This article highlights features of a process under-development that aims to recover wrought alloys for high-value applications. Chemically etched wrought aluminum scrap pieces have been separated into their respective alloy families using an optical identification system that is being developed by Alcoa. The technique has successfully completed proof-of-concept testing at Pacific Northwest National laboratory. One method of separating the mix of wrought aluminum into its alloy families combines chemical etching with an optical technique to sort the aluminum by color. John Green, vice president of technology of The Aluminum Association, believes these technologies will give automotive companies an incentive to commit to aluminum for sheet applications by ensuring that recycling wrought aluminum into higher-value applications is feasible. According to an expert, since processing recycled aluminum takes only 5 percent of the energy required to work from primary ingot, using recycled aluminum makes sense for automobiles.
Promoted as an attractive material for keeping vehicle weight down, aluminum's use in cars is expected to increase, at least for the foreseeable future, primarily because of the pressure to improve fuel economies. It's also easily recyclable, with at least 85 percent of the aluminum in cars today recovered, according to an estimate by The Aluminum Association , a Washington, D.C. based industry group. But here's the rub: Most reclaimed aluminum today goes back into lower-value auto motive castings, which comprise roughly 75 percent of the aluminum on cars, rather than higher-value wrought products. This could ultimately reduce its attractiveness as an automotive material and stymie its long-term growth prospects.
The weak link in the automotive aluminum recycling chain is sorting capability, according to some industry sources that have looked at the issue. In the current recycling infrastructure, mixed aluminum scrap, composed of castings and wrought pieces, are sold by automotive shredders to the secondary aluminum industry for recycling into foundry alloys. Castings have less stringent compositional targets and a high tolerance for impurities, and so can easily absorb mixed alloy scrap.
Today, secondary castings account for virtually all recycled automotive aluminum; higher-value wrought products are fabricated from prime ingot.
"Nearly all recovered aluminum is recycled," according to Rebecca Wyss, technical specialist at Alcoa Technical Center in Alcoa, Pa. "The question, though, is the value of the product that aluminum is recycled into. Castings have lower value than wrought products, but because the demand for castings is large and the wider compositional limits of castings allow them to absorb scrap, they have been the product of recovered aluminum. Mixed aluminum scrap contains too many alloying elements for the scrap to be absorbed into wrought product."
Although it's possible to separate cast from wrought product, so far it has not been possible to efficiently separate wrought scrap into compatible alloy families, which could then be recycled into higher-value wrought automotive applications.
With that aim in mind, a partnership of aluminum producers, automotive manufacturers, and the U.S. Department of Energy has kicked off a four-year effort to develop a technology to separate secondary, or recycled, wrought aluminum from cast, and then into its alloy families. The process will use two technologies that work hand in hand.
Wrought will be separated from cast by a thermomechanical process, followed by a second process, based on either laser or optical separation techniques. That second process will sort the secondary wrought aluminum into families, for reprocessing into higher-value automotive wrought applications, such as hoods, door panels, and some chassis parts. All three technologies have successfully undergone feasibility studies, although none has yet been used commercially.
Thermomechanical separation , also known as the "hot crush" method, was developed about 15 years ago by the U.S. Bureau of Mines, now the DOE Albany Research Center, in Albany, Ore. The hot crush system consists of a series of heating, crushing, and screening steps.
Mixed cast and wrought scrap is heated to a point that is slightly below the melt temperature of the cast al- 10ys.The heated cast alloys then undergo mechanical crushing or grinding, by hammer mill or flail mill, causing the cast alloy pieces to fracture along the weakened grain boundaries. Wrought alloys remain solid during the heating stage and do not fracture.
Heating the mix has a side benefit , because it also cleans the scrap, removing paints and coatings from the surface of the material. This eliminates the need for delacquering, normally a required step for all aluminum scrap processing.
Following the mechanical crushing stage, the mix goes through a size separation process, such as a rotary trommel, which separates smaller cast fragments from the wrought alloys. The wrought alloy pieces are then ready for further separation into their alloy families.
The hot crush method is quite effective for separating wrought aluminum from castings, giving better than 96 percent separation, according to Bill Riley, chief of technology transfer at the Albany Research Center. Although the hot crush technology has been demonstrated only with small quantities, Riley said, it can be readily scaled up. "All of the equipment used in it is known in the minerals and materials processing industries," he said.
One method of separating the mix of wrought aluminum into its alloy families combines chemical etching with an optical technique to sort the aluminum by color. The technique is being further developed in a partnership between Alcoa and Pacific Northwest National Laboratory in Richland, Wash. , under a program of the North west Alliance for Transportation Technologies. The idea behind the system is to separate the mix of alloys into five basic families for recycling back into higher-value wrought aluminum applications.
There are five alloy groups that cover the bulk of the wrought aluminum alloys used in automotive applications: • 2000 series (copper-based), frequently used in hang-on panels such as hoods; • 3000 series (manganese-based), used in radiators; • 5000 series (magnesium-based), used in some structural components; • 6000 series (magnesium- and silicon-based), used in-structural components and hang-on sheets; • 7000 series (zinc-base d), which has been used in bumpers and bumper reinforcements.
Mixed scrap will be carried on a conveyor belt through a hot chemical bath of sodium hydroxide and then dried. The etching process will impart colors to the aluminum pieces that are unique to the class of alloys they belong to. Optical sensors will be used to detect the five different colors-black, gray, silver, tan, and gold.
Following detection, a device, possibly an air jet, will shoot the individual pieces into bins. One advantage of this technique is that it offers an ability to identify alloys on a rapid scale, according to Eric Nyberg, a senior research scientist at Pacific Northwest National Laboratory, who is working on the project. Following the etching step, identifying alloys is a relatively straightforward optical challenge, in which a computer code is used to differentiate among the colors of the various pieces. He added that the chemical etching is environmentally friendly.
There are still a number of issues that must be resolved before building a demonstration system, Nyberg said. Color recognition capabilities need to be demonstrated with real- time analysis that can handle high-volume throughputs. A feedback loop has to be developed that would optimize etching temperatures and solutions, and provide information on when to change or clean the chemical bath. Lighting filters and imaging processing also must be studied to maximize discrimination among the alloys. There are also cost-related issues: the total length of the conveyor, the means of drying scrap after the chemical bath, and chemical solution life.
In early proof-of-concept tests, a conventional digital point-and-shoot camera was used to take stationary pictures of selected scrap representative of what would come out of automotive shredder residue. The aluminum pieces were laid against a uniform color background and photographed, explained Chuck Batishko, the Pacific Northwest staff scientist involved with the color sorting of the project. Digital files of the photos were created in standard image formats. From there, colorimetry values were calculated and applied to sorting algorithms, which identified the pieces according to groups based on color.
Timing will be critical in the final, continuous system, said Batishko, and the position of fragments will have to be tracked as they move down the conveyor. Ultimately, the system could use color video cameras that would track the material as it moves down the line.
The algorithms will have to be refined as the system continues to develop, said Batishko. For one thing, adjusting the chemical etching process could result in a change in hues. Although some alloys are very distinct in color after etching, others are rather close, and even overlap. "There are son1e things in the algorithms that can be tweaked," Batishko said.
Results of the identification process so far are quite positive. For example, 92.6 percent of 2036 alloy has been correctly identified in feasibility tests, while 6.8 percent was incorrectly classified as another alloy, 7003. "That gives us a pretty good basis of the effectiveness of what we found in the first phase," noted Nyberg.
In all, the method has effectively separated three of the five alloys from one another. The optical system used by Pacific Northwest was not able to distinguish between the dull black of the 7000 series and the silver of the 5000 series alloys, said Rebecca Wyss of Alcoa, who is involved in the development of the color sorting process. However, both she and Nyberg believe that there are commercially available optical systems that are sensitive enough to perform the task successfully. Wyss believes that sorting alloys into their basic groupings is sufficient for recycling wrought product back into higher-value applications.
The other current possibility for separating mixed wrought alloy scrap was developed by Alcan Aluminum Co. of Kingston, Ontario, and is based on laser-induced optical emission spectroscopy. Under this method, a stream of mixed shredded wrought scrap that has been cleaned and thermally decoated is channeled into a stream of pieces in single file. The stream passes continuously under a sensor, which detects the presence of the piece of scrap and triggers the laser. The laser would create a spark on the aluminul11 sample, and a spectrometer would then identify the sample's optical emission by comparing the signal to a referenced standard.
A feature of the system is its ability to identify samples piece by piece and at high speeds, according to Michael P. Thomas, technology manager at Alcan Global Automotive Products in Farmington, Mich. The laser method would not require an etching bath, but would need the material to be free of paint, coatings, and other treatments, which could be removed during a decoating process.
The laser and spectrometer combination also would be able to sort alloys in much finer detail than the color sorting method. Rather than categorizing them into five alloy groups, it would be able to identify individual alloys, that is, separate one type of 6000 alloy from another. According to Thomas, such a capability would result in more flexibility for the recycler.
Bringing More Value
John Green, vice president of technology of The Aluminum Association, believes these technologies will give automotive companies an incentive to commit to aluminum for sheet applications by ensuring that recycling wrought aluminum into higher-value applications is feasible.
Vehicles in North America today have about 250 pounds of aluminum, according to the Aluminum Association and other industry sources. Over the next 10 years, the average amount of aluminum could grow to more than 400 pounds, with certain "aluminum intensive" vehicles containing up to 900 pounds of aluminum each.
"As long as sheet quantities are small in relation to cast, as they are now, you can always put small quantities of sheet materials recovered into the mix of recycled cast materials," noted Green. However, mixing larger quantities of wrought alloy scrap with cast aluminum would result in lost value, he added, and could even cause compositional problems in castings.
Although mixed scrap certainly can be used in foundry applications, "Everybody would like to have segregated scrap, because it's a known quantity," said Thomas of Alcan Global. " It's much easier to handle, you know what its chemistry is, and you can use it better than if you blend it into something else. Segregated scrap is the only way of getting the scrap back into the same alloy." It's also a way of preserving expensive alloying elements, such as magnesium, he added.
Since processing recycled aluminum takes only 5 percent of the energy required to work from primary ingot, using recycled aluminum makes sense for automobiles, said Green. "The more efficiently we can do scrap sorting, the better off the industry is going to be, and the cost savings are going to come from there, too," he said.
"We as an industry have an interest in this," said Roger Heimbuch, director of materials and fastening engineering at GM North America in Detroit. The company, through the United States Council on Automotive Research, or USCAR, the government-industry consortium of the Big Three automakers, will evaluate the technology with the goal of putting in a demonstration scrap sorting facility.
If and when that should happen, recycled wrought aluminum alloys could bring more value into those products.