Selective removal of magnesium from lithium-rich brine for lithium purification by synergic solvent extraction using β-diketones and Cyanex 923.

In the production of battery-grade and high-purity LiCO, it is essential to remove magnesium impurities. The state-of-the-art solvent extraction (SX) process using Versatic Acid 10 and D2EHPA co-extracts 3.3-5.5% lithium, while removing 86-98% magnesium. Here, we demonstrate that synergic SX systems containing a β-diketone (HPMBP, HTTA or HDBM) and Cyanex 923 are highly selective for magnesium extraction over lithium (separation factor  > 1,000). The extracted magnesium and lithium complexes have the stoichiometry of [Mg∙A∙(C923)] and [Li∙A ∙(C923)] ( = 1, 2), respectively (A represents deprotonated β-diketone). The three β-diketone synergic SX systems all considerably outperformed the Versatic Acid 10 system for magnesium removal from a synthetic solution containing 24 g L Li and 0.24 g L Mg. In a three-stage batch counter-current extraction, the HPMBP and Cyanex 923 synergic SX system removed 100% magnesium with only 0.6% co-extraction of lithium. This excellent Mg/Li separation is the best result reported so far.