# Argon-oxygen decarburisation

After its invention in the 1950’s and subsequent commercialisation in the late 1960’s, the AOD process has become the most common unit process for refining of stainless steel. A key feature of this process is that the blowing mixture is diluted with inert (argon) or partially inert (nitrogen) gases in order to reduce oxidation of chromium and other alloying elements. Modern vessels are equipped with both tuyères and a top lance for gas injection. The AOD process can be divided into three main stages. The combined-blowing decarburisation stage constitutes the first part of decarburisation. The combined-blowing decarburisation stage is followed by the side-blowing decarburisation stage, which is continued until the target carbon content is obtained. The final stage of the process is the reduction stage, during which oxidised alloying elements are reduced back to the steel bath by to addition of reductants (typically ferrosilicon or aluminium) and simultaneous argon stirring.## Mathematical modelling

Since 2011, my own research has focused on the mathematical modelling of the chemical rate phenomena in the AOD process. More specifically, separate sub-models have been developed for each of the main stages of the process. The sub-models have been combined into a similator, which can be used to predict composition and temperature changes as well as the rate-controlling phenomena during AOD processing. Furthermore, the simulator features an easy-to-use graphical user interface.## Physical modelling

From 2014 to 2015, I instructed work on physical modelling of the AOD process. In this work, we have employed a 1:9 scale physical model of an 150 t AOD converter to study mixing times in the steel bath. Owing to similar kinematic viscosities, water and rapeseed oil have been used to represent liquid steel and slag. Sulphuric acid was used as a tracer substance to determine the mixing time based on pH of the water bath.## Results

### Highlights

- A law of mass action based kinetic approach was developed for mathematical treatment of parallel mass transfer limited reactions. The approach has a broad field of potential applications.
- An AOD simulator was developed based on mathematical models for side-blowing, top-blowing and reduction of slag. Experiences from application of the simulator suggest that the simulator is an effective tool for studying and optimising the operating practice. The use of the simulator has enabled realisation of significant cost-savings in actual plant practice.
- The interaction of side-blowing, top-blowing and a top slag was studied with the help of physical modelling. The results suggest that a higher side-blowing rate increased the efficiency of bath mixing, while top-blowing and top slag had an deteriorating effect, which was more pronounced if the the side-blowing rate was low.

### Publications

- E.-P. Heikkinen, V.-V. Visuri, and T. Fabritius, "On the heterogeneity of AOD slags in different stages of blowing", Proceedings of the 8th European Oxygen Steelmaking Conference, Associazione Italiana di Metallurgia, Taranto, Italy, 2018.
- V.-V. Visuri, R. Mattila, P. Kupari, and T. Fabritius, "A comparative study on refractory wear associated with fluxes for AOD slags", Proceedings of the 7th International Congress on Science and Technology of Steelmaking, Associazione Italiana di Metallurgia, Venice, Italy, no. 15, 2018.
- E.-P. Heikkinen, V.-V. Visuri, H. Suopajärvi, A. Kemppainen, M. Aula, P. Sulasalmi, and T. Fabritius, "Selected research focus areas for energy and material improvements in reduction and refining metallurgy"', Proceedings of the 2nd ISIJ-VDEh-Jernkontoret Joint Symposium, Jernkontoret, Stockholm, Sweden, pp. 24–33, 2017.
- V.-V. Visuri, M. Järvinen, A. Kärnä, P. Sulasalmi, E.-P. Heikkinen, P. Kupari, and T. Fabritius, “A Mathematical Model for Reactions During Top-Blowing in the AOD Process: Validation and Results”, Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science, vol. 48, no. 3, pp. 1868–1884, 2017.
- V.-V. Visuri, M. Järvinen, A. Kärnä, P. Sulasalmi, E.-P. Heikkinen, P. Kupari, and T. Fabritius, “A Mathematical Model for Reactions During Top-Blowing in the AOD Process: Derivation of the Model”, Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science, vol. 48, no. 3, pp. 1850–1867, 2017.
- M. Järvinen, V.-V. Visuri, E.-P. Heikkinen, A. Kärnä, P. Sulasalmi, C. De Blasio, and T. Fabritius, "Law of Mass Action Based Kinetic Approach for the Modelling of Parallel Mass Transfer Limited Reactions: Application to Metallurgical Systems", ISIJ International, vol. 56, no. 9, pp. 1543–1552, 2016.
- V.-V. Visuri, E. Isohookana, A. Kärnä, T. Haas, R. H. Eriç, and T. Fabritius, ”A Physical Modelling Study of Mixing in an AOD Vessel”, Proceedings of the 5th International Conference on Process Development in Iron and Steelmaking, Swerea MEFOS, Luleå, Sweden, no. 88, 2016.
- T. Haas, V.-V. Visuri, A. Kärnä, E. Isohookana, P. Sulasalmi, R. H. Eriç, H. Pfeifer, and T. Fabritius, "Physical Modelling of the Effect of Slag and Top-Blowing on Mixing in the AOD Process", in R. G. Reddy, P. Chaubal, P. C. Pistorius and U. Pal (eds.), Advances in Molten Slags, Fluxes, and Salts: Proceedings of the 10th International Conference on Molten Slags, Fluxes, and Salts, The Minerals, Metals and Materials Society, Seattle, WA, USA, pp. 999–1008, 2016.
- M. Järvinen, V.-V. Visuri, S. Pisilä, A. Kärnä, P. Sulasalmi, E.-P. Heikkinen and T. Fabritius, "Advanced Methods in Modelling of Metallurgical Unit Operations", in L. P. Karjalainen, D. A. Porter and S. A. Järvenpää (eds.), "Physical and Numerical Simulation of Materials Processing VII'', Materials Science Forum, vol. 762, pp. 236–241, 2013.
- V.-V. Visuri, M. Järvinen, J. Savolainen, P. Sulasalmi, E.-P. Heikkinen, and T. Fabritius, ”A Mathematical Model for the Reduction Stage of the AOD Process. Part II: Model Validation and Results”, ISIJ International, vol. 53, no. 4, pp. 613–621, 2013.
- V.-V. Visuri, M. Järvinen, P. Sulasalmi, E.-P. Heikkinen, J. Savolainen, and T. Fabritius, ”A Mathematical Model for the Reduction Stage of the AOD Process. Part I: Derivation of the Model”, ISIJ International, vol. 53, no. 4, pp. 603–612, 2013.
- V.-V. Visuri, E.-P. Heikkinen, M. Järvinen, J. Savolainen, and T. Fabritius, ”Phenomena-based model in AOD process improvement”, Proceedings of the 4th International Conference on Process Development in Iron and Steelmaking, Swerea MEFOS, Luleå, Sweden, vol. 1, pp. 225–235, 2012.
- M. Järvinen, S. Pisilä, A. Kärnä, V.-V. Visuri, T. Fabritius, T. Ikäheimonen, and P. Kupari, ”Fundamental Mathematical Modelling of AOD Process”, Proceedings of the 4th International Conference on Modelling and Simulation of Metallurgical Processes in Steelmaking, Stahlinstitut VDEh, Düsseldorf, Germany, no. 10, 2011.

### Theses instructed

- V. Kivelä, "The use of AOD process models in the development of decarburization", Master's thesis, University of Oulu, 2016.
- V. Kattilakoski, "Alternative methods for replacement of boron compounds in stabilization of secondary metallurgy slags", Master's thesis, University of Oulu, 2015.
- E. Isohookana, "A Physical Modelling Study of an AOD Converter", Master's thesis, University of Oulu, 2015.