What is an Arc Furnace?
An arc furnace, also known as an electric arc furnace (EAF), is an installation using an electric arc to heat materials, typically metals, to melting point. A typical arc furnace melts the material, or "charge" as it is known, by passing an electrical current through the charge via a set of electrodes. A combination of the heat generated by the current passage and the arcing on the charge surface melts the material. EAFs are found in research and dental prosthesis laboratories, cast iron foundries, and steel production plants and may have internal capacities ranging from a couple of pounds to 400 tons or more. Arc furnaces have several distinct advantages over other furnace types.
An electric arc furnace typically consists of a shell with side walls and a dished bowl at the bottom. A retractable roof section, through which the graphite electrodes are lowered, covers the furnace. The bowl section of the shell is lined with a layer of refractory material known as the hearth. The tap or spout used to drain off the molten metal is either mounted on the side of the shell or at the bottom of the bowl. The electrodes are generally round in cross section and made up of threaded sections which allow new sections to be added as the electrode burns away.
Either alternating current (AC) or direct current (DC) power sources may be used in an arc furnace. The average steel plant arc furnace operates on a 400 to 900 volt supply at 44,000 amperes or more supplied from transformers with ratings of approximately 60,000,000 volt-amperes (60 MVA). An arc furnace of this type can produce approximately 80 tons of molten steel per hour. Arc furnaces range in size from small units used in research laboratories holding around a pound of charge to massive steel plant installations capable of melting hundreds of tons of material. The largest of these furnaces have capacities in excess of 300 tons and use power supplies of 300 MVA or more.
The process of arc melting is a fairly simple one. Once the bowl or hearth of the furnace has been filled with feedstock, the electrodes are lowered until they contact the charge. The electric current is then applied, typically at lower voltages to start the process. The current flow through the charge in conjunction with the radiant energy from the arcing raises the heat in charge to a point where it melts. Once the charge has completely melted, its temperature and chemical composition is checked via remote probes or lances; if all is correct, the molten metal can be tapped or poured off.
The raw materials used in the furnaces are often complimented by the addition of pig iron, burnt lime, and dolomite. These additives promote the correct chemical balance in the molten steel and form surface slag which floats out impurities and insulates the charge. Arc furnace installations offer a great deal more flexibility than other furnace types; they can use an exclusive supply of scrap material and their output can be varied to suit demand. Arc furnaces can also be rapidly shut down and restarted which is not the case with blast furnaces. They are also cheaper to install and use less power to operate per ton of finished product.
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