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Electrolytic Extraction of Aluminium from Bauxite and Alumina

electrolytic extraction of aluminium
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In examining the electrolytic extraction of aluminium from bauxite and alumina, we must first briefly explain the element called aluminium and the minerals and ores that contain this very important metallic element. Aluminum is the third most plentiful element in the earth’s crust, being found abundantly as trioxosilicates(IV) in rocks and clays. The main source of aluminum is the mineral bauxite, Al2O3.2H2O, Other important minerals include kaolin, Al2O3 .2SiO2.H2O, cryolite, Na3AIF6, corundum, AI2O3, and mica, K2O.AI2O3.6SiO2.

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             Extraction of Aluminium from Bauxite  

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Although aluminum occurs so abundantly in rocks and clays, no economical process has yet been devised for extracting the metal from them, owing to their high silica content. Instead, aluminum is extraction from bauxite by electrolysis. The extraction proceeds in two stages. In the first stage, the crude bauxite is purified to yield pure anhydrous aluminum oxide, which is then electrolyzed in the second stage.

 

 

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              Purification of Bauxite       

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The bauxite is first heated with caustic soda solution under pressure to form soluble aluminate (III), NaAl(OH)4.

4Al2O3(s) + 2NaOH(aq) + 3H2O → 2NaAl(OH)4(aq)

Since the impurities in the ore, notably iron(III) oxide and the various trioxosilicates(IV), do not react with or dissolve in the alkali, they can be filtered off as a sludge. The filtrate which contains the aluminate (III) is then seeded with aluminum hydroxide crystals to induce the precipitation of aluminum hydroxide.

NaAl(OH)4(aq) → Al(OH)3(s) + NaOH(aq)

 

The aluminum hydroxide precipitate is then filtered off, washed, dried and heated strongly to yield pure aluminium oxide or alumina, A12O3, while the sodium hydroxide is concentrated and used again.

2A1(OH)3(s) → Al2O3(s) + 3H2O(1)

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In the electrolysis of alumina, the electrolytic cell is an iron container lined with graphite. The graphite lining serves as the cathode while the anodes consist of graphite rods dipping into the electrolyte which is a solution of pure alumina in molten cryolite, Na2A1F6. The electrolyte is maintained at about 950C by the heating effect of large current. This makes the process very expensive

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During the electrolysis of alumina, oxygen is given off at the anodes. As a result, the anodes are slowly burnt away as carbon(IV) oxide, and must be replaced continuously. This adds to the cost of the whole process. Molten aluminum collects at the bottom of the cell and is tapped off at intervals.

 

 

 

 

 

        The Chemistry of the Electrolysis of Alumina

Alumina consists of aluminum and oxygen ions. At the cathode the aluminum ions gain three electrons each as the cathode to become deposited as metallic aluminum.

                   3A1 + 3e → A1

At the anode the oxygen ions donate two electrons each to from atomic oxygen, which then pair off to from gaseous molecules.  

Overall electrolytic reaction can be seen below:-

4A13+(1) + 602-(1) → 4A1(s) + 3O2(g).

Aluminum is the third most plentiful element in the earth’s crust, being found abundantly as trioxosilicates (IV) in rocks and clays. The main source of aluminum is the extracted from bauxite by electrolysis. The extraction proceeds in two stages. In the first stage, the crude bauxite is purified to yield pure anhydrous aluminum oxide, which is then electrolyzed in the second stage.

 

           Electrolysis of Alumina 

The electrolytic cell is an iron container lined with graphite. The graphite lining serves as the cathode while the anodes consist of graphite rods dipping into the electrolyte which is a solution of pure alumina in molten cryolite, Na3A1F6. The electrolyte is maintained at about 950C by the heating effect of a large current. This makes the process very expensive.

During the electrolysis of alumina, oxygen is given off at the anodes. As a result, the anodes are slowly burnt away as carbon(IV) oxide, and must be replaced continuously. This adds to the cost of the whole process. Molten aluminum collects at the bottom of the cell and is tapped off at intervals.

 

 

 

            Chemistry of the Electrolysis of Alumina

In the electrolytic extraction of aluminium from alumina, we must understand the ionic composition of alumina in the electrolytic reaction process. Alumina consists of two ions. These ions are:- Aluminum and Oxygen ions.

Reaction at the Cathode: The aluminum ions gain three electrons each at the cathode to become deposited as metallic aluminum.

          413++ 3e → A1

Reaction at the Anode: The oxygen ions donate two electrons each to from atomic oxygen, which then pair off to form gaseous molecules.

          O2- → O + 2e

          O + O → O2

                         

       The Overall Electrolytic Reaction

                             4A13+(1) +6O2- (1) → 4A1(s) + 3O2(g)

   

  

       

   


About the Author

Tony Onwujiariri
Tony is an Avid Tech enthusiast that loves Scientific Inventions and Tech Products. He blogs Passionately on Science and Technology related niches and spends most of his time on Research in Content Management and SEO. Tony loves Sugar and has been in love with Don Williams since he was a toddler on Diapers.

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