Coagulation of electrosterically dispersed concentrated alumina suspensions for paste production

The effect of salt induced coagulation of concentrated alumina suspensions initially electrosterically dispersed with ammonium polyacrylate (NHPA) has been investigated. Addition of ammonium chloride caused enhanced adsorption of the NHPA onto the alumina surface. In terms of suspension characteristics minimum viscosity and minimum sediment height coincided with the attainment of complete surface coverage of the dispersing agent (3.7 mg gÿ1). Coagulation resulted in a reduction in the degree of sample homogeneity in all samples. Coagulation of suspensions initially dispersed with NHPA levels below that needed for complete surface coverage under coagulated conditions (1.4 mg gÿ1) produced viscous pastes with a yield stress suitable for processing by extrusion. At 70 wt% solids the green densities could not be distinguished from corresponding flocculated samples. Significantly, however, at 80 wt% solids a small degree of homogeneity was retained upon coagulation resulting in a higher green density than those prepared by a flocculated route. The approach of initial dispersion followed by coagulation is therefore of potential benefit in the processing of viscous, pastes such that higher degrees of homogeneity can be achieved compared to those prepared by a floc- culated route.