Imagerie et compréhension du mode de formation de granules de minerais de fer en utilisant la MicroTomographie RX

Imaging and understanding of iron ore granulation using X-ray microtomography. Evrard M., Contreras R., Pirard E. Most iron ores cannot be directly fed into blast furnaces because their particle size distribution will negatively impact the blast furnace permeability and hence the overall efficiency of the ore reduction process. The finest fraction (typ. < 6.3 mm) has to undergo a sintering process, wherein a mixture of iron ore, coke and flux (limestone, olivine,…) is heated, partially molten and transformed into pieces of sintered material with adequate size, porosity and strength characteristics to be fed into the blast furnace. The preparation of an adequate mixture involves a granulation process which is the subject of this study. Sintering and hence granulation of iron ores, is particularly important in Europe where iron ores show a downward trend in quality (finer particles, broader size range, lower grades with higher variability). In addition an increasing fraction of non-sintered material (return fines) is being added to the mix to optimize the resource efficiency while maintaining sinter quality. For most practitioners, microgranules (or micropellets) are considered as being composed of a nucleus (a particle in the range of 1 to 2 mm) surrounded by layers of the finest ore particles (typically the < 250 µm fraction). A series of experiments and 3D imaging tests have been performed in this study to better understand the granulation mechanisms. Microgranules are formed by mixing iron ore with water in a small rotating drum. Several parameters can be tuned during the granulation process: size and proportion of nucleating particles, size and proportion of fines, water addition, rotating speed, total duration,… Simple experiments using a single ore type (dominantly goethitic or hematitic) and water additions have been carried out systematically to better understand the kinetics of granulation. X-ray microtomography and subsequent 3D image analysis is used to identify and quantify the number of nuclei (when present), to measure the porosity and identify layering or cracks in the microgranule. Compared to previous studies on iron ore pellets (Farber et al. 2002; Shatokha et al. 2009; Shatokha et al. 2010), this work focuses on imaging prior to sintering. It also uses higher resolution micro-CT and definitely brings a better insight into granulation as compared to data from 2D imaging of polished blocks: less artefacts in apparent porosity due to sample preparation; good discrimination among components of the granule; clear identification of the nuclei; etc. This work is part of a larger European project (IMSIMI - Improved Sintering Mix) aiming at an optimal use of challenging input materials through carefully monitored preparation phases (mixing, granulation, etc.) and better understanding of their impact on the sintering process. Farber L., Tardos G., Michaels JN.,2002. Use of X-ray tomograzphy to study the porosity and morphology of granules. Powder Technology vol. 132, p 57-63. Shatokha V., Korobeynikov I., Maire E., Adrien J., 2009. Application of 3D X-ray tomography to investigation of structure of sinter mixture granules. Ironmaking and Steelmaking, vol. 36 (6), p 416-420. Shatokha V., Korobeynikov I., Maire E., Gremillard L., Adrien J.,2010. Iron ore sinter porosity characterisation with application of 3D X-ray tomography. Ironmaking and Steelmaking, vol. 37(5), p313-319.APGRA