Kinetic Analysis on the Rate of Stepwise Reduction of a Single Sinter with CO-CO₂-N₂ Gas Mixture

Abstract

Stepwise reduction experiments of commercial sinter under the same conditions were repeated more than 4 times over a temperature range of 700° to 1000°C.
In the analysis, reduction of calcium ferrite was not taken into consideration. Mass transfer rate from naphthalene-coated sinter was measured and the following correlation equation for estimating mass-transfer coefficient in a gas film of sinter, k_f (cm/s), was derived in consideration of shape factor :
k_f d_p^*/ D=4+0.82(ν /D)^1/3(ud_p^*/ν)^1/2
where D: diffusivity (cm²/s), d_p^* : volume equivalent diameter (cm), u : gas velocity (cm/s), ν: kinematic viscosity (cm²/s),
Values of chemical reaction rate constant k_c and effective diffusivity D_e in the unreacted-core shrinking model for one interface were so determined by trial and error that the calculated reduction curve might agree well with the experimental one. Their temperature dependencies are
k_c(^h/m)=exp[6.88-16000/(RT)](cm/s) D_e^m=exp[-0.721-4130/(RT)](cm²/s)
k_c(^m/w)=exp[5.34-12200/(RT)] D_e(^w)=exp[0.609-5470/(RT)]
k_c(^w/Fe)=exp[3.39-8560/(RT)] D_e(^Fe)=exp[3.23-9060/(RT)]
whereR: gas constant [cal/(mol·K)], T: temperature (K), h, m, w: abbreviations for hematite, magnetite and wustite.
Although microscopic observation of partially reduced samples showed the application of the single interface model was rather not suitable from the viewpoint of reduction fashion, it was good enough for evaluating the reduction rate because of satisfactory agreement between the calculated and the measured reduction curves.