How Is Aluminum Refined and What Elements Does It Contain？

Aluminum (Al) is a lightweight, silvery white metal that ranks as the third most abundant element in the Earth‘s crust, following only oxygen and silicon. However, due to its high chemical reactivity, it never occurs naturally in its pure metallic form. Instead, it is found in compounds, primarily within bauxite ore, a mixture of hydrated aluminum oxides including gibbsite (Al(OH)₃), boehmite (AlO(OH)), and diaspore.

The Refining Process in Two Stages

The journey from raw bauxite to high purity aluminum involves two distinct industrial processes.

Stage one is the Bayer process, developed in 1888. Crushed bauxite is mixed with a hot sodium hydroxide solution under pressure, dissolving the aluminum-bearing minerals while leaving impurities like iron oxides and silica behind. The resulting sodium aluminate solution is then filtered to remove the red mud residue, seeded with aluminum hydroxide crystals, and calcined at approximately 1,100°C to produce pure white alumina, or aluminum oxide (Al₂O₃). Over 90% of the world‘s alumina is now produced via this method.

Stage two is the Hall Héroult process. Alumina has a melting point above 2,000°C, making direct electrolysis impractical. The solution lies in dissolving Al₂O₃ in molten cryolite (Na₃AlF₆), which lowers the operating temperature to about 950~1,000°C. An electric current is then passed through the mixture. Molten aluminum collects at the bottom (the cathode), while oxygen combines with the carbon anodes to form CO₂. This electrolytic method remains the only industrial process for producing primary aluminum, yielding metal of 99.5~99.8% purity.

What Elements Does Aluminum Contain?

Pure aluminum itself consists solely of the element Al, with an atomic number of 13 and an atomic weight of approximately 26.98 g/mol. Commercial purity aluminum (98.8–99.7% Al) contains minor traces of iron and silicon as natural impurities. However, most applications rely on aluminum alloys, where specific elements are intentionally added to tailor mechanical properties.

For structural applications, the 6000 series (e.g., 6061) uses magnesium and silicon as its primary alloying elements, typically 0.8~1.2% Mg and 0.400~.8% Si. This alloy offers an excellent balance of moderate strength, good weldability, and superior machinability.

For high-strength demands, the 7000 series (e.g., 7075) incorporates zinc and copper as the main alloying elements, with approximately 5.16~.1% Zn and 1.2~2.0% Cu. The T6 temper of 7075 delivers nearly twice the tensile strength of 6061-T6, making it the material of choice for aerospace and high performance structural components.

Trace amounts of chromium, manganese, and titanium are also commonly present in commercial alloys, each playing a role in grain refinement and corrosion resistance. Understanding the precise elemental composition of each alloy is essential for selecting the right material for specific machining or fabrication requirements.