Manufacturing method for recycled resin pellets

Abstract

This invention provides a method for manufacturing recycled resin pellets that minimizes the reattachment of foreign matter to the waste material when it is recycled and reused, by highly removing foreign matter from the water used to wash the waste material. [Solution] A method for producing recycled resin pellets, comprising crushing waste plastic, washing it, and then melt-kneading it, wherein the washing is carried out in multiple washing steps A, B, and C, and the water used in washing steps A, B, and C is circulated in separate circulation paths for washing steps A, B, and C, and the water used in washing steps A, B, and C is reused by centrifugal separation by rotary filtration with different filtration accuracies in each circulation path.

Description

Technical Field

[0005] ,

[0004] , , ,

[0001] The present invention relates to a technology for recycling waste plastics into recycled resin pellets.

Background Art

[0002] Conventionally, plastics that have become waste materials have been sorted, collected, and recycled. For example, in the case of PET bottles and food trays, their waste materials are already being collected from general consumers and recycled into recycled resin pellets. However, at present, only a part of the resin products are recycled, and many waste resin materials are not recycled and are still disposed of by incineration, landfill, etc. From the perspective of reducing plastic waste and saving resources, it is required that various materials can be recycled.

[0003] Examples of resin products that require recycling include blue sheets and flexible containers. Blue sheets are used for covering construction sites and emergency measures for roofs damaged by typhoons, etc., and are often discarded after the construction is completed. Flexible containers are large bags for storing resin raw materials, agricultural products, etc., and many of them are one-way (disposable).

[0004] In recycling, horizontal recycling that regenerates used waste materials into the original products is ideal from the perspective of reducing waste emissions. Many blue sheets and flexible containers commonly use a fabric of yarn called flat yarn. Flat yarn is a yarn obtained by slitting a film into narrow widths and stretching it in the longitudinal direction. In the horizontal recycling of waste materials of blue sheets and flexible containers, it is required to create recycled flat yarn using the recycled resin pellets of these waste materials.

[0005] Therefore, the present inventor invented the recycled resin manufacturing apparatus described in Patent Document 1. The apparatus in Patent Document 1 focuses on the fact that waste resins exhibit large variations in MI due to UV degradation and differences in specifications. The apparatus in Patent Document 1 can separate granular recycled resin pellets into in-spec pellets that are within a predetermined viscosity range and out-of-spec pellets that are outside that viscosity range. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-81417 [Overview of the project] [Problems that the invention aims to solve]

[0007] Blue tarpaulins and flexible containers are used not only at civil engineering and construction sites, but also in various factories, farms, and livestock barns, and the waste materials have a lot of foreign matter attached to them, not just wood dust and sand. If the foreign matter attached to the waste materials is not sufficiently removed, the recycled resin pellets will contain a lot of foreign matter, making it difficult to manufacture films and stretch them. Even if the variation in MI in recycled resin pellets is suppressed using the apparatus of Patent Document 1, it is not possible to obtain recycled flat yarn that has sufficient productivity and high quality.

[0008] Many plastic molding machines, not just those used for resin pellet production or flat yarn production, are equipped with a wire mesh called a screen mesh. This screen mesh is provided to remove foreign matter, but it is mainly intended to deal with foreign matter that is unexpectedly present, and it is difficult to remove large amounts of fine particles such as sand with a screen mesh. Therefore, in the production of recycled flat yarn, it is necessary to wash the material to a state where it is almost free of foreign matter before melting and kneading it with recycled resin pellets.

[0009] Generally, cleaning uses water to wash away foreign matter, but because cleaning uses a large amount of water, it is not feasible to drain and discard the water after each cleaning cycle. It is necessary to recirculate and reuse the water used for cleaning, but if the water used for cleaning contains a lot of foreign matter, fine foreign matter that flows with the water will inevitably adhere to the waste material that is supposed to be cleaned. When circulating water, it is difficult to filter it sufficiently by simply passing it through a metal mesh filter as a filter medium.

[0010] The present invention solves the aforementioned problems and aims to produce recycled resin pellets with a high degree of foreign matter removal by highly removing foreign matter from the water used to wash the waste material, thereby reducing the re-adhesion of foreign matter to the waste material when it is recycled and reused. [Means for solving the problem]

[0011] The present invention provides a method for producing recycled resin pellets to achieve the above objective, comprising crushing waste plastic, washing it, and then melt-kneading it, wherein the washing is performed in multiple steps, the water used in the multiple washing steps is circulated in separate circulation paths for each washing step, and the water used in the washing is reused by centrifugal separation by rotary filtration with different filtration accuracies in each of the circulation paths.

[0012] According to this manufacturing method, the filtration precision is changed at each washing stage and the water is circulated. Therefore, in the washing process that focuses on removing fine foreign matter, filtration can be performed with finer precision, and even if the number of circulation cycles increases, water contamination can be suppressed. In addition, although the washing process immediately after crushing contains many fragments of waste plastic, these fragments can be easily recovered by centrifugal separation using rotary filtration, and the amount of waste plastic discarded is also reduced.

[0013] In the method for manufacturing recycled resin pellets described above, it is preferable that the washing is carried out in a plurality of steps including a specific gravity separation step and a beating step, and further, it is preferable that the circulation path for circulating the water used in the specific gravity separation step and the circulation path for circulating the water used in the beating step are different, and that the rotary filtration performed on the water used in the beating step has lower filtration accuracy than the rotary filtration performed on the water used in the specific gravity separation step.

[0014] According to this manufacturing method, beating allows for the removal of foreign matter that is difficult to remove by gravity separation, thus more effectively removing foreign matter attached to waste plastic. Although the water used for beating contains finer foreign matter, by separating the circulation paths of the water used for gravity separation and the water used for beating, and by filtering the water used for beating with a finer filtration precision, water contamination can be suppressed even after repeated circulation.

[0015] Furthermore, in the method for producing recycled resin pellets described above, it is preferable to repeat the circulation process multiple times and then purify the circulated water using a membrane separation activated sludge method.

[0016] This manufacturing method allows for the production of recycled resin pellets while maintaining the water quality necessary for washing and other processes, while preventing the need for larger equipment and reducing water usage. [Effects of the Invention]

[0017] By using the method for producing recycled resin pellets of the present invention, recycled resin pellets with minimal foreign matter contamination can be obtained. This method can be used not only for recycling blue tarpaulins and flexible containers, but also for recycling waste plastics used outdoors, such as agricultural sheets and artificial turf. It may also be used for recycling waste plastics where hygiene is a concern, such as food trays and beverage bottles. [Brief explanation of the drawing]

[0018] [Figure 1] This flowchart shows an example of the method for producing recycled resin pellets according to the present invention. [Figure 2] It is a schematic diagram showing an example of a method for manufacturing recycled resin pellets of the present invention. [Figure 3] It is a schematic diagram showing a process of purifying water used in the method for manufacturing recycled resin pellets of the present invention.

Embodiments for Carrying Out the Invention

[0019] Hereinafter, preferred embodiments for carrying out the present invention will be described. The waste plastic used in the present invention is used plastic recovered for recycling. It is not limited with respect to the material and shape, and various ones can be used. Hereinafter, as an example, a method for recycling waste plastic used indoors and outdoors in the form of a sheet, such as a blue sheet, as a raw material for the sheet again will be described.

[0020] FIG. 1 is a flowchart showing an example of a method for manufacturing recycled resin pellets according to the present embodiment. Specifically, FIG. 1 shows a process of pelletizing waste plastic that was originally used as a sheet into recycled resin (steps S100 to S105), and a process of circulating water and purifying water used in that process. FIG. 2 schematically illustrates the pelletizing process of the recycled resin and the water circulation process shown in FIG. 1. In FIG. 2, the waste plastic 10 recovered for recycling undergoes a pulverization and washing process, followed by melt extrusion and granulation to become recycled resin pellets 15.

[0021] Hereinafter, the method for manufacturing recycled resin pellets according to the present embodiment will be described along the flowchart of FIG. 1 while referring to the schematic diagram of FIG. 2. In FIG. 2, first, the waste plastic 10 is sent to the wet pulverizer 1 to perform washing A (step S100 in FIG. 1). Washing A is a process that performs both pulverization and the first washing. Water from the circulation tank A is supplied to the wet pulverizer 1, and the water drained from the wet pulverizer 1 is discharged to the circulation tank A. The circulation tank A is equipped with a centrifugal filtration device 2a, and the water supplied to the wet pulverizer 1 again passes through the centrifugal filtration device 2a. Details of the centrifugal filtration device 2a will be described later (the same applies to the centrifugal filtration devices 2b and 2c).

[0022] On the one hand, since the cleaning A only preliminarily removes foreign matters adhering to the waste plastic 10, it may only be pulverized using a dry pulverizer. These pulverization and cleaning may be performed step by step over a plurality of times respectively. Also, before pulverizing the waste plastic 10, the waste plastic 10 may be roughly shredded with a shredder, or a treatment of washing away foreign matters while keeping the waste plastic 10 in a sheet form may be performed before the cleaning A.

[0023] The present invention includes a plurality of cleaning steps, and the step after pulverizing the waste plastic 10 includes one or more cleaning steps. In the step after pulverizing the waste plastic 10, it is preferable to include two or more cleaning steps. In the present embodiment, as shown in FIG. 1, after the cleaning A (step 100) by wet pulverization, two cleaning steps of the cleaning B (step 101) by specific gravity separation and the cleaning C (step 102) by beating are carried out.

[0024] In the cleaning B (step 101 in FIG. 1) by specific gravity separation, using a water tank type specific gravity separator 4 (FIG. 2), it is cleaned by separating the target waste plastic and foreign matters by floating and sinking according to specific gravity. In FIG. 2, the pulverized waste plastic 11 is mixed with the water supplied from the circulation tank B while being rotated at high speed by the paddle screw 3a, and sent to the water tank type specific gravity separator 4. The water drained from the specific gravity separator 4 is discharged to the circulation tank B. The circulation tank B is equipped with a centrifugal filtration device 2b, and the water supplied to the paddle screw 3a again passes through the centrifugal filtration device 2b.

[0025] If simply melt-kneading and pelletizing, sufficient cleaning can be performed only with the cleaning A (step 100 in FIG. 1) by wet pulverization and the cleaning B (step 101 in FIG. 1) by specific gravity separation. However, in order to perform processing such as thinning or stretching using the melt-kneaded resin, it is preferable to remove even finer foreign matters. Therefore, in the present embodiment, the cleaning C (step 102 in FIG. 1) is carried out using a beater 5 (FIG. 2).

[0026] As the beating machine 5, a beating machine called a disc refiner can be used. A disc refiner has a structure in which bladed discs are placed facing each other at a narrow interval, and one disc is rotated at high speed, sending water and waste plastic between the discs. In this structure, foreign matter can be removed by scraping it off by applying a shearing force to the waste plastic. Beating also serves as a rinsing process, so even very fine foreign matter that is difficult to remove by specific gravity separation can be removed. In Figure 2, the waste plastic 12 processed by the specific gravity separator 4 is rotated at high speed by a paddle screw 3b, mixed with water supplied from the circulation tank C, and sent to the beating machine 5. The water drained from the beating machine 5 is discharged into the circulation tank C. The circulation tank C is equipped with a centrifugal separator 2c, and the water supplied back to the paddle screw 3b passes through the centrifugal separator 2c.

[0027] After washing by beating C (step 102 in Figure 1), dewatering is performed by pressing using a dewatering press 6 (Figure 2) (step 103 in Figure 1). Dewatering by pressing is a process to remove moisture contained in the waste plastic 13. Other methods such as centrifugal dewatering may be used instead of dewatering by pressing. However, beating makes the waste plastic 12 more likely to fibrillate due to the shear force, and it becomes easier to tear into fine fibers. The fragments of the finely torn waste plastic 13 are difficult to recover, but dewatering by pressing removes water while solidifying the waste plastic 13, so the waste plastic 14 is bundled into a certain size, making it easier to recover. In addition, because the waste plastic 14 is bundled, it is easier to feed it into the extruder 7 in the next melt mixing process.

[0028] After dewatering by pressing (step 103 in Figure 1), the recycled resin pellets 15 (Figure 2) are produced by melt mixing (step 104 in Figure 1) and granulation (step 105 in Figure 1). In Figure 2, the process from wet grinding by the wet pulverizer 1 to dewatering by pressing dewatering by the dewatering press 6 (steps 100-103 in Figure 1) is performed almost continuously. However, dewatering by pressing (step 103 in Figure 1) and melt mixing (step 104 in Figure 1) do not need to be performed continuously; in Figure 2, the waste plastic 14 can be thoroughly dried before melt mixing.

[0029] Melt mixing can be carried out as usual using the extruder 7, as shown in Figure 2. If the waste plastic 10 is clearly degraded, such as when used outdoors, viscosity adjustment is preferable. Viscosity adjustment can be done by measuring the MFR (melt flow rate) in the molten state and adding a low MFR raw material if it is higher than the standard value. Granulation (step 105 in Figure 1), which follows melt mixing (step 104 in Figure 1), is carried out using the granulator 8 (Figure 2). Granulation by the granulator 8 is preferably carried out using a pelletizer. Cutting methods such as the strand cut method and the hot cut method can be selected as appropriate.

[0030] In washing steps A, B, and C (steps 100-102 in Figure 1), a large amount of water is used in each step, so the water used in each step is reused. However, the water used for washing once contains foreign matter that was attached to the waste plastic. Therefore, if the water used for washing is reused without being re-treated, the foreign matter in the water will re-attach to the waste plastic. Ideally, the water used for washing should be purified and recycled after each use. In that case, a high purification capacity and water storage tank would be required, making the entire facility larger. Therefore, it is preferable to purify the water used for washing by repeatedly circulating it while easily and sufficiently removing the foreign matter contained in the water and keeping the degree of water contamination to a minimum.

[0031] In this embodiment, as shown in Figure 1, the washing process is divided into several washing steps, Washing A, B, and C (steps 100 to 102). In Figure 1, circulation tank A corresponds to washing A by wet grinding, circulation tank B corresponds to washing B by specific gravity separation, and circulation tank C corresponds to washing C by beating. More specifically, in Figure 2, circulation tanks A, B, and C are equipped with centrifugal separators 2a, 2b, and 2c, respectively, and each has an independent dedicated circulation path. Centrifugal separators 2a, 2b, and 2c are devices that can perform centrifugal separation by using water flow for rotational filtration. Examples of such centrifugal separators include Philstar (registered trademark: manufactured by Industria Co., Ltd.). Rotary filtration can separate not only foreign matter (dirt) such as soil and sand, but also fragments of waste plastic.

[0032] In Figure 1, the filtration accuracy in circulation tanks A, B, and C corresponds to the filtration accuracy of centrifugal separators 2a, 2b, and 2c (Figure 2). As shown in Figure 1, each filtration accuracy is set to a different value. Filtration accuracy refers to the size of particles that can be removed by filtration. Filtration accuracy corresponds to the mesh size of the filter; the smaller the value, the finer the particles that can be removed. As shown in Figure 1, the filtration accuracy in wet grinding wash A is set to 1 mm, the filtration accuracy in specific gravity separation wash B is set to 0.5 mm, and the filtration accuracy in beating wash C is set to 50 μm.

[0033] The water used in the wet grinding wash A contains not only foreign matter but also many fragments of the ground waste plastic. Therefore, in wash A, it is preferable to focus on recovering the waste plastic fragments and set the filtration accuracy of the rotary filtration in circulation tank A to be coarse. In the water used in the specific gravity separation wash B, waste plastic fragments still remain, and it is also preferable to focus on recovering them. The water circulated in circulation tank B is water that has been separated by specific gravity separator 4 (Figure 2). Therefore, the filtration accuracy of circulation tank B can be set to be smaller than that of circulation tank A. On the other hand, the beating wash C is the final stage of washing and corresponds to rinsing. Therefore, it is preferable to set the filtration accuracy with an emphasis on removing contaminants from the water.

[0034] In this embodiment, as shown in Figure 2, the circulation paths corresponding to each cleaning process A, B, and C (steps 100-102 in Figure 1) are different. Specifically, the water from circulation tank A is kept to a minimum from flowing into circulation tank B, and the water from circulation tank B is kept to a minimum from flowing into circulation tank C. Furthermore, as mentioned above, cleaning C, which involves beating, is the final stage of cleaning and corresponds to rinsing, so the filtration accuracy of circulation tank C can be set to be as low as possible compared to the filtration accuracy of circulation tank B. As a result, contaminants in the circulating water can be removed at a high level, and even after repeated circulation, foreign matter is less likely to reattach to the waste plastic.

[0035] Even with rotary filtration, foreign matter accumulates in the water used for washing. Therefore, as shown in Figure 1, it is preferable to periodically replace the water in circulation tanks A, B, and C with purified water using the pressurized flotation method and the membrane separation activated sludge method. The replacement of the water used for washing with purified water can be done, for example, by periodically injecting purified water into each circulation tank A, B, and C to dilute the water used for washing. On the other hand, if the washing processes A, B, and C and the dewatering process (steps 100-103 in Figure 1) are performed only during the daytime, it is preferable to circulate the water used for washing A, B, and C during the day and purify all the water used for washing at night when each process is stopped.

[0036] Figure 3 shows a specific example of water purification. In Figure 3, water from circulation tanks A, B, and C is collected in stock tank 20. The water from stock tank 20 is transferred to reaction tank 21, where foreign matter is coagulated using a coagulant. Then, suspended solids are removed from the water by pressurized flotation 22. Specifically, a large number of fine bubbles are generated in the water using air, and the buoyancy of the bubbles is used to make the suspended solids float to the surface. After the water treated by pressurized flotation 22 is placed in reserve tank 23, membrane separation 24 is performed using a filtration membrane to remove even finer foreign matter. The filtration membrane used for membrane separation 24 can be appropriately selected depending on the remaining foreign matter, but it is preferable to use a UF membrane (limiting filtration membrane). The water that has been purified in this way is sent to circulation tanks A, B, and C and used again for washing each of them. Water that is contaminated and cannot be completely purified is discharged after passing through disinfection tank 25.

[0037] The water purification method shown in Figure 3 employs both the so-called pressurized flotation method and the membrane separation activated sludge method, as shown in Figure 1. While various known water treatment methods can be used for water purification, waste plastic sheets used outdoors have various foreign substances attached to them, including not only soil and sand, but also oil, phosphorus, and nitrogen compounds. From the viewpoint of preventing the re-adhesion of foreign substances contained in the washing water, it is preferable that the washing water be purified as much as possible, and preferably purified to a level that is suitable for discharge. In particular, these foreign substances can be removed to a high level by using the membrane separation activated sludge method. Furthermore, in the recycling of waste plastics, oil components can cause resin burning and odor during the melting and mixing process. To more reliably remove oil components, it is preferable to use the pressurized flotation method in combination with the membrane separation activated sludge method, and it is preferable to perform water treatment by the pressurized flotation method first, followed by water treatment by the membrane separation activated sludge method.

[0038] As described above, by removing impurities from the water used for washing, the resulting recycled resin pellets contain fewer foreign substances and can be used as plastic with a purity close to that of the original raw material. When recycled resin pellets of blue tarpaulins were produced in the recycled resin pelletization process of this embodiment and used as a raw material for flat yarn, which is a raw material for blue tarpaulins, the film-forming properties when making the flat yarn were stable, and there was little yarn breakage during subsequent stretching. [Explanation of symbols]

[0039] 1. Wet grinder 2a,2b,2c Centrifugal filtration device 3a, 3b Paddle Screw 4. Specific gravity separator 5. Beating machine 6. Press and dewatering machine 7. Extruder 8 Granulator 10. Waste Plastics 15 Recycled resin pellets 22 Pressurized buoyancy 24 Membrane separation

Claims

1. A method for producing recycled resin pellets, which involves crushing waste plastic, washing it, and then melt-mixing it, The cleaning is performed in multiple steps. The water used in the aforementioned multiple cleaning processes is circulated through separate circulation paths for each cleaning process. By performing centrifugal separation by rotary filtration with different filtration accuracies in each of the aforementioned circulation paths, A method for producing recycled resin pellets, characterized by reusing the water used for the aforementioned washing.

2. The method for producing recycled resin pellets according to claim 1, wherein the washing is carried out in a plurality of steps including a specific gravity separation step and a beating step.

3. The method for producing recycled resin pellets according to claim 2, wherein the circulation path for circulating the water used in the gravity separation step and the circulation path for circulating the water used in the beating step are different, and the rotary filtration performed on the water used in the beating step has lower filtration accuracy than the rotary filtration performed on the water used in the gravity separation step.

4. A method for producing recycled resin pellets according to any one of claims 1 to 3, wherein the circulation is repeated multiple times and the circulated water is purified by a membrane separation activated sludge method.

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