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It is extremely important for scrap metal recyclers to have access to fast and reliable analytical technologies for the sorting of scrap materials. This technology helps recyclers to perform quality sorting, ensuring composition and alloy grade consistency when delivering recycled scrap to foundries and secondary aluminum producers.
By Mathieu Bauer

Lightweight metals—including alloys of mag-nesium, aluminum, and titanium—offer high strength-to-weight ratios and excellent corrosion resistance, making them ideal for a variety of applications across the automotive, aerospace, construction, packaging, and power infrastructure sectors.1 As a result, there are numerous grades of these lightweight metal alloys in circulation, each with different compositions and physical properties.

Aluminum is the most widely used of these materials, and the demand for aluminum alloys is steadily increasing. This is largely thanks to a growing global population and the trend towards replacing steel components with lighter weight metal parts to reduce weight in vehicle manufacture, to improve fuel or electricity consumption. The production of aluminum itself accounts for about 3 percent of the world’s direct industrial CO2 emissions,2 and a large share of those emissions are related to primary production, where aluminum oxide is refined from bauxite ore, then smelted into aluminum metal. Smelting is achieved using the Hall-Héroult process, an electrolytic method with high power requirements, which is often generated by the combustion of coal or natural gas.

Once produced, aluminum can be considered a sustainable metal, since it can be infinitely recycled, retaining its physical properties for new products made from recycled aluminum scrap. When compared to primary production from ores, recycling of aluminum reduces energy consumption by 95 percent.3 However, the use of scrap in the production of aluminum alloys can be challenging because most recycled aluminum involves a mixture of various grades of the metal. Accurate sorting of aluminum alloys by grade in scrap yards is therefore essential, helping to maximize the efficiency of recycling processes by allowing simple melting of the scrap to recover an alloy with the desired properties for the target application. A good example of aluminum alloy recycling is beverage cans, which can be recycled with a 100 percent recovery rate for the same application. However, in many cases, recycling is more complex, especially when scrap has originated from the same application, but uses different alloy grades.

Milestones of Handheld Alloy ID Technologies
The use of handheld X-ray fluorescence analysis (HHXRF) was first established in scrap yards about 20 years ago, allowing operators to perform rapid manual metal scrap sorting for all kinds of alloys. At the time, sorting of scrap aluminum alloys was limited, because available instruments could only differentiate between different grades based on the content of titanium and elements with higher atomic numbers, such as chromium, manganese, iron, nickel, copper, zinc, silver, tin, lead, or bismuth. This was sufficient to sort common grades of wrought alloys within the 2000, 3000, 4000, or 7000 series—as well as some alloys from the 1000, 5000, and 6000 series—but fine alloy grade identification based on magnesium and silicon was not possible. The introduction of silicon drift detectors (SDDs) in HHXRF analyzers around 15 years ago enabled the detection of both silicon and magnesium, but the measurement of magnesium remained a challenge for levels lower than 1 percent, and each measurement typically took between 15 and 60 seconds, which was unpractical in scrap operations having to sort thousands of items per day.

A breakthrough in aluminum alloy sorting came approximately a decade ago, with the introduction of handheld laser induced breakdown spectroscopy (HHLIBS) analyzers. This optical emission spectrometry method allows the sorting of aluminum alloy grades based on magnesium content within three seconds, but it does come with some drawbacks. For example, HHLIBS is more susceptible to the state and geometry of the specimen surface. In addition, the transient nature of the light emitted from the plasma induced by the laser pulses makes HHLIBS inherently less precise than HHXRF, in which the emission of X-rays is more continuous due to the high stability of the X-ray tube power supply. As a result, HHLIBS has been viewed by the scrap metal industry as a complementary technique, rather than a replacement for HHXRF analysis.

The flexibility of HHXRF was further increased following the introduction of LIBS, thanks to the development of instruments with more powerful 5W X-ray tubes and graphene windows on SDDs, offering a five- to tenfold improvement in sensitivity for magnesium. However, these devices still required sequential measurement of heavy elements—taking two to five seconds—then lighter elements in a second phase of three to 10 seconds, for a total analysis time of five to 15 seconds. While this represented significant progress, it still could not match the speed of analysis achieved by HHLIBS instruments.

New Approach for Sorting Lightweight Metals and Alloys
More recently, high end HHXRF systems (see Figure 1) have achieved a new milestone in the rapid identification of lightweight metals and alloys. In contrast to previous methods, the new approach is focused on the analysis of lightweight metals. It measures light elements such as magnesium, aluminum, and silicon—as well as transition metals from titanium to zinc—in the first phase, followed by the heavier elements in the second phase. This makes it possible to identify the most common aluminum and magnesium alloy grades within just a few seconds, enabling operators at scrap yards to sort these grades with very high throughput.

Figure 1: The Thermo Scientificâ„¢ Nitonâ„¢ XL5 Plus Handheld XRF Analyzer offers a dedicated method for lightweight alloy analysis.

This approach can be highly effective for the rapid sorting of aluminum alloys with similar concentrations of transition elements, but with different light element contents. This helps to avoid mixing casting alloys containing silicon with wrought alloys. It is also highly effective for separating twin alloys that differ only by their magnesium content, such as 3003 grade—which does not contain magnesium and is used for rigid foils, containers, or signs—and 3004 grade, which contains about 1 percent magnesium and is used for beverage cans. Another example of separating twin alloys by magnesium content is in the aerospace industry, where alloy grades 2014 and 2024 differ only in their magnesium content, but are not at all interchangeable.

This new analytical approach for HHXRF instruments is also highly effective for separating alloy grades that differ by only a few tenths of a percent in composition of light elements, such as silicon, magnesium, copper, or iron. Historically, there were significant challenges in the separation of application-specific grades, such as high value 6061, used in aerospace, marine, and automotive components—from more general-purpose grades, such as 6063 (used in window frames) or 1100 (used in cooking utensils, sheet metals, etc.). Fortunately, the new method for lightweight metal analysis allows scrap yards to separate the different grades in less than two seconds, as can be seen in Figure 2.

Figure 2:
Quantitative analysis and grade identification for aluminum alloys with similar composition can be achieved in less than two seconds.
Images courtesy of Thermo Fisher Scientific.

 

Fast and Reliable Analytical Technologies
It is extremely important for scrap metal recyclers to have access to fast and reliable analytical technologies—such as HHXRF—for the sorting of scrap materials. This technology helps recyclers to perform quality sorting, ensuring composition and alloy grade consistency when delivering recycled scrap to foundries and secondary aluminum producers. This, in turn, aids recyclers in maintaining the sale price of aluminum scrap, as well as protecting their reputations by preventing contaminants and unwanted materials from entering the recycling process. The new, dedicated method for rapid analysis of lightweight metals further contributes to the increased productivity of aluminum and magnesium alloy scrap sorting operations, thanks to a much shorter measurement time of a few seconds. | WA

Mathieu Bauer is currently working as Senior Application Scientist and Associate Product Manager at Thermo Fisher Scientific. He holds a PhD in analytical chemistry from the University of Hamburg and has more than 20 years of experience working with spectroscopy including handheld X-ray fluorescence (HHXRF). Mathieu can be reached at [email protected].

Notes
natural-resources.canada.ca/our-natural-resources/minerals-mining/mining-data-statistics-and-analysis/minerals-metals-facts/aluminum-facts/20510
www.iea.org/energy-system/industry/aluminium
www.aluminum.org/Recycling

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