Abstract

NELL, J.. Thorium in high-titania slag. J. S. Afr. Inst. Min. Metall. [online]. 2008, vol.108, n.1, pp.15-24. ISSN 2411-9717.

Radioactive elements associated with heavy mineral deposits may be removed from the ilmenite concentrate before smelting or, alternatively, from the high-titanium slag after smelting. Either way, the radioactivity removal process will involve leaching and/or flux-roasting. However, the volume of slag is approximately 50 per cent less than the volume of ilmenite concentrate and it may therefore be more efficient to remove radioactive elements from the slag. Several slags with different compositions (in terms of iron to titanium ratio, silica content and radio activity) were studied. All the slags contain iron-titanium oxide phases with M₃O₅ (M = Fe²⁺, Ti³⁺, Ti⁴⁺, Mg²⁺, Al²⁺, Cr³⁺) stoichiometry, silicate phases and a small amount of entrained metal droplets. The silicate phases have variable compositions; in all the samples it was found that droplets of a silica-rich phase (containing about 60% SiO₂) unmixes from a low-silica phase (containing about 40% SiO₂). Both the highand low-silica phases are considered to be glass, representing quenched, immiscible silicate melt phases. The silicate and oxide phases were analysed by electron microprobe to determine the distribution of thorium in the slag (the concentrations of the radioactive daughter products of the thorium decay series are too low to be detected with the microbeam analytical facilities that were available during the investigation). The results are unequivocal: the concentration of thorium in M₃O₅ oxide phases is below the detection limit of the technique (500 ppm) while the concentration in the silicate phases is at the one per cent level. Furthermore, the concentration of thorium in the low-silica phase (which also contains high concentrations of Zr and Ce) is considerably higher than in the high-silica phase. Mass-balance calculations indicate that the slags contain about 5 per cent by weight silicate phases and that the amount of thorium present in silicate phases is enough to account for the total thorium content of the slags. There is firm evidence for the concentration of thorium in silicate phases but the distribution of the radioactive daughter elements of the thorium and uranium decay series could not be determined directly. To obtain more information on the distribution of trace elements in high-titanium slag, proton induced X-ray emission spectroscopy (PIXE) was used to characterize, semi-quantitatively, the distribution of elements between coexisting silicateand oxide-phases in one of the samples. The results confirmed the partitioning of thorium to silicate phases. In addition, it was found that the low-silica phase, which contains most of the thorium, is also enriched in Zr, Ce, Nb, Ta, Ba, Y, U and Sr. Elements with large ionic radii are apparently concentrated in the low-silica phase; it is expected that the daughter products of the Th- and U-decay series would behave similarly. Therefore, to lower the radioactivity, the silicate phases must be removed from the slag. One way to do so is to subject the slag to NaOH leaching, followed by acid leaching. Another approach would be to use a modified SREP method but this runs the risk of contaminating the final slag product with impurities such as boron.

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