Effects of mechanical activation on the reduction behavior of hematite concentrate

The effect of mechanical activation on the reduction behavior of a hematite concentrate has been examined using a combination of simultaneous thermal analysis (STA), X-ray diffraction (XRD), scanning electron microscope (SEM) analysis, and laser diffraction. The samples were activated in vibratory and planetary mills. Differential thermal analysis (DTA) and thermogravimetric (TG) analysis revealed that reduction of mechanically activated and initial hematite proceeds stepwise (Fe2O3 -> Fe3O4 -> Fe). The hydrogen reduction of mechanically activated samples initiates at low temperatures compared with the initial sample. The beginning temperature (onset) of the reduction decreases from 421 degrees C in the initial sample to 330 degrees C in the mechanically activated sample, depending on the grinding intensity. Further, the reduction of hematite to magnetite in the activated samples is more pronounced due to mechanical activation. At low temperatures, the activated samples give a higher degree of conversion than the initial samples regardless of which milling device is used. Hematite reduces completely to iron metal. A comparison of mill-type effect based on stress energy (specific grinding work) suggests that the mill-type effect is confined by a stress energy of 4300 kJ/kg. After releasing 4300 kJ/kg energy, mechanical activation by the planetary mill brings about a larger decrease in onset temperature and a slightly higher degree of conversion at lower temperatures than does activation by the vibratory mill for a given stress energy. A direct relationship between the reaction characters at lower temperatures and structure sensitivity character (S/X) and stored energy can be identified after releasing 4300 kJ/kg energy. However, partial sintering of material at higher temperatures during the reduction of the mechanically activated hematite became active, and the effects of disordering of the hematite structure vanished and subsequently the reduction reaction was retarded. (c) 2006 Elsevier B.V. All rights reserved.