Solid–Liquid Phase Equilibria of a Quaternary Sulfate-Type System Containing Lithium, Rubidium, and Magnesium at 298.2 K

The aim of this study was to obtain the relationship between ion interactions and the crystallization patterns of salt species in the lithium–rubidium–magnesium sulfate system at 298.2 K. The phase equilibria of the aqueous quaternary system Li+, Rb+, Mg2+//SO42–-H2O were studied by the isothermal dissolution method at T = 298.2 K and p = 94.77 kPa. The density, refractive index, and composition of equilibrium solution were determined, on the basis of which solid–liquid phase diagrams and density/refractive index vs composition diagrams were drawn. The phase diagram consists of four quaternary invariant points and six crystallization regions, corresponding to the crystallization areas of single salts Rb2SO4, Li2SO4·H2O, and MgSO4·7H2O, as well as double salts 3Li2SO4·Rb2SO4·2H2O, Li2SO4·Rb2SO4, and Rb2SO4·MgSO4·6H2O. Notably, rubidium-containing double salts occupy more than 50% of the entire phase diagram area. The results indicate that the interactions between Li+ and Rb+ with coexisting Mg2+ and SO42– are complex, leading to the formation and precipitation of various lithium- and rubidium-bearing double salts, which hinder the effective concentrations of lithium and rubidium during the solar evaporation process in salt pans. Additionally, a multitemperature comparison of the solid–liquid phase diagrams at 273.2, 298.2, and 308.2 K reveals that temperature is also a significant factor influencing the solid-phase types and crystallization areas. For instance, the crystallization form of the double salt 3Li2SO4·Rb2SO4·2H2O changes to 3Li2SO4·Rb2SO4 at 308.2 K and the crystallization area of Li2SO4·Rb2SO4 gradually decreases, while the crystallization area of Rb2SO4·MgSO4·6H2O generally exhibits an increasing trend.