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⁺//SO₄^2––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⁺//SO₄^2––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.