Mechanically enhanced carbothermic synthesis of iron-TiN composite

Abstract

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 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 [4]. It has been reported that titanium nitride can be synthesized by reduction of titanium dioxide with carbon in a nitrogen atmosphere at 1150 8C [5]. In the work reported here, an assessment of the production of iron–TiN composite at a relatively low annealing temperature was made with an ilmenite–carbon mixture by using a pre-ball milling treatment and subsequent annealing process. The advantages expected of this production process are the inexpensive nature of the raw materials (ilmenite and graphite) and the one-step carbothermic reduction and simultaneously nitridation reaction, thus reducing the number of process steps. In this experiment, an upgraded ilmenite powder with a particle size of 200–300 mm was supplied by Westralian Sands Ltd. The chemical composition of this ilmenite was as follows: FeO, 20.8 wt %; Fe2O3, 20.0 wt %; TiO2, 53.6 wt % and impurities (MnO, SiO2, ZrO2, etc.), 3.5 wt %. A graphite powder with a purity of 99.8% or better was used as reducing agent. The weight ratio of ilmenite to graphite was 1:1, which gave excess carbon for full reduction of ilmenite to metal titanium and iron. Ball milling was performed at room temperature in a vertical planetary ball mill (ANUtech Pty Ltd, Canberra) using hardened steel balls with a diameter of 25.4 mm and a stainless steel cell. The cell was loaded with several grams of materials together with five balls and evacuated to vacuum (10y2 kPa) prior to milling. After milling, the structure of the samples was investigated by X-ray diffraction (XRD) using Co radiation (o ˆ 0.1789 nm) at room temperature. The thermal analysis was carried out in a thermogravimetric (TG) analyser where a sample was heated at a rate of 20 K miny1 under dry nitrogen flow (80 ml miny1). After each run the sample was left to cool to room temperature in the apparatus while the nitrogen flow was continued. An isothermal treatment was conducted in a thermal tube furnace under a dry nitrogen flow (60 ml miny1). Typical XRD patterns after milling and annealing are presented in Fig. 1. Fig. 1a shows that both ilmenite and graphite structures remained after 200 h of milling. However, the low intensity and the broadened shape of the graphite and ilmenite diffraction peaks suggest small crystallite size and a highly disordered structure. Indeed, it has been reported that grinding of graphite can lead to the structure disordering [6, 7] or amorphization [8]. The average grain size of the ilmenite crystallite estimated from the peak broadening by using the Scherrer formula [9] is about 45 nm, indicating a nanocrystalline structure of the milled ilmenite. Finally, no new diffraction peaks were found in this XRD pattern, suggesting that no chemical reaction occurred during the milling process. However, as shown later, the observed dramatic structural changes in both graphite and ilmenite structures strongly affect the thermal behaviour during the subsequent annealing processes. After annealing of the as-milled sample at 800 8C for 1 h under flowing nitrogen, the ilmenite phase was no longer detected by XRD (Fig. 1b) and essentially only rutile (TiO2) and iron solid solutions (Fe(C)) were obtained. The main iron solid solutions are AE-Fe(C) ‡ austenite Fe(C). Further annealing of the as-milled sample at a higher temperature (1000 8C) for 1 h leads to the formation of titanium nitride (TiN) phase, as indicated by the XRD pattern (Fig. 1c). Excess graphite was still detected in the XRD patterns (as represented by the weak diffraction peak c). Hence,