488 SMITH'S INTERMEDIATE CHEMISTRY 



slowness of the reactions necessitates the use of much higher 

 temperatures and a large excess of CO. The gases that escape 

 are therefore combustible, and are led off through openings near 

 the top of the furnace and used for pre-heating the air-blast. 



In 1920, nearly 40,000,000 tons of pig iron were produced in 

 the United States. The production in Great Britain exceeded 

 8,000,000 tons. 



- Hg iron contains 4 to 5 per cent of carbon and 

 varying amounts of silicon (as silicide of iron), phosphorus (as 

 phosphide) and sulphur (as sulphide). These impurities lower 

 the melting-point from 1510 to about 1100. The material is 

 hard and brittle. Most of it is made into wrought iron or steel, 

 but some is used in making_qbjects of cast iron, such as r_anges,. 

 stoves, pipes, and radiators^ which are not to be subjected to 

 shocks or strains. Cast iron expands on solidifying and forces 

 itself into the details of the mould. 



By adding pyrolusite Mn0 2 in the blast furnace, cast iron con- 

 taming from 20 per cent of manganese (spiegeleisen) up to 80 per 

 cent (ferromanganese), and carbon up to 6 per cent, is made for 

 use in steel manufacture. 



Wrought Iron* Wrought iron is commercially pure iron. 

 The broken pigs are placed in a reverberatory furnace (Fig. 112), 

 the hearth of which is covered with a bed of haematite ore Fe 2 3 

 and silicates. The flames and heated gases, deflected by the low 

 roof, play upon the iron and melt it. The oxygen in the hsema- 

 ^i tite combines with the carbon, phosphorus, sulphur, and silicon, 

 ^ giving the oxides. The mass is worked vigorously with iron rods 

 upon the bed of haematite (puddled), carbon monoxide escapes, 

 and the iron becomes more viscous as its melting-point rises on 

 account of the removal of the impurities. Finally, it is collected 

 in balls (blooms) on the iron rods. The treatment occupies an 

 hour and a half. To press out the slag, the blooms are first passed 



