Tri-n-butyl phosphate (TBP) reacts with nitric acid to form a hydrogen-bonded complex TBP-(HNO 3 ) x (H 2 O) y that is highly soluble in supercritical fluid carbon dioxide (SF-CO 2 ). The value of x can be up to 2.5, whereas the value of y varies between 0.4 and 0.8 determined by acid-base and Karl Fischer titrations. The protons of HNO 3 and H 2 O in the complex undergo rapid exchange and exhibit a singlet resonance peak in NMR spectra. When the complex is dissolved in a low dielectric constant solvent, small droplets of nitric acid are formed that can be detected by NMR. Phase behavior studies indicate that the complex forms a single phase with SF-CO 2 above a certain pressure for a given temperature. This CO 2 -soluble Lewis acid-base complex provides a method of introducing nitric acid in SF-CO 2 for effective dissolution of uranium dioxide, lanthanide oxides, and perhaps other metal oxides.

Dissolution behavior of U 3 O 8 and UO 2 using supercritical CO 2 medium containing HNO 3 -TBP complex as a reactant was studied. The dissolution rate of the oxides increased with increasing the HNO 3 /TBP ratio of the HNO 3 -TBP complex and the concentration of the HNO 3 -TBP complex in the supercritical CO 2 phase. A remarkable increase of the dissolution rate was observed in the dissolution of U 3 O 8 when the HNO 3 /TBP ratio of the reactant was higher than ca. 1, which indicates that the 2:1 complex, (HNO 3 ) 2 TBP, plays a role in facilitating the dissolution of the oxides. Half-dissolution time (t 1/2 ) as an indication of the dissolution kinetic was determined from the relationship between the amount of uranium dissolved and the dissolution time (dissolution curve). A logarithmic value of a reciprocal of the t 1/2 was proportional to the logarithmic concentration of HNO 3 , C HNO 3 , in the supercritical CO 2 . The slopes of the ln(1/t 1/2 ) vs. ln C HNO 3 plots for U 3 O 8 and UO 2 were different from each other, indicating that the reaction mechanisms or the rate-determining steps for the dissolution of U 3 O 8 and UO 2 are different. A principle of the dissolution of uranium oxides with the supercritical CO 2 medium is applicable to a method for the removal of uranium from solid matrices.

No abstract

A method for the direct extraction of a metal utilizing supercritical CO 2 containing tri-n-butylphosphate (TBP)-HNO 3 (TBP), which is the TBP solution of TBP -HNO 3 complex, has been developed for the recovery of Nd from its oxide. This method was applied using a flow reactor to several oxides (Nd 2 O 3 , ZrO 2 , MoO 3 , and RuO 2 ), and binary mixtures of the oxides (Nd 2 O 3 -ZrO 2 , Nd 2 O 3 -MoO 3 , and Nd 2 O 3 -RuO 2 ). Neodymium (Nd) was extracted almost quantitatively from 0.01 mol Nd 2 O 3 powder with supercritical CO 2 containing TBP -HNO 3 (TBP) at 313K and 12 MPa, while Zr, Mo, and Ru were hardly extracted from their oxides and remained in the reactor. Nd was extracted quantitatively and selectively also from the binary mixtures of the oxides. From these results, it is found that the supercritical CO 2 extraction process using TBP -HNO 3 (TBP) as reactant is feasible for the selective recovery of lanthanides directly from the various oxide mixtures. 1153