Kinetics of Reduction of Iron Oxides and Ores by Carbon

ITIHEORETICAL considerations and experimental findings point out that reduction of iron oxides and I ores by carbon proceed via the gas' phase, whenever the latter is present. The overall reaction is a coupling of the following two component reactions, viz., Gasification of carbon C(S) CO2 (g) = 2C0 (g) and Reduction of iron oxide mFe.0y(S) -fCO (g) = nFezOw [S) CO2 (g) ' Recent kinetic studies have established that the gasification reaction controls the rate of the overall process under the circumstances normally encountered. A critical evaluation of these studies have be en presented. The available information on the kinetics and mechanism of the two component reactions have also been critically reviewed. On the basis of the above, first of all the scope of further work in this area have been assessed. and. finally the productivity of the rotary kiln process of sponge iron making has been examined. Reduction of iron oxides and ores by carbon is a key metallurgical reaction. It was postulated in the nineteenth century that both "indirect" and 'direct' reduction were taking place inside an iron blast furnace. The former was meant to denote reduction by carbon monoxide, and the latter stood for reduction of iron oxides directly by carbon as follows. FeO C = Fe + CO which takes place principally in the lower portion of the stack. The "direct" reduction as per equation (1) may be called carbothermic reduction too. For quite sometime the interest in it lay in connection with blast furnaces only. Of late however, a number of alternative iron making processes. such as rotary kiln processes, electric pig iron process etc. are employing carbon reduction of iron oxides and ores. The objective of this review is: (1) to present the salient features of the kinetic aspects of reduction of oxides of iron by carbon. (2) to critically assess the situation based on the above. In doing so, the review as such shall be restric ed to the fundamental investigations alone. The reduction is kinetically a complex process and there are many contradictions, controversies etc. An effort has been made to focus on these if they are important and to resolve them as much as possible. I. TRUE DIRECT REDUCTION VERSUS REDUCTION VIA GASEOUS MEDIUM : Kinetically, a solid-solid reaction is expected to be much slower compared to a gassolid one for two reasons. First of all, the solidsolid contact area is much smaller compared to gas solid contact area. Secondly, solid-state diffusion is much slower compared to mass transfer in gases. Therefore, it was recognized even in early days that the overall reaction is perhaps taking place in two stages, viz. Reduction of iron oxides, which can be generalized as follows: mFexOy (S)-ICO (g) = nFezOw (S)-1-0O2(g) —(2) and, gasification of carbon: C(S) CO2 (g) = 2C0(g) —(3)