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.
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
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.
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
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
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.
“Ninety-five percent of the debris is removed by the
primary air classification,” Luther explained, “and five percent
or less is removed after washing and drying, at the secondary classification.”
The result is a very clean end product.