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Sorting the Wrought from the Cast in Automotive Aluminum
02-06-03
When aluminum is touted as the material of the future for automotive applications, its recyclability is one the prime attributes that is highlighted. In fact, because of the higher value of aluminum relative to other automotive materials, free market drivers already cause it to be recycled at a high rate (higher even than aluminum cans, which are recycled at a roughly 60% rate). Today the majority of aluminum use in automobiles is in the form of castings, and the recycled material can be returned to foundries for production of new castings. Projections for both the aluminum used in automobiles and the demand for secondary metal for automotive castings indicate that castings can continue to absorb the aluminum automotive scrap generated in the near term. However, as the market develops for wrought products in closure panels and aluminum body structures, it will be more profitable and increasingly desirable to have these alloys segregated and recycled back into sheet and extrusion products. In addition, it would be desirable to increase the recycling rate for automotive aluminum from 85% to 100% if possible. This need is being addressed by R&D efforts in the area of scrap sorting supported by the Department of Energy's Office of Industrial Technologies and Office of Transportation Technologies in concert with the Aluminum Automotive Alliance organized through The Aluminum Association.
First, a brief overview of the present infrastructure for recycling aluminum from end-of-life vehicles is necessary. Typically, cars proceed from the consumer to the automotive dismantler, where they are dismantled as far as economically viable for useable parts and easily sorted aluminum scrap such as wheels and radiators. The remaining portion is shredded into fist-sized pieces of varying composition. Ferrous and non-metallic scrap is separated from the non-ferrous scrap, which in turn is sent to separators to remove non-aluminum scrap, such as copper and zinc. The resulting mixed aluminum scrap is sold to the secondary industry for recycling into foundry alloys, since over 80 percent of current aluminum scrap is in the form of secondary casting alloys.
Fast forward roughly 10-15 years, when the current vehicles with much higher overall aluminum contents, as well as an increasing amount of wrought product, are at the end of their useful lives. A combination of the much higher volume of recycled material available and tighter metallurgical specifications for castings will put greater demands on the quality of metal recycled to metals casters, and in the long term the supply of recycled metal from the shredder may exceed the demand for secondary metal for aluminum castings. These factors drive the need for cost-effective methods for the separation of wrought scrap from cast scrap to ensure an efficient recycling system in the future. The need will only grow as aluminum usage continues to grow, and when aluminum-intensive vehicles utilizing high proportions of wrought products will capture a larger proportion of the market.
Recognizing the importance of automotive aluminum recycling in energy efficiency and environmental conservation, the Department of Energy's Office of Industrial Technologies and Office of Transportation Technologies are supporting R&D efforts to demonstrate advanced processes to sort and recycle aluminum scrap. Two primary methods are being investigated. Laser-induced breakdown spectroscopy is being used for separation of wrought from cast aluminum alloys, while selective etching plus color sorting is employed to subsequently separate the wrought alloys into narrower composition groupings. Each will be described in further detail below.
The selective etching plus color sorting technique utilizes treatment of the shredded pieces by a caustic etch. Since different aluminum alloys will react differently to an etch based on their alloying constituents, they can be sorted by the resulting color of the etched pieces. Alcoa has patented a process that enables separation of alloys into three distinct categories: "bright", which consists of pure Al and binary Al-Mg alloys; "gray", which consists of Al-Mg, Al-Si, and Al-Mg-Si alloys; and "dark", consisting of alloys containing Zn and Cu. Subsequent to etching, a camera performs the sorting function.
The laser-induced breakdown spectroscopy (LIBS) technique is an adaptation of the optical emission spectroscopy chemical analysis technique utilized currently for aluminum alloys. Instead of using a DC arc to vaporize a small region of the sample for analysis as in the conventional method, a laser performs this task, resulting in a higher potential rate of processing. The spectrum of each sample is analyzed and chemical composition determined, which allows piece-by-piece alloy identification. According to recent reports, up to 50 shredded pieces per second can be analyzed by this method. This technique requires a surface cleaned of all paint and protective coatings to be effective, so a decoating step is required prior to LIBS analysis.
The Huron Valley Steel Company in Belleville, IL, a company with significant interest and capabilities in the area of scrap sorting technology, has served as the principal investigator on the DOE-OIT program. They have demonstrated a proprietary process for the separation of wrought and cast alloys, and are performing pilot line studies of the LIBS and selective etching plus color sorting techniques. With recent trends pointing to a growing demand for alloy-sorted aluminum, industrial commercialization of these technologies appears imminent.
Further information on recycling and scrap sorting is available in the following publications:
- Automotive Aluminum Recycling Design Guidelines
- Automotive Aluminum Recycling, 1997
- Aluminum Industry Roadmap for the Automotive Market: Enabling Technologies and Challenges for Body Structures and Closures, May 1999
All of these publications are available from The Aluminum Association Bookstore online at www.aluminum.org.
Specific information on the Automotive Aluminum Scrap Sorting project can be obtained
from the principal investigator, Adam Gesing, at Huron Valley Steel Corporation
via e-mail at gesinga@hvsc.net or by accessing
the DOE-OIT web site at www.eere.energy.gov.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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