Shredding and Separation Technology Plastic Waste: Recover Raw Materials with the Right Technology

Author / Editor: Franz Frühauf, Hartmut Sassen / Ahlam Rais

To meet the growing needs of the plastics recycling sector, international technology Group Andritz has developed a compact recycling system which can achieve numerous benefits and additionally save transportation costs.

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Figure 01: Compact plastics recycling system by Andritz.
Figure 01: Compact plastics recycling system by Andritz.
(Source: Andritz)

There are very few materials that are discussed as much as plastics. With features like lightweight, durability and water resistance, plastic is still the most used material in the industry and everyday life. On the one hand it has an advantage, but on the other hand it leads to significant issues when it ends up in landfills. For instance, the long lifespan of its polymer challenges the environment with undeniable pollution problems. Also, the huge mountains of plastic waste formed on land as well as the oceans release microplastics which are beginning to find its way back into the food chain and on our tables.

Today, although plastic alternatives are heavily researched on, it cannot remove the plastic waste that already exists. Hence, it has to be managed by incineration, landfilling, recycling, or degradations. In this article we will focus on plastic recycling.

Plastic Waste as a Valuable Resource

Recycling technology providers are committed to conserve resources and create a cleaner world. With the right machinery, processes, and technologies, waste can be converted into valuable materials or energy sources. The benefits of this are many: Raw materials are made usable again (as a source of additional revenue or by reusing the materials within the production cycle), landfills and disposal sites will be plastic free, the need for additional plastic production is reduced, and the damage to the environment caused by burning of plastic waste is eliminated.

To meet the growing needs of the plastics recycling sector, international technology Group Andritz has developed a compact recycling system which can achieve the above mentioned benefits and additionally save transportation costs. The recycling system recovers plastics as a highly valuable secondary raw material. The final product can be seen in the form of plastic granules, also called as ‘second quality gray plastic’. It is suitable for the production of all non-transparent plastic products or for which small impurities do not matter like garden furniture, coloured plastic bags and all other sorts of common plastic materials. At present, the market price for one tonne of recycled plastic granulates is several hundred dollars.

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Meeting the Growing Demands in Plastics Recycling

The mentioned benefits of recycling plastic waste into gray plastic leads to a constantly growing industry and comes with increasing demands in terms of technology and quality, but also offers enormous potential. The key requirement is the highest possible degree of fraction purity of the plastic regranulate. In addition, specifications for Melt Flow Index (MFI), density, elasticity, and so on must be provided for every single fraction in order to verify the quality for potential customers.

Only pellets or regranulates that meet these stringent demands will have a future in the market. Furthermore, high sorting efficiency, low valuable product losses, highest throughputs, and optimised energy footprint are among the essential requirements in this industry. Andritz has combined the latest shredder technology from its recently launched Aduro product line with an all-in-one solution for separating, washing and dewatering – the Censor ACZ decanter centrifuge – to meet these demands.

The combination of the shredder technology and the decanter centrifuge provide a solution for compact and efficient plastic recycling (see Figure 01). This powerful pair is perfect for processing various types of plastic waste, especially from household waste collections.

High-Tech Shredding: The First Essential Step

As shown in Figure 01, the raw material collected is fed into an Aduro U-type shredder (Figure 02), an innovative single-shaft shredder that enables one-step processing of hard and soft plastics, and which is capable of processing bulky materials down to very small particle sizes in a single step. An adaptable and adjustable screening system makes an output size of less than 10 mm possible, which is ideal for a variety of applications.

The cutting component consists of an array of knives positioned both lower and higher and which overlap continually when cutting. This arrangement breaks down the incoming material uniformly and ensures a clean cut. Precise control of the pressure and speed of the input materials when they are pushed against the cutting knives by a pendulum pusher ensures optimum throughput and energy consumption.

Figure 02: The Andritz Aduro U Shredder.
Figure 02: The Andritz Aduro U Shredder.
(Source: Andritz)

The unique design of the pendulum pusher eliminates the need for complex sealing and avoids the ejection of materials commonly experienced with ordinary horizontal pushers. This contributes to a clean and safe work environment. A hydraulic service hatch between the pusher and rotor gives full accessibility to the wear parts and facilitates maintenance work.

Certain features, such as presettings for various materials, a protection clutch to avoid damage to motors during an overload situation, automatic safety lockout, and well-positioned service doors also contribute towards easy operation and maintenance. The shredder’s compact footprint and profile enable it to be easily installed and integrated into an existing plant or facility. Like all Aduro shredders, the U-type shredder can be equipped with the latest IIOT features to monitor machine behaviour and improve plant reliability.

Figure 03: Plastic waste in the form of wire cables, before (1) and after (2) shredding with an Aduro U Shredder.
Figure 03: Plastic waste in the form of wire cables, before (1) and after (2) shredding with an Aduro U Shredder.
(Source: Andritz)

Figure 03: Plastic waste in the form of wire cables, before (1) and after (2) shredding with an Aduro U Shredder.
Figure 03: Plastic waste in the form of wire cables, before (1) and after (2) shredding with an Aduro U Shredder.
(Source: Andritz)

Second Step in Plastic Recycling: Highly Efficient Separation

The Censor ACZ (Figure 04), sits at the core of the process pictured in Figure 01. It is a sorting centrifuge that separates, washes, and dewaters the shredded raw material simultaneously. The centrifuge is partly filled with a separation liquid, forming a liquid ring due to high-speed rotation. The plastics are fed into the centrifuge axially with make-up liquid (in household waste processing applications water is used) and impinge on the surface of the revolving liquid ring. Here, intense turbulence results in deagglomeration of the individual plastic particles and largely frees them from any adhering dirt. Any air bubbles on the surface of the particles are stripped off by the centrifugal force, which is especially important in view of the predominantly hydrophobic material.

All particles with a higher density than the liquid are forced outwards radially to the centrifuge bowl, while the lighter components float upwards inwardly. This occurs very quickly and with high selectivity in a centrifugal field creating forces over 1,000 times higher than in conventional processes. A screw conveyor rotates inside the centrifuge at a speed slightly different to that of the centrifuge bowl. Counter-rotating screw flights are fitted to a screw body and transport the separated fractions to each of the conical ends of the centrifuge.

Additional Information
"The Censor Series is a Specialised Product for Recycling of Plastics."

As design manager for decanters, Mr. Reichenbach, can you tell us what was the motivation to develop this decanter technology?

The Censor series is a specialised product for recycling of plastics. We developed it in order to offer our customers a means of designing the separation stage in a way that is space-saving but still has a high throughput. With this equipment, compact plants can be installed in place of large sink-float tanks and even perform better in terms of separation efficiency.

You mentioned that the product specialises in plastics recycling – what distinguishes it from conventional decanter centrifuges?

All decanters sediment or separate solids from liquid phases. In the vast majority of cases, there is a solid and a liquid phase. It is also quite common to separate a solid and two liquid phases. The special task accomplished by the Censor is to separate two solid phases and one liquid phase at the same process step. That means that one solid phase sinks while the second floats on the liquid phase. The liquid is normally circulated in this application and can possibly also be cleaned in order to remove any dirt and contaminants brought in from the overall process.

In which industry is this technology most popular and most used?

We have achieved particularly good results in recycling of carpeting. In this process, the individual layers are separated from one another after the carpets have been shredded. The individual solids components, in particular the yarn, are then returned to the raw material chain. Similar processes are also available for processing of old fishing nets or plastic film residue. In principle, the technology is very versatile – for example one of our latest references is an European company in the yarn sector who is marketing a recycled ‘green’ yarn successfully.

There they are lifted up above the liquid ring and thereby discharged after dewatering. After processing, the make-up liquid is discharged through nozzles located in the center of the centrifuge and circulated back into the mixing tank to be mixed with plastic and fed into the centrifuge again.

Finally, the light and heavy solids (fractions) are discharged separately. Whereas the heavy fraction can be further processed or sent to disposal, the light fraction is a highly valuable secondary raw material perfectly suitable for extrusion and injection molding. Depending on the type of plastics to be separated, the Censor ACZ achieves purity of more than 99.8 % with very low losses of valuable product.

Figure 04: The Andritz decanter centrifuge Censor ACZ with a diameter of 900 mm and a feed rate of up to 6,000 kg/h for highly efficient plastics recycling.
Figure 04: The Andritz decanter centrifuge Censor ACZ with a diameter of 900 mm and a feed rate of up to 6,000 kg/h for highly efficient plastics recycling.
(Source: Andritz)

The compact recycling system developed by Andritz consists of a slow-rotating, single-shaft shredder with an electric motor drive and a hydraulic pendulum pusher that processes plastics to a very small particle size of less than 10 mm and a decanter centrifuge that handles separation, washing and dewatering of shredded plastics in one machine.

The major advantage of this system over systems like swim-sink pools is the high degree of purity, and the few machines actually necessary for the system, since separation, cleaning and dewatering is all done by the centrifuge. It is suited for processing various types of plastic waste, and with centrifuge capacities going up to 6.000 kg/h per centrifuge, also applicable for high capacities. In short: The showcased process is a compact system that manages to produce high volumes of high-purity recycled plastics granulate – a valuable secondary raw material – and significantly reduces plastic waste.

* Franz Frühauf works in the Paper, Fiber and Recycling Division at Andritz in Graz, Austria and Hartmut Sassen works in the Separation Division at Andritz Separation in Cologne, Germany