Electrometallurgy

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electrometallurgy

[i′lek·trō′med·əl·ər·jē] (metallurgy) Industrial recovery and processing of metals by electrical and electrolytic procedures.

Electrometallurgy

the part of metallurgy encompassing industrial methods for the production of metals and alloys by means of electric current.

Electrometallurgy makes use of electrothermal and electrochemical processes. Electrothermal processes are used to extract metals from ores and concentrates and to produce and refine ferrous and nonferrous metals and alloys based on the metals extracted. In such processes, electric energy is used as a source of heat.

Electrochemical processes are commonly used in the production of ferrous and nonferrous metals by means of the electrolysis of aqueous solutions and melts. In such cases oxidation-reduction reactions occur at phase interfaces upon the passage of electric current through electrolytes. Electroplating technology, which is based on the electrochemical processes of metal, deposition on the surface of metal and nonmetal articles, occupies a special place among such processes.

Electrothermal processes encompass steel melting in arc and induction furnaces, special-purpose electrometallurgy, ore-reduction smelting (including the production of ferroalloys and mattes and the melting of pig iron in electric shaft furnaces), and the production of nickel, tin, and other metals.

Electric arc melting. Electric steel produced for subsequent conversion is melted primarily in arc furnaces with a basic lining. The important advantages of such furnaces over other steel-melting units include the possibility of heating the metal to high temperatures by means of an electric arc, the presence of a reducing atmosphere in the furnace, less waste of alloying elements, and the use of highly basic slags that substantially lower the sulfur content in the metal. Such advantages have gained arc furnaces preference in the production of various high-quality alloy steels—corrosion-resistant steels, tool steels (including highspeed types), structural steels, electrical sheet, and heat-resistant steels—as well as nickel-based alloys.

The worldwide trend in electric arc melting is toward an increase in unit melting capacities to 200–400 tons and unit transformer power to 500–600 kilovolt-amperes per ton, unit specialization (such that some units are used only for melting and others are used only for refining and alloying), a high level of automation, and the use of computers for the programmed control of the melting process. High-capacity furnaces have made the production of both alloy steel and ordinary carbon steel economically feasible. In the developed capitalist countries, carbon steel accounts for at least 50 percent of the total volume of steel produced in electric furnaces. In the USSR approximately 80 percent of all alloy metal is produced in electric furnaces.

Plasma arc furnaces with basic ceramic crucibles and capacities up to 30 tons, equipped with DC and AC plasmatrons, are finding use in the production of special steels and alloys (seePLASMA METALLURGY). Arc furnaces with acid linings are used for the production of metal for steel casting. The acid process on the whole is more productive than the basic process because of the brief melting process—the result of the shorter duration of the oxidation and reduction periods. Acid steel is less expensive than basic steel because of the lower consumption of electric power and of electrodes, the greater durability of the lining, the lower consumption of deoxidizers, and the feasibility of using the silicon-reduction process. Arc furnaces with capacities up to 100 tons are also widely used for the production of pig iron in specialized shops.

Induction melting. The production of steel in induction furnaces, primarily by remelting, usually consists in melting the charge, deoxidizing the metal, and tapping. High demands are placed on the charge materials with respect to the content of harmful phosphorus and sulfur impurities. The choice of a basic or acid crucible depends on the properties of the metal. In order to prevent reduction of the lining silica during the melting process, steels and alloys with high amounts of manganese, titanium, and aluminum are melted in basic crucibles. An important disadvantage of induction melting are cold slags, which are heated only from the metal. In some furnace designs, this disadvantage is eliminated by plasma heating of the surface of the metal-slag charge, which also permits a useful acceleration of the melting of the charge.

Pure metals, steels, and alloys for critical applications are produced in vacuum induction furnaces (seeVACUUM SMELTING). The capacities of existing furnaces range from several kg to several tens of tons. Vacuum induction melting may be accelerated by blasting with inert gases (argon and helium) and reactive gases (carbon monoxide and methane), electromagnetic stirring of the metal in the crucible, and blasting the metal with slag-forming powders.

Special-purpose electrometallurgy. Special-purpose electrometallurgy encompasses new melting and refining processes that were developed in the 1950’s and 1960’s to meet the requirements of modern technology—aerospace, jet-engine, and atomic engi-neeringand chemical machine building—for structural materials with excellent mechanical properties and high resistance to heat and corrosion. Special-purpose electrometallurgy includes arc melting in a vacuum (seeARC VACUUM FURNACE), electron-beam melting, electroslag remelting, and plasma arc melting. Such methods are used to remelt steels and alloys for critical applications, refractory metals (such as tungsten, molybedenum, niobium, and their alloys), and highly reactive metals (such as titanium, vanadium, zirconium, and their alloys).

Arc melting in a vacuum was proposed in 1905 by W. Bolton (Germany) and was first used on an industrial scale for the production of titanium by W. J. Kroll (USA) in 1940. Electroslag remelting was developed in 1952 and 1953 at the E. O. Paton Institute of Electric Welding of the Academy of Sciences of the Ukrainian SSR. Various modifications of the duplex process are used for the production of nickel-based steels and alloys for extremely critical applications; the most important of these modifications combines vacuum induction melting with vacuum arc remelting. One special-purpose technique involves vacuum melting in a crucible with a skull, with the use of an electric arc, electron beam, or plasma as the heat source. The technique uses special furnaces in which a portion of the molten metal held in a water-cooled crucible with a skull is used to produce ingots and shaped castings of highly reactive and refractory metals, such as tungsten, molybdenum, and their alloys.

Ore-reduction smelting. Ore-reduction smelting encompasses the production of ferroalloys and nonferrous metal products, such as copper and nickel mattes, lead, zinc, and slags of titanium. The process consists in the reduction of native ores and concentrates with carbon, silicon, and other reducing agents at high temperatures, usually produced by a powerful electric arc (seeORE HEAT-TREATING FURNACE). The reduction processes are usually continuous. As the melting process progresses, the charge is loaded into a bath and the products are released from the electric furnace in batches. The capacities of such furnaces may reach 100 megavolt-amperes. In Sweden, Norway, Japan, Italy, and some other countries, ore-reduction smelting is used to produce pig iron in electric blast furnaces or shaftless arc furnaces.

Electrochemical processes. H. Davy first used electrolysis for the production of sodium and potassium in 1807. By the end of the 1970’s, more than 50 metals were produced by electrolysis, including copper, nickel, aluminum, magnesium, potassium, and calcium. There are two types of electrolytic processes. The first uses the cathodic deposition of a metal from a solution obtained by hydrometallurgical methods—the leaching of ores and concentrates. Here, the electrochemical oxidation of the anion on an insoluble anode corresponds to reduction (deposition) of the metal from the solution onto the cathode.

The second type of electrochemical process is the electrore-fining of a metal from its alloy, from which a soluble anode is made. In the initial stage of the process, the metal is transferred into solution as a result of electrolytic dissolution of the anode; in the second stage the metal is deposited on the cathode. The sequence of dissolution of the metal on the anode and cathodic deposition is determined by the electromotive force series. However, under real conditions, the potential for metal separation depends significantly on the hydrogen overvoltage on the corresponding metal. The method can be used for refining zinc, manganese, nickel, iron, and other metals on an industrial scale. Aluminum, magnesium, potassium, and other metals are produced by the electrolysis of molten salts at temperatures from 700° to 1000°C. The method entails a high consumption of electric energy (15,000–20,000 kilowatt-hours per ton) compared with the electrolysis of aqueous solutions, which consumes up to 10,000 kilowatt-hours per ton.

REFERENCES

Beliaev, A. I. Metallurgiia legkikh metallov, 6th ed. Moscow, 1970.
Zelikman, A. N., and G. A. Meerson. Metallurgiia redkikh metallov. Moscow, 1973.
Edneral, F. P. Elektrometallurgiia stali i ferrosplavov, 4th ed. Moscow, 1977.

V. A. GRIGORIAN