3a.
The Extraction of Aluminium
The current method for extracting aluminium is expensive because it involves several
stages and uses large amounts of costly electrical energy. It is much more expensive than
using carbon reduction to make iron in a blast furnace.
The process of electrolysis uses of
large amounts of energy in the
extraction of a reactive metals and
makes aluminium expensive to
produce.
Aluminium is a very useful metal
but expensive to produce.
Because its position in the reactivity
series of metals, aluminium cannot
be extracted using carbonbecause
it is above carbon in the reactivity
series ie more reactive than carbon
in the series, carbon is not reactive
enough to displace aluminium from
its compounds such as aluminium
oxide.
So, if aluminium is too reactive to
be obtained by carbon reduction
of its oxide another method must
be employed which is
called electrolysis.
Aluminium is obtained from mining
the mineral bauxite which is mainly
aluminium oxide (Al2O3) and bauxite
must be purified prior to
electrolysis, adding to the
manufacturing costs.
The purified bauxite
ore of aluminium oxide is
continuously fed in. Cryolite is
added to lower the melting point and
dissolve the ore.
The ore–compound containing the
aluminium must be molten so
the ions are free to move to the
electrodes. The conducting melt is
called the electrolyte, so extracting
aluminium this way involves
theelectrolysis of molten
aluminium oxide.
Ions must be free to move to the
electrodes called the cathode (–,
negative), attracting positive ions
e.g. Al3+, and the anode (+,
positive) which attracts negative
ions e.g. O2–.
When the d.c. current is passed
through aluminium forms at the
negative cathode (metal*) and
sinks to the bottom of the tank
where it can tapped off, collected
and run into moulds to cool down
before transportation to it will be
used to make things.
At the positive anode, oxygen gas
is formed (non–metal*). This is
quite a problem. At the high
temperature of the electrolysis cell it
burns and oxidises away the carbon
electrodes to form toxic carbon
monoxide or carbon dioxide. So
the carbon–graphite electrode is
regularly replaced and the waste
gases dealt with!
It is a costly process (6x more
than Fe!) due to the large
quantities of expensive electrical
energy needed for the process.
* Two general rules:
o Metals and hydrogen (from
positive ions), form at the
negative cathode
electrode.
o Non–metals (from negative
ions), form at the positive
anode electrode.
Raw materials for the electrolysis process: ELECTRODE EQUATIONS: redox details
of the electrode processes
Bauxite ore of impure aluminium oxide Electrolysis reminders – the
[Al2O3 made up of Al3+ and O2– ions] negative electrode (–) is called the
Carbon (graphite) for the electrodes. cathode and attracts positive ions or
cations e.g. Al3+, and the positive
Cryolite reduces the melting point of electrode (+) is called the anode and
the ore and saves energy, because attracts negative ions or anions e.g.
the ionsmust be free to move to carry O2–.
the current and less energy is needed to The negative cathode electrode
melt the aluminium oxide obtained from attracts positive ions, the aluminium
the bauxite ore. ion.
Electrolysis means using d.c. At the negative (–) cathode,
electrical energy to bring about reduction occurs (electron gain)
chemical changes e.g. decomposition when the positive aluminium
of a compound to form metal deposits ions are attracted to it. They gain
or release gases. The electrical energy three electrons to change to neutral
splits the compound! Al atoms.
o Al3+ + 3e– ==> Al
At the electrolyte connections called
theanode electrode (+, attracts – ions) The positive anode attracts negative
and the cathode electrode (–, ions, the oxide ion.
attracts + ions). An electrolyte is
a conducting melt or solution of freely At the positive (+) anode,
moving ions which carry the charge of oxidation takes place (electron
the electric current. loss) when the negative oxide
ions are attracted to it. They lose
two electrons forming neutral
oxygen molecules.
o 2O2– ==> O2 + 4e–
o or
o 2O2– – 4e– ==> O2
Note: Reduction and Oxidation
always go together!
The overall electrolytic
decomposition is ...
o aluminium oxide ==>
aluminium + oxygen
o 2Al2O3 ==> 4Al + 3O2
o and is a very endothermic
process, lots of electrical
energy input!
o Note that the aluminium
oxide loses its oxygen,
therefore in this
electrolytic process the
compound aluminium
oxide is reduced to the
metal aluminium.
GENERAL NOTE ON ELECTROLYSIS:
o Any molten or dissolved material in which the liquid contains free moving
ions is called the electrolyte.
o Ions are charged particles e.g. Na+ sodium ion, or Cl– chloride ion, and their
movement or flow constitutes an electric current, because a current is moving
charged particles.
o What does the complete electrical circuit consist of?
There are two ion currents in the electrolyte flowing in opposite
directions:
positive cations e.g. Al3+ attracted to the negative cathode
electrode,
and negative anions e.g. O2– attracted to the positive anode
electrode,
BUT remember no electrons flow in the electrolyte, only in the
graphite or metal wiring!
The circuit of 'charge flow' is completed by the electrons moving around
the external circuit e.g. copper wire or graphite electrode, from the
positive to the negative electrode
This e– flow from +ve to –ve electrode perhaps doesn't make sense until
you look at the electrode reactions, electrons released at the +ve
anode move round the external circuit to produce the electron rich
negative cathode electrode.
o Electron balancing: In the above process it takes the removal of four electrons
from two oxide ions to form one oxygen molecule and the gain of three electrons
by each aluminium ion to form one aluminium atom.
Therefore for every 12 electrons you get 3 oxygen molecules and 4
aluminium atoms formed.
This means you can do mole ratio product calculations.
See electrolysis calculations in section 13. of the
Chemical Calculations pages
NOTE on RECYCLING Aluminium
o About 39% of the aluminium in foil, car components etc. is recycled aluminium.
o This makes good economics because recycling saves on costs AND allows a
mineral resource like aluminium's bauxite ore to last a lot longer – slower
depletion of the Earth's mineral ore resources will make it last longer.
Transport costs may be less (ie within UK now), but much more
importantly
mining costs are omitted – energy/machinery involved in digging out
the ore, crushing it, transporting the ore,
and the cost of actually extracting the metal from its finite ore
resource – electrolysis plant, expensive electrical energy used
o So, scrap metal merchants are doing a roaring trade at the moment.
o The savings are partly reduced by the cost off collecting waste/scrap metal and
purifying for further use.
It is estimated that recycling aluminium only uses 5% of the energy
required to extracted the same mass of aluminium from its ore – the
original aluminium extraction uses very expensive electrical energy for
the electrolysis.
The social, economic and environmental impacts of exploiting metal ores are
discussed on a separate page.
Aluminium is very useful metal and used as a lightweight construction material eg
greenhouse frames.
Aluminium can be made more resistant to corrosion by a process called anodising.
Aluminium is a reactive metal but it is resistant to corrosion. This is because aluminium
reacts in air to form a layer of aluminium oxide which then protects the aluminium from
further attack.
o This is why it appears to be less reactive than its position in the reactivity series
of metals would predict.
For some uses of aluminium it is desirable to increase artificially the thickness of the
protective oxide layer in a process is called anodising.
o This involves removing the oxide layer by treating the aluminium sheet with
sodium hydroxide solution.
o The aluminium is then placed in dilute sulphuric acid and is made the positive
electrode (anode) used in the electrolysis of the acid.
o Oxygen forms on the surface of the aluminium and reacts with the aluminium
metal to form a thicker protective oxide layer.
Aluminium can be alloyed to make 'Duralumin' by adding copper (and smaller
amounts of magnesium, silicon and iron), to make a stronger alloy used in aircraft
components (low density = 'lighter'!), greenhouse and window frames (good anti–
corrosion properties), overhead power lines (quite a good conductor and 'light'), but steel
strands are included to make the 'line' stronger and poorly electrical conducting ceramic
materials are used to insulate the wires from the pylons and the ground.
o There is a note about the bonding and structure of alloys on another page.
3b. The electrolytic extraction of the very reactive
metal sodium
The process of electrolysis uses of large amounts of
energy in the extraction of a reactive metals like sodium, potassium, magnesium
and calcium etc. and makes them expensive to produce.
Because its position in the
reactivity series of metals, sodium cannot be extracted using carbon, sodium
is above carbon and cannot be displaced by it. So, sodium is too reactive to be
obtained by carbon reduction of its oxide and another method must be
employed which is called electrolysis.
Sodium, like many of the most reactive metals, can be extracted by
electrolysis of its molten chloride. This can be done in the 'Down's Cell' shown
in the diagram.
Electrolysis reminders – the negative electrode (–) is called the cathode and
attracts positive ions or cations e.g. Na+, and the positive electrode (+) is called
the anode and attracts negative ions or anions e.g. Cl–. The ore–compound
containing the sodium (or other metal) must be molten so the ions are free to
move to the electrodes. The conducting melt is called the electrolyte.
In the molten salt the positive sodium ions migrate to the negative cathode
electrode and are reduced by electron gain to form liquid sodium atoms.
The negative cathode electrode attracts positive ions, eg the sodium ion.
At the (–) cathode: Na+ + e– ==> Na
Equally mobile in the molten chloride salt are the negative chloride ions, which
migrate to the positive anode electrode and get oxidised by electron loss to form
green chlorine gas molecules. Initially two chlorine atoms are formed and these
rapidly combine to give chlorine molecules.
The positive anode attracts negative ions, eg the chloride ion.
At the (+) anode overall: 2Cl– ==> Cl2 + 2e–
or 2Cl– ==> 2Cl + 2e– and then 2Cl ==> Cl2
Overall chemical change: 2NaCl ==> 2Na + Cl2
Other very reactive metals like lithium, potassium
and calcium can be extracted in the same way by
electrolysing their molten salts. As you can see from
the diagram on the right, all these metals are above
carbon in the reactivity series and cannot be displaced
by carbon.
Some general notes on electrolysis
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