Magnetite Particle Size Distribution and Pellet Oxidation
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MAGNETITE concentrates are widely used to produce hematite pellets for ironmaking. During firing, green pellets (magnetite concentrate with bentonite binder, rolled into approximately spherical pellets) are heated and exposed to oxygen-containing gas (using a grate-kiln process in many cases). Firing oxidizes magnetite to hematite. Pellet strength relies on the physical bonds formed between oxidized particles; fine outgrowths form on the oxidized particle surfaces,[1] favoring formation of bonds between particles. One possible cause of poor pellet strength (after firing) is the incomplete oxidation of pellet cores. Prediction of the size of unoxidized cores would require accurate information on magnetite oxidation kinetics. The oxidation of magnetite pellets at temperatures up to approximately 1200 K (or approximately 900 °C), in air or atmospheres containing similar oxygen concentrations, is controlled or partially controlled by solid-state diffusion through a hematite product layer that forms in the magnetite particles during oxidation.[2–4] Prediction of the oxidation of magnetite pellets under mixed control, where gaseous diffusion of oxygen into pellets can be partially rate-limiting, is the topic of a companion paper.[5] This article focuses on the oxidation kinetics of concentrate particles in the absence of gas mass transfer limitations, and the effect of particle size distribution on overall oxidation kinetics. Particle size effects in oxidation of magnetite pellets were previously studied by Forsmo et al.,[6] who found that finer concentrates did oxidize faster (during continuous HYEON JEONG CHO, formerly a Graduate Student with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA, is now Assistant Manager with the Technical Research Center, Hyundai Steel, Dangjin, South Korea. MING TANG, Research Assistant, and PETRUS CHRISTIAAN PISTORIUS, Professor, are with the Department of Materials Science and Engineering, Carnegie Mellon University. Contact e-mail: [email protected] Manuscript submitted December 13, 2013. Article published online July 9, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
heating in 16 pct O2). However, gaseous oxygen mass transfer undoubtedly limited the observed oxidation rate in the work of Forsmo et al.,[6] as shown by a dependence of oxidation extent on oxygen pressure. The work presented in this article specifically aimed to eliminate gaseous oxygen transfer as a rate-limiting step when considering particle size effects. To a first approximation, the effect of particle size on oxidation rate (in the absence of limitation by gaseous oxygen mass transfer) can be described by parabolic kinetics.[7] If each particle were approximately spherical, and hematite would form a continuous product layer within each oxidizing particle, then oxidation would be described by simple shrinking-core kinetics, as follows: i h ½1 t ¼ d2 = 24kp 3 2f 3ð1 fÞ2=3 where t is the oxidation time, d is the magnetite particle diameter, kp is the parabolic rate cons
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