Dissolution Behavior of Uranium Oxides with Supercritical CO₂Using HNO₃-TBP Complex as a Reactant

Abstract

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.

Aeriometallurgical Extraction of Rare Earth Elements from a NdFeB Magnet Utilizing Supercritical Fluids

“…In the case of X 4 X 7 , a simultaneous increase in the amount of chelating agent and methanol could overload the system beyond the solvation power of sc-CO 2 , thus decreasing the extraction efficiency. In the case of X 1 X 5 , an increasing temperature has previously been shown to slightly enhance extraction in systems without mechanical agitation; , however, the elevated temperature could destabilize the structure of REE–TBP–NO 3 reverse micelles, increasing their susceptibility to be disrupted by mechanical forces, thus enhancing the negative effect of increasing the agitation rate. This negative effect of an increasing agitation rate could be attributed to the unfavorable effect of shear environment on the solvation of REE–TBP–NO 3 reverse micelles by sc-CO 2 .…”

Supercritical Extraction of Lanthanide Tributyl Phosphate Complexes: Current Status and Future Directions

“…The coexistence of TBP­(HNO 3 ), TBP­(HNO 3 ) 2 , TBP­(HNO 3 )­(H 2 O), and TBP­(HNO 3 )­(H 2 O) 2 at higher acidities is corroborated by molecular dynamics simulations . These same complexes are also known to form in TBP solvent extraction systems. , …”

“…40 These same complexes are also known to form in TBP solvent extraction systems. 14,24 In addition to this work on pure oxides and hydroxides, a small number of studies have used TBP/nitric acid adducts in supercritical CO 2 to extract lanthanides from real ores or recycled materials. These include extraction from pretreated bastnasite, 50 fluorescent lamp phosphors, 52,53 nickel metal hydride batteries, 51 and neodymium−iron−boron magnets.…”

“…This was motivated by the desire for a fission product separation process that does not generate contaminated organic wastes. Researchers demonstrated the extraction of uranium and other actinides from aqueous solutions − and from solid oxides or nitrates. − In 2009, a pilot plant demonstrated the supercritical extraction of uranium from incinerator fly ash …”

Recovery of Uranium and Plutonium from Waste Matrices Using Supercritical Fluid Extraction

“…Supercritical fluid extraction of uranium and plutonium from nitric acid medium was examined using Sc-CO 2 containing tri-n-butyl phosphate (TBP) [8]. Quantitative dissolution of uranium dioxide in Sc-CO 2 containing TBP with nitric acid was reported [9][10]. Uranium dioxide and its solid solutions with neptunium, plutonium and americium dioxides were dissolved using Sc-CO 2 modified with TBP/nitric acid [11].…”