An Insight into the Mechanism and Kinetics of Reactions In BOF Steelmaking: Theory vs Practice A. K. Shukla 1) * , B. Deo 1) , S. Millman 2) , B. Snoeijer 2) , A. Overbosch 2) , and A. Kapilashrami 2) 1) Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, 208016, India 2) Tata Steel Research, Development & Technology, PO Box 10.000, 1970 CA IJmuiden, The Netherlands * Corresponding author; e-mail: shukla@iitk.ac.in A thermodynamic (equilibrium) model is developed for the BOF process. The predictions of this model show the trend of reactions when the process is considered to be at thermodynamic equilibrium. In the case of a real process, however, some tuning and adaptation becomes necessary to make more accurate predictions. A dynamic model is developed in which the kinetics of scrap dissolution is also incorporated. A comparison of the results of the equilibrium and dynamic models (made with some tuning parameters) reveals that mixing is the prime factor which can alter the course of reaction at any particular instant. Mixing is greatly affected by oxygen flow rate, lance height and the nature of scrap. The understanding of the secrets of process dynamics becomes clearer with this approach, providing a good insight into the process. Keywords: thermodynamics, Gibbs free energy, mathematical model, BOF steelmaking Submitted on 15 March 2010, accepted on 31 May 2010 Introduction The modeling of reactions in BOF steel making has been a topic of keen theoretical and practical interest over last several decades and the literature is replete with both laboratory and plant investigations [1–23]. In the present work the focus is essentially on decarburization reaction in the middle blow period of BOF. The decarburization reaction in BOF has been inves- tigated by several workers in the past [1, 2]. It has been suggested that decarburization may take place in the slag- metal emulsion, directly under the jet, on the refractory walls and within the CO gas bubbles rising in the bulk metal. In the earlier works, the slag-metal emulsion was considered to play a dominant role in decarburization. It was assumed that decarburization proceeded through the following intermedi- ate steps: 1. Ejection of metal droplets into the slag phase due to (a) the effect of impinging oxygen jet and (b) the rising CO gas bubbles which carry a thin film of metal into the slag phase. 2. The metal droplets are super saturated with oxygen at the surface in spite of their high carbon content. It is silicon dissolved in metal/droplet which prevents the nucleation of CO bubbles, till the silicon content has decreased to a low level. 3. The metal droplets (supersaturated with oxygen) generate CO at a much faster rate when they come in contact with the gas bubbles which provide additional nucleation sites. Homogeneous nucleation is also possible at very high oxygen super saturation. The emulsification of slag takes place due to the entrain- ment of gas and metal droplets in the slag. According to the above mechanism, the decarburization rate should depend strongly on the residence time of metal droplets in the slag phase. If it is considered that the steps 1 and step 3 are very fast due to high turbulence imparted by the top lance and also due to the rising CO gas bubbles, then the step 2 (oxygen transfer to the metal droplets) may become a rate controlling step. In support of this hypothesis, it has been practically observed that slag contains a large number of metal droplets. The carbon content of the droplets is also slightly less than that of the bulk metal. In the middle part of the blow, however, the slag has a much lower FeO content (close to 8% by mass) but the rate of carbon removal is still high. It suggests that another equally fast mechanism of carbon removal exists. It is possible that oxygen rich metal directly under the jet impact zone extends deep into the bulk metal bath and that CO evolves within the metal at numerous heterogeneous sites (viz. on the surface of rising gas bubbles in the metal), or on refractory lining. The rising CO gas bubbles further enhance recirculation of metal, thereby making this particular mechanism of carbon removal more effective. The laboratory investigations on decarburization kinetics, both for high and low carbon [5, 6] melts, have demonstrated the following aspects. (a) For high carbon melts at low partial pressure of oxygen 1. The rate of decarburization is independent of gas flow rate and is a linear function of oxygen partial pressure. 2. Oxygen activity in the melt is close to equilibrium with the carbon in the melt. 3. Sulphur retards the rate of decarburization due to reduction in the number of sites available for the decarburization. Nitrogen also acts in a similar way due to dissociative adsorption at surface sites. 4. The mechanism of decarburization in Fe-C melt is controlled by dissociative adsorption of oxygen. DOI: 10.1002/srin.201000123 steel research int. 81 (2010) No. 11 940 ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.steelresearch-journal.com