General Principles and Processes of Isolation of Elements

149

After studying this Unit, you will be

able to:

• appreciate the contribution of

Indian traditions in the

metallurgical processes,

• explain the terms minerals,

ores, concentration, benefaction,

calcination, roasting, refining, etc.;

• understand the principles of

oxidation and reduction as applied

to the extraction procedures;

• apply the thermodynamic

concepts like that of Gibbs energy

and entropy to the principles of

extraction of Al, Cu, Zn and Fe;

• explain why reduction of certain

oxides like Cu

O is much easier

than that of Fe

;

• explain why CO is a favourable

reducing agent at certain

temperatures while coke is better

in some other cases;

• explain why specific reducing

agents are used for the reduction

purposes.

Thermodynamics illustrates why only a certain reducing element

and a minimum specific temperature are suitable for reduction of a

metal oxide to the metal in an extraction.

General Principles and

Processes of Isolation

of Elements

General Principles and

rocesses of Isolation

of Elements

Unit

The history of civilisation is linked to the use of metals

in antiquity in many ways. Different periods of early

human civilisations have been named after metals.

The skill of extraction of metals gave many metals

and brought about several changes in the human

society. It gave weapons, tools, ornaments, utensils,

etc., and enriched the cultural life. The ‘Seven metals

of antiquity’, as they are sometimes called, are gold,

copper, silver, lead, tin, iron and mercury. Although

modern metallurgy had exponential growth after

Industrial Revolution, it is interesting to note that

many modern concepts in metallurgy have their roots

in ancient practices that pre-dated the Industrial

Revolution. For over 7000 years, India has had a

rich tradition of metallurgical skills.

The two important sources for the history of Indian

metallurgy are archaeological excavations and literary

evidences. The first evidence of metal in Indian

subcontinent comes from Mehrgarh in Baluchistan,

where a small copper bead, dated to about 6000

BCE was found. It is however thought to be native

copper, which has not been extracted from the ore.

Spectrometric studies on copper ore samples obtained

from the ancient mine pits at Khetri in Rajasthan and

on metal samples cut from representative Harappan

Objectives

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150Chemistry

artefacts recovered from Mitathal in Haryana and eight other sites

distributed in Rajasthan, Gujarat, Madhya Pradesh and Maharashtra

prove that copper metallurgy in India dates back to the Chalcolithic

cultures in the subcontinent. Indian chalcolithic copper objects were

in all probability made indigenously. The ore for extraction of metal for

making the objects was obtained from chalcopyrite ore deposits in

Aravalli Hills. Collection of archaeological texts from copper-plates and

rock-inscriptions have been compiled and published by the

Archaeological Survey of India during the past century. Royal records

were engraved on copper plates (tamra-patra). Earliest known copper-

plate has a Mauryan record that mentions famine relief efforts. It has

one of the very few pre-Ashoka Brahmi inscriptions in India.

Harappans also used gold and silver, as well as their joint alloy

electrum. Variety of ornaments such as pendants, bangles, beads and

rings have been found in ceramic or bronze pots. Early gold and silver

ornaments have been found from Indus Valley sites such as

Mohenjodaro (3000 BCE). These are on display in the National Museum,

New Delhi. India has the distinction of having the deepest ancient gold

mines in the world, in the Maski region of Karnataka. Carbon dating

places them in mid 1st millennium BCE.

Hymns of Rigveda give earliest indirect references to the alluvial

placer gold deposits in India. The river Sindhu was an important source

of gold in ancient times. It is interesting that the availability of alluvial

placer gold in the river Sindhu has been reported in modern times also.

It has been reported that there are great mines of gold in the region of

Mansarovar and in Thokjalyug even now. The Pali text, Anguttara Nikaya

narrates the process of the recovery of gold dust or particles from alluvial

placer gold deposits. Although evidence of gold refining is available in

Vedic texts, it is Kautilya’s Arthashastra, authored probably in 3rd or

4th century BCE, during Mauryan era, which has much data on

prevailing chemical practices in a long section on mines and minerals

including metal ores of gold, silver, copper, lead, tin and iron. Kautilya

describes a variety of gold called rasviddha, which is naturally occurring

gold solution. Kalidas also mentioned about such solutions. It is

astonishing how people recognised such solutions.

The native gold has different colours depending upon the nature and

amount of impurity present in it. It is likely that the different colours of

native gold were a major driving force for the development of gold refining.

Recent excavations in central parts of Ganges Valley and Vindhya

hills have shown that iron was produced there possibly as early as in

1800 BCE. In the recent excavations conducted by the Uttar Pradesh

State Archaeological Department, iron furnaces, artefacts, tuyers and

layers of slag have been found. Radiocarbon dating places them between

BCE 1800 and 1000. The results of excavation indicate that the

knowledge of iron smelting and manufacturing of iron artefacts was

well known in Eastern Vindhyas and it was in use in the Central

Ganga Plains, at least from the early 2nd millennium BCE. The quantity

and types of iron artefacts and the level of technical advancements

indicate that working of iron would have been introduced much earlier.

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General Principles and Processes of Isolation of Elements

151

The evidence indicates early use of iron in other areas of the country,

which proves that India was indeed an independent centre for the

development of the working of iron.

Iron smelting and the use of iron was especially established in

South Indian megalithic cultures. The forging of wrought iron seems

to have been at peak in India in the Ist millennium CE. Greek accounts

report the manufacture of steel in India by crucible process. In this

process, iron, charcoal and glass were mixed together in a crucible

and heated until the iron melted and absorbed the carbon. India was

a major innovator in the production of advanced quality steel. Indian

steel was called ‘the Wonder Material of the Orient’. A Roman historian,

Quintus Curtius, records that one of the gifts Porus of Taxila (326

BCE) gave to Alexander the Great was some two-and-a-half tons of

Wootz steel. Wootz steel is primarily iron containing a high proportion

of carbon (1.0 – 1.9%). Wootz is the English version of the word ‘ukku’

which is used for steel in Karnataka and Andhra Pradesh. Literary

accounts suggest that Indian Wootz steel from southern part of the

Indian subcontinent was exported to Europe, China and Arab world.

It became prominent in the Middle East where it was named as

Damasus Steel. Michael Faraday tried to duplicate this steel by alloying

iron with a variety of metals, including noble metals, but failed.

When iron ore is reduced in solid state by using charcoal, porous

iron blocks are formed. Therefore, reduced iron blocks are also called

sponge iron blocks. Any useful product can be obtained from this

material only after removing the porosity by hot forging. The iron so

obtained is termed as wrought iron. An exciting example of wrought

iron produced in ancient India is the world famous Iron Pillar. It was

erected in its present position in Delhi in 5th century CE. The Sanskrit

inscription engraved on it suggests that it was brought here from

elsewhere during the Gupta Period. The average composition (weight%)

of the components present in the wrought iron of the pillar, besides

iron, are 0.15% C, 0.05% Si, 0.05% Mn, 0.25% P, 0.005% Ni, 0.03%

Cu and 0.02% N. The most significant aspect of the pillar is that there

is no sign of corrosion inspite of the fact that it has been exposed to

the atmosphere for about 1,600 years.

Radiocarbon dating of charcoal from iron slag revealed evidence

of continuous smelting in Khasi Hills of Meghalaya. The slag layer,

which is dated to 353 BCE – CE 128, indicates that Khasi Hill region

is the earliest iron smelting site studied in the entire region of North

East India. The remnants of former iron-ore excavation and iron

manufacturing are visible even now in the landscape of Khasi Hills.

British naturalists who visited Meghalaya in early 19th century

described the iron industry that had developed in the upper part of

the Khasi Hills.

There is archaeological evidence of zinc production in Rajasthan

mines at Zawar from the 6th or 5th BCE. India was the first country

to master zinc distillation. Due to low boiling point, zinc tends to

vapourise while its ore is smelted. Pure zinc could be produced after

a sophisticated ‘downward’ distillation technique in which the vapour

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152Chemistry

was condensed in a lower container. This technique was also applied

to mercury. Indian metallurgists were masters in this technique. This

has been described in Sanskrit texts of 14th century.

Indians had knowledge about mercury. They used it for medicinal

purpose. Development of mining and metallurgy declined during the British

colonial era. By the 19th century, once flourished mines of Rajasthan were

mostly abandoned and became almost extinct. In 1947 when India got

independence, European literature on science had already found its way

slowly into the country. Thus, in post independence era, the Government

of India initiated the process of nation building through the establishment

of various institutes of science and technology. In the following sections, we

will learn about the modern methods of extraction of elements.

A few elements like carbon, sulphur, gold and noble gases, occur in free

state while others are found in combined forms in the earth’s crust.

Elements vary in abundance. Among metals, aluminium is the most

abundant. In fact, it is the third most abundant element in earth’s crust

(8.3% approx. by weight). It is a major component of many igneous

minerals including mica and clays. Many gemstones are impure forms

of Al

. For example, gems ‘ruby’ and ‘sapphire’ have Cr and Co

respectively as impurity. Iron is the second most abundant metal in the

earth’s crust. It forms a variety of compounds and their various uses

make it a very important element. It is one of the essential elements in

biological systems as well.

For obtaining a particular metal, first we look for minerals which

are naturally occurring chemical substances in the earth’s crust and

are obtained through mining. Out of many minerals in which a metal

may be found, only a few are viable to be used as source of that metal.

Such minerals are known as ores.

The principal ores of aluminium, iron, copper and zinc are given

in Table 6.1.

6.16.1

6.1

Occurrence ofOccurrence of

Occurrence of

Metals

Aluminium Bauxite AlO

(OH)

3-2x

[where 0 < x < 1]

Kaolinite (a form of clay) [Al

(OH)

]

Iron Haematite Fe

Magnetite Fe

Siderite FeCO

Iron pyrites FeS

Copper Copper pyrites CuFeS

Malachite CuCO

.Cu(OH)

Cuprite Cu

Copper glance Cu

Zinc Zinc blende or Sphalerite ZnS

Calamine ZnCO

Zincite ZnO

Metal Ores Composition

Table 6.1: Principal Ores of Some Important Metals

A particular element may occur in a variety of compounds. The

process of isolation of element from its compound should be such that

it is chemically feasible and commercially viable.

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General Principles and Processes of Isolation of Elements

153

For the purpose of extraction, bauxite is chosen for aluminium. For

iron, usually the oxide ores which are abundant and do not produce

polluting gases (like SO

that is produced in case of iron pyrites) are

taken. For copper and zinc, any of the ores listed in Table 6.1 may be

used depending upon the availability and other relevant factors.

The entire scientific and technological process used for isolation of

the metal from its ore is known as metallurgy. The extraction and

isolation of an element from its combined form involves various

principles of chemistry. Still, some general principles are common to

all the extraction processes of metals.

An ore rarely contains only a desired substance. It is usually

contaminated with earthly or undesired materials known as gangue.

The extraction and isolation of metals from ores involves the following

major steps:

• Concentration of the ore,

• Isolation of the metal from its concentrated ore, and

• Purification of the metal.

In the following Sections, we shall first describe the various steps

for effective concentration of ores. After that principles of some of the

common metallurgical processes will be discussed. Those principles

will include the thermodynamic and electrochemical aspects involved

in the effective reduction of the concentrated ore to the metal.

Removal of the unwanted materials (e.g., sand, clays, etc.) from the ore

is known as concentration, dressing or benefaction. Before proceeding

for concentration, ores are graded and crushed to reasonable size.

Concentration of ores 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 for concentration of

ore are described below.

This is based on the difference between specific gravities of the ore and

the gangue particles. It is therefore a type of gravity separation. 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 the

heavier ore particles are left behind.

This is based on differences in magnetic properties of the ore

components. If either the ore or the gangue is attracted towards

magnetic field, then the separation is

carried out by this method. For example

iron ores are attracted towards magnet,

hence, non–magnetic impurities can be

separted from them using magnetic

separation. The powdered ore is

dropped over a conveyer belt which

moves over a magnetic roller (Fig.6.1)

Magnetic substance remains attracted

towards the belt and falls close to it.

6.2.1 Hydraulic

Washing

6.2.2 Magnetic

Separation

6.26.2

6.2

ConcentrationConcentration

Concentration

of Oresof Ores

of Ores

Fig. 6.1: Magnetic separation

(schematic)

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154Chemistry

This method is used for removing gangue from sulphide ores. In this

process, a suspension of the powdered ore is made with water.

Collectors and froth

stabilisers are added to

it. Collectors (e.g., pine

oils, fatty acids, xanthates,

etc.) enhance non-

wettability of the mineral

particles and froth

stabilisers (e.g., cresols,

aniline) stabilise the froth.

The mineral particles

become wet by oils while

the gangue particles by

water. A rotating paddle

agitates the mixture and

draws air in it. As a

result, froth is formed

which carries the mineral

particles. The froth is light

and is skimmed off. It is then dried for recovery of the ore particles.

Sometimes, it is possible to separate two sulphide ores by

adjusting proportion of oil to water or by using ‘depressants’.

For example, in the case of an ore containing ZnS and PbS, the

depressant used is NaCN. It selectively prevents ZnS from coming

to the froth but allows PbS to come with the froth.

Leaching is often used if the ore is soluble in some suitable solvent.

Following examples illustrate the procedure:

(a) Leaching of alumina from bauxite

Bauxite is the principal ore of aluminium. It usually contains SiO

iron oxides and titanium oxide (TiO

) as impurities. Concentration is

carried out by heating the powdered ore with a concentrated solution

of NaOH at 473 – 523 K and 35 – 36 bar pressure. This process is

called digestion. This way, Al

is extracted out as sodium aluminate.

6.2.4 Leaching

Fig. 6.2: Froth floatation process

6.2.3 Froth

Floatation

Method

The Innovative WasherwomanThe Innovative Washerwoman

The Innovative Washerwoman

One can do wonders if he or she has a scientific temperament and is attentive to

observations. A washerwoman had an innovative mind too. While washing a miner’s

overalls, she noticed that sand and similar dirt fell to the bottom of the washtub. What

was peculiar, the copper bearing compounds that had come to the clothes from the mines,

were caught in the soapsuds and so they came to the top. One of her clients, Mrs. Carrie

Everson was a chemist. The washerwoman told her experience to Mrs. Everson. The

latter thought that the idea could be used for separating copper compounds from rocky

and earth materials on large scale. This way an invention came up. At that time only

those ores were used for extraction of copper, which contained large amounts of the metal.

Invention of the Froth Floatation Method made copper mining profitable even from the low-

grade ores. World production of copper soared and the metal became cheaper.

General Principles and Processes of Isolation of Elements

155

The impurity, SiO

too dissolves forming sodium silicate. Other

impurities are left behind.

(s) + 2NaOH(aq) + 3H

O(l) →

2Na[Al(OH)

](aq) (6.1)

The sodium aluminate present in solution is neutralised by passing

gas and hydrated Al

is precipitated. At this stage, small

amount of freshly prepared sample of hydrated Al

is added to

the solution. This is called seeding. It induces the precipitation.

2Na[Al(OH)

](aq) + 2CO

(g) → Al

.xH

O(s) + 2NaHCO

(aq) (6.2)

Sodium silicate remains in the solution and hydrated alumina is

filtered, dried and heated to give back pure Al

.xH

O(s)

1470 K

(s) + xH

O(g) (6.3)

(b) Other examples

In the metallurgy of silver and gold, the respective metal is

leached with a dilute solution of NaCN or KCN in the presence

of air, which supplies O

. The metal is obtained later by

replacement reaction.

4M(s) + 8CN

–

(aq)+ 2H

O(aq) + O

(g)

→

4[M(CN)

]

–

(aq) +

4OH

–

(aq) (M= Ag or Au) (6.4)

( )

[ ]

( )

( ) ( )

[ ]

( )

M Zn2 Zn 2Maq aq

s s

− −

+ → +

(6.5)

To extract metal from concentrated ore, it must be converted to a

form which is suitable for reduction to metal. Usually sulphide ores

are converted to oxide before reduction because oxides are easier to

reduce. Thus isolation of metals from concentrated ore involves two

major steps viz.,

(a) conversion to oxide, and

(b) reduction of the oxide to metal.

(a) Conversion to oxide

(i) Calcination: Calcinaton involves heating. It removes the volatile

matter which escapes leaving behind the metal oxide:

.xH

O(s)

(s) + xH

O(g) (6.6)

ZnCO

(s)

ZnO(s) + CO

(g) (6.7)

CaCO

.MgCO

(s)

CaO(s) + MgO(s ) + 2CO

(g) (6.8)

(ii) Roasting: In roasting, the ore is heated in a regular supply

of air in a furnace at a temperature below the melting

point of the metal. Some of the reactions involving sulphide

ores are:

Intext QuestionsIntext Questions

Intext Questions

6.1 Which of the ores mentioned in Table 6.1 can be concentrated by

magnetic separation method?

6.2 What is the significance of leaching in the extraction of aluminium?

6.36.3

6.3

ExtractionExtraction

Extraction

of Crude

of Crudeof Crude

of Crude

Metal fromMetal from

Metal from

ConcentratedConcentrated

Concentrated

OreOre

Ore

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156Chemistry

2ZnS + 3O

→ 2ZnO + 2SO

(6.9)

2PbS + 3O

→ 2PbO + 2SO

(6.10)

2Cu

S + 3O

→ 2Cu

O + 2SO

(6.11)

The sulphide ores of copper are heated

in reverberatory furnace [Fig. 6.3]. If the

ore contains iron, it is mixed with silica

before heating. Iron oxide ‘slags of

’* as iron

silicate and copper is produced in the form

of copper matte which contains Cu

S and

FeS.

FeO + SiO

→ FeSiO

(6.12)

(slag)

The SO

produced is utilised for

manufacturing H

(b) Reduction of oxide to the metal

Reduction of the metal oxide usually

involves heating it with a reducing agent,

for example C, or CO or even another metal.

The reducing agent (e.g., carbon) combines with the oxygen of the

metal oxide.

+ yC → xM + y CO (6.13)

Some metal oxides get reduced easily while others are very difficult

to be reduced (reduction means electron gain by the metal ion). In any

case, heating is required.

Some basic concepts of thermodynamics help us in understanding the

theory of metallurgical transformations. Gibbs energy is the most

significant term. To understand the variation in the temperature required

for thermal reductions and to predict which element will suit as the

reducing agent for a given metal oxide (M

), Gibbs energy

interpretations are made. The criterion for the feasibility of a thermal

reduction is that at a given temperture Gibbs energy change of the

reaction must be negative. The change in Gibbs energy, ∆G for any

process at any specified temperature, is described by the equation:

∆G = ∆H – T∆S (6.14)

where, ∆H is the enthalpy change and ∆S is the entropy change for

the process.

When the value of ∆G is negative in equation 6.14, only then the

reaction will proceed. ∆G can become negative in the following situations:

1. If ∆

S is positive, on increasing the temperature (T), the value of

T∆S increases so that ∆H < T∆S. In this situation ∆G will become

negative on increasing temperature.

2. If coupling of the two reactions, i.e. reduction and oxidation,

results in negative value of ∆G for overall reaction, the final

reaction becomes feasible. Such coupling is easily understood

6.46.4

6.4

ThermodynamicThermodynamic

Thermodynamic

Principles ofPrinciples of

Principles of

MetallurgyMetallurgy

Metallurgy

Fig. 6.3: A section of a modern

reverberatory furnace

* During metallurgy, ‘flux’ is added which combines with ‘gangue’ to form ‘slag’. Slag separates more easily from

the ore than the gangue. This way, removal of gangue becomes easier.

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General Principles and Processes of Isolation of Elements

157

through Gibbs energy (∆

) vs T plots for the formation of the

oxides (Fig. 6.4). These plots are drawn for free energy changes

that occur when one gram mole of oxygen is consumed.

The graphical representation of Gibbs energy was first used by H.J.T.

Ellingham. This provides a sound basis for considering the choice of

reducing agent in the reduction of oxides. This is known as Ellingham

Diagram. Such diagrams help us in predicting the feasibility of thermal

reduction of an ore.

As we know, during reduction, the oxide of a metal decomposes

and the reducing agent takes away the oxygen. The role of reducing

agent is to provide ∆

negative and large enough to make the sum

of ∆

of the two reactions, i.e, oxidation of the reducing agent and

reduction of the metal oxide negative.

O(s) →

xM (solid or liq) +

(g) (6.15)

If reduction is carried out by carbon the oxidation of the reducing

agent (i.e., C) will be there:

C(s) + O (g) CO(g)

→

(6.16)

There may also be complete oxidation of carbon to CO

2 2

1 1 1

C(s) + O (g) CO (g)

2 2 2

→

(6.17)

Fig. 6.4: Gibbs energy (∆

) vs T plots (schematic) for the formation of some

oxides per mole of oxygen consumed (Ellingham diagram)

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158Chemistry

On coupling (combing) reaction 6.15 and 6.16 we get:

O(s) + C(s) →

xM(s or l) + CO(g) (6.18)

On coupling reaction 6.15 and 6.17 we have

O(s) +

C(s) →

xM(s or l) +

(g) (6.19)

Similarly, if carbon monoxide is reducing agent, reactions 6.15 and

6.20 given below need to be coupled.

CO(g) +

(g) →

(g) (6.20)

Over all reaction will be as follows:

O(s) + CO(g) →

xM(s or l) + CO

(g) (6.21)

Ellingham DiagramEllingham Diagram

Ellingham Diagram

(a) Ellingham diagram normally consists of plots of ∆

vs T for the formation of oxides of

common metals and reducing agents i.e., for the reaction given below.

2xM(s) + O

(g) → 2M

O(s)

In this reaction, gas is consumed in the formation of oxide hence, molecular randomness

decreases in the formation of oxide which leades to a negative value of ∆S as a result

sign of T∆S term in equation (6.14) becomes positive. Subsequently ∆

shifts towards

higher side despite rising T. The result is positive slope in the curve for most of the

reactions for the formation of M

O(s).

(b) Each plot is a straight line and slopes upwards except when some change in phase

(s→ l or l→ g) takes place. The temperature at which such change occurs, is indicated

by an increase in the slope on positive side (e.g., in the Zn, ZnO plot, the melting is

indicated by an abrupt change in the curve) [Fig. 6.4].

line. Below this temperature, ∆

for the formation of oxide is negative so M

O is stable.

Above this point, free energy of formation of oxide is positive. The oxide, M

O will

decompose on its own.

(d) Similar diagrams are constructed for sulfides and halides also. From them it becomes

clear that why reduction of M

S is difficult.

Limitations of Ellingham Diagram

1. The graph simply indicates whether a reaction is possible or not, i.e., the tendency of

reduction with a reducing agent is indicated. This is so because it is based only on the

thermodynamic concepts. It does not explain the kinetics of the reduction process. It

cannot answer questions like how fast reduction can proceed? However, it explains why

the reactions are sluggish when every species is in solid state and smooth when the ore

melts down. It is interesting to note here that ∆H (enthalpy change) and the ∆S (entropy

change) values for any chemical reaction remain nearly constant even on varying

temperature. So the only dominant variable in equation(6.14) becomes T. However, ∆

depends much on the physical state of the compound. Since entropy depends on disorder

or randomness in the system, it will increase if a compound melts (s→ l) or vapourises

(l→ g) since molecular randomness increases on changing the phase from solid to liquid or

from liquid

to gas.

2. The interpretation of

∆

is based on K (∆G

= – RT lnK). Thus it is presumed that the

reactants and products are in equilibrium:

O + A

red

l xM + A

red

This is not always true because the reactant/product may be solid. In commercial processes

reactants and products are in contact for a short time.

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General Principles and Processes of Isolation of Elements

159

The reactions 6.18 and 6.21 describe the actual reduction of the

metal oxide, M

O, that we want to accomplish. The ∆

values for

these reactions in general, can be obtained from the corresponding

∆

values of oxides.

As we have seen, heating (i.e., increasing T) favours a negative value

of ∆

. Therefore, the temperature is chosen such that the sum of ∆

in the two combined redox processes is negative. In ∆

T plots (Ellingham diagram, Fig. 6.4), this is indicated by the point of

intersection of the two curves, i.e, the curve for the formation of M

and that for the formation of the oxide of the reducing substance. After

that point, the ∆

value becomes more negative for the combined

process making the reduction of M

O possible. The difference in the

two ∆

values after that point determines whether reduction of the

oxide of the element of the upper line is feasible by the element of

which oxide formation is represented by the lower line. If the difference

is large, the reduction is easier.

(a) Extraction of iron from its oxides

After concentration, mixture of oxide ores of iron (Fe

, Fe

) is

subjected to calcination/roasting to remove water, to decompose

carbonates and to oxidise sulphides. After that these are mixed with

limestone and coke and fed into a Blast furnace from its top, in

which the oxide is reduced to the metal. In the Blast furnace,

6.4.1 Applications

Suggest a condition under which magnesium could reduce alumina.

The two equations are:

(a)

Al + O

→

(b) 2Mg +O

→ 2MgO

At the point of intersection of the Al

and MgO curves (marked “A”

in diagram 6.4), the ∆

becomes ZERO for the reaction:

+2Mg → 2MgO +

Below that point magnesium can reduce alumina.

Although thermodynamically feasible, in practice, magnesium metal is not

used for the reduction of alumina in the metallurgy of aluminium. Why ?

Temperatures below the point of intersection of Al

and MgO curves,

magnesium can reduce alumina. But the process will be uneconomical.

Why is the reduction of a metal oxide easier if the metal formed is in

liquid state at the temperature of reduction?

The entropy is higher if the metal is in liquid state than when it is in

solid state. The value of entropy change (∆S) of the reduction process

is more on positive side when the metal formed is in liquid state and

the metal oxide being reduced is in solid state. Thus the value of ∆

becomes more on negative side and the reduction becomes easier.

Example 6.2Example 6.2

Example 6.2

SolutionSolution

Solution

Example 6.3Example 6.3

Example 6.3

SolutionSolution

Solution

SolutionSolution

Solution

Example 6.1Example 6.1

Example 6.1

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160Chemistry

Fig. 6.5: Blast furnace

[Fig. 6.5] reduction of iron oxides takes place at different temperature

ranges. A blast of hot air is blown from the bottom of the furnace

by burning coke in the lower portion to give temperature upto about

2200K. The burning of coke, therefore, supplies most of the heat

required in the process. The CO and heat move to the upper part of

the furnace. In upper part, the temperature is

lower and the iron oxides (Fe

and Fe

)

coming from the top are reduced in steps to FeO.

These reactions can be summarised as follows:

At 500 – 800 K (lower temperature range in

the blast furnace),

is first reduced to Fe

and then

to FeO

3 Fe

+ CO → 2 Fe

+ CO

(6.22)

+ 4 CO → 3Fe + 4 CO

(6.23)

+ CO → 2FeO + CO

(6.24)

Limestone is also decomposed to CaO which

removes silicate impurity of the ore as slag. The

slag is in molten state and separates out from

iron.

At 900 – 1500 K (higher temperature range

in the blast furnace):

C + CO

→ 2 CO (6.25)

FeO + CO → Fe + CO

(6.26)

Thermodynamics helps us to understand how

coke reduces the oxide and why this furnace is chosen. One of the

main reduction steps in this process involves reaction 6.27 given below.

FeO(s) + C(s) → Fe(s/l) + CO (g) (6.27)

This reaction can be seen as a reaction in which two simpler

reactions have coupled. In one the reduction of FeO is taking place

and in the other, C is being oxidised to CO:

FeO(s) → Fe(s) +

(g) (6.28)

C(s) +

(g) → CO (g) (6.29)

When both the reactions take place to yield the equation (6.27), the

net Gibbs energy change becomes:

∆

(C, CO)

+ ∆

(FeO, Fe)

= ∆

(6.30)

Naturally, the resultant reaction will take place when the right hand

side in equation 6.30 is negative. In ∆

vs T plot representing the

change Fe→ FeO in Fig. 6.6 goes upward and that representing the

change C→ CO (C,CO) goes downward. They cross each other at about

1073K. At temperatures above 1073K (approx.), the C, CO line is below

the Fe, FeO line

< . So above 1073 K in the range of

temprature 900–1500 K coke will reduce FeO and will itself be oxidised

to CO. Let us try to understand this through Fig. 6.6 (approximate values

of ∆

are given). At about 1673K (1400

C) ∆

value for the reaction:

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General Principles and Processes of Isolation of Elements

161

2FeO→ 2Fe+O

is +341 kJmol

-1

because it is

reverse of Fe→ FeO change and for the reaction

2C+O

→ 2CO ∆

is -447 kJmol

-1

. If we

calculate ∆

value for overall reaction (6.27

the value will be -53 kJmol

-1

). Therefore, reaction

6.27 becomes feasible. In a similar way the

reduction of Fe

and Fe

by CO at relatively

lower temperatures can be explained on the

basis of lower lying points of intersection of their

curves with the CO, CO

curve.

The iron obtained from Blast furnace

contains about 4% carbon and many impurities

in smaller amount (e.g., S, P, Si, Mn). This is

known as pig iron. It can be moulded into

variety of shapes. Cast iron is different from pig

iron and is made by melting pig iron with scrap

iron and coke using hot air blast. It has slightly

lower carbon content (about 3%) and is

extremely hard and brittle.

Further Reductions

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. The haematite oxidises carbon

to carbon monoxide:

+ 3 C → 2 Fe + 3 CO (6.31)

Limestone is added as a flux and sulphur, silicon and phosphorus

are oxidised and passed into the slag. The metal is removed and freed

from the slag by passing through rollers.

(b) Extraction of copper from cuprous oxide [copper(I) oxide]

In the graph of ∆

vs T for the formation of oxides (Fig. 6.4), the Cu

line is almost at the top. So it is quite easy to reduce oxide ores of copper

directly to the metal by heating with coke. The lines (C, CO) and

(C, CO

) are at much lower positions in the graph particularly after

500 – 600K. However, many of the ores are sulphides and some may

also contain iron. The sulphide ores are roasted/smelted to give oxides:

2Cu

S + 3O

→ 2Cu

O + 2SO

(6.32)

The oxide can then be easily reduced to metallic copper using coke:

O + C → 2 Cu + CO (6.33)

In actual process, the ore is heated in a reverberatory furnace after

mixing with silica. In the furnace, iron oxide ‘slags of’ as iron slicate is

formed. Copper is produced in the form of copper matte. This contains

S and FeS.

FeO + SiO

→ FeSiO

(6.34)

(Slag)

Copper matte is then charged into silica lined convertor. Some

silica is also added and hot air blast is blown to convert the remaining

Fig. 6.6: Gibbs energy Vs T plot (schematic) for

the formation of oxides of iron and

carbon (Ellingham diagram)

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162Chemistry

FeS, FeO and Cu

S/Cu

O to the metallic copper. Following reactions

take place:

2FeS + 3O

→ 2FeO + 2SO

(6.35)

FeO + SiO

→ FeSiO

(6.36)

2Cu

S + 3O

→ 2Cu

O + 2SO

(6.37)

2Cu

O + Cu

→ 6Cu + SO

(6.38)

The solidified copper obtained has blistered appearance due to the

evolution of SO

and so it is called blister copper.

The reduction of zinc oxide is done using coke. The temperature in

this case is higher than that in the case of copper. For the purpose

of heating, the oxide is made into brickettes with coke and clay.

ZnO + C

coke,

1673

Zn + CO (6.39)

The metal is distilled off and collected by rapid chilling.

We have seen how principles of thermodyamics are applied to

pyrometallurgy. Similar principles are effective in the reductions of metal

ions in solution or molten state. Here they are reduced by electrolysis or

by adding some reducing element.

In the reduction of a molten metal salt, electrolysis is done. Such

methods are based on electrochemical principles which could be

understood through the equation,

∆G

= – nE

F (6.40)

here n is the number of electrons and E

is the electrode potential of

the redox couple formed in the system. More reactive metals have large

negative values of the electrode potential. So their reduction is difficult.

If the difference of two E

values corresponds to a positive E

and

consequently negative ∆G

in equation 6.40, then the less reactive metal

will come out of the solution and the more reactive metal will go into

the solution, e.g.,

(aq) + Fe(s) → Cu(s) + Fe

(aq) (6.41)

In simple electrolysis, the M

ions are discharged at negative

electrodes (cathodes) and deposited there. Precautions are taken

considering the reactivity of the metal produced and suitable materials

are used as electrodes. Sometimes a flux is added for making the

molten mass more conducting.

Intext QuestionsIntext Questions

Intext Questions

6.3 The reaction,

+2Al → Al

+2Cr (∆

= – 421kJ)

is thermodynamically feasible as is apparent from the Gibbs energy value.

Why does it not take place at room temperature?

6.4 Is it true that under certain conditions, Mg can reduce Al

and Al can

reduce MgO? What are those conditions?

6.56.5

6.5

ElectrochemicalElectrochemical

Electrochemical

Principles ofPrinciples of

Principles of

MetallurgyMetallurgy

Metallurgy

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General Principles and Processes of Isolation of Elements

163

Aluminium

In the metallurgy of aluminium, purified Al

is mixed with Na

AlF

or CaF

which lowers the melting point of the mixture and brings

conductivity. The fused matrix is electrolysed.

Steel vessel with lining of carbon acts as

cathode and graphite anode is used. The

overall reaction may be written as:

2Al

+ 3C → 4Al + 3CO

(6.42)

This process of electrolysis is widely

known as Hall-Heroult process.

Thus electrolysis of the molten mass is

carried out in an electrolytic cell using

carbon electrodes. The oxygen liberated at

anode reacts with the carbon of anode

producing CO and CO

. This way for each

kg of aluminium produced, about 0.5 kg

of carbon anode is burnt away. The

electrolytic reactions are:

Cathode: Al

(melt) + 3e

–

→ Al(l) (6.43)

Anode: C(s) + O

2–

(melt) → CO(g) + 2e

–

(6.44)

C(s) + 2O

2–

(melt) → CO

(g) + 4e

–

(6.45)

Copper from Low Grade Ores and Scraps

Copper is extracted by hydrometallurgy from low grade ores. It is leached

out using acid or bacteria. The solution containing Cu

is treated with

scrap iron or H

(equations 6.40; 6.46).

(aq) + H

(g) → Cu(s) + 2H

(aq) (6.46)

Besides reductions, some extractions are based on oxidation particularly

for non-metals. A very common example of extraction based on

oxidation is the extraction of chlorine from brine (chlorine is abundant

in sea water as common salt) .

2Cl

–

(aq) + 2H

O(l) → 2OH

–

(aq) + H

(g) + Cl

(g) (6.47)

The ∆G

for this reaction is + 422 kJ. When it is converted to E

(using ∆G

= – nE

F), we get E

= – 2.2 V. Naturally, it will require an

external emf that is greater than 2.2 V. But the electrolysis requires an

excess potential to overcome some other hindering reactions (Unit–3,

Section 3.5.1). Thus, Cl

is obtained by electrolysis giving out H

and

aqueous NaOH as by-products. Electrolysis of molten NaCl is also

carried out. But in that case, Na metal is produced and not NaOH.

6.66.6

6.6

OxidationOxidation

Oxidation

ReductionReduction

Reduction

Fig. 6.7: Electrolytic cell for the

extraction of aluminium

At a site, low grade copper ores are available and zinc and iron scraps

are also available. Which of the two scraps would be more suitable for

reducing the leached copper ore and why?

Zinc being above iron in the electrochemical series (more reactive

metal is zinc), the reduction will be faster in case zinc scraps are

used. But zinc is costlier metal than iron so using iron scraps will be

advisable and advantageous.

Example 6.4Example 6.4

Example 6.4

SolutionSolution

Solution

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164Chemistry

As studied earlier, extraction of gold and silver involves leaching the

metal with CN

–

. This is also an oxidation reaction (Ag → Ag

or Au → Au

The metal is later recovered by displacement method.

4Au(s) + 8CN

–

(aq) + 2H

O(aq) + O

(g) →

4[Au(CN)

]

–

(aq) + 4OH

–

(aq) (6.48)

2[Au(CN)

]

–

(aq) + Zn(s) → 2Au(s) + [Zn(CN)

]

2–

(aq) (6.49)

In this reaction zinc acts as a reducing agent.

A metal extracted by any method is usually contaminated with some

impurity. For obtaining metals of high purity, several techniques are

used depending upon the differences in properties of the metal and the

impurity. Some of them are listed below.

(a) Distillation (b) Liquation

(e) Vapour phase refining (f) Chromatographic methods

These are described in detail here.

(a) Distillation

This is very useful for low boiling metals like zinc and mercury.

The impure metal is evaporated to obtain the pure metal as distillate.

(b) Liquation

In this method a low melting metal like tin can be made to flow on

a sloping surface. In this way it is separated from higher melting

impurities.

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:

Anode: M → M

+ ne

–

Cathode: M

+ ne

–

→ M (6.50)

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 → Cu

+ 2 e

–

Cathode: Cu

+ 2e

–

→

Cu (6.51)

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.

6.76.7

6.7

Refining

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General Principles and Processes of Isolation of Elements

165

(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 mobile heater

surrounding the rod of impure metal is fixed at its one end

(Fig. 6.8). 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 left behind and the impurities

pass on into the adjacent new molten zone

created by movement of heaters. The process

is repeated several times and the heater is

moved in the same direction again and again.

Impurities get concentrated at one end. This

end is cut off. This method is very useful for

producing semiconductor and other metals of

very high purity, e.g., germanium, silicon,

boron, gallium and indium.

(e) Vapour phase refining

In this method, the metal is converted into its volatile compound

which is collected and 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 named as

nickel tetracarbonyl. This compex is decomposed at higher temperature

to obtain pure metal.

Ni + 4CO

330 – 350 K

Ni(CO)

(6.52)

Ni(CO)

450 – 470 K

Ni + 4CO (6.53)

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 + 2I

→ ZrI

(6.54)

The metal iodide is decomposed on a tungsten filament, electrically

heated to about 1800K. The pure metal deposits on the filament.

ZrI

→ Zr + 2I

(6.55)

(f) Chromatographic methods

You have learnt about chromatographic technique of purification of

substances in Class XI (Unit–12).

Fig. 6.8:Fig. 6.8:

Fig. 6.8:

Zone refining process

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166Chemistry

SummarySummary

Summary

Although modern metallurgy had exponential growth after Industrial Revolution,

many modern concepts in metallurgy have their roots in ancient practices that

predated the Industrial Revolution. For over 7000 years, India has had high tradition

of metallurigical skills. Ancient Indian metallurgists have made major contributions

which deserve their place in metallurgical history of the world. In the case of zinc

and high–carbon steel, ancient India contributed significantly for the developemnt

of base for the modern metallurgical advancements which induced metallurgical

study leading to Industrial Revolution.

Metals are required for a variety of purposes. For this, we need their extraction from

the minerals in which they are present and from which their extraction is

commercially feasible.These minerals are known as ores. Ores of the metal are

associated with many impurities. Removal of these impurities to certain extent is

achieved in concentration steps. The concentrated ore is then treated chemically

for obtaining the metal. Usually the metal compounds (e.g., oxides, sulphides) are

reduced to the metal. The reducing agents used are carbon, CO or even some metals.

Column chromatography is very useful for purification of the

elements which are available in minute quantities and the impurities

are not very different in chemical properties from the element to be

purified.

Aluminium foils are used as wrappers for food materials. The fine

dust of the metal is used in paints and lacquers. Aluminium, being

highly reactive, is also used in the extraction of chromium and

manganese from their oxides. Wires of aluminium are used as

electricity conductors. Alloys containing aluminium, being light,

are very useful.

Copper is used for making wires used in electrical industry and for

water and steam pipes. It is also used in several alloys that are rather

tougher than the metal itself, e.g., brass (with zinc), bronze (with tin)

and coinage alloy (with nickel).

Zinc is used for galvanising iron. It is also used in large quantities

in batteries. It is constituent of many alloys, e.g., brass, (Cu 60%, Zn

40%) and german silver (Cu 25-30%, Zn 25-30%, Ni 40–50%). Zinc

dust is used as a reducing agent in the manufacture of dye-stuffs,

paints, etc.

Cast iron, which is the most important form of iron, is used for

casting stoves, railway sleepers, gutter pipes , toys, etc. It is used in the

manufacture of wrought iron and steel. Wrought iron is used in

making anchors, wires, bolts, chains and agricultural implements. Steel

finds a number of uses. Alloy steel is obtained when other metals are

added to it. Nickel steel is used for making cables, automobiles and

aeroplane parts, pendulum, measuring tapes. Chrome steel is used for

cutting tools and crushing machines, and stainless steel is used for

cycles, automobiles, utensils, pens, etc.

6.86.8

6.8

Uses ofUses of

Uses of

Aluminium,

Aluminium,Aluminium,

Aluminium,

CopperCopper

Copper

, Zinc, Zinc

, Zinc

and Ironand Iron

and Iron

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General Principles and Processes of Isolation of Elements

167

In these reduction processes, the thermodynamic and electrochemical concepts

are given due consideration. The metal oxide reacts with a reducing agent; the

oxide is reduced to the metal and the reducing agent is oxidised. In the two reactions,

the net Gibbs energy change is negative, which becomes more negative on raising

the temperature. Conversion of the physical states from solid to liquid or to gas,

and formation of gaseous states favours decrease in the Gibbs energy for the entire

system. This concept is graphically displayed in plots of ∆G

vs T (Ellingham diagram)

for such oxidation/reduction reactions at different temperatures. The concept of

electrode potential is useful in the isolation of metals (e.g., Al, Ag, Au) where the

sum of the two redox couples is positive so that the Gibbs energy change is negative.

The metals obtained by usual methods still contain minor impurities. Getting pure

metals requires refining. Refining process depends upon the differences in

properties of the metal and the impurities. Extraction of aluminium is usually carried

out from its bauxite ore by leaching it with NaOH. Sodium aluminate, thus formed,

is separated and then neutralised to give back the hydrated oxide, which is then

electrolysed using cryolite as a flux. Extraction of iron is done by reduction of its

oxide ore in blast furnace. Copper is extracted by smelting and heating in a

reverberatory furnace. Extraction of zinc from zinc oxides is done using coke. Several

methods are employed in refining the metal. Metals, in general, are very widely

used and have contributed significantly in the development of a variety of industries.

A Summary of the Occurrence and Extraction of some Metals is

Presented in the following Table

Metal Occurrence

Common method

Remarks

of extraction

Aluminium

Iron

Copper

Zinc

1. Bauxite, Al

. x H

2. Cryolite, Na

AlF

1. Haematite, Fe

2. Magnetite, Fe

1. Copper pyrites, CuFeS

2. Copper glance, Cu

3. Malachite,

CuCO

.Cu(OH)

4. Cuprite, Cu

1. Zinc blende or

Sphalerite, ZnS

2. Calamine, ZnCO

3. Zincite, ZnO

Electrolysis of

dissolved in

molten Na

AlF

Reduction of the

oxide with CO

and coke in Blast

furnace

Roasting of

sulphide

partially and

reduction

Roasting followed

by reduction with

coke

For the extraction, a

good source of

electricity is required.

Temperature

approaching 2170 K

is required.

It is self reduction in a

specially designed

converter. The

reduction takes place

easily. Sulphuric acid

leaching is also used

in hydrometallurgy for

low grade ores.

The metal may be

purified by fractional

distillation.

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168Chemistry

6.1 Copper can be extracted by hydrometallurgy but not zinc. Explain.

6.2 What is the role of depressant in froth floatation process?

6.3 Why is the extraction of copper from pyrites more difficult than that from its

oxide ore through reduction?

6.4 Explain: (i) Zone refining (ii) Column chromatography.

6.5 Out of C and CO, which is a better reducing agent at 673 K ?

6.6 Name the common elements present in the anode mud in electrolytic refining

of copper. Why are they so present ?

6.7 Write down the reactions taking place in different zones in the blast furnace

during the extraction of iron.

6.8 Write chemical reactions taking place in the extraction of zinc from zinc blende.

6.9 State the role of silica in the metallurgy of copper.

6.10 Which method of refining may be more suitable if element is obtained in minute

quantity?

6.11 Which method of refining will you suggest for an element in which impurities

present have chemical properties close to the properties of that elements?

6.12 Describe a method for refining nickel.

6.13 How can you separate alumina from silica in a bauxite ore associated with

silica? Give equations, if any.

6.14 Giving examples, differentiate between ‘roasting’ and ‘calcination’.

6.15 How is ‘cast iron’ different from ‘pig iron”?

6.16 Differentiate between “minerals” and “ores”.

6.17 Why copper matte is put in silica lined converter?

6.18 What is the role of cryolite in the metallurgy of aluminium?

6.19 How is leaching carried out in case of low grade copper ores?

6.20 Why is zinc not extracted from zinc oxide through reduction using CO?

6.21 The value of ∆

for formation of Cr

is – 540 kJmol

−1

and that of Al

– 827 kJmol

−1

. Is the reduction of Cr

possible with Al ?

6.22 Out of C and CO, which is a better reducing agent for ZnO ?

6.23 The choice of a reducing agent in a particular case depends on thermodynamic

factor. How far do you agree with this statement? Support your opinion with

two examples.

6.24 Name the processes from which chlorine is obtained as a by-product. What

will happen if an aqueous solution of NaCl is subjected to electrolysis?

6.25 What is the role of graphite rod in the electrometallurgy of aluminium?

6.26 Outline the principles of refining of metals by the following methods:

(i) Zone refining

(ii) Electrolytic refining

(iii) Vapour phase refining

6.27 Predict conditions under which Al might be expected to reduce MgO.

(Hint: See Intext question 6.4)

ExercisesExercises

Exercises

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General Principles and Processes of Isolation of Elements

169

Answers to Some Intext Questions

6.1 Ores in which one of the components (either the impurity or the actual ore) is

magnetic can be concentrated, e.g., ores containing iron (haematite, magnetite,

siderite and iron pyrites).

6.2 Leaching is significant as it helps in removing the impurities like SiO

, Fe

etc. from the bauxite ore.

6.3 Certain amount of activation energy is essential even for such reactions which

are thermodynamically feasible, therefore heating is required.

6.4 Yes, below 1350°C Mg can reduce Al

and above 1350°C, Al can reduce MgO.

This can be inferred from ∆G

Vs T plots (Fig. 6.4).

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