Alumina is a white granular material, a little finer than table salt, and is properly called aluminium oxide. Aluminium does not occur as a metal but must first be refined from bauxite into alumina. Approximately two tonnes of alumina are required to produce one tonne of aluminium.
Australia is the second largest producer of alumina in the world, with around 15 per cent of global production from six alumina refineries. As water is a key input for refining bauxite into alumina, Australian alumina refineries are located in areas with reliable water supply.
The Bayer refining process used by alumina refineries worldwide involves four steps:
- precipitation; and
Bauxite is finely ground in mills, then mixed with a recycled caustic soda solution and steam in digester vessels operating at high temperature and pressure. This dissolves the alumina content of the bauxite.
The solution is then cooled in a series of flash tanks.
The impurities which remain undissolved are allowed to settle as a fine mud in thickening tanks.
After several washing stages to recover caustic soda, the residue is pumped to storage dams. The solution of alumina in caustic soda is further clarified by filtration.
The bauxite residue is safely stored, in ways which meet industry best practice and required safety standards.
One technique used for bauxite residue storage in Australia is “dry stacking’”, which involves depositing and drying the residue in thin layers to a high density, thus making it very stable and unlikely to flow in the event of a containment breach.
Alumina crystals are recovered from the caustic solution by mechanically stirring the solution in open-top tanks. Crystal growth is assisted by seeding with previously precipitated alumina.
The precipitated material (called hydrate) is washed and dried at temperatures exceeding 1000 degrees Celsius. This forms the dry white anhydrous aluminium oxide powder (alumina) which is cooled and conveyed to storage.
The caustic soda is recovered and returned to the start of the process and used again.