Aluminum dross, a byproduct of the aluminum melting process, is a mixture of aluminum, its oxides, the oxides of alloying elements and less frequently halogenides, carbides and nitrides.
Dross has some good benefits, especially considering tension strength and depending on further conditioning steps can be very useful further on.
Dross is a by-product of all aluminum melting processes. Depending on scrap type, alloy treatment and melting equipment, the amount, metal content and properties of dross vary in a wide range. Dross is a mixture of aluminum, aluminum-oxide and oxides of alloying elements, additionally halogenide, carbide or nitride compounds of these metals are regularly encountered. Typically dross contains a large amount of metallic aluminum. This is mechanically fixed in a cellular structure of aluminum oxide, which has tension strength great enough to keep aluminum 20 times of its own weight bonded. As dross also contains finely dispersed gas bubbles (foam structure), it easily separates from the aluminum melt due to the reduced density.
The following process parameters have a strong influence on volume, morphology and the metal content of dross:
- Surface/volume ratio of the scrap
- Type (organic, inorganic and amount of impurities
- Melting conditions and melting equipment
- Melt composition (especially Mg, Na, Li, Ca-content)
- Refining procedure
- Way of dross removal (temperature, tools)
- Cooling parameters of removed dross
A direct comparison and evaluation of the various dross conditioning processes is difficult, as the raw material source is different. Generally modern processes are based on a fast quenching step in order to avoid an oxidation of the incorporated aluminum metal droplets. It is followed by a mechanical beneficiation step which removes as much oxide as possible. Alternatively a direct melting of the > 1 mm fraction is state of the art. The by-product has to be dumped or processed separately in a slag treatment plant, the <1 mm fraction (dross residue) can be sold to the steel industry for deoxidation purposes.
Figure 1: Formation of Dross
a) Surface oxidation of melt.
b) Crushing of oxidic skin by bath movement.
c) Sinking and floating of oxidic particles.
d) Conglomeration of oxidic particles.
e) Interspace filling up by metallic aluminum.
f) Inside oxidation of dispersed aluminum.
g) Skimming of dross from melt bath surface.
h) Metallic aluminum drip off and follow-up oxidation of solids during dross cooling
Table 1: Composition of dross
Figure 2: Dross