Iron Ore and Coke Form Pig Iron in the Blast Furnace
To create pig iron in the blast furnace we need iron ore and coke. Iron ore may contain contaminants that must be removed by adding flux. The mixture of iron ore, flux and sometimes scrap metal is called a burden.
A typical blast furnace is a cylindrical shaft furnace with a refractory lining on the inside. Hot air is blown through water cooled pipes, called tuyeres, into the bosh, the lower part of the furnace. Below, near the floor the furnace has discharge apertures which are usually sealed with refractory clay and can be opened to tap the molten iron. Just above, there are additional skimmer openings to release the slag. Usually a double bell system is used at the top of the furnace to seal gases inside while allowing to charge the furnace with iron ore, coke and flux. The gases exit the furnace at the top through dedicated pipes.
The waste gas is also called top gas and contains mostly nitrogen. Despite its low heating value it is often used to pre heat the air entering the furnace in a device called cowper.
To create ideal conditions inside the furnace the air is preheated to about 800°C and pushed through the tuyeres into the furnace at about 1.6 bar pressure.
Fluxes Remove Impurities
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 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.
Burning coke reduces the iron ore
Inside the furnace layers of coke and iron ore with flux follow each other subsequently. Coke consits almost only of carbon and reacts with oxygen in the air to form carbon dioxide. In a subsequent reaction between carbon dioxide and the carbon in the coke, carbon monoxide is formed. Therefore coke releases carbon monoxide when it isburned:These reactions increase the temperature in the lower part of the furnace to about 1600°C.
- C + O2 -> CO2
- CO2+ C -> 2 CO
The hot carbon monoxide rises into the iron ore layer above and reduces 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 according to equation 2.
This can again react to reduce the iron oxide in the next layer of iron ore. In the higher and cooler layers the carbon monoxide partly decays to carbon dioxide and carbon – this equilibrium reaction is called Boudouard reaction.
The carbon dust produced in the decay of carbon monoxide can also reduce iron oxide to metallic iron. In addition the carbon dissolves in the molten iron which reduces the melting point to about 1 100 to 1 200°C (pure iron 1 539°C).
The liquid 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 itis cleaned and re-used for other industrial purposes.
A blast-furnace works continuously
Starting and stopping the furnace is a very difficult and laborious procedure and is done very seldom. Instead the raw material is charged into the furnace 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 a torpedo ladle on a railcar. Some of these ladles 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.
Often the blast furnaces are directly connected to a steelworks, so that the molten pig iron can be brought directly to the steelworks by rail. In some cases, however, large pig iron ingots are first cast and transported to the steelworks.
Diversity of pig iron properties
Pig iron usually consists of about 92 percent iron, three to four percent carbon, and varying amounts of silicon (0.5 to 3%), manganese (0.5 to 6%), phosphorous (0.1 to 2%) 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 2%) 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.