Investigations on the carbothermic reduction of chromite ores
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RODUCTION
FERROCHROMIUM is produced by the reduction of chromite ore by carbon in the submerged-arc furnace (SAF). The kinetics of reduction of the ore is being extensively studied in many laboratories. Prereduction of the ore outside the SAF can significantly reduce the power consumption in the process. This reduction is usually carried out in rotary kilns using solid carbon-bearing materials. One of the advantages of this process is that cheaper sources of carbon can be used for reduction, whereas coke is used in the SAF for reduction. The reducibility of the ore under various conditions has to be understood to operate this process successfully. Reduction in specific power consumption, reduction of specific coke rate, and increased recoveries of metallic in the SAF are major aims of ferrochromium producers. A good understanding of the solid-state reduction process is vital for accomplishing these objectives. II. REDUCTION OF CHROMITE ORES The chromite ore is a spinel with the octahedral and tetrahedral sites occupied by cations. The octahedral sites are occupied by trivalent ions such as Cr3, Al3, and Fe3. The tetrahedral sites are occupied by divalent ions such as Fe2, Mg2, and Ca2. The relative proportions of the various cations and the oxidation state of iron in the ore vary from source to source. The nature of the gangue, the oxidation state of iron, and the (Cr/Fe) ratio are some of the major factors that distinguish the reducibility of various ores. The reduction of chromite ore, in general, can be represented by Cr2FeO4 4C → 2Cr Fe 4CO
[1]
DOLLY CHAKRABORTY, formerly with the National Metallurgical Laboratory, a Junior Research Fellow, Jamshedpur 831007, India. S. RANGANATHAN, Scientist, is with the National Metallurgical Laboratory, Jamshedpur 831 007, India. Contact e-mail: [email protected] S.N. SINHA, Professor, is with the Department of Metallurgical Engineering and Materials Science, National Institute of Technology, Jamshedpur, India. Manuscript submitted September 1, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
However, the reduction can occur either through direct interaction between the solid carbon and the ore particle or through gas/solid reaction. In the latter case, the reactions are represented by M 2O3 3CO → 2M 3CO2
[2]
3C 3CO2 → 6CO
[3]
It is possible that the carbide, rather than the metal, is formed as a product of reduction. Several investigations have been carried out to understand the reduction of chromite ore by carbon. The reduction process is controlled by the nature of the ore as well that of the reducing agent apart from other experimental conditions such as temperature, particle size, etc. Sundar Murti et al.[1] studied the reduction of synthetic chromite by carbon at 1423 to 1573 K. The authors concluded that reduction was controlled by diffusion of oxygen in the chromite. The same authors studied the reduction of a chromite ore[2] at 1513 to 1583 K. In this case, the reduction process was found to be controlled by the diffusion of oxygen in
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