Electric arc furnace

thumb|300px|An electric arc furnace during operation, c. 1975

thumb|250px|Rendering of exterior and interior of an electric arc furnace.

An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc.

Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundries for producing cast iron products) up to about 400-tonne 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 reach , while laboratory units can exceed .

In electric arc furnaces, the material inside the furnace (referred to as a charge) is directly exposed to an electric arc, and the current from the electrode terminals passes through the charge material.

Arc furnaces differ from induction furnaces, which use eddy currents to heat the charge.

History

thumb|left|[[Stassano furnace exhibited at the Museo della Scienza e della Tecnologia "Leonardo da Vinci", Milan]]

In the 19th century, a number of people had employed an electric arc to melt iron. The possibility of using an electric arc to melt metals was demonstrated by Vasily Vladimirovich Petrov in 1803. Sir Humphry Davy conducted an experimental demonstration in 1810; welding was investigated by William Haseldine 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 first successful and operational furnace was invented by James Burgess Readman in Edinburgh, Scotland, in 1888 and patented in 1889. This was specifically for the creation of phosphorus.

Further electric arc furnaces were developed by Paul Héroult, of France, with a commercial plant established in the United States in 1907. The Sanderson brothers formed The Sanderson Brothers Steel Co. in Syracuse, New York, installing the first electric arc furnace in the U.S. This furnace is now on display at Station Square, Pittsburgh, Pennsylvania.

thumb|right|A schematic cross section through a Heroult arc furnace. E is an electrode (only one shown), raised and lowered by the rack and pinion drive R and S. The interior is lined with refractory brick H, and K denotes the bottom lining. A door at A allows access to the interior. The furnace shell rests on rockers to allow it to be tilted for tapping.

Initially "electric steel" produced by an electric arc furnace 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. The Stassano electric furnace is an arc type furnace that usually rotates to mix the bath. The Girod furnace is similar to the Héroult furnace.

While EAFs were widely used in World War II for production of alloy steels, it was only later that electric steelmaking began to expand. The low capital cost for a mini-mill—around US$140–200 per ton of annual installed capacity, compared with US$1,000 per ton of annual installed capacity for an integrated steel mill—allowed mills to be quickly established in war-ravaged Europe, and also allowed them to successfully compete with the big United States steelmakers, such as Bethlehem Steel and U.S. Steel, for low-cost, carbon steel "long products" (structural steel, rod and bar, wire, and fasteners) in the U.S. market.

When Nucor—now one of the largest steel producers in the US — entered the market for long steel products in 1969, they used a mini-mill with an EAF as its steelmaking furnace, soon followed by other manufacturers. While Nucor expanded rapidly in the Eastern US, the companies that followed them into mini-mill operations concentrated on local markets for long products, where the EAF allowed the plants to vary production according to local demand. This pattern was followed globally, with EAF steel production primarily used for long products, while integrated mills, using blast furnaces and basic oxygen furnaces, cornered the markets for "flat products"—sheet steel and heavier steel plate. In 1987, Nucor expanded into the flat products market, still using the EAF production method.

Construction

thumb|right| A schematic cross-section through an EAF. Three electrodes (yellow), molten bath (gold), tapping spout at left, refractory brick movable roof, brick shell, and a refractory-lined bowl-shaped hearth.

thumb|right|A general overview of a typical AC (Alternating Current) electric arc furnace with three electrodes. All relevant hardware (except the transformer vault to left) are visible, such as water-cooled power cables, painted copper electrode arms, swing roof, refaractory-lined cylindrical melting hearth, tap spout, tilting mechanism, and exhaust ducting.

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.

The furnace is primarily split into three sections:

the shell, which consists of the sidewalls and lower steel "bowl";
the hearth, which consists of the refractory that lines the lower bowl;
the roof, which may be refractory-lined or water-cooled, and can be shaped as a section of a sphere, or as a frustum (conical section). The roof also supports the refractory delta in its centre, through which one or more graphite electrodes enter.

The hearth may be hemispherical in shape, or in an eccentric bottom tapping furnace (see below), the hearth has the shape of a halved egg. In modern meltshops, the furnace is often raised off the ground floor, so that ladles and slag pots can easily be maneuvered under either end of the furnace. Separate from the furnace structure is the electrode support and electrical system, and the tilting platform on which the furnace rests. Two configurations are possible: the electrode supports and the roof tilt with the furnace, or are fixed to the raised platform.

thumb|The roof of an arc furnace removed, showing the three electrodes

A typical alternating current furnace is powered by a three-phase electrical supply, and therefore has three electrodes. 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; the charge is heated both by current passing through the charge and by the radiant energy evolved by the arc. The electric arc temperature reaches around , thus causing the lower sections of the electrodes to glow incandescently when in operation. 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 approximately constant current and power input during the melting of the charge, even though scrap may move under the electrodes as it melts. The mast arms holding the electrodes can either carry heavy busbars (which may be hollow water-cooled copper pipes carrying current to the electrode clamps) or be "hot arms", where the whole arm carries the current, increasing efficiency. Hot arms can be made from copper-clad steel or aluminium. Large water-cooled cables connect the bus tubes or arms with the transformer located adjacent to the furnace. The transformer is installed in a vault and is cooled by pump-circulated transformer oil, with the oil being cooled by water via heat exchangers.) in the slag door, but now this is mainly done through wall-mounted injection units that combine the oxygen-fuel burners and the oxygen or carbon injection systems into one unit.

A mid-sized modern steelmaking furnace would have a transformer rated about 60,000,000 volt-amperes (60 MVA), with a secondary voltage between 400 and 900 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 80 tonnes of liquid steel in approximately 50 minutes from charging with cold scrap to tapping the furnace. In comparison, basic oxygen furnaces can have a capacity of 150–300 tonnes per batch, or "heat", and can produce a heat in 30–40 minutes. Enormous variations exist in furnace design details and operation, depending on the end product and local conditions, as well as ongoing research to improve furnace efficiency. The largest scrap-only furnace (in terms of tapping weight and transformer rating) is a DC furnace operated by Tokyo Steel in Japan, with a tap weight of 420 tonnes and fed by eight 32 MVA transformers for 256 MVA total power.

Energy density

To produce a ton of steel in an electric arc furnace requires approximately 400 kilowatt-hours (1.44 gigajoules) per short ton or about 440 kWh (1.6 GJ) per tonne. The theoretical minimum amount of energy required to melt a tonne of scrap steel is 300 kWh (1.09 GJ) (melting point ). Therefore, a 300-tonne, 300 MVA EAF will require approximately 132 MWh of energy to melt the steel, and a "power-on time" (the time that steel is being melted with an arc) of approximately 37 minutes.

Electric arc steelmaking is only economical where there is plentiful, reliable electricity, with a well-developed electrical grid. In many locations, mills operate during off-peak hours when utilities have surplus power generating capacity and the price of electricity is less. This compares very favourably with energy consumption of global steel production by all methods estimated at some 5,555 kWh (20 GJ) per tonne which is significantly lower than the conventional production route via blast furnaces and the basic oxygen furnace, which produces 2.9 tons CO<sub>2</sub> per ton of steel produced.

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 these effects, which include:

Enclosures to reduce high sound levels
Dust collector for furnace off-gas
slag production
cooling water demand
Heavy truck traffic for scrap, materials handling, and product
Environmental effects of electricity generation

Since EAF steelmaking mainly use recycled materials like scrap iron and scrap steel, as their composition varies the resulting EAF slag and EAF dust can be toxic. EAF dust is collected by air pollution control equipment. It is called collected dust and usually contains heavy metals, such as zinc, lead and dioxins, etc. It is categorized as hazardous industrial waste and disposal is regulated.

Because of the very dynamic quality of the arc furnace load, power systems may require technical measures to maintain the quality of power for other customers; flicker and harmonic distortion are common power system side-effects of arc furnace operation.

Other electric arc furnaces

thumb|Rendering of a ladle furnace, a variation of the electric arc furnace used for keeping molten steel hot

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 is lower electrode consumption per ton of steel produced, since only one electrode is used, as well as less electrical harmonics and other similar problems. The size of DC arc furnaces is limited by the current carrying capacity of available electrodes, and the 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 (LF) is used to maintain the temperature of liquid steel during processing after tapping from EAF or to change the alloy composition. The ladle is used for the first purpose when there is a delay later in the steelmaking process. The ladle furnace consists of a refractory roof, a heating system, and, when applicable, a provision for injecting argon gas into the bottom of the melt for stirring. Unlike a scrap melting furnace, a ladle furnace does not have a tilting or scrap-charging mechanism.

Electric arc furnaces are also used for production of calcium carbide, ferroalloys, and other 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, Søderberg electrodes to prevent interruptions from electrode changes.

Such a furnace is known as a submerged arc furnace, because the electrode tips are buried in the slag/charge, and arcing occurs through the slag, between the matte and the electrode. The casing and casing fins of the electrode melt the electrode paste through electrical current passing through the electrode casing and heat from the furnace.

Plasma arc furnace

A plasma arc furnace (PAF) uses plasma torches instead of graphite electrodes. Each of these torches has a casing with a nozzle and axial tubing for feeding a plasma-forming gas (either nitrogen or argon) and a burnable cylindrical graphite electrode within the tubing. Such furnaces can be called plasma arc melt (PAM) furnaces; they are used extensively in the titanium-melting industry and similar specialty metal industries.

Vacuum arc remelting

Vacuum arc remelting (VAR) is a secondary remelting process for vacuum refining and manufacturing of ingots with improved chemical and mechanical homogeneity.

In critical military and commercial aerospace applications, material engineers commonly specify VIM-VAR steels. VIM means vacuum induction melted and VAR means vacuum arc remelted. VIM-VAR steels become bearings for jet engines, rotor shafts for military helicopters, flap actuators for fighter jets, gears in jet or helicopter transmissions, mounts or fasteners for jet engines, jet tail hooks and other demanding applications.

Most grades of steel are melted once and are then cast or teemed into a solid form prior to extensive forging or rolling to a metallurgically-sound form. In contrast, VIM-VAR steels go through two more highly purifying melts under vacuum. After melting in an electric arc furnace and alloying in an argon oxygen decarburization vessel, steels destined for vacuum remelting are cast into ingot molds. The solidified ingots then head for a vacuum induction melting furnace. This vacuum remelting process rids the steel of inclusions and unwanted gases while optimizing the chemical composition.

The VIM operation returns these solid ingots to the molten state in the contaminant-free void of a vacuum. This tightly controlled melt often requires up to 24 hours. Still enveloped by the vacuum, the hot metal flows from the VIM furnace crucible into giant electrode molds. A typical electrode is about 15 feet (5 m) tall and will be in various diameters. The electrodes solidify under vacuum.

For VIM-VAR steels, the surface of the cooled electrodes must be ground to remove surface irregularities and impurities before the next vacuum remelt. Then the ground electrode is placed in a VAR furnace. In a VAR furnace, the steel gradually melts drop-by-drop in the vacuum-sealed chamber. Vacuum arc remelting further removes lingering inclusions to provide superior steel cleanliness and remove gases like oxygen, nitrogen and hydrogen. Controlling the rate at which these droplets form and solidify ensures a consistency of chemistry and microstructure throughout the entire VIM-VAR ingot, making the steel more resistant to fracture or fatigue. This refinement process is essential to meet the performance characteristics of parts like a helicopter rotor shaft, a flap actuator on a military jet, or a bearing in a jet engine.

For some commercial or military applications, steel alloys may go through only one vacuum remelt, namely the VAR. For example, steels for solid rocket cases, landing gears, or torsion bars for fighting vehicles typically involve one vacuum remelt.

Vacuum arc remelting is also used in production of titanium and other metals which are reactive or in which high purity is required.

References

External links

Recognition of first foundry as historical site
Home made small scale arc furnace using a welder (Caution with experiments!)
Electric Arc Furnace module at steeluniversity.org, including a fully interactive simulation
Process models demonstrate the EAF operation and control (MPC)
YouTube video of a small EAF in New Zealand