Investigation of the effect of polymer structure type on flocculation, rheology and dewatering behaviour of kaolinite dispersions

Abstract

The influence of an anionic polyacrylamide–acrylate copolymer (PAM) and a nonionic polyethylene oxide (PEO) polymer on the surface chemistry, shear yield stress, settling rates and consolidation behaviour of kaolinite dispersions has been investigated at pH 7.5. The magnitude of the particle zeta potential decreased with increasing flocculant concentration for both flocculants but was much greater for PEO, due to an increased adsorbed layer thickness compared with PAM. Furthermore, the adsorption studies showed a higher affinity for PEO than PAM. The optimum flocculation for improved clarification rate occurred at less than half the plateau adsorbed amount for both polymers. At similar polymer concentrations, the kaolinite floc sizes were larger and the settling rates greater in the presence of PEO than PAM. Upon shear, the consolidation of pre-sedimented kaolinite slurries significantly improved under flocculation with PEO, while PAM produced no such effect. The magnitude of yield stress was strongly dependent upon the polymer structure, with greater yield stress (stronger interparticle bridging forces) observed for PEO than for PAM flocs. The difference in shear sensitivity of the flocculated slurries may be attributed to polymer structure-related adsorption and interfacial conformation behaviour. PEO polymer chains adsorb via hydrogen bonding interactions between the ether oxygen (Lewis base) and OH groups (Bronsted acid) associated with aluminol and silanol groups, which led to flexible and compressible floc structures. For PAM, however, although the adsorption is still via hydrogen bonding between the silanol and aluminol OH groups at the particle surface and the polymer's primary amide functional groups, the interactions appear to be weakened as a consequence of electrostatic repulsion between the polymer's COO − pendant groups and the negatively charged kaolinite surface. This, in conjunction with possible steric hindrance due to PAM polymer chain branching, led to the formation of loose, open and fragile flocs. A strong correlation between the polymer structure type and flocculation, shear sensitivity and the dewatering behaviour of kaolinite dispersion was established.