Higher efficiency in the flocculation of clay suspensions by using combinations of oppositely charged polyelectrolytes

Dual systems—the highly charged polycation poly(diallyll-dimethyl-ammoniumchloride) (PDADMAC) in combination with different high molecular weight polyanions of the polyacrylamide type—were tested as flocculants for clay suspensions and natural waste waters from gravel pits. The flocculant performance was characterised by measuring the sedimentation behaviour as well as the residual turbidities and the extent of polymer adsorption. Whereas the molecular weight of the high molecular weight polycation is of lower influence, the removel of suspended particles is greatly affected by the molecular weight of the polyanion. Our experiments showed that degradation of the very high molecular weight polyanions leads to higher turbidity of the supernatant. The order of addition of the two polyelectrolytes influences the flocculation mechanism. Most effective is the addition of polycation followed by the polyanion. So a combination of patching and bridging is obtained.