Video Transcript
In this video, we will learn how
aluminum is extracted from its ore through the process of electrolysis. Why do we extract aluminum? Why is it so important? Aluminum, also known as aluminium,
has many useful applications both in industry and our daily lives. And so it is an economically
important metal. It is lightweight, strong, and
flexible. And although an oxide layer forms
on its surface after some time when aluminum is exposed to air, this protective
layer keeps the metal strong and intact. And the metal underneath is
prevented from corroding further.
Where do we get pure aluminum
from? Aluminum, the most abundant metal
in the Earth’s crust, must be extracted from an aluminum ore. Ores are solid materials such as
rocks, which occur naturally and which contain valuable metals or minerals that can
be extracted. Millions of tons of aluminum are
produced each year worldwide from its ores. The most common aluminum ore is
bauxite. Bauxite is a red-brown sedimentary
rock composed primarily of aluminum oxide, Al2O3, commonly called alumina, as well
as silicon dioxide, SiO2, or silica, the main component of sand, some iron oxide
impurities, the most notable one being iron(III) oxide, Fe2O3, which gives this ore
its distinctive red-brown color, and small amounts of white titanium dioxide,
TiO2.
The extraction of aluminum and its
separation from other compounds in its ores occurs at large smelting plants. Bauxite is first converted to
alumina in the Bayer process, and then alumina is converted to aluminum metal by the
Hall–Héroult process. In the Bayer process, bauxite ore
is first crushed, ground, and blended. Then, it is added to a hot solution
of sodium hydroxide. The sodium hydroxide reacts with
the aluminum oxide in the ore, dissolving the aluminum compounds. Insoluble impurities are then
removed. The dissolved aluminum compounds
are then cooled and precipitated and then heated again to produce alumina.
We’ve just had a brief look at the
Bayer process. The focus of this video is really
the Hall–Héroult process. The conversion of alumina to
aluminum metal is performed using electrolysis. Electrolysis is a chemical process
where electricity is passed through an electrolyte, causing it to decompose into its
constituent elements. A lot of power is required to drive
the electrolysis process, and thus it is costly to produce aluminum. Why then is this method of
electrolysis used to extract aluminum? To answer this question, we need to
know that the metal extraction method chosen for a particular metal depends on the
reactivity of that metal. This reactivity series shows some
relatively reactive metals and some less reactive metals placed in order of
decreasing reactivity relative to the reactivity of a nonmetal carbon.
The reactive metals, which include
aluminum, have very stable oxides. For example, aluminum does not
occur as a pure metal in the Earth’s crust, but rather as aluminum compounds. Aluminum oxide or alumina is a very
stable compound. It takes a lot of energy to
separate the metals from oxygen. In other words, to reduce alumina
to the pure metal and to produce oxygen takes a lot of energy. For this reason, electrolysis is
the method of choice to separate reactive metals from their oxides. Less reactive metals, however, have
less stable oxides.
An example of this is iron(III)
oxide, which is found in hematite. Less energy is required to separate
this metal from the oxygen. Typically iron metal, for example,
is extracted by reduction with carbon, since carbon is more reactive. Carbon will react with oxygen to
form carbon dioxide, for example. And this happens in a blast
furnace. So metals lower on the list are
generally reduced with carbon to get the pure metal.
Now, we know why aluminum is
extracted from its ore by electrolysis. Let’s have a look at this process
in more depth. We said that alumina is extracted
from its ore by the Bayer process and that aluminum is extracted from alumina
through electrolysis by the Hall–Héroult process. This step is what we will look at
now. The diagram shows a cross section
of the Hall–Héroult electrolytic cell. It consists of an outer layer of
heat resistant bricks, then a steel shell. The third layer is a carbon cathode
lining the cell. The carbon is either in the form of
graphite or anthracite. The cathode in an electrolytic cell
is the negative electrode. The anodes, which are positively
charged, are also made of carbon. The starting substance in the
electrolysis process is alumina.
Now, usually in electrolysis, the
starting compound is melted and then electricity is passed through it to separate
its elements. However, alumina’s melting point is
over 2000 degrees Celsius. So instead, alumina and cryolite
are mixed together, and the mixture melts at about 900 degrees Celsius. Cryolite has a lower melting point
than alumina and acts as a solvent to create an electrolyte of molten cryolite and
alumina. Less energy is used and production
costs decrease. Let’s remind ourselves what an
electrolyte is. An electrolyte is a substance with
freely moving ions, which can conduct electricity.
Now we know the setup of the cell,
how is aluminum generated? First, electricity is passed
through the electrolyte using the anodes and cathode. At the cathode, Al3+ ions gain
electrons and are reduced. We can write this half equation as
follows. Positive ions gain electrons. This is a reduction process. The acronym OIL RIG will help us
remember this. Reduction is the gain of
electrons. The liquid aluminum sinks to the
bottom of the cell and is run off and collected through an outlet. The aluminum is then cast into
blocks. Negatively charged oxide ions from
the alumina move towards the anodes. And the reaction which occurs is
shown by this half equation. This is an oxidation process.
Oxidation is the loss of electrons,
and we can see that these four electrons are being removed from the oxygen. In the process, oxygen gas is
produced, which bubbles off. Some of the oxygen reacts with the
carbon at the anodes under these very high temperatures. And so fairly regularly, the anodes
need to be replaced as they begin to disintegrate. We can add these two half equations
to get the overall electrolytic cell reaction. We can rewrite the two
equations. Next, we multiply each equation by
a suitable number so that the electrons can cancel out. If we multiply the top equation by
four and the bottom by three, we get the following. 12 electrons on the left and 12
electrons on the right can cancel out.
Now, we can add the equations
together, and we get the following overall equation: four Al3+ liquid plus six O2−
liquid giving four Al liquid plus three O2 gas. These ions we can combine to the
original starting alumina formula. We can see that for every two moles
of alumina added to the cell, four moles of aluminum are produced. Multiple electrolytic cells just
like this, sometimes over 300, are needed to produce aluminum at an economically
viable rate in industry.
Electric current of over 150000
amperes and approximately four volts per cell are needed. This is a highly energy-intensive
process requiring up to about 15 kilowatt hours per kilogram of aluminum produced,
which is about 54 megajoules of energy needed for one kilogram of aluminum. Scientists are, however,
investigating ways to make this process more efficient. Now it’s time to practice.
Which of the following is not a
reason why cryolite is part of a multielectrolyte in the extraction of aluminum? (A) Cryolite increases the amount
of electricity used. (B) Cryolite reduces the
temperature at which the aluminum melts. (C) Cryolite increases the
conductivity of the electrolyte. (D) Cryolite provides sodium ions
which help to carry the electric current. Or (E) cryolite reduces the working
temperature of the electrolytic cell.
The extraction of aluminum is a
two-step process. First, alumina is extracted from
bauxite ore in the Bayer process. Then, pure aluminum metal is
extracted from alumina in the Hall–Héroult process. This is an electrolytic
process. Electrolysis is a chemical process
where electricity is passed through an electrolyte, causing it to decompose into its
constituent elements. The question asks us about a molten
electrolyte, so we know we are looking at the electrolysis process. In the Hall–Héroult electrolytic
cell, we have a carbon cathode and a carbon anode.
Electricity is passed through an
electrolyte containing aluminum ions, which gain electrons from the cathode, are
reduced to aluminum metal. And this liquid aluminum metal
sinks to the bottom of the cell and can be collected through an outlet where it is
then cast into blocks. If the electrolyte was only
composed of molten alumina, in other words, liquid aluminum oxide, then solid
alumina would need to be heated to its melting point, which is over 2000 degrees
Celsius. This requires a lot of energy. So instead, cryolite, which is a
sodium aluminum fluoride compound, is mixed with a solid alumina. Cryolite has a lower melting point
than aluminum oxide and acts as a solvent to create an electrolyte mixture of molten
cryolite and alumina.
This mixture melts at about 900
degrees Celsius instead of over 2000 degrees Celsius, as in the case of pure
alumina. Less energy or less electricity is
therefore used. This decreases production
costs. The question asked which of the
following is not a reason why cryolite is used. Option (A), cryolite increases the
amount of electricity used, is not a reason why cryolite is used. Cryolite is used to decrease the
amount of electricity needed.
The remaining four answer options
are reasons why cryolite is added. Cryolite does reduce the
temperature at which the aluminum melts. It does increase the conductivity
of the electrolyte. It does provide sodium ions that
help to carry the electric current. And it does reduce the working
temperature of the electrolytic cell. Finally, “which of the following is
not a reason why cryolite is part of a molten electrolyte in the extraction of
aluminum?” The answer is cryolite increases
the amount of electricity used.
Here are the key points we have
learned about aluminum extraction. Aluminum is economically important
and has many useful applications. Bauxite is the most common aluminum
ore. Electrolysis must be used to
extract aluminum from its ore, since aluminum is more reactive than carbon. Cryolite is used to decrease the
temperature at which aluminum melts. And this in turn saves energy and
decreases production costs. And finally, Al3+ ions in molten
alumina are reduced to aluminum metal by gaining electrons.
