Apparatus and method for recycling flakes from shredded and washed post-consumer plastic waste

Abstract

The device for recycling flakes from shredded and washed post-consumer plastic waste comprises, viewed in the direction of processing of the plastic waste, a pre-treatment unit (2) for drying and homogenizing the flakes from the shredded and washed post-consumer plastic waste, a melt extruder (3) for melting the plastic waste dried and homogenized in the pre-treatment unit (2), a degassing extruder (5) with a connection (5a) to a vacuum source for degassing the plastic melt, a granulator (6) for granulating the plastic melt and an odor removal unit (8) for subjecting the granules to odor removal. The odor removal unit (8) has a process gas supply (10) and a gas discharge (11) for discharging an exhaust gas stream, the process gas supply (10) of the odor removal unit (8) being connected to an ozone source (12) or ozone generator (13), whereby the odor removal unit (8) is supplied with a process gas enriched with ozone.

Description

[Technical field]

[0001] The present invention relates to an apparatus and method for recycling flakes from shredded and washed post-consumer plastic waste. [Background technology]

[0002] The reuse of plastic materials plays a decisive role in terms of a sustainable circular economy. To make this possible, odorous and infectious contaminants must be removed, especially from post-consumer plastic waste. If recycled plastics are used for food packaging, they must be of particularly high purity. It must also be guaranteed that the taste, odor and consistency of the food are not affected in any way by packaging made from recycled materials.

[0003] Post-consumer plastic waste is typically processed by first shredding it into flakes or shredded material and then feeding it into a washing process, preferably achieved in a high temperature wash with the addition of caustic soda or other cleaning additives to clean the surfaces.

[0004] If a certain color purity is required for further use, optical pre-sorting of the post-consumer plastic waste before shredding is recommended, which may be combined with a sorting after washing step if necessary. The flakes thus produced can then be extruded into recycled granules. These recycled granules are then aerated for short periods (hours to days) using hot air or for longer periods (days to weeks) using ambient air to reduce the odor of the recycled granules.

[0005] In order to make a decisive contribution to a positive eco-balance, the treatment of plastic waste and its reuse in new products must be as energy-saving, resource- and environmentally-friendly and cost-effective as possible compared to the production of new materials. Decontamination of post-consumer waste is particularly energy- and time-consuming, since in conventional degassing systems the contaminated pellets are exposed to a hot air stream or kept under vacuum for several hours. A method and an installation for the decontamination of plastic waste are known from EP 2507022, in which flakes are extruded into pellets and the pellets thus obtained are deodorized by blowing hot air through an odor removal unit where the pellets remain. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent No. 2507022 [Patent Document 2] China Patent Publication No. 108641398 [Patent Document 3] Canadian Patent Application Publication No. 601047 [Patent Document 4] European Patent Application Publication No. 0602505 [Patent Document 5] European Patent Application Publication No. 3705252 [Non-patent literature]

[0007] [Non-Patent Document 1] Resources, Conservation & Recycling 161 (2020) “Development and application of an analytical method to quantify odour removal in plastic waste recycling processes” Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to provide an efficient and at the same time environmentally friendly post-consumer waste recycling process that can provide odorless recycled granules.Ideally, the present invention can improve the cleaning efficiency in the recycling of post-consumer plastics to such an extent that the recycling provides food-grade recycled granules.

[0009] It is known from CN 108641398 to wash plastic bottles in a special washing liquid and concentrate the ozone in a mixer to a temperature of 180-220° C. The plastic may then be granulated.

[0010] Canadian Patent Application Publication No. 601047 describes the treatment of PET staple fibres and PET film with ozone at room temperature to enable printing. The PET material was found to thereby maintain its flexibility and fastness, and a bleaching effect was observed. Ozone treatment was also carried out with a water vapour / ozone mixture. Tests showed that treating the PET material at temperatures between 180 and 230°C had no adverse effect on the material. However, at higher temperatures, the ozone would decompose into oxygen.

[0011] EP 0 602 505 A1 describes an ozone-forming UV emitter for treating liquids to reduce pollutants.

[0012] EP 3705252 A1 describes a method for producing plastic materials from plastic waste. The method consists of selecting industrial or post-consumer plastic waste, grinding the waste, treating the waste to reduce the odor generated by the waste, treating the waste to reduce the microbial load, tempering the waste in water, re-granulating the waste, and dehumidifying the granules. Ozonation may be used to reduce the odor and microbial load, but is not described in more detail.

[0013] A drawback of this known method is that the treatment to reduce odor and microbial load is only carried out on above-ground waste. However, it is known that especially non-polar compounds migrate into plastic materials, see for example Resources, Conservation & Recycling 161 (2020) “Development and application of an analytical method to quantify odour removal in plastic waste recycling processes”. During regranulation as well as during further processing of the regenerated granules, impurities can be released which in turn cause odours and thus prevent the use of the regranules for more valuable applications, in particular for the production of food packaging.

[0014] Thus, there remains a need for an efficient and at the same time environmentally friendly post-consumer waste treatment process that can provide odorless regenerated granules, and an apparatus for carrying out this process.

[0015] The present invention solves the posed problem by providing an apparatus for recycling flakes from shredded and washed post-consumer plastic waste having the features of claim 1 and by providing a method for recovering flakes from shredded and washed post-consumer plastic waste having the features of claim 7. Embodiments of the invention are defined in the dependent claims, the description and the drawings.

[0016] The method according to the invention for recycling flakes from shredded and washed post-consumer plastic waste comprises: drying and homogenizing the flakes from the shredded and washed post-consumer plastic waste in a gas stream, in particular in a hot gas stream; melting the dried and homogenized plastic waste to form a plastic melt; removing foreign matter from the plastic melt, if necessary; Degassing the plastic melt; granulating the plastic melt; Optionally, drying the granules; removing odor from the granules with the process gas stream; Here, the process gas stream used to remove odors from the granules is enriched with ozone or the process gas stream is provided as an ozone-enriched gas.

[0017] The device according to the invention for recycling flakes from shredded and washed post-consumer plastic waste comprises: a pre-treatment unit for drying and homogenizing flakes from the shredded and washed post-consumer plastic waste; a melt extruder disposed downstream of the pre-treatment unit for melting the plastic waste dried and homogenized in the pre-treatment unit; a degassing extruder located downstream of the melt extruder and having a connection to a vacuum source for degassing the plastic melt; a granulator for granulating the plastic melt, the granulator being disposed downstream of the degassing extruder; an odor removal unit arranged downstream of the granulation device for deodorizing the granules, The odor removal unit includes an odor removal unit having a process gas supply and a gas exhaust for exhausting an exhaust gas stream, the process gas supply of the odor removal unit being connected to an ozone source or ozone generator, thereby supplying ozone-enriched process gas to the odor removal unit.

[0018] That is, on the one hand, an ozone source (e.g. in the form of a gas cylinder filled with ozone-enriched gas, e.g. compressed air enriched with ozone) is connected to the odor elimination unit. The term "ozone source" does not imply that this source supplies pure ozone. On the other hand, an ozone generator may be connected to the odor elimination unit, in which case the ozone generator takes in the surrounding air as process gas or takes it from a process gas supply and enriches it with ozone by reacting the oxygen contained in the air under voltage or UV radiation to form ozone.

[0019] The method according to the invention for recycling flakes from shredded and washed post-consumer plastic waste comprises: drying and homogenizing the flakes from the shredded and washed post-consumer plastic waste in a gas stream, in particular in a hot gas stream; melting the dried and homogenized plastic waste to form a plastic melt; removing foreign matter from the plastic melt, if necessary; Degassing the plastic melt; granulating the plastic melt; Optionally, drying the granules; removing odor from the granules with a process gas stream; Enriching a process gas stream used to remove odor from the granules with ozone or providing the process gas stream as an ozone-enriched gas.

[0020] The present invention allows the removal of targeted odorous substances from post-consumer waste. The degree of residual odor remaining in the granules produced and treated according to the present invention can be determined in various ways. For example, various samples of the granules may be evaluated by a panel of testers trained for this task, in diluted or undiluted air. Alternatively, material-specific indicators may be defined and evaluated based on the residual amount remaining after the odor removal process is completed. Furthermore, gas chromatography analysis may be used to evaluate the cleaning efficiency of individual materials.

[0021] When recycling post-consumer plastic waste, it is desirable to install a melt filter between the melt extruder and the degassing extruder to remove foreign matter from the plastic melt.

[0022] If the granulator for granulating the plastic melt is a wet granulator, it is advantageous to provide a drying device for drying the granules, for example a drying centrifuge, downstream of the granulator so that the granules are supplied to the odor removal unit in an almost dry state.

[0023] The process gas to be enriched with ozone is preferably air, although filtered ambient air may be used if desired.

[0024] It has been shown that to achieve sufficient odor removal without significantly polluting the environment, the ozone concentration in the enriched process gas should be at least 0.1 ppm, preferably within the range of 10 ppm to 100 ppm.

[0025] In order to achieve the best odor removal effect while at the same time minimizing ozone pollution of the environment, it is preferred according to the invention to provide a regulating device for regulating the amount of ozone supplied to the odor removal unit depending on the ozone concentration in the exhaust gas flow from the odor removal unit. The absolute amount of ozone is crucial for the odor removal effect, i.e. how many ozone molecules collide in total with the granules and thus odor molecules are present (g ozone / kg plastic).

[0026] The amount of ozone delivered can be adjusted in a variety of ways, depending on the regulator. by changing the ozone concentration in the supplied process gas, or - by shortening / stopping the processing time (duration of processing), or -By manipulating the flow rate of the process gas, Be affected.

[0027] In order to make this adjustment as accurately, quickly and automatically as possible, the invention further provides an in-line measuring facility for measuring the ozone concentration in the exhaust gas stream from the odor removal unit, which in-line measuring facility controls a regulating device for adjusting the amount of ozone supplied to the odor removal unit as a function of the measured ozone concentration. As a result, on the one hand, sufficient ozone is generated or supplied to the process gas stream to remove odor-forming substances by oxidation, while at the same time the in-line measuring and controlling keeps the amount of ozone emissions low. The required ozone concentration of the supplied process gas depends on the contamination intensity and the odor intensity of the granules. The more contaminated the granules are, the more ozone is consumed during the treatment (reaction with odorous substances). The remaining ozone concentration is measured in the exhaust gas stream. If the ozone concentration in the exhaust gas stream exceeds a certain threshold value, the amount of ozone supplied to the process gas or as process gas is throttled down until the ozone concentration in the exhaust gas stream is again below the threshold value.

[0028] This control solves two problems. a.) On the one hand, excessive ozone should not be produced and subsequently released into the environment. b.) On the other hand, it should be determined when the odor removal process is complete, i.e. when the odorants have reacted with the ozone.

[0029] Essentially, the chemical process that takes place can be described as follows:

[0030] O 3 (Ozone added) + X → O-X + O 2 Where: X...odor substance OX: Oxidized odorants

[0031] If there is still enough odorant present as a reactant for ozone, the ozone can be completely reacted and consumed. The presence of oxidized odorant and oxygen in the exhaust gas stream indicates that the odor removal process is not yet complete.

[0032] However, if more ozone is supplied to the odor removal unit with the process gas than can react with the odorants carried by the granules in the odor removal unit, the ozone will leave the odor removal unit through the exhaust gas stream where it can be measured.

[0033] This may be explained simply as follows.

[0034] nO 3 (Ozone added) + X → O-X + O 2 +(n-1)O 3 (Ozone in the exhaust gas stream can be measured using a sensor) where: n ≥ 2 X: Odor substances OX: Oxidized odorants

[0035] This occurs when too much ozone is fed into the mixture removal unit. The following solution to this problem is provided:

[0036] At the start of the process, a process gas with a low ozone concentration is supplied. If no ozone is measured in the exhaust gas stream, the ozone concentration of the supplied process gas is gradually increased until a constant ozone concentration (e.g. a constant percentage of the supplied amount) is present in the exhaust gas stream. The ozone concentration of the supplied process gas is then kept constant until the odor removal process is completed, i.e. until the odorants have completely reacted with the ozone.

[0037] Alternatively or additionally, the flow rate of the process gas stream may be changed to adjust the amount of ozone. This is done by starting with a constant flow rate of the process gas stream and measuring the ozone concentration in the exhaust gas stream. If there is too much ozone, the flow rate is reduced in stages until the ozone in the exhaust gas stream is no longer measurable or falls below a certain threshold in the exhaust gas stream. The flow rate is then kept constant until the odor removal process is complete.

[0038] During the odor removal process, ideally the reaction proceeds so that the ozone is completely consumed.

[0039] O 3 (Ozone added) + X → O-X + O 2 Where: X...odor substance OX: Oxidized odorants

[0040] As the odor removal process continues, fewer odorant molecules are available as ozone reactants, leaving more ozone. The increase in ozone concentration can then be measured in the exhaust gas stream.

[0041] nO 3 (Ozone added) + X → O-X + O 2 +(n-1)O 3 (Ozone in the exhaust gas stream can be measured using a sensor) where: n ≥ 2 X: Odor substances OX: Oxidized odorants

[0042] Completion of the odor removal process may be recognized by measuring a certain increase in ozone concentration in the exhaust gas stream, or exceeding a certain threshold (e.g., a certain percentage of the supplied ozone concentration). As soon as this is detected, the ozone supply is either immediately stopped or gradually reduced according to the same principles as described above, by reducing the ozone concentration in the supplied process gas or gradually reducing the flow rate.

[0043] It should be noted that the above reaction equations are for illustrative purposes only and are simplified, as they only show the case where an odorant is oxidized once. In reality, most odorants are oxidized by ozone several times, i.e., two or more ozone molecules react with one odorant molecule. However, the principle remains the same.

[0044] In-line measurement equipment for measuring the ozone concentration in the exhaust gas stream from the odor removal unit may be provided simply to measure the ozone concentration, for example to meet official environmental requirements or to comply with country-specific regulatory values.

[0045] In order to minimize environmental pollution by ozone, in one embodiment of the invention, at least one device for at least partially decomposing the ozone contained in the exhaust gas stream is optionally provided, the device for decomposing ozone being preferably configured to thermally or catalytically treat the exhaust gas stream or to irradiate the exhaust gas stream with electromagnetic waves, the electromagnetic waves being preferably UV light having a wavelength of at least 254 nm.

[0046] The apparatus for at least partially decomposing ozone contained in the exhaust gas stream may optionally be controlled or regulated based on the ozone concentration measured in the exhaust gas stream.

[0047] The invention is explained in more detail below with reference to exemplary embodiments and with reference to the drawings. [Brief description of the drawings]

[0048] [Figure 1] 1 shows a schematic diagram of an apparatus according to the invention for recycling flakes from shredded and washed post-consumer plastic waste. [Diagram 2] 2 shows a possible embodiment of a detail of the device of FIG. 1; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] The apparatus for recycling flakes from shredded and washed post-consumer plastic waste, which is shown diagrammatically in FIG. 1, has a pre-treatment unit 2 for drying and homogenizing flakes from shredded and washed post-consumer plastic waste, where the supply of the flakes to the pre-treatment unit 2 is realized by a supply unit 1 in the form of a screw conveyor. The pre-treatment unit 2 has a gas supply 2a, preferably configured as a hot gas supply, and a gas discharge 2b. Gas, preferably hot air, is supplied through the gas supply 2a to the pre-treatment unit 2, inside which the flakes are present. The flakes are dried and homogenized using the supplied gas flow. The gas flow containing moisture and pollutants is discharged from the gas discharge 2b. Discharge from the pre-treatment unit of the flakes is carried out by a screw conveyor 2c to a melt extruder 3 arranged downstream of the pre-treatment unit 2. In the melt extruder 3, the dried and homogenized flakes from the plastic waste are melted. Downstream of the melt extruder 3 is a melt filter 4 for removing foreign matter from the plastic melt.

[0050] Downstream of the melt filter 4 is arranged a degassing extruder 5 with a vacuum connection 5a to a vacuum source not shown for degassing the plastic melt, followed by a granulator 6 for granulating the plastic melt. The granules produced in the granulator 6 are dried in a drying centrifuge 7 and then fed to an odor removal unit 8, where they are subjected to odor removal. After the odor removal treatment, the granules are discharged from the odor removal unit 8 and can then be directly processed or fed to a storage silo 9.

[0051] The odour removal unit 8 has a process gas supply 10 and a gas outlet 11 for discharging the exhaust gas stream, the process gas supply 10 of the odour removal unit 8 being connected to an ozone source 12, for example in the form of a container filled with ozone-enriched gas, for example compressed air enriched with ozone, or to an ozone generator 13 by which ozone or an ozone-enriched process gas may be supplied to the odour removal unit 8.

[0052] The regulator 20 serves to regulate the amount of ozone supplied to the odor removal unit 8 according to the ozone concentration of the exhaust gas flow from the odor removal unit 8. The regulation of the amount of ozone supplied is realized, for example, by a control valve operated by the regulator 20 in the process gas supply 10. The regulator 20 may be configured as an electronic control unit having a microprocessor, a main memory, a program memory in which the control algorithms are stored, and interfaces for communication and for operating actuators such as the aforementioned control valves. The ozone generator 13 may be configured to apply a voltage to the surrounding air so that oxygen in the surrounding air reacts to form ozone. The higher the voltage, the more ozone is produced. In this way, the ozone concentration of the gas leaving the ozone generator 13 can be regulated by controlling the voltage. The control of the ozone generator 13 to generate and vary the voltage can be realized by the regulator 20.

[0053] Alternatively, the ozone generator 13 may operate based on irradiating the drawn in ambient air with UV light, preferably having a wavelength of less than 240 nm. In such an ozone generator 13, the ozone concentration in the gases exiting the ozone generator 13 is controlled by varying the wavelength of the UV light. This control may also be performed by using the regulator 20. Shorter wavelength UV light has higher energy and therefore produces more ozone than longer wavelength UV light.

[0054] Preferably, the regulating device 20 is controlled by an in-line measuring facility 30 for measuring the ozone concentration in the exhaust gas flow from the odor removal unit 8, and the control of the regulating device 20 is performed to adjust the amount of ozone supplied to the odor removal unit 8 according to the measured ozone concentration. With such an arrangement, the process for odor removal described above may be realized with little impact of ozone on the environment at the same time. The in-line measuring facility 30 may comprise an ozone sensor installed in the gas discharge 11, although it may be located directly in or on the gas discharge 11, or alternatively remote from the gas discharge 11.

[0055] An installation 40 for at least partially decomposing the ozone contained in the exhaust gas stream is optionally provided at the gas outlet of the odor removal unit 8, the installation 40 for decomposing the ozone being preferably configured for thermally or catalytically treating the exhaust gas stream or for irradiating the exhaust gas stream with electromagnetic waves. If the ozone is decomposed by irradiating the exhausted process gas with electromagnetic waves, preferably ultraviolet light having a wavelength of at least 254 nm is used.

[0056] The process gas to be enriched with ozone is preferably air, as it is simple to handle. The ozone concentration in the enriched process gas should be at least 0.1 ppm, and preferably ranges from 10 ppm to 100 ppm.

[0057] In Fig. 2 an embodiment of an ozone gas generator 13 according to the invention for recovering ozone from ambient air is shown diagrammatically. The ozone gas generator 13 comprises an air filter 14 which contacts the ambient air and filters dust and the like from the ambient air. The ambient air thus purified is fed to an ozone generator 15 which converts a part of the ambient air into ozone according to principles known to those skilled in the art, thereby producing ozone-enriched air as process gas. The output of the ozone generator 15 is connected to the inlet or intake side of a blower 16. The blower 16 causes, on one side, the intake of air through the air filter 14 and the ozone generator 15, and on the other side blows the process gas through a heater 17 into the process gas supply 10 and through the process gas supply 10 into the odor removal unit 8. The process gas with the ozone portion consumed by the odor removal leaves the odor removal unit 8 as an exhaust gas stream through the gas outlet 11 and reaches the ozonolysis facility 40 for ozone decomposition, where the excess ozone is reduced. It should be noted that the term "ozone gas generator" should not be understood in the sense that the device generates pure ozone. Rather, the term "ozone gas generator" means that the device generates an ozone-enriched gas, in particular ozone-enriched air.

[0058] In a series of tests, the process gas flow to the odor removal unit 8 was enriched with ozone at 35 ppm, and different types of plastic post-consumer waste granules were passed through the ozone-enriched process gas in the odor removal unit 8 for 2 to 2.7 hours each. Ozone concentration 35 ppm, process gas flow rate 1600 m 3 In this case, the amount of ozone delivered was 120 g / h, which translates to 0.24 g of ozone per kg of plastic.

[0059] Then, in a blind test, two samples of the same plastic material, one passed through hot air in the conventional manner for 2-2.7 hours and the other through hot air containing 35 ppm ozone as the process gas for the same time and at the same temperature, were presented for evaluation to a trained odour test panel. Granules made from HDPE, PP and LDPE treated with ozone were rated by the test panel as significantly superior in odour intensity and pleasantness to comparison materials treated with hot air only.

Claims

1. 1. An apparatus for recycling flakes from shredded and washed post-consumer plastic waste, comprising: a pre-treatment unit (2) for drying and homogenizing the flakes from the shredded and washed post-consumer plastic waste; a melt extruder (3) arranged downstream of the pre-treatment unit (2) for melting the plastic waste dried and homogenized in the pre-treatment unit (2); a degassing extruder (5) arranged downstream of the melt extruder (3) and having a connection (5a) to a vacuum source for degassing the plastic melt; Optionally, a melt filter (4) arranged between the melt extruder (3) and the degassing extruder (5) for removing foreign matter from the plastic melt; a pelletizer (6) arranged downstream of the degassing extruder (5) for pelletizing the plastic melt; an odor removal unit (8) arranged downstream of the granulator (6) for deodorizing the granules; Optionally, a drying device (7) for drying the granules, arranged between the granulating device (6) and the odor removing unit (8), 1. The apparatus according to claim 1, wherein the odor removal unit (8) has a process gas supply (10) and a gas outlet (11) for discharging an exhaust gas stream, the process gas supply (10) of the odor removal unit (8) being connected to an ozone source (12) or an ozone generator (13), thereby supplying the odor removal unit (8) with process gas enriched with ozone.

2. an in-line measurement facility (30) for measuring the ozone concentration of the exhaust gas stream from the odor removal unit (8), 2. The apparatus of claim 1.

3. a regulator (20) for regulating the amount of ozone supplied to the odor removal unit (8) according to the ozone concentration of the exhaust gas flow from the odor removal unit, 3. Apparatus according to claim 1 or 2.

4. characterised in that the device comprises at least one facility (40) for at least partially decomposing ozone contained in the exhaust gas stream, the facility (40) being adapted to decompose ozone, preferably for thermal or catalytic treatment of the exhaust gas stream or for irradiating the exhaust gas stream with electromagnetic waves.

4. Apparatus according to any one of claims 1 to 3.

5. 4. The process gas to be enriched with ozone is air.

5. Apparatus according to any one of claims 1 to 4.

6. The ozone concentration in the enriched process gas is at least 0.1 ppm, preferably in the range between 10 ppm and 100 ppm.

6. Apparatus according to any one of claims 1 to 5.

7. 1. A method for recycling flakes from shredded and washed post-consumer plastic waste, comprising: drying and homogenizing said flakes from shredded and washed post-consumer plastic waste in a gas stream, in particular a hot gas stream; melting the dried and homogenized plastic waste to form a plastic melt; Optionally, removing foreign matter from the plastic melt; degassing the plastic melt; and granulating the plastic melt; and Optionally, drying the granules; removing odor from said granules with a process gas stream; enriching the process gas stream used to remove odors from the granules with ozone or providing the process gas stream as an ozone-enriched gas.

8. The amount of ozone supplied to the granules when removing the odor from the granules is adjusted according to the ozone concentration of the process gas at the time of discharge after the odor removal treatment of the granules. The method according to claim 7.

9. The ozone concentration of the process gas at the time of discharge after the odor removal treatment of the granules is measured by in-line measurement, and the amount of ozone supplied to the granules when removing odor from the granules is adjusted according to the measured ozone concentration. The method according to claim 8.

10. The ozone contained in the discharged process gas is at least partially decomposed, preferably by thermal or catalytic treatment of the discharged process gas.

10. The method according to any one of claims 7 to 9.

11. The ozone contained in the discharged process gas is at least partially decomposed, the decomposition of the ozone being carried out by irradiating the discharged process gas with electromagnetic waves, the electromagnetic waves being preferably UV light having a wavelength of at least 254 nm.

10. The method according to any one of claims 7 to 9.

12. 2. The process gas to be enriched with ozone is air.

12. The method according to any one of claims 7 to 11.

13. The ozone concentration in the enriched process gas is at least 0.1 ppm, preferably in the range between 10 ppm and 100 ppm.

13. The method according to any one of claims 7 to 12.

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