Removal of Zinc from Molten Aluminum by Vacuum Refining

Abstract Increasing aluminum (Al) recycling leads to greater variability of minor elements such as zinc (Zn), which can deteriorate alloy performance and limit high‑value applications. This study investigates vacuum refining as a pyrometallurgical route for Zn removal from molten Al, with emphasis on process understanding and industrial scale‑up. Laboratory experiments (100–1000 Pa, 706–826°C) and pilot trials in a siphon inert reactor (SIR) were combined with thermodynamic analysis and kinetic modeling. Under industrial‑like conditions (~ 800 mbar), Zn removal was negligible from 1 wt.%, whereas reduced pressures significantly enhanced removal efficiency, confirming the critical role of vacuum level. A three‑step mass transfer model, supported by FactSage thermodynamic calculations and Python‑based simulations, was used to quantify contributions from liquid‑phase transport, interfacial evaporation, and gas‑phase diffusion. The results show that gas‑phase mass transfer dominates under moderate vacuum (≈ 100–200 Pa), while a transition toward mixed or liquid‑phase control occurs at deeper vacuum (< 10 Pa) and elevated temperatures (> 800°C). Furthermore, process design parameters, particularly the melt surface area‑to‑volume ratio, were identified as key levers for improving refining efficiency and reducing scale‑up risk. The combined experimental and modeling approach provides practical design criteria for industrial implementation of vacuum refining, demonstrating how targeted pyrometallurgical testing can reduce the uncertainties associated with scale up and enable enhanced removal of volatile impurities from recycled Al streams.