Aluminum

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Aluminum
Chemical formula Al
Atomic Number 13 
OTP appearance silver-grey solid 
Molar Mass(g/mol) 26.981 
Melting Point(°C) 660.32 
Boiling Point(°C) 2470 
Density(g/cc) 2.7 
Coefficient of Thermal Expansion(×10-6 °C-1) 23.6
NFPA 704
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Uses

Primary

  • Structural material
  • Aluminothermic reduction of other metals

Secondary

  • Thermite
  • Alloyed with barium (BaAl4) for gas "getter"

Natural Occurrence

Hazards

Character

Production

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.

Synthesis

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.

  1. Melt a fluorinating agent (below) at 1012°C
  2. Repeat
    1. Saturate with pure aluminum oxide
    2. Electrolyze at 3-5V using a carbon electrodes producing liquid aluminum at the cathode and carbon dioxide at the anode:
      2 Al2O3 + 3 C
      {AlF
      1012°C 3-5V}
      4 Al + 3 CO2
    3. Tap off the aluminum from the bottom of the cell (it is denser than the fluoride mixture)
  3. 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

aluminum fluoride

This material, mineralized, is called rosenbergite has a melting point near 1300°C.

theory

Combining hydrofluoric acid and aluminum oxide will produce aluminum trifluoride:

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

practice

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
Fluorite: CaF2 + H2SO4 CaSO4 + 2 HF; HF + SiO2 H6SiO2F6
Fluorapatite: Ca5(PO4)3F + 5 H2SO4 5 CaSO4 + 3 H3PO4 + HF; HF + SiO2 H6SiO2F6
Hexafluorosilic acid readily converts to aluminum fluoride in the presence of aluminum oxide
H2SiF6 + Al2O3 2 AlF3 + SiO2 + H2O

Purification

Testing

Storage

Disposal

See Also

References