Method for recycling polyolefin 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 (dl), a screening step (d2), and an optical polymer-sorting process (d3).

Description

[0001] Process for recycling polyolefin containers

[0002] Field of the invention

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

[0004] State of the art

[0005] Recycling polyolefin containers is crucial for the utilization of existing resources, as a large number of containers are made from HDPE (high-density polyethylene). Therefore, recycling HDPE containers after disposal is extremely important, and a particularly efficient recycling process offers significant savings potential.

[0006] HDPE recycling streams contain PP (polypropylene), as caps and other closures are made of PP and are disposed of with the containers. According to planned EU disposal regulations, PP caps and closures will even have to be disposed of with the containers in the future. The caps are usually made of injection-molded PP. An HDPE recycling stream typically contains between 3 and 15 wt% PP, which is attributable to caps and also to so-called misplaced waste, such as PP bottles. However, only pure HDPE is valuable for the processing industry. Therefore, the PP must be separated from the HDPE as completely as possible.

[0007] Because HDPE and PP have similar densities, separation processes that exploit the different densities of the plastics to be separated (float-sink separation in water, hydrocyclones, air classifiers, etc.) are inefficient. Therefore, near-infrared container and flake sorters are used according to the state of the art. Flake sorting according to different colors is also used. Such sensor-based sorting devices are expensive to purchase and can only process a limited volume flow. Therefore, at least two expensive analysis machines must be used to achieve the necessary volume throughput of an example plant of approximately 3 mt / h. In addition to separating the interfering PP from caps, closures, and even misplaced bottles from the HDPE, near-infrared (NIR) flake sorting also aims to enrich these interfering substances in a so-called reject stream. The object of the invention

[0008] The disadvantages of the described prior art give rise to the task of improving the process for recycling polyolefin containers (in particular HDPE containers) in such a way that the sorting or depletion of PP in the recycling stream can be carried out with a reduced machine capacity and is accordingly cost-optimized, but an rHDPE quality is still generated from the NIR-sorted flakes, which can be reliably used again in container applications.

[0009] Description

[0010] The stated problem is solved in processes for recycling polyolefin containers, in particular HDPE 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 PP flakes, 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. This triple combination for sorting out PP flakes allows, in particular, PP flakes to be sorted out from the screen fraction, in which the PP flakes accumulate because they are more brittle than polyolefin flakes. As a result, only a screen fraction needs to undergo optical polymer sorting, not the entire recycling stream.

[0012] In a particularly preferred embodiment of the invention, a first and second sieve fraction of flakes are created in the sieving step. Since PP is more brittle than HDPE, the PP flakes are crushed much more than the HDPE flakes during the friction wash c. As a result, the washed PP flakes are concentrated in the < 4-6 mm range and especially in the < 3-4 mm range. Therefore, a separate fine sieve fraction is created for this fine range. The second coarse sieve fraction contains so few PP flakes that this fraction does not require optical sorting and the PP flake concentration does not need to be depleted. The flow rate of the first sieve fraction does not exceed the sorting capacity of a single near-infrared or laser sorting system. The capacity of the required polymer sorting technology can be reduced by 50-60% by creating the two sieve fractions.This eliminates the need for a second optical sorting system to sort the entire recycling stream. Investment costs and machine space requirements can be significantly reduced due to the reduced machine capacity, while the PP removal quality remains unchanged or even improved compared to sorting the entire recycling stream. The first screen fraction is 40-50 wt%, and the second screen fraction is 50-60%.

[0013] It is preferred if the first screen fraction contains flakes with a grain size of < x mm and the second screen fraction contains flakes with a grain size of > x mm. This ensures that only one flake stream is present, which has a critical PP-contaminated content and must be depleted.

[0014] The limiting particle size is preferably x=6 mm, preferably 5 mm, and particularly preferably x=4 mm. This choice of limiting particle size or screen cut reliably produces a second screen fraction that does not require a purification step involving the removal of PP flakes. The largest proportion of PP flakes is found in the first screen fraction, which is efficiently freed of PP flakes, for example, by a near-infrared or laser sorting system. This removes enough PP flakes from the first screen fraction that it can be remixed with the second screen fraction, producing rHDPE granules of sufficient quality to be used to manufacture containers again.

[0015] It has proven useful to perform polymer sorting (d3) directly after screening step (d2). This allows the first screen fraction to be immediately freed of PP contamination before further processing steps are carried out.

[0016] It has proven advantageous to separate PP from the second screen fraction by performing an additional optical polymer sorting step (d4), which is performed directly after the screening step (d2). This further improves the purity of the entire recycling stream. This is particularly useful when PP contents are atypical in this particle size range. Thus, at least the preliminary screening allows for more targeted adjustment of the NIR sorting machine parameters in (d3 and d4), thus achieving more reliable separation.

[0017] In a further embodiment of the invention, a third screen fraction with a grain size < 1 mm is created in the screening step, resulting in the first screen fraction having a grain size between 1 mm and x mm. This allows the finest screen fraction, which primarily contains PP flakes and has no value for the recycling stream for the production of rHDPE, to be used directly in other recycling processes (rHDPE recycling, rPP recycling, or recycling for PE / PP mixed applications). Color sorting d1 is expediently performed before screening step d2. Optical color sorting achieves higher resolution rates and can therefore be used more flexibly. In the case of contamination in all grain classes, the material stream can be color-sorted in advance to simplify the process technology / reduce the number of machines.

[0018] 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. This arrangement is particularly useful when a grain class is disposed of directly and color contaminants occur in this specific grain class.

[0019] It is advantageous if color sorting d1 takes place after the first and second screen fractions have been mixed. This allows color sorting d1 to be performed as the final step of flake sorting d, before the flakes are sent to extrusion or are temporarily stored prior to extrusion. This allows color sorting d1 to be performed as the final step of flake sorting d, before the flakes are sent to extrusion or are temporarily stored prior to extrusion.

[0020] The invention is also preferably characterized in that the first and second screen fractions are sorted separately in a first and second color sorting step d11, d12. This has the advantage that the first and second color sorting steps 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 parameter selection of the first and second color sorting steps to the respective flake sizes of the first and second screen fractions, thereby optimizing 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 rHDPE 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 screen fractions are partially stored temporarily in a first and second intermediate storage facility g1, g2, and the temporarily stored flakes are fed to the flake storage facility f in a defined ratio. The first and second screen fractions can therefore be stored separately before being mixed. It is also conceivable for the flakes of the first and second screen fractions not to be mixed and to be further processed separately. In a further particularly preferred embodiment of the invention, the third screen fraction is fed to a disposal facility h. The term disposal is also intended to include other utilization (recycling, rPP recycling, or recycling for PE / PP mixed applications).In this third fine screen fraction with flakes smaller than 1 mm (HDPE fines / HDPE dust), the PP content is particularly high, up to 8-10 times higher than in the other screen fractions. This is because, as already mentioned above, PP is more brittle than HDPE and is subjected to significant size reduction during friction washing. Disposing of the third screen fraction therefore also contributes to maintaining the cleanest possible recycling stream.

[0023] 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:

[0024] Further advantages and features will become apparent from the following description of four exemplary embodiments of the invention with reference to the schematic representations. They show, not to scale:

[0025] Figure 1: a first flow diagram illustrating a first embodiment of a process for recycling polyolefin containers;

[0026] Figure 2: a second flow diagram showing a second embodiment of the process in which 3 sieve fractions are produced instead of 2;

[0027] Figure 3: a third flow diagram showing a third embodiment of the process for recycling polyolefin containers and

[0028] Figure 4: a fourth flow diagram illustrating a fourth embodiment of the process for recycling polyolefin containers.

[0029] Figures 1 to 4 show four variants of an improved process for recycling polyolefin containers. HDPE containers represent the largest proportion of applications for this process, although the process is also suitable for containers made of other polyolefins. Process steps a to c are generally known. In step a, the HDPE containers sent for recycling are sorted. This is done using color sorting and near-infrared (NIR) technologies. Before the bottles are sorted, a process step for removing metals and labels can also be included. The HDPE containers can also be pre-cleaned. In step b, the HDPE containers are reduced to flakes, in particular by grinding them in a mill. The flakes are then washed in a friction wash in step c.

[0030] Typical flake sizes for HDPE after the mill are in the target range of 4 - 15 mm, depending on the mill used. However, during the washing process c, high friction is generated between the flakes, causing the flakes to break down into finer fractions. It is noticeable that polypropylene (PP) in particular is very brittle and accumulates heavily in the < 4 - 6 mm range, and especially in the < 3 - 4 mm range. The PP enters the recycling stream primarily through caps or closures on HDPE containers. The incoming recycling stream can therefore contain between 3 - 15 wt% PP. If the PP is injection-molded, it is particularly brittle. Therefore, as described below, the flakes are sieved to obtain different sieve fractions. The majority of the PP flakes are found in the sieve fraction, which contains flakes smaller than the cut-off grain size.In principle, the degree of PP depletion achieved with state-of-the-art optical polymer sorting (near-infrared, laser) is sufficient to obtain recycled HDPE flakes of good purity and quality. However, optical polymer sorting represents a bottleneck in the continuous stream, as the required flow rate is much greater than the amount a single polymer sorter can process. Therefore, the recycling stream must be split and routed through at least two sorters. This results in high investment costs and at least doubles the footprint of the sorting system.

[0031] The flake sorting d is carried out in a combination of several separation steps of the flakes, namely a color sorting d1, a sieving or screening step d2 and a polymer sorting d3.

[0032] 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 optionally a third fraction:

[0033] 1 . < x mm or 1 - x mm if a third sieve fraction is present

[0034] 2. > x mm

[0035] 3. < 1 mm, where the limit grain size is x 6 mm, preferably 5 mm, particularly preferably 4 mm.

[0036] By creating a first and second screening fraction, the smaller PP flakes are concentrated in the first screening fraction. The proportion of PP flakes in the second screening fraction is so low that they do not need to be removed from the second screening fraction using optical polymer sorting. The low volume flow of the first screening fraction can be freed of PP flakes using an optical polymer sorter without the polymer sorter reaching its capacity limits. A single polymer sorter is therefore sufficient to separate sufficient PP flakes from the entire HDPE recycling stream. This allows for significant savings in sorting costs, both because fewer machines are needed and because a sorting machine requires less space.

[0037] The second fraction > x mm is separated and optionally further purified using optical sorting systems using near-infrared or laser sources. A further optical polymer sorting stage d4 is optionally provided in the flow diagrams of Figures 1 to 4 and is indicated by the dashed arrow. The second screen fraction d2 can therefore be conveyed directly to the flake storage system f or is fed to the further polymer sorting stage d4. Depending on the origin, PP depletion in the second screen fraction is no longer necessary to produce bottle-grade rHDPE.

[0038] Depending on the material's origin, the third screen fraction < 1 mm can be considered a side stream for alternative recycling and disposed of accordingly (option in Figures 2 - 4). The third screen fraction is therefore not considered for further processing.

[0039] Before extrusion (e), the flakes must have undergone at least flake sorting (d1, d2, and d3) to ensure maximum removal of PP from the first screen fraction. The following analyses can be used to determine contaminants, especially PP:

[0040] • Dynamic differential thermal analysis (DSC) according to DIN EN ISO 11357 (in preparation for the measurement, pellets or test specimens must be created from the provided material)

[0041] • Infrared spectroscopy (NIR / FTIR) (in preparation for the measurement, pellets or test specimens must be created from the provided material)

[0042] • Infrared spectroscopy (NIR / FTIR) alternatively on flake quantities

[0043] The various polymer components in HDPE can be detected using DSC and NIR / FTIR. The contamination level depends heavily on the material origin and process genesis. Typical contamination levels on the market are as follows:

[0044] Without advanced screening:

[0045] • rHDPE white, rHDPE natural each pre-sorted at bottle level, color flake sorted, without polymer sorting: PP-% according to DSC <1.5%

[0046] • rHDPE grey (ESP) pre-sorted at bottle level, color flake sorted, without polymer sorting: PP-% according to DSC >3%

[0047] • rHDPE grey (ESP) pre-sorted at bottle level, color flake sorted, including polymer sorting: PP-% according to DSC <1.5%

[0048] Use of advanced screening:

[0049] • >4 mm: rHDPE grey (ESP) pre-sorted at bottle level, color flake sorted, without polymer sorting: PP-% according to DSC <1.5%

[0050] • >6 mm: rHDPE grey (ESP) pre-sorted at bottle level, color flake sorted, without polymer sorting: PP-% according to DSC <1.5%

[0051] • <4 mm: rHDPE grey (ESP) pre-sorted at bottle level, color flake sorted, without polymer sorting: PP-% according to DSC >3%

[0052] • <6 mm: rHDPE grey (ESP) pre-sorted at bottle level, color flake sorted, without polymer sorting: PP-% according to DSC >3%

[0053] The target PP content in rHDPE is <1-1.5% PP according to DSC. With this process, which comprises color sorting (d1), screening step (d2), and optical polymer sorting, the required capacity for polymer flake sorting technology is reduced by 50-60% by focusing polymer flake sorting on critically PP-contaminated material streams (first screening fraction).

[0054] As can be seen from Figures 1 to 4, 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 3). Color sorting d1 can take place after mixing the first and second screening fractions (Figure 3, 3rd 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 4, 4th embodiment).

[0055] The first and second fractions can be stored in a flake storage unit f in a defined mixing ratio and retrieved for extrusion e into pellets in the predetermined mixing ratio. To achieve the mixing ratio, the flake storage unit f is fed with defined mass flows of the first and second screened fractions.

[0056] 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.

[0057] Examples of ratios of sieve fractions in wt% are given in the 3 tables below depending on the limiting particle size or the sieve cut x:

[0058] Table 1 : Distribution of flakes in the three sieve fractions before sorting d

[0059] Table 2: Sorting losses of the flakes in the three sieve fractions

[0060] Table 3: Distribution of flakes in the three sieve fractions after sorting d

[0061] Key: a Container sorting b Shredding of containers into flakes c Friction washing of flakes d Flake sorting d1 Color sorting d11 First color sorting d12 Second color sorting d2 Screening step d3 Polymer sorting of the second screen fraction d4 Polymer sorting of the third screen fraction e Extrusion f Flake storage g1 First intermediate storage g2 Second intermediate storage h Disposal

Claims

1. A process for recycling polyolefin containers, in particular HDPE containers, comprising the following process steps (a) Container sorting, (b) crushing the containers into flakes, (c) Friction washing of the flakes, (d) Flake sorting, (e) Extrusion and granulation of the purified flakes, characterized in that the flake sorting (d) serves to separate PP flakes and the flake sorting (d) is a combination of - a color sorting (d1), - a sieving step (d2) and - an optical polymer sorting (d3).

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 1, characterized in that the limiting grain size is x=6 mm, preferably x=5 mm and particularly preferably x=3 mm.

5. Method according to one of claims 2 to 4, characterized in that PP is sorted out from the first sieve fraction by optical polymer sorting (d3) and the polymer sorting (d3) takes place directly after the sieving step (d2).

6. Method according to one of claims 2 to 5, characterized in that PP is sorted out from the second sieve fraction by a further optical polymer sorting (d4) and the further polymer sorting (d4) takes place directly after the sieving step (d2).

7. Method according to one of claims 2 to 6, characterized in that in the sieving step (d2) a third sieve fraction with a particle size < 1 mm is produced. as a result, the first sieve fraction has a grain size between 1 mm and x mm.

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

9. Method according to one of claims 2 to 7, characterized in that the color sorting (d1) is carried out after the mixing of the first and second sieve fractions.

10. Method according to one of claims 2 to 7, characterized in that the first and second sieve fractions are sorted separately in a first and second color sorting (d11 ,d12).

11. Method according to one of claims 2 to 10, characterized in that the purified flakes of the first and second sieve fraction are stored in a flake storage (f) in a defined ratio.

12. Method according to claim 11, characterized in that the purified flakes of the first and second sieve fraction are partially temporarily stored in a first and second intermediate storage (g1 ,g2) and the temporarily stored flakes are fed to the flake storage (f) in a defined ratio.

13. Method according to one of claims 7 to 12, characterized in that the third sieve fraction is sent to a disposal (h).

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