NCERT Textbook: General Principles and Processes of Isolation of Elements | NCERT Textbooks (Class 6 to Class 12) - CTET & State TET PDF Download

 Page 1


A few elements like carbon, sulphur, gold and noble
gases, occur in free state while others in combined forms
in the earth’s crust. The extraction and isolation of an
element from its combined form involves various
principles of chemistry. A particular element may occur
in a variety of compounds. The process of metallurgy
and isolation should be such that it is chemically feasible
and commercially viable. Still, some general principles
are common to all the extraction processes of metals.
For obtaining a particular metal, first we look for
minerals which are naturally occurring chemical
substances in the earth’s crust obtainable by mining.
Out of many minerals in which a metal may be found,
only a few are viable to be used as sources of that
metal. Such minerals are known as ores.
Rarely, an ore contains only a desired substance.
It is usually contaminated with earthly or undesired
materials known as gangue. The extraction and isolation
of metals from ores involve the following major steps:
• Concentration of the ore,
• Isolation of the metal from its concentrated ore, and
• Purification of the metal.
The entire scientific and technological process used
for isolation of the metal from its ores is known as
metallurgy.
6
After studying this Unit, you will be
able to
• explain the terms minerals,
ores, concentration, benefaction,
calcination, roasting, refining, etc.;
• understand the principles of
oxidation and reduction as applied
to the extraction procedures;
• apply the thermodynamic
concepts like that of Gibbs energy
and entropy to the principles of
extraction of Al, Cu, Zn and Fe;
• explain why reduction of certain
oxides like Cu
2
O is much easier
than that of Fe
2
O
3
;
• explain why CO is a favourable
reducing agent at certain
temperatures while coke is better
in some other cases;
• explain why specific reducing
agents are used for the reduction
purposes.
Objectives
Thermodynamics illustrates why only a certain reducing element
and a minimum specific temperature are suitable for reduction of a
metal oxide to the metal in an extraction.
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
6
Unit Unit Unit Unit Unit
© NCERT
not to be republished
Page 2


A few elements like carbon, sulphur, gold and noble
gases, occur in free state while others in combined forms
in the earth’s crust. The extraction and isolation of an
element from its combined form involves various
principles of chemistry. A particular element may occur
in a variety of compounds. The process of metallurgy
and isolation should be such that it is chemically feasible
and commercially viable. Still, some general principles
are common to all the extraction processes of metals.
For obtaining a particular metal, first we look for
minerals which are naturally occurring chemical
substances in the earth’s crust obtainable by mining.
Out of many minerals in which a metal may be found,
only a few are viable to be used as sources of that
metal. Such minerals are known as ores.
Rarely, an ore contains only a desired substance.
It is usually contaminated with earthly or undesired
materials known as gangue. The extraction and isolation
of metals from ores involve the following major steps:
• Concentration of the ore,
• Isolation of the metal from its concentrated ore, and
• Purification of the metal.
The entire scientific and technological process used
for isolation of the metal from its ores is known as
metallurgy.
6
After studying this Unit, you will be
able to
• explain the terms minerals,
ores, concentration, benefaction,
calcination, roasting, refining, etc.;
• understand the principles of
oxidation and reduction as applied
to the extraction procedures;
• apply the thermodynamic
concepts like that of Gibbs energy
and entropy to the principles of
extraction of Al, Cu, Zn and Fe;
• explain why reduction of certain
oxides like Cu
2
O is much easier
than that of Fe
2
O
3
;
• explain why CO is a favourable
reducing agent at certain
temperatures while coke is better
in some other cases;
• explain why specific reducing
agents are used for the reduction
purposes.
Objectives
Thermodynamics illustrates why only a certain reducing element
and a minimum specific temperature are suitable for reduction of a
metal oxide to the metal in an extraction.
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
6
Unit Unit Unit Unit Unit
© NCERT
not to be republished
148 Chemistry
In the present Unit, first we shall describe various steps for effective
concentration of ores. After that we shall discuss the principles of some
of the common metallurgical processes. Those principles shall include
the thermodynamic and electrochemical aspects involved in the effective
reduction of the concentrated ore to the metal.
Elements vary in abundance. Among metals, aluminium is the most
abundant. It is the third most abundant element in earth’s crust (8.3%
approx. by weight). It is a major component of many igneous minerals
including mica and clays. Many gemstones are impure forms of Al
2
O
3
and the impurities range from Cr (in ‘ruby’) to Co (in ‘sapphire’). Iron
is the second most abundant metal in the earth’s crust. It forms a
variety of compounds and their various uses make it a very important
element. It is one of the essential elements in biological systems as well.
The principal ores of aluminium, iron, copper and zinc have been
given in Table 6.1.
6.1 6.1 6.1 6.1 6.1 Occurrence of Occurrence of Occurrence of Occurrence of Occurrence of
Metals Metals Metals Metals Metals
6.2 6.2 6.2 6.2 6.2 Concentration Concentration Concentration Concentration Concentration
of Ores of Ores of Ores of Ores of Ores
Aluminium Bauxite AlO
x
(OH)
3-2x
[where 0 < x < 1]
Kaolinite (a form of clay) [Al
2 
(OH)
4
 Si
2
O
5
]
Iron Haematite Fe
2
O
3
Magnetite Fe
3
O
4
Siderite FeCO
3
Iron pyrites FeS
2
Copper Copper pyrites CuFeS
2
Malachite CuCO
3
.Cu(OH)
2
Cuprite Cu
2
O
Copper glance Cu
2
S
Zinc Zinc blende or Sphalerite ZnS
Calamine ZnCO
3
Zincite ZnO
Metal Ores Composition
For the purpose of extraction, bauxite is chosen for aluminium. For
iron, usually the oxide ores which are abundant and do not produce
polluting gases (like SO
2
 that is produced in case iron pyrites) are taken.
For copper and zinc, any of the listed ores (Table 6.1) may be used
depending upon availability and other relevant factors. Before proceeding
for concentration, ores are graded and crushed to reasonable size.
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 differences in
physical properties of the compound of the metal present and that of
the gangue. The type of the metal, the available facilities and the
environmental factors are also taken into consideration. Some of the
important procedures are described below.
This is based on the differences in gravities of the ore and the gangue
particles. It is therefore a type of gravity separation. In one such process,
Table 6.1: Principal Ores of Some Important Metals
6.2.1 Hydraulic
Washing
© NCERT
not to be republished
Page 3


A few elements like carbon, sulphur, gold and noble
gases, occur in free state while others in combined forms
in the earth’s crust. The extraction and isolation of an
element from its combined form involves various
principles of chemistry. A particular element may occur
in a variety of compounds. The process of metallurgy
and isolation should be such that it is chemically feasible
and commercially viable. Still, some general principles
are common to all the extraction processes of metals.
For obtaining a particular metal, first we look for
minerals which are naturally occurring chemical
substances in the earth’s crust obtainable by mining.
Out of many minerals in which a metal may be found,
only a few are viable to be used as sources of that
metal. Such minerals are known as ores.
Rarely, an ore contains only a desired substance.
It is usually contaminated with earthly or undesired
materials known as gangue. The extraction and isolation
of metals from ores involve the following major steps:
• Concentration of the ore,
• Isolation of the metal from its concentrated ore, and
• Purification of the metal.
The entire scientific and technological process used
for isolation of the metal from its ores is known as
metallurgy.
6
After studying this Unit, you will be
able to
• explain the terms minerals,
ores, concentration, benefaction,
calcination, roasting, refining, etc.;
• understand the principles of
oxidation and reduction as applied
to the extraction procedures;
• apply the thermodynamic
concepts like that of Gibbs energy
and entropy to the principles of
extraction of Al, Cu, Zn and Fe;
• explain why reduction of certain
oxides like Cu
2
O is much easier
than that of Fe
2
O
3
;
• explain why CO is a favourable
reducing agent at certain
temperatures while coke is better
in some other cases;
• explain why specific reducing
agents are used for the reduction
purposes.
Objectives
Thermodynamics illustrates why only a certain reducing element
and a minimum specific temperature are suitable for reduction of a
metal oxide to the metal in an extraction.
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
6
Unit Unit Unit Unit Unit
© NCERT
not to be republished
148 Chemistry
In the present Unit, first we shall describe various steps for effective
concentration of ores. After that we shall discuss the principles of some
of the common metallurgical processes. Those principles shall include
the thermodynamic and electrochemical aspects involved in the effective
reduction of the concentrated ore to the metal.
Elements vary in abundance. Among metals, aluminium is the most
abundant. It is the third most abundant element in earth’s crust (8.3%
approx. by weight). It is a major component of many igneous minerals
including mica and clays. Many gemstones are impure forms of Al
2
O
3
and the impurities range from Cr (in ‘ruby’) to Co (in ‘sapphire’). Iron
is the second most abundant metal in the earth’s crust. It forms a
variety of compounds and their various uses make it a very important
element. It is one of the essential elements in biological systems as well.
The principal ores of aluminium, iron, copper and zinc have been
given in Table 6.1.
6.1 6.1 6.1 6.1 6.1 Occurrence of Occurrence of Occurrence of Occurrence of Occurrence of
Metals Metals Metals Metals Metals
6.2 6.2 6.2 6.2 6.2 Concentration Concentration Concentration Concentration Concentration
of Ores of Ores of Ores of Ores of Ores
Aluminium Bauxite AlO
x
(OH)
3-2x
[where 0 < x < 1]
Kaolinite (a form of clay) [Al
2 
(OH)
4
 Si
2
O
5
]
Iron Haematite Fe
2
O
3
Magnetite Fe
3
O
4
Siderite FeCO
3
Iron pyrites FeS
2
Copper Copper pyrites CuFeS
2
Malachite CuCO
3
.Cu(OH)
2
Cuprite Cu
2
O
Copper glance Cu
2
S
Zinc Zinc blende or Sphalerite ZnS
Calamine ZnCO
3
Zincite ZnO
Metal Ores Composition
For the purpose of extraction, bauxite is chosen for aluminium. For
iron, usually the oxide ores which are abundant and do not produce
polluting gases (like SO
2
 that is produced in case iron pyrites) are taken.
For copper and zinc, any of the listed ores (Table 6.1) may be used
depending upon availability and other relevant factors. Before proceeding
for concentration, ores are graded and crushed to reasonable size.
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 differences in
physical properties of the compound of the metal present and that of
the gangue. The type of the metal, the available facilities and the
environmental factors are also taken into consideration. Some of the
important procedures are described below.
This is based on the differences in gravities of the ore and the gangue
particles. It is therefore a type of gravity separation. In one such process,
Table 6.1: Principal Ores of Some Important Metals
6.2.1 Hydraulic
Washing
© NCERT
not to be republished
149 General Principles and Processes of Isolation of Elements
Fig. 6.1:  Magnetic separation (schematic)
an upward stream of running water is used to wash the powdered ore.
The lighter gangue particles are washed away and the heavier ores are
left behind.
This is based on differences in
magnetic properties of the ore
components. If either the ore
or the gangue (one of these
two) is capable of being
attracted by a magnetic
field, then such separations
are carried out (e.g., in case
of iron ores). The ground
ore is carried on a conveyer
belt which passes over a
magnetic roller (Fig.6.1).
This method has been in use for removing gangue from sulphide ores. In
this process, a suspension of the powdered ore is made with water. To it,
collectors and froth stabilisers are added. Collectors (e. g., pine oils, fatty
acids, xanthates, etc.) enhance non-wettability
of the mineral particles and froth stabilisers
(e. g., cresols, aniline) stabilise the froth.
The mineral particles become wet by oils
while the gangue particles by water. A rotating
paddle agitates the mixture and draws air in
it. As a result, froth is formed which carries
the mineral particles. The froth is light and is
skimmed off. It is then dried for recovery of
the ore particles.
Sometimes, it is possible to separate two
sulphide ores by adjusting proportion of oil
to water or by using ‘depressants’. For
example, in case of an ore containing ZnS
and PbS, the depressant used is NaCN. It
selectively prevents ZnS from coming to the
froth but allows PbS to come with the froth.
The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman
One can do wonders if he or she has a scientific temperament and is attentive to
observations. A washerwoman had an innovative mind too. While washing a miner’s
overalls, she noticed that sand and similar dirt fell to the bottom of the washtub.
What was peculiar, the copper bearing compounds that had come to the clothes from
the mines, were caught in the soapsuds and so they came to the top. One of her
clients was a chemist, Mrs. Carrie Everson. The washerwoman told her experience to
Mrs. Everson. The latter thought that the idea could be used for separating copper
compounds from rocky and earth materials on large scale. This way an invention was
born. At that time only those ores were used for extraction of copper, which contained
large amounts of the metal. Invention of the Froth Floatation Method made copper
mining profitable even from the low-grade ores. World production of copper soared
and the metal became cheaper.
6.2.3 Froth
Floatation
Method
6.2.2 Magnetic
Separation
Fig. 6.2:  Froth floatation process (schematic)
© NCERT
not to be republished
Page 4


A few elements like carbon, sulphur, gold and noble
gases, occur in free state while others in combined forms
in the earth’s crust. The extraction and isolation of an
element from its combined form involves various
principles of chemistry. A particular element may occur
in a variety of compounds. The process of metallurgy
and isolation should be such that it is chemically feasible
and commercially viable. Still, some general principles
are common to all the extraction processes of metals.
For obtaining a particular metal, first we look for
minerals which are naturally occurring chemical
substances in the earth’s crust obtainable by mining.
Out of many minerals in which a metal may be found,
only a few are viable to be used as sources of that
metal. Such minerals are known as ores.
Rarely, an ore contains only a desired substance.
It is usually contaminated with earthly or undesired
materials known as gangue. The extraction and isolation
of metals from ores involve the following major steps:
• Concentration of the ore,
• Isolation of the metal from its concentrated ore, and
• Purification of the metal.
The entire scientific and technological process used
for isolation of the metal from its ores is known as
metallurgy.
6
After studying this Unit, you will be
able to
• explain the terms minerals,
ores, concentration, benefaction,
calcination, roasting, refining, etc.;
• understand the principles of
oxidation and reduction as applied
to the extraction procedures;
• apply the thermodynamic
concepts like that of Gibbs energy
and entropy to the principles of
extraction of Al, Cu, Zn and Fe;
• explain why reduction of certain
oxides like Cu
2
O is much easier
than that of Fe
2
O
3
;
• explain why CO is a favourable
reducing agent at certain
temperatures while coke is better
in some other cases;
• explain why specific reducing
agents are used for the reduction
purposes.
Objectives
Thermodynamics illustrates why only a certain reducing element
and a minimum specific temperature are suitable for reduction of a
metal oxide to the metal in an extraction.
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
6
Unit Unit Unit Unit Unit
© NCERT
not to be republished
148 Chemistry
In the present Unit, first we shall describe various steps for effective
concentration of ores. After that we shall discuss the principles of some
of the common metallurgical processes. Those principles shall include
the thermodynamic and electrochemical aspects involved in the effective
reduction of the concentrated ore to the metal.
Elements vary in abundance. Among metals, aluminium is the most
abundant. It is the third most abundant element in earth’s crust (8.3%
approx. by weight). It is a major component of many igneous minerals
including mica and clays. Many gemstones are impure forms of Al
2
O
3
and the impurities range from Cr (in ‘ruby’) to Co (in ‘sapphire’). Iron
is the second most abundant metal in the earth’s crust. It forms a
variety of compounds and their various uses make it a very important
element. It is one of the essential elements in biological systems as well.
The principal ores of aluminium, iron, copper and zinc have been
given in Table 6.1.
6.1 6.1 6.1 6.1 6.1 Occurrence of Occurrence of Occurrence of Occurrence of Occurrence of
Metals Metals Metals Metals Metals
6.2 6.2 6.2 6.2 6.2 Concentration Concentration Concentration Concentration Concentration
of Ores of Ores of Ores of Ores of Ores
Aluminium Bauxite AlO
x
(OH)
3-2x
[where 0 < x < 1]
Kaolinite (a form of clay) [Al
2 
(OH)
4
 Si
2
O
5
]
Iron Haematite Fe
2
O
3
Magnetite Fe
3
O
4
Siderite FeCO
3
Iron pyrites FeS
2
Copper Copper pyrites CuFeS
2
Malachite CuCO
3
.Cu(OH)
2
Cuprite Cu
2
O
Copper glance Cu
2
S
Zinc Zinc blende or Sphalerite ZnS
Calamine ZnCO
3
Zincite ZnO
Metal Ores Composition
For the purpose of extraction, bauxite is chosen for aluminium. For
iron, usually the oxide ores which are abundant and do not produce
polluting gases (like SO
2
 that is produced in case iron pyrites) are taken.
For copper and zinc, any of the listed ores (Table 6.1) may be used
depending upon availability and other relevant factors. Before proceeding
for concentration, ores are graded and crushed to reasonable size.
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 differences in
physical properties of the compound of the metal present and that of
the gangue. The type of the metal, the available facilities and the
environmental factors are also taken into consideration. Some of the
important procedures are described below.
This is based on the differences in gravities of the ore and the gangue
particles. It is therefore a type of gravity separation. In one such process,
Table 6.1: Principal Ores of Some Important Metals
6.2.1 Hydraulic
Washing
© NCERT
not to be republished
149 General Principles and Processes of Isolation of Elements
Fig. 6.1:  Magnetic separation (schematic)
an upward stream of running water is used to wash the powdered ore.
The lighter gangue particles are washed away and the heavier ores are
left behind.
This is based on differences in
magnetic properties of the ore
components. If either the ore
or the gangue (one of these
two) is capable of being
attracted by a magnetic
field, then such separations
are carried out (e.g., in case
of iron ores). The ground
ore is carried on a conveyer
belt which passes over a
magnetic roller (Fig.6.1).
This method has been in use for removing gangue from sulphide ores. In
this process, a suspension of the powdered ore is made with water. To it,
collectors and froth stabilisers are added. Collectors (e. g., pine oils, fatty
acids, xanthates, etc.) enhance non-wettability
of the mineral particles and froth stabilisers
(e. g., cresols, aniline) stabilise the froth.
The mineral particles become wet by oils
while the gangue particles by water. A rotating
paddle agitates the mixture and draws air in
it. As a result, froth is formed which carries
the mineral particles. The froth is light and is
skimmed off. It is then dried for recovery of
the ore particles.
Sometimes, it is possible to separate two
sulphide ores by adjusting proportion of oil
to water or by using ‘depressants’. For
example, in case of an ore containing ZnS
and PbS, the depressant used is NaCN. It
selectively prevents ZnS from coming to the
froth but allows PbS to come with the froth.
The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman
One can do wonders if he or she has a scientific temperament and is attentive to
observations. A washerwoman had an innovative mind too. While washing a miner’s
overalls, she noticed that sand and similar dirt fell to the bottom of the washtub.
What was peculiar, the copper bearing compounds that had come to the clothes from
the mines, were caught in the soapsuds and so they came to the top. One of her
clients was a chemist, Mrs. Carrie Everson. The washerwoman told her experience to
Mrs. Everson. The latter thought that the idea could be used for separating copper
compounds from rocky and earth materials on large scale. This way an invention was
born. At that time only those ores were used for extraction of copper, which contained
large amounts of the metal. Invention of the Froth Floatation Method made copper
mining profitable even from the low-grade ores. World production of copper soared
and the metal became cheaper.
6.2.3 Froth
Floatation
Method
6.2.2 Magnetic
Separation
Fig. 6.2:  Froth floatation process (schematic)
© NCERT
not to be republished
150 Chemistry
Intext Questions Intext Questions Intext Questions Intext Questions Intext Questions
6.1 Which of the ores mentioned in Table 6.1 can be concentrated by
magnetic separation method?
6.2 What is the significance of leaching in the extraction of aluminium?
6.3 6.3 6.3 6.3 6.3 Extraction Extraction Extraction Extraction Extraction
of Crude of Crude of Crude of Crude of Crude
Metal from Metal from Metal from Metal from Metal from
Concentrated Concentrated Concentrated Concentrated Concentrated
Ore Ore Ore Ore Ore
Leaching is often used if the ore is soluble in some suitable solvent.
The following examples illustrate the procedure:
(a) Leaching of alumina from bauxite
The principal ore of aluminium, bauxite, usually contains SiO
2
,
iron oxides and titanium oxide (TiO
2
) 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, Al
2
O
3 
is leached out as sodium aluminate (and SiO
2
 too as
sodium silicate) leaving the impurities behind:
Al
2
O
3
(s) + 2NaOH(aq) + 3H
2
O(l) ? 2Na[Al(OH)
 4
](aq) (6.1)
The aluminate in solution is neutralised by passing CO
2
 gas and hydrated
Al
2
O
3
 is precipitated. At this stage, the solution is seeded with freshly
prepared samples of hydrated Al
2
O
3
 which induces the precipitation:
2Na[Al(OH)
4
](aq) + CO
2
(g) ? Al
2
O
3
.xH
2
O(s) + 2NaHCO
3
 (aq)  (6.2)
The sodium silicate remains in the solution and hydrated alumina
is filtered, dried and heated to give back pure Al
2
O
3
:
Al
2
O
3
.xH
2
O(s)
1470 K
 Al
2
O
3
(s) + xH
2
O(g) (6.3)
(b) Other examples
In the metallurgy of silver and that of gold, the respective metal is
leached with a dilute solution of NaCN or KCN in the presence of
air (for O
2
) from which the metal is obtained later by replacement:
4M(s) + 8CN
–
(aq)+ 2H
2
O(aq) + O
2
(g)
 
?
 
4[M(CN)
2
]
– 
(aq) +
    4OH
–
(aq) (M= Ag or Au) (6.4)
()
[]() () () [] () ()
2
2 4
MZn 2Zn 2M aq aq CN CNss
--
+? + (6.5)
The concentrated ore must be converted into a form which is suitable
for reduction. Usually the sulphide ore is converted to oxide before
reduction. Oxides are easier to reduce (for the reason see box). Thus
isolation of metals from concentrated ore involves two major steps viz.,
(a) conversion to oxide, and
(b) reduction of the oxide to metal.
(a) Conversion to oxide
(i) Calcination: Calcinaton involves heating when the volatile matter
escapes leaving behind the metal oxide:
Fe
2
O
3
.xH
2
O(s) 

 Fe
2
O
3
 (s) + xH
2
O(g) (6.6)
ZnCO
3 
(s) 

 ZnO(s) + CO
2
(g) (6.7)
CaCO
3
.MgCO
3
(s) 

 CaO(s) + MgO(s ) + 2CO
2
(g) (6.8)
6.2.4 Leaching
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Page 5


A few elements like carbon, sulphur, gold and noble
gases, occur in free state while others in combined forms
in the earth’s crust. The extraction and isolation of an
element from its combined form involves various
principles of chemistry. A particular element may occur
in a variety of compounds. The process of metallurgy
and isolation should be such that it is chemically feasible
and commercially viable. Still, some general principles
are common to all the extraction processes of metals.
For obtaining a particular metal, first we look for
minerals which are naturally occurring chemical
substances in the earth’s crust obtainable by mining.
Out of many minerals in which a metal may be found,
only a few are viable to be used as sources of that
metal. Such minerals are known as ores.
Rarely, an ore contains only a desired substance.
It is usually contaminated with earthly or undesired
materials known as gangue. The extraction and isolation
of metals from ores involve the following major steps:
• Concentration of the ore,
• Isolation of the metal from its concentrated ore, and
• Purification of the metal.
The entire scientific and technological process used
for isolation of the metal from its ores is known as
metallurgy.
6
After studying this Unit, you will be
able to
• explain the terms minerals,
ores, concentration, benefaction,
calcination, roasting, refining, etc.;
• understand the principles of
oxidation and reduction as applied
to the extraction procedures;
• apply the thermodynamic
concepts like that of Gibbs energy
and entropy to the principles of
extraction of Al, Cu, Zn and Fe;
• explain why reduction of certain
oxides like Cu
2
O is much easier
than that of Fe
2
O
3
;
• explain why CO is a favourable
reducing agent at certain
temperatures while coke is better
in some other cases;
• explain why specific reducing
agents are used for the reduction
purposes.
Objectives
Thermodynamics illustrates why only a certain reducing element
and a minimum specific temperature are suitable for reduction of a
metal oxide to the metal in an extraction.
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
Gener Gener Gener Gener General P al P al P al P al Principles and rinciples and rinciples and rinciples and rinciples and
P P P P Pr r r r rocesses o ocesses o ocesses o ocesses o ocesses of Isolation f Isolation f Isolation f Isolation f Isolation
o o o o of Element f Element f Element f Element f Elements s s s s
6
Unit Unit Unit Unit Unit
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148 Chemistry
In the present Unit, first we shall describe various steps for effective
concentration of ores. After that we shall discuss the principles of some
of the common metallurgical processes. Those principles shall include
the thermodynamic and electrochemical aspects involved in the effective
reduction of the concentrated ore to the metal.
Elements vary in abundance. Among metals, aluminium is the most
abundant. It is the third most abundant element in earth’s crust (8.3%
approx. by weight). It is a major component of many igneous minerals
including mica and clays. Many gemstones are impure forms of Al
2
O
3
and the impurities range from Cr (in ‘ruby’) to Co (in ‘sapphire’). Iron
is the second most abundant metal in the earth’s crust. It forms a
variety of compounds and their various uses make it a very important
element. It is one of the essential elements in biological systems as well.
The principal ores of aluminium, iron, copper and zinc have been
given in Table 6.1.
6.1 6.1 6.1 6.1 6.1 Occurrence of Occurrence of Occurrence of Occurrence of Occurrence of
Metals Metals Metals Metals Metals
6.2 6.2 6.2 6.2 6.2 Concentration Concentration Concentration Concentration Concentration
of Ores of Ores of Ores of Ores of Ores
Aluminium Bauxite AlO
x
(OH)
3-2x
[where 0 < x < 1]
Kaolinite (a form of clay) [Al
2 
(OH)
4
 Si
2
O
5
]
Iron Haematite Fe
2
O
3
Magnetite Fe
3
O
4
Siderite FeCO
3
Iron pyrites FeS
2
Copper Copper pyrites CuFeS
2
Malachite CuCO
3
.Cu(OH)
2
Cuprite Cu
2
O
Copper glance Cu
2
S
Zinc Zinc blende or Sphalerite ZnS
Calamine ZnCO
3
Zincite ZnO
Metal Ores Composition
For the purpose of extraction, bauxite is chosen for aluminium. For
iron, usually the oxide ores which are abundant and do not produce
polluting gases (like SO
2
 that is produced in case iron pyrites) are taken.
For copper and zinc, any of the listed ores (Table 6.1) may be used
depending upon availability and other relevant factors. Before proceeding
for concentration, ores are graded and crushed to reasonable size.
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 differences in
physical properties of the compound of the metal present and that of
the gangue. The type of the metal, the available facilities and the
environmental factors are also taken into consideration. Some of the
important procedures are described below.
This is based on the differences in gravities of the ore and the gangue
particles. It is therefore a type of gravity separation. In one such process,
Table 6.1: Principal Ores of Some Important Metals
6.2.1 Hydraulic
Washing
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149 General Principles and Processes of Isolation of Elements
Fig. 6.1:  Magnetic separation (schematic)
an upward stream of running water is used to wash the powdered ore.
The lighter gangue particles are washed away and the heavier ores are
left behind.
This is based on differences in
magnetic properties of the ore
components. If either the ore
or the gangue (one of these
two) is capable of being
attracted by a magnetic
field, then such separations
are carried out (e.g., in case
of iron ores). The ground
ore is carried on a conveyer
belt which passes over a
magnetic roller (Fig.6.1).
This method has been in use for removing gangue from sulphide ores. In
this process, a suspension of the powdered ore is made with water. To it,
collectors and froth stabilisers are added. Collectors (e. g., pine oils, fatty
acids, xanthates, etc.) enhance non-wettability
of the mineral particles and froth stabilisers
(e. g., cresols, aniline) stabilise the froth.
The mineral particles become wet by oils
while the gangue particles by water. A rotating
paddle agitates the mixture and draws air in
it. As a result, froth is formed which carries
the mineral particles. The froth is light and is
skimmed off. It is then dried for recovery of
the ore particles.
Sometimes, it is possible to separate two
sulphide ores by adjusting proportion of oil
to water or by using ‘depressants’. For
example, in case of an ore containing ZnS
and PbS, the depressant used is NaCN. It
selectively prevents ZnS from coming to the
froth but allows PbS to come with the froth.
The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman The Innovative Washerwoman
One can do wonders if he or she has a scientific temperament and is attentive to
observations. A washerwoman had an innovative mind too. While washing a miner’s
overalls, she noticed that sand and similar dirt fell to the bottom of the washtub.
What was peculiar, the copper bearing compounds that had come to the clothes from
the mines, were caught in the soapsuds and so they came to the top. One of her
clients was a chemist, Mrs. Carrie Everson. The washerwoman told her experience to
Mrs. Everson. The latter thought that the idea could be used for separating copper
compounds from rocky and earth materials on large scale. This way an invention was
born. At that time only those ores were used for extraction of copper, which contained
large amounts of the metal. Invention of the Froth Floatation Method made copper
mining profitable even from the low-grade ores. World production of copper soared
and the metal became cheaper.
6.2.3 Froth
Floatation
Method
6.2.2 Magnetic
Separation
Fig. 6.2:  Froth floatation process (schematic)
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150 Chemistry
Intext Questions Intext Questions Intext Questions Intext Questions Intext Questions
6.1 Which of the ores mentioned in Table 6.1 can be concentrated by
magnetic separation method?
6.2 What is the significance of leaching in the extraction of aluminium?
6.3 6.3 6.3 6.3 6.3 Extraction Extraction Extraction Extraction Extraction
of Crude of Crude of Crude of Crude of Crude
Metal from Metal from Metal from Metal from Metal from
Concentrated Concentrated Concentrated Concentrated Concentrated
Ore Ore Ore Ore Ore
Leaching is often used if the ore is soluble in some suitable solvent.
The following examples illustrate the procedure:
(a) Leaching of alumina from bauxite
The principal ore of aluminium, bauxite, usually contains SiO
2
,
iron oxides and titanium oxide (TiO
2
) 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, Al
2
O
3 
is leached out as sodium aluminate (and SiO
2
 too as
sodium silicate) leaving the impurities behind:
Al
2
O
3
(s) + 2NaOH(aq) + 3H
2
O(l) ? 2Na[Al(OH)
 4
](aq) (6.1)
The aluminate in solution is neutralised by passing CO
2
 gas and hydrated
Al
2
O
3
 is precipitated. At this stage, the solution is seeded with freshly
prepared samples of hydrated Al
2
O
3
 which induces the precipitation:
2Na[Al(OH)
4
](aq) + CO
2
(g) ? Al
2
O
3
.xH
2
O(s) + 2NaHCO
3
 (aq)  (6.2)
The sodium silicate remains in the solution and hydrated alumina
is filtered, dried and heated to give back pure Al
2
O
3
:
Al
2
O
3
.xH
2
O(s)
1470 K
 Al
2
O
3
(s) + xH
2
O(g) (6.3)
(b) Other examples
In the metallurgy of silver and that of gold, the respective metal is
leached with a dilute solution of NaCN or KCN in the presence of
air (for O
2
) from which the metal is obtained later by replacement:
4M(s) + 8CN
–
(aq)+ 2H
2
O(aq) + O
2
(g)
 
?
 
4[M(CN)
2
]
– 
(aq) +
    4OH
–
(aq) (M= Ag or Au) (6.4)
()
[]() () () [] () ()
2
2 4
MZn 2Zn 2M aq aq CN CNss
--
+? + (6.5)
The concentrated ore must be converted into a form which is suitable
for reduction. Usually the sulphide ore is converted to oxide before
reduction. Oxides are easier to reduce (for the reason see box). Thus
isolation of metals from concentrated ore involves two major steps viz.,
(a) conversion to oxide, and
(b) reduction of the oxide to metal.
(a) Conversion to oxide
(i) Calcination: Calcinaton involves heating when the volatile matter
escapes leaving behind the metal oxide:
Fe
2
O
3
.xH
2
O(s) 

 Fe
2
O
3
 (s) + xH
2
O(g) (6.6)
ZnCO
3 
(s) 

 ZnO(s) + CO
2
(g) (6.7)
CaCO
3
.MgCO
3
(s) 

 CaO(s) + MgO(s ) + 2CO
2
(g) (6.8)
6.2.4 Leaching
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151 General Principles and Processes of Isolation of Elements
Fig. 6.3: A section of a modern
reverberatory furnace
(ii)  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:
2ZnS + 3O
2
 ? 2ZnO + 2SO
2
(6.9)
2PbS + 3O
2
 ? 2PbO + 2SO
2
(6.10)
2Cu
2
S + 3O
2
 ? 2Cu
2
O + 2SO
2
(6.11)
The sulphide ores of copper are heated
in reverberatory furnace. If the ore contains
iron, it is mixed with silica before heating.
Iron oxide ‘slags of’* as iron silicate and
copper is produced in the form of copper
matte which contains Cu
2
S and FeS.
FeO + SiO
2
 ? FeSiO
3
(6.12)
                  (slag)
The SO
2
 produced is utilised for manufacturing H
2
SO
4 
.
(b) Reduction of oxide to the metal
Reduction of the metal oxide usually involves heating it with some
other substance acting as a reducing agent (C or CO or even another
metal). The reducing agent (e.g., carbon) combines with the oxygen
of the metal oxide.
M
x
O
y
 + yC ? xM + y CO (6.13)
Some metal oxides get reduced easily while others are very
difficult to be reduced (reduction means electron gain or
electronation). In any case, heating is required. To understand the
variation in the temperature requirement for thermal reductions
(pyrometallurgy) and to predict which element will suit as the
reducing agent for a given metal oxide (M
x
O
y
), Gibbs energy
interpretations are made.
Some basic concepts of thermodynamics help us in understanding the
theory of metallurgical transformations. Gibbs energy is the most
significant term here.The change in Gibbs energy, ?G for any process
at any specified temperature, is described by the equation:
?G = ?H – T?S (6.14)
where, ?H is the enthalpy change and ?S is the entropy change for
the process. For any reaction, this change could also be explained
through the equation:
?G
V
 = – RTlnK (6.15)
where, K is the equilibrium constant of the ‘reactant – product’
system at the temperature,T. A negative ?G implies a +ve K in equation
6.15. And this can happen only when reaction proceeds towards
products. From these facts we can make the following conclusions:
6.4 6.4 6.4 6.4 6.4
Thermodynamic Thermodynamic Thermodynamic Thermodynamic Thermodynamic
Principles of Principles of Principles of Principles of Principles of
Metallurgy Metallurgy Metallurgy Metallurgy Metallurgy
* During metallurgy, ‘flux’ is added which combines with ‘gangue’ to form ‘slag’. Slag separates more easily from
the ore than the gangue. This way, removal of gangue becomes easier.
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