Calcium sulphate solubilities in simulated zinc processing solutions

Calcium sulphate solubilities in simulated zinc 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. In dilute ZnSO4 media, increasing H2SO4 concentrations in the range of 0.0–0.6 M H2SO4 strongly increase the solubility of calcium sulphate, but have little additional effect for acid concentrations >0.6 M H2SO4. In more concentrated zinc sulphate solutions, however, increasing acid concentrations in the range from 0.0 to 2.0 M H2SO4 slightly decrease the solubility of calcium sulphate. In dilute H2SO4 media, the solubility of calcium sulphate decreases with increasing ZnSO4 concentrations, and the decrease is most pronounced for concentrations in the range 0.0–0.2 M ZnSO4. Higher ZnSO4 concentrations further depress the solubility at temperatures below 70 °C, but have a complex effect at higher temperatures. Increasing ferric sulphate concentrations to 1.0 M Fe(SO4)1.5 in 0.3 M H2SO4–1.15 M ZnSO4 media depress the solubility of calcium sulphate to a modest extent. The addition of up to 1.25 M MgSO4 also decreases the solubility of calcium sulphate in 0.3 M H2SO4–1.15 M ZnSO4 media. In contrast, the solubility of calcium sulphate in 2.5 M ZnSO4–0.4 M MgSO4–0.18 M MnSO4 solutions, at pH values ranging from 3.6 to 4.6, is not significantly affected by the presence of up to 0.25 M Na2SO4 or up to 0.09 M (NH4)2SO4. Supporting mineralogical studies 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 was found to depend on the temperature, the H2SO4 concentration and the total sulphate concentration of the solution. Unlike the solubilities, which 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 with increasing temperature.