Sustainable Removal of Impurities from Recycled Al–Si Alloys Using Eggshell-Derived CaO Composite Flux

Impurity reduction remains a critical challenge in aluminium recycling, particularly due to tramp elements such as Sn, Pb, Fe, Ni, and excess Si and Mg in Al–Si alloys used in automotive applications. This study presents a novel, sustainable refining flux synthesized from eggshell-derived calcium oxide (CaO), combined with NaCl, MgCl2, Na2B4O7, and Na3AlF6. The flux was comprehensively characterized using XRD, FTIR, SEM-EDS, and DSC-TGA, and applied to secondary Al–Si melts at 680 ± 20°C. Post-treatment analyses (ICP-MS and OES) demonstrated significant impurity reductions: Fe (22.5%), Si (19.5%), Mg (31.7%), and Zn, Pb, and Sn reduced by over 80%. The overall aluminium purity increased from 84.2 to 87.6 wt %. Dross analysis confirmed successful impurity segregation via slag formation, chlorination, and oxide capture. The results highlight the flux’s effectiveness in enhancing melt cleanliness while valorising bio-waste, providing a scalable, eco-friendly alternative to conventional fluxes in secondary aluminium recycling.