Cracking of hematite crystals during low temperature reduction into magnetite

Degradation of iron ore sinters in the upper part of the blast furnace has often been attributed to the reduction of hematite to magnetite, but fundamental investigation of the interaction between mechanical and chemical parameters is still lacking. We therefore have devised suitable experiments with natural hematite crystals, purified from its gangue with preliminary treatments. Reductions were carried out in a thermal balance with CO/CO 2 at 350°C ⩽ T ⩽ 600°C, and the morphology of partially reduced crystals was examined with scanning electron microscopy (SEM) and optical microscopy. The reaction at 350°C and CO/CO 2 = 20 80 , involves heterogeneous nucleation with approximately spherical, cap-shaped nuclei, and cracking of hematite crystals with preferential reaction along the new surfaces obtained by these fractures. The process is very temperature sensitive; a map is given in the ( T, CO/CO 2) plane: low temperatures and low CO/CO 2 ratios favour cracking. High temperature annealing or preliminary nucleation do not improve the mechanical strength during reduction. The whole process has been analyzed, starting from the nucleation, and using a simplified fracture mechanics treatment, with due consideration of the incidence of stress corrosion cracking. The main stress was found to come from nucleation, as the shear stress at hematite-magnetite interface was negligible, so that in the 〈0001〉 direction, if not in any other, the yield stress is reached and plastic deformation occurs. The influence of T and CO/CO 2 is described and is partly accounted for, and the role of preliminary treatments (high temperature annealing and nucleation) is shown to be consistent with the above description.