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Synchrotron X-ray microprobe analysis of radioactive trace elements in mineral sands
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 897–900
Synchrotron X-ray microprobe analysis of radioactive trace elements in mineral sands R.F. Garretta,*, N. Blagojevica, Z. Caib, B. Laib, D.G. Legninib, W. Rodriguesb, A.P.J. Stampfla a
Australian Nuclear Science and Technology Organization, Private Mail Bag1, Menai, NSW 2234, Australia b Synchrotron Radiation Instrumentation CAT, Argonne National Laboratory, Argonne, IL 60439, USA
Abstract Elemental distribution maps and XANES spectra of radioactive trace elements in zircon and ilmenite mineral grains were measured using a synchrotron X-ray microprobe. The results confirm the utility of the technique for the study of trace elements in minerals, provide chemical information important for designing processes for their removal from ores, and are direct confirmation for a previously inferred model of thorium incorporation into ilmenite during weathering. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Y; 78.70.E Keywords: X-ray microprobe; Mineral trace element analysis; XANES
1. Introduction Mineral sands concentrates such as zircon and ilmenite contain low levels of U, Th and their daughter products. In recent years there has been increased pressure to lower radioactivity levels in concentrates, as radioactivity causes considerable problems in the sale of these commodities as well as in disposal of radioactive waste produced in mineral processing [1]. The long term goal of the study of these minerals is to develop a cost effective process for removal of radionuclides from minerals. A major activity within this program is *Corresponding author. Tel.: +61-2-9717-3657; fax: +61-29717-9265. E-mail address: [email protected] (R.F. Garrett).
the identification, location and speciation of the radioactivity in ores, minerals or processed materials. A much studied model mineral (zircon) and a less well understood mineral (ilmenite) were chosen to evaluate the capabilities of the synchrotron X-ray microprobe for this work. The presence of low levels of radioactivity in ores and concentrates is comparable to the presence of other undesirable impurities, the main difference being the very low mass concentration of most radionuclides, with the result that it is difficult to determine their location in mineral matrices. It is also difficult to predict their response to specific chemical treatments. The distribution and chemical speciation of these trace elements must therefore be studied in order to understand the geological processes resulting in the presence of
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 5 1 4 - 9
898
R.F. Garrett et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 897–900
the radioactive elements and the processes required for the removal of these elements. The synchrotron X-ray microprobe is uniquely capable of determining the concentration, distribution and chemical state of these trace elements. Other micro-probe techniques (electron or proton) do not have the combination of sensitivity and spatial resolution required, and do not provide chemical information. For example, the bulk concentration of thorium in Australian ilmenite is of the order of 100 ppm, while U is much lower, in the 10–30 ppm range or less [2]. Electron and proton microprobe EDS is not sensitive enough to map elements at these concentration levels: minimum detection limits for heavy elements can be as low as 20–30 ppm but the required count times are in the order of minutes per analysis point [3].
2. Experimental A number of single grain samples of zircon and ilmenite were measured at the 2-ID-D microprobe beamline [4] at SRI-CAT at the Advanced Photon Source, using a 1 mm monochromatic beam at 17.2 keV. This energy was chosen to excite the U LIII edge, but avoid exciting the Zr K edge in the zircon samples. The principal beamline components of the Xray microprobe include an undulator, a mirror, a double-crystal Si(1 1 1) monochromator, and a Fresnel zone plate to focus the X-ray beam on the sample. The zone plate was located 72 m from the undulator source, while the sample was placed about 21 cm from the zone plate, resulting in a beam spot of 2 0.2 mm at 17 keV. At the Fe K edge the focus was 1 0.14 mm. An energydispersive Ge-detector was used to measure fluorescence from the sample, which was scanned across the focused X-ray spot and spatial maps of different elements were acquired simultaneously. Samples of West Australian ilmenite and zircon, sourced approximately 300 km north of Perth, were sectioned and polished so that the sample thickness was less than 10 mm. The sections were mounted on 100 mm thick polycarbonate sheet to allow both visible light and X-ray transmission. All samples were inspected by SEM using back-
scatter imaging to locate appropriate areas of interest.
3. Results and discussion Fig. 1 shows U, Th and Pb maps from a representative zircon sample, and shows that these elements are concentrated into zoned regions, overlaying the dark bands in the SEM image. U and Th are known to mainly substitute for Zr, to form actinide orthosilicates. The presence of lead can be attributed to the natural decay of U and Th, as well as common lead incorporated during the crystallisation process. In general then the Xray microprobe results reflect the known mineralogy of this model system. The fluorescence yield from uranium thorium and lead in the zircons was calibrated by measuring a geochronological zircon standard (SL13 [5]) of the same sample thickness, using the identical microprobe geometry. The concentration range of U in Fig. 1 is between 13 and 33 ppm and Th 3 and 11 ppm. The lead concentration was much lower varying between 3 and 4 ppm. U LIII edge XANES spectra were acquired in regions of high uranium concentration. A representative spectrum is shown in Fig. 2 which
Fig. 1. Th, U and Pb elemental maps of a zircon grain, taken in the region indicated on the SEM image at top left. The images follow the zoned regions as shown by SEM backscatter image. Acquisition time: 3 s/point.
R.F. Garrett et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 897–900
Fig. 2. Uranium LIII edge XANES of a zircon mineral grain and standards. Results indicate that U in zircon is present primarily as U(VI).
indicates that uranium was predominantly present as U(IV), which has important implications for possible processing techniques: U(VI) is more amenable to removal by leaching. Fig. 3 shows synchrotron microprobe elemental maps from a weathered ilmenite grain. Current theories to account for the elevated concentration of Th in these materials include accumulation during weathering; the mechanism of such accumulation has been described but is not well understood [6]. Levins et al. [2] infer from SEM images, bulk chemistry and alpha track imaging that the thorium content of ilmenite increases with the extent of weathering of the ore and should therefore be concentrated in weathered sections of a mineral grain. Weathered parts of ilmenite are converted to leucoxene, with a reduced iron concentration, and appear dark in backscattered SEM images. The increase in thorium concentration should then be associated with Fe(III), which is formed in the weathering process and is known to be an excellent scavenger of foreign ions and colloids. The synchrotron microprobe images in Fig. 3 supply direct evidence that thorium is associated with weathered areas. Weathered regions can be seen extending several microns from cracks in the ilmenite grain. These regions have much reduced Fe concentration, and elevated Th (and Pb and Y, not shown), which supports the weathering model. The Ti distribution is almost unaffected by weathering, which is expected from
899
Fig. 3. Ti, Fe and Th elemental maps of a weathered ilmenite grain, taken in the region indicated on the SEM image at top left. It is evident that removal of Fe by the leaching process produces an increase in Th. The titanium concentration map shows uniform distribution with the exception of areas where physical cracks are confirmed by the SEM image. Acquisition time: 2 s/point.
Fig. 4. Fe K edge XANES spectrum of unweathered and weathered ilmenite compared to standards of Fe(II) and Fe(III).
solubility levels. Determinations of Fe speciation from XANES are difficult from the edge position and post edge structure, and have relied on the position of the pre-edge feature in the spectrum [7]. Fe K edge XANES spectra are shown in Fig. 4. The pre-edge peak is not evident in most of the spectra taken, and so we have not been able to quantify the Fe speciation. Nevertheless the edge position indicates that Fe in the weathered ilmenite has predominantly Fe(III) character thus
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R.F. Garrett et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 897–900
providing direct proof that the adsorption mechanism is due to a ferric oxide present in weathered zones. Droubay et al. have also measured changes in Fe speciation in different regions of an ilmenite grain, using soft X-ray absorption spectroscopy [8], but ascribed this to hematite inclusions: no cracked or weathered regions were evident in their sample.
by the Commonwealth of Australia under the Major National Research Facilities Program. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. W-31-109-Eng-38.
4. Conclusion References The results for the zircon grains studied confirm that the synchrotron X-ray microprobe has sufficient sensitivity and resolution to map the distribution and speciation of U, Th and their decay daughter products in mineral sand concentrates. The ilmenite measurements supply direct confirmation for the previously inferred weathering adsorption mechanism of Th into this system. Further work will focus on measuring the radioactive decay chain daughters of U and Th.
Acknowledgements This work was supported by the Australian Synchrotron Research Program, which is funded
[1] L. Bruzzi, M. Baroni, G. Mazzotti, R. Mele, S. Righi, J. Environ. Radioactivity 47 (2000) 171. [2] D.M. Levins, R.A. Day, P.J. McGlinn, R. Stanojevic, Proceedings of the International Conference On Radiation Protection and Radioactive Waste Management in the Mining and Minerals Processing Industries, Johannesburg, South Africa, 1995. [3] D.R. Cousens, R. Rasch, C.G. Ryan, Micron 28 (1997) 231. [4] W. Yun et al., Rev. Sci. Instrum. 70 (1999) 2238. [5] J.C. Roddick, O. van Breemen, Radiogenic Age and Isotopic Studies: Report 8; Geological Survey of Canada, Current Research (1994-F), 1-9. [6] L. Khor, T. Parks, F.J. Lincoln, J. Graham, Aust. J. Chem. 49 (1996) 847. [7] S. Bajt, S.R. Sutton, J.S. Delaney, Geochim. Cosmochim. Acta 58 (1994) 5209. [8] T. Droubay, G. Mursky, B. Tonner, J. Electron Spect. Rel. Phenom. 84 (1997) 159.
Synchrotron X-ray microprobe analysis of radioactive trace elements in mineral sands R.F. Garretta,*, N. Blagojevica, Z. Caib, B. Laib, D.G. Legninib, W. Rodriguesb, A.P.J. Stampfla a
Australian Nuclear Science and Technology Organization, Private Mail Bag1, Menai, NSW 2234, Australia b Synchrotron Radiation Instrumentation CAT, Argonne National Laboratory, Argonne, IL 60439, USA
Abstract Elemental distribution maps and XANES spectra of radioactive trace elements in zircon and ilmenite mineral grains were measured using a synchrotron X-ray microprobe. The results confirm the utility of the technique for the study of trace elements in minerals, provide chemical information important for designing processes for their removal from ores, and are direct confirmation for a previously inferred model of thorium incorporation into ilmenite during weathering. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Y; 78.70.E Keywords: X-ray microprobe; Mineral trace element analysis; XANES
1. Introduction Mineral sands concentrates such as zircon and ilmenite contain low levels of U, Th and their daughter products. In recent years there has been increased pressure to lower radioactivity levels in concentrates, as radioactivity causes considerable problems in the sale of these commodities as well as in disposal of radioactive waste produced in mineral processing [1]. The long term goal of the study of these minerals is to develop a cost effective process for removal of radionuclides from minerals. A major activity within this program is *Corresponding author. Tel.: +61-2-9717-3657; fax: +61-29717-9265. E-mail address: [email protected] (R.F. Garrett).
the identification, location and speciation of the radioactivity in ores, minerals or processed materials. A much studied model mineral (zircon) and a less well understood mineral (ilmenite) were chosen to evaluate the capabilities of the synchrotron X-ray microprobe for this work. The presence of low levels of radioactivity in ores and concentrates is comparable to the presence of other undesirable impurities, the main difference being the very low mass concentration of most radionuclides, with the result that it is difficult to determine their location in mineral matrices. It is also difficult to predict their response to specific chemical treatments. The distribution and chemical speciation of these trace elements must therefore be studied in order to understand the geological processes resulting in the presence of
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 5 1 4 - 9
898
R.F. Garrett et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 897–900
the radioactive elements and the processes required for the removal of these elements. The synchrotron X-ray microprobe is uniquely capable of determining the concentration, distribution and chemical state of these trace elements. Other micro-probe techniques (electron or proton) do not have the combination of sensitivity and spatial resolution required, and do not provide chemical information. For example, the bulk concentration of thorium in Australian ilmenite is of the order of 100 ppm, while U is much lower, in the 10–30 ppm range or less [2]. Electron and proton microprobe EDS is not sensitive enough to map elements at these concentration levels: minimum detection limits for heavy elements can be as low as 20–30 ppm but the required count times are in the order of minutes per analysis point [3].
2. Experimental A number of single grain samples of zircon and ilmenite were measured at the 2-ID-D microprobe beamline [4] at SRI-CAT at the Advanced Photon Source, using a 1 mm monochromatic beam at 17.2 keV. This energy was chosen to excite the U LIII edge, but avoid exciting the Zr K edge in the zircon samples. The principal beamline components of the Xray microprobe include an undulator, a mirror, a double-crystal Si(1 1 1) monochromator, and a Fresnel zone plate to focus the X-ray beam on the sample. The zone plate was located 72 m from the undulator source, while the sample was placed about 21 cm from the zone plate, resulting in a beam spot of 2 0.2 mm at 17 keV. At the Fe K edge the focus was 1 0.14 mm. An energydispersive Ge-detector was used to measure fluorescence from the sample, which was scanned across the focused X-ray spot and spatial maps of different elements were acquired simultaneously. Samples of West Australian ilmenite and zircon, sourced approximately 300 km north of Perth, were sectioned and polished so that the sample thickness was less than 10 mm. The sections were mounted on 100 mm thick polycarbonate sheet to allow both visible light and X-ray transmission. All samples were inspected by SEM using back-
scatter imaging to locate appropriate areas of interest.
3. Results and discussion Fig. 1 shows U, Th and Pb maps from a representative zircon sample, and shows that these elements are concentrated into zoned regions, overlaying the dark bands in the SEM image. U and Th are known to mainly substitute for Zr, to form actinide orthosilicates. The presence of lead can be attributed to the natural decay of U and Th, as well as common lead incorporated during the crystallisation process. In general then the Xray microprobe results reflect the known mineralogy of this model system. The fluorescence yield from uranium thorium and lead in the zircons was calibrated by measuring a geochronological zircon standard (SL13 [5]) of the same sample thickness, using the identical microprobe geometry. The concentration range of U in Fig. 1 is between 13 and 33 ppm and Th 3 and 11 ppm. The lead concentration was much lower varying between 3 and 4 ppm. U LIII edge XANES spectra were acquired in regions of high uranium concentration. A representative spectrum is shown in Fig. 2 which
Fig. 1. Th, U and Pb elemental maps of a zircon grain, taken in the region indicated on the SEM image at top left. The images follow the zoned regions as shown by SEM backscatter image. Acquisition time: 3 s/point.
R.F. Garrett et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 897–900
Fig. 2. Uranium LIII edge XANES of a zircon mineral grain and standards. Results indicate that U in zircon is present primarily as U(VI).
indicates that uranium was predominantly present as U(IV), which has important implications for possible processing techniques: U(VI) is more amenable to removal by leaching. Fig. 3 shows synchrotron microprobe elemental maps from a weathered ilmenite grain. Current theories to account for the elevated concentration of Th in these materials include accumulation during weathering; the mechanism of such accumulation has been described but is not well understood [6]. Levins et al. [2] infer from SEM images, bulk chemistry and alpha track imaging that the thorium content of ilmenite increases with the extent of weathering of the ore and should therefore be concentrated in weathered sections of a mineral grain. Weathered parts of ilmenite are converted to leucoxene, with a reduced iron concentration, and appear dark in backscattered SEM images. The increase in thorium concentration should then be associated with Fe(III), which is formed in the weathering process and is known to be an excellent scavenger of foreign ions and colloids. The synchrotron microprobe images in Fig. 3 supply direct evidence that thorium is associated with weathered areas. Weathered regions can be seen extending several microns from cracks in the ilmenite grain. These regions have much reduced Fe concentration, and elevated Th (and Pb and Y, not shown), which supports the weathering model. The Ti distribution is almost unaffected by weathering, which is expected from
899
Fig. 3. Ti, Fe and Th elemental maps of a weathered ilmenite grain, taken in the region indicated on the SEM image at top left. It is evident that removal of Fe by the leaching process produces an increase in Th. The titanium concentration map shows uniform distribution with the exception of areas where physical cracks are confirmed by the SEM image. Acquisition time: 2 s/point.
Fig. 4. Fe K edge XANES spectrum of unweathered and weathered ilmenite compared to standards of Fe(II) and Fe(III).
solubility levels. Determinations of Fe speciation from XANES are difficult from the edge position and post edge structure, and have relied on the position of the pre-edge feature in the spectrum [7]. Fe K edge XANES spectra are shown in Fig. 4. The pre-edge peak is not evident in most of the spectra taken, and so we have not been able to quantify the Fe speciation. Nevertheless the edge position indicates that Fe in the weathered ilmenite has predominantly Fe(III) character thus
900
R.F. Garrett et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 897–900
providing direct proof that the adsorption mechanism is due to a ferric oxide present in weathered zones. Droubay et al. have also measured changes in Fe speciation in different regions of an ilmenite grain, using soft X-ray absorption spectroscopy [8], but ascribed this to hematite inclusions: no cracked or weathered regions were evident in their sample.
by the Commonwealth of Australia under the Major National Research Facilities Program. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. W-31-109-Eng-38.
4. Conclusion References The results for the zircon grains studied confirm that the synchrotron X-ray microprobe has sufficient sensitivity and resolution to map the distribution and speciation of U, Th and their decay daughter products in mineral sand concentrates. The ilmenite measurements supply direct confirmation for the previously inferred weathering adsorption mechanism of Th into this system. Further work will focus on measuring the radioactive decay chain daughters of U and Th.
Acknowledgements This work was supported by the Australian Synchrotron Research Program, which is funded
[1] L. Bruzzi, M. Baroni, G. Mazzotti, R. Mele, S. Righi, J. Environ. Radioactivity 47 (2000) 171. [2] D.M. Levins, R.A. Day, P.J. McGlinn, R. Stanojevic, Proceedings of the International Conference On Radiation Protection and Radioactive Waste Management in the Mining and Minerals Processing Industries, Johannesburg, South Africa, 1995. [3] D.R. Cousens, R. Rasch, C.G. Ryan, Micron 28 (1997) 231. [4] W. Yun et al., Rev. Sci. Instrum. 70 (1999) 2238. [5] J.C. Roddick, O. van Breemen, Radiogenic Age and Isotopic Studies: Report 8; Geological Survey of Canada, Current Research (1994-F), 1-9. [6] L. Khor, T. Parks, F.J. Lincoln, J. Graham, Aust. J. Chem. 49 (1996) 847. [7] S. Bajt, S.R. Sutton, J.S. Delaney, Geochim. Cosmochim. Acta 58 (1994) 5209. [8] T. Droubay, G. Mursky, B. Tonner, J. Electron Spect. Rel. Phenom. 84 (1997) 159.