Osamu Tomioka

Dissolution Behavior of Uranium Oxides with Supercritical CO2 Using HNO3-TBP Complex as a Reactant

Dissolution behavior of U3O8 and UO2 using supercritical CO2 medium containing HNO3-TBP complex as a reactant was studied. The dissolution rate of the oxides increased with increasing the HNO3/TBP ratio of the HNO3-TBP complex and the concentration of the HNO3-TBP complex in the supercritical CO2 phase. A remarkable increase of the dissolution rate was observed in the dissolution of U3O8 when the HNO3/TBP ratio of the reactant was higher than ca. 1, which indicates that the 2:1 complex, (HNO3)2TBP, plays a role in facilitating the dissolution of the oxides. Half-dissolution time (t½ ) 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 t½ was proportional to the logarithmic concentration of HNO3, CHNO3, in the supercritical CO2. The slopes of the (l/t½ ) vs. ln CHNO3 plots for U3O8 and UO2 were different from each other, indicating that the reaction mechanisms or the rate-determining steps for the dissolution of U3O8 and UO2 are different. A principle of the dissolution of uranium oxides with the supercritical CO2 medium is applicable to a method for the removal of uranium from solid matrices.

New Method for the Removal of Uranium from Solid Wastes with Supercritical CO2 Medium Containing HNO3-TBP Complex

This study aims at the development of a method for the decontamination of uranium from the solid wastes containing uranium oxides, UO 2 or U 3 O 8 , using supercritical CO 2 medium containing HNO 3 -TBP complex (TBP: tri-n-butylphosphate).

Cleaning of materials contaminated with metal oxides through supercritical fluid extraction with CO2 containing TBP

Abstract Studied was an application of supercritical fluid extraction (SFE) to cleaning of the materials contaminated with metal oxides. By using supercritical CO 2 including chemical complexes of tri- n -buthylphosphate (TBP) and HNO 3 , lanthanide was recovered from oxide mixtures placed in a pressure vessel. The lanthanide oxide was dissolved with TBP-HNO 3 and extracted with TBP in supercritical CO 2 . This cleaning method reduces volume amount of liquid waste arising from treatment of the contaminated materials. Being applied to a mixture of Gd 2 O 3 and SrO, we found that Gd was selectively recovered and Sr was not found in the recovered solution. Gd was also selectively recoverd when a mixture of Gd 2 O 3 and ZrO 2 was treated. This selective nature of decontamination is an additional advantage of the SFE with TBP-HNO 3 complex. This method would be applicable to the selective cleaning of materials contamined with radioactive oxides such as UO 2 /PuO 2 .

Selective recovery of neodymium from oxides by direct extraction method with supercritical Co2 containing TBP–HNO3 complex

A method for the direct extraction of a metal utilizing supercritical CO2 containing tri-n-butylphosphate (TBP)–HNO3 (TBP), which is the TBP solution of TBP–HNO3 complex, has been developed for the recovery of Nd from its oxide. This method was applied using a flow reactor to several oxides (Nd2O3, ZrO2, MoO3, and RuO2), and binary mixtures of the oxides (Nd2O3–ZrO2, Nd2O3–MoO3, and Nd2O3–RuO2). Neodymium (Nd) was extracted almost quantitatively from 0.01 mol Nd2O3 powder with supercritical CO2 containing TBP–HNO3 (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 CO2 extraction process using TBP–HNO3 (TBP) as reactant is feasible for the selective recovery of lanthanides directly from the various oxide mixtures.

Densities of Supercritical Fluids Containing CO2 and Tri-n-butylphosphate

The densities of supercritical fluids consisting of CO2 and 1) tri-n-butylphosphate (TBP), 2) a TBP solution containing HNO3, and 3) UO2(NO3)2·2TBP were experimentally measured using a variable-volume-type view cell at 313, 323 and 333 K. The densities of the fluids increased with an increase in pressure, and decreased with an increase in temperature. The densities were expressed as a function of pressure for each mole fraction of TBP in the supercritical fluids consisting of CO2 and the organic solution at 15–30 MPa. When the mole fraction of TBP in the fluid of CO2 and UO2(NO3)2·2TBP was less than 0.01, the molar volume of the fluid calculated from its densities was smaller than that of CO2 due to negative partial molar volume of the complex, which showed characteristics of a supercritical fluid dissolving solutes.

Supercritical Carbon Dioxide Fluid Leaching (SFL) of Uranium from Solid Wastes Using HNO3-tributylphosphate (TBP) Complex as a Reactant

Supercritical carbon dioxide (CO2) leaching method (SFL), which is based on the efficient and selective dissolution of UO2 and U3O8 with supercritical CO2 containing HNO3-tributylphosphate (TBP) complex at 333 K and 15 - 20 MPa, has been developed for the removal and recovery of uranium from the solid waste contaminated by uranium oxides. The decontamination factor of UO2 or U3O8 of higher than 500 were attained by the recommended procedure, which was demonstrated using synthetic solid waste samples of a mixture of the uranium oxides and sea sand.