Installation and process for recycling LLDPE, in a linear waste stream comprising at least pieces of LLDPE film and pieces of LDPE film

Abstract

The invention relates to a linear low-density polyethylene (LLDPE) recycling installation in a linear waste stream comprising at least pieces of LLDPE film, pieces of low-density polyethylene (LDPE) film, and possible impurities, remarkable in that it successively comprises: - an optical sorting system comprising infrared detection means coupled with color analysis means for identifying LLDPE pieces and means for isolating the identified LLDPE pieces from the polyethylene piece stream; - means for washing and drying the isolated LLDPE pieces;- means for granulating washed and dried LLDPE pieces, said granulation means comprising a combination of a single-screw extruder and a twin-screw extruder disposed at the outlet of the single-screw extruder to reduce and disperse and / or melt residual impurities in the LLDPE, and to perform a clean water cut to form the granules.

Description

Title of the invention: Installation and method for recycling LLDPE, in a linear waste stream comprising at least pieces of LLDPE film and pieces of LDPE film technical field

[0001] The present invention relates to the field of polymer recyclability and circularity, and deals in particular with the recycling of films, for example packaging films, made mainly of linear low-density polyethylene, well known by the acronym LLDPE. Prior art

[0002] It is known from the prior art to recycle post-consumer film waste consisting mainly of low-density polyethylene, known by the acronym PEBD or LDPE from the English "low density polyethylene", low grade, from, for example, thick shrink covers which are found unmixed in the glass industry, or mattresses, for example.

[0003] These materials represent only a small fraction of the polyethylene waste available on the market.

[0004] To increase the fraction of polyethylene waste to be recycled, it is possible to focus on so-called post-commercial waste from, for example, the unpacking activities of shopping centers, their platforms or logistics platforms in the broadest sense.

[0005] This waste stream occurs when unpacking pallets, or grouping products, and is subject to a specific sorting instruction which is to group all PE film streams together if they are transparent.

[0006] In practice, LLDPE and LDPE streams are mixed, sometimes with impurities such as other polymers (like PP, PET, PES), printed polyethylene films, paper, and all kinds of dirt. The best streams to date consist of 98% transparent polyethylene (LDPE / LLDPE blend in varying proportions), with the remaining 2% being impurities (either other polymers, paper, wood, or printed films). However, the vast majority of these streams actually contain 5 to 20% impurities.

[0007] The variability of this source necessitates a suitable recycling facility and process in order to separate and produce sufficiently "pure" recycled LLDPE granules, i.e., containing at least 90% LLDPE, and of sufficient quality to be used in high-material packaging recycled where the specific properties of LLDPE are sought (weldability, tear and perforation resistance, stretchability, gloss for example).

[0008] The Applicant conducted tests on pellets from batches of recycled LLDPE from various suppliers. The tested LLDPE pellets contained more impurities than recycled LDPE pellets (by a factor of 5 to 10 times). These impurities are also commonly referred to as "gels" and include paper, cellulose, polymers other than LLDPE, poorly melted LLDPE, etc.

[0009] This difference is explained by its primary source, which is stretch film (LLDPE). Indeed, the stretchability and, above all, the tacky (slightly adhesive) nature of this film results in a stronger adhesion of impurities (dust, labels, etc.), which are therefore more difficult to remove during washing and shredding in its recycling process. Furthermore, these films are much thinner than other types of films on the market (by a factor of 2 to 5), which makes the potential contamination surface area to weight ratio significantly higher than other types of film waste available on the market.

[0010] Furthermore, this stretchability also traps moisture, which is a significant factor in gel formation. Water neutralizes the antioxidants incorporated during the production of virgin LLDPE (as they are essential for the use of LLDPE). These antioxidants are not present, or are present in smaller quantities, in LDPE. This difference consequently creates new constraints during the recycling of LLDPE.

[0011] Indeed, the neutralization of antioxidants upon contact with water (i.e., during the washing phase in the recycling process) leads to a degradation of the material. This degradation is reflected in a decrease in the material's melt flow index and an increase in the number of gels. Controlling these two characteristics is crucial during the recycling of post-consumer LLDPE.

[0012] Recycled LLDPE granules must have a melt flow index comparable to that of virgin LLDPE to be used in the same applications. Since recycling LLDPE degrades the material by definition (because it involves remelting the material), optimization of the recycling process is therefore sought.

[0013] In conclusion, LLDPE presents recycling constraints such that a person skilled in the art prefers in practice not to sort LLDPE and to recycle it mixed with other PE, and does not seek to extract it for recycling "pure". Description of the invention

[0014] One of the aims of the invention is therefore to remedy the problems of the prior art by proposing an installation and a method for recycling linear low-density polyethylene, known as LLDPE, in a linear waste stream comprising at least pieces of LLDPE, pieces of low-density polyethylene film, known as LDPE, and possible impurities.

[0015] The objective of the invention is that the recycled product from the installation and process of the invention is of sufficient purity and quality, i.e. comprising at least 90% LLDPE, for incorporation into plastic packaging, particularly high-content packaging, and in particular into films with a thickness of less than 50 pm.

[0016] For this purpose, an LLDPE recycling installation has been developed, in a linear waste stream comprising at least pieces of LLDPE film, pieces of low-density polyethylene film, known as LDPE, and possible impurities.

[0017] According to the invention, the installation comprises successively: - an optical sorting system including infrared detection means coupled with color analysis means, and preferably 3D shape analysis means, to identify LLDPE pieces, and means for isolating LLDPE pieces identified from the waste stream; - means of washing and drying the isolated pieces of LLDPE; - means for granulating washed and dried LLDPE pieces in the form of a combination of a single-screw extruder and a twin-screw extruder arranged at the outlet of the single-screw extruder to reduce and disperse and / or melt residual impurities in the LLDPE, and to perform a clean water cut to form the granules.

[0018] The combination of these three technologies makes it possible to obtain recycled LLDPE granules of sufficient quality to be used in packaging with a high recycled content, in particular in plastic films with a thickness of less than 50 pm.

[0019] The use of the optical sorting system coupled with the means of isolating the identified LLDPE pieces makes it possible, firstly, to eliminate impurities, but also and above all to dissociate the LDPE from the LLDPE.

[0020] In practice, optical means make it possible to detect, among the impurities, pieces of LLDPE and LDPE.

[0021] The purity level of the LLDPE fraction is correlated with the settings chosen for the isolation means. Indeed, more or less stringent criteria can be configured for the infrared and color analysis sensors to accept more or less material during positive sorting. For example, the optical sorting system can be set to consider as LLDPE all pieces for which the probability of being LLDPE is greater than 50% or 90%. Depending on the percentage chosen, the quantity and purity of recycled LLDPE will differ. Optical sorting systems are well known on the market, so they will not be described in further detail.

[0022] Twin-screw extruders are generally used to incorporate fillers or other compounds to optimize the properties of the pellets, whether recycled or not. However, the Applicant has discovered that using a twin-screw extruder, combined with a single-screw extruder, to reduce and disperse and / or melt the last residual impurities optimizes the quality of the material after granulation.

[0023] In practice, granulating LLDPE granules with a twin-screw extruder reduces the number of gels in the material by 50 to 65%. The twin-screw extruder, preferably co-rotating, reduces not only the number of gels but, more importantly, their average size and therefore their impact. This is a considerable advantage for ensuring stability during film extrusion and, where applicable, print quality of these films that conform to the 20p to 100p resolution. Although the twin-screw extruder degrades the material more than a single-screw extruder (due to its melting, filtration, and granulation functions without significant degradation), it allows for a marked improvement in the quality of the granules produced in terms of the number of gels.

[0024] The LLDPE granules from the installation of the invention are of an optimized quality to provide specific properties, such as optical and mechanical properties for example.

[0025] Preferably, the installation comprises three optical sorting systems including infrared detection means coupled with color analysis means, and preferably 3D, to identify the LLDPE pieces and means for isolating the LLDPE pieces identified from the flow of polyethylene pieces, positioned one after the other to carry out three successive sorting steps.

[0026] From the above, the invention makes it possible to obtain the purest possible LLDPE fraction, with the lowest possible LDPE fraction.

[0027] According to a particular embodiment, the means for isolating the LLDPE include compressed air ejection nozzles for performing a positive or negative sorting of the LLDPE pieces from the polyethylene stream, and thus forming an LLDPE stream to be processed.

[0028] Advantageously, the means for washing the LLDPE pieces include a wash water circuit and a clean water rinsing circuit for the LLDPE pieces. Indeed, the Applicant has observed the influence of water quality on the presence of impurities, also called "gels," such as paper, cellulose, polymers other than LLDPE, and poorly melted LLDPE. Rinsing with clean water reduces the presence of these gels. In this case, the wash water traps suspended or dissolved impurities, which will remain after evaporation / degassing during the recycling process.

[0029] The twin-screw extruder is configured to perform underwater cutting during the granulation of LLDPE, which allows for a clean cut and regular cooling of the granules to obtain a granulometry of 25 to 35 granules / gram, which allows in subsequent extrusion processes a regular flow of material without risk of friction and therefore of additional formation of angel hair and oxidation.

[0030] The invention also relates to a method for recycling LLDPE, in a linear waste stream comprising at least pieces of LLDPE film, pieces of LDPE film, and possible impurities, the method comprising successively: - a step consisting of isolating the LLDPE pieces from the polyethylene piece stream using an optical sorting system comprising infrared detection means coupled with color analysis means and, preferably, 3D shape analysis means, to identify the LLDPE pieces and means for isolating the identified LLDPE pieces; - a washing step, a rinsing step, preferably with clean water, and a drying step for the LLDPE pieces; - a granulation step of washed and dried LLDPE with a single screw extruder followed by a twin screw extruder to reduce, disperse and / or melt residual impurities. Detailed description of the invention

[0031] The invention relates to an installation and a method for recycling LLDPE.

[0032] Recycling is carried out in particular from a stream of waste cut into pieces and conveyed by a conveyor in front of a first optical sorting station.

[0033] One way to optimize the purity obtained is to implement a succession of guillotine cuts. In particular, the linear waste stream comprising at least pieces of LLDPE film, and pieces of LDPE film is conveyed in one direction and cut with a guillotine, with a pitch of approximately 10-15 cm for example, then at the outlet, the waste stream is deposited on a conveyor moving perpendicularly and subjected to a new guillotine cut, with an equivalent pitch, so as to obtain pieces of waste of 10-15 cm by 10-15 cm.

[0034] Thus the size of the waste is controlled, which makes it possible to optimize the subsequent optical sorting stages since the waste is almost uniform, unlike cutting technologies by grinding or shredding which create a distribution of very varied sizes, complicating optical sorting all the more.

[0035] The waste stream to be recycled includes pieces of LLDPE and LDPE films possibly mixed with impurities such as other polymers (type PP, PET, PES), printed polyethylene films, paper, dirt of all kinds.

[0036] The first optical sorting station has an optical sorting system which includes infrared detectors and color analysis means, and preferably 3D shape detectors.

[0037] In practice, color analysis is calibrated to detect, among impurities, pieces of LLDPE and LDPE.

[0038] These detectors thus make it possible to identify LLDPE pieces among other waste. The LLDPE pieces identified by the optical sorting system are isolated from the waste stream by means of compressed air ejection nozzles, used to perform positive sorting of the LLDPE. These nozzles selectively direct the LLDPE pieces to a separate path, thus isolating them from the rest of the waste stream. This sorting process is fast and accurate, ensuring that only the LLDPE pieces are selected for further processing.

[0039] In practice, several successive sorting steps are carried out. In a first sorting step, the impurities: non-PE, printed PE, PE colored in mass, are ejected from the flow by the compressed air ejection nozzles.

[0040] In successive second and third sorting stages, the nozzles selectively direct the LLDPE pieces to a separate path, thus isolating them from the rest of the waste stream.

[0041] Of course it is possible to modulate the negative and positive sorting, without going out of the scope of the invention.

[0042] The remaining negative flux from the second and third stages consists mainly of LDPE, a residue of LLDPE and a residue of impurities.

[0043] Having two successive positive sorting processes of LLDPE allows, on the one hand, for the recovery of a significant fraction of LLDPE, up to 90%, and, by contrast, makes the remaining LDPE stream even more "pure," allowing it to be further recycled. The different isolated LLDPE fractions are then grouped together before the washing stage.

[0044] The grouped LLDPE fractions are advantageously shredded, and any remaining metallic impurities are separated from the stream. The grouped LLDPE pieces are then conveyed, for example, by a conveyor or an air handling system, to a washing station. This washing station includes a wash water circuit to remove surface contaminants from the LLDPE pieces and a clean water rinsing circuit for the LLDPE pieces.

[0045] For example, the LLDPE pieces are passed through a water tank with rotating blades to perform washing and gravity separation of heavy impurities. The LLDPE pieces are then conveyed to a grinder into which clean water is injected.

[0046] The washed LLDPE pieces are then dried, for example in a centrifuge, and / or a compression drying system, and / or with a hot air flow to remove residual water from the LLDPE pieces. When combined, these technologies make it possible to extract most of the water trapped in the agglomerated LLDPE "balls".

[0047] In practice, rinsing water is also injected at the centrifuge inlet to complete the rinsing process. This is counterintuitive, as those skilled in the art would not consider wetting the LLDPE before drying it, particularly for energy-saving reasons. However, this step further improves the purity achieved by reducing the presence of gels. The wash water traps suspended or dissolved impurities, which will remain after evaporation / degassing during the recycling process. Rinsing with clean water in the centrifuge helps to remedy this.

[0048] Once the LLDPE pieces have been washed and dried, they are directed to a granulation station. This granulation station includes a first single-screw extruder, which melts, homogenizes and filters the main impurities from the LLDPE stream.

[0049] After this initial filtration, the molten LLDPE is, at the outlet of the single-screw extruder, fed into a second twin-screw extruder configured to disperse and / or melt any residual impurities present in the LLDPE, and also configured to perform a clean water cut to form the granules as such.

[0050] The single-screw extruder is equipped with a filter with a size of approximately 90 µm. The larger the filter size, the more unwanted gels it allows to pass through. A filter that is too fine will increase the material's residence time in the extrusion section, thus leading to an increase in the number of gels. Indeed, the longer the material remains in the extrusion screw, the more it will be heated and sheared, resulting in its degradation and consequently a greater number of gels.

[0051] The LLDPE granules obtained can then be used as raw material for the manufacture of new recycled polyethylene products, in particular for high-level incorporation into plastic packaging, and in particular into films with a thickness of less than 50 pm.

[0052] In summary, the LLDPE recycling plant described in the invention combines advanced optical sorting, washing, drying, and granulation techniques with a combination of single-screw and twin-screw extruders to efficiently recover LLDPE from a waste stream while removing impurities. This approach allows for to valorize recycled LLDPE, in a logic of upcycling and reuse at the same level of use and value, while contributing to the reduction of plastic waste.

Claims

Demands

1. Installation for recycling linear low-density polyethylene, known as LLDPE, in a linear waste stream comprising at least pieces of LLDPE film, pieces of low-density polyethylene film, known as LDPE, and possible impurities characterized in that it comprises successively: - an optical sorting system comprising infrared detection means coupled with color analysis means for identifying LLDPE pieces and means for isolating the identified LLDPE pieces from the polyethylene piece stream; - means for washing and drying the isolated LLDPE pieces;- means for granulating washed and dried LLDPE pieces, said granulation means comprising a combination of a single-screw extruder and a twin-screw extruder disposed at the outlet of the single-screw extruder to reduce and disperse and / or melt residual impurities in the LLDPE, and to perform a clean water cut to form the granules.

2. Installation claim 1, characterized in that it comprises three optical sorting systems positioned one after the other to carry out three successive sorting steps.

3. Installation of one of the preceding claims, characterized in that the means for isolating the LLDPE include compressed air ejection nozzles for performing positive or negative sorting of the LLDPE pieces from the polyethylene stream.

4. Installation according to any one of the preceding claims, characterized in that the means for washing the pieces of LLDPE comprise a washing water circuit and a circuit for rinsing the pieces of LLDPE with clean water.

5. A process for recycling linear low-density polyethylene (LLDPE) in a linear waste stream comprising at least pieces of LLDPE film, pieces of low-density polyethylene (LDPE) film, and any impurities, the process is characterized in that it successively comprises: - a step of isolating the LLDPE pieces from the polyethylene stream using an optical sorting system comprising infrared detection means coupled to

6. means of color analysis to identify pieces of LLDPE and means of isolating the identified pieces of LLDPE; - a washing step, a rinsing step, and a drying step for the LLDPE pieces; - a granulation step of washed LLDPE with a single-screw extruder followed by a twin-screw extruder to reduce, disperse and / or melt residual impurities. A process according to claim 5, characterized in that the LLDPE rinsing step is carried out with clean water.