Durable Aluminum -
Withstands age and weather
Aluminium is a silvery white and ductile member of the boron
group of chemical elements. It has the symbol Al; its atomic
number is 13. It is not soluble in water under normal
circumstances. Aluminium is the most abundant metal in the
Earth's crust, and the third most abundant element therein, after
oxygen and silicon. It makes up about 8% by weight of the Earth’
s solid surface. Aluminium is too reactive chemically to occur in
nature as a free metal. Instead, it is found combined in over 270
different minerals. The chief source of aluminium is bauxite ore.
Aluminium is remarkable for its ability to resist corrosion due to
the phenomenon of passivation and the metal's low density.
Structural components made from aluminium and its alloys are
vital to the aerospace industry and very important in other areas
of transportation and building. Its reactive nature makes it useful
as a catalyst or additive in chemical mixtures, including being
used in ammonium nitrate explosives to enhance blast power.
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Aluminium is a soft, durable, lightweight, malleable metal with
appearance ranging from silvery to dull grey, depending on the
surface roughness. Aluminium is nonmagnetic and non
sparking. It is also insoluble in alcohol, though it can be soluble
in water in certain forms. The yield strength of pure aluminium is
7–11 MPa, while aluminium alloys have yield strengths ranging
from 200 MPa to 600 MPa. Aluminium has about one-third the
density and stiffness of steel. It is ductile, and easily machined,
cast, and extruded.
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. This corrosion resistance is also often greatly
reduced when many aqueous salts are present however,
particularly in the presence of dissimilar metals.
Aluminium atoms are arranged in a face-centered cubic (FCC)
structure. Aluminium has a stacking-fault energy of
approximately 200 mJ/m².
Aluminium is one of the few metals that retain full silvery
reflectance in finely powdered form, making it an important
component of silver paints. Aluminium mirror finish has the
highest reflectance of any metal in the 200–400 nm (UV) and
the 3000–10000 nm (far IR) regions, while in the 400–700 nm
visible range it is slightly outdone by tin and silver and in the
700–3000 (near IR) by silver, gold, and copper.
Aluminium is a good thermal and electrical conductor, by weight
better than copper. Aluminium is capable of being a
superconductor, with a superconducting critical temperature of
1.2 kelvin and a critical magnetic field of about 100 gauss.
In the Earth's crust, aluminium is the most
abundant (8.3% by weight) metallic element and
the third most abundant of all elements (after
oxygen and silicon). Because of its strong
affinity to oxygen, however, it is almost never
found in the elemental state; instead it is found in
oxides or silicates. Feldspars, the most common
group of minerals in the Earth's crust, are
aluminosilicates. Native aluminium metal can be
found as a minor phase in low oxygen fugacity
environments, such as the interiors of certain
volcanoes. It also occurs in the minerals
beryl, cryolite, garnet, spinel and turquoise.
Impurities in Al2O3, such as chromium or cobalt
yield the gemstones ruby and sapphire,
respectively. Pure Al2O3, known as Corundum, is
one of the hardest materials known.
Although aluminium is the most abundant metallic element
in the Earth's crust (believed to be 7.5 to 8.1 percent), it is
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. The Washington Monument was
completed, with the 100 ounce (2.8 kg) aluminium capstone
being put in place on December 6, 1884, in an elaborate
dedication ceremony. It was the largest single piece of
aluminium cast at the time. At that time, aluminium was as
expensive as silver. Aluminium has been produced in
commercial quantities for just over 100 years.
Aluminium is a strongly reactive metal that forms a high-energy chemical bond with
oxygen. Compared to most other metals, it is difficult to extract from ore, such as
bauxite, due to the energy required to reduce aluminium oxide (Al2O3). For example,
direct reduction with carbon, as is used to produce iron, is not chemically possible,
since aluminium is a stronger reducing agent than carbon. Aluminium oxide has a
melting point of about 2,000 °C. Therefore, it must be extracted by electrolysis. In this
process, the aluminium oxide is dissolved in molten cryolite and then reduced to the
pure metal. The operational temperature of the reduction cells is around 950 to 980 °
C. Cryolite is found as a mineral in Greenland, but in industrial use it has been
replaced by a synthetic substance. Cryolite is a chemical compound of aluminium,
sodium, and calcium fluorides: (Na3AlF6). The aluminium oxide (a white powder) is
obtained by refining bauxite in the Bayer process of Karl Bayer. (Previously, the Deville
process was the predominant refining technology.)
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, its ions are free to move around. The reaction at the cathode (negative
Al3+ + 3 e− → Al
Here the aluminium ion is being reduced (electrons are added). 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 (positive electrode), oxygen is formed:
2 O2− → O2 + 4 e−
This carbon anode is then oxidized by the oxygen, releasing carbon dioxide.
O2 + C → CO2
The anodes in a reduction cell must therefore be replaced regularly, since they are
consumed in the process.
Unlike the anodes, the cathodes are not oxidized because there is no oxygen present,
as the carbon cathodes are protected by the liquid aluminium inside the cells.
Nevertheless, 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.
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-hours per 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
kA; state-of-the-art smelters
operate at about 350 kA. Trials have
been reported with 500 kA cells.
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, Ghana, the South Island
of New Zealand, Australia, the People's
Republic of China, the Middle East,
Russia, Quebec and British Columbia
in Canada, and Iceland.
Aluminium output in 2005
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
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, some being high in silica but
have the advantage of being shallow
and relatively easy to mine.
Aluminium is 100% recyclable without any loss of its natural qualities. Recovery of
the metal via recycling has become an important facet of the aluminium industry.
Recycling involves melting the scrap, a process that requires only five percent of the
energy used to produce aluminium from ore. However, a significant part (up to 15% of
the input material) is lost as dross (ash-like oxide).
Recycling was a low-profile activity until the late 1960s, when the growing use of
aluminium beverage cans brought it to the public awareness.
In Europe aluminium experiences high rates of recycling, ranging from 42% of
beverage cans, 85% of construction materials and 95% of transport vehicles.
Recycled aluminium is known as secondary aluminium, but maintains the same
physical properties as primary aluminium. Secondary aluminium is produced in a wide
range of formats and is employed in 80% of the alloy injections. Another important use
is for extrusion.
White dross from primary aluminium production and from secondary recycling
operations still contains useful quantities of aluminium which can be extracted
industrially. The process produces aluminium 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, however, the
waste has found use as a filler in asphalt and concrete.
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Although aluminium is an extremely common and
widespread element, the common aluminium minerals are
not economic sources of the metal. Almost all metallic
aluminium is produced from the ore bauxite
(AlOx(OH)3-2x). Bauxite occurs as a weathering product of
low iron and silica bedrock in tropical climatic
conditions. Large deposits of bauxite occur in
Australia, Brazil, Guinea and Jamaica but the primary
mining areas for the ore are in Ghana, Indonesia, Jamaica,
Russia and Suriname. Smelting of the ore mainly
occurs in Australia, Brazil, Canada, Norway, Russia and
the United States. Because smelting is an energy-intensive
process, regions with excess natural gas supplies (such as
the United Arab Emirates) are becoming aluminium refiners.