Production of iron
The blast furnace The chemistry of reaction

Click for larger imageIron is the most important metal in our modern society. It has been at the centre of all technological advances since the dawn of the Iron Age, some 3000 years ago. Its availability, strength and hardness have made it the most used metal.

The only native metal iron found comes from meteorites picked up off the ground. This was the only source of iron until around 1000 BC, when the first iron was produced from iron ore. Because iron is quite reactive with oxygen and water, any iron from meteorites that fall in wet conditions soon rusts away, which means that iron from this source is very rare indeed.

The first iron production was associated with fired pottery production. Scientist believe that red ochre used as decoration on pottery, released iron when heated in kilns thus the first drops of man made iron were produced. Up until then, open fires had not been hot enough to extract iron. What was required was very hot air with its oxygen removed, impossible to achieve with open fires.

The blast furnace
The modern blast furnace is simply a very large and very hot kiln. Iron ore (the most common being haematite or iron oxide (Fe2O3) is mixed with carbon (in the form of coke) and limestone and dumped into the top of a large furnace. Hot air from oil burners at the bottom of the furnace rises up through the mixture. This heats the mixture, allowing the carbon to react first with the oxygen and then the iron oxide which removes the oxygen from the iron oxide and releases liquid iron. The name "blast furnace" comes from this blast of hot air sent up though the furnace.

The limestone combines with waste materials from the iron ore and forms a slag at the bottom of the furnace. The liquid iron is poured off from the layer, which forms under the slag at the bottom of the furnace. The slag floats on top of the molten iron because the iron is denser and sinks through the liquid slag. The carbon dioxide gas produced is vented out from the top of the furnace.

The iron produced is called "pig iron" because originally the molten iron was poured into sand pits and the resulting solid iron resembled a pig! Another story is that the system looked like piglets feeding from a sow. Although modern production takes off the molten iron and combines it with other metals and carbon to form steel, the name "pig iron" has stuck.

The blast furnace process is a continual one, working 24 hours a day all year. The average blast furnace works for about 5 years after which time it is closed down, refurbished with new fire bricks and reactivated.

Blast furnace.

The chemistry of reaction
The simplified overall reaction in the blast furnace is shown in the equation:

2Fe2O3 + 3C = 4Fe + 3 CO2 Click for larger image

This does not happen in one step but rather in three separate processes, starting at the bottom of the furnace and working up to the top.

The first reaction occurs near the bottom of the furnace between carbon and oxygen from the hot air to forming carbon dioxide:

C + O2 = CO2 Click for larger image

The rising carbon dioxide further reacts with carbon forming carbon monoxide.

CO2 + C = 2 CO Click for larger image

The final reaction occurring near the top of the furnace is between the carbon monoxide and the iron ore, to produce iron and carbon dioxide.

Fe2O3 + 3CO = 2Fe + 3 CO2 Click for larger image

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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