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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)