Highly Selective Extraction of Thorium and Uranium from Ion-Absorption Type Rare Earth Ore Leachate Using a Sterically Hindered α-Aminophosphonate Extractant

The leachates derived from ion-absorption type rare earth ores (IATREO) typically contain trace concentrations of radioactive thorium (Th) and uranium (U). The inadvertent enrichment of these radionuclides during hydrometallurgical processing poses significant environmental risks and compromises the quality of rare earth (RE) products. In this study, a highly branched α-aminophosphonate extractant, di(3,5,5-trimethylhexyl)-N-(2-ethylhexyl) aminomethyl phosphonate (DTAMP), was synthesized for the selective removal of Th(IV) and U(VI) from a simulated aluminum sulfate leach liquor. Compared to its structural analogue, di(2-ethylhexyl)-N-(2-ethylhexyl) aminomethyl phosphonate (Cextrant230), DTAMP features enhanced steric hindrance, which significantly improves selectivity toward Th(IV) and U(VI) over RE(III) and Al(III). Specifically, separation factors (β) for U/REs, U/Al and Th/Al exceed 105, while βTh/REs surpasses 104 at 0.5 mol/L H2SO4, representing an improvement of 2–3 orders of magnitude over Cextrant230. Leveraging its high extraction capacity and efficient stepwise stripping capabilities, a cross-flow extraction process was proposed and validated using simulated leachate. A two-stage extraction protocol successfully reduced Th(IV) and U(VI) concentrations from initial levels of 3.20 mg/L and 1.37 mg/L to below 0.1 mg/L each, with negligible loss of REs(III). These findings underscore the potential of DTAMP as a superior extractant for the selective elimination of radioactive impurities, offering a promising pathway for cleaner REs production.