Metallurgy of aluminium from its Oxide ore

 

Metallurgy of Aluminium from its 

Oxide Ore

Ore Used and type of Metallurgy

Aluminium is the most abundant metal and the third most abundant element after O and Si  comprises about 6-7% in the earth’s crust.

Aluminium is mainly extracted from its oxide ore bauxite (impure hydrated aluminium oxide,   Al₂O₃.nH₂O or Al₂O₃.2H₂O) by electrometallurgy.

Basic Principal

The extraction of aluminium from bauxite involves the purification of the crude bauxite to yield pure anhydrous alumina (Al₂O₃) which is then electrolyzed to give 99% pure aluminium metal followed by its electrolytic refining to get 99.99% pure aluminium.

Steps of Metallurgy

The extraction of aluminium from bauxite involves following steps:

1.         Purification of Bauxite to get pure alumina.                          

2.         Electrolysis of Pure Alumina.            

3.         Electrolytic Refining of Aluminium.

1.       Purification of Bauxite Ore

1.         The chief impurities in bauxite ore are silica (SiO₂) and ferric oxide (Fe₂O₃).

2.         Both these impurities must be removed from bauxite in order to get aluminium metal of highest quality (as these impurities render the metal brittle and liable to corrosion).

 

3.         The first step in the extraction of aluminium is the preparation of alumina.

 

4.     The purification of bauxite may be carried out by different methods based on presence of impurity:

(a)       Hall’s Process

1.  This process is suitable for treating bauxite containing both impurities of Fe₂O₃ and SiO₂ in equal amount. 

2.The purification of bauxite by this method involves the fusion of Bauxite with concentrated solution  of sodium carbonate at 100°C (in autoclaves).

3.  Aluminium oxide present in bauxite goes into solution as soluble sodium aluminate while impurities of Fe₂O₃ and SiO₂ are left unaffected, remaining as residue and are filtered off after slight dilution.

4.  The solution of sodium aluminate obtained is slightly diluted and then heated upto 60°C in a current of CO₂ to form white precipitate of aluminium hydroxide.

5.  The ppt. of Al(OH)₃ are filtered, washed and ignited to give pure alumina (Al₂O₃) by loosing water.

(b) Bayer’s Process

1.         This process is suitable for treating red bauxite containing excess of Fe₂O₃. 

2.       The purification of bauxite containing excess of Fe₂O₃ is carried out by digesting or reacting finely divided bauxite ore with hot 45% concentrated caustic soda (NaOH) solution at 150°C, forming soluble sodium aluminate [NaAlO₂ or NaAl(OH)₄] leaving behind impurities of ferric oxide and silica (as they do not dissolve in alkali) which settle down as sludge or red mud and are easily removed by filtration.

 

3.       The filtrate of sodium aluminate is hydrolyzed with plenty of water giving precipitate aluminium hydroxide or aluminium oxide trihydrate (Al₂O₃.3H₂O) on cooling

4.      The ppt. of Al(OH)₃ are filtered, washed, dried and then strongly heated upto 1500°C (in a rotary kiln) to give pure alumina (Al₂O₃) by loosing water.

(c)  Serpeck’s Process

1.         This process is suitable for bauxite containing large amounts of silica.

2.         In this process bauxite is mixed with coke and heated at 1800°C in a current of N₂ gas forming aluminium nitride (AlN) which then hydrolyzed with water to give white ppt. of Al(OH)₃ [along with ammonia which is useful by product].

3.         The ppt. of Al(OH)₃ are filtered, washed with water and then ignited to get pure alumina.

4.         Silica (SiO₂) present in bauxite is reduced to silicon which volatizes off at this temperature. 

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

1.(Previously before 1886, aluminium metal is extracted from its salt AlCl₃ by heating it with sodium    metal. Sodium metal itself was expensive making aluminum even more expensive.  AlCl₃ + 3Na ¾® Al + 3NaCl. Due to the process of electrolytic extraction of aluminium, its price quickly falls).

2. The extraction of aluminium from pure and dry alumina by its electrolysis is carried out by Hall Heroult Process [discovered independently in 1886 by C.M. Hall in America and L.T. Heroult in France].

3. Alumina is a bad conductor of electricity, however it is made electrolyte by adding cryolite (Na₃AlF₆).

Principle

The principle of extraction involves the electrolytic decomposition of pure fused alumina dissolved in molten cryolite involving reduction of Al³⁺ ions at the cathode. [Alumina is a bad conductor of electricity, however it is made electrolyte by adding cryolite (Na₃AlF₆)].

Construction of Cell

1.   The electrolytic cell consists of a steel tank having 1ft thick inner lining of carbon acting as cathode.

2.   A number of suspended graphite rods acts as an anode.

3.   There is a tap hole at the bottom.

The electrolysis of alumina is conducted in a specially designed electrolytic cell made up of a steel tank (8ft x 6 ft) internally lined with thick (1 ft thick) blocks of graphite which serves as cathode (having a tap hole at the bottom). The anodes are suspended graphite rods or bars hanging in the molten charge or electrolytic mixture of pure alumina dissolved in molten cryolite with some fluorspar up to a few centimeters above the bottom.  

1.    The electrolytic mixture consists of molten alumina and molten cryolite.

2.   Cryolite furnishes the necessary ions for the conduction of electricity and thus acting as a solvent for bauxite. It also decreases the m.p of bauxite (which normally melts at 3000°C) up to 950°C.

3. A little Fluorspar (CaF₂) is also added which acts as a flux to lower fusion point of alumina and to increase the fluidity of molten aluminium.

4. To prevent the eating away of graphite anodes, the surface of electrolytic mixture is covered with some carbon powder.

 

Working of Cell

1.  a current of 5.5 (6) volt is passed through electrolyte which raises the temperature to 900-950°C at which alumina melts and dissolves in molten cryolite (the resistance of the electrolyte to the    current produces enough heat that keeps the charge in molten state) and electrolytic reduction  takes place giving 99% pure molten aluminum at the cathode which flows (sinks) down to the bottom of the cell and is tapped off from the outlet periodically. The aluminium obtained is about 99%   pure containing traces of impurities of Fe, Si, Al₂O₃ etc.

2. Oxygen gas is evolved at anodes which react with graphite anode forming CO₂. Thus graphite anodes are gradually and constantly burnt away (used up) and must be replaced by new anodes  periodically (from time to time).

3. [Actually, the raw material for this process is Al₂O₃ but cryolite furnishes AlF₃ first which liberates F₂ at the anodes that reacts with Al₂O₃ giving O₂ at anodes and forming AlF₃ again while deposits al at cathode. Thus Al₂O₃ is consumed in the process]. 

Complete Electrolytic Reaction of Extraction of Al Metal from its Oxide Ore

1. Alumina (Al₂O₃) is a bad conductor of electricity, however it is made electrolyte by adding cryolite (Na₃AlF₆). Cryolite furnishes the necessary ions for the conduction of electricity. It also acts as a solvent for bauxite, as it dissolves molten alumina. 

2.   During electrolysis, molten cryolite dissociates into its constituents; AlF₃ and NaF. AlF₃ is  electrolytically decomposed giving Al³⁺ ions at cathode and F^- ions at anode.

3.  F^- ions are oxidized and discharged at anode forming F atom, which reacts with Al₂O₃ displaces O₂ gas (as F is more electronegative than O) forming AlF₃ again along with O₂ gas (which repeats the same sequence of reactions to consume all Al₂O₃).

4.  Process is cycled repeatedly till whole of Al₂O₃ is consumed. Al³⁺ ions are reduced at cathode to  aluminium in molten state, which is collected in the bottom of the cell.

 

3.Electrolytic Refining of Aluminium (Hoope’s Process)

1. Aluminium obtained by Hall-Heroult process contains trace impurities of Fe, Cu, Si, Al₂O₃ etc.

2.Aluminium is further refined electrolytically by Hoope’s Electrolytic Process which is based on electrolysis of a fused mixture of cryolite and BaF₂ saturated with Al₂O₃.

Structure of Cell

The refining cell consists of a carbon lined iron tank, which contains three fused liquid layers of different specific gravity, one above the other:

(i) Bottom Anode Layer of molten impure Al-Cu alloy.

(ii) Middle Electrolyte Layer of molten fluorides of cryolite and BaF₂.

(iii) Top Cathode Layer of pure molten Aluminium containing hanging carbon cathodes

Working

1.         Crude aluminium is introduced at the bottom layer.

2.         On electrolysis, Al from the bottom Al-Cu alloys anode is oxidized goes into the middle layer as Al³⁺ ions leaving impurities (e.g. Fe, Cu, Ca, Si etc.) at the anode.

3.         These Al³⁺ ions from middle layer of fused cryolite are reduced at the hanging carbon cathodes and pass into the top layer as molten pure Al which is drawn off from the tapping hole at the top.

4.         Aluminium obtained is 99.99% pure.