Extraction of aluminium

Aluminum is the most abundant metal of earth's crust (8.3%) and exist as oxide and fluoride ores. The metal is extracted from bauxite ore (Al₂O₃.2H₂O). The extraction process involve three stages or steps

step 1 Purification of bauxite
step 2 Electrolytic reduction of Al₂O₃
step 3 Electrolytic purification of aluminium

1. Purification of bauxite

The bauxite ore contain iron oxide and silicon dioxide as impurities. It is purified by leaching method using concentrated sodium hydroxide solution in which bauxite dissolve forming sodium meta aluminate. On agitating this solution with freshly precipitated Al(OH)₃ for several hours Al(OH)₃ is precipitated.

NaAlO₂  +  2H₂O  ------------>  NaOH  +  Al(OH)₃

 The precipitate is filtered and dried and NaOH is concentrated and used again for leaching. The aluminium hydroxide precipitate is calcined at 1500^oC to obtain pure alumina.

2. Electrolytic reduction of pure alumina

Pure Al₂O₃ is a bad conductor of electricity and has a very high fusion temperature of 2000^oC. Any aluminium formed will vapourise at this temperature as boiling point of aluminum is only 1800^oC. Hence alumina mixed with cryolite  Na₃AlF₆ and CaF₂ with a fusion temperature of 900^oC is used as the electrolyte. The electrolysis is carried out in an iron tank lined inside with carbon acting as cathode and graphite rods dipped into the electrolyte as anode. Molten aluminium liberated at the cathode gets collected at the bottom of the tank and oxygen liberated at the anode burns away the anode as CO₂. The overall reaction may be represented as

2 Al₂O₃  ------------>  4Al (at cathode) +  3O₂ (at anode)

 C(cathode)  + O₂  --------------->  CO₂

 The anode is replaced from time to time and by adding alumina into the cell and tapping out molten aluminium from the tank the process can be continuously carried out.

3. Refining of aluminium

 In Hoope's electrolytic method of refining, three liquid layers with differing densities are used. The bottom layer is molten impure aluminium into which Cu and Si has been added to increase density. This molten layer taken is an iron tank lined with carbon is the anode. The electrolyte is a middle layer containing molten mixture of AlF₃, BaF₂ and NaF. The cathode is the upper layer containing pure molten aluminium. On electrolysis aluminium dissolve from the anode and deposit at the cathode.


 For more visit Extraction of aluminium from bauxite ore