Bolts of lightning in the laboratory…

Research in focus February 2012

Bolts of lightning are a regular occurrence at the Fraunhofer Institute for Building Physics IBP. And recently they have become even more frequent – particularly when Dr. Volker Thome leaves his office in the department of Building Chemistry, Building Biology and Hygiene and sets to work in his new laboratory. Thome is in charge of what is known as an electrodynamic fragmentation system, a machine which is currently being used by the Concrete Technology and Functional Construction Materials Group for a series of new research projects. It is this system that produces the lightning bolts, though fortunately only under controlled conditions in the laboratory. Thome describes the purpose and the target of their lab-generated lightning bolts: “We’re working on the development of innovative recycling methods for waste concrete, fiber-reinforced composites and municipal solid waste incineration bottom ash. This fragmentation system is able to disintegrate a wide range of materials into their constituent components – without destroying them.”

Thome has a particularly keen interest in the recycling of waste concrete on a grand scale, a topic he has often returned to over the course of his long career in the field of concrete technology. The latest technology to capture his attention has actually been around for a long time – several decades in fact – though it has only recently emerged from a long period of hibernation. The method of electrodynamic fragmentation was originally developed by Russian scientists in the 1940s. “But it was only recently that researchers improved the technology sufficiently to make large-scale deployment feasible. Mining companies are the biggest users of fragmentation technology because they generate large quantities of rock which have to be broken down to find the minerals they are looking for, such as lithium containing minerals or diamonds.” The ability to separate these minerals from the surrounding rock matrix without destroying them is of particular importance in the case of valuable minerals such as diamonds. In his work as a mineralogist at Fraunhofer IBP, Thome has largely been following the same basic approach. Essentially, concrete consists of mineral raw materials, for example gravel aggregate, which can only be reused in fresh concrete without difficulties if they can be cleanly separated from the cementituous binder matrix. According to estimates published by the German Construction Recycling and Waste Management Industry Working Group (ARGE KWTB), the USA, Europe and Japan produce some 900 million tons of construction waste between them each year, with Germany alone generating approximately 130 million tons of waste in 2010. Yet only 8 percent of this huge quantity of waste is recycled, while most of it is simply disposed of, or used as road metal. “That isn’t recycling,” says Thome, “it’s downcycling”. Genuine recycling would mean obtaining raw materials from waste concrete which could be re-used in cement production. “Even if we only reclaimed the high-quality aggregates from waste concrete, that alone would be enough to achieve something like a tenfold increase in the recycling rate, pushing it up to 80 percent.” As well as helping to conserve gravel pit resources, this would also ease the strain on construction waste landfill sites while also enabling the cement industry to significantly reduce its CO 2 emissions.

These are just some of the reasons for the interest now being shown in the fragmentation system which was installed at Fraunhofer IBP a few weeks ago. The "lightning bolts", or high-power pulses, generated by the machine are able to selectively separate out the constituent components of concrete, i.e. cementituous binder matrix and aggregates. “All insulators have a certain dielectric strength, which is a measure of their resistance to breakdown under applied electrical pulses,” says Thome. “The pulses will always choose to travel through air or water in preference to a solid object. However, this dielectric strength is not a constant value – it is dependent on the pulse duration.” In the case of pulses with a duration of less than 500 nanoseconds, water has a higher dielectric strength than most solids. It was this discovery – made by Russian scientists in the 1940s – that laid the foundations for creating the fragmentation system or “pulsed power processing”. To break a solid into its individual components, you submerge it in water to forcibly conduct the electrical pulses into the solid, and then let physics take care of the rest: “When the pulses strike the rock, in this case the concrete, the pre-discharges always follow the path of least resistance – along grain boundaries,” Thome continues. “You could compare it to a storm cloud, where only some of the lightning bolts actually head toward the Earth.” These pre-discharges weaken the rock along its phase boundaries. The first discharge to reach ground leads to an electrical breakdown that produces shock waves which ultimately disintegrate the concrete into its individual components, effectively ‘pulling the material apart’, says Thome: “The force of this shock wave is comparable to that of a TNT explosion.”

Compared to the conventional grinding methods used to break up waste concrete, this fragmentation method is both selective and dust-free. Researchers are currently studying the reusability of the products obtained from breaking down waste concrete as part of a national project at Fraunhofer IBP. But the recycling of waste concrete is not the only area of research in which scientists are hoping to make new advances with the aid of electrodynamic fragmentation. This method can also be used to recycle carbon fiber-reinforced polymers, which are currently either ground up or subjected to thermal treatment: “The current methods are costly and can destroy the carbon fibers. Grinding also consumes a huge amount of energy, and the wear suffered by the grinding tools causes contamination of the ground composites,” says Thome. This is another example of a situation where selective separation by means of electrodynamic fragmentation could offer a promising new alternative to conventional methods.

A further project that Thome’s team will be examining is the processing of municipal solid waste incineration (MSWI) bottom ash. MSWI bottom ash contains melted products that have similar properties to those of cement, which means they could potentially be used as a building material. However, the uses of MSWI bottom ash are severely limited due to its chemical composition. If a treatment method for bottom ash could be found to separate the melted products from the undesirable components, this would kill two birds with one stone: “As well as gaining an alternative construction material you would also end up with valuable products such as nonferrous metals. There is currently no cost-effective method of processing MSWI bottom ash in this way; but the clock is ticking, because in approximately 20 years Germany will start running short of places to dump MSWI bottom ash,” Thome warns. “So if we could find a suitable method of processing MSWI bottom ash, that would also help conserve resources and reduce landfill.”
In addition to these examples, Fraunhofer IBP researchers also have plenty of other ideas up their sleeve, as Thome explains: “We are planning to carry out numerous other projects as part of our research into the fragmentation system, but at the moment a lot of individual processes are still in the stage of development. The chemical processes which can stem from the effects of underwater discharges on solids are still relatively unexplored. They sometimes produce surprising results, which has made us even more excited about investigating and developing this method further.”

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