Aluminium

Aluminium is the most common metal in the Earth's crust, but it is locked in oxides that fire and charcoal cannot break. Since 1886 it has been made by refining bauxite to alumina, dissolving the alumina in molten cryolite and splitting it with a large direct current.

What it is

Aluminium is a light, silvery metal, about a third as dense as steel. It makes up roughly 8% of the Earth’s crust by weight, more than any other metal, yet it is almost never found as metal. It is bound to oxygen in clays, feldspars and most common rocks, and that bond is too strong for the charcoal fire that frees iron or copper.

The ore is bauxite, a reddish rock left by tropical weathering. A refinery turns it into alumina, a white oxide powder, and a smelter splits the alumina with electricity in the Hall–Héroult cell shown on the plate, still the only industrial way to make new aluminium.

How it works

From ore to metal: two to three tonnes of bauxite give a tonne of alumina, and two tonnes of alumina a tonne of aluminium.

From bauxite to alumina. In the Bayer process, crushed bauxite is digested in hot caustic soda under pressure, at 140 to 280 °C depending on the ore. The aluminium minerals dissolve; iron oxides and sand settle out as a red residue. Cooled and seeded with crystals from an earlier batch, the liquor grows aluminium hydroxide, which is roasted at up to about 1,100 °C into alumina.

From alumina to metal. Alumina melts above 2,000 °C, but a bath of molten cryolite, a fluoride of sodium and aluminium, with added aluminium fluoride, dissolves it at about 950 to 980 °C. The cell is a steel box lined with carbon blocks, which form the negative electrode. Carbon anodes hang into the bath from rods clamped to a heavy busbar.

Direct current flows from the anodes through the bath to the lining and splits the dissolved alumina. Aluminium, denser than the bath, gathers as a liquid at the bottom. Oxygen goes to the anodes and burns their carbon into carbon dioxide, about 0.4 to 0.5 kg of carbon per kilogram of metal, so worn anodes are replaced in turn. A crust of frozen bath and alumina keeps the heat in. Alumina is fed through it, and the metal is siphoned off at intervals.

The current also keeps the bath molten. Pots are wired in series, each carrying well over a hundred thousand amperes at four to five volts. Every atom of aluminium takes three electrons, so the cell uses about 13 to 15 kilowatt-hours per kilogram, less than half of which goes into the reaction itself. Power is the largest cost, so smelters stand where it is cheap, beside the dams of Canada, Norway or Iceland or near large power stations, with transformers and rectifiers to supply direct current.

In air an oxide film a few nanometres thick forms at once on the metal; it stops corrosion and re-forms when scratched. A few per cent of copper, magnesium, silicon or zinc make it strong enough for aircraft. Recycling only has to melt scrap: 8.3 gigajoules per tonne against 186 for new metal in 2019, a saving of about 95%.

How to build

Do not make or melt aluminium at home. The bath gives off toxic fluoride fumes and runs near 1,000 °C, and a drop of water trapped under molten metal flashes into steam and throws the metal out; melted cans add fumes from paints and linings. A workshop can sort the metal, work it cold, and show electrolysis at a few volts.

The magnet clings to the steel tin and ignores the aluminium can.

Sort it. A magnet attracts steel but not aluminium. To tell it from other non-magnetic metals, weigh a piece and measure the water it displaces: aluminium comes out near 2.7 grams per cubic centimetre, copper and brass above 8. Rinse off food and keep painted or coated pieces apart.

Bend and drill. Soft grades bend cold in a vice between hardwood blocks; hard alloys crack on a sharp fold, so bend them round a rounded block. To drill, centre-punch, clamp the sheet onto wood and use a sharp bit with a drop of oil: sheet held by hand can catch and spin.

Join it cold. Aluminium is hard to weld: its oxide melts above 2,000 °C, the metal at 660 °C, and it does not glow before it slumps. Use rivets, bolts or a structural adhesive, with aluminium fasteners where possible. In the wet, and above all near the sea, aluminium corrodes against copper, steel and even stainless steel; separate them with plastic washers or paint.

Show the principle. Put two pencil leads in a glass of water with a teaspoon of washing soda and connect them to a 9-volt battery. Hydrogen bubbles at the negative lead, oxygen at the positive: the current splits water as the smelter splits alumina.

Safety. No molten metal and no molten-salt electrolysis outside an industrial plant. Cut sheet is sharp: wear gloves to handle it and deburr every cut, but take the gloves off before using a drill, and protect your eyes. Washing soda irritates skin and eyes, so wear goggles. Never use table salt in the demonstration, because it gives off chlorine; keep the hydrogen away from flames, and use only a small battery, never the mains.

History and evolution

Hans Christian Ørsted obtained a tiny, impure sample from aluminium chloride in 1825. Friedrich Wöhler made a powder in 1827 and small beads in 1845.

Aluminium tableware of the Second Empire, when the metal was worth as much as silver.

In France, Henri Sainte-Claire Deville reduced the chloride with sodium and showed an ingot at the Paris exhibition of 1855. Napoleon III backed him and is said to have kept aluminium cutlery for his most important guests. In December 1884 a 100-ounce aluminium pyramid crowned the Washington Monument, at $1.10 an ounce, about the price of silver.

In 1886 two men aged 22 found the same answer. On 23 February in Oberlin, Ohio, Charles Martin Hall, with home-made batteries, obtained globules of aluminium from alumina dissolved in cryolite. Paul Héroult, who had spent his inheritance on a dynamo, filed a French patent on 23 April. Hall’s Pittsburgh company, later Alcoa, began production in 1888; Héroult’s process went to work at Neuhausen, by the Rhine Falls. Karl Josef Bayer filed his seeded precipitation of alumina in 1887 and his pressure digestion of bauxite in 1892; together they made pure alumina cheap.

A pound cost about $4.86 in 1888 and under a dollar in the early 1890s. The Wright brothers’ 1903 engine had an aluminium crankcase; heat-treated alloys such as duralumin, sold from about 1909, went into airships and aircraft. Smelters made about 74 million tonnes of new metal in 2025.