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Higher-Efficiency, High-Quality Aluminum Melting

July 31, 2005
Two R&D projects now underway are looking at ways to melt aluminum that maintain metallurgical standards, but save energy, fuel — and costs.

With demand rising for lighter metal castings from industries including automotive, internal combustion engines, appliances, and aircraft, it is no surprise that aluminum foundries and diecasters are hunting for ways to melt more efficiently. However, because they don’t want efficiency to come at the expense of product quality, there’s also a search underway for the right controls systems.

Toward that goal, the U.S. Dept. of Energy’s Office of Industrial Technologies (OIT) and numerous partners have implemented two R&D projects aiming to find ways to streamline the process of melting aluminum.

Clean aluminum meltingThe first project involves immersion heating of aluminum to reduce energy consumption and environmental impacts. Normally reverberatory furnaces are used to heat aluminum to melting temperatures with open flame, direct-fired, wall-mounted burners. In most cases, reverb furnaces have melting efficiencies that average 26-31%. Additionally, they have high-volume processing rates, along with low operating and maintenance costs. However, there are high metal oxidation rates, low efficiencies, and large floor space requirements.

OIT, together with several partners — including Aleris Recycling, Amcast Industrial Corp., DOE Albany Research Center, E3M Inc., Hydro Aluminum North America, Logan Aluminum, Metaullics Inc., Oak Ridge National Laboratory, Secat Inc., SECO/Warwick Corp., Trace Die Casting, the University of Kentucky, and Wise Alloys L.L.C. — is looking at immersion heater furnaces as a good alternative. Immersion heater furnaces heat the metal bath by direct contact. The burner is immersed in the molten aluminum and a premixed burner provides hot combustion gases that are circulated through the housing. The combustion gases never come into contact with the molten aluminum, which helps reduce oxidation losses. The submerged burner allows better heat transfer to the molten charge, which also improves thermal efficiency. Already aluminum melting efficiencies of 60-65% have been achieved by one immersion burner, and another laboratory-scale immersion electric heater is in the works that will help develop future models that could achieve efficiencies as high as 97%.

In foundries alone, the energy saved by using immersion heater furnaces is predicted to range from 2.7 to 4.2 trillion Btu/year after 10 years, assuming approximately 25% of the aluminum melted was produced through this method. This could result in savings up to $24 million annually and provide such environmental benefits as the reduction of carbon, NOx, CO, VOC, and particulate emissions.

Improved melting for diecasters

More efficient aluminum melting is also the focus of a project by researchers at Case Western Reserve University in Cleveland. Specifically, the project will look at numerous aspects of aluminum melting and handling in diecasting, including improving energy efficiencies, while also improving the quality of molten metal.

Diecasters typically use reverberatory furnaces to melt aluminum, too, but the chambers’ efficiency often is less than 10% when gas is used for the combustion fuel. From these furnaces, the molten metal is transferred to small holding furnaces, also with efficiencies of 10% or lower. Not surprisingly, the increase in fuel prices means this method of melting will continue to be rise in cost.

The energy consumption of melting in diecasting could be reduced if melting were done in partly closed electric-powered furnaces or higher-efficiency gas furnaces. The proposed study will involve manufacturers and users of melting furnaces, insulating materials, degassing and pumping equipment, filtration, and transfer ladles. In addition to CWRU, the project has a number of research partners including the North American Diecasting Assn., Cast Metals Coalition Partnership, Inductotherm, Mercury Marine, Spartan Light Metals Products, and Striko Dynarad. The hope is that by using more modern electrical melting equipment and a partial dip out procedure, it will be possible to reduce energy consumption by more than 50%.

Additionally the project hopes to improve efficiency of melting and molten metal in diecasting plants by

  • Developing in-plant monitoring methods and procedures of gas and electrical power use in melting and casting operations.
  • Training project team and plant personnel to monitor energy use and identify potential improvements.
  • Conducting energy efficiency surveys of melting and holding furnaces and molten-metal handling equipment used in diecasting.
  • Developing methods and procedures for measuring the energy used in melting and molten-metal handling operations.