General Principles and Processes of Isolation of Elements
After studying this Unit, you will be
able to:
appreciate the contribution of
Indian traditions in the
metallurgical processes,
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
O is much easier
than that of Fe
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
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.
General Principles and
Processes of Isolation
of Elements
General Principles and
rocesses of Isolation
of Elements
The history of civilisation is linked to the use of metals
in antiquity in many ways. Different periods of early
human civilisations have been named after metals.
The skill of extraction of metals gave many metals
and brought about several changes in the human
society. It gave weapons, tools, ornaments, utensils,
etc., and enriched the cultural life. The ‘Seven metals
of antiquity’, as they are sometimes called, are gold,
copper, silver, lead, tin, iron and mercury. Although
modern metallurgy had exponential growth after
Industrial Revolution, it is interesting to note that
many modern concepts in metallurgy have their roots
in ancient practices that pre-dated the Industrial
Revolution. For over 7000 years, India has had a
rich tradition of metallurgical skills.
The two important sources for the history of Indian
metallurgy are archaeological excavations and literary
evidences. The first evidence of metal in Indian
subcontinent comes from Mehrgarh in Baluchistan,
where a small copper bead, dated to about 6000
BCE was found. It is however thought to be native
copper, which has not been extracted from the ore.
Spectrometric studies on copper ore samples obtained
from the ancient mine pits at Khetri in Rajasthan and
on metal samples cut from representative Harappan
artefacts recovered from Mitathal in Haryana and eight other sites
distributed in Rajasthan, Gujarat, Madhya Pradesh and Maharashtra
prove that copper metallurgy in India dates back to the Chalcolithic
cultures in the subcontinent. Indian chalcolithic copper objects were
in all probability made indigenously. The ore for extraction of metal for
making the objects was obtained from chalcopyrite ore deposits in
Aravalli Hills. Collection of archaeological texts from copper-plates and
rock-inscriptions have been compiled and published by the
Archaeological Survey of India during the past century. Royal records
were engraved on copper plates (tamra-patra). Earliest known copper-
plate has a Mauryan record that mentions famine relief efforts. It has
one of the very few pre-Ashoka Brahmi inscriptions in India.
Harappans also used gold and silver, as well as their joint alloy
electrum. Variety of ornaments such as pendants, bangles, beads and
rings have been found in ceramic or bronze pots. Early gold and silver
ornaments have been found from Indus Valley sites such as
Mohenjodaro (3000 BCE). These are on display in the National Museum,
New Delhi. India has the distinction of having the deepest ancient gold
mines in the world, in the Maski region of Karnataka. Carbon dating
places them in mid 1st millennium BCE.
Hymns of Rigveda give earliest indirect references to the alluvial
placer gold deposits in India. The river Sindhu was an important source
of gold in ancient times. It is interesting that the availability of alluvial
placer gold in the river Sindhu has been reported in modern times also.
It has been reported that there are great mines of gold in the region of
Mansarovar and in Thokjalyug even now. The Pali text, Anguttara Nikaya
narrates the process of the recovery of gold dust or particles from alluvial
placer gold deposits. Although evidence of gold refining is available in
Vedic texts, it is Kautilya’s Arthashastra, authored probably in 3rd or
4th century BCE, during Mauryan era, which has much data on
prevailing chemical practices in a long section on mines and minerals
including metal ores of gold, silver, copper, lead, tin and iron. Kautilya
describes a variety of gold called rasviddha, which is naturally occurring
gold solution. Kalidas also mentioned about such solutions. It is
astonishing how people recognised such solutions.
The native gold has different colours depending upon the nature and
amount of impurity present in it. It is likely that the different colours of
native gold were a major driving force for the development of gold refining.
Recent excavations in central parts of Ganges Valley and Vindhya
hills have shown that iron was produced there possibly as early as in
1800 BCE. In the recent excavations conducted by the Uttar Pradesh
State Archaeological Department, iron furnaces, artefacts, tuyers and
layers of slag have been found. Radiocarbon dating places them between
BCE 1800 and 1000. The results of excavation indicate that the
knowledge of iron smelting and manufacturing of iron artefacts was
well known in Eastern Vindhyas and it was in use in the Central
Ganga Plains, at least from the early 2nd millennium BCE. The quantity
and types of iron artefacts and the level of technical advancements
indicate that working of iron would have been introduced much earlier.
General Principles and Processes of Isolation of Elements
The evidence indicates early use of iron in other areas of the country,
which proves that India was indeed an independent centre for the
development of the working of iron.
Iron smelting and the use of iron was especially established in
South Indian megalithic cultures. The forging of wrought iron seems
to have been at peak in India in the Ist millennium CE. Greek accounts
report the manufacture of steel in India by crucible process. In this
process, iron, charcoal and glass were mixed together in a crucible
and heated until the iron melted and absorbed the carbon. India was
a major innovator in the production of advanced quality steel. Indian
steel was called ‘the Wonder Material of the Orient’. A Roman historian,
Quintus Curtius, records that one of the gifts Porus of Taxila (326
BCE) gave to Alexander the Great was some two-and-a-half tons of
Wootz steel. Wootz steel is primarily iron containing a high proportion
of carbon (1.0 – 1.9%). Wootz is the English version of the word ‘ukku’
which is used for steel in Karnataka and Andhra Pradesh. Literary
accounts suggest that Indian Wootz steel from southern part of the
Indian subcontinent was exported to Europe, China and Arab world.
It became prominent in the Middle East where it was named as
Damasus Steel. Michael Faraday tried to duplicate this steel by alloying
iron with a variety of metals, including noble metals, but failed.
When iron ore is reduced in solid state by using charcoal, porous
iron blocks are formed. Therefore, reduced iron blocks are also called
sponge iron blocks. Any useful product can be obtained from this
material only after removing the porosity by hot forging. The iron so
obtained is termed as wrought iron. An exciting example of wrought
iron produced in ancient India is the world famous Iron Pillar. It was
erected in its present position in Delhi in 5th century CE. The Sanskrit
inscription engraved on it suggests that it was brought here from
elsewhere during the Gupta Period. The average composition (weight%)
of the components present in the wrought iron of the pillar, besides
iron, are 0.15% C, 0.05% Si, 0.05% Mn, 0.25% P, 0.005% Ni, 0.03%
Cu and 0.02% N. The most significant aspect of the pillar is that there
is no sign of corrosion inspite of the fact that it has been exposed to
the atmosphere for about 1,600 years.
Radiocarbon dating of charcoal from iron slag revealed evidence
of continuous smelting in Khasi Hills of Meghalaya. The slag layer,
which is dated to 353 BCE – CE 128, indicates that Khasi Hill region
is the earliest iron smelting site studied in the entire region of North
East India. The remnants of former iron-ore excavation and iron
manufacturing are visible even now in the landscape of Khasi Hills.
British naturalists who visited Meghalaya in early 19th century
described the iron industry that had developed in the upper part of
the Khasi Hills.
There is archaeological evidence of zinc production in Rajasthan
mines at Zawar from the 6th or 5th BCE. India was the first country
to master zinc distillation. Due to low boiling point, zinc tends to
vapourise while its ore is smelted. Pure zinc could be produced after
a sophisticated ‘downward’ distillation technique in which the vapour
was condensed in a lower container. This technique was also applied
to mercury. Indian metallurgists were masters in this technique. This
has been described in Sanskrit texts of 14th century.
Indians had knowledge about mercury. They used it for medicinal
purpose. Development of mining and metallurgy declined during the British
colonial era. By the 19th century, once flourished mines of Rajasthan were
mostly abandoned and became almost extinct. In 1947 when India got
independence, European literature on science had already found its way
slowly into the country. Thus, in post independence era, the Government
of India initiated the process of nation building through the establishment
of various institutes of science and technology. In the following sections, we
will learn about the modern methods of extraction of elements.
A few elements like carbon, sulphur, gold and noble gases, occur in free
state while others are found in combined forms in the earth’s crust.
Elements vary in abundance. Among metals, aluminium is the most
abundant. In fact, 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
. For example, gems ‘ruby’ and ‘sapphire’ have Cr and Co
respectively as impurity. 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.
For obtaining a particular metal, first we look for minerals which
are naturally occurring chemical substances in the earth’s crust and
are obtained through mining. Out of many minerals in which a metal
may be found, only a few are viable to be used as source of that metal.
Such minerals are known as ores.
The principal ores of aluminium, iron, copper and zinc are given
in Table 6.1.
Occurrence ofOccurrence of
Occurrence ofOccurrence of
Occurrence of
Aluminium Bauxite AlO
[where 0 < x < 1]
Kaolinite (a form of clay) [Al
Iron Haematite Fe
Magnetite Fe
Siderite FeCO
Iron pyrites FeS
Copper Copper pyrites CuFeS
Malachite CuCO
Cuprite Cu
Copper glance Cu
Zinc Zinc blende or Sphalerite ZnS
Calamine ZnCO
Zincite ZnO
Metal Ores Composition
Table 6.1: Principal Ores of Some Important Metals
A particular element may occur in a variety of compounds. The
process of isolation of element from its compound should be such that
it is chemically feasible and commercially viable.
General Principles and Processes of Isolation of Elements
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
that is produced in case of iron pyrites) are
taken. For copper and zinc, any of the ores listed in Table 6.1 may be
used depending upon the availability and other relevant factors.
The entire scientific and technological process used for isolation of
the metal from its ore is known as metallurgy. The extraction and
isolation of an element from its combined form involves various
principles of chemistry. Still, some general principles are common to
all the extraction processes of metals.
An ore rarely 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 involves the following
major steps:
Concentration of the ore,
Isolation of the metal from its concentrated ore, and
Purification of the metal.
In the following Sections, we shall first describe the various steps
for effective concentration of ores. After that principles of some of the
common metallurgical processes will be discussed. Those principles
will include the thermodynamic and electrochemical aspects involved
in the effective reduction of the concentrated ore to the metal.
Removal of the unwanted materials (e.g., sand, clays, etc.) from the ore
is known as concentration, dressing or benefaction. Before proceeding
for concentration, ores are graded and crushed to reasonable size.
Concentration of ores 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 for concentration of
ore are described below.
This is based on the difference between specific gravities of the ore and
the gangue particles. It is therefore a type of gravity separation. In one
such process, an upward stream of running water is used to wash the
powdered ore. The lighter gangue particles are washed away and the
heavier ore particles are left behind.
This is based on differences in magnetic properties of the ore
components. If either the ore or the gangue is attracted towards
magnetic field, then the separation is
carried out by this method. For example
iron ores are attracted towards magnet,
hence, non–magnetic impurities can be
separted from them using magnetic
separation. The powdered ore is
dropped over a conveyer belt which
moves over a magnetic roller (Fig.6.1)
Magnetic substance remains attracted
towards the belt and falls close to it.
6.2.1 Hydraulic
6.2.2 Magnetic
of Oresof Ores
of Oresof Ores
of Ores
Fig. 6.1: Magnetic separation
This method is used for removing gangue from sulphide ores. In this
process, a suspension of the powdered ore is made with water.
Collectors and froth
stabilisers are added to
it. 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 the 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.
Leaching is often used if the ore is soluble in some suitable solvent.
Following examples illustrate the procedure:
(a) Leaching of alumina from bauxite
Bauxite is the principal ore of aluminium. It usually contains SiO
iron oxides and titanium oxide (TiO
) 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, Al
is extracted out as sodium aluminate.
6.2.4 Leaching
Fig. 6.2: Froth floatation process
6.2.3 Froth
The Innovative WasherwomanThe Innovative Washerwoman
The Innovative WasherwomanThe 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, Mrs. Carrie
Everson was a chemist. 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 came up. 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.