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 adsorptio n of the NHPA onto the alumina surface. In terms of suspension characterist ics minimum viscosity and minimum sediment height coincided with the attain ment of complete surface coverage of the dispersing agent (3.7 mg g(-1)). C oagulation resulted in a reduction in the degree of sample homogeneity in a ll samples. Coagulation of suspensions initially dispersed with NHPA levels below that needed for complete surface coverage under coagulated condition s (1.4 mg g(-1)) produced viscous pastes with a yield stress suitable for p rocessing by extrusion. At 70 wt% solids the green densities could not be d istinguished from corresponding flocculated samples. Significantly, however , at 80 wt% solids a small degree of homogeneity was retained upon coagulat ion resulting in a higher green density than those prepared by a flocculate d route. The approach of initial dispersion followed by coagulation is ther efore of potential benefit in the processing of viscous, pastes such that h igher degrees of homogeneity can be achieved compared to those prepared by a flocculated route. (C) 2000 Elsevier Science Ltd. All rights reserved.