Davris, and 6 co-authors, reported in 2018 that the ionic liquid betainium bis(trifluoromethylsulfonyl)imide (HBetTf2N), has been applied directly as a leaching agent on a low-grade rare earth element (REE) from the Rödberg Mine ore. This orebody is in the Fen carbonatite complex ore deposit located in Norway. REEs in the Rödberg ore are mainly present in the form of fluoro-carbonates of bastnäsite group minerals. Factors studied during leaching were water ratio into ionic liquid, leaching temperature, pulp density (%w/v) and retention time. Leaching of the Rödberg ore with HbetTf2N resulted to 65–100% recovery of light REE and 40–60% recovery of heavy REE.
Additionally, > 80–90% of Ca and Mg content dissolves, whereas the HbetTf2N is selective against iron and silicon resulting in a Fe/Si rich residue. The metals dissolved into HbetTf2N after leaching are stripped with an aqueous acidic solution, regenerating the ionic liquid for reuse. Through acid stripping, REE concentration is doubled in the aqueous acidic solution, for further purification. The carboxylic acid functional group attached to the cation side of HbetTf2N is capable of dissolving bastnäsite thus enabling the development of new alternative methods for treating low-grade REE primary resources.
The authors concluded that the functionalized ionic liquid HbetTf2N found to be able of dissolving bastnäsite, which is the main REE-bearing mineral present in the Rödberg ore. Factors studied during leaching indicate that water addition into HbetTf2N had a positive effect on REE leaching, whereas increasing temperature at 90 °C and decreasing pulp density to 5% w/v, resulted in 65–100% LREE recoveries with HREE recoveries range from 40 to 60% after 24 hr. retention time. Kinetics at low pulp density indicate high-recovery yields (60–70% LREE) after 1 hr. of leaching yet REE recovery has slower kinetics at higher pulp density.
HbetTf2N leaching is selective against Fe and Si mineral phases, while high dissolution yields obtained for Ca and Mg resulted in an almost total dissolution of calcite and dolomite phases from the Rödberg ore. Metals dissolved into the IL-PLS were almost totally extracted through acid stripping using 100% excess of stoichiometric HCl, where HbetTf2N is regenerated for reuse. The regenerated HbetTf2N doesn't lose its leaching performance after 3 leaching cycles. It was found that Sc, Fe and Th form strong complexes with HbetTf2N and a bulk separation from the rest REE, Ca and Mg can be achieved. Yet IL losses should be taken into account and action on further minimizing IL losses must be implemented. Neutralization of the HCl solution after stripping resulted to a precipitate having 16.5% wt. REE content.
It can be concluded that ionic liquids have great potential to be applied in low-grade REE carbonate primary resources providing good leaching efficiency, regeneration of the leaching agent and production of an enhanced REE solution for further purification by known methods.
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