Highly Selective Recovery of Lithium from Real Precipitation Mother Liquor Using HPMBP-Cyanex 923 Synergistic Extraction System

Direct and efficient recovery of lithium from precipitation mother liquor has garnered increasing attention, but challenges remain due to the low concentration of Li+ and high Na+ excess. To address these issues, we proposed a synergistic extraction system using 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP) as the extractant and Cyanex 923 as the synergist, which exhibits high selectivity for lithium in industrial mother liquors. In a single-stage extraction, HPMBP-Cyanex 923 achieved 85% Li+ extraction with only 4% Na+ and K+ from a real mother liquor solution containing 1.46 g/L Li+, 67 g/L Na+ and 2.57 g/L K+. The total organic carbon (TOC) test showed that HPMBP-Cyanex 923 exhibited significantly lower dissolution loss (220 mg/L) compared to that of the commonly used benzoyl-1,1,1-trifluoroacetone (HBTA, 432 mg/L), indicating superior hydrophobicity, which is essential for a green extraction process. The results demonstrate that 97% Na+ in the organic phase can be scrubbed prior to Li+ stripping, and much lower acid concentration (0.2 M HCl) is required to strip the lithium due to a lower pKa value of pyrazolone compared to traditional β-diketones. Single-crystal analysis, slope analysis, and DFT calculations revealed the observation of a binuclear structure, [Li2(PMBP)2(TBPO)2], which could serve as a general structural formula for systems involving pyrazolones and an organophosphorus co-ligand with a short alkyl chain. This work presents a novel lithium extraction system that offers enhanced selectivity and hydrophobicity, along with a lower stripping acid requirement compared to traditional β-diketones. These features make it highly promising for future industrial applications.