Steel and Iron 439 



nace. The oxygen at the high furnace temperature combines 

 freely in combustion with the coke or carbon. The heat of this 

 combustion is increased greatly by the blast of heated air (cf. 

 Fig. 144), which economizes fuel and facilitates the reduction of 

 the ore. The mineral impurities or ingredients, as silica, alu- 

 mina, magnesia, phosphorus, sulfur, and lime, combine in part 

 to form a slag which is made readily fusible at the available 

 temperature by the additional limerock. The iron and slag melt 

 near the hearth of the furnace and collect in it. The molten 

 slag, which is less dense than the molten iron, floats on top. 

 Through two taps at different levels the molten iron and slag 

 are drawn off when necessary. Meanwhile the whole mass in 

 the furnace settles downward, and fresh materials are supplied 

 continually. 



The molten iron when drawn from the hearth is run into 

 great receiving ladles. In some places it is conducted directly 

 into sand or iron molds, but more commonly the molten iron 

 is put into great ladles where it is thoroughly mixed and 

 equalized before being cast in iron molds. The product is 

 the pig iron of commerce. 



The slag (Fig. 144) contains many valuable substances. It 

 may be conducted into a slag-receiving pit or into a large tank 

 of water, called the granulating pit. Slag may be crushed and 

 powdered for cement, prepared for fertilizer, or merely crushed 

 for use as ballast on railroads. 



The gases containing more or less dust are expelled through 

 the top of the furnace and conducted down through flues to 

 cleansers, which remove the dust, and perhaps some of the 

 volatilized compounds, and cyanide and alkalies. The dust 

 is a good fertilizer. The gas when cleaned is conducted to the 

 furnaces, where it is burned to heat air for the hot blast, or to 

 boilers, where it is used in generating steam. 



214. The character and uses of pig iron. Pig iron is an 

 intermediate, not a finished, product in the iron industry. In 

 appearance it is gray, mottled, or whitish in color, and crystalline 



