Reduction properties of doped hematite pellets.

The objective of the present work was to study the reduction properties of doped hematite pellets, with special reference to the properties demanded by direct reduction (DR) processes, viz: high reducibility coupled with low dimensional changes and high strength during reduction. Iron oxide ores doped with lime, silica and alumina were studied microstructurally using optical and scanning electron microscopy after compaction and sintering. A reduction-under-load apparatus has been designed comprising a strain-gauged cantilver beam at the bottom and a scaled screw-jack at the top which allows the dimensional changes of the doped iron oxide pellets to be followed during reduction under a range of different initial stresses. The reducibility of doped iron oxide pellets has been determined in hydrogen and in a hydrogen/carbon monoxide gas mixture using a thermo-gravimetric spring balance. The results showed that the reduction of iron oxide pellets in hydrogen follows three different modes of reduction, viz: homogeneous, mixed mode and topochemical. These modes are associated with varying degrees of dimensional stability and reducibility. The topochemical mode of reduction exhibits the highest dimensional stability but lower reducibility than the homogeneous mode. On the other hand, the homogeneous mode gives better reducibility but less dimensional stability. In general, lime improves dimensional stability but decreases reducibility, whereas silica and alumina both increase reducibility and decrease dimensional stability. The variation in these properties have been related in this work to the phases present in the pellet microstructure and to their morphology. Reduction in a mixture of hydrogen and carbon monoxide showed that doping agents have similar effects on reducibility as in pure hydrogen but with overall slower rates of reduction.