The raw materials needed for the production of pig-iron are iron ore and coke. Depending on the type of impurities in the iron different fluxes are added, for example limestone (for siliceous iron ore) or feldspar (for calcareous iron ore).
The pig-iron is produced in a blast-furnace which typically consists of a cylindrical steel hull and a refractory lining on the inside. The lower part of the furnace (crucible) has openings (tuyeres) through which hot air is injected. Two openings closer to the bottom are used to tap the iron and an opening above that is used to tap the slag. To tap the iron the refractory clay plug is pulled out of the hole. The raw materials are charged through two openings. The blast-furnace gas is released through another opening.
Inside the furnace layers of coke and iron ore with flux follow each other subsequently. The flux converts the impurities in the iron to easily meltable slag. The limestone for example combines with the silicates in the iron ore and forms meltable calcium silicate. Calcium silicate forms slag which collects on top of the molten iron. The air which is injected through the tuyeres (wind) is pre-warmed to about 800°C – 1 200°C and injected with slight pressure (more than 1.6 bars or 160 kPa).
Coke releases carbon monoxide when it is burned:
(1) C + O2 -> CO2 and
(2) CO2 + C -> 2 CO.
These reactions increase the temperature in the lower part of the furnace to about 1600°C. The hot carbon monoxide rises into the iron ore layer above and reduces the contained iron oxide to metallic iron. This chemical reaction can be represented by the following equation:
Fe2O3 + 3 CO -> 3 CO2 + 2 Fe.
In the following coke layer the carbon dioxide is re-converted to carbon monoxide: CO2 + C -> 2 CO which can react to reduce the iron oxide in the next layer of iron ore. In the higher and cooled layers the carbon monoxide partly decays to carbon dioxide and carbon – this equilibrium reaction is called Boudouard reaction.
The produced carbon also reduces iron oxide to metallic iron. In addition the carbon dissolves in the iron which reduces the melting point to about 1 100 to 1 200°C (pure iron 1 539°C).The “cooler” metal drips through the coke and collects at the bottom of the furnace, below the liquid slag.
The carbon monoxide/carbon dioxide mixture (blast furnace gas) pre-warms the fresh raw materials in the upper part of the furnace and escapes through the openings. Then it is cleaned and re-used for other industrial purposes.
A blast-furnace always works continuously. The raw material is refilled in small amounts every ten to fifteen minutes. Slag is removed approximately every two hours and the pig-iron is tapped about five times a day.
To tap the pig-iron the refractory clay plug is removed from one of the openings in the crucible. The liquid metal flows through a clay channeling and a skimmer which separates the slag from the iron, into a basin with refractory lining, for example on a railcar. Some of these basins can hold up to 100 tons of metal. The pig-iron is then brought to the steel mill either in liquid form or as cast iron slabs.
Common pig-iron usually consists of: about 92 percent iron, three to four percent carbon, and varying amounts of silicon (0.5 to three percent), manganese (0.5 to six percent), phosphorous (0.1 to two percent) and traces of sulfur (0.01 to 0.05 percent).
Grey pig-iron is produced when the pig-iron is cooled slowly and the carbon settles out as graphite on the raw material. This type of carbon deposition needs specific amounts of silicon (more than two percent) and manganese (less than 0.2 percent).
White pig-iron in comparison is produced when the pig-iron is cooled rapidly and the dissolved carbon forms iron carbide. The right amount of manganese (more than four percent) and silicon(less than 0.5 percent) is very important to support this process. In this case the higher amount of manganese prevents the forming of graphite. The grey pig-iron is usually used as cast-iron whereas the white pig-iron is used for steel production.