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(Bajare et al., 2012)

In the aluminium recycling process, scrap metal is remelted in furnaces with the addition of molten salt flux, which helps purify the metal, separating oxides and contaminants in a waste stream known as black dross or salt slag depending on the residual metal content.

Typically, when the material contains 5-20% Al it is known as black dross, and when it is between 5-10% it is salt cake or salt slag (Tsakiridis, 2012). Black dross, and salt slag can both be further reprocessed to recover aluminium until minimal metal content remains and a number of recyclers also use processes to recover the salt flux materials from the slag for re-use, resulting in an end material referred to as NMR, non-metallic residue (Tsakiridis, 2012).