Transient Behavior of Dephosphorization Kinetics in Oxygen Steelmaking B.K. Rout 1, G.A. Brooks 1, Z. Li 2, M.A. Rhamdhani1 Faculty of Sceince, Engineering and Technology, Swinburne University of Technology Hawthorn, Victoria, Australia, 3122 Phone: (+61) (03) 92145672 Email:
[email protected] 2
Tata Steel R&D, Swinden Technology Centre Moorgate, Rotherham, S60 3AR, UK
Keywords: BOF, kinetics, interfacial area, dephosphorization, rate constant ABSTRACT The kinetics of dephosphorization in steelmaking is a complex phenomenon, as it occurs under non-equilibrium conditions with transient rate parameters. It is clear that simple first order rate equations could not explain the behavior of phosphorus refining in steelmaking operations. In the present paper recent work on the kinetic modeling of dephosphorization in BOF process is reviewed. In particular, plant trial data has been analyzed and different equations for analyzing these data have been investigated. The nature of transient variables e.g. slag-metal interfacial area, rate constants, equilibrium driving force on kinetics of phosphorus refining has been investigated. Further a kinetic modeling approach of phosphorus incorporating the recent theory of bloated droplets is described.
1.
INTRODUCTION
Phosphorus is regarded as a harmful impurity due to its detrimental effect on mechanical properties of steel. The presence of high P makes the steel prone to embrittlement and causes degradation in electrical properties. Also steels used in thin sheet, deep drawing applications, pipeline and automobile exteriors demands low phosphorus as a critical requirement. Thus P removal is one of the prime concerns among metallurgical community since the early days of steelmaking. However, the mechanism of phosphorus reaction and its control in steelmaking operation is not clearly understood. Present day controls are based on end point steady state models which do not take into account the dynamic behavior of the process variables that affect the P reaction [1]. Therefore, the development of dynamic models should provide a better understanding and close control of phosphorus in oxygen steelmaking process. Earlier researchers applied equilibrium thermodynamics to analyze dephosphorization reaction and there have been numerous studies to determine phosphorus partition ratios and phosphate capacity of slag. Many of these models are empirical correlations based on slag composition and temperature and cannot find application outside the range of slag composition being studied. Moreover, during industrial operation it has been observed that dephosphorization does not reach equilibrium as it takes place between slag with high oxygen potential and metal with low oxygen potential. Therefore, several researchers have pointed out that a more rigorous kinetic treatment of dephosphorization must be necessary to simulate the process more accurately [2, 3]. Several researchers developed kinetic models assuming equilibrium of reactions at slag-metal interface. For example Robertson and Oguchi [4] developed a dephosphorization model based on coupling of reactions at the interface. In this model, the kinetics of reactions is described by double film theory assuming thermodynamic equilibrium at the interface. Several other researchers applied this principle of simultaneous slag/metal reactions as an effective modeling technique to simulate the refining process in multicomponent systems oxygen steelmaking.
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A recent study of coupled reaction model [5-9] includes dynamic equilibrium model, FeO generation model, scrap melting model, flux dissolution model to simulate phosphorous as well as other refining reactions during converter steelmaking. In addition, the latest model by Pahlevani et al. [9] includes the thermodynamic calculations of partitioning of P 2 O 5 in solid and liquid phase in slag. These models assume a steady state slag-metal interface and various empirical relationships were used to determine the rate parameters. Another group of Finnish researchers developed a simulation program CONSIM since the late 1980s [10]. The latest version of the program CONSIM-5 includes both thermodynamics and kinetics calculation along with mass transfer models. However, no thermodynamic calculation was made in slag reduction sub-model and several parameters of the model rely on the accuracy of measurement in the industrial scale. Jung et al. [11] have applied the principle of “Effective Equilibrium” in each reaction zone of the reactor to develop a kinetic model for BOF process. In this model, the converter was divided into many reaction zones and in each zone thermodynamic equilibrium condition was applied. The kinetics of the process was incorporated by varying the volumes of each equilibrium zone based on empirical relationships found in literature or plant measurements. This model was able to reproduce the metal composition, particularly the behavior of Mn and P reasonably well with the measured data by Cicutti et al. [12]. However, the detail modeling calculation particularly kinetics of change in each reaction zone volume has not been reported in the literature. More recently, Guo et al. [13] developed a multi zone simulation model, which includes non-equilibrium chemical reactions, flux dissolution and slag composition evolution, temperature change and scrap melting model. In this model the reaction is assumed to take place in two separate zones: oxidation (FeO generation) at jet impact zone and reduction (FeO reduction) at slag/metal interface. These workers have introduced an empirical term “Jet Oxidation Index” which is a function of process variables, in order to quantify the FeO reduction at slag/metal mixing zone. This BOF simulation model, despite the empirical nature of the relationships used, has found practical application in process control and improvement of dephosphorization efficiency in oxygen steelmaking process. All the models described above have different levels of simplification to find their application in industrial processes. However, these models are somewhat mechanistic in nature and in most cases empirical kinetic parameter such as “Jet Oxidation Index” [13], interfacial area [5-6], mass transfer coefficients [7, 14] are used in the first order rate equation for successful modeling. This indicates that the process is kinetically controlled and model developed on the basis of fundamental kinetics equations must be necessary to understand the complex nature of dephosphorization rate in oxygen steelmaking process. It has been postulated that dephosphorization reaction primarily takes place in slag-metal emulsion zone due to large number of metal droplets being ejected to the slag during blowing process. As these droplets are smaller in size their interfacial area increases up to 100 times or more than the slag/bath phase boundary [15]. Thus it is clear that significantly high interfacial area is available for the reaction during the process. Also the recent finding of “bloated droplet theory” suggests that when the metal droplets ejected into the slag-metal emulsion they become bloated due to nucleation of CO bubbles inside the droplet [16, 17]. This leads to a significant increase in residence time of metal droplets in the slag. Recently, Dogan et al. [18] developed a comprehensive model for decarburization which includes the concept of bloated droplet theory with kinetic equations. However, no researchers have combined the concept of bloated droplet theory to model the kinetic of dephosphorization in oxygen steelmaking. In the present paper kinetics of phosphorus refining reaction based on fundamental mechanism of the process has been discussed and a basic structure for this modeling approach has been presented. 2.
KINETIC MODELING OF DEPHOSPHORIZATION AND FUNDAMENTAL ISSUES
The reaction of phosphorus takes place at the interface between metal and slag, where dissolved phosphorus in the metal reacts with the oxygen in the slag. It is generally accepted that phosphorus exists as PO4 3- ion at low P concentration (
