Metals are essential for the sustainability transition and decarbonisation of society. Yet, it will be
paramount to produce them sustainably and minimise the affiliated resource and energy use and the
associated emissions. In the circular economy, the metallurgical industry should recycle existing
material stocks, and improve its utilisation of wastes, residues and side streams. This increases the
complexity of processes, as they become both (often multi-fraction) waste treatment as well as
material production processes and brings complexity to the assessment of environmental benefits.
Assessing the environmental impact of technological developments frequently is supported by life
cycle assessment (LCA). While the method is well documented, its implementation involves several
methodological choices that deserve reasoning and analysis, such as how to define the product
when former wastes are turned into new products, the selection of impact methodology when
converting emissions to environmental indicators, the definition of system boundaries and co-product
allocation and the interpretation of sensitivity and uncertainty in final outcomes.
In this exploratory study, we investigate how the variation of LCA set-up affects the environmental
burden of the system. We consider a metallurgical process where a mix of hard-to-recycle
aluminium-containing streams is used to produce aluminium cast alloys in a rotary furnace. Remelting
with salt-fluxes allows recovering metals from partly oxidised/contaminated streams, such
as dross, bottom ash and industrial shavings, but at the expense of generating significant amounts
of salt-slag/salt-cake hazardous waste. The study considers different system alternatives such as
landfilling the salt-slag residues versus valourising them into salts, aluminium concentrates,
ammonium sulfate and non-metallic-compounds to be used by the metallurgical, construction or
chemical industries. Practical recommendations are outlined to facilitate the implementation of LCA
in assessing the potential benefits of the circular economy in the metallurgical sector.