Electric arc furnace
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An electric arc furnace is a system 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). Temperatures inside an electric arc furnace can rise to approximately 2000 degrees Celsius.
History
The first electric arc furnaces were developed by Paul Héroult of France, with a commercial plant established in the United States in 1907. Initially "electric steel" was a specialty product for such uses as machine tools and spring steel. Arc furnaces were also used to prepare calcium carbide for use in carbide lamps.In the 19th century, a number of men had employed an electric arc to melt iron. Sir Humphry Davy conducted an experimental demonstration in 1810; welding was investigated by Pepys in 1815; Pinchon attempted to create an electrothermic furnace in 1853; and, in 1878 - 79, Sir William Siemens took out patents for electric furnaces of the arc type. The Stessano electric furnace is an arc type furnace that usually rotates to mix the bath. The Girod furnace is similar to the Héroult furnace.
Different from the arc type of electrothermic furnace is the induction type furnace. The Kjellin furnace and the Röchling-Rodenhauser furnace are two. The Grönwall furnace produced steel at Trollhattan, in Scandinavia.
Construction
An electric arc furnace used for steelmaking consists of a refractory-lined vessel, usually water-cooled in larger sizes, covered with a retractable roof, and through which one or more graphite electrodes enter the furnace. For a typical AC furnace three electrodes are used. Electrodes are round in section, and typically in segments with threaded couplings, so that as the electrodes wear, new segments can be added. The arc forms between the charged material and the electrode, and the charge is heated both by current passing through the charge and by the radiant energy evolved by the arc.The electrodes are automatically raised and lowered by a positioning system, which may use either electric winch hoists or hydraulic cylinders. The regulating system maintains an approximately constant current and power input during the melting of the charge, even though scrap may move under the electrodes while it melts. The mast arms holding the electrodes carry heavy bus bars, which may be hollow water-cooled copper pipes, used to convey current to the electrode holders (modern systems use 'hot arms', where the whole arm carries the current, increasing efficiency). Since the electrodes move up and down automatically for regulation of the arc, and are raised to allow removal of the furnace roof, heavy water-cooled cables connect the bus tubes with the transformer located adjacent to the furnace. To protect the transformer from the heat of the furnace, it is installed in a vault.
The refractory lined vessel has a removable roof. This unit of the furnace is often known as a shell and is separate from the electrical system. Much of the furnace shell and roof may be water-cooled. The bowl-shaped bottom of the furnace, called the "hearth", is lined with refractory bricks and granular refractory material. The furnace is built on a tilting platform so that the liquid steel can be poured into another vessel for transport in the steel making process. The operation of tilting the furnace to pour off molten steel is called "tapping". Originally, all steelmaking furnaces had a tapping spout closed with refractory that washed out when the furnace was tilted, but often modern furnaces have a bottom tap-hole on the spout to reduce inclusion of nitrogen and slag in the liquid steel. Modern plants may have two shells with a single set of electrodes that can be transferred between the two; one shell preheats scrap while the other shell is utilised for meltdown. Other DC-based furnaces have a similar arrangement, but have electrodes for each shell and one set of electronics.
A mid-sized modern steelmaking furnace would have a transformer rated about 60,000,000 volt-amperes (60 MVA), with a secondary voltage around 800 volts and a secondary current in excess of 44,000 amperes. In a modern shop such a furnace would be expected to produce a quantity of 55 metric tons of liquid steel in approximately 70 minutes from charging with cold scrap to tapping the furnace. Each batch is called a "heat".
To produce a ton of steel in an electric arc furnace requires on the close order of 400 kilowatt-hours per short ton or about 440 kW·h per metric ton (1.5 kJ/g). Electric arc furnace steelmaking is only economical where there is a plentiful supply of electric power, with a well-developed electrical grid.
Operation
- Scrap metal is delivered to a scrap bay, located next to the melt shop. Scrap generally comes in two main grades: shred (scrap light enough to have been passed through a shredder) and heavy melt (large slabs and beams), along with some direct reduced iron (DRI) or pig iron for chemical balance.
- The scrap is loaded into large scrap buckets. Care is taken to layer the scrap in the bucket to ensure good furnace operation; heavy melt is placed on top of a light layer of protective shred, on top of which is placed more shred. These layers should be present in the furnace after charging. After loading, the bucket may pass to a scrap pre-heater, which uses hot furnace off-gases to heat the scrap and recover energy to increase plant overall efficiency.
- The scrap bucket is then taken to the melt shop, the roof is swung off the furnace, and the furnace is charged with scrap. The falling scrap often raises a large amount of dust (mostly loose steel particles) during charging; if the furnace is hot, this dust is ignited and a fireball erupts out of the top of the furnace. In some twin-shell furnaces, the scrap is charged into the second shell while the first is being melted down, and pre-heated with off-gas from the active shell. Other operations have trialled pre-heating scrap on a conveyer belt (leading to continuous furnace charging) or charging the scrap from a shaft set above the furnace, with off-gases directed through the shaft. Yet other furnaces can be charged with hot (molten) metal from other operations.
- After charging, the roof is swung back over the furnace and meltdown commences. The electrodes are lowered onto the scrap, an arc is struck and the electrodes are then set to bore into the layer of shred at the top of the furnace. Lower voltages are selected for this first part of the operation to protect the roof and walls from excessive heat and damage from the arcs. Once the electrodes have reached the heavy melt at the base of the furnace and the arcs are shielded by the scrap, the voltage can be increased and the electrodes raised slightly, lengthening the arcs and increasing power to the melt. This enables a molten pool to form more rapidly, reducing tap-to-tap times. In more modern furnaces, oxygen by itself is also lanced into the scrap, combusting or cutting the steel and burning out carbon, and sometimes chemical heat is provided by wall-mounted oxy-fuel burners. Both processes accelerate scrap meltdown.
- An important part of steelmaking is the formation of slag, which floats on the surface of the molten steel. Slag usually consists of metal oxides, and acts as a destination for oxidised impurities, as a thermal blanket (stopping excessive heat loss) and helping to reduce erosion of the refractory lining. For a furnace with basic refractories (this includes most carbon steel-producing furnaces), the usual slag formers are calcium oxide (CaO, in the form of burnt lime) and magnesium oxide (MgO, in the form of dolomite and magnesite). Later in the heat, carbon (in the form of coke) is lanced into this slag layer, partially combusting to form carbon monoxide gas, which then causes the slag to foam, allowing greater thermal efficiency, and better arc stability and electrical efficiency. The slag blanket also covers the arcs, prevents damage to the furnace roof and sidewalls from radiant heat.
- Once flat bath conditions are reached, i.e. the scrap has been completely melted down, often another bucket of scrap is charged into the furnace and melted down. After the second charge is completely melted, refining operations take place to check and correct the steel chemistry and superheat the melt above its freezing temperature in preparation for tapping. More slag formers are introduced and more oxygen is lanced into the bath, burning out impurities such as silicon, sulphur, phosphorous, aluminium, manganese and calcium and removing their oxides to the slag. Metals that have a poorer affinity for oxygen than iron, such as nickel and copper, cannot be removed through oxidation and must be controlled through scrap chemistry alone (such as introducing the DRI and pig iron mentioned earlier). A foaming slag is maintained throughout, and often overflows the furnace to pour out of the slag door into the slag pit. Temperature sampling and chemical sampling (in the form of a 'chill' - a small, solidified sample of the steel) take place via automatic lances.
- Once the temperature and chemistry are correct, the steel is tapped out into a preheated ladle through tilting the furnace. Here, some alloy additions are introduced into the metal stream. Often, a few tonnes of liquid steel and slag is left in the furnace in order to form a 'hot heel', which helps preheat the next charge of scrap and accelerate its meltdown. At the same time, the furnace turnaround is started: the slag door is cleaned of solidified slag, repairs may take place, and electrodes are inspected for damage or lengthened through the addition of new segments; the taphole is filled with sand at the completion of tapping. For a 90-tonne, medium-power furnace, the whole process will usually take about 60-70 minutes from the tapping of one heat to the tapping of the next (the tap-to-tap time).
Advantages of electric arc furnace for steelmaking
The precise control of chemistry and temperature encouraged use of electric arc furnaces during World War II for production of steel for shell casings. Today steelmaking arc furnaces produce many grades of steel, from concrete reinforcing bars and common merchant-quality standard channels, bars, and flats to special bar quality grades used for the automotive and oil industry.A typical steelmaking arc furnace is the source of steel for a mini-mill, which may make bars or strip product. The steelmaking arc furnace is generally charged with scrap steel, though if hot metal from a blast furnace or direct-reduced iron is available economically, these can also be used for steelmaking.
Environmental issues
Although the modern electric arc furnace is a highly efficient recycler of steel scrap, operation of an arc furnace shop can have adverse environmental effects. Much of the capital cost of a new installation will be devoted to systems intended to reduce environmental effects. These include:- High sound levels
- Dust and off-gas production
- Slag production
- Cooling water demand
- Heavy truck traffic for scrap, materials handling, and products
- Environmental effects of electricity generation
Other electric arc furnaces
For steelmaking, direct current (DC) arc furnaces are used, with a single electrode in the roof and the current return through a conductive bottom lining or conductive pins in the base. The advantage of DC would be lower electrode consumption per ton of steel produced, since only one electrode is used, as well as less electrical harmonics and other similar problems. However, the size of DC arc furnaces is limited by the available electrodes and maximum allowable voltage. Maintenance of the conductive furnace hearth is a bottleneck in extended operation of a DC arc furnace.In a steel plant, a ladle furnace can be used to maintain the temperature of liquid steel during processing after tapping from the scrap-melting furnace. This also allows the molten steel to be kept ready for use in the event of a delay later in the steelmaking process. The ladle furnace consists of only the refractory roof and electrode system of a scrap-melting furnace, but it has no need for a tilting mechanism or scrap charging.
Electric arc furnaces are also used for production of non-ferrous alloys, and for production of phosphorus. Furnaces for these services are physically different from steel-making furnaces and may operate on a continuous, rather than batch, basis. Continuous process furnaces may also use paste-type (Soderberg) electrodes to prevent interruptions due to electrode changes.
Amateurs have constructed a variety of arc furnaces, often based on electric arc welding kits contained by silical blocks or flower pots. Though crude, these simple furnaces are capable of melting a wide range of materials and creating calcium carbide etc.
References
- H. W. Beaty (ed), "Standard Handbook for Electrical Engineers,11th Ed.", McGraw Hill,New York 1978, ISBN 007020974x
External links
- [Recognition of first foundry as historical site]
- [Home made small scale arc furnace using a welder (Caution with experiments !)]
- [Electric Arc Furnace section at steeluniversity.org, including fully interactive simulation]
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