Measurement and thermodynamic model study on solid + liquid equilibria and physicochemical properties of the ternary system MgBr2 + MgSO4 + H2O at 323.15 K

The solubility of magnesium minerals and the physicochemical properties (density and refractive index) of the ternary system MgBr2 + MgSO4 + H2O at 323.15 K were investigated using an isothermal evaporation method. It was found that there are three three-salt cosaturated invariant points, four univariant solubility isotherms and four stable crystallisation fields in the ternary system corresponding to hexahydrite (MgSO4·6H2O, Hex), tetrahydrite (MgSO4·4H2O, Tet), kieserite (MgSO4·H2O, Kie) and magnesium bromide hexahydrate (MgBr2·6H2O, Mb). Neither solid solutions nor double salts were found. The densities and refractive indices calculated from empirical equations are in good agreement with the experimental data. Combining the results from solubility measurements with the single-salt parameters for MgBr2 and MgSO4, and the mixed ion-interaction parameter θ Br,SO 4 , the parameter ψ Mg,Br,SO 4 at 323.15 K was fitted using the Pitzer and Harvie–Weare (HW) model. In addition, the average equilibrium constants of the stable equilibrium solids at 323.15 K were obtained by a method using the activity product constant. A chemical model, which combined the Pitzer parameters and the average equilibrium constants, was constructed to calculate the solid + liquid equilibria in the ternary system MgBr2 + MgSO4 + H2O at 323.15 K. The model agreed well with the equilibrium solubility data for the magnesium salts.