The solubility of gypsum and anhydrite in simulated laterite pressure acid leach solutions up to 250 °C

A theoretical and experimental investigation of the solubility of gypsum and anhydrite in simulated laterite pressure acid leach (PAL) solutions containing 0–0.3 mol/L NiSO4, 0–0.4 mol/L H2SO4, 0–0.3 mol/L MgSO4 and 0.005 mol/L Al2(SO4)3 was conducted in the temperature range of 25–250 °C. High temperature experiments were carried out using a batch titanium autoclave equipped with a dip tube for sample withdrawal through an in-situ porous titanium filter. The solubility values obtained at high temperatures were one order of magnitude smaller than those obtained at temperatures below 100 °C because gypsum (CaSO4·2H2O) transforms to anhydrite (CaSO4) at higher temperatures. A recently developed thermodynamic database for the Mixed Solvent Electrolyte (MSE) model was extended to predict the calcium sulphate phase equilibria in the systems studied. The model was successfully tested against the measured experimental data for both solids over wide ranges of composition and temperature. The use of the model developed made it possible to assess the solubility of calcium sulphate hydrates in complex aqueous pressure acid leach solutions over wide ranges of conditions.