Pilot-Scale Investigation of Liquid Aluminum Filtration through Ceramic Foam Filters: Comparison between Coulter Counter Measurements and Metallographic Analysis of Spent Filters

摘要

A pilot has been designed to study the mechanisms associated with liquid aluminum filtration through ceramic foam filters (CFFs). Before entering the filtering bowl, the liquid metal is seeded with massive artificial inclusions. A theoretical analysis of the filtration of massive inclusions shows that there are two preponderant deposition mechanisms, i.e., direct interception and sedimentation, which explain qualitatively the effect of the operating conditions on the measured filtration efficiency. Furthermore, it is observed that the filtration efficiency measured by two Coulter counters reaches a minimum value for a diameter close to 30 μm. This nonmonotonous trend can be explained by the coexistence of two populations of inclusions, intentionally added massive ones and oxide films. It is expected that oxide films are the most numerous inclusion type for diameters less than 25 μm and have an apparent volume much larger than their absolute volume (contrary to massive inclusions), leading to a higher filtration efficiency. In addition, a metallographic analysis of spent filters shows that the density of the captured massive inclusions decreases exponentially between the filter inlet and the filter outlet. The initial filter coefficient can be derived, which is equal to 25 m?1 for a 30-ppi filter grade (with a specific surface area of 1610 m?1), SiC inclusions 50 μm in diameter, and a filtration velocity of 0.9 cm·s?1.