|
 Raw Materials:
-
Iron Ore
e.g. haematite
ore (iron(III) oxide)
-
or magnetite ore
-
coke (carbon, C)
-
hot air (for the
oxygen in it)
-
limestone (calcium
carbonate)
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- Iron oxide ore is mined in many
parts of the world. Examples 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
- 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
- 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.
- This a reduction reaction, which
can be described in two ways.
- The Fe2O3 loses
its oxygen O, 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.
- 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(s) +
3CO(g) ==> 2Fe(l-s) + 3CO2(g)
- 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(s) +
3C(g) ==> 2Fe(l-s) + 3CO(g)
- or
- iron(III) oxide +
carbon ==> iron + carbon dioxide
- 2Fe2O3(s) +
3C(g) ==> 4Fe(l) + 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: Fe2O3(s)
+ 3C(s) ==>
2Fe(l-s) + 3CO(g)
- or 2Fe2O3(s)
+ 3C(s) ==> 4Fe(l-s) + 3CO2(g)
- for magnetite: Fe3O4(s)
+ 4C(s) ==>
3Fe(l-s) + 4CO(g)
- or Fe3O4(s) +
2C(s) ==>
3Fe(l-s) + 2CO2(g)
- 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
- 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
- (i) 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(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 BUT is too brittle for many applications
due to too high a carbon content from the coke.
 Copyright Dr W P Brown 2000-2010
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etc. |
Doc Brown's
GCSE/IGCSE science-CHEMISTRY REVISION
