Development of eco-friendly extraction and separation system of rare metals by using ionic liquids

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The purpose of this study is to develop eco-friendly and efficient extraction and separation system by using ionic liquids as extraction solvents. The results obtained are as follows :Ionic liquids (ILs) are molten salts composed of organic cations and organic or inorganic anions. Physicochemical properties of ILs are extensively tunable by selecting cationic and anionic partners. Therefore we synthesized a variety of imidazolium cation-based ionic liquids and clarified the relationship between the structure of the ionic liquids and their properties.Then, the ionic liquids were applied as solvents to the extraction and separation of rare earth metals with commercial extractant CMPO(octyl(phenyl(pheny)-N, N-diisobutylcarbamoylmethyl phosphine oxide). It was demonstrated that the extraction efficiency and the selectivity for individual rare earth metals were greatly enhanced compared to that in the conventional solvent system such using n-dodecane. Unique extraction mechanism in ILs system was also elucidated. Furthermore, quantitative stripping of metal ions from IL phases was succeeded using the mixture of complexing agents and the recyclability of solvent ILs was experimentally shown.Novel extractant TPEN(N, N, N', N'-tetrakis(2-pyridylmethyl)ethylenediamine) was examined for the extraction of rare earth metals. Coordination site of TPEN was protonated under the acidic conditions, and the metal ion could be easily stripped to the aqueous phase at low pH condition.Functionalized ionic liquids, which have a functional group in a molecule such as crown ether, were applied to the solvent extraction. It was shown that the extraction system without extractants could be constructed using the functionalized ionic liquid, which showed high extraction ability compared to the ordinary ionic liquids.In this study, the fundamental information was obtained to design a suitable extraction process based on ionic liquids for target ions.