QUALITATIVE AND QUANTITATIVE EVALUATION OF IRON ORE SINTERS THROUGH DIGITAL MICROSCOPY

Vale (previously CVRD – Companhia Vale do Rio Doce) evaluates the microstructure of sinters in order to understand their characteristics and behaviour in the agglomeration process, and the correlation with conventional quality parameters. Efforts on image analysis development are in progress between Vale and PUC-Rio in order to improve both the qualitative and quantitative evaluation of a representative portion of material. In the present work, digital microscopy techniques were developed allowing this kind of assessment. Employing a motorised and computer controlled optical microscope, low magnification mosaic images covering complete sinter samples were automatically obtained. The mosaics provide a powerful qualitative overview of the whole sample while allowing low resolution quantitative evaluation of the major phases. In addition, quantitative results for phase fractions were compared for the same sample regions at three magnifications (5×, 10× and 20× objective lenses). A single field (1 × 1) at the lowest magnification corresponds to mosaics of 2 × 2 and 4 × 4 images at the higher magnifications. The automated system allows perfect registration between the mosaics and the single image. Changes in phase discrimination were evaluated as a function of optical resolution, and phase fractions obtained by interactive segmentation of the image histograms were compared. While finer dark phases, eg silicates, were only visible with the best optical resolution, coarser phases such as haematite and pores were measured at all resolutions. For haematite, magnetite, ferrite and pores, the largest relative error in area fraction, between the images obtained with the 5× and 20× lenses, was 12 per cent. The results for silicate were less accurate with a relative error of 44 per cent. The results indicate that digital microscopy provides a flexible method for qualitative and quantitative evaluation of these materials. INTRODUCTION Iron ore sinters are suitable porous materials obtained by agglomeration processes of iron ore fine particles. These agglomerates are widely employed in blast furnaces to produce pig iron. Nowadays, most modern blast furnaces use iron ore sinters and they correspond to the main burden for pig iron production (Yang and Standish, 1991; Magalhães and Brandão, 2003). Thus, there is an increasing interest to research and to improve the properties of these agglomerates. The microstructural characterisation of iron ore sinters contributes to the understanding of their properties and behaviour in the agglomeration processes, as well as their use in blast furnaces. Reflected light microscopy (RLM) is typically applied to characterise iron ores and their agglomerates because it is capable of distinguishing the most common mineral phases through their distinctive reflectivities. In fact, RLM can provide fundamental information like mineralogy, porosity and texture. Vale performs a qualitative evaluation of the microstructure of iron ore sinters, by visual examination under a reflected light microscope, regarding its correlation with conventional quality parameters. Iron ore sinters have a mineralogy which is basically composed by iron oxides and hydroxides, ferrites and silicates. Their mineralogical composition constitutes an important parameter that can be specified and controlled. Thus, a quantitative evaluation of sinters is also of great interest. Therefore, efforts on RLM and digital image analysis development are in progress between Vale and PUC-Rio in order to establish qualitative and quantitative characterisation routines for sinters. Digital microscopy techniques (Paciornik and Maurício, 2004) were developed and employed to allow this kind of assessment. Sinters are very inhomogeneous materials. Thus, a representative characterisation by RLM requires either a large number of sample fields or a mosaic image covering the whole sample. Mosaic images (Bradley, Wildermoth and Mills, 2005) are very useful both for qualitative and quantitative analysis as they provide a complete view of the sample, and the obtained measurements do not suffer from statistical limitations and edge problems associated with individual fields. However, mosaic image files quickly become too big and image processing becomes impractical. In the present work usable file sizes went up to ≈170 Mb (5× objective, sinter mosaic). Equivalent mosaics at 10× and 20× would create files four and 16 times larger, all but impossible to process and analyse. On the other hand, the accurate discrimination and evaluation of the present phases might be biased by the low resolution of the 5× objective lens. Thus, higher magnifications were also employed for single fields, and for small mosaics used in multi-resolution comparison experiments. In the present work, a semi-automatic multi-resolution characterisation routine was developed and tested on sinters from Vale. The area fraction of the several present phases was measured at different magnifications/resolutions, both for individual image fields and mosaic images. The results were compared and the advantages and disadvantages of each method were evaluated. Ninth International Congress for Applied Mineralogy Brisbane, QLD, 8 10 September 2008 669 1. Associate Professor, Department of Materials Science and Metallurgy, Catholic University of Rio de Janeiro (PUC-Rio), Rua Marquês de S Vicente 225, Sala 501L, Gávea, Rio de Janeiro RJ 22453-900, Brazil. Email: julioc.alvarez@gmail.com 2. Department of Materials Science and Metallurgy, Catholic University of Rio de Janeiro (PUC-Rio), Rua Marquês de S Vicente 225, Sala 501L, Gávea, Rio de Janeiro RJ 22453-900, Brazil. Email: deturon@gmail.com 3. Department of Materials Science and Metallurgy, Catholic University of Rio de Janeiro (PUC-Rio), Rua Marquês de S Vicente 225, Sala 501L, Gávea, Rio de Janeiro RJ 22453-900, Brazil. Email: gustavoschinazi@gmail.com 4. Centre for Mineral Technology (CETEM), Av Pedro Calmon 900, Cidade Universitária, Rio de Janeiro RJ 21941-590, Brazil. Email: ogomes@gmail.com 5. Department of Materials Science and Metallurgy, Catholic University of Rio de Janeiro (PUC-Rio), Rua Marquês de S Vicente 225, Sala 501L, Gávea, Rio de Janeiro RJ 22453-900, Brazil. Email: marcosh@puc-rio.br 6. Department of Materials Science and Metallurgy, Catholic University of Rio de Janeiro (PUC-Rio), Rua Marquês de S Vicente 225, Sala 501L, Gávea, Rio de Janeiro RJ 22453-900, Brazil. Email: sidnei@puc-rio.br 7. Department of Technology and Technical Assistance for Iron Ore (DITF), Vale, Centro de Pesquisas do Miguelão, Av de Ligação 3580, Nova Lima MG 34000 000, Brazil. Email: maria.beatriz. vieira@vale.com