FACILITIES > ALUMINIUM
 
ABOUT ALUMINIUM
Aluminium is a chemical element in the boron group with symbol Al and atomic number 13. It is silvery white, and it is not soluble in water under normal circumstances.

Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal, in the Earth's crust. It makes up about 8% by weight of the Earth's solid surface. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. The chief ore of aluminium is bauxite.

Aluminium is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation.

Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials. The most useful compounds of aluminium, at least on a weight basis, are the oxides and sulfates.

Despite its prevalence in the environment, aluminium salts are not known to be used by any form of life. In keeping with its pervasiveness, aluminium is well tolerated by plants and animals. Owing to their prevalence, potential beneficial (or otherwise) biological roles of aluminium compounds are of continuing interest.
 
The properties of aluminium
Aluminium’s intrinsic properties have contributed to its popularity and varied uses.
  • Lightness: Its specific weight is 2.7 g/cm3 , which is one-third that of steel. In vehicles, aluminium reduces unnecessary weight and therefore fuel consumption.
  • Strength: Aluminium’s strength can be adapted to the application required by modifying the composition of its alloys. Certain alloys are as strong as steel.
  • Corrosion-resistance: Naturally generating a protective oxide coating, aluminium is particularly useful for protection and conservation.
  •  Conductivity: Twice as good a conductor of heat and electricity as copper (based on weight), aluminium is now playing a major role in power transmission lines.
  • Ductility: Its low density and melting point allow aluminium products to be formed up until the last stages of a product design.
  •  Reflective: As a reflector of heat and light, aluminium is suitable for such applications as solar technology and rescue blankets.
  •  Impermeable and odorless: Releasing no taste or toxins, aluminium is ideal for food and pharmaceutical packaging.
  •  Recyclability: Aluminium is 100% and infinitely recyclable with no deterioration in quality.
 
Production and Refinement
Aluminium forms strong chemical bonds with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the high reactivity of aluminum and the high melting point of most of its ores. For example, direct reduction with carbon, as is used to produce iron, is not chemically possible because aluminium is a stronger reducing agent than carbon. Indirect carbothermic reduction can be carried out using carbon and Al2O3, which forms an intermediate Al4C3 and this can further yield aluminium metal at a temperature of 1900–2000 °C. This process is still under development; it requires less energy and yields less CO2 than the Hall-Héroult process, the major industrial process for aluminium extraction. Electrolytic smelting of alumina was originally cost-prohibitive in part because of the high melting point of alumina, or aluminium oxide, (about 2,000 °C (3,600 °F)). Many minerals, however, will dissolve into a second already molten mineral, even if the temperature of the melt is significantly lower than the melting point of the first mineral. Molten cryolite was discovered to dissolve alumina at temperatures significantly lower than the melting point of pure alumina without interfering in the smelting process. In the Hall-Héroult process, alumina is first dissolved into molten cryolite with calciumfluoride and then electolytically reduced to aluminium at a temperature between 950 and 980 °C (1,740 to 1,800 °F). Cryolite is a chemical compound of aluminium and sodiumfluorides: (Na3AlF6). Although cryolite is found as a mineral in Greenland, its synthetic form is used in the industry. The aluminium oxide itself is obtained by refining bauxite in the Bayer process.

The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the refined alumina is dissolved in the electrolyte, it disassociates and its ions are free to move around. The reaction at the cathode is:
Al3+ + 3 e− → Al
 
Here the aluminium ion is being reduced. The aluminium metal then sinks to the bottom and is tapped off, usually cast into large blocks called aluminium billets for further processing.
At the anode, oxygen is formed:
2 O2− → O2 + 4 e−
 

To some extent, the carbon anode is consumed by subsequent reaction with oxygen to form carbon dioxide. The anodes in a reduction cell must therefore be replaced regularly, since they are consumed in the process. The cathodes do erode, mainly due to electrochemical processes and metal movement. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.
Aluminium electrolysis with the Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The worldwide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced (52 to 56 MJ/kg). The most modern smelters achieve approximately 12.8 kW•h/kg (46.1 MJ/kg). (Compare this to the heat of reaction, 31 MJ/kg, and the Gibbs free energy of reaction, 29 MJ/kg.) Reduction line currents for older technologies are typically 100 to 200 kiloamperes; state-of-the-art smelters operate at about 350 kA. Trials have been reported with 500 kA cells.

The Hall-Heroult process produces aluminium with a purity of above 99%. Further purification can be done by the Hoope process. The process involves the electrolysis of molten aluminium with a sodium, barium and aluminium fluoride electrolyte. The resulting aluminium has a purity of 99.99%.
Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Aluminium production consumes roughly 5% of electricity generated in the U.S. Smelters tend to be situated where electric power is both plentiful and inexpensive, such as the United Arab Emirates with excess natural gas supplies and Iceland and Norway with energy generated from renewable sources. The world's largest smelters of alumina are People's Republic of China, Russia, and Quebec and British Columbia in Canada.

In 2005, the People's Republic of China was the top producer of aluminium with almost a one-fifth world share, followed by Russia, Canada, and the USA, reports the British Geological Survey.
Over the last 50 years, Australia has become a major producer of bauxite ore and a major producer and exporter of alumina (before being overtaken by China in 2007). Australia produced 68 million tonnes of bauxite in 2010. The Australian deposits have some refining problems, some being high in silica, but have the advantage of being shallow and relatively easy to mine.
 
Aluminium Transformation
In its alloy form, aluminium can be processed in a number of ways. Usually it is extruded, cast or rolled.
  • Extrusion: A solid aluminium cylinder called a billet (available in a variety of alloys, pretreatments and dimensions), is heated and squeezed through a die with a shaped opening to create a desired profile. Extrusions are widely used in construction, road and rail applications.
  •  Casting: Using either sand casting or die casting techniques, the aluminium is shaped according to a mold.
  •  Rolling: Aluminium passes through a hot-rolling mill and is then transferred to a cold-rolling mill, which can gradually reduce the thickness of the metal down to as low as 0.05 mm. Rolled products are categorized as either foil (less than 0.2 mm thick), sheet (0.2-6 mm), or plate (thicker than 6 mm).
 
Aluminium Recycling
Fully recyclable with no downgrading of quality, aluminium is the most cost-effective material to recycle. In fact, 75% of the aluminium produced since its discovery is still in use today.

Using aluminium, industries can attain their overall recycling targets. In parallel, the aluminium industry is also constantly developing and refining its recycling processes.
 
Did you know?
  • Aluminium is so called because it is a base of “alum,” which in turn is derived from the Latin for “bitter salt.”
  •  Aluminium is the 3rd most common element on Earth after oxygen and silicon.
  •  It is possible to recycle and resell a discarded aluminium can in just 60 days.
  •  Aluminium was once considered to be a precious metal, more valuable even than gold.  It is said that Napoleon III, Emperor of France once gave a banquet where the most honored guest were given aluminium cutlery, while everyone else had gold.
  •  Aluminium is the second most plentiful metallic element on earth; an estimated 8.3% of the earth crust is composed of aluminium.
 
 
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