Destabilisation of an alkaline aqueous alumina suspension by the addition of magnesium acetate

Destabilisation of well-dispersed aqueous alumina suspensions prepared with the addition of an anionic dispersant at the inherent pH of alumina particles in water was achieved by the addition of an appropriate amount of magnesium acetate solution. The destabilisation occurred at a pH value higher than that of the IEP of alumina particles in distilled water. This may be due to either the adsorption of magnesium ions or dispersant molecules onto the surface of the alumina particles. To evaluate these two effects on the surface charge of alumina particles in aqueous slurries, the distribution of the different types of surface sites, with respect to the added magnesium acetate, was calculated. For these calculations, the multisite-complexation model for proton adsorption and the charge-distribution model for the adsorption of magnesium ions was used. According to our results, it is the adsorption of magnesium ions onto the surface of the particles that is responsible for the shift in the point of zero zeta-potential towards higher pH values.