Kinetics of reduction of hematite with hydrogen gas at modest temperatures
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I.
INTRODUCTION
IN direct
reduction processes for ironmaking, the iron oxides are reduced to metallic iron without entailing formation of liquid phases. In a broader sense the term "direct reduction" has been used to designate any ironmaking process other than the conventional blast furnace. The blast furnace method of producing iron is at present the most important because of its ability to produce large tonnages of metal at a comparatively low cost. However, because of increasing cost of constructing a modem blast furnace, lack of good quality coking coal, the availability of alternate fuels like natural gas, and finally the inflexibility in size attending a blast furnace, it has become more attractive to install direct reduction facilities in many parts of the world. It has been reported I that, with plant capacities of less than a million tons, some 33 million tons of direct-reduced iron was produced in 1980. This phenomenal increase from the 3 million tons produced in 1970 is due to the distinct advantages possessed by the direct reduction processes in the choice of fuel and installation size. Although there are many different direct reduction processes, only a few of them have been commercialized. The most notabt.e among these are: Midrex, Armco, FIOR, HYL, PUROFER, and SL/RN. Except for the SL/RN process which uses a noncoking coal, all other processes use reformed natural gas. In the gas-based direct reduction processes, the reduction of iron oxides is performed mainly by hydrogen gas and in the case of coal-based direct reduction processes, carbon monoxide is the primary reducing agent. The complexity of kinetics and mechanism of reduction of iron ores has attracted the attention of many researchers worldwide and this has led to numerous publications concerning experimental results and mathematical models attempting to interpret the results. In gas/solid reactions involving a porous solid reactant the mass-transfer processes play an important role and tend to influence significantly the kinetics of reaction especially at elevated temperatures. Thus, under such conditions it is difficult if not impossible to measure the intrinsic rate of the reaction under considY.K. RAO, Professor, and M. MOINPOUR, Graduate Research Assistant, are with the Department of Mining, Metallurgical, and Ceramic Engineering, University of Washington, Seattle, WA 98195. Manuscript submitted May 23, 1983. METALLURGICAL TRANSACTIONS B
eration, and the activation energy derived from measured rates will deviate significantly from the true value for the reaction. For the present purposes, the intrinsic rate of a heterogeneous reaction is defined as that rate which is free of mass-transfer effects. That is, the intrinsic rate is simply the rate observed when the reactant gas concentration at the reaction interface is identical to that existing in the bulk gas; in other words, the concentration gradients in the gas phase are virtually nonexistent. Data on intrinsic kinetics are far more valuable as compared to conventional rate measur
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