The adsorption characteristics of polyacrylamide flocculants on kaolin surface have been studied at 25 degreesC as a function of concentration in the presence and absence of surfactants. The adsorption density of flocculants corresponding to maximum settling rate (Gamma(optfloc)) and minimum value of specific resistance of the cake to filtration (Gamma(minSRF)) have been computed and compared with the adsorption density for monolayer coverage (Gamma(infinity)). It has been established in this study that the optimum flocculant concentration for the highest settling rate corresponds to about 50% coverage of the solid surface (i.e. Gamma(optfloc) approximate to Gamma(infinity)/2) for untreated as well as surfactant-pretreated kaolin. Floes suitable for filtration and dewatering are obtained by flocculation of either untreated or surfactant-pretreated kaolin. But in each case, the requirement of polymer concentration for achieving the minimum specific resistance to filtration (SRF) is sufficiently lower than that required for optimum flocculation. The adsorption density of polymer corresponding to minimum SRF is less than about 25% of the plateau adsorption (i.e. Gamma(minSRF) approximate to 0.25 Gamma(infinity)). This has been found to be valid for flocculation with any of the anionic, cationic or nonionic polyacrylamide flocculants used in this investigation. (C) 2002 Published by Elsevier Science B.V.
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