general principles and processes of isolation of elements

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General Principles
and Processes of
Isolation of Elements
MINERALS
A mineral is a naturally occurring substance, representable by a chemical formula, that is usually
solid and inorganic, and has a crystal structure. It is different from a rock, which can be an aggregate
of minerals or non-minerals and does not have a specific chemical composition. The exact definition
of a mineral is under debate, especially with respect to the requirement a valid species be abiogenic,
and to a lesser extent with regard to it having an ordered atomic structure. The study of minerals is
called mineralogy
ORES
An ore is a type of rock that contains sufficient minerals with important elements
including metals that can be economically extracted from the rock.[1]
The ores are extracted from the
earth through mining; they are then refined (often via smelting) to extract the valuable element, or
elements.

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GAUGUE
In mining, gangue (pronounced "gang") is the commercially worthless material that surrounds, or is
closely mixed with, a wanted mineralin an ore deposit. It is thus distinct from overburden, which is
the waste rock or materials overlying an ore or mineral body that are displaced during mining without
being processed.
Metallurgy
Metallurgy is a domain of materials science and engineering that studies the physical and chemical
behavior of metallic elements, theirintermetallic compounds, and their mixtures, which are
called alloys. Metallurgy is also the technology of metals: the way in which science is applied to the
production of metals, and the engineering of metal components for usage in products for consumers
and manufacturers. The production of metals involves the processing of ores to extract the metal
they contain, and the mixture of metals, sometimes with other elements, to produce alloys.
Metallurgy is distinguished from the craft of metalworking, although metalworking relies on
metallurgy, as medicine relies on medical science, for technical advancement
.

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OCCURRENCE OF METALS
METAL ORES COMPOSITION
ALUMINIUM BAUXITE AlOx(OH)3-2x
{Where 0<x<1}
KAOLINITE{a form of clay} {Al2(OH)4Si2O5}
IRON HAEMATITE Fe2O3
MAGNETITE Fe3O4
SIDERITE FeCO3
IRON PYRITES FeS2
COPER COPPER PYRITES CuFeS2
MALACHITE CuCO3.Cu(OH)2
CUPRITE Cu2O
COPPER GLANCE Cu2S
ZINC ZINC BLENDE OR SPHALERITE ZnS
CALAMINE ZnCO3
ZINCITE ZnO
Concentration OF ORES
Removal of the unwanted materials from the ore is known as concentration,dressing or
benefaction.It involves several steps and selection of these steps depends upon the differences in
physical properties of the compound of the metal present and that of the gangue.The type of the
metal,the available facilities and the environmental factors are also taken into consideration.Some of
the important procedures are described below:
1,Hydraulic washing
This is based on the differences in the gravitiesof the ore and the gangue particles. in one such
process,an upward stream of running water is used to wash the powdered ore. The lighter gangue
particles are washed away and heavier ores are left behind.
Hydraulic washing process is done by washing the ores with streams of water. If an ore is heavier or
denser than the gangue, then the gangue particles are washed way with the stream. The heavier or
denser ore particles remain behind and can be collected. Hydraulic washing is done for ores that
have tin or lead, as they are found to be heavier than the gangue.

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2,magnetic separation
In this process the ore is pulverized and rolled on the magnetic roller. Magnetic particles are
attracted towards belt leaving behind the non magnetic particles.
This method of concentration can be applied when the gangue and the ore particles have different
magnetic properties. For example, if the ore particles are magnetic in nature and if the gangue
particles are non – magnetic, then a strong magnet can be brought and the magnetic ore particles
can be sucked out from the powdered ore. The powdered ore is poured over a conveyer belt. One of
the rollers of the belt is made out of magnet. The magnetic roller makes the magnetic ore particles
stick on the belt and these are moved at a distance before they are collected. The gangue particles,
being non – magnetic in nature, do not get attracted to the roller and fall in a heap below the roller
itself. Iron ores like magnetite, chromite and manganese ore like pyrolusite are concentrated by this
process. Sometimes, a reverse situation may occur: the ore is non magnetic and the gangue is
magnetic. In this case also magnetic separation may be used for concentration of the non –
magnetic ore.

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3,froth floatation method
The pulverized ore is treated with an oil detergent mixture, then stirred and aerated to create a froth
that separates the detergent coated mineral from the gangue. The mineral rich froth is collected for
further processing.
This process is used for sulfide ores. Sulfide ores are first ground to powder and water is added.
Then pine oil is added and the emulsion is agitated by passing compressed air. Oil and froth float on
the surface along with the sulfide ore. The gangue particles being insoluble in oil remain at the
bottom of the water tank. The froth is removed and allowed to settle down. This is called the froth –
floating process. This process is used for sulfide ores of Cu, Pb and Zn.
Leaching
Leaching is a widely used extractive metallurgy technique which converts metals
into soluble salts in aqueous media. Compared to pyrometallurgical operations, leaching
is easier to perform and much less harmful, because no gaseous pollution occurs.
Drawbacks of leaching are the highly acidic and in some cases toxic residual effluent,
and its lower efficiency caused by the low temperatures of the operation, which
dramatically affect chemical reaction rates.
There are a variety of leaching processes, usually classified by the types of reagents
used in the operation. The reagents required depend on the ores or pretreated material
to be processed. A typical feed for leaching is either oxide or sulfide.
For material in oxide form, a simple acid leaching reaction can be illustrated by the zinc
oxide leaching reaction :
ZnO + H2SO4 → ZnSO4 + H2O

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In this reaction solid ZnO dissolves, forming soluble zinc sulfate.
In many cases other reagents are used to leach oxides. For example, in the metallurgy
of aluminium, aluminium oxide is subject to leaching by alkali solutions:
Al2O3 + 3H2O + 2NaOH → 2NaAl(OH)4
Leaching of sulfides is a more complex process due to the refractory nature of sulfide
ores. It often involves the use of pressurized vessels, called autoclaves. A good
example of the autoclave leach process can be found in the metallurgy of zinc. It is best
described by the following chemical reaction:
2ZnS + O2 + 2H2SO4 → 2ZnSO4 + 2H2O + 2S
This reaction proceeds at temperatures above the boiling point of water, thus creating
a vapor pressure inside the vessel. Oxygen is injected under pressure, making the total
pressure in the autoclave more than 0.6 MPa.
The leaching of precious metals such as gold can be carried out with cyanide
or ozone under mild conditions.
Extraction of crude metal
Ores can be concentrated by using any of the four methods - hydraulic washing, magnetic
separation, froth flotation and leaching.
Crude metal extraction concentrated ore:
The concentrated ore must be converted into an oxide, which is more suitable for reduction.
The isolation or extraction of a metal from concentrated ore involves two major steps:

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a) Converting ore into its metal oxide
b) Reducing the metal oxide into the metal
Concentrated ore can be converted into its metal oxide either by calcination (or) by roasting.
Calcination is the process of converting an ore into its oxide, by heating it strongly below its melting
point, in the presence of a limited supply of air. It is usually carried out in a reverberatory furnace.
This method is commonly used to convert carbonate ores, such as calamine and dolomite into their
oxides. Also, the hydrated ores lose water of crystallisation during calcination.
When calamine, is subjected to calcination, zinc oxide is formed and carbon dioxide is liberated.
Δ
ZnCO3(s) → ZnO(s) + CO2
Calamine Calcination Zinc Oxide Carbon Dioxide
Similarly, when dolomite is calcined, it gives calcium oxide, magnesium oxide and CO2 gas.
When hematite is calcined, it loses its water of crystallisation.
Δ
FeO3.xH2O(s) → Fe2O3(s) + xH2O(g)
Hematite Calcination Iron(III)Oxide
Roasting is also used to convert an ore into its metal oxide. It is the process of heating an ore in a
furnace with a regular supply of excess air, at a temperature below the melting point of the metal.
This process is commonly employed for sulphide ores.
Ex: Zinc sulphide is heated at about 850 °C to give zinc oxide and sulphur dioxide.
850°C
2ZnS(s) + 3O2 → 2ZnO(s) + 2SO2 (g)
Zinc Sulphide Zinc Oxide Sulphur Dioxide
Lead sulphide is roasted at about 600 °C to give lead oxide and sulphur dioxide.
The iron oxide present as an impurity in the form of gangue is removed by adding flux silica.
Gangue Slag
↑ ↑
FeO + SiO2 → FeSiO3
Iron Oxide Flux Silica Iron Silicate
Iron oxide reacts with silica and slags off as iron silicate. The slag can be separated more easily than
gangue.
Roasting (or) calcination converts an ore into metal oxide and makes it porous. The metal oxide is
porous because the volatile impurities are expelled and the moisture is removed during roasting (or)
calcination.
The metal oxide is reduced to metal by using a suitable reducing agent.
Ex: When carbon is used as the reducing agent, it combines with the oxygen of the metal oxide to
give metal and carbon monoxide (or) carbon dioxide.

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Reduction
MO + C → M + CO ↑
Carbon Monoxide
Carbon is used to reduce zinc oxide to metallic zinc. In this reaction, carbon monoxide is released.
Reduction
ZnO + C → Zn + CO ↑
Zinc Oxide Carbon Carbon Monoxide
Generally, heating is required for a metal oxide to be reduced to its metal.
Thermodynamic principles of
metallurgy
Thermodynamic principles can be applied to the ore extraction process.
The Gibbs energy equation that relates to the enthalpy and entropy of the system at a certain
temperature.
ΔG = ΔH - TΔS
ΔG = Gibbs Free Energy Change
ΔH = Change in Enthalpy
T = Temperature
ΔS = Change in Entropy
Spontaneous Process → ΔG = -ve
Non-Spontaneous Process → ΔG = +ve
According to Gibbs energy free energy can be utilized to do useful work. The term ∆H represents the
enthalpy change and ∆S is the change in entropy at temperature T.
Relation between free energy change and equilibrium constant,
ΔG = -RTlnK
The equilibrium constant K is obtained by taking the ratio of equilibrium concentrations of reactants
and products.
When a reaction proceeds from reactants to products, products are present in excess and
equilibrium constant is positive. Alternatively, the equilibrium constant is negative for a reverse
reaction.
For ∆G to be negative, the change in entropy should be positive and on increasing the temperature,
the value T∆S should exceed the enthalpy change for the reaction.
A reaction with a positive ∆G can also occur, if it is coupled with another reaction that has a large
negative ∆G value, so that the net value of ∆G would still be negative.
Ellingham diagrams are used as a tool in extraction of a metal in metallurgy to find the appropriate
conditions for reduction of ores of important metals.
In the reaction between metal oxide and carbon, carbon is the reducing agent, itself undergoes
oxidation to reduce the element.
Carbon may undergo a partial oxidation, it takes up only half mole of oxygen to form carbon
monoxide (or) it may undergo complete oxidation, takes up 1mole of oxygen to form carbon dioxide.
MxO + C → xM + CO
MxO + C → xM + CO2
MxO + ½C → xM + ½CO2

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It is a redox reaction. Redox reaction is one where oxidation and reduction take place. In this
reaction the metal oxide is reduced and carbon and carbon monoxide will oxidized.
Heating favors a negative value for ∆G. Hence the temperature is chosen such that the sum of ∆G in
the two combined redox process is negative
Ellingham diagram
Thermodynamics useful to understand the variation in temperature required for the thermal reduction
of oxides and to predict which element will suit as the reducing agent for a given metal oxide. The
Gibbs energy is the most important thermodynamic term in metal extraction.
For a spontaneous reaction the change in the Gibbs energy, ∆G, must be negative. ∆G for any
process at a given temperature is given by the equation
∆G = ∆H- T∆S
Where, ∆H = enthalpy change
T = absolute temperature
∆S = change in entropy during the reaction.
The change in the Gibbs energy when 1gram molecule of oxygen, sulphur (or) halogen is used to
form oxides, sulphides (or) halides of metals plotted against temperature. This graphical
representation is called an Ellingham diagram.
These plots are useful to determine the relative ease of reducing a given metal oxide to the metal
and also to predict the feasibility of the thermal reduction of an ore.
An Ellingham diagram normally consists of plots of change in the Gibbs energy with temperature for
the formation of oxides. An Ellingham diagram for oxides has several important features.
(i) The graphs for most metal to metal oxide reactions show a positive slope.

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Ex: 2M + O2 →2MO.
In this reaction, the entropy (or) randomness decreases from left to right due to the consumption of
gases. Hence, ∆S becomes negative. If the temperature is raised, then T ∆S becomes more
negative. So, ∆G becomes less negative.
(ii) The Gibbs energy changes follow a straight line, unless the materials melt (or) vaporise. The
temperature at which such a change occurs is indicated by an increase in the slope on the positive
side.
(iii) When the temperature is raised, a point will be reached where the graph crosses the line "∆G is
zero." Below this temperature, the free energy of formation of the oxide is negative, so the oxide is
stable. Above this temperature, the free energy of formation of the oxide is positive the oxide
becomes unstable and will decompose into the metal and dioxygen.
Any metal will reduce an oxide of another metal that lies above it in an Ellingham diagram. Ex: Al
reduces FeO, CrO and NiO in termite reaction but Al will not reduce MgO at a temperature below
1500 0C.
Limitations:
The reactants and products are in equilibrium, which is not often true.
It does not explained about the rate of the reaction
Electrochemical Principles of
Metallurgy
The principles of metallurgy are effective in the reduction of metal ions to their respective metals in
their solution (or) molten states. The reduction is carried out through electrolysis (or) using reducing
elements. Such methods are based on electrochemical principles.
Mn+ + A → M + An+
Metal ion reducing Element Metal Reduced ion
These electrochemical principles are explained in
ΔG° = -nE°F
n = Number of Electrons Gained
E° = Electrode Potential of Redox Couple
The value of E0 for a metal depends on its reactivity. Thus, it differs from metal to metal. More
reactive metals have high E0, whereas less reactive metals have low E0. It is difficult to reduce
metals that have high E0.
If the difference between the E0 values of two metals is positive, then the value of ∆G0 will be
negative.
Hence, the less reactive metal will come out of the solution and the more reactive metal will go into
the solution.
An+ + B → A + Bn+
Less Metal ion More reducing Element Less Reactive Metal More Reduced ion
Ex: The reduction of copper (II) to copper in the presence of iron.
Cun+ + Fe → Cu + Fen+
Copper(II) ion Iron Copper Iron(II) ion
In a simple electrolysis process, the positive ions always move towards the negative electrode called
the cathode.
Precautions to be taken:
 · Consider the reactivity of the metal produced in the electrolysis.

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 · Use suitable material as the electrodes.
 · Sometimes flux is needed to increase the conductivity of the molten mass.
Metallurgy of aluminium:
Addition of cryolite and fluorspar as flux to aluminium oxide because pure aluminium oxide is a bad
conductor of heat and electricity. The fused substance is used as the electrolyte.
During this process, the electrolyte is taken in a steel container lined with carbon, which acts as the
cathode. A bunch of graphite rods immersed in the electrolyte act as the anode. This process is also
known as the Hall-Heroult process. When current is passed, all the Al+3 ions move towards the
cathode. Oxygen is liberated at the anode, which reacts with the carbon atoms of the anode to form
carbon monoxide and carbon dioxide.
Cathode: Al3+ + 3e- → Al(l)
Anode : C(s) + O2- → CO(g) + 2e-
C(s) + 2O2- → CO2(g) + 4e-
Generally, copper is extracted from its low concentrated ores by a process known as
"hydrometallurgy". In this process, a solution containing Cu+2 ions is treated with a scrap of iron or
hydrogen.
Cu+2 + Fe → Cu + Fe+2
Copper(II) ion Iron Copper Ferrous(II) ion
Cu+2 + H2 → Cu + 2H+
Copper(II) ion Hydrogen Copper Hydrogen ion

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Oxidation reduction
Oxidation and reduction are two types of chemical reactions that often work together. Oxidation and
reduction reactions involve an exchange of electrons between reactants. For many students, the
confusion occurs when attempting to identify which reactant was oxidized and which reactant was
reduced.
Refining
A metal extracted by any method is usually contaminated with some impurity.For obtaining metals of
high purity,several technigues are used depending upon the differences in the properties of the
metal and the impurity.some of them listed below,
A,Distillation
B,liquation
C,Electrolysis
D,zone refiing
E,vapour phase refining
F,chromatographic methods
A,distillation
Distillation is a process of separating the component substances from a liquid mixture by
selective evaporation and condensation. Distillation may result in essentially complete separation
(nearly pure components), or it may be a partial separation that increases the concentration of
selected components of the mixture. In either case the process exploits differences in the volatility of
mixture's components. In industrial chemistry, distillation is a unit operation of practically universal
importance, but it is a physical separation process and not a chemical reaction.
Commercially, distillation has many applications. For example:
 In the fossil fuel industry distillation is a major class of operation in obtaining materials
from crude oil for fuels and for chemicalfeedstocks.
 Distillation permits separation of air into its components — notably oxygen, nitrogen,
and argon — for industrial use.
 In the field of industrial chemistry, large ranges of crude liquid products of chemical
synthesis are distilled to separate them, either from other products, or from impurities, or from
unreacted starting materials.
 Distillation of fermented products produces distilled beverages with a high alcohol content, or
separates out other fermentation products of commercial value.
An installation for distillation, especially of alcohol, is a distillery. The distillation equipment is a still

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B,liquation
Liquation is a metallurgical method for separating metals from an ore or alloy. The material must be
heated until one of the metals starts to melt and drain away from the other and can be collected.
This method was largely used to remove lead containing silver from copper, but it can also be used
to remove antimony minerals from ore, and refine tin
C,electrolytic refining
The purest copper is obtained by an electrolytic process, undertaken using a slab of impure copper
as the anode and a thin sheet of pure copper as the cathode. The electrolyteis an acidic solution of
copper sulphate. By passing electricity through the cell, copper is dissolved from the anode and
deposited on the cathode. However impurities either remain in solution or collect as an insoluble
sludge. This process only became possible following the invention of the dynamo; it was first used in
South Wales in 1869.
In this method, the impure metal is made to act as anode. A strip of the same metal in pure form is
used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same
metal. The more basic metal remains in the solution and the less basic ones go to the anode mud.
This process is also explained using the concept of electrode potential, over potential, and Gibbs
energy which you have seen in previous sections. The reactions are:

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Anode: M Mn+ + ne–
Cathode: Mn+ + ne– M
Copper is refined using an electrolytic method. Anodes are of impure copper and pure copper strips
are taken as cathode. The electrolyte is acidified solution of copper sulphate and the net result
of electrolysis is the transfer of copper in pure form from the anode to the cathode:
Anode: Cu Cu2+ + 2 e–
Cathode: Cu2+ + 2e– Cu
Impurities from the blister copper deposit as anode mud which contains antimony, selenium,
tellurium, silver, gold and platinum; recovery of these elements may meet the cost of refining.
Zinc may also be refined this way.
D,zone refining
This method is based on the principle that the impurities are more soluble in the melt than in the
solid state of the metal. A circular mobile heater is fixed at one end of a rod of the impure metal
(Fig. 6.7). The molten zone moves along with the heater which is moved forward. As the heater
moves forward, the pure metal crystallises out of the melt and the impurities pass on into the
adjacent molten zone. The process is repeated several times and the heater is moved in the same
direction. At one end, impurities get concentrated. This end is cut off. This method is very useful for
producingsemiconductor and other metals of very highpurity, e.g., germanium, silicon, boron,
gallium and indium.
E,Vapour phase refining
In this method, the metal is converted into its volatile compound and collected elsewhere. It is then
decomposed to give pure metal.So, the two requirements are:
(i) the metal should form a volatile compound with an available reagent,
(ii) the volatile compound should be easily decomposable, so that the recovery is easy.
Following examples will illustrate this technique.
Mond Process for Refining Nickel: In this process, nickel is heated in a stream of carbon
monoxide forming a volatile complex, nickel tetracarbonyl:
Ni + 4CO 330 – 350 K -------- Ni(CO)4

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The carbonyl is subjected to higher temperature so that it is decomposed giving the pure metal:
Ni(CO)4 450 – 470 K--------------- Ni + 4CO
van Arkel Method for Refining Zirconium or Titanium: This method is very useful for removing all
the oxygen and nitrogen present in the form of impurity in certain metals like Zr and Ti. The crude
metal is heated in an evacuated vessel with iodine. The metal iodide being more covalent,
volatilises:
Zr + 2I2-------ZrI4
The metal iodide is decomposed on a tungsten filament, electrically heated to about 1800K. The
pure metal is thus deposited on the filament.
ZrI4 -------> Zr + 2I2
F,chromatographic methods
Chromatography which means "color" and is the collective term for a set of laboratory techniques for
the separation of mixtures. The mixture is dissolved in a fluid called the mobile phase, which carries
it through a structure holding another material called the stationary phase. The various constituents
of the mixture travel at different speeds, causing them to separate. The separation is based on
differential partitioning between the mobile and stationary phases. Subtle differences in a
compound's partition coefficient result in differential retention on the stationary phase and thus
changing the separation.
Chromatography may be preparative or analytical. The purpose of preparative chromatography is to
separate the components of a mixture for more advanced use (and is thus a form of purification).
Analytical chromatography is done normally with smaller amounts of material and is for measuring
the relative proportions of analytes in a mixture. The two are not mutually exclusive

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