Solid-liquid phase equilibria of ternary system Na2S2O3-Na2SO4-H2O in a wide range of temperatures: Measurement and application

In order to provide a fundamental basis for treatment of wastewater from desulfurization of coke oven gas to recover pure salt, solid-liquid equilibrium data of Na2S2O3-Na2SO4-H2O system at T = (278.15, 283.15, 291.15, 333.15 or 353.15) K were measured. The experimental data was obtained using the isothermal solution saturation method and Schreinemaker's wet residue method. The solid-liquid phase diagrams of the ternary system were constructed. The ternary phase diagrams turned out to be simple eutectic type containing one invariant point, two univariant curves, and two single-salt crystallization regions. It was found that the crystallization region of Na2SO4 is larger than that of Na2S2O3 and expands with the increasing of temperature. But the crystallization region of Na2S2O3 decreases with the increasing of temperature. Through analyzing the thermodynamic characteristics of Na2S2O3 and Na2SO4, two crystallization methods for separating the mixture of Na2S2O3-Na2SO4 were put forward. A simple comparison of the two crystallization methods was carried out.