Removing the Impurities of steel

 Most of the pig Iron manufactured is used in the production of steel.  The main impurities in this steel are shown below:

Impurity	Percentage in Iron %	Percentage in Steel %
Carbon	4	0.15
Silicon	2	0.03
Sulphur	0.05-0.1	0.05
Phosphorus	0.05-0.15	0.05
Manganese	0.1	0.50

Source: Chemistry in Context

 View a pie chart of the above information. 

Basic–oxygen process

This is the most widely used method of steel making, involving the blasting of oxygen at a very high velocity into molten pig iron.

Pig iron from a blast furnace, together with steel scrap, is poured into a converter, and a jet of oxygen is then blasted into the mixture. The excess carbon in the mix and other impurities quickly burn out or form a slag, and the converter is emptied by tilting. The oxygen is blown into the charge through a lance (pipe) lowered into the furnace.  The basic–oxygen process was developed 1948 at a steelworks near the Austrian town of Linz.

 

 

(Source: Encarta 97)

 

Open-Hearth Process

Essentially the production of steel from pig iron by any process consists of burning out the excess carbon and other impurities present in the iron. One difficulty in the manufacture of steel is its high melting point, about 1370° C , which prevents the use of ordinary fuels and furnaces. To overcome this difficulty the open-hearth furnace was developed; this furnace can be operated at a high temperature by regenerative preheating of the fuel gas and air used for combustion in the furnace.  Through this method open-hearth furnaces can reach temperatures as high as 1650° C (approximately 3000° F).

The furnace is charged with a mixture of pig iron, scrap steel, and iron ore that provides additional oxygen. Limestone is added for flux and fluorspar to make the slag more fluid. 

Chemically the action of the open-hearth furnace consists of lowering the carbon content of the charge by oxidization and of removing impurities such as silicon, phosphorus, manganese, and sulfur, which combine with the limestone to form slag.  When the carbon content of the melt reaches the desired level, the furnace is tapped through a hole at the rear. The molten steel then flows through a short trough to a large ladle set below the furnace at ground level. From the ladle the steel is poured into cast-iron molds. These are called ingots, the raw material for all forms of fabricated steel.

By Martin McDevitt