Process for recycling used polyethylene-based plastics using light hydrocarbon solvent

Abstract

The present invention relates to a process for purifying a plastic raw material comprising polyethylene, comprising: a) dissolving the plastic raw material in a dissolving solvent comprising a hydrocarbyl compound having a boiling point between-15 DEG C and 100 DEG C at a dissolution temperature between 120 DEG C and 220 DEG C and a dissolution pressure between 1.0 MPa absolute pressure and 25.0 MPa absolute pressure to obtain a crude polymer solution; b) purifying the crude polymer solution, comprising: b1) separating out insoluble substances; b2) washing with a high-density solution; b3) extracting with an extraction solvent; and / or b4) adsorbing impurities; and then c) solvent-polymer separation, comprising a supercritical separation section, carried out at a temperature of between 160 DEG C and 300 DEG C and a pressure of between 2.7 MPa abs and 10.0 MPa abs, followed by at least one solvent recovery section, to obtain purified polyethylene.

Description

Technical Field

[0001] The present invention relates to a method for recycling used plastics, mainly comprising polyethylene (or PE), so as to obtain a purified polyethylene stream that can be economically upgraded, for example in the manufacture of new plastic articles. More specifically, the present invention relates to a method for purifying plastic raw materials, in particular plastic raw materials obtained from plastic waste (including polymers, and in particular polyethylene), comprising dissolving the polymer in a light hydrocarbon solvent (in particular based on (one or more) alkanes with a boiling point between -15°C and 100°C), at least one step of purifying the polymer solution obtained, so as to at least partially remove impurities (in particular additives usually used in plastic-based materials), and an optimized step of separating the purified polyethylene and the solvent, so as to be able to reuse the recovered purified polyethylene and thus economically upgrade the plastic raw material. Prior art

[0002] Plastics obtained from collection and sorting channels can be upgraded according to various channels.

[0003] "Mechanical" recycling allows for the reuse of certain waste fractions, either directly into new articles or by blending a mechanically sorted plastic waste stream with a virgin polymer stream. This type of economic upgrading is limited because mechanical sorting can improve the purity of a given type of polymer stream, but generally cannot adequately remove impurities that are at least partially entrapped in the polymer matrix, e.g., additives such as fillers, colorants, pigments, and metals.

[0004] "Chemical" recycling involves at least partial reforming of monomers through a generally complex sequence of steps. For example, plastic waste can be subjected to a pyrolysis step, and the recovered pyrolysis oil (usually after purification) can be at least partially converted, for example, into olefins by steam cracking. These olefins can then be polymerized. This type of sequence may be suitable for small amounts of sorted feedstock or sorting center waste, but it generally requires a large energy expenditure, in particular due to the high temperature treatment.

[0005] Another approach to recycling plastic waste consists in at least partially dissolving plastics, in particular thermoplastics, with the aim of purifying them by removing polymers and / or impurities other than the target compound(s) present in the raw material, such as additives (eg fillers, colorants, pigments and metals).

[0006] Therefore, several studies have proposed various methods for treating plastic waste by dissolution and purification. US2017 / 002110 describes a specific method for purifying polymer raw materials, especially polymer raw materials obtained from plastic waste, by dissolving the polymer in a solvent under specific temperature and pressure conditions and then contacting the obtained polymer solution with a solid.

[0007] WO2018 / 114047, for its part, proposes a method for dissolving a plastic in a solvent at a dissolution temperature close to the boiling point of the solvent. However, the method of WO2018 / 114047 cannot effectively handle impurities other than polymers.

[0008] US2018 / 0208736 proposes a treatment method by liquefying a thermoplastic in a solvent and then separating out insoluble substances and / or gases. The method of US2018 / 0208736 cannot effectively treat impurities soluble in the solvent.

[0009] The present invention aims to overcome these drawbacks and to participate in the recycling of plastics. More specifically, the present invention aims to propose an effective, simple and economically viable method for treating a plastic raw material based on polyethylene, in particular a plastic raw material obtained from plastic waste, so as to at least partially eliminate the impurities it contains, in particular the additives it contains and which are usually added to plastics, so as to be able to upgrade said plastic raw material and more particularly plastic waste economically. In fact, the present invention seeks to effectively separate the impurities from the polymers contained in the used plastics and in particular from the polyethylene, and to recover the purified polyethylene so as to be able to use it, for example as a polymer base in the manufacture of new plastic articles, in particular to replace virgin resins. SUMMARY OF THE INVENTION

[0011] The present invention relates to a method for purifying a plastic raw material containing polyethylene, the method comprising:

[0012] a) a dissolving step, which comprises contacting the plastic raw material with a dissolving solvent at a dissolving temperature between 120° C. and 220° C. and a dissolving pressure between 1.0 MPa and 25.0 MPa absolute pressure to obtain at least one crude polymer solution, wherein the dissolving solvent comprises at least one hydrocarbon-based compound having a boiling point between -15° C. and 100° C.;

[0013] b) a step of purifying the crude polymer solution to obtain a purified polymer solution, comprising:

[0014] b1) a sub-step of separating out insoluble matter; and / or

[0015] b2) a washing sub-step by contacting with a high density solution; and / or

[0016] b3) an extraction sub-step by contacting with an extraction solvent; and / or

[0017] b4) an impurity adsorption sub-step, by contacting with an adsorbent; then

[0018] c) a solvent-polymer separation step using at least one supercritical separation section operated at a temperature between 160° C. and 300° C. and at a pressure between 2.7 MPa absolute and 10.0 MPa absolute (P supercritical ), followed by at least one solvent recovery section, to obtain at least one purified polyethylene fraction.

[0019] The advantage of the process according to the invention is that it proposes a method for effectively treating polyethylene-based plastic raw materials, and in particular polyethylene-based plastic waste, especially plastic waste obtained from collection and sorting channels, in order to recover the polyethylene contained therein, so as to be able to recycle them to any type of application. According to the process according to the invention, it is possible to obtain a purified polyethylene stream, which advantageously contains a negligible content of impurities (in particular additives) and solvents (in particular dissolving solvents) or at least sufficiently low so that the purified polyethylene stream can be introduced into any type of plastic formulation instead of virgin polyethylene resin. For example, the purified polyethylene stream obtained at the end of the process according to the invention advantageously contains less than 5% by weight of impurities, very advantageously less than 1% by weight of impurities, and very advantageously contains less than 5% by weight of solvents (in particular dissolving solvents), preferably less than 1% by weight of solvents, preferably less than 0.1% by weight of solvents.

[0020] The process according to the invention thus proposes a simple scheme corresponding to a sequence of operations which makes it possible to remove at least a portion of the impurities, in particular at least some of the additives, from plastic waste based on polyethylene and to recover purified polyethylene, advantageously containing little or even no solvent, so as to be able to upgrade the plastic waste economically by recycling the purified polyethylene. Advantageously, the additives present in the plastic raw material may be soluble or insoluble in the solvent used in the overall process according to the invention, depending on the conditions used in the steps of the process, thus allowing an effective purification and separation of the polymer.

[0021] Furthermore, the process according to the invention proposes a sequence of operations carried out under optimal operating conditions, in particular in terms of temperature and pressure, to effectively separate impurities and solvents from polyethylene, but the operating conditions are reasonable, thus limiting the energy consumption of the process, making the process economically advantageous.

[0022] The invention also has the advantage of participating in the recycling of plastics and saving fossil resources by enabling the economic upgrading of plastic waste. In particular, the invention allows the purification of plastic waste in order to obtain a purified polyethylene fraction with a reduced content of impurities, in particular a decolorized and deodorized polyethylene fraction, which can be reused to form new plastic articles. The purified polyethylene fraction obtained can thus be used directly in formulations, as a mixture with additives such as colorants, pigments or other polymers, instead of virgin resins or as a mixture with virgin resins, to obtain plastic articles having aesthetic, mechanical or rheological processing properties, thus promoting their reuse and their economic upgrading.

[0023] The invention also enables polyethylene to be separated efficiently and advantageously at low cost from the solvent used (especially the dissolving solvent), while limiting thermal degradation of the polyethylene. Thus, the solvent used to treat the plastic raw material (especially the dissolving solvent) is at least partially recovered and can be recycled into one of the steps of the method, thereby avoiding excessive solvent consumption, thereby achieving ecological and economic advantages of the method.

[0024] The present invention therefore aims at purifying plastic raw materials, in particular plastic waste, in order to obtain purified polyethylene in order to be able to use it in any application, in particular as a replacement for virgin resins.

[0025] More specifically, the present invention aims to propose a process comprising a dissolution step, followed by at least one purification step, and then an optimized solvent / polymer separation, in order to obtain a purified polyethylene stream.

[0026] Description of the implementation plan

[0027] According to the invention, the expressions "included between ... and ... " and "between ... and ... " are equivalent and mean that the limits of the interval are included in the range of values ​​described. If this is not the case, and if the limits are not included in the described range, the invention will give such clarification.

[0028] For the purpose of the present invention, various parameter ranges of a given step, such as pressure ranges and temperature ranges, can be used alone or in combination. For example, within the meaning of the present invention, a preferred range of pressure values ​​can be combined with a more preferred range of temperature values.

[0029] Hereinafter, specific embodiments of the present invention may be described. They may be implemented separately or in combination when technically feasible, without limiting the combination.

[0030] According to the invention, the pressure is an absolute pressure and is given in MPa absolute (or MPa abs).

[0031] The terms "upstream" and "downstream" are to be understood as varying with the general flow of the fluid(s) or stream(s) under consideration in the process.

[0032] The term "additive" is a term commonly used in the field of polymers, and in particular in the field of polymer formulations. The additive introduced into the polymer formulation can be, for example, a plasticizer, a filler (which is an organic or inorganic solid compound used to change the physical, thermal, mechanical and / or electrical properties of a polymer material or to reduce its cost price), a reinforcing agent, a colorant, a pigment, a hardener, a flame retardant, a flame retardant, a stabilizer, an antioxidant, a UV absorber, an antistatic agent, etc.

[0033] The additive corresponds to at least a portion of the impurities in the plastic raw material to be processed and can be at least partially removed by the method according to the invention. Other types of impurities may be use-related impurities, such as metal impurities, paper / cardboard, biomass, polymers other than the target polyethylene (e.g. polypropylene), etc.

[0034] Thus, according to the invention, the impurities that can be at least partially removed by the process according to the invention include additives conventionally used in polymer formulations and impurities derived from the life cycle of plastic objects and materials usually associated with use and / or derived from waste collection and sorting circuits. The impurities can be metallic, organic or inorganic type; they can be packaging residues, food residues or compostable residues (biomass). These use-related impurities can also include glass, wood, cardboard, paper, aluminum, iron, metals, tires, rubber, silicone, rigid polymers, thermoplastic polymers other than polyethylene, thermosetting polymers, household products, chemicals or cosmetics, waste oils and water.

[0035] According to the invention, the polymer solution is a solution comprising a dissolving solvent and at least the target polyethylene, the target polyethylene being dissolved (i.e., in particular solvated and dispersed) in the dissolving solvent, the dissolved polyethylene being initially present in the feedstock. The polymer solution may also contain soluble impurities (dissolved in the dissolving solvent) and / or insoluble impurities (suspended in the polymer solution). Depending on the steps of the method according to the invention that have been performed, the polymer solution may therefore contain impurities in the form of insoluble particles that are advantageously suspended in the polymer solution, soluble impurities dissolved in the dissolving solvent, and / or another liquid phase that is optionally immiscible with the polymer solution.

[0036] The critical temperature and critical pressure of a solvent, in particular a dissolving solvent, are specific to the solvent and depend on the chemical properties of the solvent under consideration. For a pure substance, the critical temperature and critical pressure of a pure substance are the temperature and pressure, respectively, of the critical point of the pure substance. As is well known to those skilled in the art, below and above the critical point, the pure substance under consideration is in a supercritical form or state; it can then be referred to as a supercritical fluid.

[0037] The present invention relates to a method for purifying a plastic raw material, preferably consisting of plastic waste and containing polyethylene, comprising the following steps, and preferably consisting of the following steps:

[0038] a) a dissolving step, which comprises contacting the plastic raw material with a dissolving solvent, which contains at least one hydrocarbon-based compound, advantageously aliphatic and preferably paraffinic, having a boiling point between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, preferably between 25°C and 61°C, and very preferably between 25°C and 40°C, at a dissolving temperature between 120°C and 220°C, preferably between 130°C and 200°C, and a dissolving pressure between 1.0 MPa and 25.0 MPa, preferably between 1.0 MPa and 20.0 MPa, preferably between 5.0 MPa and 18.0 MPa, preferably between 10.0 MPa and 17.0 MPa, to obtain at least one crude polymer solution; then

[0039] b) a step of purifying the crude polymer solution, comprising at least one of the following sub-steps:

[0040] b1) a sub-step of separating off insoluble matter to obtain at least a clear polymer solution and preferably an insoluble fraction; and / or

[0041] b2) a washing sub-step, carried out by contacting with a high density solution, to obtain at least one washed polymer solution and preferably a washing effluent; and / or

[0042] b3) an extraction sub-step, carried out by contacting with an extraction solvent, to obtain at least one extracted polymer solution and preferably spent solvent; and / or

[0043] b4) an impurity adsorption sub-step, carried out by contacting with an adsorbent to obtain at least one purified polymer solution;

[0044] a purification step making it possible to obtain a purified polymer solution, which advantageously corresponds to a clarified and / or washed and / or extracted and / or refined polymer solution; then

[0045] c) a solvent-polymer separation step using at least one supercritical separation section at a temperature between 160° C. and 300° C., preferably between 190° C. and 250° C., preferably between 200 and 230° C. and at a pressure between 2.7 MPa absolute and 10.0 MPa absolute, preferably between 3.0 MPa absolute and 6.0 MPa absolute, preferably between 3.0 MPa absolute and 5.0 MPa absolute and preferably at 3.0 MPa absolute to 4.0 MPa absolute pressure (P supercritical), followed by the use of at least one solvent recovery section, in particular, the solvent recovery section is operated at a temperature between 160° C. and 300° C. and at a pressure between P supercritical and 0.000005 MPa (i.e. 5 Pa), preferably between 2.7 MPa and 0.000005 MPa, and in particular at a pressure between 1.0 MPa and 0.000005 MPa, to obtain at least one purified polyethylene fraction and advantageously a solvent fraction.

[0046] raw material

[0047] The feedstock according to the process of the invention, referred to as plastic feedstock, comprises plastic, which itself more particularly comprises polyethylene. Preferably, the plastic feedstock comprises 50 to 100 wt. %, and preferably 70 to 100 wt. % of plastic.

[0048] The plastics contained in the raw materials according to the inventive method are based on polyethylene and are generally production waste and / or waste plastic items at the end of their life, in particular household plastic waste, construction industry plastic waste, automobile or any type of transport plastic waste or electrical and electronic equipment waste. Preferably, the plastic waste is derived from collection and sorting channels. Usually, plastics or plastic materials contain polymers mixed with additives for the purpose of forming various materials and articles (injection molded parts, pipes, films, fibers, fabrics, fillers (mastics), coatings (coatings) etc.) after forming. The additives used in plastics can be organic compounds or inorganic compounds. For example, they are fillers, colorants, pigments, plasticizers, property modifiers, combustion flame retardants, etc.

[0049] Preferably, the feedstock according to the process of the invention comprises at least 80% by weight, preferably at least 85% by weight and preferably at least 90% by weight of polyethylene relative to the total weight of the plastic feedstock. The process according to the invention therefore most particularly relates to the purification and recovery of the polyethylene contained in the feedstock in order to be able to reuse it in various applications.

[0050] The plastic raw material may also contain impurities, such as polymers other than polyethylene, in particular thermoplastics, additives advantageously used to formulate the plastic material and impurities generally associated with use originating from the life cycle of the material and plastic objects and / or from waste collection and sorting circuits. The plastic raw material according to the process of the invention may contain up to 20% by weight of impurities, preferably up to 15% by weight of impurities, preferably up to 10% by weight of impurities. The plastic raw material may contain, for example, at least 5% by weight of impurities.

[0051] The plastic raw material can advantageously be pretreated before the method in order to remove at least all or some "coarse" impurities, i.e. impurities in the form of particles with a size greater than or equal to 10 mm, preferably greater than or equal to 5 mm, or even greater than or equal to 1 mm, such as wood, paper, biomass, iron, aluminum, glass, etc., and shape it, usually in the form of dispersed solids (or particles), so as to facilitate processing in the method. The pretreatment may include a grinding step, a washing step at atmospheric pressure and / or a drying step. The pretreatment can be carried out at different locations, such as at a waste collection and sorting center, or at the same location where the treatment method according to the invention is carried out. Preferably, this pretreatment can reduce the impurity content to less than 20% by weight, preferably less than 15% by weight, preferably less than 10% by weight, these percentages being given relative to the weight of the plastic raw material treated by the method according to the invention. At the end of the pretreatment, the raw material is usually stored in a dispersed solid form, such as in the form of a ground material or a powder, so as to facilitate handling and transportation to the method.

[0052] Dissolution step a)

[0053] According to the invention, the method comprises a dissolution step a), wherein the plastic raw material is placed in contact with a dissolving solvent to obtain at least one, preferably one, crude polymer solution. In particular, this step advantageously dissolves at least a portion of the polyethylene of the plastic raw material, and preferably all of the polyethylene.

[0054] The term "dissolution" is understood to mean any phenomenon leading to the production of at least one polymer solution, in particular a polyethylene solution, i.e. a liquid comprising a polymer, in particular polyethylene, dissolved in a solvent, more in particular a dissolving solvent. The phenomena involved in polymer dissolution are well known to those skilled in the art and include at least mixing, dispersion, homogenization, solvation and disentanglement of polymer chains, and more in particular polyethylene chains.

[0055] During and at the end of dissolving step a), the pressure and temperature conditions are such as to maintain the dissolving solvent at least partially, and preferably entirely, in liquid form, thereby optimizing the dissolution of the target polyethylene.

[0056] The properties of the dissolving solvent advantageously allow the use of reasonable operating conditions, and in particular temperature and pressure conditions, in particular pressure conditions, in order firstly to ensure, in the dissolving step a), and advantageously in the purification step b), that the dissolving solvent is at least partially and preferably completely maintained in the liquid phase, thereby allowing optimal dissolution of the target polyethylene and advantageously efficient purification of the polymer solution, and secondly, in the solvent-polymer separation step c), to ensure that at least part of the dissolving solvent enters a supercritical state in order to achieve stratification and thus separation of at least part of the dissolving solvent, and optionally at least partial evaporation of the residual dissolving solvent, so that a very low solvent content can be achieved in the purified polyethylene recovered at the end of the process (a content advantageously of less than 5% by weight of solvent, preferably less than 1% by weight of solvent, preferably less than 0.1% by weight of solvent, relative to the total weight of the purified polyethylene fraction). In fact, solvents consisting of very light alkanes with a boiling point below -15°C (e.g. propane) can be particularly advantageous for their relatively mild critical conditions (temperature and pressure), but require the use of high pressures to keep the dissolving solvent at least partially, and preferably completely, in liquid form throughout the dissolving step a) and the purification step b), which will require significant costs, in particular investment costs. On the contrary, the use of heavy solvents (e.g. alkanes with a boiling point above 100°C) will require very strict operating conditions in step c) in order to reach the critical conditions of the heavy solvent and to be able to obtain the solvent at least partially in a supercritical state.

[0057] Advantageously, the dissolving solvent comprises, preferably consists of, at least one hydrocarbon-based compound, advantageously aliphatic and preferably paraffinic (i.e. saturated), preferably at least one alkane, having a boiling point between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, preferably between 25°C and 61°C, and very preferably between 25°C and 40°C. Preferably, the dissolving solvent predominantly comprises, preferably at least 80% by weight, preferably at least 95% by weight, preferably 98% by weight (100% being a maximum, the percentages being expressed relative to the total weight of the dissolving solvent), hydrocarbon-based compounds, advantageously aliphatic, preferably paraffinic (or alkane), having a boiling point between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, preferably between 25°C and 61°C, and very preferably between 25°C and 40°C. Very advantageously, the critical temperature (temperature at the critical point of the pure hydrocarbon-based compound) constituting the main amount of the dissolved solvent, advantageously aliphatic, preferably paraffinic, is between 130° C. and 285° C., preferably between 158° C. and 285° C., preferably between 185° C. and 245° C., preferably between 185° C. and 230° C., and very preferably between 185° C. and 200° C. Very particularly, the critical pressure of the mainly paraffinic hydrocarbon-based compound of the dissolved solvent is between 2.5 MPa and 5.0 MPa, preferably between 2.7 MPa and 4.6 MPa, preferably between 3.0 MPa and 3.8 MPa, and most preferably between 3.0 and 3.5 MPa. According to a preferred embodiment, the dissolving solvent mainly comprises, preferably at least 80% by weight, preferably at least 95% by weight, preferably 98% by weight, aliphatic alkanes, hydrocarbon-based compounds, which are preferably linear or branched, have a boiling point between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, preferably between 25°C and 61°C, and very preferably between 25°C and 40°C, and contain 4 to 7 carbon atoms (i.e. C4-C7), preferably 5, 6 or 7 carbon atoms (C5, C6 or C7, respectively), preferably contain 5 or 6 carbon atoms (C5 or C6) and very preferably contain 5 carbon atoms (C5).

[0058] Advantageously, the dissolution step a) is carried out at a dissolution temperature of between 120° C. and 220° C., preferably between 130° C. and 200° C., very preferably between 150° C. and 200° C., and at a dissolution pressure of between 1.0 MPa absolute and 25.0 MPa absolute, preferably between 1.0 MPa absolute and 20.0 MPa absolute, preferably between 5.0 MPa absolute and 18.0 MPa absolute, preferably between 10.0 MPa absolute and 17.0 MPa absolute. More specifically, the temperature and pressure may vary throughout step a), starting from the conditions for the introduction of the plastic raw material and / or the dissolving solvent, for example from ambient conditions, i.e. a temperature between 10° C. and 30° C. and atmospheric pressure (0.1 MPa), until dissolving conditions are reached, i.e. a dissolving temperature, in particular between 120° C. and 220° C., preferably between 130° C. and 200° C., very preferably between 150° C. and 200° C., and a dissolving pressure, in particular between 1.0 MPa absolute and 25.0 MPa absolute, preferably between 1.0 MPa absolute and 20.0 MPa absolute, preferably between 5.0 MPa absolute and 18.0 MPa absolute, preferably between 10.0 MPa absolute and 17.0 MPa absolute. Very advantageously, at the end of dissolving step a), the dissolved polymer stream, in particular the polymer solution, is at the dissolving temperature and the dissolving pressure.

[0059] Limiting the temperature in step a) to a temperature less than or equal to 220° C., preferably less than or equal to 200° C., makes it possible to avoid or limit thermal degradation of the polymer, in particular polyethylene, and also to limit the energy requirements of the process, thus contributing to limiting the operating costs and carbon footprint of the process. Preferably, the dissolution temperature is greater than or equal to the melting point of polyethylene, in order to promote its dissolution.

[0060] At the same time, the dissolution pressure is advantageously greater than the saturated vapor pressure of the dissolution solvent at the dissolution temperature, so that the dissolution solvent is at least partially, and preferably completely, in liquid form at the dissolution temperature, thereby optimizing the dissolution of the target polyethylene.

[0061] Very advantageously, the dissolution temperature and pressure conditions achieved in step a) are adjusted so that the mixture (dissolving solvent+polyethylene), which may contain insoluble impurities suspended in said mixture, is homogeneous and very preferably single-phase.

[0062] Preferably, the weight ratio of the plastic raw material to the dissolving solvent (raw material / solvent) (or the ratio between the mass flow rate of the plastic raw material and the mass flow rate of the dissolving solvent at the inlet of the dissolving step a)) is between 0.01 and 2.0, preferably between 0.05 and 1.0, and preferably between 0.10 and 0.8.

[0063] Advantageously, the dissolution step a) is carried out for a residence time of 1 to 600 minutes, preferably 2 to 300 minutes, preferably 5 to 180 minutes. The residence time is understood to be the residence time at the dissolution temperature and at the dissolution pressure, i.e. the time during which the plastic material is carried out together with the dissolution solvent at the dissolution temperature and at the dissolution pressure in step a).

[0064] Advantageously, the dissolving solvent comprises and preferably consists of a supply of fresh solvent and / or a stream of recycled solvent obtained from a subsequent step of the process, preferably from the solvent-polymer separation step c).

[0065] Contacting the dissolving solvent with the plastic raw material to at least partially and preferably completely dissolve the polyethylene of the plastic raw material in the dissolving solvent can be carried out in a pipeline and / or a piece of equipment and / or between two pieces of equipment. Therefore, step a) advantageously involves at least one dissolving device, and optionally involves at least one raw material preparation device, mixing device and / or transport device. These equipment and / or device parts can be, for example, static mixers, extruders, pumps, reactors, co-current or countercurrent towers, or a combination of pipelines and equipment. Devices for transporting, in particular, fluids (such as liquids or solids) are well known to those skilled in the art. In a non-limiting manner, the transport device can include a pump, an extruder, a vibrating tube, an endless screw or a valve. Equipment and / or device parts can also include a heating system (such as an oven, an exchanger, a heating element (tracing) etc.) or be combined with it to achieve the conditions required for dissolving. The dissolving step a) can be carried out continuously, in batch mode or in a fed batch mode.

[0066] The dissolving step a) is advantageously fed with at least a plastic raw material (in particular in the form of one or more plastic raw material streams) and a dissolving solvent (in particular in the form of one or more dissolving solvent streams) via one or more transport devices. The (one or more) plastic raw material streams may be different from the (one or more) dissolving solvent streams. A portion or all of the plastic raw material may also be fed to step a) as a mixture with a portion or all of the dissolving solvent, the remainder of the solvent and / or the remainder of the raw material, where appropriate, possibly being fed to step a) separately.

[0067] During the contact of the plastic raw material with the dissolving solvent, the dissolving solvent is advantageously at least partially, and preferably completely, in liquid form, but the plastic raw material comprising polyethylene may be in solid or liquid form, optionally including suspended solid particles. The plastic raw material may also be optionally injected into the dissolving device as a mixture with the dissolving solvent in the form of a suspension in the dissolving solvent, and the preparation and injection of the suspension may be continuous or batchwise.

[0068] According to a particular embodiment of the invention, step a) may use an extruder and optionally at least one other dissolving device. In this case, the plastic raw material is optionally fed to the extruder together with at least a portion of the dissolving solvent, so that at the outlet of the extruder, at least a portion and preferably all of the target polyethylene contained in the raw material is in a molten form (or at least partially dissolved form). The plastic raw material optionally mixed with at least a portion of the dissolving solvent is then injected at least partially in a molten form (or partially dissolved form) into a dissolving device (e.g., a reactor). The plastic raw material at least partially in a molten form (and / or partially dissolved form) leaving the extruder can also be pumped by a pump dedicated to viscous fluids (commonly referred to as a melt pump or gear pump). The plastic raw material at least partially in a molten form (or partially dissolved form) can also be filtered at the outlet of the extruder using a filtering device optionally in addition to the melt pump to remove the coarsest particles; typically, the mesh size of such a filter is between 10 microns and 1 mm, preferably between 20 microns and 200 microns.

[0069] Preferably, step a) comprises an extruder into which the dissolving solvent is advantageously injected at several points in order to promote shear and therefore intimate mixing between the dissolving solvent and the plastic feedstock, which helps to dissolve the polyethylene.

[0070] Optionally, the treatment method may include an intermediate adsorption step a') during or directly downstream of the dissolution step a) and which comprises introducing an adsorbent, preferably such as alumina, silica, silica-alumina, activated carbon or bleaching earth, in the form of dispersed particles into the crude polymer solution obtained at the end of step a) or optionally during the dissolution step a). The adsorbent may then be removed during a purification step b), for example during the sub-step b1) of separating off insoluble matter and / or the washing sub-step b2). This optional adsorption step a') carried out in the presence of the adsorbent in dispersed form makes it possible to optimize the purification of the polymer solution.

[0071] The crude polymer solution obtained at the end of step a) comprises at least the dissolving solvent, the purified polyethylene that the present invention seeks to recover dissolved in the dissolving solvent. Typically, the crude polymer solution also comprises soluble impurities that are also dissolved in the dissolving solvent. The crude polymer solution may also optionally comprise insoluble impurities or compounds in the suspension. The crude polymer solution obtained at the end of step a) may also optionally comprise polymers other than the target polyethylene, such as polymers in molten form.

[0072] At the end of the dissolution step carried out under such operating conditions (in particular in terms of temperature and pressure), the polyethylene of the plastic raw material is advantageously completely or partially dissolved in the dissolving solvent; the polyethylene solution obtained (i.e. the crude polymer solution) will be able to undergo a purification step b) and a subsequent solvent-polymer separation step c) in order to recover the polyethylene from the plastic raw material in a purified form, wherein the content of impurities and residual solvents is very low, which is compatible with any type of subsequent application. The process according to the invention will therefore allow the recovery of polyethylene from plastic waste in an optimal manner and under completely rational operating conditions (in particular under a well-defined, i.e. limited, dissolution pressure), with a controllable energy consumption and therefore limited costs.

[0073] Step b) of purifying the polymer solution

[0074] The purification method according to the present invention comprises a step of purifying the crude polymer solution obtained in step a). The purification step b) comprises at least one of the following sub-steps b1), b2), b3) and b4):

[0075] b1) a sub-step of separating out insoluble matter,

[0076] b2) a washing sub-step, carried out by contact with a high-density solution,

[0077] b3) an extraction sub-step, carried out by contacting with an extraction solvent,

[0078] b4) a sub-step of adsorbing impurities, which is carried out by contacting with an adsorbent.

[0079] The various sub-steps b1), b2), b3) and b4) which may be performed in purification step b) may be operated in continuous, batch or fed-batch mode.

[0080] Preferably, purification step b) comprises at least one substep b1) of separating off insoluble matter. Purification step b) preferably comprises a plurality of (i.e. at least two) substeps selected from substeps b1), b2), b3) and b4) carried out in sequence, and preferably comprises at least one substep b1) of separating off insoluble matter and, for example, one adsorption substep b4), very advantageously carried out in the stated order. The combination of at least two substeps selected from b1), b2), b3) and b4) advantageously allows an optimal purification of the polymer solution.

[0081] The polymer solution obtained at the end of step b) is a purified polymer solution and comprises at least polyethylene dissolved in a dissolving solvent. This purified polymer solution may correspond to a clarified polymer solution obtained from sub-step b1) of separating off insoluble matter, a washed polymer solution obtained from a washing sub-step b2), an extracted polymer solution obtained from an extraction sub-step b3) or a refined polymer solution obtained from a sub-step b4) of adsorbing impurities.

[0082] Sub-step b1) of separating insoluble matter

[0083] The purification process may comprise a sub-step b1) of separating the insoluble matter by solid-liquid separation, so as to advantageously obtain at least one clear polymer solution (i.e. free of at least a part of, and preferably all of, the insoluble matter contained in the crude polymer solution) and preferably an insoluble fraction. The insoluble fraction advantageously comprises at least a part of, and preferably all of, the insoluble impurities, in particular the insoluble impurities suspended in the crude polymer solution obtained from step a).

[0084] Thus, sub-step b1) of separating off insoluble matter makes it possible to remove at least a portion of the particles of insoluble compounds in the dissolving solvent, and preferably all of them, which are present in suspension in the crude polymer solution obtained from step a) or optional step a'). Insoluble compounds (or impurities) removed in sub-step b1) of separating off insoluble matter are, for example, pigments, inorganic compounds, packaging residues (glass, wood, cardboard, paper, aluminum) and insoluble polymers.

[0085] When carrying out this separation substep b1), in addition to removing at least a portion of the insoluble impurities, this separation substep b1) advantageously makes it possible to limit operating problems, in particular such as clogging and / or corrosion of process steps downstream of this substep b1), while at the same time contributing to the purification of the plastic raw material.

[0086] The sub-step b1) of separating off the insoluble matter is advantageously carried out at a temperature of preferably between 120° C. and 220° C., preferably between 130° C. and 200° C., very preferably between 150° C. and 200° C., and at a pressure of between 1.0 MPa absolute and 25.0 MPa absolute, preferably between 1.0 MPa absolute and 20.0 MPa absolute, preferably between 5.0 MPa absolute and 18.0 MPa absolute, preferably between 10.0 MPa absolute and 17.0 MPa absolute. Very advantageously, the sub-step b1) of separating off the insoluble matter is carried out at the temperature and pressure conditions at the outlet of the dissolution step a), i.e. at the dissolution temperature and dissolution pressure as defined above.

[0087] When it is incorporated into the process, substep b1) of separating off insoluble matter is preferably fed with the crude polymer solution obtained from step a) or from the optional intermediate adsorption step a'). According to another embodiment, substep b1) can be fed with the washed polymer solution obtained from the washing substep b2).

[0088] Advantageously, sub-step b1) involves at least one solid-liquid separation stage (or solid-liquid-liquid separation stage, in particular in the case where the effluent obtained at the end of the dissolution step contains, in addition to the polymer solution and solid impurities, impurities in liquid form and with little or no solubility and / or polymers of a nature different from the target polyethylene). The solid-liquid separation stage comprises at least one solid-liquid separation device, such as a separation container, a decanter, a centrifugal decanter, a centrifuge, a filter, a sand filter, a tangential filter (in particular using a membrane) and / or a depth filter, an eddy current separator, an electrostatic separator, a triboelectric separator, preferably a decanter, a filter, a sand filter and / or an electrostatic separator. Advantageously, a self-cleaning filter can be used, which is cleaned or unblocked using a solvent stream to allow the removal of insoluble substances. Preferably, sub-step b1) involves at least one sedimentation stage, which advantageously comprises at least one decanter and / or at least one filtration stage. In sub-step b1), a filtering aid (for example diatomaceous earth or sand) can optionally be added before decantation and / or filtration.

[0089] The removal of the insoluble fraction can be facilitated by means of equipment for transporting and / or removing trace amounts of solvent that may be present in the insoluble fraction, such as a conveyor belt, a vibrating tube, a worm, an extruder or a stripper. Thus, sub-step b1) may comprise equipment for transporting and / or removing trace amounts of solvent to remove the insoluble fraction. Advantageously, at least a portion of the solvent recovered in sub-step b1) is recycled into the process.

[0090] According to a specific embodiment, the sub-step b1) of separating the insoluble matter comprises at least two, and usually less than five, solid-liquid separation equipment pieces connected in series and / or in parallel. The presence of at least two solid-liquid separation equipment pieces connected in series makes it possible to improve the removal of insoluble matter, while the presence of parallel equipment makes it possible to manage the maintenance and / or dredging operations of the equipment.

[0091] Certain insoluble compounds, in particular certain pigments and inorganic fillers, which are usually added during the formulation of polymers, may be in the form of particles with a size of less than 1 μm. This is the case, for example, with titanium dioxide, calcium carbonate and carbon black. According to one specific embodiment of sub-step b1), said sub-step b1) of separating out the insoluble substances advantageously comprises an electrostatic separator, which makes it possible to effectively remove at least a portion of the insoluble particles with a size of less than 1 μm. According to another specific embodiment of sub-step b1), sub-step b1) of separating out the insoluble substances comprises a sand filter for removing particles of different sizes, and in particular particles with a size of less than 1 μm. According to yet another specific embodiment, sub-step b1) of separating out the insoluble substances involves a tangential filter (in particular using a membrane) and / or a depth filter, optionally in the presence of a filtering aid (for example diatomaceous earth).

[0092] Depending on the nature of the raw materials, the polymer solution (preferably a crude polymer solution) fed to sub-step b1) may also optionally contain a second liquid phase, for example consisting of molten polymers, which are different in nature from the polyethylene. According to another specific embodiment, sub-step b1) advantageously comprises a solid-liquid-liquid separation stage, using equipment for separating out two liquid phases and one solid phase, preferably with the aid of at least one two-phase or three-phase separator.

[0093] Washing sub-step b2)

[0094] The purification process may optionally include a sub-step b2) of washing with a high-density solution to advantageously obtain at least one washed polymer solution and preferably a washing effluent. The washed polymer solution obtained at the end of sub-step b2) advantageously comprises the purified polyethylene dissolved in the dissolving solvent that the present invention seeks to recover. Optionally, if sub-step b2) is carried out, the washed polymer solution may further contain residual impurities that are particularly soluble in the dissolving solvent and / or optionally traces of washing solvent (i.e. traces of high-density solution).

[0095] When these two sub-steps are integrated into purification step b), the washing sub-step b2) can be integrated upstream or downstream, preferably downstream, of sub-step b1) of separating off insoluble matter.

[0096] When it is integrated into the method, the washing substep b2) is fed with a high density solution and a crude polymer solution obtained from step a) or from the optional intermediate adsorption step a'), or a clarified polymer solution obtained from b1). The polymer solution fed to the washing substep b2), in particular the crude polymer solution or the clarified polymer solution, may contain impurities in the form of suspended insoluble compounds and / or dissolved compounds. These suspended compounds or dissolved compounds can be partially or completely removed during the washing substep b2) by dissolution or precipitation and / or by entrainment in the high density solution. Therefore, when this substep b2) is performed, this substep b2) contributes to the processing of the plastic raw material, and more specifically, to the purification of the polymer solution.

[0097] Washing sub-step b2) advantageously involves placing the polymer solution, i.e., a crude or clarified polymer solution, fed to sub-step b2) in contact with a high-density solution. Advantageously, the high-density solution has a higher density than the polymer solution (i.e., a mixture of at least the target polyethylene and the dissolving solvent in which the target polyethylene is dissolved). In particular, the density of the high-density solution is preferably greater than or equal to 0.85, preferably greater than or equal to 0.9, preferably greater than or equal to 1.0, and preferably less than or equal to 1.5. The high-density solution can be an aqueous solution, which preferably comprises at least 50% by weight of water, preferably at least 75% by weight of water, and very preferably at least 90% by weight of water. Acid or alkali can be used to adjust the pH of the aqueous solution to promote the dissolution of certain compounds. The high density solution may also optionally be a solution comprising, preferably consisting of, an organic solvent (e.g. an organic solvent selected from sulfolane or N-methylpyrrolidone (NMP), optionally as a mixture with water) having a density advantageously greater than or equal to 0.85, preferably greater than or equal to 0.9, preferably greater than or equal to 1.0, and in which the polyethylene of the plastic raw material remains insoluble under the temperature and pressure conditions of substep b2). Very preferably, the high density solution is an aqueous solution, which preferably comprises at least 50% by weight of water, preferably at least 75% by weight of water, very preferably at least 90% by weight of water.

[0098] The washing substep b2) is advantageously carried out at a temperature preferably between 120° C. and 220° C., preferably between 130° C. and 200° C., very preferably between 150° C. and 200° C., and at a pressure between 1.0 MPa absolute and 25.0 MPa absolute, preferably between 1.0 MPa absolute and 20.0 MPa absolute, preferably between 5.0 MPa absolute and 18.0 MPa absolute, preferably between 10.0 MPa absolute and 17.0 MPa absolute. Very advantageously, the washing substep b2) is carried out at the dissolution temperature and dissolution pressure.

[0099] In the washing sub-step b2), when it is incorporated into the process, the mass ratio between the mass flow of high-density solution and the mass flow of polymer solution fed to sub-step b2) (high-density solution / polymer solution) is advantageously between 0.05 and 20.0, preferably between 0.1 and 10.0, and preferably between 0.5 and 3.0. The contact placed between the polymer solution and the high-density solution can be carried out at several points of the equipment used, i.e. by injecting the polymer solution and / or the high-density solution several times at different points along the equipment; the sum of the injected streams taken into account in the calculation of the mass ratio (high-density solution / polymer solution) is then carried out.

[0100] Sub-step b2) can be carried out in one or more washing equipment pieces capable of being placed in contact with the high density solution and / or in combination with a separation device, so that at least one washing effluent and a washed polymer solution can be recovered. Such equipment is well known, such as stirred reactors, static mixers, decanting mixers, two-phase or three-phase separation vessels, co-current or countercurrent washing towers, plate towers, stirred towers, packed towers, pulse towers, etc., each type of equipment may include one or more equipment pieces used alone or in combination with another type of equipment.

[0101] According to a preferred embodiment, the washing substep b2) is carried out in a countercurrent washing column, wherein on the one hand the high density solution is injected, preferably into the upper half, preferably the upper third, of the column, preferably closest to the top of the column, and on the other hand the crude or clarified polymer solution is injected, preferably into the lower half, preferably the lower third, of the column, preferably closest to the bottom of the column. According to this embodiment, at least one washed polymer solution and advantageously a washing effluent can be recovered.

[0102] According to a very specific embodiment, the stream at the inlet and / or outlet of the wash column can be split and injected at several injection points along the column and / or withdrawn at several withdrawal points along the column.

[0103] According to another embodiment, the washing substep b2) is carried out in a mixer-decanter comprising a stirring and mixing zone for bringing the high-density solution and the crude or clarified polymer solution into contact, and a decanting zone making it possible to recover the washed polymer solution and the advantageous washing effluent.

[0104] At the end of washing sub-step b2), the wash effluent advantageously obtained particularly comprises the compound dissolved in the high-density solution and / or the insoluble compound entrained in the wash effluent. The wash effluent can be reprocessed in the wash effluent treatment section, on the one hand at least partially separating the compound dissolved and / or entrained and optionally purifying the wash effluent, to obtain a purified high-density solution, on the other hand at least partially recycling a part of the purified high-density solution. The wash effluent treatment section can include one or more well-known equipment pieces for solid-liquid separation, such as a separation vessel, a decanter, a centrifugal decanter, a centrifuge or a filter. When the high-density solution is an aqueous solution, the wash effluent also can be sent outside the method, such as to a wastewater treatment station.

[0105] Extraction sub-step b3)

[0106] Step b) of the process according to the invention may comprise an extraction sub-step b3) carried out by placing in contact with an extraction solvent, so as to obtain at least one extracted polymer solution, and preferably spent solvent containing in particular impurities. The extracted polymer solution obtained at the end of sub-step b3) advantageously comprises the purified polyethylene dissolved in the dissolving solvent that the invention seeks to recover. Optionally, if (one or more) sub-steps b2) and / or b3) are carried out, the extracted polymer solution may also comprise residual impurities that are in particular soluble in the dissolving solvent and / or traces of high-density solution and / or extraction solvent.

[0107] When the extraction sub-step b3) is integrated into the process according to the invention, the extraction sub-step b3) is advantageously situated between the dissolution step a) and the solvent-polymer separation step c), preferably downstream of the sub-step b1) for separating off the insoluble substances and possibly upstream or downstream of the adsorption sub-step b4) (if the adsorption sub-step b4) is also integrated into step b)).

[0108] Advantageously, the extraction sub-step b3) is fed with an extraction solvent and a polymer solution, in particular a crude polymer solution obtained from step a), a clarified polymer solution obtained from sub-step b1), a washed polymer solution obtained from sub-step b2) or a refined polymer solution obtained from adsorption sub-step b4). Preferably, the extraction sub-step b3) is fed with an extraction solvent and a clarified polymer solution obtained from sub-step b1), a washed polymer solution obtained from sub-step b2) or optionally a refined polymer solution obtained from adsorption sub-step b4). Thus, the polymer solution fed to sub-step b3), preferably a clarified polymer solution, a washed polymer solution or a refined polymer solution, may optionally contain dissolved compounds or dissolved impurities other than polyethylene. These dissolved compounds may be partially or completely removed during the extraction sub-step b3) by placing in contact with the extraction solvent. Very advantageously, the combination of the extraction sub-step b3) with the sub-step b1) of separating off the insoluble matter and optionally the washing sub-step b2) and / or the adsorption sub-step b4) allows an improved purification of the polymer solution, taking advantage of the affinity of the impurities both for the extraction solvent and possibly for the high density solution and / or the adsorbent.

[0109] When it is incorporated in the process according to the invention, the extraction sub-step b3) advantageously involves at least one extraction stage, preferably one to five extraction stages, very preferably one extraction stage.

[0110] The mass ratio between the mass flow of the extraction solvent and the mass flow of the polymer solution (preferably clarified polymer solution, washed polymer solution or purified polymer solution) fed to sub-step b3) (extraction solvent / polymer solution) is advantageously between 0.05 and 20.0, preferably between 0.1 and 10.0, and preferably between 0.2 and 5.0. The contact between the polymer solution (preferably clarified polymer solution, washed polymer solution or purified polymer solution) fed to sub-step b3) and the extraction solvent can be carried out at several points in the extraction section, i.e. by multiple injections of polymer solution and / or extraction solvent at different points along the extraction section; the addition of the injected streams taken into account in the calculation of the mass ratio (extraction solvent / polymer solution) is then carried out.

[0111] The extraction solvent used in the extraction sub-step b3) advantageously comprises an organic solvent or a mixture of organic solvents. Preferably, the extraction solvent comprises, preferably consists of, at least one hydrocarbon-based compound, advantageously aliphatic and preferably paraffinic, preferably at least one alkane, having a boiling point between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, preferably between 25°C and 61°C, and very preferably between 25°C and 40°C. Preferably, the extraction solvent predominantly comprises, preferably at least 80% by weight, preferably at least 95% by weight, preferably 98% by weight (100% being a maximum, the percentages being expressed relative to the total weight of the dissolved solvent), preferably paraffinic aliphatic hydrocarbon-based compounds (or alkanes), having a boiling point between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, preferably between 25°C and 61°C, and very preferably between 25°C and 40°C. Very advantageously, the hydrocarbon-based compounds (advantageously aliphatic, preferably paraffinic) constituting the majority of the extraction solvent have a critical temperature (the temperature at the critical point of the pure hydrocarbon-based compounds) between 130°C and 285°C, preferably between 158°C and 285°C, preferably between 185°C and 245°C, preferably between 185°C and 230°C and very preferably between 185°C and 200°C. According to a preferred embodiment, the extraction solvent mainly comprises, preferably at least 80% by weight, preferably at least 95% by weight, preferably 98% by weight, of aliphatic chain-alkane-based compounds, which are preferably linear or branched, have a boiling point between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, preferably between 25°C and 61°C, and very preferably between 25°C and 40°C, and contain 4 to 7 carbon atoms (C4-C7), preferably 5, 6 or 7 carbon atoms (C5, C6 or C7, respectively), preferably contain 5 or 6 carbon atoms (C5 or C6) and very preferably contain 5 carbon atoms (C5).

[0112] Very preferably, the extraction solvent used in b3) is the same solvent as the dissolving solvent used in step a), optionally in a different physical state (e.g. the extraction solvent is in supercritical form relative to the dissolving solvent in liquid form), in order to facilitate the management of the solvent and in particular the purification and recycling of the solvent, in particular to the dissolving step a) and optionally to the extraction sub-step b3). Another advantage of using the same dissolving solvent and the extraction solvent, in the same or different physical state, besides facilitating the management of the solvents involved in the process according to the invention, in particular the recovery of the solvent, the treatment of the solvent and the recycling of the solvent to at least one step of the process, is to limit the energy consumption and in particular the costs resulting from the treatment and purification of the solvent.

[0113] The extraction stage(s) of b3) may comprise one or more extraction equipment pieces capable of being placed in contact with the extraction solvent and / or together with separation equipment to recover at least one spent solvent, in particular solvent containing impurities, and the extracted polymer solution. Such equipment is well known, for example stirred reactors, static mixers, decanting mixers, two-phase or three-phase separation vessels, co-current or counter-current washing columns, plate columns, stirred columns, packed columns, pulse columns, etc., each type of equipment may comprise one or more equipment pieces used alone or in combination with another type of equipment.

[0114] According to a preferred embodiment of b3), the extraction is carried out in a countercurrent extraction column, into which the extraction solvent is injected on the one hand and the polymer solution fed to substep b3) is injected on the other hand. According to this embodiment, it is possible to recover at least one extracted polymer solution on the one hand and the spent solvent, in particular containing impurities, on the other hand. Preferably, the polymer solution fed to b3), preferably the clarified, washed or refined polymer solution, is injected into the upper half of the column, preferably the upper third, which is closest to the top of the countercurrent extraction column, while the extraction solvent is injected into the lower half of the column, preferably the lower third, which is preferably closest to the bottom of the countercurrent extraction column.

[0115] The stream at the inlet and / or outlet of the countercurrent extraction column may be divided into several injection points and / or withdrawal points along the column.

[0116] According to another embodiment of b3), the extraction is carried out in a mixer-decanter which advantageously comprises, on the one hand, a stirring mixing zone for bringing the extraction solvent into contact with the polymer solution fed to sub-step b3), preferably a clarified, washed or refined polymer solution, and on the other hand a decantation zone; the decantation zone making it possible to recover the extracted polymer solution on the one hand and the spent solvent on the other hand.

[0117] Advantageously, the extraction sub-step b3) is carried out under temperature and pressure conditions different from those of the dissolution step a).

[0118] According to a preferred embodiment of b3), the extraction substep b3) involves a liquid / liquid extraction stage. Preferably, the liquid / liquid extraction stage is operated at a temperature between 120°C and 220°C, preferably between 130°C and 200°C, very preferably between 150°C and 200°C and at a pressure between 1.0 MPa absolute and 25.0 MPa absolute, preferably between 1.0 MPa absolute and 20.0 MPa absolute, preferably between 5.0 MPa absolute and 18.0 MPa absolute, preferably between 10.0 MPa absolute and 17.0 MPa absolute. In any case, in this embodiment, the temperature and pressure conditions are adjusted so that the extraction solvent is in liquid form and the dissolving solvent is preferably also in liquid form. Very advantageously, the liquid / liquid extraction, in particular when the extraction solvent is the same as the dissolving solvent, is carried out under temperature and pressure conditions different from the dissolving conditions of step a), in particular at a temperature above the dissolving temperature and / or at a pressure below the dissolving pressure, so as to be thereby in the two-phase region of the corresponding polymer-solvent mixture phase diagram.

[0119] According to another preferred embodiment of b3), the extraction sub-step b3) comprises a section of extraction under specific temperature and pressure conditions, wherein the extraction solvent is advantageously at least partially in supercritical form. This extraction can be called supercritical extraction. In this embodiment, the extraction is carried out by placing the polymer solution (preferably a clarified, washed or refined polymer solution) fed to b3) in contact with the extraction solvent, advantageously under temperature and pressure conditions capable of obtaining a supercritical phase consisting mainly (i.e. preferably at least 50% by weight, preferably at least 70% by weight, preferably at least 90% by weight) of the extraction solvent. In other words, in this embodiment, the extraction is carried out by placing the polymer solution (preferably a clarified, washed or refined polymer solution) fed to b3) in contact with the extraction solvent at least partially, preferably entirely in supercritical form. This supercritical extraction sub-step b3) advantageously allows efficient purification of the polymer solution, particularly due to the very high affinity of organic impurities (e.g. certain additives, particularly certain colorants or plasticizers) to the supercritical phase. The use of a supercritical form of the extraction solvent can also produce a significant density difference between the supercritical phase and the liquid form of the polymer solution, which facilitates stratification and separation by decantation between the two phases (i.e., between the supercritical phase and the liquid phase), thereby contributing to the purification efficiency of the polymer solution.

[0120] In this other preferred embodiment, sub-step b3) uses an extraction solvent which mainly comprises preferably at least 80% by weight, preferably at least 95% by weight, preferably 98% by weight of preferably paraffinic aliphatic hydrocarbon compounds (or alkanes) (100% is the maximum value, the percentages are expressed relative to the total weight of the dissolved solvent), and whose critical temperature is preferably between 130°C and 285°C, preferably between 158°C and 285°C, preferably between 185°C and 245°C, very preferably between 185°C and 230°C and preferably between 185°C and 200°C. Very preferably, in such a supercritical extraction sub-step b3), the extraction solvent mainly comprises preferably at least 80% by weight, preferably at least 95% by weight, preferably 98% by weight of paraffinic aliphatic hydrocarbon-based compounds, whose boiling point is between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, very preferably between 25°C and 61°C, and preferably between 25°C and 40°C, and contains 4 to 7 carbon atoms (i.e. C4-C7), preferably 5, 6 or 7 carbon atoms (respectively C5, C6 or C7), preferably contains 5 or 6 carbon atoms (C5 or C6) and very preferably contains 5 carbon atoms (C5). Very particularly, the main paraffinic aliphatic hydrocarbon-based compounds of the extraction solvent have a critical pressure between 2.5 MPa and 5.0 MPa, preferably between 2.7 MPa and 4.6 MPa, preferably between 3.0 MPa and 3.8 MPa, and most preferably between 3.0 MPa and 3.5 MPa.

[0121] Advantageously, the supercritical extraction substep b3) of this particular embodiment is carried out at a temperature preferably between 160° C. and 300° C., preferably between 190 MPa and 250° C., preferably between 200° C. and 230° C., and at a pressure preferably between 2.7 MPa absolute and 10.0 MPa absolute, preferably between 3.0 MPa absolute and 6.0 MPa absolute, preferably between 3.0 MPa absolute and 5.0 MPa absolute, and very preferably between 3.0 MPa absolute and 4.0 MPa absolute. According to a very particular mode of this embodiment of sub-step b3), the supercritical extraction is carried out at a pressure which is very advantageously between the critical pressure of the main paraffinic aliphatic hydrocarbon compounds of the extraction solvent (CP(extraction solvent)) (i.e. preferably, boiling between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, very preferably between 25°C and 61°C, and preferably between 25°C and 40°C, and containing 4 to 7 carbon atoms, preferably containing 5, 6 or 7 carbon atoms, preferably containing 5 or 6 carbon atoms and very preferably containing 5 carbon atoms, as defined above) and less than the critical pressure of the main paraffinic aliphatic hydrocarbon compounds of the extraction solvent (CP(extraction solvent)). The pressure is preferably between the critical pressure of the main paraffinic aliphatic compounds of the extraction solvent (CP (extraction solvent)) and a pressure 3.0 MPa higher than the critical pressure of the main paraffinic aliphatic compounds of the extraction solvent (i.e., CP (extraction solvent) + 3.0 MPa), preferably between the critical pressure of the main paraffinic aliphatic compounds of the extraction solvent (CP (extraction solvent)) and a pressure 1.5 MPa higher than the critical pressure of the main paraffinic aliphatic compounds of the extraction solvent (i.e., CP (extraction solvent) + 1.5 MPa), preferably between the critical pressure of the main paraffinic aliphatic compounds of the extraction solvent (CP (extraction solvent)) and a pressure 0.5 MPa higher than the critical pressure of the main paraffinic aliphatic compounds of the extraction solvent (i.e., equal to CP (extraction solvent) + 0.5 MPa), and the above pressures are absolute pressures. In any case, in this embodiment, the temperature and pressure conditions are adjusted, in particular in the adjustment section carried out in the extraction sub-step b3) upstream of the extraction section, so that the extraction solvent is at least partially in a supercritical state in the extraction section, the adjustment of the temperature and pressure of the extraction solvent in the adjustment section being advantageously carried out in a manner known to a person skilled in the art (for example, using pumps and / or valves and / or turbines and / or exchangers and / or ovens).

[0122] In a very preferred embodiment of b3), the extraction sub-step b3) involves supercritical extraction and the extraction solvent is identical to the dissolving solvent (or comprises the same main compounds and possible impurities as the dissolving solvent), except for the fact that the extraction solvent is at least partially in a supercritical phase.

[0123] Advantageously, at the end of the extraction sub-step b3), the spent solvent obtained contains in particular impurities. It can be reprocessed in the organic treatment section, on the one hand, making it possible to at least partially separate the impurities and purify the solvent to obtain a purified extraction solvent, and on the other hand, in the case where the dissolving solvent and the extraction solvent are identical, to recycle at least a portion of the purified extraction solvent to the inlet of the extraction sub-step b3) and / or to the inlet of the dissolving step a). The spent solvent can be processed according to any method known to a person skilled in the art, for example one or more of the following methods: distillation, evaporation, extraction, adsorption, crystallization and precipitation of insoluble substances, or by purging.

[0124] Adsorption sub-step b4)

[0125] Step b) of the treatment process according to the invention may comprise an adsorption sub-step b4) for obtaining at least one purified polymer solution. The purified polymer solution obtained at the end of sub-step b4) advantageously comprises the purified polyethylene that the invention seeks to recover, dissolved in a dissolving solvent.

[0126] When the adsorption sub-step b4) is incorporated into the process according to the invention, the adsorption sub-step b4) is advantageously carried out downstream of the dissolution step a) and upstream of the polymer-solvent separation step c). The adsorption sub-step b4) can be carried out upstream of the sub-step b1) for separating off the insoluble substances and / or the washing sub-step b2), and can in particular correspond to the optional intermediate adsorption step a'). Preferably, the adsorption sub-step b4) is carried out downstream of the sub-step b1) for separating off the insoluble substances and of the possible washing sub-step b2), the washing sub-step b2) itself preferably being downstream of the sub-step b1). The adsorption sub-step b4) can also be carried out, for example, upstream or downstream of the extraction sub-step b3). Thus, the adsorption sub-step b4) is carried out by placing the polymer solution fed to the adsorption sub-step b4) (in particular the crude polymer solution obtained from step a), the clarified polymer solution obtained from b1) or the washed polymer solution obtained from b2) or the extracted polymer solution obtained from b3)) in contact with one or more adsorbents.

[0127] The adsorption sub-step b4) advantageously comprises an adsorption stage operated in the presence of at least one adsorbent, preferably solid, and in particular in the form of a fixed bed, an entrained bed (or a slurry, i.e. introduced in the form of particles into the stream to be purified and entrained with the stream) or an ebullating bed, preferably in the form of a fixed bed or an entrained bed. The adsorbent(s) used in sub-step b4) are each preferably alumina, silica, silica-alumina, activated carbon, bleaching earth or a mixture thereof, preferably activated carbon, bleaching earth or a mixture thereof, preferably in the form of a fixed bed or an entrained bed, the circulation of the stream being either ascending or descending.

[0128] Advantageously, when the adsorption substep b4) is incorporated into the process, the adsorption substep b4) is preferably carried out at a temperature between 120° C. and 220° C., preferably between 130° C. and 200° C., very preferably between 150° C. and 200° C., and at a pressure between 1.0 MPa absolute and 25.0 MPa absolute, preferably between 1.0 MPa absolute and 20.0 MPa absolute, preferably between 5.0 and 18.0 MPa absolute, preferably between 10.0 MPa absolute and 17.0 MPa absolute. Very advantageously, the adsorption substep b4) is carried out at the dissolution temperature and pressure conditions, i.e. at the dissolution temperature and dissolution pressure of step a). Preferably, in the optional substep b4), the hourly space velocity (or HSV), corresponding to the ratio between the volume flow of the polymer solution fed to b4) and the volume of the adsorbent advantageously operated in b4), is 0.05 h -1 Until 10h -1 Between, preferably 0.1h -1 Up to 5.0h -1 between.

[0129] According to a particular embodiment of sub-step b4), the adsorption section may comprise one or more fixed beds of one or more adsorbents, for example in the form of an adsorption tower containing said (one or more) adsorbents, preferably at least two adsorption towers, preferably between two and four adsorption towers. When the adsorption section comprises two adsorption towers, an operating mode may be a mode of operation known according to the special terminology as "swing" operation, in which one tower is online, i.e. in use, and the other tower is on standby. When the adsorbent of the online tower is exhausted, the tower is isolated and the standby tower is put online, i.e. in use. The spent adsorbent can then be regenerated in situ and / or replaced with fresh adsorbent so that once the other tower is isolated, the tower containing the adsorbent can be put online again.

[0130] Another operating mode of this specific embodiment of b4) comprising one or more fixed beds of adsorbent is to have at least two towers operated in series. When the adsorbent of the tower located at the front is exhausted, the first tower is isolated and the spent adsorbent is regenerated in situ or replaced with fresh adsorbent. Subsequently, the tower in the last position is brought back online, and so on. This operation is called replaceable mode, or according to the term PRS for replaceable reactor system, or also called "lead and lag". The combination of at least two adsorption towers makes it possible to overcome possible and potential rapid poisoning and / or clogging of the adsorbent due to the combined effect of impurities, contaminants and insoluble substances that may be present in the material stream to be treated. The reason for this is that the presence of at least two adsorption towers facilitates the replacement and / or regeneration of the adsorbent, advantageously without stopping the method, while also controlling costs and limiting the consumption of adsorbent.

[0131] According to this particular embodiment of the adsorption sub-step b4) carried out in a fixed bed of one or more adsorbents, sub-step b4) is preferably carried out downstream of the sub-step b1) of separating off the insoluble matter and / or the washing sub-step b2), and optionally upstream or downstream of the extraction sub-step b3). Advantageously, the combination of the sub-step b1) of separating off the insoluble matter and / or the washing sub-step b2) and optionally the extraction sub-step b3) with the adsorption sub-step b4) allows to improve the purification of the polymer solution by exploiting the affinity of the residual impurities for both the adsorbent and the extraction solvent and optionally the high density solution.

[0132] According to another embodiment, the adsorption stage of b4) may comprise the addition of adsorbent particles to the polymer solution, in particular to the crude polymer solution, said particles being separable from the polymer solution by a step of removing the adsorbent particles downstream of said adsorption stage. The removal of the adsorbent particles may then advantageously correspond to the sub-step b1) of separating off the insoluble substances or to the washing sub-step b2). This implementation of the adsorption sub-step b4), i.e. by introducing adsorbent particles followed by a solid / liquid separation, advantageously corresponds to the optional intermediate adsorption step a') described previously in this specification.

[0133] Solvent-polymer separation step c)

[0134] According to the invention, the process comprises a solvent-polymer separation step c) to obtain at least one purified polyethylene fraction, and preferably a solvent fraction.

[0135] The solvent-polymer separation step c) advantageously comprises at least one supercritical separation section followed by at least one solvent recovery section, preferably one to five solvent recovery sections in series. The solvent-polymer separation step c), more particularly the supercritical separation section, in particular the first supercritical separation section, is fed with the purified polymer solution obtained from the purification step b).

[0136] Thus, the solvent-polymer separation step c) first involves at least partially, preferably predominantly or even completely separating off the solvent(s), in particular the dissolved solvent contained in the purified polymer solution fed to step c), in order to recover a polyethylene which has been at least partially, preferably predominantly and preferably completely freed from impurities and dissolved solvent and possibly other solvent(s) used in the process (i.e. extraction solvent and / or high density solution). The term "predominantly" is understood to mean at least 50% by weight, preferably at least 70% by weight, preferably at least 90% by weight, very preferably at least 95% by weight, relative to the weight of the solvent(s) contained in the purified polymer solution fed to step c), in particular the dissolved solvent and the optional extraction solvent and / or high density solution contained in the purified polymer solution fed to step c). Any method known to the person skilled in the art for separating the solvent from the polymer may be used, in particular any method capable of causing a phase change of the polymer or the solvent(s). For example, the solvent(s) may be separated off by evaporation, stripping, demixing, density differences and in particular decantation or centrifugation.

[0137] The purified polyethylene fraction obtained at the end of step c) may correspond to a concentrated polyethylene solution or a liquid (i.e. molten) purified polyethylene or a solid purified polyethylene. The solvent-polymer separation step c) may also optionally comprise a conditioning section for conditioning the recovered purified polyethylene (in solid form, and more particularly in the form of solid particles). In this possible conditioning section, the recovered purified polyethylene is advantageously cooled to a temperature below the melting point of the polyethylene, so as to obtain a fraction comprising polyethylene in solid form.

[0138] The solvent-polymer separation step c) also involves at least partially, preferably mainly and preferably completely recovering the (one or more) solvents contained in the purified polymer solution fed to step c), and in particular the dissolving solvent and the optional extraction solvent and / or the high-density solution. The term "mainly" is understood to mean at least 50% by weight, preferably at least 70% by weight, preferably at least 90% by weight, very preferably at least 95% by weight, relative to the weight of the (one or more) solvents contained in the purified polymer solution fed to step c). Thus, step c) advantageously also makes it possible to obtain at least one solvent fraction. The solvent-polymer separation step c) also optionally involves purifying the recovered solvent fraction and recycling it, in particular recycling it upstream of the dissolution step a) and possibly upstream of sub-step b2) and / or sub-step b3).

[0139] Therefore, the solvent-polymer separation step c) includes a supercritical separation section, which makes it possible to separate at least a portion of the dissolving solvent, as well as possible extraction solvents and high-density solutions, and optionally a portion of the residual impurities that are not eliminated during step b), adjusting the temperature and pressure conditions in which it is located so as to be in supercritical conditions, i.e., exceeding the critical point of the (one or more) solvents to be separated, particularly exceeding the critical point of the dissolving solvent, more particularly exceeding the critical point of the main hydrocarbon-based compounds of the dissolving solvent, which advantageously makes it possible to easily separate and recycle the solvent, particularly at least a portion of the dissolving solvent. The supercritical separation section particularly includes a fluid system, which is composed of a supercritical phase (particularly the dissolving solvent) mainly comprising a solvent and a liquid phase comprising polyethylene. The term "mainly" refers herein to at least 50% by weight, preferably at least 70% by weight, preferably at least 90% by weight, and very preferably at least 95% by weight relative to the weight of the stream (i.e. the supercritical phase) under consideration. The separation can then be referred to as a supercritical separation of (one or more) solvents. The supercritical separation of the solvent(s) allows, on the one hand, the effective separation of the solvent(s), and in particular at least a portion of the dissolved solvent, and on the other hand, the effective separation of the polyethylene or the concentration of the polyethylene solution, the supercritical separation being advantageously achieved by a significant density difference between the two phases, the supercritical phase comprising mainly the solvent, in particular the dissolved solvent, and the liquid phase comprising the polyethylene. In addition, the supercritical separation of the solvent(s) advantageously allows a significant reduction in energy and environmental costs relative to a simple evaporation of the solvent, since there is no latent heat of evaporation during entry into the supercritical state.

[0140] The supercritical separation stage is advantageously operated at a temperature between 160° C. and 300° C., preferably between 190° C. and 250° C., preferably between 200° C. and 230° C., and at a pressure (P supercritical ) between 2.7 MPa and 10.0 MPa, preferably between 3.0 MPa and 6.0 MPa, preferably between 3.0 MPa and 5.0 MPa, and preferably between 3.0 MPa and 4.0 MPa.

[0141] According to a specific embodiment, the critical separation section of step c) is between the critical pressure of the main hydrocarbon-based compounds of the dissolved solvent (CP (dissolved solvent)) and a pressure 3.0 MPa higher than the critical pressure of the main hydrocarbon-based compounds of the dissolved solvent (i.e.: CP (dissolved solvent) + 3.0 MPa), preferably between the critical pressure of the main hydrocarbon-based compounds of the dissolved solvent (CP (dissolved solvent)) and a pressure 1.5 MPa higher than the critical pressure of the main hydrocarbon-based compounds of the dissolved solvent (i.e.: CP (dissolved solvent) + 1.5 MPa), preferably between the critical pressure of the main hydrocarbon-based compounds of the dissolved solvent (CP (solvent)) and a pressure 1.5 MPa higher than the critical pressure of the main hydrocarbon-based compounds of the dissolved solvent (i.e.: CP (dissolved solvent) + 1.5 MPa). The pressure is 0.5 MPa higher than the pressure (i.e., equal to CP (dissolving solvent) + 0.5 MPa) and is carried out at a pressure (P supercritical), the pressure is absolute pressure, the main hydrocarbon-based compound of the dissolving solvent is advantageously an aliphatic, preferably a chain-alkane-based compound, and its boiling point is between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, very preferably between 25°C and 61°C, and preferably between 25°C and 40°C, and preferably contains 4 to 7 carbon atoms, preferably 5, 6 or 7 carbon atoms, preferably contains 5 or 6 carbon atoms, and very preferably contains 5 carbon atoms, as described in detail in the description of step a) above.

[0142] The supercritical separation section of step c) is preferably carried out by demixing and then decanting the liquid phase (comprising the polyethylene) and the supercritical phase (consisting of the solvent). Advantageously, the supercritical phase from the supercritical separation section constitutes at least part of the solvent fraction obtained at the end of step c). The liquid phase comprising the polyethylene is preferably sent to a solvent recovery section or a series of solvent recovery sections.

[0143] Step c) may optionally comprise one or more successive supercritical separation sections, in particular one to five, more in particular one, two or three. Thus, the liquid phase comprising polyethylene and obtained from a supercritical separation section may also be fed to another subsequent supercritical separation section, the liquid phase from the last supercritical separation section advantageously being sent to a solvent recovery section or a series of solvent recovery sections. Very preferably, step c) comprises a supercritical separation section.

[0144] Very advantageously, supercritical separation of the solvent further reduces the residual impurity content of the purified polyethylene fraction.

[0145] Preferably, the supercritical section (optionally, a series of supercritical separation sections) is followed by at least one (preferably one to five) preferably consecutive solvent recovery sections. The first solvent recovery section is fed with a liquid phase comprising polyethylene and obtained from a supercritical separation section (optionally, a series of supercritical separation sections, and in particular from the last supercritical separation section), and, in the case where the separation step comprises at least two solvent recovery sections, each subsequent solvent recovery section (i.e., starting from the second solvent recovery section) is fed with a liquid phase comprising polyethylene from the previous solvent recovery section, e.g., the second solvent recovery section is fed with a liquid phase comprising the target thermoplastic polymer from the first solvent recovery section. The liquid phase comprising polyethylene obtained from the last solvent recovery section constitutes the purified polyethylene fraction obtained at the end of step c).

[0146] The solvent-only phase or combination of phases obtained from the solvent recovery section together with the supercritical phase obtained from the supercritical separation section (optionally from a series of supercritical separation sections) constitute the solvent fraction(s) advantageously recovered at the end of step c). The solvent-only phase or combination of phases obtained from the solvent recovery section is preferably in gaseous form and can be condensed and optionally mixed with the supercritical phase obtained from the supercritical separation section, the temperature and pressure conditions of the supercritical phase advantageously having been previously adjusted to make it in liquid form.

[0147] Each solvent recovery section is implemented at a favorable temperature between 160°C and 300°C (and preferably at a temperature above the melting point of polyethylene), and at a pressure between the pressure used in the (one or more) supercritical separation sections (Psupercritical) and 0.000005 MPa (i.e. 5 Pa). Preferably, each solvent recovery section is preferably implemented at a temperature between 160°C and 300°C and at a pressure between the pressure of the previous section of step c) and 0.000005 MPa. Therefore, when step c) comprises a supercritical separation section and a plurality of (at least two) solvent recovery sections, the first solvent recovery section S1 immediately following the supercritical separation section is implemented at a pressure P(S1) between the pressure (Psupercritical) used in the supercritical separation section (preferably immediately before) and 0.000005 MPa; the second solvent recovery section S2 immediately following the solvent recovery section S1 is implemented at a pressure P(S1) between the pressure P(S1) used in the first solvent recovery section S1 and 0.000005 MPa P(S2), and so on for the subsequent sections. According to a preferred embodiment, each solvent recovery section is advantageously carried out at a temperature between 160° C. and 300° C. and at a pressure between 10.0 MPa and 0.000005 MPa, preferably between 5.0 MPa and 0.000005 MPa, preferably between 2.7 MPa and 0.000005 MPa, at a pressure of 3.7 MPa and 0.000005 MPa, at a temperature between 160° C. and 300° C., and at a pressure between 10.0 MPa and 0.000005 MPa, preferably between 5.0 MPa and 0.000005 MPa, preferably between 2 ...0.0 MPa and 0.000005 MPa, preferably between 5.0 MPa and 0.000005 MPa, preferably between 2.7 MPa and 0.000005 MPa, at a pressure between 2.7 MPa and 0.000005 MPa, at a temperature between 10.0 MPa and 0.000005 MPa, at a pressure between 2.7 MPa and 0.000005 MPa, at a pressure between 2.7 MPa and 0.000005 MPa, at a pressure between 2.7 MPa and 0.000005 MPa, at a pressure between 2.7 MPa and 0.000005 MPa, at a pressure between 2.7

[0148] In the purification process according to the invention, in particular in case several different solvents are used in the dissolution step a) and the optional extraction substep b3), step c) may comprise several solvent recovery stages, for example two, three or four solvent recovery stages, in order to recover the various solvents, in particular the dissolution solvent and the optional extraction solvent, separately, sequentially and / or continuously.

[0149] Advantageously, the supercritical separation and solvent recovery section of step c) may be operated in continuous, batch or fed-batch mode.

[0150] Very advantageously, the solvent fraction recovered at the end of step c) can be treated in an organic treatment section situated at the end of step c) in order to purify it and obtain at least one purified solvent, in particular at least one purified dissolving solvent, so as to be advantageously able to recycle it to the dissolving step a) and optionally to the washing sub-step b2) or the extraction sub-step b3). Said optional organic treatment section at the end of step c) can use any method known to a person skilled in the art, for example one or more of distillation, evaporation, liquid-liquid extraction, adsorption, crystallization and precipitation of insoluble substances, or by washing.

[0151] The process according to the invention thus makes it possible to obtain a purified polyethylene stream from plastic waste which can be used for any application, for example to replace virgin resin. The purified polyethylene stream obtained by the process according to the invention, i.e. the purified polyethylene fraction, thus has a sufficiently low content of impurities and residual solvents to enable it to be used for any application. Preferably, the purified polyethylene stream obtained at the end of the process according to the invention advantageously contains less than 5% by weight of impurities, very advantageously less than 1% by weight of impurities, and very advantageously contains less than 5% by weight of residual solvents (in particular dissolving solvents), preferably less than 1% by weight of residual solvents, preferably less than 0.1% by weight of residual solvents.

[0152] According to a preferred embodiment of the present invention, the method for purifying a plastic raw material containing polyethylene comprises and preferably consists of the following steps:

[0153] a) a step of dissolving in a dissolving solvent at a dissolving temperature of between 150°C and 250°C, preferably between 160 and 225°C, very preferably between 165 and 210°C and preferably between 170°C and 195°C and at a dissolving pressure of between 1.0 MPa absolute and 18.0 MPa absolute, preferably between 1.0 MPa absolute and 12.0 MPa absolute, preferably between 3.0 MPa absolute and 11.0 MPa absolute, preferably between 5.0 MPa absolute and 11.0 MPa absolute and very preferably between 6.0 MPa absolute and 10.0 MPa absolute, to obtain at least one crude polymer solution, the dissolving solvent comprising at least one aliphatic, paraffin-based compound having a boiling point of between -15°C and 100°C, preferably between 8°C and 100°C, preferably between 25°C and 69°C, very preferably between 25°C and 61°C and preferably between 25°C and 40°C; then

[0154] b) a step of purifying the polymer solution, comprising:

[0155] b1) a sub-step of separating off the insoluble matter to obtain a clear polymer solution and an insoluble fraction; then

[0156] b4) a sub-step of adsorbing impurities by contacting the clarified polymer solution with an adsorbent to obtain at least one purified polymer solution; then

[0157] c) a solvent-polymer separation step using at least one supercritical separation section operating at a temperature between 160° C. and 300° C., preferably between 190 and 250° C., preferably between 200 and 230° C. and at a pressure (P supercritical ) of between 2.7 MPa absolute and 10.0 MPa absolute, preferably between 3.0 MPa absolute and 6.0 MPa absolute, preferably between 3.0 MPa absolute and 5.0 MPa absolute, and preferably between 3.0 MPa absolute and 4.0 MPa absolute, followed by at least one solvent recovery section operating at a temperature between 160° C. and 300° C. and at a pressure between the pressure of the supercritical separation section (P supercritical ) and 0.000005 MPa (i.e. 5 Pa), so as to obtain at least one purified polyethylene fraction.

[0158] According to another aspect, the present invention relates to a device for purifying a plastic raw material containing polyethylene, said device comprising, preferably consisting of, the following sections:

[0159] a) a stage for dissolving the plastic raw material in a dissolving solvent, which advantageously contains at least one paraffinic aliphatic hydrocarbon-based compound, said stage being operated at a dissolving temperature between 150° C. and 250° C., preferably between 160° C. and 225° C., very preferably between 165° C. and 210° C. and preferably between 170° C. and 195° C. and at a dissolving pressure between 1.0 MPa and 18.0 MPa, preferably between 1.0 MPa and 12.0 MPa, preferably between 3.0 MPa and 11.0 MPa, preferably between 5.0 MPa and 11.0 MPa, very preferably between 6.0 MPa and 10.0 MPa, to obtain at least one crude polymer solution; then

[0160] b) a stage for purifying the polymer solution, comprising:

[0161] b1) an insoluble matter separation subsection; and / or

[0162] b2) washing the sub-stage by contact with a high density solution; and / or

[0163] b3) an extraction substage, by contacting with an extraction solvent; and / or

[0164] b4) impurity adsorption sub-stage, by contacting with an adsorbent; then

[0165] c) a solvent-polymer separation section comprising at least one supercritical separation section, which is operated at a temperature between 160° C. and 300° C., preferably between 190° C. and 250° C., preferably between 200° C. and 230° C. and at a pressure (P supercritical ) between 2.7 MPa absolute and 10.0 MPa absolute, preferably between 3.0 MPa absolute and 6.0 MPa absolute, preferably between 3.0 MPa absolute and 5.0 MPa absolute, and preferably between 3.0 MPa absolute and 4.0 MPa absolute, followed by at least one solvent recovery section, which is operated at a temperature between 160° C. and 300° C. and at a pressure between the pressure of the supercritical separation section (P supercritical ) and 0.000005 MPa (i.e. 5 Pa), so as to obtain at least one purified polyethylene fraction.

[0166] Preferably, purification section b) comprises:

[0167] b1) an insoluble matter separation sub-step to obtain a clarified polymer solution and an insoluble matter fraction; then

[0168] b4) a sub-step of adsorbing impurities by contacting the clarified polymer solution with an adsorbent to obtain at least one purified polymer solution.

[0169] The following examples illustrate the invention, particularly specific embodiments of the invention, but do not limit the scope thereof. Example

[0170] Example 1 (according to the present invention)

[0171] Dissolution step a):

[0172] A raw material from plastic waste containing 95% by weight of polyethylene (PE) is fed in the form of flakes into an extruder heated to 180°C. When leaving the extruder, the raw material is at least partially in molten form and mixed with a solvent containing 99% n-pentane preheated to 180°C at a solvent / raw material mass ratio of 9:1. The mixture of solvent and raw material is introduced into a stirred reactor and heated to 180°C and maintained at an absolute pressure of 16 MPa for a residence time of 1 hour. A polymer solution is then obtained.

[0173] The polymer solution from the dissolution step a) is then subjected to a purification step b):

[0174] The polymer solution was continuously withdrawn from the stirred reactor and passed through three filters placed in series, which were maintained at 180° C. and had cut-off diameters of 500 μm, 100 μm and 10 μm, respectively (in that order). The pressure drop across the filters was 0.05 MPa.

[0175] At the outlet of a series of filters, the clarified polymer solution passed through an adsorption section comprising a bed of activated carbon particles with a contact time of 2 hours, followed by a filter to retain the activated carbon particles. The adsorption section was operated at 180°C. This resulted in a pressure drop of 0.2 MPa.

[0176] The purified polymer solution from purification step b) is then subjected to a solvent-polymer separation step c) comprising a supercritical section:

[0177] The purified polymer solution from the adsorption stage is then heated to 210°C and a pressure slightly less than 16 MPa (solution pressure minus the pressure drop caused in each stage of the purification step b)). The polymer solution is then expanded to 4 MPa absolute pressure and injected into a decanter maintained at 4 MPa absolute pressure and 210°C with a residence time of 5 minutes. Two phases are formed: the upper phase mainly comprises n-pentane solvent in a supercritical state, and the lower liquid phase comprises polyethylene dissolved in n-pentane solvent. The upper phase is withdrawn from the upper part of the decanter.

[0178] The lower liquid phase was then subjected to evaporation of the residual solvent in two consecutive evaporation stages: first at a temperature of 210° C. and a pressure of 0.5 MPa for 5 minutes, and then at a temperature of 210° C. and a pressure of 0.01 MPa for 2 minutes.

[0179] At the end of the process, a solid A consisting of purified polyethylene (PE) was obtained at normal temperature and pressure. The solid A was analyzed.

[0180] The obtained solid A was almost colorless and almost translucent, and contained less than 5 wt % of impurities and less than 1 wt % of n-pentane.

[0181] Example 2 (not in accordance with the present invention)

[0182] In this Example 2, the same starting material was processed and the dissolution step a) and the purification step b) were carried out in the same way as the method described in Example 1.

[0183] The purified polymer solution from purification step b) is subjected to a solvent-polymer separation step which does not include a supercritical section:

[0184] The purified polymer solution from the adsorption section was maintained at 180°C and expanded to 2 MPa absolute pressure, and then injected into a decanter maintained at 2 MPa absolute pressure and 180°C for a residence time of 5 minutes. Two phases were formed: an upper gas phase composed of n-pentane solvent and a lower liquid phase containing polyethylene dissolved in n-pentane solvent. The gas phase was withdrawn from the upper part of the decanter.

[0185] The lower liquid phase was then subjected to evaporation of the residual solvent, initially at a temperature of 210° C. and a pressure of 0.5 MPa for 5 minutes, and then at a temperature of 210° C. and a pressure of 0.01 MPa for 2 minutes.

[0186] At the end of the process, at normal temperature and pressure, a solid B consisting of purified polyethylene (PE) is obtained. Solid B is analyzed.

[0187] The obtained solid B was almost colorless and almost translucent, and contained less than 5 wt % of impurities and less than 1 wt % of n-pentane.

[0188] However, the impurity content (excluding the organic compound of the dissolved solvent) in the solid B is higher than the impurity content measured in the solid A obtained in Example 1 according to the present invention.

[0189] Furthermore, according to Example 2, the energy consumption required for polymer-solvent separation is greater than that required for polymer-solvent separation in the method described in Example 1 (ie, when the polymer-solvent separation includes a supercritical phase).

Claims

1. A method for purifying a plastic feedstock containing polyethylene, the method comprising: a) a dissolving step, which comprises contacting the plastic raw material with a dissolving solvent comprising at least one hydrocarbon-based compound having a boiling point between -15°C and 100°C at a dissolving temperature between 120°C and 220°C and a dissolving pressure between 1.0 MPa absolute pressure and 25.0 MPa absolute pressure to obtain at least one crude polymer solution; b) a step of purifying the crude polymer solution to obtain a purified polymer solution, comprising: b1) a sub-step of separating insoluble matter; and / or b2) a washing sub-step, performed by contacting with a high density solution; and / or b3) an extraction sub-step, performed by contacting with an extraction solvent; and / or b4) a sub-step of adsorbing impurities by contacting with an adsorbent; then c) a solvent-polymer separation step using at least one supercritical separation section operated at a temperature between 160° C. and 300° C. and a pressure between 2.7 MPa absolute and 10.0 MPa absolute (P supercritical ), followed by at least one solvent recovery section, to obtain at least one purified polyethylene fraction.

2. The method according to claim 1, wherein the plastic raw material comprises at least 80% by weight, preferably at least 85% by weight, preferably at least 90% by weight, of polyethylene relative to the total weight of the plastic raw material.

3. The method according to claim 1 or 2, wherein the dissolving solvent comprises an aliphatic hydrocarbon compound having a boiling point between 8°C and 100°C, preferably 25°C to 69°C, preferably 25°C to 61°C and preferably 25°C to 40°C.

4. The method according to any one of the preceding claims, wherein the dissolving step a) is carried out at a dissolving temperature between 130°C and 200°C, preferably between 150°C and 200°C.

5. The method according to any one of the preceding claims, wherein the dissolving step a) is carried out at a dissolving pressure of 1.0 MPa absolute and 20.0 MPa absolute, preferably 5.0 MPa absolute to 18.0 MPa absolute and preferably 10.0 MPa absolute to 17.0 MPa absolute.

6. The method according to any one of the preceding claims, wherein the purification step b) comprises at least a sub-step b1) of separating off insoluble matter, optionally followed by a sub-step b2) of washing by contact with a high-density solution, and / or a sub-step b3) of extraction by contact with an extraction solvent, and / or a sub-step b4) of adsorbing impurities by contact with an adsorbent.

7. The process according to any one of the preceding claims, wherein the supercritical separation section in step c) is operated at a temperature between 190°C and 250°C, preferably between 200°C and 230°C.

8. The process according to any one of the preceding claims, wherein the supercritical separation stage in step c) is operated at a pressure (Psupercritical) between 3.0 MPa absolute and 6.0 MPa absolute, preferably between 3.0 MPa absolute and 5.0 MPa absolute, and preferably between 3.0 MPa absolute and 4.0 MPa absolute.

9. The process according to any one of the preceding claims, wherein step c) comprises one to five solvent recovery sections, each solvent recovery section being operated at a temperature of 160° C. to 300° C. and a pressure of the supercritical separation section (P supercritical ) to 0.000005 MPa, preferably at a pressure of 2.7 MPa to 0.000005 MPa. An apparatus for purifying a plastic raw material containing polyethylene, the apparatus comprising: a) a section for dissolving the plastic raw material in a dissolving solvent, which is operated at a dissolving temperature between 150° C. and 250° C. and a dissolving pressure between 1.0 MPa absolute pressure and 18.0 MPa absolute pressure to obtain at least one crude polymer solution; then b) a stage for purifying the polymer solution, comprising: b1) an insoluble matter separation subsection; and / or b2) a washing substage, by contact with a high density solution; and / or b3) an extraction sub-stage, carried out by contacting with an extraction solvent; and / or b4) impurity adsorption sub-stage, carried out by contact with an adsorbent; then c) a solvent-polymer separation section comprising at least one supercritical separation section, the supercritical separation section being operated at a temperature between 160° C. and 300° C. and a pressure P supercritical between 2.7 MPa absolute pressure and 10.0 MPa absolute pressure, followed by at least one solvent recovery section, the solvent recovery section being operated at a temperature between 160° C. and 300° C. and a pressure P supercritical of the supercritical separation section to a pressure between 0.000005 MPa (i.e. 5 Pa), to obtain at least one purified polyethylene fraction.