the characteristics of filter pressed kaolinite-water pastes 12/12-1-209.pdf · the characteristics...

THE CHARACTERISTICS OF FILTER PRESSED

KAOLINITE-WATER PASTES by

RICHARD WEST

State University of New York, College of Ceramics at Alfred University, Alfred, New York

A B S T R A C T

A Georgia kaolin was selected for investigation because of its large, euhedral, platelike particles and mineral composition consisting of 96 per cent kaolinite. The clay was prepared as a suspension and treated with various water-soluble compounds, with or without the addition of an organic colloid, to determine the role of the organic material in the flocculation and deflocculation characteristics of the clay as measured by the rate of filtration under pressure. Although the organic colloid acted as a strong deflocculant alone, 1 per cent of it actually increased the flocculation action of aluminum sulfate and yet increased the deflocculation action of sodium sulfate.

Water-soluble compounds were added to the clay-water slip and then either a cation exchange resin or an anion exchange resin was added to replace either the cation with H+ ~ or the anion with OH-~. The filtration characteristics of these prepared slips indicated that the anions studied - -C1- t , SO 4- ~, and PO4- 3__ play a passive role in the flocculation or the deflocculation of the clay-water slips but that these ions often have a profound effect in combination with the cations. The cations play the active role in flocculation and deflocculation. AI+3 acted as a strong flocculant and Na +x acted as a strong deflocculant. Small additions of Ca+ 2 acted as a flocculant by increasing the rate of filtration of the clay-water slip but further additions decreased the rate of filtration.

The ratio of the amount of clay to water in the slip shows an important relationship with rate of filtration. A slip with 25 per cent clay had a relatively low rate of filtration and yielded a filter cake with a higher bulk density in the wet and the dry state thau did a slip with 40 per cent clay. The lower viscosity of the slip with 2S per cent clay may allow a greater freedom for particle orientation as deposited on the filter cake than a slip with 40 per cent clay having a higher viscosity.

Electron micrographs of the top and edge of filter cakes prepared from flocculated and deflocculated clay show that the structure of both is similar with regard to parallelism of plates. However, the deflocculated clay deposits in a close-packed parallel arrangement whereas the flocculated clay deposits with some slight displacement of the plates but in a parallel arrangement.

I N T R O D U C T I O N

DURING a n d d i r e c t l y fo l lowing W o r l d W a r I I , a n u m b e r of c e r a m i c w h i t e - Wares p l a n t s w e r e f o r c e d to r e p l a c e sk i l l ed c r a f t s m e n w i t h m a c h i n e s b e c a u s e of t h e s h i f t o f m a n p o w e r t o m u n i t i o n s p l a n t s . T h e c r a f t s m e n c o u l d

209

210 TWELFTH NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

handle fairly wide variations in the workability of the clay and still produce a saleable product, although an overtone of discontent might pass through the plant during cycles of poor workability of the clay. However, the automatic forming machinery used to replace these men could not be made to handle these wide variations of workability, and so the machines either produced large quantities of high ,quality ware during cycles of good workability or large losses during cycles of poor workability (Gould and Lux, 1958, pp.98-107). The managers of these plants--as well as the suppliers of the raw materials--tried numerous theories, control tests and manufacturing techniques in an attempt to predict and control the plastic properties of the raw materials. One practical solution to these problems consisted of selecting clays with a proper blend of clay minerals and inducing more stable workability by high intensity dispersion (Gould et al., 1958, pp.124-27; West et al., 1960, pp.l-6). A simple laboratory filter press control test was developed (West, 1960, pp.413-18; Adcock and McDowall, 1957, pp.355-62) as a measure of surface area of the clays and changes in the water permeability of the clay-water pastes caused by small percentages of ions and changes in dispersion. Inasmuch as the filter press test has been a successful criterion of workability in several whitewares plants this test was selected as the basis for an investigation to study the effect of various ions on the structure of clay-water pastes. Further gains in perfection of manufacturing operations would accompany knowledge about variation of interparticle forces produced by different ions, the variation in the clay fabric of clay-water pastes produced by different ions, and changes in the clay fabric accompanying the removal of water during drying operations.

The United Clay Mines Corporation provided financial support for this project and supplied Monarch Clay used in this investigation. Thanks are extended to. G. W. Phelps, Vice President and Technical Director of the United Clay Mines Corporation, for his helpful suggestions.

T H E CLAY S E L E C T E D F O R STUDY

A Georgia kaolin was selected for this investigation because of its mineral purity and because of the large euhedral, platelike crystals which seemed desirable for examination by microscopic means. Conventional X-ray, differential thermal and chemical analyses indicated that the clay consists of slightly more than 96 per cent kaolinite with impurities of mica, plagioclase feldspar, and little or no quartz. Electron micrographs showed large well-formed crystalline plates with a thickness to diameter ratio of about 1 to 10. Particle size analyses showed 74 per cent minus 10 microns, 40 per cent minus 2 microns, 23 per cent minus 1 micron, and 2.5 per cent minus 0.2 microns.

Many different water-soluble compounds were added in varying amounts

CHARACTERISTICS OF FILTER PRESSED KAOLINITE-WATER PASTES 211

to the clay-water slip made with this clay while the viscosity was recorded with a Brabender Viscosimeter. The maximum increase in viscosity accompanied the additions of aluminum chloride or sulfate, and the maximum decrease in viscosity accompanied the additions of sodium phosphate compounds. The maximum change in viscosity was generally brought about by the addition of about 0.2 per cent of any compound.

F I L T E R P R E S S I N G B E H A V I O R OF T H E CLAY

A suspension of 25 per cent clay with 75 per cent distilled water was treated in a variety of ways with different additives to ascertain the filter pressing behavior of the clay under these conditions. In one sample, a cation exchange resin, Amberlite IR-120 obtained from Fisher Scientific Co., was mixed with the slip in an amount equal to the weight of the clay in the slip. In this case, the resin exchanges all cations for H+I. Another sample was treated in the same manner with an anion exchange resin Amberlite IRA-400 obtained from Fisher Scientific Co., so that all of the anions were exchanged for OH -1. In other samples various water-soluble sulfate compounds--aluminum sulfate, iron sulfate, and sodium sulfate-- were added in the amount of 0.2 per cent by weight of clay. Also, 0. I per cent or 1.0 per cent of an organic colloid was added to the mixtures of the clay with sulfate compounds to ascertain the role of organic material in the filter pressing behavior of the clay as the importance of both sulfate (Shell and Cortelyou, 1943, pp.179-85) and organic matter (Phelps, 1959, pp.246-50; Worrall and Green, 1953, pp.528-53; Worrall, 1956, pp.689- 705) has been stressed.

The organic colloid supplied by the United Clay Mines Corp. might be termed a sodium humate. The lignite from which the colloid was prepared contains a considerable amount of calcium and magnesium leaehates and shows a good deal of sulfate and chloride anion. The digestion was done at pH 9.5-10.0 with NaOH as the alkali. The acidic groups involved have been found to be carboxylie and phenolic hydroxyls, This accounts for increasing digestion as pH rises with phenolic hydroxyl groups coming into play and (most likely) some oxidation forming carboxylic groups.

After a five-day ageing period, the exchange resins were separated from the clay-water slip by screening through a 100 mesh sieve, and all of the samples 'were tested by the filter press test under 25 psi air pressure for a period of 30 rain. The average rates of filtration for each of the samples is shown in Fig. 1. The pH for each of the samples is shown in Table 1.

The fundamental derrivation of the Kozeny equation for industrial filtration (Carman, 1938, pp .168~8)was related to the dewatering of clays by slip casting and filter pressing (Michaels and Lin, 1954, pp. 1239- 1246; Adcock and McDowell, 1957, pp.355-62) and then applied to control over whitewares manufacturing operations (West, 1960, pp.413-19). The


q

i "!

! .- I 0

|

AddtKve None Nanlr 025g AlitS04) e 025g FelIO4.TI'I~O 0.2% NaaS04

Orgtmir Cotleld None 0.1 LO 0 O.I tO 0 0.1 tO 0 OJ

FIGURE 1.--Effect of various additives on the ra te of filtration at 25 psi of a slip containing 25 per cent clay. Per cent of ion is based upon dry

weight of clay in the slip.

T A B L E I . ~ E F F E C T OF VARIOUS ADDITIVES ON THE pH AND THE RATE OF FILTRATION

OF A SLIP CONTAINING 2 5 PER CENT CLAY

% Organic colloid] based on dry wt.

of clay

none none none 0.1 1.0 n o n e

0.1 1.0 n o n e

0.1 1.0 none 0.1 1.0

% Additive based on dry wt.

of clay

none (as received) none (CE resin) none (AE resin) n o n e

n o n e

0.2~o Alg(SO4) a 0.2% All(SO4) , 0.2% A12(SO4) 3 0.2~o FeSO~. 7H20 0.2% FeSOr 7H~O 0.2% FeSO~. 7H20 0.2% Na2SO 4 0.2% NajSO4 0.2~o NazSO4

Average rate of filtration

cm2/sec x 10-*

21.8 17.3 20.7

7.0 �9 5.6 20.0 18.6 26.7 19.0 17.4 16.0 16.8 15.5 5.4

pH

4.5 3.6 6.2 5.0 6.0 3.4 3.4 6.3 3.8 3.6 4.5 4.5 4.6 6.1

CHARACTERISTICS OF FILTER PRESSED KAOLINITE-WATER PASTES 213

clay-water shp placed in a Baroid laboratory filter press is subjected to a pressure, P, causing the deposition of successive layers to form a filter cake with the excess water being forced through the cake which has already formed. Thus, a filter cake with a volume, V, consists of a void, or liquid, fraction designated as E and a solid fraction designated as 1- -E. The thickness of the filter cake af ter a time, T, is designated as L.

A linear relationship exits between the square of the thickness of the filter cake, L ~, and the time, T, and so the term L~/T is used to provide an expression for the rate of filtration.

A program was compiled for an IBM 1620 computer for calculation of rate of filtration and the void fraction, or per cent porosity, of the filter cake. All statistical data including equations for regression lines and correlation coefficients were also programmed on the computer.

The clay was used as received from the mine without processing, other than grinding to an airfloated form. The dispersed clay seemed quite highly flocculated based on observations of viscosity, pH, and rate of filtration. The action of the ion exchange resins either replacing cations for H +1 or anions for OH -1 caused a slight lowering of the rate of filtration.

The addition of the organic colloid lowers the rate of filtration markedly with the 1.0 per cent addition. However, the effect of the organic colloid upon the action of a luminum sulfate is quite strange. The aluminum sulfate alone does change the rate of filtration slightly and the combination of 0.1 per cent organic colloid with 0.2 per cent aluminum sulfate lowers the rate of filtration slightly. However, the 1.0 per cent addition of organic colloid with a luminum chloride increases the effectiveness of flocculation of the a luminum chloride by increasing markedly the rate of filtration. This action of the organic colloid resembles an effect observed in whiteware plant practice (West and Coffin, 1960, pp.462-64). Occasionally, a feldspar becomes ground too finely allowing a slight amount of the alkali to dissolve. Normally, this soluble alkali from the feldspar will cause a deflocculation of the clay slip, lower the viscosity, and lower the rate of filtration. How- ever, if a luminum sulfate or chloride is being used in the c lay-water slip to flocculate the slip for plastic processing, the slight amount of soluble alkali increases the effectiveness of the aluminum compounds by increasing markedly the viscosity and the rate of filtration.

Iron sulfate seems less effective as a flocculant than aluminum sulfate and the addition of the organic colloid lowers the effectiveness of flocculation of the iron sulfate indicated in the graph by the lowering of the rate of filtration. The sodium sulfate does not seem to be very effective as a deflocculant but the addition of 1 per cent organic colloid causes a strong lowering of the rate of filtration of the c lay-water slip. Therefore, these experiments indicate tha t the effect of the organic colloid upon the rate of filtration of the c lay-water slip depends not only upon the amount of the organic colloid present but also upon the other ions which may be acting upon the clay and /o r the organic coUo~.

1 5


S E L E C T I V E E F F E C T O F C A T I O N S A N D A N I O N S U P O N R A T E O F F I L T R A T I O N

A large amount of work has been conducted (Brindley, 1958, pp.7-23; Harward and Coleman, 1954, pp.181-88; Hendricks, 1945, pp.625-30; Huey-Rong Hsi and Clifton, 1960, pp.269-76; Lewis, 1955, pp.54-69; Meting and Glaeser, 1954, pp.519-30; Michaels and Lin, 1955, pp.1249- 53) to show the effect of ions upon c lay-water systems with much of the work dealing with cations. However, the effect of the anions (Krebs et al., 1960, pp.260-69) cannot be disregarded as the selective addition of only cations to a system is difficult to conceive. On the other hand, the use of

3 O

N

=~20 q

g

E

o

I (

�9 No'; AP

�9 Cs

o Cd'Z* N~l

�9 F ~

= AP'

c~

0 .tO % loft

FIGURE 2.--Effect of various cations on the rate of filtration of a 25 per cent clay suspension. Per cent of ion is based upon dry weight of clay in

the slip.

CHARACTERISTICS OF FILTER PRESSED KAOLINITE--WATER PASTES 215

ion exchange resins (Harward and Coleman, 1954, pp.181-88; Ormsby, 1957) has offered a method of selectively changing all anions to an OH -1 form or of selectively changing, all cations to a H +1 form.

Ala rge quant i ty of clay-water slip was prepared from 25 per cent clay with 75 per cent distilled water and then separated into a number of individual containers. An anion exchange resin was added to some of the containers and a cation exchange resin was added to the other containers. Water-soluble compounds were added to each of the separate containers and allowed to age for a period of 5 days before removing the resin by screening through 100 mesh. The clay-water slip was then tested for rate of filtration at 25 psi air pressure for 30 min. The results of these tests are shown in Figs. 2 and 3 and in Tables 2 and 3. The results shown in Figs. 2 and 3 are average rates for each of the ion additives. Some difficulties were experienced with recharging of the ion exchange resins, as even after following the directions given in Bulletins 1E-6-55 and 1E-10-55 (revised March 1960) by the manufacturer of the resins--The Rohm and Haas Co.-- the act ivi ty of the resins varied somewhat after every recharging. The same clay-water suspension deionized by the resin after different cycles of recharging gave slightly different results for rate of filtration.

Therefore, complete series of tests were run with the same batches o~ recharged resins for closer comparison of some of the results. Also, greater importance is at tacked to some of the points on the graphs than others because they represent a larger sampling than others. Some points represent an average result of many specimens of the sample while others represent the average result with the same percentage of ion added through different compounds.

N

�9 cr

o ,~ .o#

~oae

s .2o .go ion

FIGURE 3.--Effect of various anions on the rate of filtration of a 25 per cent clay suspension.


The cat ions play an ac t ive role in changing the ra te of f i l t rat ion of the c l a y - w a t e r slip. The a l u m i n u m ion is v e r y ac t ive as a f locculant based upon the increase in the ra te of f i l t ra t ion wi th increasing a m o u n t s of this ion present. A l though the two points for the Fe +2 were i n t e rmed ia t e be tween the A1 +a and the Ca +3 the results were so va r i ed t h a t a line was not p lot ted . The Na +1 ac ted as a s t rong def locculant by lowering the ra te of f i l t ra t ion to abou t 5 • l0 - 4 cm2/sec wi th all percentages of this ion addi t ion . The points represent ing the Ca +3 addi t ions cover the range of add i t ion qui te comple te ly . Smal l percentages of Ca +3 increase the ra te of f i l t rat ion and s l ight ly g rea te r a m o u n t s cause a sharp decrease in the rate. Larger amoun t s of Ca +2 increase the ra te of f i l t ra t ion gradual ly .

A series of calcium sulfate addi t ions was run in combina t ion wi th a small percen tage of Na +1 (0.029 per cent based on d ry weigh t of clay added as N a O H to each specimen) to de te rmine in terac t ion effects be tween these two cations. The results seemed add i t ive as the Na +1 addi t ion p roduced

T A B L E 2 , - - E F F E C T O F V A R I O U S C A T I O N S O N T H E pH A N D T H E R A T E O F F I L T R A T I O N

O F A S L I P C O N T A I N I N G 2 5 P E R C E N T C L A Y

Additive Resin

one AE JC13.6H,O AE dCla.6H,O AE ;aSO4.2H,O AE ',,SO ,.2H,O AE ',aSO,.2H,O AE :aC13.2H,O AE :aSOa,2HsO AE :aClv2H,O AE :aSO,.2H~O AE :aSO,.2H,O AE ~eSO4.7H,O AE ~eClv4H,O AE ~a3PO,,12H30 AE /aaP04,12H30 AE ~a,PO 4.12H20 AE ~aC1 AE "aSO4.2H,O AE 4aOH :aSO,.2H,O AE ~aOH :aSO,.2HsO AE laOH :aSO4.2HsO AE qaOH

Ion added %ion based on dry wt.

of clay

Aver,re rate of filtration

cm*/sec • 10 -4

20.7 24.3 27.6 26.6 25.7 8.4 8.6

11.4 10.6 11.1 20.0 27.1 17.8 4.8 4,0 4.4 4.0 6.1

5.6

8.4

none none Al+a 0.022 Al+8 0.030 Ca + 2 0.002 Ca+ 2 0.009 Ca+ 2 0.025 Ca+~ 0.029 Ca +3 0.037 Ca+3 0.059 Ca+* 0.075 Ca+3 0.149 Fe+ ~ 0.040 Fe+~ 0.056 Na +1 0.020 Na+l 0.039 Na +1 0.059 Na+l 0.079 Ca +* 0.002 Na+l 0.029 Ca +* 0.009 Na+ x 0.029 Ca+* 0.037 Na +1 0.029 Ca +* 0.149 Na +1 0.029

18.8

pH

6.2 5.4 3.9 6.7 6.8 9.6 9.7 8.8

10.5 10.9 11.1 4.5 6.4 9.8

11.0 11.6 10.3 8.6

8.8

9.2

10.8


TABLE 3.~EFFECT OF VARIOUS ANIONS ON THE pH AND THE RATE OF FILTRATION OF A SLIP CONTAINING 25 PER CENT CLAY

Additive

one

aC1,.2H,O 'eClv4H,O .1C13.6H,O aC12.2H,O raC1 aSO, .2H,O .aSO,.2H,O .aSOt.2H,O 'eSOt.7H,O aSO, .2H,O aSOt.2H,O ,1,(SO,), [azSO, :aSO4.2H20 Ia3PO4.12H20 ~a3P0,.12H20 ~a3PO4.12HIO :aSO4.2H,O [a0H :aSO,.2H,O IaOH :aSO,.2H20 IaOH :aSO,.2H,O IaOH

Resin

CE CE CE CE CE CE CE CE CE CE CE CE CE CE CE CE CE CE CE

CE

CE

CE

Ion added

none C1-1 CI - ' C1-1 CI-~ C1-1 S O 4 - ' SO~ -2 S O , - ~ S O , - 2 S O , - ' SO , -* SO, -~ SO, -* SO, -* P O , - s PO,- 3

po 4 - ' S O , - 2 O H - 1 SO,-' OH-' S O , - ' OH-I SO4-~ O H - '

% ion based on dry wt.

of clay

none 0.052 0.071 0.088 0.104 0.121 0.006 0.022 0.060 0.070 0.089 0.120 0.130 0.130 0.357 0.027 0.054 0.081 0.006 0.021 0.022 0.021 0.089 0.021 0.357 0.021

Average rate of filtration

emS/see • 10-*

17.3 17.6 13.7 13.5 15.8 12.9 20.7 17.0 14.0 12.3 13.2 12.2 9.5 9.5

11.5 15.4 15.9 15.7 18.6

14.8

12.3

10.9

pH

i

3.6 3.0 2.4 1.8 2.8 2.4 4.0 4.0 3.1 2.6 2.8 2.9 2.4 2.3 2.2 3.2 3.3 3.2 4.1

i 3.5

2.9

2.3

results parallel to that of the Ca +2 alone but with a slightly lower rate of filtration.

The cations seem to play a more active role than anions as small additions in the range of 0.03 per cent of Na +1 or Ca +2 cause a marked decrease of the rate of filtration, and less than 0.03 per cent of A1 +3 causes a marked increase of the rate of filtration. All of the points for the anion additions showed considerable scatter but the grouping was quite close together in an intermediate range of rate of filtration. The lowering of the rate of filtration caused by additions to either C1-1, S04 -2 or P04 -3 was more gradual with larger amounts required for anions than for cations to ac- complish the same decrease in the rate of filtration. Although the sodium phosphate compounds showed the greatest effect in lowering the rate of filtration, the phosphate ion (P04) -3 showed the least effect in lowering the rate of filtration. The points representing S04 --~ additions cover the range of addition quite completely. Small percentages of S04 -~ increase the rate of filtration but greater amounts lower the rate of filtration. The


interaction of 0 H -1 witll S04 -2 also seems additive as the series of points representing an addition of 0.021 per cent O H - (added to each specimen and based on dry weight of clay) with various percentages of SO~ --~ is parallel with the series representing S04 -~ alone but a t a lower rate of filtration.

T H E F A B R I C OF A F I L T E R C A K E

A statistical analysis was made of all of the results of the filter press test to determine the degree of correlation which exists between rate of filtration and other physical characteristics of the filter cake. Two series of tests were run with either 25 per cent clay or 40 per cent clay in the slip a t a filter pressure of 25 psi and a t ime of 30 min. Below are the average results with the clay processed in an "as received" condition.

Per cent clay Rate of filtration Void fraction cm~/sec • 10-4 in filter cake

40 49.3 0.596

25 21.4 0.524

The filter cake formed from the slip with 25 per cent clay had a more dense structure with 52.4 per cent porosi ty as compared with 59.6 per cent porosity with the cake formed from the slip containing 40 per cent clay. A possible explanation for this might be tha t the higher clay content did not allow as much readjus tment in position of the clay particles as they were deposited on the surface of the filter cake as did the slip with 25 per cent clay. A good deal of industrial significance m a y be a t tached to this as manufacturers have found tha t clay slip with a high clay content has bet ter working and drying properties than one with low clay content. Inasmuch as the rate of filtration and rate of drying are closely related through the water permeabil i ty of the clay paste, the clay paste formed from the slip with a high bulk density would be expected to dry more rapidly than the clay paste formed from the slip with the low bulk density.

Sixteen runs were made with clay slip prepared with 40 per cent clay containing various additives, and these results are compared with 81 runs made with clay slip prepared with 25 per cent clay containing various additives. The regression lines shown in Fig. 4 show fair correlation between rate of filtration and porosity of the filter cake as the correlation coefficient is 0.66 for the runs with 25 per cent clay in the slip, and is 0.66 for the runs with 40 per cent clay in the slip. The results show tha t the void fraction of the filter cake is greater with runs having a high rate of filtration as flocculation seems to produce a more open cake. This was expected as several


0.6

t3

[

~O.S

J u

~o.4

r - 0 .66 4 0 % Clay

25% Clay r �9 0 .66 I

Rate of Filtration (cm /sic x tO "r )

FmUR• 4 . ~ P o r o s i t y of the filter cake versus ra te of f i l trat ion a t 2 5 psi.

workers (Michaels, 1958, pp.23-31; Rosenquist, 1960, pp.12-27) have conjectured about the "card house" arrangement of the clay particles in a flocculated state. A low percentage of clay in the slip produces a more open structure of cake than does a high percentage of clay in the slip at the same rate of filtration for both slips. Here again it is observed that a clay-water paste for forming whitewares will be more dense (have a lower void fraction) if a high specific gravity slip is used, and yet the rate of filtration and rate of drying of the clay paste will be higher if a high specific gravity slip is used. Thus, whitewares drying losses can be mini- mized by using high specific gravity slip as the drying shrinkage is lowered slightly while maintaining the same rate of water removal from the ware.

Ceramic manufacturers are interested particularly in changes to the clay paste during drying as a goodly share of losses are incurred during this stage of the operation. Therefore, some of the filter cakes were dried at 110~ weighed, and the volume was measured using a mercury microvolumeter (West et al., 1959, pp.26-30). The regression line in Fig. 5 showing dry bulk density versus rate of filtration was calculated from 38 samples and the correlation coefficient is 0.77. This follows the previous experience with t h e wet filter cakes as the highly deflocculated slips produced a dry cake as well as a wet cake with a higher bulk density than the filter cakes produced from highly flocculated slips.

The regression line showing volume shrinkage versus rate of filtration in Fig. 5 was calculated from 36 samples and the correlation coefficient was low (0.33). The trend of greater volume shrinkage with the cakes formed from flocculated slips seems inconclusive and may not be linear.

The coarse clay was selected for this s tudy so that a microscopic examination might help distinguish the actual fabric of the clay particles


1.55

~ 1.50

| " 1.45 m

1.40

/ /

Dry Bulk Density 1 / / ~ r . 0.TT / /

/ / / , /

/ / / /

f f f 7 ~

/ / % Volume Shrinkage / r - 0.33

/

5 I0 15 20 Rote of Filtration (crn~/sec. x I0"~

i6

15

t~

14 -= g

13

FIGURE 5.--Dry bulk density and per cent volume shrinkage on drying versus rate of filtration at 25 psi.

in a clay-water paste. The electron micrographs on Plates 1, 2 and 3 show top and side views of the dry filter cakes. The " card house" effect characterized for flocculated clays and " card pack" effect characterized for deflocculated clays does not seem clearly evident. In fact, the slight differences noted in both the wet and the dry cakes with widely different rates of filtration do not indicate large differences in the degree of disorientation with the flocculated particles as compared with the deflocculated particles. Wide variations in the rate of filtration still produce a high degree of align- ment of particles although there may be a slight C-axis displacement with the flocculated clays.

Only one large clay stack was observed with these electron micrographs and this is displayed in Plate 1. The replicas were difficult to prepare with this clay as few of the large coarse particles pulled free to leave unat tached pieces on the surface. This is evident particularly with Plate 3. However, differences can be seen between the close packed orientation of the deflocculated clay in Plate 3 and the more open but still oriented structure of the flocculated clay in Plate 2. These observations were also confirmed by a good many X-ray diffraction analyses run on top surfaces and compared with those run on edges. No significant difference was noted in the intensity variation of the basal peaks between cakes prepared with flocculated clay as compared with cakes prepared from deflocculated clay indicating a similarity of particle orientation over wide ranges of filtration rates.

PLATE 1 . - - E l e c l r o n m~crograph of top surface of filter cake. Clay was deionized wi th bo t h cat ion exchange and anion exchange resins, and t h e n

0.33 per cen t of a l u m i n u m sul fa te was added.

PLATE 2 . - - E l e c t r o n mic rograph of edge of filter cake. Clay was deionized wi th bo th ca t ion exchange and anion exchange resins, and t hen 0.33 per

cen t of a l u m i n u m sul fa te was added.

P. 2 2 0

PLATE 3 . - -Elec t ron micrograph of edge of filter cake. Clay was processed as received, 0.9. per cent Na~SO 4 plus 1 per cent organic colloid was added.

CHARACTERISTICS OF FILTER PRESSED KAOLINITE-WATER PASTES 29-1

REFERENCES

Adcock, D. S., and McDowall, I. C. (1957) The mechanism of filter pressing and slip casting: J. Am. Ceram. Sot., v,40, pp.355-62.

Brindley, G. W. (1958) Ion exchange in clay minerals: Ceramic Fabrication Processes, edited W. D. Kingery, Technology Press ot M.I.T., John Wiley and Sons, Chapman and Hall, pp.7-23.

Carman, P. C. (1938) Fundamental principles of industrial filtration: Trans. Inst. Chem. Engr., London, v.16, pp.168-188.

Gould, R. E., and Lux, J. (1958) Some experiences with the control ot plastic bodies for automatic jiggering: Ceramic Fabrication Processes, edited W. D. Kingery, Technology Press of M.I.T., John Wiley and Sons, Chapman and Hall, pp.98--107.

Gould, R., Lux, J., West, R., and Coffin, L. (1958) How to produce better whitewares with high intensity clay dispersion: Ceram. Incl., v.71, pp.124-127.

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