Stable Phase Equilibria of the Quaternary Water–Salt System Li+, Rb+, Mg2+//SO42––H2O at T = 273.2 K

The phase equilibria for the quaternary system Li + , Rb + , Mg 2+ //SO 4 2– – H 2 O at 273.2 K were studied by the isothermal dissolution equilibrium method. Based on the measured data, the space diagram, stable phase diagram, water content diagram, and the diagram of density vs composition are plotted. The stable phase diagram in the system consists of four quaternary invariant points, nine univariate curves, and six crystallization zones. For the invariant points, E 1 and E 4 belong to the commensurate type, and E 2 and E 3 belong to the incommensurate type. The order of the crystallization area is Rb 2 SO 4 ·MgSO 4 ·6H 2 O > 3Li 2 SO 4 ·Rb 2 SO 4 ·2H 2 O > Li 2 SO 4 ·Rb 2 SO 4 > MgSO 4 ·7H 2 O > Li 2 SO 4 ·H 2 O > Rb 2 SO 4 . The density of the equilibrium liquid changed regularly with the content of Rb 2 SO 4 in the solution. By comparing the stable phase diagram of the partial ternary subsystems at T = 273.2 K and T = 298.2 K, it is found that the crystallization regions of Rb 2 SO 4 increases with the decrease in temperature, which indicates that cooling is conducive to the crystallization of Rb 2 SO 4 . By comparing the stable phase diagram of the system at T = 273.2 K and T = 308.2 K, it was found that the system was composed of four invariant points, nine univariate curves, and six crystal regions. The double salt 3Li 2 SO 4 ·Rb 2 SO 4 ·2H 2 O is converted to 3Li 2 SO 4 ·Rb 2 SO 4 . The crystallization region of single salt MgSO 4 ·7H 2 O, Rb 2 SO 4 and double salt Li 2 SO 4 ·Rb 2 SO 4 decreased obviously.