Rate of reduction of ore-carbon composites: Part I. Determination of intrinsic rate constants
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I. INTRODUCTION
THE productivity and energy efficiency of the iron blast furnace have increased significantly in the past 30 years. Energy consumption has been reduced from 40 GJ/tonne hot metal in the 1960s to less than 20 GJ/tonne in the 1990s. However, it still requires coke, produces about 1.5 tonnes of CO2/tonne hot metal, and is highly capital intensive. If wood charcoal is used for iron production, net CO2 and sulfur emissions can be significantly reduced. CO2 is converted back to carbon and oxygen during tree growth and wood charcoal contains little sulfur. Wood charcoal is not appropriate to use in large blast furnaces of high productivity due to its low strength and density; commercial blast furnaces using wood charcoal are limited to 300,000 tonnes/year. A new process has been proposed using composite pellets or mixtures of wood charcoal and ore fines in a rotary hearth furnace (RHF), which are melted in a smelting unit, as schematically shown in Figure 1. In the new process, composite pellets would be prereduced up to 75 to 80 pct metallization in the RHF and charged into a smelter for final reduction, gangue separation, and melting. This process overcomes the RHF drawbacks of low productivity and high gangue in the product by doing final reduction, melting, and gangue separation in the smelting unit. It overcomes the drawback of smelters, energy generation, by preheating and doing most of the reduction in the RHF. In order to fully evaluate and optimize this process, the rate of reduction, energy requirement, and gas generation in the RHF must be known. The overall objective of this research is to develop and verify a model to compute the rate of reduction of ore-carbon pellets or mixtures including wood charcoal. The overall 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 February 19, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
pellet model presented in Part II of this series of articles includes the rates of oxidation of the carbonaceous materials, reduction of wustite, heat transfer to and within the pellet, shrinkage, and other factors. In this article, the results obtained for determining the two rate constants for different types of oxide and carbon are presented. This model will then be used in a simulation of the RHF to evaluate and optimize the proposed process. In theory, the rate of oxidation of carbon particles by CO2 can be determined directly. However, in order to avoid masstransfer effects, high gas velocities are necessary and it is never possible to ensure that such effects did not play a part in a given experiment. Moreover, at the high gas velocities necessary at elevated temperatures, the small powders of carbon and wustite would be carried away with the gas stream. Similarly, the direct measurement of the rate of re
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