Development of Polyamide Solvent Extraction Reagents for Trivalent Lanthanides
Sammanfattning: Rare earth elements (REEs), including lanthanides, are critical materials for many industrial technologies. As a result recycling of end-of-life products containing REEs might be beneficial, as this could partially satisfy the growing demands of industry for these elements. However, recycling of end-of-life products is often technically and/or commercially challenging. One possible method for the recycling of such materials is a hydrometallurgical treatment of wastes, which includes leaching of valuable metals (e.g. lanthanides) with mineral acids followed by solvent extraction for isolation and separation of the metals. As part of our efforts towards better REEs recycling, novel polyfunctional amides (polyamides) have been synthesized and tested as solvent extraction reagents for trivalent lanthanides in this work. The structure of the polyamide ligands was shown to affect largely the distribution ratios of trivalent lanthanides. It is very likely that the ability to form chelate complexes with lanthanide(III) ions is an important feature of the polyamides as solvent extraction reagents. It was demonstrated that it is possible to increase the distribution ratios of trivalent lanthanides using malonamides as extractants by increasing the denticity of the ligands. Thus the distribution ratios of lanthanide(III) ions obtained with the tetraamide 2,2’-(1,2-phenylenebis(methylene))bis(N,N,N’,N’-tetrabutylmalonamide), bearing two N,N,N’N’-tetrabutylmalonamide units on an ortho-xylylene platform, were up to one hundred times greater (under particular experimental conditions) than those with the diamide – N,N,N’N’-tetrabutylmalonamide – in two diluents – nitrobenzene and 1,2-dichlorobenzene. The increase of the distribution ratios of trivalent lanthanides obtained with this tetraamide can be attributed to the entropic effect. This conclusion follows from the studies of the stoichiometry of europium(III) extraction with the tetraamide, from the comparison of the distribution ratios of trivalent lanthanides obtained with the tetraamide and its structural isomers, and from the single-crystal diffraction studies of the neodymium(III) nitrate complex with the analogue of the tetraamide. It was shown that solvent extraction properties of the synthesized amides are sensitive to the nature of the diluent. Polar diluents promote the extraction of trivalent lanthanides with the amides. It was demonstrated that the use of the extraction chromatography resin with N,N,N’N’-tetrabutylmalonamide as an extractant and polystyrene as a support potentially can relieve the complications (e.g. toxicity, the third liquid phase formation etc.) introduced to the solvent extraction process by diluents. However further studies are needed to prove the feasibility of this alternative.
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