The effect of alum dose on the consolidation behaviour of coagulated clay dispersions

Model sludges were prepared by coagulating clay dispersions (with or without added humate) with alum and/or a cationic polymer. The sludges were consolidated by centrifuging and their compressive yield stresses calculated as a function of solids content. Both the initial solids content and the final value after dewatering decreased with increasing alum:clay ratio. The adverse effect of alum dose on dewatering was evident even at the optimum dose for effective clarification and was more pronounced for humate-containing sludges. Poor dewatering was attributed to the low density of alum sludges at the gel point and to the high yield stress of networks formed at alum doses close to the charge neutralisation point. Dispersions coagulated with cationic polyacrylamide alone dewatered to higher solids than did alum sludges, but when added after alum, the polymer had no beneficial effect on ultimate solids content. The data confirms that the addition of polymer to alum sludges is beneficial to improving the rate of dewatering only.