Mechanism of low-temperature reduction degradation of iron ore sinters

The degradation of ferrous materials under conditions of low-temperature reduction is a complex phenomenon. The onset of the process is linked to the gaseous reduction of hematite to a 'porous' form of magnetite, which results in a volume increase and the initiation of cracks. The subsequent propagation of the cracks and further reduction of hematite, which lead to the eventual breakdown of the sinter matrix, are less well understood. It is proposed that reduction degradation can be viewed as consisting of two distinct sub-processes. In the first easily accessible hematite grains are reduced, resulting in the initiation of cracks and their propagation into the surrounding sinter matrix as more of the grain is reduced. The second involves the further growth and branching of these cracks, the reduction of areas of newly exposed hematite grains along the walls of the cracks providing the energy requirements. In experimental work indentation methods were used to determine the vulnerability of sinter phases to crack propagation, and two cases are presented to demonstrate that decreases in the fracture toughness of mineral phases in the sinter result in severe deteriorations in sinter reduction degradation indices. It is considered that the proposed mechanism furthers understanding of the causes of reduction degradation, views on which are many and varied.