Aluminium encyclopaedia

Environmental protection

Environmental protection covers all measures undertaken to preserve the natural basic resources for humans, fauna and flora.

Each branch of industry has an impact on the environment, both as a result of the size of the plants as well their production processes. For a long time, industrial environmental protection, if it was practised at all, was aimed at reducing harmful effects.

The principle that one should eliminate the causes of any environmental risks and charge the costs to the polluter has become more and more accepted. It has even been found that this approach can be economical. An example from the aluminium industry involves exposure to waste gases from the fused-salt electrolysis process laden with fluorides and dust (which can be damaging to the environment). During wet cleaning by spraying with water, the aluminium dust and the fluorides enter the water. Although water treatment separates the mixture into cleaned water and sludge, the sludge has to be disposed of as hazardous waste. In dry scrubbing of the waste gases (used from 1970 onwards), aluminium oxide acts as the cleaning agent and combines with the dust and fluorides. If this aluminium oxide is returned to the fused-salt electrolysis process as a raw material, the savings made in process-related additions of fluorides to the molten metal are so large, that they pay for the operating costs of the cleaning process.

Via numerous regulations, authorities demand that the aluminium industry (as every other industry) adopts measures and installs equipment for environmental protection that necessitates a high level of research effort and substantial investments; in the case of smelters, for example, they account for about 20 per cent of the construction costs. A selection of environmental protection measures is given below:

  • Rehabilitation after bauxite mining.
  • Treatment and safe disposal of the red mud from the Bayer process used for alumina production.
  • In fused-salt electrolysis, wet or dry cleaning of the waste gases from the pots (see above) is used to keep emissions within allowable limits, likewise the cleaning of waste gases and effluent following the manufacture of anodes and cathodes. In addition, noise protection regulations are adopted for occupational health and safety.
  • During processing, waste gases and effluent have to be cleaned and noise protection regulations observed, for example during casting, forging or rolling.
  • When remelting particulate scrap resulting from recycling, pollution can occur as a result of the scrap containing impurities and thus having to be melted under a flux covering. The waste gases formed are cleaned in a filtration unit. The salt slag from remelting furnaces is water-soluble. At one time it was usually dumped but today it is being increasingly recycled: the salts and the metallic aluminium (contained in the cover fluxes) are extracted and leave about one-third behind in the form of water insoluble solids that can be disposed of in a normal manner.
  • Wherever it is possible and economical, energy-saving measures are adopted (for example for the transport of molten metal), and also contributes to environmental protection. The use of aluminium also leads to the saving of significant amounts of energy.

Waste is used as fuel in waste incineration. From an environmental point of view, however, they are mostly raw materials in the wrong place - avoidance must take priority over recycling and dumping waste.