Modern steel mills rely on two types of furnaces to produce steel. They use electric arc furnaces to produce relatively small quantities of specialty carbon steel and steel alloys from steel scrap. They rely on basic oxygen furnaces to produce very large tonnages of standard carbon steel from iron produced in blast furnaces. Both types of furnaces take about an hour to turn a load of raw material into a batch of steel.
Electric Arc Furnace
An electric arc furnace makes new steel from old steel scrap. It is a giant lidded steel kettle lined with heat-resistant ceramic refractory material. Its lid lifts up for loading with scrap. The lid also holds the three graphite electrodes that create the electric arc to melt the scrap into new steel. After loading, the electrodes are lowered into the scrap and power fed to the furnace. Electricity arcs between the electrodes, creating the heat needed to melt the steel scrap. Fluxing compounds remove impurities. To obtain additional heat, steelmakers inject pulverized coal and oxygen to supplement the electrical heat. Roughly a third of the heat in electric arc furnaces comes from the injection of fuel and oxygen.
Basic Oxygen Furnace
A basic oxygen furnace, or BOF, creates steel from molten pig iron produced from iron ore in a blast furnace, together with up to 25 percent scrap steel. This furnace works by injecting high-pressure oxygen into the molten iron to burn out excess carbon and other combustible impurities. Fluxing compounds added to the melt remove noncombustible impurities that float to the top of the melt as slag. The BOF gets the energy needed to convert iron into steel from the original heat of the molten iron together with the heat generated by burning off excess carbon and other impurities in the presence of pure oxygen.
Arc Pros and Cons
An electric arc furnace provides precise control of the internal atmosphere and temperature. It emits almost no pollution. Because it starts with scrap metal, an electric arc furnace is economical compared to other steelmaking processes for small-batch steelmaking. They can economically make batches of steel as small as 1 ton. But, electric arc furnaces require access to excessive electricity and road or rail access to bring in a steady supply of scrap metal.
BOF Pros and Cons
The basic oxygen furnace doesn't burn fuel, so it is cost efficient, but it does require the separate creation of molten iron by burning coke in a blast furnace to melt the iron from its ore. Because a BOF uses the inherent heat of molten iron as an energy source, efficient operation requires batches averaging 250 tons. Unlike the minimal emissions of an electric arc furnace, a basic oxygen furnace emits a relatively great amount of polluting gases and dust that require costly treatment through air scrubbers, electrostatic precipitators and filters before they can be released into the atmosphere. When both types of furnaces are finished, they pour the molten steel into a vessel called a "teeming ladle" that carries the melted metal to a nearby mill, where it will be cast or forged into products.