Phase equilibria in the ternary systems (MgSO4+MgB4O7+H2O) and (MgSO4+Mg2B6O11+H2O) at 273.15 K and 288.15 K: solubility and polyborate distribution

The solubility isotherms of the two ternary systems (MgSO4+MgB4O7+H2O) and (MgSO4+Mg2B6O11+H2O) were determined using the isothermal dissolution equilibrium method at 273.15 K and 288.15 K. Solubilities and acidities of the liquid phase were determined in detail experimentally. According to the experimental data, the phase diagrams for two ternary systems were given. The results indicate that the two ternary systems are the simple saturated type, and double salt or solid solution are not formed. There are one invariant point corresponding to (epsomite + hungchaoite or inderite), two isothermal dissolution curves, and two crystallization regions in two ternary systems. It was found that the crystallization region areas of magnesium borate were larger than that of epsomite, indicating that the solubility of epsomite was larger in the two systems. The presence of magnesium sulfate can prevent the transformation of hungchaoite into inderite, and magnesium sulfate has a significant solubilizing effect on both magnesium borate at the experimental temperature. The distributions of polyborate species in saturated magnesium borate solution varying with mass fraction of magnesium sulfate were calculated by the Newton iteration method. (C) 2021 Elsevier Ltd.