Solid-Liquid Phase Equilibria of Potassium-Ammonium-Magnesium Chloride System at 323.2 and 348.2 K

Considering the significant presence of ammonium in deep brine, the phase equilibriums of the quaternary systems KCl + NH4Cl + MgCl2 + H2O at 323.2 and 348.2 K were studied by the isothermal dissolution method. The composition and density of the equilibrium liquid phase were determined, and the composition of the equilibrium solid phase was identified by X-ray diffraction. At 323.2 and 348.2 K, the phase diagrams of the system consist of seven invariant points, fourteen univariate curves, and eight crystallization phase regions with the formation of solid solutions and double salts. The crystallization form of (NH4)K2Mg3Cl9<middle dot>18H(2)O is a solid solution-type double salt formed by interactions among ions in the quaternary system. By comparing the phase diagrams of the quaternary system at 273.2, 298.2, 323.2, and 348.2 K, it was found that with increasing temperature, the crystallization form of salts did not change. The crystallization areas of the solid solutions (KCl)(x)(NH4Cl)(1-x) and (NH4Cl)(x)(KCl)(1-x) first increased and then decreased with rising temperature. The crystallization phase region of (NH4)K2Mg3Cl9<middle dot>18H(2)O decreases initially and then increases, while the phase region of the potassium ammonium solid solution and the solid solution-type double salt overall increases.