B. Ya. Zilberman

New data on joint extraction of actinide nitrates and some acids with tributyl phosphate diluted with paraffins and mathematical description of the process by A.M. Rozen’s Model

Published data were systematized and new data were obtained on extraction of nitric acid, nitrates of uranium, neptunium, and plutonium in various valence states, thorium nitrate, and zirconium nitrate, and also on the effect exerted on their distribution ratios by temperature and other components present in the system, with 30% TBP in paraffin diluent used as organic phase. Data on the extraction of pertechnetic, acetic, formic, hydrazoic, hydrochloric, and dichromic acids and/or on the coextraction of their anions with U(VI) and Zr were obtained. Refined equations based on A.M. Rozen’s model were suggested for mathematical description of the extraction of the above components. These equations include the concentration extraction constants as polynomial functions of the solution ionic strength and take into account changes in the phase volumes upon mass transfer. Principles of stepwise calculation of extraction cascades in the steady-state mode, with data output in the tabular form and possibility of their transfer into MS Excel, are briefly described. The user’s interface is shown, and examples of head extractor calculation are presented.

Solubility and coprecipitation of barium and strontium nitrates in HNO3 solutions and multicomponent systems

The solubility of Ba(NO3)2 and Sr(NO3)2 in HNO3 solutions at 25–95°C is characterized by the power dependence on the total concentration of the nitrate ion with the exponent for Ba(NO3)2 equal to −2 in ∼9 M HNO3 and −6 in more concentrated acid solutions. The latter exponent is also characteristic of the more soluble Sr(NO3)2 throughout the examined range of HNO3 concentrations. In strongly acidic solutions, Ba(NO3)2 coprecipitates with Sr(NO3)2. The solubility curve for Ba(NO3)2 in NH4(Na)NO3 solutions suggests formation of a double salt, whereas in UO2(NO3)2 solutions the dependence is the same as in HNO3 solutions.

Extraction of zirconium with tributyl phosphate from nitric acid solutions

The extraction of zirconium with tributyl phosphate (TBP) in organic diluent was studied. The dependence of the distribution ratio of Zr on the TBP concentration in the organic phase and also the IR spectra of Zr-TBP extracts obtained at different concentrations of Zr and HNO3 in the aqueous phase show that at least two kinds of extractable zirconium nitrate solvates with TBP are formed in extraction with 30% TBP. The solvate formed at HNO3 concentration not exceeding 3 M contains water molecules. The extract-able species formed at higher nitric acid concentration in the aqueous phase is anhydrous zirconium nitrate disolvate with TBP.

Prospects for development of a process for recovering technetium from spent fuel of nuclear power plants

Comperative analysis is made of the Tc behavior in different stages of spent fuel reprocessing, to make a decision on the process allowing separation of Tc as an individual product. The electrochemical and sorption processes for separating Tc from nitric acid solutions are tested. The electrochemical process presents significant difficulties, and it appears more favorable to recover Tc as an individual product in the first extraction cycle. Based on the fractionation principle, a scheme can be suggested in which Tc is separated along with Np in the first cycle and then recovered using ion exchange or extraction.

Effect of some reducing and complexing agents on the extraction behavior of technetium in the TBP-HNO3 system

The effect of a series of complexing and reducing agents on the extraction-chemical behavior of technetium as applied to extraction splitting of uranium and plutonium in the Purex process was examined. Kinetic parameters of the catalytic decomposition of N2H5NO3 under the action of Tc in the presence of these agents were evaluated. Variation of the ratio of the oxidized and reduced Tc species in the course of the process and in the hydrazine-free systems was determined. Reagents preventing oxidation of the reduced technetium and decomposition of N2H5NO3 in nitric acid solutions (acetohydroxamic acid, hydroxylamine, ascorbic acid, etc.), inhibiting the reduction of Tc with hydrazine nitrate (H2O2, HN3, etc.), and known as complexing agents toward quadrivalent actinides but indifferent to Tc were considered. The possibilities of using these reagents for modifying the Tc behavior in the course of SNF extraction reprocessing with the aim to direct this element to different process streams were discussed.

Extraction of molybdenum from supersaturated solutions in nitric acid with tributyl phosphate solutions

Data were obtained on the composition of saturated Mo solutions in HNO3 solutions of various concentrations at various temperatures, with a maximum at 5 M HNO3. Extraction of Mo from such solutions of low acidity with concentrated solutions of TBP in xylene at room temperature was studied, and the IR spectra of the extract were measured. The extractability of Mo in the supersaturation region sharply decreases with a decrease in the TBP concentration and with an increase in the acidity of the aqueous phase. The kinetics of the Mo extraction is described by a first-order rate equation. The process apparently involves formation of molybdic acid in the aqueous phase by the reaction MoO2(OH)+ + H2O = H2MoO4 + H+. It is assumed that H2MoO4 dissolves in water present in TBP, instead of precipitating from the aqueous phase.

Formation of molybdenum and zirconium precipitates in concentrated uranyl nitrate solutions

The influence of the keeping time, temperature, and concentrations of U and HNO3 on the residual content of Mo and Zr in the solution was examined. An increase in the preliminary keeping time favors more complete precipitation. An increase in the uranium concentration to 800 g l−1 and/or in the temperature to 105°C leads to growing formation of precipitates of molybdic acid (in the absence of Zr) or zirconium molybdate. The presence of zirconium promotes, and an increase in the acidity impedes the process.

Extraction of Mo from solutions in HNO3 and other mineral acids with solutions of dibutyl hydrogen phosphate in a diluent

Extraction of Mo from HNO3 solutions with solutions of HDBP in xylene and CCl4 in a wide range of Mo concentrations was studied. The Mo distribution ratios are considerably higher with CCl4 diluent compared to xylene, but the extractant capacity in both cases is the same and corresponds to the ratio HDBP: Mo = 2. The active species in the Mo extraction is the HDBP dimer. In the first step, an acidic molybdenyl salt with HDBP of the composition MoO2(DBP)2(HDBP)2, exhibiting certain secondary extraction properties toward rare-earth elements, is formed in all the cases. The dependence of the Mo extraction on the aqueous solution acidity passes through a minimum at 3–4 M HNO3. The subsequent increase in the Mo distribution ratios is associated with the simultaneous extraction of HNO3 (or nitrate ion) whose concentration in the extract is considerably lower than the Mo concentration. With an increase in the loading of the extract with molybdenum, the acidic molybdenum salt of HDBP undergoes restructurization, probably associated with additional coordination of H2MoO4 to it. The dependence of the Mo distribution ratio on the acid concentration in the extraction from sulfuric acid solutions passes through a minimum at 3–4 M H2SO4, which correlates with the first step of the acid dissociation, and in the extraction from HClO4 the dependence passes through a minimum and a maximum. In the extraction from hydrochloric acid solutions, the Mo extractability decreases with an increase in the acid concentration, owing to complexation in the aqueous phase. The nature of processes occurring at various loadings of the extract with molybdenum are discussed.

Extraction of long-lived radionuclides from nitric acid solutions using an extractant based on dibutyl hydrogen phosphate and chlorinated cobalt dicarbollide

Chlorinated cobalt dicarbollide (CCD), when added to concentrations of 0.25 M to a solution of dibutyl hydrogen phosphate (HDBP) in m-nitrobenzotrifluoride (MNBTF), increases the distribution ratios of trace amounts of Eu and Am without changing the slope (tan α ∼ 2) of their dependences on HDBP concentration in the 0–1.5 M range. At [CCD]/([CCD] + DBPA]) = 0.2–0.22, the synergistic effect is observed in the entire range of HDBP concentrations in extraction of these elements from 1.0 and 2.5 M HNO3. In this case, HDBP suppresses the extraction of Cs with CCD in the area below the synergistic maximum, where antagonism is observed in the extraction of Cs. Polyethylene glycol (PEG, Slovafol-909) was added to the extraction mixture to improve the extraction of Sr. The extremum is attained at its concentration in the solvent with HDBP of ∼0.033–0.065 M, which is smaller than that in the absence of HDBP by a factor of 1.5–2.5. With increasing concentration of HDBP in the HDBP-CCD-PEG-MNBTF extraction system, the slopes for Eu and Am are 1.3 and 0.6, whereas the slopes for Cs and Sr decrease nonlinearly and amount to −1.8 and −1.3, respectively. With increasing concentration of HNO3, D for Eu, Am, and Cm decreases in proportion to the HNO3 concentration to the power of −3 irrespective of the PEG concentration, and for Cs and Sr, to the power of −2 in the presence of PEG, whereas in the PEG-free systems the dependences are nonlinear. The synergistic extractant is characterized by higher (by an order of magnitude) solubility of metal solvates as compared to the HDBP-MNBTF system (concentration of Eu in the extractant >0.163 M). The extractant containing HDBP (1.1 M), CCD (0.23 M), and Slovafol-90 (0.065 M) in MNBTF is suggested for combined recovery of rareearth (REE) and transplutonium elements (TPE) and of Cs and Sr from high-level waste (HLW) after reprocessing of spent nuclear fuel (SNF) with high burn-up.

Synergic extraction of Ce(III) from HNO3 solutions with a mixture of CCD and HDBP ZS in a polar diluent

The extraction of Ce(III) from HNO3 solutions with a mixture of chlorinated cobalt dicarbollide (CCD) and dibutyl hydrogen phosphate (HDBP) or its zirconium salt (HDBP ZS) in m-nitrobenzotrifluoride was studied by the method of isomolar series. A considerable synergic effect S was observed, reaching a maximum in a mixture of HDBP with CCD at the molar ratio HDBP : CCD = 7 : 1 (S = 100). The highest Ce(III) distribution ratios DCe are observed with mixtures of CCD and HDBP ZS (1 : 6 and 1 : 4). An increase in the HNO3 concentration causes changes in DCe and S to different extents depending on the type of HDBP ZS. The synergic effect was attributed to the formation of coordination compounds that are more hydrophobic than the initial compounds and have stronger bonds of Ce(III) with the ligands.