Reduction of titania by methane-hydrogen-argon gas mixture
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I. INTRODUCTION
TITANIUM -containing minerals are widely distributed and abundant. With an estimated average concentration in the earth’s crust of 0.6 pct, titanium is 60 times as abundant as copper or nickel.[1] Titanium is mainly utilized in the form of titanium dioxide as white pigment, and the demand for titanium dioxide pigment is growing steadily.[2] Titanium minerals are also processed to produce metallic titanium and titanium compounds, particularly titanium carbide and nitride, which are used in the manufacture of composite materials or as catalysts. Titania ferrous ores, which are found throughout the world, often in large deposits,[3,4] are becoming a viable alternative source of iron ores as conventional iron ore reserves are dwindling. This stimulates an interest in research in the processing of the titania-containing minerals. Titania is reduced by methane-containing gas to titanium carbide, which can be a valuable product itself or processed further to produce sponge titanium. Titanium carbide is easier to chlorinate than titania. Titanium tetrachloride can be formed from titanium carbide at low temperature, 350 to 420 K.[5] Nieberlein showed[6] that carbothermally reduced titanium minerals that contain titanium carbide and suboxides can be chlorinated easily at 670 to 770 K. Synthesis of titanium carbide can be implemented using different technologies. Commercially, it is primarily produced by reduction of titania with carbon (carbothermic reduction) in a temperature range of 1970 to 2370 K.[7] However, this reaction proceeds extremely slowly due to inadequate contact area between solid particles, and practically does not proceed to completion. A much faster process involves a direct carburization of metallic titanium or titanium hydride by combustion synthesis (self-propagation).[8] However, this method is expensive because of the cost of
GUANGQING ZHANG, PhD Student, and OLEG OSTROVSKI, Associate Professor, are with the School of Materials Science and Engineering, the University of New South Wales, Sydney 2052, Australia. Manuscript submitted March 8, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
metallic titanium, which is used as a charge material. Titanium carbide can also be produced by the gaseous pyrolysis of TiCl4 in a carbon-containing atmosphere (chemical vapor deposition technique). This process is not only expensive, but also very corrosive.[9] Therefore, development of a new, efficient process for synthesis of titanium carbide is of great importance. Oyama et al.[10] reduced titania into titanium carbide by hydrocarbons using temperature-programmed synthesis. They demonstrated that the temperature-programmed synthesis of refractory carbides is superior to traditional isothermal methods, and that the synthesis temperature for groups IV through VI carbides and nitrides is approximately two-thirds the melting temperature of the parent oxide (2145 K for TiO2[11]). They used the only gas composition, 95 vol pct H2 and 5 vol pct CH4. The aim of the work of Oyama et al. was to examine t
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