AN INVESTIGATION ON THE SYNTHESIS OF THE URANIUM MINERAL BRANNERITE

Transcription

1 AN INVESIGAION ON HE SYNHESIS OF HE RANIM MINERAL BRANNERIE Mohamad Hassan Amin, James ardio, Fiona Charalambous and Suresh K. Bhargava* Advanced Materials & Industrial Chemistry Group, School of Applied Sciences, RMI niversity, Melbourne, VIC-3001, Australia * suresh.bhargava@rmit.edu.au ABSRAC In this study, the synthesis of brannerite was investigated using a novel method and was compared with the results of previous methods. his involved studies on the influence of starting materials, i/ ratio, temperature and time of synthesis. Improved knowledge on the effects these parameters have on the synthesis of brannerite is very important for the development of methods for extracting uranium from this mineral which may become a significant source of uranium in future years. Synthesised brannerite was characterised using BE N 2 adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and diffuse reflectance Fourier ransformed Infra-red spectra. INRODCION he ever-increasing worldwide use of electricity has led to a greater need to find more efficient and environmentally friendly avenues to decrease the dependency on carbonbased energy sources. Nuclear technology has therefore rapidly come into the forefront as an economic and environmental solution. he most widely processed uranium mineral is uraninite due to its relatively high concentration and ease of extraction. However, due to the increased mining and eventual exhaustion of high grade uranium sources, there is a greater interest on extracting uranium from more refractory minerals. Brannerite, a uranium titanate mineral (i 2 O 6 ), is one such alternative which exists in a number of uranium ore deposits. Brannerite has also attracted attention as a minor phase in ceramic formulations designed for immobilization of high-level nuclear waste such as surplus uranium and plutonium (Zhang et al. 2003, Finnie et al. 2003). Despite the importance of brannerite both as an increased source of fissionable uranium and as an actinide-host in titanate waste forms, the properties of this mineral are not well understood. his is because high purity brannerite is extremely difficult to synthesize and pure samples of natural brannerite are hard to obtain, as it is usually present in ores in low concentrations as finely disseminated grains. During the synthesis of brannerite, the uranium is almost always partially oxidised and so the product may contain (IV), (V), (VI) as well as io 2 (Helean et al. 2003). ranium has been found to exist as (IV) only in pure i 2 O 6 (Vance et al. 2001). Due to the multivalent character of uranium, which can exist in these oxidation states along with the rather unstable (III) state, it is difficult to fix the valence state of uranium in brannerite (Bera et al. 1998). As aliovalent uranium is possible in synthetic brannerite, a pure i 2 O 6 can only be synthesised by the control of oxygen partial pressure, temperature, time and i/ ratio. Previous methods have been investigated to synthesize pure brannerite. Zhang et al. (2003) utilised the alkoxide/nitrate route which involves drying stoichiometric mixtures of and i compounds and calcining in argon at 750 C for 1 h. he calcines were then wet-milled for 2h and then dried, before being hot-pressed at 1260 C for 2h under 21MPa in graphite dies. However the brannerite product contained minor rutile

2 inclusions (approx 5% io 2 and trace amounts of reduced i oxide) and O 2 (<0.1%) (Zhang et al. 2003). Donaldson et al. (2005) heated a powdered mixture of high purity O 2 and 99.9% pure io 2 (anatase) for 300 h in a gas mixture of 5%: 95% CO: CO 2 at 1623K. he actual composition of uranium-containing compound was found to be 0.95 i 2.05 O 6 with 0.31mass% contamination of O 2 (Donaldson et al. 2005). Both methods require extensive procedures and use of high temperatures and pressures. hey also had minor impurities as byproducts. It is therefore of great interest to find an easier and faster alternative to synthesize pure brannerite. In this study, the synthesis of brannerite was investigated using a novel method and was compared with the results of previous methods. he influence of starting materials, i/ ratio, temperature and time of synthesis were studied. Improved knowledge on the effects of these parameters on the synthesis of brannerite is very important for the development of methods for producing brannerite which can be used for studying uranium extraction and actinide containment. EXPERIMENAL ranyl nitrate hexahydrate (O 2 (NO 3 ) 2.6H 2 O), titanium oxide (io 2 ), ranyl acetate (O 2 (CH 3 COO) 2 2H 2 O) (BDH, 97.5%), titanyl sulphate dihydrate (ioso 4.2H 2 O) (BHPB, 97%), were used as received. MilliQ water was used for material synthesis. Brannerite (i 2 O 6 ) was prepared using a dry method, which hereafter is referred to as method. ranyl nitrate hexahydrate was mixed with titanium oxide using varying i/ ratios. he mixtures obtained were then calcined in Ar/H 2 (95% Ar/ 5% H 2 ) at different temperatures and times. BE surface areas of products were measured by N 2 adsorption desorption at 77K using a Micromeritics ASAP 2010 instrument. Before measurements, samples were degassed under vacuum at room temperature for 30 mins with slow degassing and then were followed with fast mode at 280 C overnight. Powder XRD patterns of products were obtained with a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ= 1.542Å ) operated at 40 kv and 35mA. X-ray photoelectron spectroscopy (XPS) (hermo Scientific K-Alpha XPS spectrometer) was used to investigate the oxidation states of the elements in the products. All binding energies were referenced to the C 1s line at 285.0eV. ransmission Electron Microscopy (EM) (JEOL 2010 EM) was used to identify the phase features of products. F Infrared spectra were recorded with a Fourier transform infrared (FIR) spectrometer (Spectrum 2000 Perkin Elmer) with a diffuse reflectance attachment on flat surfaces of finely powdered samples. RESLS AND DISCSSION Effect of i/ ratio he effect of i/ ratio was investigated using the following i/ ratios: 2, 3, 4, 5, 5.5, 6 and 6.5, using a calcination temperature of 1150 C for a period of 30h. X-ray diffraction spectra for the obtained products are shown in Fig.1. All X-ray patterns consisted of diffractions lines of brannerite (i 2 O 6 ), titanium oxide (io 2 ) and uranium oxide (O 2 ) phases and were clarified by referring to PDF cards , and respectively. With the increase of i/ ratio, the Bragg peaks of rutile and uranium oxide reduced gradually while the formation of brannerite increased with increasing i/ ratio up to i/=6 and then decreased, as shown in Fig.1. Among all the i/ ratios tested, the sample with a i/ ratio of 6 showed the best result (in 2

3 terms of purity), but still contained some unreacted uranium oxide and io 2 phases. hus, the effect of temperature was investigated. Effect of calcination temperature he effect of calcination temperature was investigated for a mixture containing a i/ ratio of 6 at 1050, 1100, 1150 and 1200 C for 30h. X-ray diffraction spectra for calcined products are shown in Fig.2. All X-ray patterns consisted of diffraction lines of brannerite (i 2 O 6 ), titanium oxide (io 2 ) and uranium oxide (O 2 ). With the increase in temperature, the Bragg peaks of rutile and uranium oxide reduced gradually while the formation of brannerite increased with increasing temperature. Among all the temperature tested, the sample calcined at 1200 C showed the best result, but a small trace of uranium oxide and io 2 phases were still detectable in the XRD pattern. Effect of calcination time he effect of calcination time, ranging from 10 to 30h, was determined by calcining a sample containing a i/ ratio of 6. It was found that the Bragg peaks for uranium oxide and io 2 phases were only present in the sample heated at 1200 C for 20h in very small quantities as shown in Fig.3. his result indicates that the formation of brannerite is most likely complete in 20h at 1200 C and that the prolonging of calcination time may have led to the appearance of diffraction lines of titanium oxide (io 2 ) and uranium oxide (O 2 ) due to breakdown of brannerite. he valence state(s) of uranium in the surface of the brannerites was studied using X-ray photoelectron spectroscopy (XPS). Although XPS is a surface technique with a probing depth of approximately 5 nm, this technique has been used to determine the surface oxidation state of in uranium compounds and has the ability to discriminate between different oxidation states and chemical environments (Charalambous et al. 2010). In the current study, valuable information can be obtained as we are only comparing the surface composition of the two products. Fig.4 shows the survey and 4f XPS spectra of the synthesised brannerite (i/=6, 1200 C, 20h). he 4f XPS of the sample shows the main peaks at 380.1eV for the 4f 7/2 and 391.1eV for the 4f 5/2 suggesting the presence of single valance state corresponding to (IV) of according to the literature (Allen et al and Chadwick 1973). Additionally, the XPS spectrum of 4f 5/2 shows a satellite peak at 397.1eV (the 4f 5/2 satellite peaks are marked as * in Fig.4). Satellites appear mostly due to the transition from -O bonding band to the empty states above the Fermi level. Satellite for (IV) standard matches well with the literature value. o obtain further valence state information, diffuse reflectance infrared Fourier transform spectroscopy (DRIFS) was conducted on the synthesised brannerite (i/=6, 1200 C, 20h). he DRIFS spectrum of the synthesised brannerite is shown in Fig.5. sample shows no significant spectral features over this wavelength range, although a weak band was present at 1670nm, can be assigned to (IV) (Vance et al. 2001). No absorption peaks can be observed by DRIFS in i 2 O 6 pure brannerite. In i 2 O 6 pure brannerite, the absence of observable absorption peaks was previously interpreted (Vance et al. 2001) as showing that there are no isolated 4+ ions, but a more likely possibility is that because the (IV) site has a centre of inversion (2/m point symmetry), electric dipole transitions are forbidden, although weak vibronic absorptions might be expected (Vance et al. 2001). EM micrograph of the synthesised brannerite was obtained to identify and distribution of phases. From the EM micrograph of the synthesised brannerite specimen 3

4 (i/=6, 1200 C, 20h) shown in Fig.6 it was found that there is almost only one homogeneous phase distributed throughout the sample. Fig.6 (b) shows the typical elemental analysis pattern of this phase. his result confirmed the XRD pattern obtained for this sample that showed only traces of non-brannerite phases. he BE of synthesised brannerite (i/=6, 1200 C, 20h) was finally investigated and a summary of characteristics such as BE surface area, pore volume and BJH pore diameter are shown in able1. CONCLSIONS he purity of the brannerite produced using the new method used in this study was influenced by the starting i/ ratio, reaction temperature and reaction time. he highest purity brannerite (based on XRD analysis) was obtained using the following conditions: i/=6, = 200 C, t=20 h. XPS analysis revealed that the brannerite produced using the aforementioned conditions contained uranium in the (IV) oxidation state. EM-EDX analysis of the prepared brannerite showed it to be highly homogenous, whilst BE analysis showed it had a very low surface area. ACKNOWLEDGEMENS he authors would like to thank the Australian Research Council and BHP-Billiton for supporting this project through the Australian Research Council Linkage Projects scheme. able 1: Surface areas, pore volume and BJH pore of synthesised brannerite. S BE (m 2 /g) V (cm 3.g -1 ) BJH (nm) S BE : BE surface area; V: pore volume calculated by the N 2 amount at the highest P/P o (~0.99); D BJH : pore diameter, calculated by BJH method 4

5 =Brannerite =io 2 =ranium oxide i/=6.5 Intensity (cps) i/=6 i/=5.5 i/=5 i/=4 i/=3 i/= theta (degrees) Fig. 1. XRD patterns of calcined mixture of uranyl nitrate and titanium oxide with various i/ ratios at 1150 C for 30h. =Brannerite =io 2 =ranium oxide 1200 C Intensity (cps) 1150 C 1100 C 1050 C theta (degrees) Fig. 2. XRD patterns of calcined mixture of uranyl nitrate and titanium oxide with i/ ratio of 6 at various temperatures for 30h. 5

6 =Brannerite =ranium oxide =io 2 30h Intensity (cps) 25h 20h 15h 10h theta (degrees) Fig. 3. Effect of calcination time on synthesis of brannerite at 1200 C. Counts/s (a) O 1s i 2p3 4d5 i 2s 4f7 5s 5d5 i3s 5p3 6s Counts/s (b) * 4f 7/2 4f 5/ Binding Energy (ev) Binding Energy (ev) Fig.4. Survey (a) and 4f (b) X-ray photoelectron spectrum of synthesised brannerite (i/ = 6, 1200 C, 20 h), with 4f 5/2 satellite marked as *. 6

7 Absorbance Wavelength (nm) Fig. 5. Absorbance versus wavelength for diffuse reflectance spectrum of the synthesised brannerite (i/=6, 1200 C, 20h). a 2000 b i In tensity i 1µm Energy (kev) Fig.6. (a) EM micrograph of synthesized brannerite and (b) the X-ray Spectroscopy of the homogeneous phase in the sample (i/=6, 1200 C, 20h). REFERENCES Allen, G.C., Crofts, J.A., et al X-Ray Photoelectron Spectroscopy of Some ranium Oxide Phases. Published on 01 January 1974 on doi: /d Bera, S., Sali, S.K., Sampath, S., Narasimhan, S.V., Venugopal, V., Oxidation state of uranium: an XPS study of alkali and alkaline earth ranates. Journal of Nuclear Materials, 255, pp Charalambous, F.A., Ram, R., ardio, J., Bhargava, S. K., Characterisation and Dissolution Studies of Varying Forms of Brannerite, ranium rd International Conference on ranium and 40th Annual Hydrometallurgy Meeting, 15 Aug Aug 2010, Saskatoon, Saskatchewan, CANADA. Chadwick, D., ranium 4f Binding energies studied by X-ray photoelectron spectroscopy. Chemical physics letters, 21(2), pp

8 Donaldson, M. H., Stevens, R., Lang, B.E., Boerio-Goates, J., Woodfield, B.F., Putnam, R. L., Navrotsky, A., Heat capacities and absolute enropies of i 2 O 6 and Cei 2 O 6. Journal of hermal Analysis and Calorimetry, 81, pp Finnie, K., Zhang, Z., Vance, E., Carter, M., Examination of valence states in the brannerite structure by near-infrared diffuse reflectance and X-ray photoelectron spectroscopies. Journal of Nuclear Materials, 317, pp Helean, K.B., Navrotsky, A., Lumpkin, G. R., Colella, M., Lian, J., Ewing, R.C., Ebbinghaus, B., Catalano, G Enthalpies of formation of -, h-, Ce-brannerite: implications for plutonium immobilization; J. Nucl. Mater, 320, pp Vance, E.R., Watson, J.N., Carter, M.L., Day, R.A., Begg. B.D., Crystal chemistry and stabilization in air of brannerite, I 2 O 6. J. Am. Chem. Soc., 84(1), pp Zhang, Y., homas, B.S., Lumpkin, G.R., Blackford, M., Zhang, Z., Colella, M., Aly. Z., Dissolution of synthetic brannerite in acidic and alkaline fluids. J. Nucl. Mater. 321, pp.1-7. BRIEF BIOGRAPHY OF PRESENER Dr Amin is currently working in the Industrial Chemistry & Advanced Materials Group in the school of Applied Science, RMI niversity, Melbourne, Australia. Previously he worked as a A/Professor in Azad niversity of Yazd and Materials and Energy Research Centre in ehran ( ). He was an A/Professor visiting fellow at the niversity of New South Wales, Australia ( ). Dr Amin was awarded his Ph.D. Degree in Material Engineering (Ceramics) from the Material and Energy Research Centre (Ministry of Science, Research and echnology, ehran) in