Manufacture of Pig Iron Concentration Calcination Smelting Blast furnace Chemistry of reactions in Blast furnace. 15CY101: Chemistry SRM University 2

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1 Manufacture of Pig Iron Concentration Calcination Smelting Blast furnace Chemistry of reactions in Blast furnace. 15CY101: Chemistry SRM University

2 Iron is very reactive and is found in nature in form of its oxides, carbonatesandsulphates. The ores are: i) Haematite (Fe O 3 ) ii) Magnetite (Fe 3 O 4 ) iii) Iron Pyrites (FeS ) iv) Siderite (FeCO 3 ) v) Limonite (Fe O 3 -xh O, in which x is typically around 1.5) The main iron ore ishaematite (iron(iii)oxide-fe O 3 ). The iron ore containsimpurities, mainlysilica (silicon dioxide). Since iron is below carbon in the reactivity series, iron in the ore is reduced to iron metal byheatingwithcarbon (coke). 15CY101: Chemistry SRM University 3

3 Other raw materials needed to reduce iron from the ores are 1) Coke ) Limestone Coke is ahighcarbonfuel produced by heating bituminous coal in a limited oxygen atmosphere for several hours, followed by water spraying in special quenching towers. Cokeservestwofunctions in the reduction process: It is a fuel that supplies heat for the chemical reactions; and It produces carbon monoxide(co)to reduce the iron ore. Limestone is a rock containing high proportions of calcium carbonate(caco 3 ). The limestone is used in the process as a flux to react with and remove impurities in the molten iron as slag 15CY101: Chemistry SRM University 4

4 Pig iron is the firstproduct in the process of converting iron ore into useful metal Iron orebecomespigironwhenimpuritiesareburnedout in a blast furnace Pig iron will have some impurities but hashighmetalcontent. The composition is as follows Carbon 3 to 4 % Phosphorus 0.3 to 1.7 % Silicon 1 to 3 % Sulphur Under 1.0 % Manganese 0.1 to 1 % Iron Remainder Pig iron is therawmaterial for all iron and steel products 15CY101: Chemistry SRM University 5

5 Ore Blast Furnace Pig Iron Cupola Puddling Furnace Bessemer Converter O.H. Furnace Electric Furnace Oxygen Processing Cast Iron Wrought Iron Steel 15CY101: Chemistry SRM University 6

6 There are three major commercial forms of Iron. They differ in their carbon content. 1.CastIron It contains -5% Carbon and 1 3% Silicon along with traces of other impurities likesulphur,phosphorus,manganese etc.. Wrought Iron 3. Steel It is thepurestform of Iron and contains carbon to the extent of0.5% It contains0.5to1.5%of carbon along with varying amount of other elements. 15CY101: Chemistry SRM University 7

7 Pig iron is classified into three grades based on chemical composition 1) Basic pig iron ) Foundary pig iron 3) Ferroalloys Basic Pig Iron It is used for steel making Must be low in silicon (1.5 % max) Must be low in sulphur (0.04 %) Phosphorus less than 1 % Manganese 1 to % Carbon 3.5 to 4.4 % 15CY101: Chemistry SRM University 8

8 Foundary Pig Iron It is used for production of iron casting Contains Si 0.5 to 3.5 % Mn 0.4 to 1.5 % P to 0.9 % C 3 to 4.5 % S up to 0.05 % Fe - remainder 15CY101: Chemistry SRM University 9

Ferroalloys Ferroalloys arealloysofpigiron, each rich in one specific element It is used as additives, in iron and steel industries, to control or alter the

9 Ferroalloys Ferroalloys arealloysofpigiron, each rich in one specific element It is used as additives, in iron and steel industries, to control or alter the properties of iron and steel Examples Ferromanganese pig iron contains 74 to 8 % Mn Ferrosilicon Pig iron contains 5 to 17 % Si 15CY101: Chemistry SRM University 10

10 The ore is crushed in crushers and is broken to small pieces. It is concentrated withgravityseparation process in which it is washed with water toremoveclay,sand, etc. 15CY101: Chemistry SRM University 11

11 The ore is then heated in absence of air(calcined). This results in decomposition of carbonates into oxides and then ferrous oxide is converted into Ferric Oxide. FeCO 3 FeO + CO 4 FeO + O Fe O 3 15CY101: Chemistry SRM University 1

12 The concentrated ore is mixed with calculated quantity ofcoke,limestone and the mixture is put in theblastfurnace from top. 15CY101: Chemistry SRM University 13

13 It is atallcylindrical furnace made of steel. It is lined inside withfirebricks. It isnarrowatthetop and has an arrangement for the introduction of ore and outlet for waste gases. Heated with help of HotGases. 15CY101: Chemistry SRM University 14

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15 Named as averyhightemperatureis developed inside it by means of forcingablastofheatedair Height is about 30 metres and interior diameter is8metres To permit proper flow, it is made narrow at the top and bottom Outside of the furnace is constructed of steel plates There is a well or hearth at the bottom of the furnace in which molten iron collects prior to its tapping Hearth is lined with refractory carbon brick 15CY101: Chemistry SRM University 16

16 Thewidestpart of the furnace is at the top of the bosh and its diameter depends upon the output Bosh is thesmeltingzone and the hottest part of the furnace The raw materials after weighing in the desired proportions are raised to the top of the blast furnace through mechanical charging equipment A double bell arrangement is provided at the top of to check the escape of gas from the top of the furnace 15CY101: Chemistry SRM University 17

17 The materials are charged first on the upper bell which is then lowered allowing them to fall on the lower bell After closing the upper bell the lower bell is lowered to enable the materials to fall on to the stack line. Ore, coke and limestone are charged in relation at a rate adequate to maintain the stackline in the correct height 15CY101: Chemistry SRM University 18

18 At the top of the hearth (above molten metal and slag) are situated thetuyeres through which blast of heated air is introduced There are may be 8-18 tuyeres round the furnace but usually there are 10 Tuyeres are located peripherally near the top of the hearth and they are the inlets that admit the heated air necessary for combustion and chemical reaction They are made of Copper or bronze 15CY101: Chemistry SRM University 19

19 Following Chemical Reactions Take Place in a Blast Furnace i) Formation of Carbon Monoxide: Near the bottom of the furnace, coke burns in air to form Carbon Dioxide and a lot of heat is produced. We get a temperature of about 1875 K. This CO further reacts with more coke and is reduced to CO. C + O C + CO CO CO Heat 15CY101: Chemistry SRM University 0 +

20 ii) Reduction of Haematite to Iron: In the upper part of the furnace, the temperature is between 975K to 1075K. Here Haematite is reduced to Iron by CO. ThismoltenIron is collected at thebottom of the furnace. Fe + + O + 3CO Fe 3 CO 3 15CY101: Chemistry SRM University 1

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22 Blast furnace is essentially an apparatus for heating the ore in the presence of an excess of carbon, principally in the form of carbon monoxide (CO) CO is thereducingagent CO combines with the oxygen present in the ore and thus reduces the ore into iron Fe O + CO 3 + 3CO Fe 3 The descending iron ore is converted in to pig iron by ascending stream of hot reducing gas, mainly CO, CO 15CY101: Chemistry SRM University 3

23 1. It acts asflux to remove sand from Haematite in form of liquid Slag. In the middle of the furnace, the temperature is about K. Here Limestone decomposes to produce calcium oxide (CaO) and CO. This CaO reacts with reacts with silica (sand) present in the ore to form slag(casio 3 ). CaCO 3 + heat CaO + SiO CaO + CO CaSiO 3. Slag is lighter than molten iron so itfloatsovermolten iron andprotects it from oxidising back into its oxides. 15CY101: Chemistry SRM University 4

Pyrometallurgy of iron is still the most important pyrometallurgical process economically.

1 Pyrometallurgy of iron is still the most important pyrometallurgical process economically. Prehistorically, iron was prepared by simply heating it with charcoal in a fired clay pot. Coke is coal that