The blast furnace reduces iron ore into pig iron, also called hot metal — the chief ingredient used to make the steel.

A blast furnace utilizes primarily coke, iron ore and limestone to produce pig iron. It is a continuously operating process where alternating layers of iron ore pellets, coke and limestone are charged into the top of the blast furnace. Inside the blast furnace, the three key ingredients are heated by hot blast air heated to 1,000–1,200°C, which is blown in through the tuyeres at the bottom of the furnace. Additional fuels such as oil, natural gas and pulverized coal are also added to the hot blast to aid in the efficiency of the blast furnace. The carbon of the coke is burned by the hot blast to form CO. The CO reacts with the oxygen of the iron ore to create CO2. Through the heat of the blast furnace and additional carbon from coke, the CO2 breaks back down into CO. The process repeats itself from the bottom of the blast furnace to the top, at which time the CO and CO2 is captured in the offgas system. The offgas of the blast furnace is sent through a dropout chamber to remove particulate matter from the gas. The hot gas is then circulated through a series of refractory checkers in a blast furnace stove, thereby heating the checkers. A valve is switched and cool air is then circulated through these pre-heated checkers, which pre-heat the air that is then sent to the tuyeres of the blast furnace creating the hot blast. The cooled, cleaned offgas is then sent for further processing or for energy generation.

The reactions within the blast furnace from bottom of the furnace to the top are:

O2 (via hot blast air) + 2C => 2CO

2FeO + CO => CO2 + Fe (to be tapped as hot metal)

CO2 + C (from coke) => 2CO

Fe2O3(iron ore) + CO => 2FeO + CO2

The iron ore reduces to elemental iron with approximately 4% dissolved carbon, 0.7% SiO2 (impurity from the iron ore) and 0.01% sulfur (impurity from the coke). On regular intervals, the blast furnace taphole is drilled out to allow the molten iron to tap into a runner and down into a torpedo car or molten iron ladle for transportation to the steelmaking facilities — typically to a basic oxygen furnace, but an electric arc furnace can also utilize molten iron. As the molten iron travels down the runner, a refractory gate redirects the slag on top of the molten iron to a slag pot. The slag is cooled and processed as road or construction aggregate.

Other methods of producing hot metal include:

Siemens Corex: http://www.industry.siemens.com/verticals/metals-industry/en/metals/ironmaking/corex/Pages/home.aspx