An electric arc furnace is a system that heats the 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.
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.
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. 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. 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 in the liquid steel.
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. Electric arc furnace steelmaking is only economical where there is a plentiful supply of electric power, with a well-developed electrical grid.
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 econmically, 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
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 effects of arc furnace operation on a power system.
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. The advantage of DC would be lower electrode consumption per ton of steel produced, since only one electrode is used. 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 use 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.
References
- H. W. Beaty (ed), "Standard Handbook for Electrical Engineers,11th Ed.", McGraw Hill,New York 1978, ISBN 007020974x
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