Dewatering properties of dual-polymer-flocculated systems

Abstract

This work examines the effect of dual-polymer flocculation on the compressive yield stress and hindered settling function of positively charged alumina suspensions as measured by a pressure filtration technique. The primary aggregates were formed using a short-chain poly(acrylic acid) (PAA) under two different shear mixing conditions at pH 5. This resulted in two types of aggregates with different mass fractal dimension and size. These aggregates were further flocculated using either a cationic or an anionic long-chain polymer at the same pH. A filtration study of these dual-polymer-induced aggregates showed that there may exist an optimum dosage for the second polymer, which results in the lowest final cake moisture. Dual-polymer flocculation was found to result in lower compressive yield stresses than that found for the case of single, high-molecular-weight polymer flocculation. This gave rise to lower final cake moistures for the dual-polymer-flocculated systems (28.3% w/w) compared to the case of the single, high-molecular-weight polymer-flocculated system (34.7% w/w). The filtration rate was high for the dual-polymer systems in most cases. In addition, the polymer dosage required to achieve good supernatant clarity was decreased from that of the single-polymer case when dual polymers were used. The use of dual-polymer flocculants of opposite charge was found to give an improvement in final cake moisture over that achieved using dual polymers of like charge. The structural characteristics of the primary aggregates were also found to have a significant influence on the dewatering properties of the dual-polymer-induced aggregates. Under the conditions used, large, open primary aggregates led to final flocs with lower cake moisture and faster filtration rate. It is envisaged that optimisation of industrial dewatering processes in this way can result in considerable savings in transportation and enhanced product value.