Rare earth elements (REEs) are becoming more and more significant as they play crucial roles in many advanced technologies. Therefore, the development of optimized processes for their recovery, whether from primary resources or from secondary sources, has become necessary, including recovery from mine tailings, recycling of end-of-life products and urban and industrial waste. Ionic solvents, including ionic liquids (ILs) and deep-eutectic solvents (DESs), have attracted much attention since they represent an alternative to conventional processes for metal recovery. These systems are used as reactive agents in leaching and extraction processes. The most significant studies reported in the last decade regarding the recovery of REEs are presented in this review.

To produce ultra low-sulfur gasoline and meet the new regulations, the improvement of the HDS reaction by the removal of inhibitors, like nitrogen-containing compounds (N-compounds) commonly found in middle distillates used for diesel feed, could be worth considering. Liquid–liquid extraction using 1-butyl-3-methyl-imidazolium chloride (BMIm⁺Cl⁻) was found to be a very promising approach for the removal of neutral N-compounds with high selectivity towards sulfur. BMIm⁺Cl⁻ was first evaluated using synthetic solution with dibenzothiophene and carbazole as model compounds, and a high selectivity for N-compounds was found. In order to explain the observed selectivity, the extraction of heterocyclic aromatic compounds from a hydrocarbon mixture by BMIm⁺Cl⁻ was studied as the function of the toluene/n-dodecane ratio. The extraction of sulfur-containing compounds and basic N-compounds decreased when the mass fraction of toluene increased, whereas the extraction of proton donor N-compounds was found to be almost independent of the mass fraction of toluene. We thus assumed that the selective extraction of indole and carbazole by BMIm⁺Cl⁻ could be explained both by electron pair acceptor properties of the imidazolium ring and by the binding of the hydrogen-donor group with chloride anion of the RTIL. BMIm⁺Cl⁻ was then evaluated using straight-run diesel feed, containing 13400 ppm S and 105 ppm N. An extraction of up to 50% of the N-compounds was obtained in one step, whereas the sulfur concentration reduction was only 5%. The selectivity of the extraction process towards heterocyclic aromatic compounds and polyaromatic compounds using BMIm⁺Cl⁻ was further emphasized by two-dimensional GC-MS analysis of the extracted compounds.