Stable Phase Diagram of the Quaternary Water-Salt System K+, Rb+, Cs+//SO42--H2O at T=298.2 K

The aqueous quaternary system K+, Rb+, Cs+//SO42--H2O is one of the most important subsystems of the sulfate-type salt lake brine located in the Qinghai-Tibet Plateau. The system was investigated at T = 298.2 K by the isothermal dissolution equilibrium method. The space phase diagram, the projected phase diagram, the water content diagram, and the physicochemical properties versus composition diagrams were plotted based on the measured data. The projected phase diagram consists of four invariant points (including three commensurate points and one incommensurate point), nine univariant curves, and six crystallization fields corresponding to three single salts K2SO4, Rb2SO4, and Cs2SO4 and three solid solutions [(K-x, Rb1-x)(2)SO4], [(K-x, Cs1-x)(2)SO4], and [(Rb-x, Cs1-x)(2)SO4]. No double salt was formed in this system at 298.2 K. The size order of the crystalline phase region of salts is [(K-x, Rb1-x)(2)SO4] > [(K-x, Cs1-x)(2)SO4] > K2SO4 > [(Rb-x, Cs1-x)(2)SO4] > Rb2SO4 > Cs2SO4, which indicates that it is hardly possible to directly separate rubidium and cesium as single salt Rb2SO4 and Cs2SO4 from the sulfate-type old brine. The type of three solid solutions was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the three solid solutions [(K-x, Rb1-x)(2)SO4], [(K-x, Cs1-x)(2)SO4], and [(Rb-x, Cs1-x)(2)SO4] are the complete solid solution in which the solute atoms are dissolved in the solvent lattice while still maintaining the solvent type, and isomorphism phenomenon is observed in the three solid solutions [(K-x, Rb1-x)(2)SO4], [(K-x, Cs1-x)(2)SO4], and [(Rb-x, Cs1-x)(2)SO4].