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Material World
Compound · Al2O3

Aluminium oxide

Industrial alumina — the refined intermediate between bauxite and aluminium metal, and a major ceramic in its own right.

Abrasive Refractory Insulator

Aluminium oxide is the same composition as the mineral corundum, but they are kept as separate entities here for a reason that is practical rather than pedantic: industrial alumina is a refined powder produced from bauxite by the Bayer process, and its sources, grades, uses and supply chain have essentially nothing in common with a gem-quality corundum crystal.

Almost all of it is consumed making aluminium metal. The remainder is a significant ceramic, abrasive and refractory.

Processing

Extracted from bauxite by the Bayer process: the ore is digested in hot sodium hydroxide, which dissolves the aluminium hydroxides and leaves iron oxides and silicates behind as the residue known as red mud. Aluminium hydroxide is precipitated from the cooled liquor and calcined to the oxide.

Uses

Most alumina produced is never used as alumina: it is fed to the Hall–Héroult cells and reduced to aluminium metal, and that single destination accounts for the great majority of world production.

The remainder — specialty or non-metallurgical alumina — is a major engineering ceramic in its own right. It is the substrate for electronic circuits, the insulator in spark plugs, the abrasive in sandpaper and grinding wheels, and the wear-resistant lining of chutes and pipes. High-purity alumina makes sapphire windows, LED substrates and the bearing surfaces of hip implants, and activated alumina is a desiccant and catalyst support.

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.

has the same composition as

  • Corundum — mineral · identical composition, entirely different sources and supply chains

is sourced from

  • Bauxite — rock · dissolved out with hot caustic soda in the Bayer process

is an input to

is used as

  • Abrasive — application
  • Refractory lining — application · the standard high-alumina refractory where conditions are severe

is a component of

  • Technical ceramic — material · alumina is the most-used engineering ceramic
  • Refractory brick — material · the general-purpose workhorse — raising alumina content above fireclay's raises the service temperature and the price together

is produced by

  • Bayer process — process · crystallised from the liquor and calcined, ready for the smelter

is an alternative to

  • Zirconia — compound · the two workhorse technical ceramics. Alumina is harder, cheaper and more thermally conductive; zirconia is several times tougher, because a crack triggers a phase change that closes it

contains

Sources

  • Material World
    Our own writing
  • PubChem
    US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Aluminium oxide 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

  • 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 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
  • 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
  • Aluminium oxide → is produced by (crystallised from the liquor and calcined, ready for the smelter) → Bayer process → takes as input (hot concentrated caustic soda dissolves the aluminium hydroxides) → Sodium hydroxide → is sourced from (electrolysis of brine, which produces chlorine in fixed proportion) → Halite
  • 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
  • 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
  • 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

These are the most distinct paths back. Aluminium oxide can be traced through others besides.

Downstream — what it becomes

  • Aluminium oxide → is an input to (dissolved in molten cryolite and electrolysed) → 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
  • Aluminium oxide → is a component of (the general-purpose workhorse — raising alumina content above fireclay's raises the service temperature and the price together) → Refractory brick → is an input to (or installed unfired as a monolithic castable and cured by the furnace's own first heat) → Firing → produces (as sintering, from refined powders rather than clay) → Technical ceramic → is used in (the honeycomb monolith the metals are dispersed across, which supplies the surface area and survives the thermal cycling) → Catalytic converter → is used as (the largest catalytic application by metal value there is) → Catalysis
  • Aluminium oxide → is a component of (alumina is the most-used engineering ceramic) → Technical ceramic → is used in (the honeycomb monolith the metals are dispersed across, which supplies the surface area and survives the thermal cycling) → Catalytic converter → is used as (the largest catalytic application by metal value there is) → Catalysis
  • Aluminium oxide → is used as → Abrasive
  • Aluminium oxide → is used as (the standard high-alumina refractory where conditions are severe) → Refractory lining
  • Aluminium oxide → is a component of (alumina is the most-used engineering ceramic) → Technical ceramic → is used as (alumina and silicon carbide are the dominant manufactured abrasives) → Abrasive

These are the most distinct paths onward. Aluminium oxide ends up in others besides.