Mechanically activated carbothermic reduction of ilmenite
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I.
INTRODUCTION
THE mineral ilmenite is a naturally occurring iron titanate (nominally FeTiO3) and is abundant in nature. The commercial grades of ilmenite contain 45 to 658 pct TiO2[1] and are regarded as a huge resource to the production of rutile (TiO2), which can be used directly as pigment or for the manufacture of titanium. However, extraction of the iron from the ilmenite has been, and still is, an expensive undertaking. Metallurgical processing involves high-temperature processing and often hot acid leaching, with associated environmental problems.[2,3] Therefore, the search for an environmentally friendly, cost-effective extraction method remains a strong industrial imperative. In a commonly used extraction process (‘‘Becher process’’),[4,5] ilmenite is reacted with coal in an iron reduction kiln at 1100 7C. This carbothermic reaction reduces the iron in the ilmenite to the metallic form. After cooling the products of the reaction, the iron can be rusted out in slurry form with ammonium chloride acting as a catalyst for rusting. The remaining iron compounds are removed by leaching with sulfuric acid. In order to increase the efficiency of the preceding reduction process, the carbothermic reactions have been extensively investigated.[6–10] The chemical reactions that occur during the reduction process can be conveniently separated into a solid-state reduction (typically from 860 7C to 1000 7C): FeTiO3 1 C → Fe 1 TiO2 1 CO (g)
[1]
a gaseous reduction (typically above 1000 7C): FeTiO3 1 CO (g) → TiO2 1 Fe 1 CO2 (g)
[2]
with CO generation (the Boudouard reaction): C 1 CO2 → 2CO
[3]
and a reduction of rutile (typically above 1200 7C): 3TiO2 1 CO (g) → Ti3O5 1 CO2 (g)
[4]
Y. CHEN, Senior Research Scientist, T. HWANG and M. MARSH, Research Assistants, and J.S. WILLIAMS, Professor and Head of the Department, are with the Department of Electronic Materials and Engineering, Research School of Physical Science and Engineering, The Australian National University, Canberra, ACT0200, Australia. Manuscript submitted March 8, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
The kinetics of these reactions can be illustrated by thermogravimetric curves obtained by heating a mixture of ilmenite and graphite in a thermogravimetric analyzer. A schematic example illustrating typical behavior is shown in Figure 1. The decrease in sample weight in the low-temperature range is relatively slow and corresponds to the solid-state reduction (Reaction [1]) in which oxygen diffusion to the surfaces of ilmenite particles is thought to be the rate limiting step. The rapid weight loss in the hightemperature range corresponds to gaseous reduction reactions (Reactions [2], [3], and [4]). For prolonged, complete reduction at higher temperatures than that in Figure 1, the final stable phase is titanium carbide (TiC). In an attempt to increase the rate of reduction, catalysts, such as alkali metal carbonates, have been used during the reduction process. As a consequence, the reduction temperature can be reduced by about 200 7C.[11]
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