Selective leaching of rare earth elements from bauxite residue (red mud), using a functionalized hydrophobic ionic liquid

In this study the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide (HbetTf2N) has been applied directly on leaching bauxite residue, for achieving selective dissolution of rare earth elements (REE) against iron. Several factors have been examined such as leaching temperature, retention time, pulp density and water addition into the ionic liquid. It was found that moderate water addition, elevated temperatures and increased retention time provide up to 70-85% REE extraction yields with respective Fe extraction yield < 3%. Sc recovery showed a different leaching behavior from the rest of the REE with its extraction yield not exceeding 45%. Furthermore, no dissolution occurs for Ti and Si whereas 30% maximum extraction of Al was obtained, followed by almost total dissolution of Ca and Na content. This work shows that most REE and a fraction of Sc are associated with Calcium- and Aluminium-bearing phases in the bauxite residue, while the majority of Sc is associated probably with the remaining Fe and Ti-bearing phases. The metals dissolved into hydrophobic ionic liquid can be extracted with an acidic medium while the IL leaching agent is regenerated and after separation can be reused to a new portion of bauxite residue. Enabling consecutive scrubbing-stripping on the ionic liquid pregnant solution an enhanced acidic Sc solution with reduced level of impurities is obtained.