Influence of unadsorbed and weakly adsorbed flocculants on separation properties of kaolin suspensions

This paper presents the results of studies on the influence of unadsorbed or physisorbed flocculant on filtration and dewatering characteristics of kaolin suspension. The unadsorbed flocculant in the suspension has been estimated by measuring the desorption of the respective flocculants on washing after their equilibrium adsorption. The presence of such species in the system has been invariably found to hinder filtration and dewatering behaviour. Their removal from the suspension by washing has led to a substantial improvement in filtration behaviour evident by the reduction in specific resistance of the cake to filtration (SRF).The flocculation and dewatering results have been explained in terms of the nature of polymer adsorption and conformation. It has been shown that cationic flocculant, which adsorbs predominantly by physisorption, and is in most coiled form after adsorption is most efficient in enhancing filtration behaviour. On the other hand, non-ionic polyacrylamide flocculant, which is predominantly chemisorbed on kaolin and exhibits a more stretched conformation, is most suitable for enhancing sedimentation rate. The anionic flocculant, for which the conformation is most stretched, exhibits lowest sedimentation rate and poor filtration behaviour. In either case, removal of the excess unadsorbed flocculant from the liquid leads to improvement in filtration behaviour.