Solubility Experiments and Thermodynamic Modeling for the Quaternary System MgF₂–MnF₂–MnSO₄–H₂O and Its Subsystems at 298.15 K

The solubility data of the MgF₂–MnF₂–MnSO₄–H₂O quaternary system and its subsystems are crucial for the removal of magnesium ions from a MnSO₄ aqueous solution as the MgF₂ precipitate. However, these data recorded at 298.15 K have not been reported, except for the MnF₂–MnSO₄–H₂O system. In this study, the solubility isotherms of the MgF₂–MnF₂–H₂O, MgF₂–MnSO₄–H₂O, and MgF₂–MnF₂–MnSO₄–H₂O systems at 298.15 K and MnF₂ solubility in pure water at various temperatures were systematically measured. The results show that MgF₂ solubility initially decreases sharply followed by a gradual decrease as the MnF₂ concentration increases. MgF₂ solubility rises with increasing MnSO₄ concentration in pure MnSO₄ and the MnF₂-saturated MnSO₄ aqueous solution. MgF₂ solubility in the MnF₂-saturated MnSO₄ aqueous solution decreases to 2.3–6.5% of its value in the pure MnSO₄ aqueous solution at a certain MnSO₄ concentration. MnF₂ solubility increases with increasing temperatures in the MnF₂–H₂O system. A Pitzer–Simonson–Clegg model incorporated in the ISLEC software was used to simulate and predict the solubility curves of the MgF₂–MnF₂–MnSO₄–H₂O quaternary system and its subsystems. The model satisfactorily represented the solubility data of the subbinary and subternary systems of the MgF₂–MnF₂–MnSO₄–H₂O quaternary system within the analysis error, except for the solubility isotherms of the Mg(Mn)SO₄·7H₂O, Mg(Mn)SO₄·4H₂O, and Mg(Mn)SO₄·H₂O solid solutions. The predicted solubility data of the MgF₂–MnSO₄–H₂O system at 298.15 K agree well with the experimental data with a slight deviation of the MgF₂–MnF₂–MnSO₄–H₂O system. Based on the established model, the variation of MgF₂ solubility curves with the addition of anhydrous MnF₂ in MnSO₄ aqueous solutions at different concentrations was predicted. The study provides theoretical guidance for removing magnesium ions from a MnSO₄ aqueous solution via MgF₂ precipitation.