Mathematical Modeling of the Kinetics of Carbothermic Reduction of Iron Oxides in Ore-Coal Composite Pellets
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INTRODUCTION
IN the search for a new ironmaking process to replace the blast furnace, the goal is sustainable development,[1,2] to be achieved by the elimination of coke ovens and high-temperature agglomerations in the interest of environmental protection and economic efficiency. From the points of view of economics and availability, this new process should be based on coal as the source of energy and reductant, rather than natural gas, oil, electricity, etc. In the 1970s, it was hoped that smelting reduction processes in which fine ore and coal could be used directly without high-temperature preparations would realize such dreams. After several costly tests of different designs at pilot-plant scales, the common drawbacks appeared to be a high coal rate, difficulties in process control, refractory corrosion, and potentially high capital costs. In the last decade, ironmakers have focused their attention on rotary hearth furnace (RHF) operations by recycling waste oxides generated in steel plants, but had limited success. Coal- and iron-bearing materials are mixed in proper proportions and agglomerated at ambient temperature, without firing. This satisfies the basic requirement of a new process using ore and coal without high-temperature preparatory steps. At the present time, it seems that an ore-coal composite agglomerate appears to be the favorite starting material in new processes designed by individual inventors.[3,5] KANG SUN, formerly Research Associate, with the Department of Materials Science and Engineering, McMaster University, presently Technical Specialist, with RTI International Metals, Inc., 1000 Warren Ave., Niles, OH 44446. Contact e-mail: [email protected] W.-K. LU, Professor Emeritus, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada. Manuscript submitted October 30, 2006. Article published online January 9, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
Up to the present time, most kinetic studies of iron ore reduction carried out at the laboratory scale are designed with the reduction of indurated pellets of iron ore with the blast furnace in mind. The typical procedure of study begins by measuring the kinetics of reduction using the weight-loss method under isothermal conditions and a steady flow of a reducing gas of a constant composition. The second step is proposing a mechanistic model and mathematical rate expressions to define kinetic parameters. Next, by finding the best fit of the theoretical curve and experimental data points, the values of kinetic parameters are extracted from the raw data and compared with those reported in the literature. It is obvious that the same approach could not be used for kinetic studies of the reduction of iron oxides in orecoal composite agglomerates. There are many solid phases present and very large and changing temperature gradients in the system. The number of chemical reactions is too great to track and to measure. In Sun’s doctoral thesis, he has attempted to investigate this complex system.[4] He simplified
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