The solubility of calcium sulphate in simulated copper sulphate electrorefining solutions

The solubility of calcium sulphate in simulated copper sulphate electrorefining media was determined in experiments carried out on heating and cooling from 20 to 95 °C. In solutions containing 1.5 M H2SO4, the solubility of gypsum (CaSO4·2H2O) decreases with increasing CuSO4 concentrations to at least 1.3 M. In 0.7 M CuSO4 media, increasing H2SO4 concentrations to 1.0 M significantly increase the solubility of gypsum, but higher acid concentrations reduce the solubility. The solubility of gypsum in 0.60 or 0.94 M CuSO4–1.5 M H2SO4 media decreases with increasing NiSO4 concentrations to 0.5 M. In contrast, increasing concentrations of pentavalent arsenic, to 20 g/L As(V), have no significant effect on the solubility. At temperatures > 60 °C, where anhydrite is thermodynamically stable, the solubility decreases as the CuSO4 and H2SO4 concentrations increase. In the absence of anhydrite seed, the transition from gypsum to anhydrite begins at temperatures above 60 °C, and is complete within two days at > 80 °C. Anhydrite seed significantly promotes the reaction, and complete transformation occurs within two days at 65 °C. In all instances, the densities of the calcium sulphate-saturated solutions increase systematically with increasing concentrations of any of the major solution species and decrease slightly with increasing temperature.