Solid–Liquid Equilibria of Aqueous Ternary System Na+, Mg2+//SO42––H2O at 288.2, 303.2, and 318.2 K

The phase equilibria of the ternary system Na+, Mg2+//SO42––H2O at 288.2, 303.2, and 318.2 K were investigated by the isothermal dissolution method. The solubility, density, and refractive index of the equilibrium liquid phase were measured, and the phase diagrams and density/refractive index versus composition diagrams were drawn. According to the multitemperature comparison of Na+, Mg2+//SO42––H2O at 288.2, 298.2, 303.2, 318.2, 323.2, and 348.2 K, it has found that it is a simple system at 288.2 K, while a complex system with double salt bloedite (Na2SO4·MgSO4·4H2O) formed at 298.2–323.2 K and double salts 6Na2SO4·7MgSO4·15H2O and 3Na2SO4·MgSO4 formed at 348.2 K. The solid phase of sodium sulfate and magnesium sulfate changes along with the increase of temperature: for sodium sulfate, Na2SO4·10H2O (288.2–303.2 K) and Na2SO4 (303.2–348.2 K); for magnesium sulfate, MgSO4·7H2O (288.2–303.2 K), MgSO4·6H2O (318.2, 323.2 K), and MgSO4·H2O (348.2 K). The crystallization area of Na2SO4·MgSO4·4H2O increases with the increase in temperature, which can lead to the coprecipitation of sodium and magnesium from salt lakes. Additionally, this temperature-dependent behavior is significant in the cooling crystallization method used to produce Na2SO4·10H2O and MgSO4·7H2O from the double salt Na2SO4·MgSO4·4H2O.