Aluminum

General properties

Where aluminum comes from

How aluminum is obtained

Uses

Chemistry and compounds

Resources

Aluminum is the metallic chemical element of atomic number 13. Its symbol is Al; its atomic weight is 26.98; its specific gravity is 2.70; its melting point is 1, 220.5°F (660°C); and its boiling point is 4, 566.2°F (2, 519°C).

Aluminum is a metal in group 13 of the periodic table. Its atoms consist of a single stable isotope,²⁷Al. Known as aluminium in other English-speaking countries, it was named after alum, one of its salts that has been known for thousands of years and was used by the Egyptians, Greeks, and Romans as a mordant —a chemical that helps dyes stick to cloth.

General properties

Aluminum is a light-weight, silvery metal, familiar to every household in the form of pots and pans, beverage cans, and aluminum foil. It is attractive, nontoxic, corrosion-resistant, non-magnetic, and easy to form, cast, or machine into a variety of shapes. It is one of the most useful metals we have; four million tons of it are produced every year in the United States alone—a production rate that among metals is second only to that of iron.

Pure aluminum is relatively soft and not the strongest of metals, but when melted together with other elements such as copper, manganese, silicon, magnesium, and zinc, it forms alloys with a wide range of useful properties. Aluminum alloys are used in airplanes, highway signs, bridges, storage tanks, and buildings. Aluminum is being used more and more in automobiles because it is only one-third as heavy as steel and therefore decreases fuel consumption.

Where aluminum comes from

Aluminum is the third most abundant element in Earth’s crust, after only oxygen and silicon, and it is the most abundant of all metals. It constitutes 8.1% of the crust by weight and 6.3% of all the atoms in the crust. Because it is a very active metal, aluminum is never found in the metallic form, but only in a wide variety of earthy and rocky minerals, including feldspar, mica, granite, and clay. Kaolin is an especially fine, white aluminum-containing clay that is used in making porcelain.

Aluminum oxide, Al₂ O₃, often called alumina, does not melt until over 3, 632°F (2, 000°C), and is used to line furnaces. Other forms of alumina are corundum and emery, which are very hard and are used as abrasives. Among the many other mineral forms in which aluminum is found are several semiprecious gemstones, including garnet (Fe₃ Al₂ Si₃ O₁₂), beryl (Be₃ Al₂ Si₆ O₁₈), and ruby and sapphire, which are Al₂ O₃ containing impurities of chromium and iron, respectively. Artificially made rubies and sapphires are used in lasers.

How aluminum is obtained

As a highly reactive metal, aluminum is very difficult to separate from the other elements with which it is combined in its minerals and compounds. In spite of its great abundance on Earth, the metal itself remained unknown for centuries. In 1825, some impure aluminum metal was finally isolated by H. C. Oersted by treating aluminum chloride, AlCl₃, with potassium amalgam—potassium dissolved in mercury. Then in 1827, H. Wöhler obtained pure aluminum by the reaction of metallic potassium with AlCl₃. He is generally given credit for the discovery of this element.

At this time, it was still very expensive to produce aluminum metal in any quantity, and for a long time it remained a rare and valuable metal. In 1852, aluminum was selling for about $545 a pound. The big breakthrough came in 1886, when Charles M. Hall, a 23-year-old student at Oberlin College in Ohio, and Paul L-T. Héroult, another college student in France, independently invented what is now known as the Hall or Hall-Héroult process. It consists of dissolving alumina in melted cryolite, Na₃ AlF₆, a common aluminum-containing mineral, and then passing an electric current through the hot liquid. Molten aluminum metal collects at the cathode (negative electrode) in a process called electrolysis. Not long after the development of this process, the price of aluminum metal plummeted to around 30 cents a pound.

In the production of aluminum today by the Hall-H´roult process, the aluminum oxide is dissolved in a molten mixture of sodium, calcium, and aluminum fluorides, which melts at a lower temperature than cryolite. The aluminum oxide is in the form of bauxite, a white, brown, or red earthy clay; it was first found near Les Baux, France, in 1821 by P. Berthier, and is now the main source of all aluminum. It is mined in various parts of Africa and in France, Surinam, Jamaica, and the United States—mainly in Alabama, Arkansas, and Georgia. The world’s supply of bauxite appears to be immense enough to last for hundreds of years at the rate it is being mined today.

Uses

In spite of the fact that aluminum is very active chemically, it does not corrode in moist air the way iron does. Instead, it quickly forms a thin, hard coating of aluminum oxide. Unlike iron oxide or rust, which flakes off, the aluminum oxide sticks tightly to the metal and protects it from further oxidation. The oxide coating is so thin that it is transparent, so the aluminum retains its silvery metallic appearance. Sea water, however, will corrode aluminum unless it has been given an unusually thick coating of oxide by the anodizing process.

When aluminum is heated to high temperatures in a vacuum, it evaporates and condenses onto any nearby cool surface such as glass or plastic. When evaporated onto glass, it makes a very good mirror, and aluminum has largely replaced silver for that purpose because it does not tarnish and turn black, as silver does when exposed to impure air. Many food-packaging materials and shiny plastic novelties are made of paper or plastic with an evaporated coating of bright aluminum. The “silver” helium balloons that we see at birthday parties are made of a tough plastic called Mylar, covered with a thin, evaporated coating of aluminum metal.

Aluminum conducts electricity about 60% as well as copper, which is still very good among metals. Because it is also light in weight and highly ductile (can be drawn out into thin wires), it is used instead of copper in almost all of the high-voltage electric transmission lines in the United States.

Aluminum is used to make kitchen pots and pans because of its high heat conductivity. It is handy as an air- and water-tight food wrapping because it is very malleable; it can be pressed between steel rollers to make foil (a thin sheet) less than a thousandth of an inch thick. Claims are occasionally made that aluminum is toxic and that aluminum cookware is therefore dangerous, but no clear evidence for this belief has ever been found. Many widely used antacids in the drug store contain thousands of times more aluminum (in the form of aluminum hydroxide) than a person could ever get from eating food cooked in an aluminum pot. Aluminum is the only light element that has no known physiological function in the human body.

Chemistry and compounds

Aluminum is an unusual metal in that it reacts not only with acids, but with bases as well. Like many active metals, aluminum dissolves in strong acids to evolve hydrogen gas and form salts. In fact, cooking even weakly acidic foods such as tomatoes in an aluminum pot can dissolve enough aluminum to give the dish a “metallic” taste. Aluminum also dissolves in strong bases such as sodium hydroxide, commonly known as lye. Most oven cleaners, which are designed to work on steel and porcelain, contain sodium or potassium hydroxide; the user must take care not to get it on any aluminum parts of the range because it will cause adverse effects. Some commercial drain cleaners contain lye mixed with shavings of aluminum metal; the aluminum dissolves in the sodium hydroxide solution to produce bubbles of hydrogen gas, which add a mechanical clog-breaking action to the grease-dissolving action of the lye.

Hydrated aluminum chloride, AlCl₃·H₂ O, also called aluminum chlorohydrate, is used in antiperspirants because, like alum (potassium aluminum sulfate), it has an astringent effect—a tissue-shrinking effect—that closes up the sweat-gland ducts and stops perspiration.

Over one million tons of aluminum sulfate, Al₂ (SO₄)₃, are produced in the United States each year by dissolving aluminum oxide in sulfuric acid, H₂ SO₄. It is used in water purification because when it reacts with lime (or any base), it forms a sticky precipitate of aluminum hydroxide that sweeps out tiny particles of impurities. Sodium aluminum sulfate, NaAl(SO₄)₂·12H₂ O, a kind of alum, is used in “double-acting” baking powders. It acts as an acid, reacting at oven temperatures with the sodium bicarbonate in the powder to form bubbles of carbon dioxide gas.

See also Metal production; Metallurgy.

Resources

BOOKS

Kirk-Othmer Encyclopedia of Chemical Technology. 5th ed. New York: John Wiley & Sons, 2005.

Lide, David R., ed. CRC Handbook of Chemistry and Physics, 87th Edition. Boca Raton, FL: CRC Press, 2006.

Snyder, C.H. The Extraordinary Chemistry of Ordinary Things, With Late Nite Labs. 4th ed. New York: John Wiley & Sons, 2004.

Robert L. Wolke

Aluminum

The metallic element aluminum is the third most plentiful element in the earth's crust, comprising 8% of the planet's soil and rocks (oxygen and silicon make up 47% and 28%, respectively). In nature, aluminum is found only in chemical compounds with other elements such as sulphur, silicon, and oxygen. Pure, metallic aluminum can be economically produced only from aluminum oxide ore.

Metallic aluminum has many properties that make it useful in a wide range of applications. It is lightweight, strong, nonmagnetic, and nontoxic. It conducts heat and electricity and reflects heat and light. It is strong but easily workable, and it retains its strength under extreme cold without becoming brittle. The surface of aluminum quickly oxidizes to form an invisible barrier to corrosion. Furthermore, aluminum can easily and economically be recycled into new products.

Background

Aluminum compounds have proven useful for thousands of years. Around 5000 b.c., Persian potters made their strongest vessels from clay that contained aluminum oxide. Ancient Egyptians and Babylonians used aluminum compounds in fabric dyes, cosmetics, and medicines. However, it was not until the early nineteenth century that aluminum was identified as an element and isolated as a pure metal. The difficulty of extracting aluminum from its natural compounds kept the metal rare for many years; half a century after its discovery, it was still as rare and valuable as silver.

In 1886, two 22-year-old scientists independently developed a smelting process that made economical mass production of aluminum possible. Known as the Hall-Heroult process after its American and French inventors, the process is still the primary method of aluminum production today. The Bayer process for refining aluminum ore, developed in 1888 by an Austrian chemist, also contributed significantly to the economical mass production of aluminum.

In 1884, 125 lb (60 kg) of aluminum was produced in the United States, and it sold for about the same unit price as silver. In 1995, U.S. plants produced 7.8 billion lb (3.6 million metric tons) of aluminum, and the price of silver was seventy-five times as much as the price of aluminum.

Raw Materials

Aluminum compounds occur in all types of clay, but the ore that is most useful for producing pure aluminum is bauxite. Bauxite consists of 45-60% aluminum oxide, along with various impurities such as sand, iron, and other metals. Although some bauxite deposits are hard rock, most consist of relatively soft dirt that is easily dug from open-pit mines. Australia produces more than one-third of the world's supply of bauxite. It takes about 4 lb (2 kg) of bauxite to produce 1 lb (0.5 kg) of aluminum metal.

Caustic soda (sodium hydroxide) is used to dissolve the aluminum compounds found in the bauxite, separating them from the impurities. Depending on the composition of the bauxite ore, relatively small amounts of other chemicals may be used in the extraction of aluminum. Starch, lime, and sodium sulphide are some examples.

Cryolite, a chemical compound composed of sodium, aluminum, and fluorine, is used as the electrolyte (current-conducting medium) in the smelting operation. Naturally occurring cryolite was once mined in Greenland, but the compound is now produced synthetically for use in the production of aluminum. Aluminum fluoride is added to lower the melting point of the electrolyte solution.

The other major ingredient used in the smelting operation is carbon. Carbon electrodes transmit the electric current through the electrolyte. During the smelting operation, some of the carbon is consumed as it combines with oxygen to form carbon dioxide. In fact, about half a pound (0.2 kg) of carbon is used for every pound (2.2 kg) of aluminum produced. Some of the carbon used in aluminum smelting is a byproduct of oil refining; additional carbon is obtained from coal.

Because aluminum smelting involves passing an electric current through a molten electrolyte, it requires large amounts of electrical energy. On average, production of 2 lb (1 kg) of aluminum requires 15 kilowatt-hours (kWh) of energy. The cost of electricity represents about one-third of the cost of smelting aluminum.

The Manufacturing
Process

Aluminum manufacture is accomplished in two phases: the Bayer process of refining the bauxite ore to obtain aluminum oxide, and the Hall-Heroult process of smelting the aluminum oxide to release pure aluminum.

The Bayer process

• 1 First, the bauxite ore is mechanically crushed. Then, the crushed ore is mixed with caustic soda and processed in a grinding mill to produce a slurry (a watery suspension) containing very fine particles of ore.
• 2 The slurry is pumped into a digester, a tank that functions like a pressure cooker. The slurry is heated to 230-520°F (110-270°C) under a pressure of 50 lb/in² (340 kPa). These conditions are maintained for a time ranging from half an hour to several hours. Additional caustic soda may be added to ensure that all aluminum-containing compounds are dissolved.
• 3 The hot slurry, which is now a sodium aluminate solution, passes through a series of flash tanks that reduce the pressure and recover heat that can be reused in the refining process.
• 4 The slurry is pumped into a settling tank. As the slurry rests in this tank, impurities that will not dissolve in the caustic soda settle to the bottom of the vessel. One manufacturer compares this process to fine sand settling to the bottom of a glass of sugar water; the sugar does not settle out because it is dissolved in the water, just as the aluminum in the settling tank remains dissolved in the caustic soda. The residue (called "red mud") that accumulates in the bottom of the tank consists of fine sand, iron oxide, and oxides of trace elements like titanium.
• 5 After the impurities have settled out, the remaining liquid, which looks somewhat like coffee, is pumped through a series of cloth filters. Any fine particles of impurities that remain in the solution are trapped by the filters. This material is washed to recover alumina and caustic soda that can be reused.
• 6 The filtered liquid is pumped through a series of six-story-tall precipitation tanks. Seed crystals of alumina hydrate (alumina bonded to water molecules) are added through the top of each tank. The seed crystals grow as they settle through the liquid and dissolved alumina attaches to them.
• 7 The crystals precipitate (settle to the bottom of the tank) and are removed. After washing, they are transferred to a kiln for calcining (heating to release the water molecules that are chemically bonded to the alumina molecules). A screw conveyor moves a continuous stream of crystals into a rotating, cylindrical kiln that is tilted to allow gravity to move the material through it. A temperature of 2,000° F (1,100° C) drives off the water molecules, leaving anhydrous (waterless) alumina crystals. After leaving the kiln, the crystals pass through a cooler.

The Hall-Heroult process

Smelting of alumina into metallic aluminum takes place in a steel vat called a reduction pot. The bottom of the pot is lined with carbon, which acts as one electrode (conductor of electric current) of the system. The opposite electrodes consist of a set of carbon rods suspended above the pot; they are lowered into an electrolyte solution and held about 1.5 in (3.8 cm) above the surface of the molten aluminum that accumulates on the floor of the pot. Reduction pots are arranged in rows (potlines) consisting of 50-200 pots that are connected in series to form an electric circuit. Each potline can produce 66,000-110,000 tons (60,000-100,000 metric tons) of aluminum per year. A typical smelting plant consists of two or three potlines.

• 8 Within the reduction pot, alumina crystals are dissolved in molten cryolite at a temperature of 1,760-1,780° F (960-970° C) to form an electrolyte solution that will conduct electricity from the carbon rods to the carbon-lined bed of the pot. A direct current (4-6 volts and 100,000-230,000 amperes) is passed through the solution. The resulting reaction breaks the bonds between the aluminum and oxygen atoms in the alumina molecules. The oxygen that is released is attracted to the carbon rods, where it forms carbon dioxide. The freed aluminum atoms settle to the bottom of the pot as molten metal.

  The smelting process is a continuous one, with more alumina being added to the cryolite solution to replace the decomposed compound. A constant electric current is maintained. Heat generated by the flow of electricity at the bottom electrode keeps the contents of the pot in a liquid state, but a crust tends to form atop the molten electrolyte. Periodically, the crust is broken to allow more alumina to be added for processing. The pure molten aluminum accumulates at the bottom of the pot and is siphoned off. The pots are operated 24 hours a day, seven days a week.

• 9 A crucible is moved down the potline, collecting 9,000 lb (4,000 kg) of molten aluminum, which is 99.8% pure. The metal is transferred to a holding furnace and then cast (poured into molds) as ingots. One common technique is to pour the molten aluminum into a long, horizontal mold. As the metal moves through the mold, the exterior is cooled with water, causing the aluminum to solidify. The solid shaft emerges from the far end of the mold, where it is sawed at appropriate intervals to form ingots of the desired length. Like the smelting process itself, this casting process is also continuous.

Byproducts/Waste

Alumina, the intermediate substance that is produced by the Bayer process and that constitutes the raw material for the Hall-Heroult process, is also a useful final product. It is a white, powdery substance with a consistency that ranges from that of talcum powder to that of granulated sugar. It can be used in a wide range of products such as laundry detergents, toothpaste, and fluorescent light bulbs. It is an important ingredient in ceramic materials; for example, it is used to make false teeth, spark plugs, and clear ceramic windshields for military airplanes. An effective polishing compound, it is used to finish computer hard drives, among other products. Its chemical properties make it effective in many other applications, including catalytic converters and explosives. It is even used in rocket fuel—400,000 lb (180,000 kg) is consumed in every space shuttle launch. Approximately 10% of the alumina produced each year is used for applications other than making aluminum.

The largest waste product generated in bauxite refining is the tailings (ore refuse) called "red mud." A refinery produces about the same amount of red mud as it does alumina (in terms of dry weight). It contains some useful substances, like iron, titanium, soda, and alumina, but no one has been able to develop an economical process for recovering them. Other than a small amount of red mud that is used commercially for coloring masonry, this is truly a waste product. Most refineries simply collect the red mud in an open pond that allows some of its moisture to evaporate; when the mud has dried to a solid enough consistency, which may take several years, it is covered with dirt or mixed with soil.

Several types of waste products are generated by decomposition of carbon electrodes during the smelting operation. Aluminum plants in the United States create significant amounts of greenhouse gases, generating about 5.5 million tons (5 million metric tons) of carbon dioxide and 3,300 tons (3,000 metric tons) of perfluorocarbons (compounds of carbon and fluorine) each year.

Approximately 120,000 tons (110,000 metric tons) of spent potlining (SPL) material is removed from aluminum reduction pots each year. Designated a hazardous material by the Environmental Protection Agency (EPA), SPL has posed a significant disposal problem for the industry. In 1996, the first in a planned series of recycling plants opened; these plants transform SPL into glass frit, an intermediate product from which glass and ceramics can be manufactured. Ultimately, the recycled SPL appears in such products as ceramic tile, glass fibers, and asphalt shingle granules.

The Future

Virtually all of the aluminum producers in the United States are members of the Voluntary Aluminum Industrial Partnership (VAIP), an organization that works closely with the EPA to find solutions to the pollution problems facing the industry. A major focus of research is the effort to develop an inert (chemically inactive) electrode material for aluminum reduction pots. A titanium-diboride-graphite compound shows significant promise. Among the benefits expected to come when this new technology is perfected are elimination of the greenhouse gas emissions and a 25% reduction in energy use during the smelting operation.

Where to Learn More

Books

Altenpohl, Dietrich. Aluminum Viewed from Within: An Introduction into the Metallurgy of Aluminum Fabrication (English translation). Dusseldorf: Aluminium-Verlag, 1982.

Russell, Allen S. "Aluminum." McGraw-Hill Encyclopedia of Science & Technology. New York: McGraw-Hill, 1997.

Periodicals

Thompson, James V. "Alumina: Simple Chemistry—Complex Plants." Engineering & Mining Journal (February 1, 1995): 42 ff.

Other

Alcoa Aluminum. http://www.alcoa.com/ (March 1999).

Reynolds Metals Company. http://www.reynoldswrap.com/gbu/bauxitealumina/ (April 1999).

—LorettaHall

Aluminum

Aluminum is the metallic chemical element of atomic number 13. Its symbol is Al, its atomic weight is 26.98, its specific gravity is 2.70, its melting point is 1,220.5°F (660°C), and its boiling point is 4,566.2°F (2,519°C).

Aluminum is a metal in group 13 of the periodic table . Its atoms consist of a single stable isotope , 27Al. Known as aluminium in other English-speaking countries, it was named after alum, one of its salts that has been known for thousands of years and was used by the Egyptians, Greeks, and Romans as a mordant—a chemical that helps dyes stick to cloth.

General properties

Aluminum is a light-weight, silvery metal, familiar to every household in the form of pots and pans, beverage cans, and aluminum foil. It is attractive, nontoxic, corrosion-resistant, non-magnetic, and easy to form, cast, or machine into a variety of shapes. It is one of the most useful metals we have; four million tons of it are produced every year in the United States alone—a production rate that among metals is second only to that of iron .

Pure aluminum is relatively soft and not the strongest of metals, but when melted together with other elements such as copper , manganese, silicon, magnesium , and zinc, it forms alloys with a wide range of useful properties. Aluminum alloys are used in airplanes, highway signs, bridges , storage tanks, and buildings. Aluminum is being used more and more in automobiles because it is only one-third as heavy as steel and therefore decreases fuel consumption.

Where aluminum comes from

Aluminum is the third most abundant element in the earth's crust, after only oxygen and silicon, and it is the most abundant of all metals. It constitutes 8.1% of the crust by weight and 6.3% of all the atoms in the crust. Because it is a very active metal, aluminum is never found in the metallic form, but only in a wide variety of earthy and rocky minerals , including feldspar, mica, granite, and clay. Kaolin is an especially fine, white aluminum-containing clay that is used in making porcelain.

Aluminum oxide, Al2O3, often called alumina, does not melt until over 3,632°F (2,000°C), and is used to line furnaces. Other forms of alumina are corundum and emery, which are very hard and are used as abrasives . Among the many other mineral forms that aluminum is found in are several semiprecious gemstones, including garnet (Fe3Al2Si3O12), beryl (Be3Al2Si6O18), and ruby and sapphire, which are Al2O3 containing impurities of chromium and iron, respectively. Artificially made rubies and sapphires are used in lasers.

How aluminum is obtained

As a highly reactive metal, aluminum is very difficult to separate from the other elements that are combined with it in its minerals and compounds. In spite of its great abundance on Earth , the metal itself remained unknown for centuries. In 1825, some impure aluminum metal was finally isolated by H. C. Oersted by treating aluminum chloride, AlCl3, with potassium amalgam—potassium dissolved in mercury. Then in 1827, H. Wöhler obtained pure aluminum by the reaction of metallic potassium with AlCl3. He is generally given credit for the discovery of this element.

But it was still very expensive to produce aluminum metal in any quantity, and for a long time it remained a rare and valuable metal. In 1852, aluminum was selling for about $545 a pound. The big breakthrough came in 1886, when Charles M. Hall, a 23-year-old student at Oberlin College in Ohio, and Paul L-T. Héroult, another college student in France, independently invented what is now known as the Hall or Hall-Héroult process. It consists of dissolving alumina in melted cryolite, Na3AlF6, a common aluminum-containing mineral, and then passing an electric current through the hot liquid. Molten aluminum metal collects at the cathode (negative electrode) in a process called electrolysis . Not long after the development of this process, the price of aluminum metal plummeted to around 30 cents a pound.

In the production of aluminum today by the Hall-Héroult process, the aluminum oxide is dissolved in a molten mixture of sodium , calcium , and aluminum fluorides, which melts at a lower temperature than cryolite. The aluminum oxide is in the form of bauxite, a white, brown, or red earthy clay; it was first found near Les Baux, France, in 1821 by P. Berthier, and is now the main source of all aluminum. It is mined in various parts of Africa and in France, Surinam, Jamaica, and the United States—mainly in Alabama, Arkansas, and Georgia. The world's supply of bauxite appears to be immense enough to last for hundreds of years at the rate it is being mined today.

Uses

In spite of the fact that aluminum is very active chemically, it does not corrode in moist air the way iron does. Instead, it quickly forms a thin, hard coating of aluminum oxide. Unlike iron oxide or rust, which flakes off, the aluminum oxide sticks tightly to the metal and protects it from further oxidation. The oxide coating is so thin that it is transparent, so the aluminum retains its silvery metallic appearance. Sea water , however, will corrode aluminum unless it has been given an unusually thick coating of oxide by the anodizing process.

When aluminum is heated to high temperatures in a vacuum , it evaporates and condenses onto any nearby cool surface such as glass or plastic. When evaporated onto glass, it makes a very good mirror, and aluminum has largely replaced silver for that purpose because it does not tarnish and turn black, as silver does when exposed to impure air. Many food-packaging materials and shiny plastic novelties are made of paper or plastic with an evaporated coating of bright aluminum. The "silver" helium balloons that we see at birthday parties are made of a tough plastic called Mylar, covered with a thin, evaporated coating of aluminum metal.

Aluminum conducts electricity about 60% as well as copper, which is still very good among metals. Because it is also light in weight and highly ductile (can be drawn out into thin wires), it is used instead of copper in almost all of the high-voltage electric transmission lines in the United States.

Aluminum is used to make kitchen pots and pans because of its high heat conductivity. It is handy as an air- and water-tight food wrapping because it is very malleable; it can be pressed between steel rollers to make foil (a thin sheet) less than a thousandth of an inch thick. Claims are occasionally made that aluminum is toxic and that aluminum cookware is therefore dangerous, but no clear evidence for this belief has ever been found. Many widely used antacids in the drug store contain thousands of times more aluminum (in the form of aluminum hydroxide ) than a person could ever get from eating food cooked in an aluminum pot. Aluminum is the only light element that has no known physiological function in the human body.

Chemistry and compounds

Aluminum is an unusual metal in that it reacts not only with acids, but with bases as well. Like many active metals, aluminum dissolves in strong acids to evolve hydrogen gas and form salts. In fact, cooking even weakly acidic foods such as tomatoes in an aluminum pot can dissolve enough aluminum to give the dish a "metallic" taste . But aluminum also dissolves in strong bases such as sodium hydroxide , commonly known as lye. Most oven cleaners, which are designed to work on steel and porcelain, contain sodium or potassium hydroxide; the user must take care not to get it on any aluminum parts of the range because it will cause adverse effects. Some commercial drain cleaners contain lye mixed with shavings of aluminum metal; the aluminum dissolves in the sodium hydroxide solution to produce bubbles of hydrogen gas, which add a mechanical clog-breaking action to the grease-dissolving action of the lye.

Hydrated aluminum chloride, AlCl3•H2O, also called aluminum chlorohydrate, is used in antiperspirants because, like alum (potassium aluminum sulfate ), it has an astringent effect—a tissue-shrinking effect—that closes up the sweat-gland ducts and stops perspiration.

Over one million tons of aluminum sulfate, Al2(SO4)3, are produced in the United States each year by dissolving aluminum oxide in sulfuric acid , H2SO4. It is used in water purification because when it reacts with lime (or any base), it forms a sticky precipitate of aluminum hydroxide that sweeps out tiny particles of impurities. Sodium aluminum sulfate, NaAl(SO4)2•12H2O, a kind of alum, is used in "double-acting" baking powders. It acts as an acid, reacting at oven temperatures with the sodium bicarbonate in the powder to form bubbles of carbon dioxide gas.

See also Metal production; Metallurgy.

Resources

books

"Aluminum." Kirk-Othmer Encyclopedia of Chemical Technology. 4th ed. Suppl. New York: John Wiley & Sons, 1998.

Braungart, Michael and William McDonough. Cradle to Cradle: Remaking the Way We Make Things. North Point Press, 2002.

Lide, D.R., ed. CRC Handbook of Chemistry and Physics Boca Raton: CRC Press, 2001.

Snyder, C.H. The Extraordinary Chemistry of Ordinary Things. 4th ed. New York: John Wiley and Sons, 2002.

Robert L. Wolke

Aluminum

Aluminum is the third most abundant element in the earth's crust , ranking only behind oxygen and silicon . It makes up about 9% of the earth's crust, making it the most abundant of all metals . The chemical symbol for aluminum, Al, is taken from the first two letters of the element's name.

Aluminum has an atomic number of 13 and an atomic mass of 26.98. Aluminum is a silver-like metal with a slightly bluish tint. It has a melting point of 1,220°F (660°C), a boiling point of about 4,440°F (2,450°C), and a density of 2.708 grams per cubic centimeter. Aluminum is both ductile and malleable.

Aluminum is a very good conductor of electricity , surpassed only by silver and copper in this regard. However, aluminum is much less expensive than either silver and copper. For that reason, engineers are currently trying to discover new ways in which aluminum can be used to replace silver and copper in electrical wires and equipment.

Aluminum occurs in nature as a compound, never as a pure metal. The primary commercial source for aluminum is the mineral bauxite, a complex compound consisting of aluminum, oxygen, and other elements. Bauxite is found in many parts of the world, including Australia , Brazil, Guinea, Jamaica, Russia, and the United States. In the United States, aluminum is produced in Montana, Oregon, Washington, Kentucky, North Carolina, South Carolina, and Tennessee.

Aluminum is extracted from bauxite in a two-step process. In the first step, aluminum oxide is separated from bauxite. Aluminum metal is produced from aluminum oxide.

At one time, The extraction of pure aluminum metal from aluminum oxide was very difficult. The initial process requires that aluminum oxide first be melted, then electrolyzed. This is difficult and expensive because aluminum oxide melts at only very high temperatures. An inexpensive method for carrying out this operation was discovered in 1886 by Charles Martin Hall, at the time, a student at Oberlin College in Ohio. Hall found that aluminum oxide melts at a much lower temperature if it is first mixed with a mineral known as cryolite. Passing electric current through a molten mixture of aluminum oxide and cryolite, produces aluminum metal.

At the time of Hall's discovery, aluminum was a very expensive metal. It sold for about $10 per pound—so rare and was displayed at the 1855 Paris Exposition next the French crown jewels. As a result of Hall's research, the price of aluminum dropped to less than $.40 per pound).

Aluminum was named for one of its most important compounds, alum, a compound of potassium, aluminum, sulfur, and oxygen. The chemical name for alum is potassium aluminum sulfate, KAl(SO₄)₂.

Alum has been widely used by humans for thousands of years. It was mined in ancient Greece and then sold to the Turks who used it to make a beautiful red dye known as Turkey red. Alum has also been long used as a mordant in dyeing. In addition, alum was used as an astringent to treat injuries.

Eventually, chemists began to realize that alum might contain a new element. The first person to actually produce aluminum from a mineral was the Danish chemist and physicist Hans Christian Oersted (1777-1851). Oersted was not very successful, however, in producing a very pure form of aluminum.

The first pure sample of aluminum metal was not made until 1827 when the German chemit Friedrich Wöhler heated a combination of aluminum chloride and potassium metal. Being more active, the potassium replaces the aluminum, leaving a combination of potassium chloride and aluminum metal.

Aluminum readily reacts with oxygen to form aluminum oxide: 4Al + 3O₂ → 2Al₂O₃. Aluminum oxide forms a thin, whitish coating on the aluminum metal that prevents the metal from reacting further with oxygen (i.e., corrosion ).

The largest single use of aluminum alloys is in the transportation industry. Car and truck manufacturers use aluminum alloys because they are strong, but lightweight. Another important use of aluminum alloys is in the packaging industry. Aluminum foil, drink cans, paint tubes, and containers for home products are all made of aluminum alloys. Other uses of aluminum alloys include window and door frames, screens, roofing, siding, electrical wires and appliances, automobile engines, heating and cooling systems, kitchen utensils, garden furniture, and heavy machinery.

Aluminum is also made into a large variety of compounds with many industrial and practical uses. Aluminum ammonium sulfate, Al(NH₄)(SO₄)₂, is used as a mordant, in water purification and sewage treatment systems, in paper production and the tanning of leather, and as a food additive. Aluminum borate is used in the production of glass and ceramics.

One of the most widely used compounds is aluminum chloride (AlCl₃), employed in the manufacture of paints, antiperspirants, and synthetic rubber. It is also important in the process of converting crude petroleum into useful products, such as gasoline, diesel and heating oil, and kerosene.

See also Chemical elements; Minerals

ALUMINUM

ALUMINUM, the most useful of the nonferrous metals, was first isolated in metallic form in 1825 by Hans Christian Oersted in Denmark. The metal remained a laboratory curiosity until 1854, when Henri Sainte-Claire Deville discovered a process using metallic sodium as a reductant that led to the first commercial production of aluminum. The price of the metal fell from $545 per pound in 1852 to $8 in 1885, and uses for the lightweight metal began to increase greatly. Emperor Napoleon III of France, for example, considered outfitting his army with lightweight aluminum armor and equipment, but the price of the metal remained too high for widespread use.

In 1886, an American, Charles Martin Hall, and a Frenchman, Paul Héroult, independently discovered that aluminum could be produced by electrolyzing a solution of aluminum oxide in molten cryolite (sodium aluminum fluoride). The electrolytic process won immediate acceptance by the commercial industry and in 2002 remained the sole commercial method used for making aluminum.

Hall's invention led to the formation of the Pittsburgh Reduction Company in 1888. This company, now known as Alcoa (for Aluminum Company of America), initially produced fifty pounds of aluminum per day, becoming by the turn of the twentieth century the world's largest producer of aluminum, a position it still enjoys in 2002. A more diverse aluminum industry developed in Europe. Within ten years, firms operated in Switzerland, Germany, Austria, France, and Scotland—all having obtained rights to Héroult's patents to make the metal. By 1900 total world production was about 7,500 short tons; American production was 2,500 tons.

The advent of the airplane in World War I greatly increased demand for the lightweight metal. In 1918 the primary capacity in the United States had grown to 62,500 short tons; world production amounted to 143,900 tons. Steady growth of the aluminum industry continued, and in 1939 the United States produced 160,000 tons of the 774,000 tons produced worldwide. The airplane became a key factor in waging World War II, and aluminum production throughout the world tripled; in the United States it grew sixfold. Another major period of growth in the industry took place during the Korean War, when the United States produced almost half of the world total of 3,069,000 tons. In 1972 total world production of aluminum came to some 12 million tons, but the American share, produced by twelve companies, had dropped to 34 percent, or 4,122,000 tons. By 2000, the aluminum industry in the United States operated more than three hundred plants in thirty-five states, employed more than 145,000 people, and produced an average of 11.5 million tons of aluminum annually.

Aluminum is the most abundant metallic element in the earth's crust. It is made from the mineral bauxite (hydrated aluminum oxide), which is found in plentiful supply throughout the tropical areas of the world. Five countries, Jamaica, Surinam, Guyana, Guinea, and Australia, mined about 61 percent of the world's supplies in 1972, with the remainder coming from twenty-two other countries. At the end of the twentieth century, the U.S. aluminum industry relied to a roughly equivalent degree on production from domestic ore materials (34.3 percent of production in 2000), imported ingots and mill products (33.5 percent), and recycled scrap materials (32.2 percent).

The great growth in the use of aluminum metal indicates its versatility. It has a unique combination of useful properties: lightness, good thermal and electrical conductivity, high reflectivity, malleability, resistance to corrosion, and excellent tensile strength in alloyed form. It is extensively employed in building and construction, where each new house uses almost four hundred pounds of the metal for such items as windows, doors, and siding. Another major market is transportation: the average automobile uses almost eighty pounds of aluminum, and truck and railroad car bodies use aluminum extensively because each pound of weight saved permits an extra pound of revenue-producing payload. The aerospace industries are also large consumers of aluminum. There are many electrical applications because it is one-third as heavy and roughly two-thirds as conductive as copper. Applications for the metal are also growing rapidly for containers and packaging, where it is used in cans, foil, and frozen-food containers. Indeed, the metal's versatility suggests countless possible applications.

BIBLIOGRAPHY

Van Horn, Kent R., ed. Prepared by engineers, scientists, and metallurgists of Aluminum Company of America. Aluminum. Vol. 2, Design and Application. Metals Park, Ohio: American Society for Metals, 1967.

Kenneth B.Higbie/c. w.

See alsoAircraft Industry ; Automobile Industry .