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The Bessemer Process Of Steel Making

 The Bessemer Process consists in blowing air through molten pig iron contained in a special furnace known as converter which shaped like a huge concrete mixer. The converter is made of steel plates lined inside with a refractory material. The type of refractory lining used depends upon the character of the steel - making process, i.e; upon the acid process or basic process. 




In the acid process, the converter is lined with silica brick which is known in the refractory trade as "acid". The acid process does not eliminate phosphorus or Sulphur from the metal. In the basic process, the converter is lined with dolomite, which is known as "basic". It removes Phosphorus and to some extent Sulphur.

Three clearly distinctive stages may be noted in the conversion of normal pig iron steel.

These are :

(1) The slag Formation or Blowing period , 

(2) The Brilliant flame Blowing Period ,

(3) The reddish Smoke Period. 

The first stage commences as soon as the blast is put on under a pressure of about 2 to 2.5 atm (200 to 250 k N/m2) gauge, and the vessel turned upright. The blast is introduced through the tuyeres in the bottom. The oxygen of the blast oxidizes the iron to ferrous oxide. As a slag , the ferrous oxides mixes with the metal. Having a greater affinity for oxygen , the silicon and manganese which separate as oxides ( insoluble in the molten metal ) go into the slag. These reactions are accompanied by the evolution of a large amount of heat which raises the temperature from 1250*C to 1525*C. This stage lasts for three or four minutes, and may also be called the silicon blow. 

The second stage commences after the oxidation of Iron, silicon and manganese, When the metal has reached a sufficiently high temperature. This creates favorable conditions for intensive burning of carbon from the molten bath. The dissolved carbon is oxidized by the ferrous oxide of the slag. The gas evolved during this stage is rich in carbon monoxide which burns at the nose of the converter with a dazzling white flame. It takes about eight to twelve minutes , as a rule, to eliminate the carbon.

The third stage begins when the flame "drops" - a sign that the carbon has been practically removed from the charge. This stage lasts for one or two minutes after which the converter is turned to the horizontal position. The additions of deoxidizes such as ferromanganese , ferro silicon , or aluminum are made to the metal bath to eliminate the oxygen and to bring the manganese and silicon content of the steel to the specific value. 

The daily production of a converter for 25 to 30 tones per heat in size is 1200 tons. This is enough to cater for the output of  a big blast furnace. 


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