Fe-Si particles on the surface of blast furnace coke

Abstract

This study investigates the surface of unpolished samples of blast furnace (BF) coke drilled from the tuyere zone, which hosts Fe-Si particles (mostly Fe₃Si) that vary in size, shape, depth of submersion (penetration) into the coke matrix, and contact features with the surface. Based on the shape of the particles and the extent of their contact with the coke matrix, they have been grouped into three major types: (I) sphere-like droplets with limited contact area, (II) semi-spheres with a larger contact area, and (III) irregular segregations with a spherical surface, which exhibit the largest contact area among the three types of particles. Considering the ratio between the height (h) of the particles and half of their length at the surface level (l) along the cross-section, these three types can be characterized as follows: (I) h > l, (II) h ≈ l, and (III) h < l. All the three types of particles can be found near each other. The shape and the extent of the contact depend on the degree of penetration of the material into the matrix, which is a function of the composition of the particles. Type (I) particles were initially saturated with Si at an earlier stage and, for that reason, they can react less with carbon in the coke matrix than type (II) and (III), thereby moving faster through the coke cone. Thermodynamic calculations have shown that the temperature interval of 1250–1300°C can be considered the starting point for Si entering into molten iron under quartz-dominated coke ash. Accordingly, the initial pick-up of Si by molten iron can be assumed to be mineral-related. In terms of BF practice, better conditions for sliding Fe-Si droplets through the coke cone are available when they come into contact with free SiO₂ concentrated into small grains, and when the SiO₂/ΣMe _x O _y mass ratio in the coke ash is high.