Investigations into a ternary liquid-liquid-solid extraction system for partitioning of lithium, with a view to potential isotopic enrichment applications

The fractionation of lithium isotopes is of great importance to the nuclear industry. Solvent extraction (SX) methods using benzo-15-crown-5 ether (B15C5) are one of the more mature and easily scalable technologies to achieve this. However, the isotopic enrichment (α) possible for SX has not been significantly improved for >40 years. Here, we present an investigation into a novel three-phase (liquid-liquid-solid) system, combining B15C5, dissolved in an imidazolium ionic liquid, with two organosilicas with ethyl/butyl phosphonic acid (EBPsil) and sulfonic acid (SULFsil) functional groups. The aqueous Li mass transfer was investigated, initially in two-phase systems, which suggested that Li was favourably extracted as complexes involving two of the extracting ligands. The B15C5 organic phase produced the highest distribution coefficient (K D ), followed by SULFsil. In three-phase experiments, it was found that increasing the effective ligand concentration in either the liquid or solid phase increased Li transfer into that phase, but not at the expense of transfer into the other non-aqueous phase. Li extraction could not be well controlled by initial aqueous pH, due to buffering by contact with the ionic liquid in the organic phase. The addition of Mg 2+ as a competing ion was strongly suppressive for solid-phase adsorption, but not SX. A maximum α of 1.035 ± 0.004 was achieved for 6 Li enrichment in the organic phase, at a Li/B15C5 ether molar ratio of ~2.5. Surprisingly, increasing the organosilica mass in the system appeared to enhance the α value for the organic phase, which is contrary to almost every other study involving Li isotope fractionation via cation-exchange. This result was believed to be due to the hydrophilicity of the silica-based matrix and suggests that a pathway is possible to significant improvement in isotope fractionation efficiency, using a three-phase methodology.