:■« -^v^o^ y " '*bv^ :M^^^ ^^^s "-o ^. O > .'. ^^Ad^ f, *bv" • 'MM' \^/ .'A'-' %/ .*M&'- \./ »'M- %.** .*M^-t \-/ 'A ^*/ v^'^y V*^*/ V^"> %*^^*/ V'^'/ '^^ v;s* .A o Vol "J"' ^. .^ »: ..*" .v'V ^bv' A. o t tv^mSS' • \0 v" . aA » » • o^ ^^ aO . '•l.^^'i- "^ A^ ..«•'» ■r ■^ -o.-.i-t.'o^ \;^-;/ \'9/ \W-\'^' "V*^-'*/ V*^\/ ^^^^'^^^o'^ "V^^^:^'* ^^^' "H ^^~ \/ /^^' V^^ yM£^^ S..^"^ /Jife« ^-...^ ^ ^ 1 "VV^ * o ^ ,-.^\.-j^%\ o°^.ii^%>o /\v:^/\ 0^ ':^o^ <^J. 'o , I. IC ^^^2 Bureau of Mines Information Circular/1984 Review of Desliming Methods and Equipment By Christopher H. Roe UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 8972 Review of Desliming Methods and Equipment By Christopher H. Roe UNITED STATES DEPARTMENT OF THE INTERIOR William P. Clark, Secretary BUREAU OF MINES Robert C. Norton, Director Library of Congress Cataloging in Publication Data: Roe, Christopher H Review of desliming methods and equipment. (Information circular ; 8972) Bibliography: p. 47-48. Supt. of Docs, no.: I 28.27:8972. 1. Tailings (Metallurgy)— Dewatering. I. Title. II. Series: In- formation circular (United States. Bureau of Mines) ; 8972. TN2&5.U4- [TN535] 622s [622'.79l 83-600383 CONTENTS Page Abstract 1 Introduction 2 Acknowledgments 2 Chemical treatment • • • 2 Advantages and disadvantages of chemical treatment 3 Gravitational methods • 3 Background ..........*. • 3 Conventional thickeners 5 High-rate thickeners 6 Advantages and disadvantages of conventional and high-rate thickeners.. 7 Multiple-plate thickeners 8 Types of multiple-plate thickeners 10 Advantages and disadvantages of multiple-plate thickeners 11 Sizing thickeners 11 Settling rate basis 11 Solids flux basis 13 Centrifugal sedimentation 15 Background. 15 Solid-bowl centrifuges 15 Screen-bowl centrifuges 17 Disk centrifuges 18 Centrifuge performance 18 Advantages and disadvantages of centrifuges 19 Sizing centrifuges 20 Sigma concept 20 Pilot plant testing 21 Filtration equipment 22 Background 22 Filter presses 22 Advantages and disadvantages of filter presses 23 Continuous pressure filters 24 Belt filter presses 24 Advantages and disadvantages of belt filter presses 26 Vacuum filtration equipment 26 Drum vacuum filters 26 Cake discharge methods 27 Equipment modifications 27 Performance 28 Advantages and disadvantages of drxim vacuum filters 29 Rotary disk vacuum filters 29 Advantages and disadvantages of rotary disk vacuum filters 30 Horizontal continuous vacuum filters 30 Rotary table vacuum filters..... «... 30 Horizontal belt vacuum filters 31 Advantages and disadvantages of horizontal vacuum filters 32 Selecting and sizing filtration equipment 32 Laboratory testing 33 Factors affecting filtration 35 Hydrocyclones 37 Background 37 Advantages and disadvantages of hydrocyclones 38 Sizing hydrocyclones 38 \ ii CONTENTS — Continued Page Thermal dewaterlng 39 Background 39 Thermal dryer operation 39 Advantages and disadvantages of theimal dryers 40 Sizing thermal dryers 41 Current Bureau of Mines research on desllmlng methods 43 Electroklnetlc methods 43 Background 43 Application 43 Current research and use 44 Trommel screen 44 Description of the method and equipment 44 Test results. 45 Conclusions 45 Discussion 45 References 47 Appendix A. — Mathematical terms 49 Appendix B, — Manufacturers of dewaterlng equipment as of October 1982 54 Appendix C, — Available dewaterlng equipment listed by manufacturer as of October 1982 57 Appendix D. — Equipment efficiency 59 ILLUSTRATIONS 1. Cross section of gravity thickener showing zones of the sedimentation process 4 2. Plan and cross-section views of conventional thickener 5 3 . Cross section of high-rate thickener 7 4. Diagram of particle path In ideal settling tank 8 5 . Settling basin containing 10 parallel plates 8 6. Settling basin containing 10 plates set at angle of 60° above horizontal.. 9 7 . Cross section of countercurrent multiple-plate thickener 10 8. Front and side cross sections of typical crossflow multiple-plate thickener 10 9. Graph of A-B Interface height versus time for batch settling test 12 10. Graph of total solids flux versus solids concentration showing relation- ships of the various components 14 11. Graph of settling flux versus solids concentration showing operating line and limiting solids handling capacity •. 15 12. Cross section of solid-bowl centrifuge 16 13. Cross section of screen-bowl centrifuge 17 14. Cross section of disk centrifuge..... 18 15. Cross section of recessed-plate filter press 23 16. Cross section of belt filter press 24 17. Front and side views of drum vacuum filter 27 18. Schematic drawings of various discharge methods for drum vacuum filters... 28 19. Front and side views of disk vacuum filter.. 30 20. Plan and cross-section views of horizontal rotary vacuum filter 30 21. Cross section of typical horizontal belt vacuum filter 31 22. Typical laboratory Installation for vacuum leaf tests 33 23. Representative curve for cake formation rate versus feed solids concentration 35 ill ILLUSTRATIONS—Continued Page 24. Representative curve for cake formation rate versus cycle time 36 25. Cross section of hydrocyclone 37 26. Graph of pressure drop versus throughput capacity for two hydrocyclones . . 39 27. Simplified cross sections showing operation of drum, suspension, and mul- t ilouver thermal dryers 40 28. Simplified cross sections showing operation of vertical tray, continuous carrier , and f luidized-bed thermal dryers 40 29. Two configurations for electrokinetically dewatering slimes 43 30. Diagram showing operation of rotary trommel 45 31. Chart showing generalized capabilities of commonly used dewatering equip- ment with respect to solids cake, moisture content, and particle size.. 46 D-1. Example of a grade efficiency curve showing the relationship of X50, xgs, and Xmax 59 TABLE 1. Classification of selected vacuum filters 33 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT °F degree Fahrenheit lb/in2 pound per square inch ft foot min minute ft2 square foot min/rev minute per revolution ftVh cubic foot per hour pet percent gal/h gallon per hour qt quart gal/min gallon per minute rev/min revolution per minute h hour ton/d ton per day in inch V/in2 volt per square inch kW kilowatt wt pet weight percent lb pound yr year lb/h-ft-2 pound per hour per square foot REVIEW OF DESLIMING METHODS AND EQUIPMENT By Christopher H, Roe ABSTRACT This Bureau of Mines report reviews the various methods of removing the liquid from tailings slurries that contain very fine grained solids. Gravitational settlement, centrifugation, filtration, and thermal drying methods are discussed in detail. Chemical additives, electrokinetic de- watering, and the rotary trommel screen are also mentioned. Lists of dewatering equipment and suppliers are given in the appendixes to assist the planner who must choose the most efficient, economic, and practical method of dewatering very fine grained mill tailings. ^Mining engineer, Spokane Research Center, Bureau of Mines, Spokane, WA. INTRODUCTION The disposal of wet fine-grained wastes from a milling operation can be a diffi- cult challenge for the plant operator. He or she must consider the stability of the material after deposition, environ- mental consequences, and, above all, eco- nomic constraints. Waste material or tailings with a minimum of material smaller than 200 mesh (0.0029 in) can be piled up or used as backfill with few complications because it drains water easily and is relatively stable. On the other hand, materials with a high percentage of particles smaller than 200 mesh present far more problems for water drainage and stability. The small size allows intermolecular attraction be- tween water molecules and solid particles to influence the hydraulic and structural characteristics of the material. As a result, when these fine tailings are sat- urated, they have low permeabilities and have little or no shear strength (16).^ Current tailings disposal practice is to mix these fine tailings or slimes (35, p. 1026) with large volumes of water and to pipe the resulting slurry to settling ponds. In these ponds, the solid parti- cles settle out of the slurry and the re- maining liquid is decanted out of the pond. This disposal method works well; however, it requires large ponds. Also, as a consequence of the small particle size, these slimes may not become com- pletely settled for years and may present a possibility of structural failure. These problems can be mitigated or eliminated if the water content is sub- stantially reduced prior to disposal. Many different methods have been devel- oped for separating small-sized solids from liquids, A cursory evaluation of these methods was done by the Bureau of Mines. This report presents the results of the study in a format that should as- sist mill operators who must establish a tailings disposal system or upgrade an existing dewatering circuit for metal or nonmetal mining operations; dewatering coal slimes is reviewed in a report done under contract to the Bureau of mines ( 14 ) . Many of the conclusions in this report are based on experience from the coal processing, power generation, and sewage treatment industries in the hope that a sharing of knowledge will be bene- ficial to those interested in dewatering slimes. The descriptions of each dewatering method, along with the lists of manufac- turers and equipment in appendixes B and C, should assist mill operators in making decisions about the most appropriate and economic methods of tailings dewatering to investigate further. ACKNOWLEDGMENTS The author thanks Ken Miyoshi, mill superintendent, Western Nuclear Corp., Wellpinit, WA, for reviewing this paper and offering considerable technical advice. CHEMICAL TREATMENT This paper discusses the physical meth- ods of separating solids and liquids. It must be acknowledged, however, that any discussion of solid-liquid separation must include some comments on chemical ^Underlined numbers in parentheses re- fer to items in the list of references preceding the appendixes. additives used in this process. Other Bureau projects are investigating the various aspects of chemical additives, so only a brief review of these substances is included in this report. Chemical treatment is often the first step in slurry concentration. Chemical additives (or reagents) are added to a slurry to promote formation of more eas- ily separated solid masses (24). For many of the dewatering devices evaluated in this report, chemical pretreatment is frequently used or may even be neces- sary to remove as much water from the solids as possible prior to mechanical dewatering. For many years, glue, gums, starch, lime, and similar additives were used as flocculating aids to improve the separa- tion rate of small solids from liquids (10). These substances were reasonably successful but increased the volume of the solids that had to be transported and disposed after dewatering (24). With the introduction of polymers or polyelectrolytes , great improvements oc- curred. Settling rates for solids in- creased by a factor of 10, 20, or more, and solids that could not previously be thickened were responsive to the addition of polymers (10). In addition, lower doses of these additives were needed to produce the desired thickening. As a re- sult, the volume of the solids did not increase and production rates improved (24). ADVANTAGES AND DISADVANTAGES OF CHEMICAL TREATMENT The proper application of chemical ad- ditives will — 1. Increase separation efficiency. 2. Increase throughput. 3. Require a minimum mixing equipment. investment for There are, however, several disadvan- tages to using chemical additives — 1. They can be very expensive. GRAVITATIONAL METHODS 2. A large concentration of some addi- tives may be needed to produce the re- quired results. 3. Not all slurries are responsive to the chemical additives (J_). Chemical additives are often needed to increase the efficiency or throughput of a dewatering system. The use of chemi- cal additives must be carefully consid- ered because they are expensive to pur- chase and will increase the processing cost. BACKGROUND This section describes equipment known as thickeners because of their capability to concentrate or thicken the solids from a slurry that has a very low solids content. In gravitational methods, the force of gravity causes the solids to settle and separate from the liquid. In the mining industry, flocculants are add- ed to the slurry which cause the solids to form larger masses or "floes" that settle at an accelerated rate. With respect to dewatering slimes , thickening equipment is often used to concentrate the fine particles for further processing by other dewatering equipment which will then produce the final dewatered product. Over the years, nomenclature has been adopted that is specific to thickener functions in the mineral processing in- dustries. The solid-liquid mixture that is to be separated by sedimentation is known as the feed. The sediment ed ma- terial having a solids content higher than that of the feed is the underflow. The clarified liquid from which solids have been removed is the overflow. This terminology is used even for equipment where the overflow does not migrate over a weir or the underflow does not emerge from the bottom of the sedimentation de- vice (J7). Regardless of name or variation in de- sign, all gravitational equipment depends on sedimentation to produce the thickened product. During initial sedimentation, solid particles in a nonturbulent solu- tion move downward under the influence of gravity relative to the liquid. The velocity of this movement increases until the upward force of drag, caused by the viscosity of the liquid, equals the down- ward force of gravity on the particles. These particles then fall at a constant velocity, called the terminal or free- settling velocity. In addition to gravity, the size of the reacting force is dependent on the particle diameter and density and the solution density and viscosity. The magnitude of the terminal settling velocity can be shown as — v= = _ Gx2(Ds-Di) J8, (1) where Vg = the terminal settling veloc- ity, feet per second, G = the gravitational constant, feet per second per second, X = the particle diameter, inch, Ds = the particle density, slugs per cubic foot, D| = the liquid density at a spe- cified temperature, slugs per cubic foot. boundary conditions such as the smallest particle size just completely retained by a given device ( 26 ) . As sedimentation continues, the concen- tration of the solids increases through the process of "hindered settling" and then continues into the "compression phase." In this phase, a further concen- tration of the sediment occurs and an in- crease in the concentration of solids takes place; however, the process pro- ceeds at a slower rate. This slowing down is due to the fact that during the exchange of solid matter for water, the water does not reach the top relatively unhindered but has to pass through in- creasingly narrowing capillaries between the more densely packed particles. Also, the friction between the touching parti- cles slows down the compression process (22). As the sedimentation process reaches equilibrixim in the tank, four zones will be present (fig. 1): 1. Clear solution zone. 2. Feed zone. and y = the viscosity of the liquid at a specified temperature, pound-seconds per square foot, J = a correction factor for par- ticle shape, dimensionless (26). 3 Where nonspherical particles are con- cerned, Vs alters by factor (J), which is less than 1.0. Where there is a low den- sity of particles, Vg also decreases by a factor that is a function of the particle concentration. The purely mathematical description of sedimentation is imprecise owing to variations in particle diameter, shape, and distribution for any given slurry. Therefore, the above formula will be used for the determination of ^A list of used in this dix A, all mathematical symbols report is given in appen- 3. Transition zone. 4. Compression zone ( 13 , p. 27-71). The clear solution zone contains the clarified solution of the overflow. The feed zone has the solids concentration of the unsedimented feed. The transition zone has a higher concentration of solids in hindered settling. The compression Feed Clear water zone Transition zone Compression zone FIGURE 1. - Cross section of gravity thickener showing zones of the sedimentation process (13), zone has the highest concentration of solids in compression and is the origin of the underflow. Gravity thickeners consist of open tanks with a feed inlet at the top and a means of collecting the sludge at the bottom by a rake. As the contents are slowly stirred by the rake rotation, the solids settle and are drawn off the bot- tom in a continuous underflow. Gases es- cape from the surface, and clear solution is removed by the overflow weir (24), Chemicals are added to the feed to aid settling, and, in the mining industry, many gravitational thickeners produce un- derflows having over 40 pet solids with recoveries approaching 100 pet. Three general types of thickeners will be discussed in this report: convention- al, high-rate, and multiple-plate thick- eners.. The following sections discuss each of these in detail. CONVENTIONAL THICKENERS Conventional thickeners use the sedi- mentation principle for liquid-solid sep- aration. They are much larger than high- rate or multiple-plate thickeners and have several characteristic design fea- tures such as — 1. Cylindroconical shape. 2. Annular overflow weir, 3. Walkway and feed pipe support. 4. Feed well. 5. Drive mechanism and rake, 6. Underflow cone or trench (17), Conventional devices are typified by a cylindrical upper portion attached to a shallow conical section having the apex oriented downward (fig, 2). The width or diameter of these thickeners is much larger than the depth. Most conventional thickeners are equipped with an annular overflow weir, which may be located in- ternal or external to the tank and which Overflow Feed well ] — Walkway Annular overflow launder Walkway Rake • — U nde r f lo w Underflow pump FIGURE 2. - Plan and cross-section views of conventional thickener (17). is equipped with a froth baffle. Usual- ly, overflow is regulated through a notched weir so that adjustments can be made to compensate for uneven tank set- tlement in the subjacent soil. Most thickeners have a walkway to the center of the thickener. The walkway usually serves as support for the piping that carries the feed to the center of the machine. The feed pipe terminates at a device located at the center of the thickener, which is called the feed well or center well. The function of the feed well is to dissipate the kinetic energy of the incoming feed and to form a zone of quiescence conducive to sedimentation. Feed wells are manufactured in many shapes and sizes because recent infor- mation shows that the feed introduction method and the feed well shape greatly affect sedimentation behavior. The func- tion of the rakes in a thickener is to gently move the sedimented solids from the periphery towards the center discharge point. The movement of the rakes is provided by a drive mechanism. After the sediment ed solids have been moved toward the center of the tank by the rake, they are removed from the thickener through a cone or trench lo- cated near the bottom center ( 17 ) . Although they are all based on the same sedimentation process, a variety of con- ventional thickeners are being manufac- tured which differ from each other in their design. For instance, the feed pipe support may extend from the edge of the tank to the center or may span comr- pletely across the diameter of the tank. The rake arms in some thickeners are rigidly attached to a central vertical shaft or lattice, while in other designs, the arms may be suspended from cables. Drive mechanisms also vary from worm-spur gear combinations to hydraulically oper- ated push-pull arrangements for rotating the rake arms about the central axis. Each of these thickener configurations is designed to assist the process of con- tinuous sedimentation by steadily remov- ing the consolidated solids and the clear liquid to make room for the introduction of more feed material (17). HIGH-RATE THICKENERS Recently, several manufacturers have introduced high-capacity or high-rate thickeners , which have much smaller tanks than conventional thickeners. The lat- eral area for a conventional installation ranges from 5 to 10 ft^ for each ton of solids thickened per day, but the area of high-rate units may be as low as 0.3 to 0.6 ft^ for each ton of solids per day These high-capacity thickeners have smaller tanks because they discharge the feed directly into the bed of settled solids and use chemical additives to hasten the flocculation of the sediments. The efficiency of the flocculation de- pends on how thoroughly the flocculant and the slurry are mixed. The many dif- ferent high-capacity thickeners on the market use various mechanisms or designs to mix the flocculant with the slurry. High-rate thickeners, while smaller than conventional models, can often meet or exceed the performance of the larger units. In one such comparison between a conventional and a high-rate thickener, the latter unit produced underflow solids concentrations equal to or greater than those produced by the conventional unit. This was done even at feed input rates 10 times that of the conventional thickener. These results, though, were dependent on the slurry solids being responsive to the chemical flocculant used during the trials (10). Figura 3 shows a typical high-capacity thickener. The chemical additives are combined with the incoming feed and previously thickened solids in the mix- ing chamber. This mixture is thorough- ly churned by the blades of the mixing mechanism. The high concentration of the solids causes all chemical reactions to occur quickly and completely so that flocculation is considerably improved. This mixture moves from the reaction chamber to the clarified zone, where the flocculated solids quickly settle, to be used again with the incoming feed. A portion of the solids is collected inward by the rakes and evacuated as underflow. As in conventional thickeners, the clear solution migrates upward through the cir- culating sludge bed and exits over the overflow weir ( 23 , p. 19-52) . Gravity plays only a part in the solids-liquid separation in this thicken- er. It would be more precise to refer to this as a filtration unit in which the filter media is a suspended sludge bed. This is true because as the solids set- tle, the pore spaces between them become more restricted and trap other solids be- ing carried along with the water migrat- ing upward (17) . The horsepower required to operate these thickeners is approximately equiva- lent to the horsepower for conventional units having a similar capacity. The drive unit employed by high-rate thicken- ers is usually a hydraulic arrangement (17). Drive unit £l\|^^^Overf Feed inflow Thickened solids Rake arms Chemical additives low weir Mixing chamber Mixing mechanism -Underflow FIGURE 3. - Cross section of high-rate thickener (23). ADVANTAGES AND DISADVANTAGES OF CONVENTIONAL AND HIGH-RATE THICKENERS Thickeners have been used extensively by the minerals industry for concentrat- ing slurries, and over the years many design improvements have been made that enable them to be very reliable and effi- cient for thickening operations. Gravi- tational thickeners, in general, may be advantageous to an operation because they — 1. Are operation. capable of continuous 2. Are capable of processing slurries with a variety of solids concentrations and size ranges, 3. Have fairly low maintenance and operational costs for the mechanical equipment. 4. Provide a kneading action by the rake mechanism which is beneficial to the compression process (26), In addition, high-rate thickeners have some advantages over conventional thick- eners, such as — 1, Smaller lateral space requirements (i.e, , lower installation costs), 2. Greater throughput of solids based on available area for settlement (10). Gravitational thickeners, though, do have some drawbacks , which include — 1, Requirements for large spaces, 2, Necessity for large watertight basins. 3. Sensitivity to persistent strong winds, especially for very large units. 4. Expensive operation if large amounts of flocculants are needed to obtain the desired concentration (10). MULTIPLE-PLATE THICKENERS Innovations have been made to reduce the space needed for gravity thickeners. First among these is the multiple-plate thickener, which uses a series of evenly spaced inclined plates positioned in a settling tank. Multiple-plate thickeners are in use by the minerals industry around the world for clarifying, classifying, and thicken- ing. In the United States, this equip- ment is being used in the coal industry for dewatering the waste products of coal cleaning (7^). The multiple-plate thickener can be used to accomplish both free settling and hindered settling; however, for sim- plicity, only free settling will be dis- cussed in describing the theory of opera- tion. For an ideal settling basin, the thickener feed enters at one end of the basin, flows uniformly along its length at velocity V| , and exits at the other end (fig. 4). Any one particle will set- tle at velocity Vg. The actual trajec- tory of the particle is indicated by the vector Vy. If the trajectory takes the particle to the bottom of the basin be- fore it reaches the far end, then the particle is assumed to have been removed from the liquid. A particle starting at rea (Ap) Height idth KEY Ap Area of settling basin Qs Volumetric flow of incoming slurry V Flow velocity Vg Settling velocity Vy Particle velocity FIGURE 4. - Diagram of particle path in ideal settling tank (7). the top must settle to the bottom at velocity Vg in the same time or less than the velocity of the liquid, V| , in the basin. Thus, the feed quantity, Qg, di- vided by the settling area, Ap , of the basin is known as the overflow rate or surface loading and is often expressed as gallons per minute per square foot. Based on this relationship, all particles are removed that have a settling rate equal to or greater than the overflow rate. It should be noted that the height or detention time of the basin is not one of the main parameters that affect the separation efficiency (7). Area ( Ap) Feed rate—' ( 10 QJ ■ KEY Ap Area of each plate Qs Volumetric flow of incoming slurry FIGURE 5. i ?^1^ Overflow ■•>^-tV«.--f,|- 1llH- will be known or fixed, but the settling flux comr- ponent, Fg, and solids concentration. X 3 (0 o _l o m _i < 1- o Batch flux curve Bulk transport SOLIDS CONCENTRATION. Css FIGURE 10. - Graph of total solids flux versus solids concentration showing relationships of the various components (32). 'ss » will be unknown. Using values for F-j-s and Cgs ^^^ picking values for Cgu* equation 11 can be used to obtain cor- responding values for Fg. A graph of the settling flux, Fg, versus concentration, Css» ^^^ then be constructed, as shown in figure 10 (32, p. 97). If values of Cgs and the corresponding Fg are plotted as shown in figure 11, the result is a straight line with a slope of -Fg/Csu and an intercept of Fg i . This is referred to as the operating line and represents the thickening characteristics for a particular thickener at any speci- fied underflow rate. The batch flux curve can also be plotted, and the inter- sections of the operating line and the batch flux line indicate those solids concentrations that satisfy both rela- tionships (32, p. 97). In figure 11, an operating line has been plotted for a thickener that is functioning within its limits. In this example, only one zone of concentration exists at Cgsi to produce an underflow concentration, Cgui* If» however, the slurry concentration and the underflow concentration are increased to Css2 and Csu2» respectively, the operating line intercepts the tangent to the batch flux curve and a second zone of concentration 15 -SS1 ^SS2 SOLIDS CONCENTRATION, C35 FIGURE 11. - Graph of settling flux versus sol- ids concentration showing operating line and lim- iting solids handling capacity (32). begins to appear with a concentration of Cs2« This condition represents the lim- iting solids handling capacity of a thickener at a particular underflow rate (32, p. 97). In practice, the necessary data are ob- tained by running tests using a thickener and the slurry to be thickened. Altering the applied flux, Fts» ^^^ the underflow concentration, v^su> dlcatlon of the will provide limiting flux After the maximum value of C BACKGROUND s u an in- value . has been determined, a tangent to the batch flux curve can be drawn. The area of the tank can then be determined by dividing the maximum rate of solids loading by the limiting flux. Experience has shown, though, that the optimal thicken- er throughput will be about 90 pet of the values calculated ( 32 , p. 98). As with conventional thickeners , the use of a pilot plant is also necessary for determining the best size for a high- capacity thickener and for establishing the amount of flocculant needed for proper thickening. The properties of the slurry and the many available flocculants will vary considerably, so the best flow rates and flocculant injections should be determined experimentally. Multiple-plate thickeners should also be sized according to results of labora- tory and pilot plant testing. The re- sults will aid the designer in determin- ing the settling rate of the solids so that the overflow rate and effluent qual- ity can be established. The sludge vol- ume can also be determined in order to establish the underflow solids concentra- tion. Finally, the need for chemical pretreatment can be evaluated (7). CENTRIFUGAL SEDIMENTATION Centrifugal sedimentation depends on the density difference between solids and liquids where the particles are subjected to centrifugal forces that make them move radially outwards or Inwards through the liquid, depending on whether they are heavier or lighter than the liquid ( 32 , p. 125). Centrifuges are compact ma- chines and are capable of producing high liquid clarification and solids concen- tration. Most of the units available today are designed for continuous opera- tion (24). Centrifuges have relatively modest cap- ital costs, but they may be expensive to operate because of the need for chemical conditioning in most applications, high power consumption, and extensive mainte- nance requirements. Lower speed units consume less power and, as a result, have fewer wear problems. Maintenance diffi- culties can be greatly reduced if the construction materials are specified to match the abrasive or corrosive charac- teristics of the slurry to be handled (24). A large variety of centrifuges are available on the market; however, several types are used primarily for clarifying and not dewaterlng, so they are not in- cluded in this report. Solid-bowl, screen-bowl and disk centrifuges are of interest in dewaterlng slimes and are discussed in the following sections. SOLID-BOWL CENTRIFUGES Solid-bowl, scroll, or decanter cen- trifuges consist of a horizontally rotat- ing chamber that has one end tapered into 16 a cone. The slurry is admitted through axial feed tubes and removed radially out of the bowl, A screw or scroll mechanism rotates in the same direction as the bowl but 5 to 100 rev/min faster or slower than the bowl and thus can push the sol- ids along the length of the chamber. The speed of the bowl rotation can vary from 1,600 to 6,000 rev/min. The solids are collected by the scroll towards the tapered end of the bowl, while the solu- tion overflows a weir at the other end. The tapered section serves as a drying zone or beach area prior to the cake dis- charge ( 24 , 32 , p, 139), Figure 12 shows a simplified cross section of a typical solid-bowl centrifuge. With regard to the machine design, a number of variations are available in the contour of the centrifuge shell, scroll flight angle and pitch, beach angle and length, conveyor speed, and feed posi- tion. An alternative to the liquid over- flow outlet is an internally mounted tube for skimming off the liquid (^2, p, 140) . Specially designed models of this cen- trifuge have been used to dewater very fine slurries in hydrocyclone circuits for recovering and dewatering deslimed coal. Other solid-bowl centrifuges have been used to dewater froth flotation tailings after thickening in a static thickener (18), Another major area of application for this type of centrifuge is in the classification of solids such as kaolin clay and titanium oxide ( 32 , p. 140), Polyelectrolytes are widely used for the flocculation of the solids to be Rotating solid bowl Rotating scroll ^ / /J- Drive sheave Gear unit Liquid discharge FIGURE 12, - Cross section of solid- bowl centrifuge (3), dewatered in solid-bowl centrifuges. The point of addition varies , depending on the type of the polyelectrolyte and slur- ry. Anionic polyelectrolytes are usually introduced upstream from the centrifuge, while cationics are added within the cen- trifuge because they react very quickly with the slurry ( 32 , p, 140), Wilson and Miller ( 36 ) compared the ef- fectiveness of solid-bowl centrifuges to that of disk vacuum filters and commented on the parameters that affect their use in coal dewatering. The solid-bowl cen- trifuge and the disk vacuum filter pro- duced similar results on the same slurry. The solids from each of the two dewater- ing machines contained 20 pet minus 325 mesh with 20 pet surface moisture and a centrate or filtrate of about 0,1 to 0,2 pet solids. The products from these units are very similar in nature, so the choice between the devices should be made on the basis of handling character- istics, throughput, floor space, cost, and overall circuit or layout considera- tions. If the centrifuge is chosen, in- plant adjustments to feed rates, speed, and pool depth can be made to obtain the proper balance of throughput, moisture, maintenance cost, and effluent clarity (36), Wilson and Miller also found that steam-heating the feed to 110° F resulted in a 4-pct reduction in product moisture; however, higher temperatures did not fur- ther improve results. The use of heat may be economically feasible as a means of reducing the product moisture if an inexpensive source of heat is available (36), One of the principal advantages of this machine is that it can dewater dilute slurries. In plants that use shaking ta- bles , no dewatering screens are required between the table and the centrifuge as would be needed when a screen-type ma- chine is used. The solid-bowl unit will require more horsepower, though, because it must accelerate the water as well as the solids during the dewatering opera- tion (18, p, 12-20), 17 SCREEN-BOWL CENTRIFUGES Screen bowls , also called basket or perforate units, are a second type of centrifuge. Positive-discharge machines are screen-bowl centrifuges with trans- port devices and are the most common type of centrifuge found in the minerals industry today. These units have two elements that rotate about a vertical or horizontal axis. These elements con- sist of an outside conical screen frame and an inside solid cone that carries spiral hindrance flights. A gear ar- rangement produces a differential speed in the two rotating elements so that they both rotate in the same direction, but the screen element moves slightly faster than the cone carrying the spirals. The operation is similar to that of the solid-bowl centrifuge. The slurry en- ters the machine at the top and falls on the apex of the cone. The centrifugal force developed by the rotating cone throws the solid-liquid mixture against the screen. The water passes through the perforations and is collected in an effluent chamber. Meanwhile, the flights spiral downward and the solid material is gradually transported to the bottom of the screen. The conical shape of the basket causes the solids and water to be subjected to zones of increasing centrifugal force ( 18 , p. 12-16). Fig- ure 13 shows a cross section of a typical screen-bowl centrifuge. Another type of screen-bowl centrifuge is the vibrating-basket type that is fre- quently being installed in new plants. This centrifuge has either a vertical or horizontal basket that vibrates in such a manner as to cause the solids to move through the machine. This vibration tends to loosen the bed of particles so that they are free-draining and only mod- erate force is required to effect thor- ough dewatering. Because of the low speed generally used in these centri- fuges, the moisture content of the solids is usually higher than that produced by the transport-type unit; however, wear and horsepower are low and solids degradation is minimal. The principal difference between the horizontal and Feea Rotating screen bowl Rotating cone with spiral flights Liquid discharge Screen bowl support arms Solids discharge Gear drive mechanism FIGURE 13, - Cross section of screen-bowl centrifuge (5), vertical screen-bowl types is that the horizontal axis machine requires less headroom than the vertical device ( 18 , pp. 12-16 to 12-19), Wilson and Miller also conducted tests on screen-bowl units. They found that the positive-discharge screen-bowl centrifuge provided 4 to 6 pet lower moisture in the solids product than the solid-bowl cen- trifuge. Thus, for similar sized centri- fuges , the screen bowl is preferable to the solid bowl for this particular in- stance. As with the solid-bowl centri- fuge, they found that the screen-bowl centrate imist be bled out of the plant to a pond or backed up by a secondary recov- ery system. Because of the recovery re- quirement, this type of centrifuge should operate for maximum moisture reduction as opposed to maximum effluent clarity to fully benefit from this costly system (36). Further test results indicated that on- ly minor moisture reduction was obtained by steam heating the feed, so heating was not recommended for this type of centri- fuge. Surface-tension-reducing chemicals and a flocculant were also tested; how- ever, both approaches were ineffective in reducing moisture (36). 18 DISK CENTRIFUGES The last type of centrifuge to be dis- cussed is the disk centrifuge. Its con- struction is similar to that of the ver- tical screen-bowl unit; however, instead of using just one cone, it uses multi- ple cones for dewatering (fig. 14). The basic idea of increasing the settling capacity by using a number of disks in parallel is the same as the multiple- plate principle in gravity sedimentation (312, p. 141). The slurry enters the unit at the top, and the solids are forced to the outer circumference by centrifugal action, then removed through the outer rim. The clar- ified water is channeled upward through passages between the disks. These cen- trifuges can handle inflows of up to 3,300 gal/min containing low-density par- ticles up to 0.1 in in diameter and con- centrations to about 1 pet solids. The output can be up to 6 pet solids , or even up to 10 pet solids if chemical additives are used (24). Disk centrifuges are operated at speeds up to 12,000 rev/min, depending on the bowl diameter. The bowls usually have Feed rp-^ Liquid Rotating disks Solids Rotating screen bowl FIGURE 14, - Cross section of disk centrifuge (32). equal dimensions of height and diameter for optimum capacities, and the angle of the cones is usually between 35° and 50°, which is large enough to facilitate the sliding of the particles on the disk sur- faces ( 32 , p. 143). There are several variations in this design, which include recirculating the solids discharge, a facility for wash- ing before discharge, and a paring tube for pressurized solids discharge ( 32 , pp. 143-145). Disk centrifuges are very effective for dewatering fine-grained solids and are often used for dewatering kaolin clay (22, p. 144). CENTRIFUGE PERFORMANCE The performance and efficiency of a centrifuge depend on a number of factors. The more important factors are — 1. Centrifuge rotation speed. 2. Diameter. 3. Length. 4. Beach angle and length (for hori- zontal centrifuges) . 5. Feed point of slurry. 6. 7. 8. Feed point of flocculants. Scroll rotation speed differential and pitch (for positive-discharge units) . Pool depth (for (11, p. 4-3). solid-bowl units) Increasing the bowl rotation speed usually increases the solids recovery. There may be an increase in the solids cake concentration; however, the increase of the fines in the cake tends to in- crease the cake moisture. Higher speeds also increase both maintenance and op- erating costs of the centrifuge ( 11 , p. 4-3). 19 Centrifuge-bowl diameters generally range from 6 to 50 in, and bowl lengths are generally from two to four times the bowl diameter. Bowl speed is normally a function of the bowl diameter because the effects of speed and diameter determine the resulting centrifugal force acting on the slurry. Typical values for cen- trifugal force range from 1,000 to 4,000 times the force of gravity; the higher centrifugal forces are associated with the smaller bowl diameters ( 11 , p. 4-3). The bowl length will affect the cen- trate clarity, A longer bowl increases the residence time of the slurry as it travels from one end of the bowl to the other. This increased time allows the finer particles sufficient time to sep- arate from the liquid (2^, p. 11-35). In solid-bowl units, the beach angle and beach length will affect both the percent solids in the final cake and the torque needed to move the solids out of the centrifuge at a constant scroll dif- ferential speed. The longer the cake is allowed to dewater on the beach, the higher the cake solid concentration, and the higher the torque requirements for discharge ( 11 , p. 4-4) , The feed entry point into the bowl will influence the percent solids in the cake and the solids recovery. For solid- bowl centrifuges , recovery will be im- proved if the feed entry point is near the beach because the slurry particles have a longer distance to migrate to the end of the machine where the liquid exits (JJ., p, 4-4), Chemicals may be added to the slurry to accelerate the flocculation of fine par- ticles that do not immediately separate from the liquid. The high degree of agi- tation and mixing within the centrifuge generally necessitates large polymer doses to effect an increase in the solids recovery. In one instance, a dosage range of 0,1 to 0,3 lb polymer per ton of solids increased the capacity of a cen- trifuge 5 to 10 pet at the same solids recovery (11, p, 4-9), In positive-discharge units , changes in the scroll rotation speed and pitch will affect the solids recovery and cake dry- ness. Small residence times allow only the heavier and larger solids to be re- moved, while the finer solids remain sus- pended in the liquid. This factor will be particularly important when dewatering slimes. Most solid-bowl centrifuges are provid- ed with an adjustable pool depth setting so that the liquid level in the bowl can be changed after Installation. An in- crease in the pool depth will improve the percentage of the solids that are sep- arated from the liquid. This improvement in solids recovery is due to an increased residence time and a reduction in the agitation within the centrifuge at deeper pool depths; however, deeper ponds will increase the amount of moisture in the cake because of the reduced dewatering time on the beach (11, p, 4-4), ADVANTAGES AND DISADVANTAGES OF CENTRIFUGES A main advantage of the centrifuge is its operational flexibility. Within its design limits, a centrifuge can be fed a slurry at various rates and still provide a consistent solids product. If the feed rate exceeds the design limits of the unit, the excess solids appear in the liquid; however, the quality of the dis- charged cake does not deteriorate and the percent solids remains relatively con- stant (JJ_, p. 4-4), A disadvantage of the centrifuge is abrasive wear on the scroll and other interior parts , which results in high maintenance costs. In recent years, scrolls have been manufactured with im- proved materials such as tungsten carbide on the wearing surfaces. Operational results indicate an order of magnitude improvement in the life of these com- ponents (3; JJ_, p, 4-4), Another draw- back of the centrifuge is that the feed slurry may need to be prethickened. Although centrifuges can dewater a wide range of slurries , a very low feed solids 20 content means that the centrifuge must process large volumes of slurry input. The number of centrifuges needed in a de- watering circuit is directly proportional to the volume of feed slurry to be pro- cessed. Consequently, the slurry should be prethickened by sedimentation, hydro- cycloning, or other means prior to cen- trifugation. The final circuit configu- ration will be the result of compromising the performance and economy of the pre- thickening and centrifuge equipment ( 11 , p. 4-7). SIZING CENTRIFUGES When centrifuging a slurry, it is not necessarily true that an increased force will decrease the moisture content of the product. Materials that deform, break, or degrade will not be dewatered proportionally to the applied forces. It should also be realized that horse- power, wear, maintenance, and degrada- tion will accelerate with increased forces applied in the machine. While a large centrifugal force developed by a machine may be an indication that it is sturdily built, this force should not be the only criterion used in selecting a centrifuge to dewater any material ( 18 , p. 12-15). For any particle traveling in a circu- lar motion about a point, the centrifugal acceleration is Ac =^, (12) Centrifugal acceleration is then ex- pressed as multiples of the gravitational acceleration: and Ac ^ Vp2 G GR^' (13) p ^ Vp2^ (2TrRcN)2 ^ Att^R^N^ ^ GRc GRc G * where Vp = 2 RcN, feet per second, N = the number of revolutions per second, TT = 3.1416, Fc = the centrifugal acceleration expressed as multiples of gravitational acceleration, dimensionless , and G = the gravitational accelera- tion, feet per second per second (J^, p. 12-14). SIGMA CONCEPT The "sigma concept" has been widely used in the field of centrifugal sedi- mentation for the last 30 yr. It is a simplified representation of machine performance in terms of the particle size, the total volumetric rate, and an index of the centrifuge size. The sigma concept characterizes a centrifuge's ability to separate solids from liquids and is widely used in industry ( 32 , p. 130). where Ac = centrifugal acceleration, feet per second per second. Vp = the linear peripheral ve- locity, feet per second, and Re = the radius of curvature, feet (18, p. 12-14). The volumetric throughput of a centri- fuge can be expressed as — Qc = 2 V. 27rL3zf(|Rcb2 +^Rsz2)], (15) where Qc = the volumetric throughput, cubic feet per second. 21 Vs = the terminal settling veloc- ity of the solids, feet per second, 0) = the angular velocity of the solids, radians per second, G = the acceleration of gravity, feet per second per second, jsz ~ the length of the settling zone, feet. because different total efficiencies can be obtained for a given cut size, depend- ing on the size distribution of the sol- ids. The best method of describing the performance of a centrifuge is by using the grade efficiency curve, which is briefly discussed in appendix D. This requires many tests, together with deeper theoretical considerations, but the re- sults will provide more meaningful and reliable predictions of total efficien- cies for different slurries (32, p. 133). and Rsz = the radial length from the rotational axis to the set- tling zone surface, feet. s-cb the radial length from the rotational axis to the cen- trifuge bowl surface, feet (11, p. 4-5). This equation is composed of two parts. The first component, Vg , describes the settling characteristics of the solids. The remainder of the equation represents the machine variables that effect separa- tion efficiency, such that — 2 = 2itLc R sz ')' (16) where Z = the sigma value for a par- ticular centrifuge configu- ration (11, p. 4-5). PILOT-PLANT TESTING The centrifuge evaluation procedure, generally used in pilot testing, relates the percent of solids recovered and cake solids concentration to the feed rate at various operating conditions. The recovery of feed solids in the centri- fuge cake is determined from the mea- surements of flow rates and concentra- tions entering and leaving the centri- fuge. By combining the material balance equations and the definition of recov- ery, the following simplified equation relates the recovery to various solids concentrations : R sc _ "sc (Wss - Wsi) where R Wss (W sc Wsl)* (17) sc = the percent recovery of solids in the cake, Equation 16 is the basic expression of the sigma concept, which gives an esti- mate of the maximum flow rate that will allow solids of a particular size to sep- arate from the liquid. Sigma is a con- stant containing factors pertaining only to the centrifuge and can be thought of as the theoretical capacity factor. It is expressed in terms of area and facili- tates comparison between the performances of geometrically and hydrodynamically similar centrifuges processing the same slurry (32, p. 131). A shortcoming of the sigma concept is that the cut size or smallest particle separated from the liquid is not suitable as a criterion for separation efficiency Wsc = the weight percent of sol- ids in the cake, = the weight percent of sol- ids in the centrate. W s I and Wgs = the weight percent of sol- ids in the fed slurry ( 11 , pp. 4-6 to 4-7). This equation simplifies the testing pro- cedure so that the solids recovery can be calculated from concentration measure- ments made on the three process streams entering or leaving the centrifuge. Thus, flow rate measurements need not be made to calculate the recovery of solids in the cake (11, p. 4-7). 22 FILTRATION EQUIPMENT BACKGROUND Filtration is a process where solids are separated from liquids by passing a slurry through a permeable filter medium that retains the solids. To cause the fluid to flow through the filter medium, a pressure drop has to be applied. The pressure drop can be achieved by various means, including gravity, vacuum, or di- rect pressure (32, p. 171). If the liq- uid is induced to flow through the medium by hydrostatic head, it is called gravity filtration. If higher than atmospheric pressure is applied upstream from the filter, the process is called pressure filtration; if lower than atmospheric pressure is applied downstream from the filter, it is referred to as vacuum fil- tration (20, p. 1473). There are two types of filtration sys- tems available: surface filters and depth filters. Surface filters are used for cake filtration, where the solids are deposited in the form of a cake on the surface of a relatively thin filter medium. Depth filters are used for deep- bed filtration, where particle deposi- tion takes place within the medium (32, p. 173). Surface filtration is the pro- cess in common use by the minerals indus- try and is the only one discussed in this report. At the commencement of surface filtra- tion, the slurry particles that are the same size as or larger than the openings of the filter medium are held at these openings and create smaller passages , which remove even smaller particles from the slurry. A filter cake is formed which, in turn, functions as a more effi- cient filter for the subsequent filtra- tion (32, p. 172). filter equipment is usually expressed as the yield in pounds of dry solids per square foot of filter area per hour (24). The filter medium is probably the sin- gle most important component of a filter that affects equipment performance. Many different materials are now available as filter media. Filter cloth can be com- posed of duck, chain, and twill weaves, as well as felt made of cotton, ny- lon, polyester, polypropylene, and other natural or synthetic materials. Filter cloths and screens made of steel, stain- less steel, and other alloys are also on the market (29). Filter paper is often required as the filter medium to obtain high solids retention. Filter paper requires the support of filter cloth, screens, or per- forated metal sheets to prevent its breaking. The filter paper can be easily removed from the support filter medium (29). Covering the surface of the filter cloth or filter paper with a filter aid is known as a precoat and is often re- quired to prevent blinding or clogging the filter medium. Materials such as di- atomaceous earth, paper pulp, or perlite can be very effective. A filter aid can also be added to the slurry as a body feed to produce a filter cake that re- mains relatively permeable during the filtration cycle and gives a good overall filtration rate (29). Various combina- tions of filter cloth, filter paper, and precoating are capable of filtering out particles as small as 0.0002 in in diame- ter (3a). The following sections discuss the fil- tration equipment commonly used today. In practice, solids are deposited on the filter medium as a cake, which is re- moved from the filter medium by a mechan- ical method such as scraping. After the cake is removed, the filter is cleaned by spray washing, then put into position to receive more solids. The performance of FILTER PRESSES Filter presses are batch units that use higher than atmospheric pressures and a series of filter plates to separate the solids from the liquid. They are also called pressure, plate-and-f rame, or 23 press filters. Although this type of de- vice is well known, it has had little at- tention or application in the U.S. miner- als industry because of its batch process nature, high capital cost, and associated high labor requirements. In spite of these drawbacks, the filter press should receive close attention because of its separation efficiency (24). There are two types of filter press systems available — the high-pressure sys- tem and the low-pressure system. The first type uses a pressure of up to 225 Ib/in^ , while the second type uses a max- imum pressure of 100 Ib/in^ . Research has shown that the higher filter pressure does not provide a significant benefit over the lower pressure unit with respect to dryer filter cakes or shorter cycles on chemically conditioned slurries ( 22 ) . Pressure filtration is a fairly simple process. Initially, the slurry is chemr- ically treated and then pumped into the press (fig. 15). The filtrate flow from a large press can be 10,000 to 15,000 gal/h, so that this part of the cycle is often called the fast fill. During this phase, the cake chambers of the filter press collect the major amount of slurry solids. As the chambers become progres- sively filled with solids, the pressure inside the press rises and the filtrate flow rapidly decreases. This portion of the cycle is the cake consolidation Filtrate outlet Feed Inlet Filtrate channels Filter medium Filter cake Recessed plates FIGURE 15, - Cross section of recessed-plate filter press (9, 22). phase, where slurry solids are being forced under pressure into the cake cham- bers. This compacting action displaces more water from the loosely formed slurry cake and enables the press to produce harder and drier cakes than other means of dewatering (6^, 22, 24). At a predetermined low-flow condition, pumping is stopped and the feed holes that carried the unconsolidated sludge into the press are blown clear. The stack of filter plates is undamped, and the individual plates are mechanically separated, allowing the filter cake be- tween adjacent plates to drop out of the chambers. The cakes then fall against breaker bars that reduce them to a con- venient size before they are transported for final disposal (6^, 9^). Although pressure filtration is a batch process, a series of presses can be or- ganized to work in a semi continuous oper- ation. This is done by having one press being filled, while a second is being airblown and a third is being unloaded or waiting to be filled again ( 25 ) . Filter presses find wide application in the process industries for the separation of slow-settling solids from liquids when the solids content is 1 to 10 pet and the f ilterability is poor. The use of these presses depends mainly on the particle size and on the quantity of solids in the slurry feed (22, p. 211). The capacity of the press is dependent on such factors as the characteristics of the material being filtered, operating temperature, pressure, filter medium being used, and type and size of filter press. There is no mathematical formula or scientific method for determining the exact filtra- tion capabilities or rates. The most re- liable method is to use past experience or records and to make controlled tests on laboratory-sized equipment (29). ADVANTAGES AND DISADVANTAGES OF FILTER PRESSES Filter presses are efficient at de- watering very fine grained solids such as slimes and consume only moderate amounts 24 of power during their operation. Labor requirements are high, however, and the batch operation may be a limitation In some facilities. Chemical pre treatment Is frequently needed to obtain the neces- sary solids capture. Also, large space requirements and the possible need to treat the decant liquors may restrict use (24) . Despite these problems , a well-de- slgned system of multiple filter presses might be the answer to difficult slime dewatering problems. CONTINUOUS PRESSURE FILTERS Although they are not well known, some pressure filters are available that work continuously. These units operate simi- larly to a drum or disk vacuum filter, except that the entire mechanism is en- closed in a pressurized tank. The cake is removed from the filter medium by a blade under superatmospherlc pressure. The dislodged cake is throttled out of the tank to atmospheric pressure by a self-sealing screw conveyor or a series of receivers and pressure locks ( 23 , p. 19-75). The operating pressure in these ma- chines is normally 100 Ib/ln^ with a pressure drop of 40 Ib/in^ across the filter medium. Filter areas from 4 to 700 ft^ are available. The cost of con- tinuous pressure filters can be two to four times that of a vacuum filtration unit having the same filter area ( 23 , p. 19-75). Continuous pressure filters are advan- tageous because they work continuously and do not require close operator super- vision. These units are also capable of much higher filtration rates for low- compressibility solids than are vacuum filters. There are drawbacks, too, be- cause these machines are mechanically complex, do not allow access for any maintenance during operation, and have problems with lubrication. As mentioned before, these units are more expensive than similarly sized vacuum filters (23, p. 19-75). BELT FILTER PRESSES Belt filter presses have been manufac- tured in Europe since the mldsixtles and have been used mostly in pulp or paper plants and for sewage sludge dewatering. Only in the last several years have these machines been used by the minerals indus- try. Experimental work with the belt press indicates that this machine pro- duces a drier cake with lower polymer consumption than centrifuges or similar dewatering devices. The belt press cake also has a relatively high shear strength, which is important if the cake Is to be transported to a waste dump (28). The belt filtration process is composed of three operational stages: chemical conditioning of the feed slurry, gravity drainage to a nonfluid consistency, and compaction of the dewatered solids. Fig- ure 16 depicts a simple belt press and shows the location of the three stages. Although present-day belt presses may be very complex pieces of equipment, they follow the same concepts indicated in this figure. Good chemical conditioning is of primary Importance to successful and consistent dewatering by the belt filter press. The recent developments in high-grade polymeric reagents have encouraged the development of belt press- es to their present performance levels (19). Upper belt wash Chemically treated feed Drive rollers Dry cake FIGURE 16, - Cross section of belt filter press (28), 25 The first stage in belt press dewater- ing is to add a flocculating agent to the slurry in a conditioning zone (12). The resultant mixture must be allowed to set under low shear conditions to allow floc- culation to take place. If this mixture is disturbed during f locculation, more chemical agents may be needed to obtain the necessary degree of flocculation and any economic advantage will be lost. The conditioning zone should have an adjusta- ble residence time which can be increased or decreased to suit the slurry being treated. Correct conditioning liberates the free water originally contained in the slurry and also facilitates the cap- ture of the finest particles present by causing them to flocculate with the larger particles. This is necessary since water removal will be carried out on belts whose openings are larger than the smallest slurry particles (19). The next stage after chemical condi- tioning is gravity dewatering, which al- lows the free water to drain from the solid particles. The slope of the belt will normally be set at 5° to 15° from horizontal to assure proper distribution of the solids across the belt as the wa- ter is separated from the slurry. At the end of the drainage section, the slurry will lose about 50 pet of its free water. At this time, the formation of an even surface cake is essential for the suc- cessful completion of the following stage in the cycle. The even surface prevents uneven belt tension or distortion, and the rigidity of the semicake enhances distribution through the process (19) . Following the gravity dewatering stage, the semicake is sandwiched between the carrying belt below and the covering belt above. The semicake is squeezed between the two belts and subjected to flexing in opposite directions as it passes around various rollers. This action in- creases water release and allows greater compaction of the cake (19). At least one manufacturer has developed a high- performance belt press that has a low and a high belt pressure stage. This machine is basically the same as other belt presses except that in the high-pressure stage, an additional belt is added to in- crease the pressure on the solids cake and remove more water ( 12 ) . Finally, the dried cake passes from be- tween the two belts and is carried away from the machine by truck or conveyor belt for ultimate disposal (19). The material and construction of the filter belts themselves are very impor- tant. Belt filter presses do not utilize the belt fabric as a filter medium but as a support for the self -filtering solid- liquid mass. The mesh size of the belt fabric is not critical because very large floes will be produced by the polyelec- trolytes in the first stage of operation. Texture, however, is a significant fea- ture of the belts because it influences the adhesion of the cake to the belt and the physical strength of the belt. Cur- rently, the available belts are those used in the papermaking industry, al- though a wider variety of special-purpose woven belts are becoming available. Most belts are woven from polyester monofila- ment, and many weaving patterns are available (19), A major problem with the belt press is that an uneven layer of sludge causes the belt to deviate from the desired tracking line. This occurs because the filter belts usually have short lengths and widths. Direct side pressure should not be used because guide rollers would cause serious wear on the edges of the belt. One system that has been devised uses a delicately balanced sensor flap which rests lightly on the edge of the belt. This flap is attached to the spindle of a rotary directional fluid-flow control valve. Displacement of this valve causes hydraulic fluid to flow to either one or the other end of a cylinder which sup- ports one end of the belt roller. The cylinder's movement causes the roller to slew across the line of travel of the belt, and belt tracking is induced rather than forced (19), 26 The openings in the belt are easily plugged by the solids particles so the belt must be washed continuously. The cont£imination of these perforations de- pends on the belt tension, pressure on the cake, complexity of belt weave, type of sludge being treated, and quantity and type of polyelectrolyte being used. Washing is done using water; however, the volume and pressure used vary from manu- facturer to manufacturer. The current trend is to recycle the filtrate as belt washing water. This is satisfactory if the spray system is well designed to al- low cleaning in the event of a blockage. At low solids concentrations the volume of filtrate produced will be adequate for belt washing. ADVANTAGES AND DISADVANTAGES OF BELT FILTER PRESSES Belt filter presses have several advan- tages, which include — 1. Continuous operation. 2. Low power consumption, as low as 3.7 kW according to one manufacturer (12). 3. Low consumption of chemical addi- tives; one manufacturer claims his unit uses only 25 to 50 pet that used by cen- trifuges for some applications (12). The disadvantages of belt filter press- es include — 1. Problems with proper belt tracking (19). 2. Shortened belt life if very coarse solids are dewatered. 3. Difficulty in dewatering slurries having a very low solids content ( 28 ) . VACUUM FILTRATION EQUIPMENT Vacuum filtration equipment has been in use in this country for over 100 yr, and continuous vacuum machines represent the most prevalent type of filtration equip- ment seen in the mineral industry today. Their simple operation and need for a minimum of auxiliary equipment has made them popular for solid-liquid separation in a variety of industrial applications. Continuous vacuum filters separate the solids and liquid in a slurry by placing the filter in the slurry and applying suction behind the filter so that the water and solids are drawn to it. The solids are collected on the filtration surface, while the water is drawn through the filter and separated from the solids. The particles accumulate to form a cake on the filtering surface that is gradual- ly lifted from the slurry. The cake is removed from the filter by various means, and the filter is returned to the slurry to repeat the process ( 18 , p. 12-51). Vacuum filters must have several items of auxiliary equipment in order to func- tion properly. An agitator is needed to keep the slurry solids in suspension un- til they are drawn to the filter medium ( 32 , p. 226). A vacuum pump provides the subatmospheric pressure to draw the liq- uid through the filter. A receiver with a barometric leg is located between the filter and the vacuum pump and separates the liquid from air drawn through the fi- lter. Last, a filtrate pump removes the liquid from the receiver and forces it to the disposal point or back into the circuit for reuse ( 19 ) . The following sections describe the filtration equipment found in the min- erals industry. DRUM VACUUM FILTERS Drum vacuum filters are probably the most common type of continuous vacuum filters in use. They consist of a hori- zontal cylinder or drum whose circumfer- ence holds the filter medium. The drum rotates in a slurry tank, and a vacuum inside the drum draws the liquid through the unit, leaving the solids captured on the filter surface. Scrapers or other devices then remove the cake from the drum, and the medium reenters the tank, enabling operation to be continuous ( 24 ) . Figure 17 shows the front and side views of a typical drum vacuum filter. 27 SIDE VIEW FRONT VIEW Flltsr cake FIGURE 17» - Front and side views of drum vacuum filter (20). When a precoat method is used, an advancing-knife mechanism must be pre- cisely positioned to dislodge the cake and a minimum amount of the precoat ma- terial from the drum surface. This is an exacting procedure because the knife- advance system must not move more than several thousandths of an inch. The knife system has to be constructed so that its linear integrity, with respect to the drum face, is absolute and any de- viations from the correct distance must be extremely small (20, p. 1477). Vacuum drums can handle throughputs up to 250 gal/min, particles as small as 0.0020 in, and solids loadings between 8 and 10 pet. The average yield is between 2 and 10 lb of dry solids per square foot per hour (24) . Drum vacuum filters have been widely used in industry because of their adapta- bility with respect to filter media and cake discharge methods. A variety of filter media can be easily used with a drum filter because the material itself is attached around the circumference of the cylinder. This configuration simpli- fies installation, inspection, mainte- nance, and removal of the filter medium. (24). CAKE DISCHARGE METHODS In addition to the variety of filter media available, there are many methods of removing the cake from the filter sur- face of a drum vacuum filter. The scraper-blade method of discharge is com- monly used where the cake is friable and has poor mechanical qualities. A cake that is fairly thick and is not strongly held to the filter cloth could be easily dislodged by a blade. This blade could be steel, rubber coated, or made of other materials compatible with the par- ticular corrosive or abrasive qualities of the filtrate or the solids. In many cases, a pressure reversal or blow is sufficient to loosen the cake from the filter medium, and the blade itself only guides the loose cake to the discharge chute (20, p. 1476). A roller discharge device may be used where a thin cake of sticky or thixotro- pic material is formed on the drum. This system uses a roller positioned close to the drum filter at the point of cake dis- charge. The cake is transferred from the filter to the roller and then removed from the roller by a cutting blade ( 20 , p. 1476). The belt discharge arrangement uses a filter cloth that winds around the drum to form the cake but leaves the drum to carry the solids to the dis- charged area. The cloth or belt passes over a small-diameter roller which causes the cake to separate from the filter. This system can be used for a cake hav- ing mechanical strength, for a thin cake, or where intensive washing of the filter is necessary to maintain the cloth openings. After the belt has passed over the discharge roll, it re- turns to the drum through a belt-washing system (20, p. 1476). Some of the vari- ous discharge methods are illustrated in figure 18. EQUIPMENT MODIFICATIONS Drum vacuum filters have been modified to separate solids and liquids under ad- verse conditions. For example, these ma- chines can be fitted with simple hoods which limit the escape of poisonous or foul-smelling vapors. They can be adapt- ed for complete sealing and for operation in a nitrogen environment; however, this complicates access to the internal parts , 28 Compressed air line Vacuum lines KNIFE DISCHARGE Atmospheric port Vacuum lines ROLLER DISCHARGE Atmospheric port Vacuum lines BELT DISCHARGE Vacuum lines PRECOAT Slurry FIGURE 18, - Schematic drawings of various dis- charge methods for drum vacuum filters (20), and the drum design must be such that the vacuum system and the cake receiving sys- tem are isolated to prevent vapor loss (20, p. 1481). The drum vacuum filter can also be mod- ified with accessories that improve the quality of the cake with respect to its washing and drying characteristics. This is possible because the cake moves through the washing and drying zones in the form of a continuous sheet and be- cause the cake and filter medium are ade- quately supported on the drum shell. The filters can be fitted with simple rollers which extend the full width of the filter drum and can be arranged to eliminate irregularities or cracks in the cake prior to washing and drying. The wash waters and air are therefore applied to a uniform surface and will not short cir- cuit or channel the deposited solids. The cake compression system may also have a "wash blanket" draped over the cake, which further limits any tendency for the air or wash water to channel the solids. These blankets allow the wash water to be used near the point where the cake emerges from the slurry ( 20 , p. 1481). Drum vacuum filters can also be de- signed so that the trough and hood are thermally insulated. If needed, the unit can be equipped with heat exchange equip- ment for heating or cooling the cake ( 20 , p. 1481). The basic design of the drum vacuum filter has undergone little change; how- ever, with the development of new and improved construction materials, many options have become available. Drums can now be fabricated in a variety of metals, plastics, and rubber for hand- ling corrosive materials. Internal pip- ing and valves have also been improved by the use of corrosion- and abrasion- resistant materials, which greatly reduce shutdowns for repair of leaks or loss of vacuum. Tanks and agitators can also be fabricated of special materials, and tank linings have greatly reduced wear (20 , p. 1475). PERFORMANCE For any given feed condition, the per- formance of drum vacuum filters can be optimized by the drum speed, the vacuum, and the percentage of drum surface sub- merged in the feed slurry. Most drum filters have controls for the manual ad- justment of these variables, and some models have automatic adjustments which are actuated by changes in the quality and quantity of the feed or cake ( 20 , p. 1480). A drum filter should be operated with the greatest degree of submergence and at the highest drum speed in order to maxi- mize the throughput of solids. It should be remembered that any increase in the submergence reduces the proportion of the drum area available for washing and dry- ing. Drum submergence above 40 pet ne- cessitates the use of seals on the drum shaft where it passes through the trough. No matter what combinations of drum speed, vacuum, and submergence are used, the sum effect must produce a cake that can be completely and easily removed from the drum. If this cannot be achieved, the filter medium will quickly deterio- rate and its life will be severely short- ened (20, pp. 1480-1481). 29 ADVANTAGES AND DISADVANTAGES OF DRUM VACUUM FILTERS The advantages of the rotary drum vac- uiua filter are — 1. Continuous operation, which results in low operating labor costs. 2. Many design and operational varia- tions available for a wide range of sus- pensions of divergent nature. 3. Clean operation. 4. Low maintenance costs. 5. Effective washing and dewatering (32, p. 226). 6. Provides a filtrate with a low sus- pended solids concentration. 7. Does not require skilled personnel, 8. Has low maintenance requirements for continuously operating equipment (19). The disadvantages of the rotary drum vacuum filter are — 1. High capital cost. 2. Limitations imposed by vapor pres- sure of hot or volatile liquids. 3. Incapable of handling products that form explosive or inflammable gases under vacuum. 4. Unsuitable slurries. for quick-settling 5. Tendency for cloth blinding due to thin cakes and short cycles , although this may be alleviated by the applica- tion of a belt or string discharge ( 32 , p. 227). 6. Auxiliary equipment such as vacuum pumps is very loud. 7. Consumes the largest amount of en- ergy per unit of slurry dewatered, in most applications. 8. Produces wetter cakes if blowback is used, and greater filter medium wear if blowback is used in conjunction with a scraper knife (32^, p. 227). ROTARY DISK VACUUM FILTERS The principle of operation for the rotary disk vacuum filter is the same as that for the rotary drum filter. The disks are oriented in a vertical plane and are composed of several pie-shaped sectors which fit into a central pipe for support of the disks and for transport of the filtrate. These sectors can be re- moved without disturbing the others in the same disk, and at slow speeds, it is possible to change a sector while the filter is still in operation. The filter medium cloth can be slipped over each sector and fastened at the innermost end of each sector ( 20 ) . As the disks ro- tate, they go through pickup and dewater- ing operations similar to those carried out on the drum filters. At the dis- charge point , the cake is removed by means of wires or knives. Figure 19 shows two simplified views of a disk filter. Disk vacuum filters are avail- able with areas from 0.5 ft^ to approxi- mately 3,300 ft^; for large areas, as many as 12 disks are used in a single unit (32, p. 229). Owing to the vertical disk orientation, the wide variety of discharge methods used on drum filters cannot be applied to disk filters. Thus, their application is somewhat more limited than that of the drum filters, but a disk filter will oc- cupy only one-third the floorspace of a drum filter having the same filter medium area and is less expensive ( 20 , p. 1477). Disk filter provide more efficient agi- tation than drum filter agitators. In some applications a disk filter may be more cost efficient (18, p. 12-63). 30 SIDE VIEW FRONT VIEW Filter cakes Compressed air port Vacuum Scraper blades Slurry level Filter surface Filter sector FIGURE 19, - Front and side views of disk vacuum filter (13). ADVANTAGES AND DISADVANTAGES OF ROTARY DISK VACUUM FILTERS The advantages of using a rotary disk vacuum filter are — the liquid in the slurry feed passes through a filter, leaving the solids de- posited on top of the media. These units can be characterized as flat-bed, high- capacity filters which lend themselves to granular, fast-filtering materials and high-specific-gravity concentrates ( 20 , p. 1478). ROTARY TABLE VACUUM FILTERS The rotary table units have a circular shape, the filter medium and supports ro- tate about a central axis. The feed slurry is deposited along the radius of the unit and rotates while it is being subjected to vacuum dewatering, washing, and drying, finally being removed from the filter medium by mechanical means (20, p, 1478) , Figure 20 shows a cross section of a typical horizontal table filter. 1, Low capital costs per unit area. 2, Large filter areas with minimum f loorspace. 3, Rapid medivim replacement ( 32 , p. 229). The disadvantages of the rotary disk vacuum filtration system are — 1, Difficulties in washing the cake. 2, Difficulties in discharging very thin cakes, 3, Inflexible operation, 4, High rate of medium wear with a scraper discharge, 5, Unsuitability for noncoherent cakes (32, p, 229). HORIZONTAL CONTINUOUS VACUUM FILTERS These vacuum filters use a horizontal filter surface in the form of a table, a belt, or multiple pans in a circular ar- rangement. The operating principle of the horizontal filter is the same as that of the rotary drum or disk filter, where The rotary table machines permit a choice of cake thickness, washing time, and drying cycle where 4- to 5-in-thick cakes can often be handled. Sharp sep- arations between countercurrent wash waters are also possible because of the horizontal drainage configuration. Ro- tary table filters, though used in indus- try, are better suited for dewatering free-draining solids and not sticky or thixotropic slimes because of the diffi- culties in removing the solids from the filter medium (20, p. 1478). Discharge zone TOP VIEW SIDE VIEW Wash bar Filter cake Filler mediun and table FIGURE 20. - Plan and cross-section views of hor- izontal rotary vacuum filter (20). 31 HORIZONTAL BELT VACUUM FILTERS The horizontal belt vacuum filter uses an endless belt of filter fabric support- ed by a slotted or perforated endless belt. Both belts travel over one or more vacuum zones. The slurry is deposited onto the filter at one end, wash water is applied at one or more points along the path of belt travel, and the cake is dumped at the other end. The support belt and the filter are parted and di- rected along separate lines of pulleys. The filter is washed and rejoins the sup- port belt just ahead of the slurry depo- sition point. Figure 21 shows a cross section of a typical horizontal belt vac- uum filter ( 15 ; 20, p, 1478). The horizontal belt machines have a high capacity per square foot of area un- der vacuum, similar to the horizontal ro- taries. They are well adapted for a countercurrent discharge circuit and en- able the cake to be flooded with wash solvent so that it can be steeped in the wash liquid. They are suited for coun- tercurrent leaching or washing ( 20 , p, 1478). Horizontal belt filters have been manu- factured for more than 30 yr, but recent- ly have they been used for large- tonnage applications that require filter cake washing. For example, the largest unit available in 1950 had only 40 ft^ of active filter area, while modern machines have over 900 ft^ of filter area. Many mechanical improvements have also been made on these machines , which have im- proved their reliability. The biggest single improvement, though, was the abil- ity to manufacture a continuous drainage belt to very close tolerances , which spurred the development of large-capacity units (20^, p, 1479), The horizontal filter shows several ad- vantages because the slurry deposition on the horizontal filter belt eliminates the constant slurry agitation necessary for rotary types. In the horizontal filter, the cake travels as a ribbon of unwashed cake which is gradually washed to the re- quired purity, either concurrently or countercurrently (19) , Another advantage of the horizontal machine is its ability to resist filter medium blinding. Most slurries contain, in addition to some medium-sized mate- rial, a fraction of fines. No matter how efficient the agitation with the rotary drum, these fines will always be concen- trated near the surface within the trough. Consequently, the fine material is sucked against the filter cloth of the rotary drum filter and causes blinding before the large particles reach the cloth. This does not happen on hori- zontal belt filters because the biggest particles reach the filter cloth first. Slurry-] Wash dam , a a m y r- Wash water Wash dam ^ Air box -^ Discharged liquid-* X Dewatered slurry *^o, scharged solids Filter cloth ^Carrier belt ^Filter cloth wash FIGURE 21. - Cross section of typical horizontal belt vacuum filter (]5, 20). 32 followed by the finer particles. As a result, the larger particles act as a precoat for the finer solids (19). Another concern with the rotary drum and belt filters involves maintaining a vacuum. Many cakes, after being de- watered, shrink and crack. This allows air to pass freely through the cake, which reduces the vacuum and the dewater- ing effect on the cake. On the rotary drum filter, this is remedied by using expensive blankets , rakes , or squeeze rollers. On the belt filter, a simple sheet of impervious material such as polyethylene trailed over the cake is usually adequate to maintain the maximum vacuum (19). ADVANTAGES AND DISADVANTAGES OF HORIZONTAL VACUUM FILTERS The advantages of the horizontal vacuum filters are — 1. Excellent wash capability. 2. Flexible operation. 3. High-volume operation for fast-set- tling solids (22, p. 230). The disadvantages of the horizontal vacuum filters are — 1. Requirements for large floorspace. 2. High initial costs. 3. Unsuitability for slow settling solids ( 32 , p. 230). SELECTING AND SIZING FILTRATION EQUIPMENT In the selection of filtration equip- ment, the job requirements must be com- pared to those associated with the equip- ment characteristics. Job-related fac- tors include slurry character, production magnitude, process conditions, performr- ance requirements, and permissible mate- rials of construction. The equipment- related factors are the type of cycle (batch or continuous) , driving force (gravity, pressure, or vacuum), produc- tion rates of largest and smarllest units, separation sharpness, washing capability, dependability, feasible materials of con- struction, and costs. This last item must include depreciation (installed cost plus expected equipment life) , mainte- nance, operating cost (labor, services, and filter media) , and penalty of product loss (if any). In addition, considera- tion must be given to preconditioning and the use of filter aids (20^, p. 1485). The suitability of the most common types of filters for various classes of slur- ries is summarized in table 1. Continuous filters are the most desira- ble when the process to which they con- tribute is a steady-level, continuous one; however, the rate at which the cake forms and the magnitude of production rate will probably be the critical fac- tors. For example, the use of a rotary vacuum filter is not practical if a 0.1- in-thick cake will not form under normal vacuum in less than 5 min and if more than 50 ft^/h of wet cake is to be pro- duced. The production use of batch fil- ters is harder to define, although they have been used in some processes that turn out 200 ton/d of dry solids. Occa- sionally, equipment flexibility and high filtering pressures will become more important than other factors that would otherwise dictate continuous equipment. Small-scale tests are essential for esti- mating the filtration rate, the washing characteristics, and other important fea- tures. Filtration is essentially an art rather than a science, and experience with the various aspects of vacuum fil- tration will help in better approaching the selection of equipment and evaluating test results (20, p. 1485). 33 TABLE 1. - Classification of selected vacuum filters (20, p. 1481) (Performance index: 1 = very poor or negligible; 9 = highest possible performance) Maximum area, ft2 Slurry Relative performance Type classification' Cake dryness Cake washing Filtrate A B C D E clarity Horizontal belt^ ••••••. 900 160 860 860 860 860 3,230 X X X X X X X X X X X X X X X 5-8 4-7 5-8 5-6 5-8 NA 2-3 7-8 8-9 6 5 6 6 1 6 Horizontal rotary table^ Rotary drum: Knife discharge^ •••••••••••••• 7 8 Roller discharge^ ....•...•.•.. 8 Belt discharge^ «•••••••••••••• 7 Precoat^ .••■•.••.•■...••...••. 9 Rotary disk^ 6 NA Not available. 'a — High solids concentration (>20 pet), free draining, fast settling, high filtra- tion rates. B — Rapid cake formation, reasonably fast settling solids. C — Lower solids concentration, slow thin cake formation, difficult to discharge. D — Low solids concentration, slow cake formation, very poor strength properties. E — Very low solids concentration, solids usually blind normal filter media. 2 For free-draining materials where good washing is necessary. ^Wide range of types and sizes. Generally suitable for most slurries of types B and/or C. Can be fitted with various devices to improve washing and cake drying. ^Suitable for slurries that blind most filter media. ^Large throughput for small floorspace. LABORATORY TESTING Extensive research has been done to de- velop laboratory procedures for determin- ing the filterability of slurries. The filter-leaf test is the commonly used method of estimating the necessary filter area for a particular slurry. In this method, a test leaf is used which is cov- ered with a filter medixim identical to that intended for the full-scale filter. Figure 22 shows the typical apparatus Ring stand with platform to hold test leaf while drying Air bleed to reduce vacuum at leaf To vacuum source \ Sh utoff valve —^ i= Timer Rubber hose Stirring rod Gage reading in cubic feet per minute to measure air velocity during dry time Zl Filtrate flask, Iqt or larger Wash liquor ' — Slurry container, if required in 2-qt capacity graduated beaker FIGURE 22. - Typical laboratory installation for vacuum leaf tests (20). 34 needed for a leaf test, for the test follows: The procedure 1, Condition 2 qt of sludge for fil- tration. The sludge should be thickened to the same concentration as the produc- tion slurry. operation. The test results will provide filtration parameters for the cake forma- tion, drying, and washing portions of the filtration cycle. The filter-leaf test is easy to perform; however, several pre- cautions should be observed to assure accurate results: 2, Apply the desired vacuum to the filter leaf and immerse in the sample for 1-1/2 min while maintaining sample agita- tion. The test leaf is usually inserted upside-down in the slurry to simulate the cake formation zone of a drum filter. 1, Representative should be used. slurry samples 2. The test should be repeated 5 to 10 times to observe any filter medixim blinding. 3, Bring the leaf to a vertical posi- tion and allow it to dry under vacuum for 3 min. This simulates the cake draining and drying part of the cycle, 4, Blow off the cake for 1-1/2 min, which gives a total drum cycle of 6 min. To discharge the cake, the leaf is disconnected from the vacuum, and air pressure of not more than 2 Ib/in^ is applied, 5, Dry and weigh cake to determine percentage moisture, which can be com- puted from the equation — f _ Wds ■Lev ~ -C V At I (18) where fcv = the filter cake formation rate, pounds per square foot per hour, W(js = the dry weight of the sol- ids cake, pounds, tcv = the cycle time, hours. 3, The sample must be continually agi- tated to assure that it is homogeneous, 4, The vacuum must be regulated so that it does not vary during the test. The vacuum should be the same as that in- tended for use in the full-scale opera- tion U, p, 11-29), After the tests have been completed, the results can be analyzed. A graph of the moisture content of the filter-leaf test cakes versus a correlating factor should be constructed. The correlating factor is calculated from: — where Fw = V a V td ^(fe;) (19) Fv = the filter cake correlating factor, dimensionless , Vav = the volume of airflow through the cake per unit area of filtering surface, cubic feet per minute per square foot. and At I = the filter test leaf area, square feet (2^, p, 11-29), The test can be easily modified for other cycle times or discharge mechanisms. Filter leaves and testing instructions are available from most filter manufac- turers. It may be necessary to adjust the results obtained by a factor to coiiq)ensate for partial medium blinding and for scaling over a long period of and t(jv = the drying time, minutes, Pdc = the pressure differential, pounds per square inch, gage, Wdst = the weight of the dry cake solids for a given cake thickness , pounds per square foot (2^; JL8^, p. 11- 31). 35 A decreasing moisture correlation indi- cates that the moisture content de- creases; and as the air rate through the cake per unit of filtering area is in- creased, the vacuum differential, or the length of the drying time, is increased. On the other hand, if the cake thickness and the cake weight are increased, the moisture content increases. Knowing the percentage of available drying time of the filter cycle and using the design formation, such as the proper cake thick- ness for a given type of filter, the vac- uum level, and the airflow rate through the cake, it is possible to predict for each cycle time the discharged filter cake moisture content expected from the full-scale filter. The filter area pro- vided in the design should be for the maximum solids removal rate plus a 5- to 15-pct safety factor (2, p. 11-31). FACTORS AFFECTING FILTRATION Efficient vacuum filtration is influ- enced by many variables, of which some can be controlled and others cannot. The following items represent many of the factors that affect the final moisture content of a filter product: 1. Cake thickness. 2. Pressure drop across cake. 3. Drying time. 4. Volume of air or gas per minute per square foot of filtering area. 5. Viscosity of filtrate. 6. Surface tension of filtrate. 7. Filter medium. 8. Size distribution of solids, 9. Permeability of cake. 10. Specific gravity of dry solids. 11. Inherent moisture of dry solids. 12. Surface properties and other char- acteristics of solids. 13. Type of filter and construction. 14. Homogeneity of cake formation. 15. Temperature of solids and gas ( 18 , p. 12-53). Two other conditions — feed solids concen- tration and cycle time — are important in vacuum filtration because they influence many of the above factors. The feed solids concentration is very important in the filtration process; con- sequently, a thickening device often pre- cedes the filter to ensure that the feed solids concentration is consistent with economic and efficient operation. A gen- eral plot of dry cake output versus feed solids concentration reveals a curve, as shown in figure 23 ( 18 , p. 12-54), Each slurry has its own characteristic filtration curve, which must be experi- mentally determined. In this example, the slurry exhibits a sharp incremental rate increase above 35 pet solids. Con- trolling the solids concentration be- tween the limits of 45 and 55 pet solids 200 < c z t- I < z O xi 150 100 — 50 10 20 30 40 50 60 FEED SOLIDS CONCENTRATION, wt pet FIGURE 23, - Representative curve for cake forma- tion rate versus feed solids concentration (18), 36 will require less filtration area and the resulting filter operating costs will be reduced. Above 58 pet solids, this slurry becomes relatively viscous and transportation to the filter will be difficult. There is a point of inflec- tion at about 55 pet solids where the curve becomes asymptotic. This indicates that further slurry thickening above 55 pet solids is impractical and uneconomi- cal because it produces only a slight increase in the filtration rate ( 18 , pp. 12-54 to 12-55). The other important consideration for filtration operations is the cycle time. In this discussion, cycle time will be concerned with rotary vacuum filters, al- though the same principles apply to other types of filters. The cycle time of a continuous vacuum filter is the amount of time the filter takes to make one complete revolution and is given in terms of minutes per revolu- tion. During each cycle, there are three phases of filter operation: cake forma- tion, cake dewatering, and cake dis- charge. At the end of each cycle, the filter discharges a certain amount of cake per given filter area. With these data, the dry cake formation rate can be expressed in pounds per hour per square foot of filtering area. A log-log plot of dry cake formation rate versus cycle time for an arbitrary feed solids concen- tration is shown in figure 24 ( 18 , p. 12- 55). For this example, the slope of the curve is -0.5, based on an assumption that solids concentration and cake com- pressibility remain constant. This rela- tionship can be mathematically expressed as — t tcvo - the old minutes , cycle time. = f, cvn (20) where ^cvo ~ the old cake formation rate, pounds per square foot per minute, fcvn - the new cake formation rate, pounds per square foot per minute. and tcvn ~ the new cycle time, min- utes (18, p. 12-55). Vacuum filters are normally equipped with variable-speed drives operating within a range of 1.5 to 9.0 min/rev. Thus, for any filter area, cake output can be doubled, tripled, or halved, as the situation requires ( 18 , p. 12-55). Cycle time also affects the filter cake moisture content and dischargeability. As a general rule, the filters should be sized for a cycle time of at least 3 min/ rev and preferably 4 min/rev. Cycle times less than 3 min/rev will increase the cake moisture and produce thin cakes, which are difficult to remove from the filter medium. Difficult cake discharge can mean sizable increases in filter maintenance costs ( 18 , pp. 12-55 to 12- 56). 200 100 < cc 50 z o I- < (T O u. UJ < o 10 Slope -0.^^^^ ^^ 10 CYCLE TIME, min/rev FIGURE 24, - Representative curve for cake forma- tion rote versus cycle time (18). 37 HYDROCYCLONES BACKGROUND Cyclones have found wide use in indus- try for solids classification and concen- tration. The cyclones designed for liq- uids are called hydrocyclones , hydraulic cyclones, or hydroclones. The basic sep- aration principle used in cyclones is centrifugal sedimentation, where the sus- pended particles are subjected to a cen- trifugal force which causes them to sep- arate from the fluid. Cyclones have no moving parts , and the necessary vortex motion is provided by the fluid itself (32, p, 101). Hydrocyclones , by themselves , are not capable of producing a dry solids product because they use liquids for their opera- tion. Cyclones are useful, however, in concentrating the solids content of a slurry ahead of another dewatering ma- chine, such as a filter or centrifuge. The cyclone consists of a short cylin- drical section attached to an inverted, truncated, conical section. The apex or bottom of the conical section is called the underflow orifice, A central over- flow orifice or vortex finder is fitted to the base of the cone, and a feed orifice is attached tangent ially to the cylindrical body section. Figure 25 shows the cross section of a typical hydrocyclone. The slurry enters at high Overflow (liquid) Slurry feed Feed orifice Overflow orifice or vortex finder Conical body section Underflow orifice pressure through the tangential feed ori- fice into the cylindrical section, where a rotating force field is established. The solids in the slurry are settled to the side wall by this force, slide down the inclined wall to the apex of the cone, and exit through the underflow ori- fice. The liquid portion of the feed travels to the center of the cone with some of the finest solids and exits through the overflow orifice, A vortex of air extends throughout the length of the cyclone (^8, p, 12-26), A cyclone is a simply constructed de- vice; however, its principles of opera- tion are complex and there are many vari- ables that must be carefully evaluated to produce the desired separations. The most significant variables are — 1, Cyclone diameter, 2, Cyclone cone angle. 3, Feed, overflow, and underflow ori- fice sizes. 4, Length of cylindrical section, 5, Feed pressure, 6, Feed concentration. 7, 27), Particle size (18, pp. 12-26 to 12- ■ Underflow (solids) FIGURE 25. - Cross section of hydrocyclone (1_8). The hydrocyclone diameter is the most important factor influencing the applica- tion and efficiency of a cyclone because the smaller the particle to be dewatered, the smaller the cyclone diameter that must be used. For most applications, the cyclone manufacturer will determine the size and cone angle of the cyclone to be used (^8, p. 12-27). The orifice size is another important factor that influences cyclone perform- ance. The underflow orifice will de- termine the concentration and flow of the thickened solids from the cyclone. Enlarging the underflow orifice Increases the flow rate and the percentage of fines 38 in the underflow. The larger underflow orifice also allows more liquid to pass through it and decreases the concen- tration of the product, A larger over- flow orifice increases the total flow, the concentration of solids , and the max- imum particle size contained in the over- flow of the cone, A secondary effect is that the underflow of the cone increases in solids concentration and contains a larger percentage of coarse sizes ( 18 , p, 12-27), Changes to the feed orifice affect the volume processed by the cone because as the area of the feed orifice is in- creased, there is an accompanying propor- tional increase in the flow to the cy- clone. The additional flow reduces the retention time of the slurry within the cone and causes the cyclone to reject coarser materials to the overflow. As a result, when a cyclone feed is increased, the cyclone underflow orifice should also be increased to accommodate the higher tonnage of material fed to the cyclone (JJ^, p, 12-28), The feed inlet can be either rectangu- lar or circular; however, a rectangular inlet with its long side parallel with the axis of the cyclone produces better results. The top of the feed orifice should be flush with the top of the cy- clone to eliminate a dead space which would cause short circuiting of the feed (32^, p. 113). In the mineral industry it is common practice to have several vortex finders with different diameters or nozzles which can be put into the exit pipe. This ena- bles the operator to change the length of the vortex finder, when needed. An in- crease in the length of the vortex finder improves the efficiency of removal of the coarse particles but decreases the effi- ciency for the finer particles ( 32 , p. 114), The feed pressure affects the volume processed and the relative efficiency of the cyclone. The total flow to a cyclone will vary proportionally to the square root of the pressure. Increasing the feed pressure causes the underflow con- centration to increase and become finer in size, while the overflow discharge in- creases and the overflow solids also be- come finer ( 18 ) , The interior surface of the cyclone should be as smooth as possible to pro- mote good material flow. Abrasion re- sistance should be built into a cyclone if it is to be operated with abrasive solids, A wide range of construction ma- terials, such as steel, nylon, ceramics, polyurethane , and rubber, are available (32, p, lU), ADVANTAGES AND DISADVANTAGES OF HYDROCYCLONES Hydrocyclones have been used for many solid-liquid separation applications be- cause of the following advantages: 1, High capacity, 2, Simple operation, 3, Compact design, which uses a mini- mum of floorspace, 4, Relatively low capital costs. 5, Low maintenance and processing costs (8^), Hydrocyclones do have a drawback, how- ever, because the smallest particles in the slurry will be carried away with the overflow. Proper cyclone design can min- imize this loss, but clarification of this liquid may be necessary, and this possibility should be recognized by the mill operator, SIZING HYDROCYCLONES The designer of a hydrocyclone instal- lation will be concerned with the size and number of cyclones needed. This will be based on the desired separa- tion efficiency and flow rate. Several small-diameter cyclones working in paral- lel are more efficient than one large cyclone handling the same capacity ( 32 , p, 114), 39 A simple, straightforward method of sizing hydrocyclones is to first deter- mine the requirements of the installation in terms of particle size, amount of throughput, or pressure differential be- tween the inlet and overflow orifices ( 32 , p. 114). Knowing this information, manufacturers' bulletins can be consult- ed. Operational data for hydrocyclones are often presented in graphic form; fig- ure 26 shows a plot of pressure drop ver- sus throughput capacity for two represen- tative hydrocyclones. As can be seen, these two cyclones are designed to handle very small solids. 1 25 _ 1 1 Model / -w 1 r" l_ 1 00 / 90 A Model B — 80 r@ / — 70 — / / — 60 — / / — 50 — L f@ — 40 1 m / - 30 - / — 25 - / — 20 J@ ) ^ - 1 5 — / - 1 1 1 %i 1 1 1 1 KEY ^, Particle size X 10"^ etc. inch for 95- pcf separation efficiency for a given pressure drop and tfiroughput capacity 0.6 0.8 1.0 2 4 8 8 1 THROUGHPUT C A P ACIT Y , ga I / tnln FIGURE 26.- Graph of pressure drop versus through- put capacity for two hydrocyclones (8), More sophisticated methods can be used to determine the diameter, cone angle, and other dimensions for a particular situation. Their application, however. is very complex and a reference such as Svarovsky (32, pp. 106-118) should be consulted for a complete theoretical discussion. THERMAL DEWATERING BACKGROUND The use of heat is another method of separating liquids from solids. Thermal processes can be used to dry a slurry or just the thickened solids; however, fuel consumption will increase proportionally with the increase in the moisture pres- ent. It is, therefore, cost effective to use thermal drying after a substantial amount of moisture has already been re- moved by other methods such as centrifu- gation or filtration. There are basically three methods of heat transfer for drying: by convection or direct contact between the wet solids and hot air. Over the years, many different configurations of convection dryers have been available for the minerals industry. Now, six basic types exist, as follows: 1. Drum. 2. Suspension or flash, 3. Multi louvre. 4. Vertical tray. 5. Continuous carrier. 1 . Convection — the direct contact of particles with warm air. 2. Conduction — the direct contact of particles with a heated shell of a dryer or other heated particles. 3. Radiation — heat radiating from a hot surface to the particles ( 13 , p. 27- 75). 6. Fluidized bed (^, p. 13-7). In each of these dryers, the wet solids are separated from each other and sub- jected to a flow of hot gases or air. In the drum-type dryer, the solids enter one end of a rotating cylinder through which the hot air is blowing. The solids are tumbled the length of the cylinder and exit the other end (13, p. 27-76). THERMAL DRYER OPERATION Dryers used commercially for drying minerals commonly utilize heat transfer In the suspension or flash dryer, the wet solids are carried upward through a vertical duct by a blast of superheated air for a very brief time. The air is 40 usually heated to 1,200° F, but the sol- ids are in contact with the hot air for only a fraction of a second and are not changed chemically ( 18 , p. 13-39). The louvered arrangement has a series of specially designed flights which carry the solids upward through a flow of hot air. The vertical tray, on the other hand, has a series of shelves placed in a terrace arrangement. The solids are fed in at the top, and a vibrating action causes them to fall from one shelf to the next through heated air. The continuous carrier type uses a vibrating inclined screen to support the solids as they tum- ble through the hot airstream ( 18 , pp. 13-39 to 13-43). The f luidized-bed dryer uses a slightly different approach and has seen wider use in the past two decades. In this type of dryer, the solids are fed into a heater box and subjected to a high-velocity flow of hot air. The violent bubbling action of the solids is similar to that of boil- ing water, leading to the description of the dryer as a f luidized bed (4^) . Fig- ures 27 and 28 show simplified cross sec- tions of the various thermal dryers. Rotating drum Dry product DRUM Vertical draft tube SUSPENSION Falling material Dry produc MULTILOUVER Wet feed FIGURE 27, - Simplified cross sections showing operation of drum, suspension, and multilouver ther- mal dryers (18), Wet feed Vibrating tray Hot air Wet feed Vibrating screen VERTICAL TRAY Dry product CONTINUOUS CARRIER Dry product ADVANTAGES AND DISADVANTAGES OF THERMAL DRYERS Thermal dewatering equipment has sev- eral advantages , which include — 1. Ability to reduce the moisture con- tent of slimes to 6 pet or less. Wet feed ►>:..; FLUIDIZED BED T ^Perforated plate 2. Minimal labor requirements. 3. Capability to operate continuously as long as feed material is available, 4. Low maintenance costs. FIGURE 28, - Simplified cross sections showing operation of vertical tray, continuous carrier, and fluidized-bed thermal dryers (18), too low, the powdery product may be dif- ficult to handle or transport (1). There are, however, several disadvan- tages, such as — 2, The fuel demands can become very high (4). 1. The moisture content of the product must be carefully monitored. If it is 3. A mechanical dewatering device is usually needed ahead of a thermal dryer. 41 SIZING THERMAL DRYERS As wet solids are dried, they go through three stages with respect to moisture loss (13, p. 27-75) : 1. Warming period. 2. Constant-rate period. 3. Falling-rate period. The constant-rate and falling-rate periods have the greatest impact on the drying time required for a particular material. The warming period will be significant if radiant heat from sur- rounding surfaces is negligible ( 13 , p. 27-75). The rate of liquid evapora- tion during the constant rate period can be estimated using mass transfer or heat transfer equations as follows: Ec = Ma Ky (Hai - Ha) A^j (mass transfer), (21) 17 _ hy (Ta - Ti) Ad ^'-- ^^ (heat transfer) , (22) where Ec = the rate of evaporation, pounds per hour. Ma = the molecular weight of air, pounds per pound-mole. Ky = a mass transfer coefficient, pound-moles per square foot per hour. Ha = the humidity of the ambient air, pounds of water per pound of dry air. Ha I = the humidity of the air at the solid-air interface, pounds of water per pound of dry air. A(j = the drying area, feet. square and h„ = a I heat transfer coefficient, Btu per square foot per hour per °F. For airflow parallel with surface of solid, hy = (0.0128 Vam)°*®. For airflow perpendicular to surface of solid, hy = (0.37 Vam)°-^^. Vam = the A±r mass velocity, pounds per square foot per hour. Tg = the temperature of the ambi- ent air, "F, Ti = the temperature of the air at the solid-air interface, "F, and X| = the latent heat of water at the temperature , T | , of the solid-air interface, Btu per pound (22, p. 27-75). Equations 21 and 22 are valid while the surface of each particle is saturated. When this condition is no longer the case, thermal drying enters the falling rate period (_33, p. 17-02). If the total average moisture content (Xfav) of ^^y particle is composed of the average free moisture (Xfav) ^^d chem- ically bound or equilibrium moisture (Xeq), the rate of liquid evaporation can then be estimated by the liquid diffusion equation, which is — _ Xfav Xeq _ Xf Xti - X av eq Xfi = !^G""'^|"""'^-)' (23) where "^ ~ ( T ) » e = Dmt, _ "m«-n Xti = S2 the rate of liquid evapora- tion during the falling rate period, pounds of water per pound of dry solid, the initial total mois- ture content of the solids , pounds of water per pound of dry solid. 42 ■■tav ~ ■■eq the average total moisture content during the falling rate period, pounds of water per pound of dry solid, the equilibrium moisture con- tent of the solids, pounds of water per pound of dry solid. decreased. To design for the percent open area, a factor called the discharge coefficient must be determined with the aid of a single-orifice test plate that has the same cross-sectional characteris- tics as the proposed full-scale dryer. Information obtained from the test plate can be used in the following equation to determine this coefficient: Xfi = the initial free moisture content at the beginning of the falling rate period, pounds of water per pound of dry solid. Ch = qa Ma P op 4825 Ao Pdo Tk (24) where C^ = the discharge coefficient, dimensionless , Xfav = the average free moisture content during the falling rate period, pounds of water per pound of dry solid, e = Euler's number - 2,71828, Dm = the diffusivity value of moisture through a solid, square feet per hour, tn = any arbitrary time after the beginning of the falling rate period, hours. qa = the airflow through the ori- fice plate for the air tem- perature and pressure used during the test, cubic feet per minute. Ma = the molecular weight of air, 29, Pop = the mean absolute pressure in the orifice, pounds per square inch , Aq = orifice area, square feet. and S = one-half the thickness of the layer of solids in the dry- er, feet (13, p, 27-75). P(jo = the pressure differential across the orifice, inches of water. These equations are helpful in estimat- ing the rate of evaporation of moisture and will enable the designer to determine the total drying time, feed rate, air- volume requirements, and other important factors concerning thermal dryers. When a considered orifice the plate most appli to 10 or Pressure than this ing gases p, 13-18). f luidized-bed dryer is being the pressure drop across the plate must be evaluated and size designed accordingly. For cations, a pressure drop equal 12 in of water is adequate, differences slightly higher should be used if the fluidiz- are as hot as 1,200° F (18, The pressure drop will increase as the percent open area in the plate is and Tk = the temperature, (18, p, 13-17), kelvins After the discharge coefficient has been determined for a particular orifice configuration, the volume of air needed to pass through the orifice to provide the necessary pressure drop can be calcu- lated. The total number of orifices for the full-scale dryer can then be found if the designer knows the volume of air needed to maintain the pressure drop for one orifice and the total volume of air needed for drying the solids (18, p. 13- 19). These equations will provide approxi- mate values for sizing thermal dryers, but as with other equipment , it is recom- mended that pilot-plant testing be done. 43 CURRENT BUREAU OF MINES RESEARCH ON DESLIMING METHODS The bulk of this review has been con- cerned with well established methods of dewatering slimes; however, at least two Bureau of Mines innovations for dewater- ing slimes should also be brought to the attention of the mining industry: Elect rokine tic methods and the rotary trommel. ELECTROKINETIC METHODS Background The Bureau is currently developing a method of dewatering slimes using an electrokinetic potential. This dewater- ing technique is intended to be used at the place of the slimes disposal rather than in the mill or other processing lo- cation. This method takes advantage of the electrical surface charge on the solid particles in a water suspension. Bureau of Mines research has shown this technique to be generally successful for treating siliceous mine tailings from several north Idaho metal mines , thicken- er underflow from two Appalachian coal preparation plants, and materials from numerous other coal and metal mine sites (30). Not all slimes or sludges can be de- watered by this method, however, because the sludge from an acid mine drainage treatment plant and scrubber sludge from a large coal-fired power plant were not responsive to this technique. The physi- cal properties of the slurries that af- fect their response to treatment include electrical conductivity, particle-size distribution, water content, and surface charge density. Chemical properties also influence behavior , but their importance is specific to each sample and difficult to characterize (30) . Application The electrokinetic phenomena of elec- trophoresis and electro-osmosis are prin- cipally responsible for the effects ob- served. Electrophoresis is the migration of small electrically charged particles through a stationary liquid due to an ex- ternal electrical potential. Electro- osmosis, on the other hand, is the mi- gration of liquid through a stationary porous solid as a result of an external electrical potential. Particle sedimentation can be acceler- ated by imposing a properly oriented electrical field on a slurry. For exam- ple, in a slurry composed of negatively charged solids, the anode or positive electrode is located at the bottom of the slurry, and the cathode or negative elec- trode is positioned at the surface (fig. 29). Practically speaking, the anode can be a section of abandoned steel track or wire mesh placed on the floor of the fill area prior to the slurry deposition. The cathode can be wire mesh positioned at the slurry surface and suspended from wood floats or cables attached to the roof of the mine opening. A relatively large direct voltage of 2 to 6 V/in^ of tailings surface area is applied to the electrodes, and the negatively charged solids begin migrating downward to the positively charged anode. The electro- phoretic migration will effectively ac- celerate the settling of these solids (30). INITIAL PARTICLE SEPARATION Elec trode polarity -Top electrode (cathode) suspended from a suitable buoyant float ■ C lear water yar'ticle migratio ^ Bottom - Slurry electrode ( anode) FINAL SOLIDS DEWATERING Electrode p olarity Top electrode (anode) [fit f Water migration 1 \Mn iM -Consolidated solids -Sand drain Bottom electrode (cathode) FIGURE 29. - Two configurations for elec- trokinetically dewatering slimes (30). 44 When sedimentation is complete, further drainage can be stimulated by reversing the polarity so that the bottom electrode becomes the negatively charged cathode. The water Immediately adjacent to the negatively charged solid particles will contain excess dissolved positive ions that, in effect, give this water a posi- tive charge. In an electrical field es- tablished between two separated elec- trodes burled in a slurry, the solid particles will not move appreciably be- cause of their relatively dense packing, but the water will be carried toward the negatively charged cathode by the vis- cous drag of the migrating positive ions. This movement of water by direct current potential is electro-osmosis. As the wa- ter migrates to the cathode, the liquid can be removed by using slotted pipes or gravel drains. Electrophoretic flow is relatively in- dependent of pore size and is particu- larly attractive for dewatering dense slurries of fine particles where hydrau- lic flow of water through the sediment is negligible because of small pore size. Feasibility of electrokinetlc dewater- ing for a particular slurry is best determined by direct testing in the laboratory. The complex Interaction of factors affecting the efficiency of the process has prevented the use of physical properties alone as reliable predictors of performance. Change in application methods, such as current density, current reversal, electrode configuration, or settlement time, can also have unique and important effects on the response of a given slurry (30) . Current Research and Use Field tests conducted in two Idaho mines demonstrated that the electrokinet- lc process can effectively dewater and denslfy unclassified mill tailings or slimes for use as backfill, with moderate power consumption. One mine is preparing to use the process to dewater slimes un- derground as a regular operating proce- dure, and another mine plans to use it when space occupied by old slime deposits needs to be recovered for other purposes (30). TROMMEL SCREEN Description of the Method and Equipment The Bureau is also doing extensive re- search on the trommel screen for dewater- ing slurries. In this method, the slurry is first mixed with a flocculating agent to agglomerate the small particles into much larger masses. Next, the treated slurry passes over a hydrosleve to remove some of the liquid. The remaining wet solids then go into the upper end of a long inclined cylinder or trommel made of steel screen. The trommel rotates about its long axis and allows any remaining liquid to pass through the screen while the dewatered solids move through the cylinder and exit through the lower end (fig. 30) (27_, 37). The most important aspect of this meth- od is adding the proper flocculant to the slurry. It must cause the solids to form sufficiently large masses that will not pass through the screen with the liquid but will remain on top of the screen sur- face. At the same time, the flocculating agent must be potent enough so that a minimum amount of the chemical will pro- duce the required f locculation. Exten- sive testing has indicated that polyethy- lene oxide (PEO) is suitable for use with many different slurry types in the trom- mel screen. This flocculating agent is a water-soluble polymer having a nominal molecular weight of 8 million. This agent causes slurry solids to flocculate within minutes and works well with the hydrosleve and trommel screen (27, 37). 45 Chemically treated slurry Rotating trommel Liquid discharge Solid discharge FIGURE 30, - Diagram showing operation of rotary trommel (27), and 9°. The slurry moved over the hydro- sieve and through the trommel by gravity (27, 37). Test Results Test results using this method indicate that phosphatic and coal-clay slurries can be successfully dewatered. In one particular test, a coal-clay slurry had 23.9 wt pet solids, of which over 70 pet were smaller than 325 mesh. A solution containing 0.125 wt pet PEG was added to the slurry at a dose of 0.78 lb PEG per ton of dry solids. The dewatered product had 60.1 wt pet solids (37). This im- pressive value is representative of the results obtained so far. Conclusions The hydrosieve used for this research was constructed of stainless steel and was 8 ft long. The first 4 ft had screen openings of 0.04 in, and the last 4 ft had openings of 0.02 in. The hydrosieve was inclined at an angle of 58° from horizontal (37) . The trommel screen was also composed of stainless steel but had 10-mesh openings. The trommel had a length of 36 in and a diameter of 6 in. The angle of inclina- tion from horizontal was between 3° and Research on the trommel screen is on- going, so no information is available on production costs per ton of dried solids. The equipment is fairly simple in design and would not represent a substantial capital investment; however, this method does require chemical pretreatment that would significantly affect the cost of dewatering slimes. Nonetheless, this method shows promise for mineral industry use, and research is continuing on the refinement of the trommel screen. DISCUSSION In recent years , economic pressures have caused milling operations to max- imize the separation of economic min- erals from the waste material. This has been done by grinding the ore to much smaller particle sizes. This fine grinding, while increasing the mineral extraction, has also posed a serious disposal problem for the waste materi- al. As practically all mineral bene- ficiation involves the use of water, the resulting solid-liquid mixture or slurry must be properly disposed of in accordance with the current environmental regulations. There is an abundance of dewatering equipment available that can separate the solids from the water with varying de- grees of efficiency. Physical separation methods , such as gravity thickeners , cen- trifuges , filters , thermal dryers , and cyclones , all reduce the water content or increase the solids content of the slur- ry. Although these items were discussed separately, they can be used in conjunc- tion with each other to produce a solids mass with an acceptably low moisture con- tent. Figure 31 shows the relative capa- bilities of common pieces of dewatering equipment in terms of the slurry particle size. 46 o a UJ < O CO o _l o CO LU QC ID I- CO 100.0 80.0 60.0 40.0 30.0 25.0 20.0 15.0 10.0 8.0 6.0 4.0 3.0 2.5 2.0 325 200 TYLER MESH 100 48 28 20 14 10 8 1.5 — 1 \ r 6 T Gravity thickeners Hydrocyciones Drum vacuum filters Solid-bowl centrifuges Electrokinetic methods Disk vacuum filters Belt presses Filter presses Horizontal vacuum filters Drum vacuum filters Solid-bowl centrifuges Vibrating-basket centrifuges Positive-discharge basket centrifuges 1.0 Thermal dryers 1 1 1 0.001 0.002 0.005 0.01 0.02 0.05 PARTICLE SIZE, in 0.1 0.25 FIGURE 31. - Chart showing generalized capabilities of commonly used dewatering equipment with respect to solids cake, moisture content, and particle size (6, 18, 25, 28, 30-31). The Bureau is continuously doing re- search that will benefit the mining in- dustry. Classical dewatering methods are being improved, and new methods are being devised. Electrokinetics and the rotary trommel are but two ongoing Bureau proj- ects that should help the mining industry reduce costs and increase efficiency. Despite the amount of knowledge we have concerning dewatering, its practice is still an art. Each mine generates waste material which, in one way or another, is different from the waste of any other mine. Variations in mineral content and physical properties must be evaluated carefully so that the right combination of equipment and methods will produce a sufficiently dry product. This paper was prepared in order to give the mill opera- tor or owner an overview of what dewater- ing equipment is available. An awareness of the different dewatering practices will enable such operators to evaluate alternatives that perhaps would not have been considered otherwise. This paper should also serve as a basis for further detailed research into the many ways of meeting the desliming challenge. 47 REFERENCES 1. Anderson, J. C. Coal Waste Dis- posal To Eliminate Tailings Ponds. Min. Cong. J., v. 61, No. 7, July 1975, pp. 42-45. 2. Baker, M. , Jr., Inc. (Beaver, PA). FGD Sludge Disposal Manual. Rept. FP-977 (prepared for the Electric Power Research Institute, Palo Alto, CA) , Jan, 1979, 536 pp. 3. Bird Machine Co., Inc. (South Walpole, MA). Bird Centrifugals: for Clean Coal and Refuse Dewatering. 1980, 6 pp. 4. Casili, J. T. Heat Drying Sludge From Ponds. Min. Cong. J., v. 61, No, 1, Jan. 1975, pp. 34-37. 5. Centrifugal and Mechanical, Inc. (St. Louis, MO). CMI Model EBW. Undat- ed, 4 pp. 6. Coal Age. Improved Equipment Available Now. V. 85, No. 1, Jan. 1980, pp. 56-61. 7. Cook, R, L. , and J. J. Childress. Performance of Lamella Thickeners in Coal Preparation Plants. Min. Eng. , v. 30, No. 5, May 1978, pp. 566-571. 8. Dorr-Oliver, Inc. (Stamford, CT). Cyclones. 1976, 28 pp. 9. Duriron Co., Inc., Filtration Sys- tems Division (Angola, NY). Durco Quadra Press Filters. Bull. EF/21, 1980, 7 pp. 10. Emmett, R. C, and R. P. Klepper. Technology and Performance of the Hi- Capacity Thickeners. Min. Eng., v. 32, No. 8, Aug. 1980, pp. 1264-1269. 11. Envirotech Corp. (Salt Lake City, UT), Sludge Dewatering for FGD Products. Rept. FP-937 (prepared for the Electric Power Research Institute, Palo Alto, CA, Apr. 1979, 260 pp. 12. Fischer, M. C, and M. G. Schill. The Dewatering of Fine Coal Refuse With a Continuous High Performance Belt Filter Press. Pres, at Fall Meeting, Soc. Min. Eng,, AIME, Tucson, AZ, Oct. 18, 1979, 11 pp.; available from the authors, Parkson Corp., Fort Lauderdale, FL. 13. Given, I. A. (ed,). SME Mining Engineering Handbook. Society of Mining Engineers of AIME, 1973, 2666 pp. 14. Jacobsen, S. P., W. Roushey, and E. L. Rau. Coal Waste Dewatering Systems (contract J0205012, CO Sch, Mines Res, Inst.). BuMines OFR 114-81, 1981, 133 pp.; NTIS PB 81-244501. 15. Joy Manufacturing Co. , Denver Equipment Division (Colorado Springs, CO) . Denver Horizontal Belt Vacuum Fil- ter. Bull. F 18-B103, 1979, 11 pp. 16. Kealy, C. D. , R. A. Busch, and M. M. McDonald. Seepage-Environmental Analysis of the Slime Zone of a Tailings Pond, BuMines RI 7939, 1974, 89 pp. 17. Keane, J. M. Sedimentation: The- ory, Equipment, and Methods. World Min., V. 32, No. 12, Nov. 1979, pp. 44-51. 18. Leonard, J. W. , and D. R. Mitchell (ed.). Coal Preparation. AIME, 3d ed. , 1968, 877 pp. 19. Mcllvaine Co. (Northbrook, IL) . The Liquid Filtration Manual. 1980, pp. 451-464. 20. Moos, S. M. , and R. E, Dugger, Vacuum Filtration: Available Equipment and Recent Innovations. Min. Eng., v. 31, No. 10, Oct, 1979, pp. 1473-1486. 21. Parkson Corp. Fort Lauderdale, FL). Lamella Gravity Set tiers/ Thicken- ers. Bull. LT-103, 1979, 10 pp. 22. Perrin, W. R. Co. , Ltd. (Houston, TX) . An Introduction to Filter Presses for Effluent and Sludge Dewatering. Un- dated, 16 pp. 48 23. Perry, R. H. , and C. H. Chilton (eds.). Chemical Engineers Handbook. McGraw-Hill, 5th ed. , 1973, 1958 pp. 24. Pollution Equipment News. Select- ing Sludge Thickening and Dewatering Equipment. V. 13, No. 5, Oct. 1980, pp. 78-82. 25. Schlitt, W. J., B. P. Ream, L. J. Haug, and W. D. Southard. Precipitating and Drying Cement Copper at Kennecott's Bingham Canyon Facility. Min. Eng. , V. 31, No. 6, June 1979, pp. 671-678. 26. Schlitter, W. E., and W. Markl. Cross-Flow Lamella Thickeners. Min. Mag., V. 134, No. 4, Apr. 1976, pp. 261- 297. 27. Smelley, A. G. , and I. L. Feld. Flocculation Dewatering of Florida Phos- phatic Clay Wastes. BuMines RI 8349, 1979, 26 pp. 28. Soderberg, R. , and K. R. Dorman. Sludge Dewatering by Belt Press. Min. Cong. J., V. 65, No. 8, Aug. 1979, pp. 29-32. 29. Sperry, D. R. and Co. (North Aurora, XL). Sperry Filter Presses. Catalog 12, undated, 29 pp. 30. Sprute, R. 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Smelley, Preliminary Studies on the Dewatering of Coal-Clay Waste Slurries Using a Flocculant. BuMines RI 8636, 1982, 15 pp. 49 APPENDIX A. —MATHEMATICAL TERMS Ac = The centrifugal acceleration of a particle in a centrifuge. Acs = The cross-sectional area of a gravitational settling basin. Aj = The drying area of a thermal dryer. A I = The inclination angle above horizontal of the plates in a multiple-plate thickener. Aq = The area of the orifices in the constriction plate of a fluidized-bed dryer. Ap = The top surface area of each plate in a multiple-plate thickener. At I = The area of a vacuum filter test leaf. Cave - The average solids concentration in the compression zone of a gravitational thickener. C(j = The discharge coefficient that characterizes the airflow through an orifice or constriction plate of a fluidized-bed dryer. Cs2 = The secondary solids concentration produced when a hypothetical gravitational thickener processes a slurry having the maximum solids concentration permis- sible for that particular thickener, Cse - The solids concentration in a settling test after an arbitrary time, t^. Css - The solids concentration of the slurry prior to dewatering or thickening. Cssi = The solids concentration of a hypothetical slurry as shown on a batch flux versus solids concentration graph. Css2 ~ The solids concentration of a hypothetical slurry that is greater than Cggi. Csu = The solids concentration of the underflow or thickened solids. Csui = The solids concentration of the underflow produced by a hypothetical gravita- tional thickener processing an arbitrary slurry having a solids concentration CssI • Csu2 ~ The solids concentration of a hypothetical underflow that is greater than Csu I • D| = The density of the slurry liquid at a specified temperature. Dm = The diffusivity value of moisture through a solid during thermal drying. Ds = The density of the solid particles in a slurry, Ec = The rate of evaporation during the constant-rate drying period during thermal drying. Ef = The evaporation rate during the falling rate period during thermal drying. 50 Ef = The total efficiency for a particular piece of solid-liquid separation equipment. e = Euler's number, which has a value of 2.71828... Fbt = The bulk transport flux component based on the underflow from a particular gravitational thickener. Fc = Centrifugal force expressed as multiples of gravitational force. Fg = The settling flux component for a particular gravitational thickener. Fg I = An intercept point on the batch flux axis of a hypothetical batch flux versus solids concentration graph for a gravitational thickener. F-i-s = The total solids flux for a particular gravitational thickener. Fy = The filter cake correlating factor for vacuum filters. fcv = The filter cake formation rate for a vacuum filter. fcvn - The iiew cake formation rate for a vacuum filter. fcvo ~ The old cake formation rate for a vacuum filter. G = The gravitational acceleration at the earth's surface. G(x) = A gravimetric separation function that describes the separation efficiency of a piece of solid-liquid separation equipment. Hg = The humidity of the ambient air around a thermal dryer. Ha be - The height of the interface between zones A and B in a slurry settling test after an arbitrary time, tg. Hgbo - The original height of the interface between zones A and B in a slurry set- tling test. Hg I = The humidity of the air at the solid-air interface in a thermal dryer. hy = A heat transfer coefficient for thermal drying. J = A correction factor for particle shape. Ky = A mass transfer coefficient for thermal drying. Lp = The length of each plate in a multiple-plate thickener. Ls2 = The length of the settling zone in the direction of slurry transport in a centrifuge. Mg = The molecular weight of air. Mg = The mass of all solids that have been separated from the liquid of a slurry. 51 Mf = The total mass of all solids in a slurry before solid-liquid separation. N = The number of revolutions per second for a centrifuge bowl. P(jc = The pressure differential across the cake of a vacuum filter. Pdo = The air pressure differential across the orifice plate of fluidized-bed dryers. Pop = The mean absolute air pressure in the orifice of a constriction plate in a fluidized-bed dryer. Qc = The volumetric flow of slurry through a centrifuge. Qs = The volumetric flow of a slurry through a settling zone. qa = The airflow through the orifice plate under controlled air pressure and tem- perature conditions in a fluidized-bed dryer. Re = The radius of curvature of a particle in a centrifuge. Rcb = The radial distance from the rotational axis to the inside surface of the bowl in a centrifuge. Rsc = The percent recovery of solids in the cake produced by a centrifuge. Rsz = The radial distance from the rotational axis to the surface of the settling zone in a centrifuge. S = Equal to one-half the thickness of the layer of solids in a thermal dryer. Sf = The subsidence rate for solids in the bottom of a settling tank. Sgs = The average specific gravity of the solids in a slurry. Sgsc ~ The average specific gravity of the solids in compression. Sg,y = The specific gravity of water, which is 1.0. Sp = The spacing between plates of a multiple-plate thickener. Tg = The ambient air temperature around a thermal dryer. Ti = The air temperature at the solid-air interface in a thermal dryer. Tk = The temperature in Kelvins. tcv = The cycle time for vacuum filters. tcvn - The new cycle time for a vacuum filter. tcvo = The old cycle time for vacuum filter, t(jv = The drying time for a vacuum filter cake. 52 te = An arbitrary amount of time after the beginning of a settling test for gravi- tational thickeners. ths = The holding time necessary for the solids to settle from the entering feed solids concentration to the underflow concentration, tn = An arbitrary time after the falling-rate drying has begun in a thermal dryer, Vam = The air mass velocity through a thermal dryer, Vav = The volume of air passing through a vacuum filter cake per unit of filter sur- face area, Vsc - The volume of solids in compression in a gravitational thickener, Vjjt = The bulk transport velocity of the solids through a gravitational thickener. V| = The linear velocity of a slurry particle moving through a settling zone. Vp = The linear peripheral velocity of a particle in a centrifuge, Vg = The terminal settling velocity of solids through a liquid, Vy = The resultant velocity vector of a settling slurry particle as a consequence of its forward motion and the downward pull of gravity. The dry weight of the solids cake obtained during a vacuum filter leaf test. The dry weight of solids for a given cake thickness on a vacuum filter. The weight ratio of water to solids in the discharge of a gravitational thickener. The weight ratio of water to solids in the slurry. The weight percent of solids in the cake. The weight percent of solids in the liquid after liquid-solid separation. The weight percent of solids in a slurry prior to liquid-solid separation. The equilibrium moisture content of the solids during the falling-rate period of thermal drying, Xfav = The average free moisture content during the falling-rate period of thermal drying. Xf i = The initial free moisture content of the solids at the beginning of the falling-rate period during thermal drying. Xfav = The average total solids moisture content during the falling-rate period of thermal drying, Xt i = The initial total moisture content of the solids at the beginning of the falling-rate period of thermal drying. Wds Wdst Wrd Wrs Wsc Wsl Wss Xen 53 X = The diameter of solid particles in a slurry, Xmax = The largest particle size on a grade efficiency curve that is capable of being separated from the liquid of a slurry by a particular piece of dewatering equipment. X50 = The particle size on a grade efficiency curve representing a 50-pct probabil- ity of being separated from the liquid of a slurry. X98 = The particle size on a grade efficiency curve that represents 98-pct separa- tion efficiency. a = A factor used in the liquid diffusion equation for the falling-rate drying period of thermal drying. g = A factor used in the liquid diffusion equation for the falling-rate drying period of thermal drying. Xj = The latent heat of water at the temperature of the solid-air interface in a thermal dryer, y = The viscosity of the liquid in a slurry at a specified temperature, TT = 3.1416. Z = A characteristic value that describes the machine variables for a particular centrifuge. 0) = The angular velocity of a particle undergoing centrifugal acceleration. 54 APPENDIX B. —MANUFACTURERS OF DEWATERING EQUIPMENT AS OF OCTOBER 1982 (24. 34)1.2 AFL Industries, Inc. 3661 West Blue Heron Blvd. Riviera Beach, FL 33404 Al f a-Laval , Inc . 2115 Linwood Ave, Fort Lee, NJ 07024 Baker-Perkins, Inc. 1000 Hess St. Saginaw, MI 48601 The Leon J. Barrett Co. Box 551 Worcester, MA 01613 Bird Machine Co. , Inc. 100-T Neponset St. South Walpole, MA 02071 Calgon Corp. Box 1346-C Pittsburgh, PA 15230 Carus Chemical Co. , Inc. 1500 Eighth St. LaSalle, IL 61301 C-E Bauer Box 968 Springfield, OH 45501 C-E Raymond 200 West Monroe St. Chicago, IL 60606 Dedert Corp. 20000-T Governor's Dr. Olympia Field, IL 60461 Denver Equipment Div. Joy Manufacturing Co. Box 340 Colorado Springs, CO 80901 Donaldson Co. Liquid Systems Div. 1400 West 94th St. Minneapolis, MN 55431 Dorr'-Oliver, Inc. 79 Havemeyer Lane Stamford, CT 06904 Duriron Co. , Inc. Filtration Systems Div. 9542 Hardpan Rd. Angola, NY 14006 Environmental Elements Corp. Box 1318 Baltimore, MD 21203 Envirotech Corp. 3000 Sand Hill Rd. Menlo Park, CA 94025 ERC/Lancy Div. , Dart & Kraft Co. 525 West New Castle St. Zelienople, PA 16063 Centrifugal and Mechanical Industries, Inc. 146 President St. St. Louis, MO 63118 Clow Corp. 1211 W. 22d St. Oak Brook, IL 60521 Filpaco Industries, Inc. 3837 West Lake St. Chicago, IL 60624 FMC Corp., Materials Handling Systems Div. 3400 Walnut Colmar, PA 18915 1 Reference to specific equipment suppliers does not imply endorsement by the Bureau of Mines. ^This list is as complete as possible based on the information available at the time this paper was written. No responsibility can be taken for omissions or changes in listings. 55 Heyl and Patterson Dept. 10 Box 36 Pittsburgh, PA 15230 Industrial Filter & Pump Manufacturing 5900 West Ogden Ave. Cicero, XL 60650 Infilco Degremont, Inc. Dept. T-R Box 29599 Richmond, VA 23288 lU Conversion Systems , Inc. Dept. T-R 115 Gibraltar Rd. Horsham, PA 19044 JWI, Inc. Box 9A Holland, MI 49423 Keene Corp. Filtration Div. 1571 Forrest Ave. LaGrange, GA 37743 Komline-Sanderson Engineering Corp. 100 Holland Ave. Peapack, NJ 07977 Krebs Engineers 1205 Chrysler Dr. Menlo Park, CA 94025 Krofta Engineering Corp. 101-T Yokun Ave. Lenox, MA 01240 Lakos Separators 1911 North Helm Box 6119 Fresno, CA 93703 Larox OY Box 29 SF-53101 Lappeenranta 10 Finland Lavin/Guinard International, Inc. 500 Davisville Rd. Hatboro, PA 19040 McNally-Pittsburg Manufacturing Corp. Third at Walnut St. Pittsburg, KS 66762 Nalco Chemical Co. 2901 Butterfield Rd. Oak Brook, IL 60521 National-Standard Co. Perforated Metals Div. Drawer 507 Carbondale, PA 18407 Netzsch, Inc. 119 Pickering Way Pickering Creek Industrial Park Ext on, PA 19341 Parkson Corp. 2727-T NW 62d St. Fort Lauderdale, FL 33309 Passavant Corp, Carson Rd. Box 2503 Birmingham, AL 35201 William R. Perrin Co., Ltd. 432 Monarch Ave. Aj ax , Ont . Canada LIS 2G7 Serfilco Div. Service Filtration Corporation 1234 Depot St. Glenview, IL 60025 SFS Div. , BINAB USA, Inc. 15271 NW 60th Ave. Miami Lakes, FL 33014 D. R. Sperry and Co. 112-T North Grant St. North Aurora, IL 60542 56 Star Systems Filtration Div. 101 Kershaw St. Box 815 Timmonsville, SC 29161 Transamerica Delaval, Inc. Condenser and Filter Div. Front St. Florence, NJ 08518 Tretolite Div. , Tretolite Corp. 369 Marshall Ave. St. Louis, MO 63119 Vara International, Inc. 1201-T 19th PI. Vara International Plaza Vero Beach, FL 32960 WEMCO Div., Envirotech Corp, 1796 Tribute Rd. Box 15619 Sacramento, CA 95813 Western States Machine Co. 1716 Fair grove Ave. Hamilton, OH 45012 Zimpro, Inc. Dept. MZ Military Road Rothschild, WI 54474 57 APPENDIX C. —AVAILABLE DEWATERING EQUIPMENT LISTED BY MANUFACTURER AS OF OCTOBER 1982 (24) Equipment and manufacturer^ »^ Gravitational thickeners: AFL Denver Dorr-Oliver Envlrotech Industrial Filter & Pump Larox Parkson SFS Centrifuges: Alf a-Laval. Baker-Perkins Barrett Bird Machine Centrifugal and Mechanical... Dedert Donaldson Dorr-Oliver Envlrotech lU Conversion Lavln MET Pro National Standard WEMCO Western States Hydrocyclones : C-E Bauer Dorr-Oliver Krebs Engineers Lakos Larox WEMCO Filters: AFL Bird Machine C-E Bauer Clow Denver Dorr-Oliver Durlron Environmental Elements Envlrotech ERC/Lancy Fllpaco Industrial Filter & Pump Inf llco lU Conversion JWI Komllne-Sanderson Krofta See footnotes at end of table. Type Multiple plate. Conventional. Conventional and high rate. Do. Multiple plate. Conventional. Multiple plate. Do. Basket, disk, and solid bowl. Pusher. Basket and solid bowl. Do. Basket. Do. Solid bowl. Basket, disk, and solid bowl. Horizontal. Solid bowl. Basket and solid bowl. Solid bowl. Basket. Do. Do. Undifferentiated. Do. Do. Do. Do. Do. Belt press and gravity. Horizontal belt, belt, and drum. Gravity and rotary. Filter press and horizontal belt. Horizontal belt. Belt, disk, and drum. Filter press. Do. Drum, filter press, gravity, and horizontal belt, Filter press. Do. Do. Horizontal belt. Drum. Filter press. Vacuum belt, belt press, drum, and gravity. Belt and gravity. 58 APPENDIX C. —AVAILABLE DEWATERING EQUIPMENT LISTED BY MANUFACTURER AS OF OCTOBER 1982 (24)— Continued Equipment and manufacturer^ '^ Type Filters — Continued: Larox Netzsch Inc Parkson Passavant Perrin. Serfilco Sperry Star Systems Transamerica-Delaval Vara Zimpro Thermal dryer or incinerator; C-E Raymond Dedert. Dorr-Oliver FMC , Heyl and Patterson , Komline- Sanderson , McNally-Pittsburg , Chemical treatment aids: Calgon Carus ( ERC/Lancy , Industrial Filter & Pump.., lU Conversion , Keene , Nalco , Passavant , Tretolite , ' See appendix B for comple ^This list is as complete time this paper was written, in listings. Disk, filter press, and drum. Filter press. Belt press. Belt press and filter press. Horizontal belt and filter press. Gravity and vacuum belt. Filter press. Do. Gravity and belt press. Gravity. Filter press. Rotary, fluidized bed, and suspension. Drum and suspension. Fluidized bed. Drum, fluidized bed, and louvre. Fluidized bed. Suspension. Fluidized bed and vertical tray. Polyelectroly tes . Potassium permanganate. Lime and polyelectrolytes. Lime. Lime and polyelectrolytes. Do. Polyelectrolytes . Lime and polyelectrolytes. Polyelectrolytes . te name and address, as possible based on the information available at the No responsibility can be taken for omissions or changes 59 APPENDIX D. —EQUIPMENT EFFICIENCY Less than perfect performance of sep- aration equipment can be characterized by the separation efficiency. The grade ef- ficiency concept can be applied to solid- liquid separation equipment whose per- formance does not change with time if all operational variables are kept constant. Hydrocyclones , centrifuges, and gravita- tional thickeners are examples of such equipment. This concept is not widely used in filtration because the efficiency changes with the amount of solids col- lected on the face of the filter medium. For filtration, though, it is helpful to determine the grade efficiency of the clean medixom, which influences the ini- tial retention characteristics of the filter and can be used for filter rating (32, p. 31).'' TOTAL EFFICIENCY The total efficiency for dewater- ing equipment can be determined by the equation M< Mt' (D-1) Additional information must be known about the particle-size distribution of the feed solids , the density of the sol- ids , and such operational data as flow rate, temperature, type of fluid, and solids input concentration. A single value for the total efficiency cannot be used to represent the separation capabil- ity of the equipment for any materials other than those actually tested ( 32 , pp. 34-35). GRADE EFFICIENCY The efficiency of separation equipment, however, can be characterized by a gravi- metric grade efficiency function, G(x) . This is a probablistic mathematical ex- pression, based on mass efficiency, which describes the particle trajectories dur- ing the separation process. A grade ef- ficiency function can be developed for each type of separation equipment that describes the efficiency of separation for a range of particle sizes. This information can be graphed as an S-shaped curve, such as the one shown in fig- ure D-1. This graph is often referred to where Ef = the total equipment effi- ciency, Mg = the mass of all solids sepa- rated from a slurry liquid, and Mf = the mass of all slurry sol- ids prior to solid-liquid separation (32, p. 33). The performance of most available sepa- rational equipment is predominantly size dependent, so the total efficiency de- pends on the size distribution of the feed solids and is not suitable for the general criterion of efficiency. Conse- quently values of total efficiency stated by equipment manufacturers may not be en- tirely accurate concerning the separa- tional capability of their equipment. 'Underlined numbers in parentheses re- fer to items in the list of references preceding the appendixes. u a >■" o z UJ o u. u. Ui z o H < < Q. UJ (0 QC < Q. too 98 80 60 50 40 KEY Particle size having 5 0-p ct separation efficiency — X Particle size having 9^ 90T3ct separation efficiency/ Particle size having ^ 10 0-pct separation efficiency _i 20 50 PARTICLE SIZE,(dimensionless) FIGURE D-1. - Example of a grade efficiency curve showing the relationship of Xjq, X93, and X.., (32). 60 as the partition probability curve be- cause it shows the probability for each particle size of either being separated or remaining with the fluid ( 32 , p. 35). PARTICLE SIZE PARAMETERS The grade efficiency curve can be used for determining several operational pa- rameters for a particular piece of equip- ment. The particle corresponding to the 50-pct probability is called the equi- probable size, X30, and is used as the minimum cutoff size or cut size of the particular type of equipment. This cut size is independent of the feed material, and its determination requires a knowl- edge of the entire grade efficiency curve (32, p. 35). In any separation operation, there will be a particle size larger than the grade efficiency, which is 100 pet. This is the largest particle remaining in the overflow after separation of the maximum particle size that would have a chance to escape and is called x^ax ( 32 , p. 38). If the particle trajectories in the separator can be approximated, the most unfavorable conditions of separation are taken for detenaining this limit of sepa- ration. It is difficult to determine the limit of separation accurately, so the size corresponding to 98-pct efficiency, X98, is used, which gives an easily de- fined point. This size is called the approximate limit of separation and is widely used in filter rating ( 32 , p. 38). The relationship of X50, xgs, and x^ax is shown in figure D-1. The concept of grade efficiency is helpful in determining the application of a particular piece of equipment for a particular dewatering or desliming opera- tion. 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