Experimental Study on the Chemical Stability of Phosphate-Bonded Al2O3-Based Ceramic Foam Filters (CFFs)

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Production of high-quality aluminum products requires an extensive melt treatment process, even more so with the increasing focus on recycling and sustainability. Filtration is a commonly used process segment for removal of non-metallic inclusions in aluminum, and ceramic foam filters (CFFs) are often used as the filtration media. In the present study, the chemical stability of phosphate-bonded Al2O3-based CFFs has been investigated. Three filters with different chemical compositions have been submerged into pure aluminum (with traces of Mg) and in an aluminum-magnesium melt (~ 2 wt pct Mg) at 730 °C. In addition to filter characterization before and after exposure to molten metal, using various imaging and X-ray techniques, the melt itself was analyzed by spark optical emission spectroscopy. The generation of phosphine gas was also measured by the use of Dräger tubes, and thermodynamic calculations performed using FactSage™. The phosphate-bonded filters were observed to react with the magnesium present in the molten aluminum even at very low magnesium concentrations (0.00035 wt pct), and as the magnesium concentration increased the severity of the degradation became more and more evident. The exposure time proved to have detrimental effect on the filter structure, with pieces of the filter struts broken off causing melt contamination. Severe filter degradation also resulted in color changes with accompanying diffusion of magnesium and phosphorus to and from the filter, respectively. Moreover, phosphine gas was released in amounts exceeding recommended exposure limits when the filter came in contact with the humidity in the air after testing. Good agreement was established to exist between the results from the thermodynamic calculations performed and the experimental results.