Metal Separation in the Processing of Secondary Fuels - the Extraction of Potential Recycables and Equipment Protection

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Source: Steinert Elektromagnetbau GmbH

The treatment of solid domestic and commercial refuse and the extraction of secondary fuels from these materials will be subject to stricter regulations due to new landfill regulations coming into force.

In this way, it will be possible to further increase the recycling quotas and minimize bottlenecks in processing capacity. There are grounds for believing that the competition between traditional treatment in a waste incineration plant and waste reuse as a secondary fuel will have a restraining influence on the prices for waste disposal.

The quality requirements to which such a secondary fuel is subject have been made more precise in recent years, in order to enable environmentally friendly and interference-free use. Metals and their separation have played an important role here, for a variety of reasons.

• The absence of metals is an important property from the user’s point of view, as traditional users of fossil fuels are not set up to deal with metals.
• In addition, metals have considerable value, which should be retained and exploited.
• as substitute fuels are compacted to ease transport and charge handling the presses has to be protected. Metals would cause long-term interference to this process and would increase its costs.

Recent years have seen considerable investments in the production of secondary fuels. The plants are extremely large and process vast quantities of materials.

The process consists of crushing, sieving, sorting by density, metal separation and material separation. These sorting steps are carried out for each of the three or four size fractions, in order to achieve the maximum quality. The subsequent bricketting in ring roller presses or similar turns the previously extremely loose material into a free-flowing, easy to charge fuel.

Conventional metal separators in this area are the suspension magnets for iron separation and eddy current separators for the extraction of non-ferrous metals. About 95% of the iron and 90% of the non-ferrous metals can be recovered in this way. These units’ sharpness of separation and reliability of operation are ensured by sophisticated design details such as the eccentric pole system in the eddy current separator and the special electrical coils wound from aluminium strips in the magnetic separator.

Although magnetic separators can also handle larger burden depth with great success, non-ferrous separation requires single-particles layers. Due to the extremely large volumes, several eddy current separators are frequently operated in parallel. It is not unusual for up to nine metal separators to be operated in a traditional waste processing system! Throughput is in the region of 6–8 t/h and metre of working width in the non ferrous section.

The protection of the press tools from left metals is, of course, of special interest for the producers of secondary fuels. Alongside iron and aluminium, stainless steel can, due to physical reasons (low electrical conductivity, high density, weakly magnitisable) only be separated to a limited extent by magnetic separation and eddy current separators. What’s more, this material is worked in large pieces. Metals like this in the material to be pressed are fatal to pressing tools. These stainless steels can now be separated, thanks to a new type of unit with completely new sorting criteria. The sorting criteria are based on electrical induction, which is why this separator is known as the Inductive Sorting System ISS®. The principle of the ISS®: A large number of small metal detectors recognize the exact location of the metal parts, which are then blown out of the flow of materials by small, computer-controlled compressed air nozzles.

The sensitivity can be set over wide ranges. The number of incorrect ejections is maintained within tight limits. As this technology is also subject to continuous improvement at STEINERT, a set-up appropriate to the plant design in question can always be found.

STEINERT has successfully sold around 70 such ISS® units in the metal industry. It is thus a tried and aproved technology that reliably stands up to the harsh conditions of a shredding plant. Two ISS® units have been sold for substitute fuel processing in Germany and have carried out their task to the complete satisfaction of the customers. Another such plant has recently gone into production. This particular sale was preceded by comprehensive experiments and long-term tests. STEINERT’s in-house technical centre makes such steps possible with other customers’ materials and other separators as well.

The market has accepted these technologies with great interest, as they make it possible not only to further improve the product quality and the application potential of substitute fuels, but also to make the operations more reliable and therefore more economical.

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