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

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⁺//SO₄^2--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 SO₄^2- 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.