Blast furnace burden softening and melting phenomena: Part III. Melt onset and initial microstructural transformations in pellets

The softening and melting of the ferrous burden in the blast furnace occur in the cohesive zone, affecting the furnace's performance. This work was designed to better understand these phenomena. In this article, the onset of melting and the evolution of the microstructure before softening were analyzed using a confocal scanning laser microscope (CSLM). Experiments were performed with industrial or synthetic pellets prereduced to wustite, placed over a metallic iron liner, and heated at either 1 degrees C/min or 3 degrees C/min under argon. The samples showed exudation of liquid, which coated the liner at temperatures above 1150 degrees C. The melt onset for FeO-SiO2 synthetic samples occurred at similar temperatures, regardless of the experimental conditions. The melt onset temperature for the basic samples was lower than the acid samples. A sudden increase of the amount of liquid occurred first for the acid samples. A transition in the microstructure of the burden from heterogeneous "as-received" to a partially molten homogeneous structure with wustite particles permeated by an intergranular (slag) phase was observed in the industrial samples. These results suggest that the initial liquid does not directly influence softening; rather, it acts to improve mass transport and to push the system to a semisolid.