A singular approach

Mixed aluminum scrap grades, including preconsumer clippings and solids, or postconsumer grades such as taint/tabor, twitch and zorba, often find homes in domestic secondary smelting operations or are exported. But newer technology known as LIBS, or laser-induced breakdown spectroscopy, enables these grades to be separated by alloy family or even by specific alloys at scale, allowing material to be put to its best use, enabling true circularity.

LIBS uses a laser to create plasma on a sample’s surface and a sensor to analyze the light that plasma emits to determine the chemical composition of the aluminum. The technology has been used at laboratory scale for decades and was introduced to hand-held scrap analyzers in the 2010s, but it only recently has been used to sort mass quantities of aluminum at industrial scale.

Earlier this decade, Robert Broughton, president of Steinert US, the Walton, Kentucky-based U.S. subsidiary of Germany-based Steinert GmbH, says LIBS was seen as too inconsistent in its performance and not accurate enough to sort material effectively at industrial scale. Now, he says, the technology “has passed over any sort of quality barrier.”

Broughton says today’s equipment can start with a “very poor-quality” product and upgrade it to furnace-ready. “It’s no longer a technical question; it’s really down to what’s the highest value economic situation.”

Steinert introduced its LSS, or line sorting system, LIBS unit in 2018 and Plasmax in 2023. Broughton says LSS can perform six positive sorts, while Plasmax can perform three.

Other companies with LIBS-based sorting technology include Austin AI, based in Austin, Texas; Italy-based SGM Magnetics SpA; and the Germany-based Tomra Recycling Sorting business unit of Tomra Systems ASA, based in Norway.

The last step

Depending on the infeed material, LIBS is commonly used after other sorting technologies have been employed.

According to Tomra Americas Metals Segment Manager Giuseppe Granara, the X-Tract, Tomra’s XRT (X-ray transmission) technology, which separates material by its atomic density, is commonly used prior to its Autosort Pulse “dynamic” LIBS technology to remove contaminants such as heavy metals, inerts, composite materials and light alloys from taint/tabor or zorba.

“As a second step, it facilitates the separation of high-density alloys (cast aluminum with high heavy metal content) from low-density alloys (wrought aluminum and cast aluminum with low heavy metal content), preparing premium twitch for further refinement using dynamic LIBS technology,” he says of the X-Tract.

The Autosort Pulse can separate different aluminum alloys, including 1xxx and 3xxx series as well as 5xxx and 6xxx series, which he says are identical in appearance and density and have long posed a challenge for traditional methods.

“This laser-induced breakdown spectroscopy solution is an in-house developed, highly advanced laser technology and the result of years of research,” Granara adds. “Thanks to the machine’s bulk infeed system and powerful combination of sensors, dynamic LIBS allows for separation based on alloys’ elemental composition at high throughput.”

SGM Magnetics Managing Director Didier Haegelsteen says, “By basing the sorting decision on the chemical analysis of the metal piece, LIBS sorting technology offers huge possibilities in terms of sorting option varieties and excels where other technologies, such as X-ray transmission or wet media separation, come across limits.”

“The XRT reduces the copper and zinc content to less than 0.1 percent, and the LIBS reduces silicon to less than 0.5 percent,” Haegelsteen explains. “Even though LIBS can do both processes, the capital cost of the technology on a per-ton-per-hour capacity is significantly more than the XRT,” therefore, he says, cascading the two technologies is the better option.

SGM started by developing its own technology for separating aluminum alloys, the X-ray backscattering sorter (XRF-BS). While its sorting accuracy was good, Haegelsteen says the 1.5-ton-per-hour throughput was “by far too little.”

“On deciding to go back to the LIBS technology, we came across Cleansort and, after making our technological due diligence, we came to the conclusion that they had a significant edge in terms of LIBS sorting technology for metals,” he says of the German company SGM decided to partner with. “Our common goal is to offer the world market the most advanced LIBS technology for sorting metals, guaranteeing customers the best proximity, service and application expertise.”

Proper preparation and feeding

According to Judit Jeney, the managing director at Austin AI Europe, based in Budapest, Hungary, the whole system must be optimized to achieve the desired results when sorting with LIBS.

“A key point is the size range,” she says, noting optimal performance with material ranging from 40 millimeters to 140 millimeters, or 1.5 inches to 5.5 inches, in size. “We can handle smaller or bigger material, but the efficiencies are better with a more homogeneous size range.”

Granara also suggests sizing material into fractions that range from 10 millimeters to 50 millimeters (slightly less than half an inch to slightly less than 2 inches) and 40 millimeters to 150 millimeters (roughly 1.5 inches to less than 6 inches) to achieve the best performance.

Contacts also indicate that while throughput is higher with larger pieces, there is such a thing as too large.

With obsolete scrap, material larger than 6 inches “potentially has a lot of attachments or compounds” and has not been liberated enough to create high-purity products, Broughton says.

Jeney says how material is fed to the LIBS sorter also is important, with Austin AI using a two-step vibratory feeder to distribute the material across the width of the feeder plates to provide singular distribution of the pieces, enabling analysis of each one as it’s “free falling” along the company’s unique chute design into the laser path.

While paint and dirt generally are not problematic, Broughton says some coatings can prove challenging.

“LIBS is a high-powered laser, but there are some reflective coatings that are used in street signs and on other materials that difficult to effectively burn away,” he says. “But, by and large, that’s not a majority of the pieces.”

Regarding Tomra’s Autosort Pulse, Granara says, “The system offers multiple scanning modes within its software, enabling variable penetration depths into materials or allowing multipoint data acquisitions, significantly improving sorting accuracy even on material that is painted or coated.”

Haegelsteen says it’s important to understand how the laser cleans the surface of the material to be sampled, adding that some manufacturers burn multiple points on the surface of a single piece and some perform multiple burns in the same spot to remove the undesired coating before doing the LIBS spectrographic reading.

“The problem with the latter is that by making multiple burns in the same spot you end up with residue from the burning of the coating around the reading spot, which keeps interfering with the reading,” he says. “In the case of Cleansort, cleaning is not done by burning a single spot but by laser ablating a surface and then focusing on the center of the ablated surface, far enough from its edges and consequently from the residual material. This guarantees that surface is clean from both coating and its cleaning residue.”