IRON, NICKEL, COBALT 



417 



The blast furnace (Fig. Ill) is an iron structure 40 to 100 feet 

 high, lined with fire-brick. A circular pipe delivers a blast of 

 pre-heated compressed air to several nozzles 

 (tuyeres) near the foot. The ore, coke and lime- 

 stone are admitted at the top. Reduction of the 

 ore occurs continuously as the solid materials, 

 passing downwards, become hotter and hotter 

 through combustion of the coke. The melted 

 iron and slag (immiscible) finally collect hi two 

 layers in the Jiearth or jjrucjhla-at the bottom. 

 From time to time the slag is allowed to flow 

 from an opening near the top of the crucible, 

 and the iron from a similar opening at the bottom. 

 Plugs of wet clay close the openings and are in- 

 stantly baked hard. The iron is taken in ladles 

 to other parts of the plant, or is cast into " pigs " 

 in steel moulds and chilled in water. 



FIG. Ill 



Reactions in the Blast Furnace. The actions (see equa- 

 tions, above) are both reversible and the carbon dioxide formed 

 tends to react to reproduce the original materials. At any partic- 

 ular temperature it is necessary therefore to keep the proportion 

 of C0 2 to CO in the furnace gases below a certain value in order 

 to prevent the reversal of the reactions. This proportion of COa 

 to CO is regulated, however, by a third reversible reaction: 



C + CO 2 ^ 2CO. 



At high temperatures (above 1000) this reaction is almost com- 

 plete in the forward direction. As the temperature falls, the pro- 

 portion of CO 2 in the equilibrium mixture increases rapidly. 



If all three reactions had time to attain equilibrium conditions 

 in the blast furnace, reduction of Fe 3 4 to FeO, and of FeO to Fe, 

 would occur at about 645 and 700 respectively, and the residual 

 gases would contain very little CO. In practice, however, the 



