Aluminium

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Background

Physically, chemically and mechanically aluminium is a metal like steel, brass, copper, zinc, lead or titanium. It can be melted, cast, formed and machined much like these metals and it conducts electric current. In fact often the same equipment and fabrication methods are used as for steel.

Light Weight

Aluminium is a very light metal with a specific weight of 2.7 g/cm3, about a third that of steel. For example, the use of aluminium in vehicles reduces dead-weight and energy consumption while increasing load capacity. Its strength can be adapted to the application required by modifying the composition of its alloys.

Aluminium corrosion Resistance

Aluminium naturally generates a protective oxide coating and is highly corrosion resistant. Different types of surface treatment such as anodising, painting or lacquering can further improve this property. It is particularly useful for applications where protection and conservation are required.

Aluminium electrical and Thermal Conductivity

Aluminium is an excellent heat and electricity conductor and in relation to its weight is almost twice as good a conductor as copper. This has made aluminium the most commonly used material in major power transmission lines.

Aluminium reflectivity

Aluminium is a good reflector of visible light as well as heat, and that together with its low weight, makes it an ideal material for reflectors in, for example, light fittings or rescue blankets.

Aluminium ductility

Aluminium is ductile and has a low melting point and density. In a molten condition it can be processed in a number of ways. Its ductility allows products of aluminium to be basically formed close to the end of the product’s design.

Aluminium impermeable and Odourless

Aluminium foil, even when it is rolled to only 0.007 mm thickness, is still completely impermeable and lets neither light aroma nor taste substances out. Moreover, the metal itself is non-toxic and releases no aroma or taste substances which makes it ideal for packaging sensitive products such as food or pharmaceuticals.

Aluminium recyclability

Aluminium is 100 percent recyclable with no downgrading of its qualities. The re-melting of aluminium requires little energy: only about 5 percent of the energy required to produce the primary metal initially is needed in the recycling process.

It is decorative. It is easily formed, machined, and cast. Alloys with small amounts of copper, magnesium, silicon, manganese, and other elements have very useful properties.

Strength depends on purity. 99.996 per cent pure aluminium has a tensile strength of about 49 megapascals (MPa), rising to 700 MPa following alloying and suitable heat treatment.
Although not found free in nature, Aluminium is an abundant element in the earth's crust.
A key property is low density. Aluminium is only one-third the weight of steel.
Aluminium and most of its alloys are highly resistant to most forms of corrosion. The metal's natural coating of aluminium oxide provides a highly effective barrier to the ravages of air, temperature, moisture and chemical attack.
Aluminium is a superb conductor of electricity. This property allied with other intrinsic qualities has ensured the replacement of copper by aluminium in many situations.
Aluminium is non-magnetic and non-combustible, properties invaluable in advanced industries such as electronics or in offshore structures.
Aluminium is non-toxic and impervious, qualities that have established its use in the food and packaging industries since the earliest times.
Other valuable properties include high reflectivity, heat barrier properties and heat conduction. The metal is malleable and easily worked by the common manufacturing and shaping processes.

Physical Properties

Density / Specific Gravity (g.cm-3 at 20 °C)	2.70
Melting Point (°C)	660
Specific heat at 100 °C, cal.g-1K-1 (Jkg-1K-1)	0.2241 (938)
Latent heat of fusion, cal.g-1 (kJ.kg-1)	94.7 (397.0)
Electrical conductivity at 20°C (% of international annealed copper standard)	64.94
Thermal conductivity (cal.sec-1cm-1K-1)	0.5
Thermal emmisivity at 100°F (%)	3.0
Reflectivity for light, tungsten filament (%)	90.0

These properties can be very significantly altered with the addition of small amounts of alloying materials. Aluminium reacts with oxygen to form a microscopic (0.000000635cm) protective film of oxide, which prevents corrosion.
Aluminium in massive form is non-flammable. Finely divided particles will burn. Carbon monoxide or dioxide, aluminum oxide and water will be emitted. This is a useful property for making rocket fuel.

ALUMINIUM

This page starts by looking at the extraction of aluminium from its ore, bauxite, including some economic and environmental issues. It finishes by looking at some uses of aluminium.

Extracting aluminium from bauxite
Introduction
Aluminium is too high in the electrochemical series (reactivity series) to extract it from its ore using carbon reduction. The temperatures needed are too high to be economic.
Instead, it is extracted by electrolysis. The ore is first converted into pure aluminium oxide by the Bayer Process, and this is then electrolysed in solution in molten cryolite - another aluminium compound. The aluminium oxide has too high a melting point to electrolyse on its own.

Aluminium ore
The usual aluminium ore is bauxite. Bauxite is essentially an impure aluminium oxide. The major impurities include iron oxides, silicon dioxide and titanium dioxide.

Note: Bauxite actually contains one of a variety of hydrated aluminium oxides some of which you can write as Al2O3,xH2O. Since this is in itself a simplification, for UK A level purposes we normally just treat it as impure Al2O3.

Purifiying the aluminium oxide - the Bayer Process
Reaction with sodium hydroxide solution
Crushed bauxite is treated with moderately concentrated sodium hydroxide solution. The concentration, temperature and pressure used depend on the source of the bauxite and exactly what form of aluminium oxide it contains. Temperatures are typically from 140°C to 240°C; pressures can be up to about 35 atmospheres.
High pressures are necessary to keep the water in the sodium hydroxide solution liquid at temperatures above 100°C. The higher the temperature, the higher the pressure needed.
With hot concentrated sodium hydroxide solution, aluminium oxide reacts to give a solution of sodium tetrahydroxoaluminate.

Note: You may find all sorts of other formulae given for the product from this reaction. These range from NaAlO2 (which is a dehydrated form of the one in the equation) to Na3Al(OH)6 (which is a different product altogether).
What you actually get will depend on things like the temperature and the concentration of the sodium hydroxide solution. In any case, the truth is almost certainly a lot more complicated than any of these.
The version I am using is perfectly acceptable and is consistent with the aluminium chemistry you will find elsewhere on the site.

The impurities in the bauxite remain as solids. For example, the other metal oxides present tend not to react with the sodium hydroxide solution and so remain unchanged. Some of the silicon dioxide reacts, but goes on to form a sodium aluminosilicate which precipitates out.
All of these solids are separated from the sodium tetrahydroxoaluminate solution by filtration. They form a "red mud" which is just stored in huge lagoons.

Precipitation of aluminium hydroxide
The sodium tetrahydroxoaluminate solution is cooled, and "seeded" with some previously produced aluminium hydroxide. This provides something for the new aluminium hydroxide to precipitate around.

Note: This all starts to be a bit of a nightmare if you try to get at the truth of what is happening. There are two separate issues here - both glossed over by most sources. If you like your life to be simple, ignore the rest of this note!
You may well find that the product is quoted as Al2O3,3H2O. If you add everything up, that is the same as 2Al(OH)3. So which is it? I suspect that it probably isn't exactly either of these. The aluminium hydroxide originally precipitated doesn't have that formula - it's a simplification. And it will rearrange to form some sort of hydrated oxide, but whether it is as simple as the other formula suggests, I doubt!
The second thing is to wonder why this reaction happens at all. If you have done any aluminium chemistry, you may recognise that what normally happens is exactly the reverse of this reaction. So why is it going the other way this time? I suspect (although I don't know for sure) that it is an effect of lowering the temperature on the position of equilibrium. If the formation of the NaAl(OH)4 is endothermic, it would be favoured by high temperatures (the conditions under which the NaAl(OH)4 is formed during the first stage of the reaction). If you lowered the temperature (by cooling the reaction mixture after filtration), it would favour the exothermic change - the reverse.
If you know this explanation to be wrong, please get in touch with me via the address on the about this site page.

Formation of pure aluminium oxide
Aluminium oxide (sometimes known as alumina) is made by heating the aluminium hydroxide to a temperature of about 1100 - 1200°C.

Conversion of the aluminium oxide into aluminium by electrolysis
The aluminium oxide is electrolysed in solution in molten cryolite, Na3AlF6. Cryolite is another aluminium ore, but is rare and expensive, and most is now made chemically.

The electrolysis cell
The diagram shows a very simplified version of an electrolysis cell.

Although the carbon lining of the cell is labelled as the cathode, the effective cathode is mainly the molten aluminium that forms on the bottom of the cell.
Molten aluminium is syphoned out of the cell from time to time, and new aluminium oxide added at the top.
The cell operates at a low voltage of about 5 - 6 volts, but at huge currents of 100,000 amps or more. The heating effect of these large currents keeps the cell at a temperature of about 1000°C.

The electrode reactions
These are very complicated - in fact one source I've looked at says that they aren't fully understood. For chemistry purposes at this level, they are always simplified (to the point of being wrong! - see comment below).
This is the simplification:
Aluminium is released at the cathode. Aluminium ions are reduced by gaining 3 electrons.

Oxygen is produced initially at the anode.

However, at the temperature of the cell, the carbon anodes burn in this oxygen to give carbon dioxide and carbon monoxide.
Continual replacement of the anodes is a major expense.

Note: That all seems fairly obvious, and until I was doing the research for this page, I thought it was fairly obvious too. It's a pity it turns out to be wrong! It is currently believed that neither cathode nor anode reaction happens like this. There is a reaction between the aluminium oxide and the cryolite to produce a range of complex ions involving aluminium, oxygen and/or fluorine. It is the various complexes present which gain or lose electrons at the electrodes, rearranging themselves again in the process. I'm not giving details of this because I understand that there is still some uncertainty involved.
For exam purposes, you will have to use the untrue versions above because these are what are in virtually all of the textbooks at this level and are what examiners will almost certainly expect. However, if you ever meet this bit of chemistry again at a higher level, you should be aware that you will have to rethink it.

Some economic and environmental considerations
This section is designed to give you a brief idea of the sort of economic and environmental issues involved with the extraction of aluminium. I wouldn't claim that it covers everything!

Note: This is deliberately brief because a lot of it is just common sense, and you will probably already have met it in detail in earlier chemistry courses, in geography, in general studies, or wherever.
If you aren't sure about the various environmental problems like acid rain, global warming and the like, the very best site to find out about them is the US Environmental Protection Agency.

Economic considerations
Think about:

The high cost of the process because of the huge amounts of electricity it uses. This is so high because to produce 1 mole of aluminium which only weighs 27 g you need 3 moles of electrons. You are having to add a lot of electrons (because of the high charge on the ion) to produce a small mass of aluminium (because of its low relative atomic mass).
Energy and material costs in constantly replacing the anodes.
Energy and material costs in producing the cryolite, some of which gets lost during the electrolysis.

Environmental problems in mining and transporting the bauxite
Think about:

Loss of landscape due to mining, processing and transporting the bauxite.
Noise and air pollution (greenhouse effect, acid rain) involved in these operations.

Extracting aluminium from the bauxite
Think about:

Loss of landscape due to the size of the chemical plant needed, and in the production and transport of the electricity.
Noise.
Atmospheric pollution from the various stages of extraction. For example: carbon dioxide from the burning of the anodes (greenhouse effect); carbon monoxide (poisonous); fluorine (and fluorine compounds) lost from the cryolite during the electrolysis process (poisonous).
Pollution caused by power generation (varying depending on how the electricity is generated.)
Disposal of red mud into unsightly lagoons.
Transport of the finished aluminium.

Recycling
Think about:

Saving of raw materials and particularly electrical energy by not having to extract the aluminium from the bauxite. Recycling aluminium uses only about 5% of the energy used to extract it from bauxite.
Avoiding the environmental problems in the extraction of aluminium from the bauxite.
Not having to find space to dump the unwanted aluminium if it wasn't recycled.
(Offsetting these to a minor extent) Energy and pollution costs in collecting and transporting the recycled aluminium.

Uses of aluminium
Aluminium is usually alloyed with other elements such as silicon, copper or magnesium. Pure aluminium isn't very strong, and alloying it adds to it strength.
Aluminium is especially useful because it

has a low density;
is strong when alloyed;
is a good conductor of electricity;
has a good appearance;
resists corrosion because of the strong thin layer of aluminium oxide on its surface. This layer can be strengthened further by anodising the aluminium.

Anodising essentially involves etching the aluminium with sodium hydroxide solution to remove the existing oxide layer, and then making the aluminium article the anode in an electrolysis of dilute sulphuric acid. The oxygen given of at the anode reacts with the aluminium surface, to build up a film of oxide up to about 0.02 mm thick.
As well as increasing the corrosion resistance of the aluminium, this film is porous at this stage and will also take up dyes. (It is further treated to make it completely non-porous afterwards.) That means that you can make aluminium articles with the colour built into the surface.

Some uses include:

aluminium is used for	because
aircraft	light, strong, resists corrosion
other transport such as ships' superstructures, container vehicle bodies, tube trains (metro trains)	light, strong, resists corrosion
overhead power cables (with a steel core to strengthen them)	light, resists corrosion, good conductor of electricity
saucepans	light, resists corrosion, good appearance, good conductor of heat

http://www.almolink.org/notes/aluminium.doc
Many thanks to Author : Alrick Moodie www.almolink.org/

Aluminium is the most abundant metal in the Earth's crust. Bauxite, the main ore of aluminium, is an impure form of aluminium oxide. To make aluminium, purified aluminium oxide (Al2O3) is dissolved in molten cryolite (Na3AlF6) at 900°C and then electrolysed.

Research about aluminium recycling

Quotes form source: http://www.mpma.org.uk

Recycling one kilogram of aluminium can save up to 8 kilograms of bauxite, four kilograms of chemical products and 14 kilowatt hours of electricity.
Anything made of aluminium can be recycled repeatedly: not only cans, but aluminium foil, plates and pie molds, window frames, garden furniture and automotive components are melted down and used to make the same products again.

Used aluminium cans can be recycled to make new aluminium cans, aluminium windows can be recycled to make new aluminium windows and old aluminium engine blocks to make new ones.

The recycling rate for aluminium cans is already above 70% in some countries.
The aluminium industry has set up various schemes to encourage recycling in many countries.

Aluminium beverage cans can be profitably recycled by individuals and groups and most countries have a national can recycling association which offers advice, support, and can put collectors in touch with purchasing organisations.
Process scrap at all stages is meticulously collected and sorted by alloy by all aluminium companies and most customer organisations. Unlike other metals, scrap aluminium has significant value and commands good market prices.

The London Metal exchange quotes aluminium scrap prices.
Aluminium companies have invested in dedicated state of the art secondary metal processing plants to recycle aluminium. In the case of beverage cans, the process uses gas collected from burning off the volatile substances in can coatings to provide heat for the process. Every last bit of energy is used.
Used beverage cans are normally back on supermarket shelves as new beverage cans in 6-8 weeks in those countries which have dedicated can collecting and recycling schemes.
In Europe, the aluminium beverage can meets the minimum targets set in the European directive on Packaging and Waste. Sweden (92 per cent) and Switzerland (88 per cent) are the European can recycling champions. The European average is 40 per cent, a ten per cent increase since 1994.
The recycling of aluminium beverage cans eliminates waste. It saves energy, conserves natural resources, reduces use of city landfills and provides added revenue for recyclers, charities and local town government. The aluminium can is therefore good news for the environment and good for the economy.
The aluminium can is 100% recyclable; there are no labels or covers to be removed.
Today's aluminium can requires about 40% less metal than the can made 25 years ago; hence the need for less energy and less raw material per can.
Cans made from aluminium are worth 6 to 20 times more than any other used packaging material.
Aluminium is the only packaging material that more than covers the cost of its own collection and processing at recycling centres.

Source: http://www.mpma.org.uk/pages/userdata/mpma/AluminiumRecyclingFact1.doc

Author: was not indicated in the source document

Aluminium

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