Aluminium – It’s the most abundant metal in the Earth’s crust, yet it entirely escaped our notice until 1825.
You might say it was hidden in plain sight. Aluminium is a highly reactive metal, meaning it readily undergoes chemical reactions with other elements and compounds to form different substances. As a result, nearly all of the naturally occurring aluminium atoms on Earth ended up tucked away in the molecules of more than 270 different minerals, including gemstones like emeralds and rubies. So, while it’s actually 8.2 per cent of the Earth’s crust, making it the most common metal and third-most common element (behind oxygen and silicon), you would never know it’s there without investigating on the chemical level.
The search was on in the mid-1700s, when chemists began experimenting with alum, a class of abundant chemical compounds. Alum compounds, such as potassium aluminium sulphate, were well known, going back at least to the Ancient Greeks and Romans, who used them as an astringent to close wounds and a mordant to bind dye to cloth. Early chemical investigation of alum suggested that the compound included an unknown metal.
The trouble was that 18th-century chemists had no way to separate the mystery element from the rest of the atoms in the compound. In 1825, the Danish chemist Hans Christian 0rsted finally devised a chemical reaction that could extract it, but his process could only yield minuscule amounts at a time, making thorough experimentation difficult. Following up on 0rsted’s discovery, the German chemist Friedrich Wohler developed a more effective process, and by 1845, he had produced enough aluminium to demonstrate its basic properties. However, the method of extraction was still far too troublesome and slow to support wide-scale production.
In 1854, the French chemist Henri Etienne Sainte-Claire Deville refined the process further, reducing the price from USD 1,200 per kilogram to USD 40, which was a huge drop, but still very expensive. That all changed in the 1880s, thanks to two key technological leaps.
In 1886, American chemist Charles Martin Hall and French chemist Paul LT Heroult both independently invented a process for extracting aluminium from aluminium oxide. The Hall-Heroult process relies on electrolysis, a means of breaking down chemical compounds into component elements using an electric current. The basic idea is to conduct electricity from a positive terminal (an anode) to a negative terminal (a cathode) via liquid or molten material. Each terminal attracts and repels charged atoms (ions). The positively charged anode attracts negative ions and repels positive ions, and the cathode vice versa.
Scientists had tried to produce aluminium through electrolysis since the 1800s, but had no luck. Hall and Heroult’s breakthrough was first dissolving aluminium oxide in molten cryolite (sodium aluminium fluoride). Applying an electric current to this material draws the positive aluminium ions to the cathode, which is typically the vat itself, made from iron lined with graphite.
Hot on their heels in 1888, Austrian chemist Karl Josef Bayer found a way to extract aluminium oxide from bauxite, a naturally occurring ore found in abundance in layers Just below the Earth’s surface. Geologists drill core samples in likely areas and, on locating bauxite, they clear the ground above with bulldozers. Australia leads global bauxite mining, producing one-third of the total ore.
Together, the Hall-Heroult cost-effective process and the Bayer process, both still in use, ushered in what could be called the “Aluminium Age’. The metal’s properties made it an instant hit. It’s lightweight – about a third the weight of steel – but still strong. It’s also very ductile, meaning it’s easy to draw into a wire or flatten into a sheet, and it’s malleable, making it relatively simple to bang it into just about any shape.
Add to that exceptional conduction of heat and electricity, and you’ve got an incredibly versatile metal. But aluminium’s greatest trick may be its resistance to corrosion. Like iron.
aluminium is highly reactive to oxygen in the air, but the result of the oxidation reaction is very different. Oxygen and iron react to produce a flaky layer of rust, which falls away, revealing a lower layer of iron, which then oxidises to form yet more rust. In contrast, when aluminium encounters oxygen, the oxidation reaction produces an incredibly hard transparent oxide compound that essentially surrounds the aluminium with a shield that protects it from oxygen and other elements. And best of all, if this protective layer happens to get damaged, it will very quickly reform, reconstructing the shield.
Most aluminium products are actually made from an aluminium alloy – a combination of two metals. The combinations accentuate and amplify certain properties. For example, alloying aluminium with copper improves strength, while an alloy of aluminium and manganese improves resistance to corrosion.
You can turn aluminium into an infinite variety of products, through a number of manufacturing processes. You can cast it into any shape that you want by pouring it into a mould and then letting it cool. You can roll it into malleable sheets, up to a minuscule 0.15 millimetres (0.006 inches) thick. You can forge it to make it super-strong. You can machine it (cutting away material) to produce screws, bolts and other hardware. Finally, you can force it through a die to extrude it into a particular shape, including thin wire.
Aluminium also boasts another major superpower over many other metals: recyclability. Recycling programmes use old aluminium cans to make new ones, at about 30 per cent the cost of making them from scratch. They shred old cans into pieces, melt them in a furnace, form rectangular blocks called ingots, then roll out the ingots into thin sheets from which new cans are cut; believe it or not, this whole process can take just 60 days. Old car parts can undergo a similar process. Thanks to recycling, two-thirds of the aluminium ever produced is still in use today.
Aluminium extraction step-by-step
2. Crusher – A crusher breaks the ore into smaller pieces, in preparation for the Bayer process, which separates an aluminium compound from the bauxite.
3. Digester – The digester mixes the bauxite with caustic soda (sodium hydroxide), which dissolves the aluminium oxide to form liquid sodium aluminate. Clarification, or filtering, enables impurities to be removed from the solution.
4. Precipitation – The addition of aluminium hydroxide causes the sodium aluminate to precipitate into a solid.
5. Heating – The last stage of the Bayer process is heating the solid sodium aluminate. This removes the water, forming aluminium oxide – a fine white powder better known as alumina.
6. Smelting – The first step of the smelting – extracting the pure aluminium – is dissolving the alumina with molten sodium aluminium fluoride, also called cryolite, at 1,000″C (1,832″F).
7. Electrolysis – Running a current through separates the component chemicals in the molten material. The negative cathode terminal attracts positively charged aluminium ions, which are reduced to pure aluminium metal.
Where you can find aluminum?
Rocket fuel – While you might not be surprised to hear that NASA’s space shuttles are made mainly from aluminium, what you may not have realised is that they are also powered by aluminium inside the solid rocket boosters (SRBs). When burned with oxygen, atomized aluminium powder makes for a great fuel. Aluminium powder accounts for about 16 per cent of SRB fuel.
ASM Space Lattice – Aluminium’s high strength-to-weight ratio makes it an excellent dome material. Geodesic dome inventor Buckminster Fuller designed this 76m (250ft)-diameter, 80-ton aluminium structure for the American Society for Metals headquarters in Ohio, USA.
Airstream trailers – The quintessential camping trailer took its design from Twenties aeroplane fuselages. Inventor Wally Byam opted for malleable aluminium which he could shape into a fuel-efficient, aerodynamic form.
Ravensbourne College building – Aluminium’s weather resistance and sculptural flexibility make it a popular material for building facades. Ravensbourne’s building on London’s Greenwich peninsula is covered in 28,000 aluminium tiles.
Top of the Washington Monument – When the monument was approaching completion in 1884, the lead engineer selected the novel, relatively rare aluminium for its 23cm (9in) lightning rod pyramid.
ISS – Built by Boeing, the US Destiny Laboratory module is a major component of the ISS. The 8.5m (28ft) pressurised unit is made from aluminium and represents the heart of the space station. Aluminium forms part of the outer debris shield too, which is tough enough to vaporize small particles of space junk.
Airbus A380 – Aluminium has become the most important material in aerospace history. The world’s largest commercial aircraft is 61 per cent aluminium alloy!
Burj Khalifa hotel – The world’s tallest manmade structure is also the highest installation whose architectural cladding consists of an aluminium and glazed facade. The total weight of the aluminium used is the same as five Airbus A380s, and the surface area of the curtain wall is 132,190m2 (1,422,880ft2).
Morning coffee – Nespresso’s airtight coffee capsules are made of aluminium to keep the product fresh, away from air, light and humidity.
Pots and pans – Much modern cook ware includes aluminium, which boasts excellent thermal conductivity. But possible links to neurodegenerative disease have made it somewhat controversial.
Automobiles – Aluminium keeps this all-electric car lightweight, while still strong and rigid. Each car begins life as a 9,072kg (20,000lb) aluminium coil, which is stamped into sections.
Computers – Many of Apple’s devices are made of anodized aluminium, which not only polishes and toughens a product, but also provides a way of adding colour via oxidation, as seen in multicolored iPods.
Kitchen foil – As a natural barrier to light, oxygen, moisture and just about anything airborne, including bacteria, flexible aluminium sheets are great food protectors.
Drinks cans – On top of being light and cheap, the king of aluminium products is 100 per cent recyclable. 113,204 cans are recycled every minute.