Rate of Iron Carbide Formation from Reduced Iron in CO-H2-H2S Mixtures under Pressurized Conditions

Abstract

Under pressurized conditions up to 0.6 MPa, the rate of the iron carbide formation from completely reduced iron in CO-H 2 -H 2 S mixtures was measured by gravimetric technique. The fractional conversion for carbide formation, f q , was defined as the weight gain divided by the weight of carbon for the complete conversion to cementite. First, the compositions of the partially reacted samples were analyzed by Mössbauer absorption spectroscopy. Then, the fractional conversion calculated from the composition, f q M , was compared with f q . The value of f q agreed reasonably with that of f q M in the range f q Ͻ0.8, from which the gravimetric method is proved to be valid to the measurements for the rate of iron carbide formation. The carbide formation curves, f q versus t relation, were not influenced by the weight of the sample and the flow rate of the reaction gas in the conditions; 50 mg, more than 3.33ϫ10 Ϫ6 m 3 /s. Cross sectional view indicated the homogeneous distribution of cementite regions in the reduced iron particles partially converted to cementite. On the other hand, the rate was very sensitive to the conditions of the carburization reaction on the pore surface. The rate increased with increasing the content of carbon monoxide up to 80 to 90 %, where the rate was the maximum, and then gradually decreased with further increasing the content. The rate increased with the total pressure, but decreased with the activity of sulfur. The rate controlling step of the iron carbide formation was discussed on the basis of the experimental results. The rate controlling step was assumed to be the carburization reactions on the pore surface of reduced iron particles.

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

“…[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, H 2 , and H 2 S. 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.…”

“…When the carbon activity, controlled by the CO content in the gas, becomes larger than unity, cementite forms through Eq. [7]. The chemical equilibrium in the system is affected by temperature, gas composition, and the chemical potentials of the species inside the pellet which are influenced by mass transfer in the solid phase.…”

“…Similar results were obtained and discussed in the previous work. [11] Carbon deposition during reduction has been reported in other works which used gas mixtures containing H 2 and CO. [1,2,6,7,9] Formation of graphite takes place on the surface of pellets in atmospheres with high carbon potential. Reduced iron acts as a catalyst for carbon deposition; therefore, the amount of deposited carbon is enhanced as reduction proceeds.…”

“…[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][2][3][4][5][6][7][8][9][10] Hayashi and Iguchi performed studies of carbide formation under different experimental conditions in H 2 -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.…”

“…The rate of Fe 3 C 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 CH 4 -H 2 -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).…”