X-ray microtomography for characterisation of cracks in iron ore pellets after reduction

Abstract

This work presents a method, based on X-ray microtomography and three-dimensional (3D) image analysis, of characterising and quantifying crack distribution in iron ore pellets. The aims have been to verify the method and to determine to what extent crack propagation contributes to the decrease in compressive strength that occurs during reduction at 500°C as haematite transforms into magnetite. Raw materials known to cause disintegration problems were selected in order to promote crack propagation. Pellets displayed crack lengths of sizes roughly corresponding to half the pellet diameter already before reduction and, during reduction, a further crack propagation of ～50% occurred. Through estimations by finite element analysis of the crack size and the pellet geometry, it has been possible to determine that this crack growth most likely is a mechanism that contributes to the decrease in compressive strength. The decrease of ～90% that was experimentally determined to occur after 30 min of reduction is, however, too large to be explained by crack propagation alone. The study shows that the proposed techniques allow 3D imaging of iron ore pellets and characterisation of cracks. The scans are non-destructive and can be carried out repeatedly, which allows a specific sample to be studied at different stages during a process. Through future use of the proposed method, our aim is to reach a deeper understanding of the mechanisms behind low temperature disintegration of iron ore pellets and the performance of LKAB olivine pellets inside the blast furnace.