Comparative Studies of Silicon Dissolution in Molten Aluminum Under Different Flow Conditions Part II: Two-Phase Flow
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ent of a high efficiency alloying process is short assimilation time. To make Al-Si alloys, metallurgical grade Si (MGSi) is generally used in the form of crushed pieces (5 to 10 cm) that are dropped onto the surface of molten Al. The solid Si floats on the Al, but is mostly submerged. Some form of stirring is required to continuously expose the pieces to fresh molten Al, to speed up the dissolution process, and to obtain a uniform melt composition. The addition of Si can be a lengthy process, resulting in lost production time, energy loss as heat when adding Si and stirring the melt, and material losses due to dross formation. Significant savings could be achieved if the dissolution time could be reduced by only 10 pct.[1] The assimilation rate of a solid addition in melting/ holding furnaces can be improved by various methods. One can raise the flame temperature of the furnace, as this increases the solubility of Si into the Al. However, MEHRAN SEYED AHMADI, R&D Design Engineer, formerly with the Department of Materials and Science and Engineering, University of Toronto, Toronto, ON, Canada, is now with the Venmar CES Inc., Saskatoon, SK, Canada. STAVROS A. ARGYROPOULOS, Professor Emeritus, is with the Department of Materials and Science and Engineering, University of Toronto. Contact e-mail: stavros. [email protected] MARKUS BUSSMANN, Associate Professor, is with the Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, ON, Canada. DON DOUTRE, Technical Leader, Recycling and Molten Metal Processing, is with the Novelis Global Technology Center, Kingston, ON, Canada. Manuscript submitted April 23, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
this approach has many drawbacks; for instance, it can cause overheating of the metal near the flame, leading to excessive oxidation and increasing metal loss. Another approach to accelerating the rate of Si dissolution is mechanical stirring using a steel blade or an impeller to circulate the molten metal. This produces a higher bulk velocity in the liquid; however, the use of either device entails considerable energy loss, since the furnace doors must be open. A more sophisticated method is electromagnetic stirring with the furnace doors closed, which can be very efficient, but is costly for new installations, and difficult to retrofit on old furnaces.[2] Another method to increase the dissolution rate of Si is gas injection: bubbles move quickly through the liquid, agitate the flow, and result in bubble-induced stirring. Gas stirring can be used by itself or with mechanical stirring. Gas injection is relatively economical, and has been extensively used in the steelmaking industry for homogenization and inclusion removal.[3,4] The implementation of gas injection is feasible in the Al industry, as evidenced by the fact that, for example, hydrogen is removed from liquid Al by purging with Ar and/or N2 through a simple tube.[5] This is the second of two papers on the dissolution of Si in molten Al. In the first,[6] the dissolution rate of Si was exa
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