Plasma Metallurgy

the extraction from ores, the smelting, and the processing of metals and alloys in plasma reactors and plasma furnaces; also the use of plasma heating to increase the efficiency of existing methods of smelting. The development of plasma metallurgy began in the 1950’s in the USSR, Japan, the USA, the German Democratic Republic, the Federal Republic of Germany, and other countries.

When the ores, for example, oxides, are processed, they undergo thermal dissociation in the plasma. The ores either are fed into the plasma jet in the form of powders or are mixed with conducting materials, such as carbon, to form consumable electrodes of plasmatrons. Reverse reactions are prevented by the inclusion of reducing agents, such as carbon or hydrogen; by the sharp “quenching” of the gaseous dissociation products at the outlet from the plasma reactor; or by the obtaining of intermediate products, such as chlorides. An important task in the processing of complex compounds is the separation of the resulting products.

The smelting of steels and alloys is performed in plasma arc furnaces. An inert atmosphere and the absence of the sources of metal contamination that are usually encountered in arc melting permit the obtaining of pure metals from normal charges, which contain large quantities of waste materials. For example, special low-carbon stainless steels of high quality can be produced by this method. The partial replacement of argon by nitrogen in the plasma-forming gas or directly in the furnace atmosphere yields metal alloyed with nitrogen without the use of nitrided alloys.

Metals and alloys can be remelted to increase their purity or for alloying purposes in plasma arc furnaces with metallic water-cooled crystallizers. The thorough refining of a metal is promoted by the flow of an inert or a reducing gas, a large contact surface of the metal with the gas phase, and the treatment of the metal with the slag. Crystallization of metals in such plasma arc furnaces can be controlled by the separate regulation of the metal’s melting rate and the heat flux through the bath. The various forms of the process that have been adopted—either separately or in combination—by industry include refining re-melting in an atmosphere of inert gases, the combination of remelting with hydrogen-plasma deoxidizing of a metal or with saturation of the metal with nitrogen, and plasma arc remelting with slag. Operation of the process under a normal or higher than normal pressure prevents the loss of volatile alloying elements, such as chromium and manganese, and the saturation of the melt with nitrogen. If the process is carried out under reduced pressure, there results a more thorough degassing of the metal, for example, titanium. Remelting is performed in plasma arc furnaces to increase the quality of special alloyed steels, precision alloys, heat-resistant alloys, and refractory metals. Additional uses are the production of austenitic steels with a high nitrogen content unattainable by other methods of smelting and the reduction of losses of volatile and readily oxidizable elements.

The use of plasma arc heating in induction melting lessens the time required for smelting a charge and substantially improves metal refining because of the superheating of the slag by the arc. Plasmatrons can be used as auxiliary heat sources in blast and open-hearth furnaces and in heat-treating furnaces for semifinished products. Other uses include the growth of single crystals.