1. Purification of bauxite by Baeyer's process
In the Baeyer's process, the bauxite ore is heated with concentrated NaOH solution under pressure (Aluminum is purified by leaching method). The alumina dissolves as sodium meta aluminate. The other materials present in the ore are left as insoluble part. This solution is filtered in ore are left as insoluble part. This solution is filtered off. From the solution Al(OH)3 is precipitated by adding freshly prepared Al(OH)3 to the cold dilute solution and agitating.
Al2O3 + 2NaOH -------> 2NaAlO2 + H2O
NaAlO2 + 2h2o -------> NaOH + Al(OH)3
The precipitated Al(OH)3 is dried and ignited to get pure alumina.
2Al(OH)3 --------> Al2O3 + 3H2O
2. Electrolysis of alumina
The alumina is dissolved in a mixture of molten cryolite and fluorspar which lowers the melting point. It is then electrolysed in a rectangular steel tank with carbon lining, which serves as cathode. Anode is set of thick carbon rods suspended from the top into the fused Al2O3. The temperature is maintained between 1200 and 1310k. Oxygen is evolved at the anode.
Al2O3 -------> 2Al3+ + 3O2-
At cathode: 2Al3+ + 6 electron -------> 2Al
At cathode: 3O2- --------> 1.5 O2 + 6 electron
Aluminium formed at the cathode gets collected at the bottom of the electrolytic cell from where it is removed periodically.
The metal obtained by this method is about 99% pure. Further purification is carried out by Hoop's electrolytic method. The electrolytic refining of aluminum is carried out in Hoop's cell. The cell consist of iron tank having a lining of carbon. It has three layers of molten liquids with different densities. Molten impure aluminum in the bottom layer which along with the carbon lining acts as the cathode. The middle layer is a mixture of molten fluorides of sodium, barium and aluminum in the molten fluorides of sodium, barium and aluminum in the molten state. This act as the electrolyte. Top layer consist of molten pure aluminum in which a number of carbon electrodes are suspended. These carbon electrodes act as the cathode.
When electric current is passed, the aluminium ions from the middle layer move to the top layer and are discharged at the cathode as pure aluminium. At the same time, an equivalent amount of aluminum from the bottom layer migrates to the middle layer leaving behind the impurities. The pure aluminum is removed from the tapping hole from time to time.
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