Regeneration, micronisation and grinding plastics
There are three ways of conditioning or packing recently sorted plastic waste. Depending on their nature, their final intended use and particular industrial requirements applying to them, products will experience quite different processing in recycling factories before be resold. The time has come, therefore, to look at the various final operations in the transformation of plastic materials, as done at Paprec.
Regeneration, also known as extrusion or granulation, is a plastic recovery process that enables recycling companies to produce top quality granules. Treated for a long time, purified, homogenised and ready to be mixed with virgin plastic granules or used as they are, this secondary raw material (or compound) offers the highest possible level of quality and is therefore the most demanding product created in Paprec recycling factories.
Although machines and the method of controlling them change with the treatment capabilities of various sites and their specialisations, or due to the nature of the plastics treated, the general principle remains the same: ground material is placed in a plastics extruder (i.e. in a heated duct fitted with a screw, filtration systems and mechanical knives) to be softened again, have any residual pollutants removed by aspirating any gases released by the effect of heat, conditioned in the form of long dough-like segments, cooled and then cut by knives to get back to the structure of a virgin granule. Once polycondensed and resold, it can start a second life. It is worth pointing out that similar extrusion machines are used in the manufacture of plastics.
At Paprec, regeneration is used to recycle polystyrene, PVC, PET, polyethylene and polypropylene. And to gain a better understanding of how to recycle plastics by using this recycling technique, we should look more closely at the PET plastic bottle recycling process on a latest-generation line :
Having been sorted, reduced in a plastic grinder, washed and dried, the PET flakes are sent to two homogenisation silos, where they are continually agitated as material is added. At the end of the line is an aerodynamic separator that separates materials according to their size/weight. At the exit from the separator, the final refining stage, the flakes are sorted by infra-red cameras and undesirable items are removed by blowing. The selected material is then sent to four quality-control silos where a representative sample is taken of the average quality in the silo. Depending on the result, the flakes will be stored in big-bags or in an intermediate silo to be fed into the extruder.
But before they are actually extruded, plastics to be recycled are first preheated in a dedicated machine, then taken by a second mechanical feed screw to two hoppers for hot-air-drying. Then the plastic is deposited at the entrance to the extruder machine's screw, which, by turning against the walls of a duct heated by electrical elements, softens it considerably. The PET dough continues its path along the screw, where any impurities are released (all of which are converted into gases) by pump systems attached to the extruder.
Then come the phases where the material is extruded and cut into plastic rods (or monofilaments) and water-cooled before being filtered and granulated using pre-calibrated grids to produce the size of product required. Once conditioned, it will be stored in an intermediate silo for quality control, carried out by laboratories that, using special small extrusion machines, are able to simulate on a small scale plastics manufacturing methods similar to those that will be used by the end-user of the recycled plastic materials. In this way, the goods are checked to ensure conformity (colour, grade, density, tensile modulus, degree of pollution, impact-resistance, etc.).
After they have spent a certain time in the reactor, the granules are evacuated and again sent to intermediate silos. Once the silos are full, the material is stored in big-bags. It should also be pointed out that, using computer systems developed internally, Paprec ensures the that its products are traceable from the arrival of waste in the factory until the recycled materials are sold, with the various waste treatment stages being entered in a database.
We should note that, where waste management and, more specifically, the extrusion of thermoplastics, are concerned, Paprec, in partnership with a specialist company, has succeeded in innovating and developing a unique extrusion machine capable of treating packaging and other heavily inked films.
The micronisation of plastics consists in pulverising the flakes until an extremely fine powder is obtained. Of a quality lower than that found in regenerated products (since impurities have not been removed and the plastics have simply reduced to a minimum size) but still useful and costing less to produce, it is still greatly prized by the plastics industry and often mixed with top-quality materials to manufacture multi-layer products by plastic rotary moulding (a combination of pure material for the upper layers and micronised binders for the intermediate parts).
At Paprec, it is PVC and PET that are most often treated in this way. Mixtures of the ground material to be micronised are first loaded into silos to be homogenised before being conveyed pneumatically to micronisers. These two-plate machines (one fixed and the other moving) blow the flakes onto the walls by centrifugal force before a rotor smashes the edges off the material. Then, during the final powder micronisation stage, it is sieved down to the desired particle size (often less than 300 microns in diameter) and stored in tanks.
In some cases, and for some identified product categories (such as polycarbonates and methyl polymethacrylate), Paprec sells the plastic waste, simply ground and washed, to industrial companies, which then incorporate it as it is into their plastic parts manufacturing systems, or sell it to compounding specialists who turn the tonnages supplied into top-quality granules. For certain generic products (PE, PP and polystyrene), quantities of material may be crushed in a grinder on a given site and then sent in big-bags to a second group recycling factory, capable of regenerating or micronising them.
As with other processes, the grinding method changes, depending on the type of plastic. PVC, for example, is dry-crushed in a screened grinding machine that has a cylinder and two knives. This latest-generation plastics grinder can process four tonnes an hour, grind materials heavily loaded with glass fibres and extract the fine fraction of treated materials by means of a dust removal system. But Paprec Group also uses other methods, such as grinding under water which, unlike a knife grinder, also allows materials such as PET to be washed.
The plastic recycling cycle is a step-by-step process. Discover the different steps :
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