UNIT - 6 General Principles & Processes of Isolation of Elements
Learning Outcomes
The learners will be able to
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Explain the terms minerals, ores, calcination, roasting, refining, etc.
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Understand the principles of oxidation & reduction in isolation of elements
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Apply the uses of thermodynamics parameters like Gibbs energy & entropy in extraction of metals.
Minerals
The compounds of metals in which metals occur in nature are called minerals.
Ores
The minerals from which metal can be conveniently and economically extracted are called ores. While all ores are minerals but all minerals are not ores.
Gangue
An ore is usually contaminated with undesired materials known as gangue.
Metallurgy
The entire scientific and technological process used for isolation of the metal from its ores is known as metallurgy.
The extraction & isolation of metals from ores involve the following major steps:
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Concentration of the ore.
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Isolation of the metal from its concentrated ore
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Purification of the metal
Classification of Ores
Ores may be divided into following types
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Oxides: Haematite,
; Limonite, ; Magnetite or Loadstone, ; Bauxite, ; Cassiterite or Tinstone, ; Corundum, ; Diaspore, ; Pyrolusite, ; Zincite, ; Rutile, ; Cuprite or Ruby copper, -
Carbonates: Magnesite,
; Dolomite, . ; Cerussite, ; Calamine or Smithosonite, ; Siderite, ; Malachite or Basic copper carbonate, . Azurite, . ; Limestone or Calcite (Marble, Chalk, Slate), -
Sulphides : Iron pyrites,
; Galena, PbS; Zinc blende, ZnS ; Cinnabar, ; Chalcopyrites or Copper pyrites, CuFeS; Silver glance orArgentite, ; copper glance, ; Pentlandite, (Ni, Fe) S , -
Halides : Sylvine,
; Camallite, . ; Common salt or Rock salt, ; Horn silver, ; Cryolite, ; Fluorite or Fluorspar, -
Sulphates: Anglesite,
; Gypsum, . ; Barytes, ; Epsomite or Epsom salt, ; Kieserite, . . -
Silicates : Asbestos,
; Talc, ; Willemite, ; Felspar, Mica,
I) Concentration of ores:
Removal of unwanted impurities (gangue) e.g. sand, clay etc. from the ore is known as concentration, benefaction or dressing of the ore
1) Levigation.
It is the process of separating the lighter gangue particles from the heavier ore by washing in a current of water. This is also called gravity separation or hydraulic washing and is generally used for oxide ores and carbonate ores. This method is based on the difference in densities of the ore particles and impurities. For example, haematite, tin stone and native ores of
2) Froth-Floatation
This process is used for the concentration of sulphide ores and is based upon preferential wetting of the ore particles by the oil used as a foaming agent (pine oil and fatty acids) and gangue particles by water. As a result, the ore particles become light and rise to the top in form of froth while the gangue particles become heavy and settle down. Thus adsorption phenomenon is involved in this method. Cresols and aniline are used as froth stabilizers.
Ethyl xanthate and potassium ethyl xanthate are used as collectors (to make the ore water repellant) and copper sulphate is used as an activator while sodium and potassium cyanides are used as depressants. In case of sulphide minerals containing two or more metals such as galena (PbS) is separated from sphalerite (ZnS) and iron pyrites
3) Electromagnetic Separation
This method is used when either the ore or the impurities associated with it are magnetic in nature. For example, cassiterite
4) 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. For example, in Baeyer’s process bauxite is leached with a hot concentrated solution of
II) Isolation of the metal from its concentrated ore
The concentrated ore must be converted into a form which is suitable for reduction. Oxides are easier to reduce, thus non-oxide ores are first converted into an oxide ore and then it reduce to metal by suitable agent.
a) Conversion to oxide:
i) Roasting
Roasting is the process in which the ore is heated strongly, below its melting point, in presence of excess of air. This process is used for the conversion of sulphide ores to their respective metal oxides (de-electronation of ores).
ii) Calcination
Calcination is the process in which the ore is heated strongly, below its melting point, either in absence or in a limited supply or air. This process is used for the conversion of the carbonates and hydrated oxide ores to their respective oxides.
b) Reduction of oxide to the metal:
i) Reduction by Carbon-Smelting
Smelting is the process of extraction of a metal from its roasted or calcined ore by heating it with powdered coke in presence of a flux.
ii) Flux is a substance which combines with gangue (earthy impurities) still present in the roasted or the calcined ore to form a fusible product called slag.
Acidic fluxes: For basic impurities like those of lime, or oxides of iron present in the ore, acidic fluxes like silica
Basic fluxes:: For acidic impurities like those of silica
Thus silica
iii) Reduction by Aluminium-Goldschmidt Aluminothermic Process
Aluminothermic process involves the reduction of oxides
The mixture of metallic oxide such as
iv) Reduction by Precipitation (Hydro-metallurgy)
Hydrometallurgy is the process of dissolving the metal or its ore by the action of a suitable chemical reagent followed by recovery of the metal either by electrolysis or by the use of a suitable precipitating agent (displacement method). For example,
v) Reduction by Mg
This method is applicable for the reduction of
III) REFINING OF CRUDE METALS
Refining or purification of crude metals.
Refining of metals is the process of obtaining the metals in the pure state from crude metals. Refining is done by the following methods:
1) Liquation
Liquation is used for the refining of such metals as are readily fusible (i.e. having low melting points) such as
2) Distillation
Distillation is used for the refining of such metals which have low boiling points such as
3) Poling
This method is employed when the impure metal contains impurities of its own oxide. For example,
4) Cupellation
Cupellation is used for the refining of such metals which contain impurities of other metals which form volatile oxides.
5) Electro-refining
Metals like
6) Zone-refining
This method is based upon the principle of fractional crystallization, ie. difference in solubilities of impurities in molten (more soluble) and solid state of metal Semiconductors like silicon, germanium and gallium are purified by this method.
7) Van Arkel Method - Vapour Phase Refining
This method is used for preparing ultrapure metals required in space technology in this method, the impure metal is first converted into a volatile stable compound
generally iodide at lower temperature (leaving behind the impurities) which is then decomposed at a higher temperature to give the pure metal. Metals like titanium, zirconium, thorium and uranium are purified by this method.
This process is also called vapour-phase refining.
8) Chromatography
The method is based on the principle that the different components of a mixture are adsorbed to different extents on an adsorbent.
9) Electrolytic Reduction-Electrometallurgy
Electrometallurgy is the process of extracting highly electropositive (active) metals such as
For example, Na is obtained by the electrolysis of fused NaCl in Down’s cell and Al by the electrolysis of fused
where
Since
The above reaction is made use of in the extraction of copper from low grade ores and scraps.
Varieties of Iron
- Pig iron.
It is the impurest form of iron. It is obtained directly from the blast furnace and contains about
- Cast iron
It contains 3\% carbon. It is extremely hard but brittle. Its melting point is the same as that of pig iron.
- Wrought iron
It is the purest form of iron. Carbon is partly present as graphite and partly as iron carbide.
- Steel
The carbon content of various steels lies between those cast iron and wrought iron.
The choice of the reducing agent, however, depends upon the reactivity of the metal. Oxides of very reactive metals like
Extraction of Cast Iron
Cast iron is usually extracted from haematite. The ore after concentration by gravity separation process is calcined to remove moisture, impurities of
The ore becomes sintered (porous) and hence is more suitable for reduction to metallic state. In case of carbonate ore (siderite), during calcination, it is converted into ferric oxide
However, in case of sulphide ore (iron pyrites), concentration is carried out by roasting:
The next step is smelting where the calcined ore ( 8 parts) is mixed with limestone ( 1 part) and coke (4 parts) and is then fed from the top into a blast furnace while preheated air at about
Thermodynamic Principles of Metallurgy
For any process, Gibbs free energy change
where
If
The criterion of feasibility of a reaction at any temperature is that the
A reaction with
Each plot is a straight line except when some change in phase (solid
Reducing behaviour of carbon: When carbon reacts with dioxygen, three types of reactions are possible:

For reaction (i) the volume of

The three lines cross at
Effect of Temperature on the Free Energy Change of the overall reduction process
We know that for any process,
Since on increasing the temperature, the values of
Theory of reduction process:
Thermodynamics helps us to understand how coke reduces iron oxide to metal. One of the main reduction steps in the process is
This redox reaction can be divided into the following two half reactions one involving reduction and the other oxidation.
Reduction:
Oxidation:
The net free energy change of these two combined reactions is
Naturally, the resultant reaction, i.e., Eq. (i) will take place only when the
From the Ellingham diagram, it is evident that on increasing the temperature. the curve for the reaction
In contrast, at temperatures below
At 500-800 K (lower temperature range in the blast furnace)
At 900-1500 K (higher temperature range in the blast furnace)
Reactions taking place in the furance.
Near the bottom of the furnace (zone of combustion,
But the further reduction of
However, direct reduction of iron ores (haematite, magnetite etc.) left unreduced around
Limestone which acts as flux, decomposes at
At the lower part of the furnace (zone of fusion, 1423-1673 K) the spongy iron melts and dissolves some carbon,
The molten slag being less dense floats over the surface of the molten iron. The molten iron is tapped off from the furnace and is then solidified to give blocks of iron called Cast iron or Pig iron.
Solved Examples
1) In view of the sign of
Which oxidation states are more characteristic for lead and tin?
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For lead +2 , for tin +2
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For lead +4 , for tin +4
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For lead +2 , for tin +4
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For lead +4 , for tin +2
Show Answer
Hint: In the given question, in the first equation,
In the second equation,
Thus the correct option is (3).
2) Which of the following metal is leached by cyanide process?
Show Answer
Hint: Leaching by cyanide process is also known as Mac Arthur Forest Cyanide Process. It is used to concentrate Silver and Gold ore. The other metals mentioned are concentrated by other methods.
Thus (1) is the correct option.
3) Which of the following ore is best concentrated by froth floatation method?
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Magnetite
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Galena
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Cassiterite
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Malachite
Show Answer
Hint: Froth floatation method is used for the concentration of sulphide ores. The formulae of the given ores are:
Magnetite
Galena PbS
Malachite
Out of these ores, only Galena is a sulphide ore.
Thus (2) is the correct option.
4) During the process of electrolytic refining of copper some metals present as impurity settle as amide mud. These are
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and -
and -
and -
and
Show Answer
Hint: The anode mud may have impurities like antimony, selenium, tellurium, silver, gold and platinum. The recovery of these elements may meet the cost of refining.
Thus the correct option is (3).
5) Heating
Show Answer
Hint: The given reaction is used to recover copper from copper matte the copper obtained is called blister copper as when the metal solidifies, the dissolved sulphur dioxide escapes producing blisters on the metal surface.
Thus (1) is the correct option.
PRACTICE QUESTIONS
Question 1- Roasting process is applied to which of the following ores?
a) Galena
b) Iron pyrites
c) Copper glance
d) All
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Answer:- dQuestion 2- Which of the following metal can be extracted by smelting?
a)
b)
c)
d) All
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Answer:- bQuestion 3- The chemical reagent used for leaching of gold and silver ores is
a) sodium hydroxide
b) potassium cyanide
c) potassium cyanate
d) sodium thiosulphate.
Show Answer
Answer:- bQuestion 4- The most abundant ore of iron is
a) haematite
b) limonite
c) magnetite
d) siderite.
Show Answer
Answer:- aQuestion 5- The chemical composition of ‘slag’ formed during smelting process in the extraction of copper is
a)
b)
c)
d)
Show Answer
Answer:- dQuestion 6- The incorrect statement among the following is
a) hydrogen is used to reduce
b) zirconium is refined by van Arkel method
c) the sulphide ore galena is concentrated by froth floatation process
d) in the metallurgy of iron flux used is
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Answer:- dQuestion 7- Which of the following is used in thermite welding ?
a)
b)
c)
d)
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Answer:- bQuestion 8- Zinc metal is refined by
a) fractional crystallization
b) fractional distillation
c) electrolysis
d) both by (b) and(c).
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Answer:- dQuestion 9- The froth floatation process is based upon
a) magnetic properties of gangue
b) specific gravity of ore particles
c) preferential wetting of ore particles by oil
d) preferential wetting of gangue particles by oil
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Answer:- cQuestion 10- in Goldschmidt aluminothermic process, reducing agent used is .
a) coke
b) Al powder
c)
d)
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Answer:- bQuestion 11- Cupellation process is used in the metallurgy of
a)
b)
c)
d)
Show Answer
Answer:- bQuestion 12- The method used for refining of iron is called
a) bessemerisation
b) electrolysis
c) cupellation
d) Liquation
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Answer:- aQuestion 13- When copper pyrites is roasted in excess of air, a mixture of
a)
b) lime stone, which is a basic flux
c)
d)
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Answer:- aQuestion 14- The most convenient method for the extraction of silver from silver glance is
(a) leaching
(b) hydrometallurgy
(c) smelting
(d) roasting
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Answer:- bQuestion 15- Carbon cannot reduce
(a) free energy change for the formation of
(b)
(c) carbon has higher affinity towards oxygen than iron
(d) iron has higher affinity towards oxygen than carbon
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Answer:- dQuestion 16- From the Ellingham graphs on carbon, which of the following statements is false?
(a)
(b)
(c)
(d)
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Answer:- cQuestion 17- Which is incorrect statement?
(a) Below
(b) At
(c) Above
(d) Above
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Answer:- dQuestion 18- Copper metal is refined by
(a) liquation
(b) cupellation
(c) bessemerisation
(d) poling
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Answer:- dQuestion 19- The temperature of blast furnace to produce iron from its ore
(a)
(b)
c)
d)
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Answer:- cQuestion 20- Which of the following factors is of no significance for roasting sulphide ores to the oxides and not subjecting the sulphide ores to carbon reduction directly?
a)
b) Metal sulphides are thermodynamicaliy more stable than
c)
d) Metal sulphides are less stable than the corresponding oxides.
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Answer:- aQuestion 21- The process of extracting metals by electrolysis of their oxides; hydroxides or chlorides in the fused state is called
a) electrometallurgy
b) electro-refining
c) zone-refining
d) hydrometallurgy.
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Answer:- aQuestion 22- According to Ellingham diagram, the oxidation reaction of carbon to carbon monoxide may be used to reduce which one of the following oxides at the lowest temperature?
a)
b)
c)
d)
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Answer:- bQuestion 23- The reduction of zinc oxide with coke occurs at temperature.
a) greater than that for
b) less than that for
c) less than that for
d) equal to that for
Show Answer
Answer:- aQuestion 24-
a)
b)
c)
d)
Show Answer
Answer:- c