Extraction Mechanism of Lithium from the Alkali Solution with Diketonate-Based Ionic Liquid Extractants

Four diketonate-based functional ionic liquids (ILs) with trialkylmethylammonium ([A336]⁺) cation were synthesized, and the synergistic extraction systems containing IL and thetrialkylphosphineoxide (TRPO) were developed to separate Li⁺ from the solution generated during the Li₂CO₃ production by the reaction of LiCl and Na₂CO₃. Compared to their precursors 1,3-diketones, over the wide pH range of 1.26–10.17, ILs showed higher Li⁺ extractability from the solution with a high Na/Li mass ratio. Among the four ILs, the IL with 2-thenoyl-trifluoroacetonate (TTA^–) anion showed the best Li⁺ separation ability. Under the optimized conditions, [A336]­TTA was up to 83.26%, which was much higher than that of its precursor (71.50%). The results of slope analysis and characterizations indicated that the extracted Li⁺ species using the extraction systems with or without TRPO were Li₂CO₃·2­[A336]­TTA·TRPO and Li₂CO₃·4­[A336]­TTA, respectively. Finally, a three-stage extraction process was validated, and the recovery efficiency of Li⁺ from the solution provided by Qinghai CITIC Guoan Science and Technology Development Co., Ltd. (CITIC) was up to 97.16%, indicating that the extraction system of [A336]­TTA + TRPO was efficient for separating lithium from the alkaline aqueous solution.