Kinetics of carbide formation from reduced iron in CO-H2-H2S mixtures

Abstract

Gravimetric measurements were made for the kinetics of the carbide formation from reduced iron by preventing the decomposition of iron carbides and the deposition of carbon by utilizing the effect of sulfur. Particles of a hematite ore were completely reduced in the H 2 -H 2 S mixture and converted to iron carbide in a CO-H 2 -H 2 S-CO 2 -H 2 O-Ar or CO-CO 2 -COS-N 2 mixture at 873 to 1073 K. The carbide formation kinetics was drastically accelerated by the addition of a few percent H 2 to CO and reached the maximum at about 20 pct H 2 . From morphology observation, it was assumed that the carburization reaction on the surface of pores in the reduced iron determines the carbide formation kinetics, and the carburization rate, df /dt, is expressed as the product of a function of carburization rate, g( p i , T ), and the relative reaction surface area at the time t, 1 Ϫ f . Applying the integrated rate equation, Ϫln(1 Ϫ f ) ϭ g( p i , T )t, to the experimental results, the value of g( p i , T ) was obtained. By analyzing the carbide formation rate, it was assumed that the elementary reactions determining the carbide formation rate are CO ϩ H(ad) → [C] ϩ OH(ad) and OH(ad) ϩ H 2 ϭ H 2 O ϩ H(ad), except in the low ratio of p H 2 /p CO , where the rate determining elementary reaction shifts to O(ad) ϩ CO → CO 2 . The overall rate Eq. [22] has been proposed for the carbide formation from the reduced iron in CO-H 2 -H 2 S-CO 2 -H 2 O mixtures.

Kinetics of Decomposition and Re-oxidation Resistance of .THETA.- and .CHI.-Iron Carbides at Elevated Temperatures and Influence of Their Formation Conditions

“…on iron carbide production and its kinetics have been studied in several previous studies [1][2][3][4][5][6][7][8][9][10] involving the present authors' for the last decade. The iron carbides, q-iron carbide (cementite, Fe 3 C) and c-iron carbide (super iron carbide, Fe 5 C 2 ), are thermodynamically meta-stable.…”

Kinetics of Cementite Formation from Reduced Iron in CO-H2-H2S Mixtures under Pressurized Conditions

“…In the present authors' previous study 13) which was conducted in CO-H 2 -H 2 S mixtures under the total pressure of 0.03 to 0.1 MPa by mixing nitrogen to the reaction gas, the value of g(p i ,T ) was proportional to 1.7 power to the total pressure of the gas mixtures containing 20 % and more of hydrogen. The reason why the pressure dependence of g(p i ,T ), i.e.…”

“…1) Since then, fundamental studies [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] concerning iron carbide production from iron ore have been actively performed. Since iron carbides are thermodynamically meta-stable, however, it is not formed until the carbon activity of the reaction gas is increased beyond unity.…”

“…In our previous studies, [7][8][9]13) the rate of carbide formation from completely reduced iron ore in H 2 -H 2 S mixtures was accurately measured in CO-H 2 -H 2 S mixtures by gravimetric technique at 973 K. But, the measurements of the carbide formation rate were made only under total pressures 0.1013 MPa (1 atm) or less.…”

Microstructure Evolution of Iron Carbide during Reaction with Steam at Elevated Temperatures

“…By using reduced iron that was also derived from hematite ore and possessed a porous microstructure, Iguchi and his colleagues carried out a systematic investigation on the carburizing process of the porous iron with mixed gases containing carbon monoxide and shed light on the influence of microstructure over the formation mechanism of iron carbide. [7][8][9][10] To date, however, little has been understood about the relationship between microstructure and reaction kinetics for the iron carbide-steam reaction although a previous research implied that the porosity of the hematite ore-derived iron carbide can vary through the reaction with steam at temperatures lower than 1 073 K. 11) In the present investigation, which was meant as the first step to understand the correlation between microstructural features and reaction kinetics for the iron carbide-steam reaction, cross-sectional microscopy was employed to examine the microstructure evolution of the ore-derived iron carbide brought by the reaction with steam in a temperature range between 393 K and 1 373 K. Phase transformations that occurred in the iron carbide through the reaction were also analyzed by X-ray diffractometry (XRD) in combination with thermogravimetry (TG) and were associated with the microstructure variation of the iron carbide.…”