Comportamento de amolecimento e fusão de cargas ferrosas através de abordagem experimental e modelo termodinâmico

Abstract

The softening and melting properties of the iron-bearing materials play a decisive role in the formation of the cohesive zone. The position and thickness of this region greatly affect blast furnace gas flow distribution and heat-transfer efficiency, which influences furnace permeability and productivity. During the reduction, softening and melting of the iron-bearing materials, major microstructure changes occur. The understanding of such transformations and its relation with the softening and melting phenomena is essential to the development of new raw materials, technologies, and models. In this context, the present work sought to investigate the phenomena of the softening and melting process for three iron-bearing materials through softening and melting under load tests, and to develop a thermodynamic model to approach the phase evolution of the materials throughout its reduction, softening and melting. The experimental part of this work characterized the softening and melting properties of samples of lump ore, acid pellet, and sinter through conventional softening and melting under load experiments. Moreover, interrupted experiments were carried out, based on contraction and pressure drop levels, to obtain samples in specific moments of the softening and melting process. The products obtained and the samples as received were characterized by apparent and true density (used to calculate the open, closed and total porosity), X-ray diffraction, reflected light microscopy and scanning electron microscopy (with energy dispersed spectroscopy). Furthermore, a thermodynamic model was developed using FactSage thermodynamic software and macro-processing. The model was constructed using a series of equilibrium stages and mathematical operators (splitters) to determine flow directions of streams and to consider kinetic inhibitions. From the experimental results, three main regions of reduction were characterized, namely: solid/gas reduction, reduction retardation, and melting reduction. In the first reduction region, reduction occurred following the shrinking core model with the increasing of samples open porosity. On the reduction retardation region, a sharp decrease in open porosity was identified, which reflected on the diminishing of rate of indirect reduction. At the region of melting reduction, the slag (rich in FeO) initially present in the particles’ core exuded to the bed, leading to a sharp increase in reduction rate due to slag contact with the reducing gas and coke. At the beginning of softening, the microstructure of the material was comprised of pseudo-globular wüstite interspersed with slag. As heating progressed and bed contraction increased, that structure coalesced to form a globular shape wüstite in a well-connected liquid slag matrix. Regarding the thermodynamic model, based on literature data, the methodology applied was capable of obtaining iron-bearing materials reduction degrees in very good agreement to the experimental data used. The splitters used to partly consider kinetic inhibitions showed a close relation with the rate of reduction. Moreover, the calculated profiles of slag quantity showed a close relationship with the softening and melting behaviors evaluated from softening and melting experiments. The calculated first liquid formation temperature was similar to that of when the sample beds attained 10% contraction, although the level of similarity between those parameters depended on the level of heterogeneity of each raw material. In addition, as temperature and consequently the amount of liquid increased, samples appeared to get closer to equilibrium conditions. In especial, the liquid fractions were quite similar to the experimental profiles of pressure drop, which qualitatively determine the most important characteristics of the cohesive zone.