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Numerical Simulation of Slag Foaming with Reaction Kinetics in Oxygen Steelmaking

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Numerical Simulation of Slag Foaming with Reaction Kinetics in Oxygen Steelmaking NUMERICAL SIMULATION OF SLAG FOAMING WITH REACTION KINETICS IN OXYGEN STEELMAKING A Thesis Submitted for the Degree of Doctor of Philosophy By Anuththara Kirindigoda Hewage Faculty of Science, Engineering and Technology Swinburne University of Technology Melbourne, Australia 2017 Declaration I, Anuththara Kirindigoda Hewage, declare that the work in this thesis presented for the degree of Doctor of Philosophy is entirely my work. The work is original and to the best of my knowledge, does not contain any material that has been accepted for any other academic award, or previously published or written by another person, except where due reference is made in the thesis. Further, I warrant that I have obtained, where necessary, permission from the copyright owners to use any third party copyright material reproduced in the thesis, or to use any of own published work in which the copyright is held by another party. This work was carried out during the period from March 2013 to September 2016 under the supervision of A/Prof. Jamal Naser and Prof. Geoffrey Brooks. ----------------------------------------------------- Anuththara Kirindigoda Hewage Certification This is to certify that the above statement made by the candidate is correct to the best of our knowledge. Associate Professor Jamal Naser Professor Geoffrey Brooks ii Research abstract Foaming is an important phenomenon that is commonly encountered when gas is blown through a viscous liquid. Foams are a common occurrence in oxygen steelmaking which is produced by trapping the gases in the slag layer. With the progression of the blow, the quantity of slag as well as the gases generated increase, and consequently, the slag foaming also increases. Hence, slag foaming has to be properly controlled for a continuous and efficient production process. Thus, a thorough understanding of foams and the foaming process is necessary to optimize the process by minimizing the slag foaming. Traditionally, the uninterrupted operation of the steelmaking process is the sole responsibility of the operator, which is recently surpassed by models developed via experimental studies and theoretical/computational analysis. There are several models available in the literature focusing on different aspects of the steelmaking process including refining kinetics and slag foaming using different approaches. Among those models, the CFD model developed by Sattar[1] to predict foaming and decarburization in oxygen steelmaking was selected as the predecessor work of the present study. This CFD model developed by Sattar.[1] was able to predict foam height, the population of the ten bubble classes, decarburization, and heat generation in the oxygen steelmaking process. Despite the better performance of this model, several inaccuracies were identified such as lack of consideration of refining kinetics and incomplete criteria for formation, drainage, and collapse of foam, which established the background for the present study. The primary goal of the present study was to improve and extend the model developed by Sattar[1] with a semi-empirical kinetics model to calculate and predict the refining behavior of steel and more enhanced criteria for formation, drainage, and collapse of foam. In the present study, the CFD model was improved and extended in three main stages. In the first stage, a semi-empirical kinetics model was developed and incorporated into the CFD foaming model to predict the refining behavior of main impurity elements (i.e. C, Si, Mn, and P). The kinetics model was based on first order diffusion kinetics, and the iii performance of this model was analyzed using the data reported in the IMPHOS study[2] before incorporating into the CFD model. The kinetics model was then incorporated into the main CFD model, and the model predictions on the removal behavior of C, Si, Mn, and P were compared with the observed data reported in IMPHOS[2]. The removal behavior of all the four elements was captured reasonably well by the kinetic model incorporated in the CFD foaming model. In the second stage, the CFD model was further enhanced by improving the criteria for formation and collapse of foam and incorporating continuous foam drainage. At this stage of the CFD model, it consisted of three phases: liquid phase (slag and metal components), gas phase (gas bubbles, O2 and CO) and foam phase (foam bubbles and gas and liquid in foam). The foam phase was produced from liquid and gas phases using the same formation criteria developed by Sattar[1]. The predicted foam height at this stage of the model was representative of the average foam height measured in the IMPHOS study[2]. In the third stage, the three phases CFD model was further enhanced to a four phases model by introducing slag as a separate phase. The aim of this stage of the CFD model was to produce foam from slag, upgrading the foaming model to replicate slag foaming in oxygen steelmaking. The four phases included in the CFD model were the gas phase (gas bubbles, O2 and CO), the liquid phase (hot metal components), the slag phase (oxides and fluxes) and the slag foam phase (foam bubbles, liquid, slag and gas in foam). Once the slag phase was formed, it produced the foam phase in combination with the gas phase, and the same foaming model used in the three phases model was employed in this stage replacing the liquid phase with the slag phase. The slag foam height predictions of the four phases CFD model were in good agreement with the observed foam height data measured in the IMPHOS study[2]. The scope of the present study was limited to improving the CFD model developed by Sattar[1] in terms of its predictions on the removal behavior of C, Si, Mn and P and the evolution of slag foam height by incorporating sub-models for removal kinetics and foam drainage, and improving the existing sub-models for formation and collapse of foam. Introducing the slag as a separate phase and producing foam from slag and gas was iv carried out as part of improving the model predictions on foam height evolution. The other sub-models such as bubble break-up and coalescence which were in the initial CFD model were not modified in the present study. Even though the predictions of the present CFD model were in reasonably good agreement with the observations reported in IMPHOS pilot plant trials[2], further improvements to the present CFD model are possible. Suggestions for such improvements include incorporating a more rigorous kinetics model instead of the semi- empirical kinetics model, use of more sophisticated criteria for creating the slag phase and calculating the evolution of physical properties of slag and slag foam via more rigorous relationships. In conclusion, the results obtained from the CFD model are a strong step towards developing a general CFD model for oxygen steelmaking process, and therefore, the present study represents a significant contribution to knowledge in that regards. A general CFD model of oxygen steelmaking process as such will be able to predict every aspect of the process facilitating the operators to pre-define the optimal process conditions and optimum required raw material quantities and energy requirements. v Acknowledgements A life episode in a foreign country entirely dedicated to obtaining the doctoral qualification is always a significant, challenging and interesting experience in one’s career. At this point of submission of my thesis, I would like to recall the empowering and comforting help I received from all parties those took part in my personal life and my academic life. So I extend my sincere gratitude to all of them on behalf of their encouragement and support for completing my doctoral degree with success. First, I would like to acknowledge my supervisors for their great support on my research and career development during the candidacy. Dr. Jamal Naser, my primary supervisor, gave me this opportunity and his supervision always encouraged and motivated me in achieving the goals. His technical expertise and the experience in the field was a huge strength for me throughout the candidacy. Professor Geoffrey Brooks, my co-supervisor, showed me the technical perspective of the research and encouraged me to deliver the results in a way that is suitable for the industry. His friendly and encouraging advice and critics refined my understanding of the research issues and improved my skills in communicating the research effectively. Secondly, I would like to extend my gratitude to my colleagues in High-Temperature Processing (HTP) group, especially, Abdus Sattar, Naoto Sasaki, and Bapin Rout. Abdus Sattar was the author of the predecessor work of the present study, and he helped me a lot in understanding his work and the software with which I needed to work. Naoto Sasaki and Bapin Rout helped me in learning more about the oxygen steelmaking process with their industry experience and collaborated in my research work. So I would like to thank them and the other members of the HTP group including Neslihan Dogan, Shabnam Sabah, Mehedi Mohammad, Jaefer Yenus and Epma Putri as well for their contribution and encouragement towards the success of my research. Further, the assistance I received from Arafat Bhuiyan in getting familiar with the new environment in the faculty at the start of the degree and throughout the degree was immense, and I extend my sincere gratitude to him. Also, I acknowledge the support I received from the other members of the CFD group under the supervision of Dr. Jamal Naser, especially, Rezwanul Karim and Hamid Sarhan. vi Furthermore, the guidance and support I received from the staff of the Faculty of Science, Engineering and Technology and other related departments at the Swinburne University of Technology were also invaluable for improving the skills I required to conduct and communicate my research efficiently and effectively and for improving my career and employability.
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