Kinetics of reduction of MnO in powder mixtures with carbon
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TRODUCTION
THE reduction of manganese oxide by solid carbon in a porous mixture of particles is the basis of the industrial production of ferromanganese. It also is a case of direct carbothermic reduction of an oxide occurring in a quiescent packed bed or nonflow system.[1] Carbon monoxide is both a product of the overall reaction and a reaction intermediate. An extensive series of experimental measurements of the kinetics of the reaction between solid MnO and graphite has been performed previously.[2] It was found that the solid product is the carbide Mn5C2. Accordingly, the oxide-reduction step is 5MnO 1 9CO 5 Mn5C2 1 7CO2
[1]
The intermediate CO2 then is consumed by the Boudouard reaction, which also regenerates CO: 7CO2 1 7C 5 14CO
[2]
The sum of Eqs. [1] and [2] is the overall reduction reaction: 5MnO 1 7C 5 Mn5C2 1 5CO
[3]
The possible rate-controlling steps in the carbothermic reduction of metal oxides have been the subject of many investigations.[1] The Boudouard reaction, Reaction [2] on the surface of the carbon particles, the diffusion of the CO through the porous product layer forming on the oxide particles, or outward diffusion of the product CO through the pore system of the reaction mixture have been identified as the possible slow steps.[1,18] More recently the present authors have identified ‘‘intrinsic transport’’ as another possible rate-controlling step.[3] This is a slow-diffusion step of
W.J. RANKIN, formerly Director, G.K. Williams Cooperative Research Centre for Extractive Metallurgy, Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3052, Australia, is Market Development Manager, CSIRO Minerals, Clayton, Victoria 3052, Australia. J.R. WYNNYCKYJ, formerly Professor of Chemical Engineering and Extractive Metallurgy, is Professor Emeritus, Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted September 18, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
the gaseous intermediate CO2 within the pore system of the solid reactant mixture between the oxide particles, where it forms by Reaction [1], to the carbon particles, where it is consumed by Reaction [2]. With respect to Reaction [3] specifically, Rankin and van Deventer[2] have obtained comprehensive experimental results on conversion vs time at a series of temperatures, different reactant particle sizes, and initial reactant-ratio conditions. Their tentative conclusion was that the Boudouard reaction was rate controlling. In this article, their experimental results are re-examined by treating their data according to model equations for the aforementioned ratelimiting steps, including the intrinsic-transport model. It is shown that the intrinsic-transport model is applicable to the MnO 1 C system. The latter is able to best account for the observed kinetics in most cases, in particular where the Boudouard reaction fails to do so.
II.
REVIEW OF THEORETICAL BACKGROUND
MnO is a relatively stable oxide, i.e., is relatively low on the Ellingham d
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