Into the Light

BHS-Sonthofen, a manufacturer of shredder light processing plants based in Sonthofen, Germany, has brought together various parties involved in recovering material downstream of the auto shredder and has developed a number of concepts for post-shredder systems that are based on field studies and long-term experience. Site research at plants run by companies such as Comet Sambre, Scholz and GDE shows that implementing economical post-shredder separating and sorting technology is possible.

The recovery of nonmetallics downstream of the auto shredder is a complex and multifaceted subject. The composition and material properties of the shredder light fraction (SLF) largely depend on the composition of the input materials, the shredder type, the shredder’s throughput, the burden depth of the material and dedusting system settings. As a result, some important SLF parameters, such as bulk density, total organic carbon (TOC) content, moisture and heavy metal contamination, can vary considerably over time. SLF is a heterogeneous material and, therefore, the method for processing this material must be flexible.

For operators of post-shredder plants, compliance with legislative mandates is important, but so is economic efficiency. However, the processing of shredder light fractions is relatively independent of raw material market prices; a more important factor is the cost saving potential of the post-shredder plant. Landfilling or incinerating SLF is a fixed cost that has the tendency to increase over time. Consequently, the economical operation of a post-shredder plant hinges on:

A detailed analysis of SLF composition illustrates the possible fluctuations in percentage of the metals present. Essentially there are three main groups:

These figures show that it is necessary to separate the individual material groups, which will make it easier to dispose of them or to feed to an additional downstream process. The metallic components can be recycled, the mineral fraction can continue to be deposited in landfills, and the combustible materials can be used for various combustion processes.

In principle this goal seems to be simple and achievable. SLF, however, is a highly entangled and intermingled material that is difficult to separate.

As the name suggests, SLF are lightweight particles, such as rubber, textile and plastics, that are separated from the main metals stream by a dedusting system and/or zig-zag classifier. The name also reflects the grain size distribution curve of SLF, which indicates a high proportion of fine-grained and/or dusty material. Research and analysis shows us that there also are significant irregularities in the gradation of SLFs.

Research has revealed that sorting SLF without prior mechanical stress will not provide the desired result. In particular the fine grained and/or dusty material is highly susceptible to accumulating in foam material or clinging to plastic or metal components. Thus, it is extremely difficult to isolate these adhesions using only screening, classifying and separation to obtain a satisfying product quality. Moreover, these impurities often are a "knock out" criterion for various disposal routes.

For this task, equipment manufacturers such as BHS Sonthofen offer selective crushing machines for disaggregating and cleaning the material. For primary treatment of the SLF, machines like the Rotorshredder have proven to be effective. In the case that additional fine grain size processing is requested or necessary, The BHS Rotor Impact Mill is one potential solution.

The patented Rotorshredder consists of a cylindrical working chamber with double wall housing. In the middle of the machine is a vertical shaft that is equipped with pairs of crushing tools. The inner cylinder wall is made up of a solid bar grate designed to ensure that the crushed material leaves the machine as quickly as possible, which saves energy and wear costs. The input material is fed into the machine from the top. As the material enters the working chamber, it hits the hammer-shaped crushing tools where it is exposed to intense impact as well as to punching and shearing forces. The material then leaves the working chamber through the bar grate only a few seconds later. This continuous operation enables processing of high amounts of SLF (up to 30 tons per hour). As the material falls down, it passes a variable number of crushing levels that are designed to achieve an optimal disaggregation effect.

The Rotor Impact Mill was originally used for rock processing and has its origins in the production of fine-grained minerals (e.g., sand). The input material is fed from the top into the central duct. On contact with the fast spinning rotor, it is highly accelerated outward by centrifugal forces against a wall. Then the horseshoe-shaped impellers catch the material and shear it against the anvil ring. Brittle, hard materials (such as rock and glass) are greatly reduced in size (pulverized) by the impact effect, while ductile materials (such as copper and aluminum) are deformed by the shear action, and composite materials are disaggregated and separated.

The primary objective of the aforementioned SLF processing method is to produce intermediate products for sorting and classifying. The disaggregated and untangled material can be separated by using corresponding screening, classifying and sorting equipment.

Depending on the degree of sorting applied to the SLF stream, various end fractions emerge that have defined and largely homogeneous properties. These fractions can be disposed of using a predetermined disposal route that may vary depending on the region or country.

For the recovery of metals from SLF, many shredder plants install an over-belt magnet and additional nonferrous metal separation equipment, such as eddy current separators. But because of the heterogeneous composition of the SLF material, the level of separation achieved can be insufficient, resulting in deteriorated quality and diminished yields of recovered material.

The dedusting and/or the zig-zag classifier systems on some large auto shredder systems often use a low suction level to minimize the loss of metals into the SLF. However, by installing a further crushing system on the SLF, operations can potentially maximize metal recovery and enhance product quality as well. As a result, the ASR (auto shredder residue) can be sucked off to a greater extent, positively influencing ferrous scrap quality.

During the past years, the processing of shredder light material has undergone further development in particular by combining size-reduction technology with corresponding classifying and sorting techniques. The individual components were altered in a targeted manner and adapted to the respective task. This upgrade enabled the push to the dry mechanical processing of SLF to such an extent that large scale post-shredder plants evolved.

Experience shows that metal content in SLF is not the key element for the economic efficiency of post-shredder plants. The option to select alternative and less expensive disposal methods for defined material flow rates provides much greater economic potential. As a result, SLF reprocessing is only partly exposed to the risk of fluctuating prices of raw material markets while the cheaper disposal options provide a high cost savings potential to the operators of shredder facilities.