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Separation of zirconium from hafnium in sulfate medium using solvent extraction with a new reagent BEAP
Hydrometallurgy 169 (2017) 607–611
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Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Separation of zirconium from hafnium in sulfate medium using solvent extraction with a new reagent BEAP
MARK
She Chena,b, Zhifeng Zhangb, Shengting Kuangb, Yanling Lib, Xiaowen Huanga,⁎, Wuping Liaob,⁎ a
School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China State Key Laboratory of Rare Earth Resource Utilization, ERC for the Separation and Purification of REs and Thorium, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Zirconium Hafnium Solvent extraction BEAP Sulfate medium
A new extractant, bis(2-ethylhexyl)-1-(2-ethylhexylamino)propylphosphonate (BEAP or B) was synthesized to extract and separate zirconium and hafnium from sulfate medium. Under optimum conditions, the separation factor of Zr over Hf was found to be 6.8. The extracted complexes were determined to be ZrO(HSO4)2·3B and HfO (HSO4)2·2B. The extractions of zirconium and hafnium are both exothermic. A counter-current operation was conducted, in which the content of HfO2 was decreased from 3% to 0.6% in the ZrO2 product. It is suggested that much more extraction and scrubbing stages are needed to reach the goal of 0.01% HfO2 in ZrO2 product.
1. Introduction Zirconium and hafnium usually co-exist in nature. Zirconium is widely used as the container and structural material of nuclear reactor due to its low thermal neutron capture cross-section. Differently, hafnium is an important control material for thermonuclear reaction because of its high neutron capture cross-section. Due to their diverse neutron capture properties, it is crucial to separate them before they are used in nuclear energy. For example, the content of hafnium in nuclear zirconium should be < 0.01%. Due to their similar chemical properties, the separation of zirconium and hafnium is still a challenge (Mukherji, 1970; Marczenko and Balcerzak, 2002; Xu et al., 2010; Xu et al., 2015a; Zhang, 2004, 2007). Many methods such as fractional crystallization (Niemand and Crouse, 2015), fractional precipitation (Schumb and Pittman, 2002), ion exchange (Huffman and Lilly, 1951; Benedict et al., 1954), silica gel adsorption (Hansen and Gunnar, 1949), molten salt distillation (Spink, 1980) and solvent extraction (Fischer and Chalybaeus, 1947; Cox et al., 1958) have been studied for the separation of zirconium and hafnium. Furthermore, only solvent extraction is used in industry with MIBK and TBP as the extractants (Fischer and Chalybaeus, 1947; Fischer et al., 1948; Sommers and Perrine, 2002). However, there are some disadvantages concerned the extraction systems of these two extractants. The MIBK system leads to solvent loss, atmospheric pollution and poor working environment due to the defects that the waste stream contains high concentrations of ammonium, cyanides and organic by-products with low flash point, high vapour pressure and solubility in the aqueous
⁎
Corresponding authors. E-mail addresses: [email protected] (X. Huang), [email protected] (W. Liao).
http://dx.doi.org/10.1016/j.hydromet.2017.04.001 Received 21 December 2016; Received in revised form 1 April 2017; Accepted 1 April 2017 Available online 04 April 2017 0304-386X/ © 2017 Published by Elsevier B.V.
phase (Snyder and Lee, 1992; Taghizadeh et al., 2008; Xu et al., 2010; Banda and Man, 2015). For the TBP system, the extractant has a high aqueous solubility and is not stable for long time operation, the equipments are seriously corroded, and the emulsified phenomenon is found in continuous production (Levitt and Freund, 2002). So other extractants such as Cyanex 923 were studied. Gupta et al. (2005) extracted zirconium from zircon leaching solution using 0.1 mol/L Cyanex 923 solution in toluene and recovered around 98% Zr with the purity of 98%. Nayl et al. (2009) studied the extraction and separation of Zr and Hf from nitrate medium by some Cyanex extractants and found that Cyanex 925 is more efficient than Cyanex 921 and Cyanex 923. Lee and Wang (Lee et al., 2015; Wang and Lee, 2016a) studied the extraction and separation of Zr and Hf using di-2-ethylhexylphosphoric acid (D2EHPA) from sulfate solutions and found that the loading capacity of D2EHPA is low (only 10 g/L Zr and 0.2 g/L Hf with 0.05 mol/L D2EHPA) and the efficient stripping needs strong acidic solutions (4 mol/L H2SO4). Synergistic extraction was also applied for the separation of zirconium and hafnium (Rezaee et al., 2011; Banda et al., 2012; Xu et al., 2015b; Xu et al., 2016; Wang and Lee, 2016b). Although, synergistic extraction can overcome some shortcomings of the sole extractant, the fatal problem is that it's so hard to maintain a ratio of multiple extractants for long time operations. In the present work, a new neutral extractant bis(2-ethylhexyl)-1-(2ethylhexylamino)propylphosphonate (BEAP) was synthesized and applied for the extraction and separation of zirconium and hafnium from sulfate medium. The extraction stoichiometry was deduced and thermal parameters were calculated.
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2. Experimental 2.1. Chemicals and materials Bis(2-ethylhexyl)phosphite (Shanghai Rare-earth Chemical Co., Ltd.), 2-ethylhexylamine (Shanghai Aladdin Industrial Co.) and propanal (Beijing Chemical Co., Ltd) were used as raw materials for the synthesis of BEAP without further purification. The extractant was dissolved in n-heptane to the required concentrations. The stock solutions of Zr and Hf were prepared by dissolving the corresponding salts in the corresponding mineral acid solutions. All work solutions were obtained by diluting the stock solutions. The initial concentrations of Zr and Hf were fixed to 1.0 × 10− 2 mol/L (unless otherwise stated). All other reagents were of analytical reagent grade. 2.2. Apparatus Nuclear magnetic resonance spectra were determined using a Bruker AV 600 M for 1H NMR (600 MHz) and FT-IR spectra (KBr pellets) using a Bruker Vertex 70 Spectrometer. The metal concentrations were determined by an inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer Optima 8000).
Fig. 2. Effect of acid concentration on the extraction of metal ions.[Zr4 +] = [Hf4 +] = 1.0 × 10− 2 mol/L, [BEAP] = 5.0 × 10− 2 mol/L.
2.3. Synthesis of bis(2-ethylhexyl)-1-(2-ethylhexylamino) propylphosphonate (BEAP)
3. Results and discussion
brium, respectively.
3.1. Extraction of Zr and Hf from different acidic media
BEAP (Fig. 1) was synthesized according to the method in the literature (Stoikov et al., 2000; Kuang et al., 2017; Lu et al., 2017). A mixture of 30.6 g (0.1 mol) bis(2-ethylhexyl)phosphite, 14.2 g (0.11 mol) 2-ethylhexylamine, 6.4 g (0.11 mol) propanal, 80 mL anhydrous toluene and 0.2 g p-toluenesulfonic acid were placed in a singleneck round-bottom flask (250 mL) equipped with a magnetic stirrer, a Dean–Stark trap and a reflux condenser. The mixture was heated with stirring for 6 h in an oil bath at 130 °C. At the end of the reaction 0.1 g of K2CO3 was added into the solution, and the mixture was refluxed for 15 min for the removal of the catalyst. The mixture was then cooled, washed with water (3 × 100 mL) and dried. The organic solution was evaporated in a vacuum on the rotary evaporator to remove the excess reagents and solvent until no fraction was steamed out. The obtained product was an oily compound with a yield of 90% (42.80 g) and a purity of 97.80%. 1H NMR (600 MHz, CDCl3, ppm): δ 4.01 (m, 4H, CH2), 2.61 (m, 1H, CH), 2.36 (d, 2H, CH2), 2.0 (s, 1H, NH), 1.56–1.71 (m, 3H,CH), 1.48 (m, 2H, CH2), 1.25–1.35 (m, 24H, CH2), 0.89 (t, 21H, CH3).
The extraction of Zr and Hf using BEAP dissolved in n-heptane was investigated from various acidic media including H2SO4, HCl and HNO3 solutions. In the experiments, the initial concentrations of Zr and Hf in the aqueous solutions were both kept at 0.01 mol/L, and the concentration of BEAP was maintained at 0.05 mol/L. The ranges of acid concentration were 0.05–6 mol/L for H2SO4, 0.05–5 mol/L for HCl, and 0.05–4 mol/L for HNO3. The phase ratio (O/A) was fixed at 1:1 with a constant total volume. As shown in Fig. 2, the extractant demonstrated higher extraction ability toward Zr than Hf in all three studied acid systems. Furthermore, the extraction of Zr and Hf by BEAP from different acidic media increased in the order of HCl < HNO3 < H2SO4. In the studied acidic range, the maximum of the extraction percentage of Zr reached 48.0% for the HCl system, 68.9% for the HNO3 system and 90.7% for the H2SO4 system, while that of Hf was 32.5% for the HCl system, 55.5% for the HNO3 system and 59.1% for the H2SO4 system. In the H2SO4 medium, the extraction of Zr first increased with the increasing H2SO4 concentration when CH2SO4 was lower than 0.59 mol/L. Then it decreased with the further increasing acid concentration. Similar trend was also observed for the extraction of Hf. Differently, in the HCl and HNO3 media, the extraction of Zr and Hf changes slightly with the increasing acid concentration. The separation factor (Table 1) of Zr over Hf reached a maximum of 6.77 at the H2SO4 concentration of 0.59 mol/ L while that was kept < 2 for the HNO3 system and < 3.2 for the HCl system. It seems that better separation of Zr from Hf can be obtained with sulfate medium. However, when the H2SO4 concentration is > 1.0 mol/L, the extraction of both Zr and Hf decreased rapidly with the
2.4. Experimental procedures The aqueous and organic phases, 5 mL, each were mixed and shaken using oscillator for 20 min to reach equilibrium at 298 ± 1 K (except the temperature experiment). The phases were then separated by gravity. The metal concentrations in the aqueous phase were determined by ICP-OES. The metal concentrations in the organic phases were obtained by mass balance. Distribution ratios (D) were calculated by D = [M](o) / [M](a), where [M](o) and [M](a) represent the concentrations of the metal ions in the organic and aqueous phases at equili-
Table 1 Separation factors (βZr/Hf) at different acid concentrations. β CH2SO4(mol/L) 0.24 0.59 1.09 1.32 2.41 3.40
Fig. 1. Molecular structure of bis(2-ethylhexyl)-1-(2-ethylhexylamino)propylphosphonate (BEAP).
608
βZr/Hf 2.88 6.77 6.27 5.55 2.84 2.09
CHCl(mol/L) 0.50 1.14 1.73 2.34 2.96 3.58
βZr/Hf 3.14 2.30 2.14 2.17 2.02 1.87
CHNO3(mol/L) 0.58 1.38 2.17 2.99 3.75 4.54
βZr/Hf 1.65 1.61 1.72 1.81 1.85 1.77
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increasing acid concentration and the difference between the extraction of Zr and Hf became smaller, indicating that the separation of Zr and Hf was more difficult. 3.2. Extraction stoichiometry of Zr4 + and Hf4 + with BEAP The extraction of metal ions from the aqueous solution with low sulfuric acid concentration using BEAP can be expressed by the following equations (Xiong et al., 2006): Kex
MO2+ +2HSO‐4 + n B(O) ⎯⎯⎯→ MO(HSO4 )2⋅n B(O)
(1)
where “B” represents “BEAP”, “M” Zr or Hf and “n” the numbers of the extractant involved in the reaction. The equilibrium constant Kex is defined as follows:
Kex =
[MO(HSO4 )2⋅n B](o) n [MO2+] [HSO‐4 ]2 [B](o)
(2)
Taking logarithms:
log D = log Kex + 2 log [HSO‐4] + n log[B](o)
(3)
Fig. 4. Infrared spectra of BEAP and the chelates H2SO4·B and ZrO(HSO4)2·3B.
where
D=
[MO(HSO4 )2⋅n B](o) [MO2+]
Table 2 Characteristic IR spectral data for BEAP and the extracted Zr complexes.
(4) Probable assignment
When the original metal concentration and sulfuric acid concentration were fixed, the effect of the extractant concentration on the extraction of Zr and Hf was investigated. As shown in Fig. 3, the plots of logD versus log[B] give two straight lines with the slopes of 3.04 and 2.01 for Zr and Hf, respectively. Therefore, n would equal to 3 and 2, suggesting that three and two extractant molecules were involved in the chelates of Zr and Hf, respectively. The extraction of Zr and Hf from the aqueous solution with low sulfuric acid concentration using BEAP can be expressed by the following equations. Kex
ZrO2+ +2HSO‐4 +3B(O) ⎯⎯⎯→ ZrO(HSO4 )2⋅3B(O) Kex
HfO2+ +2HSO‐4 +2B(O) ⎯⎯⎯→ HfO(HSO4 )2 ⋅2B(O)
Wave number, cm− 1 BEAP
ν CeH, ν CeH δasCH3 −, δsCH3 − νaseCH2 −, νseCH2 − δ NeH ν P]O νas PeOeC, νs PeOeC νasSO42 −, νsSO42 − ν HSO4− as
s
2960, 1460, 2930, 1632 1246 1046, – –
2876 1382 2859
1014
H2SO4·B
ZrO(HSO4)2·3B
2960, 2876 1460, 1382 2930, 2859 1607 1259(broad) 1046, 1014 –, 589 –
2960, 2876 1460, 1382 2930, 2859 1600 1259 1046, 1014 1142, 589 663, 631
(5) ZrO(HSO4)2·3B (dried under an infrared lamp) are shown in Fig. 4. Table 2 gives wave numbers and probable assignments. It is found that the stretching vibration of the P]O group at 1246 cm− 1 becomes broader after the extraction of H2SO4 and zirconium, which might be due to the coordination of phosphoryl oxygen atom and H2SO4 and/or the metals. The overlap of the vibration of sulfate ions with the P]O vibration might also contribute to broadening. Notedly, the band at 1142 cm− 1 associated with the asymmetric stretching vibration of sulfate ion appeared obviously for ZrO(HSO4)2·3B complex. Furthermore, a band at 1632 cm− 1, which can be assigned to the NeH stretching vibration, is shifted to 1607 cm− 1 for the H2SO4·B complex and 1600 cm− 1 for the ZrO(HSO4)2·3B complex, respectively, which indicates that nitrogen atom maybe also take part in the coordination. The bands at 663 and 631 cm− 1 demonstrated that HSO4− ions were involved in the chelate of zirconium.
(6)
It is well known that sulfate ions have a stronger tendency to form complexes with zirconium and hafnium (Wang and Lee, 2016a). When sulfuric acid concentration in the aqueous solution is relatively high (> 2 mol/L), the decrease in the extraction of Zr and Hf can be attributed to the formation of anionic metal complexes. FT-IR spectra of the extractant BEAP and the chelates H2SO4·B and
3.3. Effect of temperature The effect of temperature on the extraction of Zr4 + and Hf4 + with BEAP has been studied in the range of 298–320 K. The enthalpy change (ΔH) can be determined according to the following equation:
Δ log D −ΔH = 2.303R Δ1 T
(7)
Eq. (4) was derived from Eq. (3) and the Van't Hoff equation. The relationship between logD and 1000/T is shown in Fig. 5. The plots give the slopes of 3.28 and 0.60 for Zr4 + and Hf4 + extraction, respectively. Δ H was found to be − 62.80 KJ/mol for Zr4 + extraction and − 14.49 KJ/mol for Hf4 + extraction, indicating that the extractions of Zr4 + and Hf4 + with BEAP are both exothermic processes.
Fig. 3. Effect of extractant concentration on the extraction of metal ions.[Zr4 +] = [Hf4 +] = 1.0 × 10− 2 mol/L, CH2SO4 = 0.75 mol/L.
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Fig. 6. Loading of BEAP (30% in n-heptane).
Fig. 5. Effect of temperature on the extraction of metal ions[Zr4 +] = [Hf4 +] = 1.0 × 10− 2 mol/L, [BEAP] = 5.0 × 10− 2 mol/L, CH2SO4 = 0.75 mol/L.
The change of Gibbs energy, Δ G, and the change of entropy, ΔS of the extraction system at 298 K can also be determined using the Gibbs–Helmholtz equation:
ΔG = −RTlnKex
(8)
ΔG = ΔH − T ΔS
(9)
ΔS =
ΔH − ΔG T
(10)
Δ G and Δ S can be calculated to be − 8.49 kJ/mol and 182.86 J/ (mol K) for the Zr4 + extraction, respectively. While for the Hf4 + extraction, Δ G and Δ S were determined to be − 4.88 kJ/mol and 14.41 J/(mol K), respectively. The minus values of Δ G implied that the extraction reactions of both Zr4 + and Hf4 + are spontaneous. The positive values of Δ S indicate an increase in the randomness of the investigated systems. 3.4. Loading capacity and stripping studies Fig. 7. Effect of HNO3 concentration for stripping percentage.
30% BEAP (v/v) in n-heptane was repeatedly contacted with the same volume of aqueous solutions containing 0.125 mol/L Zr or 0.320 mol/L Hf and 0.75 mol/L H2SO4. The cumulative concentration of the metal in the organic phase was calculated by summing the concentration difference in each contact. It was found that the organic phase was nearly saturated with the metal after three contacts for Zr4 + and two contacts for Hf4 + (Fig. 6). The extraction capacity of 30% BEAP in n-heptane is 26.80 g/L for Zr and 25.15 g/L for Hf. The stripping of metal ions from the loaded organic phase was performed. Although both Zr and Hf can be stripped from loaded organic phase by nitric acid solutions, the stripping percentages of Zr and Hf were discriminated (Fig. 7). Therefore, the separation of both metals would be possible during the stripping. As shown in Fig. 7, both Zr and Hf can be stripped by nitric acid. The stripping percentages increase with the increasing nitric acid concentration. When the stripping solution is a solution of 2 mol/L nitric acid, the stripping rate of Zr reaches 81.7%. The maximum of stripping rate of Hf reaches 45% at about 1.25 mol/L nitric acid. Then the stripping of Zr was nearly kept unchanged with the increasing nitric acid concentration until its concentration is 4 mol/L. After that, the stripping decreased. The stripping of Hf decreased with the further increasing nitric acid concentration. The optimal stripping solution concentration would be in the range of 2–4 mol/L nitric acid. Other stripping agents such as oxalic acid and hydrochloric acid were also studied. The results demonstrated that oxalic acid can strip the loaded Zr and Hf but can't separate them in the stripping process.
When HCl was used as the stripping agent, the stripping of Zr and Hf increased from 25.7% to 41.3% and 30% to 39.8%, respectively, when the HCl concentration increased from 0.5 to 1.5 mol/L. However, it should be noted that when the concentration of hydrochloric acid was higher than 1.5 mol/L, some kinds of cationic polymer of Zr and Hf formed and the phase separation became difficult (Wang et al., 2013). Therefore, 3 mol/L nitric acid was chosen as the stripping agent for the loaded Zr and Hf.
3.5. Purification of zirconium Based on the above results, a process was designed to extract and purify zirconium from a mixed solution of zirconium and hafnium. The optimized parameters of the process such as the stages for extracting, scrubbing and stripping, and the ratio of the flow rates of organic phase (S), feed (F), and scrubbing solution (W) were calculated according to the literature method (Xu et al., 1985). The experiment was conducted using 9-stages counter-current extraction with 30% BEAP (v/v) in nheptane as organic phase, 7-stages scrubbing with 0.75 mol/L H2SO4 solution as the scrubbing solution and 4-stages stripping with 3 mol/L HNO3 solution as the stripping agent. The flow rate ratio of organic phase (S), feed (F), and scrubbing regent (W) were optimized to be 1: 1: 1. The simple flow sheet was shown in Fig. 8. The initial metal concentrations of the mixed solution was fixed to 0.40 mol/L contain610
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Ceram. Soc. 71 (12), 4158. Huffman, E.H., Lilly, R.C., 1951. Anion exchange of complex ions of hafnium and zirconium in HCl-HF mixtures. J. Am. Ceram. Soc. 73 (6), 2902–2905. Kuang, S., Zhang, Z., Li, Y., Wu, G., Wei, H., Liao, W., 2017. Selective extraction and separation of Ce(IV) from thorium and trivalent rare earths in sulfate medium by an α-aminophosphonate extractant. Hydrometallurgy 167, 107–114. Lee, M.S., Banda, R., Min, S.H., 2015. Separation of Hf(IV)-Zr(IV) in H2SO4 solutions using solvent extraction with D2EHPA or Cyanex 272 at different reagent and metal ion concentrations. Hydrometallurgy 152, 84–90. Levitt, A.E., Freund, H., 2002. Solvent extraction of zirconium with tributyl phosphate1. J. Am. Ceram. Soc. 78 (8), 1545–1549. Lu, Y., Zhang, Z., Li, Y., Liao, W., 2017. Extraction and recovery of cerium(IV) and thorium(IV) from sulphate medium by an α-aminophosphonate extractant. J. Rare Earths 35, 34–40. Marczenko, Z., Balcerzak, M., 2002. 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Fig. 8. Flow sheet for the extraction and separation of Zr and Hf by BEAP.
ing 97% Zr and 3% Hf. The mixed solution also contained 0.75 mol/L H2SO4. After about 25 circuits, the extraction process reached equilibrium. The content of HfO2 in the ZrO2 product was decreased from 3% to 0.6% after the counter-current extraction using separating funnels. Therefore, this preliminary indicated that the extractant BEAP can be used for the effective separation of zirconium and hafnium. It would be possible to get nuclear zirconium with < 0.01% HfO2 by using much more extraction and scrubbing stages. 4. Conclusions BEAP (B) was used to separate zirconium from hafnium with a separation factor of 6.8 under optimum conditions. The content of HfO2 in ZrO2 product was decreased from 3% in the feed to 0.6% in the Zr product after counter-current operation. The extracted complexes can be determined to be ZrO(HSO4)2·3B and HfO(HSO4)2·2B for Zr and Hf, respectively. The extraction reaction was an exothermic process. Thermodynamic parameters, ΔG, ΔH and ΔS, were calculated to be − 62.80 KJ/mol, − 8.49 KJ/mol and 182.86 J/(mol K) for Zr4 + extraction, respectively, and − 14.49 KJ/mol, −4.88 KJ/mol and 14.41 J/ (mol K) for Hf4 + extraction, respectively. It is suggested that much more extraction and scrubbing stages are needed to obtain nuclear zirconium product containing < 0.01% HfO2. Acknowledgements This work was supported by 973 program (2012CBA01206), the National Natural Science Foundation of China (Nos. 21521092, 51222404) and the strategic priority research program of CAS (XDA02030100). References Banda, R., Man, S.L., 2015. Solvent extraction for the separation of Zr and Hf from aqueous solutions. Sep. Purif. Rev. 44 (3), 243–249. Banda, R., Lee, H.Y., Lee, M.S., 2012. Separation of Zr from Hf in acidic chloride solutions
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Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Separation of zirconium from hafnium in sulfate medium using solvent extraction with a new reagent BEAP
MARK
She Chena,b, Zhifeng Zhangb, Shengting Kuangb, Yanling Lib, Xiaowen Huanga,⁎, Wuping Liaob,⁎ a
School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China State Key Laboratory of Rare Earth Resource Utilization, ERC for the Separation and Purification of REs and Thorium, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Zirconium Hafnium Solvent extraction BEAP Sulfate medium
A new extractant, bis(2-ethylhexyl)-1-(2-ethylhexylamino)propylphosphonate (BEAP or B) was synthesized to extract and separate zirconium and hafnium from sulfate medium. Under optimum conditions, the separation factor of Zr over Hf was found to be 6.8. The extracted complexes were determined to be ZrO(HSO4)2·3B and HfO (HSO4)2·2B. The extractions of zirconium and hafnium are both exothermic. A counter-current operation was conducted, in which the content of HfO2 was decreased from 3% to 0.6% in the ZrO2 product. It is suggested that much more extraction and scrubbing stages are needed to reach the goal of 0.01% HfO2 in ZrO2 product.
1. Introduction Zirconium and hafnium usually co-exist in nature. Zirconium is widely used as the container and structural material of nuclear reactor due to its low thermal neutron capture cross-section. Differently, hafnium is an important control material for thermonuclear reaction because of its high neutron capture cross-section. Due to their diverse neutron capture properties, it is crucial to separate them before they are used in nuclear energy. For example, the content of hafnium in nuclear zirconium should be < 0.01%. Due to their similar chemical properties, the separation of zirconium and hafnium is still a challenge (Mukherji, 1970; Marczenko and Balcerzak, 2002; Xu et al., 2010; Xu et al., 2015a; Zhang, 2004, 2007). Many methods such as fractional crystallization (Niemand and Crouse, 2015), fractional precipitation (Schumb and Pittman, 2002), ion exchange (Huffman and Lilly, 1951; Benedict et al., 1954), silica gel adsorption (Hansen and Gunnar, 1949), molten salt distillation (Spink, 1980) and solvent extraction (Fischer and Chalybaeus, 1947; Cox et al., 1958) have been studied for the separation of zirconium and hafnium. Furthermore, only solvent extraction is used in industry with MIBK and TBP as the extractants (Fischer and Chalybaeus, 1947; Fischer et al., 1948; Sommers and Perrine, 2002). However, there are some disadvantages concerned the extraction systems of these two extractants. The MIBK system leads to solvent loss, atmospheric pollution and poor working environment due to the defects that the waste stream contains high concentrations of ammonium, cyanides and organic by-products with low flash point, high vapour pressure and solubility in the aqueous
⁎
Corresponding authors. E-mail addresses: [email protected] (X. Huang), [email protected] (W. Liao).
http://dx.doi.org/10.1016/j.hydromet.2017.04.001 Received 21 December 2016; Received in revised form 1 April 2017; Accepted 1 April 2017 Available online 04 April 2017 0304-386X/ © 2017 Published by Elsevier B.V.
phase (Snyder and Lee, 1992; Taghizadeh et al., 2008; Xu et al., 2010; Banda and Man, 2015). For the TBP system, the extractant has a high aqueous solubility and is not stable for long time operation, the equipments are seriously corroded, and the emulsified phenomenon is found in continuous production (Levitt and Freund, 2002). So other extractants such as Cyanex 923 were studied. Gupta et al. (2005) extracted zirconium from zircon leaching solution using 0.1 mol/L Cyanex 923 solution in toluene and recovered around 98% Zr with the purity of 98%. Nayl et al. (2009) studied the extraction and separation of Zr and Hf from nitrate medium by some Cyanex extractants and found that Cyanex 925 is more efficient than Cyanex 921 and Cyanex 923. Lee and Wang (Lee et al., 2015; Wang and Lee, 2016a) studied the extraction and separation of Zr and Hf using di-2-ethylhexylphosphoric acid (D2EHPA) from sulfate solutions and found that the loading capacity of D2EHPA is low (only 10 g/L Zr and 0.2 g/L Hf with 0.05 mol/L D2EHPA) and the efficient stripping needs strong acidic solutions (4 mol/L H2SO4). Synergistic extraction was also applied for the separation of zirconium and hafnium (Rezaee et al., 2011; Banda et al., 2012; Xu et al., 2015b; Xu et al., 2016; Wang and Lee, 2016b). Although, synergistic extraction can overcome some shortcomings of the sole extractant, the fatal problem is that it's so hard to maintain a ratio of multiple extractants for long time operations. In the present work, a new neutral extractant bis(2-ethylhexyl)-1-(2ethylhexylamino)propylphosphonate (BEAP) was synthesized and applied for the extraction and separation of zirconium and hafnium from sulfate medium. The extraction stoichiometry was deduced and thermal parameters were calculated.
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2. Experimental 2.1. Chemicals and materials Bis(2-ethylhexyl)phosphite (Shanghai Rare-earth Chemical Co., Ltd.), 2-ethylhexylamine (Shanghai Aladdin Industrial Co.) and propanal (Beijing Chemical Co., Ltd) were used as raw materials for the synthesis of BEAP without further purification. The extractant was dissolved in n-heptane to the required concentrations. The stock solutions of Zr and Hf were prepared by dissolving the corresponding salts in the corresponding mineral acid solutions. All work solutions were obtained by diluting the stock solutions. The initial concentrations of Zr and Hf were fixed to 1.0 × 10− 2 mol/L (unless otherwise stated). All other reagents were of analytical reagent grade. 2.2. Apparatus Nuclear magnetic resonance spectra were determined using a Bruker AV 600 M for 1H NMR (600 MHz) and FT-IR spectra (KBr pellets) using a Bruker Vertex 70 Spectrometer. The metal concentrations were determined by an inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer Optima 8000).
Fig. 2. Effect of acid concentration on the extraction of metal ions.[Zr4 +] = [Hf4 +] = 1.0 × 10− 2 mol/L, [BEAP] = 5.0 × 10− 2 mol/L.
2.3. Synthesis of bis(2-ethylhexyl)-1-(2-ethylhexylamino) propylphosphonate (BEAP)
3. Results and discussion
brium, respectively.
3.1. Extraction of Zr and Hf from different acidic media
BEAP (Fig. 1) was synthesized according to the method in the literature (Stoikov et al., 2000; Kuang et al., 2017; Lu et al., 2017). A mixture of 30.6 g (0.1 mol) bis(2-ethylhexyl)phosphite, 14.2 g (0.11 mol) 2-ethylhexylamine, 6.4 g (0.11 mol) propanal, 80 mL anhydrous toluene and 0.2 g p-toluenesulfonic acid were placed in a singleneck round-bottom flask (250 mL) equipped with a magnetic stirrer, a Dean–Stark trap and a reflux condenser. The mixture was heated with stirring for 6 h in an oil bath at 130 °C. At the end of the reaction 0.1 g of K2CO3 was added into the solution, and the mixture was refluxed for 15 min for the removal of the catalyst. The mixture was then cooled, washed with water (3 × 100 mL) and dried. The organic solution was evaporated in a vacuum on the rotary evaporator to remove the excess reagents and solvent until no fraction was steamed out. The obtained product was an oily compound with a yield of 90% (42.80 g) and a purity of 97.80%. 1H NMR (600 MHz, CDCl3, ppm): δ 4.01 (m, 4H, CH2), 2.61 (m, 1H, CH), 2.36 (d, 2H, CH2), 2.0 (s, 1H, NH), 1.56–1.71 (m, 3H,CH), 1.48 (m, 2H, CH2), 1.25–1.35 (m, 24H, CH2), 0.89 (t, 21H, CH3).
The extraction of Zr and Hf using BEAP dissolved in n-heptane was investigated from various acidic media including H2SO4, HCl and HNO3 solutions. In the experiments, the initial concentrations of Zr and Hf in the aqueous solutions were both kept at 0.01 mol/L, and the concentration of BEAP was maintained at 0.05 mol/L. The ranges of acid concentration were 0.05–6 mol/L for H2SO4, 0.05–5 mol/L for HCl, and 0.05–4 mol/L for HNO3. The phase ratio (O/A) was fixed at 1:1 with a constant total volume. As shown in Fig. 2, the extractant demonstrated higher extraction ability toward Zr than Hf in all three studied acid systems. Furthermore, the extraction of Zr and Hf by BEAP from different acidic media increased in the order of HCl < HNO3 < H2SO4. In the studied acidic range, the maximum of the extraction percentage of Zr reached 48.0% for the HCl system, 68.9% for the HNO3 system and 90.7% for the H2SO4 system, while that of Hf was 32.5% for the HCl system, 55.5% for the HNO3 system and 59.1% for the H2SO4 system. In the H2SO4 medium, the extraction of Zr first increased with the increasing H2SO4 concentration when CH2SO4 was lower than 0.59 mol/L. Then it decreased with the further increasing acid concentration. Similar trend was also observed for the extraction of Hf. Differently, in the HCl and HNO3 media, the extraction of Zr and Hf changes slightly with the increasing acid concentration. The separation factor (Table 1) of Zr over Hf reached a maximum of 6.77 at the H2SO4 concentration of 0.59 mol/ L while that was kept < 2 for the HNO3 system and < 3.2 for the HCl system. It seems that better separation of Zr from Hf can be obtained with sulfate medium. However, when the H2SO4 concentration is > 1.0 mol/L, the extraction of both Zr and Hf decreased rapidly with the
2.4. Experimental procedures The aqueous and organic phases, 5 mL, each were mixed and shaken using oscillator for 20 min to reach equilibrium at 298 ± 1 K (except the temperature experiment). The phases were then separated by gravity. The metal concentrations in the aqueous phase were determined by ICP-OES. The metal concentrations in the organic phases were obtained by mass balance. Distribution ratios (D) were calculated by D = [M](o) / [M](a), where [M](o) and [M](a) represent the concentrations of the metal ions in the organic and aqueous phases at equili-
Table 1 Separation factors (βZr/Hf) at different acid concentrations. β CH2SO4(mol/L) 0.24 0.59 1.09 1.32 2.41 3.40
Fig. 1. Molecular structure of bis(2-ethylhexyl)-1-(2-ethylhexylamino)propylphosphonate (BEAP).
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βZr/Hf 2.88 6.77 6.27 5.55 2.84 2.09
CHCl(mol/L) 0.50 1.14 1.73 2.34 2.96 3.58
βZr/Hf 3.14 2.30 2.14 2.17 2.02 1.87
CHNO3(mol/L) 0.58 1.38 2.17 2.99 3.75 4.54
βZr/Hf 1.65 1.61 1.72 1.81 1.85 1.77
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increasing acid concentration and the difference between the extraction of Zr and Hf became smaller, indicating that the separation of Zr and Hf was more difficult. 3.2. Extraction stoichiometry of Zr4 + and Hf4 + with BEAP The extraction of metal ions from the aqueous solution with low sulfuric acid concentration using BEAP can be expressed by the following equations (Xiong et al., 2006): Kex
MO2+ +2HSO‐4 + n B(O) ⎯⎯⎯→ MO(HSO4 )2⋅n B(O)
(1)
where “B” represents “BEAP”, “M” Zr or Hf and “n” the numbers of the extractant involved in the reaction. The equilibrium constant Kex is defined as follows:
Kex =
[MO(HSO4 )2⋅n B](o) n [MO2+] [HSO‐4 ]2 [B](o)
(2)
Taking logarithms:
log D = log Kex + 2 log [HSO‐4] + n log[B](o)
(3)
Fig. 4. Infrared spectra of BEAP and the chelates H2SO4·B and ZrO(HSO4)2·3B.
where
D=
[MO(HSO4 )2⋅n B](o) [MO2+]
Table 2 Characteristic IR spectral data for BEAP and the extracted Zr complexes.
(4) Probable assignment
When the original metal concentration and sulfuric acid concentration were fixed, the effect of the extractant concentration on the extraction of Zr and Hf was investigated. As shown in Fig. 3, the plots of logD versus log[B] give two straight lines with the slopes of 3.04 and 2.01 for Zr and Hf, respectively. Therefore, n would equal to 3 and 2, suggesting that three and two extractant molecules were involved in the chelates of Zr and Hf, respectively. The extraction of Zr and Hf from the aqueous solution with low sulfuric acid concentration using BEAP can be expressed by the following equations. Kex
ZrO2+ +2HSO‐4 +3B(O) ⎯⎯⎯→ ZrO(HSO4 )2⋅3B(O) Kex
HfO2+ +2HSO‐4 +2B(O) ⎯⎯⎯→ HfO(HSO4 )2 ⋅2B(O)
Wave number, cm− 1 BEAP
ν CeH, ν CeH δasCH3 −, δsCH3 − νaseCH2 −, νseCH2 − δ NeH ν P]O νas PeOeC, νs PeOeC νasSO42 −, νsSO42 − ν HSO4− as
s
2960, 1460, 2930, 1632 1246 1046, – –
2876 1382 2859
1014
H2SO4·B
ZrO(HSO4)2·3B
2960, 2876 1460, 1382 2930, 2859 1607 1259(broad) 1046, 1014 –, 589 –
2960, 2876 1460, 1382 2930, 2859 1600 1259 1046, 1014 1142, 589 663, 631
(5) ZrO(HSO4)2·3B (dried under an infrared lamp) are shown in Fig. 4. Table 2 gives wave numbers and probable assignments. It is found that the stretching vibration of the P]O group at 1246 cm− 1 becomes broader after the extraction of H2SO4 and zirconium, which might be due to the coordination of phosphoryl oxygen atom and H2SO4 and/or the metals. The overlap of the vibration of sulfate ions with the P]O vibration might also contribute to broadening. Notedly, the band at 1142 cm− 1 associated with the asymmetric stretching vibration of sulfate ion appeared obviously for ZrO(HSO4)2·3B complex. Furthermore, a band at 1632 cm− 1, which can be assigned to the NeH stretching vibration, is shifted to 1607 cm− 1 for the H2SO4·B complex and 1600 cm− 1 for the ZrO(HSO4)2·3B complex, respectively, which indicates that nitrogen atom maybe also take part in the coordination. The bands at 663 and 631 cm− 1 demonstrated that HSO4− ions were involved in the chelate of zirconium.
(6)
It is well known that sulfate ions have a stronger tendency to form complexes with zirconium and hafnium (Wang and Lee, 2016a). When sulfuric acid concentration in the aqueous solution is relatively high (> 2 mol/L), the decrease in the extraction of Zr and Hf can be attributed to the formation of anionic metal complexes. FT-IR spectra of the extractant BEAP and the chelates H2SO4·B and
3.3. Effect of temperature The effect of temperature on the extraction of Zr4 + and Hf4 + with BEAP has been studied in the range of 298–320 K. The enthalpy change (ΔH) can be determined according to the following equation:
Δ log D −ΔH = 2.303R Δ1 T
(7)
Eq. (4) was derived from Eq. (3) and the Van't Hoff equation. The relationship between logD and 1000/T is shown in Fig. 5. The plots give the slopes of 3.28 and 0.60 for Zr4 + and Hf4 + extraction, respectively. Δ H was found to be − 62.80 KJ/mol for Zr4 + extraction and − 14.49 KJ/mol for Hf4 + extraction, indicating that the extractions of Zr4 + and Hf4 + with BEAP are both exothermic processes.
Fig. 3. Effect of extractant concentration on the extraction of metal ions.[Zr4 +] = [Hf4 +] = 1.0 × 10− 2 mol/L, CH2SO4 = 0.75 mol/L.
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Fig. 6. Loading of BEAP (30% in n-heptane).
Fig. 5. Effect of temperature on the extraction of metal ions[Zr4 +] = [Hf4 +] = 1.0 × 10− 2 mol/L, [BEAP] = 5.0 × 10− 2 mol/L, CH2SO4 = 0.75 mol/L.
The change of Gibbs energy, Δ G, and the change of entropy, ΔS of the extraction system at 298 K can also be determined using the Gibbs–Helmholtz equation:
ΔG = −RTlnKex
(8)
ΔG = ΔH − T ΔS
(9)
ΔS =
ΔH − ΔG T
(10)
Δ G and Δ S can be calculated to be − 8.49 kJ/mol and 182.86 J/ (mol K) for the Zr4 + extraction, respectively. While for the Hf4 + extraction, Δ G and Δ S were determined to be − 4.88 kJ/mol and 14.41 J/(mol K), respectively. The minus values of Δ G implied that the extraction reactions of both Zr4 + and Hf4 + are spontaneous. The positive values of Δ S indicate an increase in the randomness of the investigated systems. 3.4. Loading capacity and stripping studies Fig. 7. Effect of HNO3 concentration for stripping percentage.
30% BEAP (v/v) in n-heptane was repeatedly contacted with the same volume of aqueous solutions containing 0.125 mol/L Zr or 0.320 mol/L Hf and 0.75 mol/L H2SO4. The cumulative concentration of the metal in the organic phase was calculated by summing the concentration difference in each contact. It was found that the organic phase was nearly saturated with the metal after three contacts for Zr4 + and two contacts for Hf4 + (Fig. 6). The extraction capacity of 30% BEAP in n-heptane is 26.80 g/L for Zr and 25.15 g/L for Hf. The stripping of metal ions from the loaded organic phase was performed. Although both Zr and Hf can be stripped from loaded organic phase by nitric acid solutions, the stripping percentages of Zr and Hf were discriminated (Fig. 7). Therefore, the separation of both metals would be possible during the stripping. As shown in Fig. 7, both Zr and Hf can be stripped by nitric acid. The stripping percentages increase with the increasing nitric acid concentration. When the stripping solution is a solution of 2 mol/L nitric acid, the stripping rate of Zr reaches 81.7%. The maximum of stripping rate of Hf reaches 45% at about 1.25 mol/L nitric acid. Then the stripping of Zr was nearly kept unchanged with the increasing nitric acid concentration until its concentration is 4 mol/L. After that, the stripping decreased. The stripping of Hf decreased with the further increasing nitric acid concentration. The optimal stripping solution concentration would be in the range of 2–4 mol/L nitric acid. Other stripping agents such as oxalic acid and hydrochloric acid were also studied. The results demonstrated that oxalic acid can strip the loaded Zr and Hf but can't separate them in the stripping process.
When HCl was used as the stripping agent, the stripping of Zr and Hf increased from 25.7% to 41.3% and 30% to 39.8%, respectively, when the HCl concentration increased from 0.5 to 1.5 mol/L. However, it should be noted that when the concentration of hydrochloric acid was higher than 1.5 mol/L, some kinds of cationic polymer of Zr and Hf formed and the phase separation became difficult (Wang et al., 2013). Therefore, 3 mol/L nitric acid was chosen as the stripping agent for the loaded Zr and Hf.
3.5. Purification of zirconium Based on the above results, a process was designed to extract and purify zirconium from a mixed solution of zirconium and hafnium. The optimized parameters of the process such as the stages for extracting, scrubbing and stripping, and the ratio of the flow rates of organic phase (S), feed (F), and scrubbing solution (W) were calculated according to the literature method (Xu et al., 1985). The experiment was conducted using 9-stages counter-current extraction with 30% BEAP (v/v) in nheptane as organic phase, 7-stages scrubbing with 0.75 mol/L H2SO4 solution as the scrubbing solution and 4-stages stripping with 3 mol/L HNO3 solution as the stripping agent. The flow rate ratio of organic phase (S), feed (F), and scrubbing regent (W) were optimized to be 1: 1: 1. The simple flow sheet was shown in Fig. 8. The initial metal concentrations of the mixed solution was fixed to 0.40 mol/L contain610
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by using TOPO and its mixture with other extractants. J. Radioanal. Nucl. Chem. 298 (1), 259–264. Benedict, J.T., Schumb, W.C., Coryell, C.D., 1954. Distribution of zirconium and hafnium between cation-exchange resin and acid solutions. The column separation with nitric acid-citric acid mixture. J. Am. Ceram. Soc. 76 (8), 2036–2040. Cox, R.P., Peterson, H.C., Beyer, G.H., 1958. Separating hafnium from zirconium. Solvent extraction with tributyl phosphate. Ind. Eng. Chem. 50 (2), 141–143. Fischer, W., Chalybaeus, W., 1947. Die Trennung des Hafniums vom Zirkonium durch Verteilung. Z. Anorg. Chem. 255, 79–100. Fischer, W., Chalybaeus, W., Zumbusch, M., 1948. Die Prparative Gewinnung Reiner Hafniumverbindungen Durch Verteilung. Z. Anorg. Chem. 255, 277–286. Gupta, B., Malik, P., Mudhar, N., 2005. Extraction and recovery of zirconium from zircon using Cyanex 923. Solvent. Extr. Ion. Exch. 23 (3), 345–357. Hansen, R.S., Gunnar, K., 1949. The preparation of hafnium-free zirconium. J. Am. Ceram. Soc. 71 (12), 4158. Huffman, E.H., Lilly, R.C., 1951. Anion exchange of complex ions of hafnium and zirconium in HCl-HF mixtures. J. Am. Ceram. Soc. 73 (6), 2902–2905. Kuang, S., Zhang, Z., Li, Y., Wu, G., Wei, H., Liao, W., 2017. Selective extraction and separation of Ce(IV) from thorium and trivalent rare earths in sulfate medium by an α-aminophosphonate extractant. Hydrometallurgy 167, 107–114. Lee, M.S., Banda, R., Min, S.H., 2015. Separation of Hf(IV)-Zr(IV) in H2SO4 solutions using solvent extraction with D2EHPA or Cyanex 272 at different reagent and metal ion concentrations. Hydrometallurgy 152, 84–90. Levitt, A.E., Freund, H., 2002. Solvent extraction of zirconium with tributyl phosphate1. J. Am. Ceram. Soc. 78 (8), 1545–1549. Lu, Y., Zhang, Z., Li, Y., Liao, W., 2017. Extraction and recovery of cerium(IV) and thorium(IV) from sulphate medium by an α-aminophosphonate extractant. J. Rare Earths 35, 34–40. Marczenko, Z., Balcerzak, M., 2002. 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Fig. 8. Flow sheet for the extraction and separation of Zr and Hf by BEAP.
ing 97% Zr and 3% Hf. The mixed solution also contained 0.75 mol/L H2SO4. After about 25 circuits, the extraction process reached equilibrium. The content of HfO2 in the ZrO2 product was decreased from 3% to 0.6% after the counter-current extraction using separating funnels. Therefore, this preliminary indicated that the extractant BEAP can be used for the effective separation of zirconium and hafnium. It would be possible to get nuclear zirconium with < 0.01% HfO2 by using much more extraction and scrubbing stages. 4. Conclusions BEAP (B) was used to separate zirconium from hafnium with a separation factor of 6.8 under optimum conditions. The content of HfO2 in ZrO2 product was decreased from 3% in the feed to 0.6% in the Zr product after counter-current operation. The extracted complexes can be determined to be ZrO(HSO4)2·3B and HfO(HSO4)2·2B for Zr and Hf, respectively. The extraction reaction was an exothermic process. Thermodynamic parameters, ΔG, ΔH and ΔS, were calculated to be − 62.80 KJ/mol, − 8.49 KJ/mol and 182.86 J/(mol K) for Zr4 + extraction, respectively, and − 14.49 KJ/mol, −4.88 KJ/mol and 14.41 J/ (mol K) for Hf4 + extraction, respectively. It is suggested that much more extraction and scrubbing stages are needed to obtain nuclear zirconium product containing < 0.01% HfO2. Acknowledgements This work was supported by 973 program (2012CBA01206), the National Natural Science Foundation of China (Nos. 21521092, 51222404) and the strategic priority research program of CAS (XDA02030100). References Banda, R., Man, S.L., 2015. Solvent extraction for the separation of Zr and Hf from aqueous solutions. Sep. Purif. Rev. 44 (3), 243–249. Banda, R., Lee, H.Y., Lee, M.S., 2012. Separation of Zr from Hf in acidic chloride solutions
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