Radionuclides in the aluminium industry
Bauxite is the most important raw material for aluminium. The production of aluminium metal comprises two basic steps of refining, which consists of alumina production from bauxite by treating (dissolving) the raw material with caustic soda, and reduction, i.e. production of aluminium metal from alumina by an electrolytic process.
The bauxite ore, containing up to 55% of alumina, is red-coloured by its iron content (10 to 25%) and contains various amounts of crystalline water and silicon dioxide, as well. When the ore is treated with aqueous caustic soda, the amphoteric aluminium hydroxide and the acidic silicon dioxide will be dissolved leaving the precipitation of iron and titanium oxides as so-called red sludge. This sludge contains the essential parts of uranium, thorium and their decay products.
The raw material (bauxite) shows often high Th-232 and U-238 levels. Red sludge as waste respectively red sludge bricks as by-products, show activity concentrations of Th-232, U-238 and Ra-226. Typical activity concentrations in feed materials of metal smelters are respectively 300 Bq.kg^-1 for Th-232 and 300 Bq.kg^-1 for U-238.