Studies On The Metastable Phase Equilibrium Of The Quinary System Na⁺,Rb⁺,Cs⁺,Mg²⁺//Cl^--H₂O And Its Partial Subsystems At 298.2 K And 323.2 K

The Luojiaping underground brine is riched in valuable components such as Rb⁺,Cs⁺,Mg²⁺,etc,and has high development value.In order to extract beneficial elements from brine,it is necessary to study the phase equilibria of Luojiaping typical underground deep brine system.The component solubilities,densities and refractive indices of the equilibrated solution in the quinary system Na⁺,Rb⁺,Cs⁺,Mg²⁺//Cl^--H₂O,the quaternary system Na⁺,Rb⁺,Mg²⁺//Cl^--H₂O and Na⁺,Cs⁺,Mg²⁺//Cl^--H₂O at 298.2 K and 323.2 K were measured with an isothermal evaporation method.Based on the experimental data,the space phase diagram,the plane projection phase diagram,density vs composition diagram,and refractive index vs composition diagram were constructed.In order to ensure the reliability of data,the uncertainty analysis of data is carried out.At the confidence level of 95%,the confidence intervals of each ion concentration are as follows:C?Cl^-?=2.1117±0.0178 mol/L?C?Na⁺?=0.2650±0.0043 mol/L?C?Rb⁺?=1.5712±0.0236 mol/L?C?Cs⁺?=0.9294±0.0162mol/L?C(Mg²⁺)=0.6899±0.0064 mol/L.The quaternary system Na⁺,Rb⁺,Mg²⁺//Cl^--H₂O is of complex type system at298.2 K and 323.2 K,with double salt rubidium carnallite RbCl·MgCl₂·6H₂O formed.Its metastable phase diagram consists of two invariant points,five univariate curves and four crystallization fields.In the temperature range?298.2-323.2?K,the crystallization area of RbCl decreases with the increase of temperature,so low temperature is beneficial to precipitate RbCl in the system coexisted with chloride of sodium,rubidium and magnesium.According to the phase diagram of the quaternary system Na⁺,Cs⁺,Mg²⁺//Cl^--H₂O at 298.2 K,it can be seen that the phase diagram consists of two invariant points,five univariate curves and four crystallization fields.This system is of a complex type,with a double salt CsCl·MgCl₂·6H₂O formed.Comparisons between the metastable phase diagrams of this quaternary system at 298.2 K and 323.2 K show that the number and type of invariant points,crystalline and univariate curves has no changes with the change of temperature,whereas the crystal region size is affected by the temperature.The crystallization area of CsCl decreases with the increase of temperature,indicated that CsCl can be precipitated from the coexisting salt solution by changing the temperature.The quinary system Na⁺,Rb⁺,Cs⁺,Mg²⁺//Cl^--H₂O is of complex type system,with two double salt RbCl·MgCl₂·6H₂O and CsCl·MgCl₂·6H₂O,and one solid solution?Rb,Cs?Cl formed.Its phase diagrams at 298.2 K and 323.2 K consist of 4invariant points,13 univariate curves and 7 crystallization fields.In the temperature range?298.2-323.2?K,the crystallization area of CsCl,RbCl·MgCl2·6H2O,and?Rb,Cs?Cl increases with the increase of temperature.It is suggested that higher temperature is beneficial to the precipitation of RbCl·MgCl2·6H2O,?Rb,Cs?Cl,and CsCl in the system coexisted with chloride of sodium,rubidium,cesium and magnesium.