Solid–Liquid Equilibria in the Quinary System Na⁺, K⁺//Cl^–, Br^–, SO₄^2––H₂O at 323.15 and 348.15 K

Aiming at the comprehensive utilization of bromine resources in bittern, the stable solid–liquid equilibria in the quinary system Na⁺, K⁺//Cl^–, Br^–, SO₄^2––H₂O at 323.15 and 348.15 K were studied using the isothermal dissolution equilibrium method. The corresponding phase equilibrium data were measured, and with which the simplified equilibrium dry-salt phase diagrams saturated with Na₂SO₄ and K₂SO₄, respectively, Na⁺ content diagrams and K⁺ content diagrams were plotted. As a result, the quinary system saturated with Na₂SO₄ at both two temperatures contains an invariant point, three univariant curves, and three crystallization regions including the single-salt KBr, the double-salt Na₂SO₄·3K₂SO₄, and the solid solution Na­(Cl, Br). The crystallization areas of Na₂SO₄·3K₂SO₄, Na­(Cl, Br), and KBr decrease in sequence. By comparison, the phase diagrams of the quinary system saturated with K₂SO₄ at both 323.15 and 348.15 K are much simpler and have no invariant points, instead of one univariant curve and two crystallization regions (Na₂SO₄·3K₂SO₄ and K­(Cl, Br)). The crystallization area of Na₂SO₄·3K₂SO₄ is essentially as large as that of K­(Cl, Br).