Sorting plastic waste

Sorting, the first step in recycling plastic waste after collection, separates, cleans and prepares materials intended for recycling. Depending on its nature, sorted plastic waste will be processed directly on the site of the recycling company responsible for receiving it or be taken to a specialist recycling centre. Since there are many categories of materials and many processes, varying according to the type of product, we are going to identify the main methods used by Paprec, follow the route taken by materials in recycling centres and detail the treatment methods specific to the various types of plastic waste.

MANUAL SORTING

In many cases, and for a large number of materials, after the material has been checked for conformity, this stage allows an initial waste sorting operation to be carried out, thus facilitating future automatic processing. So, things such as defective parts, scrap and off-cuts from industry, various polypropylene (PP), polystyrene (PS) and polyethylene (PE) products, collected post-consumption, are placed on belts and dismantled by operators who remove the coarsest pollutants (for example, rubber, screws and rivets from waste bin bags) and carry out an initial chromatic separation of materials. The same is true for films, for example, which are divided into two flows: coloured and natural, which are removed by hand and redirected to grinding machines. In some cases, simple visual verification is required before sending products to the different recycling lines.

 

  • For PET (plastic bottles and tops), Paprec, a leader in plastic recycling in France, has installed fully automated waste sorting systems. After the unloading of the bales resulting from selective sorting, waste is placed on a conveyor and sent to a machine that splits the bales. Once free from their binding, the bottles are pushed by blades on a toothed shaft, which separates them, facilitating their routing to conveyor belts. There are two successive magnets, or Overbands, which remove any pollution (extracting any ferrous wire that might be present).

 

  • With regard to PVC (mainly industrial plastic waste, joinery mouldings from doors and windows), manual waste sorting consists of removing materials thrown randomly into skips and bins by customers and separating the various colours at the entrance to the grinders, the aim being to isolate and process as much white plastic waste as possible. After grinding, which we will talk about later, the plastic goes to a hopper or silo, before being sent to a stage where metals are removed by Overband. Thus, metal parts (such as aluminium particles) are captured and separated from PVC granules.

POST-GRINDING WASTE SORTING

Grinding (or reducing products to flakes) is an essential phase in the recycling process for various types of plastic waste and most often takes place downstream of the sorting phase. Polypropylene products, for example, are fully sorted before being ground. But when processing PVC waste, Paprec grinds the material before sending it to the optical sorting machines. A large cylinder fitted with knives turns the materials at different speeds, depending on the nature of the product and chops it against a fixed part fitted with contra-rotating blades. A perforated grille in the bottom of the machine allows granules reduced to the desired size (no larger than 10 mm) to fall through. Dust removal systems also allow the fine fraction (smaller than 1 mm) to be extracted, thus improving the quality of the soon-to-be-regenerated product.

OPTICAL WASTE SORTING

Used today by Paprec Group to process flows of PVC or PET household plastic waste, optical sorting is fully automated. It enables separation and pollution removal to be refined, optimising the plastic waste recycling process and, via capillary action, ensuring a higher final quality.
 

  • For PVC, this first waste recovery phase is divided into two stages. The recently ground material, with any metal removed, is first placed on a conveyor belt, taken to a suction system and finally sent to a hopper whose function is to feed the optical waste sorting machine's hermetic chamber. Cameras, set to run a target program, analyse the spectrum of the material spread across a grid. In this way, non-conforming material fragments (such as door or window seals) are identified and then blown out by a system of air nozzles. Two special cameras are also responsible for checking all sides of the granules to identify any coloured materials, which, once detected, will be ejected into a dedicated channel. The conforming material is redirected into a second channel, recovered, put in big-bags (handled by forklift trucks) and stored in silos, so that it can be homogenised prior to being extruded. In some cases, the material is automatically sent to extrusion machines by suction or mechanical distribution.

 

  • As for PET bottles that have had any pollution removed, these are first sent to a pre-washing cylinder, the purpose of which is to remove labels from the flow of bottles, using cold water. They then all move to a double bouncing screen (two sloping vibrating belts that typically separate hollow bodies from flat bodies), the labels are blown to a waste storage area and the recyclable waste plastic bottles are then routed to three optical sensors that identify the colour and nature of the material. Undesirable fractions are then sent through again to provide the best possible sort. We should also note that this automated plastic sorting phase on whole bottles is fully controlled by Paprec Group operators.

FLOTATION WASTE SORTING

Another automatic waste treatment process, sorting by flotation, can also be considered as a first-stage washing of materials. Let's look again at PET plastic bottles: once they have had any coloured or undesirable materials removed, the flow of bottles is split in two to feed underwater grinders, which then reduce the bottles to flakes. They are then mixed with water to facilitate transport to the flotation separation phase.
 

 
The ground material is immersed in a large tank filled with a static liquid (a mixture of water and various additives, such as chlorine and calcium), which separates the PET (which sinks because it has a specific gravity greater than 1) from the plastic tops (which float because they have a specific gravity of less than 1). We should note that for this type of waste sorting to work, recyclable waste must be ground and reduced to a size less than 10 mm but also that the difference in density between the materials to be separated must be at least 0.2 g per cm3.
 
 

This sorting method is also used for a large number of plastic materials other than PET. Thus, for example, products in the mixture made of polyethylene, polypropylene or other miscellaneous ground plastics have many of the pollutants they might contain (such as metal particles) removed and are thus purified to make their future regeneration easier.

DENSITIMETRIC WASTE SORTING

To improve the recovery of plastic waste, Paprec also sorts several types of bulk waste (such as mixtures of polypropylene and polyamide collected from industrial companies) using densimetric tables in its plastic recycling factories, the purpose of which is to sort the plastic by separating the heavy fraction of materials from the light fraction by means of a dry method. They are placed on a plate (vibrating, inclined and fitted with fans) which sucks up the lighter products and evacuates the heavy fraction in contact with the plate.

WASHING

Today, sources of top-quality materials are drying up. To recover new quantities and innovate in the field of waste treatment, Paprec has installed a washing line intended to purify rigid polyethylene, polypropylene, polystyrene and PVC products (for example, post-industrial waste bins at the end of their life, stretch films, worn-out big-bags, car bumpers and various post-consumption plastics such as physiological saline solution vials). This machine, a combination of several types of equipment (a shredder, grinder and centrifuge) specially designed to improve the recycling of plastic, operates in the following way :
 

  • At the head of the line, a shredder transforms plastic waste into fractions, which are then routed to 10 m3. washing tanks filled with cold water containing no additives. The aim is then to strip out unwanted products, specifically adhesive tape, labels and dust, or solid pollutants such as sand and stones. After immersion, the materials are removed to a horizontal centrifuge where the friction between pieces of product removes impurities or, more accurately expels them through a grid with 3 mm holes. Then, only the structure of the plastic remains and this is sent to an underwater grinding phase. Once that stage is complete, the material goes to a second centrifuge, a vertical one this time, where the friction from knives removes any final residues. The products are then spun and dried by compression and are then ready for shredding, grinding again and baling (or being put into big-bags).

 

  • At Paprec, an automatic washing line is also used to refine plastic bottle recycling and, more particularly, waste sorting and the removal of pollution from PET. Actually, after being ground under water, the plastic bottle flakes for recycling are removed from the liquid tank by a screw whose function is to take them to a friction washing cylinder. Like in a washing machine or in the washing line detailed above, the flakes rub against each other to remove impurities. Another screw then pours the PET into a rinsing tank identical to the pre-washing one (see section on plastic sorting by flotation), but one which uses hot water; the purpose of this is to complete the purification of the product. The rinsed PET is then spun in a centrifuge and hot-air-dried. Once dried, the flakes are then sent to a vibrating screen, to arrive at a final particle size. At that stage, the flakes meeting the specification are pneumatically transferred to the part of the line called conditioning, another phase in waste treatment.

 
The plastic recycling cycle is a step-by-step process. Discover the different steps :
 

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