Effect of Experimental Conditions on Cementite Formation During Reduction of Iron Ore Pellets
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THE growing interests in application of direct reduced iron (DRI) in electric arc furnace (EAF) have prompted several investigations of carburization of iron and formation of iron carbide during gaseous reduction of iron oxides.[1–5] The problems associated with metal dusting, a corrosion process taking place in iron and steels used in atmospheres with high carbon activity, have further emphasized the importance of studies in this subject.[6] Preceding works have shown that factors such as temperature, gas composition, total pressure in the reactor, and the residence time have great impact on the rate of formation and stability of carbides.[1–10] Hayashi and Iguchi performed studies of carbide formation under different experimental conditions in H2-CO gas mixtures with addition of sulfur.[1,2] They reduced and carburized four types of hematite ore with different compositions and concluded that carbide formation in the applied conditions was not influenced by the chemical composition of the ores. Addition of small amounts of sulfur to the gas mixture decreased the amount of carbon deposition and stabilized the cementite phase.[1] In another work, the rate of carburization was enhanced by application of higher gas pressures; however, it resulted in larger amounts of free carbon on the ore particles.[2] Similar results were obtained by Iguchi et al.[7] from carburization of iron in atmospheres containing CO, H2, and H2S. The rate of cementite formation increased at higher pressures and at higher MANIA KAZEMI, Ph.D. Student, and DU SICHEN, Professor, are with the Department of Materials Science and Engineering, Royal Institute of Technology, 10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted April 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
temperatures in the fluidized bed. They discovered that the chemical reactions at the pore surface of reduced iron control the carburization rate. Although the presence of sulfur was beneficial to cementite formation and decreasing the carbon deposition, applying high sulfur activities in the gas decreased the rate of carbide formation. The rate of Fe3C formation was largely influenced by the variations of CO content in the gas phase and maximum rate was achieved with 80 pct CO in the mixture.[7] The decomposition and stability of cementite formed in CH4-H2-Ar atmosphere during reduction of iron ore at temperatures between 773 K and 1223 K (500 °C and 950 °C) were studied by Longbottom et al.[3] Cementite decomposition took place at all temperatures. They found out that under the experimental conditions applied, the cementite phase was most stable at temperatures between 1003 K and 1023 K (730 °C and 750 °C). The decomposition rate increased at temperatures below 873 K (600 °C) and above 1023 K (750 °C).[3] Hwang et al. examined the carburization of pure iron powder, iron from reduction of hematite and iron sheets in H2-CO atmospheres. They pointed out that the carburization kinetics are largely affected by the surface area of the iron powders and
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