Kaolinite flocculation

MICHAELS, A. S. and Bolger, J. C. Ind. Eng. Chem. Fundamentals 1 (1962) 24. Settling rates and sediment volumes of flocculated kaolinite suspensions. 

A recent paper by Lairdinvestigated the efficacy of HPAM flocculation of kaolinite, illite and quartz by carrying out visible absorption experiments. He concluded that HPAM more effectively flocculates kaolinite than quartz or illite. This was also the conclusion of previous work by Allen et al. who studied the adsorption of HPAM onto kaolinite, quartz and feldspar at various HPAM concentrations and solution pH by X-ray photoelectron spectroscopy (XPS). Much of the previous work on polyacrylamide adsorption onto aluminosilicates monitored the adsorbed amount by viscometry, carbon analysis and radiotracer techniques. These methods rely on following adsorption by subtraction from that detected in solution. 

Figure 11. Effect of pH on the rigidity modulus of flocculated kaolinite.

Michaels A.S., Baker W.E., Bixler H.J. and Vieth W.R., 1967. Permeability and washing characteristics of flocculated kaolinite filter cakes, Ind. Eng. Chem. Fundam., 6,33-40. 

Adsorption on Kaolinite. As for polyacrylamides, adsorption of XCPS on kaolinite is conducted as a function of S/L and the results extrapolated to S/L=0. However, the S/L dependence of XCPS adsorption on kaolinite is considerably less than that for HPAM. This is due to the flat conformation of the adsorbed molecules of semirigid xanthan (25) compared to the more extended conformation of flexible HPAM (27). The absence of loops and tails in the adsorbed XCPS layer thus diminishes the probability of flocculation of particles by polymer bridging. The slight dependence in adsorption on S/L may therefore be attributed to coagulation of particles induced by Ca. ... 

Adsorption of nonionic and anionic polyacrylamides on kaolinite clay is studied together with various flocculation properties (settling rate, sediment volume, supernatant clarity and suspension viscosity) under controlled conditions of pH, ionic strength and agitation. Adsorption and flocculation data obtained simultaneously for selected systems were correlated to obtain information on the dependence of flocculation on the surface coverage. Interestingly, optimum polymer concentration and type vary depending upon the flocculation response that is monitored. This is discussed in terms of the different properties of the floes and the floe network that control different flocculation responses. Flocculation itself is examined as the cumulative result of many subprocesses that can depend differently on system properties. 

Flocculation is indeed dependent on polymer adsorption, and there are hypotheses correlating the two phenomena, but often these have been put forth without detailed measurement of the two phenomena simultaneously (10-13). In this paper, flocculation is investigated as a function of polymer and solution properties and hydrodynamic conditions by measuring different properties of the system, including adsorption, using well characterized kaolinite and polymer samples prepared specifically for this purpose. Also, the role of concentration and charge density of polyacrylamide and polyacrylamide-polyacrylic acid co-polymers in determining kaolinite flocculation is examined under controlled hydrodynamic conditions. 

Figure 1. Adsorption and flocculation obtained with Na-kaolinite at pH 4.5 as a function of dosage of polyacrylamide (PAM 0.4-0).

Figure 7. Diagram illustrating the effect of polymer charge density on various flocculation responses of Na-kaolinite at pH 4.5.

Figure 9. Flocculation of Na-kaolinite at pH 4.5 as measured by the settling rate and the supernatant clarity as a function of particle surface coverage by 33% hydrolyzed polyacrylamide (HPAM 0.4-33).

The presence of excess salt, particularly of bivalent inorganics can reduce the electrostatic repulsion between the anionic polymer and the kaolinite particles and enhance flocculation (24,25). The optimum flocculation of fine coal and coal refuse (which contained 13 to 65% clays) obtained by Lewellyn and Wang(24) with hydrolyzed polyacrylamide containing 70% acrylate in fact might have resulted from the use of recycled water which contained 55 ppm Ca and 30 ppm Mg. Other works in literature have reported maximum flocculation of kaolinite to occur with 30% hydrolyzed polyacrylamide (5,6,10,26,27). Such differences in reported results could easily result from variations in water chemistry, originating either with the water used or with dissolved organics released by the mineral itself. 

Flocculation using polyacrylamide and hydrolyzed polyacrylamides was investigated by studying a number of flocculation responses of Na-kaolinite under controlled chemical and hydrodynamic conditions. ... 

Flocculation was correlated with both adsorption density and estimated surface coverage for the nonionic and 33% hydrolyzed polyacrylamides. Maximum settling rate was obtained with the nonionic flocculent at 0.1 and with the hydrolyzed sample at 0.2 surface coverage. Supernatant clarity showed a maximum at a surface coverage of Na-kaolinite by the hydrolyzed polyacrylamide of 0.1. At higher surface coverages (such as 0.5) considered in the past to be optimum for flocculation, complete dispersion was obtained with both the nonionic and the anionic polymer. 

Schofield RK, Samson HR (1954) Flocculation of kaolinite due to the attraction of opposite charged crystal faces. Discuss Faraday Soc 18 135-145 Schofield RK, Samson HR (1953) The defiocculation of kaolinite suspensions and the accompanying change-over from positive to negative chloride adsorption. Clay Miner BuU 2 45-51 Schulten HR (2001) Models of humic structures association of humic acids and organic matter in soils and water. In Qapp CE et al. Humic substances and chemical contaminants. Soil Science Society of America, Madison, Wl, pp 73-88... 

An AFM and XPS Investigation of the Selective Flocculation of Kaolinite from a Mineral Mixture... 

This paper contributes to the literature by quantifying anionic polymer adsorption onto the clay minerals kaolinite, feldspar, mica and quartz by X-ray photoelectron spectroscopy (XPS). XPS measures the sorbed amount directly rather than by a subtraction technique. This enables an insight into how effective selective flocculation is for obtaining kaolinite from a mineral mixture. Atomic force microscopy (AFM) is also used to image polymer adsorption onto mineral surfaces and the effectiveness of this technique applied to mineral surfaces is discussed here. 

SEM images of the monodispersed minerals are shown in Figure 1. Unfortunately it was not possible to obtain kaolinite particles in the desired size range (10-20 pm) due to flocculation occuring in the sedimentation process. It was not desirable to chemically treat the surface of the kaolinite to prevent this, as this would affect later results. To this end, the kaolinite was used untreated, as received, with the smaller size fraction shown below. Monodispersed minerals will allow direct comparison in HPAM adsorption isotherms. 

Figure 4 SEM images of a kaolinite-quartz-mica mineral mix after flocculation by HPAM. The kaolin covers other minerals encapsulating them inside the floes...

It is seen here that HPAM strongly adsorbs to kaolinite, causing flocculation, and also has affinity to the feldspar surface. Little or no adsorption was observed on the quartz or mica surface. Any adsorption mechanism must account for this. 

XPS can be used to quantify HPAM adsorption onto minerals at various polymer bulk concentrations. It is seen here that kaolinite has twice the affinity for HPAM than feldspar at pH 9.0 and 50 ppm. Little or no adsorption was monitored on the surface of quartz or mica. Imaging XPS to monitor selective adsorption of mineral mixes proved difficult. Flocculating a mineral mixture of kaolinite, mica and quartz caused the kaolin floes to encapsulate the other minerals. This created a layer of kaolin on the quartz and mica prohibiting polymer mapping on their surfaces. It is shown that the effectiveness of the kaolin recovery is more strongly affected by encapsulation of other minerals during flocculation rather than the selective adsorption process. 

Galactomannans have been used as flocculents (for the purification of ores, for example) and a study has been made of the gum structure by electron microscopy. The smallest fiber-diameter for an evaporated film of guar gum was found260 to be 4 nm, but, when a kaolinite-guar gum flocculate was examined, the diameter of the smallest fiber that was attached to the kaolin was found to be 10 nm. It is suggested that flocculation is caused by polymer bridging of the clay particles. 

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

Kretzschmar, R., Hesterberg, D., and Sticher, H. (1997). Effects of adsorbed humic acid on surface charge and flocculation of kaolinite. Soil Sci. Soc. Am. I. 61,101-108. 

Hinckley believes the differences between the two types of clays is largely a function of the environment of deposition and post-depositional leaching. The poorly crystallized hard clays were believed to have been deposited in a marine environment where face to face flocculation occurred. The well-crystallized soft clays were presumably flocculated edge to face in a fresh-water environment. The more porous soft clays were more thoroughly leached and re-crystallized than the less porous hard clays. Like other kaolinite studies this one does not provide any specific information about the actual chemical composition of the kaolinite minerals. 

Clay colloids provide a good example of the kinds of structures that can be formed upon flocculation. The association of plate-like clay particles is complicated by the fact that minerals such as montmorillonite, illite, and kaolinite can exhibit different surface charges at different particle surfaces. 

---