September 2004
-View the list of manufacturers at the bottom of the page

Plastics destined for recycling can be sorted either as whole bottles and large pieces, or as flake. Each method has its proponents, and some recyclers may choose to employ both technologies in order to produce the cleanest product possible.

Additional sorting is critical when the plastic stream may contain metal, paperboard, and other municipal contaminants commonly included in material from curbside collection programs. Overhead magnets, manual sorting, and air classifiers are often employed to “clean” the plastic stream prior to the plastic classification and separation process.

How well that cleaning is done may affect the choice of machinery chosen for the next separation step; some sorting machinery can detect metallic and other contaminants, while others only identify specific plastic resins. Others go a step beyond to identify individual plastic colors.

In many operations, the majority of the sorting is done while the pieces are whole, and the flake sorting is used to “polish” the final product, and to remedy any mistakes made in the first sort. Flake sorting also removes HDPE bottlecap rings that remain on whole bottles.

In both whole and flake sorting, a wash/dry cycle is common. During this step, labels can be washed off the bottles and removed from the plastic stream, and residual organic material is washed away as well.

Magnetic Separations Systems, Inc. (MSS) manufactures two machines used for plastic sorting, the Sapphire and the Aladdin. Felix Hottenstein, sales director, explained that the Sapphire sorts by type of plastic using near infrared (NIR) detectors. A compressed air pulse is used for separation, with two output streams from a single input.

The Aladdin does everything the Sapphire does, and it also separates by color using full-spectrum color detection and NIR. The Aladdin can produce three output streams from one input.

Typically, bottles come in as baled product and must be un-baled before sorting. Whole unbaled bottles should be flattened or perforated to prevent rolling. “Round bottles can roll around; if the bottle rolls, the timing between identification and ejection can be affected,” Hottenstein explained.

These machines can also be equipped with a metal detector to remove aluminum cans and other metallic contamination from the plastic stream. Hottenstein added that the newest MSS machines, “always use the latest, state of the art technologies, so when there are new plastics coming out, we can upgrade the software or hardware.”

While plastic separation is a somewhat mature technology, typically using near infrared spectroscopy and visible light sensors as well as the less-technological methods such as float/sink tanks, there are other aspects of the machinery that have improved over the years. For example, all of MSS’s newer machines have modem hookups, so MSS can diagnose, adjust the machines, and upgrade the software without having to send a technician to the customer’s location.

National Recovery Technologies (NRT) manufactures a variety of machines for the plastic recycler including the Flake Analyzer which “provides a rapid analysis of large flake samples” according to John Thomson, engineering manager at NRT.

While the Flake Analyzer is often used to evaluate the quality of shipments, it is also used for sorting flake based on the type of plastic.

The VinylCycle machine, introduced in 1991 and a proven workhorse, sorts PVC from PET in a stream of crushed or whole plastic bottles. Thomson explained, “While this is a mature technology, it has proven reliable over the years; the PVC cannot ‘hide’ behind another bottle when x-ray transmission is used [for identification].” Even small traces of PVC in a batch of PET can cause problems in the recycling process.

When other materials need to be sorted from the plastic stream, Thomson recommends the MultiSort IR. It can identify metal, paperboard, and other contaminants and remove them from the recyclable plastics, while also identifying the desired plastic by resin. It does not sort by color or transparency.

Like MSS, the newer NRT machines have modem hookups for diagnostic checkups and adjustments.

Another NRT choice for recyclers is the MultiSort ES. This machine also sorts by color and can identify a wide range of colors, tints, and transparencies. For example, it can be adjusted to sort natural (clear) HDPE from the colored material in the stream. The MultiSort ES performs one sorting procedure at a time, but colors can be selected and grouped. For example, PET and HDPE are separated, while clear PET is separated from all of the colored PET.

For multiple sorts, MultiSort ES machines can be joined together with independent separation steps at each machine.

Polymer Recovery Systems, Inc. (PRSI) makes an air classification system as well as float/sink tanks for further separation of post-consumer and post-industrial plastics. Project engineer Adrian Luther said that this system results in clean end product, with labels and other contaminants removed.

The air classification begins with plastic flake that has been ground to a ¼” to ½” size. The flake is fed into the air classifier where it falls against a rising column of air. The lighter paper and debris are removed by the air stream, while the heavier plastic flake falls into a float/sink tank. There, the material is sorted based on the specific gravity. In the case of whole, shredded pop bottles, the HDPE bottle cap would sink, while the PET bottle would float.

Air velocity in the air stream can be adjusted via a slide gate to fine-tune the separation. The base solution in the float/sink tank can also be adjusted.

From the float/sink tank, the separated material would go to a washing/drying system, where labels would be loosened and organic materials would be removed. Next is another air classification system that is “essentially the same as the first one,” according to Luther. This second air separation removes any paper that might have been missed during the first process, as well as labels separated during the washing process.