Metastable Phase Equilibrium in the Aqueous Quaternary System (NaCl + MgCl2 + Na2SO4 + MgSO4 + H2O) at 323.15 K

Experimental studies on the metastable solubilities and physicochemical properties (density, viscosity, pH. and refractive index) of the aqueous quaternary system (NaCl + MgCl2 + Na2SO4 + MgSO4 + H2O) at 323.15 K were determined with the method of isothermal evaporation. On the basis of experimental data, the metastable phase diagram and the diagram of physicochemical properties versus composition were plotted. There are six invariant points, twelve univariant curves, and seven crystallization zones corresponding to halite, thenardite, vanthoffite (3NaSO(4)center dot MgSO4), astrakanite (NaSO4 center dot MgSO4 center dot 4H(2)O), hexahydrite (MgSO4 center dot 6H(2)O), tetrahydrite (MgSO4 center dot 4H(2)O), and bischofite (MgCl2 center dot 6H(2)O). There is no solid solution except two double salts of vanthoffite and astrakanite which existed in the metastable system. When compared with the stable equilibrium phase diagram at 328.15 K. the crystallized zones of loeweite (6Na(2)SO(4)center dot 7MgSO(4)center dot 15H(2)O) and kieserite (MgSO4 center dot H2O) disappeared; the areas of halite and hexahydrite increased: and the areas of thenardite, astrakanite, and vanthoffite decreased. It is indicated that the metastable phenomena of sodium sulfate and magnesium sulfate are significant. The physicochemical properties (density, viscosity. pH. and refractive index) in the metastable equilibrium system change regularly with the mass fraction change of the magnesium ion.