| Aluminium |
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| 13Al |
|
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| Aluminium in the periodic table |
| Appearance |
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silvery gray metallic
Spectral lines of aluminium |
| General properties |
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| Name, symbol,number | aluminium, Al, 13 |
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| Pronunciation | UK  i//
al-ew-min-ee-əm;
|
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| Element category | poor metal |
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| Group, period,block | 13, 3, p |
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| Standard atomic weight | 26.9815386(13) |
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| Electron configuration | [Ne] 3s2 3p1
2, 8, 3
|
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| History |
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| Prediction | Antoine Lavoisier[1] (1787) |
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| First isolation | Friedrich Wöhler[1] (1827) |
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| Named by | Humphry Davy[1] (1807) |
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| Physical properties |
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| Phase | solid |
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| Density(near r.t.) | 2.70 g·cm−3 |
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| Liquid density atm.p. | 2.375 g·cm−3 |
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| Melting point | 933.47 K, 660.32 °C, 1220.58 °F |
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| Boiling point | 2792 K, 2519 °C, 4566 °F |
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| Heat of fusion | 10.71 kJ·mol−1 |
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| Heat of vaporization | 294.0 kJ·mol−1 |
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| Molar heat capacity | 24.200 J·mol−1·K−1 |
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| Vapor pressure |
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| P (Pa) | 1 | 10 | 100 | 1 k | 10 k | 100 k |
| at T (K) | 1482 | 1632 | 1817 | 2054 | 2364 | 2790 |
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| Atomic properties |
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| Oxidation states | 3, 2[2], 1[3]
(amphoteric oxide) |
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| Electronegativity | 1.61 (Pauling scale) |
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Ionization energies
(more) | 1st: 577.5 kJ·mol−1 |
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| 2nd: 1816.7 kJ·mol−1 |
| 3rd: 2744.8 kJ·mol−1 |
| Atomic radius | 143 pm |
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| Covalent radius | 121±4 pm |
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| Van der Waals radius | 184 pm |
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| Miscellanea |
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| Crystal structure | face-centered cubic
|
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| Magnetic ordering | paramagnetic[4] |
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| Electrical resistivity | (20 °C) 28.2 nΩ·m |
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| Thermal conductivity | 237 W·m−1·K−1 |
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| Thermal expansion | (25 °C) 23.1 µm·m−1·K−1 |
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| Speed of sound(thin rod) | (r.t.) (rolled) 5,000 m·s−1 |
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| Young's modulus | 70 GPa |
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| Shear modulus | 26 GPa |
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| Bulk modulus | 76 GPa |
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| Poisson ratio | 0.35 |
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| Mohs hardness | 2.75 |
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| Vickers hardness | 167 MPa |
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| Brinell hardness | 245 MPa |
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| CAS registry number | 7429-90-5 |
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| Most stable isotopes |
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| Main article: Isotopes of aluminium |
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|
· ref |
Aluminium is remarkable for the metal's low
density and for its ability to resist
corrosiondue to the phenomenon of
passivation. Structural components made from aluminium 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 aluminium, at least on a weight basis, are the
oxides and
sulfates.
Despite its prevalence in the environment, aluminium
salts are not known to be used by any form of life. In keeping with its pervasiveness, aluminium is well tolerated by plants and animals.
[6] Owing to their prevalence, potential beneficial (or otherwise) biological roles of aluminium compounds are of continuing interest.
Characteristics
Etched surface from a high purity (99.9998%) aluminium bar, size 55×37 mm
Physical
Aluminium 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 aluminium serves as a good reflector (approximately 92%) of
visible lightand an excellent reflector (as much as 98%) of medium and far
infrared radiation. The
yield strength of pure aluminium is 7–11
MPa, while
aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.
[7]Aluminium has about one-third the
density and
stiffness of
steel. It is easily
machined,
cast,
drawn and
extruded.
Chemical
Corrosion resistance can be excellent due to a thin surface layer of
aluminium oxide that forms when the metal is exposed to air, effectively preventing further
oxidation. The strongest aluminium alloys are less corrosion resistant due to
galvanic reactions with alloyed
copper.
[7] This corrosion resistance is also often greatly reduced by aqueous salts, particularly in the presence of dissimilar metals.
Owing to its resistance to corrosion, aluminium is one of the few metals that retain silvery reflectance in finely powdered form, making it an important component of
silver-colored paints. Aluminium 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.
[10]- 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.
[11]
Isotopes
Aluminium has many known
isotopes, whose mass numbers range from 21 to 42; however, only
27Al (
stable isotope) and
26Al (
radioactive isotope,
t1/2 = 7.2×10
5 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. Aluminium isotopes have found practical application in dating
marine sediments, manganese nodules, glacial ice,
quartzin
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 10
5 to 10
6year time scales.
[12] 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.
[13]
Production and refinement
Bauxite, a major aluminium ore. The red-brown colour is due to the presence of
ironminerals.
Aluminium forms strong
chemical bonds with oxygen. Compared to most other metals, it is difficult to extract from ore, such as
bauxite, due to the high reactivity of aluminium and the high melting point of most of its ores. For example, direct reduction with
carbon, as is used to produce
iron, is not chemically possible because aluminium is a stronger reducing agent than carbon. Indirect carbothermic reduction can be carried out using carbon and Al
2O
3, which forms an intermediate Al
4C
3 and this can further yield aluminium metal at a temperature of 1900–2000 °C. This process is still under development; it requires less energy and yields less CO
2 than the
Hall-Héroult process, the major industrial process for aluminium extraction.
[19] Electrolytic smelting of alumina was originally cost-prohibitive in part because of the high melting point of alumina, or aluminium oxide, (about
2,000 °C(3,600 °F)). Many minerals, however, will dissolve into a second already molten mineral, even if the temperature of the melt is significantly lower than the melting point of the first mineral. Molten
cryolite was discovered to dissolve alumina at temperatures significantly lower than the melting point of pure alumina without interfering in the smelting process. In the
Hall-Héroult process, alumina is first dissolved into molten cryolite with
calcium fluoride and then electrolytically reduced to aluminium at a temperature between 950 and 980 °C (1,740 to 1,800 °F). Cryolite is a chemical compound of aluminium and
sodium fluorides: (Na
3AlF
6). Although cryolite is found as a mineral in
Greenland, its synthetic form is used in the industry. The aluminium oxide itself is obtained by refining bauxite in the
Bayer process.
The electrolytic process replaced the
Wöhler process, which involved the reduction of anhydrous aluminium chloride with
potassium. Both of the
electrodes used in the electrolysis of aluminium oxide are carbon. Once the refined alumina is dissolved in the electrolyte, it disassociates and its ions are free to move around. The reaction at the
cathode is:
- Al3+ + 3 e− → Al
Here the aluminium ion is being
reduced. The aluminium metal then sinks to the bottom and is tapped off, usually cast into large blocks called
aluminium billets for further processing.
At the
anode, oxygen is formed:
- 2 O2− → O2 + 4 e−
To some extent, the carbon anode is consumed by subsequent reaction with oxygen to form carbon dioxide. The anodes in a reduction cell must therefore be replaced regularly, since they are consumed in the process. The cathodes do erode, mainly due to electrochemical processes and metal movement. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.
World production trend of aluminium
Aluminium electrolysis with the
Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The worldwide average specific energy consumption is approximately 15±0.5
kilowatt-hoursper kilogram of aluminium produced (52 to 56
MJ/kg). The most modern smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg). (Compare this to the
heat of reaction, 31 MJ/kg, and the
Gibbs free energy of reaction, 29 MJ/kg.) Reduction line currents for older technologies are typically 100 to 200
kiloamperes; state-of-the-art smelters operate at about 350 kA. Trials have been reported with 500 kA cells.
[citation needed]
The Hall-Heroult process produces aluminium with a purity of above 99%. Further purification can be done by the
Hoope process. The process involves the electrolysis of molten aluminium with a sodium, barium and aluminium fluoride electrolyte. The resulting aluminium has a purity of 99.99%.
[20][21]
Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Aluminium production consumes roughly 5% of electricity generated in the U.S.
[22] Smelters tend to be situated where electric power is both plentiful and inexpensive, such as the
United Arab Emirates with excess natural gas supplies and
Iceland and
Norway with energy generated from
renewable sources. The world's largest
smelters of alumina are People's Republic of China, Russia, and
Quebec and
British Columbia in
Canada.
[22][23][24]
Aluminium spot price 1987 2012
In 2005, the People's Republic of China was the top producer of aluminium with almost a one-fifth world share, followed by Russia, Canada, and the USA, reports the
British Geological Survey.
Over the last 50 years, Australia has become a major producer of bauxite ore and a major producer and exporter of alumina (before being overtaken by China in 2007).
[23][25] Australia produced 68 million
tonnes of bauxite in 2010. The Australian deposits have some refining problems, some being high in silica, but have the advantage of being shallow and relatively easy to mine.
[26]
Applications
General use
Aluminium is the most widely used
non-ferrous metal.
[37] Global production of aluminium in 2005 was 31.9 million tonnes. It exceeded that of any other metal except
iron (837.5 million tonnes).
[38] Forecast for 2012 is 42–45 million tonnes, driven by rising Chinese output.
[39]
Aluminium is almost always alloyed, which markedly improves its mechanical properties, especially when
tempered. For example, the common
aluminium foils and beverage cans are alloys of 92% to 99% aluminium.
[40] The main
alloying agents are copper,
zinc,
magnesium,
manganese, and
silicon (e.g.,
duralumin) and the levels of these other metals are in the range of a few percent by weight.
[41]
Aluminium slabs being transported from a smelter
Some of the many uses for aluminium metal are in:
- Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles, etc.) as sheet, tube, castings, etc.
- Packaging (cans, foil, frame of etc.)
- Construction (windows, doors, siding, building wire, etc.).[42]
- A wide range of household items, from cooking utensils to baseball bats, watches.[43]
- Street lighting poles, sailing ship masts, walking poles, etc.
- Outer shells of consumer electronics, also cases for equipment e.g. photographic equipment, MacBook Pro's casing
- Electrical transmission lines for power distribution
- MKM steel and Alnico magnets
- Super purity aluminium (SPA, 99.980% to 99.999% Al), used in electronics and CDs.
- Heat sinks for electronic appliances such as transistors and CPUs.
- Substrate material of metal-core copper clad laminates used in high brightness LED lighting.
- Powdered aluminium is used in paint, and in pyrotechnics such as solid rocket fuels andthermite.
- Aluminium can be reacted with hydrochloric acid or with sodium hydroxide to produce hydrogen gas.
- A variety of countries, including France, Italy, Poland, Finland, Romania, Israel, and the former Yugoslavia, have issued coins struck in aluminium or aluminium-copper alloys.[44][45]
- Some guitar models sport aluminium diamond plates on the surface of the instruments, usually either chrome or black. Kramer Guitars and Travis Bean are both known for having produced guitars with necks made of aluminium, which gives the instrument a very distinct sound.
Aluminium alloys in structural applications
Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system (
ANSI) or by names indicating their main alloying constituents (
DIN and
ISO).
The strength and durability of aluminium alloys vary widely, not only as a result of the components of the specific alloy, but also as a result of heat treatments and manufacturing processes. A lack of knowledge of these aspects has from time to time led to improperly designed structures and gained aluminium a bad reputation.
One important structural limitation of aluminium alloys is their
fatigue strength. Unlike steels, aluminium alloys have no well-defined
fatigue limit, meaning that fatigue failure eventually occurs, under even very small cyclic loadings. This implies that engineers must assess these loads and design for a
fixed life rather than an infinite life.
Another important property of aluminium alloys is their sensitivity to heat. Workshop procedures involving heating are complicated by the fact that aluminium, unlike steel, melts without first glowing red. Forming operations where a
blow torch is used therefore require some expertise, since no visual signs reveal how close the material is to melting. Aluminium alloys, like all structural alloys, also are subject to internal stresses following heating operations such as welding and casting. The problem with aluminium alloys in this regard is their low
melting point, which make them more susceptible to distortions from thermally induced stress relief. Controlled stress relief can be done during manufacturing by heat-treating the parts in an oven, followed by gradual cooling—in effect
annealing the stresses.
The low melting point of aluminium alloys has not precluded their use in rocketry; even for use in constructing combustion chambers where gases can reach 3500 K. The
Agena upper stage engine used a regeneratively cooled aluminium design for some parts of the nozzle, including the thermally critical throat region.
AYORINDE AYODEJI.
