An aluminium melt always contains impurities. Even during the Bayer process, it is not possible to keep oxides of metals like sodium away from aluminium oxide completely, and from there they then find their way into primary aluminium via fused-salt electrolysis. In addition, aluminium oxide that has been used for cleaning waste gases from the electrolysis process is also used as a raw material today (see emissions) and this brings with it fluorides and metal oxides. During the production of recycling aluminium (also called "secondary aluminium" because it uses secondary raw materials), impurities (from lacquers and fouling on the scrap) enter the melt. And finally the surface of the melt comes into contact with air, whereby aluminium oxidises (combines with oxygen), but also absorbs hydrogen.
Depending on the application, for quality assurance purposes it is necessary to reduce the quantities of specific impurities in the melt to below certain limits, for example the hydrogen content from 10-100 cubic centimetres per kilogram of melt in most cases to less than 1.5-2 cubic centimetres, and the sodium content from an average 15 parts per million to 2-10. There is a range of specific cleaning processes available and these are carried out immediately prior to casting, to prevent impurities forming again.
- With sedimentation one allows the melt to stand still for some time. Impurities heavier than aluminium settle on the bottom while lighter ones rise to the surface and mix with the oxidised aluminium.
- During filtration, the melt flows through sintered alumina or through foamed ceramic filters in which impurities from about 20 thousandths of a millimetre upwards are removed.
- During flushing with inert gases, gases such as argon are injected via hollow lances or rotary jets in such a way that the gases rise up through the melt as bubbles that are as small as possible and at the same time stir the melt. In doing so, they transport non-metallic impurities to the surface.
- During flushing with active gases such as chlorine, the gases react (hence the expression "active") with a part of the impurities in the melt (for example with calcium to form calcium chloride or magnesium to form magnesium chloride), and this reduces the content of these impurities. The treatment lasts a few minutes and the amount of chlorine used is up to 0.5 per cent of the weight of the melt. Salts such as sodium chloride also form as reaction products, and one has to remove these at the end of the active-gas flushing treatment using an inert gas. The salts and the toxic chlorine require fume extraction for industrial safety and waste-gas cleaning for environmental protection.
If one produces a vacuum in the melting crucible, the hydrogen appears as bubbles at the surface and is extracted there. Such "vacuum degassing" is effective and is not harmful to the environment.
As a result of melt purification, metal oxides, salts and impurities always collect on the surface. Together with the oxidised aluminium they form "dross". This is skimmed off and recycled to save costs because of its metallic aluminium content.