The UK has an ambitious target to construct the Spherical Tokamak for Energy Production (STEP) prototype fusion power plant by 2040. For this purpose, it requires more than 100 tonnes of isotopically pure lithium-6. This isotope undergoes neutron capture to yield He-4 and H-3. It is widely considered the most feasible method of tritium breeding to support future fusion reactors.
Lithium is not a scarce element and natural lithium is mined in great quantities, primarily for electric vehicle battery production. However, isotopic separation is chemically challenging. Li-6 enrichment on an industrial scale has only been achieved with the COLEX process, which was used historically in America (now banned) and currently by Russia and China. This process uses mercury amalgam and is environmentally unviable in the UK.
An alternate strategy is a solvent extraction process, using crown ether extractants, dissolved in a suitable solvent. The [crown ether/Li-6]+ complex is slightly more energetically favourable than the equivalent Li-7 complex and this leads to an isotopic enrichment ratio of up to 1.057, which approaches that of COLEX.
Optimising the conditions to achieve maximal Li-6 enrichment, whilst retaining reasonable overall Li extraction, is a difficult balancing act and there are also pragmatic considerations around the choice of solvent, relating to cost, volatility and toxicity. Ionic liquids, such as 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] [BMIm][NTf2], have more recently been explored for improved Li organic solubility. However, this technology is unproven beyond the laboratory scale.
This work will utilise centrifugal contactors, commonly used for process intensification in the nuclear, hydrometallurgical and chemical industries, to advance the technological readiness level (TRL) of Li-6 enrichment via crown ether solvent extraction. It will detail some of the challenges encountered during the scale-up attempt, which are not commonly considered in laboratory experiments.
