Overview of Key Processes
Chemical recycling is the broad term used to describe a range of emerging technologies in the waste management industry which allow plastics to be recycled back into raw material building blocks. It is not energy recovery or incineration.
As a complementary technology, chemical recycling is there to help increase the range of plastics that are currently recyclable and therefore drive-up recycling rates rather than to divert material from mechanical recycling facilities.
Below is an overview of the key technologies...
For definitions please see the glossary.
Contents
1. Plastics Recycling Family Tree
2. Depolymerisation e.g. Solvolysis
1. Plastics Recycling Family Tree
2. Depolymerisation e.g. Solvolysis
This is the reverse of the polymerisation reaction and yields a single-monomer molecule or a shorter polymer fragment (called Oligomers). The monomers are identical to those used to produce the polymer so similar quality to virgin. This process can only be used for PET, PBT, Polycarbonate, and polyamides.
This process heats polymer in the absence of oxygen to produce a range of hydrocarbon raw materials. A distillation process (which utilises the different boiling points of different fractions) can be used to make the hydrocarbon vapours into products ranging from heavy wax to light oils and gases. The combination of heavy to light outputs can be varied with changes to process time and temperatures. The products can be used in the same way as oil using conventional refining technologies. A catalyst can be used to allow a lower temperature and time to be used.
This process involves heating mixed plastic waste material to very high temperatures (~500 to 1,300°C) with limited oxygen. This partial oxidation produces a ‘Syngas’ (a mixture of hydrogen, carbon monoxide and some carbon dioxide). The ‘Syngas’ can produce a variety of chemicals (methanol, ammonia, hydrocarbons, acetic acid) for plastics production as well as fuel and fertiliser.
Gasification of mixed waste has been used for some time.
The process applies heat and pressure to waste plastic to generate a plastic melt stream which is then mixed with steam and heated to supercritical conditions (> 373 °C, > 220 bar). Polymers are broken down (cracked) into shorter-chain hydrocarbons in this environment. The reaction mixture is then passed through a reactor system and depressurised, with products then fractionated based on boiling points. The process is flexible and produces a range of liquid and gaseous hydrocarbons which can be used as feedstocks to make a range of products, such as new plastics and other chemicals.
Complementary to these technologies and mechanical recycling is dissolution...
Dissolution recycling is a purification process through which the polymer present in a mixed plastics waste is selectively dissolved in a solvent, allowing it to be separated from the waste and recovered in a pure form without changing its chemical nature. Several industrial examples of such technology already exist and apply to different polymers, such as polyvinyl chloride (PVC), polystyrene (PS), nylon (PA) or polypropylene (PP), to separate them from mixed, multi-material waste.
