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Solvent extraction separation of uranium(VI) and thorium(IV) with neutral organophosphorus and amine ligands
Fuel 116 (2014) 595–600
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Solvent extraction separation of uranium(VI) and thorium(IV) with neutral organophosphorus and amine ligands M. Eskandari Nasab ⇑ Faculty of Mineral Processing Engineering, Zarand College, Bahonar University, P.O. Box 14579-73869, Kerman 7616914111, Iran
h i g h l i g h t s The effects of key parameters on the separation of U(VI) and Th(VI) were studied. Three different acids and neural amine extractants were simultaneously studied. HNO3 and HCl provide the optimum conditions for extracting thorium and uranium. The extraction mechanisms of uranium and thorium were studied.
a r t i c l e
i n f o
Article history: Received 20 April 2013 Received in revised form 26 July 2013 Accepted 15 August 2013 Available online 2 September 2013 Keywords: Uranium Thorium Separation Aliquat 336 TOA
a b s t r a c t Taguchi’s method was used to determine the optimum conditions for the separation of uranium and thorium using neutral extractants. The experimental conditions were studied in the concentration ranges of 0.01–5 M for three different acids (nitric acid, hydrochloric acid, and sulfuric acid) using tributylphosphate (TBP), trioctylamine (TOA), and tricaprylyl methyl ammonium chloride (Aliquat 336) as the extractants. The optimum condition for the separation of thorium from uranium was obtained at 5.0 M nitric acid solution using Aliquat 336. Under this condition, thorium was extracted selectively from uranium with the extraction efficiency and separation factor of 97.5% and 33.07, respectively. The equilibrium equations were also determined. The results showed that the optimum combination for uranium was extraction from 2.0 M hydrochloric acid solution by Aliquat 336. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The nuclear technology is an attractive option to obtain energy because of increasing energy demands and the threat of global warming due to CO2 emission caused by coal and hydrocarbon burning. At the present time, the nuclear power production is based on the uranium fuel cycle. Natural uranium contains only 0.7% of fissile isotope uranium-235. However, a large number of power reactors in the world are based on the enriched uranium fuel, which contains more than about 2% of uranium-235. Another fissile isotope particularly suitable for thermal reactors is uranium233, which is obtained by the neutron irradiation of thorium-232. Thus, uranium and thorium are the two most vital elements for nuclear energy industry [1]. Their natural sources such as monazite and thorite generally contain a sizeable fraction of uranium and thorium. Since the disposal of industrial waste effluents containing uranium and thorium has serious hazard impacts on the environment then the reprocessing of irradiated thorium to separate and ⇑ Tel.: +98 3412448226. E-mail address: [email protected] 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.08.043
purify uranium-233 is an important aspect of thorium fuel cycle, and it greatly influences the economics of the fuel cycle. Consequently, the separation and purification of uranium and thorium is of great importance. The methodology adopted for the separation of these metal ions from different ores and nuclear fuel reprocessing has always attracted the attention of separation scientists. Separation of uranium and thorium is relatively difficult because thorium does not display variable oxidation states. Thus, it should be used the difference in the complexation behavior of thorium and uranium to obtain the desired separation factors. In the conventional solvent extraction processes (i.e., Thorex and Amex process) [2,3], the recovery and separation of uranium and thorium are accomplished through a variety of methods after chemical attacking the mineral with sulfuric acid or sodium hydroxide [4]. In the Thorex process, 4 M nitric acid solution was contacted with 30% (v/v) TBP in n-dodecane has been used to separate thorium and uranium from each other and from fission products [2]. The Amex process was first developed in 1959 [3]. In this process, thorium and uranium are extracted in two solvent extraction cycle. In the first cycle, thorium is extracted with a primary amine, while
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uranium is extracted in the second cycle using secondary or tertiary amine. However, these processes have certain limitations largely due to the low solubility of Th(N03)4. x.TBP (where x = 3 or 4) in n-dodecane, low separation factor of uranium(VI) over thorium(IV), interference caused by hydrolytic degradation products of TBP and also need to multistage extraction and stripping process [5]. The disadvantages of these processes could be largely avoided if it were feasible to separate uranium and thorium, using another extractant. Previous reports have used other extractants for uranium/thorium separation [6–13]. In our previous studies [12–14], some cationic extractants such as Cyanex302 [bis(2, 4, 4-trimethylpentyl) thiophosphinic acid] and Cyanex272 [bis (2, 4, 4-trimethylpentyl) phosphinic acid] were studied as the alternative for TBP in the Thorex process. The results showed that the greatest separation factor (3.76) for the separation of thorium from uranium was obtained from 1.0 M sulfuric acid solution using Cyanex302. According to these results, uranium could be selectively extracted from thorium at 5.0 M nitric acid and its separation factor was almost equal to that obtained by Cyanex302. Therefore, two individual processes such as extraction by TBP (the first step) and Cyanex302 (the second step) was proposed to separate thorium from uranium [14]. Also, Hammad et al. [15] proposed two individual extraction process for separating thorium and uranium. They separated uranium from Egyptian monazite alkaline hydrous oxide cake concentrate dissolved in concentrated HCl solution by anion exchanger column containing Amberlite IRA-400. Afterward, thorium in the effluent solution was purified by extraction from nitric acid medium by TBP diluted in kerosene. The long chain amines had shown to be excellent extractants for a great number of anionic metallic complexes, which are extensively used to extract thorium and uranium [3,16,17]. The successful development of the solvent extraction processes using alkyl amines for recovering and separating uranium and thorium stared since 1958 [3]. Hughes and Singh [16] extracted thorium from sulfate leach solution by 0.1 M Adogen-383 (secondary amine) diluted in kerosene. Amaral and Morais reported that Prime JM-T (primary amine) extracts preferably thorium, while Alamine336 (tertiary amine) extracts uranium [17]. In the present study, the extraction and separation of uranium and thorium have been investigated with neutral extractants such as trioctyl amine (TOA) or quaternary ammonium salt (tricaprylyl methyl ammonium chloride or Aliquat-336) as an alternative for TBP using Taguchi method. 2. Experimental
emission spectroscopy (ICP-OES) analyzer from Liberty 220 Varian. Mechanical shaker was used to contact organic and aqueous phases. 2.3. Solvent extraction experiments Solvent extraction experiments (Table 1) were carried out using a mechanical shaker. The equal volumes of two phases were mixed. The constant parameters including organic to aqueous volume ratio, temperature, contact time of two phases, and phase volumes were adjusted at 1:1, 25 °C, 10 min and 10 ml, respectively. The two phases were separated by separatory funnel and the concentrations of the metallic ions in aqueous phases before and after extractions were determined using ICP. Then, the distribution ratio, D, and extraction percent, E, were calculated from Eqs. (1) and (2), respectively:
D¼
ðC 0 CÞ C0
ð1Þ
E¼
D ðD þ V a =V o Þ
ð2Þ
Here C0, C, Va and Vo are the concentrations of metallic ions in the aqueous phases before and after extractions, aqueous and organic phase volume, respectively. 2.4. Experimental design The Taguchi method has been proposed as powerful method of experimental design [18] and has been used to determine the best experimental design in solvent extraction processes [13]. In this method, the signal to noise (S/N) ratio is used to determine the best experimental conditions. Because of the purpose of this work, the S/N ratio (the higher the better) was used to evaluate the response of each experiment [18] through the following equation:
ðS=NÞi ¼ 10 log
! 1X 1 n i y2i
ð3Þ
where, n is the number of tests and yi is the experimental value in the Ith experiment. In the present study, three factors at three levels, i.e. acid concentration, acid type and extractant were considered. For three parameters each at three levels, the traditional full factorial design would require 27 experiments. However, in the current design (Taguchi L9 orthogonal array) the required experiments are only nine. The levels of combinations of these experiments have been shown in Table 1. The results reported in this article are the mean values of two replications at each set of experiments.
2.1. Reagents TBP (Tri-n-butyl phosphate), TOA (tri-octyl amine) and Aliquat336 [(R3NCH3)+Cl, R = octyl/decyl] were produced by Cytec, Canada. Kerosene, from Fluka, was used as a diluent for economic and practical reasons. Stock aqueous solutions of thorium(IV) and uranium(VI) were prepared by dissolving appropriate amounts of Th(NO3)4.5H2O (Merck) and UO2(NO3)2.6H2O (Merck) in distilled water. 1 cm3 of concentrated nitric acid was added to 100 cm3 of solution to prevent hydrolysis. The initial metallic ion concentrations were maintained at 104 mol dm3 for all experiments. All other chemical reagents used were of analytical grades. 2.2. Apparatus The concentrations of thorium and uranium in the aqueous phases were determined by an inductively coupled plasma optical
3. Results and discussion 3.1. The optimum process conditions Fig. 1 shows the S/N ratios for the extractions of uranium and thorium. According to Taguchi’s method, a high influence on extraction or separation factor is shown by a high S/N ratio. According to Fig. 1, the optimal combinations were the extraction of thorium from 5.0 M sulfuric acid by TOA or Aliquat-336. For uranium, the maximum extractions were obtained from 5.0 M hydrochloric acid. Also, TOA or Aliquat-336 is the best extractant for extraction of uranium. Fig. 2a indicates that the average distribution coefficients of uranium and thorium increase with increasing acid concentration (except for the extraction of uranium at 5.0 M). Sato reported that the preferred extraction mechanisms of metals by the amine
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M. Eskandari Nasab / Fuel 116 (2014) 595–600 Table 1 Taguchi’s L9 orthogonal array design for extractive separation of uranium and thorium by neutral extractants. Test no
1 2 3 4 5 6 7 8 9
Extraction parameters
SN ratio
Extraction (%)
Distribution coefficient (D)
Separation factor
Acid conc. (M)
Acid type
Solvent type
U
Th
U
Th
U
Th
DTh/DU
DU/DTh
1 1 1 3 3 3 5 5 5
HNO3 HCl H2SO4 HNO3 HCl H2SO4 HNO3 HCl H2SO4
TBP TOA Aliquat-336 TOA Aliquat-336 TBP Aliquat-336 TBP TOA
8.9 25.4 27.3 38.1 27.0 26.5 39.8 27.4 32.6
9.3 35.1 13.2 29.8 38.9 18.2 34.8 37.7 27.0
2.9 57.1 4.6 30.8 87.9 8.1 53.8 76.9 22.5
0.4 18.6 23.2 80.7 22.5 21.1 97.5 23.4 39.6
0.03 1.33 0.05 0.45 7.28 0.09 1.16 3.33 0.29
0.00 0.23 0.30 4.19 0.29 0.27 38.44 0.31 0.66
0.12 0.17 6.29 9.39 0.04 3.02 33.07 0.09 2.26
8.38 5.83 0.16 0.11 25.06 0.33 0.01 10.91 0.44
Table 2 Instability constants of uranium and thorium complexes with studied anions [21].
U
Th
*
Fig. 1. The S/N ratio for the extraction of thorium and uranium by different acids and neutral extractants.
Fig. 2. The mean distribution coefficients for the extraction of thorium and uranium.
extractants is anion exchange [19], while solvation mechanism for extracting by TBP [20]. Therefore, with increasing acid concentration, the neutral or anionic complexes of metals will be formed and the extraction equations (e.g. Eq. (2)) tend to right. The complex stability constants in Table 2 show that at higher acid concentrations, thorium forms neutral complexes. While, for uranium this
Ion type
Instability constant
NO3ClSO4-2 NO3ClSO4-2
K1 = 25.1*; K2 = 1*; K3 = 0.13 K1 = 1.3; K2 = 6.6*; K3 = 50.2 K1 = 1.9*; K2 = 3.3 *105 K1 = 0.16; K2 = 0.48; K3 = 1.3; K4 = 1.8* K1 = 0.04; K2 = 10.5; K3 = 1.4; K4 = 5.5* K1 = 6.3 * 103; K2 = 3.5 *104⁄
Neutral complex.
mechanism is valid only at low content of ion concentrations. Therefore, at higher acid concentrations, extraction of uranium is slightly decreased. Fig. 2b shows that the average distribution coefficient of thorium from nitric acid medium is larger than that of other ones due to more hydrophobic properties of nitrate species [22]. While, for uranium the maximum distribution coefficient was obtained from hydrochloric acid medium. Table 2 shows that uranium forms more strong anionic complexes with hydrochloride ions. Therefore, hydrochloric solution gives more extractable uranium species. Also, according to the results showed in Table 1, the extraction of uranium is much higher than thorium only at hydrochloric media. The effect of extractant type has been shown in Fig. 2c. From Fig. 2c it was observed that the extractions of both metal ions with Aliquat 336 are higher than other ones. Since the extractions of metal ions by amine extractants follow anion exchange (Eq. (2)) or solvation mechanism [23,24] then the increasing of metal extractions by Aliquat 336 may be due to the increasing basic property by the alkyl group, which improves the cation–anion ion pair and increases the stability of the complex formed in extraction process. Habashi reported that quaternary ammonium compounds such as Aliquat 336 are more basic extractants, which makes them more effective than other amine extractants due to their ability to extract metal ions at all acidic ranges of solutions [25]. But other amine extractants such as TOA can only be used in acidic or neutral conditions. The separation factors were considered in order to evaluate the optimum conditions for the separation of thorium and uranium. The mean of separation factors for separating thorium from uranium or direct process (DTh/DU) are shown in Fig. 3. The optimum separation factor was obtained from 5.0 M nitric acid solution by organic phase containing Aliquat-336. Under this condition, a separation factor of 33.07 and thorium extraction of 97.5% were obtained. These conditions are in applicable in real solvent extraction process because in addition to maximum possible separation factor, the thorium extraction is optimum. This optimum condition shows higher separation ability of Aliquat 336 than those of the extractants used in the Amex and Thorex processes [2,3]. Since the optimum conditions were used during the principal experiments (experiment 6), then there was no need to perform confirmatory experiments.
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chiometric relation of thorium extraction from nitrate solution by Aliquat 336 was studied. In the case of the extractions of metal ions by quaternary ammonium salts, anion exchange is thought to be the preferred mechanism involved and the extraction reaction can be written as Eq. (2). Alternative mechanism has also been proposed by Coleman et al. in the study of the extraction of uranium(VI) from sulfate solutions [24]. They suggested that extraction of uranium(VI) occurs via adduct formation. In the discussion of the anion exchange behavior of thorium, it was found that thorium forms anionic complexes in the high acid concentration that can be extracted by anion exchange extractants and resins [26]. Therefore, it seems that the preferred mechanism for extraction of thorium from nitrate medium by Aliquat 336 follows anion exchange mechanism, which can be written as:
mR3 CH3 Nþ Cl þ ðMðNO3 ÞðmþnÞ Þm () ðR3 CH3 NÞm MðNO3 ÞðmþnÞ
þ mCl
Fig. 3. The mean separation factor for extracting thorium from aqueous solution containing uranium.
ð4Þ
where, R3CH3N+Cl- and (M) are Aliquat 336 and metal ion, respectively To determine the extraction equilibrium of the system, the main extracted species should be considered. Over the years, the hydrolysis and complexation behavior of thorium(IV) has been studied [26–29]. Ekberg et al. show that Th(OH)+3 is the dominant hydrolysis species at the lower pH values [27]. Also, Moulin et al. [28] showed that thorium complexes are formed in the perchlorate media with the combinations of perchlorate and hydroxide ions. Since according to the investigations of Zebroski et al. [29], the stability constants of thorium complexes with nitrate anions are higher than those with perchlorate anions then thorium nitrate complexes were formed in the aqueous nitrate medium. Consequently, the extractable complex of Th(NO3)62 dominates the extraction system. The NMR spectra of Thorium Nitrato Complexes were studied by Marsh et al. [30]. They reported the presence of thorium hexa nitrato complex at higher nitric acid concentrations. Therefore, the proposed extraction equilibrium for thorium can be represented by the following reaction:
2R3 CH3 Nþ Cl þ ðThðNO3 Þ6 Þ
2
() ðR3 CH3 NÞ2 ThðNO3 Þ6
þ 2Cl
Fig. 4. The mean separation factor for extracting uranium from aqueous solution containing thorium.
ð5Þ
To verify our proposed mechanism in Eq. (5), the log–log plot of thorium distribution coefficient vs. extraction parameters were studied. The equilibrium constant for the extraction process in Eq. (4) is kex, where m
It is clear from Table 1 that the extractions of uranium in some experiments are higher than those of thorium (i.e., experiments 1, 2, 5, and 8). But this order of metal extractions is reversed in other experiments. Therefore, the separation factors of uranium over thorium (reverse process) were also considered (Fig. 4). According to Fig. 4, Aliquat-336 is also the best extractant for the selective extraction of uranium over thorium. But the optimum condition of the aqueous medium was achieved when 3.0 M hydrochloric acid was used (experiment 5). Under this condition, the extraction of uranium was 87.9% at the separation factor of about 25. Therefore, the separation of thorium and uranium using neutral extractants is applicable by two completely different processes. In the first process, thorium is selectively extracted from uranium, while in second process thorium is remained in the aqueous solution. 3.2. Equilibrium equations Since the optimum conditions for the separation of thorium from uranium were achieved from nitrate solutions, then the stoi-
kex ¼
½ðR3 CH3 NÞm MðNO3 Þmþn ½Cl
ð6Þ
m
½R3 CH3 N Cl ½ðMðNO3 ÞðmþnÞ Þm þ
By taking the log of Eq. (6) and rearranging it:
log D ¼ log kex m log½Cl þ m log½R3 CH3 Nþ Cl
ð7Þ
The log–log plot of thorium distribution coefficients vs the concentrations of Aliquat-336 in Fig. 5 shows the slope of 1. This slope suggests that the thorium complex was extracted with two molecules of Aliquat 336 (or m = 2). Thus, the proposed extraction mechanism in Eq. (5) is supported by the slope analysis of the experimental results. Since the optimum conditions for separating uranium from thorium was obtained from hydrochloric solution then the extraction mechanism of uranium from this acidic medium by Aliquat 336 was also studied. The Pourbaix diagram of uranium species [31] is shown in Fig. 6. It is clear that uranyl ion, UO2+2, is formed in oxidizing strong acid solutions that used in our study. Also, many previous researchers reported the aqueous species of uranium as monomeric divalent cations, UO2þ 2 [6].
M. Eskandari Nasab / Fuel 116 (2014) 595–600
Fig. 5. Effect of Aliquat 336 concentrations on the extraction of thorium from 5.0 M nitric acid medium.
599
Fig. 7. Effect of Aliquat 336 concentrations on the extraction of uranium from 3.0 M hydrochloric acid medium.
Fig. 8. Effects of HCl and HNO3 concentrations on the stripping of uranium and thorium from organic solvent containing Aliquat 336.
3.3. The optimum stripping conditions The stripping of uranium and thorium from loaded organic solvent containing aliquat-336 were studied using HNO3 and HCl as stripping agents. Stripping experiments were carried out by O/A = 1 and temperature of 25 °C. Fig. 8 indicated that 6.0 M HCl provides the optimum conditions for the selective stripping of thorium over uranium. 4. Conclusions Fig. 6. The Pourbiax diagram for uranium at the pressure and temperature of 105 Pa. and 298 °C, respectively [31].
The slope of log D for uranium extraction vs. [Aliquat 336] in Fig. 7 indicates that only one molecule of the extractant participates in the extraction process (m = 1). Therefore, the overall extraction reaction can be presented as:
R3 CH3 Nþ Cl þ ðUO2 Cl3 Þ () R3 CH3 NUOþ2 2 Cl3 þ Cl
ð8Þ
Abrao [32] reported a similar extractable complex of uranium (i.e., UO2 Cl3 ) during the extraction by tertiary amine tri-n-octyl amine. For the extraction of anionic metal complexes by high molecular weight quaternary ammonium salts, the complexation reactions at the aqueous–organic interface seem to provide the most realistic mechanism because quaternary ammonium salts are surface active reagents [33]. Therefore, in the extractions of uranium and thorium by Aliquat 336, the extraction processes include diffusion of the reagent to the interface, complexation at the interface and finally diffusion of the complex formed away from the interface.
The solvent extraction and separation of thorium and uranium from different acidic media by neural organophsphorus and amine extractants were investigated. The optimum conditions for the separation of thorium from uranium were nitric acid concentration of 5.0 M using Aliquat 336. Under this condition, in addition to obtain a maximum separation factor of 33.07, the recovery of thorium was almost quantitative. The optimum condition shows higher separation ability of Aliquat 336 than those of the extractants used in the conventional Amex and Thorex processes. For separating uranium from thorium, the results showed that Aliquat 336 is also provided the optimum conditions, while 3.0 M hydrochloride acid solution was the optimum aqueous solution. Therefore, at least two completely different processes could be used to separate thorium and uranium by Aliquat 336. The stoichiometric relations for the extraction of thorium from nitric acid solution and uranium from hydrochloride solution showed that thorium and uranium were extracted in the forms of anionic complexes through anion exchange mechanisms. Acknowledgements The author would like greatly wish to thank Mrs. Z. Moslemi for revising the English language of the manuscript.
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References [1] Seaborg GT, Katz JJ. The actinide elements, Vol 14A. New York: McGraw-Hill; 1957. [2] Habashi F. Handbook of extractive hydrometallurgy, vol. III. New York: WILEYVCH; 1997. [3] Crouse DJ, Brown KB. Recovery of thorium, uranium, and rare earths from monazite sulfate liquors by amine extraction (Amex) process. , Technical Report ORNL-2720. USA: Oak Ridge National Laboratory; 1957. [4] Gupta CK, Krishnamurthy N. Extractive metallurgy of rare earth elements. Int Mater Rev 1994;5:204–7. [5] Sato T. Extraction of uranium (VI) and thorium from nitric acid solutions by trin-butyl phosphate. J Appl Chem 1965;15:489–95. [6] Gupta B, Malik P, Deep A. Extraction of uranium, thorium and lanthanides using Cyanex-923: their separations and recovery from monazite. J Radioanal Nucl Ch 2002;252:451–6. [7] Meera R, Varma R L, Reddy MLP. Enhanced extraction of thorium (IV) and uranium (VI) with 1-phenyl-3-methyl-4-pivaloy-5-pyrazolone in presence of various neutral organophosphorus extractants. Radiochem Acta 2004;92:17–23. [8] Zhou ZM, Wang WC, Wang JS, Du HF, Ye ML. Kinetic studies on the stripping of thorium(IV) and uranium(IV) from tributylphosphate-kerosene with dilute nitric acid solution. J Radioanal Nucl Ch 1998;231:167–9. [9] Sahu SK, Chakravortty V, Reddy MLP, Ramamohan TR. The synergistic extraction of thorium (IV) and uranium (VI) with mixtures of 3-phenyl-1-4– benzoyl-5-isoxazolone and crown ethers. Talanta 2000;51:523–30. [10] Yu S, Ma L, Bao B. Synergistic extraction of U(VI) and Th(IV) with 4- benzoyl2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol-3-thione (HBMPPT) and tri-noctylphosphine oxide (TOPO) in toluene. J Radioanal Nucl Ch 1999;240:219–21. [11] Koladkar DV, Dhadke PM. Extraction and separation of Th(IV) and U(VI) from nitric acid media using PIA-8 and HDEHP. J Radioanal Nucl ch 2002;253:297–302. [12] Eskandari nasab M, Sam A, Alamdar milani S. Determination of optimum process conditions for the separation of thorium and rare earth elements by solvent extraction. Hydrometallurgy 2011;106:141–7. [13] Eskandari nasab M, Sam A, Alamdar milani S. Determination of optimum process conditions for solvent extraction of thorium using Taguchi method. J Radioanal Nucl ch 2011;287:239–45. [14] Eskandari nasab M, Alamdar milani S, Sam A. Extractive separation of Th(IV), U(VI), Ti(IV), La(III) and Fe(III) from Zarigan ore. J Radioanal Nucl ch 2011;288:677–83. [15] Hammad AK, Ibrahim EE, Hammad MR. Separation and purification of thorium and uranium from their concentrate. Arab J Nucl Sci Appl 1986;19:19–24.
[16] Hughes KC, Singh R. The isolation of thorium from monazite by solvent extraction Part I. Hydrometallurgy 1980;6:25–33. [17] Amaral JCBS, Morais CA. Thorium and uranium extraction from rare earth elements in monazite sulfuric acid leach liquor through solvent extraction. Miner Eng 2010;23:498–503. [18] Ranjit K. Design of experiments using the Taguchi approach: 16 steps to product and process improvement. USA: Wiley; 2011. [19] Sato T. Extraction of uranium from nitric acid solutions by long-chain aliphatic Amines. J Appl Chem 1965;15:92–8. [20] Alibrahim M, Shlewit H. Solvent extraction of uranium (VI) by tributyl phosphate/dodecane from nitric acid medium. Period Polytech Chem 2007;51:57–60. [21] Lurie J. Handbook of analytical chemistry. Mascow: Mir Publisher; 1975. p. 283–90. [22] Cathbert FL. Thorium Production Technology. USA: National Lead Company of Ohio; 1958. p.104–120. [23] Ritcey GM, Aahbrook AW. Solvent extraction. Amsterdam: Elsevier; 1984. [24] Coleman CF, Brown KB, Moore JG, Crouse DJ. Solvent extraction with alkyl amines. Ind. Eng. Chem. 1958;50:1758–62. [25] Habashi F. A texbook of hydrometallurgy. Canada: Laval University; 1993. p. 430–40. [26] Hyde EK. The radiochemistry of thorium. Berkeley (California): Lawrence radiation laboratory, University of California; 1960. p. 10–27. [27] Ekberg C, Albinsson Y, Comarmond MJ, Brown PL. Studies on the Complexation Behavior of Thorium(IV) 1. Hydrolysis Equilibria. J Solution Chem 2000;29:63–86. [28] Moulin C, Amekraz B, Hubert S, Moulin V. Study of thorium hydrolysis species by electrospray-ionization mass spectrometry. Anal Chim Acta 2001;441:269–79. [29] Zebroski EL, Alter HW, Heumann FK. Thorium complexes with chloride, fluoride, nitrate, phosphate and sulfate. J Am Chem Soc 1951;73:5646–50. [30] Marsh SF, Veirs DK, Jarvinen GD, Barr ME, Moddy EW. Molecularly engineered resins for plutonium recovery. Los Alamos Sci. 2000;26:454–63. [31] National Institute of Advanced Industrial Science and Technology, vol. 419. Atlas of Eh–pH diagrams. Japan: Report of Geological Survey; 2005. p. 264–67. [32] Abrão A. Technology of uranium purification. Impurities decontamination from uranyl chloride by extraction with tri-n-octylamine in the presence of thorium as compexing ligand. Brazil. Technical report of Instituto De Energia Atomia; 1976. p. 1–13. [33] Scibona G, Conte A, Scuppa B, Danesi PR. Proceedings of international solvent extraction conference. London: Society of Chemical Industry; 2011.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Solvent extraction separation of uranium(VI) and thorium(IV) with neutral organophosphorus and amine ligands M. Eskandari Nasab ⇑ Faculty of Mineral Processing Engineering, Zarand College, Bahonar University, P.O. Box 14579-73869, Kerman 7616914111, Iran
h i g h l i g h t s The effects of key parameters on the separation of U(VI) and Th(VI) were studied. Three different acids and neural amine extractants were simultaneously studied. HNO3 and HCl provide the optimum conditions for extracting thorium and uranium. The extraction mechanisms of uranium and thorium were studied.
a r t i c l e
i n f o
Article history: Received 20 April 2013 Received in revised form 26 July 2013 Accepted 15 August 2013 Available online 2 September 2013 Keywords: Uranium Thorium Separation Aliquat 336 TOA
a b s t r a c t Taguchi’s method was used to determine the optimum conditions for the separation of uranium and thorium using neutral extractants. The experimental conditions were studied in the concentration ranges of 0.01–5 M for three different acids (nitric acid, hydrochloric acid, and sulfuric acid) using tributylphosphate (TBP), trioctylamine (TOA), and tricaprylyl methyl ammonium chloride (Aliquat 336) as the extractants. The optimum condition for the separation of thorium from uranium was obtained at 5.0 M nitric acid solution using Aliquat 336. Under this condition, thorium was extracted selectively from uranium with the extraction efficiency and separation factor of 97.5% and 33.07, respectively. The equilibrium equations were also determined. The results showed that the optimum combination for uranium was extraction from 2.0 M hydrochloric acid solution by Aliquat 336. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The nuclear technology is an attractive option to obtain energy because of increasing energy demands and the threat of global warming due to CO2 emission caused by coal and hydrocarbon burning. At the present time, the nuclear power production is based on the uranium fuel cycle. Natural uranium contains only 0.7% of fissile isotope uranium-235. However, a large number of power reactors in the world are based on the enriched uranium fuel, which contains more than about 2% of uranium-235. Another fissile isotope particularly suitable for thermal reactors is uranium233, which is obtained by the neutron irradiation of thorium-232. Thus, uranium and thorium are the two most vital elements for nuclear energy industry [1]. Their natural sources such as monazite and thorite generally contain a sizeable fraction of uranium and thorium. Since the disposal of industrial waste effluents containing uranium and thorium has serious hazard impacts on the environment then the reprocessing of irradiated thorium to separate and ⇑ Tel.: +98 3412448226. E-mail address: [email protected] 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.08.043
purify uranium-233 is an important aspect of thorium fuel cycle, and it greatly influences the economics of the fuel cycle. Consequently, the separation and purification of uranium and thorium is of great importance. The methodology adopted for the separation of these metal ions from different ores and nuclear fuel reprocessing has always attracted the attention of separation scientists. Separation of uranium and thorium is relatively difficult because thorium does not display variable oxidation states. Thus, it should be used the difference in the complexation behavior of thorium and uranium to obtain the desired separation factors. In the conventional solvent extraction processes (i.e., Thorex and Amex process) [2,3], the recovery and separation of uranium and thorium are accomplished through a variety of methods after chemical attacking the mineral with sulfuric acid or sodium hydroxide [4]. In the Thorex process, 4 M nitric acid solution was contacted with 30% (v/v) TBP in n-dodecane has been used to separate thorium and uranium from each other and from fission products [2]. The Amex process was first developed in 1959 [3]. In this process, thorium and uranium are extracted in two solvent extraction cycle. In the first cycle, thorium is extracted with a primary amine, while
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uranium is extracted in the second cycle using secondary or tertiary amine. However, these processes have certain limitations largely due to the low solubility of Th(N03)4. x.TBP (where x = 3 or 4) in n-dodecane, low separation factor of uranium(VI) over thorium(IV), interference caused by hydrolytic degradation products of TBP and also need to multistage extraction and stripping process [5]. The disadvantages of these processes could be largely avoided if it were feasible to separate uranium and thorium, using another extractant. Previous reports have used other extractants for uranium/thorium separation [6–13]. In our previous studies [12–14], some cationic extractants such as Cyanex302 [bis(2, 4, 4-trimethylpentyl) thiophosphinic acid] and Cyanex272 [bis (2, 4, 4-trimethylpentyl) phosphinic acid] were studied as the alternative for TBP in the Thorex process. The results showed that the greatest separation factor (3.76) for the separation of thorium from uranium was obtained from 1.0 M sulfuric acid solution using Cyanex302. According to these results, uranium could be selectively extracted from thorium at 5.0 M nitric acid and its separation factor was almost equal to that obtained by Cyanex302. Therefore, two individual processes such as extraction by TBP (the first step) and Cyanex302 (the second step) was proposed to separate thorium from uranium [14]. Also, Hammad et al. [15] proposed two individual extraction process for separating thorium and uranium. They separated uranium from Egyptian monazite alkaline hydrous oxide cake concentrate dissolved in concentrated HCl solution by anion exchanger column containing Amberlite IRA-400. Afterward, thorium in the effluent solution was purified by extraction from nitric acid medium by TBP diluted in kerosene. The long chain amines had shown to be excellent extractants for a great number of anionic metallic complexes, which are extensively used to extract thorium and uranium [3,16,17]. The successful development of the solvent extraction processes using alkyl amines for recovering and separating uranium and thorium stared since 1958 [3]. Hughes and Singh [16] extracted thorium from sulfate leach solution by 0.1 M Adogen-383 (secondary amine) diluted in kerosene. Amaral and Morais reported that Prime JM-T (primary amine) extracts preferably thorium, while Alamine336 (tertiary amine) extracts uranium [17]. In the present study, the extraction and separation of uranium and thorium have been investigated with neutral extractants such as trioctyl amine (TOA) or quaternary ammonium salt (tricaprylyl methyl ammonium chloride or Aliquat-336) as an alternative for TBP using Taguchi method. 2. Experimental
emission spectroscopy (ICP-OES) analyzer from Liberty 220 Varian. Mechanical shaker was used to contact organic and aqueous phases. 2.3. Solvent extraction experiments Solvent extraction experiments (Table 1) were carried out using a mechanical shaker. The equal volumes of two phases were mixed. The constant parameters including organic to aqueous volume ratio, temperature, contact time of two phases, and phase volumes were adjusted at 1:1, 25 °C, 10 min and 10 ml, respectively. The two phases were separated by separatory funnel and the concentrations of the metallic ions in aqueous phases before and after extractions were determined using ICP. Then, the distribution ratio, D, and extraction percent, E, were calculated from Eqs. (1) and (2), respectively:
D¼
ðC 0 CÞ C0
ð1Þ
E¼
D ðD þ V a =V o Þ
ð2Þ
Here C0, C, Va and Vo are the concentrations of metallic ions in the aqueous phases before and after extractions, aqueous and organic phase volume, respectively. 2.4. Experimental design The Taguchi method has been proposed as powerful method of experimental design [18] and has been used to determine the best experimental design in solvent extraction processes [13]. In this method, the signal to noise (S/N) ratio is used to determine the best experimental conditions. Because of the purpose of this work, the S/N ratio (the higher the better) was used to evaluate the response of each experiment [18] through the following equation:
ðS=NÞi ¼ 10 log
! 1X 1 n i y2i
ð3Þ
where, n is the number of tests and yi is the experimental value in the Ith experiment. In the present study, three factors at three levels, i.e. acid concentration, acid type and extractant were considered. For three parameters each at three levels, the traditional full factorial design would require 27 experiments. However, in the current design (Taguchi L9 orthogonal array) the required experiments are only nine. The levels of combinations of these experiments have been shown in Table 1. The results reported in this article are the mean values of two replications at each set of experiments.
2.1. Reagents TBP (Tri-n-butyl phosphate), TOA (tri-octyl amine) and Aliquat336 [(R3NCH3)+Cl, R = octyl/decyl] were produced by Cytec, Canada. Kerosene, from Fluka, was used as a diluent for economic and practical reasons. Stock aqueous solutions of thorium(IV) and uranium(VI) were prepared by dissolving appropriate amounts of Th(NO3)4.5H2O (Merck) and UO2(NO3)2.6H2O (Merck) in distilled water. 1 cm3 of concentrated nitric acid was added to 100 cm3 of solution to prevent hydrolysis. The initial metallic ion concentrations were maintained at 104 mol dm3 for all experiments. All other chemical reagents used were of analytical grades. 2.2. Apparatus The concentrations of thorium and uranium in the aqueous phases were determined by an inductively coupled plasma optical
3. Results and discussion 3.1. The optimum process conditions Fig. 1 shows the S/N ratios for the extractions of uranium and thorium. According to Taguchi’s method, a high influence on extraction or separation factor is shown by a high S/N ratio. According to Fig. 1, the optimal combinations were the extraction of thorium from 5.0 M sulfuric acid by TOA or Aliquat-336. For uranium, the maximum extractions were obtained from 5.0 M hydrochloric acid. Also, TOA or Aliquat-336 is the best extractant for extraction of uranium. Fig. 2a indicates that the average distribution coefficients of uranium and thorium increase with increasing acid concentration (except for the extraction of uranium at 5.0 M). Sato reported that the preferred extraction mechanisms of metals by the amine
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M. Eskandari Nasab / Fuel 116 (2014) 595–600 Table 1 Taguchi’s L9 orthogonal array design for extractive separation of uranium and thorium by neutral extractants. Test no
1 2 3 4 5 6 7 8 9
Extraction parameters
SN ratio
Extraction (%)
Distribution coefficient (D)
Separation factor
Acid conc. (M)
Acid type
Solvent type
U
Th
U
Th
U
Th
DTh/DU
DU/DTh
1 1 1 3 3 3 5 5 5
HNO3 HCl H2SO4 HNO3 HCl H2SO4 HNO3 HCl H2SO4
TBP TOA Aliquat-336 TOA Aliquat-336 TBP Aliquat-336 TBP TOA
8.9 25.4 27.3 38.1 27.0 26.5 39.8 27.4 32.6
9.3 35.1 13.2 29.8 38.9 18.2 34.8 37.7 27.0
2.9 57.1 4.6 30.8 87.9 8.1 53.8 76.9 22.5
0.4 18.6 23.2 80.7 22.5 21.1 97.5 23.4 39.6
0.03 1.33 0.05 0.45 7.28 0.09 1.16 3.33 0.29
0.00 0.23 0.30 4.19 0.29 0.27 38.44 0.31 0.66
0.12 0.17 6.29 9.39 0.04 3.02 33.07 0.09 2.26
8.38 5.83 0.16 0.11 25.06 0.33 0.01 10.91 0.44
Table 2 Instability constants of uranium and thorium complexes with studied anions [21].
U
Th
*
Fig. 1. The S/N ratio for the extraction of thorium and uranium by different acids and neutral extractants.
Fig. 2. The mean distribution coefficients for the extraction of thorium and uranium.
extractants is anion exchange [19], while solvation mechanism for extracting by TBP [20]. Therefore, with increasing acid concentration, the neutral or anionic complexes of metals will be formed and the extraction equations (e.g. Eq. (2)) tend to right. The complex stability constants in Table 2 show that at higher acid concentrations, thorium forms neutral complexes. While, for uranium this
Ion type
Instability constant
NO3ClSO4-2 NO3ClSO4-2
K1 = 25.1*; K2 = 1*; K3 = 0.13 K1 = 1.3; K2 = 6.6*; K3 = 50.2 K1 = 1.9*; K2 = 3.3 *105 K1 = 0.16; K2 = 0.48; K3 = 1.3; K4 = 1.8* K1 = 0.04; K2 = 10.5; K3 = 1.4; K4 = 5.5* K1 = 6.3 * 103; K2 = 3.5 *104⁄
Neutral complex.
mechanism is valid only at low content of ion concentrations. Therefore, at higher acid concentrations, extraction of uranium is slightly decreased. Fig. 2b shows that the average distribution coefficient of thorium from nitric acid medium is larger than that of other ones due to more hydrophobic properties of nitrate species [22]. While, for uranium the maximum distribution coefficient was obtained from hydrochloric acid medium. Table 2 shows that uranium forms more strong anionic complexes with hydrochloride ions. Therefore, hydrochloric solution gives more extractable uranium species. Also, according to the results showed in Table 1, the extraction of uranium is much higher than thorium only at hydrochloric media. The effect of extractant type has been shown in Fig. 2c. From Fig. 2c it was observed that the extractions of both metal ions with Aliquat 336 are higher than other ones. Since the extractions of metal ions by amine extractants follow anion exchange (Eq. (2)) or solvation mechanism [23,24] then the increasing of metal extractions by Aliquat 336 may be due to the increasing basic property by the alkyl group, which improves the cation–anion ion pair and increases the stability of the complex formed in extraction process. Habashi reported that quaternary ammonium compounds such as Aliquat 336 are more basic extractants, which makes them more effective than other amine extractants due to their ability to extract metal ions at all acidic ranges of solutions [25]. But other amine extractants such as TOA can only be used in acidic or neutral conditions. The separation factors were considered in order to evaluate the optimum conditions for the separation of thorium and uranium. The mean of separation factors for separating thorium from uranium or direct process (DTh/DU) are shown in Fig. 3. The optimum separation factor was obtained from 5.0 M nitric acid solution by organic phase containing Aliquat-336. Under this condition, a separation factor of 33.07 and thorium extraction of 97.5% were obtained. These conditions are in applicable in real solvent extraction process because in addition to maximum possible separation factor, the thorium extraction is optimum. This optimum condition shows higher separation ability of Aliquat 336 than those of the extractants used in the Amex and Thorex processes [2,3]. Since the optimum conditions were used during the principal experiments (experiment 6), then there was no need to perform confirmatory experiments.
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chiometric relation of thorium extraction from nitrate solution by Aliquat 336 was studied. In the case of the extractions of metal ions by quaternary ammonium salts, anion exchange is thought to be the preferred mechanism involved and the extraction reaction can be written as Eq. (2). Alternative mechanism has also been proposed by Coleman et al. in the study of the extraction of uranium(VI) from sulfate solutions [24]. They suggested that extraction of uranium(VI) occurs via adduct formation. In the discussion of the anion exchange behavior of thorium, it was found that thorium forms anionic complexes in the high acid concentration that can be extracted by anion exchange extractants and resins [26]. Therefore, it seems that the preferred mechanism for extraction of thorium from nitrate medium by Aliquat 336 follows anion exchange mechanism, which can be written as:
mR3 CH3 Nþ Cl þ ðMðNO3 ÞðmþnÞ Þm () ðR3 CH3 NÞm MðNO3 ÞðmþnÞ
þ mCl
Fig. 3. The mean separation factor for extracting thorium from aqueous solution containing uranium.
ð4Þ
where, R3CH3N+Cl- and (M) are Aliquat 336 and metal ion, respectively To determine the extraction equilibrium of the system, the main extracted species should be considered. Over the years, the hydrolysis and complexation behavior of thorium(IV) has been studied [26–29]. Ekberg et al. show that Th(OH)+3 is the dominant hydrolysis species at the lower pH values [27]. Also, Moulin et al. [28] showed that thorium complexes are formed in the perchlorate media with the combinations of perchlorate and hydroxide ions. Since according to the investigations of Zebroski et al. [29], the stability constants of thorium complexes with nitrate anions are higher than those with perchlorate anions then thorium nitrate complexes were formed in the aqueous nitrate medium. Consequently, the extractable complex of Th(NO3)62 dominates the extraction system. The NMR spectra of Thorium Nitrato Complexes were studied by Marsh et al. [30]. They reported the presence of thorium hexa nitrato complex at higher nitric acid concentrations. Therefore, the proposed extraction equilibrium for thorium can be represented by the following reaction:
2R3 CH3 Nþ Cl þ ðThðNO3 Þ6 Þ
2
() ðR3 CH3 NÞ2 ThðNO3 Þ6
þ 2Cl
Fig. 4. The mean separation factor for extracting uranium from aqueous solution containing thorium.
ð5Þ
To verify our proposed mechanism in Eq. (5), the log–log plot of thorium distribution coefficient vs. extraction parameters were studied. The equilibrium constant for the extraction process in Eq. (4) is kex, where m
It is clear from Table 1 that the extractions of uranium in some experiments are higher than those of thorium (i.e., experiments 1, 2, 5, and 8). But this order of metal extractions is reversed in other experiments. Therefore, the separation factors of uranium over thorium (reverse process) were also considered (Fig. 4). According to Fig. 4, Aliquat-336 is also the best extractant for the selective extraction of uranium over thorium. But the optimum condition of the aqueous medium was achieved when 3.0 M hydrochloric acid was used (experiment 5). Under this condition, the extraction of uranium was 87.9% at the separation factor of about 25. Therefore, the separation of thorium and uranium using neutral extractants is applicable by two completely different processes. In the first process, thorium is selectively extracted from uranium, while in second process thorium is remained in the aqueous solution. 3.2. Equilibrium equations Since the optimum conditions for the separation of thorium from uranium were achieved from nitrate solutions, then the stoi-
kex ¼
½ðR3 CH3 NÞm MðNO3 Þmþn ½Cl
ð6Þ
m
½R3 CH3 N Cl ½ðMðNO3 ÞðmþnÞ Þm þ
By taking the log of Eq. (6) and rearranging it:
log D ¼ log kex m log½Cl þ m log½R3 CH3 Nþ Cl
ð7Þ
The log–log plot of thorium distribution coefficients vs the concentrations of Aliquat-336 in Fig. 5 shows the slope of 1. This slope suggests that the thorium complex was extracted with two molecules of Aliquat 336 (or m = 2). Thus, the proposed extraction mechanism in Eq. (5) is supported by the slope analysis of the experimental results. Since the optimum conditions for separating uranium from thorium was obtained from hydrochloric solution then the extraction mechanism of uranium from this acidic medium by Aliquat 336 was also studied. The Pourbaix diagram of uranium species [31] is shown in Fig. 6. It is clear that uranyl ion, UO2+2, is formed in oxidizing strong acid solutions that used in our study. Also, many previous researchers reported the aqueous species of uranium as monomeric divalent cations, UO2þ 2 [6].
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Fig. 5. Effect of Aliquat 336 concentrations on the extraction of thorium from 5.0 M nitric acid medium.
599
Fig. 7. Effect of Aliquat 336 concentrations on the extraction of uranium from 3.0 M hydrochloric acid medium.
Fig. 8. Effects of HCl and HNO3 concentrations on the stripping of uranium and thorium from organic solvent containing Aliquat 336.
3.3. The optimum stripping conditions The stripping of uranium and thorium from loaded organic solvent containing aliquat-336 were studied using HNO3 and HCl as stripping agents. Stripping experiments were carried out by O/A = 1 and temperature of 25 °C. Fig. 8 indicated that 6.0 M HCl provides the optimum conditions for the selective stripping of thorium over uranium. 4. Conclusions Fig. 6. The Pourbiax diagram for uranium at the pressure and temperature of 105 Pa. and 298 °C, respectively [31].
The slope of log D for uranium extraction vs. [Aliquat 336] in Fig. 7 indicates that only one molecule of the extractant participates in the extraction process (m = 1). Therefore, the overall extraction reaction can be presented as:
R3 CH3 Nþ Cl þ ðUO2 Cl3 Þ () R3 CH3 NUOþ2 2 Cl3 þ Cl
ð8Þ
Abrao [32] reported a similar extractable complex of uranium (i.e., UO2 Cl3 ) during the extraction by tertiary amine tri-n-octyl amine. For the extraction of anionic metal complexes by high molecular weight quaternary ammonium salts, the complexation reactions at the aqueous–organic interface seem to provide the most realistic mechanism because quaternary ammonium salts are surface active reagents [33]. Therefore, in the extractions of uranium and thorium by Aliquat 336, the extraction processes include diffusion of the reagent to the interface, complexation at the interface and finally diffusion of the complex formed away from the interface.
The solvent extraction and separation of thorium and uranium from different acidic media by neural organophsphorus and amine extractants were investigated. The optimum conditions for the separation of thorium from uranium were nitric acid concentration of 5.0 M using Aliquat 336. Under this condition, in addition to obtain a maximum separation factor of 33.07, the recovery of thorium was almost quantitative. The optimum condition shows higher separation ability of Aliquat 336 than those of the extractants used in the conventional Amex and Thorex processes. For separating uranium from thorium, the results showed that Aliquat 336 is also provided the optimum conditions, while 3.0 M hydrochloride acid solution was the optimum aqueous solution. Therefore, at least two completely different processes could be used to separate thorium and uranium by Aliquat 336. The stoichiometric relations for the extraction of thorium from nitric acid solution and uranium from hydrochloride solution showed that thorium and uranium were extracted in the forms of anionic complexes through anion exchange mechanisms. Acknowledgements The author would like greatly wish to thank Mrs. Z. Moslemi for revising the English language of the manuscript.
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