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CHEMISTRY : Metal Cation Identification


Information on ALUMINIUM




  1. General Information

  2. Occurence Uses and Properties

  3. The Metal and its Alloys

  4. History of the Metal

  5. 5) Compounds
  6. Back to Main Metal List




General Information



Aluminium (Al), is a silvery white metal of main Group IIIa (boron group) of the periodic table. It has a melting point of 660 C (1,220 F) and a density of 2.7 grams per cubic centimetre. The most abundant metallic element, it constitutes 8.1 percent of the Earth's crust. In nature it occurs chemically combined with oxygen and other elements. In the pure state it is soft and ductile, but it can be alloyed with many other elements to increase strength and provide a number of useful properties. Alloys of aluminum are light, strong, and formable by almost all known metalworking processes. They can be cast, joined by many techniques, and machined easily, and they accept a wide variety of finishes.

In addition to its low density, many of the appliCATions of aluminum and its alloys are based on its high electrical and thermal conductivity, high reflectivity, and resistance to corrosion. It owes its corrosion resistance to a continuous film of aluminum oxide that grows rapidly on a nascent aluminum surface exposed to air.

Pure aluminum (99.996 percent) is quite soft and weak; commercial aluminum (99.0 to 99.6 percent pure) with small amounts of silicon and iron is hard and strong. Ductile and highly malleable, aluminum can be drawn into wire or rolled into thin foil. The metal is only about one-third as dense as iron or copper. Though chemically active, aluminum is nevertheless highly corrosion-resistant because in air a hard, tough oxide film forms on its surface.

Aluminum is an excellent conductor of heat and electricity. Its thermal conductivity is about one-half that of copper; its electrical conductivity, about two-thirds. It crystallizes in the face-centred cubic structure. All natural aluminum is the stable isotope aluminum-27. Metallic aluminum and its oxide and hydroxide are nontoxic.

Aluminum is slowly attacked by most dilute acids and rapidly dissolves in concentrated hydrochloric acid. Concentrated nitric acid, however, can be shipped in aluminum tank cars because it renders the metal passive. Even very pure aluminum is vigorously attacked by alkalies such as sodium and potassium hydroxide to yield hydrogen and the aluminate ion. Because of its great affinity for oxygen, finely divided aluminum, if ignited, will burn in carbon monoxide or carbon dioxide with the formation of aluminum oxide and carbide; but, at temperatures up to red heat, aluminum is inert to sulfur.



Occurrence, uses, and properties.



Aluminum occurs in igneous rocks chiefly as aluminosiliCATes in feldspars, feldspathoids, and micas; in the soil derived from them as clay; and upon further weathering as bauxite and iron-rich laterite. Bauxite, a mixture of hydrated aluminum oxides, is the principal aluminum ore. Crystalline aluminum oxide (emery, corundum), which occurs in a few igneous rocks, is mined as a natural abrasive or in its finer varieties as rubies and sapphires. Aluminum is present in other gemstones, such as topaz, garnet, and chrysoberyl. Of the many other aluminum minerals, alunite and cryolite have some commercial importance.

Crude aluminum was isolated (1825) by Hans Christian Ørsted by reducing aluminum chloride with potassium amalgam. Sir Humphry Davy had prepared (1809) an iron-aluminum alloy by electrolyzing fused alumina (aluminum oxide) and had already named the element aluminum; the word later was modified to aluminium in England and some other European countries. A German chemist, Friedrich Wöhler, using potassium metal as the reducing agent, produced aluminum powder (1827) and small globules of the metal (1845) from which he was able to determine some of its properties.

The new metal was introduced to the public (1855) at the Paris Exposition at about the time that it became available (in small amounts at great expense) by the sodium reduction of molten aluminum chloride. When electric power became relatively plentiful and cheap, almost simultaneously Charles Martin Hall in the United States and Paul-Louis-Toussaint Héroult in France discovered (1886) the modern method of commercially producing aluminum: electrolysis of purified alumina (Al2O3) dissolved in molten cryolite (Na3AlF6). During the 1960s aluminum moved into first place, ahead of copper, in world production of nonferrous metals. For more specific information about the mining, refining, and production of aluminum, see Industries, Extraction and Processing: Aluminum.

Aluminum is added in small amounts to certain metals to improve their properties for specific uses, as in aluminum bronzes and most magnesium-base alloys; or, for aluminum-base alloys, moderate amounts of other metals and silicon are added to aluminum. The metal and its alloys are used extensively for aircraft construction, building materials, consumer durables (refrigerators, air conditioners, cooking utensils), electrical conductors, and chemical and food-processing equipment.




History


Before 5000 BC people in Mesopotamia were making fine pottery from a clay that consisted largely of an aluminum compound, and almost 4,000 years ago Egyptians and Babylonians used aluminum compounds in various chemicals and medicines. Pliny refers to alumen, known now as alum, a compound of aluminum widely employed in the ancient and medieval world to fix dyes in textiles. By the 18th century, the earthy base alumina was recognized as the potential source of a metal.

The English chemist Humphry Davy in 1807 attempted to extract the metal. Though unsuccessful, he satisfied himself that alumina had a metallic base, which he named "alumium" and later changed to "aluminum." The name has been retained in the United States but modified to "aluminium" in many other countries.

A Danish physicist and chemist, Hans Christian Ørsted, in 1825 finally produced aluminum. "It forms," Ørsted reported, "a lump of metal which in color and luster somewhat resembles tin."

A few years later Friedrich Wöhler, a German chemist at the University of Göttingen, made metallic aluminum in particles as large as pinheads and first determined the following properties of aluminum: specific gravity, ductility, colour, and stability in air.



Chemical compounds



Aluminum oxide
Aluminum oxide exists in several different crystallographic forms, of which corundum is most common. Corundum is characterized by a high specific gravity (4.0), a high melting point (about 2,050 C, or 3,700 F), great insolubility, and hardness.

Aluminum oxide is the major ingredient in the commercial chemicals known as aluminas. Of the pure, inorganic chemicals, aluminas are among the largest volume produced in the world today. Rubies and sapphires are crystalline, nearly pure varieties of alumina, coloured by small amounts of impurities. Synthetic rubies and sapphires are made commercially by fusing a mixture of high-purity aluminum oxide with colouring agents in an oxyhydrogen blowpipe flame. Most are cut and drilled to form tiny "jewel" bearings in watches and various precision measuring instruments.

Activated alumina is a porous form of aluminum oxide from which much of the chemically combined water has been driven off at temperatures low enough to avoid sintering. It is chemically inert to most gases, nontoxic, and will not soften, swell, or disintegrate in water. It has the ability to adsorb and hold moisture without change in form or properties, and it has high resistance to shock and abrasion. Activated alumina is used in oil, chemical, and petrochemical industries as a dehydration agent and purifier in the manufacture of gasoline, petrochemicals, natural gas, and hydrogen peroxide.

Calcined alumina is aluminum oxide that has been heated at temperatures in excess of 1,050 C (1,900 F) to drive off nearly all chemically combined water. In this form, alumina has great chemical purity, extreme hardness (9 on the Mohs hardness scale, on which diamond is 10), high density, and a high melting point (slightly above 2,050 C [3,700 F]). It possesses good thermal conductivity, heat and shock resistance, and high electrical resistivity at elevated temperatures. This combination of properties makes calcined alumina useful in abrasives, glass, porcelains, spark plugs, and electrical insulators, but the greatest quantity of calcined alumina is used to obtain aluminum.

Tabular alumina is aluminum oxide that has been heated to temperatures above 1,650 C (3,000 F). Composed of tabletlike crystals, it has high heat capacity and thermal conductivity as well as exceptional strength and volume stability at high temperatures. For these reasons, a major use of tabular alumina is in the production of high-quality refractories, the materials used for lining industrial furnaces. High-alumina refractories are used in the metal and glass industries in boiler installations, in large furnaces and kilns for smelting metals and firing glass, pottery and porcelain, and in the manufacture of building bricks.

Most refractories are produced in the form of brick, bonded and fired in furnaces. Some castable refractories are made in the form of mortars, usually tabular alumina with calcium aluminate cement as a binder. These mortars, called grog, are sprayed under pressure to form the linings of the steel industry's electric and basic oxygen furnaces, ladles, and coke ovens and for steam boilers, rotary kilns, and many other high-temperature appliCATions.

Fused aluminas are used in special refractories for the glass industry. Fused alumina is calcined alumina that is melted in electric-arc furnaces, cooled, crushed, and recast into desired shapes. In another appliCATion, industrial processes requiring hot gases use a unique heat-transfer device called a pebble heater. Gases to be heated are passed through a bed of tabular alumina balls that have been heated to extreme temperatures. In still another appliCATion, an aluminous insulating material is formed by melting alumina and silica in an electric furnace and subjecting the molten mixture to high-velocity gases to produce fine white fibres.




The metal and its alloys

A ductile, silvery white metal usually with dull lustre owing to a surface film of aluminum oxide, aluminum is light, weighing approximately one-third as much as an equal volume of copper or steel. It is corrosion-resistant, is an excellent conductor of heat and electricity, reflects both light and radiant heat, is nonmagnetic, does not readily absorb neutrons, can be safely used with foods and medicines, and can be formed by all known metalworking processes.

Aluminum can be joined by welding, brazing, soldering, adhesive bonding, riveting, stitching, or stapling and by means of a number of mechanical assemblies such as nuts and bolts, screws, and nails. It can be given a wide variety of mechanical finishes by grinding, polishing, buffing, abrasive blasting, and burnishing. A variety of chemical finishes can be used, such as alkaline or acid etches, bright dips (these give an extremely shiny finish to metal), chemical milling, and immersion plating. It is suited to an electrochemical process called anodizing. Or it can be electroplated with other metals or given organic coatings such as paint, lacquer, and plastic films. Aluminum can be finished by porcelain enameling or metallizing.

High-purity aluminum (99.9 percent) is relatively soft and has a fairly low tensile strength of about 50 megapascals (500 kilograms per square centimetre, or 7,000 pounds per square inch) in the annealed condition. (Annealing involves heating and then cooling slowly to make the metal less brittle.) By alloying and proper thermal and mechanical treatment, however, it can be made much harder and stronger, with tensile strengths as high as 700 megapascals. Unlike some other metals, the strength and ductility of aluminum increase at very low temperatures. Upon melting, the solid metal expands about 7 percent in volume, the solidifiCATion shrinkage being 6.6 percent of the liquid volume. Hydrogen is the only gas known to be appreciably soluble in molten aluminum; its solubility increases with temperature but becomes nearly zero when the metal freezes.

Aluminum may act as a base to form salts with acids or as a weak acid to form salts with strong alkalies. It is stable in air because of a thin, transparent oxide film that forms on exposure to air, protecting the aluminum from further oxidation and reaction. Growth of this natural oxide film is self-limiting--that is, when a thin layer is formed, further growth is halted. Molten aluminum is protected in air by a thicker oxide coating, which also deters further oxidation. Finely divided atomized or flake aluminum mixed with air and ignited will explode violently. Aluminum reacts rapidly with boiling water to liberate hydrogen and form aluminum hydroxide.

In its superpure condition (99.99 percent), aluminum lacks strength and hardness but is formable, weldable, corrosion-resistant, and an excellent conductor of electricity. Superpure aluminum has many appliCATions: in chemical equipment, in reflectors, as a CATalyst in making gasoline, in fine jewelry, and in electronic components. Most aluminum used today, however, is alloyed with other elements to increase strength.

The most common alloying elements are manganese (Mn), magnesium (Mg), copper (Cu), zinc (Zn), and silicon (Si). (Lithium [Li] is added to some of the newest alloys for the aerospace industry.) Smaller amounts of chromium (Cr), zirconium (Zr), vanadium (V), titanium (Ti), boron (B), tin (Sn), bismuth (Bi), and lead (Pb) may be added for particular purposes. Iron is present as an impurity.

Aluminum alloy products may be cast in a foundry into their final shape through sand-casting, permanent-mold-casting, or die-casting, or they may be cast into cylinders or rectangular blocks that are worked, or wrought, into products such as sheet, plate, forgings, or extrusions.

Reference: Encyclopædia Britannica, Inc. 1994-2000 ©




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