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ALAN S. MICHAELS Soil Stabilization Laboratory, Massachusetts Institute of Technology, Cambridge, Mass.
DefloccuIation of Kaolinite
Deflocculation plays an important role in many diverse industrial applications, and like many commonplace physical phenomena it i s well utilized but little understood. The physicochemical explanation provided here should do much to promote further research and hence more efficient use of the dispersion mechanism
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mEcnvE deflocculation in aqueous media of clays and other particulate mineral solids has gained importance over the past few years in such diverse areas as ceramics, oil-well drilling, soil stabilization, detergents, latex paints, and coatings for paper and cathode ray tubes. A host of deflocculating agents or dispersants have appeared which vary widely in chemical nature and specificity of action-lignosulfonates, tannins (such as quebracho), aromatic-aldehyde sulfonates, synthetic polyelectrolytes, modified starches and cellulosics, and condensed alkali phosphates. Of these, alkali (sodium) polyphosphates have been among the most widely used because of their broad range of utility and their relatively low cost. The ability of polyphosphates to disperse various mineral solids in water has been described frequently (6, 20, 23, 37, 35), along with certain anomalies in behavior. However, few attempts have been made (25) to elucidate the dispersion mechanism or the conditions under which dispersion occurs. The interaction of polyphosphates with one common mineral (kaolinite) was studied to find a physicochemical explanation for deflocculation.
by Alkali Polyphosphates
Deflocculation as a n Electrokinetic Process. The physicochemical factors influencing the stability of aqueous colloidal dispersions have been well delineated and correlated by Overbeek (26) and others (37). The ability of certain ionizing substances to disperse a flocculated aqueous suspensoid system into primary particles is due primarily to an increase in electrokinetic potential of the particles, inasmuch as an increase in electrophoretic mobility of particles invariably results from such treatment. This increase in potential may arise from replacement of polyvalent cations by monovalent cations-e.g., sodiumin the diffuse double layer via a metathetical precipitation or chelation process, or from selective adsorption on the solid surface of an ionized deflocculant component with consequent increase in charge-density. Those deflocculants which operate by double-layer cation exchange are usually active with all anionic colloids capable of dispersion, while those which operate by adsorption may be highly specific in their dispersive action. Because appreciable electrokinetic potentials can be developed only in solutions of low ionic strength, deflocculating agents are effective only in systems essentially free of foreign salts, and maximum deflocculation occurs at relatively low concentration levels (6). Kaolinite a n d Its Aqueous Dispersions. The crystallographic structure and chemical, physical, and colloidal properties of kaolinite, Alt(OH)&Os, have been exhaustively studied by Gruner (77), Brindley ( 5 ) , and others (23, 24). The basic crystalline unit is a sheet composed of an octahedrally coordinated layer of alumina (gibbsite) condensed with a hexagonal planar
network of silica tetrahedra. In the primary particle, these sheets are stacked together to produce a roughly hexagonal leaflet about 0.1 micron (about 150 sheets) thick and 1 micron in length or breadth. Kaolinite exhibits cation exchange (to the extent of 1 to 10 meq. per 100 grams), and in the sodium form it displays a moderately high (-24 mv.) electrokinetic potential (8). This cation exchange capacity was formerly attributed to anionic sites at the ruptured platelet edges (73), but it is now believed to result from a small amount of isomorphous substitution of lowervalency ions-e.g., aluminum for silicon or possibly magnesium for aluminumin the crystal lattice. Under alkaline conditions, ionization of surface hydroxyls associated with aluminum can contribute appreciably to the cation exchange capacity as manifested by the marked increase in cation fixation with increasing pH. At moderately low pH (29) the edges of kaolinite flakes contain positively charged sites which attract anions from solution and are responsible for “edge to-face” flocculation in neutral or acidic solutions. These cationic sites evidently result from coordination of aluminum ions at the edges with water (rather than hydroxyl ion) when hydroxyl ion concentration is low. Van Olphen (25) postulated similar positively charged sites on montmorillonite edges to account for the thixotropic character of its dilute dispersions, and for the destruction of thixotropy when traces of alkali salts of polyacids-e.g., sodium carboxymethylcellulose sodium metaphosphate-were added. This picture is strongly supported by the work of Thiessen (32), whose electron microVOL. 50, NO. 6
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JUNE 1958
951
A
-B
-c /
Schematic view of a kaolinite flake edge A. 6. C.
Individual lattice layers Chinks or discontinuities in individual layers Single sheet, curled exfoliations
photographs showed negatively charged colloidal gold deposited on kaolinite platelet edges and positively charged gold deposited on the platelet faces. The following mechanisms for deflocculation of kaolinite can be proposed : Replacement of polyvalent by monovalent cations in the diffuse double layer associated with the platelet faces. Elimination (via polyanion retention in the double layer) of positively charged edge sites. Adsorption of anions on the platelet faces. Adsorption of anions on the platelet edges in excess of the number of cationic sites initially present. Of these four possibilities, the last would have the most potent deflocculating effect, as it would render all parts of the particle surface anionic and cause interparticle repulsions for all possible configurational positions. I t will be shown that alkali polyphosphates probably operate in this fashion, and their powerful deflocculating action on kaolinite thus explained. Condensed Alkali Polyphosphates. The structure and properties of the sodium salts of polyphosphoric acid have been elaborated by Van Wazer and others (33-36),Green (70), and others. Sodium polyphosphates are linear (cyclic for some of lower molecular weight) polymeric structures composed of linked phosphorus-oxygen tetrahedra, ~P,03,+I. with the general formula TVa,&+ I n aqueous solution, the compounds
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ionize to yield an extended anionic chain surrounded by a swarm of sodium counter ions. Linear polyphosphates containing three or more phosphorus atoms can sequester or chelate polyvalent metal ions-e.g., calcium, aluminum, iron-as soluble complexes with small dissociation constants. The ability of polyphosphates to deflocculate kaolinite and other minerals increases with molecular weight: The dimer (pyrophosphate) is relatively ineffective, the trimer (tri-polyphosphate) considerably better, and the tetramer (tetraphosphate) still more active. Beyond the tetramer, there is little increase in effectiveness with chain length. Sodium polyphosphates undergo slow hydrolysis (reversion) to orthophosphate in solution, a process accelerated by reduction (but not by moderate elevation) of pH. Sodium or calcium ions accelerate hydrolysis, while magnesium suppresses it (70). Lower molecular weight species hydrolyze to orthophosphate more rapidly. Water-insoluble hydroxides of many metals (of significance here is aluminum) greatly accelerate polyphosphate hydrolysis (7-3). The chelating ability of linear polyphosphates suggests that cation exchange might be the primary cause of their deflocculating action; however, homoionic sodium kaolinite is far less deflocculated than the same clay after polyphosphate treatment. Furthermore, treatment of kaolinite with polyphosphate increases cation exchange capacity sub-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
stantially as determined by leaching with ammonium acetate and subsequent analysis (ammonium ion) (Table I). Clearly alkali polyphosphates increase the density of anionic sites on the kaolinite, but the precise nature of these new sites remains to be determined. Experimental
T o examine the kaolinite-polyphosphate interaction more carefully, two sets of experiments were performed: one involved natural kaolinite and a commercial alkali polyphosphate and the other a specially prepared calcium kaolinite and highly purified sodium tripolyphosphate. Natural Kaolinite and Quadrafos. Unpurified Bath, S. C., kaolinite was suspended in freshly prepared, distilled water solutions of technical sodium “tetraphosphate” (Quadrafos, Rumford Chemical Works). Cation exchange capacity of the kaolinite was 2.24 rt 0.2 meq./100 grams as calcium and magnesium; the tetraphosphate analyzed 63.5 to 64% phosphorus pentoxide and 36.0 to 36.57, sodium oxide, and the mole ratio of sodium was 1.29. Sodium tetraphosphate is a phosphate glass of wide molecular weight distribution (33), containing roughly 10% tripolyphosphate and 20% tetraphosphate, the remainder being primarily higher linear polyphosphates. The suspensions were aged with intermittent agitation for periods of time varying from 4 hours to several days at 22’ C., then centrifuged, and the supernatant solutions were analyzed. Orthophosphate was determined via colorimetric measurement of the phosphomolydate complex; polyphosphates via hydrolysis with strong acid at boiling temperature, followed by orthophosphate analysis; and calcium and magnesium by titration with sodium Versenate and Eriochrome Black T indicator (27).
The clay phase from these equilibrations was treated with 6 N sulfuric acid t o release retained phosphates, and the extracting solution was analyzed immediately for ortho- and polyphosphates as above. Calcium Kaolinite and Sodium
Table 1. Effect of Polyphosphate Treatment on Cation Exchange Capacity of Kaolinite Cation Exchange Capacity, Treatment, Grams R/Ieq./100 Kaolinite Na Tetraphosphate/ Grams Sample 100 Grams Kaolinite Kaolinite A
B
0.0 0.5 0.0 0.5
6.2 8.4 4.2 6.7
f 0.8 f 0.5 rt 0 . 4 f 0.4
DEFLOCCULATION O F K A O L I N I T E Tripolyphosphate. Purified, fractionated (1 to 2 micron), Bath, S. C., kaolinite was converted to the calcium form by leaching with calcium acetate solution followed by washing with ethanol. Cation exchange capacity of the converted clay was 3.4 & 0.3 meq. per 100 grams. Purified, crystalline sodium tripolyphosphate ( N a ~ P ~ 0 1 ~ . 6 H ana~0 lyzed to contain 98.7 & 0.4% phosphorus as tripolyphosphate) was dissolved in distilled water, and weighed amounts of clay were added to these solutions. Suspensions were agitated for 6 to 48 hours; they were then centrifuged and the supernatant solutions analyzed. Ortho-, pyro-, and tripolyphosphate were determined by the butanol-extraction procedure of Martin and Doty (27) and Van Wazer and Karl-Kroupa (36), sodium was determined via flame photometry, and calcium via oxalate precipitation and permanganate titration. No attempt was made to analyze the solid phase in these studies. While the majority of the experimentally determined values reported are the results of individual measurements, random replication of data indicated a reproducibility of approximately f5%.
Results and Discussion Hydrolysis of Polyphosphates by Kaolinite. The first phenomenon noted was that kaolinite greatly accelerated hydrolysis of both sodium tetraphosphate (Table 11) and sodium tripolyphosphate. While little change in orthophosphate concentration in solution occurred on aging, a substantial) amount of orthophosphate was adsorbed by the clay over a 7-day period; whether hydrolysis occurred before or after adsorption cannot be ascertained from these data. Significantly, the extent of hydrolysis depended on the ratio of phosphate added to clay present and not on the concentration of phosphates in solution. Hydrolysis was also self-limiting, as little further change was noticed after 7 days. With sodium tripolyphosphate (Table 111) hydrolysis occurred even more rapidly, no polyphosphates being detected in solution after only 6 hours of contact with kaolinite, when initial polyphosphate concentration was about 1.6 mmoles of phosphorus per 100 grams of clay. In the absence of kaolinite, hydrolysis of either polyphosphate was undetectable in the same time interval. Sorption of Polyphosphates by Kaolinite. After brief contact with sodium "tetraphosphate" solutions, natural kaolinite was well deflocculated by even the most dilute of these. Fairly constant amounts of orthophosphate and polyphosphate were retained, irrespective of the initial polyphosphate solution concentration (Table
Table II. Hydrolysis of Sodium Tetraphosphate b y Kaolinite"
Initial Soln. Tetraphosphate, weight %
Final S o h Free HPO4,
Vol. Total P added, added, HPO4, Mmoles ml. pmoles pmoles P 2 days 7 days
0.1
125 500 2000 500 2000
1.12 4.50 18.0 0.4 18.0 71.8 " 100 grams of kaolinite; 22O
Table 111.
0.05 0.22 0.85 0.85 3.5
