4-7 metric tons of bauxite = 2 metric tons of alumina = 1 metric ton of aluminium.
Bauxite contains, in addition to alumina, silica, iron oxide and titanic dioxide. The Bayer process is used to extract alumina from the bauxite:
- Alumina is separated from the bauxite by using a hot solution of caustic soda (sodium hydroxide) and lime (calcium oxide).
- The mixture is pumped into high-pressure containers and heated.
- The caustic soda dissolves the alumina, which precipitates from the saturated solution. The alumina is then washed and heated to drive off water.
- The precipitate is filtered and washed to remove and recover the caustic solution.
- Everything else is filtered away, and the alumina is dried to a white powder.
- Most alumina refineries are located close to the bauxite mine, or at the nearest harbor, where the alumina can be easily shipped to aluminium production plants.
How does alumina production affect the environment?
When alumina is extracted from bauxite, the process creates about an equal amount of bauxite residue, also known as red mud. This needs to be managed properly to protect the environment, and is a challenging part of alumina production.
The aluminium industry is moving from storage in lagoons to dry stacking, because this is much more efficient and also safer for the environment. Hydro uses only dry stacking to store bauxite residue.
There are different storage methods for the bauxite residue produced by alumina refineries. This model is based on reports from 17 refineries representing 44 percent of global production in 2007 (Source: Review of current bauxite residue management, disposal and storage: Practices, Engineering and Science. CSIRO document DMR-3608, May 2009).
Other environmental issues related to alumina production:
- Energy consumption – a lot of energy is needed to make alumina.
- Water management – process water as well as surface water from precipitation and run-off from the bauxite residue deposits are cleaned before being discharged to the environment.
- Impacts from the plant’s infrastructure and the bauxite residue disposal – when the bauxite residue deposits are full, they will be replanted with local species.
Meet Alunorte: The world’s largest alumina refinery
Alunorte has a total production capacity of almost 6.3 million metric tons per year, which is about 7 percent of the world’s production.
The plant is one of the most energy-efficient plants in the world. Coal and heavy fuel oil are the main energy sources for electricity and steam generation.
The plant emits about 3.8 million metric tons of CO2 per year. Looking at the amount of CO2 emitted per ton produced, Alunorte is among the best third of the world’s producers.
The plant operates with comparatively low water consumption (2.3 cubic meters/metric ton alumina).
Alunorte uses the dry stacking method for bauxite residue. This means that the residue can be stacked in geotechnically stable piles instead of in lagoons. At the end of use, the stockpiles are covered through replanting.