Kinetics of the crystallisation of barium sulphate. Effect of additives, stirring rate and barium: sulphate ratio on the rate of precipitation

Abstract

The kinetics of the crystallisation of barium sulphate has been studied by following the changes in ionic conductivity in supersaturated solutions containing both stoichiometric and non-stoichiometric concentrations of the lattice ions at 298 K. The rate of growth showed a quadratic dependence upon the relative supersaturation which suggests a surface-controlled growth mechanism. The effect of sodium tripolyphosphate and several organic phosphonates on the growth kinetics have been studied at several additive concentrations. The retardation effect of these additives has been attributed to the simultaneous chelation of an active-site metal ion and adsorption on the surface through a phosphonate/sulphate exchange. In contrast, low concentration levels of additives such as chromate and ferrocyanide anions enhanced the rate of growth markedly. The latter effect was interpreted in terms of a dynamic adsorption/desorption mechanism.