Aluminum family

The aluminum family consists of elements in Group 13 of the periodic table: boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl). The family is usually named after the second element, aluminum, rather than the first, boron, because boron is less typical of the family members than is aluminum. Boron is a metalloid (an element that has some of the properties of metals and some of the properties of nonmetals), while the other four members of the family are all metals.

Aluminum

Aluminum is a lightweight, silvery metal, familiar to every household in the form of pots and pans, beverage cans, and aluminum foil. It is attractive, nontoxic, corrosion-resistant, nonmagnetic, and easy to form, cast, or machine into a variety of shapes. It has a melting point of 660°C (1,220°F) and a boiling point of 2,519°C (4,566°F).

Aluminum is the third most abundant element in Earth's crust after oxygen and silicon, and it is the most abundant of all metals. It constitutes 8.1 percent of the crust by weight and 6.3 percent of all the atoms in the crust. Because it is a very active metal, aluminum is never found in its metallic form. Rather, it occurs 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.

Known as aluminium in other English-speaking countries, the element was named after the mineral alum, one of its salts that has been known for thousands of years. Alum was used by the Egyptians, Greeks, and Romans as a mordant, a chemical that helps dyes stick to cloth.

Properties and uses. Pure aluminum is relatively soft and not the strongest of metals. When melted together with other elements such as copper, manganese, silicon, magnesium, and zinc, however, it forms alloys (a substance composed of two or more metals or of a metal and a nonmetal) with a wide range of useful properties. Aluminum alloys are used in airplanes, highway signs, bridges, storage tanks, and buildings. The world's tallest buildings, the World Trade Center towers in New York, are covered with aluminum. Aluminum is being used more and more in automobiles because it is only one-third as heavy as steel and therefore decreases fuel consumption.

In spite of the fact that aluminum is chemically very active, 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. (During the anodizing process, a piece of aluminum is oxidized in order to create on its surface a coating of aluminum oxide, which is able to take dyes, unlike plain aluminum.)

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. Aluminum has largely replaced silver in the production of mirrors 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-colored helium balloons popular at birthday parties are made of a tough plastic called Mylar™, covered with a thin, evaporated coating of aluminum metal.

Aluminum is one of the best conductors of electricity, with a conductivity about 60 percent that of copper. Because it is also light in weight and highly ductile (able to 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 airtight and watertight food wrapping because it is very malleable; it can be pressed between steel rollers to make foil (a thin sheet) less than one-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 over-the-counter antacids 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.

Production. As a highly reactive metal, aluminum is very difficult to separate from 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 Danish physicist Hans Christian Oersted (1777–1851) by treating aluminum chloride with potassium amalgam (potassium dissolved in mercury). Then, in 1827, German chemist Hans Wöhler (1800–1882) obtained pure aluminum by the reaction of metallic potassium with aluminum chloride. He is generally given credit for the discovery of elemental aluminum.

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. This process consists of dissolving alumina (aluminum oxide) in melted cryolite, a common aluminum-containing mineral, and then passing an electric current through the hot liquid. Molten aluminum metal collects at the cathode (negative electrode). Not long after the development of this process, the price of aluminum metal plummeted to about 30 cents a pound. The process used to extract aluminum from its ores today is essentially the same as that developed by Hall and Héroult 150 years ago.

Boron

Elemental boron occurs in a variety of forms, ranging from clear red crystals to a black or brown powder to a transparent black crystal that is nearly as hard as diamond. The element is never found free in nature but is extracted commercially from minerals such as borax, ulexite, colemanite, and kernite. Boron is a relatively rare element, constituting about 0.001 percent of Earth's crust. It ranks number 38 in abundance, after nitrogen, lithium, and lead, but before bromine, uranium, and tin.

Properties and uses. The physical properties of boron are somewhat difficult to determine since the element occurs in so many different forms. The melting point of its most stable form is given as 2,180°C (3,900°F) (the second highest after carbon); its boiling point is about 3,650°C (6,600°F).

Chemically, boron is a fascinating element. One text on the chemical elements claims that the inorganic chemistry of boron is "more diverse and complex than that of any other element in the periodic table." The element forms five types of compounds: (1) metal borides (a metal plus boron), (2) boron hydrides (boron plus hydrogen), (3) boron trihalides (boron plus a halide; a halide is a simple halogen compound), (4) oxo compounds (boron plus complex oxygen radicals; a radical is a group of atoms that behaves as a unit in chemical reactions but is not stable except as part of the compound), and organoboron compounds (boron combined with an organic, or carbon-containing, component).

Boron itself has relatively few uses aside from its role in nuclear reactors as a neutron absorber and in alloys as a hardening agent. (Nuclear reactors are devices used to control the energy released from nuclear reactions.) It is also used in the manufacture of semiconductors. (Semiconductors are substances that conduct an electric current but do so very

poorly.) Probably its best known compound, borax, is used as a water softening agent, in the production of glasses and ceramics, and as an herbicide. A compound derived from borax—boric acid—is used as an eyewash and in the production of heat-resistant glass.

Two boron compounds of special interest are boron carbide and boron nitride. Both are used as refractories, substances that are highly resistant to heat. The melting point of boron carbide is about 2,350°C (4,230°F) and that of boron nitride, over 3,000°C (5,400°F). When boron nitride powder is compressed at very high pressures, it produces a hard crystalline material that is as hard as natural diamonds.

Gallium, indium, and thallium

For most of its history, gallium was best known for one unusual physical property: it has a melting point of 29.76°C (85.6°F), less than that of the human body. If you were to hold a lump of gallium metal in your hand, therefore, it would melt.

In spite of this fact, gallium and its compounds have traditionally had few uses—until recently. In the 1970s, a compound of gallium called gallium arsenide was found to have semiconductor properties. Gallium arsenide has also been used extensively in light-emitting diodes (LEDs), which are used in the electronic displays of calculators, watches, and CD players.

Neither indium nor thallium has many commercial applications. The former element is used largely in making alloys and in the production of transistors and photo cells. A radioactive isotope of the latter, thallium-201, is used in medical diagnostic studies, especially those involving the function of the circulatory system.

[See also Periodic table; Transistor ]

ALUMINUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Aluminum is found in Row 2, Group 13 of the periodic table. The periodic table is a chart that shows how the chemical elements are related to each other. Elements in the same column usually have similar chemical properties. The first element in this group is boron. However, boron is very different from all other members of the family. Therefore, group 13 is known as the aluminum family.

Aluminum is the third most abundant element in the Earth's crust, falling behind oxygen and silicon. It is the most abundant metal. It is somewhat surprising, then, that aluminum was not discovered until relatively late in human history. Aluminum occurs naturally only in compounds, never as a pure metal. Removing aluminum from its compounds is quite difficult. An inexpensive method for producing pure aluminum was not developed until 1886.

SYMBOL
Al

ATOMIC NUMBER
13

ATOMIC MASS
26.98154

FAMILY
Group 13 (IIIA)
Aluminum

PRONUNCIATION
uh-LOO-min-um

Today, aluminum is the most widely used metal in the world after iron. It is used in the manufacture of automobiles, packaging materials, electrical equipment, machinery, and building construction. Aluminum is also ideal for beer and soft drink cans and foil because it can be melted and reused, or recycled.

Discovery and naming

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

No one is sure when alum was first used by humans. The ancient Greeks and Romans were familiar with the compound alum. It was mined in early Greece where it was sold to the Turks. The Turks used the compound to make a beautiful dye known as Turkey red. Records indicate that the Romans were using alum as early as the first century B.C.

These early people used alum as an astringent and as a mordant. An astringent is a chemical that causes skin to pull together. Sprinkling alum over a cut causes the skin to close over the cut and start its healing. A mordant is used in dyeing cloth. Few natural dyes stick directly to cloth. A mordant bonds to the cloth and the dye bonds to the mordant.

Over time, chemists gradually began to realize that alum might contain a new element. In the mid-1700s, German chemist Andreas Sigismund Marggraf (1709-82) claimed to have found a new "earth" called alumina in alum. But he was unable to remove a pure metal from alum.

The first person to accomplish that task was Danish chemist and physicist Hans Christian Oersted (1777-1851). Oersted heated a combination of alumina and potassium amalgam. An amalgam is an alloy of a metal and mercury. In this reaction, Oersted produced an aluminum amalgam—aluminum metal in combination with mercury. He was unable, however, to separate the aluminum from the mercury.

Today, aluminum is the most widely used metal in the world after iron.

Pure aluminum metal was finally produced in 1827 by German chemist Friedrich Wöhler (1800-82). Wöhler used a method perfected by English chemist Sir Humphry Davy (1778-1829), who succeeded in isolating several elements during his life-time. (See sidebar on Davy in the calcium entry.) Wöhler heated a mixture of aluminum chloride and potassium metal. Being more active, the potassium replaces the aluminum, as shown by the following:

The pure aluminum can then be collected as a gray powder, which must be melted to produce the shiny aluminum that is most familiar to consumers.

After Wöhler's work, it was possible, but very expensive, to produce pure aluminum. It cost so much that there were almost no commercial uses for it.

A number of chemists realized how important it was to find a less expensive way to prepare aluminum. In 1883, Russian chemist V. A. Tyurin found a less expensive way to produce pure aluminum. He passed an electric current through a molten (melted) mixture of cryolite and sodium chloride (ordinary table salt). Cryolite is sodium aluminum fluoride (Na₃AlF₆). Over the next few years, similar methods for isolating aluminum were developed by other chemists in Europe.

The most dramatic breakthrough in aluminum research was made by a college student in the United States. Charles Martin Hall (1863-1914) was a student at Oberlin College in Oberlin, Ohio, when he became interested in the problem of producing aluminum. Using homemade equipment in a woodshed behind his home, he achieved success by passing an electric current through a molten mixture of cryolite and aluminum oxide (Al₂O₃).

Hall's method was far cheaper than any previous method. After his discovery, the price of aluminum fell from about $20/kg ($10/lb) to less than $1/kg (about $.40/lb). Hall's research changed aluminum from a semi-precious metal to one that could be used for many everyday products.

What's in a name?

In North America, aluminum is spelled with one i and is pronounced uh-LOO-min-um. Elsewhere in the world, a second i is added—making it aluminium—and the word is pronounced al-yoo-MIN-ee-um.

Physical properties

Aluminum is a silver-like metal with a slightly bluish tint. It has a melting point of 660°C (1,220°F) and a boiling point of 2,327-2,450°C (4,221-4,442°F). The density is 2.708 grams per cubic centimeter. Aluminum is both ductile and malleable. Ductile means capable of being pulled into thin wires. Malleable means capable of being hammered into thin sheets.

Aluminum is an excellent conductor of electricity. Silver and copper are better conductors than aluminum but are much more expensive. Engineers are looking for ways to use aluminum more often in electrical equipment because of its lower costs.

Chemical properties

Aluminum has one interesting and very useful property. In moist air, it combines slowly with oxygen to form aluminum oxide:

The aluminum oxide forms a very thin, whitish coating on the aluminum metal. The coating prevents the metal from reacting further with oxygen and protects the metal from further corrosion (rusting). It is easy to see the aluminum oxide on aluminum outdoor furniture and unpainted house siding.

Aluminum is a fairly active metal. It reacts with many hot acids. It also reacts with alkalis. An alkali is a chemical with properties opposite those of an acid. Sodium hydroxide (common lye) and limewater are examples of alkalis. It is unusual for an element to react with both acids and alkalis. Such elements are said to be amphoteric.

Aluminum also reacts quickly with hot water. In powdered form, it catches fire quickly when exposed to a flame.

Aluminum: Precious metal?

B efore chemists developed inexpensive ways to produce pure aluminum, it was considered a somewhat precious metal. In fact, in 1855, a bar of pure aluminum metal was displayed at the Paris Exposition. It was placed next to the French crown jewels!

Aluminum is an excellent conductor of electricity.

Occurrence in nature

The abundance of aluminum in the Earth's crust is estimated to be about 8.8 percent. It occurs in many different minerals.

Bauxite, a complicated mixture of compounds consisting of aluminum, oxygen, and other elements, is the primary commercial source for aluminum.

Large reserves of bauxite are found in Australia, Brazil, Guinea, Jamaica, Russia, and the United States. The largest producer of aluminum metal is the United States; states that produce the most aluminum are Montana, Oregon, Washington, Kentucky, North Carolina, South Carolina, and Tennessee.

Isotopes

Only one naturally occurring isotope of aluminum exists, aluminum-27. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Aluminum has six radioactive isotopes. A radioactive isotope gives off either energy or subatomic particles in order to reduce the atomic mass and become stable. When the emission produces a change in the number of protons, the atom is no longer the same element. The particles and energy emitted from the nucleus are called radiation. The process of decaying from one element into another is known as radioactive decay.

No radioactive isotope of aluminum has any commercial use.

Extraction

Aluminum production is a two-step process. First, aluminum oxide is separated from bauxite by the Bayer process. In this process, bauxite is mixed with sodium hydroxide (NaOH), which dissolves the aluminum oxide. The other compounds in bauxite are left behind.

The aluminum oxide is then treated with a process similar to the Hall method. There is not enough natural cryolite to make all the aluminum needed, so synthetic (artificial) cryolite is manufactured for this purpose. The chemical reaction is the same with synthetic cryolite as with natural cryolite. About 21 million metric tons of aluminum were produced in 1996 by this two-stage process.

Uses

Aluminum is used as pure metal, in alloys, and in a variety of compounds. An alloy is made by melting and then mixing two or more metals. The mixture has properties different from those of the individual metals. Aluminum alloys are classified in numbered series according to the other elements they contain.

The 1000 classification is reserved for alloys of nearly pure aluminum metal. They tend to be less strong than other alloys of aluminum, however. These metals are used in the structural parts of buildings, as decorative trim, in chemical equipment, and as heat reflectors.

The 2000 series are alloys of copper and aluminum. They are very strong, are corrosion (rust) resistant, and can be machined, or worked with, very easily. Some applications of 2000 series aluminum alloys are in truck paneling and structural parts of aircraft.

The 3000 series is made up of alloys of aluminum and manganese. These alloys are not as strong as the 2000 series, but they also have good machinability. Alloys in this series are used for cooking utensils, storage tanks, aluminum furniture, highway signs, and roofing.

Alloys in the 4000 series contain silicon. They have low melting points and are used to make solders and to add gray coloring to metal. Solders are low-melting alloys used to join two metals to each other. The 5000, 6000, and 7000 series include alloys consisting of magnesium, both magnesium and silicon, and zinc, respectively. These are used in ship and boat production, parts for cranes and gun mounts, bridges, structural parts in buildings, automobile parts, and aircraft components.

The largest single use of aluminum is in the transportation industry (28 percent). Car and truck manufacturers like aluminum and aluminum alloys because they are very strong, yet lightweight. Companies are using more aluminum products in electric cars. These cars must be lightweight in order to conserve battery power. General Motors, Ford, and Chrysler have all announced advanced new car designs in which aluminum products will be used more extensively. Aluminum producers also plan to make a wider variety of wheels for both cars and trucks.

Twenty-three percent of all aluminum produced finds its way into packaging. Aluminum foil, beer and soft drink cans, paint tubes, and containers for home products such as aerosol sprays are all made from aluminum.

Fourteen percent of all aluminum goes into building and construction. Windows and door frames, screens, roofing, and siding, as well as the construction of mobile homes and structural parts of buildings rely on aluminum.

The remaining 35 percent of aluminum goes into a staggering range of products, including electrical wires and appliances, automobile engines, heating and cooling systems, bridges, vacuum cleaners, kitchen utensils, garden furniture, heavy machinery, and specialized chemical equipment.

Compounds

A relatively small amount of aluminum is used to make a large variety of aluminum compounds. These include:

aluminum ammonium sulfate (Al(NH₄)(SO₄)₂): mordant, water purification and sewage treatment, paper production, food additive, leather tanning

aluminum borate (Al₂O₃B₂O₃): production of glass and ceramics

aluminum borohydride (Al(BH₄)₃: additive in jet fuels

aluminum chloride (AlCl₃): paint manufacture, antiperspirant, petroleum refining, production of synthetic rubber

aluminum fluorosilicate (Al₂(SiF₆)₃): production of synthetic gemstones, glass, and ceramics

aluminum hydroxide (Al(OH)₃): antacid, mordant, water purification, manufacture of glass and ceramics, waterproofing of fabrics

aluminum phosphate (AlPO₄): manufacture of glass, ceramics, pulp and paper products, cosmetics, paints and varnishes, and in making dental cement

aluminum sulfate, or alum (Al₂(SO₄)₃): manufacture of paper, mordant, fire extinguisher system, water purification and sewage treatment, food additive, fireproofing and fire retardant, and leather tanning

Health effects

Aluminum has no known function in the human body. There is some debate, however, as to its possible health effects. In the 1980s, some health scientists became concerned that aluminum might be associated with Alzheimer's disease. This is a condition that most commonly affects older people, leading to forgetfulness and loss of mental skills. It is still not clear whether aluminum plays any part in Alzheimer's disease.

Some authorities believe that breathing aluminum dust may also cause health problems. It may cause a pneumonia-like condition currently called aluminosis. Again, there is not enough evidence to support this view.

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 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

melting point: 660.32°C
boiling point: 2,519°C
density: 2.70 g/cm³
most common ions: Al³⁺

Aluminum is a silvery-white metallic element discovered in 1825 by Danish chemist Hans Christian Ørsted. It is the most abundant metal found in Earth's crust, comprising 8.3 percent of the crust's total weight. Its content in seawater, however, is as low as 0.01 gram per metric ton (0.01 part per million). The key isotope of aluminum is ²⁷Al with a natural abundance of 100 percent, but seven other isotopes are known, one of which is used as a radioactive tracer (²⁶Al).

Aluminum is not found in its metallic state in nature; it is usually found as silicate, oxide, or hydrated oxide (bauxite). Its extraction from ore is difficult and expensive; aluminum is therefore commonly recycled, the energy of recycling being a mere 5 percent of the energy needed to extract the metal.

Aluminum is lightweight, ductile , and easily machined. It is protected by an oxide film from reacting with air and water, and is therefore rust-resistant. It is one of the lightest metals but is quite tough and most helpful in metallurgy , transportation (e.g., aircraft, automobiles, railroad cars, and boats), and architecture (e.g., window frames and decorative ornaments). It is also used in the manufacture of cooking gear because it is a good conductor of heat. Aluminum foils as thin as 0.18 millimeter (0.007 inch) are a household convenience, protecting food from spoiling and providing insulation. Aluminum-made beverage cans are widely manufactured; more than 100 billion are produced each year. The average human body contains about 35 milligrams (0.0012 ounce) of aluminum, but no known biological role has been established for it; it is, however, suspected to be a factor in the development of Alzheimer's disease.

see also Electrochemistry.

Jean-Claude Bünzli

Bibliography

Altenpohl, Dietrich G., and Kaufman, J. G. (1998). ALUMINUM: Technology, Applications, and Environment (A Profile of a Modern Metal, Sixth edition). Washington, DC: Minerals, Metals and Materials Society.

Farndon, John (2001). Aluminum. Tarrytown, NY: Benchmark Books.