Phase transformations during fluidized bed reduction of New Zealand titanomagnetite ironsand in hydrogen gas
Direct reduction of iron ore in a fluidized bed reactor is usually limited by the onset of particle sticking at temperatures ≳ 800 °C. This paper examines the phase and microstructural evolution of New Zealand titanomagnetite (TTM) ironsand during fluidized bed reduction by hydrogen gas at temperatures ranging from 750 °C to 1000 °C. All samples were characterized using quantitative X-ray diffraction and scanning electron microscope/energy dispersive spectroscopy. No sticking was observed at any point during this series of experimental reduction reactions. At higher temperatures sticking appears to be prevented by the formation of a Ti-rich oxide shell around each partially-reduced particle. At lower temperatures, no shell is observed to form, but temperatures are not high enough to drive particle sticking/agglomeration. The reduction mechanism appears to vary within different temperature regimes. At high temperatures (950 °C and 1000 °C), the reduction of TTM proceeds via the wüstite phase during the initial reduction stage, although the TTM is not completely converted to wüstite. The formation of wüstite results in an enrichment of Ti within the residual TTM phase. This is observed as an increase in the TTM lattice parameter, which is stretched by Ti substitution into the spinel. For reactions at low temperatures (750 °C and 800 °C), the TTM is directly reduced to metallic iron without any evidence of the appearance of the wüstite phase. At ‘intermediate temperatures’ (850 °C and 900 °C), a mixed behaviour is observed with the appearance of low levels of short-lived wüstite during the reduction reaction. Overall, these results indicate that the fluidized bed processing of NZ TTM ironsand in hydrogen can be completed over a wide temperature range without incurring sticking.
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