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1, , Metallurgy, Concepts, , (i) What is metallurgy (ii) Minerals and Ores, (iv) Processes involved in extraction of metals, , (iii) Occurrence of metals, , Introduction, Copper and iron were the first metals put to extensive use by early man. Copper mixed, with tin called bronze was so widely used for many years that this period came to be, known as the ‘Bronze Age’. The increasing use of metals in day to day life aroused the, interest of man in their properties and the sources from which they could be recovered., This gave birth to a new branch in chemistry called metallurgy., , The science that deals with procedures used in extracting metals from their, ores, purifying and alloying metals and creating useful objects from metals, is called metallurgy., , Metallurgy is also the practice of removing valuable metals from an ore and refining the, extracted raw metals into a purer form., Mineral and Ore, Natural materials found inside the earth containing metals in their combined states ( as a, single compound or as a mixture of compounds ) mixed with non – metallic impurities of, Earth and rock ( called gangue ) are termed minerals. These metals do not occur in the, native form and need to be extracted for use. All minerals are not suitable for extraction, of metals. Minerals from which the metal can be extracted easily and economically are, called ores. Ores contain metal compounds with a lower percentage of impurities. Thus, all ores are minerals but all minerals are not ores., Occurrence of metals, Metals occur in nature in the free as well as in the combined states. The most unreactive, metals i.e. which are not affected by air and water, like silver, gold and platinum are, generally found in the free state. In other words, elements which have low chemical, reactivity generally occur native or free or in metallic state and those which are, chemically reactive or affected by air and water generally occur in combined state e.g., halogens, chalcogens.

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2, , Ores can be divided into four general groups as follows., (i) Native ores – These ores contain metals in the free state. e.g. Ag, Au, Pt, Hg, Cu etc., These are usually found associated with rock or alluvilial materials like clay, sand etc., Sometimes, lumps of pure metals are also found in them. These are termed nuggets., Iron is found in free state as meteorites which also have 20% to 30 % nickel., (ii) Sulphurised and arsenical ores - These ores consist of sulphides and arsenides in, simple and complex forms of metals. Some examples of this group are PbS, ZnS,, Ag2S, NiAs, CuFeS2, 3Ag2S. Sb2S3 etc., (iii) Oxide ores - In these ores, metals are present as their oxides or oxysalts such as, carbonates, nitrates, sulphates, phosphates, silicates etc. The examples include Fe2O3,, Al2O3, BeO.Al2O3, MnO2, CaCO3, FeO.TiO2 , NaNO3, BaSO4 , Zn2SiO4, Ca3(PO4)2, etc., (iv) Halide ores - Metallic halides are very few in nature. Chlorides are more common., The examples include common salt, NaCl, , Carnallite, KCl, MgCl2.6H2O ,, Fluospar, CaF2 , Horn silver, AgCl etc., An overview of the processes, The various processes involved in the extraction of metals from their ores and their, subsequent refining are known as metallurgy. An overview of various processes involved, during metallurgy is given below.

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3, Enrichment of ore, This is also called as concentration or dressing of the ore. Ore is an impure metal, containing large amount of sand and rocky material. The impurities like sand, rocky, materials, limestone, mica etc. is called gangue or matrix. These impurities must be, removed from the ore before the extraction of metal. Sometimes the ore appears in the, form of big lumps. So it becomes necessary to break those lumps into small pieces., Crushing and grinding of the ore : These ores occur in nature as huge lumps. They are, broken to small pieces with the help of crushers or grinders. These pieces are then, reduced to fine powder with the help of a ball mill or stamp mill. This process is called, pulverisation., Depending upon the nature of the ore, one or more of the following steps are taken to, concentrate the ore. These are mostly physical methods of concentration., Concentration or dressing of the ore : These ores are usually obtained from the ground, and therefore contain large amount of unwanted impurities, e.g., earthing particles, rocky, matter, sand, limestone etc.. It is essential to separate the large bulk of these impurities, from the ore to avoid bulk handling and in subsequent fuel costs. The removal of these, impurities from the ores is known as concentration. The concentration is done by, physical as well as chemical methods, (1) Hydraulic washing ( Gravity separation ) : In this process, the ore particles are, poured over a hydraulic classifier which is a vibrating inclined table with grooves and a, jet of water is allowed to flow over it. The denser ore settles in the grooves while the, lighter gangue particles are washed away. This method is used for concentration of heavy, oxide ores of lead, tin, iron etc. The hydraulic washing method is shown in the following, figure., , Fig.1 - Hydraulic washing

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4, (2) Froth floatation - This method is especially used for sulphide ores. The method, employs a mixture of water and pine oil which is made to agitate with the ore., A mixture of water, pine oil, detergent and powdered ore is first taken in a tank. A blast, of compressed air is blown through the pipe of a rotating agitator to produce froth. The, sulphide ore particles are wetted and coated by pine oil and rise up along with the froth, (froth being lighter). The gangue particles wetted by water sink to the bottom of the tank, ( water being heavier ). Sulphide being more electronegative attracts the covalent oil, molecules. The gangue being less electronegative is attracted by the water. The froth, containing the sulphide ore is transferred to another container, washed and dried. Thus, sulphide ore is separated from the gangue. The froth floatation process is shown in the, following figure., , Fig.2 – Froth floatation process, (3) Magnetic separation – Magnetic ores like pyrolusite ( MnO2) and chromite, ( FeO.Cr2O3 )are enriched by this method by making use of the difference in the, magnetic properties of the ore and gangue particles. The powdered ore is dropped on to, leather or brass conveyer belt, which moves over two rollers one of which is magnetic., When the ore passes over the magnetic roller, it sticks to the belt due to the force of, attraction and falls nearer to magnetic roller. The gangue falls in a normal way under the, influence of gravity. The magnetic ore and gangue thus form two separate heaps., Following figure shows the magnetic separation method.

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5, , Fig.3 – Magnetic separation process, (4) Leaching – It means washing. In this process, the ore is washed with some suitable, reagent ( solvent ) so that the main metal passes into its salt solution. This solution is, separated and subjected to further treatment like precipitation. It is then treated further to, recover the metal. This is a chemical method of concentration., Conversion of concentrated ore to oxide, It is easier to obtain a metal from its oxide form as compared to its sulphide, carbonate or, any other form. Therefore, prior to reduction usually the metal is converted to its oxide, form. Following methods are used to convert the concentrated ore to its oxide form., 1) Calcination – It is a process in which the ore is heated strongly in absence of air. The, ore is heated at a temperature well below its melting point. The ore gets thermally, decomposed, undergoes phase transformation and eliminates the volatile impurities like, moisture, carbon dioxide etc. Since the ore becomes porous and compact , it easily, undergoes further chemical reactions. This method is generally used for carbonate and, hydrated ores., 2) Roasting – It is a process wherein the ore is heated either alone or with some other, material in excess of air below the fusion point of the ore. Usually, this method is used, for sulphide ores. In roasting, definite chemical changes take place to form oxide or, chloride of the metal. Ores of metals like zinc, lead, copper and nickel, when roasted in, air, are converted to their oxides. Ores of some metals like lead may get partially, oxidized and converted to sulphate. In such case, it is called sulphating roasting or partial, roasting. Ores of metals like silver and gold are mixed with common salt and are heated, in air. They are converted to their chlorides which are easy to reduce. This type of, roasting is called chlorinating roasting. The purpose of roasting is to convert the ore in a, form suitable to reduce. The gaseous product of sulphide roasting, sulphur dioxide, is, often used to produce sulphuric acid.

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6, Reduction of oxide to a metal, One or more of the following methods can be used to obtain a metal from its oxide., (i) Heating – Less reactive metals like silver and mercury can be obtained by heating, their oxides alone. These metals are placed at the bottom of the electrochemical series., 2 HgO, , → 2 Hg + O2, , ;, , 2 Ag2O, , → 4Ag + O2, , (ii) Carbon reduction - Some metals like lead, copper, zinc and iron can be obtained by, reducing their oxides by carbon. When the oxides of these metals are heated with coke,, the oxides are reduced to a metal. Carbon has more affinity for oxygen than the metals, have it for oxygen hence carbon extracts the oxygen leaving behind the free metal., PbO + C → Pb + CO, ZnO + C → Zn + CO, , ;, ;, , CuO + C → Cu + CO, Fe2O3 + 3 C → 2 Fe + 3CO, , (iii) Use of carbon monoxide – Oxides of metals like PbO, CuO, FeO can be reduced by, carbon monoxide at high temperature to give the corresponding metals., CuO + CO → Cu + CO2 :, , FeO + CO → Fe + CO2 ; PbO + CO → Pb + CO2, , (iv) Use of aluminium – Oxides of metals like ZnO, Cr2O3 and MnO2 can not be, reduced by carbon because these metals have a greater affinity for oxygen than carbon., An active metal like aluminium ( in the form of powder ) is required to reduce the oxides, of these metals. The reduction of a metal oxide by heating with aluminium is called, aluminothermy. In this process, lot of heat is evolved and hence the metal may melt in, the container. It is tapped from the bottom of the container., Cr2O3 + 2 Al → Al2O3 + 2 Cr + Heat ; 3MnO2 + 4Al → 2 Al2O3 + 3 Mn + Heat, (v) Electrolysis – The metals like iron, zinc, lead, chromium, manganese lie in the, middle of the electrochemical series. They are somewhat active. So their oxides can be, reduced by carbon or carbon monoxide or reactive metals like aluminium or sodium,, calcium . But the metals like sodium, magnesium, calcium, aluminium which are placed, in the top of the electrochemical series i.e. which are very active, can not be obtained by, the reduction of their oxides by ordinary reducing agents. So they are obtained by passing, an electric current through the purified molten ore. An electric current is passed through, the molten oxide or chloride of the metal. The metal gets deposited at the cathode from, where it is separated.

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7, Refining of metals, (i) Liquation – It is a technique for separating constituents of an ore, a metal, or an alloy, by partial melting. The technique is used when the melting temperature of the metal is, lower than that of the impurity and the impurities are not miscible with the metal. Metals, like Bi, Sn, Pb, Hg etc. are purified by this technique. The sloping floor of the, reverberatory furnace is used to melt the crude metal, when pure metal flows down and, impurities are left behind., , Fig. 4 - The liquation method, Electro- refining - Electro-refining of metals is a process of obtaining pure metal from, the impure one by the process of electrolysis., Process – The process of electro-refining of metals involves the following steps :, (i) The electrolyte is usually an aqueous solution of the salt of the metal with some, corresponding acid, if necessary., (ii) A thick block of impure metal is made as the anode., (iii) A thin rod or sheet of pure metal is made as the cathode., (iv) The metal cations being positive, migrate towards the cathode and get discharged., (v) At anode, the atoms of the metal lose electrons, form cations and enter the solution., (vi) The less electropositive impurities in the anode, settle down at the bottom and are, removed as anode mud while the more electropositive impurities pass into the solution., (vii) Anode finally disintegrates while the cathode gains in weight due to the collection of, pure metal. This way pure metal is obtained., Distillation - Metals like zinc and mercury which boil at low temperature are purified by, this method. The impure metal is taken in iron retort and heated strongly above the, boiling point of the metal. At the boiling point, vapours of the metal are produced which, are led to a condenser. By condensation of the vapours, pure metal is obtained., Activity 1 – Collect 2 to 3 ores from your teacher. Suggest the steps and draw a flow, sheet type diagram to recover the metal from its ore.

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8, , Test your understanding :, 1) What is the difference between a mineral and an ore ?, 2) What are the methods used to concentrate an ore ?, 3) What is the purpose of the following processes ?, (i) Calcination (ii) Roasting (iii) Magnetic separation (iv) Liquation, 4) Which reducing agents are used in metallurgy ?, , Aluminium and iron are two important metals which are used as construction materials., So also they find many applications in domestic life. Hence we will study the metallurgy, of these two metals here., , Metallurgy of aluminium, Concepts, , (i) Occurrence of aluminium (ii) Purification of bauxite (iii) Electrolysis of alumina, (iv) Refining of aluminium (v) Physical and chemical properties of aluminium, (vi) Uses of aluminium (vii) Alloys of aluminium, , Occurrence, Aluminium is the most abundant ( 8.13 % ) metallic element in the earth’s crust and after, oxygen and silicon, the third most abundant of all elements in the crust. Because of its, strong affinity to oxygen, it is not found in nature in the elemental state but only in, combined forms such as oxide or silicate., Aluminium occurs in igneous rocks chiefly as alumino silicate in feldspar, feldspathoids,, and mica; in the soil derived from them as clay and upon further weathering as bauxite, and iron-rich laterite. Bauxite, a mixture of hydrated aluminium oxides, is the principal, aluminium ore., Extraction of aluminium, The extraction of aluminium is called elctrometallurgy. It deals with the use of, electricity for smelting or refining of metals. In electrometallurgy, the electrochemical, effect of an electric current brings about the reduction of metallic compounds and thereby, the extraction of metals from their ores ( electro-winning ) or the purification of the, metals ( electro-refining.)

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9, The principal aluminium ore is bauxite Al2O3 . 2H2O. It is essentially an impure, aluminium oxide. The major impurities include iron oxide, silicon dioxide and titanium, dioxide., The extraction of aluminium, in principle , should be easy because the ore occurs in, oxide form which can be reduced by a suitable reducing agent to give the metal., However, in practice, aluminium oxide can not be reduced that easily. Aluminium has, great affinity for oxygen hence it can not be reduced by usual reducing agents, Aluminium is too high in the electrochemical series ( it is a highly reactive element ) so it, can not be reduced by hydrogen or carbon. If at all reduced by carbon, the temperature, required for the reduction is very high. This does not make the process economic. Hence, aluminium is obtained by the electrolysis of pure alumina., Purification of bauxite, Bauxite contains iron oxide or silica as major impurity. The bauxite containing iron oxide, as major impurity is called red bauxite and the bauxite containing silica as major, impurity is called white bauxite. Iron and silicon both make aluminium metal brittle and, liable for corrosion hence they must be eliminated. If bauxite contains iron oxide, Fe2O3, as the major impurity, it is purified by Baeyer’s process or Hall’s process. . If it, contains silica , SiO2 as the major impurity, it is purified by Serpek’s process., (i) Serpek’s process – This process is used when bauxite ore contains appreciable, amount of silica (above 7 %) and low amount of Fe2O3 ( less than 1 %) .Powdered, bauxite is mixed with carbon and heated up to 18000C in a current of nitrogen ., Aluminium from bauxite is converted to aluminium nitride while silica is reduced to, silicon., Al2O3 .n H2O + 3C + N2 → 2 AlN + 3 CO + n H2O, SiO2 + 2C → Si ↑ + 2 CO ↑, Silicon volatilizes at this temperature. Aluminium nitride is hydrolyzed with hot water. It, precipitates aluminium hydroxide., AlN + 3 H2O → Al(OH)3 ↓ + NH3, The precipitate of Al(OH)3 is washed, dried and ignited at about 15000C to get pure, alumina., 2 Al(OH)3 → Al2O3 + 3 H2O ↑, b) Baeyer’s process – This process is used when bauxite ore contains appreciable, amount of Fe2O3 ( 7 to 10 % ) and low amount of silica ( less than 1 % ). The ore is, first calcined and then finely ground. It is then digested with a hot and strong solution, of caustic soda ( 45 % ) in an autoclave under 80 lb. pressure at 1500C for 2 to 8 hours., At this stage, aluminium oxide dissolves in NaOH to form sodium meta aluminate

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10, ( NaAlO2) while ferric oxide and titanium dioxide remain undissolved.. They are then, removed by filtration., Al2O3 + 2 NaOH → 2 NaAlO2 + H2O, Sodium meta aluminate ( soluble ), Silica dissolves in the form of silicate. After filtration, sodium meta aluminate solution, is diluted with water, slowly cooled and then mixed with a little freshly precipitated, aluminium hydroxide which acts as a nucleus for precipitation of aluminium, hydroxide.( Alternatively CO2 can be passed till the solution becomes acidic ) It is, then digested. Sodium meta aluminate, NaAlO2 hydrolyses to give precipitate of, aluminium hydroxide., NaAlO2 + 2 H2O → NaOH + Al(OH)3 ↓, Aluminium hydroxide precipitate is then washed, dried and ignited to get pure, alumina ( Al2O3 ) .The filtrate containing caustic soda is concentrated and used again., 2 Al(OH)3 → Al2O3 + 3 H2O ↑, c) Hall’s process - This process is used for low grade bauxite ores. In this process,, bauxite ore is fused with sodium carbonate, Na2CO3 to give water soluble sodium meta, aluminate , NaAlO2 leaving behind Fe2O3 and SiO2., Al2O3 + Na 2CO3 → 2 NaAlO2 + CO2 ↑, The fused mass of sodium meta silicate is extracted with water and filtered. The, impurities Fe2O3 and SiO2, remain on the filter paper. The filtrate containing NaAlO2 is warmed and CO2 is passed, through it, when Al(OH)3 is precipitated., 2 NaAlO2 + CO2 + 3 H2O → 2 Al(OH)3 ↓ + Na2CO3, The precipitate is filtered, washed and ignited to obtain pure alumina., 2 Al(OH)3 → Al2O3 + 3 H2O ↑, Pure alumina, Electrolysis of pure alumina, Aluminium can be obtained by electrolysis of pure alumina but it offers two problems., (i) Pure alumina is a poor conductor of electricity and melts at about 20000C., (ii) When fused alumina is electrolyzed at 20000C ,the metal formed vapourises as its, boiling point is 18000C., Aluminium is usually prepared by Hall- Heroult process. Alumina is fused with, cryolite Na3AlF6. Alumina dissolves in cryolite. Cryolite lowers the temperature of the, mixture. Small amount of CaF2 and AlF3 are also added to lower the temperature of the, mixture. Pure alumina melts at 20000 C while the mixture melts at about 9500C The

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11, charge consists of cryolite ( 85 % ), CaF2 ( 5 % ), AlF3 ( 5 % ) and Al2O3( 5 % ) .The, electrolysis is carried out at temperature of 9500C and with a voltage of 5.5 volts in a, graphite lined steel tank which acts as a cathode. The anodes are made of graphite. The, Al2O3 is added from feeder at the top. Some coke is thrown on the surface of charge to, control the oxidation of the metal. The electrode reactions are complicated and their exact, nature is not known . The simplified mechanism of electrode reactions is given below., Na3AlF3 → 3 NaF + AlF3 ;, , 4 AlF3 →, , 4Al 3+ + 12 F-, , At anode → 2 Al2O3 + 12 F- → 4 AlF3 + 3 O2 + 12 e- ; 4C + 3O2 → 2 CO2 + 2 CO, At cathode → 4 Al3+ + 12 e- → 4 Al, Diagram –, , Fig. 5 - Electrolysis of alumina, Refining of aluminium metal, Metal produced by Hall – Herouit’s process is almost 99.9 percent aluminium, and it contains small amounts of iron, silicon from the bath and some alumina and, carbon. So Hoope’s electrolytic refining process is used to refine the metal., Hoope’s process – In this process, fused salt electrolyte is used. The cell uses three, liquid layers of different densities., (i) The bottom anode layer consists of impure aluminium., (ii) The middle layer consists of cryolite , alumina and barium fluoride acting as, electrolyte., (iii) The top cathode layer is of pure metal. This aluminium layer is connected with, graphite electrode to the mains.

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12, , Fig.. 6 - Hoope’s cell for refining aluminium, , The cell is made of iron box. It is lined from inside with carbon. The cell is shown in, Fig. 6 . On passing electric current, aluminium from the middle layer passes into the top, layer and equivalent amount of aluminium passes from the bottom layer to the middle, layer. From time to time , pure aluminium is removed from the top and aluminium of, lower purity is added to the bottom layer. Thus, there is transfer of aluminium from the, base to the top while impurities are left behind. Pure aluminium is tapped from the top., The refined aluminium has purity of 99.99 %., Physical and Chemical Properties of Aluminium, (i) Aluminium is a white metal with a slight bluish tinge. In moist air, it becomes dull, owing to the formation of superficial protective layer of its oxide., (ii) It is a light metal ( sp. Gravity 2.7 ) which melts at 6580 C and boils at 18000C., (iii) It is malleable and ductile especially between 1000C and 1500C. Near about its, melting point, it becomes brittle and can be ground to powder., (iv) It is an excellent conductor of heat and electricity., (v) It is tough and has a moderate tensile strength., (vi) Finely divided aluminium or thin aluminium foil burns readily in air or oxygen when, heated, forming aluminium oxide ( with little nitride AlN also ) liberating much heat., 4 Al + 3 O2 → 2 Al2O3 + Heat, (vii) Action of acids - Aluminium is above hydrogen in the activity series and it, displaces hydrogen from non-oxidising acids like HCl and dilute H2SO4., 2 Al + 6 HCl → 2 AlCl3 + 3 H2, 2Al + 3 H2SO4 → Al2(SO4)3 + 3 H2

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13, But hot concentrated H2SO4 gives SO2, 2 Al + 6 H2SO4 → Al2(SO4)3 + 3 SO2 + 6 H2O, Concentrated HNO3 makes aluminium passive and with dilute HNO3, it produces, ammonium nitrate but no gas is evolved., 8 Al + 30 HNO3 → 8 Al(NO3)3 + 3 NH4NO3 + 9 H2O, (viii) Action of alkalies - Aluminium is an amphoteric metal so it reacts with acids as, well as alkalies. ( The reaction with acids are given above ), ., 2 Al + 2 NaOH + 2 H2O → 2 NaAlO2 + 3 H2, (ix) Action of non-metals - Heated aluminium directly combines with halogens, carbon, nitrogen and sulphur., a) 2Al + 3 Cl2 → 2 AlCl3, c) 2 Al + N2 → 2 AlN, , b) 4 Al + 3 C → Al4C3, d) 2 Al + 3 S → Al2S3, , (x) Reducing action - At high temperature, aluminium has a strong affinity for oxygen, and hence it reduces oxides of iron, manganese etc., Fe2O3 + 2 Al → Al2O3 + 2 Fe + Heat, Uses of aluminium, (i) Since it is lighter and has high tensile strength, aluminium is used in making body of, air-ships and motor cars., (ii) On account of its good electrical conductivity, it is used for making electrical, transmission cables., (iii) On account of its good thermal conductivity, it is used in making cooking utensils., (iv) Since it resists corrosion, it is used in aluminium paints., (v) Aluminium foils are used in wrapping cigaretts, confectionary items etc., (vi) Aluminium is used as a deoxidizer and for removing blow holes in metallurgy., (vii) It is used in thermite welding and in the aluminothermic process., (viii) Salts of aluminium such as alum are used as mordants in dyeing industries.

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14, Alloys of aluminium, Alloy, Composition, Properties, Uses, -----------------------------------------------------------------------------------------------------------1) Magnalium, 98 % Al, 2 % Mg, Hard, tough, light,, For making balance, Can be excellently, beams, light, worked on lathe, instruments, articles, -----------------------------------------------------------------------------------------------------------2) Duralium, 95 % Al, 4 % Mn,, Resistance to, For making airships,, 0.5 % Mg, corrosion, highly, aeroplanes etc., ductile, light, -----------------------------------------------------------------------------------------------------------3) Aluminium, 10 t0 12 % Al,, Resistance to, For making utensils,, Bronze, 88 to 90 % Cu, corrosion, readily, jewellery, decorative, fusible, strong, articles , coins, -----------------------------------------------------------------------------------------------------------4) Nickeloy, 95 % Al, 4% Cu,, Extremely light,, For making airships, 1 % Ni, great mechanical, strength, -----------------------------------------------------------------------------------------------------------5) Alnico, 50% steel, 20%Al,, For making, 20% Ni, 10% Cu, permanent magnets, ------------------------------------------------------------------------------------------------------------, , Activity 2 - Suggest the names of three alloys of aluminium which you use in day to, day life., Test your understanding, 1) Name the ore and give its composition from which aluminium is extracted., 2) Name the processes used to purify aluminium ore., 3) What is the composition of electrolyte when molten alumina is subjected to, electrolysis ?, 4) Draw a diagram of Hoop’s cell for refining aluminium metal., 5) Give the names , composition and uses of any two alloys of aluminium., , Metallurgy of Iron

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15, , Concepts, , (i) Occurrence of iron (ii) Commercial forms of iron (iii) Manufacture of cast iron, (iv) Blast furnace (v) Physical and chemical properties of cast iron, (vi) Manufacture of steel (vi) Alloys of iron, , Occurrence, Next to aluminium, iron is the most abundant metal in earth’s crust. Iron is the fourth, most abundant ( about 5 % ) metallic element in the earth’s crust . Because of its strong, affinity to oxygen, it is not found in nature in the elemental state but only in combined, forms such as oxide. Iron is easily attacked by humid atmosphere. It is generally found, associated with other metals like copper, cobalt and nickel. Principally iron occurs as, oxides, much less as sulphide and sometimes as the carbonate., The chief sources of iron are –, (i) Red Haematite – Fe2O3, Brown Haematite or Limonite – 2 Fe2O3 . H2O,, Magnetite – Fe3O4, (ii) Siderite – or Spathic Iron ore - FeCO3, (iii) Iron pyrites – FeS2, (iv) Chalcopyrites – CuFeS2, Commercial forms of iron, There are three commercial forms of iron ., (i) Cast or Pig iron – It is the most impure form of iron. It contains about 1.5 % to 4.5, % carbon. Other impurities like Si, P, Mn and S are present upto about 1.5 %., (ii) Wrought or Malleable iron – It is the purest form of iron. It contains about 0.2 %, carbon., (iii) Steel – It is an alloy of iron with carbon and other elements like manganese, silicon, and phosphorus. It is midway between cast and wrought iron as far as impurities are, concerned. It contains 0.1 to 1.5 % carbon., The three varieties differ from each other mainly in their carbon content., The first step in the extraction of iron is the production of pig or cast iron which is, subsequently used in making wrought iron or steel.

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16, Extraction of iron, The extraction of iron is pyrometallurgy. It is the extractive metallurgy which consists, of the thermal treatment given to minerals or ores to recover the metal. The process, involves chemical reactions at elevated temperature., The process of extraction of iron is fundamentally very simple as it consists essentially of, the reduction of iron oxide by carbon. But as molten iron dissolves carbon and other, impurities, iron obtained is impure and is known as pig iron or cast iron. The ore ( red, haematite or hydrated oxide or carbonate ) is calcined in shallow kilns to remove, moisture, carbon dioxide etc. The ore thereby becomes porous and is then more easily, reduced in the blast furnace., , Iron is normally extracted from its oxide ore called haematite and rarely extracted from, carbonate ore called siderite. Iron pyrite is an important source of sulphur and therefore, it is not used in the extraction of iron. The extraction of iron involves following steps., Manufacture of cast ion, It is done in following steps., , (i) Washing and concentration or dressing of the ore, Haematite ore is washed with water. It is subjected to magnetic separation. The, ore being magnetic in nature, falls apart as a separate heap. This way the ore becomes, rich in oxide of iron. It is then broken into small pieces of 1” to 2” size, screened and, shifted. This helps to remove gangue. Due to washing, silicious impurities are removed., The ore is thus concentrated., (ii) Preliminary roasting and calcinations, The concentrated ore is roasted and calcined with a little coal in shallow kiln, ( furnace ) in excess air. Following changes take place during roasting and calcinations., (i) Moisture escapes as steam and organic matter present burns off to give CO2 and, sulphur and arsenic are oxidized to form their volatile oxides SO2 and As2O3 respectively., (ii) Ferrous oxide is converted to ferric oxide which avoids formation of ferrous silicate, in the slag during smelting and (iii) The mass becomes porous and thus makes it more, suitable for reduction to metallic iron. Following reactions take place., Fe2O3. 3 H2O → Fe2O3 + 3 H2O ↑ ; FeCO3 → FeO + CO2 ↑ ;, 4 FeO + O2 → 2 Fe2O3

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17, (iii) Reduction or smelting in a Blast Furnace, The roasted and calcined ore ( 8 parts ) is mixed with coke ( 4 parts ) which acts, as a reducing agent and limestone ( 1 part ) which acts as a flux. The mixture is, introduced in a tall Blast Furnace. The blast furnace has two functions (i) to reduce the, ore to metallic iron and (ii) to remove the impurities in the form of slag., Description of Blast Furnace, a) It consists of an outer shell which is made of steel plates riveted ( fastened ) together., From inside it sis lined with fire bricks. ( Refer Figure ), b) It is about 15’ to 120’ tall. ( The height varies from place to place. ) It is 15’ to 24’ in, diameter at the wider end near the bottom. ( boshes, lower cone ) It is kept in a vertical, position with the help of iron columns., c) In its lower part, the furnace gradually narrows down below the boshes. The lower part, of the furnace is called hearth (floor ) or crucible where molten iron and slag is, collected., d) The mouth of the furnace i.e. the top is closed by a double cup and cone arrangement, through which the mixture ( called charge ) of the calcined ore, limestone and coke is, fed from time to time. The hot gases escape through the flue. ( Refer diagram ), e) The cone is made of iron and is kept tight against the top of the furnace being, counterpoised by weights., Diagram –, , Fig. 7 -, , Blast Furnace

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18, f) A blast of hot and dry air, obtained by hot gases, escaping from the blast furnace itself, and freed from dust in a scrubber , is blown into the furnace , just above the hearth, by a number of water-cooled pipes, called tuyeres or twyers ( pipe rings)., g) Near the bottom of the furnace, there are two outlets on opposite sides, one for the, removal of slag ( called slag hole ) and the other for taking out molten metal ( called, metal hole or tap hole )., h) The temperature of the furnace is not constant in all the parts. It is maximum at the, hearth ( about 17000C ) and decreases slowly towards the throat of the furnace ( about, 3000C ), The Process, The charge ( roasted ore + coke + limestone in the proportion 8: 4:1 ) is, introduced in the furnace from the top. Simultaneously, the furnace is lit and a hot blast, of air is passed through the tuyeres. The different heat zones of the blast furnace are, shown in the figure., Reactions in the blast furnace, Following chemical reactions take place in different zones of the blast furnace., (i) Zone of reduction – ( 3000C to 8000C - i.e. dull red heat ), This is the uppermost zone of the blast furnace. It is called the zone of reduction. Here, the iron oxide from the charge is reduced by carbon monoxide to spongy iron., Fe2O3 + 3 CO → 2 Fe + 3 CO2 ↑, The reduction of Fe2O3 actually takes place in following three stages., a) Conversion of ferric oxide to ferroso - ferric oxide, 3 Fe2O3 + . CO → CO2 + 2 Fe3O4, b) Conversion of ferroso – ferric oxide to ferrous oxide., Fe3O4 + CO → CO2 + 3 FeO, c) Conversion of ferrous oxide to metallic iron., FeO + CO → CO2 + Fe, When the spongy iron falls in the middle region ( zone of heat absorption ), limestone,, CaCO3 decomposes to give CaO ( lime ) and CO2. Lime thus obtained acts as a flux., It combines with silica to form a fusible ( meltable ) slag., CaCO3 → CaO + CO2 ; CaO + SiO2 → CASiO3 ( slag ), (ii) Zone of heat absorption – ( 8000C to 12000C – i.e. bright red heat ), This is the middle part or zone of the blast furnace. In this zone, the ascending CO2 is, reduced to carbon monoxide when it reacts with carbon ( coke )., CO2 + C → 2 CO - 39 kcal, As the reaction is endothermic, the temperature in this region falls and comes in the, range 8000C – 10000C.

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19, (iii) Zone of combustion – ( 13000C to 15000C – i.e. white heat ), This is the zone near the tuyeres . Here the carbon burns to form CO2 producing, tremendous amount of heat., C + O2 → CO2 ↑ + 97 kcal, The heat evolved raises the temperature to 15000C. As the hot gases go up and meet, the descending charge, the temperature falls slowly to about 13000C., (iv) Zone of fusion – (15000C to 19000C ), In this zone, the spongy iron melts and dissolves some carbon, phosphorus and silica., The molten iron collects at the bottom of the furnace while the fusible slag floats on, it and protects the iron from oxidation. The layers of molten iron and slag are, withdrawn through separate tapping holes from time to time., The process is economical as it is continuous one. The waste gases containing, about 25% CO, 15% CO2, 56% N2 and 4% H2 are let out through the outer pipe. These, are burnt with air to produce heat which is used for preheating the air blast passed, through the tuyeres. The blast furnace can work day and night for years together. Iron so, obtained is known as Pig Iron. It is remelted in a vertical furnace ( known as cupola )and, can be cast or poured into moulds. It is then called cast iron. Thus cast iron is obtained, after remelting pig iron., Varieties of cast iron, When pig iron in the blast furnace is suddenly cooled, crystalline cast iron is, obtained. It is known as white cast iron. In this form, carbon is present in the combined, state as iron carbide. It is very hard and white in colour. On the other hand, if molten iron, is slowly cooled in sand moulds a graphite coloured iron is formed. It is known as grey, cast iron. In this type, a part of carbon separates out as graphite and gives grey colour to, the metal. It is softer and more coarse grained than the white form., Products of Blast Furnace, The products of blast furnace are (i) Pig iron (ii) Slag (iii) Flue gases ., (i) Pig iron – Average composition of pig iron is : a) Iron – 92 to 95 % , b) Carbon – 2.5, to 4.5 % c) Silicon – 0.7 to 3% d) Phosphorus –0.5 to 1 % e) Manganese - 0.2 to1 %, f) Sulphur – 0.1 to 0.3 %, , (ii) Slag - It is mostly calcium silicate containing some amount of aluminium silicate. It, Contains 55 % SiO2 , 30 % CaO and 15 % CO2 and 15 % Al2O3. It is useful for road, making and cement manufacture.

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20, , (iii)Flue gases - The gases leaving the blast furnace through the flue are known as flue, gases. The average composition of the gaseous mixture is a) CO – 25 % b) CO2 –, 10 % c) N2 – 58 to 60 % d) H2 – 1 to 2 % Due to the appreciable proportion of, carbon monoxide, the gaseous mixture has a fuel value and hence it is used for, heating the air blast in Cowper’s stoves., Major part of cast iron is used to manufacture steel. It is also used for casting metal, objects such as pipes, railings, weights and heavier parts of machinery., Physical and Chemical Properties of Cast Iron, (i) Since cast iron is impure, it has melting point ( about 12000C ) lower than that of pure, iron ( about 15300 C)., (ii) On solidifying, it expands, (iii) It is harder due to the presence of carbon and silicon., (iv) Due to sulphur, it is brittle when red hot ( red short ) and due to phosphorus, it is, brittle when cold ( cold short )., (v) It can not be welded and can not be permanently magnetized., (vi) Action of air - Pure iron is not affected by dry air. In presence of moist air and, carbon dioxide, it begins to rust readily forming reddish brown hydrated ferric oxide., When heated in air, it is oxidized and gets covered with thick bluish black scales of, ferroso – ferric oxide Fe3O4., (vii) Action of steam – When steam is passed over iron heated to 8000C to 10000C, iron, is oxidized to Fe3O4., 3 Fe + 4 H2O → Fe3O4 + 4 H2 ↑, (viii) Action of acids - Non-oxidising acids react with iron and form ferrous salts, evolving hydrogen. Hot and concentrated sulphuric acid oxidizes iron forming a mixture, of ferrous and ferric sulphates with the evolution of SO2., Fe + 2 H2SO4 → FeSO4 + SO2 + 2 H2O, 2 FeSO4 + 2 H2SO4 → Fe2(SO4 )3 + SO2 + 2 H2O, Cold and dilute HNO3 forms a mixture of ferrous nitrate and NH4NO3., 4 Fe + 10 HNO3 → 4 Fe(NO3 )3 + NH4NO3 + 3 H2O, Highly concentrated and pure nitric acid of specific gravity 1.45 has apparently no action, on iron. Iron is made passive in such acid. It is suggested that passivity of iron is due to, the formation of a thin protective invisible oxide film on the surface of iron. Iron also, becomes passive by other oxidizing agents like dichromates, nitrates, etc. Passive iron is, not affected by dilute acids. Passivity is lost when such a passive metal is rubbed or, treated with reducing agents.

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21, (ix) Other reactions - Iron is not affected by alkalies. When heated with (a) sulphur it, forms ferrous sulphide and (b) dry chlorine it forms ferric chloride., Fe + S → FeS, , ;, , 2 Fe + 3 Cl2 → 2 FeCl3, , Wrought Iron, It is the purest form of iron. The carbon content in it is very low (less than 0.2, %).It has very small amount of impurities. It melts at a higher temperature ( 15000C), than cast iron. It is obtained by purifying cast iron by the process known as puddling, ( i.e. stirring ) It is grey in colour. It is soft, ductile and malleable. It is used to prepare, chains, wires, bolts, couplings for railway carriages etc., Uses of iron, 1) Major part of cast iron is used to manufacture steel., 2) Cast iron is used for casting metal objects such as pipes, railings, weights, heavier, parts of machines etc., 3) Iron is used as material of construction., , Manufacture of steel, Steel is manufactured from pig or cast iron.. Iron containing 0.1 to 1.5 % carbon is called, steel. It is manufactured from cast iron by burning out carbon, silicon, phosphorus and, sulphur. Then a calculated quantity of carbon is added to it to get a required quality of, steel. Following methods are used for the manufacture of steel from pig or cast iron., (i) Bessemer process (ii) Open Hearth process (iii) L. D .process, 1) Bessemer process, Bessemer process was invented in 1855 by an English steel maker Henry, Bessemer. The process is carried out in a special kind of egg-shaped or pear-shaped, furnace. The furnace is called Bessemer converter., Bessemer converter is 20 feet high and 10 feet in diameter. This is made of steel, plates. It is lined with silica ( SiO2 ) or a mixture of lime (CaO ) and magnesia ( MgO), depending upon the nature of impurities present in cast iron. If the impurities are basic,, like MnO, then a lining of silica bricks is used and the process is known as Acid, Bessemer process. On the other hand, if the impurities are acidic like P2O5 or SO2, then, a lining of CaO and MgO is used and the process is known as Basic Bessemer process., Bessemer converter is provided with a number of fine holes at the bottom through which, a hot blast of air can be forced in fine jets. It is supported on two horizontal arms, ( trunnions ) so that it can be tilted in a vertical plane. The converter can hold a charge of, 20 tons at a time. The Bessemer converter is shown in the following figure.

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22, , Figure 8 - Bessemer Converter, a) Acid Bessemer Process - First the converter is turned into a horizontal position. Then, molten pig or cast iron containing little or no phosphorus is introduced into it. The, converter is then brought almost to a vertical position and a hot blast of air is introduced., The air is used to oxidize the impurities. Silicon and manganese are partly oxidized and, their oxides pass into slag. Sulphur burns off as SO2. A portion of iron is also oxidized., The iron oxide ( Fe2O3 ) formed oxidizes Mn and Si, if any. The iron oxide now reacts, with the carbon present evolving CO which burns at the mouth of the converter with a, blue flame. When the flame dies out, the molten metal in the converter becomes wrought, iron containing little Fe2O3. To get steel, the converter is again tilted, the blast of hot air, is stopped and a calculated quantity of molten spiegeleisen ( an alloy of Fe, C and Mn ), is added. The hot blast of air is turned on again for few minutes for thorough mixing. The, molten steel is then poured out and cast into moulds. The reactions taking place in the, Bessemer converter are summarized below., Si + O2 → SiO2 ;, , 2 Mn + O2 → 2 MnO ;, , MnO + SiO2 → MnSiO3, , 2 C + O2 → 2 CO ;, , 4 Fe + 3 O2 → 2 Fe2O3 ;, , Fe2O3 + 3 C → 2 Fe + 3 CO, , b) Basic Bessemer process ( Thomas Gilchrist process ) - This process is only used to, treat pig iron containing phosphorus. A charge of limestone and coke is first introduced, into the converter. The hot blast of air is turned on. The molten cast iron containing, phosphorus is then added and the blast continued., Silicon and manganese are first oxidized and pass into slag. Then phosphorus and carbon, are oxidized simultaneously. Carbon monoxide burns at the mouth of the converter., Phosphorus pentoxide reacts with lime and forms a slag containing calcium phosphate. It, is known as Thomas Slag and is used as a fertilizer. The molten iron is poured out into a, ladle ( deep spoon with long handle ) separated from the slag and then mixed with the, charge of requisite amount of spiegeleisen for recarburisation and deoxidation to form, steel. The reactions taking place in the converter are summarized below., P4 + 5 O2 → P4O10, , ;, , 6 CaO + P4O10 → 2 Ca3(PO4)2 ( Thomas Slag )

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23, Merits of Bessemer process, (i) Time required for the production of steel is less, (ii) Production cost is low., Demerits of Bessemer process, (i) Steel produced is of poor quality, (ii) Loss of iron in slag is more., 2) Open hearth process ( Siemens - Martine process ), Most of the high grade steel is made by this process. This is a modern method of, manufacture of steel. The furnace used for this purpose is shown in following figure. The, process is carried out in a large shallow hearth. When the hearth ( floor ), is lined with, silica, the process is called Acid Open Hearth process. When the hearth is lined with, lime and magnesia, the process is called Basic Open Hearth process. The furnace is, heated by means of producer gas ( a mixture of CO + N2 with little amount of H2 , CO2, and CH4 ) and air. There are two generators – one for the hot air and one for the producer, gas. The spent hot gases are made to leave the hearth through two other generators., Meanwhile, the first two generators get cooled and the producer gas and air are switched, over to the second set of generators and the spent hot gases are led through the first two, generators. The process, thus, works alternatively, , Fig. 9 – Open hearth furnace, a) Acid Open Hearth Process - When pig iron contains no phosphorus, then this, method is used. Pig or cast iron, rusted steel scrap and iron ore are introduced on, to the hearth of the furnace. It melts due to hot producer gas. Iron oxide ( Fe2O3 ), acts as an oxidizing agent.

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24, Fe2O3 + 3 C → 2 Fe + 3 CO ↑ ; 2 Fe2O3 + 3 S → 4 Fe + 3 SO2↑, The oxides of Mn (i.e. MnO ) and Si ( i.e. SiO2 ) react with each other to form slag, (MnSiO3 ). The amount of carbon is adjusted to get desired quality of steel., b) Basic Open Hearth Process - When pig iron contains phosphorus, then this method, is used. Carbon, sulphur and silicon get oxidized to their respective oxides., Phosphorus is oxidized to form P2O5 which combines with CaO ( lime ) to form slag., The reactions are as follows., 10 Fe2O3 + 12 P → 20 Fe + 3 P4O10 ;, SiO2 + CaO → CaSiO3 ( Slag ), , P4O10 + 6 CaO → 3 Ca3 ( PO4)2 ( Slag ), ;, , SiO2 + MnO → MnSiO3 ( Slag ), , A small quantity of charge is taken out after certain interval of time and analysed for, carbon content. When a chemical test indicates that metal contains the minimum required, amount of carbon, then calculated quantity of spiegeleisen is added. A little aluminium, or ferrosilicon is added to the molten steel which is drawn out from the furnace. This, removes any dissolved O2 and N2 in the molten steel., Comparison of Bessemer and Open Hearth process, Open hearth process has many advantages over Bessemer process. A comparison of both, these processes is given in the following table., , Bessemer Process, , Open Hearth Process, , (i) The oxidation of impurities is, carried out by hot blast of air., (ii) It is restricted to pig iron of a, particular composition., (iii) On account of internal heating,, the temperature can not be, controlled very well., (iv) Since there is no regenerative, system of heat, fuel can not be, saved., , (i) The oxidation of impurities is, carried out by Fe2O3., (ii) It is adapted to pig iron of any, composition., (iii) On account of external heating,, the temperature can be, controlled very well., (iv) Since there is regenerative, system of heat, fuel can be, saved ., , (v) The process is not continuous, and the charging is difficult., (vi)The steel is not of good quality., , (v) The process is continuous and, the charging is easier., (vi) The steel is of uniform and, superior quality., (vii) It is slow and takes twenty, hours for its completion., , (vii)It is quick and takes only ten, minutes for its completion.

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25, (viii)Approximately 15 % of iron, is lost in slag., , (viii)The loss of iron is not more, than 4 %, , 3) L.D. ( Linz and Donawitze ) Process, In this process, highly pure oxygen is blown from the top of the converter. A jet of, 99.5 % pure oxygen is blown at supersonic speed on to the molten bath of pig or cast iron, when all the impurities including phosphorus and sulphur are burnt away at high, temperature, leaving low carbon content and high quality steel. The density of the metal, is higher than impure metal. Therefore, it sinks to the bottom. The process requires about, 30 to 40 minutes. The process is much useful for getting low carbon steel. The converter, for l. D. process is shown in the following figure., , Fig. - 10 - L.D. Process, Advantages of L.D. process, (i) Due to liberation of large amount of heat, scrap iron can be used as starting material., (ii) Carbon and phosphorus are removed at the same time., (iii) Superior quality of steel is obtained., (iv) Process is very cheap and quick., (v) Nitrogen content in the finished steel is very low., Alloy Steels, N, 1, , Metal added Alloy Steel, Manganese Manganese, steel, , Composition Properties, 12 to 15 %, Very hard and, Mn, resistant to, wear, , Uses, For rock crushing, machinery, armour, plates, rail road tracks

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26, , 2, , Nickel, steel, , Nickel steel, , 3.2 % Ni, , 3, , Chromium, , 11.5 % Cr, , 4, , Nickel, , Chromium, steel, Invar, , 5, , Chromium, , Chromium, steel, , 1.5 to 2 %, Cr, , 6, , Vanadium, and, chromium, Tungsten, , 7, 8, , Chromium, – vanadium, steel, Tungsten, steel, Molybdenum Molybdenu, m steel, , 36 % Ni, , Increased, elasticity and, hardness, Resists, corrosion, Coefficient of, expansion, equals that of, glass, Extremely hard, , 0.15 %V +, 1% Cr, , Good tensile, strength, , 14 to 20%, W, 0.3 to 3 %, Mo, , Extremely hard, and strong, Retains, hardness even, at high, temperature, , etc., For making armour, plates, cables,, automobile parts, For utensils, cycle and, automobile parts, For clock pendulums, and measuring tapes, , For cutting tools and, crushing machinery,, armour piercing bullets, For springs, shafts,, axles and frames, For drills and high, speed tools, For cutting tools and, axles, , Activity 3 – (i) Find out the places in India where (a) you get iron ore. (b) manufacture, of cast iron or pig iron is done. (ii) Mention names of three alloys of iron which you use, in day to day life., Test your understanding, 1) How does iron occur in nature ? Give any three source of iron., 2) What are the commercial forms of iron ? In which respect do they differ ?, 3) Draw and describe the blast furnace and the reactions taking place in various zones in, it during the manufacture of cast iron., 4) Name the processes used to manufacture steel., 5) Give the names, composition and uses of any two alloy steels ., , References / Figures / Diagrams :, 1) Fig. 1 - Hydraulic washing, http://www.tutorvista.com/topic/iron-sulphide-separate, 2) Fig. 2 – – Froth floatation process, http://cbseportal.com/exam/Important-Topics/Chemistry-Froth-Floatation-Process