NCERT Solution / Notes Class 12 Chemistry Chapter 6 General Principles and Processes of Isolation of Elements

NCERT Solutions, Question Answer and Mind Map for Class 12 Chemistry Chapter 6, “General Principles and Processes of Isolation of Elements,” is a study material package designed to help students understand the principles and methods used for the extraction of metals from their ores.

The NCERT Solutions provide detailed explanations and answers to the questions presented in the chapter. The solutions cover all the topics in the chapter, including concentration of ores, extraction of crude metal from concentrated ore, refining of crude metal, and thermodynamic and electrochemical principles of metallurgy. They also provide tips on how to answer different types of questions, including short answer, long answer, and multiple-choice questions.

The question-answer section of the chapter covers a wide range of topics, from the various methods used for the concentration of ores to the extraction and refining of metals such as copper, iron, and zinc. It also includes questions on the principles of metallurgy, including thermodynamics and electrochemistry.

The mind map provides a visual representation of the key topics covered in the chapter, allowing students to understand the connections between different concepts and ideas. The mind map covers the different methods of concentration of ores, the extraction of crude metal from concentrated ore, and the refining of crude metal.

NCERT Solution / Notes Class 12 Chemistry Chapter 6 General Principles and Processes of Isolation of Elements with mind map PDF Download

Principles of Metallurgy

Thermodynamic Principles of Metallurgy:

• Some theories of thermodynamics help us in understanding the theory of metallurgical transformations.
• For any process, Gibbs free energy change (ΔG) is given by the equation,

ΔG = ΔH – TΔS

Where,

ΔH = The enthalpy change

ΔS = Entropy change

T = Absolute temperature

• This free energy change is also related to the equilibrium constant K of the reactant product system which is given by the equation,
• ΔG^o = -RTInk
• If ΔG^o is negative, then K will be positive. This means that the reaction will proceed towards products. From this, we can draw the following conclusions:

Ellingham Diagram:

1. The graphical representation of Gibbs energy was first used by H.J.T. Ellingham.
2. This representation provides a basis for considering the choice of reducing agent in the reduction of oxides. This is known as Ellingham diagram.
3. Such diagrams help us in predicting the feasibility of thermal reduction of an ore.
4. Ellingham diagram normally consists of plots of Δ_fG^o vs T for the formation of oxides of elements. These diagrams can also be constructed for sulphides and halides of elements.
5. Consider the reaction involving formation of metal oxide.
6. In this reaction, oxygen gas is consumed and metal oxide is formed.
7. Because gases have higher entropy than liquids and solids ΔS becomes negative. Thus, if temperature is increased, TΔS becomes more negative. Also, ΔG becomes less negative.
8. This means that Δ_fG^o against T lines have positive slope for the reactions of above type.
9. Each line is a straight line except some change in phase which takes place. The temperature at which such change occurs is indicated by an increase in slope on positive side.
10. For example, in the Zn-ZnO plot, at boiling point of zinc, there is an abrupt increase in the positive slope of the curve.

Limitations of Ellingham Diagram:

1. Kinetics of reduction:
   • Ellingham diagrams are based on thermodynamic concepts. Hence, these diagrams suggest whether the reduction of the given metal oxide with a particular reducing agent is possible or not.
   • It does not tell anything about the kinetics of the reduction process.
2. Reactant/product equilibrium:

• The interpretation of ΔG^o is based upon the equilibrium constant K. Thus, it is presumed that the reactants and products are in equilibrium.

• But, this is not always true because the reactant or product may be solid.

Theory of pyrometallurgy:

“The process of extracting a metal by reduction of its oxide with carbon in the form of coke, charcoal or carbon monoxide is called smelting.”

• During reduction, the oxide of the metal decompose

• The reducing agent takes away the oxygen given by the metal oxide.

• By reversing the equation (1), we will get the oxidation reaction. 

• If instead of partial oxidation of C to CO, complete oxidation of C to CO₂ occurs, then the oxidation of the reducing agent may be represented as

• If CO is used instead of C as a reducing agent, the oxidation of the reducing agent may be represented as

• Subtracting eq. (3) from each of the three equations, (2), (4) and (5), we have,

• These equations (6), (7) and (8) describe the actual reduction of the metal oxide, M_xO to the free metal, M.
• More heating gives the more negative Δ_rG^o value. Therefore, the temperature is chosen such that the sum of Δ_rG^o in the two combined redox processes is negative.
• In Δ_rG^o vs T plots, this is indicated by the point of intersection of the two curves. After that point, the Δ_rG^o value becomes more negative for the combined process including the reduction of MxO.
• If the difference between two Δ_rG^o values is large, then reduction is easier.

Applications of Pyrometallurgy:

Extraction of iron from its oxides:

• Theory of reduction process:

1. Oxide ores of iron, after concentration through calcination/roasting, are mixed with limestone and coke and fed into a blast furnace from its top.
2. The oxide is reduced to the metal. The reduction takes place as follows

3. Here, we can see two different reactions, one is reduction of FeO and other is oxidation of C to CO,

3. The net free energy change becomes,

3. The reaction will take place only when the Δ_rG is negative.
4. If we plot the graph of ΔG^o against T for the given reaction, then we can observe that at 1073K or above, the C, CO line is much below the Fe, FeO line.
5. This means that the coke will reduce FeO to Fe and itself will be oxidized to CO.
6. Below 1073K, the CO, CO₂ line lies below Fe, FeO line. Hence, in this region, CO reduces the oxides of iron.

• Reactions taking place in furnace:

1. Zone of combustion:

Near the tuyeres, where small pipes through which a blast of hot air is introduced, coke burns to form carbon dioxide.

Because the reaction is exothermic, lot of heat is produced and temperature is around 2170K.

2. Zone of heat absorption:

This is lower part of furnace and temperature is between 1423K–1673K. The CO₂ formed, meets the descending charge. The coke present in the charge reduces CO₂ to CO.

This reaction is endothermic. Therefore, the temperature gradually falls to 1423K.

3. Zone of slag formation:

It is the middle part of the furnace. The temperature is around 1123K. In this region, limestone decomposes to form CaO and CO₂. This CaO combines with silica to form fusible calcium silicate slag.

4. Zone of reduction:

This is the upper part of the furnace. The temperature is between 500K-900K. Here, ores are reduced to Fe by CO.

In the middle part of furnace, the temperature is 900-1500K. At a temperature above 1073K, FeO reduces to Fe by carbon. 

Finally, the direct reduction of iron ore takes place completely to iron by carbon above 1073K.

5. Zone of fusion:
   1. This is the lower part of furnace.
   2. Temperature here is between 1423K-1673K. In this region, spongy iron with impurities and CaSiO₃ slag melts.
   3. Both molten iron and molten slag forms two separate layers.
   4. The molten slag is lighter. Hence, it forms upper layer while molten iron is heavier and forms lower layer.
   5. The iron obtained from this furnace contains 4% carbon and many impurities in small amount. This is called pig iron.

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

1. In the Ellingham diagram of formation of Cu₂O from Cu, it can be seen that the (Cu, Cu₂O) curve is at the top, while (C, CO) and (CO, CO₂) lines lie below in the temperature range of 500K-600K. Hence, it is very easy to reduce cuprous oxide to metallic copper.
2. But most of the ores of copper are sulphides. Therefore, the sulphide ores are first roasted in the reverberatory furnace to convert them into oxides.

3. These oxides can then be reduced to metallic copper using coke as a reducing agent.

4. In the actual process, the sulphide ore is roasted in the reverberatory furnace where the copper pyrites is converted into a mixture of FeS and Cu₂S.

5. Iron is more reactive than copper. Hence, FeS is preferentially oxidized to FeO than Cu₂S to Cu₂O. The Cu₂O is then combined with FeS and converted back to Cu₂S.

6. Therefore, the roasted ore mainly contains Cu₂S and FeO along with some unreacted FeS.
7. The roasted ore is then mixed with silica and some powdered coke and heated strongly in a blast furnace. During this, FeO combines with silica to form fusible ferrous silicate slag.

8. At the furnace’s temperature, the entire solid melts and two layers of molten solids are formed. The slag, being lighter, forms upper layer and can be withdrawn from slag hole time to time.
9. The lower molten layer is called copper matte. It consists of Cu₂S and FeS.
10. This copper matte is then transferred to the Bessemer converter where the impurities such as As, S and Sb escape as their respective volatile oxide. Also the Fes is oxidized to FeO which combines with silica to form FeSiO₃ slag.

11. That slag melts and floats on the top of the molten mass from where it is removed. When the whole iron is removed as a slag, some of the cuprous sulphide undergoes oxidation to form cuprous oxide which then reacts with cuprous sulphide to form copper metal.

12. The copper metal formed is then cooled so as to remove SO₂. Some gas bubbles are trapped during this solidification giving blister like appearance to the metal, Hence, they are called as blister copper.
13. The blister copper is finally purified by electrolytic refining or poling.

Extraction of zinc from zinc oxide:

1. From the Ellingham diagram of formation of ZnO from Zn, we can see that the intersection of (Zn, ZnO) and (C, CO) curves lies at a higher temperature than that of (Cu, Cu₂O) and (C, CO) curves. Therefore, reduction of ZnO with coke is carried out at a higher temperature than that of Cu₂O.
2. Above 1270K, Δ_fG^o for ZnO is higher than that of CO₂ and CO. Hence, above this temperature, Δ_fG^o for reduction of ZnO by carbon is negative, and therefore ZnO is easily reduced by coke.
3. For the purpose of heating, the oxide is made into briquettes with coke and clay and heated above 1270K.

4. It is noted that the Δ_fG^o of CO2 from CO is always higher than that of ZnO. Hence, CO cannot be used for reduction of ZnO to Zn.
5. Because the boiling point of Zn is low (1180K), the metal is distilled off and collected by rapid chilling.

Electrochemical Principles of Metallurgy:

• The principles of thermodynamics can also be applied in the reduction of metal ions in the solutions or molten state. These reductions are usually carried out either by electrolysis or by adding some suitable reducing agent.
• 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,

Where, 

n = Number of electrons involved in the reduction process

E^o = Standard electrode potential of redox couple (M/Mⁿ⁺) present in the system

• Highly reactive metals have large negative values of electrode potential. So, their reduction is difficult.
• If the difference of two E^o values corresponds to a positive E^o and consequently a negative ΔG^o , then the less reactive metal will come out from the solution and more reactive metal will go into the solution.

• In simple electrolysis, the Mⁿ⁺ ions are discharged at the cathode and deposited there.

• Depending upon the reactivity of the metal produced, the materials of the electrode are selected. Sometimes, a flux is added for making the molten mass more conducting.

“A substance which chemically combines with the gangue which may still be present in the roasted or the calcined ore to form an easily fusible material called the slag is known as the flux.”

“The process of extraction of metals by electrolysis of their fused salts is called as

electrometallurgy.”

Applications of Electrolysis to Metallurgy:

Extraction of Aluminium from Alumina:

1. Fused alumina (Al₂O₃) is a bad conductor of electricity. Therefore, Cryolite (Na₃AlF₆) and Fluorspar (CaF₂) are added to alumina which makes the alumina, a good conductor of electricity and also reduces the melting point of mixture around 1140K.

“The process of obtaining aluminium by electrolysis of a mixture of purified alumina and cryolite is called Hall and Heroult process.”

2. The electrolysis of the molten mass is carried out in an electrolytic tank made of iron using carbon electrode.
3. The molten electrolyte is covered with a layer of powdered coke to prevent oxidation and loss of heat due to radiation. The temperature of tank is maintained at about 1173k.
4. The reactions taking place during electrolysis are

5. For each kg of aluminium produced, 0.5 kg of carbon anode is burnt away. Because of this, the anodes have to be replaced time to time.
6. The aluminium metal liberated at cathode melts at high temperature of the tank.
7. The molten aluminium is heavier than the molten electrolyte and hence it sinks to the bottom of the tank from where it is withdrawn time to time through the tapping hole.
8. The metal obtained from this process is 99.95% pure.

Copper from Low Grade Ores and Scraps:

1. Copper is extracted by hydrometallurgy from low grade ores.
2. The low grade ores are leached by treating with an acid when copper metal goes into a solution as Cu²⁺ ions.
3. The solution containing Cu²⁺ ions is then treated with scrap iron or H₂ gas.

1. Because E^o of Fe²⁺/Fe (-0.44V) or that of H⁺/H₂ (0.0V) redox couple is lower than that of Cu²⁺/Cu (+0.34V), Fe and H₂ can displace Cu from Cu²⁺ ions.

Extraction of Non-Metals by Oxidation:

1. Non metals occur in a combined state in its reduced form. Therefore, they are generally extracted or isolated by oxidation of their compounds.
2. Consider an isolation of chlorine from brine.

3. Because oxidation cannot be carried out by ordinary chemical methods, it is accomplished by electrolysis.
4. During electrolysis, Cl₂ is liberated at anode and H₂ at the cathode while NaOH is obtained in the solution.
5. Electrolysis of molten NaCl can be carried out but here Na metal is liberated at cathode and Cl₂ at anode.

Extraction of Metals by Oxidation and Reduction:

1. Extraction of gold and silver involves leaching of the metals. This is an oxidation reaction because during leaching process, Ag is oxidized to Ag⁺ and Au to Au⁺ which are then combined with CN^– ions to form their respective soluble complexes.

2. The metals are then recovered from these complexes by reduction or displacement methods using more electropositive zinc metal.

In these above displacement reactions, zinc acts as a reducing agent.

3. From the above theory, we can conclude that the extraction of gold and silver occurs by hydrometallurgy.

“The process of extraction of metals by dissolving the ore in a suitable reagent followed by precipitation or displacement of the metal by a more reactive or a more electropositive metal is called hydrometallurgy.”

Refining:

• Metals which even after extraction method contain some impurities are called crude metals. The impurities present in crude metals are

• The crude metals are, therefore, purified or refined. The method used for purification of metals depends upon the nature of metal and the nature of the impurities to be removed.

The process of purifying the crude metal is called refining.

• The common methods used for refining of metals are

Distillation process:

1. This method is employed for purification of volatile methods such as zinc, mercury and cadmium.
2. The impure metal is heated in an iron retort and the vapours are condensed in a separate receiver.
3. The pure metal distils over, leaving behind the non-volatile impurities in retort.

Liquation process:

1. This method is used for purification of metals whose melting point is lower than those of impurities present in it.
2. In this process, a crude metal is heated in an inert atmosphere of carbon monoxide on the sloping hearth of a reverberatory furnace.
3. The metal melts and flows down into the receiver leaving behind the impurities on the hearth.
4. Metals such as tin and lead are purified by this method.

Liquation process

Electrolytic Refining:

• A large number of metals such as copper, silver, gold, lead, nickel, chromium and zinc are refined by this method.
• In this method, the impure metal is converted into a block which acts as an anode while cathode is made by a pure strip of the same metal.
• These electrodes are dipped into the solution of double salt of metal.
• When an electric current is passed, metal ions from electrolyte are deposited at cathode in the form of pure metal while metal from the anode goes into the electrolyte solution as metal ions.

• In case of electrolytic refining of copper, crude copper metal acts as an anode while thin sheet of pure copper acts as a cathode. An electrolyte takes in the copper sulphate solution acidified with sulphuric acid.

• The impurities of iron, nickel, zinc and cobalt present in copper are passed into the solution as soluble sulphates.
• The other impurities such as antimony, tellurium, selenium, gold and silver are less electropositive, and hence they do not get affected by the electrolytic solution and settle down under the anode as anode mud or anode sludge.
• The copper obtained from this method is 99.95% pure.

Zone Refining:

1. This method is very useful for obtaining metals of high purity. For e.g. germanium, silicon, gallium and boron.
2. This method is based upon the principle that the impurities are more soluble in the molten state than in the solid state of the metal.
3. In this method, an impure metal is converted into a bar which is heated at one end with a moving circular heater, so that it forms a molten zone.
4. Because of the slow moving heater along the length of the rod, the pure metal crystallizes out of the melt, whereas the impurities pass into the adjacent molten zone.
5. This process is repeated a number of times till the impurities are completely removed.
6. This process is usually carried out in an inert atmosphere to avoid oxidation of metal.
7. The metals obtained from this method are highly pure.

Vapour-phase Refining:

In this method, a crude metal is separated from impurities by first converting it into a suitable volatile compound by heating it with a specific reagent at a low temperature and then decomposing the volatile compound at some higher temperature to get a pure metal.

Thus, the two requirements are

Distillation process:

1. This method is employed for purification of volatile methods such as zinc, mercury and cadmium.
2. The impure metal is heated in an iron retort and the vapours are condensed in a separate receiver.
3. The pure metal distils over, leaving behind the non-volatile impurities in retort.

Liquation process:

1. This method is used for purification of metals whose melting point is lower than those of impurities present in it.
2. In this process, a crude metal is heated in an inert atmosphere of carbon monoxide on the sloping hearth of a reverberatory furnace.
3. The metal melts and flows down into the receiver leaving behind the impurities on the hearth.
4. Metals such as tin and lead are purified by this method.

Liquation process

Electrolytic Refining:

• A large number of metals such as copper, silver, gold, lead, nickel, chromium and zinc are refined by this method.
• In this method, the impure metal is converted into a block which acts as an anode while cathode is made by a pure strip of the same metal.
• These electrodes are dipped into the solution of double salt of metal.
• When an electric current is passed, metal ions from electrolyte are deposited at cathode in the form of pure metal while metal from the anode goes into the electrolyte solution as metal ions.

• In case of electrolytic refining of copper, crude copper metal acts as an anode while thin sheet of pure copper acts as a cathode. An electrolyte takes in the copper sulphate solution acidified with sulphuric acid.

• The impurities of iron, nickel, zinc and cobalt present in copper are passed into the solution as soluble sulphates.
• The other impurities such as antimony, tellurium, selenium, gold and silver are less electropositive, and hence they do not get affected by the electrolytic solution and settle down under the anode as anode mud or anode sludge.
• The copper obtained from this method is 99.95% pure.

Zone Refining:

1. This method is very useful for obtaining metals of high purity. For e.g. germanium, silicon, gallium and boron.
2. This method is based upon the principle that the impurities are more soluble in the molten state than in the solid state of the metal.
3. In this method, an impure metal is converted into a bar which is heated at one end with a moving circular heater, so that it forms a molten zone.
4. Because of the slow moving heater along the length of the rod, the pure metal crystallizes out of the melt, whereas the impurities pass into the adjacent molten zone.
5. This process is repeated a number of times till the impurities are completely removed.
6. This process is usually carried out in an inert atmosphere to avoid oxidation of metal.
7. The metals obtained from this method are highly pure.

Vapour-phase Refining:

In this method, a crude metal is separated from impurities by first converting it into a suitable volatile compound by heating it with a specific reagent at a low temperature and then decomposing the volatile compound at some higher temperature to get a pure metal.

Thus, the two requirements are

This method is described by the following two processes:

1. Mond Process:
   1. It is used for refining of nickel.
   2. When impure nickel is heated in a presence of CO at 330K-350K, it forms volatile nickel tetracarbonyl complex leaving behind the impurities.
   3. The nickel tetracarbonyl is then heated to a higher temperature and undergoes thermal decomposition giving pure nickel. 

2. Van Arkel Method:
   1. This method is very useful for preparing ultra-pure metal by removing all oxygen and nitrogen impurities from metals such as zirconium and titanium.
   2. In this method, crude zirconium is heated in an evacuated vessel with iodine at 870K to form zirconium tetraiodide.
   3. It is then separated and decomposed by heating over a tungsten filament at 2075K to get pure zirconium

Chromatographic methods:

1. Chromatography is the most versatile and modern method for separation, purification and testing the purity of elements and their compounds.
2. This method is based upon the principle that different components of mixture are adsorbed to different extent on an adsorbent.
3. It consists of two phases, stationary phase and mobile phase. The stationary phase can be either solid or liquid on solid support while the mobile phase can be a liquid, gas or super critical fluid such as CO₂.
4. There are several types of chromatography depending upon the physical state of the stationary phase and mobile phase. Let us discuss one of the many, column chromatography.

Column Chromatography:

1. It is a widely used technique.
2. In this, an adsorbent such as alumina or silica gel is packed in a column. This forms a stationary phase.
3. The mixture to be separated is dissolved in a suitable solvent and applied to the top of the column. This forms a mobile phase.
4. Suppose a mixture of three different components say A, B and C is applied to the column for separation. Also, suppose that the component A is more strongly adsorbed, B is moderately adsorbed and C is weakly adsorbed.
5. While elution, three different components began to separate and form three different coloured bands, if the mixture is coloured.
6. The C is weakly adsorbed, and hence moves faster down the column, followed by B and A which moves slowly down the column.
7. These are collected in different flasks. Evaporation of solvent will gives us the desired component.
8. If the mixture is colourless, a column is extracted with suitable solvent and collected in different flasks. The suitable chemical and physical methods are used to get the desired component.
9. This technique is especially suitable for those elements which are available in very minute quantities and the impurities are not very much different in chemical properties from the elements to be purified.

Uses of Aluminium, Copper, Zinc and Iron:

Modes of Occurrence of Elements and Metals

Modes of Occurrence of Elements

The elements generally occur in the free state (called native state) or in the combined state. This is mainly because of different chemical reactivities of elements.

1) Native State

The elements which have very low reactivity and are not attacked by oxygen or air, moisture, carbon dioxide or other non-metals occur in the free state, called native state.

For example: carbon, sulphur, nitrogen, noble gases, metals like gold, silver, platinum, etc. occur in nature in the native state.

Therefore, these elements are also called native elements.

2) Combined State

The elements which are reactive and have a tendency to combine with oxygen or air, moisture, carbon dioxide and non-metals like carbon, nitrogen, sulphur, phosphorus, arsenic, halogen, etc. occur in the combined state.

These elements occur in the crust of the earth in the form of their compounds. In the combined state the non-metals are usually found in the reduced form and the metals in the oxidised form.

Among the metals, only a few metals, such as silver, gold, platinum, etc. occur in native state. Non-metals such as carbon and sulphur also occur in native state as well as in combined state.

Abundance of Elements

Most metals occur in the combined states. The most common forms of metals in the combined state are oxides, carbonates, sulphides, silicates, halides, sulphates, arsenides, phosphate, etc.

Among metals, aluminium is the most abundant. It is the third most abundant element in earth’s crust (8.31% by wt). It occurs widely as a constituent of rocks and soils. It is a major component of many igneous minerals including mica and clay.

Iron is the second most abundant metal in earth’s crust. It forms a variety of compounds which have various important uses. This makes iron a very important element. Iron is also one of the essential elements in biological systems.

Mineral and Ores

Most of the metals have a tendency to react with moisture, oxygen, sulphur, halogens, etc. and therefore, occur in the crust of the earth in the form of their compounds such as oxides, sulphides, halides, silicates, carbonates, nitrates, phosphates, etc.

The naturally occurring chemical substances in which metals occur in the earth’s crust are called minerals. The mineral, from which the metal can be economically and conveniently extracted, is called an ore.

The type of mineral from which the metal can be extracted is decided on the basis of

profitability.

For example: Metals are not generally extracted from silicate minerals because of the difficulties during extraction.

2) Aluminium occurs in the earth’s crust in the form of two minerals, bauxite (Al₂O₃.2H₂0)

and clay (Al₂O₃.2SiO₂.2H₂0). Aluminium can be conveniently and economically extracted from bauxite, while it has not been possible to extract aluminium from clay by some easy and cheap method.

3) The main minerals of copper are copper glance (Cu₂S), cuprite (Cu₂O), copper pyrites (CuFeS₂), malachite [CuCO₃ Cu (OH)₂], etc. but copper can be conveniently extracted from copper pyrites. Therefore, the ore of copper is copper pyrites. 

Metal	Name	Composition
Aluminium	Bauxite	AlOx(OH)3-2x
	Feldspar	KAlSiO8
	Cryolite	Na3AlF6
	Kaolinite	Al2(OH)4Si2O5
Iron	Haematite	Fe2O3
	Magnetite	Fe3O4
	Siderite	FeCO3
	Iron Pyrites	FeS2
	Limonite	Fe2O3.3H2O
Copper	Copper glance	Cu2S
	Copper pyrite	CuFeS2
	Malachite	CuCO3.Cu(OH)2
	Cuprite	Cu2O
	Azurite	2 CuCO3. Cu(OH)2
Zinc	Zinc blende	ZnS
	Calamine	Zn2SiO4
	Zincite	ZnO.Fe2O3
	Willemite	ZnO.Fe2O3
Manganese	Pyrolusite	MnO2
	Braunite	Mn2O3
Calcium	Limestone	CaCO3
	Gypsum	CaSO4.2H2O
Magnesium	Magnesite	MgCO3
	Dolomite	CaCO3.MgCO3
Lead	Galena	PbS
	Cerrusite	PbCO3
Mercury	Cinnabar	HgS

Extraction of Copper and Zinc

Occurrence and Extraction of Copper

Occurrence of Copper

It occurs as native copper as well as in combined state.The main ores of copper are:

• Copper glance Cu₂S
• Copper pyrites CuFeS₂
• Malachite Cu (OH) ₂CuCO₃
• Cuprite or (Rubby copper) Cu₂O
• Azurite 2CuCO₃.Cu(OH)₂

Extraction of Copper

It is mainly extracted from copper pyrites (CuFeS₂). The sulphide ore is usually of very low grade and contains iron sulphide, gangue and smaller quantities of arsenic, antimony, selenium tellurium, silver, gold and platinum.

The various steps involved in the extraction are:

1) Crushing and concentration

The ore is crushed in jaw crushers and then finally powdered.

It is concentrated by froth floatation process.

In this process, the finely powdered ore is mixed with water and some pine oil in a tank.

The mixture is agitated by blowing compressed air into it.

The particles are preferentially wetted by oil and rise to the surface of the tank in the form of a froth (a foam) from where these are skimmed off.

The silicious and earthy impurities are preferentially wetted by water and sink to the bottom of the tank.

2) Roasting

The concentrated ore is roasted i.e., heated strongly in the presence of excess air in a reverberatory furnace. During roasting the following changes occur:

a) Moisture is removed from the ore and it becomes dry.

b) The impurities of sulphur, arsenic, antimony and phosphorus are removed as their volatile oxides.

S + O₂ —> SO₂

P₄ + 5O₂ —–> 2P₂O₅

4As + 3O₂ → 2As₂O₃

4Sb + 3O₂ → 2Sb₂O₃

(iii) Copper pyrites is converted to ferrous sulphide (FeS), cuprous sulphide(Cu₂S) which are partially oxidised

2CuFeS₂ + O₂ —–> Cu₂S + 2FeS + SO₂

Copper pyrites

2FeS + 3O₂ → 2FeO + 2SO₂

2CuS + 3O₂ → 2Cu₂O + 2SO₂

3) Smelting

The roasted ore is mixed with some powdered coke and sand and is heated strongly in a blast furnace.

The blast furnace is made up of steel and is lined inside with fire bricks. A blast of hot air is introduced at the lower part of the furnace.

The following changes occur during smelting :

(a) Most of the ferrous sulphide gets oxidised to ferrous oxide which combines with silica (flux) to form fusible slag.

2FeS + 3O₂ → 2FeO + 2SO₂

FeO + SiO₂ → FeSiO₃

Ferrous silicate (slag)

The slag being lighter floats and forms the upper layer. It is removed through the slag hole from time to time.

(b) During roasting or in the blast furnace if any oxide of copper is formed, it combines with FeS and is changed back into its sulphide.

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂

Cu2O + Fes → Cu₂S + FeO

Ferrous oxide thus formed again combines with silica to form more slag.

FeO + SiO₂ —–> FeSiO₃

As a result of smelting, two separate layers are formed at the bottom of the furnace. The upper layer consists of slag and is removed as a waste. The lower layer of molten mass contains mostly cuprous sulphide and some traces of ferrous sulphide. It is called matte and is taken out from the taping hole at the bottom.

4) Bessemerisation

The molten matte from the blast furnace is transferred into a Bessemer converter.

The vessel is made of steel and is lined inside with lime or magnesium oxide. A hot blast of air mixed with sand is blown into the molten matte. During this process :

(i) traces of ferrous sulphide present in the matte is oxidised to FeO which combines with silica to form slag.

2FeS + 3O₂ —-> 2FeO + 2SO₂

FeO + SiO₂ —-> FeSiO₃ 

(ii) copper sulphide is partially oxidised to cuprous oxide which further reacts with remaining copper sulphide to form copper and sulphur dioxide.

2 Cu2S + 3O2 → 2Cu₂O + 2SO₂

Cu₂S + 2Cu₂O —-> 6 Cu + SO₂

After the reaction has been completed, the converter is tilted and the molten copper is poured into sand moulds. On cooling, sulphur dioxide, nitrogen and oxygen escape from the metal. The copper thus, obtained is about 99% pure and is known as blister copper. The name blister comes from the fact that as the metal solidifies, the dissolved SO, escapes producing blisters on the metal surface.

5) Refining

a) Poling: The blister copper is purified by heating it strongly in a reverberatory furnace in the presence of excess of air. The impurities are either removed as volatile oxides or converted into slag.

Some of the copper also changes to cuprous oxide. This is reduced back to copper by stirring the molten metal with green poles of wood. The hydrocarbons present in these freshly cut poles reduce cuprous oxide to copper which is about 99.5% pure. 

b) Electrolytic refining: In this method, a thin sheet of metal is made as cathode and the block of crude metal is made as anode.

Both the electrodes are placed in an acidified copper sulphate solution. When electric current is passed through the solution impure copper from anode goes into the solution and pure copper from the solution gets deposited on the cathode.

At anode Cu -2e¯ ——-> Cu²⁺

At cathode Cu²⁺ + 2e‾ →Cu

The impurities of zinc, nickel, iron, etc. get collected below the anode as anode mud.

Prolonged exposure of copper pyrites to air and rain leads to the formation of dilute solution of copper sulphate. Copper can be precipitated from this solution by the addition of scrap iron. It is then refined electrolytically.

Occurrence and Extraction of Zinc

Zinc does not occur in native form because it is a reactive metal. The main ores of zinc are:

• Zinc blende ZnS
• Calamine ZnCO₃
• Zincite ZnO
• Franklinite ZnO.Fe₂O₃
• Willemite Zn₂SiO₄

The principal ore of zinc is zinc blende.

Extraction: Zinc can be extracted from zinc blende by the following steps:

1) Concentration: 

The ore is concentrated by froth floatation process.

2) Roasting: 

Concentrated ore is roasted in the presence of excess of air at about 1200 K to convert zinc sulphide into zinc oxide.

2ZnS + 3O₂ ——> 2ZnO + 2SO₂

Sulphur dioxide gas evolved in this process may be used in the manufacture of sulphuric acid.

3) Reduction: 

Zinc oxide is reduced to zinc by heating with crushed coke at 673 K in vertical fire clay retorts.

ZnO + C —–> Zn + CO

The vapours of zinc formed are collected and condensed.

4) Refining: 

The impure metal is refined by fractional distillation or by electrolytic method.

(i) By fractional distillation: 

Impure zinc contains impurities of cadmium (b.p. = 1073 K), lead (b.p. 2024 K) and iron (b.p. 3273 K). The boiling point of zinc is 1183 K. The impure metal is distilled when zinc and cadmium with low boiling points distil over leaving behind lead and iron.

The boiling points of zinc and cadmium are also different and therefore, the mixture of zinc and cadmium is again subjected to fractional distillation when low boiling cadmium distils leaving behind zinc metal in the distillation flask.

(ii) By electrolytic refining: 

In this process, the impure zine is made anode while a plate of pure zinc is made the cathode. The electrolyte is zinc sulphate containing a small amount of dilute sulphuric acid. On passing current, zinc from the electrolyte is deposited at the cathode while an equivalent amount of zinc from anode goes into the electrolyte. Therefore, pure zinc is obtained on cathode.