Blast furnace burden softening and melting phenomena: Part III. melt onset and initial microstructural transformations i
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VEN though direct reduced iron (DRI) production increased in the past decade and smelting processes are maturing, the blast furnace is still the predominant process for the reduction of iron ore. The cohesive zone has a significant effect on the efficiency and productivity of the furnace. In this region, the metallic burden softens, drips liquid, and melts. In the meanwhile, the metallic burden loses permeability, forcing the gas flow through the coke layers. The deformation of the ferrous burden unit is the macroscopic result of softening. As the temperature increases, the liquid fraction increases until the burden unit cannot hold, resulting in the dripping of liquid. The final step is the final meltdown. This article is part of a series aimed at studying the underlying phenomena that occur in the cohesive zone. Two previous articles presented the observation of the interaction between bulk pellets during softening and melting[1] and the microstructural transformations after softening and before melting.[2] In this article, the importance of the presence of liquid in the early stages of softening will be discussed, as well as its implications in the transformations of the microstructure that are involved in the early stages of softening. In the present article, these observations are complemented with those performed with the confocal scanning laser microscope (CSLM) at lower temperature. To validate some of these observations, experiments were performed using synthetic FeO-SiO2 pellets. These experiments were PAULO F. NOGUEIRA, formerly Graduate Student with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, is Senior Researcher, with Ferrous Products R&D, Cia. Vale do Rio Doce CVRD, Vitoria, ES, 29090-900 Brazil. Contact e-mail: [email protected] RICHARD J. FRUEHAN, Professor, is with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted July 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
designed to provide a better understanding of the phenomena occurring at the microscopical level that leads to the formation of a significant amount of liquid in the pellet. As demonstrated by Bakker,[3] the onset of liquid is the key factor leading to softening. The initial oxide liquid will form at the interface of lowest melting point, usually between an iron oxide particle and another oxide. It depends on the phases present and their distribution. After the first melt is formed, it will tend to wet the ore particles due to the reduction of the interfacial energy.[3] This rate of liquid spreading would depend on the viscosity of the slag formed. Furthermore, as the liquid spreads, it may encounter other oxide particles and would react with them if it lowers the system energy, and thus pushes the system toward equilibrium. The burden unit would become a semisolid material when the liquid phase covers the oxide particles, acting a
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