Selective separation of radioactive thorium and uranium from scandium using N-heterocyclic carboxamide ligands

Scandium is a valuable strategic resource widely dispersed in nature and frequently coexists with other minerals, some of which contain radionuclides, particularly uranium and thorium. It is of great importance to remove these radioactive elements in a green and efficient manner to manufacture superior scandium products. The present study synthesized a range of N-heterocyclic carboxamide ligands and undertook a full investigation of their efficacy in extracting and separating uranium and thorium from scandium. Of these, two demonstrated preferential extraction of radioactive elements in HNO3, while the extraction of scandium was observed in HClO4. Under optimum conditions, 95.3% of uranium and 70.7% of thorium were extracted with only a 3.6% loss of scandium after a single-stage extraction, and the other rare earth elements were scarcely extracted. In addition, almost all the uranium and thorium in a simulated solution were depleted following a five-stage extraction process when treated with the phenanthroline-derived carboxamide ligand, resulting in a scandium oxide product with a 99.99% purity. The coordination mechanism of the extraction was identified using slope analysis, NMR titration, and UV-vis titration. All the evidence indicates that U(VI) and Th(IV) complexes with the phenanthroline-derived carboxamide ligand have a stoichiometry ratio of 1:1, whereas, in the case of scandium, there are both 1:1 and 1:2 species. This work enables the preparation of high-purity scandium with an ultra-low level of radioactivity and provides a new idea for the efficient processing of rare earth minerals.