Polymers flocculation, alumina

Starch is a polysaccharide found in many plant species. Com and potatoes are two common sources of industrial starch. The composition of starch varies somewhat in terms of the amount of branching of the polymer chains (11). Its principal use as a flocculant is in the Bayer process for extracting aluminum from bauxite ore. The digestion of bauxite in sodium hydroxide solution produces a suspension of finely divided iron minerals and siUcates, called red mud, in a highly alkaline Hquor. Starch is used to settle the red mud so that relatively pure alumina can be produced from the clarified Hquor. It has been largely replaced by acryHc acid and acrylamide-based (11,12) polymers, although a number of plants stiH add some starch in addition to synthetic polymers to reduce the level of residual suspended soHds in the Hquor. Starch [9005-25-8] can be modified with various reagents to produce semisynthetic polymers. The principal one of these is cationic starch, which is used as a retention aid in paper production as a component of a dual system (13,14) or a microparticle system (15). 

Figure 7 Destabilisation/flocculation of a fine alumina suspension at pH 11 by polymer combinations, (a) Cationic only (1.07mgL ) (b) cationic (1.07mg) followed by anionic (2.14mgL ) (c) anionic only (2.14mgL ) (d) anionic (2.14mgL )followed by cationic (1.07mgL )...

Flocculation of alumina suspensions obtained by the sequential addition of polystyrene sulfonate (M j, = 4600) and cationic polyacrylamide (M, = 4,000,000) at pH 4.5 is compared in Figure 7.33 with that obtained using single polymers. While the anionic polystyrene sulfonate had only a minor effect, cationic polyacrylamide did not produce any flocculation. However, when used together, both polymers adsorb completely. This coadsorption is attributed to the interaction of complexes between cationic polyacrylamide and the polystyrene sulfonate at the solid-liquid interface. The mechanism of the superior flocculation obtained with the dual polymer system is illustrated schematically in Figure 7.34. The anionic polystyrene sulfonate adsorbs on alumina surface and acts as an anionic anchor for the adsorption of the long-chain cationic polymer, which ultimately provides interparticle bridging and excellent flocculation. 

FIGURE 7.33 Flocculation response of alumina suspension with dual polymer systems. , C-PAM alone 0, PSS alone O, PSS and C-PAM A, C-PAM after PSS , PSS after C-PAM. (From Yu, X. and Soma-sundaran, P, Colloids Surf., 81, 17, 1993. With permission.)... 

FIGURE 7.36 Flocculation and zeta potential of alumina particles as a function of polymer concentration. (From Das, K.K. and Somasundaran, P., Colloids Surf. A, 182, 25, 2001. With permission.)... 

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

The mechanism behind destabilization with macromolecules is very dependent on the size of the molecule. Polymers of lower molecular mass can show a strong affinity to the oil/water interface, adsorb irreversibly and destabilize in this way. Another route of destabilization is flocculation. Flocculation is an aggregation process in whieh droplets form three-dimensional clusters, each droplet retaining its individual identity. In order to model the importance of flocculation in the destabilization of model systems, one can investigate a-alumina dispersions (52). 

In a later development, Somasundaran et al. ]57] developed a PBM for aggregation by polymers in shear environments. The D LVO theory was extended for this case, as discussed in the previous section, by using the modifled expression for van der Waals attraction for particles covered with polymers and the expression for bridging attraction or steric repulsion derived from the scaling theory [25]. Their model was tested qualitatively with experimental data for the flocculation of colloidal alumina suspensions in the presence of PAA and was found to reproduce the observed experimental trends [60] reasonably well. 

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