New methods for reprocessing residues from aluminium recycling

Extract of a presentation by Dr Heribert Summer (AluMonte GmbH, Austria).

When remelting used aluminium scrap, a flux comprising a mixture of 90% sodium chloride and 10% sodium fluoride is usually used, in quantities between 3-70%. The aluminium scrap is added to the molten flux in such a way that the flux combines with the impurities in the melt and protects the scrap from exposure to air. Without the flux layer, the aluminium would react with oxygen at these temperatures, namely about 800 °C, to form aluminium oxide.

The remelting process involving aluminium scrap and cover flux produces both recycled aluminium and so-called ‘dross’ comprising cover flux, impurities and metal oxides (of aluminium and other metals present). When the dross has a metal content of more than 45%, the term ‘skimmings’ is used otherwise it should be referred to as ‘dross’. Aluminium can be recovered from the dross or skimmings. Fluxes used during remelting can also be recovered.

During skimming in hearth or rotary drum furnaces during primary or secondary processing, 10–80% of the aluminium is lost. This aluminium can be present in the form of large lumps down to fine grains smaller than 2 mm and is incorporated mechanically in the dross, which may or may not contain flux (where flux is used, the dross can contain up to 20% flux). Such dross or skimmings can now be processed using the ALDROS system (aluminium dross processing system).

The liquid phase is first separated from the hot dross, i.e. molten aluminium is extracted by applying pressure. After squeezing, a rotating ladle transfers the hot dross in a hermetically sealed atmosphere to a cooling drum. Inside the drum, the hot dross is cooled indirectly by water in a double-walled tube. The cooled dross is then subjected to milling: this produces an aluminium-containing granulate of differing particle sizes, which is separated out via a vibrating screen. At the end of this screening process, the residue is dust, which mainly contains oxides and is drawn off via a filter. This so-called ball mill dust can either be used to recover the flux that it contains, and the residual dust is then used as a source of aluminium oxide in the cement industry or in the production of steel, or it can be dumped.

The granulated particles produced during ball milling, which can contain up to 90% aluminium, are remelted in a rotary drum furnace; thanks to this recycling process (the REDROSS process), up to 90% of the aluminium from the dross/skimmings can be recovered. For the user, recovery of the major part of the aluminium tied up in the dross means profit maximisation, while costs for processing the flux-containing dross or disposing of it by dumping are reduced.