Metallurgy
General Principles of extraction of Metals
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Metallurgy
Definition:
The scientific and technological process used for the
extraction/isolation of the metal from its ore is called as metallurgy.
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Common Ores
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Definitions
The compound of a metal found in nature is called a mineral.
The minerals from which metal can be economically and
conveniently extracted are called ores.
An ore is usually contaminated with earthy or undesired materials
known as gangue. Hence all minerals are not ores but all ores are
minerals .
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Definitions (contd.)
Mineral - Compound of Metal
Ore - Mineral from which metal can be extracted commercially and
profitably.
Gangue/Matrix - Impurities present with an ore - dirt/dust or other
minerals
FLux - Something that is added to the ore to remove gangue
particles.
Flux Combines with gangue/matrix to form fusible slag. (fusible -
molten)
FLUX + GANGUE = SLAG
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Extraction
Process of Extraction
The main steps involved in the extraction of metals are
• Crushing and grinding of ore (Large Particle to small)
• Concentration of ore (removal of gangue particles)
• Conversion to Metal Oxide (from carbonate, sulphate, sulphide
to oxide)
• Reduction of ore / Extraction of Metal from Metal Oxide (MO
to M)
• Refining (purifying)
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Crushing and Grinding
The ore is first crushed by jaw crushers and ground to a powder by
reduction equipments like ball mills and stamp mills.
Reason
• Easier to transport
• Easier to process
• Removes dust and dirt
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Concentration of Ore
Removal of the unwanted materials (e.g., sand, clays, etc.) from the
ore is known as concentration, dressing or benefaction.
It involves several steps and selection of these steps depends upon
the di↵erences in physical properties of the compound of the metal
present and that of the gangue
During metallurgy,
‘flux’ is added which combines with ‘gangue’ to form ‘slag’.
Slag separates more easily from the ore than the gangue.
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Common Methods of Ore Dressing
• Hydraulic washing or Gravity separation or Levigation method
• Magnetic separation/electromagnetic separation
• Froth floatation method (sulphides)
• Leaching
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Reduction
Reduction
The concentrated ore must be converted into a form which is
suitable for reduction.
Usually the carbonate/sulphate/sulphide ore is converted to oxide
before reduction.
Oxides are easier to reduce.
Thus isolation of metals from concentrated ore involves two major
steps as given below.
• Conversion to oxide,
• Reduction of the oxide to metal.
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Common Methods
Roasting - (Sulphide ores)
Roasting: In roasting, the ore is heated in a regular supply of air in
a furnace at a temperature below the melting point of the metal.
Some of the reactions involving sulphide ores are:
2 ZnS + 3 O2 ! 2 ZnO + 2 SO2
2 Cu2 S + 3 O2 ! 2 Cu2 O + 2 SO2
Purpose of roasting:
• To convert the sulphide into oxide and sulphate
• To remove impurities like S, As, Sb.
• To remove moisture
• To Oxidise easily oxidisable substances
The processing of strong heating of the ore in presence of excess
amount of air below its melting point.
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Common Methods(Contd.)
Calcination - (Carbonate Ores)
Calcination: Calcinaton involves heating when the volatile matter
escapes leaving behind the metal oxide:
ZnCO3 (s) ! ZnO(s) + CO2 (g)
CaCO3 · MgCO3 (s) ! CaO(s) + MgO(s ) + 2 CO2 (g)
Purpose of calcination
• Remove the volatile impurities
• To remove moisture
• Make the mass porous
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Reduction of Oxide to Metal
There most common ways in which metal is extracted from its oxide
are :
• Reduction with C
• Reduction with CO
With less Common methods being
• Reduction with hydrogen
• Self reduction ( 2 HgO + HgS ! 3 Hg + SO2 )
• Reduction with some other metal
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Ellingham Diagram
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Ellingham Diagram (Contd)
Ellingham diagram normally consists of plots of G vs T for
formation of oxides of elements i.e., for the reaction, 2 xM(s) +
O2 (g) ! 2 MxO(s)
Some Key Points:
• In this reaction, the gaseous amount (hence molecular
randomness) is decreasing from left to right due to the
consumption of gases leading to a –ve value of S.
• Subsequently G shifts towards higher side despite rising T.
The result is +ve slope in the curve for most of the reactions
shown above for formation of MxO(s).
• Each plot is a straight line except when some change in phase
(s to liq or liq to g) takes place.
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Ellingham Diagram (contd.)
• The temperature at which such change occurs, is indicated by
an increase in the slope on +ve side (e.g., in the Zn, ZnO plot,
the melting is indicated by an abrupt change in the curve).
• There is a point in a curve below which G is negative (so
MxO is stable). Above this point, MxO will decompose on its
own.
• In an Ellingham diagram, the plots of G for oxidation (and
therefore reduction of the corresponding species) of common
metals and some reducing agents are given.
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Example on Ellingham Diagram
Q. When can Magnesium reduce Aluminium.
Q. When can Aluminium reduce Magnesium.
Q. Can copper be used as a reducing agent for Al ?
Q. At what Temp will the reaction C + 2 ZnO ! CO2 + 2 Zn be
spontaneous?
3 Mg + Al2 O3 ! 3 MgO + 2 Al
3 MgO + 2 Al ! 3 Mg + Al2 O3
Cu + Al2 O3 ! CuO + Al
OR
CuO + AlCu + Al2 O3 ! Cu + Al2 O3
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Limitations of Ellingham Diagram
• The graph simply indicates whether a reaction is possible or
not i.e., the tendency of reduction with a reducing agent is
indicated. This is so because it is based only on the
thermodynamic concepts. It does not say about the kinetics of
the reduction process.
• It assumes the reactents and products are in equilibrium which
is not true, in case of solids and liquids.
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Refining
A metal extracted by any method is usually contaminated with some
impurity. For obtaining metals of high purity, several techniques are
used depending upon the di↵erences in properties of the metal and
the impurity. Some of them are listed below.
• Distillation
• Liquation
• Electrolysis
• Polling
• Zone refining
• Vapour phase refining
• Chromatographic methods
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Other Methods of Refining
Liquation
Distillation In this method a low melting
This is very useful for low metal like tin can be made to
boiling metals like zinc and flow on a sloping surface of a
mercury. The impure metal is reverberatory furnace leaving
evaporated to obtain the pure behind the less fusible
metal as distillate. impurities. . In this way it is
separated from higher melting
impurities.
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Other Methods of Refining
Electrolytic refining
In this method, the impure metal is made to act as anode. A strip
of the same metal in pure form is used as cathode. They are put in
a suitable electrolytic bath containing soluble salt of the same metal.
The more basic metal remains in the solution and the less basic ones
go to the anode mud.
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Other Methods of Refining
Zone refining
Th is method is based on the principle that the impurities are more
soluble in the melt than in the solid state of the metal.
A circular mobile heater is fixed at one end of a rod of the impure
metal. The molten zone moves along with the heater which is
moved forward. As the heater moves forward, the pure metal
crystallises out of the melt and the impurities pass on into the
adjacent molten zone.
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Other Methods of Refining
Vapor Phase Refining
In this method, the metal is converted into its volatile compound
and collected in some vessel.
It is then decomposed to give pure metal.
The two requirements for this are :
• the metal should form a volatile compound with an available
reagent,
• the volatile compound should be easily decomposable, so that
the recovery is easy.
Mond process for refining nickel: In this process, nickel is heated in
a stream of carbon monoxide forming a volatile complex, nickel
tetracarbonyl. Other important process incude van-Arkel method
for refining of Zirconium and Titatium.
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Other Methods of Refining
Polling
Green logs of wood are used to stir molten metal( usually copper ).
These green logs remove volatile organic impurities.
The escaping gases cause uneven rough metal surface ( and is called
as blister copper )
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Other Methods of Refining
Chromatography
This method is based on the principle that di↵erent components of
a mixture are di↵erently adsorbed on an adsorbent.
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Extraction of Iron i
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Extraction of Iron ii
Important Points
• Gangue - Silicon Dioxide
• Flux - limestone
• Slag - CaSiO3
• Actual Reducing agent
• Reactions at Each Level
• Di↵erences between Pig Iron and Wrought Iron
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Extraction of Aluminium i
Hall-Herolt’s Process
In the metallurgy of aluminium, purified Al2O3 is mixed with
Na3 AlF6 or CaF2 which lowers the melting point of the mix and
brings conductivity.
The fused matrix is electrolysed. Steel vessel with lining of carbon
acts as cathode and graphite anode is used.
Thus electrolysis of the molten mass is carried out in an electrolytic
cell using carbon electrodes. The oxygen liberated at anode reacts
with the carbon of anode producing CO and CO2.
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Extraction of Aluminium ii
2 Al2 O3 + 3 C ! 4 Al + 3 CO2
Use of Cryolite(NaAlF6) and Florspar(CaF2) - To lower the melting
point and increase conductivity.
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Leaching
Also called as hydrometallurgy.
Extraction of silver, gold, Al, Cu (from low grade ores) etc.
Most common method for extraction of silver and gold is the Mac
Arthur Forest Method
It requires the use of NaCN or KCN in the presence of air/oxygen.
4M(S) + 8CN(aq) + 2H2 O(aq) + O2(g ) ! 4[M(CN)2 ](aq) + 4OH(aq)
(M = Ag or Au)
2[M(CN)2 ](aq) + Zn(s) ! [Zn(CN)4 ]2(aq) + 2M(s)
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Recap
• Di↵erence between mineral and ore
• Flux + =
• Steps in Reduction
• One example of self reduction
• One use of Zone refining
• What is Zinc Blende
• What is Calamine
• Formula for Haematite and Magnetite.
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Common Questions i
Which of the following occurs in native state ? ( Navtive State -
elemental, uncombined)
(A) Gold (B) Silver (C) Platinum (D) Sodium
In which of the following case you have to use electrometallurgy?
(A) Sodium (B) Copper (C) potassium (D) gold
What is the formula for :
(A) Hematite (B) magnetite (C) calamine (D) galena
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Common Questions ii
Leaching is used in which of the following cases?
(A) Gold (D) high grade copper
(B) silver (E) low grade copper
(C) aluminium (F) Potassium
For sulphide ores which of the following are commonly used?
(A) Calcination (C) Levigation
(B) Roasting (D) Froth Flotation
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RECAP
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SOME COMMON QUESTIONS
JEE Questions i
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JEE Questions ii
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JEE Questions (contd)
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JEE Questions (contd)
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JEE Questions (contd)
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END
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