Solid-Liquid Phase Equilibria of Aqueous Quaternary System Na+, K+, Mg2+//SO4 2<bold>-</bold> -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<middle dot>10H(2)O, and MgSO4<middle dot>7H(2)O and double salts Na2SO4<middle dot>3K(2)SO(4), Na2SO4<middle dot>MgSO4<middle dot>4H(2)O, and K2SO4<middle dot>MgSO4<middle dot>6H(2)O 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<middle dot>10H(2)O, MgSO4<middle dot>7H(2)O, and K2SO4<middle dot>MgSO4<middle dot>6H(2)O transformed into Na2SO4, MgSO4<middle dot>6H(2)O, and K2SO4<middle dot>MgSO4<middle dot>4H(2)O, respectively. At 298.2 and 323.2 K, the crystallization region area of the potassium salt composed of Na2SO4<middle dot>3K(2)SO(4), K2SO4, and K2SO4<middle dot>MgSO4<middle dot>nH(2)O 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<middle dot>3K(2)SO(4) was the largest; potassium may be more easily precipitated from this system in the form of Na2SO4<middle dot>3K(2)SO(4). With the increase in temperature from 298.2 to 323.2 K, the crystallization region of Na2SO4<middle dot>MgSO4<middle dot>4H(2)O significantly increased, while the crystallization regions of Na2SO4<middle dot>nH(2)O and MgSO4<middle dot>nH(2)O 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.