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Making iron
 
How does the blast furnace work?
The Blast Furnace is a large steel structure about 30 metres high. It is lined with refractory firebricks that can withstand temperatures approaching 2000oC. The furnace gets its name from the method that is used to heat it. Pre-heated air at about 1000oC is blasted into the furnace through nozzles near its base.

The largest Blast Furnaces in the UK produce around 60 000 tonnes of iron per week. The blast furnace at Redcar, which is one of the largest in Europe, has produced up to 11 000 tonnes per day (77 000 tonnes per week) but is currently running at 8000 tonnes per day. This is equivalent to all the iron needed for about 5 cars every minute.

Photograph of blast furnace
Redcar number 1 blast furnace.
The hot air blast to the furnace burns the coke and maintains the very high temperatures that are needed to reduce the ore to iron. The reaction between air and the fuel generates carbon monoxide. This gas reduces the iron (III) oxide in the ore to iron.

iron (III) oxide + carbon monoxide iron + carbon dioxide
Fe2O3(s) + 3 CO(g) 2 Fe(s) + 3 CO2(g)

Because the furnace temperature is in the region of 1500oC, the metal is produced in a molten state and this runs down to the base of the furnace.

The furnace temperature is also high enough to decompose limestone into calcium oxide

calcium carbonate calcium oxide + carbon dioxide
CaCO3(s) CaO(s) + CO2(g)

This oxide helps to remove some of the acidic impurities from the ore

calcium oxide + silica calcium silicate
CaO(s) + SiO2(s) CaSiO3(l)

Diagram of blast furnace with roll over highlights
Diagram of blast furnace with roll over highlights
Diagram of blast furnace with roll over highlights Diagram of blast furnace with roll over highlights
Diagram of blast furnace with roll over highlights
The impurities are removed react with calcium oxide to make a liquid slag that floats on top of the molten iron. The slag is collected after the denser iron has been run out of a tap hole near the bottom of the furnace.

The production of iron in a Blast Furnace is a continuous process. The furnace is heated constantly and is re-charged with raw materials from the top while it is being tapped from the bottom. Iron making in the furnace usually continues for about ten years before the furnace linings have to be renewed.

The energy costs of the operation are kept to a minimum by collecting and cleaning the hot gas that leaves the furnace. This gas contains a lot of carbon monoxide. It can be re-used as a fuel for other steelmaking processes or to heat up the air blast to the furnace.

Refining iron
The metal that leaves the Blast Furnace contains between 4% and 5% of carbon. This much carbon makes a very hard but brittle metal which is not much use. The next step in the production of steel is to reduce the levels of carbon and other impurity elements in the hot metal.
 
   
Question 7.

Suggest why the Blast Furnace metal contains such a high percentage of carbon.