General Principle and Process of Isolation of Elements
Minerals:
-
These are naturally occurring chemical substances in the in the earth crust obtainable
by mining.
Ores : -
only few minerals are used as sources of
that metal. Such minerals are known as ores.
Gangue:
-
ores are usually contaminated with earthly or undersides materials known as
gangue.
Metallurgy:
-
The entire scientific and technological process used for isolation of the
metal from its ores is known as metallurgy.
Concentration
of the ore: -
The removal of earthy and siliceous
impurities (i.e. gangue or matrix) from the ores is called concentration of
ores.
(i)
Hydraulic
washing: -
The processes by which lighter
earthy particles are free from the heavier ore particles by washing with water.
(ii)
Magnetic
separation: -
This method of concentration is employed
when either the ore or impurities
associated with it are magnetic in nature.
Example:
- chromate.
(iii)
Froth
floatation: -
This method is based upon the fact
that surface of sulphide ores is preferentially wetted by oils while that of
gangue is preferentially wetted by water.
(iv)
Leaching:
-
This process consists in treating the powdered ore with a suitable reagent
which are selectively dissolved the ore but not the impurities.
1) Leaching of Alumina
from bauxite: -
ore of aluminum, bauxite usually
contain SiO2, iron oxide, Tio2,
as impurities .the concentration is carried out by digesting the powdered ore
with the concentrated solution of
NaOH at 473 to 523 K and 35-36
bar pressure. This way Al2O3 is leached out as sodium aluminates
behind.
·
3H2O+Al2O3+2NaOH
----> 2Na[Al(OH)4]
The aluminates in solution is neutralized by passing
Co2 gas and hydrated Al2O3
is precipitated.
·
2Na[Al(OH)4]+CO2 ----> Al2O3*xH2O +
2NaHCO3
NaHCO3 remains
in the solution and hydrated alumina is filtered, dried and heated to give back
pure Al2O3.
2) 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 O2) from which the metal is obtained later
by replacement.
4[M(CN)2]- + Zn ----> [Zn(CN)4]2- + 2M
Extraction
of crude metal from concentrated Ore :-
The concentrated are must be converted into a form
which is suitable for reduction. The process used to obtain metals in Free
State from concentrated ores is called extraction. It involves two chemical processes-
1. Conversion of the ore into metallic
oxide or de-electronation of ores: -
Metal usually
present as hydroxides, carbonates and sulphides depending upon the nature of
the minerals present in the ore, the following two present in the ore, the
following two methods are used for conversion of ore into their respective
oxides.
(a) Calcinations:
-
It is the process of converting on ore into is oxide by heating it strongly
below its melting point either in absent or limited supply of air during
calcinations following change occur
(i)
Moisture is driven out
(ii)
Volatile impurities of S, As and P are removed as their volatile oxides
(iii)
Water is remove from hydrated oxides and
hydroxide ores
(iv)
Carbonate ore are converted into their
respective oxides by lose of carbon dioxide
(b) Roasting:
-
It is the process of converting an ore into its metallic oxide by heating
strongly at temperature insufficient to melt in excess of air. This process is
usually used for sulphide ores. Following change occur during roasting.
(i)
Moisture
is removed
(ii)
Organic matter is destroyed
(iii)
Non metallic impurities like that of S,
P and Ar are oxidized and removed as volatile gases.
S8
+ 8O2 ---->
8SO2
(Sulphur dioxide)
P4
+ 5O2 ---->
P4O10
(Phosphorus pentaoxide)
4As
+ 3O2 ---->
2As2O3
(Arsenious oxide)
(iv)
Ores are generally converted into
metallic oxides
2ZnS
(Zinc sulphide) + 3O2 ----> 2ZnO
(Zinc oxide) + 2SO2
2PbS
(Lead oxide) + 3O2 ---->
2PbO (Lead oxide) + 2SO2
2Cu2S
(Cuprous sulphide) + 3O2 ----> 2Cu2O (Cuprous oxide) + 2SO2
Like
calcinations, roasting is also carried out in a reverberatory furnace
Thermodynamic
principles of metallurgy: -
For any process, Gibbs free energy change (∆G)
is given by
∆G = ∆H - T∆S
When
∆H is the enthalpy change and ∆S is the entropy change and T is the absolute change
for any reaction
∆Gฯด
= -RT ln K
Where,
k is the equilibrium constant of the reactant – product system at the
temperature T, A negative ∆G implies a +ve K in equation and this can happen
only when reaction proceeds towards product.
Applications:-
(a) Extraction of iron from
its oxides -
Oxides ores of iron after
concentration through calcinations/ roasting are mixed with limestone and coke
and fed into a blast blast-furnace from its top. Here, the oxides is reduced to
the metals
FeO + C ----> Fe + CO
And FeO is reduced and
C is oxidized to CO.
FeO
----> Fe + 1/2O2
C
+ 1/2O2 ----> CO
At 500-800 K
3Fe2O3
+ CO ----> 2Fe2O4
+ CO2
Fe3O4 + 4CO ----> 3Fe +
4CO2
Fe2O3 +CO ----> 2FeO + CO2
At 900-1500 K
C + CO2 ----> 2CO
FeO +CO ----> Fe +CO2
Limestone is also
decompose to CaO which remove silicate impurities of the ore as a slag
CaCO3 ----> CaO + CO2
CaO + SiO2 ---->
CaSiO3 (slag)
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.
(b) Extraction of copper
from cuprous oxide: -
Most of the ores are sulphide and
some may also contain iron . The sulphide ores are roasted/ melted to give
oxides.
2Cu2S + 3O2 ----> 2Cu2O + 2SO2
The oxide then can be
easily reduced to metallic copper using coke.
Cu2O + C ----> 2Cu + CO
And slag of iron
silicate is produced
FeO + SiO2 ---->
FeSiO3 (slag)
Following reaction also
takes place
2FeS + 3O2 ---->
2FeO + 2SO2
FeO + SiO2 ---->
FeSiO3
2Cu2S + 3O2 ----> 2Cu2O + 2SO2
2Cu2O + 2Cu2S
----> 6Cu + SO2
(c) Extraction of zinc from
zinc oxide: -
The reduction of zinc oxide is done
by using coke
The
metal is distilled off and collected by rapid chilling
(d) Extraction of Aluminum:
-
Alumina Al2O3 is mixed with Na3AlF6
or CaF2 which lowers the melting point of the mixture and brings
conductivity. Fused mixture is electrolyzed. Steel cathode and graphite anode
is used. The graphite anode is useful for the reduction of metal, the overall
reaction are as follow-
2Al2O3
+ 3C ---->
4Al + 3CO2
The
electrolytic reactions are –
Cathode:
Al3+ + 3e- ----> Al
Anode:
C + O2- ----> CO + 2e-
C + 2O2- ----> CO2 + 4e-
(e) Extraction of copper
low grad ores and scraps :-
Copper
is extracted by hydro metallurgy from low grade
ores it is leached out using acid or bacteria the sol containing Cu2+ is treated with scrape iron or hydrogen.
Cu2+
+ H2 ----> Cu + 2H+
Oxidation
reduction: -
Besides reduction, some extraction
are based on oxidation particularly for non-metals. For extraction of chlorine
from brine.
2Cl-
+ 2H2O ---->
2OH- + H2 + Cl2
Refining:
-
The process of purifying the crude
metals is called refining. Some methods of refining are as follow.
a)
Distillation:
-
This is very useful for low boiling metals like
zinc and mercury. The impure metals are evaporated to obtain the pure metals as
distillate.
b)
Liquation:
-
In this method a low melting metal like tin can be made to flow on a sloping surface,
in this way it is separated from higher melting impurities.
c)
Electrolytic
refining: -
In this method the impure metal is
made to act as anode a strip of some metal in pure form is used as cathode.
They are put in a suitable electrolytic bath containing soluble salt of the
some metals the more basic metal remains in the solution and the less basic
ones go to the anode mud.
Anode:
M ----> Mn+
+ ne-
Cathode: Mn+ + ne- ----> M
d)
Zone
refining: -
In this method a circular mobile
heater is fixed at one end of a rod or the impure metal, as heater more
forward, the pure metal crystallises out of the melt and impurities pass on
into the adjacent molten zone.
e)
Vapour
phase refining :-
The crude metals is free from impurities by first converting it into a
suitable volatile compound by heating it with specific reagent at a lower
temperature and then decomposing the volatile compound at some higher
temperature to give the pure metal.
Ex:
- (1) Mond Process and (2) van arkel
method
f)
Chromatographic
method: -
The method is based upon the principle that the
different components of a mixture are adsorbed to different extents of an
adsorbent.
For example in column chromatography,
different components of the mixture are adsorbed to different extents depending
upon their polarity.