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Immersion Melting Approaches for Aluminum
12-26-02
Conventional melting of aluminum is done in gas or oil-fired reverberatory furnaces with radiant burners, which tend to have low energy efficiency. An alternative is to employ conductive heat transfer for melting by immersing the heating source in the molten aluminum, with potentially much higher thermal efficiency. This has not been possible, however, due primarily to heater material limitations. Two projects underway in the Department of Energy's Office of Industrial Technologies portfolio are aimed at solving this problem.
The main attributes needed for a successful immersion heating system are the ability
to efficiently transmit heat, retain mechanical stability, and not contaminate
the melt. Typically a ceramic material has been identified for this application,
but conventional monolithic ceramics do not have adequate thermal shock resistance.
Recent work under DOE-OIT's Continuous Fiber Ceramic Composites program involves
development of a ceramic composite material consisting of silicon carbide fibers
in a nitride-bonded silicon carbide matrix. The desired tubular shape for an immersion
heating unit is fabricated by filament winding the fibers and then nitriding them
to form the composite. To date, 36 inch long tubes have been fabricated and tested.
These tubes exhibit uniform heating, are thermal shock resistant, are not wetted
by the molten aluminum, and are not chemically attacked. While the ceramic matrix
composite tubes will surely be significantly higher in initial cost than conventional
ceramic tubes. The increased life should provide some advantage over the longer
term. For example, an immersion tube using this material survived over 1,000 hours
and 31 casting cycles in an aluminum casting furnace at one industrial site and
for 1,752 hours at 47 casting cycles at another. The next step is multi-tube testing
at an automotive aluminum casting facility. Further information on this development
is available from the material developers, Textron Systems Division at www.systems.textron.com
or from the CFCC homepage at www.eere.energy.gov/industry/aluminum/pdfs/immers_tubes.pdf.
An alternative approach, also focusing on materials of construction for high heat flux immersion heaters, is being carried out by Apogee Technologies, Inc. under the Aluminum Industries of the Future program in the Department of Energy's Office of Industrial Technologies. The heater designs under consideration are based on a highly conductive, impact resistant ceramic coating on a metallic sheath and a highly thermally conductive dielectric integral coupling medium between the sheath and the heat-producing element. This allows heat transfer by conduction to be the dominant mode, rather than the particle-to-particle radiation heat transfer that prevails in conventional immersion heaters with compacted powder heating media. The composite refractory coating is resistant to corrosive attack by the molten aluminum but is sufficiently thin to provide a high heat flux. The specification, production, and evaluation of the thin coating are key technical elements of the project. The target heat flux for these heating units is 70,000 Btu/hr-ft2, which would make the heaters practical for very large scale applications.
In this program, the high heat flux immersion heaters are an enabling technology
for the development of an isothermal melting system, labeled ITM for short. This
system is a multi-bay flow system in which metal is withdrawn from the hearth
and returned at essentially the same temperature (i.e. isothermal) after solid
aluminum has been added and melted in a specially designed charge bay. If such
a system can be developed, it is projected that it would offer significant energy
savings, reduction in dross formation, lower melt loss, and lower capital cost
than a conventional reverberatory furnace. Demonstration of each of the technology
elements for the ITM system as well as system integration on a commercial scale
are all part of this ambitious undertaking. Further information on this project
can be obtained by contacting Apogee Technology, Inc. at apotecki@aol.com or visiting
the Aluminum Industries of the Future web site at www.eere.energy.gov/industry/aluminum.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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