Abstract
Process for reducing the iron content in 356 secondary aluminum alloy is developed in the current study. The Fe-rich intermetallic compounds (or sludge) were removed by sedimentation of liquid aluminum through alloy addition. Effects of alloy additions were first surveyed by thermodynamic evaluation. The sedimentation temperature and time were designed according to the solidification curves of the secondary aluminum alloys. In this study, combinations of 0–2 wt% of manganese (Mn) and chromium (Cr) were added to the melts for isothermal sedimentation. Fe-rich sludges of Al(FeMn)Si, Al(FeCr)Si, and Al(FeMnCr)Si compounds were formed, and sedimented. Mn tends to increase the sludge particle size, while Cr addition has the effects of refining the precipitates and increasing aspect ratios of the precipitate morphology. Although chromium demonstrated higher iron removal capability than manganese, when more than 1 wt% Cr is added, the Al13Cr4Si4 phase forms without further removing Fe. When both Mn and Cr additions are excessive, Mn- and Cr-rich phases are formed at the center of the sludge phase. Therefore, excessive Mn and Cr cannot further increase iron removal rate. For optimum iron removal conditions, 1 wt% Mn- and 1 wt% Cr-added melts can remove 73.8% of Fe from the 356 secondary alloy and recover 77.3% secondary alloy by gravity separation method.
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