Aluminum
Last updated: November 1, 2007.
Suppose you had to design the perfect
material—what would it be
like? You'd probably want it to be plentiful and relatively
inexpensive, strong and lightweight, easy to combine with other
materials, resistant to heat and corrosion, and a good conductor of electricity. In short, you'd probably come
up with a material like aluminum
(spelled aluminium in some
countries).
It's the commonest metal in Earth's crust, the third most
plentiful chemical element on our planet (only oxygen and silicon exist
in greater quantity), and the second most popular metal for making
things (after iron/steel). We all see
and use aluminum every day without even thinking about it. Disposable
drinks cans are made from it and so is cooking foil. You can find this
ghostly grey-white metal in some pretty amazing places, from jet engines in airplanes to the hulls of
hi-tech warships. What makes aluminum such a brilliantly useful
material? Let's take a closer look!
Photo: The US Navy's High Speed Vessel (HSV) 2
Swift is
a "wave-piercing" aluminum-hulled catamaran: the two prongs on the bow
(front end)
slice through the waves for greater speed. The lightweight aluminum
hull means the ship weighs less, so it can go faster and operate closer
to the shore in only 3.7 m (12 feet) of water.
Photo by Michelle R. Hammond courtesy of US
Navy.
What's it like?
Aluminum is soft, lightweight, fire-proof and heat-resistant, easy
to work into new shapes, and able to conduct electricity. It reflects
light and heat very effectively and it doesn't rust. It reacts easily
with other chemical elements, especially oxygen, and readily forms an
outer layer of aluminum oxide if you leave it in the air. We call these
things aluminum's physical and chemical properties.
Aluminum really comes into its own when you combine it with other
metals to make aluminum alloys
(an alloy is two or more metals mixed together to make a new material
that combines the best properites of the original metals). A few of the
metals commonly used to make aluminum alloys include boron, copper,
lithium, magnesium, manganese, silicon, tin, and zinc. You mix aluminum
with one or more of these depending on the job you're trying to do.
What's it used for?
Pure aluminum is very soft. If you want to make something stronger
but still lightweight,
hard-wearing, and able to survive the high temperatures in an airplane
or car engine, you mix aluminum and
copper. For food packaging, you don't need anything like the same
strength,
but you do need a material that's easy to shape and seal. You get
those qualities by alloying aluminum with magnesium or magnesium.
Suppose you want to carry
electricity over long distances from power
plants to homes and factories. You could use copper, which is
generally the best conductor (carrier) of electricity, but it's heavy
and expensive. Aluminum might be an option, but it doesn't carry
electricity so readily. One solution is to make power cables from
aluminum alloyed
with boron, which conducts electricity almost as well as copper but is
a great deal lighter and less droopy on hot days. Typically, aluminum
alloys contain 90-99 percent aluminum.
Photo: The experimental aluminum Ford Sable
car, produced in 1995, was 180 kg (400 lbs) lighter than a comparable
steel-bodied car and considerably more energy efficient.
Photo courtesy of US Department of Energy (DOE).
How's it made?
Aluminum reacts so readily with oxygen that you never naturally find
it in its pure form. Instead, compounds of aluminum exist in huge
quantities in Earth's crust as an ore (raw rocky material) called bauxite.
This is the common name for
hydrated alumina, a substance typically made from about two thirds
aluminum oxide (chemical formula Al2O3) with one
third water molecules
(H2O) locked into its crystal
structure. Depending on where on Earth
it's
found, bauxite also contains a range of different impurities such as
iron oxide, silicon oxide, and titanium oxide.
If you want to turn bauxite into aluminum to make useful things like
cans, cooking foil, and space rockets,
you've got to get rid of the impurities and the water and split the
aluminum atoms from the oxygen atoms they're locked onto. So making
aluminum is actually a multi-stage process.
First, you dig the bauxite from the ground, crush it up, dry it (if
it contains too much water), and purify it to leave just the aluminum
oxide. Then you use an electrical technique called electrolysis
to
split this into aluminum and oyxgen. (Electrolysis is the opposite to
what happens inside a battery. In a
battery, you have two different metal connections inserted into a
chemical compound and complete a circuit between them to generate
electricity. In electrolysis, you pass electricity, via two metal
connections, into a chemical compound, which then gradually splits
apart into its atoms.) Once separated out,
the pure aluminum is cast into blocks known as ingots, which can be
worked or shaped or used as a raw material for making aluminum alloys.
Photo: It's much cheaper and more
environmentally friendly to recycle used aluminum
than to dig bauxite from the ground and process it.
These squashed mats of cans are called biscuits. They're ready to melt
down and recycle.
Photo courtesy of US
Airforce.
Making usable, shiny aluminum from dirty lumps of bauxite that
you've dug from the ground is a lengthy, dirty, incredibly
energy-intensive process. That's why the aluminum industry is so keen
on recycling things like used drink cans.
It's far quicker, cheaper, and easier to melt these down and reuse them
than it is to process bauxite. It's also much better for the
environment because it saves a huge amount of energy.
A brief history of aluminum
Photo: Building an aluminum boat.
This high-speed alumnium boat, known as the Littoral Surface
Craft-Experimental (LSC-X) or X-Craft,
is shown here during construction in Freeland, Washington.
Photo by Jesse Praino courtesy of US
Navy.
- 1746: German chemist Andreas Marggraf
(1709–1782) realizes that alum (a natural aluminum compound used
for dying textiles since ancient times) contains an unknown metal. It's
aluminum, of
course, but he doesn't know that.
- 1809: English chemist Sir Humphry Davy (1778–1829) names this metal
"alumium" and (later) "aluminium",
but is unable to separate it out.
- 1825: Danish chemist and electrical pioneer Hans
Christian Øersted (1777–1851) turns
aluminum oxide into aluminum chloride and then uses potassium to turn
the chloride into pure aluminum. Unfortunately, he cannot repeat the
trick a second time!
- 1827: German chemist Friedrich Wöhler (1800–1882) also makes a small
quantity of aluminum by heating
aluminum oxide with potassium metal.
- 1855: French chemist Henri Sainte-Claire
Deville (1818–1881) uses sodium to separate out
aluminum. Since sodium is cheaper and easier to obtain than potassium,
Deville is
able to produce more aluminum—enough to make an ingot. He puts this
on display at a public exhibition in Paris, France. Deville's new
method means aluminum starts to become more widely available and the
price begins to fall.
- 1886: Working independently, the American team of Charles Martin Hall (1863–1914) and his sister
Julia Brainerd Hall (1859–1925) and Frenchman Paul-Louis-Toussaint
Héroult
(1863–1914) discover the modern method of splitting aluminum oxide with
electrolysis to make pure aluminum. Their highly efficient technique,
known as the Hall-Héroult process, is still used to produce most
of the world's aluminum today.
- 1888: Austrian chemist Karl Bayer
(1847–1904) finds a less expensive way of turning bauxite into
aluminum oxide—the raw material needed for the Hall-Héroult
process.
Together, the Bayer and Hall-Héroult processes drastically
reduce the
price of aluminum, enabling the metal to be used in much greater
quantities.
- Early 1900s: First aluminum recycling programs.
- 1913: Aluminum foil first produced.
- 1920s: Modern aluminum alloys begin to appear.
- 1925: American Chemical Society officially changes the name from
"aluminium" to "aluminum" in the United States.
- 1957: First aluminum power lines are introduced.
Further reading
