A mathematical model of aluminum depth filtration with ceramic foam filters: Part I. Validation for short-term filtratio
- PDF / 877,921 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 102 Downloads / 257 Views
I. INTRODUCTION
THE filtration of aluminum has become widely spread as a final refining technique used before casting the metal. The removal of solid inclusions improves metal fluidity, resulting in greater castability.[1] Furthermore, the obtained microstructure leads to enhanced mechanical properties, e.g., strength and ductility are increased providing better formability and machinability, and also, an inclusion-free metal favors a low tool wearing. Melts that have a maximum inclusion concentration of ,10 ppm in volume are generally filtered using ceramic foam filters. When these melts flow through a foam filter at typical melt superficial velocities from 5 to 15 mm/s, depth filtration occurs.[2] This mode of particle removal consists in trapping the particles in the interior of the filter itself rather than on the inlet surface, as it happens with the other two modes of filtration, i.e., sieving and cake filtration, which are outside the scope of the present work. The particle concentration, C, observed in the solidified metal that remains within spent filters after depth filtration, follows an exponential profile.[3,4] This profile can be derived by integrating the following first-order rate expression. C 5 2lC z
[1]
for the limits C 5 Ci at z 5 0 and C 5 C0 at z 5 L, where l is a filtration coefficient, with dimension of reciprocal length. In general, this coefficient changes during filtration
F.A. ACOSTA G., Assistant Professor, and A.H. CASTILLEJOS E., Professor, are with the Centro de Investigacio´n y de Estudios Avanzados del IPN-Unidad Saltillo, Apdo Postal 663, Saltillo, Caoh., 25000 Me´xico. Manuscript submitted September 3, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
because it depends on the amount of particles accumulating within the filter. The filtration period where l remains constant is known as initial or short-term filtration, while that period where the accumulation of particles starts to play an effect on the value of the filtration coefficient is known as aging or long-term filtration. The present article is focused to show the results of a study on the initial filtration period characterized by an initial filtration coefficient, l0. For this case, the corresponding filtration efficiency is given by the following equation:
h 5 [1 2 exp (2l0 L)] 3 100
[2]
The previous equation results from substitution of the integrated Eq. [1] into the definition of filtration efficiency given as h 5 (Ci 2 Co)/Ci 3 100, and it establishes the dependence of the filtration efficiency with the filter thickness L. However, it should be noticed that the filtration coefficient does not depend on this thickness but on the pore size and geometry and on the flow and particle properties as well. Equation [2] has been used by Mutharasan et al.[5] to determine l0 for ceramic filters used for filtration of TiB2 from aluminum in laboratory tests. They found that this coefficient decreases sharply with the melt superficial velocity in the range from 1 to ,3 mm/s, but it becomes nearly constant at higher veloc
