Interactions between Magnetite Oxidation and Flux Calcination during Iron Ore Pellet Induration
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TE (Fe3O4) has been an important feedstock for the world steel industry for the last 50 years and will continue to be for the foreseeable future. The oxidation of magnetite is a reaction of great importance for the production of iron ore pellets due to its exothermic nature reducing the fuel requirements for their induration (heat hardening). Due to this fact, there have been many studies of the oxidation of ground magnetite rolled into pellets.[1–5] The common consensus of the literature is that, on their own in an oxidizing environment, magnetite pellets oxidize following a ‘‘shrinking core’’ style reaction above 400 °C. This is due to the diffusion of oxygen to the reaction front through the porous matrix being the controlling resistance. As the temperature increases above 1300 °C, usually accompanied by a drop in oxygen potential in industrial pellet furnaces, the hematite product becomes less stable thermodynamically, and in extreme cases reversion to hematite can occur. The presence of divalent ions such as calcium and magnesium in the magnetite lattice can also retard oxidation as they are incompatible with the corundum crystal structure of hematite.
ANDREW R. FIRTH is formerly Research Scientist, CSIRO Minerals. Contact e-mail: andrew.fi[email protected] JOHN F. GARDEN, Project Scientist, is with CSIRO Minerals, Pullenvale, Queensland, Australia. Manuscript submitted May 25, 2007. Article published online August 5, 2008. 524—VOLUME 39B, AUGUST 2008
Early magnetite oxidation studies[1,2] were performed at a time when the majority of pellets being produced were ‘‘acid’’ pellets. Acid pellets do not have any fluxes, such as limestone, dolomite, or olivine, added to improve their metallurgical properties, and the gangue minerals present were typically quartz and bentonite clay. Over the 1990s, however, almost all pellets produced in the western world had fluxes added, especially carbonate fluxes such as limestone and dolomite. At a small number of deposits, ‘‘self-fluxing’’ of the magnetite concentrate occurs due to the presence of dolomite and calcite in the ore body, but this is rare. Carbonate fluxes generate CO2 as they calcine, especially around 800 °C. The calcination reaction is thermally activated, so it occurs simultaneously across the pellet. This means that CO2 is being generated and released from the fluxed pellet at the same time during firing that O2 is trying to diffuse through the pellets to the oxidation interface. In previous mathematical models of the pellet induration process,[6–8] however, the calcination and oxidation reactions are dealt with separately with no gas interaction assumed in the pellets. In order to test the validity of the assumption that the gas generated during calcination does not affect oxidation, a mathematical model of a pellet undergoing induration has been developed using MATLAB.* This *MATLAB is a trademark of Mathworks Inc., Natick, MA.
model includes both the calcination and oxidation reactions, as well as diffusion of gases within the pore METALLURGICAL AND MATERIALS TRANSACTI
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