Electric arc furnace and Construction

An Electric Arc Furnace (EAF) is a furnace in which materials are heated by an electric arc.Industrial electric arc furnaces range in size from small units with a capacity of about 1 ton (used in foundries for the production of cast iron products) to about 400 ton units for secondary steelmaking.Electric arc furnaces used by research labs and dentists may have a capacity of only a few tens of grams. Industrial arc furnaces can reach temperatures of 1,800 °C (3,300 °F), while laboratory units can exceed 3,000 °C (5,400 °F).In an electric arc furnace, the charge (the material that enters the furnace to be heated, not to be confused with the electrical charge) is directly exposed to the arc, and current from the furnace terminals passes through the charge.Electric arc furnaces differ from induction furnaces in that the charge is heated by eddy currents.

Construction 

Electric arc furnaces for steelmaking consist of a refractory-lined, usually water-cooled, larger size vessel covered with a retractable roof through which one or more graphite electrodes enter the furnace.The furnace is mainly divided into three parts:

• The outer shell, consisting of side walls and a lower steel "bowl";

• Hearth, consisting of refractory material arranged in a lower bowl;

• The roof, which can be refractory-lined or water-cooled, can be part of a sphere or a truncated cone.The roof also supports a refractory delta at its center through which one or more graphite electrodes enter.The hearth may be hemispherical, or in the case of eccentric bottom tapping furnaces (see figure below), the hearth is in the shape of half an egg.In modern smelters, the furnace is usually raised from the ground so that ladles and slag pots can easily be moved under the ends of the furnace. Separated from the furnace structure are the electrode holders and electrical system, as well as the inclined platform on which the furnace sits.Two configurations are possible: electrode supported and roof tilted, or fixed on a raised platform.A typical AC electric stove is powered by a three-phase power supply and therefore has three electrodes.The electrodes are circular in cross-section and are usually segments with threaded joints so that new segments can be added as the electrodes wear out. An arc is formed between a charged material and an electrode; the charge is heated by the current flowing through the charge and by the radiant energy produced by the arc.The arc temperature reaches approximately 3,000 °C (5,400 °F), causing the lower part of the electrode to glow incandescently as it operates.The electrodes are automatically raised and lowered by a positioning system, which can use electric winch cranes or hydraulic cylinders.A regulating system maintains an approximately constant current and power input during charge melting, even though scrap may move under the electrodes as it melts.The mast arm that holds the electrodes can carry a heavy duty busbar (which can be a hollow water-cooled copper tube that feeds the current to the electrode holder) or a "hot arm" where the entire arm carries the current, increasing efficiency.The thermal arm can be made of copper clad steel or aluminum.Large water-cooled cables connect the bus ducts or bus arms to the transformer close to the furnace.The transformer is installed in the vault and is cooled by a pump circulating transformer oil which is cooled by water through a heat exchanger.

The furnace is built on a sloping platform so that the molten steel can be poured into another container for transport.The operation of tilting the furnace to pour molten steel is called "tapping".Originally, all steelmaking furnaces had a taphole closed with refractory material which was washed away when the furnace was tilted, but modern furnaces often have an eccentric bottom taphole (EBT) to reduce molten steel nitrogen and slag inclusions.These furnaces have a taphole that runs vertically through the hearth and casing, and is off-center in the narrow "nose" of the egg-shaped hearth.When it's off, it's filled with refractory sand, such as olivine. Modern plants probably have two shells and a set of electrodes that can be transferred between the two; one shell preheats the waste, while the other is used for melting.Other DC based furnaces have a similar arrangement, but each housing has electrodes and a set of electronics.AC furnaces typically exhibit a pattern of hot and cold spots around the hearth perimeter, with the cold spots located between the electrodes.Modern furnaces have oxy-fuel burners installed in the side walls and use them to provide chemical energy to cold spots to heat the steel more evenly.Additional chemical energy is provided by injecting oxygen and carbon into the furnace; in the past, this was done with lances (hollow mild steel pipes) in the slag door, but now this is mostly done with wall-mounted injection units, This injection unit combines an oxy-fuel burner and an oxygen or carbon injection system into one unit.

A medium-sized modern steelmaking furnace will have a transformer rated at approximately 60,000,000 volt-amps (60 MVA), with a secondary voltage between 400 and 900 volts and a secondary current in excess of 44,000 amps.In a modern plant, the furnace is expected to produce 80 tons of molten steel in about 50 minutes, from charging with cold scrap to exiting the furnace.In comparison, basic oxygen furnaces have a capacity of 150-300 tons per batch, or "heat", and can generate heat in 30-40 minutes.Furnace design details and operation vary widely depending on the final product and local conditions, as well as ongoing research to improve furnace efficiency.The largest scrap furnace (in terms of tap weight and transformer rating) is a once-through furnace operated by Tokyo Steel Corporation, Japan, with a tap weight of 420 tonnes and fed by eight 32 MVA transformers for a total of 256 MVA.