Synergistic extraction of Ce(IV) and Th(IV) with mixtures of Cyanex 923 and organophosphorus acids in sulfuric acid media

Separation and Purification Technology 118 (2013) 487–491

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Synergistic extraction of Ce(IV) and Th(IV) with mixtures of Cyanex 923 and organophosphorus acids in sulfuric acid media Hui Tong a,b, Yanliang Wang a, Wuping Liao a, Deqian Li a,⇑ a b

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China School of Natural Sciences, Wuhan University of Technology, Wuhan 430070, Hubei, PR China

a r t i c l e

i n f o

Article history: Received 28 November 2012 Received in revised form 23 July 2013 Accepted 27 July 2013 Available online 6 August 2013 Keywords: Synergistic extraction Cerium Thorium Rare earth Cyanex 923

a b s t r a c t The synergistic effect of mixtures of Cyanex 923 and organophosphorus acids including P204, P507 and Cyanex 272 in extraction of Ce(IV) and Th(IV) from sulfuric acid solutions has been investigated. Under the experimental conditions, the extraction of Ce(IV) presented an significant synergistic effect with the mixed extraction systems of Cyanex 923–P204, Cyanex 923–P507 and Cyanex 923–Cyanex 272. On the contrary, the extraction of Th(IV) showed significant antagonistic effects in the mixed systems. The separation factor for Ce(IV)/Th(IV) reached as 36.8 at a mole fraction of 70% Cyanex 923 in the Cyanex 923– P507 system. It is shown that the mixed extraction systems not only enhance the extraction efficiency of Ce(IV) but also improve the selectivity of Ce(IV) over Th(IV). Furthermore, a fractional extraction process (including 4 stages of extraction, 2 stages of scrubbing and 2 stages of stripping) with the mixed Cyanex 923–P507 to separate Ce(IV) from the bastnasite leach solution has been designed. Cerium solution of Ce2(SO4)3 and CeF3 nanoparticle were obtained with 99.8% and 99.9% purity, respectively. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Panxi bastnasite discovered in the 1980s in Sichuan province of China is a major rare earth resource. The concentrate contains 60– 70% REO with CeO2/REO being about 50%, 0.2–0.4% Th and 6–10% F [1,2]. At present, a process consisting of oxidizing roasting combined with HCl selective leaching and alkali conversion process is commonly used in the bastnasite treatment [3,4]. Some problems gradually emerge with the HCl selectively leaching step during the industrial practice such as low purity (95–99%) and recovery (about 70%) of the CeO2 product, difficulty in recovering radioactive Th and poisonous F, etc. In order to overcome the drawbacks above, a clean metallurgical process of separating Ce, F, Th and RE from Panxi bastnasite has been developed [5]. After roasting in air, bastnasite concentrate is decomposed with dilute sulfuric acid directly with a decomposition rate over 92%. An organophosphine oxide reagent, Cyanex 923 was employed for separating Ce(IV) from the bastnasite leach solution. Meanwhile, F(I) could be extracted into organic phase with Ce(IV) and recovered as CeF3 nanoparticles. Synergistic solvent extraction systems not only enhance the extraction efficiency and improve selectivity among metal ions, but also improve the solubility of the extracted complexes in the organic phase, eliminate emulsification and the formation of third

⇑ Corresponding author. Tel.: +86 431 85262036; fax: +86 431 85698041. E-mail address: [email protected] (D. Li). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.07.039

phases. These systems have gained more and more attention in recent years [6,7]. Binary synergistic extraction systems including mixture of alkaline–acidic extractants [8–13], acidic–acidic extractants [14–17] and neutral organophosphorus-acidic extractants [18–22] have been studied comprehensively. Huang et al. [23– 25] investigated the synergistic effects of RE(III) ions in sulfuric acid medium using di(2-ethylhexyl) phosphoric acid (P204) and 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (P507) as acidic–acidic synergistic extraction system. However, it is difficult to separate Ce(IV) from Th(IV) by P507 or P204–P507 mixture in the bastnasite leach liquor. In this study, the extraction mechanism of organophosphorusacidic extraction systems consisting of Cyanex 923 and organophosphorus acids including P204, P507 and Cyanex 272 were compared. Synergistic effect of Ce(IV) and Th(IV) with these systems were discussed.

2. Experimental 2.1. Reagents and apparatus Cyanex 923 and Cyanex 272 were kindly supplied by CYTEC Canada Inc. and purified as previously reported [26]. P204, P507 and sulfonated kerosene were kindly provided by Shanghai Rareearth Chemical Co., Ltd. of China. In order to simulate actual working conditions, the extractants were used without further purification in the experiments of fractional extraction. Stock solution of

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Fig. 1. Effect of H2SO4 concentration on the extraction of Ce(IV) with single extractant .Organic phase: [Cyanex 923, P204, P507 or Cyanex 272] = 0.30 mol/L. Aqueous phase: [Ce4+] = 0.0505 mol/L.

Ce(IV) were prepared from Ce(HSO4)4 of analytical grade. Stock solution of Th(IV) was prepared by dissolving ThO2 in concentrated sulfuric acid. The concentration of Ce(IV) and free H+ in aqueous phase were determined by titration with standard Fe2+ and NaOH, respectively [27]. Thorium was determined by EDTA titration [28,29]. Mixture of rare earth elements were accurately determined by inductively coupled plasma optical emission spectrometer (ICP-OES, Thermo icap 6000 series). 2.2. Solvent extraction process Extraction equilibrium experiments were carried out by shaking equal volumes (5 mL) of aqueous and organic solutions for 30 min in equilibrium tubes at room temperature. Fractional extraction was carried out by ‘‘funnel method’’ [30] to simulate multistage counter-current extraction in the laboratory. 3. Results and discussion 3.1. Extraction behavior of Ce(IV) and Th(IV) by single extractant In order to find the optimum aqueous acidity for separation of Ce(IV) and Th(IV), the effect of H2SO4 concentration with these single extractant systems of Cyanex 923, P204, P507 and Cyanex 272 was investigated and the results are shown in Figs. 1 and 2. It was found that Cyanex 923 is an excellent extractant to extract Ce(IV) in the full range of H2SO4 concentrations tested. The extraction of both Ce(IV) and Th(IV) with P204 and P507 decrease sharply with increasing H2SO4 concentration. P507 showed higher extraction of Ce(IV) (Fig. 1) but lower extraction of Th(IV) (Fig. 2) than P204. Cyanex 272 is not suitable for extraction of Ce(IV) and Th(IV) when H2SO4 concentration was higher than 0.5 M.

was obtained with the Cyanex 923 fraction of 0.6–0.8 in the C923–P507 system. On the contrary, the extraction of Th(IV) showed a significant antagonistic effect in the mixed systems. The minimum synergistic effect, RTh(IV),min, was found in the Cyanex 923 fraction range 0.5–0.8 in the mixed extraction systems. The separation factor for Ce(IV)/Th(IV), bCe(IV)/Th(IV), was also calculated and the results are shown in Table 2. The values of bCe(IV)/ Th(IV) in the Cyanex 923 fraction of 0.6 reached 14.4, 36.8 and 16.3 with C923–P204, C923–P507 and C923–C272, respectively, while the corresponding bCe(IV)/Th(IV) value was 3.9 with the system containing only Cyanex 923. It is concluded that the extraction efficiency of Ce(IV) was enhanced and the selectivity of Ce(IV) from Th(IV) was improved with the mixed extraction systems. Considering higher separation factor and potential lower reagent costs, C923–P507 was chosen to further investigate the extraction mechanism of Ce(IV). The exaction equilibrium of Ce(IV) from sulfuric acid solutions with mixture of Cyanex 923 and P507 can be expressed as in the following equation:

CeðHSO4 Þ4 þ xðHLÞ2ðoÞ þ yBðoÞ $ CeðHSO4 Þ4m  H2xn L2x  yBðoÞ þ nHþ þ mHSO4

ð1Þ

where ‘‘o’’ denotes the organic phase, (HL)2 the dimeric species of acidic phosphate P507, B neutral phosphorus extractant Cyanex 923. Considering charge balance of H+ and HSO 4 , the value of m should be equal to that of n. So Eq. (1) can be rewritten as Eq. (2).

CeðHSO4 Þ4 þ xðHLÞ2ðoÞ þ yBðoÞ $ CeðHSO4 Þ4z  H2xz L2x  yBðoÞ þ zHþ þ zHSO4

ð2Þ

The distribution ratio of Ce (Dmix) and equilibrium constants (Kmix) of the synergistic system can be expressed as Eqs. (3) and (4), respectively.

Dmix ¼

½CeðHSO4 Þ4z  H2xz L2x  yBðoÞ  ½CeðHSO4 Þ4 

K mix ¼

Dmix ½Hþ  ½HSO4 z ½ðHLÞ2 xðoÞ ½ByðoÞ

ð3Þ

z

ð4Þ

Taking logarithms,

log Dmix ¼ log K mix  z log½Hþ   z log½HSO4  þ x log ½ðHLÞ2 ðoÞ þ y log ½BðoÞ

ð5Þ

3.2. Synergistic extraction of Ce(IV) and Th(IV) with the mixture systems The synergistic effects for Ce(IV) and Th(IV) with the mixed extraction systems including Cyanex 923–P204 (C923–P204), Cyanex 923–P507 (C923–P507) and Cyanex 923–Cyanex 272 (C923– C272) are compared in Figs. 3 and 4, respectively. The synergistic enhancement coefficients, R = Dmix/(DC923 + Di), i = P204, P507 or Cyanex 272, were calculated (Table 1). The extraction of Ce(IV) presented a significant synergistic effect in the mixed extraction systems. For example, the maximum synergistic effect, RCe(IV),max,

Fig. 2. Effect of H2SO4 concentration on the extraction of Th(IV) with single extractant. Organic phase: [Cyanex 923, P204, P507 or Cyanex 272] = 0.30 mol/L. Aqueous phase: [Th4+] = 0.0444 mol/L.

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Table 2 Comparison of the separation factors for Ce(IV)/Th(IV) with the mixture systems. Fraction (C923)

C923

C923–P204

C923–P507

C923–C272

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

– 6.55 5.49 4.33 4.14 3.97 3.75 3.94 4.41 5.02 5.72

0.50 1.01 2.23 3.43 4.38 6.01 9.44 14.4 16.9 11.2 5.72

2.83 4.36 6.84 8.92 15.2 23.0 31.2 36.8 36.1 20.2 5.72

5.05 14.5 20.0 20.5 22.8 22.9 18.7 16.3 13.8 8.73 5.72

Fig. 3. Synergistic extraction of Ce(IV) with the mixture systems at different mole fractions of Cyanex 923. Organic phase: [Cyanex 923] + [P204, P507 or Cyanex 272] = 0.3 mol/L. Aqueous phase: [Ce4+] = 0.0505 mol/L, H2SO4 = 1.0 mol/L.

Fig. 5. Effect of Cyanex 923 concentration on the extraction of Ce(IV) with C923– P507 mixture. Organic phase: [P507] = 0.15 mol/L in C923–P507 mixture. Aqueous phase: [Ce4+] = 0.0994 mol/L, [H2SO4] = 0.951 mol/L.

Fig. 4. Synergistic extraction of Th(IV) with the mixture systems at different mole fractions of Cyanex 923. Organic phase: [Cyanex 923] + [P204, P507 or Cyanex 272] = 0.3 mol/L. Aqueous phase: [Th4+] = 0.0444 mol/L, H2SO4 = 1.0 mol/L.

Table 1 Synergistic enhancement coefficient of Ce(IV) and Th(IV) with the mixture systems. Fraction (C923)

RCe(IV)

RTh(IV)

C923– P204

C923– P507

C923– C272

C923– P204

C923– P507

C923– C272

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

1.00 1.54 2.83 3.36 3.22 3.13 3.44 3.77 4.00 2.29 1.00

1.00 1.38 2.03 2.83 4.81 6.24 7.11 7.84 6.66 3.75 1.00

1.00 2.58 3.49 4.00 4.24 4.23 3.85 3.19 2.54 1.60 1.00

1.00 0.96 0.91 0.89 0.81 0.78 0.75 0.75 0.82 0.92 1.00

1.00 0.96 0.91 0.90 0.86 0.80 0.74 0.77 0.80 0.92 1.00

1.00 1.01 0.91 0.85 0.78 0.75 0.77 0.77 0.82 0.92 1.00

For determining the values of x, y and z, the slope analysis method was used. The effects of concentrations of Cyanex 923, P507 and free H+ on the extraction of Ce(IV) with C923–P507 mixture were studied. As shown in Figs. 5 and 6, the plots of log Dmix versus log [Cyanex 923] and log [P507] with slope of about 2.01 and 1.17, indicating x = 1, y = 2, respectively. The plot of log Dmix versus log [H+]free (Fig. 7) with slope of about 0.84 can be

explained that x mole of P507 dimer release one mole H+ during the synergistic reaction. In other word, z = 1. By determining the relationship between the change of free H2SO4 concentration, DH2SO4, and the change of extracted complex concentration, DCe(org), another evidence is emerged to support H2SO4 is a product during the synergistic reactions. As shown in Fig. 8, H2SO4 released increases with increasing Ce(org). The slopes obtained from the plots of log DCe(org) against logDH2SO4indicate that the Ce complex formed shows first order depended on H2SO4 released. Therefore,

Fig. 6. Effect of P507 concentration on the extraction of Ce(IV) with C923–P507 mixture. Organic phase: [C923] = 0.15 mol/L in C923–P507 mixture. Aqueous phase: [Ce4+] = 0.0994 mol/L, [H2SO4] = 0.951 mol/L.

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H. Tong et al. / Separation and Purification Technology 118 (2013) 487–491 Table 3 Technical parameters used in the fractional extraction simulation. Materials

Composition

Volume ratio

Organic phase

0.525 mol/L C923 0.315 mol/L P507 in sulfonated kerosene 126 g/L REO 91.8 g/L CeO2 0.706 g/L ThO2 13.2 g/L F 0.5 mol/L H2SO4

32

5

1.0 g/L H3BO3 0.21 mol/L H2O2

15

Feed

Scrubbing solution Stripping solution

11

from Th(IV) using the mixed C923–P507 system significantly improved by the addition of P507 to Cyanex 923.

+

Fig. 7. Effect of H concentration on the extraction of Ce(IV) with C923–P507 mixture. Organic phase: [C923 + P507] = 0.30 mol/L. [C923]: [P507] = 1:1. Aqueous phase: [Ce4+] = 0.130 mol/L.

4. Conclusions Comparing the extraction of Ce(IV) and Th(IV) with the mixed extraction systems including C923–P204, C923–P507 and C923– C272, the extraction of Ce(IV) presented a significant synergistic effect, while the extraction of Th(IV) showed a significant antagonistic effect. The separation factor for Ce(IV)/Th(IV) was enhanced obviously with the addition of organophosphorus acids to Cyanex 923. By slope analysis method, the exaction equilibrium of Ce(IV) from sulfuric acid solutions with mixtures of Cyanex 923 and P507 can be expressed as follow:

CeðHSO4 Þ4 þ ðHLÞ2ðoÞ þ 2BðoÞ $ CeðHSO4 Þ3  HL2  2BðoÞ þ H2 SO4 A fractional extraction process with the mixed C923–P507 to separate Ce(IV) from enriched cerium sulfate leaching solution has been designed. High purity cerium products with low ThO2 were obtained. Fig. 8. Relationship between DH2SO4 and DCe(org) during the synergistic reactions. (a) Effect of [Cyanex 923]; (b) effect of [P507]; and (c) effect of [H+]free.

the synergistic extraction of Ce(IV) with C923–P507 mixture in the range of H2SO4 concentrations tested can be described by the following equation:

CeðHSO4 Þ4 þ ðHLÞ2ðoÞ þ 2BðoÞ $ CeðHSO4 Þ3  HL2  2BðoÞ þ H2 SO4

Acknowledgements The authors wish to thank Shanghai Rare-earth Chemical Co., Ltd. of China and CYTEC Canada Inc. for supplying extractants. This project is supported by the National Program entitled ‘‘basic research on high efficiency utilization of rare earth elements in the field of environmental protection’’ (2004CB719506), Natural Science Foundation of China (20371046; 50574080).

ð6Þ References

3.3. Fractional simulation Based on the thermodynamic parameters of extraction of Ce(IV) and Th(IV) with C923–P507 in sulfuric acid media, a new separation process including 4 stages of extraction, 2 stages of scrubbing and 2 stages of stripping was developed for the separation of cerium from the bastnasite leach solution. The concentrate was roasted in air, selectively leached by HCl, and then leached by H2SO4. The feed solution was prepared by diluting the sulfate leaching solution and adjusting H2SO4 concentration. A series of separatory funnels were employed to simulate counter-current extraction. Table 3 shows the technical parameters for separating Ce(IV) from sulfate leaching solution in detail. Organic phase is stable after recycling many times during the fractional extraction test. Ce2(SO4)3 solution and CeF3 nanoparticle were obtained with 99.8% purity and 99.9% purity, respectively. ThO2 in cerium products was lower than 10 ppm. It was confirmed that the separation of Ce(IV)

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