Method for recycling polyolefin containers

Abstract

The invention relates to a method for recycling polyolefin containers, comprising the following method steps: (a) sorting the containers, (b) size-reducing the containers to flakes, (c) washing, (d) air sizing, (e) sorting the flakes, (f) granulating the flakes in an extruder, and (g) controlling the odor of the granular material. The washing step (c) is carried out with an alkaline medium having a temperature of less than 60°C and an alkaline medium concentration of less than 0.5 wt.-%.

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] A processing method for producing polyolefin recyclates is known from the prior art. In this process, polyolefin-containing residues are crushed into flakes. The flakes are subjected to washing in an alkaline medium, sorting, granulation, and degassing in an extruder. The resulting granules can be used to manufacture new consumer product packaging.

[0006] The wash is carried out with water at 80°C (hot wash) and a caustic soda concentration of 1 to 3 wt%. The advantage of these washing conditions is that the flakes are reliably freed of impurities, label residue, and adhesive residue. However, these washing conditions also have disadvantages:

[0007] Polyolefins, especially HDPE (high-density polyethylene) and PP (polypropylene), are non-polar polymers. During recycling, they are washed with polar media such as water, alkalis, and acids. Washing alkalis are widely used. Washing non-polar plastics with a polar washing alkali directly results in contaminants that are less polar than water or the caustic soda dissolving more rapidly in the polymer and thus contaminating it. These contaminants can lead to unpleasant odors and discoloration. These contaminants can also contaminate food products filled in recycled containers or cause skin irritation when the container is touched. While washing cleans the surface of the flakes, the material itself becomes further contaminated by the washing process.

[0008] Before washing at the recycling plant, the polyolefin flakes to be washed contain substances that can only be removed through chemical degradation in the form of an etching reaction. This etching can be achieved using alkalis or acids. Alkalis are commonly used, usually 1 to 3% sodium hydroxide solution, because they are inexpensive to produce. Large molecules, which cannot penetrate the polyolefin matrix due to their size, are significantly degraded by alkalis. This creates small molecules that penetrate deep into the matrix and contaminate the polymer. This also creates contamination in the polymer matrix, as described in the last paragraph. The higher the temperature of the wash liquor, the more effectively the lye etches the flake surfaces, but the higher the contaminant concentration in the polymer matrix.

[0009] The polyolefins contain adhesives, labels, and sleeves with inks, coatings, and other types of decoration, which detach easily or more effectively from the polyolefins at elevated temperatures. These residues partially crosslink further during the hot wash or release plasticizers, thus becoming brittle. The resulting fine particles are almost impossible to separate using optical separation mechanisms. Sleeves made of PETG or other polyesters such as PLA curl up during hot washes, enclosing and curling the polyolefin flakes. Separation using the sink-and-float process is therefore often no longer possible. Polyester particles to be sorted out reach their glass transition point at temperatures above 60°C, become sticky, and adhere to the recycling plant's equipment. The adhesion of the sticky polyester particles makes separation more difficult.

[0010] Object of the invention

[0011] The disadvantages of the described prior art give rise to the object of creating an improved processing method in which the quality of the purified polyolefin flakes, in particular their polymer matrix contamination with impurities and their surface contamination with label and adhesive residues, is improved.

[0012] Description

[0013] The stated problem is solved in a process for recycling polyolefin 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.

[0014] The invention is characterized in that the washing step is carried out with a caustic solution with a caustic temperature of less than 60°C and a caustic solution concentration of less than 0.5 wt%. The flakes, which are still cold after the cold wash, absorb fewer contaminants because the material inside is still cold, and the migration of odorous substances into the material is significantly lower in the cold state. Due to the low caustic solution concentration, large molecules of contaminants located on the surface of the flakes (e.g., adhesive residues) are degraded to a lesser extent. The unwanted migration of small molecules into the polymer matrix is ​​thus significantly reduced.

[0015] The "mild" alkali parameters surprisingly lead to a significant improvement in product quality with regard to odor, discoloration, and skin contact irritation: By lowering the temperature, the usual polyolefin contaminations in the concentrations of butyric acid, nonenal, nonanal, nonalactone, and limonene can be reduced by 50%. By further lowering the alkali concentration, migrated contaminants are reduced by a further 10 to 20%.

[0016] In a preferred embodiment of the invention, the lye has a temperature between 40°C and 60°C. At this low temperature range, the tendency of small molecules to migrate into the polyolefin matrix is ​​significantly reduced. The cleaning performance of the lye is still sufficient above 40°C, since flakes still retaining label and adhesive residues after the washing step can be detected and separated in a flake sorter.

[0017] It is preferred if, in the washing step, the lye is processed and purified in a lye treatment plant, and the washing step is carried out partly with fresh lye and partly with lye treated in the lye treatment plant. This results in fewer dissolved contaminants in the lye that could undesirably migrate into the polyolefin matrix. The purity of the lye therefore improves the quality of the flakes in terms of odor.

[0018] It has proven advantageous to use wet shredding with wash water as a first washing step, with the wash water being treated in a water treatment plant. This allows for a rough cleaning of the containers and the flakes simultaneously with the shredding of the containers into flakes.

[0019] Advantageously, the washing step is a second washing step, whereby the flakes are cleaned twice: during the wet comminution and during the washing step. In a further preferred embodiment of the invention, the lye is fed countercurrently to the comminution after the lye preparation. Countercurrent flow of the lye without prior treatment is economically problematic. The lye is therefore regenerated in the lye preparation and kept in circulation for as long as possible. By partially feeding the regenerated lye into the comminution, the cleaning performance of the wash water is increased.

[0020] In a further preferred embodiment of the invention, flake sorting is an optical sorting system with color and / or polymer detection. Surprisingly, the sorter detects label and adhesive residues, since such contaminated flakes have a different color and / or a different polymer than the flakes not sorted out by the sorter.

[0021] In another particularly preferred embodiment of the invention, flakes containing label and adhesive residues are sorted out during flake sorting. Due to the selected wash liquor parameters regarding temperature and concentration, more label, film, and adhesive residues remain on the flakes than when the liquor parameters according to the prior art are 80°C and a liquor concentration of at least 1%. The flakes containing contaminant residues can be very effectively separated by flake sorting, which completely compensates for the disadvantage of washing with cold and mild liquor.

[0022] Further advantages and features will become apparent from the following description of an embodiment of the invention with reference to a schematic flow diagram.

[0023] Figure 1 shows a flow diagram of a process according to the invention for recycling polyolefin containers derived from collected post-consumer waste, designated collectively by reference numeral 11. The process is used to produce PCR (Post Consumer Recycled) granules, which in turn can be used to produce recycled polyolefin containers. This requires higher requirements for characteristics such as particle contamination, color, and odor than when the granules are used to produce construction products, such as sewage pipes.

[0024] The flow diagram of the process for recycling polyolefin containers shows the process steps (a) to (g) in a predefined sequence. In the first process step (a), the discarded and collected polyolefin containers 13 (input material) are sorted. The containers are sorted by color and polymer. This allows other polymers still present in the polyolefin recycling stream to be sorted out, creating different color classes.

[0025] In step (b), the containers are reduced to flakes in a wet cutting mill with a water circuit 27 including water treatment (i), and the flakes are fed to a washing step (c). The increased quality requirements regarding granule contamination, odor, and color can surprisingly be met using a lye 15 with a lye temperature of less than 60°C, and particularly preferably between less than 60°C and 40°C. The lye concentration is less than 0.5% to minimize the degradation of large molecules. Contrary to the general belief that only a hot wash with a 2% lye at 80°C leads to the desired granule quality, the required quality can also be achieved with the present "mild" lye parameters.

[0026] The low lye temperature and low lye concentration prevent degradation products that are detrimental to quality (large molecules are degraded less) and reduce the tendency of contaminants to migrate into the polyolefin matrix. Washing step (c) is carried out partly with fresh lye 15 and partly with lye 24 prepared in lye preparation (k) to prevent degradation products or contaminants from penetrating the flakes. However, these washing conditions lead to increased label and film residues and adhesive residues adhering to the flakes after washing step (c). Washing step (c) also includes a sink-float separation including final rinsing of the lye residues.

[0027] A part of the used lye is fed in countercurrent 17 to the wet crushing unit (b), which is fed with >5m 3Circulating water per megaton of plastic waste is used. Shredding thus acts as the first washing step, efficiently separating coarse contaminants. The circulating water 27 must be treated using suitable process technology (i), preferably with a sedimentation tank with an upstream grit trap, before being returned to the shredder's water circuit 27.

[0028] The flake stream is freed from residues 19 of free labels, tags, and foils (sleeves) in an air separation stage (d). The flakes can also be dried in the air separation stage (d).

[0029] In flake sorting (e), a highly sensitive sorter is used, which has color recognition and / or polymer recognition. The sorter separates flakes 21 with a color different from the main color and flakes made of foreign polymers. Surprisingly, flakes with label and film residues and adhesive residues can also be sorted out in flake sorting (e), as these have a different color or a foreign polymer, which the sorter detects. The flakes washed with low-concentration and low-temperature alkali and optically sorted exhibit a surprisingly high quality with regard to unwanted migration into the polymer matrix and impurities on the surface. In an extrusion (f), the flakes are extruded into pellets or granules.Degassing of the recycled polyolefin material can also take place in the extruder, as it is melted, and the heating can serve to separate migrated contaminants. In a final step (g), the odor treatment of the granules or pellets takes place. The final granules 23, which leave the odor treatment (g), can be packaged in big backs or temporarily stored in silos.

[0030] Legend:

[0031] 11 Processes for recycling polyolefin containers

[0032] 13 Collected polyolefin containers

[0033] 15 lye

[0034] 17 Countercurrent

[0035] 19 Remnants of labels and foils

[0036] 21 flakes with different colors and polymers

[0037] 23 Final granulate

[0038] 25 Caustic solution cycle

[0039] 27 Wash water circuit shredder a Color sorting of containers b Shredding of containers into flakes c Hot washing step of flakes including float-sink separation c1 First washing step c2 Second washing step d Air separation e Flake sorting f Extrusion g Odor treatment h Caustic solution treatment i Water treatment shredder

Claims

1. A process (11) for recycling polyolefin containers comprising the following Procedural steps (a) container sorting, (b) crushing the containers into flakes, (c) washing step, (d) wind sifting, (e) flake sorting, (f) granulating the flakes in an extruder and (g) Odour treatment of the granules, characterized in that the washing step (c) is carried out with a lye (15) having a lye temperature of less than 60°C and a lye concentration of less than 0.5 wt%.

2. Method according to claim 1, characterized in that the lye (15) has a temperature between 40°C and 60°C.

3. Method according to claim 1 or 2, characterized in that in the washing step (c) the lye (15) is processed and cleaned in a lye processing unit (h) and the washing step (c) is operated partly with fresh lye (15) and partly with lye processed in the lye processing unit (k).

4. Method according to one of the preceding claims, characterized in that the comminution is a wet comminution (b) with washing water and acts as a first washing step (c1), the washing water being treated in a water treatment (i).

5. The method according to claim 4, characterized in that the washing step is a second washing step (c2).

6. Process according to one of claims 3 to 5, characterized in that the lye is fed to the comminution (b) in countercurrent (17) after the lye preparation (h).

7. Method according to one of the preceding claims, characterized in that the flake sorting (e) is an optical sorting with color recognition and / or polymer recognition.

8. Method according to claim 7, characterized in that in the flake sorting (e) flakes (21) with label residues and adhesive residues are sorted out.

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