Extraction of Aluminium:

Important ores of Aluminium:

(1) Corundum (Al₂O₃)

(2) Diaspore (Al₂O₃.H₂O)

(3) Bauxite (Al₂O₃.2H₂O)

(4) Gibbsite (Al₂O₃.3H₂O)

(5) Cryolite (AlF₃.3NaF) or Na₃AlF₆

(4) Alunite (K₂SO₄.Al₂(SO₄)₃.4Al(OH)₃

(5) Spinel (MgOAl₂O₃)

(6) Felspar (K₂O.Al₂O₃.6SiO₃) or KAlSi₃O₈

(7) China Clay or Kaoline [Al₂O₃.2SiO₂.2H₂O]

(A) Leaching of Bauxide ore:

(B) Purification of Bauxite:

(1) Baeyer’s Process: By Bayer’s process commercially it is being carried out (for red bauxite) In other words this process is applied to the bauxite ore containing ferric oxide (red bauxite Fe₂O₃) as chief impurity. Ore roasted to convert ferrous oxide to ferric oxide.

(2) Hall’s Process: This process is also applied to the bauxite ore containing ferric oxide (red bauxite Fe₂O₃) as major impurity. This process ore is fused with Na₂CO₃.

(3) Serpeck’s Process: This process is applied to the bauxite ore containing Silica (White bauxite SiO₂) as major impurity.

 NOTE:Silicone Volatile at this temperature and removed easily.

Electrolytic reduction of Al₂O₃:

Electrolysis of molten mixture: HALL HEROULT PROCESS:

Cathode           :           Iron-tank lined with bricks Carbon

Anode              :           Graphite rods (Carbon)

Electrolyte       :           Molten [Al₂O₃ (5%), Na₃AlF₆ (85 %), CaF₂ (5%), AlF₃ (5%) ]

Temperature    :          900 degree centigrade 

Reactions at Electrodes:

According to the first theory the following reaction occurs:

As Cryolite has greater electrochemical stability it does not dissociate. It only increases the dissociation of  Al₂O₃  But the second theory states that, Cryolite undergoes electrolytic dissociation first then Al⁺³  goes to the cathode, produced F₂ at anode then reacts with Al₂O₃   produces .

LEACHING: HYDROMETALLURGY:

The use of an aqueous solution to extract metals from their ores is known as Hydrometallurgy. One early example of hydrometallurgy is a process used to obtain gold. Gold occurs in its elemental state, but often as very small particles mixed with other substances. The gold can be separated out of the mixture by selectively dissolving it into solution, a process called leaching. Hydrometallurgy use for extraction of Cu, Ag, Au etc. Hydrometallurgy used two famous processes (1) MacArthur-Forrest Process (2) Leaching

(1) MacArthur-Forrest Process:

(A) Extraction of Gold:

In the Mac Arthur process, solid gold reacts with sodium cyanide to form a soluble gold complex. The impurities are filtered out of the solution and the gold is reduced back to elemental gold with a reactive metal such as zinc.

(B) Extraction of Silver:

Metallic Ag is dissolved from its ore in dilute NaCN solution, and the solute so obtained is treated with scrap Zn when Ag is precipitated. Air is blown into the solution oxidize Na₂S. Leaching the metals like silver, gold with CN^- is an oxidation reaction

Here Zn act as reducing agent

Leaching has been practiced for many years and often results in the contamination of streams and rivers with cyanide. New alternatives, using the thiosulfate ion (S₂O₃^-) , are being investigated to replace it.

(C) Extraction of Copper:

Different acid, base, and salt solutions are sometimes used to selectively separate out metal-bearing minerals. For example, sulfuric acid is used to separate the copper and iron from the mineral chalcopyrite, CuFeS₂, and a sodium chloride solution is used to separate the lead from the insoluble mineral anglesite, PbSO₄.

Hydrometallurgy is often more economical than Pyrometallurgy, due to the high energy costs associated with the elevated temperatures needed for calcination and roasting.

(2) Leaching:

Leaching is often used if the ore is soluble in some suitable solvent like acids, bases and suitable chemical reagents. For example Al, Ag, and Au ore and low grade copper ore.

Leaching of alumina from bauxite:

The principal ore of aluminium, bauxite, usually contains SiO₂, iron oxide and titanium oxide (TiO₂) as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at 473-523 K and 35-36 bar pressure. This way, Al₂O₃ is leached out as sodium aluminate (and also SiO₂ as sodium silicate) leaving behind the impurities, iron oxide and titanium oxide.

The aluminate in solution is neutralised by passing CO₂ gas and hydrated Al₂O₃ is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated Al₂O₃ which induces the precipitation

The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al₂O₃:

This step comprises the Bayer’s process.

PYROMETALLURGY:

Once the mineral is separated from the gangue, the elemental metal is extracted from the mineral. Several different techniques can achieve this separation. In Pyrometallurgy, heat is employed to extract a metal from its mineral. Different heating conditions have different effects on the mineral.

(1) The Pyrometallurgy based on Thermodynamics principles, and generally Cu, Sn, Pb, Fe, and Hg extracted by Pyrometallurgy.

(2) Pyrometallurgy carried out by following process

(A) Roasting of ore:

(B) Calcination of ore:

(C) Smelting of ore: (Gangue, Flux, Slag )

           

(A) Roasting of ore: The ore is heated strongly below its melting point in presence of excess of air which removes impurities of non – metals at their volatile oxides.

(1) Roasting is exothermic process once started it does not required additional heating.

(2) Roasting employed for Sulphide ores.

(3) Chemical Conversion of ore takes place.

(4) Roasting is carried out in Reverbatory furnace

The process of roasting required the following:

(1) Conversion of sulphide ores to their respective oxides.

(2) Conversion of sulphide ores to their Sulphate.

Note: Some time roasting may not bring about complete oxidation

(3) Roasting at high temperature: the sulphide ore of some of the metal like Cu, Pb, Hg, Sb, etc when heated strongly in the free supply of air or O₂ are reduced directly to the metal rather than to the metallic oxide for example.

The reduction of the sulphide ore directly into metal by heating it in air or O₂ is called  Self reduction, Auto reduction, Air reduction etc and the SO₂ produced is utilised  for manufacturing of Sulphuric acid.

Consequence of Roasting:

(1) Sulphide Ore is converted into oxide/sulphate which may be further decomposed into metal oxide, example Sulphur dioxide

(2) Organic Matter is burnt away.

(3) Impurities of sulphure, phosphorous, arsenic and antimony are oxidised into the respective volatile oxide.

(4) When concentrated tine Stone ore (SnO₂) is heated strongly in a free supply of air (roasting), the impurities of CuS and FeS present in the ore are converted into CuSO₄ and FeSO₄ respectively.

(B) Calcination:   (C) Smelting:  (1) Flux: ,  (2) Slag:

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ROASTING : PYROMETALLURGY:

The ore is heated strongly below its melting point in presence of excess of air which removes impurities of non – metals at their volatile oxides.

(1) Roasting is exothermic process once started it does not required additional heating.

(2) Roasting employed for Sulphide ores.

(3) Chemical Conversion of ore takes place.

(4) Roasting is carried out in Reverbatory furnace

The process of roasting required the following:

(1) Conversion of sulphide ores to their respective oxides.

(2) Conversion of sulphide ores to their Sulphate.

Note: Some time roasting may not bring about complete oxidation

(3) Roasting at high temperature: the sulphide ore of some of the metal like Cu, Pb, Hg, Sb, etc when heated strongly in the free supply of air or O₂ are reduced directly to the metal rather than to the metallic oxide for example.

The reduction of the sulphide ore directly into metal by heating it in air or O₂ is called  Self reduction, Auto reduction, Air reduction etc and the SO₂ produced is utilised  for manufacturing of Sulphuric acid.

Consequence of Roasting:

(1) Sulphide Ore is converted into oxide/sulphate which may be further decomposed into metal oxide, example Sulphur dioxide

(2) Organic Matter is burnt away.

(3) Impurities of sulphure, phosphorous, arsenic and antimony are oxidised into the respective volatile oxide.

(4) When concentrated tine Stone ore (SnO₂) is heated strongly in a free supply of air (roasting), the impurities of CuS and FeS present in the ore are converted into CuSO₄ and FeSO₄ respectively.

CALCINATION : PYROMETALLURGY:

Ore is heated strongly in absence or limited supply of air (air is used here as carrier of heat and moisture).Calcination process temperature is below melting point of treated ore. This process of calcination brings about the following changes.

(1) The Carbonate ore gets decomposed to form the oxide of the metals for example

(2) Water of crystallisation present in the hydrated oxide ores are gets lost as moisture as

Consequence of Calcination:

(1) Volatile products like water vapour / CO₂ are removed and ore become porous

(2) Organic impurities decomposes into smaller volatile products

Note:  Both calcination and Roasting are generally carried out in a Reverbatory furnace. In case of roasting, the air holes are kept open while they are partially or completely closed during calcination.