Particle interactions in aqueous kaolinite suspensions

Particle Interactions in Aqueous Kaolinite Suspensions III. SedimentationVolumes I A N E R I C M E L T O N ~ AND B R I A N RAND Department of Ceramics, Glasses and Polymers, University of Shefeld, Sheffield SIO 2TZ, England

Received May 10, 1976; accepted August 19, 1976 Sediment volumes of suspensions of homoionic "Al-free" Na kaolinite, Standard Porcelain, and a natural kaolinite sample have been followed as a function of pH and NaC1 concentration. The addition of NaC1 decreases the sediment volume at low pH values and increases it at high. The results are interpreted in terms of the effect of the electrolyte upon the density of packing of particles within floes and average floc size. It is suggested that the Bingham yield stress is a more sensitive measure of particle-particle interactions than the sedimentation volume. INTRODUCTION A systematic investigation of the effect of p H and concentration of indifferent electrolyte (NaC1) upon the rheological properties of dilute suspensions of natural (1) and carefully prepared Na kaolinite (2) samples has established that the model of particle-particle interactions proposed earlier (2), and based on the views of Schofield and van Olphen, appears to hold for these systems. The sedimentation behavior of these suspensions has also been investigated and is interpreted here in terms of the electrical double layers developed at the kaolinite/water interfaces. An extensive study of the settling rates and sediment volumes of flocculated kaolinite suspensions has been reported by Michaels and Bolger (3), who also developed a model of the sedimentation process. They assumed that the kaolinite particles were coagulated into floes which were themselves grouped together into units known as aggregates, there being a continuous network of such aggregates throughout the suspension when hindered settling was observable. Equations were a Present address: Tinsley Wire Industries Ltd., Shepcote Lane, Sheffield 9, England.

developed enabling floc and aggregate volume concentrations and the average aggregate size to be obtained from the sediment volumes and settling rates. Suspensions were investigated at p H values of 4 and 6 and after the addition of 0.06 M NaC1, which was considered to be a sufficiently high electrolyte concentration to promote "card-pack," or face-face, aggregates. However, in the previous study by Rand and Melton (2) it was shown that, for a sodium kaolinite sample not contaminated with soluble salts, NaC1 concentrations greater than 0.15 M were required to bring about face-face coagulation and that 0.06 M NaC1 was not a high enough 1 : 1 electrolyte level to coagulate fully the suspension into the edgeedge mode at p H values around 9. Hence, it was decided to carry out a limited study of the sedimentation properties of the kaolinite samples previously investigated by rheological techniques, and to compare the sediment volumes of suspensions coagulated in the different modes of particle interaction described earlier. The sediment volumes were determined over a much wider range of pH and NaC1 concentrations than was investigated by Michaels and Bolger, but settling rates and the effects of container dimensions were not

331 C o p y r i g h t (~) 1977 by A c a d e m i c Press, Inc. All rights of reproduction in a n y form reserved.

Journal of Colloid and Interface Science, Vol.

69, No. 2, J u n e 15, 1977 I S S N 0021-9797

332

MELTON AND RAND

"AI free" kaolinite ....o

I

+' O - 0 6 M N o C I

---e

i i, tl

+ 0 ' 1 7 M.NoCI + 0.33 M.NaCI ~. 0 - 6 8 M. NaCI

I

~ I'O

...... ~

.--...~..- A

--..o 16

M.NoCI

14'

x

x.

x

~o

i

"',

' l

%

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o

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4

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I

I

I

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I

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6

7

B

9

IO pH

FIG. 1. Effect of pH and NaC1 upon the sediment volumes of "Al-free" Na kaolinite su½pensions (9 wt~o). further explored. The electrolyte concentration was extended up to 1.0 M NaC1 and hence the results should be of relevance to the sedimentation of kaolinite in marine environments. EXPERIMENTAL The clays used in this study have all been described previously (1, 2). They were homoionic Na kaolinite (Al-free), its natural raw kaolinite precursor, and Standard Porcelain, a commercial china clay, Clay suspensions of concentration 9.1 and 4.75 wt(~ were prepared by adding the required quantity of dry clay to distilled water. The suspensions were all deflocculated with N a O H and shaken under identical conditions to ensure complete dispersion. The p H and the electrolyte (NaCI) concentration were then

adjusted, and the suspensions were transferred to stoppered 25-ml graduated cylinders of equal bore, inverted end over end 25 times, and allowed to sediment. The p H changed during the first sedimentation, so after equilibrium was attained the cylinders were inverted end over end a further 25 times and then allowed to resediment. When the sediment volume showed no change in a period of 24 hr its value was noted. All measurements were carried out at room temperature, 22 ~ I°C. RESULTS Figures 1 and 2 show the sediment volumes of the Al-free homoionic Na kaolinite at the two solid concentrations investigated, as a function of p H and NaC1 concentration. The

Journal o f Colloid and Interface Science, V o l , 60, N o . 2, J u n e 15, 1 9 7 7

SEDIMENTATION VOLUMES OF KAOLiNITES

333

'~1 free" kaolinite 1,2-

----o --.e

, ,n

-~O.06M.NoCl 40.17 MNoCI

........ v ---.O ' J

. , '

+0.33 40"68 ~I'0

M.NaCI M NaCI MNaCI

I0"

o 8"

~o o

x

% e'~,

v .........

-

- ..~ll e

. . , . .

............ d~-~..... 0"~ ~'w-"

O•

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4"

2

3

i

4

;

i

6

i

7

~

,~,

; pH

FIG. 2. Effect of pH and NaC1 upon the sediment volumes of "Al-free" Na kaolinite suspensions (4.8 wt%).

highest value of p i t at which the sediment volume could be observed in these suspensions, in the absence of added electrolyte, was at 7.3, which is about the value at which the yield stress-pH curves all coincide in the previous study (2), i.e., the edge surface isoelectric point. Above this value the suspensions were deflocculated and did not fully sediment over a period of weeks. However, after the addition of NaC1, sedimentation occurred at high pH. I t is clear from these figures that the effect of NaCI upon the sediment volume is similar to its effect upon the Bingham yield stress; i.e., it lowers the sediment volume at p H values below about 7.3 and initially increases it above this p H value.

One important difference between the sedimentation results and the Bingham yield stress changes (2), is that in the former case there is no common intersection of the curves at p H 7.3. This point is illustrated more clearly in Fig. 3, where the sediment volumes are plotted at selected pH' values as a function of NaC1 concentration. At p H 7.3 the sediment volume is initially decreased upon the addition of electrolyte, but becomes independent of p H at NaC1 concentrations above 0.1 M. Figures 4 and 5 show sediment volumes for the raw kaolinite and Standard Porcelain clay, respectively. These are similar to the homoionic clay except that, as in the case of the rheological investigations (1), the data are shifted slightly to lower p H values.

Journal of Colloid and Interface Science,

Vol. 60, N o . 2, J u n e 15, 1 9 7 7

334

MELTON AND RAND variation in sediment volumes. The suspension can be regarded as consisting of flocs whose average size and density are determined by the solid concentration (3) and the electrolyte environment, which determine the nature of the particle-particle interactions. These flocs will, of course, be much larger than the flow units which determine the plastic viscosity, but, since the internal structures of both are determined largely by the nature of the particle-particle bond, their variations with pH and ionic strength should be similar. Firth and Hunter (4, 5) have shown that the floc density increases as the ~" potential increases. This results in a decrease in the net energy of attraction between particles and a greater number of particle-particle contacts within the floc are required to prevent disruption by shear forces. Michaels and Bolger (3) have shown that the sediment volume can be expressed by the equation

9w% &.8w% x ¥ pH5 + & pH7 n ; pH7.4 prig-5

\

°

% ~12

% >o 8 • E 4-

o!2

0'6

o'.~

o-8

Molarity NoCI

FIG. 3. Effect of pH and NaC1 upon the sediment volumes of "Al-free" Na kaolinite suspensions (4.8 and 9 wt%). DISCUSSION The variation in sediment volumes, brought about by changes in the electrolyte environment in the suspension, resembles quite closely the variation of plastic viscosity observed earlier, which was explained (2) in terms of the flow units, or flocs, which existed under high shear conditions. A similar explanation can be proposed to explain the

----o ~..e ___~

V~ = (VoCfKd~K/0.62) + b,

Vs = equilibrium sediment volume, V0 = initial volume of suspension, Cm = ratio of floc to kaolinite volume concentrations, q~K = kaolinite volume concentration. R O W kclolinit¢ , ,i "~ O . O b M , NQCI ii n 4" O- I M, NQCI ii II *0.6 M. NQCI

~0 x

% u6

E

o ~

=. E

----~ - ~

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\ r_

j

)- -o . . . . .

I

o

I

.

FIG. 4. Effect of pH and NaCI upon the sediment volumes of untreated kaolinite suspensions (4.8wt%). dournal oJ Colloid and Interface Science,

V o l . 6 0 , N o . 2, J u n e

El-1

where

1,5, 1 9 7 7

335

S E D I M E N T A T I O N V O L U M E S OF K A O L I N I T E S

This equation considers the settling mass to be made up of spherical incompressible flocs. The sediment is made up of two regions. In the lower region the pressure of the overlying sediment forces the flocs into a random closepacked arrangement of volume fraction 0.62. Above this lies a nonuniform lower-density zone of volume b. Michaels and Bolger showed that plots of V~ against V0 at constant kaolinite volume concentration were linear, demonstrating the constancy of b and CfK. They determined the floc-to-kaolinite volume concentrations of flocculated suspensions. Both b and CfK were found to vary slightly with the kaolinite volume concentration. However, this variation was small. In this work measurements were taken at different kaolinite volume concentrations, but if the variation of CIK and b with q~K can be neglected, to a first approximation, then the results can be used to calculate CfK, which is a measure of the floc size. Thus, it is assumed that the variation in floc size with pH and electrolyte concentration is large compared with the variation of CfK and + b with 4~. CfK values were therefore calculated from the expression

V0 \ ~ K 1

[ 221

~-

~

~

A No NaCI o +0.017 MINOCI

. ÷o.ox . . . .

~\

,sJ /

2

q~K2

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%

10 4

I " ]Z .

I 5

I 6

f 7

8

pH

FIG. 5. Effect of p H and NaC1 upon the sediment volumes of suspension of Standard Porcelain (9 w t % ) .

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D

I

4

I

,

/

"--m O. --J"

I

6

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8

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10

pH

FIG. 6. Effect of pH and NaCl upon the floc-kaolinite volume ratio of suspensions of "Al-free" Na kaolinite. - - O - - , No NaCI addition; - - 0 . - , -I-0.06 M NaCl; - - D - - , -1-t-0.17 M NaC1; - - V - - , -I-0.33 M NaCI; - - & - - , -I-0.68 M NaC1.

V1 and V2 are the sediment volumes at kaolinite volume fractions q~KI and (~K2, respectively. The plots of floc-kaolinite volume ratio against p H (Fig. 6) show similarities with the sediment volumes. At low p H values and low electrolyte concentrations the largest flocs are obtained when the particles are coagulated in edge-face modes. Under these conditions there is a large energy barrier to face-face flocculation, and the edge-face attractive energy is high. Both of these factors will favor voluminous flocs. As the p H approaches 7.3 the edge surface potential becomes less positive and the edge face linkage is less strong, presumably resulting in slightly more dense flocs, which at the edge isoelectric point are held together by edge-edge van der Waals forces only. Above this p H value the flocs are too small to result in appreciable sedimentation, although there still exists a certain amount of structure since a yield stress can be measured up to a p H value of 8.8 (2). Thus, the Bingham yield stress would appear to be a more sensitive measure of the structure within a suspension than sediment volume or plastic viscosity. The effect of NaC1 at low pH is to decrease the edge-face attractive energy and compress the face double layer. Both of these effects will

Journal of Colloid and Interface Science, Vol. 60, No. 2, June 15, 1977

336

MELTON AND RAND

lead to an increase in the floc density, as observed. At the edge isoelectric point this compression of the face double layer should also lead to an increase in floc density. Above this pH value the sediment volume will be expected initially to increase with increasing NaCI concentration as the energy barrier to edge-edge interaction is lowered, but this presumably occurs at concentrations lower than those investigated here. Further increases in NaC1 concentration lower the edge and face Stern potentials and increase the attractive energy of interaction between particles in either face-face or edge-edge associations. The effect of this on the edge-edge interaction would perhaps tend to favor more voluminous flocs, but its effect on the faces allows a closer approach in this mode which would tend to increase the density of the flocs. Perhaps these two effects cancel each other and at high pH there is no significant change in sediment volume or CfK as the NaC1 concentration is increased in the range 0.06-1.0 M. In the limit as the particles within the flocs are allowed to coagulate in the face-face orientation (above 0.12 M, as deduced earlier (2)) the flocs may be envisaged as consisting essentially of small packets of kaolinite particles stacked in the face-face mode, the packets themselves being linked together through edge-edge and edgeface linkages. The natural raw kaolinite and the Standard Porcelain clay show effects of pH and NaC1 upon the sediment volumes similar to those shown by the homoionic Na kaolinite. The values of the sediment volume are lower for the natural kaolinite than for the homoionic kaolinite because of its smaller average particle size, whereas the fine Standard Porcelain shows the highest sediment volumes of all. Once again the results can be explained by invoking the model of particle-particle interactions which follows from the double-layer structure of kaolinite proposed by Schofield and Samson (6) and van Olphen (7). It is not

possible, however, to use the sediment volumes as a precise measure of the isoelectric point of the kaolinite edge surface as discussed earlier. The Bingham yield stress is a more sensitive measure of structural changes. Dollimore and Horridge (8) have attempted to do this. They suggested that the maximum in their sediment volume-pH curve (at pH 5.8) obtained by coagulating the kaolinite with a nonionic polymer was attributable to an edge-edge structure and, therefore, was located at the edge isoelectric point. It was pointed out previously (1) that the isoelectric point could be as low as 5.8 if the kaolinite had e.vperienced a leaching operation at some time in its history. However, in view of the results shown here it seems more likely that this maximum is a result of the development of the edge-face structure to its fullest extent, and not a measure of the edge isoelectric point at all. This, of course, assumes, as did Dollimore and Horridge, that the sediment volume is determined primarily by the basic electrochemistry of the kaolinite particles and not by the "bridging" polymer itself. ACKNOWLEDGMENT One of us (I.E.M.) gratefully acknowledges the receipt of a maintenance award from the Science Research Council. REFERENCES 1. MELTON, I. E. AND RAND, B., J. Colloid Interface Sei. 60, 321 (1977). (Part II this series). 2. RAN'O, B. AND MELTON, I. E., J. Colloid Interface Sci. 60, 308 (1977). (Part I of this series), 3. MICHAELS,A. S. ANDBOLCER,J. C., Ind. Eng. Chem. Fundamentals 1, 24 (1962). 4. FmTrL B.!A., J. Colloid Interface Sci. 57, 257 (1976). 5. FmT~, B. A. ANDHUNXER,R. J., J. Colloid Interface Sci. 57, 266 (1976). 6. SCHOFIELD, R. K. AND SAMSON, H. R., Discuss. Faraday Soc. 18 135 (1954). 7. VAN OLPrIEN, H., "Introduction to Clay Colloid Chemistry." Interscience, New York, 1963. 8. DOLLIMORE, D. AND HOgRIDGE, T. A,, J. Colloid Interface Sci. 42, 581 (1973).

Journal of Colloid-and Interface Science, Vol. 60, No. 2, June 15, 1977