CaF2(s) Solubility Isotherm in the CaF2–CaSO4–MSO4–H2O (M = Zn, Mn, Mg) Quaternary Systems and Their Subsystems at Various Temperatures

A ZnSO4 electrolyte solution with impurity salts such as MnSO4 and MgSO4 is often used in zinc hydrometallurgical processes over a wide temperature range. CaSO4·2H2O(s) scaling and fluorine corrosion are two encountered problems in the process. Understanding the CaF2(s) solubility behavior in MSO4 (M = Zn, Mn, Mg) aqueous solutions at various temperatures helps remove Ca2+ and F– ions from the solution. However, CaF2(s) solubility data, especially at temperatures >298 K, have not been published yet. In this work, CaF2(s) solubilities in the CaF2–CaSO4–MSO4–H2O (M = Zn, Mn, Mg) quaternary systems and its subsystems of the CaF2–MSO4–H2O (M = Zn, Mn, Mg) at 348.15 K and the CaF2–CaSO4–MSO4–H2O (M = Zn, Mg) quaternary systems and the CaF2–MgSO4–H2O ternary system at 298.15 K are systematically measured. The results show that the CaF2(s) solubility in the MSO4 (M = Zn, Mn, Mg) aqueous solution of a certain concentration at 348.15 K is considerably higher than that at 298.15 K. At 298.15 and 348.15 K, the salting-in effect of MSO4 (M = Zn, Mn, Mg) on CaF2(s) solubility is enhanced with the order ZnSO4 < MnSO4 < MgSO4. At both temperatures, CaF2(s) solubilities in CaSO4·2H2O(s)- or CaSO4(s)-saturated MSO4 (M = Zn, Mn, Mg) aqueous solutions are generally lower than those in pure MSO4 (M = Zn, Mn, Mg) aqueous solutions. Particularly, the salting-in effect of MgSO4 on CaF2 is so strong that CaF2(s) dissolves completely and is converted to CaSO4·2H2O(s) and MgF2(s) in a concentrated MgSO4 aqueous solution. Hence, this study indicates that high temperatures and high salt concentrations increase the CaF2(s) solubility, making the removal of Ca2+ and F– ions from the system challenging. Further, the existence of impurity salts MnSO4 and MgSO4 makes it more difficult.