Phase Equilibria of the Aqueous Quaternary System Li+, Rb+, Cs+//SO42–-H2O at T = 273.2 K

The phase equilibria of the quaternary system Li+, Rb+, Cs+//SO42–-H2O at T = 273.2 K was investigated by the isothermal dissolution method. The solubility and density of the system were measured and the related phase diagram, water content diagram, and density vs composition diagram were plotted. The solid phase of the quaternary invariant point was identified by the X-ray diffraction method and the solid solution morphology was analyzed by scanning electron microscopy. Results show that the stable phase diagram consists of 6 invariant points, 13 uninvariant curves, and 8 crystallization regions (corresponding to single salts Li2SO4·H2O, Rb2SO4, and Cs2SO4; double salts 3Li2SO4·Rb2SO4·2H2O, 3Li2SO4·Cs2SO4·2H2O, Li2SO4·Rb2SO4, and Li2SO4·Cs2SO4; and solid solution [(Rb, Cs)2SO4]). Also, the crystallization region of double salt 3Li2SO4·Rb2SO4·2H2O is the largest, which means that it is easiest to precipitate from this system. Meanwhile, rubidium and cesium easily form a solid solution in the sulfate system, which increases the difficulty of separating the two elements. Comparing the stable phase diagram of the ternary subsystems at T = 273.2 K and T = 298.2 K, it is found that the crystallization regions of Rb2SO4, Cs2SO4, [(Rb, Cs)2SO4], and 3Li2SO4·Cs2SO4·2H2O increase with the decrease of temperature, indicating that cooling is conducive to the crystallization of Rb2SO4, Cs2SO4, [(Rb, Cs)2SO4], and 3Li2SO4·Cs2SO4·2H2O.