Formation Of Third Phase Research Articles

Extraction and third phase formation behavior of Eu(III) IN CMPO–TBP extractants present in room temperature ionic liquid

The extraction behavior of europium(III) from nitric acid medium by a solution of n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) and tri-n-butylphosphate (TBP) in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (bmimNTf2), was studied. The distribution ratio (DEu) of Eu(III) in CMPO–TBP/bmimNTf2 was measured as a function of various parameters such as the concentrations of nitric acid, europium, CMPO, and TBP and temperature. The distribution ratio of europium increases with the increase in the concentration of nitric acid reaches a maximum value around 1M followed by decrease. Loading of europium in CMPO–TBP/bmimNTf2 increases with the increase in Eu(III) concentration without leading to the formation of third phase, which is usually observed in conventional diluents. The stoichiometry of metal–solvate in organic phase varies from 1:3 (Eu:CMPO) at trace levels to 1:1 at higher europium loadings. The viscosity of the TBP–CMPO/bmimNTf2 system at various temperatures and the enthalpy change accompanied by the extraction of europium(III) were determined and reported in this paper.

Third-Phase Formation in the Extraction of Phosphotungstic Acid by TBP in n-Octane

The solvent extraction of 12-phosphotungstic acid, also known as 12-tungstophosphoric acid—H3PW12O40, the so-called Keggin heteropolyacid—by 0.73 M (20%v/v) tri-n-butyl phosphate (TBP) in n-octane under conditions comparable to those used previously for the extraction of conventional inorganic mineral acids is described. A simplified phase diagram for the pentanary system comprised of H3PW12O40, HNO3, H2O, TBP, and n-octane reveals an extremely low initial concentration of H3PW12O40 (1.1 mM) at the LOC (limiting organic concentration) condition, far lower than the most effective third-phase-forming inorganic acid, namely HClO4. The results from small-angle neutron scattering (SANS) indicate that the interparticle attraction energy—U(r) calculated through application of the Baxter sticky sphere model to the SANS data at the LOC condition—does not approach the −2 k B T value associated with phase splitting in previous studies of TBP third-phase formation. The third-phase formation model based on attractive interactions between polar cores of reverse micelles, successfully developed for TBP and other extraction systems does not apply to the extraction of H3PW12O40. Rather, the separation of a third-phase from the TBP organic phase stems from the limited solubility of the heavy and highly polar H3PW12O40–TBP species in the alkane diluent.

SANS Study of HCl Extraction by Selected Neutral Organophosphorus Compounds in n-Octane

The extraction of HCl by tri(2-ethylhexyl) phosphate (TEHP), tri-n-octyl phosphate (TOP), and tri-n-octylphosphine oxide (TOPO) in n-octane was investigated by liquid-liquid distribution of acid and water and small-angle neutron scattering (SANS) measurements. No formation of a heavy organic phase (third phase) was observed with TEHP and TOP under the experimental conditions used, whereas for 0.4 M TOPO the HCl limiting organic concentration (LOC) at 23°C was 0.32 M (with 5.1 M HCl in the equilibrium aqueous phase). For higher HCl concentrations in the aqueous phase, the organic phase splits into a light and a heavy layer. For TEHP and TOP, the SANS results, interpreted using the Baxter model for hard spheres with surface adhesion, indicated the formation of only small reverse micelles with little intermicellar attraction. For TOPO, the scattering signals suggested the formation of much larger and strongly interacting micelles. The critical values of the stickiness parameter, τ−1, and the interaction potential energy, U(r), for the LOC sample in the TOPO system were consistent with the model for third-phase formation previously developed for tri-n-butyl phosphate (TBP). According to this model, organic phase splitting is due to van der Waals interactions between the polar cores of reverse micelles formed by the extractants in the organic phase.

Comparative Studies on Third-Phase Formation in the Extraction of Thorium Nitrate by Tri-n-Butyl Phosphate and Tri-n-Amyl Phosphate in Straight Chain Alkane Diluents

This paper reports the results of the studies on third-phase formation during the extraction of thorium nitrate from solutions with near-zero free acidity by 1.1 M solutions of tri-n-butyl phosphate (TBP) and tri-n-amyl phosphate (TAP) in n-octane, n-decane, n-dodecane, n-tetradecane, and n-hexadecane. The concentration of Th(NO3)4 in the organic phase in biphasic regions and the distribution of Th(NO3)4 between the diluent-rich phase (DP) and the third phase (TP) in triphasic regions for the extraction from its solution with near-zero free acidity by the aforesaid solvents has been investigated as a function of equilibrium aqueous-phase thorium nitrate concentration at 303 K. Densities of organic phases in the biphasic and triphasic regions have been measured as a function of equilibrium aqueous-phase thorium nitrate concentration at 303 K. Also discussed are the variations of the ratios of the volume of the diluent-rich phase to that of the third phase as a function of the equilibrium aqueous-phase Th(NO3)4 concentration and chain length of the diluent in the extraction of Th(IV) from its solutions with near-zero free acidity by the above solvents at 303 K.

Development of T2EHDGA Based Process for Actinide Partitioning. Part I: Batch Studies for Process Optimization

N,N,N′,N′-tetra-2-ethylhexyl diglycolamide (T2EHDGA) has been used for the extraction of actinides, lanthanides, fission products, and structural elements from Pressurized Heavy Water Reactor High Level Waste (PHWR-HLW). In view of third-phase formation under loading conditions, iso-decanol, tri-n-butyl phosphate (TBP), and N,N-di-n-hexyl octanamide (DHOA) were evaluated as the phase modifiers along with T2EHDGA. The loading of lanthanides (e.g. Nd(III), used as surrogate for trivalent minor actinides, Am(III) and Cm(III) and rare earth elements) in the organic phase sharply increased with the addition of iso-decanol as compared to other modifiers (TBP and DHOA). A minimum concentration of phase modifiers needed to avoid third-phase formation for 0.1 M T2EHDGA was 5% iso-decanol, 20% TBP or 20% DHOA. The most promising system, viz. 0.1 M T2EHDGA and 5% (v/v) iso-decanol was evaluated as extractant for actinide partitioning. The distribution behavior of various metal ions, viz. Am, Pu, U, Eu, Sr, Pd, Cs, Tc, Fe, and Mo has been studied from nitric acid as well as from synthetic PHWR-HLW using the optimized organic phase. Extraction of Am (tracer) from synthetic PHWR-HLW suggested a quantitative recovery of minor actinides in four contacts, while stripping (with 0.01 M HNO3) was quantitative in two contacts at O/A = 1. The testing of the optimized extraction system in mixer-settlers with simulated HLW is in progress.

Studies on the Use of N,N,N´,N´-Tetra(2-ethylhexyl) Diglycolamide (TEHDGA) for Actinide Partitioning. I: Investigation on Third-Phase Formation and Extraction Behavior

Diglycolamides have emerged as an interesting class of extractants for actinide partitioning from high-level waste (HLW). N,N,N´,N´-tetraoctyl diglycolamide (TODGA) has been extensively studied for lanthanide-actinide co-extraction behavior. The present work deals with a branched isomer of TODGA, that is, N,N,N´,N´-tetra(2-ethylhexyl) diglycolamide (TEHDGA). TEHDGA was studied for the extraction of 241Am and third-phase formation. The effect of using different phase modifiers on the prevention of the formation of a third phase during nitric acid extraction by TEHDGA along with the acid uptake behavior by TEHDGA in the presence of the modifiers was studied. The modifiers used for this purpose were di(n-hexyl)octanamide (DHOA), isodecanol, and n-decanol. The effect of the modifiers on the uptake of 241Am as a function of acid concentration and as a function of modifier concentration was also examined. DHOA was found to be a suitable modifier, in spite of its high acid uptake. The uptake of lanthanides Ce, La, Eu, Gd, and Nd and elements such as Fe, Ni, Mn, Mo, Ru, Sr, and Cs with DHOA-modified TEHDGA–n-dodecane solvent systems were investigated. The results obtained indicated that, while DHOA-modified TEHDGA/n-dodecane extracted lanthanides and actinides, it did not show any significant uptake of other elements. Thus, the TEHDGA-DHOA/n-dodecane solvent system can be used effectively for the partitioning of lanthanides and actinides from HLW.

Studies on the Extraction of Actinides by Dibutylbutyl Phosphonate

Dibutylbutyl phosphonate (DBBP) was synthesized by the Michaelis Becker reaction, and was characterized by elemental analysis, IR, and 31P NMR. The extraction of U(VI), Th(IV), Pu(IV), Fe(III), and Am(III) by 1.1 M DBBP in n-dodecane as a function of nitric acid concentration was studied, and the results are compared with the extraction behavior of these ions with tributyl phosphate (TBP) and diamylamyl phosphonate (DAAP) in n-dodecane. The physical properties of the extractant such as the density, surface tension, viscosity, aqueous solubility, and phase-disengagement time with 1.1 M DBBP/n-dodecane have been measured and compared with those of 1.1 M TBP/n-dodecane and 1.1 M DAAP/n-dodecane. Studies on the third-phase formation behavior of 1.1 M DBBP/n-dodecane with U(VI) and Th(IV) nitrates in nitric acid medium have been carried out, and the results are compared with the TBP and DAAP systems. Studies were also carried out to study the U-Fe separation by DBBP impregnated on XAD-4 resin.

Third-phase formation in the extraction of thorium nitrate by N,N-dihexyloctanamide

N,N-dihexyloctanamide (DHOA) is an alternative candidate to tri-n-butylphosphate (TBP) for the reprocessing of spent nuclear fuels including those based on thorium. This paper reports the thirdphase formation behavior of Th(IV) with varying phase modifiers, diluents, extractant/nitrate ion concentration and temperature using DHOA/n-dodecane as solvent. The Th(IV)-LOC (limiting organic concentration) values increased with increasing concentration of the alcohols (modifiers) and that of DHOA in the organic phase. No third phase was observed when diethyl benzene (DEB) and decahydronaphthalene (decalin) were used as diluents. There was a sharp increase in Th(IV)-LOC value from 39.6 g/L (1M HNO3) to 48.6 g/L (1M HNO3 + 1M NaNO3) beyond which a saturation behavior was observed for Th(IV)-DHOA/n-dodecane system. The Th(IV)–LOC values increased with temperature but decreased with the aqueous phase acidity.

Extraction of U(VI) by Tri-n-Amyl Phosphate Under High Solvent Loading Conditions

Tri-n-butyl phosphate (TBP) is being employed as the extractant for the processing of nuclear materials all over the world. However, some of its limitations such as third-phase formation, aqueous solubility, chemical and radiation degradation, etc., affect the performance of TBP-based extraction processes. Hence, there is a need to identify alternative extractants that do not possess the disadvantages of TBP but retain the desirable properties of TBP. In this connection, higher homologues of TBP such as tri-n-amyl phosphate (TAP) and some of its isomers are considered to be important for various solvent extraction processes. Batch extraction studies have been carried out on the extraction of U(VI) by 1.1 M solutions of TBP and TAP in n-dodecane as well as heavy normal paraffin (HNP), and the results are reported in this paper. Extraction of U(VI) by 1.1 M TAP/HNP under high solvent loading conditions and subsequent stripping of U(VI) by 0.01 M HNO3 from loaded 1.1 M TAP/HNP in a countercurrent mode were also carried out with an ejector mixer-settler. This paper describes the results of these mixer-settler runs. Results revealed the suitability of higher homologues of TBP for reprocessing applications.

Small angle neutron scattering study of U(VI) third phase formation in HNO3/DHDECMP-n-dodecane system

Small angle neutron scattering studies (SANS) were carried out to understand the formation of third phase in DHDECMP-dodecane-UO2(NO3)2/HNO3 system. It was observed that third phase formation takes place due to the formation of UO2(NO3)2.DHDECMP reverse micelles in the dodecane phase. SANS data obtained were interpreted with particle interaction model using Baxter sticky spheres model.

Application of the Baxter Model for Hard Spheres with Surface Adhesion to SANS Data for the U(VI)−HNO3, TBP−n-Dodecane System

Small-angle neutron scattering (SANS) data for the tri-n-butyl phosphate (TBP)−n-dodecane, HNO3−UO2(NO3)2 solvent extraction system have been interpreted using the Baxter model for hard spheres with surface adhesion. The increase in the scattering intensity in the low Q range observed when increasing amounts of HNO3 or UO2(NO3)2 are transferred into the organic phase has been interpreted as arising from interactions between solute particles. The SANS data have been reproduced using a 12−16 Å diameter of the hard sphere, dhs, and a 5.6kBT−7.1kBT stickiness parameter, τ-1. When in contact with an aqueous phase, TBP in n-dodecane forms small reverse micelles containing three TBP molecules. Upon extraction of water, HNO3, and UO2(NO3)2, the swollen micelles interact through attractive forces between their polar cores with a potential energy of about 2kBT and an effective Hamaker constant of about 4kBT. The intermicellar attraction, under suitable conditions, leads to third-phase formation. Upon phase splitting, most of the solutes in the original organic phase (water, TBP, HNO3, and UO2(NO3)2) separate in a continuous phase containing interspersed layers of n-dodecane.

Sulfuric Acid Extraction with TrialkylamineEffect of Xylene and n-Octanol as Modifiers

The equilibrium of sulfuric acid extraction with trialkylamine was measured at 25 °C. Xylene, n-octanol, and their mixtures with a paraffinic solvent were used as diluents. The effects of the diluent composition and of the amine content in the organic phase on extraction equilibrium were investigated. An increase of the xylene content in the diluent shifted the limit of third-phase formation to higher concentrations of acid in the organic phase. No third-phase formation was observed with pure xylene. With n-octanol, the third phase formed only at the highest amine and lowest modifier contents used. The shapes of the amine loading curves indicated preferential formation of the 1:2 acid/amine complex at lower acid concentrations and of the 1:1 complex and its dimer (2:2) in the high-concentration range. At lower acid contents, a positive synergetic effect of the two modifiers on the equilibrium was observed. In systems with n-octanol, the physical extraction with the modifier contributed to the overall acid extraction. The experimental results were correlated with a mathematical model incorporating chemical reactions of acid/ amine complex formation and nonspecific interactions of species present in the organic phase.

USE OF A MIXTURE OF TRPO AND TBP FOR THE PARTITIONING OF ACTINIDES FROM HIGH-LEVEL WASTE SOLUTIONS OF PUREX ORIGIN AND ITS COMPARISON WITH CMPO AND OTHER PHOSPHORUS-BASED EXTRACTANTS

The partitioning of long-lived alpha-emitting actinides, present in high-level radioactive waste (HLW) solutions from PUREX reprocessing of spent nuclear fuels, has been carried out using a commercially available, reasonably inexpensive, trialkylphosphine oxide (TRPO), Cyanex®-923 alone or its mixture with tri-n-butylphosphate (TBP) in dodecane. The studies have been carried out from nitric acid as well as from three types of synthetic HLW solutions. With 30% TRPO alone, third-phase formation was observed when HLW in 3.0 M HNO3 was taken as such. With the mixture of TRPO and TBP, it has been shown that the HLW need not be diluted 10 times, but only the acidity has to be reduced from ∼3 to ∼1 M (without substantial dilution) to achieve an efficient separation of the minor actinides. The results thus obtained using TRPO has been compared with those already reported while using dihexyl-N,N-diethylcarbamoylmethyl phosphonate, diisodecyl phosphoric acid, and octyl(phenyl)N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO). The mixture of 0.2 M CMPO and 1.2 M TBP in dodecane (TRUEX solvent) seems to be a superior extractant for the partitioning of minor actinides from HLW solutions.

Transient- and Steady-State Concentration Profiles in a DIAMEX-like Countercurrent Process for An(III) + Ln(III) Separation

A new version of the DIAMEX (DIAMide EXtraction) process is reported to separate trivalent Am,Cm (An) and lanthanides (Ln) from a PUREX (Plutonium Uranium Refining by EXtraction by tributyl phosphate) process raffinate at a high aqueous/extractant flow-rate ratio while taking care of prevent third-phase formation. The process has been modeled by a computer program and tested on a 16-stage mixer/settler battery to perform extraction, scrubbing, and stripping to obtain an Am (representing the mixture of Am + Cm) decontamination factor (feed/raffinate) higher than 103 and a Mo + Zr fission product content in the product lower than 1 wt% of that of Am. Transient and steady-state concentration profiles of the solutes have been calculated and experimentally determined, and good agreement was generally observed.

An Attempt to Theoretically Predict Third-Phase Formation in the Dimethyldibutyltetradecylmalonamide (DMDBTDMA)/Dodecane/Water/Nitric Acid Extraction System

ABSTRACT The formation of a third phase in solvent extraction (due to splitting of the organic phase into two layers) often occurs when the aqueous phase is highly concentrated in acids. This has been reported with the extraction system dimethyldibutyltetradecylmalonamide (DMDBTDMA)/n-dodecane/water/nitric acid, both in the presence and absence of metal ions. Whereas many experimental efforts have been made to investigate the effects of different parameters on third-phase formation, very few attempts have been made to predict this phenomenon on theoretical grounds. Because the part played by aggregation of the extractant molecules is recognized, we propose a new predictive approach based on the use of the Flory-Huggins theory of polymer solutions, which had been successfully applied for the prediction of phase separation phenomena in nonionic surfactant solutions. We show that this model can provide an excellent prediction of the demixing curve (in the absence of metal ions) when establishing the relation between the interaction parameter ×12 calculated from this theory and the nitric acid content of the aqueous phase. Apparent values of the solubility parameter δ2 of the diamide extractant at different acid loadings have been calculated, from which the effect of the nature of the diluent can also be very nicely predicted.

An Attempt to Theoretically Predict Third-Phase Formation in the Dimethyldibutyltetradecylmalonamide (DMDBTDMA)/Dodecane/Water/Nitric Acid Extraction System

ABSTRACT The formation of a third phase in solvent extraction (due to splitting of the organic phase into two layers) often occurs when the aqueous phase is highly concentrated in acids. This has been reported with the extraction system dimethyldibutyltetradecylmalonamide (DMDBTDMA)/n-dodecane/water/nitric acid, both in the presence and absence of metal ions. Whereas many experimental efforts have been made to investigate the effects of different parameters on third-phase formation, very few attempts have been made to predict this phenomenon on theoretical grounds. Because the part played by aggregation of the extractant molecules is recognized, we propose a new predictive approach based on the use of the Flory-Huggins theory of polymer solutions, which had been successfully applied for the prediction of phase separation phenomena in nonionic surfactant solutions. We show that this model can provide an excellent prediction of the demixing curve (in the absence of metal ions) when establishing the relation...

A SETCHENOW TYPE MODEL FOR Pu(lV) THIRD PHASE FORMATION IN NITRIC ACID - BIS(2-ETHYLHEXYL) SULPHOXIDE/DODECANE BIPHASIC SYSTEM AT 298.15 K

ABSTRACT Formation of third phase (second organic phase) due to limiting solubility of tetravalent actinide solvates in organic solvent has been studied in a qualitative manner by several researchers. Quantitative relations for interactions among various solutes for second organic phase formation were not reported in open literature. In this contribution, an empirical model for Pu(IV) third phase formation in biphasic Pu(NO3)4-H2O-HNO3-0.4 M BESO/dodecane system at 298.15 K is reported. This model is similar to Setchenow model for salting in/out. The reported model could accommodate effect of inextractable nitrates and diluent modifiers on the limiting organic concentration of Pu(IV).

Extraction of citric acid with tertiary amine—third-phase formation

The conditions of third-phase formation in the system made up of aqueous solution of citric acid-solution of trialkylamine in a binary diluent were studied. As binary diluents mixtures of octanol-1 or chloroform with n-heptane were used. The experiments were carried out in water-free systems, at 25°C under the limiting condition of amine fully loaded with acid, thus simulating extraction form highly concentrated aqueous phase. The data could be presented as pseudo-ternary equilibria and were correlated using the extended method, devised by Rod ( Chem. Engng J. 1976, pp. 105–110).

Amides as Phase Modifiers for N,N′-Tetraalkylmalonamide Extraction of Actinides and Lanthanides from Nitric Acid Solutions

The N,N′-tetraalkylmalonamides are a class of compounds under development for transuranic (TRU) separations under high nitric acid conditions. There are several issues that challenge the further development of these ligands. One is the development of improved synthetic procedures that lend themselves to commercial scale-up. Another major issue is the third-phase formation that occurs when the N,N′-tetraalkylmalonamides are contacted with medium-to-high nitric acid concentrations in hydrocarbon solvents. To address the synthesis issue we have developed a new synthetic approach for preparing these materials. Third-phase formation can be eliminated by addition of diluent modifiers such as tributylphosphate (TBP). TBP is inappropriate if a nonphosphate-containing process stream is required. Amides have been proposed as alternatives for TBP in a variety of applications because of their ease of synthesis and the variety of substituents that can be generated. We have been able to develop an amide phase-modified system that extends the working process range of alkylmalonamides (0.5 M) in dodecane (unbranched hydrocarbon) from 3.5 M to 7.5 M nitric acid and in Isopar H (branched hydrocarbon) from 4.0 M to 10.0 M nitric acid using 1.0 M di-2-ethylhexylacetamide/0.5 M alkylmalonamide. The Kd values were comparable to extraction with alkylmalonamide in Isopar H or hydrogenated tetrapropylene (TPH) solvents. The overall extraction system was more robust than the phase-unmodified system allowing for greater temperature and acid concentration fluctuations without third-phase formation.

A REVIEW OF THIRD PHASE FORMATION IN EXTRACTION OF ACTINIDES BY NEUTRAL ORGANOPHOSPHORUS EXTRACTANTS

ABSTRACT Data are reviewed on the formation of third phase in the extraction of acti-nide(IV.VI) nitrates by neutral organophosphorus extractants, mainly tributyl phosphate. The data are critically evaluated and the effect of variables on the third phase formation is discussed. The variables are the concentrations of nitric acid and the extractant, temperature, the nature of diluent, addition of modifiers and the ionic strength of the aqueous phase. Also discussed are systems involving two extracted actinide ions.