Metastable phase equilibria for the quaternary system containing potassium, magnesium, rubidium and chloride at 323.15 K

By employing the method of isothermal evaporation, the metastable phase equilibria of the quaternary systems KCl + RbCl + MgCl2 + H2O were researched at 323.15 K. In this paper, the solubilities and physicochemical properties, such as refractive indices and densities of the equilibrated solution, were determined. Through analysis of the experiments data, the metastable phase diagram, the water content diagram and physicochemical properties versus composition diagrams of the quaternary system were plotted. The study results indicated that in the metastable phase diagram, there were four invariant points (H1, H2, H3, H4), nine univariant curves, and six crystallization fields. And the six crystallization fields were magnesium chloride hexahydrate (MgCl2·6H2O) and potassium chloride (KCl) and rubidium chloride (RbCl) and a rubidium and magnesium chloride double salt named carnallite (RbCl·MgCl2·6H2O) and a potassium and magnesium chloride double salt named carnallite (KCl·MgCl2·6H2O) and a solid solution of potassium and rubidium chloride [(K, Rb)Cl]. The solid solution [(K, Rb)Cl] had the largest crystallization field. This showed that only by using evaporation and crystallization methods at 323.15 K, was it difficult to separate potassium from rubidium in chloride solution. The physicochemical properties of the quaternary system change regularly with the changes of composition in aqueous solutions.