Aluminum is a non-ferrous metal of low density, widely used in engineering because of its excellent strength-to-weight ratio, high conductivity and resistance to corrosion. It is widely used in sectors such as construction, transportation, aeronautics and packaging.
Technical interest then arises in understanding how is aluminum made? and, in addition, what makes it so resistant to oxidation? By analyzing the aluminum manufacturing process, from bauxite to the final metal, it becomes evident how chemical and metallurgical engineering has optimized its structural properties, consolidating it as a strategic material in manufacturing and industrial sustainability.
Origin of aluminum: Bauxite as a raw material
Bauxite is the main and practically exclusive mineral from which aluminum is extracted on an industrial scale; it is not a simple mineral, it is a heterogeneous sedimentary rock, rich in hydrated minerals of aluminum oxides, such as gibbsite (Al(OH)₃), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)). Its coloration can vary from reddish and brown to whitish, depending on the content of iron oxides, silica and other impurities.
The main reserves of this mineral are found in equatorial and tropical regions, such as Australia, Guinea, Brazil, China and Jamaica, and are extracted by open-pit mining, where the top layers of soil are removed to expose the deposit. The extracted material is transported to processing plants, marking the beginning of a complex industrial chain that constitutes the aluminum manufacturing process.
Key stages in the aluminum manufacturing process
Aluminum manufacturing involves a sequence of chemical and electrochemical processes that transform bauxite into pure metal, structured in two fundamental stages: the first is the refining of bauxite to obtain alumina (aluminum oxide, Al₂O₃) through the Bayer Process; the second, the electrolytic reduction of alumina to produce metallic aluminum through the Hall-Héroult Process.
Each phase requires operational precision, thermodynamic control and physicochemical parameters that directly affect the quality of the metal obtained.
Bauxite extraction and initial preparation
The initial phase of the aluminum manufacturing process focuses on obtaining the bauxite, which, once extracted from the mine, is transported to the conditioning plants, where it is subjected to crushing operations to reduce the size of the rocks and facilitate its operational handling; subsequently, the fragmented material is subjected to a mechanical grinding process, which transforms it into a fine powder, increasing the specific reaction surface of the particles.
This physical conditioning is decisive in optimizing the efficiency of the alkaline solution in the next chemical stage. The larger the contact surface, the more efficient is the selective extraction of the aluminum compounds, and the better is the yield of the industrial transformation; where the particle size characteristics and the mineralogical purity of the input material condition the overall efficiency of the subsequent metallurgical process.
Alumina refining using the Bayer process
The refining of bauxite to obtain alumina (aluminum oxide, Al₂O₃) is carried out using the Bayer Process, developed at the end of the 19th century and still in use due to its efficiency. In this stage, this mineral is ground and mixed in pressurized digesters with a hot and concentrated solution of sodium hydroxide (NaOH), reaching temperatures between 150 and 200 °C under controlled pressure.
This combination favors the dissolution of the hydrated aluminum oxides, forming sodium aluminate, while insoluble impurities, such as iron oxides and silica, are separated as “red mud” through decantation, a highly alkaline waste whose environmental management is a significant challenge. This sludge is composed mainly of residual iron oxides, silica, titanium dioxide and aluminum oxides.
The supernatant solution rich in sodium aluminate is filtered to remove solid residues and then cooled in a controlled manner; precipitation of aluminum hydroxide (Al(OH)₃) is induced by inoculation with fine crystals of the same compound; the precipitate is washed to remove traces of caustic soda and transferred to rotary or fluidized bed furnaces for calcination.
During calcination, at temperatures of 1000 to 1200 °C, the water of hydration is removed and high purity anhydrous alumina is obtained in the form of a white granular powder. This aluminum oxide is the direct input for the electrolysis stage in the process of obtaining metallic aluminum.
Primary aluminum production: Hall-Héroult electrolysis
The second stage of aluminum manufacture is carried out through the Hall-Héroult process (developed in 1886), by means of the electrolytic reduction of alumina to obtain pure metallic aluminum. The reaction is carried out in electrolytic cells, where a steel tank internally coated with carbon acts as a cathode. On this structure, consumable carbon anodes are installed, which actively participate in the electrochemical reaction during operation.
Due to the high melting point of alumina (greater than 2000 °C), molten cryolite (Na₃AlF₆) is used as a flux, which allows the alumina to be dissolved and the temperature to be reduced to a range of 950-980 °C; with the alumina dissolved in the electrolyte, a high intensity direct current is applied, (more than 200 kA), which reduces the Al³⁺ ions at the cathode, and generate the liquid aluminum; at the same time, the oxygen ions react with the carbon anodes, forming carbon dioxide (CO₂) and degrading the anodes, which must be replaced periodically.
This process consumes a high energy potential of approximately 13,500 to 15,000 kWh per ton of primary aluminum, which drives the industry to continuously seek innovations in energy efficiency, such as the development of inert cells or the use of renewable energy sources.
The liquid aluminum, which is denser than the electrolyte, accumulates at the bottom of the tank and is extracted by siphon or vacuum; it is then transferred to holding furnaces for homogenization and alloying, preparing it for further processing.
Smelting, alloying and casting
The primary aluminum collected, with a purity of 99.7 to 99.9 %, is used in applications that require specific metallurgical characteristics. However, in most cases, its properties are adjusted by a process called controlled alloying, which allows the material to be adapted to different operating conditions, such as thermal loads and mechanical stresses.
To this end, alloying elements such as copper, magnesium, silicon, zinc, manganese or lithium are incorporated to modify the microstructure of the aluminum and optimize key properties such as mechanical strength, malleability, hardness and corrosion resistance.
These alloys are classified into standardized series from 1000 to 8000. For example, the 2000 and 7000 series are used in aeronautics for their high strength, being found in fuselage and wing components; the 8000 series alloys with lithium are an example of innovation, allowing greater weight reduction in aircraft and aerospace structures due to their lower density and higher elastic modulus.
Finally, liquid aluminum in its pure or alloyed state is poured into molds to obtain semi-finished products such as extrusion billets, rolling billets or specific casting alloys; these formats support processes such as extrusion, rolling or die casting, as well as advanced techniques such as 3D printing of aluminum, obtaining useful geometric shapes and specific properties for each industry.
To complement the information on the stages of the aluminum manufacturing process, the following technical video illustrates the stages from bauxite extraction to electrolytic reduction through the Hall-Héroult process. It passes through fundamental stages such as alkaline digestion, alumina calcination, and alloy formation. Source: History of Simple Things.
How Is aluminum made?
Table 1. Classification of aluminum alloys by series
| Grade | Main alloy | Characteristics |
| 1000 Series | 99%. It is not an alloy, it is aluminum with the presence of impurities | High conductivity, excellent chemical resistance |
| 2000 Series | Copper (Cu) | High strength, low corrosion resistance |
| 3000 Series | Manganese (Mn) | Good corrosion resistance, weldability |
| 4000 Series | Silicon (Si) | Low coefficient of expansion, good fluidity |
| 5000 Series | Magnesium (Mg) | Excellent corrosion resistance, weldability |
| 6000 Series | Magnesium (Mg) + Silicon (Si) | Good strength, weldability and extrusion |
| 7000 Series | Zinc (Zn) | Highest mechanical strength, heat treatable |
| 8000 Series | Various elements (lithium, iron, etc.) | Special properties, specific applications |
Does aluminum rust? Understanding its chemical resistance
The question of whether aluminum rusts requires a precise, technical answer. Although aluminum reacts with oxygen, unlike iron, it does not undergo progressive or destructive corrosion. Instead of forming a heterogeneous corrosion layer on the surface, it immediately generates a thin, stable passive layer of aluminum oxide (Al₂O₃) upon contact with air or water. This protective layer, typically between 2 and 10 nanometers thick, acts as a barrier against further oxidation.
This film is uniform, dense and strongly adherent, which prevents the penetration of oxygen and other corrosive agents. It functions as a protective barrier that isolates the metal from the aggressive environment and is also self-healing; in case of mechanical or chemical damage, it regenerates spontaneously in the presence of oxygen.
Due to this natural passivation, aluminum retains its structural integrity and surface appearance, even in aggressive environments. However, in extreme pH conditions (very acidic or very alkaline) or in the presence of agents such as chlorides (common in marine or salty environments) or mercury, the coating can degrade, facilitating the appearance of localized corrosion.
In highly demanding applications or specific corrosive environments, this material is often subjected to additional surface treatments, such as aluminum anodizing; which forms a thicker and more porous oxide layer that can be sealed, increasing corrosion resistance, surface hardness and even allowing decorative finishes.
Structural properties of aluminum
The main characteristics of this mineral include:
- Low density and high strength-to-weight ratio: With a density of approximately 2.7 g/cm³, it is an exceptionally lightweight material. This characteristic, combined with its excellent strength-to-weight ratio, makes it an optimal material for designs requiring lightness and high efficiency, outperforming steel or copper in applications where mass reduction is required.
- Corrosion resistance (passivation): Its durability is due to the spontaneous formation of a surface layer of aluminum oxide when exposed to air. This layer acts as a passive barrier that protects the metal from corrosion, maintaining its chemical stability even in aggressive environments.
- Low temperature behavior: aluminum maintains its mechanical strength at low temperatures, making it perfectly functional in cryogenic environments.
- Malleability and ductility: It has high malleability and ductility, facilitating forming processes such as extrusion, rolling and casting without affecting its internal properties.
- High conductivity: It has an electrical conductivity close to 60% of that of copper, as well as high thermal conductivity, properties that make it suitable for heat dissipation and electrical conduction applications.
- Unlimited recyclability: Aluminum can be recycled unlimitedly without loss of physical-mechanical properties. The production of secondary aluminum requires only 5% of the energy used in the primary process; this energy saving translates into a reduction of CO₂ emissions, consolidating it as a strategic material for industrial decarbonization and circular economy.
Conclusions
The aluminum manufacturing process, from the thermochemical and hydrometallurgical approach, constitutes a highly specialized chain to transform bauxite into functional metal. This procedure is developed in stages such as alkaline digestion, aluminum hydroxide precipitation, calcination to alumina and electrolytic reduction in Hall-Héroult cells, each of which requires strict control of physicochemical variables that determine the purity, energy efficiency and final properties of the product.
A technical understanding of how is aluminum made allows the engineering applied in each intermediate transformation and its effect on the structural and chemical behavior of the material to be assessed. Although aluminum reacts with oxygen, the spontaneous formation of a surface layer of Al₂O₃ acts as a protective barrier, inhibiting the progression of the oxidative process.
References
- https://waykenrm.com/blogs/types-of-aluminum-alloys/
- https://en.wikipedia.org/wiki/Hall%E2%80%93H%C3%A9roult_process
- https://en.wikipedia.org/wiki/Bayer_process
