Aluminum hydroxide

 

Aluminum hydroxide

Chemical formula	Al(OH)3
OTP appearance	white amorphous solid
Molar Mass(g/mol)	78
Density(g/cc)	2.42
Melting Point(°C)	300
Boiling Point(°C)	decomp
Solubility in water(g/L)	10-5
NFPA 704	0 1 0

Uses

Primary

• Key intermediate in the production of aluminum

Natural occurrence

• Aluminum hydroxide does occur naturally as the mineral gibbsite

Production

Extraction

The mineral gibbsite is primarily aluminum hydroxide

Synthesis

From bauxite

Bayer

1. Combine bauxite with sodium hydroxide and heat. The silicon dioxide and aluminum oxide will dissolve, leaving other impurities as slag

   Al₂O₃ + 2 NaOH

   {

   →

   170-180°C}

   2 NaAlO₂(aq) + H₂O

   SiO₂ + 2 NaOH

   {

   →

   170-180°C}

   Na₂SiO₃(aq) + H₂O

2. Filter the solution
3. Discard the residue, containing mostly
4. Cool the filtrate, precipitating aluminum hydroxide.

   NaAl(OH)₄ → NaOH + Al(OH)₃(s)

5. Filter the solution
6. Discard the filtrate, largely a solution of sodium silicate and sodium aluminum silicate: Na₂SiO₃ + Na₂Al₂Si₁₄O₃₂•3H₂O.
7. Residue is aluminum hydroxide.

Deville

1. Calcine bauxite, sodium carbonate and carbon at 1200°C, producing sodium aluminate and other materials

   Al₂O₃ + Na₂CO₃

   {

   →

   1200°C}

   2 NaAlO₂ + CO₂

2. Cool
3. Dissolve the calcine in a sodium hydroxide solution, which dissolves only the sodium aluminate, leaving the other materials as solids
4. Filter
5. Discard residue
6. Bubble carbon dioxide through the solution, precipitating aluminum hydroxide and regenerating the sodium carbonate.

   2 NaAlO₂(aq) + CO₂(g) → 2 Al(OH)₃(s) + Na₂CO₃(aq) + H₂O(l)

7. Filter
8. Discard filtrate
9. Residue is aluminum hydroxide.