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 (ΔSd, ΔHd, and ΔGd) 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.