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7/31/2019 Engineering Materials Smelting of Iron Lect05
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E n g i n e e r i n g M a t e r i a
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Production of Iron
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OutlineBlast furnace chargeIron OresFluxMetallurgical Coke
Coke Manufacturing &its Quality
General view of Blast
Furnace plantSketch of Blast Furnace
Construction and
working of Blast Furnace
Charging Mechanism inBlast Furnace
Smelting operation
Chemical Reactions inBlast Furnace
Formation and tappingof slag
Pig Iron- Properties &uses
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Blast Furnace Charge
BF charge consists of the following:
Iron ore ; iron oxide in lump or sinter form
Fuel ; coke
Flux ; limestone
In addition Hot air is admitted through the tuyeres
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The most important variety of iron ore are:
Nomenclature Composition Fe %age Location
Magnetite(Black oxide)
Fe 3O4 72.4 Makarwal, Swabi, Chitral (PAK),Land of midnight sun (Sweden),Pennsylvania (USA), Siberia (Russia)
Hematite (red,brown &black)
Fe 2O3 40-65Kala bagh, Chnari, Chaghi, Chalghazi(PAK),Lake superior, Utah (USA), Lorraine(France), Amazon basin (Brazil)
Limonite &otherhydroxide types of ores
2Fe 2O3 . H 2Oto
Fe 2O3 . 3H 2O
20-55 Not found in PAK,Britains East coast
Siderite FeCO 3 Low inFe
Hazara, Mardan (PAK),South Wales, Staffordshire & west York
Iron Pyrite (yellow) FeS 42Mardan, Chitral (PAK)
Iron Ores
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Flux
Flux is a material, which is added to thecontents of a Blast furnace or a Cupola forthe purpose of purging the metal of impurities, and of rendering the slag moreliquid.
The flux most commonly used in iron andsteel furnaces is limestone , which is chargedin the proper proportions with the iron oreand fuel.
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Metallurgical Coke
Coke is a solid carbonaceous materialderived from destructive distillation of low-ash, low-sulfur, high coking property
bituminous coal . The volatile constituents of the coal
including
water, coal-gas, and
coal-tar
are driven off by baking in an airless oven at temperatures ashigh 1000 C.
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Metallurgical Coke Metallurgical coke is produced by
carbonization of coal at high temperaturesto produce a macroporous carbon material of high strength and relatively large lumpsize .
Notes:
Metallurgical cokes must have a high strength tosupport heavy loads in the blast furnace withoutdisintegration.
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Coke Manufacturing Process Bituminous coal is pulverized by ball mill into fine
powder.Fine coal is heated in coke oven battery at 700 900C in absence of air. This process is called
Carbonization. After soaking at this temperature for about 7 8hrs ., it is allowed to cool in the furnace.
Due to coking property of bituminous coalpowdered coal converts into lump size andbecomes very hard and highly porous.
After cooling, the coke produced is crushed intodesired size.
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Quality Testing of Coke
Following parameters are determined in thetesting laboratory for justifying the quality of thecoke.
Strength of the coke,
fixed Carbon (88-92%),
sulphur content and
ash content
By-products like tar may also be collected
during carbonization process.
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General view of Blast Furnace plant
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Sketch of Blast Furnace Plant
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Sketch of Blast Furnace
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Construction and working of Blast Furnace Blast furnace consists of a vertical steel shell ~100 ft
high & ~ 21 ft dia and lined with refractory material . It has a charging arrangement (Bell & Cone) at the
top . Raw material is charged by a skip car throughthis charging mechanism into the blast furnace.
It has a means of running off Pig iron & Slag at thebottom.
Air is blown in near the bottom of the furnace. Thisforced draught increases the speed of combustion &maintains the necessary high temperature .
Cooling water is circulated around the Bosh area in
order to protect the furnace from high temp.
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Various zones inBlast Furnace
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Charging Mechanism in Blast Furnace
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Smelting operation
Smelting consists of two operations :a - Chemical reduction of iron from its oxides byCO, which is formed by the combustion of C in the
coke with the O 2 of the air blast admitted throughthe tuyeres near the base.
b - The liquefaction of the gangue by means of flux (limestone) to form a fusible slag which will runfrom the furnace.
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Smelting operation
During the production of pig iron (smelting
operation) large amount of gases are evolved ,
which contain about 30% CO having considerable
calorific value. The exit gases are collected , led through down-
comer pipe to a cleaning chamber and electrostatic
dust precipitator & then used as fuel gas in Cowper Stoves in order to pre-heat the air blast to the
furnace.
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Chemical Reactions in Blast Furnace The air , which is blown at the bottom of the
furnace causes partial combustion of the coke :2C + O 2 = 2CO + Heat 1
CO , which is powerful reducing agent , risesthrough the charge, & chemically reduces theiron oxide :-
Fe 2O 3 + 3CO = 2Fe +3CO 2 2
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Reduction sequence of iron oxide is as
Fe 2O3 Fe 3O4 FeO Fe , when temp is > 570 C
Fe 2O3 Fe 3O4 Fe, when temp is < 570 C
The equation # 2 takes place in the upper part of the furnace, where the temp is too low to melt the
iron formed. The iron remains as a spongy mass until it moves
down into the lower part of the furnace, where it
melts & runs down over the hot coke, dissolving C,S, Mn, P & Si as it goes.
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SLAG : The slag is a liquid mixture of ash,
flux, and other impurities.Formation and tapping of Slag At the time of the reduction of iron oxide,
the gangue , which is composed of silica,combines with lime to produce slag as :-
SiO 2 + 2CaO = 2CaO.SiO 2(Calcium silicate slag)
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SLAG
Calcium silicate slag has much lower melting pointthan silica and can run from the furnace quite easily .
The furnace is tapped at regular intervals & the pigiron is run either into large ladles for transference,while still molten, to steel making plant, or alternatively, cast into pigs for subsequent use in the
cupola or open hearth furnace.
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Slag (Continue)
Limestone has an other function
the removal of sulphur. This entails having an excess of lime in the slag
and is often impracticable with low grade ores,
since they contain so much silica, which must beneutralized first.
In British practice, the sulphur is removed at a
later stage by adding soda-ash to molten pig ironas it runs from the blast furnace into the ladle:-
FeS + Na 2CO 3 = FeO + Na 2S + CO 2
(Iron sulphide Soda ash joins the slag
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Tapping of Slag
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Pig Iron - Properties & uses
Definition :- Itis a high-carbon iron made by reduction of
iron ore in the blast furnace .
Metallic iron, which is a product of reduction of ironore, appears in the lower part of stack & in thebosh.
It contains ~ 1 % C and found to in spongy form. Asthis material sinks down in the B.F. and is heatedfurther, iron dissolves C in increasing quantities.
This reduces the melting temp & it melts and drawsoff to the hearth in the form of drops.
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Pig Iron (Continue)
Pig iron is really a complex alloy . In additionto Fe, it contains up to 10 % of other elements , mainly, C, Mn, Si, S & P.
Total amount of C in the pig iron is usuallybetween 3 4% & is present either
as compound, iron carbide (Fe 3C) or
as un-combined carbon in the form of graphite . Pig iron low in S & high in P may be used in
the manufacture of acid steel & high-duty
iron castings.
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Thanks
Bl t f di
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Blast furnace diagram
1. Hot blast from Cowperstoves
2. Melting zone ( bosh )3. Reduction zone of ferrous
oxide ( barrel )4. Reduction zone of ferric
oxide ( stack )5. Pre-heating zone ( throat )6. Feed of ore , limestone ,
and coke 7. Exhaust gases8. Column of ore, coke and
limestone9. Removal of slag 10. Tapping of molten
pig iron 11. Collection of waste gases
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