Evaluation of the use of recycled slag in the BOF process

Abstract

Steelmaking converters increasingly face challenges from the use of higher phosphorous ores in the Blast-Furnace and the need to reduce their environmental impact on all aspects, including the volume of generated solid residues. According to CSN's 2023 integrated report, approximately 648.000 tons of steelmaking slag were produced at its plant in Brazil in 2023. Although part of this volume is used as road ballast, this destination does not consume all of the production, making it necessary to study sustainable routes for the use of this by-product. One possibility of reducing the total amount of slag generated in the melt shop while maintaining or improving the converteŕs ability to dephosphorize is to recycle specially adjusted recovered slag mixtures, adding them to the converter to enhance early formation of a fluid slag. In this work, we use a recycled slag mixture regularly produced at CSN, with 40% CaO and around 24% total Fe + MnO. Two aspects of this slag mixture were evaluated to estimate its potential as an early addition to the converter, replacing part of the usual lime addition. First, we investigate storage and handling properties, including the slag mixture stability with time. Then, the effect of the early addition of this recycled slag product to the converter is evaluated. After preliminary studies, tests were performed in a total of 255 heats, (corresponding to a total of 530,000 tons). In these tests, additions of the recycled slag varied in the range of 500–3000 kg/heat. The results of these heats were compared both with standard heats and heats to which an addition of sodalite was made to improve dephosphorization. The comparison included dephosphorization results, slag behavior in the converter and overall lime consumption. The results indicate that the use of the recovered slag mixtures has no effect on stable converter operation, produce effective dephosphorization, and form slags with the desired basicity and theoretical viscosity. With excessive additions of the recycled slag, flame formation was observed during the end of the blow: to prevent potential cooling problems of the gas collection equipment, the addition was limited to 1300  kg/heat. In this scenario, a potential decrease in lime consumption of 2  kg/t of steel was observed. The yearly estimate of the use of this technique would result in savings of 9000 tons of lime, and consequently the generation of CO₂ associated with lime production, and the corresponding reduction in the amount of solid residue.