Hydrogen Reduction Kinetics of Magnetite Concentrate Particles Relevant to a Novel Flash Ironmaking Process
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A novel ironmaking technology is under development at the University of Utah. This technology produces iron directly from magnetite concentrate by gas-solid flash reduction. Hydrogen is the main reducing agent in this process that takes place at temperatures above 1473 K (1200 °C). The direct use of concentrates allows bypassing the problematic pelletization/sintering and cokemaking steps required in blast furnace ironmaking. The research on the gaseous reduction of iron oxide in the form of a pellet or sinter the size of which is typically much larger than the size of the single concentrate particle has been widely studied. Themelis and Gauvin[1,2] derived a rate equation from the published data of twenty independent studies on the reduction of iron oxide particles by hydrogen with particle sizes ranging from 70 to 42,000 lm in diameter. Those data were obtained at temperatures of 873 K to 1273 K (600 °C to 1000 °C) under the conditions of negligible boundary layer resistance to gas transport. The global rate equation covering particles of this wide size range, represented by the shrinking-core rate equation, had an activation energy value of 12.6 kJ, which is rather low for chemical reaction, indicating that the measured rates might contain mass transfer effects. However, less work has been done on the reduction of oxide particles in the size range of magnetite concentrates. In one study, the hydrogen reduction of ferric oxide particles made from carbonyl iron varying in size from 5 to 102 lm was conducted in a gas conveyed HAITAO WANG, Graduate Student, and H.Y. SOHN, Professor, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] Manuscript submitted April 6, 2012. Article published online October 18, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
system.[3] The experiments were carried out in the temperature range of 773 K to 1373 K (500 °C to 1100 °C). The results for the larger particles confirmed the validity of the rate equation of Themelis and Gauvin.[1,2] However, for particles below 31 lm, the experimental data did not satisfy the above-mentioned rate equation and in fact the rate decreased with particle size, contrary to the size effect expected by the shrinkingcore rate equation. Ozawa and Tanaka[4] further studied the hydrogen reduction of crushed fine hematite ore in a gas conveyed system. The hematite ore contained 63.5 pct total iron and the mean particle diameter was about 18 lm. The reduction degree in 5-seconds time ranged from 20 to 80 pct with different feeding rates at 973 K to 1173 K (700 °C to 900 °C). The rate depended on the square of the unreduced fraction, and the rate constant decreased linearly with an increase in the partial pressure of water vapor. Others investigated the kinetics of reduction of magnetite by pure hydrogen and the mixture of hydrogen and inert gas (He, N2, and Ar) in the temperature range 511 K to 993 K (238 °C to 720 °C).[5–7] Extremely thin (89 lm) magnetite specimens were used in
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