Solid-Liquid Stable Equilibrium of the Aqueous Quaternary System NH4SCN-(NH4)2S2O3-(NH4)2SO4-H2O at 303.15 K

The salts of thiocyanate, thiosulfate, and sulfate exist in a variety of industrial processes such as textile, coking, and electroplating. The effluents from these industries cannot be discharged directly owing to their severe environmental pollution. In addition, there is an opportunity to recover significant commercial value from these effluents. The phase equilibria and phase diagram play a major role in separating and recovering useful resources. In this paper, the phase diagrams of the system NH4SCN-(NH4)(2)S2O3-(NH4)(2)SO4-H2O and its subsystems at 303.15 K were determined by an isothermal dissolution equilibrium method. In the phase diagram of the quaternary system, there is one invariant point, three univariant curves and three crystallized regions. The invariant point is cosaturated with ammonium thiocyanate, ammonium sulfate, and ammonium thiosulfate (NH4SCN + (NH4)(2)SO4 + (NH4)(2)S2O3). The three crystallization regions correspond to ammonium sulfate ((NH4)(2)SO4), ammonium thiocyanate (NH4SCN), and ammonium thiosulfate ((NH4)(2)S2O3). The first one is the largest area, but the latter two are much smaller ones. The phase diagram can be utilized to separate and recover NH4SCN, (NH4)(2)SO4, and (NH4)(2)S2O3 selectively and individually from the desulfurization effluents. The result of this paper shows that the environmental pollutant can be converted to the valuable product by a simple, economical, and green process.