Top blowing attributes

• Energetic supply of oxygen
• Control of slag formation
• Control of oxygen distribution.
• Simultaneous removal of C and P.
• Inadequate stirring of slag /metal phases.

In bottom blown steelmaking, all oxygen is injected through the bottom tuyeres .Though this technology provides efficient bath stirring and enhanced carbon removal, but it is difficult to distribute oxygen within the bath and also to control slag formation.

As compared with top blowing, bottom blowing changes somewhat the course of reaction of carbon, manganese, phosphorous oxidation, resulting in a different degree of oxidation of bath.

Bottom blowing increases sharply the intensity of bath stirring and increases the gas/metal area by 10-20 times the values of typical of top blowing. Rate of decarburization is higher than that of top blowing.

As in top blowing the intensity of decarburization at higher concentration of carbon is determined by the rate of supply of the oxidant. With a carbon content of less than 0.1% in bath, mass transfers become rate controlling which favours in iron oxidation. Since bottom blowing provides favourable conditions for decarburization reaction, the metal and slag have a lower degree of oxidation. For example, concentration of FeO in slag is not less than 15-25% during the entire blow  in top blown steel-making and increases even more by the end of blowing, whereas in the bottom blown converter process it does not exceed 5-7% and raises up to 15-25%at low concentration of carbon. Since the slag in bottom blown converter has a low degree of oxidation at most during the whole operation, condition are unfavourable for dephosphorization reaction. Only toward the end phosphorous removal occurs.

Since the slag in the bottom blown converter oxidizes much less than in the top blowing, concentration of Mn in steel is higher in the course of a heat.

The advantages of pure top and pure bottom are coupled and a new technology is developed under the name combined top blowing of oxygen and bottom stirring. The advantages of such a technology when compared with pure top blowing like reduced FeO content of slag and O content of steel etc. are obvious.

Several process technologies for combined blowing were developed under different names. These technologies differ in

• Amount of inert gas
• Type, number and arrangement of bottom tuyeres and porous plugs.

The reader may see the references given at the end of lecture to get technological details.