Refining
process
Generally the metal
extracted from its ore contains some impurities such as unreacted oxide ore,
other metals, nonmetals etc... Removal of such impurities associated with the
isolated crude metal is called refining process. In this section, let us
discuss some of the common refining methods.
This method is employed
for low boiling volatile metals like zinc (boiling point 1180 K) and mercury
(630 K). In this method, the impure metal is heated to evaporate and the
vapours are condensed to get pure metal.
This method, is employed
to remove the impurities with high melting points from metals having relatively
low melting points such as tin (Sb; mp= 904 K), lead (Pb; mp=600 K), mercury
(Hg; mp=234 K), and bismuth (Bi; mp=545 K). In this process, the crude metal is
heated to form fusible liquid and allowed to flow on a sloping surface. The
impure metal is placed on sloping hearth of a reverberatory furnace and it is
heated just above the melting point of the metal in the absence of air, the
molten pure metal flows down and the impurities are left behind. The molten
metal is collected and solidified.
The crude metal is
refined by electrolysis. It is carried out in an electrolytic cell containing
aqueous solution of the salts of the metal of interest. The rods of impure
metal are used as anode and thin strips of pure metal are used as cathode. The
metal of interest dissolves from the anode, pass into the solution while the same
amount of metal ions from the solution will be deposited at the cathode. During
electrolysis, the less electropositive impurities in the anode, settle down at
the bottom and are removed as anode mud.
Let us understand this
process by considering electrolytic refining of silver as an example.
Cathode : Pure silver
Anode : Impure silver
rods
Electrolyte : Acidified
aqueous solution of silver nitrate.
When a current is passed
through the electrodes the following reactions will take place
Reaction at anode
Ag (s) → Ag+ (aq) + 1e-
Reaction at cathode
Ag+ (aq) + 1e-
→ Ag (s)
During electrolysis, at
the anode the silver atoms lose electrons and enter the solution. The
positively charged silver cations migrate towards the cathode and get
discharged by gaining electrons and deposited on the cathode. Other metals such
as copper, zinc etc.,can also be refined by this process in a similar manner.
This method is based on
the principles of fractional crystallisation. When an impure metal is melted
and allowed to solidify, the impurities will prefer to be in the molten region.
i.e. impurities are more soluble in the melt than in the solid state metal. In
this process the impure metal is taken in the form of a rod. One end of the rod
is heated using a mobile induction heater which results in melting of the metal
on that portion of the rod. When the heater is slowly moved to the other end
the pure metal crystallises while the impurities will move on to the adjacent
molten zone formed due to the movement of the heater. As the heater moves
further away, the molten zone containing impurities also moves along with it.
The process is repeated several times by moving the heater in the same
direction again and again to achieve the desired purity level. This process is
carried out in an inert gas atmosphere to prevent the oxidation of metals .
Elements such as germanium (Ge), silicon (Si) and galium (Ga) that are used as
semiconductor are refined using this process.
In this method, the
metal is treated with a suitable reagent which can form a volatile compound
with the metal. Then the volatile compound is decomposed to give the pure
metal. We can understand this method by considering the following process.
The impure nickel is
heated in a stream of carbon monoxide at around 350 K. The nickel reacts with
the CO to form a highly volatile nickel tetracarbonyl. The solid impurities are
left behind.
Ni (s) + 4 CO (g) → Ni(CO)4
(g)
On heating the nickel
tetracarbonyl around 460 K, the complex decomposes to give pure metal.
Ni(CO)4 (g) → Ni (s) + 4 CO (g)
This method is based on
the thermal decomposition of metal compounds which lead to the formation of
pure metals. Titanium and zirconium can be purified using this method. For
example, the impure titanium metal is heated in an evacuated vessel with iodine
at a temperature of 550 K to form the volatile titanium tetra-iodide.(TiI4).
The impurities are left behind, as they do not react with iodine.
Ti (s) + 2I2
(s) → TiI4 (vapour)
The volatile titanium
tetraiodide vapour is passed over a tungsten filament at a temperature aroud
1800 K. The titanium tetraiodide is decomposed and pure titanium is deposited
on the filament. The iodine is reused.
TiI4 (vapour)
Ti (s) + 2I2 (s)
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