Influence of flocculant overdosing on solid/liquid separation properties

Abstract

The concentration of flocculant plays an important role in deciding the success or failure of solid/liquid separation process of suspended industrial wastes. A starvation dosage leads to incomplete separation whereas excess dosage been invariably found to hinder filtration and dewatering behaviour. The operating personnels often tend to overdose a flocculation and dewatering process in an industry without realizing the consequence of such overdosing. This paper makes in attempt to study the result of an anionic, cationic and nonionic flocculant overdosing by studying the influence of unadsorbed or physisorbed flocculant on filtration and dewatering characteristics of kaolin suspension. The estimation of unadsorbed flocculants have been done by measuring the desorption of the respective flocculants on washing after their equilibrium adsorption. Removal of the unadsorbed flocculants that do not participate in the flocculation process, 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.