Removal of Inclusions from Aluminum Through Filtration
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cleanliness has been in the limelight during the last three decades and remains a concern in the aluminum casting industry. In general, cleaning aluminum and its alloys refers to minimizing contaminants such as dissolved gasses (especially hydrogen), nonmetallic inclusions (such as oxides, carbides, and nitrides), a variety of intermetallic compounds, and alkali and alkaline-earth elements such as sodium, lithium, and calcium. These contaminants enter the molten aluminum through the ore and raw materials used in the extractive metallurgical processes, through the refractory materials and the atmosphere during aluminum production, and even through the refining processes. The common types of inclusions in aluminum have been reported to be oxides, nitrides, carbides, fluorides, and borides.[1] Extensive research has resulted in a significant improvement in our present understanding of the various aspects of these contaminants, and in many foundries, melt-cleaning practices have been established and are used routinely. However, with the ever-increasing demand for improved metal properties, the requirements for molten metal cleanliness have become extremely stringent. Various methods such as sedimentation, flow LUCAS NANA WIREDU DAMOAH, PhD Student, and LIFENG ZHANG, Assistant Professor, are with the Department of Material Sciences and Engineering, Missouri University of Science and Technology, 223 McNutt Hall, Rolla, MO 65409-0330. Contact e-mail: [email protected] Manuscript submitted August 26, 2008. Article published online April 16, 2010. 886—VOLUME 41B, AUGUST 2010
transport, bubble flotation, filtration, and electromagnetic force are being used to remove inclusions from molten aluminum. Investments in filtration systems are relatively cheap, and hence, every effort at making them even more efficient must be explored. AlF3 is used to purify the molten aluminum by removing dissolved impurities such as Na and Ca through powder fluxing[2,3] and granular bed filtration.[4] It was reported that ~98 pct of Na and Ca were removed from aluminum through AlF3 granular bed filtration.[4,5] The use and the evaluation of the efficiency of ceramic foam filters (CFFs) in the removal of nonmetallic inclusions from molten aluminum have been studied widely in the literatures.[6–14] Reports from these studies indicated that CFFs are capable of achieving high aluminum filtration efficiencies and the formation of bridge-like structures of inclusions at the top area of the filter contributes to the high efficiencies. It is well known that CFFs also remove inclusions smaller than the pore size of the filter in a deep-bed filtration mode. However, there is inadequate information to explain the removal mechanism of smaller particles within the filters. Furthermore, using the existing Al2O3 CFFs coated with AlF3 to purify molten aluminum has the potential to remove both dissolved impurities and nonmetallic inclusions simultaneously. The current study observed the morphologies of inclusions commonly found in aluminum in both two and three dimensions, and experiment
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