Solid–Liquid Metastable Phase Equilibria in the Ternary Systems KCl + NH4Cl + H2O and NH4Cl + MgCl2 + H2O at 298.15 K

Abstract

The metastable phase equilibria in the ternary systems KCl + NH4Cl + H2O and NH4Cl + MgCl2 + H2O were investigated at 298.15 K using an isothermal evaporation method. The metastable phase diagrams and the physicochemical properties versus composition diagrams were constructed on the basis of the experimental data. In the phase diagram of the ternary system KCl + NH4Cl + H2O, there are three invariant points and five crystallization fields, corresponding to potassium chloride (KCl), ammonium chloride (NH4Cl), a solid solution based on potassium chloride [(K, NH4)Cl], a solid solution based on ammonium chloride [(NH4, K)Cl], and a crystallization field of two kinds of solid solution [(K, NH4)Cl + (NH4, K)Cl] precipitation. Comparisons between the metastable and stable phase diagrams at 298.15 K show that the crystallization forms of salts are all the same, whereas the size of crystallization regions has slight changes. The ternary system NH4Cl + MgCl2 + H2O is a complex type system with a double salt ammonium carnallite (NH4Cl·MgCl2·6H2O) formed at 298.15 K. Comparisons between the metastable and the stable phase diagrams at 298.15 K and 323.15 K show that the crystallization forms have not changed, while the crystallization fields have slight changes. The density coefficient Ai and refractive index coefficient Bi of salts KCl, NH4Cl, and MgCl2 at 298.15 K were obtained by fitting. The calculated values of densities and refractive indices agree well with the experimental values, with a maximum relative error better than 0.010.