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50.7 EXTRACTION OF ALUMINIUM

Aluminiumis one of the elements of Group 13 of the periodic table. Although aluminium is a reactive metal according to the electrochemical series, it is rendered unreactive due to the formation of a oxide film on its surface.

OCCURRENCE

Aluminium is quite reactive element and hence does not occur in nature in the native form. Aluminium is the most abundant metal and the third most abundant element in the earth’s crust. The important ores of aluminium are:

(i) Bauxite, Al2O3.2H2O

(ii) Cryolite, Na3 AlF6

(iii) Feldspar, KAlSiO3O8

(iv) Mica, KAISiO10 (OH)2

 EXTRACTION (From bauxite)

Aluminium metal is extracted from bauxite is a two stage process.

Stage 1. Involves extraction of alumina (AIP3) from bauxite.

Stage 2. Involves extraction of pure aluminium from (Al2O3) by its electrolysis in molten cryolite [Na3AlF6J]

1. Purification of Bauxite. Bauxite contains SiO2, iron oxides and titanium (IV) oxide as impurities.Bauxite is digested with a hot concentrated (45%) solution of sodium hydroxide at about 473-523 K and 35-36 bar pressure.

Alumina dissolves to form sodium tetrahydroxoaluminate (III), Na[Al(OH)4J leaving behind iron oxide and TiO2.

AlO3 + 2NaOH + 3H2O   →        2Na[Al(OH)4

Silica (SiO2) also dissolves in sodium hydroxide to form soluble sodium trioxosilicate(IV), Na2SiO3.

SiO2 + 2NaOH                 →        2Na2SiO3 + H2O

The impurities are filtered out and CO2 is bubbled. Through the filtrate containing sodium tetrahydroxoaluminate(IlI) and sodium trioxosilicate(IV). At this stage the solution may be seeded with freshly precipitated aluminium hydroxide. Aluminium hydroxide precipitates leaving behind

sodium trioxosilicate(IV) in solution. This is filtered and the precipitate of AI(OH)3 is heated at 1473 K to obtain pure alumina.

Na[AI(OH)4 + CO2       →        NaHCO3 + Al(OH)3 ↓

1473 K

2Al(OH)3                   →        Al2O3 + 3H2O

 

2. Electrolysis of Pure Alumina (Hall-Heroult Process).

Aluminium is obtained from alumina by the process of electrolysis. This method is known as HaU-Heroult process. Purified alumina is dissolved in molten cryolite (Na3AlF6) and is electrolysed in an iron tank lined inside with carbon (Fig. 50.3). Carbon lining serves as cathode while a number

of carbon rods dipping in the fused electrolyte serve as anode. Cryolite improves the electrical conductivity of the cell as alumina is a poor conductor. Moreover, cryolite lowers the
melting point of the mixture to about 1250 K. Other impurities such as CaF2, NaF and AIF3 may also be added. The temperature of the electrolyte is maintained between 1200-1300 K.

On passing electric current, aluminium is liberated at the cathode and gets collected at the bottom of the tank from where it is removed. Oxygen liberated at the anode combines with the carbon of the anode to produce carbon monoxide which either burns or escapes out. The reactions taking place

during electrolysis are:

At cathode:      A13+ (melt) + 3e-                 →        7 Al (l)

At anode:            C(s) + 02- (melt)                    →    CO(g) + 2e-

C(s) + 202- (melt)                   →          CO2(g) + 4e-

Since during electrolysis, the carbon electrodes get consumed, they have to be replaced periodically. For each kg of aluminium about 0.5 kg carbon is burnt away.

The extraction of aluminium requires large amount of electrical energy. About 15,000 kWh are needed to produce 1 tonne of aluminium. For example, the smelter at Tema, Ghana, owned by the Volta Aluminium Company, uses about

2,760,000 MWh electricity to produce nearly 200,000 tonnes of aluminium annually. The company receives power from the Akosombo power station of the Volta River hydroelectric dam. Due to its proximity to the power station, the company receives sufficient electrical energy at relatively cheap rate.

50.7-EXTRACTION-OF-ALUMINIUM

 

USES OF ALUMINIUM

Aluminium is a soft metal with a density of 2.7 g cm-3 Aluminium can be easily extruded through dies to form various shapes.

1. It is used for making angles used in windows.

2. It is a good conductor of electricity. Since it is not as good conductor as copper, thicker cables of
aluminium are used for transmission of electricity.

3. Aluminium forms many useful alloys e.g., magnalium (AI and Mg), duralumin (AI, Cu, Mg and Mn). Aluminium alloys are used in aircraft and other transportation vehicles.

4. In the form of finely-divided powder, aluminium is used in antirust paints.

5.Aluminium foil is used for wrapping cigarettes, confectionery, etc.

6.Aluminium is used to produce metals such as chromium and manganese from their ores (aluminothermic process)

 

Cr2O3 + 2Al                 →                    Al2O3 + 2Cr

3MnO2 + 4Al               →                    2Al2O3 + 3Mn

7. Aluminium utensils are extensively used for household purposes.