Electric Arc Furnace
An Electric Arc Furnace (EAF) is a furnace that heats charged material by means of an electric arc.
Arc furnaces range in size from small units of approximately one ton capacity (used in foundries for producing cast iron products) up to about 400 ton units used for secondary steelmaking. Arc furnaces used in research laboratories and by dentists may have a capacity of only a few dozen grams. Industrial electric arc furnace temperatures can be up to 1,800 °C (3,272 °F), while laboratory units can exceed 3,000 °C (5,432 °F). Arc furnaces differ from induction furnaces in that the charge material is directly exposed to an electric arc, and the current in the furnace terminals passes through the charged material.
Unlike the basic oxygen route, the EAF does not use hot metal. It is charged with “cold” material. This is normally steel scrap (recycled goods made from steel which have reached the end of their useful life). Other forms of raw material are however available which have been produced from iron ore. These include Direct Reduced Iron (DRI) and iron carbide, as well as Pig Iron, which is iron from a blast furnace which has been cast and allowed to go cold, instead of being charged straight into a Basic Oxygen Furnace (BOF) vessel.
Steel scrap (or other ferrous material) is first tipped into the EAF from an overhead crane. A lid is then swung into position over the furnace. This lid contains electrodes which are lowered into the furnace. An electric current is passed through the electrodes to form an arc. The heat generated by this arc melts the scrap. The electricity needed for this process is enough to power a town with a population of 100,000.
During the melting process, other metals (ferro-alloys) are added to the steel to give it the required chemical composition. As with the basic oxygen process, oxygen is blown in to the furnace to purify the steel and lime and fluorspar are added to combine with the impurities and form slag.
After samples have been taken to check the chemical composition of the steel, the furnace is tilted to allow the slag, which is floating on the surface of the molten steel, to be poured off. The furnace is then tilted in the other direction and the molten steel poured (tapped) into a ladle, where it either undergoes secondary steelmaking or is transported to the caster.
The modern electric arc furnace typically makes 150 tonnes in each melt, which takes around 90 minutes.
Special quality steels. A vast range of special quality steels is made in electric arc furnaces by adding other metals to form steel alloys. The most commonly-known of these is stainless steel, which has chromium and nickel added to form a corrosion-resistant steel. There are very many others however: the very hard steels used to make machine tools, the steels specially-formulated to make them suitable for engineering, steels developed to survive for decades the hostile environment of nuclear reactors, light but strong steels used in aerospace, extra tough steels for armour plating – to name but a few.
Electric Arc Furnace Documents