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Fundamental Investigation of Kinetics of Ferro-Silicon Reactions in Cupola Scrap Melting Processes

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Fundamental Investigation of Kinetics of Ferro-Silicon Reactions in Cupola Scrap Melting Processes The University of New South Wales Faculty of Science School of Materials Science and Engineering Fundamental Investigation of Kinetics of Ferro-Silicon Reactions in Cupola Scrap Melting Processes A Thesis in Materials Science and Engineering By Pedro Javier Yunes Rubio Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY March 2013 To my family and wife CERTIFICATE OF ORIGINALITY I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, nor material which to a substantial extent has been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due to acknowledgment is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged. Signed _______________________ Pedro Javier Yunes Rubio COPYRIGHT STATEMENT I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provision of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorize University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied for a partial restriction of the digital copy of my thesis or dissertation. Signed ______________________ Date ________________________ AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation has occurred and if there are any minor variations in formatting, they are the result of conversion to digital formats.’ Signed ________________________ Date __________________________ iii ACKNOWLEDGEMENTS I would like to express my deepest sense of gratitude and appreciation to my supervisor Professor Veena Sahajwalla for her unrelenting support and guidance throughout my research and her willingness and availability to discuss several aspects of the project. Her assertiveness and appropriate guidance as well as openness for constructive discussions were very encouraging and intellectually stimulating. I also owe an enormous debt of gratitude to Associate Professor Rita Khanna for her constructive discussions, her guidance and continuous encouragement and support to get the work completed. I wish to thanks the support from Russell Bush, Technology Manager from Tyco Water for his contribution and technical expertise and his valuable suggestions. I also wish to acknowledge the financial support and interest from the Australian Research Council. I am extremely grateful to Mr. N. Saha - Chaudhury for his great help in making things happen in our daily laboratory work. His opportune advice and support in my experimental work is highly appreciated. I would like to sincerely thank to Professor Chris Sorrell and Dr. Haiping Sun for their valuable and constructive coaching during my research work. And last but not least, I wish to thank to my family for their constant encouragement and inspiration throughout the course of my project. Very special thanks to my wife Marcela for her support and inspiration to achieve this result. iv LIST OF PUBLICATIONS Journal Papers 1- Yunes, P. J., Hong, L., Saha-Chaudhury, N., Bush, R. and Sahajwalla, V. Dynamic Wetting of Graphite and SiC by Ferrosilicon Alloys and Silicon at 1550 °C. ISIJ International, 2006. 46 (11): p. 2006. 2- Yunes, P. J., Saha-Chaudhury, N. and Sahajwalla V. Carbon Dissolution Occurring during Graphite-Ferrosilicon Interactions at 1550 °C. 2009. 49 (12) p. 1868. 3- Yunes, P. J., Khanna, R., Saha-Chaudhury, N. and Sahajwalla, V.Simultaneous Decarburisation and Oxidation Reactions occurring in Silicon and Ferrosilicon alloys at 1823 K. Steel Research International, 2013. 84 (1) p. 40. Conference Paper 4- Yunes, P. J. and Sahajwalla, V. Kinetics of Carbon Dissolution in Ferrosilicon Alloys, International Symposium of Research Students on Materials Science and Engineering, Chennai, India, December 20-22, 2004. v 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 vi 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 chemical kinetics. 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. vii TABLE OF CONTENTS PAGE Certificate of Originality ii Copyright Statement iii Acknowledgments iv List of Publications v Abstracts vi Table of Contents viii List of Figures xii List of Tables xviii Chapter 1 Introduction 1 1. Introduction 2 1.2 Scope of the project 4 Chapter 2 Literature Review 6 2. Literature Review 7 2.1 Outline of the Cupola Furnace 7 2.1.1 Zones of the Cupola Furnace 8 2.2 Study of the phase diagrams. 10 2.2.1 Phase diagram Fe – Si system. 10 2.2.2 Phase diagram Si – C system. 11 2.2.3 Fe-Si-C ternary diagram. 13 2.3 Reaction kinetics 15 2.4 Chemical reaction and Mass Transfer 17 2.4.1 Previous studies of carbon dissolution in iron. 18 2.4.2 Previous studies of Carbon dissolution in silicon and 21 ferrosilicon viii 2.5 Kinetics of oxidation 24 2.5.1 Previous studies of decarburization in iron. 25 2.5.2 Previous studies of decarburization in silicon and 32 Ferrosilicon. 2.6 Wettability 38 2.6.1 The system of liquid iron and graphite. 41 2.6.2 Wettability of liquid iron on alumina 44 2.6.3 Wettability of silicon and ferrosilicon on SiC 45 2.7 Summary of the literature review. 46 Chapter 3 Experimental
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