase. The two liquid phases are caused to run countercurrently in the bowl and the wanted product is leached from one phase into the other. Separate designs are available each for heavy and light raffinate. This type of decanter is often used in the pharmaceutical industry.

Counter-currentSeparation Zone extraction Separation Zone Main Bea,ring

\

Separating Disc ~

P

u

m

Centrtpetal

p

Feed Tube

Cyclo Gear

I Distributor Holes Discharge

Main Bearing

/

/

'

\

Discharge(Extract)'

Regulating Ring Distributor Conveyor

Main Bearing

Counter-currentSeparat=on Zone extraction Separation Zone - " l ' - - - -Separatmg - ~ _ _ Dtsc /CentripetalPurnp F~dTu~

\ Cycio Gear

E-L Discharge

Bowl

l

Conveyor

\ Discharge (Extract Regulating Ring Main Bearing Distrtbutor Distnbutor Holes

Figure 2.71. Two t~lpes of a countercurrent extractor decanter (By cour.tesy of Westfalia ).

90 SpecialFeatures 2.4.1.4 Decanters for temperature and pressure extremes

Many processes, particularly in the petrochemical industry, require the decanter to operate at elevated t e m p e r a t u r e s and pressures, say up to 400~ and 10 atm. In these cases special designs and materials are used for bearing seals and lubricants. For the higher pressures, mechanical seals will be used. These decanters are made in both horizontal and vertical construction, although those that supply the vertical design claim that this design is the most reliable for the application, while others would hotly dispute it. 2.4.7.5 The cantilevered bowl

In the early 1960s a r a n g e of horizontal bowl decanters was produced with just one bearing cartridge with two m a i n bearings such that the bowl was cantilevered from these bearings. At least one m a n u f a c t u r e r offers such a machine as interchangeable with a horizontal basket centrifuge, using the same drive assembly. The bowls of these decanters generally have small length-to-diameter ratios, nearer 2 than 3. The design allows the free end to be open or at least very accessible. Maintenance, relatively, is much easier. With large diameters, large conveyor hubs are possible allowing high conveyor torques. Generally these decanters have been used where clarification is easy and where high solids volume or torques are required. 2.4.1.6 The "'hubless " conveyor

A design has been patented (1992) by Alfa I,aval, which involves a decanter with a conveyor with a much reduced hub diameter [1]. The conveyor consists of a ribbon flight m o u n t e d onto a set of longitudinal radial vanes welded to the hub. The feed zone area is completely open. Thus, the vanes spanning the feed zone hold the front and rear sections of the conveyor together. The design allows the pond surface to be brought very close to the centre line. This allows much easier admission of the feed and the power consumption for accelerating the feed to bowl speed is m u c h less. The main disadvantage is that the g-level at the pond surface is appreciably reduced. 2.4.'1.7 Thickening decanter

In a thickening operation, where all t h a t is wanted is partial removal of some liquid, both of the two m a i n flow options are possible, but the type of control with each could be different. One control option is to run the bowl with a pond level close to the level of solids discharge (conventionally called " n e u t r a l " ) and to control the differential at a precise low level. A second option is to run the pond m u c h shallower and operate the differential m u c h higher and less

Decanter Design

Radial Vane Supports for Ribbon Conveyor

91

Ribbon Conveyor

Ribbon Flight Section 'A' - 'A' P=.--Radial Vanes

Figzlre 2.72. The "hubless" conve!lor.

precisely. The high differential prevents too m u c h drying and scrolls liquor out with the solids. In the first option some baffling on the conveyor (see Section 2.4.6.1) will probably be necessary. However, if necessary, the cake could be allowed to over thicken, by setting the decanter to dewater, without any need for fine control, with some feed bypass mixed in with the cake at discharge, to reduce dryness to the desired level. This enhances overall capacity and can reduce overall flocculant consumption w h e n used, as the by-pass requires no flocculant. This option is best used with those slurries that are more readily dewatered. The thickening decanter will differ slightly from the dewatering decanter in the casing. In the thickening decanter, any c o n t a m i n a t i o n into the centre c o m p a r t m e n t needs to be drained to the cake end, rather t h a n the centrate end as in the dewatering design. In dewatering the cake remains solid and will not flow and c o n t a m i n a t e the centrate, whereas in the thickener both process streams are fluid, and any losses are better directed to the thickened cake. The "B" range of decanters m a n u f a c t u r e d by KHD (now Bird Humboldt) are unique in having one, or possibly two, nozzles in the bowl wall, allowing the discharge of the bulk of the thickened product. The remainder of the product is scrolled up the beach, which is m u c h steeper than normal (as m u c h as 45~ Discharge from the nozzle can be m a d e intermittent by providing a zero pitch to the conveyor for less t h a n one t u r n at the nozzle location, thus covering the nozzle for a fraction of each conveyor turn.

92

Special Features

Figure 2.7 3. Nozzle discharge thickening decanter.

An interesting development [4] is offered by Bird, in which the conveyor in the clarifying zone, normally the cylindrical section, is replaced by rakes. Moreover, the clarifying section is made slightly conical, up to about 5 ~ semiincluded angle, narrowing towards the front hub. The principle of this design is that the clarifying zone is free from turbulence created by a conventional conveyor, enabling the conveyor to be rotated faster to provide a higher than normal capacity. The sediment in a thickening application tends to behave similar to a liquid and thus in this decanter will fall down the 5 ~ slope into the path of the conveyor at the start of the beach (Figure 2.74). 2.4.1.8 The dual beach decanter

KHD (now Bird Humboldt) manufactures a decanter with a beach at each end of a short cylindrical section. This decanter is designed to separate light and heavy solids, particularly plastics, from a liquid. A baffle disc is m o u n t e d on the conveyor at the centre to prevent floating solids passing from the feed entry side to the heavy solids discharge end. On the feed side of the baffle, an extra small diameter flight section is added, with reversed pitch, between the main flight and conveyor hub (see Section 2.4.10.2). This extra flight is for scrolling light solids to the light solids beach. The clarified liquid decants into tubes, or nozzles, inserted into the bowl wall adjacent to the central baffle disc on the heavy solids side (Figure 2.76).

Decanter Design

Bowl

FEED 1tJ

I

93

~ Conveyor

Conveyor

Rake Assembly -~ 9

i

9

h r - T I - 71- -~o~ol-~JL

,!

9

l,

II Beach

Clarifier

Heavier Phase

Bowl CONVEYOR

NO CONVEYOR

BEACH

CLARIFIER -

Figure 2.74. Tapered bowl thickening decanter ( Bzl courtesy of Bird). Gearbox

Adjustable Impeller

!

9

,

Centrate Discharge Solids

Figure 2.75. The Flottweg Sedicanter ( B!! r 2.4.2 C e n t r i p e t a l

of Flottweg).

pump

Discharge of clarified liquid from a decanter can be achieved in a completely different way using a centripetal pump. Fitted at the front hub, a centripetal pump converts the rotational velocity of the centrate to pressure on discharge from the bowl. Some control of pond height m a y be effected with the pump, while the bowl is at speed, using back pressure. Such a pump requires extra power. The centrifugal energy of the liquid is converted to kinetic energy for

94 Special Featz~res

i r~lF .

.

.

.

.

.

~m ~,_..

,,..:-:_

~.

Figz~re 2.76. A dztal beach decanter (By courtesz3 of Bird Hzmzboldt ).

\ \ \ \ \

\ \ \

Liquid Centripetal Pump

Figz~re 2.77. A centripetal pzmzp.

flow in the external pipe against a pressure head. It is capable of providing a pressurised flow to the following process operations with reduced air entrainment.

Decanter Design

95

2.4.3 Skimmer pipe An alternative to the centripetal p u m p is the skimmer pipe, by m e a n s of w h i c h the centrate is skimmed or scooped from the pond surface in the bowl. The position of the skimmer in the bowl can be altered from the outside while the bowl is rotating, and so afford a m e a n s of pond depth a d j u s t m e n t during operation. This is done using a lever on the outside of the casing attached to a pipe positioned over and t h r o u g h an a n n u l a r plate mounted on the front face of the front hub of the bowl. The skimmer pipe thus dips into the bowl pond

[ Regulating D~sc ~

-

~

Skimmer Pipe---.__..._

I i ! I

! I

9 -J 9

__J

!

i

I L~

Sktmmer Pipe Radtal Adjustment Position

Separating Plate

Collecting Chamber

Regulating Tube Figure 2.78. A skimmer pipe.

96 SpecialFeatures inside the a n n u l a r plate. The skimmer also has the penalty of requiring extra power for operation. 2.4.4 Centrate weir design

Over the years there have been m a n y designs to facilitate o p t i m u m c e n t r a t e discharge rate and clarity. Some of these are described in Section 2.3.4.2. Other design e n d e a v o u r s have been made to e n h a n c e the area and length of the weir for discharge, to make height c h a n g e easier or even while the bowl is at speed, and to overcome washout problems with very deep ponds during start-up. 2.4.4.1 Cup dam

An advance on the m o o n dam design (see Section 2.3.4.2) is the cup design. Here the whole dam bulges out from the hub in the shape of a part of a sphere. This maximises weir length. 2.4.4.2 Notched dam

The notched dam [5] is used to overcome washout from the cake discharge port during the start up of deep pond bowls. The inner edge of each dam plate is notched to bring the pond level below the cake discharge ]eve] at low flow rates. Thus, during start-up, the bow] is fed at low feed rate. When the cake has built up sumciently to reach the baffle and form a sea], the feed rate is increased to bring the pond level to its full height. As an alternative to notching the dam, a small hole can be drilled in each dam to produce the same effect.

Centrate 9Discharge ,_~_/X//,///;'//~

....

,,_~~L._Z

]-11

DaplaCover ~ ~ I

Cup Dam

Figure 2.79. A cup dan1.

~. y/~]

Decanter Design

A

B

C

D

E

F

97

Figure 2.80. Notched dams and equivalents.

2.4.4.3 Inflatable d a m

Recently an inflatable dam has been patented [6]. Here a rubber toroid is inflated by the hydraulic pressure of a stream of control water fed under its surface. The degree of inflation is managed by controlling the flow or head of control water. This inflatable dam sits just inside the hub to part cover the discharge ports. The control of the inflatable dam is useful in combating the w a s h o u t at start up, previously described. Alternatively the dam can be used solely as a seal to be made after start up, with reliance on mechanical weir plates thereafter. The inflatable darn is also useful in three-phase separation control, and in the control of thickening applications.

2.4.5 Noise suppression Any protuberance on the bowl will induce noise, and the faster the bowl then the louder will be the noise. Impellers at the cake discharge are major producers of noise. Even bolt heads in the bowl can emit noise. Recessing the bolts helps to abate noise, but to improve the abatement one technique is to cover all major bolt heads with metal rings secured by m u c h smaller flush bolts. These can be smaller as they do not have such a great load to bear. This action is more frequently taken on the larger high-speed decanters.

98

Special Features

m

Figure 2.81. Inflatable dam.

Decanters open to the atmosphere produce more noise than closed ones, and thus closing or sealing the casing outlets reduces noise levels. Resonant vibration of various static components, particularly in the casing, can produce irritating noises and thus care has to be taken to avoid flimsy and unsupported structures. The ultimate noise suppression is the use of a purposely constructed noise suppression hood. These hoods can be very effective, but they do have

Decanter Desiqn

99

disadvantages apart from the cost. It can be a major exercise to initiate maintenance. There are advantages in being able to see and hear the m a c h i n e to diagnose, or be alerted to, any problem. 2.4.6 Bowl baffles

The use of appropriately designed and positioned baffles, in a decanter bowl, can direct process streams in the directions required. Processes otherwise impossible are made possible, such as three-phase separation, and the use of pond levels above the solids discharge level. Baffles are used for both liquid and cake as well as floating solids. 2.4.6.1 Cake baffle disc

The cake baffle disc was patented more than 15 years ago by the then Pennwalt/Sharples company [ 7]. It has been variously k n o w n as the Lee disc (after the inventor), a BD disc, biological disc or a baffle disc. Together with a negative ring dam (pond level higher t h a n the cake discharge) it was k n o w n at one time as Centriseal. The baffle disc comprises a plain disc welded to the conveyor hub somewhere between the feed zone and the cake discharge. On c o u n t e r c u r r e n t decanters the usual location for the disc is at the foot of the beach. However, more recently, the disc has been seen situated in several positions on the beach from bottom to top. Ideally there should be a m i n i m u m distance, say one pitch, between the disc and the feed zone. This is to avoid turbulence from incoming feed at the periphery of the disc. The clearance between the tip of the disc and the conveyor outside diameter will be anything from a few millimetres up to about half of the pond

Cake Baffle D~sc

Figure 2.82. A cake baffle disc.

100

SpeciaI Features

depth. The clearance quoted is often disc tip to bowl wall. However, w h e n the smaller clearances are used, the former has the most practical significance. This is because the difference between the two, which is the heel, is relatively static. The function of the disc is to form a seal with the scrolling cake to prevent the escape of feed or clarified liquor w h e n the liquid level is higher t h a n the cake discharge level. This technique is often used in the dewatering of effluent sludges. The baffle disc can experience considerable axial forces, and thus a substantial thickness needs to be used with good welding where it crosses the flight. In some instances a dog-leg is used where the disc is attached to the flight. This can produce u n w a n t e d paddling of cake, by virtue of the axial c o m p o n e n t of the dog-leg. 2.4.6.2

Baffle

cone

The baffle cone has the same function as the cake baffle disc. In place of the disc at the foot of the beach, a cone the full length of the beach or a shortened one at the foot is substituted. Feed is admitted to the bowl, inside the cone. This has the a d v a n t a g e of preventing disturbance to the cake scrolling past the feed zone. An extra

i.,i- t

,..-

-.,,

WTI m

--

-

Figure 2.83. A baffle cone.

r |

-

.

I 81

w

leiitlire of'tht. cone i s that it allows tlocculant to bc addcd inside thc apex o f t h e cone. which ensiires ;in intimatc and uniform mix with the f e d as il lli)ws iiridtB r the feed zone. When using tlocculani thc mile h a s hern cititrwn t o produce Inrgc increases in cffificicncy over the siinplt. disc. The gaps under cones arc usually around half t.he poiid tli?plti. Howcvcr. much smaller gaps havc bccn uscd in vegetahlr oil refining (in a thrcc-phasc dccaiitcr). Uniquely in v e g e ~ a b l eoil refining a doi~blccoiiccnrric conr has hccn uscd where the caustic s o d a wash is fed hctwccn thc cones. 'Ihc wiisli exits a t thc pcriphcry or the cones under t h e surface ofthc oil.

'Ihis disc is the same as thc cake baffle disc hut titted closc 10 t h u co1itriil.e discharge. 11 is only siiited to thc ~ ~ ~ l ~ i t e r c u rdecanter. reiit As its [ i a [ ~ w implies, the tloatcr disc traps floating solids and prevents thcm dischiirging with the clarified centrate. 'I'hc cenlrate dips undcr thc disc before disr:h;irging, thus allowing solids with air attat:hetl lo h ; ~ v emarc time to deaerate. Floating solids build up on the surface ol' I h e pond bctwccn the floater disc: and the beach. Eventually thc floating solids build up sufficicnrly to be carried o n t h e back ofthc hcavy solids scrolling u p l.hz he;ic,h.

The iloaterdisc can cause problems with venting of gas frnni the howl. gases ciitraincd or drawn in by thc fccd zunc. 'lo o ~ ~ r tthis ~ ~sorr~e t ~ snlall ~ t vent holcs can bc drilled at the root of thc disc above the pond surface. 2.4.6.4 C'onvPying baffle

The rwnveyiog hat'flc IH1 is :I p;iteril by Alf;q h V i i I j 11 substitutes for t.he cake bafflc disc, I t consists of a similar disc. split along a radius and stretched to a largcr pitch than thc standat-d conveyor pitch. This largcr pitch can bc as much as doublc thc standard pitch. One end of thc conveying baffle is wcldcd 1 0the coiivtfyiiip Pdce of the coiivevor while the other end is welded to the back face of' the conveyor, t.0 the rear of the first wrld. Thus the coiiveyirig baffle winds itself between [.he main coriveyor at a larger pit.ch eriabliiip the faster scrolling of the cakc nearcr the axis until it cnrnes up against. the rear o f the main flight. If thc conveying flight is positioned nn t.he he;ir:h t.hen Ihe outside edge needs lo be (apered to suit the profile of thc beach. Sometimes it will be round that this t;iperirig is such that the clearancc bctwccn the baMe and the bowl or beach is made t.o reduct. lowards tiit: rear.

Thc longitudinal baffle 191 perfortiis tlre S ~ I WI'iinction as the rakc baffle disc h u t in Corm is a rcctangular- plate fitted. h y wt!ldirig- between two adiaccni flights just bchind the feed znnc. Wliilc his d t v i c r i s simplr a n d eilsy to fit, it can be difficult to iisif. Rping longitudirially fitted. i t can C . ~ I I S Punwantcd paddling of t h e howl rontgnts, which intrrfm-rs w i t h thr hydraulic equilibrium i n the II(-IwI. Fur easy applications rnt!c:h:inlcnl simplicity Outweighs practical diRiculties. A hrrn this hijfflr has becn iiscd which is hiiiged. The idea behind this is lo h;ivr :iiilonialic adjustment of gap height to cope with varying solids hiids. This Iornliit may h a w problcms v v i t l i sealing of the sides, and very ntigittitrc porids. IS it could a l l o w ~ a~hout.

104 Specinl Fentrrrts

2.4.7 Clarification enhancement

Tn all but t h e last 10 or 1 5 years the majority of dccantcr applications have been limited hy [.heir ability to clarify the liquor. Scale up of thcsc centrifuges would bc bawd on thcir claritication ability. 'lhus, a lot o f I.he earlier dcvclopnieilt has bccn concentrated 011 enhancing clarificai.ion capacii y. Bowl speeds havc bccn incrcascd, bowls lcngthened and solids disr:hiirge areas increased to ensure solids chlokitkg does : i d inhibit clxificntinn rnlnme. Apart from thcsc obvious mcthods there are some 111eoh;tnic:;~l mm1ific:;jtions which can hclp, Because o f the very high capdc:ii.ies possible through n rclatlvely smoll volrime. velocities in ii decanler can be very high. producing vcry high Reynolds numbers. Ti,rrbulent ilow i s not the bcst rcgimc to scttlc solids. $u incans h a w bccn dcvclopcd lo incrwsc thc clarification Icngth. reduce the cffcctive settling height and dccrcasc thc mean vclocity of the liquor to be clarificd, 'lhis fcaturc is :\Is0 known as Sigma cnhanccmcnt, 2.4,7:1

C!ud3;-axi;rl i l 0 l Y

With a riorrnal hrlical cwrlvrynr the clarified liqur:ir m u s t wind its way a r o u n d thc helix of the flight before discharge. The velocity ot't.he centra1.e i s thus much larger then i t would be if it could flow axially. Moreover tht! velocily must hc that much extra to overcome the screwing ellect ol'the ccinwyor. To allow axial flow on iiiaiiy modern conveyors. holcs or "wrindows" are (:UI in ihr I1ight.s.'1'0 rnaintain a rcasonablc strcngth to the flight the open area [or flow is about 5 0 % of the cross-sectinnal area availahlc to t h e depth of t h e wiiidnws. Nntur;illy t h e windows canriol he cul to the periphery of thc flight.

as a

reasonable depth of flight must bc maintaincd to scroll thc cab. The

nmoiinl ofribbon flight lek will d e p ~ r i du p o n the scrnlllrig capacity required. This qu;isi-;jxi;il Ilow Ceat.ure car1 he addetl afi.er t.he coriveyor is made alheii not without cosi. A hrt.ter desigri is possible wheri the Seeat.ure is iricluderl in !.he initial manufacture. A low-cost dcsign is possible by simply cutting out thc

windows in thc flights before welding on, Wowcver a n alternative dcsign i s to

wcld pillws to thc conveyor h u b , onto which is welded a ribbon flight. Becausc the pillars can bc iiiadc with greater strength a much greater open area for axial flow is possible.

?..1.7.2 Fully d a l lluw This k a t iirc has thrt saint: f'uricl.icm ijs [.lie rl~~itsi-iixiiiltlow. In this design. longitudiiial radial vanes a r c wclded t o thc conscyor hub. say 6 to 12 i n number. and a ribbon tlight is wclded on the outer edge of the vanes. 7'he

viines extend Irom the leed zone to t'hc:front hub. In this design axial tlow of liquor is assured wit,liniit a n y cirrurrifrrcritiaI ~ r t ' i l r slip, e The liquor cannot rotate around h e helix due t o the build-up of cake in lront ol'the flight scaling u n d e r thc vanes. One problem with this dcvice is t.he Iwssiblc imc'vcn distribution nf flow b c t ~ c c uthc vanes. 'I'o ~ v t r ~ ( ' m!his, i s ii small nuinbcr of hulcs in 1 1 i r viines will allow some flow bclween the rhanncls. It i s iiiipnrtarit to rncike thc numbcrofchanncls rnatrh the number of'f'ccd ports in the crrnvcyor. 2.4.7..3 Vancs

In desigri this is similar to thc fully axial conveyor. exccpt that the vanes arc angled Lo the r a d i i nnd thcrc are many riinrP of thcm. I n present designs thc iiuniber o f vanes are 48 to 9 6 or i m t e depending iipoii t h e size uf decanter. They extend from just forward oCi.he feed zonc to t h e front huh. 'l'lie principle of thcsc vanes is (hiit they act like discs in ;i disc centrifuge separator. Ry virtue of the 17arie ariy,le the cffcctiye set.~liiigrlistancc fnr t.hy solids i s rnlirli rcduced.

106

Special Features

U "+U "L1 nU "El "El

nU "U

l:i~lure 2 . 9 0 .

Beach Conveyor Assembly Removed

,tl~(lm lh'd vt~ne conve!lor.

Theoretically the clarification capacity of a vaned conveyor can be doubled or increased even more. In practice the increase is s o m e w h a t less due to channelling of the centrate, and its not passing t h r o u g h the vanes with a uniform velocity distribution. The vaned conveyor is not suitable for high solids loads or for those solids that tend to be at all sticky. The precise level of solids that is too high depends upon the settings of the decanter required. "Too h i g h " is when the level of solids in the bowl start to reach into the vanes and impair their functioning. It is for those slurries with a small a m o u n t of very fine free-flowing solids, particularly inorganic solids, although natural and artificial proteins have been processed very satisfactorily. This design is competitive with the more expensive high g, high-speed disc stack centrifuge, giving higher solids handling, drier cakes and acceptable clarities. 2.4.7.4 Discs

A decanter conveyor with a disc stack [10] has the same function as a conveyor with angled vanes. The disc stack comprises a stack of say 5 0 - 1 0 0 conical discs very much the same as a stack of discs in a disc centrifuge. The

Decanter Design

10 7

Disk Stack

| t

t

i /

t'

' :

/ |:

!

|i

,

| -

I,-"-I

I: ',1 I"--"1

tDIscStack Assembly I on Conveyor Hub ](less R,bbon Flights)

Figure 2.91. A decanter conveyor with disc stack.

stack is fitted around the conveyor hub at the front end, surrounded by a ribbon flight. The liquor is forced to flow between the discs from the periphery to the centre and then out over the front hub dams. The disc stack e n h a n c e s the centrate clarity for a given capacity, or alternatively e n h a n c e s the decanter's capacity for a given centrate clarity. 2.4.8 Conveyor rake A typical positive rake angle of a conveyor flight would be 1 ~ Sometimes this is automatically provided by the design of the tile wear protection. The reasoning behind such a raked flight is t h a t it will tend to lift the cake from the bowl and in so doing reduce conveyor torque. Thus, this would not be used on soft cake/low torque applications but more on torque-producing, stiff, cohesive cakes.

108

SpeciaI Features

Variable Negative Rake Conveyor

Figure 2.92. A conveyor with variable negative rake (By courtesy oJNoxon ).

A negative rake would be used on soft sludges, those that tend not to produce sufficient friction for efficient scrolling. It is sometimes used to provide shearing of the cake. With moisture physically bound in some cakes, it is considered that some shearing of the cake will release some of the bound moisture. Notwithstanding that, shearing of clay-like cakes such as effluent sludges is necessary to provide escape paths for released moisture. One manufacturer, Noxon. has patented a progressive ( n e g a t i v e ) r a k e d conveyor to enhance cake dryness [1 1 ]. The rake of this conveyor increases from the front end of the conveyor for the full length to the cake discharge.

2.4.9 Conveyor tiles There are m a n y innovations with regard to conveyor tiles fitted to the flight tips. These are not only associated with the type of materials used, but also in the method by which they are secured. Many of these innovations are subject to patents [12]. Ceramic or tungsten carbide plates are riveted, brazed, or epoxy bonded to backing plates, which are then welded to the conveyor flight. The ceramics or tungsten carbide, while being highly abrasion resistant, can be themselves highly abrasive. They are the materials used for cutting tools. Thus it is important that they are very carefully secured lest they break loose and cause dangerous wear to the bowl itself. As an added safety measure some ceramics are uniquely locked by dovetails onto their backing plates before brazing or bonding or whatever [13]. The tips of the ceramics or tungsten carbide are generally chamfered to provided m i n i m u m radial contact. The m o u n t i n g of the ceramic often, purposely, provides a rake to the tile. A different form is the Alfa Laval plough tile [14], which, with its ploughing action, provides an efficient turning over of the cake. Very high scrolling efficiencies are claimed for this tile with exceptional benefits for cake dryness on some applications. See Figure 2.5 7 for other tile designs.

Decanter Design

109

. ": . :

Figure 2.93. A plough tile.

2.4.10 Conveyor pitch Generally manufacturers will have a standard pitch for each bowl diameter (about 5 ~ pitch angle) plus at least one other pitch generally smaller, a little more than half the standard. Very approximately these would be 30% and 20% of the inside bowl diameter, respectively. Some manufacturers have experimented with wider pitches. 2.4.10.1 Variable pitch

Many manufacturers have experimented with variable conveyor pitches. These have been used to enhance dryness in effluent dewatering. The pitch starts wide at the centrate discharge end, narrowing progressively along the

I

~l~-/ll(((t~Zi~b/~,i(/~lil_~ o~ -

Figure 2.94. A variable pitch conveyor.

110

SpecialFeatnres

Figure 2.9 5. The KHD dual pitch conveyor (By courtesy of Bird Hmnbohtt ).

bowl and, sometimes, up the beach. This theoretically provides an increasing pressure on the cake as it travels along the bowl. A patent by KHD (now Bird Humboldt) specifies a standard pitch in the cylindrical section of the bowl with a n a r r o w e r pitch on the beach [15]. This obviates the need for a baffle at the foot of the beach as the n a r r o w pitch produces a deeper cake forming the seal. The n a r r o w pitch provides a back pressure on w h i c h the standard pitch can work. An alternative use for the variable pitch is to improve centrate quality. For this the pitch of the flight adjacent to the front hub is increased, as much as double that for the rest of the conveyor. This design is particularly useful for applications in which the decanter can become heavily loaded with solids. The increased pitch tends to keep the centrate discharge region of the bowl relatively free of solids, thus reducing r e - e n t r a i n m e n t of solids as the centrate streams towards the weir.

2.4.10.2 Reverse pitch A most novel design comes from Japan [16], and has been in existence for some time, but as far as is known has never been exploited to any large extent. This consists of a countercurrent decanter with an inner flight with a negative pitch. The driest cake will be found nearest the bowl and so the inner flight conveys the wetter cake in the opposite direction, to prolong its residence time. There will be some reducing of the outer diameter of the inner flight to allow the forward cake flow to reduce to nothing towards the front hub.

2.4.11 Counterbalance and scraper flights At the rear h u b there is a finite clearance between the hub and the end of the conveyor. Depending upon the precise design, this clearance can be quite large. The reaction of the conveyor can shift the conveyor forward a further

Decanter Design

111

Reverse Flights Figure 2.96. A reverse pitch conveyor.

Front Scraper Blade

Extra Counter Balance

Rear Scraper Blade

Figure 2.97. Scraper blade and counter balance flights.

small distance. With the designs that have radial holes in the beach section for cake discharge, the dead zone beyond the cake discharge can be even larger. In this dead area, between the end of the conveyor and the inside face of the rear hub, cake can build up and become very compacted and eventually cause considerable wear. To combat this problem a small flight with negative pitch is often welded 180 ~ from the main flight end. This not only scrolls cake back into the path of the main flight, but also serves to c o u n t e r b a l a n c e the nonsymmetry of a single lead helical flight. At the larger front end, some c o u n t e r - b a l a n c i n g to the end of the flight is required, due to the n o n - s y m m e t r y of a single lead conveyor, w h i c h is often done by welding on a small flight section with positive pitch. This will be a 60 ~ segment with centre line positioned 90 ~ from the end of the main flight.

112

SpeciaI Features

However, particularly on the larger bowls, this needs some special a t t e n t i o n w h e n balancing. While perfectly balanced on the balancing m a c h i n e , considerable out of balance can occur w h e n the bowl is filled with process material. This is caused by n o n - s y m m e t r i c a l b u o y a n c y effects of single-lead conveyors. This has to be taken into a c c o u n t w h e n dry balancing. Final balancing should be checked with the bowl filled with liquid. 2.4.12 Feed z o n e

There can almost be an infinite n u m b e r of feed zone designs with different shaped inlets and outlets, different n u m b e r of outlets, erosion protection, accelerators and linings. One special feed zone design has come about from advanced development and a patent (Alfa Laval) for a decanter centrifuge [17] has been issued. This is the soft inlet feed zone. This is used on sludges with fine, discrete particles. It comprises an open volume around the feed tube. Feed builds up in the feed zone and is accelerated by vanes at a small radius. The combined action from the friction on the inside of the feed zone and the accelerating vanes will bring the feed up to bowl speed, before discharging into the pond. The design is such that the feed has to get to bowl speed before it can reach the feed zone discharge ports. This minimises shock, and thus minimises reduction of particle or agglomerate size. Many feed zones have their discharge ports following the helix of the flight. This can allow build up of solids in dead areas. This is overcome with the "inline" feed zone that is possible when using a baffle cone (see Section 2.4.6.2).

) F igtire 2.9 8. A soft il~let feed zone.

Decanter Design

113

The feed ports can be symmetrically placed around the conveyor hub under the cone with floc ports equi-spaced between the feed ports (see Figure 2.80). 2.4.13 The reslurry collector

Sometimes it is necessary to reslurry the cake discharged from the decanter. This is generally for washing out contaminants from the cake, as in lactose production. It is sometimes done in effluent thickening, and when a greater dryness than is required is produced by the decanter for easy control. Then the cake is back mixed with a feed by-pass which reduces overall flocculant consumption, because the by-pass does not need flocculant. The by-passing material is, of course, 100% recovery. The reslurrying can be done in a separate mixer after the decanter, but more often the slurrying liquor is added into the decanter cake discharge and, because of the turbulence therein, it reduces the mixing required thereafter. A more efficient system has been devised where the reslurrying liquor is added to an inner collector surrounding the cake discharge fitted with outer impellers. This provides very efficient mixing, obviating any need for further mixing outside the decanter.

Spray Nozzle Bars

Bowl

Reslurry Collector Cake Discharge Figure 2.99. A reslurry collector.

114

Special Features

2.4.14 CIP

The CIP feature is a very useful and necessary option for the decanter. It is used in many processes using foodstuffs, pharmaceuticals and biochemical products. To be able to clean a decanter to high levels of hygiene standard, without the need for dismantling, allows the use of decanters when otherwise it would be impossible. There are varying degrees of sophistication offered for CIP. A donkey motor with a clutch may be fitted to the main motor to drive the bowl at low speed, slow enough to allow liquor to tumble inside the bowl, rather than rotate with the bowl. Sometimes a similar design but with a smaller motor is also fitted to the back-drive system. Today, however, the donkey motors have been mostly superseded by frequency inverter drives on the main drives to allow the necessary speed changes as required. During CIP the bowl and conveyor are rotated at this low speed for a set period after which there is the option to reverse the conveyor for a set period and then do the same thing in the reverse direction. During the CIP process cleaning or sterilising liquor is fed into the bowl and sprayed onto the outer surfaces of the bowl from spray bars fitted to the casing. It is useful during the CIP process to be able to empty the bowl. To this end, spring-loaded drain valves will be fitted into the bowl wall. At speed, the valves close with centrifugal force against the spring. Below a set speed the spring force is greater than centrifugal force and the valves open. 2.4.15 The Rotodiff

The Rotodiff is a device supplied by Viscotherm of Z/irich, Switzerland. It is a device, a hydraulic system, which substitutes for both gearbox and back-drive system on a decanter. The Rotodiff, which is manufactured in a large number of sizes and torque ratings, is fitted in place of the gearbox. Depending on the specification, it could be one or more rows of pistons. For a specific torque rating the Rotodiff is lighter than a standard gearbox and thus the Rotodiff offers higher maximum torques for a given decanter size. The Rotodiff is a sophisticated, rotating radial piston, hydraulic motor and rotary seal device, powered by a stationary variable speed pump unit and control system. The central system effects similar differential control to that possible with the standard gearbox and back-drive system. However, with a Rotodiff system, the differential speed is independent of the bowl speed. Being hydraulic, it can be easily made explosion proof, and systems are supplied in which both back-drive and main drive are hydraulic to satisfy explosion proof requirements. When using a Rotodiff, the differential can readily be reversed to assist with unblocking a bowl. However, this procedure must be done with care lest conveyor flights are bent at the front, pushing against the front hub.

Decanter Design 115

/ ,,-/, ,, ,/ / / ' ,' ,,/. / !'

9 9 :. -."7

,' .

9

j

.,/

,.."

/' /t

,"

,y,.

/"

i

O| ,,

'1,0 "....

Z" / /

,.,,,~. . -...

:. -..-...L~

Figz~re 2.100. The Viscotherm Rotodiff s!lstem (B!t coz~rtes~of Viscotherm).

Extra high pressure units ( 2 0 0 - 3 5 0 bar) have recently been developed, which enable reduced sizes and costs for the same duty. Naturally, maintenance requirements are those normally associated with hydraulic systems.

2.4.16 Power regeneration Conventional back-drive systems on decanters perform a braking duty. As such many of them have the ability to regenerate power. Although an eddy current brake is unable to do this, AC, DC, and inverter motors, and hydraulic systems are. The braking process causes the electric motor to act as a generator and so returns power to the grid. In the case of the DC and inverter motor, the power regenerated is usually considered "dirty" unless electric filters are fitted which smooth out unwanted harmonics. A better, t h o u g h more expensive, approach is to fit DC or inverter drives to both main drive and back-drive and use for both a common DC bus. The power regeneration is then simple and clean, and the main motor then draws from

116

Special Features

the grid only the net power required by the decanter. This system is subject to a recent patent [18]. 2.4.17 Dual main drive motor

The power required to accelerate the high inertia bowl to speed can be considerably higher t h a n that required to rotate the bowl at speed. Thus, to start the decanter, the main drive motor need special consideration. This could be one of a n u m b e r of alternatives, a soft start motor, a specially wound motor, a star-delta starter, or a motor with a fluid coupling. A patented alternative (Alfa Laval) to these methods, used on large decanters, is a dual set of motors [19]. Both motors are energised for start up and one is switched off when the bowl is at speed. 2.4.18 Floating conveyor

To get two c o n c e n t r i c masses rotating at different speeds, at 2 0 0 0 to 3 0 0 0 g or more, requires some sophisticated engineering and careful dedicated construction. Some decanters have now been built to produce as m u c h as 10 0 0 0 g, requiring even higher levels of accuracy. A new approach, which has been successful, has been to fabricate the conveyor, such that it floats in the process material held in the bowl. The hub of the conveyor is made of thingauge steel and sealed to make the overall density of the conveyor less than unity. The feed zone of this design thus operates flooded. Floating conveyors avoid the natural frequency problem associated with long bowls. 2.4.19 Decanter controls

With modern electronic technology, a host of different controls is possible for the decanter, all of w h i c h can be integrated with the rest of the plant, with, if necessary, a u t o m a t i o n , remote control and telemetering. The control of the main drive is relatively simple and straightforward. The control of the back-drive system has required dedicated development. Today the back-drive systems can control the decanter at a fixed differential, at a fixed torque or cake dryness or with some hybrid control system. The hybrid system, say, controls at a fixed differential, until a pre-set torque limit is reached and then at that torque level until the differential changes to a pre-set level. With main drive and back-drive systems under control, it remains to control centrate quality. For this, a good centrate monitor is required, capable of assessing the level of suspended solids in the centrate. This has been difficult but there are a few reliable devices now available on the market [20]. The monitor is then coupled via a PID controller to the polymer pump speed control. It is now c o m m o n to couple the centrate and back-drive controls to a PLC together with signals to and from other parts of the decanter plant, such

Decanter Design

117

as the polymer make-up system, off-take conveyors, and pumps and feed tanks. It is not a great step to go from here with a fully integrated system that controls and monitors the whole plant with safety warnings, level alarms, m a i n t e n a n c e prompts and a complete regular costing and audit of the plant performance. A feed solids monitor and flow meter will record the feed processed. The polymer plant is able to indicate polymer consumption. Electric meters will advise on power consumption. Feeding into the PLC costs of the various commodities allows the PLC to calculate r u n n i n g costs. Various control algorithms will be available in the PLC to control the plant to a m a x i m u m running cost, a m i n i m u m cake dryness or, say, a m i n i m u m feed rate within certain limits of other parameters. Many decanter plants operate today for many hours unattended and therefore there is a high demand for good automatic control systems.

\ @

,~

MBH ~

M~

T

Tr

DIFFERENTIALSPEED CONTROL ena., ~ 2

@

o~

#: A l ~ l . ~

|

Figure 2.101. An Alfa Laval back-drive controller.

2.5 References 1 2 3

4 5 6 7 8 9 10 11

12 13 14 15

L Shapiro. (Alfa Laval) Decanter centrifuge with conveyor capable of high speed and higher flow rates. US Patent 5354255, 11 October 1994 N F Madsen. (Alfa Laval) Decanter Centrifuge. US Patent 4828541, 9 May 1989 H Hemfort. (Westfalia) Continuously operating solid-jacket counterflow centrifugal extractor. US Patent 4147293.3 April 1979; W Ostkamp, K H Brunner, F Wibbelt. (Westfalia) Continuously, completely jacketed, countercurrent centrifugal extractor. US Patent 4451247, 29 May 1984 A H Shapiro. (Bird Machine) Conveyorless clarifier. US Patent 5067939, 26 November 1991 P LaMontagne. (Pennwalt) Centrifuge employing variable height discharge weir. US Patent 4575370, 11 March 1986 J W Caldwell. (Alfa Laval) Inflatable dam for a decanter centrifuge. US Patent 5257968, 2 November 1993 C Y Lee. (Pennwalt) Centrifuge apparatus. US Patent 3795361, 5 March 1974 B Madsen. (Alfa Laval) Decanter centrifuge with helical- rib baffle. US Patent 6024686, 15 February 2000 R E High, A J Samways. (Pennwalt) Centrifuge apparatus. US Patent 3934792, 27 January 1976; R E High. Decanter centrifuge. (Decanter Pty). GB Patent 2182869, 28 May 1987 S Suzuki. (Kotobuki Techrex) Sedimentation centrifuge containing screw conveyor with fins. US Patent 5310399, 10 May 1994 L A Larson. (Noxon) Decanter centrifuge. World Patent WO93/22062, 11 November 1993; G L Grimwood, G C Grimwood. (Broadbent) Decanting centrifuges with improved compression. US Patent 5584791, 17 December 1996 F Brautigam. (Pennwalt)Centrifuge apparatus. US Patent 3764062, 9 October 1973; L Shapiro. (Pennwalt) Hard surfacing for a centrifuge conveyor. US Patent 4328925, 11 May 1982 D Locke, JW Trueman. (Alfa Laval) Screw conveyor for centrifuges. GB Patent 2273253, 15 June 1994 J W Caldwell. (Pennwalt) Conveyor flight configuration. US Patent 4449967, 22 May 1984 R Schilp, W Epper. (KHD) Solid bowl worm centrifuge. US Patent 5545119, 13 August 1996

Decanter design

119

16 IHI (Ishikawajima-Harima Heavy Industry), Utility Model JP 1880613 17 B Madsen. (Alfa Laval) Decanter centrifuge with energy dissipating inlet. US Patent 5374234, 20 December 1994 18 J L Cooperstein. (Alfa Laval) Variable frequency centrifuge control. US Patent 5203762, 20 April 1993 19 J L Cooperstein. (Alfa Laval) Decanter centrifuge having dual motor drive. US Patent 5342279, 30 August 1994 20 J G Joyce. (Alfa Laval) Turbidity measurement. US Patent 5453832, 26 September 1995

This Page Intentionally Left Blank

CHAPTER 3 Applications The decanter centrifuge is a major item of processing equipment in m a n y industrial applications, and this chapter illustrates this range of use. The driving forces in the decanter marketplace result almost entirely from the needs of these industrial applications, so their importance to a handbook on the decanter is easy to see. The decanter has no place, however, in domestic, institutional or commercial (business) applications, which are covered by separation equipment of quite different kinds.

3.1 Application Classes In addition to reviewing decanter applications by specific industrial end-use, it is necessary first to recognise that there are ways of looking at decanter applications in a more general sense. Mechanical liquid/solid or liquid/liquid separations serve two broad purposes: 9 a utility function, such as the cleaning of cooling water or of a recycled hydraulic fluid; and 9 a process function, such as the recovery of crystallised salt or the polishing of beer. Utility applications are found t h r o u g h o u t the whole of the m a n u f a c t u r i n g and service industries, and their characteristics are dictated by the n a t u r e of the application, and more often t h a n not are independent of the n a t u r e of the end-use sector, such as power generation, brewing or the production of pharmaceuticals, in which they are found. Utility applications are. on the whole, of similar levels of importance t h r o u g h o u t industry and commerce. Process applications, on the other hand, are usually end-use sector specific, with important variations imposed by operating temperatures and pressures, by the corrosiveness of the liquid or abrasiveness of the solids in the system, and by the individual process needs, such as cake dryness or centrate clarity. Process applications may be an absolutely vital part of the end-use process, or may have only a small part to play. Decanter applications certainly obey this differentiation. The n u m b e r of different utility applications is small, a l t h o u g h the main one, the dewatering of waste sludges, is an e n o r m o u s part of the decanter market. The great variety of decanter uses occurs in the very wide range of process applications. This variety of end-use separation applications can be grouped into five broad types: clarification, classification, thickening, dewatering and w a s h i n g according to the main purpose of the separation process. It is, of course, very probable that more t h a n one of these may be involved in a particular application. The decanter separates suspended solids (or immiscible liquid droplets) from a liquid stream, so that one prime purpose for the decanter is the

Applications 123 clarification of this liquid stream as far as possible free from the suspended material. The decanter can be operated so as to give a high degree of clarification, although it is not always possible to achieve high degrees of clarification and of dewatering of the solids at the same time. It is not often t h a t a decanter is used only to clarify a liquid, and it is not by any m e a n s the best equipment for clarifying a slurry that contains only a small a m o u n t of solids in suspension. Waste slurry processing, although not only a clarification process, does require the most effective removal of solids t h a t is possible. In clarification mode, extra efforts m u s t be made to ensure t h a t no cross-contamination between the exit streams is allowed. As far as possible, then, clarification aims at a complete separation of solids from the liquid stream. The next purpose, by contrast, aims specifically to leave some solids in the exit liquid. In the classification of solids by a decanter, a slurry of solid particles of mixed particle size, or, less often, of mixed densities, is treated in such a way that a specific fraction is removed as separated solid, leaving a well-defined fraction of the original solids still in suspension. This mode of operation is particularly relevant to the processing of kaolin (china clay), and it also finds a place where the decanter is used to remove oversize material, ahead of a more efficient clarifier, which might interfere with the final separator's operation (e.g. which might block the nozzles of a disc centrifuge). The decanter is a very efficient m e a n s of effecting classification by particle size. The other intentionally partial separation occurs with the thickening of slurries, where only some of the suspending liquid is removed, to leave a thicker suspension to be discharged for subsequent processing. Thickening also can be used in waste sludge treatment, where subsequent dewatering is u n d e r t a k e n by other types of equipment. It is also possible to achieve a final slurry thickness by dewatering the solids more than necessary, and then mixing them with feed liquid to the required degree, thus reducing the flocculant load. Sludges are thickened, rather t h a n fully dewatered, if they are to be pumped or barged for final disposal. The bulk of decanter centrifuge applications involve the recovery of the suspended solids, usually because the solids are wanted for subsequent processing, but also to make any subsequent treatment easier or less costly. Where the next stage is not affected by the presence of some of the liquid phase, then the purpose of the separation is usually the efficient dewatering of the solids to as dry a state as is feasible and economically justifiable. This is certainly the case in the dewatering of waste sludges, where the discharged solids will be dumped onto land, incinerated, or thermally dried (prior to use as fertilizer, or to incineration). The lowest w a t e r content in a sludge reduces the cost of transport, or of the energy needed for drying and incineration. If the separated solids are to be treated chemically, or otherwise used in a subsequent process, such that purity is important, then they may need to be washed free of the original liquid. Washing of the separated solid can either be

124

Application Classes

done on the beach of a n o r m a l decanter or on the extended cylindrical screen section of a screen-bowl decanter. W a s h i n g on the beach is well suited to fibrous solids, and to most crystalline solids, whilst some crystals or hard g r a n u l a r materials (such as coal) can be washed efficiently on the screen-bowl section. If the solids are not easily dewatered, but need washing, then it may be necessary to reslurry the discharged solids, outside the decanter, and dewater them in a second machine. In m a n y cases of slurry dewatering, and especially where the slurry is a waste needing t r e a t m e n t prior to safe disposal, the decanter can dewater such slurries to a high level of dryness. The extra-dry solids decanter has become an important feature of the decanter market, both to allow it to compete for business with some designs of filter, and to provide a good means of processing waste sludges ahead of incineration or land disposal. A further modification of these five basic uses is the three-phase decanter, in which two immiscible liquids exist as well as the suspended solids that are involved in the separation. Here good clarification of both liquids is usually required, as well as " d e w a t e r i n g " of the solids from both liquids.

3.2 Application Analysis The actual uses of the decanter can be found in almost all of m a n u f a c t u r i n g industry, in process terms, and beyond it as well for utility (mainly waste treatment) uses. It is a help to see these application sectors in context, as is s h o w n in Table 3.1. This table gives an a r r a n g e m e n t of the main items in the totality of h u m a n economic endeavour, as exemplified by the divisions of the UK Standard Industrial Classification. (This classification is similar to those used for various purposes by the statistical offices of the United Nations, the European Union, and the US and m a n y other governments.) The items in Table 3.1 have been selected to show the range of sectors of interest to the decanter centrifuge, with those of significant interest in bold text. The emboldened categories are those in which decanters play an important part in the process of that category. It should be remembered that decanters also have a vital role in the t r e a t m e n t of waste slurries, which can arise from almost any of the m a n u f a c t u r i n g sectors. It is a moot point as to w h e t h e r or not some waste sludge processing steps rank as "utility" or "process" applications, especially in the production of freshwater from raw water. Quite certainly, such t r e a t m e n t in the municipal waste treatment sector is a "process" one, hence the highlighting of "Other c o m m u n i t y services" The emboldened categories in Table 3.1 do not imply that all parts of these sectors use the decanter, but a key use will be found somewhere in each one. The remainder of this chapter is devoted to brief descriptions of these key applications, with reference to the features of the decanter that make it valuable for that application.

126

ApplicationAnalysis

Table 3.1. Standard industrial classification, 1992 SIC section

CA CB DA DB DC DD DE DF DG DH DI DJ DK DL DM DN

Activity

Agriculture, hunting and forestry Fishing and fish farming Mining and quarrying Energy materials Other mining and quarrying Manufacturing Food products, beverages and tobacco Textiles and textile products Leather and leather products Wood and wood products Pulp, paper, printing and publishing Processed energy products Chemicals and chemical products Rubber and plastic products Other non-metallic mineral products Basic metal production and fabrications Machinery and equipment Electrical equipment Transport equipment Other manufacturing Electricity, gas and water production and supply Construction Wholesale and retail trade Hotels and restaurants Transport. storage and communication Financial services Other business services Public administration and defence Education Health and social work Other community services Domestic services

Source" National Statistics. Extracts from National Statistics are Crown Copyright and may only be reproduced by permission.

3.3 Waste Sludge Processing One of the largest applications for the decanter centrifuge, certainly in terms of machines sold, is the dewatering of waste slurries arising from industrial processing, from raw water treatment, and from the t r e a t m e n t of municipal sewage. Almost all industrial m a n u f a c t u r i n g processes generate liquid wastes containing suspended solids; even an industry as apparently unconnected with liquid processes in its operations as m a c h i n e r y manufacture. Machining uses complex liquids for cooling and lubricating machine tools, which then become contaminated with dirt and metal scrap. Machine tool fluids are usually valuable e n o u g h to need recycling, and so they must be cleaned before re-use. This is also true of m a n y industrial wastes, that the liquid, once cleaned, or the separated solids, once dewatered (and, perhaps, washed) can be recycled profitably. However, the main bulk of waste slurries have to be dewatered in order that the suspending liquid can be discharged to a river or lake w i t h o u t polluting it, and the solids can be sent for final disposal in as small a bulk and as safe a condition as possible. Waste sludges, once adequately dewatered, can be sent to landfill, or for soil improvement or fertiliser use, or to incineration. For most of these final destinations, a high degree of w a t e r removal is beneficial, and the decanter's ability to achieve high drynesses has led to its wide acceptance in the treatment of waste slurries. 3.3.1 Industrial wastes

The wastes produced in m a n u f a c t u r i n g processes vary in composition as widely as does the nature of the slurry-producing process. The range of modifications to the basic decanter structure enables it to cope with this wide variation in feed compositions. Whether the ultimate destiny of the final sludge is to be recycled within the factory, or to be sent away for landfill, there is usually sound a r g u m e n t in favour of discharging it from the decanter in as dry a state as possible. The materials from which the decanter is made will have to be chosen well to match the likely corrosive or abrasive nature of the slurry components.

128

Waste Sludge Processing

The variety in the n a t u r e of sludges from industrial sources can be seen from the following list of manufacturing and other processes creating waste slurries in need of treatment: manufacture of acetylene manufacture of aniline steel works' blast furnace operation processing of cellulose coal processing wastes extraction of coffee decortication (debarking) of wood flotation sludges from the de-inking of recycled waste paper dye and pigment processing wastes sludges from electrolytic and electrochemical processes fish transport water wet processing for the desulphurisation of flue gases foundry operations insecticide production iron ore scrubber slurries milk processing paper mill operations nuclear fuel and spent fuel processing slaughterhouse wastes tanneries manufacture of TV tubes all of which, and more besides, use decanters to dewater the slurries. In particular, it should be noted that, like the dust scrubbers from iron making, an increasing demand for better air pollution prevention is increasing the n u m b e r of wet scrubbers in operation. Each scrubber installation produces a slurry of the removed dust, which becomes a potential decanter application. For any waste which is produced carrying an oily liquid as well as water, then the oil can be recovered separately from the water and the suspended solids, by the use of the three-phase decanter. This will occur especially in the oil refining and blending processes, but also in several food industry applications. The treatment of refinery slop oils is an important decanter application. The increasing need to treat sludges of all kinds, especially toxic ones, is leading to the development of total sludge treatment systems, which aim to destroy all organic matter and leave a neutral inorganic sludge. Wet air oxidation was the first of these, and this has recently been joined by Kemira's "Krepro" and Chematur's "Aqua-Critox" processes (the latter using supercritical water oxidation). The wet sludges from these processes could be

Applications 129

sent to landfill, but they may be concentrated enough to justify recycling. The decanter would then be the means for the dewatering of the final sludge. 3.3.2 Water treatment sludges

The boundary between what is a direct-line-of-process slurry, and what is a "utility" application, is not easily drawn in the case of the treatment of raw water, surface or ground, to produce drinking water, or water for process use. Raw water is allowed time to settle out most of its suspended dirt, but coagulants may be used to hasten the process. The resultant slurry is, without doubt, a waste material, especially if the dirt content is high. If a high load of coagulant like alum is used in the treatment, then there may be a case for the recycling of the separated solids, for re-use, after suitable processing. As well as the removal of sediment, waterworks may also soften the water, and may use lime for that purpose (as opposed to fixed bed ion exchange), again producing a sludge needing dewatering in a decanter. Waste lime slurries are frequently recycled for conversion back to reusable material. Selective recovery of the calcium carbonate from the waste will enable such calcining to be undertaken without the heavy inert load of the unseparated waste. For waterworks applications, decanters normally will be operated with polymer addition facilities, at alternative admission points. They will need full erosion protection for the flights, using tiles. They will have some kind of cake baffle, possibly a cone. and a variable speed back-drive, with good differential and torque control. They will be operated with deep neutral ponds, with axial flow (i.e. with flight windows, such that the liquid flows parallel to the axis, rather than around the helical space), and with provision for wash-out prevention at start-up. 3.3.3 Municipal sewage treatment

Probably the fastest growing, in market size terms, of all decanter applications, the treatment of sewage sludges is a vital part of the developed world's attempts to improve its relationship with its environment. Now that dumping of sewage sludges at sea has finally stopped, the sewage from 1520% of the world's population is now fully treated, giving rise to vast quantities of sludges. The full sewage treatment process has three main stages: 9 primary treatment, involving simple sedimentation, normally unaided; 9 secondary treatment, which is usually biological, over trickle beds, or in an activated sludge process; and 9 tertiary treatment, which involves a range of polishing operations.

130 Waste Sludge Processing There may be screens ahead of primary treatment (as well as grit and grease traps), from which the screenings may be macerated and added to the primary sludge. There will certainly be sludges produced by the primary and secondary treatment, and there may also be sludges from some of the tertiary processes. Many sewage treatment works use sludge digesters, which ferment the various sludges anaerobically, reducing the sludge volume considerably, but not eliminating it. The dewatering tasks at a sewage treatment works can thus include: 9 primary sludge, possibly mixed with screenings, which is a comparatively easy task: 9 secondary (biological) sludge, which is not easily dewatered, and so is usually mixed with primary sludge, to give a third variety: 9 mixed sludge; and 9 digester sludge. The decanter can handle all of these sludges, with varying degrees of separation efficiency. The use of coagulants and flocculants in the treatment of municipal sludges is now commonplace, and, indeed, the decanter would not be the efficient dewatering machine that it is today in the absence of modern, synthetic flocculants. Secondary sludges are soft, and not easily dewatered, so that the role of the decanter in the processing of such sludges is more as a thickener, than as a dewatering tool, probably ahead of digestion. For this duty, the decanter will need full erosion protection, preferably with tiles on the conveyor flights. The decanter will operate at maximum speed with axial flow, with a deep neutral pond, and a pond setting just shallower than neutral. It should have a backdrive with good differential control, and a standard ratio, low torque gear box. Wash-out prevention and a cake baffle are necessary. An inflatable weir dam is a good idea, and can prevent wash-out. Sigma enhancement (i.e. the use of vanes, similar to the disc stack of a disc centrifuge) can be advantageous, but polymer addition may not be necessary, depending upon the degree of dryness required. If digestion is not an option, then the secondary sludge could be mixed with the primary sludge and screenings, and this mixed sludge (or primary on its own) can then be efficiently dewatered by the decanter. A resultant dryness of 18-22% can readily be achieved. With dry solids operation, this can be increased to 25-30%, or even 3 0 - 3 5 % solids (an appearance of being solid). The relative proportions of primary to secondary sludge, and of municipal to industrial, will affect the final figures. Uncommonly, figures of 4 0 - 5 0 % have been experienced. The specification for a decanter to handle primary sludge alone, or mixed primary and secondary, or digester sludges, would be as follows:

Applications

131

9 9 9 9 9

polymer addition essential, with the ability to add internally; full erosion protection essential, using flight tiles; cakebaffle useful; back-drive required, with good differential control; standard bowl speed sufficient (g levels up to 2 5 0 0 depending upon precise design); 9 w a s h - o u t prevention necessary on start-up (such as notched weir plates); 9 deep neutral pond (better performance than shallow pond); and 9 axial flow. For the more abrasive sludges, a casing wear liner will probably be needed, while for the most abrasive sludges, hard surfacing should be added to back and front of the flights around the feed entry into the pond. For dry solids operation, the above applies, plus: 9 negative pond operation: high-ratio (three-stage if necessary) high torque gearbox, with good torque control: and 9 a cake baffle or restriction of some form, or its equivalent, is essential.

9

The processes, such as those already mentioned (in Section 3.3.1), which treat whole waste slurries and destroy all organic matter, will find a place in municipal sludge treatment once they become established, considerably extending the decanter market.

3.4 Energy Materials Production The production of raw materials for energy g e n e r a t i o n - coal, oil and gas offers considerable scope for the use of decanters, at least for coal and oil. The manufacture of gas is a little used process now t h a t natural gas is so freely available. When this phase passes, as gas availability reduces, then the gasification ofcoals and heavy oils will also offer good applications (as will the eventual need to convert coals into liquid fuels). In coal processing, the mined coal must be washed free of impurities. The run-of-mine coal contains the clay and rock that are within the coal seam, as well as roof and floor material. Coal itself is a relatively low-density material, compared with the impurities, and can be separated by sedimentation in a liquid of appropriate density. An increasing proportion of the coal that is mined is crushed to quite small particle size, partly to enable the removal of as much sulphur as possible. For such small sizes, the decanter provides an excellent means of separation, also enabling the settlement fluid to be recycled. Whatever the size of coal being processed, there will always be a need to separate residual coal from the washings, for which the decanter is a standard separating device (as well as for the final dewatering of waste liquids the coal tailings). Fine coal may also be separated bv means of froth flotation, and the dewatering of flotation concentrates is a n o t h e r good application for decanters. In the production of petroleum or natural gas from u n d e r g r o u n d sources, the initial well holes must be drilled, often into very deep strata. The search for new oil resources also requires the drilling of m a n y trial holes, more often dry than successful. For such drilling, the drill head must be cooled and lubricated, and so a lubricating fluid (the "drilling mud") is pumped down to the drill bit. Not only does the drilling mud cool and lubricate the bit, but it also carries the rock cuttings back to the surface, it prevents water leaking into the well from water-bearing rock, and it prevents the collapse of the hole behind the drill. These duties require that the drilling mud be a complex liquid, basically a suspension of clay (bentonite) in water, but dosed with a range of special chemicals that allow it to cope with the r a n g e of rock compositions to be encountered, and to have the right density. The preparation of a drilling

Applications

133

mud is a size classification task, for w h i c h the decanter is very well suited. The used liquid is often too expensive to discard, so needs t r e a t m e n t for the removal of rock fragments for w h i c h duty, again, the decanter is a very good choice. For either of these duties, the decanter will need to have full erosion protection, but can r u n at moderate or low speeds. It will also need an explosion proof motor (and back-drive equipment if this is used).

3.5 Processed Fuels The production of n a t u r a l gas results in an adaptable and easily used fuel, and further processing is often quite unnecessary. The processing of coals and oil, on the other hand, are major industries. The carbonization (coking) of coal is a largely dry process, but the byproduct tars and phenols do require cleaning, for which the decanter is used. The modern petroleum refinery has a range of uses for the decanter, as well as the various waste streams that need treatment. One of the most i m p o r t a n t is in the preparation and recovery of catalyst for cracking and reforming uses.

-

3.6 Minerals Extraction and Processing .

-

-.

-~

-_

.

ing, ol' metiilli(: ;in(l non-met.nl1ic minpr;ils :ire m:ijor applicat.iciri areas lor tho decarikr. This cl:issitic:ition

The extraction, iivd ~libseqiienl pro(

obviously overlaps considerably with the production of inorganic chcmicals (CQvcrCd in Scction 3.9.1)- and the trcatment of calcium carbonatc differs littlc. whether it be intended as a chcmical or for other usos. 'I'ypical riiiiierals involving the decanter in ttieir procrssirig iricliidr [:ijl(.itin1 carbonate and chloride. clays, gypsuiii and potash slimes. A sigiiificant part of the wct cxtractioii processes fcjr rriiriurals involves either the rernoval of oversixe rriaterials from ;1 suspension or the division of a suspensiori into t w o Erat:tioris by piirticlr sizc. Far thesc purposes. the decarilcr, opcratirig in classitic:iit inn mnde, i s a n ideal proccssing tool. Thc processing of china clay (kaolin) rcprcsents one of tlie r ~ l c l r suses ~ for t h e dec-;inter. K;iolin occurs naturally as a fine white powdery deposit. (:omposed largely of the mineral kaolinilc, in ptirticle sizes iri t tic range from 0 . 1 to 1 0 pm or morc. It i s uscd as a filler arid coiiting in the making of paper. iiS the basic malerial for c r r a i i i i c warc. as a filler (or robhcr, and in paints and m a n y other products. Each of thcse may need ;i cl:iy with i1 difl'crcnt particle sizc. and thc decanter is ablc lo provide the necessarv classificatiori o f particle sizc, a typical cut point beirig 2 pin. The kmlin is washed out idits rlidtive rock. largc rnatcrial is settled o u t . and t hen the slurry is sized i n a ctin1plc.a system of cyclones and decantrrs. It is ii largs applicatioii a n d :I good rnarket f(Jr decanlers. Other minerals - rnet;illic or non-mctallic - arr processed similarly. sud1 as bentonite. ( - h i l l k , mica. and thoso rniiieriils produced I'm use 21s pigrlirnts it1 paiiils. For these dutics. thc dccanter should be ablc lo operate a1 dilltrtnt speeds. arid h:iw ;I variablc spccd back-drive. I?uII wear protcciion is r s s e n t i n l . 'I'hc. use ol flocculants m a y be iirrrssiiry t'cir smie iiiirit.raI processing xtivities. A great deal or mincral processiiig ends with n conctntrated slurry r ~ t th e rcquired product, and this is usunlly drwiilcrcd to ii cohesive cilkr i n a decanter, and may be washed as well. on t h e beach of a standard dt:canter. uc in a scrccn-bowl machine.

3.7 Food and Food By-Products In most of the applications discussed so far, cleanliness of operation has not been a vital aspect of the design. In those that remain, food, beverages, and some parts of the chemicals sector, sanitary operation is usually vital to the process. Thus, for most of the remaining applications, the decanter should have the clean-in-place facility built in to its design. The production of foodstuffs, finished foods, and their by-products covers many different activities:

meat and meat products; fish and fish products; fruit and vegetables; vegetable and animal oils and fats; dairy products; grain milling and starch; prepared animal feeds: bread, pastry goods and cakes: sugar, cocoa and confectionery; pasta and other farinaceous products: tea and coffee: and soups and other food products. with those components of the industry important to the decanter in bold type. As with other major industries, there are many components of the food industry, using water in the processing, which give rise to liquid wastes carrying material in suspension. It is unlikely that these wastes can be processed for full recycle, but several can recover some value (for animal feed, for example). All will need dewatering, before final discharge. Where fat or oil are involved, as well as water, then the three-phase decanter can be used in the dewatering process. 3.7.1 Meat and meat products processing The main processing of meat from the slaughterhouse to the butcher's counter has no use for decanters (other than in waste treatment), but they

Applications

13 7

have a major role in the processing of meat by-products. These are the nonmeat (but not necessarily inedible) materials collected during the slaughtering process. Those that use the decanter in their subsequent treatment include: 9 bones and rendered meat, for use as bone and meat meal in animal feeds and fertilizers; * blood, for use as blood meal; 9 gelatin, obtained from high-collagen products such as pork snouts, pork skin, and dried rendered bone, for use in confections and jellies; 9 edible fats are used as lard, tallow, shortenings, and cooking oils: and 9 inedible fats, used in soap and candle manufacturing, and in various industrial grease formulations. Hormones and other pharmaceutical products such as insulin, heparin, and cortisone are obtained from various glands and tissues, while gelatin is also used for pharmaceutical preparations. These are discussed further in Section 3.9.4. A major application lies in the rendering of animal fats, edible or inedible, wet or dry. Early rendering processes involved the heating of the raw fat to break the cell walls and release the fat as liquid, followed by the separation of the hot fat from the residual cell material (greaves). Several centrifugal separation processes were developed in the 1960s, for which the cells are ruptured in special disintegrators under close temperature control. The protein tissue is separated from the liquid phase in a decanter, following which a second centrifuge separates the fat from the aqueous protein l a y e r or the whole separation may be undertaken in a three-phase decanter. Compared with conventional rendering, the centrifugal methods provide a higher yield of better-quality fat, and the separated protein has potential as an edible meat product, as opposed to its earlier use as animal feed or fertilizer. A decanter for use in rendering processes would require full hard surfacing, and some dry beach, but would need to be simple and robust, to match the characteristics of the industry.

3.7.2 Fish processing As with meat, the main production of whole fish does not offer any scope for the decanter, but the bulk production of fish meal certainly does. Fish meal is a coarsely ground powder, high in protein, made from the cooked tlesh of fish. Though formerly important as a fertilizer, fish meal is now primarily used in animal feed. The oily fish, such as menhaden, anchovy, herring, sand eel and pilchard, are the main source of fish meal and its companion product, fish oil. To be processed into meal, chopped fish is fed by a screw conveyor t h r o u g h long steam cookers. The cooked mash is then pressed to remove water and oil.

138

Foodand Food By-Products

The press effluent is separated in a three-phase decanter into oil, water, and residual meal. In addition to its three-phase design, the fish meal decanter will have full erosion protection, with standard tiles, and a variable speed back-drive. The fishing industry has to deal with a raw material with high nutrient content, but a low rate of utilisation of fish in normal fillet production. These factors, together with the growing worldwide awareness of the limited supply of natural fish stocks, have all led the fishing industry to develop procedures for better utilisation of the original fish, and for the development of edible products from underutilised species. Surimi was developed in Japan long ago, when it was discovered that washing minced fish flesh, followed by heating, resulted in a natural gelling of the flesh. Combined with other ingredients, the surimi could provide a range of "ready-to-eat" products. Modern surimi production involves continuously operating lines with automated machinery for heading, gutting, and deboning of the fish; mincing, washing, and pressing (to remove water); and heating of the flesh, or freezing of the flesh for later processing into various end-products. The decanter has a part to play in the treatment of the presswater, in an application of growing interest. Recently, Alfa Laval successfully introduced a decanter that can replace the press itself. This simplifies the process and reduces the footprint needed for the process line, which is critical for on board plants.

3.7.3 Fruit and vegetable products The processing of fruits and vegetable material includes a number of operations where the decanter has a part to play. These include: 9 the extraction of oils from seeds, such as rape. corn (maize), cottonseed. sunflower, safflower, groundnut (peanut), flax (linseed), castor, tung, soybean and sesame: 9 the extraction of oils from fruits such as olive, palm (and palm kernel) and coconut; 9 the extraction of starch from corn (maize), wheat, tapioca and potatoes: 9 the separation of gluten from starch; and 9 the canning of whole or cut fruit and vegetables. Not included here are the processing of fruits and grains into juice or fermented beverages, which is covered in Section 3.8. nor the onward conversion of oils into soaps, which is covered in Section 3.9.3. Vegetable seed oils are a very important component of the modern diet, and their production has increased to become a significant application for the decanter. The separated seeds (some, such as cottonseed or corn, being a byproduct of other processing) are rendered, usually by mechanical grinding, but also by cooking, to form an oily paste. This paste may be mechanically

1.0 uxtrude ihe oil (a1,id sorxie water), or separated directly in a decanter. After the oil i s extracted from the oilseeds, the residual meal. of c:nke, which h a s a high protein contcnt (high enough that it frequcnlly det.errrIiries t h e valrlt: o r t.he o i l nrnp), is usually nsed i j s n protein concentrate to feed livcstock and poultry. If thc cakc is poisonous. as with castor beans and tung nuts, it is uscd as fertilincr. Dccantcrs for vcgctable oil processing must be able 1.0 opera1.e at. eIeviIt.ed teI11peratures. 'rhey should h a v e ;j (:ombinal.ion of' $1 dry beach with a bac.kdrive for setting the differerilial. In the processing of olive oil. the degree oCpiirity ot'the oil is very important. such ihat. only the minimum of degradation can be allowed. 'The whole f r u i t . irlcluding the si.ones, ;ire c,nrnrninute,d in grinders or hammer mills. a i d 1 h e resullarii. paste is t.han stirred until the oil appears as large droplels. The nil ilnd olive juice arc then separated from the resir1il;il solids ("pomacc"), cithcr by a hydrnulir or mechaiiical press, or i n a d e r x n l e r . The separated cakc is ed, to extract further oil. which is used I'or industrial purposcs. rather t.tian cdihlc, and the oil/juicc mixture is scparated in a disc oentrifiigE. 4 more modcrn vcrsion of tlic prirniiry separation uses a threc-phase dec:inter, to achicvc both scparatioris a1 once, with very little degradation in oil quality. The pomaco can he wiishrt!. in thc decanter, with s u r w recycled juice, to iiicrtasc the oil rerrioval, or the fccd liquor inay he diluled with watcr tbr the same purpose. 'lliwe is, in some parts of the industry, ;1 tendency now tu return to thc two-phasc dccariter. Palm fruit is p r w e s w i in much the sariit: wiiy as Oliiics, cxcept i . h i i ~the kerncls may be st:par;itrd first. thcn broken l o enablc thc palm kernel nil tn bc extracted separately. Thc palm fruit may be cooked before 1.11~. o i l is lihcrated. 'l'ht. olive oil applicatim is ;i very important one for the d e m n t c r , and sotile duuanter rnariul'iicturinE compstriirs exist just to satisfy t . 1 olive ~ oil market. The olive arid palm oil dccenter will normally he of three-phasr: desigri (alt.hoiighsome processes now use a standard decanter, dischargirig 1 he juicc w i h I.he cake). It will ust: rnoderatcly high bowl speeds ( t o yrodut:o 2 500 to SOOOg). and full crosion proIwtion (because {.he fruit is n c r f r h m , u g h l y cleaned. a n d beuausc the stoncs are quite iihriisivc). It will prnbably have a lixed s p ~ back-drive. d with tixcd capacity, ijlthough difkrerit.ial spccd utirilrt)l is irn optional feature. Starch is primarily dcrivcd h r r l corn (maize). The cleancd grains ;Ire soaked to soften them, t hcn mcchariically rendered to liberate ihc corn germ. which is thc oil coril;jining cornponerit.. The remainder ol' i h e kerncl corlt.ilirls starch. gluten and tlbrc. which ;ire progrcssively srparated. and the starch is then classified. Thc dccanter h;js somc part i r i hot.h the scparalion a n d c.lassific a t io r i s1.agr s llaririeries largcly deal irr whole or cut fruit and vegctablcs, but. t.herc is a good applicet.inn for dccanters in wastc liquid prowising, yielding arl edible cake, which can bc rccgcled iri1.o !ood products.

pressEd

+

140 Foodand Food By-Products

3.7.4 Other food processing Decanters are used in o t h e r parts of the food industry, for example: 9 for separation, classification and washing in the production of casein and lactose in milk processing plants: 9 for the preparation of edible protein from soybeans, and from single-cell protein fermentations; 9 in the processing of coffee beans, and the production of instant tea and coffee; and 9 in the production of soups. Decanters in casein production will often have a centripetal pump discharge to combat foam, and axial flow, as well as clean-in-place systems. Lactose production uses a double-lead conveyor, with on-the-beach rinsing, and a reslurry rinse solids collector, as well, again, as CIP. Protein separation and clarification employ erosion protection on the first stage extraction of the n a t u r a l products, as well as m a x i m u m operating speeds, and a variable speed brake. Naturally, they also have a hygienic finish, and CIP. Sigma e n h a n c e m e n t is sometimes employed.

3.8 Beverages The beverage industry divides into two main parts ~ the making of alcoholic and non-alcoholic drinks. For non-alcoholic beverages, the main decanter application is in the separation of fruit and vegetable juices from their crushed pulp. In alcoholic beverage production, from grain sources and from grapes and other fruit, there is a major decanter application in the separation of the processed grains from the liquid product. In the case of brewing, the fermented barley is separated for use as an animal feedstuff, as brewers' grains. From the distillation of fermented materials, the stillage from the bottom of the still is also processed to separate the spent grains for feedstuff use, as distillers' (dried) grains. The decanter is an efficient device in both applications for the production of a well-dewatered cake. For brewing mash and stillage treatment, the decanter should have erosion protection, with special tiles to allow for the high operating temperatures in stillage processing. It should have directional feed nozzles on shallow pond machines, and a cake baffle with deeper ponds. Deep pond machines are increasingly being used, when higher capacities are employed, and when Sigma enhancement is used, to obtain m a x i m u m benefit from these characteristics. The decanter should operate at maximum bowl speed, and at close to neutral pond.

3.9 The Chemicals Industry The chemicals sector covers a wide range of important application areas for the decanter. The full spectrum is: industrial gases dyestuffs a n d p i g m e n t s f e r t i l i z e r s a n d n i t r o g e n compounds other basic i n o r g a n i c c h e m i c a l s p l a s t i c s in p r i m a r y f o r m synthetic rubber in primary form other basic o r g a n i c c h e m i c a l s

pesticides and other a g r o - c h e m i c a l s paints, varnishes and other coatings, printing inks, and mastics pharmaceuticals, medicinal chemicals and botanical

products soap a n d d e t e r g e n t s , c l e a n i n g a n d p o l i s h i n g m a t e r i a l s , p e r f u m e s a n d toiletries explosives glues a n d g e l a t i n e e s s e n t i a l oils photographic chemicals and materials recording media other miscellaneous chemicals where, once again, the sectors of significant interest to the decanter, i.e. where most decanters are used, are in bold type. The first of the above groups of industries is concerned with bulk chemicals, and the other two with fine chemicals and pharmaceuticals. The whole chemicals sector represents a major market for the decanter, with over 200 different chemicals processed by its means although most of the applications are very similar. The petrochemical and plastics products are perhaps the most important in terms of numbers of machines sold.

Applications

14 3

3.9.1 Bulk inorganic chemicals Apart from the production of industrial gases, the inorganic chemicals industry is full of decanter applications, although the b o u n d a r y between w h a t is an inorganic chemical and w h a t is a mineral is not easily drawn. Many of the mineral applications already discussed find identical processes within the inorganic chemical sector. The decanter will be used either for dewatering and w a s h i n g of g r a n u l a r material and crystals, or for the classification of materials, where particle size is an important characteristic of the product (such as in the m a n u f a c t u r e of pigments for paints). Even the industrial gas sector m a y find a use for decanters if the " h y d r o g e n e c o n o m y " ever comes into being. The conversion of h y d r o g e n - c o n t a i n i n g raw materials could be big business in a decade or two. The m a n u f a c t u r e of titanium dioxide, for example, by the sulphate route, involves the sulphuric acid digestion of the oxide ore (ilmenite), followed by the separation of unreacted solid from the resultant slurry. This is an application for a decanter, made from corrosion-resistant steels, and equipped with full erosion protection. The produced dioxide needs a decanter again for wet size classification. Similar applications occur in the m a n u f a c t u r e of phosphoric acids. The decanter will need a high-torque, lowratio gear box. Among the other bulk inorganic chemicals whose m a n u f a c t u r e includes the use of a decanter are a l u m i n i u m and m a g n e s i u m hydroxides, iron oxides and salts, calcium carbonate and other salts, silicon and silicates, caustic soda, sodium carbonate and other salts, graphite, u r a n i u m "yellow cake", gypsum, and zinc oxide and salts. Calcium carbonate is a particularly difficult material to treat in a decanter, in t h a t it needs very high torque to achieve an adequate level of cake dryness.

3.9.2 Bulk organic chemicals Bulk organic chemicals include all the precursors to plastics, as well as the relatively simple chemicals such as acetone or benzene or organic acids. Also in this category, of course, are the major industries devoted to the m a n u f a c t u r e of polymeric materials of all kinds. (The term " p e t r o c h e m i c a l s " is also covered here to mean those chemicals made from petroleum as a raw material, although the products are the same as if made from any other raw material.) Not surprisingly, the main decanter applications lie in the wet polymerisation of m o n o m e r s to form: polyolefins (polyethylene and polypropylene); polystyrene; and polyvinyl and polyvinylidene polymers (acetate, alcohol and chloride).

144 The Chemicals Industry The decanter is used to dewater the polymer granules, an operation which is often more difficult t h a n it sounds. This is especially true with PVC, which can produce torsional vibrations ("chatter") of the conveyor, overcome by fitting a very stiff conveyor. PVC separation decanters have multi-lead conveyors (at least two leads, sometimes three), with conveyors whose flights are polished, with chamfered edges. The machines operate with shallow ponds, and have a large capacity cake discharge on the beach. There is a hightorque, low-ratio gear box. Poly vinyl alcohol uses decanters with beach rinsing, and a cake baffle, operating with a shallow pond. Polystyrene requires a high-speed decanter, also with polished and chamfered-edged conveyor flights. Operation is with a shallow pond, and there must be a close tolerance on the fit of the conveyor in the bowl, to prevent the entrapment of the plastic granules between the conveyor and the bowl. The casing venting must be such that there is no reentrainment of moisture in the separated solids, which can have as little as 1% moisture. Other petrochemicals, especially terephthalic acid and PTA, and nylon, as well as polyethylene and polypropylene, require flameproof equipment, and use the vertical decanter, which can be pressurised, and will operate at higher temperatures. These also have polished flights, and chamfered flight tips, without hard surfacing. Another important organic chemical is p-xylene, whose production uses decanters suitable for low temperature operation, and fitted with a screen bowl. 3.9.3 Fine and household chemicals

The main application for decanters in the fine chemical sector is in the manufacture of soap and detergents. In soap manufacture, natural fats and oils (such as tallow or vegetable oils) are treated with hot alkali solution, such as caustic soda, producing a sodium fatty acid salt (the soap) and glycerin (or glycerol). If industrially produced fatty acids are used in place of natural fats or oils, the reaction with caustic soda yields soap and water instead of soap and glycerin. After the saponification reaction is complete, the suspension of soap in excess caustic soda solution is centrifuged to separate the two liquid phases and the residual solid material. Special designs have been developed which enable the washing of the oil within the bowl. Originally a tubular centrifuge application, this now provides a good market also for the threephase decanter. 3.9.4 Pharmaceutical and medicinal chemicals

A large proportion of the products of the pharmaceutical industry is made in batch processes, for which the decanter, quintessentially a continuous

Applications

145

machine, is not wholly suited. However, m a n y materials are extracted from natural products, such as insulin or blood fractions, and the fermentation routes to antibiotics have long been continuous processes. These certainly do use the decanter. As an example, insulin is made from animal pancreases, by rupture of the gland material by mincing. The minced material is extracted with acidified alcohol, and the gland residue is separated in a decanter, for further extraction with more acidified alcohol. The final alcoholic extract is clarified, and processed further, with eventual production by crystallization. Many antibiotics, of which penicillin is one example, are made by fermentation of organic materials, and the decanter is used to separate the product liquor from the fermentation residues. Blood, whether from h u m a n s (for transfers) or from animals (for blood meal fertilizer) has to be fractionated or dewatered. This is a good application for a decanter, provided that it is of hygienic design. Fractionation of blood plasma proteins requires very high-speed operation, and the 10 000 g machines have been developed expressly for this application. Special machines have also been developed to permit the extraction of valuable materials from one liquid to another.

3.10 Other Applications Many other individual applications for decanters exist, outside the classifications already covered. In the agricultural sector, decanters are used to dewater pig manure, as well as other farming wastes. In the mechanical engineering sector, as well as the cutting and lubricating oil recycle systems, there is the removal of accumulated impurities from electroplating and electrochemical machining solutions. The recycle of waste oils (as opposed to their treatment prior to disposal) is becoming an important process for the decanter, as the cost of all kinds of oils has started once again to rise. The recycling of plastics is of rapidly growing concern, and the double beach (one at each end) decanter is used to separate plastic granules into two fractions by density.

CHAPTER 4 Decanter Theory The decanter centrifuge lends itself to a wide range of theoretical treatments, both process and mechanical. Reif and Stahl [1] observed that the decanter incorporates three "extensive fundamental problems", dewatering of the solids, clarification of the liquor, and conveyance of the cake produced. These three major subjects will be covered here, together with separate sections to deal with specific decanter processes, such as thickening, classification, threephase separation and the latest technology of "dry solids" operation. Separate sections concerning allied topics, such as particle size technology and fluid flow will also be included briefly. A few important mechanical aspects will also be covered, such as resonance, maximum bowl speed, and bearing and gearbox life. Firstly some basic decanter theories will be expounded.

148

Decanter Theory

Vc

$

Fi~lure 4.1. D!tnamics of a particle movinq in a circle.

time t - 0

4.1 Basic Theories . - _ ~- _

-L

--.--

--."-

As with every specialist subject. it i s easy for ihe cenlriTugt: erigirieer to ;qss~rnethiqt his basic t,heories are iiniversally iindersiood. To e n s u r e i.hat.(.his work is comptchcnsiblc to thc widcst possible readership. a few basic c.odccpts will bc covcred. Thcsc will includc 6-forcc, diffcrcntial spccd. and mass

balances across the dccanter. 4.1.1 Acreleration force

'rhe celltrifugal acceler;it.ion Ibrce, commnnly known a s g-farce. is thc basic motive force for scparat.irig the solids from the liquid in any scdimeiiting cciitrifuge. 'I'hus, iri :I haridbook about t.he decantcr no apdogits arc needed wlieri covcririg g-force as the first basic concept. Consider Pigiire 4.1. A particle. of mass H I . rotales at a tatigeril.iiil velocity. v ~ . ,a i d arigular velocity, CJ, in a circle of radius, r . After a lime, t , t h r particle has moved to ii paint un thc circle radius, r . which subtends a n angle, -;. cvtiere 7 2 JI., from its position at time f-0, Lhe extreme right or thc horixontal diameter oCi.ht!circle. At time t , the horiznntal distance ofthe particle Iroiii the centre ol'the circle i s s, and at timc t=O it w a s r: (4.1)

s = r.cos(7)

The horizontal ;ic:celeration of the particle towards thc centre is diffcrcntial of s:

Iht:

second

(4-3) = -w?r.ros( w t )

A t tirrie f.=O:

(4.4)

150

Basic Theories

d2S

dt 2

= --c02 r.cos(O)

= -co2r

(4.5)

(4.6)

Thus, anything moving in a circle of radius r, at an angular velocity ~, will experience an acceleration towards the centre of the circle of ~2r. In the centrifuge, it is the liquid that moves round in a circle, and the particles in suspension are free to move relative to the liquid. Thus, relative to the liquid, the suspended particles experience an acceleration, ~2r, radially outwards. Thus, the gravitational force, F, on a particle of mass m, is the product of its mass and acceleration, where: F ~- m r ~ 2 (4.7) In centrifuge parlance the term "g" (or g-level) is often used. This is the number of times the acceleration in the centrifuge is greater than that due to gravity alone. To differentiate between "g", which is dimensionless, and the acceleration due to gravity, having dimensions of LT -2, the ratio of the two accelerations "g" will be denoted here by gc, and that due to gravity simply by g. Thus: gr =

ad2t '

g

(4.8)

Note that the g-level within a centrifuge will thus vary, proportional to the radius, t h r o u g h o u t the depth of the liquid, and is proportional to rotational bowl speed squared. (To calculate gc using a simple expression, use rpm 2 x diameter in m m / 1 . 7 8 9 x 106; for example, rotating at 3000 rpm at 450 mm diameter, gc would be 32 x 4 5 0 / 1 . 7 8 9 - 2264. 4.1.2 Differential

The difference in rotational speed between the bowl and the conveyor is commonly referred to as the conveyor differential speed, N. Conveyor differential speed is calculated from a knowledge of the rotational bowl speed, S, the gearbox pinion speed, Sp, and the gearbox ratio, RGB: N = ( S - Sp) RGB

(4.9)

When an epicyclic gearbox is used, the conveyor rotates at a speed less than the bowl speed, while with a Cyclo gearbox the conveyor rotates at a speed above the bowl speed. This fact can have an effect on process performance

Decanter Theory

151

with short bowls, if the feed is not fully accelerated to bowl speed on entry [2]. With the conveyor rotating faster t h a n the bowl, the liquor has to get up to speed to find its way to the liquor discharge hub. With conveyors rotating slower than the bowl, the liquor could wind its way a r o u n d the helix of the conveyor to the centrate discharge ports, w i t h o u t ever getting up to bowl speed. Note, to effect scrolling in the required direction, the flight helix on a conveyor using an epicyclic gearbox would be left-hand, and with a Cyclo gearbox it would be r i g h t - h a n d (assuming conventional equipment and operation). With modern technology, the speeds of the bowl and gearbox pinion can be continuously measured with t a c h o m e t e r s or proximity probes, and their signals fed to a simple PLC to work out, and even control, the differential. The PLC would need the gearbox ratio p r o g r a m m e d in to execute this duty.

4.1.3 Conveyor torque Conveyor torque, T, is a m e a s u r e of the force exerted by the conveyor in moving the separated solids t h r o u g h the bowl, up the beach and out of the decanter. It equals the pinion torque, T~,,times the gearbox ratio: T = Rc~ x Tp

(4.10)

It is not easy to measure conveyor torque directly, whereas pinion torque can usually be obtained from i n s t r u m e n t a t i o n on the pinion braking system. This signal is often given to a PLC for control purposes. Conveyor torque is a vital measure in the control of modern decanter systems.

4.1.4 Process performance calculations Consider the two-phase decanter separation system in Figure 4.2. Input is the feed at a rate of Qf, of density pf, with a solids fraction xf, and an additive, often a flocculant, of density pp, at a rate of Qp, with a solids fraction Xp. There are two products, cake at a volumetric tlow rate Q,~, at density p.~, and solids fraction xs, and a centrate at flow rate 01, at density pl, with a solids fraction of xl. From m e a s u r e m e n t s of some of the eight process parameters mentioned, it is required to assess the performance of the decanter. It is normal to monitor the feed rate, Qf, and the additive rate, Qp, with flow meters. Periodically, gravimetric analyses are conducted on samples of feed, cake, centrate and, if necessary, the additive. Performance is judged by how high is the solids recovery, R, and how low is the flocculant dose, PD, w h e n this is used. Recovery is the percentage of solids in the feed that reports to the cake discharge. Flocculant dose, sometime referred to as polymer dose, is the a m o u n t of dry polymer used per unit dry solids in the feed, usually expressed as kg/t db (kilograms per tonne dry basis).

152

Qf x,.

Basic Theories

qp

1

Q,

Q,

Xs

X! Figure 4.2. Decanter Mass Balance.

As an intermediate parameter in the calculations, it is necessary to calculate the centrate rate, Ol, by conducting a total and a solids mass balance across the decanter. Total mass balance: @PS + GPp = O~p~ + O~pJ

(4.11)

Qfpyxf + QpppXp = QspsXs + Olp~xj

(4.12)

Solids mass balance:

Eliminating Qsps from equations (4.11) and (4.12): OlPl - OfPf (Xs - xf ) + QPPP (Xs -- Xp)

(xs-

(Xs-

(4.13)

Recovery of solids is calculated by subtracting the percentage loss of solids in the centrate from 100. Thus:

R=loo

1

Qlx,p,~

(4.14)

Polymer dosage is given by: PD = GXpPp

(4.1 5)

Polymer dose levels are frequently expressed in kg/t db, with the dry basis measure applying to both solids rate and polymer rate. During these calculations, one must take care with the units used. Volumetric flow rates are invariably measured and the density terms are often ignored as they are usually close to unity. However, the density terms must be used when density values are significantly above unity. The gravimetric analyses of the samples should all be total solids (i.e. samples are evaporated to dryness, and thus measure suspended and dissolved solids), which all except

Decanter Theory

153

the centrate generally are. However, centrates are generally analysed in terms of suspended solids. Any dissolved solids in the centrate c a n n o t be considered a measure of a decanter's inefficiency, as it is suspended solids w h i c h it separates, and any dissolved solids attached to the cake by virtue of its moisture content represents a bonus. Dissolved solids in the centrate are usually low and can be ignored, but w h e n they are not, then they should be included in the equations w h e n calculating centrate rate.

4.2 Particle Size Distribution Very few process slurries contain particles of uniform size. A large proportion of slurries, processed by decanters, contain solids which have a particle size distribution which conforms closely to a logarithmic probability distribution. The logarithmic probability equation was derived by Hatch and Choate [3] in 1929: dz

d{ln(d) }

exp[

Z =

~

In(a,,)

.

-

{ln(d) - ln(d~) }2] , 2 {ln(a.)}-

(4.16)

where Z is the total number of particles: d is the particle diameter: ag is the geometric standard deviation: dg is the geometric mean diameter: and z is the number of particles less than diameter d. Integrating this equation gives the formula for a cumulative number distribution: C,, - 5 + 5 erf x/21n(~rg)

(4.17)

where Cn is the cumulative fraction of the number of particles below size d: and erfis a tabulated integral from - 1 to + 1. It can be shown, by using equation (4.16), that the equation for the cumulative weight distribution is similar:

C,,. = ~ + ~erf x/2 In(;.) -

x/2

(4.18)

where Cw is the cumulative weight or volume of particles below size d. Inverting and simplifying equation (4.18): In(d) - al + a2erf -1 (2C,,. - 1) where al and a2 are constants, functions ofdg and ag.

(4.19)

Decanter Theory 155

Since Hatch and Choate first published their equation, special graph paper has been developed and printed whereby plotting the cumulative percent of particles by n u m b e r or weight, oversize or undersize, against particle size, results in a straight line. The mathematics of the distribution are such that one can readily transfer between weight and number distributions, and even area and diameter distributions [4]. The diameter at the 50% line on the graph gives the geometric m e a n diameter for the type of distribution plotted, be it number, weight or whatever. The geometric standard deviation, which is the same for all types of distribution, is given by the size for w h i c h 8 4 . 1 3 % of the n u m b e r , weight, etc., is smaller, divided by the geometric mean size: d84,13 erq -- d5o

dso -

- - - dl 5.87

(4.20)

The relationship between the various means is given by" dgu, - dge3(in ~ 4 , - d.qe2On~

(4.21)

)2

d ql - d~e(In (70)2

(4.22)

(4.2 3)

where dgw is the geometric mean for a weight distribution: dgs is the geometric mean for an area distribution" dg] is the geometric mean for a diameter distribution: and dg is the geometric mean for a number distribution. Figure 4.3 shows a typical distribution plotted as a number, area, and weight distribution on the specially scaled graph paper. From a grapb such as this, the two basic parameters, ~rg and dg, can readily be obtained and from these more pertinent information can also be obtained. For example, the total surface area. Av, of the solids ir~ the slurry can be calculated: 6

At -- dT,, .e} ~M~ ~-~

(4.24)

This parameter is useful for paints and pigments, giving the covering power of the solids. It is also useful in assessing relative flocculant demand, as this is proportional to the surface area of the particles. In general, the decanter removes the coarsest or densest particles from the slurry, leaving a n l y the finest or least dense in the centrate. Knowing the required percentage solids removal from the slurry, the recovery, one can read the desired cut point from the distribution graph. This then gives an appreciation of the feasibility of the desired separation. Experience will tell whether the decanter would be capable of achieving the required cut point. For instance, a cut point of 2 - 5 Bm would be feasible on a decanter with most

156

Particle Size Distribution

Particle Size8 II ? J

1 I llllll[ I illIllil

' ,- : : m : : ~

I III',I

'

,,

,

Microns II 11

I IlIlllllllll I lll[llllIll!

! I

i i::::iiiiiiiii

1 I |II11|11111111|1111111111111111t111111111111Ii11111111111tl I ill|llllllllilllllllillllllll||lllllllll|lllllllI|ll|ll|llll!

| I I

i i ii~ii~iii~ii~iiTiiii~iiiiii~:~iiiiiiiiiiii!.'.~!!

II

Weight

iiii',iil]iil]',iii]iiil}iiii',',',iii?iiii', -

[ 1

IIIIIIII1111 Illlllllllll

i

I i:]~!!!!

I I

] [

! i I

[ II11[I] [[1:1111

l lllilili 1 llllllll I ill',llll

!

I I

I

[11',1111

11111III

!! !!!!!!!!!fill

I i i[]i!!iii!!!:~,!!!!!!:~![!!iiiii!i!i~Numberolatribution

[! 11

1 i

II 11 ll

1 II,II]1111li ! 111111111111

11

l lllllltltlll

11

I [lllllllllll~r

: ..............

o~ . . . . .

I l 1111111 I Illlllll

1 I [

I lllllllllIli 1111111111111 l lllllll~llll

~

1 I 11111Jlpr'][lllllll]iJ~rlll111][111[lll]ll,llJ~r-[l [ | lllJ41r"illl[lllll]~1"ll]111111111111111111L,Fllll[11|1

HI|

I i lh~FlllllllllllL~lltllllllllllllllilllJ'll I 12P1111111lllU~lllllllllillllllllllJ,~lll|l|llllllIl|lllllll l~'31111111J/llllllllllllllllllilllL,Kllllll_llllllllllllllllili

Ill

I

I!! [

[

~ | . . . . . . . . . . .

!%1111 F I

I

lllllllllllllll

i

i[iiii 11 1111,111111[

Ii ~ 11

1

l

11Iillt11111

1 It

I

I

IlllIlllllll

[

l I

1FIlllJ~FIIIIlllllllllllllllllbl~lltlllllllllllllllttllllill

,'1111 'i1 b.,~gllll:,lllllll:l, llll~,lllll;lllllllllll',lllllllllllllll~ll ! i tlllll,,l[llll,|lJIIr-llllil~l,llllllll,llltll|I|illl

II I I 1 Ill il I l l l l l l l Illllllllllll

I1 :1 ::llll:l~llll i1111111,11

I !1 l I1 II 1 II

06

.-" iI ,ii~!iiii ii liiiiiiiiiiiii ~:llllll II l:;lllllllll

il i: '!~i/!.L"4~i!!!!iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii[iiiiiiiii, l,~'-,~l:',llllll:tlllllllllllll:;f|;:l:',',l[llllllll:lllllll,lll

i li :iiiiiiii::i:!i l',',',:lll',l:

:

:

I

~

', l ~

I ~|l',;lll

l|

I [ll~l',lll~ll

', l ~ " l ~ , l l ~ l l l l l ~ ; l l ~ l t l ~ l l ~ ; ' , l l ; ; l l l l l l l ~ ' , l l ' , l f i l l ~ l l ; l ' , t ' ~ l : l

o. iI i1 :iil~ i! i iiiiiiiiiiii II Jl;~iiiiiiii[iiiiii:iO~r I]illll 11 1 I I 1 1 1 1 1 1 ~"i ! 1 I1111,11111111,11111/1111,,11,1I,III1,11 03

..................... ! 1 : I111111 :

9

tt .............................................. Iv I 1 ] l l l l l l l l l l , l l l l ] l l l l l l l l l l l l l l l l l l l l l l l l l l l l i , l l l l l i l l l l l l [ l

1 1 1 llllllll]]ll

I

[~'~'',ll::

I:

]' l : l l l : l l ~ l l l

i

1 i iii111

l l

l

i i i llll

r1-Tli

o~ I -t-i H-tfttt--tt--t tIH-tfH+Ht--i !--I ~ll!llll ;l [ llllllllll~ , I, i~,: .................... I 1[ 11JJ ] I [ 1 1 1 1 1 1 1 1 T I I 1

o~

I I I I i I

1 !~L:IIII I II1:1111

1 1 l 1 llllllll|ll II 1 1111111[1111

I ~1111111 1 :111III! 1 fill!

11 11

ool

o01 oi

,

I

lll;llll

II

I lillllltlltl

l

;

:Illll:',',l:lll;:llllll~l:ll:;::l~:lllll'~:;l:',lllllllllllll'

1

l

1 [ 1 I I Ill

I I 1

os

~

I I I

2

I I I l I l~]Illllll

:

IllIllllllll

lllllIlll~;',

I I

111t11111

I

lllllill~

i

I

I I 1 llllll ! ~ I I1 I1111111]

| |

ii :"iiii~.i ! ::lllll ~

:I

lll;ll~:

|

|

.

i 1i -+i;Hiiiii~-4i--iiiiii;-Ft1 I,, Ill,,, I 1 Illlll ,

l I

t t ]

1 1 [ 1" I

'

l

.... I ...... t f t 1 !11111111I]11 :llllllll:ll

......... ~4 I I i11111! I l :l

:','l:.'.~:

/ /

[

1

II1111111I[

I

I111[I I

1

1 I

1 1

IIIIIil1111 1 1 I 1 IIIII1! I I 11111IIl11111 11 IIIl11! 1

1 I

1 I

I111IIIII11| Iil11IIIIIIII

I

[ I

J

I

~-1~+~.~!i~Hf~+f~-~.~+H~.~HfH~-H-~~-HH-~-~-~--~-~-~-HI~fH~ I I ii;;ii..) f--t--- _1 . . . . . l ..... ; l f l l l l l l l l l l [ l l ~ l ......... l l ; ~ [ l - ...... I II~ ............... ,,,,,, .... i llllll~ ....... If|||,If|Ill, ................ llll:llll,,l~,,l' [ l 1 1I !III; .... t11111'1 IIIIII!I t I II I illlllllllllllllllll]lllllllllllIllillllllllllllllllllllll 111,1111,1I [ i l Illllll 1 1 | 1 IJ lllllll;llllllllllllllllllllllllllllllllllllllllll]lllll 1 lllllllllIIllllllllll[lllllllllll]~lllllllllllllllllllllll I

: 1] II

lI[llllltllllitttltllllillllllilltflllllllllti1111111IIIt|ill

1 IIllllllllllllllllllllIllllIlllllllllllllllllllIllllllllll'll 1 l~llllllllllltlltlllllllllllllllllllllllllllIIll[lllltllllill ~ l~llltl

,11,!I lllllIl~Ill~ I , t lIl.tlll O2

11 I I [ L L I I I I I I I l I I I I I I I I ]

1

] ~

111111111- l !1111111 ? ] IillllllT !

~ I llll~llll 1

1 lllllllIllll I IIIIIIIFHII

iIII1['1

1,,I,1 I l l l . . . . . . I ......

ltlit

I

111111'[ 11111[

II lllll:ll~

I I

:

, 1 1 1 1 11

1 I I

o.I n7

I lllllllllllllllll],~"llllllllllllllllllllll[llllllllllllllllll]

1--1 l-!

i !! !!!!!!!! [-| [Iii, ~ 11[II;I[

111111111111

1111LJ,'FIIIllllllllllllllllllLi#Illlllllllllllllllllllllllllill

, I 111[lllllllll 11 i I I I I I I I I l ] l l . ~ r

....

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

"llllllllllllllllllll

!11111i1 llllll[[

i it~iiii':i'~!

Distribution

ii:~::~i i:~iiii:,iiiiil i i iiii::::ii';iiii::ii~:iiiiii~iiiiiii~i::ii:~ii..A.~O.~:.~.u, ~

:lllllll II l ll:::l":lll : I ::llll:'lllllllllll',::lllllll~,~llll:l',l .... .1| ........ l:l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

:

l 11 I ]1

s

Io

~,:~lIlu~l~ll~II~l~l~llI.Illll~l~ ~o

3o

,IO

Io

II)

Iio

f

1

111111111111

I

I

If

I1 11

Iti111;

Io

Is

Iii

II

Ill

ii

IIIIIL~I t 11IIilliill]

~I~ ~ l~l~l~l~.,l~;+~. IIII

i I

I

llelm

Cumulative % Undersize

Figure 4.3. Cumulative weight, area and number log probabilit!l distributions.

slurries, but, say, less than O.1 pm would probably be impossible, however high the density of the particles. The cut point size is the smallest particle size that has to be settled in the decanter. Technically 50% of particles of that size settle and 50% are lost in the centrate: above that size the separational efficiency increases and below it vice versa. In consequence, the size distributions in both the cake and the centrate will also exhibit logarithmic probability distributions. Figure 4.4 depicts examples of weight frequency distributions for feed, cake and centrate. Note that the centrate and cake lines intersect at the cut point size, and at a frequency level half that of the feed at that size. Hence, there is a 50% split between cake and centrate at the cut point. These frequency distributions are plotted as cumulative weight distributions in Figure 4. S.

16 14

12

10

8 6

4 2 0

10

158

Particle Size Distribution

Particle

lo

Size - Microns

9I I iiiiiiii ii iiiiiiiiiiill

o _ JI ?

.

I

iiiillll Illlllll !i!!!!!"

]l |! "-

ii iiiiiiiiiiiiiiii!iiiiiiiii!*,i!ii'

i iillllllllli ] 111]11111111 ' '''t~!ii~-';.;

1 1 t

i~!!!!!!!ii!iiiiiiiiii~~

2~t~iiiiill i ii ~,iiiiii i i

1 i illllllllillllllillllllll]il,ii ]II]IIIIIIIIIIIlIIIIAd"IdPTI[! i; llllllllllllllll~lllll I IIl]llllllllllllllltltllll]ll ! !!!ii~!!~.iiiii!!!iii~iiii.!!!:!!,!!!!!!!..!i~2~TF_7..r~ii.::iir--~-

I Idr t l l l l l i l l l l l I.,SI IIIIILIIIIII i !!!;.i.;'i!!.;i

9 [ I i!iiiill i! i iiii~iiii!i ] i!ii iiii~iiiiiiiiiiilHHiiiiiiiiiiiiii~g-~;~iiiiiilL-~

:1

! [ !!!!!!!! i! [l[!!!!!ii',i! I

] ii!!!!!!

ii

]

i1 i| iiiiiiiii!iiii!iiiiiltt [1111111 II [ ]1111111|i11 I i ;fill;;, ,r l l ' : l l P , ~';"~:,: t...llllllh~ I ~r iiIIIIII, l

l-

' i I

t I

! I I I

l 1 i ]

,

,

lil~ii~ lllllll

;l i llill;;;;il; '|-t |III;HIIIll ,'T' t t l i t l l : l : ' , : ; ~ p ~ - H ~ - H i ~ iPl IIII',iiii~Ii

t ................ ~......... I I illllllllll]lll.,d~lidl~l

[

i i i,;,;,;~tl;;,~..~-;;~,,"-;;;; I I : l;;l;:~3,--'~,ill,l~;,',ll:l ', : ; ; l : : ' 2 " , l ' r : ' ; : ' , , ' ' : ; ] ; : " ; ; l H - H ~ - i - + - ~ ~ ` ~ ' ~ ' ' ',.,i.-~'GL~-%l',llllll"iiiiilllll~ i

t I 1 ' - ' " " ' ~ I I luHeLy,,"-L-,I"III

i!1illi 1111111 II11111 IIIIIII

I

I I il[lli, 9 I I11],ili'11 I I IAl~r"llllllll l.Ai~r]ll-IPrll',ll

I

ll~l

[ t

,,,,,,~,,.~,,,,,,,,

,,

1 1 1 1

*' .... ii ................

........ iit~lllillifi4.1.,"il ............. FeedlllllllllHl~,-'~!HICenirllte

Illliil[iliill~llllllArllllillll,~llilllliilllllllllllllll I11111 I II il 1 iJ~rl I ] ; ~ 1 i I lllllllllllllllllJ~rllllll":lll~l'.lllllllllllllllllllllll IllllllllJilJllllA~lllll]ll!!lllll]lllillllllllllllllllllll

,,,,,,I,,,,,,~

111

,, ,,ILillli,,,,,

:

iii

III]I',Ii:}IIIIIIIAPlllIIIIIIIIII|Illll ;iiii~i, :::::::::::::::::::::::::::: :[lllili I',II~,,-]II',',t::::H',II::::I~',;lllil!i""--m'll:;;;:;;H;;;l;ll;l;;~; ~ ~ ~ ` ~ ' 4 q ' ~ ~ ' ~ ~ ~";,',IIIIIIIIIIIIII',',iiiiiIII'II

lllil~:~liilll IIIIIIIIIIIIII

lllllllllllllllll

,,,,

]

| I

] !

i ~ i i i i ~ ! ! ! ! ! i ! !i ~!!!!!! !

: : : :.:. :. .:.:. :. .:.:. :. .:.:. .:.:.:. .:.:.:. .:.:. :. . . . . . . . . . . . .: .: .: .: " t t ] - ~: : : : : ~,~

I. . . .I. . .l .l .l l. |.;.'.,.l.l .l .l l. l.l.l.l.l l l l

lliL|ll[ llllllll "iiiiiii

i i iiiiiiiiiiii i ii iiiii~ii i

! i ii!!!!!!iii!!!!!liii!!!!!![][illiis

I. . . .l .l.t.l. :. .:.:. i : : l :

l I1 I I1 i ii

l , : : I I I

I I I 1

~ .: :.: . . . "::I:::: . . . . . . . .:

.......................... 1 Illllllllll] [ | [ ; " ; I I

;;;;;;;;i;;; "i:i::::::~', ;;ii"-:;I:i;; lllllllllll;l IIIIIIIIIII'~

; ;; ~ ,', ' ;: II I II

ill|fll ;;;;~;; ::;;;:i ;;":'': llllllll I',IIIII'

I 1 1 I

ilIIIII1Tlll II1 l I I l l l l I!111111]!i; III11111111

I 1 I I

I111111 I111Iii Illllll 1111111

ii i i ,,, I Ii il II I!

:::::::::::~

llliiIHili! lllllllllllll

i II lilllll II IIIIIII II il 11 11

:

[ i ; ; ~ ~ ; ; I '

1 -: ; : I

i : I I I I

o,,~I l'lllllll II,, ~,:~::',::,::, iiii:!!iiiiiiiiilb:!!!!!!!!iiiiiii ii ~iiiiii i i IIIIIIIIIIIII I ,~,,:,',:iliil!~iii~iiiiiill I IIIIII,, ....... , IIIIIIIII I .................................... ,:,,.iii~,, .... ,,,, ...... , .... ~ .... i iiiiiiiiiiiii o. ........................................ 1

07 ! !

..

,,,:::,,,,

!!!!_~ !I ! !!!!!!!!!!!!

! ! !!!!!!!!!!!N!!!!!!!!!!!!!-7!!!;!!i!!!!!!!!!!!!!!!!!!!!!!!!l!!

i i 111117,1111!111iiiii~i i

I

t

I

o, i

I

IIIiiIII

!1I I Iiii!!!!!!!~! lllllllllill

! I! l!!!!!!!!!!!!!!!!!!!l!!!!!!:!!!!!i!!!!!!!!!!!!ii!!!!!!!!!!!i:ii llllllllllllllllllllllllli{lllili~lllllllllllHillllllll~lll i i iii!!!!iiiiiiiiiiii!!!iiiii:,!iiiiiT!!!!!!!!i!!!i!iii!!!!!!ill

i 'iii!!!! !t iiii,i!!!!!!!l f] IIIIIII If1 / IIIIIIIIII[II / I lillll i lllllllllill !!

i11111

/ "

t

'

lll.:li:

I

7

II;IIIII

i ! !

T [1IIilil I llllllll ] ]1111111

L~-I

~

o,

! !! !!!!!!!. ! '

!i iiiiiii~iiiiiiiiiiiiiiiiii:71iii!iiiiiiiiiiiiii~iiiiiiiii~il

o, i I !!!!!!~

o.$

! !!!!!!!!!!!:

iJiiiiii~ ii:iiiiiiiiii~ii

iiiiii

I I1 Illlllllllllllllllllllllll,:ililllllllllllllllllllllllllllllll llllliiillilllllllilliiill illllilllillillllllilllllilllilll

I1

I ll]lllllllll

I i

1 ! lllllllllllllllllllllllli'.lllliI'illllllllllllllllllllllll

'.I

I lll;l',ili:il

;

i

ti

I IIIIIIIIIIII

l

I lllll;llllllll',lll;lll~.lll;ilillli',lll',lllllllI:ll",',',',lll;l.

1 1 1

llllll lllllllllllllllllllllillll 1 i II111II111I[IIil111111111;-11111.'.11111111111111111II11III]I l llllllFlllllllllllllllllll.;llliI:;llllllllllllllllllllllll~l 1 I1lI1111111111111111111111;;11111 {llllllllllllllllllllllllll

-ii i iiiiiiiiiiii

9 ~ l'"" If I fill'HI ''|' I I IIIII'"I'"'"'IIII I 11 1111111 III! ,

001

00S 0.1

,11,,

,

01

011

Illl

I

i

-i i-i-i i-iiiiiiiiiiiiiiiiiiiiiii~;i~-ii-i4iiiiii4-i-ii-bii~iiiii-iiiiii

t 111111111111 SIIIIIIIII~I.I] I illlllllllll

[i, lI'i I'"" ll~ II,'I"' 1[111

I I1 IIIIIIIIIIIII illllllili!i

I I I

iilliiiiiiiiilllliiillllll:~l;lil;iiiiiiiiiiiliiiiiilliiiiill

'ii "iI iiiiii" i!I I Ii liilIIIIIIN i1 i] iiiiilNiiiiiiiiiiiiiiiiii: 1 lllllll 111111111111 ! lllllllllllllllllllilllll 1I II 1I

i !I I!!!!!!!!!!!!! !! I!!.!!!!!! ! !: IIIIII~III: I II I~IiII~ I ! i iiii!!!ii!!i ! !! iiiili!! ! "

~'

. tt

,

.

IFIIIIII,I, ~ .

';0

.

. lie

iiiii'.ii[ii[iiii[iiiiiiiiiiiiii~i

I1111,: le

4O

l le

l0

I!I,

~

Cumulative % Undersize

i Se

lllllillll

IIl i IIIitlii Ull!lll ii

iiii i i1 i

II : 9

lllill;iiiii

i

;I

iiiiiiii

I

;

I

l',l'.lllltlll

I

II

lllll;ll

I

:

1 i I

llllllllllll I111]111111I Illliilllll',

I 1 1 I

i i i- i, iiiiii~iii '

I

II111111 11 '||''|~'''' :ll'"lllll""""l"lllZl !11111; II1 I11

.

1

I

i

I

i ii

1

iiiiili i . 4

9

i I[ 11il}iiiiiiill i ii iilliiii i "9 lJlll I I I 1 I 1 I111111 1 1 111 I I I '|I~I I |'ill'",,

,

~

1

,

SO

'iS

II

tl

l

I I1 1 II I I1

II

I111111 1 l 1111II11 1 11111111 [

""I"I I '

I111 I I

,

I Ill

ill

Figure 4.5. Cumulative ~veight distributions for examples offeed, cake and centrate.

l

' ltll

9 9 9

4.3 Clarification The separation of solid particles and agglomerates from the suspending liquors, within a decanter centrifuge, has invited n u m e r o u s theories. Few of these theories have provided exact results, which has given the opportunity for m a n y more. This is no reflection on those providing the theories, as the system, let alone the processes used, are quite complex. In a decanter, liquid and solids flow in a helical path within a cylindrical vessel with a conical end. The rotational velocity can vary from the bowl wall to the pond surface. Many theories start by assuming discrete spherical particles that settle in a l a m i n a r flow regime, when often this is far from w h a t actually happens. The expression of liquid from the cake can be as a result of one or more mechanisms. It could be by filtration, compaction, or simple drainage against the scrolling action. Nevertheless over the years a n u m b e r of theories have been proposed, which allow a reasonable fit with m u c h of the data, or specific categories of data. This chapter gives some of the more c o m m o n and usable theories.

4.3.1 Sigma theory For more t h a n 40 years the clarification of liquor in decanter centrifuges, in fact in most sedimentation-type centrifuges, has been characterised by the Sigma theory of Ambler [ 5]. This theory assumes that spherical particles settle in a laminar flow regime according to Stokes' law [6], in a cylindrical bowl rotating at constant angular velocity. Since the first publication by Ambler, several variations have been used which are approximations, or developments, of the original. Refer to Figure 4.6. Consider the smallest particle in the feed sludge that has to be separated, the cut point size, de. This particle has a density, Ps, and settles in a liquor of density, PL, and viscosity, r/L. The feed slurry enters the decanter at a rate of Qf, at a pond radius, rl, at point X at time t=0. By the time the particle traverses the clarifying length of the centrifuge, L, in time t=te, the particle m u s t settle to a radius r 2, at point Y, the bowl internal radius, if it is to be collected by the conveyor. The centrifuge rotates at a constant a n g u l a r

velocity, w.It is assumcd that the fluid i n the bowl also rotates unil'ormly at angular velocity w , and travels along llie howl in plug flow. It is furthcr assumed that the particle being considered is homogeneous and spherical. settling i n a lairiitiar flow regime. A t time t;f. the particle has a radial vclucity \',and a constant axial velocity dv,,It is also assumed that the particle travels a negligible distance from the pond sur!ace before it reaches its terminal velacltg. The axial velocity is given by:

Thus: X L

t,, - - (rf

-

(4.2 h )

Qf

'I'he radial velocity at any time t is given by: \Tr

=

7 W-

\rs

n

I'

--

(4.2 7)

where 17, is t h e Stokes settlirig velocity, given by:

(4.1 x) where rl, is the parfiule size at the cut point; p s is the dcnsity of thc pilrticle; pL is the density d t h liquid: ~ and r/is the viscosity of thc liquid.

Now: ?,

dr

-- dt

(4.29)

Siihstifutirig equation (4.29)into (4.27) and integrating between the limits

of r=rl to rL ;ind !=(.I tot,:

I
'rllc terms (111 the right-haad side of equation (,4.31j consist of vs. which is solely i1 limction o f t h e process material, nnd t h e remainder olthe terms, that ;ire snlttly fiinct.ions ol' t.he ccnt.rifuge. T h e w I;itt.er lerrns are cnllixtively knnwn as Sigma. I:, the c:larific:ation c:iipiic:ityofthE:c:crilrifiige:

(4.32) Sigma has units of arcs. which is consisterit with rior~.r:erilriTi~gnlclarifying cquipment. Frorncquations(4.31) and (4.52):

Bquatiuii (4.32)is \.he eqiiation For Sigma prcierred todijy. i I n d is particularly recorr~~iieridd when deep pond dccanlers ( r lir.?'-'O,h5) are uscd. Whcii Amhler- first derived his formula. only shallow pnnds ( r ,/ r ! > 0 . 7 5 ) were wed. and he uscd different starting assumptions l'or his derivation. With a shallow pond it is assumed that 11ir incomiiig iced distributes itselr cvenly t.hrvughuut the depth or the pond, iri the annular plant: o f t h e puirit of rnlry. The theory thcn develops thc snmc cquatioiis in the some way, assuming that half ofthc particles of the srnallcst size that have l o be separatcd will be removed, This is consisterit wi1.h Ihe definition ofcut pnirli. The last particle of the hall'ol'the smallcst particles 10 he setilcd will star[ a ! il radiiis r, at thc fccd point, at which half of all particles will start inward arid Iialfoulward oftliis point ir'i this plane, S o :

whmce: ( 4 . 35 )

NOW substituI.ing equation (4.29)into e q u i l t i ~ n( 4 . 2 7 )agaiti. hiil this timc integratirlg het.ween t h e limits of r = r l a i d r=rx and t=O t o t=t,:

(4.36)

Solving equation (4,361, eliminating rx using equalion 14.75) and eliminating t, using equation (4.26);

(4. .3 7) uj

(4.38)

= 2u,.c

whcrc Z this time is the truc Arnblcr Sigma given by: (4.39) Cornpare 1: o f rqiial.irm (4.3’4) with E of equaiion ( 4 . 3 2 ) . Nole I.hc extra numeral 2 in ryuatiori ( 4 . 3 X ) cornpared wilh equaiiori ( 4 . 3 3 ) . This is t o he cxpcctcd if all thc particlcs that h ; ~ v cto be separatcd h a w the advantage of starting at half pond depth! I t will also be wen in the literature 171 that approximations are sometimes made for 1 tie logarithmic term in equation (4.321to givc: (4.40)

With shallow ponds it Is sometimes considered that the g-fnrce is cnnst.arit, iri which case equation (4.27) would be rewritten; (4.41)

Substituting equation (4.29) i n t o ( 4 . 4 1)and irikgraliag:

(4.42) Eliminating t , I‘rorn equations (4,421 and (4.26)and rearrangirrg gives:

SJ =

7&2(rl

2Y

+ Ul)

(I-;

.-

rf)

. (r2 - h )

.v>

(4.43)

Decanter Theory

163

= 7rLg:.(r2 + rl)Vs

(4.44)

= 7rLg ,~.DAv. Vs

(4.4 5)

where g'c is the mean g-level in the pond; and DAy is the average pond diameter. Alternatively equation (4.43) may be written: V , Os = ~-TrOc.V~

(4.46)

where Ar is the pond depth: and V is the pond volume. For this derivation of clarification capacity, it is readily deduced that" E = 7rL

W2

r~ - r~

(4.47)

g r2 - - r l

In the graph of Figure 4.7 the various formulae for Sigma developed so far (equations (4.32), (4.39), (4.40) and (4.47)) are compared for various ratios ofrl/r2. The common factor 7rLw2/g is removed and r2 is taken as unity, for the graphical comparison. By means of Figure 4.7, a number of observations may be made. The expansion of the logarithmic term to give an easier formula for Ambler's Sigma is a very acceptable approximation. The even simpler formula last developed above is also acceptable for shallow ponds (radii ratio greater than 0.75). However, there is a significant difference for the formula used for deep ponds. Notice that with zero pond radius, the shallow pond versions of Sigma have finite Sigma values while the deep pond version has a zero value. This is because a particle starting at the centre line will experience no g to initiate its fall, while those which by definition start half way into the pond, or are subjected to a mean g throughout the pond will always have a finite settling rate. However for practical designs the radius ratio will always be appreciably over zero, generally in the range 0.4-0.8. It will be seen from the various Sigma formulae that increasing the length of the bowl increases Sigma pro rata. Thus, in this respect Sigma is additive. Some like to include the Sigma value of the beach in their formula [8], especially when feeding on the beach. For this, using equations (4.46) and (4.40): E = 2 7 r w 2 Lc

r~+

r~

+

(r~+3r,,'2+

)

48)

where Lk is the wetted beach axial length" and Lc is the cylindrical length of the bowl.

164

Clarification

2.50 i

I I 1

2.00 U

~" 1.50 ell

o v~

J

9 o a'~

DID r~

a''~

1

I

i

9

~. 1.00 oO~ o

m

~

'

o

.r

0.50 o

D e e p Pond.

I'

Simplified. Equn. 4.40

[

!. . . .

Ambler.

i I

t

!

....

0.2

0.4 Pond

Radius/Bowl

I

Equn. 4.32

Equn.

4.39

,

Approximate. Equn. 4.47 l

i

1

,

0.6

0.8

1

0.00 0

i

I I. . . . . .

I

I

"

I

Radius

Fignre 4.7. Graph comparing the various.fornz.hu' for Sigma at vario.s pond depths.

Considering the assumptions used in the derivation, and the approximations used, one could question whether the use of the simpler equation (4.47) would not suffice, for use with shallower pond machines at least. It is the ratio of Sigma values which is used when scaling from one decanter size to another. Using equation (4.48), the ratio will be little affected as the extra term will increase by approximately the same ratio with geometrically similar machines. Another expression, in place of Sigma, uses an empirical formula taking a nominal bowl radius, the ~ bowl radius, r~ (which equals three quarters of r2). This expression [2 ] is termed the "area equivalent", Ae~ and is determined by"

Ae3 =

r2

L~ +

cota

(4.49)

where a is the beach angle ( half included). More often the abbreviated form is used, which ignores beach volume and uses clarifying length"

(4.50)

is then used as a scale-up factor, in p l i i c t : of Sigrna. Its usc is simply a mdttcr ofchoice arid habit. A]] thc formulac? iridicate that a bcttcr clarifici1I.ioriq i a c i t y is achicvcd at the shallowest pond depth, whereas in practice it is generally t.he opposite. 'rherefore, thc simple formula is gelierally considcred sufiicieot for p r ~ r i i c ~ l purposes. Howcver, when scalirig kern one macliinc to another, it is imperative t,hat i:he s ar ~i efurmula is L I S for ~ hnt,h rnachiries. It is also rcc.orniiicnded that one should riot noritlally scale hrtwettri machiries of dissiniilar geornctry. h-1

4.3.1.7 Usirig sigma

11 is unusual 10 use aiiy of tbcsc formulac 10 computc the cilpiicity of';] single machine. Their rriost effective and reliable use is in scaling data Irom OPC geometrically sirIiilar machitie to aiiothcr and assessing rclative

perform ii (1 ces ,

Eliminating \',Criir.i~cyuittiniis( 4 . 2 8 ) and ( 4 . 3 3 ) :

&. 4 (Ps 01,h/ c 1X.lI -

-

( 4..5 1

or (4.52) 'I'aking Inger-ithmsoflinth sides n f s q u a t i o r i ( 4 . 5 L ) :

L

l I l ( 9

y

ln(d,)

( 4 . S '3 )

T I is known from equation ( 4 . 1'7) that thc pcrccntage over nr uiidrr size is ii Ir)gari1hrnir: pi-ob;ihiliI.y i'i.inr.tion of pal-tick diameter. 'I'hus. combining equations (4.19 ) and ( 4 . 531, a logarithmic prohahilily relationship between Qr/Y nnd solids recovcry is obtaincd:

I'liitting ur/C againsl solids recovery will f h u s give a good correlation [ 9 ] . l'lottirig on logarithrnic probability papw will produce a straight linc 1101. 'l'he saiiie straight lirie is obtained for data from different decanters. prcfcrably

166

Clarification

of the same geometry. However, it m u s t be noted t h a t this only applies to process materials with solids exhibiting a skew Gaussian (logarithmic probability) distribution. W h e n scaling from one m a c h i n e to another: Qf2 = ~Z2

(4.55)

where the subscripts 1 and 2 refer to centrifuges 1 and 2, respectively. W h e n machines of different geometry are used then one needs to take into account the relevant efficiency, (, of each design, when: Q f 2 __

~2 Y~2

(4.56)

4.3.2 Sigma enhancement The use of conical discs, or angled vanes, on the conveyor will theoretically e n h a n c e the Sigma value, the clarification capacity, of the centrifuge. To estimate the Sigma value of a stack of conical discs, the formula used for disc stack centrifuges m a y be employed [ 11 ]: ED =

27rnD

~d2

3

g

.(r~ -- r~). cot O

(4.5 7)

where ED is the Sigma value for the disc stack; nD is the n u m b e r of discs; r ~ is the outside radius of the discs; and 0 is the half included angle of the discs. The total Sigma value for the centrifuge is obtained by adding ED to the Sigma value calculated for the conveyor section between the feed zone and the discs. There is little published on the effect of longitudinal angled vanes, but the equation is derived in a similar fashion to t h a t used for the disc stack centrifuge: nvLvw 2

~

7rLvw 2

Ev = 2g. c o t , (r~ - r~) -~- ~ r ~ g

(4.58)

where Ev is the Sigma value for the vanes; Lv is the length of the vanes: nv is the n u m b e r of vanes; and ~bis the angle between the vane and a radius. If the vanes do not extend the full length between the feed zone and the centrate discharge, t h e n the Sigma of the plain section needs to be added to Ev to obtain the total Sigma value for the centrifuge. Caution is needed in using these extended Sigma values, particularly for the angled vanes. This is because flow t h r o u g h the vanes or discs can channel, to

Decanter Theory 167

take the easiest path. This will reduce the effectiveness of the devices. Good designs, therefore, will endeavour to ensure even distribution of the flow across the vane and disc openings. Even then as liquor flows from the outer edge of vanes or discs, towards the centrifuge axis for discharge at the weir lips, considerable changes in kinetic energy occur. This can cause very complex flow patterns, turbulence and Coriolis effects.

4.3.3 Flocculant requirement Chapter 5 is devoted to flocculants. However, it is appropriate at this j u n c t u r e to mention the need for polymers in some process applications, particularly effluent applications which are a large m a r k e t for the decanter. In these applications, without a polymer flocculant, it would not be possible to employ a decanter. It is clear from equation (4.28), Stokes' law, that the settling velocity of a particle, Vs, is proportional to the square of its diameter. Thus doubling the particle diameter will increase vs by a factor of four. This results in greater separation efficiency. The objective of flocculants is to c h a n g e the electrochemical forces on the surface of the particles, so as to bind t h e m together such that they act as one large particle. Once flocculated, these particles must be handled carefully so as not to break them up mechanically. This is especially true w h e n processing them in a decanter. In most applications, the a m o u n t of polymer used is just sufficient to flocculate a sample of the feed. The a m o u n t necessary, as assessed in the laboratory, is generally the a m o u n t used in practice on the centrifuge, plus or minus a small fraction. However, recently there has been considerable development in decanters and their use in obtaining extra-dry cakes from compressible sludges, particularly effluents. In these instances, the consumption of polymer has increased considerably. The a m o u n t of flocculant needed increases as the extent of dryness required in the cake increases, and it increases exponentially. The a m o u n t of flocculant required also increases with the feed rate to the centrifuge [ 12 ]. In practice, on a "dry solids" application, the polymer used will be two to three times t h a t which would be used on a standard application with the same process material. There has not been a theoretical formula proposed to quantify polymer demand. However, the available data suggest a format similar to equation (4.59): P D - kl + k2.e (~:'-k~

(4.59)

where kl, k2, and k3 are constants. Practical data can be very erratic, as it is easy to overdose w h e n striving for extra dryness. When assessing the m i n i m u m polymer requirement, it is necessary carefully to adjust all operating parameters, to ensure performance is at the limit, without contingency levels added.

4.4 Classification Classification, the fractionation or separation of particles by size, could be considered as merely inefficient clarification. The cut, or desired classification, is adjusted by altering the centrifuge's efficiency. This is most easily done by altering the feed rate or bowl speed. However, adjustment of pond depth or differential may, in certain circumstances, be used. In a thick suspension hindered settling occurs, when there is a tendency for the larger particles, which should settle, to get held up by the dense concentration of the smaller particles. In these circumstances higher differentials could be used, to agitate the suspension and so release the heavier particles. The disadvantage of this is that the cake or "heavy fraction" tends to be wetter, as a result of the higher differential, and thus entrains larger quantities of the smaller particles. To correct this, a shallow pond is selected to allow release of liquid containing the smaller particles on the dry beach. In some classification applications, the required cut point is very sharp and the rheology of both separated phases is such that they remain quite fluid. In this type of application the pond used would be relatively deep, and separation would be akin to a liquid/liquid separation, using a hydraulic balance under some form of baffle. Very occasionally there will be found a classification application where it is required to separate two distinctly different particles, such as in the refining of minerals. In these cases the two different substances to be separated may have markedly different densities. This is particularly acceptable and quite advantageous when the denser material comprises the larger-sized particles. However, if this is not so, one must consider a combination of density and particle size for the cut point of each of the two substances, in relation to Stokes' law. One could visualise the situation of a large, low-density particle settling faster than a high-density, small particle. Thus for such a process to be feasible: 2 dc2h(Psh - Pf) > dc,(PslPl)

(4.60)

where dch iS the required cut point size of the heavy fraction; dd is the required cut point size of the light fraction; Psh is the density of the heavy solids; and P~l is the density of the light solids.

Decanter Theory 169

Each of the two solid constituents will have their own size distribution from which a cut point size can be chosen to give the desired purity of product and yield. Poor efficiencies can occur in some classification applications, due to n a t u r a l agglomeration of particles. In these applications, the use of dispersants is quite common. Dispersants have the opposite effect to flocculants, and can be equally powerful.

4.5 Three-Phase Separation Decanter three-philse operalion involves t:he sep;rrai:ion ol‘ two immiscible liquids from i t solid. The two immiscible liquids are generally nil and water. This could bc a wastc oil application or thc separation of a vcgctablc oil. such as palin or olivc oil. To put tlic decanter into operation two weir heights or cquivalcnt h a w to he set relative lo the solids discharge level, as illustrated in Figure 4.X. Firs1,ly the weir height, governed by radius r l , is set to fix the extent of the dry beach rcquired heforc solids arc dischargcd at radius r.+ The radius I’h has t.heri t o he set. Lo create a hydraulic balance between the two liquid phases, to m:iiritairi Ihe equilibrium line at rr, where required. Thc prcssurc at a n y radius. I’, in ii rotatirig ceritrifuge is given by ! I , . where:

(4.h 1)

Thus, in the three phase ceritrifuge, thc pressure at the cquilihrium line i s

P, where: (4.h2)

where pl is t h e dcnsity of the light phasr; arid pi, i s the dcnsity nf the heavy phase. The choice ole-line position dcpcnds upon R number orfactors. ‘I’licvolumc nf each phase in the bcrwl could bc chosen in propurtion to the volumes of each in t.he feed. ‘Thco approximiiiely the valuc of Q/Cfor each phase would he the same. Howcvcr. if the separa1.ion of one phasc from another i s relatively more difiicult than vicc versa. then extra vnlumc could bc given 1 . 0 orit: phase in the bowl to improve its clarification efticiericy. Alternatively. the purity or o r i f phase may be more important than the other. arid then bias would be given to t,he morc important phase. Nevertheless. care has to be takcn in setting the ttline in order not to allow breakthrough of one phiist: ii1t.o the other. When thc flow rate o f one or morc of the phases is high, cresting over the weirs call movc the e-line cc-)nsiderablp. and adjusl.ment of the wcir hcights

Figfin, 4.8. Hydraulic bnlattw it1 thrcc-phase sepnmtioti.

Decanter Theor!! 171

172

Three-Phase Separation

will become necessary. Back-pressure from a centripetal pump or skimmer pipe will require recalculation of weir settings. Working with three-phase separation will require revision of the formulae used for performance evaluation. Not only will there be interest in solids recovery and clarity (absence of solids), for both liquid phases, there will be an avid interest in what has happened to the oil. How free of water is the oil? What is the recovery of the oil? How much oil is left in the cake and the water phase? In some of the three-phase applications, water is added to dislodge the oil from the solids. With water addition there are two input streams, feed and water, and three outlet streams, light liquid phase (oil), heavy liquid phase (water) and cake (solids). Each stream is analysed for the three elements, oil, water and solids. Four of the five streams are monitored for flow rate. The cake rate would be difficult to measure accurately. To analyse performance, a mass balance across the centrifuge is performed for the three separate elements and the total mass, after which the cake rate is eliminated. Formulae are then developed for pertinent recoveries and purities. It is not necessary to develop these here as the pertinent formulae will depend on the application, and in any case the development is similar to that already shown for two-phase separation (see Section 4.1 ).

.

. .

4.6 Thickening

.

...-

'I'he decantcr thickening process i s ;I part.ia1 removal of liquid froni a slurry, It is similar to clarification a n d cliis.;iI~caI.ion.The main difftrcncc is in thc wag t.tle (:cnirifuge is operated a n d controllcd. By definition t.here i s not t h e same interest iri petting t h e cakc SO dry. This oitcn cnnhles extra c i ~ p i ~ c i ttoy he achieved.

SimiI:irIy t o c:IassiIic:at.ioii. there ar-e t w o ways to opcratc thc decanter. Without i.he riced for close differential control. the pond IS set just below the c:~kedischarge level and t h e differential aild f w d ratc ~ I * set C hiEh. SO that with some cresting A wet r a k e is virlually washed o u t . Thc afterriative is to sef lhr pond finely, iind il biltile disc o r (:OIIC is huilt i r i l o I h e u m v t y u r . Very ilcc:i1rilt.e c:nnt.rnl of' t h e difftrcntial urisiircs thii thickness or cakt. required. T h e p o n d setting is tincly tuncd to creatc a hydraulic halancc bctween the ccntrate and sllRhtly heavier cakc. Hawevcr, with the crcst which incvitably occurs with high throughputs. thc pond setting is usually slightly positivc. I n decanter terminology a positive pond is one wherc thc liquid discharge is at ii larger diamctcr t h a n t h c solids dischargc, and a negative p r i d is the opposi t.e. M a n y thickcning applications are with process materials that do not fully dewaler, or arc tlifiir:ult I(.) tlew;it,er. Neverthelcss, pcrhaps surprisingly. sume ofI.herni r:ari be Ihirkened without t h e use nltlocculants. o r at Icasl with rrlur:h reduced yuaritii ies. Occ;isioiinlly, process muterials arc rcquircd to be thickened that nnrrriiilly d e w a k r readily. With these. flocculant is usually rcquirecl. b u t ttiercr is more of ;I problem to prevent complctc dcwatwing. Afiaio there a r e two control options. One riiher L I S ~ Shigh diftcrcntials a n d dccp ponds or o w carefully controls the dirferential. A further nltcrnntivt. is 10 fully dcwater, without too dry a cake. and t h e n back-mix wi1.h ;I feed by-pass with the c.ake discharge. This last mcthnd has advan1agr.s in rcduced overall polymcr consumption a n d high overall recovery ;is t.he by-pass is 1oOw, rccovcry without polymer usage. 'I'ht! coril.rol lor this method is by mcalls of control 1t 11g t h c hy - pass rate . 'I'hicketiiiig control is a milt.t.er n i balancing t h c volurnctric scrullirig rat.t! with the a m o u n t of solids being fed 1.0 t h e decanter. The r a t c of solids into i he decanter is the product of the feed rale. Or. a n d the l e d solids content. I,.The

174

Thickening

scrolling rate, Ss, of a particular decanter would be proportional to the differential, N. In a thickening process one might expect the scrolling rate to be an inverse function of cake solids content, because as the "cake" becomes thicker it gets more viscous and the scrolling efficiency reduces. Thus: N

s~-

Xs

(4.63)

Dividing the scrolling rate by the solids feed rate gives an empirical "thickening factor", ~, where: N = (4.64)

O.sxsx~

As the factor 9 is increased, cake dryness decreases and solids recoveries increase, and vice versa. Very good correlations can be found between cake dryness and ~, and with solids recovery and 9 for fixed pond depths and fixed polymer dosages.

4.7 Conveying In the past, the vast majority of decanter applications were limited by the clarification capacity of the centrifuge. With the development of the decanter, better designs, higher g-levels, and better knowledge of the technology, today many applications are governed by what happens at the other end of the machine. Performance is now often limited by how efficiently the solids can be dewatered, and how efficiently and at what rate the solids can be discharged. 4.7.1 The Beta theory

The decanter capacity, related to a solids conveying limitation, is indicated by" Oj p f 3 z f O( N P n l r 2 ( r 2 psXs

-

rl)

(4.65)

where xf is the solids concentration in the feed; Xs is the solids concentration in the cake; pr is the density of the feed; Ps is the density of the cake; N is the differential between conveyor and bowl; P is the pitch of the conveyor: and nl is the n u m b e r of leads or flights on the conveyor. This is k n o w n as the Beta theory, derived by Vesilind [ 13 ], where: fl -- 2 7 r N P n , r2 (r2 -

rl)

(4.66)

Thus for solids conveying limitation, scaling from one machine to another:

Qf2

=

32

(4.67)

where the subscripts 1 and 2 refer to the centrifuges 1 and 2, respectively. The Beta value calculated by the above formula does not take into account scrolling efficiency, nor the fact that the depth of solids between flights reduces to a m i n i m u m at the point of discharge. However, if scaling is between machines of similar geometry, efficiencies will be similar and dimensions of

176 Conveying the flight area at discharge will be proportional to that in the cylindrical section. Thus, equation (4.65) will be valid. One of the basic assumptions for equation (4.65) is that the cake fills the space between the flights, from the scrolling surface to the pond surface, throughout the length where capacity is being considered. With non-cohesive cakes, those that tend to be fluid or creamy, a head of cake touching the conveyor hub at discharge will not be possible and here the more appropriate formula would be, instead of equation (4.65): Of pfxf cx NPnlr2 PsXs

(4.68)

When thickening is involved, equation (4.68) is certainly more appropriate. If liquor levels are close to or above the solids discharge level, some fluid mechanics technology may need to be invoked with some attention to cresting heights. 4.7.2 Conveying on the beach

It is important in decanter work to have a good understanding of the factors that affect the mechanism and efficiency of scrolling [10], particularly up the beach. Consider the general case of a single particle, mass m, being pushed up the beach, angle a, by a flight of the conveyor. Consider the forces on that particle (in Figure 4.9). Constructing a vector diagram of these forces, there is the normal force, FN, perpendicular to the face of the flight, the scroll friction at right-angles to this, the weight of the particle resolved down the beach parallel to the bowl axis, and finally the friction from the beach. The direction of the beach friction is indicative of the direction of travel of the particle up the beach, at angle 0 to the axis. It is worth considering what factors minimise 0 and thus maximise scrolling efficiency. Reducing scroll friction by polishing and smoothing is one factor. Maximising beach friction is another: this is generally done by ribbing or grooving, which effectively polarises the friction, to stop slippage when cake rotates with the conveyor. This means that it is easier for the cake to slide up along the grooves, which are in line with the centrifuge axis, than to shear over itself at right-angles to the grooves. Reducing the acceleration force on the particle, the weight resolved down the beach, by reducing bowl speed or decreasing beach angle will improve scrolling efficiency but will have adverse effects on other features of the process. The same goes for reducing conveyor pitch. It will be noticed that the weight resolved down the beach has a buoyancy component. When the particle leaves the pond for the dry beach this buoyancy effect is lost. Instantaneously the weight resolved down the beach

Decanter Theory

o ~ ,\

177

Direction # of Motion r ,, . ,, .m g .sin (x Fs

m ' g = ~d___~(p_ pt)gr

m'g .s"

1 S w _._..Jl ~s - 1 + tan~-tanO - P

S.~

Best ~s = p = co,,,2~

Figure 4.9. Force Vector Diagram.

markedly increases. It can increase so much with some materials, that scrolling ceases. This is where the use of a baffle disc feature can be effective. A baffle disc is fitted onto the hub of the conveyor at the foot of the beach to restrict excessive cake flow, and the pond level is raised above the solids discharge level. Therefore, theoretically the cake is below the pond surface right up to the point of discharge, thus m a i n t a i n i n g any beneficial buoyancy effects. Scrolling efficiency is also maintained, and enhanced by means of the differential hydraulic pressure across the disc.

4.7.3 Dry solids conveying When a decanter is operated to obtain the driest cake from a compressible sludge, the decanter bowl will be virtually full of cake [9], from front to rear, with next to no volume of clear s u p e r n a t a n t . The dryness of the cake, as will be seen in Section 4.9.3, with a constant torque, is inversely proportional to the volume of the cake. In a dry solids decanter, there is generally a restriction, for example a baffle disc, against which the conveyor compresses the cake. Sometimes the cake discharge aperture forms the restriction or acts as an extra restriction in series

178

Conveying

with the baffle. The maximum possible t h r o u g h p u t of the decanter is proportional to a combination of these restriction areas, to the conveyor differential and also to the cake dryness. It is proportional to cake dryness because the dryer the cake, the greater is its density and thus the mass per unit volume being conveyed. Thus: QfR pfxf = PNAr PsXs

(4.69)

where R is solids recovery: xf is feed solids content; Xs is cake solids content: Ar is a function of the area of the conveyor restriction; and P is conveyor pitch. It is generally found that, with a properly operated dry solids decanter, the scrolling capacity at the point of maximum restriction, the smallest area, is greater than that calculated using equation (4.69), but less than that predicted by the equation at any other restriction in series.

4.8 Conveyor Torque Whcn conveying solids in a "non dry solids" modc, thc majority of thc torquc is uscd i n conveying solids up thc bcach. This tfirquc is given by [ 101: (4.70)

where To is heel torque; A, is wel. area ol'heach: A, is dry iire:i rIl'he:ich;and k., is a corisl.arit. greater t h a n 1. deperiding upon t.hr type ot'snlici. The heel torque is given by: I'o hfm (4.71 ) whcrc A,, is the total surfacc ai-ca ofbowl and beach. In thc case of campressiblc cake and where the driest solids are needed, ii i s neccssary to work out what pressure is required or1 the machirie hri ng scaled to. and rriodulate the dilTtlrerit.iai1.0produce the torque to providc:that. In this type of process. hydraulic pressure aids s c r o l h g , and the torque is not solely applied to thc Bcach section. 'I'liis is discussed more lully in Scctian 4.9.3.

4.9 Dewatering and Washing The washing process is included in this section, as the extent of washing feasible will deperid upon the arriouiit of dewatering possible aflerwards. 4.9.1 Solids dewatering

To attempt to quantify the wtent. of dewatering within :i decanter it i s necessary to appreciate the type of dewatcrina taking place. Even t.hen i t may be difficult. l'here arc a nuinbcr of ways that moisture separates frnrn t . h coke. It could be by fillr:ition t.hrough the cake back into the pond. oiicc thc cake has left the wet beach. 11 r:ouId hc hy simple drainage. through or ovcr thc calic bcd agoinst the scrnlling art ion of !.he conveyor. Squeezing out thc moistui-c, by compaction of thc cake. is another alt.ernat.ive. In conventional dewatering, drying on ii dry heaoh is used frequently, where cotiveyor diffcrcntial is minimised wit.hiri ltie lirriits of the required scrolling capacity, arid the pond levcl is minirnised l o increasc dry beach length, In h e dt!waiering of coiiipressiblc cakes, ;IS m u c h pressure as pmsiblc is put on thc cake. before ndvrrsely affecting capacity or ccntrate clw-ily. In dry solids operation, Which will hr discussed in more dctail in other sections, it has heen reported [ l ] that increasing cake height, and thus pressure in the centrifuge howl. improves dryiicss capahility. 'J'hus. for such applications. i t would he ndvaiilageous to maximise pond depih. '1'0 estimatc thc prcssure within the pond, refer t o equation (4.61 For 11 cnkc drairluge dewatering. scale up would be by nnc of the cake ( : o r ~ v e y a ~ i cformulac. e equ;jtions ( 4 . h 5 ) o r (4.6K).and pond d e p h arid conveyor differential would he judiciously adjusted to ensure [.heresirletice timc in the crit.icnl areas was kcpt the s m i e , Thc coarser and rncire crystalline mi~t.erialswill rely 011 intcrstitia! drairlagt., whcn it will be a matter of ensuring t.hat [.hesolids coiivcyance ri1t.e0 1 1 Ihe dry beach is less than the drninagc velocity. Herr t.he final moisture will be a l'iinotioii of the surface area o f the cake, Firlcr ljut still particulate rrialerials will take advantage of rcsidence t.ime on the dry beach.

Decanter Theory

181

The more cohesive organic materials, such as clay-like substances and municipal effluents, can drain by d e c a n t a t i o n with the squeezing action mentioned. Dewatering will c o m m e n c e u n d e r the liquor surface. Thus, it will be appreciated that there c a n n o t be a generalised equation for dewatering as there is for clarification. As with clarification though, special designs and devices can be incorporated, especially with respect to the conveyor, to e n h a n c e the dewatering capability of the decanter.

4.9.2 Washing In some decanter applications it is required to remove, from the solids, some dissolved impurities in the liquid held in the cake. This is achieved by spraying rinse liquid onto the solids as they are conveyed up the beach. Admitting the rinse too far up the beach can cause problems, by w a s h i n g the cake back into the pond, or producing too wet a cake. W a s h i n g too far down the beach risks poor w a s h i n g efficiency, w h e n rinse by-passes the cake, by streaming over the surface of the supernatant. To maximise rinsing efficiency, it is necessary to keep the cake flooded with rinse liquor, but not to add excess unless it flows t h r o u g h the cake, rather t h a n over it. The ideal location for admitting the rinse, therefore, is at the junction between the wet and the dry beach. For o p t i m u m location of the rinse nozzle(s) it is necessary to have a good appreciation of the cake profile around the wet and dry beach junction. Consider the idealised system depicted in Figure 4.1(). Feed enters the system at rate Of with suspended solids content xf and dissolved impurity content yr. Cake is discharged at the right at rate 0,~ with solids content x's and impurities y~.

Qf

Q,, x,.=o y~-O

x~: y~

e~ a

_0_0 0 u u|

9

-

0

QI

QA

X~: y~

x,:y, Figure 4.10. Rinsing.

182

Dewatering and Washing

Rinse flows countercurrently at rate Qw with solids content Xw = 0 and impurities Yw = 0. The centrate, which includes the spent rinse, flows out at the left at rate Ql with solids content xl (which will be assumed equal to zero) and impurities content Yl. In the system considered, the cake remains completely flooded without excess liquor above it. Impurity level in the feed is If where: If = lO0.y/%wb = l o 0 . Y f % d b xl

(4.72)

Similarly, in the cake: (4.73)

Is = l o 0 . YS%db Xs

Volumetric dry solids flow is QsDwhere: (4.74)

Ps

where ps is the solids density and ps is the cake density. The voidage flow in the cake at discharge is therefore Qv where (4.75)

O,, = Os - OsD

Assuming that the solids are impervious, and without surface adsorption, then Ow must equal or exceed Qv to remove all the impurity. If Qw is less than Qv then the impurity level of the solids as they emerge from the pond, Ie, will be proportional to the difference between these two figures, assuming plug flow: le --

lO0. Yf. Q Qv" ~--' xs 0,,

~0

(4.76)

After emergence from the pond, further dewatering takes place on the dry beach, assuming that there is a dry beach. Thus, the impurity level of the discharged cake will reduce to Is where Ix = Ie o-Ss Ps(1 - xs) 9 " O,, "Pl

(4.77)

In practice solids are not impervious, and diffusion has to be relied upon to reduce impurity levels. Consider a modification of Figure 4.10, as in Figure 4.11, to include diffusion.

Decanter Theory

183

Ow

Of

c , = y,, = 0

el

................................._~ Diffusion Q,

I

C 3 = C~

O. C2

Figure 4.11. Rinsing with diffusion.

The c o n c e n t r a t i o n s cl to c4 are the impurity c o n c e n t r a t i o n s in the liquor, as s h o w n in Figure 4.11. Thus: c3 - (1 - xl)

(4.78)

,l]s c2 =~1~ - x ~ )

(4.79)

Yf t'~ = (1 -- Xf)

(4.80)

c4 -

y,,, -

0

(4.81)

Now: (cs - c4).0,,.

-

(cl

-

c2)(l

-

x.~).Os

(4.82)

If the diffusion process is 100% efficient: c3 - cl Substituting equations rearranging"

(4.81)

r162

and

(]

(4.83)

(4.83)

-

x~)G

into

-

(1 - x~)O~

0,,,

equation

(4.82)

and

(4.84)

However, the diffusion process is seldom, if ever, 100% efficient. The mass transfer factor, JD, for this type of situation is given by [ 14]"

184

Dewateringand Washing

]D = -hD - --(Sc) 0"67 l/c

(4.85)

where Sc is the Schmidt n u m b e r , a dimensionless group: Sc = n pD

(4.86)

and h D iS the mass transfer coefficient; Uc is the superficial velocity of the rinse; 7/is the rinse viscosity; D is the diffusivity of the impurity: and p is the density of the rinse. The mass transfer factor, JD. is a function of the modified Reynolds number, Rein, where: uc4P Re,, = (1 - e)r/

(4.87)

where d p is the characteristic size of the bed, i.e. the typical m e a n pore size; and e is the cake voidage. To estimate the total mass transfer of impurity, Na, the following basic equation is used: Na = hDA,,Ac

(4.88)

where Ac is the surface area of the cake bed: and Ac is the mean concentration difference between the cake particle surface and the rinse. From equation (4.88) it is seen t h a t mass transfer will decrease as pond level increases, because the surface area of the bed, Ac, decreases. Washing efficiency should be unaffected by g-level, if all other parameter values are held constant, unless a higher g-level enables a lower residual moisture level in the cake, and thus a proportionally lower level of impurities. As differential is increased, the layer of cake becomes thinner, and therefore the superficial velocity of rinse liquor proportionally increases. However, the Reynolds n u m b e r remains essentially constant as the characteristic size of the bed decreases, which in t u r n means t h a t the mass transfer factor remains constant. Thus, from equation (4.85), the mass transfer coefficient will be proportional to superficial velocity. This means that washing efficiency should improve with differential as is found in practice. The theory would suggest that w a s h i n g efficiency should be unaffected by feed rate. However, there comes a point, w h e n feed rate is increased, at which the thickness of the cake on the beach is such that the rinse c a n n o t flood the bed because of the high g field. This then invalidates the theory. To m a i n t a i n a constant bed thickness as feed rate increases would require a pro r a t a increase

Decanter Theory

185

in bowl diameter. This suggests that the capacity of a decanter, limited by washing efficiency, would be proportional to its diameter. 4.9.3 Solids compaction

Many centrifuge manufacturers have expended significant development effort over the past 10 years or more to improve the cake product dryness from decanters when employed on compressible sludges, particularly effluent sludges. The efforts have not been wasted in that several suppliers now offer special ranges of decanters for "dry solids". It has been found [15] that, in "dry solids" operation, the cake dryness produced is proportional to the torque developed by the conveyor or vice versa. It has been shown [12] that:

where xs is cake dryness; T is cake yield stress: T is conveyor torque; and V is pond volume in bowl. To improve dryness in a dry solids decanter, conveyor differential is reduced, and thus throughput has to be reduced, as a result of which the cake compacts and gets dryer, resulting in increased torque. Pushing this reduction too far will result in overspill of the solids into the centrate. The question for the centrifuge specialist is " w h a t is the limit of dryness achievable, assuming that the practical torque limit of the centrifuge is not reached?" The maximum dryness achievable, without producing dirty centrate, will improve with bowl speed and pond depth and with reduced feed rate.

4.10 Dry Solids Operation --

--_

It has just beeii showti thal the perhrrnance of it dry solids dec,antcr is related l a convcyclr torque achievable. bowl spzed, pond depth, and tlocculant usagc. Once h e ciikt: dryness has been fixcd. i t is uscful to be able to assess the

maximum r:;ipacit,ypossihlc on a givcn dccantcr. It i s shown in Section 4 . 3 . 1 that thc clarification capacity of a decaiiler is thc pruduct of the Stokcs settling velocity (eyuation (4.28)) arid t.he Sigma value of the centrifuge (cquation ( 4 . 3 2 ) ) '['he . Stiikes velocity is a functinn of the process material's physical parameters. while Sigma is A furit:l.ion o f mec:hiinisal t'eiitiires ol'i he cent,ril'iigc. Equation (4,301 can be rearranged to give: (4.90)

In

:i dry solids decanter t,he howl is l'u11 [ I h] of solids and the assuiiiylicrtis made ror the derivation or Sigma art: hardly relevant. 'I'he solids are much coarser, heirig Ilocs, with very little distance. if any. to fall. arid thc scyararion process is one of solids comp:~c~.iori, with ~ h clarified u liquor having to Mter through cvcr-shrinking chiinnels. Corner-Walker [I(>] has used 1l'Arcy's equation to arrivc at the following lormula:

L).I = K 1' c'

(4.9 1)

whew K, is i\ function ( i f t he process sludge parameters: (4.92)

and kuv is i i I 1 average cake permcahility. Then:

Decanter Theory 187

where Z' is the Sigma scale-up value for the compaction process of "dry solids". Note from equations (4.90) and (4.93): ~P_/ = { 1 -- ( r l / r 2 ) 2 } ~

(4.94)

An alternative approach to this version of compaction will now be made, which is based on conventional compaction theory [18] where processing capacity is found to be a function of volume rather than area. Consider the compaction of a concentrated compressible sludge in a cylinder in a field of 1 g, as illustrated in Figure 4.12. After time t the interface which develops between the settling sludge and the clear supernatant will be at a height H above the base of the cylinder. This interface will have a velocity v. After an infinite time, settlement will cease when v will be zero, and the height of the interface will be at height H~. At this point the weight of the cake is no longer sufficient to express any more liquid from between the pores of the cake. The rate of sedimentation is given [ 18] approximately by the expression: dH v----~(H-H~) dt

(495)

In this system, it is the compressive forces of the weight of the solids that are forcing the liquid at an ever-reducing rate, up through the ever-reducing spaces between the particles. As in most fluid flow systems, this flow rate will be proportional to g, as will be seen in the laboratory bottle spin centrifuge. The total volumetric flow of supernatant, O, which will equal the volumetric shrinkage of the cake, will be proportional to the cross-sectional area, A, of the cylinder. Introducing this area, and the proportionality constant, into equation (4.9 5), the following expression results: Q=ksA(H-H~)

where ks will have units ofT- ~ and is a constant for a given sludge.

\

Figure 4.12. Solids compaction in a 1gfield.

(4.96)

188

Dry Solids Operation

The height components of equation (4.96), H and H:~. when multiplied by a density are equivalent to pressure heads. Consider now compaction in the centrifuge. In the decanter centrifuge the geometry is quite different, as illustrated in Figure 4.13. Nevertheless, the equivalent pressure heads may be derived using equation (4.61 ) for pressure, Pr, developed at radius r within a centrifuge. In equation (4.61) an expression is given for the pressure developed at radius r with liquor above to a level of radius rl. In the present case, it is required to know the pressure of the head of cake, at the bowl wall radius, r2, with the surface of the cake at radius, !"1. Thus, r needs to be replaced by r 2, and rl by tin equation (4.61 ). Thus, the term/-/in the 1 g mode will be equivalent in the centrifuge to: ~2

2g (r~ - !"2)

(4.97)

and H~ will be equivalent to: ~,2

2--g (r~ - r~)

(4.98)

where r~ is the radius to which the sludge would settle in infinite time. Substituting equations (4.97) and (4.98)into (4.96) the following equation is obtained: ~d2

(21 = k6A-~fl [(,'~ - r 2) - (r 2 -/'2,~)]

(4.99)

where Q~ is the centrate rate that equals the cake volume reduction rate: and k~ is a constant for the sludge and system. It is k n o w n that w h e n a decanter is operated to its limit, it will be full of solids, so: r = rl (4.100) and thus: A = 2 7rrl L where L is the length of the bowl where compaction takes place.

Figure 4.13. Compaction in the decanter.

(4.101 )

Also cake dryness. the dry weighl. per unit weighl of cake expressed as a pcrccntagc. is. within the range ol’cake drynesses heing considered, very close to bcing inversely proportional to cake volume. This is because the cake dcnsity is close to unity, and so if for instance the cake is compressed to half its volume the dryness will bc very close to double that which it was before. ‘I’hus: x,TxL(r;- r;) = . Y & T q r ;

-

r;!)

(4.1(12)

which redur:es I.U:

w h e r e xS is t.he operaling cake dryn,essand X, is the dryness of the cake after infinite time. Wtiilr: we iire considcring thc continuous centrifugr s y s k r n , rx and T~ are hypothetical valucs. which occur after irifiriile lirne at‘ter Feed and discharges have been arrested, Substituting equations (4. 100).(4.1 0 1 ) itnd (4.103)into cquation (4.99):

(4.104)

(4.105 )

where I,’ is Itir pond volume. thc part nf t h e howl in which compaction takes place. arid: (4.1Oh) wheregI is thc crntrifuge g-level at thc poiid surface Thus, cquatinn (4.104)Irliiy he rewrittcn:

(4.107)

( 4 . 1 08)

where Q, is thc cakc ratc:

(4.109) where

R is th e solids rccovery aiid xr is t.he solids fraction iri i.he Iked,

190 Dry Solids Operation For acceptable performance: R~ 1

(4.110)

Substituting equations (4.11 O) and (4.109) into equation (4.108): (4.111) Substituting equation (4.111 ) into (4.10 7 ): (4.112) Thus, the highest possible feed rate, Qr, is proportional to g-level multiplied by volume times a function of xs, x~ and xr. In any single system, x~ and Xr will be constants. If xs is plotted as ordinate against Of/glV, then a line cutting the ordinate axis at a dryness of x~: will result, with a negative slope. Over the range of drynesses generally tested the line will be close to a straight line. The term gl V is referred to as "g-volume", or as g-Vol in equations, in the scalingup calculations of Chapter 7. With centrifuges of similar type and geometry, it should be possible to scale performance from one machine to another. Where geometries are dissimilar, the parameter x~: is liable to vary, as it will be a function of the g developed within the centrifuge and the depth of pond used. Shallow pond machines are not able to produce as m u c h pressure at the bowl wall as deep pond machines and thus the ultimate dryness achievable will be less. Some fundamental work [19] has indicated the relationship between a compressible cake's dryness and its yield stress. As a moist cake is subjected to a stress, or pressure, there is an equilibrium level of moisture for each load value. The graphical results of this, for one particular sludge, are shown in Figure 4.14. It cannot be said how closely this relationship is translatable to any other effluent, but it is anticipated that similar sludges will behave similarly, with perhaps some adjustments to the constants of the equation for the line. The slope of this line is 0.2 6. A figure of 0.2 5 will be taken here to formulate firstorder assessments of performance. Thus approximately: x~ cx ~

(4.113)

where Pc is the pressure to which the cake is subjected. This pressure will be proportional to density difference between solids and liquor, and also to cake voidage. Nevertheless, this pressure will be proportional to the total pressure, and so using total liquid pressures in comparing two systems will be valid, where the solids occupy the entire pond volume of the decanter.

Decanter Theory

191

2.00 1.80 1.60

f

A

1.40

o

0

o

1.20 1.oo

"6 0.8O 0.6o

I

F J i

t~ J

J

F

i

0.40

0.20 o.o0

0.00

1

iF 1,

,1

1.00

200

300

1

I

4.00

5.00

1 600

7.00

Log10 (Yield Pressure Pa) Figure 4.14. Cake !lield stress.

Thus, when comparing or scaling capacities of two decanters, the pressures at the bowl walls should be the same or the value of x~ should be adjusted in the light of the different pressures. Not only will the ordinate intercept change, but so will the slope of the line. The slope of the line could also be affected by changes in k6. which is constant for the sludge and system. If the geometry of the centrifuge changes, or the g-level, then this could affect the compaction e n h a n c e m e n t effect of the conveyor enumerated by k6. Equation (4.104) can be differentiated with respect to r] to find the optimum ratio of r ] / r 2 for maximising feed rate. The value of this optimum ratio is 1 / x/~ -- O. 5 8. When polymer is used, as invariably is the case, a slight adjustment to equation (4.1 1 2 ) is needed. The subject of the equation then becomes the total flow to the decanter, Qt, where: Ot = Q / + Op

(4.114)

where Qp is the polymer feed rate. The feed solids Xf needs to be replaced by x'f where" 9,

o~

(4.115)

4 J l Fluid Dynamics 13y desigri t.he de(:ir~ler1i;Indles very high throughputs relative to the small space i l oct:iipies. Mrjrpover. [.he tlow is nc11 simply in one end. and straight

through and out thc othcr end. Flow can bc under. over and around baffles: it call bc a hellcal path around thc convcyor flights. or axially through holes in the conveyor, or a combiiiation ofboth. The axiiil velocity of the feed into the decanter liiis l o he converted i o a rg(ation:i,l velOr:ity in a very short time. This c a n I:LIUSP ct)risidrr:ihle turhulrnce. iInd help is required outside the Iced Z(IIIP t~ keep the howl contents up t o t;pecd, if not to get it fully t o s p e d . Thc rotational speed of

liquor at the pond surfacc can slipbclow that at thc bowl wall. '1'0 maintain f l o w down thc bowl a n d ovcr thc wcirs. a hydrnulic hend builds lip wilh a Cresi. Tri this secliori s o m e of Ihese phenoniena will

be exanlined morc closcly.

4.11.1 Reynolds number

Thc dcgrcc of turbulence in pipes a n d c.h;inriels is charar.terised by the valut: of the Keynolds number. For a pipc:

( 4 . 1 1h )

H T ~ ~ C 'Ill, T C i' s thc

pipe diarnrtrr: a i i d I I is the velocity in the pipc.

In thc decantcr with axial tlow; 11

=

Of

In EI channel, the pipe diaIneter. Up, i s substituted by diameter, &,:

n

(4.117)

TT(I.4 - 1-4,

4A Ill

P

ii

hydruulic

mean

(4.118 )

where A is the cross scctionaj area of the channel; ant1 p is [ h e wetled pcrimeter of the channel. Thus, for n non-circular pipe (jr channel: P lldr II Rp = -

(4.11 9 )

17

For an annulus; d,,, = 2(r?

r] )

(4.120)

[lowever 11ieaririulus of the pond in a decanter does nut have its iririrr surfare " w e t k d " , arid thus the hydraulic mean diameter becomes:

(4.123) If thc vnlucs ford,,, from cquatinn (4,12 1 ) and velocity from equation (4.11 7 ) are introduced into t h e Reynolds numbcr in cquation [,4.IIYI. this w t i u l r l imply axial flow. For helical flow: 1' = 1 ' + L(r.7

- 1.1)

(4.122)

(4.113 ) Thus:

and:

EqUatioIis (4.12 4 ) ;inti ( 4 , 1 2 5 1ran bc substituted iri1.o equation ( 4 . 1 19) to find the K~'yrrold~ riurnher lor helical flow. Once, the value ofthe Rtynolds number is kno w n , for whattver type offlow is used, the level of turbulcncc can be asscssrd. With a Reynolds iiutiitwr below 2 0 0 0 i h r Ilow would bc laminar. I t will be lourid [ h a t the flow in many. il'noi most. of all practical cases is in the turbulent regime. In a11 decanters, with solids iiioviiig radially o u t a n d liquid moving radially in, itc,c.elc.rationand dccc,leration occur. respectively. Without any mecliar1ic;tl

194 FluidDynamics device to do this, viscous drag of the pond is the only means by which these actions can be accomplished. If the viscosity is low, considerable turbulence can occur, affecting cresting, interface location, stability, and sedimentation and re-entrainment of settled solids.

4.11.2 Moving layer In a pond of an operating decanter centrifuge, there often tend to be two distinct liquid layers. The upper or surface layer, the moving layer, moves rapidly and turbulently towards the discharge weirs. Under this moving layer, the pond is quiescent, allowing solids to settle under a laminar flow regime, and then to compact. This is a simplistic picture, as the shape of the conveyor and its movement adds to the complexity. It is sometimes useful to estimate the depth of the moving layer to know w h e n it is liable to disturb and re-entrain sedimented solids. It will be appreciated that the thickness of the moving layer will depend upon whether the flow is axial or helical. Research has shown that for axial flow: h,,, ~x

4q;

(4.126)

where hmis the thickness of the moving layer. It will be seen that the formula is independent of path length, the clarifying length. It has also been found that moving layer thickness closely follows cresting height (see Section 4.11.3). Thus, the shape, size and number of weir plates used can affect the moving layer thickness. The moving layer thicknesses found in helical flow are greater than those calculated using equation (4.12 6).

4.11.3 Cresting The level difference between centrate and cake discharges can be quite critical when optimising process performance. When the level difference is small, the degree and consistency of the centrate cresting can play an important part in the process performance optimising. The crest height, the pond surface level above the actual weir height, is a function of the centrate rate, the total weir width and centrifuge g-level, as well as physical constants of the liquor such as viscosity and density. Crest height is hc, given by: (4.127)

Decanter Theory

195

where co is a constant and generally approximately 0.415; and B is the total length of weirs. This equation is derived from the Francis formula [20]. Experimental data show that, due to the interrupted n a t u r e of the discharge weir, the calculated value of crest height needs to be increased by 35% for axial flow and 90% for helical flow. With a 360 ~ internal weir, B would be the full circumference, and thus would cause the least cresting. 4.11.4 Feed zone acceleration

Feed zones are designed to accept the m a x i m u m possible feed rate, and bring it up to bowl speed with the m i n i m u m of splashing and rejection. Bringing the feed up to the angular velocity of the bowl is not necessarily enough. As the process material flows out of the feed zone to the pond, it has a constant linear velocity fixed by its a n g u l a r velocity at its point of exit from the feed zone. To m a i n t a i n its a n g u l a r velocity extra linear velocity is required as the radius increases [21 ]. The power required to bring the feed material up to bowl speed at the pond surface is Pp, where:

PP - Or p.r~2 ,~

(4.12 8 )

Power available in the feed stream at the pond surface is PA. where: 1

P a -- -~ Of Pf~X r-~

(4.129)

The power lost on entry is thus the difference between equations (4.124) and (4.125), and this is dissipated in heat and turbulence on entry. Thus to minimise turbulence and power loss, it is necessary to design the decanter with the pond surface as close as practicably possible to the centre line. Nevertheless, other process considerations may require the taking of a different view.

4.12 Power Consumption The total power input required by a decanter centrifuge comprises a number of separate power components: PT -- PP + PwI: + Ps + PB

(4.130)

where PT is the total power required by the decanter: Pp is the power required to accelerate the process material to the bowl speed at the discharge radius; PWF is the power to overcome windage and friction: Ps is the power required for conveying: and PBis the power for braking. From equation (4.128): Pp - O.rpfw2,'d

(4.131)

where rd is the process discharge radius. Naturally, if cake and centrate are discharged at different radii, then these two power components have to be calculated separately. The windage and friction component is given by: PWF = k7 + k8w + k,~,,'2

(4.132)

where k7, ks and k9 are constants. Pwv can be calculated with difficulty but is more generally derived practically in the factory by measuring the power absorbed for different bowl speeds. The conveying component is given by: Ps = N T

(4.133)

where N is the conveyor differential; and T is the conveyor torque. Similarly, the braking component is: PB

--

SpTp

(4.134)

where Sp and Tp are the pinion speed and torque, respectively. In some types of backdrive the braking power can be regenerated, so that the total power used is reduced.

Decanter Theory 197

4.12.1 Main motor sizing The p o w e r of the m a i n m o t o r will be based on the c a l c u l a t i o n of PT from e q u a t i o n ( 4 . 1 3 0 ) , while its physical size will be influenced by its s t a r t i n g r e q u i r e m e n t s . Motor m a n u f a c t u r e r s rate their m o t o r s o n the basis of the m a x i m u m p o w e r delivered at the m o t o r shaft, PM. This h a s to be g r e a t e r t h a n PT to cater for frictional losses in the drive belts a n d fluid coupling, if used. Thus, the m o t o r p o w e r is PM: PM .r

= PT

(4.135)

w h e r e (D is the fluid drive efficiency; and (B is the efficiency of the drive belts. The p o w e r used, h o w e v e r , will be g r e a t e r t h a n PM, d u e to losses in the m o t o r itself and losses in some c o n t r o l gear w h e n used, s u c h as an inverter. P o w e r is lost w i t h i n a m o t o r due to a n u m b e r of factors, w h i c h include: 9 iron losses in the m a g n e t i s i n g material, p r o d u c i n g h e a t in the m o t o r rotor and stator; 9 friction in the rotor bearings; 9 e n e r g y needed to drive a cooling fan, internally a t t a c h e d to the m o t o r shaft: 9 w i n d a g e losses; and 9 copper losses (the p o w e r lost due to the resistance of the w i n d i n g s , s o m e t i m e s referred to as the I-'R losses). These five factors combine to give a m o t o r efficiency. CM, of less t h a n unity. Extra p o w e r is also necessarily supplied to the motor, w h e n the p o w e r factor is less t h a n unity. The p o w e r factor will n e v e r be unity, a n d is a m e a s u r e of how m u c h the c u r r e n t lags or leads the applied voltage. It is m e a s u r e d as the cosine of the p h a s e angle b e t w e e n c u r r e n t a n d voltage. W h e n an i n d u c t i o n m o t o r is c o n n e c t e d to an AC electrical supply, w h e t h e r the m o t o r does useful work or not. a c u r r e n t is d r a w n to excite the m o t o r . This c u r r e n t , i n s t a n t a n e o u s l y on start-up, lags 9 0 ~ out of phase with the voltage, a n d is reactive c u r r e n t , or so-called idle or wattless c u r r e n t . The power factor increases as the m o t o r accelerates. W h e n the m o t o r is put to work. it will take in addition to its excitation current, a c u r r e n t according to the a m o u n t of work to be done. The p o w e r factor will increase and will be m a x i m u m w h e n the m o t o r works at its full power rating. T h u s the power t a k e n from the m a i n s supply will be Pc w h e r e : FpPc.4~r = P~I w h e r e Fp is the p o w e r factor.

(4.136)

198 PowerConsumption To combat the anomaly of a low power factor, the installation of a capacitance bank, ideally directly across the motor windings, causes the motor current to reach its m a x i m u m value closer to w h e n the voltage does in the alternating cycle. Therefore, a suitably designed capacitor added to an induction motor will reduce the lag of current, by any desired a m o u n t . Generally, in industry, because the cost of small capacitors is high, it is more economical and expedient to install large banks of capacitors at the supply source, and automatically switch in and out various sets of capacitors as the d e m a n d fluctuates. Moreover, a leading current, w h i c h is possible if the capacitor is too large, increases wattless current as m u c h as a lagging current. Motor m a n u f a c t u r e r s supply motors in standard increments of power. Thus, after power demand for the decanter is calculated, the next larger size is specified. Motor m a n u f a c t u r e r s can supply tables of efficiency and power factors for ranges of loading. Also available are performance curves for their motors, giving output torque against rotational speed. The selected size of motor, for economic reasons, needs to be as near as possible to the power d e m a n d e d by the centrifuge. Details of the installation need to be considered in the motor specification. These factors would include the ambient temperature, w h e t h e r the installation is indoors or out, and w h e t h e r any hazards exist, such as flammable materials in use, and w h e t h e r the motor will need to be hosed down. The installed electrical services need to be assessed to ensure that they are adequate for the method of starting contemplated. It is important that supply cables be adequately sized to minimise voltage drops before reaching the main motor. The power supplied to the motor reduces proportionally to the square of any voltage drop. Nevertheless c u r r e n t will increase to compensate for the drop in voltage, increasing the heating and losses. Moreover regulations restrict voltage drops to a total of 4%. If reduced voltage starting is used, it is important that the reduced starting torque is never less than the sum of the friction and windage torque. Since the torque available to accelerate the bowl is equal to the difference between the motor torque and friction and windage torque, the motor may not reach full speed in a reasonable time, unless care is taken. 4.12.2 Main motor acceleration

Most decanter rotating assemblies have high inertias, which can require several minutes' acceleration [22], or run-up time, ta. If the run-up time is too short, drive belts will slip and wear out prematurely, or even break. If the runup time is too long, then the motor could overheat and burn out. The r u n - u p time is:

ta=

~M(IM + lP) ( T a - Ti)

(4.13 7)

Decanter Theory

199

where ]M is the inertia of the motor; lp is the inertia of the decanter at the motor; 02M is the motor speed; Ta is the m o t o r torque; and Tl is the reactive torque of the decanter. The decanter inertia is given by" 022

lp - -~M lo

(4.138)

where ]D is the inertia of the rotating assembly. Both Ta and Tl vary with speed, and not linearly. Examples of motor and decanter torque/speed curves are shown in Figure 4.15. To use equation (4.13 7), Ta and Tl are averaged over the speed range from zero to full motor speed. Given the inertia at the motor shaft, the equations in this section are used to determine w h e t h e r the torque of the chosen motor is sufficient to accelerate the decanter bowl smoothly to speed, w i t h o u t slippage of the belts. The t h e r m a l limits and the torque limit of the n u m b e r of drive belts used and their cross-sections have to be checked, with the pre-set diameter of the smallest pulley taken into account. Causing the belts to slip will end in their failure, while producing copious a m o u n t s of dust in the belt guard, which could be an explosion hazard. 'I .

600

.

.

.

.

~'--~-

-l 250

-~.., "%

"O

200 "0 m

m 500 O .J

l

- - - Current Star iI l!

IJ. 300

C9

200

o

:

C~urrent DOL

400 0

~

--- " Torque DOL I t__ t'--" "" Torque Star *. . . . . . . . .

"~..-.-

.

X

i'

I

'

15o

i

t,~

loo # ". ~ . .

.. "

1oo

0"

.

'

0

o

50

Full Load ~ o d 0

20 Motor

40 Speed

60 % of Full

80 Speed

Figure 4.15. Motor and decanWr torque~speed curves.

100

4.13 Mechanical Design The design of a good and reliable decanter centrifuge requires a thorough knowledge of most mechanical engineering disciplines such as machine dynamics, strength of materials, bearing design, and gearbox design. Some of the more important fundamental aspects of the mechanical design of decanter centrifuges will now be discussed. The need for a careful mechanical design can be illustrated by examining the energy accumulated in a decanter centrifuge in operation. The rotational energy in a medium size decanter with rotational inertia of 50 kg m 2. rotating at 3 6 0 0 rpm, will be 3.55 M]. This energy corresponds to the kinetic energy of a vehicle weighing 9.2 tons travelling at 100 km/h. Furthermore, it can be shown that the rotational energy of a decanter centrifuge will increase with the fourth power of the diameter, when the centrifugal force at the bowl wall, go, and the length/diameter ratio, )~, are kept constant. The ratio of the diameters of the largest to the smallest industrial bowl is over 10. Thus, the ratio between the rotational energy of the largest and the smallest decanter centrifuge on the market is over 104. With the high energy involved, failure of one of the major rotating components of a decanter centrifuge can cause severe damage, both to the decanter and its surroundings. For all decanter designs a risk analysis, evaluating all possible failure modes, must be carried out. A European standard [23] deals with the foreseen risks for centrifuges in normal operating environments. The standard gives requirements for design, verification, and installation of centrifuges. The high risk of failure of a decanter requires a high quality, both of design and production, as well as periodic inspection during use, to ensure that unanticipated deterioration of materials of construction has not occurred. From the energy comparison above, it is seen that the risk increases with size, and a design which is adequate for a small laboratoryscale decanter, may be extremely dangerous on a large industrial scale.

thc matcrtal of the bowl shell. The cylindrical part of the bowl shell wiIl be, for normal decanter designs. the part of the bowl subjected to the highest stress levels. The trlaxinlum pressure 011 the bowl shell is calculated using equation ( 4 b 1) : I

whew PLmis the maximum pressurc at thc bowl wall: and phq is the maxiniarn bulk density of process mstcrial evcr likely in tbc decantcr, ;is t.he Defining t, as the wall thickness of the decanter bow1 shell and dcnslty of the bowl rnatcrial. the average tangential stress i r i the howl sheli can bc, exprcssed. for a straiEht cylinder. as: (4.140)

where fJ2 is the aCt.iiill prrssure i1.t the bowl wall: and n1is the mean t.angentia1 strcssin thebowl wall. The fnrrnc~lais equivalent to the well-known pressure vesst.1 tormuiae For thiri cylindrical shclls. To ciisure safety againsl. f:iilure, thc tangential stress rr~ust be below B ccrtain allowable s h e w . According to the European EngineerinR l>irec,tivc. the tangcnt.ial stress shiill be kept bclow h h ' % OT the yield strenEth and 44% of the uli.irna1.estrength of the bowl materinl, at the maximum allowcd opcrating teniperiilure, It is readily observed that the first term of cquation (4.14111 will decrease with tw, a n d the second tcrni will increase with t w , Unlike the sit.uation for a pressure vessel, simply intreesing the thicknm ofthe howl shell will not always reduce the risk of failure. On t h e graph of Ftgurc 4.l h . the relationship between the maximum oht.ein;tble gfarce. and the decankr lntcrnal diameter, tor differcnt process densitics is shown to il1ustr:Jt.r Ihls. Thc g-force is c.iilcnlatcd by assuming a bowl thickncss of 10uhof 1 tic! internal radius. 'I'he ;jllowablc stress is set. to 2 4 0 MPa, which corresponds to thc stress limil ol'a duplcx stainless s k e l i1t 1w c . Of course, ol.her howl materials, such as titanium and aluminium. will give d if kren t v a1u es . 'I'he pressure insidc the bowl will also create an axial fnrcc. acting o n the cnd hubs. The maximum axial force, F,, is found by taking the m e a n pressurc in the pond as half the pressure c:IIcuhtCd from eq11ilt.ion (4*140),;jnd multipl.yina by t h e cross-sect.iana1arca of t h e pond. Thus: r y

(4.141)

On a l a r w dccanter. the axial form on the end hubs will bc greater t h a n lo6 N. The cnd hubs, and axial fixings, on the rotor must tfiereforc be designed to

Mechanical Design

202

9000 O

~

,~

"

8000

~

~ .]L "

~i

1

i

i \!

-

,

'

', t ~,

t ,

i ,x,i

ooo eooo

~ , ".

"

9 ~

,

,

!

~

t

'.

!

3000

i

!

I

';

2000

l

1 I

[ '

'

~

100

200

o

I

0

',

'

300

~

i

l

-

Density

1.2

i

;

i

,

~

1

=

.

t

! i

i I

I" " ' ' ' ~ ' ' "

;

~

i

~

!

~

~.-..=

9

'

-.

-i

. ~

9 '

;

500

600

700

Diameter

.

,~"---! .

.I

I

:.

800

900

-.

u

.

.

~

i 1000

t

,, '

; ,

, t

,

~,

, ".

400 Bowl

3.6~

, ~

9

Density

I ~

~

" :'

Process

r-..

',

'

-Process

i'--..; i i i!

4000

1000

: I ~,......

2 i

,

1 1100

, 1200

mm

Figure 4.16. Example of the relationship between bowl radius, max g-force and cake density for one material and one relative bowl shell thickness.

withstand this force with a sufficient safety margin. Components of the decanter may be subjected to several other design-dependent static loads, which must be considered by the designer. One example is the axial load, acting on the conveyor, caused by conveying the solids. The decanter will also be subjected to cyclic loads, which can cause m e c h a n i c a l fatigue damage on both the rotating assembly and on the stationary parts. Among the cyclic loads which must be considered by the designer are the bending forces on the shafts, caused by the weight of the rotor, the loads from belt drives, unbalance forces from the rotor, and cyclic loads from frequent starts and stops, or intermittent loading with process material. On complicated hub geometries, often it will be necessary to make a finite element calculation of the stresses to make a proper fatigue evaluation. The notch sensitivity and ductility of the material m u s t be considered. Quality assurance procedures during m a n u f a c t u r i n g , such as X-raying of critical welds and die penetrant testing of castings, must be maintained. 4.13.2

Critical

speeds

The n a t u r a l frequencies and critical speeds of a decanter will depend on its actual configuration. A conventional decanter centrifuge consists of a flame holding the double rotor - - conveyor and bowl - - in rigid bearings. The m a i n motor can be attached to the rotor flame either by a rigid connection or flexibly through vibration isolators. Further the motor can be attached to a sub-flame or to a n o t h e r part of the supporting structure. These factors are more fully described in Section 2.1.

Decanter Theory

203

At speeds below operating speed the main flame and the rotor can be considered as one rigid body. If the decanter flame is m o u n t e d on soft vibration isolators the decanter assembly will have six natural frequencies and associated vibration modes below the operating speed. The n a t u r a l frequencies are determined by the spring stiffness of the vibration isolators and the mass and inertia of the total system. When the main motor also is supported on the decanter flame by vibration isolators, it will have six additional natural frequencies below the operating speed. The important critical speed for a decanter is the lowest speed at which there is significant flexible deformation of the rotor. This speed is called the first rotor critical speed. Decanters will always have a certain unbalance, both due to the handling of solids from the process and due to wear on the rotor. Operating the decanter close to, or just above, the flexible critical speed of the rotor will result in high vibration levels and very high stresses in the rotor components. The critical value of the rotor speed will therefore be an upper limit for the operating speed, and the decanter must be operated below this speed with a safe margin. The first rotor critical speed will mainly be a function of conveyor geometry, bowl geometry, gearbox weight, main bearing stiffness and conveyor bearing stiffness. The first rotor critical speed will decrease with the length of the decanter. The critical speed of a decanter can be calculated by using a finite element method and verified by measurements. It is normal practice to test decanters at a speed 15-20~ above operating speed, to verify the design integrity, and such an over-speed test can also reveal if the operating speed is close to a critical speed. These factors influencing the first rotor critical speed have been more fully covered by Madsen [24].

4.13.3 Liquid instability problems Often very large vertical and horizontal vibrations are seen in some speed intervals on decanters when they are started and stopped with liquid inside the bowl. The vibration frequencies in the instability intervals correspond to rigid body natural frequencies of the decanter, but the vibrations are not caused by unbalances. Rather, they are due to interaction between the liquid inside the bowl and the decanter. The vibrations which occur in some instability speed ranges are subsynchronous, i.e. the vibration frequency is a fraction (normally about 0.7) of the actual operating speed. If, for example, the decanter is vibrating at an operating speed of 1000 rpm, the vibration frequency will be around 700 rpm. The vibrations are usually harmless, but as very large forces could be acting on the foundations of the decanter, the manufacturer must supply information on the magnitude of these forces, and the foundation must be designed to withstand these forces. By having a constant flow of water to the decanter during starting and stopping, the instability vibrations can be suppressed.

204

MechanicalDesign

The complicated d y n a m i c p h e n o m e n o n , w h i c h is related to all rotating cylinders with an internal a n n u l u s of liquid, has been dealt with in a n u m b e r of publications [25, 26].

4.13.4 Length/diameter ratio In general the bowl strength, the first rotor critical speed, and the m a x i m u m permissible speed of the m a i n bearings control the m a x i m u m speed at which a decanter can be operated. It is argued [24] t h a t a long, slender decanter centrifuge will give advantages with respect to overall economy, power c o n s u m p t i o n and process performance. For a centrifuge with the L/D ratio above 3, the critical speed will often be the m a i n factor controlling the m a x i m u m obtainable speed and it can therefore be desirable to increase the critical speed in order to obtain a high L/D ratio w i t h o u t sacrificing the m a x i m u m operating speed. W h e n a decanter bowl, for calculation purposes, is approximated to a beam, its n a t u r a l frequency is inversely proportional to the square of its length. In that g-force is proportional to the square of bowl speed, and it is necessary to keep resonance frequency above bowl speed, the m a x i m u m bowl speed is proportional to its length to the fourth power. To obtain g-forces in the range 20()0-3()O0, generally required for commercial decanters, the m a x i m u m length-to-diameter ratio, for the most frequently used designs, has to be restricted to a little over 4.0 [24]. In order to increase the critical speed of the rotor a n u m b e r of different modifications can be made to the rotor system. By supporting one or both main bearings in a flexible pillow block the first critical speed of the rotor can be turned into a low speed rigid-body motion for the rotor. It can then be operated supercritically with respect to this critical speed. Other modifications are the floating conveyor and the separately supported gearbox. These sorts of modifications have been utilised by Alfa Laval in producing a decanter with an L/D of over 5 which can operate with up to 10 0 0 0 g. How these modifications extend the possible L/D ratio and clarification capacity was graphed by Madsen, for 2 5 0 m m diameter bowls, and reproduced in Figure 4.17.

4.13.5 Bearing life One of the most frequent reasons for breakdown of decanters is failure of one of the m a i n bearings. The operating conditions of decanters are often very arduous, and there can be a high load on the bearings. The failure of a main bearing on a properly designed decanter will not lead to a dangerous situation, but it can cause d a m a g e to other parts of the decanter, and expensive downtime.

'I'he bearing lire is defiried a s h e r i u r n h r ~of rrvolu~.iorisor number of' h o l m at constarit speed a bearing will operala bclore i t I'ijils. Acr:ortling to thc international statidard [ 2 71. and based o n t h e assiimpliori that i h e beoring will rail hy laligiie. t.he expected life of ii hearing is calculatcd by the sitiiple f( I rm11 1 ;-I : (4.1 4 2 )

whcrc L I r l is thc expccted life nieasiired i r i 10" rc-volutions: C is the dyriarriis load capacity of t h e hearing, a characteristic figurc for the hearing, determined by the m a n u l a c l u r e r in accordancc with thc IS(.)standard; Ce is the equivalent d y n a t n i r load, calculatcd f'ram the dynamic a n d static loads; and 11,'isa nurnhur depending upon thc hcaring type (e.g. for hall hearings w = 3 afid for rnllrr twaririgs \I' = 1 O/ 3 ) . Both C a n d a r r expressed in a unit offoroc. 'l'licr I , , , ) lire is d s u sometilncs referred Lo a s t h e R l o lifc. For a macliine rohl.ing ;it ;Ic,onstant speed. ti. in revolulions per minutc. tile expccted life cat1 be expressed in expected hours nfnpersiion, L I O h :

where Lloh is thc cxpcctcd life i n hours; arid n is the number of rcvolutiotis per mi nu te.

206

MechanicalDesign

This simple formula was developed around 1950 and was based on data for bearing fatigue failure. Since the formula was published, considerable progress has been made both in the knowledge of bearing failure, in bearing manufacturing, and in lubrication, which is not reflected by this formula. The formula is based on the assumption of fatigue failure, although it is in fact not the most frequent failure mode for bearings. More realistic methods for calculation of bearing life have since been developed, which both account for the improved quality of bearings, for the bearing environment and for the lubrication conditions. In 1977 the same ISO standard [27] introduced an adjusted life-rating Lloah formula:

Lloah--b,.b2.b3.(~--~)'"

(4.144)

b2

where bl is a constant accounting for reliability; is a constant accounting for the material used; and b3 is a constant accounting for environmental conditions. The key to avoiding failure of bearings is proper maintenance and lubrication. By monitoring and analysing vibrations measured with sensors directly on the bearing housings of a rotating machine, bearing faults can often be detected before they lead to failure. Several systems for detection of bearing faults, by continuous vibration monitoring, are available, and some decanter manufacturers offer their own specialised systems. For critical installations, and installations with several decanters, such monitoring systems can be a good investment, to avoid inconvenient bearing failures, damage to the machine, and unnecessary downtime. 4.13.6 Gearbox life

The decanter manufacturer will often quote the expected life of the gearbox. This will be based on the fatigue life of the gear teeth, which is proportional to the ninth power of the torque encountered. Thus, one has to be extremely careful not to overload the gearbox above its torque rating. An 8~ increase of torque over its rating will halve the expected life of the gearbox. 4.13.7 Feed tube

Each component of the decanter has its own natural frequency, even the stationary components, which could resonate sympathetically if this frequency is close to the bowl speed. The feed tube is a good example, being a long, thin tube. Apart from the inverse square relationship with length, resonance frequency is also proportional to the fourth power of diameter in its simplest form. Unless care is taken the feed tube can be caused to resonate like a tuning fork. The design engineer thus endeavours to maximise diameter and

Decanter Theory 207

minimise the length of the feed tube. Other t e c h n i q u e s employed include tapering the feed tube and making it of lighter materials. Of course the double concentric tube used, when flocculant is added, helps to increase the natural frequency.

4.14 Nomenclature

Symbol

Description

tl 1

Constant Constant Cross-sectional area Total surface area of bowl and beach Surface area of cake bed Dry beach area Centrifuge "area equivalent" Function of restriction area in the conveyor Wet beach area Constant Constant Constant Total length of weirs Expected bearing life in revolutions Weir discharge coefficient Impurity concentration in feed mother liquor Impurity concentration in cake moisture Impurity concentration in centrate Impurity concentration in wash liquor Dynamic load capacity of bearing Equivalent dynamic load of bearing Cumulative fraction by number of particles below sized Cumulative weight or volume of particles below size d Particle diameter Cut point size Required cut point size of the heavy fraction Required cut point size of the light fraction Geometric mean diameter (number basis)

A AB

A~ AD

Ae~_ 4 A~ Aw bl b, b3 B BI() r C1 C C3 C4

C CE Cn C Vr

d dc dch

dcl dg

Dimensions

L2 L2 L2 L2

L2 L2 L2

L

MLT -2 MLT -2

Decanter Theory 209

dgl dgs dgw dm dp d15.87 d so d84.13 D DAy Dp e F FB FN Fs Fp Fx g g~ gc I gl

g-Vol hD hm H If 1D ]D JM kav kl

kl k2 ks k4 ks k6 k7 ks k9

Geometric m e a n diameter (length basis) Geometric m e a n diameter (area basis) Geometric m e a n diameter (weight/volume basis) Hydraulic m e a n diameter Characteristic size of cake bed 15.87% of all particles are below this diameter Median diameter 8 4 . 1 3 % of all particles are below this diameter Diffusivity Mean diameter of pond Pipe diameter Cake voidage Force Beach friction Normal force from conveyor flight Scroll friction Power factor M a x i m u m axial force Acceleration due to gravity Acceleration n u m b e r of times greater t h a n gravity Mean value ofgc in pond Centrifuge g-level at the pond surface g-volume, product of g l and V Mass transfer coefficient Thickness of moving layer Height of interface at time t Height of interface after infinite settling time Impurity concentration % in cake at exit from pond Impurity concentration % in feed Impurity concentration % in cake Mass transfer factor Inertia of the rotating assembly Inertia of the main motor Average cake permeability Constant Constant Constant Constant Constant Constant Constant Constant Constant Constant

L L L L L L L L L2T-1 L L MLT -2 MLT -2 MLT -2 MLT -2 MLT -2 LT-2

L~ LT-2 L L L

ML 2 ML 2 L2

Z-1 T-1 ML2T-3 ML2T-2 ML2T-1

210

Nomenclature

Kp

L Lc Lk Lv Llo Lxoh L aOah m n

nD nv

nl N

Na P P Pc PA PB Pc PD PM Pp Ps PT Pwr P2 P2m

Of Qf~ Of2

Ol (2p (L

QsD

(2,

Qw V t"d t"x t"1 F2

Product c o n s t a n t for compaction Clarifying length Cylindrical length of the bowl Wetted beach length Length of vanes on a conveyor Expected bearing life in revolutions Expected bearing life in h o u r s Expected bearing life in h o u r s - adjusted formula Mass Revolutions per minute of bearing Number of discs in a disc stack Number of vanes in a vane stack Number of leads or flights on the conveyor Differential Mass transfer rate Perimeter length Conveyor pitch Pressure in the cake Power available in the feed stream in the bowl Power required for braking Power taken from mains supply Polymer dose Total power absorbed by the main m o t o r Power required to accelerate the process stream Power required for conveying Total power absorbed by the decanter Power to overcome windage and friction Pressure at bowl wall Maximum pressure at bowl wall Volumetric flow rate of feed Volumetric flow rate of feed to decanter 1 Volumetric flow rate of feed to decanter 2 Volumetric flow rate of centrate Volumetric flow rate of flocculant Volumetric flow rate of cake Volumetric flow rate of solids in cake Total volumetric flow rate of flocculant and feed Volumetric flow rate of cake voidage Volumetric flow rate of rinse Radius Radius of discharge of process stream Radius used in Ambler Sigma derivation Pond radius Bowl inside radius

LT-a L L L L T T

M T-1

T-1

MT-a L L ML-1T-2 ML2T-3 ML2T-3 ML2T- 3 ML2T - 3 ML2T -3 ML2T-3 ML2T - 3 ML2T- ~ ML-1T-2 ML-IT-2 L3T-1 L3T-1 L3T-1 L3T-1 L~T-1 L3T-1 L~T-1 L3T-a L3T-1 L3T-1 L L L L L

Decanter Theory

r3 too

R

Re Rein ROB S Sl 82

S Sc Sp Ss t

te tw

T Ta Tl Tp To u Uc v Va Vc Vr Vs

V W Xf X ft Xl .Yp ~s ~Coo

Yr Yl Ys Yw Z

Z

Ouside radius of a disc or vane stack Three quarters bowl radius Radius to which the sludge would settle in infinite time Solids recovery Reynolds n u m b e r Modified Reynolds number Gearbox ratio Distance Distance Distance Bowl speed Schmidt number Pinion speed Volumetric scrolling rate Time Time for particle to traverse decanter Thickness of bowl shell Conveyor torque Motor torque Reactive torque of decanter Pinion torque Heel torque Velocity Superficial velocity Settling velocity of interface Axial velocity Tangential velocity Radial settling velocity Stokes settling velocity Pond volume Constant Solids fraction in feed Solids fraction in total feed and flocculant Solids fraction in centrate Solids fraction in flocculant Solids fraction in cake Solids fraction in cake after infinite compaction time Impurity fraction in feed Impurity fraction in centrate Impurity fraction in cake Impurity fraction in rinse Number of particles less t h a n diameter d Total number of particles

L L L T-1 7-1

LST-1 T T L ML2T-2 ML2T-2 ML2T-2 ML2T-2 ML2T -2 LT-1 LT-1 LT-1 LT-1 LT-1 LT-1 LT-1 L3

211

212

/31 /32

-7 Ac Ar

CB ~D ~s C,

q r/L 0 2 71"

P Pb Pr Pl PM Pp Ps PS Psh Psi O'g (7t

s s

Y]2 T

0

~,0M

Nomenclature

Beach angle Beta value: the scrolling capacity Beta value; the scrolling capacity of centrifuge 1 Beta value; the scrolling capacity of centrifuge 2 Angle rotated after time t Mean concentration difference Pond depth Bowl separational efficiency Efficiency of the drive belts Fluid drive efficiency Scrolling efficiency Separational efficiency of decanter 1 Separational efficiency of decanter 2 Viscosity Viscosity of supernatant The half included angle of a disc stack Ratio of bowl length to diameter Pitch angle Universal constant Density Density of bowl material Density of feed Density of centrate Density of supernatant Maximum bulk density of process material Density of flocculant Density of cake Density of solids Density of the heavy solids Density of the light solids Geometric standard deviation Average tangential stress in bowl shell Sigma; equivalent settling area of centrifuge Sigma value for a disc stack Sigma value for a vane stack Modified Sigma for a compaction process Sigma for bowl 1 Sigma for bowl 2 Cake yield stress Cake path angle up the beach Angle between a vane and the radius Thickening factor Angular velocity Motor speed

o

L3T-a L3T-1 L3T-1 o

L

ML-1T-a ML-1T-a o

o

ML-3 ML- 3 ML-3 MLML-3 ML-3 ML-3 ML-3 ML- 3 ML-3 ML-3 ML-1T-2 L2 L2 L2 L2 L2 L2 ML-aT-2 o o

L-3 T-1 T-1

4.15 References 1

F Reif, W Stahl. Transportation of moist solids in decanter centrifuges. Chem

Eng Prog 85(11) (1989) 57-67 2

B Madsen. Flow and sedimentation in decanter centrifuges. IChemE

Symposium Series 113 (1989) 301-17 3 4 5 6 7 8 9

l0 11 12 13 14 15 16 17 18

T Hatch, S P Choate. Description of the size properties of non-uniform particulate substances. Harvard Engineering School, Publ. No. 35 (1928-29) 369--87 F A Records. The Performance of a 4" micronizer. AWRE series 0 reports Number O41/61, Feb. 1962 C M Ambler. The evaluation of centrifuge performance. Chem Eng Prog 48(3) (1952) 150-8 G G Stokes. On the effect of the internal friction of fluids on the motion pendulum. Trans Cam Phil Sot" 9 (1851) 8 G A Frampton. Evaluating the performance of industrial centrifuges. Chem Proc Eng 44(8)(1963) 402-12 C M Ambler. Theory of scaling up laboratory data for the sedimentationtype centrifuge. J Biochem MicrobiolTechnol Eng I (1959) 185-205 S Yano. Experimental studies of separational efficiencies in centrifugal sedimenters. Proceedings of the first China - J a p a n joint international conference on filtration and separation, China, Nov. 1991. Chinese Mechanical Engineering Society & Society of Chemical Engineering, Japan FA Records. Recent advances in sludge processing. Aqua Enviro, University of Leeds, 19 Nov. 1991 A Lavanchy, F W Keith. Centrifugal separation. Kirk Othmer Encyclo Chem Technol & Engng 2nd Edn,Vol. 4, p. 719 N Corner-Walker, FA Records. Filtration+Separation 37(8) (2000) PA Vesilind. Scale-up of solid bowl centrifuge performance. ] Envirmlln Eng Division, ASCE, April 1974 Coulson, Richardson. Chem Eng I (1962) 254 W W-F Leung. Torque requirement for high-solids centrifugal sludge dewatering. Filtration+Separation 35 (1998) 883 (Figure lb) NCorner-Walker. Filtration+Separation 37 (2000) 28-32 E A Relter, 1~ Schilp. Solid-bowl centrifuges for wastewater sludge treatment. Filtration+Separation 31 (5) (1994) Coulson, Richardson. Chem Eng 2 (1956) 515

214 References

19 20 21 22 23 24 25 26

27

] Eiken, B Madsen, ] Oppelstrup. Private communication Perry. Chemical Engineers' Handbook. 3 rd edition 409 W W-F Leung, A H Shapiro. Improved design of conical accelerators for decanter and pusher centrifuges. Filtration+Separation 33 (1996) 735 Electro Courier IV (1976), No. 2 European Standard EN12547. Centrifuges- common safety requirements, CEN 1999 N F Madsen. Slender decanter centrifuges. I Chem E Symposium Series 113 (1989) 281-99 J A Wolf Jr.Whirl dynamics of a rotor partially filled with liquid. ASME ]App Mech December (1968) 678-82 F Ehric. A state-of-the-art survey in rotor dynamics - nonlinear and selfexcited vibration phenomena. Proceedings 2nd International Symposium on Transport Phenomena, Dynamics and Design of Rotating Machinery, Hemisphere Publishing Corporation (1989) ISO 281, Rolling bearings- dynamic load and life ratings

CHAPTER 5 Flocculation Formulae have been presented in Chapter 4, for estimating settling velocities of suspended particles. From these formulae it will be seen that the settling velocity is proportional to the second power of particle size. A simple calculation will show that, with very small organic particles, say less than 1 ()~m, settling rates will be very low, only a few centimetres per hour. With this sort of settling rate some e n h a n c e m e n t is required. The use of a decanter centrifuge would increase the rate by a factor of a few thousand. However with particle sizes even lower, say nearer 2 or 3 gm, particularly with the lower density particles of n a t u r a l materials, even this level of e n h a n c e m e n t is not sufficient to allow efficient separation with a decanter centrifuge. This is when flocculant aids are necessary to increase the size of the particles.

Figure 5.1 Anautomatic Polymer make-up system (By courtsey of Tomal).

. c

5.1 The Principle of Flocculation To form n picture or I.he ability of a sludge tu settlc, takc a sample in n ghss beirker ijnd allow il to set1.k oil a 1)ench. under thc iilflucnce of gravity. As ;I rule of thumb. if a distinct interl'nct: develops heiween Lhe settling solids arid a clear supernatant, and good set.tltrment occ11rs within approximakly half a n hour. then it is probable that the sludge will reidily separa1.e i r i a decarrter centrifuge. without any floc,c,ulaiitaddition. By good settlement, is mcitnt the solids scttlina least 7 0 % of thc distance that t.hey would ;icbieoe, g i w n iniinitc timc. ~

J

J

I:ine particles in tiii aqueous susperisiori t i w mc-)stfrrqueiitly coated with an electrical r:liargc [ 11. 'l'hrre are ii nurnhcr of reasons Tor 1 h c i elc!ctrir:al charges. mainly associated will1 the 1vater.s ability to ioriisc chcrriicals. Ioriisation, arid thus flocculaiion. ;ire not possible in non-aqueous suspensions. The polarity of'iht. (:hitrges on a mass of'suspended particlcs, normally the siirne thrniighout the mass, ciiuses the particlcs to rcpcl anc anothcr, The siiiallcr thc particlcs. tlicii thc greatcr is the effect of the repelling forces. Wherl particlcs iirc a s small as about 0.1 prri o r less. the suspensiori heromes cdloidi\l, and thcri no $cttlcmciit takes place. Were these small particles ahle I O iippronch closer to one another. small molecular altractive fcirces, called Vijn d r r W a a l s forces, would cvcntually becotne greater than the eltxtricill torces, allowinE agglomcration to occur. l'tiiis, iri sornt! flowulittion proccsscs. prc-trcatmetit is etriployed 1.0 hrcak d o w n t h e repulsion Ihrres, to initiarc agglomeration. Simple agitation [a]uitn sorrieliii1t.s iriitia1.e agglomeri\t.ion. b u t rnorc gsncrally the electrical chargp on t h e piirliclrs h a s to bc rcdiiccd or nuutralised by ptl, adjustnienl or by treating wi1.h a n inorganic coagulant. Adjiishienl o l pH would bc by use of mineral acids or alkalis. This sornctirncs has thc added advantage of precipit,at.ing kinw,ant.eddiss~il\~ed sails. Howcvcr. thcrc arc timcs when salts are dissolved by I.he pH chiinge. This would bc a disadvantage if the purpose o f the treat.mrn1 is t o remove these substances. when dissolving them wnuld rnakt. ii niort- dific:dt. Thc inorganic coagulants used are sa1t.s ol rniiltivillent metals such as calcium, iron or aluminium. These can itt t.he sitme time adjust thc pH.

218

The Principle of Flocculation

sometimes more than required. Occasionally more complex chemicals, such as polyaluminium chloride (PAC), are used. The processes just described are generally termed coagulation, and they achieve a certain degree of agglomeration by adjusting the environment within which the particles exist, and the nature of the particles themselves. Flocculation is a different process, in which the particles are actually bound together in a larger agglomerate, by electro-chemical attraction to a special flocculant molecule. Coagulation pre-treatments can be sufficient on their own for some processes, such as gravity thickening, but for the decanter centrifuge the resultant agglomerates are neither large enough nor sufficiently strong to withstand the turbulent entry through the feed zone. For the decanter, therefore, a much stronger agglomeration is required. This is achieved by the use of flocculants after the pre-treatment, or, on most occasions, by using flocculants on their own. Flocculants have been used in the process industries for more than a century. For instance, isinglass, a fish product, has been used in the settlement of lees in beer and wine making. Starch, tannin and chitin are also products which have been used for a very long time. However, it has been the development of polymers for use as flocculants in the past 40 years that has opened up a large market for decanter centrifuges. The polymeric flocculants, also known as polyelectrolytes, are manufactured with a variety of ionic charges, both negatively charged anionic and positively charged cationic. They are also available in a large range of molecular weights, from a few hundred thousand to 30 million or more. The lowest molecular weight polymers tend to work by coating the suspended particles and neutralising their charge, to allow close approach to one another and thus agglomeration (i.e. a coagulation process). It is the much higher molecular weight polymers that have found wide use with the decanter centrifuge. These are mostly polyacrylamides. They are very longchain molecules with charges along their length. The suspended particles attach themselves to these charged sites while other polymer molecules attach themselves to other sites on the same particles, and other particles as well. Thus a network of polymer molecules and suspended particles builds to form agglomerates or flocs. The bonding of these flocs is much stronger than in the case of the natural agglomerates, or those formed by pre-treatment with inorganic coagulants. These long, linear polymer molecules can exhibit overdosing effects when excess polymer blocks new charged sites for further agglomeration. This results in reduced size and strength of flocs. New polymers have therefore been developed in the past decade, which are non-linear, with cross-linking between polymer chains. These polymers do not suffer from overdosing problems. Because of the cross-linking of the molecular chains tend to be more difficult to dissolve, and are therefore supplied as fine powder dispersions in

Flocculation

219

oil. Another advantage of the cross-linked polymer is its ability to reform flocs after they have been broken. Polyelectrolytes are formed by co-polymerising various proportions of cationic or anionic monomers with nonionic monomers. The relative proportions of the ionic and nonionic monomers dictate the relative ionic charge strength of the final polymer, from fully cationic, to nonionic, to fully anionic. Rarely, polymers are made with both anionic and cationic charges.

5.2 Polymer Solution Make-Up Flocculants from polymer suppliers come in several forms. A large a m o u n t is supplied in solid form, as powder, granules, or beads. An equally large a m o u n t is supplied as solutions, emulsions, or more c o m m o n l y dispersions, in concentrations from a r o u n d 15% up to 50% of polymer in the dispersant. The solid products require the most care w h e n being dissolved. W i t h o u t such care, pockets of undissolved powder will form, surrounded by a partially dissolved jelly-like mass, making further solution impossible. These pockets w h e n small are sometimes known as "fish eyes".

5.2.1 Dissolving solid polymers There are a n u m b e r of ways to dissolve the solids, depending upon the quantity to be prepared. For the purpose of this discussion, three orders of magnitude of batch size will be considered. These are a quantity of 1 O0 to 200 ml for laboratory evaluation, 25 to 100 1 for small-scale decanter tests, and 1 m 3 or more for plant use. The small laboratory samples m a y be prepared by slowly sprinkling a weighed powder sample onto a stirred measured volume of water, and then agitating the sample for up to half an hour. W i t h o u t meticulous care, quantities of the powder will agglomerate before dissolving, after which getting t h e m into solution is impossible. To overcome this problem and aid dissolution, the aliquot of solids can be dispersed into a small quantity of volatile solvent, in which the polymer is insoluble. This is slowly poured into the measured quantity of water with stirring. The solvent is then allowed to evaporate, while the polymer dissolves into the water. Methanol is a c o m m o n reagent used with acrylamide polymers. However, here again care has to be taken, as methanol will dissolve some polymers t h a t are not based on polyacrylamides. For the medium-size sample, the last method m a y be used with a little upscaling. This time the mixing vessel is a quarter-filled with w a t e r and stirred. The m e a s u r e d quantity of powder is then sprinkled into the r e m a i n d e r of the water as it is squirted, u n d e r pressure, into the stirred vessel, which is finally

Flocculation

221

made up to the required volume. A solvent dispersant can be used if necessary, but this has generally been unnecessary. Usually, production quantities are made up automatically. The standard automatic make-up plant will consist of a mixing vessel, into which w a t e r is admitted at a constant rate. The solid polymer is metered out from a hygroscopically secure hopper, using a screw feeder, into the incoming stream of make-up water. Some automatic systems use an air blower to convey the polymer, entering the air stream via a venturi, to a mixer, where the water enters with a cyclone action to keep the powder a w a y from the mixer walls. From the mixer the product falls into a stirred ageing vessel. The physical nature of solid polymer is such that it requires a finite time, at least half an hour, to fully dissolve and for the molecular chain to u n w i n d and become fully functional. Once made up in the mixing vessel, the solution is aged with gentle stirring for the requisite time, usually at least half an h o u r and up to one hour, before it is transferred to a second tank used for feeding to the decanter. An example of an automatic make-up system is s h o w n in Figure 5.1. An alternative automatic system uses a series of at least four stirred vessels with overflow from one to another. A low-level probe in the final tank triggers the start of make-up in the first. The total volume of the four or more tanks ensures a mean residence time sufficient for the required ageing period. The shortcoming of this system is that ageing time is not uniform. The age of the solution at discharge will vary, from almost zero up to several times the mean. It is possible to calculate this age distribution [3]. The system is simple and offers fully continuous polymer solution make-up. If the size and n u m b e r of tanks are chosen carefully, the age distribution of the preparation presents no serious problem, apart from the overall size of the system.

5.2.2 Diluting dispersions To make up the dispersions is generally much easier. Two volumes have to be measured out, albeit of considerably different sizes. These are the w a t e r itself and the polymer as supplied, which as stated will be a n y t h i n g from 15 to 50% active ingredient. They both have to be measured or metered to ensure the correct ratio for the desired concentration of the active ingredient. The use of a surface-active agent to aid dispersion is sometimes made. This is to ensure an intimate mix of the polymer in the water, by reducing surface tension. It also helps to minimise the formation of "fish eyes" small globules of undispersed polymer surrounded by a viscous layer of dissolved polymer of high concentration. This problem is not so prevalent with dispersions. Ageing is still required although not necessarily for so long. The automatic systems make use of a small metering pump for the polymer dispersion. Some automatic systems are designed to handle both solid-grade polymers and dispersions.

222 PolymerSolution Make-Up To meter the two flows, a flow meter, a variable orifice meter or eddy current or sonic flow meter, for the water is generally used, and a metering pump, generally a progressive cavity pump, is used for the polymer. Unlike for solid polymer make-up, for dispersions the make-up plant is the same basic design for all sizes unless it is to be made up by hand, which is possible for small test runs. 5.2.3 Final flocculant solution characteristics

Nothing has yet been said about the concentration of polymer required, as it is to be fed to the centrifuge. A level of O. 1% of active ingredient is very common and could be considered the norm. It is more common to quote solution strengths in terms of active ingredient, but polymers, which are supplied in solution or as a dispersion, are sometimes quoted in terms of percentage of "as supplied". Care must then be taken when assessing performance. Because of the physical nature of the solid polymer, some suppliers add small amounts of chemicals to enhance the flow properties or aid dissolution. When assessing polymer dosage these additives also need to be taken into account. This again emphasises the need to record polymer dosage as kilograms of active ingredient per ton of dry solids in the feed, when assessing relative performance levels. It should be appreciated that all solid-grade polymers will contain a small amount of moisture. Conversely to what has been said, this moisture ought to be included with active ingredient figures when measurements are made. For instance, sometimes it is found necessary to check the concentration of polymer solutions by gravimetric analysis, which is done by evaporating a sample to dryness. Then it should be remembered to analyse a sample of the raw polymer in the same way. This is so that the solution is recorded correctly as weight percent of the solid polymer as supplied, rather than as true active ingredient. Percentage active ingredient is satisfactory so long as the percentage active ingredient in the supply is known, and appreciated by those concerned. Solid-grade, high molecular weight polyacrylamides can, reasonably easily, be made into solutions of up to about 1% by weight. Above this concentration, the solids become very difficult to dissolve. More normally the maximum strength of make-up is 0.5 to 0.7%. At these concentrations the solutions are extremely viscous, and they would then be further diluted before use, either in line on the way to the decanter or in a separate mixing tank. Two extreme examples of polymer viscosity are shown in Figure 5.2. These are not necessarily typical of the polymers specified, but just examples to show the viscosity extremes that could be encountered. The viscosity of a polyelectrolyte solution is a measure of not only its concentration, but also of the molecular weight of the polymer. Moreover, violent agitation, or circulating the solution under pressure t h r o u g h an

Flocculation

12000

_ , -- ~ o m c ;

10000

"~

+

MW; ~

m~m

MW; ~

~fiomc; m ~ m

8000

~

6000

~

c~. i

223

i i,

j

-' '

I

o

i

4000

t

i

2000 m m i m

0

...~ , ..... 0

0.2

?'" 0.4

Polymer

~ I

m O

~

i 0.6

Concentration

'

o

--

"

i

~

, I"'

0.8

1

1.2

% w/w

Figure 5.2. Viscosity ranges of polyelectrolyte solutions.

orifice, will reduce the efficiency of the flocculant by breaking the molecular chains, which will reduce the viscosity of the solution. Polymer solutions lose their potency with age. The more dilute the polymer, the quicker is the deterioration. Making up with hard water also makes deterioration more rapid. Thus, in preference, it is better to make up with soft water as concentrated as possible, and only dilute to the required dilution just before use. Figure 5.3 shows some results of work testing the effect of water hardness. The dewaterability of the sludge was gauged by the a m o u n t of s u p e r n a t a n t produced after a set spin in a laboratory bottle centrifuge. Note the correlation of the fall in polymer solution viscosity with increased water hardness and resulting decrease in polymer efficiency. The more dilute the polymer, the more efficiently it works, providing optimum mixing with the process liquor. However the bigger the difference between the viscosities of the polymer solution and the process liquor, the more difficult is the mixing. The higher viscosity component tends not to break up and disperse so easily as the other liquor. Another factor that affects the choice of the polymer concentration is the quantity to be used, compared with the quantity of process liquor. Once the size of the decanter has to be increased to cope with the extra volumetric load of the flocculant solution, then dilution has gone too far. The flow of flocculant solution should be not more than, say, 10 or 1 5% of the feed rate, on this basis.

224

Polymer Solution Make-Up 160 140 g~

120

I

\

100 e~

o e~

J

I

i

\

Viscosity" O. 1 % soln. 75

{

~4~ at

,ha

t

80

80

I J

i ~

r

t,

60

%---.-,.,

.......

70

tj

t

T

'

65 y~

40 20

i 0

!

1 O0

200

300

60 400

Total Hardness TS mg/! Figure 5.3. The effect of water hardness on polyelectrolyte solutions.

5.3 Polymer Choice The first choice to make in selecting a polymeric flocculant is its ionic charge, w h e t h e r cationic or anionic, or even nonionic. The pH and type of sludge dictate this. Anionic polymers are more effective in alkaline solution, while cationic polymers prefer acid solutions. In general, pH should be no more t h a n one or two units away from 7. However, w h e n using p r e - t r e a t m e n t chemicals, the usable pH r a n g e can double, as some of the flocculants can be more effective in the presence of the chemical. Anionic and nonionic polymers are better for flocculating inorganic slurries, such as minerals. Cationic flocculants are more frequently used with organic sludges, such as sewage. Nonionic polymers would be the first choice with sludges which are very acidic. In decanter applications, the higher molecular weight polymers predominate. Nevertheless, occasionally some lower molecular weight polymers are found very effective w h e n used on some decanter centrifuges. Cross-linked polymers are finding frequent use on decanters, with both municipal and, particularly, industrial effluents. W h e n a sample of sludge arrives in the laboratory for flocculant assessment, there are a n u m b e r of c o m m o n tests w h i c h are conducted. Its settling rate u n d e r gravity is observed and its pH is measured. A small sample, about 50 ml, is spun in a laboratory test-tube centrifuge for a fixed time, say 5 to 10 minutes. The g-level at the tip of the centrifuge will be of the order of 1 5 0 0 to 2 0 0 0 g. The volume of sediment will be recorded. The a m o u n t of solids in the feed will be analysed gravimetrically. Normally this is done as a total solids measure, by evaporating a weighed sample to dryness. However, occasionally the suspended solids content requires to be k n o w n . Then, besides a total solids analysis of the whole sample, a total solids content will be measured on a sample of s u p e r n a t a n t or filtrate, and the suspended solids will be obtained by difference. In m a n y sludges, such as municipal effluents, the dissolved solids are 0.1 to 0.3% w/w or less, and a relatively small fraction of the total. However, in some other sludges, the dissolved solids are a significant fraction of the total, and differentiating between total and suspended solids is important when assessing the centrifuge performance. Centrifuges separate suspended solids, but c a n n o t be

226 PolymerChoice expected to remove dissolved solids. Using, for calculations, total solids in the feed and suspended solids in the centrate is common practice, e n h a n c i n g the perceived separational efficiency of the centrifuge, but generally only marginally. However, this should not be done when the dissolved solids are high relative to the amount of suspended solids in the feed. Other analyses on the sludge, such as particle size distribution measurement of the suspended solids, viscosity, density, and chemical and biological oxygen demand, are all done on particular sludges from time to time, but generally do not affect the choice of flocculant. They all help to build a picture of the sludge to be processed, help to compare the sludge with others from past experience, and will be useful for particular aspects of the separation. For example, size distribution is important in classification, and density is important when assessing the m a x i m u m safe operating speed of the centrifuge. The analyses mentioned so far all help to form a picture of the sludge to compare with past experience. This experience will enable the choice of a range of likely polymers that might be effective. A selection of a n y t h i n g from 10 to 20 polymers would be made, and preferably fresh samples of the flocculant will be made. Alternatively, already prepared concentrated solutions will be diluted to provide, say, 100 ml samples at 0.1% concentration. Alternatively, for expediency and economy, old (but not too old) samples will be used initially, and then a narrower range of fresh polymer samples will be made up on the basis of the initial assessment. The initial assessment is made by adding aliquots of each polymer to separate measured quantities of sludge. There are many ways of performing this initial polymer evaluation, both empirical and rigorous [4], each depending upon the preferences of the analyst, but many are similar with the same objective. One example follows. The 10 or 20 polymer samples (10() ml) are placed in line on the bench. In front of each polymer container a 100 ml measuring cylinder of sludge is placed. A sample of approximately 20 ml of the first polymer is placed in a 200 ml beaker together with the sludge sample. These samples are mixed, by pouring them into a second 200 ml beaker and then successively pouring back and forth from one beaker to the other five or six times. This is repeated for each of the polymer samples in fresh beakers. Each of the products is then examined and assessed and compared. One will be looking for the largest stable floc with the cleanest supernatant. If no, or just poor, flocculation has taken place, a further 20 ml of each polymer is added and the procedure repeated. Naturally, the quantities mentioned would be adjusted before starting, given experience from earlier analyses. If flocculation after the second addition does not produce a result, a decision has to be made whether to restart the assessment with reduced polymer quantities because of overdosing, or whether to induce some granulation with a pre-treatment or by pH adjustment.

Flocculation

227

From the initial assessment, a selection of, say, three to six polymers will be made. Any polymer which does not perform on the bench will not perform in the decanter. Any polymer which does perform on the bench is a candidate for the centrifuge, but it is not a guarantee that it will work well there. The retest of the narrow range of polymers will be started as before with 1 O0 ml of sludge for each of the polymer samples. This time smaller samples of polymer are accurately measured by pipette and added successively to the sludge sample. The size of the polymer sample, 1, 2, or, say, 5 ml, is chosen depending upon the results of the initial assessment. At each addition, the size of the floc and the clarity of the supernatant are recorded. The number of pours to form the flocs could also be recorded. From this retest, one to three polymers would be selected for further evaluation and/or testing on the decanter, on the basis of clearest supernatant, largest and strongest flocs, and the quickest formation of flocs. Any bench retesting would be to refine further the amount of polymer required, to see how strong the flocs are, how long they take to form, and whether they have the ability to reform once broken. Further laboratory tests may be instituted to assess the dewaterability of the flocs produced. There are a number of techniques employed for this. A flocculated sample probably would be recentrifuged in the laboratory bottle spinner to observe any change in the settled volume. A favoured device for assessing dewaterability is the CST, capillary suction test, apparatus [5,6], as depicted in Figure 5.4. A piece of filter paper is placed over two concentric circular electrodes embedded into the surface of a fiat Perspex plate. A small open-ended cylinder of about 10 ml capacity is placed at the centre of the filter paper concentric with the electrodes. A sample of flocculated sludge is poured into the cylinder, after which supernatant liquor seeps outwards through the pores of the filter paper. An electronic device detects when the second electrode is reached. A similar test on the untreated sludge may be conducted. A CST time of a few hundred seconds would be typical for a raw effluent sludge. A good flocculated sludge would give a CST time ofless than 15 seconds, or even below 10 seconds. An alternative or adjunct to the CST is a laboratory filter press. A fixed pressure is applied to a cylinder of sludge and the quantity of filtrate is measured. The cake dryness and thickness are also measured and compared. The ability of a sludge to dewater, which is what the CST and filter press are measuring, is a measure of the dryness achievable. In that different polymers give different CST values, it follows that different polymers can be instrumental in achieving different cake drynesses on the decanter. This is found to be so in practice.

228

Polymer Choice

9

~.-~:, m

Fiqure 5.4. A CST apparatus.

5.4 Pretreatment If laboratory tests show that flocculation is difficult, impossible, or requires excessive quantities of flocculant, then a primary coagulant may be considered. This adds an extra dimension to the laboratory tests and will invariably require flocculant reassessment due to the resultant change of pH. Quite often, the ionic activity of the flocculant required would change, from cationic to anionic or vice versa. Common coagulants that are used include aluminium sulphate, ferric sulphate and chloride, lime, and polyaluminium chloride, all except lime, being multivalent inorganic chemicals. The optimum amount of addition may cause the pH to move beyond the normal range to between 3 and l l. The optimum a m o u n t of coagulant, which could be in the range of 5 0 - 3 0 0 kg/ ton, can produce a matrix of fine, weak granules. However, the proof of the optimum quantity is whether good flocculation can then be achieved, and preferably at less cost than with the coagulant. The use of a pre-treatment with a coagulant does have disadvantages apart from the extra cost. Extra equipment is required to control the addition. Some of the chemicals can be quite corrosive. An extra solids load is placed on the effluent system. Extra pH adjustment may be required for the clarified liquid after separation, and before disposal. Another form of pre-treatment, which is rather unusual, is the addition of a small dose of anionic polymer to initiate granulation followed by a larger dose of cationic polymer. On the decanter centrifuge, the anionic polymer is added upstream of the centrifuge while the cationic polymer is added into the bowl. This type of treatment would not be chosen lightly as it would require duplication of all the make-up addition and control equipment.

5.5 Admitting Flocculant to the Decanter The laboratory tests will advise the operator as to the quantities of polymeric flocculant that will be required. The equipment will be set up capable of supplying in excess of this q u a n t i t y by at least 50% and perhaps 300% w h e n dry solids operation is planned. It will be seen in subsequent chapters that three times the normal a m o u n t of polymer can be used in dry solids operation w h e n the driest cakes are required. W h e n applying the flocculant to the sludge, there has to be good intimate mixing, otherwise excess polymer will be needed to ensure full flocculation. Sufficient time has to be allowed to ensure o p t i m u m flocculation before settlement commences, to maximise the use of the sedimentation and compaction zones in the decanter. Flocculation m u s t not be too early, such t h a t major breakage of the flocs occurs before sedimentation commences. W h a t is the normal or correct addition point varies depending upon the design of the centrifuge, the type of sludge being processed and how the centrifuge is being operated. For a slow-speed decanter it often suffices to admit the polymer into the feed zone. For the high-speed centrifuge it is mostly necessary to admit the flocculant into a separate chamber, the floc zone, for it to enter the pond separately and mix with the feed there. Flocculant can be added before the centrifuge but, as just stated, with the higher speed centrifuges it is often necessary to add it inside the bowl to avoid break-up of the flocs on entry. Figure 5.5 shows the effect of inline dosing. In this particular test some polymer was added in-line, and the more polymer added in-line the more was the total polymer needed to m a i n t a i n centrate clarity. It will be seen that cake dryness increases by m u c h less t h a n 1% for the addition of an extra 70% polymer. This is for a cationic polymer addition to digested sludge in a high speed 73 7 m m diameter bowl decanter. This is not m e a n t to infer t h a t this happens universally. For a different process stream with a different design of m a c h i n e , set differently, the reverse could occur. The graph is to demonstrate that the point of admission for the flocculant can be critical, and steps always m u s t be taken to check it on new applications. As another example, anionic polymers tend to be slower acting t h a n the cationics and, even with the high-speed centrifuges, often have to be added u p s t r e a m of the centrifuge.

Flocculation

24

i

23.5 ~:

i

io

23

, .

231

i

~

9

1

~ ~2.~ e

22

t

~~ 21.5 1 a 21

I

20.5

f

20, 0.00

2.00

4.00

6.00

8.00

10.00

I n - L i n e P o l y m e r D o s e k ~ t db

'8t

-~ 17 16

ii

15 o 14

/ Y

13 ~ 12

J. O

~

11

~ 10 ,.I' o

[-

9

0.00

2.00

4.00

6.00

8.00

10.00

In-Line Polymer D o s e kg/t db Figure ~. 5. l~ffect qf in-line polynzer addition.

Adding the flocculant u p s t r e a m of the d e c a n t e r is u s u a l l y done by t e e i n g into the feed line, and any inefficiencies in m i x i n g are o v e r c o m e at e n t r y into the centrifuge, ttowever, c o n s i d e r a b l e w o r k has been d o n e i n d i c a t i n g the i m p o r t a n c e of mixing [7]. In s o m e plants, in-line static mixers have been used. Some studies h a v e been c o n d u c t e d into the g r o w t h and b r e a k a g e of tlocs, and a m e t h o d has been developed to m e a s u r e the extent of floc b r e a k i n g in a centrifuge [8]. The cross-linked polymers h a v e the ability of r e f o r m i n g flocs, and do not greatly exhibit the p h e n o m e n o n of o v e r d o s i n g . These c a n often be added u p s t r e a m . Some o p e r a t o r s prefer adding this polymer u p s t r e a m , b e c a u s e it is claimed t h a t some b r e a k a g e of flocs on e n t r y to the c e n t r i f u g e is beneficial in releasing m o r e w a t e r . This is confirmed by a lot of w o r k [ 9 - 1 2 ] . w h i c h has

232

Admitting Flocculant to the Decanter

s h o w n correlations between agitation, floc size and floc density. It has been s h o w n that floc density markedly decreases with increase in size. With dry solids operation, extra polymer allows extra dryness a n d / o r extra capacity. The polymer requirement increases exponentially with capacity. Flocculant requirement in dry solids operation is covered again in the next chapter, but no-one has given a satisfactory explanation as to w h y this requires extra polymer, and one can only guess. Extra dryness requires extra torque on the conveyor putting more stress on the cake. Flocs tend to be larger and stronger as more polymer is added. Excess flocculant could be beneficial in assisting with re-flocculation of any damaged and broken flocs. The increased floc stability with increased polymer dosage is s h o w n in the g r a p h in Figure 5.6, where stability is measured by the n u m b e r of pours, from one beaker to another, needed to break up the formed flocs. The value of extra dryness varies from plant to plant and c o u n t r y to country. When thermal dewatering is required after centrifuging, it pays to d e w a t e r as m u c h as possible mechanically in the decanter, which is m u c h cheaper than t h e r m a l costs. However, if the cake is to be incinerated then there is no economic advantage in drying in the decanter m u c h beyond its dryness when it will burn autothermically. 160 140 o

120

-~ 100 U s_

m

80

0

"" 60

i

t_

o

,

40

"

,

,

i i i

I |

L

10

12

14

/ !

20 |

2

4

6

8

P o l y m e r D o s e kg/t db

Figure 5.6. Increase qffloc stability with polztmer dosage.

16

5.6 Flocculant Suppliers There have been several large m a n u f a c t u r e r s of polyelectrolytes. However, the polyelectrolyte industry is no different to many others, with take-overs and buy-outs, such that today there are just four major manufacturers. These are Ciba (with the former Allied Colloids), Cytec (formerly Cyanamid with the former BTI company), Floerger, and Stockhausen. Nevertheless, there are a large n u m b e r of other major companies supplying flocculants u n d e r their own brand names, having obtained supplies from one of the main manufacturers. These supplies can be to the manufacturers' or their own specitication, and then perhaps blended to their own special recipes. The flocculant market is a very competitive one. Flocculants are sold at several thousands of pounds sterling per ton, with some users p u r c h a s i n g several tons every few months. Having a product that performs at (). 1 or 0.2 kg/ton less than anyone else can be a major economic advantage. On the other hand a polymer which can be shown to produce 1 or 2% extra dryness or 1 or 2% extra recovery, in some fields, is well worth the extra cost of the polymer. It would be useful for the reader to have a table of polymers for each supplier, with molecular weight, ionic charge and cost, with equivalences between the suppliers. However, apart from the commercial reticence of suppliers to participate, such a table would soon be out of date. New and more effective products are continually being introduced to the market. T u r n o v e r and competition enable modification of the price. Moreover. the determination of the very high molecular weights is difficult, and for some, conjectural. If one product enables good separational performance, most polymer suppliers will be able to offer their "equivalent". or w h a t they would assess as something more efficient. Most suppliers will provide a user with an outline of their products, w h i c h are anionic and which are cationic, and their relative molecular weight. They will also make suggestions as to w h e r e the user should make a selection for the application. Most suppliers would offer a service, to bona fide users, to conduct a flocculant assessment on a process liquor sample from the user. It is w o r t h w h i l e noting that some suppliers market their products with different identification numbers and names in different countries.

234

Flocculant Suppliers

In some applications, a lower efficiency but m u c h lower cost m a y be preferable for the user, r a t h e r t h a n a high-efficiency, high cost product. Each user must evaluate the economics of operational factors specific to the application.

5.7 Low-Toxicity Polymers There is a d e m a n d for " n o n - t o x i c " flocculants for use in potable w a t e r treatment and similar applications. The toxic component of polymers is the residual acrylamide m o n o m e r content. To enable a formal classification of low toxicity, the acrylamide content must be 0.025% or less for use in most European countries (0.05% in the USA). Some standard polymers meet this requirement, but extra work, and thus extra cost, is required to provide the certification and enforce the quality control. When considering the q u a n t i t y of m o n o m e r that contaminates the clarified liquor from a decanter, it needs to be appreciated that most of the polymer added to the system ends up in the cake. This is unless there is a vast a m o u n t of overdosing or there is little or no flocculation. There has been interest in using flocculants for foodstuffs, particularly animal feeds. However, because of the public concern about c o n t a m i n a t i o n of the food chain, there has been very little use in this area. Most m a n u f a c t u r e r s offer a range of low-toxicity-grade polymers.

5.8 Applications A large percentage of flocculants used with decanter centrifuges is for the treatment of municipal and industrial effluents. Nevertheless, there are other large applications such as mineral processing. Whereas in effluent t r e a t m e n t polymer usage is generally in the range 2 . 5 - 5 kg/t db, minerals require generally less than 1 kg/t db. In coal washing, for instance, 0 . 2 - 0 . 3 kg/t db is not u n k n o w n . Dry solids operation on the decanter often demands m u c h more polymer, even as much as 1 5 - 2 0 kg/t db for the driest cakes. Surprisingly, good separation can be achieved in the decanter on a good (SVI, settled volume index, of 120 or less) activated municipal effluent. without the use of a tlocculant. This is possibly due to the h o m o g e n o u s n a t u r e of an activated sludge. Industrial wastes include sludges from paper mills, de-inking plants, tanneries, creameries, potable w a t e r w o r k s and m a n y food processing plants. For these the choice of polymer will vary from plant to plant, due to large variations in the make up of the sludges which can occur. Most municipal sludges can be separated on the decanter, the most common of which are: 9 9 9 9 9 9 9

primary; co-settled humus, or activated, and primary; mixed and secondary, aerobic or anaerobic digested; oxidation ditch; carousel: frothflotated: and whole aerated.

Specially treated sludges, such as biological and limed sludges, can also be separated on the decanter.

5.9 Performance There are m a n y ways in which the performance of dosing flocculants m a y be represented graphically. Figure 5.7 show the results from a 737 m m d i a m e t e r decanter in 1990, thickening a 2% digested sewage. It shows the effect of increasing polymer dose and changing feed rate on centrate quality. An alternative presentation is shown in Figure 5.8, where each line represents a particular polymer dose level, and the effect of cake dryness with feed rate is examined. This work is again on digested sludge from a different plant using a 425 mm diameter decanter. In dry solids work it is more usual to plot dryness against polymer dosage for fixed feed rates, as in Figures 5.9 and 5.1 (). Figure 5.11 shows the effect of not only a change in feed rate, but a change in the polymer specification. This clearly demonstrates that the choice of polymer can influence the final dryness. Figure 5.12 summarises the effects of the various variables of d e c a n t e r dewatering with a flocculant. 8000

i

T

7000

y= -

.~______~--- "--65 m3/h

6000 g~

["

"0

5000

o r~

4000

em I

40 51) 85 m3/h]

x 9

-t~X

3000 2000 1000

II

0 2

2.5

3

" 1

3.5

z

P o l y m e r D o s e kg/t db Figure 5.7. Centrate solids against polymer dose for various feed rates.

4.5

238

Performance 32 l 30*6ke~ 28

ms kg/t db

L

9 10 kg/t r

26

~24 m 22

2O 18

I

0

10

20

;0

40

Feed Rate m 3 / h Figure 5.8. Cake dryness againstfeed ratefor various polymer dosages.

28-

1

26

......t-

t

24

"-"-4V-

t O

22

I (

~20 ,.~ m 18

1

16 ~14 0

2

4

6

$

10

12

14

P o l y m e r D o s e k ~ t db Figure 5.9. Cake dryness against polymer dosage: DS operation.

16

Flocculation 32.

I

I

30.

f

28

/.

~ 26 ~

'

/

I

9

, Y ,h'// ' / _.//.,,

24

I

/

22

.~ 20 ~J

/

t 9 7 m]/h

18 161

140

2

~"t i

1

4

6

9 14 m3Pa

]

A 28 m3/h

I

1

8

10

12

14

I

l

1

16

18

20

22

Polymer Dose kg/t db Figure 5.10. Cake dryness against polymer dosagefor various feed rates.

30 .... I

28

r~

I

9

I~

26

I

~' I

24 .,g

9 20 m3/h: Polymer A 1

22

1 30 m3/h Polymer A 9 20 m3/h Polymer B

0

2

4

9 30 m3/h Polymer B

I t

I ....

20

6

8

10

12

14

!

1

16

18

20

Polymer Dose k ~ t db Figure 5.1 1. Cake dryness against polymer dosage for two feed rates and two polymers.

239

240

Performance 36 I "

34 INCFLEASING TC!RQUE

,\

32

____~i~

~ . ~ ~

30 z

!

24

FEED

RATE i !

22

'i 16 0

2

. 4

I 6

.. 8

POLYMER

! 10 DOSAGE

12 k g / t db.

Figure 5.12. Summary of effects of parameters in DS operation.

14

5.10 References G M Moody. Pre-treatment chemicals. Filtration+Separation, April 1995 R Hogg. The role of mechanical agitation in flocculation and dispersion of mineral particles. XVIth International Mineral Proc. Congress, Stockholm, 1988 3 JAWWA. Jan 1975, p. 52 4 R Hogg, P Bunnail, H Suharyono. Chemical and physical variables in polymer-induced flocculation. AIChemE Symposium Solid/Liquid Separation in Industry, Pittsburgh, July 1991 5 CST apparatus made by Triton Electronic, Bigods Hall, Great Dunmow, Essex, CM6 3BE, UK. J Institute of Water Pollution Control 2 (1968) 6 C P Werle, J T Novak, W R Knocke, J H Sherrard. Mixing intensity and polymer sludge conditioning. ] Env Eng 110 (5) (1984) 7 R Hogg, A C Maffei, D T Ray. Modelling of Flocculation Process for Dewatering System Controller, Society for Mining Metallurgy and Exploration, Littleton, CO. 199() 8 D J Bell, K H Brunner. A method for the evaluation of floc break-up in centrifuges. Filtration+Separation, July/August 1983 9 R C Klimpel, C Dirican, R Hogg. Measurement of agglomerate density in flocculated fine particle suspensions. Particulate Sci Technol 4 (1986) 45-59 10 R C Klimpel, R Hogg. Evaluation of floc structures. Colloids and Surfaces 5~ (1991) 279-88 11 R Hogg, R C Klimpel, D T Ray. Agglomerate structure in flocculated suspensions and its effect on sedimentation and dewatering. Minerals and Metallurgical Processing May (1987) 108 12 A DAndreadakis. Physical and chemical properties of activated sludge. Floc Wat Res 27 (12)(1993) 1707-14 l 2

This Page Intentionally Left Blank

CHAPTER 6 Test Work and Data In spite of the copious theories for the decanter centrifuge, it is not possible to predict performance with any particular process material simply from knowledge of pertinent physical p a r a m e t e r values, that is, without prior knowledge of the process material's behaviour on a decanter. Test work is essential on any new, or u n k n o w n , sludge and on any sludge, such as effluents, that can vary widely in composition a n d / o r quality. The decanter chosen for any test work could be a small laboratory machine, a n y t h i n g from l O0 to 2 50 mm in diameter, or an industrial size, up to the full size expected. Occasionally, but rarely, a decanter larger t h a n necessary is used, w h e n the data need to be scaled down. This would occur when the larger m a c h i n e happens to be more readily available. Occasionally again, when there is some previous experience available, it is sufficient to conduct a few laboratory analyses to compare with the previous experience before a performance prediction is given. The main object of test work is to be able accurately to predict performance and to size equipment necessary for full-scale operation. Naturally, w h e n installed, new equipment will need to be tested to confirm the performance predicted or guaranteed. Decanter m a n u f a c t u r e r s and specialists also conduct test work not only to demonstrate the advertised performance of the decanter, but also to assess new designs and features, and new process sludges. The ancillary equipment required for a test plant is essentially the same type, w h a t e v e r the size of the test decanter. However, it is easier to make do on the smaller size plant. While a bucket and stopwatch will suffice with a small plant, i n s t r u m e n t a t i o n is preferred on the larger sizes. Similarly, small collector bins can be used with the smaller equipment, but some mechanised conveying system would be required on a large decanter, unless it could be m o u n t e d over a large hopper. For a good test facility, one t h a t will yield reliable data, it is necessary to have it as well instrumented as possible, with a u t o m a t e d flow and m e a s u r e m e n t of feed and off-take streams. It is necessary to keep the operator

244

Test Work and Data

flee, to observe and record the performance, unfettered from having to control and conduct the h u s b a n d r y of the plant. Such a system is described more fully in the next section.

Flocculant[ i i

f'

~ow

Decanter

/___k

Polymer Pump

Feed Pump _

te l

Make-U Tank

Cake F(qure 6.1. A decanter test facilit~l.

6.1 Test Equipment Figure 6.1 is a sketch of the flow and i n s t r u m e n t a t i o n of a full test facility, including polymer make-up and addition equipment. The main equipment around the decanter test machine includes the process feed supply, as well as the polymer make-up system, and the cake and centrate off-takes. Each pertinent piece of equipment is described in turn below. The decanter needs to be steadfastly mounted, level, on a firm base, and sufficiently high to allow good access to couple up discharge off-take facilities. Within the decanter's own start-up and control gear usually will be instruments for continuously monitoring the bowl speed, the brake speed, and the brake or conveyor torque. Ideally the conveyor torque or differential will be automatically controlled by a simple PLC controller. The feed vessel needs to be stirred to ensure that a uniform sample is supplied to the decanter, unless one can be sure that the contents are uniform and no settlement will occur over the period of the test work. Often the vessel will be the process plant itself and all that is needed is a tee in the process pipeline. The feed pump needs to be a variable speed metering type. Usually this is a progressive cavity type, although with some sludges variable speed gear pumps have been used successfully. Valve-restricted centrifugal pumps have been used, but these have generally proved quite unsatisfactory. The same can be said of using a pressure head and restrictor. With slurries, intermittent build-up of solids occur on the upstream side of the valve, making control of the test work impossible. With the smaller decanter, only a small feed tank and stirrer are necessary when remote from the feed source. However, care needs to be taken to ensure that variations from batch to batch are minimal, or that the tank size is sufficient to reduce the n u m b e r of feed batches necessary. A calibration of the feed pump is useful, to facilitate a speedy rate setting, and is a useful check of the flow meter calibration, but is not a recommended alternative, as most pumps will wear and lose their calibration with time. The polymer make-up vessel, on larger a u t o m a t e d plants, will be part of an integrated make-up system. It will have an automatic controller governing the raw polymer feeder rate, feeding from the hopper and controlling the

246 TestEquipment feeder speed, the time the feeder is on, the polymer ageing time and transfer pump actuation. It will also act upon signals from the various level probes. The controller will further control the opening of the water valve, and count the batches of polymer used. A second vessel receives the aged polymer, and acts as the polymer supply tank for the decanter. [n smaller test facilities the polymer will be made up m a n u a l l y and batchwise. In these smaller tests the size of the m a k e - u p vessel needs to be large e n o u g h to ensure a sufficient supply for several test runs. The polymer vessel could be duplicated to ensure not r u n n i n g out during a test. However, the practicalities of c h a n g i n g vessels during a test have to be considered. These include prevention of air getting into the feed line and the consistency between batches. The polymer feed p u m p also needs to be of the variable speed and metering type. The progressive cavity type is the first choice, but peristaltic designs are quite acceptable. Piston pumps have been used, occasionally, with alleviators to smooth the flow. So long as the pulse frequency is high, there is not too m u c h need for an alleviator. A good calibration of the polymer pump, whichever type is chosen, is very useful in conducting the tests, and can be a n alternative to the flow meter, as the polymer p u m p does not so readily w e a r and lose calibration with polymer. The peristaltic pump is expected to wear, of course, as it can wear on the outside of its tubing. On large-scale tests, it is necessary to employ a continuous device to remove the discharged cake from the test ~irea, because of the sheer volume which accumulates in a short time. This device will probably be a simple belt conveyor or perhaps a screw conveyor. Manually removed hoppers, or buckets, can suffice on the smaller tests. Discharged centrate is usually directed to drain but on smaller facilities where i n s t r u m e n t a t i o n is limited it m a y be necessary to resort to m e a s u r i n g the centrate using a bucket and stopwatch technique. The pipe work for the test rig needs to be adequately sized and engineered to ensure free flow of the process materials. Access must be allowed for sampling the four process streams, feed, polymer, centrate and cake. These four samples would be gravimetrically analysed for solids content. There are a few alternatives to the system so far described, depending upon the particular application. Some applications will not n e e d t h e polymer system. On thickening applications the cake will be fluid and thus a tank receiver will be required. For thickened cake an off-take pump, controlled by a level probe, could be used. It would be useful to monitor the thickened cake rate, a l t h o u g h not essential. This could be done by measuring the fill rate of the receiver. Some polymer systems use liquid polymer. Then the polymer powder feeder would be replaced by a very small metering pump. This p u m p would need calibration for monitoring, as a flow meter is impracticable here. Three-phase decanter tests generally do not require polymer, a l t h o u g h some waste oil processes have used polymer. However, by definition, a second

Test Work and Data

247

liquid discharge is present. It is necessary to m e a s u r e the feed flow and one of the two liquid discharge rates, usually the oil or light phase, to enable a mass balance across the decanter. In three-phase work, extra analyses are required, not only of the extra liquid discharge, but also for light phase content of feed, cake and the two liquid discharges. In classification processes, particle size analyses are usually necessary on two or more of the streams. Where the decanter is used to separate ores, by virtue of density difference, e.g. calcium fluoride from barytes, chemical analyses are also conducted on two or more process streams. The precise design of the test decanter needs to be adapted to the process to be tested. Oil/water separation requires a three-phase design, solvents require flameproof electrics, effluents need abrasion protection. While m a n u f a c t u r e r s will have a pool of test decanters of basic design, it is essential that they are able to adapt them for special application testing. For instance, they will have special decanter conveyors that can be introduced, w h e n necessary, and will be able to make small changes to them where necessary, such as the addition of a floater disc for floating solids. With a test decanter it is essential to be able to adjust bowl speed, differential and pond depth to suit particular applications or difficulties as they arise.

6.2 Test Procedures The test equipment is put together as shown in Figure 6.1, or as near as possible to that, w h i c h the available facilities will allow. The ancillary equipment should be sized to cope with the m a x i m u m t h r o u g h p u t likely, down to a capacity of, say, half or less. of that desired or expected. A t u r n down ratio of 1 - 10 is desirable on the variable t h r o u g h p u t capacity pumps. The start-up of the test equipment will rely heavily on past experience with the process selected, or with a process considered similar or comparable. This experience will allow a selection of parameter values, to give the decanter the best possible start, so that the operator can be satisfied that a viable performance is possible. A preliminary run is conducted, to confirm this performance, after w h i c h a p r o g r a m m e for the tests is planned. Commercially, test work is mostly conducted ad hoc, with a view to achieving a commercially successful objective as quickly as possible. Thereafter, small adjustments would be made to improve upon the success. The parameters chosen for investigation will depend upon the objectives, the type of process and how close to the objectives the preliminary test comes. Also the time available will condition choices to be made. A technical test series would be planned differently from a commercial test, under conditions where more time is usually available. With a given test decanter the n u m b e r of variables at the disposal of the test engineer is limited. With a simple dewatering application w i t h o u t flocculant the main variables would be: 9 9 9 9

pond depth: feed rate; bowlspeed; and differential speed a n d / o r conveyor torque.

W h e n using flocculant the extra parameters would be: 9 9 9 9

polymer type: polymer dosage: polymer strength; and polymer addition point.

now1 speed a s a pararnetcr for investigation is used sparjngly. and is generally b r o u g h i in frlr ir~vestigatiori last. ivhen earlier results havc suggested a benefit. i a chariging the bowl speed from thc standard. Pond depth is usually fixed early in the test programme. and only chanjicd aftcr carly data have been analysed t.o show t.hat a pond dept.11(:harlgr: woul(l be of bencfit. Nevcrthelcss, some tests will investigate a wide range of pond dcpths, whcn it has been difficult t.o achicve good centrnte good rake a t the same timc. Whctl illvcstigating a parameter. whosc value c,an be infinitely variable,

such as fccd rate, one should vary it logarithmically rather than ari~.hrrietically.For iristarice, I , 2 , 4. 8 . I h m'/h or 1 , 1.5, 2 , .3, 5 , 7 , 10 rn"/h., rather thari, say, 2. 4 , b , 8,10. Hy this rneiiris a wider pictureof trends may be obtailled with fewer data points. For polymer dosage. it. is g:tjner:JIIy not possihle to be so precise, iis t,he riurnerictil valuc of the polgrricr dose is not kriown until a f k r the analyses hijvc been c,ornplctc. a:, fced solids uoricerltraliori can v:iry from test to test. Fnr a properly conducted lest a rriinirriurn o f l i v r dnta points a r e rcquired for B graph. However, ifthere is not a well-ddined trend. then thcrc will be a iieed for several times that narnbcr of r i ; i ~ . apoints. I!errl rate is most often thc first paranwter t o hr irivestigaled. Thc capacity o f t h e dccantcr is nf fundamental iiitercsl to t hc iiscr. Thc pond lcvul and h ~ ~ w l s p e d will havc hccii fixcd during I hr prelimimiry tcsting. If floccular~lis to be used, thcn a saft rinsing 1t.vt.l will have been detcrmincd and used duririg the prcliminary tests. afi.cr bench valuation of likely p o l y w r s l o bc uscd, 'I'he s d c dosing l w c l will bc such iis to pcrriiit a wide range of' k c t i ratcx and convcyor diffcrentials. without a n y appreriable deterioration i i i ccntrate quality. A series ol'lesls at. say. five differeIit l w d rates will bc cutidut1t.d ijnd smiplcs of feed. (xn1,riiIf:and cake will be taken lor solids aiialyscs. Each test ruri will be conducted with. a s n w r as can he jvdgacl, tilt. samc flocculant dos:igc Icvcl. 'Ihe setting ol'diffcrcntial speed and/or lorquc h r each ruri w i l l depend upon the type of tcst bcing condur:ted. For a silllple drwatcring ~ i r s l ,;I tixed diffcrenlial speed may bc chosen. Ilowevcr, if it is thoiipht that t h c dccantvr may be solids capacity lirniled. thcn the same (tied r;itr/diRtrential speed ral.io could be chosen lor earh test. F(Jr dry solids oper;ilion. ;I tixcd torque would hc morc likely t o he rhossn. AIterri~lilirtIy.for each test t h c minimurn difftrential is found. where thc hest dryness isarhievcd w i 1ho II t dct cr i or at io n o C ce n t r ti t. c gu aht y . For each tcst r u n , the pararnelcr values are set, i ~ n the d decanter is allowed to run [tor a set lime after cquilibriurn has been achieved. 'I'ht.set time would ideally be a l.irrie siifficicnt for there t o have hccn a miriiiriurri 0 1 three bowl voluinc uhariges, since achicving equilibrium. T h m . this time is calculated b y dividing tcit.;il howl holding capacity hy feed rate. or in the case of dry solids operation. by dividing the total bowl holding capacity by the volumetric cake discharge ralc, ;it its discharge dryness.

250

Test Procedures

Once the decanter has r u n at equilibrium for the desired time, sampling can commence. Sampling is often quite cursory, w i t h o u t too m u c h attention to detail, and generally this does not detract from the result. However, nothing is lost in introducing a little method and science: 9 always take the sample in the same order with the same time interval, taking the feed sample first; 9 fill each sample jar with small aliquots over a period of a few seconds, and mix the sample before closing the lid of the sample jar: and 9 do not leave off lids of sample jars, w h i c h would otherwise allow evaporation. Once the first test series has been completed, the data should be graphed and examined to see w h e t h e r an i m p r o v e m e n t in performance is required, or is likely to be achieved by altering one of the other parameters. For instance, if better dryness were required, the test series could be repeated with a lower differential, higher torque or higher bowl speed. For better c e n t r a t e quality, a deeper pond might be chosen. Once a set of data has been obtained which correlates, the flocculant consumption, if used, needs to be optimised. Alternative polymers might be examined if centrate quality had been difficult to m a i n t a i n or if the quantity needed was considered excessive. The relative flow rates of polymer solution, and feed, would be assessed to see w h e t h e r the polymer concentration needs to be adjusted to make it m i n i m u m strength, w i t h o u t causing it to be a large fraction of the total flow. This should not be more than, say, 10 or 15%. Polymer tends to be most efficient w h e n it is most dilute. Moreover it is easier to get a uniform mix of two liquids w h e n they are both of comparable size. However the larger the volume the flocculant is, then the greater is the clarification capacity lost unnecessarily to the clean flocculant. The location for admitting the polymer may be questioned, and considered for introduction further upstream, if flocculation in the centrate has been observed, or if extra dryness is required at the expense of extra polymer in dry solids work. Having decided polymer type, concentration and addition point, the operator can u n d e r t a k e a series of tests of polymer dosage, at the o p t i m u m feed rate found in the first series. Other parameters m a y be tested as spot tests, or as series, depending upon objectives, time available and the results achieved so far. Testing a three-phase decanter will take a slightly different course, because of the extra product stream. The prime objective in the preliminary testing will be to fix the o p t i m u m differential height, between the levels of the two liquid discharges, which can vary with feed rate. The o p t i m u m differential height will be w h e n the efficiency of separation of the two liquid phases is m a x i m u m . If differential height is too small, then heavy liquid phase content will be too high in the light phase discharge. With too large a differential height, light

Test W o r k and Data

2 51

phase content will be lost into the heavy phase discharge. Cresting over the weirs affects the o p t i m u m differential between the weir heights of the two liquid discharges. Hence, changing feed rates will move the e-line in the bowl between the two phases, in or out, depending upon the relative proportions of the two phases in the feed. Thus the objective in preliminary test work, with the three-phase machine, will be to fix the o p t i m u m feed rate and weir heights, or at least fix the weir height for a limited range of feed rates. Conveyor differential would then be the main p a r a m e t e r tested, together with some of the process parameters such as feed temperature. Thickening is a n o t h e r different test series. In the preliminary test work, the objective would be to set the pond level high e n o u g h to get good centrate, and at such a level that the solids discharge thickness can be controlled by conveyor differential speed adjustment. The test work proper would concentrate on a series of ranges of differential for fixed feed rates. Depending upon the initial results, a d j u s t m e n t of pond depth or bowl speed may be considered. The use of flocculants in thickening, as usual, adds a n o t h e r dimension to the test work, and probably will widen the range of pond depths that can be considered. It will also enable higher feed rates. Classification work is similar to thickening. No flocculant is used in classification but occasionally dispersants are used. The use of dispersants is not so complex as the use of flocculants. Dispersants are usually simply added to the feed tank. Turbulence on entering the decanter is an asset rather t h a n a hindrance. The preliminary work will be to select weir height, such t h a t conveyor differential is able to control cake dryness from wet to dry. The test work will be planned to investigate cut point and efficiency for a p e r m u t a t i o n of feed rates and differentials. In classification, conveyor differential is a critical parameter. Too high a differential will create turbulence and carry over of coarse solids into the fines. It can also give a wet cake, where in some processes the wetness will contain fines. Too low a differential will increase hindered settling, allowing fines to be trapped amongst the coarse solids. Testing different decanter designs is best done with two decanters r u n n i n g side-by-side, both being fed from the same feed tank. However, circumstances do not always allow this, and then consecutive testing is employed. This always poses the question as to w h e t h e r any differences found could be due to a change in feed quality. Feed quality is always questioned w h e n performancecannot be explained, and thus this p a r a m e t e r needs to be eliminated wherever possible. W h e n testing different decanter designs, the performance of one usually will already be known. However, if possible, it should be re-tested alongside the new design, to guard against feed change effects or errors in the first series. The exact design of the tests will, naturally, depend upon the new design feature being investigated, and the objective of the new feature. A special conveyor pitch to improve capacity would concentrate on a test series permutating feed rate and differential. A different flocculant zone would invite a test programme series with a range of flocculant doses.

6.3 Test Log It is always preferable to record too much data rather t h a n too little. Data recorded and found of no value shortly after the tests, can prove invaluable in other work, years later. P a r a m e t e r values of the decanter need to be recorded in full, particularly of any special features that have been added after the original m a n u f a c t u r e . Often these details will be encompassed, for brevity, if not confidentiality, in a serial n u m b e r of the machine, plus the date of the test to fix the last modification date. All the design p a r a m e t e r s discussed in Chapter 2 ought to be available for cross-referencing with the test data. For the test series the following decanter details need to be known: 9 9 9 9 9 9 9

Bowl inside diameter: Conveyor pitch and n u m b e r of leads: Baffle diameter (if used) and position: Clarifying length; Cake discharge diameter: Beach angle; Conveyor hub diameter at cake discharge.

If some of these details are not k n o w n at the time of the test, recording the bowl and conveyor (if possible) serial numbers will enable the details to be found later. It is good practice to record serial numbers a n y w a y , for later reference. For each test r u n the following machine p a r a m e t e r values should be recorded where appropriate: 9 9 9 9 9 9

Bowl speed; Pond diameter; Conveyor differential; Light/heavy phase diameter (three-phase); Conveyor torque; Current and power of the motors.

Test Work and Data

2 53

From the decanter data, various pertinent other data can be calculated such as holding volume d o w n s t r e a m of the baffle disc, scaling factors such as Sigma, and m a x i m u m scrolling rates. Process data also need recording, such as feed source, and for each r u n the following need to be recorded: 9 9 9 9 9 9 9

Feed %solids; Centrate % solids; Cake%solids; Polymer % solids; Feed rate; Polymer rate; Polymer addition point.

Any density m e a s u r e m e n t s of any of the process streams are useful additions. For three-phase work, oil content of each phase needs to b e recorded, plus the solids analysis of the light phase product. From each set of test run data, the volumetric rates of each process s t r e a m can be calculated as necessary, plus other pertinent figures such as t o r q u e / volume for dry solids work, polymer dose kg/ton db for flocculant work, gvolumes. An example of a results sheet is shown in Figure 6.2. It is displayed to include polymer addition. For three-phase test work, oil analyses would need to be added, and data for two liquid discharge streams also would need to be recorded. The polymer addition would be omitted where none is used, but would be adapted for the use of rinse. Figure 6.2 is shown as a general result sheet to cover as wide a range of applications as possible. Naturally it can be, and is, adapted for special applications such as classification or, say, refining work, w h e n interest, perhaps, will be more in size distributions and chemical analyses, respectively.

Test Log

254

Results Sheet Machine Location Process

/

Machine 1.Run Number 2.Date. 3.Time Machine Conditions .4.Bowl Speed rpm. 5.Pond dia ram. ,6.Conveyor diff rpm. ,7.Conveyor torque kNm

/

9

,,,

,

9

..

,

!

i

,Feed Conditions

.

,9.Feed 4n=.~Rate Solids m3/h" ~_ ,,,,.re~, ~ w/w d.s. ,Additive Conditions " ,11 .Type. ,12.Concentration %w/w. ,13.Addition point, ,14.Rate m3/h. ,15.Dilution m~/Th.

, I

. i

. . . . .... . .

:

D.Solids Recoven/%w/w. E.C..ake Rate k~)/h w.b. F.Q/I; m ~ G.TN N/cm 2

.

' ,

;' i i

Product cond!tions 116.Cake Solids %w/w. 117.Centrate Solids mg/I. A.Centrate Rate m3/h. B.Polymer O0se kg/tonne..,:

.

;

I

"

i

.

....

.

.

,

,7 ; :

I

l

i

[

I _

,i

,,

....

.

.

.

.

,,,

Figure 6.2. An example of a result sheet.

6.4 Some Test Data Some actual test data are given in the next sections to cover as wide a range of applications as possible, and also to support various aspects of the theories developed in Chapter 4. In order not to overload the reader with too m a n y superfluous figures the tables of data c o n c e n t r a t e on the pertinent figures. The information is gleaned from records covering several decades, and does not necessarily represent the m a x i m u m performance achievable today. In a n y case, most of the process materials will vary considerably in quality from plant to plant. Nevertheless, these data are useful in d e m o n s t r a t i n g the range of performances that have been achieved on the decanter centrifuge, and the trends of performance as p a r a m e t e r values have been altered. It also will provide material for Chapter 7, which is concerned with the scaling of test data.

6.4.1 Spent grain Years ago, the spent grain from distilleries was a waste product. Today, thanks to the decanter, it is a valuable product used for animal feed. Moreover, the lower moisture content of the spent grain cake means better value to the farmer, and also m e a n s extra yield in the main distillery process. Reduced suspended solids in the centrate enables higher concentrations to be produced by the evaporators that follow the separation process. Thus, apart from decanter capacity, the distillery is interested in low suspended solids in the centrate, and the best cake dryness. The data are tabulated in Table A. 1 of the Appendix. In Figure 6.3 is plotted solids recovery against feed rate for three different conveyor differentials. It can be seen that recovery reduces as feed rate increases. The lowest differential also causes a reduction in recovery. This is undoubtedly due to choking of the bowl. Figure 6.4 d e m o n s t r a t e s the reduction of cake dryness with increased differential. It also shows that better dryness, for a given differential, is achieved at higher feed rates. Thus cake dryness has to be balanced against the extent of recovery, for a given capacity. Figures 6.5 and 6.6 show the effect of conveyor differential and feed rate on conveyor torque. From these graphs it can be appreciated that increasing feed

256

Some Test Data 90.0

85.0 80.0

ee

=e 75.0

A

70.0 e,, w 65.0

,ih,

,=,,=

eDiff. 13.2 RPM t ==Diff. 18.2 RPM I 9Diff. 23.2 RpMJ

60.0 55.0

,qlp

50.0 0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

Feed Rate mSlh

Figure 6.3. Graph - R e c o v e r y v Feed Rate - Spent Grain.

30.0

1

i

.

280

I

26.0

gll

=

24.0

E 22.0

a

@

tg

20.0

-[ 94.6 m31h l | 9g.Om3/h 18.0 - 9 918m3/h 16.0 10.0

~, 12.0

14.0

I 16.0

' 18.0

20.0

22.0

24.0

C o n v e y o r Differential R P M

Figure 6.4. Graph-Cake Dryness v Differential S p e e d - Spent Grain.

26.0

Test W o r k and Data

1.80

T T

1.60

~X

'

1.40 ~

1.20

1.00

~

0.80

0.60

0.40 I

1

I

4.0

6.0

8.0

0.20

I

0.00

2.0

0.0

!

1 l&Diff" 23.2RPM~

10.0

12.0

14.0

t

16.0

18.0

20.0

Feed Rate mS/h Figure 6.5. Graph - Conveyor Torque v Feed Rate - Spent Grain.

1.80

it

1.60

~,

I

I

i -,t.l ~\

1.40

i

E z 1.20 Q =I

" 1.00

I.-

.~ 0.80

cO 0.60 tO

L,,0m3,,/ /

o.4o /

99.0m3/h /

/

0.20 ~-& 13"8m3/hI 1 e18ma/h I /

o.oo|

10.0

I

12.0

I

14.0

'~~-4--~

"~,

t

,

16.0

1

18.0

---- ~"

,

20.0

22.0

24.0

Conveyor Differential RPM Figure 6.6. Graph - Conveyor Torque v Differential Speed - Spent Grain.

26.0

257

258

Some Test Data

rate, and increasing cake dryness by reducing conveyor differential, will eventually increase the conveyor torque to the gearbox limit. 6.4.2 A g r i c u l t u r a l products

Various valuable food products can be extracted from fruits and vegetables by mashing and separation, leaving a cellulose by-product usable for animal fodder. Some of these fruits and vegetables lend themselves well to separation on a decanter centrifuge. The data tabulated in Table A.2 of the Appendix are from early development work on one such product. In this process the centrate is the main product. Thus the cake needs to be as dry as possible to enhance yield of the product. The cake is compressible and thus "dry solids" technology could be applied. The first ten runs were conducted on a laboratory decanter of 150 mm diameter. The following year, tests were conducted on a small plant with a pilot plant size decanter of 425 mm bowl diameter. The plant capacity was limited, but results (runs 13-29) were sufficiently encouraging to warrant work on a larger plant the following year (runs 31-34). Figure 6.7 shows a graph of cake dryness against torque/bowl volume. Considering the time lapse between the tests, and the likelihood of variation in feed quality, the correlation is quite good. The graph in Figure 6.8 shows the variation in dryness with differential on the larger decanter, for a fixed feed rate.

24.0

L I

22.0 20.0 18.0

t

J

16.0 at 14.0 el

12.0 Q 10.0 0 ,Ig

m

1 !

/. r

8.0 6.0

~ i

4.0

l

I I '

i

~e 150mm Bowl Dia 9425 mm Bowl Dia 1.5 m3/h 99425mm Bowl Dia High Capacity,

2.0

0.00

,,

I

0.0

0.50

1.00

1.50

,

I

1

2.00

2.50

,

......

3.00

TorqueNolume Nlcm 2

Figure 6.7. Graph-Cake

Drynessv Torque/Volume- Agricultural Product.

Test Work and Data

24.0

22.0 20.0 18.0

~ 16.o. 14.0

m m r 9

12.0

a

10.0

m

== 8.0-

1~=1=] 7k 1

1

II

II i I

J

!

1.0

2.0

1 I

t

6.0

0.0 0.0

I I

I 1

4.0

2.0

2 59

=

'

lJ 3.0

!1 4.0

t 5.0

6.0

==1.5 m3/h 1

!

7.0

8.0

Conveyor Differential RPM Figure 6 . 8 . Graph -

Cake Dryness v Differential Speed - Agricultural Product.

The laboratory decanter was limited by its small gearbox torque, and by its relatively small pond depth. However, its performance was sufficiently e n c o u r a g i n g to w a r r a n t the larger scale tests and was confirmed in practice. The last laboratory test recorded stretched the limits of the decanter to demonstrate the feasibility of extra dryness. Because of the lower recoveries of the last three test runs, these are not included in the graphs. For the first pilot plant series, no centrate samples were analysed, but adjudged "good" and so a nominal figure is used for the sake of the calculations. 6.4.3 Lime sludge classification

There is interest in lime sludge classification in both wastewater and potable water t r e a t m e n t plants. If the m a g n e s i u m hydroxide content of a lime sludge can be suitably reduced, the residual calcium carbonate can be recalcined to calcium oxide. Too m u c h m a g n e s i u m hydroxide will prevent the slaking of the lime. The restrictions and rising cost of disposal of spent lime makes recovery and recycling attractive. In the decanter separation process, it is required to obtain high calcium carbonate recovery, say above 80%, while keeping m a g n e s i u m hydroxide recovery (in the cake) as low as possible, say below 50%. Figures 6.9 and 6.10 show graphs of recovery of the two constituents against t h r o u g h p u t , on a 150 m m diameter laboratory machine at two different pond levels. The difference of performance between deep and shallow

260

Some Test Data 100.0 90.0

80.0 70.0 60.0 o

50.0

'10 0

9

40.0

IL

m m

m

30.0 20.0

9150ram Bowl Dia Pond 1 4 3 : c a C 0 3

10.0

9 150ram Bowl Dia Pond 143:Mg(OH)2 I I ,.

0.0 0.00

0.50

1.00

1.50

2.00

2.50

Feed Rate m3/h

Figure 6.9. Graph - Recovery v Feed Rate (Pond 14 3 ) - Lime Sludge Classification.

100.0

,

90.0

f

80.0

v~..@

70.0 60.0 50.0 n, O

.m,

40.0 30.0 l

20.0

I

10.0 0.0 0.00

., 1 l 9150ram Bowl Dia Pond 130:CaCO3

9150mm Bowl Dia Pond 1 3 0 : M g ( O H ) 2 T

1

0.50

1.00

1.50

2.00

2.50

F e e d R a t e mS/h

Figure 6.10. Graph- Recovery v Feed Rate (Pond 1 3 0 ) - Lime Sludge Classification.

Test Work and Data

2 61

ponds, if any in this case, is marginal. The indications are that adequate performance at capacities beyond that tested is achievable. Figure 6.11 is a similar graph for a pilot scale decanter, of 356 m m bowl diameter, where similar performances are shown. Data for this work are tabulated in Table A. 3 of the Appendix.

6.4.4 Clay classification Efficient separation of clay at very small particle sizes (1-2 lam) is required to produce good-quality products for the paper coating industry. Data in Table A.4 of the Appendix are from work using a decanter with a 425 mm bowl diameter. The six runs s h o w n in Table A.4 are a sample from a test series of nearly 100 runs. Large n u m b e r s of particle size analyses on feed and centrate solids were necessary to complete the work. Only an outline of the data is given here to demonstrate the principle of the data development, and this is shown in Figures 6.12 to 6.1 7. Figure 6.12 is a graph, on log-probability scale, of feed and centrate particle size distributions for a run at 10 m S/h. From this graph are calculated frequency distributions for feed and centrate solids, knowing the solids recovery for the run. These are plotted on the graph in Figure 6.13, together with the cake size distribution, which is the difference between the first two curves. Note that the cut point for the run is indicated, and recorded, for where the cake and centrate lines intersect. This is the size at which there is a 5(): 50 split of particles shared between cake and centrate. 100.0

I

90.0

I

80.0

1 j

70.0 ae ~,

I

I

60.0

I

50.0 = .,...

t 1

40.0 30.0

f

20.0

I

10.0 0.0 0.00

I

L___

5.00

93,56mm Bowl Dia Pond 286:CaCO3 9356mm Bowl Dia Pond 286:Mg(OH)2

10.00

15.00

20.00

.I

i

25.00

Feed Rate m=lh

Figure 6. ] 1. Graph- Recovery v Feed Rate (Pond 2 8 6 ) - Lime Sludge Classification.

Some Test Data

262

10

//

=L

.= m e

o 1: I Q.

[ : Fenee~-~e 0.1

0.02

0.0

0.2

1.5

7.5

25

C%

50

75

92.5

98.5

99.8

90.[]8

100

k+erf't((C-50)/50) Cumulative % Undersize Figure 6.12. Graph

-

Particle Size v C u m u l a t i v e % Undersize

- Clay Classification.

70.0 60.0 50.0 u 40.0 r

--e--Feed ] --B-- Centrate ! Cake

==

g 30.0

\

,,.

20.0 10.0

O. 1

1

cut Point

Particle Size p

Figure 6.13. Graph - Size Frequency Distribution - Clay Classification.

10

Test W o r k and Data

263

The efficiency of separation for any particle size can be calculated by taking the ratio of the heights of cake and feed at the chosen size. The relationship between separational efficiency and particle size is shown on the graph in Figure 6.14 for 10 m3/h feed rate. This type of graph may be plotted for other capacities as well. The recovery of total solids is plotted against feed rate on the graph in Figure 6.15. These data are required when calculating frequency distributions from the cumulative weight graphs. Once the cut points are obtained from sufficient runs at different feed rates, these can then be plotted against feed rate, or more properly against centrate rate, which is the determining parameter for cut point. Cut point against centrate rate is shown with the graph in Figure 6.16. To complete the picture, the graph in Figure 6.17 indicates the sort of variation of product purity that could be expected with the material tested. Thus, from the graph it can be seen that at a rate of approximately 12 t/h from the size of decanter tested, 90% of particles in the centrate product will be less than 2 ~m and 70% less than 1 ~m.

6.4.5 Waste activated sludge thickening Thickening waste activated sludge (WAS) is a common application for the decanter in the municipal waste industry. Unusually for the decanter in sewage applications, it often does not require polymer for good performance with this 10

::I.

0 i

_e

.X

1

.?

1

I

a.

i

''/ / 0.1

0.0

002

0.2

I/ I

i]

i 0/11

i 1.5

i 7.5

J i

i t I

9F~I]

l

25

R%

50

75

92.5

98.5

J i

9O.8

k+erf'4((R-,50)/50) R e c o v e r y Figure 6 . 1 4 . Graph - P a r t i c l e Size v R e c o v e r y - Clay Classification.

9O.98 IO0

264

Some Test Data

70.0 60.0 50.0

~M

~' 40.0 n, 30.0 m ,,,,1=

20.0 10.0 0.0 5

10

15

20

25

Feed Rate mSlh Fiqure 6 . 1 5 . Graph - R e c o v e r y v Feed R a t e - Clay C l assi f i cat i on.

3.5

i I

t

2.5 :1

,,c

,.==, O

r

1

I 1

2 1.5

I J

/

I

i

i

A

0.5

0

1

"

~

0

i

I

t

I

1

5

10

15

20

Centrate Rate t/h Figure 6 . 1 6 . Graph - C u t P o i n t v C e n t r a t e R a t e - Clay C l assi f i cat i on.

25

Test Work and Data

i

100 o

90 v e~

o

265

80

o0

io

~

6o

| ~o

i

5o

1 micron

!- -

40

5

10

15

- 2 mic o

I

20

Centrate Rate t/h Fiflllre 6 . 1 7 .

Graph -

Centrate Solids Composition v Centrate Rate - Clay Classification.

material. Invariably, a good decanter will not require polymer when thickening WAS if the settled volume index (SVI) is approximately 1 O() or less. The SVI should not be confused with the SSVI, the stirred settled volume index. Table A. :3 in the Appendix contains a sample of data from a test series on thickening municipal WAS in a 7 3 7 mm diameter decanter bowl. Salient data from this table are plotted on the graphs in Figures 6.18 and 6.19. From these graphs it will be seen that conveyor differential controls both dryness and solids recovery. Feed rate also has a marked effect on both cake dryness and recovery. Increasing bowl speed with a slight decrease in pond depth makes the cake dryness a little more sensitive to conveyor differential change. Recovery is unaltered. Extra pond depth may have helped recovery, but this may have impaired dryness control. 6.4.6 Digested sludge thickening

Thickening digested sewage sludge requires different techniques from that required for WAS. Digested sludge requires polymer flocculant in the decanter, and because it is possible to over thicken, even dewater control has to be tighter. Cake dryness is controlled by the conveyor differential. The exact differential for a required cake dryness, or thickness, also depends upon the amount of solids (Qfxxr) being fed to the centrifuge. Scrolling efficiency is affected by the precise cake solids content being produced. Thus a good way of correlating digested sludge thickening results from a decanter is to plot solids recovery, and cake solids, separately, against the empirical thickening factor qJ (N/[ Qf.xf.Xs)).

266

Some Test Data

100.0

[ l I

95.0

II

1 L

!

II

I

,,

,~ o

90.0 85.0 m ...,.

o

i

t

l

80.0

1 j~

!

I

1

r

I h

I

I f

70.0 0.0

i 2.0

8

1

4.0

6.0

o, 2545 RPM; Pond 237; 40 m3/h [] 2545 RPM; Pond 237; 70 m3/h

8.0

10.0

i

1

1

12.0

14.0

16.0

I

l

18.0 20.0

,

22.0

Conveyor Differential RPM Figure 6.18. Graph- R e c o v e r y v Differential Speed - W A S T h i c k e n i n g .

6.0

l

..L

i

84' ~ i

5.0

I ! l i

4.0 lit

L"

3.0

i ; I

a O

a

tO

2.0

0.0

. t

El

~

z

f

!

t

i

~

0.0

( I

l

i

1.0

] 1 t I t

t

2.0

i

I

t

4.0

II

'

~

\..

1

'

t

I

t

I i I

i I

!

,

I

1

92300 RPM; Pond 235; 40 m31h 92300 RPM; Pond 235; 70 m3/h

I i

( I

i I

6.0

8.0

10.0

[ ] 2545 RPM; Pond 237; 70 m3/h 12.0

t

i

14.0

16.0

1 18.0

20.0

Conveyor Differential RPM

Figure 6.19. Graph - Cake D r y n e s s v Differential Speed - W A S T h i c k e n i n g .

22.0

Test Work and Data

267

Some decanter digested thickening data are tabulated in Table A.6 in the Appendix, and plotted on the graphs in Figures 6.20 and 6.21. While the correlations in this data set are not perfect, the trends are clearly visible. Examining the data shows that 2 . 5 - 3 . 0 kg/t polymer is sufficient for this particular sludge. 6.4.7 Lactose w a s h i n g

Production of lactose from milk requires the washing out of a n u m b e r of impurities such as sulphate salts. This can be accomplished using the decanter centrifuge employing a rinse feature on the beach. Rinse is applied within the centrifuge t h r o u g h the outer tube of a concentric feed tube. Brief data are given in Table A. 7 in the Appendix. These data are plotted in the two graphs in Figures 6.22 and 6.23. The work was conducted on two sizes of decanter, one of 356 mm bowl diameter, and the other of 6 0 0 m m bowl diameter. Figure 6.22 shows the washing of one specific impurity and indicates the effect of differential on the smaller machine. The second graph indicates the relative washing efficiencies of the two sizes of decanter, once optimised, with all analysed impurities averaged for several runs. This second graph will be used to demonstrate calculations in a later chapter. Notice that the initial impurity level of the larger machine is always the lowest. The reason for this is not known, but could be due to its longer beach, enabling a lower moisture level. Alternatively the smaller machine run at a higher g level

100.0

J

...A.

95.0 90.0

]

I

1

1 I

I

t

80.0 75.0

L

70.0

J

I

1.0

2.0

3.0

4.0

I

I

9

I

1

i i

0.0

I

;:

I

i

85.0

i

5.0

t

6.0

7.0

Psi (N/(Qx#~)} "100

Figure 6.20. Graph-

Recovery v Factor Psi- Digested Sludge Thickening.

8.0

268

Some Test Data 18.0 I

16.0

.I

L

I

14.0

~

12.0

I I I

T

!

J

I

I

t

i

t

1.0

2.0

3.0

m 10.0

|

~

8.0

a

6.0

o ,x

......

4.0 2.0 0.0

A

,e 9 J I

I

t J 1 I l

l

4.0

5.0

6.0

7.0

84 ,

0.0

8.0

Psi {N/(Qx#~)} "100

Fiqure 6.21. Graph - Cake Dryness v Factor Psi - Digested Sludge Thickening.

4.s

i

l

1

t

I

1

t ,

I

~ 2.5 ~

m

!

I

~. 1-5 ]4, ! 35,6mm Bowl Dia: 23 RPM Diff. m

I

]

I

1 t n 356mm Bowl Dia: 36 RPM Diff.

9356mm Bowl Dia: 46 RPM Diff. 0.5 'ie 600mm Bowl Dia: 49 RPM Diff. J t

0

0

I

2.5

l

5

( I

7.5

10

I

12.5

W a s h i n g Rate ( W a s h / F e e d * 100)

Fi~3ure 6.22. Graph- Specific Impurity v Washing R a t e - L a c t o s e Washing.

15

Test W o r k and Data

I

4.5

269

I

P

4 3.5

~

3

~2.5 Q.

E el Ir

2 1.5 0.5 I 0

F-~o356mm Bowl Dia 60 RPM Diff. ]J II 600mm Bowl Dia: 49 RPM Diff. j~

2.5

Figure 6 . 2 J. (Jraph -

I

5 7.5 10 Washing Rate (Wash/Feed * 100)

t

12.5

15

General Impurity v Washing Rate- Lactose Washing.

could have captured more fines, which, relatively, would attract a higher impurity level due to their larger specific surface area. 6.4.8 Coal railings dewatering

Sample data are given in Table A.8 in the Appendix. With such a small a m o u n t of data there is nothing to be graphed. Coal tailings require the use of an anionic flocculant, albeit a relatively small amount, which is usually admitted upstream of the decanter, as in this case. The first four runs recorded were conducted at a standard bowl speed, giving just under 240()g gravitational field. The last six runs were w h e n using a much lower gravitational field, under 1 ()OOg. It will be seen that just as good a dryness was achieved at the low g level, with the benefit of reduced polymer usage and cleaner centrate. It is apparent that with this relatively heavy process material, the high g produced high torques and low scrolling efficiencies, even with extra pond depth. Lower differentials were possible at the lower g level, to compensate for the smaller g in obtaining the required cake dryness.

6.4.9 Dry solids (DS) dewatering Obtaining extra-dry cake using a decanter (DS operation) is a relatively new technique for the decanter, h a v i n g been developed only since approximately

2 70

Some Test Data

1985. Whereas, hitherto, dewatering a digested sewage sludge would have produced a cake with, at best, 1 8 - 2 0 % solids, today well over 30% is possible. W h e n operating properly in DS mode, a decanter will be virtually full of cake, from end to end, and from bowl wall to pond surface. The conveyor will be pushing the cake towards the discharge ports against a restriction, a baffle or a n a r r o w i n g of the conveyor pitch, or against the nip between conveyor hub and the beach. The first principle of DS operation to appreciate is t h a t the volumetric wet cake rate is directly proportional to the conveyor differential. Thus, if the solids input rate doubles, say, then the differential has to double if the cake dryness is to remain the same. If the cake dryness increases, t h e n the differential has to reduce proportionally to the decrease in volume of the cake. The graphs in Figures 6.24 and 6.25 d e m o n s t r a t e the proportionality of wet cake rate to differential, for six sizes of decanter. Note t h a t the proportionality is not directly a function of the size of the decanter, but a function of the crosssectional area available for scrolling, at the most restricted point. If the pond level is not set sufficiently deep then the scrolling efficiency is impaired, as is demonstrated by the graphs in Figures 6.26 and 6.2 7. Once the pond is sufficiently deep, no extra scrolling capacity is obtained, as seen in Figure 6.2 7. Scrolling capacity is also dependent, as would be expected, on conveyor pitch. This is seen in Figure 6.28, where capacity is seen to increase a little more than the ratio of the pitches.

4.00

......

I

3.50

,oo

I

~

"

" 'J

=

.x

/Xl

1.50

I

Y_~=

'"

1 ....

1.00 9

0.50

]e

1 50mm Bowl Dia. 9425mm Bowl Dia.

9450mm Bowl Dia.

0.00 0.0

2.0

4.0

6.0 Conveyor

8.0 Differential

10.0 RPM

Figure 6.24. Graph- C a k e R a t e v D i f f e r e n t i a l S p e e d - DS.

12.0

14.0

Test W o r k and Data

2 71

16.0 14.0 12.0

../

!~9 10.0

I

Y

Q.

E

8.0

tm

6.0 4.0

2.0

0.0

OA J ~ " ,,,~

S

o

E

~ 0.0

1.0

i O 575mm Bowl Dia. ! 9737mm BowlDia. It& 1016mm Bowl Dia.

J

2.0

3.0 Conveyor

4.0 Differential

5.0

6.0

7.0

RPM

Figure 6 . 2 5 . Graph - C a k e R a t e v D i f f e r e n t i a l Speed - I)S.

0.20

1 I

L l

t

.

0.16

" •0.12

l 1

9 0.08 m

~ I

I I

l

/

i

I

,

i ,/

!

!/

I l

l

/

t

I/

Jf

I !

~

,

t

"/I .,

ii J-'~"

I ~..-

i

o

0.04 i

0.00 0.0

I

l

2.0

4.0

~

I I

6.0 Conveyor

4)Pond 106

I

8.0 Differential

10.0 RPM

Figure 6 . 2 6 . Graph - C a k e R a t e v D i f f e r e n t i a l Speed - DS.

J

12.0

14.0

272

Some Test Data 7.0-[ 6.0

....

I

I

1

5.0

i| .

.

2.0

.

.

0.0

.

B,,I

o.

I -,,

0.0

.

1.0

+

2.0

3.0

i'e P~na 2i~

!

4.0

I.~~

5.0

6.0

7.0

Conveyor Differential RPM

Figure 6.2 7. Graph- Cake Rate v Differential S p e e d - DS.

1.40

+t

1.20

t

I

9

t

, J

1.00

t

a. 0.80 Q

I~ 0.60

O

,,

,~ s

JI o

0.40

9127mm Pitch Pitch 9200mm ,.+

0.20 0.00

I

I 0.0

1.0

2.0

3.0

' 4.0

5.0

Conveyor Differential RPM

Figz~re 6.28. Graph-Cake Rate v Differential S p e e d - D S .

6.0

Test Work and Data

2 73

Scrolling rate is a volumetric rate function. As the graphs have been plotted with tph scrolling rate, some small variation can be expected from sludge to sludge, due to density changes, and this is seen in the graph in Figure 6.29. The next feature or principle of DS operation is that cake dryness is proportional to the ratio of conveying torque/pond volume. At the limit, cake dryness will tail off, increasing only logarithmically with torque/volume. These relationships are shown in the graphs plotted in Figures 6 . 3 0 - 6 . 3 2 . Note that, on the same sludge, there is good correlation between different sizes of decanter, and different designs as diverse as those with different pitches. The correlation of polymer dosage and cake dryness using DS data is covered in Chapter 5, with graphed examples in Figures 5.7-5.11. One further example is shown in Figure 6.33. With test work which is not so closely controlled it is difficult to obtain such good definition as shown here. In ad hoc test work polymer dosage is often kept high to ensure good centrate and maximum dryness. With this sort of data, one has to look at the maximum dryness achieved at each level of polymer dosage, before centrate quality is lost. At best a cloud of points will be seen on the graph with a well-defined upper limit, with the shape of the curves similar to those in Figures 5.9-5.11. Note that separate curves will be obtained for each feed rate. Finally, for DS work a correlation is needed for cake dryness and centrifuge capacity. This is done by graphing cake dryness against the function feed rate/ g-volume. A line is then drawn through the points of maximum dryness. This line then defines the threshold between clean and dirty centrate. To enable

4.00

1

1 !

3.50 3.00

l

II

!

Q.

t

0

t

=c 1 590 r

i

o

I I

t=

I

I

n,,

J

I I

I

1~ 2.00

t

1.00 0.50

I

9

0.0

i

vm-

0o0

1.0

2.0

I

I

1

3.0

! I

JI

I

4~ 2.50

l

4.0

5.0

I

i

7.0

8.0

DJgestecl 9 ~ DAF

9Primary ~--

t 6.0

I I

9.0

10.0

C o n v e y o r Differentia R P M

Figure 6.2 9. Graph- C a k e R a t e v D i f f e r e n t i a l S p e e d - DS.

11.0

12.0

274

SomeTest Data

27.0 25.0 j=

&..

I !

23.0

i

&&

w

21.0 tt

19.0

E

_]

a 17.0 o .lg m ro 15.0

"9

13.0

I

I

jrl,'

i

~Ilil -v

1

11.0

0.50

0.(I:)

1.00

1.50

2.00

....

~

9150ram Bowl Dia: 106ramPond 9 l~Omm Br Dia: 110mm Pond Dia &425ram Bowl Dia: 220mm Pond Dia l

1

3.00

3.50

1

2.50

Conveyor Torque/Volume

4.(]0

N/cm =

Figure 6.30. Graph-Cake Dryness v T o r q u e / V o l u m e - DS.

36.0

1

;

"'

I

!

"

'

i

t

34.0 32.0

I

9

-

9

;~ 30.0

~ 28.0

m ti

O ,al r

(.)

9 26.0 24.0 22.0

I --"~i

i 20.0 ! 0.00

i

/

,,, [ 0.50

I ,l-:~7~mm~0.: B.,..~,,.

-I 1.00

1.50

I -~7~mr,, ~ !

~ 2.00

D':'~,, D"c

~-' I

Bowl_Dia: B ~ , ,e,Cone, ,,,,

i,

2.50

4.00

3.00

Conveyor Torque/Volume N/cm = Figure 6.31. Graph-Cake Dryness v Torque/Volume- DS.

3.50

Test Work and Data 30.0 28.0 26.0 24.0

=

22.0-

C

9 20.0

aO

18.0

I

o

9

i

16.0.

./

f

u/

;r

, i

i

14.0

9

12.0

9

10.0

9

-,

!

,i

'0 127mm Pitch 9200ram Pitch

L

0.00

1.00

2.00

3.00

!

,

4.00

5.00

Conveyor Torque/Volume Nlcm=

Figure 6.32. Graph - C a k e Dryness v Torque/Volume- DS.

30.0 28.0 26.0

r,&

/'

~ 24.0 ~ 22.0 I

= 20.0 C a0

18.0

9

/

m

o 16.0

I.~sm~h 1l=8 m3~ L

14.0

10.0

I

[

0.00

5.00

10.00

15.00

>9.3 m3/h

20.00

Polymer Dose kg/t db

Figure 6. J J. Graph - Cake Dryness v Polymer Dose - DS.

[

A

25.00

2 75

276

Some Test Data

scale-up of data, the data for different sizes of decanter need to be coincident on the graph. This is not so for the two sizes plotted in Figure 6.34. This is because the slope of the line and the co-ordinate intercept are dependent upon the maximum dryness achievable, x~, which is a function of the depth of pond in the bowl. In the 152 mm diameter bowl the pond is very shallow. A better correlation is shown in Figure 6.35 for three different bowl designs, involving different beach angles and baffle types, and two different sizes. The data series in the graph in Figure 6.35 are separated, for easier definition, in Figures 6 . 3 6 - 6 . 3 9 . It can be seen from these that all but the smaller machine lines are coincident. The slight difference of the smaller one is probably due to its slightly reduced pond depth. 30.0 28.0 26.0

i

t

152mm Bowl Oia. !

~

n

~

-

~

!: 425mm BowlDia.j - -

]

24.0 Ig

22.0

I

- 20.0

00 II

o

"t

18.0

!

16.0

--.-......

t

-

14.0 12.0 10.0 0.00

)

J

0.05

O.10

O.15

0.20

Q4/g-Vol h "I Figure 6.34. Graph -

Cake Dryness v Feed Rate/g-Volume - DS.

0.25

Test W o r k and Data

35.0

"

~~176

......

25.0

~

"-.

I

.

.

.

.

I.',J

.

.

.

20.0

.

~..

.

.

....

lo.o

0.00

"

t

I

e 575mm Bowl DJa Type A I ! u 575mm Bowl DJa Type B ) J i ii 9575mm Bowl Dia Type C

,

i

I I

0.05

.

.

'

.

0m 15.0

2 77

.

0.10

.

.

0.15

0.20

l ii

0.25

Q~g-Vol h "4 Figure 6.3 5. Graph - C a k e D r y n e s s v F e e d R a t e / g - V o l u m e - DS.

35.0

30.0

=

25.0

m

-t

e~ 2 o . 0 o

15.0

10.0 0.00

I ~-~575mmB~~DiaTypeA1 0.05

0.10

0.15

0.20

Ch/g-Vol h "4 Figure 6 . 3 6 . Graph - C a k e D r y n e s s v F e e d R a t e / g - V o l u m e - I)S.

0.25

278

Some Test Data 35.0

....

)

30.0

m

I

25.0

= a

20.0

m

15.0

! !m 575mm Bowl Oia Type B J I

10.0 0.00

0.05

O.10

O.15

0.20

0.25

Q4g-Vol h "t

Figt~re 6.37. Graph- Cake Dryness v Feed R a t e / g - V o l u m e - DS.

35.0

)

30.0

1

) I i

~e 25.0 m

=

E,

0

20.0

m

u

1

I

,

I

15.0

10.0 0.00

1

J

0.05

0.10

I

i {" 5zsm~ ~,~, D~. ryp. c l l

0.15

,

,

]

0.20

Qg/g-Vol h 'n

Figure 6.38. Graph-Cake Dryness v Feed Rate/g-Volume-DS.

0.25

Test Work and Data 35.0

i

!

I

"

30.0 "'-"-t

25.0 m

= o

0

20.0

m

15.0 Jt

10.0 0.00

IJ It ~ 425mm Bowl Dia Type A 1J

J 0.05

J

0.10

0.15

J

0.20

Qt/g-Vol h "t

Figure 6.39. (;raph- Cake Dryness v Feed R a t e / g - V o l u m e - I)S.

0.25

279

This Page Intentionally Left Blank

CHAPTER 7 Calculations and Scaling Each test run in any test p r o g r a m m e requires a n u m b e r of basic calculations depending upon the source of the data. The formulae required are given in Chapter 4. Which calculations are necessary depend upon w h a t instrumentation is installed on the test facility. For instance, if centrate rate is measured then the feed rate needs to be calculated, and if the feed rate is measured then centrate rate has to be calculated. Both these rates are needed to calculate solids recovery, a n o t h e r of the calculations needed. Some of the early models of decanter were equipped only to indicate gearbox pinion speed, rather t h a n specifically conveyor differential. This meant that differential speed had to be calculated, and calculated using the nominal bowl speed without a bowl speed measurement, or with just a one-off measurement. With this calculation it has to be borne in mind that bowl speed can vary by 1 O0-2()0 rpm with change ofload, and belt slip, if any. This could lead to a major percentage error on differential calculation if the differential is low. Early decanters using eddy c u r r e n t brakes were not able to indicate conveyor torque continuously. A reading of brake speed, and a n o t h e r ofbrake current, had to be recorded and used w h e n referring to a brake calibration chart, to obtain the brake torque. Conveyor torque could be obtained by multiplying this figure by the gearbox ratio. A typical eddy current brake calibration chart is shown in Figure 7.1. When flocculants are used the polymer dosage level has to be calculated. On special applications unique calculations will need to be performed. In threephase work the mass balance needed to work out centrate rates is more complex. Oil recoveries and losses will need to be assessed. With three mass balances, solids, oil and water, some imbalance is to be expected due to experimental error, and care is needed to ensure that these errors are not allowed to affect the reliability of the result. In classification work mass balances may be needed on a host of size ranges. When plotting cumulative data sets against size, the size used needs to be at

282

Calculations and Scaling 140.0

[

i

I

-

l

120.0.

1.75 Amps 100.0

-

__

_

i

1.50 Amps E

80.0 ,

Z o o" po J< (I; =_ m

,

1.25 A m p s I

60.0

/,

40,0

I/

il 20.0

o.o

1~ ,

, ,

0

F

500

.

,!

I

!

I

1000

1500

!

......

i

~

.

2000

.

.

7 0 5 ,A m p s

.

.

jo 0 .

2500

Brake Speed RPM

Figure 7.1. A typical Eddy Current Brake calibration.

3000

3500

Calculations and Scaling

283

one end of the size range, the end depending upon w h e t h e r the data are cumulative undersize or oversize. Test data may indicate that the size of test machine is adequate for the duty envisaged. Alternatively, the m a c h i n e tested would be too big, which is not usual. More usually the test data have to be scaled to a larger size of decanter. W h e n the data need to be scaled to a n o t h e r decanter size, other calculations may need to be performed for each run, such as conveyor torque/volume, feed rate/Sigma, wet solids r a t e / c o n v e y o r differential speed, as well as the Sigma value itself if the data involve changes of bowl speed. As will be seen, these intermediate calculations help with the scale-up. When contemplating the scaling of data, one has to consider w h a t is the limitation to the performance on the test machine, because different limitations require different scaling techniques. Moreover if the scale-up factor is large, then the scaling may introduce another limitation, if the scale factor for the second limitation is smaller t h a n the first. The main decanter performance limitations are as follows: 9 9 9 9 9 9

centrate clarity; cakedryness-non-DS: cake d r y n e s s - DS: scrolling volumetric capacity; scrolling torque; main motor power.

7.1 Basic Calculations For a set of example calculations, w h i c h have to be conducted on each test run, the data from ru n 26 of Table A.6 in the Appendix will be used. Data" Feed rate Feed solids Flocculant rate Polymer concentration Centrate solids Cake solids

19.6 m 3 / h 2.6% w / w 0.98 m 3 / h 0.13% w / w

Bowl speed Pinion speed Gearbox ratio Bowl diameter

3 1 5 0 rpm 6 5 0 rpm 125 425 mm

1350 ppm 10.7% w / w

Clarifying length Pond diameter

800 mm 257 mm

It will be assumed that the densities of feed, polymer solution, and centrate are unity. Thus, from equation (4.13), c e n t r a t e rate" (10.7-2.68) (10.7-0.13) Qt = 19.6 + 0.98 (6.9 - 0.135) (10.7 - 0.135) 19.6x8.02 0 . 9 8 x 10.57 = + 10.565 10.565 = 14.88 + O.98 Ol ~" 15.9

m 3/h

(15.9x0.135)

From equation (4.14), r e c o v e r y :

= 100(1 - 0 . 0 4 0 9 ) R ~ 95.9% From equation (4.15) p o l y m e r

PD-PD-

0.98 x 0.13 19.6 x 2.68 2.43 kg/t.db

x 1000

dosage:

Cnlrirlntians nvd ScnIing

2 85

where d h indicates dry basis and wb would indicate wet basis. Thus, in this case the polymer dose is quoted in kilograins of active polymer (dry) per ton of dry solids in t h e feed. From eqrialion (4.9),conveyor differential:

‘I’lie data in Table A.6 are fur a thickening application. For thickeriirig. ii is sornelirnes useful LO calculate psi, the thickening factor.

l h r n equation (4.59),psi;

* = 19.6

20 x 2.68 x

10.7

Note the mixture of units ( r p m divided hy tn3$l. For thc purist. the answer miry be indtiplicd by 2 x x MJ, tc> givt: uriit,sofm-’, Psi is uscd fur cornparison purposes. and therefore. 50 lorig a s [he same units arc uscd throughout, t.he choice of units is immatcriel. Thc Sigma value fur llie decaqtes normally would be obtained horn the decantcr manufacturer. h u t Tor demonstration purposw A v;Iluc wqll be estlmatcd hcrc. Frntii cquaiion ( 4 , # ) .centrifuge g-level:

>

= 330.. x

21.25 481

-

1:ro111equation ( 4 . 3 2 ) .Sigma:

114576 400 x 0 . 5 0 3 = z 7 v 0 0 x 569.45

= 27900 x

c = 1.59 x

-

~

1o’cm’

286

Basic Calculations

Thus, 0 / ~ " 19.6 x l O 0 0 x l O 0 0 x 10 1.59 x 107 Q/E = 12.3 mm/h -

For the purposes of demonstration a pinion torque of 10 Nm will be assumed. In that this was a thickening application, where there would be little interest in torque m e a s u r e m e n t , the pinion torque, in all probability, would have been m u c h lower. However, this figure is at the lower end of w h a t would be experienced in a dewatering application. From equation (4.10), c o n v e y o r torque: T = 125 x 1 0 = 1 2 5 0 N m T = 1.25 kNm

Clarifying v o l u m e : 800 7152 72 4 0 0 (42 -25 ) x 10 = 7 1 9 9 0 cm 3

V--

V-

71.99 1

Thus, T~V: T/V

=

T/V-

1.25 1000 x 1 ()0 x 71.99 1000 1.74 N/cm 2

Q/g-Vol: 19.6 x 1000

Q/g-Vol = 2 3 5 7 x 71.99 Q/g-Vol- 11.55 x 10 -2 h -1 Thus, the necessary factors have been calculated should a scale-up be required from this one set of data. Naturally, in a real situation calculations would not be conducted both for thickening and DS. They are done by way of example here. With a 10.7% cake, it is unlikely to be a DS application. However, were it to have been a DS application, and the dryness was considered adequate and, for instance, double the capacity was being sought, then a decanter with twice the g-volume would be required. The g-level would probably be chosen similar, thus the bowl volume would need to be double, and therefore to m a i n t a i n the same dryness a gearbox of twice the torque

Calculations and Scaling

287

would be required. More normally these calculations are conducted for each run, and performance levels plotted against them, such that the optimum performance can be chosen and scale-up is made from there.

7.2 Three-Phase Calculations Similar calculations need to be c o n d u c t e d on a n y m o n i t o r e d r u n in threephase work. A sample set of calculations is given here. Data: Feed rate Feed solids Feed oil Feed w a t e r Rinse w a t e r rate Cake solids Cake oil Cake w a t e r Oil rate Oil w a t e r Oil solids Oil

4 m 3/h 39% w/w 24%w/w 3 7% w / w 1 mS/h 50% w / w 6 % w/w 44% w/w 0.8 m 3 / h 1% w / w 1% w / w 98% w/w

Bowl d i a m e t e r Oil discharge d i a m e t e r W a t e r discharge d i a m e t e r Cake discharge d i a m e t e r Bowl speed Clarifying l e n g t h

42 5 m m 2 70 m m 274mm 264 mm 3150 rpm 750 m m

Effluent solids Effluent oil Effluent w a t e r Oil density

4.4% w / w 0.4% w / w 95.2% w / w 0.85

First a total mass balance, followed by a solids mass balance, is conducted. One of the two u n k n o w n s , solids rate or effluent rate, is eliminated by substituting from one equation into the other, and the two u n k n o w n s are calculated. Thus, from an equation similar to e q u a t i o n (4.11 ), the total mass b a l a n c e is: FeedRate + W a t e r R a t e = CakeRate + E f f l u e n t R a t e + OilRate 4 + 1 = CakeRate + EffluentRate + 0.8 x 0.85

whence 4.32 = CakeRate + EffluentRate The solids mass b a l a n c e is: 4 • 0.39 + 0 - 0.5 x CakeRate + 0.044 • EffluentRate + 0.8 • 0.85 x 0.01 1.56 = 0.5 x CakeRate + 0.044 • EffluentRate + 0.0068

Calculations and Scaling

whence 1.5532 - 0.5 x

CakeRate

+

0.044 x

By substitution" Cake rate - 3 . 0 0 2 7 t/h E f f l u e n t r a t e - 1 . 3 1 7 3 t/h

289

EffluentRate

,-,3.o t/h ,,~1.3 t/h

This assumes that the densities of effluent, feed, and cake are all unity. This is not quite true, but within the experimental error associated with this type of work, this is acceptable. Recovery of solids

=

3 . 0 0 2 7 x 0.5 x 100~96.2% 4 x 0.39

R e c o v e r y o f oil

0.80 x 0.98 x 100 ,~ 81.7% 4 x0.24 Oil l o s s in c a k e

3.002 7 x ().06 x 100~,18.8% 4 x 0.24 Oil l o s s in w a t e r

1.3173 x 0 . 0 0 4 x 1 O0 -~, 0.~/o 4 x ().24 Note that the oil recovery and oil losses do not add up precisely to 10()%. This is due to experimental error, and sometimes can be much larger. In three-phase work, it can be useful to determine the approximate position of the e-line (equilibrium line). Using equation (4.61 ): 0 " 8 5~x ~2 2 g [ ' 7 -

( 22~] -0 )

- l"Oxa~2 ~2ff ['7-

which simplifies to" 0 . 8 5 ( r ~ - 1352 ) = 1.O(r~- 1372 ) O.15r~ = 1 8 7 6 9 r,. - -

V/

~3277.75 O. 1 5

15491.25

,~, 1 4 8

Thus, e - l i n e d i a m e t e r " = 2 9 6 mm

mm

= 296

mmDia

(_~)2]

290

Three-PhaseCalculations

This gives a depth of oil of ( 2 9 6 - 2 7 0 ) / 2 - 13 m m over a w a t e r d e p t h of ( 4 2 5 - 2 9 6 ) / 2 - 64.5 mm. A s s u m i n g it is just as easy to s e p a r a t e the oil from the w a t e r as it is to s e p a r a t e the w a t e r from the oil, w h i c h is n o t necessarily so, it is w o r t h c a l c u l a t i n g the Q/E for each phase. For the light phase, from e q u a t i o n (4.32), Sigma: E

7r x 7 5 0 x 3302 ---

981 x 10

= 26155.9 x :

(2962 - 2702) X

202 l n ( 2 9 6 / 2 7 0 ) 14716

400 x 0.0919 1.05 • 107cm 2

Thus, light phase Q/E: 0.8 0/z

-

Q/E -

1 . 0 5 x 100.76

x lO00x

lO00x

10

mm/h

For the h e a v y p h a s e (flow rate - 1 . 2 0 1 8 ) , Sigma" E=Trx

7 5 0 x 3302 x ( 4 2 5 2 - 2 9 6 2 ) 981 x 10 2021n(425/296)

= 26155.9 x

9 3 O09

400 x 0.361 7 = 26155.9 x 642.86 -)

E = 1.68 x 107 cmThus,

heavy phase

Q/E-

O/E: 1.2018 1 . 6 8 x 107 • 1 0 0 0 •

1000•

10

O / E - 0.72 m m / h

With Q/E values being so similar for the two phases, it w o u l d seem t h a t the differential pond setting is o p t i m u m , b a r r i n g a n y cresting effects or backpressure effects from any d i s c h a r g e device. If it were n e c e s s a r y , say, to i m p r o v e the quality of the oil p h a s e at the e x p e n s e of w a t e r effluent quality, t h e n the w a t e r d i s c h a r g e d i a m e t e r w o u l d need to be increased very slightly, to increase the depth of the oil level in the pond.

7.3 Classification Calculations -

_-

As a11 example of thc calculations needed for a decanter test run On a cIassitic;jt.ion d u t y . the data uscd for Figures 6.1 2 - 6.1.7 will h e er~ipluyed.'The dat.a are given in. arid adjacent to, Tablc 7 . 1 . The parl.icle size analyses frdkil Tablc 7 . 1 arc plott,ed in Figerr: h.1 2 . Prom this graph frequency dist.rihutions are calculated. 'I'he data abovc arc cctabulated, ciilculi~tingthe percentage it1 each size interval, and dividing that percentage by the size i n k r v a l . 'I'Iiese figures. thus calculatcd, give the rclativc frequcncy for each size interval. The freqrirric:ies iri t.hr c:eril.raf.c! distributioe arc thcn multiplied by (1-solids recovery ;IS ii Itect.ion),to make the frequcncics in thc ccntratc distribution correspond to those in the feed. by virtue o f the particles lost in the cake. 'J'hesefigures are tabulated in Table 7.2,

Table 7.1.PArlii:lc s i x ;rn;rlyscs

_..-..,.

,, ,

.,- , -,

Cum. 9:, undcrsiec feed

r1111r.

'%I

1 I).O 9.0

07.4 96.9

0 9 ,!I 7

8.0

9f1.1

7.0 6.0 5.U

95.1)

99.9 1 99.88 9 9 .7 h

,

,

Particlr sizc

(pn) ..

4.0

3.0 1.o 1.5 I ,I) 0.8 0.0 0.4

0.3 0.2 0.1

,

.

.

--

widcrsiac

centrale ~

93.h

Y1.3 88.0 X2.h

7 2 .o h 3 .c i

50.0 42.6 '3'1.6

22.0 15.2

8.0 2.5

F w d rate Howl dlamr:tr:r iliirifylng lerlgth Cakc discharge diamcter Pond diameter

YS.Yh

Krrwl spct:d Solids rrtwvt.ry I)i k r c nIi al

9Y.53

99.00 97.60 93.00

SG of feed Feed solids Cakc solids

H7.00 73.00 ~13.00 49.00 30.00

C c r ~ t r . a ~u co l i d s

1X.hO 8.20 1.20 . .. .

. . ...

292

Classification Calculations

Table 7.2. Frequency distributions Mean size (~tm)

Size interval (pm)

9.50 8.50 7.50 6.50 5.50 4.50 3.50 2.50 1.75 1.25 0.90 0.70 0.50 0.35 0.25 0.15 0.05

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5 O. 5 0.2 0.2 0.2 0.1 0.1 0.1 0.1

Feed % in interval

Feed (%/~tm)

0.5 0.8 1.1 1.4 2.3 3.3 5.4 10.6 8.4 13.6 7.4 9.0 11.6 6.8 6.6 6.1 2.5

0.5 0.8 1.1 1.4 2.3 3.3 5.4 10.6 16.8 27.2 37.0 45.0 58.0 68.0 66.0 61.0 25.0

Centrate % in interval

Centrate (%/~tm)

0.01 0.03 0.05 0.12 0.23 0.53 1.40 4.60 6.00 14.00 10.00 14.00 19.00 11.40 10.40 7.00 1.20

0.01 0.03 0.05 0.12 0.23 0.53 1.40 4.60 12.00 28.00 50.00 70.00 95.00 114.00 104.00 70.00 12.C)0

Centrate (%/~tmx(1-R)) 0.01 0.02 0.03 0.07 0.13 0.31 0.82 2.70 7.03 16.41 29.30 41.02 55.67 66.80 60.94 41.02 7.03

By t a k i n g t h e difference b e t w e e n the two f r e q u e n c y (%/lam) c o l u m n s , t h e size f r e q u e n c y d i s t r i b u t i o n for the cake is o b t a i n e d . This is s h o w n in Table 7.3. T h e t h r e e f r e q u e n c y d i s t r i b u t i o n s are plotted on the g r a p h in Figure 6 . 1 3 . The cut p o i n t is o b t a i n e d as the particle size at w h i c h t h e cake a n d c e n t r a t e d i s t r i b u t i o n s intersect. As, at this point, t h e f r e q u e n c i e s for the t w o d i s t r i b u t i o n s are e q u a l a n d total t h a t for the feed, it follows t h a t this f r e q u e n c y is half t h a t in the feed, w h i c h is t h e definition of c u t point. T h e cut p o i n t for e a c h flow rate is similarly o b t a i n e d , after w h i c h the g r a p h of flow r a t e a g a i n s t c u t p o i n t m a y be plotted, as in Figure 6 . 1 6 . F r o m t h e f r e q u e n c y d i s t r i b u t i o n s , the s e p a r a t i o n a l efficiency for e a c h a n d a n y size m a y be o b t a i n e d by t a k i n g t h e ratio of t h e frequencies, cake to feed, a n d m u l t i p l y i n g by 1 0 0 . This c a n be d o n e for e a c h feed r a t e tested. T h e efficiency plot for 10 m 3/h is s h o w n in Figure 6 . 1 4 . Figure 6 . 1 5 gives t h e r e c o v e r y for e a c h feed rate, w h i c h is n e e d e d to c a l c u l a t e t h e c e n t r a t e f r e q u e n c y c u r v e , a n d t h u s t h e cake f r e q u e n c y curve.

~

un

u~

~

~

C

0

uq

u~

0

~

~

C

C

~

0

0

~.

N

,_,.

,_,~ 0

0"

,-t

,..,.

N

i

,..,.

7.4 Washing The data depicted in Figure 6.23 will be taken as the d a t a source for a demonstration calculation. The associated data are as follows: 5500 kg/h 8.125%w/w 37.5%w/w 25% w / w 7.5%

Feed rate Feed impurity level Feed suspended solids Cake moisture content Wash rate

Figure 4.11 (the rinsing with diffusion diagram) is reproduced as Figure 7.2, with specific figures, the derivations for which are given below. W a s h rate:

7.5 0W

---

100

x 5500-

412.5 kg/h

Assuming full recovery of solids. D r y C a k e Rate" 37.5 QsXs = x 5 5 0 0 = 2 0 6 2 . 5 kg/h.db 100

Q, =5500kg/h

Q.=412.5~ c,=y. =0

Q.~2750k~ c2=?

Figure 7.2. Rinsingwith diffusion-Massflows.

Calculations andScaling 295

Wet c a k e rate: 100 (100-25)

x 2062.5 - 2750 kg/h.wb

Impurity l e v e l of feed: If-

8.125%.wb

= 8.125 x

1 O0

37.5 !1 - 2 1 . 6 7%.db Based on the liquor in the feed, i m p u r i t y c o n c e n t r a t i o n : I()0

cl - 8 . 1 2 5 x

(100-

37.5)

= 13.0%

This last i m p u r i t y level. 13 %. will be the s a m e for the cake after d e c a n t i n g the excess liquor, without rinse. However, based on the solids, c a k e i m p u r i t y

level ( w i t h o u t rinse): ( 2 7 5 ( ) - 2()62.5)

Is- 21.67 x

(5500 - 2062.S)

I~ -- 4.3 3%.db Compare this figure with the ordinate intercept on the g r a p h in Figure 6.2 3. It will be seen from this g r a p h t h a t this figure agrees extremely well with the practical result.

Cake m o i s t u r e d i s c h a r g e rate" O.~ps(1 - xs) - 2 7 5 0 -

2 0 6 2 . 5 - 6 8 7 . 5 kg/h

Rinse rate: 7.5 0,,,- looX

~500

O , , , - 4 1 2 . 5 kg/h Thus, with perfect rinsing, c a k e i m p u r i t y level w i t h rinse:

Is=

(687.5 - 4 1 2 . 5 ) 687.5

x4.33=

1.73%.db

296

Washing

This is the lowest impurity level possible with 7.5% rinse. In practice (Figure 6.23), for the smallest decanter tested, the level is 2.3%. Thus, r i n s i n g efficiency"

_- ( 4 . 3 3 - 2 . 3 ) (4.33-1.73)

x 100

= 78% To determine the effect t h a t any possible p a r a m e t e r change m a y have, it is necessary to determine the effective mass transfer coefficient for these conditions. Impurity c o n c e n t r a t i o n in m o i s t u r e of cake" c2 =

2062.5

x 2.3 = 6.9%

687.5

Thence the concentration in the centrate liquor can be obtained using equation (4.77). C e n t r a t e liquor i m p u r i t y level: (13 -

6.9) x 687.5

c3 = 6.1 x

-

412.5(c3

- O)

687.5

412.5

= 10.17%

The l o g a r i t h m i c c o n c e n t r a t i o n difference: Ac=

(6.9-0)-(13-10.17) ln[(6.9

Ac-

- 0)/(13

-

IO.17)]

4.57%

From equation (4.88)" 687.5 • ( 1 3 - 6.9) hDA,, -IO0 x 4.57 = 9.18 kg/h.% conc.diff The question that now could be posed is whether, by increasing conveyor differential on the smaller decanter, the cake impurity level could be lowered to m a t c h that achieved on the larger model, as s h o w n in Figure 6.23; i.e. the impurity level needs to be lowered from 2.3% to 2.0%, still with 7.5% rinse rate. Increasing the differential from 60 rpm to, say, 75 rpm, increases the superficial velocity ofrinse over the cake by 7 5 / 6 0 = 1.25.

This increase in siiprrfirial velocity will incrrijsr t h e mass transfer coefficient by thc Same proportion (see q u a t i o i l (4.85 ) ) .Thus. revised h d , : = 1 . 2 5 x 9.1 8

=

1 I .5 ~ ~ / h . % ~ O t l C . d ~ /

The impurity lcvcls in the cake ciin now nnly be back calculated by iteration. 'I'ablc 7.4 gives figures obtained in the it.eraf,i()ri,from hid^ a resdt c a n he iriterpolakd. llsing l'able 7.4, it will bc seen that a diflerential of 7 5 rpm (1r)ok along the row for which hoAc is nearest to 1 1 . 5 ) only rcduccs thc cake impurii.y level frtim 2 . 3 to 2.1 7 . barely half thc reduction rcquired. To achieve the 2.03: tignre, t.he ccinveyor d i l k e n l i a l would need lo be doubled. assuming that.it. were prar:t.icahle. 'lable Z4.iterative clilculations for cake impurity levels __

..

Cake moisture iinpurity ( % )

-

.. -

. . . .

Mi (kg/h )

Cake impurity i% db) ,

_".

Ceri t ml t: liquor impurity ('%,1

5.2 5,5

1.71

i3. h 1

13 . 0 0

1.8.5

h.0 6.5

2.00

51.50 48.14

2.1;

44.07

12.50 11.h; 1O.H 1

7.0

2.33 Z.iO

41.15 17.H 7

2.67

14.36

-f . -3 n.o ..

-

.. -.....

0.011 ',ON

'.Y

15.55

10.00

3.09 1.Y5 4.72

0.17

5.4')

8.33

8.53

.......

'4.79 I1.JI

H.74 (3.89

4.03 .......

-

7.5 The Probability Scale Sometimes it is necessary to plot on a log-probability graph but the probability scale is not available as such in a spreadsheet. The probability scale can be calculated using the r i g h t - h a n d side of equation (4.19 t. The probability scale, therefore, will be proportional to e r f - l ( 2 C x - 1 ) , where Cx is a percentage figure for which the scale is required. The m a t h e m a t i c a l term, erf(x), is a tabulated integral, w h i c h may be obtained from any good m a t h e m a t i c a l book of tabulated transcendental functions. For the spreadsheet a look-up table will have to be created of Cx against e r f - ~ ( 2 C ~ - l ) , and a simple formula introduced to interpolate linearly between values. This is how the graphs in Figures 6.12 and 6.14 were created. The look-up table used is shown in Table 7.5. The probability scale is frequently used in decanter work and it is useful to know how to create such a scale w h e n a ready-made one is not available.

Calculations and Scaling

299

Table 7.5. Look-up table for Cx against erf-l(2Cx-1) Percent Cx

E r f - l ( 2 C x - 1)

0 0.01 0.05 0.1 0.2 0.5 1 2 5 10 20 30 40 50 6O 7O 80 9O 95 98 99 99.5 99.8 99.9 99.95 99.99 1 ()0

0.00 O.37 0.68 0.82 0.97 1.18 1.36 1.55 1.84 2.05 2.33 2.63 2.82 3.00 3.18 3.37 3.68 3.95 4.16 4.45 4.65 4.82 5 .()4 5.19 5.32 5.63 6.()()

7.6 Scale-Up of Centrate Clarity Limiting Applications Scaling up between two decanter sizes is generally best done w h e n there is geometrical similarity between them. This means the same beach angle, the same conveyor pitch angle, and the same conveyor and feed zone designs. When there are differences then the scale-up may not be reliable. A centrate clarity limiting application is characterised by a fall off in centrate quality, w h e n feed rate is increased, independent of conveyor differential, once pond depth has been optimised. Spent wash dewatering, discussed in the previous chapter, is one such example. Note the proviso concerning differential. Referring to Figure 6.3, the conveyor differential needed to be at least 18 rpm for the centrate to be unaffected, and raised even higher at the higher capacities. To scale any of the capacities tested to another size of decanter, or the same decanter with a different bowl speed, the ratio of Sigma values simply would be used. The best dryness achievable on the test machine is taken from Figure 6.4, using the minimum differential necessary to achieve the best recovery, at the rate chosen, shown on the graph in Figure 6.3. Translating the optimum differential to the larger machine is usually not done by calculations, but by trial and error adjustment of differential, when commissioning the larger decanter. However, if necessary, calculations of cake scrolling rate and cake residence time may be made to ensure that the necessary differential range required on the larger machine is available. The clay and lime classification applications, covered in Chapter 6, are also examples of data which would be scaled by Sigma ratios. The lime classification is interesting in that it involves two materials of suspended solids with different densities. In this application it was required to produce a cake with less t h a n 60% m a g n e s i u m hydroxide, to prevent slagging during calcining. For economic reasons, it was necessary to recover at least 85% of the calcium hydroxide. From Figure 6.11 it will be seen that, on the test machine, any capacity between 12.5 and 20 mS/h would achieve the objective. This capacity range would be scaled proportionally to Sigma.

Calculations and Scaling

301

The lowest capacity would achieve 90% recovery of calcium hydroxide, and the highest 85 %.

7.7 Simple Dewatering and Torque Scale-Up While the majority of applications will be scaled by Sigma, there are occasions when high conveyor torques are experienced during test work. Then it will be necessary to scale the torques experienced, to the proposed larger machine, not only to ensure that this will not be limiting but also to estimate the power required for the drive motor. Some "dry solids" applications (covered in Section 7.9) will have their capacities limited by gearbox torque rating. When scaling these applications to larger decanter sizes, it is important that the torque rating available is greater than the scaled-up torque. The graph in Figure 6.6 gives torque data for the spent wash dewatering application. These data need to be converted to a form suitable for scaling. From equation (4.70) it will be seen that torque is proportional to feed rate and inversely proportional to differential. Therefore, the data of Figure 6.6 should be re-plotted as torque against feed rate/conveyor differential speed. This is done in Figure 7.3. Here it will be seen that most of the data, for the lowest two differentials tested, form a straight line 1.1 kNm/(m3/h/ rpm dill). As an example, these torque data will be scaled to a decanter of 737 mm bowl diameter. Data:

Bowl speed Bowl diameter Pond diameter Cake discharge diameter Clarifying length Conveyor pitch Wetted area of bowl Beach area Conveyor differential Feed rate

rpm mm mm mm mm mm m2 m2 rpm m3/h

Test decanter 3150 42 5 261 264 12 O0 12 7 2.2 0.5 13 16

Large decanter ? 73 7 480 483 22 60 54 6.7 1.4 ? ?

Calculations and Scaling 1.80 1.60

./

9

303

j.

1.40 E z 1.20 .ar Q =1

" 1.00

I-

>"

J

-

0.80 0.60

0.40

t 9Diff. 13.2 RPM

0.20

0.00

9

==Diff. 18.2 RPM

I I

i& Diff. 23.2 RPM ..--.------__~]~ I

,

0.00

0.20

0.40

0.60

0.80

Feed Rate/Differential

l

J

1.00

1.20

1.40

Q/N

Figure 7.3. Conveltor Torque vs. Feed Rate:D(fJerential Ratio.

Firstly t h e bowl speed for the l a r g e r m a c h i n e n e e d s to be c a l c u l a t e d , a s s u m i n g t h a t it will need to h a v e the s a m e g-level as t h e test m a c h i n e . For t h e test m a c h i n e , b o w l speed:

S - 330 rad/sec

g-level (equation (4.8)): 3302 g'

981

425 x 20

g,. = 2 3 5 7

For t h e l a r g e r m a c h i n e , b o w l s p e e d :

S =

2357 x 981x 737

= 250 rads/sec

S .~ 2 4 0 0 r p m

20

304

SimpleDewatering and Torque Scale-Up

The feed rate will be scaled by Sigma, which thus needs to be calculated for each machine, using equation (4.32). For the test machine, Sigma: E

7r x 1200 x 3302 ----

981 x 10

F,42s,[~,_yO__)

2_,261, ]i_2_ff)2

X

ln(425~ k2611

41849.5 x 281.26 0.4876 E = 2.41 x 107 cm 2 For the larger model, Sigma: E3

7r x 2 2 6 0 x 2502 --

981 x 10

[ ( ~ ) 2 - (~~176 21 X

ln(~)

4 5 2 3 4 . 5 x 781.9 0.4288 E = 8.25 x 107 cm 2 Feed rate: 8.25 x 16 = 54.8 m3/h Of - 2.41 Differential is the next parameter value to be calculated. In the absence of any other information, initially at least, the differential for the larger decanter would be fixed to have the same cake thickness, in the bowl, as the test machine. Cake thickness will be proportional to feed rate, and inversely proportional to conveyor pitch and differential. Thus, for the larger machine,

conveyor differential: 54.8 127 425 • x x13 16 254 737 N ~ 13 rpm

N-

which, conveniently but coincidentally, is the same as the smaller machine. Torque is scaled up using equation (4.70). Conveniently again, there is no dry beach, and so there is no need to use the factor k4. However, the heel torque needs to be estimated using equation (4.71). The heel torque for the smaller decanter is the ordinate intercept on the graph in Figure 7.3, which is approximately 0.3 kNm. Thus, for the larger decanter, h e e l torque: 6.7 2357 737 To = 2.2 • 2 3 5 7 • 4 2 5 • 0.3 To ~ 1.6 kNm

Calculations and Scaling

T o r q u e on the test m a c h i n e , at the specified feed rate, is 1.6 5 kNm. On t h e l a r g e r m a c h i n e , torque above heel torque: 1.4 ( T - To) = 54.8 x 4 2 5 x 0.5 x ( 1 . 6 5 - 0.3) 16 737 ( T - To) = 7.5 kNm

Total conveying torque" T-

7.5 + 1.6 = 9.1 kNm

305

7.8 Main Motor Sizing The data for the spent wash dewatering, used in the last section, will also be used to estimate the motor size required for the large decanter example. Equations (4.130)-(4.134) are used to calculate the components of process power, except the power consumed to overcome windage and friction. This component would be obtained from the manufacturer of the decanter, or by measurement in the field. The m a n u f a c t u r e r supplies the figures of 4.4 and 3 3.6 kW, windage and friction, for the two sizes of decanter, operating at the speeds specified. The figure for the test decanter is not needed for calculations, but is given for comparison purposes. The centrate and cake discharge at approximately the same diameter and therefore it is not necessary to work out the components of power for these two streams separately. Were the discharge diameters to be markedly different, then the rate for each stream would need to be calculated, and equation (4.131 ) would then need to be used for each stream. For the larger decanter, assuming process densities are close to unity, p r o c e s s a c c e l e r a t i o n p o w e r , substituting the appropriate values calculated into equation (4.131) (w = 250, Qr = 54.8, rd=480/2/10):

Pp= 5 4 28 x 2 5 0 2 . x ( 428x0l 0 )

1000

3 6 0 0 x l O O x 100

= 54.8 x 10 ~ W

PI'- 54.8 kW From equation (4.133 ), c o n v e y i n g p o w e r : 27r Ps = 13 x 9.1 x 1000 x 6--0 = 12.4 x 103 Nm/s

Ps = 12.4 kW For the braking power, it is necessary to know the gearbox ratio, which will be taken as 98 (from the manufacturer).

Calculations and Scaling

307

From equation (4.9), p i n i o n s p e e d : Sp = 2 4 0 0 - ( 1 3

x 98)

Sv=1126rpm and from equation (4.10), p i n i o n torque" Tp = 9.1 x 1 0 0 0 / 9 8 Tp = 92.9 Nm From equation (4.134), b r a k i n g p o w e r " PB -- 92.9 x 1126 x

27l"

60

= 11 x 1 0 3 N m / s P ~ - - 11 kW From equation (4.130), the decanter requires at the bowl pulley, t o t a l power" PT-

3 3.6 + 54.8 + 12.4 + 11.0

PT-- l l l . 8 k W The precise motor specification will depend upon a n u m b e r of other factors such as the type of drive and starter, and how m u c h contingency, or expansion, for which the user wants to cater. The losses between the motor and the centrifuge, such as belt friction, and fluid coupling losses if applicable, all need to be taken into account. However, the motor will be no smaller than 125 kW, the next standard size above the power so far calculated. If a much larger size of motor is chosen, the power factor for the motor efficiency will be reduced, imposing a greater penalty on the cost of electricity.

7.9 DS Scaling Table A.9 in the Appendix gives a set of 14 data points for an unspecified effluent, which will be used to demonstrate scaling of DS data. The requirement is to specify a size of decanter that can process the same sludge at 50 m 3 / h to give a cake of 30% dryness or better with the m i n i m u m of polymer usage, and good centrate. The salient data for the test machine and two larger machines w o r t h y of consideration are given below. Decanter Bowl diameter Clarifying length Bowl volume Nom. scrolling rate Max. conv. torque Gearbox ratio Bowl speed Cake discharge dia. Pond dia.

mm mm 1 tph/rpm kNm rpm mm mm

Test 42 5 800 86 0.28 2.7 125 3150 264 252

No. 1 575 2000 385 1.00 16 267 2900 326 306

No. 2 737 1550 460 2.00 20 254 2400 483 463

The pond used for the test m a c h i n e was 6 mm above neutral. For the larger machines 10 m m above neutral has been chosen as that is k n o w n to be a good working level in practice. First the g-volume needs to be calculated for each model. Notice that, in the g-Vol calculations for DS work, it is the g level at the pond surface that is used rather t h a n g at the bowl wall, which is more commonly used in centrifuge work. Using equations (4.105) and (4.106), for the test machine, g-volume:

(2

7r x 3 1 5 0 ) 2 60

g-Vol =

86 x

g-Vol =

120.2 m s

252 20 x 1 0 0 0 x 981

Calculations and Scaling

309

For larger machine no. 1, g-volume: --

g-Vol 385 x

(2rrx2900) 60

2x

306

20 x 1 0 0 0 x 981

g-Vol = 553.8 rn 3 and for the larger machine no. 2, g-volume"

g-Vol-460x

(2

7r x 2 400'~ 2 6~ J X20x

463 1000x981

g-Vol- 685.7 m 3

From these calculations it will be seen that the g-level at the pond surface on each machine is, respectively, 1400, 1440, and 1490, which are all very similar, indicating that scale up should be straightforward, providing the geometry of the test machine and the two larger machines are similar. The data from the test decanter are plotted in graphs in Figures 7.4-7.7. The figures calculated above are used with these graphs to assess the likely performance of the larger machines proposed for the duty. For Figure 7.4 scrolling rates are plotted against differential. This indicates that the test scrolling rate is 0.28 t p h / r p m , which is what is expected. Therefore, no adjustment is needed for this parameter for the larger machines. 2.50

2.00

I I I I t

,Q

.= 1.50 Q.

I I I

'ZI

Q @

.x 1.00 q ro

I

0.50

I

J

/

J

1

9

I I

,

I O: ;cries1 ] I

0.00 0.0

2.0

4.0

6.0

Conveyor Differential RPM Figure 7.4. Cake Rate vs. Differential.

8.0

10.0

310 DSScaling In Figure 7.5 test data of cake dryness are plotted against t o r q u e / v o l u m e . It shows that a t o r q u e / v o l u m e figure of 2.0 N / c m 2 will be sufficient to produce a cake of 30%. Thus, for no. 1 machine, c o n v e y o r torque" T = 2.0 x 3 8 5 / 1 0 0 = 7.7 k N m and for no. 2 machine, c o n v e y o r

torque"

T = 2.0 • 4 6 0 / 1 0 0 - 9.2 k N m These torques are well within the capability of the size of m a c h i n e s selected. It should be noted that if drier cakes will be obtained in the future with development of equipment, techniques or chemicals, the conveyor torque will increase. Reserve gearbox torque capacity will permit such i m p r o v e m e n t and also result in longer gearbox life. The m a x i m u m capacity possible may be gauged from the graph in Figure 7.6. Here cake dryness test data are plotted against feed rate/g-volume. Above the line drawn on the graph, centrates are liable to be dirty, as were the centrates for the points with open symbols. Below the line good performance can be expected. For the required 50 m3/h on machine no. 1, O / g - v o l u m e :

36.0

I fl

34.0 32.0

~e

J

30.0

m

:28.0

~ 28.o

,I f

24.0 22.0 20.0 0.00

l I

I t

i

0.50

1.00

1.50

1 2.00

2.50

Conveyor Torque/Volume Nlcm =

Figure 7.5. Dryness vs. Conveyor Torque:Volume Ration

3.00

3.50

Calculations and Scaling 311

g-Vol

= 5 0 / 5 5 3 . 8 t7-1

O_ = 9.0 x 10 -2 h -1

g-Vol

and for m a c h i n e no. 2, O / g - v o l u m e "

g-Vol

0 g-Vol

= 5 0 / 6 8 5 . 7 h -1 = 7.3 x 1 0 -2 h -1

From Figure 7.6, it will be seen t h a t for a 30% cake these two points are comfortably below the line. From this g r a p h also, it will be seen t h a t 50 m 3 / h on m a c h i n e no. 1 c o r r e s p o n d s to a b o u t 11 m 3 / h on the test m a c h i n e a n d for no. 2, a b o u t 8.5 m 3/h on the test m a c h i n e . This estimate is from k n o w i n g t h a t the abscissa values for the points are 3, 5, 7, 10 m ~/h, etc. The p o l y m e r dose r e q u i r e m e n t needs to be estimated now. The g r a p h in Figure 7.7 plots cake dryness a g a i n s t p o l y m e r dose, for two test rates of 7 and 15 m~/h. It is necessary to i n t e r p o l a t e along the 30% line for 11 a n d 8.5 40.0

I

I

38.0

1

//

1

I

36.0 34.0 lid lid r 9

32.0

I I l

30.0

Q 9 28.0

.'>..

i

9

Poo=" Centrate

, -"

I u

~

26.0

9

24.0 22.0 20.0 0.0

20

4.0

6.0

8.0

10.0

12.0

14.0

Q~Jg-Vol l O ~ h "~

Figure 7.6. CakeDryness vs Feed Rate:g-Vol Ratio.

16.0

18.0

20.0

312

DS Scaling

m3/h. Ideally more data would be desirable, but from Table A.9 it is k n o w n t h a t 31% cake was achieved at 10 m3/h at a polymer dose of 9.5 kg/t. Thus a r o u g h estimate for the dosage on decanters no. 1 and no. 2 would be, respectively, 9 and 6 kg/t db. A further small test might be initiated to obtain more precise estimates. As is often the case, a choice needs to be made between capital and revenue expenditure. A larger and thus a more expensive m a c h i n e will consume less polymer t h a n the smaller machine. The pond depth relative to the bowl diameter is larger for no. I decanter and smaller for no. 2. This would suggest there is some performance in h a n d with the estimate for no. 1 and suggest some caution with the estimate for no. 2. This would bring the performance levels for both closer, and thus the choice would probably be biased towards the smaller of the two. It remains to check that the conveyor differentials needed for the two scaled-up machines are within the working range of the gearboxes specified. W e t c a k e rate: OsPs = 50 x 3 / 3 0 x 1.0 tph Qsps = 5 tph

No. I d i f f e r e n t i a l " N-

5/1.0-

5 rpm

38.0 36.0 34.0

f

32.0 i

tIr

V

30.0

a~" 28.0 a 26.0 (..1

f

7

22.0

JeT'm31h ! II 15m3/h

.1

20.0 0.00

I

.S

1

24.0

....t

2.00

4.00

6.00

8.00

10.00

Polymer Dose kg/t db

Figure 7.7. Cake Dryness vs. Polymer Dose.

12.00

14.00

16.00

Calculations and Scaling

313

No. 1 d i f f e r e n t i a l r a n g e : = 0 to

2900 267

= 0 to 1 0 . 9

rpm

No. 2 d i f f e r e n t i a l :

N-

5 / 2 . 0 = 2.5 r p m

No. 2 d i f f e r e n t i a l r a n g e :

= 0 to

2400 = 0 to 9.4 r p m 254

This concludes two very that the required dryness machines at the desired conveyor torques and w h a t

satisfactory scale-ups. The scale-ups have s h o w n can be achieved with both of the two larger capacity. The calculations have s h o w n w h a t differentials would be needed on each m a c h i n e .

This Page Intentionally Left Blank

CHAPTER 8 Instrumentation and Control The first production decanters were virtually devoid of i n s t r u m e n t a t i o n and control, apart from the m a i n motor starter. Today, i n s t r u m e n t a t i o n and controls are m a n y [ l ], and can be quite sophisticated. The present t e n d e n c y is for full automation, to minimise the need for h u m a n intervention, and reduce labour costs. Improved safety standards have encouraged the development of some useful, and reliable, instruments. The development of small, affordable controllers themselves has enabled the introduction of some m u c h needed process instruments. Hitherto an expensive process i n s t r u m e n t could not be justified to be used merely as a monitor. When a decanter is automated, a u t o m a t i o n of a lot of the associated equipment is also necessary, together with interlocking. For instance, it would be inadequate to have a decanter operating automatically u n a t t e n d e d if failure of the cake off-take system could occur w i t h o u t c o m m u n i c a t i o n of the fact to the decanter control system. Figure 8.1 depicts an i n s t r u m e n t and flow diagram for a decanter plant using flocculant, with alternative cake discharge flows for thickening and dewatering. It is not possible to cover every eventuality with one diagram, but this one covers the majority of usual situations. The i n s t r u m e n t a t i o n s h o w n is not necessarily always used, but is t h a t which the plant engineer would consider useful, were it possible. The equipment that could be controlled automatically, or is controlled in standard plants, is marked. Each of these possible instruments will be discussed in turn, after outlining the various modules of a decanter plant.

!

.,.

fell +I"+ [| |

6

| ~

ljy~ +

1~--~ (~l DewateHng

j| Thickening

Fi#ure 8.1. An instrument and.flow alia#ram for a decanter plant.

I

II

..,

C E L n P Q

Control Input Electrical Amps/Watts Level Count Pressure Flow Rate

t T 9 O

Time Torque Solids Concentration !Temperature

s

[~

Kt?

Speed

8.1 Decanter Plant Modules A fully equipped decanter centrifuge plant will normally have several distinct modules within it: 9 9 9 9 9

the the the the the

flocculant system; process slurry feed system; decanter itself; centrate off-take system; and cake discharge system.

The flocculant system for a decanter plant, particularly for the larger plants, is usually supplied as a separate entity, with its own control system. Some of these control systems can be quite sophisticated, with dosing controlled from a feed solids concentration sensor. Nevertheless, there is no reason w h y this control system could not be coupled into the main control system. There are a n u m b e r of types of polymer make-up system. The one represented in Figure 8.1 is the usual dual tank batch make-up system for solid-grade polymers. It basically comprises a powder hopper with a screw feeder, discharging into a stirred vessel. The volume of water is controlled by level probes in this vessel. The contents are stirred for a fixed time, to allow the polymer to dissolve and age to its full potency. After the required ageing time, it is automatically transferred to the polymer supply vessel w h e n actuated by a low-level signal from this second tank. The polymer pump is controlled from the decanter control system. The feed will be supplied from the main plant. This could simply be a tee into a pipeline of the plant, or more usually from a storage tank. A variable speed pump, usually a progressive cavity type, feeds the process slurry to the centrifuge. The rate is fixed m a n u a l l y or by a plant controller. The decanter system itself hardly needs further description. The main motor and back-drive motors are the main control inputs. Larger decanters m a y have a separate oil lubrication system for the main bearings, in which oil flows, t e m p e r a t u r e s and pressures are monitored. The centrate off-take system is generally a large pipe to drain, or to a receiver vessel. Occasionally the decanter will be fitted with a centripetal or

318

Decanter Plant Modules

skimmer pump, when a pressurised discharge will occur, which may have to be released below the liquid product surface in the receiver, to prevent or reduce foaming. In three-phase decanters a second light phase discharge will be present, the flow of which will also need measuring. Dewatered cake is often discharged onto a belt conveyor, straight into a hopper, or perhaps into a screw conveyor or elevator. Where decanters employ negative pond operation, ponds deeper than the cake discharge level, u n w a n t e d liquid discharge from the cake outlet can occur during start-up. This can produce an unpleasant mess on belts, causing them to slip, and will contaminate the product. This is sometimes prevented using notched weir plates, or special start-up and shut down procedures. Alternatively, devices are fitted under the cake discharge to feed the wet cake back to the feed vessel. These devices could be, alternatively, a flap diverter, or a hopper that is automatically moved under the discharge at start-up. The u n w a n t e d liquor discharge is then pumped back to the feed tank. A further alternative is to angle the belt conveyor, such that liquid flows back down the belt into a hopper, while solids convey upwards on the belt. With all these devices some flush may be required after the wash-out has ceased. Thickened cake discharge can simply be into a hopper which is emptied by a pump actuated by level probes in the hopper. However, modern technology often requires the discharge to be monitored for solids content, if not rate. For this a small stirred buffer tank is used. A sample from this tank is pumped and recycled continuously to provide a continuous sample. The stirred buffer tank is sized to smooth out major fluctuations which can occur in the decanter discharge, due to hold-up in the casing.

8.2 Instrumentation This section is separated into the various categories of ins trume nts , such as flow meters, solids co n c e n tratio n meters and timers. Lastly, controllers will be covered. 8.2.1 Flow meters

Flow meters for aqueous slurries are reliable, accurate and seldom require adjustment after initial calibration. Moreover they are a m e n a b l e for connection to PLCs, computers and controllers. The most c o m m o n models used on decanter plants are eddy current and ultrasonic type. Flow meters are used on the feed line after the feed pump and similarly on the polymer line. Flow m e a s u r e m e n t is employed on the oil lubrication lines, but is usually of the rotameter, or variable orifice type. This m e a ns that they are used for indication only, and are not readily coupled into the control system, unless simply as alarm features. Ideally, a flow meter should be fitted on the thickened cake recycle line. This is because the solids monitor works on the principle that the cake solids concentration is a function of viscosity, which in turn is monitored as a pressure drop w h e n flowing. T h u s the flow rate also affects the pressure drop, and therefore must be kept constant. However, often it is found t h a t plant users rely on the constant rate from a metering pump, m a k i n g periodic adjustments to flow or calibration should the pump wear. The total flow of thickened cake is usually obtained by calculation, but a check can be made by m e a s u r i n g the time intervals between discharge of the sump tank. This would be h o w the oil flow is measured from a three-phase decanter. If dilution w a t e r is used, this is generally measured with a r o t a m e t e r variable orifice meter. However if this flow has to be integrated into a control system then an electronic method, as used for the feed, will be necessary. 8.2.2 Solids concentration meters

These monitors tend to be the most expensive instruments, but enable the most sophisticated type of process control. W i t h o u t them "live" m e a s u r e m e n t

320

Instrumentation

of, for example, solids recovery, polymer dosage, cake and centrate rates, and product quality would not be possible. Some laboratory analyses take a few hours to perform, by which time the plant could be w a y off the control desired. A few companies offer devices that can continuously measure solids content of feed flows. Various principles have been used, including the coriolis effect and the use of a radioactive source. The method using a radioactive source has proved reliable, but there is resistance to using it where a watercourse is involved, and moreover there are stringent regulations with regard to the disposal of the instrument once it is at the end of its useful life. Nevertheless the suppliers naturally offer a comprehensive service. Light reflection or transmission is another method that is in use. Centrate solids concentration m e a s u r e m e n t is an i m p o r t a n t p a r a m e t e r for decanter control. Several such instruments are available to measure in this range. However, one problem presented by centrate from a decanter, on m a n y applications, is the copious production of bubbles or foam in the flow. These bubbles are read, by many instruments, as solids, thus preventing the use of such devices. De-aeration of a sample flow of the centrate has met with a modicum of success. Some decanter m a n u f a c t u r e r s developing their own i n s t r u m e n t [2] have obtained more success. Continuous m e a s u r e m e n t of solids, or moisture content, of dewatered cake, as far as is known, has not been practised on decanters yet. However, infrared devices, which can measure moisture content of products on conveyor belts, w h e n positioned about 30 cm above the belt. have been reported. The solids content of the flocculant solution would be a useful parameter in any integrated control system. However, the solids are dissolved, and the concentrations involved are a fraction of 1%, with an accuracy requirement probably down to 0.001%. Moreover viscosity of solutions covers a wide range, and is dependent upon a n u m b e r of other factors, not the least of which is temperature. Therefore, it is not practicable to measure polymer concentration directly. However, there is no reason w h y the powder feeder could not be calibrated for the powder in use, and its on-time measured. With liquid polymer make-up, the on-time of a calibrated raw liquid polymer pump would be measured.

8.2.3 Level probes There is no great need to measure levels in the plant, but merely to have an indication of w h e t h e r a tank or hopper is empty, full, or in between. This is achieved by conductivity, or sonic, probes. They are fitted to the two polymer tanks, to initiate a new batch make-up, and to actuate transfer before the polymer supply tank empties. Probes could be employed in the polymer powder hopper to guard against r u n n i n g out during operation. Smaller plants will not use powder probes, and rely on a system using several days' supply.

Instrumentation and Control

321

8.2.4 Speed probes It is particularly necessary to measure the speed of rotation of the decanter bowl and the gearbox pinion shaft. Occasionally a t a c h o m e t e r will be built into the braking device. More generally, bowl and pinion speeds are measured by proximity probes, acting on a protuberance or castellation, on a spigot, hub or shaft. The speeds of the feed and polymer pump, and also the cake sample pump, are useful t h o u g h not absolutely necessary to measure, as comparing this speed with a calibration speed will indicate the onset of wear. Measuring the speed of the polymer screw feeder has already been mentioned. Another useful speed monitor would be on the solids conveyor driven shaft. All that is needed here is an indication t h a t the shaft has stopped, for instance if the belt should break. In a n o n - a t t e n d e d plant it is essential to know if the off-take system ceases to function, so that the feed m a y be arrested. It is worth noting that the majority of d o w n t i m e of a decanter plant is caused by failures in ancillary equipment, rather t h a n the decanter itself.

8.2.5 Temperature probes The temperature of the lubricating oil from the bearings is usually m e a s u r e d with thermocouples. The t e m p e r a t u r e of the feed is only measured if this is an operating parameter. The t e m p e r a t u r e of m o t o r windings are usually monitored by thermistors, connected to a safety cut-out system in the motor control gear. Obtaining a direct reading of motor winding t e m p e r a t u r e would be unusual.

8.2.6 Torque measurement Conveyor torque today is an essential part of decanter control. However, direct reading of conveyor torque is very difficult to achieve. Even direct reading of pinion torque is difficult, but could be done using strain gauges on the pinion shaft. However, the most usual method is to use a calibration of the braking device. The control device for the brake will give a read-out, on request, of the braking torque.

8.2.7 Timers Timers are integral parts of some of the control systems. They are used in the starter of the main motor, to switch from star to delta operation. They are used in control systems, for the sequential start-up and shut-down of ancillary equipment. Timers are used for the ageing of the polymer, and the on-time of

322

Instrumentation

the feeder. A timer would be used to me a s u re the fill time of the cake sump, to check cake rate. However, a l t h o u g h run-time meters can be fitted to most motors, this is usually, if at all, only on the main motor. 8.2.8 Counters

Counters are used to count batches of polymer made up, to keep an overall check on usage. Cumulative flow is often found in electronic flow meters, to keep account of total flows t h r o u g h the plant. 8.2.9 Electrical meters

The c u r r e n t to the main motor is often monitored to prevent overloading. It also gives an indication of the power being consumed, a l t h o u g h a better device for this is the wattmeter. 8.2.10 Bearing monitors

Interest is now being placed in instruments that monitor the health of bearings in operation. P r e m a t u r e failure can be predicted before expensive d a m a g e occurs. These i n s t r u m e n t s are not yet in wide use.

8.3 Controlled Equipment The control strategy for a decanter plant often will hinge on experience, the user's requirements, w h a t is available and the extent of control required. One of the main decisions to make is regarding the flocculant control. The option for flocculant control is w h e t h e r to have a feed-forward control, requiring a feed solids meter, or w h e t h e r to have feedback control using a centrate solids monitor. With feed-forward control, the flocculant rate is modulated according to the level of solids in the feed. The ratio of flocculant to feed solids may have to be trimmed occasionally should the quality of the feed vary. With feedback control the control performance is independent of feed quality. Nevertheless some centrate monitors can be badly affected by aeration and foam which can occur with some polymers and feeds. The extent of the sophistication of the control will depend upon how m u c h of the plant is required to be incorporated into the decanter system. The good functioning of feed tank levels, off-take pumps and conveyors all may need to be brought into the strategy with appropriate interlock controls. To devise a control strategy for a decanter plant, it is necessary to k n o w what devices are available to the controller. These maybe on/off devices, or devices which can be varied in output by the controller. 8.3.1 On/off devices

These will include the stirrers in the polymer system and thickened cake sump. Also included will be complete module systems, such as the polymer system, the oil lubrication system, and perhaps the cake off-take system. The decanter main motor is also a controlled on/off device, although a variable speed main motor can be employed. The pumps actuated by the level probes on the polymer system, and the sump discharge, are also on/off devices, as are the belt conveyor, the cake diverter, and the polymer screw feeder. In a completely m a n u a l plant, even the feed and polymer pumps could be on/off, and merely controlled on or offby safety interlocks.

324 ControlledEquipment 8.3.2 Variable output devices

These are mainly the feed and polymer pumps, and the decanter brake torque or speed. However, in special cases, the actual bowl speed could be a part of a control strategy. The pond depth itself, using the inflatable dam, could be used in a thickening control strategy. The polymer feeder could be used in a control system, if wide ranges of feed concentration were to be anticipated. As far as is known, this has not yet been used.

8.4 Controllers Modern electronic technology has provided industry with a wide choice of small, user-friendly, cost-effective controllers, with proportional integral and derivative (PID) control action. These can be used individually on the input flows, or integrated into a master controller. It would not be u n u s u a l to have one each on the feed and polymer pumps. A signal from the flow meter would be supplied to the controller, which would adjust the speed of the pump to give a flow agreeing with the set point entered by the operator. The set point could be set alternatively by a master controller. A separate controller is supplied with each polymer make-up system. W h e n energised, it will, according to inputted set points, control the on-times of make-up water and polymer feeder. It will control the ageing time in the makeup tank, w h e n the stirrer is switched on and w h e n switched off. Transfer of aged polymer will only be allowed when the supply tank is below a certain level, when the transfer pump is energised, and afterwards de-energised. This is a simple but very effective system. There are some variations from m a n u f a c t u r e r to manufacturer. The polymer is very hygroscopic and difficult to dissolve, and if not handled properly, can create an inordinate mess. One m a n u f a c t u r e r supplies an air blower to transfer the dry powder into a cyclone wetting chamber, to minimise the onset of glue-like deposits in the lines. The polymer control system can be a u g m e n t e d with a feed solids meter, to give "feed-forward" control, fixing the kg/t db polymer usage to an operator set point. The main motor controller is a separate controller, and depends upon the type of installation and motor. The m o t o r could be AC, DC or inverter type. Rarely, it could be a hydraulic motor. The starter could be DOL (direct-online), particularly if a fluid coupling is fitted, it could be a soft-start inverter system, or a DC system. With an inverter system thc back-drive, also an inverter type, could be connected t h r o u g h the DC bus to allow power regeneration. The starter itself could be actuated by a separate master system. Undoubtedly there will be interlocks with the starter, to cause it to de-energise with certain scenarios. All the controllers are important, but the most important controller for the process is the one controlling the gearbox pinion shaft brake. This PLC

326

Controllers

(programmable logic computer) will be required to control the brake, either to give a set conveyor differential, or a set o u t p u t torque. Whilst this duty, as specified, seems simple, the overall duty expected makes it, internally, quite complex. In certain circumstances it is required to control speeds close to zero and even to reverse speed. It is expected to be suitable for the complete r a n g e of a m a n u f a c t u r e r ' s decanters, and yet expected to control each within safe limits. Moreover it needs to be appreciated that reducing o u t p u t torque allows a lower differential, which increases torque! Thus to allow a higher conveyor torque, the controller effectively has to reduce its o u t p u t torque. Nevertheless, excellent controls have been established on several t h o u s a n d s of installations. A good brake controller will be required to indicate: 9 9 9 9 9 9 9 9

Bowl speed: Conveyor differential speed: Brake or conveyor torque; T o r q u e h i g h / l o w alarm; Differentialhigh/low alarm: Status (start-up or running); Mode of control (torque/differential): Set point.

Access is needed to the operating parameters, with an encrypted code to prevent unauthorised tampering. One such controller is shown in Figure 8.2. Only after using such an instrument can the extent of the needs for such a device be appreciated. The operating parameters may include: 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

Entry code; Modes permitted: Upper and lower alarm limits; Set points: Setpointlimits; PID settings: Secondary PID settings for two-stage control; Sense of alarms (normally on or off): Gearbox ratio; Pulses per revolution for probes; Pulley ratios for speed recalculations; Control ramp rate; Calibration of external signals; Parameters for transmission of data; Parameters for computer communication; Brake t o r q u e / c u r r e n t calibration reference.

Instrumentation and Control

327

This is a very brief synopsis of w h a t could be 60 or more s e p a r a t e parameters.

00@0 0000 0@00 0000

~.: Alfa Laval

Figure 8.2. An

Alfa Laval Automatic Backdrive Controller (,4 BC).

8.5 Integrated Controller With separate controllers already provided to control separate functions of the plant, it is an obvious next step to integrate them into one master controller, or to supply a master controller to supervise the individual controllers. This is being demanded increasingly. Some large plants demand a central remote control room, with mimic diagrams, all controlled by one central, large industrial computer. Some decanter manufacturers already have their own integrated controller, all with varying degrees of sophistication. Some of the duties of an integrated controller are described below. All the signals available, shown in the diagram in Figure 8.1, need to be continuously fed to the controller and converted to digital figures. It should be possible to display any of these figures on request. The figures then need to be processed, according to the relationships in Chapter 4, to provide figures of: 9 9 9 9 9 9 9 9

Solids recovery; Polymer dosage: Torque/volume; Feed rate/g-volume; Centrate rate; Cake rate; Cake rate/differential; Power usage on the decanter and the total.

These should all be displayable. A cost display should be possible, once application itemised cost data are inputted. The data required for an effluent would include the cost of power at various times of the day, cost of effluent disposal, polymer cost, and cake disposal cost. Other costs that may be included would be, for example, amortisation of capital. The processor would then work out the plant running costs for display, or periodic print out. The controller processor would have in-built control algorithms for the plant manager to select. Control could be to minimise overall cost, maximise

Instrumentation and Control

329

dryness, minimise revenue costs, or maximise t h r o u g h p u t . It could also be on the basis of keeping the feed tank d o w n to a certain level. Priorities would need to be set for the various performance factors, such as solids recovery, dryness, cost and throughput. Maximum and m i n i m u m levels for each would need to be set. The controller would be set on a c o n t i n u o u s loop to conduct the calculations, perform control adjustments, display and if necessary print results, and act as an a n n u n c i a t o r for alarms and m a i n t e n a n c e schedules. The control method could be a simple "hill climbing" technique w h e r e small adjustments of one variable at a time are made, and performance checked. The adjustment continues so long as performance improves and a step back is made once a deterioration is detected. The next variable is then adjusted in the same way. Adjustment steps could then be reduced once all variables have been used. The process is then repeated. An alternative control method, which is a newly developed technique, uses a technique called fuzzy logic [ 3 ].

8.6 CIP The equipment used for the CIP feature is described in Section 2 . 4 . 1 4 . A small PLC, or an adjunct to one of the s t a n d a r d controllers, is required to supervise the CIP operation. An operator giving the start c o m m a n d to the PLC or equivalent will initiate the CIP sequence of events. The feed will be stopped. Then the main motor and back-drive system will be de-energised, and allowed to coast down to the required CIP speed, when the main drive donkey motor will be energised and take over to rotate the bowl at such a speed as to generate slightly less than l g (about 70% of lg). The more sophisticated systems will also have a donkey motor to rotate the gearbox pinion. Timers will control the duration of the low-speed r u n n i n g and other timers will open valves to admit cleaning fluid into the bowl and into the spray bars on the casing. Some systems will periodically reverse the back-drive donkey motor to reverse the conveyor differential. This feature must be used with caution, as reversing the scrolling could jam solids between the front bowl hub and the conveyor, and ultimately bend conveyor flights. The program in the PLC will dictate the duration of the CIP, the duration of each phase, and how m a n y times the conveyor, and if necessary the bowl, are reversed. The program will also dictate when, and for how long, the cleaning tluids are applied. The CIP feature is a valuable asset in food and p h a r m a c e u t i c a l processing. The ability to keep the decanter clean and hygienic, w i t h o u t the need to dismantle it, has enabled the use of decanters in processes h i t h e r t o impossible. Decanters can be run for m a n y m o n t h s without dismantling, with acceptable standards of cleanliness. With suitable designs of decanter, the CIP procedure can be used, where necessary, for sterilisation, instead of, or with, chemical cleaning.

8.7 References l 2 3

W Leung, P Wardell, L Hales. (Baker Hughes Inc.) Method and apparatus for controlling and monitoring continuous feed centrifuge. US Patent 5948271, 1 December 1995 J G Joyce (Alfa Laval) Turbidity measurement. US Patent 5453832, 6 March 1991 C yon Altrock, B Krause. Fuzzy logic application note: optimization of a water treatment system, http://www.fuzzytech.com/e.a.dek.htm

This Page Intentionally Left Blank

CHAPTER 9 The Decanter Market In a total world market for liquid/solid separat.iriri eqiiiprnent. of' about $ 6 billioris (coveririg ;ill appliciitions, domestic and institutional as well a s hdUStrial), the decanter has come to be an important cornponeat. with ;t market share of ahoui 1 0 ' K (11' that figurc. This chapter looks briefly at. lhe market hi-decanters, tiow i l is made up, and how it is expeckd I,{) develop.

9.1 Market Characteristics The decanter centrifuge is an i m p o r t a n t processing tool, but is by no m e a n s cheap, so the decision to invest in a new decanter is one that has to be taken with care. The m a r k e t is characterised by the presence in it of a few large suppliers, with m a n y years of experience and with wide ranges of types of decanter available. There is then a group of smaller, general suppliers, plus a handful of niche m a r k e t suppliers (mostly to olive oil production and similar applications). There is certainly e n o u g h experience available in the m a r k e t place, to enable any potential p u r c h a s e r to obtain satisfactory quotations for a new m a c h i n e from a n u m b e r of competitive suppliers. The purchase of a new decanter is very strongly influenced by the intended process duty, and almost all such purchases are made only after careful analysis by the supplier of the required performance, and. possibly, after some kind of trial with the customer's process liquor. Trials m a y involve the installation of a temporary test decanter, and ancillary plant, as a static or mobile rig. Such a test may be for an extensive period, to cover all the likely variations in process slurry characteristics. The test rig could be a full size, pilot scale, or laboratory installation. Experience of a particular application by the supplier makes the selection process one which can be approached with confidence, and the potential p u r c h a s e r would do well to enquire as to the level of relevant experience available. The major m a n u f a c t u r e r s have sales or subsidiary c o m p a n y offices in most, if not all, the larger industrial countries, and local to m a n y of the large decanter markets. These offices are usually staffed with very competent sales engineers, able to convert the supplier's wealth of experience into a preliminary quotation quite easily. Such a start m u s t usually then be followed by the trial process already mentioned.

9.2 Market Trends As is s h o w n in some detail in this handbook, the decanter is an extremely versatile processing device, by virtue of the m a n y different items of its makeup that can be changed to suit the process needs. In this way, the decanter has been able to meet a wide range of process challenges over the past halfcentury. The main trend in the market place can thus be expected to be a steady improvement in detailed design, to enable the decanter to meet further such challenges. These improvements will spread to all the main suppliers, so t h a t the choice ofdecanter source will still remain wide. The major application growth will continue to be in the processing of waste slurries, and this duty requires as inexpensive a machine as possible, a l t h o u g h coupled with quite advanced specifications, in order to achieve high dryness figures in the discharged solids. The trends identitied in Chapters 1 and 2 will impact on the market, but the major marketing input continues to be to let the world of the process industries know w h a t a useful thing the decanter is. and how it can solve so many liquid/solid processing problems.

9.3 Market Size Estimates The estimation of the size of the decanter market is beset by the usual problems faced by any attempt to e n u m e r a t e a market: definition of scope and avoidance of double counting being two of the most difficult. Currency variations can have a marked effect on size estimation, especially where historical data are being extrapolated. Market size estimations m a y be approached from two directions: top-down and bottom-up. The d o w n w a r d s approach starts with national or international data, for trade and production, and uses these to derive components of the market. This method is made difficult by the lack of c o m m o n identity a m o n g categories of data, and by the omission, certainly from national production data, of most small companies. The existence of a single supplier in a national market m a y also be a sufficient reason to omit the figures from published statistics. The upwards approach starts with individual components of the particular market, and aggregates t h e m to arrive at an overall figure. These components may be the sales into particular end-uses, or the sales by individual manufacturers. It is in this method that the problems of scope and double counting are most likely to occur. Many companies, for instance, do not differentiate between m a c h i n e - o n l y sales, and all of the ancillary work t h a t is done to make up a final sales contract, or of the size of after-sales work. The result of the inadequacies of each of these two approaches is that both have to be used together, to derive an estimate that appears to satisfy both. There may then be, as is the case with the decanter, other published m a r k e t size estimates, which can be used to corroborate the results of the direct analyses. These published data rarely agree very closely, once (and if) a c o m m o n basis can be established, but they do give outer limits to a calculated figure, and present some confidence as to the results of the work. 9.3.1 Overall decanter market size

By means, then, of the methods just outlined, a total world market for the decanter centrifuge has been derived of $625 millions for 2 0 0 0 , at a mid-year value of the US dollar. This figure has a probable accuracy of + 10%.

it relates to the final salc to thc end-user. at the pricc paid by that

customer:

4

i t covers the sale or whole decanlers only, :ind r i o t any aft.er-s;ilrs work; i t covers f.he sale a1. the i.irne of t h e whole machine of ;iny si;jnd;ird supply of sp:ire parts, but not of any spares for that machine said later: i

d

it ex~liidesill1 xiditional equipment sold with thc dccantcr that is not necessary fur the safe and cficient operatioil o r t h e riiacliirw.

This salcs valuc corresponds to a tigurc iri t h e regioii o f 1500 10 .3000 Ibr I he number of decantercelltrifuges t o he sold in 20(')0. It is expected that the decaliter rnarkei, w h i c h hiis hcen growing q u i t e strongly in size sirice the eiid o f t h e recession o l t h c early 1 Y Y O s , will continue t h i s growth p;ittrrri, Over the next tive pears. indccd. t l ~ c markct is t-xprt.tet1 tu grow at brlwyerii 4 iind 4.5%pcr aiiiiuiii (i.c. cnm brt abl y iri cixrt?ss ol' thc expected iiicreasr in gross dnint?stic product tigurcs).

3 j . l ' i L for all water arid w;ir;te water t r e a t m e n t . industrial as M C I I a s rri i i n I ci pal : (1. 3% lor fuel malt'rral extraction atid processing; 4

I I

a

1 9 . 1'XI for food arid b r v w i g c proccssing: 1 3.4'%,for minerah arid hulk inorganic chcrnicala: 1 0.4'K for h l h organic chcinicals and petrochemicals: 8.i'%,fur finechuinirals and pharmaceutical?; iind 7.[1'%for other applications.

338

Market Size Estimates

9.3.4 Suppliers' market shares The suppliers of decanters to the world m a r k e t are mentioned in Chapter 1, w h i c h includes a list of most of the m a n u f a c t u r e r s k n o w n to be producing decanters in 2 0 0 0 . If allowance is made for the companies not listed in Chapter 1 (believed to be all small ones), then the major holdings of m a r k e t share are: 9 9 9 9 9

36.8% by Alfa Laval and Tomoe; 16.8% by Baker Process (Bird Machine and Bird Humboldt); 9.6%byFlottweg; 8.8% by Westfalia; and 6.4% by Pieralisi.

Shares of between 1.5 and 3.0~ are held by Broadbent (with Tanabe), Guinard, and Siebtechnik. and of between 0.5 and 1.5~ by A m e n d u n i , Centriquip, Centrisys. Hiller, Hutchison-Hayes. Noxon, and P e n n w a l t India. This leaves a market share of 6.1% held by the other, unspecified companies. It can be seen that the five largest companies hold almost four-fifths of the total market.

CHAPTER 10 Suppliers' Data This chapter lists the main rnari~1I'a~:~i~rt:rs ofd(:r:arllers, together with details of thcir company structiirc and of their ranges of decanter centrifiige. This is lint an cxhaustivr list. but it includes data alrcady in the public domain, issued by manufacturers in their brochures. a n d augmented by data supplied by some ofthc companies Ibr thc purposcs of'this book. 'L'hc coveragc hcre is intcndcd to be that of all of the main manufacturers. plus as ~iiaiij'othcrs a s could bc located. 1Pcit each r i i a n ~ i f a ~ t u r are e r given the siilicnl r i 1 c l . s iil>(>11i i1.s Iieatiq~iiirters irrirl ut ticr ilrlrlresses, plus its mariufat:tutirig rurlpci. arid othcr iriforrn;ilion d u s t : to t h e rtwdtir. lktails ofits drcarilcr rriodels art! I hrri t i ~ h u l u ~ e t l . 'I'his iriforniu~ivri is prvvirled lu enitble the reader to develop some parnmctcrs ofchoicc when a ncw purchase of zi decanter is to be undertaker]. The data glvcn should not, h o w c w r , bc uscd Tor design iiiid spec-ilicaliori purposes. b u t niorc for initial studies a s t o what c o i ~ l dlie p s s i t ~ l r .o r frir corn par is(>n stud i cs. F i11a 1 rcc onim e 11d a t i o 11s s11 o u Id ;I 1w ;Iy s hr! so 11 g h t from thc prcftrrcd suppliers, Thc ent.rics arc in :Ilph~ibetic:il nrrlcr, and no attempt has been rnadc by the authors tu be rritriclivc in any cntry. IJnder the heading "company uwnership", rriutition is rn:itic! of m:ijor clwncrr;lIip by another soriipariy. or 01 thc cxistonce of m a j o r cqiiity holders. Otherwisc. ownership is assuo~edt.o hr private o r public: sharc ownership. according to type oTc.orrlp:iny. khtrics u n d c r "othcr main biisiacsscs" refer t,o oI,}ier rjon-dscanter aotivitirs ofthc riameti c.ornpilny. whilc "othcr company c'oriricctioris" refbr to business associations spccific l o the decanter husiriess. The data given under thc hcading "decailler sales" art! eithttr siipplicd by t.hr ~niin~Il'ilct1irer in qiiestion. or cstimated by t h y authors. Set. Chapter 9 lor discussion on market size estimation. In the model tabulation. all combinations of (iritcrrial) howl diameter and length tire includcd ;IS far as possible. 'Ihe column headed "howl length" rcfcrs to the cylindrical bowl lerigl.11 or ihe sedimentation zonc. whitc the t h t b

340 Suppliers'Data "total inside length" includes the beach, a length for the cake discharge, and the screen section in screen-bowl machines. The column headed "beach dia" refers to the diameter at the cake discharge. The "total liquid volume" is the bowl's liquid holding capacity with the pond set at neutral. The maximum bowl speed tabulated is that quoted by the manufacturer for a m a x i m u m process density of 1.2 kg/1, unless otherwise stated. The location of the feed zone dictates the clarifying length. Often this is located at the foot of the beach, but this is by no means universal and is seldom so on co-current designs. This feature, together with the design and location of baffles and restrictions, is considered an intimate design detail by most manufacturers and is only known to the users after purchase. The ranges of gearbox ratios and torque ratings are those that are known to have been used. or which have been quoted by the manufacturer. The maximum and minimum ratios do not necessarily correspond to the maximum and minimum torque ratings, respectively. A specific ratio and torque within the range cannot arbitrarily be quoted. For precise figures the supplier must be approached directly. In the column "design", "A" corresponds to counter-current operation, while "B" signifies co-current: "H" refers to horizontal mounting, while "V" is vertical. A and B together, or H and V together, signify that the size is available in both variants. A reasonably complete set of data is given for the range of decanters made by Alfa Laval, which company decided to support the production of this handbook by making free of its design data, so as to enable the reader to put much of the body of the text into some real context. Other manufacturers did not feel able to support this gesture, and so the data supplied are considerably fewer, but at least give some idea of the relative strengths of the production ranges.

Alfa Laval, Sweden Name of company Alfa Laval Separation AB Headquarters address Hans Stahles vfig, S- 14 7 8() Tumba, Sweden Phone: (+46)8 5306 5000: Fax: (+46) 8 5303 3589 UK address Doman Road, Camberley, Surrey GU 15 3DN Phone: 0 1 2 7 6 6 3 3 8 3 : F a x : 0 1 2 7 6 61088 US address 955 Mearns Road, Warminster, PA 18974, USA Phone: (+ 1) 215-443 4000: Fax: (+ 1) 21:5-443 4112

ner s ti ip hl ;i j or i t J' h o Id i n g hy 1nd I I st r i K ;i p i t a I ( Sw cdcn

c'oiii p ii ny CIIV

h ti la 11 cc' by Te t rii La I:a 1 (Switzttrl;ind 1 'I'y p c of c 0m pa I? 1' Spec i t i Ii st cen t r i f'uge m a nuCa c t u re r . Th c Shar plcs Corpora t i c m now par 1 of Alt'a Laval, was cine of the world's first f w o major manuhaclurcrs uf dcca 11 t c'rs Othcr main busiiicsscs IXsc stack ccntrifugcs, tuhular bowl ccntrifuE,cs. filtration cciitrifugcs Alfa Laval company also has 'I'hermal arid I k w divisions ( )t h t'r corn pa II y con 11 ec t io 11s Associat.eclmanufacturer: 'Iomoe. 'I'okyo. J apari

k c a r i t e r iriariufacturing sites C;irnhPrley. U K Coprn tiagen (Suhorg). llerlmatk W ;i rm i 11s I Er 1.J S A

Pune, India Decanter range Over h O models. i1viiil:ible in range or v;iriarits. w i t h rhree-phase and scrccn-bowl designs. plus vertical tiiachincs Dccantcr salcs Over 1000 un its sold a n n u a l l y . Sales in region of$130 illillions i'orccasl for 3000. i n c h d i n g I h o s r suld by 'I'omoe. Sales specialisatioti Very wide range nf applications. strong in wastc treatnicnt

Suppliers'Data

343

Machine details Model ref

8o~,1 dta

,

(mm)

I

2 3 4 5 6 7 8

9 I0 I1 12 13 14 i5 16 17 18 19 20 21 22 23 24 25 2e 27 28 2~ .3(3 31 32 33 34 35 30 37 38 30 40 41 42 43 4.4 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 60 67

152 152 152 18-1 230 250 249 310 353 353 353 353 353 353 356 356 356 356 356 356 356 356 356 425 425 425 425 425 450 450 450 4 82 451) 450 4")8 418 480 480 508 50g 549 575 575 575 010 610 670 620 6"10 610 615 615 635 ~35 635 73 .) 737 73") 73 "~ 73 "~ 889 1016 1016 1016 ll)lb 1016 12"0

Bowl length' ~mm) .

.

Beach a n g ~ {deg ) . . 10 (I I00 10 0 10 0

Total ms)de length (nun)

Tota'l bqu:d vol ' ( ! )

Max bowl Power load Gearbox mu0 speed ffpm) range tkW) range

Gearbox torque range (kNm}

Mac~nc desqztr

353 353 50~ 430

2 2 3 5

6000

tq 2-0 9

A/H MH A/a,' A/H MH

8 0

! 42

700

12

10 0 82 82 85 20 0 85 20 0 85 =-~30 10 0 I0 0 I0 o I011 I0 0 10 0 17 0 53 100 I0 0 II 0 15 0 I00 O g5 20 0 21) t'q 20 0 85 85 B ") 82 10 0 20 0 I0 0 I0 0 It, 0 10 0 20 0 20 3 l0 O 15 0 80 80 16 0 16 0 I(]0 II 0 10 0 17 0 IC o 100 tO 0 15 o IO o 15 0 1130 I0 Ot IC o 15 (1 1(2 0 15 0 [5 o

180 102 12.8 202 198 202 198 202 108 264 241 264 241 264 241 212 254 200 264 230 108 2O4 208 266 258 258 258 266 266 274 274 203 263 381 3gl 349 326 326 326 381 285 17~ 375 352 352 483 381 483 391 483 55o 483 343 483 343 572 762 610 406 610 400 50~,

550 i 275 1627 860 86O 1160 1160 1460 1460 572 572 704 794 1264 1264 12o4 1264 12o4 1264 1204 1264 1772 1772 !,,t~ 1460 IglO 2360 1910 2~60 1984 2448 2035 2t)'~5 i186 1946 2266 2~0 2440 3015 1930 1030 1850 7640 1845 2615 16-'6 1676 2286 2256 2946 2337 2397 2737 30.18 30,tg 3302 "~556 3556 3556 44 ;0 4470 __5334 . . . .

tO 3h 8,2 "~5 47 55 67 74 87 I8 20 28 32 49 57 ~0 47 68 86 98 109 130 104, III 137 1~5 23~ 15 n 2O4 l~7 246 212 233 93 151 "t2 380 ~,~15 "86 2")8 358 247 4(~5 3%4 503 187 2.58 272 .~2 35O 365 46l 62t 634 go I 10(34 1104 14~6 1934 1900 2255-) ~'~2')

15

.......

'Beach dta : (ram) . . . 110 112 119 121

_

95-98

2

6O00

98

0

80(IO bOO0 62O0 tO00 9 8500 76r 4000 4000 4000 4000 4(~X) 4000 4000 4000 4000 4O00 4flO0 4000 4000 40(~ 4000 31250 3,b50 3~50 3650 3050 3250 3250 3250 2900 3250 2qO0 2")00 2700 3650 M1511 9 3130 3350 I050 2900 2000 2t,hO 285O 2g50 27,~; 370(I 270O 27(R) %(I~K) 3000 3000 30[')0 2700 2700 _'27~1 271g) 2800 ?6(141 2150 2(hX) 20:kq 20LX) 23',,)0 21',)0 .,:')o

95 95 25-55 94 92 92 57-159 57-159 57-159 57-159 57-159 57-159 52-125 52-125 52-125 52-i~ 52-125 52-125 52- I LS 52-]25 52-125 52-125 52-125 52-125 52-125 52-I 2b 57 57-159 57-159 57 ISq 57-150 169 57-50 57- 59 57-380 9"I-380 49-98 49-98 91,), 207 380 267 380 2127- %36 49 98 49--Q~ 4(i 45 45 45 48--325 48-325 48-325 48-'~25 48-325 80 48--325 48-325 48 325 48- 325 81~95 't0--95 47-80 80-95 60.--484 4?-95 47-o5 . . . . . . .

0o 09 (19 09 I5 15 2 5-3 5 2 5-3.5 2 5-3.5 2 5-3 5 2 5-3 5 2 5-3 5 8-2 7 8-2 7 B-2,7 ,'1-2 7 B-2 7 8-2 7 ~-2 7 .R-2 7 fl-2 7 8-2 7 8-2 7 B-2 7 8--2 7 8-2 7 5--6 0 2.5-0.0 25-00 2 5-6 0 60 t 5-B 0 t.5-g 0 ~, 5 - I 0 0 "LS-10 0 10 2 10 2 10 I0 0--i6 0 I0 0 16 0 16 O- 16 0 I0 2 IC 70 70 7.0 7o 20, 0--24 0 2t} (I.-24 U 20 0-24 0 20, 0--24 0 20 0--24 0 34 0 2C o. 24 O 20 (I,-24 0 20 0--24 0 2(3 0--24 t) 20 .)-.3-1 0 2C 4-34 0 20, &-3.4 0 2G 4-34 0 ~,4 0 56 6 20 4 ?n _4. . . .

A/H MH A/H A/H A/H NH A/H A/H A/H A/H A/H MH/V A/H A/H/V A/H A/H A/H A/H ~H A/H A/H A/H AJH A/)! A~H A/H A/H

A/H A/H A/H A/H A/H A/H A/H ~H .VII A/14 ndH A/lt adH A/H~" A/H A/H A/ll A/WV Jdtt MH All! A/tt A/H A/I! A/H A/It g/H

A,'tt

~H .....

In this. and all ()tiler tables of data in Chapter 1(). the following notes apply: 1. The column headed "bowl length" refers to the internal, cylindrical bowl length, of the sedimentation zone. while the "total inside length" includes the beach, a length for the cake discharge, and the screen section in screen-bowl machines. 2. The column headed "beach dia" refers to the diameter at the cake discharge. ~. The "total liquid volume" is the bowl's liquid holding capacity with the pond set at neutral. 4. In the column "Design". " A " corresponds to counter-current operation, while "B" signifies co-current: "H" refers to horizontal mounting, while "V" is vertical. A and B together, or H and V together, signify that the size is available in both variants.

344 Sz~ppliers' Data

Amenduni, Italy Name of company Amenduni Nicola Srl Headquarters address Via delle Mimose 3. Z I, 1-70026 Modugno (BA), Italy Phone: (+39) 080 531 4910: Fax: ( + 3 9 ) 0 8 0 531 4 9 2 0 UK address None US address None Company ownership Private company Type of company Specialist equipment manufacturer Other main businesses Other company connections None Decanter manufacturing sites Modugno Decanter range Four models with two diameters Decanter sales Sales in region of $4 millions forecast for 20()() Sales specialisation Olive oil extraction Machine details Beach dta." (mn))

Total mstde length [nun)

Gearbox t~lrque rangc (kNm)

Machine des0gn ~

Bowl dla. Bowl (mm j length ~

I 9(~)

.393

1()41

A,q-I

2. 9,01

430

14()()

MH

3 992/C

"430

1670

MH

4 ~)21S

450

1930

.AdI-I

,ram)

Beach an~ie (dog. t

Total hqmd Max bowl Power load Gearbox v u i 'q ] ~ .~peed qrpnn~range (kW') rauo

Mndel ret. nu

range

Bird Humboldt, Germany Name of company Bird Humboldt Headquarters address Dillenburger Strasse 100, D- 5110 5 K61n. Germany Phone: (+49) 0221 822 6500; Fax: (+49) 0221 822 6169 UK address Euroby Ltd

Suppliers'Data

345

31 High Street. Wootton Bassett, Wilts SN4 7AF Phone: 0 1 7 9 3 8 4 8 1 0 4 : Fax: 0 1 7 9 3 8 4 8 1 0 4 US address Bird Machine Co Inc 1 O0 Neponset Street, South Walpole, MA 020 71 Phone: (+1) 5 0 8 - 6 6 8 O400: Fax: (+1) 5 0 8 - 6 6 8 6 8 5 5 Company ownership Subsidiary of Baker Hughes group, as part of Baker Process Type of c o m p a n y Specialist equipment m a n u f a c t u r e r Other main businesses Waste water t r e a t m e n t systems, t h e r m a l sludge t r e a t m e n t ( " C e n t r i d r y " disc and rotary driers), filter presses, gravity separation systems Other company connections Within Baker Process, sister c o m p a n y to Bird Machine, South Walpole, USA Decanter m a n u f a c t u r i n g sites Cologne, Germany Decanter range The range of supply is that of the KHD/Humboldt Wedag c o m p a n y . acquired by Bird Machine Co. There are a dozen basic models, available in several different bowl lengths, with co-current as well as c o u n t e r - c u r r e n t tlow, three-phase and screen-bowl versions. Also includes "Censor" Twin beach machine, for separation of solids by density Decanter sales For 2()()() these are estimated at $1 ()5 millions, to cover all Baker Process decanter sales Sales specialisation Very wide range of applications, special success in waste t r e a t m e n t Machine details Mendel ref n,~

Bowl d,a (ram,

I "

] 5o 2'q)

350

Bowl length' (ram)

Bcach angle (deg)

Bench din. z (ram)

Total Inside length t mm~

Total h q u , d Max bowl Powerioa-d ~,ol. ~ {I) speed ~tpm~range ( k W ) .,

Gearbox rauo range

Gearbox Iorque rdnge ~k N m )

Macn,ne deslgn a

A/El

I (~51)

~4(~)

22 15/3. t}

A/H .a~tl-I

~9 5

451) 530

I ~50 1590

3000 2r

18.5/'~5 22/75

MH MH

5a

530

2280

2200

30/75

A/H

7

6(~) ~50

2500

I ~2 451L6~)

q I0

7a ,~

650 750

22~0

2800

MH AJH

551160

A/H A/H

76O 900

250 300

A/H A/I-!

1i

1000

400

A/H

I2

1 I IX~

40(I

A/I-!

I~

t 21)0

400

A/I'I

I..4

141~t

500 . . . . . .

A/H

.

2300

346 Suppliers'Data Broadbent, UK Name of company Thomas Broadbent & Sons Ltd Headquarters address Queen Street South. Huddersfield, W Yorks HD ] 3EA Phone: O1484 4 2 2 1 1 1 ; Fax: 0 1 4 8 4 516142 UK address As above US address Broadbent Incorporated PO Box 18 5249, 2 6 8 4 Gravel Drive. Fort Worth, TX 76118 Phone: (+1) 817-595 2411; Fax: (+1) 817-595 0415 Company ownership Publically owned Type of company Specialist equipment manufacturing company Other main businesses Batch and continuous basket centrifuges: laundry equipment Other company connections Associated manufacturer: Tanabe. Tokyo, Japan Decanter manufacturing sites Huddersfield, UK Decanter range Standard machines including three-phase separators, plus screen-bowl designs Standard machines as 17 models, in 1 () different diameters Decanter sales Sales in region of $20 millions forecast for 2()00, including those sold by Tanabe Sales specialisation Company history heavily sugar oriented. Decanters sold strongly into the chemicals sector, and for waste treatment, especially in flue-gas desulphurisation processes

Suppliers'Data

Figure 10.2. A Broadbent centrifzLqe installation.

Machine details Model ref no

Bowl d0a (ram)

Bog l length I (,rim)

Beach anglc qdeg.)

Beach dna : (ram)

Total ms]de length (ram)

Total hquid Max bowl Power load vol ' ( I ) speed (rpm)range tkW)

I

t 50

3(x)

5/7 5

2

7(~) 7(10 i I00

7 5115

4

225 450 45o

5 6

5sO 6tW)

! 375 9.51)

30/60 ~0/60

i

btW)

15o0

t~(x}

2100

30/t)() 10/75

750 "'~(] o00

I~75 220t)

55/I I(.)

II

I ),ILK)

r

12

othl

l(kO0

2500

] 10/! 5(}

I~

1500

75~00

9 I(I

15130 15130

%011 I(1

14

1200

21013

1101350

15

120O

30(0")

I I (11350

16 17

1200

36(K)

1101350

I4(X)

18f10

I I 0/350

Centriquip, UK Name of company Centriquip Limited Headquarters address Derby Road. Clay Cross. Derbys $4 5 9AG Phone: 0 1 2 4 6 2 5 2 6 0 0 ; Fax: 0 1 2 4 6 252601 UK address As above US address

Gearbox ratm range

Gcarbox torquc range (kNm)

Machine destgn 4

347

348

Suppliers' Data

None Company ownership Private company Type of company Specialist decanter manufacturer Other main businesses None Other company connections None Decanter manufacturing sites Clay Cross, UK Decanter range A range of models, based on three sizes of machine, with variable specifications, to suit c u s t o m e r ~ i n d u s t r y requirements Decanter sales Sales in region of $6 millions forecast for 2000 Sales specialisation Waste sludge treatment for industrial and municipal applications

Figure 10. 3. A Cetztriquip centrilzlge itlstallation.

Suppliers'Data 349

Machine details Model rcf . . . . Bowl dla. (mm) I CO3(~) 2 CQ40r 3 CQ5 _f~,

....

Bowl B e ~ t ; "" Beach-dia? Total' inside ro'tal liquid length I angle (dell.) (ram) ienllth (ram) vol. ~ (i) (mm)

Max bowl P . . . . speed load (rpm) range (kW)

Gearbox ratio range

Gearbt, x Mach,ne torque design ~ range LkNm)

] 8.5 45 45

Centrisys, USA Name of company Centrisys Corporation Headquarters address 9586 58th Place. Kenosha. WI :33144. USA Phone: (+1) 2 6 2 - 6 5 4 6006; Fax:(+1) 262-654 6063 UK address David Hollier Ltd Upperthorpe, Westwoodside. Doncaster DN9 2AQ US address As above Company ownership Private company Type ofcompany Specialist decanter manufacturer Other main businesses Service facility for all major brands of centrifuge Other company connections None Decanter manufacturing sites Kenosha, USA Decanter range Six models, available as two- and three-phase machines, with variable specifications Decanter sales Sales in region of $ 7 millions forecast for 2()()() Sales specialisation Water and waste water sludges

3 50

Suppliers'Data

Machine details "Iviodc! ref no.

"Bowl dia. "'Bowl ' (ram) length' (mm~

Beach Imgic -"Beach ~a ~ (deS ~ (mitt) .

.

.

Tocli reside Tor hqmd voi' Max bowl Pow.~" C_~ar~ Gearbox ~bc]uac" lcngth (ram) ( } A speed (rpm) load nmgc rauo range tm'que rtnlp: design'

.

(,kw)

...

i~m.)

.

1 CS 10-,t

250

830

15

160

1000

27

6000

"7 5/18 5

I 0-,I..4

AJ]-[

2 CS 1,1-4

350

1040

15

215

1450

68

4500

|8.5/30

2.0--8 9

A/H

3 CS ls-a

450

L280

15

260

1800

14.8

3600

22/45

4 4--8.9

A/H

4 CS 21-4

330

1880

15

330

2300

387

3200

30/56

4.4-12,9

AJH

5 C5 26.-4

660

2340

15

410

2650

531

2830

56/! 12

8.9--16 7

AJH

6 CS 30-4

7~0

2700

15

475

3050

814

2650

112/186

8 9-35.0

A/H

,~-,

t

"~

~, i ~

''

k',

Figure 10.4. A Centris!ls mobih' lest rig.

Flottwcg, Germany

Name of company Plottwcg GrnhFl Headquarters address rndustriestrasse 6-8, n-X413 7 Vilsbibutrg, Cermiiny Phone: ( + 4 9 ) 0 8 7 4 1 301-0;I:ax:(+49)US7413 0 1 3 0 0 Uk'iiddrcss Krauss-Maffci (UK) Ltd E u r o p Houlcvard. Gemini nusirless ['ark. Warrington, Cheshire WA.5 5TR Phone: c) 1 9 2 5 h44 1OI) : Fax: 0 19 2.5 G 44 1 7 X US address Krauss Maffei ('orporation, Process l'cchnology Llivision P()Box h 2 7 0 , Florence, K Y 41022-h270 Company ownership A rrieiiibcr of thc Krauss-Mnffri Grniip. withiil Atecs. owned by the Siemens-Bosch consortium (Crrmany 1 'I'yyc of colllp~llly A specialist decantcr manufacturer ( )I h I:r 111 ai 11 b usi n csscs

I k l t presses a r c supylicd by Flollweg. a n d combined systcims with thr! disc st:lcIi r.rntril'uges madu h y Vcroncsi Other r:r)mpijny ~ o r ~ r i e c l i o r i s Closti ;issocialiori w i t h VeIonesi inside tlic Krauss-Maffci group I)rca n ter mar I 11 I'a c t 11 ring sites V i lshi bu r g , Germ i i n y I k c a n t e r r a n ge Thirtccii modcls. based on eight bowl diametcrs, available with variable spcciticalinns. including thrsc-phasc operation, a n d dcsigiis For optimal

sludgc thickcning and high dry solids (ipcratiun TlwrC arc four distinct modcl rangcs, cacb with its ow11 particular process sepii rat i II11c a pa hi 1it y :

I1ec;inter siiles Sales in region oI5hO miillions 1orcc;ist lor 2 0 0 0 Sales specialisation Very wide range or ;ipplir:;itions, n o piirticular speciality, althouEh not17 emphiisising wiisle treiilment

3 52

Suppliers'Data

Machine details Bea'cla dr'a? Total Model ref. no ROW[ dia. Bowl "" Beach (ram) length t angle (deg.) (mm) inside length (mm) (ram) I Z 23-3 2 Z 23-4 3 Z 32-3 4 Z 32..4 5 Z 40-3 6 Z 40-4 7 KVZ-51 g Z53-4 9 Z 6E-2 l0 Z 6E-3 l I Z 6E-4 12 Z 73-4 13 Z 92-4

23O 230 32O 320 420 420 500 5"~0 620 620 620 730 920

Total liquid Max bowl Power " C_learboxGearbox vol. 3 (I) speed (rpm) load ratio torque range range range (kNm) (kW) 7.5/15 7.5/18.5 15130 |5/30 22/45 22/45 30190 22/55 45/132 451132 45t 132 37/110 90/250

Figzlre 10. 5. A Flottwe~.I cetztrffzlge installation.

Gennaretti, Italy Name of company Genaretti SpA Headquarters address Via Roncaglia 10, 1-60035 Jesi (AN), Italy Phone: (+39) 0731 200200: Fax" (+39) 0731 2 0 0 1 2 4

Machine desngtl 4

Suppliers'Data

353

UK address None US address None Company ownership Private company Type of company Specialist decanter manufacturer Other main businesses None Other company connections None Decanter manufacturing sites Jesi. Italy Decanter range Two basic models, with a range of specifications for different applications Decanter sales Sales in region of $4 millions fl~recast fl)r 2()()() Sales specialisation Range of industrial applications, but mainly involved in animal and vegetable products Machine details ,_ Model ref. nt:. Bowl dr.',. Bc~wl (ttun) length ~ (nun) 1357 2 5(X)

Be:,ch Beach dla.: Total angle ~deg.) {Jmn) inside length ( mm )

Total hquid Max howl Power vol ~ ( i ) .,,peed (rpml load range

(kW) 4(X)() 3(X)O

15130 45155

Gearbox ralm range

Gcarb~x torque range ~'l,N m l

Machine destsn'

A/H A/H

Guinard, France

Name of cmupany Guinard Centrifugation Headquarters address 2-4 ave de l'Europe. F-7814() V61izy, France P h o n e : ( + 3 3 ) ] 3 9 2 6 ( ) 5 5 ( ) : F a x : ( + 3 3 ) l 3926()56() UK address Andritz Ltd, Unit B, Sheepbridge Centre, Sheepbridge Lane, Chesterfield, Derbys $41 9RX Phone" ()1246 26()66(): Fax" ()1246 26()76() US address Andritz-Ruthner Inc 1010 Commercial Blvd South, Arlington. TX 76001 Phone: (+1) 817-465 5611" Fax: (+1) 817-468 3961

354 Sz~ppliers' Data Company ownership Subsidiary of Andritz group (Austria) Type of company Specialist separation equipment manufacturer Other main businesses Press belt filters, disc stack centrifuges, basket centrifuges. Other company connections Only within Andritz group Decanter manufacturing sites Chateauroux, France Decanter range Seven basic models, with some design variability Decanter sales Sales in region of $12 millions forecast for 2 OO0 Sales specialisation Coupled with Andritz equipment, strongly oriented towards waste treatment Machine details Model tel nt~. Bo~.'l daa Bowl (nm11 length ~ i rnm I 1 D2 2 D3 3Da 4 D5 5 D6 6 D7

2b{)

7 DI()

ItKIO

Beach Beach dta : Total angle ~deg ) (nun} inside length (ram)

Total 'hqu'Jd Max bowl P~)~,er vol.' ( 1~ speed (rpml luad rant~e (kW)

Gearbox ratto range

Ge~,x torque range (kNml

Math,he cteslgn'

A/[-I. A/H A/H A/H A/H A./H 4201)

2 twJ~)

AJH

Hiller, Germany Name of company Hiller GmbH Headquarters address Schwalbenholzstrasse 2. D-84 ] 37 Vilsbiburg, Germany Phone: ( + 4 9 ) 0 8 7 4 1 48~ Fax: ( + 4 9 ) ( ] 8 7 4 ] 4304 IJK address Dirk European Holdings Ltd 29-3 ] Woodchurch Lane, Prenton. Birkenhead. Wirral CH42 9PJ Phone" 0 1 5 1 - 6 0 8 8552" Fax" O151-608 7579 US address None Company ownership Type of company Specialist decanter manufacturer, until 1998 made exclusively for Humboldt

Suppliers'Data

355

Other main businesses Hydraulic motors for decanters Other company connections None outside Dirk group Decanter manufacturing sites Vilsbiburg, Germany Decanter range Five models in two basic types Decanter sales Manufacturing capacity for 200 centrifuges per year Estimated sales for 2000 of $8 millions Sales specialisation Food (especially olive oil) and waste treatment Machine details Model ref no. Bov,'i dta. Bowl (ran:) tenglh t I ram)

Bcach Beach d=a 2 Total angle (deg.) (ram) mslde length

Total h q m d Max bowl

I D31 2 D37

308 "~72

135q 1634

P o w e r Gearbox speed ( r p m ) l o a d ratm range range (kW) 4500 II 4~K) 18 5/22

3 D45 4 D54

450 544

! qtr 2359

35(X1 3200

3LV37 37155

A/H A/H

5 D6~

b60

2847

2900

75/9()

A/H

[ nml)

vol. ~ ( l )

Gearbox torque lange (kNm~

Hutchison-Hayes, USA Name of company Hutchison-Hayes Separators lnc Headquarters address 352() East Belt. Houston. TX 77()1 5. USA Phone: (+1) 71 3-455 96()(): Fax: (+1) 71 3-455 7753 UK address None [IS address As above Company ownership Private company Type of company Specialist engineering company Other main businesses Reconditions centrifuges, supplies range of disc stack centrifuges Other company connections None Decanter manufacturing sites Houston, USA

Machine design 4

A/H A/H

356

Suppliers'Data

Decanter range Five models in two different diameters Decanter sales Sales in region of $ 5 millions forecast for 2 000 Sales specialisation Food industry (dairy and milk, meat and fish processing, edible oils), fuels and lube oils, petrochemicals Machine details M, ldel ret no. Bowl d,a B r (mm~ Icngth f I mm ) 11430

356

762

Beach Beach dia." Total angle ldeg.) Cram) inside length (.ram,j 10.0

Total liquid Max bowl Power G e m ' b o x vo[ ~ { I i speed (rpm)load ratio range range (kW; .... ~

Gearbox torque range (kNm) '

Machine design 4

2 1433

356

838

8.5

A/H

4 1456 5 5500

356 406

1422 1397

8.5 10 0

A/H A/H

,~

Noxon, Sweden

Name of company Noxon AB Headquarters address Sisj6 Kullegata 6. S-421 32 Vfistra Fr61unda. Sweden Phone: ( + 4 6 ) 3 1 748 5400; Fax: ( + 4 6 ) 3 1 748 5419 UK address Waterlink (UK)Ltd Prickwillow Road. Ely. Cambs CB 7 4TX Phone: ()1353 645700: Fax: 1353 6 4 5 7 0 2 US address Waterlink Inc 410() Holiday Street N W. Canton. OH 44 718-2 532 Phone: (+ 1 ) 330-649 4000: Fax: (+ 1) 3 30-649 4008 Company ownership Part of Waterlink Inc. through European Water and Wastewater Division/Waterlink AB Type of company Specialist decanter manufacturer Other main businesses None Other company connections None outside the Waterlink group Decanter manufacturing sites Kungsbacka. Sweden

9

9

b~

~

--_2,

u

,..,.

"'F ~

~

~...o

9

,....

o

'x

,,.,,,,

--u

O

~"<

=

~

,,.,. ]

oo

.. ......

-,.] ~.]

J

._,... ,,,31

+

Z ~

,Ji

o

,-.., >

..

~./!

M o.

-...,,

,.,., ,,,-,.

",i., I

o

~

1.0

"1

~

o1 o. 0

~-"

..,.,., i,,,a ~

,.o L.. ~

~,,~,-, ~

'x

o - - ~ u o

'-'I.

='

"lP

0~

'c ~''

~ '~'

]'~,, V~

K

--

r

o =

~

B

t..

~.]

o

=u

l::Z.,

u

~~-~

9

,~"

~,.,o

B

9

:~

~

,,-..,.,

358

Suppliers'Data

Sales specialisation Wide industrial coverage, inherited from Sharples progenitor Machine details "ModeLr~f .o

Bowlcha (ram;

Bowl

length' , Imm)

Beach ~mgle Beach ~a.:

(deg)

(ram;

,,,

Total ,ns,~ Tolal

Max bok, I pow~ load Gearbox Gearbox Machine length (ram) hqmd vol ~ speed rrpm) raage (kW) mtm range torquer'aagc design' (I) .. (kNm)

i P-600

152

tO

119

353

2

6000

95

0.2

2 P- 1500

250

l0

180

559

10

4000

94

0.9

A/H

3 P-2fR)O

356

10

264

572

IA

4000

52-125

I g-24

A/H

4 P-3000

356

i0

264

794

28

4000

52-125

I .R-2.4

A/H

5 P-3400

156

I0

264

1264

49

40(0

52-125

1.8-2.4

A/H

6 P M 20000

356

I0

241

794

32

40,30

52-125

I &-2.4

A/H

7 PM 30000

356

10

241

1264

57

4000

52-125

1.8-2.4

A/H

8 PM 35000

42~

10

264

1264

86

3650

52-125

I g-24

AsH

.~IH

Pieralisi, Italy Name of company Gruppo Pieralisi Headquarters address Via Don Battistini 1. I-6()0 3 5 ]esi {AN), Italy Phone: (+39)()731 2311;Fax: ( + 3 9 ) 0 7 3 1 2 3 1 2 3 9 UK address Kirton Engineering Ltd Old Station Close. Shepshed, Leics LE 12 9 NI Phone'()1 509 5()4565" Fax" ()15()9 60()()1 ] US address None Company ownership Private company Type of company Specialist engineering company Other main businesses Vertical basket centrifuges, belt press filters Other company connections None Decanter manufacturing sites lesi, Italy Decanter range Four basic models, with a number of specification variables Decanter sales Sales in region of $40 millions forecast for 2 0 0 0 Sales specialisation Primarily an olive oil decanter maker, which has diversified into other industry sectors

Suppliers'Data

359

Machine details Model ref. no.

.

l B.;y,

.

2 Baby 2 3 FP61R)/M

Brwl dia. (ram) .

.

23., 232 353

Bowl Beach Icngth ~ angle qmm) (deg.) .

_~64 773 925

4 FP600 RS/M 353 5 FP600 2RS&t 353 6 Jumbo I 470

1228 1525 1189

7 Jumbo 2 8 Jumbo 3

1589 1998

470 470

9 Jumbo 4

470

2406

I0 Giant I

700

205 I

! ! Giant 2 12 Grant ~

700 7()(I

25 ! 3 2975

Beach din." (mm)

Total Lnslde Total length hquid (mm) vol. ~ (I)

Max bowl speed 9(.rpm)

Power load range (kW)

520O 5200 4100

5.5 7.5 !1

I9 1 0 0

1i

A/I-I A/H A/H A/H

41 O0

i5 30

AIH AJH

3350

Gearbox n~tio range

Gearbnx torque range (kNml

Machine design 4

3350

37

AJH

3350

45

A/H

335O 2000

45

A/H

2000 2000

55

A/H

75 90

A/H A/H

Siebtechnik, Germany Name of c o m p a n y Siebtechnik GmbH Headquarters address Platanenallee 46. I)-4 54 78 Mfilheim an der Ruhr. Germany Phone: (+49) ()2()8 58()1-()(): Fax: (+49) ()2()8 58()1 3()() IlK address TEMA (Machinery)I,td 3 Great Central Way, Woodford Halse, N o r t h a n t s NN 11 3 PZ Phone" ()1 327 2626()()' Fax" ()1 327 2 6 2 5 7 1 US address Tema Systems Inc 78()6 Redsky 1)rive. Cincinnati, OH 4 5249 Phone: (+1) 51 3-489 7811: Fax: (+1) 51 3-489 4 8 1 7 Company ownership Private c o m p a n y Type of c o m p a n y Specialist engineering company, for mechanical separation equipment Other main businesses Horizontal conical basket centrifuges (sliding discharge, worm discharge and vibratory), pusher centrifuges, screening equipment and size reduction equiprnenl Other company connections None Decanter m a n u f a c t u r i n g sites Mfilheim. Germany Cincinnati, USA

360

Suppliers'Data

B

~ B

~

m

~

i

v

%

Figure 10.6 A Siebtechnik centrifuge installation.

Decanter range Six different models of decanter-type centrifuge: tunnel design, pedestal bearing design, and overhung-mounted decanters, plus decanters with second larger diameter cone (Twin-Cone Decanter), and larger diameter screening drum (Turbo-Screen Decanter), as well as an overhung design worm-screen centrifuge with a decanting area ("Conthick"). Some models with cantilevered bowl are interchangeable with conical basket filtration machines. Decanter sales Sales in region of $12 millions forecast for 2000, including those sold by Tema Sales specialisation Wide range of industrial applications, especially within the chemical and pharmaceutical industries, as well as industrial waste water applications. Major focus on individually designed centrifuges based on application requirements

Suppliers'Data

361

Machine details Model ref no.

Bowl dia. (ram)

Bowl length I (ram)

Beach angle (dog.)

Beach dia. " lmm)

Total inside length (ram)

Total Max liquid l~,wl vol. 3 ( 1) speed (rpm)

Power load range (kW)

Gearbox Gearbox ratio range torque range (kNm)

Machine design 4

Tunnel design (TSt, pedestal bearing design (DZ) and overhung-mounted (T3YF) 1 2 3 4

210 3(~} 3o0 420

5 6

500 600

7 8 9

710 850 1000

Twin-Cone (TWC; and Turbo-Screen (TSD) i0 1I 12 13

300/450 50t)/'/00 6O0/75O 8(~)/I 0 ~

Conthwk r 14 15 !6

240/180 320/240 4501340

17 !8

600145 (.) 800/600

19

20

I()()()17 50

12001000

Westfalia, Germany Name of company Westfalia Separator AG Headquarters address Werner-Habig-Strasse 1. D- 593()20elde. Germany Phone: ( + 4 9 ) 0 2 5 2 2 77-()" Fax: (+49t ()2522 7 7 2 4 8 8 tJK address Habig House. Old Wolverton. Milton Keynes MK 12 SPY Phone: ()19()8 313366:Fax: ()19()8 3 1 1 3 8 4 [JS address 10() Fairway Court. Northvale. N] ()764 7 Phone: (+1) 201-767 39()0:Fax: (+1) 201-767 3416 Company ownership Part of GEA Mechanical Separation Division. itself a subsidiary of Metallgesellschaft Type of company Specialist centrifuge manufacturer Other main businesses Disc stack centrifuges

362

Sztppliers ' Data

Other company connections None Decanter manufacturing sites Oelde, Germany Niederahr, Germany Chfiteau-Thierry, France Decanter range Three ranges of three, four, and five models respectively, but new range appearing Decanter sales Sales in region of $ 5 5 millions forecast for 2 0 0 0 Sales specialisation Very wide industrial application, with modern emphasis on waste treatment Machine details Model ref m, Bowl dia. Bo'wl' ' Beach angle (rnnt) length ~ (dcg.i ~'mm)

,

Beach dia. 2 Total Total hquid Max b~ (mml inside "~ol."(!) speed (rpm) length _. (nwn) .

Power load range (kW)

Gc,'u'0ox ratio rnnge

Gearbox Iorq tie range (kNm~

Machine design 4

I CB300 2 CA450

A/F! A/H

3 CA505

A]H AIH

4 CA755

5 MD43. 6 MD44 7 MD54

4(X)

400 500

1211(I Ir 20(KI

AJF1 A/H

HIM3

4L~

1200

A/H

v HD44

400

16tKb

AJI-I

1o HD54

5r

2000

A/H

I I itD8 ~

~(l~)

2400

A/H

12 HD84

~s(M)

3200

l 3 AD0504

3(~.)

i 2(1(I

44(X)

14 ADi22!)

458

2015

3(Xk')

18 5 35

15 AD 2040

510

2(14o

35(X)

9O

A/H AlE

A/H

CHAPTER 11 --..

.-

Glossary

of Terms The plate. or target. with vanes in the lked zone. opposite the feed lube exil, which accelerates the ikctd strtslrn up to bowl speed An org:anic chemical. wt1ic.h when polymerised acls as :I Ilo~~r~ul;lling agent A syntmym used f’or conveylnl: baffle A description fur the ionic charge of ;I polyclcctrc~lytc. and which is ncgalive Arrangcmcnt of indicators OHa PlL’or intcgratcd contrtjllcr to display r)periitintl:ll

A(.(-t*lpr;llor Arrylamide Active bafilc Anionic Annunciiitot Anti-vibration

Arithmetic

fiXitIt

flCJ%

mounl

mean

condition

and

fuIlctionirlg

oflhta

pl;lnt

A mer:h;inic;~l device, fitted undct- the dccantrr, to isolal~~any vibrations of’thc decanter from tlic surrounrling environment. Also called ;I vibration isolalot Themcan ofa set ofr~ numhcrs. ohtaincd by ;Idding all the numbers togt’lh(~r and dividing by 11 The now

ofrl;Iriiicd liquid along the cylindrical part ofthr howl when it flnws parallel with the axis, ruablcd by windows or holt>s irl the conveyor flights (as distinct Cram the norma\ helical Ilow partcrn) A systc+m(electrical. hydraulir rrr mechanical) IO t4’rt.t control althc conveyor ditkrential S[W’l

364

Glossar!! qf Terms

Backing plate

Baffle

Baffle cone

Baffle disc BD Beach

Beach angle

Beach length Beach liner

Beta Bowl

Bowl assembly

The support plate of steel, compatible with the process and for welding to the flight tip, on w h i c h a ceramic tile is mounted, by brazing or bonding A restriction, generally in the form of a disc or cone, on the hub of the conveyor, used to impede the m o v e m e n t of cake or solids, or to redirect the m o v e m e n t of the liquid flow. It can also be a plate, c o m p a r t m e n t a l i s i n g the centrifuge casing, minimising crossc o n t a m i n a t i o n of discharged products A conical baffle built onto the conveyor hub, shrouding the feed zone. and impeding the flow of cake travelling from the bowl's cylindrical section onto the beach. Sometimes called a BD cone A disc built onto the conveyor hub to baffle scrolled cake. Also known as a cake baffle Baffle disc Conical section of the rotating assembly, up which the cake is scrolled before discharge. Sometimes referred to as the extension Semi-included angle of the beach cone. It is the angle subtended by the beach inner surface to the decanter axis The axial length of the beach A covering applied to the inner surface of the beach, which m a y be plain, roughened, ribbed or grooved, to aid scrolling or for wear protection A scrolling capacity function The hollow cylindrical and conical shell of the decanter rotating assembly. Alternatively. it is the vessel, including hubs and the beach. which holds the process being separated. As such. it excludes the conveyor and gearbox. Sometimes the whole rotating assembly is referred to as the bowl or bowl assembly. For example, "Mount the bowl in the frame", m e a n s get the rotating assembly m o u n t e d in the casing, onto its frame The assembled bowl, hubs, beach and conveyor with m a i n bearings and pillow blocks

A formed met.iil sheet. inserted into thcborv! cglilider. to act as a scrotlirig aid, or for wear protection. It could he Cull or part c p f I.hr cglitidrical length. I t could be plain. ribbed, gnjljved or w i h ii special surface finish 'rhe t ~ o l l ocylirider ~,~ section w h i r h fits around the c:orivryvr

'['he inside surfacc oTthc bowl shcll A compnrtmcnt within the convcyt)r h u h between fccd and flooculakit zones. l o prevent splashing from tht! feed chamber into the flocculant chamber Synonymous wit11 b i l k rbaniber The sedirnented residuc from thc centrifuged

rlakr baffle disc

Cake yicltl strcss

t'iln I

C'iintilevcrcd clcciinter

C' a pi 1liar y siI I'Iinn test

ret:d iz disc m t w n f d on t h e C O ~ ~ V hub. C ~ O ~ genrrally at the ]unction betwccn the bowl shell a n d bcach, t o restrici tlie flow ofcakc. and provide a back pressure. i t can also bc littcd tinywhcru u p [lie h r x - h o r intu t h e c y 1in d r ic al sect i ( )n Thc minitriurri prtwurc that can cxprtss mnisiurc frtim t h u cake. The cakc yield stress d u p c d s iipon t h e solidsct}nccntr~tir,nin thc caks, a n d increascs rapidly iis that concentration increases Often used instcad d * ' r a k t I . ' . h i t whcn it is. i t usually itifdrs a positive ri3ke. e.nabling litting of the cake. Srr?rake A short horizontal decantel wirh i.hr maih bearirigs pliir-ed at t h e same end ot't hc b o w l , suu h t h a t 1.hr rot i1t in g assciiibl y is cii n t i ICVU rcd

from t hosc bearings A test, or apparatus.whir:h a s s ~ ~ s s cthc s drw;itrrability oC;i sludgti by measut'irrgi I s ;tbilily and speed to release moisture. Often used to asscss the eKe,ctivrness of floccu lants 'I'he plate or shcct rrietal housing for the b o w l ;~sscmbly.which collects. and kceps wp;irate, t.hc discharges from the hcach and liqiiiil outlet A plate welded inside the casing to coinparttrit.ul.;ilise it and kccp separ;! te the d i ffc rcn t bo w 1 d i sr h a r ges

366 Glossary of Terms Casing gutter

Cationic Centrate Centrifugal force Centripetal force Centripetal pump

Ceramic

Chatter

CIP

Clarification e n h a n c e m e n t

Clarifying length

A ledge welded to the inner edge of the upper half of a casing baffle, to prevent splashings r u n n i n g down the baffle surface and onto the bowl A description for the ionic charge of a polyelectrolyte, and which is positive The clarified liquor discharged from a decanter The gravitational force produced by rotation at speed The reactive force to a centrifugal force A p u m p fitted to a decanter's front hub, which converts centrifugal energy in the centrate to pressure energy, and thus causes flow under pressure in a discharge pipe line A non-organic, non-metallic compound. Often a refractory, used for lining furnaces, but in decanter work is used for its abrasion resistance. For example, alumina, silicon nitride Torsional vibrations of the d y n a m i c system consisting ofbowl, conveyor and the friction between the conveyor and the product. This m a y result in metal fatigue and p r e m a t u r e failure of gearboxes Clean-in-place. The features on a decanter which allow it to be cleaned hygienically without the need for dismantling. The operation of such features. This involves operating the decanter for set periods at just below 1 g, forward and backwards, and flushing it with a cleaning agent. The outside of the bowl and the inside of the casing are cleaned by spray nozzles Longitudinal vanes welded to a decanter conveyor, at an angle to its radii, from the conveyor hub to the conveyor ribbon flight. These increase the theoretical Sigma of the decanter. Alternatively discs can be substituted for the vanes. Also k n o w n as Sigma enhancement The distance between the middle of the feed zone exit and the front hub

Classification

y'hc dc.cantcr process whcrcby a fccd stream is separ;;~edin1.o t.wo streams containing particles otdistinctly different sizes. or of (1i ( k e n t I:ci m posi t inn by s i rt us of density difkrcncc A shroud placed over the cake discharge from thc bcacli, to direct the cake flow and prercnt sticking to the casing A pr(Jpriei;iry type ofhard surfacing materi;d A cake whit:h can bcsqueezed to express moisture

ronr Coriveying baffle

Corivcyor Colivtayor cliffcrcntial

C'onvtsynr h u b

More oftcn rcfcrred to a s [.hebench A complcx forin of cake baRle, comprising a flight section with larger pitch. sirctching from thr front o r o n e main flight to the r w r of mother The dcvicc that movcs the scttlt~lsolids in thc bowl The differtwcr in speed belwtrrn howl and c u nvcyor 'I'hc central supporling tube ol'the coriveyor. whic:h hoL1sC'S thc krd iind floccularil or rinse zii r i PS, ii nd t hc co 11 vr y or bc a r i ti g a SSP m b 1i cs The rotational reaction uf t h e c o n w y o r to thP resistance c ' d c a k t ~movoriic'rii, exprwsed i l l wrms offorw t.imcs r a d i u s at which the I'orcc is acting

C o 11 n t crs h aft 1)at:k -4 rive

l'resting

CSI' Curnuliitivc distrihutinn

A back-drivc systciii whereby the pinion shaft 1s rotated by i l bell ctJ1lrlWted 10 a shaft. w h i c h in tiirn i s tolaled by a p u l l ~ y011r h e main rotating assrrnhly. I t cflects :i tixcd diffcrciitial.hut this r u n he changed by dtcririg the sizes oi'thc pulleys Thc l t ~ rofpo11d I ;iho\rc the weir pIatc 1)eigIit. bcing th;it nCccssilry to p r o d u t y thc dist.h;lrg:l. flow 1 0m a t c h ttic i n p u t rate See capillary suclion tcsl A particle sizc dislribution. I r it by weight. v ~ l u u i esurface . o r cross-sectional area, number 01' whalevcr. the percentage for a parlicular size is thc percentage of thc distribution wliich is below or above that size

Cyclo I'ylindricnl lcngth

A hulbous weir plate, which archcs out from the front hub surfact to provide extra weir edge lcngth A term allotted to a particular type of gearbox Thc distance froin the root oCtht: beach l o the

fl'Orlt hub Darn Dam plarc

Dewatering

Tjifferrntinl

ni 1 11t i on water Dirnciisioiilcss group

Short for dam plate Weir plate. OW of ij set of plates boitcd to thc froat h b b to coiilrol the pond depth l h e decanter process whereby the solids arc sep;ir;iled from the siispendinE liquor. to give H low-moisture cake and ii ccntratc with low yolids contkvt Con w y u r difkrcntia 1 Water used to rcducc the strength orthe floccularnt Solution. gcncratly addcd i n the flocculant line A set Oiproccss parameters. w h i ~ hr,v h e r i va riou st\! m u 1ti plied together andiut d i Bided. 11as iio d i m e n sion s , p ro1.i d i n g c'o Ii s i st r 11t II11 it Y :ire iist?d

13isr.s

1)uublt. lead Uovutail t i l e

A nnzzleftted t n r a c h o f t h e e x i t p c i r t s o f t h t l w d zone. a r i d l i l t e d radially. Thr exit o i e a ~ h norile faces :ixi:illy or tangentially. s u c h that thc feed from the feed zone discharFcs onto thc back 01-Front face o f t h e adjacent flight or \aiigeniiaIIv intu the helical channel. towards the rear o r front of the bowl C'oriicaI plates iitted around thc ctmvcyoi- h u h towards the ctiiitrat.e discharge to e i i h a n c ~ cliirili(;;itiori L:ilpii[;it.y.A ribhi)[\ [light is iittiiched 10 the o u ~ s i d rperiphery of t h e discs Two cquispaced ccrntinuo~irtlighlson lhr cn11 vc y o r h 11b A typp of'tilrsonstruc:tiori. Tlir cerainic wear plate has a triangular spigot on its hack h c c raating with a triangular rcccss on the front face ofrhe backing platc to which it is bonded or brazed. 11 provitks support arid extra safety should t h e bond or braze fail

Dry beach

US

ns m o ~ i e 135 operation Uual angle bench

L)uplex steel

Epicyclic

Er f

A spring-loadcd plug titted to t.hr h > w ld1d3 usually in pairs fitted 1 8 0 ,apart. It is used for c ~ work p to allow the liowl ID drairi durirtg clealling, opening at low h v l speed a n d r:losing as spccd increases The portjo11 of the beach ahove t.he lrvet OL' thc pond Ury solids. The description of a dccantrr dcsign. its method ofoperation or itscake product. whereby extra-dry cake is iichieved The control mcthod for US opera! ion Operating ti dccanter in t?S mode A beitch which has two distinct semi-included ;~nglcs.such ttiar. ifscross-scction is co111:ave o r convc'x A decailtcr with a hviich at both etids. I:sed tor separating riglit z i i d heavy solids from ii suspension h type ol'strcl uscd for twwl c:onstructiori. whcrt: high chloride. ct:)riiciiland etrviitrd t ~ ~ ~ ~ p e r i i cxist t u r c in s the fccd or wlirre high stt'erigth is r c q u i r d A circular wcir 1)1;1tcwith a11 ccc-rntrichole. liwally fitled in rqual numbers with Tour or iriiirr to a set. Hy rotating t h v pliitc in its recess, (1 iFierent pcr nrl heights m :I y be o b t ai tird . Sr)mctimes cillled a universitl dam A description ol'thc inorc! common type of gParbnn. built with two or thrce st ugcs. Each stage consists of a "sun" fit'ar\vhcel s 11rro u I 1ded by two (.I r t h r w ' pl a r I r t ' ' geai*wliec+ls rulinirll: o n i i tmr11t.d track 011 ihc ii'liier wall oftlir gearbox. The planer wheels arc mountcd on a c;irricr plale to whiuli is centrally mounted t.hc piiiiori shaft for the sun wheel of the nexi. stage or 1 0engage L ~ P conveyor itself A mathematical lunction. called on error fuiicl ion, wheTe:

Integratkon is not possible m;ithcmatically. and t.hercfore t h e function has to be tahulatcd

370

GlossaryofTerms

Feed chamber Feed nozzle Feed tube

Feed zone

Flight

Flight tip Floater disc

Floating conveyor

Floc chamber Flocculant Flocculant chamber

Floc feed tube Flush water Frame

S y n o n y m o u s with feed zone A short tube, often lined with erosion protection, fitted to each exit of the feed zone A pipe secured to the frame and inserted along the bowl axis to the feed zone, to carry the feed stream into the centrifuge A c h a m b e r in the hub of the conveyor, where the feed is received from the feed tube and b r o u g h t up to speed, before discharging into the pond The metal plate welded radially to the conveyor to provide the scrolling surface of the conveyor. It is one c o n t i n u o u s plate w o u n d end to end on the conveyor hub. Erroneously, one 360 ~ turn of flight is often considered one flight. One talks of "adjacent flights", meaning the part of the flight 360 ~ from the last one The outer periphery of the flight A disc fitted at the front end of the conveyor hub, dipping in the pond, to prevent floating solids discharging with the centrate A conveyor whose overall density is less than the suspending liquor of the feed. It thus floats in the pond. Used on some 10 ()00g decanters. Also called a swimming conveyor Same as flocculant c h a m b e r A reagent which causes fine particles in suspension to agglomerate A c o m p a r t m e n t behind the feed zone in the conveyor hub, sometimes separated by a buffer chamber, used for receiving the flocculant before passing into the pond. Sometimes referred to as the polymer c h a m b e r or polymer zone. In certain designs it can double as a rinse chamber A feed tube with a concentric outer tube for conveying flocculant or rinse into the bowl The water used to wash out the decanter on shutting down The cast iron, or fabricated steel, base on which the rotating assembly is m o u n t e d

Friction factor

Froril

Front hiib

Full axial flow

(;a u ssia I 1 d i st r i b u Ii (11 i

A dimcnsionlcss ~ r o u piiivolving . fric.tion, fluid velocity and derisity, used in fluid rriechariic:sa d related 1.0 Reynrllds number Conventionally the end n l t h e d e c a n k r where t~hec e n t r a k discharges The tlange, or end plate, which encloses thc howl a t t h e centrate end, and includes t h e spindles. which suppnrt thc front main and front conveyor bearings. It also supports the

ccntratc weir platcs Flclw along t h e bowl which is axial, a n d cannot deviate f r w it. This fl(iw is achieved bet,wem radial vaiics or Sigrrla eiihancernent vanes A partic:lrr size distribution havinE a symmetricitl bell shape about

Geom u t r i c st a I I d ;I rd drvi;i\inn g-Forw

g-l,evel

Heel

Ihc mean. Sornctimcs known as t h e nnrmal or ilorrr~al probability d ist r ihu t ion. 'I't 1t: rri ;I t h em at I cs [or thc CLIWL' wcrc dcveIoped hy thc mat hcma t ici a n r'k hloivrc. h u l also associated wit11 i t were ol,tit:r g r r a t mathcmaticians. ( i a u s s arid 1,;ipl;icc An cncl(ised metal cylinder- rnnl:lining a system ofgears irnlliurstid in oil, Ti is inciunted on onc C k l d of t h u rotating ;issrmhlv iind e k c t s the rntaticiri crf the conveyor ui a slightly differtiit speed to thrbowl Sce "chattcr" Tho i n w r i u f a set of'n nuiiilxrs, ohloined 13s multiplying all t h e nurribers logether and taking t h e 11th root A special sCilodi\rd d t v i a t i o ~ used i in a skcw Caussian distributiuri T h e outward tudial forcc produccd hy rotation in a circlc Tlic numhcr o1'rimes t1iecentrilug;il ticld is greater than earth's gravity The ;ixial slots machined in the bowl wall or beach to emourage keying to assis1 scrolling rfficicncg The thin layer ol'ixke betwccn Low l/hach and CoklVcyOr d w to thc necessary rnechailicaI clcarance

372

Glossaryof Terms

Heel torque

High baffle

Hindered settling

Horizontal decanter Hub

Hubless conveyor

Impellers

Inertia

Inflatable dam

Intlatable seal

In-line dosing Integrated controller

Inverter

The conveyor torque developed by the heel after feeding and the bowl has emptied, apart from the heel Named after the inventor, R High. Another n a m e for the longitudinal cake baffle, between two flights The settling of particles in a fluid, w h e n they are so close that they impede one a n o t h e r ' s settling rate A decanter whose axis is horizontal w h e n installed The central part of the conveyor. Alternatively the end flanges of the bowl/beach, each having a central spindle for supporting m a i n bearings on the outside and conveyor bearings on the inside A conveyor which has no central hub. or just a very small diameter one. The flights are held on radial vanes joined onto a small diameter pipe support Small blades affixed to the outside of the beach at the cake discharge, to impel the cake a w a y from the discharge area. or around and out of a collector The property of a rotating body, which opposes any change of rotational speed. A function used to calculate r u n - u p times of the rotating bodies An elastomeric device fitted inside the front hub, to control pond depth by hydraulic inflation Similar to an inflatable dam, but used to control wash-out by acting on valve ports at a larger radius than the pond surface Admitting flocculant in the feed line s o m e w h e r e before the decanter A PLC or industrial computer used to supervise the overall r u n n i n g of the decanter plant, either directly or by supervising other controllers An electrical device which modifies the m a i n s electrical frequency to control the speed of an electrical motor

lrlverter motor 1,;lrnhda

Lcad

l.,og probability

Mriiri

frame

M ii ss t r a n sfc i- c oc Fficic 11 t

hl c t cr i 11 g p LI 1'11p

Oil nozzle

orriega

374

Glossary of Terms

pH

Pillow block Pinion shaft

Pinion torque Pitch Pitch angle

Plough tile PLC Polyacrylamide Polyelectrolyte Polymer

Polymer chamber Polymer dosage Polymer dose Polymer utilisation baffle

Polymer zone Pond Pond level Pond volume

Power factor

A physical chemistry term used to denote the acidity or alkalinity of an aqueous liquor. Numerically equal to the logarithm of the negative of the molar concentration of the hydrogen ions The housing for a main bearing The small shaft of the gearbox to which is attached a torque arm or a back-drive motor or brake. Sometimes referred to as the sun wheel shaft or input shaft The torque on the pinion shaft The distance between consecutive sections of the conveyor flight 90 ~ less the angle subtended by the flight tip and the conveyor axis. Alternatively it is the arctangent of it x conveyor diameter/pitch A tile with a plough-shaped profile, used for lifting and rotating the cake while scrolling Programmable logic computer. Used to control various decanter plant devices A type of flocculant A generic name for a class of flocculants In decanter work synonymous with flocculant, unless it refers to a particular decanter feed See flocculant chamber Synonymous with polymer dose The amount of polymer used per unit solids in the feed A disc fitted to the conveyor hub just down stream of the feed zone. It dips slightly in the pond and prevents the flocculant skimming o v e r the surface of the pond with poor utilisation See flocculant chamber The process liquor held in the bowl or the volume occupied by it The radial depth of liquor in the bowl The actual volume of the pond. In DS work it sometimes means just the volume between front hub and cake baffle The degree by which electrical current leads or lags the voltage. Numerically it is the cosine of the phase angle

Glossary of Terms

Probability

Psi

Racetrack Rake

Rear Rear hub

Recovery Rennbahn Reslurry collector

Reverse pitch

Reynolds number Ribbon flight Ribs

375

For the purpose of this book it is a particular m a t h e m a t i c a l scale used in particle size distribution work A factor used in thickening work to indicate the control position between high dryness, poor centrate, and wet cake, good centrate A close-fitting collector for the solids phase. Also called a R e n n b a h n The angle the flight subtends to a perpendicular from the axis. Can be positive, which lifts the cake, then sometimes called cant, or it can be negative when cake will be pressed against the bowl wall. In some designs the flights are " c a n t e d " in the beach section, to provide zero rake on the beach; i.e. the flights are perpendicular to the beach. Tiles are designed to give the conveyor flight tips a cant to improve scrolling efficiency Conventionally the end of the decanter where the cake discharges The flange, or end plate, which encloses the bowl at the cake end, and includes the spindles which support the rear main and rear conveyor bearings The percentage of the suspended solids in the feed w h i c h reports to the cake discharge See racetrack A collector with rinse nozzle connections to slurry the cake on exit from the bowl. Used for c o u n t e r - c u r r e n t w a s h i n g in some processes Refers to a conveyor with an inner flight as well as the main outer flight. The inner flight has opposite pitch, so as to scroll surface cake towards the front hub to increase its residence time in the bowl A dimensionless group, used in fluid dynamics, heat and mass transfer A flight of small height, welded to pillars or vanes attached to the conveyor hub Metal strips sometimes welded, or spot-welded, axially to the bowl or beach, or onto a liner thereon. Used sometimes in place of grooves

376

Glossary of Terms

Ring dam Rinse Rinse c h a m b e r

Rinse zone Rotodiff Saddle

Schmidt n u m b e r Screen bowl Screw Scroll Semi-included angle Sigma Sigma e n h a n c e m e n t

Single piece dam

Skew Gaussian distribution Skimmer p u m p

Soft inlet

Spigot

Another term for weir plate Fluid, usually water, applied to the cake on the beach to wash out u n w a n t e d impurities The c o m p a r t m e n t in the hub of the conveyor behind the feed c h a m b e r into w h i c h the rinse is applied before spraying onto the cake. A buffer chamber is sometimes interposed between rinse and feed chambers. The floc c h a m b e r is sometimes used as a rinse c h a m b e r Same as rinse c h a m b e r The n a m e used by the Viscotherm Company for their hydraulic gearbox A wear insert placed over the castellations of the beach that form the cake discharge. In profile it has the shape of a horse's saddle. The shape spreads the wearing area A dimensionless group used in mass transfer A decanter which has a perforated bowl section on the end of the beach S y n o n y m o u s with conveyor S y n o n y m o u s with conveyor Half the apex angle of a cone. Used to describe the beach angle A scale-up factor for decanters, indicating the clarification ability A m e a n s of increasing the clarification capacity of a bowl, using discs or vanes t see clarification e n h a n c e m e n t ) A type of weir plate, made in one piece, which covers all centrate discharge points to the same level A log-probability distribution A special pipe a t t a c h m e n t close to the bowl front hub. which enables centrate to be skimmed off the pond surface, obviating the need for a weir plate. The skimmer can be adjusted while the bowl is at speed, which effectively alters the pond height A type of feed zone that enables feed to be brought up to speed and enter the pond w i t h o u t turbulence A short raised step to afford m a t i n g in a recess of another component

Spindle Split bcach Split dam

Standard deviation

Stellite Stokcs settling vclocity

Sub-frame

A sinall solid or hollow tube on the axis of a r:ompc:)rierltwhich rnt,ates.Siiialler than a hub A diial ;Inglc beach A type ofweirp1;il.rfor t h e whole front h I i h , madc in two identical halves A rnathcinatical tcrm to indicate the spread of a sct of numbcrs. Thc dlfferciic,c:bclwccii each number and the arithmetic niean of them all, is squared. All the squared figures are tlieri added together and divided by ltle t.ot.al number, Finally thv square root i s taken of Ihisnumber A propriet.ary n a m e for i t t y p e OF hard surfacing Thc tcriiiiiial falling vclucity uf'a particle in a gravitational ficld of I g H S predicted by Stokcs' law A fabricated frame f i t t d iindt?r t . h niairi ~ frame to support it arid thc rriairi r I i o l r r a n d hark-

SlIIlM~tleei stlart

drive systems Svnoriynious will1 pinion shalt

Supccficial vclocity

Thc avcragc t~ulocityof a fluid through it hr:d of particles. averaged over t h y wt~iolt.a r u r > T

t11r bod See fliratirig cwriveyor A cylindrical bar ai1actietl t o one r n d ola corivryor o n lrlrarr decanters. tensioned to support the t h r u s t reaction of thc c o n r c y o r

caused by the torquc nccdcd to scroll tht. cake A decantcr prnccss whcrc dewatcring t.:ikrs

p l a ~ tbut . the cake phase re~iiiiirisliquid A decanter. or process. in which the feed is

'I'nrque Torque overload a r r ~ i

'I'tiple lead Ilniversal darn IJpper rasing

split into three phases, gent.riiI\y ii cake and a light a n d heilvy liquid A repl;ic-r;iblcassembly fixed to tliu ctinvryor llight tips 1 0comh;it erosion Tangential fbtcc times distance An :irm which can be mounted on a gearhox pinion shaft, which is kcpt at rest bv a torque

overload mcchanism. 'Hie n i r c h a n ism releases thc ar1u when an overroad o(:(:urs Three separate flights on a conveyor liuh Another name for an eccentric dam The uppcr half ofa casing. usually hinged to allow access lo the bowl

378

Glossary of Terms

Van der Waal forces Vanes

Vertical decanter Vibration isolator Viscotherm Wash Wash out Wear insert

Weir Weir plate Wet beach Windage Windage and friction

Window

Name given to small inter-particulate forces at the molecular level Longitudinal plates fixed to the conveyor hub supporting the flights. Used for full axial flow, the hubless conveyor, and Sigma enhancement A decanter whose axis is vertical w h e n installed Same as anti-vibration m o u n t Name of the company which supplies the Rotodiff Same as rinse The u n w a n t e d discharge of feed from the cake discharge ports A fabrication which is inserted into another to combat erosion. Found in the feed zone around the cake discharge and other areas prone to wear The discharge of liquid over a lip A replaceable plate on the front hub face to control the pond height That part of the beach which is below the pond level The flow of air induced by the rotation of the centrifuge Name given to the power component arising from windage and the various frictional forces in the decanter system A hole in a conveyor flight, which allows the clarified liquor to flow axially (otherwise, it would flow helically, between the flights)

Appendix This ;ippt?ndix l.ahulai.es all t h e data used i r i the previous chapt.ers, arid sorneof'whir:h i s f.he sour(:t: data for graphs iisetl i r i iht: illustrnlioris.

380

Appendix

Table A.1 Spent grain data used in Section 6.4.1

Machine Condltlons

4.Bowl Speed rpm. 51Pond dia ram. 61Conveyor diff' rpm. 7.Conveyor torque kNm Feed Conditions

9.Feed Rate m3/h. 10.Feed Solids % w/w d.s. Product Conditions

16.Cake Solids %w/w. 7.Centrate Solids m~/I. A.Centrate Rate m3/h. D.Solids Recovery %w/w. E.Cake Rate k~/h w.b.

,l,

425 1200 Distilliary Spent Wash

Decanter Bowl Diameter mm Clarifyin 9 Length mm Process

3150 261 23.2 0.37

3150 261 18.2 0.871

3150 261 13.2 0.66

3150 261 23.2 0.50

3150 261 18.2 0.87

3150 261 13.2 1.06

4.6 5.50

4.61 5.44

4.6 '-" 4.15

9.0 3.96

9.0 4.30

4.48

18.8 11100 3.5

24.8 13200 3.8

23.5 11000 4.0

16.6 10600 7.3

22.2 10700 7.6

25.6 12900 7.8

84.8 1.14

80.0 0.81

77.1 0.63

78.2 1.68

78.9 1.38

75.0 1.18

,,

9.6

Appendix

381

Table A.] (contd.) Decanter Bowl Diameter mm I Clarifvino Len~lth Lenoth mm mml Clari~in~l Process

Machine i .Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia mm. 6.Conveyor diff' rpm. 7.Conveyor torque kNm

425 1200 Distilliary Spent Wash

1 12 3150 261 23.2 0.60 ,

3150 261 18.2 0.99

3150 261 13.2 1.56

3150 261 23.2 0.75

3150 261 18.2 1.37

Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w/w d.s. Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids m~l. A.Centrate Rate m3/h.

D.Sohds Recovery %w/w. E.Cake Rate k~/h w.b.

3150" 261 13.2 1.75 ,

18.0 3.81

24.5 13300 .....i"1.6

26.3 14600 11.8

27.9 15400 12.4

21.9 14700 15.9

25.5 17200 16.3

28.1 18000 16'6

77.4

75.0 1.97

66.4 1.36

66.9 2.15

60.2 1.67

56.4 1.38

2.16

|

|

382

Appendix

Table A.2 Agricultural product data used in Section 6.4.2 Decanter Bowl Dia. mm

150

Clarifying Length mm

220 Agricultural Product

Process

Machine

1.Run Number

Machine Conditions

4.Bowl Speed rpm. 5.Pond dia mm. 6.Conveyor diff' rpm. 7.Conveyor torque kNm

~ooo 112 1.5 D.01'

50OOI 1121

3.0 ..... O.Ol

,.sooo 112! 1.5 0.01

0.011

5O00 112 s.o 0.01

680 0.1

5OOO1

5OOO

112 i

112

5.01 O.Ol i

3.01

I Feed Conditions

Product Conditions l~6.Cake Solids %w/w. ~17.Centrate Solids m~l/i. A.Centrate Rate m3/h.

11.4 600 0.2

10.7 650 0.3

10.9 680 0.3

12.3 730 0.1

11.0 660 0.1

D.Solids Recovery %W/W. E.Cake Rate k~/h w.b: F.Q/T. mm/h G.TN N/cm2

93.0 0.02 1.27 0.48

92.4 0.02 1.41 0.41

92.7 0.02 1.42 0.59

92.1 0.01 0.63 0.51

92.9 0.01 0.63 0.28

.,

,,

9.1

92.8 0.01 0.64 0.35

Appendix 383 Table A.2 (contd.) Decanter Bowl Dia. mm Clarifying Length mm Process

150 220 Agricultural Product

Machlne 1.Run Number Machlne Conditions 4.Bowl Speed rpm. &Pond dia mm. &Conveyor diff" rpm. 7.Conveyor torque kNm

5OOO 112 3.0 0.01

Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w / w d.s.

0.4 0.87

Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids mg/I. A.Centrate Rate m3/h.

D.Solids Recovery %w/w. E.Cake Rate kg/h w.b. F.Q/Z m m / h G.T/V N/cm2

12 = 5000 112 5.0 0.02

5OOO 112 10.0 0.01

5000 1121 5.0 0,03

9.9 650 0.3

13.0 3990 0.8

12.0 1880 0.7

18.0 1890 1.1

93.1 0.03 1.81 0.51

68.0 0.05! 4.28 0.72,

85.3 0.07 4.09 0.60

86.1 0.07 6.21 1.15

Appendix

384

Table A.2 (contd.) 425

Decanter Bowl Dia. mm .....Clarifying Length mm Process ~

800

Agricuitural Product

Machine

1.Run Number

M a c h i n e Conditions ......

4.Bowl Speed rpm, :5.Pond dia mm. 6,Conve}/or diff' rpm, 7.Conveyor torque kNm

3400 267 7.0 0,29

3400 267 7.0 0.29

3400 267 5,0 0.36

3400! 267; 5.0 0.36

3400 267 3,0 0.35

,27

127

,43

154

3400 267 3,0 '6,54

~Feed Conditions

Product Conditions

16.Cake Solids %w/w. 17,Centrate Solids mg/I, A.Centrate Rate m3/h.

1.0

1 .I

D.Solids Recover}/%w/w. E.Cake Rate kg/h w.b, F.Q/~ mm/h G.T/V N/cm2

98,7 0.48 0,60 0.33

98.7 0,44 0.60 0.33

.

98.7 0.4A 0,60 0.42

98,6 0.39 0.60 0.42

1 .I

I.I

98,6 0.36 0,60 0.41

98.6 0.36 0.60 0.62

Appendix 385 Table A.2 (contd.) Decanter Bowl Dia. mmJ Clarifying Length mm Process

425 800 Agricultural Product

2t

21

22

23

24

3400 267 1.0 1.34

3400 267 ! 1,0 1,21

3400 262 3.0 0,37

3400 262 1,0 1,21

3400 262 0.5 2.31

3400 262 1,0 1.50

Feed Conditions 9.Feed Rate m3/h, 10.Feed Solids % w / w d.s,

1.5 3,74

1,5 3.74

1,5 3.74

1,5 3.74

1,5 3,74

1.5 3.74

Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids mg/I. A.Centrate Rate m3/h.

19.1 700 1,2

19.3 700 1.2

14,9 700 1.1

18.4 700 1.2

20.9 700 1.2

19.1 700 1.2

D,Solids Recovery %w/w. IE,Cake Rate kg/h w.b. F,Q/~ mm/h G.T/V N/cm2

98,5! 0,29: 0.60 1.55

98,5 0.29 0.60 1.41

98.6 0.37 0,60 0,44

98.5 0,30 0.60 ~ 1,41

98.5 0,26 0,60 ! 2.68

98.5 0.29 0.60 1.74

Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor diff' rpm. 7.Conveyor torque kNm

21

19

20

2i

21

2i

2

386

Appendix

Table A.2 (contd.) Decanter Bowl Dia. mm Clarifying Length mm Process

425 800 Agricultural Product

.,

Machine

0 30

I.Run Number

Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor diff' rpm, 7.Conveyor torque kNm

3400 262 1.0i 1.50

3400 262 0.6 2.06

3400 262 0.7 2,10

3400 262 0.8 1,00

3400 262 0.9 1.00

16.Cake Solids%w/w. 17.Centrate Solids mg/l. A.Centrate Rate m31h,

20.0 700 1.2

22.9 700 1.3

23.3 700 1.3

19.9 700 1.2

20.5 700 1.2

D.Solids Recovery %wlw, E,Cake Rate kglh w,b, F,QIZ: m m l h G.T/V N/cm2

98.5 0.28 0,60 1,74

98.4 0.24 0,60 2.39

98.4 0.24 0.60 2.44

98,5 0.28 0.60 1.16

98.5 0.27! 0.60 1.16

Feed Conditions

Product Conditions

Appendix 387 Table A.2 (contd.)

Decanter Bowl Dia. mm Clarifying Length mm Process

Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5,Pond dia ram, 6,Conveyor diff' rpm. 7.Conveyor torque kNm

425 800 Agricultural Product

.,

3400 268 18.0 2.25

3400 268 18.0 2,25

3400 268 12.0 1.50

3400 268 6.0 0.75

16.Cake Solids %w/w. 17,Centrate Solids mg/I. A.Centrate Rate m3/h.

20.9 600 ~ 12.2!

20.1 5OO 9,3

18.2 390 9.3

15.6 340 9.1

D.Solids Recovery %w/w. _E.Cake Rate kg/h w,b. F.Q/Z mm/h G,T/V N/cm2

98.1 1.83 5.60 2.61

98,4 1.47! 4,321 2.61

98.8 1.66 4.40 1.74

99.0 1.94 4.40 0.87

iFeed Conditions

388

Appendix

Table A.3 Lime sludge classification data used in Section 6.4.3 Decanter Bowl Diameter m m Clarifying Len~h Procen

Machine

1.Run Number

,

Machine Conditions

4.Bowl Speed rpmo 5.Pond dia mm. Feed Conditions

9.Feed Rate m3/h. Product Conditions

D.CaC03 Solids Recovery %w/w. F.M~I(OH)2 Solids Recovery %w/.w.

150

170 Lime Sludge Classification

1

1J

1I

, 5000 143

6 5000 143

,,

5000 143

0.6 I

5OOO 143

92.5J 64.0 !

87.5 55.0

.,87.5I

51 .oi

I

5000 143 1.5

1.2 I

I

92.5 92.0

5OOO 9143

86.5 45.5

86.5 45.0

Appendix 389 Table A.3 (contd.) 150 170 Lime Sludge Classification

Decanter Bowl Diameter mm Cladfyin~l Len~Ith Process

Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram. Feed Conditions 9.Feed Rate m3/h. Product Conditions D.CaC03 Solids Recovery %w/w. F.Mg(OH)2 Solids Recovery %w/w.

~1 5000 143

"l ~9.0 :10.0

~1 5000 143

~1

,:1

5000 143

5000 143

~0] ~01 ~i

~!

81.5 42.0

5000 143

1;I

81.5 54.0

79.0 46.0

79.0 40.0

390

Appendix

Table A.3 (contd.) 150 170 Lime Sludge Classification

Decanter Bowl Diameter mm Clarifying Length Proceu

IMachine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia mm.

1 18

5000 130

5000 130

5000 ,3o

5OOO 130

5000 130

Feed Conditions 9.Feed Rate m3/h. Product Conditions D.CaC03 Solids Recovery %w/w, F,Mg(OH)2 Solids Recovery %w/w.

1.4

96.0 43.0

94.0 47.0

91.5 53.0

87.0 55.0

85.0 45.0

83.0 51.0

Appendix

Table A.3 (contd.) 150 17O Lime Sludge Classification

Decantei Bowl Diameter mm i

cla.~i.~ Le.~th

,,.

Process

L

,,

Machine

1.Run Number

!Machine Conditions

14.Bow,IS~eed r p m . 5.Pond dia ram.

"

1

1

1

5ooo 130

5o00 130

" ~,o6o 130

85.0

8O.0

79.0 ... 35.'0t

Feed Conditions , ,,

9.Feed Rate m3/h. Product Conditions'

D.CaCO3 Solids Recovery %w/w. |F.Mg(OH)2 Solids Recovery %w/w. I ! !

33.0t . . . . . 38.0!

221 1

.I

I

391

392

Appendix

Table A.3 (contd./ 356 870 Lime Sludge Classification

Decanter Bowl Diameter mm Clarifying Length Proce•

Machine I .Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram. Feed Conditions

9.Feed Rate m3/h. Product Conditions

D.CaCO3 Solids Recovery %w/w. F.Mg(OH)2 Solids Recovery %w/w.

21

28 3250 286

37j 97.8 73.0

32~ I

325oj 286l

1

8:~'J 94.0 83.0

9"8!

2861

I

,.

21

30

32sol

325o

2861

I

_

93.5 84.O

286

I 56.0

. ,

2

29

61

I

55.0

Appendix Table A.3 (contd.)

356 870 Lime Sludge Classification

Decanter Bowl Diameter mm Clarifyin~ Length Proceu

Machine

1.Run Number

Machine Conditions

4.Bowl Speed rpm. ~5.Pond dia ram.

2 3250 286

3250 286

17'7I

20"4I

88.sl

86.0 48.0

Feed Conditions

9.Feed Rate m3/h. Product Conditions

D.CaCO3 Solids Recovery %w/w, F.Mcj"(OH)2Solids Recovery %w/w,

52.0

393

394

Appendix

Table A.4 Clay classification data used in Section 6.4.4 Decanter Bowl Diameter mm Clarifying Length mm Process

.

425

!

780

Clay Classification

Machine

1.Run Number

Machine Conditions

4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor diff' rpm,

;Feed C:ondltlon$

9.Feed Rate m3/h. 10.Feed Solids % w / w d.s.

11.%<21~

12.%<1p 14.Rate t/h.

Product Conditions

16.Cake Solids %w/w. 17.Centrate Solids % w/w A.Centrate Rate m3/h. B.%<2F iC%< 1t~ D.Solids Recovery %w/w. E.Cake Rate t/h w.b. F,Cake S.G, G.Yield <2is ..

.,,

3150 232 32

232 32

228 32

228 51

226 32

3150 226 32

16.0 21.4 73 50 18.5

20.0 20.3 73 50 22.9

12.0 19.0 68 50 13.6

8.0 18.5 68 50 9.0

6.0 13.8 70 49 6.6

10.0 13.8 69 48 10.9

58.8 16.0 17.8 85 60 34.7 2.33 1,58 76.00

60.1 17.0 23.8 83 64 22.0 1.71 1.6[] 86.00

59.3 14.0 13.5 89 70 35.0 1.51 1.58 85.50

58.8 12.8 8.7 89 74 40.0 1,13 1.58 78.7C

58.8 7.7 6.5 97! 80 51.0 0.79 1.59 68.00

58.8 8.8 11.4 94 76 42.C] 1.1g 1.5~ 78.0C

.=

..

. . . . .

.....

Appendix

395

Table A.5 WAS thickening data used in Section 6.4.5 737 1550 W A S Thickening WAS

Decanter Bowl Diameter mm: '

ClarlfyingLength m m I Process!

"Machine

'1,Run Number

'Machine Conditions

4.Bowl Speed rpm. 5,Pond dia mm. 6,Conveyor diff' rpm. 7.Conveyor torque kNm

" ' !

2300 345 20.0

2300 345 10.0

2300 '345 15.0i -.

~3001 345, 15.0i .0.00,

r9.Feed Rate m3/h. lO.Feed Solids % w/w d.s. ,,

Additive Conditions

11.Type. :14.Rate m3/h. 15.Dilution m3/h.

4o.ol

:~o.oI

'~0.01

i oot

.,1.o9t

,ooloI olooI

O0 '" olo0I

O0

Product Conditions

.,.

23001 345 I 5.0 0.11

_

2300 345 10.0 0.61

|

|

Feed Conditions

.=

O0

70.0 !

1tl I 0

70.0~ 1.14[

0I

70.0 1,15

O0 _ 010

0 0.0 0,0

2920 49.9 O.0 81.9 20.10 11.13

,,

16.Cake Solids % w / w 17.Centrate Solids m~l/l. A.Centrate Rate m31h. B.Polymer Dose kg/tonne.

3,5 1210 28.7 0.0

D.Solids Recovery %w/w, E.Cake Rate k~l/h w,b. F.Q/.~ mm/h

~i.32

_

92,0 6.36

5.8

5.4

2.8

1770 33.7 0.0

.28~

,.

1320 32.7 0.0

47.1 0.0

4.3 " 3660 56.3 i, 0.0

85.9 6.34 6.36

90.1 7.30 6.36

82.6 22.85 11.13

74.2 13.70 "'11.13

3.3-

396

Appendix

Table A.5 (contd.) Decanter Bowl Diameter mm Clarifying Length mm Process

737

1550

WAS Thickening

Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5,Pond dia ram. ,, 6.Conveyor diff' rpm, 7.Conveyor torque kNm

2545 347 20,0 0.86

Feed Conditions [9.Feed Rate m3/h. 10.Feed Solids % w/w d.s.

40.0 1.37

r

Additive Conditions 11.Type. 114.Rate m3/h. 15.Dilution m3/h.

2545 347 15.0 0.61

2545 347 10.0 0.53

2545 347 18,0 0.85

2545

347 18.0 0.94 40.0 1.22

0 0.0 0.0

0 0,0 0.0

0 0.0 0.0

0 0.0 0.0

16.Cake Solids%wlw. 17.Centrate Solidsmg/l.

A.Centrate Rate m3/h, B.Polymer Dose kg/tonne.

4.9 1690 29.8 0.0

3.5 3550 47.3 0.0

4.3 3850 52.7 0.0

D.Solids Recovery %w/w. E.Cake Rate kg/h w.b. F.Q/E. mm/h

90.8 10.18 5.19

82.5 22,74 9.09

78.7 17.26 9,09

Product Conditions

2545 347 12.0 0.56

0 0.0 0.0

0.0

3460 46.4 0.0

4.1 3770 50.9 0.0

5.9 1500 32.6 0.0

83.5 23.62 9.09

80.1 19,12 9.09

90.0 7.42 5.19

3.4

84

0 0.0

Appendi.x"

39 7

Table A.5 (contd.) 737 1550 WAS Thickening

Decanter Bowl Diameter mm Clarifyin~l Length mm Process

Machine

!

~1,Run Number Machine Conditions

4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor dill' rpm. ,7.Conveyor torque kNm

, i

~

2545 3471 20.0 0.99

2545 347 15.0

2545 347 22.0 0.87

2545 347 21.0 0.87

2545 347 19.8

4O.O 1.24

9.Feed Rate m3/h. 110.Feed Solids % w / w d.s.

| |

|

Additive Conditions

11 .Type. ;14.Rate m3/h. i15.Dilution m3/h. 1

, 1

,Product Conditions

16.Cake Solids %w/w. 17.Centrate Solids mcj/I. A.Centrate Rate m3/h. B.Polymer Dose k~/tonne, D.Solids r~ecovery %w/w. E.Cake Rate kg/h w.b. F.Q/:~ mm/h .

.

!

J

F e e d Conditions

.

2545 347 19.8

.

|

, , i

.

E

0 0.0 0,0

0.0

0

0.0

0.0 O0

0.0 0.0

o 0.0 0.0

o 0.0 0.0

4.9 920 31.5 0.0

3.4 1680 50.2 0,0

36 666 28.4 0.0:

3.9 770 29.7 0.0

5,0 1030 31,8 0.0

5,4 1170 31.4 0.0

93,5 8.52 5.19

88.8 19.81 9.09!

95.7 11.60 5 19

94.6 10.31 5.19

92.6 8.18 5.19

92.6 8.55 5.19

0

0

398

Appendix

Table A.6 Digested sludge thickening data used in Sections 6.4.6 and 7.1 Decanter Bowl Diameter mm Clarifying Length mm Process

JMachine Conditions

4.Bowl Speed rpm. 5.Pond dia mm. 6.Conveyor diff' rpm.

J

425 8OO Thickening Digested Sewage .

.

.

.

.

3150 257 12.0

3150 257 13.0

3150 257 15.0

Z63 o. I 0 B 1.0

Z63 o, I 0 B 1.0

Z63 o, I 0 B 1.3

14.5 1400 15.5 3.3

10,8 700 14.6 3.2

6.4 1000 12.8 3.9

92.8

96.7 2.77 5.67 3.92

96.1 4.86 5.67 7.30

3150 257 14.0

3150, 257 14.0

3150! 257 14.0

Z63 O.I 0 B 0.8

Z63 o. I 0 B 1.0

Z63 o, I 0 B 1,5

8.7 1000 14.1 2.4

9.7 1450 14.7, 3.1

11.5 675 16.6 4,7

95.5 3.42 5.81 5.15

93.3 3.07 5.81 4.54

96,7 2.68 5.81 3.85

Feed Conditions 9.Feed Rat( 10.Feed Sol

Additive Conditions

l i.Type.._

12.Concentration %w/w. 13.Addltion point. _ 14.Rate m3/h. .

.

.

.

,.

,,

Product Conditions n

16.Coke Solids %w/w. 17.Centrate Solidsmg/l. A.Centrate Rate m3/h. B.:Pol y mer Dose kg/tonne. ,,,

.....

. . .

D.SolIds Recovery .%.w/w. E.Cake Rate kg/h w.b. i~'.Q/~ mm/h G.Psi * 100

1.93

5 67

, ,

,,.

.

_ _

.

. .

, ,

.

.

, .

Appendix 399 Table A.6 (contd.)

Machine l.Run N u m b e r Machine Conditions 4. Bowl Speed rpm.

3150 257 15.0

5.Pond dia mm, 6.Conveyor diff' rpm.

I

425 800 Thickening Digested Sewage

Decanter Bowl Diameter mm Clarifying Length mm Process

3150 257 12.0

3150 257 16.4

3150 257 16,4

3150 257 14.0

3150 257 13.01 I

Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w / w d.s. Additive Condltions

111~.Type. ! 12.Concentration %w/w. :13,Addition point. ~14.Rate m3/h. Product Conditions

116.CakeSolids %w/w. _

17.CenlTate Solids m~l/I. A.Centrate Rate m3/h. B.Polymer Dose k~l/tonne.

1

D.Solids Recovery %w/w. E.Cake Rate k~l/h w.b. IF.Q/~ mm/h rG.PsJ "

I00

_ .

Z63 0.10 B 0.8

Z63 0.10 0.7

1,3

1.0

0.8

8.1 1425 15,6 2.5

9.3 1350 13,8 2.6

104 1450 11.1 2.9

7.6 1150 18.2 2.9

7.0 950 14.8 2,6

94.1 3.84 5.81

94.0 2.52 4.4g 5,14

93.9 3.86 7.1g 3.69

95.1 4.36 6.29 6.18

96.3 4,03 5.12 6.85

5.61

B

Z63 010 B

Z63 0.10 B

Z63 0.10 B

Z63 0.10 B

0.8

7.6

1150 112"59. 94.6 3.26 5.12

Appendix

400

Table A.6 (contd.) 425

Decanter Bowl Diameter m m .

.

.

.

.

Clarifying Length m m

800

Thickening Digested Sewage

Process

Machine

I .Run Number

Machine Conditions

4.Bowl Speed rpm.

5.Pond dia ram. 6.Conveyor dill' rpm.

I

13

3150!

2571 20.01

I

2571 17.6i

!

2571 16.0!

3 501

2571 15.01

!

2571 13.0 !

257 12.0

Feed Conditions

Addltive Conditions

11.Typel

12.Concentration %w/w. 13.Addltlon point.

Z63

Z63

Z63

Z63

Z63

Z63

0. I0 B

0. I0 B

0. I0 B

0.10 B

0. I0 BI

0. I0 B

16.Cake Solids %w/w, 17.Centrate Solids m~I/I. ' A.Centrate Rate m3/h, B.Polymer Dose k~l/tonne.

10.5 1300 21.4 2.5

9.6 1000 17.5 3.6

9,2 1000 15.9 2.8

8.2 1000 12.9 2.9

6,8 950 10.4 2.9

12.0 1100 i' 1.9: 2.8

D.Solids Recovery %w/w. E.Cake Rate k~l/h w.b. F.Q/~ mm/h G.Psi * 100

95,6 5.80 8.85 3.01

95.4 3.821 6.91 4.79

95.9 4,03 6.50 4.49

95,7 3.51 5.39 6.03

96.0 3.45 4.56 7.79

94.9 2.02 4.56 3.92

14.Rate m3/h.

Product Conditions

1.6

1.4

1.1

0.9

0.7

0.7

Appendix 40 ] Table A.6 (contd.) 425 80O Thickening Digested Sewage

Decanter Bowl Diameter mm Clarifying Length mm Process

I

Machine 1.Run Number Machlne Conditions 4.Bowl Speed rpm. !5.Pond dla mm. 6.Conveyor diff' rpm.

3150 257 15.0

Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w/w d.s.

19.0 2.17

Addltlve Cor~llflons

11.Type. 12.Concentration %w/w. 13.Addition point. 14.Rate m3/h.

24 3150 257 15.0

3150 257 15.0

31501 257 15.01

3150 257 15.0

3150 257 13.0

16.8 1.73

Z92 0.13 B 1.0

Z92 0.13 B 1.0

Z92 0.13 B 1.3

Z92 0.131 B 1.5

Z92 0,13 B 0.8

Z92 0.13 B 0.5

16.Cake Solids%w/w. 17.Centrate Solidsmg/l. A.Centrate Rate m3/h. B.Polymer Dose kg/tonne.

12.0 1250 16.7 3.0

9.3 1050 14.3 3.7

11.7 750 15,3 4.8

9.7 850 15.2 6.0

8,4 1100 13.8! 2.91

8.3 950 14.0 2.2

D.SolIds Recovery %w/w. IE.Cake Rate k~l/h w.b. F.Q/~ mm/h G.Psi ~ 100

94.9 3.25 8.57 3.03

95.5 3.46 I 5.81 4.78

96.5 2.69 5.81 3.91

95.9 3.11 5.81 4.94

95.3 3.70 5.81 5.51

95.4 3.32 5.81 5.37

Product Condltlons

402

Appendix

Table A.6 (contd.) Decanter Bowl Diameter m m Clarifying Length m m Process

425 800 Thickening Digested Sewage

Machine

1.Run Number

Machine Conditions

4..Bowl Speed rpm. i5.Pond dia mm, 6.Conveyor dlff'rpm.

3150 257 21,5

Feed Conditi-0ns

31 50 257 20.0

31 50 257 20,0

3150 257 21.0

3150 257 21.5

3150 257 16.0

9.Feed Rate m3/h. 10.Feed Solids% w / w d.s.

19'.2 2.61

Addltlve Condlfions 11 .Type. 12.Concentration %w/w. 13.Addition point. 14.Rate m3/h.

Z92 0.13 B 0.8

Z92 0.13

Z92i 0.13

Z92 0.13

1.0

1.1

1.3

1.4

0.8

16.Cake Solids %w/w. 17.CentTate Solids mg/I. A.Centrate Rate m3/h. B.Polymer Dose kg/tonne.

9.1 1800 14.7 1.9

I0.7~ 1350 15,9 2.4

9.6 1500 17.4 2.6

8.0 1450 18.6 2.9

6.9 1600 20.0 3.6

8.5 1400 14.9 2.7

D.Solids Recovery %w/w. E.Cake Rate kg/h w.b. F.Q/~, mm/h G.Psi * 100

94.'7 ! 5.18 6.64 4.70

95.9 4.70 6.78 3.56

95.1 5.28 7.47 3.88

94.9 6.26

93.5 6.56 8.71 6.34

93.9 3.80 6.22 5.50

Product Conditions

....

..

18.0 1.91

B

B

B

,=,

8.16

4.93

Z92 0.13 B

Z92 0.13

B

Appendix 40 3 Table A.6 (contd.) 425 800 Thickening Digested Sewage

Decanter Bowl Diameter mm Clarlfyln~ Length mm Process

Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5,Pond dla mm. 6.Conveyor diff' rpm.

1 36 31 50 257 14.0

31 50 257 15,0

Feed conditions 9.Feed Rate m3/h. 10.Feed Solids % w/w d.s.

15.2 1.81

Additive Conditions 11 .Type. 12.Concentration %w/w. 13.Addition point. 14.Rate m3/h.

Z92 0.13 B 0.6

Z92 0.13 B 1.1

Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids mg/I, A.Centrate Rate m3/h. B.Polymer Dose kg/tonne.

6.9 1500 12.1 2.7

12.6 1450 18.7 3.2

D.Sollds Recovery %w/w. E.Cake Rate kg/h w.b. F.Q/~ mm/h G.Psi " 100

93.4 3.69 5.25 7.35

93.7 3.18 7.19 2.78

31 50 257 16.0

3150 257 17.0

3150 257 18.0

3150 257 19,0

20.8 2.33

Z92 0.13 B 1.3

Z92 0.13 B 1.3

Z92 0.13 B 1.3

Z92 0.13 B 1.3

12.2 18.7 3.6

12.6 1100 19.0 3.8

8.9 1200 17,6 3.7

7,9 95O 16.1 3.2

94.6 3.36 7.19 3.02

94.9 3.07 7.19 3.29

95.0 4.46 7.19 4.85

96.9 5.95 7.19 4.98

1250

404

Appendix

Table A.6 (contd.)

[

425 800 Thickening Digested Sewage

Decanter Bowl Diameter m m Clarifying Length mm Process

,

I

1 42 Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram.

6.Conveyor diff',.,rpm.

31 50 257 20.0

31 50 257 10.0

3150 257 8.0

3150 257 12.0

3150 257~ 12.0

3150 257 12.0

Z92 0.10 B 0.5

Z92; 0.10 B 0.5

Z92 0.10

1.3

Z92 0.10 B 1.0

Z92 0.10 B 1.3

14.1 4300 17.3 2.7

13.0 3300 15.2 2.5

13.8 2400 15.7 ~ 3.3

84.0 2.74

87.7 2.72 5.88 2.26

Feed Conditions

Additive Conditions 11.Type. 12.Concentration %w/w. 13.Addition point. 14.Rate m3/h. "

Z92 0.13 B 1.3

Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids mg/I. A.Centrate Rate m3/h. B.Polymer Dose k~l/tonne.

6,9 " 1150 16.6 4,0

12.41 2150 9.3~ 1.6

14.3 3000 10.0 1.8

D.Solids Recovery %w/w. E.Cake Rate kg/h w.b. F.Q/E mm/h G.Psi ~ 100

95. I 5.40 7.19 7.44

93.8 2.40 3.87 2.53

89.2i 1.70 3.87 2.02

....

B

..

..,

,..

6.50

1.83

~.~ 2.48 5.88 2.27

Appendix 40 5 Table A.6 (contd.) 425

Decanter Bowl Diameter mm Clarifying Length mm Process

800 Thickening Digested Sewage

1 43

Machine ~.Run Number Machine Conditions [4.Bowl Speed rpm. 5.Pond dia ram. 6.Convey..0r diff' rpm.

3150 257 10.0

Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w/w d.s.

15.3 2.16

Additive Conditions 11.Type. -i 2.Concentration %w/w, 13.Addition point. 14.Rate m3/h.

Z92 0.10 B 0.8

Z92 0.10 B 0.8

Z63 0.10 B 0.8

Z63 0.10 B 0.8

Z63 0.10 B 1.0

Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids m~l/I. A.Centrate Rate m3/h. B.Polymer Dose k~l/tonne.

13.0 38001 13.91 2.3

13.6 1700 12.9 2.7

14.5 2250 13.0 2.7

13,5 1200 11,0 3.0

15,0 1600!

D.Sollds I~ecovery %w/w. E.Cake Rate k~l/h w.b. F.Q/Z mm/h G.Psl " 100

84.0 2.11 5.2g 2.33

92.2 1.88 4.84 2.91

89.3 1.68 4.84 2.52

94.6 1.72

91.9 1.40 4.15 2.63

,,,

.,.

3150 257 11.0: ,,,.

3150 257 10.0

3150 257 10.0

3150 257

9.0

3150 257 8.0

12.0 1.91

4.15

3.01

11.6 4.4

Z63 0.10 B 1.0

....

15.2 1600 11.6 4.4

'91.9 1.38 4.15 2.30

406

Appendix

Table A.7 Lactose washing data used in Section 6.4.7 General washing

Specific Washing Bowl Dia. Mm Wash - ' ~ . ~ e r e n t i a l iO/oof Feed 0 2.5 5 7.5 10 12.5 15

356 23RPM "

4.3

4.1 3.7 3.6 3.1 3

.i

356 36RPM

356 46RPM 4

3.7 3.2 3 2.7 2.5

3.8

3.4 3.1 2.7 2.5 2.1

600 49RPM 3.'3

2.9 2.4 1.9 1.8 1.7

356 60RPM

i|

4.3 3.5 2.8 2.3 2 1.8 1.7

"

600 49RPM 318 3.1 2.5 2 1.7 1.6 1.5

Appendix Table A.8 Coal tailings data discussed in Section 6.4.8 Decanter Bowl Diameter mm Clarifyin~l Len~Ith mm Process

425 560 Coal Tailings Dewatering

Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor diff' rpm. 7.Conveyor torque kNm

3150 264 39.0 0.99

Feed Conditions 9,Feed Rate m3/h. 10,Feed Solids % w/w d.s,

8.0 35.40

Additive Conditions 11.Type. 12.Concentration %w/w, 13.Addition point. 14.Rate m3/h, 15.Dilution m3/h. Product Conditions 16,Cake Solids %w/w, 17,Centrate Solids m~l/I. A.Centrate Rate m3/h, B.Polymer Dose k~/tonne,

D.Solids Recovery %w/w. E.Cake Rate k~l/h w,b. F.Q/s mm/h G,T/V N/cm2

I I

3150 264 39.0 0.57

3150 264 51.0 1.04

3150: 264 41.0[ 0.36

M3127 0.10 D 1.2 0,0

M3127

M3127l

0.10 D 1.8 0.0

0.10 D 1.5 0.0:

M3127 0.10 D 1.2 0.0

66,0 78000 5,6 0.4

62.3 1210 5,3 0.8

64,4 1920 6,4 0,6

63.0! 1300 5.6 0,5

84.7 3.63 5,37 1.15

99.7 3,48 4,70 0,66

99.5 4.15 6.04 1,21

99.7 3.54 5.37 0.42

407

408

Appendix

Table A.8 (contd.) 425 560 Coal Tailings Dewatering

Decanter Bowl Diameter m m Clarifying Length m m Process .

_

I

Machine

I .Run Number

Machine Conditions

[4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor dill' rpm, 7.Conveyor torque kNm

2000 270 26.0 0.73

2000 270 26.0: 1.30

M3127 0.10 D 1.5 0.0

M3127 0,10 D 1.5 0.0

2000 270 26.0 0,73

2(~30 270 26.0 0,68

2000 270 26.0 0.78

2000 270 26.0 0,60

M3 i ; 2 7 0,10 D 1.2

M3127 0.10 D 1.2

M3127 0,10 D 1.2

0.0

o.o

0.o

~.8

65.2

63,2

Feed Conditions

9.Feed Rate m3/h. 10.Feed Solids % w/w d,s, Additive Conditions

11 .Type. 12.Concentration %w/w. 13,Addition point. 14.Rate m3/h. 15.Dilution m3/h. ,

,

Product Conditions

16.Cake Solids %w/w. 17.Centrate Solids mg/l. A,Centrate Rate m3/h. B.Polymer Dose k~l/tonne. D.Solids Recovery %w/w. E.Cake Rate kg/h w.b. F.Q/Z: mm/h G.T/V N/cm2 .

.

.

.

61.5 480 4.6 .. 0.5 ,.L

.

99,9 4.85 13.33 0.85

,,

'1'213127" 0.10 D 1,5 ..

9

'

o.0

61.4 510 5.5 0.6

64.5

99.9 4.23 11.66 1.51

99.9 4,13 14.99 0.85

560 5.4 0.6

,

"

320 4,8 0.4

~.9

4.65 13.33 0.79

,,,

165 4.4 0.31

~00.0

5,10 0.00 0.91

350 6.9 0.4 99,9

4.86 0,00 0,70

APl;endi.v 4(]9 Table A.9 Data for DS scaling used in Section 7.9

Decanter Bowl Diameter

425 800 Unspecified Effluent

ClarifyingLength Process

J

....

,

Machine

1.Run Number

Machine Conditions

4. Bowl Speed rpm. 5.Pond dia mm. 6.Conveyor diff' rpm. 7.Conveyor torque kNm

Feed C~ondition$

9.Feed Rate m3/h. 10.Feed Solids % w / w d.s.

AdditiVe Conditions

3150 252 1.0 2.70

3150 252 1.6 2.70

3150 252 2.2 2,30

3150 252 3.3 1.80

3.0 3.11

5.0 3.21

7.0 2,99

10.0 2.95

.

.

.

.

.

.

3150 252 5.7 1.10

3150 252 8.7 0,40

15.0 3.05

22.0 2.88

....

11.Type. 12.Concentration %w/w. 13.Addition point. 14.Rate m3/h. 15.Dilution m3/h.

Z78FS40 0.15 B 0.5 0.0

Z78FS40 0.15 B 0.8 0.0

Z78FS40 0.15 , ,

B

Z78F.~40 0.15 . . .

0.0

B ."i ,9 0.0

1.2

Z78FS40 0.15

B

3,0

Z78FS40 0.15

B

4.8

0.0

0.o

25.3 650 24.3 11.4

_ =

:Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids mg/I. A,Centrate Rate m3/h. B.Polymer Dose kg/tonne.

34.0 550 3.2 7.2

34.2 425 5.4 7.7

33.0 485 7.6 8.6

31.1 620 10,9 9.5

_ _

28.2: 495! 16.4 9.8

D.SolicJs Recovery %w/w. E,Cake Rate kg/h w.b. F.Q/Z mm/h G.T/V N/cm 2 H Q/~-,.V.ol *100h "1

98,1 0.27 1.40 3.14 2.50

98,6 0.46 2.33 3.14 4.16

98.2 0.62 3.26 2.67 5.82

97.7 0.93 4.66 2.09 8.32

98.2 1.59 6.99 1.28 12.48

,

.

. ,

I

,,

,.,

_

10.25 ~ 0.461

18.36

410

Appendix

Table A.9 (contd.) Decanter Bowl Diameter Clarifying Length Process

I

425 800 Unspecified Effluent

Machine Condltions 4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor diff' rpm. 7.Conveyor torque kNm

31 50 252 6.7 0.10

3150 252 6,0 0.60

3150 252 5.4 1.50

3150 252 5.0 1.60

31"50 252 2.7 0.80

3150 252 2.4' 1.40

Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w / w d.s.

15.0 2.97

15.0 2.97

15.0 3.14

15.0 3.07

7.0 2.91

7,0 2.84

Additive Conditions 11.Type. 12.Concentration %w/w. 13.Addition point. 14.Rate m3/h. 15.Dilution m3/h. Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids m~t/I. A.Centrate Rate m3/h. B.Polymer Dose k~l/tonne. D.Sollds Recovery %w/w. E.Cake Rate k~I/h w.b. F.Q/~. mm/h G,T/V N/cm 2 H Q/~l-Vol *lOOh"~

,

.""

Z78FS40 0.15 B 1.4 0.0

Z78FS40 0.15 B 2.3 0.0

24.1 800 14.6 4.7

26.2 445 15,6 7.7 ~

97,4 1.80 6.99 0.12 12.48

98.4 1.67 6.99 0,70 12.48

Z78FS40 0.15 B 4.2 0.0

Z78FS40 0.15 B 4.6 0.0

,,

Z78FS40 0.15 B 0.3 0.0

Z78FS40 0.15 B 0.4 0.0

.....

29.5 1400 17.7 13.4

30.1 200C 18.2 15.0!

2'7..2 590 6.6 2.4

29.1 380 6.7 3.2

94.7 1.51 6.99 1.74 12.48

92.1 1.41 6.99 1.86 12.48

98.1 0,.73 3.26 0.93 5.82

98.7 0.67 3.26 1.63 5.82

Appendix Table A.9 (contd.) Decanter Bowl Diameter Clarifying Length Process

I Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor diff' rpm. 7.Conveyor torque kNm Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w/w d.s. Additive Conditions 11.Type. 12.Concentration %w/w. 13.Addition point. 14.Rate m3/h. 15.Dilution m3/h.

=,,

425 800 Unspecified 15ffluent

I

3150 252 2.4 2.00

3150 252 2.4 2.50

7.0 3.02

7.0 3.04

Z78FS40 0.15 B 0.8 0.0

Z78FS40 0.15 B 1.7 0.0

Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids mg/i. A.Centrate Rate m3/h. B.Polymer Dose k~l/tonne.

31.5 425 7.2 6.0

33.8 1500 8.1 11.8

D.Solids Recovery %w/w. E.Cake Rate k~l/h w.b. IF.Q/y mm/h G.T/V N/cm 2 H O/g-Vol * 100h"1

98.6 0.66 3.26 2.32 5.82

94.3 0.59

411

This Page Intentionally Left Blank

Editorial Index A

abrasion see wear protection AC rnotors 4 5 . 1 1 5. 197. 325 acceleration force 149-5() accelerator blades/vanes 3 2 . 6 8 acetone 143 acetylene 128 acidic sludges 22 5 acrylamide content, polymers 2 35:see also polyacrilamide additive chamber 29. :~ 3.34 additives, to polymeric tlocculants 222 agglomeration 21 7 agitation 21 7 agricultural products, test data 258 -9 agricultural wastes 146 agro-chemicals 142 air in-tlow, feed tube ~9: see also windage air pollution prevention 12 8 alarm systems 11 7. 319 alcoholic beverages 141 : see also spent grain alumina tiles 6 4 . 7 2 alunlinium 2() 1 aluminium (coagulant) 129.21 7- 1 8 aluminium hydroxide 14 aluminium sulphate 229 aniline 128 animal fats 1 37 animal feed136. 1 4 1 . 2 3 5 . 2 5 5 , 2 5 8 antibiotics 14 5 applications 5.1 3. 121---45 classes 1 2 2 - 4 . 1 2 :, early 7-9 market estimates ~ ~7 Aqua-Critox 128 arc welding 72 area equivalent 1 6 4 - 5 automatic control 1 1 6 - 1 7 . 3 1 5 . ~2 3- 3() axial tlow see flow. axial axial load/thrust 2 7 - 8 . "50.65.201 B

buck-drive 1 7 . 4 5 - - 6 . 8 2 - 4 . 1 2 9 , 1 3 1 , 1 9 6

control 116. 321. 3 2 5 - 7 . 3 3 ( ) baffle cone l()(I-l()l. 1 1 2 - 1 3 . 129. 173 bat'fle discs 6 6 . 9 2 . 9 9 - 1 ( ) ( ) . 1()1-3. 173, 177.253 baffle guttering 78 baffles casing 4 1 . 7 7 - 8 conveying 1 ()2- 3 conveyor 9 1 . 9 2 . 9 9 - 1 ()3.27(l hinged 1()3 longitudinal 1() 3 in three-phase separation 87 8 see also cake baffles: grooves bar screen 86 beach 2 . 2 5 - 8 . 5 9 - 6 4 angle 26. 47. 59-61. 1 6 4 . 2 7 6 . 3()() conveying theory 1 7 6 - 7 , 1 79 dual beach 92. 146 liner 2 8 . 5 4 . 6 4 . 7 1 pitch 11 () Sigma value 163 wall thickness 64 w a s h i n g 124.1 35.181 bearing supports, frame 38 bearings 2, 1 7 . 5 8 - 9 . 9 ( ) conveyor 2 9 - 3(). 3 4 - 6 . 7 3 failure 2()4- 6. 322 frame 37-8, 76 front hub 2 4 - 5 life 2 ( ) 4 6 monitoring 322 rear hub 2~- 8, standards 2()6 vertical decanters 4 9 - 5() belts 1 7 . 4 5 , 199. 321. ~23 bentonite 1 32.1 35 Beta theory 1 7 5 - 6 beverages 141 : see also spent grain biochemical industry 9.1 3 . 1 1 4 biological disc 99 biological sludge 2 36 Bird Machine Company 7 . 8 blast furnace operation 128

414

Index

blood 145 bolt heads 97 botanical products 142 bowl 2, 7, 9, 2 1 - 5 . 5 4 - 9 . 2 76 baffles 9 9 - 1 0 3 casing 2 fabrication 54 hubs, see front hub: rear hub liner 2 2 . 2 3 . 5 4 , 56-8, 71 shell loading 2 0 0 - 2 0 2 special variants 86-95, 104 speed 2 0 0 - 2 0 2 . 2 4 7 . 2 4 8 , 2 4 9 , 2 6 5 , 2 8 1 . 303 unblocking 114 wall/nozzles 2.17, 2 0 . 2 1 - 2 . 5 6 - 7 , 9 1 . 2 9 wear protection 71 braking power, calculation 306-7 braking torque 83. 321. 324. 326 braking see also back-drive brewing 141 : see also spent grain bubbles, in flow 320 buffer chamber 30 buffer tank 318 C cake 17-18 dryness 108.1 ~)9--10. 1 4 3 . 1 7 3 - 4 . 1 7 7 8.18(), 2 2 7 . 2 37 and tlocculants 2 30.2 32 test work 255 see also dry solids reslurrying 11 3 shearing 108 sticking 5 4 . 6 4 . 7 8 cake baffles 9 9 - 1 ( ) 0 . 1 2 9 . 1 3 0 , 131 cake collectors 78--9 cake discharge 2 6 . 2 8 . 5 6 . 6 1 - 4 , 78-9, 113, 246 casing41, 54 diameter 2 8 . 1 7 7 height 56 liners 78 system/module 318 cake rate calculation 289 cake size frequency distribution 292. 293 calcining 1 2 9 , 3 0 0 calcium I c o a g u l a n t ) 2 1 7 - 1 8 calcium carbonate 135,143, 259 calcium hydroxide 300-301 canning 138, 139 capacitors 198 capillary suction test (CST) 22 7-8 carbide 49, 64 carbon fibre 39 carbon steel 18.21, 53 casein 140 casing 17, 18, 38.40--43, 54, 7 7 - 8 baffles 41, 77-8, 87

seals42, 51, 52, 7 9 - 8 0 wear resistance 71, 131 cast iron 37 castellation 64 casting, of bowl 54 catalyst preparation and recovery 134 caustic soda 101.143, 144 cellulose 128, 258 cement industry 7 centrate 1 9 - 2 0 monitoring 116. 3 2 0 , 3 2 3 quality 110, 1 1 6 - 1 7 , 2 3 7 , 2 5 0 , 3 0 0 - 3 ( ) 1 rate calculation 1 5 2 , 2 8 4 centrate discharge by centripetal pump 9 3 - 4 casing 42, 4 3 . 7 9 and floater disc 1 () 1-2 system/module 317-18 test facility 246. 247 centrate weirs see front hub weirs/dams centrifugal force 2 . 3 . 2 7 : see also g-force centrifuges 4 . 7 special variants 86-93 standards 200 centripetal pump 9.55, 9 3 - 4 . 1 7 2 . 3 1 7 Centriseal 9 . 9 9 ceramic materials 3 2 . 4 9 . 6 4 . 7 2 . I t)8 chalk 1 35 chemicals industry 142- 5 china clay tkaolinl 7. 123.1 35 chitin 218 chlorides, in process material 53 CIP(clean-in-placel 9.1 3.47, 114, 1 36. 14~). 33O clarification 5.9~). 9 2 . 1 2 3 conveyorless 92 enhancement 1()4-7 scale-up 30()-3()1 theory 159-67 volume calculation 286 classification 5, 123, 1 3 9 . 2 2 6 calculations 2 8 1 - 2 . 2 9 1 - 3 and scaling up 3~)()--3()1 test work 2 4 7 . 2 5 9 - ~6 theory 168-9 see also particle size clays 132.1 34. 1 8 1 . 2 6 1 - 3 . 300 clean-in-place (CIP) 9, 13, 47, 1 1 4 . 1 3 6 . 140.330 co-current flow 1 9 - 2 0 . 5 1 - 3 , 65 coagulants 7. 1 2 9 , 1 3 0 . 2 1 7 - 1 8 . 2 2 9 coal processing wastes 128, 1 3 2 . 1 3 4 coal tailings 269 coal washing 86,124. 132. 236 coating materials 71-3 coatings industry 142 cobalt-based alloys 71 codes of practice/standards 4 9 , 2 0 0 . 2 0 6

Index

coffee production processes 1 2 8 . 1 4 0 Colmonoy 71 compaction theory 18 5 . 1 8 6 - 9 1 conductivity probes 320 control algorithms 117, 328-9 control systems 9 . 1 1 6 - 1 7 . 3 1 5 . 3 2 3- 3() test equipment 24 5-7 controllers 1 1 6 . 3 1 5 , 32 5-7 integrated 32 7-8 conveying theory 1 7 5 - 8 conveyor 2 9 - 3 6 . 6 4 - 7 3 . 144. 2 4 7 , 3 0 0 tloating 1 1 6 conveyor baffles 9 1 . 9 2 . 9 9 - 1 0 3 . 270 conveyor bearings 2 9 - 3 0 . 3 4 - 6 , 61, 73 conveyor differential speed 150-51, 196, 251. 255. 2 6 5 . 2 7 ( ) calculation 1 5 0 , 2 8 1 . 285 and scaling up 3()(). 3()2.3()4.3()9, 31213 conveyor flights 2 . 2 9 - 3 1 , 3 7 . 6 6 - 7 . 9 2 clarification enhancement 104-9 wear protection 7 1 - 3 . 1 1 1 . 129.13(). 131. 144: see also tiles conveyor hub 1 7.29- 3(). 58.66.9() clarification e n h a n c e m e n t 1 0 5 - 9 . 1 6 6 - 7 c o m e y o r pitch 31.92.1 ( ) 9 - 1 2 . 2 5 1 . 2 7 ( ) . 3()() conveyor rake 1()7 8 conveyor seals 29. 34 6 . 7 3 convevortorque 1()7. 1 8 5 . 2 4 5 . 2 5 5 - 8 . 27 1. 326 calculation 1 51. 286 measurenlent 151. 1 79. 286. 321 and scaling up 3{)2-5. ~;1() see also scrolling torque conveyor vanes 1 ( ) 5 - 7 . 1 6 6 Coriolis effect 167. 32() corrosion-resistant materials 53 - 4 . 7 2 . 1 4 3 cortisone 1 37 costing 11 7. 328-9 counter balancing 1 l()-I 2 countercurrent flow 19-2(), 5 1 - 2 . 6 5 , 89. 1()1. l 1()

counters 3 2 2 countershaft system 8 2 . 8 4 cream processing 4 . 7 . 2 36 cresting 2 3. 176. 192. 194-5. 251 crystalline solids 124, 180 cut point 1 5 5 - 6 . 2 6 1 - 3 , 2 9 2 cyclic loads 2()2 Cyclo gearbox 36. ~7.73, 7 4 . 7 5 - 6 conveyor differential speed 15(I-51 D

dam plates see front hub weirs/dams D'Arcy's equation 186 l)C motors 46. 8(). 8 3 . 1 1 5 . 325 De Laval. G. 6

415

de-aeration, centrate solids concentration measurement 320 de-inking plants 1 2 8 . 2 3 6 decanter plants, instrumentation 3 1 5 - 2 2 decanters 4 . 7 orientation 2 . 1 9 . 4 7 - 5 1 , 9 0 special variants 8 6 - 9 3 , 24 7 standards 200 deliquoring see dewatering Denmark. manufacturers 11 density 2 , 1 4 6 , 152. 1 6 8 , 1 9 4 . 226 desulphurisation processes 128 detergents industry 142 dewaterability 22 7 dewatering 5 . 9 1 . 1 2 3 coal tailings, test data 269 flotation concentrates 1 32 in mineral processing 13 5 scaling up 3(12-5 solids 18()-81. 2 6 9 - 7 9 : see also dr)' solids spent grain 141. 255-8.3()(). 3()2 test procedures 2 4 8 , 2 4 9 theory 18()-81 see also waste sludge processing differential height 2 5(I-51 differential speed 15()-51. 2 4 7 . 2 58: s e e a l s o conveyor differential speed diffusion 182- 3 digester sludges 1 3(}-~ 1. 265 -8 direct-on-line motors 45.8(). 325 directional ff'ed nozzles 67. 141 disc-stack centrifuges 4.1 ()6- 7 discharge vents/ports 17, 3 2 . 4 2 - ~. 67 8. 112-13 dispersants (use in classification) 1 6 9 . 2 5 l dispersions (polymeric tlocculant ~22(). 221)

distilleries 141. 255 donkey motors 114.33() drain valves 114 drainage, dewatering by 18(). 181 drilling mud I 32-3 drive bells 199 drive motor 2.1 7 . 4 3 - 5 . 8 ( I - 8 2 acceleration 4 4 - 5 . 1 9 8 -9 control 116. 32 3. 325 dual 116 installation 198 mounting 811, 81- 2 sizing 197-8, 3 0 6 - 7 speed 4 4 - 5 timers 321-2 torque 4 4 - 5 : see also braking torque drysolids (DS) 9 , 1 8 6 - 9 1 . 335 basic calcualtions 2 8 6 - 7 compaction theory 185 conveying theory 1 7 7 - 8 . 1 7 9 tlocculant addition 167.22(). 2 38.24()

416

Index

scaling up 302. 308-13 test work 2 4 9 , 2 5 8 , 2 6 5 . 2 6 9 - 7 9 dyestuffs and pigments 128, 135. 141. 142, 155 E

e-line (equilibrium line t calculations 170. 289-90 eddy current brake 4 6 . 8 3 , 8 5 . 1 1 5 calculations 2 8 1 , 2 8 3 eddy current flow meter 319 effluent flocculants 33, 1 6 7 . 2 3 6 municipal 129-31. 2 2 5 . 236. 2 6 3 - 5 rate calculation 289 test facility 2 4 7 thickening 113, 130. 2 6 3 - 8 see also dewatering: waste sludge elastomers 51, 54 electrical meters 3 2 2 electrolytic and e l e c t r o c h e m i c a l sludges 128, 146 energy materials production 132-3 equilibrium line (e-line) calculations 1 70. 289-9() erf 1 54. 2 9 8 - 9 erosion protection see hard surfacing: tiles European Engineering Directive 2{)1 European standards 2{){) Expamet 56 F

failure, mechanical 2()() 207 fats. rendering 137 fatty acids 144 feed ports 17. 32 feed pump 245 feed quality 251 feed rate 1 8 4 . 2 4 8 . 2 4 9 , 2 5 1 and scaling up 3()2 test work 2 4 8 , 2 4 9 . 251. 255. 263. 292 feed tube 17, 31, 3 8 - 9 . 5 1 . 7 6 - 7 tloc feature 76. 207 resonance 206-7 rinse feature 267 seals 39 feed vessel 2 4 5 . 3 1 8 feed zone 29, 30, 31-2, 6 7 - 9 , 3 0 0 acceleration 195 admitting flocculants 230, 231 liner 54 special variants 112-13 test equipment 2 4 5 , 2 4 6 wear protection 71-3 fermented materials 141 ferric chloride 229 ferric sulphate 229 fertilizers 127, 142, 145

fibrous solids 124 filters (presses) 4 . 2 2 7 filtration 86. 180 first critical rotor speed 203 fish eyes 220, 221 fish oil industry 7 fish processing 137-8 fish transport water 128 flame spraying 71-2 floater disc 1 0 1 - 2 floating conveyor 116 floc feature, feed tube 76.230, 231 floc zone 2 9 . 3 0 , 3 4 . 6 9 - 7 0 . 2 3 0 . 251 flocculant system/module 317 flocculants 29, 30.33, 3 4 . 6 9 - 7 1 . 101.130. 135,217-18 admitting to decanter 2 3 0 - 3 2 . 248 performance 2 3 7 - 4 0 selection 22 5-8. 2 4 8 . 2 5{) solids content measurement 320 suppliers 2 33-4 see also polymeric tlocculants flocculation 2 1 5 - 4 0 applications 2 36 pre-treatment 2 1 7 - 1 8 theory 1 6 7 . 1 7 3 . 2 1 7 - 1 9 see also flocculants flotation concentrates 1 32 flow 1 9 - 2 0 . 5 1 - 3 . 1 6 7 axial 104-5. 192. 193. 194 sewage treatment 12'). 1 3(). 131 co-current 19-20, 5 l - 3 countercurrent 19- 2(). 51-2 fully axial 1 ()5 helical 1 9 2 , 1 9 3 . 194 laminar 193. 194 pressurised 94 quasi-axial 1{)4-5 flow diagram, decanter plant 315, 316 tlow meters 1 51, 1 5 2 . 2 2 2 . 3 1 9 . 3 2 2 fluid coupling 8 0 - 8 1 , 1 1 6 . 307. 325 fluid dynamics 1 7 6 . 1 9 2 - 5 food and food by-products 9 . 1 3 . 1 1 4 . 1 2 8 . 136-40.235.236.258-9,330 foundry operations 128 fractionation 145" see also classification frame 1 7, 3 7 - 4 0 . 7 6 - 7 bearings 3 7 - 8 . 7 6 France 10. 11 frequency distributions 2 9 1 - 3 frequency inverter drives 114 friction, beach 176 friction power component 196, 197, 198, 306 front hub 2 2 - 5 . 5 4 - 5 . 9 3 weirs/dams 1 7 . 2 2 - 3 , 55-6, 5 7 . 8 7 . 9 6 - 7 froth flotation 1 3 2 . 2 3 6 fruit and vegetable products 1 3 8 - 9 . 1 4 1 ,

Index

2 58-9" set' also vegetable oils fuses, high rupture (HRC)45 fuzzy logic 329 G

g-force 1 4 9 - 5 0 g-level calculation 150. 2 8 5 , 2 8 6 . 303. 308-9 gasification 1 32 gaskets 21.42, 54.78: see also seals gearbox 3.8, 9, 1 7 . 1 9 . 3 6 - 7 , 55, 73-6. 131 and back-drive 4 5, 325-6 Cyclo 3 6 . 3 7 . 7 3 . 7 4 . 7 5 - 6 conveyor differential speed 150-51 epicyclic 3 6 - 7 . 4 5 . 7 5 . 7 6 conveyor differential speed 15()-51 hydraulic 36.46 life 206.31 () Maun type 73- 5 ratio 3()6-7 Rotodiff 36.76 gelatinle) 1 37. 142 Germany 1(). 11 glass fibre 39 glues 142 gluten extraction 1 38.1 ~9 grain, dewatering see spent grain graphite 14 gravimetric analyses 1 51.152- 3. 2 2 2 , 2 2 5 6 gravitational field 2 69 gravity separation 3.4 Greece 11 grooves and baffles 22, 28, 64, 176: seealso baffles gypsum 1 34.14 3 tt

hard surt'acing 6 4 . 7 1 - 3.1 31.1 37 hard water 22 3. 224 Hastelloy 53.72 HI)PE 78 heel 17.18 torque 1 79 heparin 1 37 high rupture fuses {ttRC) 45 high-temperature applications 4 7 . 5 3 4 . 7 3 . 9(1. 141 high-velocity oxide fuel iHV()Ft 72 horizontal decanters see orientation hormones 137 hubs. see conveyor hub: front hub: rear hub HVOF (high-velocity oxide fuel) 72 hydraulic balance, three-phase separation 17O-71

hydraulic conveyor drives 4 6 . 8 3 hydraulic motors 80; see also Rotodiff hydrocyclones 3.4

417

hydrogen economy 143 I

impellers 7 8 - 9 . 9 7 imperforate basket centrifuges 4, 7 impurity calculations 1 8 2 - 4 . 2 6 7 - 9 , 2 9 4 - 7 India 11 industrial classification 1 2 5 - 6 industrial wastes processing 12 7 - 9 . 2 3 6 inertia 1 9 8 - 9 , 2 0 0 infrared instrumentation 320 inline dosing 23(), 231 inorganic chemicals 142, 1 4 3 , 2 2 9 inorganic sludges, flocculation 225 inorganic solids, processing 106 insecticides 128 instability 203 instrumentation 9 . 1 1 6 - 1 7 . 3 1 5- 3() test equipment 245-7 insulin I 37. 145 inverter motors 4 5 . 8 3 . 1 1 5 . 3 2 5 ionic activity 217. 229 iron coagulants 21 7-18. 229 iron compounds, manufacture 14 3 iron ore scrubber slurries 128 isinglass 218 IS() standards, bearings 2()6 Italy 1(). 11

I

Japan 11 12 K

kaolin (china clay} 7.12 3.1 35 knurling/ribbing 2 2 . 2 8 . 5 6 - 7 , 64.1 76 Krepro 128 L labvrinths 78 lactose 14(). 267-9 landfill 1 2 7 . 1 2 9 Lee disc 99 level probes 32() Liedbeck. A. 6.7 light retlection/transmission instrumentation 32() lime ~coagulant ) 2 2 9 lime sludge/slurry 129.2 36.2 59-61. 3()() liners beach 2 8 . 5 4 . 6 4 . 7 1 bowl 2 2 . 2 3 , 5 4 . 5 6 - 8 . 7 1 cake discharge 78 feed zone 54 logarithmic probability 154. 165-6. 2 9 8 - 9 low-temperature applications 8 6 , 1 4 4 lubrication 19, 2 4 - 5 . 2 7 - 8 . 5 9 . 9 0 . 206, 317,319 monitoring 317,319. 321

418

Index

pillow blocks 38 systems 24, 38, 323 M

machine tool fluids 127 magnesium hydroxide 1 4 3 . 2 59. 300 main drive see drive motor maintenance 117. 206 manufacturers 1 0 - 1 2 . 334. 3 3 7 , 3 3 9 - 6 2 early decanters 6-11 polymeric flocculants 233 market 333-8 mass balance calculations 152. 2 4 7 . 2 8 1 - 2 . 288 mastics 142 materials ofconstruction 9 . 1 8 . 2 1 . 3 7 . 3 9 . 4 9 . 5 3 4.64 for hard surfacing 7 1 - 3 . 1 3 1 . 1 3 7 Maun type gearbox 73-5 meat/meat products 1 2 8 . 1 3 6 - 7 mechanical design 2()~-2(~7 mechanical engineering applications 146 medicinal chemicals 1 4 2 . 1 4 4 - 5: see also pharmaceuticals meters 116, 319-22: see also tlow meters methanol 22() mica 1 35 milk and cream 4 . 7 . 1 2 8 . 1 4 ( ) . 2 3 6 . 2 6 7 - 9 minerals extraction and processing 1 3 5 . 1 6 8 tlocculants 22 5 . 2 3 6 modules 3 1 7 - 1 8 . 3 2 3 molecular weight, tlocculants 2 1 8 . 2 33 monitoring 1 1 6 - 1 7 , 2 ( ) 6 . 3 1 7 - 2 2 test facility 24 5-7 motors AC46. 115. 197. 325 D C 4 6 . 8 ( ) , 8 3 , 1 1 5 . 325 direct-on-line 45.8(), 325 donkey 114. 330 hydraulic 8(): see also Rotodiff inverter 4 5 . 8 3 . 1 1 5 . 325 soft start 8 0 . 1 1 6 . 325 star-delta 4 5 . 8 0 , 116 starter 4 4 . 4 5 . 325 three-phase 80 variable speed 8 2 - 3.323 see also back-drive: drive motor (main drivel mounting, drive motor 8(). 81-2 mud. drilling 1 32-3 multi-lead conveyors 6 6 , 1 4 4 municipal sewage treatment 1 2 9 - 31.225. 236,263-5 N

negative pond operation 5 6 . 1 3 1 , 318 negative ring dam 99

neoprene 2 1 , 4 2 . 5 4 nickel-based alloys 71 noise 38, 4 9 . 5 5 . 7 9 , 9 7 - 9 nomenclature 2 0 8 - 1 2 nuclear power processes 1 3 . 1 2 8 nylon 144 O

oil. water and solids, separation 8 6 - 8 . 1 7 0 72; see also three-phase decanters: waste oils oil production and refining 13, 128, 132. 134 oils, vegetable 101. 138-9. 142, 17(). 334 olive oil 139, 1 7 0 . 3 3 4 on/offdevices 32 3-4 ore processing 1 3 . 1 2 8 . 1 4 3 organic chemicals 1 4 2 . 1 4 3 , 181 organic sludges, flocculation 225 orientation 2, 19, 4 7 - 5 1 , 9 ( ) overload protection 4 5 . 8 2 oxidation treatments 128 P

paints 135. 1 4 2 . 1 5 5 palm fruit oil 139.17() paper mill operations 128.2 36 para-xylene 86.144 particle dvnamics 1 4 8 . 1 4 9 - 5() particle s i z e 1 5 4 - 8 . 168 9. 2 2 6 . 2 4 7 . 2 6 1 3.291 : see also classification patents, early 6-9 Pecker. ].S. 7 penicillin 14 5 performance calculations 1 51 - 3 . 1 7 2 . 2 8 1 313 perfumes 142 pesticides 142 petrochemical industry 9t). 1 3 2 - 3 . 1 4 2 , 143-4 pH 21 7 - 1 8 . 2 2 5 , 2 2 6 . 229 pharmaceuticals 8 9 . 1 1 4 . 1 37. 142. 144-5, 33O phenols 134 phosphoric acids 143 PI1) controller 1 1 6 . 3 2 5 pigments and dyestuffs 128.1 3 5 . 1 4 1 . 1 5 5 pillow blocks 37, 76.2()4 pipe work. test facility 246 pitch angle see conveyor pitch pitting 53 plasma spraying 72 plastics processing 9 2 , 1 4 2 . 143-4. 146 PLC (programmable logic computerl 1 1 6 1 7 , 1 5 1 . 2 4 5 . 325-6. 330 polyacrylamide flocculants 2 1 8 - 1 9 , 2 2 0 polyaluminium chloride (PACt 218. 229 polyelectrolytes see polymeric flocculants

Index

polymeric flocculants 2 1 8 - 1 9 , 2 2 0 addition 7 , 1 2 9 . 1 3 0 . 1 3 1 . 1 9 1 , 2 4 8 , 2 5 3 . 265 concentration measurement 320 consumption measurement 117 cross-linked 2 1 8 - 1 9 . 2 2 5 . 2 3 1 dosage 1 5 1 - 2 . 1 5 5 , 167, 222, 2 3 1 - 2 . 2 3 7 - - 4 0 , 2 4 8 , 2 7 3 . 317 calculation 1 5 2 . 2 8 1 . 2 8 4 - 5 and scaling up 3 1 1 - 1 2 electrical charges 2 1 7 - 1 9 , 225 monitoring 3 2 0 . 3 2 1 . 322, 3 2 3 - 4 selection 2 2 5 - 8 solution make-up 22(I-24, 317 solution strengths 2 2 2 - 4 test facility 2 4 5 - 7 , 2 4 8 - 5 1 polymerisation processes 14 3-4 polyolefins 14 3, 144 polystyrene 143, 144 polyvinyis 143, 144 pond 2 . 2 2 - 3 pond settings 2 3 . 5 5 - 6 . 9 3 . 1 9 ( ) . 2 4 7 , 2 4 8 . 249. 259, 29(). :112 deep 141, 161. 2 5 9 - 6 1 . 265 negative 5 6 . 9 9 , 1 3 1 , 3 1 8 neutral 9(). 129.13(). 1 ]1. 141 shallow 9(), 1 3(), 131. 1 4 1 , 1 6 3 . 1 9 ( ) . 259-61 pond volume 2 7 ports 1 7. 3 2 . 4 2 - 3.67 - 8 . 1 1 2 - 1 3 potash 1 35 power consumption 117. 196-9. 3(16-7. 322 power losses 195, 197, ](17 power regeneration 115-16, 325 pre-liocculation treatment 21 7-18, 2 2 5 . 2 2 6 , 2 2 9 : see also coagulants pressurised applications 47, 49, 90 pressurised tlow 94 printing inks 1 2 8 , 1 4 2 . 2 ] 6 probability, logarithmic 154, 1 6 5 - 6 , 2 9 8 - 9 probes 3 2 0 - 2 1 process acceleration power 306 process applications 122-4 process data. test log 253 process instruments 315-16. ] 19--22 process performance see performance calculations proportional integral and derivative (PIDI controller 116. 325 protein processing 1 0 6 . 1 3 7 , 1 4 0 proximity probes 321 psi (thickening factor) 1 7 4 , 2 6 5 - 8 , 2 8 5 P T A 144 P T F E 4 2 . 5 1 , 54.78, 80 pumps 9 . 3 8 . 2 4 5 - 7 . 3 1 7 - 1 8 . 319. 323. 324 centripetal 9, 55, 93-4, 1 7 2 . 3 1 7 speeds 321

419

purity 123--4; see also impurity calculations PVC linings 54 PVC separation 144 R

racetrack ( R e n n b a h n 1 7 8 - 9 radioactive source instrumentation 320 rakes, in clarifying zone 92, 1 (17-8 raw water treatment 12 7 . 1 2 9 re-suspension 51, 52 rear hub 2 5 . 2 6 - 8 . 6 1 , 1 1 0 - 1 2 recovery, oil 289: see also solids, recovery recycling 127, 128. 129. 1 3 9 , 1 4 6 . 2 3 6 , 259 Rennbahn (racetrack) 78-9 rendering 137 reslurry 11 3, 124. 140 resonance 3 8 - 9 , 76, 98.2()6 results sheet 2 5 3 - 4 reverse pitch 92.11(1 Reynolds n u m b e r 1 0 4 . 1 8 4 . 1 9 2 - 4 ribbing 2 2 . 2 8 , 5 6 - 7 . 6 4 , 1 76 rinse nozzles 181 rinse zone 29.3(). 3 3. 34. 69 -71 rinsing, see washing risk analysis 2()() Ritsch, H.P. 7 rotameters 319 rotating assembly 17.19, 21- 37 Rotodiff 36, 7 6 . 1 1 4 - 1 5 rubber 54, 7 3, 77.78. 142 g

safety 117, 2()(), 2()6, 315 sampling 2 5() sanitary performance 9, 136 saponitication 144 scaling 2 5 3 , 2 7 6 , 2 8 2 . 3()()-] 1 3 Schmidt n u m b e r 184 scraper blades 11 O- 12 screen-bowl decanter 86, 124, 135 screens, sewage treatment 13() scroll Iscrew conveyer) 2: set' also conveyor scrolling aids/efficiency 2 3 . 2 8 , 5 6 . 5 8 , 71, 1 7 6 . 1 7 9 , 2 6 5 . 269.27() scrolling rate 1 7 3 - 4 . 2 5 3 . 273. 3()(), ]()9 scrolling reversal 3 ]() scrolling torque 2 8 . 5 8 . 6 6 : see also conveyor torque seals 19, 21, 54, 59.9(1 beach 2 6 - 8 bowl 2 1 . 2 2 . 2 5 casing 42, 5 1 . 7 8 . 7 9 - 8 0 conveyor 29, 34-6, 73 feed tube 39 vertical decanters 49-51 see also gaskets sedimentation principle 3

420

Index

separation 3-5, 8 . 5 2 , 1 2 2 - 4 low-temperature 86 partial 123 three-phase 8 6 - 8 , 89, 9 7 , 1 0 1 , 1 2 4 settled volume index (SVI) 265 settling ability 217 velocity 1 5 9 - 6 1 , 2 1 5 see also clarification sewage treatment 1 2 9 - 3 1 , 2 2 5, 2 3 6 , 2 6 3 5: see also effluent Sharpies Corporation 7 . 8 . 9 short circuit protection 45 Sigma 1 5 9 - 6 7, 2 53 calculation 1 6 1 , 2 8 5 - 6 . 2 9 0 and "dr), solids" 1 8 6 - 7 e n h a n c e m e n t 1 0 4 . 1 3 0 . 140. 141. 1 6 6 - 7 and scaling up 3()(). 304 silicon carbide 72 silicon/silicates, processing 14 3 skimmer pipe 8 7 . 9 5 - 6 . 1 7 2 skimmer pump 9 . 3 1 8 slaughterhouse wastes 1 2 8 . 1 3 6 - 7 sludge, digesters 13(): see also waste sludge slurry dewatering 124 soap industry 1 4 2 , 1 4 4 soft start motor 8(). 116. 325 soil improvement 127 solids compaction .see dry solids solids concentration meters 319 -2() solids rec(wery 5. 2 5 5 - 6 3 , 2 6 5 rate calculation 1 52. 284 solvents 8(). 24 7 sonic probes 32() soybean protein 14() speeds 2 critical 2()2-3. 204 measurement 1 51. 2 4 5 - 7 , 3 2 1 spent grain 141. 2 5 5 - 8 , 3()(). 302. 3()6 spindles 2 2 . 4 2 spot tests 25() spray nozzles, floc zone 71 stainless steels 9, 1 8 . 2 1 . 4 9 . 5 3 . 7 2 duplex 5 3 . 5 4 Standard Industrial Classitication 1 2 5 - 6 standards/codes of practice 49.2(1(). 2()6 star-delta motors 4 5, 8(). 116 starch, as tlocculant 218 starch extraction 1 3 8 . 1 3 9 starter motors 44, 45, 325 steel 2 9 . 3 7 , 64" see also c a r b o n steel" stainless steel steel works 128 Stellite 5 4 . 6 4 . 7 1 , 78 sterilising 1 1 4 . 3 3 0 : see also CIP stillage treatment 141 stirred settled volume index (SSVI) 265 stirring 3 1 7 . 3 1 8 , 24 5, 32 3

Stokes'Law 1 5 9 , 1 6 0 . 167. 168. 186 stress corrosion cracking (SCC) 5 3 - 4 stresses 2 0 0 - 2 0 7

sub-frame 17, 39, 4 3, 80 sulphate salts 2 6 7 supercritical water oxidation 128 suppliers 334, 33 7: see also m a n u f a c t u r e r s surfacing 64, 7 1 - 3 . 131. 137 surimi 138 Sweden 12 Switzerland 12 T t a c h o m e t e r 321 tanneries 128, 2 36 tannin, as flocculant 218 tars 134 tea, instant 140 telemetering 116 temperature extremes see high-temperature: low-temperature temperature probes 32 l tension bar 2 7 - 8 . 6 5 terephthalic acid 144 test data 2 5 5 - 7 9 test equipment 24 3- 7 test log 2 5 2 - 4 test procedures 2 4 8 - - 3 1 . 3 3 4 thermistors 4 5. 321 thermocouples 321 thickening 5, 6 1 . 9 ( ) - 9 2 . 9 7 . 1 2 3 basic calculations 2 8 4 - 7 effluent 113.1 3(). 2 6 3 - 8 factor {psi) 1 7 4 . 2 6 5 - 8 . 2 8 5 test work 2 51 theory 1 7 3 - 4 three-phase decanters 8 6 - 8 . 8 9 . 9 7 . 1 ()1. 1 2 4 , 1 2 8 . 136, 1 39. 144 calculations 2 8 1 , 2 8 8 - 9 ( ) instrumentation 319 separation theory 17()-72 testwork 2 4 7 . 2 5()-"51.253 three-phase motors 8() tiles 6 4 . 7 2 , 1 0 8 . 129, 13() timers 321-2. 330 timing belts 1 7, 45 titanium dioxide m a n u f a c t u r e 14 :~ titanium materials 4 9 . 5 3 . 201 torque 27, 4 4 - 5 , 1 79: see also b r a k i n g torque: conveyor torque toxic sludge treatment 1 2 8 - 9 toxicity, of polymeric flocculants 2 35 trials 334: see also test procedures tubular bowl centrifuges 4, 7 tungsten carbide 28, 6 4 . 7 1 , 7 2 . 108 turbulence 5 2 . 7 8 . 9 2 , 1 ( ) 4 . 1 1 3 , 1 6 7 . 1 9 2 5.251 TV tube m a n u f a c t u r e 128

Index

two-phase decanter, process performance calculations 1 5 1 - 3 U

UK 1 0 . 1 2 ultrasonic flow meter 319 u r a n i u m "yellow cake" 143 u r e t h a n e / s y n t h e t i c rubber 73, 7 8 . 1 4 2 USA 1 0 . 1 2 utility applications 122 V Van der Waals forces 217 vanes, c o n v e y o r hub 1 0 5 - 7 . 1 6 6 variable orifice meters 319 variable o u t p u t devices 324 variable speed motors 8 2 - 3 . 3 2 3 varnishes 142 vegetable and fruit products 1 3 8 - 9 . 1 4 1 . 258-9 vegetable oils 101, 1 38-9. 1 4 2 , 1 7 0 . 334 vents 4 2 - 3 . 8 0 . 1 ( ) 2 . 1 1 2 - 1 3 vertical decanters 2 - 3 . 1 9 . 4 7- 51 vibration 3(). 3 9 . 7 7 . 2 ( ) 3 - 4 : seealso resonance vibration isolators 1 7.39-4(). 43,8(). 82. 2O 3 vibration monitoring 2()6 viscosity 194, 319 oftlocculants 222- 3. 32() Viton 54

421

Von Bechtolsheim, C. 6 tV wash-out prevention 129, 130, 131 washing 5 . 1 2 3 - 4 . 1 3 5 . 1 8 1 - 5 calculations 2 9 4 - 7 test data 2 6 7 - 9 see also rinse nozzles: rinse zone waste oils processing/recycling 1 4 6 . 1 7 0 . 247 waste sludge 8, 1 3 . 1 2 2 . 1 2 3 . 1 2 7 - 3 1 , 2 5 9 67.335 waste activated sludge (WASt 2 6 3 - 5 see also effluent water addition 172 water hardness 223. 224 water (potable). treatment 12 7, 129, 236. 259 wattmeter 322 wear protection 3 3 . 5 4 . 6 4 . 7 1 - 3 . 1 31. 247: see also conveyor flights weight distribution 1 5 4 - 8 weirs (centrate) see front hub weirs/dams welding, double 31 welding rods 72 w i n d a g e 4 2 - 3 . 8 ( ) . 87. 196. 3()6 and friction power component 1 9 6 . 1 9 7 . 198. 3()6 wood. decortication 128 Z zinc oxide and salts 14 3

This Page Intentionally Left Blank