SOLUBILITY MEASUREMENT AND SOLID-LIQUID EQUILIBRIUM MODEL FOR THE TERNARY SYSTEM MgBr2 + MgSO4 + H2O AT 288.15 K

The solubility of magnesium minerals and the refractive index of the ternary system MgBr2 + MgSO4 + H2O at 288.15 K were investigated using an isothermal dissolution method. It was found that there are two invariant points in the phase diagram and the solubility isotherm of this ternary system consists of three branches, corresponding to equilibrium crystallization of Epsomite (MgSO4 center dot 7H(2)O, Eps), hexahydrite (MgSO4 center dot 6H(2)O, Hex) and magnesium bromide hexahydrate (MgBr2 center dot 6H(2)O, Mb). Neither solid solutions nor double salts were found. The refractive indices calculated from empirical equation 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 theta(Br),(SO4), the parameter Psi(Mg), (Br), (SO4) at 288.15 K was fitted using the Pitzer theory and Harvie-Weare (HW) approach. In addition, the average equilibrium constants of the stable equilibrium solids at 288.15 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 288.15 K. The model agreed well with the equilibrium solubility data for the magnesium salts.