Solubility Isotherms of Gypsum, Hemihydrate, and Anhydrite in the Ternary Systems CaSO₄ + MSO₄ + H₂O (M = Mn, Co, Ni, Cu, Zn) at T = 298.1 K to 373.1 K

The solubilities of anhydrite in the ternary systems CaSO₄ + MSO₄ + H₂O (M = Co, Ni) were determined through isothermal solution saturation at 348.1 K and 363.1 K. At low bivalent metal sulfate concentrations, anhydrite solubility decreases until it eventually reaches a minimum. Anhydrite solubility subsequently increases with the amount of heavy metal sulfate to a maximum. At this point, further increases in the concentration of metal sulfate decreases the solubility of anhydrite until saturation of the added bivalent metal sulfate. A Pitzer thermodynamic model was selected to predict isopiestic data including calcium sulfate solubilities of the ternary systems CaSO₄ + MSO₄ + H₂O (M = Mn, Co, Ni, Cu, Zn) from 298.1 K to 373.1 K. The functional dependencies of the MSO₄ (M = Ni, Cu, Zn) ion interaction parameters with temperature were determined, and the third virial parameters were given. The calculated solubilities are in agreement with the available experimental data. Using the Pitzer model and parameters, the solubility isotherms of metastable solid-phase hemihydrate, as well as gypsum and anhydrite, in the CaSO₄ + MSO₄ + H₂O (M = Mn, Co, Ni, Cu, Zn) systems were predicted over a wide range of temperatures and concentrations.