Extractive separations of lithium isotopes with benzo-15-crown-5 and ionic liquids: a comparative study between stirred vessels and small channel contactors
The separation of lithium and the fractionation of the isotopes by solvent extraction was studied, using benzo-15-crown-5 as an extractant dissolved in a diluent of an ionic liquid and anisole. Equilibrium experiments were carried out in stirred vessels while continuous flow experiments were performed in small channels with 0.5 mm diameter. In the flow channels about 8-19% extraction was reached in approximately 1 min with an apparent fractionation factor of 1.032 (±0.005). In comparison, in stirred vessels 10–25% extraction was achieved with a fractionation of 1.026 (±0.002) over an hour. An increase in residence time in the channels increased the extraction percentage but reduced the lithium fractionation. When the ionic liquids were diluted with anisole, equilibrium was reached at an increased rate while the extraction percentage was not affected in stirred vessels. Density Functional Theory (DFT) calculations suggested that the fractionation of the Li isotopes is attributed to different vibrational frequencies of the Li-O bonds with the extractant in the organic solution and with water, while the isotope exchange was spontaneous. The ionic liquid helped the isotope fractionation, while its dilution with anisole did not significantly affect it.