Effects of surface charge and shear during orthokinetic flocculation on the adsorption and sedimentation of kaolin suspensions in polyelectrolyte solutions

Suspensions of negatively charged kaolin clay as a model system have been flocculated by adsorbing nonionic, cationic, and anionic polyelectrolytes under shear. Test results show that the maximum rate of settling increases with charge density of the cationic polymer being used. Both cylinder test and jar test using low shear conditions tend to underestimate the polymer dose requirements for the orthokinetic flocculation process. Under prolonged shear conditions, the optimum polymer doses required for maximum turbidity removal or minimum residual turbidity coincide with those required for zero zeta-potential or plateau adsorption of polymer. An explanation of the experimental observations based on the charge density of polymers and the shear conditions is given. It is suggested that both polymer bridging and charge neutralization mechanisms may operate in the adsorption/flocculation process. The relative importance between them in determining the optimum polymer doses was analyzed with supplementing rheological measurements of such flocculated systems. It was pointed out why a higher charge density polymer produces a flee structure with higher sensitivity for a shear field.