Solid–Liquid Phase Equilibria of Aqueous Quaternary System Na+, K+, Mg2+//SO42––H2O at T = 298.2 and 323.2 K

The phase equilibria of aqueous quaternary system Na+, K+, Mg2+//SO42––H2O at T = 298.2 and 323.2 K were studied by the isothermal dissolution method. The density, refractive index, and composition of the equilibrium solution were determined, on the basis of which the phase diagrams and density/refractive index vs composition diagrams were drawn. There were four quaternary invariant points and six crystallization regions corresponding to single salts K2SO4, Na2SO4·10H2O, and MgSO4·7H2O and double salts Na2SO4·3K2SO4, Na2SO4·MgSO4·4H2O, and K2SO4·MgSO4·6H2O in the quaternary system Na+, K+, Mg2+//SO42––H2O at 298.2 K. When the temperature of the system was increased to 323.2 K, the crystalline form of some salts changed Na2SO4·10H2O, MgSO4·7H2O, and K2SO4·MgSO4·6H2O transformed into Na2SO4, MgSO4·6H2O, and K2SO4·MgSO4·4H2O, respectively. At 298.2 and 323.2 K, the crystallization region area of the potassium salt composed of Na2SO4·3K2SO4, K2SO4, and K2SO4·MgSO4·nH2O was more than 72% of the whole crystallization region area, which indicated that potassium was most easily crystallized from the system, at which the crystallization region of Na2SO4·3K2SO4 was the largest; potassium may be more easily precipitated from this system in the form of Na2SO4·3K2SO4. With the increase in temperature from 298.2 to 323.2 K, the crystallization region of Na2SO4·MgSO4·4H2O significantly increased, while the crystallization regions of Na2SO4·nH2O and MgSO4·nH2O significantly reduced. The sodium and magnesium in the system can be removed according to the change in the crystallization regions of sodium and magnesium with temperature, which provides favorable conditions for the preparation of K2SO4.