Aluimium Electrolysis

1. ALUMINIUMALUMINIUM
Extraction and usesExtraction and uses

2. BACKGROUND
Aluminium is the most common metal in the Earth’s 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.

3. 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.

4. 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 isn’t 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.

5. 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 …………..

6. LYNEMOUTH, just north of Newcastle, is one of two primary
Aluminium production plants in the UK. There is another on
Anglesey in N. Wales.
Can you identify the three main features marked?
Electrolysis plant and
rolling mills
Coal-fired power
station dedicated
to the plant
Waste residues, largely
iron oxides

7. 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.
Again, can you see the two main features – not waste this time!
Smelting and
electrolysis plant
with rolling mills
Hydroelectric power
station
Water from reservoir below
Ben Nevis

8. 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

9. Typical mining operations for bauxite

10. 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 2072o
C,
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 1000o
C.

11. 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

12. 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.
Keeping the electrolysis cell molten

13. 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- => Al(l)
The positively charged anode, as blocks in the surface of the cell,
Attracts the negatively charged oxide ions…..
2O2-
(l) => 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.

14. Watch the TV/Video
Link your knowledge of the process
to what it looks like in real life!

15. 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.

16. Examples of anodised aluminium.
Benefits of anodised aluminium
1. Increased corrosion resistance.
2. Dyed products available

17. Uses of aluminium
Aluminium is low density, strong, a good conductor of heat and
good conductor of electricity.
1. Electricity cables along overhead pylons.
2. Aircraft industry
3. 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.