Study of removing radioactive thorium(IV) from high-purity scandium(III) products with di-(2-ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP) in nitric acid solution

Abstract

Scandium (Sc) is a critical rare earth element. Its compounds are of great concern due to their excellent properties. However, the application is limited by the presence of radioactive impurity thorium (Th). The study aims to remove the radioactive Th(IV) from high-purity Sc2O3 by DEHEHP. The extraction behavior of Sc(III) from HNO3 solution using DEHEHP was also investigated. The effects of HNO3, extractant and salting-out reagent (LiNO3) concentrations on the Sc(III) extraction were studied. The extraction mechanism and thermodynamic parameters were determined. Among the three extractants: Cyanex 923, TBP and DEHEHP, DEHEHP showed the greatest separation ability for Sc(III) and Th(IV) from HNO3 solution. The effects of HNO3, LiNO3, and initial Sc(III) concentrations as well as phase ratio on the separation factor βTh/Sc were investigated. It was indicated that βTh/Sc reached as high as 196 after separation by 30% (v/v) DEHEHP at 3.0 mol L−1 HNO3. The McCabe-Thiele diagram for Th(IV) was plotted. After the six-stage counter-current extraction experiment, the concentration of Th(IV) in the raffinate decreased to 20 mg L−1 from the solution containing 7.71 g L−1 Sc(III) and 692 mg L−1 Th(IV) without the co-extraction of Sc(III). Finally, Sc2O3 of purity >99.99% was obtained.