Revisiting the Solid–Liquid Phase Equilibrium of the Li2SO4 + Rb2SO4 + H2O and Li2SO4 + Cs2SO4 + H2O Systems

Solid–liquid phase equilibrium data of the Li2SO4 + Rb2SO4 + H2O and Li2SO4 + Cs2SO4 + H2O systems are of importance for the design of alkali metal element extraction processes from sulfate leaching solution of lepidolite. Considering the serious inconsistency of the reported solid phases, especially double salts and solid solutions, phase diagrams of the two systems were reinvestigated experimentally. The results on the Li2SO4 + Rb2SO4 + H2O system at 298.15 K show five solids, i.e., Rb2SO4, Rb2SO4·Li2SO4, Rb2SO4·3Li2SO4·2H2O, Rb2SO4·5Li2SO4·4H2O, and a solid solution with the end members Rb2SO4·5Li2SO4·4H2O and Li2SO4·H2O. For the Li2SO4 + Cs2SO4 + H2O system at 298.15 K, the results also show five solids, i.e., Cs2SO4, Cs2SO4·Li2SO4, Cs2SO4·3Li2SO4·2H2O, Cs2SO4·5Li2SO4·4H2O, and Li2SO4·H2O, while there is no solid solution phase with the end members Cs2SO4·5Li2SO4·4H2O and Li2SO4·H2O. Moreover, the single crystal structures of Rb2SO4·3Li2SO4·2H2O and Cs2SO4·3Li2SO4·2H2O were determined.