Solid-Liquid Phase Equilibria of the Pb²⁺, Ca²⁺, Mg²⁺//Cl^--H₂O Quaternary System and Its Subsystems at 303.2 K

The solid-liquid phase equilibria of the ternary systems Pb²⁺, Ca²⁺//Cl^--H₂O, Pb²⁺, Mg²⁺//Cl^--H₂O, and Ca²⁺, Mg²⁺//Cl^--H₂O were investigated at atmospheric pressure and T = 303.2 K using the isothermal dissolution equilibrium method. Additionally, solid phase equilibria of the quaternary system Pb²⁺, Mg²⁺, and Ca²⁺//Cl^--H₂O were determined, and the corresponding stable phase diagrams and density-composition diagrams were constructed. The results indicate that the phase diagrams of Pb²⁺, Ca²⁺//Cl^--H₂O mainly consist of a ternary invariant point, two solubility curves, and four crystalline regions, while there are two ternary invariant points, three solubility curves, and six crystalline regions in the Pb²⁺, Mg²⁺//Cl^--H₂O and Ca²⁺, Mg²⁺//Cl^--H₂O systems. The results of the density-versus-w(CaCl₂) plots of the various ternary systems confirm that the density of the equilibrium solution tends to go upward with the increase in the mass fraction of CaCl₂. The density of various ternary systems reaches the maximum and equilibrium at the corresponding invariant point, and there is no significant change with the further increase in the CaCl₂ mass fraction. Furthermore, the phase diagram of the Pb²⁺, Mg²⁺, Ca²⁺//Cl^--H₂O quaternary system includes two invariant points, five isothermal dissolution curves, and five crystalline regions. The order of the relative areas of the crystalline regions for the five salts is PbCl₂ > CaCl₂·2MgCl₂·12H₂O > 2PbCl₂·3MgCl₂·18H₂O > MgCl₂·6H₂O > CaCl₂·4H₂O.