"Emissions" is the term used to describe gaseous, liquid and solid substances released into the air, waters and the ground. "Immissions" are the effects of such emissions on the environment. Allowable emissions (i.e. the limiting values for each individual substance) are determined in such a way that the immisions they cause do not exceed specific limits that in the opinion of legislators pose a risk for human beings and the environment. The limiting values and their monitoring are laid down in the legislation on environmental protection. Gaseous, liquid and solid emissions are generated during the extraction and processing of aluminium. The classical emissions are fluorides. Fluorine is a component of aluminium fluoride, which is needed for the extraction of aluminium by fused-salt electrolysis.
Gaseous hydrogen fluoride is formed mainly by the reaction of molten aluminium fluoride with water vapour in the air and with hydrocarbon residues in the anodes. Dry scrubbing with aluminium oxide, introduced in the 1970s, fulfils all of the requirements with respect to eliminating fluoride emissions with a removal rate of 99 per cent, and fluorides are no longer an immision problem today.
Emissions and residues are produced during various stages of the extraction and processing of aluminium:
- anodising: alkali and acid vapours
- Bayer process for the extraction of aluminium oxide: red mud (about one kilogram for every kilogram of aluminium oxide produced), carbon dioxide and dust
- fused-salt electrolysis: organic substances (during the production of anodes); carbon dioxide and sulphur dioxide (due to burning off of the anodes), gaseous and dust-like fluorides
- extrusion: oil vapours
- remelting: organic substances (as a result of burning off of lacquers)
- rolling of strip and foil: organic substances (as a result of the decomposition of rolling oil).