Microstructures of industrial sinters from Quadrilatero Ferrifero’s iron ores, Minas Gerais State, Brazil

Sinters correspond to the main burden of blast furnaces for pig iron production. The industrial sinters studied were produced with iron ores from Quadrilatero Ferrifero, which are hematite-rich ores. They are composed of four main phases: hematite, magnetite, silicoferrite of calcium and aluminium (SFCA) and silicates. Hematite corresponds either to a relict phase (pure Fe2O3) or to a restructured mineral; the relict phase can be, not rarely, related to specific fabrics of each iron ore mix employed in the industrial sintering process, while the restructured hematite represents particles grown by coalescence and/or partially reequilibrated (Al2O3-bearing). Magnetite is associated to SFCA, which can replace it, and it is also related to MgO-bearing silicate cores, which are favourable sites to develop it. SFCA is an important phase in sinters and, in controlled amount, it contributes to a good mechanical strength of these agglomerates. SFCA common types are massive, columnar and acicular. Pseudomorphs of massive SFCA after magnetite have been observed. The columnar type occurs at the relict hematite edges and in association with magnetite. Intergrowths of thin SFCA and silicates are identified too. Silicates composition ranges from Ca-rich (dicalcium silicate) to Fe–Ca-bearing, both with significant phosphorus content. These microstructural aspects play an important role in evaluating industrial sinters quality such as mechanical strength parameters and behaviour during reduction stages.