Plastic cleaning method and apparatus

Abstract

To provide a plastic cleaning method that removes contaminants from plastic using superheated steam treatment, the method effectively removing contaminants in a significantly reduced processing time.SOLUTION: There is provided a plastic cleaning method for performing superheated steam treatment by introducing plastic 7 into a superheated steam treatment region into which superheated steam flows, the superheated steam treatment region being maintained at a temperature of 100°C or higher and being provided on a transportation path. The superheated steam treatment region is formed of a high-temperature treatment region A supplied with relatively high-temperature superheated steam and a low-temperature treatment region B supplied with relatively low-temperature superheated steam. The high-temperature treatment region A and the low-temperature treatment region B are continuous. The plastic 7 is continuously transferred from the high-temperature treatment region A to the low-temperature treatment region B to perform superheated steam treatment, and the treated plastic 7 is continuously removed from the low-temperature treatment region B.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and apparatus for cleaning plastics, and more particularly to a method and apparatus for cleaning plastics to remove impurities from used plastics. [Background technology]

[0002] From the viewpoint of environmental protection in recent years, there is a demand for recycling various plastic molded products such as films and containers. When recycling plastics, not only is it possible to reuse plastic molded products as they are without damaging their shape, but it is also possible to crush them, recover them in the form of granules or pellets, and mold them again as recycled plastics.

[0003] In any form of plastic recycling, it is necessary to remove impurities from the plastic that have become contaminated through use. A common method for removing impurities from plastics is washing with an organic solvent or water.

[0004] For example, Patent Document 1 discloses a method of removing impurities by contacting polyethylene with an organic solvent and purifying the solution. Furthermore, Patent Document 2 discloses a method of crushing packaging materials (plastic films) and then chemically cleaning them with acid or alkali for recycling, and Patent Document 3 discloses a method of separating foreign matter and plastics by washing with water.

[0005] However, when an organic solvent is used as a medium to remove impurities from plastics, there are problems of environmental pollution and high costs required for exhaust systems and waste liquid treatment of the organic solvent, etc. Also, when an acid or alkali is used as a cleaning medium, the problem of environmental pollution can be avoided, but the equipment becomes heavy and large.

[0006] On the other hand, when water is used as the cleaning medium, there is no problem with waste liquid treatment. However, in terms of cleaning ability, water is far inferior to organic solvents, and is insufficient for cleaning oily stains such as grease and oil adhering to the surface of plastics. Furthermore, it is almost impossible to remove sorbed substances absorbed inside the plastics.

[0007] Therefore, it would be most preferable if oil stains and sorbed substances inside plastics could be effectively removed by washing with water. From this perspective, in Patent Document 4, the present applicant has proposed a method for removing impurities from plastics by washing with superheated steam as a washing medium.

[0008] The method of Patent Document 4 uses water, so it does not have a negative impact on the environment, does not require large-scale exhaust equipment or waste liquid treatment equipment, and can cleanly remove oily stains such as grease and oils attached to the plastic surface, as well as impurities such as oily stains (i.e., sorption components) that have soaked into the plastic, in the same way as when an organic solvent is used as a cleaning medium. However, there is still room for improvement in that it takes a long time to clean. In other words, there was a problem that the odor was not sufficiently removed unless the superheated steam treatment was performed over a sufficient period of time. In particular, when the used plastic was mainly made of polypropylene, it took a long time to remove the odor. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Special Publication No. 2020-511560 [Patent Document 2] Japanese Patent Application Publication No. 6-31733 [Patent Document 3] Japanese Patent Application Publication No. 6-173182 [Patent Document 4] WO2022 / 124015 issue Summary of the Invention [Problem to be solved by the invention]

[0010] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a method and apparatus for removing contaminants from plastics by superheated steam treatment, which can effectively remove contaminants in a significantly shortened treatment time. [Means for solving the problem]

[0011] As a result of conducting numerous experiments and studies into the above-mentioned problems, the inventors have discovered that in order to effectively remove contaminants from used plastics by superheated steam treatment and to obtain plastics that are almost odorless in a short period of time, it is effective to remove the plastics that have been kept at high temperatures for cleaning from the superheated steam treatment area not immediately, but rather to allow the temperature to drop appropriately while undergoing superheated steam treatment, and then remove them from the superheated steam treatment area, thereby completing the present invention.

[0012] That is, according to the present invention, a superheated steam treatment area maintained at a temperature of 100° C. or more is provided in a conveying path, and plastics are introduced into the superheated steam treatment area through which superheated steam flows to perform cleaning, comprising the steps of: The superheated steam treatment area is formed of a high-temperature treatment area to which relatively high-temperature superheated steam is supplied and a low-temperature treatment area to which relatively low-temperature superheated steam is supplied, the high-temperature treatment area and the low-temperature treatment area being continuous with each other, The plastic is continuously transferred from the high temperature processing area to the low temperature processing area for superheated steam processing; continuously removing treated plastic from said low temperature treatment zone; A method for cleaning plastics is provided.

[0013] In the method of the present invention, the following means are preferably used: (1) Superheated steam having a temperature of 100°C to 150°C is supplied to the low-temperature treatment area, and superheated steam having a temperature 20°C or more higher than that of the superheated steam supplied to the low-temperature treatment area is supplied to the high-temperature treatment area. (2) The passage time of the plastic through the high-temperature treatment area is set to be longer than the passage time through the low-temperature treatment area. (3) The plastic to be processed is used plastic. (4) The oxygen concentration in the superheated steam treatment area is adjusted to 5% or less throughout.

[0014] According to the present invention, there is also provided a plastic washing apparatus including a superheated steam treatment chamber to which a steam supply pipe and a steam exhaust pipe are connected and which is maintained at a temperature of 100° C. or higher, and a plastic transport member passing through the superheated steam treatment chamber, the superheated steam treatment chamber is provided with a high-temperature treatment region located on an inlet side of the plastic and supplied with relatively high-temperature superheated steam, and a low-temperature treatment region located on an outlet side of the plastic and supplied with relatively low-temperature superheated steam, the steam supply pipe is connected to each of the high temperature processing area and the low temperature processing area; A plastic cleaning device is provided, comprising: In such a plastic washing device, (5) The steam exhaust pipe is connected to each of the high-temperature treatment area and the low-temperature treatment area; is preferred. Effect of the Invention

[0015] The plastic washing method of the present invention involves supplying plastics to be treated (specifically used plastics) to a superheated steam treatment zone maintained at a temperature of 100°C or higher and washing with superheated steam, but this treatment zone is divided into a high-temperature treatment zone and a low-temperature treatment zone, and the plastics are continuously passed from the high-temperature treatment zone to the low-temperature treatment zone and continuously removed. That is, following the superheated steam treatment in the high-temperature treatment zone, the plastics are continuously removed while the treatment temperature is lowered in the presence of superheated steam, so there is no need for a dehydration process as a post-process, washing is performed effectively in a very short time, and not only are contaminants effectively removed, but plastics with almost no odor are obtained. This type of washing process effectively removes foreign matter, and not only contaminants are removed, but also odorous components generated by the decomposition of the plastic or contaminants are preferably washed (deodorized).

[0016] For example, in Experimental Example 9, which used a batch-type processing furnace described below, when used plastics were washed with superheated steam at 240°C for 5 minutes in a processing chamber and then taken out as is, the results showed that contaminants could not be sufficiently removed and there was a strong odor (evaluated at room temperature). The amount of contaminants was evaluated based on the amount of aldehyde.

[0017] In addition, in Experimental Example 10, which used a similar batch-type treatment furnace, after 5 minutes of treatment at 240°C, the temperature inside the treatment chamber was lowered to 150°C, and when the plastic was removed from the treatment chamber, it took about 20 minutes to lower the temperature to 150°C, so the residence time of the plastic in the treatment chamber was very long, at about 25 minutes in total. Also, although there was a tendency for improvement compared to Experimental Example 9, the removal of pollutants was still insufficient and the odor was still strong.

[0018] Based on the results of Experimental Examples 9 and 10, it was presumed that the longer the temperature was maintained at 150°C or above, the more effective the removal of pollutants and odors would be. However, the results of Experimental Examples 1 and 2 contradicted this presumption.

[0019] That is, in Experimental Example 1 described later, in accordance with the present invention, the treatment chamber was divided into a high-temperature treatment area and a low-temperature treatment area, and superheated steam set at a temperature of 220°C was supplied to the high-temperature treatment area, while superheated steam set at a temperature of 150°C was supplied to the low-temperature treatment area, and cleaning with superheated steam was carried out continuously. The conveying speed of the plastic was set so that the treatment time (passage time) in the high-temperature treatment area was 5 minutes, and the treatment time (passage time) in the low-temperature treatment area was 2 minutes, and the total treatment time in the treatment chamber was a fairly short 7 minutes. In Experimental Example 1, compared to Experimental Examples 9 and 10, the results showed that contaminants were sufficiently removed and there was almost no odor (evaluated at room temperature). That is, because the superheated steam treatment was performed while the plastic was continuously conveyed, the treatment time was shortened, but contrary to the above assumption, the results showed that contaminants were sufficiently removed and there was almost no odor even if the time at which the plastic was held at 150°C or higher was short.

[0020] Based on this, in Experimental Example 2, the outlet temperature of the low-temperature treatment area was set even lower, at 100°C, and the plastic was washed at the same conveying speed as in Experimental Example 1. Again, the results were the same as in Experimental Example 1, with sufficient removal of contaminants and almost no odor. Furthermore, the temperatures in the high-temperature and low-temperature treatment areas, as well as the conveying speed, were varied, but in all cases, contaminants were effectively removed in a short time and odor generation was effectively suppressed compared to the case where a batch-type treatment furnace was used.

[0021] From these findings, the inventors have concluded that, although the cleaning time at high temperatures is important in cleaning with superheated steam treatment, if plastics kept at a relatively high temperature are kept in an atmosphere with a high oxygen content for a long time, the deterioration of the plastic will progress, and the amount of pollutants (acids and aldehydes) that cause odors will increase, causing the generation of odors.In other words, the present invention has succeeded in sufficiently removing pollutants in a short time and effectively suppressing the generation of odors by continuously performing superheated steam treatment while preventing the plastics from being kept in the atmosphere at a temperature above a certain level. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram showing a cleaning device for carrying out the method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] <Cleaning equipment> In FIG. 1, which shows a cleaning apparatus for carrying out the cleaning method of the present invention, the cleaning apparatus, designated by 10, has a high-temperature treatment chamber A (high-temperature treatment area) and a low-temperature treatment chamber B (low-temperature treatment area) arranged side by side in series, and a continuous conveying device 1 is provided so that the chambers pass through the interior of the chambers.

[0024] The continuous conveying device 1 consists of a pair of conveying rollers 3, 3 and a conveying belt 5 stretched between the conveying rollers 3, 3. The conveying belt 5, driven by the rotation of the conveying rollers 3, 3, continuously conveys the plastic 7 to be treated from the high-temperature treatment chamber A to the low-temperature treatment chamber B, and then removes it from the low-temperature treatment chamber B to the outside.

[0025] 1, the high-temperature processing chamber A and the low-temperature processing chamber B communicate with each other, and no partition is provided between them. Each processing chamber is provided with its own steam supply pipe (9a and 9a') and steam exhaust pipe (9b and 9b'). That is, in the high-temperature processing chamber A, high-temperature steam is supplied from the steam supply pipe 9a and exhausted from the steam exhaust pipe 9b, and in the low-temperature processing chamber B, low-temperature steam is supplied from the steam supply pipe 9a' and exhausted from the steam exhaust pipe 9b'. The superheated steam introduced forms a high-temperature atmosphere in the high-temperature processing chamber A, and a low-temperature atmosphere compared to the high-temperature processing chamber A is formed in the low-temperature processing chamber B.

[0026] 1, the high-temperature treatment chamber A is provided with a steam exhaust pipe 9b, and the low-temperature treatment chamber B is provided with a steam exhaust pipe 9b', but depending on the total capacity of the cleaning device 10 and the steam supply speed from the steam supply pipe 9a or 9a', it is also possible to use only one steam exhaust pipe. However, in order to ensure that high-temperature superheated steam is sprayed onto the plastic 7 in the high-temperature treatment chamber A and low-temperature superheated steam is sprayed onto the plastic 7 in the low-temperature treatment chamber B, it is preferable to provide a steam exhaust pipe 9b for exhausting high-temperature superheated steam in the high-temperature treatment chamber A and a steam exhaust pipe 9b' for exhausting low-temperature superheated steam in the low-temperature treatment chamber B, with dedicated steam exhaust pipes provided for each chamber, as shown in FIG.

[0027] In addition, as long as the continuous transport of the plastic 7 is not impaired, by providing dedicated steam exhaust pipes 9b, 9b' and at the same time providing a partition (not shown) between the high-temperature processing chamber A and the low-temperature processing chamber B, the processing temperature and processing time in each processing chamber A, B can be controlled more precisely.

[0028] Furthermore, as necessary, inert gas such as nitrogen can be appropriately supplied and discharged from these steam supply pipes 9a and 9a', so that the atmosphere in the high-temperature processing chamber A and the low-temperature processing chamber B can be replaced with the inert gas, and the oxygen concentration in the chambers can be further reduced. The supply of the inert gas is not limited to the above-mentioned method as long as it can reduce the oxygen concentration in the high-temperature processing chamber A and the low-temperature processing chamber B, and a method of supplying the inert gas from a source other than the steam supply pipes 9a and 9a' may also be used. Other examples of cleaning equipment include a series of multiple high-temperature treatment chambers (high-temperature treatment areas) with different temperature settings, each of which is provided with its own steam supply pipe (and even a steam exhaust pipe), allowing each chamber to be set to an arbitrary temperature depending on the plastic being treated.

[0029] In addition, the above-mentioned cleaning device 10 is structured to be heated by a heater or the like so that the atmosphere in the high-temperature processing chamber A and the low-temperature processing chamber B is maintained at 100°C or higher, condensation of the supplied superheated steam is prevented (especially at the inlet and outlet sides of the device), and the temperature sprayed onto the plastic 7 in each processing chamber A, B is maintained at the set temperature.

[0030] <Cleaning medium> In the present invention, impurities are removed from plastics using the above-mentioned cleaning device 10, and superheated steam (i.e., water) is used as the cleaning medium for this purpose. This superheated steam has a lower dielectric constant due to heating compared to water at 100°C or less, and for example, superheated steam with a relative dielectric constant of 60 or less, preferably 40 or less, more preferably 25 or less, and even more preferably 10 or less is used.

[0031] That is, the relative dielectric constant of water at room temperature is remarkably higher than that of organic solvents, and it is a unique polar solvent, which has poor affinity (oil repellency) with organic components (oil components such as fats and oils), which is the reason why dirt is difficult to remove. On the other hand, it is known that the relative dielectric constant decreases as the temperature increases. As can be understood from this, the dielectric constant is reduced by heating, and cleaning is performed with water (superheated steam) that is in a state close to that of an organic solvent. For example, the above-mentioned Patent Document 4 also provides a detailed explanation of the measurement method of the relative dielectric constant, but when water is heated to about 200°C, its relative dielectric constant becomes about 35, which is close to the relative dielectric constant of methanol at room temperature, and when heated to about 300°C, its relative dielectric constant becomes about 20, which is close to the relative dielectric constant of acetone at room temperature, and when further heated near the critical point, it becomes the same as non-polar organic solvents such as chloroform and ethyl ether.

[0032] As can be understood from the above explanation, the superheated steam used as a cleaning medium is in a state close to an organic solvent due to its non-polarity caused by its low dielectric constant, and furthermore, due to this low dielectric constant, the water becomes high-temperature steam exceeding 100° C. When in contact with such high-temperature water, the polymer chains that make up the plastic loosen, and the low-dielectric water easily penetrates into the plastic. According to the present invention, by using water vapor, which has been made to have a low dielectric constant and become nearly non-polar like an organic solvent, and which has been heated to a high temperature to reduce the dielectric constant, as a cleaning medium, impurities adhering to the surface of plastics and impurities sorbed within the plastics can be effectively removed.

[0033] Furthermore, in the present invention, the water used as the cleaning medium may be in a state called superheated steam, subcritical water, or supercritical water, for example, in the high-temperature treatment chamber A, by heating for reducing the dielectric constant as described above.

[0034] <Plastics to be cleaned> In the present invention, the plastic to be cleaned is a so-called used plastic, and any plastic can be used as long as it is not decomposed by cleaning with heated steam as described above. However, preferred examples include olefin resins that are widely recycled by sorting and recovery, such as low-density polyethylene, linear low-density polyethylene, medium or high-density polyethylene, polypropylene, poly-1-butene, and poly-4-methyl-1-pentene. Of course, random or block copolymers of α-olefins such as ethylene, propylene, 1-butene, and 4-methyl-1-pentene may also be used, and cyclic olefin copolymers disclosed in JP-A-2007-284066 and the like can also be suitably used. This is because such olefin resins are extremely stable chemically, and when cleaning with high-temperature water with a low dielectric constant, not only is there little risk of hydrolysis, but they can also be easily recovered.

[0035] Furthermore, when the used plastic contains ethylene-vinyl alcohol copolymer (EVOH), the effect of the present invention is maximized. EVOH is a resin that is widely used in the field of packaging materials as a gas barrier resin, and prevents oxidative deterioration of packaged materials by blocking the permeation of oxygen. However, it is susceptible to thermal degradation, and has the property of generating odorous components such as acids and aldehydes due to thermal degradation. However, according to the treatment method of the present invention, the generation of odorous components can be effectively prevented even when EVOH is contained.

[0036] In the present invention, the used plastics are obtained by collecting waste plastic molded products (such as containers and lids), separating them from other molded products of different materials as appropriate, and removing as much foreign matter as possible through crushing, washing, separation by specific gravity, etc. In the treatment described below, there are no particular limitations, but from the standpoint of transportability, processability, etc., it is preferable for the plastics to be crushed into a flake or granular form, and in particular, those with a mesh size of 10 mm or less, more preferably 8 mm or less, and even more preferably 6 mm or less.

[0037] <Plastic cleaning> The above-mentioned cleaning of plastics, i.e., the cleaning process for removing contaminants, is carried out using the cleaning apparatus 10 described above. That is, by continuously driving the conveyor 1 (conveyor belt 5), used plastics 7 are continuously introduced into the cleaning device 10, and the plastics are cleaned by spraying superheated steam onto them in the cleaning device 10, and the cleaned plastics are continuously removed from the cleaning device 10. By such continuous processing, the present invention makes it possible to obtain plastics from which contaminants have been removed in an extremely short period of time.

[0038] In the present invention, the above-mentioned cleaning device 10 is divided into a high-temperature treatment chamber A and a low-temperature treatment chamber B. While the plastic 7 is being transported, a cleaning process using relatively high-temperature superheated steam is carried out in the high-temperature treatment chamber A, and then a cleaning process using relatively low-temperature superheated steam is carried out in the low-temperature treatment chamber B.

[0039] In such a process, the high-temperature processing chamber A can be structured so that it is kept at a high temperature using a heater or the like, while the low-temperature processing chamber B can be structured so that the interior atmosphere is cooled using a heat exchanger, so that the temperature of the water vapor supplied can be lower than that of the high-temperature processing chamber A.

[0040] In the present invention, low-temperature treatment chamber B is set to supply water vapor at a relatively lower temperature than that in high-temperature treatment chamber A, for example, water vapor at a temperature preferably between 100°C and 150°C, more preferably between 100°C and 135°C, and even more preferably between 100°C and 120°C, and to spray it onto the continuously transported plastics 7. In other words, by setting the treatment temperature in low-temperature treatment chamber B low, deterioration of the plastics 7 when they are removed from the cleaning device 10 is suppressed, and the generation of odors can also be effectively suppressed.

[0041] Moreover, the high-temperature treatment chamber A is set so that superheated steam having a higher temperature is sprayed thereto than in the low-temperature treatment chamber B. For example, superheated steam having a temperature at least 20° C. higher, preferably 35° C. higher, and more preferably 50° C. higher than that of the superheated steam supplied in the low-temperature treatment chamber B is supplied and sprayed onto the continuously transported plastics 7. By spraying superheated steam maintained at such a high temperature, sorbed substances such as oil stains adhering to the plastic surface and oil stains that have penetrated into the interior of the plastic can be effectively washed and removed.

[0042] 1, the high-temperature treatment chamber A is connected to the low-temperature treatment chamber B, so that there is an overall temperature distribution in the high-temperature treatment chamber A and the low-temperature treatment chamber B, and in particular, the atmospheric temperature at the outlet side of the low-temperature treatment chamber B is the lowest. In consideration of such a temperature distribution, it is preferable that the maximum temperature in the high-temperature treatment chamber A is set to 300° C. or less, more preferably 260° C. or less, and further preferably 230° C. or less. If this maximum temperature is higher than necessary, the plastic 7 may decompose, or it may become difficult to adjust the temperature in the next low-temperature treatment chamber B, making it difficult to complete the treatment in a short time.

[0043] As can be understood from the above description, in the present invention, the low-temperature treatment chamber B connected to the high-temperature treatment chamber A is supplied with water vapor having a lower temperature than the superheated steam supplied to the high-temperature treatment chamber A to perform the cleaning process, thereby effectively suppressing deterioration of the plastic 7. For example, as in Example 12 of Patent Document 4, when the low-temperature treatment chamber B is not provided and the temperature of the treatment chamber A is lowered and cooled after the treatment in the high-temperature treatment chamber A, the process becomes a so-called batch process rather than a continuous process, and cooling takes a long time, making it impossible to complete cleaning in a short time.

[0044] In addition, in the low-temperature treatment chamber B, the plastics 7 maintained at high temperatures are cleaned with superheated steam without coming into direct contact with the atmosphere with a high oxygen concentration. That is, if the plastics 7 were to come into contact with the atmosphere at a high temperature, it would deteriorate, and as shown in the experimental example described later, aldehydes would be produced, which would result in the generation of odors. Therefore, in this low-temperature treatment chamber B, cleaning is performed with superheated steam, so the temperature at the outlet must be maintained at at least 100°C to prevent condensation of the superheated steam. Also, if the temperature at the outlet is excessively high, the temperature of the plastics 7 cleaned in the high-temperature treatment chamber A will not be sufficiently reduced, and the plastics 7 treated with superheated steam will come into contact with the atmosphere at a high temperature, causing deterioration of the plastics 7, increasing the amount of aldehyde produced, and causing a strong odor.

[0045] In the present invention, the high-temperature treatment chamber A and the low-temperature treatment chamber B are not separated by a partition or the like, but are connected to each other. For this reason, the two chambers A and B cannot be strictly distinguished from each other. Basically, the temperature gradient from the high-temperature treatment chamber A to the low-temperature treatment chamber B can be confirmed by measuring the temperature of the upper surface of the conveyor belt 5 running through the treatment chambers A and B or its vicinity with a thermocouple or the like in advance laboratory tests, thereby confirming the atmospheric temperature (or temperature gradient) in the high-temperature treatment chamber A where the treatment with high-temperature superheated steam is performed, and the atmospheric temperature (or temperature gradient) in the low-temperature treatment chamber B where the treatment with low-temperature superheated steam is performed.

[0046] In the present invention, in order to effectively clean and remove contaminants attached to the surface of the plastic or sorbed inside, it is preferable to make the residence time in the high-temperature treatment chamber A long and the residence time in the low-temperature treatment chamber B short. The low-temperature treatment chamber B is an area in which the main purpose is to suppress the deterioration of the plastic 7 rather than to remove contaminants, and it is not necessary to set the residence time long, as long as the residence time required to lower the temperature to a certain level is ensured. For example, the residence time in the high-temperature treatment chamber A can be set long by positioning the steam supply pipe 9a for supplying high-temperature superheated steam closer to the outlet side than the center. In addition, the residence time in the high-temperature treatment chamber A can be set long by providing multiple high-temperature treatment chambers (high-temperature treatment areas) in series and providing each high-temperature treatment chamber with a dedicated steam supply pipe (and even a steam exhaust pipe). Furthermore, the residence time in the two chambers A and B can be set arbitrarily by installing a transport device in each of the high-temperature treatment chamber A and the low-temperature treatment chamber B and connecting them. Also, although not strictly, the temperature gradients in the high-temperature treatment chamber A and the low-temperature treatment chamber B can be measured by the above-mentioned method, and the position where the temperature drops sharply can be set as the boundary line X between the high-temperature treatment chamber A and the low-temperature treatment chamber B. Based on the boundary line X thus formed, the residence time in the high-temperature treatment chamber A and the residence time of the continuously conveyed plastic 7 in the low-temperature treatment chamber B can be set. In order to perform effective cleaning in a shorter time, it is preferable to set the residence time in the high-temperature treatment chamber A (passage time of the plastic 7) set in this way to be longer than the residence time in the low-temperature treatment chamber B. For example, it is preferable that the residence time in the low-temperature treatment chamber B is about 95 to 10% of the residence time in the high-temperature treatment chamber A.

[0047] Furthermore, the oxygen concentration in the treatment with superheated steam in the above-mentioned high-temperature treatment chamber A and low-temperature treatment chamber B is adjusted to 5% or less throughout, more preferably 3% or less, and further preferably 1% or less. At this time, by appropriately adjusting the amount of superheated steam supplied (kg / h) to the high-temperature treatment chamber A and low-temperature treatment chamber B to which the superheated steam is supplied, a state in which the oxygen concentration is maintained low can be obtained. In addition, it is also preferable to replace the atmosphere in these chambers with an inert gas such as nitrogen gas and to perform the treatment while flowing the inert gas, as necessary. This allows the superheated steam treatment to be performed in an environment with a lower oxygen concentration, making it possible to more reliably prevent the deterioration of the plastic 7.

[0048] Plastics from which impurities (organic impurities such as oily stains such as grease) have been removed in a shorter time by the above-mentioned washing are less likely to deteriorate and less likely to produce odors, and so can be pelletized by melt extrusion, either alone or after being melt-kneaded with virgin plastic, using an extruder or the like, and then molded again as recycled plastics, and reused as packaging containers, etc. Of course, the washed plastics can also be reused as recycled products as they are.

[0049] As described above, the present invention does not require large-scale exhaust equipment or waste liquid treatment equipment, and can remove contaminants from used plastics in an extremely short time, similar to the case of using organic solvents, making it an extremely useful technology for recycling plastics. EXAMPLES

[0050] The present invention will be further illustrated by the following experimental examples, but the present invention is not limited to these examples. The various samples used in the following experiments, and the measurements and evaluations are as follows:

[0051] (1) Sample to be cleaned Sample A was used as the plastic to be cleaned. This was flakes made by crushing a high-density polyethylene bottle after it had been filled with edible oil. The sample was left untreated and the aldehydes were measured in the same manner as in the experimental example described below. As a reference, the same measurement was also carried out on sample A0, which was flakes obtained by crushing a high-density polyethylene bottle not filled with oil.

[0052] (2) Measurement of aldehydes The aldehydes contained in the washed plastics were pretreated by solid-phase microextraction as described below, and then measured by gas chromatography-mass spectrometry. Pretreatment by solid-phase microextraction was carried out as follows. A 2 g sample was placed in a headspace vial, and an SPME fiber (CAR / PDMS, 85 μm film thickness, manufactured by Supelco) was inserted as an adsorbent. The vial was then stored at 40°C for 15 minutes to allow the impurity components (aldehydes) to be adsorbed onto the SPME fiber, thereby carrying out pretreatment.

[0053] This SPME fiber was measured using a gas chromatography mass spectrometer (GC / MS GC-7890A, MSD-5975C, manufactured by Agilent Technology) to desorb the components adsorbed on the SPME fiber, and the components separated by gas chromatography were detected and analyzed by mass spectrometry to qualitatively and quantitatively analyze the impurity components.

[0054] The measurement conditions are as follows. column: Agilent Technology J&W DB-WAX (60m x 0.25mm, 0.25μ film thickness) Carrier gas: Helium Detector: MSD Ionization method: EI Oven temperature: 45°C to 220°C at a rate of 7°C / min and hold for 15 minutes Inlet temperature: 250℃ Injection mode: splitless Mass spectrum range: 27-600

[0055] <Experimental Example 1> The above-mentioned sample A was used as the plastic to be cleaned. Sample A was placed on the conveyor 1 (conveyor belt 5) and treated by passing through the high-temperature treatment chamber A filled with superheated steam at 220°C for 5 minutes, and then passed through the low-temperature treatment chamber B filled with superheated steam at 150°C for 2 minutes, and sample A was removed from the cleaning device 10. In this case, the treatment time was 7 minutes. The amount of superheated steam supplied was 20 kg / h for a sample charge of 10 g. The obtained sample was pretreated by solid-phase microextraction and aldehydes were measured by gas chromatography mass spectrometry. The position of the boundary line X between the high-temperature processing chamber A and the low-temperature processing chamber B was determined by measuring the temperature distribution on the conveyor belt 5 and determining the portion where the temperature suddenly dropped from 220°C to 150°C. The evaluation results are shown in Table 1.

[0056] <Experimental Example 2> The same treatment and measurement as in Experimental Example 1 were carried out, except that the temperature of the superheated steam in the low-temperature treatment chamber B was set to 100° C. The evaluation results are shown in Table 1. The boundary line X between the high temperature processing chamber A and the low temperature processing chamber B was almost the same as in the first experimental example.

[0057] <Experimental Example 3> The same treatment and measurement as in Experimental Example 1 were carried out, except that the superheated steam temperature in the high-temperature treatment chamber A was set to 200° C., and the superheated steam temperature in the low-temperature treatment chamber B was set to 100° C. In this example, too, the boundary line X between the high-temperature treatment chamber A and the low-temperature treatment chamber B was almost the same as in Experimental Example 1. The evaluation results are shown in Table 1.

[0058] <Experimental Example 4> The same treatment and measurement as in Experimental Example 1 were carried out, except that the superheated steam temperature in low-temperature treatment chamber B was set to 100° C. and the treatment was carried out by passing the superheated steam through low-temperature treatment chamber B for 10 minutes. The evaluation results are shown in Table 1.

[0059] <Experimental Example 5> The same processing and measurement as in Experimental Example 1 were carried out, except that the low-temperature processing chamber B was not used (the entire cleaning apparatus 10 was filled with superheated steam at 220°C) and sample A was immediately removed from the cleaning apparatus 10. In other words, the entire cleaning apparatus 10 was the high-temperature processing apparatus A, and the low-temperature processing chamber B was not provided. The evaluation results are shown in Table 1.

[0060] <Experimental Example 6> The same treatment and measurement as in Experimental Example 3 were carried out, except that the entire cleaning equipment 10 was filled with superheated steam at 200°C, sample A was passed through it for 5 minutes, and then immediately removed. In this example, the entire cleaning equipment 10 was the high-temperature treatment equipment A, and no low-temperature treatment chamber B was provided. The evaluation results are shown in Table 1.

[0061] <Experimental Example 7> The entire cleaning apparatus 10 was filled with superheated steam at 150° C., and sample A was passed through it for 7 minutes and then removed, but the same treatment and measurement were carried out as in Experimental Example 1. In other words, the entire cleaning apparatus 10 was the high-temperature treatment apparatus A, and the low-temperature treatment chamber B was not provided. The evaluation results are shown in Table 1.

[0062] <Experimental Example 8> The cleaning device 10 was entirely filled with superheated steam at 100° C., and sample A was passed through it for 7 minutes and then removed, but the same treatment and measurement as in Experimental Example 7 were carried out. That is, in this example, the entire cleaning apparatus 10 is a high-temperature processing apparatus A, and a low-temperature processing chamber B is not provided. The evaluation results are shown in Table 1.

[0063] <Experimental Example 9> (Batch Type) Sample A was placed in a batch reactor filled with superheated steam at 240°C and treated for 5 minutes. After completion of the treatment, the reactor was opened at the treatment temperature and sample A was taken out. In this case, the treatment time was 5 minutes. The amount of superheated steam supplied was 20 kg / h for a sample charge of 10 g. The obtained sample was subjected to measurement of aldehydes by gas chromatography mass spectrometry in the same manner as in Experimental Example 1. The evaluation results are shown in Table 1.

[0064] <Experimental Example 10> (Batch Type) Sample A was placed in a batch reactor filled with superheated steam at 240°C, treated for 5 minutes, and then cooled to 150°C in the superheated steam atmosphere before opening the reactor and removing the sample. The measurements were carried out in the same manner as in Experimental Example 9. The evaluation results are shown in Table 1. At this time, it took 20 minutes to lower the temperature to 150° C., and the total treatment time was 25 minutes. In Table 1, the low-temperature treatment chamber B is described as the cooling chamber B.

[0065] [Table 1] [Explanation of symbols]

[0066] 1: Continuous conveying device 3: Transport roller 5: Conveyor belt 7: Processing plastic 9a, 9a': Steam supply pipe 9b, 9b': Steam exhaust pipe 10: Cleaning equipment A: High temperature treatment chamber B: Low temperature treatment room X: Boundary line between high-temperature processing chamber A and low-temperature processing chamber B

Claims

1. A method for cleaning plastics, comprising the steps of: providing a superheated steam treatment area maintained at a temperature of 100° C. or more in a conveying path; introducing plastics into the superheated steam treatment area through which superheated steam flows; The superheated steam treatment area is formed of a high-temperature treatment area to which relatively high-temperature superheated steam is supplied and a low-temperature treatment area to which relatively low-temperature superheated steam is supplied, the high-temperature treatment area and the low-temperature treatment area are continuous with each other, The plastic is continuously transferred from the high temperature processing area to the low temperature processing area for superheated steam processing; continuously removing treated plastic from said low temperature treatment zone; A method for cleaning plastics, comprising:

2. 2. The method for cleaning plastics according to claim 1, wherein the low-temperature treatment area is supplied with superheated steam at a temperature of 100°C to 150°C, and the high-temperature treatment area is supplied with superheated steam at a temperature 20°C or more higher than that of the superheated steam supplied to the low-temperature treatment area.

3. 3. The method for washing plastics according to claim 2, wherein the time for which the plastics pass through said high temperature treatment zone is set longer than the time for which the plastics pass through said low temperature treatment zone.

4. 2. The method for washing plastics according to claim 1, wherein the plastics to be treated are used plastics.

5. 2. The method for cleaning plastics according to claim 1, wherein the oxygen concentration in the superheated steam treatment zone is adjusted to 5% or less throughout the zone.

6. A plastic washing device comprising a superheated steam treatment chamber to which a steam supply pipe and a steam exhaust pipe are connected and which is maintained at a temperature of 100°C or higher, and a plastic transport member passing through the superheated steam treatment chamber, the superheated steam treatment chamber is provided with a high-temperature treatment region located on an inlet side of the plastic and supplied with relatively high-temperature superheated steam, and a low-temperature treatment region located on an outlet side of the plastic and supplied with relatively low-temperature superheated steam, the steam supply pipe is connected to each of the high temperature processing area and the low temperature processing area; A plastic cleaning device characterized by the above.

7. 7. The plastic cleaning apparatus according to claim 6, wherein said steam exhaust pipe is connected to each of said high temperature processing area and said low temperature processing area.

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