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Abstract
An in-depth study has been carried out on the reduction, softening and melting behavior
of olivine based pellets in the experimental blast furnace. The aim of the project was to
develop a fundamental understanding of the reduction mechanisms of olivine based
pellets and to develop a correlation between the reduction rate and the softening
behavior in the cohesive zone of blast furnace. The carburization characteristics of
reduced iron were also investigated by examining olivine pellet and coke samples
excavated and probed from an experimental blast furnace as well as the experimental
investigations of pure hematite and heat treated coke.
X-ray diffraction analysis was used to successfully determine the reduction degree of
olivine pellets in different parts of the experimental blast furnace. These results were
found to be consistent with assessments of reduction degree based on a detailed
chemical analysis. The average reduction degree of iron oxide was seen to increase as
the pellets descended towards lower zones of the EBF. Up to 75% reduction was
completed before the pellet had reached the cohesive zone; remaining reduction was
completed within the cohesive zone. Coke Lc showed a linear variation with
experimental temperature above 11000C; a correlation was established to estimate
furnace temperature as a function of EBF depth.
The reduction degree of iron ore pellet showed a linear correlation with distance from
the stock line of the EBF to the upper part of cohesive zone. But an abrupt increase in
reduction rate was observed in the cohesive zone, a result observed in both EBF and
experimental studies. The presence of olivine did not have much influence on the
reduction degree of iron ore pellets for temperatures below 1100oC in the upper shaft
zone of the EBF. However, olivine was found to increase the rate of reduction in the
advanced stages of reduction in the cohesive zone for temperatures in excess of 1100oC.
This effect was attributed to the formation of increased amount of molten iron oxide
within the pellet.
The initial melt formation and acceleration of the reduction rate in the cohesive zone of
the EBF were also investigated. From the comparison between the reduction degree of
excavated olivine pellets in this study and previous studies of EBF, it was found that the
excavated pellets were located in the cohesive zone and increase of reduction rate in this
zone could be verified by not only the change of gas composition, but also initial melt
formation containing FeO phase.
The chemical composition of slag phases of excavated pellet samples were located on
the line between 2FeO·SiO2 and 2MgO·SiO2 in the FeO-MgO-SiO2 ternary phase
diagram. This result is in good agreement with the assumption that the initial slag
formation of the olivine pellets had proceeded from the fayalite and after reduction the
FeO contents in the slag phase had decreased and eventually precipitated as slag with a
higher melting point. From the analysis of carbon contents of the excavated pellets in
cohesive zone, it was observed that the carburization of iron pellets began after the
complete reduction. This study has established that the reduction rate of iron pellet is
the rate controlling step for carburization irrespective of the carburization reaction by
the solid carbon.