The Extraction of Aluminium from Bauxite

Sources:

1. http://entrancechemistry.blogspot.sg/2011/05/extraction-of-aluminium-from-bauxite.html
2. http://www.chemguide.co.uk/inorganic/extraction/aluminium.html
3. http://sam.davyson.com/as/physics/aluminium/siteus/extraction.html
4. http://en.wikipedia.org/wiki/Bauxite
5. http://en.wikipedia.org/wiki/Bayer_process
6. http://en.wikipedia.org/wiki/Hall%E2%80%93H%C3%A9roult_process
7. http://en.wikipedia.org/wiki/Red_mud

About Bauxite: 

Bauxite is an aluminium ore that contains a mixture of aluminium oxides( Al(OH)₃, γ-AlO(OH), and  α-AlO(OH etc.) as well as iron oxides,silicon dioxide and titanium oxide.

As aluminium is high up in the reactivity series, a pure sample of aluminium cannot be obtained by reduction with carbon. Furthermore, aluminium forms a stable compound with oxygen and cannot be displaced easily.

Therefore Aluminium has to be purified with two different methods, the Bayer Process or Electrolysis.

Bayer Process:

Industrial set-up for the Bayer Process.

During the Bayer Process, crushed bauxite is broken down or digested by a solution of hot concentrated NaOH solution, according to this reaction: Al₂O₃ + 2 NaOH + 3 H₂O → 2 NaAl(OH)₄

_{In the digester Sodium tetryhydroxoaluminate will be dissolved as solution while the other impurities in the ore will not react and remain insoluble.} 

Next All of these solids are filtered out. The residue is known as 'red mud'.

Then sodium tetryhydroxoaluminate is cooled in the precipitator. Crystallisation takes place, producing a precipitate of Aluminium hydroxide, Al(OH)3, or alumina  and sodium hydroxide.

NaAl(OH)₄ → Al(OH)₃ + NaOH

Aluminium Hydroxide is then purified in the Rotary Kiln by heating to temperatures between 1000 to 1200 degrees celsius. Aluminium hydroxide is then decomposed into aluminium oxide and water.

2 Al(OH)₃ → Al₂O₃ + 3 H₂O and is collected for electrolysis.

Electrolysis or the Hall-Heroult process:

simplified Electrolysis cell.

Aluminium Oxide is first dissolved in a mixture of molten cryolite, Na3AlF6, and Aluminium fluoride to lower the melting point of aluminium oxide in the set-up shown above. A negative current will be passed through the carbon lining of the 

 set-up while a positive current will be passed through the carbon anodes. This electrolyses the aluminium oxide, cryolite 

 and the aluminium fluoride,  causing liquid aluminium to be formed in the cathode. Meanwhile oxygen gas reacts with 

 carbon from the anode to form carbon dioxide. 

 Liquid Aluminium is taken out with a siphon and is transported elsewhere to be cast into aluminium ingots.

Environmental impact:

Red Mud expelled in the early part of the experiment is accumulated in small containments or disposed of in the lakes and rivers. As the mixture is a strong alkali, it would adversely affect the pH of the soil, leading to complications in its disposal.

We believe that as red mud is a mixture of the waste products from bauxite, such as haematite and silicon dioxide, it can be taken away for iron to be produced instead of dumped.

Research by: Wei Liang and Radhika

Write up by: Elizabeth