Aluminum (or aluminum) is a chemical element in the boron group with symbol Al and atomic number 13. It is silvery white, and it is not soluble in water under normal circumstances.
Aluminum is the third most abundant element (after oxygen and silicon), and the most abundant metal, in the Earth’s crust. It makes up about 8% by weight of the Earth’s solid surface. Aluminum metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. The chief ore of aluminum is bauxite.
Aluminum is remarkable for the metal’s low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminum and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials. The most useful compounds of aluminum, at least on a weight basis, are the oxides and sulfates.
Despite its prevalence in the environment, aluminum salts are not known to be used by any form of life. In keeping with its pervasiveness, aluminum is well tolerated by plants and animals. Owing to their prevalence, potential beneficial (or otherwise) biological roles of aluminum compounds are of continuing interest.
Physical Aluminum is a relatively soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is nonmagnetic and does not easily ignite. A fresh film of aluminum serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminum is 7–11 MPa, while aluminum alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminum has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded.
Aluminum atoms are arranged in a face-centered cubic (fcc) structure. Aluminum has a stacking-fault energy of approximately 200 mJ/m2.
Aluminum is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper’s density. Aluminum is capable of being a superconductor, with a superconducting critical temperature of 1.2 Kelvin and a critical magnetic field of about 100 gauss (10 milliteslas).
Corrosion resistance can be excellent due to a thin surface layer of aluminum oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminum alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is also often greatly reduced by aqueous salts, particularly in the presence of dissimilar metals.
Owing to its resistance to corrosion, aluminum is one of the few metals that retain silvery reflectance in finely powdered form, making it an important component of silver-colored paints. aluminum mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3,000–10,000 nm (far IR) regions; in the 400–700 nm visible range it is slightly outperformed by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper.
Aluminum is oxidized by water to produce hydrogen and heat:
2 Al + 3 H2O → Al2O3 + 3 H2
This conversion is of interest for the production of hydrogen. Challenges include circumventing the formed oxide layer which inhibits the reaction and the expenses associated with the storage of energy by regeneration of the Al metal.
Aluminum has many known isotopes, whose mass numbers range from 21 to 42; however, only 27Al (stable isotope) and 26Al (radioactive isotope, t1/2 = 7.2×105 y) occur naturally. 27Al has a natural abundance above 99.9%. 26Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminum isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites.
The ratio of 26Al to 10Be has been used to study the role of transport, deposition, sediment storage, burial times, and erosion on 105 to 106 year time scales. Cosmogenic 26Al was first applied in studies of the Moon and meteorites. Meteoroid fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding drastically reduces 26Al production, and its decay can then be used to determine the meteorite’s terrestrial age.
Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.[1
Aluminum is theoretically 100% recyclable without any loss of its natural qualities. According to the International Resource Panel’s Metal Stocks in Society report, the global per capita stock of aluminum in use in society (i.e. in cars, buildings, electronics etc.) is 80 kg. Much of this is in more-developed countries (350–500 kg per capita) rather than less-developed countries (35 kg per capita). Knowing the per capita stocks and their approximate lifespans is important for planning recycling.
Recovery of the metal via recycling has become an important use of the aluminum industry. Recycling was a low-profile activity until the late 1960s, when the growing use of aluminum beverage cans brought it to the public awareness.
Recycling involves melting the scrap, a process that requires only 5% of the energy used to produce aluminum from ore, though a significant part (up to 15% of the input material) is lost as dross (ash-like oxide). The dross can undergo a further process to extract aluminum.
In Europe aluminum experiences high rates of recycling, ranging from 42% of beverage cans, 85% of construction materials and 95% of transport vehicles. Recycled aluminum is known as secondary aluminum, but maintains the same physical properties as primary aluminum. Secondary aluminum is produced in a wide range of formats and is employed in 80% of alloy injections. Another important use is for extrusion. White dross from primary aluminum production and from secondary recycling operations still contains useful quantities of aluminum that can be extracted industrially. The process produces aluminum billets, together with a highly complex waste material. This waste is difficult to manage. It reacts with water, releasing a mixture of gases (including, among others, hydrogen, acetylene, and ammonia), which spontaneously ignites on contact with air; contact with damp air results in the release of copious quantities of ammonia gas. Despite these difficulties, the waste has found use as a filler in asphalt and concrete.