Metastable Phase Equilibria in the Aqueous Ternary Systems KCl + MgCl₂ + H₂O and KCl + RbCl + H₂O at 323.15 K

Abstract

The metastable phase equilibria in the ternary systems KCl + MgCl 2 + H 2 O and KCl + RbCl + H 2 O were studied at 323.15 K using an isothermal evaporation method. The solubilities, densities, and refractive indices of the equilibrated solution were measured. The crystalloid forms of the solid phase were determined using chemical analysis and an X-ray diffraction method. On the basis of the experimental data, the metastable equilibrium phase diagrams and the physicochemical properties vs composition in the ternary systems at 323.15 K were plotted. In the metastable phase diagram of the ternary system KCl + MgCl 2 + H 2 O at 323.15 K, there are two invariant points, three univariant curves, and three crystallization fields, corresponding to potassium chloride (KCl), magnesium chloride hexahydrate (MgCl 2 ·6H 2 O), and a potassium and magnesium chloride double salt named carnallite (KCl·MgCl 2 ·6H 2 O). The salt KCl has the largest crystallization field and can be easily separated from the mixed aqueous solution containing potassium and magnesium chloride using an evaporation method. In the metastable phase diagram of the ternary system KCl + RbCl + H 2 O, there are two invariant points and three crystallization fields, corresponding to the single salts potassium chloride (KCl) and rubidium chloride (RbCl) and a solid solution of potassium and rubidium chloride [(K, Rb)Cl]. The crystallization field of the solid solution (K, Rb)Cl almost occupies all the phase region; thus, it is difficult to separate potassium from rubidium in chloride solution by only using evaporation and crystallization methods at 323.15 K.