Method for recycling polyester containers

Abstract

The invention relates to a method for recycling polyester containers, in particular PET containers, said method comprising the following method steps: (a) sorting the containers, (b) comminuting the containers to produce flakes, (c) friction washing the flakes, (d) sorting the flakes, and (e) extruding and granulating the cleaned flakes. The flake-sorting process (d) is used to separate flakes which have a foreign polymer that is different from the polyester, and the flake-sorting process (d) is a combination of a colour-sorting process (d1), a screening step (d2), and a polymer-sorting process (d3, d4).

Description

[0001] Process for recycling polyester containers

[0002] Field of the invention

[0003] The invention relates to a method for recycling polyester containers according to the preamble of claim 1.

[0004] State of the art

[0005] PET is one of the most widely used plastics in the world, and recycling this type of plastic is particularly important for the use of existing resources.

[0006] Some sources of PET recycling streams contain non-PET components (e.g., PVC, PS, PA, PE, PP), which is due to regional markets and their production and waste treatment infrastructures. Different packaging designs are the result of specific visual or technical requirements imposed by production and / or customers. For example, the materials used for bottles, sleeves, and closures can lead to such non-PET contamination and non-bottle applications, some of which may be present in the incoming bale material.

[0007] These non-PET components significantly impair the quality of the PET flakes extruded to produce rPET bottles and containers. However, to date, it has not been possible to improve the detection devices in a PET recycling stream from a mixed collection to such an extent that the chlorine content in the cleaned and sorted stream prior to extrusion is significantly below a concentration of 30 ppm. The chlorine content is typically determined using X-ray fluorescence analysis.

[0008] Object of the invention

[0009] The disadvantages of the described prior art give rise to the task of improving the process for recycling polyester containers (especially PET bottles and PET trays) in such a way that as much non-polyester, especially PVC, as possible is removed from the recycling stream, thus enabling the use of the resulting recycled polyester in the food sector. Description

[0010] The stated problem is solved in a process for recycling polyester containers, in particular PET containers, by the features stated in the characterizing portion of patent claim 1. Further developments and / or advantageous embodiments are the subject of the dependent patent claims.

[0011] The invention is preferably characterized in that the flake sorting step d serves to separate flakes that contain a foreign polymer different from polyester, and the flake sorting step d is a combination of a color sorting step d1, a screening step d2, and a polymer sorting step d3, d4. This triple combination for sorting out flake contamination allows, in particular, flakes containing PVC to be sorted out from the flake stream. These are particularly undesirable in the flakes before they are extruded into pellets, since the pellets cannot meet the requirements for food packaging if they contain PVC.

[0012] In a particularly preferred embodiment of the invention, at least a first and second sieve fraction of flakes are created in the sieving step. Especially in the second sieve fraction, which has small flakes (smaller than the limiting particle size of the sieve arrangement), PVC and chlorine accumulate particularly high levels, which can be removed as completely as possible from the sieve fractions. The high proportion of PVC flakes in the fine fraction is due to the fact that PVC is more brittle than PET and is significantly reduced in size during friction washing.

[0013] It is preferred if the first screen fraction comprises flakes with a grain size > 1 mm, and the second screen fraction comprises flakes with a grain size < x mm. This produces fractions that can be treated differently after screening step d2 to remove as much contaminant, especially PVC, as possible from the flake stream.

[0014] In a further preferred embodiment of the invention, a third screen fraction is created in screening step d2, which comprises flakes with a grain size of < 1 mm, thus resulting in a second screen fraction with a grain size between 1 and x mm. This creates an ultrafine fraction with a particularly high concentration of foreign polymers, particularly PVC. This fraction can be recycled or disposed of without further sorting steps. The limiting grain size is expediently x=5 mm, preferably 4 mm, and particularly preferably x=3 mm. This results in fractions that can be subjected to different polymer sorting types d3, d4 in order to achieve a maximum sorting rate for contaminants.

[0015] It has proven useful to perform polymer sorting (d3, d4) directly after the screening step (d2). This allows the second and third screening fractions to be immediately freed of contamination before further processing steps are carried out.

[0016] In another particularly preferred embodiment, polymer sorting d3, by which foreign polymers are sorted out in the second screen fraction, is a non-optical polymer sorting process, in particular an electrostatic polymer sorting process. This is because PVC accumulates more strongly in the second fraction and optical sorting reaches its resolution limits at this grain size. Electrostatic polymer sorting therefore significantly increases the degree of purification of the flake stream. Centrifuges, air classifiers, etc. are also conceivable for non-optical polymer sorting.

[0017] The polymer sorting d4, which removes foreign polymers in the first sieve fraction, is preferably an optical polymer sorting process. For these grain sizes, especially those above 3 mm, optical sorting works reliably. Near-infrared or laser technology is preferably used for optical detection.

[0018] Color sorting (d1) is preferably performed before screening step (d2). Optical color sorting offers higher resolution rates and can therefore be used more flexibly. For contamination in all grain sizes, the material stream can be color-sorted in advance to simplify the process technology and reduce the number of machines required.

[0019] According to a further embodiment of the invention, it is also conceivable for the color sorting d1 to take place after the polymer sorting d3, d4. This arrangement is particularly useful when a grain class is disposed of directly and color contaminants occur in this specific grain class.

[0020] It proves advantageous if the color sorting d1 takes place after the first and second screen fractions have been mixed. This allows the color sorting d1 to be carried out as the final step of the flake sorting d, before the flakes are extruded or temporarily stored prior to extrusion. The invention is also preferably characterized in that the first and second screen fractions are sorted separately in a first and second color sorting d11, d12. This has the advantage that the first and second color sorting d11 and d12 can be carried out particularly accurately, since two screen fractions are color-sorted separately. Accordingly, it is also possible to adapt the first and second color sorting to the respective flake sizes of the first and second screen fractions and thus optimize color recognition.

[0021] In a further preferred embodiment of the invention, the cleaned flakes of the first and second screen fractions are stored in a flake storage unit f in a defined ratio. This allows the flake composition required for the production of the rPET pellets to be stored and fed to the extrusion unit e at any time.

[0022] In a further preferred embodiment of the invention, the cleaned flakes of the first and second screening fractions are partially temporarily stored in a first and second buffer storage unit g1, g2, and the temporarily stored flakes are fed to the flake storage unit f in a defined ratio. The first and second screening fractions can therefore be stored separately before being mixed. It is also conceivable that the flakes of the first and second screening fractions are not mixed and are further processed separately.

[0023] It is preferred if the foreign polymer is PVC. PVC is a predominant contaminant, especially in mixed collections, and is particularly problematic, since the rPET produced with too high a PVC content cannot be used for the production of food containers. This process allows the production of rPET granules with such purity that the quality requirements for use in the food industry are reliably met.

[0024] In a further particularly preferred embodiment of the invention, the third screen fraction is sent for disposal h. The term disposal should also be understood to include other utilization, e.g. thermal utilization or chemical recycling. In this third fine screen fraction with flakes smaller than 1 mm (PET fines / PET dust), the PVC content is particularly high and up to 90 times higher than in the other screen fractions. This is because PVC is more brittle than PET and is very extensively shredded during friction washing. The utilization of the third screen fraction is therefore extremely efficient in order to obtain the cleanest possible flake stream. It has proven advantageous if friction washing c is carried out at temperatures > 55 °C. Flakes from collected PET bottles are reliably freed from residual bottle contents and other contaminants during hot washing.

[0025] If the containers to be recycled are PET trays, the flakes must be subjected to a “cold wash” with a washing temperature below 55°C to prevent the flakes from becoming even more finely divided at higher washing temperatures.

[0026] In a further preferred embodiment of the invention, electrostatic polymer sorting d3 is carried out by charging the polyester flakes and foreign polymer flakes to different degrees and separating them into different flake streams in a high-voltage electric field. In combination with the screening step, this achieves extremely precise separation of foreign polymers.

[0027] Further advantages and features will become apparent from the following description of three exemplary embodiments of the invention with reference to the schematic representations. They show, not to scale:

[0028] Figure 1: a first flow diagram showing a first embodiment of a method for recycling PET containers;

[0029] Figure 2: a second flow diagram showing a second embodiment of the process for recycling PET containers

[0030] Figure 3: a third flow diagram showing a third embodiment of the process for recycling PET containers and

[0031] Figure 4: a functional diagram of electrostatic polymer sorting.

[0032] Figures 1 to 3 show three embodiments of an improved process for recycling PET containers. PET containers represent the largest proportion for the application of this process, although the process is also suitable for containers made of other polyesters. The containers are in particular PET bottles or PET trays. The process steps a to c are basically known. In step a, the PET bottles or PET trays fed into the recycling collection are sorted. This is done using color sorting and near-infrared (NIR) technology. Before the bottles are sorted, a process step for removing metals and labels can also be included. The PET bottles can also be pre-cleaned. In step b, the PET bottles are shredded into flakes, in particular by grinding them in a mill.In step c, the flakes are washed in a friction wash, with a washing temperature exceeding 55°C for efficient cleaning. If PET trays are involved, washing step c must be performed in a "cold wash" at a washing temperature below 55°C, because higher washing temperatures undesirably cause the flakes from PET trays to become even more finely divided.

[0033] Typical flake sizes for PET after the mill are in the target range of 4–12 mm, depending on the mill used. However, during the washing process (c), high friction is generated between the flakes, causing them to break down into finer fractions. It is noticeable that PVC, in particular, is very brittle and accumulates strongly in the < 4 mm range, and especially in the < 1 mm range. The chlorine content in flakes < 1 mm (PET fines / PET dust) is up to 90 times higher, but at least 10 times higher, than in "standard" PET flakes. Therefore, the flakes are sieved, as described below, to obtain different sieve fractions.

[0034] Flake sorting d is carried out in a combination of several flake separation steps, namely color sorting d1, sieving or a sieving step d2 and polymer sorting d3, d4. This allows foreign polymer flakes (unwanted contaminants in the flake stream) that differ from polyesters, especially PET, to be removed from the flake stream to a greater extent than was previously possible with state-of-the-art separation methods. PVC, in particular, is considered to be a foreign polymer that must be removed from the flakes to the greatest possible extent. Only by removing the PVC-containing flakes as completely as possible can the recycled PET flakes be processed to produce containers for filling food.

[0035] Color sorting (d1) is typically performed using color cameras, sometimes in combination with near-infrared (NIR) imaging, and usually takes place in a specially designed sorting system. Color sorting (d1) eliminates flakes that could impair the desired color of the containers made from the recycled granulate. The ground, (hot-)washed, and color-sorted flakes are separated into their size composition using a mechanically driven screen or two screens in the d2 screening step to ensure the best possible further processing for all flake sizes. The initial fraction is divided into a first, second, and third fraction:

[0036] 1. > x mm fraction

[0037] 2. 1 - x mm fraction

[0038] 3. < 1 mm fraction where the limiting grain size is x 5mm, preferably 4mm and particularly preferably 3 mm.

[0039] The third fraction < 1 mm is considered a side stream for alternative recycling and disposed of accordingly if the foreign polymer is PVC (disposal h), since PVC is particularly concentrated as a contaminant in the third fraction. This fraction is therefore not considered for further processing.

[0040] The second fraction, 1 - x mm, is handled separately and further purified using electrostatic polymer sorting (d3). This purification step cannot be performed with optical systems, as they reach their resolution limits. Polymer sorting (d3) can remove a large portion of the contaminants in the second fraction. It is also conceivable that polymer sorting (d3) is a density separation, or that electrostatic separation is supplemented by density separation. This allows the fine PVC contaminants to be removed from the second fraction. Density separators can be air classifiers, dust collectors, or hydrocyclones.

[0041] The fraction > x mm is handled separately and further purified using optical polymer sorting (d4). For flakes larger than the limit size x, contaminants, especially PVC flakes, can be detected using NIR or laser detection technologies and removed via compressed air discharge. Color sorting (d1), screening step (d2), and polymer sorting (d3, d4) are performed by separate machines as described above.

[0042] Before extrusion (e), the flakes must undergo flake sorting (d1, d2, and d3 or d4) to ensure maximum removal of contaminants, especially PVC flakes. The flakes cleaned in this way meet the quality requirements for use in the food industry. The PVC content or various chemical elements in the flake stream can be detected using X-ray fluorescence analysis (XRF). For PVC contamination, XRF can be used to determine the chlorine content, thus indirectly determining the PVC content.

[0043] Common contamination levels on the market are as follows: rPET from the deposit stream (DE): 15 - 25 ppm chlorine rPET from the mixed collection: > 30 ppm chlorine (usually 30 - 70 ppm chlorine)

[0044] Common recycling processes: rPET from mixed collection without novel flake sorting: > 30 ppm chlorine

[0045] The following values ​​can be achieved with this process: rPET from mixed collection including innovative flake sorting: < 25 ppm chlorine.

[0046] As can be seen from Figures 1 to 3, polymer sorting d3 or d4 takes place directly after screening step d2. According to the first embodiment, as shown in Figure 1, color sorting d1 takes place before screening step d2. However, color sorting d1 can also take place after polymer sorting d3, d4 (Figure 2). Color sorting d1 can take place after mixing the first and second screening fractions (Figure 2, 2nd embodiment), or the first and second screening fractions are each subjected to a first and second color sorting (d11, d12) separately before being mixed (Figure 3, 3rd embodiment).

[0047] The first and second screen fractions can be stored in a flake storage unit (f) in a defined mixing ratio and retrieved for extrusion into pellets in the predetermined mixing ratio. To achieve the mixing ratio, the flake storage unit (f) is filled with defined mass flows of the first and second screen fractions.

[0048] The cleaned flakes from the first and second screen fractions can be temporarily stored in a first and second buffer storage unit (g1, g2) by diverting side streams. The temporarily stored flakes are fed to the flake storage unit f in a defined ratio. This allows the first and second screen fractions to be temporarily stored separately.

[0049] Examples of sieve fraction ratios are given in the 3 tables below for PET bottles:

[0050] Table 1 : Distribution of flakes in the three sieve fractions before sorting d

[0051] Table 2: Sorting losses of the flakes in the three sieve fractions

[0052] Table 3: Distribution of flakes in the three sieve fractions after sorting d

[0053] Examples of sieve fraction ratios are given in the 3 tables below for PET trays:

[0054] Table 4: Distribution of flakes in the three sieve fractions before sorting d

[0055] Table 5: Sorting losses of the flakes in the three sieve fractions.

[0056] Table 6: Distribution of flakes in the three sieve fractions after sorting d

[0057] Figure 4 shows a functional diagram of the electrostatic polymer sorting system d3. In combination with the preceding screening step d2, the electrostatic sorting system d3 contributes to a particularly high separation efficiency for foreign polymers. The electrostatic separation device is designated overall by reference numeral 11.

[0058] During electrostatic sorting (d3), the mixture of PET and non-PET is fed via a feeder 13 to a vibrating chute 15. The vibrating chute 15 is vibrated by a vibrating motor 17. The vibrating chute 15 acts as a charging unit (vibrating chute) that electrically charges the flake stream. The electrical charge is generated by friction between the various flakes, which is provoked in a very small space during vibration. Due to the friction, charges / electrons near the surface are exchanged, and the PET becomes partially positively charged, while the PVC becomes more negatively charged.

[0059] This different charge subsequently becomes the sorting criterion. For sorting, an external high-voltage electric field is applied that attracts the PET particles and repels the PVC particles (or vice versa, depending on the polarity of the voltage field). For this purpose, the charged flake stream is passed over a belt 19, for example, between a positive electrode 21 and a neutral electrode 23. In the example shown, the high-voltage field is therefore applied externally using a rotating roller that is positively charged and forms the positive electrode 21. The flake stream is split by the negatively charged PVC flakes 25 being attracted to the positive electrode (cathode) 21 and the positively charged PET flakes 27 being repelled by the cathode. If even more positively charged PET-G (glycol-modified PET) 29 is present in the flake stream, it is repelled even more strongly by the cathode.For this purpose, two or three flake streams can be formed, which are spatially separated by partition walls 31.

[0060] Based on the triboelectric series shown in Table 7, it is also possible to estimate which other polymers this process is suitable for. The greater the difference between two polymers in the diagram above, the better their separation is possible. As a positive side effect, PA, for example, can also be separated in the present process, as it has a much higher positive charge capacity than PET.

[0061] PUR Polyurethan

[0062] PMMA polymethyl methacrylate

[0063] PC polycarbonate

[0064] PA polyamide

[0065] ABS Acrylonitrile Butadiene Styrene

[0066] PS polystyrene

[0067] PE Polyethylen

[0068] PP Polypropylen

[0069] PET Polyethylene terephthalate

[0070] RUC chlorinated rubber

[0071] PVDC Polyvinylidene chloride

[0072] PVC Polyvinyl chloride

[0073] PTFE Polytatrafluoroethylene , — ,

[0074] Table 7: Triboelectric series of different polymers Key: a Container sorting b Crushing of the containers to flakes c Friction washing of the flakes d Flake sorting d1 Color sorting d11 First color sorting d12 Second color sorting d2 Sieving step d3 Polymer sorting of the second sieve fraction d4 Polymer sorting of the first sieve fraction e Extrusion f Flake storage gl First intermediate storage g2 Second intermediate storage h Disposal

[0075] 11 Electrostatic separator

[0076] 13 feeders

[0077] 15 vibrating trough

[0078] 17 Vibration motor

[0079] 19 volumes

[0080] 21 Positive electrode

[0081] 23 Negative electrode

[0082] 25 PVC flakes

[0083] 27 PET flakes

[0084] 29 PET-G flakes

[0085] 31 partition walls

Claims

1. A process for recycling polyester containers, in particular PET containers, comprising the following process steps (a) Container sorting, (b) crushing the containers into flakes, (c) Friction washing of the flakes, (d) Flake sorting and (e) Extrusion and granulation of the purified flakes, characterized in that the flake sorting (d) serves to separate flakes which contain a foreign polymer different from the polyester and the flake sorting (d) is a combination of - a color sorting (d1), - a sieving step (d2) and - a polymer sorting (d3,d4).

2. Method according to claim 1, characterized in that at least a first and second sieve fractions of flakes are produced in the sieving step.

3. Method according to claim 2, characterized in that the first sieve fraction comprises flakes with a grain size > x mm and the second sieve fraction comprises flakes with a grain size < x mm.

4. Method according to claim 3, characterized in that in the sieving step (d2) a third sieve fraction is produced which has flakes with a grain size < 1mm and thereby the second sieve fraction has a grain size between 1 and x mm.

5. Method according to claim 3 or 4, characterized in that the limiting grain size is x=5mm, preferably 4mm and particularly preferably x=3mm.

6. Method according to one of the preceding claims, characterized in that the polymer sorting (d3,d4) takes place directly after the sieving step (d2).

7. Method according to one of the preceding claims, characterized in that the polymer sorting (d3), by which foreign polymers in the second The sieve fraction is sorted out; this is a non-optical polymer sorting process, in particular an electrostatic polymer sorting process.

8. Method according to one of the preceding claims, characterized in that the polymer sorting (d4), by which foreign polymers are sorted out in the first sieve fraction, is an optical polymer sorting.

9. Method according to one of the preceding claims, characterized in that the color sorting (d1) takes place before the sieving step (d2).

10. Method according to any one of claims 1 to 8, characterized in that the color sorting (d1) is carried out after the polymer sorting (d3,d4).

11. Method according to claim 10, characterized in that the color sorting (d1) is carried out after the mixing of the first and second sieve fractions.

12. Method according to one of claims 1 to 8 or 10, characterized in that the first and second sieve fraction are sorted separately in a first and second color sorting (d11 ,d12).

13. Method according to one of claims 2 to 12, characterized in that the purified flakes of the first and second sieve fraction are stored in a flake storage (f) in a defined ratio.

14. Method according to claim 13, characterized in that the purified flakes of the first and second sieve fraction are partially stored in a first and second intermediate storage unit (g1 ,g2) and the temporarily stored flakes are fed to the flake storage unit (f) in a defined ratio.

15. Method according to one of the preceding claims, characterized in that the foreign polymer is PVC.

16. Method according to one of claims 4 to 15, characterized in that the third sieve fraction is sent to a disposal (h).

17. Method according to one of the preceding claims, characterized in that the friction washing (c) is carried out at temperatures > 55 °C. A method according to any one of claims 1 to 16, characterized in that the friction washing (c) is carried out at temperatures < 55 °C. A method according to any one of claims 7 to 18, characterized in that the electrostatic polymer sorting (d3) is carried out by charging the polyester flakes and foreign polymer flakes to different strengths and separating them into different flake currents in a high-voltage electric field.

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