Phase Diagram Construction And Valuable Elements Separation Of Carbonate Salt Lake Brine System Before And After CO₂ Carbonation

Carbonate salt lake brine is rich in many valuable elements such as sodium,potassium,lithium and boron which can be separated initially by natural evaporation.However,in the middle and late stages of evaporation,the salts present a complex law of crystallization.Sodium and potassium are easy to form potassium and sodium double salt,lithium carbonate（Li₂CO₃）has small enrichment space,and precipitates almost simultaneously with potassium salt.Aimed at these issues,this article selects the Zabuye salt lake as the research object,put forward a method of introducing CO₂to transfer the original carbonate system into bicarbonate system to separate valuable elements efficiently.In the first part,the phase behavior of the ternary system K₂CO₃-Na₂CO₃-H₂O before and after carbonation was studied.Phase diagrams of the system K₂CO₃-Na₂CO₃-H₂O at 298.2K and the system KHCO₃-NaHCO₃-H₂O at 298.2K and 313.2K were drawn.The composition,density,conductivity and pH value of the solution were determined.By comparison,it is found that the carbonate system contains double salt in the form of Na₂CO₃·K₂CO₃·H₂O（6-12）,while the bicarbonate system is a simple ternary system,without any double salt crystallization field.Phase equilibrium of the system KHCO₃-NaHCO₃-H₂O was obviously affected by temperature.These findings provide a basis for effective separation of sodium and potassium by CO₂ carbonation.The final recovery rate of potassium salt reached 30.5%,and the purity reached 99%.Compared with the extraction of potassium single salt by evaporation alone,the recovery rate increased by nearly 6 times.In the second part,lithium and potassium salts in Li₂CO₃-K₂CO₃-H₂O system were separated by CO₂ carbonation and evaporation.The changes of phase and solution composition in the system before and after carbonation were investigated.The maximum solubility of Li⁺in LiHCO₃ solution obtained from carbonation in pure Li₂CO₃ solution was 14.1g/L,while the maximum solubility of Li⁺in LiHCO₃ and KHCO₃ co-saturated solutions was 5.8g/L.Trough evaporation,theultimate maximum concentration of Li⁺was 16g/L.This state-of-art was traced via isotherm evaporation on the quaternary system of Li⁺,K⁺//CO₃^2-,HCO₃^--H₂O system.In the last part,boron was introduced and a continuous process for extracting potassium,boron and lithium from carbonate subtype brine of Zabuye salt lake was carried out.The concentration region of boron in the initial brine suitable for the above process was obtained and the mass fraction of boron（expressed in B₂O₃）is in the range of w（B₂O₃）=0.66%to w（B₂O₃）=4.24%.The extraction efficiency（E%）can reach the maximum value of 98%when w（B₂O₃）was 0.66%.The recovery of boron was 80.26%.Thermal decomposition and evaporation temperatures are 70°C and 25°C,respectively.The recoveries of lithium and potassium were 80.31%and 75.33%.Moreover,the phase transition of Li⁺,K⁺//CO₃^2-,B₄O₇^2--H₂O quarternary system after carbonation and the impact of boron on the carbonation process of carbonate system were studied.Finally,the scheme of the valuable elements separation in the complex system of Li⁺,Na⁺,K⁺//CO₃^2-,B₄O₇^2--H₂O was determined.