Application of thermodynamic and kinetic principles in the reduction of metal oxides by carbon in a plasma environment
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INTRODUCTION
ENERGY consumption by the iron and steel industry alone amounts to approximately 11 pct of the world's total energy consumption. 1 Furthermore, if it is also taken into account that oxide reduction smelting processes use mainly higher quality organic fuels, i.e., coking coals and natural gas, the development of alternative technologies which rely on more abundant lower grade coals merits careful consideration. One such technology which has emerged recently and which could in the future prove to be an alternative to conventional ferrous and nonferrous smelting processes is plasma reduction and smelting. By using plasma in metallurgical processes, the energy required to initiate the metallurgical reaction, e.g., heat energy required to attain the minimum reduction temperature, is separated from the energy required to sustain the carbothermic reduction of the metal oxides. Thus, the application of plasma technology provides an extra degree of freedom which is not available within conventional metallurgical processes. However, the high cost of electrical energy must be counterbalanced by fully utilizing the improved reaction kinetics potentially available in a plasma environment. The application of thermal plasma systems to the production of iron, steel, ferroalloys, and acetylene has been the subject of intensive research and engineering developments over the past decade. These developments have been reported in the literature in the form of both review articles and technical publications. 2-7 The attraction of plasma as a processing alternative lies in (i) its independence of oxygen potential, (ii)its flexibility in using low grade reductant materials, (iii) high energy density processing with resultant smaller reactor vessels, and (iv) excenent pollution control. K. UPADHYA, formerly Postdoctoral Fellow with Mineral Research Center, University of Minnesota, is Assistant Professor, University of Illinois, Chicago, IL. J. J. MOORE, Professor and Associate Director, and K.J. REID, Professor and Director, are with the Mineral Resources Research Center, University of Minnesota, 56 East River Road, Minneapolis, MN 55455. Manuscript submitted February 7, 1985.
METALLURGICALTRANSACTIONS B
Due to points (iii) and (iv) above, there is a strong potential for greatly decreased capital cost compared with existing reduction smelting technology. What is plasma? A gas at room temperature consists of molecules, each usually having two or more atoms combined. When the gas is heated sufficiently, the molecules begin to dissociate into individual atoms. At still higher temperatures ionization begins and a plasma state is reached. To sustain a stable plasma, a small degree of ionization is required, e.g., 3 pct, and the plasma is therefore electrically conducting. The electrical conductivity of a plasma medium is close to that of a molten salt or slag phase, i.e., 10 to 102 ohm -1 cm -1 and is four orders of magnitude less than that for solid metals, i.e., 106 ohm-1 cm-l. Diatomic gases such as hydrogen and nitrogen,
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