Page 188 - Materials Chemistry, Second Edition

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Sustainability of Municipal Solid Waste Management
before shredding it as this improves its quality. Usually washing takes place
after cutting (cutting refers to splitting the plastics into two parts to ease the
washing process) and sometimes even after shredding to obtain better
results. Foreign materials like paper and covers are removed before cleaning.
Washing can be carried out either manually or mechanically. Manually, the
plastics are placed in a drum cut in half and the water is stirred with a
paddle. The water is heated if the waste is greasy; soap and caustic soda
are added to help remove the more difficult grease (Lardinos and van de
Klundert, 1995).
As with washing, the plastics can be dried either manually or mechan-
ically. Manually, the plastics are left to dry in the sun. This method works
best for plastic films, which are hung on lines to dry. Shredded plastics are
centrifuged and left to further dry in the sun. If left without being cen-
trifuged, the plastics will need a longer time to dry, will require more stirring
2
and a space of around 15–20 m for 300 kg of shredded plastics (Lardinos and
van de Klundert, 1995).
In developing countries, plastics are cut in two halves and washed in a
hot water basin using burners. For 1 ton of plastics, 25 kg of caustic soda
3
should be added to 2 m of warm water. The plastics are then rinsed in cold
water in two different basins to remove the caustic soda; they are then cen-
trifuged and left to dry for 2–4 hours in the sun.

The second stage – plastics are prepared for further
reprocessing
This stage involves size reduction in which waste plastics undergo cutting,
shredding, agglomeration, and pelletizing. This process increases the cost of
plastics waste since it eases their use in the manufacturing process and
decreases transportation cost (Lardinos and van de Klundert, 1995). The
smaller the size of the shredded plastics, the more regular their shape (as in
the case of pelletizing) leading to wider market demand and higher price.
The first material transformation step is cutting the plastics into
smaller pieces, as is the case with large bottles, cans, and buckets. These
waste plastics are usually cut by a circular saw or with a bandsaw (Lardinos
and van de Klundert, 1995). In most developing countries, the sorted and
washed plastics are cut into two pieces by large scissors fixed on a wooden
base. It is estimated that an average of three workers can cut up to 1 ton of
sorted plastics per day.
Waste plastics cut in two or more pieces are fed into a shredder for fur-
ther cutting. The plastic is cleaned before shredding and sometimes gets
further cleaning after shredding according to manufacturer demand. The
shredder machine used for thin film plastics is different from that used for
rigid plastics. For rigid plastics a horizontal cutting machine is used where
the blades are rotating on a horizontal axis and the shredded plastics pass
through a grid into a collecting tray (Lardinos and van de Klundert, 1995).