Aki Kärnä

A Detailed Single Bubble Reaction Sub‐Model for AOD Process

A new detailed model that describes the chemical reactions, mass transfer and heat transfer taking place on the surface of a single gas bubble in liquid steel is presented in this paper. By using this model, locally occurring small scale physical and chemical mechanisms can be effectively studied. This information is required later in developing a simplified reaction sub-model to be used in CFD simulation of an operating AOD vessel. To demonstrate the capabilities of the new model, the behaviour of a single bubble under two example conditions was simulated. In the case of high carbon content of the steel, here 1%, a contribution analysis showed that the major fraction of the oxygen goes to oxidize dissolved C. When 50% of the carbon in the bath is burned and if the same gas composition (90% O2, N2) is still used, the main product is initially Cr2O3, indicating that the gas composition should have been changed if this had been a real process in question. To verify, a series of O2/N2 ratios 0.1…0.95 were simulated at 50% C conversion to see how more optimal product yield can be obtained. In addition, time dependent profiles of temperature and all species in and around the bubble are presented. The results presented here are applicable only to a local position in the AOD vessel. To be applicable to a whole AOD vessel, the model should be implemented as a source term into CFD software or a corresponding process simulation tool. This will be our future work.

A Mathematical Model for Reactions During Top-Blowing in the AOD Process: Derivation of the Model

In an earlier work, a fundamental mathematical model was proposed for side-blowing operation in the argon–oxygen decarburization (AOD) process. The purpose of this work is to present a new model, which focuses on the reactions during top-blowing in the AOD process. The model considers chemical reaction rate phenomena between the gas jet and the metal bath as well as between the gas jet and metal droplets. The rate expressions were formulated according to a law of mass action-based method, which accounts for the mass-transfer resistances in the liquid metal, gas, and slag phases. The generation rate of the metal droplets was related to the blowing number theory. This paper presents the description of the model, while validation and preliminary results are presented in the second part of this work.

A Mathematical Model for the Reduction Stage of the CAS-OB Process

This paper proposes a novel method for modeling the reduction stage of the CAS-OB process (composition adjustment by sealed argon bubbling–oxygen blowing). Our previous study proposed a model for the heating stage of the CAS-OB process; the purpose of the present study is to extend this work toward a more comprehensive model for the process in question. The CAS-OB process is designed for homogenization and control of the composition and temperature of steel. During the reduction stage, the steel phase is stirred intensely by employing the gas nozzles at the bottom of the ladle, which blow argon gas. It is assumed that the reduction rate of the top slag is dictated by the formation of slag droplets at the steel-slag interface. Slag droplets, which are generated due to turning of the steel flow in the spout, contribute mainly by increasing the interfacial area between the steel and slag phases. This phenomenon has been taken into account based on our previous study, in which the droplet size distribution and generation rate at different steel flow velocities. The reactions considered between the slag and steel phases are assumed to be mass transfer controlled and reversible. We validated the results from the model against the measurements from the real CAS-OB process. The results indicate that the model accurately predicts the end compositions of slag and steel. Moreover, it was discovered that the cooling rate of steel during the gas stirring given by the model is consistent with the results reported in the literature.

Advanced Methods in Modelling of Metallurgical Unit Operations

This paper summarizes and discusses our recent work on modelling of several steelmaking processes. The work started by developing a detailed sub-model for a single gas bubble reacting in liquid steel. The key feature in this model was an approach based on LOMA, Law of Mass Action, which was employed for defining the chemical rate of a reaction in a robust way. The bubble reaction model was then coupled with a new simulator concept for the AOD process, Argon-Oxygen Decarburization. After a successful validation, the same approach was used to model chemical reactions and chemical heating of liquid steel in the CAS-OB process, Composition Adjustment by Sealed Argon Bubbling Oxygen Blowing, using a supersonic lance. Finally, a new model was developed and implemented into the existing AOD process model for slag reduction with slag droplets. The purpose of this paper is to present a generalised framework for applying and validating the LOMA approach into modelling of metallurgical unit operations. In addition, the use of Computational Fluid Dynamics (CFD) in the validation and verification work is discussed.

Supersonic Lance Mass Transfer Modelling

Numerical models of steelmaking processes are essential tools for process development and optimisation. A usable model is detailed enough to provide reliable results and not to slow to run. In order to make a fast and accurate model of a single process, all model parameters must be known well. This can be achieved by first simulating detailed models from which the parameters are obtained.In many converter processes oxygen is delivered into melt by supersonic top lance blowing. When such process is modeled, a model describing mass transfer from the lance into melt surface is needed.This paper describes numerical modeling of mass transfer by supersonic lances. Lance flow CFD models are used to determine mass transfer coefficients for typical lance applications. Models are validated with supersonic nozzle data and wall impinging jet mass transfer data from literature. The results are later used in fast process simulation models.

A Mathematical Model for Reactions During Top-Blowing in the AOD Process: Validation and Results

In earlier work, a fundamental mathematical model was proposed for side-blowing operation in the argon oxygen decarburization (AOD) process. In the preceding part “Derivation of the Model,” a new mathematical model was proposed for reactions during top-blowing in the AOD process. In this model it was assumed that reactions occur simultaneously at the surface of the cavity caused by the gas jet and at the surface of the metal droplets ejected from the metal bath. This paper presents validation and preliminary results with twelve industrial heats. In the studied heats, the last combined-blowing stage was altered so that oxygen was introduced from the top lance only. Four heats were conducted using an oxygen–nitrogen mixture (1:1), while eight heats were conducted with pure oxygen. Simultaneously, nitrogen or argon gas was blown via tuyères in order to provide mixing that is comparable to regular practice. The measured carbon content varied from 0.4 to 0.5 wt pct before the studied stage to 0.1 to 0.2 wt pct after the studied stage. The results suggest that the model is capable of predicting changes in metal bath composition and temperature with a reasonably high degree of accuracy. The calculations indicate that the top slag may supply oxygen for decarburization during top-blowing. Furthermore, it is postulated that the metal droplets generated by the shear stress of top-blowing create a large mass exchange area, which plays an important role in enabling the high decarburization rates observed during top-blowing in the AOD process. The overall rate of decarburization attributable to top-blowing in the last combined-blowing stage was found to be limited by the mass transfer of dissolved carbon.

Physical Modelling of the Effect of Slag and Top-Blowing on Mixing in the AOD Process

The argon-oxygen decarburization (AOD) process is the most common process for refining stainless steel. High blowing rates and the resulting efficient mixing of the steel bath are characteristic of the AOD process. In this work, a 1:9-scale physical model was used to study mixing in a 150 t AOD vessel. Water, air and rapeseed oil were used to represent steel, argon and slag, respectively, while the dynamic similarity with the actual converter was maintained using the modified Froude number and the momentum number. Employing sulfuric acid as a tracer, the mixing times were determined on the basis of pH measurements according to the 97.5% criterion. The gas blowing rate and slag-steel volume ratio were varied in order to study their effect on the mixing time. The effect of top-blowing was also investigated. The results suggest that mixing time decreases as the modified Froude number of the tuyeres increases and that the presence of a slag layer increases the mixing time. Furthermore, top-blowing was found to increase the mixing time both with and without the slag layer.