~ IRIlssosl t Bureau of Mines Report of Investigations/19B3

~

Alumina Miniplant Operations­Separation of Aluminum Chloride Liquor From Leach Residue Solids by Classification and Thickening

By Roy T. Sorensen, Dwight L. Sawyer, Jr., and Theodore L. Turner

UNITED STATES DEPARTMENT OF THE INTERIOR

Report of Investigations 8805

Alumina Miniplant Operations­Separation of Aluminum Chloride Liquor From Leach Residue Solids by Classification and Thickening

By Roy T. Sorensen, Dwight L. Sawyer, Jr.,

and Theodore L. Turner

UNITED STATES DEPARTMENT OF THE INTERIOR

James G. Watt, Secretary

BUREAU OF MINES

Robert C. Horton, Director

UNIT OF M~ASURE AllllREVIATIONS USED IN THIg REPORT

atm atmo~phere hr hour

Be' llaume' in inch

deg degree L liter

·C de'gree Celsius lb pound

ft foot Ib/f t 3 pound per euble foot

ft/hr foot per hour lb/hr pound per hour

ft/min foot per minute lb/ton pound per short ton

f t 2 square foot min minute

ft 2/lb square foot per pound mL milliliter

ft 2/tpd square foot per short ton pet percent per day

rpm revolution per minute g gram

sec second gal gallon

Library of C9ngress Cataloging in Publicatio' Data:

SorcIlS{Hl, Roy T Alumina mitlipiant ol)crations-S<'l'arntjon oj alumiHlIllI chloride liq·

lIlH' from leach residue solids by c]nssificarioll ami thickening.

(IlUr'L'IlU of Min"s report elf ;11\,'"~ti!latiolls ; 8805)

IlIcllltil'S biblillglaphical rl'i'uc:nces.

Stlrt. of Docs. nu,: I 28,23:880').

I. Aluminum chloride. 2. Ktl"lin. :\. Leaching. <i. Sepamlion (Tc(:hl1olo!!},). I • .silwy(~r, Dwigbt L. n. TnrHcI', Theodore 1.. HI. Title. IV. SCI'je,.,: ill'pnfl of invesrigations (United Stl1tes. BllreaLl of Mines) ; BBO~,

TN2~~.lJ41! ['1'1'245,:\4] 6228 [669'.,722J 83·600202

CONTENTS

Ab s t rae t ••••••••••••.••••••••••••••• I •••••••••••••••• I ••••••••••••••• II •••••••••

Introduction ................................................................... ' Background •••..•.••...•.........••••••......••....•...•.....••.•..•••.•... Miniplan't test program ••••••••••••••••

Acknowledgments ..•••.••....••.•••......•.....•.....•.....•••.•.•.......•....... Materials, equipment, and procedures ..........................................•

Calcined kaolin feed to leaching reactors ••••••••••••••••••••••••••••••••• RCI feed to leaching reactors ••••••••••••••••••••••••••••••••••••••••••••• Flocculant •••••••••••••••••••••••••••• Miniplant leaching reactors ••••••••••• Classifier section •••••••••••••••••••• Thickener section .••• ' ••••••••••••••••• Mixing tanks ......................... . Miniplant operation ••••••••••••.•••••• Bench-scale testing procedures ••••••••

General procedures •......................•..............•............ Process monitoring testing procedures •••••••••••••••••••••••••••••••• Independent laboratory study testing procedures ••••••••••••••••••••••

Experimental results ••.•••••••••••••••••••• Classifier-thickener circuit ••••••••••

Test parameters •••••••••••••••••• Best fit material balance •••••••• Classifier separation •••••••••••• Thickener compression •••••••••••• Predicted material balance •••••••

•••••••••••••••••••••••••••••• ID •••••

Waste disposal ............................................... 0 co ••••••

Miniplant thickener area and flocculant requirements ••••••••••••••••• Thickener-only circuit ............................................... II •••

Tests on miniplant leached slurry.............. • ••••••••••••••••• Tests on bench-scale leached slurry •••••••••••••••••••••••••••••••••• Predicted thickener-only material balance................. • ••••••

Summary and conclusions ..................................................... 0 ••••

1-2. 3.

ILLUSTRATIONS

Flowsheet of clay-Hel process ......•..................... II ••••••••••••••••

Four-classifier and five-thickener solids-liquid separation circuit ••••••• Settling curve and Kynch analysis for miniplant test 3 classifier 1

1 2 2 2 3 3 3 4 4 4 4 5 5 5 5 5 7 8 8 8 8

11 11 14 17 18 18 19 19 20 21 22

2 6

overflow .............................................. I • • • • • • • • • ••• •• • • • 7 4. Settling curve and Kynch analysis for miniplant test 3 reactor discharge

5. 6. 7.

slurry .................................................................. . Material balance for solids-liquid separation in miniplant test 1 ••••••••• Material balance for solids-liquid separation in miniplant test 3 ••••••••• Effect of leaching and classifier circuits on residue particle size, tests

7 9

10

1 and 3.......................................................................... 12 8. Distribution of solids between classifier 1 overflow and sand products by

size fraction, test 1 .............................. ' ............•.•.•.•....•. 12 9. Distribution of solids between classifier 1 overflow and sand products by

size fraction, test 2 ................................... 0 •••••••••••••••• 12 10. Distribution of solids between classifier 1 overflow and sand products by

size fraction, test 3..................................................... 13

ii

ILLUSTRATIONS--Continued

11. Distribution of solids between classifier 1 overflow and sand products by size fraction, test 4................................................... 13

12. Measured and calculated values for thickener underflow, test 1............ 14 13. Predicted best circuit material balance for solids-liquid separation in

miniplant test 1........................................................ 15 14. Predicted best circuit material balance for solids-liquid separation in

miniplant test 3........................................................ 16 15. Measured and calculated values for thickener underflow, test 3............ 17 16. Flocculant addition versus thickener area requirements for bench-scale

settling of miniplant tests 1 and 3 classifier 1 overflow............... 19 17. Flocculant addition versus thickener area requirements for bench-scale

settling of miniplant tests 1 and 3 leaching reactor discharge.......... 20 18. Flocculant addition versus thickener area requirements in terms of minus

150-mesh solids for bench-scale settling of miniplant test 3 products... 20 19. Effect of fines in thickener feed on final settled solids and design set-

tled solids.. ...... ... ............... .................. .....•..... ...... 21

TABLES

1. Screen analyses of calcined kaolin miniplant feed......................... 4 2. Screen analyses of calcined kaolin used in independent laboratory test

program. . . . . . . . . . . . . . . . . . . . . . . . . • . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . 4 3. Classifier specifications................................................. 5 4. Thickener specifications.................................................. 5 5. :t1i.niplant test schedule ............. G ..................... I-I.............. 11 6. Classifier 1 performance summary.......................................... 11 7. Settling tests using miniplant test 3 thickener feed slurries............. 17 8. Predicted final products from solids-liquid separation--miniplant tests 1

and 3 .•........................................ II. II...................... 18 9. Average flocculant addition per thickener in miniplant tests 1 and 3...... 19

10. Summary of thickener parameters and requirements from bench-scale settling tests on both mini plant and bench-scale leaching slurry................. 21

11. Effect of thickener underflow solids and number of stages on estimated pregnant liquor alumina recovery and content in a thickener-only circuit 22

ALUMINA MINIPLANT OPERATIONS-SEPARATION OF ALUMINUM CHLORIDE LIQUOR FROM LEACH RESIDUE SOLIDS BY CLASSIFICATION AND THICKENING

By Roy T. Sorensen, 1 Dwight L. Sawyer, Jr.,2 and Theodore L. Turner 3

ABSTRACT

The Bureau of Mines has investigated the recovery of cell-grade alu­mina by RCI leaching of calcined kaolin in the alumina miniplant at its Boulder City (NV) Engineering Laboratory.

Classification and thickening were used for separating aluminum chlo­ride leach liquor from the siliceous residue generated in the acid leaching step of the ciay-RCI process. Coarse solids were classified from fines at 115 mesh and countercurrently washed in three additional spiral classifiers. Fines were treated in a conventional five­thickener, countercurrent decantation circuit. When this method was applied to the solids-liquid separation of slurry from RCI leaching of minus lO-mesh calcined kaolin, 75 pct of the residue reported as classifier sands and 25 pct as thickener underflow. Predicted individ­ual classifier and thickener area requirements were 6 and 25 ft 2 !tpd, respectively. Total thickener circuit flocculant requirement was 2.4 lb!ton solids. Aluminum chloride pregnant liquor was produced that analyzed 8.4 pct A1 203 • and contained more than 97 pct of the alumina in the liquid phase of the slurry. Bench-scale settling tests were used to correlate fines content of leach residue to thickener area and flocculant requirements.

'Metallurgist. 2Supervisory chemical engineer. 3Research chemist. All authors are with the Boulder City Engineering Laboratory, Bureau of Mines,

Boulder City, NV.

2

INTRODUCTION

BACKGROUND

In 1975, the Bureau of Mines con­structed and operated an alumina mini­plant to study the production of cell­grade alumina from kaolinitic clay by the clay-HCI process. The process flowsheet shown in figure 1 includes a solids­liquid separation step in which solid residues in the leaching slurry are separated from the aluminum chloride liq­uor and washed countercurrently. Ini­tially, filtration or thickening were considered suitable, although equipment requirements needed to be established. 4

Earlier miniplant-scale research using nitric acid instead of HCI indicated se­vere cloth blinding and excessive filter area requirements when pressure or vacuum filtration was used. :onsequently, two thickeners in series were selected for solids-liquid separation in the prelimi­nary testing in the clay-HCI miniplant. They were sized on the basis of bench­scale settling tests on leach slurry con­taining considerable quantities of fine solids.

In actual operation of the miniplant, packing of coarse residue particles in the thickeners caused plugging of the underflow pumping system. To eliminate the plugging condition, two screw classi­fiers were added to remove coarse solids from the thickener feed. Results from this circuit indicated that a classifier­thickener circuit was feasible, and a new circuit was designed and installed in the miniplant. Five thickeners in series were considered sufficient to indicate operational problems in any multiunit circuit and to predict alumina recovery in relation to the number of units. It was estimated that alumina recovery in four classifiers was the same as in five thickeners.

4peters, F. A., P. W. Johnson, and R. C. Kirby. Methods for producing Alu­mina From Clay. An Evaluation of Five Hydrochloric Acid Processes. BuMines RI 6133, 1962, 68 pp.

MINI PLANT TEST PROGRAM

The first miniplant study (test 1) cov­ered in this report was designed to demonstrate and evaluate the use of a four-classifier and five-thickener solids-liquid separation circuit in the clay-HCI process for the production of cell-grade alumina from kaolinitic clay, and to develop a material balance in terms of liquor, residue, and soluble alumina. A second study (designated tests 2, 3, and 4 of this report) was

Raw kaolin

__ .I:Igl_Q..a~ ____ ,

L-~~~ I r

r

r

r' r

I I r

r

Heat

Heat

Mud and sand residue

KEY --liquors, solids --;:;--:- Vor,orsrgas ~ Study mea

FIGURE 1. • Flowsheet of clay.Hel process.

conducted to determine the effect on alu­minum recovery of circuit modifications intended to decrease· the generation of fines by particle-to-particle attrition during solids-liquid separation, and to compare circuit response using minus 10-mesh calcined kaolin as the baseline leaching reactor feed with a minus B-mesh calcined kaolin feed. Thickener area and flocculant requirements in both studies were to be determined by bench-scale settling tests.

3

The results of these two miniplant studies indicated that an evaluation of the effect of using calcined kaolin finer than minus 10 mesh on leaching slurry settling characteristics was required also. Therefore, the additional bench­scale investigation was conducted by an independent laboratory to compare set­tling characteristics of slurries pro­duced by leaching different sizes and types of calcined kaolin with HCl.

ACKNOWLEDGMENTS

The authors wish to acknowledge the valuable assistance provided by Travis Galloway, research engineer, Reynolds Aluminum Co., during the second mini plant study, and by N. Kelly Brown, formerly

supervisor of testing, Envirotech Re­search Center, Envirotech Corp., for di­recting the independent, bench-scale set­tling evaluation.

MATERIALS, EQUIPMENT, AND PROCEDURES

CALCINED KAOLIN FEED TO LEACHING REACTORS

The raw kaolin used in the mini plant was from the Reedy Creek Mine of the Thiele Kaolin Co., Sandersville, GA, and was crushed and screened to provide a mi­nus 20- plus 4-mesh product; the minus 20-mesh fines were pelletized to about 3lB-in on a conventional pelletizing disk. The mixed material was calcined in a rotary kiln, and was crushed and screened to the desired size. Screen analyses of the calcined kaolin feed products for the miniplant program are given in table 1. Calcined kaolin ana­lyzed 42.7 pct acid-soluble alumina in test 1 and 40.4 pct in tests 2 through 4.

In the first miniplant study, calcined kaolin feed was elevated to an Acrison5

weightometer by means of a screw eleva­tor. To decrease the formation of fines by particle-to-particle attrition, the elevator was replaced by drum hoisting and a horizontal belt conveyor for mini-plant tests 2 through 4.

5 Reference to specific equipment, prod­ucts, trade names, or manufacturers does not imply endorsement by the Bureau of Mines.

The calcined kaolin nmterials that were used in the independent laboratory pro­gram were prepared by two methods. The samples designated minus 10 mesh, minus 20 mesh, and minus 35 mesh were prepared by stage-crushing 1/4-in calcined kaolin pellets to size. A "misted" material was produced by mixing a minus 1B-mesh raw kaolin clay with a fine mist of water while tumbling the clay on a pelletizing disk.6 The water addition was sufficient to cause the finest particle~ to adhere to the coarser ones but was insufficient for pellet formation. Size increase in­curred by the coarse particles was negli­gible. The material was calcined in a laboratory furnace at 7500 C without agi­tation. To obtain the same fines content as that caused by particle-to-particle attrition in fluidized-bed calcination system, calcined material was agitated in an unheated fluidized bed reactor for a similar time period. Screen analyses of the materials before leaching are pre­sented in table 2.

6schaller, J. L., D. B. Hunter, and D. L. Sawyer, Jr. Alumina Miniplant Operations--production of Misted Raw Kao­lin Feed. BuMines RI 8712, 1982, 20 pp.

4

TABLE 1. - Screen analyses of calcined kaolin miniplant feed

(Cumulative weight retained, percent)

Tyler standard Nominal feed size, mesh sieve, mesh Minus 10, Minus

tests 1 and 3 test 10 •••••••••••• 6 29 14 .....•...... 29 46 20 .•..•.•...•. 49 59 28 ......•..... 62 70 35 .•.•....•.•. 71 77 48 .......•.... 78 83 65 ...••....... 85 88 100 •.••.•••••• 90 92 150 ••••••••••• 94 95

HCl FEED TO LEACHING REACTORS

8, 2

/ Commercial 20 Be muriatic acid (31 pct

HCl) was diluted to 25 pct HCl before mixing with calcined kaolin. Both water and acid additions to the leach reactor train were metered by diaphragm-type pumps. Acid addition was set at 105 pct of the stoichiometric amount estimated for dissolving the acid-soluble alumina and iron in the calcined kaolin feed material.

FLOCCULANT

SEPARAN MGL synthetic polymer, a high­molecular weight, water-soluble rtonionic polyacrylamide, was added as a 0.2-pct

solution. The manufacturer was Dow Chem­ical Co., Midland, MI.

MINI PLANT LEACHING REACTORS

Leaching of calcined kaolin was carried out in three jacketed 50-gal, 24-in-ID, glass-lined steel reactors arranged in cascade. Hot oil circulating within the jacket provided temperature control. Acidic vapors from the boiling leaching liquors were condensed by water-cooled reflux condensers and the condensate re­turned to the reactors. The reactor overflows were located 16 in up the ket­tle sidewall, and gave about 23 gal ef­fective volume per reactor. Stirrers were three-blade retreating curve design, glass-coated steel, and operated at 120 rpm.

The leaching slurry from the final re­actor was cooled from 113 0 to about 60 0 C before discharging to the first classi­fier. A stirred mixer with a commer­cially pure titanium submerged cooling coil, a water-jacketed pipe, and a con­ventional glass condenser served as heat exchangers.

CLASSIFIER SECTION

Flared-tank screw classifiers were used in the classifier circuit and were rub­ber lined. Classifier specifications are given in table 3.

TABLE 2. - Screen analyses of calcined kaolin used in independent laboratory test program

(Cumulative weight retained, percent)

Tyler standard Nominal feed size, mesh sieve, mesh Minus 10 10 by 100 Minus 18 1 Minus 20 20 by 100 Minus 35 35 by 100

14 ••••...•. It •• 33 36 NM NM NM NM NM 20 •••••••••••• 55 59 NM NM NM NM NM 28 •••.......•. 68 73 22 29 34 NM NM 35 ••••• ., •••••• 77 82 53 51 60 NM NM 48 ............ 84 90 71 67 79 50 66 65 •••••••••••• 89 96 81 77 91 64 85 100 ••••••••••• 93 NM 80 85 NM 75 NM 150 ••••••••••• 96 NM 94 91 NM 83 NM NM Not measured. 1 Misted.

TABLE 3. - Classifier specifications

Classifier number. ••. 1 4 Cylindrical tank section ID •••••• in •. 18 10 10 10

Tank length (overflow to discharge) ••• ft •. 10 9 7.5 7.5

Pool area ••••••• ft 2 •. Freeboard length.ft •• Slope ••••••••••• deg •• Screw diameter ••• in •• Screw speed ••••• rpm •• Screw speed, peripheral •• ft/min ••

9.6 6.4 7.1 6.9 3.5 3 2.5 2.5

29 21 30 30 16 8 9.5 9.5

0.09 0.40 0.86 0.86

0.42 0.84 2.13 2.13

THICKENER SECTION

The thickeners were approximately 5-ft ID by 5 or 6 ft high and had rubber­covered rakes and shafts. The rakes plowed inward to a small conical section on a flat-bottomed tank. Thickener spec­ifications are given in table 4.

TABLE 4. - Thickener specifications

Thickener number •.. _._.~_1 __ 0~r __ -2-+_3, 4, or 5 (1) (2) Tank construction • •••

Tank diameter ••.. in •. Depth •...•....... ft •. Area ••..••....•. ft 2 .. Rake speed ....•. rpm •. Rake speed,

58 60 6 5

18.4 19.5 0.38 0.53

peripheral .. ft/min.. 5.6 TFiber-reinfo;~;~;lastic-.--~--2Rubber-lined steel.

7.8

MIXING TANKS

Flocculant was mixed with process streams (fig. 2) in 3- and 7-gal, fiber reinforced plastic, center-stirred, cy­lindrical tanks. In the smaller mixer, the flocculant solution was diluted by the thickener overflow. In the larger mixer, the diluted solution was mixed with the advancing thickener underflow. Mixing time varied between 5 and 10 min. Mixing was accomplished by slow-speed stirring (about 60 rpm) with large perfo­rated two-bladed impellers of 90 0 pitch, 8-in diameter by 1-1/2 in high, which were located several inches from the bot­tom of the tanks.

5

Initially, the larger mixers were used for mixing the advancing classifier sands with the returning classifier overflow between each classification stage. To decrease production of fines by particle­to-particle attrition, these mixers were replaced in the second study by inclined, baffled chutes. For the same reason, the smaller mixer initially used for prepar­ing the leach reactor feed slurry by cold premixing of acid solution with calcined kaolin was replaced by an inclined sluice tube.

MINI PLANT OPERATION

In the circuit shown in figure 2, the leaching reactor discharge, after cool­ing in a heat exchanger, was split in classifier 1 into a sands and an overflow product. The overflow product, which contained most of the fine residue, was mixed with flocculant and discharged into thickener 1. The classifier 1 sands were washed countercurrently in three classifiers, while the thickener 1 under­flow was washed countercurrently in four

thickeners. Thickener 1 overflow consti­tuted the raw pregnant liquor prod­uct from the combined circuit; where­as, thickener 5 underflow and classi­fier 4 sands were the waste materials (tailings) .

BENCH-SCALE TESTING PROCEDURES

General Procedures

Bench-scale tests were divided into two categories--process monitoring tests on miniplant slurries and an independent laboratory study on laboratory-leached slurries.

Settling was carried out in 2-L gradu­ated cylinders with a four-wire stirrer run at 0.1 rpm. The stirrer consisted of four parallel 12-gage titanium wires (17-in long) connected at the top and bottom and diagonally spaced 2-3/8 in apart. The stirrer travel encompassed a 2-3/8-in-diam hollow cylinder within the 3-in-diam cylindrical graduate and ex­tended to 1/2 in above the bottom of the

6

Calcined kaolil .--.-----.-----.----_ fo fume scrubber

Water HCI acid

Wash Flocculant water

I··~ . --~~=j Thickener underflow

Classifiers

Classifier sand

Polishing filter and solvent extraction

Pregnant liquor storage

FIGURE 2. - Four-classifier and five-thickener solids-liquid separation circuit.

graduate. The graduate was kept in a hot water bath during the settling period. Interface heights were measured every 30 or 60 sec during initial settling and less frequently when the settling rate began to slow. Height measurement con­tinued for 16 to 24 hr. On completion of the tests, the total sanple was weighed. The clear liquor was decanted and its specific gravity measured. The solids were filtered,· washed, dried, and weighed.

The settling area requirements were calculated by a standardized Kynch test method. 7 The interface height was

7Kynch, G. J. A Theory of Sedimen­tation. Trans. Faraday Soc., 48, 1952, p. 166.

plotted versus time. Examples are given in figures 3 and 4. Tangents were ex­tended to the extremities of the curve. The angle formed by the tangents was bi­sected. The point at which the bisector intersects the curve is the critical point, Cpo A line tangential to the curve is drawn through Cpo A final con­centration line is drawn at the final in­terface height of the test. If the com­pression had stopped, the line was labeled Doo. If cessation of compression was not established definitely, the line was labeled as of the time of the last reading, D16 hr or D24 hr. The percent solids of the compressed slurry was cal­culated from the liquor density, solids density (estimated 2.3), and dry solids weight. The volume height line for the design solids (Ddeslgn) was calculated

1.2

1.1

... -f-.: ::t: (l}.

UJ ::t:

tJl 7

u <t I.i.: IX tJl

.6 I--~

.5

.4

,::\

.2

o 20

2,7 pct solids

Feed: classifier I overflow Flocculent addition 0.54Ib/fon Design oreo: 35,95 flGI fpd'

Gr oduote base

40 60 80

SETTLING TIME ,min

100

FIGURE 3 •• Settling curve and Kynch analysis for miniplant test 3 classifier 1 overflow.

and plotted on the curve. The design solids was always less than the final in­dividual test solids or average replicate group final solids.

The intersection of the curve tangent and the Ddeslgn line is designated Tu. Its value on the time axis is converted from minutes to days. The design unit area (Adeslgn) in square feet of thick­ener area per tons of solids per day is calculated from the values obtained and the following equation:

Adesfgn '"

where

= initial slurry height,

ton =

7

1.4·->:----,,--..------,-------,-----.-----, '--.Ho

1.3

12

1.0-

,9

.8

0fced' 11.7 pcl solids

F ccd reoctar discharge Floccufont addition' 0.17 Ib/tan De sign oreo: 4.84 fiG I I pd

,7 \ Cp

~ ~~~=-~~~~ ,6 ____ ...!\-...:-= ~_ 4.0 deslgn=25 pot solids Tu

.5~~.-O-OO-=2-8-~-SO-lid-S--~---------~

.4

.3

100

SETTLING TIME, mill

FIGURE 4. & Settling curve and Kynch analysis for miniplant test 3 reactor discharge slurry.

and

1.33 = design factor.

The procedure was supplied by the con­tractor that made the independent labora­tory study.

Process }ionitoring Testing Procedures

Settling tests were made on the follow­ing types of miniplant slurries:

1. Reactor 3 discharge.

2. Classifier 1 overflow.

3. Thickener 1, 2, 3, 4, and 5 feed.

A 1,450~L sample of reactor 3 dis­charge slurry was diluted with 550 mL of a mixture of 0.2 pct SEPARAN }IGL solu­tion and actual miniplant thickener 2

8

overflow. If 10 mL of the flocculant solution was used, 540 mL of the thick­ener overflow solution was used. These volumes were set by the anticipated mate­rial balance flows. The thickener 2 overflow represented the countercurrent wash. The ratio of the volumes was close to the ratios of the actual miniplant flowstreams. The diluted flocculant so­lution and the reactor 3 discharge sample were blunged for 60 sec in a 2-L graduate before the settling test was started.

For settling tests on classifier 1 overflow, the ratio of classifier over­flow to the mixture of flocculant solu­tion and thickener 2 overflow was 1,470 mL to 530 mL, and was close to that of plant flowstreams.

The constant temperature bath was main­tained at 80 0 C for settling tests on the reactor 3 discharge; this is the tempera­ture expected in the first thickener han­dling leach slurry in a commercial opera­tion. The temperature vas maintained at 40° C for settling tests on the classi­fier 1 overflow; this is the temperature expected in the first thickener following heat exchange treatment and classifica­tion of leach slurry in a commercial operation.

In the tests run on feed slurry of the individual thickeners, approximately 2,000-mL samples were taken. No floccu­lant was added and settling was carried out in a 40° C constant temperature bath.

Independent Laboratory Study Testing Procedures

Settling tests in the independent labo­ratory study were carried out on

acid-leached slurries from batch leach­ing of different sizes and types of calcined kaolin. A 4-L glass resin ket­tle with a hemispherical bottom was used as the leaching reactor. The 3/4-by 4-in flat bladed, crescent-shaped, paddle-type impeller was set about 1 in above the bottom of the reactor and ro­tated at 185 rpm, which was the minimum speed for keeping the solids in suspen­sion. The charge was 1,655 g of 26-pct HCI and 460 g of calcined kaolin. After addition of solids to the boiling liq­uor, leaching was carried out at boil­ing (about 107° C) for 30 min. The re­sulting slurry had a volume of about 1,520 mL and was diluted before each set­tling test with 380 mL of liquor contain­ing about 6.7 pct Al 203 (commercial alu­minum chloride) to simulate a system using a countercurrent wash. The rela­tive quantities of these bench-scale products and their concentrations were similar to those in the rniniplant opera­tion. For instance, miniplant thickener 2 overflow averaged about 6.3 pct A1 203 . The 0.2-pct flocculant was added and mixed by blunging with an inverted rub­ber stopper attached to a glass rod (five up-and-down strokes). An injection of one-fifth of the flocculant to the face of the stopper on each upstroke was made with a hypodermic syringe connected to a thin plastic tube that directed the solution to the face of the stopper. A thin titanium metal sheet over the end of the stopper dispersed the floccu­lant to the sides of the stopper. After mixing, which required about 15 sec, the plunger was removed. A stirrer operating at 0.1 rpm was inserted and settling begun.

EXPERIMENTAL RESULTS

CLASSIFIER-THICKENER CIRCUIT

Test Parameters

Two rniniplant studies were made cover­ing four different sets of test condi­tions shown in table 5. As described in the "Materials, Equipment, and

Procedures" section, a number of circuit modifications were made and process throughput was increased in the second study (tests 2, 3, and 4) to decrease production of fines by particle-to­particle attrition. Only one set of con­ditions (test 1) was used in the first study. In the second study, the major

DilullDn HZO '-, .. ----.-,~-

10660lb Q.601b

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I 4.031bA1203

\ flocClJbtI . !."~" A~03. \

' 0.60 ib . t .~---, . !:-Iquor

322..05Ib 16. I I Ib A.lz 03 !'l00'. AI2.03

Solias 30.55111

8.66% I I

Li,\1Jor "]73101b

869 Ib AI20.~ 500')1. AllO,

Solids 30551b 1469 '1'.

1

L.!9~<JL 148.35'b

7 42111 Alz03

Leoching mactor dISCharge ··Llq~or--~~-- -.~

4385"""lb 41B81bAlz03 955%Al z03 ~a 56311b I I 74"1.

, Liquor 48S 50!b

41 7!'llb A1z03 8.54 '1'0 AIZO)

Solids 3Cl.551b

589 "I"

flacculanl 5.00 r~ AIZ03 I~-T'--- ""'<>-"--"'<>"-,

Lt~

35640lb 22491b AIZ03

6.31"'q A120~ SQUd§

30551b 790°;"

~ 3 ! i

LiquQr 19""<>35Ib

12 081b Ay"O;o 6 31 % A1z03

Sotids -3-6~55 Ib

3,17'1'. i

Liquor

165051tl 10.42 It! AllO,

, Floccu\CI!11 A'lOJ : 4-60~lb~ --J-"-~----~

410.7010 30.90 Ib AI:z03

731% AIC!O:3 Sclias ·"30.55th

5 9170

2.06.2.3Ib I 5De Ib Alz03

7.3! % AIZ03

Ib 12..90'1.

Liquor 447slb

3 821b AI2,03 B 54'0 Alz03

Solids -37.29111

45.44% I-"-'-~'i

• • I..lquor Y3~461b

5 17 Ib AI" 03 3.88% Al203

Soh!!2 41491b 23.72 'Y.

~~ql!~ BB68lb

BASIS

Calcined kaolin .. ___ .... _____ .' Ib ." 1 00

Hel. 25% .... _ ••.. _ Ib ._.397

Flocculam, SEPARAN MGl. __ % 0.2

38751b 1.!')Olb AI'i!03 3.88 % A\20'l

Solids :.;i96 4520~n

~~'-- ....

I 351b A120;3 1.52 'I'D Alz03

. i Uquor

Lj

t Lrquor

4.201b £l.52%

"_ ~nofil ,iJ..!l.!!cr, 456.301b

37081b AlzO;, ~U:3 % AI20J

AlzC3 recovery -8a53-~"

241~051b I':IS8Ib Alz03 B 13 % Alz03

SOlids 36~55111

11.21%

IZVOlb I. 891b AIl03 I 52 % AI2.03

SOlids 34351b

21.60 "I.

Liq~or -,.-..... '--.--~-

L)quor

8595ib 3911) A1z03

.45'r"AI2.0 3 .§.~

2,.39Ib 2..71 'l'.

36.02. Ib .54", AIZ 03

1 52 '. Aiz 0 3 SOlids ''3Ql5111

4556 'YD r-······_··· .... ,,~-~_I~~ 116.891b

.54 Ib AI'lOJ 45% AIZO:3

Solids -'3cli511l

2050%

FIGURE 5 •• Material balance for solids-liquid separation in miniplant test I. Best fit of sample analyses and flow measurements,

~11\Jll~r: ,I:1,z 0 80571b

Waste --- liquor --30.94Ib

.141b AI!03

.451'. AlzO;3 Soilds ~7 76 1U 47. 29 °1.

AJ~ 0310,5 . -33%

9

10

, Waste llijUor

32.56lb 421b Alj10,

1.26% AI20J 5.l:I.fuU.

1457ib 30.91 %

1112 OJ 1055

110 '0

F locculGnl Q.46Til-

. , LlqlJor

43,701b I 151bAI203 264 %Alz03

50110s 14-:-571b

2500%

Ib 5.5511.1 AI20J 4.44 % AI20,

Sollds 1457

10.45% '-"'-'.--''T'-- __ Li quol 78.221b

U I,

f Uql.lor 46641h

2,081b Al20J 4.44% A 203

SOlids -iq:"57Ib

23,80 %

3.48 Ib AI20j Flcccuklnf 444 % AleO,

2221b ---r--­Liquor 159261b

10,09 Ib Alz03 6.34% Alz03

Solid, ~71b

8.38 %

Liquor -ST,30Ib

3.891b AI203 634% A'Z03

Solids -'4,57Ib

19.20 %

BASIS

Calcined kaolin _______________ Ib 100

Hel, 25% ___________ ~ _________ Ib __ 346

Flocculant, SEPARAN MGL _____ " 0.2

Liquor . 97.12'lb

2,591b1l1203 :265%AJ20~

Sol;ds "'1-:7 Ib

1,7 %

. ___ ._ PregnCJnt liquor

Liquor -593110

3.031b AIZOJ 5,II%AI20~

Solids 48~62Ib

45.05 '%

, ~----L'quor 148,73 Jb

3.941b Alz03 2.65'1" A120"

Solids 4 7 ,191b 24.84%

liq..or '59.431b

,91 1b AI203 I.Of%AI203

Sollds .541b 60 %,

I 445.301b ~ 37.3Blb A120:,!!

, liquor -7fJ811b

, 840"10 AI203

6 621b Alz03 8.40%AI,,03

Solids 14571D

r 5. fiO "Ill

Liquor -51.03Ib

1.35fb A!20J 2.65% A120~

So:iris -5(fS7Ib

48.16 ". r liquor i3:3J61b

I .351b A1l!0:s 1.01'1'uA!203

Solids lf7.411b

2026%

FIGURE 6, c Material balonce for solids-liquid separafion in miniplont test 3. Best fit of sample anal~'ses and flow meaSliremenls.

W~9te

- llifuor 43.72 1b

441b AI20J I 01% AI203

Solids 46,87 I b 51.73'ru

AI loss

1 1

~ J

TABLE 5. - Minip1ant test schedule

Test and study .•.. 1 , I 2, II 3, II 4, II Duration ..•• days .. 10 2 6 1.5 Minip1ant circuit. U M M M Calcined kaolin feed:

Nominal top 10 8 10 10 size ••... mesh ••

Rate •••.• 1b/hr •• 100 173 175 243 M Mod1f1ed to ffilnimize production of

fines by partic1e-to-partic1e attrition. U Unmodified.

sampling and process evaluation effort was made for test 3. Test 2, which used minus 8-mesh calcined clay, was aborted after 2 days because of classification problems. Test 4 was run for 1-1/2 days to investigate circuit capacity. Fresh water addition to the thickeners was set about equal to the amount of liquor ex­pected in the thickener 5 underflow. To obtain a steady operation, the wash addi­tion to the classifiers had to be set considerably above the amount of liquor in the classifier 4 sands.

Best Fit Material Balance

Solids-liquid separation material bal­ances were made by best-fitting the aver­age circuit flow rates and sample analy­ses to the constraints of the f10wsheet in figure 2. The balances for tests 1 and 3 are presented in figures 5 and 6, respectively, and are on a basis of 100 1b/hr of calcined kaolin feed to the leaching section. The higher process throughput in test 3 and the circuit mod­ifications to decrease partic1e-to­particle attrition resulted in increas­ing the aluminum recovery in the preg­nant liquor from the solids-liquid sepa­ration from 88.5 pct in test 1 to 97.8 pct in test 3. A1 203 contents in the pregnant liquors were 8.1 and 8.4 pct, respectively.

The increase in recovery resulted from the smaller amount of solids reporting to the thickeners and the higher thickener underflow densities in test 3. This is because the higher the underflow or sands fraction solids in a countercurrent

11

washing system, the higher the washing efficiency or solute (in this case, alu­minum chloride reported as A1 203 ) recov­ery. T~ese best fit balances represent the best estimate of average circuit con­ditions during the tests. Because thick­eners 2 through 5 were improper<ly oper­ated in test 1, and thickener pulp was pasty and contaminated with coarse parti­cles; in test 3, it was necessary to mod­ify these balances to predict results under proper operating conditions. The predicted balances are discussed later in this report.

Classifier Separation

Classifier 1 separated both the fine residue solids and the pregnant liquor from coarse solids. Both classifier functions were dependent on residue sol­ids particle size distribution, leaching slu~ry throughput, and slurry viscosity. Classifier performance is summarized in table 6.

The minus ISO-mesh fines fraction of the leaching slurry solids fed to the classifier section varied from a high of 36 pct in test 1 to a low of 16 pct in test 4. Fines production was decreased owing to circuit modification and higher throughput, which also resulted in a

TABLE 6. - Classifier 1 performance summary

Tes t 1 •••••••••••••••••••• 1 2 3 Feed solids:

Rate ••••••..•••• 1b/hr •• 58 107 107 Minus 150 mesh •••• pct •• 36 224 21

Overflow, settling ve10cit y3 •••.••. ft/min •• 41 65 60

Split, solids to thickener: Pct of total •••.••••..• 51 277 25 Size 4 •••••••.•••• mesh •• 115 220 ll5

Area requirement ft 2 /tpd solids •• 12 7 7

4

150 16

81

24 100

5 lSee table 5 for test conditions. 2Extreme1y viscous classifier feed

slurry. 3Average upward flow velocity in pool

area. 4Ty1er standard sieve.

12

smaller fraction of the SQlids reporting to the thickener section. The decrease in fines from test 1 to test 3 is illus­trated in figure 7 and resulted in a cor­responding decrease of from 51 to 25 pct in the percentage of residue reporting to the thickeners. The least fines were contained in the classifier feed to test 4, when only 24 pet of the residue re­ported to the thickeners.

The classification size made in each test is illustrated by bar graphs (figs, 8 through 11) that were developed from flow rates in the material balances and screen analyses of the c1aasifier prod­ucts. The bars compare flow rates of

... o Q

o w z <t I­W a:

j::

100 I

g[) L -0-.0

1% so - -!J-

-0-

7D

60

~ 50 3:::

20

10

Test

1

3

Product

Calcinod kaolin feed

Leaching reactor 3 discharge

1 Composite 3 classifier

discharge

o 10 14 20 28 35 48 65 laO 150

Tesl

SCREEN SIZE, mesh (Tyler slal1dard siove)

FlGURE 7. ~ Effect of leaching and classifier circuits on residue particle sizel tests 1 and 3.

~ o

25

KEY

Clossitisr I sand

ClasslfiGr 1 "vertlo\\' U

u: 15 rn Cl ::J a tIJ

w :::J Q f,fJ LU II:

B by 20 20 by 4B 48 by 100100 by 150 Minus 150

SIZE FRACTION, m~9h [Tyler standard slevol

FIGURE 8. g Distribution of solids between

(:Iassifier 1 overflow ond sand products by size fract ion I test 1.

70,-----------~~~----------------------~ KEY Classfie, I sand ~

Classifier I overllow C 60

.c:.

'" '" E

0 (\j

.... c ... '" '" ~ .i1

~ u

0 ~ 0

'" !

I

10 I

I

6 by 14 14 byZO 20byZ8 28by~5 Minus 35

SIZE FRACTION, mesh I Tyler stondord siolll!)

FIGURE 9. - Di~tributjon of solids between clas­sifier 1 overflow and sand products b)' size fraction, test 2.

l: "-~

w f-<l; cr:

3: 0 ..J IL

IJ)

0 ::i 0 ({)

ILl :J D

iii ILl a:

40,---------------------------------------,

15

10

o 8 by2.0

KEY Classifier I sand ~

Classifier I overflow D

.r:. '" OJ

E

~ --0 -0. '" ~

'" ;;:: 'iii ..., c

(3

0 I()

0 1.0

I I

I

2.0 by 48 48 by 100 100 by 150 Minus 150

S IZ E FRACTION, mesh (Tyler standard sieve)

FIGURE 10 •• Distribution of solids between clas­

sifier 1 overflow and sand products by size fraction, test 3.

specific size fractions in classifier 1 overflow with the same size fraction in classifier 1 sand. The bar graphs were used to determine the mesh size of the classifier split shown previously in table 6. The size at which particles reported to both overflow and sand in equal amounts was the split size. (In both tests 1 and 3 these equal amounts occurred in the 100- by 150-mesh frac­tion whose median size can best be repre­sented by the Tyler fourth-root-of-2 series size of 115 mesh. In test 4, the equal amount fraction must be coarser than 100 by 150 mesh but finer than 65 by 100 mesh and can be best represented by 100 mesh.) Sharply defined splits were made in all tests except 2. In test 2 a poorly defined split at 20 mesh was obtained, and 77 pct of the residue reported to the thickeners. The ineffi­cient classification was caused by a very viscous reactor discharge slurry. Since the minus 8-mesh calcined kaolin feed was

13

110,----------------------------,

KEY

Classifier I sooo ~

Closs~ier I overflow

o 8 by 48 48by65 65 by 100 IOObyl50 Minus 150

SIZE FRACTION, mesh (Tyler standard sieve I

FIGURE 11. Q Distribution of solids between

classifier 1 overflow and sand products by size fractionl test 4.

too coarse for the reactors, solids built up and particle-to-particle attrition increased. The presence of very coarse material in the thickeners decreased overall Alz03 recovery, caused underflow pumping problems, and negated the pre­classifica~ion step.

Classifier 1 pool area requirements pertain to the actual miniplant pool area which, under the conditions listed in table 6, provided the solids splits described in figures 8 through 11. The decrease in classifier 1 pool area re­quirements from test 1 to 3 of 12 to 7 ft 2 per ton of solids treated per day reflected the corresponding increase in classifier throughput shown in table 6. This increase occurred without coarsen­ing the size of the classifier split and was due to the decrease in fines content of classifier 1 feed from test 1 to test 3.

Assuming that particle size settling velocity in a classifier varies with the square of the particle diameter,S the classifier pool area requirements can

A. M. Principles of Mineral Dressing. McGraw Hill Book Co., Inc., New York, 1939, p. 217.

14

be decreased by increasing the size of splits as follows:

Mesh of split •••••••• 115 100 65 48 Percent of classifier pool area require-ment at 115-mesh split ••••••••••••••• 100 71 36 18

In confirmation, the coarser (lOa-mesh) size split in test 4 accompanied a great­er classifier input, which corresponded to an area requirement of 5 ft 2 per ton of solids per day, or about 71 pct of the 7 ft 2 of pool area used per ton per day in test 3.

On a basis of relative size of the ac­tual miniplant classifiers, classifiers 2, 3, and 4 pool area requirements would be about three-fourths of that of classi­fier 1. On a basis of holding rising overflow velocities in classifiers 2, 3, and 4 the same as in classifier 1, about one-fourth of the classifier 1 pool area would be required.

Thickener Compression

Miniplant thickener operation, test­ing, and evaluation were correlated to test 1 (first study) and 3 (second study) conditions.

Alumina recovery in the thickener sec­tion was proportional to the percent sol­ids in the underflow streams. Estimates of the underflow solids under test 1 con­ditions are given in figure 12. The val­ues obtained in the "best fit" material balance were a compromise between ob­served solids and the rest of the mini­plant test measurements and were low. The low percent solids in thickeners 3 through 5 were due to an inability to pump the underflow at slow rates so that a buildup of settled solids in the thick­eners could occur. Underflow pumping rates slow enough to maintain a 1-1/2-ft depth of settled pulp could be maintained in thickeners 1 and 2 because slurry in these thickener stages inherently settled to lower percent solids. Lower solids gave greater pump discharge volume when

equal solids flow rates were maintained between thickeners. The measured thick­ener 1 underflow was 13.3 pct solids, was attained with the test 1 circuitry, and represents the expected value when oper­ating with a sufficient inventory of set­tled solids.

When test 1 sampling was completed, an on-off timer was installed on thickener 5 underflow pump, and the inventory was in­creased from the 3 in it had held for days to 1 ft by maintaining slow pumping rates. The underflow held steady at 22 pct solids for several hours at an equi­librium pumping rate. If timers had been used, 22 pct solids could have been sus­tained in test 1 and also higher solids in thickeners 3 and 4.

Only the 13 pct solids for thickener 1 underflow and the 22 pct solids for thickener 5 were obtained with adequate pulp inventories. These two values and interpolated percent solids for the in­termediate thickeners (fig. 12) are the only estimates for predicting thickener operation at adequate pulp inventories. A predicted material balance (fig. 13) developed by modifying the test 1 data and using these new values should provide a more reliable estimate of expected solids-liquid separation than the best fit material balance (fig. 5).

(; a.

ui 0 :::; a C/)

::: a ...J LL II: w 0 z :J II: UJ Z w ~ Q

:i: I-

28

26 (') Mlniplanl samplo average /' t; Bench-scale reseltling lest /"

24 '" Best lit rna lorial balance "

0 Prodicted material balance ,/ 22 /~

,/'/ 20

4'~'-' lB

./ 16 .p-.--if'

.~' 14 .~ . ~ .

12

10 2 3 4 5 6

THICKENER NUMBER

FIGURE 12 .• Measured and calculated values

for thickener underflow, percent solids, test 1.

r £locculanf . 060lb

liquor 123:74 Jb

5.22 ib 1'11,,03 422% Alz 0 3

SolidS 30551b 19 eo,<

lllocMlng rE'OOor cischarge ·····Liq\J~~-~-· ... - -._ ..... ~ ....

i i 3 I

""~ \/

1 Uquor 14:n)3

6141b Ai203 569% A120!

Solids '30.5510

.760 % I

43B,57ib 41 88!1l AI:a03 955 %Alz0:3

Solids 5SJJ Ib 1174%

Ib 589 %

.--'T'-... ~-- Lig~ 160.12 ib

9,r21b AlzO;, 5,69 % AI2 03

Floccu/orrt 4,601b "r"

Liquor 367,381b

25 381b 111203 6.91% AI203

Solid, 30.551b

7.68

169.12.lb 1 1.66lb All! 03

6,91'Y" AI 203 Solids ~51b

15,30%

BASIS

Calcined kaolin Ib 100

-'1"~-.. --- LI~ 44.7810

HCI, 25% __ _

I hiquor 94.711b

3.6SibA1203 3.68%AI:.;03

Solid§ 9.531b 9,14%

1

.3 S21b AizO" 8 54% AI'! 0;,

Solids

Floccullll1t, SEPARAN MGL ___ % ___ 0.2

:n

202,66 ib Ib 11120;, 'l'o 111203

Ip 13.10 '0

~lQuor

88681b 35 10 A 12.0;,

152'.11120)

420lb 452%

I

38751b ! 50lb AI203 3.88 'Yo A120;,

SJ'liQ3. 3196

45,20'l'. ~~"-,

L'quor :24.70 Ib

U! 91b Alz03 1.52 % AI2,03

Solid~ 34351b

21.60 % ___ LI'l.~~

36.021b .541b A I 203

1.52 % AI2 0:3 Solids ~5 Ib

85.951b 45.56 %

E!!!gnonf Ii~o(

48!HiOlh 39. i 9 Ib AI20l 802 % AI~OJ

A I 2. O~ reCO\'!1fY

-93.58%-~

,3910 AI203 .45 %A'203

Ib 2,71%

t-·_··_·· Liqu(}r

116.891b 54 Ib A12l'3

,45,% AIZ03 Solids -30151b

20.50')'.

FIGURE 13 •• Predictod best circuit moterial balance for so ids-liquid separation in minipiClnt test 1,

IS

ao 1371b

,\1 II ',1

il 1i F

16

Dilution H20

47.581b

Ib 1.49 Ib AIl03 1.62'1;, AllO;,

Solids

0.461b

l4.571b 13.70 r-- _~~~_ ... Liquor

M.271b Ib

Ikl\Sor . 37.461b

.611b AIZO) 1,62'7'0 A1203

Solids 1457tb

2800% All/. 03 loss ~ 1.60%-

tb 3,99 Ib A!Z03 3.40"'" Alz03

Ib 11.05 % u,t

Liquor 43,701b

I 491b Alz03 3.40 'YD AIZ 03

SOlids !4:571b

25DO'J(,

't -- ~iCluor 72.631b 250lb Alz03

Floa:uk:mt 3.40% AlzO,3 0,501b-~-t

liquor 145,241b

7.27b All' 0", 5.00 D/o Alz 0 3

Solids ~14-:S? Ib

9.12%

3 t

Liquor -6210 Ib

3111b AJp03 5.00% Alz03

;;J~.t:::"1L! AIz03 9.98% Alz03

Solids 61441b 13,82 'Yo

Ib

, Liquor 484,25 Ib

43.40 Ib Alz03 8,96'Yo Alz03

Solids 64-]0Ib

r 1,75%

37.79 Ib Alz03 % Alz03

Liquor IOI}'Olb

5,17IbAlzO;, 5.11 "oAlz03

SoliCl!i

BASIS

Calcined kaolin ______ ._ .. _____ Ib ." 1 00

Hel. 25%. __ .",. _________ , _____ Ib 345

Flocculant, SEPARAN MGL ____ %_ 0.2

Ib b 334%

~6 Ib 2,93 % 820 Ib Alz03

5,11°10 AI203

[

A_ !:!quor I 8:3.14 Ib

4,16 Ib Alz03

IFlocculont 5,OO%AZOJ [ 2 22 I b ... t " """""','

Li ql. or 174,851b

11.721b A203 6. 7 00/0 AI203

Solids t , 14,571b 7.69 oro ,--~~0 ___ Liquor

Ib 4.77 Ib 41203 6.70% AI203

103.72.10 6,951b A1Z03

F1oc.culant 6 ~O % All! 03 4 .62ii; --"-1

. , Liquor 529891b

44,751b AI203 844% AI203

Solids 14.571b 268%

Uquor

97121b 2.591bAlz 0 3 2.65% AI20J

Solids T.751b I %

!:Iquor 59,311b

3,03 Ib A120:3 5.11 % AI203

Solids 48621b

14873 Ib A120;, Alz 0 3

Liquor 'S9431b

,glib Alz03 1,01 % Alz03

SDlids , ,541b

60%

Liquor 5f031b

I ,351b Alz03 2.65'1'0 Alz03

Solids 50,871b 48.16 ·10

fill( , Liquor 133-:-161b

1 351b 41203 1.01°,(, Alz03

SDlids 47.41 Ib 26.26%

FIGURE 14 •• Predicted best circuit material bulunce for solids-liquid separation in minipiant test 3.

Waste ,-liquor

43.721u .441b Alz03

J .01% Ai'203 So ids

46.87 III 5173%

412°3 loss -I',ls°/"-

An additional estimate of underflow solids was obtained by resettling 2-L samples of thickener underflows (fig. 12). Resettling of thickener 1 underflow gave an estimate for percent solids from thickener 2, and thickener 2 underflow resettling for thickener 3, etc. These data closely agreed with the underflow percent solids in the predicted material balance (fig. 14).

In the second study, all thickener un­derflow pumps were equipped with on-off timers so that slow pumping rates could be maintained to permit pulp inventories of 2 to 2-1/2 ft per thickener.

At the end of test 3 of the second study and when most of the minus 8-mesh material from test 2 had gone through the thickener circuit, bench-scale settling tests were made on feed to individual thickeners (table 7). The final under­flow solids observed in the tests agreed with the average underflow solids from plant sampling and those obtained in developing the best fit material balance (fig. 15). However, miniplant thickener 4 and 5 underflows had a pasty consist­ency similar to their counterparts in the settling tests. Steady pumping of

TABLE 7. - Settling tests using miniplant test 3 thickener feed slurries'

Solids, pct Average Thick- Feed Thickener thickener area ener slurry underflow requirements, 2

(design) ft 2 /tpd 1 ••••••• 2.4 14.0 26.74 2 ••••••• 6.7 17.0 26.08 3 ••••••• NR NR NR 4 ••••••• 11.7 25.0 24.04 5 ••••••• 14.8 28.0 32.99

Av •••• NAp NAp 25.21 NAp Not appllcable. NR Not run. , Included miniplant recycle wash liquor

and miniplant flocculant addition. 2Kynch method of analyses was used.

The area requirements are in terms of tons of total solids to the thickener; would be 1.15 times as high in terms of minus 150-mesh solids, and 0.15 times as high in terms of calcined kaolin leaching feed.

17

these products required a degree of oper­ator attention intolerable in a commer­cial operation. Bench-scale observations indicated that a small amount of liquor markedly increased the fluidity of these products. Decreasing the underflow per­cent solids in a material balance for predicted operation (fig. 14) to the level of the Ddeslgn values (table 7) would provide an underflow that is fluid enough to be tolerated in a commercial operation.

Predicted Material Balance

Predicted solids-liquid separation for the classifier-thickener circuit under the conditions of test 1 and 3 are pre­sented in figures 13 and 14. Data summa­rized in table 8 show that predicted alu­mina recovery increased from 93.6 pct in test 1 to 97.3 pct in test 3. A1 203 contents in the pregnant liquors were 8.1 and 8.4 pct, respectively. As in the best fit material balance, the higher recovery in test 3 was due to increases in the solids fraction reporting as clas­sifier sands and in the thickener under­flow solids. To attain a 99-pct Al 203

r:; 0.

en 0 :J 0 en :!: 0 ...J lJ.. a:: w 0 Z :J

a:: w z w :.:: u :i: t-

32

30

28

26

24

22

20

18

16

14

12

10

. // "

/ / / '

/ '

/ / ./// ~ ..

"P' " y /'

/" KEY

/ 0 Miniplant sample average if/" 8 Bench-scale settling test. Do;>

[J Best iit material balance

2

o Bench-scale settling test, Ddesign

3 4

THICKENER NUMBER

5

FIGURE 15. ~ Measured and ca Iculated values

for thickener underflow, percent solids, test 3.

18

recovery, it can be calculated with the test 3 material balance as a basis, that five classifiers and seven thickeners are required. A pregnant liquor product con­taining 8.6 pct Alz 03 would be obtained.

Waste Disposal

The waste product from the solids­liquid separation consisted of thickener 5 underflow and classifier 4 sand. Half of the residue solids reported to the thickener 5 underflow in test 1 and one­quarter in test 3. As shown in table 8, thickener 5 underflow was 22 pct solids and classifier 4 sand was 47 pct solids in test 1 and significantly higher in test 3, 28 and 52 pct solids, respective­ly. The composite tailings in test 1 contained 29 pct solids and occupied 69 pct more volume than the as-mined clay. The composite tailings from test 3 contained 43 pct solids and theoretically occupied only 11 pct more volume than the as-mined clay if no air was entrapped in the mixture.

The thickener 5 underflow was very pasty during test 3 when its solids content approached or exceeded 30 pct. As the composited waste product from test 3 containing 43 pct solids could probably

not be pumped, especially not as a thick­ener underflow, a thickener-only circuit could not provide a waste product as com­pact as that obtained in test 3. Because of the handling problem at high solids content in the thickener underflow, the combined tailings volume predicted from a hypothetical five-classifier and seven­thickener circuit should be at least 111 pct of the aS4mined clay volume.

Miniplant Thickener Area and Flocculant Requirements

In both miniplant studies, flocculant addition was first set by preliminary settling tests, modified by visual obser­vation, and then held at the rates shown in table 9. The settling area require­ments for test 3 as shown in table 7 are from settling tests on thickener feed that had the flocculant added in the miniplant. The relationship between thickener 1 area requirements and floccu­lant addition as shown in figure 16 is from bench-scale settling tests on clas­sifier 1 overflow that had the flocculant added at the bench. With the 1.3 Ib SEPARAN MGL per ton of solids added in the miniplant, thickener 1 requirements were 27 ft Z per ton of solids per day in test 3 (table 7). If the flocculant were

TABLE B. - Predicted final products from solids-liquid separation-­miniplant tests 1 and 3

Liquor, pct' Slurry Test Product Alz03 A1z03 Solids, Percent of

analyses distribution pctl mined clay volume Z

1 •••• Raw pregnant liquor •••••• B.O 93.6 0.01 NAp Classifier 4 sand ••..•.•• .5 .3 47.3 45 Thickener 5 underflow •••• 2.4 6.1 22.0 121 Composite waste product •• 1.9 6.4 29.5 169

3 •••• Raw pregnant liquor •••••• 8.4 97.3 .01 NAp Classifier 4 sand •••••••• 1.0 1.1 51.7 46 Thickener 5 underflow •••• 1.6 1.6 2B.0 65 Composite waste product •• 1.3 2.7 43.1 111

NAp Not applicable. lBased on predicted material balances, figures 13 and 14. zBased on 76 Ib/ft3 of dry kaolin in place as mined and 1.163 Ib of dry kao­

lin per pound of calcined kaolin. These are calculated from values supplied by D. White, former Georgia State Liasion Officer, Bureau of Mines, and from Dana's Manual of Mineralogy, 7th ed., 1951. Residue solids density of 2.3 and liquor densities estimated from A1z03 analyses were used.

TABLE 9. - Average flocculant addition per thickener in miniplant tests 1 and 3, 'pound SEPARAN MGL per ton of solids thickened

Thickener Test 1 Test 3 1 ••••...••.... 0.58 1. 27 2 ••••••••••••• .60 .61 3 ••.•••••••••• .24 .14 4 ••••••••••••• .08 .25 5 ••••••••••••• .08 .12

Tot ale •••••• 1.58 2.39

added at the bench, figure 16 shows that the area requirements of 27 ft 2 per ton of solids per day is attained with only 0.07 lb/ton. The longer retention time in the miniplant mixer (5 min versus 1 min) and/or the different miniplant mix­ing agitation (stirrer versus blunger) may have caused less efficient conditions in the miniplant.

Bench-scale settling tests on individ­ual mini plant thickener feeds (table 7) show that the thickeners have almost identical area requirements of 25 ft 2 /tpd at the miniplant flocculant additions shown in table 9.

THICKENER-ONLY CIRCUIT

Tests on Miniplant Leached Slurry

Preclassification was required in the miniplant before thickening of leaching slurry from minus 10~esh calcined kaolin because the thickener underflow pumps could not handle the coarsest material. Larger scale equipment or even a finer sized leaching feed would make the use of classifiers optional. To evaluate a thickener-only circuit, bench-scale set­tling tests were made on unclassified mini plant leaching reactor discharge pro­duced during tests 1 (study I) and 3 (study II) and are summarized in figure 17. Lower fines content of the reactor discharge in test 3 than in test 1 re­sulted in a drastic decrease in the bench-scale flocculant requirements and the calculated (Kynch method) area requirements.

19

250r------r~--~~-----,------,_----_,

~ z w ::!E w a:: S ~ a:: «

150

li! 100 « a:: w z w ~ u :r: I-

50

KEY

Test Ddesign

-0-. I 13,1

----.0r- 3 14,0

~----------,6

O~----~------~------L------~----~ 2 3 4

FLOCCUl/\NT ADDITION ,Ib SEPARflN MGL per ton or solids

FIGURE 16.· Flocculant addition versus thickener area requirements for bench-scale settling of mini­plant tests 1 and 3 classifier 1 overflow.

5

A comparison with testing on classifier 1 overflow (figure 17 with figure 16) in­dicates an appreciable decrease in thick­ening requirements when the classifiers were eliminated. Underflow solids were higher when pre classification was elimi­nated (20 to 25 pct solids as opposed to 13 to 14 pct solids). These differences were due to the amounts of fines in the thickener feed. The fines in the thick­ener feed were critical to the relation between flocculant addition and thick­ener area requirement. If the SEPARAN MGL addition in pounds per ton of minus 150~esh solids is plotted against thick­ener area requirements in square feet per ton of minus 150~esh solids per day, figure 18, the classifier 1 overflow

0.1 0.2 0.3 0.4 0.5 06 07 0.8 09 1.0 1.1

FLOCCULANT ADDITION, I b SEPARAN MGL per ton of solids

FIGURE 17.· Flocculantaddition versus thickener area requirements for bench-scale 5ettl ing of mini­plant tests 1 and 3 leaching reactor discharge.

settling curve coincides with the leach­ing reactor discharge settling curve. Settling area and flocculant requirements are a function of the minus lsO-mesh fraction in the thickener feed slurry and will not be affected by previous removal of coarse material. In terms of tons of minus ISO-mesh solids fed to the thick­ener, 1.5 Ib of flocculant per ton corre­sponds to a thickener area requirement of 15 ft 2 /tpd. A flocculant addition of about 0.2 Ib of SEPARAN MGL per ton of calcined kaolin fed to the leaching cir­cuit corresponds to a thickener require­ment of 2 ft 2 /tpd.

Percent solids in the thickener under­flow are inversely proportional to the percent minus ISO-mesh solids in the thickener feed. If the final settled solids (Doo) from the settling tests and the design settled solids (Ddeslgn) are plotted as a function of the minus

<n ~ (5 <n

£: <n CD

~ a U1 ~

<n :::J c ·E u e N; ci) f-Z ill ::;: ill a: :5 0 ill cr: <C ill cr: <C cr: ill Z ill ;,:: S! J: f-

150

140 KFY -...,f.I,- - Classifier 1 overllow

130 ----0-- Leaching reactor dischargo

120

110

100

90

80

70

60

50

40

30

20

10

o . z", , ' ........ -- - --. ------8

O~-----L------~-----7------7-----~ 5

FLOCCULANT ADDITION, Ib SEPARAN MGL per Ion 01 minus 150-m8sh solids

FIGURE 18.· Flocculantadditionversusthickener area requirement~ in terms of minus 15D-mesh solids for bench-scale settling of mini plant test 3 products.

ISO-mesh in the thickener feed, a curvi­linear relationship is established as shown in figure 19.

Tests on Bench-Scale Leached Slurry

Figure 19 includes data from tests on bench-scale leached slurry. In these tests, calcined kaolin of different sizes (table 2) were batch leached in a bench­scale reactor. Samples with the minus 100-mesh material screened out were tested to evaluate a circuit in which fines are removed by air classification and returned to the calcination operation after pelletizing. Settling tests were made on the leaching slurry, using the Kynch technique of analysis. The solids

... t:J 0.

uj

g 30 o en ~ o --'

Final settled sol ids (D= )

~ 25 w o z ~

0: W aJ 20 ~ (j

i I-

15

Design settled solids (DdeslQn )

10~ ____ -b ______ ~ ____ ~ ____ ~L-____ ~

o 20 40 60 80 100

FINES IN THICKENER FEED, pet minus 150 mesh

FIGURE 19. - Effect of fines in thickener feed on Einal settled solid5 and design settled solids.

21

in the slurries from the bench-scale leaches varied considerably in fines con­tent as is shown in table 10 along with a summary of the settling tests. F1occu-1ant and thickener area requirements for the bench-scale leached minus 10-mesh calcined kaolin were less than for the minip1ant leached slurry (table 10). Less fines were produced in bench-scale leaching.

F1occu1ant and thickener area require­ments in table 10 were higher when set­t1ingthe finer sized material except for the minus 18-mesh misted material for which minimal thickener area and f1occu-1ant requirements were obtained. Screen­ing out fines before leaching decreased f1occu1ant requirements but not thickener area requirements.

Predicted Thickener-Only Material Balance

Alumina recovery in a thickener-only solids-liquid separation circuit is de­pendent on the number of thickeners, the

TABLE 10. - Summary of thickener parameters and requirements from bench-scale settling tests on both mini plant and bench-scale leaching slurry

Nominal calcined kaolin size mesh .•

Feed fines content pct minus 150 mesh ..

F10ccu1ant requirements 1b SEPARAN MGL per ton solids ..

Area requirements ft 2/tpd solids ••

Design underflow density pet solids ••

36 21

0.5 0.5

20 1

20 25i

Bench-scale reactor

6 6 15 19 1

0.23 0.04 0.17 0.27 0.06

0.86 0.80 2.00 2.41 0.85

35 35 35 35 35 ,

CLASSIFIER 1 OVERFLOW SETTLING Feed fines content

pct minus 150 mesh .. 92 87 NR NR NR[: NR Floccu1ant requirements 1b SEPARAN MGL per ton solids •• 1.5 1.0 NR NR NR NR NR

Area requirements ft 2 /tpd solids •• 21 20 NR NR NR NR NR

Design underflow density pct solids •• 13.1 14.0 NR NR NR NR NR

NR Not run. l Uase1ine. 2Misted.

3

0.04

2.18

35

NRI NR

NR

NR

35 by 100

6

0.08

1.15

35

NR

NR

NR

NR

22

underflow solids content, and the dilu­tion wash. Table 11 compares the alumina recovery and content in the pregnant liq­uor predicted for thickening systems with 28 and 35 pct underflow solids. Ten thickeners are required to attain 99 pct Al 2 0 3 recovery at 28 pct solids underflow but only seven at 35 pct underflow. All multistage settling tests on HC1-1eached kaolin have indicated a significant in­crease in underflow percent solids with each thickening stage. The bench-scale

testing on minip1ant leached slurry indi­cated underflows of 20 to 25 pct solids. The 28- to 35-pct solids levels represent estimates for the underflow after seven or more stages.

TABLE 11. - Effect of thickener underflow solids and number of stages on esti­mated pregnant liquor alumina recovery and content in a thickener-only circuit

Underflow Stages 1 Al2 03 , pct solids, pct AnalYSl.S Recovery 28 ••• 0 ••••• 7 8.4 96.7 28 ......... 10 8.6 99.0 35 ••••••••• 7 8.6 99.0 35 ......... 10 8.7 99.7

'Washing water and f10ccu1ant additions set at about same amount as in c1ay-HC1 acid minip1ant test 3 (138 Ib per 100 1b calcined kaolin leached).

SUMMARY AND CONCLUSIONS

Solids-liquid separation tests were made in the c1ay-HC1 minip1ant to eval­uate a four-classifier and five-thickener circuit at feed rates of from 100 to 243 Ib/hr of calcined kaolin. Material bal­ances were made for two different test conditions. Test 1 was performed at a feed rate of 100 1b of minus 10-mesh cal­cined kaolin per hour, while test 3 used the same leaching circuit, a feed rate of 173.5 1b/hr and inclined chutes in lieu of stirred mixers. Decreased production of fines by attrition in test 3 improved alumina recovery in the pregnant liquor as shown in the following, in percent:

Al 2 03 Al 2 03 Test recovery content

1 ••• 88.5 8.1 3 ••• 97.8 8.4

Both batch and minip1ant-sca1e ancil­lary testing indicated that percent sol­ids in the thickener underflow would have been higher in test 1 if adequate settled solids inventory in the thickeners had been maintained. Conversely, percent solids in the thickener underflow would have been less in test 3 if the settled solids had not been contaminated with coarse solids from the preceding test. Predicted final residue solids which are corrected for thickener operating conditions are

Test

1 3

1 3

1 3

Unit

Classifier • . . do ....

Thickener · .. do III •••

Composite · .. do ....

Solids, pct

47 52

22 28

28 43

Predicted material balances based on revised discharge solids provided more meaningful estimates of alumina recovery in the pregnant liquor as follows, in percent:

Al 2 03 Al 2 03 Test recovery content

1 ••• 93.6 8.1 3 ••• 97.3 8.4

The combined tailings predicted from test 3 occupied 11 pct more volume than the as-mined clay compared with 67 pct more in test 1. The differences are re­lated to the decrease in fines production which decreased the amount of solids re­porting to the thickeners. The solids splits are summarized below for the two comprehensive tests (1 and 3) and two shorter tests (2 and 4).

Calcined kaolin, Solids to Test lb/hr thickener, pct

1 ••• 100 52 2 ••• 175 77 3 ••• 173 26 4 ... 243 24

In test 2, calcined kaolin too coarse (minus 8 mesh) for adequate suspension in the leaching reactors caused excessive fines production and prevented efficient classification. Operating the system with minus 10-mesh calcined kaolin at a higher feed rate (243 lb/hr versus 173 lb/hr) did not change the solids split significantly.

The particle size of classifier split was 115 mesh (Tyler) for the two compre­hensive tests (1 and 3), but increased to about 20 mesh with the minus 8-mesh cal­cined kaolin (test 2). Increasing the minus 10-mesh calcined kaolin feed rate to 243 Ib/hr increased the particle size of split from 115 to 100 mesh.

Classifier 1 area requirements for the classifier-thickener circuit are depen­dent on the size of separation, as shown in the following:

Split, mesh (Tyler sieve) •• 115 Area, square feet per ton

100

per day .......•••• ,....... 7 5

Classifiers 2, 3, and 4 ,area requirements would be three-fourths of these values on a basis of the ratio of mini plant classi­fier 1 pool area to classifier 2, 3, or 4 pool area or one-fourth of these values on a basis of equal rising velocities in the classifier pool.

Flocculant requirements scale tests miniplant. in table 10

addition and thickener area were developed from bench­on slurry produced in the

The requirements summarized indicated the following:

1. Low fines content in the leaching slurry required low thickener area and thickener flocculant additions.

*U.S. GOVERNMENT PRINTING OFFICE:1983-605-015/67

23

2. Settling fine solids in the classi­fier 1 overflow required more thickener area and more flocculant per ton of sol­ids settled than for thickening the leaching slurry directly.

3. Flocculant addition and area re­quirements per ton of calcined kaolin treated were the same for classified and unclassified feeds. If the classifiers were eliminated, no additional thickener area would be required.

4. Percent soilds in the thickener un­derflow were shown to vary inversely with the percent fines in the thickener feed.

5. Bench-scale reactor discharge from leaching minus 10-mesh calcined clay con­tained less fines than the miniplant leaching reactor discharge (miniplant test 3), required less flocculant, and gave greater thickener percent solids.

6. Minus 18-mesh misted calcined kao­lin leaching feed resulted in signifi­cantly lower flocculant requirements than for minus 10-, 20-, or 35-mesh calcined kaolin.

7. Minus 20-mesh and calcined kaolin leaching increased thickener area additions.

minus 35-mesh feed required and flocculant

The mini plant tests demonstrated that thickener underflow percent solids increased significantly with each suc­cessive thickening stage. Predicted sol­ids for test 3 varied from 14 pct for thickener 1 underflow to 28 pct for thickener 5.

At the predicted underflow percent sol­ids, thickener area requirements were similar for each thickener and averaged 25 ft 2 /tpd solids or 3.7 ft2/tpd calcined kaolin. Total flocculant addition was 2.4 Ib/ton of solids (0.35 Ib/ton cal­cined kaolin), of which 1.3 Ib/ton was added to the first thickener.

INT.-EJU.Orc tvlINES,PGH.,PA. 27D65