Stable Phase Equilibrium of the Quaternary System Li₂SO₄ + Cs₂SO₄ + MgSO₄ + H₂O at 298.2 K

Abstract

To understand the thermodynamic characteristics of the brine located in Qaidam Basin salt lake, the stable equilibrium solubilities, densities, and refractive indices in the quaternary system Li2SO4 + Cs2SO4 + MgSO4 + H2O were studied at T = 298.2 K using the isothermal dissolution method. Chemical analysis and X-ray diffraction (XRD) were applied to determine the crystalloid forms of the solid phases. In this aqueous equilibrium system at 298.2 K, two double salts (3Li2SO4·Cs2SO4·2H2O and Li2SO4·Cs2SO4) were generated from lithium sulfate and cesium sulfate, and one double salt was produced between cesium and magnesium sulfate with the form of Cs2SO4·MgSO4·6H2O. No complex salts (such as double salt or solid solution) are observed between lithium sulfate and magnesium sulfate. The stable phase diagram corresponds to three single salts (Li2SO4·H2O, Cs2SO4, and MgSO4·7H2O) and three types of double salts (3Li2SO4·Cs2SO4·2H2O, Li2SO4·Cs2SO4, and Cs2SO4·MgSO4·6H2O). It exhibits nine univariant curves, six crystallization areas, and four invariant points (including two commensurate points E1 and E4 and two incommensurate points E2 and E3). The crystallization region of salt Cs2SO4 is the smallest, whereas the double salt Li2SO4·Cs2SO4 is the largest, indicating that cesium is difficult to be directly separated as cesium sulfate from the brine in the Qaidam Basin.