The Blast Furnace

What is a Blast Furnace? The purpose of a blast furnace is to reduce and convert iron oxides into liquid iron called "hot metal". The blast furnace is a huge, steel stack lined with refractory brick. Iron ore, coke and limestone are put into the top, and preheated air is blown into the bottom.

Why does Iron have to be extracted in a Blast Furnace??? Iron can be extracted by the blast furnace because it can be displaced by carbon. This is more efficient method than electrolysis because it is more cost effective The fact that iron-oxide can be reduced by carbon along with the abundance of iron-oxide in earth’s crust (4th most abundant element), makes iron so economical

The Method Three substances are needed to enable to extraction of iron from its ore. The combined mixture is called the charge. Iron ore, haematite - Fe2O3 often contains impurities such as sand (SiO2) Limestone (calcium carbonate). Coke - mainly carbon The charge is placed in the blast furnace. The blast furnace is around 30 metres high and lined with fireproof bricks. Hot air is blasted through the bottom.

Principle reactions of iron-making Oxygen in the air reacts with coke to give carbon dioxide: C(s) + O 2(g)  CO2(g) The limestone breaks down to form carbon dioxide: CaCO3(s)  CO2 (g) + CaO(s) Carbon dioxide produced in 1 + 2 react with more coke to produce carbon monoxide: CO2(g) + C(s)  2CO(g)

IRON MAKING IS THUS A REDUCTIVE PROCESS The carbon monoxide reduces the iron in the ore to give molten iron: 3CO(g) + Fe2O3(s)  2Fe(l) + 3CO2(g) The limestone from 2, reacts with the sand to form slag (calcium silicate): CaO(s) + SiO(s)  CaSiO3(l) Other slag forming reactions also occur IRON MAKING IS THUS A REDUCTIVE PROCESS

Sections of a blast furnace Main sections are (from bottom to top) Hearth Bosch Stack Charging is done through the top The tuyeres through which hot air is blasted are situated at the joining of bosch and hearth A slaghole and a taphole is provided at the hearth

Outline of operation The burden is charged in layers One layer comprises coke and the other one is iron ore+flux and so on A bed of coke is maintained in the tuyere region all the time The hot air blasted through the tuyeres, reacts with the coke, burning them, and produces carbon monoxide This zone experiences the highest temperature (1800-2000 C) because combustion takes place here The hot gases generated rise through the furnace and gives up its sensible heat to the charge that is filling the stack column

Outline of operation As the gas rises its temperature drops because its heat is taken away by the charge. A temperature gradient is created, and so the temperature is highest at the tuyere region and lowest at the top The alternately charged burden thus, experiences different temperatures depending on where it is located in the furnace The level of the entire burden periodically drops as metal and slag are tapped from the bottom This means that burden charged at the top (where T is low) gradually experiences higher T as it drops periodically

Outline of operation When it drops sufficiently and the temperature is suitable, iron-oxide reduction begins As it drops further the ore is eventually fully reduced to metallic iron and thereafter picks up some extra carbon (carburization – this also lowers the melting point of solid iron) At some point the temperature is high enough to cause melting and the metallic drops trickle through the bed of coke and so does the liquid slag, which has formed by now The molten liquids finally settle at the hearth where slag and metal forms separate layers due to their specific gravity being different The coke that was charged from the top does not take part in any significant reaction until it reaches the tuyere region Here it joins the coke bed already present and serves to replenish it, which is continually burning

Detailed reactions (see figure on next page) Logical order of reactions (Note: these reactions DO NOT occur in the same place. They occur in different parts but reaction products of one reaction may take part in another one) Burning of coke Reduction, carburization and melting of iron oxides by gas produced in 1 Decomposition of limestone by heat generated from 1 Slag formation by reactants made available from 2 and 3 Slag dissociation (reverse of 4) Reduction reactions made possible because of 5 Impurity pick up due to reactions in 6

1. Iron ore + limestone sinter 2. Coke 3. Elevator 4. Feedstock inlet 5. Layer of coke 6. Layer of sinter pellets of ore and limestone 7. Hot blast (around 1200 °C) 8. Removal of slag 9. Tapping of molten pig iron 10. Slag pot 11. Torpedo car for pig iron 12. Dust cyclone for separation of solid particles 13. Cowper stoves for hot blast 14. Smoke outlet (can be redirected to carbon capture & storage (CCS) tank) 15. Feed air for Cowper stoves (air pre-heaters) 16. Powdered coal 17. Coke oven 18. Coke 19. Blast furnace gas downcomer

About the stoves Something that increases the thermal efficiency of the furnace greatly is the use of gas coming out of the furnace itself (Blast burnace gas-BFG) This gas contains some percentages of unburnt CO (~20%), and so can be used as a fuel This gas is burnt inside the cowper stoves (which works like regenerators) heating them up Once heating is complete, BFG is shut off and fresh air is then passed through the hot stoves This heated air is then sent to the furnace which we have referred to as the hot blast before When the T of the stove drops too much fresh air is shut off and fuel is passed again Thus one stove cannot continuously supply hot air, because it works in a cyclic heating-cooling operation In fact three or more stoves are generally required because the time it takes to heat up one stove is not the same as the time it takes to cool it So two stoves would be inadequate and a third would be needed