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A previous ACARP project (C14074, Coke Mineral Matter Reactions) presented the results of a study of the role of mineral matter and mineral matter reactions in the degradation of coke through their influence upon coke reactivity and coke strength.
The results of the study showed that mechanisms previously proposed to explain coke weakening in the lower part of the blast furnace were not applicable and further discussions with steel industry personnel showed that the mechanism of coke degradation in the lower part of the blast furnace is poorly understood and an important research area. The aim of this study was to examine the nature of mineral reactions in the lower part of the blast furnace and their effect upon coke degradation through the application of quantitative mineralogy. The role of temperature and gas composition was also assessed in this study in order to determine the relative effect of these two variables on coke degradation.
As noted in the first study, the postulated mechanism of coke weakening by silica reduction is not supported by this research. Silicon carbide was present only as minor phase, indicating that the quantitative reduction of silicon oxide that has been observed when heating cokes in inert atmospheres does not occur in blast furnaces and hence the loss in coke strength is unlikely to be due to the formation of silicon carbide. Conditions in the lower part of the blast furnace are strongly reducing, especially in the raceway as indicated by the presence of significant iron silicide. A possible mechanism of coke degradation in the lower part of the blast furnace is by iron catalysed graphitisation in strongly reducing regions leading to the formation of easily abraded coarsely crystalline graphite.
The results of the microstrength tests show the feed coke appears to be the weakest, and the raceway coke the strongest. The Phase 2 Bosh cokes show a progressive increase in strength with decreasing particle size.
The variation in microstrength shows a positive correlation with increasing amounts of fine anisotropic component and increasing Lc values, indicating increasing coke crystallinity, although the trend is not as evident as for the petrographic data. Both of these observations would suggest that temperature is an important factor in determining coke strength.
Chemistry may also be an important factor as, in the Phase 2 bosh samples, the increasing strength with decreasing particle size is correlated with decreasing silica contents and increasing iron, calcium, magnesium and potassium contents. The relationship with mineralogy is less clear.
Unlike the correlation shown by the furnace cokes of increasing microstrength and degree of crystallinity, there is no evidence of a simple relationship between the coke crystallinity values and the coke microstrength of the annealed cokes.