Solvent extraction of boron from mildly alkaline salt lake brine in Tibet, China

Since the level of resource depletion is maintained at a high level, the recovery of boron from salt lake brine has become an effective way to meet the increasing demand for boron. This study investigates the optimization of boron extraction from the weakly alkaline brine of the Laguocuo Salt Lake (LGCSL) in Tibet, China, a representative of Tibetan weakly alkaline salt lakes. We evaluated the efficacy of 2,2,4-trimethyl-1,3-pentanediol (TMPD) as an extractant within a solvent mixture of 2-butyl-1-octanol (C₁₂–OH) and sulfonated kerosene. The extraction performance was systematically assessed through single-stage and multi-stage counter-current extraction experiments, examining variables such as extractant type, concentration, pH, temperature, and the presence of co-existing ions. Our results demonstrate that optimal boron extraction is achieved under conditions of pH 8.0, an organic-to-aqueous phase ratio (O/A) of 1 : 1.5, and lower temperatures. Under these parameters, single-stage extraction efficiency surpassed 83%, while a three-stage process achieved an impressive 98.61% efficiency. Stripping experiments identified sodium hydroxide (NaOH) as an effective stripping agent, with a concentration of 0.3 mol L⁻¹ and a phase ratio of 2 : 1 at room temperature yielding high stripping efficiency and significant boron concentration enrichment. To elucidate the extraction mechanism, Raman spectroscopy was employed to characterize the structural interactions between TMPD and boron complexes in the organic phase. Additionally, the influence of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions, prevalent in alkaline brines, on boron extraction was investigated. These ions were found to affect the extraction efficiency, likely through competitive interactions or complex formation, highlighting the necessity of their consideration in optimizing the extraction process. This study provides both theoretical insights and practical experimental data essential for the efficient recovery of boron from weakly alkaline salt lake brines.