Role of Heat Transfer in Early Stage Decarburization of DRI in Slag
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IN the past four decades, the use of direct reduced iron (DRI) as a high-quality, iron-bearing feedstock has been investigated for virtually all major iron and steelmaking processes including as a coolant in a basic oxygen furnace,[1] scrap replacement and impurity diluter in electric arc furnace (EAF),[2–8] and prereduced iron feed for a blast furnace.[9] In particular, DRI production continues to increase as a major feedstock in EAF steelmaking.[10] The growing role of DRI in today’s steel industry arises from two facts: the increasing price of steel scrap and the rising demand for high-quality steels that requires dilution of the contaminations from scrap. It has been well recognized that the kinetics of the slag–DRI interactions plays a significant role in the process throughput, as well as cleanliness of the steel (i.e., the removal of nitrogen, etc.). Several attempts have been made to establish a model that quantifies the DRI–slag reaction kinetics. Sadrnezhaad and Elliott[11] introduced a heat-transfer–based model for the DRI/slag reaction. When compared with experimentally measured rates, they concluded that the model underestimates the rate. It was justified that this is because their model did not take into consideration the agitation caused by gas evolution inside the molten slag and the resultant increased rate of heat transfer. Later, a similar model was given by Goldstein and Fruehan.[12] However, they used an empirical relationship to include the effect of agitation on the reaction kinetics. MANSOOR BARATI, Professor, is with the Department of Materials Science and Engineering, University of Toronto, Toronto, ON M5S 3E4, Canada. Contact e-mail: [email protected] ERFAN SHARIFI, formerly Graduate Student, Department of Materials Science and Engineering, University of Toronto, is now Engineer at Process Research Ortech Inc., Mississauga, ON, Canada. Manuscript submitted August 25, 2011. Article published online April 3, 2012. 680—VOLUME 43B, AUGUST 2012
In recent experiments involving the immersion of DRI pellets into slag, Li and Barati[13] observed a two-stage behavior in the gas evolution measurements. A rapid initial gas evolution period lasting 20 to 30 seconds was followed by a slower but longer regime that continued until the carbon content of the DRI was consumed almost completely. They explained that the gas release process involves a reaction between carbon of DRI and FeO, Reaction [1], where FeO is supplied first from DRI itself and then once fully consumed, from the FeO of slag. The different nature of the two steps gives rise to a sudden change in the gas evolution rate. FeO þ C ¼ Fe þ COðgÞ
½1
In a more recent study,[14] the authors investigated the effect of the DRI carbon content and the pellet preheating temperature on the reaction kinetics. They observed a similar behavior for various experimental conditions and noted that most of the pellet carbon is removed during the first step, although its duration is shorter than the second step. Therefore, it is critical to determi
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