Influence of sintering conditions on the reduction behaviour of pure hematite compacts

This paper is centered upon possible effects of the peak temperature attained during sintering upon subsequent reduction of hematite. To this end, pure hematite compacts were prepared and sintered at various temperatures in the 1100–1400 °C range, and analysed microscopically before and after reduction. The microstructure undergoes marked changes during high temperature sintering (1370–1400 °C). This change was found to be associated with magnetite formation and subsequent re-oxidation during cooling. Fine cracks (discontinuities) developing along grain boundaries and the coarse pores and irregular shape of re-oxidised hematite, associated with the earlier formation of magnetite, influenced reduction behaviour particularly at 550 °C, resulting in expansion and severe loss of strength. In isothermal reduction tests of pure hematite tablets conducted between 400 and 1200 °C, four types of reduction behaviour were observed. Expansion and strength after reduction were both affected by such changes of the reduction mechanism. Reduction behaviour was influenced by both the reduction temperature and nature of the microstructure prior to reduction.