Extraction of Trivalent Lanthanides and Americium by Tri-n-octylphosphine Oxide from Ammonium Thiocyanate Media

Abstract

An investigation of the solvent extraction of trivalent lanthanides and Am³⁺ from ammonium-thiocyanate media by tri(n-octyl)phosphine oxide (TOPO) in toluene has been completed. This system is of interest both for its potential as a means of separating transplutonium actinides from fission-product lanthanides and for inherent interest in thiocyanate-based solvent extraction systems. Partitioning was monitored using radiotracer techniques where appropriate, and ICP-OES or ICP-MS for others. The extraction behavior of all members of the lanthanide series (except for Pm) plus Y have been investigated. Conditional enthalpies (all exothermic) were determined (for selected systems) from the temperature dependence of the extraction reaction. A comparison with nitrate media shows higher extractive power of TOPO in contact with thiocyanate media, arising at least in part from the lower heat of the phase transfer of thiocyanate (relative to nitrate). The moderate tendency of HSCN to partition into the extractant phase has been profiled. Slope analysis indicates that TOPO solvation decreases from four (M(SCN)₃TOPO₄) for the light members of the series to three (or less) for the heavy lanthanide ions; Am³⁺ is extracted with four TOPO molecules. Despite the decrease in Ln:TOPO stoichiometry across the series, extraction is generally flat for the light lanthanides and increases from Gd³⁺ to Lu³⁺. The extraction of Am³⁺ from mildly-acidic ammonium-thiocyanate media was found to be at least 10 times stronger than that of the lanthanides between La³⁺ and Gd³⁺.     

Synergistic extraction and separation study of rare earth elements from nitrate medium by mixtures of sec‐nonylphenoxy acetic acid and 2,2′‐bipyridyl

BACKGROUND: Synergistic extraction has been proven to enhance extractability and selectivity. Numerous types of synergistic extraction systems have been applied to rare earth elements, among which sec‐nonylphenoxyacetic acid (CA100) has proved to be an excellent synergistic extractant. In this study, the synergistic enhancement of the extraction of holmium(III) from nitrate medium by mixtures of CA100 (H₂A₂) with 2,2′‐bipyridyl (bipy, B) in n‐heptane has been investigated. The extraction of all other lanthanides (except polonium) and yttrium by the mixtures in n‐heptane has also been studied. RESULTS: Mixtures of CA100 and bipy have significant synergistic effects on all rare earth elements, for example holmium(III) is extracted as Ho(NO₃)₂HA₂B with the mixture instead of HoH₂A₅, which is extracted by CA100 alone. The thermodynamic functions, ΔH^o, ΔG^o, and ΔS^o have been calculated as 2.96 kJ mol^?1, ? 6.23 kJ mol^?1, and 31.34 J mol^?1 K^?1, respectively. CONCLUSION: Methods of slope analysis and constant molar ratio have been successfully applied to study the synergistic extraction stoichiometries of holmium(III) by mixtures of CA100 and bipy. Mixtures of these extractants have also shown various synergistic effects with other rare earth elements, making it possible to separate them. Thus CA100 + bipy may be used to separate yttrium from other lanthanides at appropriate ratios of the extractants. Copyright © 2011 Society of Chemical Industry     

Extraction and Separation of Zr(IV) and Hf(IV) from Nitrate Medium by Some CYANEX Extractants

Abstract

A solvent extraction study has been carried out to extract and separate zirconium and hafnium from nitrate medium by using some phosphine oxide extractants (CYANEX 921, CYANEX 923, and CYANEX 925) in kerosene. The influence of the different factors affecting the extraction process was studied in detail. Apparently the rate of extraction of Zr(IV) and Hf(IV) in CYANEX 921, CYANEX 923, and CYANEX 925 is reasonably fast. The extraction increases with increasing temperature, suggesting that the reaction is endothermic. The stripping percent of Zr(IV) and Hf(IV) by 0.5 M HNO₃ from the loaded organic phase after two stages reached 97.5% and 10.2%, respectively, which lead to good separation of the two metals. Under the optimum conditions, the extraction of zirconium was about 90, 87.6, and 91.6% and separation factors equal to 17, 21.4, and 40.7 were obtained for CYANEX 921, CYANEX 923, and CYANEX 925, respectively. The results obtained reveal that 2.0 M nitric acid is the optimum acid concentration for the separation of Zr(IV) and Hf(IV) and 0.4 M CYANEX 925 performs more efficient separation compared with other organophosphorus extractants.     

SOLVENT EXTRACTION OF TRIVALENT LANTHANIDES AND YTTRIUM FROM NITRATE MEDIA BY DIALKYL SULPHOXIDES

The solvent extraction of the trivalent rare-earth metals (the lanthanides and yttrium, Ln³⁺) from nitrate media by xylene solutions of some dialkyl sulphoxides (R₂SO, where R=alkyl) was studied. The salting-out effect of added metal nitrate (5.00 M) increases in the order: NH₄NO₃>NaNO₃>LiNO₃. Extraction is independent of pH in the range from 2 up to at least 4, but decreases gradually below pH 2 due to the competing extraction of nitric acid.

Vapour-pressure osmometry data indicate that the dialkyl sulphoxides are not appreciably self-associated under the conditions studied (0.10 to 0.60M in xylene or toluene solutions at 30°C), which permits the use of organic-phase concentration terms (in place of activities) in the slope-analysis studies. The distribution ratios for the extraction of yttrium, gadolinium and lutetium show a third-order dependence on the concentration of the sulphoxide in the organic phase, with a slightly higher dependence (3.3) for lanthanum. In addition, the mole ratio of nitrate to metal in the loaded organic phase is close to 3, which suggests that the stoichiometry of the extracted complexes is Ln(NO₃)₃(R₂SO)₃.

Increasing the steric bulk of the substituent alkyl groups (R) in the sulphoxides causes a marked decrease in metal extraction, and a reasonable correlation exists between the extraction of a given metal and the sum of the steric parameters of the alkyl groups in each of the sulphoxides used. In traversing the lanthanide series, the extraction increases in the interval La to Sm, and decreases thereafter to the end of the series. The behaviour of yttrium most closely resembles that of thulium or ytterbium.     

Hydration counteracts the separation of lanthanides by solvent extraction

The extraction of lanthanides from aqueous nitrate solutions by quaternary ammonium nitrate ionic liquids (e.g., [A336][NO₃]) shows a negative sequence (i.e., light lanthanides are more efficiently extracted than heavy lanthanides), which conflicts with the lanthanide contraction. In this study, we explored the origin of the negative sequence by investigating the extraction of lanthanides from ethylammonium nitrate by [A336][NO₃]. The extraction shows a positive sequence, which is converted to a negative sequence with the addition of water. The transformation from positive to negative sequences reveals that the negative sequence is caused by the hydration of lanthanide ions: hydration of lanthanide ions counteracts the extraction. Therefore, the use of solvents that have weak solvation with lanthanide ions might enhance the separation of the elements by solvent extraction.     

A Novel Solvent System Involving Terpyridine and Cyanex-301 for Am(III) and Eu(III) Separation from Weakly Acidic Feeds

Separation of trivalent actinides and lanthanides is a challenging task and has a great relevance in the nuclear fuel cycle. Bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex-301) show high selectivity for the trivalent actinides over the lanthanides at pH 3 or higher and N-donor ligands were reported to enhance the selectivity. 2,2?:6?,6”-Terpyridine (terpy), on the other hand, has shown to be quite effective at lower pH values and the combination of Cyanex 301 and terpy was evaluated in the present study, for the first time, for the separation of Am(III) from Eu(III), representative actinide and lanthanide elements, respectively at pH 2.0.

Thermodynamic parameters (enthalpy, entropy, and free energy) for the two phase extraction were also determined from the distribution studies at variable temperatures. Extraction of both Am³⁺ and Eu³⁺ was favored by negative enthalpy of extraction. More negative ΔG value indicated that Am³⁺ extraction was more favoured as compared to Eu³⁺ extraction using this solvent system. Effect of diluent composition on the extraction of Am³⁺ and Eu³⁺ was also studied in the present work.     

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.     

Extraction of Lanthanides(III) and Am(III) by Mixtures of Malonamide and Dialkylphosphoric Acid

Abstract

N,N′‐dimethyl‐N,N′‐dioctylhexylethoxymalonamide, DMDOHEMA, and di‐n‐hexylphosphoric acid, HDHP, are the extractants of reference for the French DIAMEX–SANEX process for the separation of trivalent actinide ions from the lanthanide ions. In this work, the extraction of Eu³⁺ and Am³⁺ by the two extractants, alone or in mixtures, has been investigated under a variety of experimental conditions. The two cations are extracted by HDHP as the M(DHP · HDHP)₃ complexes with an Eu/Am separation factor of ～10. With DMDOHEMA, Eu³⁺ and Am³⁺ are extracted as the M(NO₃)₃(DMDOHEMA)₂ disolvate species with an Am/Eu separation factor of ～2. The metal distribution ratios measured with a mixture of the two reagents indicated that almost all lanthanides are extracted equally well. The extraction of Eu³⁺ and Am³⁺ by HDHP‐DMDOHEMA mixtures exhibits a change of extraction mechanism and a reversal of selectivity taking place at ～1 M HNO₃ in the aqueous phase. Below this aqueous acidity, HDHP dominates the metal extraction by the mixture, whereas DMDOHEMA is the predominant extractant at higher aqueous acidities. Some measurements indicated apparent modest antagonism between the two extractants in the extraction of Eu³⁺ and synergism in the extraction of Am³⁺. These data were interpreted as resulting from the formation in the organic phase of mixed HDHP‐DMDOHEMA species containing two HDHP and five DMDOHEMA molecules.     

Solvent Extraction of Copper from Nitrate Media with Chelating LIX‐Reagents: Comparative Equilibrium Study

Abstract

Comparative experimental studies were carried out on extraction of copper(II) cations from aqueous acid nitrate media using four LIX‐reagents, representatives of different extractant classes: LIX 984N‐I, LIX 860N, LIX 84‐I and LIX 65N. As a diluent, liquid hydrocarbon undecane was used. The extraction behavior of the LIX‐reagents was compared based on an analysis of the influence of the main factors on the two‐phase mass transfer process: aqueous pH‐value, initial copper and extractant concentrations, and temperature. The experimental data received were used in the calculation of important parameters characterizing the efficiency of copper extraction from nitrate media with different LIX reagents: distribution ratios D, concentration extraction constants K _ex, pH_0.5‐values, and thermodynamic parameters such as enthalpy, entropy, and free energy changes (ΔH ⁰, ΔS ⁰, ΔG ⁰‐values).     

Extraction Studies of Lanthanide(III) Ions with N,N′‐Dimethyl‐N,N′‐diphenylpyridine‐2,6‐dicarboxyamide (DMDPhPDA) from Nitric Acid Solutions

Abstract

The new diamide compound, N,N′‐dimethyl‐N,N′‐diphenylpyridine‐2,6‐dicarboxyamide (DMDPhPDA), was synthesized and the distribution ratios of lanthanides from 1 to 5 M nitric acid solutions into DMDPhPDA CHCl₃ solution were determined. The extraction mechanism of lanthanide with DMDPhPDA was discussed based on the slope analysis of acid and ligand concentration dependencies and the variation of distribution ratio along the lanthanides series. The number of DMDPhPDA molecules in extracted complexes increase from 3 for lighter lanthanides to 4 for heavier lanthanides. From the previous EXAFS study of a complex similar in structure, Ln(III) would form an inner‐sphere complex with the two DMDPhPDA molecules and an outer‐sphere complex with the third and/or fourth DMDPhPDA molecules in addition to an inner‐sphere complex. Nitric acid concentration has more influence on the distribution ratio and the difference of distribution ratio among lanthanides than the ligand concentration.     

The Separation of Am from Lanthanides by Purified Cyanex 301 Extraction

Abstract

Separation factors of tracer amounts of Am from micro lanthanides (La, Ce, Pr, Nd, and Sm) by purified Cyanex 301 extraction in nitrate media have been determined: SF_Am/La ～ 3500, SF_Am/Ce,pr ～ 1000, SF_Am/Nd ～ 1900, and SF_Am/Sm～ 4500, with an average value >2300. The distribution ratio decreases with increasing lanthanide concentration in the aqueous phase. In the presence of a macro amount of Pr + Nd (0.1 ～ 0.6 M) the separation factors SF_Am/Eu and SF_Am/pr+Ndare about 4.7 × 10³ and 2.1 × 10³, respectively. The results of the countercurrent fractional process show that by using three extraction stages and two scrubbing stages, >99.99% Am can be separated from a tracer amount of Eu with <0.1% extraction of Eu. Using six extraction stages, >99.99% Am and <0.6% macro amount of Pr ± Nd are extracted into the organic phase.     

The Partitioning of Americium and the Lanthanides Using Tetrabutyldiglycolamide (TBDGA) in Octanol and in Ionic Liquid Solution

Separations among the lanthanides and the separation of Am from the lanthanides remain challenging, and research in this area continues to expand. The separation of adjacent lanthanides is of interest to high-tech industries because individual lanthanides have specialized uses and are in short supply. In nuclear fuel cycle applications Am would be incorporated into fast-reactor fuels, yet the lanthanides are not desired. In this work, the diglycolamide N,N,N′,N′-tetrabutyldiglycolamide (TBDGA) was investigated as a ligand for lanthanide and Am solvent extraction in both molecular and room-temperature-ionic-liquid (RTIL) diluents.The RTIL [C₄MIM][Tf₂N] showed very high extraction efficiency for these trivalent metals from low nitric acid concentrations, while the molecular diluent 1-octanol showed high extraction efficiency at high acid concentrations. This was attributed to the extraction of ionic nitrate complexes by the RTIL, whereas 1-octanol extracted neutral nitrate complexes. TBDGA in RTIL did not provide adequate separation factors for Am/lanthanide partitioning, but 1-octanol did show reasonable separation possibilities. Lanthanide intergroup separations appeared to be feasible in both diluents, but with higher separation factors from 1-octanol.     

Studies on the solvent extraction of rare earths from nitrate media with a combination of di‐(2‐ethylhexyl)phosphoric acid and sec‐octylphenoxyacetic acid

BACKGROUND: Di‐(2‐ethylhexyl)phosphoric acid (D2EHPA, H₂A₂) has been used extensively in hydrometallurgy for the extraction of rare earths, but it has some limitations. Synergistic extraction has attracted much attention because of its enhanced extractabilities and selectivities. In the present study, sec‐octylphenoxyacetic acid (CA12, H₂B₂) was added into D2EHPA systems for the extraction and separation of rare earths. The extraction mechanism of lanthanum with the mixtures and the separation of lanthanoids and yttrium were investigated. RESULTS: The synergistic enhancement coefficient for La³⁺ extracted with D2EHPA + CA12 was calculated as 3.63. La³⁺ was extracted as La(NO₃)₂H₂A₂B with the mixture. The logarithm of the equilibrium constant was determined as 0.80. The thermodynamic functions, ΔH, ΔG, and ΔS were calculated to be 4.03 kJ mol^?1, ? 1.96 kJ mol^?1, and 20.46 J mol^?1 K^?1, respectively. The mixtures have synergistic effects on Ce³⁺, Nd³⁺, and Y³⁺, with an especially strong synergistic effect on Y³⁺. Neither synergistic nor antagonistic effects on Dy³⁺ and weak antagonistic effects on Lu³⁺ were found. CONCLUSION: Mixtures of D2EHPA and CA12 exhibit evident synergistic effects when used to extract La³⁺ from nitric solution. The stoichiometries of the extracted complexes have been determined by graphical and numerical methods to be La(NO₃)₂H₂A₂B with the mixture. The extraction is an endothermic process. The mixture exhibits different extraction effects on rare earths, which provides possibilities for the separation of Y³⁺ from Ln³⁺ at a proper ratio of D2EHPA and CA12. Copyright © 2008 Society of Chemical Industry     

EXTRACTION OF SOME LANTHANIDES AND ACTINIDES BY 15-CROWN-5 THENOYLTRIFLUOROACETONE MIXTURE IN CHLOROFORM

The extraction of Eu³⁺, Tm³⁺, and Yb³⁺ by a mixture of thenoyltrifluoroacetone (HTTA) and 15-Crown-5 (15C5) in chloroform from a 0.1-ionic strength acetate buffer was investigated. Large enhancement in the extraction for these lanthanides was obtained when a mixture of the two extractants was used. Experimental data indicated that this enhancement is a result of the formation of a lanthanide adduct in the organic phase of the type Ln(TTA) ³° 2CE, where CE refers to a crown ether. The synergistic factors (S.F.), mixed extraction constants and formation constants of the different adducts in the organic phase were found to take the sequence Eu³⁺ > Tm³⁺ > Yb³⁺.

Using 12-Crown-4 (12C4), no enhancement in the extraction was observed. This was explained in terms of the small cavity size of 12C4 relative to 15C5.

The same system comprising HTTA-15C5 was also studied for the extraction of Pu⁴⁺, Am³⁺, Nd³⁺, and Er³⁺. Moderate enhancement was obtained for the extraction of Am³⁺ while no increase in the extraction was found for Pu⁴⁺.The metal concentration in the aqueous phase was found to largely affect the S.F. values for Nd³⁺ and Er³⁺.     

Solvent extraction studies of Sm(III) from nitrate medium and separation factors of rare earth elements with mixtures of sec‐octylphenoxyacetic acid and 1,10‐phenthroline

BACKGROUND: Liquid–liquid extraction is widely used for the separation of rare earths, among which synergistic extraction has attracted more and more attention. Numerous types of synergistic extraction systems have been applied to rare earths with high extraction efficiency and selectivities. In the present study, mixtures of sec‐octylphenoxyacetic acid (CA12, H₂A₂) and 1,10‐phenanthroline (phen, B) have been used for the extraction of rare earths from nitrate medium. The stoichiometry of samarium(III) extraction has been studied using the methods of slope analysis and constant molar ratio. The possibility of using synergistic extraction effects to separate rare earths has also been studied. RESULTS: Mixtures of CA12 and phen display synergistic effects in the extraction of rare earth elements giving maximum enhancement coefficients of 5.5 (La); 13.7 (Nd); 15.9 (Sm); 24.5 (Tb); 45.4 (Yb) and 12.3 (Y). Samarium(III) is extracted as SmHA₄B₃ with mixtures of CA12 and phen instead of SmHA₄ when extracted with CA12 alone. The calculated logarithm of the equilibrium constant is 6.0 and the thermodynamic functions, ΔH, ΔG, and ΔS, have been calculated as 4.3 kJ mol^?1, ? 33.7 kJ mol^?1 and 129.7 J mol^?1 K^?1, respectively. CONCLUSION: Mixtures of CA12 and phen exhibit synergistic effects on rare earth elements. Graphical and numerical methods have been successfully used to determine their stoichiometries. The different synergistic effects may provide the possibility of separating yttrium from heavy lanthanoids at an appropriate ratio of CA12 and phen. Copyright © 2010 Society of Chemical Industry     

Trivalent Lanthanide/Actinide Separation Using Aqueous-Modified TALSPEAK Chemistry

TALSPEAK is a liquid/liquid extraction process designed to separate trivalent lanthanides (Ln³⁺) from the minor actinides (MAs) Am³⁺ and Cm³⁺. Traditional TALSPEAK organic phase is comprised of the monoacidic dialkyl bis(2-ethylhexyl)phosphoric acid extractant (HDEHP) in diisopropyl benzene (DIPB). The aqueous phase contains a soluble aminopolycarboxylate diethylenetriamine-N,N,N’,N”,N”-pentaacetic acid (DTPA) in a concentrated (1.0–2.0 M) lactic acid (HL) buffer with the aqueous acidity typically adjusted to pH 3.0. This process balances the selective complexation of the actinides by DTPA against the electrostatic attraction of the lanthanides by the HDEHP extractant to achieve the desired trivalent lanthanide/actinide group separation. In this study, the aqueous phase has been modified by replacing the lactic acid buffer with a variety of simple and longer-chain amino acid buffers. The results show successful trivalent lanthanide/actinide group separation with the aqueous-modified TALSPEAK process at pH 2. The amino acid buffer concentrations were reduced to 0.5 M (at pH 2), and separations were performed without any effect on phase-transfer kinetics. Successful modeling of the aqueous-modified TALSPEAK process (p[H⁺] 1.6–3.1) using a simplified thermodynamic model and an internally consistent set of thermodynamic data is presented.     

Formation of W/O Microemulsions in the Extraction of the Lanthanide Series by Purified Cyanex 301

The formation of water-in-oil (W/O) microemulsions during the extraction of the series of trivalent lanthanides Ln(III) by bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HC301, also known as purified Cyanex 301) was studied. The phenomena in the formation of W/O microemulsions were similar in the extraction of all Ln(III) by HC301 at a high neutralization degree (50%), according to the measurement of the distribution ratios of Ln(III) and the concentrations of Na⁺ and NO₃^- in the organic phase, IR spectroscopy, and dynamic light scattering (DLS). W/O microemulsions also formed at a low neutralization degree (15%) for the extraction of heavy Ln(III). The coordination environment of the representative heavy lanthanide Ho(III) in the extracted complexes was monitored by absorption spectroscopy and extended X-ray absorption fine structure (EXAFS). Unlike the light lanthanide, Nd(III) and Ho(III) in the organic phase did not directly coordinate with the HC301 anions regardless of whether W/O microemulsions formed, which further demonstrated the different extraction behavior of HC301 toward the light lanthanides and the heavy lanthanides.     

EXTRACTION OF Se,Y, AND LANTHANIDES BY QUATERNARY AMMONIUM SALTS

The separation of Sc and Y from the lanthanides by extraction with various quaternary ammonium salts was investigated. Aliquat-336,methyltrioctylammonium nitrate (MTOA), methyldibutylhexadecylammonium nitrate (MDBHDA) and tributylhexadecylammonium nitrate (TBHDA) were used as extractants. The synthesis of MDBHDA and TBHDA from industrial products is described. From the results obtained in the extraction studies of REE as a function of the parameters of aqueous and organic phases, we have determined the optimal separation conditions. The salting-out action in these systems of a great number of cations such as Al³⁺, Fe³⁺, Mg³⁺, Cu³⁺, Ni³⁺, Li³⁺, Cs³⁺, was investigated. To determine the functional dependences of the separation factors for the REE, the viscosity of the water layer around the ion was used.     

Synergistic Extraction of Lanthanides(III) by Mixtures of N‐p‐Methoxybenzoyl‐N‐phenylhydroxylamine and 1,10‐Phenanthroline

Abstract

We conducted a study on the equilibrium extraction behavior of the trivalent lanthanide ions (M³⁺), La, Pr, Eu, Ho, and Yb, from tartrate aqueous solutions into chloroform solutions containing N‐p‐methoxybenzoyl‐N‐phenylhydroxylamine (Methoxy‐BPHA, HL) and 1,10‐phenanthroline (phen). The synergistic species extracted was found to be {ML₂(phen) (HL)}⁺(1/2)Tar^2?, where Tar^2? is tartrate ion. The extraction constants were calculated. The extraction separation behavior and extractability of lanthanides are discussed in comparison with the self‐adducted chelate, ML₃(HL)₂, which was extracted in the absence of phen, and synergistic extraction by mixtures of other extractants such as 2‐thenoyltrifluoroacetone, and neutral donors.     

EXTRACTION SYSTEMS USING BIS-1,2-0ICARBCILYLC0BALTATE ANO POLYOXONIUM COMPOUNDS FOR LANTHANIOE SEPARATIONS

ABSTRACT

The extraction of rare earths (lanthanides, Y, Sc) by voluminous bis-1,2-dicarbollylcobaltate anions disolved in nitrobenzene and in a nitrobenzene - carbon tetrachloride mixture has been investigated and the exchange extraction constants for both solvents, individual extraction constants anddG for ion transfer across the water - nitrobenzene phase boundary has been determined. Extraction decreases with increasing atomic number of the lanthanide.

The influence of several polyoxonium compounds on the distribution ratios and the extraction selectivity has been investigated. In the extraction systems with bis-1,2-dicarbollylcobaltate - 18-crown-6 in nitrobenzene, synergism was found for the light lanthanides but antagonism was observed for the heavy ones. The overall separation factor is Ctla/10 compared to in the absence of crown la/²