Where Found (Encyclopedia of Global Resources)
Aluminum is the most abundant metallic element in the Earth’s crust, comprising 8.3 percent of the mass in the crust by weight. As an element it is exceeded in crustal abundance only by oxygen and silicon. Commercially, the most important aluminum ore is bauxite, a mixed aluminum oxide hydroxide with a composition that varies with climate. Large reserves of this mineral exist, typically found in thick layers with little topsoil or overburden so that it can be easily mined. Worldwide reserves are tremendously large, notably in Australia, Africa, Brazil, and countries in Central America.
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Primary Uses (Encyclopedia of Global Resources)
By far the main use of aluminum in its metallic form is as a structural material in the construction and transportation (particularly aircraft) industries. Another major use is as a container material, of which the soft drink can is the most widely recognizable example. Aluminum is a more effective conductor per unit of mass than copper, so it is a more versatile material for power lines. Electrical transmission lines thus account for a sizable fraction of total world production as well.
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Technical Definition (Encyclopedia of Global Resources)
Aluminum (atomic number 13) is a member of the boron group (Group III) of the periodic table of the elements. In terms of chemical and physical properties, the metallic element aluminum is more like boron than the other elements in the group. There is only one stable, naturally occurring isotope of aluminum, with an atomic weight of 26.98154. The pure solid exists in a single crystalline form in which every aluminum atom in the solid-state lattice is surrounded by twelve others at equal distances. Aluminum has a density of 2.699 grams per cubic centimeter. It has a melting point of 660.37° Celsius and a boiling point of 2,467° Celsius.
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Description, Distribution, and Forms (Encyclopedia of Global Resources)
Aluminum is the most abundant metallic element accessible in the Earth’s crust. Of the other metallic elements, only iron and copper display abundances approaching that of aluminum. Aluminum is a constituent of igneous minerals such as feldspar and mica. When these ores weather, they tend to generate clays such as kaolinite and vermiculite. These materials are widespread in the Earth’s crust. In addition, aluminum may be found in rarer minerals such as cryolite, spinel, beryl, turquoise, and corundum. Aluminum compounds are important as precious minerals as well. The presence of a slight trace of a transition metal impurity in the aluminum oxide crystalline lattice typically imparts color to the solid. The ore of central commercial importance in the primary extraction of aluminum is bauxite, a mixed aluminum oxide and hydroxide first discovered in 1821. It is generated when silica and other materials are leached by weathering from silicates of aluminum.
An aluminum compound of some importance is lithium aluminum hydride. This compound is an effective reducing agent and functions as a hydrogenating agent. It was a mainstay in organic synthesis until supplanted by organometallic hydrides that are less expensive to produce and easier to manipulate. Commercial production of lithium aluminum hydride dates from around 1950. Within twenty years the compound had displayed reactivity with more than sixty types...
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History (Encyclopedia of Global Resources)
Although aluminum is the most abundant metallic element accessible on Earth, it was not isolated in its elemental form until the early 1800’s. Even then, for most of the nineteenth century, its rarity made it a precious metal with an expensive price tag and uses centering on decorative rather than practical applications. Chemical thermodynamics, the study of energy relationships in chemical reactions, provides a basis for understanding why isolating aluminum was so difficult and why a pyrometallurgical technique could not be used.
Most metallic elements, with the exception of gold, silver, and a few others, tend to react with oxygen in the atmosphere or with other elements and form compounds rather than remain in their elemental, metallic state. To isolate the metal from the compound in its elemental form, the metal must receive electrons in a process called reduction. Typically this process requires heat energy, so the reduction (or smelting) must be done at elevated temperatures. Some elements, such as copper and lead, can be reduced at a relatively low, easily attainable temperature. Because of their ease of extraction, these elements have been known for a long time. Other elements, such as iron, require a much higher temperature for the reduction reaction to proceed at an appreciable rate. These higher temperatures are much more difficult to obtain, requiring a higher level of technology, especially in terms of furnace...
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Obtaining Aluminum (Encyclopedia of Global Resources)
The difficulty in extracting aluminum from its ores, coupled with the interesting properties of the metal, made the quest for an economical means of production a target of many prospective inventors. The key discoveries needed to realize large-scale production were made almost simultaneously in 1886 by two young men, Charles Martin Hall in the United States and Paul Héroult in France. Both men were in their early twenties. The key to the success of these individuals lay in the extractive technique they utilized (an electrochemical method) and critical experimental modifications that they made. Hall correctly deduced that, at the high operating temperatures of his apparatus, impurities from the clay container he was using as a reaction vessel may have been causing undesired side reactions to take place, thus preventing aluminum formation. He eliminated the clay vessel and instead used one lined with graphite, a form of elemental carbon. The graphite also served as one of the electrodes in the electrochemical cell. With these modifications, Hall was able to produce large pieces of aluminum. These pieces were kept by the company Hall founded and dubbed the “crown jewels.” Commercial production escalated rapidly, resulting in another tremendous downward shift in price, from around twelve dollars per kilogram to about seventy-five cents per kilogram only fifteen years later.
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Uses of Aluminum (Encyclopedia of Global Resources)
Aluminum is of great importance in modern society. Many of the engineering accomplishments of the twentieth century would have been impossible without the availability of aluminum. Modern airliners would be impossible to construct without it. The importance of aluminum is based on its great strength, light weight, and resistance to corrosion. This resistance occurs because typically a thin film of chemically inert aluminum oxide forms on the surface of the metal, shielding it from further corrosion. This protective layer can be artificially produced through an anodization process. Production of differing oxide layer thicknesses provides a variety of appearances and properties in the anodized materials. An oxide layer on the order of 15 micrometers in thickness gives sufficient corrosion protection for exterior structural use. Aluminum’s appearance makes it a desirable external feature. The first skyscraper to be clad with aluminum was built in the 1950’s. The limited tensile strength of aluminum means that alloys typically offer better service as structural components. There is no doubt that, after the construction industry, the aviation industry is the greatest beneficiary of aluminum’s benefits. Aluminum and aluminum alloys make possible strong, lightweight airframes.
In addition to their major applications when in the metallic state, aluminum compounds are utilized in industrial processes and to make materials...
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Further Reading (Encyclopedia of Global Resources)
Altenpohl, Dietrich. Aluminum—Technology, Applications, and Environment, a Profile of a Modern Metal: Aluminum from Within. 6th ed. Washington, D.C.: Aluminum Association, 1998.
Büchel, Karl Heinz, Hans-Heinrich Moretto, and Peter Woditsch. Industrial Inorganic Chemistry. 2d rev. ed. Translated by David R. Terrell. New York: Wiley-VCH, 2000.
Geller, Tom. “Aluminum.” Chemical Heritage 25, no. 4 (Winter, 2007/2008): 32-36.
Greenwood, N. N., and A. Earnshaw. “Aluminium, Gallium, Indium, and Thallium.” In Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Krebs, Robert E. The History and Use of Our Earth’s Chemical Elements: A Reference Guide. Illustrations by Rae Déjur. 2d ed. Westport, Conn.: Greenwood Press, 2006.
Massey, A. G. “Group 13: Boron, Aluminum, Gallium, Indium, and Thallium.” In Main Group Chemistry. 2d ed. New York: Wiley, 2000.
Totten, George E., and D. Scott MacKenzie, eds. Handbook of Aluminum. New York: M. Dekker, 2003.
Walker, Jearl. “Retracing the Steps by Which Aluminum Metal Was Initially Purified Back in 1886.” Scientific American 255, no. 2 (August, 1986): 116.
Natural Resources Canada. Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews. www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/com-eng
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Aluminum (Chemical Elements)
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.
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Aluminum (World of Earth Science)
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 poundo 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 (How Products are Made)
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.
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.
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
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.
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 fuel00,000 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.
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
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.
Thompson, James V. "Alumina: Simple Chemistryomplex Plants." Engineering & Mining Journal (February 1, 1995): 42 ff.
Alcoa Aluminum. http://www.alcoa.com/ (March 1999).
Reynolds Metals Company. (April 1999).