Effect of MgO and Al2O3 variations in oxygen steelmaking (BOF) slag on slag morphology and phosphorus distribution

Abstract

Operational data of BOF and the slag samples for different starting conditions of phosphorus (0.06–0.26%P) and silicon content (0.3–1.2%Si) of hot metal have been analysed. The contribution of parameters which are well known to affect phosphorus distribution at tap, such as basicity, temperature, FeO content of slag, slag mass etc., is investigated through models of the ionic theory of slag, optical basicity, regular solution approach, and molecular theory of slag. The best overall results are obtained by the model based on the molecular theory of slag in which several operational parameters are also incorporated. Investigations of different slag samples, based on optical, SEM, EPMA and X-ray studies, reveal the effect of MgO and Al2 O3 on slag morphology and phosphorus distribution in different phases. It is important to consider the phosphorus distribution ratio in the solid and liquid part of the slag. The solid part of the slag, which is mostly dicalciumsilicate, can contain up to 5% phosphorus. The phosphorus content of the liquid part of the slag may depend upon the phosphorus content of the hot metal or the phosphorus load of the slag. It is found that incorporation of the effect of dicalciumsilicate in the model improves the accuracy of prediction. For better process control the addition of iron ore towards the end of the blow must be avoided while treating high phosphorus hot metal or during the production of ultralow phosphorus steels.