blast furnace

The blast furnace is used to extract iron from a mixture of iron ore, coke, and limestone. The less dense impurities float above the molten iron and are tapped off as slag. The molten iron sinks to the bottom of the furnace and is tapped off into moulds referred to as pigs. The iron extracted this way is also known as pig iron.

Smelting furnace used to extract metals from their ores, chiefly pig iron from iron ore. The temperature is raised by the injection of an air blast.

In the extraction of iron the ingredients of the furnace are iron ore (iron(III) oxide), coke (carbon), and limestone (calcium carbonate). The coke is the fuel and provides the agent (carbon monoxide) for the reduction of the iron ore.

C + O2 → CO2

CO2 + C → 2CO

At the high temperature of the furnace the iron oxide is reduced to the metal.

Fe2O3 + 3CO → 2Fe + 3CO2

The limestone is decomposed to quicklime (calcium oxide), which combines with the acidic impurities of the ore to form a molten mass known as slag (calcium silicate).

CaCO3 → CaO + CO2

CaO + SiO2 → CaSiO3

The principle of reducing ferrous oxides by carbon has been known for thousands of years, but the present blast furnace was introduced around 1400. The fuel was originally charcoal and the resulting iron was either used for casting or converted to wrought iron or steel. Production increased with the use of coke as fuel in the 18th century.