Effect of low concentrations of silica, alumina, lime, and lithia on the magnetic roasting of hematite

Abstract

The conversion of hematite to magnetite was investigated in a mixture of H2, H2O, and N2. In the temperature range of 500° to 700°C, the magnetic roast reaction gives a sigmoidal kinetic curve with a finite induction time. The induction time decreases with rise in temperature and increases in the presence of alkali and alkaline earth oxides. The magnetic roast reaction was also studied in the presence of low concentrations of silica (quartz structure) and alumina (5.0 and 4.3 wt pct, respectively, which correspond to 8.4 × 10-4 g atom of Si or Al per gram mixture). The addition of SiO2 to hematite decreases the induction time. At temperatures below 550°C, alumina increases the induction time; at higher temperatures it has about the same effect as silica. For pellets containing SiO2, a maximum in the relative decrease in induction time, A, was observed at 578°C; for pellets containing A12O3, there was a steady increase in A with increasing temperature. Because the α β quartz transition occurs at 575°C, the enhanced surface activity at 578°C in the presence of quartz is attributed to the Hedvall effect of solid-state chemistry. The induction period of the magnetic roast reaction was exceptionally prolonged in the presence of lithia. Mixing of hematite with silica, alumina, and lithia (8.4 × 10-4 g atom, respectively, per gram mixture) was found to eliminate the beneficial effect of quartz by inhibiting its α β transformation.