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Determination of optimum process conditions for the separation of thorium and rare earth elements by solvent extraction
Hydrometallurgy 106 (2011) 141–147
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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Determination of optimum process conditions for the separation of thorium and rare earth elements by solvent extraction M. Eskandari Nasab a,⁎, A. Sam a, S.A. Milani b a b
Faculty of Mineral Processing Engineering, Bahonar University, Kerman 7616914111, Iran Jaber Ibn Hayan Research labs, Tehran, P. O. Box 14893-836, Iran
a r t i c l e
i n f o
Article history: Received 21 August 2010 Received in revised form 13 December 2010 Accepted 14 December 2010 Available online 23 December 2010 Keywords: Thorium Extraction Rare earth elements Cyanex272 Taguchi
a b s t r a c t The solvent extraction of thorium, some rare earth elements (including lanthanum, cerium and yttrium) and iron was separately investigated in order to determine their optimum separation conditions. The experimental conditions were studied in the concentration range of 0.01–5 M for three acids using Cyanex272, Cyanex302 and TBP (HA) as extractants by Taguchi's method. It was found that Cyanex272 could separate thorium and rare elements more efficiently compared with the conventional process (TBP). The optimum acid concentration in the case of Cyanex272 was determined as 0.5 M HNO3. The graphical method demonstrated that the composition of the extracted thorium complex was as Th(OH)2(NO3)A.HA. Furthermore, the stripping experiments showed that using a mixture of 1 M H2SO4 and 2.7 ⁎ 10−4 M EDTA could provide optimum conditions for the selective stripping of thorium. As a result, thorium was extracted selectively from 0.5 M nitric acid solution of Zarigan ore with the extraction efficiency of 83%. Also yttrium was separated from lanthanides by synergistic mixture containing Cyanex272 and TBP. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Rare earth elements are used in a wide range of products and processes such as ceramics, carbon arc lamps, alloys, catalysts, etc. In addition, the use of thorium as a replacement of natural uranium could increase substantially the total nuclear energy resources in the future. Monazite, bastnasite and xenotime are natural deposits of lanthanides containing thorium. Therefore, the methods for the extraction and separation of these metal ions from different ores have always attracted the attention of separation scientists. It may also be used to decontaminate the rare earth concentrates from thorium-bearing radioactive material or to produce nuclear fuel. The conventional solvent extraction technology uses TBP/HNO3 contacted counter currently with 40% (v/v) TBP in kerosene. Under these conditions, the distribution coefficients for thorium and rare earth elements were 1 and 0.02, respectively (Habashi, 1997). Preston and Preez (1990) reported that the neutral organophosphorus reagents like TBP were only applicable in the bulk recovery of the rare earth elements from nitrate solutions, but the separation factors for adjacent rare earth pairs were generally small, ranging from 1.2 to 2.2. In another process, the purification of individual rare earth elements and other elements of monazite and bastnasite was carried out using a variety of extractants including acidic and neutral organophosphorus compounds, amines and carboxylic acids (McGill,
⁎ Corresponding author. Tel.: +98 2166591428. E-mail address: [email protected] (M.E. Nasab). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.12.014
1993). These processes are relatively expensive due to a large number of mixer–settler stages, high cost of reagent and emulation problems. The disadvantages of these processes could largely be eliminated if it is feasible to separate these elements using another extractant. The organophosphorus compounds have been studied as an important group of extractants for solvent extraction and separation of thorium and rare earth elements (Jain, 2006; Saji and Reddy, 2003; Siekierski and Fidelis, 1975; Sun et al., 2005; Wang et al., 2002; El-Hefny and Daoud, 2004; Gupta et al., 2002; Karve and Gaur, 2006; Jia et al., 2008). Among them, the acidic extractants have shown very high distribution coefficients for thorium extraction from nitrate media at pH=3. For example, Karve and Gaur (2006) reported the distribution coefficient of thorium in 0.001 M HNO3 as 110.10. Some researchers have used the synergistic effects of chelate reagents to improve the separation of rare earth elements with acidic extractants. Sun et al. (2005) and Jia et al. (2008) respectively investigated the synergistic effects of sec-octylphenoxyacetic acid for the separation of rare earth elements by Cyanex272 and D2EHPA. For the neutral extractants, the use of Cynex923 by Gupta et al. (2002) has resulted in high extraction and separation efficiency of thorium. The aforementioned methods had also certain limitations such as complex formation of TBP with fluoride, sulfate, phosphate and carbonate anions, emulation formation of D2EHPA, poor selectivity of Cyanex921 and need to multistage stripping for Cyanex923. Consequently, the search for better extractants and conditions continues. Koh et al. (2005) reported that Cyanex302 [bis(2,4,4-trimethylpentyl) thiophosphinic acid] was much better than Cyanex272 [bis(2,4,4-trimethylpentyl) phosphinic acid] in extracting metallic ions at low pH area. Taguchi's orthogonal array design method is an efficient means for evaluation of solvent extraction
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process. Some investigations have tried to determine the optimum conditions in solvent extraction processes (Eskandari et al., 2010; Taghizadeh et al., 2008). Taghizadeh et al. (2008) have investigated the optimum process conditions for the extraction and separation of zirconium and hafnium using orthogonal array L9. The experimental conditions were studied in the range of 0.1 to 2.0 M for three different acids and TBP, D2EHPA or Cyanex272 as extractant. They reported the optimum extraction of zirconium as 71% when 2.0 M nitric acid and Cyanex272 were used. In the present work, the extraction behaviors of thorium, lanthanum, cerium, yttrium and iron are investigated with Cyanex272 or Cyanex302 as an alternative for common solvent extraction process (TBP) using Taguchi's method. Then, the study is continued to develop a process to separate thorium and rare earths elements from Zarigan leach solution. 2. Material and methods 2.1. Experimental design Fig. 1. S/N graphs for extraction of thorium, lanthanum, cerium, yttrium and iron.
The Taguchi's orthogonal array design is a powerful effective method for evaluation and improvement of laboratory and continuous process efficiencies (Antony and Antony, 2001). 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 (Ranjit, 2001) through the following equation:
ðS =NÞi = −10 log
1 1 ∑ n i y2i
!!
CeCl3·7H2O, YCl3·6H2O and FeCl3·6H2O (all from Merck) were used as the sources of Th(IV), La(III), Ce(III), Y(III) and Fe(III), respectively. Real experiments were carried out from the leach solution of Zarigan ore. The leachate solution was obtained in our previous works in Jaber Ibn Hayan research labs of Tehran. All chemicals used in this study were of analytical grade and applied without further purification.
ð1Þ 2.3. Procedures
where, n is the number of tests and yi is the experimental value in the Ith experiment. In the present study, three factors (acid type, acid concentration and extractant type) were considered in three levels. The traditional full factorial design would require 27 experiments for these three factors each at three levels. However, in the current design (Taguchi's L9 orthogonal array), only nine experiments are required. The extraction factors and their selected levels are presented in Table 1. The results are the mean values of two applications at each set of experiments. 2.2. Chemicals and reagents Cyanex272 and Cyanex302, both from Cytec (Canada), and TBP from Merck (Germany) were used as extractants. Kerosene was supplied by Fluka. Sulfuric, nitric, and hydrochloric acids (all from Fluka) and EDTA (from Merck) were used as stripping reagents and complexing agents, respectively. Th(NO 3 )4 ·5H2 O, LaCl 3·7H2O,
The aqueous phases were prepared by dissolving appropriate amounts of thorium, lanthanum, cerium, yttrium or iron powder in a solution containing 0.01 to 5.0 M of each acidic medium. The concentration of each metal ion was 10−4 M. The necessary concentration of Cyanex272 or Cyanex302 for an acceptable level of thorium extraction was determined through the preliminary tests. The concentration of each extractant was 0.1 M. All of these concentrations are excess of the stoichiometric quantities of metallic ions. Extraction experiments were carried out from mixed metal solutions according to Table 1. Each experiment was accomplished by extracting all the elements individually and the optimum conditions for each metal ion were calculated. Unless otherwise stated, the experiments were carried out at the phase ratio of O/A = 1 and the temperature of 25° C. Equal volumes (10.0 mL) of organic solutions and aqueous solutions were mixed and shaken using a mechanical shaker for 10 min. Then the organic and aqueous phases were separated by a separatory funnel. Stripping experiments were carried out using different concentrations of three acids (sulfuric, nitric and
Table 1 Taguchi's L9 orthogonal array design for separation of thorium, rare earth elements and iron using solvent extraction. Parameters
Results
Test No
Acid conc. (M)
Acid type
Solvent type
Th extraction (%)
1 2 3 4 5 6 7 8 9 Confirmatory
0.01 0.01 0.01 1 1 1 5 5 5 0.01
HNO3 HCl H2SO4 HNO3 HCl H2SO4 HNO3 HCl H2SO4 HNO3
Cyanex302 Cyanex272 TBP Cyanex272 TBP Cyanex302 TBP Cyanex272 Cyanex302 Cyanex272
97.4 97.4 10.4 94.6 4.9 5.4 19.3 16.7 47.7 98.7
a
Separation factor = DTh/DM (M: other metal).
Separation factorsa Th/La
Th/Ce
Th/Y
Th/Fe
118.5 134.5 0.49 50.63 0.16 0.11 0.48 0.51 0.66 190.8
226.6 198.6 0.78 60.98 0.26 0.14 0.79 0.70 0.77 279.9
25.9 2.19 0.09 11.6 0.04 0.03 0.15 0.13 0.28 3.5
4.23 0.68 0.36 16.04 0.10 0.10 0.46 0.07 0.71 1.3
M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147
143
Fig. 4. The amounts of thorium separation factors from lanthanum, cerium, yttrium and iron at different nitric acid concentrations: [extractant] = 0.1 M Cyanex272 in kerosene.
Fig. 2. Mean of means for separation factors.
hydrochloric) for 5 min. All aqueous solution analyses were accomplished using ICP-OES. 3. Results and discussions 3.1. Optimization of extraction parameters Fig. 1 shows the S/N ratio for the extraction efficiency of thorium, lanthanum, cerium, yttrium and iron. According to Taguchi's method, a high influence on extraction or separation factor is shown by a high S/N ratio. As shown in Fig. 1, the optimum combination for extraction of thorium was extraction from 0.01 M nitric acid solution by Cyanex272 in kerosene. Also Cyanex272 is also the best extractant for extraction of other studied elements, while for both iron and yttrium, the optimum extraction percents were obtained from 0.01 M hydrochloric acid. For lanthanum and cerium, the optimum extraction condition was 5.0 M sulfuric acid. Therefore, the separation factors were considered in order to evaluate the amount of thorium separations from other elements. Fig. 2 shows that the highest separation factors for separation of thorium from cerium and lanthanum were obtained with 0.01 M nitric acid using Cyanex272. The optimum conditions for separation of thorium from yttrium and iron were the extractions from 0.01 M HNO3 using Cyanex302 (experiment 1) and 1.0 M HNO3 by Cyanex272 (experiment 4), respectively. Since the optimum conditions for thorium separation from cerium and lanthanum were not obtained in the main experiments, one confirmatory experiment was carried out with their optimum separation levels. The results
Fig. 3. Extraction of thorium, iron, lanthanum, cerium and yttrium versus nitric acid concentration by Cyanex272.
confirmed the optimum conditions because they gave the thorium separation factors from cerium and lanthanum higher than those in the main experiments (Table 1). More experiments were carried out for a comprehensive understanding of the extraction process of thorium and rare earth elements. The results are shown in Figs. 3, 4, 5 and 6. Fig. 3 shows the extraction efficiencies of thorium, yttrium, cerium, lanthanum and iron from 0.0015 to 7 M nitric acid by 0.1 M Cyanex272 diluted in kerosene. It is clear that the extraction amounts increase in the following order: La (III) b Ce(III) b Y(III) b Fe(III) b Th(IV), which is in consistency with the observations of Jia et al. (2008) for some rare earth elements with D2EHPA. The increasing order of the extraction of metallic ions from Th(IV) to La(III), shown in Table 2, is in accordance with the increasing of the charge to radius ratios (z/r) (except for Fe(III)). The relationship between extraction percent and charge to radius ratio is against the Born equation (Choppin et al., 2004), which indicates that for the same ligands, with constant structural effects, the stability constants of the cations are related to “z/r”. Less extraction percent of iron with cationic extractant has also been reported in some of previous works. For instance, Da Silva et al. (2008) obtained similar results when they conducted the extraction of iron with D2EHPA. It is clear from Fig. 3 that the extraction of thorium starts to reduce from 0.5 M nitric acid concentration. But for other elements, the extractions are decreased with the increasing of nitric acid concentration up to 0.5 M. Earlier investigations on the extraction of thorium reported that the extraction of thorium and rare earth elements with acidic extractants at low acid concentrations followed the cationic exchange mechanism (Jia et al., 2008; El-Hefny and Daoud, 2004; Karve and Gaur, 2006; Jia et al., 2008): +n M
aq:
m þ +ð ðHAÞ2 Þorg: ⇔ ðMðAn Þ×ðm−nÞHAÞorg: +ðnH Þorg: 2
ð2Þ
Fig. 5. Comparison between thorium separations from yttrium versus nitric acid concentration using Cyanex272 and Cyanex302 extractants: [each extractant] = 0.001 M in kerosene.
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M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147 Table 2 Cationic charge to radius ratio for the studied metal ions Choppin et al. (2004). Ion
Th(IV)
Fe(III)
Y(III)
Ce(III)
La(III)
Z/ra
4
4.6
3.3
3
2.8
a
Fig. 6. Extraction of thorium, iron, lanthanum, cerium and yttrium vs. nitric acid concentration using TBP.
where, m is the number of extractant HA and n is the charge of thorium aqueous complex. According to this extraction mechanism, the equilibrium tends toward left with the increasing of acid concentration, hence, extraction percent is decreased at low acid concentrations. The extraction amounts of lanthanides (La(III), Ce(III) and Y(III)) were increased at the highest acid concentrations due to change in their extraction mechanisms from cation exchange to solvation, which is, in turn, because of the formation of neutral complex with nitrate anions. El-Hefny and Daoud (2004) reported that the extractable complexes of thorium by acidic extractants contain stronger ligands of hydroxide (Lurie, 1975) at low nitric acid area. This is why the extraction of thorium started to reduce at higher acid concentrations. Therefore, the reasonable separation factors were achieved only in 0.5 M nitric acid (Fig. 4). As shown in Fig. 4, the selective extractions were decreased in the order; Th(IV)/La(III) b Th(IV)/Ce(III) b Th(IV)/Y(III) b Th(IV)/Fe(III). Also the separation factors of thorium from lanthanum and cerium in 0.01 M HNO3 were higher than those in 1 M and 5 M HNO3, while the thorium separation factor from iron in 1 M HNO3 was more than in other acid concentrations. These results are in good agreement with the trends obtained by Taguchi's method for the nitrate media and acid concentrations. The results of separating thorium from yttrium by Cyanex272 and Cyanex302 extractants in nitric acid medium are shown in Fig. 5. Since Cyanex302 is a stronger acid than Cyanex272, and the degree of acidic strength of the studied lanthanides increases from yttrium to lanthanum, thereafter, according to HSAB1 theory (Choppin et al., 2004), Cyanex302 gives lower yttrium extraction amount than Cyanex272 does. Fig. 5 clearly shows that Cyanex302 is more efficient than Cyanex272 for separating thorium and yttrium, while this order is reversed for separating thorium from less acidic lanthanides such as cerium and lanthanum. These results are also in accordance with those obtained by Taguchi's method. The separation curves of all studied elements were plotted using graphical representations in Excel. Whereby, the optimum nitric acid concentrations for mutual separations of all elements were determined (the values are presented in parentheses of Table 3). As Table 3 obviously shows, the optimum nitric acid concentration for thorium separation from lanthanides and iron by Cyanex272 is 0.5 M, which is clearly more than 0.001 M, reported in the previous investigations as the optimum condition for solvent extraction of thorium alone (El-Hefny and Daoud, 2004; Karve and Gaur, 2006). Table 3 also shows that the separation factor of 2.4 was obtained for separating cerium from lanthanum in 1.5 ⁎ 10−3 M nitric acid, which is more than that obtained by Jia et al., 2009 with Cyanex301.
1
Hard–soft acid and base.
Unit of r = 10 nm.
The effect of HNO3 concentrations on the extraction of elements by 0.1 M TBP is given in Fig. 6. The extraction of thorium was found to decrease with the increase in nitric acid concentration, while the extractions of other elements were increased (except in 2 M). Therefore, thorium can only be separated from other elements at higher nitric acid concentrations by extracting them in TBP and leaving thorium in the aqueous phase (reverse process). The results showed that the separation factors of thorium by TBP were lower than those by Cyanex272 and Cyanex302. These results confirmed Taguchi's method results as well (Fig. 2). Since 0.5 M nitric acid was determined as the optimum nitric acid concentration for thorium separation from rare earth elements, therefore, to investigate the synergistic effects of mixed ligands on the separation of rare earth elements (La(III), Ce(III) and Y(III)) from raffinate, their extractions from 0.5 M nitric acid solutions with mixtures of Cyanex272 and TBP (summation of extractants concentrations = 1.5 M) were also studied. Fig. 7 shows the extraction amounts of Ln+ 3 and Y+ 3 at different ratios of the extractant mixtures. The results demonstrate that the mixtures have synergistic effects on all of the studied rare earth elements. The separation of Ln+ 3 and Y+ 3 from each other is known to be difficult because of their similar chemical properties. As Fig. 7 shows, the different extraction behaviors of Ln+ 3 and Y+ 3 with extractant mixtures could be considered for their separation from each other. Table 4 shows the separation factors between Ln+ 3 and Y+ 3 at different extractant mixtures. As shown, the optimum separation between Ln+ 3 and Y+ 3 was obtained by an extractant mixture containing 1.2 M Cynaex272 and 0.3 M TBP. Therefore, the mixture of Cyanex272 and TBP could be used for separating Y+ 3 from Ln+ 3. It is clear form Table 4 that Cyanex272 is better than the conventional process (i.e., TBP) for separating Ln+ 3 and Y+ 3. Our primary studies for comparing the effects of different acidic extractants such as Cyanex272, D2EHPA and Cyanex302 at different pH areas showed that D2EHPA was the best extractant for extracting lanthanum and cerium from aqueous solutions (Table 5). 3.2. Effect of various reagents on the stripping of thorium, lanthanum and iron Since appreciable amounts of iron could be extracted in the organic phase along with thorium (Fig. 3), the stripping behaviors of thorium, iron and lanthanum (as the representatives of lanthanides) were investigated using varying concentrations of different acids. Fig. 8 shows that 1 M H2SO4 and 2 M HCl were effective for the selective stripping of thorium and lanthanum from iron, while 0.5 M HCl and 2 M HNO3 could be used for the selective stripping of iron from other elements. The complex stability constants of sulfate ions with thorium and lanthanum are stronger than with iron (Lurie, Table 3 Separation factors of the studied elements and the best acid concentrationa for their separation from nitrate media using Cyanex272.
La(III) Ce(III) Y(III) Fe(III) Th(IV) a
Ce(III)
Y(III)
Fe(III)
Th(IV)
2.36(0.0015)
2.1(0.1) 1.9(0.1)
119.7(0.0015) 281.9(0.0015) 30.6(0.1)
277.9(0.5) 228.4(0.5) 151.5(0.5) 43.2(0.5)
Values in the parenthesis.
M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147
145
Table 5 The optimum conditions for the extraction of lanthanum and cerium from aqueous solution. Extractant
D2EHPA Cyanex302 Cyanex272
pH
2 4 3
Extraction (%) La
Ce
92.7 50.3 22
95 55 28
words, the following extraction equation was used to determine the average number of released hydrogen: Fig. 7. Extraction of Ln+ 3 and Y+ 3 with Cyanex272 and TBP: [Ln+ 3]=[Y]=10−4 M, [Cyanex272]+[TBP]=1.5 M, [HNO3]=1 M.
1975), while iron forms stronger complexes with nitrate and hydrochloride ions. These results show the same trends obtained in Fig. 8.
+n
ðMÞ
+
m þ ðHAÞ2 ⇔ MAn ðm−nÞHA+nH 2
where, M and n represent the thorium complex in the aqueous phase and its charge, respectively. As mentioned before, the relationship between the distribution ratio D and the extraction constant Kex could be described as:
3.3. Extraction equilibrium logD = logKex + To establish the extraction equilibrium of the system, the main extractable species should be considered. In aqueous nitrate solution, numerous hydrolysis species of thorium have been reported (Baes and Mesemer, 1976; Moulin et al., 2001). Therefore, the method of slope analysis was used in the present study to determine the thorium extraction equilibrium from the optimum nitric acid concentration (0.5 M). According to Fig. 3, the extraction of thorium from 0.5 M nitric acid solution follows the cation exchange mechanism. In addition, Cyanex272 forms the dimmeric species (HA)2 in aliphatic diluents (Biswas and Singha, 2007). Thus the general equation of extraction could be written as: m ðHAÞ2 ⇔ ThðOH Þp ðNO3 Þk Að4−p−kÞ 2 þ ×ðm−4 + p +kÞHA+ ð4−p−K ÞH +4
ðThÞ
−
−
+p OH +k NO3 +
−
ð3Þ
in the extractable complex of where, p and k imply (OH ) thorium in aqueous solution and m is the amount of the extractant HA in the extracted complex. The extraction constant of this equilibrium is obtained by the following relations: h
i ð4−p−kÞ ThðOH Þp ðNO3 Þk Að4−p−kÞ ×ðm−4+ p+kÞ Hþ Kex = : + 4 k Th ½OH − p NO− ðHAÞ2 m=2 3 Kex =
− p
D − k ðHAÞ2 m=2 NO3
ð8Þ
KExp +1 þ ThðOH Þ2 ðNO3 Þ + ðHAÞ2 ⇔ ThðOHÞ2 ðNO3 ÞA:HA+H
ð9Þ
Despite the cation exchange mechanism of thorium extraction by Cyanex272 extractant in 0.5 M HNO3 (Eq. (9)), the extracted thorium complex was solvated by the extractent HA. This explains that the basicity of oxygen in Cyanex272 was more as compared with that of the reacting oxygen of water. Therefore, Cyanex272 could be replaced
ð5Þ
þ ð p + k−4Þ
½OH ½H
ð4Þ
h i m þ log½H2 A2 −nlog H 2
The slope of log D vs. [H+] in Fig. 9 indicates that only one hydrogen ion was released during the extraction process (n = 1). ElHefny and Daoud (2004) found the same extractable species (n = 1) for thorium extraction by Cyanex301 and Cyanex302 under the same conditions. Also Moulin et al. (2001) reported the thorium complexes with the charges of +1 at the pH range of 0–3 in perchloric medium (Table 6). In this table, thorium complexes have been shown as the combinations of perchlorate and hydroxide ions. Since the stability constants of thorium complexes with nitrate anions were higher than those with perchlorate anions, then thorium nitrate complexes were formed in the aqueous nitrate medium. Therefore, the extraction equilibrium for thorium could be expressed as:
and (NO−3)
ð7Þ
By taking the log of Eq. (5), the following equation is obtained: −
logD= logKex +k log ½NO3 +
h i m þ log ðHAÞ2 + ðk−4Þ log H −14p ð6Þ 2
The slope analysis of the experimental data of thorium extraction by Cyanex272 is shown in Fig. 9, where the log–log relations between D and the variables [(HA)2] and [NO− 3 ] gave the slopes 1. In other
Table 4 Separation factors between Ce+ 3, La+ 3and Y+ 3 with different extractant mixtures.
Y/La Y/Ce
C1.5
C1.2 T0.3
C0.9 T0.6
C0.6 T0.9
C0.3 T1.2
T1.5
17.2 16
304 126.7
191.7 88.5
11.4 52
51.9 25.9
13.3 6.3
C = Cyanex272, T = TBP, indices show the concentration of each extractant.
Fig. 8. Stripping of thorium, iron, lanthanum from the organic phase with different acids at different concentrations.
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M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147 Table 7 Composition of the aqueous solutions of Cyanex 272 process.
Feed Raffinate Scrubbed solution Stripped solution
Fig. 9. Log D versus log [Cyanex272], log [H+] and log [NO− 3 ] for the extraction of thorium.
with the coordination water of thorium complex. El-Hefny and Daoud (2004) confirmed the presence of Cyanex301 and Cyanex302 as solvating agents during the extraction of thorium. 3.4. Separation of thorium from Zarigan ore The leach solution of pre-concentrated Zarigan ore was precipitated at pH = 4.4 with ammonia. Then 0.5 M HNO3 was used to dissolve the thorium cake. Table 7 indicates the composition of the final aqueous solution, which was used for the real experiments. Ten milliliters of this solution was equilibrated with an equal volume of 0.1 M Cyanex272. The results in Table 7 show that thorium was quantitatively extracted in the organic phase, leaving the lanthanide fraction (lanthanum, cerium and yttrium) and iron behind (about 85– 90%) in the aqueous phase. Iron was scrubbed from the organic phase by 1 M nitric acid (3 vol) and then thorium was recovered from the organic phase with the mixture of 1 M H2SO and 2.7 ⁎ 10−4 M EDTA. EDTA was used because it forms a very strong complex with thorium (Nanda et al., 2002). The organic phase was regenerated by washing it with double distilled water. The thorium stripped solution was precipitated at pH = 3.8. Then it was calcined to ThO2. The recoveries of thorium and rare earth elements to the final organic and aqueous phases were 83 and 88%, respectively. Afterward, raffinate was processed using the mixture of Cyanex272 (1.2 M) and TBP (0.3 M). Under this condition, 90% of yttrium was extracted into the organic phase. Finally, lanthanum and cerium were extracted from the final raffinate using D2EHPA, which resulted in about 90% of lanthanide extraction into the organic phase. Meanwhile, the extraction process was offered as thorium extraction by Cyanex272 (first step), synergistic extraction of yttrium from raffinate with a mixture of Cyanex272 and TBP (second step) and extraction of lanthanum and cerium form the final aqueous phase with D2EHPA (third step). 4. Conclusions The present work proposed a convenient liquid–liquid extraction method for separation of thorium, rare earth elements and iron. Taguchi's method was used to study the effects of acid type, acid concentration and the type of extractants on the selective separation of thorium, rare earth elements and iron. The optimum conditions for
Table 6 Thorium complexes at different pHs (Moulin et al., 2001). Species (%) pH
[Th(ClO4)3]+
[Th(ClO2)(OH)]+
[Th(ClO4)(OH)2]+
[Th(OH)3]+
0 1 2 3
100 95 42 10
0 5 37 35
0 0 18 40
0 0 3 15
Th(IV)
Fe (total)
La(III)
Ce(III)
Y(III)
184.2 20.4 2 152
3632 3097 428 25.4
268.3 240.85 0 24.2
311.1 258.8 25 23.2
129.4 114.2 5 8
thorium separations from lanthanum and cerium were 0.01 M using Cyanex272. For separating thorium separations from yttrium and iron, the optimum conditions obtained were 0.01 M HNO3 using Cyanex302 and 1 M HNO3 using Cyanex272, respectively. Detailed separation studies using Cyanex272 showed that 0.5 M nitric acid was the optimum acid concentration for separation of thorium. The slope analysis method determined the stoichiomerty of the extracted complexes as Th(NO3)(OH)2A.HA. Mixture of 1 M sulfuric acid and 2.7 ⁎ 10−4 M EDTA was found to be as the best thorium stripping reagent from the loaded Cyanex272. Separation of thorium from Zarigan ore by the use of Cyanex272 showed high operation efficiency. The extractant mixture containing 1.2 M Cyanex272 and TBP showed different extraction effects for Ce+ 3, La+ 3 and Y+ 3, providing the possibility for the separation of Y+ 3 from the lanthanides. D2EHPA provided the optimum conditions for separation of lanthanum and cerium form the final raffinate. Acknowledgements The authors wish to greatly thank Dr. M. Ghannadi Maragheh for providing the possibility of conducting this research. We also would like to appreciate M.R. Khatami and Z. Moslemi for reviewing and also Ibrahim Parvin for the final English revision of this manuscript. References Antony, J., Antony, F.J., 2001. Teaching the Taguchi method to industrial engineersl. Work Study 50, 141–149. Baes, C., Mesemer, R., 1976. The Hydrolysis of Cations. Wiley and Sons Inc., New York. Biswas, R.K., Singha, H.P., 2007. Solvent extraction of Cu(II) by purified Cyanex. Indian J. Chem. Technol. 272, 269–275. Choppin, G.R., Musikas, C., Rydberg, J., Sekine, T., 2004. Solvent Extraction Principle and Practice. Marcel Dekker Inc., Ney York. Da Silva, G.C., Da Cunha, J.W., Dweck, J., Afonso, J.C., 2008. Liquid–liquid extraction (LLE) of iron and titanium by bis-(2-ethyl-hexyl) phosphoric acid (D2EHPA). Miner. Eng. 21, 416–419. El-Hefny, N.E., Daoud, J.A., 2004. Extraction and separation of thorium(IV) and praseodymium (III) with Cyanex 301 and Cyanex 302 from nitrate medium. J. Radioanal. Nucl. Chem. 261, 357–363. Eskandari, M, Sam, A., Alamdar Milani, S, 2010. Determination of optimum process conditions for solvent extraction of thorium using Taguchi method. J. Radioanal. Nucl. Chem., doi:10.1007/s10967-010-0857-1(article). Gupta, B., Malik, P., Deep, A., 2002. Extraction of uranium, thorium and lanthanides using Cyanex-923: Their separations and recovery from monazite. J. Radioanal. Nucl. Chem. 251, 451–456. Habashi, F., 1997. Handbook of Extractive Metallurgy, Vol. 3. Germany, Wiely-VCH, Heidelberg. Jain, R., 2006. Chemistry of lanthanoids. Department of Chemistry. Hindu college, Dehli 110052, India. Jia, Q., Li, Z., Zhou, W., Li, H., 2008. Studies on the Solvent Extraction of Rare Earth Elements from Nitratemedia with a Combination of di-(2-ethylhexyl)phosphoric acid and sec-octylphenoxyacetic acid. Wiley Interscience Publisher, pp. 565–569. www.interscience.wiley.com. Jia, Q., Tong, S., Li, Z., Zhou, W., Li, H., Meng, S., 2009. Solvent extraction of rare earth elements with mixtures of sec-octylphenoxy acetic acid and bis(2, 4, 4-trimethylpentyl) dithiophosphinic acid. Sep. Purif. Technol. 64, 345–350. Karve, M., Gaur, C., 2006. Liquid–liquid extraction of Th(IV) with Cyanex302. J. Radioanal. Nucl. Chem. 270, 461–464. Koh, M., Park, K., Yang, D., Kim, H., Kim, H., 2005. The synergistic effect of organophosphorus and dithiocarbamate ligands on metal extraction in supercritical CO2. Bull. Korean Chem. Soc. 26, 423–427. Lurie, J., 1975. Handbook of Analytical Chemistry. Mir publisher, pp. 283–290. McGill, I., 1993. Rare earth elements. Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 22. VCH, Weinheim, pp. 607–649. Moulin, C., Amekraz, B., Hubert, S., Moulin, V., 2001. Study of thorium hydrolysis species by electrospary-ionization mass spectrometry. Anal. Chem. Acta 269–279.
M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147 Nanda, D., Oak, M.S., Maiti, B., Chauhan, H.P., Dutta, P.K., 2002. Selective and uphill transport of uranyl ion in the presence of some base metals and thorium across bulk liquid membrance by di (2-ethylhexyl) phosphoric acid. Sep. Sci. Technol. 37, 3357–3367. Preston, J., Preez, A., 1990. Handbook on the Physics and Chemistry of Rare Earth Elements. Amsterdam, pp. 883–894. Ranjit, K., 2001. Design of Experiments using the Taguchi Approach: 16 Steps to Product and Process Improvement, A Wiley–Interscience Publication. John Wiley & Sons Inc., USA. Saji, J., Reddy, M.L.P., 2003. Selective extraction and separation of titanium(IV) from multivalent metal chloride solutions using 2-Ethylhexyl phosphonic acid mono-2ethylhexyl ester. Sep. Sci. Technol. 38, 427–441.
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Siekierski, Z., Fidelis, I., 1975. Extraction Chromatography of Lanthanides. Elsevier Scientific, New York, USA. Sun, X., Zhao, J., Meng, S., Li, D., 2005. Synergistic extraction and separation of yttrium from heavy rare earth elements using mixture of secoctylphenoxyacetic acid and bis(2, 4, 4-trimethylpentyl)phosphinic acid. Anal. Chim. Acta 533, 83–88. Taghizadeh, M., Ghasemzadeh, R., Ashrafizadeh, S.N., Saberyan, K., Ghanadi, M., 2008. Determination of optimum process conditions for the extraction and separation of zirconium and hafnium by solvent extraction. Hydrometallurgy 90, 115–120. Wang, J., Wu, G., Liu, L., Liu, J., Chen, J., 2002. Study of D2EHPA in vitriol medium of separating non-RE impurities from RE-chloride by extraction. Chin. rare earth elements 23, 69–70.
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Determination of optimum process conditions for the separation of thorium and rare earth elements by solvent extraction M. Eskandari Nasab a,⁎, A. Sam a, S.A. Milani b a b
Faculty of Mineral Processing Engineering, Bahonar University, Kerman 7616914111, Iran Jaber Ibn Hayan Research labs, Tehran, P. O. Box 14893-836, Iran
a r t i c l e
i n f o
Article history: Received 21 August 2010 Received in revised form 13 December 2010 Accepted 14 December 2010 Available online 23 December 2010 Keywords: Thorium Extraction Rare earth elements Cyanex272 Taguchi
a b s t r a c t The solvent extraction of thorium, some rare earth elements (including lanthanum, cerium and yttrium) and iron was separately investigated in order to determine their optimum separation conditions. The experimental conditions were studied in the concentration range of 0.01–5 M for three acids using Cyanex272, Cyanex302 and TBP (HA) as extractants by Taguchi's method. It was found that Cyanex272 could separate thorium and rare elements more efficiently compared with the conventional process (TBP). The optimum acid concentration in the case of Cyanex272 was determined as 0.5 M HNO3. The graphical method demonstrated that the composition of the extracted thorium complex was as Th(OH)2(NO3)A.HA. Furthermore, the stripping experiments showed that using a mixture of 1 M H2SO4 and 2.7 ⁎ 10−4 M EDTA could provide optimum conditions for the selective stripping of thorium. As a result, thorium was extracted selectively from 0.5 M nitric acid solution of Zarigan ore with the extraction efficiency of 83%. Also yttrium was separated from lanthanides by synergistic mixture containing Cyanex272 and TBP. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Rare earth elements are used in a wide range of products and processes such as ceramics, carbon arc lamps, alloys, catalysts, etc. In addition, the use of thorium as a replacement of natural uranium could increase substantially the total nuclear energy resources in the future. Monazite, bastnasite and xenotime are natural deposits of lanthanides containing thorium. Therefore, the methods for the extraction and separation of these metal ions from different ores have always attracted the attention of separation scientists. It may also be used to decontaminate the rare earth concentrates from thorium-bearing radioactive material or to produce nuclear fuel. The conventional solvent extraction technology uses TBP/HNO3 contacted counter currently with 40% (v/v) TBP in kerosene. Under these conditions, the distribution coefficients for thorium and rare earth elements were 1 and 0.02, respectively (Habashi, 1997). Preston and Preez (1990) reported that the neutral organophosphorus reagents like TBP were only applicable in the bulk recovery of the rare earth elements from nitrate solutions, but the separation factors for adjacent rare earth pairs were generally small, ranging from 1.2 to 2.2. In another process, the purification of individual rare earth elements and other elements of monazite and bastnasite was carried out using a variety of extractants including acidic and neutral organophosphorus compounds, amines and carboxylic acids (McGill,
⁎ Corresponding author. Tel.: +98 2166591428. E-mail address: [email protected] (M.E. Nasab). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.12.014
1993). These processes are relatively expensive due to a large number of mixer–settler stages, high cost of reagent and emulation problems. The disadvantages of these processes could largely be eliminated if it is feasible to separate these elements using another extractant. The organophosphorus compounds have been studied as an important group of extractants for solvent extraction and separation of thorium and rare earth elements (Jain, 2006; Saji and Reddy, 2003; Siekierski and Fidelis, 1975; Sun et al., 2005; Wang et al., 2002; El-Hefny and Daoud, 2004; Gupta et al., 2002; Karve and Gaur, 2006; Jia et al., 2008). Among them, the acidic extractants have shown very high distribution coefficients for thorium extraction from nitrate media at pH=3. For example, Karve and Gaur (2006) reported the distribution coefficient of thorium in 0.001 M HNO3 as 110.10. Some researchers have used the synergistic effects of chelate reagents to improve the separation of rare earth elements with acidic extractants. Sun et al. (2005) and Jia et al. (2008) respectively investigated the synergistic effects of sec-octylphenoxyacetic acid for the separation of rare earth elements by Cyanex272 and D2EHPA. For the neutral extractants, the use of Cynex923 by Gupta et al. (2002) has resulted in high extraction and separation efficiency of thorium. The aforementioned methods had also certain limitations such as complex formation of TBP with fluoride, sulfate, phosphate and carbonate anions, emulation formation of D2EHPA, poor selectivity of Cyanex921 and need to multistage stripping for Cyanex923. Consequently, the search for better extractants and conditions continues. Koh et al. (2005) reported that Cyanex302 [bis(2,4,4-trimethylpentyl) thiophosphinic acid] was much better than Cyanex272 [bis(2,4,4-trimethylpentyl) phosphinic acid] in extracting metallic ions at low pH area. Taguchi's orthogonal array design method is an efficient means for evaluation of solvent extraction
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process. Some investigations have tried to determine the optimum conditions in solvent extraction processes (Eskandari et al., 2010; Taghizadeh et al., 2008). Taghizadeh et al. (2008) have investigated the optimum process conditions for the extraction and separation of zirconium and hafnium using orthogonal array L9. The experimental conditions were studied in the range of 0.1 to 2.0 M for three different acids and TBP, D2EHPA or Cyanex272 as extractant. They reported the optimum extraction of zirconium as 71% when 2.0 M nitric acid and Cyanex272 were used. In the present work, the extraction behaviors of thorium, lanthanum, cerium, yttrium and iron are investigated with Cyanex272 or Cyanex302 as an alternative for common solvent extraction process (TBP) using Taguchi's method. Then, the study is continued to develop a process to separate thorium and rare earths elements from Zarigan leach solution. 2. Material and methods 2.1. Experimental design Fig. 1. S/N graphs for extraction of thorium, lanthanum, cerium, yttrium and iron.
The Taguchi's orthogonal array design is a powerful effective method for evaluation and improvement of laboratory and continuous process efficiencies (Antony and Antony, 2001). 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 (Ranjit, 2001) through the following equation:
ðS =NÞi = −10 log
1 1 ∑ n i y2i
!!
CeCl3·7H2O, YCl3·6H2O and FeCl3·6H2O (all from Merck) were used as the sources of Th(IV), La(III), Ce(III), Y(III) and Fe(III), respectively. Real experiments were carried out from the leach solution of Zarigan ore. The leachate solution was obtained in our previous works in Jaber Ibn Hayan research labs of Tehran. All chemicals used in this study were of analytical grade and applied without further purification.
ð1Þ 2.3. Procedures
where, n is the number of tests and yi is the experimental value in the Ith experiment. In the present study, three factors (acid type, acid concentration and extractant type) were considered in three levels. The traditional full factorial design would require 27 experiments for these three factors each at three levels. However, in the current design (Taguchi's L9 orthogonal array), only nine experiments are required. The extraction factors and their selected levels are presented in Table 1. The results are the mean values of two applications at each set of experiments. 2.2. Chemicals and reagents Cyanex272 and Cyanex302, both from Cytec (Canada), and TBP from Merck (Germany) were used as extractants. Kerosene was supplied by Fluka. Sulfuric, nitric, and hydrochloric acids (all from Fluka) and EDTA (from Merck) were used as stripping reagents and complexing agents, respectively. Th(NO 3 )4 ·5H2 O, LaCl 3·7H2O,
The aqueous phases were prepared by dissolving appropriate amounts of thorium, lanthanum, cerium, yttrium or iron powder in a solution containing 0.01 to 5.0 M of each acidic medium. The concentration of each metal ion was 10−4 M. The necessary concentration of Cyanex272 or Cyanex302 for an acceptable level of thorium extraction was determined through the preliminary tests. The concentration of each extractant was 0.1 M. All of these concentrations are excess of the stoichiometric quantities of metallic ions. Extraction experiments were carried out from mixed metal solutions according to Table 1. Each experiment was accomplished by extracting all the elements individually and the optimum conditions for each metal ion were calculated. Unless otherwise stated, the experiments were carried out at the phase ratio of O/A = 1 and the temperature of 25° C. Equal volumes (10.0 mL) of organic solutions and aqueous solutions were mixed and shaken using a mechanical shaker for 10 min. Then the organic and aqueous phases were separated by a separatory funnel. Stripping experiments were carried out using different concentrations of three acids (sulfuric, nitric and
Table 1 Taguchi's L9 orthogonal array design for separation of thorium, rare earth elements and iron using solvent extraction. Parameters
Results
Test No
Acid conc. (M)
Acid type
Solvent type
Th extraction (%)
1 2 3 4 5 6 7 8 9 Confirmatory
0.01 0.01 0.01 1 1 1 5 5 5 0.01
HNO3 HCl H2SO4 HNO3 HCl H2SO4 HNO3 HCl H2SO4 HNO3
Cyanex302 Cyanex272 TBP Cyanex272 TBP Cyanex302 TBP Cyanex272 Cyanex302 Cyanex272
97.4 97.4 10.4 94.6 4.9 5.4 19.3 16.7 47.7 98.7
a
Separation factor = DTh/DM (M: other metal).
Separation factorsa Th/La
Th/Ce
Th/Y
Th/Fe
118.5 134.5 0.49 50.63 0.16 0.11 0.48 0.51 0.66 190.8
226.6 198.6 0.78 60.98 0.26 0.14 0.79 0.70 0.77 279.9
25.9 2.19 0.09 11.6 0.04 0.03 0.15 0.13 0.28 3.5
4.23 0.68 0.36 16.04 0.10 0.10 0.46 0.07 0.71 1.3
M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147
143
Fig. 4. The amounts of thorium separation factors from lanthanum, cerium, yttrium and iron at different nitric acid concentrations: [extractant] = 0.1 M Cyanex272 in kerosene.
Fig. 2. Mean of means for separation factors.
hydrochloric) for 5 min. All aqueous solution analyses were accomplished using ICP-OES. 3. Results and discussions 3.1. Optimization of extraction parameters Fig. 1 shows the S/N ratio for the extraction efficiency of thorium, lanthanum, cerium, yttrium and iron. According to Taguchi's method, a high influence on extraction or separation factor is shown by a high S/N ratio. As shown in Fig. 1, the optimum combination for extraction of thorium was extraction from 0.01 M nitric acid solution by Cyanex272 in kerosene. Also Cyanex272 is also the best extractant for extraction of other studied elements, while for both iron and yttrium, the optimum extraction percents were obtained from 0.01 M hydrochloric acid. For lanthanum and cerium, the optimum extraction condition was 5.0 M sulfuric acid. Therefore, the separation factors were considered in order to evaluate the amount of thorium separations from other elements. Fig. 2 shows that the highest separation factors for separation of thorium from cerium and lanthanum were obtained with 0.01 M nitric acid using Cyanex272. The optimum conditions for separation of thorium from yttrium and iron were the extractions from 0.01 M HNO3 using Cyanex302 (experiment 1) and 1.0 M HNO3 by Cyanex272 (experiment 4), respectively. Since the optimum conditions for thorium separation from cerium and lanthanum were not obtained in the main experiments, one confirmatory experiment was carried out with their optimum separation levels. The results
Fig. 3. Extraction of thorium, iron, lanthanum, cerium and yttrium versus nitric acid concentration by Cyanex272.
confirmed the optimum conditions because they gave the thorium separation factors from cerium and lanthanum higher than those in the main experiments (Table 1). More experiments were carried out for a comprehensive understanding of the extraction process of thorium and rare earth elements. The results are shown in Figs. 3, 4, 5 and 6. Fig. 3 shows the extraction efficiencies of thorium, yttrium, cerium, lanthanum and iron from 0.0015 to 7 M nitric acid by 0.1 M Cyanex272 diluted in kerosene. It is clear that the extraction amounts increase in the following order: La (III) b Ce(III) b Y(III) b Fe(III) b Th(IV), which is in consistency with the observations of Jia et al. (2008) for some rare earth elements with D2EHPA. The increasing order of the extraction of metallic ions from Th(IV) to La(III), shown in Table 2, is in accordance with the increasing of the charge to radius ratios (z/r) (except for Fe(III)). The relationship between extraction percent and charge to radius ratio is against the Born equation (Choppin et al., 2004), which indicates that for the same ligands, with constant structural effects, the stability constants of the cations are related to “z/r”. Less extraction percent of iron with cationic extractant has also been reported in some of previous works. For instance, Da Silva et al. (2008) obtained similar results when they conducted the extraction of iron with D2EHPA. It is clear from Fig. 3 that the extraction of thorium starts to reduce from 0.5 M nitric acid concentration. But for other elements, the extractions are decreased with the increasing of nitric acid concentration up to 0.5 M. Earlier investigations on the extraction of thorium reported that the extraction of thorium and rare earth elements with acidic extractants at low acid concentrations followed the cationic exchange mechanism (Jia et al., 2008; El-Hefny and Daoud, 2004; Karve and Gaur, 2006; Jia et al., 2008): +n M
aq:
m þ +ð ðHAÞ2 Þorg: ⇔ ðMðAn Þ×ðm−nÞHAÞorg: +ðnH Þorg: 2
ð2Þ
Fig. 5. Comparison between thorium separations from yttrium versus nitric acid concentration using Cyanex272 and Cyanex302 extractants: [each extractant] = 0.001 M in kerosene.
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M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147 Table 2 Cationic charge to radius ratio for the studied metal ions Choppin et al. (2004). Ion
Th(IV)
Fe(III)
Y(III)
Ce(III)
La(III)
Z/ra
4
4.6
3.3
3
2.8
a
Fig. 6. Extraction of thorium, iron, lanthanum, cerium and yttrium vs. nitric acid concentration using TBP.
where, m is the number of extractant HA and n is the charge of thorium aqueous complex. According to this extraction mechanism, the equilibrium tends toward left with the increasing of acid concentration, hence, extraction percent is decreased at low acid concentrations. The extraction amounts of lanthanides (La(III), Ce(III) and Y(III)) were increased at the highest acid concentrations due to change in their extraction mechanisms from cation exchange to solvation, which is, in turn, because of the formation of neutral complex with nitrate anions. El-Hefny and Daoud (2004) reported that the extractable complexes of thorium by acidic extractants contain stronger ligands of hydroxide (Lurie, 1975) at low nitric acid area. This is why the extraction of thorium started to reduce at higher acid concentrations. Therefore, the reasonable separation factors were achieved only in 0.5 M nitric acid (Fig. 4). As shown in Fig. 4, the selective extractions were decreased in the order; Th(IV)/La(III) b Th(IV)/Ce(III) b Th(IV)/Y(III) b Th(IV)/Fe(III). Also the separation factors of thorium from lanthanum and cerium in 0.01 M HNO3 were higher than those in 1 M and 5 M HNO3, while the thorium separation factor from iron in 1 M HNO3 was more than in other acid concentrations. These results are in good agreement with the trends obtained by Taguchi's method for the nitrate media and acid concentrations. The results of separating thorium from yttrium by Cyanex272 and Cyanex302 extractants in nitric acid medium are shown in Fig. 5. Since Cyanex302 is a stronger acid than Cyanex272, and the degree of acidic strength of the studied lanthanides increases from yttrium to lanthanum, thereafter, according to HSAB1 theory (Choppin et al., 2004), Cyanex302 gives lower yttrium extraction amount than Cyanex272 does. Fig. 5 clearly shows that Cyanex302 is more efficient than Cyanex272 for separating thorium and yttrium, while this order is reversed for separating thorium from less acidic lanthanides such as cerium and lanthanum. These results are also in accordance with those obtained by Taguchi's method. The separation curves of all studied elements were plotted using graphical representations in Excel. Whereby, the optimum nitric acid concentrations for mutual separations of all elements were determined (the values are presented in parentheses of Table 3). As Table 3 obviously shows, the optimum nitric acid concentration for thorium separation from lanthanides and iron by Cyanex272 is 0.5 M, which is clearly more than 0.001 M, reported in the previous investigations as the optimum condition for solvent extraction of thorium alone (El-Hefny and Daoud, 2004; Karve and Gaur, 2006). Table 3 also shows that the separation factor of 2.4 was obtained for separating cerium from lanthanum in 1.5 ⁎ 10−3 M nitric acid, which is more than that obtained by Jia et al., 2009 with Cyanex301.
1
Hard–soft acid and base.
Unit of r = 10 nm.
The effect of HNO3 concentrations on the extraction of elements by 0.1 M TBP is given in Fig. 6. The extraction of thorium was found to decrease with the increase in nitric acid concentration, while the extractions of other elements were increased (except in 2 M). Therefore, thorium can only be separated from other elements at higher nitric acid concentrations by extracting them in TBP and leaving thorium in the aqueous phase (reverse process). The results showed that the separation factors of thorium by TBP were lower than those by Cyanex272 and Cyanex302. These results confirmed Taguchi's method results as well (Fig. 2). Since 0.5 M nitric acid was determined as the optimum nitric acid concentration for thorium separation from rare earth elements, therefore, to investigate the synergistic effects of mixed ligands on the separation of rare earth elements (La(III), Ce(III) and Y(III)) from raffinate, their extractions from 0.5 M nitric acid solutions with mixtures of Cyanex272 and TBP (summation of extractants concentrations = 1.5 M) were also studied. Fig. 7 shows the extraction amounts of Ln+ 3 and Y+ 3 at different ratios of the extractant mixtures. The results demonstrate that the mixtures have synergistic effects on all of the studied rare earth elements. The separation of Ln+ 3 and Y+ 3 from each other is known to be difficult because of their similar chemical properties. As Fig. 7 shows, the different extraction behaviors of Ln+ 3 and Y+ 3 with extractant mixtures could be considered for their separation from each other. Table 4 shows the separation factors between Ln+ 3 and Y+ 3 at different extractant mixtures. As shown, the optimum separation between Ln+ 3 and Y+ 3 was obtained by an extractant mixture containing 1.2 M Cynaex272 and 0.3 M TBP. Therefore, the mixture of Cyanex272 and TBP could be used for separating Y+ 3 from Ln+ 3. It is clear form Table 4 that Cyanex272 is better than the conventional process (i.e., TBP) for separating Ln+ 3 and Y+ 3. Our primary studies for comparing the effects of different acidic extractants such as Cyanex272, D2EHPA and Cyanex302 at different pH areas showed that D2EHPA was the best extractant for extracting lanthanum and cerium from aqueous solutions (Table 5). 3.2. Effect of various reagents on the stripping of thorium, lanthanum and iron Since appreciable amounts of iron could be extracted in the organic phase along with thorium (Fig. 3), the stripping behaviors of thorium, iron and lanthanum (as the representatives of lanthanides) were investigated using varying concentrations of different acids. Fig. 8 shows that 1 M H2SO4 and 2 M HCl were effective for the selective stripping of thorium and lanthanum from iron, while 0.5 M HCl and 2 M HNO3 could be used for the selective stripping of iron from other elements. The complex stability constants of sulfate ions with thorium and lanthanum are stronger than with iron (Lurie, Table 3 Separation factors of the studied elements and the best acid concentrationa for their separation from nitrate media using Cyanex272.
La(III) Ce(III) Y(III) Fe(III) Th(IV) a
Ce(III)
Y(III)
Fe(III)
Th(IV)
2.36(0.0015)
2.1(0.1) 1.9(0.1)
119.7(0.0015) 281.9(0.0015) 30.6(0.1)
277.9(0.5) 228.4(0.5) 151.5(0.5) 43.2(0.5)
Values in the parenthesis.
M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147
145
Table 5 The optimum conditions for the extraction of lanthanum and cerium from aqueous solution. Extractant
D2EHPA Cyanex302 Cyanex272
pH
2 4 3
Extraction (%) La
Ce
92.7 50.3 22
95 55 28
words, the following extraction equation was used to determine the average number of released hydrogen: Fig. 7. Extraction of Ln+ 3 and Y+ 3 with Cyanex272 and TBP: [Ln+ 3]=[Y]=10−4 M, [Cyanex272]+[TBP]=1.5 M, [HNO3]=1 M.
1975), while iron forms stronger complexes with nitrate and hydrochloride ions. These results show the same trends obtained in Fig. 8.
+n
ðMÞ
+
m þ ðHAÞ2 ⇔ MAn ðm−nÞHA+nH 2
where, M and n represent the thorium complex in the aqueous phase and its charge, respectively. As mentioned before, the relationship between the distribution ratio D and the extraction constant Kex could be described as:
3.3. Extraction equilibrium logD = logKex + To establish the extraction equilibrium of the system, the main extractable species should be considered. In aqueous nitrate solution, numerous hydrolysis species of thorium have been reported (Baes and Mesemer, 1976; Moulin et al., 2001). Therefore, the method of slope analysis was used in the present study to determine the thorium extraction equilibrium from the optimum nitric acid concentration (0.5 M). According to Fig. 3, the extraction of thorium from 0.5 M nitric acid solution follows the cation exchange mechanism. In addition, Cyanex272 forms the dimmeric species (HA)2 in aliphatic diluents (Biswas and Singha, 2007). Thus the general equation of extraction could be written as: m ðHAÞ2 ⇔ ThðOH Þp ðNO3 Þk Að4−p−kÞ 2 þ ×ðm−4 + p +kÞHA+ ð4−p−K ÞH +4
ðThÞ
−
−
+p OH +k NO3 +
−
ð3Þ
in the extractable complex of where, p and k imply (OH ) thorium in aqueous solution and m is the amount of the extractant HA in the extracted complex. The extraction constant of this equilibrium is obtained by the following relations: h
i ð4−p−kÞ ThðOH Þp ðNO3 Þk Að4−p−kÞ ×ðm−4+ p+kÞ Hþ Kex = : + 4 k Th ½OH − p NO− ðHAÞ2 m=2 3 Kex =
− p
D − k ðHAÞ2 m=2 NO3
ð8Þ
KExp +1 þ ThðOH Þ2 ðNO3 Þ + ðHAÞ2 ⇔ ThðOHÞ2 ðNO3 ÞA:HA+H
ð9Þ
Despite the cation exchange mechanism of thorium extraction by Cyanex272 extractant in 0.5 M HNO3 (Eq. (9)), the extracted thorium complex was solvated by the extractent HA. This explains that the basicity of oxygen in Cyanex272 was more as compared with that of the reacting oxygen of water. Therefore, Cyanex272 could be replaced
ð5Þ
þ ð p + k−4Þ
½OH ½H
ð4Þ
h i m þ log½H2 A2 −nlog H 2
The slope of log D vs. [H+] in Fig. 9 indicates that only one hydrogen ion was released during the extraction process (n = 1). ElHefny and Daoud (2004) found the same extractable species (n = 1) for thorium extraction by Cyanex301 and Cyanex302 under the same conditions. Also Moulin et al. (2001) reported the thorium complexes with the charges of +1 at the pH range of 0–3 in perchloric medium (Table 6). In this table, thorium complexes have been shown as the combinations of perchlorate and hydroxide ions. Since the stability constants of thorium complexes with nitrate anions were higher than those with perchlorate anions, then thorium nitrate complexes were formed in the aqueous nitrate medium. Therefore, the extraction equilibrium for thorium could be expressed as:
and (NO−3)
ð7Þ
By taking the log of Eq. (5), the following equation is obtained: −
logD= logKex +k log ½NO3 +
h i m þ log ðHAÞ2 + ðk−4Þ log H −14p ð6Þ 2
The slope analysis of the experimental data of thorium extraction by Cyanex272 is shown in Fig. 9, where the log–log relations between D and the variables [(HA)2] and [NO− 3 ] gave the slopes 1. In other
Table 4 Separation factors between Ce+ 3, La+ 3and Y+ 3 with different extractant mixtures.
Y/La Y/Ce
C1.5
C1.2 T0.3
C0.9 T0.6
C0.6 T0.9
C0.3 T1.2
T1.5
17.2 16
304 126.7
191.7 88.5
11.4 52
51.9 25.9
13.3 6.3
C = Cyanex272, T = TBP, indices show the concentration of each extractant.
Fig. 8. Stripping of thorium, iron, lanthanum from the organic phase with different acids at different concentrations.
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M.E. Nasab et al. / Hydrometallurgy 106 (2011) 141–147 Table 7 Composition of the aqueous solutions of Cyanex 272 process.
Feed Raffinate Scrubbed solution Stripped solution
Fig. 9. Log D versus log [Cyanex272], log [H+] and log [NO− 3 ] for the extraction of thorium.
with the coordination water of thorium complex. El-Hefny and Daoud (2004) confirmed the presence of Cyanex301 and Cyanex302 as solvating agents during the extraction of thorium. 3.4. Separation of thorium from Zarigan ore The leach solution of pre-concentrated Zarigan ore was precipitated at pH = 4.4 with ammonia. Then 0.5 M HNO3 was used to dissolve the thorium cake. Table 7 indicates the composition of the final aqueous solution, which was used for the real experiments. Ten milliliters of this solution was equilibrated with an equal volume of 0.1 M Cyanex272. The results in Table 7 show that thorium was quantitatively extracted in the organic phase, leaving the lanthanide fraction (lanthanum, cerium and yttrium) and iron behind (about 85– 90%) in the aqueous phase. Iron was scrubbed from the organic phase by 1 M nitric acid (3 vol) and then thorium was recovered from the organic phase with the mixture of 1 M H2SO and 2.7 ⁎ 10−4 M EDTA. EDTA was used because it forms a very strong complex with thorium (Nanda et al., 2002). The organic phase was regenerated by washing it with double distilled water. The thorium stripped solution was precipitated at pH = 3.8. Then it was calcined to ThO2. The recoveries of thorium and rare earth elements to the final organic and aqueous phases were 83 and 88%, respectively. Afterward, raffinate was processed using the mixture of Cyanex272 (1.2 M) and TBP (0.3 M). Under this condition, 90% of yttrium was extracted into the organic phase. Finally, lanthanum and cerium were extracted from the final raffinate using D2EHPA, which resulted in about 90% of lanthanide extraction into the organic phase. Meanwhile, the extraction process was offered as thorium extraction by Cyanex272 (first step), synergistic extraction of yttrium from raffinate with a mixture of Cyanex272 and TBP (second step) and extraction of lanthanum and cerium form the final aqueous phase with D2EHPA (third step). 4. Conclusions The present work proposed a convenient liquid–liquid extraction method for separation of thorium, rare earth elements and iron. Taguchi's method was used to study the effects of acid type, acid concentration and the type of extractants on the selective separation of thorium, rare earth elements and iron. The optimum conditions for
Table 6 Thorium complexes at different pHs (Moulin et al., 2001). Species (%) pH
[Th(ClO4)3]+
[Th(ClO2)(OH)]+
[Th(ClO4)(OH)2]+
[Th(OH)3]+
0 1 2 3
100 95 42 10
0 5 37 35
0 0 18 40
0 0 3 15
Th(IV)
Fe (total)
La(III)
Ce(III)
Y(III)
184.2 20.4 2 152
3632 3097 428 25.4
268.3 240.85 0 24.2
311.1 258.8 25 23.2
129.4 114.2 5 8
thorium separations from lanthanum and cerium were 0.01 M using Cyanex272. For separating thorium separations from yttrium and iron, the optimum conditions obtained were 0.01 M HNO3 using Cyanex302 and 1 M HNO3 using Cyanex272, respectively. Detailed separation studies using Cyanex272 showed that 0.5 M nitric acid was the optimum acid concentration for separation of thorium. The slope analysis method determined the stoichiomerty of the extracted complexes as Th(NO3)(OH)2A.HA. Mixture of 1 M sulfuric acid and 2.7 ⁎ 10−4 M EDTA was found to be as the best thorium stripping reagent from the loaded Cyanex272. Separation of thorium from Zarigan ore by the use of Cyanex272 showed high operation efficiency. The extractant mixture containing 1.2 M Cyanex272 and TBP showed different extraction effects for Ce+ 3, La+ 3 and Y+ 3, providing the possibility for the separation of Y+ 3 from the lanthanides. D2EHPA provided the optimum conditions for separation of lanthanum and cerium form the final raffinate. Acknowledgements The authors wish to greatly thank Dr. M. Ghannadi Maragheh for providing the possibility of conducting this research. We also would like to appreciate M.R. Khatami and Z. Moslemi for reviewing and also Ibrahim Parvin for the final English revision of this manuscript. References Antony, J., Antony, F.J., 2001. Teaching the Taguchi method to industrial engineersl. Work Study 50, 141–149. Baes, C., Mesemer, R., 1976. The Hydrolysis of Cations. Wiley and Sons Inc., New York. Biswas, R.K., Singha, H.P., 2007. Solvent extraction of Cu(II) by purified Cyanex. Indian J. Chem. Technol. 272, 269–275. Choppin, G.R., Musikas, C., Rydberg, J., Sekine, T., 2004. Solvent Extraction Principle and Practice. Marcel Dekker Inc., Ney York. Da Silva, G.C., Da Cunha, J.W., Dweck, J., Afonso, J.C., 2008. Liquid–liquid extraction (LLE) of iron and titanium by bis-(2-ethyl-hexyl) phosphoric acid (D2EHPA). Miner. Eng. 21, 416–419. El-Hefny, N.E., Daoud, J.A., 2004. Extraction and separation of thorium(IV) and praseodymium (III) with Cyanex 301 and Cyanex 302 from nitrate medium. J. Radioanal. Nucl. Chem. 261, 357–363. Eskandari, M, Sam, A., Alamdar Milani, S, 2010. Determination of optimum process conditions for solvent extraction of thorium using Taguchi method. J. Radioanal. Nucl. Chem., doi:10.1007/s10967-010-0857-1(article). Gupta, B., Malik, P., Deep, A., 2002. Extraction of uranium, thorium and lanthanides using Cyanex-923: Their separations and recovery from monazite. J. Radioanal. Nucl. Chem. 251, 451–456. Habashi, F., 1997. Handbook of Extractive Metallurgy, Vol. 3. Germany, Wiely-VCH, Heidelberg. Jain, R., 2006. Chemistry of lanthanoids. Department of Chemistry. Hindu college, Dehli 110052, India. Jia, Q., Li, Z., Zhou, W., Li, H., 2008. Studies on the Solvent Extraction of Rare Earth Elements from Nitratemedia with a Combination of di-(2-ethylhexyl)phosphoric acid and sec-octylphenoxyacetic acid. Wiley Interscience Publisher, pp. 565–569. www.interscience.wiley.com. Jia, Q., Tong, S., Li, Z., Zhou, W., Li, H., Meng, S., 2009. Solvent extraction of rare earth elements with mixtures of sec-octylphenoxy acetic acid and bis(2, 4, 4-trimethylpentyl) dithiophosphinic acid. Sep. Purif. Technol. 64, 345–350. Karve, M., Gaur, C., 2006. Liquid–liquid extraction of Th(IV) with Cyanex302. J. Radioanal. Nucl. Chem. 270, 461–464. Koh, M., Park, K., Yang, D., Kim, H., Kim, H., 2005. The synergistic effect of organophosphorus and dithiocarbamate ligands on metal extraction in supercritical CO2. Bull. Korean Chem. Soc. 26, 423–427. Lurie, J., 1975. Handbook of Analytical Chemistry. Mir publisher, pp. 283–290. McGill, I., 1993. Rare earth elements. Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 22. VCH, Weinheim, pp. 607–649. Moulin, C., Amekraz, B., Hubert, S., Moulin, V., 2001. Study of thorium hydrolysis species by electrospary-ionization mass spectrometry. Anal. Chem. Acta 269–279.
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