Influence of Coating and De-Coating on the Coalescence of Aluminium Drops in Salt

In a rotary furnace for aluminium recycling and dross treatment, a salt flux is added which protects against oxidation and captures non-metallic impurities. Furthermore, the salt has to promote the coalescence of the metal drops in the dross. This work investigates the coalescence of molten aluminium for different types of scrap. One hundred discs were stamped from aluminium alloy sheets with and without coating. They were melted, covered in NaCl-KCl-Na3AlF6 molten salt, in an induction furnace at 790 degrees C. The solidified aluminum droplets were extracted by leaching the salt with water. The fraction of coalesced drops and the average diameter were determined to evaluate the coalescence efficiency. The effect of various de-coating temperatures was studied. The results show that the coalescence is negatively affected by coating. Long holding times has no effect. Complete coalescence off all discs are achieved with uncoated scrap. The drops coalesce if the temperature of the combustion reaction for the coating is attained.