Process and Mechanism Study on Lithium Extraction from Alkaline Solution by 2-hydroxy-4-Octyloxybenzophenone and Trioctyl Phosphate Synergistic System

As the new energy industry advances, extracting lithium from secondary resources such as process tail liquor has become an effective approach to meet the ever-growing demand for lithium resources. In this work, a synergistic extraction system composed of 4-octyloxy-2-hydroxybenzophenone (HOOBP) and trioctyl phosphine (TOP) was used to recover lithium from alkaline high-sodium tail liquor. The key influencing factors for lithium extraction were investigated at first. The optimal organic phase composition (0.3 mol/L TOP and 0.2 mol/L TOP) and single-stage extraction conditions (COH- = 0.4 mol/L) were determined, with the lithium extraction efficiency of 79% and the Li/Na separation coefficient ~ 200. The process parameters for multistage extraction, scrubbing, and stripping were optimized, and a full-process scheme consisting of "four-stage extraction, three-stage scrubbing, and three-stage stripping" was designed. A total of 250 h of process verification was conducted on 10 stages mixer-settler unit, the lithium extraction efficiency remained stably above 98%. The lithium concentration in the resultant lithium-enriched solution reached over 25 g/L, with impurity ions below 0.1 g/L, and the purity of the prepared Li2SO4·H2O was 99.88%. At the molecular level, the complex configuration was elucidated by Fourier transform infrared spectroscopy (FT-IR) and density functional theory (DFT) calculations. This research lays a theoretical and experimental foundation for lithium recovery from complex alkaline lithium-containing resources, and provides a promising technical route for practical applications.