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Particle interactions in aqueous kaolinite suspensions
Particle Interactions in Aqueous Kaolinite Suspensions II. Comparison of Some Laboratory and Commercial Kaolinite Samples I A N E R I C M E L T O N I AND B R I A N R A N D
Department of Ceramics, Glasses and Polymers, University of Sheffield, Sheffield SIO 2 TZ, England Received May 10, 1976; accepted August 19, 1976 Flow curves of dilute suspensions of a natural kaolinite sample and two commercial kaolinite samples have been measured as a function of pH and NaCI concentration. The results have been compared with five specially prepared batches of sodium kaolinite. Two of the untreated and two treated samples were found to be contaminated with a specifically adsorbed aluminum species, and they also had edge isoelectric points lower than the specially prepared Na kaolinites which were "aluminum free." Possible reasons for this shift are discussed and comments are made about the preparation process. The effect of NaCI and pH upon the Bingham yield stress of the kaolinite samples can be explained in terms of the modes of particle interaction discussed previously in a detailed study of the "Al-free" Na kaolinite, but the pH values and added NaC1 concentrations at which edge edge interactions are fully developed vary according to the edge isoelectric point and degree of contamination of the clay with soluble salts. Finally, the results of this study are compared with those of others. INTRODUCTION T h e surface chemistry and the rheological and sedimentation properties of aqueous clay suspensions are frequently investigated using n a t u r a l clay minerals which have been carefully purified and converted to a homoionic state, a procedure which is essential in a t t e m p t s to a p p l y the theory of colloidal stability quantit a t i v e l y or s e m i q u a n t i t a t i v e l y to a description of the particular c l a y / w a t e r system under investigation. I n such studies it is hoped t h a t the clay particles have been cleansed of any specifically adsorbed impurities, gel coatings, or mineral impurities. However, it is also highly desirable that, once an understanding of the behavior of such controlled systems has been obtained, the n a t u r a l materials themselves be investigated and compared with the materials used in the model systems in order to establish the roles of the adsorbed impurities or contaminating materials. Montmorillonite,
present to only a v e r y small extent in kaolinires, can have a profound effect upon their rheological properties; as can the organic m a t t e r in a ball clay or fireclay (1). Too frequently this comparison is not made, or the behavior of the n a t u r a l clay is assumed to be the same as t h a t of the clay in the model system. For example, van Olphen (2) suggested t h a t edge-face structures existed in montmorillonite sols only below an electrolyte (NaC1) concentration of about 10--~ M (pH unspecified) b u t it is not necessarily correct to assume t h a t such structures exist in commercial bentonite systems, where the electrolyte concentration m a y be higher than this value. Hence caution is to be advised in interpreting the results obtained with u n t r e a t e d clays. Specifically adsorbed impurities m a y exist at the k a o l i n i t e / w a t e r interface for a v a r i e t y of reasons.
1Present address: Tinsley Wire Industries Ltd., Shepcote Lane, Sheffield 9, England.
1. N a t u r a l clays have frequently been exposed at some time to acidic conditions under 321
Copyright ~ 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.
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MELTON AND RAND
which there may have been a small degree of dissolution of the lattice to produce aluminum and/or silicate species which may subsequently be readsorbed at one or more of the surfaces. Michaels and Bolger (3) compared the pH dependence of the apparent viscosity of an acid-bleached kaolinite and its salt-washed product. The original clay was found to deflocculate at a slightly lower pH value, an effect which was attributed to the action of a specifically adsorbed cationic aluminum species and an anionic silicate species. At low pH values these species were considered to be adsorbed at basal faces and edges, respectively, so lowering the strength of the edge-face linkage. Bundy and Murray (4) also suggest that soluble aluminum species and colloidal hydrous alumina can markedly affect the flow properties of kaolinite suspensions. 2. Certain clays, particularly ball clays and fireclays, are associated with appreciable quantities of organic materials in the form of rather ill-defined lignites or humus (5). Such species are known to impart a deflocculating effect (1) to the clays, presumably by a combination of ionic and "steric" stabilization. 3. In the beneficiation of clays, flocculating and deflocculating ionic species of either organic or inorganic character, and of varying molecular weight, may be added at different stages in the process. Any such species remaining on the clay surfaces after drying will obviously affect the behavior of the clays on subsequent redispersion. A previous paper (6) reported fundamental investigations of the flow behavior of dilute aqueous suspensions of a carefully purified homoionic Na kaolinitic clay sample. The variations in Bingham yield value and plastic viscosity of the suspensions with changing electrolyte concentration and pH were interpreted in terms of the electrical double layers developed at the basal and edge surfaces, and a value was suggested for the isoelectric point of the edge surface (pH = 7.3). Similar investigations have also been carried out on the raw clay from which the homoionic sample was
prepared and on two commercial china clays. These results are described here. In addition to these it was discovered that variations in the preparative procedure for obtaining the homoionic clay resulted in slightly different properties. These results are also discussed. EXPERIMENTAL
Materials. The kaolinite sample investigated most fully was supplied by the Steetley Co. Ltd. (Worksop, England) and was purified (as described earlier (6)) by sedimentation and hydrocycloning to produce a clay of size fraction 42% < 2 m ESD, designated as "raw kaolinite." This clay sample was further processed (6) by acidified salt washes, following the Schofield and Samson (7) method, to remove specifically adsorbed impurities. Five separate batches of these homoionic sodium kaolinites, each of about ½ kg, were prepared. Three batches showed similar rheological properties (batches 1, 2, and 4) and two (3 and 5) exhibited slight differences. Of these two anomalous batches, one had been left as a suspension in 1 M NaC1 at pH 4 for a month and was not subsequently washed to as low an aluminum level as the others. The other batch had also been left as a suspension (at pH = 5) for 2 weeks during the final washing procedure to reduce the sodium concentration. After ten treatments with acidified 1 M NaC1 the supernatant fluid above batch 2 had an aluminum level of about 2 X 10-4 M. At this stage it, too, had been left standing (6 weeks), after which the aluminum concentration had risen considerably. A further series of washes with 1 M NaC1 at pH 3-4 enabled the aluminum concentration to be reduced again to 10-4 M, and the clay was subsequently treated as usual. The properties of this clay were similar to those of batches 1 and 4, for which the preparation was carried out on a continuous 24-hr cycle. The other clays were both supplied by ECCLP Ltd. They were Supreme Kaolin (a fine particle size china clay used for paper coating) and Standard Porcelain, a fine-
Journal of Colloid and Inlerface Science, Vol. 60, No. 2, June 15, 1977
323
COMPARISON OF KAOLINITE SAMPLES TABLE I Properties and Preparation Procedure of Kaolinite Samples Investigated % Particles <20 ~m ESD
Kaolinite
Crystallinity index (8)
40
R a w kaolinite (Steetley)
Conduct i v i t y of suspension (gmho)
Soluble cationic species
Specifiea!ly adsorbed cations
Edge isoelectric point
1.26
27
K +, N a +, Ca 2+, Mg2 +
A13+ (12 #g ml i), C a 2+, Mg2 +
6.7
68
N a +, K +, a little Ca ~+
AI 3+ (24 #g ml 0, C a 2+, Mg2 +
5.6
?
?
6.2
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Approx. 70
1.21
Suprenle Kaolin
Approx. 90
--
--
Homoionic sodium kaolinites (from Steetley Kaolinite) Batch N a + and a trace of K * 2 3 4 5
80
1.32
20
grained, plastic china clay blend used in the ceramic industry in the production of electrical porcelain, porcelain, and bone-china tableware. Table I summarizes the properties of the kaolinite samples investigated. The soluble impurities were determined by dispersing 1 g of the kaolinite in 10 ml of distilled water and after 24 hr measuring the conductivity of the supernatant solution, which was also analyzed by the atomic absorption method for the major cationic species. The specifically adsorbed ions
/ I
2
3
l,
q FIG. 1. Titration curves. O, Homoionic Na kaolinite,
batch I; A, raw kaolinite; e , Standard Porcelain.
7.3
O A13+ (15 #g ml-~) O A134- (?)
7.3 6.6 7.3 6.5
were determined similarly, except that distilled water was replaced by 1 M NaCI solution at pH 3. Rheological measuremenls. These were carried out as described earlier (6). Titration. Some of the clay samples, as 9 wt% suspensions, were titrated against analar NaOH and HC1. The suspensions were continually stirred and the pH was monitored using an EIL model 7050 pH meter. The Pallman effect (9) was found to be negligible. Eleclrophorelic mobility. Selected clays were examined by the technique of microelectrophoresis at an ionic strength of 10.3 M in sodium nitrate over the pH range 3-11. The apparatus used was of the type described by Smith (10) and consisted of a rectangular cell fitted with blacked Palladium electrodes. The electrophoretic mobility was calculated as the mean of 40 readings, 10 in each direction at each stationary level.
oH!meq,100, I A4
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RESULTS Figure 1 shows the titration curves for three of the clays. That of Standard Porcelain is significantly different in shape from the other two, resembling the results of Michaels and Bolger (3) for an acid-bleached kaolinite
Journal of Colloid and Interface Science, Vol. 60, No. 2, J u n e 15, 1977
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MELTON AND RAND
A Batch (1) • Raw kaolinite 5-0
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FIG. 2. Electrophoretic mobility-pH curves for kaolinites. sample. The inflection point at low pH values and the extra alkali required to neutralize the positive edges at about pH 7 can be attributed to the presence of a specifically absorbed aluminum impurity. This is consistent with the chemical evidence presented in Table I, which shows that aluminum could be removed in larger quantities from this clay than from the other samples. Although there is no evidence for aluminum on the raw kaolinite from the titration data, aluminum could be removed by acidified salt washing. Of all the clays investigated, only the homoionic sodium kaolinites, batches 1, 2, and 4, showed no aluminum in the washings with acidified 1 M NaC1. The pH dependence of the electrophoretic mobility of the raw kaolinite and its "Al-free" homoionic sodium product (batch 1) is shown in Fig. 2. The variation in mobility is very similar to that observed by Flegmann (11) and other workers (12, 13). The slight difference in the magnitude of the electrophoretic mobility of the clays at high pH values may not be significant. The effect of pH and electrolyte concentration upon the Bingham yield values of suspensions of the three "A1 free" homoionic Na kaolinite samples has been discussed in detail in the first paper of this series (6). Batches 3 and 5 of the treated Na kaolinites gave slightly different results (Fig. 3). The point of de-
flocculation is shifted to a lower pH (pH 7.4 for batch 3) as are the crossover points in the curves with and without electrolyte (pH values of 6.6 and 6.5, respectively). A similar result was obtained with the raw kaolinite itself (Fig. 4), with an apparent edge isoelectric point at a pH value of 6.7. The effect of a wider range of NaC1 concentrations was investigated at pH values of 4.7, 6.7, and 9.5 (Fig. 5). Except for a shift to lower pH values, it behaves like the "Al-free" homoionic Na kaolinite, at least at pH values lower than 7. The behavior at high pH values is slightly 1.0_
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Journal of Colloid and Interface Science, Vol. 60, No. 2, June 15, 1977
COMPARISON OF KAOLINITE SAMPI,ES
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obtained, crossing the original yield stress-pH curve (at low electrolyte concentration) at different points. Figure 8 shows the dependence of the yield stress upon electrolyte concentration at various fixed pH values. Again, these data are similar to those for the "Al-free" homoionic Na kaolinite if it is assumed that the isoelectric point of the edge surface is displaced to a pH value of 5.6 and that less electrolyte is required to bring about the face-face structure. All the clays which have an edge surface isoelectric point lower than the value of 7.3 are associated in some way with an aluminum species which can be washed off the clay surfaces by 1 M NaC1 (Table I). The clay which appeared to have the largest amount of aluminum associated with it and which also had the lowest edge isoelectric point was Standard Porcelain. After treatment with acidic 1 M salt solution the concentration of aluminum in the washings was of the order of 8 X 10.4 M. This concentration of aluminum was added as A1C13 to a sample of the batch 4 homoionic Na kaolinite (Fig. 9), but it did not alter the edge isoelectric point. The effect of the AIC1, is similar to that of the NaC1 in that it slightly increased the yield stress at high pH values and lowered it at low values. After the
Journal of Colloid and Interface Science, Vol. 60, No. 2, J u n e 15, 1977
326
MELTON AND RAND
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addition of 0.068 M NaC1 the yield stress-pH curve of this clay is identical with that of the homoionic clay after the addition of the same electrolyte concentration. Similarly, the batch 5 Na kaolinite, after immersion in HC1 at pH 1.6 for 2 weeks, shows no change in the pH dependence of the Bingham yield stress, or in the effect of NaCl additions (Fig. 10). The general trend, which was established in the previous study, that the plastic viscosities of the suspensions changed in the same way as the yield stress, as a result of changes in the suspension environment, seemed to apply to this study also.
DISCUSSION
The most striking feature of these results is that all the clays investigated show the same general response to changes in the pH at low ionic strength and to changes in the electrolyte concentration at pH values below 7 as did the "Al-free" homoionic Na kaolinite investigated earlier (6). The differences are in the points of intersection of the Bingham yield stress pH curves (edge isoelectric point) and in the pH value of complete deflocculation in the absence of added electrolyte. The results for the Standard Porcelain clay Slan:lard Porceloin 10-4 M. NaCL
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COMPARISON OF KAOLINITE SAMPLES are slightly different from the others and from the previous investigation in that the common intersection of yield stress pH curves is obtained only at added NaCI concentrations below 0.05 M and the yield stress at this point is much higher than in the other systems. The lower electrolyte concentration can be reconciled with the proposed explanation if the clay already contained soluble salts. Table I shows that the concentration of soluble salts resulting from this clay was higher than that of the other kaolinites studied. The higher yield stress at the isoelectric point is probably a result of the fine particle size of this clay. The Supreme Kaolin, which is also a fine clay, has a yield stress at the isoelectric point intermediate between the yield stresses of the Standard Porcelain and the raw kaolinite. The yield stress is decreased by about 75% when the edge-edge structure is changed to face-face, whereas for raw and homoionic kaolinites the decreases are of the order of about 20-40%. This difference is probably again due to the different particle size distributions of the different clays. The finer Standard Porcelain probably consists of thinner platelets. Hence, when face-face aggregates are formed there is a greater reduction in the number of linkages. The important difference, however, among all the clays reported here and the "Al-free" homoionic Na kaolinite investigated previously (6), is the shift of the edge isoelectric
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point to lower pH values, which was also observed by Michaels and Bolger (3). In a study of the electrophoretic and flotation characteristics of alumina silicates such as sillimanite and andalusite, Harman and Fuerstenau (14) found that the isoelectric point was shifted to lower pH values by treatment of the minerals with acid prior to the electrophoretic measurement. This behavior, which is similar to the effect found here, was attributed to the selective dissolution of aluminum from the crystals with a resultant
E
0.1
7
FIG. 9. Effect of A1C13 on homoionic Na kaolinite (batch 4) suspensions (9 wt%). Al-free Na kaolinite in (O) 10-4M NaC1, (O)0.068 M NaC1, ( [ ] ) 8 X 10-4 M A1CI,~ in 10-4 M NaC1, (m) 8 X 10-4 M A1CI,~ in 0.068 M NaC1.
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FIG. I0. Effect of pH and NaCI on the Bingham yield stress of "Al-free" Na kaolinite (batch 5) after aging in HCI.
Journal of Colloid and Interface Science, Vol. 60, No. 2, June 15, 1977
328
MELTON AND RAND "~E E~
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FIO. 11. Comparison of the pH dependence of rheological parameters of kaolinite suspensions. ®, "Al-free Na kaolinite [Rand and Melton(6)] ; V, "saltwashed" Na kaolinite [Michaels and Bolger (3)]; V, Na-kaolinite FFlegmann et al. (23)]; • and N, Na kaolinite, volume concentrations, respectively, of 0.019 and 0.036 [Nicol and Hunter (24)]; • and A, A1kaolinite, H-kaolinite, respectively [Flegmann et al. (23)]; @, H-kaolinite [Street (25)]; O, acid-bleached kaolinite l-Michaels and Bolger (3)]; IN, Na kaolinite contaminated with traces of A13+[this study].
lowering of the aluminum-to-silicon ratio at the surface. Since silica has a lower isoelectric point than alumina the isoelectric point shifted to lower pH values. It is possible that a similar effect has occurred with the kaolinite samples described in this study, since clays which showed a shift in edge isoelectric point were contaminated with aluminum species. Martin (15), Cashen (16), and Buchanan and Oppenheim (17) have observed dissolution of kaolinite in aqueous suspensions, the effect being greater at pH 2.25 than at 6 in the latter case. Buchanan and Oppenheim also found that about 20 times as much aluminum as silicon was dissolved and that the leached kaolinite showed greater negative electrophoretic mobilities. The dissolution of kaolinite to produce the aluminum which can be removed by 1 M Journal of Colloid and Interface Science,
NaC1 solutions may well have been predominantly from the edge surfaces, which are likely to be the most reactive, and a silica-rich edge might result. Specific adsorption of ions can also alter the isoelectric points of oxides (18). However, Wiese and Healey (19) have clearly shown that the isoelectric point of TiO2 is shifted to higher pH values (up to 9) in the presence of soluble aluminum complexes. It seems very unlikely therefore that these species could be responsible for the observed shift in the edge isoelectric point and they are probably specifically adsorbed on the basal surfaces only. This is supported by the lack of effect of A1CI~ in solution on the edge isoelectric point (Fig. 9). Solubility data for kaolinite and gibbsite have been reviewed recently by Curtis and Spears (20), from whose work it can be shown that at pH 4 kaolinite is in equilibrium with 10-4 M A P + only if the concentration of silicic acid is about 10-5 M. It is possible that the levels of silicic acid in the washings were below this value, resulting in attack of the crystal and further release of AP+. It is also clear from these equations that at low pH values more aluminum than silicon is to be expected in the supernatant liquid at equilibrium, in accordance with the findings of Buchanan and Oppenheim (17). The effect this is likely to have on the edge isoelectric point may well depend upon the initial condition of that surface before immersion, i.e., on whether it is already silica rich. In this work and that of others who have used this washing technique, only aluminum concentrations in the washings have been monitored. It is suggested that the amounts of silicic acid should also be controlled in an attempt to minimize the dissolution reaction. This might lead to the preparation of kaolinitic samples in which surface characteristics are more precisely controlled. Part I of this series (6) shows a list (Table I) of values of the isoelectric point of the kaolinite edge surface and it was shown that the value of 7.3 obtained with the "Al-free" homoionic Na kaolinite was in good agreement
Vol. 60, No. 2, June 15, 1977
COMPARISON OF KAOLINITE SAMPLES with a number of other investigations where the clays were given a similar pretreatment. Dollimore and Horridge (21), however, quoted a value of 5.8 for a commercial china clay. It is likely that this clay also has exchangable aluminum at the surface, since it was not given any special pretreatment, and this may account for the lower isoelectric point obtained. Street and Buchanan (13), on the basis of electrokinetic measurements, suggested (but did not conclusively show) that the isoelectric point of the edge surface of a hydrogen kaolinite was obtained after the addition of 6 meq/100 g of NaOH, which corresponds to a pH value of about 7.5. This seems fairly high for a sample which will undoubtedly be contaminated with a large quantity of aluminum and, on the basis of the work presented here, the value might be expected to be somewhat lower. A recent investigation by Tschapek et al. (22) purports to determine the point of zero charge of kaolinite (i.e., the point where adsorbed H + balances the net negative charge due to lattice substitution) by a titration technique. The clays used were all in the hydrogen form and the shape of the titration curves clearly show the presence of aluminum at the surface. It seems unlikely that their value for the "point of zero charge" carries any fundamental significance since the quantities of adsorbed hydrogen and hydroxyl ions will be a function of the amount of adsorbed aluminum. Curves showing the pH dependence of rheological parameters of kaolinite samples from various studies are compared in Fig. 11. Differences in particle size distribution and volume fraction of clay account for the different magnitudes of the yield stress. The pH dependence of the salt-washed Na kaolinite of Michaels and Bolger (3) is similar to the "AIfree" homoionic Na kaolinite investigated here, whereas the Na kaolinite investigated by Flegmann el al. (23) exhibits a large yield stress even at pH values around 10. Nicol and Hunter (24) also observed high yield values at
329
pH values above 7, but these results were for systems maintained at a much higher ionic strength ( 1 0 2 M), and edge-edge coagulation will have set in. However, their yield stress-pH curves are different from the "Al-free" Na kaolinite investigated by Rand and Melton (6), even at 1.7 X 10-2 M NaC1, showing maxima at pH values between 5.5 and 6.5 and a sharp drop in yield stress at pH values lower than at the maximum. The reason for these discrepancies is not clear, but it may lie in the method of preparation of the clay. The hydrogen and aluminum kaolinites of Flegmann el al. and the hydrogen clay of Street (25) have maxima in similar pH regions and appear to be similar in shape. These curves are significantly different from the curves obtained in this study for clays contaminated with small quantities of exchangable aluminum. It may be that the hydrogen and aluminum clays contain colloidal alumina which can promote mutual coagulation. Attempts in this study to create a hydrogen clay did not succeed in significantly altering the rheological properties of the clay, which was already contaminated with a small amount of aluminum. The situation is confused, there being little real agreement between the results of different workers even where careful preparative techniques have been used to convert the clays to homoionic sodium forms. Further experimentation is called for, perhaps by a collaborative effort among different schools using slightly different preparative techniques and clays of different origin. Meanwhile these studies are being continued and extended. ACKNOWLEDGMENT One of us (I.E.M.) gratefully acknowledges the receipt of a maintenance award from the Science Research Council. REFERENCES 1. NORTON,F. H., "Fine Ceramics," p. 60. McGrawHill, New York, 1970. 2. VAN OLPHEN, H., in "Proceedings of the 4th National Conference on Clays and Clay Minerals," p. 204. Pergamon, Oxford, 1956.
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MELTON AND RAND
3. MICHAELS, A. S. AND BOLGER, T. C., Ind. Eng. Chem. Fundamentals 3, 14 (1964). 4. BUNDY, W. M. AND MURRAY, H. H., Clays Clay Minerals 21, 295 (1973). 5. WORRAL, W. E. AND GREEN, C. V., Trans. Brit. Ceram. Soc. 52, 528 (1953). 6. RAND, B. AND MELTON, L E., J. Colloid Interface Sci. 60, 308 (1977). (Paper I of this series.) 7. SCHOFIELD, R. K. AND SAMSON, H. R., Discuss. Faraday Soc. 18, 135 (1954). 8. HINCKLEY, D. N., in "Proceedings of the l l t h National Conference on Clays and Clay Minerals," p. 229. Pergamon, Oxford, 1963. 9. VAN OLPnErZ, H., "Introduction to Clay Colloid Chemistry," p. 199. Interscience, New York, 1963. 10. SMITH,A. L., in "Dispersion of Powders in Liquids" (G. D. Parfltt, Ed.), p. 39. Elsevier, New York/ Amsterdam, 1969. 11. FLEGMANN, A. W., Ph.D. thesis, University of Cambridge, 1967. 12. VESTIER,D., Sci. Term 14, 289 (1969). l]. STREET, N. AND ]~UCHANAN,A. S., Aust. J. Chem. 9, 450 (1956).
14. HARMAN,T. J. S. ANDFUERSTENAU,D. W., Trans. Soc. Min. Eng. A I M E 235, 367 (1956). 15. MARTIN, R. W., in "Proceedings of the 5th Conference on Clays and Clay Minerals," p. 25. Pergamon, Oxford, 1958. 16. CASHEN,G. H., Trans. Faraday Soc. 55, 477 (1959). 17. BUCHANAN,A. S. AND OPPENHEIM, R. C., Aust. J. Chem. 21, 2367 (1968). 18. LYKLEMA,T., Discuss. Faraday Soc. 52, 318 (1971). 19. WIESE, G. R. AND HEALY, T. W., J. Colloid Interface Sci. 51, 434 (1975). 20. CURTIS, C. D. AND SPEARS, D. A., Clays Clay Minerals 19, 219 (1971). 21. DOLLIMORE,D. AND HORRIDGE, T. A., J. Colloid Interface Scl. 42, 581 (1973). 22. TSCHAI~EK~M.~ TCIIEICHVILLI~L.~ AND WASOWSKI~ C., Clays Clay Minerals 10, 219 (1974). 23. •LEGMANN, A. W., GOODWIN,J. W., ANDOTTEWILL, R. H., Proc. Brit. Ceram. Soc. 13, 31 (1969). 24. NlCOL, S. K. AND HUNTER, R. J., Aust. J. Chem. 23, 2177 (1970). 25. STREET,N., Aust. J. Chem. 9, 467 (1956).
Journal of Colloid and Interface Science, Vol. 60, No. 2, June 15, 1977
Department of Ceramics, Glasses and Polymers, University of Sheffield, Sheffield SIO 2 TZ, England Received May 10, 1976; accepted August 19, 1976 Flow curves of dilute suspensions of a natural kaolinite sample and two commercial kaolinite samples have been measured as a function of pH and NaCI concentration. The results have been compared with five specially prepared batches of sodium kaolinite. Two of the untreated and two treated samples were found to be contaminated with a specifically adsorbed aluminum species, and they also had edge isoelectric points lower than the specially prepared Na kaolinites which were "aluminum free." Possible reasons for this shift are discussed and comments are made about the preparation process. The effect of NaCI and pH upon the Bingham yield stress of the kaolinite samples can be explained in terms of the modes of particle interaction discussed previously in a detailed study of the "Al-free" Na kaolinite, but the pH values and added NaC1 concentrations at which edge edge interactions are fully developed vary according to the edge isoelectric point and degree of contamination of the clay with soluble salts. Finally, the results of this study are compared with those of others. INTRODUCTION T h e surface chemistry and the rheological and sedimentation properties of aqueous clay suspensions are frequently investigated using n a t u r a l clay minerals which have been carefully purified and converted to a homoionic state, a procedure which is essential in a t t e m p t s to a p p l y the theory of colloidal stability quantit a t i v e l y or s e m i q u a n t i t a t i v e l y to a description of the particular c l a y / w a t e r system under investigation. I n such studies it is hoped t h a t the clay particles have been cleansed of any specifically adsorbed impurities, gel coatings, or mineral impurities. However, it is also highly desirable that, once an understanding of the behavior of such controlled systems has been obtained, the n a t u r a l materials themselves be investigated and compared with the materials used in the model systems in order to establish the roles of the adsorbed impurities or contaminating materials. Montmorillonite,
present to only a v e r y small extent in kaolinires, can have a profound effect upon their rheological properties; as can the organic m a t t e r in a ball clay or fireclay (1). Too frequently this comparison is not made, or the behavior of the n a t u r a l clay is assumed to be the same as t h a t of the clay in the model system. For example, van Olphen (2) suggested t h a t edge-face structures existed in montmorillonite sols only below an electrolyte (NaC1) concentration of about 10--~ M (pH unspecified) b u t it is not necessarily correct to assume t h a t such structures exist in commercial bentonite systems, where the electrolyte concentration m a y be higher than this value. Hence caution is to be advised in interpreting the results obtained with u n t r e a t e d clays. Specifically adsorbed impurities m a y exist at the k a o l i n i t e / w a t e r interface for a v a r i e t y of reasons.
1Present address: Tinsley Wire Industries Ltd., Shepcote Lane, Sheffield 9, England.
1. N a t u r a l clays have frequently been exposed at some time to acidic conditions under 321
Copyright ~ 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.
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MELTON AND RAND
which there may have been a small degree of dissolution of the lattice to produce aluminum and/or silicate species which may subsequently be readsorbed at one or more of the surfaces. Michaels and Bolger (3) compared the pH dependence of the apparent viscosity of an acid-bleached kaolinite and its salt-washed product. The original clay was found to deflocculate at a slightly lower pH value, an effect which was attributed to the action of a specifically adsorbed cationic aluminum species and an anionic silicate species. At low pH values these species were considered to be adsorbed at basal faces and edges, respectively, so lowering the strength of the edge-face linkage. Bundy and Murray (4) also suggest that soluble aluminum species and colloidal hydrous alumina can markedly affect the flow properties of kaolinite suspensions. 2. Certain clays, particularly ball clays and fireclays, are associated with appreciable quantities of organic materials in the form of rather ill-defined lignites or humus (5). Such species are known to impart a deflocculating effect (1) to the clays, presumably by a combination of ionic and "steric" stabilization. 3. In the beneficiation of clays, flocculating and deflocculating ionic species of either organic or inorganic character, and of varying molecular weight, may be added at different stages in the process. Any such species remaining on the clay surfaces after drying will obviously affect the behavior of the clays on subsequent redispersion. A previous paper (6) reported fundamental investigations of the flow behavior of dilute aqueous suspensions of a carefully purified homoionic Na kaolinitic clay sample. The variations in Bingham yield value and plastic viscosity of the suspensions with changing electrolyte concentration and pH were interpreted in terms of the electrical double layers developed at the basal and edge surfaces, and a value was suggested for the isoelectric point of the edge surface (pH = 7.3). Similar investigations have also been carried out on the raw clay from which the homoionic sample was
prepared and on two commercial china clays. These results are described here. In addition to these it was discovered that variations in the preparative procedure for obtaining the homoionic clay resulted in slightly different properties. These results are also discussed. EXPERIMENTAL
Materials. The kaolinite sample investigated most fully was supplied by the Steetley Co. Ltd. (Worksop, England) and was purified (as described earlier (6)) by sedimentation and hydrocycloning to produce a clay of size fraction 42% < 2 m ESD, designated as "raw kaolinite." This clay sample was further processed (6) by acidified salt washes, following the Schofield and Samson (7) method, to remove specifically adsorbed impurities. Five separate batches of these homoionic sodium kaolinites, each of about ½ kg, were prepared. Three batches showed similar rheological properties (batches 1, 2, and 4) and two (3 and 5) exhibited slight differences. Of these two anomalous batches, one had been left as a suspension in 1 M NaC1 at pH 4 for a month and was not subsequently washed to as low an aluminum level as the others. The other batch had also been left as a suspension (at pH = 5) for 2 weeks during the final washing procedure to reduce the sodium concentration. After ten treatments with acidified 1 M NaC1 the supernatant fluid above batch 2 had an aluminum level of about 2 X 10-4 M. At this stage it, too, had been left standing (6 weeks), after which the aluminum concentration had risen considerably. A further series of washes with 1 M NaC1 at pH 3-4 enabled the aluminum concentration to be reduced again to 10-4 M, and the clay was subsequently treated as usual. The properties of this clay were similar to those of batches 1 and 4, for which the preparation was carried out on a continuous 24-hr cycle. The other clays were both supplied by ECCLP Ltd. They were Supreme Kaolin (a fine particle size china clay used for paper coating) and Standard Porcelain, a fine-
Journal of Colloid and Inlerface Science, Vol. 60, No. 2, June 15, 1977
323
COMPARISON OF KAOLINITE SAMPLES TABLE I Properties and Preparation Procedure of Kaolinite Samples Investigated % Particles <20 ~m ESD
Kaolinite
Crystallinity index (8)
40
R a w kaolinite (Steetley)
Conduct i v i t y of suspension (gmho)
Soluble cationic species
Specifiea!ly adsorbed cations
Edge isoelectric point
1.26
27
K +, N a +, Ca 2+, Mg2 +
A13+ (12 #g ml i), C a 2+, Mg2 +
6.7
68
N a +, K +, a little Ca ~+
AI 3+ (24 #g ml 0, C a 2+, Mg2 +
5.6
?
?
6.2
S t a n d a r d Porcelain
Approx. 70
1.21
Suprenle Kaolin
Approx. 90
--
--
Homoionic sodium kaolinites (from Steetley Kaolinite) Batch N a + and a trace of K * 2 3 4 5
80
1.32
20
grained, plastic china clay blend used in the ceramic industry in the production of electrical porcelain, porcelain, and bone-china tableware. Table I summarizes the properties of the kaolinite samples investigated. The soluble impurities were determined by dispersing 1 g of the kaolinite in 10 ml of distilled water and after 24 hr measuring the conductivity of the supernatant solution, which was also analyzed by the atomic absorption method for the major cationic species. The specifically adsorbed ions
/ I
2
3
l,
q FIG. 1. Titration curves. O, Homoionic Na kaolinite,
batch I; A, raw kaolinite; e , Standard Porcelain.
7.3
O A13+ (15 #g ml-~) O A134- (?)
7.3 6.6 7.3 6.5
were determined similarly, except that distilled water was replaced by 1 M NaCI solution at pH 3. Rheological measuremenls. These were carried out as described earlier (6). Titration. Some of the clay samples, as 9 wt% suspensions, were titrated against analar NaOH and HC1. The suspensions were continually stirred and the pH was monitored using an EIL model 7050 pH meter. The Pallman effect (9) was found to be negligible. Eleclrophorelic mobility. Selected clays were examined by the technique of microelectrophoresis at an ionic strength of 10.3 M in sodium nitrate over the pH range 3-11. The apparatus used was of the type described by Smith (10) and consisted of a rectangular cell fitted with blacked Palladium electrodes. The electrophoretic mobility was calculated as the mean of 40 readings, 10 in each direction at each stationary level.
oH!meq,100, I A4
O
RESULTS Figure 1 shows the titration curves for three of the clays. That of Standard Porcelain is significantly different in shape from the other two, resembling the results of Michaels and Bolger (3) for an acid-bleached kaolinite
Journal of Colloid and Interface Science, Vol. 60, No. 2, J u n e 15, 1977
324
MELTON AND RAND
A Batch (1) • Raw kaolinite 5-0
A
//~//~"X
/./
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'~ 4.0
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FIG. 2. Electrophoretic mobility-pH curves for kaolinites. sample. The inflection point at low pH values and the extra alkali required to neutralize the positive edges at about pH 7 can be attributed to the presence of a specifically absorbed aluminum impurity. This is consistent with the chemical evidence presented in Table I, which shows that aluminum could be removed in larger quantities from this clay than from the other samples. Although there is no evidence for aluminum on the raw kaolinite from the titration data, aluminum could be removed by acidified salt washing. Of all the clays investigated, only the homoionic sodium kaolinites, batches 1, 2, and 4, showed no aluminum in the washings with acidified 1 M NaC1. The pH dependence of the electrophoretic mobility of the raw kaolinite and its "Al-free" homoionic sodium product (batch 1) is shown in Fig. 2. The variation in mobility is very similar to that observed by Flegmann (11) and other workers (12, 13). The slight difference in the magnitude of the electrophoretic mobility of the clays at high pH values may not be significant. The effect of pH and electrolyte concentration upon the Bingham yield values of suspensions of the three "A1 free" homoionic Na kaolinite samples has been discussed in detail in the first paper of this series (6). Batches 3 and 5 of the treated Na kaolinites gave slightly different results (Fig. 3). The point of de-
flocculation is shifted to a lower pH (pH 7.4 for batch 3) as are the crossover points in the curves with and without electrolyte (pH values of 6.6 and 6.5, respectively). A similar result was obtained with the raw kaolinite itself (Fig. 4), with an apparent edge isoelectric point at a pH value of 6.7. The effect of a wider range of NaC1 concentrations was investigated at pH values of 4.7, 6.7, and 9.5 (Fig. 5). Except for a shift to lower pH values, it behaves like the "Al-free" homoionic Na kaolinite, at least at pH values lower than 7. The behavior at high pH values is slightly 1.0_
z
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Journal of Colloid and Interface Science, Vol. 60, No. 2, June 15, 1977
COMPARISON OF KAOLINITE SAMPI,ES
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obtained, crossing the original yield stress-pH curve (at low electrolyte concentration) at different points. Figure 8 shows the dependence of the yield stress upon electrolyte concentration at various fixed pH values. Again, these data are similar to those for the "Al-free" homoionic Na kaolinite if it is assumed that the isoelectric point of the edge surface is displaced to a pH value of 5.6 and that less electrolyte is required to bring about the face-face structure. All the clays which have an edge surface isoelectric point lower than the value of 7.3 are associated in some way with an aluminum species which can be washed off the clay surfaces by 1 M NaC1 (Table I). The clay which appeared to have the largest amount of aluminum associated with it and which also had the lowest edge isoelectric point was Standard Porcelain. After treatment with acidic 1 M salt solution the concentration of aluminum in the washings was of the order of 8 X 10.4 M. This concentration of aluminum was added as A1C13 to a sample of the batch 4 homoionic Na kaolinite (Fig. 9), but it did not alter the edge isoelectric point. The effect of the AIC1, is similar to that of the NaC1 in that it slightly increased the yield stress at high pH values and lowered it at low values. After the
Journal of Colloid and Interface Science, Vol. 60, No. 2, J u n e 15, 1977
326
MELTON AND RAND
~E 2.0
•
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0
IK
.
+0"00 M.NoCI
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6
7
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addition of 0.068 M NaC1 the yield stress-pH curve of this clay is identical with that of the homoionic clay after the addition of the same electrolyte concentration. Similarly, the batch 5 Na kaolinite, after immersion in HC1 at pH 1.6 for 2 weeks, shows no change in the pH dependence of the Bingham yield stress, or in the effect of NaCl additions (Fig. 10). The general trend, which was established in the previous study, that the plastic viscosities of the suspensions changed in the same way as the yield stress, as a result of changes in the suspension environment, seemed to apply to this study also.
DISCUSSION
The most striking feature of these results is that all the clays investigated show the same general response to changes in the pH at low ionic strength and to changes in the electrolyte concentration at pH values below 7 as did the "Al-free" homoionic Na kaolinite investigated earlier (6). The differences are in the points of intersection of the Bingham yield stress pH curves (edge isoelectric point) and in the pH value of complete deflocculation in the absence of added electrolyte. The results for the Standard Porcelain clay Slan:lard Porceloin 10-4 M. NaCL
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FIG. 7, Effect of p H and NaC1 on the B i n g h a m yield stress of Standard Porcelain suspensions (9 w t % ) . Journal of Colloid and Interface Science, Vol. 60, No. 2, June 15, 1977
COMPARISON OF KAOLINITE SAMPLES are slightly different from the others and from the previous investigation in that the common intersection of yield stress pH curves is obtained only at added NaCI concentrations below 0.05 M and the yield stress at this point is much higher than in the other systems. The lower electrolyte concentration can be reconciled with the proposed explanation if the clay already contained soluble salts. Table I shows that the concentration of soluble salts resulting from this clay was higher than that of the other kaolinites studied. The higher yield stress at the isoelectric point is probably a result of the fine particle size of this clay. The Supreme Kaolin, which is also a fine clay, has a yield stress at the isoelectric point intermediate between the yield stresses of the Standard Porcelain and the raw kaolinite. The yield stress is decreased by about 75% when the edge-edge structure is changed to face-face, whereas for raw and homoionic kaolinites the decreases are of the order of about 20-40%. This difference is probably again due to the different particle size distributions of the different clays. The finer Standard Porcelain probably consists of thinner platelets. Hence, when face-face aggregates are formed there is a greater reduction in the number of linkages. The important difference, however, among all the clays reported here and the "Al-free" homoionic Na kaolinite investigated previously (6), is the shift of the edge isoelectric
,E 2 ' 5 a z t\ 20 \~ ~
a pH o pH [] pH
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point to lower pH values, which was also observed by Michaels and Bolger (3). In a study of the electrophoretic and flotation characteristics of alumina silicates such as sillimanite and andalusite, Harman and Fuerstenau (14) found that the isoelectric point was shifted to lower pH values by treatment of the minerals with acid prior to the electrophoretic measurement. This behavior, which is similar to the effect found here, was attributed to the selective dissolution of aluminum from the crystals with a resultant
E
0.1
7
FIG. 9. Effect of A1C13 on homoionic Na kaolinite (batch 4) suspensions (9 wt%). Al-free Na kaolinite in (O) 10-4M NaC1, (O)0.068 M NaC1, ( [ ] ) 8 X 10-4 M A1CI,~ in 10-4 M NaC1, (m) 8 X 10-4 M A1CI,~ in 0.068 M NaC1.
"O
u
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^
7
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ot
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FIG. I0. Effect of pH and NaCI on the Bingham yield stress of "Al-free" Na kaolinite (batch 5) after aging in HCI.
Journal of Colloid and Interface Science, Vol. 60, No. 2, June 15, 1977
328
MELTON AND RAND "~E E~
•
,I~ ~ I
! 24
~i-y
i /
~/
Na -
kaolinites
i("~'[B
s.o
o~
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,\
,
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_10 _0
6 pH 8 10 H-or AI contaminated kaotinites
4
6
8
10
pH
FIO. 11. Comparison of the pH dependence of rheological parameters of kaolinite suspensions. ®, "Al-free Na kaolinite [Rand and Melton(6)] ; V, "saltwashed" Na kaolinite [Michaels and Bolger (3)]; V, Na-kaolinite FFlegmann et al. (23)]; • and N, Na kaolinite, volume concentrations, respectively, of 0.019 and 0.036 [Nicol and Hunter (24)]; • and A, A1kaolinite, H-kaolinite, respectively [Flegmann et al. (23)]; @, H-kaolinite [Street (25)]; O, acid-bleached kaolinite l-Michaels and Bolger (3)]; IN, Na kaolinite contaminated with traces of A13+[this study].
lowering of the aluminum-to-silicon ratio at the surface. Since silica has a lower isoelectric point than alumina the isoelectric point shifted to lower pH values. It is possible that a similar effect has occurred with the kaolinite samples described in this study, since clays which showed a shift in edge isoelectric point were contaminated with aluminum species. Martin (15), Cashen (16), and Buchanan and Oppenheim (17) have observed dissolution of kaolinite in aqueous suspensions, the effect being greater at pH 2.25 than at 6 in the latter case. Buchanan and Oppenheim also found that about 20 times as much aluminum as silicon was dissolved and that the leached kaolinite showed greater negative electrophoretic mobilities. The dissolution of kaolinite to produce the aluminum which can be removed by 1 M Journal of Colloid and Interface Science,
NaC1 solutions may well have been predominantly from the edge surfaces, which are likely to be the most reactive, and a silica-rich edge might result. Specific adsorption of ions can also alter the isoelectric points of oxides (18). However, Wiese and Healey (19) have clearly shown that the isoelectric point of TiO2 is shifted to higher pH values (up to 9) in the presence of soluble aluminum complexes. It seems very unlikely therefore that these species could be responsible for the observed shift in the edge isoelectric point and they are probably specifically adsorbed on the basal surfaces only. This is supported by the lack of effect of A1CI~ in solution on the edge isoelectric point (Fig. 9). Solubility data for kaolinite and gibbsite have been reviewed recently by Curtis and Spears (20), from whose work it can be shown that at pH 4 kaolinite is in equilibrium with 10-4 M A P + only if the concentration of silicic acid is about 10-5 M. It is possible that the levels of silicic acid in the washings were below this value, resulting in attack of the crystal and further release of AP+. It is also clear from these equations that at low pH values more aluminum than silicon is to be expected in the supernatant liquid at equilibrium, in accordance with the findings of Buchanan and Oppenheim (17). The effect this is likely to have on the edge isoelectric point may well depend upon the initial condition of that surface before immersion, i.e., on whether it is already silica rich. In this work and that of others who have used this washing technique, only aluminum concentrations in the washings have been monitored. It is suggested that the amounts of silicic acid should also be controlled in an attempt to minimize the dissolution reaction. This might lead to the preparation of kaolinitic samples in which surface characteristics are more precisely controlled. Part I of this series (6) shows a list (Table I) of values of the isoelectric point of the kaolinite edge surface and it was shown that the value of 7.3 obtained with the "Al-free" homoionic Na kaolinite was in good agreement
Vol. 60, No. 2, June 15, 1977
COMPARISON OF KAOLINITE SAMPLES with a number of other investigations where the clays were given a similar pretreatment. Dollimore and Horridge (21), however, quoted a value of 5.8 for a commercial china clay. It is likely that this clay also has exchangable aluminum at the surface, since it was not given any special pretreatment, and this may account for the lower isoelectric point obtained. Street and Buchanan (13), on the basis of electrokinetic measurements, suggested (but did not conclusively show) that the isoelectric point of the edge surface of a hydrogen kaolinite was obtained after the addition of 6 meq/100 g of NaOH, which corresponds to a pH value of about 7.5. This seems fairly high for a sample which will undoubtedly be contaminated with a large quantity of aluminum and, on the basis of the work presented here, the value might be expected to be somewhat lower. A recent investigation by Tschapek et al. (22) purports to determine the point of zero charge of kaolinite (i.e., the point where adsorbed H + balances the net negative charge due to lattice substitution) by a titration technique. The clays used were all in the hydrogen form and the shape of the titration curves clearly show the presence of aluminum at the surface. It seems unlikely that their value for the "point of zero charge" carries any fundamental significance since the quantities of adsorbed hydrogen and hydroxyl ions will be a function of the amount of adsorbed aluminum. Curves showing the pH dependence of rheological parameters of kaolinite samples from various studies are compared in Fig. 11. Differences in particle size distribution and volume fraction of clay account for the different magnitudes of the yield stress. The pH dependence of the salt-washed Na kaolinite of Michaels and Bolger (3) is similar to the "AIfree" homoionic Na kaolinite investigated here, whereas the Na kaolinite investigated by Flegmann el al. (23) exhibits a large yield stress even at pH values around 10. Nicol and Hunter (24) also observed high yield values at
329
pH values above 7, but these results were for systems maintained at a much higher ionic strength ( 1 0 2 M), and edge-edge coagulation will have set in. However, their yield stress-pH curves are different from the "Al-free" Na kaolinite investigated by Rand and Melton (6), even at 1.7 X 10-2 M NaC1, showing maxima at pH values between 5.5 and 6.5 and a sharp drop in yield stress at pH values lower than at the maximum. The reason for these discrepancies is not clear, but it may lie in the method of preparation of the clay. The hydrogen and aluminum kaolinites of Flegmann el al. and the hydrogen clay of Street (25) have maxima in similar pH regions and appear to be similar in shape. These curves are significantly different from the curves obtained in this study for clays contaminated with small quantities of exchangable aluminum. It may be that the hydrogen and aluminum clays contain colloidal alumina which can promote mutual coagulation. Attempts in this study to create a hydrogen clay did not succeed in significantly altering the rheological properties of the clay, which was already contaminated with a small amount of aluminum. The situation is confused, there being little real agreement between the results of different workers even where careful preparative techniques have been used to convert the clays to homoionic sodium forms. Further experimentation is called for, perhaps by a collaborative effort among different schools using slightly different preparative techniques and clays of different origin. Meanwhile these studies are being continued and extended. ACKNOWLEDGMENT One of us (I.E.M.) gratefully acknowledges the receipt of a maintenance award from the Science Research Council. REFERENCES 1. NORTON,F. H., "Fine Ceramics," p. 60. McGrawHill, New York, 1970. 2. VAN OLPHEN, H., in "Proceedings of the 4th National Conference on Clays and Clay Minerals," p. 204. Pergamon, Oxford, 1956.
Journal of Colloid and Interface Science, Vol. 60, No. 2, J u n e 15, 1977
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MELTON AND RAND
3. MICHAELS, A. S. AND BOLGER, T. C., Ind. Eng. Chem. Fundamentals 3, 14 (1964). 4. BUNDY, W. M. AND MURRAY, H. H., Clays Clay Minerals 21, 295 (1973). 5. WORRAL, W. E. AND GREEN, C. V., Trans. Brit. Ceram. Soc. 52, 528 (1953). 6. RAND, B. AND MELTON, L E., J. Colloid Interface Sci. 60, 308 (1977). (Paper I of this series.) 7. SCHOFIELD, R. K. AND SAMSON, H. R., Discuss. Faraday Soc. 18, 135 (1954). 8. HINCKLEY, D. N., in "Proceedings of the l l t h National Conference on Clays and Clay Minerals," p. 229. Pergamon, Oxford, 1963. 9. VAN OLPnErZ, H., "Introduction to Clay Colloid Chemistry," p. 199. Interscience, New York, 1963. 10. SMITH,A. L., in "Dispersion of Powders in Liquids" (G. D. Parfltt, Ed.), p. 39. Elsevier, New York/ Amsterdam, 1969. 11. FLEGMANN, A. W., Ph.D. thesis, University of Cambridge, 1967. 12. VESTIER,D., Sci. Term 14, 289 (1969). l]. STREET, N. AND ]~UCHANAN,A. S., Aust. J. Chem. 9, 450 (1956).
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Journal of Colloid and Interface Science, Vol. 60, No. 2, June 15, 1977