Polythermal phase equilibria in the Na2CO3-LiCl-H2O ternary system and crystallization of lithium carbonate

Understanding the phase equilibria and solubility behavior in multicomponent salt-water systems is essential for the development of efficient separation and resource recovery technologies, especially for lithium-containing compounds. However, reliable phase equilibrium data for the Na2CO3–LiCl–H2O system remain limited over a wide range of temperatures and concentrations. In this study, solubility relationships and phase transformations in the sodium carbonate–lithium chloride–water system were systematically studied using a visual polythermal method over a wide range of temperatures and compositions. The resulting phase diagram describes distinct crystallization regions corresponding to ice, Na2CO3∙10H2O, Na2CO3∙7H2O, LiCl∙5H2O, LiCl∙2H2O, LiCl∙H2O, and the newly identified Li2CO3 phase. The formation and stability of the new lithium carbonate phase were confirmed using combined chemical and physicochemical analysis, including infrared spectroscopy and X-ray diffraction, ensuring the reliability of the obtained equilibrium data. Characteristic absorption bands in the IR spectrum observed at 1437.03 cm-1 and 869.93 cm-1 were assigned to carbonate functional groups, providing further evidence for the formation of Li2CO3. The polythermal diagram shows that a significant portion of the system is occupied by crystallization fields, indicating the initially low solubility of lithium carbonate. This behavior highlights the possibility of selectively separating Li2CO3 from saturated solutions through controlled evaporation. Overall, the results provide fundamental thermodynamic insights and practical recommendations for lithium recovery processes and the development of effective separation strategies in aqueous carbonate-chloride systems.