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metallurgy 1

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METALLURGY II 1 Mineral : The metallic compounds occurring in nature are called minerals. 2. Ore : A naturally occurring mineral from which a metal can be conveniently extracted, is called an ore. 3. Matrix or Gangue : The unwanted impurities which are associated with ore are called gangue or matrix. Stone, sand, clay, etc., constitute gangue. 4. Metallurgy : The processes involved in the extraction of pure metals from their ore are collectively called metallurgy. 5. Dressing of ore : The process or processes involved, which give the ore such a physical form, so that gangue can be easily removed from it, is called dressing of ore. Handpicking, grinding, crushing and pulverising are the common methods employed during the dressing of ore. 6. Concentration of ore : The various processes which help in the removal of gangue from dressed ore, thereby increasing the percentage of metal in ore, are called concentration of ore. 7. Concentration by electromagnetic process : In this process, the iron particles and its compounds are removed from the ore, when pulverised ore is allowed to move over a belt, driven by an electromagnetic pulley. 8. Concentration by gravity process : In this process the pulverised ore is placed over a wide sloping table. A jet of water is directed against the ore, when the lighter particles of gangue are washed off, leaving behind ore. 9. Concentration by froth floatation process : This process is used for the concentration of sulphide ores. The pulverised ore is churned in a mixture of water and pine oil with compressed air, when foam is formed. The sulphide particles stick to foam and rise up. The foam is removed and from it, the concentrated ore is removed. 1 Key Points 10. Calcination : The process of heating the ore in a limited supply of air, such that temperature is not sufficient to melt the ore, is called calcination. 11. Objectives achieved during calcination : (i) It removes moisture from ore (ii) It makes the ore porous (iii) It expels volatile impurities (iv) It decomposes carbonate ores to their oxides (v) It removes water of crystallization from their hydrated salts. 12. Roasting : The process of strongly heating the ore in excess of air is called roasting. 13. Objectives achieved during the roasting of ore : In addition to all the objectives achieved during calcination, it oxidises sulphide ores to oxide ores. 14. Extraction of metal from a concentrated ore : It is achieved by reduction of ore by any one of the following methods : (i) Electrolytic process (ii) By heating with coke or carbon monoxide (iii) By heating with aluminium powder. 15. Purification of metal obtained by reduction of ore : It is carried out by any of the following methods, depending upon the nature of metal and the nature of impurities. (a) Distillation (b) Liquation (c) Polling (d) Oxidation of impurities (e) Electrolytic refining. 16. Ores of Aluminium : Bauxite (Al2O3.2H2O), corundum (Al2O3), cryolite (Na3AlF6) and alunite [K2SO4.Al2 (SO4)3. 4A1(OH)3] are common ores. 17. The purification of bauxite is carried out by Baeyer s process. In this process, bauxite is digested with sodium hydroxide. The digested mixture is then treated with water to form aluminium hydroxide. The aluminium hydroxide is then roasted to obtain pure alumina (A12O3). 18. The electrolyte in the extraction of aluminium consists of 1 part of alumina, 3 parts of cryolite and one part of fluorspar in the fused state. 2 Key Points 19. During electrolysis of above mixture, aluminium collects at the cathode and oxygen is liberated at the anode. The oxygen reacts with carbon anodes to form carbon dioxide. Al3+ + 3e Al; O2 2e O; C + 2O CO2 20. Alumina cannot be directly electrolysed on account of the following reasons : (i) In solid state, it is a bad conductor of electricity. (ii) In fused state, it conducts electricity. However, its melting point is 2050 C. At this temperature, aluminium metal vaporises. 21. Cryolite lowers the m.p. of alumina to 850 C. At this temperature, molten aluminium is formed, which sinks below the electrolyte. Thus, oxidation of aluminium does not take place. 22. Physical properties of Aluminium : (i) It is a white metal which is malleable and ductile. (ii) It has a density of 2.7 g cm 3. It is a very good conductor of heat and electricity. 23. Chemical properties of Aluminium : (i) Dry air does not react with aluminium. However, moist air, rapidly tarnishes it on account of the formation of very thin layer of aluminium oxide. (ii) Pure water does not react with aluminium. However, sea water slowly reacts with it, with the liberation of hydrogen gas. (iii) It reacts with dilute hydrochloric acid and sulphuric acid with the formation of respective salts and hydrogen. 2Al + 6HCl 2AlCl3 + 3H2 Nitric acid, being a powerful oxidising agent, forms a protective layer of Al2O3 and hence the aluminium goes passive. (iv) Aluminium dissolves in concentrated and boiling hot solution of caustic alkalis to form hydrogen gas. heat 2Al + 2NaOH + 2H2O 2NaAlO2 + 3H2 (g) (v) Aluminium reacts with non-metals like chlorine and sulphur to form their binary compounds. 2Al + 3Cl2 2AlCl3 3 Key Points 24. 25. 26. 27. (vi) A mixture of aluminium powder and ferric oxide on igniting produces a large amount of heat and ferric oxide is reduced to iron. Fe2O3 + 2Al 2Fe + Al2O3 + Heat Uses of Aluminium : (i) It is used for making cheap household appliances. (ii) It is used in electric transmission wires. (iii) It is used for making lightweight alloys. (iv) It is used in the packaging industry. (v) It is used as a reducing agent. The chief ores of zinc are : (i) Zincite (ZnO) (ii) Zinc blende (ZnS) (iii) Calamine (ZnCO3). Physical properties of Zinc : (i) Pure zinc is a bluish grey metal which rapidly tarnishes in air due to formation of the protective layer of zinc oxide. (ii) At the room temperature, it is a hard crystalline solid, brittle in nature. It becomes fairly malleable and ductile between 150 C to 200 C. (iii) It melts at 419 C and boils at 907 C. (iv) Its density is 7.2 g cm 3 and it is a good conductor of heat. Chemical properties of Zinc : (i) Zinc metal does not react with dry air. However, when heated strongly in air, it catches fire to form zinc oxide. (ii) Pure water does not react with zinc. However, if steam is passed over red hot zinc, it forms zinc oxide and hydrogen gas. (iii) Zinc on heating with sulphur, forms zinc sulphide. (iv) Zinc in red hot state, reacts with chlorine to form zinc chloride. (v) Zinc reacts with dil. HCl and dil. H2SO4 to form their respective salts and hydrogen. (vi) Zinc dissolves in strong caustic alkali solutions to liberate hydrogen gas. (vii) Zinc displaces less active metals from their salt solutions. 4 Key Points 28. Uses of Zinc : 1. It is used for galvanising iron sheets. Galvanising protects iron from rusting. 2. It is extensively used in dry cell industry. 3. It is used in making alloys like, brass, bronze, bell metal, etc. 4. It is used as a reducing agent in the laboratory. 29. Ores of Iron : Haematite (Fe2O3), Limonite (Fe2O3.H2O), Magnetite (Fe3O4) and Siderite (FeCO3) are the ores of iron. 30. Cast iron and its physical properties : Cast iron is the most impure form of iron. It contains between 2.5 % to 5 % of carbon and from 1% to 2% of impurities, such as sulphur, phosphorus and silicon. (a) It is dark grey in colour and is very hard and brittle. (b) If molten cast iron is suddenly cooled, it changes to white pig iron. (c) It is not easily acted upon by moist air and hence does not easily rust. 31. Uses of Cast iron : 1. It is used in the manufacture of wrought iron and steel. 2. It is used for making (i) drain pipes (ii) manhole covers (iii) lampposts (iv) immovable parts of machines. 32. Steel and its manufacture by Bessemer process : Steel is an alloy of iron, containing carbon between 0.1 % to 1.5 %. Special properties to steel can be imparted, by adding to it small amount of other metals, such as nickel, chromium, vanadium, manganese, etc. Molten cast iron is poured in Bessemer converter and hot air from tuyeres is blown in, when impurities react with oxygen to form their volatile oxides which are blown out along with slag. A flame appears at the mouth of Bessemer converter because of the presence of specks of carbon present in the gases. When the flame dies, it is an indication that all impurities have been blown out. Now spiegeleisen (an alloy of 74 % iron, 20 % manganese 5 Key Points 33. 34. 35. 36. and 6% carbon) is added in calculated amount. The molten mass is stirred when it forms steel. Hardening of steel : When steel is heated to a temperature of 750 C to 800 C and then suddenly plunged into cold water or cold oil, the process is called hardening of steel. Tempering of steel : The process of heating the hardened steel to some fixed temperature and then cooling it slowly is called tempering or annealing of steel. Case hardening of steel : The process of heating mild steel with powdered carbon in clay moulds, such that its upper surface becomes very hard, but inner surface remains soft and spongy is called case hardening of steel. Chemical properties of Iron : 1. Dry air has no action with iron. However, moist air reacts with iron to form rust. 2. Pure water has no action with iron. However, when steam is passed over red hot iron, it forms magnetic oxide of iron and hydrogen. 3. Iron on heating with sulphur forms iron (II) sulphide. 4. When chlorine is passed over red hot iron, it forms iron (III) chloride. 5. Iron reacts with dil. HCl and dil. H2SO4 to form respective salts and hydrogen gas. 6. Iron reacts with very dilute nitric acid to form nitric oxide gas, ferrous nitrate and water. 7. Iron displaces less active metals from their salt solutions. 6 Key Points

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