About Aluminium [Al] Element

Characteristics

An: 13 N: 14

Am: 26.981538 (2) g/mol

Group No: 13

Group Name: Metals

Block: p-block Period: 3

State: solid at 298 K

Colour: silvery Classification: Metallic

Boiling Point: 2792K (2519^oC)

Melting Point: 933.47K (660.32^oC)

Superconducting temperature: 1.175K (-271.975^oC)

Density: 2.70g/cm³

Discovery Information

Who: Hans Christian Oersted

When: 1825

Where: Denmark

Name Origin

Latin: alumen (alun). "Aluminium" in different languages.

Sources

Most plentiful metal in earth’s crust (7.5% - 8.1%), but virtually never occurs in free form, so rare that it was once considered a precious metal more valuable than gold! Obtained by electrolysis from bauxite (Al₂O₂).

The most important aluminium ore. It was named after the village "Les Baux-de-Provence" in southern France, where it was first discovered in 1821 by geologist Pierre Berthier.

Primary reserves are found in Surinam, Jamaica, Ghana, Indonesia and Russia. Annual production is around 15 million tons.

Abundance

Universe: 50 ppm (by weight)

Sun: 60 ppm (by weight)

Carbonaceous meteorite: 9300 ppm

Earth’s Crust: 82000 ppm

Seawater: Atlantic surface: 9.7 x 10^-4 ppm, Atlantic deep: 5.2 x 10^-4 ppm, Pacific surface: 1.3 x 10^-4 ppm, Pacific deep: 1.3 x 10^-5 ppm

Human: 900 ppb by weight,0 ppb by atoms

Uses

Kitchen utensils, building decorations, electrical transmission (not nearly as conductive as copper, but cheaper) as well as packaging (can, foil etc.). Aluminium alloys form vital components of aircraft and rockets as a result of their high strength to weight ratio. Alloys containing copper, magnesium, silicon, manganese and other metals are much stronger and more durable than aluminium, making aluminium useful in the manufacture of aircraft and rockets.

Most electronic appliances that require cooling of their internal devices (like transistors, CPUs - semiconductors in general) have heat sinks that are made of aluminium due to its ease of manufacture and good heat conductivity. Copper heat sinks are smaller although more expensive and harder to manufacture.

Aluminium oxide (Al₂O₃), alumina, is found naturally as corundum (rubies and sapphires), emery, and is used in glass making. Synthetic ruby and sapphire are used in lasers. Powdered aluminium is a commonly used silvering agent in paint. Aluminium flakes may also be included in undercoat paints, particularly wood primer - on drying, the flakes overlap to produce a water resistant barrier.

History

The ancient Greeks and Romans used aluminium salts as dyeing mordants and as astringents for dressing wounds. In 1761 Guyton de Morveau suggested calling the base alum alumine. In 1808, Humphry Davy identified the existence of a metal base of alum, which he at first named alumium and later aluminum.

Friedrich Wöhler is generally credited with isolating aluminium (Latin alumen, alum) in 1827 by mixing anhydrous aluminium chloride with potassium. The metal, however, had indeed been produced for the first time two years earlier - but in an impure form - by the Danish physicist and chemist Hans Christian Orsted. Therefore, Orsted can also be listed as the discoverer of the metal. Further, Pierre Berthier discovered aluminium in bauxite ore and successfully extracted it. The Frenchman Henri Etienne Sainte-Claire Deville improved Wöhler’s method in 1846 and described his improvements in a book in 1859, chief among these being the substitution of sodium for the considerably more expensive potassium.

Aluminium was selected as the material to be used for the apex of the Washington Monument, at a time when one ounce (30 grams) cost twice the daily wages of a common worker in the project; aluminium was a semiprecious metal at that time.

Notes

It is the second most malleable metal (gold being first) and the sixth most ductile.

Hazards

Aluminium is one of the few abundant elements that appears to have no beneficial function in living cells, but a few percent of people are allergic to it - they experience contact dermatitis from any form of it: an itchy rash from using antiperspirant products, digestive disorders and inability to absorb nutrients from eating food cooked in aluminium pans. Aluminium powder is flammable. Reacts very exothermically with halogens.

Aluminium Compounds

Aluminium boride AlB₂

It is used as a grinding compound to replace diamond or corundum. It is also used as a coating to strengthen aluminium.

Aluminium chlorohydrate AlnCl_(3n-m)(OH)_m A group of aluminium salts used in deodorants and antiperspirants and as a flocculant in water purification.

Aluminium gallium indium phosphide AlGaInP

A semiconductor material used in the manufacture of light-emitting diodes of high-brightness red, orange, green, and yellow colour, to form the heterostructure emitting light. It is also used to make diode lasers.

Aluminium gallium nitride AlGaN

Used to manufacture light-emitting diodes operating in blue to ultraviolet region, where wavelengths down to 250 nm (far UV) were achieved. It is also used in blue semiconductor lasers. It is also used in detectors of ultraviolet radiation.

Aluminium gallium phosphide AlGaP

A semiconductor material used in the manufacture of green light-emitting diodes.

Aluminium monostearate Al(OH)₂C₁₈H₃₅O₂

It is used to form gels in the packaging of pharmaceuticals, and in the preparation of variuos colours for cosmetics. It is usually safe in commercial products, but aluminium may accumulate in the body.

Aluminium nitrate Al(NO₃)₃

It is used in tanning leather, antiperspirants, corrosion inhibitors, extraction of uranium, petroleum refining, and as a nitrating agent.

Aluminium sulfate Al₂(SO₄)₃.16H₂O

Used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes impurities to coagulate which are removed as the particulate settles to the bottom of the container or more easily filtered.

When dissolved in a large amount of neutral or slightly-alkaline water, aluminum sulfate produces a gelatinous precipitate of aluminum hydroxide, Al(OH)₃. In dyeing and printing cloth, the gelatinous precipitate helps the dye adhere to the clothing fibers by rendering the pigment insoluble.

Yttrium aluminium garnet Y₃Al₅O₁₂

A synthetic crystalline material of the garnet group, used as the active laser medium in various solid-state lasers..

It also finds use in jewellery as a diamond simulant. Coloured variants are faceted and valued (as synthetics) for their clarity, durability, high refractive index and dispersion.

Reactions of Aluminium

Reactions with water

A thin layer of oxide prevents aluminium being attack by water.

Reactions with air

A thin layer of oxide prevents aluminium being attack by air. Aluminium will burn in oxygen with a brillian white flame to form aluminium trixode.

4Al + 3O_{2 (g)} --> 2Al₂O_{3 (s)}
 

Reactions with halogens

Aluminium reacts vigorously with all halogens to form aluminium halides.

2Al _(s) + 3Cl_{2 (g)} --> 2AlCl_{3 (s)}

2Al _(s) + 3Br_{2 (l)} --> 2AlBr_{3 (s)}

2Al _(s) + 3I_{2 (l)} --> Al₂I_{6 (s)}
 

Reactions with acids

Aluminium dissolves readily in dilute suphuric acid for form solutions containing the Al(III) ion and hydroden gas.

2Al _(s) + 3H₂SO_{4 (aq)} --> 2Al³⁺ _(aq) + 2SO₄^2- _(aq) + 3H_{2 (g)}

The reaction with dilute hydrochloric acid also yield the same Al(III) ion and hydrogen gas.

2Al _(s) + 6HCl _(aq) --> 2Al³⁺ _(aq) + 6Cl^- _(aq) + 3H_{2 (g)}
 

Reactions with bases

Aluminium dissolves in sodium hydroxide to yield hydrogen gas and aluminates of the form [Al(OH)₄]^-.

2Al _(s) + 2NaOH _(aq) + 6H₂O _(l) --> 2Na⁺ _(aq) + 2[Al(OH)₄]^- + 3H_{2 (g)}

Occurrence and Production of Aluminium

Metal production and refinement

Although aluminium is the most abundant metallic element in Earth’s crust (believed to be 7.5% to 8.1%), it is very rare in its free form, occurring in oxygen-deficient environments such as volcanic mud, and it was once considered a precious metal more valuable than gold. Napoleon III, Emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the other guests had to make do with gold ones!

Aluminium is a reactive metal that is difficult to extract from ore, aluminium oxide (Al₂O₃).

Direct reduction - with carbon, for example - is not economically viable since aluminium oxide has a melting point of about 2,000^oC. It is extracted by electrolysis; the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. By this process, the operational temperature of the reduction cells is around 950 to 980^oC. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a mixture of aluminium, sodium, and calcium fluorides: (Na₃AlF₆). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process.

The electrolytic process replaced the Wohler 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 ore is in the molten state, its ions are free to move around. The reaction at the cathode - the negative terminal - is;

Al³⁺ + 3e^- --> Al

Here the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off.

At the positive electrode (anode), oxygen is formed:

2O^2- --> O₂ + 4e^-

This carbon anode is then oxidised by the oxygen, releasing carbon dioxide. The anodes in a reduction must therefore be replaced regularly, since they are consumed in the process:

O₂ + C --> CO₂

Unlike the anodes, the cathodes are not oxidised because there is no oxygen present at the cathode. The carbon cathode is protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.

Aluminium electrolysis with the Hall-Heroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The world-wide average specific energy consumption is approximately 15 kilowatt-hours per kilogram of aluminium produced from alumina. (52 to 56 MJ/kg). The most modern smelters reach approximately 12.8 kWh/kg (46.1 MJ/kg). Reduction line current for older technologies are typically 100 to 200 kA. State-of-the-art smelters operate with about 350 kA. Trials have been reported with 500 kA cells.

Recovery of the metal via recycling has become an important facet of the aluminium industry. Recycling involves melting the scrap, a process that uses only five percent of the energy needed to produce aluminium from ore. Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public consciousness.

Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Smelters tend to be situated where electric power is both plentiful and inexpensive, such as South Africa, the South Island of New Zealand, Australia, China, the Middle East, Russia, Quebec and British Columbia in Canada, and Iceland.

In 2004, the People’s Republic of China was the top world producer of aluminium. Over the last 50 years, Australia has become a major producer of bauxite ore and a major producer and exporter of alumina. Australia produced 62 million tonnes of bauxite in 2005. The Australian deposits have some refining problems, being high in silica but have the advantage of being shallow and relatively easy to mine.

Isotopes of Aluminium

²⁶Al [13 neutrons]

Abundance: synthetic

Half life: 7.17 x 10⁵ years [ beta+ ]

Decay Energy: 1.17 MeV

Decays to ²⁶Mg.

Half life: 7.17 x 10⁵ years [ Electron Capture ]

Decay Energy: ? MeV

Decays to ²⁶Mg.

Half life: 7.17 x 10⁵ years [ Gamma Radiation ]

Decay Energy: 1.8086 MeV

Decays to ?.

Meteorite research has also shown that ²⁶Al was relatively abundant at the time of formation of our planetary system. Most meteoriticists 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. 

²⁷Al [14 neutrons]

Abundance: 100%

Stable with 14 neutrons

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