METHOD FOR RECYCLING POLYOLEFIN CONTAINERS

Abstract

A method for recycling polyester containers, in particular PET containers, said method including the following method steps: sorting the containers, comminuting the containers to produce flakes, friction washing the flakes, sorting the flakes, and extruding and granulating the cleaned flakes. The flake-sorting process is used to separate flakes which have a foreign polymer that is different from the polyester, and the flake-sorting process is a combination of a color-sorting process, a screening step, and an optical polymer-sorting process.

Claims

1.-13. (canceled)

14. A method for recycling polyolefin containers, in particular HDPE containers, said method comprising the following method steps: sorting the containers, comminuting the containers to produce flakes, friction washing the flakes, sorting the flakes, extruding and granulating the cleaned flakes, wherein the flake-sorting process is used to separate flakes and the flake-sorting process is a combination of: a color-sorting process, a screening step, and an optical polymer-sorting process.

15. The method in accordance with claim 14, wherein in the screening step at least a first and a second screened fraction of flakes are produced.

16. The method in accordance with claim 15, wherein the first screened fraction comprises flakes with a grain size <x mm and the second screened fraction comprises flakes with a grain size >x mm.

17. The method in accordance with claim 14, wherein the limit grain size is x=6 mm.

18. The method in accordance with claim 15, wherein PP is sorted out from the first screened fraction by the optical polymer-sorting process and the polymer-sorting process takes place directly after the screening step.

19. The method in accordance with claim 15, wherein PP is sorted out from the second screened fraction by a further optical polymer-sorting process and the further polymer-sorting process takes place directly after the screening step.

20. The method in accordance with claim 15, wherein in the screening step a third screened fraction with a grain size <1 mm is created and thereby the first screened fraction has a grain size between 1 mm and x mm.

21. The method in accordance with claim 14, wherein the color-sorting process takes place before the screening step.

22. The method in accordance with claim 15, wherein the color-sorting process takes place after the mixing of the first and second screened fractions.

23. The method in accordance with claim 15, wherein the first and second screened fractions are sorted separately in a first and a second color-sorting process.

24. The method in accordance with claim 15, wherein the cleaned flakes of the first and second screened fractions are stored in a flake storage facility in a defined ratio.

25. The method in accordance with claim 24, wherein the cleaned flakes of the first and second screened fractions are partially stored temporarily in a first and a second intermediate storage facility and the temporarily stored flakes are fed to the flake storage facility in a defined ratio.

26. The method in accordance with claim 20, wherein the third screened fraction is fed to a disposal facility.

Description

[0022] Further advantages and features will become apparent from the following description of four exemplary embodiments of the invention with reference to the schematic drawings. In the figures, in a representation that is not to scale:

[0023] FIG. 1: is a first flow diagram showing a first embodiment of a method for recycling polyolefin containers;

[0024] FIG. 2: is a second flow diagram showing a second embodiment of the method, in which three screened fractions are produced instead of two;

[0025] FIG. 3: is a third flow diagram showing a third embodiment of the method for recycling polyolefin containers; and

[0026] FIG. 4: is a fourth flow diagram showing a fourth embodiment of the method for recycling polyolefin containers.

[0027] FIGS. 1 to 4 show four embodiments of an improved method for recycling polyolefin containers. HDPE containers represent the largest proportion for the application of this method, although the method is also suitable for containers made of other polyolefins. Method steps a to c are basically 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 step for removing metals and labels may also be provided. The HDPE containers can also be pre-cleaned. In step b, the HDPE containers are comminuted into flakes, in particular ground in a mill. In step c, the flakes are washed in a friction wash.

[0028] 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, a high level of friction is generated between the flakes, causing the flakes to break down into finer particles. It is noticeable that polypropylene (PP) in particular is very brittle and accumulates strongly in the range <4-6 mm and in particular in the range <3-4 mm. 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. When PP is injection molded, it is particularly brittle. Therefore, as described below, the flakes are screened to obtain different screened fractions. The majority of PP flakes are present in the screened fraction, which contains flakes which are smaller than the limit grain size. In principle, the degree of PP depletion using optical polymer sorting (near-infrared, laser) in accordance with the prior art is sufficient to obtain recycled HDPE flakes with good purity and quality. However, optical polymer sorting represents a bottleneck in the continuous stream because the required flow rate is much greater than the amount that a polymer sorter can process. Therefore, the recycling stream must be divided and passed through at least two sorters. This causes high investment costs and at least doubles the space required for the sorting system.

[0029] The flake-sorting process d is carried out in a combination of multiple separation steps of the flakes, namely a color-sorting process d1, a sieving or screening step d2 and a polymer-sorting process d3.

[0030] Color sorting d1 is usually carried out using color cameras, sometimes in combination with near-infrared (NIR), and usually takes place in a specially designed sorting system. Color sorting d1 sorts out flakes that could affect the desired color of the containers made from the recycled granulate.

[0031] The ground, (hot) washed & color-sorted flakes are split up into their size composition using a machine-driven screen or two screens in the screening step d2 in order to be able to apply the best possible further treatment for all flake sizes. The initial fraction is divided into a first, second and an optional third fraction: [0032] 1. <x mm or 1-x mm if a third screened fraction is present [0033] 2. >x mm [0034] 3. <1 mm
wherein the limit grain size is x 6 mm, preferably 5 mm, particularly preferably 4 mm.

[0035] By creating a first and second screened fraction, the smaller PP flakes are enriched in the first screened fraction. In the second screened fraction, the proportion of PP flakes is so low that they do not need to be removed from the second screened fraction by means of optical polymer sorting. The small volume flow of the first screened fraction can be freed from the PP flakes using an optical polymer sorter without the polymer sorter reaching its capacity limits. A single polymer sorter is therefore sufficient to sort out sufficient PP flakes from the entire HDPE recycling stream. This allows significant savings in terms of sorting costs, on the one hand because the number of machines can be reduced and on the other hand because a sorting machine requires less space.

[0036] The second fraction >x mm is separated and optionally further purified by optical sorting systems using near-infrared or laser sources. A further optical polymer-sorting process d4 is optionally provided in the flow diagrams of FIGS. 1 to 4 and shown by the dashed arrow. The second screened fraction d2 can therefore be conveyed directly into the flake storage facility f or is fed to the further polymer-sorting process d4. Depending on the origin, PP depletion in the second screened fraction is no longer necessary to produce bottle-grade rHDPE.

[0037] Depending on the origin of the material, the third screened fraction <1 mm can be considered a side stream for other recycling and disposed of h accordingly (option in FIGS. 2-4). The third screened fraction is therefore not taken into account for the further process.

[0038] Before extrusion e, the flakes must have passed through at least the flake-sorting stages d1, d2 and d3 to ensure maximum removal of PP from the first screened fraction.

[0039] The following analyses can be used to determine contamination, in particular PP: [0040] Differential scanning calorimetry (DSC) in accordance with DIN EN ISO 11357 (in preparation for the measurement, pellets or test specimens must be created from the material provided) [0041] Infrared spectroscopy (NIR/FTIR) (in preparation for the measurement, pellets or test specimens must be created from the material provided) [0042] Infrared spectroscopy (NIR/FTIR) alternatively on flake quantities

[0043] The different polymer components in HDPE can be detected using DSC and NIR/FTIR. The contamination level depends very much 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-% in accordance with DSC <1.5% [0046] rHDPE gray (ESP) each pre-sorted at bottle level, color flake-sorted, without polymer sorting: PP-% in accordance with DSC >3% [0047] rHDPE gray (ESP) each pre-sorted at bottle level, color flake-sorted, including polymer sorting: PP-% in accordance with DSC <1.5%

[0048] Use of advanced screening: [0049] >4 mm: rHDPE gray (ESP) pre-sorted at bottle level, color flake-sorted, without polymer sorting: PP-% in accordance with DSC <1.5%. [0050] >6 mm: rHDPE gray (ESP) pre-sorted at bottle level, color flake-sorted, without polymer sorting: PP-% in accordance with DSC <1.5%. [0051] <4 mm: rHDPE gray (ESP) pre-sorted at bottle level, color flake-sorted, without polymer sorting: PP-% in accordance with DSC >3%. [0052] <6 mm: rHDPE gray (ESP) pre-sorted at bottle level, color flake-sorted, without polymer sorting: PP-% in accordance with DSC >3%

[0053] The target value of PP in rHDPE is <1-1.5% PP in accordance with DSC.

[0054] With the present method comprising a color-sorting process d1, a screening step d2 and an optical polymer-sorting process, the required capacity for polymer flake-sorting technology is reduced by 50-60% by focusing the polymer flake sorting on critically PP-contaminated material stream components (first screened fraction).

[0055] As can be seen from FIGS. 1 to 4, the polymer-sorting process d3 or d4 takes place directly after the screening step d2. In accordance with the first exemplary embodiment, as shown in FIG. 1, the color-sorting process d1 takes place before the screening step d2. The color-sorting process d1 can also take place after the polymer-sorting process d3, d4 (FIG. 3). The color-sorting process d1 can take place after the mixing of the first and second screened fractions (FIG. 3, 3rd exemplary embodiment) or the first and second screened fractions are each subjected to a first and second color-sorting process (d11, d12) before they are mixed (FIG. 4, 4.sup.th exemplary embodiment).

[0056] The first and second fractions can be stored in a defined mixing ratio in a flake storage facility f and retrieved for extrusion e into pellets in the predetermined mixing ratio. To establish the mixing ratio, the flake storage facility f is filled with defined mass flows of the first and second screened fractions.

[0057] The cleaned flakes of the first and second screened fractions can be temporarily stored in a first and a second intermediate storage facility (g1, g2) by diverting side streams. The temporarily stored flakes are fed to the flake storage facility f in a defined ratio. This means that the first and second screened fractions can also be stored separately from each other.

[0058] Examples of ratios of the screened fractions in wt. % are given in the three tables below depending on the limit grain size or screen cut x:

TABLE-US-00001 TABLE 1 Distribution of flakes in the three screened fractions before sorting Proportion before sorting Screen cut at x mm <1 mm 1 x mm >x mm x = 4 mm 1.0% 30% 69% x = 5 mm 1.0% 40% 59% x = 6 mm 1.0% 50% 49%

TABLE-US-00002 TABLE 2 Sorting losses of the flakes in the three screened fractions Sorting losses (relative to the respective fraction) Screen cut at x mm <1 mm 1 x mm >x mm x = 4 mm 100% 4.5% 2.0% x = 5 mm 100% 4.0% 1.8% x = 6 mm 100% 3.0% 1.5%

TABLE-US-00003 TABLE 3 Distribution of flakes in the three screened fractions after sorting Final flake proportion Screen cut at x mm <1 mm 1 x mm >x mm x = 4 mm 0% 29.8% 70.2% x = 5 mm 0% 39.9% 60.2% x = 6 mm 0% 50.4% 50.1%

LIST OF REFERENCE SIGNS

[0059] a Container sorting [0060] b Comminuting the containers to produce flakes [0061] C Friction washing the flakes [0062] d Flake sorting [0063] d1 Color sorting [0064] d11 First color sorting [0065] d12 Second color sorting [0066] d2 Screening step [0067] d3 Polymer sorting of the second screened fraction [0068] d4 Polymer sorting of the third screened fraction [0069] e Extrusion [0070] f Flake storage facility [0071] g1 First intermediate storage facility [0072] g2 Second intermediate storage facility [0073] h Disposal