Plastics circular economy: reduction of contaminants in recycled material and process effluents

The manufacture of secondary raw materials from a waste stream requires the cleaning of:

• The product stream from ‘risk cycle substances’. These are substances within the material matrix: both intentionally added substances (IAS), such as additives; and non-intentionally added substances (NIAS), such as those resulting from the decomposition of additives.
• The process exhaust and wastewater effluent.

Once this has been completed, the secondary raw material can be fed into the relevant production step of the plastic lifecycle. Read more about DESOTEC’s work in the plastics and composites industry here.

As the world strives to meet its net-zero goals, the pressure is on to increase the carbon recovery from waste plastics through the production of high quality secondary raw materials (to prevent downcycling).

State-of-the-art mechanical recycling techniques are mostly extrusion-based, in which the polymer remains intact. This technology works very well for relatively pure waste streams like PET.

However, increasingly complex multi-layered plastic composites are being developed for packaging, textiles, functional materials etc, which require more sophisticated recycling technologies to reach virgin-like secondary raw material quality.

Furthermore, IAS such as dyes, plasticisers or fillers can constitute a significant weight fraction of the plastic product. These additives are necessary for functionality, but must be removed when producing recycled plastic secondary raw material for the direct replacement of virgin material, especially if that recycled material is to be used in another application.

For short-lived plastic products such as packaging, the additives found in plastic waste streams typically comply with current chemicals regulations (REACH/ CLP in the EU, and TSCA in the USA).

However, some plastic products for construction or consumer goods have much longer lifespans. Thus, substances like bisphenol-A that have already been phased out may enter the recycling process today. To prevent accumulation of these substances of (very) high concern (SVHC), also called risk cycle substances, the recycled plastic product stream needs to be cleaned.

Table 1: (Non-)intentionally added substances to plastic material and their hazards to humans

The key component of plastic products is a polymer or combination of polymers made from repeating chemical monomer units. Further chemicals are added as processing aids such as lubricants or as additives such as plasticisers, flame retardants, heat and light stabilisers or pigments. These include molecules such as phthalates, paraffins, bisphenols, polyfluoroalkyl substances (PFAS), alkylphenols, polyaromatic hydrocarbons (PAH), biocides, talc, and clay.

Fig. 2: Comparison of typical temperatures used in plastic recycling/ valorisation approaches vs. boiling temperature of typical plastic additives. The right column indicates in which product phase the (decomposed) Non-intended (NIAS) and the Intended Added Substances (IAS) can mainly be found.

Fig. 2 shows that not all NIAS/ IAS can be fully removed into the vapor phase through typical process parameters employed in extrusion (mechanical recycling) to yield an additive-free recyclate.

For recycling technologies which employ a solvent (delamination, depolymerisation), removal of (N)IAS from the recyclate is a function of the solubility of the (N)IAS in the used solvent.

With thermal technologies (pyrolysis, liquefaction) (N)IAS will evaporate into the desired vapor phase - partially or fully decomposed. Without catalytic vapor treatment, these modified molecules will be found in the condensed oil phase. At even higher temperatures (gasification, incineration), the (N)IAS will also evaporate but will mainly be oxidised into CO₂ (incineration) or to CO (gasification). Very stable compounds like PFAS may not be altered and then found in the gas/ product phase.

Both solvent-requiring (solvolysis and depolymerisation) and thermolytic (pyrolysis and liquefaction) processes result in liquids that contain impurities, causing unwanted colours, odours, and even substances of concern, therefore reducing the value of the recycled raw material and potentially damaging downstream equipment or processes. Furthermore, plastic waste streams from different sources can have very different degrees of contamination, or be relatively clean. This may make it difficult to calculate the business case to decide on a purification technology.

Catalytic hydrotreatment: Plastic pyrolysis oil may be cleaned using a catalytic hydrotreater. This technology is well known and proven, but may be a very expensive solution, especially for smaller applications. Larger refineries will most probably already have a hydrotreater in place but, depending on the level of contaminants, this may be very expensive to operate in terms of hydrogen consumption and catalyst deactivation.

Caustic wash: Another possibility is a caustic wash. Currently, its effectiveness on an industrial scale with plastic pyrolysis oil is under study. This treatment results in a highly organics-contaminated aqueous waste stream which itself needs treatment beyond a simple waste water treatment plant.

Activated carbon treatment solutions: Activated carbon filters offer a viable alternative in both solvent-requiring and thermolytic plastic recycling applications, purifying the liquid and oil for the next stage of the recycling process. Activated carbon is relatively agnostic to different organic contaminants or even fluctuating concentrations thereof. End-of-line product quality control can be sufficient for control of the activated carbon purification process. When the activated carbon comes in mobile filters, investment costs are very minimal and operating costs directly reflect the intake of contaminants with the waste stream. Fixed costs are minimalised.

Our market research shows that there is an ever-growing number of plastics recycling projects with new technologies in Europe and North America where, for example, pyrolysis is being implemented, and a similar number of solvolysis/ depolymerisation applications. These projects involve multiple stakeholders, including brand owners, waste handlers, technology developers, oil companies, and machine manufacturers.

Many customers in the European plastic pyrolysis oil, delamination and depolymerisation space (and a few customers in the USA) are already lab-testing our solutions, including industrial-sized pilot tests with DESOTEC’s commercial filters.

DESOTEC solutions for pyrolysis oil: Although some companies initially believed that activated carbon works only with water, our results suggest that our filtration solutions are capable of reducing levels of heteroatom-containing contaminants in oils. A reduction of 40% and more of nitrogen-, oxygen- and chlorine-compounds was achieved for both post-industrial and post-consumer waste pyrolysis.

Key to this is high quality renewable-based activated carbon with a beneficial pore size distribution and surface groups. Specialised filters can cope with the typical process parameters – in some cases, the filtration has to be undertaken at an elevated temperature to make the oil or even wax phase pumpable.

DESOTEC solutions for depolymerisation and solvolysis: We also have lab tests underway with a considerable number of companies in this area. Here, the liquid to be treated is often protic, in most cases water (sometimes with a catalyst), e.g. for PET. However, other solvents or even sets of solvents (also aprotic) are also used.

The activated carbon grades used in these applications depend on the solvent, degree of pollution and required product purity level. A 99% colour (pollutant) reduction, for example, was achieved in solvent-based recycling of dark textile waste. In such a scenario, specialised filters would be employed for elevated temperatures and potentially higher/ lower pH values.