Metals and metallurgy
The periodic table consists of 118 elements, sorted by atomic number, atomic weight,
symbols, discovered year and the group. The elements have been broadly divided into metals
and non-metals on the basis of their physical and chemical properties. Metals have low
ionisation energies, low electron affinities and low electronegativities which are responsible
for their high electrical and thermal conductivities, high malleability and ductility, metallic
lustre and hardness. Out of the 118 elements, 80 elements are metals. In the present chapter,
we shall study the occurrence and general methods used for the extraction of the metals from
its ores.
Occurrence of Metals
Metals occur in nature in free or combined state. A metal is said to occur in free state
if it is found in nature in it’s metallic form. In combined state, metals occur as compounds
with other elements usually non-metallic. The earth’s crust is the biggest source of metals
although soluble salts of metals are also found in the seawater.
Noble metals have little or no affinity for oxygen and can resist the attack of water
and other chemical reagents. They occur in the free, i.e. the native state. Thus, metals such as
silver, gold and platinum occur in nature in native state along with some alluvial impurities
such a s clay and sand. Sometimes lumps of pure metals known as nuggets are also found.
The standard electrode potentials (reduction potentials) of these metals are always positive
which means that the ions of such metals have a tendency to accept electrons and get reduced
to their corresponding metals.
The standard potentials of active metals are negative which means these metals
always have a tendency to get oxidised to their ions. The ions of these metals are reluctant to
get reduced to their metals. That is why these metals occur in nature in the combined state,
i.e. in the form of compounds. These compounds are known as minerals.
The minerals from which metals can be conveniently and economically extracted
are referred to as ores.
Active metals occur in the form of the following types of ores:
Sulphide ores: Metals such as iron, mercury, copper, etc., occur as their sulphides, e.g., iron
pyrites (FeS), galena (PbS), cinnabar (HgS), copper pyrites (CuFeS), zinc blende (ZnS), etc.
Oxide ores: Metals such as iron, aluminium, manganese, tin, iron, zinc, etc., occur as their
oxides, e.g., haematitie (Fe2O3), bauxite (Al2O3.2H2O), pyrolusite (MnO2), casserite (SnO2),
zincate (ZnO2), etc.
Carbonate ores: Some important carbonate ores are of manganese, iron, copper, zinc and
lead, e.g., magnesite (MgCO3), dolomite (CaCO3, MgCO3), siderite (FeCO3), calamine
(ZnCO3), etc.
Halide ores: Amongst the halides, chloride ores are the most common. Carnalite
(KCl.MgCl2.6H2O), rock salt (NaCl) and horn silver (AgCl) are common among the halide
ores. Bromides and iodides of potassium and magnesium are present in small quantities in sea
water. The fluoride ores include fluorspar (CaF2) and cryolite (AlF3.3NaF).
1
,Sulphate ores: Many sulphide ores get converted into sulphates by atmospheric oxidation.
The common sulphate ores are barytes (BaSO4) and anhydrite (CaSO4).
Metallurgy
The process of extracting metals from their ores is called metallurgy. The process of
metallurgy depends on the nature of the ore from which the ore is to be extracted. It is not
possible to scheme out a universal scheme for extraction of all metals since the extraction of
each metal involves individual procedure depending upon its nature and properties.
Various Steps Involved in Metallurgical Processes: The various steps involved in the
extraction of pure metals from their respective ores are as follows:
1. Concentration of the Ore: Metallic ores are often found mixed with rocky material,
e.g. quartz, felspar, mica and other silicates. These impurities are known as gangue.
Before the ore is subjected to metallurgical processes, it is necessary to remove these
unwanted impurities. This operation is known as concentration of the ore. The
common methods of concentration are as follows:
a. Hand picking: The ore is separated from the main stock in a sufficient degree of
purity by simply picking it by hand and then eliminating the rocky material
adhered to the ore by breaking with hammer.
b. Gravity separation: The ore can be concentrated by taking advantage of the
differences in the specific gravity of the impurities and the gangue particles. The
grounded ore is subjected to agitation with water or washed with a running stream
of water. The running water takes with it the lighter gangue particles and the
heavier ore particles settle at the bottom.
c. Magnetic separation: This method is used for separating a magnetic ore from its
non-magnetic impurities by means of magnetic separator. The ore is first crushed
to powder which is carried over a moving belt passing through a magnetic roller
and get stuck to the surface of the belt while the non-magnetic gangue particle fall
aside. The ore particles then fall off as a separate heap when the belt moves a little
away from the magnetic roller. This method is applicable only to magnetic ores.
Chromite (FeO.Cr2O3) and pyrolusite (MnO2) are ores which are magnetic and
therefore can be concentrated by magnetic method.
Fig. 1.1: Magnetic separation (schematic)
2
, d. Electrostatic separation: This method is based on the fact that particles which
are good conductors of electricity become electrically charged as soon as they are
brought into an electrostatic field and are consequently repelled by the electrode
carrying like charge. Lead sulphide and zinc sulphide ores which occur together in
nature are separated by this method. The mixture ore is grounded and then fed
upon a roller in a thin layer and subjected to an electrostatic field. Lead sulphide,
being a good conductor gets charged immediately and is thrown away from the
roller due to electrical repulsion. The zinc sulphide ore which is a poor conductor,
drops vertically from the roller.
e. Froth floatation: This method is based on the principle of preferential wetting of
the solid surfaces by liquids. Metallic sulphides are wetted by certain oil like pine
oil but not by water.
The grounded ore is put in water and then little amount of pine oil is added. The
mixture is agitated violently with air (Fig. 1.2) when froth is formed at the air-
water interface. The ore particles which are preferentially wetted by oil rises up to
the with the froth while the gangue particles wetted by water remains below the
froth and after sometime settles down. The froth separates out, settles down as
shown in the left of the figure. This process is the Froth floatation process.
Fig. 1.2: Froth Floatation Technique
f. Leaching: This is a chemical method for concentration of ores. In this method, the
powdered ore is treated with a suitable solvent which may dissolve the ore but not
the impurities. The impurities are filtered off and the ore is recovered from the
solution by a suitable chemical method.
Bauxite is the principal ore of aluminium. It usually contains SiO 2, iron oxides and
titanium oxide (TiO2) as impurities. Concentration is carried out by heating the
powdered ore with a concentrated solution of NaOH at 473 – 523 K and 35–36
bar pressure. This process is called digestion. This way, Al2O3 is extracted out as
sodium aluminate. The impurity, SiO2 too dissolves forming sodium silicate.
Other impurities are left behind.
Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2Na[Al(OH)4](aq).
3
The periodic table consists of 118 elements, sorted by atomic number, atomic weight,
symbols, discovered year and the group. The elements have been broadly divided into metals
and non-metals on the basis of their physical and chemical properties. Metals have low
ionisation energies, low electron affinities and low electronegativities which are responsible
for their high electrical and thermal conductivities, high malleability and ductility, metallic
lustre and hardness. Out of the 118 elements, 80 elements are metals. In the present chapter,
we shall study the occurrence and general methods used for the extraction of the metals from
its ores.
Occurrence of Metals
Metals occur in nature in free or combined state. A metal is said to occur in free state
if it is found in nature in it’s metallic form. In combined state, metals occur as compounds
with other elements usually non-metallic. The earth’s crust is the biggest source of metals
although soluble salts of metals are also found in the seawater.
Noble metals have little or no affinity for oxygen and can resist the attack of water
and other chemical reagents. They occur in the free, i.e. the native state. Thus, metals such as
silver, gold and platinum occur in nature in native state along with some alluvial impurities
such a s clay and sand. Sometimes lumps of pure metals known as nuggets are also found.
The standard electrode potentials (reduction potentials) of these metals are always positive
which means that the ions of such metals have a tendency to accept electrons and get reduced
to their corresponding metals.
The standard potentials of active metals are negative which means these metals
always have a tendency to get oxidised to their ions. The ions of these metals are reluctant to
get reduced to their metals. That is why these metals occur in nature in the combined state,
i.e. in the form of compounds. These compounds are known as minerals.
The minerals from which metals can be conveniently and economically extracted
are referred to as ores.
Active metals occur in the form of the following types of ores:
Sulphide ores: Metals such as iron, mercury, copper, etc., occur as their sulphides, e.g., iron
pyrites (FeS), galena (PbS), cinnabar (HgS), copper pyrites (CuFeS), zinc blende (ZnS), etc.
Oxide ores: Metals such as iron, aluminium, manganese, tin, iron, zinc, etc., occur as their
oxides, e.g., haematitie (Fe2O3), bauxite (Al2O3.2H2O), pyrolusite (MnO2), casserite (SnO2),
zincate (ZnO2), etc.
Carbonate ores: Some important carbonate ores are of manganese, iron, copper, zinc and
lead, e.g., magnesite (MgCO3), dolomite (CaCO3, MgCO3), siderite (FeCO3), calamine
(ZnCO3), etc.
Halide ores: Amongst the halides, chloride ores are the most common. Carnalite
(KCl.MgCl2.6H2O), rock salt (NaCl) and horn silver (AgCl) are common among the halide
ores. Bromides and iodides of potassium and magnesium are present in small quantities in sea
water. The fluoride ores include fluorspar (CaF2) and cryolite (AlF3.3NaF).
1
,Sulphate ores: Many sulphide ores get converted into sulphates by atmospheric oxidation.
The common sulphate ores are barytes (BaSO4) and anhydrite (CaSO4).
Metallurgy
The process of extracting metals from their ores is called metallurgy. The process of
metallurgy depends on the nature of the ore from which the ore is to be extracted. It is not
possible to scheme out a universal scheme for extraction of all metals since the extraction of
each metal involves individual procedure depending upon its nature and properties.
Various Steps Involved in Metallurgical Processes: The various steps involved in the
extraction of pure metals from their respective ores are as follows:
1. Concentration of the Ore: Metallic ores are often found mixed with rocky material,
e.g. quartz, felspar, mica and other silicates. These impurities are known as gangue.
Before the ore is subjected to metallurgical processes, it is necessary to remove these
unwanted impurities. This operation is known as concentration of the ore. The
common methods of concentration are as follows:
a. Hand picking: The ore is separated from the main stock in a sufficient degree of
purity by simply picking it by hand and then eliminating the rocky material
adhered to the ore by breaking with hammer.
b. Gravity separation: The ore can be concentrated by taking advantage of the
differences in the specific gravity of the impurities and the gangue particles. The
grounded ore is subjected to agitation with water or washed with a running stream
of water. The running water takes with it the lighter gangue particles and the
heavier ore particles settle at the bottom.
c. Magnetic separation: This method is used for separating a magnetic ore from its
non-magnetic impurities by means of magnetic separator. The ore is first crushed
to powder which is carried over a moving belt passing through a magnetic roller
and get stuck to the surface of the belt while the non-magnetic gangue particle fall
aside. The ore particles then fall off as a separate heap when the belt moves a little
away from the magnetic roller. This method is applicable only to magnetic ores.
Chromite (FeO.Cr2O3) and pyrolusite (MnO2) are ores which are magnetic and
therefore can be concentrated by magnetic method.
Fig. 1.1: Magnetic separation (schematic)
2
, d. Electrostatic separation: This method is based on the fact that particles which
are good conductors of electricity become electrically charged as soon as they are
brought into an electrostatic field and are consequently repelled by the electrode
carrying like charge. Lead sulphide and zinc sulphide ores which occur together in
nature are separated by this method. The mixture ore is grounded and then fed
upon a roller in a thin layer and subjected to an electrostatic field. Lead sulphide,
being a good conductor gets charged immediately and is thrown away from the
roller due to electrical repulsion. The zinc sulphide ore which is a poor conductor,
drops vertically from the roller.
e. Froth floatation: This method is based on the principle of preferential wetting of
the solid surfaces by liquids. Metallic sulphides are wetted by certain oil like pine
oil but not by water.
The grounded ore is put in water and then little amount of pine oil is added. The
mixture is agitated violently with air (Fig. 1.2) when froth is formed at the air-
water interface. The ore particles which are preferentially wetted by oil rises up to
the with the froth while the gangue particles wetted by water remains below the
froth and after sometime settles down. The froth separates out, settles down as
shown in the left of the figure. This process is the Froth floatation process.
Fig. 1.2: Froth Floatation Technique
f. Leaching: This is a chemical method for concentration of ores. In this method, the
powdered ore is treated with a suitable solvent which may dissolve the ore but not
the impurities. The impurities are filtered off and the ore is recovered from the
solution by a suitable chemical method.
Bauxite is the principal ore of aluminium. It usually contains SiO 2, iron oxides and
titanium oxide (TiO2) as impurities. Concentration is carried out by heating the
powdered ore with a concentrated solution of NaOH at 473 – 523 K and 35–36
bar pressure. This process is called digestion. This way, Al2O3 is extracted out as
sodium aluminate. The impurity, SiO2 too dissolves forming sodium silicate.
Other impurities are left behind.
Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2Na[Al(OH)4](aq).
3
