Indirect reduction of steel
Note: Numerous details have been left out for the sake of clarity. The information below is not sufficient for the production of high-quality metal.
Pig iron (which is high-carbon iron similar to cast iron, but not intended as a final product) is produced in a blast furnace and then has excess carbon removed in an oxygen furnace.
Oxygen is blown into the molten pig iron at supersonic speeds, where it burns with carbon (a stronger reducing agent than iron) to produce carbon dioxide gas (which escapes) and heat. The reaction is highly energetic and serves to agitate the molten metal (mixing it) and also heats it (molten pig iron comes out of the blast furnace at less then the melting point of steel). The Bessemer converter, the original 19th century oxygen furnace, had a lining of acidic materials. The modern furnace is known as the basic oxygen furnace, due to its basic lining. This process lowers the carbon content of the metal to the desired value.
After the composition is further fine-tuned, the steel can be cast into various shapes.
Steel can also be produced from wrought iron or other low-carbon iron. If the iron is alternately heated in a furnace burning charcoal or coke (or graphite, diamond or other high-purity forms of carbon, though this does not make economic sense) and the slag expelled by hammering, the iron will eventually absorb enough carbon to become steel. This is the pre-industrial method and is known as steeling.
The wrought iron for this process would itself have been converted from pig iron (see wrought iron for details), making it a three-step process of adding carbon, removing carbon and then adding carbon again.
Comparison with direct reduction
- Uses more energy overall.
- Using hydrogen gas as the main reducing agent inherently implies direct reduction.
- L. Sims - The backyard blacksmith: Traditional techniques for the modern smith 2006. ISBN 978-0-7858-2567-8 p. 46