Smelter emissions
Fluorides
Most smelters operated by IAI members now have powerful scrubbing equipment which removes up to 99% of all emissions from the pots. The materials removed by the scrubbing can then be re-used in the smelting process. As a result, current average levels of emission to atmosphere are as low as 1kg (0.5kg for the new modern plants) of fluoride per tonne of aluminium. This compares with 3.9 kg per tonne in 1974.
There are two main types of fluoride emissions:
- inorganic fluorides
A mixture of the inorganic fluoride compounds (NaF, AlF3 and Na3AlF6 - as particulates, and HF - as a gas), can have a local effect in the area around a smelter (unlike PFCs which do not have any local effects but a global effect as a greenhouse gas).
The development in fluoride reductions is best divided into three generations as follows:
| 1st generation plants | 1940-1955 | 12-15 kg per tonne; | |
| 2nd generation plants | 1955-1975 | 2-6 kg per tonne; | |
| 3rd generation plants | 1975-today | 0.3-1 kg per tonne. |
Since the fluoride emissions have reduced to their current levels, vegetation previously affected in plant environments has quickly recovered. Most aluminium smelters are surrounded by environmental control zones - normally farmland, and the environment in these is closely monitored.
Polycyclic aromatic hydrocarbons (PAHs)
These substances are emitted during the manufacture of anodes for modern "pre-bake" aluminium smelters, and during the electrolytic process itself in the older "Söderberg" type facilities.
Pre-bake anodes are made from petroleum coke and pitch. These are by-products from the petroleum and steel process and are baked in either gas- or oil-fired ovens. Anodes are progressively consumed during use and are eventually replaced. The “anode butts” are then recycled.
PAH emissions from Söderberg facilities have been dramatically reduced by the introduction of "dry anode technology" at many locations as well as through other process improvements and alterations in the raw materials used.
Sulphur dioxide
This is generated from the sulphur content at fossil-fueled power stations, and from the other parts of the aluminium production process - steam generation in alumina plants, ovens in anode plants, and anode consumption in the pots. The remedy is to use low sulphur fuel and coke if available, and wet scrubbers to remove the particles from the air.
Carbon dioxide
Carbon dioxide is a feature of all metal processing that produces metal by electrolysis from ores containing oxides. The gas forms when the carbon in the anode combines with the oxygen in aluminium oxide during the smelting process. It is therefore an unavoidable by product of the aluminium smelting process. Over the past 10 years the aluminium industry has reduced its carbon dioxide output by around 10 per cent through the development and use of better production techniques.
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