The recycling of secondary aluminum enjoys high priority in regard of ecologically and economically aspects. To ensure a comparable product quality of the secondary and primary aluminum appropriate and effective procedures for the removal of for example non-metallic inclusions from the molten secondary aluminum have to be applied. A simple, relatively cheap, and efficient process to reduce non-metallic inclusions is the usage of ceramic foam filters during the casting process.
The study focuses on the effect of the filter surface chemistry on the filtration behavior. Five different oxide filter surface chemistries (alumina (Al2O3), spinel (MgAl2O4), mullite (3Al2O3·2SiO2), silica (SiO2) and titanium dioxide (TiO2)) were tested with the aluminum alloy AlSi7Mg with regard to their filtration behavior.
During these trials the amount and size of the non-metallic inclusions with LiMCA (Liquid metal cleanliness analyzer) measurements were determined. This test set-up allows the simultaneous use of two LiMCAs (before and after the filter) which allows the calculation of the filtration efficiency.
The comparison of the total filtration efficiency showed the best results for the Al2O3 filter with a total efficiency of 94.5%. The TiO2 filter possesses with 85.4% the lowest filtration efficiency. The evaluation of the filtration efficiency in dependence on the inclusion size provides further insights. The Al2O3 and the MgAl2O4 filters show comparable filtration efficiency in dependence on the inclusion size. At inclusion sizes of >90 µm the filtration efficiencies are 100%.