How does steelmaking work?

Steel is an alloy of mostly iron and carbon. Pure iron is soft and weak: the carbon is essential for its strength. However, a carbon content higher than about 2% makes it brittle. So, steelmakers mix up the proportions according to the client’s needs. Adding other elements such as manganese, chromium and nickel can improve its properties, making it harder or more corrosion resistant. 

There are two main steps to steel production. The first is extracting iron from iron ore mined from the earth, which is typically done in a blast furnace. A mixture of iron ore, coke (a type of coal that has been baked to drive off impurities such as tar), and limestone are fed into the top of the blast furnace in a specific order. This is called ‘charging’ the furnace. At the same time, a stream of air heated to about 1,200°C by a device called a hot stove (the ‘blast’) is supplied to the bottom of the furnace.

As the column of coke, iron ore and limestone descends towards the blast, the coke ignites. The burning fuel in the furnace raises the temperature to about 2,200°C and produces carbon monoxide. Oxygen from the iron ore reacts with the gas to produce carbon dioxide, leaving molten iron, which flows out of the bottom of the blast furnace. Impurities from the ore react with the limestone and coke ash to form slag, which floats on top and can be removed. Because of an excess of carbon, some of the CO₂ is reduced back to carbon monoxide and emitted from the top of the furnace. 

The coke also plays a structural role within the column of materials added to the furnace: ensuring the blast passes evenly through the iron ore. To reduce costs and lessen the coke needed, pulverised coal is often injected into the blast furnace with the hot blast. At this stage, the carbon content of the molten iron is at about 4%, so must be decreased. 

Next, the hot metal is treated to remove impurities such as sulphur. It is then added to a vessel containing some scrap steel (known as a basic oxygen furnace) – whose role is to lower the temperature, while boosting the iron content. Then, a water-cooled pipe lowered into the vessel blows oxygen onto the liquid metal at about twice the speed of sound. This oxidises the carbon in the hot metal, forming carbon monoxide and CO₂. After 20 minutes or so, steelworkers check whether the liquid steel has reached the desired chemical composition. It may then be refined further, for example, by adding alloying agents.

Liquid steel is then cast into slabs and can be cut and rolled to form different shapes depending on its ultimate use. Large panels may be used for cars, thin sheets to form tin cans, or bars and beams for construction.

In the overall process, the blast furnace accounts for over 70% of CO₂ emissions due to the chemical reduction process and the fuel required to heat the furnace. The basic oxygen furnace is less energy intensive. Typically, it does not consume extra energy, as oxidising the carbon in the molten iron creates heat.

In the UK, blast furnaces are no longer being used and have largely been replaced by electric arc furnaces, which run on electricity rather than coking coal.