Behavior of direct reduced iron and hot briquetted iron in the upper blast furnace shaft: Part II. A model of oxidation
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UCTION
IN some countries, direct reduced iron (DRI) and hot briquetted iron (HBI) have partially replaced the traditional blast furnace (BF) ferrous burden consisting of lump ore, sinter, and pellets. It is generally believed that the use of DRI/HBI will reduce the coke rate in BF and will increase its productivity. However, as will be shown, the metallic iron in the DRI/HBI can be oxidized in the upper exchanger in the blast furnace, which produces a gas richer in CO/H2 but may not result in the reduction in the coke rate as much as anticipated. The FeO in the periphery of DRI could also affect softening and melting, in particular, the interaction with the adjacent pellets. Both DRI and HBI also contain a small amount of iron oxide phases before being charged into the BF due to oxidation of metallic iron in air during storage, handling, and transportation.[1,2,3] In modern furnaces, operating on basic sinter and oxygen enriched air blast, the gas composition is highly oxidizing to iron above the thermal reserve zone.[4] Because the reactions at the higher temperatures in the BF are affected by the burden characteristics in the low-temperature zone, it would be useful to understand the changes in the state and morphology of iron present inside DRI and HBI before these materials reach the cohesive zone in the BF. Part I of this series of articles[5] concentrated on identifying the kinetics and mechanism of oxidation of metallic iron in DRI/HBI at 400 °C to 900 °C, which is the temperature range in which oxidation of DRI/HBI in the upper BF shaft is expected. The initial rate of oxidation at high temperatures is governed by a limited mix control of chemical kinetics and porous mass transfer of gas. The work presented in this article is intended to provide a better understanding of the behavior of DRI/HBI in the BF shaft. A P. KAUSHIK, Research Engineer, is with Steelmaking and Refractories Process Research, Mittal Steel USA R&D Centre, E. Chicago, IN 46312. R.J. FRUEHAN, U.S. Steel Professor and Director of Center of Iron and Steelmaking Research (CSIR), is with the Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted December 1, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
model of oxidation of DRI/HBI was developed, which was then subjected to the laboratory tests.
II.
EXPERIMENTAL
The DRI and HBI samples chosen for this work were the same as those described in Part I of this series.[5] The chemical composition and microstructure of ‘‘as-received’’ DRI/HBI samples were also previously shown. To develop the model of oxidation of materials in CO:CO2 gas mixtures, the material was oxidized using an ultra-high-purity grade (99.999 pct) CO2 in a temperature profile, as shown in Figure 1. For this purpose, the DRI/HBI samples were sieved in a size range of 12.5 to 16 mm in industrial screens and then dried in a box furnace at 200 °C. The sample was then wrapped into a mesh of platinum wire, weighed, and then suspended with a platinum chain in a tube fu
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