Solubility Measurement of the Systems MF2 (M = Ca, Mg, Zn) + ZnSO4 + H2O and MF2 (M = Ca, Mg) + ZnF2 + H2O at 298.15 K

Solubility of sparingly soluble salts, i.e., CaF2(s), MgF2(s), and ZnF2(s), in ZnSO4 aqueous solution is necessary for the process design to remove Ca2+, Mg2+, and F- ions from zinc hydrometallurgical systems. However, these data have been unavailable in any literature up to now. In this work, the solubilities of MF2(s) (M = Ca, Mg, Zn) in ZnSO4 and MF2(s) (M = Ca, Mg) in ZnF2 aqueous solutions have been elaborately measured at 298.15 K. The results showed that the CaF2(s) and MgF2(s) solubilities increase with the increasing ZnSO4 concentration; for example, their solubility in 2 mol.kg(-1) ZnSO4 aqueous solution increases to about 6 times of that in pure water. The ZnSO4 has a slightly stronger salt-in effect on CaF2(s) than on MgF2(s). However, the ZnF2(s) solubility in ZnSO4 aqueous solution decreases at first and then increases with increasing ZnSO4 concentration. As expected, ZnF2 possesses strong salting-out effect on the solubility of CaF2((s)) and MgF2((s)). A remarkable character is that the CaF2(s) solubility goes through a minimum point and then increases again with the ZnF2 addition; inversely, the MgF2(s) solubility decreases with the ZnF2 addition with time. Their different solubility characters have been interpreted from the aspect of ion association interaction. An ion association model has been applied to predict the CaF2(s) and MgF2(s) solubility in ZnF2 aqueous solution. The predicted results agree with the experimental ones quite well, which indicates that the ion association dominates the excess properties of these concerned systems.