COEXTRACTION OF URANIUM AND TECHNETIUM IN TBP-SYSTEMS

Abstract

The coextraction of technetium with uranium in TBP/HNO systems has been studied. The extraction mechanism,

has been found to agree well with experimental data at 5 M HNO,.The extraction constant, K_TC,e for this reaction Is given by

where T is temperature (K) and C_T total TBP concentration (%). This equation is valid for 5 M HNO, DSFS–30% TBP, and from 20 to 60°C, with an average error of 2·8% in log K_TC,e     

Solvent Extraction Chemistry and Kinetics of Zirconium

Abstract

The solvent extraction behavior of zirconium in the HN03-tributyl phosphate (TBP) system can be explained based on the existence of four principal aqueous species, Zr⁴⁺, ZrOH³⁺, Zr₃(OH)⁸⁺ ₄, and oxo-polymers. The Zr⁴⁺ and ZrOH³⁺ species are extractable and are in equilibrium with inextractable Zr₃(OH)⁸⁺ ₄. The oxo-polymers are formed by heat, are-inextractable, and are not: in equilibrium with the other species. The aqueous equilibria and their equilibrium quotients have been previously determined. In the present study, these equilibria were used along with both tracer and macro zirconium concentrations (oxo-polymers excluded by extraction and back scrubbing) to determine the distribution equilibrium constants for both the Zr⁴⁺ and ZrOH³⁺ ions. The four equilibrium constants give excellent fits to both tracer and macro-zirconium distribution data.

The concentrations of the extractable zirconium species which are calculated from the equilibria have been used to begin examining the extraction kinetics of zirconium in the HNO₃-TBP system. In relatively concentrated nitric acid, approximately 3 M and greater, Zr⁴⁺ ion predominates, and the rate of extraction of zirconium increases as approximately the second power of the TBP concentration. In low acid (1 M and less) ZrOH³⁺ ion predominates, and the rate of extraction of zirconium increases as approximately the third power of the nitrate concentration. This is in significant contrast with the behavior of uranium, which shows only a small dependence of the extraction rate on TBP concentration, and no dependence on nitrate concentration. This suggests that operation of a kinetic separations system at low TBP and nitrate concentrations will significantly improve separations over those achieved at equilibrium.     

ZIRCONIUM EXTRACTION INTO OCTYL(PHENYL)-N,N- DIISOBUTYLCARBAMOYLMETHYL PHOSPHINE OXIDE AND TRIBUTYL PHOSPHATE†

ABSTRACT

Classical slope analysis techniques were used to determine the octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) and nitrate stoichiometrics for the extraction of zirconium by CMPO diluted with diisopropylbenzene (DIPB). The equilibrium constant for the extraction of zirconium by CMPO was also determined using classical slope analysis techniques. The extraction of zirconium by TBP in n-dodecane was used as a control to verify the zirconium species as Zr⁺⁴, and to verify the experimental methodology. Equilibrium [CMPO]_org and [TBP]_org concentrations were determined by accounting for the extraction of HNO₃ into both TBP and CMPO solvents. Nitric acid dissociation and aqueous phase activity coefficients were also taken into consideration. Organic activity coefficients, Zr⁺⁴ activity coefficients, Zr⁺⁴ hydrolysis, and consumption of TBP or CMPO by water were neglected. Nitrate and CMPO dependencies for the extraction of zirconium have been determined from this work to be:

An equilibrium constant of 1.13 × 10⁵ ± 1.48 × 10⁴ at 25^° C was also determined for this reaction.     

SOLVENT EXTRACTION OF SOME TERVALENT LANTHANOIDS WITH 4-ACYL-3-PHENYL-5-ISOXAZOLONES

ABSTRACT

The solvent extraction of some tervalent lanthanoid ions (Ln³⁺) with five 4-acyl-3-phenyl-5-isoxazolones (AcyPI or HL),i.e. 4-acetyl-, 4-benzoyl-, 4- (4-toluoyl)-, 4- (4-fluorobenzoyl) - and 4-(4-nitrobenzoyl)-3-phenyl-5-isoxazolones (API,BPI, TPI, FBPI and NBPI respectively) into benzene was studied in the presence of tri-n-butylphosphate (TBP). The extracted species were (Ln³⁺) (L^-)₃ (TBP)₂ for AcyPI.     

Extraction relationship of Li+ and H+ using tributyl phosphate in the presence of Fe(III)

ABSTRACT

During the extraction of lithium from high Mg-containing salt lake brines by tributyl phosphate (TBP) in the presence of Fe(III), H⁺ is used to stabilize Fe(III). However, the distribution ratio of H⁺ (D_H) is 4–6 times higher than that of Li⁺ (D_Li), which affects the extraction of Li⁺ significantly. In this study, the competition mechanism between H⁺ and Li⁺ was investigated by spectral analysis and thermodynamic equilibrium. The extracted species are determined as HFeCl₄ · 2TBP and LiFeCl₄ · 2TBP for H⁺ and Li⁺, respectively. The apparent equilibrium constants are K_H = 799.8 and K_Li = 120.6, respectively. Both equilibrium constants and the distribution ratios for H⁺ and Li⁺ extraction show that extraction of H⁺ is stronger than Li⁺.     

Solvent Extraction and Spectrophotometric Studies on Synergistic Extraction of Plutonium(IV) by Mixtures of Thenoyltrifluoroacetone (HTTA) and Tri-n-butylphosphate (TBP) in Benzene

Abstract

The synergistic extraction of Pu(IV) from perchloric acid solutions into mixtures of thenoyltrifluoroacetone (HTTA) and tri-n-butylphosphate (TBP) in benzene was investigated by solvent extraction methods. The adduct responsible for synergism was found to be Pu(TTA)₄·TBP. The adduct formation between Pu(TTA)₄ and TBP in the benzene phase was also investigated by spectrophotometry. The equilibrium constants for the equilibria involved were obtained both by solvent extraction and by spectrophotometric methods.     

Separation of phenylacetic acid using tri-n-butyl phosphate in hexanol: Equilibrium and kinetics

Equilibrium and kinetics studies are required to design the continuous extraction process for the acid-extraction system. In the present study, an attempt has been made to investigate the equilibrium and kinetics parameters for the reactive extraction of phenylacetic acid (PAA) with tri-n-butyl phosphate (TBP) in hexanol. The equilibrium results show that the formation of the (1:1) PAA–TBP complex in the organic phase with an overall equilibrium complexation constant (K_e) was 78.74 and 29.15 m³.kmol^?1 for TBP concentrations of 0.734 and 1.464 kmol.m^?3, respectively. The mass transfer coefficients (k_L) for PAA were found to be in the range of 3.7 × 10^–5–6.2 × 10^–5 m.s^?1. Based on the Hatta number (H_a = 8.48), the reaction was found to be fast chemical reaction (regime 3) with the order of reaction as 0.77 and 0.36 with respect to PAA and TBP, respectively. The rate constant of the reaction was obtained as 0.017 kmol.m^?3.s^?1.     

Recent R&D Studies Related to Coprocessing of Spent Nuclear Fuel Using N,N-Dihexyloctanamide

Abstract

The PUREX process has undergone several modifications to address the issues of high burn up, fewer solvent extraction cycles, and reduced waste arisings. Advanced fuel cycle scenarios have led to a renewed international interest in the development of separation schemes for co-recovering U/Pu from spent fuels. Completely incinerable N,N-dihexyloctanamide (DHOA) has been identified as a promising candidate for the reprocessing of spent fuels. Batch extraction studies were carried out to evaluate DHOA and TBP for the coprocessing (co-extraction and co-stripping) of U and Pu from spent fuel under varying concentrations of nitric acid and of uranium as well as under simulated pressurized heavy water reactor spent fuel feed conditions. At 50 g/L U in 4 M HNO₃, D_Pu values for 1.1 M DHOA and 1.1 M TBP solutions in n-dodecane were 7.9 and 3.8, respectively. In contrast, significantly lower D_Pu value at 0.5 M HNO₃ (4 × 10^?3) for DHOA as compared to TBP (4 × 10^?2) suggested that it was a better choice for coprocessing of spent nuclear fuel. This behavior was attributed to the change in stoichiometry of extracted species at lower acidity vis-a-vis the higher acidity. These studies suggest that plutonium fraction can be enriched with respect to uranium contamination in the product stream. DHOA displays better extraction behavior of plutonium and stripping behavior of uranium under simulated feed conditions. DHOA appears distinctly better than TBP with respect to fission product/structural material decontamination of U/Pu.     

Role of Acetohydroxamic Acid in Selective Extraction of Technetium and Uranium Employing N,N-Dihexyloctanamide as Extractant

Straight chain N,N-dihexyloctanamide (DHOA) has been identified as a promising alternate extractant to tributyl phosphate (TBP) for the reprocessing of uranium based spent fuels. The present work compares extraction behavior of technetium using DHOA and TBP solutions in n-dodecane, under varying experimental conditions such as acidity (0.5–6 M HNO₃); extractant concentration (1.1 and 1.5 M), and uranium loading (50 g/L, relevant for Pu rich spent fuel feed solutions). The effect of acetohydroxamic acid concentration on U, Pu, Np, and Tc extraction behavior has also been investigated. Pu(IV)-AHA interaction and its influence on extraction using TBP and DHOA extractants has been studied spectrophotometrically. The experimental data suggest that 1.1 M DHOA is better than 1.1 M TBP with respect to co-extraction of Tc and U, and U decontamination with respect to Np/Pu.     

Coupled Transport of Zr(IV) through Tri-n-butylphosphate-Xylene-Based Supported Liquid Membranes

Abstract

The transport of Zr(IV) through tri-n-butylphosphate-xylene-based liquid membranes, supported in a polypropylene hydrophobic microporous film, has been studied. The concentration of HNO₃ in the feed solution and tri-n-butylphosphate (TBP) carrier in the membrane were varied, and the flux and permeability coefficients were determined. The optimum conditions found for maximum flux were determined to be 10 mol/dm³ HNO₃ and 2.93 mol/dm³ TBP with a flux value of 12.9 × 10^?6 mol · m^?2 · s^?1. The solvent extraction study revealed that 1.25 to 3.5 protons are involved in zirconium transport, and that two molecules of TBP are involved in the complex formation. The value of protons involved varies with acid concentration. The zirconium ion transport is coupled with nitrate ions transport.     

Extraction of HNO3 and some ions with tributyl phosphate (tbp)-mixed trialkyl phosphine oxides (trpo)/kerosene

Tributyl phosphate (TBP) and trialkyl phosphine oxides (TRPO) are important extractants. They are widely used in industrial extraction processes, especially in the nuclear power industry. However, both TBP and TRPO suffer from several disadvantages. TBP has a low extractability for trivalent transuranium elements such as Am³⁺ and Pu³⁺ while TRPO has low loading capacity for HNO₃ and UO₂ ²⁺. The extraction of HNO₃ and 20 other ions of importance in the nuclear power industry was studied using TBP-TRPO/kerosene. The loading capacity of UO₂ ²⁺ and HNO₃ in TBP-TRPO/kerosene was determined. The synergistic extraction characteristics of the mixture for Am³⁺ and TcO₄ m were studied. The influence of high-concentration UO₂ ²⁺ on the extraction of Am³⁺, Eu³⁺, Pu⁴⁺, and TcO₄ m was investigated. The experimental results show that TBP-TRPO/kerosene mixtures display both a high extractability for a number of ions and a high loading capacity for UO₂ ²⁺ and HNO₃.     

DISTRIBUTION BEHAVIOUR OF U(VI), Th(IV) AND Pa(V) FROM NITRIC ACID MEDIUM USING LINEAR AND BRANCHED CHAIN EXTRACTANTS

ABSTRACT

The extraction behaviour of 1M solutions of tri-2-ethylhexyl phosphate (TEHP), di-2-ethyl hexyl isobutyramide (D2EHIBA), tri-n-butyl phosphate (TBP) and di-n-hexyl hexanamide (DHHA) in n-dodecane towards U(VI), Th(IV) and Pa(V) in the presence of 220 g/L of Th from nitric acid medium has been studied. The limiting organic concentrations (LOC) of thorium (g/L) for 1 M TBP and 1 M DHHA are evaluated as 31, 20 ( at 1 M HNO₃) and 25,13 (at 4 M HNO₃) respectively. The distribution ratio (D) values of U(VI), Th(IV) and Pa(V) in the presence of thorium (220 g/L) at. 1 M HNO₃ suggest that branching in the alky group of amides suppresses the extraction considerably. In view of the selective extraction of U over Th by 5 % TBP in THOREX process at 4 M HNO₃, distribution behaviour is also studied employing a lower concenfration (0·18 M) of extractant for comparison purpose, Separation factor (S. F.) values for U(VI) over Th(IV) under different experimental conditions consistently varied in the order: D2EHIBA > DHHA > TEHP > TBP. The quantitative extraction of ²³³U from a synthetic mixture containing ²³³U (10^?5 M). ²³³Pa (10^?11 M) and thorium (220 g/L) at 1 M HNO₃ using 1 M solution of D2EHIBA in n-dodecane is achieved in three stages, Stripping and reusability studies of D2EHIBA have also been carried out.     

Solvent Extraction and Emulsion Separation in Magnetic Fields

Abstract

In two-phase emulsion separations, it is customary to employ large settling volumes (for mixer-settling apparatus) or large centrifugal forces (for centrifugal contactors). Improvement can sometimes be achieved by using an extractant with magnetic properties in the presence of a variable field. In the work reported in this paper, two different extractants (D-2EHPA and TBP) were employed in magnetic field experiments. These compounds are both stable and resistant to acid (15% H₂SO₄) and basic (NaOH, pH = 10) media. The test results for extraction of Cu²⁺ (with D-2EHPA) and UO₂ ²⁺ (with TBP) from aqueous media were positive. The emulsion separation for these two systems in the presence of a magnetic field was 160 times faster than in the gravitational field alone.     

Synergistic Extraction of Dysprosium and Aggregate Formation in Solvent Extraction Systems Combining TBP and HDBP

During treatment of nuclear fuel in the Plutonium/URanium EXtraction (PUREX) process, the extractant tri-n-butyl phosphate (TBP) is known to degrade to dibutylphosphoric acid (HDBP), which increases the extraction of metal ions, thereby inhibiting their stripping from the organic phase. To better understand this phenomenon, we investigated how mixtures of TBP and HDBP influenced the extraction of metal, nitric acid, and water, and correlated the results to aggregated structures in the organic phase. The mole ratios of TBP-HDBP mixtures had a non-linear effect on the extraction of Dy³⁺ and water from 0.2 M HNO₃, indicating synergism. In 2 M HNO₃, the TBP:HDBP mole ratio had a more linear relation to Dy³⁺ and water extraction, so the synergistic effect was less pronounced than in the low acid system. The extraction of nitric acid showed no synergistic effect and follows closely what would be expected in a system using TBP only. The small-angle X-ray scattering (SAXS) data of the 0.2 M acid system showed maximum contrast at a TBP:HDBP mole ratio of 0.25, so that the synergistic mixture is also the most aggregated at 0.2 M acid. The 2 M acid system also showed that the mixed system is more aggregated than the end members, although this does not result in peak extraction. Previous studies of synergistic extraction of metal cations explain the enhanced extraction by increased dehydration of the metal ion. Although our data do not rule out the formation of mixed complexes according to the classical mechanism of synergism, our evidence of increased water extraction and aggregate formation in systems combining TBP and HDBP are complementary to the metal-centric dehydration aspects of the process. The findings in this study give insights into the complex chemistry of solvent extraction, providing a possible link between formation of aggregates in the organic phase and synergistic extraction.     

Thorium Ions Transport across Tri-n-butyl Phosphate—Benzene Based Supported Liquid Membranes

Abstract

Transport of Th(IV) ions across tri-n-butyl phosphate (TBP) benzene based liquid membranes supported in microporous hydrophobic polypropylene film (MHPF) has been studied. Various parameters such as variation of nitric acid concentration in the feed, TBP concentration in the membrane, and temperature on the given metal ions transport have been investigated. The effects of nitric acid and TBP concentrations on the distribution coefficient were also studied, and the data obtained were used to determine the Th ions—TBP complex diffusion coefficient in the membrane. Permeability coefficients of Th(IV) ions were also determined as a function of the TBP and nitric acid concentrations. The optimal conditions for the transport of Th(IV) ions across the membrane are 6 mol·dm^?3 HNO₃ concentration, 2.188 mol·dm^?3 TBP concentration, and 25°C. The stoichiometry of the chemical species involved in chemical reaction during the transport of Th(IV) ions has also been studied.     

SOLVENT EXTRACTION OF SOME TERVALENT LANTHAN01DS WITH 5-BR0M0-, 3,5-DIBROMO- AND 5-NITROSALICYLALDEHYDE ACETOHYDRAZONES

ABSTRACT

5-Bromo-, 3,5-dibromo- and 5-nitrosalicylaldehyde acetohydrazones (BSAH, DBSAH and NSAH, respectively, or H₂L) were synthesized. The extraction of lanthanoid ions (Ln3+) including praseodymium, europium and ytterbium ions into 1,2-dichloroethane with the synthesized hydrazones in the presence of both tri-n-butylphos-phate (TBP) and perchlorate has been investigated. The extracted species were (Ln³⁺)(HLP^?)₂(TBP)₃(CI₄ ^?) for all the hydrazones. The acid dissociation constants, Ka, and the partition coefficients, K_o, of the hydrazones and the extraction constants, K_ex, of the lanthanoid complexes were determined. The introduction of electron-withdrawing bromo or nitro groups to the 3- and/or 5-position of the benzene ring in salicylaldehyde acetohydrazone (SAH), the mother compound of the synthesized hydrazones, was very effective for increasing not only the K_ex value of SAH but also its K_D value and the K_ex values of its lanthanoid complexes. Taking into consideration the above-mentioned three constants, of the synthesized hydrazones DBSAH was the most balanced and recommend-able extractant for the lanthanoids.     

AKUFVE STUDIES ON EXTRACTION BEHAVIOUR OF NEPTUNIUM FROM SIMULATED HLW SOLUTIONS BY 30% TBP

ABSTRACT

The extraction behaviour of neptunium from 3 M nitric acid as well as simulated pressurised heavy water reactor high level radioactive waste (PHWR-HLW) solution by 30% TBP/dodecane was studied using AKUFVE. Np(IV)/Np(V) was oxidised to Np(VI) using oxidising agents, such as K₂Cr₂O₇, VO₂ ⁺ and NaNO₂. Stripping of neptunium from the loaded TBP phase was studied using reducing agents like hydrogen peroxide, ascorbic acid, hydroxyl-amine hydrochloride and hydrazine sulphate. Results of these extraction and stripping studies have been discussed in this paper.     

CALORIMETRIC STUDIES OF THE SYNERGISTIC EFFECT. THE REACTION OF UO2 2+, Th 4+ AND Nd 3+ TTA COMPLEXES WITH TBP AND TOPO IN BENZENE

Titration calorimetry was used to investigate the thermodynamics (ΔG, ΔH, ΔS) of the addition reactions between M(TTA)_n(H₂O)_m (where M = UO₂ ²⁺, Th⁴⁺, Nd³⁺) and TBP or TOPO in dry and in water saturated benzene solvent. The adduct species formed were UO₂((TTA)₂ S, Th(TTA)₄ S, and Nd(TTA)₃ S_n (n = 1,2).The thermodynamic values complemented by water analysis data, allow evaluation of the relative importance of enthalpic and entropic contributions of ΔG, of the role played by water and of the changes in coordination number which occur during the reactions with the adduct ligands.     

Pyridinedicarboxylic Acid Diamides as Selective Ligands for Extraction and Separation of Trivalent Lanthanides and Actinides: DFT Study

This article presents a general approach to solving the urgent practical problem of separation of 4f-(lanthanides, Ln³⁺) and 5f-elements (actinides, An³⁺) very similar in properties based on the DFT quantum-chemical supercomputer simulation of Ln³⁺ and An³⁺ complexes with polydentate nitrogen-containing heterocyclic ligands. The method allows to calculate the geometry parameters of ligands and complexes and the metal to ligand binding energies with accuracy, permitting a direct comparison of calculation results with the experimental data, and estimate selectivity factors for separation of Eu³⁺/Am³⁺ model pair cations (SF_Am/Eu) in extraction experiments on a semi-quantitative level.

The applicability of the method and the approach demonstrated by DFT-modeling (nonempirical PBE functional, extended relativistic full-electron basis set) of a large series of diamides of pyridine-2,6-dicarboxylic (dipicolinic) acid (L) with different substituents at the amide nitrogen atoms and in the pyridine cycle, as well as their complexes [LM]³⁺, (H₂O)_nM(NO₃)₃ (n = 3, 4), and LM(NO₃)₃ (M = Eu, Am).

Based on the theoretical analysis a new model is proposed that describes the mechanism of Ln³⁺ and An³⁺ extraction in two-phase system highly acidic water solution-organic solvent, according to which the formation of An³⁺ and Ln³⁺ complexes occurs at the water/organic interface as a substitution reaction of hydroxonium ion in a cavity of a protonated ligand for the metal cation.

Calculation results confirm the experimentally established higher extraction ability of dipicolinic acid diamides containing one aryl and one alkyl substituent at the amide nitrogen atoms compared to the N,N,N′,N′-tetraalkyl diamides (“effect of anomalous aryl strengthening”). Based on the simulation results the structure of the modified ligand L suggested that it should ensure maximum An³⁺/Ln³separation selectivity in the series of dipicolinic acid diamides.