Selective separation of lithium from the high magnesium brine by the extraction system containing phosphate-based ionic liquids

The high acidity of stripping is one of the major problems that hinder the industrialization of the tributyl phosphate (TBP) + FeCl 3 system for lithium extraction from the salt lake brines with high Mg/Li ratio. In this study, a novel solvent extraction system, trialkylmethylammonium di(2-ethylhexyl)orthophosphinate ([N 1888 ][P 507 ]) + TBP + FeCl 3 , was developed to significantly reduce the difficulty of stripping and simplify the extraction process. The conditions of extraction and stripping were systematically studied in the present work. The results showed that Li + extraction efficiency could reach 61% under the optimal extraction parameters and Li + stripping ratio was 90% with a relative mild conditions of O/A = 10:1, [HCl] aq  = 1.5 mol·L –1 . Also, combined with experimental results, FT-IR analysis, and UV analysis, the possible mechanisms of lithium extraction and stripping were put forward. Li + was extracted from the aqueous phase through the formation of [Li·2TBP][FeCl 4 ] adduct, meanwhile [N 1888 ][P 507 ] was just slightly involved in the extraction step. In the stripping procedure, the interaction between [FeCl 4 ] − and [Li·2TBP] + might be destroyed, and new possible complexes, [Li·2TBP] + , and [FeCl 4 ·TBP·P 507 ] 2− /[FeCl 4 ·P 507 ] 2− were formed in the organic phase. Additionally, the extraction efficiency of Li + remained above 70% while the organic phase was reused for six cycles. Compared with the traditional system, the addition of [N 1888 ][P 507 ] resulted in a slight decrease of Li + extraction efficiency (about 5%), the stripping acidity significantly reduced from 6 mol·L –1 HCl to 1.5 mol·L –1 HCl. This new extraction system showed a promising performance in selective extracting and easy stripping in lithium separation from the salt lake brines with high Mg/Li ratio.