General Principles and Processes of Isolation of Metals Chapter - Chemistry JEE Main

The methods and separation of metals from their sources are discussed in this chapter. Metals are constantly extracted from their mineral sources. Mining is used to collect these ores, which are present in the earth's crust. The extraction and purification of metals is the subject of this chapter. This chapter describes a variety of procedures and processes that are specific to specific metals. This is a crucial chapter because it teaches you how these metals that you use every day are refined. The majority of naturally occurring components are classified as minerals, which contain metal compounds and are found in the earth's crust. 

Ore is defined as a natural rock or silt containing one or more valuable minerals, primarily metal-bearing minerals, that may be extracted, processed, and sold for a profit. As a result, these ores may be thought of as minerals from which metal can be extracted cheaply and quickly. Metals are not usually present in their free state, and some metals, such as potassium, sodium, calcium, and magnesium, are found in a mixed state. General principles and processes of isolation of metals in chemistry aim to teach the students about the various processes of extraction of metals from ores. This article provides an example of the types of questions that might be asked about this subject in the JEE exam. 

JEE Main Chemistry Chapters 2024 

Important Topics of General Principles and Processes of Isolation of Metals Chapter

• Principles of Metallurgy

• Extraction

• Various refining methods

Some Important Definitions 

Metallurgy 

• The earth's crust contains a large diversity of metals. Minerals are naturally occurring metallic compounds combined with sand, soil, and rocks that have a certain chemical makeup. Ore is a metallic compound with a reasonably high metal concentration that may be utilised to extract certain elements in a practical and cost-effective manner.

• The definition of metallurgy is "the discipline of chemistry concerned with the extraction of metals in their pure state from their ore."

• Occurrence of Metals

• Native Ores: They contain the metal in the free state. Silver, gold, and platinum occur as native ores.

• Oxidised Ores: They consist of oxides or oxysalts, such as carbonates, phosphates, sulphates, and silicates of metals.

• Sulphurised Ores: They consist of sulphides of metals like iron, lead, zinc, and mercury.

• Halide Ores: They consist of halides of metals.

Extraction of Ores

• The extraction process is used to reduce costs and to get the purest form of metal possible.

• Metal extraction from ores must be made easier, better, and more cost-effective for the industry.

• This is the method an extraction process is constructed, and it is based on the principles of metallurgy used to verify the inorganic chemical characteristics of the elements of ore.

• Impurities such as pebbles, sands, and limestone are common in ore. It's called gangue. Flux is a chemical that is introduced to ore in order to eliminate the impurities that are there. The interaction of gangue and flux in ores then results in the creation of slag, a fusible substance.

Steps Involved in Metallurgy

The following principles are involved in metallurgy:

(1) Crushing and Grinding: This includes crushing ores into fine powder in a crusher, which is the basic process in metal metallurgy.

(2) The Concentration of Ores: The ores mined from the earth's crust contain a huge number of undesired impurities known as gangue, which include quartz, silicates, sand, feldspar, mica, and other minerals. The dressing is the process of removing undesirable contaminants from ore and is also known as the Concentration of ore because it steadily raises the amount of metal.

(3) Reduction to Free Metal:  Reduction is the process of heating metal oxides to convert them to metal.

(4) Purification or Refining of the Metal: The resulting metal is processed using a variety of techniques.

Methods Involved in the Concentration of Ores

It is referred to as the process of the removal of gangue from ore. There are a number of methods for the concentration of ores and the methods are based on the properties of the ore.

These methods are described below.

Physical Methods

• Hydraulic Washing: Ores are passed through an upward stream of water in this process, which separates the lighter gangue particles from the heavier metal ore. This is a gravity separation technique as well.

• Magnetic Separation: The separation is carried out using the magnetic characteristics of either the ore or the gangue in this process. The ore is crushed into fine bits and then fed onto a conveyor belt that passes over a magnetic roller in this procedure. The magnetic ore stays on the belt, while the gangue falls off.

• Froth Flotation Method: Gangue is discovered to be extracted from sulphide ores using this approach. The ore is first pulverised, after which it is suspended in water. Stabilisers are introduced to the collectors and froths. Pine oils, fatty acids, and other collectors improve the non-wettability of the metal component of the ore, allowing it to form a froth, while froth stabilisers (cresols, aniline, and other compounds) keep the froth in place. The oil used wets the metal, and the water wets the gangue even more. Paddles and air are used to continually stir up the suspension in order to generate the foam. In order to recover the metal, the frothy metal is scraped off the top and dried.

Chemical Methods 

• Calcination: It is a process that involves the heating of ore in the absence of air in order to remove water from hydrated oxide at temperatures below the melting point.

• Roasting: It is a process that involves the roasting of ores to the temperatures below their melting points, that too mainly in the presence of air.

• Leaching: When the ore is determined to be soluble in a solvent, this procedure is utilised. The powdered ore is subsequently dissolved in a chemical solution, most often a strong NaOH solution. The chemical solution dissolves the metal in the ore, and the chemical solution may be recovered and separated from the gangue. This method is used to extract aluminium from bauxite ore.

Various Reduction Processes

Some of the methods commonly used to get free metal from the concentrated ore are given below:

• Smelting: Smelting is the process of removing metal in a state of fusion. The ore is combined with carbon derived from the preceding processes and heated in a suitable furnace in this procedure. During the procedure, a sufficient flux is supplied to convert the non-fusible gangue to fusible slag. The metal can be obtained in molten form or as condensed vapours when the metallic oxide is reduced by carbon. This method is used to extract metals such as tin, zinc, and lead. 

Flux is a chemical used in the smelting process to transform infusible silicons or earthy impurities into slag, a fusible material. Slag is made up of impurities and flux. The slag has a low melting point and density and is incompatible with the metal. The slag floats on top of the metal, shielding it from oxidation. The slag hole is used to remove it from the furnace. If the impurities in the ore are acidic (SiO2), a basic flux, such as CaO, MgO, FeO, and so on, is added; if the impurities are basic (CaO, FeO, and so on), an acidic flux (SiO2) is utilised.

• Goldschmidt Aluminothermic Process: When there are oxides that cannot be easily reduced by carbon, the reduction method is utilised. Metallic oxide ore is combined with aluminium powder, also known as thermite, and placed in a steel crucible lined inside with a refractory substance, which is then fired by a magnesium ribbon. A variety of metals, such as chromium and manganese, may be produced in a very pure condition on a commercial scale using this method.

• Self-reduction Process: This procedure is also known as the auto reduction or air reduction procedure. Sulphide ores containing less electropositive metals such as Hg, Pb, Cu, and others are heated in air to convert a portion of the ore to oxide or sulphate, which interacts with the remaining sulphide ore to produce the metal and sulphur dioxide. In this procedure, no external reducing agent is utilised.

• Electrolytic Reduction Process: Alkali and alkaline earth metals, zinc, and aluminium are all extracted using this method. The substance that will be used to make a metal is heated first and then electrolyzed. To reduce the melting point of the material consumed, some additional salt is sometimes added.

Various Refining Methods 

Metals obtained as above are usually impure and need purification. The following are some of the processes used in metal refining.

• By Poling: Greenwood poles are used to stir the molten metals. Wood, when exposed to the high temperatures of molten metals, produces hydrocarbons such as methane, which is formed by the reduction of any oxide present in the metal, such as copper oxide in blister copper. In the instance of tin, the impurities oxidise and float as scum on the molten metal, which is then removed.

• By Cupellation: When impurities are oxidised and blown away, the impure metal is heated in a blast of air. When impure silver is heated in the air, the lead in it oxidises to litharge (PbO) and is blown away, leaving a gleaming silver surface.

• By Liquation: This method is used to refine easily flammable metals such as lead and tin. The impure metal is heated in a reverberatory furnace's sloping hearth. Impurities are left behind as the metal melts and flows down.

• By Fractional Distillation: The separation of cadmium from zinc is done using this method. In zinc metallurgy, the metal is inextricably linked to cadmium. When the initial component of the condensate contains cadmium, the impure zinc is combined with powdered coke and heated, while zinc is produced in the succeeding sections.

• Mond's Process: This process is used to purify nickel. The volatile chemical nickel carbonyl is generated when impure nickel is heated with carbon monoxide at 60–80°C. At 180°C, nickel carbonyl decomposes to pure nickel and carbon monoxide, which may be reused.

• Van Arkel Process: This technique is commonly used to obtain ultrapure metals. The impurities are unaffected as the impure metal is transformed into a volatile chemical. The volatile molecule is subsequently electrically decomposed to provide pure metal. This process has been used to purify Ti, Zr, Hf, Si, and other materials.

• Zone Refining or Fractional Crystallisation: This process refines semiconductor elements such as Si, Ge, Ga, and other similar elements. It is possible to acquire metals that are extremely pure. The approach relies on the difference in impurity solubility between the molten and solid states of metals. A moveable heater is wrapped around a metal rod. The heater is pushed across the rod carefully. The metal melts at the point of heating, and the pure metal crystallises as the heater advances from one end of the rod to the other, while the impurities pass through the neighbouring melted zone.

Thermodynamic and Electrochemical Principles in Metal Extraction

The extraction of metals from their ores involves a combination of thermodynamic and electrochemical principles. These processes are essential in understanding how metals are obtained from their naturally occurring compounds. Let's delve into the key concepts and mechanisms involved:

1. Thermodynamic Principles:

Gibbs Free Energy (ΔG):

The thermodynamic feasibility of a chemical process, such as the reduction of metal ores, is determined by the Gibbs free energy change (ΔG). If ΔG is negative, the process is spontaneous and thermodynamically favorable. For metal extraction, the reduction of metal oxides to pure metal is often the critical step.

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. The 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. 

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

If ΔG⁰ < 0, then the 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 a low cost. For this purpose, metal oxides and sulfides are reduced by the best suitable reducing agents through spontaneous reactions. This Ellingham diagram plays an important role to select the best suitable reducing agent. Thus, in a way, the Ellingham diagram relates thermodynamics and Metallurgy. 

Suppose for a reaction A + B 🡪 C, 

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 –

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 the following reactions of metal oxides –

2Ag2O Δ🡪 4Ag + O2

2HgO Δ🡪 2Hg + O2

2ZnO Δ🡪 2Zn + O2

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

For C🡪 CO2, ΔG⁰ = 0 so the 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. The reaction is given below –

ZnO + C 🡪 Zn + CO

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

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

Redox Reactions:

The extraction of metals frequently involves redox (reduction-oxidation) reactions. These reactions involve the transfer of electrons from one substance to another. The metal ore (usually an oxide or sulfide) is reduced (gains electrons), while another substance, typically carbon or hydrogen, is oxidized (loses electrons). The feasibility of these reactions depends on their standard reduction potentials (E°) and the standard electromotive force (EMF) of the cell.

Ellingham Diagram:

The Ellingham diagram is a graphical representation of the Gibbs free energy of formation for metal oxides as a function of temperature. It helps in predicting the temperature at which a particular metal oxide can be reduced. The metal oxides with lower negative slopes on the diagram are easier to reduce and, therefore, have higher thermodynamic feasibility for extraction.

Salient Features of Ellingham Diagram 

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

Following are the salient features of it –

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

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

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

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

Applications of Ellingham Diagram

Few applications of the 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 the reduction of metal oxides.

• It is used to find out the feasibility of the thermal reduction of 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 a few limitations as well which are listed below –

• It ignores the reaction kinetics meaning it does not provide any information about the 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.

2. Electrochemical Principles:

Electrolysis:

Electrolysis is a key electrochemical process used in the extraction of metals like aluminum. In the Hall-Héroult process, aluminum oxide (Al2O3) is dissolved in molten cryolite (Na3AlF6), creating an electrolytic cell. When an electric current is passed through the cell, aluminum ions migrate to the cathode, gain electrons, and deposit as aluminum metal, while oxygen ions migrate to the anode, lose electrons, and form oxygen gas.

Galvanic Cells:

In certain metal extraction processes, such as the extraction of zinc from zinc sulfide ore, a galvanic cell is used. A typical example is the Daniell cell, where zinc metal (Zn) is oxidized to form Zn^2+ ions in the anode compartment, while copper ions (Cu^2+) are reduced to form copper metal (Cu) at the cathode. The flow of electrons between the anode and cathode generates electrical energy.

Electrorefining:

Electrorefining is used to purify metals obtained through reduction processes. In the case of copper, for example, crude copper is made the anode in an electrolytic cell. As electric current passes through the cell, copper ions are oxidized at the anode and deposited as pure copper metal at the cathode.

Standard Electrode Potentials (E°):

The feasibility of electrochemical processes is assessed by comparing the standard electrode potentials (E°) of the involved half-reactions. A positive cell potential (E°cell) indicates that the cell is capable of generating electrical energy. The Nernst equation allows for the calculation of cell potential under non-standard conditions.

Overpotential:

In practice, electrochemical processes may face challenges due to overpotential, which is the difference between the actual potential required for a reaction to proceed and the standard potential. Overpotential can affect the efficiency of metal extraction by increasing energy consumption.

JEE Main General Principles and Processes of Isolation of Metals Solved Examples

Example 1: Extraction of zinc from zinc blende is achieved by:

A. Electrolytic reduction

B. Roasting followed by reduction with carbon

C. Roasting followed by reduction with another metal

D. Roasting followed by self-reduction

Solution: Zinc is derived from zinc blende, which is a kind of mineral (ZnS). The ore is concentrated by using pine oil in the froth flotation process. The concentrated ore is next roasted at a high temperature of 1200K in the presence of air. Zinc sulphate (ZnSO4) is generated from a zinc blender when the roasting temperature is lower than this temperature. When the roasted ore is reduced by carbon, the zinc sulphate is transformed back to zinc sulphide. So, the correct option is B.

Key point to remember: Roasting is the process of heating sulphide ore to a high temperature in the presence of air or oxygen. It is an intermediate step in the processing of certain ores.

Example 2: Malachite is an ore of

(a) iron

(b) copper

(c) zinc

(d) Sliver

Solution: The formation of malachite is by the weathering of copper ore bodies in the vicinity. The formula of Malachite is Cu2CO3(OH)2. So from the formula of Malachite, we can understand that it is an ore of copper.

Key point to remember: To know the formula of Malachite.

Solved Questions from the Previous Year Question Papers

Question 1: Which of the following ores is concentrated by the chemical leaching method?

(a) Cinnabar

(b) Argentite

(c) Copper pyrites

(d) Galena

Solution: In the metallurgy of silver, the respective metal is leached with a dilute solution of NaCN or KCN in the presence of air, which supplies O2. The metal is obtained later by replacement reaction.

The reactions involved are as follows:

4Ag+8CN−+2H2​O+O2→4[M(CN)2​]−+4OH−

2[Ag(CN)2]−+Zn→[Zn(CN)4​]2−+2Ag

Hence, Argentite being ore of silver is concentrated by leaching. So, the correct option is (b).

Trick: Familiarity with the Leaching process.

Question 2: Which one of the following ores is best concentrated by the froth-flotation method?

(1) Magnetite

(2) Cassiterite

(3) Galena

(4) Malachite

Solution: Froth-floatation method: In this method, gangue is removed  from sulphide ores. The ore is first powdered and its suspension is created in the water. To this collectors and froth, stabilisers are added. The oil wets the metal and the water wets the gangue. Paddles and air constantly stir up the suspension to create the froth. This frothy metal is skimmed off the top and dried, to recover the metal.

Since this method is used for the concentration of sulphide ores and here the only sulphide ore present is Galena (PbS). Therefore, option (3) is the answer.

Trick: To know the process of Froth-floatation.

Question 3: Common impurities present in bauxite are

(a) CuO

(b) ZnO

(c) CaO

(d) SiO2

Solution: Iron oxides such as hematite and goethite, Fe2O3, sand silicon dioxide SiO2, clay mineral kaolinite, and a minor quantity of TiO2 are all present in bauxite. So, the correct option is (d).

Trick:  Iron oxides (goethite and hematite), silicon dioxide, the clay mineral kaolinite, and minor quantities of anatase are the most common impurities in bauxite.

Practice Questions

Question 1: Sulphide ore is concentrated by:

(a) Froth flotation process

(b) Magnetic separation

(c) Gravity method

(d) Calcination method

Answer: (a) Froth flotation process.

Question 2: An ore of tin-containing FeCrO4 is concentrated by

(a) Gravity separation

(b) Magnetic separation

(c) Froth floatation

(d) Leaching

Answer: (b) Magnetic separation

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Conclusion

Thus, the study of the extraction of metals from ores is important as it allows one to use the minerals present deep inside the earth's crust. A mineral is a naturally occurring mixture of metals and non-metals. Minerals can be used to extract metals. An ore is a naturally occurring solid substance from which precious metal or minerals may be extracted economically. Impurities or unwanted particles known as gangue pollute ore. Minerals are all ores, but the opposite is not true.

A variety of concepts are involved to carry out the extraction of various metals from their respective ores and further help in understanding a phenomenon in detail. Hence, this is important not only for competitive exams like JEE or NEET but also for a better understanding of the extraction process of metals.