Stable Phase Equilibria of the Quaternary Water-Salt System Li⁺, Rb⁺, Mg²⁺//SO₄ ^2--H₂O at T = 273.2 K

The phase equilibria for the quaternary system Li⁺, Rb⁺, Mg²⁺//SO₄ ^2- - H₂O 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, E₁ and E₄ belong to the commensurate type, and E₂ and E₃ belong to the incommensurate type. The order of the crystallization area is Rb₂SO₄·MgSO₄·6H₂O > 3Li₂SO₄·Rb₂SO₄·2H₂O > Li₂SO₄·Rb₂SO₄ > MgSO₄·7H₂O > Li₂SO₄·H₂O > Rb₂SO₄. The density of the equilibrium liquid changed regularly with the content of Rb₂SO₄ 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 Rb₂SO₄ increases with the decrease in temperature, which indicates that cooling is conducive to the crystallization of Rb₂SO₄. 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 3Li₂SO₄·Rb₂SO₄·2H₂O is converted to 3Li₂SO₄·Rb₂SO₄. The crystallization region of single salt MgSO₄·7H₂O, Rb₂SO₄ and double salt Li₂SO₄·Rb₂SO₄ decreased obviously.