General principles and processes of isolation of elements

GENERAL PRINCIPLES AND PROCESSES
OF ISOLATION OF ELEMENTS
UNIT 6
CLASS 12

OCCURANCE OF METALS
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.

Minerals are naturally occurring chemical substances in the
earth’s crust obtainable by mining.
Minerals

ORE
Ores are those mineral from which metal can be
extracted easily and economically.

Gangue
It is the earthy or undesirable contaminating mineral
which present in the ore .

Concentration of the ore,
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 its ores is called
metallurgy and it involve the following major steps:
Concentration of the ore
Isolation of the metal from its concentrated ore, and
Purification of the metal.

Principal Ores of Some Important Metals

Concentration of Ores
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.
Hydraulic Washing
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, 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.

Magnetic Separation
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

Froth Floatation Method
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.

Depressants
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.
Leaching
It is achemical seperation process in which a suitable solvent or
reagent is used in which the ore is soluble but impurities are not.
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 O2) from which the metal is obtained later by replacement:
4M(s) + 8CN–(aq)+ 2H2O(aq) + O2(g) → 4[M(CN)2]– (aq) + 4OH–(aq)
(M= Ag or Au)
Pure Ag and Au can be obtained by replacement process.
4[M(CN)2]– + Zn [Zn(CN)4] 2– + M

Leaching of alumina from bauxite
The principal ore of aluminium, bauxite, usually contains SiO2, iron oxides 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 SiO2 too as sodium silicate) leaving the impurities behind:
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2Na[Al(OH) 4](aq)
The aluminate in solution is neutralized 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:
2Na[Al(OH)4](aq) + CO2(g) → Al2O3.xH2O(s) + 2NaHCO3 (aq)
The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give
back pure Al2O3:
Al2O3.xH2O(s) 1470 K Al2O3(s) + xH2O(g)

Reaction involve in leaching of alumina by Baeyer’s process

Extraction of Crude Metal from Concentrated Ore
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. Thus isolation
of metals from concentrated ore involves two major steps viz.,
(a)conversion to oxide
(b) reduction of the oxide to metal.
Conversion to oxide
Calcination involves heating when the volatile matter escapes leaving behind the
metal oxide:In this process ore is heated in absence of air.
Fe2O3.xH2O(s) Fe2O3 (s) + xH2O(g)
ZnCO3 (s) ZnO(s) + CO2(g)
CaCO3.MgCO3(s) CaO(s) + MgO(s ) + 2CO2(g)
Calcination

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 + 3O2 → 2ZnO + 2SO2
2PbS + 3O2 → 2PbO + 2SO2
2Cu2S + 3O2 → 2Cu2O + 2SO2

Roasting of copper pyrites (CuFeS2)
The sulphide ores of copper are heated in reverberatory furnace. If the ore contains iron, it is
mixed with silica before heating. Iron oxide mix with silica to form iron silicate(slag)
( FeSiO3)and copper is produced in the form of copper matte which contains Cu2S and FeS.
Cu2S then undergo roasting in presence of O2 and gets converted to oxides.
2Cu2S+3O2 2Cu2O+2SO2

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.
Reduction of the metal oxide

EXTRACTION OF IRON
Oxide ores of iron, after concentration
through calcination/roasting are mixed with
limestone and coke and fed into a Blast
furnace from its top. Here, the oxide
is reduced to the metal.
In the Blast furnace, reduction of iron
oxides takes place in different temperature
ranges..
Hot air is blown from the bottom of the
furnace and coke is burnt into give
temperature upto about 2200K in the lower
portion itself.

The burning of coke therefore supplies most
of the heat required in the process. The CO
and heat moves to upper part of the furnace.
In upper part, the temperature is lower and the
iron oxides(Fe2O3 and Fe3O4) coming from the
top are reduced in steps to FeO. Thus, the
reduction reactions taking place in the lower
temperature range and in the higher
temperature range,

These reactions can be summarised as follows:
At 500 – 800 K (lower temperature range in the
blast furnace) –
At
At 900 – 1500 K (higher temperature range in the blast furnace):

General principles and processes of isolation of elements

Limestone is also decomposed to CaO which removes
silicate impurity of the ore as slag. The slag is in molten
state and separates out from iron.
The iron obtained from Blast furnace contains about 4%
carbon and many impurities in smaller amount (e.g., S,
P, Si, Mn). This is known as pig iron and cast into variety
of shapes. Cast iron is different from pig iron and is
made by melting pig iron with scrap iron and coke using
hot air blast.
It has slightly lower carbon content (about3%) and is
extremely hard and brittle.

In the metallurgy of aluminium, purified Al2O3 is mixed
with Na3AlF6 or CaF2 which lowers the melting point of
the mixture and brings conductivity. The fused matrix is
electrolysed.
Steel vessel with lining of carbon acts as cathode and
graphite anode is used.
The overall reaction may be written as:
2Al2O3 + 3C 4Al + 3CO2
Metallurgy of aluminium
This process of electrolysis is widely known as
Hall-Heroult process

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. This way for each kg of aluminium
produced, about 0.5 kg of carbon anode is burnt away. The
electrolytic reactions are:
Cathode: Al3+ (melt) + 3e– Al(l)
Anode: C(s) + O2– (melt) CO(g) + 2e–
C(s) + 2O2– (melt) CO2 (g) + 4e

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.

Copper is refined using an electrolytic method. Anodes are of
impure copper and pure copper strips are taken as cathode. The
electrolyte is acidified solution of copper sulphate and the net result
of electrolysis is the transfer of copper in pure form from the anode
to the cathode:
Anode: Cu Cu2+ + 2 e–
Cathode: Cu2+ + 2e– Cu
Impurities from the blister copper deposit as anode mud which
contains antimony, selenium, tellurium, silver, gold and platinum;
recovery of these elements may meet the cost of refining. Zinc may
also be refined this way

Zone refining
This method is based on the principle that the impurities are more soluble in the melt
than in the solid state of the metal. A mobile heater surrounding the rod of impure metal is
fixed at its one end. 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 left behind and the
impurities pass on into the adjacent new molten zone created by movement of heaters. The
process is repeated several times and the heater is moved in the same direction again and
again.Impurities get concentrated at one end. This end is cut off. This method is very useful
forproducing semiconductor and other metals of very high purity, e.g., germanium, silicon,
boron, gallium and indium.

(e) Vapour phase refining
In this method, the metal is converted into its volatile compound
which is collected and decomposed to give pure metal. So, the two
requirements are:
(i) the metal should form a volatile compound with an available
reagent,
(ii) 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 named as nickel tetracarbonyl. This compex is decomposed at
higher temperature to obtain pure metal.

van Arkel Method for Refining Zirconium or Titanium:
This method is very useful for removing all the oxygen and nitrogen present in the
form of impurity in certain metals like Zr and Ti. The crude metal is
heated in an evacuated vessel with iodine. The metal iodide being
more covalent, volatilises:
Zr + 2I2 ZrI4
The metal iodide is decomposed on a tungsten filament, electricallyheated to about 1800K.
The pure metal deposits on the filament.
ZrI4 Zr + 2I2

Chromatographic methods
Column chromatography is very useful for purification of the
elements which are available in minute quantities and the impurities
are not very different in chemical properties from the element to be
purified.

USES OF ALUMINIUM
i. Aluminium foils are used as wrappers for chocolates.
ii. The fine dust of the metal is used in paints and
lacquers.
iii. Aluminium, being highly reactive, is also used in the
extraction of chromium and manganese from their
oxides.
iv. Wires of aluminium are used as electricity
conductors.
v. Alloys containing aluminium, being light, are very
useful.

Copper is used for making wires used in
electrical
industry and for water and steam pipes.
It is also used in several alloys that are rather
tougher than the metal itself, e.g., brass (with
zinc), bronze (with tin) and coinage alloy (with
nickel).
Zinc is used for galvanising iron.
ii. It is also used in large quantities in batteries,
as a constituent of many alloys, e.g., brass,
(Cu 60%, Zn 40%) and german silver (Cu 25-
30%, Zn 25-30%, Ni 40–50%).
iii. Zinc dust is used as a reducing agent in the
manufacture of dye-stuffs, paints, etc
USES OF ZINC
USES OF COPPER

i. Cast iron, which is the most important form of iron, is used
for casting stoves, railway sleepers, gutter pipes , toys, etc.
ii. It is used in the manufacture of wrought iron and steel.
iii. Wrought iron is used in making anchors, wires, bolts, chains
and agricultural implements.
iv. Steel finds a number of uses.
v. Alloy steel is obtained when other metals are added to it.
vi. Nickel steel is used for making cables, automobiles and
aeroplane parts, pendulum, measuring tapes, chrome steel
for cutting tools and crushing machines, and stainless steel
for cycles, automobiles, utensils, pens, etc.
USES OF IRON

General principles and processes of isolation of elements

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