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.

The equilibrium data of quaternary system phosphoric acid + sodium chloride + water + tri-n-butyl phosphate (TBP) was discussed at 298.15 K and atmospheric pressure. It was investigated experimentally in the mass concentration range 0–15% of NaCl and 4–24% of H3PO4 in the initial aqueous phase. The experimental results show that the separation factor of H2O and the distribution coefficient of H3PO4 increase when the content of H3PO4 increases. The separation factor of NaCl decreases after an increase to a maximum value. Moreover, it was found out that the mass ratio of ternary system H3PO4–NaCl–H2O to the saturation point. It would separate easily when the mass percent of NaCl occupies more than 12% in the system. The experimental tie-line data was correlated with the nonrandom two-liquid activity coefficient model.

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