The stable equilibrium solubility, density, and refractive index of the quaternary system Li+, Na+, Mg2+//SO4 2––H2O were studied at T = 323 K by the isothermal dissolution method. The results show that at T = 323 K, a sodium–magnesium sulfate mineral astrakhanite (Na2SO4·MgSO4·4H2O) and a lithium–sodium sulfate double salt (Li2SO4·Na2SO4) are formed in this quaternary system 323 K, but no double salt or solid solution formed between lithium sulfate and magnesium sulfate. The phase diagram consists of seven isothermal dissolution curves, five regions of crystallization, and three invariant points, among which one belongs to an commensurate point (H3) and two belong to incommensurate points (H1 and H2). Its five crystallization fields correspond to single salts Li2SO4·H2O, Na2SO4, hexahydrite (MgSO4·6H2O), and two double salts astrakhanite (Na2SO4·MgSO4·4H2O) and Li2SO4·Na2SO4. Hexahydrite (MgSO4·6H2O) has the smallest crystal phase region, while lithium sulfate monohydrate (Li2SO4·H2O) has the largest crystal region. At equilibrium, the refractive index and density in the solution change regularly with the change in concentration of sodium sulfate. Comparisons of the phase diagrams of Li+, Na+, Mg2+//SO4 2––H2O at different temperatures show that the temperature is the main factor for the salt crystals, not only the crystallization form but also the crystal water content.

The stable phase equilibrium of the quaternary system Li+, Na+, Rb+// SO4 2– – H2O at T = 298.2 and 323.2 K (P = 94.5 kPa) was studied by an isothermal dissolution equilibrium method. The solubilities, density, and refractive index of equilibrated solution were determined. It is found that the projected phase diagrams of the quaternary system at 298.2 and 323.2 K (P = 94.5 kPa) both contain four invariant points, nine univariant curves, and six crystallization regions. By comparing the phase diagrams at 298.2 and 323.2 K, it is found that at 298.2 K, the region of single salt Rb2SO4 is the largest one and that of Li2SO4·H2O is the smallest one. At T = 323.2 K, the single salt Na2SO4·10H2O transforms into Na2SO4 and the lithium sodium double salt is transformed from 3Na2SO4·Li2SO4·12H2O to Li2SO4·Na2SO4. Meanwhile, the maximum phase region shifts from Rb2SO4 to Li2SO4·Na2SO4. This indicates that with the rise of temperature, the solubilities of Rb2SO4 and Li2SO4·Na2SO4 changed significantly and the lithium sodium double salt will be crystallized out from the system in the form of Li2SO4·Na2SO4, with a higher lithium sodium ratio. In other words, it is unfavorable to extract rubidium sulfate from sulfate brine, while it has obvious promoting effects on extracting lithium sulfate.

The aqueous quaternary system K + , Rb + , Cs + //SO 4 2− −H 2 O 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 K 2 SO 4 , Rb 2 SO 4 , and Cs 2 SO 4 and three solid solutions [(K x , Rb 1−x ) 2 SO 4 ], [(K x , Cs 1−x ) 2 SO 4 ], and [(Rb x , Cs 1−x ) 2 SO 4 ]. No double salt was formed in this system at 298.2 K. The size order of the crystalline phase region of salts iswhich indicates that it is hardly possible to directly separate rubidium and cesium as single salt Rb 2 SO 4 and Cs 2 SO 4 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 , Rb 1−x ) 2 SO 4 ], [(K x , Cs 1−x ) 2 SO 4 ], and [(Rb x , Cs 1−x ) 2 SO 4 ] 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 , Rb 1−x ) 2 SO 4 ], [(K x , Cs 1−x ) 2 SO 4 ], and [(Rb x , Cs 1−x ) 2 SO 4 ].

The stable-phase equilibria of the aqueous quaternary system K + , Rb + , Cs + //SO 4 2− −H 2 O was investigated at T = 323.2 K using the isothermal dissolution equilibrium method. The solubility, density, and refractive index of equilibrium solutions were determined using the chemical/instrument analysis method, the specific gravity bottle method, and the WYA Abbe refractometer, respectively. The solid phases were identified by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that besides three single salts (K 2 SO 4 , Rb 2 SO 4 , and Cs 2 SO 4 ), three complete solid solutions are formed in this system at 323.2 K, namely,, and [(Rb x , Cs 1−x ) 2 SO 4 ], increasing the difficulty of separation potassium from the solution composed of rubidium, cesium, and sulfate. The stable-phase diagram of the quaternary system K + , Rb + , Cs + //SO 4 2− −H 2 O at T = 323.2 K consists of four invariant points, nine univariant curves, and six crystallization regions. The sequence of the size of the salt crystal region isThe phase diagrams at different temperatures (T = 298.2 K and T = 323.2 K) show that temperature is an important factor affecting salt crystallization, especially for the solid solution. As the temperature increases, the crystallization region of [(K x , Cs 1−x ) 2 SO 4 ] conspicuously decreases, whereas the crystallization regions of [(K x , Rb 1−x ) 2 SO 4 ] and [(Rb x , Cs 1−x ) 2 SO 4 ] increase slightly.