The influence of flocculant adsorption kinetics on the dewaterability of kaolinite and smectite clay mineral dispersions

Abstract

Polymer mediated flocculation is central to efficient water recovery from and impoundment volume reduction of mineral waste tailings, particularly where clay minerals comprise a significant proportion. In this study, the adsorption kinetics of four high molecular weight polymeric flocculants onto colloidal, negatively charged kaolinite and smectite clay substrates under orthokinetic conditions has been investigated. For both kaolinite and smectite surfaces, the adsorption rate decreased in the sequence: non-ionic polyacrylamide homopolymer (PAM N) > anionic polyacrylamide-acrylate copolymer (PAM A) > non-ionic polyethylene oxide (PEO) > anionic polyacrylamide 2-acrylamido 2-methylpropane sulphonate copolymer (PAM S). Upon flocculation under laminar flow conditions, optimum subsidence rates for both clay types decreased according to flocculant type in the order PAM S > PEO > PAM A > PAM N, implying slower polymer adsorption rate facilitated higher floc settling behaviour. Calculation of first order rate constants for the adsorption of PAM N, PAM A and PAM S onto kaolinite at 25 °C and 50 °C and subsequent estimation of an activation energy in the range 14–22 kJ mol−1 polymer suggest that the overall energy is distributed among the transition state hydrogen bonds which are PAM polymer structure independent. Rationalisation of the results suggests that under industrially relevant conditions, flocculants with relatively slower tails’ particle attachment kinetics facilitate improved bridging performance and markedly enhanced settling behaviour.