The Aluminium Story is a story of energy.
Bayer Process (aluminium refining)
Key input materials are the ores bauxite gibbsite (Al(OH)3), boehmite (γ-AlO(OH)), or diaspore (α-AlO(OH)), caustic soda (NaOH) and calcined lime (CaO). World average consumption of these inputs is outlined below:
|Raw Material||kg per tonne of alumina|
|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.
Hall-Héroult Process (aluminium smelting)
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 that are an input to the reduction process.
The remaining 200 kg of the mass of baked anodes come from recycled anode butts, an output of the reduction process.
|Raw Material||kg per tonne of baked anode|
|Pet coke||~ 650|
Alumina (Al2O3), aluminium fluoride (AlF3) and baked anodes are the key inputs to the smelting process; 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. 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|
|Baked anodes||~ 450|
|Aluminium fluoride||< 20|
|Cathode carbon||< 10|
|Refractory materials||< 10|
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 a water input of, on average, 2.6 m3 per tonne of ingot. The majority of this water input is for cooling purposes in the ingot casting process, such that 1.2 m3 are discharged in an unaltered state per tonne of ingot. The net freshwater consumption of the smelting process is therefore around 1.4 m3 per tonne of of aluminium.
“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.”