Bessemer converter is a pear-shaped furnace, 20 feet high and 10 feet in diameter. It is made of steel plates and is lined inside with brick to resist heat. There are number of holes at the base, called tuyeres, for air circulation. The converter can be rotated on a horizontal central axis.
Process
The Bessemer converter is turned into a horizontal position and molten pig iron is poured into it. A blast of hot air is sent through the tuyeres.The converter is rotated so that it is vertical and hot air is blown continuously. In its upward travel through the converter, the air oxidises the impurities in pig iron.
Manganese and silicon are oxidised to the respective oxides during the first 5 to 10 minutes. Carbon monoxide is liberated at a later stage and it burns with a blue flame at the mouth of the converter. When all the carbon is oxidised, the blue flame dies out. To convert pig iron to steel, the required amount of carbon is then added. This is in the form of an alloy of iron called spiegeleisen. It contains carbon and manganese besides iron. The resulting product is manganese steel, which is removed by tilting the converter.
Note:
>Adding a little aluminium or silicon-iron alloy to molten steel helps in removing dissolved carbon monoxide or nitrogen from the molten metals.
>Gas must not be left behind as an impurity. The presence of carbon monoxide or nitrogen can cause defects like blowholes in castings.
Slag Formation
The impurities like manganese oxide and silicon dioxide formed during the initial stages of the Bessemer process are removed when they react with each to form a slag.
Note:-
If the cast or pig iron contains phosphorus as an impurity ,the converter should be lined with lime (CaO) and magnesia (MgO) instead of silica.Some lime is also added to the charge.
