The oxidant used in this steelmaking process is oxygen. Air is blown into the molten pig iron to oxidize impurities such as silicon and manganese. In the process of oxidation, a lot of heat is released (containing 1% silicon can increase the temperature of pig iron by 200 degrees Celsius), which can make the furnace reach a high enough temperature. Therefore, converter steelmaking does not require additional fuel.
Converter steelmaking is carried out in a converter. The shape of the converter is like a pear, with refractory bricks on the inner wall, and many small holes (tuyere) on the side of the furnace. The compressed air is blown into the furnace from these small holes, which is also called a side-blown converter.
In the beginning, the converter is horizontal, and liquid pig iron at 1300 degrees Celsius is injected into it, and a certain amount of quicklime is added, then air is blown and the converter is turned to stand upright.
At this time, the surface of the liquid pig iron reacts violently to oxidize iron, silicon, and manganese (FeO, SiO2, MnO,) to form slag, and the convection of molten steel and slag makes the reaction spread throughout the furnace.
After a few minutes, when only a small amount of silicon and manganese remains in the molten steel, the carbon begins to oxidize, producing carbon monoxide (exothermic) and causing the molten steel to boil violently.
A huge flame appeared at the mouth of the furnace due to the burning of the overflowing carbon monoxide. Finally, phosphorus also oxidizes and further produces ferrous phosphate.
Ferrous phosphate reacts with quicklime to form stable calcium phosphate and calcium sulfide, which together become slag.
When the phosphorus and sulfur gradually decrease, the flame retreats and the brown steam of iron tetroxide appears at the furnace mouth, which indicates that the steel has been smelted.
At this time, the blasting should be stopped immediately, and the top converter should be turned to the horizontal position, the molten steel should be poured into the ladle, and the deoxidizer should be added for deoxidation. The whole process only takes about 15 minutes. If the air is blown in from the bottom of the furnace, it is a low-blown converter.
With the development of oxygen production technology, oxygen top-blown converters (and side-blown converters) are now commonly used. This kind of converter blows high-pressure industrial pure oxygen, and the reaction is more violent, which can further improve the production efficiency and the quality of steel.