Development of an MSE-based chemical model for the solubility of calcium sulphate in mixed chloride–sulphate solutions

A recently developed mixed-solvent electrolyte (MSE) model to describe the chemistry of calcium sulphate in aqueous multi-component sulphate solutions has been extended to complex mixed chloride–sulphate solutions. The model accurately represents the solubility of calcium sulphate solid phases, namely dihydrate, hemihydrate and anhydrite, in mixed chloride–sulphate solutions containing H2SO4, Fe2(SO4)3, HCl, NaCl, Na2SO4, MgCl2, MgSO4, CaCl2, etc., over a wide range of temperatures and ionic strengths. It predicted the solubility of calcium sulphate from 20 to 95 °C in simulated nickel processing solutions containing NiSO4, H2SO4, Fe2(SO4)3, Na2SO4 and LiCl. It was found that increasing Na2SO4 or LiCl concentrations in NiSO4–H2SO4 solutions do not have a significant effect on the calcium sulphate solubility. The model was shown to be a practical tool for assessing the scaling potential of calcium sulphate over a wide range of temperatures and concentrations where experimental data are not available.