Process

Hall–Héroult process

The electrolytic reduction of alumina that made aluminium a commodity rather than a precious metal.

The Hall–Héroult process dissolves alumina in molten cryolite at around 950 °C and passes a large electric current through it, depositing aluminium metal at the cathode. Cryolite is the key: it lowers alumina's melting point from over 2,000 °C to something workable.

Charles Hall and Paul Héroult arrived at it independently in 1886, both aged 22. Within a few years the price of aluminium had fallen by more than 95 percent.

Economic significance

The process is extremely electricity-intensive, and that has shaped the geography of the industry ever since: aluminium smelters are built where power is cheap, often beside hydroelectric schemes, rather than near the bauxite. Aluminium is sometimes described as solid electricity, which overstates the case but not by much.

Uses

This is how essentially all primary aluminium is made — every tonne not recovered from scrap. Alumina is dissolved in molten cryolite at around nine hundred and sixty degrees and electrolysed, aluminium collecting at the cathode while the carbon anodes are consumed producing carbon dioxide.

Because the process is defined by its electricity consumption, smelters are sited where power is cheap and reliable rather than near the ore, and aluminium is sometimes described as solidified electricity. That also makes recycling extraordinarily attractive: remelting scrap uses a small fraction of the energy of primary production, and the metal is not degraded by it.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

Aluminium oxide — compound · dissolved in molten cryolite and electrolysed

produces

Aluminium — element · collects at the cathode while the carbon anodes are consumed

succeeded

Molten salt electrolysis — process · the Hall–Héroult process is molten salt electrolysis specialised to alumina dissolved in cryolite

Sources

Material World
Our own writing

Questions this page answers

What is produced from Hall–Héroult process?

Where it comes from, and what it becomes

Follow Hall–Héroult process back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is sourced from (partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt) → Peridotite → is composed of (the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition) → Olivine
Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is produced by (crystallised from the liquor and calcined, ready for the smelter) → Bayer process → takes as input (the ore, dissolved selectively so the iron and silica are left behind) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is composed of (an early-crystallising constituent of basaltic magma) → Olivine
Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is composed of (an early-crystallising constituent of basaltic magma) → Olivine
Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is composed of (calcium-rich plagioclase is a defining constituent of basalt) → Plagioclase
Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is composed of (carried up as xenocrysts in alkali basalt and recovered from the gravels the basalt weathers into, rather than mined from the rock itself) → Sapphire
Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is composed of (lining vugs and geodes in weathered basalt flows, which is where nearly all of the world's amethyst comes from — the cavity is a gas bubble the lava left behind) → Amethyst

These are the most distinct paths back. Hall–Héroult process can be traced through others besides.

Downstream — what it becomes

Hall–Héroult process → produces (collects at the cathode while the carbon anodes are consumed) → Aluminium → is an input to (the cheapest reducing metal and the commonest, and the aluminium in thermite) → Metallothermic reduction → produces (magnesium reducing boron oxide, which gives an impure amorphous boron — the crystalline element needs a different route entirely) → Boron → is a component of (about one per cent, and structurally essential — the compound is Nd₂Fe₁₄B, and without the boron it does not form) → Neodymium magnet → is used as (the strongest in commercial use, and the reason a motor, a hard drive and an earbud can be small) → Permanent magnets
Hall–Héroult process → produces (collects at the cathode while the carbon anodes are consumed) → Aluminium → is a component of (the base, and what the alloy exists to make useful) → Duralumin → is used as (aircraft structure, which it was developed for and dominated for half a century) → Structural engineering
Hall–Héroult process → produces (collects at the cathode while the carbon anodes are consumed) → Aluminium → is used as → Electrical conduction
Hall–Héroult process → produces (collects at the cathode while the carbon anodes are consumed) → Aluminium → is used as (in structures where the weight of the structure itself is the load that matters — aircraft, superstructures, long-span roofs) → Structural engineering
Hall–Héroult process → produces (collects at the cathode while the carbon anodes are consumed) → Aluminium → is used as (the drinks can and the foil laminate layer, where an impermeable barrier a few microns thick is worth the metal) → Packaging
Hall–Héroult process → produces (collects at the cathode while the carbon anodes are consumed) → Aluminium → is an input to (the cheapest reducing metal and the commonest, and the aluminium in thermite) → Metallothermic reduction → produces (magnesium reducing beryllium fluoride, and the reason beryllium is expensive before anyone accounts for how carefully it has to be handled) → Beryllium → is used as (copper-beryllium alloys, for non-sparking and spring applications) → Alloying

These are the most distinct paths onward. Hall–Héroult process ends up in others besides.