Solid–Liquid Equilibrium for the Ternary Systems (Na₂SO₄ + NaH₂PO₄ + H₂O) and (Na₂SO₄ + NaCl + H₂O) at 313.15 K and Atmospheric Pressure

Abstract

Solid–liquid equilibrium in the ternary systems (Na2SO4 + NaH2PO4 + H2O) and (Na2SO4 + NaCl + H2O) at 313.15 K and atmospheric pressure was investigated by using isothermal solution saturation and moist residue method. The solubility and thermodynamic properties of the solution were determined. The phase diagrams and the diagrams of thermodynamic properties versus composition were constructed. It turned out that there was one invariant point, two uninvariant curves, and two crystallization regions. In the phase diagram of the ternary system (Na2SO4 + NaH2PO4 + H2O) at 313.15 K, the crystallization region of Na2SO4 was obviously larger than the NaH2PO4·2H2O crystalline region. When Na2SO4 was 11.57 wt % and NaH2PO4 was 44.41 wt %, this system reached saturation at 313.15 K. In the phase diagram of the ternary system (Na2SO4 + NaCl + H2O) at 313.15 K, the crystallization region of Na2SO4 was significantly larger than the crystallization region of NaCl. When NaCl was 22.56 wt % and Na2SO4 was 5.00 wt %, this system reached saturation at 313.15 K. The research results could be used for the optimization solvent extraction process of sodium dihydrogen phosphate in industrial production.