Recovered co-products can be recycled during the steelmaking process or sold for use by other industries. Use of co-products supports the sustainability of the steel industry. It prevents landfill waste, reduces CO2 emissions and helps preserve natural resources.
The sale of co-products is also economically sustainable. It generates revenues for steel producers and forms the base of a lucrative worldwide industry. Some companies report a by-products utilization and recycling rate as high as 99%.
The main co-products from iron and crude steel production are slags, process gases, dust and sludge.
More than 400 million tonnes of iron and steel slags are produced each year. Slags are a mixture of silica, calcium oxide, magnesium oxide, and aluminium and iron oxides. During smelting, slagging agents and fluxes (mainly limestone or dolomite and silica sand) are added to the blast furnace or steelmaking furnace to remove impurities from the iron ore, steel scrap and other ferrous feeds. As the slags are lighter than the liquid metal, they float and can be easily removed.
Slags are recognised as marketable products. The worldwide average recovery rate for slag varies from over 80% for steelmaking slag to nearly 100% for iron-making slag. There is still much potential to increase the recovery and use of slags in many countries, especially for environmental and economic benefits.
Gases from ironmaking and steelmaking, once cleaned, are used internally, reducing the demand for externally-produced electricity. Coke oven gas contains about 55% hydrogen and may prove an important hydrogen source in the future.
The dust and sludge removed from the gases consist primarily of iron and can be used again in steelmaking. Iron oxides that cannot be recycled internally can be sold to other industries for various applications, from Portland cement to electric motor cores.
The EAF route may create zinc oxides that can be collected and sold as a raw material. In the BF-BOF route, cleaning the coke oven gas creates valuable raw materials for other industries including ammonium sulphate (fertiliser), BTX (benzene, toluene and xylene – used to make plastic products), and tar and napthalene (used to make pencil pitch which in turn is used to produce electrodes for the aluminium industry, plastics and paints).