Aluminum

 

Aluminum

Chemical formula	Al
Atomic Number	13
OTP appearance	silver-grey solid
Molar Mass(g/mol)	26.981
Density(g/cc)	2.7
Melting Point(°C)	660.32
Boiling Point(°C)	2470
Coefficient of Thermal Expansion(×10-6 °C-1)	23.6
NFPA 704	1 0 1

Uses

Primary

• Structural material
• Aluminothermic reduction of other metals

Secondary

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

Natural Occurrence

• Elemental aluminum does not occur naturally.
• Aluminum hydroxide occurs naturally in gibbsite and bauxite

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

      {AlF₃ 

      →

      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