Metallurgy Notes for IIT JEE PDF

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Metallurgy is one of the simplest topics to score marks in JEE. Students can expect questions that are direct and less time-consuming. The entire scientific and technological process used for the extraction of metal from its ores is known as metallurgy. The important topics include metals and their extraction processes, ores and minerals, and refining methods, which include liquidation, distillation, electro-refining, zone refining, and vapour phase refining.

Table of Contents

• Concentration of Ore
• Conversion of Concentrated Ore into Metal Oxide
• Conversion of Metal Oxide into Metal
• Purification
• Aluminium
• Extraction of Iron
• Steel
• Refining of Metals
• Extraction of Copper
• Important Ores of Various Metals
• Solved Examples
• Practice Problems
• Frequently Asked Questions

The process of extracting metals from their ores is called metallurgy. The extraction process involves the following steps:

• Concentration of ore
• Conversion of concentrated ore into metal oxide
• Conversion of metal oxide into metals
• Purification of metals

Concentration of Ore

The metal ores contain non-metals, sand, clay, lime and rock materials. These impurities are called gangue. The removal of impurities from the ore is called the concentration of the ore. Following are the methods of concentration.

(a) Electromagnetic separation

This process is used only when impurities or metallic ore are magnetic in nature. The finely powdered ore is dropped over electromagnetic rollers. At the other end, the magnetic portion of the ore is attracted by the magnetic roller and falls nearer to the roller, while non-magnetic impurities fall away from the roller.

(b) Gravity separation

This process is based on the fact that specific gravities of metallic ore and earthy impurities are different. The finely powdered ore is agitated with water or washed with a running stream of water on a willey table. The ore particles will be left on the table, and lighter impurities will be washed away.

(c) Froth floatation process

This method is used for sulphide ores since they are preferentially wetted by oils. The finely powdered ore is introduced into water to which a small quantity of a suitable oil is added. Water is then agitated with a stream of air. The ore, which is preferentially wetted by oil, rises to the surface along with the foams, while the gangue, which is wetted by water, remains in the water at the bottom. The foam, along with the ore, is separated. This is known as the froth floatation process. Two types of substances are used in this process, and they are frothing agents and flotation agents. Eucalyptus oil and pine oil are generally used as frothing agents. Sodium ethyl xanthate or sodium amyl xanthate are used as flotation agents.

(d) Leaching or chemical method

This method depends upon the chemical nature of the ore. For example, bauxite ore of aluminium contains impurities of iron oxide, titanium oxide and silica impurities. Bauxite is heated in a solution of sodium hydroxide at 150-2000°C. It dissolves in the solution, forming a soluble complex. The impurities remain insoluble, and they are removed by filtration.

Al₂O₃ + 6NaOH → 2Na₃AlO₃ + 3H₂O

The filtrate is boiled with water; aluminium hydroxide gets precipitated, which on heating at high-temperature, changes to alumina.

Na₃AlO₃ + 3H₂O → Al(OH)₃ + 3NaOH

2Al(OH)₃ → Al₂O₃ + 3H₂O

Conversion of Concentrated Ore into Metal Oxide

(a) Calcination

In this process, the concentrated ore is heated to expel the organic matter and moisture in the ore. The ores are heated in the absence of air below the melting point. Then, the ores are converted into metal oxide. This method is used to remove water from hydrated ores and CO₂ from carbonate ores. This process is generally carried out in a reverberatory furnace.

CaCO₃ → CaO + CO₂

Limestone

Al₂O₃.2H₂O (bauxite) → Al₂O₃ + 2H₂O

2Fe₂O₃.3H₂O (Limonite) → 2Fe₂O₃ + 3H₂O

(b) Roasting

This method is usually done with sulphide ores. Sulphide ores alone or in the presence of other substances are heated in the presence of air below their melting points. The impurities like arsenic, phosphorus, sulphur, etc., are removed in the form of volatile oxides. This is carried out in a blast furnace or reverberatory furnace.

2ZnS (Zinc blende) + 3O₂ → 2ZnO +2SO₂ ↑

2PbS (Galena) + 3O₂→ 2PbO + 2SO₂ ↑

Conversion of Metal Oxide into Metal

(a) Carbon reduction method

Smelting – The roasted or calcined ore is mixed with a suitable quantity of coke, and the reducing substance and the flux are heated to a high temperature above the melting point of the metal. The metal oxide reduces to metal, and the flux combines with impurities (gangue) to form an easily fusible product called slag. Slag is not soluble in molten metal. It is lighter and can be easily skimmed off from the surface of the molten metal. This process is done in a reverberatory furnace or blast furnace.

SnO₂+ 2C→ Sn + 2CO↑

(Tin oxide) Tin

PbO (Lead oxide) + C → Pb + CO↑

ZnO (Zinc oxide) + C → Zn + CO↑

Flux: Metal ore contains some alkaline or acidic impurities which form fusible products by combining with some substances. These substances are added during smelting and are called flux. The fusible product is called slag. Basic flux is used for acidic impurities, and acidic flux is used for basic flux or impurities.

CaO + SiO₂ → CaSiO₃

CaO is the basic flux, SiO₂ is the acidic impurity, and calcium silicate is the slag.

MnO + SiO₂ → MnSiO₃

MnO is a basic impurity, SiO₂ is an acidic flux, and manganese silicate is the slag.

Usually, the slag is a silicate or phosphate and melts before the melting point of the metal.

(b) Goldschmidt thermite process (Alumino thermite process) – In this process, a mixture of metal oxide and aluminium powder (thermite) is ignited in a closed crucible by means of alighted magnesium ribbon having ignition mixtures (Mg + BaO₂ ) at one end. It is an exothermic reaction that liberates a large amount of heat. The temperature of the crucible reaches 3000°C. At this temperature, the metal oxide is reduced to metal by aluminium.

Cr₂O₃ + 2Al → 2C + Al₂O₃

3Mn₃O₄ + 8Al → 9Mn + 4Al₂O₃

(c) Precipitation method: More reactive element (more electropositive) replaces the less reactive (less electropositive) element from its salt solution. For example, iron can precipitate Cu from copper sulphate solution, while copper cannot precipitate iron from iron sulphate solution. This principle is used in the precipitation method. Silver can be extracted from its ores by using this theory. If zinc is added to a solution of silver, silver precipitates out.

Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S

2Na[Ag(CN)₂] + Zn → 2Ag + Na₂[Zn(CN)₄]

(d) Electrolytic method: When metal oxides, chlorides or hydroxides in a fused state are electrolysed, and the metals will deposit.

(d) Electrolytic method: When metal oxides, chlorides or hydroxides in a fused state are electrolysed, the metals will deposit at the cathode. The reduction of active elements such as alkali metals, alkaline earth metals and the oxides of Al with carbon is difficult because these form carbides with carbon at high temperatures.

Purification

The following methods can be used for the purification of metals.

(a) Liquation: We use this method when the impurity is less fusible than the metal itself. The impure metal is placed on the sloping hearth of the furnace and heated. The metal melts and drains away, leaving behind impurities. An example is tin.

(b) Oxidation method: We use this method when the impurity has a greater affinity with oxygen as compared to metal. The impurities are oxidised to form scum. The scum is skimmed off. Metal oxides are used as oxidising agents. For example, copper oxide is added to impure copper.

(c) Electrolysis: Most of the metals are purified electrolytically. A thin pure metal rod or strip is used as a cathode. The metal salt is taken as the electrolyte. The impure metal rod is used as an anode. On electrolysis, pure metal is deposited at the cathode. Metals like Cu, Ag, Al, Zn, Cr etc., are purified by this method.

(d) Distillation: The metals with low boiling points can be purified by distillation.

For example, Zn, Cd, Hg etc.

Aluminium

In the metallurgy of aluminium, purified Al₂O₃ is mixed with Na₃AlF₆ or CaF_2, which lowers the melting point of the mix and brings conductivity. The fused matrix is electrolysed. The cathode is a steel vessel with a lining of carbon, and the anode used is graphite. The reaction is given below:

2Al₂O₃ + 3C → 4Al + 3CO₂

This process of electrolysis is called the Hall-Heroult process.

Important Ores of Aluminium:

• Mica: KH₂Al₂(SiO₃)₄
• Corundum: Al₂O₃
• Alumstone or Alunite: K₂SO₄.Al₂(SO₄)₃ 24H₂O
• Bauxite: Al₂O₃.2H₂O
• Cryolite: Na₃AlF₆
• Feldspar: KAlSi₃O₈

Extraction of aluminium from bauxite

(i) Baeyer process

Finely powdered bauxite is digested with an aqueous solution of sodium hydroxide in an autoclave at 1500°C and 80 atmospheric pressure. Alumina present in bauxite dissolves as sodium meta aluminate. The other impurities remain insoluble, and these are filtered out.

Al₂O₃.2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O

The filtrate containing sodium meta aluminate is agitated with freshly precipitated Al(OH)₃ for 36 hours. Sodium meta aluminate hydrolyses to Al(OH)₃, NaAlO₂ + 2H₂O → NaOH + Al(OH)₃.

Then Al(OH)₃ is washed, filtered, dried and heated at about 15000°C, and alumina is obtained.

(ii) Electrolytic reduction of alumina

Pure Al₂O₃ on electrolytic reduction gives aluminium.

The melting point of Al₂O₃ is about 2000°C. This is reduced by mixing fused cryolite (Na₃AlF₆) + fluorspar (CaF₂) into it. The melting point of this mixture is about 900°C. This mixture is a good conductor of electricity. The inner carbon lining of the cell is used as the cathode, and the carbon rods as the anode. The fused mixture of alumina + Cryolite + Fluorspar is taken in the cell, whose temperature is maintained at about 900 to 10000°C. This mixture is covered by carbon powder which maintains the temperature of the cell. The carbon anodes are dipped in the mixture. Aluminium gets free at the cathode, while oxygen is at the anode. The aluminium obtained is about 99.8% pure.

(iii) Hall’s method

In this method, bauxite ore is fused with Na₂CO₃, and the sodium meta aluminate is formed and extracted with water. Carbon dioxide gas is passed through the sodium meta aluminate solution at a temperature of 50-600°C, and Al(OH)₃ gets precipitated. Al(OH)₃ thus obtained is filtered, dried and heated in a furnace at about 15000°C to obtain Al₂O₃.

Purification of Aluminium

Hoope’s method – Aluminium is refined electrolytically. The refining cell consists of three fused layers.

(i) The bottom anode layer of impure Al + Cu + Si

(ii) The middle electrolyte layer of cryolite (Na₃AlF) + BaF₂

(iii) The top cathode layer of pure aluminium. On passing the electric current, pure Al from the bottom layer rises to the upper layer.

Uses of Aluminium

Aluminium is used in a wide range of applications in the field of transport, construction, etc.

The dust of this metal is used in paints.

Aluminium foils are used in the food industries for wrapping food.

It is used in the extraction of manganese and chromium from its oxides.

It is used in the manufacturing of storage cans. Since it is a soft metal, it can be moulded into any shape.

Aluminium is the most commonly used metal after iron.

Extraction of Iron

Some of the ores of iron are as follows.

• Haematite – Fe₂O₃
• Iron pyrites – FeS₂
• Magnetite – Fe₃O₄
• Limonite – Fe₂O₃.3H₂O
• Copper pyrites – CuFeS₂

Various forms of iron:

Cast iron or pig iron – The least pure form of iron. It is brittle and cannot be hammered. It contains 2.2 to 4.4% of carbon.

Steel – It comes in between cast iron and wrought iron. It is malleable, and it contains 0.25 to 2% of carbon.

Wrought iron – The purest form of iron. It is malleable, and it contains 0.1 to 0.25% of carbon.

Extraction of cast iron

The following reactions take place in the blast furnace.

Zone of combustion

C + O₂ → CO₂

CO₂ + C → 2CO

Zone of reduction

Fe₂O₃ + 3CO → 2Fe + 3CO₂

FeO +CO → Fe + CO₂

Zone of slag formation

CaCO₃ → CaO + CO₂

CaO + SiO₂ → CaSiO₃ (fusible slag)

Zone of fusion

Molten iron is heavier than molten slag. The two liquids are periodically tapped off. The molten iron tapped off from the furnace is solidified into blocks called ‘pigs’.

Steel

Steel is prepared from cast iron. It can be made in the following ways:

i) Directly from ore.

ii) By mixing the required amount of cast iron and wrought iron.

iii) By reducing the amount of carbon in cast iron.

iv) By adding some carbon to wrought iron

Production of steel from cast iron

Acidic Bessemer process

The Bessemer is lined with silica.

In the acidic Bessemer, hot air oxidises Mn. Silicon and Carbon impurities are present in cast iron.

2Mn + O₂ → 2MnO

Si + O₂→ SiO₂

MnO combines with SiO₂ to give slag MnSiO_3, which is separated. Carbon is oxidised to carbon monoxide, which burns with a blue flame at the mouth of the converter.

When the whole of the carbon is oxidised, the blue flame suddenly stops.

The required amount of carbon is then added, usually in the form of an alloy of Fe+Mn + C, called spiegel. This process is called spiegeleisen. Manganese steel is the resulting product. It is harder and possesses much greater tensile strength.

Alloys of Steel

Heat Treatment of Steels

By proper heat treatment, the hardness and elasticity of steel can be controlled.

Tempering: Hard steel is heated to 2700°C and then allowed to cool slowly. It becomes comparatively less hard and less brittle.

Annealing: In this process, steel is heated to redness and then allowed to cool slowly. It turns soft and malleable. Annealing is used to reduce hardness, increase ductility and help eliminate internal stresses.

Hardening: In this process, steel is heated to redness and suddenly quenched by plunging into water or oil. It becomes extremely hard and brittle.

Refining of Metals

Electro-refining: The impure metal is converted into a block that forms the anode, while the cathode is a rod or plate of pure metal. These electrodes are suspended in an electrolyte which is the solution of a soluble salt of the metal, usually a double salt of the metal. The metal ions from the electrolyte are deposited at the cathode in the form of pure metal when an electric current is passed. An equivalent amount of metal dissolves from the anode and goes into the electrolyte solution as a metal ion. The insoluble impurities settle down below the anode as anode mud. The soluble impurities present in the crude metal anode go into the solution.

Zone refining: Usually, this method is used for preparing extremely pure metals. It is based upon the principle that when a molten solution of the impure metal is allowed to cool, the pure metal crystallises out, while the impurities remain in the melt. This is also known as fractional crystallisation.

Cupellation and polling: Cupellation works on the principle that noble metals do not undergo oxidation, and they are chemically unreactive. Cupellation is a refining process used when ores or metals are present under high temperatures. The noble metals are separated from impurities using a cupel which is a flat, porous dish. This method is also used for testing fresh metals and coin making.

Polling is used for refining metals that contain impurities of their own oxide. We use polling generally to get rid of oxide impurities of the metal like copper oxide, CuO2 or tin oxide SnO2. In this process, a reducing agent like diesel or natural gas reacts with copper oxide and reduces it to copper. The polling process is done in two stages, first oxidation and then reduction.

Van Arkel method: It is the process of refining metal by converting it into its volatile compound and then decomposing it to get pure metal. This method is used for purifying titanium. The metal is converted to its iodide form, which is highly volatile and unstable, leaving behind impurities. The unstable compound thus formed is decomposed to get the pure metal.

Ti(s) + 2I₂(s) → TiI₄(g) → Ti + 2I₂(g)

Extraction of Copper

Copper is found in nature in a free and combined state. The ores are as follows:

• Copper glance: Cu₂S
• Copper pyrites (Chalopyrites): CuFeS₂
• Malachite: [Cu₂CO₃(OH)₂] 
• Cuprite or Ruby copper: Cu₂O
• Azurite: Cu(OH)₂.2CuCO₃

Extraction of copper from copper pyrites

(a) Concentration: The ore is concentrated by froth flotation.

(b) Roasting: The impurities of arsenic and antimony are removed as volatile oxides.

2CuFeS₂ + O₂ → Cu₂S+ 2FeS + SO₂

2Cu₂S + 3O₂ → 2Cu₂O+ 2SO₂

2FeS + 3O₂ → 2FeO + 2SO₂

(c) Smelting: Sand and coke are mixed with roasted ore. It is then smelted in a blast furnace. FeO reacts with SiO₂ and gives ferrous silicate slag. Cu₂S and FeS are oxidised soon.

FeO + SiO₂ → FeSiO₃ (slag)

Cu₂O reacts with FeS to give cuprous sulphide and FeO. Again, FeO reacts with SiO₂ and gives slag FeSiO₃.

Cu₂O +FeS → Cu₂S + FeO

FeO + SiO₂ → FeSiO₃ (slag)

The slag is removed. Molten Cu₂S and a small amount of FeS remain below the surface of the slag, which is called matte. This is taken out from the furnace.

(d) Bessemerisation: Molten matte is transferred to a Bessemer converter. The Bessemer converter is a pear-shaped furnace with an inner lining of CaO or MgO. A blast of air mixed with sand is blown through the molten matte. The reaction is as follows:

2Cu₂S +3O₂ → 2Cu₂O + 2SO₂

Cu₂S + 2Cu₂O→ 6Cu + SO₂

The resulting molten copper is cast into ingots. It is about 98% pure copper. A blistered appearance is formed due to the escape of SO₂ gas during the cooling of molten copper. It is called blister copper. This copper contains about 2% mixture of Ag, Au, Ni, Zn, Pb, Sn, etc.

(e) Refining – Polling: The blistered copper is heated in the reverberatory furnace. Cuprous oxide present in it oxidises the impurities. The impurities of sulphur and arsenic are separated as volatile oxides, and the other impurities as slag. This copper still has the impurity of cuprous oxide. The molten copper is then agitated with green wood bamboo. Cuprous oxide is reduced to copper by the hydrocarbons obtained from the greenwood bamboo.

Alloys of Copper

Uses of Aluminium, Copper, Zinc and Iron

Aluminium foils are used as wrappers for chocolates. The fine dust of the metal is used in paints and lacquers. It is also used in the extraction of chromium and manganese from their oxides. Wires of aluminium are used as electrical conductors. Alloys containing aluminium, being light, are very useful.

In the electrical industry, copper is widely used for making wires and for water and steam pipes. It is also used in several alloys that are tougher than the metal itself, for example, 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 as a constituent of many alloys, for example, 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 dyestuffs, paints, etc.

Cast iron is used for casting stoves, railway sleepers, gutter pipes, 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, pendulums, measuring tapes, chrome steel for cutting tools and crushing machines, and stainless steel for cycles, automobiles, utensils, pens, etc.

Important Ores of Various Metals

Solved Examples

Question 1:

Calamine is

1) CaCO₃

2) MgCO₃

3) ZnCO₃

4) CaCO₃ + CaO

Solution:

Calamine is an ore of zinc. It is zinc carbonate ZnCO₃.

Hence option 3 is the answer.

Question 2:

Extraction of metal from the ore cassiterite involves

1) Carbon reduction of an oxide ore

2) Self-reduction of a sulphide ore

3) Removal of copper impurity

4) Removal of iron impurity

5) Both (1) and (4)

Solution:

Tin is obtained by the reduction of the ore cassiterite with coal in a reverberatory furnace. Limestone is added to produce a slag with impurities, which can be removed.

SnO₂ + 2C → Sn + 2CO.

Crude tin, thus obtained, is contaminated with iron, lead, and other metals. Then, it is remelted on an inclined furnace. This process is called liquation. The easily fusible tin metals are away, and the less fusible impurities are left behind. Molten tin is finally stirred with green poles of wood in contact with the air. In this process, any remaining metal impurities are oxidised, forming scum, which rises to the surface and is removed. This process is called poling.

Hence, option 5 is the answer.

Question 3:

Which one of the following statements is false?

1) During roasting, moisture is removed from the ore

2) The ore is freed from almost all non-metallic impurities

3) Calcination of ore is carried out in the absence of any blast of air

4) The concentrated zinc blende is subjected to calcination during its extraction by pyrometallurgy

5) Both (2) and (4)

Solution:

Metallic ore contains gangue particles like sand, clay, and quartz, and during extraction by pyrometallurgy, zinc blende is subjected to roasting to remove impurities.

Hence option 5 is the answer.

Question 4:

Gallium arsenide is purified by

1) Froth floatation process

2) Van-Arkel method

3) Zone-refining method

4) Electrolytic method

Solution:

Gallium arsenide is purified by the zone-refining method.

Hence option 3 is the answer.

Question 5:

In the froth floatation process for the purification of ores, the ore particles float because

1) They are light

2) Their surface is hydrophobic, i.e. not easily wetted by water

3) They bear an electrostatic charge

4) They are insoluble

Solution:

In the froth flotation process, for the purification of ores, the ore particles float because their surface is not easily wetted by water (hydrophobic). This method is based on the difference in the wetting properties of ores and gangue particles with water and oil.

Hence, option 2 is the answer.

Practice Problems

1. Among the following statements, the incorrect one is

1) Calamine and siderite are carbonates

2) Argentite and cuprite are oxides

3) Zinc blende and iron pyrites are sulphides

4) Malachite and azurite are ores of copper

2. The method of zone refining of metals is based on the principle of

1) Greater mobility of the pure metal than that of the impurity

2) Higher melting point of the impurity than that of the pure metal

3) Greater noble character of the solid metal than that of the impurity

4) Greater solubility of the impurity in the molten state than in the solid

3. In a mixture of PbS, Zns, and FeS_2, each component is separated from the other by using the reagents in the following sequence in the froth floatation process

1) Potassium ethyl xanthate, KCN

2) Potassium ethyl xanthate, KCN, NaOH, copper sulphate, acid

3) KCN, CuSO₄, acid

4) None of the above

4. PbS can be separated from ZnS by the electrostatic separation method. The property utilised in this method is

1) PbS is a good conductor, and ZnS is a poor conductor of electricity

2) PbS is a bad conductor, and ZnS is a good conductor of electricity

3) Both PbS and ZnS are good conductors

4) Both PbS and ZnS are bad conductors

5. Pick the incorrect statement.

1) Asbestos and willemite are silicate minerals

2) Anglesite and barytes are sulphate minerals

3) Sylvine and fluorspar are halide minerals

4) Calamine and zincite are the minerals of calcium

6. Which of the following is correct?

1) Tin stone is magnetic in nature

2) Wolframite is non-magnetic in nature

3) Wolframite is FeWO₄

4) Cassiterite and rutile are sulphides of the metals

7. The purpose of smelting an ore is to

1) Reduce it

2) Oxidise it

3) Obtain an alloy

4) Separate volatile impurities

8. Which of the following cannot be obtained by electrolysis of the aqueous solution of their salts?

1) Ag

2) Mg and Al

3) Cu

4) Cr

9. An ore of tin-containing FeCrO₄ is concentrated by

1) Magnetic separation

2) Froth floatation

3) Electrostatic method

4) Gravity separation

Metallurgy Important Topics

Metallurgy Important Questions