Characteristics of an oxa-diamide–HNO3 extractant in the supercritical fluid extraction of uranium

Abstract

An extractant is required in the recovery process to drive the uranium to a stage that enables it to be extracted using the extraction solvent. This paper proposes the composition of a composite extractant, N,N,N′,N′-tetrabutyl-3-oxapentane-diamide–HNO3 (TBODA–HNO3) as an extractant, to successfully achieve the objective using supercritical carbon dioxide (sc-CO2). The composite TBODA–HNO3 extractant has a chemical composition of TBODA(HNO3)1.0(H2O)1.5. The U(IV) in the UO2 containing solid phase is directly oxidized to U(VI) in the form of $$ {\rm UO}_{2}^{2 + } $$ in sc-CO2, which contains a CO2-soluble TBODA–HNO3 extractant at 200 atm and 50 °C. The resulting $$ {\rm UO}_{2}^{2 + } $$ /TBODA complex can be consequently extracted using acetone-modified sc-CO2. The chemical composition of the $$ {\rm UO}_{2}^{2 + } $$ /TBODA complex, which can be extracted by nonpolar sc-CO2, is proposed in the form of an ion pair: [UO2(TBODA)2]2+–2( $$ {\rm NO}_{3}^{ - } $$ ).