Ultra-low dosage flocculation of alumina using polyacrylic acid

Flocculation experiments conducted with alumina suspensions using polyacrylic acids of different molecular weights showed that flocculation and dispersion can be obtained with both low and high molecular weight polymers depending on the dosage and other solution conditions. While as expected, low molecular weight (5000 g mol−1) polyacrylic acid effectively dispersed alumina suspensions at normal dosages, higher molecular weight moieties (50 000 g mol−1 and above) caused flocculation. Surprisingly, marked flocculation was obtained with parts per billion level polymer dosage. At such ultra-low dosages, a minimum in flocculation was observed at ∼pH 4.5 close to the reported pKa of polyacrylic acid. Zeta potential results did not reveal any significant change in the electrokinetic properties of alumina particles due to the presence of low dosage of polyacrylic acid, only at higher dosages charge reversal occurred. Partial charge neutralization coupled with hydrogen bonding is postulated to be the mechanism for the flocculation obtained at the ultra-low dosages. At the highest dosages, crowding of the polymer segments at the solid/liquid interface results in poor flocculation especially in the acidic pH range.