Solvent Extraction of Li+ using Organophosphorus Ligands in the Presence of Ammonia

In this work, the selective extraction of Li+ with the aid of organophosphorus ligands (H-OP) including phenylphosphonic (H-PHO), phenylphosphinic (H-PHI) and bis(2-ethylhexyl) phosphoric (H-BIS) acids in the absence and presence of ammonia was studied. Adding NH3 to the aqueous phase resulted in significant improvement in the % extraction of Li+ into the organic phase containing H-OP ligands. The highest % extraction values obtained in the case of H-PHO, H-PHI, and H-BIS were 43.2%, 45.7%, and 90.0%, respectively. Two mechanisms were inferred; the first was that the extraction equilibrium reaction of LiCl + H-OP Li-OP + HCl shifted forward due the reaction of the produced HCl with NH3. The second mechanism was that the Li+/NH4+ exchange of NH4-OP (produced from the reaction of H-OP with NH3) was easier than Li+/H+ exchange of H-OP itself. Competitive extraction experiments indicated that the selectivity factors of Li+ over Na+ and K+ were strongly dependent on the concentration of H-OP ligands which suggested that aggregation of ligand molecules via hydrogen bonding is the limiting factor for selectivity.