Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

General Principles and Processes of
Isolation of Elements
Earth crust is the source of many elements
...

Aluminium
is the most abundant metal of earth crust and iron comes second
...
5%, Fe-4
...
2%, Na-2
...
3%, Mg-2
...

Native State
Elements which have low chemical reactivity or noble metals having least
electropositive
character are not attacked by oxygen
...
These elements,
therefore,
occur in the free state or in the native state, e
...
, Au, Ag, Pt, S, O, N, noble gases,
etc
...
, occur in nature combined form
as their
compounds such as oxides, carbonates sulphides
...

Hydrogen is the only non-metal which exists in oxidised form only
...

Every mineral is not suitable for the extraction of the metal
...

Thus, all ores are minerals but all minerals are not ores
...

Metallurgy
The entire scientific and technological process used for isolation of the metal from
its ores is
known as metallurgy
...
Pyrometallurgy Extraction of metals takes place at very high temperature
...
are extracted by this method
...
Bydrometallurgical process In this method, metals are extracted by the use of
their
aqueous solution
...

3
...
, are extracted from their molten
salt
solution through electrolytic method
...

The process
of grinding the crushed ore into fine powder with the help of the stamp mills is
called
pulverisation
...
g
...
clays, etc
...
It can be carried out by various
ways depending
upon the nature of the ore
...
The lighter impurities are washed away
...
this
method is based on the difference in the densities (specific gravities) of ore and
gangue
...

Froth Floatation
This method is used for the concentration of sulphide ores
...
As a
result
...
Thus
...

The froth can be stabilised by the addition of stabilisers (aniline or cresols)
...

Depressants These are used to prevent certain types of particles from forming the
froth with
air bubbled, e
...
, NaCN can be used as a depressant in the separation of ZnS and
PbS ores
...

Collectors It increasesthe non-wettability of ore particles by water, e
...
, pine oils,
xanthates
and fatty acids
...
e
...
, chromite, FeCr2O4, containing magnetic SiliCIOUS
gangue and
wolframite FeWO4, Containing cassiterite, 8nO4 (non-magnetic impurities) can be
separated by
this method
...

Chemical Method-Leaching
Leaching is the process in which the ore is concentrated by chemical reaction with
a suitable
reagent which dissolves the ore but not the impurities, e
...
, bauxite is leached with
a hot
concentrated solution of NaOH which dissolves aluminium while other oxides
(Fe2O3, TiO2,
SiO2), remain undissolved and noble metals (Ag and Au) are leached with a dilute
aqueous
solution of NaCN or KCN in the presence of air
...
Thus, isolation of crude metal from concentrated ore involves two major
steps:
1
...

2
...

Conversion to Oxides
(i) Calcination It is the process of converting an ore into its oxides by heating it
strongly,
below its melting point in a limited supply of air or in absence of air
...


Calcination is used for metal carbonates and hydroxides and is carried out in
reverberatory
furnace
...

below its melting point m excess of air
...
Roasting helps to remove the
non-metallic
impurities and moisture
...

Acidic refractories : SiO2 and SiO2 + Al2O3
Basic refractories : CaO and MgO
Neutral refractories : Graphite, chromites
...

Heavy metals like Cu
...
So, etc
...

Reduction of the Oxides to Metal
The roasted or the calcined ore is then converted to the free metal by reduction
...

Metals which are low in the activity series (like Cu, Hg, Au) are obtained by
heating their
compounds lD air: metals which are in the middle of the activity “cries (like Fe
...
g
...

(i) Smelting (reduction with carbon) The process of extracting the metal by fusion
of its oxide

ore with carbon (C) or CO is called smelting
...


During smelting a substance
...
This slag is insoluble in the molten metal and is lighter than the
molten metal
...

Acidic flux For basic impurities, acidic flux is added
...
g
...

The obtained slag is used in road making as well as in the manufacturing of cement
and
fertilizers
...

iii) Reduction by aluminium It is known as alumino thermic reduction or Gold
Schmidt
thermite process
...

e
...
, Cr2O3 + 2Al → Al2O3 + 2Cr
Mixture of the oxide and Al i
...

(iv) Auto reduction This is used for reduction of sulphide ores of Pb, Hg, Cu, etc
...


(v) Reduction by Mg
TiCl4 + 2Mg → 2MgCl2 + Ti (Kroll’s process)
vi) Electrolytic reduction or electrometallurgy It is the process of extracting highly
electropositive (active) metals such as Na, K, Ca, Mg, Al, etc by electrolysis of
their oxides,
hydroxides or chlorides in fused state, e
...
, Mg is prepared by the electrolysis of
fused salt of
MgCl2 (Dow’s process)
...

For the spontaneous reduction of an oxide, halide or sulphide by an element, the
essential
condition is that there is a decrease in the free energy of the system (-ve ΔG)
...
ΔG
can be
given as
ΔG = ΔH – TΔS
where, ΔH = enthalpy change;
ΔG = Gibbs free energy
T = temperature;
ΔS = entropy change
For the reduction of a metal oxide with a reducing agent, the plot of ΔG° against
temperature is
studied, which is called Ellingbam diagram
...
All the plots slope upwards since ΔG° becomes more positive when temperature
increases,
i
...
, stability of oxides decreases
...
A metal will reduce the oxide of other metals which lie above it in Ellingham
diagram, i
...
,
the metals for which the free energy of formation (ΔG°f) of their oxides is more
negative can
reduce those metal oxides which has less negative ΔG°f
3
...

Mg can reduce A12O3 below 162 K but above 1023 K, Al can reduce MgO
...
CO is more effective reducing agent below 1073 K and above 1073 K
...
g
...

Coke reduces ZnO above 1270 K
...
Pb, Hg, Bi) than the impurities, The impure metal is placed on the sloping
hearth and is

gently heated
...

(ii) Distillation This is useful for low boiling metals such as Zn, Hg
...

(iii) Cupellation
This method is used when impure metal contains impurities of other metals which
form volatile
oxides
...
g
...

Chemical Methods
(i) Poling This method is used when the impure metal contains impurities of Its
own oxide,
e
...
, CU2O in blister copper and SnO2 in impure Sn
...
At this high temperature
...

(ii) Electro-refining
In this method, impure metal forms the anode while the cathode is a rod or sheet of
pure metal
...


On passing electricity, the pure metal gets deposited on the cathode while the
insoluble
impurities settle down below the anode as anode mud or anode sludge
...

(iii) Zone-refining This method is based upon the principle of fractional
crystallisation, i
...
,
difference in solubilities of impurities in molten and solid state of metal
...

Elements of very high purity are obtained by this method
...

After this, the volatile compound is decomposed by heating to some higher
temperature to give
pure metal
...
g
...
)

(b) Mond’s process It is used for refining of nickel
...
The
impure metal
is dissolved in a suitable solvent and the solution is allowed to run slowly into an
adsorbent
column packed with alumina (Al2O3)
...
These are then eluted with suitable eluent (solvent)
...

weakly adsorbed component is eluted first and the strongly adsorbed component is
eluted
afterwards
...
Metal Aluminium (AI)
Occurrence
1
...
XH2O

2
...

Neutral flux is the neutral compound added to the ore to decrease its melting point
and to make
it conducting, e
...
, CaF2, cryolite (Na3AlF6) etc
...
Metal Iron (Fe)
Occurrence
1
...
Magnetite – FE3O4
Common method of extraction Reduction of the oxide with CO and coke in blast
furnace
...
g
...

Cast iron is different from pig iron and is made by melting pig iron with scrap iron
and coke
using hot air blast
...

Wrought iron or malleable iron is the purest form of commercial iron and is
prepared from
cast iron by oxidising impurities in a reverberatory furnace lined with haematite
...

Fe2O2 + 3C → 2Fe + 3CO
3
...
Copper pyrites – CuFeS2
2
...

Cu2S + FeS is called matte
...

4
...
Zinc blen de or sphalerite-ZnS
2
...
Zincite – ZnO
Common method of extraction Roasting followed by reduction with coke
...

97-98% pure zinc is called spelter
...
Metal Nickel (Ni)

Occurrence
1
...
Kupfernickel – NiAs
3
...
Ni) As
Common method of extraction Roasting followed by Refining is done by Mond’s
Process
...




---