Supersolubility and solubility of lithium phosphate in sodium carbonate solution

Abstract

The tail liquid generated from lithium carbonate production in salt lake brine is termed lithium-bearing mother liquor. This mother liquor exhibits a complex composition, with the Li⁺ concentration typically around 1.5 g L⁻¹, representing a significant lithium resource. Preparing lithium phosphate (Li₃PO₄) from this mother liquor is critical for efficient lithium recovery. However, the lack of data on the thermodynamic behavior and Li₃PO₄ crystallization in such complex solutions has hindered the high-efficiency recovery of lithium resources. In this study, the solubility of Li₃O₄ in sodium carbonate solutions was determined using the dynamic dissolution equilibrium method. The effects of temperature and sodium carbonate concentration on solubility were analyzed, and experimental data were correlated using an exponential equation. Results indicated that the solubility of Li₃PO₄ in pure water and sodium carbonate solutions increases with temperature and sodium carbonate concentration. The supersolubility of Li₃PO₄ in LiCl–Na₂CO₃ electrolyte solutions was measured via turbidimetric analysis, and the metastable zone width (MSZW) was determined. The supersolubility of Li₃PO₄ significantly decreased with rising temperature. In contrast, supersolubility initially increased and then decreased with higher Na₂CO₃ concentrations, with reactant concentration being the decisive factor driving the crystallization reaction. Furthermore, the MSZW narrowed at elevated temperatures. Thermodynamic functions (ΔS_d, ΔH_d, and ΔG_d) for the dissolution process were calculated via the van't Hoff equation, confirming that Li₃PO₄ dissolution is a spontaneous and endothermic process. Based on solubility and supersolubility data, a novel process was developed to prepare battery-grade Li₃PO₄ (purity: 99.80%) from salt lake mother liquor. The results of Raman, FTIR, TG and SEM suggested that the prepared lithium phosphate was pure phase. This study provides fundamental physicochemical data and theoretical insights for the efficient separation and extraction of lithium resources from lithium precipitation mother liquor.