Phase Equilibria and Slag Formation in the Magnetite Core of Fluxed Iron Ore Pellets
Andrew Robert Firth, John Frederick Garden, Jeff Donald Douglas
Distinctly different morphologies of pellet microstructure develop in the oxidised hematite shell and the reduced magnetite core during induration. Individual pellet induration experiments in an air atmosphere showed that slag formed at lower temperatures in the magnetite core, with molten slag forming between 1100 and 1150°C compared to 1200°C in the oxidised outer rim. The amounts of slag formed were not large enough for accurate compositions to be determined, however. Experiments were then conducted with pellet sized magnetite compacts, held at a temperature between 1075 and 1225°C for 20 min in a nitrogen atmosphere containing 0.5% oxygen (pO2=0.005 atm). The aim was to retain magnetite as the stable iron oxide and to generate larger pools of molten slag. These tests showed that a eutectic iron calcium silicate melt can form within the silica particles at temperatures as low as 1100°C. Surrounding these silica particles, a melt-like morphology appears, although the remaining slag has a wollastonite (CaSiO3) composition. As wollastonite is not molten at the temperatures studied the iron oxide from the slag would have been absorbed by the surrounding magnetite. Above 1175°C, the molten eutectic slag can be observed to fill in gaps between the magnetite particles after dissolving nearly all the calcium and silicon. This behaviour suggests that the oxygen partial pressure varies greatly within the magnetite core of iron ore pellets and has a strong influence on slag formation. Formation of slag in iron ore pellets is important as sufficient slag must be formed to bond the pellet, but not so much that the core becomes too heavily sintered, pulling away from the shell and reducing pellet strength.
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