ore (iron(III) oxide) the source of iron.
or magnetite ore
coke (carbon, C),
both fuel and reducing agent.
hot air (for the
oxygen in it) to burn the coke
carbonate) to remove certain impurities like silica.
ore is used to make iron and steel eg haematite and magnetite. Iron is
produced (extracted) 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. Coke is a cheap and readily made
Iron is not a very reactive metal, so, 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
- Iron oxide ore is mined in many
parts of the world.
- Examples of rich or high quality ores are
- and magnetite formula Fe3O4.
- A solid mixture of magnetite/haematite ore,
coke and limestone is continuously fed into the top of the blast
- The double role and function of coke
- 1st Coke function (i) As a fuel
- The coke is ignited
at the base and
air blown in to burn the coke (carbon) to form carbon dioxide in an
reaction (C gains O).
- The heat energy is needed from this very
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 ==>
- 2nd Coke function (ii) As a reducing
- At high temperatures the carbon dioxide formed,
reacts with more coke (carbon) to form carbon
- carbon dioxide + carbon ==> carbon monoxide
- Note: that carbon dioxide, CO2,
is reduced by oxygen
loss to the carbon, and the carbon is oxidised by oxygen, O gain to
- 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 for more advanced students
- the ion Fe3+ gains three electrons to form Fe,
- noting that electron gain is
- 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
- 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.
as in the two reactions above, oxidation and reduction always go
- Other possible iron ore reduction
reactions are direct reduction of the iron oxide by carbon
- iron(III) oxide +
carbon ==> iron + carbon monoxide
- Fe2O3 +
3C ==> 2Fe + 3CO
- Fe2O3(s) +
3C(g) ==> 2Fe(l/s) + 3CO(g)
- 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
+ 3C ==>
2Fe + 3CO
+ 3C(s) ==>
2Fe(l/s) + 3CO(g)
- iron(III) oxide + carbon ==> iron
+ carbon dioxide
+ 3C ==> 4Fe + 3CO2
+ 3C(s) ==> 4Fe(l/s) + 3CO2(g)
- for magnetite
- 'iron tetroxide' + carbon ==>
iron + carbon monoxide
+ 4C ==>
3Fe + 4CO
+ 4C(s) ==>
3Fe(l/s) + 4CO(g)
- 'iron tetroxide' + carbon ==>
iron + carbon dioxide
- Fe3O4 +
3Fe + 2CO2
- Fe3O4(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
- 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
- There are two ways to show the
formation of the waste slag, which is mainly calcium
- (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 +
- (iii) CaO + SiO2 ==>
- (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
- 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 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 monoxide + oxygen ==>
- 2CO + O2
- 2CO(g) + O2(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
- Iron from a blast furnace is ok for
very hard cast iron objects BUT is too brittle for many applications
due to too high a carbon content from the coke.
2b. STEEL MAKING
Why do we need to
convert most iron into steel alloys?
Most metals in everyday use are alloys.
Iron is a good conductor of heat and can be bent or hammered into
shape (malleable), quite strong physically – made stronger when alloyed with
This makes iron useful as structural material and for making things that must allow heat to pass through easily
and useful construction materials.
Steel is an alloy
because it is a mixture of a metal (iron) with other elements
(carbon and perhaps other metals too).
Iron from the
furnace contains about ~96% iron with ~4% of impurities including
carbon, silica and phosphorus.
In this state the cast iron
is too hard and too brittle for most purposes.
Cast iron is hard and can be
used directly for some purposes eg manhole covers, ornamental railings because of its
strength in compression and is very hard wearing.
However, if all the
impurities are removed, the resulting very pure iron is too soft for any
Therefore, strong useful
steel is made by controlling the amount of carbon and selected metals to
produce an alloy mixture with the right physical properties fit for a
particular application e.g. steel for car bodies, chrome stainless
steel, extremely hard and tough tungsten–iron steel alloys etc.
The real importance of alloys is
that they can be designed
to have properties for specific uses in terms of eg compression/tensile strength
or corrosion reduction ie less susceptible to rusting.
steels (0.1% carbon) are easily shaped for car bodies
High–carbon steels (1.5% carbon,
often with other metals too) are
hard wearing and inflexible and can be used for cutting tools, bridge
Stainless steels have
chromium (and maybe nickel) added and are much resistant to corrosion (from
oxygen/water) than iron or plain steel which readily rust. Objects made of
iron or plain steel, particularly those exposed to the weather, regularly
have to be painted or coated with some other protective layer from the
effects of water and oxygen.
The properties of iron
can be altered by adding small quantities of other metals or carbon to
make steel, one of the useful metal alloys
in widespread use today.
Steels are alloys since they are
mixtures of iron with
other metals or with non–metals like carbon or silicon.
(1) Molten iron from the
blast furnace is mixed with recycled scrap iron
Then pure oxygen is
passed into the mixture and the non–metal impurities
such as silicon or phosphorus are then converted into
acidic oxides (the BOS oxidation process) ..
Calcium carbonate (a
base) is then added to remove the acidic
oxide impurities (in an acid–base reaction). The salts produced by this reaction form a slag which can
be tapped off separately.
produce pure iron.
of carbon and/or other metallic elements such as titanium, manganese
or chromium are then added to make a wide range
of steels with particular properties.
Because of the high
temperatures the mixture is stirred by bubbling in unreactive
Economics of recycling
scrap steel or ion: Most steel consists of >25% recycled
iron/steel and you do have the 'scrap' collection costs and
problems with varying steel composition* BUT you save enormously
because there is no mining cost or overseas transport costs AND
less junk lying around! (NOTE: * some companies send their own
scrap to be mixed with the next batch of 'specialised' steel they
order, this saves both companies money!). More on this below.
NOTE on RECYCLING Iron
About 42% of iron/steel in
goods/components manufactured in iron/steel is recycled iron/steel,
whether it be steel pans, car bodies, bridge girders, stainless steel
This makes good economics
because recycling saves on several costs AND allows a mineral resource
like iron's haematite or magnetite ore resources to last a lot longer – slower depletion of
the Earth's mineral ore resources will make it last longer.
So, scrap metal merchants
are doing a roaring trade at the moment.
The savings are partly
reduced by the cost off collecting waste/scrap metal.
There are particular
problems due to the varying composition of alloys, but if the
composition is known, or obtained from chemical analysis, the different
compositions can be blended together to a desired alloy composition.
social, economic and environmental impacts of
exploiting metal ores including RECYCLING are discussed on a separate page.
WHERE NEXT? Other
associated KS4 Science GCSE/IGCSE chemistry web pages on this site
equations: blast furnace equations C + O2 ==> CO2 CO2 + C ==> 2CO Fe2O3
+ 3CO ==> 2Fe + 3CO2 Fe2O3 + 3C ==> 2Fe + 3CO 2Fe2O3 + 3C ==> 4Fe + 3CO2
2Fe2O3 + 3C ==> 4Fe + 3CO2 Fe3O4 + 4C ==> 3Fe + 4CO Fe3O4 + 2C ==> 3Fe +
2CO2 CaCO3 + SiO2 ==> CaSiO3 + CO2 2CO + O2 ==> 2CO2 steel making Si +
O2 ==> SiO2, or 4P + 5O2 ==> P4O10
Extracting iron from its ores with carbon and limestone in a blast furnace, how
to change iron into steel, steelmaking