Ultra-low dosage flocculation of alumina using polyacrylic acid

Flocculation experiments conducted with alumina suspensions using polyacryl ic acids of different molecular weights showed that flocculation and disper sion can be obtained with both low and high molecular weight polymers depen ding on the dosage and other solution conditions. While as expected, tow mo lecular weight (5000 g mol(-1)) polyacrylic acid effectively dispersed alum ina suspensions at normal dosages, higher molecular weight moieties (50 000 g mol(-1) and above) caused flocculation. Surprisingly, marked flocculatio n was obtained with parts per billion level polymer dosage. At such ultra-l ow dosages, a minimum in flocculation was observed at similar to pH 4.5 clo se to the reported pK(a) of polyacrylic acid. Zeta potential results did no t reveal any significant change in the electrokinetic properties of alumina particles due to the presence of low dosage of polyacrylic acid, only at h igher dosages charge reversal occurred. Partial charge neutralization coupl ed with hydrogen bonding is postulated to be the mechanism for the floccula tion obtained at the ultra-low dosages. At the highest dosages, crowding of the polymer segments at the solid/liquid interface results in poor floccul ation especially in the acidic pH range. (C) 2001 Elsevier Science B.V. All rights reserved.