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What is a Blast Furnace? Steel Foundations

A blast furnace reduces iron ore into pig iron, also called hot metal, in the ironmaking process, which is the chief ingredient used in the steelmaking process.

A blast furnace primarily utilizes metallurgical coke, iron ore, and “burnt” lime to produce pig iron. It is a continuously operating process where alternating layers of iron ore pellets, coke and limestone are charged through the top of the blast furnace.

Using recycled heat from the blast furnace, outside air is heated to 1,000°C – 1,200°C in the heat exchanger of the hot blast stove.  This hot blast air is blown in through pipes called tuyeres at the bottom of the blast furnace and heats the three key ingredients to start the chemical reactions needed for ironmaking.  Additional fuels such as oil, natural gas and pulverized coal are also added to the hot blast air to aid in the efficiency of the blast furnace.

Once the chemical reaction is started, the carbon in the metallurgical coke is burned by the hot blast air to form carbon monoxide (CO), which removes the oxygen from the iron ore and creates carbon dioxide (CO2).  Through the heat of the blast furnace and the carbon from coke additions, the carbon dioxide (CO2) breaks back down into carbon monoxide (CO), allowing it to react with more iron ore. This chemical reaction repeats itself from the bottom of the blast furnace to the top. The result of the chemical reaction is that all the oxygen is removed from the iron ore, which is reduced to elemental iron with approximately 4% dissolved carbon, 0.7% SiO2 (impurity from the iron ore) and 0.01% sulfur (impurity from the coke).

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 CO and CO2 gases created at the blast furnace are called offgas and they are collected at the top of the furnace to recycle the heat.  After being cleaned of particulates in a dust catcher the offgas passes through one of the hot blast stoves to heat its internal refractory chambers.  Once the inside of the blast stove is properly heated, outside air is brought in and heated by the hot refractory chambers, so it can go into the blast furnace at a very high temperature.  There are always two blast stoves to ensure that the cycle of heating and cooling is continuous.

On regular intervals, the blast furnace taphole at the bottom of the furnace is drilled out to allow the molten iron to flow into a runner and down into a torpedo car or ladle for transportation to steelmaking facilities, where it will be further processed in a basic oxygen furnace or an electric arc furnace. As the molten iron travels down the runner, a gate redirects the slag formed on top of the molten iron to a slag pot. The slag is cooled and processed as road or construction aggregate.

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