The Reaction Behavior of Direct Reduced Iron (DRI) in Steelmaking Slags: Effect of DRI Carbon and Preheating Temperature
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TRODUCTION
IN an earlier study, Li and Barati[1] have described the behavior of direct reduced iron (DRI) in slags with various FeO contents and basicities. X-ray fluoroscopy observations provided a qualitative insight into the mechanism of melting and decarburization of DRI. This work is an extension of the previous investigation, aiming to examine the effect of DRI carbon content, slag temperature, and DRI pellet preheating temperature on the kinetics of decarburization of the DRI. The behavior of DRI in steelmaking slags has been the subject of several other studies. Sadrnezhad and Elliott[2] conducted an experimental study on the melting rate of DRI pellets in CaO-FeO-SiO2 slags. The experiments involved immersing DRI pellets inside the slag to measure the rate of gas evolution. The measurements were carried out by observing the number of gas bubble sites on the surface of molten slag. They found two peaks at 1023 K and 1273 K (750 C and 1000 C) in the gas evolution vs temperature curves and justified that the peaks corresponded to the reduction of magnetite to wustite (Reaction [1]) and wustite to iron (Reaction [2]), respectively. This conclusion was based on the assumption that the DRI pellet contained a considerable quantity of magnetite. However, because commercial DRI pellets contain little magnetite, other
ERFAN SHARIFI, Formerly Graduate Student, Department of Materials Science and Engineering, University of Toronto, Toronto, ON M5S 3E4, Canada, is now Process Engineer, Process Research Ortech, Mississauga, ON L5K 1B3, Canada. Contact e-mail: erfan. sharifi@utoronto.ca MANSOOR BARATI, Assistant Professor, is with the Department of Materials Science and Engineering, University of Toronto. Manuscript submitted April 8, 2010. Article published online July 13, 2010. 1018—VOLUME 41B, OCTOBER 2010
researchers[1,3,4] considered Eq. [2] the dominant reaction in DRI–slag interactions. Fe3 O4 þ C ¼ 3FeO þ COðgÞ
½1
FeO þ C ¼ Fe þ COðgÞ
½2
The carbon shown in these reactions can be as free, dissolved, or Fe3C. It has been reported[5,6] that more than 70 pct of the carbon in the DRI is in the form of iron carbide. However, it is likely that this metastable compound may decompose at high DRI-slag reaction temperatures, resulting in carbon being dissolved in iron. Sadrnezhad and Elliott[2] also discussed that neither the pellet size nor the heating rate of DRI pellets affect the volume of gas evolved from the reaction; instead, the gas evolution depends directly on the chemical composition of pellet and slag. In another study, Goldstein et al.[3] released DRI pellets into the typical steelmaking slag (40CaO-41SiO2-9FeO-10Al2O3 wt pct). The reaction was visualized using X-ray fluoroscopy. The reaction started by an initial incubation period of 8 to 10 seconds followed by a 20- to 30-second rapid gas evolution. On the one hand, they concluded that the behavior of DRI in slag is similar to that of Fe-C drops. On the other hand, for Fe-C/slag reactions, Mulholland et al.[7] observed a gas layer around the Fe-C drops. T
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