Solid-Liquid Equilibrium in the Quaternary System Na+//Cl–, SO42–, SeO32––H2O at 268.15 and 288.15 K

The isothermal dissolution equilibrium method was employed to acquire phase equilibrium data for the quaternary system Na⁺//Cl^–, SO₄^2–, SeO₃^2––H₂O at 268.15 and 288.15 K. The construction of the stereo phase diagram, dry salt projection diagram, and water content diagram was based on the measured data. The results show that the equilibrium phase diagram of the quaternary system at the two temperatures is composed of two invariant points, five univariate curves, and four crystal zones, and no compound salt or solid solution is formed, but the composition of invariant points and crystal zones is different. Among them, the crystallization area of Na₂SO₄·10H₂O is the largest, while that of Na₂SeO₃·5H₂O is the smallest, indicating that Na₂SeO₃·5H₂O has a higher solubility and is less likely to crystallize and precipitate in the saturated solution of the system. Meanwhile, Na₂SeO₃ exhibited a significant salting-out effect on both Na₂SO₄ and NaCl. In the multitemperature comparison phase diagram, the phase diagram configuration gradually simplifies with the increase in temperature. Hydrated salts undergo dehydration phenomena. At 323.15 K, Na₂SO₄·10H₂O and Na₂SeO₃·5H₂O become anhydrous salts. Temperature is the key factor regulating the crystallization morphology and phase equilibrium laws of the salts in this quaternary system.