arc heating[′ärk ‚hēd·iŋ]
The heating of matter by an electric arc. The matter may be solid, liquid, or gaseous. When the heating is direct, the material to be heated is one electrode; for indirect heating, the heat is transferred from the arc by conduction, convection, or radiation.
At atmospheric pressure, the arc behaves much like a resistor operating at temperatures of the order of thousands of kelvins. The energy source is extremely concentrated and can reach many millions of watts per cubic meter. Almost all materials can be melted quickly under these conditions, and chemical reactions can be carried out under oxidizing, neutral, or reducing conditions.
In a direct-arc furnace, the arc strikes directly between the graphite electrodes and the charge being melted. These furnaces are used in steelmaking, foundries, ferroalloy production, and some nonferrous metallurgical applications. Although an extremely large number of furnace types are available, they are all essentially the same. They consist of a containment vessel with a refractory lining, a removable roof for charging, electrodes to supply the energy for melting and reaction, openings and a mechanism for pouring the product, a power supply, and controls. The required accessory components include water-cooling circuits, gas cleaning and extraction equipment, cranes for charging the furnace, and ladles to remove the product. Because the electrodes are consumed by volatilization and reaction, a mechanism must be provided to feed them continuously through the electrode holders.
In submerged-arc furnaces, the arcs are below the solid feed and sometimes below the molten product. Submerged-arc furnaces differ from those used in steelmaking in that raw materials are fed continuously around the electrodes and the product and slag are tapped off intermittently. The furnace vessel is usually stationary. Submerged-arc furnaces are often used for carbothermic reductions (for example, to make ferroalloys), and the gases formed by the reduction reaction percolate up through the charge, preheating and sometimes prereducing it. Because of this, the energy efficiency of this type of furnace is high. The passage of the exhaust gas through the burden also filters it and thus reduces air-pollution control costs.
Although carbon arcs are plasmas, common usage of the term plasma torch suggests the injection of gas into or around the arc. This gas may be inert, neutral, oxidizing, or reducing, depending on the application and the electrodes used. Plasma torches are available at powers ranging from a few kilowatts to over 10 MW; usually they use direct-current electricity and water-cooled metallic electrodes.
Direct-current carbon arc furnaces operate on the basis that a direct-current arc is more stable than its alternating-current counterpart, and can, therefore, be run at lower current and higher voltage by increasing the arc length. This reduces both the electrode diameter and the electrode consumption compared to alternating-current operation at similar powers. Tests have also shown that injecting gas through a hole drilled through the center of the electrode further increases stability and reduces wear. Powdered ore and reductants may be injected with this gas, reducing the need for agglomerating the arc furnace feed.
In most cases, direct-current carbon arc furnaces have one carbon electrode, with the product forming the second electrode. The current is usually removed from the furnace through a bottom constructed of electrically conducting material. Several direct-current plasma furnaces with powers ranging from 1 to 45 MW are in operation.