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Cannot handle metals

* Advantages and disadvantages are considered representative for each class of equipment but may not be valid in all cases as technology is steadily evolving.

* Advantages and disadvantages are considered representative for each class of equipment but may not be valid in all cases as technology is steadily evolving.

Size reduction equipment considerations

Size reduction equipment considerations

Figure 14.14. Factors to consider in selecting size reduction equipment for a plastics recycling plant. Adapted from reference 10.
Figure 14.15. Closeup of a rotary grinder showing placement of cutters.

counterrotating knives, the rotary grinder's teeth chip away the material, almost like a punch-and-die operation. As a result, the first cut produces a much smaller particle size, approximately the size of the teeth, and little material is recycled back around the rotor. This approach produces fewer fines and much more uniform particles in a more efficient manner than many other traditional size reduction techniques.

Because the cutting tooth design used on this machine is similar to the tooth design used on metal milling machines, it is effective in cutting through some amount of metal typically found attached to plastics in many durable goods. Because each tooth has four independent cutting surfaces, the machine can be refitted with new cutting surfaces four times before a new set of cutters is needed; cutters can be rotated and flipped without dismantling the machine by removing one socket head screw. Both hardened steel and carbide cutters are available.

One further, and significant, advantage of rotary grinders is the hopper and ram design. Most rotary grinders have very large hoppers that can accommodate large parts and even bales of materials. More unique, however, is the horizontal ram that is used to push material into the rotor. The ram is usually driven by hydraulics, and the load is monitored such that the ram keeps a constant force of material on the rotating shaft. It also senses when the load drops too low, and it should reverse its direction to allow more material to fall in front of it for another cycle. Rotary grinders are equipped with screens to allow recirculation of oversized particles. Today, rotary grinders have found an important place in plastics recycling of all types.

Removal of Nonplastic Materials Nonplastic materials or low-volume, nonrigid plastic materials that can be present in plastics recycling streams before and after the primary size reduction step include ferrous metals, nonferrous metals, foams, film, rubber, labels, paint and coatings, metallic foils, glass, rocks, sand, dirt, and the like. The removal of all of these materials is required if the objective of the recycling operation is to produce recyclates capable of competing with virgin resins in value-added applications. Size-reduced plastics must therefore be subjected to several additional processing steps in order to prepare plastics free of nonplastic contaminants (see Fig. 14.13). Although for simplicity only dry processing may be desirable, in practice some separations require the use of a fluid medium (wet processing) for reasons of effectiveness and efficiency.

Metals Removal Ferrous metals, including low-grade stainless steel and nickel alloys, can be removed using various types of magnets. Overhead belt magnets, magnetic pulleys, and drum magnets are common. Specialized devices are used to remove final traces of magnetic materials as part of a final polishing step. Nonfer-rous metals are removed using eddy current separators and/or electrostatic separators. These are fully commercial technologies that have the advantage of being dry processes. Neither technology provides a perfect separation, and both eddy current and electrostatic separators can be expensive ($100,000+). Fluidized beds have also been used to separate nonferrous metals as part of a plastics recycling operation, but experience is still limited compared to eddy current and electrostatic methods. With added complication due to water treatment requirements, wet processes such as elutriation, heavy media tanks, and mineral jigs are also used to remove nonferrous metals and glass. Some recycling processes starting with very contaminated feedstocks such as automotive shredder residue place a heavy media tank very early in the process in order to drop out most metal, rocks, glass, and sand that could damage size reduction equipment [74].

Lights Removal Air classification methods are used to remove light contaminants such as dust, film and foam fragments, and paper glass powder in a recycling operation in the absence of water [1, 10]. Two common types of air classifier systems, cyclone separators and multiaspirators, are shown in Figures 14.16 and 14.17. Other types of air classifiers are air knives, elutriators, zig-zag classifiers, and air tables. Air classifiers are rather simple equipment where control is often more art than science, and the equipment must be "tuned" for each stream of material. Separations of materials are based on differences in terminal velocities in an airstream and are highly dependent on particle size and shape.

The Kongskilde aspirator operates like a cyclone in that the heavy particles in a rotating airstream are thrown outward against the wall of the aspirator chamber and fall out the bottom. The lighter particles are drawn up and out of the aspirator by the moving air stream. The "light" particles are not necessarily the lightest in strict particle mass. Rather, they have the lowest terminal velocity. Therefore, a more dense particle with a large surface area-to-mass ratio, and therefore more drag, can be carried into the light stream, while a less dense particle with a small surface area-to-mass ratio, and less drag, can report to the heavy stream. To help break up and evenly distribute the commingled feed stream into the airstream, Kongskilde incorporates an axial fan inside its aspirator. This lower fan design helps to disperse the feed stream, allowing the light materials to be vacuumed up quickly by the external fan. Although this does help break up and distribute the feed, it is also prone to plugging during operation, especially when using particles larger than 1 or 2 inches.

Feedstock inlet

Heavy cut

Figure 14.16. Diagram of a cyclone-type air classification system. Source: MBA Polymers, Inc. for the American Plastics Council.

Feedstock inlet

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