Iron Refining in Recycled Al-Si Alloys by Intermetallic Precipitation

Aluminium production is expected to increasingly depend on recycled aluminium, driven by lower energy costs and the need to reduce carbon footprints as environmental regulations and policies become stricter. One of the main limitations of recycling aluminium, mainly aluminium-silicon (Al-Si) alloys, is the high iron (Fe) concentration. Fe forms the β-Al5SiFe phase, which has a plate-like morphology with sharp edges that severely degrades both castability and the mechanical properties of the cast Al. A promising approach for Fe refining is through the addition of Fe-bearing intermetallic particle formers, such as manganese (Mn), chromium (Cr) and strontium (Sr). The main aim of this research is to develop a quantitative understanding of Fe removal in low-Fe Al-Si alloys by investigating the formation and sedimentation behaviour of Fe-bearing intermetallic particles under different Mn, Cr, and Sr combinations. To do so, a series of small-scale experiments was conducted using 8 kg of molten Al-11Si-0.5Fe alloy. The results demonstrated that, for a low-Fe Al-Si alloy, a relationship between Fe removal efficiency and the Fe-bearing intermetallic particles was established by calculating the Fe removal potential. When comparing the Fe removal potential with the clean aluminium yield, a clear trade-off was noticed. The higher Fe removal efficiency results in lower clean aluminium yield (Mn-Cr addition) and vice versa (Mn-Sr/Mn-only additions). To further understand the effect of cooling rate on Fe-bearing intermetallic particles, a physics-based growth-sedimentation model was developed and calibrated using experimental results to quantitatively describe the coupled evolution of particle size and settling behaviour at different cooling rates. The model showed that the 1.5Mn0.6Cr addition results in a lower yield due to increased resistive forces arising from viscosity, tortuosity, particle-particle interactions, and drag. While in 1.5Mn and 1.2Mn0.3Sr, a higher yield was obtained due to their compact morphology and more effective sedimentation.