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 1500oC 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 2000oC. Any aluminium formed will vapourise at this temperature as boiling point of aluminum is only 1800oC. Hence alumina mixed with cryolite  Na3AlF6 and CaF2 with a fusion temperature of 900oC 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.

Extraction of aluminum

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

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