Blast furnace burden softening and melting phenomena: Part I. Pellet bulk interaction observation
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I. INTRODUCTION
THE blast furnace is the dominant route for the production of primary iron, despite the increase in Direct Reduced Iron (DRI) production and the development of smelting processes. The shape and position of the cohesive zone in the blast furnace affect the gas flow pattern, which has an effect on the efficiency and productivity of the process. This zone is the region where the metallic burden loses its permeability, and gas flow occurs only through the coke layers. The loss in permeability is not necessarily caused by the liquid phase in the ferrous burden unit (pellet or sinter), although liquids do increase the pressure drop. The solid phases can be deformed due to the weight of the burden, and occupy the gaps between the ferrous burden pieces. In the cohesive zone, different phenomena occur at the same time, the softening and melting of the oxide phase and the softening and melting of the metallic iron phase. The strength of the metal phase will depend on its carbon content and on its cross-sectional area. The softening and melting of the oxide phase will be affected by the quantity of the nonferrous oxides present, i.e., slag formers, along with their distribution, morphology, and chemistry. It will also depend on the reduction degree, since it affects the availability of iron oxide as a slag former. These processes are rather complex, so they will be subdivided into steps. The first oxide melt will form at the interface of lowest melting point, usually between an iron oxide particle and another oxide. It thus is heavily dependent on the microstructure, i.e., the phases present and their distribution. After the first melt is formed, it will tend to wet the ore PAULO F. NOGUEIRA, formerly Graduate Student with the Center for Iron and Steelmaking Research, Carnegie Mellon University, is Researcher, Institute for Technology Research, IPT, São Paulo, SP 05508-901 Brazil. Contact e-mail: [email protected] RICHARD J. FRUEHAN, Professor, is with the Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted January 7, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
particles due to the reduction of the interfacial energy.[1] This liquid film will slow the reduction kinetics since it inserts another resistance to the reaction, transport through the liquid slag film. This is often called reduction retardation. In the vast majority of blast furnaces, the metallic burden reaches the softening temperature with a reduction degree higher than 50 pct. Therefore, it can be assumed that there is a porous solid iron shell confining the solid and liquid oxides. As the liquid phase covers the solid oxide particles, it becomes a semisolid material, where the liquid slag acts as a lubricant for the remaining particles.[1] Thus, after the liquid slag in the core has wet all the oxide particles, the core has a very reduced mechanical strength and the resistance to deformation will be determined by the iron shell.[1] The ability of this metallic shell to hold the
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