Metallurgy
Metallurgy
The process used for extraction of metal in their pure form from their ores is called metallurgy.
Physical properties of metals
1)
Physical State: Under ordinary temperature metals stay in solid state. However metals like
mercury and gallium are exception, which are in liquid state even at room temperature.
2)
Lustre: - Metals have a shiny reflecting surface this properties of metal called lustre.
3)
Hardness: Generally metals are hard, not soft. Exception: Sodium and potassium are
soft and can be easily cut by a knife.
4)
Ductility: When a metal is pulled through a hole its wire is formed. This property of
metal is called ductility
5)
Malleability: Take an iron nail. Place it on a platform and keep on hammering it. After
sometime we will see a thin sheet forming. This property is called malleability
of metals.
6)
Conduction of Heat: Metals are good conductors of heat. Silver, copper and aluminium are the
best conductors of heat
7)
Conduction of Electricity: Metals are good conductors of electricity. Lead is an exception, which is
neither a good conductor of heat nor a good conductor of electricity.
8)
Density: Metals have high density Sodium, potassium and lithium are exception, having
lower density then water. The density of lithium is only 0.53 g/cc.
9)
Melting point & Boiling Point:
Generally metals have high melting points and boiling points.
Exceptions: Hg, Ga, Na, K.
10)
Sonority: Metals are sonorous. They produce sound on striking.
Chemical properties of metals
Metals are reactive. They lose electrons easily and
become positively charged ions. That is why metals are called electropositive
elements.
Reaction of metals with oxygen
Metals combine with oxygen on heating in air and metal
oxides are formed. Sodium and potassium are very reactive metals. Sodium metal
combines with oxygen in the air even at room temperature and forms sodium
oxide.
- On exposure to air sodium readily catches fire. Therefore, to prevent accident in the laboratory or elsewhere it is kept in kerosene
Oxides of some metals are soluble in water. They react
with water to form alkali.
We know that magnesium oxide is formed on burning
magnesium ribbon in the air.
Magnesium oxide reacts with water to form an alkali,
called magnesium hydroxide.
Also Read Chemical reaction and equation
Reaction of metals with water
Sodium and potassium metal react rapidly and vigorously
with water and liberates hydrogen gas.
Calcium reacts with water slowly and less vigorously. The
hydrogen gas released in this reaction collects on the surface of the metal in
the form of bubbles and the metal floats on water.
The metals; aluminium, iron and zinc do not react with
cold or hot water, but they react with steam to form their oxides. Hydrogen gas
is released in this reaction.
Reaction of metals with Acid
When samples of aluminium, magnesium, iron or zinc are
treated with dilute sulphuric or hydrochloric acid, sulphate or chloride salts
of metals are formed. Hydrogen gas is liberated in this reaction. The
reactivity of these metals can be indicated by the following sequence.
Mg > Al > Zn > Fe
Reaction of metals with nitric acid
Nitrate salts of metals are formed on reaction of metals
with nitric acid. Various oxides of nitrogen (N2O, NO, NO2) are also formed in accordance with the concentration of
nitric acid.
Also Read Carbon Compound
Define the following terms
Minerals: -
The naturally occurring compounds of metal along with other impurities
are known as minerals.
Ores: -
The minerals from which the metal can be
separated economically are called ores.
Gangue: -
Ores contain metal compound with some of impurities like soil, sand, rock
material etc. these impurities are called as gangue.
Roasting: -
The
sulphide ores are first converted into oxides by heating strongly in excess of
air. This process is called as roasting.
Calcination: -
Carbonate ores are strongly heated
in a limited supply of air to transform them into oxides. This process is
called calcination.
Extraction of Aluminium.
Symbol : Al
Colour : Silver white
Atomic number : 13
Electronic configuration: 2, 8, 3
Valency : 3
Aluminium being reactive metal does not occur in nature
in Free State. Aluminium is extracted from its ore bauxite (Al2O3nH2O). Bauxite
contains 30% to 70% of Al2O3 and remaining part
is gangue. It is made up of sand, silica, iron oxide etc. There are two steps in
the extraction of aluminium.
1) Concentration of bauxite ore 2) Electrolytic reduction of alumina
1) Concentration of bauxite ore:-
Bauxite is the main ore of aluminium. Silica (SiO2), ferric oxide (Fe2O3) and titanium oxide (TiO2) are the
impurities present in bauxite. Separation of these impurities is done by
leaching process using either Bayer’s method or Hall’s method. In both these
methods finally the concentrated alumina is obtained by calcination.
In the Bayer’s process:-
1) In the Bayer’s process the ore bauxite is heated with concentrated solution of caustic soda (NaOH) at 140 to 150°C under high pressure for 2 to 8 hours in a tank called digester tank.
2) Electrolytic reduction of alumina:-
1) In this method electrolysis of molten mixture of alumina (melting point > 2000°C) is done in a steel tank.
2) The tank has a graphite lining on the inner side. This lining does the work of a cathode. A set of graphite rods dipped in the molten electrolyte works as anode.
3) Cryolite (Na3AlF6) and fluorspar (CaF2) are added in the mixture to lower its melting point up to 1000°C.
4) Aluminium is deposited on the cathode on passing electric current. Molten aluminium being heavier than the electrolyte, is collected at the bottom of the tank.
5) It is taken out from there from time to time, Oxygen gas is liberated at the anode.
The electrode reactions are as shown below.
- The liberated oxygen reacts with the anodes to form carbon dioxide gas. The anodes have to be changed from time to time as they get oxidised during the electrolysis of alumina.
Also Read Chemical reaction and equations
What is corrosion?
Ans. The Slow process of
decay or destruction of metal due to the effect of air, moisture, acid, base,
or other chemicals on it is called corrosion.
1)
Iron reacts with
moist air and a deposit of reddish substance (Fe2O3 H2O) is formed on it. This substance is
called rust.
2)
Carbon dioxide in
moist air reacts with the surface of copper vessel. Copper loses its lustre due
to formation of greenish layer of copper carbonate (CuCO3) on its surface. This is called patination of copper.
3)
On exposure to air, silver articles turn blackish
after some time. This is because of the layer of silver sulphide (Ag2S) formed by the reaction of silver with hydrogen
sulphide in air.
4)
By oxidation of
aluminium, a thin layer of aluminium oxide forms on it.
Prevention of corrosion
The prevention of corrosion can be achieved by various
ways. Some of these methods are as follows.
1. To fix a layer of some substance on the metal surface
so that the contact of the metal with moisture and oxygen in the air is
prevented and no reaction would occur between them.
2. To prevent corrosion of metals by applying a layer of
paint, oil, grease or varnish on their surface. For example, corrosion of iron
can be prevented by this method.
Corrosion can be prevented by putting a layer of noncorrodible
metal on a corrodible metal. This can be done in many ways.
1) Galvanizing
In this method a
thin layer of zinc is applied to prevent corrosion of iron or steel. For example,
shining iron nails, pins, etc. In this method corrosion of zinc occurs first
because zinc is more electropositive than iron. After a few rainy seasons the
zinc layer goes away and the inner iron gets exposed. Then iron starts rusting.
2) Tinning
In this method a layer of molten tin is
deposited on metals. We call this as ‘kalhaee’.A greenish layer forms on the surface of a
copper or brass vessel. This greenish layer is poisonous. If buttermilk or
curry is placed in such a vessel it gets spoiled. Tinning is done to prevent
all such damages.
3) Anodization
In this method metals like copper, aluminium
are coated with a thin and strong layer of their oxides by means of
electrolysis. For this the copper or aluminium article is used as anode. As
this oxide layer is strong and uniform all over the surface, it is useful for prevention
of the corrosion of the metal.
4) Electroplating
In this method a less reactive metal is
coated on a more reactive metal by electrolysis. Silver plated spoons, gold
plated ornaments are the examples of electroplating.
5) Alloying
Alloy is the homogenous mixture formed by
mixing a metal with other metals or non-metals in certain proportion is called
an alloy.
For example, bronze is an alloy formed from
90% copper and 10 % tin.
Stainless steel does not get stains with air
or water and also does not rust.
It is an alloy made from 74% iron, 18%
chromium and 8% carbon. In recent times various types of alloys are used for
minting coins.
No comments: