Kinetics and Mechanism of Decarburization and Melting of Direct-Reduced Iron Pellets in Slag
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TRODUCTION
DIRECT-REDUCED iron (DRI) is a major iron feedstock for many of today’s steelmaking operations. Driven by the rising price of scrap and the market’s demand for high-quality clean steel, there has been a trend toward increasing the DRI charge into electric-arc furnaces (EAFs), as a substitute for steel scrap. As a result, DRI production has experienced a substantial increase in the last few decades, from 0.8 million tons/ year in 1970 to over 67 million tons/year in 2006[1] and is expected to grow even more rapidly in the next decade. More than 95 pct of the DRI produced is used in the EAF; the proportion of DRI in the metallic feed typically exceeds 50 pct. In some cases, 100 pct DRI has been used in the EAF. In recent years, numerous advancements have emerged in DRI-based steelmaking technology. Charging hot DRI to EAF to save the sensible heat of the DRI and reduce the energy consumption is among these.[2–7] It has been reported that DRI can be conveyed to the EAF at 700 °C and, as a consequence, the electrical energy consumption is reduced approximately 120 kWh/ton of steel, which constitutes approximately 20 to 30 pct savings in energy compared to existing coldfed furnaces. Nevertheless, the advantages of the hotcharging technology cannot be completely realized as a result of different operating modes of the EAF and JIANGHUA LI, formerly Postdoctoral Fellow, Department of Materials Science and Engineering, University of Toronto, is Research Engineer, AK Steel Corporation, Middletown, OH, USA. MANSOOR BARATI, Assistant Professor, is with the Department of Materials Science and Engineering, University of Toronto, Toronto, ON, Canada M5S 3E4. Contact e-mail: [email protected] Manuscript submitted June 2, 2008. Article published online December 17, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
DRI-producing processes, such as Midrex and HYL, where the former is a batch process and the latter is operated continuously. A novel steelmaking technology, the continuousreduced iron steelmaking process (CRISP), has been introduced[8,9] to overcome the plant availability mismatch between the two processes, by melting DRI and producing semifinished steel in a continuous fashion. The technical aspects of the CRISP have been presented in detail elsewhere.[10–12] Briefly, the process employs a large stationary EAF to continuously melt DRI and scrap and decarburize the steel with the addition of minimum or no gaseous oxygen. Steel and slag are tapped periodically and the appropriate slag chemistry and long residence time will be used to lower the steel carbon to the desired levels. The critical aspects of the process are the mechanisms and rates of decarburization and the melting of the DRI pellets as they are fed from the direct reduction module and pass through the slag phase. For example, the feeding rate of the DRI and slag composition must be carefully controlled, to ensure the prompt melting of the DRI pellets, the decarburization of steel to the desired level, and good foaming, to protect the sidewall
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