A Novel System Based on TOP and NaBPh4 for Selective Extraction of Lithium Ions from Salt Lake Brines with High Mg/Li Ratios

Abstract

Solvent extraction has become a common technique for recovering lithium ions from brines with high Mg/Li ratios because this process is inexpensive, readily automated and easily scaled-up. The present work examined a new extraction system intended to improve the lithium ion extraction efficiency and the separation of lithium and magnesium while addressing problems associated with the use of FeCl₃ as a coextracting agent. This system employed trioctyl phosphate as the extractant, NaBPh₄ as the coextraction agent and 2-octanone as the diluent. The proportion of trioctyl phosphate, molar ratio of NaBPh₄ to lithium ions and the organic/aqueous phase ratio were all optimized through experiments to give a single-stage Li⁺ extraction efficiency of 83.60%. In this process, the organic phase was washed with a solution of LiCl and NaCl to reduce the concentrations of Mg²⁺ and K⁺ to less than 1 mg/L. After three subsequent extractions of the organic phase with a Na₂CO₃ or NaHCO₃ solution, the Li⁺ extraction efficiency was as high as 99%. During ten extraction cycles, the β_{Li⁺/Mg²⁺} value was maintained between 6000 and 7000, indicating good stability. Finally, the extraction mechanism was determined using infrared and nuclear magnetic resonance spectroscopy and these analyses confirmed the selective extraction of Li⁺. This new extraction system does not require acids or bases for regeneration of the organic phase, and the long-term operational stability of the organic phase is greatly improved.