Aluminium oxide
Aluminium oxide is the stable form of aluminium in air. Its α mineral (corundum) is one of the hardest and most refractory materials known; its red variety is ruby, its blue variety sapphire.
Physical properties
Structure
Detailed description
Al₂O₃ exists as several polymorphs, the most stable being the α form (corundum). In this phase, each aluminium atom is coordinated by six oxygens in an octahedron; octahedra share edges and vertices in a close-packed hexagonal arrangement of the oxygen sublattice. This architecture accounts for its exceptional hardness (9 Mohs), its extreme melting point and its chemical inertness.
In nature: corundum in metamorphic rocks, ruby and sapphire (with traces of Cr³⁺ or Ti⁴⁺/Fe²⁺), emery. A minor but ubiquitous component of bauxites.
Industrially extracted from bauxite via the Bayer process: high-pressure digestion in sodium hydroxide, precipitation of aluminium hydroxide, then calcination at ~1100 °C to obtain α-alumina. Still the dominant worldwide process for producing metallic aluminium.
Uses and applications
- Main precursor of metallic aluminium (Hall-Héroult electrolysis)
- Technical ceramics, abrasives (powder), emery papers
- Substrate and encapsulation in microelectronics
- Catalyst support (reforming, hydrotreating)
- Orthopaedic and dental implants (hip prosthesis heads)
- Gemmology: synthetic rubies and sapphires (Verneuil process)
Constituent elements
References
Related processes
Industrial processes involving this compound.
- MetallurgyOutput
Bayer process
Refining of pure alumina (Al₂O₃) from bauxite ore by selective dissolution in hot concentrated caustic soda. The mandatory step upstream of Hall-Héroult electrolysis — no Bayer, no aluminium metal.
- ElectrolysisInput
Hall-Héroult process
Electrolysis at 950-980 °C of alumina dissolved in molten cryolite (Na₃AlF₆) to produce aluminium metal. Universal since 1886 — it alone consumes ~3 % of world electricity.