Study On Multi-Temperature Phase Equilibrium Of Li⁺,Na⁺//Cl^--H₂O Ternary System

High-quality lithium chloride is in short supply in the market,and China is rich in Salt Lake lithium resources,therefore,it is imperative to produce high-purity lithium chloride directly from Salt Lake brines.The key to industrial production of high-quality lithium chloride is the removal of impurities,of which the most difficult is the removal of sodium ions.The investigation showed that the ternary water-salt system Li⁺,Na⁺//Cl^--H₂O lacked solubility data and physical property data in the high-temperature region,and the research on how the impurity sodium chloride affected the kinetic process of lithium chloride crystallization had not been reported.Therefore,this system was selected as the research system to provide basic data support and theoretical guidance for industrial removal of sodium chloride impurities and to reveal the mechanism of the effect of impurity sodium chloride on the nucleation process of lithium chloride.On the one hand,the thermodynamic phase diagram of the Li⁺,Na⁺//Cl^--H₂O system was studied in this paper.The saturated solubility data and the corresponding liquid phase density were determined by the isothermal solution equilibrium method at 348.15 K,358.15 K,and 363.15 K,and the corresponding phase diagrams were drawn.No complex salts were found in the phase diagrams at three temperatures,which were composed of one saturation point,two solubility curves,and two crystallization regions,corresponding to sodium chloride and lithium chloride monohydrate,respectively.Combined with the phase diagrams of other temperatures in the literature,it could be seen that the crystallization region of sodium chloride was larger in the middle and high-temperature range,and increased slightly with the increase in temperature.In contrast,the crystallization region of lithium chloride（hydrated）was always very small and almost did not change with temperature.Based on the phase equilibrium data,the lithium chloride solution with higher purity could be obtained by evaporation of lithium chloride solution containing sodium chloride and precipitating the sodium chloride.At the same time,the modified BET model was used to predict the phase diagram of the system,which was in good agreement with the measured phase diagram.The accuracy of the experimental density data was verified by comparing the measured density with the literature data.Finally,the apparent molar volume was calculated by using the empirical formula based on Pitzer’s theory,which enriched the apparent molar volume data of the system.On the other hand,the effect of sodium chloride content on the nucleation metastable zone of lithium chloride during cooling crystallization at 348.15 K was investigated.It was found that when the mass concentration of sodium chloride in the saturated solution was 0.05%,the metastable zone width（MSZW）of lithium chloride increased most obviously,while the MSZW decreased gradually with the rise of the concentration of sodium chloride.When the content of sodium chloride was 0.30%,the MSZW of lithium chloride was the same as that without sodium chloride,and the effect of sodium chloride almost disappeared.When the impurity content was 0.05%,the inhibition on nucleation was the greatest,and then the inhibition decreased with the increase of impurity concentration.The reason may be that the hydration energy and the radius of hydrated ions of sodium and lithium ions are different.Once sodium ions enter the solution,the original interactions between lithium ions and water molecules are broken,which greatly hinders the migration and attachment of ions.As a result,the MSZW of lithium chloride increases,while with the increase of sodium ions,the interactions between species in the solution gradually reach a certain equilibrium,and this hindrance decreases gradually.By selecting different models to quantitatively describe the metastable zone（MSZ）,it was known that in Sangwal’s theory,the increase of sodium chloride generally reduced the solid-liquid interface energy（12%～18%）,while the pre-exponential factor related to kinetics decreased significantly,with a maximum decrease of 80.63%.The narrowing of MSZW mainly depended on the decrease of solid-liquid interface energy of lithium chloride in the presence of sodium chloride.To some extent,the research results of this paper could explain the effect of sodium chloride impurities on the nucleation process of lithium chloride.In contrast,models such as the Nyvlt model had been proved to be not suitable for the application of this system.This study provides basic data support and theoretical guidance for the production of high-quality lithium chloride in the industry.