Behavior of direct reduced iron and hot briquetted iron in the upper blast furnace shaft: Part I. Fundamentals of kineti
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ast furnace (BF) is the major iron producing unit and will remain the same for the foreseeable future. In some countries, direct reduced iron (DRI) and hot briquetted iron (HBI) have partially replaced the traditional BF ferrous burden consisting of lump ore, sinter, and pellets. The DRI or sponge iron is an industrial name for the metallized ferrous material produced by solid-state reduction of iron ore. The term HBI refers to the briquettes that are manufactured by compressing reduced iron ore fines, DRI, iron ore pellets, and sometimes scrap. Most of the DRI and HBI produced are over 90 pct metallized, and 85 pct or more total iron is present in the form of metallic iron phase with the remaining as residual unreduced wustite. It is considered that any form of prereduced ferrous burden and metallic iron decreases the coke rate and increases the productivity of the BF.[1] In addition, charging of DRI and HBI into the BF was largely initiated to meet the increased demand of steel. The high-temperature properties of ferrous materials are crucial for BF productivity. These properties in turn are dependent on the history of the burden in low-temperature zones in the BF, particularly in the granular zone and thermal reserve zone. Further, due to the varying gas and temperature profile in the upper BF shaft, knowledge of the state of iron (that is, the amount of wustite) and its morphology in DRI as it reaches the cohesive zone becomes P. KAUSHIK, Research Engineer, is with Steelmaking and Refractories Process Research at Mittal Steel USA R&D Center, East Chicago, IN. R.J. FRUEHAN, U.S. Steel Professor and Director of Center for Iron and Steel Making Research (CSIR), is with Carnegie Mellon University, Pittsburgh, PA. Manuscript submitted November 16, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
essential. This is necessary as the change in overall FeO content will change the softening behavior of the burden and its thermophysical properties, which, to some extent, govern the temperature at which the liquid material exudes from the burden. For lump iron ore and pellets, studies[2] have shown that, at the end of the thermal reserve zone, the state of iron oxide in the burden is a mix of wustite and metallic iron as they are reduced during their descent in the BF. This may not be necessarily true for the ferrous material, which is present in the form of metallic iron such as DRI and HBI. It is generally believed that the use of DRI/HBI will reduce the coke rate in the BF. Moreover, HBI provides mechanical strength to the bed of the ferrous burden in the BF, thus allowing the usage of formed coke and dense coals. However, as will be shown, the metallic iron in DRI/ HBI can be oxidized in the upper exchanger of the BF by Reactions [1a] and [1b], which produce a gas richer in CO or H2, but may not result in the reduction in the coke rate as much as believed. The FeO in the periphery of DRI could also affect softening and melting, in particular the interaction with the adjacent pellets. Fe 1 CO2 ¼ FeO 1 CO
[1a]
Fe 1 H2 O 5
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