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Uranium stripping from tri-n-butyl phosphate by hydrofluoric acid solutions with hydrazine addition
Hydrometallurgy 157 (2015) 179–183
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Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Uranium stripping from tri-n-butyl phosphate by hydrofluoric acid solutions with hydrazine addition S.Yu. Skripchenko ⁎, A.L. Smirnov, A.M. Pastukhov, M.V. Chernyshov Department of Rare Metals and Nanomaterials, Institute of Physics and Technology, Ural Federal University, 620002 Ekaterinburg, St. Mira, 19, Russian Federation
a r t i c l e
i n f o
Article history: Received 21 February 2015 Received in revised form 1 August 2015 Accepted 19 August 2015 Available online 25 August 2015 Keywords: Solvent extraction Uranium stripping Tri-n-butyl phosphate Hydrazine hydrate Uranium tetrafluoride Hydrofluoric acid
a b s t r a c t The process of uranium stripping from tri-n-butyl phosphate by hydrofluoric acid solutions with the addition of hydrazine was investigated in the temperature range of 20 to 60 °C. Uranium was selectively precipitated in the form of a hydrazine uranyl fluoride complex when stripped from TBP by HF solutions with the addition of hydrazine. The uranium precipitation increased with increasing N2H4/U molar ratio in the range of 1 to 3 and with increasing holding time. Uranium tetrafluoride was obtained by the thermal decomposition of the hydrazine uranyl fluoride complex in a hydrogen stream. The uranium content in UF4 is 76%. The obtained uranium tetrafluoride is a high-purity product and meets all requirements imposed on UF4 at conversion plants. Thus, the stripping method provides a reduction of uranium processing operations to uranium tetrafluoride. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The aim of any uranium-refining method is to obtain nuclear grade material, which could be used as a reactor fuel. The decrease in the content of elemental impurities to acceptable values by a single operation at the present stage of refining technology development is not feasible. The problem of obtaining a nuclear grade product is achieved by combining two or more refining methods. Mainly, the flowsheets use a combination of solvent extraction and preparation of uranium tetrafluoride. Solvent extraction consists of the dissolution of yellow cake in nitric acid and the selective extraction of uranium using tri-n-butyl phosphate (TBP). The purified uranium is usually stripped from the organic phase into the aqueous phase by water or dilute nitric acid. The resulting uranyl nitrate solution is converted to UO3 in two steps, evaporation and denitration. UO3 is subsequently reduced with hydrogen to UO2, then converted to UF4 with hydrogen fluoride at elevated temperatures (Benedict et al., 1981; Edwards and Oliver, 2000; Morss et al., 2010; Wilson, 1996). The main disadvantage of this approach is the multi-step process of producing uranium tetrafluoride and the use of complicated, expensive and highly energy-consuming operations, such as evaporation, denitration, and the reduction of UO3 to UO2. Reducing the number of operations is possible through the use of hydrogen peroxide or ammonium carbonate as stripping agent. The application of hydrogen peroxide or ammonium carbonate solutions for ⁎ Corresponding author. E-mail address: [email protected] (S.Y. Skripchenko).
http://dx.doi.org/10.1016/j.hydromet.2015.08.015 0304-386X/© 2015 Elsevier B.V. All rights reserved.
stripping from loaded TBP results in the precipitation of uranium in the form of uranium peroxide or ammonium uranyl tricarbonate, respectively, which can readily be converted to UO3 (Chegrouche and Kebir, 1992; Singh and Gupta, 2000; Smirnov et al., 2013). The operations of evaporation and denitration can be also eliminated by the use of the precipitation method. The introduction of ammonia, hydrogen peroxide or ammonium carbonate into the uranyl nitrate solution from solvent extraction causes the precipitation of uranium in the form of ammonium diuranate, uranium peroxide or ammonium uranyl tricarbonate, respectively. These products are then calcined to form UO3. Ammonium uranyl tricarbonate can also be converted to UO2 in an inert or reducing atmosphere (Benedict et al., 1981; Gupta and Singh, 2003; Mishra et al., 2013). However, the number of operations could be significantly reduced by obtaining uranium fluorides directly from the stripping stage. Highpurity uranium tetrafluoride is precipitated from loaded TBP by the use of hydrofluoric acid stripping solutions if the organic phase contains the uranium as a tetravalent uranium compound. In this case, the prior reduction of U(VI) to U(IV) in feed process solutions using aluminum (or electrolysis) is required (Ellis et al., 1962). This paper describes the application of a hydrofluoric acid solution with hydrazine addition as an agent for the stripping of uranium from loaded TBP. This method differs from traditional stripping methods and has several advantages. Uranium is stripped from the loaded organic phase through direct precipitation in the form of the hydrazine uranyl fluoride complex. This product is readily converted to uranium tetrafluoride by thermal decomposition in a reducing atmosphere. The final product meets all requirements imposed on UF4 at conversion plants.
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Table 1 Elemental impurities in ammonium polyuranate, μg/g U. B Ca Fe K Mg Mo Na
136 1435 118 50 364 1731 3414
P S Si Th V Al Pb
1390 19714 1775 45 71 648 b10
Li Sn Mn Ni Cu Cl Cr
1.5 b10 18 22 3.2 133 37
Ti W Sb Ta Bi Nb Sr
196 3.2 1.5 0.3 2.4 1.4 2.5
The use of this stripping method eliminates certain processing steps from uranium refinement, including evaporation, the denitration stage and the reduction of UO3 to UO2.
2. Experimental The loaded organic phase containing 84 g/L U(VI) and 0.27 mol/L HNO3 was prepared through the selective extraction of uranium from uranyl nitrate solution (100 g/L U(VI), 1.59 mol/L HNO3) using 30% (by volume) tri-n-butyl phosphate in kerosene (ShellSol D90). The extraction was conducted in a separatory funnel at room temperature (20–25 °C). The volumetric ratio between the organic and aqueous phases (O/A) was 1. The uranyl nitrate solution was prepared by the dissolution of ammonium polyuranate in nitric acid solution. The process was continued until the complete dissolution of uranium concentrate. Ammonium polyuranate was obtained from uranium ore at the JSC Dalur (Kurgan region, Russia). JSC Dalur produces uranium using the acid in-situ leaching (ISL) method. The recovery of uranium from ISL solutions includes the following process steps: uranium sorption on strong base anionic resins, uranium desorption using ammonium nitrate as eluting reagent, and ammonium polyuranate precipitation by ammonium bicarbonate. The X-ray phase analysis revealed that the ammonium polyuranate used in this study contains the following phases: 2UO3·NH3·3H2O and (NH4)4(H2O)[(UO2)2(SO4)O2]2. The predominant phase is 2UO3·NH3·3H2O (97.25%). The humidity of the ammonium polyuranate was 31.2%; the uranium content in the dry product was 70.94%. The elemental impurity contents in the ammonium polyuranate are presented in Table 1. The stripping procedures were performed in a polypropylene tube (50 mL) at 20–60 °C. In all the experiments, the volumetric ratio between the organic and aqueous phases (O/A) was 1. The uranium was stripped from the loaded organic phase by hydrofluoric acid solutions both with and without hydrazine. The stripping solutions were prepared using distilled water, concentrated hydrofluoric acid, and hydrazine hydrate shortly prior to the experiments. All chemicals used in the study were reagents of chemically pure grade. The stripping experiments were performed using the following procedure. Equal volumes (20 mL) of loaded organic phase and stripping solution were placed in a polypropylene tube and stirred using a rotary mixer (ELMI Intelli Mixer RM-2) at 90 rpm. All three phases were then separated. First, the organic phase was separated using a separatory funnel. The uranium precipitate was then collected from the aqueous
Fig. 1. X-ray powder diffraction pattern of uranium compounds: 1 — UO2F2N2H42HF ·1.25H2O; 2 — UF4·2.5H2O; 3 — mixture of UF4 and UO2 (×); 4 — UF4.
phase by vacuum filtration, washed with ethanol and dried at room temperature. The thermal decomposition of uranium compounds was performed in a pipe furnace at a temperature of 450–600 °C in vacuum and in a hydrogen stream. The uranium concentration in the aqueous phase was determined using ICP-AES (Optima 4300 DV Perkin Elmer). The uranium compounds obtained by stripping were analyzed using ICP-AES. The uranium concentration in the organic phases was calculated based on the mass balance. X-ray diffraction patterns of uranium compound powders were obtained on a STADI-P (STOE, Germany). Qualitative phase analysis of the diffraction patterns was conducted by search/match techniques using a powder diffraction database (ICDD PDF-2, 2009). Quantitative phase analysis of uranium compounds was performed by the Rietveld method using X-ray powder diffraction data. The particle size distribution of uranium tetrafluoride was determined using an Analysette 22 laser particle sizer. All infrared spectra of the uranium compounds were measured in the range of 400–4000 cm− 1 using a Bruker VERTEX 70 spectrometer. The measurements of IR spectra of uranium compounds were performed on solid-phase samples using the KBr pellet technique. Mixtures of KBr and the samples were pressed into transparent pellets (d = 13 mm) at 600 MPa.
Table 2 The effect of HF concentration on uranium distribution during stripping from loaded organic phase. HF concentration, mol/L
0.5 1.0 1.3 1.5 2.0 4.0 6.0
Uranium distribution, % Organic phase
Aqueous phase
27.2 11.4 4.1 1.2 0.8 0.5 0.3
72.8 88.6 95.9 98.8 99.2 99.5 99.7
Fig. 2. Infrared spectra for uranium compounds: 1 — UO 2 F 2 N 2 H 4 2HF ·1.25H 2 O; 2 — UF4 ·2.5H 2O; 3 — UF4 .
S.Y. Skripchenko et al. / Hydrometallurgy 157 (2015) 179–183 Table 3 The effect of N2H4/U molar ratio on uranium distribution during stripping from loaded organic phase by a solution containing 6 mol/L HF. N2H4/U molar ratio
0.3 1.0 1.5 2.0 2.5 3.0 3.5 4.5 6.0 9.0
Aqueous phase
Precipitate
0.5 0.5 0.4 0.2 0.1 0.1 0.1 0.1 0.1 0.1
99.5 48.7 14.8 13.3 9.1 6.1 8.6 12.3 99.9 99.9
– 50.8 84.8 86.5 90.8 93.8 91.3 87.6 – –
All experiments were performed at least in duplicate, and the average error was less than 1.0%. 3. Results and discussion The effect of the HF content on the uranium stripping was studied at room temperature (20–25 °C). The uranium was stripped from the loaded organic phase using hydrofluoric acid solutions (0.5–6 mol/L). All other parameters were kept constant in these tests. The results of these experiments are presented in Table 2. The results indicate that uranium stripping increases as the HF content in the solution increases. Sahoo and Satapathy (1964) reported that during the reaction of uranyl salts with hydrazine hydrogen fluoride in the presence of hydrofluoric acid, uranium fluorides are precipitated. The thermal destruction of these compounds leads to the formation of uranium tetrafluoride. The loaded organic phase contains uranium as the UO2(NO3)2·2TBP complex, so the process of uranium stripping from TBP by hydrofluoric acid solutions with hydrazine addition was studied. The use of a hydrofluoric acid solution with the addition of hydrazine hydrate for stripping results in uranium precipitation. The experiments were performed using a stripping solution that contained 2–7 mol/L HF and 0.3–9 mol N2H4/mol U. The phase contact time was 0.15 h after stirring was stopped. The XRD pattern of the obtained uranium compound is shown in Fig. 1(1). Comparison of the measured diffraction pattern with the powder diffraction database (ICDD PDF-2, 2009) gave no results. Therefore, X-ray phase analysis of hydrazine uranyl fluoride complexes prepared using the experimental technique described by Sahoo and Satapathy (1964) was performed. Measured diffraction patterns were compared with the diffraction pattern of uranium compound obtained by stripping. According to the results, the uranium is precipitated in the form of UO2F2N2H42HF ·1.25H2O during stripping from the organic phase by hydrofluoric acid solution with the addition of hydrazine hydrate. The IR spectrum of the hydrazine uranyl fluoride complex obtained by stripping is shown in Fig. 2(1). Comparison of IR spectra with the spectra of uranium compounds (Batsanov and Gagarinskii, 1964; Tsvetkov et al., 1973; Varga et al., 2011), hydrazine (Durig et al., 1966) Table 4 The effect of HF concentration on uranium distribution during stripping from loaded organic phase by a solution containing 3 mol N2H4/mol U. HF concentration, mol/L
2 3 3.5 4 5 6 7
Table 5 The effect of holding time on uranium distribution during stripping from loaded organic phase by a solution containing 3 mol N2H4/mol U and 6 mol/L HF. Holding time, h
Uranium distribution, % Organic phase
Uranium distribution, % Organic phase
Aqueous phase
Precipitate
0.6 0.4 0.3 0.1 0.1 0.1 0.1
99.4 99.6 99.7 26.6 15.2 6.1 5.0
– – – 73.3 84.7 93.8 93.9
181
Uranium distribution, %
0 4 12 24
Organic phase
Aqueous phase
Precipitate
0.1 0.1 0.1 0.1
6.1 5.5 5.2 4.6
93.8 94.4 94.7 95.3
and some metal halides with hydrazine (Athavale and Iyer, 1967; Earnshaw et al., 1964; Glavič and Slivnik, 1970; Sacconi and Sabatini, 1963) allowed the identification of several major regions. The band at 432 cm− 1 is due to the asymmetric stretching vibrations of the U–F. The sharp peak at 598 cm−1 is assigned to the asymmetric rocking of the NH2 groups. The two bands with maxima at 917 and 788 cm− 1 are due to the symmetric and asymmetric stretching vibrations of the uranyl group, respectively. The strong bands at 1124 and 1156 cm−1 are attributed to the NH2 twisting vibration. The two weak bands at approximately 1340 and 1382 cm−1 are assigned to the wagging vibrations of the NH2 groups. The bands in the 1500–1700 cm−1 region are due to the NH2 bending vibrations (at 1522 cm− 1 and 1567 cm− 1) and the vibrational bending mode of H2O (at 1630 cm−1). The combination bands in the 2200–3600 cm−1 region are characterized by the O–H and NH2 stretching vibrations. The peak at 2143 cm−1 is assigned to uncompensated interference from carbon dioxide in the atmosphere of the sample compartment. The extent of uranium precipitation from the loaded organic phase was found to increase as the N2H4/U molar ratio increased in the range of 1–3 (Table 3). The hydrofluoric acid concentration in the stripping solution should be no less than 4 mol/L to achieve uranium precipitation (Table 4). Uranium is not precipitated at N2H4/U molar ratios greater than 5 but is almost completely extracted into the aqueous phase. The uranium is precipitated from these solutions during holding for several days. The X-ray phase analysis revealed that uranium precipitated as UF4·2.5H2O. The XRD pattern is shown in Fig. 1(2). The precipitation of uranium tetrafluoride is due to the recovery of U(VI) to U(IV) with hydrazine in the presence of hydrofluoric acid (Anan'ev et al., 2001). The infrared spectrum of the obtained uranium tetrafluoride is shown in Fig. 2(2). The band at 459 cm−1 is associated with the asymmetric U–F stretching vibrations. The sharp peak at 956 cm− 1 is due to the asymmetric vibrations of the U_O in the uranyl ion. The bands with maxima at 1087, 1118 and 1219 cm−1 are attributed to the NH2 twisting vibration. The weak band at 1420 cm−1 is assigned to the NH2 wagging vibrations. The bands at 1539 and 1587 cm−1 are due to the NH2 bending vibrations. The peak at 1647 cm−1 is assigned to the vibrational bending mode of H2O. The combination bands in the 2800–3600 cm−1 region are due to the O–H and NH2 stretching vibrations. The bands of NH2 and uranyl groups indicate that the uranium partially precipitated as a hydrazine uranyl fluoride complex. The uranium precipitate contains trace amounts of the hydrazine uranyl fluoride complex, as it was not found by X-ray diffraction analysis. It is known that temperature is important in uranium stripping (Morss et al., 2010). Therefore, uranium was stripped from the loaded Table 6 Elemental impurities forming volatile fluorides in uranium tetrafluoride, μg/g U. Element
UF4
ASTM C787-11
Element
UF4
ASTM C787-11
Element
UF4
ASTM C787-11
Sb As B Br Cl
0.2 0.1 0.1 1.6 1.3
1 3 1 5 100
Cr Mo Nb P Ru
3.5 0.3 0.1 4.7 0.1
10 2 1 50 1
Si Ta Ti W V
16 0.2 0.7 0.8 0.2
100 1 1 2 2
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Table 7 Elemental impurities forming nonvolatile fluorides in uranium tetrafluoride, μg/g U. Al Fe Na Ba Pb Sr Be
3 21 7 b2 0.1 b1 b1
Li Th Bi Mg Sn Cd Mn
b1 b3 b2 b4 b10 b2 b1
Zn Ca Ni Zr K Cu Ag
b2 6 3 0.7 5 0.1 b1
organic phase at different temperatures (20–60 °C) with the other parameters held constant. Tests were performed using a solution that contained 3 mol N2H4/mol U and 6 mol/L HF. The phase contact time was 0.15 h after stirring was stopped. According to the results, the amount of uranium stripped from the loaded organic phase increases slightly with temperature in the range of 20 to 60 °C. Thus, the contents of hydrofluoric acid and hydrazine in the stripping solution are the major parameters that influence the extent of uranium precipitation. A study (Smirnov et al., 2013) showed that increasing the contact time of the phases increases the degree of uranium precipitation from the organic phase. Therefore, the effect of the phase contact time after stirring (i.e., the holding time) on uranium stripping from TBP was studied. The stripping studies were performed at temperatures from 20–25 °C using hydrofluoric acid solutions (6 mol/L) with hydrazine hydrate added (3 mol N2H4/mol U). Samples of the organic and aqueous phases were drawn hourly, and their uranium content was analyzed. The results indicate that the amount of uranium stripped from the loaded organic phase increases as the holding time is increased (Table 5). In addition, some part of the uranium is precipitated in the form of UF4·2.5H2O by the recovery of U(VI) to U(IV) with hydrazine in the presence of hydrofluoric acid (Anan'ev et al., 2001). The hydrazine uranyl fluoride complex obtained by stripping was calcined at a temperature of 450–600 °C in a vacuum and in a hydrogen stream. According to the results of the X-ray phase analysis, the final product of thermal decomposition in a vacuum is a mixture of UF4 and UO2 (Fig. 1(3)), at 55–60% and 40–45%, respectively. The X-ray diffraction analysis revealed that the final product of thermal decomposition of the hydrazine uranyl fluoride complex in a hydrogen stream is UF4. The XRD pattern is shown in Fig. 1(4). The uranium content in the uranium tetrafluoride is 76%. The infrared spectrum is shown in Fig. 2(3). This spectrum contains several absorption bands in the range of 400–4000 cm−1. Two broad bands with maxima at 3530 cm− 1 and 3191 cm− 1 are due to stretching vibrations of O–H. The sharp peak at 1613 cm−1 is associated with the vibrational bending mode of H2O. The absorption bands at 1011 cm−1 and 939 cm−1 are related to the symmetric and asymmetric vibrations of the uranyl group. All these bands are due to the presence of a UO2F2 ·nH2O impurity in
Fig. 3. Particle size distribution of uranium tetrafluoride.
the sample (Nyquist and Kagel, 1971). The content of UO2F2 ·nH2O in the samples does not exceed 2–3%. The obtained uranium tetrafluoride is a high-purity product and can be used to produce UF6 corresponding to ASTM C787-11 (ASTM, 2011). According to ASTM C787-11, only impurities forming volatile fluorides are separately normalized (Table 6). Impurities forming non-volatile fluorides are normalized by the total number, which should not exceed 300 μg/g U. The uranium tetrafluoride content of these impurities is not more than 100 μg/g U (Table 7). The size distribution of the particles of uranium tetrafluoride is shown in Fig. 3. The mean particle diameter of uranium tetrafluoride is 20–25 μm. The tap density is 2.7 g/cm3. Analysis of the data suggests that the uranium tetrafluoride meets all requirements imposed on this product at conversion plants. 4. Conclusions The results of this work clearly indicate that uranium can be effectively stripped from a loaded organic phase using hydrofluoric acid solution with the addition of hydrazine hydrate. The use of a hydrofluoric acid solution with the addition of hydrazine hydrate for stripping results in the precipitation of uranium in the form of a hydrazine uranyl fluoride complex. The HF concentration in the stripping solution and the N2H4/U molar ratio should be 6 mol/L and 3, respectively, to achieve efficient uranium precipitation from a loaded organic phase (approximately 94%). The hydrazine uranyl fluoride complex obtained in this work was converted to UF4 by thermal decomposition in a hydrogen stream. The uranium tetrafluoride obtained under these conditions is a highly pure product with a uranium content of 76% and a mean particle diameter of 20–25 μm. According to studies, this UF4 can be used to produce UF6 that corresponds to ASTM C787-11. Thus, the stripping method provides a reduction of uranium processing operations to produce high-purity uranium tetrafluoride. Acknowledgments This research project has been supported by UrFU under the Framework Programme of the development of UrFU through the “Young Scientists UrFU” competition. References Anan'ev, A., Shilov, V., Afonas'eva, T., Mikhailova, N., Milovanov, A., 2001. Catalytic reduction of U(VI) with hydrazine and formic acid in HNO3 solutions. Radiochemistry 43 (1), 39–43. ASTM, 2011. Standard Specification for Uranium Hexafluoride for Enrichment (ASTM Standard C787-11). ASTM International, West Conshohocken, PA, USA. Athavale, V., Iyer, P., 1967. Studies on some metal-hydrazine complexes. J. Inorg. Nucl. Chem. 29 (4), 1003–1012. Batsanov, S., Gagarinskii, Y.V., 1964. An optical study of uranium tetrafluoride crystal hydrates. J. Struct. Chem. 4 (3), 356–361. Benedict, M., Pigford, T.H., Levi, H.W., 1981. Nuclear Chemical Engineering. McGraw-Hill, New York. Chegrouche, S., Kebir, A., 1992. Study of ammonium uranyl carbonate re-extraction– crystallization process by ammonium carbonate. Hydrometallurgy 28 (2), 135–147. Durig, J., Bush, S., Mercer, E., 1966. Vibrational spectrum of hydrazine-d4 and a Raman study of hydrogen bonding in hydrazine. J. Chem. Phys. 44 (11), 4238–4247. Earnshaw, A., Larkworthy, L., Patel, K., 1964. Magnetic and other properties of chromium(II) hydrazine halides. Z. Anorg. Allg. Chem. 334 (3–4), 163–168. Edwards, C., Oliver, A., 2000. Uranium processing: a review of current methods and technology. JOM 52 (9), 12–20. Ellis, D.A., Grinstead, R.R., Long, R.S., Magner, J.E., 1962. Preparation of high purity UF4, US Patent, US3030175. Glavič, P., Slivnik, J., 1970. Studies on the uranium tetrafluoride-hydrazinium (1 +) fluoride and the uranium tetrafluoride-hydrazine systems. J. Inorg. Nucl. Chem. 32 (9), 2939–2949. Gupta, C., Singh, H., 2003. Uranium Resource Processing: Secondary Resources. Springer, Netherlands. ICDD PDF-2, 2009. Powder Diffraction Database. International Centre for Diffraction Data, Philadelphia, USA. Mishra, B., Gubel, N.R., Bhola, R., 2013. Uranium Processing, Uranium Processing and Properties. Springer, pp. 123–172. Morss, L.R., Edelstein, N.M., Fuger, J., 2010. The Chemistry of the Actinide and Transactinide Elements. Springer, Netherlands.
S.Y. Skripchenko et al. / Hydrometallurgy 157 (2015) 179–183 Nyquist, R.A., Kagel, R.O., 1971. Infrared Spectra of Inorganic Compounds (3800–45 cm−1). Academic Press, New York. Sacconi, L., Sabatini, A., 1963. The infra-red spectra of metal (II)-hydrazine complexes. J. Inorg. Nucl. Chem. 25 (11), 1389–1393. Sahoo, B., Satapathy, K., 1964. Preparation of uranium tetrafluoride by the thermal decomposition of hydrazine uranyl fluoride complexes. J. Inorg. Nucl. Chem. 26 (8), 1379–1380. Singh, H., Gupta, C.K., 2000. Solvent extraction in production and processing of uranium and thorium. Miner. Process. Extr. Metall. Rev. 21 (1–5), 307–349. Smirnov, A., Skripchenko, S.Y., Rychkov, V., Pastukhov, A., Shtutsa, M., 2013. Uranium stripping from tri-n-butyl phosphate by hydrogen peroxide solutions. Hydrometallurgy 137, 18–22.
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Tsvetkov, A.A., Seleznev, V.P., Sudarikov, B.N., Gromov, B.V., 1973. Infrared spectra of uranyl fluoride complexes with water and hydrogen fluoride. Russ. J. Inorg. Chem. 18 (3), 783–786. Varga, Z., et al., 2011. Characterization and classification of uranium ore concentrates (yellow cakes) using infrared spectrometry. Radiochim. Acta 99 (12), 807–813. Wilson, P.D., 1996. The Nuclear Fuel Cycle: From Ore to Wastes. Oxford University Press, Oxford.
Contents lists available at ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Uranium stripping from tri-n-butyl phosphate by hydrofluoric acid solutions with hydrazine addition S.Yu. Skripchenko ⁎, A.L. Smirnov, A.M. Pastukhov, M.V. Chernyshov Department of Rare Metals and Nanomaterials, Institute of Physics and Technology, Ural Federal University, 620002 Ekaterinburg, St. Mira, 19, Russian Federation
a r t i c l e
i n f o
Article history: Received 21 February 2015 Received in revised form 1 August 2015 Accepted 19 August 2015 Available online 25 August 2015 Keywords: Solvent extraction Uranium stripping Tri-n-butyl phosphate Hydrazine hydrate Uranium tetrafluoride Hydrofluoric acid
a b s t r a c t The process of uranium stripping from tri-n-butyl phosphate by hydrofluoric acid solutions with the addition of hydrazine was investigated in the temperature range of 20 to 60 °C. Uranium was selectively precipitated in the form of a hydrazine uranyl fluoride complex when stripped from TBP by HF solutions with the addition of hydrazine. The uranium precipitation increased with increasing N2H4/U molar ratio in the range of 1 to 3 and with increasing holding time. Uranium tetrafluoride was obtained by the thermal decomposition of the hydrazine uranyl fluoride complex in a hydrogen stream. The uranium content in UF4 is 76%. The obtained uranium tetrafluoride is a high-purity product and meets all requirements imposed on UF4 at conversion plants. Thus, the stripping method provides a reduction of uranium processing operations to uranium tetrafluoride. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The aim of any uranium-refining method is to obtain nuclear grade material, which could be used as a reactor fuel. The decrease in the content of elemental impurities to acceptable values by a single operation at the present stage of refining technology development is not feasible. The problem of obtaining a nuclear grade product is achieved by combining two or more refining methods. Mainly, the flowsheets use a combination of solvent extraction and preparation of uranium tetrafluoride. Solvent extraction consists of the dissolution of yellow cake in nitric acid and the selective extraction of uranium using tri-n-butyl phosphate (TBP). The purified uranium is usually stripped from the organic phase into the aqueous phase by water or dilute nitric acid. The resulting uranyl nitrate solution is converted to UO3 in two steps, evaporation and denitration. UO3 is subsequently reduced with hydrogen to UO2, then converted to UF4 with hydrogen fluoride at elevated temperatures (Benedict et al., 1981; Edwards and Oliver, 2000; Morss et al., 2010; Wilson, 1996). The main disadvantage of this approach is the multi-step process of producing uranium tetrafluoride and the use of complicated, expensive and highly energy-consuming operations, such as evaporation, denitration, and the reduction of UO3 to UO2. Reducing the number of operations is possible through the use of hydrogen peroxide or ammonium carbonate as stripping agent. The application of hydrogen peroxide or ammonium carbonate solutions for ⁎ Corresponding author. E-mail address: [email protected] (S.Y. Skripchenko).
http://dx.doi.org/10.1016/j.hydromet.2015.08.015 0304-386X/© 2015 Elsevier B.V. All rights reserved.
stripping from loaded TBP results in the precipitation of uranium in the form of uranium peroxide or ammonium uranyl tricarbonate, respectively, which can readily be converted to UO3 (Chegrouche and Kebir, 1992; Singh and Gupta, 2000; Smirnov et al., 2013). The operations of evaporation and denitration can be also eliminated by the use of the precipitation method. The introduction of ammonia, hydrogen peroxide or ammonium carbonate into the uranyl nitrate solution from solvent extraction causes the precipitation of uranium in the form of ammonium diuranate, uranium peroxide or ammonium uranyl tricarbonate, respectively. These products are then calcined to form UO3. Ammonium uranyl tricarbonate can also be converted to UO2 in an inert or reducing atmosphere (Benedict et al., 1981; Gupta and Singh, 2003; Mishra et al., 2013). However, the number of operations could be significantly reduced by obtaining uranium fluorides directly from the stripping stage. Highpurity uranium tetrafluoride is precipitated from loaded TBP by the use of hydrofluoric acid stripping solutions if the organic phase contains the uranium as a tetravalent uranium compound. In this case, the prior reduction of U(VI) to U(IV) in feed process solutions using aluminum (or electrolysis) is required (Ellis et al., 1962). This paper describes the application of a hydrofluoric acid solution with hydrazine addition as an agent for the stripping of uranium from loaded TBP. This method differs from traditional stripping methods and has several advantages. Uranium is stripped from the loaded organic phase through direct precipitation in the form of the hydrazine uranyl fluoride complex. This product is readily converted to uranium tetrafluoride by thermal decomposition in a reducing atmosphere. The final product meets all requirements imposed on UF4 at conversion plants.
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S.Y. Skripchenko et al. / Hydrometallurgy 157 (2015) 179–183
Table 1 Elemental impurities in ammonium polyuranate, μg/g U. B Ca Fe K Mg Mo Na
136 1435 118 50 364 1731 3414
P S Si Th V Al Pb
1390 19714 1775 45 71 648 b10
Li Sn Mn Ni Cu Cl Cr
1.5 b10 18 22 3.2 133 37
Ti W Sb Ta Bi Nb Sr
196 3.2 1.5 0.3 2.4 1.4 2.5
The use of this stripping method eliminates certain processing steps from uranium refinement, including evaporation, the denitration stage and the reduction of UO3 to UO2.
2. Experimental The loaded organic phase containing 84 g/L U(VI) and 0.27 mol/L HNO3 was prepared through the selective extraction of uranium from uranyl nitrate solution (100 g/L U(VI), 1.59 mol/L HNO3) using 30% (by volume) tri-n-butyl phosphate in kerosene (ShellSol D90). The extraction was conducted in a separatory funnel at room temperature (20–25 °C). The volumetric ratio between the organic and aqueous phases (O/A) was 1. The uranyl nitrate solution was prepared by the dissolution of ammonium polyuranate in nitric acid solution. The process was continued until the complete dissolution of uranium concentrate. Ammonium polyuranate was obtained from uranium ore at the JSC Dalur (Kurgan region, Russia). JSC Dalur produces uranium using the acid in-situ leaching (ISL) method. The recovery of uranium from ISL solutions includes the following process steps: uranium sorption on strong base anionic resins, uranium desorption using ammonium nitrate as eluting reagent, and ammonium polyuranate precipitation by ammonium bicarbonate. The X-ray phase analysis revealed that the ammonium polyuranate used in this study contains the following phases: 2UO3·NH3·3H2O and (NH4)4(H2O)[(UO2)2(SO4)O2]2. The predominant phase is 2UO3·NH3·3H2O (97.25%). The humidity of the ammonium polyuranate was 31.2%; the uranium content in the dry product was 70.94%. The elemental impurity contents in the ammonium polyuranate are presented in Table 1. The stripping procedures were performed in a polypropylene tube (50 mL) at 20–60 °C. In all the experiments, the volumetric ratio between the organic and aqueous phases (O/A) was 1. The uranium was stripped from the loaded organic phase by hydrofluoric acid solutions both with and without hydrazine. The stripping solutions were prepared using distilled water, concentrated hydrofluoric acid, and hydrazine hydrate shortly prior to the experiments. All chemicals used in the study were reagents of chemically pure grade. The stripping experiments were performed using the following procedure. Equal volumes (20 mL) of loaded organic phase and stripping solution were placed in a polypropylene tube and stirred using a rotary mixer (ELMI Intelli Mixer RM-2) at 90 rpm. All three phases were then separated. First, the organic phase was separated using a separatory funnel. The uranium precipitate was then collected from the aqueous
Fig. 1. X-ray powder diffraction pattern of uranium compounds: 1 — UO2F2N2H42HF ·1.25H2O; 2 — UF4·2.5H2O; 3 — mixture of UF4 and UO2 (×); 4 — UF4.
phase by vacuum filtration, washed with ethanol and dried at room temperature. The thermal decomposition of uranium compounds was performed in a pipe furnace at a temperature of 450–600 °C in vacuum and in a hydrogen stream. The uranium concentration in the aqueous phase was determined using ICP-AES (Optima 4300 DV Perkin Elmer). The uranium compounds obtained by stripping were analyzed using ICP-AES. The uranium concentration in the organic phases was calculated based on the mass balance. X-ray diffraction patterns of uranium compound powders were obtained on a STADI-P (STOE, Germany). Qualitative phase analysis of the diffraction patterns was conducted by search/match techniques using a powder diffraction database (ICDD PDF-2, 2009). Quantitative phase analysis of uranium compounds was performed by the Rietveld method using X-ray powder diffraction data. The particle size distribution of uranium tetrafluoride was determined using an Analysette 22 laser particle sizer. All infrared spectra of the uranium compounds were measured in the range of 400–4000 cm− 1 using a Bruker VERTEX 70 spectrometer. The measurements of IR spectra of uranium compounds were performed on solid-phase samples using the KBr pellet technique. Mixtures of KBr and the samples were pressed into transparent pellets (d = 13 mm) at 600 MPa.
Table 2 The effect of HF concentration on uranium distribution during stripping from loaded organic phase. HF concentration, mol/L
0.5 1.0 1.3 1.5 2.0 4.0 6.0
Uranium distribution, % Organic phase
Aqueous phase
27.2 11.4 4.1 1.2 0.8 0.5 0.3
72.8 88.6 95.9 98.8 99.2 99.5 99.7
Fig. 2. Infrared spectra for uranium compounds: 1 — UO 2 F 2 N 2 H 4 2HF ·1.25H 2 O; 2 — UF4 ·2.5H 2O; 3 — UF4 .
S.Y. Skripchenko et al. / Hydrometallurgy 157 (2015) 179–183 Table 3 The effect of N2H4/U molar ratio on uranium distribution during stripping from loaded organic phase by a solution containing 6 mol/L HF. N2H4/U molar ratio
0.3 1.0 1.5 2.0 2.5 3.0 3.5 4.5 6.0 9.0
Aqueous phase
Precipitate
0.5 0.5 0.4 0.2 0.1 0.1 0.1 0.1 0.1 0.1
99.5 48.7 14.8 13.3 9.1 6.1 8.6 12.3 99.9 99.9
– 50.8 84.8 86.5 90.8 93.8 91.3 87.6 – –
All experiments were performed at least in duplicate, and the average error was less than 1.0%. 3. Results and discussion The effect of the HF content on the uranium stripping was studied at room temperature (20–25 °C). The uranium was stripped from the loaded organic phase using hydrofluoric acid solutions (0.5–6 mol/L). All other parameters were kept constant in these tests. The results of these experiments are presented in Table 2. The results indicate that uranium stripping increases as the HF content in the solution increases. Sahoo and Satapathy (1964) reported that during the reaction of uranyl salts with hydrazine hydrogen fluoride in the presence of hydrofluoric acid, uranium fluorides are precipitated. The thermal destruction of these compounds leads to the formation of uranium tetrafluoride. The loaded organic phase contains uranium as the UO2(NO3)2·2TBP complex, so the process of uranium stripping from TBP by hydrofluoric acid solutions with hydrazine addition was studied. The use of a hydrofluoric acid solution with the addition of hydrazine hydrate for stripping results in uranium precipitation. The experiments were performed using a stripping solution that contained 2–7 mol/L HF and 0.3–9 mol N2H4/mol U. The phase contact time was 0.15 h after stirring was stopped. The XRD pattern of the obtained uranium compound is shown in Fig. 1(1). Comparison of the measured diffraction pattern with the powder diffraction database (ICDD PDF-2, 2009) gave no results. Therefore, X-ray phase analysis of hydrazine uranyl fluoride complexes prepared using the experimental technique described by Sahoo and Satapathy (1964) was performed. Measured diffraction patterns were compared with the diffraction pattern of uranium compound obtained by stripping. According to the results, the uranium is precipitated in the form of UO2F2N2H42HF ·1.25H2O during stripping from the organic phase by hydrofluoric acid solution with the addition of hydrazine hydrate. The IR spectrum of the hydrazine uranyl fluoride complex obtained by stripping is shown in Fig. 2(1). Comparison of IR spectra with the spectra of uranium compounds (Batsanov and Gagarinskii, 1964; Tsvetkov et al., 1973; Varga et al., 2011), hydrazine (Durig et al., 1966) Table 4 The effect of HF concentration on uranium distribution during stripping from loaded organic phase by a solution containing 3 mol N2H4/mol U. HF concentration, mol/L
2 3 3.5 4 5 6 7
Table 5 The effect of holding time on uranium distribution during stripping from loaded organic phase by a solution containing 3 mol N2H4/mol U and 6 mol/L HF. Holding time, h
Uranium distribution, % Organic phase
Uranium distribution, % Organic phase
Aqueous phase
Precipitate
0.6 0.4 0.3 0.1 0.1 0.1 0.1
99.4 99.6 99.7 26.6 15.2 6.1 5.0
– – – 73.3 84.7 93.8 93.9
181
Uranium distribution, %
0 4 12 24
Organic phase
Aqueous phase
Precipitate
0.1 0.1 0.1 0.1
6.1 5.5 5.2 4.6
93.8 94.4 94.7 95.3
and some metal halides with hydrazine (Athavale and Iyer, 1967; Earnshaw et al., 1964; Glavič and Slivnik, 1970; Sacconi and Sabatini, 1963) allowed the identification of several major regions. The band at 432 cm− 1 is due to the asymmetric stretching vibrations of the U–F. The sharp peak at 598 cm−1 is assigned to the asymmetric rocking of the NH2 groups. The two bands with maxima at 917 and 788 cm− 1 are due to the symmetric and asymmetric stretching vibrations of the uranyl group, respectively. The strong bands at 1124 and 1156 cm−1 are attributed to the NH2 twisting vibration. The two weak bands at approximately 1340 and 1382 cm−1 are assigned to the wagging vibrations of the NH2 groups. The bands in the 1500–1700 cm−1 region are due to the NH2 bending vibrations (at 1522 cm− 1 and 1567 cm− 1) and the vibrational bending mode of H2O (at 1630 cm−1). The combination bands in the 2200–3600 cm−1 region are characterized by the O–H and NH2 stretching vibrations. The peak at 2143 cm−1 is assigned to uncompensated interference from carbon dioxide in the atmosphere of the sample compartment. The extent of uranium precipitation from the loaded organic phase was found to increase as the N2H4/U molar ratio increased in the range of 1–3 (Table 3). The hydrofluoric acid concentration in the stripping solution should be no less than 4 mol/L to achieve uranium precipitation (Table 4). Uranium is not precipitated at N2H4/U molar ratios greater than 5 but is almost completely extracted into the aqueous phase. The uranium is precipitated from these solutions during holding for several days. The X-ray phase analysis revealed that uranium precipitated as UF4·2.5H2O. The XRD pattern is shown in Fig. 1(2). The precipitation of uranium tetrafluoride is due to the recovery of U(VI) to U(IV) with hydrazine in the presence of hydrofluoric acid (Anan'ev et al., 2001). The infrared spectrum of the obtained uranium tetrafluoride is shown in Fig. 2(2). The band at 459 cm−1 is associated with the asymmetric U–F stretching vibrations. The sharp peak at 956 cm− 1 is due to the asymmetric vibrations of the U_O in the uranyl ion. The bands with maxima at 1087, 1118 and 1219 cm−1 are attributed to the NH2 twisting vibration. The weak band at 1420 cm−1 is assigned to the NH2 wagging vibrations. The bands at 1539 and 1587 cm−1 are due to the NH2 bending vibrations. The peak at 1647 cm−1 is assigned to the vibrational bending mode of H2O. The combination bands in the 2800–3600 cm−1 region are due to the O–H and NH2 stretching vibrations. The bands of NH2 and uranyl groups indicate that the uranium partially precipitated as a hydrazine uranyl fluoride complex. The uranium precipitate contains trace amounts of the hydrazine uranyl fluoride complex, as it was not found by X-ray diffraction analysis. It is known that temperature is important in uranium stripping (Morss et al., 2010). Therefore, uranium was stripped from the loaded Table 6 Elemental impurities forming volatile fluorides in uranium tetrafluoride, μg/g U. Element
UF4
ASTM C787-11
Element
UF4
ASTM C787-11
Element
UF4
ASTM C787-11
Sb As B Br Cl
0.2 0.1 0.1 1.6 1.3
1 3 1 5 100
Cr Mo Nb P Ru
3.5 0.3 0.1 4.7 0.1
10 2 1 50 1
Si Ta Ti W V
16 0.2 0.7 0.8 0.2
100 1 1 2 2
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Table 7 Elemental impurities forming nonvolatile fluorides in uranium tetrafluoride, μg/g U. Al Fe Na Ba Pb Sr Be
3 21 7 b2 0.1 b1 b1
Li Th Bi Mg Sn Cd Mn
b1 b3 b2 b4 b10 b2 b1
Zn Ca Ni Zr K Cu Ag
b2 6 3 0.7 5 0.1 b1
organic phase at different temperatures (20–60 °C) with the other parameters held constant. Tests were performed using a solution that contained 3 mol N2H4/mol U and 6 mol/L HF. The phase contact time was 0.15 h after stirring was stopped. According to the results, the amount of uranium stripped from the loaded organic phase increases slightly with temperature in the range of 20 to 60 °C. Thus, the contents of hydrofluoric acid and hydrazine in the stripping solution are the major parameters that influence the extent of uranium precipitation. A study (Smirnov et al., 2013) showed that increasing the contact time of the phases increases the degree of uranium precipitation from the organic phase. Therefore, the effect of the phase contact time after stirring (i.e., the holding time) on uranium stripping from TBP was studied. The stripping studies were performed at temperatures from 20–25 °C using hydrofluoric acid solutions (6 mol/L) with hydrazine hydrate added (3 mol N2H4/mol U). Samples of the organic and aqueous phases were drawn hourly, and their uranium content was analyzed. The results indicate that the amount of uranium stripped from the loaded organic phase increases as the holding time is increased (Table 5). In addition, some part of the uranium is precipitated in the form of UF4·2.5H2O by the recovery of U(VI) to U(IV) with hydrazine in the presence of hydrofluoric acid (Anan'ev et al., 2001). The hydrazine uranyl fluoride complex obtained by stripping was calcined at a temperature of 450–600 °C in a vacuum and in a hydrogen stream. According to the results of the X-ray phase analysis, the final product of thermal decomposition in a vacuum is a mixture of UF4 and UO2 (Fig. 1(3)), at 55–60% and 40–45%, respectively. The X-ray diffraction analysis revealed that the final product of thermal decomposition of the hydrazine uranyl fluoride complex in a hydrogen stream is UF4. The XRD pattern is shown in Fig. 1(4). The uranium content in the uranium tetrafluoride is 76%. The infrared spectrum is shown in Fig. 2(3). This spectrum contains several absorption bands in the range of 400–4000 cm−1. Two broad bands with maxima at 3530 cm− 1 and 3191 cm− 1 are due to stretching vibrations of O–H. The sharp peak at 1613 cm−1 is associated with the vibrational bending mode of H2O. The absorption bands at 1011 cm−1 and 939 cm−1 are related to the symmetric and asymmetric vibrations of the uranyl group. All these bands are due to the presence of a UO2F2 ·nH2O impurity in
Fig. 3. Particle size distribution of uranium tetrafluoride.
the sample (Nyquist and Kagel, 1971). The content of UO2F2 ·nH2O in the samples does not exceed 2–3%. The obtained uranium tetrafluoride is a high-purity product and can be used to produce UF6 corresponding to ASTM C787-11 (ASTM, 2011). According to ASTM C787-11, only impurities forming volatile fluorides are separately normalized (Table 6). Impurities forming non-volatile fluorides are normalized by the total number, which should not exceed 300 μg/g U. The uranium tetrafluoride content of these impurities is not more than 100 μg/g U (Table 7). The size distribution of the particles of uranium tetrafluoride is shown in Fig. 3. The mean particle diameter of uranium tetrafluoride is 20–25 μm. The tap density is 2.7 g/cm3. Analysis of the data suggests that the uranium tetrafluoride meets all requirements imposed on this product at conversion plants. 4. Conclusions The results of this work clearly indicate that uranium can be effectively stripped from a loaded organic phase using hydrofluoric acid solution with the addition of hydrazine hydrate. The use of a hydrofluoric acid solution with the addition of hydrazine hydrate for stripping results in the precipitation of uranium in the form of a hydrazine uranyl fluoride complex. The HF concentration in the stripping solution and the N2H4/U molar ratio should be 6 mol/L and 3, respectively, to achieve efficient uranium precipitation from a loaded organic phase (approximately 94%). The hydrazine uranyl fluoride complex obtained in this work was converted to UF4 by thermal decomposition in a hydrogen stream. The uranium tetrafluoride obtained under these conditions is a highly pure product with a uranium content of 76% and a mean particle diameter of 20–25 μm. According to studies, this UF4 can be used to produce UF6 that corresponds to ASTM C787-11. Thus, the stripping method provides a reduction of uranium processing operations to produce high-purity uranium tetrafluoride. Acknowledgments This research project has been supported by UrFU under the Framework Programme of the development of UrFU through the “Young Scientists UrFU” competition. References Anan'ev, A., Shilov, V., Afonas'eva, T., Mikhailova, N., Milovanov, A., 2001. Catalytic reduction of U(VI) with hydrazine and formic acid in HNO3 solutions. Radiochemistry 43 (1), 39–43. ASTM, 2011. Standard Specification for Uranium Hexafluoride for Enrichment (ASTM Standard C787-11). ASTM International, West Conshohocken, PA, USA. Athavale, V., Iyer, P., 1967. Studies on some metal-hydrazine complexes. J. Inorg. Nucl. Chem. 29 (4), 1003–1012. Batsanov, S., Gagarinskii, Y.V., 1964. An optical study of uranium tetrafluoride crystal hydrates. J. Struct. Chem. 4 (3), 356–361. Benedict, M., Pigford, T.H., Levi, H.W., 1981. Nuclear Chemical Engineering. McGraw-Hill, New York. Chegrouche, S., Kebir, A., 1992. Study of ammonium uranyl carbonate re-extraction– crystallization process by ammonium carbonate. Hydrometallurgy 28 (2), 135–147. Durig, J., Bush, S., Mercer, E., 1966. Vibrational spectrum of hydrazine-d4 and a Raman study of hydrogen bonding in hydrazine. J. Chem. Phys. 44 (11), 4238–4247. Earnshaw, A., Larkworthy, L., Patel, K., 1964. Magnetic and other properties of chromium(II) hydrazine halides. Z. Anorg. Allg. Chem. 334 (3–4), 163–168. Edwards, C., Oliver, A., 2000. Uranium processing: a review of current methods and technology. JOM 52 (9), 12–20. Ellis, D.A., Grinstead, R.R., Long, R.S., Magner, J.E., 1962. Preparation of high purity UF4, US Patent, US3030175. Glavič, P., Slivnik, J., 1970. Studies on the uranium tetrafluoride-hydrazinium (1 +) fluoride and the uranium tetrafluoride-hydrazine systems. J. Inorg. Nucl. Chem. 32 (9), 2939–2949. Gupta, C., Singh, H., 2003. Uranium Resource Processing: Secondary Resources. Springer, Netherlands. ICDD PDF-2, 2009. Powder Diffraction Database. International Centre for Diffraction Data, Philadelphia, USA. Mishra, B., Gubel, N.R., Bhola, R., 2013. Uranium Processing, Uranium Processing and Properties. Springer, pp. 123–172. Morss, L.R., Edelstein, N.M., Fuger, J., 2010. The Chemistry of the Actinide and Transactinide Elements. Springer, Netherlands.
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