S
e
l
f
r
e
du
c
t
i
o
n
f
o
r
C
u
(
E
x
t
r
a
c
t
i
o
n
o
f
C
U
f
r
o
m
c
o
pp
e
r
g
l
a
n
c
e
.
)
2
C
u
2
O
+
C
u
2
S
——
A
→
6
C
u
+
S
O
2
(c)
Metal displacement method
In this method, a water soluble compound is obtained from the ore. The aqueous solution of the compound is
reacted with a more electropositive metal which displaces, the metal from the solution.
Ex.
(i)
Extraction of Cu fro low grade (lean) ore
C
u
C
O
3
.
C
u
(
O
H
)
2
+
di
l
u
t
e
a
c
id
—→
C
u
+
2
——
S
c
r
—
a
p
e
—
F
e
—
→
C
u
+
F
e
+
2
Malachite
(aq) (s)
(aq)
Ex.
(ii)
Separation of Ag by Complex formation (Cyanide process).
2Na[Ag(CN)
2
] + Zn → Na
2
[Zn(CN)
4
+ 2Ag
(d)
Electrolytic reduction
This process is mainly used for the extraction of highly electropositive metals.
Electrolysis is carried out in large cells and a small amount of another suitable electrolyte is added which:
(i)
Lowers the melting point of the main electrolyte
(ii)
Enhances its conductivity
(iii)
Reduces corrosion troubles
Ex.
Na, K, Mg, Ca, Al, etc.
e.g. Manufacture of metallic sodium (Down's process)
Molten NaCl containing a little CaCl
2
is electrolysed between graphite anode and iron cathode. The
various reactions that take place are
On Fusion : NaCl
Na
+
+ Cl
–
(Ions become mobile)
On Electrolysis : At Cathode :
Na
+
+ e
–
→
Na (reduction)
(Metallic sodium)
At Anode : 2Cl
–
→ Cl
2
(g) + 2e
–
Note : Hall heroult process :
This process is used for extraction of Al from alumina. The extraction of Al from
Al
2
O
3
is quite difficult because –
(i)
Fusion temperature of Alumina is quite high (2050°C). Even more than boilling point of Al (1150°C).
(ii)
It is a bad conductor of electricity.
To overcome these difficulties we mix some amount of neutral flux [Na
3
AlF
6
+ CaF
2
]. Neutral flux provide free
ions to the solution which decreases the fusion temperature of Alumina from 2050°C to 950°C.
(e)
Alumino thermite process :
In this process those metal oxide will be reduced which required high temperature
and at high temperature carbon react with metal to from metal carbide.
In this process we use aluminium as a reducing agent due to
(i)
Al has greater affinity towards oxygen as it forms most stable oxide (Al
2
O
3
)
(ii)
This reaction is highly exothermic in nature and once it starts it will continue till all the metal oxide reduces
to metal.
For Cr, Mn, Fe :
Cr
2
O
3
+ Al ——→ Al
2
O
3
+ 2Cr
Fe
2
O
3
+ Al ——→ Al
2
O
3
+
2Fe
Commercial use of this method is in aluminothermite weldings.
Note :-
(Fe
2
O
3
: Al) mixture in 3 : 1 is used.