Solid-Liquid Equilibria of Aqueous Ternary System Na+, Mg2+//SO4 2--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<middle dot>MgSO4<middle dot>4H(2)O) formed at 298.2-323.2 K and double salts 6Na(2)SO(4)<middle dot>7MgSO(4)<middle dot>15H(2)O and 3Na(2)SO(4)<middle dot>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<middle dot>10H(2)O (288.2-303.2 K) and Na2SO4 (303.2-348.2 K); for magnesium sulfate, MgSO4<middle dot>7H(2)O (288.2-303.2 K), MgSO4<middle dot>6H(2)O (318.2, 323.2 K), and MgSO4<middle dot>H2O (348.2 K). The crystallization area of Na2SO4<middle dot>MgSO4<middle dot>4H(2)O 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<middle dot>10H(2)O and MgSO4<middle dot>7H(2)O from the double salt Na2SO4<middle dot>MgSO4<middle dot>4H(2)O.