Aluminum
Jenna Khan
4/9/98
Material Science
Period 5
Aluminum is a silver-white, ductile metal that was discovered by Oersted in 1825.
Aluminum is amphoteric, lightweight (specific gravity of 2.70), a good conductor
of electricity (better conductor weight for weight than copper), and not easily corroded
because an oxide film rapidly forms when it is exposed to oxygen because it is strongly
electropositive and extremely reactive. Aluminum occurs naturally and is always
found combined with other elements such as sodium, potassium, iron, calcium, and
magnesium. It makes up approximately 8% of the earth's surface and is the third most abundant
chemical and the most abundant metallic constituent in the crust of the earth. Bauxite,
an impure hydrated aluminum oxide, is the commercial source of aluminum and its compounds because it is more difficult and expensive to extract aluminum from other
ores.
Bauxite is "derived from sedimentary clays that have been changed though long-continued
leaching by ground water or by a process of natural beneficiation of igneous rocks"
(Finch, 492) and can be white, gray, yellow, or red. In the United States it can
be found in Georgia, Alabama, Mississippi, and Arkansas but these deposits are minimal
and there are no known deposits in Canada so the major producers of bauxite are Jamaica
and Surinam. In 1989 the estimated world production of primary aluminum was 18 million metric tons with approximately 4 million metric tons being produced in the United
States, and the price of aluminum was less than $2 per kg. "U.S. consumption, by
major markets, consisted of containers and packaging, 31 percent; building and construction, 20 percent; transportation, 24 percent; electric equipment, 10 percent; consumer
durables, 9 percent; and miscellaneous, 6 percent. In 1989, recycled aluminum accounted
for over 20 percent of total aluminum consumption in the United States." (Encarta)
To extract aluminum from bauxite the ore is first mixed with a caustic soda solution
and then heated in an autoclave to dissolve the alumina (aluminum oxide). The impurities
which remain solid are filtered away and the remaining alumina is precipitated into a crystalline form and excess water is removed in a rotary kiln. The alumina
is then dissolved into molten cryolite and the pure aluminum is removed in electrolysis
with the molten aluminum collected at the cathode and oxygen being produced at the
anode. This deposit is poured into molds and cooled into pigs. The pigs are remelted
to remove any remaining impurities and then poured to form ingots. Methods of extraction
can produce aluminum which is 99.99 percent pure.
Aluminum is a versatile element which can be used is a variety of ways ranging from
gum wrappers to tanks but is generally used as some type of alloy. Some of the forms
it is used in are aluminum nitrate, aluminum oxide, aluminum hydroxide (used in antacids), aluminum chlorohydrate (used in deodorants), and aluminum sulfate (used to treat
drinking water). Aluminum has a high strength-to-weight ratio and weighs significantly
less than steel ("A given volume of aluminum weighs less than one-third as much as the same volume of steel." (Encarta)) which makes it useful in the construction
of aircrafts, railroad cars, and automobiles, and for other applications in which
mobility and energy conservation are important. Aluminum also has high heat conductivity
and can be used as cooking utensils and the pistons of internal-combustion engines.
Wire is often made of aluminum, especially for long-distance, high-voltage transmissions,
even though aluminum has only 63 percent of the electrical conductance of copper
for wire of a given size, because it weighs less than half as much. When the thickness
of the wire is increased so that the conductance is comparable the aluminum wire
is still lighter. Aluminum is an excellent insulator and is used for aluminum siding
and storm windows. Many household products are also made of aluminum such as foil and
various containers, bottles, and pans because it is easy to form and does not generally
contaminate food. It is also used in paints and fireworks and in the production
of glass, rubber, and ceramics. The metal is also used in low-temperature nuclear reactors
because it absorbs relatively few neutrons and at cryogenic temperatures because
it becomes stronger and retains its toughness as it gets colder. Aluminum's corrosion resistance in salt water also makes it useful in boat hulls and other aquatic devices.
Armor plates for tanks, personnel carriers, and other military vehicles are now
being made with new aluminum alloys.
The abundance and accessibility of aluminum causes everyone to be exposed to at least
low levels of from the environment or treatments. Aluminum is not a necessary substance
for the body and people are often exposed through the consumption of foods, water, or other substances such as antacids which contain aluminum, but very little comes
from cooking utensils. Breathing aluminum dust in the workplace is also another
common source of exposure. In 1990 the total releases of aluminum fume or dust by
379 facilities were 3,680,952 pounds, which ranked 64th of the TRI's 322 chemicals. Low
levels of exposure from these sources or from skin contact is not considered dangerous
but breathing air with high levels of aluminum can cause respiratory problems such
as asthma, coughing, or pulmonary fibrosis. People considered most at risk are refinery
and metal industry workers, people with chronic kidney failures that require long-term
hemodialysis treatment (dialysis fluid contains aluminum, and because the patients' kidneys are diseased, the aluminum can build up and they can develop bone or brain
diseases which may be caused by the excess aluminum), infants fed a diet of high
aluminum-contaminated formula, people who consume large quantities of of antacids,
and people who live in close proximity to sources of industrial emissions or hazardous waste
sites. Aluminum is not classified as a carcinogen even though it has produced cancer
in laboratory mice. (GVAL) It is also associated with seizures, is neurotoxic, and
is considered a possible etiologic factor in Alzheimer's and other neurological disorders.
It has been linked with Alzheimer's disease because people with the disease have
high levels of aluminum in their brains, but it is not clear whether the aluminum
causes the disease or if a buildup occurs when people already have the disease. A
study which exposed rats prenatally to aluminum indicates that "chronic exposure
to aluminum impairs glutamate-induced activation of nitric oxide synthase and nitric
oxide-induced activation of guanylate cyclase" (Chronic ...) which may contribute to the neurotixicity
of aluminum. Aluminum is also dangerous in the form of aluminum sulfate because
when it is heated to decomposition it emits toxic fumes. Aluminum chlorohydrate in deodorants can also cause certain people to get rashes. There is no evidence
that aluminum has any effect on reproduction of people or animals, but studies show
that aluminum does effect certain fish. Smallmouth bass are highly sensitive to
elevated aluminum concentrations (100 ug 1^-1) at the low pH of 5.5 common in low-alkalinity
waters. "The most toxic forms of aluminum are inorganic monometric forms abd aluminum
toxicity to fish had shown to be highest at pH of 5.0-5.5 where these forms predominate." (Necessary ...) The addition of aluminum to the water appears to make the
low pH more toxic which causes deformities and abnormal swimming behavior in the
smallmouth bass.
The EPA requires spills of 5,000 pounds or more of aluminum sulfate to be reported
and has other regulations for the pesticide aluminum phosphide. The EPA recommends
0.2 parts of aluminum per million parts of water as the maximum concentration for
drinking water because of taste and odor concerns. The Occupational Safety and Health administration
set the maximum concentration limit for aluminum dust in the air of a workplace to
15 milligrams per cubic meter of air for an 8-hour workday over a 40-hour week, while The National Institute for Occupational Safety and Health recommended that
the concentration not exceed 10 milligrams per cubic meter for up to a 10-hour workday
over a 40-hour work week. The FDA confirms the general safety of aluminum cooking
utensils, aluminum foil, antacids, and other aluminum products.
Works Cited
"Al-alloys in the Atmosphere: Materials Performance" -
http://www.metalogic.be/MatWeb/reading/air/alat.htm
"Aluminum Alloys: Performance in Various Corrosives" -
http://www.metalogic.be/MatWeb/reading/mat-envs/aleee.htm
"Aluminum and Aluminum Compounds Chemical Backgrounder," Environment
Writer, July 1, 1997 - http://www.nsc.org/ehc/ew/shems/aluminum.htm
"Aluminum," Microsoft (R) Encarta. Copyright (c) 1994 Microsoft Corporation. Copyright
(c) 1994 Funk & Wagnall's Corporation.
"Aluminum Sulfate. N-Hydrate" -
http://www.chem.utah.edu/MSDS/A/ALUMINUM_SULFATE%2C_N-
HYDRATE
"Aluminum," ToxFAQs, September 1995 -
http://www.healthfuture.com/aluminum.htm
"Chronic Exposure to Aluminum Impairs Neuronal Glutamate-Nitric Oxide-Cyclic
GMP Pathway", Journal of Neurochemistry -
http://www.jneurochem.com/issues/v70n4/v70n4/v70n4p1609.html
Johson, Carl and William Weeks, Metallurgy
, American Technical Publishers, Inc., USA, 1977.
Knopf, Alfred, Rocks and Minerals
, Random House, Inc., New York, 1988.
"Necessary Water Quality Conditions for Smallmouth Bass," McMaster University -
http://www.science.mcmaster.ca/Biology/Harbour/SPECIES/BASS/PH.HTM
The Voumne Library
, The Southwestern Company, Tennessee, 1988.
"Toxic Chemicals in Vaccines," GVAL -
http://www.pages.prodigy.com/gval/toxic.htm
Weaver, John, Physical Elements of Geography
, McGraw Hill Book Company, Inc.,
New York, 1957.
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