Coalescence of Clean, Coated, and Decoated Aluminum for Various Salts, and Salt–Scrap Ratios

Abstract

In a rotary furnace for aluminum recycling and dross treatment, a salt flux covers the molten metal to prevent new oxidation and to capture oxides, and impurities contained in the scrap. Furthermore, the salt must promote the coalescence of the metal drops trapped in the dross. This work deals with the coalescence of aluminum droplets obtained by melting scrap with different impurity contents under salt fluxes characterized by various compositions. The scrap was stamped out from AA3000 coil with and without coating in the form of disks. One hundred disks, covered by molten flux, were melted in a ceramic crucible placed in an induction furnace at 790 °C. Thermal decoating treatments at various temperatures, i.e., 400, 500 and 600 °C, were performed to reduce the amount of impurities on the scrap surface. Two salts already used in industry were compared with an experimental one. The salts are here referred to as (1) Industrial salt (2) Recycled salt and (3) Cryolite salt, respectively. CaF2 was added to the Industrial and to the Recycled salts while the experimental one, Cryolite salt, contained Na3AlF6, as suggested by the name. The amount of salt ranged from 0 to 4 times the mass of the scrap. The coalescence efficiency has been evaluated through a factor that ranges from zero (no coalescence) to more than 20 (all drops coalesce to one big drop) considering the fraction of coalesced drops, and their average diameters. The coalescence behavior is negatively affected by the coating, but a suitable decoating treatment and fluxing can lead to the coalescence of the aluminum drops. A complete coalescence is achieved by melting clean disks under the flux that contains cryolite, giving a coalescence factor of 21, while the drops do not coalesce if CaF2 salt is added. The same results are obtained if coated disks are decoated at a temperature higher than the combustion temperature of the coating, i.e., 600 °C. Industrial salt gives a coalescence factor lower than 3, while Recycled salt gives a coalescence factor of 4. No positive effects are obtained adding CaF2 amounts ranging between 2 and 6 wt%.