Method for cleaning plastic waste

Abstract

The invention relates to a method for cleaning pre-comminuted plastic waste, in particular plastic flakes. The method has: • a pre-treatment step in which pre-comminuted plastic waste is pre-treated in a pre-treatment device, said pre-treatment including the process of heating the plastic waste to a cleaning temperature using a heating device of the pre-treatment device and / or adding at least one cleaning additive and mixing the plastic waste with the at least one cleaning additive using the pre-treatment device, • a cleaning step in which the plastic waste pre-treated in the pre-treatment device is cleaned in a cleaning device, said cleaning device comprising two cleaning tools which are rotated relative to each other, wherein the plastic waste is guided together with a cleaning liquid through a working gap between mutually facing working surfaces of the cleaning tools, comprising cleaning teeth, and impurities are removed from the plastic waste by means of friction, and • a drying step in which the plastic waste cleaned in the cleaning device is dried in a drying device.

Description

[0001] Process for cleaning plastic waste

[0002] The invention relates to a method for cleaning pre-shredded plastic waste, in particular plastic flakes.

[0003] For example, traditional washing technologies with cold or hot process water are used for cleaning. One such cleaning process, which involves the product to be cleaned passing through several cleaning tanks with the addition of cleaning agents, which are intended in particular to separate ink from the plastic waste, is known from EP 2 832 459 B1. However, hot melt adhesives, printing inks, and metal vapor deposition, for example, are not completely removed. Complete removal is defined as residual contamination of less than 10 ppm, which only causes minimal VOCs (Volatile Organic Components) during further thermal processing, such as extrusion or melting. In traditional hot caustic soda washes, printing inks, for example, are only removed if specially adapted surfactants are used for the caustic soda and the flakes are penetrated in the wash solution for a long time.The detachment of hot-melt adhesives is also problematic. The residual adhesion levels required by the market, at least less than 100 ppm, are generally not achieved. This becomes noticeable during the application of the recycled materials, namely through gels in the film or a yellow tint. Further serious consequences include outgassing during granule extrusion or during further processing in LSP (liquid state polymerization), which condenses PET in a high vacuum.

[0004] The consequences of inadequate purification from merely "clean" to "highly pure" severely limit the use of recyclates and thus their marketing. At the same time, polymer quality requirements are increasing, comparable to those of primary plastics. Products made from recyclates must be largely free of adhering printing inks, various VOCs that burden degassing and melt filtration in the extruder, and extrinsic contamination, as such residual impurities can lead to adverse changes in polymer properties during mechanical and processing, as well as to undesirable color and odor changes. The contaminants described above would make the intended approval of the recyclates for use as food packaging impossible if they are not removed during the purification process.

[0005] EP 2 094 462 B1 discloses a process for separating cellulose and other adhering substances during the recycling of waste plastics, particularly mixed plastics. In this process, films and pieces of thicker plastic parts from possibly pre-sorted plastic waste are mechanically pre-shredded into flakes or particles of a predetermined size. The shredded material is fed into a disc refiner together with water without first producing a compacted material or an agglomerate from the flakes. Contaminants are largely abraded from the flakes by the interacting discs of the disc refiner and subsequently present as separate substances that can be separated from the plastic parts using a suitable separation process. A similar process is known from EP 2 094 461 B1.

[0006] EP 2 734 302 B1 discloses a further method for removing contaminants from plastic scrap using a disc refiner. The disc refiner is not a toothed disc refiner, but rather has a plurality of cleaning ribs extending between an inner and outer edge of the cleaning surfaces on the cleaning discs, wherein a plurality of cleaning webs extending transversely to the direction of extension of the cleaning ribs are arranged between at least some adjacent cleaning ribs. At least one flank of the cleaning ribs is inclined or curved relative to the axial direction of the respective cleaning disc, and the cleaning webs each rise in a ramp-like manner and have a lower height than the cleaning ribs.In this way, the plastic shreds are subjected to low mechanical stress during cleaning by pulling them between the discs, in particular between the cleaning ribs, and bending, folding or bunching of the plastic shreds, which can lead to inadequate cleaning, is avoided.

[0007] Furthermore, EP 3 057 751 B1 discloses a device and a method for cleaning plastic, in particular plastic scrap, during plastic recycling. A toothed disc refiner is used, comprising a central inlet for introducing the plastic to be cleaned into the working gap and an outlet provided at the outer edge of the working gap for the cleaned plastic together with abraded impurities and water. The outlet has an outlet pipe through which water is pumped, flowing laterally past the working gap and directed tangentially to the working gap during operation. This water exerts a suction effect on the working gap, so that the cleaned plastic is conveyed into the outlet pipe. In this way, in addition to the input and cleaning consistency, the discharge consistency can also be flexibly adjusted, in particular independently of the cleaning consistency.In order to achieve maximum energy efficiency and cleaning effect, it is therefore possible to set a high solid consistency in the working gap and, at the same time, to set a suspension with a low solid consistency that is easy to convey or pump downstream of the working gap.

[0008] Based on the prior art described above, the invention is therefore based on the object of providing a method of the type mentioned above with which plastic waste can be cleaned as extensively as possible of a wide variety of contaminants. The invention solves this problem by independent claim 1. Advantageous embodiments can be found in the dependent claims, the description, and the figures.

[0009] For a method of the type mentioned above, the invention solves the problem by the following steps:

[0010] • a pretreatment step in which pre-shredded plastic waste is pretreated in a pretreatment device, wherein the pretreatment comprises heating the plastic waste with a heating device of the pretreatment device to a cleaning temperature and / or the addition of at least one cleaning additive and the mixing of the plastic waste with the at least one cleaning additive by the pretreatment device,

[0011] • a cleaning step in which the plastic waste pretreated in the pretreatment device is cleaned in a cleaning device, wherein the cleaning device comprises two cleaning tools driven in rotation relative to one another, wherein the plastic waste is guided together with a cleaning liquid through a working gap between mutually facing working surfaces of the cleaning tools with cleaning teeth, wherein impurities are removed from the plastic waste by friction,

[0012] • a drying step in which the plastic waste cleaned in the cleaning device is dried in a drying device.

[0013] The inventive method is used for cleaning pre-shredded plastic waste, for example, plastic flakes or plastic shreds shredded from plastic films. In principle, plastic flakes can be produced by shredding thin-walled hard plastics or films, etc. The contaminants to be removed can be, in particular, surface deposits, such as cellulose from label residues, hot-melt adhesives, organic contamination from food residues, printed ink layers, optionally with printed seals, metallized surfaces, adhesive labels made of paper, plastic films, or metal foils, etc. The inventive method is used for cleaning pre-shredded plastic waste. The plastic waste can be provided already shredded.However, it is also possible for the process according to the invention to comprise comminution of the plastic waste prior to carrying out the cleaning process. The comminution of plastic waste, as mentioned in this application, can be carried out, for example, by mechanical comminution devices such as shredders or cutting mills.

[0014] In the pretreatment step, the pre-shredded plastic waste is pretreated in a pretreatment device for optimal subsequent cleaning. In particular, no cleaning of the plastic waste takes place in the pretreatment step. In particular, the pretreatment step preferably comprises essentially no friction and essentially no or only minimal agitation of the plastic waste. The pretreatment comprises heating the plastic waste with a heating device of the pretreatment device to a cleaning temperature and / or the addition of at least one cleaning additive and the mixing of the plastic waste with the at least one cleaning additive by the pretreatment device. During the pretreatment step, a liquid such as water can also be added. This can in particular already be the cleaning liquid used in the cleaning step.Pretreatment serves to act on the surfaces of the plastic waste, for example, with a tempered wash solution at a defined temperature and for a defined exposure time. As mentioned, during pretreatment, essentially no friction is exerted on the surfaces of the plastic waste, for example, through agitation. Rather, the task of the pretreatment device is to soften the structure of any contaminants adhering to the surfaces of the plastic waste, such as a printed layer or an adhesive layer, and to neutralize the van der Waals forces. The purpose of pretreatment is therefore to reduce the physical binding forces that cause the contaminants to adhere to the plastic surface.The removal of contaminants, such as printing inks, essentially does not occur during the pretreatment step, and this is also undesirable in order to prevent the migration of contaminants to the interfaces of the plastics. The pretreatment device can be thermally insulated to keep the plastic waste heated by the heating device at the specified cleaning temperature for a specified time. After pretreatment, the plastic waste is optimally prepared for subsequent cleaning.

[0015] The method according to the invention accordingly further comprises a cleaning step in which the pretreated plastic waste is cleaned in a cleaning device. The cleaning device forms, in particular, a toothed disc cleaner and comprises two mutually facing cleaning tools, which can be, for example, cleaning discs. At least one of the cleaning tools is driven in rotation by means of a rotary drive, so that a relative rotation occurs between the cleaning tools. The cleaning tools each have a working surface provided with cleaning teeth. The working surfaces are annular, in particular circular, with an annular, in particular circular, working gap being defined between the opposing working surfaces.The cleaning teeth on the opposing working surfaces mesh with each other, forming a male tool and a female tool. The teeth on the working surfaces are individual teeth and are therefore not connected to each other by ribs or the like. The cleaning tools can be adjusted axially relative to each other using a suitable adjustment device, thereby adjusting the width of the working gap. In this way, depending on the plastic waste to be cleaned, the meshing cleaning teeth can achieve the desired friction, ensuring that contaminants can be optimally removed from the appropriately pre-treated plastic waste. The plastic waste is cleaned by friction. In particular, contaminants are abraded from the surfaces of the plastic waste.For this purpose, it is possible to adjust a very small distance between the cleaning teeth of the opposing cleaning tools by means of appropriate axial adjustment, right up to the point where they come into contact with each other. For example, it is possible for the cleaning teeth to come into contact with the base of the opposing cleaning tool. A small distance can remain between the tooth flanks of the cleaning teeth, so that the plastic waste is forced over the tooth flanks to achieve an optimal cleaning result. The axis of rotation of the at least one rotatably driven cleaning tool, in particular the cleaning disc, can simultaneously be the axis of symmetry of the cleaning tool, in particular the cleaning disc. An electric drive, for example, can be used as the rotary drive.

[0016] The cleaning device has an inlet which can, for example, open centrally into the working gap. The plastic waste to be cleaned is fed into the working gap via this inlet, for example together with the cleaning fluid. However, separate feeding of the cleaning fluid and the plastic waste into the working gap is also possible. The plastic waste to be cleaned is fed through the working gap together with the cleaning fluid, which can be introduced separately or together with the plastic waste. The cleaning fluid can, in particular, be water. This also applies to the other embodiments mentioned below. The plastic waste is sheared between the cleaning tools and is thus evenly distributed in the working gap. In doing so, the plastic waste passes through the numbered rows formed by the cleaning teeth.The friction exerted by the cleaning teeth on the plastic waste rubs off or loosens contaminants on the surfaces. The plastic waste, along with the loosened deposits or contaminants and the cleaning fluid, is discharged from the working gap of the cleaning device via an outlet for further processing. Combined with the previous pretreatment, virtually any contaminant can be reliably and largely removed from the plastic waste in this cleaning step.

[0017] Following the cleaning step, a mechanical liquid separation can be performed, for example, i.e., a separation of the plastic waste from the cleaning liquid. The liquid separation can, in particular, take place immediately, for example, within less than one minute, preferably less than 30 seconds, after exiting the outlet of the cleaning device. This reliably prevents recontamination due to the cleaned plastic remaining in the cleaning device for too long. If a liquid separation is provided for in this application, this can be done, for example, using a phase separator and / or a centrifuge.

[0018] This is followed by a drying step in which the plastic waste, which has been cleaned in the cleaning device and, if applicable, separated from the cleaning fluid and thus the separated contaminants, is dried in a drying device. Drying brings the cleaned plastic waste into a state that is easy to process further. After drying, the plastic can, for example, have a residual moisture content of less than 5%, preferably less than 3%. After the drying step, a screening step, for example an air screening step, can follow in which three-dimensional plastic flakes are separated from thin film flakes. This facilitates further processing. Using the method according to the invention, a purity of the cleaned plastic waste of less than 100 ppm residual contamination, preferably less than 50 ppm, and even more preferably less than 20 ppm, can be achieved.This is achieved by combining the pretreatment step with appropriate temperature control and / or exposure to cleaning additives, but in particular without exerting friction on the plastic waste, with subsequent cleaning by friction in a suitable cleaning tool. Thus, a precise exposure time in combination with a defined temperature and / or a defined cleaning additive can be achieved in the pretreatment step without contaminants, in particular the plastic flakes, being removed from the plastic waste. In prior art processes, such as those described in EP 2 832 459 B1, the plastic waste is cleaned in agitation containers, in which temperature control and addition of cleaning additives also take place.This does not allow for defined cleaning, as it cannot be guaranteed that the essentially random agitation of the plastic waste in the agitation tanks, for example, by stirring, exposes all plastic particles to the same cleaning effect. The result is incomplete cleaning of individual plastic particles. State-of-the-art technology addresses this problem by using higher cleaning temperatures, larger addition quantities of cleaning additives, and longer exposure times. However, this is undesirable from an energy and environmental perspective.

[0019] According to the invention, however, the pretreatment step does not serve to clean the plastic waste, but merely as a defined pretreatment for the subsequent friction cleaning in the cleaning device. Energy consumption, residence time, and the need for cleaning additives can be optimized, with correspondingly beneficial effects on operating costs and environmental compatibility. The cleaning device's cleaning tooth geometry ensures that all plastic particles are exposed to essentially the same cleaning friction. This ensures consistently optimal cleaning results for all plastic particles. The pretreatment step, which serves only as a pretreatment step, prevents premature dissolution of contaminants such as printing inks or coatings into the liquid used in the pretreatment step, which would impair the effectiveness and reuse of cleaning additives.Intensive processing of the liquid used in the pretreatment step can be avoided.

[0020] The solids consistency of the suspension of plastic waste and cleaning fluid in the cleaning step is also important. For optimal cleaning, the solids content of the plastic waste to be cleaned in the working gap relative to the cleaning fluid present in the working gap should be less than 5 wt.% for plastic waste in the form of films and less than 10 wt.% for plastic waste in the form of PET or rigid plastics.

[0021] According to one embodiment, the pretreatment device can have a plurality of pretreatment containers that receive the plastic waste during pretreatment. In this way, the desired temperature and / or the desired exposure time of any cleaning additives can be set in a defined manner. Thus, unlike with a continuously flowing product batch, the batch-wise pretreatment of the plastic waste in a plurality of pretreatment containers easily ensures a desired minimum residence time for temperature control or exposure to cleaning additives. The plastic waste to be cleaned can be fed from the pretreatment containers for further processing one after the other and after the predetermined residence time. According to a further embodiment, the pretreatment device can have at least one pretreatment screw that conveys the plastic waste during the pretreatment step.The pretreatment screw conveys the plastic waste, in particular together with a liquid, for example the cleaning liquid used in the subsequent cleaning step. The pretreatment screw can be completely filled with the liquid. A defined residence time for the plastic waste can be set using the pretreatment screw. The pretreatment screw can comprise the heating device and / or thermal insulation. The pretreatment screw can also comprise at least one feed device for the at least one cleaning additive. However, it is also possible for the at least one cleaning additive to be fed to the plastic waste before it enters the pretreatment screw and / or for the plastic waste to be heated before it enters the pretreatment screw.

[0022] According to a further embodiment, the pretreatment device can have at least one pretreatment pipe system, in particular a pipe bundle system, through which the plastic waste is conveyed during the pretreatment step. The plastic waste is also conveyed through the pretreatment pipe system, for example by means of a pump together with a liquid, for example the cleaning liquid used in the subsequent cleaning step. The pretreatment pipe system can be completely filled with the liquid. A defined residence time for the plastic waste can also be set via the pretreatment pipe system. The pretreatment pipe system can comprise the heating device and / or thermal insulation. The pretreatment pipe system can also comprise at least one feed device for the at least one cleaning additive.However, it is also possible for the at least one cleaning additive to be added to the plastic waste before it enters the pretreatment pipe system and / or for the plastic waste to be heated before it enters the pretreatment pipe system. The pretreatment device can further comprise a stirred tank in which the plastic waste is stirred into a suspension. This ensures that the action of cleaning additives and, if appropriate, temperature control is achieved particularly reliably and evenly. Furthermore, it is possible for the stirred tank or one of the pretreatment tanks or several of the pretreatment tanks to have a heating device and / or thermal insulation. It is also possible for the stirred tank or one of the pretreatment tanks or several of the pretreatment tanks to have feed devices via which the at least one cleaning additive is fed directly into the pretreatment tank(s).

[0023] According to a further embodiment, the cleaning temperature can be at least 40°C and no more than 90°C, preferably no more than 70°C, and furthermore preferably no more than 60°C. It has been shown that with the process according to the invention, the desired cleaning effect is achieved even at cleaning temperatures that are significantly lower than those used in the prior art, and optionally also with a significantly reduced use of cleaning additives and significantly reduced residence times. Some plastics, such as amorphous PET, crystallize even at relatively low temperatures in the range of 70°C to 80°C and become correspondingly brittle. The use of brittle recyclates is generally not possible or severely restricted. By making cleaning possible even at lower cleaning temperatures of less than 70°C, preferably no more than 60°C, crystallization of even problematic plastics can be reliably avoided.A reduced use of cleaning additives is also desirable from an environmental point of view.

[0024] The at least one cleaning additive may comprise at least one surfactant and / or at least one sodium hydroxide solution and / or at least one complexing agent. Such cleaning additives enable the desired pretreatment to be carried out particularly effectively.

[0025] As already explained, the cleaning tools of the cleaning device can be cleaning discs. The plastic waste can be introduced into the working gap through an inlet opening centrally into the working gap and can be removed from the working gap through an outlet provided at the outer edge of the working gap. In particular, the inlet can open into the working gap in the axial direction of the cleaning discs.

[0026] According to a further embodiment, the width of the working gap can narrow radially outwards starting from the inlet in a first section, and the width of the working gap in a second section arranged radially outside the first section can be constant. The first section and the second section can in particular be a first annular section and a second annular section. The inlet zone formed by the first section serves to receive the quantity of plastic waste material at the inlet. The working gap in the first section, which is initially wider and then narrows outwards, leads to homogenization and even distribution of the plastic waste, even if the inlet consistency, i.e. the proportion of plastic waste per unit of time and / or per volume of process liquid, fluctuates considerably. This homogenization and even distribution is crucial for the subsequent cleaning success.Optimum cleaning can only be achieved if the plastic waste is transported as evenly and individually as possible along the rows of teeth formed by the opposing cleaning teeth to the outlet.

[0027] The second section, in which the working gap width remains constant, continues to be of crucial importance. Only the combination of the narrowing first section and the constant second section produces the optimum cleaning result. The first section ensures the necessary homogenisation and equalisation of the plastic waste, while the second section, with a constant working gap width, ensures sufficient physical stress on the plastic waste for an optimum cleaning effect. In this way, the plastic waste, in particular plastic flakes, are aligned parallel to the flanks of the cleaning teeth in the inlet zone formed by the first section, preventing any accumulation or clumping of plastic waste, which would be detrimental to cleaning. The necessary processing of the plastic waste for cleaning takes place in the working zone formed by the second section.

[0028] Plastic flakes produced from plastic film, in particular, are difficult to dose for cleaning purposes because they have a high dry and wet volume and a low bulk density. The design of the working gap with the first section and the resulting inlet zone allows the cleaning tools to easily accommodate even sinusoidally fluctuating dosages without clogging, for example. The design of the working gap also reliably breaks down sandwich or film packages, for example, created during the shredding of plastic film.

[0029] For optimal cleaning, it is crucial that plastic waste, particularly that produced from film by pre-shredding, is passed over the cleaning teeth with its entire surface so that the surfaces are thoroughly cleaned. Pre-shredding of the plastic waste is particularly important because the unfolding of accordion-folded plastic flakes by the cleaning tools according to the invention and the disintegration of film packages into individual flakes can lead to an elongation of the pre-shredded plastic waste. In practice, for example, an average grain size of the pre-shredded plastic waste of no more than 50 mm, preferably no more than 25 mm, for example in the range of 20 to 25 mm, has proven effective.This allows complete friction of the entire surface of the plastic waste in the working zone formed by the second section to be achieved with practical dimensions of the cleaning teeth. The friction exerted by the opposing cleaning teeth achieves complete removal of contaminants, including particularly difficult-to-clean printing inks or metallized surfaces. This enables a cleaning result with extrinsic residual contamination of less than 100 ppm, preferably less than 10 ppm. Thus, the recyclates produced according to the invention can be used like primary plastics for the manufacture of plastic parts.

[0030] The cleaning device can further comprise means for axially adjusting the cleaning tools relative to one another, so that the width of the working gap is adjustable, wherein the cleaning teeth of the cleaning tools can be brought into contact with one another in the second section. This configuration allows the cleaning tools or cleaning teeth to achieve sufficient friction for cleaning, even for thin film waste. The cleaning teeth of the opposing cleaning tools can, in particular, engage one another in a form-fitting manner.

[0031] The second section can be directly connected to the first section. This optimizes the transition between the inlet zone formed by the first section and the working zone formed by the second section.

[0032] The second section can extend to the outer edge of the working gap. The first section and the second section together can cover the entire working gap. However, it would also be conceivable to provide an inlet section, for example, between the inlet and the first section, on which, for example, no cleaning teeth are arranged. The working surfaces of the cleaning tools, which have the cleaning teeth, can be conical in the first section and parallel to one another in the second section. In this way, the first and second sections can be designed particularly simply.

[0033] Furthermore, the cleaning teeth on the surface sections of the working surfaces of the cleaning tools forming the first section can be arranged at a greater distance from one another than on the surface sections of the working surfaces of the cleaning tools forming the second section. The density of the cleaning teeth per unit area is therefore lower in the first section than in the second section. This further simplifies the infeed of the plastic waste and further improves the homogenization, uniformization, and separation of the plastic waste, while in the working zone formed by the second section, the cleaning teeth arranged more closely to one another ensure particularly effective cleaning.

[0034] The cleaning tool can basically be designed as described in the parallel, unpublished German patent application 10 2022 117 372.0 of the present applicant.

[0035] According to a further embodiment, the cleaned plastic waste can be dried in the drying device by means of pressing and / or air drying. This achieves particularly effective drying.

[0036] According to a further embodiment, it can be provided that a pre-cleaning step is carried out before the pre-treatment step, in which the shredded plastic waste is cleaned in a pre-cleaning device, wherein the pre-cleaning device comprises two cleaning tools driven to rotate relative to one another, wherein the plastic waste is guided together with a cleaning liquid through a working gap between mutually facing working surfaces of the cleaning tools with cleaning teeth, wherein contaminants are removed from the plastic waste substantially without friction.

[0037] The cleaning fluid can also be water. Pre-cleaning of the plastic waste pretreated in the pre-treatment facility can be advantageous, depending on the type of plastic waste to be cleaned, among other things. A basic distinction is made between plastic waste from the consumer sector, so-called post-consumer raw materials (PCR), and plastic waste from the industrial sector, so-called post-industrial raw materials (PIR). PCRs, in turn, are differentiated according to their origin, namely their origin from households, agriculture, or retail. PCR raw materials exhibit very different levels of contamination. These are usually significantly higher than 50,000 ppm. Contamination also varies widely, primarily cellulose from adhesive labels, food residues, glue, mineral deposits from soil and clay, but also printing inks.PIR raw materials typically exhibit an average contamination level of 50,000 ppm. The contamination is more limited and almost exclusively comprises printing inks, primers, binders, and sealants. The pre-cleaning step is particularly important for achieving the desired cleaning result with PCR plastic waste. With PIR plastic waste, the pre-cleaning step can further improve the cleaning result, but is not absolutely necessary. Pre-cleaning in the pre-cleaning step is carried out without cleaning additives. Pre-cleaning can also be carried out without temperature control, particularly with unheated cleaning fluid. The solids consistency should be similar to that in the cleaning step. Pre-cleaning significantly reduces contamination, allowing for further improved subsequent cleaning in the cleaning step. Pre-cleaning is also used to separate flake packets.The pre-cleaning tool can be designed in principle like the cleaning tool, but during the pre-cleaning process, essentially no friction is exerted on the surfaces of the plastic waste. In particular, the distance between the cleaning teeth of the pre-cleaning tool can be selected to be larger than that of the cleaning tool in the cleaning step. The focus is on forming a suspension of the plastic waste and the detached contaminants, such as paper labels, into individual cellulose fibers. Accordingly, there is a distance between the intermeshing cleaning teeth of the pre-cleaning tool. The pre-cleaning tool can, in particular, be designed as described in the parallel, unpublished German patent application 10 2022 117 371.2 of the present applicant.After the pre-cleaning step, a liquid discharge can again take place, as already explained above for the cleaning step.

[0038] According to a further embodiment, the pre-cleaned plastic waste can be further shredded in a shredding step after the pre-cleaning step and before the cleaning step. The further shredding in the shredding step can take place before the pre-treatment step. As explained above, the grain size of the plastic flakes for cleaning in the cleaning step should not exceed 50 mm, preferably not exceeding 25 mm. This allows complete removal of extrinsic contamination. With larger plastic flakes, there is a risk that not the entire surface is exposed to the cleaning friction, for example, with folded plastic flakes. The desired grain size is generally determined by the height of the cleaning teeth of the cleaning tool.To avoid excessive fines, which could lead to yield losses in subsequent steps, the flake size should not be less than 5 mm. During the pre-cleaning process, adjacent plastic sections may also become folded, resulting in longer plastic strips after the pre-cleaning process that are difficult to clean in the subsequent cleaning process. Such long plastic strips, for example, are shredded in the shredding step to a size suitable for the subsequent main cleaning process. As explained above, shredding can be performed, for example, in a mechanical shredding device such as a shredder or a cutting mill.

[0039] According to a further embodiment, the plastic waste can be rinsed with a rinsing liquid after the pre-cleaning step and before the cleaning step. Rinsing can, in turn, take place before the pre-treatment cut. The rinsing liquid can, in particular, be water. Rinsing can take place in full flow. Rinsing can take place before or after further shredding. Rinsing with rinsing liquid minimizes the carryover of the already dissolved contaminants caused by the pre-cleaned plastic waste into the subsequent steps.

[0040] The cleaning fluid used in the pre-cleaning step can be fed into a pre-cleaning fluid circuit. The cleaning fluid used in the cleaning step can also be fed into a cleaning fluid circuit. Such fluid circuits can minimize fluid consumption. Only a small amount of fluid needs to be added from an external supply to the ongoing process, namely to compensate for relatively small fluid losses caused by the cleaning fluid being carried along by the plastic waste to be cleaned. In practice, a fresh fluid consumption of less than 1 m 3 / ton of product to be cleaned. Low fluid consumption reduces operating costs and is desirable from an environmental perspective.

[0041] The cleaning fluid used in the pre-cleaning step and / or the cleaning fluid used in the cleaning step can be further filtered. Such filter devices, such as ultrafiltration devices, can remove contaminants from the circulating fluid, preventing recontamination through the fluid circulation and thus ensuring the cleaning effect over the long term.

[0042] According to a further embodiment, the pre-cleaning fluid circuit can be separated from the cleaning fluid circuit. This separation of the "dirty" pre-cleaning fluid circuit from the "clean" cleaning fluid circuit, acting as a "firewall," minimizes contamination between the fluid circuits and thus recontamination of, for example, already pre-cleaned plastic waste during the cleaning step. Due to the secure separation between the fluid circuits, no cleaning fluid—and thus, in particular, the contaminants contained therein, removed during the pre-cleaning process—is transferred from the pre-cleaning process.If further shredding is performed with cleaning fluid, this can also be done with the pre-cleaning fluid circuit or in a separate shredding fluid circuit. This is then separated from the cleaning fluid circuit.

[0043] According to a further embodiment, it can be provided that after the cleaning step and before the drying step, a post-cleaning step is carried out in which the plastic cleaned in the cleaning device is further cleaned in a post-cleaning device without the addition of cleaning additives. The post-cleaning device comprises two cleaning tools driven in rotation relative to one another, the plastic waste being guided together with a cleaning fluid through a working gap between mutually facing working surfaces of the cleaning tools with cleaning teeth, any cleaning additives remaining after the cleaning step being removed from the plastic waste. The cleaning fluid can again be water. The post-cleaning device can be designed like the pre-cleaning device.During the post-cleaning process, essentially no friction is exerted on the plastic waste already cleaned in the cleaning step. The primary purpose of post-cleaning is to remove any cleaning additives added during the pre-treatment step, particularly through hydrodynamic turbulence. Of course, further removal of contaminants is also possible to a limited extent. Post-cleaning is typically performed without cleaning additives. Post-cleaning can also be performed without temperature control, particularly with unheated cleaning fluid. After the post-cleaning step, the fluid can be removed again, as explained above.

[0044] The cleaning fluid used in the post-cleaning step can be fed into a post-cleaning fluid circuit. The cleaning fluid used in the post-cleaning step can be filtered. Ultrafiltration can be used, in particular, to achieve optimal post-cleaning results. The post-cleaning fluid circuit can be separated from the cleaning fluid circuit to avoid contamination between cleaning steps.

[0045] An embodiment of the invention is explained in more detail below with reference to the figures. Figures 1a to 1e schematically illustrate different method steps of the method according to the invention.

[0046] Unless otherwise stated, the same reference symbols in the figures refer to the same objects.

[0047] Figure 1a illustrates a pre-cleaning step of the process according to the invention. Pre-shredded plastic waste to be cleaned, particularly in the form of plastic flakes, is fed to a feed silo 12 via a plastic inlet 10. From the feed silo 12, the plastic waste to be cleaned enters a heavy-duty trap 14, in which any coarse heavy material contained in the plastic waste is separated from the plastic waste to be cleaned and removed. From the heavy-duty trap, the plastic waste enters a feed hopper 16 of a pre-cleaning device. A cleaning fluid, such as water, is also fed to the feed hopper 16 via a first fluid inlet 17. From the feed hopper 16, the plastic waste, together with the cleaning fluid, enters a working gap formed between two cleaning discs 18, 20 of the pre-cleaning device.For this purpose, the pre-cleaning device has an inlet opening centrally into the working gap. One of the cleaning discs 18, 20, for example, is driven in rotation via a rotary drive 22. The cleaning discs 18, 20 have cleaning teeth on their surfaces defining the working gap. Due to the relative rotational movement between the cleaning discs 18, 20, the plastic waste is conveyed over the cleaning teeth and is pre-cleaned in the process. Due to the rotational movement, the plastic waste reaches an outlet provided on the outer edge of the working gap, through which the plastic waste is fed to a mechanical liquid removal device 24. In the example shown, a liquid pump 26 is also arranged in the area of ​​the outlet, which pumps liquid, in particular water, supplied via a second liquid inlet 28 tangentially past the working gap and its outlet.The liquid pump 26 can also be formed by a Venturi nozzle, wherein the liquid jet conveyed tangentially past the working gap and its outlet exerts a suction effect on the working gap due to the Venturi effect, so that the plastic waste is conveyed out of the working gap and further, as described in EP 3 057 751 B1. The liquid jet pumped by the liquid pump 26 or the Venturi nozzle transports the mixture of pre-cleaned plastic waste and cleaning liquid emerging from the outlet to the liquid separation device 24. Therein, a mechanical liquid separation takes place by separating the pre-cleaned plastic waste from the liquid. The separated liquid exits via the liquid outlet 30. Preferably, the exited liquid is filtered and then fed back to the first and second liquid inlets 17, 28 in a circuit.The pre-cleaned plastic waste exits through an outlet 32 ​​and reaches an inlet 34 of a comminution device shown in Fig. 1b. The pre-cleaning device shown in Figure 1a exerts essentially no friction on the plastic waste. The pre-cleaning device can be designed as described in the parallel, unpublished German patent application 10 2022 117 371.2 of the present applicant.

[0048] Figure 1b illustrates a possible comminution step of the method according to the invention after the pre-cleaning step and before the subsequent cleaning step. The pre-cleaned plastic waste fed in via inlet 34 enters a wet shredder 36, to which liquid, such as water, is further fed via a liquid inlet 38. In the wet shredder 36, the plastic waste is further mechanically comminuted to an optimal size for the subsequent cleaning step, as explained above. From the wet shredder 36, the mixture of further comminuted plastic waste and liquid in turn passes to a mechanical liquid separator 40, in which the further comminuted plastic waste is separated from the liquid. The further comminuted plastic waste exits via outlet 42, and the separated liquid exits via liquid outlet 44.The leaked liquid can, if necessary, be filtered and recycled back to the liquid inlet 38.

[0049] The following pretreatment and cleaning steps will be explained with reference to Figure 1c. The pre-cleaned and further shredded plastic waste flows from the outlet 42 to an inlet 46 of a pretreatment device comprising several pretreatment containers 48. A cleaning liquid, such as water, is also supplied to the pretreatment containers 48 via a liquid inlet 50. The cleaning liquid is supplied upstream of the liquid inlet 50 from a liquid treatment unit 52 to a mixing and heating container 54 of the pretreatment device. The cleaning liquid in the mixing and heating container 54 can be heated to a predetermined cleaning temperature via a heating device 56, for example, a hot steam or gas heating device, and a heat exchanger 60.At least one cleaning additive, for example, at least one surfactant and / or at least one sodium hydroxide solution and / or at least one complexing agent, can be added from a cleaning additive container 58 to the cleaning liquid contained in the mixing and heating container 54. The mixture of cleaning liquid and optionally the at least one cleaning additive, heated to the desired cleaning temperature depending on the process, is fed via the liquid inlet 50 to one of the pretreatment containers 48, where it is mixed with the plastic waste fed in via the inlet 46. As soon as one pretreatment container 48 is filled, the next pretreatment container 48 is filled.As soon as the mixture of plastic waste and cleaning fluid, as well as optionally the at least one cleaning additive, has reached a defined residence time, the pretreated mixture of plastic waste and cleaning fluid, and optionally with the at least one cleaning additive, is fed to an inlet 64 of the cleaning device via a pump 62. In this way, the pretreatment containers 48 are filled and emptied one after the other.

[0050] Alternatively or in addition to the pretreatment containers 48, the pretreatment device could also comprise a pretreatment screw conveying the mixture or a pretreatment pipe system conveying the mixture.

[0051] The cleaning device corresponds in its basic design to the pre-cleaning device explained in relation to Figure 1a. In particular, the cleaning device also comprises two cleaning discs 66, 68 which define a working gap, in particular a circular ring, into which the plastic waste is fed via a central inlet, together with the cleaning fluid and the at least one cleaning additive. The cleaning discs 66, 68 in turn have cleaning teeth on their surfaces defining the working gap, and at least one of the cleaning discs 66, 68 is driven in rotation by means of a rotary drive 70. As a result, the plastic waste is conveyed from the central inlet through the working gap and is forced over the cleaning teeth of the cleaning discs 66, 68.The plastic waste, together with the cleaning fluid and the at least one cleaning additive, is conveyed to an outlet provided at the outer edge of the working gap and from there to a further mechanical fluid removal device 72, as illustrated by arrow 74 in Figure 1c. The cleaning of the previously pre-cleaned, further shredded, and specifically and precisely pre-treated plastic parts in the pre-treatment device occurs through friction caused by contact with the cleaning teeth of the cleaning discs 66, 68. In particular, the contaminants remaining on the surface of the plastic waste are thereby abraded by friction. Due to the pre-cleaning, further shredding, and pre-treatment of the plastic waste, the plastic waste is essentially completely cleaned.It has been shown that, in particular due to the pre-cleaning, further comminution, and pre-treatment according to the invention, as well as the use of the pre-cleaning device and cleaning device designed as a toothed disk cleaner, it is possible to work with significantly lower cleaning temperatures of less than 70°C, preferably less than 60°C, compared to the prior art, while achieving optimal cleaning results. The use of cleaning additives has also been reduced compared to the prior art. The cleaning device can be configured as in the unpublished German patent application 10 2022 117 372.0 of the present applicant. The liquid separation device 72, in turn, separates the cleaning liquid with the at least one cleaning additive and the contaminants contained therein, abraded from the plastic waste, from the cleaned plastic waste, which exits via the outlet 76.The separated cleaning liquid with the at least one cleaning additive and the abraded contaminants exits via the liquid outlet 78 and is fed from there to the water treatment 52, as illustrated in Figure 1c by the arrow 80. In the water treatment 52, the cleaning liquid is separated from the contaminants and the at least one cleaning additive during filtration, in particular ultrafiltration, so that the purified cleaning liquid can be fed back to the mixing and heating tank 54 in a circuit, as illustrated in Figure 1c by the arrow 82.

[0052] The cleaned plastic waste emerging from outlet 76 then reaches the inlet 84 of a post-cleaning device shown in Figure 1d. From inlet 84, the plastic waste enters a feed hopper 86, to which a cleaning liquid, in particular water, is also fed via a liquid inlet 88. From feed hopper 86, the mixture of cleaned plastic waste and cleaning liquid enters a particularly circular working gap formed between two cleaning discs 90, 92 of the post-cleaning device, again in particular via a central inlet. At least one of the cleaning discs 90, 92 is driven in rotation via a rotary drive 94, and the cleaning discs in turn have cleaning teeth on their surfaces delimiting the working gap.Due to the relative rotational movement between the cleaning discs 90, 92, the plastic waste supplied together with the cleaning fluid is subjected to a post-cleaning process, which again occurs essentially without friction. In particular, any cleaning additives remaining on the cleaned plastic waste from the cleaning step are removed from the plastic waste. Via an outlet provided at the outer edge of the working gap, the post-cleaned plastic waste, together with the cleaning fluid and any separated cleaning additives, reaches a first mechanical fluid removal device 96. The post-cleaning device can be designed like the pre-cleaning device shown in Figure 1a.Also located at the outlet of the post-cleaning device is a liquid pump 98, which also conveys cleaning liquid tangentially past the working gap and its outlet via the liquid inlet 88, thereby conveying the post-cleaned plastic waste together with the cleaning liquid containing the cleaning additives from the working gap to the first liquid discharge device 96. The liquid pump 98 can again be formed by a Venturi nozzle, wherein the liquid jet conveyed tangentially past the working gap and its outlet exerts a suction effect on the working gap due to the Venturi effect, so that the plastic waste is conveyed out of the working gap and further, as described in EP 3 057 751 B1. The liquid separated from the post-cleaned plastic waste in the first liquid discharge device 96 is discharged via a liquid outlet 100, as shown in Fig.Id by arrow 102. The plastic waste dried in the first liquid removal device 96 then passes to a second mechanical liquid removal device 104, as illustrated in Fig. Id by arrow 106. There, any remaining residual liquid is separated from the plastic waste and also discharged via the liquid outlet 100, as illustrated in Fig. Id by arrow 107.

[0053] The further dried, post-cleaned plastic waste exits via outlet 108 and from there reaches an inlet 110 of the drying device shown in Fig. 1e. In the drying device, the plastic waste is dried for subsequent reuse. Liquid discharged via outlet 100 can in turn be recirculated, optionally filtered, back to the liquid inlet 88. The drying step according to Figure 1e initially comprises mechanical drying in a mechanical liquid separation device 112. Liquid separated therein is discharged via a liquid outlet 114. The plastic waste then passes through a thermal drying device 116, where it is dried to a residual moisture content of less than 5%, preferably less than 3%. Following the drying step, screening, in particular air screening, can take place in a screening device 118.This allows three-dimensional plastic flakes to be separated from thin film flakes. The finally cleaned and dried plastic waste exits via outlet 120 and can be fed for further processing.

[0054] List of reference symbols:

[0055] Plastic inlet

[0056] Feed silo

[0057] Heavy goods trap

[0058] Feed hopper first liquid inlet, 20 cleaning disc

[0059] rotary drive

[0060] Liquid discharge device

[0061] Liquid pump second liquid inlet

[0062] Liquid outlet

[0063] Outlet

[0064] inlet

[0065] wet shredder

[0066] Liquid inlet

[0067] Liquid discharge device

[0068] Outlet

[0069] Liquid outlet

[0070] inlet

[0071] Pretreatment tank

[0072] Liquid inlet

[0073] Liquid treatment

[0074] Mixing and heating tank

[0075] Heating device

[0076] Additional cleaning container

[0077] heat exchanger

[0078] pump

[0079] Inlet, 68 cleaning discs

[0080] rotary drive

[0081] Liquid discharge device

[0082] Arrow

[0083] Outlet

[0084] Liquid outlet

[0085] Arrow

[0086] Arrow 4 Inlet 6 Feed hopper 8 Inlet 0, 92 Cleaning discs 4 Rotary drive 6 First liquid discharge device 8 Liquid pump

[0087] 100 liquid outlet

[0088] 102 Arrow

[0089] 104 second liquid discharge device

[0090] 106 Arrow

[0091] 108 Outlet

[0092] HO inlet

[0093] 112 Liquid discharge device

[0094] 114 Liquid outlet

[0095] 116 Drying device

[0096] 118 screening device

[0097] 120 outlet

Claims

Claims:

1. A process for cleaning pre-shredded plastic waste, in particular plastic flakes, characterized by: • a pretreatment step in which pre-shredded plastic waste is pretreated in a pretreatment device, wherein the pretreatment comprises heating the plastic waste with a heating device (56) of the pretreatment device to a cleaning temperature and / or the addition of at least one cleaning additive and the mixing of the plastic waste with the at least one cleaning additive by the pre-treatment device, • a cleaning step in which the plastic waste pretreated in the pretreatment device is cleaned in a cleaning device, wherein the cleaning device comprises two cleaning tools (66, 68) driven in rotation relative to one another, wherein the plastic waste is guided together with a cleaning liquid through a working gap between mutually facing working surfaces of the cleaning tools (66, 68) with cleaning teeth, wherein impurities are removed from the plastic waste by friction, • a drying step in which the plastic waste cleaned in the cleaning device is dried in a drying device (116).

2. Method according to claim 1, characterized in that the pretreatment device has a plurality of pretreatment containers (48) which receive the plastic waste during the pretreatment.

3. Method according to one of the preceding claims, characterized in that the pretreatment device has at least one pretreatment screw which conveys the plastic waste during the pretreatment step. Method according to one of the preceding claims, characterized in that the pretreatment device has at least one pretreatment pipe system through which the plastic waste is conveyed during the pretreatment step. Method according to one of the preceding claims, characterized in that the pretreatment device comprises a stirred tank in which the plastic waste is stirred. Method according to one of the preceding claims, characterized in that the cleaning temperature is at least 40°C and not more than 90°C, preferably not more than 70°C, more preferably not more than 60°C. Method according to one of the preceding claims, characterized in that the at least one cleaning additive comprises at least one surfactant and / or at least one sodium hydroxide solution and / or at least one complexing agent.Method according to one of the preceding claims, characterized in that the cleaning tools (66, 68) of the cleaning device are cleaning discs (66, 68), and in that the plastic waste is introduced into the working gap through an inlet opening centrally into the working gap and is guided out of the working gap through an outlet provided at the outer edge of the working gap. Method according to claim 8, characterized in that the width of the working gap, starting from the inlet, varies radially in a first section. Direction towards the outside narrows and that the width of the working gap is constant in a second section arranged radially outside the first section. Method according to one of the preceding claims, characterized in that the cleaned plastic waste is dried in the drying device (116) by means of pressing and / or air drying. Method according to one of the preceding claims, characterized in that before the pretreatment step, a pre-cleaning step is carried out in which the shredded plastic waste is cleaned in a pre-cleaning device, wherein the pre-cleaning device comprises two cleaning tools (18, 20) driven in rotation relative to one another, wherein the plastic waste, together with a cleaning liquid, is conveyed through a working gap between mutually facing working surfaces of the Cleaning tools (18, 20) with cleaning teeth are used, whereby contaminants are removed from the plastic waste substantially without friction. Method according to claim 11, characterized in that after the pre-cleaning step and before the cleaning step, further comminution of the pre-cleaned plastic waste takes place in a comminution step. Method according to one of claims 11 or 12, characterized in that the cleaning fluid used in the pre-cleaning step is conducted in a pre-cleaning fluid circuit and / or that the cleaning fluid used in the cleaning step is conducted in a cleaning fluid circuit. Method according to claim 13, characterized in that the cleaning liquid used in the pre-cleaning step and / or the cleaning liquid used in the cleaning step is filtered. Method according to one of claims 13 or 14, characterized in that the pre-cleaning liquid circuit is separated from the cleaning liquid circuit. Method according to one of claims 11 to 15, characterized in that the plastic waste is rinsed with a rinsing liquid after the pre-cleaning step and before the cleaning step.Method according to one of the preceding claims, characterized in that after the cleaning step and before the drying step, a post-cleaning step is carried out in which the plastic cleaned in the cleaning device is further cleaned in a post-cleaning device without the addition of cleaning additives, wherein the post-cleaning device comprises two cleaning tools (90, 92) driven in rotation relative to one another, wherein the plastic waste is guided together with a cleaning liquid through a working gap between mutually facing working surfaces of the cleaning tools (90, 92) with cleaning teeth, wherein any cleaning additives remaining after the cleaning step are removed from the plastic waste. Method according to claim 17, characterized in that the cleaning liquid used in the post-cleaning step is guided in a post-cleaning liquid circuit.Method according to claim 18, characterized in that the cleaning liquid used in the post-cleaning step is filtered.

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