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 impuritysalts suchas MnSO4 and MgSO4 is often used in zinc hydrometallurgicalprocesses over a wide temperature range. CaSO4 center dot 2H(2)O(s) scaling and fluorine corrosion are two encountered problemsin the process. Understanding the CaF2(s) solubility behaviorin MSO4 (M = Zn, Mn, Mg) aqueous solutions at various temperatureshelps remove Ca2+ and F- ions from thesolution. However, CaF2(s) solubility data, especiallyat 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 theCaF(2)-CaSO4-MSO4-H2O (M = Zn, Mg) quaternary systems and the CaF2-MgSO4-H2O ternary system at 298.15 K are systematicallymeasured. The results show that the CaF2(s) solubilityin the MSO4 (M = Zn, Mn, Mg) aqueous solution of a certainconcentration at 348.15 K is considerably higher than that at 298.15K. At 298.15 and 348.15 K, the salting-in effect of MSO4 (M = Zn, Mn, Mg) on CaF2(s) solubility is enhanced withthe order ZnSO4 < MnSO4 < MgSO4. At both temperatures, CaF2(s) solubilities in CaSO4 center dot 2H(2)O(s)- or CaSO4(s)-saturatedMSO(4) (M = Zn, Mn, Mg) aqueous solutions are generally lowerthan 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 completelyand is converted to CaSO4 center dot 2H(2)O(s) and MgF2(s) in a concentrated MgSO4 aqueous solution. Hence,this study indicates that high temperatures and high salt concentrationsincrease the CaF2(s) solubility, making the removal ofCa(2+) and F- ions from the system challenging.Further, the existence of impurity salts MnSO4 and MgSO4 makes it more difficult.

Automated summary

In zinc hydrometallurgical processes, a ZnSO4 electrolyte solution with impurity salts such as MnSO4 and MgSO4 is commonly used. However, scaling of CaSO4·2H2O(s) and corrosion by fluorine pose problems in the process. This study investigates the solubility of CaF2(s) in MSO4 (M = Zn, Mn, Mg) aqueous solutions at different temperatures to aid in the removal of Ca2+ and F- ions. The results show that the solubility of CaF2(s) is significantly higher at 348.15 K compared to 298.15 K. The salting-in effect of MSO4 (M = Zn, Mn, Mg) on CaF2(s) solubility is observed, with MgSO4 exerting the strongest influence. The presence of impurity salts MnSO4 and MgSO4 further complicates the system.