Electrometallurgy

Summary

Electrometallurgy includes electrowinning, electroforming, electrorefining, and electroplating. The electrowinning of aluminum from its oxide (alumina) accounts for essentially all the world's supply of this metal. Electrorefining of copper is used to achieve levels of purity needed for its use as an electrical conductor. Electroplating is used to protect base metals from corrosion, to increase hardness and wear resistance, or to create a decorative surface. Electroforming is used to produce small, intricately shaped metal objects.

Definition and Basic Principles

Electrometallurgy is that part of metallurgy that involves the use of electric current to reduce compounds of metals to free metals. It includes uses of electrolysis such as metal plating, metal refining, and electroforming. (Electrolysis in this context does not include the cosmetic use of electrolysis in hair removal.)

Background and History

The field of electrometallurgy started in the late eighteenth century and was the direct result of the development of a source of electric current, the cell invented by Alessandro Volta of Pavia, Italy. Volta placed alternating disks of zinc and copper separated by brine-soaked felt disks into a long vertical stack, essentially inventing the battery. In March 1800, Volta described his "pile" in a letter to the Royal Society in London. His primitive device could produce only a small electric current, but it stimulated scientists all over Europe to construct bigger and better batteries and to explore their uses. Luigi Valentino Brugnatelli, an acquaintance of Volta, used a voltaic pile in 1802 to do electroplating. British surgeon Alfred Smee published Elements of Electrometallurgy in 1840, using the term "electrometallurgy" for the first time.

Michael Faraday discovered the basic laws of electrolysis early in the nineteenth century and was the first to use the word "electrolysis." Both Sir Humphry Davy and Robert Bunsen used electrolysis to prepare chemical elements. The development of dynamos based on Faraday's ideas made possible larger currents and opened up industrial uses of electrolysis, most notably the Hall-Héroult process for the production of aluminum around 1886.

The Aluminum Company of America (known as Alcoa since 1999) was founded in 1909 and became a major producer of aluminum by the Hall-Héroult process. The companies Rio Tinto, China Hongqiao Group, RUSAL, and Norsk Hydro also became major producers of aluminum. Magnesium was first made commercially in Germany by I. G. Farben in the 1890s and later in the United States by Dow Chemical. In both instances, electrolysis of molten magnesium chloride was used to produce the magnesium.

How It Works

Electrolysis involves the passage of an electric current through a circuit containing a liquid electrolyte, which may be a water solution or a molten solid. The current is carried through the electrolyte by migration of ions (positively or negatively charged particles). At the electrodes, the ions in the electrolyte undergo chemical reactions. For example, in the electrolysis of molten sodium chloride, positively charged sodium ions migrate to one electrode (the cathode) and negatively charged chloride ions to the other electrode (the anode). As the sodium ions interact with the cathode, they absorb electrons from the external circuit, forming sodium metal. This absorption of electrons is known as reduction of the sodium ions. At the anode, chloride ions lose electrons to the external circuit and are converted to chlorine gas—an oxidation reaction.

The chemical reaction occurring in electrolysis requires a minimum voltage. Sufficient voltage also must be supplied to overcome internal resistance in the electrolyte. Because an electrolyte may contain a variety of ions, alternative reactions are possible. In aqueous electrolytes, positively charged hydrogen ions are always present and can be reduced at the cathode. In a water solution of sodium chloride, hydrogen ions are reduced to the exclusion of sodium ions, and no sodium metal forms. The only metals that can be liberated by electrolysis from aqueous solution are those such as copper, silver, cadmium, and zinc, whose ions are more easily reduced than hydrogen ions.

An electrochemical method of liberating active metals depends on finding a liquid medium more resistant to reduction than water. The use of molten salts as electrolytes often solves this problem. Salts usually have high melting points, and it can be advantageous to choose mixtures of salts with lower melting temperatures, as long as the new cations introduced do not interfere with the desired reduction reaction. The choice of electrode materials is important because it can affect the purity of the metal being liberated. Usually, inert electrodes are preferable.

In electroplating, it is necessary to produce a uniform adherent coating on the object that forms the cathode. Successful plating requires careful control of several factors, such as temperature, concentration of the metal ion, current density at the cathode, and cleanliness of the surface to be plated. The metal ions in the electrolyte may need to be replenished as they are being depleted. Current density is usually kept low, and plating is done slowly. Sometimes, additives that react with the metal ions or modify the viscosity or surface tension of the medium are used in the electrolyte. Successful plating conditions are often discovered by experiment and may not be understood in a fundamental sense.

Electroforming consists of depositing a metal plate on an object with the purpose of preparing a duplicate of the object in all its details. Nonconducting objects can be rendered conductive by coating them with a conductive layer. The plated metal forms on a part called a mandrel, which forms a template and from which the new metal part is separated after the operation. The mandrel may be saved and used again or dissolved away with chemicals to separate it from the new object.

In electrorefining, anodes of impure metals are immersed in an electrolyte that contains a salt of the metal to be refined—often a sulfate—and possibly sulfuric acid. When current is passed through, the metal dissolves from the anode and is redeposited on the cathode in a purer form. Control of the applied voltage is necessary to prevent the dissolution of less easily oxidized metal impurities. Material that is not oxidized at the anode falls to the bottom of the electrolysis cell as anode slime. This slime can be further processed for any valuable materials it may contain.

Applications and Products

Electrowinning of Aluminum.Aluminum, the most abundant metal in the Earth's crust, did not become readily available commercially until the development of the Hall-Héroult process. This process involves electrolysis of dry aluminum oxide (alumina) dissolved in cryolite (sodium aluminum hexafluoride). Additional calcium fluoride is used to lower the melting point of the cryolite. The process runs at about 960 degrees Celsius and uses carbon electrodes. The alumina for the Hall-Héroult process is obtained from an ore called bauxite, an impure aluminum oxide with varying amounts of compounds such as iron oxide and silica. The preparation of pure alumina follows the Bayer process: The alumina is extracted from the bauxite as a solution in sodium hydroxide (caustic soda), reprecipitated by acidification, filtered, and dried. The electrolysis cell has a carbon coating at the bottom that forms a cathode, while the anodes are graphite rods extending down into the molten salt electrolyte. As aluminum forms, it forms a pool at the bottom of the cell and can be removed periodically.

The anodes are consumed as the carbon reacts with the oxygen liberated by the electrolysis. The alumina needs to be replenished from time to time in the melt. The applied voltage is about 4.5 volts, but it rises sharply if the alumina concentration is too low. The electrolysis cells are connected in series, and there may be several hundred cells in an aluminum plant. The consumption of electric power amounts to about 15,000 kilowatt-hours per ton of aluminum. (A family in a home might consume 150 kilowatt-hours of power in a month.) Much of the power goes for heating and melting the electrolyte. The cost of electric power is a significant factor in aluminum manufacture and makes it advantageous to locate plants where power is relatively inexpensive, for example, where hydropower is available. Although some other metals are produced by electrolysis, aluminum is the metal produced in the greatest amount, at tens of millions of metric tons per year.

Aluminum is the most commonly used structural metal after iron. As a low-density strong metal, aluminum tends to find uses where weight saving is important, such as in aircraft. When automobile manufacturers try to increase gas mileage, they replace the steel in vehicles with aluminum to save weight. Aluminum containers are commonly used for foods and beverages and aluminum foil for packaging.

Alcoa developed a second electrolytic aluminum process that involves electrolysis of aluminum chloride. The aluminum chloride is obtained by chlorinating aluminum oxide. The chlorine liberated at the anode can be recycled. Also the electric power requirements of this process are less than for the Hall-Héroult process. This aluminum chloride reduction has not been used as much as the oxide reduction.

Electrowinning of Other Metals. Magnesium, like aluminum, is a low-density metal and is also manufactured by electrolysis. The electrolysis of molten magnesium chloride (in the presence of other metal chlorides to lower the melting point) yields magnesium at the cathode. The scale of magnesium production is not as large as that of aluminum, amounting to several hundred thousand metric tons per year. Magnesium is used in aircraft alloys, flares, and chemical syntheses.

Sodium metal comes from the electrolysis of molten sodium chloride in an apparatus called the Downs cell after its inventor J. C. Downs. Molten sodium forms at the cathode. Sodium metal was formerly very important in the process for making the gasoline additive tetraethyl lead. As leaded fuel is no longer sold in the United States, this use has declined, but sodium continues to be used in organic syntheses, the manufacture of titanium, and as a component (with potassium) in high-temperature heat exchange media for nuclear reactors.

Lithium and calcium are obtained in relatively small quantities by the electrolysis of their chlorides. Lithium is assuming great importance for its use in high-performance batteries for all types of applications but particularly for powering electric automobiles. A lightweight lithium-aluminum alloy has been used in the National Aeronautics and Space Administration's Ares rocket and the external tank of the space shuttle.

Electrorefining Applications. Metals are often obtained from their ores by pyrometallurgy (sequences of heating) and the use of reducing agents such as carbon. Metals obtained this way include iron and copper. Iron can be refined electrolytically, but much iron is used for steel production without refining.

Copper is used in applications where purity is important. Pure copper is ductile and an excellent electrical conductor, so it must be refined for use in electrical wiring. Copper anodes (blister copper) are suspended in a water solution containing sulfuric acid and copper sulfate with steel cathodes. Electrolysis results in the dissolution of copper from the anode and the migration of copper ions to the cathode, where purified metal is deposited. The result is copper of 99.9 percent purity. A similar procedure is used in recycling copper. Other electrorefined metals include aluminum, zinc, nickel, cobalt, tin, lead, silver, and gold. Materials are added to the electrolyte to make the metal deposit more uniform—glue, metal chlorides, levelers, and brighteners may be included. The details of the conditions for electrorefining vary depending on the metal.

Electroforming Applications. The possibility of reproducing complicated shapes on both large and small scale is an advantage of electroforming. Objects that would be impossible to produce because of their intricate shapes or small sizes are made by electroforming. The metal used may be a single metal or an alloy. The metals used most often are nickel and copper. The manufacture of compact discs for recording sound makes use of electroforming in the reproduction of the bumps and grooves in a studio disk of glass, which is a negative copy, and is then used to make a mold from which plastic discs can be cast. Metal foil can be electroformed by using a rotating mandrel surrounded by a cylindrical anode. The foil is peeled off the mandrel in a continuous sheet. The electroforming of copper foil for electronic applications is the largest application of electroforming.

Electroplating Applications. Plating is done to protect metal surfaces from corrosion, to enhance the appearance of a surface, or to modify a surface's properties in other ways such as to increase hardness or reflectivity. Familiar applications include chromium plating of automobile parts, silver plating of tableware and jewelry, and gold plating of medals and computer parts. Plating can also be done on nonmetallic surfaces such as plastic or ceramics. The manufacture of circuit boards requires a number of steps, some of which involve the electroplating of copper, lead, and tin. Many switches and other electrical contacts are plated with gold to prevent corrosion. The metals involved in commercial electroplating are mostly deposited from an aqueous electrolyte. This excludes metals such as aluminum or magnesium, which cannot be liberated in water solution. If a molten salt electrolyte is used, aluminum plating is possible, but it is seldom done.

Careers and Course Work

The path to a career in electrometallurgy is through a bachelor's or master's degree in chemistry or chemical engineering, although research careers require a doctorate. The coursework involves a thorough grounding in physical science (chemistry, physics), two years of calculus, and computer science and engineering principles courses. Most large state universities offer programs in chemistry and chemical engineering. A few universities offer specialized work in electrometallurgy. Major metal-producing companies often also offer summer internships and even scholarships for students.

Multiday personal development courses in electroplating are widely available through groups such as the National Society for Surface Finishing.

Social Context and Future Prospects

The electrometallurgy industry, like many industries, poses challenges for society. Metals have great value and many uses essential to modern life, but electrometallurgy consumes huge amounts of energy and uses many unpleasant chemicals. Additionally, aluminum plants emit carbon dioxide and fluorine compounds. However, using electricity to produce metals remains the cleanest and most efficient method.

Electrometallurgy continues to become more efficient and less polluting. New techniques permit the acquisition of additional metals by electrometallurgy. Titanium metal continues to be made by reducing its chloride by sodium metal. Still, the Fray Farthing Chen (FFC) Cambridge process announced in 2000 shows promise as an electrolytic method for obtaining titanium. The process involves the reduction of titanium oxide by electrically generated calcium in a molten calcium chloride medium. Titanium is valued for its strength, lightweight, high-temperature performance, and corrosion resistance. These qualities make it essential in jet engine turbines. Titanium is also stable in the human body and can be used to make artificial knees and hips.

Other industry trends include further efforts to create and implement energy-friendly, low-emission metal-making methods. Boston Metal revolutionized the decarbonization of steelmaking with its Molten Oxide Electrolysis (MOE) system. Using all grades of iron ore, the MOE produces green steel cost-competitively. The MOE technology won the company the Fast Company’s 2024 World Changing Ideas Awards.

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