Softening behaviors of acid and olivine fluxed iron ore pellets in the cohesive zone of a blast furnace

A ferrous burden loses its permeability in the cohesive zone of a Blast Furnace (BF) which has an effect on the flow of reducing gases. Iron ore pellets with various chemical compositions have different softening properties. Due to the occurrence of numerous simultaneous phenomena the clarification of different variables is difficult. In this study the effect of Reduction Degree (RD) on the softening behavior of individual acid and olivine fluxed iron ore pellets was experimentally tested under inert conditions. The acid pellet softened rapidly at 1150°C and reached about 40% contraction at 1200°C. The olivine fluxed pellet softened gradually in the range of 1150 and 1350°C and reached 30-35% contraction. The RDs of 50-70% for acid and 50-65% for olivine fluxed pellet had no significant effect on the softening behavior. However, the highest contraction-% was reached with the lowest RD. The results indicate that softening of the pellets is caused by the softening of the pellet core. The early softening of the acid pellet was attributed to high SiO2 content and formation of fayalite slag with high wöstite solubility. The superior properties of the olivine fluxed pellet were attributed to the low SiO₂ content and favorable effects of fluxes to prevent wöstite dissolution. FactSage V6.4-software and its FToxid-database was used to compute the phase equilibrium of a pre-reduced pellet with a quaternary FeO-SiO₂-CaO-MgO system in the core region. The computed phase equilibrium provided additional information about the effects of different components in the phase equilibrium.