Need to Know: Isothermal Melting

Oct. 15, 2005
for High-Quality Aluminum, and Energy Savings

Aluminum foundries, permanent molders, and diecasters can benefit from adopting isothermal melting—ITM for short—claims Edward Eckert, president of Apogee Technology Inc.

The firm is focused on developing technologies for metal treatment, melting, and related applications, particularly for the aluminum industry.

Eckert’s company has done the development work over three years to make the isothermal melting concept a practical application. In this effort, Apogee received financial assistance from the U.S. Dept. of Energy’s Industrial Technologies Program (DOE-ITP) to develop ITM. Project partners are Aleris International (formerly Commonwealth Aluminum), Newport, OH, and Drexel University, Philadelphia.

Eckert reported at the Ohio Technology Showcase in Cleveland on September 28 that the first commercial ITM unit is rated for melting at 5,000 lb/hr operating around the clock.

Among ITM’s advantages: It produces excellent metal quality with low dissolved gas and suspended solids. In the process, it offers energy savings and significantly reduced dross formation. As a highly efficient heating operation, ITM requires only one-fifth the floor space that a comparable conventional melting system would require. Finally, it’s expected that commercial ITM units will be available at relatively low capital cost.

As explained by Eckert, ITM is accomplished in a multi-bay flow system. Electronically controlled immersion heaters raise the temperature of molten aluminum in the heating bay. Molten aluminum drawn into the heating bay provides the energy to melt the solid aluminum charge. Metal is withdrawn and returned to the hearth at approximately the same temperature — hence, the designation “isothermal melting.”

Heat transfer to the melt and charge occurs by flowing metal around a strategically designed array of submerged heaters. The result is that ITM’s electric-resistance high-flux heat source requires no combustion air and eliminates emissions. Consequently, the maximum melt temperature experienced in the ITM process under the most aggressive melting conditions is projected to be less than 90°F above the desired holding temperature.

In addition, a patented moderate heat flux (25 W/in2) system consisting of electric-resistance heaters is embedded into special thermally conductive refractory panels. The purpose of this system is to offset holding heat losses. It does not contribute to melting heat requirements.

In commercial settings today, most aluminum is melted in reverberatory furnaces that use natural gas or fuel oil burners. These furnaces use radiant heating as the dominant heat-transfer mechanism and they have thermal efficiency of less than 30%. Their emissions create environmental concerns and their combustion gases are detrimental to metal quality. They promote oxidation and hydrogen adsorption.

Peak melt surface temperatures in reverberatory melting can exceed 2,000°F. This results in an increase in oxidation rate by a factor of 64, compared to a bulk temperature of 1,350°F.

Additionally, the dewpoint of typical products of combustion is equivalent to saturated air at 75°F, further exacerbating oxidation and establishing a high partial pressure of monatomic hydrogen.

Eckert pointed out that reverberatory and similar melting processes depend on downstream remedial metal treatment to remove oxide inclusions and dissolved hydrogen.

Conductive heat transfer from immersion heaters can provide thermal efficiencies as high as 97% and it avoids detrimental combustion gas contact with aluminum.

New materials and construction techniques allow immersion heaters to be built with high heat flux (approximately 70,000 Btu/hr-ft2.) These new heater designs 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 coupling media. The composite refractory coating is resistant to corrosive attack by molten aluminum, yet sufficiently thin enough to provide a high heat flux.

High-flux heaters incorporated into the ITM are practical for very large scale applications, Eckert reports.