Figure 14.18. Density ranges and overlap for compounded plastic resins used in durable applications. Source: American Plastics Council.

resins have been commonly used in automotive interiors, for example, complicating their straightforward separation using density methods alone [49]. Some of the advantages and disadvantages of sink-float classifiers are summarized below.

Sink-Float Tanks or Classifiers



Most commonly practiced technology — well known

Can be very inexpensive

Few moving parts

Discrete separations are possible (if particles are well wetted

Plastics have density distributions rather than discrete values Must use some media in water — salts are commonly used Some media loss will occur Water treatment issues related to media

Reliable wetting of particles can be a problem

Must control density of bath hydrocyclones Hydrocyclones are an economical and effective tool for separating mixed plastics and for removing many contaminants from a target plastic [75, 77]. The basic components of a hydrocyclone are shown in Figure 14.19, and Figure 14.20 shows a schematic demonstrating the working principle behind the separation. The motive force for effecting hydrocyclone separations is again density differential, and the greater the difference in density, the higher probability of separating two dissimilar components. The shape of the particles to be




Figure 14.19. General arrangement of a hydrocyclone useful in high-throughput plastic-plastic separations [77].

separated in a hydrocyclone is also an important consideration. Since one hydrocyclone cannot guarantee close-tolerance separations, it is common practice to install hydrocyclones in series.

Hydrocyclones lie between sink-float tanks and centrifuges with respect to cost and complexity [77]. They represent a realistic density separation tool because they are relatively inexpensive, require very little space, suffer from fewer material-wetting problems than sink-float systems, and most importantly can be operated at extremely high throughputs. A single 10-inch cyclone can theoretically sort materials at rates in excess of 5000 lb/h. Though multiple stages and pumps are usually required for challenging separations, the overall capital costs can be very reasonable particularly if water alone is used as a separation medium.

Extensive testing of the fundamental performance of hydrocyclones for the separation of engineering plastics has been reported [77]. Initial testing was performed on a hydrocyclone test stand. The test stand could accommodate up to three hydrocyclones, was used to demonstrate the cyclone capabilities,

Overflow, lights

Underflow, heavies

Figure 14.20. Schematic representation of spiral flow in a hydrocyclone [77].

and experiment with new concepts. A next-generation design was then built to test the most promising approaches suggested from the test stand studies. The research elucidated the fundamental parameters of the cyclone and demonstrated new configurations that are more efficient. Areas of study included the use of variable-flow/pressure control, variable apex and vortex combinations, multistep cyclones, elevated cyclones, separation curves, and statistical analysis.

This research led to a greater understanding of the capabilities and fundamental operating parameters for hydrocyclones and was used to help design a pilot separation line, the purpose of which was to demonstrate the application of hydrocyclone separations to a wide variety of plastic recycle streams [10]. Some of the advantages and disadvantages of hydrocyclones are summarized here:




High throughputs are possible Moderately simple in design Much less expensive than centrifuges

Particle wetting less of a problem compared to sink-float

• Must use some media in water

• Some medial loss will occur

• Water treatment issues (media)

• Need to pump to drive cyclone

• particle size and shape important

Table 14.15 lists several manufacturers of hydrocyclones in the United States.

centrifuges Centrifuges represent perhaps one of the most sophisticated density separation tools and can make very accurate separations at reasonably high throughputs [75, 81]. The primary drawback to centrifuges is their relatively high purchase and maintenance costs. Furthermore, only rather small particles can be fed to most centrifuges, so size reduction expenses can be high as well. However, the high centrifugal forces can overcome particle shape effects. Thus centrifuges have been shown to have utility for the separation of plastics in the form of rigids, films, and fibers. Some of the advantages and disadvantages of centrifuges are summarized here:



• Most discrete density separations are possible due to high g-factors

• Particle shape of little importance (film and rigids can be separated in same system)

• Fairly compact footprint


• Must use some media in water (most use some type of salt)

• Some media loss will occur

• Water treatment issues (media)

• Much more expensive than sink-float or hydrocyclone

A rough comparative estimate of cost and throughput capacity for hydrocyclones, centrifuges, and sink-float systems is shown here [77]:

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