High-Voltage Solutions for Material Recycling

In Switzerland, there are currently 29 waste-to-energy plants (WTE), which incinerate all non-recyclable waste in an environmentally friendly way.

However, not everything ends up in flames: around 100 kg of slag containing unburned components is generated per person per year. The slag is usually landfilled, but it still contains valuable materials. The “Waste-to-Value” specialist Selfrag AG is addressing this problem: the company has built a pilot plant in Full-Reuenthal, Aargau, where a special process is used to recover resources such as metals—e.g., iron, aluminum, or copper—and minerals from the slag. “Spoons, two-franc coins, screws—there’s hardly anything that doesn’t end up in the trash. We have made it our mission to recover as many valuable materials as possible from the slag,” explains Jürgen Kalke, an electrical engineer at Selfrag.

A Sophisticated Special Treatment

In Full-Reuenthal, Selfrag currently processes 35,000 tons of slag annually from the waste-to-energy plants in Turgi, Renergia Luzern, and Buchs. The so-called raw slag is dumped into the material bunker and transported by crane into the “Grizzly,” a sizable screening sieve, where the largest pieces are retained. From there, it moves via conveyor systems into the modular “Waste-to-Value” plant. Light metal parts are removed using magnetic separators, and the slag is sieved until the target fraction, with components no larger than 40 mm, remains. The rest is sorted by hand, revealing unburned material, which is sent back to the waste-to-energy plant. “Our goal is to recover metals and ceramic materials amounting to 50% of the delivered raw slag,” explains Kalke.

Thanks to its self-developed high-voltage fragmentation, Selfrag can recover around 20% more valuable materials from the slag than other plants.

The target fraction then passes through several dry and wet screens and is classified by size and density. One fraction enters the heart of the plant: two 3.5-ton generators, developed and patented by Selfrag, further separate the material using lightning-like high-voltage pulses. With a capacitor system, they reach a voltage of 200 kV and discharge up to 20 times per second. These discharges occur in water and generate a very high current flow. The current travels along the grain boundaries of the materials, selectively separating them, aided by the shockwave. “The high-voltage fragmentation makes our processing unique and allows us to recover about 20% more material than other plants,” emphasizes Kalke.

Siemens has been supporting us for a long time. Typically, electronic components are not certified for use in high-voltage environments, so we subject the parts to these rigorous tests ourselves—the Siemens products perform very well in this context.

Josef Mullis, Electrical Engineer, SF Elektro-engineering

Step-by-step approach in a challenging environment

Recovering valuable materials is complex and demanding—so it’s hardly surprising that the entire plant is equipped with over 900 field devices, such as motors, valves, and sensors. “Siemens has been supporting us for a long time. Typically, electronic components are not certified for use in high-voltage environments, so we subject the parts to these rigorous tests ourselves—the Siemens products perform very well in this context.”

The engineering of the plant was handled by the automation company SF Elektro-engineering AG. “We took care of everything—from electrical planning and material procurement to programming the software for the controllers, drives, and WinCC, as well as the interfaces to external systems, all the way to commissioning,” summarizes Josef Mullis, Electrical Engineer at SF Elektro-engineering, describing the ambitious project. That the plant was completed on schedule is thanks to the foresight of the Siemens Solution Partner from Flums: “During and after the pandemic, we stocked our warehouse with the most important components, which allowed us to supply Selfrag promptly,” recalls Mullis. Kalke confirms that this proactive planning was a major advantage. The modular design of the plant also proved effective: “While the electricians were still working on wiring in one area, we were already able to test other plant modules. This step-by-step approach also proved to be a great advantage during the three-month commissioning period. One challenge was the harsh environmental conditions in the large hall: dust, moisture, and acidic fumes in the air had to be kept away from the switchgear using appropriate protective measures.”

he production plant is controlled by a Safety CPU 1512F and an S7-1517. Additional Siemens controllers are installed in three modules and the filter presses within the plant.