Rate of reduction of ore-carbon composites: Part II. Modeling of reduction in extended composites
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I. INTRODUCTION
IN Part I of this series of articles,[1] a new process for ironmaking was proposed using a rotary hearth furnace (RHF) in combination with a smelter. In the new process, composite pellets of iron oxide and carbon would be partially reduced in a rotary hearth furnace, and the final reduction, melting, and gangue separation would be done in the smelting unit. The rate constants for oxidation of wood charcoal and graphite by CO2 and reduction of wustite by CO were measured at temperatures relevant to the RHF. In this article, a model for the reduction of composite pellets is developed taking into account the combined kinetics of carbon oxidation and wustite reduction along with heat transfer and pellet shrinkage. The modeling for this system was done coupling the equations of continuity for solid and gas with an enthalpy balance applied to a control volume inside the pellet. Experimentally, measurements of mass change, pellet size, and composition of off-gas during reduction were done in order to validate the model. The final model was used in a process model of the RHF.[22] A number of models can be found in the literature, which are related to reduction in composites of iron oxides and carbon.[2,3,6–9] The main features commonly found in the reduction models are reaction rate laws, temperature change, gas transport, and changes in size of specimen. A summary of the approaches found in the literature for each of these items is given in Table I. The interested reader is referred to the original authors for the specifics of their individual contributions. In here, only an outline of the major differences in approach between authors is given. O.M. FORTINI, Senior Research Engineer, is with the U.S. Steel Research and Technology Center, Monroeville, PA 15146. R.J. FRUEHAN, Professor, is with the Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: fruehan@ andrew.cmu.edu Manuscript submitted April 7, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
Reaction rate laws: The generally accepted mechanism of reduction in composites of iron oxides and carbon consists of two elementary reaction steps: carbon oxidation and reduction of iron oxides. The two steps were discussed in Part I of this series of articles. Consistent with this reaction mechanism, Rao[2] and Tien and Turkdogan[3] presented models for reduction in composites where the overall kinetics is controlled by the oxidation of carbon. This should be the case at lower temperatures, where rates of reduction of iron oxides are considerably faster than rates of carbon oxidation. Rao used a rate law in the original form derived from the mechanisms of Gadsby et al.[4] and Reif,[5] while Tien and Turkdogan employed a simplified version to account for the effects of poisoning by CO. Sohn and Szekely[6] suggested the use of a form similar to that used by Tien Turkdogan with the addition of a reversible term for both steps of carbon oxidation and reduction of the iron oxides. In the modeling of reac
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