High-Efficiency Removal of Calcium and Magnesium from Lithium-Concentrated Solution via Counter-Current Extraction Using Di-(2-ethylhexyl)phosphinic Acid

A high loss ratio of lithium (Li⁺) and low removal efficiency of calcium (Ca²⁺) and magnesium (Mg²⁺) remain a major challenge so far in obtaining high-purity lithium products from a lithium-concentrated solution. In this work, a state-of-the-art separation and purification process including saponification, extraction, scrubbing, stripping, and precipitation was proposed, which achieved the ultrapurity lithium concentration. Compared with the previously reported di(2-ethylhexyl)phosphoric acid (P204, D2EHPA) system, the selected extractant di(2-ethylhexyl)phosphinic acid (P227) showed superior separation ability with a lower loss ratio of Li⁺. Meanwhile, the stripping acidity of the loaded organic phase was low, and the selective stripping of Li⁺ can be achieved by controlling the phase ratio. In the ternary mixed system, the removal efficiency of Ca²⁺ and Mg²⁺ reached 99.81% and 100%, respectively, while only 0.71% Li⁺ was extracted. The yield and purity of Li⁺ at an aqueous phase exit were 98.44% and 99.99%, respectively, indicating the successful production of the ultrapurity lithium-concentrated solution. Furthermore, in the simulated actual solutions, the 100% removal efficiencies of both Ca²⁺ and Mg²⁺ and only 2.09% Li⁺ loss demonstrated that our system possesses the potential of high-performance practical applications. More importantly, the high-purity Li₂CO₃ product was therefore obtained by precipitation treatments. These findings take a step toward developing an ultrahigh ion-selective technique for efficient lithium purification from a lithium-concentrated solution that could serve as industrially applicable separation platforms.