关键词: Phase equilibria, Solubility, Hydrate, Deep brine, Double salt, Solid solution

Abstract: In order to obtain the crystalline forms of the salts of the potassium, ammonium, calcium coexisting chloride system, the phase equilibria relationship of quaternary system K⁺, NH₄⁺, Ca²⁺//Cl^--H₂O at 298.2, 323.2, and 348.2 K was studied by isothermal dissolution equilibrium method. The solubility and density of equilibrium liquid phases of the system were experimentally determined; X-ray powder diffractometer was used to determine the compositions of the equilibrium solid phase at the quaternary invariant point. It is found that the quaternary system is a complex system at these three temperatures. The phase diagram at 298.2 K consists of three invariant points, seven univariate curves and five crystalline phase regions, forming the solid solutions (NH₄Cl)_x(KCl)_1-x and (KCl)_x(NH₄Cl)_1-x; while at 323.2 and 348.2 K the phase diagram consists of five invariant points, eleven univariate curves and seven crystalline phase regions, the double salts (KCl·CaCl₂) and (2NH₄Cl·CaCl₂·3H₂O), solid solutions (KCl)_x(NH₄Cl)_1-x and (NH₄Cl)_x(KCl)_1-x were formed. Among them, the crystalline phase region of solid solution (KCl)_x(NH₄Cl)_1-x is the largest at three temperatures, indicating that it is the easiest to crystallize in this system. Comparing the phase diagrams of the quaternary system at 298.2, 323.2, and 348.2 K, it can be seen that the crystalline form of CaCl₂ changes with the increase of temperature: CaCl₂·6H₂O at 298.2 K, CaCl₂·2H₂O at 323.2 and 348.2 K. From 323.2 to 348.2 K, the crystalline phase regions of (KCl·CaCl₂) and (2NH₄Cl·CaCl₂·3H₂O) increased gradually.

Key words: Phase equilibria, Solubility, Hydrate, Deep brine, Double salt, Solid solution