The addition of silica particles (4.3, 19, 43, 105, and 285 nm) to stable alumina dispersions (310nm) was found to aggregate the alumina causing the large aggregates to settle out. The oppositely charged silica particles adsorb onto the alumina particle surface resulting in charge neutralisation, bridging and aggregation. The effect of particle size ratio and surface coverage on the amount of silica required to produce the clearest supernatant was investigated. It was found that as the silica particle size increases the number of particles needed for optimum aggregation decreased, although the total amount of silica (wt.%) increased. Below a particle size ratio of about 0.025, the number of silica particles exceeded the calculated half surface coverage number, while at particle size ratios near 1.0 the number of silica particles needed was only about 25% of that calculated for half surface coverage. The median point of the range of concentrations that produced clear supernatants correlated with zero zeta potential.
Colloids and Surfaces a-Physicochemical and Engineering Aspects Vol. 255, no. 1-3, p. 85-90