Minerals
The metallic compounds generally seen in the earth’s crust are called minerals.
Example : Bauxite (Al2O 3 2H2O), Cryolite (Na3AlF6), Clay (Al2O3 2SiO2 2H2O) etc. are some of the
minerals of aluminium.
Ore
A mineral from which a metal is economically, easily and quickly extracted, is called the ore of the metal.
Metallurgy
It involves all the processes leading to the separation of a pure metal from its ore.
There are three important stages in metallurgy.
1.Concentration of ores
- Extraction of metal from concentrated ore
- Refining of metals
I. Concentration of ores
The process of removing the impurities (gangue) from the ore obtained from the earth’s crust is
termed concentration of the ore. Depending on the nature of the ore and the impurities, there are
different methods of concentration. - Levigation or hydraulic washing
When the impurities are lighter and the ore particles are heavier,the lighter impurities are
removed by washing in a current of water
e.g.concentration of oxide ores, concentration of the ores of gold.
Froth floatation
This process is used when the impurities are heavier and the ore particles are lighter .
Sulphide ores are usually concentrated by this method.
Magnetic separation
If either the ore or the impurity has magnetic nature, concentration is done by this method.
This method is used for the concentration of magnetite, ore of iron and also to separate
iron tungstate, the magnetic impurity from tin stone (SnO2 ), the non-magnetic ore of tin.
4.Leaching
On adding the ore to a suitable solution, a chemical reaction takes place and the ore dissolves.
The insoluble impurities are filtered off. The pure ore is separated from the filtrate by a
chemical reaction.
Bauxite, the ore of aluminium is concentrated by this method.
II.Extraction of metals from concentrated ore
It has usually two stages.
a) Conversion of the concentrated ore into its oxide.
b) Reduction of the oxide.
(a) Conversion of concentrated ore into its oxide
i)Calcination : Calcination is the process of heating the concentrated ore in the absence of air at
temperature below its melting point. Carbonates and hydroxides of metals decompose to form
their oxides.
ii) Roasting : Roasting is the process of heating the concentrated ore in a current of air at a
temperature below its melting point. When the concentrated ore is subjected to roasting, the
moisture present in it is removed as vapour. Sulphide ore combines with oxygen to form oxide.
e.g. Cu2S ore is converted to Cu2O by roasting.
b) Reduction of the oxide
The process of extraction of metal from the oxide is reduction. Suitable reducing agents can be used
for this purpose.
During the process of the production of metal, electricity, carbon, carbon monoxide etc. are used as
reducing agents on the basis of the reactivity of the metal.
Electricity is used as the reducing agent to extract highly reactive metals like sodium, potassium
and calcium from their ores.
III.
III. Refining of metals
The metal obtained by reduction may contain other metals, metal oxides and small quantities of non
metals as impurities. Refining of metals is the process of removal of these impurities to get the pure
metal. Depending on the nature of metals and the impurities present in them, different methods are
used for the refining of metals. Some methods are given below.
a. Liquation
Low melting metals like tin and lead may contain other high melting metals or metal oxides as
impurities. On heating such metals on the inclined surface of a furnace, the pure metal melts and
flows down leaving the impurities behind. This process is termed liquation.
b. Distillation
This method is used for the refining of metals with low boiling points such as zinc, cadmium and
mercury. When the impure metal is heated in a retort, the pure metal alone vapourises. The vapours
are condensed to get the pure metal. This method is termed distillation.
c.Electrolytic refining
Electrolytic refining is the process of refining a metal by the electrolysis of a solution of the salt of
the metal, using a small piece of pure metal as the negative electrode and the impure metal as the
positive electrode. Copper can be refined by this method
Industrial production of iron( Day 1)
• Minerals of iron- Haematite, magnetite, iron pyrites etc.
• Iron pyrites – fool’s gold. Reason : It has a yellow brazen colour which resembles gold.
• Ores of iron- Haematite(Fe2O3) , Magnetite(Fe3O4)
Iron is industrially prepared mainly from haematite.
Impurities having low density are removed by washing.
Magnetic separation is also employed .
Roasting . During roasting , impurities like sulphur, arsenic, phosphorous etc. are removed as
their gaseous oxides. Water is also expelled along with this.
[Washing. Magnetic separation. Roasting ]But the gangue, silica ( silicon dioxide ) present in the ore is not
removed.
Haematite is converted into iron by using the blast furnace.
Blast of hot air is passed through the bottom of the furnace.
That is why this furnace is called blast furnace.
Process
Raw materials : Haematite(Fe2O3), limestone(CaCO3) and coke(C).
Hematite, limestone and coke are fed into the furnace through a special arrangement at the top of the furnace.
Reactions
C + O2 CO → 2 + Heat
CO2 + C + Heat 2CO →
Fe2O3 + 3CO 2Fe + 3CO → 2
The reduction of haematite into iron is done mainly by this carbon monoxide.
(CO is the reducing agent)
Calcium carbonate decomposes to give calcium oxide and carbon dioxide at high temperature in
the furnace.
CaCO3 CaO + CO → 2
This calcium oxide (flux) reacts with SiO2 (gangue) in the ore to form easily melting calcium
silicate(slag).
CaO+ SiO2 CaSiO → 3
Flux + Gangue → Slag
If the gangue is acidic in nature, basic flux is to be used.
If the gangue is basic in nature, acidic flux is to be used.
The molten slag being less dense, floats over the molten iron.
Pig iron
The molten iron obtained from the blast furnace is called pig iron.
It contains 4% carbon and other impurities like manganese, silicon, phosphorus etc.
Extraction of Aluminium
Main Ore – Bauxite
Aluminium is industrially produced through two important stages.
They are
1.Concentration of bauxite
- Electrolysis of concentrated alumina.
1.Concentration of bauxite
Bauxite is concentrated through the leaching process. Impure bauxite is added to hot concentrated
NaOH solution, where it gets converted into sodium aluminate. Impurities are then filtered off. A
small quantity of freshly prepared aluminium hydroxide precipitate is added and diluted with water,
to get more amount of Al(OH)3 precipitate. The precipitate is separated, washed and then heated
strongly to get alumina.
Summary
Leaching
● Hot Concentrated NaOH + Impure Bauxite→ Sodium aluminate.
● Sodium aluminate + small quantity of freshly prepared aluminium hydroxide → More amount
of Al(OH)3 precipitate
● The precipitate is separated, washed and then heated strongly → Alumina
For Electrolysis :
The Concentrated alumina (Al2O3) + Molten cryolite (Na3AlF6)
●For Electrolyte :
The Concentrated alumina (Al2O3) + Molten cryolite (Na3AlF6)
●Why Cryolite is added? :
- To reduce the melting point of alumina
- To increase its electrical conductivity.
●Reactions while electricity is passed through it :
Al2O3 2Al →
3+ + 3O2-
● C athode
Carbon lining
●Anode
Carbon rods
●Reaction at the negative electrode(cathode) / Reaction of aluminium ion
Al3+ + 3ē → Al
●Reaction at the positive electrode(anode) / Reaction of oxide ion
2O2- O → 2 + 4ē
C + O2 CO → 2
●In this process carbon anodes are replaced from time to time. Why?
Answer: Here carbon rods are the anode. At the anode, the following reaction occurs
2O2- → O2 + 4ē
C + O2 → CO2
This slowly burns away the anode. Hence anodes need to be replaced periodically.
