The way a twin-shaft shredding unit works is fairly simple: Two tools moving in opposite directions draw in the collected waste material for shredding and crush it all together into smaller pieces. If it is purely about volume reduction, for example for easier transport, this can be achieved with conventionally designed shafts.
Some applications, however, require specific particle sizes for their processes. Plants that recover energy from waste wood, for example, are usually designed with a standardized particle size in mind. Oversized particles in the material stream can block the conveyors or safety systems such as rotary valves. In addition, too many fines negatively affect combustion and very small particles might lead to lumps jamming the facility. Therefore, if the required size is not obtained in the first shredding step, the material must be treated further.
The crux of the matter is that additional processing is costly, reduces the recycler’s profit margins and, in the worst case, leads to customer complaints. To avoid these problems, cutting systems can provide a solution. For example, Lindner offers a patented SF synchronous fine-cutting system that can be used as a solution to these problems.
The easiest way to achieve smaller particle sizes would be to reduce the gap between the individual cutting tools. Conventional shaft pairs crush the material with rippers acting in parallel. If, in this case, the effective working range were extended, the amount of force required and thus the energy consumption would increase significantly. However, if the angle between the knives is changed, they act just like scissors, effectively applying force to a much smaller area. This increases the individual tools’ impact while maintaining the same energy consumption.
Thanks to the arrangement of the knives, rippers and small blocks on Lindner’s SF cutting unit, the material is shredded up to four times in one pass, which results in smaller particles with a low percentage of fines. Since less force is needed, the shafts are subject to less stress, which reduces maintenance and increases the cutting unit’s service life, according to Lindner.
Looking at the entire shaft, it appears that the cutting edges are not aligned in parallel. Thanks to this arrangement, the two shafts act just like two Archimedean screws rotating in opposite directions. What the ancient Greeks used to transport water over great heights with the help of gravity positively affects the way the shredding unit pulls in the material. According to Lindner, this aggressive intake increases the throughput enormously and also mixes the final fraction once again for an output that is as homogeneous as possible.
What needs to be proven
Past experiences show that, with just one single pass, Lindner’s shredders from the Urraco or Miura series equipped with these tools produce a standardized P100 particle size according to ISO 17225-1 with fewer than 4 percent fines during waste wood shredding. This standard basically describes the percentages to be achieved as well as maximum specifications for the main fraction, oversized particles and fines.
During the production of these standardized particles, depending on the configuration, Lindner states that its twin-shaft shredders achieve 20 up to 120 metric tons per hour.
This article was provided by Lindner, which is a shredding solutions provider based in Spittal an der Drau, Austria. The company manufactures machines and system components that are exported to almost 100 countries. In addition to stationary and mobile shredders for waste processing, Lindner also offers complete systems for plastics recycling, solid recovered fuel and waste wood processing. The shredders can be used to process municipal solid waste, commercial and industrial waste, waste wood, plastics, packaging material, paper and light scrap, among other materials.