34
UNIT III
EXTRACTIVE METALLURGY
Extraction of Uranium from Pitchblende
Important ores of Uranium are:
(i)
(ii)
(iii)
(iv)
)
Pitchblende, (
Carnotite,
Calcium Carnotite,
(
) (
Autunite,
)
The most important ores of Uranium are Pitch blende and Carnotite
Extraction from chief ore pitchblende:
(i) Pitchblende is concentrated by gravity process, followed by roasting. The roasted ore is
digested with dil.
when uranium passes into the aqueous solution.
⁄
(ii) Sodium Carbonate is added to Uranyl Sulphate
Carbonate.
solution to form sodium Uranyl
(
(iii)
The solution containing sodium Uranyl carbonate on treatment with HCl forms Uranyl
Chloride
(
(iv)
)
)
Metal ions such as Cu and Pb are removed as precipitates of their sulphides by the action
of
The solution is then treated with excess of ammonia and ammonium diuranate is
formed.
This ammonium diuranate upon strong ignition in air gives uranium oxide.
(
)
This uranium oxide is then reduced with Aluminium
35
BLOCK DIAGRAM
(
)
discarded
(
)
36
Extraction of Uranium from carnotite
The ore is dissolved in conc.
. The solution thus obtained is neutralised by NaOH and
then treated with
. Ba and Ra sulphates are precipitated and U and V sulphates are present
in soluble state in the solution
(i)
This filtrate is treated with excess of boiling solution of
, Uranyl and vanadyl
sulphate gets converted into sodium uranyl carbonate and sodium vanadate
respectively.
(
)
⁄
(ii)
The resulting solution is treated with
precipitates uranium as
(
)
(
(iii)
followed by NaOH. This treatment
(
)
)
This sodium di uranate is then converted into ammonium di – urinate on treatment
with NH4OH and ammonium diuranate on strong ignition in the presence of air
produces Uranium oxide.
(
(
(iv)
)
)
This uranium oxide can be reduced with Al to yield pure Uranium metal.
37
BLOCK DIAGRAM
(
)
(
)
Sodium diuranate
(
Ammonium diuranate
(
)
Ignition
Uranium
)
38
Extraction of Thorium from Monazite
Occurrence: Thorium is found in India as the monazite sand at Travancore. It is also found
in Srilanka
The important ores of thorium are,
)
1. Monazite (
) and (
)
2. Thorianite (
)
3. Thorite (
Extraction from monazite sand
(i) The ore which is concentrated by electromagnetic separation method is heated with
conc.
at
, the resultant solution contains Thorium sulphate, Lanthanum
and Cerium sulphates.
(
(
)
)
(
) (
)
(ii) The solution now containing phosphoric acid is treated with NaOH so that
(
) is precipitated while La and Ce phosphates are left in the solution itself.
(
)
(iii) The precipitated (
and Ce as impurities.
(
) is heated with
)
to form crude Thoria with La
(
)
(iv) The crude thoria is now treated with oxalic acid and ammonium oxalate to give
Thorium oxalate remains in solution while La and Ce oxalates are precipitated.
(
) is evaporated and ignited to produce
(v) The solution containing
which is dissolved in HNO3 to form Thorium nitrate . This solution on evaporation
and cooling yields crystals of pure Th(NO3)4.4H2O . This on ignition gives thoria
(ThO2)
(vi) Resultant Thoria is now heated with carbon in presence of
to give
.
This on reduction with Ca gives Th.
.
39
BLOCK DIAGRAM
Ignited
HNO3
40
Extraction of Tungsten
Occurrence: Tungsten is a rare element. It is found combined with minerals, usually with
tin ores.
The most important minerals are
(i)
(ii)
(iii)
Wolframite (Fe, Mn) WO4
Scheelite ( Ca WO4)
Hubernite (Mn WO3)
Extraction of tungsten from wolframite
(i)
The wolframite ore concentrated by electromagnetic separation method is heated with
Na2CO3 and NaNO3 in air to give sodium tungstate, Na2WO4 (Soluble) and a mixture
of Fe2O3 and MnO2 (insoluble)
(
)
→
(ii)
The fused mass is extracted with water when Na2WO4 goes into solution leaving
behind
and
as precipitates.
(iii)
The solution containing Na2WO4 is treated with HCl and the resulting tungstic acid is
ignited to give WO3.
(iv)
The Oxide of tungsten obtained is then treated with Al to give tungsten
WO3 + 2
+ W.
41
BLOCK DIAGRAM
+NaNO3
42
ALLOYS
Purpose for alloying
The purpose of making alloys is
To increase the hardness of the metal
To lower the melting points of the metal
To resist the corrosion of the metal
To modify chemical activity of the metal
To increase the tensile strength of the metal
PROPERTIES AND COMPOSITION OF SOME IMPORTANT ALLOYS
Brass:
It is an alloy of copper and zinc. Its composition is Cu = 60 – 90% and Zn = 10 –
40%. They posses greater strength, durability and low melting point and good corrosion
resistance.
The property of this alloy varies with the percentage of Zn.
Bronze:
Bronze is a copper tin based alloy. If Zn is added to it Gun metal is formed. Generally
Bronze contains Cu – 80 to 95% and Sn – 5 to 20 %. Bronze is strong and is highly corrosion
resistant. Coinage bronze has a percentage composition of Cu – 89 to 92% and Sn – 8 to 11
%. It is used in the manufacture of coins, statues and ornaments.
Duralumin:
Aluminium is the main constituent of duralumin. It has a percentage composition of 94%
Al, 4% Cu and 0.5% each of Fe, Mn, Si and Mg. it is used for making nuts, bolts, tubes,
sheets and non magnetic parts of instruments and air ships.
Nichrome:
It is a Nickel Chromium alloy and has a composition of Ni – 55 to 78%, Cr – 5 to
23%, Mn – 0.7 to 1.5%, Si – 0.4 to 1.3%, Al – 0.2 %. It is highly chemical resistant.
As this alloy has high electrical resistance it is used for making heating elements in
electric irons, heaters and other electrical appliances. The alloy is used for making
heating elements in a muffle furnace
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Dutch metal:
It is an alloy of copper and zinc. The composition of dutch metal is 80% Cu and 20% Zinc. It is
suitable for drawing and forming operations. It is used for making cheap jewellery, battery caps,
flexible hoses, tubes, etc.
Bearing alloys
Bearing alloys are used for ball bearings or roller bearings . Bearing take most of the wear and
thus saves the axle that runs on them. Sn and Pb are two important bearing alloy metals .
However, Sn is superior to lead because of its corrosion resistance and thermal conductivity .
Bearing alloys usually contain a hard metal which resist wear and a soft metal which distributes
the load.
Properties of bearing alloys
A bearing or anti friction alloy should have the following properties:
Low co – efficient of friction
Non – corrosive
Ability to withstand continuous bearing pressure and impact
High melting point and high thermal conductivity
Babbit metal:
The composition of babbit metal is Sn - 80 to 90%, Cu - 3 to 10%, Sb - 8 to 12%. It is
extensively used for making bearings. It is specially suitable for constant turning tools such as
saw blades.
composition and uses of i). Invar and ii). Solder
Name
% Composition
Solder
Sn - 63%
Pb - 37%
It is used for soldering
electronic components
Ni – 36%
C – 0.5%
Fe-63.5%
It is used for making
pendulum in clocks
Invar
Uses
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Composition, properties & uses of Alnico
Alnico is an alloy of aluminium, Nickel, cobalt. It has a composition of Aluminium =
8 – 12 %, Nickel = 14 – 28 %, Cobalt = 5 – 35 %. It is highly magnetic and is used for
making permanent magnets.
POWDER METALLURGY
Powder metallurgy may be defined as the technique of manufacturing metal powder,
consolidating them and fabricating into desired shapes of articles with little or no melting.
FABRICATION OF ARTICLES THROUGH POWDER METALLURGY
Powder metallurgy involves the following operations in sequence.
1.
2.
3.
4.
Preparation of the metal powder
Mixing or Blending
Compacting
Sintering
1) PREPARATION OF THE METAL POWDER
The main methods used in obtaining metals in the powdered form are,
a.
b.
c.
d.
e.
Mechanical Pulverization
Atomization
Reduction of metal oxides
Electro – deposition
Decomposition
a. Mechanical pulverization
Metals are pulverized by the use of counter rotating plates or rapidly moving
hammers. This is followed by ball milling.
I th m thod call d ‘shotti g’ a liquid m tal is pass d through a orific a d cool d
by dropping into water. The small spherical metal pieces are powdered further by
pulverization. Mg powder is prepared by pulverization, whereas aluminium powder is
obtained by shotting and pulverizing.
b. Atomization
In the atomization process, the liquid metal is forced through a nozzle at high pressure.
The jet of the liquid is broken down by a blast of compressed inert gas. In the modified
atomization process the jet of liquid metal is allowed to strike a rapidly rotating disc and the
45
fine droplets are immediately cooled to obtain a fine powder. Low melting metals like tin,
lead, zinc etc are powdered by this method
c. Reduction of metal oxide
Metal powders can be obtained by the reduction of their metal oxides.
Example: Tungsten metal can be obtained in the powdered form by reduction of its
oxide by
→
(
)
The powdered metals so obtained are sponge like and are suitable for cold processing.
Spongy iron is produced by reduction of iron ore by charcoal at a constant
temperature. Metallic powders of metals with high melting points are obtained by this
method.
d. Electro – deposition
If electro plating conditions are suitably adjusted, metals may be made to get deposited
on electrodes in the form of spongy or brittle deposits which can be scraped off and
ground to a fine powder.
e. Decomposition
Nickel can be converted into nickel tetra carbonyl by reacting with CO at
( ) when heated to
decomposes to give powdery Nickel metal.
(
) →
(
. Thus
)
In the same way Fe powder can be made by decomposition of
(
) .
2) MIXING OR BLENDING
Metal powders in desired proportions are uniformly mixed in order to get best results,
especially when an alloy is to be made. To ensure proper mixing electrically operated double
cone mixers are used.
For special purposes, lubricants, volatilizing agents or non – metallic powders are also
mixed with the metallic powders. Particles having similar size, shape & density blend well to
give excellent results.
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3) COMPACTING THE POWDER
The metallic powder or the blended metallic powder is pressed into the required shape
using a suitable die. This is known as the compacting process.
The powder is taken in the cavity of the bottom die and compressed with the
top die. The pressure used depends on the nature of the material. The dies are made of high
grad hard d st l with fi fi ish. Th articl obtai d aft r compacti g is call d as “gr
compact”.
By compacting
The density of the green compact is increased
Adhesion and cold weld of the powder is produced
Required porosity & strength can be created by adjusting the pressure
There are two groups of compacting methods,
Pressure Techniques
Die compacting
Isotactic compaction
Continuous method
Non – Pressure Techniques
Slip casting method
Gravity method
4) SINTERING
Heating the compacted material in a furnace under controlled conditions is called as
sintering. Sintering is done at a temperature just below the melting point of the main
component of the mixture.
During this process a part of the mixture (low melting component) melts and bond
the remaining particles into a coherent mass. (it should be noted that in sintering only a part
of the mixture melts, the main body remains a solid)
After reaching the sintering temperature, the article is allowed to remain at the same
temperature for some time. This is known as soaking.
47
A sintering cycle consists of heating, soaking and cooling. The sintering is usually
carried out in an atmosphere of hydrogen.
The following 3 stages are involved in sintering
In the first stage, the newly formed bond area grows in size and pore area shrinks.
In the second stage, neck formation takes place
In the third stage, there is rounding of the central pore
Application of powder metallurgy
Powder metallurgy techniques are used to produce self lubricating alloy bearings.
Tungsten filament is produced from tungsten powder which is used for production of
electric bulbs.
Refractory composites are produced when high melting metal powders are mixed with
ceramic oxides, carbides, nitrides etc.
Powder metallurgy techniques are used in surgical implants which require replacement
of bony skeleton of the human body with alloy metal structures.
Advantages of Powder Metallurgy
This method saves cost & labour
Rate of production is high
Dimensional accuracy & good surface finish is obtained. Hence, products hardly require
sizing
Life of the components is longer
Products are free from defects such as voids, blow holes etc.,
No material is wasted
Porous articles can be produced
Impossible parts such as super hard cutting tools can be produced.
Uses:
Porous bearings, bulb filaments, gears, cams, gas turbines, cutting tools, magnetic materials etc.,
can be made.
48
Limitations of powder metallurgy
Limitations of powder metallurgy:
It is cost effective only for mass production.
The machine parts so produced are sometimes not strong enough or as tough as the
metals produced by other techniques such as casting, forging etc.,
Big machineries cannot be produced by powder metallurgy techniques.
In many machine parts, final polishing, electroplating etc., has to be done.