How is aluminium produced? Which raw materials are used and what processes are required in order to change the raw materials into pure aluminium?
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Before we look at the actual production process, it’s good to know where the process begins: with extraction. Aluminium is extracted from bauxite. Bauxite is the name for the ore in which around 30 to 54% of the aluminium oxide is contained. Because of the fact that aluminium has a tendency to bind with oxygen, there is very little pure aluminium found naturally. Aluminium is only available in the form on an oxide, i.e. metal bound with oxygen. This aluminium oxide has to be extracted from the aforementioned bauxite.
Bauxite is primarily extracted in (sub) tropical areas because this bauxite contains the highest quantities of alumina. Alumina is the name for pure aluminium oxide. The extraction of alumina from the bauxite usually takes place in the vicinity of the bauxite extraction site to save on transport costs.
The alumina is extracted from the bauxite by means of the so-called Bayer process, named after the inventor of the process Karl Bayer (in 1887). A large quantity of bauxite is finely milled. This bauxite is then poured into a large container to which heated caustic soda is gradually added. Caustic soda is a solution of corrosive soda and water. The caustic soda dissolves the aluminium oxide that is found in the bauxite. The other elements of the bauxite do not dissolve.
The liquid mixture of caustic soda and the dissolved aluminium is referred to as ‘slurry’. The remaining substance from the bauxite is called ‘red mud’. The slurry is separated from the red mud via filters. Red mud is disposed of as waste. The slurry is then cooled and transferred to precipitation tanks, also called precipitators. Small white crystals then form in the liquid and fall to the bottom. From there, they are pumped to another tank where the crystals can continue to grow.
The crystals are then taken to an oven where they are converted to alumina at a temperature of 1000℃. This alumina looks like a fine white powder and can be further processed to obtain pure aluminium.
The alumina is transported to the primary smelters, or aluminium factories. They are often located where cheap and relatively clean energy can be generated. These primary smelters turn the alumina into pure aluminium using the so-called Hall-Héroult process. This process is named after the two inventors who both came up with the process in around 1886, without having collaborated with one another.
Alumina comprises aluminium and oxygen and the aim of the Hall-Héroult process is to separate the two. The alumina is poured into a reductive container. This container is filled with the fluoride salt cryolite, which is then heated to a temperature of 1000℃. The interior of the container is covered with graphite. There are also two carbon anode-blocks hanging in the container.
Between the anode-blocks and the graphite, an electrical charge is generated with the low voltage (4-5 Volt), but a massive amperage of 220,000-340,000 Ampère. This electric current separates the oxygen from the aluminium. The oxygen binds to the carbon on the anode-blocks and is removed in the form of CO2. The pure aluminium sinks to the bottom where the liquid is drained off. The liquid aluminium is then poured into forms so that it can be further processed for a range of applications.
The conversion of alumina into pure aluminium requires a great deal of energy to generate the power. For this reason, alumina is processed at locations where the energy required can be generated simply and relatively cleanly. Often, the primary smelters at these locations use hydropower stations.
The extraction of alumina from the bauxite creates one waste product, a red substance called ‘red mud’. On a small scale, the red mud is used to colour roofing tiles, as a filler for asphalt roads and for water purification.
However, most of the red mud is not economically usable. This red sludge is stored in basins with an impenetrable clay base. It must stay there from 5 to 10 years for the remaining caustic soda to be neutralised. CO² from the air or rainwater ensures that the remaining caustic soda is neutralised into a residue of soda and water.
During the electrolysis process, the waste and emissions are primarily fluoride emissions and rubble that is created when replacing the covering layer in the container. When this layer is replaced, a rubble that contains fluoride is created. Projects have already begun to extract this fluoride from the rubble for reuse. These projects are expected to be perfected between 2023 and 2028 and will then be used on a large scale. The replacement of anode-blocks also creates waste in the form of carbon blocks. These blocks are supplied as fuel and raw materials to the steel and cement industries.
The complete production process from aluminium is a fascinating procedure. Starting with the extraction of the bauxite, through to the conversion of alumina and ultimately to the entire electrolysis process. The end product: pure aluminium, ready to be further processed by post-processing companies.
Aluminium has many characteristic properties which make it a very versatile metal, with a range of applications. Aluminium is therefore very recyclable. Recycling aluminium costs just 5% of the energy costs that are required to make primary aluminium.
Read all about the various features which make aluminium sustainable here:
Would you like to know more about the special properties of aluminium? Read all about the various properties of the material here:
Bauxite is a mixture of aluminium oxide and iron oxide and it is the primary source of aluminium. When mixed with Caustic Soda and heated under pressure (Bayer process) it becomes Aluminium Hydroxide, which after washing and heating (calcining) converts into pure aluminium oxide called Alumina. Electrolysis transforms alumina into aluminium; this is a smelting process that involves two metal electrodes being submerged into a molten material containing positive and negative ions, one electrode is a positive terminal (anode) and the other electrode is a negative terminal (cathode). Baked Anodes are made from the mixture of Calcined Petroleum Coke (made by calcination of Green Petroleum Coke ) and binding material called Coal Tar Pitch. Alumina is dissolved into molten cryolite at 1,000 degrees C. Cryolite (Crushed Bath) and Aluminium Fluoride are added to lower the temperature of the electrolysis and to increase the conductivity of the molten material in the cell (pot). When electrical current passes through the molten material at the cathode end aluminium ions are reduced to aluminium metal and at the anode end carbon is oxidized to form carbon dioxide gas. Finally molten aluminium sinks to the bottom of the pot and is drained through a plug.