Fundamental investigation of kinetics of ferro-silicon reactions in cupola scrap melting processes

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Copyright: Yunes Rubio, Pedro Javier
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Abstract
This work investigates high temperature interactions of silicon and ferrosilicon alloys with graphite as well as the reactions occurring in the presence of oxidising gases. These reactions play a key role in the scrap-melting cupola process. Using the sessile droplet method, the dynamic wetting of synthetic graphite by liquid ferrosilicon alloys containing 24.7 and 74 %Si and silicon (98.5 % Si) at 1550 °C was investigated. Silicon 98.5% and ferrosilicon alloys containing 74 and 24.7% Si showed good wetting behaviour (θ < 90º) with synthetic graphite at 1550 °C. Full wetting was observed for silicon 98.5 and ferrosilicon 74 within the first 90 seconds. However, the final contact angle value appeared higher for the low-silicon ferroalloy and remained steady around 70 degrees during the 2 hours-run. The role of the interfacial product formed and its relationship with the dynamic wetting phenomena was also investigated. The formation of SiC at the interface appeared 30 seconds after melting for Si 98.5, while this was observed after 60 seconds for FeSi 74, and after 30 minutes for FeSi 24.7. Further wettability investigations carried out on SiC substrates showed trends similar to the ones observed on synthetic graphite. Full wetting was observed for Si 98.5 and FeSi 74 after 80 and 90 seconds, while FeSi 24.7 showed a different pattern, since the contact angle decreased rapidly during the first 10 minutes and remained steady around 40 degrees after that time. The dynamic wetting appeared to be strongly dependent on the rate of formation of SiC at the ferrosilicon-graphite interface. A kinetic mechanism has been developed for the carbon dissolution phenomena in ferrosilicon alloys. The overall rate constants at 1550 °C for Si 98.5, FeSi 74 and FeSi 24.7 were determined to be 3.8, 3 and 3.9 x 10-3 (s-1) respectively. These did not vary significantly across samples under investigation. A rapid increase of carbon pickup was observed during the initial few minutes and remained fairly constant later on. The faster rate observed in the case of FeSi 24.7 was explained on the basis of delayed formation of SiC interfacial product which had a retarding effect on the overall process and dictated carbon transfer. In depth and detailed investigations were carried out on the effect of the alloy composition, oxygen partial pressure and flow rate on interactions at 1550 °C . Significant differences were observed in the weight gain and carbon loss between these three alloys; both decarburisation and silicon oxidation reactions were found to occur simultaneously. There was a clear evidence for two rate regimes: the rate of decarburisation was found to be much higher during the initial 2 minutes and a much slower rate was observed in later stages for all specimens. These rate regimes were explained in terms of the extent of surface coverage with the reaction product silica. No significant effect was found on the decarburization rates when the proportion of oxidizing gas (CO2) was increased from 20 to 100%, indicating that mass transfer in the gas phase was not a dominant rate controlling step compared to chemicalkinetics. The net weight gain in these alloys was found to be due to the combined influence of decarburization (weight loss due to the generation of a gaseous product) and silicon oxidation (weight gain due to silica formation on the sample surface). The results of this investigation showed that the silicon losses in the cupola process can be better managed by using lower grade ferrosilicon alloys as well as an adequate air blowing regime.
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Yunes Rubio, Pedro Javier
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Publication Year
2013
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PhD Doctorate
UNSW Faculty
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