Iron ore is used to make iron and steel eg haematite and magnetite. It is produced in a blast furnace by reducing iron oxides with carbon and it is the carbon that removes the oxygen from the iron oxides – the carbon is known as the reducing agent.
Because its position in the reactivity series of metals, iron can be extracted using carbon in a blast furnace because iron is below carbon (iron is less reactive than carbon). Therefore, iron can be displaced from its eg its oxides, by heating with the theoretically 'more reactive' carbon in a sort of displacement reaction.
Iron oxide ore is mined in many parts of the world.
Examples of rich or high quality ores are haematite Fe2O3 and magnetite Fe3O4.
A solid mixture of magnetite/haematite ore, coke and limestone is continuously fed into the top of the blast furnace.
The double role and function of coke (carbon)
1st Coke function (i) As a fuel
The coke is ignited at the base and hot air blown in to burn the coke (carbon) to form carbon dioxide in an oxidation reaction (C gains O).
The heat energy is needed from this very exothermic reaction to raise the temperature of the blast furnace to over 1000oC to effect the ore reduction. The furnace contents must be he
carbon + oxygen ==> carbon dioxide
C + O2 ==> CO2
C(s) + O2(g) ==> CO2(g)
2nd Coke function (ii) As a reducing agent
At high temperatures the carbon dioxide formed, reacts with more coke (carbon) to form carbon monoxide
carbon dioxide + carbon ==> carbon monoxide
CO2 + C ==> 2CO
CO2(g) + C(s) ==> 2CO(g)
Note: that carbon dioxide, CO2, is reduced by oxygen loss to the carbon, and the carbon is oxidised by oxygen, O gain to carbon dioxide.
The carbon monoxide is the molecule that actually removes the oxygen from the iron oxide ore.
REDOX definition reminders – reduction is a process of oxygen loss (or electron gain) and oxidation is a process of oxygen gain (or electron loss).
This a reduction reaction, which can be described in two ways.
The Fe2O3 loses its oxygen O (reduction), or Fe3+ gains three electrons to form Fe.
The carbon monoxide, CO, is known as the reducing agent because it is the oxygen (O) remover and gets oxidised to carbon dioxide in the process (CO gains oxygen).
This frees the iron, which is molten at the high blast furnace temperature, and trickles down to the base of the blast furnace and run off.
An example of the main reduction reaction is ...
iron(III) oxide + carbon monoxide ==> iron + carbon dioxide
Fe2O3 + 3CO ==> 2Fe + 3CO2
Fe2O3(s) + 3CO(g) ==> 2Fe(l/s) + 3CO2(g)
Note on state symbols (l/s), iron is initially formed as a liquid and then obviously solidifies on cooling.
note, as in the two reactions above, oxidation and reduction always go together!
Other possible iron ore reduction reactions are direct reduction of the iron oxide by carbon itself ...
iron(III) oxide + carbon ==> iron + carbon monoxide
Fe2O3 + 3C ==> 2Fe + 3CO
Fe2O3(s) + 3C(g) ==> 2Fe(l/s) + 3CO(g)
or
iron(III) oxide + carbon ==> iron + carbon dioxide
2Fe2O3 + 3C ==> 4Fe + 3CO2
2Fe2O3(s) + 3C(g) ==> 4Fe(l/s) + 3CO2(g)
The iron is initially formed in its liquid state because of the high temperatures of the blast furnace (well over 1000oC) but when cooled it is cast into solid ingots, or the liquid iron can be transported directly in special insulated 'torpedo' wagons to a steel making plant on the same industrial site complex..
At the highest temperatures in a blast furnace the reactions can be written as a direct reduction of the oxide with carbon, and, carbon monoxide (CO) can be formed as well as carbon dioxide (CO2) e.g.
for haematite
iron(III) oxide + carbon ==> iron + carbon monoxide
Fe2O3 + 3C ==> 2Fe + 3CO
Fe2O3(s) + 3C(s) ==> 2Fe(l/s) + 3CO(g)
or
iron(III) oxide + carbon ==> iron + carbon dioxide
2Fe2O3 + 3C ==> 4Fe + 3CO2
2Fe2O3(s) + 3C(s) ==> 4Fe(l/s) + 3CO2(g)
for magnetite
'iron tetroxide' + carbon ==> iron + carbon monoxide
Fe3O4 + 4C ==> 3Fe + 4CO
Fe3O4(s) + 4C(s) ==> 3Fe(l/s) + 4CO(g)
or
'iron tetroxide' + carbon ==> iron + carbon dioxide
Fe3O4 + 2C ==> 3Fe + 2CO2
Fe3O4(s) + 2C(s) ==> 3Fe(l/s) + 2CO2(g)
Name note: The correct name for iron tetroxide is diiron(III)iron(II) oxide, but I wouldn't worry about it!
I'm afraid the chemistry of the blast furnace can get very complicated indeed!
The role of limestone in the extraction of iron
The original ore contains acidic mineral impurities such as silica (SiO2, silicon dioxide).
These react with the calcium carbonate (limestone) to form a molten slag, the main ingredient being calcium silicate.
There are two ways to show the formation of the waste slag, which is mainly calcium silicate.
(i) calcium carbonate + silica ==> calcium silicate + carbon dioxide
(i) CaCO3 + SiO2 ==> CaSiO3 + CO2
Reaction (i) is a sort of displacement reaction i.e. the less volatile high melting/boiling silicon dioxide (silica) displaces the more volatile gaseous carbon dioxide.
However, this is sometimes shown in two stages, i.e. reactions (ii) and (iii):
(ii) CaCO3 ==> CaO + CO2
(iii) CaO + SiO2 ==> CaSiO3
(ii) is the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide, and the reaction needs a high temperature of over 900oC, but that's no problem in the blast furnace!
(iii) is the combination of the basic calcium oxide and the acidic silicon dioxide to form calcium silicate.
However, whichever way you represent the reaction, its all the same in the end.
If you 'add up' chemical reactions (ii) and (iii) you get (i), check for yourself.
The molten slag forms a layer above the more dense molten iron and they can be both separately, and regularly, drained away. The iron is cooled and cast into pig iron ingots OR transferred directly to a steel producing furnace.
The waste gases and dust from the blast furnace must be appropriately treated to avoid polluting the environment.
The highly toxic carbon monoxide can actually be burnt to provide a source of heat energy, and in the exothermic reaction it is converted into relatively harmless carbon dioxide.
carbon monoxide + oxygen ==> carbon dioxide
2CO + O2 ==> 2CO2
2CO(g) + O2(g) ==> 2CO2(g)
Acidic gases like sulphur dioxide from sulphide ores, can be removed by bubbling through an alkali solution such as calcium hydroxide solution ('limewater') where it is neutralised and oxidised to harmless calcium sulphate. Cleaning a gas in this way is called 'gas scrubbing'.
Any contaminated water must be purged of harmful chemicals before being released into a river or recycled via water treatment plant.
The waste slag is used for road construction or filling in quarries which can then be landscaped.
Iron from a blast furnace is ok for very hard cast iron objects B
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