METHOD FOR RECYCLING POLYOLEFIN CONTAINERS

Description

Field of the invention

[0001] The invention relates to a process for recycling polyolefin containers according to the preamble of claim 1 State of the art

[0002] Polyolefins are among the most widely used plastics in the world, and recycling them is particularly important for the utilization of existing resources. Impurities in the material lead to detrimental discoloration, an unpleasant odor, and impurities that prevent further use in the food industry. State-of-the-art recycling systems are closed recycling loops in which, for example, HDPE milk bottles are reused to produce milk bottles. Recycling is only successful because a clearly defined input stream without the usual contaminants is used for recycling. Given the usual contaminants found in, for example, completely empty detergent containers, fuel canisters, shampoos, and other non-food packaging, this high-quality, contamination-free recycling is very complex, if at all possible.In particular, the strong smell deters many packaging manufacturers from using polyolefin regenerates.

[0003] State-of-the-art processes involve deodorizing recycled granules by passing them through an air stream, a vacuum, a nitrogen stream, or steam. Steam is particularly effective for odor removal, as disclosed in WO 2013 / 072035 A1. Odor removal is generally more effective the higher the temperature used for deodorization. However, an upper limit is reached at 120 to 130°C, as the granules melt and stick together above this temperature.

[0004] Various options for odor removal from recycled granules are also disclosed in DE 10 2016 116742 A1, EP 2 384 873 A1, WO 92 / 22380 A1, KR 101 229 089 B1, WO 2012 / 117250 A1, and the technical article "Research & development to improve the recyclability of plastic milk bottles", Wrap Project MDP025-003 dated January 1, 2022. Odor removal can be achieved by treatment at temperatures between 50 and 155 °C. Applying a vacuum can positively influence the separation of migrating substances. The separation of migrating substances can also be achieved by treatment with an organic solvent or water, or by degassing in the vacuum zone of an extruder.

[0005] For cleaning, polyolefin packaging, either as a bottle or ground into flakes, is also intensively washed to remove adhesives, labels, sleeves, or residues from the contents. This washing process usually involves a cold pre-wash followed by a hot post-wash (approximately 40°C to 90°C). This temperature is specifically designed to help remove hot-melt adhesives, such as those used for applying labels. A 1% to 3% NaOH solution is usually used as the wash water, which, with the help of surfactants, is intended to intensify the washing process. However, the washing process can only remove contaminants on the surface of the material, not contaminants that have penetrated deep into the material. Object of the invention

[0006] The disadvantages of the described prior art give rise to the task of removing unpleasant odors from recycled polyolefins as completely as possible so that a mixed stream of different polyolefin packaging can be recycled and the flakes and containers newly produced from the mixed stream have no or very low odor pollution. Description

[0007] The stated problem is solved in a process for recycling polyolefin 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.

[0008] The invention is preferably characterized in that the cleaning step comprises a percolation c1 of the flakes with caustic soda at a temperature < 60 °C and, following step c1, a first deodorization c2 of the flakes with steam. The flakes are transferred from the cold wash to a caustic soda percolation reactor, in which the flakes are wetted on the surface with cold caustic soda (temperature < 60 °C) in order to dissolve adhesive residues and surface contaminants and to make them easier to remove with the steam. Subsequently, the mixture of flakes with residual caustic soda is steamed with a high proportion of steam in the steam deodorization in order to entrain the volatile and odor-causing substances with the steam and thus obtain flakes with very low odor. The combination of cold caustic soda, steam, and heat only on the surface ensures very good cleaning results.These can be further improved if the steam supply is carried out in several stages, which leads to the entrainment of the contaminating substances being carried out even more effectively.

[0009] The flakes, 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. The steam condensing on the surface of the flakes interacts with the excess steam flowing past, so that water and other contaminants, even if they have a higher boiling point than water, are carried away by the released steam droplets.

[0010] The energy used in the steam treatment is not wasted, as the flakes are heated by the steam, thus reducing the energy required for melting and granulation. The excess water condensed on the flakes by the steam, along with the adhesive residue, is removed from the flakes before entering the extruder and granulating. Ideally, the extruder for granulation has a degassing system to further remove small, odorous molecules.

[0011] Using hot steam instead of hot lye has the additional positive effect that large organic molecules react with the lye and break down into many small, odorous organic compounds. However, these reactions do not occur with steam. The saponification of fats and oils is well known; the lye breaks them down to form small, odorous molecules. Hydrolysis also affects other large molecules such as proteins and carbohydrates. Lye can attack many organic compounds and break them down into small, odorous molecules. The hotter the lye, the more intense the reactions and the greater the exposure to odorous molecules in the hot wash, which can penetrate deep into the polyester matrix.

[0012] In a particularly preferred embodiment of the invention, the steam in step c2 entrains the caustic soda from the surface of the flakes. As a result, the steam and caustic soda, which are contaminated with impurities and adhesive residues, are removed from the flakes together.

[0013] Conveniently, the contaminated caustic soda and the contaminated steam are separated in a separation process h. This allows the steam to release its thermal energy in a heat exchanger before being fed into wastewater treatment.

[0014] It is advantageous to perform a second deodorization f of the regranulate after step e. This further improves the quality of the regranulate. For example, the second deodorization can take place in a conventional degassing system. Steam, nitrogen, and air can serve as degassing media, with the inflow being designed in several parts or stages. This has the advantage that the heat profile is more homogeneous across the bed height, making degassing more sustainable and effective. It is also possible for the extruder for the granulation to be equipped with a degassing system to additionally remove small, odorous molecules.

[0015] For optimal results, the wastewater from the intensive wash, the contaminated caustic soda, and the contaminated steam are fed into a wastewater treatment plant. The steam is first condensed in a heat exchanger, allowing all three wastewaters to be fed into a common wastewater treatment plant. All three wastewaters contain similar levels of contamination, namely dissolved adhesive residues and odorous dissolved substances.

[0016] In a further preferred embodiment of the invention, flake sorting (d) is performed before percolation (c1), after percolation (c1), or after the first deodorization (c2). These embodiments of the different positions of flake sorting in the process sequence have the effect of producing extremely pure color fractions, thus largely avoiding so-called "cross-contamination."

[0017] It has proven advantageous to perform intensive washing b with water at a temperature of < 60°C. This roughly cleans the containers or flakes and removes water-soluble contaminants. After the intensive washing, the flakes are at a low temperature, preventing odorous substances from penetrating the polymer matrix.

[0018] In another particularly preferred embodiment of the invention, the contaminated steam in a heat exchanger transfers its residual heat to water intended for steam generation for preheating. The excess steam and its energy, as well as the small, odorous molecules it contains, are passed through the heat exchanger, and the energy is used for further steam production. The condensate, including the odorous substances and contaminants, is fed to a wastewater treatment plant.

[0019] It has proven useful to evaporate the preheated water into steam in a steam generator. This allows for a particularly energy-efficient production of steam to absorb the odorous molecules.

[0020] The invention is also preferably characterized in that the flakes are preheated for step e by step c2. The melting of the flakes during extrusion is therefore as energy-efficient as possible, further reducing the overall energy requirement of the recycling process.

[0021] It is advantageous to rinse and dewater the flakes after step c or d. The goal of this step is to prevent caustic soda residues from being carried over into the subsequent process steps and to ensure that the flakes can be processed as dry as possible.

[0022] Further advantages and features will become apparent from the following description of an embodiment of the invention with reference to the schematic representations. These are not to scale: Figure 1: Flow diagram of a recycling process in a first embodiment; Figure 2: Flow diagram of the recycling process in a second embodiment; and Figure 3: Flow diagram of the recycling process in a third embodiment.

[0023] In the Figures 1 to 3 A flow diagram for the recycling of polyolefin containers is shown. The starting material for all three embodiments is bales of collected polyolefin containers, particularly bottles, which have been sent to a recycling center.

[0024] The bales are opened and the containers are separated onto a conveyor belt (step bb, bale breaking). In step a, the containers are fed into a color sorting system. The color-sorted containers are washed in an intensive wash (b) with cold water (temperature < 60°C). This takes place in a friction washer. The containers are shredded into flakes and mechanically separated from the wash water (step fs, flake separation, density separation).

[0025] In step c1, the flakes are wetted with cold caustic soda in a percolation process. This process dissolves or dissolves adhesive residues from labels and other contaminants on the container surface. The cold caustic soda has a temperature of < 60 °C.

[0026] In contrast to the usual cleaning step with hot 1 to 3% caustic soda at 40 to 90 °C, the caustic soda is cold, at a temperature of < 60 °C. The hot caustic soda reliably dissolves so-called hot melt adhesives, especially when the washing process is intensified with surfactants. However, the hot washing process has the disadvantage of increasing the odor of the recycled flakes: When the polyolefin flakes are heated in the hot solution together with the residues in the polyolefin packaging, the adhesives, and the food residues, small organic compounds migrate into the polyolefin matrix at this high temperature, causing an even more intense odor. The migration of these small molecules into the material depends on the temperature and drives small, odorous molecules into the material as a negative effect of the surface cleaning.Particularly affected are odorous organic acids such as butyric acid, valeric acid, aldehydes octanal, nonanal, decanal, undecanal, dodecanal, lactones, saturated mineral oil hydrocarbons (MOSH) and aromatic mineral oil hydrocarbons (MOAH).

[0027] Another disadvantage of the hot cleaning step is that organic structures not only dissolve easily in lye, but also large organic molecules react with the lye and break down into many small, odorous organic compounds. The saponification of fats and oils is well known; the lye breaks them down and transforms them into small, odorous molecules. Hydrolysis also affects other large molecules such as proteins and carbohydrates: A hot lye can attack many organic compounds and break them down into small, odorous molecules. The hotter the lye, the more intense the breaking reactions and the greater the exposure to odorous molecules in the hot wash. These broken-up small molecules, in addition to the existing small molecules, ensure increased migration into the polyolefin matrix and a correspondingly greater odor load in the recycled flakes or granules.

[0028] Following step c1, the flakes undergo a first deodorization step c2 with steam. During this step, the flakes are exposed to a high steam concentration to entrain the volatile and odor-causing substances with the steam and to clean the flakes of adhesive residues and other contaminants. This results in clean flakes with very low odor. Therefore, it is more advisable not to wash flakes with hot water after intensive washing b, but rather to wet them with NaOH (percolation, c1) and clean them with steam (c2) to significantly improve the odor of the regranulates and products derived from them.

[0029] The cleaned flakes are sorted in a flake sorting system d and extruded in step e and granulated into regranulates. Figures 2 and 3As shown, flake sorting (d) can also be performed before step (c1) or (c2). This results in extremely pure color fractions, thus largely avoiding so-called "cross-contamination." Since the flakes are preheated by steam, they require only a minimal amount of heat input during melting in the extrusion (e). The regranulates can be subjected to a second deodorization (f) in addition to the steam treatment (c2), for example, using a degassing system (venting system).

[0030] After step c or d, the flakes are rinsed and dewatered k. This prevents caustic soda residues from being carried over into the subsequent process steps and allows the flakes to be processed as dry as possible.

[0031] The steam, along with the caustic soda entrained by the surface of the flakes, enters a separation unit h. In the separation unit h, the steam is separated from the caustic soda. The caustic soda, like the wash water from the intensive wash, is fed to a wastewater treatment unit g. The steam is passed through a heat exchanger i, where it releases heat and condenses. The contaminated and condensed steam is also fed to the wastewater treatment unit g.

[0032] In heat exchanger i, water is preheated by the used steam to generate steam. The preheated water is converted into steam in a steam generator j, which is fed to step c2. Legend:

[0033] bbOpen bales aContainer sorting bIntensive washing fsFlake separation c1Percolation with cold caustic soda c2First deodorization with steam dFlake sorting eExtrusion and granulation fSecondary deodorization gWastewater treatment hSeparation iHeat exchanger jSteam generator kRinsing and dewatering

Claims

1. A method for recycling polyolefin containers, comprising the following method steps: a) container sorting, b) intensive wash with comminution of the containers to form flakes, friction wash and density separation, c) cleaning step, d) flake sorting, e) extrusion and granulation of the cleaned flakes, characterized in that the cleaning step comprises: - c1) a percolation of the flakes with cold sodium hydroxide solution at a temperature of < 60 °C, and - c2) a first deodorization of the flakes with steam carried out subsequently to step c1).

2. The method according to claim 1, characterized in that the steam in step c2) entrains the sodium hydroxide solution from the surface of the flakes.

3. The method according to claim 1 or 2, characterized in that, in a separation h), the contaminated sodium hydroxide solution and the contaminated steam are separated from one another.

4. The method according to any one of the preceding claims, characterized in that, after step e), a second deodorization f) of the regranulates is carried out.

5. The method according to one of the preceding claims, characterized in that the wastewater of the intensive wash, the contaminated sodium hydroxide solution and the contaminated steam are supplied to a wastewater treatment g).

6. The method according to any of the preceding claims, characterized in that the flake sorting d) is carried out before percolation c1), after percolation c1) or after the first deodorization c2).

7. The method according to any of the preceding claims, characterized in that the intensive washing b) is carried out with water at a temperature < 60 °C.

8. The method according to any of the preceding claims, characterized in that, in a heat exchanger i), the contaminated steam releases its residual heat to water provided for steam generation for preheating.

9. The method according to claim 8, characterized in that the preheated water is evaporated to steam in a steam generator j).

10. The method according to any of the preceding claims, characterized in that the flakes are preheated by the step c2) for the step e).

11. The method according to any of the preceding claims, characterized in that, after the step c) or d), rinsing and draining k) of the flakes takes place.