Effect of the structure of cationic polymers on the flocculation and the electrophoretic mobility of crystalline silica

Abstract

A number of cationic polyethyleneimine polymers with molecular weights ranging from about 200 to 32000 were prepared and characterized. Their behavior as flocculants for crystalline silica was examined by means of filtration and electrophoretic methods. The charge on the silica, which is negative, was changed to positive by the addition of the positively charged polymers. The concentration of polymer at which the charge reversed, 0.03–0.1 p.p.m. for 0.04 weight per cent silica dispersion was quite close to the concentration at which the refiltration rate of the silica reached a maximum at pH of 4.0, 6.0, and 9.0. Further addition of cationics rapidly increased the electrophoretic mobility to a high positive value which remained essentially constant even up to high concentrations. With polymers up to 5200 molecular weight an increase in concentration beyond the optimum had but little effect on refiltration, but with higher molecular weight polymers an increase soon caused redispersal of the flocs and very poor refiltration. Flocculation and good filtration of silica by cationic polymers occur at much lower molecular weights than is required with anionic and nonionic polymers, and are the result of reversal of the negative charge on the silica by the positively charged polymer. This is followed by a bridging between particles by the polymer molecules. The effect of chain length and polymer charge on the charge reversal and bridging is discussed.