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Aluminium uses and extraction

Aluminium is an attractive, corrosion resistant lightweight shiny metal. It has many desirable properties and uses, a few of which are illustrated below:

Low density

Aluminium and its alloys are low density, corrosion resistant alloys with an excellent strength to weight ratio and excellent fatigue resistance which makes them ideal for use in the aviation and car industries. Aluminium alloys are the primary material used to make alloy wheels for cars and the body panels in many sports and electric cars. Compared to steel, aluminium alloys are significantly lighter. This translates to better fuel efficiency, improved handling.

Plane made of aluminium alloys. Aluminium alloys are used to produce alloy wheels for cars.

Electrical and thermal conductivity

Copper metal is used as an electrical conductor in the electrical wires in our home and while aluminium is not as good an electrical conductor as copper it is significantly lighter. The low density of aluminium is a major factor in favour of its use for overhead power lines and cables, particularly for long distance transmission cables on pylons where the electrical cables can run for many miles. Aluminium is also much cheaper than copper, making it a more economical choice for power lines. Even though aluminium conducts electricity less efficiently than copper (around 61% as conductive), its weight advantage makes up for it. Thicker aluminium wires can be used to achieve the same conductivity as thinner copper wires, but they'll still weigh less.

Aluminium is a good thermal conductor and transfers heat well compared to many other materials. This makes it useful in applications where heat dissipation is important, like heat sinks in electronics or pots and pans for cooking.

Aluminium is a lightweight or low density metal which is used to manufacture of electricity transmission cables. Aluminium is used as a heat sink in many electrical devices and components to dissicipate heat. Aluminum is a good thermal conductor and transfers heat well compared to many other materials. This makes it an ideal material forcooking pots and pans.

Appearance and corrosion resistance and common uses

Aluminium is a corrosion resistant metal due to the fact that it forms a thin, invisible and tough layer of aluminium oxide on its surface when it comes into contact with air or oxygen. This layer thin layer of aluminium oxide acts as a shield, protecting the aluminium metal underneath from further oxidation. This naturally resistant to corrosion makes aluminium suitable for use in harsh weather conditions. It can withstand rain, wind, and extreme temperatures without deteriorating. This corrosion resistance along with its low density makes it an ideal material to use as cladding on buildings, as shown in the image below.

However aluminium is a fairly soft metal and is prone to dents and scratches, especially from hail, strong winds, or flying debris. These imperfections can detract from the building's appearance and may require panel replacements which could be an expensive business. Recently there have been some fires in high-rise building which spread quickly, some types of aluminium cladding, particularly Aluminium Composite Panels (ACPs) with flammable cores have raised fire safety concerns. Strict regulations and fire safety tests are crucial when choosing ACPs to ensure they meet building codes and reduce the risks associated with fires in high-rise buildings and offices.

Aluminium is also an attractive metal which can be painted, anodised or powdered coated and it can also be formed into various shapes and panels, allowing for creative modern looking architectural designs in buildings and offices. Aluminium being a good thermal conductor can help prevent a mobile phone from overheating

Aluminium is a fairly strong, lightweight and attractive metal which can offer good impact protection makes it an ideal material for items such as mobile phone cases. Being a lightweight material means it won't add significant bulk to your phone, making it comfortable to carry around. Since aluminium is a good thermal conductor this can help draw heat away from your phone, particularly beneficial during activities that generate heat, such as gaming or video conferencing. This can help prevent overheating and improve the performance of the mobile phone. Aluminium can be machined and finished in various ways, allowing for sleek and stylish phone case designs. Aluminium can also be anodized for a variety of colours or left with a natural metallic finish and finally unlike some other metals, aluminium doesn't typically interfere with mobile cell phone signals to a significant degree. This ensures you maintain good connectivity.

Perhaps the best known use for aluminium is as cooking foil. As mentioned above aluminium metal conducts heat very efficiently this ensures that any foods wrapped in it during cooking cook evenly. Aluminium foil acts as a barrier to light, air, and moisture. This is crucial for food storage as it helps prevent spoilage caused by light or air exposure and locks in moisture to keep food from drying out, ideal to ensure your sandwiches don't dry out!

Aluminium is a lightweight, 
attractive corrosion resistant material which making it ideal as a material to clad buildings. Aluminium is a lightweight attractive metal making it ideal for use as a mobile phone case. Aluminium foil is used to help cook foods evenly.

Extracting aluminium from its ore

Aluminium is the third most abundant element in the Earth; its main ore is called bauxite. Bauxite contains a high proportion of aluminium compounds, chiefly aluminium oxide (Al2O3). The aluminium oxide contains positively charged aluminium ions (Al3+) and negatively charged oxide ions (O2-) ions. It is named after a village in southern France called Les Baux, where it was first discovered in 1821.

Despite the fact that aluminium is the third most common element on Earth in the late 19th century it was more valuable than silver or gold and it was considered a rare metal. The reason for this is that before the discovery of electrolysis aluminium was extracted by carrying out a displacement reaction using sodium or potassium on aluminium compounds, this made it a very expensive metal indeed.

However with the discovery of the electrolytic process the cost of aluminium fell dramatically. However the extraction of aluminium by electrolysis was not as straightforward as might have been expected at first. In order to extract a metal from its ore by electrolysis the metal ore needs to be molten (melted to form a liquid) or dissolved to form a solution. However bauxite is not soluble in many solvents and it also has a very high melting point, in excess of 20000C. This made the extraction of aluminium from its ore difficult and expensive.

The solution to the problem was found by two scientists working independently from each other, Charles Hall an American scientist and Paul Heroult a French scientist found that aluminium oxide would dissolve in cryolite (Na3AlF6), a rare mineral found in Greenland.

If cryolite is heated to around 9500C then aluminium oxide will dissolve in it and form a solution which conducts electricity. This made it possible to extract aluminium by electrolysis from its ore cheaply and following this the cost of aluminium metal plummeted. Aluminium oxide (Al23+O32-) dissolves in the molten cryolite inside the cell where temperatures are around 9500C. When it dissolves its giant lattice structure is broken down and the aluminium and oxide ions are free to move. This is exactly the same as what would happen if an ionic compound was dissolved in water, only in this case it is dissolved in molten cryolite. The image below shows a typical cell used to extract aluminium.

Labelled diagram of the Hall-Heroult cell used to extract aluminium metal from its ore.

The cell is lined with a layer of graphite; this graphite is made the cathode. This negatively charged cathode will attract the positively charged aluminium ions and reduce them to aluminium atoms:

At the cathode

At the cathode the aluminium ions will be reduced (gain electrons) and form aluminium metal. Aluminium melts at 6600C and since the cell temperature is maintained around 9500C the aluminium produced will be a liquid. It collects at the bottom of the cell where it will be siphoned out when required. The half-equation for the cathode reaction is shown below:
The reduction of aluminium ions at the cathode in the Hall-Heroult cell

At the anode

The oxidation of oxide ions at the anode to form oxygen gas during the extraction of aluminium metal At the positively charged anode the negatively charged oxide ions (O2-) are attracted and they are oxidised (lose electrons). The half equation for this oxidation reaction is:
O2-O + 2e
However since oxygen gas is a diatomic gas so the equation must be multiplied by two. This gives:
2O2-O2 + 4e
 
Two oxide ions are oxidised on the anode surface and the two oxygen atoms produced join to form a molecule of oxygen. However to slightly complicate this we have to bear in mind that the anodes are made of graphite (a form of carbon) and they are at 8500C and so with oxygen gas being produced on their surface they will simply react with it, they slowly burn away and have to be regularly replaced.
graphite (carbon)(s) + oxygen(g)→ carbon dioxide(g)
C + O2 → CO2

Overall equation:

cathode: Al3+ + 3e → Al
anode: 2O2- → O2 + 4e
The aluminium ions gain 3e and the oxide ions lose 4 electrons, to balance these equations in terms of electrons simply multiply one equation by x3 and the other equation by x4 to give 12 electrons in each equation:
cathode: 4Al3+ + 12e → 4Al
anode: 6O2- → 3O2 + 12e
The electrons cancel out, there are 12 on each side of the equations, so combining both equations this leaves.
4Al3+ + 6O2- 4Al + 3O2
this equation is the same as:
2Al2O34Al + 3O2
This equation represents the decomposition of aluminium oxide during electrolysis.

Costs

One of the major costs in extracting aluminium is the cost and energy required to continually make new anodes as the old one burn away and turn into carbon dioxide gas. The other major consideration in aluminium extraction is the vast amount of electricity required to extract aluminium from its ore. Huge electrical currents in excess of 150 000 amps are used and so a cheap source of electricity nearby is required. A hydro-electric power station would be ideal!

Key points

Practice questions

Check your understanding - Questions on aluminium extraction.

Quick Quiz- Check your understanding - Aluminium quick quiz

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