XANES and micro-XRF Investigations of Ilmenite and Derived Products

P. Kappen, J.V. Dubrawski1 and P.J. Pigram

Centre for Materials and Surface Science and Department of Physics, La Trobe University, Victoria 3086, Australia 1 Iluka Resources Ltd, Level 23 / 140 St Georges Terrace, Perth, WA 6000, Australia

In the mineral sands industry ilmenite (FeTiO3) is an important source mineral for the production of synthetic rutile, which is processed to become titania (TiO2) a white pigment used in industrial quantities. Natural ilmenite contains a range of impurities, including MnO and traces (up to few 100ppm) of radionuclides such as U and Th. Purification processes are employed to remove Fe and other impurities from the ilmenite, resulting in synthetic rutile. Treatment steps include FeTiO3 reduction in a kiln, Fe removal via aeration, and Fe and Mn extraction by acid leaching. In the final product, U and Th levels can remain undesirably high, leading to product quality issues. These problems arise because the distribution and chemistry of these elements (and their daughter nuclides) in ilmenite and its products are poorly understood. To date, only a few studies have been published on radionuclides in ilmenite [1-3] providing some information on structure and elemental composition. A recent µ-XRF/XANES analysis of Th in ilmenite has indicated that thorium is preferably located in weathered parts of the mineral [4].

We report on XRF, XANES and µ-XRF/XANES investigations of ilmenite from all four processing steps: feed ilmenite (FI), reduced ilmenite (RI), leach feed (LF), and synthetic rutile (SR). The experiments were conducted at beamlines C and L at HASYLAB, using Si(111) double crystal monochromators. Micro-XRF/XANES maps and scans (Th-LIII edge) were recorded at beamline L using a polycapillary (spot size ~20 µm) and a single-pixel Si-Drift-Detector (with collimator; 1 mm diameter). Large beam spot (few mm2) XRF and XANES data were collected at beamline C using a 7-element Si(Li) fluorescence detector. For µ-XRF/XANES, mineral grains were embedded in resin on a glass substrate and polished to 70-100 µm thickness, thus exposing a cross section through the grains; samples RI and SR were polished both sides. For large beam XANES and XRF, pellets were pressed using ~130 mg of finely ground mineral material with cellulose as a binder. XRF data were analysed using the software GeoPIXE II [5], and XANES data were handled using XANDA Dactyloscope [6].

In Figure 1, normalised XRF data (large beam) of the four minerals samples are shown for the region around Th-Lα and U-Lα fluorescence. It can be seen that upon kiln reduction, the fluorescence intensity of Th-Lα drops (black blue curves). The intensities of the other fluorescence lines are also reduced, which may be due to a higher carbon content in the sample, as the kiln process involves coal as a reducing agent. The decrease in Th fluorescence intensity indicates a drop in Th concentration as expected for reduced ilmenite. Interestingly, after aeration (LF) and leaching (SR), the intensity of Th-Lα increases by about factor of 10. This result indicates that Th is enriched during aeration and leaching, leaving an increased thorium content in the synthetic rutile. Similar observations can be made for uranium. In RI, uranium cannot be detected, while in FI, LF, and SR some U is present.

Figure 1: Large beam XRF data of processed ilmenite mineral sands.

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Selected elemental maps (Fe, Ti, Mn, Th, U) of the processed minerals are presented in Figure 2. It can be observed that after aeration (LF), less Fe is present, which is consistent with expectations from the purification process. Also, as above, Th and U signals are prominent in FI, LF, and SR, while in RI little Th and no U were detected. These results are consistent with the XRF data in Figure 1. Furthermore, when uranium is present it is largely associated with Th, indicating that during the purification radionuclides do not tend to separate on a micro-scale. The elemental maps also show that in the cases of FI and RI, thorium is predominately not associated with Mn. However, after aeration (LF) and leaching (SR), some partial correlation between Th and Mn can be observed. This finding may appear to indicate an importance of the role of Mn in the purification process. On the other hand, XANES and µ-XANES data (not presented here), show that the chemical environment around Th is not altered during processing. All spectra acquired are very similar to a reference spectrum of Th as ThO2 in monazite. Thorium occurs here in its highest valence state, and this state is stable throughout all processing steps. Hence, any changes in association of Th and Mn have no impact on the chemistry of Th.

We wish to thank Karen Rickers, Gerald Falkenberg, and Edmund Welter (HASYLAB) for their support during the beamtimes. We are also grateful to Asaf Raza (University of Melbourne) for polishing samples for µ-XRF analyses. This study was kindly supported by the State Government of Victoria, Australia, via the Synchrotron Industry Access Program of the Department of Innovation, Industry and Regional Development (DIIRD).

References [1] P.E. Jackson, J. Carnevale, H. Fuping et al., J. Chromatogr. A.. 671 (1-2), 181 (1994). [2] A. Filippidis, P. Misaelides, A. Clouvas et al., Environ. Geochem. Health 19 (2), 83 (1997). [3] I.K. Sedler, A. Feenstra, and T. Peters, Eur. J. Mineralogy 6 (6), 873 (1994). [4] R.F. Garrett, N. Blagojevic, Z. Cai et al., NIM-A 467 (Part 2), 897 (2001). [5] C.G. Ryan, B.E. Etschman, and D.R. Cousens, GeoPIXE II, http://nmp.csiro.au/GeoPIXE.html. [6] K.V. Klementiev, XANDA Dactyloscope, http://www.desy.de/~klmn/xanda.html.

Figure 2: Micro-XRF maps (Fe, Ti, Th, U, Mn) of processed ilmenite. Rows from top to bottom: feed

ilmenite (FI), reduced ilmenite (RI), leach feed (LF), synthetic rutile (SR). Each image is individually scaled to fit an 8-bit temperature colour-scale with light colours representing higher fluorescence inteneities.

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