Minerals Engineering 19 (2006) 232–236 This article is also available online at: www.elsevier.com/locate/mineng Oxidation of high-titanium slags in the presence of water vapour P. Chris Pistorius *, Tebogo Motlhamme Department of Materials Science and Metallurgical Engineering, University of Pretoria, Lynnwood Road, Pretoria, Gauteng 0002, South Africa Received 3 May 2005; accepted 26 May 2005 Available online 19 July 2005 Abstract High-titanium slag can be oxidised by exposure to oxygen or water vapour, since the slag contains trivalent titanium and divalent iron; such oxidation is used in slag upgrading processes. The presence of water vapour may increase the rate of oxidation. To test this, samples of crushed high-titanium slag were oxidised in various mixtures of oxygen, argon, and water vapour, in a fluidised bed, at 800 °C, for up to 2 h. The presence of water vapour did increase the degree of oxidation, without changing the nature of the reaction products, which were rutile, pseudobrookite with increased iron content, and some anatase. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Extractive metallurgy; Pyrometallurgy; Oxidation 1. Background High-titanium slags are an important feedstock for the production of TiO2 pigment. The slags are produced by carbothermic reduction of ilmenite; reduction serves to reduce the FeO content of the slag, by forming metallic iron. Some reduction of TiO2 to Ti2O3 also occurs. This is illustrated by the slag composition which is given in Table 1 (this is the composition of the slag used in this work, and is typical of such slags). The simultaneous reduction of titanium to the trivalent form (while metallic iron is produced) has several important implications for subsequent processing of the slag, of which only two are mentioned here. Firstly, the FeO and Ti2O3 contents of the slag are interrelated in such a way that the solidified slag consists mainly of a single phase, which follows the M3O5 stoichiometry, and is often referred to as ‘‘pseudobrookite’’ (Pistorius, 2002; Zietsman and Pistorius, 2004). Secondly, Ti2O3 is * Corresponding author. Tel.: +27 12 420 3182; fax: +27 12 362 4304. E-mail address: chris.pistorius@up.ac.za (P.C. Pistorius). 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.05.016 readily oxidised during subsequent processing (after tapping from the ilmenite smelter). Oxidation of trivalent titanium affects several slag processing steps: oxidation causes exothermicity during carbochlorination (Pistorius and le Roux, 2002), it is the basis of processes to upgrade the titanium content of the slag (Borowiec et al., 1998; van Dyk and Pistorius, 1999), and low-temperature oxidation (around 400 °C) causes decrepitation of the solidified slag (Bessinger et al., 2001; de Villiers et al., 2004). Oxidation of Ti2O3 changes the phase balance in the slag: for oxidation above 550 °C, rutile (and anatase in some cases) precipitates (and the remaining pseudobrookite is enriched with iron) as titanium is oxidised from the trivalent to the tetravalent form (de Villiers et al., 2004). For this case where oxidation gives rutile and pseudobrookite as products, Fig. 1 shows the predicted change in phase percentages as the slag is oxidised. Note that, in this figure, 100% oxidation is taken to correspond to conversion of all trivalent titanium to the tetravalent form (for the slag in the present work, this corresponds to 2.8% of mass gain). After complete oxidation of the trivalent titanium, further oxidation is possible, of the iron in the slag (from the 233 P.C. Pistorius, T. Motlhamme / Minerals Engineering 19 (2006) 232–236 Table 1 Composition of the slag used in this work (mass percentages; determined by a combination of X-ray fluorescence and titration for Ti3+) Species TiO2 Ti2O3 FeO MnO Al2O3 MgO V2O3 Cr2O3 SiO2 CaO Fe0 Mass % 58.8 25.2 10.0 1.72 1.25 1.00 0.36 0.16 1.59 0.14 0.19 composition in Table 1. The approximation involves taking all divalent cations other than Ca2+ as equivalent to Fe2+ with respect to phase stability, and all trivalent cations as equivalent to Ti3+; Si4+ and Ca2+ are assumed not to be present in the phases shown (this is similar to the approach followed before, e.g. Pistorius (2002) and de Villiers et al. (2004)). While Figs. 1 and 2 reflect the phase changes which are generally observed experimentally upon oxidation of the slag, ilmenite is predicted to form as an equilibrium phase during oxidation (Fig. 3). 1 Mole fraction of phase 0.8 M3 O 5 0.6 0.4 Rutile 0.2 1 a 800 °C 0 20 40 60 80 100 Percentage oxidation Fig. 1. Predicted change in the relative amounts of rutile and pseudobrookite (‘‘M3O5’’) in the slag, as a function of the degree of oxidation of the slag (100% oxidation is taken to be where all the trivalent titanium has been converted to the tetravalent form). The mole fractions are calculated on the basis of the numbers of cations in the phases; only M3O5 and rutile considered. divalent to the trivalent form; completion of this reaction corresponds to a total mass gain of 3.9%). This progression of phases is also shown in the ternary diagrams in Fig. 2, with the trajectory of the slag composition during oxidation indicated. Note that the composition which is plotted in Fig. 2 is an approximation of the slag Mole fraction of phase 0 M3O5 0.8 0.6 Rutile 0.4 Ilmenite 0.2 0 0 40 60 80 100 Percentage oxidation b FeO FeO1.5 n utio ol id s O4 M3 FeO1.5 FeO 20 sol id ite n lme sol M 3 O5 solid solution tion u sol i M3O5 line rutile-M3O5 initial composition tie line oxidation path TiO1.5 TiO2 Fig. 2. Change in phase relationships in high-titanium slag during oxidation above 550 °C, based on previous experimental observations (Borowiec et al., 1998; Bessinger et al., 2001; de Villiers et al., 2004). The initial equivalent composition of the slag in the present work is shown, and the change in slag composition during oxidation. TiO1.5 TiO2 M 3 O5 solid solution Fig. 3. Equilibria in high-titanium slags at 800 °C, considering M3O5, rutile, ilmenite, and spinel (M3O4) as possible phases (FeO–TiO2– TiO1.5 equilibria calculated with FactSage; FeO–TiO2–FeO1.5 equilibria from Haggerty (1976)). (a) Change in the amounts of phases as functions of the degree of oxidation of the slag (100% oxidation is taken to be where all the trivalent titanium has been converted to the tetravalent form). The mole fractions are calculated on the basis of the numbers of cations in the phases. (b) Ternary diagram (compositions plotted as mole fractions), showing some tie lines as broken lines. P.C. Pistorius, T. Motlhamme / Minerals Engineering 19 (2006) 232–236 Even without oxidation, the phases in the slag are predicted to change upon reheating, because M3O5 is unstable at temperatures below approximately 1300 °C, tending to transform to rutile and metallic iron through a disproportionation reaction (Eriksson et al., 1996). However, this transformation appears to be quite slow, and our previous observations indicated that it only occurs after it is triggered by slight oxidation of the slag. The work presented here investigated whether water vapour affects the phase changes during oxidation. While much of the laboratory work on oxidation of high-titanium slags appears to have been performed in air or oxygen, water vapour is likely to be present in industrial practice (because the oxidising gas is typically the combustion product of a hydrocarbon fuel). The oxidation rate of titanium metal is higher in water vapour than in oxygen (Wouters et al., 1997), and the aim of the present work was to test whether high-titanium slag would also oxidise faster in the presence of water vapour. 2. Experimental work Crushed high-titanium slag, with the composition as shown in Table 1, was used for oxidation experiments. The 425–600 lm size range was used. Samples with a mass of 30 g were oxidised in a fluidised bed in a silica reactor (with inner diameter 34 mm), which was heated in a vertical tube furnace. Samples were heated in an inert fluidising gas (argon or nitrogen) after introduction into the reactor, to allow the sample temperature to equilibrate with the furnace (this took approximately 15 min). The fluidising gas was then changed to the oxidising mixture (with compositions as indicated below), and the sample oxidised for 1 h or 2 h. At the end of this time, the reactor with the sample was removed from the furnace, and the sample cooled under an inert gas before removal and weighing. Changes in mass, phase composition (as studied by X-ray diffractometry (XRD)) and microstructure (studied by scanning electron microscopy) were used to characterise the degree of oxidation and its effects. All experiments were repeated at least once. The gases which were used were Ar containing 15% O2, Ar containing 15% H2O, and Ar containing 15% O2 and 15% H2O. The oxygen and argon flow rates were fixed with mass flow controllers. The same equipment as described by Pistorius et al. (2003) was used to control the water vapour content of the gas—first saturating the gas with water vapour at some 90 °C, and then condensing out the excess water in a condensor which was held at a controlled temperature, of 51 °C. The gas flow rate was sufficiently high to ensure that mass transfer was not a limitation (the room-temperature gas flow rate was more than 2 dm3/min in all cases). Oxidation was performed at atmospheric pressure (which is approximately 0.86 atm in Pretoria). Before oxidation, the slag consisted mostly of pseudobrookite, with a small amount of rutile; quantification of the X-ray diffractogram by Rietveld refinement indicated that the slag contained approximately 88% M3O5, 8% rutile, and less than 2% of anatase (which is in approximate agreement with the phase composition of unoxidised slag as shown in Fig. 1, with 13% rutile/ anatase). 3. Results and discussion The increases in sample mass as a result of oxidation in the three different gas mixtures are shown in Fig. 4. (Note that full oxidation of the trivalent titanium in the slag to the tetravalent form would cause a 2.8% increase in mass.) The substantial oxidation which occurred in the oxidising gases caused clear changes in microstructure and phase composition. As Fig. 5 shows, iron-enriched regions (appearing brighter with backscattered electron imaging) formed within the slag, together with rutile and anatase (as indicated by XRD). The approximate percentages of phases in the slags are summarised in Table 2. In the Ar + H2O mixture the mass gain was smaller, and the most obvious microstructural change was the precipitation of metallic iron particles. Microscopically, iron precipitation appeared more prominent for slags exposed to Ar + H2O than for the other gases, and this observation was confirmed by XRD (which showed a higher metallic iron peak for Ar + H2O; see Table 2). The observation of iron precipitation together with slag oxidation tallies with previous observations that this (disproportionation) reaction only occurs if the slag is slightly oxidised, presumably because of a nucleation barrier. The disproportionation reaction can be written as follows: 3 Percentage mass increase 234 Ar-15%O2-15%H2O Ar-15%O2 Ar-15%H2O 800°C 2 1 0 0 30 60 90 120 t (min) Fig. 4. Increases in slag mass, for high-titanium slag samples exposed to three different oxidising gases at 800 °C. 235 P.C. Pistorius, T. Motlhamme / Minerals Engineering 19 (2006) 232–236 Fig. 5. Microstructural changes in slag samples after exposure to different gas atmospheres for two hours at 800 °C. The gas atmospheres were (a) Ar + H2O (main observation is precipitation of metallic iron—bright points in the image), (b) Ar + O2 (porosity appeared, with rutile/anatase, and pseudobrookite which is enriched in iron) and (c) Ar + O2 + H2O (observations as for (b)). Scanning electron micrographs, with back-scattered electron imaging. Regions which are richer in iron appear brighter in these images. Table 2 Approximate percentages of phases in the slags before oxidation, and after 1 and 2 h of oxidation at 800 °C (percentages obtained from Rietveld refinement of XRD spectra) Slag treatment M3O5 Rutile Anatase Fe0 Hematite Ilmenite Unoxidised Ar–15%H2O; 1 h Ar–15%O2; 1 h Ar–15%O2–15%H2O; 1 h Ar–15%H2O; 2 h Ar–15%O2; 2 h Ar–15%O2–15%H2O; 2 h 88 64 55 53 52 44 43 8 24 26 28 33 34 34 2 8 14 15 11 18 19 0.2 0.9 0.7 0.5 1.2 0.6 0.5 0.1 0.2 1.3 1 0.9 1.5 1.2 1.4 3.1 3.2 2.4 2.2 2.4 2.6 FeTi2 O5 þ Ti3 O5 ! Fe þ 5TiO2 ð1Þ As Eq. (1) indicates, the ratio of the increase in mass percentage of rutile/anatase to that of metallic iron should be 7.2:1 (since 5 moles of TiO2 form for 1 mole of Fe). The actual increase in rutile/anatase after oxidation in water vapour is much more than this (given that the metallic iron content increased by approximately half a percent— Table 2). The large increase in rutile/ anatase content (and the mass increase of 1.2% after 2 h) for the slag exposed to Ar–15% H2O hence indicate that water vapour is significantly oxidising. In line with previous experimental observations, little or no ilmenite was observed in the oxidised slags (Table 2)—even though a large amount of ilmenite is predicted as an equilibrium phase (see Fig. 3). The results show that water vapour itself oxidises high-titanium slag, and also increases the oxidation rate of the slag in oxygen-containing gas. The mechanism for this increase is not clear. The appropriate mass transfer correlations for a fluidised bed show that oxidation was clearly not controlled by mass transfer in the gas phase. This means that the effect of water vapour must act through the kinetics of the interfacial reaction, or that of diffusion (of anions and cations) in the solid phases. Experimentally distinguishing which of these is ratedetermining would not be simple, because the development of porosity during oxidation means that the surface area and diffusion distances are not well defined. However, the practical implication of this is clear: the presence of water vapour can decrease the processing time which is required to oxidise slag (during slag upgrading, for example). Similar considerations may hold during the oxidative roasting of ilmenite; this will be investigated in further work. 4. Conclusion For oxidation at elevated temperatures—as used in slag upgrading processes—the presence of water vapour should decrease the required residence time. Despite its predicted presence as an equilibrium phase, no significant amount of ilmenite formed during oxidation; the only products were rutile, pseudobrookite, and some anatase. These products are not changed by the presence of water vapour, although there is an indication that more iron formed by disproportionation in the oxygen-free Ar–H2O mixture. Acknowledgements The assistance of Johan de Villiers and Sabine Verryn with X-ray diffraction is gratefully acknowledged. The research was made possible by support by Kumba 236 P.C. Pistorius, T. Motlhamme / Minerals Engineering 19 (2006) 232–236 Resources and the Technology and Human Resources for Industry Programme (THRIP) managed by the National Research Foundation (NRF) and financed by the dti. This material is based on work supported by the NRF under Grant Number 2053355. References Bessinger, D., Geldenhuis, J.M.A., Pistorius, P.C., Mulaba, A., Hearne, G., 2001. The decrepitation of solidified high titania slags. Journal of Non-Crystalline Solids 282 (1), 132–142. Borowiec, K., Grau, A.E., Guegin, M., Turgeon, J.-F., 1998. Method to upgrade titania slag and resulting product. 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