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. Particularly, the continuous extractive fractionation of the lithium isotopes was investigated in small channels for the first time. Depending on the composition of the diluent, 4–25 % of the lithium was extracted in stirred vessels with a fractionation factor of 1.026 (±0.002) over an hour. In comparison, in the small channel contactors with 0.5 mm diameter, about 8–19 % extraction was reached in approximately 1 min with an apparent fractionation factor of 1.032 (±0.005). An increase in residence time in the channels increased the extraction percentage but reduced the degree of lithium fractionation. When the ionic liquids were diluted with anisole, equilibrium was reached faster 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. The results also revealed that the ionic liquid facilitates the isotope fractionation, while dilution of the ionic liquid with anisole did not affect fractionation.