Physico-chemical interactions

Physico- chemical interactions of molten bath with oxygen jet depends on the lance profile i.e. change of lance height during the blow. The lance profile is specific to each converter and depends on converter profile, hot metal composition, oxygen flow rate, hot metal chemistry and steel of desired composition. Nevertheless, in all converters initial lance distance is such as to promote iron oxidation so that dissolution of CaO commences. The idea is to create a basic and limy slag at the early part of the blow to onset dephosphorization. Shallow jet penetration covers the larger bath surface and is favorable more for iron and silicon oxidation. Small amount of carbon may be removed. Once slag is formed, lance is lowered. Oxygen jet penetrates into the bath and carbon reaction favours because oxygen is available now deep into the bath. At the same time, force of the oxygen jet creates metal droplets and as a consequence three phase dispersion of gas/slag /metal droplets are formed which enhance the rate of decarburization. Figure 12.2 shows the lance profile and the accompanying physico-chemical interactions

We note that at higher lance distance, oxygen jet penetration into bath is shallow and slag formation occurs. (See 12.2 a)  As the lance distance is decreased, jet penetrates deep into the bath, carbon reaction commences, CO forms, droplets are produced which together leads to the formation of a three phase dispersion consisting of gas bubbles/slag/metal droplets (See 12.2 b and c).  In this state of blow, both carbon and phosphorus removal occur at a faster rate. Formation of three phase dispersion is a characteristic feature of the top blown steelmaking. Three phase dispersion creates conditions for faster removal rates of C and P. Foaming of slag has to be controlled to avoid expulsion of slag, which can be controlled by controlling C reaction with FeO in slag. Reaction between C and FeO of slag in slag will not allow CO bubbles to grow. Smaller size gas bubbles can be trapped easily in slag as compared to larger sizes. Slag may foam and may be expelled from the converter.