Metastable Phase Equilibria In The Quinary System Li⁺,Na⁺,Cs⁺?Rb⁺?,Mg²⁺//Cl^-H₂O And Its Partial Subsystems At 298.2 K And 323.2 K

Sichuan basin is abundant with underground brine resources.The brine is riched in rare and valuable components such as Li⁺,Rb⁺,Cs⁺,etc,and has high development value.The various alkali and alkali earth metal elements in the brine are in coexistence state,and the ion interactions are complex,which makes it difficult to extract and separate beneficial elements.Therefore,it is necessary to study the phase equilibria of typical underground deep brine system in China.In this paper,according to the characteristics of typical underground deep brine,the metastable phase equilibria of the quinary system Li⁺,Na⁺,Cs⁺?Rb⁺?,Mg²⁺//Cl^--H₂O and its partial subsystems at298.2 K and 323.2 K has been done by using an isothermal method.The equilibrium liquid phase compositions,densities and refractive indices of the equilibrated solution are also measured experimentally.The corresponding metastable phase diagrams?space diagrams and planar projection diagrams?,multi temperature phase diagrams,water content diagrams,density vs composition diagrams and refractive index vs composition diagrams were plotted.The quaternary system Li⁺,Na⁺,Cs⁺//Cl^--H₂O at 298.2 K and 323.2 K belongs to the simple system,there is no double salt or solid solution formed.LiCl has a salting out effect on NaCl and CsCl.Comparisions between the metastable phase diagrams and solubility data of the quaternary system at 298.2 K and 323.2 K show that the configuration of the phase diagram is basically consistent.With the increase of temperature,the solubility of NaCl and LiCl has not obvious change,while the solubility of CsCl increases obviously.Based on the phenomena in the above study,CsCl can be separated by the thermosol-cooling crystallization.The system Na⁺,Cs⁺,Mg²⁺//Cl^--H₂O at 323.2 K is of a complex system with double salt Cs-Car?CsCl·MgCl₂·6H₂O?formed in the quaternary system.MgCl₂shows a salting out effect on NaCl.Compared with the metastable phase diagrams of the quaternary system at 298.2 K and 323.2 K,the crystalline form of each salt and the type of invariant point are the same.Along with increases of the temperature,the area of the crystallization field of CsCl and CsCl·MgCl₂·6H₂O decreases.Therefore,the precipitation of CsCl and CsCl·MgCl2·6H2O can be achieved by changing the temperature.The metastable phase diagram of the quinary system Li⁺,Na⁺,Cs⁺,Mg²⁺//Cl^--H₂O at 298.2 K and 323.2 K are divided into six parts,which correspond to six crystallization zones,by three invariant points and ten isothermal dissolution curves.There are double salt Li-Car?LiCl·MgCl₂·7H₂O?and CsCl·MgCl₂·6H₂O formed.LiCl shows a salting out effect on CsCl and MgCl₂.Compared with the metastable phase diagrams of the quinary system at 298.2 K and 323.2 K,the crystalline form of each salt has not change at different temperatures,while the area of the crystallization field of each salt changes obviously.According to the results of the phase diagram,the relationship between the solubilities of LiCl·MgCl₂·7H₂O and CsCl·MgCl₂·6H₂O was obtained,and the results are consistent with Christov'research results.The metastable phase diagrams of the quinary system Li⁺,Na⁺,Rb⁺,Mg²⁺//Cl^--H₂O at 298.2 K and 323.2 K are composed of three invariant points,six crystallization fields and ten univariant curves.LiCl shows a salting out effect on RbCl and MgCl₂.Compared with the metastable phase diagrams of the quinary system at 298.2 K and323.2 K,with the increase of temperature,the area of the crystallization field of RbCl decreases,and the area of the crystallization field of Rb-Car?RbCl·MgCl2·6H2O?increases.It indicates that under the saturation of salt NaCl,high temperature is beneficial to the enrichment of RbCl in solution.Results show that the various alkalis and alkaline earth chlorides salt-water system in the brine are complex,and various carnallite type double salts can be easily formed.The crystallization field of salt NaCl is larger,and the crystallization field of double salt LiCl·MgCl₂·6H₂O is smaller.The crystallographic form of inorganic salts and the crystallization rules of salts obtained form the phase equilibria research enriched the thermodynamic data of salt-water system,which can provide the theoretical direction to establish the best chemical production process and technology for the comprehensive utilization of typical deep underground brine in China.