Plastic-cleaning method and apparatus

Abstract

A plastic-cleaning method in which a superheated steam treatment area held at a temperature of 100°C or higher is provided on a conveyance path, and a plastic 7 is introduced into the superheated steam treatment area through which superheated steam is flowed and a superheated steam treatment is carried out, said plastic-cleaning method being characterized in that: the superheated steam treatment area is formed from a high-temperature treatment area A to which superheated steam of a relatively high temperature is supplied and a low-temperature treatment area B to which superheated steam of a relatively low temperature is supplied; the high-temperature treatment area A and the low-temperature treatment area B are contiguous, and the superheated steam treatment is carried out with the plastic 7 being continuously transported from the high-temperature treatment area A to the low-temperature treatment area B; and the treated plastic 7 is continuously removed from the low-temperature treatment area B.

Description

Plastic cleaning method and device

[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.

[0002] In recent years, from the viewpoint of environmental protection, there has been a demand for the recycling of various plastic molded products such as films and containers. When recycling plastics, not only can the plastic molded products be reused as they are without damaging their shape, but they can also be crushed and recovered in the form of granules or pellets, which can then be molded again as recycled plastics.

[0003] Regardless of the form in which plastics are reused, it is necessary to remove impurities from the plastics that have become contaminated during use. A common method for removing impurities from plastics is to wash them with organic solvents 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. Patent Document 2 discloses a recycling process in which packaging material (plastic film) is crushed and then chemically washed with an acid or alkali, and Patent Document 3 discloses separating foreign matter from plastic 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, waste treatment of organic solvents, etc. Furthermore, when an acid or alkali is used as a cleaning medium, the problem of environmental pollution can be avoided, but the equipment becomes large and bulky.

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

[0007] Therefore, it would be most desirable 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 adversely affect the environment, does not require large-scale exhaust equipment or waste liquid treatment equipment, and can cleanly remove not only oily stains such as grease and oils adhering to the plastic surface, but also impurities such as oily stains (i.e., sorbed components) that have soaked into the plastic, just as when an organic solvent is used as a cleaning medium. However, there is still room for improvement in that the cleaning takes time. That is, there is a problem that odors are not sufficiently removed unless the superheated steam treatment is performed for a sufficient period of time. In particular, when the used plastic is mainly composed of polypropylene, it takes a long time to remove the odor.

[0009] Special table 2020-511560 publication JP 6-31733 publication JP 6-173182 publication WO 2022 / 124015 number

[0010] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method and apparatus for removing contaminants from plastics by superheated steam treatment, which can effectively remove contaminants in a significantly reduced treatment time.

[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 zone not immediately, but by allowing the temperature to drop appropriately while the plastics are being treated with superheated steam, and then removing the plastics from the superheated steam treatment zone, and have thereby completed the present invention.

[0012] That is, according to the present invention, there is provided a method for washing plastics, in which a superheated steam treatment area maintained at a temperature of 100°C or higher is provided in a conveying path, and plastics are introduced into the superheated steam treatment area through which superheated steam is flowing to perform washing, wherein the superheated steam treatment area is formed from 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, and the high-temperature treatment area and the low-temperature treatment area are continuous, and the plastics are continuously transported from the high-temperature treatment area to the low-temperature treatment area to undergo the superheated steam treatment, and the treated plastics are continuously removed from the low-temperature treatment area.

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

[0014] According to the present invention, there is also provided a plastic washing apparatus comprising a superheated steam treatment chamber connected to a steam supply pipe and a steam discharge pipe and maintained at a temperature of 100°C or higher, and a plastic transport member passing through the superheated steam treatment chamber, wherein the superheated steam treatment chamber comprises a high-temperature treatment zone located on the plastic inlet side and supplied with relatively high-temperature superheated steam, and a low-temperature treatment zone located on the plastic discharge side and supplied with relatively low-temperature superheated steam, and the steam supply pipe is connected to each of the high-temperature treatment zone and the low-temperature treatment zone. In such a plastic washing apparatus, it is preferable that (5) the steam discharge pipe is connected to each of the high-temperature treatment zone and the low-temperature treatment zone.

[0015] The plastics cleaning method of the present invention involves feeding plastics (specifically, used plastics) to a superheated steam treatment zone maintained at a temperature of 100°C or higher and cleaning them with superheated steam. This treatment zone is divided into a high-temperature treatment zone and a low-temperature treatment zone, and the plastics are continuously passed through the high-temperature and low-temperature treatment zones and continuously removed. 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. This eliminates the need for a subsequent dehydration process, effectively cleaning the plastics in a very short time, effectively removing contaminants, and producing plastics with little odor. This cleaning process effectively removes foreign matter and contaminants, while also effectively cleaning (deodorizing) odorous components resulting from the decomposition of the plastic or contaminants.

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

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

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

[0019] Specifically, in Experimental Example 1 described below, in accordance with the present invention, a treatment chamber was divided into a high-temperature treatment zone and a low-temperature treatment zone, and superheated steam set at 220°C was supplied to the high-temperature treatment zone, while superheated steam set at 150°C was supplied to the low-temperature treatment zone. Continuous superheated steam cleaning was performed. The conveying speed of the plastic was set so that the treatment time (passing time) in the high-temperature treatment zone was 5 minutes, and the treatment time (passing time) in the low-temperature treatment zone was 2 minutes, resulting in a fairly short total treatment time of 7 minutes. Experimental Example 1 demonstrated sufficient removal of contaminants and virtually no odor compared to Experimental Examples 9 and 10 (evaluated at room temperature). Although the superheated steam treatment was performed while the plastic was continuously conveyed, shortening the treatment time, contrary to the above assumption, sufficient removal of contaminants and virtually no odor were observed even when the time spent at temperatures above 150°C was short.

[0020] For this reason, in Experimental Example 2, the temperature on the outlet side of the low-temperature treatment area was set even lower at 100°C, and the plastics were washed at the same conveying speed as in Experimental Example 1, and the results were again the same as in Experimental Example 1, with sufficient removal of contaminants and almost no odor. Furthermore, the temperature in the high-temperature treatment area or low-temperature treatment area, as well as the conveying speed, were variously changed, and in all cases, contaminants were removed in a short time and odor generation was effectively suppressed compared to when 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 oxygen-rich atmosphere for a long period of 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 period of time and effectively suppressing the generation of odors by continuously performing superheated steam treatment while preventing plastics from being kept in the atmosphere at a temperature above a certain level.

[0022] 1 is a diagram showing an outline of a cleaning device for carrying out the method of the present invention.

[0023] <Cleaning Apparatus> In FIG. 1 , which shows a cleaning apparatus for carrying out the cleaning method of the present invention, this cleaning apparatus, designated by the reference numeral 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 thereof.

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

[0025] 1, the high-temperature treatment chamber A and the low-temperature treatment chamber B are connected to each other, with no partition or the like between them, and each treatment 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 treatment chamber A, high-temperature steam is supplied from the steam supply pipe 9a and exhausted from the steam exhaust pipe 9b, while in the low-temperature treatment 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 creates a high-temperature atmosphere in the high-temperature treatment chamber A, and a lower-temperature atmosphere in the low-temperature treatment chamber B compared to the high-temperature treatment chamber A.

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

[0027] Furthermore, 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, it is possible to more precisely control the processing temperature and processing time in each processing chamber A, B.

[0028] Furthermore, as necessary, these steam supply pipes 9a and 9a' can also be used to supply and exhaust an inert gas such as nitrogen, thereby replacing the atmosphere in the high-temperature treatment chamber A and the low-temperature treatment chamber B with the inert gas and further reducing the oxygen concentration in the chambers. The supply of the inert gas is not limited to the above-described method, and a method of supplying the gas from a source other than the steam supply pipes 9a and 9a' is also possible as long as it is possible to reduce the oxygen concentration in the high-temperature treatment chamber A and the low-temperature treatment chamber B. Another example of a cleaning device may be one in which multiple high-temperature treatment chambers (high-temperature treatment zones) with different temperature settings are arranged in series, and each treatment chamber may be provided with its own steam supply pipe (and even a steam exhaust pipe) so that the temperature can be set to any desired value depending on the plastic being treated.

[0029] Furthermore, the cleaning device 10 is structured 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 (particularly at the inlet and outlet sides of the device), and the temperature of the steam sprayed onto the plastic 7 in each processing chamber A, B is maintained at the set temperature.

[0030] <Cleaning Medium> In the present invention, superheated steam (i.e., water) is used as the cleaning medium to remove impurities from plastics using the above-mentioned cleaning device 10. This superheated steam has been heated to have a lower dielectric constant than water at 100°C or less, and for example, superheated steam having 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 dielectric constant of water at room temperature is significantly higher than that of organic solvents, making it a uniquely polar solvent. This means that it has poor affinity (oil-repellent properties) for organic components (oils and other oily components), making it difficult to remove dirt. On the other hand, it is known that the dielectric constant decreases as the temperature increases. As can be understood from this, cleaning is performed using water (superheated steam) that is in a state similar to that of an organic solvent, with the dielectric constant reduced by heating. For example, Patent Document 4, mentioned above, provides detailed explanations of methods for measuring the dielectric constant. When water is heated to about 200°C, its dielectric constant becomes about 35, which is close to that of methanol at room temperature. When heated to about 300°C, its dielectric constant becomes about 20, which is close to that of acetone at room temperature. Further heating, near the critical point, makes its dielectric constant comparable to that of nonpolar organic solvents such as chloroform and ethyl ether.

[0032] As can be understood from the above explanation, superheated steam used as a cleaning medium is non-polarized by reducing its dielectric constant, making it similar to an organic solvent. Furthermore, due to this low dielectric constant, water becomes high-temperature steam exceeding 100°C. When such high-temperature water comes into contact with plastic, the polymer chains constituting the plastic loosen, making it easier for the low-dielectric water to penetrate into the plastic. According to the present invention, by using water vapor, which has been reduced in dielectric constant to a non-polar state similar to an organic solvent and heated to a high temperature to reduce the dielectric constant, as a cleaning medium, impurities adhering to the plastic surface and impurities sorbed within the plastic 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, due to heating for reducing the dielectric constant as described above.

[0034] <Plastics to be Cleaned> In the present invention, the plastics to be cleaned are so-called used plastics. Any plastics can be used as long as they do not decompose when cleaned with heated steam as described above. However, preferred examples include olefin-based resins, which are widely recycled through separate collection 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. Cyclic olefin copolymers such as those disclosed in JP-A-2007-284066 can also be suitably used. Such olefin-based resins are highly chemically stable, and when cleaned with high-temperature water having a low dielectric constant, they are not only unlikely to undergo hydrolysis or the like, but can also be easily recovered.

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

[0036] In the present invention, the used plastics are collected waste plastics of various plastic molded products (e.g., containers, lids, etc.), appropriately separated from other molded products of different materials, and subjected to crushing, washing, separation by specific gravity, etc. to remove as much foreign matter as possible. In the treatment described below, there are no particular restrictions, but from the viewpoints of transportability and processability, crushed plastics in the form of flakes or granules are preferred, and those with a mesh diameter of 10 mm or less, more preferably 8 mm or less, and even more preferably 6 mm or less are particularly preferred.

[0037] <Washing of Plastics> The above-mentioned washing of plastics, i.e., the washing process for removing contaminants, is carried out using the above-mentioned washing device 10. That is, by continuously driving the conveying device 1 (conveyor belt 5), used plastics 7 are continuously introduced into the washing device 10, and the plastics are washed by spraying superheated steam onto them within the washing device 10, and the washed plastics are continuously removed from the washing device 10. By this continuous process, 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, the cleaning process is carried out in the high-temperature treatment chamber A using relatively high-temperature superheated steam, and then the cleaning process is carried out in the low-temperature treatment chamber B using relatively low-temperature superheated steam.

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

[0040] In the present invention, low-temperature treatment chamber B is configured to supply steam at a temperature relatively lower than that of high-temperature treatment chamber A, for example, steam 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 conveyed 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 odor generation can also be effectively suppressed.

[0041] The high-temperature treatment chamber A is set so that superheated steam having a higher temperature is sprayed thereto than that sprayed thereto 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 to the low-temperature treatment chamber B is supplied and sprayed onto the continuously conveyed plastics 7. By spraying superheated steam maintained at such a high temperature, sorbed substances such as oily stains adhering to the surface of the plastics or oily stains that have penetrated into the interior of the plastics can be effectively cleaned and removed.

[0042] 1, since the high-temperature treatment chamber A is connected to the low-temperature treatment chamber B, there is an overall temperature distribution within the high-temperature treatment chamber A and the low-temperature treatment chamber B, with the atmospheric temperature at the outlet side of the low-temperature treatment chamber B being the lowest. In consideration of this temperature distribution, it is preferable that the maximum temperature within the high-temperature treatment chamber A be set to 300°C or less, more preferably 260°C or less, and even more preferably 230°C or less. If this maximum temperature is higher than necessary, it may cause decomposition of the plastic 7 or make it difficult to adjust the temperature in the subsequent low-temperature treatment chamber B, making it difficult to complete the treatment in a short time.

[0043] As can be understood from the above explanation, in the present invention, the cleaning process is carried out by supplying water vapor having a lower temperature than the superheated steam supplied to the high-temperature treatment chamber A to the low-temperature treatment chamber B connected to the high-temperature treatment chamber A, thereby effectively suppressing deterioration of the plastic 7. For example, as in Example 12 of Patent Document 4, if 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, this results in a so-called batch process rather than a continuous process, which requires a long time for cooling and makes it impossible to complete cleaning in a short time.

[0044] Furthermore, in this low-temperature treatment chamber B, the plastic 7 maintained at a high temperature is cleaned with superheated steam without direct contact with atmospheric air with a high oxygen concentration. Contact with atmospheric air at a high temperature would cause deterioration of the plastic 7, resulting in the production of aldehydes and the resulting odor, as demonstrated in the experimental example described below. Therefore, since cleaning is performed with superheated steam in this low-temperature treatment chamber B, the temperature at the outlet must be maintained at at least 100°C to prevent condensation of the superheated steam. Furthermore, if the temperature at the outlet is excessively high, the temperature of the plastic 7 cleaned in the high-temperature treatment chamber A will not be sufficiently reduced, and the plastic 7 treated with superheated steam will come into contact with atmospheric air at a high temperature, causing deterioration of the plastic 7, resulting in the production of a large amount of aldehydes and 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 mutually connected. For this reason, it is not possible to strictly distinguish between the two chambers A and B. However, 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 using a thermocouple or the like in advance laboratory tests, thereby confirming the ambient temperature (or temperature gradient) in the high-temperature treatment chamber A where treatment with high-temperature superheated steam is performed, and the ambient temperature (or temperature gradient) in the low-temperature treatment chamber B where treatment with low-temperature superheated steam is performed.

[0046] Furthermore, in the present invention, in order to effectively clean and remove contaminants adhering to the surface of the plastic or sorbed inside, it is preferable to lengthen the residence time in the high-temperature treatment chamber A and shorten the residence time in the low-temperature treatment chamber B. The low-temperature treatment chamber B is a region whose primary purpose is to prevent deterioration of the plastic 7 rather than to remove contaminants, and so a long residence time is not necessary; it is sufficient to ensure a residence time sufficient for the temperature to drop to a certain level. For example, the residence time in the high-temperature treatment chamber A can be lengthened by positioning the steam supply pipe 9a for supplying high-temperature superheated steam closer to the outlet than the center. Furthermore, the residence time in the high-temperature treatment chamber A can be lengthened by providing multiple high-temperature treatment chambers (high-temperature treatment zones) in series and providing each high-temperature treatment chamber with its own dedicated steam supply pipe (and even steam exhaust pipe). Furthermore, the residence times in the high-temperature treatment chamber A and the low-temperature treatment chamber B can be freely set by installing a transport device in each chamber and connecting them. Alternatively, 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 method described above, and the position where the temperature drops sharply can be used 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. A structure in which the residence time (passage time of the plastic 7) in the high-temperature treatment chamber A set in this manner is longer than the residence time in the low-temperature treatment chamber B is preferable for more effective cleaning in a shorter time. For example, it is preferable that the residence time in the low-temperature treatment chamber B be approximately 95 to 10% of the residence time in the high-temperature treatment chamber A.

[0047] Furthermore, the treatment with superheated steam in the above-mentioned high-temperature treatment chamber A and low-temperature treatment chamber B is preferably performed with an oxygen concentration adjusted to 5% or less throughout, more preferably 3% or less, and even more preferably 1% or less. 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 achieved. It is also preferable to replace the atmosphere in these chambers with an inert gas such as nitrogen gas and perform the treatment while flowing the inert gas, as necessary. This allows the superheated steam treatment to be performed with a lower oxygen concentration, more reliably preventing 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 cleaning method are less likely to deteriorate and produce less odor, and therefore can be pelletized by melt extrusion using an extruder or the like, either alone or after being melt-kneaded with virgin plastic, and then molded again as recycled plastics, and reused as packaging containers, etc. Of course, the plastics after cleaning 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, in the same way as when organic solvents are used, making it an extremely useful technology for recycling plastics.

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

[0051] (1) Sample to be cleaned Sample A was used as the plastic to be cleaned. This is flakes obtained by crushing a high-density polyethylene bottle after filling it with edible oil. The sample was left untreated and subjected to measurements of aldehydes in the same manner as in the experimental example described below. For reference, sample A0, which is flakes obtained by crushing a high-density polyethylene bottle that had not been filled with oil, was also subjected to the same measurements.

[0052] (2) Measurement of Aldehydes The aldehydes contained in the washed plastics were measured by gas chromatography-mass spectrometry after pretreatment using solid-phase microextraction as described below. Pretreatment using solid-phase microextraction was performed as follows: 2 g of sample was sealed in a headspace vial, and SPME fiber (CAR / PDMS, 85 μm film thickness, manufactured by Supelco) acting as an adsorbent was placed inside. The sample was then stored at 40°C for 15 minutes, allowing impurity components (aldehydes) to be adsorbed onto the SPME fiber, thereby completing the 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 identify the impurity components.

[0054] The measurement conditions were as follows: Column: DB-WAX (60 m x 0.25 mm, 0.25 μm film thickness) manufactured by Agilent Technology J&W Carrier gas: Helium Detector: MSD Ionization method: EI Oven temperature: Raise from 45°C to 220°C at a rate of 7°C / min and hold for 15 minutes Injection port temperature: 250°C Injection mode: Splitless Mass spectrum range: 27 to 600

[0055] Experimental Example 1: The plastic to be cleaned was the aforementioned sample A. Sample A was placed on the conveyor device 1 (conveyor belt 5) and treated by passing it through the high-temperature treatment chamber A filled with superheated steam at 220°C for 5 minutes. It was then treated by passing it through the low-temperature treatment chamber B filled with superheated steam at 150°C for 2 minutes, after which sample A was removed from the cleaning device 10. In this case, the treatment time was 7 minutes. The superheated steam was supplied at a rate of 20 kg / h for a sample load of 10 g. The obtained sample was pretreated by solid-phase microextraction and subjected to aldehyde measurement by gas chromatography-mass spectrometry. The boundary line X between the high-temperature treatment chamber A and the low-temperature treatment chamber B was determined by measuring the temperature distribution on the conveyor belt 5 and determining the location where the temperature rapidly 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 superheated steam temperature 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 treatment chamber A and the low-temperature treatment chamber B was almost the same as in Experimental Example 1.

[0057] Experimental Example 3 The same treatments and measurements 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, the boundary line X between the high-temperature treatment chamber A and the low-temperature treatment chamber B was also approximately 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 no low-temperature processing chamber B was 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 apparatus 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 apparatus 10 was the high-temperature treatment apparatus A, and no low-temperature treatment chamber B was provided. The evaluation results are shown in Table 1.

[0061] Experimental Example 7 The same treatment and measurement as in Experimental Example 1 were carried out, except that 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. In other words, the entire cleaning apparatus 10 was the high-temperature treatment apparatus A, and no low-temperature treatment chamber B was provided. The evaluation results are shown in Table 1.

[0062] Experimental Example 8 The same treatment and measurement as in Experimental Example 7 were carried out, except that the entire cleaning apparatus 10 was filled with superheated steam at 100°C, and sample A was passed through it for 7 minutes and then removed. That is, in this example, the entire cleaning apparatus 10 was the high-temperature treatment apparatus A, and no low-temperature treatment chamber B was provided. The evaluation results are shown in Table 1.

[0063] Experimental Example 9 (Batch Method) Sample A was placed in a batch-type reactor filled with superheated steam at 240°C and treated for 5 minutes. After completion of the treatment, the reactor was opened while still at the treatment temperature, and Sample A was removed. In this case, the treatment time was 5 minutes. The superheated steam water supply rate was 20 kg / h for a sample charge of 10 g. The obtained sample was subjected to aldehyde measurement 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-type 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. Measurements were carried out in the same manner as in Experimental Example 9. The evaluation results are shown in Table 1. In this case, it took 20 minutes to cool the temperature to 150°C, and the total treatment time was 25 minutes. In Table 1, the low-temperature treatment chamber B is referred to as cooling chamber B.

[0065]

[0066] 1: Continuous conveying device 3: Conveying roller 5: Conveying belt 7: Plastic to be treated 9a, 9a': Steam supply pipe 9b, 9b': Steam exhaust pipe 10: Cleaning device A: High temperature treatment chamber B: Low temperature treatment chamber X: Boundary line between high temperature treatment chamber A and low temperature treatment chamber B

Claims

1. A method for cleaning plastics in which a superheated steam treatment area maintained at a temperature of 100°C or higher is provided in a transport path, and plastics are introduced into the superheated steam treatment area through which superheated steam flows to perform superheated steam treatment, the superheated steam treatment area being formed from 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, the plastics being continuously transported from the high-temperature treatment area to the low-temperature treatment area to undergo superheated steam treatment, and the treated plastics being continuously removed from the low-temperature treatment area.

2. A method for cleaning plastics as described in claim 1, wherein superheated steam at a temperature of 100°C to 150°C is supplied to the low-temperature treatment area, and superheated steam at a temperature at least 20°C higher than the superheated steam supplied to the low-temperature treatment area is supplied to the high-temperature treatment area.

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

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

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

6. A plastic cleaning 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, wherein the superheated steam treatment chamber comprises a high-temperature treatment area located on the inlet side of the plastic and to which relatively high-temperature superheated steam is supplied, and a low-temperature treatment area located on the exhaust side of the plastic and to which relatively low-temperature superheated steam is supplied, and wherein the steam supply pipes are connected to each of the high-temperature treatment area and the low-temperature treatment area.

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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