An Advanced TALSPEAK Concept for Separating Minor Actinides. Part 2. Flowsheet Test with Actinide-spiked Simulant

An Advanced TALSPEAK (trivalent actinide–lanthanide separations by phosphorus-reagent extraction from aqueous complexes) counter-current flowsheet test was demonstrated using a simulated feed spiked with radionuclides in annular centrifugal contactors. A solvent comprising 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP] or PC88A) in n-dodecane was used to extract trivalent lanthanides away from the trivalent actinides Am³⁺ and Cm³⁺, which were preferentially complexed in a citrate-buffered aqueous phase with N-(2-hydroxyethyl)ethylenediamine-N,N´,N´-triacetic acid (HEDTA). In a 24-stage demonstration test, the trivalent actinides were efficiently separated from the trivalent lanthanides with decontamination factors >1000, demonstrating the excellent performance of the chemical system. Clean actinide and lanthanide product fractions and spent solvent with very low contaminations were obtained. The results of the process test are presented and discussed.     

An Advanced TALSPEAK Concept for Separating Minor Actinides. Part 1. Process Optimization and Flowsheet Development

A solvent extraction system was developed for separating trivalent actinides from lanthanides. This “Advanced TALSPEAK” system uses 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) to extract the lanthanides into an n-dodecane-based solvent; the actinides are retained in a citrate-buffered aqueous phase by complexation to a polyaminocarboxylate ligand. Several aqueous-phase ligands were investigated, and N-(2-hydroxyethyl)ethylenediamine-N,N’,N’-triacetic acid (HEDTA) was chosen for further study. Batch distribution measurements indicate that the separation of americium (Am) from the light lanthanides increases as the pH increases. However, previous investigations indicated that the extraction rates for the heavier lanthanides decrease with increasing pH. Therefore, a balance between these competing effects is required. An aqueous phase at pH 2.6 was chosen for further process development, because this offered optimal separation. Centrifugal-contactor single-stage efficiencies were measured to characterize the system’s performance under flow conditions, and an Advanced TALSPEAK flowsheet was designed.     

Advanced TALSPEAK Separations Using a Malonate Buffer System

The extraction behavior of lanthanides and americium has been evaluated under Advanced TALSPEAK (Trivalent Actinide Lanthanide Separation by Phosphorus-reagent Extraction from Aqueous Komplexes) conditions using malonic acid as the aqueous buffering agent. The extractant 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) was used as an organic phase liquid cation exchanger in n-dodecane diluent, while N-(hydroxyethyl)-ethylenediaminetriacetic acid (HEDTA) served as a selective aqueous holdback reagent. Extractions conducted from malonate media exhibit a pH profile that flattens as the concentration of malonate is increased up to 1.0 M malonate. This relatively flat extraction behavior from pH 2.5–4.0 is reminiscent of previous studies on Advanced TALSPEAK in lactate media. The extraction kinetics with other carboxylic acid buffers as well as the effects of varying HEDTA, HEH[EHP], and malonate concentration are compared.     

Combining CMPO and HEH[EHP] for Separating Trivalent Lanthanides from the Transuranic Elements

Combining octyl(phenyl)-N,N-diisobutyl-carbamoylmethylphosphine oxide (CMPO) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) into a single process solvent for separating transuranic elements from liquid high-level waste is explored. The lanthanides and americium can be co-extracted from HNO₃ into 0.2 mol/L CMPO + 1.0 mol/L HEH[EHP] in n-dodecane. The extraction is relatively insensitive to the HNO₃ concentration within 0.1–5 mol/L HNO₃. Americium can be selectively stripped from the CMPO/HEH[EHP] solvent into a citrate-buffered N-(2-hydroxyethyl)ethylenediaminetriacetic acid solution. Separation factors >14 can be achieved in the range pH 2.5–3.7, and the separation factors are relatively insensitive to pH—a major advantage of this solvent formulation.     

Investigation of the Impacts of Gamma Radiolysis on an Advanced TALSPEAK Separation

The advanced TALSPEAK process is a selective solvent extraction that utilizes 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) to separate lanthanide elements from trivalent actinides, which are held back in the aqueous phase by N-hydroxylethyl-N,N’,N’-ethylenediamine triacetic acid (HEDTA) buffered by citric acid. Gamma irradiation of an experiment containing Eu(III) and Am(III) as representative lanthanide and actinide elements resulted in higher distribution ratios of both and separation factors which decreased in an exponential fashion with increasing dose. Analysis of the reagents showed that the HEDTA concentration also decreased in an exponential fashion, strongly suggesting that degradation was correlated with loss of separation selectivity. In contrast, the concentration of citrate was unaffected, and while the concentration of HEH[EHP] did decrease, its dose-dependent kinetic profile indicated that it was not limiting partitioning. A second set of experiments were conducted using a citrate concentration that was 7.5 X higher, with the expectation that citrate would protect the HEDTA by scavenging radiolytically formed OH radicals. HEDTA degradation was significantly mitigated at higher gamma doses, but the Eu-Am separation was worse than in the low citrate experiments, presumably because at the high citrate concentrations, the Eu-citrate complexes formed in abundances competitive with the Am complexes, and are more effectively held back in the aqueous phase.     

Mixed Monofunctional Extractants for Trivalent Actinide/Lanthanide Separations: TALSPEAK-MME

The basic features of an f-element extraction process based on a solvent composed of equimolar mixtures of Cyanex-923 (a mixed trialkyl phosphine oxide) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) extractants in n-dodecane are investigated in this report. This system, which combines features of the TRPO and TALSPEAK processes, is based on co-extraction of trivalent lanthanides and actinides from 0.1 to 1.0 M HNO₃ followed by application of a buffered aminopolycarboxylate solution strip to accomplish a Reverse TALSPEAK selective removal of actinides. This mixed-extractant medium could enable a simplified approach to selective trivalent f-element extraction and actinide partitioning in a single process. As compared with other combined process applications in development for more compact actinide partitioning processes (DIAMEX-SANEX, GANEX, TRUSPEAK, ALSEP), this combination features only monofunctional extractants with high solubility limits and comparatively low molar mass. Selective actinide stripping from the loaded extractant phase is done using a glycine-buffered solution containing N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) or triethylenetetramine-N,N,N’,N’’,N’’’,N’’’-hexaacetic acid (TTHA). The results reported provide evidence for simplified interactions between the two extractants and demonstrate a pathway toward using mixed monofunctional extractants to separate trivalent actinides (An) from fission product lanthanides (Ln).     

Effect of HEH[EHP] impurities on the ALSEP solvent extraction process

In solvent extraction processes, organic phase impurities can negatively impact separation factors, hydrolytic performance, and overall system robustness. The resulting inconsistent performance can affect the process-level viability of a separation concept, and thus knowledge of the impurities present, their effects on the process, and how to remove them are vital. Deleterious impurities may be introduced into a system from reagent synthesis, or result from degradation via radiolysis and hydrolysis during use. In this work, the acidic extractant, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP])—proposed for application in extractive processes aimed at separating trivalent minor actinides from lanthanides and other fission products—is characterized with respect to its common impurities and their impact on Am(III) stripping in the Actinide Lanthanide SEParation (ALSEP) system. To control impurities in HEH[EHP], existing purification technologies commonly applied for the acidic organophosphorus reagent were assessed and a new chromatographic purification method specific to HEH[EHP] is presented.     

Dimerization of 2-Ethylhexylphosphonic Acid Mono-2-ethylhexyl Ester (HEH[EHP]) as Determined by NMR Spectrometry

ABSTRACT

The aggregation state of extractant molecules is an important consideration in characterizing or modeling solvent extraction systems used in hydrometallurgy. For example, previously reported efforts to measure organic-phase dimerization constants have required the presence of an additional phase, either aqueous phase or gas phase. NMR spectroscopy can be used to probe organic-phase inter-molecular interactions without requiring the presence of an additional phase, making it possible to study the simplest organic system, that contain only extractant and diluent. In this report, the dimerization of 2-ethylhexylphosphonic acid mono-2-ethylhexl ester (HEH[EHP]) in n-dodecane and in toluene was investigated by two different NMR-based methods: chemical shifts and Diffusion Ordered SpectroscopY (DOSY). The chemical-shift analysis requires monitoring the chemical shift of the acidic proton as the concentration of HEH[EHP] changes. DOSY is a 2D NMR technique used to probe the size of molecules. The size of diffusing species was related back to the average aggregate molecular weight via a calibration curve. Because the DOSY method had not been used in this manner before, a validation of the method using the interaction constant between HDEHP and CMPO was performed. After this validation, DOSY was applied to HEH[EHP] dimerization. DOSY results demonstrated that the chemical shift of the acidic proton was the peak most affected by the dimerization state of the HEH[EHP]. All other peaks, including that of ³¹P, were affected more significantly by the changing dielectric constant of the solution. The dimerization constants determined were significantly lower than those reported in prior literature, possibly due to the effect of water in the literature values.     

Selective Extraction of Scandium by a Long Alkyl Chain Carboxylic Acid/Organophosphonic Ester Binary Extractant

ABSTRACT

The organophosphorus extractant 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) is widely used for solvent extraction of rare earth elements in an acidic leaching solution, but stripping of the loaded metals is difficult because of the high affinity between the metals and the extractant. Adding 100 times the concentration of neodecanoic acid (Versatic 10) to the extractant solution reduces the extraction ability for scandium (Sc) and enables quantitative stripping of the loaded Sc with a mild acidic solution, such as a 1 M mineral acid. The loading capacity is dependent on the PC88A concentration, so PC88A is the main species responsible for Sc extraction in the binary mixture. Nuclear magnetic resonance spectroscopy (NMR) of the organic phases suggests a potential interaction between the mixed extractants through the change in the hydrogen bonding. This causes an antagonistic effect and facilitates efficient Sc stripping from the extractant solution, so this method could be used for scandium recovery in industry.     

Carrier Mediated Transport of Thorium from Nitric Acid Medium using 2-Ethyl Hexyl Hydrogen 2-Ethyl Hexyl Phosphonate (PC88A)/N-Dodecane as Carrier

The present study deals with the application of supported liquid membrane (SLM) technique for the separation of thorium from nitric acid medium using 2-ethyl hexyl hydrogen 2-ethyl hexyl phosphonate (PC88A) as a carrier and aqueous ammonium carbonate as a receiving phase. The effects of feed acidity, nature of strippant, and membrane pore size and membrane thickness on the transport of thorium have been studied in detail. Transport behavior of uranium (²³³U) and fission products from a radioanalytical laboratory waste is also studied. Stability of the membrane against the leaching of the extractant and stability of the membrane support have been investigated. An attempt has been made to model the physicochemical transport of thorium in SLM and establish the mechanism of thorium transport. Transport of thorium increased from 25% to about 96% using 0.75 M PC88A in n-dodecane as carrier and 2 M ammonium carbonate as stripping phase as the feed acidity decreased from 4 M HNO₃ to 0.5 M HNO₃. Optimum conditions obtained from this study were applied to recover thorium and ²³³U from analytical waste generated in the laboratory.     

强酸性阳离子交换树脂催化合成氯乙酸异辛酯的研究

对采用７３２强酸性苯乙烯系阳离子交换树脂作催化剂合成氯乙酸异辛酯进行了实验研究，提出了适宜的反应条件。实验结果表明，该法酯化率高、过程简便、产物颜色浅，优于浓硫酸作催化剂。     

Liquid membrane transport and separation of Zn2+ and Cd2+ from sulfate media using organophosphorus acids as mobile carriers

The transport and separation of Zn²⁺ and Cd²⁺ from binary sulfate solutions in a supported liquid membrane using di(2‐ethylhexyl)phosphoric acid (D2EHPA) and 2‐ethylhexylphosphonic acid mono‐2‐ethylhexyl ester (PC88A) as mobile carriers was studied. Batch solvent extraction experiments were conducted to obtain the reaction stoichiometries. Experiments were performed at different metal concentrations (1.4–14.5 mol m^?3), metal concentration ratios (0.4–9.2), pH (2–5), and carrier concentrations (0.1–0.6 mol dm^?3). A mass transfer model was proposed that considers diffusion in the aqueous feed and strip stagnant layers, and within the membrane. The interfacial reactions were assumed to approach equilibrium instantaneously. It was shown that the proposed model was applicable for binary Zn²⁺/Cd²⁺ systems (standard deviation, 5%). The larger separation factors of Zn²⁺ over Cd²⁺ with PC88A than D2EHPA under equilibrium (batch solvent extraction) and non‐equilibrium (liquid membrane) conditions were also evaluated and discussed. Copyright © 2003 Society of Chemical Industry     

Separation of Carrier Free 90Y from 90Sr by Hollow Fiber Supported Liquid Membrane Containing Bis(2-ethylhexyl) Phosphonic Acid

The present article gives a comparative account of the efficiency of carrier-free ⁹⁰Y separation from ⁹⁰Sr by solvent extraction, flat sheet-supported liquid membrane (FSSLM) and hollow fiber-supported liquid membrane (HFSLM) methods using bis(2-ethylhexyl) phosphonic acid (PC88A) as the carrier extractant. The major focus of this work has been to develop the HFSLM method for the separation of Y(III) on a relatively large scale. The feed and receiver phase conditions were optimized by carrying out batch solvent-extraction studies. The extraction of Sr(II) by PC88A was negligible in the acidity range of 0.01–3 M HNO₃, whereas the extraction of Y(III) was significantly large at lower acidity (≤0.1 M HNO₃) with a separation factor (SF = D_Y/D_Sr) of 8.5 × 10⁴. HFSLM studies suggested selective and efficient transport of Y(III) into 3 M HNO₃ from a feed solution containing a mixture of Y(III) and Sr(II) at 0.1 M HNO₃. On the other hand, transport of Sr(II) was negligible in the receiver phase. The purity of the separated ⁹⁰Y was ascertained by paper chromatography and by half-life measurement. The radiation stability of the carrier was excellent as studied up to 1000 KGy dose.     

Studies on the Separation and Recovery of Uranium from Phosphoric Acid Medium Using a Synergistic Mixture of (2-Ethylhexyl)phosphonic Acid Mono 2-Ethyl Hexyl Ester (PC-88A) and Tri-n-octylphosphine Oxide (TOPO)

This paper deals with studies on the extraction of uranium(VI) from phosphoric acid medium using (2-ethylhexyl)phosphonic acid mono 2-ethylhexyl ester and tri-n-octylphosphine oxide individually as well as from their synergistic mixture. Different extraction parameters were investigated. With an increase in phosphoric acid concentration in the aqueous phase, the distribution ratio (Du) was found to decrease in all the cases. Synergism was observed when a mixture of PC-88A and TOPO was used. The synergistic mixture in the mole ratio of 4:1 (1.80 M PC-88A: 0.45 M TOPO) in xylene was found to be most suitable for uranium extraction. Among the various strip liquors used, 5% (w/v) solution of (NH₄)₂CO₃ was found to be the most suitable. Using a mixture of 1.8 M PC-88A and 0.45 M TOPO as the extractant system and 0.5 M ammonium carbonate as the stripping agent, uranium recovery was found to be better than 97% ± 3% in multiple contacts, (n = 2) from actual Davies Gray Waste while in case of wet phosphoric acid more than 52% ± 3% (n = 3) only could be recovered where n is the number of contacts.     

Studies on the Separation and Recovery of Thorium from Nitric Acid Medium using (2-ethyl hexyl) Phosphonic Acid,Mono (2-ethyl hexyl) Ester (PC88A)/N-Dodecane as Extractant System

This paper deals with the studies on the separation and recovery of thorium and 233-uranium from nitric acid medium using (2-ethyl hexyl) phosphonic acid, mono (2-ethyl hexyl) ester/n-dodecane as an extractant system. The different extraction parameters were investigated. The distribution ratio of thorium decreased with increase in nitric acid concentration. The optimum solvent concentration for quantitative separation of thorium from aqueous feed solution was 0.75 M of PC88A whereas dodecane was the most suitable of diluents with an organic to aqueous phase ratio of 1:1. Among the various strippants used, 2 M solution of (NH₄)₂CO₃ was found to be the most suitable for back extraction of thorium. The developed method was used to recover thorium and ²³³U from radioanalytical waste generated during thorium analysis by ethylene diamine tetraacetic acid (EDTA) titremtric method and recoveries for both Th and U were more than 85%.