The influence of carbon deposition on the reduction kinetics of commercial grade hematite pellets with CO, H 2 , and N 2
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INTRODUCTION
THE purpose of this investigation
is to examine the rate at which commercial grade hematite pellets react with CO, CO2, and H2 mixtures over the temperature range 500 ~ to 1000 ~ which corresponds to the region where carbon deposition onto the solid surface may accompany the reduction process. Problems of this type are of obvious technological interest, because of their relevance to both the direct reduction processes used for sponge iron production and also to the operation of the iron blast furnace. In direct reduction systems carbon deposition may be desirable under certain circumstances; but in any case it would be helpful to define the precise conditions under which such carbon deposition would take place at sufficiently fast rates to have an influence on the overall operational characteristics of the system. Under other circumstances carbon deposition may inhibit the reduction process and may thus be undesirable. The decomposition of CO and attendant carbon deposition may also be thermodynamically feasible in the iron blast furnace. Regarding previous work, the reduction kinetics of hematite pellets with various reducing gases has been extensively studied and will not be reviewed here.~'2'3 The rate of carbon deposition from CO/CO2 mixtures has been studied by numerous investigators, including Schenck et al.,~ Turkdogan and Vinters, 2 and othersfl ~ It is noted that most of these researchers studied carbon deposition kinetics onto metallic iron surfaces, which N. TOWHIDI is Professor and Head, Department of Metallurgy, University of Teberan, Teheran. Iran. J. SZEKELY is Professor of Materials Engineering and Associate Director of the Center for Materials Processing, Massachusetts Institute of Technology, Cambridge, MA 02139. The work was done while Dr. Towhidi was a Visiting Scientist at MIT. Manuscript submitted November 3, 1981.
METALLURGICAL TRANSACTIONS B
have been prepared either by direct reduction or by some other means. There appears to be general consensus among the investigators that the range of gas compositions and temperatures over which carbon deposition occurs does indeed correspond to that defined by thermodynamic considerations. However, the actual rate at which carbon deposition occurs was found to be markedly influenced by both the nature of the solid iron surface (composition and mode of preparation) and the possible impurity content of the gas. ~.2 Furthermore, there appears to be general acceptance of the views, first put forward by Schenck,' that the formation of an iron phase which would act as a catalyst is a necessary pre-condition of extensive carbon deposition. Furthermore, Haas et al. 3 and Gaballah et al. 4 found that even small quantities of hydrogen have led to a dramatic increase in the carbon deposition rate. Another interesting point should be noted, that as suggested by Turkdogan, on carbon deposition onto iron most of the carbon could be converted into cementite, and since cementite would not be a catalyst for the decomposition of CO, this could inhibi
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