Metastable Phase Equilibrium in the Aqueous Ternary System (MgCl2+ MgSO4+ H2O) at 308.15 K

The metastable solubilities and the refractive indices in the aqueous ternary system (MgCl2 + MgSO4 + H2O) at 308.15 K were determined with the method of isothermal evaporation, and the metastable phase diagram and the diagram of refractive index versus composition were plotted. Four two-salt cosaturated points of epsomite (MgSO4·7H2O, Eps) + hexahydrite (MgSO4·6H2O, Hex), Hex + pentahydrite (MgSO4·5H2O, Pen), Pen + tetrahydrite (MgSO4·4H2O, Tet), and Tet + bischofite (MgCl2·6H2O, Bis), five isothermal univariant curves, and five metastable crystallization fields corresponding to Eps, Hex, Pen, Tet, and Bis were formed in the metastable ternary system. A comparison of the stable and metastable phase diagrams of the ternary system at 308.15 K shows that one more metastable crystallization field is formed. Neither a solid solution nor double salts are found. The solubility predictions of the metastable ternary system at 308.15 K based on Pitzer's ion-interaction model and its extended Harvie−Weare (HW) model are presented, and the calculated results were in good agreement with the experimental results.