Tateo Usui

Abstract:

Rates of gaseous reduction of commercial sinter with CO-CO2-N2 or H2-H2O-N2 gas mixture were measured under single particle and fixed bed situation at a constant temperature. By these data analyses using the unreacted-core shrinking (UCS) models for one and three interface(s), rate parameters; namely chemical reaction rate constants and intra-particle effective diffusivities in the models were evaluated. Fixed bed packed with the sinter was also reduced under rising temperature conditions with stepwise gas concentration change, which was roughly simulated as in a blast furnace. The reduction rate was analyzed by using UCS model for three interfaces with the pre-determined rate parameter values; the comparison between the experimental reduction curve and the computed one showed rough agreement but not so precise and this discrepancy was considered by the existence of quaternary calcium ferrite (abbreviated by CF), which is reported the complex crystalline mineral produced from Fe2O3, CaO, SiO2 and Al2O3. In all the previous analyses for reduction reaction of iron oxides in a blast furnace, sinter was treated as pure iron oxides (hematite and magnetite); the existence of CF was disregarded. Afterward, final fractional reduction of sinter in hematite to magnetite stage was found out to be about 70 % at lower reduction temperatures, which was caused by the irreducibility of CF in this region and verified with XRD analysis. By changing reduction temperature and reducing gas composition, we determined the border line between CF (= 'Fe2O3') and 'Fe3O4'. Equilibrium relations for 'Fe3O4' / 'FeO' and 'FeO' / 'Fe' were reported by Prof. Maeda, et al, where 'Fe2O3', 'Fe3O4', 'FeO' and 'Fe' designate hematite, magnetite, wustite and iron stages of CF, respectively. Reduction steps for CF can be written as: CF (= 'Fe2O3') a†’ 'Fe3O4' a†’ 'FeO' a†’ 'Fe', which are much the same as those for pure iron oxides. However, a reported variation of gas composition with temperature measured in a blast furnace shows that the gas composition in the thermal reserve zone is only a little higher than the wustite/iron equilibrium, the reduction potential of which is less than 'FeO' / 'Fe' equilibrium and hence 'FeO' cannot be reduced to 'Fe'. Therefore, gaseous reduction model for sinter has been developed in consideration of CF reaction process; UCS model for six interfaces has been proposed to take into account reaction processes of CF as well as pure iron oxides. Trial comparison of the calculated reduction curve with our previously reported experimental data mentioned above under simulated blast furnace conditions has shown rather reasonable agreement. The present model will play an important role in analyzing the reduction rate of sinter, in which CF is existing or even intentionally increasing to suppress the bad effect of SiO2.