Improved Solid-liquid Separations with Temperature Responsive Flocculants: a Review

Recent progress in the development of temperature responsive flocculants based on poly (N-isopropyl acrylamide) (PNIPAM) is reviewed. The polymer switches between a flocculant at temperature above the Lower Critical Solution Temperature (LCST) (32C for PNIPAM) and a dispersant at temperature below the LCST. Both batch sedimentation and continuous pilot scale operations have been conducted. Sedimentation rates, sediment and underflow concentration and supernatant and overflow clarity have been investigated. Comparison to conventional poly acrylamide (PAM) based flocculants is favourable. In batch sedimentation, the sediment can be significant consolidated after cooling to below the LCST resulting in improved solidliquid separation. The influence of polymer architecture, (homopolymers and charged random and block co-polymers) is discussed in relation to the secondary consolidation upon cooling as well as the ability to add the polymer at temperature above the LCST. In continuous operation, the enhanced consolidation of the underflow is limited by the relatively short residence times of the sediment bed within the thickener. The amount of time required for the liquid to permeate through the relatively deep sediment bed is greater than the residence time in the thickener. It appears the more significant benefit of the flocculants may be in the significantly reduced underflow rheology of the cooled thickened sediment.