The solubility of calcium sulphate in simulated nickel sulphate–chloride processing solutions

Calcium sulphate solubilities in simulated nickel sulphate–chloride processing solutions, as well as the densities of the corresponding saturated solutions, were determined on heating and cooling in a series of experiments carried out from 20 to 95 °C. The solubility of calcium sulphate decreases progressively with increasing NiSO4 concentrations up to 1.4 M NiSO4, in a base solution containing 0.15 M H2SO4, 0.2 M Fe(SO4)1.5 and 0.3 M LiCl, but the decrease is most pronounced for concentrations in the range from 0.0 to 0.3 M NiSO4. In the base solution containing 1.3 M NiSO4, increasing acid concentrations in the range from 0.0 to 0.7 M H2SO4 and increasing ferric sulphate concentrations to 1.0 M Fe(SO4)1.5 slightly depress the solubility of calcium sulphate at temperatures below ∼60 °C, but have the reverse effect at higher temperatures. Increasing chloride concentrations systematically depress the solubility of calcium sulphate in 1.3 M NiSO4–0.20 M Fe(SO4)1.5–0.15 M H2SO4 solutions, and the effect is nearly independent of the temperature. The presence of up to 0.5 M Na2SO4 in the base solution slightly depresses the solubility of calcium sulphate at all temperatures; however, the presence of both Na2SO4 and Fe(SO4)1.5 results in the precipitation of sodium jarosite at temperatures > 80 °C. Supporting X-ray diffraction analyses showed that the solubility depends strongly on whether the saturating solid phase is gypsum (CaSO4·2H2O) or anhydrite (CaSO4). The transition from gypsum to anhydrite is promoted by elevated temperatures, by high acid and salt concentrations, and by the presence of anhydrite seed. Unlike the solubilities, which often varied in a complex manner, the densities of the calcium sulphate-saturated solutions increased systematically with increasing concentrations of any of the solution species and decreased slightly, in a nearly linear manner, with increasing temperature.