Effect of Experimental Conditions on Cementite Formation During Reduction of Iron Ore Pellets

Metallurgical and Materials Transactions B, Aug 2016

Experiments have been carried out to study the effect of temperature, gas composition, residence time, and type of iron ore pellets on formation of cementite during gaseous reduction of hematite. Industrial iron ore pellets have been reduced isothermally in a gas mixture with H2 and CO as main components. The presence of Fe3C in the partially reduced pellets shows that reduction and cementite formation take place at the same time. The maximum content of cementite is identified in the samples reduced by H2-CO at 1123 K (850 °C). The decrease in the carbide content due to addition of 1 pct CO2 to the initial gas mixture reveals the major influence of carbon potential in the gas atmosphere. Further increase of CO2 content increases the Fe3C. The variations of the amount of cementite with the CO2 content suggest that both the thermodynamics and kinetics of cementite formation are affected by the gas composition. Cementite decomposes to graphite and iron particles in reducing and inert atmospheres as the residence time of pellets at high temperature is increased above 60 minutes.

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Effect of Experimental Conditions on Cementite Formation During Reduction of Iron Ore Pellets

Effect of Experimental Conditions on Cementite Formation During Reduction of Iron Ore Pellets MANIA KAZEMI 0 DU SICHEN 0 0 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 Experiments have been carried out to study the effect of temperature, gas composition, residence time, and type of iron ore pellets on formation of cementite during gaseous reduction of hematite. Industrial iron ore pellets have been reduced isothermally in a gas mixture with H2 and CO as main components. The presence of Fe3C in the partially reduced pellets shows that reduction and cementite formation take place at the same time. The maximum content of cementite is identified in the samples reduced by H2-CO at 1123 K (850 C). The decrease in the carbide content due to addition of 1 pct CO2 to the initial gas mixture reveals the major influence of carbon potential in the gas atmosphere. Further increase of CO2 content increases the Fe3C. The variations of the amount of cementite with the CO2 content suggest that both the thermodynamics and kinetics of cementite formation are affected by the gas composition. Cementite decomposes to graphite and iron particles in reducing and inert atmospheres as the residence time of pellets at high temperature is increased above 60 minutes. I. INTRODUCTION 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 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 by mass transfer of carbon in the iron sheet. The iron obtained from gaseous reduction of Fe2O3 by H2 had larger surface area and porosity than the pure iron powder. Therefore, the carburization rate was higher in the reduced iron powder.[ 8 ] Grabke et al. conducted fundamental studies on the kinetics and mechanism of carbide formation and carbon deposition in iron in gas mixtures containing CO or CH4 and traces of sulfur.[ 6 ] They concluded that the rate of carburization was higher in atmospheres containing CO than in CH4. They also observed that the presence of sulfur prevented graphite nucleation and decomposition of cementite. Due to the importance of the carbon and cementite contents in DRI, the present work has been performed focusing on t (...truncated)


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Mania Kazemi, Du Sichen. Effect of Experimental Conditions on Cementite Formation During Reduction of Iron Ore Pellets, Metallurgical and Materials Transactions B, 2016, pp. 3519-3526, Volume 47, Issue 6, DOI: 10.1007/s11663-016-0780-0