|OTP appearance||silver-grey solid|
|Coefficient of Thermal Expansion(×10-6 °C-1)||23.6|
- Structural material
- Aluminothermic reduction of other metals
- Alloyed with barium (BaAl4) for gas "getter"
- Elemental aluminum does not occur naturally.
- Aluminum hydroxide occurs naturally in gibbsite and bauxite
While aluminum oxide is plentiful on earth, the aluminum-oxygen bonds are extremely strong, thus it is very difficult to reduce to pure aluminum by simple means.
- AKA Hall-Heroult process
Direct electrolysis of aluminum oxide is impractical because the melting temperature of aluminum oxide (2072°C) is beyond the range of normal furnaces. Sodium aluminum hexafluoride, however, has a melting point of 1012°C and aluminum oxide will dissolve in molten cryolite. The mixture has a combined melting point of about 1000°C. Electrolysis then cleaves the aluminum from the oxygen. The oxygen combines with the graphite of the anode producing carbon dioxide which exits the system as a gas, and leaves the molten cryolite to absorb additional bauxite.
- Melt a fluorinating agent (below) at 1012°C
- Until all the aluminum oxide is converted
Production of fluorination agents
sodium aluminum hexafluoride
This material, mineralized, is called cryolite. It does occur naturally, but is somewhat rare. It has a melting point near 950°C. It is mainly produced by combining sodium hydroxide and aluminum oxide
- 6 NaOH + Al2O3 + 12 HF → 2 Na3AlF6 + 9 H2O
This material, mineralized, is called rosenbergite has a melting point near 1300°C.
- 2 Al2O3 + 6 HF → 2 AlF3 + 3 H2O
In practice, it is much easier to do a "one pot" mixture of fluoride minerals, sulfuric acid, and aluminum oxide
We start with with sulfuric acid and fluorine-containing silica minerals, producing hexafluorosilic acid:
- Fluorine-bearing minerals and sulfuric acid, generating F2or HF which combine with silicon dioxide to give rise to H2SiF6
- Hexafluorosilic acid readily converts to aluminum fluoride in the presence of aluminum oxide
- H2SiF6 + Al2O3 → 2 AlF3 + SiO2 + H2O