Refining and smelting both require a series of raw materials that help turn bauxite into pure aluminium. From caustic soda to water, here’s how it happens.
Aluminium refining
The Bayer Process is the most commonly used method of obtaining alumina from bauxite, and is explored in detail here. The key input materials are the ores bauxite gibbsite, boehmite, or diaspore, caustic soda, and calcined lime. The global average consumption of these inputs is as follows:
| Raw Material | kg per tonne of alumina |
|---|---|
| Bauxite | ~ 3,000 |
| Caustic soda | ~ 100 |
| Calcined lime | ~ 50 |
The Bayer Process also requires water; the global average input of fresh being 2.6 m3 per tonne of alumina. However, the process tends to discharge a high proportion (1.2 m3) of this water in an unaltered form, meaning the average net “freshwater consumption” for the Bayer Process is 1.4 m3 per tonne of alumina. Seawater input and output is balanced (average 3.2 m3 in and out).
Bauxite consumption per tonne of alumina has increased in recent years (up from around 2,750 in 2005) due to declining quality of bauxite resources. However, the energy efficiency of the Bayer process has improved over that period, indicating that the industry is developing innovative technologies to extract alumina from bauxite in increasingly cost-effective ways.
Aluminium smelting
The Hall-Héroult Process is the industrial method for smelting primary aluminium, and you can read more about it here. The prebake anode production process requires calcined petroleum coke and liquid coal tar pitch, which, along with heat from combusted fuels, forms the baked anodes used in the reduction process.
The remaining 200kg of the mass of baked anodes comes from recycled anode butts, an output of the reduction process.
| Raw Material | kg per tonne of baked anode |
|---|---|
| Pet coke | ~ 650 |
| Pitch | ~ 150 |
Alumina, aluminium fluoride, and baked anodes are key inputs to smelting. However, other strategic input materials include pot lining materials (cathode carbon), refractory items, and steel. The steel is reused and recycled rather than consumed in the process, while the cathode carbon and refractory materials are also recycled to a given degree at the end of their life.
| Raw Material | kg per tonne of primary aluminium |
|---|---|
| Alumina | ~ 2,000 |
| Baked anodes | ~ 450 |
| Aluminium fluoride | < 20 |
| Cathode carbon | < 10 |
| Refractory materials | < 10 |
| Steel | < 5 |
Primary aluminium ingot casting still has an element of scrap usage. Ninety percent of the aluminium for primary ingot production is liquid metal from the reduction process, with 5% remelted scrap from the ingot production process, 2.5% from external scrap and around 2% alloying elements .
The primary aluminium production process has an average water input of 2.6 m3 per tonne of ingot. Much of this water input is for cooling purposes in the ingot casting process, such that 1.2 m3 is discharged in an unaltered state per tonne of ingot. Therefore, the net freshwater consumption of the smelting process is around 1.4 m3 per tonne of aluminium.
Executive Vice President and
Chief Commercial Officer
Alcoa Corporation
“Aluminium consumption is expected to nearly double between 2010 and 2020, driven largely by China and emerging markets. To meet such demand, supply of the metal will have to grow at almost double the global GDP growth rate, along with secure supplies of high quality and low cost raw materials.”