CHEMISTRY MASTER: UNIT 10 METALLURGY

Syllabus

Mode of occurrence of metals
Concentration of ores
General principles of extraction of metals from ores
Refining of metals
Extraction of sodium, aluminium, iron and copper
Manufacture of steel
Different types of steel , Heat treatment and uses

All material objects are composed of elements. There are about 118 elements known so far out of which 92 occur in nature while the rest are man-made. The elements are broadly classified into metals and non-metals. About two-thirds of the elements are metals.

Occurrence of elements

The elements generally occur in native and combined states. The elements like oxygen, nitrogen , noble gases, gold, platinum etc occur in the free state or native state (uncombined form). On the other hand, most of the elements occur in the combined state and are found in nature in the form of their compounds. The reason for this lies in the different chemical reactivities of the elements.

The most common forms of elements in the combined state are oxides, carbonates, sulphides , halides, sulphates etc. The list of few elements and their nature of occurrence is given in the TABLE.

Elements and their occurrence

*Combined state*	*Element*	*Name of the mineral*
Oxides	Fe Al Mn	Haematite(Fe₂O₃) Magnetite(Fe₂O₃.FeO) Bauxite (Al₂O₃.2H₂O) Pyrolusite (MnO₂)
Carbonates	Ca Mg Cu Zn	Calcite (CaCO₃) Dolomite (CaCO₃MgCO₃) Malachite [CuCO₃Cu(OH)₂] Calamine (ZnCO₃)
Silicates	Mg, Si Al, Si	Calcium magnesium silicate (asbestos) (CaSiO₃.MgSiO₃) China clay Al₂O₃.SiO₂.2H₂O
Sulphides	K,Al,Si Fe Cu Hg Zn Pb	Mica sheets Iron pyrites FeS₂ Copper pyrites CuFeS₂ Cinnabar HgS Zinc sulphide ZnS Lead sulphide (Galena) PbS
Halides	Na Al K, Mg Ca	Common salt NaCl Cryolite Na₃AlF₆ Carnallite KCl MgCl₂ 6H₂O Fluorite CaF₂
Sulphates	Ba Pb Ca	Barytes BaSO₄ Anglesite PbSO₄ Gypsum CaSO₄ 2 H₂O

Earth as a source of elements

Earth is a source of a large number of elements. The distribution of these elements in different parts of the earth is different. Earth consists of three main parts : Atmosphere, hydrosphere and lithosphere.

1. Atmosphere : The gaseous mixture surrounding the earth is called atmosphere. The main constituents of atmosphere are nitrogen (78.09%), oxygen(20.95%) and other gases ( < 1%).

2. Hydrosphere : It covers about 80% of the earth’s surface in the form of sea or ocean. There are large number of elements present in sea water.

3. Lithosphere : The solid phase of the earth is called lithosphere. Almost all the elements , metals as well as non-metals are present in lithosphere. It consists of different types of rocks. The distribution of the elements in the earth’s crust is shown in Fig.

It is observed that silicon and oxygen are the main constituents of earth and constitute about 75% of the earth’s crust.

OCCURRENCE OF METALS

Minerals and ores

The natural substances in which metals occur either in the native or combined state are called minerals. The mineral, from which the metal can be economically and conveniently extracted is called an ore. For example, aluminium occurs in earth’s crust in the form of two minerals, bauxite (Al₂O₃.2 H₂O) and clay (Al₂O₃. 2 SiO₂ . 2 H₂O).Out of the two , 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. Therefore, the ore of aluminium is bauxite. All ores are minerals, but all minerals are not ores.

EXTRACTION OF METALS (METALLURGY)

The process of extraction of the metal in the free state from its ore is called metallurgy. The ores are generally associated with non-metallic impurities of earth and rocks. These are called gangue or matrix. Since the nature of the ore and also the properties of different metals are different, therefore it is not possible to have the universal scheme which may be applicable to all metals. However, some common steps involved in the metallurgical operations are :

i) Concentration of the ore.

ii) Isolation of metal

iii) Purification or refining of the metal.

A general scheme of various metallurgical operations employed for the extraction of metals from ores is given below.

i) Concentration or benefication of the ore

The ore obtained from the earth’s crust is associated with rocky and silicon impurities. It is quite essential to get rid of these impurities so that they may not cause any interferences in the process of extraction. The removal of impurities from the pulverised ore is called concentration or benification of the ore.

(a) Crushing and grinding of the ore

Most of the ores are obtained from the crust of the earth in the form of huge lumps. These lumps have to be converted into powdered form so that the chemical changes which have to take place at the later stages may become convenient. These are further pulverised in Stamp mill or Ball mill.

(b) Levigation or Gravity separation

This method is usually applicable to oxide ores in which the ore particles are heavier than impurities. The powdered ore is washed with running stream of water. The lighter impurities are washed away leaving behind the heavier ore particles.

This process is generally used for the concentration of sulphide ores. The finely powdered ore is taken in a mixture of water and pine oil. The oil acts as a frothing agent. The contents are kept agitated by the blast of air. As a result of agitation , the froth is produced. The ore particles are preferentially wetted by the oil and are carried to the surface by the foam. The gangue material which is preferentially wetted by water sinks to the bottom of the tank. Sometimes the other reagents are also added to the tank which increases the binding force between the particles of the ore and foam. Such reagents are called collectors. The foam at the surface of the tank is transferred to the other tank where it is washed with water to recover the ore particles.

Froth floatation process

(a) Magnetic separation

The method is usually employed for the separation of magnetic impurities from non-magnetic ore. For example, tungstates of iron and maganese from tinstone are separated by this method. The powdered ore is dropped over the belt revolving round the rollers, one of which is magnetic.

Magnetic separation

The magnetic rollers attracts the magnetic part of the ore and they are collected in the form of heap near it. The non-magnetic part of the ore flies off and forms a heap away from the impurities.

(b) Liquation

This method is used for concentrating ores having low melting points than the impurities. The powdered ore is heated on a slopping floor of the furnace. The temperature is raised to just above the melting point of the ore. The ore melts and flows down the floor while the infusible impurities are left behind. This method is used for concentrating ores of antimony.

Liquation

This is a chemical method of concentration. In this method the powdered ore is treated with certain reagents in which the ore is soluble but the impurities are not soluble. The impurities left undissolved are removed by filtration.

Leaching method is used for concentrating ores of aluminium, silver , gold etc. For example, the ore of aluminium , bauxite is concentrated by this method. The ore is leached with strong solution of sodium hydroxide when Al₂O₃ dissolves in alkali to form soluble sodium meta-aluminate. The solution is filtered and the filtrate is diluted with water and agitated for two to three hours to convert it into aluminium hydroxide.

Al₂O₃ + 2 NaOH ® 2 NaAlO₂ + H₂O

sodium aluminate

NaAlO₂ + 2 H₂O ® Al(OH)₃ + NaOH

ppt

The precipitate is separated by filtration. It is dried and upon heating gives pure alumia.

Leaching is used to concentrate silver and gold ores.

Production of metal from the concentrated ore

The process of extraction of metal from the concentrated ore depends upon the nature of the ore as well as the nature of the impurities present in the ore. Before the concentrated ore is subjected to final metallurgical operations in order to get the metal in the free state, the preliminary chemical treatment may be necessary. The objective of this preliminary chemical treatment is :

(a) To get rid of impurities which would cause difficulties in later stages ; and

(b) To convert the ore into oxide of the metal.

The process employed for preliminary treatment are calcination and roasting.

(i) Calcination

It is a process of heating the ore in a limited supply of air below its melting point. The process involves :

· The removal of volatile impurities,

· The removal of moisture,

· The decomposition of any carbonate into oxide.

2 Fe₂O₃ . 3 H₂O ® 2 Fe₂O₃ + 3 H₂O

CaCO₃ ® CaO + CO₂

MgCO₃ ® MgO + CO₂

PbCO₃ ® PbO + CO₂

CuCO₃.(OH)₂ ® 2 CuO + H₂O + CO₂

CaCO₃ MgCO₃ ® CaO + MgO + 2 CO₂

(ii) Roasting

Roasting is the process of heating the ore strongly in the presence of excess of air. This process of heating the ore is employed when the oxidation of the ore is required. As a result of roasting,

· Moisture is driven away.

· Volatile impurities are removed.

· The impurities like sulphur, phosphorus, arsenic etc are removed as oxides.

· The ore undergoes oxidation to form metal oxides.

2 PbS + 3 O₂ ® 2 PbO + 2 SO₂

2 ZnS + 3 O₂ ® 2 ZnO + 2 SO₂

Both calcination and roasting processes are carried out in special type of furnace called reverberatory furnace.

Section of a modern reverberatory furnace

After the preliminary treatment, the ore may be subjected to reduction process by one of the following methods depending upon its nature.

1. Smelting : Reduction with carbon

In this process , the roasted or calcined ore is mixed with suitable quantity of coke or charcoal(which acts as reducing agent) and is heated to a high temperature above its melting point. During reduction, an additional reagent is also added to the ore to remove the impurities still present in the ore. The additional reagent added is called flux. Flux combines with the impurities to form a fusible product called slag.

Flux + gangue ® Slag

The selection of flux depends upon the nature of impurities. If the impurities are acidic in nature, the flux is basic, i.e., lime (CaO). On the other hand, for basic impurities, an acidic flux, silica (SiO₂) is used.

For example,

The metal oxide , during reduction process, gets reduced to metal.

Fe₂O₃ + 3 C ® 2 Fe + 3 CO

PbO + C ® Pb + CO

SnO₂ + 2 C ® Sn + 2 CO

The process of reduction with carbon is carried out in the blast furnace.

Production of iron from the oxide ore

The extraction of iron , the most important industrial metal is carried out in a blast furnace (Fig) . The charge consists of iron ores (haematite, Fe₂O₃ and magnetite, Fe₃O₄), coke and lime stone which is heated with a blast of air (Fig). As the exothermic reaction proceeds a composition and temperature gradient is set up in the furnace. Below 1123 K, CO reduces the ores to FeO. Reduction to Fe by CO can occur at about 1123 K and that direct reduction ( in which the reducing agent is C ) can occur above 1123 K. In this region lime stone also decompose to give CaO and CO₂ and slag formation takes place.

Blast furnance

2. Reduction with Aluminium

Certain metal oxides such as Cr₂O₃ and Mn₃O₄are not easily reduced with carbon. In such cases aluminium is used as reducing agent because it is more electropositive than chromium or manganese. The process of reduction of oxides with aluminium is called aluminiothermy. Some examples are :

Cr₂O₃ + 2 Al ® Al₂O₃ + 2 Cr

3 Mn₃O₄ + 8 Al ® 4 Al₂O₃ + 9 Mn

3. Reduction with Mg : In some cases , the reduction is carried out by alkaline earth metals like magnesium.

TiCl₄ + 2 Mg ® 2 MgCl₂ + Ti

4. Reduction with hydrogen or water gas

Hydrogen is a very efficient reducing agent. However, its use is limited to those ores where carbon and other reagents are not suitable. It is because of its expensive nature. The roasted ore is heated in small trays in the current of hydrogen gas. For example, tungsten and indium can be obtained by reduction of their oxides with H₂.

WO₃ + 3 H₂ ® W + 3 H₂O

In₂O₃+ 3 H₂ ® 2 In + 3 H₂O

Similarly, extraction of nickel is carried out from its oxide by the use of water gas.

2 NiO + CO + H₂ ® 2 Ni + CO₂ + H₂O

water gas

5. Electrolytic reduction

The highly electropositive elements such as alkali metals, alkaline earth metals and aluminium cannot be extracted by carbon reduction methods. They are extracted by electrolysis of fused salts. The process of extraction of metals by the use of electrolysis is called electrometallurgy.

These metals which occur as chlorides or oxosalts are converted into their chlorides. When electric current is passed through a fused chloride , the Mⁿ⁺ ions are discharged at the negative electrode (cathode) and deposited. In the electrolysis of brine (NaCl) using mercury cathodes , Na⁺ is discharged at the cathode and forms an amalgam. This takes place in preference to the liberation of H₂(expected from electrochemical series) due to high overvoltage at Hg cathode.

Metals other than the s-block can also be extracted by electrolysis of fused compounds, e.g. fused Al₂O₃ mixed with Na₃AlCl₆is used for the production of Al.

6. Leaching or Hydrometallurgy

Some metals like gold and silver are extracted from their concentrated ores by leaching. They are dissolved in suitable reagents like acids or bases leaving behind insoluble impurities. The metal is recovered from the solution by precipitation or crystallisation. For example, silver ore is leached with dilute solution of sodium cyanide. Silver dissolves forming a complex, sodium argentocyanide. The solution is further treated with scrap zinc to get the precipitate of silver.

Similarly, native gold is leached with potassium cyanide solution and is recovered from the solution by addition of scrap zinc.

4 Au + 8 KCN +O₂ +2H₂O ® 4 K[Au(CN)₂]+ 4 KOH

2 K[Au(CN)]₂ + Zn ® K₂[Zn(CN)₄ ]+ 2 Au

Purification or refining of crude metals

The metals obtained by any of the above methods still contains some impurities like :

· Other metals originally present in the ore.

· Unrreduced oxides or sulphides of metals.

· Residual slag or flux and

· non-metals like carbon, silicon, phosphorus etc.

The process of removal of impurities from the crude metal is termed as refining. The method used for refining of the metal depend upon the nature of impurities. Some of the methods are :

1. Distilation : This is useful for low boiling metals. Examples are purification of zinc and mercury.

2. Liquation : The process is applied to the metals having low melting points such as tin or lead. The impure metal is placed on the sloping hearth maintained at the temperature above the melting point of the metal. The metal melts and flows down the sloping hearth into receiver leaving behind the solid impurities.

3. Poling : In this method, the impure metal is melted and the molten metal is stirred with the logs of green wood. The impurities are removed either as gases or they get oxidised and form a scum over the surface of molten metal.

4. Electrolysis

This method is widely used in the refining of copper, silver. Gold, lead, zinc , aluminium etc. The impure metal is made the anode and a pure strip of the same metal the cathode in a suitable electrolytic bath. The process takes place as:

Anode : M ® Mⁿ⁺ + n e

Cathode : Mⁿ⁺ + n e ® M

The net result is the transfer of pure metal from anode to cathode. The applied voltage is such that more electropositive metals (impurity) remain as ions in the bath whereas the less electropositive metals(impurities) remain unionised and fall down as anode mud. Thus in electrolytic refining of copper, more basic metals like zinc remain in the solution as cations, whereas less basic ones like Te, Ag, Au etc go into the anode mud.

Electrolytic refining

5. Cupellation : This is an oxidation method for the refining of certain metals. This method is used for refining those metals in which the impurities have greater tendency to get oxidised than the metal itself. Silver is refined by this method. The impure metal is fused in small boat shaped dishes made of bone ash called cupels. The cupels are heated in a suitable furnace by blast of air blown over them. The lead (impurity) is easily oxidised to lead monoxide(PbO) and is carried away by the blast, while pure silver is left behind.

6. Zone refining : This method is particularly used when the metals are required in high degree of purity. In this method, a metal rod is placed inside a small high frequency induction furnace. A narrow zone of metal is melted. The furnace is slowly moved along the rod. The pure metal crystallises out of the melt while impurities remain in the melt which moves along with the melted zone of the rod with the movement of the furnace. The process is repeated several times. The end of the rod where the impurities have collected is cut off. This method is employed for the purification of germanium, silicon, gallium, etc. which are used in semiconductors.

Zone refining of metals

7. Vapour phase refining

(i) Van arkel method : This method is used for getting ultra pure metals. In this method, the metal is converted to a volatile unstable compound (e.g. iodide) taking care that the impurities are not affected during compound formation. The compound thus obtained is decomposed to get pure metal. This method is used for the purification of metals like titanium and zirconium.

(ii) Mond process : Nickel when heated in a stream of carbon monoxide forms volatle nickel carbonyl, Ni(CO)₄. The carbonyl vapour when subjected to still higher temperature undergoes thermal decomposition giving pure metal.

8. Chromatographic method

The principle of separation and purification by chromatography is that the various components of a mixture are differently adsorbed on an adsorbent. In column chromatography an adsorbent (eg. Al₂O₃ ) is packed in a glass column. The mixture to be separated or purified , taken in a suitable solvent , is applied on the top of the column. The components of the mixture get adsorbed on the column. They are then eluted with a suitable eluent (solvent). The weakly adsorbed component is eluted first followed by more strongly adsorbed and so on. The method is especially suitable for such elements which are available only in minute quantities and the impurities are not very much different in their chemical behaviour from the element to be purified.

SODIUM

Extraction : Sodium occur in nature as:

(i) Rock salt deposits. : NaCl

(ii) Sea water

(iii) Sodium carbonate : Na₂CO₃

(iv)

Chile

salt petre NaNO₃

(v) Albite (Soda feldspar) : NaAlSi₃O₈

(vi) Borax : (Na₂B₄O₇. 10 H₂O)

Sodium is obtained by the electrolytic reduction of fused mixture of sodium chloride(40%) and calcium chloride (60%) at 1123 K, in Downs cell.

Down’s cell

The cell consists of a steel tank lined with fire bricks. A circular graphite anode is placed in the centre of the cell. This is surrounded by a ring shaped steel cathode. The anode and cathode are separated by a steel gauze diaphragm . The steel gauze diaphragm prevents the contact of sodium (liberated at cathode) from the chlorine (liberated at anode) otherwise they will combine together to form sodium chloride. The anode is covered by a conical hood which provides the outlet for the escape of chlorine gas. The cathode is provided with a circular trough attached to a pipe connected to a reservoir to collect molten sodium produced in the process.

The melting point of NaCl is 1080 K, which can be lowered to 850 K by the addition CaCl₂ to it. A fused mixture of 40% NaCl and 60% CaCl₂ is taken in the cell and electric current is passed through it. The following reactions take place.

On account of electrolysis , sodium is liberated at the cathode. Due to high temperature in the cell , it is in the molten state and rises up in the pipe attached to the circular trough kept on the cathode and collects in the receiver. Chlorine is liberated at the anode and is drawn out from the hood kept at the anode. The sodium metal obtained by this method is 99.8% pure. It contains some calcium (less than 1%) which separates almost completely if the metal is allowed to cool slowly. Chlorine is obtained as a byproduct in this process.

USES

1. An alloy of sodium with mercury (sodium amalgam) is used in the preparation of a number of organic compounds.

2. Sodium vapour lamps are used for lighting.

3. Sodium is used as reagent to detect the presence of nitrogen, sulphur and halogens in organic compounds.

4. It is used to prepare a number of compounds like NaOH, KOH, NaCN , NaNH₂ etc.

5. In the molten state it is used in nuclear reactors as heat transfer medium.

6. An alloy of sodium-potassium is used in high temperature thermometers.

ALUMINIUM METAL EXTRACTION

Occurrence : Aluminium occurs widely as a constituent of rocks and soils. The main ores of aluminium are given below :

(i) Bauxite : Al₂O₃ . 2 H₂O

(ii) Cryolite : Na₃AlF₆

(iii) Feldspar : KAlSi₃O₈

(iv) Mica : KAlSi₂O₁₀(OH)₂

Extraction of aluminium

Aluminium is normally extracted from bauxite ore, Al₂O₃ . 2 H₂O. It involves two steps :

(i) Purification of Bauxite

(ii) Electrolysis of alumina

In the first stage, pure alumina(Al₂O₃) is obtained from bauxite and in the second stage , electrolysis of Al₂O₃ in molten cryolite (Na₃AlF₆) is carried out to obtain aluminium metal.

(i) Purification of Bauxite : Bauxite contains SiO₂, iron oxides and titanium (IV) oxides as impurities. The bauxite ore is digested with a concentrated solution of sodium hydroxide at 473-523 K and 35 – 36 bar pressure. Aluminium oxide and silica dissolve to form sodium aluminate and sodium silicate respectively leaving behind iron oxide and TiO₂ which are filtered off.

Al₂O₃(s) + 2 NaOH(aq) + 3 H₂O ® 2 Na[Al(OH)₄](aq)

The filtrate containg sodium aluminate and sodium silicate is diluted and seeded with freshly precipitated aluminium hydroxide leaving behind sodium silicate in solution.

The aluminium hydroxide is filtered, dried and calcined at 1473 K to yield pure alumina.

(ii) Electrolysis of fused alumina : Aluminium is obtained from alumina by electrolysis ; this is known as Hall – Heroult Process. The modern electrolysis process uses synthetic cryolite , Na₃AlF₆. Typical electrolyte composition ranges are Na₃AlF₆ (80 – 85%), CaF₂ ( 5 – 7%). AlF₃ ( 5 – 7%) , Al₂O₃( 2 – 8% intermittenly recharged). The electrolysis of this mixture is carried out in an electrolytic cell (Fig) using carbon electrodes.

Electrolysis of fused alumina

The oxygen liberated at the anode reacts with carbon anode producing CO and CO₂. The overall reactions may be written as :

Cathode : Al³⁺(aq) + 3 e^-® Al(ℓ)

Anode : C(s) + O²^- (melt) ® CO(g) + 2 e^-

C(s) + 2 O²^- (melt) ® CO₂(g) + 4 e^-

For each kilogram of aluminium produced , over 0.5 kg of carbon anode is burnt away. Because of this anodes need to be replaced periodically.

Refining of aluminium

Molten aluminium obtained from the above step is refined by Hoope’s process. In this process , impure aluminium is refined electrolytically. The electrolytic cell (Fig) contains three liquid layers of different specific gravities. The upper most layer consists of pure molten aluminium having carbpm electrodes dipping in it. This layer acts as cathode. The middle layer consists of fluorides of sodium , barium and aluminium. This layer serves the purpose of electrolyte. The bottom layer consists of impure aluminium (in the molten state) having a carbon electrode. This layer acts as anode.

Refining of aluminium by Hoope’s process

On passing electric current, aluminium ions from the fused electrolyte pass into the upper most layer and get discharged. Pure aluminium thus set free collects in this layer. At the same time, an equivalent quantity of aluminium from the bottom layer goes into the middle layer. The impurities are left in the bottom layer itself. Impurities are left in the bottom layer itself. Impure aluminium is added from time to time to the bottom layer. Aluminium obtained from the top layer is 100% pure.

Iron

Iron is the second most abundant metal occuring in earth’s crust. It is a reactive metal and does not occur in free state. In combined state , it occurs as oxides, carbonates and sulphides. The common ores of iron are :

(i) Haematite : Fe₂O₃(red oxide of iron)

(ii) Magnetite : Fe₃O₄ (magnetic oxide of iron)

(iii) Limonite : Fe₂O₃ . 3 H₂O (hydrated oxide of iron)

(iv) Iron pyrites : FeS₂ (v) Siderite : FeCO₃

Commercial variety of iron

There are three varieties of iron :

1. Cast iron or pig iron

It contains 2 to 4.5% of carbon , along with impurities such as sulphur, silicon, phosphorus, manganese etc. It is the least pure form of iron. It is brittle and cannot be welded.

2. Wrought iron

It is the purest form of iron and contains carbon and other impurities not more than 0.5% . It is malleable and and can be welded.

3. Steel

It contains 0.5 to 1.5% carbon along with small amounts of other elements such as manganese, chromium, nickel, etc. and other impurities. It comes in between cast iron and wrought iron and exhibits intermediate properties.

Extraction of iron

The cast iron is usually extracted from its oxide ore (haematite). This process involves the following steps:

1. Concentration

The ore is crushed in jaw crushers and is broken to small pieces of about one inch in size. The crushed ore is concentrated by gravity separation process in which it is washed with water to remove clay, sand, etc.

2. Calcination

The concentrated ore is then calcined (heated strongly in presence of limited supply of air ) in a reverberatory furnance . During this process, the following changes take place:

(i) Moisture is removed.

(ii) The impurities such as sulphur, phosphorus and arsenic are converted to their gaseous oxides which is volatile and escape.

S + O₂ ® SO₂

4 As + 3 O₂ ® 2 As₂O₃

P₄ + 5 O₂ ® P₄O₁₀

If some ferrous carbonate is present , it changes to oxide.

FeCO₃ ® FeO + CO₂

4FeO + O₂ ® 2 Fe₂O₃

Ferric oxide

This prevents the loss of iron due to the formation of ferrous silicate (slag) during smelting.

(iii) The entire mass becomes porous which helps in the reduction process at a later stage.

3. Smelting

The calcined ore is reduced with carbon, i.e is smelted in the blast furnace (Fig).

The calcined ore (8 parts) is mixed with coke(4 parts) and lime (1 part) and is introduced from the top through the cup and cone arrangement. At the same time , a blast of hot air is blown upwards with the help of tuyers arrangement. The added coke serves as a fuel as well as a reducing agent while added lime seves as a flux.

Blast furnace for manufacture of cast iron

The following reactions take place in the furnace :

(i) Combustion zone

At the base, coke burns to produce carbon dioxide which starts rising upward during the reaction. The reaction is exothermic and heat produced raises the temperature to about 1775 K. This region is called combustion zone.

C + O₂® CO₂ : DH = - 393.4 kJ

(ii) Fusion zone

As carbon dioxide rises upward, it comes in contact with layers of coke and gets reduced to carbon monoxide.

C + CO₂® 2 CO : DH = + 163.2 kJ

This is an endothermic reaction and therefore the temperature is lowered to 1475 – 1575 K. The iron produced in the upper region melts here. Any Fe₂O₃ if present undergoes reduction by hot coke to iron. This region is called fusion zone.

Fe₂O₃ + 3 C ® 2 Fe + 3 CO + heat

(iii) Slag formation zone

In the middle portion of the furnace, the temperature is about 1075 to 1275 K. In this region lime stone decomposes to produce lime(CaO) and carbondioxide(CO₂). The lime thus produced acts as a flux and combines with silica (present as impurity) to produce slag.

CaCO₃ ® CaO + CO₂

(lime stone)

CaO + SiO₂ ® CaSiO₃

Flux (lime)

The molten slag forms a separate layer (being lighter) above the molten iron. This region is called slag formation zone.

(iv) Reduction zone

The temperature near the top of the furnace is of the order of 875 K. The oxides of iron are reduced by carbon monoxide to iron.

Fe₂O₃ + 2 CO ® 2 FeO + CO₂

FeO + CO ® Fe + CO₂

The region of the furnace is called reduction zone.

The spongy iron produced in the reduction zone moves slowly and melts in the fusion zone. It dissolves some carbon , silicon and phosphorus and forms the lower layer at the base of the furnace. It is removed from the tapping hole from time to time. The iron thus obtained is called cast iron or pig iron.

Preparation of wrought iron

Wrought iron is the pure form of iron and contains less than 0.5% impurities. The cast iron obtained above contains about 2.5 – 5% carbon and other impurities such as S, P, Si and Mn. In order to convert cast iron into wrought iron, the percentage of carbon and that of other impurities has to be decreased. This is done by heating the cast iron on the hearth of a reverberatory furnace (known as puddling furnace) with haematite (Fe₂O₃). The haematite supplies the oxygen and oxidises carbon, silicon, manganese and phosphorus present in cast iron to carbon monoxide (CO) , silica (SiO₂), manganese oxide(MnO) and phosphorus pentoxide (P₂O₅) respectively.

Thus,

Whereas CO and SO₂ escape, MnO and silica (SiO₂) combine to form manganous silicate(MnSiO₃) as slag.

Similarly, phosphorus pentoxide combines with Fe₂O₃ to form ferric phosphate slag.

2 Fe₂O₃ + P₄O₁₀® 4 FePO₄

ferric phosphate (slag)

Wrought iron thus prepared contains about 0.2% of carbon and some traces of P and Si in the form of slag.

Steel

Steel may be broadly classified as mild steel (0.1 – 0.5% C ) and hard steel (0.6 – 1.5% C). Nowadays , bulk of pig iron is converted into steel. The mild steel is cheaper than the wrought iron and stronger and more workable than cast iron ; it has also the advantage over both in that it can be hardened by heating to redness and then cooled rapidly (quenching ) in water and ‘tempered ‘ by reheating to 473 K to 573 K and cooling more slowly. The hardness , resilience and ductility can be controlled by varying the temperature and rate of cooling as well as the precise composition of the steel.

Alloy steel

Alloy steel with their enormous variety of physical properties are prepared by the addition of the appropriate alloying metal or metals( e.g., Mn, Cr, Ni , W) . Thus, stainless steel contains 18% of chromium ; tungsten steel (which is very hard) about 5% of tungsten ; manganese steel (which is very tough) about 13% of manganese .

Steel Making Processes

(i) Bessemer Process

The molten pig iron is fed into a converter at a temperature of about 1473 K. A blast of oxygen diluted with either steam or carbondioxide is blown through the converter. Oxygen reacts with impurities and raises the temperature to 2173 K. Carbon is oxidised to carbon monoxide which burns off at the mouth of the converter; oxides of silicon and manganese form slag. After about ten minutes , the flame dies down indicating that all carbon has been removed. The flame is stopped, slag is tapped off and then other metals (Mn, Cr, Ni and W) may be added towards the end of the operation to produce the required type of steel.

(ii) Open-Hearth Process

In this process , a mixture of molten pig iron , scrap steel , iron ore and lime stone is heated on a shallow hearth furnace by producer gas. The furnace is adapted for different types of pig iron feed by using acidic or basic lining. The impurities are oxidised by the iron oxide present which form a slag by combining with the lining. Thus,

3 C + F₂O₃ ® 2 Fe + 3 CO

Oxides of P and Si + lining (CaO + MgO) ®

Phosphate and silicate slag

Towards the end (after about 10 hours) an alloy of Mn, Fe and C (speiegelesin) is added together with alloying metals.

The Open-Hearth process has advantages over Bessemer’s process largely because of greater ease with which the composition of the steel can be controlled and greater fuel economy.

(iii) The Oxygen Top-Blowing Process

In this process , liquid iron from the blast furnace is charged into a converter, scrap steel is added and jet of oxygen is blown through retractable steel ‘lance’ into or over the surface of the liquid metal. The impurities are oxidised and with the addition of lime form slag, which is usually removed by tilting the converter. When steel of desired composition is obtained, the oxygen is turned off and the molten steel is poured into laddler for casting into ingots.

The oxygen top-blowing process

(iv) The Electric Arc Process

A charge of scrap steel and turnings is fed into the furnace and is melted by electric arc struck between adjustable carbon electrodes. Again acidic or basic linings are employed for scrap differing in phosphorus content. This method is widely used in the manufacture of alloys and other high quality steels such as stainless steel and high-speed cutting steel.

Electric arc process

(v) The High-Frequency Induction Process

A charge of alloy scrap of known composition, together with iron is fed into the furnace. Alternating current at 500-2000 Hz passes through the insulated water-cooled copper coils. The resulting magnetic field sets up steady current, which generates heat. The circulation of the metal caused by these currents produces strong stirring effect. The induction furnace is capable of producing high quality alloy steels containing tugsten , vanadium , chromium , manganese, molybdenum, cobalt and nickel for making ball-bearing, magnets, dies and tool steel etc.

High –frequency induction process

Properties of steel

The properties of steel and its hardness depends upon its its carbon contents and heat treatment.

(a) Properties based on carbon conents

Based on carbon content, there are three types of steel :

(i) Mild steel : It has the least percentage of carbon (0.1 – 0.5%). It is used for the manufacture of wires and sheets.

(ii) Medium steel : It contains 0.2 to 0.5% carbon and is harder than mild steel. It is used for constructing rails, wheels and also used in buildings.

(iii) Hard steel : The carbon contents vary from 0.6 to 1.5% . It is very hard and it is used in making parts of machines.

(b) Properties based on Heat Treatment

The hardness of steel can be increased or decreased by heat treatment as described below :

(i) Quenching : If a steel article is heated to redness and then suddenly cooled by plunching into water or some oil, the steel becomes hard and brittle. This treatment is called quenching or hardening of steel.

(ii) Tempering : When the quenched or hardened steel is heated to 500 – 575 K temperature and kept at that temperature for some time and then cooled slowly, the steel obtained becomes slightly less hard and tough. This process is called tempering of steel and steel thus obtained is called tempered steel. The properties of tempered steel depend on the temperature and time of tempering.

(iii) Annealing : If quenched steel is heated to temperature below red hot and then allowed to cool slowly, it becomes soft. This process is called annealing.

Passive Iron

When a piece of iron is dipped in concentrated nitric acid, a reaction takes place which stops after some time completely. The iron does not appear to undergo any change in appearance. But it becomes unreactive. It is due to the formation of a thin insoluble and invisible Fe₃O₄ film on the surface which prevents its further reactions.

COPPER

Occurrence

Copper does not occur abundantly in nature (about 1 x 10^-⁴ % of the earth’s crust). The chief ores of copper are :

(i) Copper glance Cu₂S

(ii) Copper pyrites CuFeS₂

(iii) Malachite Cu(OH)₂ CuCO₃

(iv) Cuprite Cu₂O

(v) Azurite 2 CuCO₃ Cu(OH)₂

Extraction of copper

Copper is mainly extracted from copper pyrites (CuFeS₂). The various steps involved in the extraction are :

1. Crushing and concentration : The ore is crushed in jaw crushers and finely powdered. It is concentrated by froth floatation process. In this process , finely divided ore is mixed with water and some pine oil in the tank. The mixture is agitated by blowing compressed air into it. The particles are preferentially wetted by oil and rise to the surface of tank in the form of 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.

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

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

S + O₂ ® SO₂

P₄+ 5 O₂® P₄O₁₀

4 As + 3 O₂ ® 2 As₂O₃

4 Sb + 3 O₂ ® 2 Sb₂O₃

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

2 CuFeS₂ + O₂ ® Cu₂S + 2 FeS + SO₂

2 FeS + 3 O₂ ® 2 FeO + 2 SO₂

2 Cu₂S + 3 O₂ ® 2 Cu₂O + 2 SO₂

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.

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

2 FeS + 3 O₂ ® 2 FeO + 2 SO₂

FeO + SiO₂ ® FeSiO₃

Flux ferrous silicate (slag)

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

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

2 Cu₂S + 3 O₂ ® 2 Cu₂O + SO₂

Cu₂O + FeS® Cu₂S + FeO

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

FeO + SiO₂ ® FeSiO₃

flux Slag

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. The lower layer of the molten mass contains mostly cuprous sulphide and some traces of ferrous sulphide. It is called matte and is taken out from the tapping hole at the bottom.

(iv) Bessemerisation : The molten matte from the blast furnace is transferred into a Bessemer converter (Fig).

Bessemer converter

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

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

2 FeS + 3 O₂ ® 2 FeO + 2 SO₂

FeO + SiO₂ ® FeSiO₃ (slag)

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

2 Cu₂S + 3 O₂ ® Cu₂O + 2 SO₂

Cu₂S + 2 Cu₂O ® 6 Cu + SO₂

After the reaction has completed, the converter is tilted and the molten copper is poured into sand moulds. 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 sulphur dioxide escapes producing blisters on metal surface.

(iii) Refining : The blister copper is purified as follows :

(a) Poling : The blister copper is purified by heating it strongly in a reverberatory furnace in 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. Further purification is done by electrolytic refining.

(b) Electrolytic refining : The crude copper is further purified by electrolytic method. In this method, a thin sheet of metal is made as cathode and 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 - 2 e- ® Cu²⁺

At cathode

Cu²⁺ + 2 e- ® Cu

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

Alternate method

Prelonged exposure of copper pyrites to air and rain leads to formation of a dilute solution of copper sulphate, from which the metal is precipitated by addition of scrap iron. It is always refined electrolytically.

STEEL AND IMPORTANT ALLOYS

Alloys are homogeneous mixtures of two or more metals and a metal and a non-metal. Transition metals have a good tendency to form alloys.

Steel obtained by addition of some other one or more metals such as Cr, V, Ti, Mo, Mn, Co or Ni to carbon steel are called alloy steels. The metals are added to obtain some special properties to steel. Some special alloy steel , their composition , properties and uses are given below.

Uses

Amalgam is an alloy of a metal with mercury. Sodium amalgam is used as a reducing agent. An amalgam of tin is used for coating mirror and mercury-silver-tin alloy is employed for dental filling. Other uses of mercury include the production of mercury drugs and detonators. It is used in thermometers, botton cells , vaccum pumps and fluorescent lamps.

Some important alloy steels

Name	Percentage composition	Poperties	Uses
1. Stainless steel	Fe = 73 , Cr = 18, Ni = 8	Resistance to rusting	Utensils, cycle and automobile parts, cutlery
2. Nickel steel	Fe=96-98, Ni = 2 - 4	Hard, elastic and rust proof	Cables, automobiles and aeroplane parts, armour plates, gears
3. Invar	Fe = 64, Ni = 36	Low expansion on heating	Meter scales, measuring instruments, clock pendulms
4. Chrome steel	Fe = 98 , Cr = 1.5 – 2.0	High tensile strength	Cutting tools such as files, cutlery
5. Tungsten steel	Fe = 94 , W = 5 and C	Hard , resistant to corrosion	High speed cutting tools, springs
6. Silicon steel	Fe = 85 , Si = 15	Hard and resistant to acids	Pumps and pipes for carrying acids
7. Alnico	Fe = 60 , Al = 12, Ni = 20, Co = 5	Strongly magnetic	Permanent magnets
8. Manganese steel	Fe = 86 , Mn = 13 and C	Extremely hard, resistant to wear and tear	Rock crushers, burglar proof safes, rail road tracks

Alloys of Transition Metals

Alloy	Percentage composition	Uses
Alloys of copper
Brass	Cu = 60 , Zn = 40	Utensils, condenser tubes, catridge caps
Aluminium bronze	Cu = 90, Al = 10	Coins, picture frames, golden powder for paints, cheap jewellery
Bronze	Cu = 90 , Sn= 10	Coins, statues, control valves
Bell metal	Cu = 80, Sn = 20	Bells, gongs
Gunmetal	Cu = 88, Sn = 10, Zn = 2	Gears, bearings, castings
German silver	Cu = 25 – 50, Zn = 25 – 35, Ni = 10 - 35	Utensils, resistence wire
Phosphor bronze	Cu = 95, Sn = 4.8, P = 0.2	Springs, electrical equipments
Monel metal	Cu = 30, Ni = 67, Fe and Mn = 3	Acid pumps and acid containers
Gold copper alloy	Au = 90 , Cu = 10	Gold coins, jewellery, watch cases, spectacle rims
Constantan	Cu = 60, Ni = 40	Electrical apparatus
Alloys of silver
Coinage silver	Ag = 90, Cu = 10	Silver coins
Silver solder	Ag = 63, Cu = 30, Zn = 7	Soldering and for joining metals
Dental alloy (dental amalgam)	Hg = 52, Sn = 12.5, Cu = 2, Zn = 0.5	For filling teeth
Palladium silver	Ag = 40, Pd = 60	Potentiometer and winding of some special instruments

Some important alloys of Aluminium

Name	Percentage composition	Poperties	Uses
1. Magnalium	Al 95% : Mg 5%	Very tough, strong , can be worked on a lathe,

QUESTIONS

1. What are minerals and ores ?

2. How elements are originated in the crust of earth ?

3. How , the elements higher than Fe were produced ?

4. Describe the main sources of earth.

5. Describe the chemical composition of core and mantle.

6. Write different types of igneous rocks and their chemical composition.

7. Describe the different types of ore.

8. What is meant by the term metallurgy ? Name the various steps.

9. Why is iron abundant element on earth and why are the elements with higher atomic numbers increasingly rare ?

10. Copper and silver are below hydrogen in the electrochemical series and yet they are found in the combined state as sulphides in nature. Coment.

11. Describe the principle of froth flotation process ? What is the role of stabiliser and depressant ? Give one example each.

12. Describe the principle of the following process in detail :

(i) Mond’s process (ii) Zone refining

(iii) Electrolytic refining

13. You are provided with samples of some impure metals such as zinc, copper and germanium. Which methods would you recommend for the purification of each of these metals ?

14. Name the chief forms of the occurrence of the following in the earh’s crust : (a) Al (b) Ca (c) Na (d) Pb

15. Discuss some of the factors which need consideration before deciding on the methods of extraction of the metal from its ore.

16. The choice of reducing agent in a particular case depends on thermodynamic factor. How far do you agree with this statement ? Support your opinion with examples.

17. Which is a better reducing agent at 710°C , C or CuO ?

18. Indicate the temperature at which carbon can be used as a reducing agent for FeO.

19. Is it true that under certain conditions Mg can reduce SiO₂ and Si can reduce MgO ?

20. Giving appropriate example, explain how the reactivity of a metal is related to its mode of occurrence in nature.

21. Name three metals which are obtained by the reduction of their oxides though they do not occur as such in earth’s crust.

22. Outline the principles of refining of metals by the following methods :

(i) Electrolytic refining (ii) Zone refining (iii) vapour phase refining.

23. Predict the modes of occurrence of the following three types of metals :

(i) Highly reactive ( e.g. Na)

(ii) Moderately reactive ( e.g. Fe)

(iii) Noble metal ( e.g. Au)

24. How do non-metals occur in nature ? How are they extracted/isolated from their natural sources ?

25. Name the process from which chlorine is obtained as a bye product.

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

27. Name the ores of tin, iron and aluminium. What methods are employed for the concentration / purification of their ores ?

28. Name three ores which are concentrated by froth floatation process. What is meant by a depressant ?

29. What is the thermodynamic consideration in the choice of a reducing agent in metallurgy ?

30. Carbon monoxide is more effective agent than carbon below 983 K but above this temperature the reverse is true. How would you explain this ?

31. Describe the principle of extraction of each of the following :

(i) Sn from SnO₂ (ii) Zn from ZnO (iii) Cr from Cr₂O₃.

32. Which metals are generally extracted by the electrolytic processes ? What positions these metals generally occupy in the periodic table ?

33. Name the main steel plants which are operated by the Steel Authority of India.

34. Name the metals which ae associated with the following terms in their extraction from their ores :

(i) Bessemer converter (ii) Blast furnance (iii) Aluminothermic process (iv) Magnetic separation.

35. What do you understand by the following terms ?

(i) Roasting (ii) Calcination (iii) Smelting

36. Which method would you suggest for the separation of the metals in the following mixtures ?

(i) zinc and iron (ii) Copper and magnesium

(iii) Rare earths . Give reasons.

37. Why donot metals occur as nitrates in nature ?

38. Sea water contains ions such as Na⁺, K⁺ and Mg²⁺ ions but it does not contain Cu²⁺, Ba²⁺ and Pb²⁺ ions. Why ?

39. What are ores and minerals ? Illustrate with a suitable example.

40. What is flux and slag as applied to metallugy ?

41. Copper and silver are below hydrogen in electrochemical series and yet they are found in the combined state as sulphides in nature. Comment.

42. Describe the principle of each of the following processes in detail :

(a) Mond’s process (b) Zone refining (c) Electrolytic refining

43. You are provided with samples of some impure metals such as zinc, copper and germanium. Which methods could you recommend for the purification of each of these metals ?

44. Describe some of the factors which need consideration before deciding of extraction of metal from its ore.

45. Name the process from which chlorine is obtained as a by-product. What will happen if an an aqueous solution of NaCl is subjected to electrolysis ?

UNIT 10 METALLURGY

Some important alloy steels

Alloys of Transition Metals

Alloys of copper

Alloys of silver

PLUS TWO

QUESTIONS