S. I. Stepanov

Solvent extraction of uranium(VI) peroxo-carbonate complexes from carbonate solutions by methyltrioctylammonium carbonate

245 Uranium(VI) forms a series of carbonate com plexes, which are soluble in aqueous solutions of ammonium or alkali metal (M) carbonates. Among such complexes, M2[UO2(CO3)2] and M4[UO2(CO3)3] are the most stable ones [1]. These compounds are readily extracted by quaternary ammonium salts to form corresponding compounds with the quaternary ammonium cation R4N + in the organic phase. The chemistry of extraction of U(VI) carbonate com pounds with quaternary ammonium salts has been comprehensively described [2].

Chemistry of the CARBEX process: Identification of absorption bands of the ligands in the electronic spectra of aqueous solutions of Na4[UO2(CO3)3]

On the basis of consideration of hydration, hydrolysis, dissociation, polymerization, and ligand exchange that occur in aqueous solutions of U(VI) complexes, a new approach to the assignment of absorption bands of the ligands in electronic spectra of uranium(VI) carbonate complexes in the range 190–400 nm has been suggested. For the Na4[UO2(CO3)3] complex, the following assignment of absorption bands has been made: Na3[UO2(CO3)3]–, 258 nm; Na2[UO2(CO3)3]2–, 300 nm; and Na4[UO2(CO3)3], 330 nm.

Chemistry of the CARBEX process: Identification of absorption bands of the ligands in the electronic spectra of aqueous solutions of Na4[UO2(O2)CO3)2]

On the basis of consideration of dissociation, hydration, association, and ligand exchange, the assignment of absorption bands in the electronic spectra of aqueous solutions of the Na4[UO2(O2)CO3)2] complex has been performed. It has been demonstrated that the absorption in the range 190–400 nm is caused by the oxygen atoms of the O22- and CO32- groups and hydration water molecules of dissociated and neutral complex species Na3[UO2(O2)(CO3)2]–, Na2[UO2(O2)(CO3)2]2–, and Na4[UO2(O2)(CO3)2].

NMR study of U(VI) extraction from carbonate-fluoride solutions with methyltrioctylammonium fluoride

Mixed U(VI) carbonate fluoride complexes are formed in aqueous solutions by the reaction of ura nyl fluorides with alkali metal carbonates or by the reac tion of uranyl carbonates with alkali metal fluorides. The following mixed complexes have been described in the literature [1]: Na2[UO2(CO3)F2], Na3[UO2(CO3)F3], Na4[UO2(CO3)F4], and Na3[UO2(CO3)2F2]. Each of these complexes can be efficiently extracted with qua ternary ammonium salts.

Physicochemical principles of preparation of U(VI) carbonate solutions for extraction reprocessing in the Carbex process

Physicochemical principles of preparation of U(VI) carbonate solutions in the step of oxidative dissolution of U3O8 and UO2 in the Carbex process are considered. Carbonate solutions with the U(VI) concentration higher than 100 g L–1, suitable for subsequent final purification of uranium by extraction, can be prepared under the conditions of formation of U(VI) carbonate–peroxide complexes in the course of dissolution with prevention of hydrolysis of U(VI) compounds. The behavior of impurities simulating some fission products in the course of oxidative dissolution was studied, and the decontamination factors of U(VI) from the chosen simulated fission products were determined.

High-temperature interaction of components of spent uranium nuclear fuel with alkali metal carbonates in the course of voloxidation in the Carbex process

Interaction of UO2, U3O8, ZrO2, Nb2O5, CeO2, Ce2O3, Pr6O11, Eu2O3, Yb2O3, Tb7O12, and Re metal with lithium, sodium, and potassium carbonates in the temperature interval 300–900°С was studied using methods of thermal analysis and X-ray diffraction. The major products of the high-temperature reactions are alkali metal uranates and diuranates, and in the case of ZrO2, Nb2O5, and lanthanide oxides, oxides of metals in higher oxidation states and salts of the corresponding metal-containing acids, formed by reactions of the oxides with sodium carbonate.

Carbonate extraction-based refining of uranium. Separation of U(VI), Ce(IV), and Ln(III) from aqueous carbonate solutions with methyltrioctylammonium carbonate

The extraction of U(VI), Ce(IV), La(III), Nd(III), Sm(III), and Y(III) from an aqueous solution of Na2CO3 (0.25 mol/L) resulting from oxidative dissolution of U(IV) in the presence of H2O2 into a solution of methyltrioctylammonium carbonate (0.25 mol/L) in toluene. It was found that βU(VI)/Ln(III) values vary from ~8 to 3290 as the O : W ratio changes from 2 : 1 to 10 : 1, while βU(VI)/Ce(IV) varies from ~1.5 to 10, which allows for the extraction separation of U(VI) from Ce(IV) in a 8- to 10-stage counter-current extraction cascade and from Ln(III) in 2- to 3-stage cascade under the same conditions.

Chemistry of the CARBEX process. Identification of the absorption bands of the ligands in the electronic spectra of U(VI) extracts with methyltrioctylammonium carbonate

The electronic absorption bands of extracts of the Na4[UO2(O2)(CO3)2] complex with methyltrioctylammonium (MTOA) carbonate were assigned taking into account hydration, hydrolysis, dissociation, polymerization, and ligand exchange, which occur in aqueous and organic solutions. It was shown that the extractable compound, (R4N)4[UO2(O2)(CO3)2], present in low concentrations in the organic phase partly dissociates by one step to give the (R4N)3[UO2(O2)(CO3)2]– anions, while at high concentrations, it is converted to polynuclear complex.

Chemistry of the CARBOFTOREX process. Identification of the absorption bands of the ligands in the electronic spectra of aqueous solutions of uranyl fluoride

The electronic absorption bands of aqueous solutions of the [UO2F2(H2O)n] complex were assigned taking into account dissociation, hydration, association, and ligand exchange. The absorption in the range of 190–400 nm was found to be related to the formation of cationic, neutral, and anionic complex species, [UO2F2(H2O)n].

Chemistry of the CARBOFTOREX process. Identification of the absorption bands of the ligands in the electronic spectra of aqueous solutions of U(VI) fluoride carbonate complexes

The electronic absorption bands of U(VI) fluoride carbonate and fluoride hydroxide complexes were assigned taking account of dissociation, hydration, association, and ligand exchange. The absorption in the range of 190–400 nm was found to be related to the formation of neutral and dissociated anionic U(VI) fluoride carbonate and fluoride hydroxide complexes and the polynuclear Na2n[(UO2–O–UO2)F4(OH)2n–1 ∙ kH2O] complex.