Influence of polymer adsorption and conformation on flocculation and dewatering of kaolin suspension

This paper reports the results of the investigation on flocculation and dewatering characteristics of kaolin suspension by high molecular weight polyacrylamide flocculants of different ionic nature in the absence and the presence of some surfactants. The presence of surfactants in the system has been investigated in two modes of their addition. In the first mode, the kaolin suspension was pretreated with surfactants before flocculation with the polymers. In the second mode, the suspension was treated simultaneously by both polymer and the surfactant from their mixed solutions. The results of separation properties have been interpreted on the basis of physico-chemical properties of the particle surface such as surface charge, adsorption and conformation of the polymers. The conformation of the adsorbed polymer has a marked influence on the flocculation and hence on the separation properties of kaolin suspensions. The polymer conformations can be modified through pH adjustment as well as through the presence of surfactants. For ionic flocculants, a high value of hydrodynamic radius (R-H) or adsorbed layer thickness (ALT) have been found to be detrimental to flocculation because of hindrance to close approach of polymer coated particles to each other. But for non-ionic flocculant where the repulsive forces are minimum, an expanded segments of the polymer chain has more probability of attaching simultaneously with more particles thereby facilitating flocculation by bridging. In most of the cases, pre-adsorbed surfactants on kaolin further improves both settling rates and filtration characteristics on flocculation. Best flocculation of both bare as well as surfactant pretreated kaolin have been found to occur at about 50% of the monolayer coverage (i.e. C-opt floc approximate to 0.50C(m)). On the other hand, separation by filtration was found to be best at a surface coverage close to or <25% of the adsorption density for monolayer coverage (i.e. C-min SRF approximate to 0.25C(m)). (C) 2003 Elsevier B.V. All rights reserved.