Plastic Waste Treatment and Recycling
Energy-efficient mixers and dryers are widely acknowledged to support manufacturers’ sustainability goals by reducing production related emissions. But many people don’t realize that they also play a role in circularity thanks to being embedded in processes that turn plastic waste into valuable raw materials. This article takes a closer look at how mixing and drying technology supports various polymer recycling routes so that the growing mountain of plastic waste can be valorized and reused.
Today’s high-performance mixers and dryers are known to be designed with energy efficiency in mind. Efficient heat transfer, low specific energy input and the ability to recover process heat all contribute not only to lower operating costs, but also to more sustainable manufacturing operations. What is less visible, however, is their contribution to sustainability thanks to their role within processes that recycle or upgrade waste streams.
The Benefits of Recycling
“Recycling is another important aspect of many companies’ sustainability efforts nowadays, besides energy,” says Marc Jacobs, Team Manager CMM at Hosokawa Micron BV. “This often makes economic sense. After all, if you can reuse material from within your own process, you immediately reduce your raw-material consumption and also waste disposal costs.”
As a leading designer and builder of mixing, drying and agglomeration systems for powders, particles and bulk solids based in Doetinchem, the Netherlands, Hosokawa Micron has been supporting recycling in various industries for many years. Around a decade ago, for example, the company adapted one of its Nauta mixers for a biscuit manufacturer. Thanks to a relatively small change to the mixing screw, any non-mixed dough could be extracted at the bottom of the drum and fed back in at the top. “This ‘internal recycling loop’ kept more material in the process, thus significantly reducing waste for the customer,” explains Jacobs.
Plastic Production has Doubled
The polymer industry is under particular scrutiny when it comes to waste reduction and recycling. According to the ‘Global Plastics Outlook’ report published by the Organisation for Economic Cooperation and Development (OECD), global plastic production has more than doubled in the past two decades (increasing from 234 million tonnes in 2000 to 460 million tonnes in 2019).
This is creating more plastic waste than traditional disposal systems can handle, and posing environmental and climaterelated threats. To minimise such impacts by reducing plastic waste, many governments are introducing stricter regulations stimulating companies to keep recycle or upcycle polymers. For example, the European Union’s new Packaging and Packaging Waste Regulation (PPWR) requires all packaging on the EU market to be designed for recycling by 2030, and mandates specific recycled content targets – ranging from 10% to 35% – for all plastic packaging.
Besides mechanical recycling, which is the most common route for plastic recycling and involves sorting, cleaning, shredding, remelting and reforming plastic waste while preserving the original polymer structure, there are several other techniques available for reusing plastics. These include dissolution and pyrolysis/gasification, both of which are discussed in more detail below.
Dissolution of Plastic-rich Waste
Whether plastic waste can be recycled or upgraded, and which process is most suitable, depends on the composition of the material involved. Plastic-rich waste streams (comprising more than 80% polymer) can be recycled in a dissolution process. In this case, a solvent is applied to the waste to dissolve the polymer, allowing any undissolved contaminants to be mechanically separated and removed. The polymer is then recovered by evaporating the solvent under vacuum. “Our vacuum dryers can be used in this process, either based on our Nauta technology or our conical paddle dryer (CPD). Operating at reduced pressure and low temperatures prevents thermal degradation while still achieving high residual solvent removal,” comments Jacobs. The resulting polymer material is then passed through an extruder to prepare it for pelletization.
Homogeneous Mixing of Performance Additives
Manufacturing the actual performance additives used to upgrade the polymer material following dissolution also involves a classical Nauta powder mixer. For example, a high-performance, industrial-grade mixer from Hosokawa Micron has been a critical component of a compounding line at a European manufacturer of polymer additives active in the international plastics and recycling industry.
Within this production line, peroxide masterbatches are produced, among other products, which are used in the recycling sector to reduce the viscosity of polymers. “To ensure consistent product quality, the polymer additives must be mixed into a fully homogeneous blend,” explains Raymond Fels, Application Engineer CMM at Hosokawa Micron. “This mixing process must also take place without excessive heat generation, as the ingredients used are temperature-sensitive.”
The Hosokawa Micron Nauta mixer used in this application combines efficient homogenisation with relatively low heat input. In addition, the installation is easy and safe to operate, reliable in continuous feeding to extrusion lines, and requires minimal maintenance in industrial production environments.
Pyrolysis for Mixed-Plastic Waste
In the case of mixed or contaminated plastic waste streams, the pyrolysis (or ‘gasification’) route offers a solution. This entails heating the waste in the absence of oxygen and converting it into oil and gas, leaving a solid residue behind. This residue contains mineral components such as calcium carbonate and titanium dioxide, which makes upgrading it for reuse more complex – but not impossible.
“Pyrolysis is also the route for obtaining recovered carbon black (rCB) from rubber tyres,” comments Jacobs. In that process, after pyrolysis, the granules are pre-crushed and then finely ground. They then enter the pelletizing system, which consists of a mixer to add the binding solution and a dryer to convert the wet carbon black pellets into easy-to-handle, dry and stable pellets.
“Together with our sister companies, Hosokawa Alpine AG and Hosokawa Solids, we provide all the equipment needed for this process and have already developed a number of cost-effective total systems for carbon black recovery,” explains Jacobs.
“It’s still early days for this technology in plastic recycling. But based on how the rCB market has evolved over the past 15 years, taking tyre recycling from technically possible to economically viable, I see no reason why similar things couldn’t happen within polymer recycling,” he adds.
Improving Sustainability and Profitability
“Polymer waste treatment and recycling is such a complex area that there is no one-size-fits-all solution,” says Fels. At Hosokawa Micron, he and the company’s other technical specialists are always open to hearing customers’ ideas, and discussing and exploring together how mixing and drying technology can contribute to solving their challenges.
“Drawing on our process knowledge, we can determine which of our solutions are most suitable for a specific application, and then run pilot-scale testing in our Test Centre,” he continues.
“Polymer recycling is a very exciting, fast-moving and urgent topic. Our mixers and dryers are already embedded in some process steps at various companies involved in polymer recycling. We are keen to help them and others make the entire process even more sustainable and more profitable, for a better future for all,” concludes Jacobs.