Lower density plastics s
Other Higher non-plastic density materials plastics
Figure 14.13. Diagram showing important processing operations in a plastics recycling plant designed to handle complex multimaterial streams. Source: American Plastics Council.
step in the plastics recycling operation. This is the first critical step in a plastics recycling plant and especially important in a plant designed to handle plastics from end-of life durables. Figure 14.13 shows key processing steps in a plastics recycling plant capable of handling complex, contaminated, mixed plastic feed-streams. For complex material steams, more than one size reduction step may be required. Depending on the degree of contamination, a coarser, more robust shredding operation might be needed, followed by contaminate removal, final cleaning, and then granulation. In the case of plastics from end-of-life durables, due to extensive multimaterial contamination that often occurs, size reduction becomes perhaps the most critical step in the entire plastics recycling operation. In the case of plastic bottle recycling one can often turn directly to granulation to produce a size-reduced flake. Table 14.12 provides a summary of several size reduction technologies for different plastics recycling applications.
Size reduction is used to increase bulk density, lowering storage requirements and shipping/transport costs, ease material handling and conveying, and liberate foreign materials. There are many specific challenges associated with durables recycling: large and widely varied parts, often significant amounts and sizes of metal, thick wall sections, tough engineering plastics, high modulus but brittle plastics, high rubber content sometimes present, film plastics sometime present, wide range of cutting and fracture behaviors, extremely wide range of foreign materials, well-adhered foreign materials (labels, foams, fabrics, laminates, metal foils, etc.), hardened metals, high dust/fluff loadings, and numerous different material types used in many different applications leading to more equipment cleanouts.
Desired attributes of size reduction equipment for the recycling of plastics from end-of-life durable goods include: accommodates large amounts of metal, handles tough engineering plastics at reasonable throughputs, liberates molded-in and well-adhered materials, does not imbed or encapsulate foreign materials, produces uniform particle shapes and sizes, is safe for wide range of operators, accommodates very large parts or bales of materials, provides high throughput to power requirement ratio, minimizes fines generation, can be enclosed or evacuated, is reasonably priced, needs low maintenance, is easy to clean for material switch-overs, produces low noise, and has reasonable power requirements . Figure 14.14 summarizes the many factors that must be considered in selecting a size reduction system.
As summarized in Table 14.12, the types of equipment available to meet these challenges include: hammer mills, ring mills, sheer shredders with screens, four-shaft shear shredders with screens, rotary grinders, and granulators. Depending on the types of feedstocks coming into a plastics recycling plant, there may be a need for several different stages of size reduction and liberation, each involving a different type of equipment. In some applications, wet or underwater grinding may be desirable.
Rotary grinders deserve special mention. In early APC-sponsored studies in collaboration with MBA Polymers and wTe Corporation, rotary grinders (originally developed in Europe for the wood chipping industry) were found to work for a wide variety and size of plastic parts, tolerate moderate amounts of metal, and provide acceptable throughputs, especially when a ram was used to help push and hold the parts against the rotor and teeth [10, 49, 73]. Rotary grinders employ a rotating drum that contains numerous square teeth arranged such that the diagonal axis is parallel with the direction of rotation of the rotor. Figure 14.15 shows a closeup of a rotary grinder showing one type of tooth configuration. Unlike a shredder that shears material into strips as it is pulled through stacks of
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