Production Of Aluminium most abundant element in the Earth

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Aluminium is the 3rd most abundant element in the Earth’s crust and is present as 7.3% by mass. In nature it is only stable when bonded to other elements, particularly in the form of silicates and oxides as it is too reactive to occur free in nature. By 1808 its existence was first established and it took many years to release the metal from its ore and many more years to produce a viable, commercial production process. By 1886 Charles Hall discovered a cheap method of extracting aluminium from its ore.

Aluminium Extraction vs. Iron and Zinc Extraction

Aluminium extraction differs from that of other elements such as zinc and iron for a number of reasons, which all relate to its properties. Zinc and iron production is quite a similar process, whereby both procedures employ the use of a blast furnace in the presence of carbon (coke) and other reducing compounds to reduce the prepared ores into the element being purified. Aluminium ores are prepared by treatment with concentrated sodium hydroxide to produce aluminium oxide. This method is used as aluminium is amphoteric (reacts with acids and bases) but not other compounds are not so they remain insoluble (e.g. iron oxide). The blast furnace method is not employed for aluminium extraction because aluminium on its own is very reactive and when it forms compounds like aluminium oxide it is very stable. Therefore it is very difficult to reduce it using standard compounds normally used in extractions, even at very high temperatures. That is why the electrolysis method is used for aluminium extraction.

Aluminium Extraction

Aluminium is produced industrially through the electrolysis of molten aluminium oxide. The chief aluminium ore is termed bauxite and contains aluminium in the form of hydrated oxide (Al2O3.xH2O). Mines have been built all over the world in order to collect bauxite (see figure 1) and once bauxite is mined, the first step in the extraction of aluminium is to treat the ore to release aluminium oxide. This is achieved due to the amphoteric nature of aluminium, on which purification is based:
Bauxite: impure Al2O3

Add NaOH (to produce dissolved Al2O3)
Al2O3(s) + 2OH-(aq) + 3H2O(l)2[Al(OH)4]- (aq)

Filtrate produced containing Al(OH)4 – and SiO3 2-

Al(OH)3 precipitate produced
2[Al(OH)4]- + CO22Al(OH)3(s) + CO32-(aq) + H2O(l)

2Al(OH)3(s) Al2O3 + H2O

Once pure aluminium oxide, also termed alumina, is generated, the next process is the use of electrolysis to produce pure aluminium.

As discussed earlier, during the 19th century, the problems associated with aluminium extraction were associated with its readiness to form compounds and the stability of the compounds formed. In order for electrolysis to occur, aluminium oxides would have to be melted into the molten solid form and the electrolysis process would have to use molten solids rather than aqueous solutions, due to their high reactivity. The problem was that aluminium salts have a very high melting point (2000°C). A solution was found: to dissolve the aluminium oxide in molten cryolite (Na3Alf6), giving the conducting mixture a lower melting point of 850°C. This temperature needs to be maintained in the cell by passing current through the electrolyte, as aluminium will form at this temperature.

Aluminium is purified from aluminium oxide therefore, by adding the dissolved aluminium oxide in molten cryolite to the electrolysis cell, aluminium is discharged at the graphite lining of the cell (see figure 2), which also acts as the cathode (formed by the thick carbon or graphite lining of the pot/cell):
Al3+ + 4e- Al (Cathode)

The molten aluminium is collected at the bottom of the cell and is the tapped off over specific periods of time. Oxygen is collected at the anodes (made up of petroleum coke and pitch) and reacts with the carbon anodes to generate oxides of carbon, due to the high temperatures involved:
2O2- O2 + 4e- (Anode)

Locations of Aluminium Production
Aluminium ore, bauxite occurs mainly in tropical and sub-tropical areas such as Africa, West Indies, South America and Australia, it also occurs in some European countries, such as Greece and Hungary. Thus, bauxite mines are present in these areas. Aluminium smelting (the electrolysis procedure) uses up a lot of energy and on average, around the world, it takes around 15.7 kWh of electricity to produce one kilogram of aluminium from alumina. This is why the world’s smelters are located in areas which have access to large amounts of power resources, such as hydro-electric, natural gas, coal and nuclear sources, as well as where energy is cheap.

The Main Uses for Aluminium
The uses for aluminium are vast, it is used in a number of industries from transport, electrical and construction to packaging. This is due to the fact that this metal resists corrosion and has a low density, it also is completely resistant to corrosion and can be used as a thermal conductor and insulator. Therefore, it has been used in the production of foil and cooking pans, fire-fighter suits contain aluminium to reflect radiant heat. As it is a good conductor it is used in overhead cables. It is used in the commercial aviation industry, aluminium makes up about 80 per cent of an aircraft’s weight, as well as the road, rail and sea travel industries in the production of vehicles. In addition it is used largely for the protection, storage and preparation of food and drinks. All kinds of aluminium products are used now in construction and in residential renovation of homes.

Excess aluminium in the body has been linked to Alzheimer’s disease, however, under normal circumstances the body does not accumulate large doses of aluminium it is only when the natural barriers in the body limiting the absorption of aluminium are overcome e.g. where the ability of the kidneys to excrete aluminium is damaged, accumulation of aluminium in the body may occur.

Environmental issues
A number of environmental concerns have been raised with regards to the effects of the production of aluminium on the environment. Firstly, areas where hydroelectric power is generated are usually places of great natural beauty so conservationists have a problem with the construction of power plants in these areas. Secondly due to the requirement of large amounts of electricity in the production process, (electricity consumption averages 15.95 kWh per kg of molten metal), the main problem is the emissions generated when coal fired power plants are used, emitting substantial amounts of greenhouse (CO2) gases into the atmosphere, as well as reducing the amount of the non-renewable energy sources. The Aluminium Industry is continually looking to reduce energy consumption and emissions via a more efficient production of aluminium and recycling. Smelter emissions also pose a problem for the environment leading to global warming and climate change. These emissions include: polycyclic aromatic hydrocarbons, sulphur dioxide, carbon dioxide, inorganic fluorides, perfluorocarbons:
tetrafluoromethane (CF4) and hexafluoroethane (C2F6). Greenhouse gas emissions are also produced from energy consumed in producing lime and caustic soda. With regards to cathode waste, such as old linings, which consist of old refractory bricks and carbon and material from the electrolytic bath, landfill is used but precautions are taken to ensure that the waste material cannot leach out of the site through rainfall.

Recycling Aluminium: Benefits and Problems
A number of recycling schemes have been set up all over the UK and in the US and once people are made aware of the benefits of recycling to the environment more and more appear to be recycling.
Scrap aluminium separated into New Scrap and Old Scrap. New scrap is that surplus material that arises during the manufacture and fabrication of aluminium alloys, e.g. trimmings from the edges of sheet aluminium. Old scrap is material which has been used by the consumer and subsequently discarded such as, used beverage cans.

The scrap aluminium which has been collected is loaded into a furnace, which melts the aluminium completely. This molten metal is then cast or processed using techniques such as: alloying it with other materials to make an array of metals with different properties. The main alloying ingredients are iron, silicon, zinc, copper and magnesium. Aluminium can be rolled into plate, sheets, or wafer thin foils and it can be cast into a variety of shapes.

Recycling Aluminium provides a number of benefits, particularly to the environment.Since the material can be recycled indefinitely without loss of quality, and the high essential value it holds, different schemes have been developed for the recovery of used aluminium in all major European countries. Also, all aluminium products retain some worth; this allows it to be possible to create value by recycling them into new products. Banishing waste is another benefit. There are very few disadvantages to aluminium recycling, such as; costs to the government for the setting up of recycling schemes for public awareness as well as development of recycling centres etc.