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(1) Ores of iron : Haematite Fe2O3, Magnetite (Fe3O4) Limonite (Fe2O3.3H2O), Iron pyrites (FeS2) Copper pyrities (CuFeS2) etc.
(2) Extraction : Cast iron isextracted from its oxides by reduction with carbon and carbon monoxide in a blast furnace to give pig iron.
Roasting : Ferrous oxide convert into ferric oxide.
Fe2O3.3H2O → Fe2O3 + 3H2O; 2FeCO3
→ 2FeO + 2CO2
4FeO + O2 → 2Fe2O3
Smelting : Reduction of roasted ore of ferric oxide carried out in a blast furnace.
(i) The reduction of ferric oxide is done by carbon and carbon monoxide (between 1473k to 1873k) 2C + O2 → 2CO
. It is a reversible and exothermic reaction. Hence according to Le-chatelier principle more iron will be produced in the furnace at lower temp. Fe2O3 + CO → 2FeO + CO2
(iii) FeO + C ————————→ Fe + Co
endothermic reaction
The gases leaving at the top of the furnace contain up to 28% CO and are burnt in cowper's stove to pre-heat the air for blast
Varieties of iron : The three commercial varieties of iron differ in their carbon contents. These are;
(1) Cast iron or Pig-iron : It is most impure form of iron and contains highest proportion of carbon (2.5–4%).
(2) Wrought iron or Malleable iron : It is the purest form of iron and contains minimum amount of carbon (0.12–0.25%).
(3) Steel : It is the most important form of iron and finds extensive applications. Its carbons content (Impurity) is mid-way between cast iron and wrought iron. It contains 0.2–1.5% carbon. Steels containing 0.2–0.5% of carbon are known as mild steels, while those containing 0.5–1.5% carbon are known as hard steels.
Steel is generally manufactured from cast iron by three processes, viz, (i) Bessemer Process which involves the use of a large pear-shaped furnace (vessel) called Bessemer converter, (ii) L.D. process and (iii) open hearth process, Spiegeleisen (an alloy of Fe, Mn and C) is added during manufacture of steel.
Heat treatment of steels : Heat treatment of steel may be defined as the process of carefully heating the steel to high temperature followed by cooling to the room temperature under controlled conditions. Heat treatment of steel is done for the following two purposes,
(i) To develop certain special properties like hardness, strength, ductility etc. without changing the chemical composition.
(ii) To remove some undesirable properties or gases like entrapped gases, internal stresses and strains. The various methods of heat treatment are,
(a) Annealing : It is a process of heating steel to redness followed by slow cooling.
(b) Quenching or hardening : It is a process of heating steel to redness followed by sudden cooling by plunging the red hot steel into water or oil.
(c) Tempering : It is a process of heating the hardened or quenched steel to a temperature much below redness (473–623K) followed by slow cooling.
(d) Case-hardening : It is a process of giving a thin coating of hardened steel to wrought iron or to a strong and flexible mild steel by heating it in contact with charcoal followed by quenching in oil.
(e) Nitriding : It is a process of heating steels at about 700oC in an atmosphere of ammonia. This process imparts a hard coating of iron nitride on the surface of steel.
Properties of steel : The properties of steel depend upon its carbon contents. With the increase in carbon content, the hardness of steel increases while its ductility decreases.
(i) Low carbon or soft steels contain carbon upto 0.25%.
(ii) Medium carbon steels or mild steels contain 0.25–0.5% carbon.
(iii) High carbon or hard steels contains 0.1 – 1.5 percent carbon.
(iv) Alloy steels or special steels are alloys of steel with Ni, Cr, Co, W, Mn, V etc., For example
(a) Stainless steel (Fe = 73%, Cr = 18%, Ni = 8% + C) is resistant to corrosion and is used for making ornamental pieces, cutlery etc.
b) Invar (Fe = 64%, Ni = 36%) has small coefficient of expansion and is used for making metre scales, pendulum rods and watches.
(c) Manganese steel (Fe = 86%, Mn 13% + carbon) is very hard and resistant to wear and hence is used for making rock drills, safes etc.
(d) Tungsten steel (Fe = 94%, W = 5% + carbon) is quite hard and is used for making high speed cutting tools.
(e) Permalloy (Fe = 21%, Ni = 78% + carbon) is strongly magnetised by electric current but loses magnetism when current is cut off. It is used for making electromagnets, ocean cables etc.
Properties of iron
(1) Dry or moist air has no action on pure iron but impure iron when exposed to moist air is covered with a layer of rust Fe2O3 + Fe(OH)3. However, finely divided pure iron burns in air or oxygen forming Fe3O4 (magnetic oxide of iron).
3Fe + 2O2 → Fe3O4
(2) Iron decomposes steam at red heat
3Fe + 4H2O ——————→ Fe3O4 + 4H2
Steam
(3) Action of acids: Iron reacts with dil. HCl and dil. H2SO4 liberating hydrogen. with hot conc.H2SO4, it gives SO2, with dil.HNO3, it gives NH4NO3 and moderately conc. HNO3 reacts with iron forming NO2.
Cold conc. HNO3 makes iron passive due to the deposit of a thin layer of iron oxide (Fe3O4) on the surface.
Hot conc. HNO3 reacts with iron liberating NO.
Fe + 4HNO3 (hot conc.) → Fe(NO3)3 + NO + 2H2O
(4) Iron does not react with alkalies.
(5) It displaces less electropositive metals (e.g., Cu, Ag etc.) from their salts
CuSO4 + Fe → FeSO4 + Cu
(6) Finely divided iron combines with CO forming penta carbonyl
Fe + 5CO → Fe(CO)5
(7) Iron does not form amalgam with Hg.
(8) Iron is the most abundant and most widely used transition metal. |
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