Development of Dynamic Models for Oxygen Steelmaking

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1 Development of Dynamic Models for Oxygen Steelmaking Geoffrey Brooks, Neslihan Dogan, M. Akbar Rhamdhani, Morshed Alam and Jamal Naser Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, P.O Box 218, Hawthorn, Victoria 3122, Australia Abstract The development of useful models for predicting and controlling Oxygen Steelmaking is very difficult because of the complex multi-phase physics and chemistry of the process. In particular, the relationship between the physics of droplet generation and behaviour in the emulsion with the physico-chemical behaviour of the droplets has been difficult to quantify. Recently, researchers at Swinburne University of Technology have been developing models that link the dynamic physics associated with the lance with the chemical processes. This has resulted in a more comprehensive understanding of the process dynamics of the oxygen steelmaking and a fully predictive model for the de-carburisation kinetics is under development. Result and discussion on how this work impacts on the practical operation of steelmaking furnaces will be discussed in the paper. Introduction Oxygen steelmaking remains the dominant route for steel production. 1) In recent years, Electric Arc Furnace (EAF) steelmaking has tended to duplicate aspects of oxygen steelmaking with high speed injection of gases and increased productivity resulting from the increase of chemical energy input into the operations. 2) Whilst, Oxygen Steelmaking is an impressive technology, in terms of productivity, product quality and reliability, there are still many aspects of the process that are difficult to optimise. This difficulty reflects partially the extreme conditions within the process (i.e. it is hard to measure any process parameters) but also theoretical difficulties in establishing the appropriate models to deal with the high transient conditions within the reactor. Four basic configurations of oxygen steelmaking currently exist, namely: i) top blown basic oxygen steelmaking which uses a lance to inject the oxygen, ii) top blown basic oxygen steelmaking with inert bottom gas stirring, iii) bottom blown steelmaking using shrouded tuyere technology, and iv) combined top and bottom blown basic oxygen steelmaking using a combination of tuyere and lance injection of oxygen. Whilst there are some differences in chemistry and operation of these processes, all use pure oxygen to oxidize impurities from pig iron, and high speed gas injection to generate emulsions with large interfacial areas 1

2 between slag, metal and gases. Top blown oxygen steelmaking is the dominant arrangement in industry at this time and the basic process layout is shown schematically in Fig. 1. Fig-1: Schematic representation of top-blowing oxygen steelmaking The major reactions in oxygen steelmaking are presented in Table I. The dissolution of oxygen into the metal via gas and slag phase (1-4), decarburization through dissolved Oxygen pick up by the metal: Table I The major reactions in oxygen steelmaking. O 2 (g) = 2O (1) (FeO) = Fe + O (2) (Fe 2 O 3 ) = 2(FeO) + O (3) CO 2 (g) = CO(g) + O (4) Oxidation of elements in the metal: C + O = CO(g) (5) Fe + O = (FeO) (6) Si + 2O = (SiO 2 ) (7) Mn + O = (MnO) (8) 2P + 5O = (P 2 O 5 ) (9) 2

3 oxygen (5), and oxidation of Fe, Si, Mn, and P (6-9) are the most important reactions for the overall kinetics of the steelmaking process. These reactions take place in three distinct regions within a top blown oxygen steelmaking vessel: Region 1, Metal-Oxygen. This is the region where oxygen, injected at supersonic speeds, is in direct contact with the molten metal. In this region oxygen pick up is via reaction (1), and reactions (5) to (9) follow from this initial pick-up of oxygen. The size of this impact region is a function of flowrate, lance height, lance geometry and physical properties of the slag and metal 1). This region is sometimes referred to as the hot zone i.e. referring to the extreme heat generated in this region. Region 2. Slag-Metal. This is the region where the stirred bath of metal is in direct contact with emulsified slag above. In this region, oxygen for reactions (5) to (9) is supplied from two sources, (i) the oxygen dissolved at the oxygen metal interface and subsequently transferred to the slag-metal interface by the intense stirring in the metal bath and (ii) oxygen transferred from the emulsified slag through reactions (2-3). Region 3. Slag-Metal-Gas Emulsion. In this region, an FeO rich slag is in contact with droplets of molten metal. Gas generated from decarburization from both the metal-oxygen and slag-metal regions, as well as gas generated from decarburization in the emulsion, results in a highly stirred system with significant quantities of gas hold-up in the emulsion. In this region, oxidation of the metal occurs via reaction (6) before the various oxidation reactions, reaction (5) and reactions (7-9), take place. Estimates of the fraction of total metal in the emulsion vary from 1 to 78% and estimates of interfacial area vary from 8 to 250 m 2 /tonne 27). This variation in fraction of metal emulsified and interfacial area is also reflected in the wide range of droplet sizes recovered from samples, 0.23 mm to 25.4 mm 27). In addition, to these steelmaking reactions, there are also other important physical/chemical processes taking place that also affect the process, particularly flux dissolution, scrap melting and refractory dissolution 1). However, it would be correct to say that overall productivity of the process is set by the kinetics of the steelmaking reactions described above and not by flux dissolution, scrap melting and the degradation of the refractory, important as these processes are to costs, process stability and stable operation. This brief description of the process illustrates a number of important points in the modelling of oxygen steelmaking, namely, (i) the process is highly dynamic, (ii) the kinetics of major reactions are strongly connected to droplet generation via injection and (iii) there is currently limited understanding of how much decarburisation occurs in the Region 1 compared to Region 3. The first author of this paper and his co-workers at McMaster University, CSIRO and now Swinburne University of Technology (Subagyo, Pan, Naser, Rhamdhani, Dogan, Alam, Coley and Irons) have been working over the last decade to further quantify the dynamics of oxygen steelmaking process, in particular, they have attempted to develop mathematical models to predict the interaction between the jet with the bath, the generation of droplets, the trajectory and residence of droplets in the emulsion, the decarburisation kinetics in the emulsion, the kinetics of scrap melting and the dissolution of flux. 18,22,24,26&27) 3

4 All of this work is part of a along term effort to develop a global model of oxygen steelmaking that links process dynamics with measurable outputs from the process using sound physics and chemistry. 26 In this paper, we will summarise two components of the work, namely the development of the supersonic jet from the lance and the generation of droplets through the interaction between the jet and the bath. These two areas illustrate some of the challenges and different techniques that can be used to model the steelmaking process. Development of Supersonic Gas Jet Supersonic gas jets are widely used in Basic Oxygen Furnace (BOF) and Electric Arc Furnace (EAF) steelmaking for oxidizing the dissolved impurities from the liquid iron inside the furnace. Supersonic gas jets are preferred over subsonic jets because of high dynamic pressure associated with it which results in higher depth of penetration and better mixing. De Laval nozzles are used to accelerate gas jets to supersonic velocities of around 2.0 Mach in steelmaking 1). When a supersonic jet exits from a Laval nozzle, it interacts with the surrounding gas to produce a region of turbulent mixing as shown in Fig-2. This process Fig-2 Regions of a supersonic jet exiting from a Laval nozzle 10) results in an increase in jet diameter and decrease in jet velocity with increasing distance from nozzle exit. A supersonic jet emitting from a nozzle can be divided into two different regions: 1) a coherent region where the velocity of the gas is supersonic and is equal to the nozzle exit velocity and 2) a fully developed flow region. The length of the coherent region is also known as the potential flow core length or coherent length. Various experimental and numerical investigations of the behaviour of supersonic oxygen jet after emerging from the 4

5 Laval nozzle have been reported in the literature 2-10). At room ambient temperature the potential core length of the supersonic jet was found to be around 10 times the nozzle exit diameter. However, these results are not transferable as the temperature inside a steelmaking furnace is much higher (1800K) than the room temperature. Sumi et al. 4) studied experimentally the behaviour of supersonic oxygen jet at three different ambient temperatures: 285K, 772K and 1002K. Their results showed that the velocity attenuation of the jet was restrained and the potential core length was extended under high ambient temperature condition. Present authors performed computational fluid dynamics (CFD) modelling to investigate the supersonic gas jet characteristics at different ambient temperatures and validated their results against the experimental results of the Sumi et al 4). The experimental 4) and CFD 10) results of the axial velocity distribution of supersonic jet at different ambient temperatures are shown in Fig-3: Fig-3 Axial Velocity distribution of the supersonic jet at different ambient temperatures. 10) The supersonic Laval nozzle that was used in both CFD and experimental studies has an exit diameter of d e =9.2mm. The mass flow rate through the Laval nozzle was 150Nm 3 /hr and the exit Mach number was M e =1.72. Fig-3 shows that the potential core length of the supersonic jet increases at high ambient temperature conditions. At steelmaking temperature of around 1800K, the potential core length is around 2.5 times than that at room ambient temperature. This occurs because at high ambient temperatures the density of the ambient fluid is low. Therefore, the mass addition to the jet from the surrounding medium is low which reduces the growth rate of turbulent mixing region. As a result, the velocity decreases more slowly and potential core length of the jet increases at high ambient temperatures. Fig-4 shows the dynamic pressure distribution on the centre axis of the jet. As expected, dynamic pressure of the jet is higher at high ambient temperatures. Dynamic pressure of the jet is more important characteristic than the velocity. Because, though at high ambient temperatures the impact velocity of the jet on the liquid melt is higher, the density of the jet is 5

6 lower. Dynamic pressure of the jet takes into account both the velocity and density changes. The higher the dynamic pressure, the bigger the momentum transfer to the liquid bath which will result in higher depth of penetration in the liquid bath. This in turn enhances the mixing of oxygen and liquid melt and therefore improves the decarburization rate. Fig-4 also shows that as the distance from the nozzle exit increases, the relative difference between the dynamic pressure distribution of the jet at different ambient temperatures decreases. If the distance between the liquid bath and nozzle exit is more than 60 nozzle exit diameter then the effect of high ambient temperature is negligible because the dynamic pressure of the impinging jet on the liquid bath would be almost same at all ambient temperatures. Fig-4 Axial dynamic pressure distribution of the supersonic jet at different ambient temperatures 10) Droplet Generation The supersonic gas jets generate droplets upon impingement on liquid melt inside the steelmaking furnace. Droplet generation has both beneficial and detrimental effects in steelmaking. It is a crucial part of the process kinetics of oxygen steelmaking because it contributes to large interfacial area during the blow which in turn affects the mass transfer between metal and slag. 1) On the other hand, it may cause wearing of refractories, skulling on the mouth of the vessels and lances which can result in loss of productions ) Several experimental studies and mathematical models investigating the influence of the intensity of jet momentum on the metal droplet generation rate have been established in the past three decades. 12, 14-21) Subagyo and co-workers defined a dimensionless blowing number ( N B ) based on Kelvin-Helmholtz instability theory. This dimensionless number relates the jet momentum intensity and the properties of liquid metal and given by the following equation, 6

7 N B ρ U 2 = G G (1) 2 σgρ L where ρ G is the density of gas(kg/m 3 ), U G is the critical gas velocity(m/s), σ is the surface tension of the liquid (N/m), and ρ L is the density of the liquid(kg/m 3 ). Subagyo et al. 22) evaluated their results against the experimental study of Standish and He, 14, 19) which was undertaken at cold temperatures. The results were in good agreement with previous studies, as beseen in Fig-5. The Blowing number can be related to the rate of droplet generation per unit volume of the blown gas based on the previous cold and hot models 18). The correlation is given in Eq. (2). R F B G 3. 2 = (N B ) [ (NB) ] (2) Here R B refers to droplet generation rate (kg/min) and F g refers to volumetric flow rate of blown gas (Nm 3 /min). Fig-5 The rate of droplet generation as a function of blowing number 18) In order to quantify the influence of high ambient temperature on the droplet generation rate, the present authors calculated the blowing for the supersonic jet at different ambient temperatures using the calculated velocity distributions. 10) Fig-6. shows the variation of 7

8 Fig-6 Variation of blowing number with nozzle bath distance at different ambient temperatures. blowing number with distance from the nozzle exit (it is assumed as the distance between nozzle exit and liquid bath) at different ambient temperatures. The surface tension of the liquid melt was taken as 1.9 N/m assuming that the liquid melt is iron. 23) The variation of surface tension with temperature and composition was not considered because the transfer of jet momentum on liquid surface is the dominant factor for droplet generation compared to the changes in liquid properties. 24) The density of the liquid melt was taken as 7030 Kg/m 3. It is seen from the Fig-6. that the blowing number is higher for gas jets at high ambient temperatures. The higher the blowing number, the greater the droplet generation rate ( as per equation (2). Fig-7 shows the predictions of the blowing number as a function of lance dynamics to analyse the droplet generation under the given operating conditions for 200 tonne topblowing Fig-7 Blowing number as a function of lance height and blowing time 24) 8

9 oxygen steelmaking process. (Oxygen flow rate: 620 Nm 3 /min, 6-head lance with an inclination angle of 17.5, supply pressure:10 atm) The lance height is the only variable changing with time and the other blowing conditions remain constant in the industrial data reported by Cicutti et al. 25) The lance height was decreased gradually and kept constant after 7 min until the end of the blow. As shown in Fig-7, the decrease in lance height increases the blowing number thereby droplet generation rate as the blowing progresses. The calculated blowing number as a function of lance height ranges from 4.8 to 6.7. The predictions of blowing numbers in the present calculations establish good agreement with those reported by Subagyo et al. 18) and Dogan et al. 24) Based on the blowing number calculations, the droplet generation rate can be predicted simultaneously. The droplet generation rate is very important because the amount and number of metal droplets generated in the slag-gas-emulsion provides information on the size of interfacial area during the blow which in turn affects the mass transfer and overall kinetics between metal and slag. A global model of oxygen steelmaking including the kinetics of scrap melting, flux dissolution, slag chemistry, temperature profile of the system, formation and residence of metal droplets in the emulsion, kinetics of decarburization reaction in different reaction zones has been developed by the present authors. 26) In this model, total decarburization rate in the emulsion zone is obtained by the summation of decarburization rates of individual metal droplets. The generated droplets, whose residence time is smaller than given time-step, are returning from the emulsion zone. Based on this boundary condition, decarburization rate can be calculated using; M e dc dt = n i= 1 mi i 100 t t + t t ( C C ) i (3) Here the number of the droplets in the emulsion zone is represented by n, m i is weight of a single droplet (kg) and C is the carbon content of droplet (mass%). The global model is validated against the industrial data reported by Cicutti et al. 25) Fig-8 shows the predictions of decarburization rate in emulsion phase with respect to blowing number. During the blow, decarburization rate increases at 4 min then 7 min. The increase in the decarburization rate is due to the variations in the lance height. As the lance is decreased, more droplets are ejected through the emulsion zone and more of the decarburization reaction takes place in the emulsion zone. At the 7 th minute, the rate reaches peak level during the main blow and decreases back towards the end of the blow since the metal droplets contain less carbon and the driving force for the reaction between metal droplets and slag is decreased. So, overall decarburization of the process begins to decrease as decarburization in the emulsion decreases. 9

10 Fig-8 Model predictions of decarburization rate in the emulsion as a function of blowing time and the blowing number 26) Further work is currently underway to link this decarburisation model with decarburisation in the hot zone (Metal-Oxygen region in Fig-1), which in turn will be linked to flux dissolution, and scrap melting into a global model of the oxygen steelmaking process. 26) Also, we are currently validating aspects of the model against new industrial data. This work will be published in the open literature. Conclusions The modelling of oxygen steelmaking is difficult because of the complex multi-phase physics and chemistry of the process. In our work, a range of mathematical techniques have been used to model key aspects of the process, including computational fluids dynamics, semiempirical techniques and step forward solutions to ordinary differential equations. Further work to link decarburisation in the emulsion zone and in the hot zone, flux dissolution and scrap melting are underway. References 1. B. Deo and R. Boom, "Fundamentals of Steelmaking Metallurgy", Prentice Hall, Upper Saddle River, NJ, B. Allemand, P. Bruchet, C. Champinot, S. Melen, and F. Porzucek, "Theoretical and experimental study of supersonic oxygen jets. Industrial application in EAF," La Revue de Metallurgie, Vol.98, No.6, 2001, pp R. Imai, K. Kawakami, S. Miyoshi, and S.-i. Jinbo, "Effects of Blowing Conditions on Blowing Reactions in LD converter," Nippon Kokan Technical Report-Overseas, Vol.8, No.1968, pp I. Sumi, Y. Kishimoto, Y. Kikichi, and H. Igarashi, "Effect of high temperature field on supersonic oxygen jet behaviour," ISIJ International, Vol.46, No.9, 2006, pp Y. Tago and Y. Higuchi, "Fluid Flow Analysis of Jets from Nozzles in Top Blown Process," ISIJ Int., Vol.43, No.2, 2003, pp

11 6. J. C. Lau, P. J. Morris, and M. J. Fisher, "Measurements in subsonic and supersonic free jets using laser velocimeter," J. Fluid Mech., Vol.93, No.1979, pp K.-i. Naito, Y. Ogawa, T. Inomoto, S.-y. Kitamura, and M. Yano, "Characteristics of Jets from Top-Blown Lance Converter," ISIJ Int., Vol.40, No.1, 2000, pp H. Katanoda, Y. Miyazato, M. Masuda, and K. Matsu, "Numerical visualization of supersonic jets discharged into high-temperature surrounding gas," in The 10th Int. Symp. on Flow Visualization, August 26-28, 2002, Kyoto, The visualization society of Japan. 9. D. S. Tandra, A. Kaliazine, D. E. Cormack, and H. N. Tran, "Numerical Simulation of Supersonic Jet Flow Using a Modified k-epsilon Model," International Journal of Computational Fluid Dynamics, Vol.20, No.1, 2006, pp M. Alam, J. Naser, and G. A. Brooks, "CFD Simulation of Supersonic Oxygen Jet Behaviour at Steelmaking Temperature," Metallurgical and Materials Transactions B, (Published online 4th February, 2010). 11. P. McGee and G. A. Irons, "The penetration of oxygen lance jets in foaming slags," Iron and Steelmaker (I and SM), Vol.29, No.1, 2002, pp J. M. Luomala, T. L. J. Fabritius, E. O. Virtanen, T. P. Siivola, and J. J. Harkki, "Splashing and spitting behaviour in the combined blown steelmaking," ISIJ International, Vol.42, No.9, 2002, pp K. D. Peaslee and D. G. C. Robertson, "Fluid dynamics of inclined jetting on a slag/metal bath," in EPD Congress Proceedings, February 27-March 31,1994, Pennsylvania, TMS, pp. pp Q. L. He and N. Standish, "A model study of Droplet Generation in BOF Steelmaking," ISIJ Int., Vol.30, No.4, 1990, pp S. C. Koria and K. W. Lange, "Disintegration of iron-carbon drop by high velocity gas jet," Ironmaking Steelmaking, Vol.10, No.4, 1983, pp K. D. Peaslee, D. K. Panda, and D. G. C. Robertson, "Physical Modeling of Metal/Slag/Gas Interactions and Reactions," in Proc. of 76th Steelmaking Conference 1993, p D.-J. Min and R. J. Fruehan, "Rate of reduction of FeO in slag by Fe-C drops " Metall. Mater. Trans.B, Vol.23, No.1, 1992, pp Subagyo, G. A. Brooks, K. S. Coley, and G. A. Irons, "Generation of Droplets in Slag-Metal Emulsions through Top Gas Blowing," ISIJ International, Vol.43, No.7, 2003, pp N. Standish and Q. L. He, "Drop generation due to an impinging jet and the effect of bottom blowing in the steelmaking vessel," ISIJ International, Vol.29, No.6, 1989, pp A. Chatterjee and B. A.C., "Break-up of a liquid surface by an impinging gs jet," Journal of Iron and Steel Institute, Vol.210, No.March 1972, pp K. D. Peaslee and D. G. C. Robertson, "Model studies of splash, waves, and recirculating flows within steelmaking furnaces," in Steelmaking Conference Proceedings, 1994Iron & Steel Soc of AIME, pp Subagyo, G. A. Brooks, K. S. Coley, and G. A. Irons, "Generation of Droplets in Slag-Metal Emulsions through Top Gas Blowing," ISIJ Int., Vol.43, No.7, 2003, pp R. I. L. Guthrie, Engineering in Process Metallurgy, Oxford University Press, New York, 1989,pp. 24. N. Dogan, G. Brooks, and M. A. Rhamdhani, "Analysis of droplet generation in oxygen steelmaking," ISIJ Int., Vol.49, No.1, 2009, pp C. Cicutti, M. Valdez, T. Perez, J. Petroni, A. Gomez, R. Donayo, and L. Ferro, "Study of Slag- Metal Reactions In An LD-LBE Converter," in 6 th International Conference on Molten Slags, Fluxes and Salts, 2000, Stockholm-Helsinki, p N. Dogan, G. A. Brooks, and M. A. Rhamdhani, "Development of a Comprehensive Model for Oxygen Steelmaking," AISTech 2010, Pittsburgh, USA, Subagyo, G.A. Brooks, and K.S. Coley, Interfacial Area in Top Blown Oxygen Steelmaking, Ironmaking Conference Proceedings, ISS, 2002, Warrendale PA, USA, pp