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ELSTAD, H., ERIKSEN, J.M., HILDAL, A., ROSENQVIST, T., and SEIM, S. Equilibrium between titania slags and metallic iron. The 6th International Heavy Minerals Conference `Back to Basics', The Southern African Institute of Mining and Metallurgy, 2007.

Equilibrium between titania slags and metallic iron

H. ELSTAD*, J.M. ERIKSEN, A. HILDAL*, T. ROSENQVIST, and S. SEIM

*TINFOS TTI, Tyssedal, Norway TINFOS TTI, Department of Material Science and Engineering (MS&E), Norwegian University of Science and Technology (NTNU), Norway Department of Material Science and Engineering (MS&E), Norwegian, University of Science and Technology (NTNU), Norway

Introduction

The titanium industry demands high titanium feedstock for their processes. As rutile resources are limited, ilmenite has become the most important titanium source. Ilmenite smelting is an important part of upgrading ilmenite to high titanium feedstock. In the year 2005, upgrading of ilmenite by smelting, producing a slag with 80­90% TiO 2 , accounted for approximately 40% of the feedstock to the TiO2 pigment industry. In ilmenite smelting molten iron is produced together with a slag, which in addition to titanium oxide contains iron oxide as well as various impurity oxides of manganese, magnesium, silicon, aluminium, etc. Titanium is present mainly as TiO2 but partly also as Ti2O3. It is known that the concentration of Ti 2 O 3 increases with decreasing concentration of FeO, but this dependency is affected by the nature of the impurity oxides. Sigurdson and Moore 1 showed that slags high in CaO and MgO had a lower concentration of Ti2O3 than slags high in SiO2 for the same amount of FeO. Pistorius2 showed in 2002 that if the impurity oxides MnO and MgO are counted, on a molar basis, as FeO, and Cr2O3, V2O3 and part of the Al2O3*), as Ti2O3, industrial slags will have a composition close to that of the M3O5 phase, where M denotes the elements Ti, Fe, Mn, etc, and which has the pseudobrookite structure. However, for equilibrium with liquid metallic iron he computed, by means of a computer program FACT, that the slag equilibrium composition is represented by a curved line in the ternary phase diagram FeTiO3-TiO2-Ti2O3, but with a significantly higher concentration of trivalent titanium than actually observed in the industrial slags, and he discussed various mechanisms by which the slag during solidification could change its composition to a lower Ti2O3 content, but the possibility that the computed compositions might be wrong was not mentioned. Tranell3 found in her research on titaniferous silicate slags a lower Ti3+/ Ti4+ ratio than that which is computed by the FACT program. The purpose of the present investigation was to study the equilibrium composition of Fe-Ti-O slag in equilibrium with liquid metallic iron, as well as the effects of impurities such as SiO2, CaO and MgO on the slag composition.

Industrial slags from TINFOS Titan and Iron KS (TTI) will also be discussed and their compositions will be plotted in the ternary phase diagram. These are slags produced from Tellnes ilmenite, quite high in magnesium and iron.

Experimental

In the past, equilibrium studies of the Fe-Ti-O slag system were difficult when it came to producing the slag. It was difficult to find a suitable crucible for the meltings. The highly corrosive slag would attack all known refractories, and the metal would attack metallic crucibles such as those made of platinum and molybdenum. In the present investigations the slag-metal combination was prepared in a high-frequency induction furnace with a vertically segmented water-cooled copper crucible. The crucible is shown in Figures 1a and 1b. It was designed by the Australian Nuclear Science and Technology Organisation (ANSTO) with power from a 75 kVA highfrequency generator (750 kHz) supplied by Farfield Electronics Pty, Ltd, Australia. This enabled the slag-metal combination to be melted with turbulent stirring and is expected to be isothermal. The entire temperature gradient

*An

amount of Al2O3 corresponding to 1/3 of the SiO2 content is assumed to go into the silicate glass phase and is subtracted from the total Al2O3 content.

Figure 1a. 3-D sketch of the water-cooled copper crucible

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the slags E.4­E.6 were cooled more slowly and solidified completely after about 20 minutes. The slags were analysed by X-ray fluorescence (XRF) for their metallic elements, and by wet chemical methods for their contents of Ti2O3 as well as for droplets of metallic iron, which invariably were found suspended in the slag. Furthermore, their content of acid-insoluble oxide was determined, which gave a measure of the amounts of rutile phase in the slag. These analyses were carried out by the TINFOS Titan & Iron K/S Company. Furthermore, the button of metallic iron in the bottom of the slag ingot was weighed and in some cases analysed. Slag samples were also examined by X-ray diffraction (XRD) and microprobe analysis.

Results (Fe-Ti-O)

For slags without impurity oxides the results are given and discussed elsewhere4, and are summarized in Figure 2. Here the two parameters NFe = nFe/(nFe + nTi) and NO = nO/(nFe + nTi) were calculated from the chemical analyses, the letter n denoting the molar concentration of the element in the slag, manganese being counted as iron. In Figure 2 different symbols denote the top, middle and bottom of the slag ingot, and the curve for iron -saturation is drawn between the points for the top and bottom parts. Also shown are the compositions from pilot plant smelting tests on rather pure ilmenite as reported by Swinden and Jones5 and Pistorius and Coetzee6, as well as the compositions for equilibrium with metallic iron as computed by Pistorius2. The present saturation curve agrees closely with the one previously proposed by Rosenqvist7. All the slags were examined by X-ray diffraction (XRD) and, with the exception of slag E.1, also by microprobe analysis, with the results given in Table I. Here TiO2-x denotes a phase, which may be either a reduced rutile, or a Magnéli phase (TinO2n-1) with some FeO in solid solution. For all slags except E.3 the dominating phase was an M3O5 phase. If the X-ray spacings are compared with those given by Eriksson et al. 8 it appears that it is a solid solution between FeTi2O 5 and Ti3O 5. For experiment E.3 Ti 3O 5

Figure 1b. Cross-section of melting furnace

is believed to be within the freeze lining, about 1 mm thick. Similar studies for oxide melts are described by Hong et al.11. About ½ kg of slag-metal could be melted in each experiment. Slags with a molar ratio Ti2O3/FeO of unity were obtained by melting pure TiO2 (laboratory grade) with a surplus of metallic iron, in which case the reaction: [1] occurred. Slags with lower or higher Ti2O3/FeO ratios were obtained by addition of either some Fe2O3 or some metallic titanium, in which case the additional reactions: [2] [3] occured. Furthermore, some slags were prepared with the addition of either SiO2, MgO or CaO (slags S.1­S.6). For experiments E.1­E.4 metallic iron was added in the form of soft steel washers, which contained about 1% manganese as the main impurity. This gave rise to a few tenths of a per cent MnO in the slag, and made it possible to see how manganese distributed among the different slag phases. For experiments E.5, E.6 and S.1­S.6 only enough steel washers were used to initiate the slag melting, the remainder being added as high purity electrolytic iron. Iron was added to give a molar ratio O/M of about 1.5, which ensured saturation with metallic iron. The melting time was about 15 minutes for experiments E.1-E.6, and about 25 minutes for S.1­S.6. In order to prevent oxidation the meltings were carried out in an argon atmosphere in a gas-tight chamber, and the slags were allowed to solidify in the crucible under argon. As the freezing of the slag took place from the water-cooled bottom of the crucible and upwards, samples for analysis were in most cases cut from the bottom and/or top part of the solidified slag ingot and were analysed separately. Slags E.1­E.3 and S.1­S.6 were cooled rapidly by turning off the electric power, whereas

Figure 2. Composition of slags. Also shown are slag compositions given by Swinden and Jones5 and Pistorius and Coetzee6 and computed line for saturation with metallic iron at liquidus temperature according to Pistorius2, dashed line, and the stoichiometric M3O5 composition dotted line, Eriksen, Robles 4 and Rosenqvist

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TableI XRD and microprobe analyses of slags without impurities

Slag no. Microprobe analyses, atomic %, normalized to 100% XRD phases Ti Fe Mn O NFe NO M3O5 Rutile (trace) Fe (trace) M3O5 Rutile (trace) Fe (trace) Ti3O5 Ti2O3 Rutile (trace) Fe (trace) M3O5 TiO2-x Fe (trace) M3O5 M3O5 M2O3 Rutile Fe(tr) M3O5 M2O3 Rutile (tr) Fe(tr) M3O5 Ti3O5 (tr) TiO2-x (tr) M3O5 Ti3O5 TiO2-x (tr) 30.0 2.0 37.6 39.0 39.0 31.3 34.6 34.0 28.7 21.8 32.6 29.3 21.3 0.0 33.9 35.8 35.0 33.9 7.0 92.3 <0.1 0.4 90.5 7.1 1.0 4.0 9.1 17.1 1.6 8.9 18.3 97.3 3.5 1.9 2.3 2.0 0.13 0.00 0.05 0.20 0.50 0.05 0.02 0.05 0.04 0.90 0.01 0.02 0.12 0.00 0.02 0.02 0.01 0.01 62.0 3,0 0.19 1.68 -

E.1

The M2O3 phase was for experiment E.3 practically pure Ti2O3, and for experiment E.5 practically pure FeTiO3. In both cases the MnO concentration was found to be significantly higher in the M2O3 phase than in the adjacent M3O5 phase. This shows that manganese is preferentially dissolved in the M 2 O 3 phase, which agrees with the findings of Grey et al.10.

E.2

E.3B

62.3 <0.001 1.65 60.4 0.02 1.53 5.0 61.5 64.0 62,0 62.1 60.2 65.8 61.8 60.0 2.7 62.5 62.4 62.0 63.2 0.18 0.03 0.11 0.24 0.45 0.05 0.23 0.46 0.09 0.05 0.06 0.06 1.60 1.80 1.63 1.64 1.51 1.92 1.61 1.50 1.67 1.66 1.66 1.76 -

E.4T

E.4B E.5T

E.5B

E.6T

E.6B

Figure 3a. Back-scatter microprobe pictures for slag E.4 bottom. Light grey: M3O5. dark grey: TiO2-x phase

denotes a phase (anosovite) with a structure slightly different from that of pseudobrookite. This slag also contained an M 2O 3 phase, which was practically pure Ti2O3. X-ray diffraction and microprobe analysis of slag E.6 showed the occurrence of both the M 3 O 5 phase (pseudobrookite), with N Fe 0.1 and the Ti 3O 5 phase (anosovite), with NFe 0.05. The microprobe values give the average of values obtained by a small number (2­3) of point analyses, and may not be fully representative for the phase as a whole. Furthermore, the pieces examined may not be fully representative for the entire slag due to segregation during freezing. For experiment E.4 backscatter pictures for the bottom and top parts are given in Figure 3. The bottom part appears to be mainly the M3O5 phase with only small amounts of TiO 2-x , whereas the top part shows a typical eutectic structure with lamellas of M3O5 and TiO2-x phases. The composition of the TiO 2-x phase shows that this was practically free of iron (NFe = 0.03) and had an NO value of 1.8 corresponding to the M5O9 composition. Microprobe Xray pictures from the bottom part of E.4 showed that the M3O5 phase was not homogenous, the centre of the grains being low in iron and high in titanium, whereas the opposite was the case for the fringes of the solid solution grains. Evidently the composition of the solid solution had shifted towards a higher FeTi 2 O 5 concentration during solidification.

Figure 3b. Back-scatter microprobe pictures for slag E.4 top. Light grey: M3O5. Dark grey: TiO2-x phase

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The X-ray diffraction diagrams and the large amounts of acid insolubles for slags E.4­--E.6, which were cooled slowly, showed that these contained significant amounts of rutile or TiO2-x4. In contrast, industrial slags contain very little rutile. The small amounts of titanium and oxygen found in the metallic iron droplets may possibly be caused by contamination from adjacent oxide phases. Melting point determination The temperature during melting and solidification in the induction furnace could not be measured. Instead, small pieces of the slag, about 3 mm cross-section, were heated in a `sessile drop furnace' in argon, and the fusion of the slag was observed optically. The furnace temperature was measured with a two-colour optical pyrometer, calibrated against the melting points of pure iron and copper. The slag pieces rested on a substrate, which, for the first measurements, was either graphite or vitreous carbon, the latter being known to be extremely inert. As long as the slag was solid and had only point contact with the substrate, reaction with the substrate was not expected, and the initial melting temperature, the solidus temperature, could be observed. As soon as a liquid phase appeared, this reacted strongly with the substrate, regardless whether it was graphite or vitreous carbon. Microprobe analysis of the sample after the melting point experiments showed that practically all iron oxide had been reduced to metal. In contact with the graphite or vitreous carbon an oxycarbide phase with about equal amounts of TiO and TiC had been formed. The remaining part of the slag had been reduced to practically pure Ti 3 O 5 + Ti 2 O 3 and had a melting temperature of about 1710­1740°C for all slags. The lower value may represent the eutecticum between Ti3O5 and Ti2O3. One melting point experiment was done with a molybdenum substrate on a slag from experiment E.4 Top. Initial melting occurred at about 1675°C, and complete melting at 1718°C. Microprobe analysis of the slag after the experiment showed that practically all the iron and manganese in the slag had diffused into the molybdenum substrate forming a solid solution. The slag had been enriched in TiO 2 and had a composition with about NO = 1.74 and NFe = 0.06. Another melting point measurement on slag E.4T on a TiO 2 substrate in a shallow graphite bowl gave initial melting at 1620°C and complete melting at 1720°C. The first of these probably corresponds to the eutective groove between M 3O 5 and the TiO 2-x phase, the second to the liquidus surface. Microprobe analysis showed that the slag had reacted with the TiO2 substrate and the graphite bowl to give an intimate mixture of titanium oxide, carbon and some metallic iron with no well-defined phases. For slag E.5 T on a TiO2 substrate a sudden and almost complete melting occurred at 1540°C. This is believed to correspond closely to the ternary peritecticum between M2O3, M3O5 and TiO2-x. Further heating was stopped, and the solidified slag was examined by microprobe analysis, which showed the occurrence of a TiO 2-x phase with NO = 1.86 corresponding to Ti7O13 and an M3O5 phase with NO = 1.67 and NFe = 0.14. Slag E.6 T on a TiO2 substrate showed initial melting at 1675°C and complete melting at 1725°C. Microprobe analysis after the melting point determination showed an almost iron free slag with NO = 1.60. Evidently reduction by the graphite bowl had taken place. Slag E.6 B collapsed suddenly at 1710°C without there being any noticeable

solidus melting. This is believed to represent the eutecticum between Ti2O3 and Ti3O5. Phase diagrams The present results were combined with those of Eriksson et al.8 to estimate the ternary Fe-Ti-O phase diagram and its liquidus surface, which is shown in Figure 4. As all experiments, with the exception of experiment E.3, showed the occurrence of either rutile or a TiO2-x phase, it may be concluded that this phase is formed during solidification. The small and variable amounts of iron droplets found in the slag are believed to have been in suspension and have not been part of the oxide melt at its liquidus (syntectic) temperature. This is supported by the fact that its concentration increases from the top to the bottom of the ingot, and is larger in the rapidly than in the slowly cooled slag. The top sections for the experiments E.2, E.4 and E.6 most likely represent the composition of the eutectic groove between the M3O5 and the rutile or TiO2-x phase. According to Eriksson et al.8 Ti3O5 melts congruently at 1718°C and its eutecticum with TiO2-x occurs at 1665°C and N O = 1.73, and with Ti 2 O 3 at about 1680°C and NO = 1.62. As mentioned above the latter eutecticum is more likely to be around 1710°C. Furthermore, FeTi2O5 melts peritectically at 1455°C, and FeTiO 3 melts peritectically at 1377°C. As mentioned above the top part of slag E.5 is believed to correspond closely to the ternary peritecticum between M2O3, M3O5 and rutile with a melting temperature of about 1540°C. Also shown in Figure 4 is the 1500°C isotherm given by Pesl and Eric9. As seen, this shows a much wider melting region than found in the present study. Figure 5 shows a suggested melting point diagram for the Fe-TiO2 composition line. It should be pointed out that this is not a true binary diagram as the composition of the M3O5 phase changes from a somewhat higher Ti3O5 content at the syntectic temperature of about 1710°C to a somewhat higher FeTi2O5 content at the eutectic temperature of about 1620°C. At the same time the NO value increases from maybe 1.63 to 1.67. According to this diagram a slag melted at, say 1750°C, will on cooling to the syntectic temperature first eject a small amount of iron droplets. At the syntectic temperature a M3O5 phase with a Ti3O5 surplus and a small oxygen

TiO2 1857°C 0.0 2.0

0.1

1.9

0.2 NFe 0.3 FeTi2O5 1455°C 0.4

1.8 NO 1665°C 1.7 Ti3O5 1718°C 1710°C 1.6

0.5 1377°C FeTiO3 0.0

0.2

0.4

0.6

0.8

1.5 1.0 Ti2O3 1842°C

Figure 4. Proposed ternary melting diagram for slags without impurities

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Figure 5. Proposed pseudobinary section Fe-TiO2 as a function of temperature

S.1 and S.3 showed traces of rutile, while slags S.2 and S.3T showed traces of an M2O3 phase (ilmenite). Slag S.3 contained also an oxide phase TiO 2-x , which will be discussed later. It is evident that the three impurity oxides behaved in three different ways during solidification of the slags. Thus in slag S.1 silicon was present exclusively in the silicate phase which, on the average, contained in atomic per cent: 28.5 Si, 2.9 Ti and 0.6 Fe. In weight per cent this means that the silicate inclusions contained 86% SiO2, 11.6% TiO2 and 2.4% FeO. As the slag as a whole contained 4.3% SiO2 this means that also 0.6% TiO2 and 0.1% FeO were present in the silicate inclusions. These were subtracted from the total analyses given in Table III, and the NFe and NO values for the oxide melt were calculated to 0.19 and 1.68 respectively, and are shown in Figure 6. Similar correction was made for slag S.6. For slags S.2 and S.5 microprobe analyses showed that all magnesium in the slag substituted for iron in the M3O5 and M2O3 phases. Therefore, for these slags the parameters NFe and NO give the ratios (nFe+nMg)/(nTi+nFe+nMg) and nO/ (n Ti +n Fe +n Mg ) respectively. It is also evident that magnesium, contrary to manganese, is preferentially enriched in the M3O5 rather than in the M2O3 phase. For slag S.3 and S.4 calcium was present mainly in the perovskite phase, which on the average contained in atomic per cent: 21.2 Ti, 0.2 Fe, 19.7 Ca and 58.9 O, i.e. close to

deficiency will precipitate. On further cooling an M3O5 phase with decreasing Ti3O5 content and increasing oxygen content will precipitate, and the slag will solidify at the eutectic temperature of about 1620°C. Under equilibrium conditions the M 3 O 5 phase will decompose below about 1325°C to give metallic iron and rutile or some Magnéli phase 8 . In practice this decomposition is very slow and the phase combination M3O5 + TiO2-x is retained at room temperature.

Effects of impurities

Three slags were prepared with a Ti2O3/FeO molar ratio of unity and three with a Ti2O3/FeO molar ratio of three and with addition of 5% of respectively SiO2, MgO and CaO, and with enough iron to ensure metal saturation. The melting time was about 25 minutes, and the slags were cooled rapidly in the crucible. The analytical results for representative samples of the slags are shown in Table II. It will be noticed that the slags S.1 and S.6 (with SiO2) contained the highest content of Ti 2 O 3 and slag S.3 (with CaO) contained the lowest content of FeO. Slag S.4 (with CaO) contained the highest amounts of insolubles. Xray diffraction and microprobe analyses (Table III) showed for all slags mainly the M3O5 phase and traces of metallic iron. Slags S.1 and S.6 showed in addition a silicate glass phase, and S.3 and S.4 showed perovskite, CaTiO3. Slag

Figure 6. Composition of slags with added impurities and of industrial slags from Tinfos Titan & Iron K/S. For slag S.1 and S.6 the SiO2 content formed a separate silicate glass phase. For slags S.2 and S.5 MgO is counted as FeO. For slag S.3 and S.4 the CaO content is assumed present as CaTiO3. For TTI slags the procedure by Pistorius2 is applied

Table II Composition of slags with impurities in weight%, and solidus temperature

Slag no. S.1 S.2 S.3 S.4 S.5 S.6 TiO2 tot 81.6 81.6 79.7 88.4 93.1 91.1 FeO tot 18.4 16.3 14.0 6.8 5.5 8.9 SiO2 4.3 5.1 MgO 5.1 5.6 CaO 6.0 5.7 Ti2O3 as TiO2 26.1 17.1 21.8 31.9 34.3 43.6 Fe met 0.9 1.0 0.2 0.2 0.8 0.8 Insoluble 0.9 0.1 0.9 7.0 0.1 0.4 Solidus temp (C) 1491 1547 1382 1420 1735 1575

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Table III Microprobe analyses in atomic%, normalized to 100% for slags with impurities

Slag No S.1 T* Phases M3O5 Silicate Fe (tr) M3O5 Rutile Silicate Fe (tr) M3O5 M2O3 Fe (tr) M3O5 M2O3 Fe (tr) M3O5 TiO2-x Rutile Perovskite M2O3 M3O5 TiO2-x Rutile M3O5-x TiO2-x Perovskite M3O5-x TiO2-x Perovskite M3O5-x Fe (tr) M3O5-x Fe (tr) M3O5-x Silicate Fe (tr) M3O5-x Silicate Fe (tr) Ti 30.8 2.0 1.8 32.3 31.4 3.8 2.6 29.3 21.8 2.6 29.8 21.2 2.9 29.9 29.4 32.8 20.5 20.7 31.0 29.2 32.7 36.3 34.1 21.8 36.1 34.3 22.0 32.3 0.7 33.5 3.2 37.0 2.7 3.4 37.5 3.2 4.3 Fe 5.1 0.5 96.9 4.4 2.1 0.7 96.3 4.1 15.6 96.3 3.1 16.3 96.0 9.5 3.7 1.2 0.6 17.5 5.4 3.7 0.9 2.0 0.9 0.4 2.5 0.8 0.2 1.3 98.3 1.6 95.5 2.6 0.4 94.7 1.9 0.3 92.8 0.1 33.1 0.3 0.1 32.8 0.7 4.0 0.0 3.8 0.1 Si 29.5 Mg Ca O 64.0 68.1 1.2 63.2 66.1 68.6 1.1 4.2 1.0 4.5 1.8 0.36 3.11 0.10 19.70 0.50 0.10 3.00 0.15 0.03 19.40 0.12 0.18 20.00 62.5 61.5 1.1 62.6 60.7 1.0 62.7 63.6 65.9 59.3 61.4 63.6 64.1 66.3 61.7 64.6 58.4 61.2 64.0 57.7 62.2 1.0 61.2 1.2 60.4 63.9 1.7 60.5 63.8 2.3 NFe 0.14 NO 1.78

S.1 B

0.12 0.06

1.72 1.97

27.5

S.2 T

0.22 0.43 0.20 0.46 0.24 0.11 0.03 0.46 0.15 0.11 0.03 0.05 0.03 0.06 0.02 0.14 0.14 0.07

1.66 1.60 1.67 1.55 1.60 1.92 1.94 1.61 1.75 1.95 1.97 1.61 1.85 1.58 1.78 1.64 1.62 1.53

S.2 B

S.3 T

S.3 B

S.4 T

S.4 B

S.5 T S.5 B S.6 T

S.6 B

0.05

1.53

*The

XRD diagram for slag S.1 showed exclusively the M3O5 pattern whereas microprobe analysis gave NO 1.75 corresponding to an M4O7 composition. The reason for this is not clear. For slag S.2 N e and NO are calculated by including MgO, on a molar basis, in the FeO value. F For slags S.4­S.6 XRD showed mainly the M3O5 phase whereas microprobe analyses gave NO values between 1.53 and 1.64 i.e. less than the stoichiometric composition (NO = 1.67). It is conceivable that the M3O5 phase may exist with an oxygen deficiency, and may be denoted M3O5-x.

the stoichiometric CaTiO3 composition. This phase was found mainly in the top part of the slag ingot, which shows that it was formed during the final solidification. Furthermore, these slags contained a phase TiO2-x with NO 1.90, with significantly less iron and more calcium than in the adjacent M3O5 phase. This indicates that some Magnéli phase may take some CaO into solid solution with correspondingly less FeO. For simplicity, for these slags the NFe and NO values in Figure 6 were calculated by assuming all CaO tied up as CaTiO 3 . Nevertheless, these slags showed, even after subtraction of CaTiO3, higher NO values than the others. Evidently CaO has stabilized Ti 4+ in the slag. The melting temperature of slag S.1 (with SiO2) was studied on a single crystal silicon carbide substrate. Initial

melting, the solidus temperature, was observed at about 1490ºC. As soon as liquid phase was established this reacted rapidly with the silicon carbide substrate under vigorous gas evolution to give a metallic melt and an oxide which did not melt below 1720ºC. Subsequent microprobe analysis showed that the metallic phase contained, in atomic per cent, around 50 iron, about 35 silicon, 1­4 titanium and 15­17 carbon. The oxide phase was almost pure rutile. The melting temperature of slag S.2 (with MgO) was studied on a TiO2 substrate in a shallow graphite cup. Initial melting occurred at about 1550ºC and the slag was completely melted at 1685ºC. Microprobe analysis showed no traces of a metallic phase. The oxide phases were rutile and M3O5 with low Fe contents.

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Slag S.3 (with CaO) on a TiO2 substrate showed initial melting at 1382ºC and was completely melted at about 1525ºC. Microprobe analysis showed that the metallic phase contained, in atomic per cent, 3.9 Ti, 2.1 O and 93.2 Fe. Furthermore, the slag contained a M3O5 phase with about 6.5 at% iron, and a rutile phase which contained ~1 at% iron. For slag S.4 (with CaO) the melting temperature was studied on a TiB2 substrate. Initial melting occurred at about 1420ºC and complete melting above 1700ºC. Slag S.5 (with MgO) was also studied on a TiB 2 substrate, and showed initial melting at about 1735ºC. Complete melting was not reached, even at 1800ºC. Thus all impurities tend to lower the eutectic temperature between the M3O5 and rutile phases shown in Figures 4 and 5. The largest freezing point depression is found for slags with CaO addition, the lowest with addition of MgO. This means that the impurities tend to stabilize the liquid oxide phase, and also extend its range of stability to lower oxygen contents. This may be the reason why industrial slags show mainly the M3O5 phase with only traces of rutile (acid insolubles) Figure 5. Addition of MgO and CaO tends to stabilize the Ti4+ ion with corresponding less Ti3+.

Table IV Slags from Tinfos Titan & Iron K/S

TTI 1 TiO2 tot FeO Ti2O3 as TiO2 MgO MnO Al2O3 V2O5 Cr2O3 Others Fe met NFe NO XRD phases 81.7 6.9 27.1 5.8 0.58 1.7 0.35 0.11 5.14 0.23 0.190 1.665 M3O5 Rutile (tr) Fe (tr) TTI 2 78.6 11.2 20.8 5.8 0.56 1.7 0.35 0.13 5.19 0.33 0.231 1.656 M3O5 Rutile (tr) Fe (tr) TTI 3 79.7 9.1 23.9 5.9 0.56 1.6 0.35 0.13 5.16 0.46 0.212 1.660 M3O5 Rutile (tr) Fe (tr)

Acknowledgement

The authors are grateful to Sean Gaal for assistance with preparing the slags and melting point determinations

Slags from Tinfos Titan & Iron K/S

Three slags from Tinfos Titan & Iron K/S were examined. These were taken during tapping of the furnace by dipping a cold steel bar into the slag flow. The slag solidified and cooled rapidly, and no noticeably oxidation was expected. The composition of the slags is given in Table IV. The NFe and NO values were calculated by the procedure given by Pistorius2, by counting MnO and MgO, on a molar basis, as FeO, and V2O5, Cr2O3 and part of Al2O3 as Ti2O3. This gave for the three slags NFe from 0.190 to 0.231 and NO values from 1.656 to 1.665. Within the limits of analytical accuracy the NO values all agree with the stoichiometric composition M3O5. These values are plotted in Figure 6. All slags showed almost exclusively the M3O5 phase with only traces of rutile and metallic iron. Thus the procedure suggested by Pistorius2 is well suited to predict the phase combination for industrial slags.

References

1. SIGURDSON, H. and MOORE, C.H. Petrology of High Titanium Slags, Trans AIME, vol. 185, 1949. pp. 914­919. 2. PISTORIUS, P.C. The relationship between FeO and Ti 2 O 3 in ilmenite smelter slags, Scand. J. Met. vol. 31, 1949. pp. 120­125. 3. TRANELL, G. SINTEF Materials Technology. Trondheim. Norway. Private communication. 4. ERIKSEN, J.M., ROBLES, E.C., and ROSENQVIST, T. Equilibrium between liquid Fe-Ti-O slags and metallic iron, Steel Research Int., 2007, In print. 5. SWINDEN, D.J. and JONES, D.G. Arc-furnace smelting of Western Australian beach sand ilmenite, Trans. Instn. Min. Met. 1978: p. C87: pp. C83­87. 6. PISTORIUS, P.C. and COETZEE, C. Physicochemical Aspects of Titanium Slag production and Solidification, Met. and Mater. Trans. vol. 34 B, 2003. pp. 581­588. 7. ROSENQVIST, T. Ilmenite Smelting, Trans. Techn. Univ. Kosice, vol. 2, 1992. pp. 40­46. 8. ERIKSSON, G., PELTON, A.D., WOERMANN, E., and ENDER, A. Measurement and Thermodynamic Evaluation of Phase Equilibria in the Fe-Ti-O System, Ber. Bunsenges. Phys. Chem., vol. 100, 1996. pp. 1839­1849.

Summary

The equilibrium between titania slags and liquid iron was studied in an induction furnace with a multiple segmented water-cooled copper crucible. The slags were either pure Fe-Ti-O melts or melts with addition of about 5% of either SiO2, MgO or CaO, and the Ti2O3/FeO ratio of the slags was varied by addition of either metallic Ti or Fe2O3. The composition of the slag in equilibrium with liquid iron was found to follow an arched curve in the FeTiO3-TiO2-Ti2O3 ternary diagram, and with its apex slightly above the composition of the FeTi2O5-Ti3O5 solid solution line (the M3O5 phase). The melting temperature in the Fe-Ti-O system was studied, and showed a eutectic groove between the M3O5 phase and rutile or Magnéli phases (TinO2n-x). Addition of about 5% of respectively SiO2, MgO or CaO showed that these lowered the eutectic temperature and shifted the composition of the slag in equilibrium with iron closer to the M3O5 line. SiO2 formed a separate silicate glass phase with small amounts of TiO2 and FeO. MgO went into solid solution in the M 3 O 5 phase where it substituted for FeO. CaO solidified mainly as perovskite, CaTiO3, whereas small amounts went into solid solution in a Magnéli phase.

EQUILIBRIUM BETWEEN TITANIA SLAGS AND METALLIC IRON

41

9. PESL, J. and ERIC, R.H. High-Temperature Phase relations and Thermodynamics in the Iron-TitaniumOxygen System, Met. and Mater. Trans. B, vol. 30B, 1999. pp. 695­705. 10. GREY, I.E., REID, A.F., and JONES, D.G. Reaction sequences in the reduction of ilmenite: part 4: interpretation in terms of the Fe-Ti-O and Fe-TiMn-O phase diagram, Trans. Inst. Min. Met. 1974.

pp. C83, pp. 105­111. See also Grey, I.E., Li, C., and Reid, A.F. Phase equilibrium in the system MnOTiO 2- Ti 2O 3 at 1473 ºK Journ. Solid State Chem. vol. 17, 1976. pp. 343­352. 11. HONG, S.W., MIN, B.T., SONG, J.H., and KIM, H.D. Application of cold crucible for melting of UO 2 /ZrO 2 mixture, Materials Science and Engineering A357, 2003. pp. 297­303.

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