Studies On Phase Equilibria Of The Quaternary System Mg²⁺,zn²⁺, Pb²⁺?k⁺?//cl^--H₂O And Some Ternary Subsystems At 323 K

In this paper,the phase equilibria of the quaternary systems Mg²⁺,Zn²⁺,Pb²⁺?K⁺?//Cl^--H₂O and the related ternary sub-systems at 323 K were studied by isothermal dissolution equilibrium method.The determination of the ions in the equilibrium liquid phase is mainly carried out by chemical analysis method supplemented by atomic absorption method.X-ray powder diffraction was used for qualitative analysis of equilibrium solid phases,and wet residue method was used for the identification of solid phases in ternary systems.The phase diagrams of the two quaternary systems mentioned above and five unreported ternary systems at 323 K were obtained through experiments,which are K⁺,Mg²⁺,Zn²⁺//Cl^--H₂O,Pb²⁺,Mg²⁺,Zn²⁺//Cl^--H₂O,KCl-PbCl₂-H₂O,KCl-ZnCl₂-H₂O,MgCl₂-PbCl₂-H₂O,MgCl₂-ZnCl₂-H₂O and PbCl₂-ZnCl₂-H₂O,respectively.At the same time,the water content diagrams of two unreported quaternary systems are also plotted.The ternary system KCl-PbCl₂-H₂O is a complex system with three invariant points,four solubility curves and four solid phase crystallization zones?corresponding to KCl,KCl·2PbCl₂,KCl·PbCl₂ and PbCl₂,respectively?.With the increase of the concentration of KCl,two different complex salts formed,which were KCl·2PbCl₂and KCl·PbCl₂,respectively.The solubility of the two complex salts was relatively small.The ternary systems KCl-ZnCl₂-H₂O,MgCl₂-PbCl₂-H₂O,MgCl₂-ZnCl₂-H₂O all have double salt formation and contain two invariant points,three univiarate curves and three solid phase crystallization zones.Zinc chloride has a strong solubilizing effect on potassium chloride in the ternary system KCl-ZnCl₂-H₂O,whose corresponding crystallization zones are KCl,2KCl·ZnCl₂ and ZnCl₂,respectively.With the addition of zinc chloride,the mass fraction of potassium chloride increases gradually.When the mass fraction of potassium chloride reaches its maximum value and zinc chloride is added,the solubility of potassium chloride begins to decrease again and the complex salt of chemical formula 2KCl·ZnCl₂formes.In ternary system MgCl₂-PbCl₂-H₂O,the solubility of lead chloride increases with the increase of the concentration of magnesium chloride,and a complex salt3MgCl₂·2PbCl₂·18H₂O forms,and the corresponding crystallization fileds of this system are MgCl₂·6H₂O,3MgCl₂·2PbCl₂·18H₂O and PbCl₂.With the formation of the complex salt 3MgCl₂·2PbCl₂·18H₂O,the solubility of lead chloride decreases gradullay.In the ternary system MgCl₂-ZnCl₂-H₂O,a double salt with the chemical formula MgZnCl₄·6H₂O is produced,the three solid phase crystallization zones are MgCl₂·6H₂O,MgZnCl₄·6H₂O and ZnCl₂.The ternary system PbCl₂-ZnCl₂-H₂O is a simple ternary system with only one invariant point and two univariate curves,and the corresponding solid phases at the invariant point are PbCl₂ and ZnCl₂.The quaternary system Pb²⁺,Mg²⁺,Zn²⁺//Cl^--H₂O consists of three invariant points,seven univariate curves and five crystallization zones,which are corresponding to PbCl₂,ZnCl₂,MgZnCl₄·6H₂O,3MgCl₂·2PbCl₂·18H₂O and MgCl₂·6H₂O?Bis?.Among them,the area of the crystallization zone?PbCl₂?is the largest and almost occupies the whole phase diagram,which indicates that lead chloride is easier to crystallize and precipitate in solution.Compared with the phase diagram of Mg²⁺,Pb²⁺,Zn²⁺//Cl^--H₂O at 373 K,the crystallization zone of MgZnCl₄·6H₂O with that of 3MgCl₂·2PbCl₂·18H₂O was found.The quaternary system K⁺,Mg²⁺,Zn²⁺//Cl^--H₂O includes four invariant points,nine univariate curves,and six solid phase crystallization regions,which are ZnCl₂,2KCl·ZnCl₂,MgZnCl₄·6H₂O,MgCl₂·6H₂O?Bis?,KMgCl₃·6H₂O?Car?and KCl,respectively.Potassium chloride is subjected to the salting-out effect of magnesium chloride,resulting in the solubility of potassium chloride greatly reduces,and the crystallization zone greatly increases.Compared with the phase diagram at 373 K,the shape and size of the crystallization zone are similar,except that the crystallization zone of MgZnCl₄·6H₂O at 323 K replaces that of MgZnCl₄·5H₂O at 373 K.