Solubility Phase Diagram of the Quaternary System Li⁺, Mg²⁺//Cl^–, SO₄^2––H₂O at 298.15 K: Experimental Redetermination and Model Simulation

Abstract

Solubility isotherms for the ternary system MgCl 2 –MgSO 4 –H 2 O and the quaternary reciprocal system Li +, Mg 2+ //Cl –, SO 4 2– –H 2 O were determined at 298.15 K by an isothermal dissolution method. In the ternary phase diagram, there are six solubility branches corresponding to the solid phases MgSO 4 · n H 2 O (s) ( n = 7, 6, 5, 4, 1) and MgCl 2 ·6H 2 O (s) . In the quaternary equilibrium phase diagram, there are 16 solubility co-saturated lines corresponding to the solid phases MgSO 4 · n H 2 O (s) ( n = 7, 6, 5, 4, 1), MgCl 2 ·6H 2 O (s), Li 2 SO 4 ·H 2 O (s), LiCl·MgCl 2 ·7H 2 O (s), and LiCl·H 2 O (s) . This report describes for the first time that the equilibrium solid phases MgSO 4 ·H 2 O (s) and MgSO 4 ·4H 2 O (s) have been found to exist in this quaternary system. However, the phase field of MgSO 4 ·H 2 O (s) overlaps with the phase fields of MgSO 4 ·4H 2 O (s) and MgSO 4 ·5H 2 O (s), which indicates that MgSO 4 ·4H 2 O (s) and MgSO 4 ·5H 2 O (s) are metastable phases; MgSO 4 ·H 2 O (s) is a relatively more stable phase in both the ternary and quaternary systems. A Pitzer–Simonson–Clegg thermodynamic model was used to simulate the properties of the sub-binary and subternary systems and to predict the solubility phase diagram of the quaternary system. The results of the modeling are in reasonable agreement with the experimental data.