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

Abstract

A ZnSO 4 electrolyte solution with impurity salts such as MnSO 4 and MgSO 4 is often used in zinc hydrometallurgical processes over a wide temperature range. CaSO 4 •2H 2 O(s) scaling and fluorine corrosion are two encountered problems in the process. Understanding the CaF 2 (s) solubility behavior in MSO 4 (M = Zn, Mn, Mg) aqueous solutions at various temperatures helps remove Ca 2+ and F − ions from the solution. However, CaF 2 (s) solubility data, especially at temperatures >298 K, have not been published yet. In this work, CaF 2 (s) solubilities in the CaF 2 −CaSO 4 −MSO 4 − H 2 O (M = Zn, Mn, Mg) quaternary systems and its subsystems of the CaF 2 − MSO 4 −H 2 O (M = Zn, Mn, Mg) at 348.15 K and the CaF 2 −CaSO 4 −MSO 4 − H 2 O (M = Zn, Mg) quaternary systems and the CaF 2 −MgSO 4 −H 2 O ternary system at 298.15 K are systematically measured. The results show that the CaF 2 (s) solubility in the MSO 4 (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 MSO 4 (M = Zn, Mn, Mg) on CaF 2 (s) solubility is enhanced with the order ZnSO 4 < MnSO 4 < MgSO 4 . At both temperatures, CaF 2 (s) solubilities in CaSO 4 •2H 2 O(s)-or CaSO 4 (s)-saturated MSO 4 (M = Zn, Mn, Mg) aqueous solutions are generally lower than those in pure MSO 4 (M = Zn, Mn, Mg) aqueous solutions. Particularly, the salting-in effect of MgSO 4 on CaF 2 is so strong that CaF 2 (s) dissolves completely and is converted to CaSO 4 •2H 2 O(s) and MgF 2 (s) in a concentrated MgSO 4 aqueous solution. Hence, this study indicates that high temperatures and high salt concentrations increase the CaF 2 (s) solubility, making the removal of Ca 2+ and F − ions from the system challenging. Further, the existence of impurity salts MnSO 4 and MgSO 4 makes it more difficult.