IntroductionAluminium is the third most abundant element in the earth's crust, only oxygen and silicon are present in greater amounts. Whilst chemical compounds of aluminium have been used from earliest times, it was only in 1807 that the scientist Sir Humphrey Davey predicted that it should exist. Today more aluminium is produced each year than any other non ferrous metal. Indeed it is difficult to believe that aluminium was first isolated about 160 years ago and produced in industrial quantities for only the past 100 years.
ApplicationsThe attractive properties of aluminium and its many alloys have led to their use in packaging, buildings, transport and electrical applications. They all have low density and good thermal and electrical conductivity, they are non toxic and can be wrought and cast by all conventional, and a few less usual, methods. Most have good corrosion resistance and can be profitably recycled.
UsageSome 20 million tonnes of aluminium are produced each year, a figure certain to grow with the increase in demand for lighter, more durable, energy efficient and recyclable goods.
Aluminium can, depending upon the application, replace many different materials including copper, zinc, tin plate, steel, stainless steel, titanium, paper, wood. concrete and composite materials on both technical and economic considerations.
ExtractionOreThe principal ore of aluminium is bauxite, widely abundant with huge deposits in Australia, the Caribbean, Africa, China and South America. It is normally mined by open cut techniques.
The bauxite is purified by the Bayer process which involves heating in caustic soda. The aluminium trihydrate dissolves leaving a residue of insoluble iron and titanium oxides. The aluminium trihydrate is dried to produce alumina. The iron and titanium oxide residue is called `red mud'. Two tonnes of bauxite yield one tonne of alumina.
SmeltingToday aluminium is extracted by essentially the process developed, simultaneously but separately, by Hall in the USA and Heroult in France over 100 years ago.
The process is electrolytic with the alumina dissolved in cryolite (sodium aluminium flouride) forming the electrolyte of a `cell' or `pot'. The pot, which operates at 950°C, consists of a steel shell lined with carbon, which acts as the cathode, the molten electrolyte and consumable carbon anodes suspended in the electrolyte. High amperages at low voltages are passed through the pot depositing pure aluminium at the cathode. A modern smelter pot is typically 10 metres long, 4 metres wide and 1.5 metres high running at 180,000 amps to produce about 1 tonne of aluminium each twenty four hours. Operation is continuous and a smelter producing say 250,000 tonnes per annum will have several hundred pots. Approximately 14,000 kilowatt hours of electricity are required to produce one tonne of aluminium from two tonnes of alumina.
EnvironmentPotentially the mining and smelting of aluminium can have major environmental impact. The industry is very conscious of this and its efforts and achievements in rehabilitation of open cut sites and restoring flora and fauna have won awards from the United Nations Environment Programme. Red mud disposal areas are now successfully revegetated. Emissions from the pot line are passed through scrubbing systems to ensure that environmental requirements are met.
RecyclingThe economic and environmental reasons for recycling most materials today are strong, but for aluminium they are absolutely compelling. It will progressively undermine the `aluminium is too expensive' philosophy as more and more aluminium is recycled. The reasons are simple, although it takes 14,000 KWH to produce 1 tonne of aluminium, it only takes 700 KWH, 5% of this, to remelt that same tonne when it is recycled.
There is no difference in quality between virgin and recycled aluminium and its alloys. Beverage cans become new beverage cans on a 3 to 6 month cycle, cylinder head castings become new castings on a 10 to 20 year cycle and structures become new structures on perhaps a 50 year cycle.
At the end of its life, whether 6 months or 50 years, the product needs to be located, collected, dismantled where necessary and fed back into the production system. At the time of writing it is estimated that 60% of the metal in use is being recycled, as new products are developed with recycling as an intrinsic design feature this will surely grow.
Future DevelopmentsImprovements are continually sought in both the product and processing.
Product Development.Substantial improvements in the properties of conventionally produced alloys are restricted by metallurgical factors such as the solubility of alloying elements in solid aluminium and by the relatively slow cooling rates inherent in casting processes, although some fine tuning will continue. Alternative, less conventional methods, are being used to produce 'new materials' with superior properties. Techniques such as spray casting, vapour deposition, particle and fibre reinforced aluminium composites and aluminium/non metallic laminates are being increasingly used to make pistons, brake components, airframe parts and even armour plate cost effective.
Process Development.Continues to improve the conventional production processes to allow higher rolling and extrusion speeds without loss of quality, the production of thinner cast and extruded sections and improved, strength and ductility in castings.