BOOK REVIEW

ABSTRACT

A previous paper reported the effect of increasing concentrations of hydrofluoric, oxalic, sulfuric and phosphoric acids at three fixed HCl or HNO₃ concentrations on the retention of Am(III), Th(IV) or Np(IV), and U(VI) by the new chelating ion-exchange resin Diphonix?. In this paper the metal uptake data have been analyzed by calculating the effect that complex formation in solution has on the overall metal uptake by the resin. The calculations performed have allowed us to conclude that in most cases the uptake of Am(III) and U(VI) by Diphonix can be explained by assuming that only the uncomplexed metal cation is sorbed. With tetravalent actinides, on the contrary, the uptake data can only be explained if the simultaneous sorption of the uncomplexed cation and of neutral metal complexes is assumed. Indirect evidence has also been obtained on the existence of anionic fluoride complexes of Np(IV). In all other cases, with minor exceptions, the complex formation data available in the literature seem to describe accurately the behavior of the investigated systems.     

ACIDS. PART 1. P,P"-DI(2-ETHYLHEXYL) METHANEDIPHOSPHONIC ACID

ABSTRACT

Two novel extractants, P,P'-di(2-ethylhexyl) methanediphosphonic acid (H2DEH[MDP]) and P.P'-dioctyl methanediphosphonic acid (H2DO[MDP]) have been synthesized at high purity and yield. H₂DEH[MDP] was selected for metal extraction studies because of its better physical properties. An investigation of the extraction of alkaline earth cations, Fe(III) and representative tri-, tetra- and hexavalent actinide ions from nitric acid solutions into o-xylene solutions of H₂DEH[MDP] at different concentrations was performed. With a few exceptions, the acid dependencies of the extraction of the above metal species strongly resembles those measured in the uptake of the same metals by the chelating ion exchange resin Diphonix®, which contains gem-diphosphonic acid groups chemically attached to a polymeric matrix. H₂DEH[MDP] exhibits practically no selectivity among the alkaline earth cations. The almost lack of acid dependency observed with Fe(lll) and tetra- and hexavalent actinides indicates that these ions are chelated by H₂DEH[MDP] mostly through the P=0 groups of the extractant. With Fe(lll) and the actinides, variable slopes of the extractant dependencies were measured, their values being strongly dependent on the acidity of the aqueous phase. Typically, the slope of the extractant dependency has a value of about two if the metal is extracted from high acidity solutions. This value becomes progressively lower as the aqueous acidity is reduced, tending to almost zero for acidities around 0.1 M. This phenomenon has been attributed to the formation of metal complexes having different stoichiometrics, ligand protonation and solubility in the aqueous phase.

H₂DEH[MDP] possesses an extraordinary affinity for actinides and Fe(III). In analogy with the Diphonix resin, this property can be exploited for actinide preconcentration and separation from complex matrices of analytical interest and from mixed waste solutions.     

EXTRACTION BY N,N'-TETRAALKYLMALONAMIDES II.

Suitable N.N'-tetraalkylmalonamldes for Am(III) extraction from concentrated HNO₃ aqueous liquors have been selected. Extraction mechanisms of Eu(III) and Am(III) by NN'-dlmethyldioctylmalonamide (DMDOMA) have been investigated. The distributions ratios of Am(III) and Eu(III) between 1M DMDOMA in t-butylbenzene and per chloric acid are higher than those obtained In nitrate media. Extraction data for Pu)IV), U(VI), Fe(III), Zr(IV) are reported. From these results It can be concluded that the NN'-dlmethyldlal-kylmalonamldes are good extractants for the actinides contained in the radioactive wastes.     

UPTAKE OF METAL IONS BY A NEW CHELATING ION EXCHANGE RESIN. PART 8 : SIMULTANEOUS UPTAKE OF CATIONIC AND ANIONIC SPECIES* *

ABSTRACT

The uptake of tri-, tetra- and hexa-valent actinides as well as of pertechnetate anions by some new multifunctional ion exchange resins has been investigated. The new resins, identified as Diphonix A, for anionic Diphonix, contain the same geminally substituted diphosphonic acid groups bonded to a styrenic-based polymer matrix as the regular Diphonix resin, plus strong base anion exchange groups such as the tetraalkylammonium (Diphonix A - Type 1 resins) or the quaternized pyridinium (Diphonix A - Type 2 resins) groups. Our uptake measurements have shown that the Type 2 Diphonix A resins are as effective as the regular Diphonix resin in the rapid uptake of actinides from acidic solutions, while at the same time sorbing pertechnetate anions in a manner comparable to existing commercial anion exchange resins. The failure of the Type 1 Diphonix A resins to perform equally well has been explained as a consequence of the mutual interaction of adjacent diphosphonic acid groups and tetraalkylammonium groups. Uptake data have also been obtained with Se(IV). The behavior of the Diphonix A resins toward Se(IV) uptake in acidic solutions is comparable to that of commercial anion exchange resins.     

Diphonix® Resin: A Review of Its Properties and Applications

Abstract

The recently developed Diphonix® resin is a new multifunctional chelating ion exchange resin containing geminally substituted diphosphonic acid ligands chemically bonded to a styrene-based polymeric matrix. Diphonix can be regarded as a dual mechanism polymer, with a sulfonic acid cation exchange group allowing for rapid access, mostly non-specific, of ions into the polymeric network, and the diphosphonic acid group responsible for specificity (recognition) for a number of metal cations. The Diphonix resin exhibits an extraordinarily strong affinity for actinides, especially in the tetra- and hexavalent oxidation states. Therefore the resin has potential for applications in TRU and mixed waste treatment and characterization, and in the development of new procedures for rapid actinide preconcentration and separation from environmental samples. Metal uptake studies have been extended to alkaline earth cations, to transition and post-transition metal species, and to metal sorption from neutral or near neutral solutions. Also the kinetic behavior of the resin has been investigated in detail. In view of the above applications the influence of the most commonly occurring matrix constituents (Na, Ca, Al, Fe, hydrofluoric, sulfuric, oxalic and phosphoric acids) on the uptake of actinide ions has been measured. This review paper summarizes the most important results obtained in the studies on the properties of the Diphonix resin and gives an overview of the applications already in existence or under development in the fields of mixed waste treatment, actinide separation procedures, treatment of radwaste from nuclear power and fuel processing plants, and removal of iron from copper electrowinning solutions.

     

UPTAKE OF METAL IONS BY A NEW CHELATING ION-EXCHANGE RESIN. PART 1: ACID DEPENDENCIES OF ACTINIDE IONS*

ABSTRACT

The uptake of several actinide ions [U(VI), Pu(IV), Np(IV), Th(IV] and Am(DI)) from nitric and hydrochloric acid solutions, and of U(VI) from near-neutral solutions by the new chelating ion-exchange resin, Diphonix^TM, has been investigated. Diphonix is a polyfunctional resin containing sulfonic and gem-diphosphonic acid groups chemically bonded in a styrene-divinylbenzene polymeric network. Comparison of the acid dependencies of the actinide ions uptake measured with Diphonix with those obtained using a commercial sulfonic -type resin and a resin containing both sulfonic and monophosphonic aCid groups, hat Shown that Diphonix binds the actinides via a different kind of chemical interaction, involving the.formation of chelate complexes through the phosphoryl groups of the gem-diphosphonic acids. As a consequence, Diphonix is superior to other resins in extracting actinide ions from very acidic solutions. A better performance of Diphonix is also observed with the uptake of uranium from neutral solutions. Conditions for efficient stripping of actinide species from the resin have been found.     

Extraction Behavior of Actinides and Metal Ions by the Promising Extractant,N,N,N′,N′-Tetraoctyl-3,6-dioxaoctanediamide (DOODA)

Abstract

The extraction of actinides, fission products, some non-nuclear elements, and nitric acid by N,N,N′,N′-tetraoctyl-3,6-dioxaoctanediamide (DOODA-C₈) in dodecane was extensively studied. Also studied was the extraction of HNO₃ and Nd(III) by the tetradodecyl analog of DOODA-C₈ in dodecane. Both extractants contain two ether oxygen atoms in the backbone chain carrying the two amide groups and can thus act as tetradentate ligands. The extractability of actinides decreases in the order Pu(IV) > U(VI), Am(III) > Np(V) in the extraction from nitric acid and Pu(IV) > Am(III) >> U(VI) in the extraction from perchloric acid. Ions of di-, tri-, tetra-, hexa-, and heptavalent metals strongly differ in the extractability by DOODA-C₈ but, except for lanthanides(III), there is no visible correlation of their distribution ratios with ionic radii. Due to the efficient extraction of actinides, weak extraction of fission products, and sufficient extraction capacity, DOODA-C₈ is a promising extractant for the recovery of minor actinides from high-level radioactive wastes.     

UPTAKE OF METAL IONS BY A NEW CHELATING ION-EXCHANGE RESIN. PART 4 : KINETICS

Abstract

The rate of uptake of several actinide ions [Am(III), U(VI), Th(IV), Np(IV) and Pu(IV)] and of some transition-metal ions [Co(II), Zn(II), Fe(III) and Cr(III)] at tracer concentration level, from solutions of various compositions, by the new chelating ion-exchange resin, Diphonix^Tm, has been investigated. Diphonix is a polyfunctional resin containing sulfonic and gem-diphosphonic acid groups chemically bonded in a styrene-divinylbenzene polymeric network. It binds actinide and other ions through the formation of chelate complexes with the phosphoryl groups of the gem-diphosphonic acids. The experiments discussed in this work have allowed us to establish the paramount importance of the presence of the sulfonic groups in obtaining practically useful rates of metal ions uptake. Comparison of the kinetic behavior of Diphonix with that of commercial sulfonic-type resins has shown that Diphonix reacts with the investigated ions as rapidly as do the other resins. Conditions for efficient and rapid stripping of all the investigated cations, including Cr(III), have been found.     

THE EXTRACTION OF SELECTED ACTINIDES IN THE ( III) (IV) AND ( VI) OXIDATION STATES FROM HYDROCHLORIC ACID BY Oπ D( iB) CMPO: THE TRUEX-CHLORIDE PROCESS∗

The extraction of tetravalent Th, Np, and Pu and hexavalent U from hydrochloric acid was studied using octyl(phenyl)-N,N-diiso-butylcarbamoylmethylphosphine oxides, 0π D(iB)CMP0 or CMPO, dissolved in tetrachloroethylene (TCE). A 0·5 MCMPO solution in TCE was found to be an extremely effective extractant for the tetravalent actinides and U(VI) from moderate to concentrated HC1. Extractant dependencies of the distribution ratios of Th(IV), U(VI), and Pu(IV) were 3rd, 2nd, and 3rd power, respectively, indicating the following species: ThCl₄’3CMPO, UO₂Cl₂2CMP0, and PuCl₄»3CMPO. The distribution ratios of a variety of non-actinide elements, including selected alkali, alkaline earths, Al, transition and post-transition metal ions were also measured from 2 Mand 6 M_ HC1. Based on the distribution ratios of tri-, tetra-, and hexavalent actinides from chloride media, a generic actinide extraction/recovery process was developed for the removal of actinides from chloride salt wastes. The process is called TRUEX-Chloride.     

UPTAKE OF METAL IONS BY A NEW CHELATING ION-EXCHANGE RESIN. PART 2: ACID DEPENDENCIES OF TRANSITION AND POST-TRANSITION METAL IONS*

ABSTRACT

Diphonix^(tm)is a new dual-mechanism polyfunctional resin containing sulfonic and gem-diphosphonic acid groups. In Part 1 of this series the effectiveness of Diphonix in removing actinide ions from very acidic solutions was demonstrated. In this paper we report on the uptake of various transition and post-transition metal ions with Diphonix and two other resins for comparison. The results show that Diphonix has a very high affinity for Fe(III) and Cr(III) in very acidic solutions. From neutral solutions Diphonix exhibits a high selectivity for lead and transition metals over calcium. Conditions for efficient stripping of the investigated ions have been found.     

Extraction Chromatography of Actinides Using Cyanex‐923 as Stationary Phase

Abstract

Cyanex‐923 has been used as the stationary phase in the extraction chromatographic separation of actinide ions from simulated high‐level waste (SHLW). Chromosorb‐W was used as the solid inert support. The uptake behaviour of U(VI), Pu(IV), and Am(III) was studied by batch studies. Breakthrough capacity of the column for Am(III) in the presence of macro amount of Eu(III) has been determined in three successive cycles of loading and elution. The influence of interfering ions like Fe(III), Cr(VI), Al(III), and Ni(II), etc. has been investigated. Effect of column parameters like diameter and height on Am(III) elution profile has also been studied.     

Extraction of Trivalent Europium and Americium from Nitric Acid Solution with Bisdiglycolamides

Two bisdiglycolamides (BisDGAs) of N,N,N′′′,N′′′-tetrabutyl-N?,N′′-ethidene bisdiglycolamide (TBE-BisDGA) and N,N,N′′′,N′′′-tetrabutyl-N?,N′′-m-xylylene bisdiglycolamide (TBX-BisDGA) were synthesized. Their extraction behaviors of Eu(III) and Am(III), as well as nitric acid were investigated from nitric acid medium by using n-octanol as diluent. Nitric acid is extracted as the form of HNO₃·(BisDGAs)_0.6 by BisDGAs and the conditional acid uptake constants of TBE-BisDGA and TBX-BisDGA were 0.26 and 0.10, respectively. The distribution ratios of Eu(III) and Am(III) increased with the increase of nitric acid and extractant concentration, whilst decreased with temperature rise. TBX-BisDGA had a stronger extraction power for Eu(III) and Am(III) than TBE-BisDGA. Both of the extractants displayed a higher affinity toward Eu(III) than Am(III). In the examination of the acidity range from 0.5 to 5.0 M, a maximum separation factor SF_{Eu(III)/Am(III)} can reach 8.0 at 3.0 M HNO₃ for TBX-BisDGA; and 10 at 4.0 M HNO₃ for TBE-BisDGA, respectively. Slope analyses showed that Eu(III) and Am(III) are extracted as di-solvated species by TBX-BisDGA or TBE-BisDGA. The extraction mechanism was described and the apparent extraction equilibrium constant as well as Gibbs free energy change, enthalpy change and entropy change were presented. In addition, their Eu(III) complexes were analyzed by using infrared spectra.     

ACTINIDE PARTITIONING FROM HLW IN A CONTINUOUS DIDPA EXTRACTION PROCESS BY MEANS OF CENTRIFUGAL EXTRACTORS

ABSTRACT

An experiment on actinide partitioning from real high level waste (HLW) was performed in a continuous process by extraction with diisodecylphosphoric acid (DIDPA) using a battery of 12 centrifugal extractors installed in a hot cell. The HNO₃ concentration of the HLW was adjusted to 0.5 M by dilution. The extraction section had 8 stages, and H₂O₂ was added to extract Np effectively. After extraction, Am and Cm were back-extracted with 4 M HNO₃ in 4 stages and Np and Pu were stripped with 0.8 M H₂C₂O₄ in 8 stages.

The actinides, except Np, were extracted from HLW with a very high yield. Although only 84 % of the Np were recovered in the present experiment, the recovery would be improved to 99.7 % by increasing the temperature to 45°C, the number of stages from 8 to 16 and the H₂O₂ concentration from 1 M to 2 M. Long-lived Tc and the main heat and radiation emitters Cs and Sr were not extracted and were thus separated from the actinides with high decontamination factors.

About 98 % of Am and Cm were recovered from the loaded solvent in the first stripping step with 4 M HNO₃. About 86 % of Np and about 92 % of Pu were back-extracted with 0.8 M H₂C₂O₄. These incomplete recoveries would be improved by increasing the number of stages and by optimizing the other process parameters.     

Extraction of Actinide(III,IV, V,VI) Ions and TcO4 − by N,N,N′,N′‐Tetraisobutyl‐3‐Oxa‐Glutaramide

Abstract

The extraction behavior of U(VI), Np(V), Pu(IV), Am(III), and TcO₄ ^? with N,N,N′,N′‐tetraisobutyl‐3‐oxa‐glutaramide (TiBOGA) were investigated. An organic phase of 0.2 mol/L TiBOGA in 40/60% (V/V) 1‐octanol/kerosene showed good extractability for actinides (III, IV, V VI) and TcO₄ ^? from aqueous solutions of HNO₃ (0.1 to 4 mol/L). At 25°C, the distribution ratio of the actinide ions (D _An) generally increased as the concentration of HNO₃ in the aqueous phase was increased from 0.1 to 4 mol/L, while the D _Tc at first increased, then decreased, with a maximum of 3.0 at 2 mol/L HNO₃. Based on the slope analysis of the dependence of D _M (M=An or Tc) on the concentrations of reagents, the formula of extracted complexes were assumed to be UO₂L₂(NO₃)₂, NpO₂L₂(NO₃), PuL(NO₃)₄, AmL₃(NO₃)₃, and HL₂(TcO₄) where L=TiBOGA. The enthalpy and entropy of the corresponding extraction reactions, Δ_r H and Δ_r S, were calculated from the dependence of D on temperature in the range of 15–55°C. For U(VI), Np(V), Am(III) and TcO₄ ^?, the extraction reactions are enthalpy driven and disfavored by entropy (Δ_r H<0 and Δ_r S<0). In contrast, the extraction reaction of Pu(IV) is entropy driven and disfavored by enthalpy (Δ_r H>0 and Δ_r S>0). A test run with 0.2 mol/L TiBOGA in 40/60% 1‐octanol/kerosene was performed to separate actinides and TcO₄ ^? from a simulated acidic high‐level liquid waste (HLLW), using tracer amounts of ²³⁸U(VI), ²³⁷Np(V), ²³⁹Pu(IV), ²⁴¹Am(III) and ⁹⁹TcO₄ ^?. The distribution ratios of U(VI), Np(V), Pu(IV), Am(III) and TcO₄ ^? were 12.4, 3.9, 87, >1000 and 1.5, respectively, confirming that TiBOGA is a promising extractant for the separation of all actinides and TcO₄ ^? from acidic HLLW. It is noteworthy that the extractability of TiBOGA for Np(V) from acidic HLLW (D _Np(V)=3.9) is much higher than that of many other extractants that have been studied for the separation of actinides from HLLW.     

SELECTIVE EXTRACTION OF Am(III) OVER Eu(III) BY 2,6-DITRIAZOLYL- AND 2,6-DITRIAZINYLPYRIDINES1

ABSTRACT

2,6-Di(5-alkyl-1,2,4-triazol-3-yl)-pyridine and 2,6-di(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine type compounds extract Am(III) 2-bromohexanoate and nitrate from acidic solutions (≤1 M HNO₃) with an appreciable efficiency and selectivity, yielding Am(III)/Eu(III) separation factors of ≤150.     

A COUNTER CURRENT EXPERIMENT FOR THE SEPARATION OF TRIVALENT ACTINIDES AND LANTHANIDES BY THE SETFICS PROCESS

ABSTRACT

The SETFICS process, a variation of TRUEX process, was developed for the recovery of Am and Cm from acidic waste solution and the separation of actinides (III) and light lanthanides. The process uses the general TRUEX solvent as the extracting reagent and a DTPA-sodium nitrate solution for selective stripping of actinides(III). The basic flow sheet is composed of four steps: extraction-scrubbing; acid stripping; actinide(III) stripping; and lanthanide stripping.

To demonstrate the usefulness of the SETFICS process, a counter current experiment was conducted using a real TRUEX product solution. Americium and curium were successfully recovered with a solution of 0.05 M DTPA-4 M NaNO₃ (pH 2.0). Although the actinide(III) product solution contained Sm and Eu, the decontamination factor of ¹⁴⁴Ce/²⁴¹Am was 72, and most of the light lanthanides, specifically La, Ce, Pr, Nd, were removed. At least 80 % of the lanthanides were separated from the Am and Cm end products. In order to minimize the acidity in the actinide(III) stripping step, the nitric acid which extracted with the trivalent metals was previously removed with a solution of 0.5 M NaNO₃ (pH 2.0) in the “acid stripping” step.     

Selected Alkyl(phenyl)-N,N-dialkylcarbamoylmethylphosphine Oxides as Extractants for Am(III) from Nitric Acid Media

Abstract

A new series of neutral bifunctional extractants, alkyl(phenyl)-N,N-dialkylcarbamoylmethylphosphine oxides, has been prepared and studied as extractants for Am(III) from nitric acid media. Two types of alkyl(phenyl)-N,N-dialkyl CMPO compounds were prepared, one containing N,N-diethyl groups and the other containing N,N-diisobutyl groups. The N,N-diethyl series contained hexyl(phenyl) and 6-methylheptyl(phenyl) derivatives, abbreviated HφDECMPO, and 6-MHφDECMPO, respectively. The N,N-diisobutyl series contained the n-octyl(phenyl), 6-methylheptyl(phenyl), and the 2-ethylhexyl(phenyl) derivatives, abbreviated OφD[IB]CMPO, 6-MHφD[IB]CMPO, and 2-EHφD[IB]CMPO, respectively. Third power extractant dependencies for the extraction of Am(III) from 0.5 and 3 M HNO₃ were obtained at low (<0.25 M) concentrations of extractant, but higher power dependencies were obtained above 0.25 M extractant from 3 M HNO₃. The HφDECMPO, 6-MHφDECMPO, 6-MHφD[IB]CMPO, and OφD[IB]CMPO [all 0.5 M in diethylbenzene (DEB)] are significantly better extractants than DHDECMPO for Am(III) from 1 to 6 M HNO₃. These same extractants have lower D _Am values than DHDECMPO at low acidities. HφDECMPO and OφD[IB]CMPO also have better selectivity for Am(III) over Fe(III) than DHDECMPO. HφDECMPO in DEB has a strong tendency toward the formation of a second liquid organic phase on extracting macroconcentrations of Nd(III) and U(VI) from 3 M HNO₃; however, this behavior is substantially diminished with the OφD[IB]CMPO and 6-MHφD[IB]CMPO compounds.     

Tuning the Extractive Properties of Purex Solvent using Room Temperature Ionic Liquid

An Aliquat-336 based ionic liquid, namely, tri-n-octylmethylammonium bis(2-ethylhexyl)phosphate ([A3636]⁺[DEHP]^?) was prepared and studied for the extraction of U(VI), Pu(IV), and Am(III) from nitric acid medium. Since the ionic liquid, [A336]⁺[DEHP]^? was miscible in n-dodecane (n-DD), the extraction of these actinides in the PUREX solvent, 1.1 M tri-n-butylphosphate (TBP) in n-dodecane (n-DD), was investigated in the presence of small concentrations of ionic liquid. The distribution ratio of U(VI) and Am(III) in 0.03 M [A336]⁺[DEHP]^?/n-DD decreased with increase in the concentration of nitric acid; whereas the extraction of Pu(IV) initially increased, it reached a maximum at 4 M nitric acid followed by the decrease. The extraction of actinides in ionic liquid medium decreased in the order Pu(IV) > U(VI) >> Am(III), indicating the feasibility of modifying the extractive properties of TBP/n-DD to favor Pu(IV) extraction. Therefore, the extraction of Pu(IV) in a solution of TBP – [A336]⁺[DEHP]^? in n-DD was also studied. The distribution ratio of Pu(IV) increased with increase in the concentration of ionic liquid and decrease in the concentration of TBP in organic phase. The distribution ratio of Pu(IV) determined in the presence of small concentration of ionic liquid in 1.1 M TBP/n-DD was always much higher than that observed in 1.1 M TBP/n-DD. In contrast to this, the distribution ratio of U(VI) decreased by the addition of ionic liquid and Am(III) was inextractable even in the presence of ionic liquid.     

PURIFICATION OF THORIUM FROM URANIUM-233 PROCESS RESIDUE

ABSTRACT

Thorium-229 can be used to produce ²¹³Bi. Researchers in phase I human trials are investigating the use of antibodies labeled with ²¹³Bi for selectively destroying leukemia cells. Other types of cancer may potentially be treated using similar approaches. Crude ²²⁹Th was liberated from Rachig rings by sonication in 7.5 M HNO₃ followed by filtration. Contaminants, included significant levels of uranium, a number of other metals, and radiolytic by-products of di-(2-butyl) phosphoric acid extractant (which was used in the original separation of ²³³U from thorium). Thorium was selectively retained on Reillex HPQ anion-exchange resin from 7.5 M HNO₃ at 65°C, where U(VI), Ac(III), Fe(III), Al(III); Ra(II), and Pb(II) were eluted. Thorium and uranium isotherms on Reillex HPQ are reported. The thorium was then easily eluted from the bed with 0.1 M HNO₃. To overcome mass transfer limitations of the resin, the separation was conducted at 65°C. The resin stood up well to use over several campaigns. Other researchers have reported that HPQ has excellent radiological and chemical stability. The eluted thorium was further purified by hydroxide precipitation from the organic contaminants. This process yielded 65 mCi of ²²⁹Th.

     

Comparative Evaluation of Tri-n-butyl Phosphate (TBP) and Tris(2-ethylhexyl) Phosphate (TEHP) for the Recovery of Uranium from Monazite Leach Solution

Separation of U(VI) from Th(IV) and rare earth elements (REEs) present in monazite leach solution (nitric acid medium) has been studied using tris(2-ethylhexyl) phosphate (TEHP) and tri-n-butyl phosphate (TBP) dissolved in n-paraffin as solvents under varying experimental conditions such as nitric acid, extractant and metal ion concentrations etc. There is an increase in distribution ratio of U(VI) (D _U ) with increase in aqueous phase acidity up to 5 M HNO₃ beyond which a decrease is observed. Typically for 1 × 10^?3 M U(VI), the D_U values increase from 8 (0.5 M HNO₃) to 80 (5 M HNO₃) for 1.1 M TEHP, and from 2 (0.5 M HNO₃) to 43 (5 M HNO₃) for 1.1 M TBP in n-paraffin. The separation factors of U(VI) (β: D_U/D_M) over metal ions (M) such as Th(IV) and Y(III) (chosen as a representative of heavy REEs) are better for TEHP than TBP at all nitric acid concentrations. Batch solvent extraction data have been used to construct the McCabe-Thiele diagrams for the recovery of U(VI) employing TEHP as the extractant. A process flow sheet has been proposed with 0.2 M TEHP in n-paraffin as solvent for the recovery of U(VI) from simulated monazite leach solution in HNO₃ medium.