ALUMINIUM

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ALUMINIUM

• Extraction and uses

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BACKGROUND Aluminium is the most common metal in
the Earths crust. It comprises approximately
7.5 of the crust by mass. Aluminium is very
reactive and is found in a range of
oxide, hydroxide and silicate minerals.
THE REACTIVITY SERIES
Aluminium is higher than carbon in the reactivity
series so cannot be extracted by carbon
reduction, unlike less reactive metals such as
zinc, iron and lead. For those metals above
carbon, electrolysis is used.
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ELECTROLYSIS
Electricity
Lyse/lysis meaning to move
For electrolysis we need a system we can pass a
current through.
What is current?
A strict definition would say that current is a
flow of electrical charge through a conductor.
The important point for electrolysis is that this
charge may be as electrons or as charged ions.
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What systems can undergo electrolysis?

1. Aqueous solutions of ions

Aluminium compounds found in ores that are
very insoluble as we shall see later. This means
that dissolving isnt an option.
2. Melting the ore to produce a molten liquid
Molten liquids can produce ions that may
be separated. This is used for aluminium.
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The process of aluminium electrolysis is not new
The present industrial method of production was
discovered simultaneously and independently in
1886 by Paul-Louis Héroult in France and Charles
Hall of the United States. Their method is the
basis for the world aluminium industry today
Before we look at the ores, we should look at
the main features of processing plants ..
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LYNEMOUTH, just north of Newcastle, is one of two
primary Aluminium production plants in the UK.
There is another on Anglesey in N. Wales.
Electrolysis plant and rolling mills
Coal-fired power station dedicated to the plant
Waste residues, largely iron oxides
Can you identify the three main features marked?
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There are a number of secondary aluminium plants
in the UK where waste aluminium is separated,
smelted and recovered. This plant is near Fort
William in Scotland.
Water from reservoir below Ben Nevis
Hydroelectric power station
Smelting and electrolysis plant with rolling mills
Again, can you see the two main features not
waste this time!
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Aluminium Ore
BAUXITE
Bauxite is not a single composition, more a range
of hydrated aluminium oxides and hydroxides.
It is found as a weathering product in soils in
hot sub-tropical and tropical climates. Main
resources are in N. Africa, S. America, SE Asia
and N. Australia
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Typical mining operations for bauxite
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The chemistry of bauxite processing
The complex mixture that makes up bauxite is
first processed to produce pure aluminium oxide,
alumina (Al2O3)
(For GCSE you do not need to know how, however
this Is a requirement for AS/A2)
So we need molten alumina to do electrolysis
. ..however, this would require a temperature
over 2072oC, the melting point of alumina. This
is far too high for an economic process.
Alumina is melted in another chemical, cryolite
(Na3AlF6), which lowers the melting point to
around 1000oC.
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The electrolysis process for aluminium
Draw this, noting the bubbles. We will discuss
the chemistry of these later. Note the tap hole
for molten aluminium. This is a CONTINUOUS process
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Keeping the electrolysis cell molten
The energy to keep the alumina and cryolite
molten comes from a 100000A current flowing
through the 6V electrolysis cell. The energy to
provide the initial melting is enormous so
these run continuously for many years before
finally being taken out of service.
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Aluminium Electrolysis Chemistry
The negative cathode, around the edges of the
cell, attracts the positively charged aluminium
ions. These gain electrons and aluminium which
fall to the base of the tank as molten metal
Al3(l) 3e- gt Al(l)
The positively charged anode, as blocks in the
surface of the cell, Attracts the negatively
charged oxide ions..
2O2-(l) gt O2(g) 4e-
Oxygen is not evolved though. This process is at
1000oC with a carbon anode. This anode burns away
producing carbon dioxide. This is the gas given
off from the process. The carbon anodes need
regular replacement to keep the cells
operational.
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Watch the TV/Video Link your knowledge of the
process to what it looks like in real life!
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ANODISING ALUMINIUM
Aluminium is very unreactive for a metal so high
up the reactivity series. This is due to the ease
with which it forms a protective oxide layer.
Anodising makes this oxide layer even stronger.
Step 1. The original oxide layer is removed with
sodium hydroxide.
Step 2. The aluminium is made into the anode in
dilute sulphuric acid. Any metal can make the
cathode but usually this is made of aluminium as
well.
Step 3. Oxygen, evolved at the anode, reacts with
the aluminium to form a 0.02mm oxide film. Much
thicker than before anodising.
Step 4. At this stage, the oxide is porous and
dyes can be added. Further treatment makes the
oxide solid and resistant.
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Examples of anodised aluminium.

• Benefits of anodised aluminium
• Increased corrosion resistance.
• Dyed products available

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Uses of aluminium
Aluminium is low density, strong, a good
conductor of heat and good conductor of
electricity.

• Electricity cables along overhead pylons.
• Aircraft industry
• Vehicles particularly where weight is an issue
  eg. Trams,
• trains etc.
• 4. Cooking pans this includes the appearance of
  the metal as
• a factor in choice, to complement the low
  density, strength and
• heat conducting aspects.
• Examples of anodised aluminium are now also seen
  in many
• applications.