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