Part 1. Plastic waste sorting
We are committed to delivering information regarding the plastic industry through this web page that can be easily read by a general audience and helpful people in the field. In any case, we will try to cover the stay of the art in the different topics. Our first entry is oriented to plastic waste management.
We will cover1:
a) Sorting plastic waste,
b) Mechanical recycling,
c) Chemical recycling, and
d) Waste disposal.
Before covering the first topic, we will contextualize the plastic waste problem in an introduction.
Introduction
10 % of the fossil hydrocarbons are upgraded to polymers (350 Mt/annum); most are used for packaging, and some end up as consumer waste at a rate of 1 -2 Kg /capita.day. The 10 wt % of municipal solid waste is plastic. 60 % of them are polyesters HDPE (high-density polyethylene), LDPE (low-density polyethylene), LLDPE (linear low-density polyethylene), and PP (polypropylene), 40 %: PET (polyethylene terephthalate), PVC (polyvinyl chloride), PS (polystyrene) and others. 12 % of the spent plastic is recycled, 25 % incinerated, and 60 % ends up in the environment. Recycling not only contributes to carbon recycling but also to energy saving. Its success depends on the quality and the purity of the waste. Therefore, some steps are involved: Reuse, Reprocessing (Mechanical treatment), Depolymerization (chemical treatment), Conversion to hydrocarbons, and energy recovery. Post-consumed plastics are heterogeneous and undefined in quality. For instance, in plastic bottles: 75-90 wt % dominant polymer; 5-15 wt % foreign polymer and 5-15 wt % Residue. The last two come from CAP/Lid and labels. Therefore, waste sorting is essential for recycling.
Waste sorting
The following are the typical sorting steps:
Size separation (using sieves).
Elimination of foreign materials.
Sorting of plastic materials and sizing and granulation into plastic recycle.
The elimination of foreign materials is performed by gravity under flowing air, sink float using liquids or magnets to separate ferrous material. The sorting of plastic materials is performed by the density difference of the plastics and further refined using a magnetic or electrical field. IR detector (NIR: near-infrared reflectance spectroscopy2) is also used on a conveyor to detect different plastics. Fluorescent pigment (tracer based3) is also used to detect plastics using UV light. Digital watermarks (code embedded) are used to label plastic bottles with detailed information. Next level: Robotic sorting using artificial intelligence is being used to sort plastic bottles.
1.- J-P. Lange ACS.Sustain. Chem.Eng.. (2021) 9, 47, 15722–15738
2.- S. Zhu, H. Chen, M. Wang, X.Guo, Y. Lei, G.Jin. DOAJ (2019) 2, 77-81
3.- E.G. Karakolis, B. Nguyen, J.B. You, C.M. Rochman, D. Sinton Environ. Sci.Technol. (2019) 6,6 334-340
