Aluminium (Al) is a metal with a density of 2812.5 kg/m³ and is characterised by its high resistance to corrosion, as well as being a good conductor of electricity and heat, it is easily machined. It melts at 660 ºC. This metal comes only from the extraction of the ore, called bauxite. It is first transformed into bauxite by a process called Bayer and then into aluminium metal by electrolysis.
The name bauxite comes from the French town of Les Baux, where it was first mined. The world's main deposits are in Australia, Brazil and Africa. Bauxite is a mineral that is very rich in aluminium, up to 30% by mass, compared to the 20% of aluminium silicates found in clays and coals. As a practical example, we can say that, with four tonnes of bauxite, we will obtain two tonnes of alumina and, finally, one tonne of aluminium.
The Bayer process starts with crushing the bauxite and washing it with a hot sodium hydroxide solution at high pressure and temperature. The soda dissolves the aluminium compounds, which, being in a strongly basic medium, hydrate.
Types of aluminium
- Series 1000 1XXX 99 % or more aluminium.
- Series 2000 2XXX Copper (Cu).
- Series 3000 3XXX Manganese (Mn).
- Series 4000 4XXX Silicon (Si).
- Series 5000 5XXX Magnesium (Mg).
- Series 6000 6XXX Magnesium (Mg) and Silicon (Si).
- Series 7000 7XXX Zinc (Zn).
- Series 8000 8XXX Other elements.
Some of its most important processes are:
The extrusion process consists of applying pressure to the aluminium cylinder (billet) and passing it through a mould (die) to achieve the desired shape. Each type of profile has a suitable mould called "die", which determines its shape. The billet is heated (to approximately 500 °C, the temperature at which the aluminium reaches a plastic state) to facilitate its passage through the die, and is introduced into the press.
The machining process for aluminium on chip removal machines is less than that required for steel, which means less energy consumption and less tool wear.
The recycling of aluminium is tremendously important, due to the high cost of obtaining it because of the energy consumption required, which is solved by its low recycling cost, taking into account that aluminium is the third most common element in the earth's crust.
Aluminium is 100% recyclable without changing its physical qualities and requires only 5% of the energy needed to produce primary aluminium.
Recycled aluminium is called secondary aluminium, maintaining the same properties as primary aluminium. The recycling of secondary aluminium requires specialised techniques, which vary depending on the type of scrap to be melted. In addition, in order to carry out an optimal aluminium recycling process, a thorough selection and separation of the scrap must be carried out in order to achieve the desired alloy. Thus, for example, when the raw material is shavings or fine scrap, a dryer-slitter is used to obtain maximum melting performance. This takes place in vortex furnaces.
If, on the other hand, we start from slag or low-quality scrap, it is melted in rotary furnaces. The salt slag generated is sent to a cooling system and conveyed by conveyor belts to storage silos and then to salt recovery plants. The molten aluminium is transferred to reverberatory holding furnaces, equipped with an agitation system using porous plugs, which homogenises the alloy and temperature of the liquid aluminium. Finally, the alloyed metal is transferred to a casting machine to obtain ingots or poured directly into ladles for transport in liquid form by road.
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