Aluminum
by Chris Woodford. Last updated: September 27, 2021.
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—and that's also the official
IUPAC spelling).
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 gray-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: Aluminum is a wonderfully weather-proof material.
At the Federal Building and U.S. Courthouse, Wheeling, West Virginia, it features
prominently in the striking windows and other internal features.
Photo by Carol M. Highsmith, courtesy of Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
What's aluminum 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.
Photo: The experimental aluminum Ford Sable
car, produced over 25 years ago in 1995, was 180 kg (400 lbs) lighter than a comparable
steel-bodied car and considerably more energy efficient.
Today, as fuel economy becomes ever more important, full-bodied aluminum cars are commonplace.
A new Ford F-150 truck, with a full aluminum body, is a whopping 39 percent (320kg or 700lbs) lighter than its predecessor,
according to the Aluminum Association.
Photo courtesy of US Department of Energy (DOE).
Alloys
Aluminum really comes into its own when you combine it with other
metals to make aluminum alloys
(an alloy is a metal mixed together with other elements to make a new material
with improved properties—it might be stronger or it might melt at a higher temperature). 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.
Composites
Aluminum can be combined with other materials in a quite different way
in composites (hybrid materials made from two or more materials that retain
their separate identity without chemically combining, mixing, or dissolving). So, for example, aluminum can act as the "background material" (matrix) in what's called a metal matrix composite (MMC), reinforced with particles of silicon carbide, to make a strong, stiff, lightweight material suitable for a wide variety of aerospace, electronic, and automobile uses—and (crucially) better than aluminum alone.
What's aluminum used for?
Chart: Aluminum consumption in the United States. Transportation (planes, ships, trucks, and cars) is now by far the biggest single use for the metal and its alloys. Source: US Geological Survey,
Mineral Commodity Summaries: Aluminum. January 2021.
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.
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.
How is aluminum 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.
The world currently has about 55–75 billion tons of bauxite resources—enough to
meet demand "well into the future" (according to the US Geological Survey's Mineral
Commodity Summaries, January 2021).
Photo: Ready for recycling: These squashed mats of aluminum cans are called biscuits. They're ready to melt
down and recycle. According to the Aluminum Association, nearly 70 percent of the aluminum ever mined is still in use today, thanks to effective recycling programs. It's much cheaper and more environmentally friendly to recycle used aluminum than to dig bauxite from the ground and process it: recycling saves about 95 percent of the energy that would be needed to make brand new aluminum.
Photo courtesy of US Air Force.
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 oxygen. (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.
Making usable, shiny aluminum from rocky 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.
Chart: Why recycling aluminum makes sense. The amount of energy it takes to recycle metal for reuse (orange bars) is a fraction of what it takes to produce virgin metal in the first place (blue bars), but the difference is much greater for aluminum (center) than for either steel (left) or copper (right) because it's so hard to extract and refine aluminum in the first place. Data source: "Table 7.11 Embodied energy of selected materials" in Energy and Carbon Emissions by Nicola Terry, UIT Cambridge, 2011, based on data from the Inventory of Carbon and Energy (ICE) by the Sustainable Energy Research Team, University of Bath.
A brief history of aluminum
Photo: Building an aluminum boat.
This high-speed aluminum 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.
Who discovered aluminum, how, and when? Here's the story as it happened...
- 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.
- 1893: Studebaker launches an aluminum farm wagon for the Chicago World's Columbian Exposition.
- 1899: A Dürkopp sports car with an aluminum body is unveiled at the Berlin International Motor Show.
A few years later, the
Pierce Arrow Motor Car Company produces its cars with cast aluminum bodies.
- 1901: Motor pioneer Carl Benz produces the first aluminum car engine.
- 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.
- 1946: Aluminum is used for the bodywork of the lightweight, mass-produced
Panhard Dyna X.
- 1957: The first aluminum power lines are introduced.
- 1959: Coors produces the first all-aluminum drinks can.
- 1975: Daniel Cudzik invents the stay-on ring-pull tab for drinks cans.
- 1990: The International Union of Pure and Applied Chemistry (IUPAC) officially adopts "aluminium"
as its spelling.
- 1994: The Audi A8
sets new standards in lightweight car production with an aluminum body framework weighing just 249kg (almost half the
weight of a comparable steel shell).
- 2015: Ford launches an all-aluminum bodied version of its hugely popular F-150 truck.
