The valorization of plastic solid waste (PSW) by primary to quaternary routes: From re-use to energy and chemicals

Polymers are the most versatile material in our modern day and age. With certain chemicals and additives (pigments, concentrates, anti-blockers, light transformers (LTs), UV-stabilizers, etc.), they become what we know as plastics. The aim of this review is to provide the reader with an in depth analysis regarding the recovery, treatment and recycling routes of plastic solid waste (PSW), as well as the main advantages and disadvantages associated with every route. Recovery and recycling of PSW can be categorized by four main routes, i.e. re-extrusion, mechanical, chemical and energy recovery. Re-extrusion (primary recycling) utilizes scrap plastics by re-introducing the reminder of certain extruded thermoplastics (mainly poly-α-olefins) into heat cycles within a processing line. When plastic articles are discarded after a number of life cycles, mechanical recycling techniques present themselves as a candidate for utilizing a percentage of the waste as recyclate and/or fillers. Collectively, all technologies that convert polymers to either monomers (monomer recycling) or petrochemicals (feedstock recycling) are referred to as chemical recycling. The technology behind its success is the depolymerization processes (e.g. thermolysis) that can result in a very profitable and sustainable industrial scheme, providing a high product yield and a minimal waste. Nevertheless, due to their high calorific value and embodied energy, plastics are being incinerated solely or in combination with municipal solid waste (MSW) in many developed countries. This review also presents a number of application and technologies currently being used to incinerate plastics. Cement kilns and fluidized beds are the two most common units used to recover energy from PSW or MSW with high PSW content. It is concluded that, tertiary (chemical methods) and quaternary (energy recovery) are robust enough to be investigated and researched in the near future, for they provide a very sustainable solution to the PSW cycle.