Some observations and insights on BOS refining

Abstract

Selected IMPHOS heats, have been used to make observations on decarburising and dephosphorising performance, scrap melting and slag foaming characteristics during BOS refining. If it is assumed that decarburisation takes place solely in the slag/metal emulsion then maximum metal residence time in the emulsion is just under 9 seconds and at peak decarburisation time, the maximum amount of metal in the emulsion is ∼50% of the total metal content in the converter. To evaluate the effects of changes in slag component chemistry on phosphorus refining it is necessary to account for changes in slag weight, which can change substantially throughout a heat and be significantly different heat-to-heat. Dephosphorising performance depends on the thermodynamic stability of slag phases that are able to take up phosphorus and the distribution of phosphorus between these thermodynamically stable phases. The application of proprietary thermodynamic models such as MTDATA and FACTSage has helped to clarify such events. Skull build-up on the scrap pile is at a maximum when the bulk bath temperature is ∼1460°C. At this time, the solid scrap and skull component of the bulk bath makes-up just over 60% of all the metal charged to the converter. All scrap and skull is melted out at a bulk bath temperature of ∼1610°C. The stability of the foamy slag/metal emulsion changes over the period of the blow. Slag height increases with an increase in FeO(tot)wt-% and decreases with a decrease in decarburisation rate and the collapse of the foamy slag.

Keywords:

• BOS refining
• Steelmaking
• Phosphorus
• Decarburising
• Scrap melting
• Slag foaming
• Thermodynamics
• IMPHOS

This paper is a revised and extended version of a presentation made at the International Symposium of NMD-ATM 2012, Jamshedpur, India, which will be published in a Special Issue of Transactions of the Indian Institute of Metals.

The authors wish to acknowledge the helpful discussions on decarburisation with Professor Ken Coley at McMaster University.

The authors also wish to acknowledge that this work was carried out with a financial grant from the Research Fund for Coal and Steel of the European Community.

IMPHOS (Improved Phosphorus Refining) is available for Free Download from the EU Bookshop: http://bookshop.europa.eu/en/imphos-pbKINA25005/