Principle of extraction of Aluminium and the Alums.

Aluminium.

It is the most abduent metal. The most important ore of aluminium is bauxite. Aluminium is also the occur in a large amount in aluminosilicate rocks such as feldspars, and micas. It is the third most abduent element after oxygen and silicon by weight in earth's crust.When these aluminium rocks weather they form clay minerals or other metamorphic rocks.There is no easy or economical method of extracting aluminium from feldspars, micas or clays.Hence, aluminium was discovered in relatively last period and most abduent in the earth crust.

Extraction of aluminium.

Aluminium is usually extracted from the bauxite ore Bauxite is a hydrated aluminium oxide.i,e Al₂O₃.xH₂O (X=1,2,3).

Principle.

Extraction of aluminium from bauxite ore involve the following two steps.

1. Purification of ores (Bayer's process).
2. Electrolytic reduction of Al₂O₃ into Al.(Hall Heroult process).

1. Purification of ore (Bayer's process)

In this methods, the impure ore containing Fe2O3 and SiO2 (as impurities) is treated with concentration NaOH at 150 degrees C., As a result, the Al₂O₃.3H₂O ore dissolve forming sodium meta-aluminate (NaAlO2) while the impurities remain undissolved (insoluble). The impurities are removed by filtration.

$$Al_2O_3.3H_2O+NaOH \overset{150^{\circ}{c}}{\rightarrow}NaAlO_2+H_2O+impurities$$

$$NaAlO_2=Soluble$$

$$Impurities= insoluble$$

This method of purification of the ore is also called leaching. From the solution of sodium meta-aluminate, Al(OH)3 is precipitated either by seeding the solution with freshly precipitated AL(OH)3 or by passing CO2 gas through a solution which lowers the P.H. value.

$$NaAlO_2+H_2O\xrightarrow[]{seeding\, with \,AlOH_3\,hydrolysis}AlOH_3↓$$

Or,

$$NaAlO_2+CO_2+H_2O→AL(OH)_3↓+NA_2CO_3$$

The precipitate of aluminium hydroxide is separated, washed, dried, and finally ignited to get pure alumina Al₂O_3.

$$2Al(OH)_3→Al_2O_3+3H_2O$$

$$Al_2O_3=Pure$$

2. Electrolytic reduction of Al2O3 into Al (Hall Heroult Process).

In this process, aluminium is mixed with following three fluxes.

1. Cryolite (Na₃AlF₄)
2. Fluorspar.
3. AlF_3.

These fluxes have following advantages.

1. These fluxes act as added impurities which decrease the melting point of Al₂0₃ from 2015degreeCto about 950degreec.
2. These fluxes increase the electrical conductivity of electrolytic cell as Al₂O₃ is a poor conductor.

A typical electrolytic mixture consists of 85% cryolite, 5% fluorospar, 5% AlF_3, 5%Al₂O_3.

This mixture is melted in a graphite lined steel tank at 150degree C. When electricity is passed through the electrolyte is passed through the electrolytic mixture AlF₃ dissociated as.

AlF₃→ Al³⁺ + 3F-

Aluminium is liberated at the cathode and fluorine is liberated as an anode. The fluorine reacts immediately with Al₂O₃ to form AlF₃ which further undergo electrolysis to produce Al.

At cathode.

$$Al^3++3e^- →Al$$

At anode.

$$F^- →F+e^-$$

$$2F →F_2$$

$$Al_2O_3+F_2 →AlF_3+O_2↑$$

Molten aluminium is drained out periodically from the tap hole and more Al₂O₃ is added for further electrolysis.

Uses.

Alumina is lighter metal, the good conductor of electricity and resistant to corrosion. Hence, is widely used for different purpose.

• It is used as structural in the manufacture of the aeroplane, ships, cars, buses, and heat exchange.
• It is used to make utensils.
• It is used to make electric power cables.
• Finely divided aluminium is used to make aluminium paint.
• It is used to make foil which is used to wraps foods.
• It is used in buildings to make a window, door and the locks.
• It is used to make a variety of alloys.
• It is used to make containers, tubes, for toothpaste.
• It is used to make Amul gum which is used as reducing agent.

Alums.

All alum are salts.

All salts are not alums.

Alums are double salts which are represented by general formula [M(H₂O)₆] , M[I], [M(H₂0)6SO₄)₂],M[III]. Where M=[I] are monovalent cations like Na⁺, K⁺, NH4⁺, etc and M=(III) are Al³⁺, Cr³⁺, Fe³⁺ etc.

Examples.

Potash alum= [K(H₂O)₆] [Al(H₂O)₆] (SO4)₂

=K₂SO₄Al₂(SO4]₃.24H₂O

Chrom alum= [K(H₂O]₆ [Cr(H₂O]₆] (SO4)₂

K=[I]

Cr=[III]

=K₂SO₄Cr(SO4)₃.24H₂O

Chrome are highly soluble in water and dissociated completely in solution into their constituent ions.

$$K_2SO_4Al_2(SO_4)_3.24H_2O→K^+(aq)+SO_4^{2-}(aq)+Al^{3+}(aq)$$

Aluminium.

It is the most abduent metal. The most important ore of aluminium is bauxite. Aluminium is also the occur in a large amount in aluminosilicate rocks such as feldspars, and micas. It is the third most abduent element after oxygen and silicon by weight in earth's crust.When these aluminium rocks weather they form clay minerals or other metamorphic rocks.There is no easy or economical method of extracting aluminium from feldspars, micas or clays.Hence, aluminium was discovered in relatively last period and most abduent in the earth crust.

Extraction of aluminium.

Aluminium is usually extracted from the bauxite ore Bauxite is a hydrated aluminium oxide.i,e Al₂O₃.xH₂O (X=1,2,3).

Principle.

Extraction of aluminium from bauxite ore involve the following two steps.

1. Purification of ores (Bayer's process).
2. Electrolytic reduction of Al₂O₃ into Al.(Hall Heroult process).

1. Purification of ore (Bayer's process)

In this methods, the impure ore containing Fe2O3 and SiO2 (as impurities) is treated with concentration NaOH at 150 degrees C., As a result, the Al2O3.3H2O ore dissolve forming sodium meta-aluminate (NaAlO2) while the impurities remain undissolved (insoluble). The impurities are removed by filtration.

$$Al_2O_3.3H_2O+NaOH \overset{150^{\circ}{c}}{\rightarrow}NaAlO_2+H_2O+impurities$$

NaAlO₂₌ Soluble

Impurities= insoluble.

This method of purification of the ore is also called leaching. From the solution of sodium meta-aluminate, Al(OH)₃ is precipitated either by seeding the solution with freshly precipitated AL(OH)₃ or by passing CO₂ gas through a solution which lowers the P.H. value.

Or,

$$NaAlO_2+CO_2+H_2O→AL(OH)_3↓+NA_2CO_3$$

The precipitate of aluminium hydroxide is separated, washed, dried, and finally ignited to get pure alumina Al₂O_3.

$$2Al(OH)_3→Al_2O_3+3H_2O$$

Al₂O₃₌ Pure

2. Electrolytic reduction of Al2O3 into Al (Hall Heroult Process).

In this process, aluminium is mixed with following three fluxes.

1. Cryolite (Na₃AlF₄)
2. Fluorspar.
3. AlF_3.

These fluxes have following advantages.

1. These fluxes act as added impurities which decrease the melting point of Al₂0₃ from 2015degreec to about 950degreec.
2. These fluxes increase the electrical conductivity of electrolytic cell as Al2O3 is a poor conductor.

A typical electrolytic mixture consists of 85% cryolite, 5% fluorospar, 5% AlF₃, 5%Al2O3.

This mixture is melted in a graphite lined steel tank at 150degree C. When electricity is passed through the electrolyte is passed through the electrolytic mixture AlF3 dissociated as.

AlF₃→ Al³⁺ + 3F-

Aluminium is liberated at the cathode and fluorine is liberated as an anode. The fluorine reacts immediately with Al2O3 to form AlF3 which further undergo electrolysis to produce Al.

At cathode.

$$Al^3++3e^- →Al$$

At anode.

$$F^- →F+e^-$$

$$2F →F_2$$

$$Al_2O_3+F_2 →AlF_3+O_2↑$$

Molten aluminium is drained out periodically from the tap hole and more Al2O3 is added for further electrolysis.

Uses.

Alumina is lighter metal, the good conductor of electricity and resistant to corrosion. Hence, is widely used for different purpose.

• It is used as structural in the manufacture of the aeroplane, ships, cars, buses, and heat exchange.
• It is used to make utensils.
• It is used to make electric power cables.
• Finely divided aluminium is used to make aluminium paint.
• It is used to make foil which is used to wraps foods.
• It is used in buildings to make a window, door and the locks.
• It is used to make a variety of alloys.
• It is used to make containers, tubes, for toothpaste.
• It is used to make Amul gum which is used as reducing agent.

Alums.

All alum are salts.

All salts are not alums.

Alums are double salts which are represented by general formula [M(H₂O)₆] , M[I], [M(H₂0)6SO₄)₂],M[III]. Where M=[I] are monovalent cations like Na⁺, K⁺, NH4⁺, etc and M=(III) are Al³⁺, Cr³⁺, Fe³⁺ etc.

Examples.

Potash alum= [K(H₂O)₆] [Al(H₂O)₆] (SO4)₂

=K₂SO₄Al₂(SO4]₃.24H₂O

Chrom alum= [K(H₂O]₆ [Cr(H₂O]₆] (SO4)₂

K=[I]

Cr=[III]

=K₂SO₄Cr(SO4)₃.24H₂O

Chrome are highly soluble in water and dissociated completely in solution into their constituent ions.

K₂SO₄Al(SO4)₃.24H₂O → K⁺(aq) + SO₄^--(aq) + Al₃+(aq)

.All double salts are not alums. Only those double salts satisfy above general formula are called alums. Hence, all alums are not double salts but double salts are not alums. Example, potash alum K₂SO₄, Al₂(SO₄)₃.24H₂Ois a double salt as well as alum.However, Mohr's salt FeSO₄(NH₄)2SO₄.6H₂O is a salt but no alum.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.

K₂SO₄Al(SO4)₃.24H₂O → K⁺(aq) + SO₄^--(aq) + Al₃+(aq)

.All double salts are not alums. Only those double salts satisfy above general formula are called alums. Hence, all alums are not double salts but double salts are not alums. Example, potash alum K₂SO₄, Al₂(SO₄)₃.24H₂Ois a double salt as well as alum.However, Mohr's salt FeSO₄(NH₄)2SO₄.6H₂O is a salt but no alum.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.