IRON, COBALT, AND NICKEL 



325 



The cast iron formed in blast furnaces is not always of the same 

 quality. When slowly cooled it is soft, has a grey colour, and is not 



the carbonic anhydride with the incandescent charcoal, so that the reduction in the blast 

 furnace is without doubt brought about by the formation and decomposition of carbonic 

 oxide and not by carbon itself thus, Fe 2 O 3 + 3CO = Fe2 + 8C0 2 . The reduced iron, on 

 further subsidence and contact with carbon, forms cast iron, which flows to the bottom 

 of the furnace. In these lower layers, where the temperature is highest (about 1,800), 



15-0 1 



Flo. 93. Vertical section of a modern Cleveland blast furnace capable of producing 300 to 1,000 tons 

 of pig iron weekly. The outer casing is of riveted iron plates, the furnace being lined with re- 

 fractory fire-brick. It is closed at the top by a cap and cone ' arrangement, by means of which 

 the charge can be fed into the furnace at suitable intervals by lowering the moveable cone. 



the foreign matter of the ore finally forms slag, which also is fusible, with the aid of 

 fluxes. The air blown in from below, through the so-called tuyeres, encounters carbon 

 in the lower layers of the furnace, and burns it, converting it into carbonic anhydride. 

 It is evident that this develops the highest temperature in these lower layers of the 

 furnace, because here the combustion of the carbon is effected by heated and compressed 

 air. This is very essential, for it is by virtue of this high temperature that the 

 process of forming the slag and of forming and fusing the cast iron are effected 



