JOURNAL OF MATERIALS SCIENCE LETTERS 16 (1997) 37–39 Mechanically enhanced carbothermic synthesis of iron–TiN composite Y. CHEN Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia We have previously reported [1–3] that high energy ball milling has an enhanced role on carbothermic reduction of mineral ilmenite (FeTiO3) to rutile (TiO2). Ball milling of a mixture of ilmenite with carbon at room temperature can dramatically lead to a high reduction rate and a low temperature required (c) c for full reduction of ilmenite to rutile and iron during the subsequent annealing process. Titanium carbide can also be produced at a relative low temperature (1000 8C) if sufficient carbon is supplied c [4]. It has been reported that titanium nitride can be (b) synthesized by reduction of titanium dioxide with Intensity (arbitrary units) carbon in a nitrogen atmosphere at 1150 8C [5]. In (a) c i i the work reported here, an assessment of the i i i i i production of iron–TiN composite at a relatively low annealing temperature was made with an 20 40 60 80 100 ilmenite–carbon mixture by using a pre-ball milling 2è (degrees) treatment and subsequent annealing process. The advantages expected of this production process are Figure 1 XRD patterns for the ilmenite–graphite mixture after (a) 200 h milling and subsequently annealed under nitrogen flow at the inexpensive nature of the raw materials (ilmenite 3 (b) 800 8C for 1 h and (c) 1000 8C for 1 h. (), Æ-Fe; ( ), ª-Fe(C) and graphite) and the one-step carbothermic reduc- (austenite); (c), graphite; (i), FeTiO3 (ilmenite); (e), TiO2; ( ), TiN. tion and simultaneously nitridation reaction, thus reducing the number of process steps. In this experiment, an upgraded ilmenite powder ilmenite and graphite structures remained after 200 h with a particle size of 200–300 mm was supplied by of milling. However, the low intensity and the Westralian Sands Ltd. The chemical composition of broadened shape of the graphite and ilmenite this ilmenite was as follows: FeO, 20.8 wt %; Fe2O3, diffraction peaks suggest small crystallite size and 20.0 wt %; TiO2, 53.6 wt % and impurities (MnO, a highly disordered structure. Indeed, it has been SiO2, ZrO2, etc.), 3.5 wt %. A graphite powder with reported that grinding of graphite can lead to the a purity of 99.8% or better was used as reducing structure disordering [6, 7] or amorphization [8]. The agent. The weight ratio of ilmenite to graphite was average grain size of the ilmenite crystallite 1:1, which gave excess carbon for full reduction of estimated from the peak broadening by using the ilmenite to metal titanium and iron. Ball milling was Scherrer formula [9] is about 45 nm, indicating a performed at room temperature in a vertical nanocrystalline structure of the milled ilmenite. planetary ball mill (ANUtech Pty Ltd, Canberra) Finally, no new diffraction peaks were found in this using hardened steel balls with a diameter of XRD pattern, suggesting that no chemical reaction 25.4 mm and a stainless steel cell. The cell was occurred during the milling process. However, as loaded with several grams of materials together with shown later, the observed dramatic structural five balls and evacuated to vacuum (10ÿ2 kPa) prior changes in both graphite and ilmenite structures to milling. After milling, the structure of the samples strongly affect the thermal behaviour during the was investigated by X-ray diffraction (XRD) using subsequent annealing processes. After annealing of Co radiation (º 0.1789 nm) at room temperature. the as-milled sample at 800 8C for 1 h under flowing The thermal analysis was carried out in a thermo- nitrogen, the ilmenite phase was no longer detected gravimetric (TG) analyser where a sample was by XRD (Fig. 1b) and essentially only rutile (TiO2) heated at a rate of 20 K minÿ1 under dry nitrogen and iron solid solutions (Fe(C)) were obtained. The ÿ1 flow (80 ml min ). After each run the sample was main iron solid solutions are Æ-Fe(C) austenite left to cool to room temperature in the apparatus Fe(C). Further annealing of the as-milled sample at a while the nitrogen flow was continued. An isother- higher temperature (1000 8C) for 1 h leads to the mal treatment was conducted in a thermal tube formation of titanium nitride (TiN) phase, as furnace under a dry nitrogen flow (60 ml minÿ1). indicated by the XRD pattern (Fig. 1c). Excess Typical XRD patterns after milling and annealing graphite was still detected in the XRD patterns (as are presented in Fig. 1. Fig. 1a shows that both represented by the weak diffraction peak c). Hence, 0261-8028 5 1997 Chapman & Hall 37 ilmenite is reduced by graphite to titanium oxide and The second slow weight loss of the 200 h milled iron during the first stage of annealing, and sample (above 950 8C) was probably caused by nitridation of titanium oxides occurs during the high nitridation reaction. Therefore, the longer a sample temperature range. This reaction sequence is in good is milled, the more rapidly it exhibits weight loss agreement with the literature [5]. However, in the during annealing. As has been mentioned previously, case of pre-ball milling treatment, the full nitridation the weight loss profile during annealing is indicative temperature (1000 8C) is lower than that of a purely of the various stages of carbothermic reaction. We thermally activated process. Thus, pre-ball milling conclude that extensive milling leads to a faster solid treatment of ilmenite–graphite mixtures reduces the state reduction and a lower temperature for the onset annealing temperature required for full nitridation. of gaseous reduction. This effect can be seen clearly from the following The above results were confirmed by XRD TG results. analysis of the corresponding annealed samples. Three samples with the same ratio of ilmenite to The XRD pattern of the as-mixed sample after graphite were first milled for 0 (as-mixed), 50 and heating to 1200 8C shows that only some of the 200 h then analysed in a TG apparatus at a rate of ilmenite has been reduced to TiO2 and Fe(C) (Fig. ÿ 20 8C minÿ1 under nitrogen flow (80 ml min 1). 3a). By contrast, in the case of the 50 h milled Variations of sample weight during heating (TG sample (the XRD pattern of the as-annealed sample curves) were monitored and are shown in Fig. 2. For is shown in Fig. 3b), not only all of the ilmenite has the as-mixed sample (Fig. 2a), a significant weight been reduced to ª-Ti3O5 and Fe(C), but also a small loss was observed above 850 8C and a rapid loss fraction of TiN has formed. The weak XRD peaks started at about 1100 8C. This is a typical TG curve suggest a small grain size of the ª-Ti3O5 phase. For for the carbothermic reduction reaction of ilmenite the 200 h milled sample, after heating to 1100 8C, [10–12]. The decrease in sample weight during the TiN and Æ-Fe phases were detected in the XRD low temperature range was relatively slow and pattern (Fig. 3c). These results clearly indicate that, corresponded to the solid state reduction: Fe- for the milled samples, both carbothermic reduction 3 TiO3 C Fe TiO2 CO (g). The rapid weight and nitridation reaction have a lower reaction loss during the high temperature range was due to temperature and high kinetics. This is consistent gaseous reduction reactions such as FeTiO3 CO with the TG analysis results. (g) 3 TiO2 Fe CO2 (g) with CO generation (the The relation between full nitridation temperature 3 Boudouard reaction): C CO2 2CO and a reduc- and milling time is presented in Fig. 4. The tion of rutile (typically above 1200 8C) to give, temperature for full nitridation was determined by 3 for example: 3TiO2 CO (g) Ti3O5 CO2 (g). using the XRD analysis on the samples firstly milled Comparing these three TG curves, it is evident that for different times and subsequently heated to longer milling time leads to a higher rate of weight different temperatures in a TG apparatus under loss during the low temperature range (solid state flowing nitrogen. This figure indicates that the full reduction) and a lower temperature onset of the nitridation temperature decreases with the increase in second stage (presumably the gaseous phase reac- pre-milling time. tion). The onset temperature (indicated by arrows) of the second stage reduction for samples milled for 0, 50 and 200 h was approximately 1100 , 950 and 850 8C, respectively. The weight loss during con- tinuous heating to 1200 8C was 7.1 wt % and c (a) i i i 18.0 wt % for the 0 h and 50 h milled samples, i i i respectively. A total of 21.6 wt % was lost for the 200 h milled sample during heating to only 1100 8C. c 5.0 (a) (c) 0.0 (b) 25.0 Intensity (arbitrary units) 210.0 215.0 (b) c 220.0 (c) 20 40 60 80 100 225.0 2è (degrees) Variation of sample weight (wt%) of sample weight Variation 230.0 200 400 600 800 1000 1200 Figure 3 XRD patterns for ilmenite–graphite samples milled for different times and subsequently heated in the thermogravimetric Temperature (°C) analyser under nitrogen flow. (a) As-mixed/1200 8C, (b) 50 h milling/ 3 1200 8C and (c) 200 h milling/1100 8C. (), Æ-Fe; ( ), ª-Fe(C) Figure 2 Thermogravimetric curves for ilmenite–graphite mixtures (austenite); (c), graphite, (i), FeTiO3 (ilmenite); (e), TiO2;(s), ª- milled for (a) 0 h, (b) 50 h and (c) 200 h under Ar flow.