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In combined blown basic oxygen steelmaking converter bottom stirring plays an important role in mixing within the bath. Better mixing within the bath and improved mass transfer between slag and metal is believed to cause better dephosphorisation. Normally an equal amount of gas in passed from each tuyere in bottom stirring throughout the blow. Here a novel bottom stirring scheme has been proposed and investigated where different amount of gas was injected from different tuyeres in last 3 to 5 min of the blow. The investigation was carried out with respect to mixing and mass transfer in a scaled-down physical model having appropriate similarity with the actual steelmaking vessel. The scheme, differential flow bottom stirring, basically redistributes the total bottom gas flow in a manner so that a linear flow gradient is imposed across the bath. It was found that the differential flow bottom stirring scheme gives 30-35 % improvement in mixing compared to uniform flow through all the bottom tuyeres. The scheme also gave better mass transfer rates than the conventional stirring. A 30 % increase in the mass transfer rate was observed. Plant trials with the scheme were conducted to assess the impact on dephosphorisation within the practical BOF vessel. Improved BOF performance in terms of phosphorous removal was obtained in the plant trials. Comparatively lower turn-down phosphorous and improved phosphorous partition was achieved with a saving in bottom gas injection amount.

A Novel Bottom Stirring Scheme to Improve BOF Performance through Mixing and Mass Transfer Modelling