General Principles and Processes of Isolation of Elements

We get minerals and ores in abundance in the earth’s crust. Some ores have been proven themselves a great resource for mankind. Such as iron obtained from ore of iron (Hematite) built the foundation of industrial revolution. On the other hand, aluminium was a crucial strategic resource for aviation during World War I and World War II. Still, aluminium metal dominates in the various fields of the market due to its unique properties and easy and cost - effective extraction. Out of total naturally occurring elements 70% are metals. Metals occur in nature in free as well as in combined states. Generally, reactive metals occur in combined states in the form of oxides, sulfides, carbonates etc. The metals in the middle of the reactivity series such as zinc, iron, lead etc. are moderately reactive and are found in earth’s crust in the form of oxides, sulfides, carbonates etc. Out of these metal compounds, pure metal can be economically extracted from some of their compounds. 

Metallurgy techniques have been used in India since ancient times. First evidence of metallurgy in India was found in Mehrgarh in Baluchistan, dated 6000 BCE. Gold, copper, silver, lead, tin, iron and mercury are the seven metals of antiquity. 


What are Ores and Minerals? 

Minerals are naturally occurring, homogeneous inorganic solid substances having a definite chemical composition and characteristic crystalline structure, color and hardness. For example, copper pyrite (CuFeS2), calamine (ZnCO3) etc. are minerals.

A mineral from which metal can be economically extracted in a maximum amount is called an ore. Some examples of ores or metals are listed below in tabular form –


List of Some Common Ores of Metals

Fe2O3

Hematite

Ore of iron 

CuFeS2

Copper pyrite 

Ore of copper 

MnO2

Pyrolusite 

Ore of manganese 

PbS 

Galena 

Ore of lead 

Al2O3.2H2O

Bauxite 

Ore of aluminum 

AuTe2

Calaverite 

Ore of gold (It mostly occurs as free metal or with silver and mercury; ore of gold are comparatively rare)

ZnS

Zinc blend 

Ore of zinc 

AgCl 

Horn silver 

Ore of silver 

HgS 

Cinnabar 

Ore of mercury


Difference Between Ores and Minerals 

S. No.

Ores 

Minerals 

1.

These are those naturally occurring homogeneous inorganic solid substances from which metal can be economically extracted. 

These are naturally occurring homogeneous inorganic solid substances which have a definite chemical composition.

2.

All ores are minerals. 

All minerals are not ores. 

3. 

Zinc blend, bauxite, cinnabar etc. are examples of ores. 

Clay (it’s not an ore), horn silver, cryolite etc. are examples of minerals. 


Extraction of Metals from Ores 

The process of extraction of pure metals from their ores, is called Metallurgy. Methods are used in the extraction of metals from their ores on the basis of their reactivity. For the extraction of highly reactive metals from their ores electrolysis is used and for extraction of metals of medium reactivity calcination, roasting and reduction methods are used. While metals of low reactivity are extracted from their ores by roasting and refining. Although mostly low reactive metals are found in free state such as gold is generally found in free state or with silver and mercury (As amalgam). Before applying all these methods of extraction of metals according to their reactivity, concentration of ores is done. Ores are found in nature with many impurities such as rock particles, sand and other impurities. These impurities in an ore are called matrix or gauge. The removal of matrix or gauge from the ore is called concentration ore. 

Several steps are involved in the extraction of pure metal from ores. Major steps are as follows –

  • Concentration of the ore 

  • Isolation of the metal from its concentrated ore

  • Purification of the metal 

Below is given a flow diagram of various steps involved in extraction of various metals on the basis of their reactivity –

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Concentration of Ores 

First ores are crushed and grinded and then gauge or matrix is removed from the powdered ore. This is called concentration of ores. Ores of highly, moderately, and low reactive metals are 1st concentrated. This step is common for ores of all types of metals. It is carried out by various methods depending upon the nature of the ore. It is generally carried out by following 4 methods –

  • Gravity separation 

  • Froth floatation method 

  • Electromagnetic separation 

  • Leaching method 

Gravity separation, froth floatation and electromagnetic separation methods are physical methods while leaching method is a chemical method of concentration of ores. 

  • Gravity Separation – It is based on the difference in the weight of metal and matrix. So, due to gravity the heavier one settled down at the bottom. Generally, metals are heavier than matrix, so they get separated at the bottom. This method of separation is also called hydraulic washing as crushed ore particles are mixed with an upward stream of water.

  • Froth Floatation Method – It is the method to separate hydrophobic materials from hydrophilic. It is based on the difference in the wetting properties of the ore and the gauge particles. It is generally used for sulfide ores. 

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  • Electromagnetic Separation – This method is based on the magnetic properties of the metal particles and gauge particles. Powdered ore is placed on the roller belt which is a magnetic belt. So, the magnetic material gets attracted towards the belt and stays on that and thus gets separated. Generally, it is used for iron and manganese ores. 

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  • Leaching – It is based on the solubility of ore in the suitable solvent. 

Leaching of Al2O3 from bauxite – It is carried out by Hall’s method. It was invented in 1886 by the American chemist Charles Martin Hall. It takes place by following three steps –


Step 1. Conversion of Impure Bauxite into Sodium Aluminate – The ore is fused to red heat with sodium carbonate and formation of sodium aluminate takes place. Reaction involved is given below –

Al2O3.2H2O + Na2CO3 + heat 🡪 2NaAlO2 +2H2O +CO2


Step 2. Conversion of Sodium Aluminate into Aluminium Hydroxide – 

2NaAlO2 +3H2O +CO2 🡪 Na2CO3 + 2Al(OH)3


Step 3. Conversion of Aluminium Hydroxide into Pure Alumina – 

2Al(OH)3 1100⁰C 🡪 Al2O3 + 3H2O

Then metals are extracted from concentrated ores by various specific procedures based on the reactivity of the metal ore.


Extraction of Highly Reactive Metals 

Metals which are present at the top of reactivity series are highly reactive metals. As these metals have more affinity towards oxygen than carbon so ores of these metals cannot be reduced to their oxides as extraction of these metals from their oxides will be more difficult and expensive. These metals are obtained from their ores by electrolytic reduction. For example, sodium metal is obtained by the electrolysis of its molten chloride. In this process metals are deposited at cathode (negatively charged electrode) whereas chlorine is liberated at the anode (positively charged electrode). 

Aluminum oxide (Al2O3) is a stable oxide, so, in its metallurgical process, an electrolytic cell is used and aluminium is obtained through electrolysis. For this purpose, the Hall – Heroult process is used. 

Hall – Heroult Method – Alumina is highly stable oxide and melts at 2050⁰C. That's why alumina cannot be directly electrolyzed. Its electrolysis is done with cryolite (3 parts by weight) and fluorspar (1 part by weight). In this process for electrolysis an iron tank lined with heat resistant material and has a sloping floor, provided with an outlet for tapping molten aluminium metal is used. Gas carbon or graphite are used as cathode and thick carbon rods are used as anode. Coke powder covering is used to prevent burning of carbon anodes and to prevent heat loss from molten electrolyte. A direct current of 100 A is passed through the electrolyte and the temperature is maintained at 950⁰C. In this process sodium, calcium and aluminium ions are formed which migrates towards the cathode. However, only aluminium ions reach the cathode due to their lower position in the electrochemical series. Thus, pure aluminium get deposited at cathode and melts due to 950⁰C temperature of the electrolyte, as it is heavier than electrolyte, so it gets deposited at the base of the electrolytic tank. While at anode nascent oxygen forms which reacts with carbon of coke and forms carbon mono oxide which reacts with atmospheric oxygen and forms carbon dioxide. Although nascent oxygen formed at anode reacts with carbon of carbon-anode as well. That’s why carbon – anodes are consumed gradually and need to be replaced time to time. 

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Electrolysis of Fused Alumina 

Reactions involved in the electrolysis are given below –

Al2O3 ↔ 2Al+3 + 3O-2

Na3AlF3 ↔ 3Na+ + Al+3 + 6F-

CaF2 ↔ Ca+2 + 2F-


At Cathode –

2Al+3 + 3e− → Al


At Anode –

O-2 - 2e− 🡪 [O]

2[O] 🡪 O2

C(coke) + [O] 🡪 CO

2CO + (air)O2 🡪 2CO2

Thus, pure aluminium metal is obtained from the chief ore of aluminium which is bauxite. 


Extraction of Moderately Reactive Metals 

Metals which are present at the middle of reactivity series are moderately reactive metals such as iron, zinc, lead etc. These metals are generally found in nature as carbonates or sulfides ores. So, these are 1st changed into their oxides as it is easier to extract these metals from their oxides. It is done by following two methods –

  • Roasting 

  • Calcination 

Sulfide ores are changed into their oxides by roasting. In this process sulfide ores are heated strongly in the presence of excess of air. 

Carbonate ores are changed into their oxides by calcination. In this process carbonate ores are heated strongly in the presence of a limited amount of air.


Extraction of Low Reactive Metals 

Metals which are present at the bottom of reactivity series are low reactive metals such as mercury, copper etc. The oxides of these metals can be reduced to metals by heating alone. For example, HgS (Cinnabar) is heated in air, first it converts into its oxide (HgO) and when HgO is further heated at 300 ⁰C, it gets reduced to mercury. 


Refining 

A metal extracted by any method either calcination/roasting or electrolysis, some impurities remain in it. For obtaining a highly pure metal, various other techniques are used based on the differences in properties of metals and the impurities. Very specific methods are used according to the metal and the impurity present in it.

If metal and impurity have differences in boiling points, then distillation method is used. It is a very useful method for metals which has low boiling points such as Zn and Hg etc. 

Another general method which is used in refining of metals is liquation in which impure metals are made to flow on a sloping surface. In this process low melting metals get separated from higher melting impurities. 

Other common methods of refining are electrolytic refining and zone refining. In electrolytic refining, impure metal is used as an anode while same pure metal is used as cathode. During electrolysis pure metal transfers from the anode to cathode and impurities get deposited as anode mud. 

Zone refining works on the principle that the impurities are more soluble in the molten state than in the solid state of metal. So, in this method mobile induction coil heaters are made to move on the rod of impure metal. A molten zone is formed on the rod which moves with the heater.  As the heater moves forward, pure metal crystallizes and leaves behind while impurities move forward with the molten zone due to their more soluble nature in molten state. 

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Apart from these methods, chromatography technique is also used in the purification of elements when there is very minute difference in the chemical properties of impurities and metals. Chromatography is based on the fact that different components of a mixture have different affinity towards stationary phase. Components get physically separated by distributing between stationary phase and mobile phase. The forces by which components get adsorbed by stationary phase are weak forces and nonionic forces such as hydrogen bond or Van der Waals forces. The various components of a mixture travel at different speeds and get separated. The separation is based on differential partitioning between the mobile and stationary phases. Component’s different partitioning coefficients result in differential retention on the stationary phase which results in their separation on stationary phase. 


Thermodynamic Principle of Metallurgy 

Thermodynamic principles help us to make the metallurgical techniques more efficient and help us to understand the reasoning behind them. For example, in the reduction of oxides of metals, the most suitable reducing agent should be used to get the best results. Now the best suitable reducing agent can be decided by the help of thermodynamics. Ellingham diagram provides the base for the choice of reducing agent in the reduction of oxides. 


Ellingham Diagram 

Ellingham diagram is one of the important topics of Chemistry for Class XII board exams. Before discussing the Ellingham diagram, let us understand the term Gibbs free energy. 

Gibbs free energy is a thermodynamic potential that can be used to calculate the maximum of reversible work that may be performed by a thermodynamic system at a constant temperature and pressure.

It is represented by G. It is measured in joules. Equation for Gibbs free energy is given below –

ΔG = ΔH - TΔS

If the reactants and products are all in their standard states, then the equation is written as –

ΔG⁰ = ΔH⁰ - TΔS⁰

The concept of Gibbs free energy was given by American scientist Josiah Willard Gibbs in the 1870s. It was used to call available energy. 

Gibbs free energy helps in prediction of the spontaneity of the chemical reaction and in determination of useful work that can be extracted from it. 

If ΔG⁰ < 0, then reaction will be spontaneous.

If ΔG⁰ = 0, the reaction will be at equilibrium 

If ΔG⁰ > 0, then the reaction will be non-spontaneous. 

In metallurgy, we want metal oxides and sulfides to get reduced to get pure metal at low cost. For this purpose, metal oxides and sulfides are reduced by best suitable reducing agents through spontaneous reactions. In this Ellingham diagram plays an important role to select a best suitable reducing agent. Thus, in a way Ellingham diagram relates thermodynamics and metallurgy. 

Suppose for a reaction A + B 🡪 C, 

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Values above the ΔG⁰ = 0 will be positive while below the ΔG⁰ = 0 will be negative. General representation is shown below for your better understanding –

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If ΔG⁰ > 0 for A + B 🡪 C then this reaction will be spontaneous. 

If we reverse the reaction –

C 🡪 A + B, then ΔG⁰ > 0 so the reaction will be non-spontaneous. 

Now if ΔG⁰ > 0 for A + B 🡪 C then this reaction will be non – spontaneous.

If we reverse the reaction –

C 🡪 A + B, then ΔG⁰ > 0 so the reaction will be spontaneous. 

For example, suppose a graph is given for following reactions of metal oxides –

2Ag2O Δ🡪 4Ag + O2

2HgO Δ🡪 2Hg + O2

2ZnO Δ🡪 2Zn + O2

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As you can see in the graph that silver oxide and mercuric oxide need lower temperature to get reduced while zinc oxide require higher temperature. Providing higher temperatures for reduction of metal oxides is an expensive technique for industries to get metals from their oxides. So, for those metal oxides which require high temperature for reduction are reduced by suitable reducing agents. Now here Ellingham diagram comes in picture as it gives the information about the best suitable reducing agent for a specific metal oxide. Zinc oxide is reduced by carbon. 

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For C🡪 CO2,ΔG⁰ = 0 so reaction will be at equilibrium. 

For CO 🡪 CO2, ΔG⁰ > 0 so the reaction will be non-spontaneous. 

For C🡪 CO, ΔG⁰ < 0 so the reaction will be spontaneous. 

So, we use carbon as a reducing agent for zinc oxide. Reaction is given below –

ZnO + C 🡪 Zn + CO

Thus, we can say for spontaneous reaction graph between ΔG⁰ and T will be –

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For non-spontaneous reaction graph between  ΔG⁰ and T will be –


Salient Features of Ellingham Diagram 

Ellingham diagrams were 1st constructed by Harold Ellingham in 1944.

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Ellingham Diagram 

 Following are the salient features of it –

  • It is a plot of  ΔG⁰ in kJ/mol of oxygen and temperature of formation of oxide.

  • As ΔG⁰ becomes less negative at high temperatures so each line of converting metals into metal oxide slope upwards. 

  • Each plot line is straight line except some lines where the change in phase such as solid 🡪 liquid or Liquid 🡪 Gas etc. takes place. 

  • With increase in temperature slope lines cross ΔG⁰ = 0 which means for them  ΔG⁰ > 0 . Theoretically this happens in case of mercury, silver and gold. 


Applications of Ellingham Diagram

Few applications of Ellingham diagram are listed below –

  • It is used to evaluate the ease of reduction of metal oxides and sulfides. 

  • In metallurgy it is used to predict the equilibrium temperature between metal, oxide and oxygen. It also predicts the reaction of metals with nitrogen, sulfur and non-metals.

  • By Ellingham diagram we can predict the condition under which an ore can be reduced to its metal.

  • It is used for finding the best suitable reducing agent for reduction of metal oxides.

  • It is used to find out the feasibility of thermal reduction of an ore.

  • As we know, the Ellingham curve for aluminium lies below most metals such as Fe, Cr etc. which indicates that Al can be used as the reducing agent for oxides of all these metals. 


Limitations of Ellingham Diagram 

It has few limitations as well which are listed below –

  • It ignores the reaction kinetics means it does not provide any information about kinetics of the reduction reaction.

  • The analysis is thermodynamic in nature. It means the reactions which are predicted by the Ellingham diagram can be very slow. 

  • It assumes that the reactants and products are in equilibrium, but it is not always the case.


Uses of Al, Cu, Zn and Fe

Aluminium 

  • It is widely used in packaging. Aluminium foils are used in food packaging in the food industry and in packaging of medicines in the medicinal field. 

  • Aluminium has an unbeatable strength to weight ratio which makes it very useful in the transport industry. It is used in the trains, buses, auto parts, bicycles etc. Network of high speed railway lines in Shinkansen in japan uses a lot of aluminium. It is also known as ‘winged metal’ as it is an ideal metal for use in aircrafts. 50 – 90% of the space shuttles include aluminium alloys. 

  • Aluminium is widely used in construction. Aluminium is resistant to corrosion, so it is an ideal material for construction of buildings. 

  • It is used in electrical supplies. Mostly Al is used for the long distance power lines. 

  • It is used in the body of mobile phones, laptops, iPhones, MacBooks etc. 

  • It is used in furniture, decorative items, home interiors, chairs, lamps, picture frames etc. 

  • Aluminium has been used since ancient times. Its compound alum has been known since the 5th century BCE for dyeing.  


Copper 

  • It is used in utensils. 

  • It is also used in construction of buildings, monuments, decorative items etc. 

  • It has applications in plumbing and roofing materials. 

  • It is used in many alloys. 

  • It is used in printed circuit boards. 

  • It is also used in manufacturing of clutches, door handles, pipes etc. 

  • It is used in many musical instruments such as gongs, bells etc. 

  • It is an important component of brass, phosphor bronze etc. 

  • It is used in electrical busbars. 

  • Many bacteria and other microorganisms do not grow on copper. That’s why it is used in lining parts of the ships. 

  • It has antimicrobial properties. 

  • It is used in production of solar panels, wind turbines etc. 

  • Since ancient times, copper has been used in treatment of many diseases. 


Zinc 

  • Zinc is the 4th most widely consumed metal in the world. Due to its anticorrosive properties, it is used in many alloys with other metals.

  • It is used in automobiles. 

  • Zinc is used in formation of zinc oxide which is used in rubber manufacturing. 

  • It is also used in manufacturing of protective skin ointment. 

  • It is used in multivitamin tablets. 

  • It is a component of electrochemical cells and many batteries. 

  • Many compounds of zinc are of great use such as in production of dyes, paints, ointments, shampoos, diet supplements etc. 

  • It is used in galvanizing other metals to prevent them from rusting. Galvanized steel (from zinc) is used for car bodies, lamps etc. 


Iron 

  • Iron is the highest used metal in the world, accounting for over 90% of worldwide metal production. 

  • It is widely used in construction of buildings and bridges all over the world.

  • It is used as a catalyst in many reactions. 

  • It is found in our blood in hemoglobin. Thus, it is an important element for life. 

  • It is used in metallurgy as well. 

  • Its many compounds are used in many fields. For example, iron chloride is used in purification of water.

  •  It is an important part of multivitamin tablets, diet supplements etc. 

  • It is an integral part of our diet. Its deficiency causes many diseases such as anemia etc. 

This ends our coverage on the summary of the unit “General Principles and Processes of Isolation of Elements”. We hope you enjoyed learning and were able to grasp the concepts. You can get separate articles as well on various subtopics of this unit such as Extraction of aluminium, Froth floatation method etc. on Vedantu website. We hope after reading this article you will be able to solve problems based on the topic. We have already provided detailed study notes or revision notes for this unit, which you can easily download by registering yourself on Vedantu website. Here in this article we have discussed the unit in a summarized way with the emphasis on important topics of the unit.  If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.