Aluminium Production and Processing
Aluminium is the third most abundant element in the Earth’s crust after oxygen and silicon. Although it occurs very frequently, it is never found in its pure form. Therefore, the material cannot simply be mined from nature, but must be produced in a complex process.
What raw material lies behind aluminium? Which processes are required to obtain pure aluminium in the end?
The Basis: Bauxite
In nature, aluminium is found only in aluminium compounds. The highest aluminium concentration is found in so-called bauxites. These are weathering products of limestone and silicate rock.
In bauxite, the proportion of aluminium oxides is often more than 50 percent. This is the raw material for aluminium production. The largest mining areas are located in Australia, China, Guinea, Brazil and India.
In such locations, the raw material contains the largest amount of alumina, i.e. pure aluminium oxide. Alumina is usually extracted directly at the bauxite mining site. This saves transport costs. But how does it work?
The Conversion: Alumina
Through the Bayer process, invented in 1887 and named after Karl Bayer, pure aluminium oxide is extracted from bauxite. This is also called alumina.
Bauxite is finely ground in large quantities. It is then placed in a large container and mixed with heated sodium hydroxide solution. This is a solution of caustic soda and water that dissolves the aluminium oxide in the bauxite.
Aluminium slurry is the liquid mixture of sodium hydroxide solution and the aluminium oxide dissolved in it. Red mud refers to the undissolved bauxite components.
Using filters, the aluminium slurry is separated from the red mud. The latter is disposed of. The aluminium slurry is cooled and pumped into silos. Small white crystals form in the liquid and sink to the bottom.
These crystals are transferred into another silo, where they can continue to grow. At a temperature of around 1000 °C, the crystals are converted in a furnace into alumina, i.e. pure aluminium oxide. A fine white powder is produced.
Extraction: The Electrolysis Process
The alumina then moves on to the primary smelting furnaces. The so-called aluminium smelters are often located in places where inexpensive and comparatively clean energy can be generated.
Pure aluminium is produced from alumina using the Hall-Héroult process. The process is named after two researchers who independently invented this method in 1886.
Alumina consists of aluminium and oxygen. In the Hall-Héroult process, these two components are separated by fused-salt electrolysis. The alumina is placed in a reduction vessel.
This vessel contains the fluoride salt cryolite, heated to around 1000 °C. The inside of the vessel is often lined with graphite. An electrical voltage is generated between the carbon anode blocks and the graphite.
Although this voltage is low, around 4 to 5 volts, it has an extremely high current strength of 220,000 to 340,000 amperes. The electric current separates the oxygen from the aluminium.
The oxygen binds to the carbon of the anode blocks and is discharged in the form of CO₂. The pure aluminium sinks to the bottom and is drained off in liquid form. The liquid aluminium is then cast into moulds and can be further processed for various purposes.
The Material: Aluminium
The mining of bauxite, the conversion into alumina and the electrolysis process show how complex aluminium production is. First, pure aluminium oxide is extracted from the raw material bauxite. The oxide is then dissolved in molten cryolite and processed into aluminium by fused-salt electrolysis.
The end product, pure aluminium, has many useful properties. It makes the material versatile, easy to process and highly recyclable. Recycling aluminium requires only around five percent of the energy needed to produce primary aluminium.
Important Steps in Aluminium Production
- Bauxite mining: extraction of the aluminium-containing raw material.
- Bayer process: conversion of bauxite into alumina or aluminium oxide.
- Fused-salt electrolysis: separation of aluminium and oxygen in the Hall-Héroult process.
- Casting: liquid aluminium is cast into moulds.
- Further processing: aluminium can be processed into sheets, plates, profiles, tubes or components.
- Recycling: aluminium can be reused in an energy-efficient way.
Conclusion
Aluminium is a technically sophisticated material with a complex production chain. From bauxite through alumina to electrolysis, several process steps are required before pure aluminium is produced.
It is precisely this combination of low weight, good workability, corrosion resistance and high recyclability that makes aluminium one of the most important materials for industry, construction, mechanical engineering, vehicle manufacturing and many other applications.
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Frequently Asked Questions About Aluminium Production
What is aluminium made from?
Aluminium is mainly produced from bauxite. From this raw material, alumina or aluminium oxide is first extracted. Pure aluminium is then produced by electrolysis.
What is bauxite?
Bauxite is an aluminium-containing rock and the most important raw material for aluminium production. It often contains a high proportion of aluminium oxides.
What is the Bayer process?
In the Bayer process, pure alumina or aluminium oxide is extracted from bauxite. The ground bauxite is treated with heated sodium hydroxide solution and then filtered, cooled and further processed.
How is pure aluminium produced?
Pure aluminium is produced using the Hall-Héroult process. Aluminium oxide is dissolved in molten cryolite and separated into aluminium and oxygen by fused-salt electrolysis.
Why is aluminium easy to recycle?
Aluminium is highly recyclable. Recycling requires only around five percent of the energy needed to produce primary aluminium.
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