Formation Of Third Phase Research Articles

An Advanced Solvent for the Caustic-Side Solvent Extraction of Cesium from Nuclear Waste: Comparing Lipophilic Guanidines for Improved Hydrolytic Stability

ABSTRACT An advanced solvent has been developed for application in the extraction of cesium from alkaline nuclear waste. Four lipophilic guanidines have been compared to arrive at a more hydrolysis-resistant suppressor component of the solvent. Plans call for deployment of the Next-Generation Caustic Side-Solvent Extraction (NG-CSSX) process at industrial scale for separation of radioactive Cs-137 from the legacy salt waste stored at the US Department of Energy Savannah River Site (SRS). In the solvent used in NG-CSSX, an alkyl guanidine “suppressor” component facilitates efficient stripping of the loaded cesium from the solvent. However, as typical of guanidine compounds, the suppressor previously used in the NG-CSSX solvent, N,N‘,N’’-tri(3,7-dimethyloctyl)guanidine (TiDG), suffers from hydrolytic degradation under process conditions. Recently, the sterically hindered alkyl guanidine N,N’-dicyclohexyl-N’’-(10-nonadecyl)guanidine (DCNDG) has been found to offer 8 to 44 times greater hydrolysis resistance than TiDG. Here, the process performance characteristics of the NG-CSSX solvent incorporating DCNDG are determined in comparison with TiDG and two other guanidine suppressors. The solvent has been adjusted in density to accommodate use in the SRS Salt Waste Processing Facility and tested under aggressive bench-scale conditions intended to increase plant throughput. Experiments include the effect of ageing at normal and off-normal temperatures of the distribution of Cs+ through the NG-CSSX process, the fate and effects of guanidine degradation products, suppressor capacity, coalescence times for aqueous-solvent dispersions, tendency for emulsification, third-phase formation, and degree of protonation of the guanidine in stripping. While the chosen structures of the four compared guanidines mainly effect differences in stability, subtle differences in other system properties such as emulsion formation and suppressor capacity provide insight into the function of this important solvent component. In comparison with TiDG and two other guanidines, DCNDG provides greatly increased stability while not compromising the excellent functional properties of the NG-CSSX solvent.

Insight into the interaction between zirconium dibutyl phosphate complex and diluents: An experimental and computational study

In solvent extraction processes, there are challenges caused by the formation of the third phase or interfacial precipitation. The branched alkanes have been used as diluents to disrupt the third-phase formation, while there is little study about the effect of diluents on the appearance of precipitation. Here, we report the inhibiting effect of the novel hyperbranched alkane diluent (HAD) on the formation of zirconium-dibutyl phosphate (Zr-DBP) precipitation and investigate the formation mechanism by the density functional theory (DFT) theory. The extraction experiments show that HAD as the diluent has a remarkable inhibiting ability to form Zr-DBP precipitation, compared with n-dodecane and kerosene. The spectra show no noticeable difference in the composition and structures of the precipitations formed in the three diluents. The energetics from the DFT calculations suggest the P-OH and P=O ligands on one HDBP molecule can bridge two Zr-DBP complexes into a dimer and then aggregate into the precipitation. Besides, the interactions between diluent-diluent molecules play a significant role in precipitation formation. The straight-chain n-dodecanes have a strong intermolecular dispersion force and tend to attract each other but not the Zr-DBP complex. However, the branched alkanes with weak intermolecular interactions have a better dispersive capacity for Zr-DBP complexes, due to the much higher dispersion force of diluent-complex interaction than that of the diluent-diluent interaction. This work provides a new insight into the formation of interfacial crud from the interactions between solutes and diluents of the organic phase in solvent extraction.

Solubility of di-(2-ethylhexyl)phosphoric acid (D2EHPA) in aqueous electrolyte solutions: Studies relevant to liquid-liquid extraction

Di-(2-ethylhexyl)phosphoric acid (D2EHPA) is an acidic extractant frequently used in solvent extraction for metal refining. Only limited information is available on the amount and type of impurities present in commercially available D2EHPA and their effect on liquid-liquid extraction. In this paper, these impurities were identified and the importance of pre-conditioning the extractant before carrying out extractions is demonstrated. Furthermore, the solubility of D2EHPA in aqueous solutions was investigated as a function of pH, concentration of extractant and anion present in the aqueous phase (chloride, bromide, iodide, sulfate, methanesulfonate, thiocyanate, and nitrate). The density of the organic phases and the sodium and water uptake in the organic phase were determined as a function of the different anions present in the aqueous phase. The type of anion was found not to have a large influence on the solubility of D2EHPA in the aqueous phases. Low concentrations of salt in the aqueous phase or high pH values resulted in significant losses of D2EHPA to the aqueous phase. A very sharp increase in water content in the organic phase was observed around pH 5 (pH 6 for the nitrate system), accompanied by a significant increase in extraction of sodium ions, which explains the observed increase in density of the organic phase, and the tendency to third-phase formation. The results from this fundamental study help to better understand the behavior of liquid-liquid extraction systems in which D2EHPA is used as extractant and can help to prevent third-phase formation and losses of extractant to the aqueous phase. Moreover, the impurities present in commercially available D2EHPA and their impact on liquid-liquid extraction processes are determined.

Aggregation Behavior of Nitrilotriacetamide (NTAmide) Ligands in Thorium(IV) Extraction from Acidic Medium: Small-Angle Neutron Scattering, Fourier Transform Infrared, and Theoretical Studies.

Two tripodal amides obtained from nitrilotriacetic acid with n-butyl and n-octyl alkyl chains (HBNTA(LI) and HONTA(LII), respectively) were studied for the extraction of Th(IV) ions from nitric acid medium. The effect of the diluent medium, i.e., n-dodecane alone and a mixture of n-dodecane and 1-decanol, onto aggregate formation were investigated using small angle neutron scattering (SANS) studies. In addition, the influence of the ligand structure, nitric acid, and Th(IV) loading onto ligand aggregation and third-phase formation tendency was discussed.The LI/LII exist as monomers (aggregarte radius for LI: 6.0 Å; LII:7.4 Å) in the presence of 1-decanol, whereas LII forms dimers (aggregarte radius for LII:9.3 Å; LI does not dissolve in n-dodecane) in the absence of 1-decanol. The aggregation number increases for both the ligands after HNO3 and Th(IV) loading. The maximum organic concentration (0.050 ± 0.004 M) of Th(IV) was reached without third-phase formation for 0.1 M LI/LII dissolved in 20% isodecanol +80% n-dodecane. The interaction of 1-decanol with LII and HNO3/Th(IV) with amidic oxygens of LI/LII results in shift of carbonyl stretching frequency, as shown by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) studies. The structural and bonding information of the Th-LI/LII complex were derived from the density functional theoretical (DFT) studies. The molecular dynamics (MD) simulations suggested that the aggregation behavior of the ligand in the present system is governed by the population of hydrogen bonds by phase modifier around the ligand molecules. Although the theoretical studies suggested higher Gibbs free energy of complexation for Th4+ ions with LI than LII, the extraction was found to be higher with the latter, possibly due to the higher lipophilicity and solubility of the Th-LII aggregate in the nonpolar media.

Extraction of Uranium in Nitric Media with Novel Asymmetric Tetra-Alkylcarbamide.

The use of tetra-alkylcarbamides as novel ligands: N,N-butyl-N’,N’-hexylurea (L1: ABHU), and N,N-butyl-N’,N’-pentylurea (L2: ABPU), for the solvent extraction and complexation behaviors of uranium(VI) was synthesized and investigated in this study. The effects of HNO3 and NO3− concentrations in the aqueous phase on the distribution ratio of U(VI) were examined. Under 5 mol/L HNO3 concentration, DU reached 5.02 and 4.94 respectively without third-phase formation. During the extraction, slope measurements and IR spectral analysis revealed that the U(VI) complexes are a form of UO2(NO3)2·2L for both ligands. In addition, thermodynamic studies showed that the uranium extraction reaction was a spontaneous exothermic reaction. The deep structural analysis of the complexes was realized with DFT calculation. The bond length, bond properties, and topology of the complexes were discussed in detail to analyze the extraction behavior. This study enriches the coordination chemistry of U(VI) by tetra-alkylcarbamides, which may offer new clues for the design and synthesis of novel ligands for the separation, enrichment, and recovery of uranium in the nuclear fuel cycle.

Solidification and grain formation in alloys: a 2D application of the grand-potential-based phase-field approach

Solidification is a significant step in the forming of crystalline structures during various manufacturing and material processing techniques. Solidification characteristics and the microstructures formed during the process dictate the properties and performance of the materials. Hence, understanding how the process conditions relate to various microstructure formations is paramount. In this work, a grand-potential-based multi-phase, multi-component, multi-order-parameter phase-field model is used to demonstrate the solidification of alloys in 2D. This model has several key advantages over other multi-phase models such as it decouples the bulk energy from the interfacial energy, removes the constraints for the phase concentration variable, and prevents spurious third-phase formation at the two phase interfaces. Here, the model is implemented in a finite-element-based phase-field modeling code. The role of various modeling parameters in governing the solidification rate and the shape of the solidified structure is evaluated. It is demonstrated that the process conditions such as temperature gradient, thermal diffusion, cooling rate, etc, influence the solidification characteristics by altering the level of undercooling. Furthermore, the capability of the model to capture directional solidification and polycrystalline structure formation exhibiting various grain shapes is illustrated. In both these cases, the process conditions have been related to the growth rate and associated shape of the dendritic structure. This work serves as a stepping stone towards resolving the larger problem of understanding the process–structure–property–performance correlation in solidified materials.

Diluent effects on the stability range of w/o micellar systems and microemulsions made with anionic extractants

Here we present a series of complete phase prisms for water, an organic diluent and di-(2-ethylhexyl) phosphoric acid (HDEHP), one of the most widely used double-branched lipophilic surfactants in hydrometallurgy. Partial or total titration with sodium hydroxide evidence that the mole fraction of the counter-cation “Z” is the variable that controls the packing and spontaneous curvature of the curved film formed by this extractant. Penetrating solvents such as toluene and iso-octane and the non-penetrating solvent dodecane as well as common hydrotropes acting as co-solvents, are considered. The three classical cuts of the phase prism are shown. The regions for which liquid–liquid extraction is possible are determined, as well as the location of the liquid crystals at the origin of the often observed third-phase formation. It is shown that profoundly different trends are obtained when replacing the common solvents currently used in hydrometallurgical processes with hydrotropes.

Hydrofluorocarbon Diluent for CMPO Without Third Phase Formation: Extraction of Uranium(VI) and Lanthanide(III) Ions

ABSTRACT The extraction behaviors of trivalent lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and hexavalent uranium with octylphenyl(,-diisobutylcarbamoylmethyl)phosphine oxide (CMPO) from a nitric acid medium were studied using hydrofluorocarbon (1,1,1,2,2,3,4,5,5,5- decafluoropentane: HFC-43) as a nonflammable diluent. It was revealed that HFC-43 exhibits high solubility of CMPO even in the absence of tributylphosphate (TBP), which has typically been necessary to prevent third-phase formation in the conventional TRUEX process using -dodecane as a diluent. Distribution ratios have been studied as a function of aqueous HNO concentrations, CMPO concentrations, and ionic radii of lanthanide(III) ions. The stoichiometry of the extracted coordination complex was examined based on the slope analysis, and it was determined that two to three CMPO molecules are coordinated to one lanthanide or uranium ion. The feasibility of the process is indicated by the distribution ratio, which is comparable to the conventional system using -dodecane as diluent. In addition, the safety benefits of applying the nonflammable solvent HFC to the TRUEX process are also presented.

The Coordination Chemistry and Stoichiometry of Extracted Diglycolamide Complexes of Lanthanides in Extraction Chromatography Materials

ABSTRACT Industrial rare earth element (REE) separations predominantly utilize solvent extraction processes tailored toward conventional resources such as bastnäsite, monazite, and ion adsorption clays. Advances in diglycolamide (DGA) chemistry have shown effective extraction characteristics for REE separations. However, limitations associated with traditional DGA solvent extraction techniques, such as third-phase formation and gelling, have hindered commercial viability. By supporting DGA extractants on porous resins such as polystyrene divinyl benzene (PS-DVB), the desirable combination of solvent extraction selectivity and ease of operation of sorbent columns can be achieved. To design a low-cost model for such solid-supported DGAs, extraction characteristics as influenced by the underlying coordination chemistry must be explored to achieve efficient functional systems. Within this study, we report novel DGA resin materials, each incorporating one of the DGAs N,N,N’,N’-tetra-(1-octyl)-3-oxapentane-1,5-diamide (TODGA), N,N′-dimethyl-N,N′-dioctyl-3-oxapentane-1,5-diamide (DMDODGA), and 2,2ʹ-oxybis(1-(3-(((2-ethylhexyl)thio)methyl)-4-methylpyrrolidin-1-yl)ethan-1-one) (DEHPDGA). The affinity of DGAs across the lanthanide (Ln) series was evaluated for both hydrochloric acid and nitric acid media with varying Ln feed concentrations to study distribution ratios and loading characteristics. Focusing on dysprosium, extended X-Ray Absorption Fine Structure (EXAFS) and density functional theory (DFT) calculations were also utilized to explore coordination chemistry and their effects on ligand performance. The general trend for both acid media resulted in DMDODGA having the highest extraction strength of all three DGAs at varying acid concentrations. Coordination-chemistry analysis supported by loading data, DFT calculations, and EXAFS results under forced loading conditions posited less than the expected 3:1 ligand-to-metal coordination.

Studies on the aggregation behavior of mineral acid and Zr(IV) loaded Tris(2-methylbutyl) phosphate and tri-n-alkyl phosphate systems using small angle neutron scattering

ABSTRACT The third-phase phenomenon has been observed during the extraction of certain metal ions and mineral acids by 30% (v/v) tri-n-butyl phosphate (TBP) in n-dodecane in the solvent extraction stage of spent nuclear fuel reprocessing. At molecular scale, it has been predicted that the self-organization of the extracted polar species to form reverse micelle-like structure is primarily responsible for third-phase formation. In this regard, the aggregation behavior of solutions of three different trialkyl phosphates in n-alkanes loaded with monoprotic mineral acids (HClO4, HCl and HNO3) as well as tetravalent Th and Zr has been evaluated using Small Angle Neutron Scattering (SANS). The aggregation of TBP has been compared with two of its five carbon analogues i.e. tri-n-amyl phosphate (TAP) and tris(2-methylbutyl) phosphate (T2MBP), which have gained its attention in the course of development of an alternate extractant for the reprocessing process. The effects of trialkyl phosphate structure, diluent chain length and extractant concentration on the aggregation have been emphasized. The contribution of the polar (anionic and cationic component) as well as the apolar part of the reverse micelle toward the phase splitting behavior has been analyzed. The SANS results are in agreement with the earlier reported solvent extraction data.

Liquid–Liquid Extraction of Actinides from Nitric Acid Feeds Using Two Hexa-n-alkylnitrilotriacetamides

ABSTRACT Solutions of two nitrilotriacetamide (NTA) ligands, viz., N,N,N’,N’,N”,N”-hexa-n-butylnitrilotriacetamide (HBNTA) and N,N,N’,N’,N”,N”-hexa-n-hexylnitrilotriacetamide (HHNTA) in 20% isodecanol–80% n-dodecane were employed for the extraction of U(VI), Np(IV), Pu(IV), and Am(III) from nitric acid feed solutions. Limitations of ligand solubility and third-phase formation when contacted with nitric acid solutions led to the choice of the diluent composition of 20% isodecanol and 80% n-dodecane. The results of the solvent extraction studies indicated poor extraction of the metal ions with HBNTA as compared to that with HHNTA as the extractant and hence, major part of this work involved HHNTA. Slope analysis suggested extraction of 1:1 (ML) species with Np(IV) and Pu(IV) at 3 M HNO3 and mixed 1:2 (ML2) and 1:3 (ML3) species with U(VI) at pH 2. At 0.5 M HNO3, the observed trend for extraction was Pu(IV) > Np(IV) >> Am(III) > U(VI).

Third phase formation behaviour of tris(2-methylbutyl) phosphate and tri-n-alkyl phosphates in the extraction of mineral acids and tetravalent metal ions

Abstract The formation of third phase is a detrimental phenomenon witnessed in the nuclear separation processes during the extraction of certain metal salts by an organic solution, which comprises of a neutral extractant, tri-n-butyl phosphate dispersed in a non-polar diluent, n-dodecane (n-C12H26). In the present work, a comparative analysis on the third phase formation behaviour of three trialkyl phosphates, TBP, its linear and branched higher homologues tri-n-amyl phosphate and tris(2-methylbutyl) phosphate (T2MBP), respectively, have been provided which will be useful for the identification of an extractant with minimum third phase formation tendency. The organic phase splitting behaviour during the extraction of three mineral acids (HClO4, HCl and HNO3) and two tetravalent metal nitrates (Th(IV) and Zr(IV)) by trialkyl phosphates has been investigated to understand the influence of anionic and cationic components, respectively, in third phase formation of trialkyl phosphates. The effect of structure of the alkyl groups of trialkyl phosphate and chain length of diluents on third phase formation during Zr(IV) extraction from HNO3 media have also been studied. Overall, the third phase formation behaviour of T2MBP was found to be lower both for the extraction of mineral acid and Zr(IV), thereby indicating its potentiality for applications in various solvent extraction processes.

Recovery of germanium and indium from leaching solution of germanium dross using solvent extraction with TOA, TBP and D2EHPA

Germanium and indium are considered by many economies as critical raw materials. Therefore, investigation on possible methods of selective extraction of these elements from natural sources is getting increasing attention. The paper presents the results of solvent extraction tests of real sulphate solution obtained by leaching of dross and containing 5.15 g/dm3 Ge, 1.52 g/dm3 In and 5.81 g/dm3 Zn. In the first stage initial solution was doped with tartaric acid (citric and ascorbic acid were also tested) and germanium was extracted with trioctylamine(TOA)-tributyl phosphate (TBP) system. It was found that addition of complexing agent was necessary to achieve germanium extraction with TOA. Then, germanium was stripped with sodium hydroxide solution. Germanium extraction yield was >99%, while its stripping yield was 66–69%. Moreover, indium and zinc extraction yields were < 1%, therefore the operation was very selective. Additionally, it was found that stripping may be performed only if NaOH concentration was in the range between 10 and 20%. The reason was precipitation of germanium from aqueous phase at low NaOH (<10%) concentrations and formation of third phase and emulsion at higher NaOH levels (>20%). In the second stage, indium was recovered from the raffinate obtained after germanium extraction. Di-(2-ethylhexyl) phosphoric acid (DEHPA/D2EHPA) was used as an extractant and hydrochloric acid as a strippant. Indium extraction and stripping yields were > 98%, while zinc extraction yield was <1% allowing selective separation of In form Zn. It was found that solvent extraction using investigated system (3-stage germanium extraction, 3-stage germanium stripping, 2-stage indium extraction and 2-stage indium stripping) may be applied for selective recovery of germanium and indium.

Correction to “Understanding the Effect of the Phase Modifier n-Octanol on Extraction, Aggregation, and Third-Phase Appearance in Solvent Extraction”

Phase modifiers are often added to solvent extraction processes to avoid the third-phase formation. While this important issue was attributed to sticky interactions between reverse aggregates, structural effects of phase modifiers remain ambiguous. As they are similar to reverse hydrotropes, phase modifiers may act as cosurfactants or cosolvents in the organic phase in a solvent extraction system. We therefore applied an innovative small-angle scattering approach coupled with surface tension measurements on the industrially applied AMEX process to evaluate how phase modifiers repel the third phase and affect the extraction properties. We first confirmed that adding 1-octanol has a small influence on the extraction performance. By varying the scattering contrast of the solution with deuterated 1-octanol, we found that 1-octanol is located both in the solvent, acting as a cosolvent and diluting the aggregates, and in an outer shell of the aggregates. Further surface tension measurements demonstrated that instead of penetrating till the core of the aggregates as a cosurfactant, 1-octanol only penetrates their shell and forms a shielding barrier avoiding the coalescence of aggregates.

How Phase Modifiers Disrupt Third-phase Formation in Solvent Extraction Solutions

ABSTRACT The structure of complex organic solutions based on N,N,N’,N’-tetraoctyldiglycolamide (TODGA) used in actinide separation processes were characterized after water and neodymium extraction. The organic phases were characterized using FTIR spectroscopy, ESI-MS spectrometry, Small and Wide Angle X-ray Scattering (SWAXS) and molecular dynamic simulations. The role of the “phase modifier” like 1-octanol (used in the innovative SANEX process) or the “co-extractant” like DMDOHEMA (used, for example, in the EURO-GANEX process) added to the organic phase to avoid third-phase formation was investigated at molecular and supramolecular scales. Without metallic cations in 0.2 mol/L TODGA solution in n-heptane, TODGA molecules are organized in monomers and small aggregates containing mainly two or three TODGA molecules. After Nd(NO3)3 extraction, complexes or aggregates are formed in which each neodymium cation is surrounded in average by 2 to 4 TODGA, 3 nitrate ions and 1 to 2 water molecules. Polymetallic species (aggregates) may also be formed. Adding 5%Vol 1-octanol or 0.5 mol/L DMDOHEMA to 0.2 mol/L TODGA solution in n-heptane leads to the formation of mixed aggregates containing TODGA and octanol molecules or TODGA and DMDOHEMA molecules. These molecules both interact directly with the extracted metallic cation in the organic phase, modifying the structure of the neodymium species, and improving their solubility in this organic phase. At least one TODGA molecule remains in the cation first coordination sphere in both cases and preserves its significant role in the complexation/selectivity of the cations. But it is clear that both 1-octanol and DMDOHEMA have a role in the speciation of the cations in the organic phase: 1-octanol replaces water molecules in the first coordination sphere of Nd, acting as a co-surfactant; DMDOHEMA drives the organization and DMDOHEMA molecules partially replace TODGA in the first coordination sphere of Nd.

Understanding the Effect of the Phase Modifier n-Octanol on Extraction, Aggregation, and Third-Phase Appearance in Solvent Extraction.

Phase modifiers are often added to solvent extraction processes to avoid the third-phase formation. While this important issue was attributed to sticky interactions between reverse aggregates, structural effects of phase modifiers remain ambiguous. As they are similar to reverse hydrotropes, phase modifiers may act as cosurfactants or cosolvents in the organic phase in a solvent extraction system. We therefore applied an innovative small-angle scattering approach coupled with surface tension measurements on the industrially applied AMEX process to evaluate how phase modifiers repel the third phase and affect the extraction properties. We first confirmed that adding 1-octanol has a small influence on the extraction performance. By varying the scattering contrast of the solution with deuterated 1-octanol, we found that 1-octanol is located both in the solvent, acting as a cosolvent and diluting the aggregates, and in an outer shell of the aggregates. Further surface tension measurements demonstrated that instead of penetrating till the core of the aggregates as a cosurfactant, 1-octanol only penetrates their shell and forms a shielding barrier avoiding the coalescence of aggregates.

Microvolume Screening of Extraction and Phase Behavior in a Liquid-Liquid Microsystem.

Spontaneous formation of a third immiscible phase during liquid-liquid solvent extraction presents an enormous technical challenge for industry. Insight from current empirical investigations is greatly limited by the lack of methodologies that simultaneously report the progress of the extraction, third-phase onset time, and chemical and physical nature. The microfluidic strategy presented here answers this challenge by supporting an optically transparent submicroliter organic-phase film in a micropillar array surrounded by the aqueous phase. To demonstrate, we used 1 M Cyanex 572 in Shellsol D70 (organic phase) to extract Yb3+ and Dy3+ from a pH 2 aqueous phase. Real-time optical tracking confirmed that the visual onset of third-phase formation is consistent with the cessation of extraction (at the loading limit). Spectroscopic analysis of the solid-like third phase was carried out successfully. The new analytical approach offers a step change in speed and efficiency for reagent development, process control, and fundamental studies of complex phase behavior in reactive multiphase systems.

Extraction Behaviour of Tris(2-methylbutyl) Phosphate with Fission Products and Heavy Metal Ions

ABSTRACTSystematic screening of various symmetric trialkyl phosphates has led to the conclusion that tris(2-methylbutyl) phosphate (T2MBP), an organophosphate having branched amyl groups, can also be adopted as an alternate extractant to tri-n-butyl phosphate (TBP) for the selective extraction of U(VI) and Pu(IV) during the nuclear fuel reprocessing. In this context, a detailed study on the extraction efficiency of 1.1 M T2MBP/n-dodecane for the extraction of two important nuclear fuel elements, U(VI) and Pu(IV) has been carried out. The cross-current extraction study with U(VI), Pu(IV), and mixed U(VI)–Pu(IV) feed solutions has also been carried out to investigate the extraction and stripping behaviour of the respective metal ions by T2MBP. Results indicated that in contrast to TBP-based systems, there was no occurrence of third-phase formation during the extraction of tetravalent plutonium by T2MBP up to its threshold limit. In addition, its extraction behaviour with certain important fission product elements was also taken up as a part of the study. It has been observed that the extraction of trivalent lanthanides (La, Ce, Nd, Gd), Zr and Tc using T2MBP is marginally lower than is seen for TBP, indicating the possibility of achieving better decontamination factor during reprocessing.

Role of Phase Modifiers in Controlling the Third-phase Formation During the Solvent Extraction of Trivalent Actinides

ABSTRACTDiglycolamides have been proposed for partitioning of trivalent actinides from high-level liquid waste (HLLW). Third-phase formation is an undesirable event during the course of solvent extraction of trivalent actinides from HLLW into the solution of N,N,N’,N’-tetra(2-ethylhexyl)diglycolamide (TEHDGA) in n-dodecane (n-DD). Polar reagents such as tri-n-butyl phosphate (TBP), and N,N-dihexyloctanamide (DHOA) have been added to the TEHDGA phase, as phase modifiers in significant concentration, to overcome the third-phase formation. To understand the role of these phase modifiers in controlling the third-phase formation, the extraction behaviour of nitric acid and the trivalent representative metal ion Nd(III) was studied in a binary solution containing TEHDGA and phase modifier in n-dodecane. The organic phase obtained after extraction was subjected to dynamic light-scattering studies to examine the aggregation behaviour of the reverse micelles formed upon extraction and to unravel the unique role of TBP and DHOA in controlling the third-phase formation. The study revealed that the addition of these modifiers brought down the average size of aggregates and their distribution in organic phase below the limiting aggregate size for third-phase formation and increased the dispersion of aggregates in the n-dodecane phase. Among the two phase modifiers proposed for trivalent actinide separation from HLLW, TBP has been identified as a promising reagent for minimizing the third-phase formation.

An insight into third-phase formation in the extraction of thorium nitrate by tris(2-methylbutyl) phosphate and tri-n-alkyl phosphates

ABSTRACTThird phase (TP) formation in the extraction of Th(IV) by tris(2-methylbutyl) phosphate (T2MBP) from HNO3 media was studied and compared with tri-n-butyl phosphate (TBP) systems. Concentrations of Th(IV) and HNO3 and densities of organic phases were determined as a function of Th(NO3)4 concentration. Extractant concentrations and volume ratios in the triphasic regions were also measured. The extraction experiments show lower TP formation tendency of T2MBP. The aggregation behaviour of Th(IV) loaded solutions of tri-n-amyl phosphate (TAP), T2MBP and TBP in n-C8D18 and n-C12D26 were investigated using small angle neutron scattering technique and found to be in correlation with TP formation trends.