Abstract

The recycling of aluminum provides high efficiency in terms of ecologic and economic aspects. To ensure high quality products for both secondary and primary aluminum, appropriate and effective procedures have to be applied for the removal of non-metallic inclusions.  A simple, relatively cheap, and efficient process to reduce non-metallic inclusions is the usage of ceramic foam filters during the casting process.

The present 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.

The filtration experiments were conducted on a laboratory scale with a filtration pilot plant at Constellium C-TEC (Voreppe, France), which allows appropriate filtration durations (40 to 76 min) using a 730 kg aluminum furnace. During these trials the amount and size of the non-metallic inclusions were quantified with LiMCA (Liquid metal cleanliness analyzer). The test set-up allows the simultaneous use of two LiMCAs (before and after the filter) from which the filtration efficiency can be calculated.

All filter natures were quite efficient in terms of inclusions removal (85 to 95%). The test results showed the coatings to rank in the following sequence (from highest to lowest): Al2O3, MgAl2O4, 3Al2O3·2SiO2 and TiO2. The analysis of the removal efficiency as function of the inclusion size shows that the Al2O3 and the MgAl2O4 filters are comparable (efficiency closed to 100% for inclusions larger than 90 µm) while the 3Al2O3·2SiO2 filter possesses the lowest filtration efficiency for the larger inclusions (> 110 µm). In these trials, the 3Al2O3·2SiO2 filter showed the highest filtration efficiencies for smaller inclusions (< 60 µm). The TiO2 filter showed the lowest filtration efficiencies.

The spent filters were metallographically analyzed at the SEM in order to observe the interactions between the inclusions and the filter surface. Only limited interaction between inclusions and the four different coating natures were observed, regardless of the chemistry of the coating.