Shredders have to adjust to market fluctuations

By Gerhard Wirsig (Dipl.-Ing. FH), freelance journalist, Düsseldorf

The high global demand for steel in recent years had a major impact on the complex supply channels for the raw materials. It is just like a combustible gas being introduced to one end of an air duct system and then ignited. An explosive flame darts through the pipelines. Once the gas is switched off, the advancing deflagration is extinguished and negative pressure is created as the area cools down. A bold simile? Let us examine the market.

The high demand for steel caused steel prices to rise. The prices for ore and coking coal experienced an almost simultaneous increase. Equally the demand for processed scrap steel picked up. Some quantities of scrap were diverted from their traditional channels of distribution and found new customers. During this period steelworks tended to reduce their quality standards for processed scrap metal so that they could procure the required volumes. Shredding plants throughout the world were operating at full capacity. The pull of demand soon reached the scrap metal suppliers. Increasingly the scrap delivered to shredding plants was of an inferior quality and contained higher levels of contamination. The processing plants found themselves in a catch-22 situation. They had to process inferior material while working flat out, which resulted in greater wear, more repairs and therefore longer downtimes. Any companies still producing scrap that exceeded the quality requirements could not expect to be rewarded for their efforts.

Poor scrap being stockpiled

When the demand for steel and the prices for this commodity collapsed, the warehouses were still full of unprocessed mixed scrap, some of moderate quality. This material is still waiting to be processed. The steelworks switched to tightening the quality screws when it came to purchasing scrap. With an excess supply of processed scrap, they can now insist on buying only top grade material.

Shredding plants are the principal interface between supplying mixed scrap and purchasing scrap from the steelworks. The crisis has shown that they have to cope with fluctuations at the upstream end (input) and at the downstream end (output). Contrary to frequent assumptions, the increased value is derived less from crushing the material in the hammer mill, but rather from reprocessing the recovered metal. The profit is generated by sorting the material as carefully as possible into steel scrap, non-ferrous scrap, SLF (shredder light fraction) and SHF (shredder heavy fraction).

The Düsseldorf-based mechanical engineering company Metso Lindemann GmbH has developed its own design philosophy, which allows it to construct shredders that are flexible, low-maintenance and durable. A complex scenario emerged in an interview with Peter Wilbert, product manager for metal shredders.

De-dusting is split and adjustable

Power Zerdirator® shredders offer application flexibility and as such are designed to be adjustable so that they can process various grades of steel and aluminium. The adjustability of the de-dusting unit plays a key part in this. In the systems currently produced, the de-dusting process is split. Dust is removed from the Zerdirator's outgoing air flow in the conventional way using a combination of cyclone and Venturi washer. This system is designed to be resistant to explosion shock pressure. In a decoupled process, dust is removed from the separating drum, where no severe air shocks occur.

There is a strong counter-flow of air in the separating drum, which separates the SLF and also entrains some light sheet cuttings. To break up material compositions and adhesions, the material being processed in the shredder is flung onto baffle plates in free flight as soon as it enters the separating drum. The aim is to achieve an action that inputs the material in a wide-ranging ballistic curve. Efficiency would be reduced if the material were to impact the baffles at one particular point. The counter-flow of air in the drum that also covers the inlet zone passes into a cyclone with a downstream dry filter. A specific trend in material cleaning can be discerned at Lindemann. Traditional air separators are being replaced by the more efficient separating drums, and this currently applies to all models from 1,250 to 7,000 HP. The material is fed into the separating drum at an adjustable speed, quantity and trajectory to suit the specific material being processed.

Light sheet flies away

Contaminated mixed scrap requires the two decoupled dust removal units to deliver superior cleaning efficiency. However, the air flows should not be so strong that any more valuable metal than is absolutely unavoidable is entrained with the SLF. This would also make disposal of the SLF more costly. Consequently the extraction capacity is adjusted using ventilators that are controlled by frequency converters, and using butterfly valves. If scrap aluminium is being processed in the shredder, the air flows in the de-dusting unit have to be restricted. Otherwise too much light metal would enter the SLF, from where it could only be removed with difficulty.

The increased amounts of dirt (contamination) in the scrap result in the wear of some dust removal components and in increased cleaning requirements. Areas particularly subject to intensive wear are therefore lined with special abrasion-resistant materials. In addition, the ventilators are equipped with traversing devices that make them easy to open and to clean. These measures should achieve a marked decrease in the downtimes required for cleaning and maintenance.

During the commissioning of a Lindemann shredder, series of measurements are carried out on site so that the system as a whole can be correctly adjusted. This also involves storing setting parameters for the separate processing of aluminium.

Since the de-dusting units are critical components of the overall system, they are designed by Lindemann and only manufactured by approved sub-contractors. The design process is based on experience gained from 750 industrial shredding plants installed throughout the world. Lindemann, by its own account, is the only company able to deliver a complete package: feeder units, Power Zerdirator®, de-dusting system, conveyor belts and separation technology.

Operators also buy mixed-manufacturer shredding plants

However, in a free market operators sometimes like to build their plants using units from different manufacturers. Peter Wilbert at Lindemann remarked that "a situation can occur where customers buy the shredder from us, and the other parts of the plant from our competitors. We have noticed that other suppliers have provided downstream units that are too small for our Power Zerdirators, as they often underestimate the output of our shredders. Increasingly we are asked to replace the existing dust removal unit with a Lindemann de-duster. We recently replaced downstream units for customers in Austria, Holland and Denmark."

Image 1: Special lining materials provide increased protection against wear

Image 2: Decoupled dust removal units for the Zerdirator® and the separating drum

Special requirements are the rule

Customer requirements can vary greatly in practice. For instance, a company in Scotland replaced a 3,000 HP shredder that was 13 years old with a new 5,000 HP Zerdirator®. However, the existing conditions for approval had to be complied with when the new hammer mill was fitted, which meant that the maximum height of the plant could not exceed 15 m. By working together with the customer and the authorities, the geometry of the de-duster and modify the pipeline arrangement.

In another case a competitor's shredder model was to be replaced by a Lindemann Zerdirator®. The customer stipulated that the existing foundations and available space were to be used. The solution was to fit an adapter frame to the Power Zerdirator® so that the foundations could be used.

Images 3 and 4: New machine adapted to existing structural conditions

Double magnet prevents entrainment

One of the most recent installations was set up in the Benelux region. A series ZZ300x300 Power Zerdirator® was equipped with a newly designed magnetic separator. Two controllable electromagnetic drums were fitted in series downstream of the separating drum. The reason for incorporating a double unit is that in large-scale shredding plants with high peak throughput values of up to 400 t/h material can be entrained if there is only one magnetic drum. This effect occurs if the layer of shredded material on the conveyor belt is too thick. Non-magnetic parts are then carried along by the magnetic adhesion between other steel and iron parts. The additional cleaning process carried out with the second magnetic drum allows valuable non-ferrous metals to be recovered, when they would otherwise be entrained in the stream of ferrous metals.

Image 5: Two magnetic drums fitted in series prevent the entrainment of non-ferrous metals

Image 6: Derek plant (Belgium)

 Scrap is eating up the shredder

The nature of the system is that shredders are slowly eroded from the inside. Steel is used to crush steel. However, it also happens that scrap vehicles are misused for the disposal of rubbish and waste. As a result, mineral materials will also get into the hammer mill. The hammer heads bear the main load of crushing the steel. Consequently they are hardened.

To increase the endurance and breaking strength of the cast-steel hammers, Lindemann uses a hardening gradient that changes with the hammer profile. The faces used to crush the scrap metal are extremely wear-resistant, while the hammer eye is softer, so as to cushion the impact on the hammer axles. The company manufactures its own hammers. Given the tough working conditions associated with continuous operation, other machine components that are not directly exposed to abrasion can wear out too. If the spring assemblies on the rotor suspension set as a result of age, misalignment of the hammer axles could occur. To even out these height differences between the hammer axle and the removable hammer head, the hammer mount on the Zerdirators® is equipped with flexible heads. This facilitates the removal and replacement of hammers and protective caps.

Figure 7: Hammer configuration

The hammer mill of a shredder has a rotor shaft with fixed discs. The hammer axles pass through the entire disc assembly near the outer perimeter of the discs. The free-swinging hammers are suspended from the hammer axles between the rotor discs. During operation centrifugal forces align the hammers outwards. They crush the incoming scrap until it is small enough to fall through the grid below. If the impact of the hammers is insufficient to break up a solid piece of scrap metal immediately or fling it against the baffles on the rotor housing, the hammer will move back in pendulum fashion.

43,000 t of shredder in three years

Once the components have been manufactured in Düsseldorf they have to be transported to the customer, a journey that often involves large distances, before the shredding plant can be assembled. "The transportation of any large and heavy load represents a logistical challenge," commented Bert Mesmann, shipping manager at Metso Lindemann. Approximately 2,110 trucks have been used to move the shredding plants over the past three years. They have shifted 43,000 t of freight in total. Wilbert added that the logistical co-ordination of plant design, manufacture, transportation and assembly is carried out successfully despite several shredders being manufactured simultaneously for a number of clients.

Figure 8: Abnormal load: a shredder unit is transported

Sisters help each other

Since Texas Shredders was integrated in the Metso Corporation, Lindemann has reported a considerable increase in its US market shares. The co-operation is working well in the other direction as well. The first 8,000 HP shredding plant manufactured by Metso Texas Shredder was recently assembled and commissioned in Poland. A project team based in Düsseldorf was able to provide its sister company with support in drawing up the CE documentation.

Contact:
Metso Lindemann GmbH
Peter Wilbert
Erkrather Strasse 401
D - 40231 Düsseldorf
T: +49 (0)211-2105-431
F: +49 (0)211-2105 80 431
E: peter.wilbert@metso.com