Liquid-liquid extraction of Li+ using mixed ion carrier system at room temperature ionic liquid

An environmentally benign technique for the separation and recovery of lithium (Li+) from aqueous streams, containing mixed metal ions was developed via liquid-liquid extraction (LLE). Hydrophobic room temperature ionic liquids (RTIL) were tested as the main extracting solvents. To increase the metal extraction, a proton-ionizable agent bis(2-ethylhexyl) phosphoric acid (DEHPA) was added into the RTIL. To enhance the metal uptake selectivity, three Li+-selective neutral ion carriers such as 6-hydroxy-dibenzo-14-crown-4, dibenzo-14-crown-4, and tri-n-octyl-phosphine (TOPO) were also used and tested as extractant additives. Among the tested RTILs, phosphonium-based CYPHOS IL 109 was the most stable extractant as it exhibited the lowest loss when contacted with water. Addition of proton-ionizable agent DEHPA in CYPHOS IL 109 afforded a high extraction of multivalent cations with negligible recovery of monovalent metals. On the other hand, the addition of neutral ion carrier TOPO in DEHPA/CYPHOS IL 109 extractant resulted in a selective Li+ uptake which was maximized when the pH of the aqueous solution was maintained at 13, with TOPO/DEHPA molar ratio = 8. The overall process could be a two-stage LLE system wherein the unwanted multivalent cations are initially removed using DEHPA/CYPHOS IL 109 followed by the selective extraction of Li+, using TOPO/DEHPA/CYPHOS IL 109 system.