SMELTING OF IRON

Outline Iron Ores

General view of Blast

Furnace plant

Sketch of Blast Furnace

Construction and

working of Blast Furnace

Blast Furnace Charge

Metallurgical Coke

Coke Manufacturing &

its Quality

Flux

Charging Mechanism in

Blast Furnace

Smelting operation

Chemical Reactions in

Blast Furnace

Formation and tapping

of slag

Pig Iron- Properties &

uses

The most important variety of iron ore are:

Nomenclature Composition Fe %age Location

Magnetite

(Black oxide)

Fe3O4 72.4 Makarwal, Swabi, Chitral (PAK),

Land of midnight sun (Sweden), Pennsylvania(USA), Siberia(Russia)

Hematite (red, brown & black)

Fe2O3 40-65 Kala bagh, Chnari, Chaghi, Chalghazi (PAK),

Lake superior, Utah (USA), Lorraine (France), Amazon basin (Brazil)

Limonite & other hydroxide types of ores

2Fe2O3 . H2O to

Fe2O3 . 3H2O

20-55 Not found in PAK,

Britain’s East coast

Siderite FeCO3 Low in Fe

Hazara, Mardan (PAK),

South Wales, Staffordshire &west York (UK)

Iron Pyrite (yellow)

FeS 42 Mardan, Chitral (PAK)

Iron Ores

General view of Blast Furnace plant

Sketch of Blast Furnace Plant

Sketch of Blast Furnace

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 through this charging mechanism into the blast furnace.

• It has a means of running off Pig iron & Slag at the bottom.

• Air is blown in near the bottom of the furnace. This forced 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.

Blast furnace diagram

1. Hot blast from Cowper stoves 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 coke7. Exhaust gases8. Column of ore, coke and

limestone9. Removal of slag10. Tapping of molten

pig iron11. Collection of waste gases

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

Metallurgical Coke• Coke is a solid carbonaceous material derived from

destructive distillation of low-ash, low-sulfur 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 as high 1000° C.

• Metallurgical coke is produced by carbonization of coal at high temperatures to produce a macroporous carbon material of high strength and relatively large lump size.

Notes:

• Metallurgical cokes must have a high strength to support heavy loads in the blast furnace without disintegration.

Coke Manufacturing Process Bituminous coal is pulverized by ball mill into fine

powder.

Fine coal is heated in coke oven battery at 700–900 ° C in absence of air. This process is called Carbonization.

After soaking at this temperature for about 7–8 hrs., it is allowed to cool in the furnace.

Due to coking property of bituminous coal powdered coal converts into lump size and becomes very hard and highly porous.

After cooling, the coke produced is crushed into desired size.

Coke Manufacturing Process

• Strength of the coke,

• fixed Carbon (88-92%),

• sulphur content and

• ash content

is determined in the testing laboratory for

justifying the quality of the coke.

• By-products like tar may also be collected during

carbonization process.

Quality Control and Standards

Coking coal used should have

low ash content,

low sulphur and

should be highly coking in nature for better quality of coke.

Other properties like

porosity,

abrasive strength and

hardness

must be checked in the laboratory for maintaining the

quality of Metallurgical coke.

Flux

• Flux is a material, which is added to the contents of a Blast furnace or a Cupola for the purpose of purging the metal of impurities, and of rendering the slag more liquid.

• The flux most commonly used in iron and steel furnaces is limestone, which is charged in the proper proportions with the iron ore and fuel.

Charging Mechanism in Blast Furnace

Smelting operation

Smelting consists of two operations :

a- Chemical reduction of iron from its oxides by

CO, which is formed by the combustion of C in the

coke with the O2 of the air blast admitted through

the tuyeres near the base.

b- The liquefaction of the gangue by means of flux

(limestone) to form a fusible slag which will run

from the furnace.

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.

Chemical Reactions in Blast Furnace• The air, which is blown at the bottom of the furnace

causes partial combustion of the coke:

2C + O2 = 2CO + Heat 1• As the CO, which is powerful reducing agent,

rises through the charge, it chemically reduces the iron oxide :-

Fe2O3 + 3CO = 2Fe +3CO2 2

• Reduction sequence of iron oxide is as

Fe2O3 Fe3O4 FeO Fe, when temp is > 570˚ C

Fe2O3 Fe3O4 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 therefore 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.

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 :-

2CaO + SiO2 = 2CaO.SiO2

(Calcium silicate slag)• Calcium silicate slag has much lower melting point

than silica and can run from the furnace quite easily.• The furnace is tapped at regular intervals & the pig

iron 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.

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 be neutralized first.

• In British practice the sulphur is removed at a later

stage by adding soda-ash to molten pig iron as it

runs from the blast furnace into the ladle:-

FeS + Na2CO3 = FeO + Na2S + CO2

(Iron sulphide Soda ash joins the slag

in the pig iron)

Tapping of Slag

Pig Iron - Properties & uses

Definition:- “It is a high-carbon iron made by reduction of iron ore in the blast furnace”.

• Metallic iron, which is a product of reduction of of iron ore, appears in the lower part of ‘stack’ & in the ‘bosh’.

• It contains ~ 1 % C and found to in spongy form. As this material sinks down in the B.F. and is heated further, iron dissolves C in increasing quantities. This reduces the melting temp & it melts and draws off to the ‘hearth’ in the form of drops.

Pig Iron… (Continue)

– Pig iron is really a complex alloy. In addition to Fe, it contains up to 10 % of other elements, mainly, C, Mn, Si, S & P.

– Total amount of C in the pig iron is usually between 3 – 4% & is present either as compound, iron carbide (Fe3C) 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.

Thanks