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EXTRACTION OF IRON:

Important Ores of Iron:

SN
Ores
Formula
1
Haematite (Red)
Fe2O3
2
Magnetite     
Fe3O4
3
Siderite or Spathic
FeCO3
4
Iron Pyrite (fool’s Gold)
FeS2
5
Goethite
FeO(OH)
6
Limonite  
2Fe3O4.3H2O

Others Ores

7
Ilmenite  (FeO+TiO2)
FeTiO3
8
Chromite (FeO+Cr2O3)
FeCr2O4

Extraction process of iron: Iron is extracted from if oxide ores especially form the magnetite, Haematite, and Limonite ores. The extraction involves the following process.
(1) Enrichment of concentration of ores:
(A)Dressing or Benefaction of the ore: After mining, Iron ore is first broken into small pieces of 3 – 5 cm in size.
(B)Magnetic and Gravity Separation: Use due to the presence of metal carbonate
(2) Conversion of ores into oxide ores:
(A) Calcination: Heating ore in the absence of oxygen below melting point moisture and CO2 are removed.
(B) Roasting: Heating of ore in the presence of oxygen below melting point. During roasting P, S, C, As, Sb etc. are oxidized to the respective oxides and removed.
(i) Fe3O4 is decomposed to ferrous oxide (FeO) and ferric oxide (Fe2O3).
(ii) Ferrous oxide reacts with silica to form ferrous silicate at high temperature.
(ii) The conversion of FeO into Fe2O3 will prevent the formation of FeSiO3 Thus mass of the ore becomes porous causing the increase in the effective surface area.
(3) Smelting: The calcined ore is mixed with Limestone (Flux) and Coke (a reductant) and smelted in Blast furnace.
Reaction in Blast furnace: blast furnace is used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. The blast furnace is a huge, steel stack lined with refractory brick, where iron ore, coke and limestone are dumped into the top, and pre heated air is blown into the bottom.

 (1) Zone of combustion: (900 -1500 K): This is the higher temperature range found at lower part of blast furnace
(2)  Zone of reduction: (500 – 800 K): This is lower temperature range found at upper part of blast furnace.
(3)  Zone of slag formation: (1070-1270 K): Middle part of blast furnace:
Other impurities like P4O10 and SiO2 and MnO2 also reduced
(4) Zone of fusion:  (1570 – 2170 K): lower part of furnace.
Molten iron is heavier than molten slag. The two liquids are periodically tapped off and solidified into blocks called pigs. And thus Iron obtained from blast furnace is also called pig Iron.

Type of Iron:
(1) Pig Iron: Composition:
                                
SN
Impurities
%
1
Carbon (C)
3- 4.3
2
Silicon (Si)
1-2.0
3
Manganese (Mn)
0.5-2.0
4
Phosphorous (P)
0.05-2.0
5
Sulphure (S)
0.05-1.0

(2) White Cast Iron: When re molted pig iron is suddenly cooled, white cast iron is results. In this form of cast iron carbon is found to be combined form as cementite (Fe3C).
(3) Grey Cast Iron: However when re molted pig iron is slowly cooled, Grey cast iron is results. In this form of cast iron carbon is found to be combined form of Graphite.

(4) Wrought iron:
This is done by heating cast iron with haematite (Fe2O3) which oxidises C to CO, S to SO2 Si to SiO2, P, P4O10 and Mn to MnO . Where CO and SO2 escapes, manganese oxide (MnO) and Silica (SiO2combine to form slag.
Similarly phosphorus pentoxide combines with haematite to form ferric phosphate slag.
Manufacturing Process:
Casts iron takes in Puddling furnace and melted by hot blast of air. The chemical reactions which occur are:-

On removing impurities, the melting point rises and it becomes a semi solid mass. The metal is taken out of the furnace in the form of balls which are then beaten under hammer to separate out the slag. The product thus formed is thus called wrought iron.

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]
(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:

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.

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