Method for Recycling Used Plastics Based on Polyethylene Using Light Hydrocarbon Solvents

Abstract

A method for purifying a plastic raw material containing polyethylene, comprising: a) dissolving the plastic raw material in a dissolving solvent containing a hydrocarbon compound having a boiling point of 15°C to 100°C at a dissolving temperature of 120°C to 220°C and a dissolving pressure of 1.0 to 25.0 MPa absolute pressure to obtain a crude polymer solution; b) purifying the crude polymer solution; b1) separating insoluble substances, and / or b2) washing with a concentrated solution, and / or b3) extracting with an extraction solvent, and / or b4) adsorbing impurities, then c) separating the solvent polymer at a temperature of 160°C to 300°C and a pressure of 2.7 to 10.0 MPa using a supercritical separation section, and then obtaining purified polyethylene through at least one solvent recovery section.

Description

Technical Field

[0001] The present invention relates to a method for recycling used plastics mainly containing polyethylene (PE) to obtain a stream of purified polyethylene, which stream can be economically upgraded, for example, in the manufacture of new plastic articles. More particularly, the present invention relates to a method for purifying a plastic feedstock, especially one obtained from plastic waste, the plastic feedstock containing a polymer, especially polyethylene, the method comprising dissolving the polymer in a light hydrocarbon solvent having a boiling point of -15°C to 100°C, especially one based on one or more alkanes, at least one step of purifying the resulting polymer solution to at least partially remove impurities, especially additives conventionally used in plastic-based materials, and an optimized step of separating the purified polyethylene from the solvent, and the recovered purified polyethylene can be reused, and thus the plastic feedstock can be economically upgraded.

Background Art

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

[0003] By "mechanical" recycling, certain waste can be partially reused directly in new objects or by mixing the mechanically sorted plastic waste stream with the stream of unused polymer. This type of economic upgrade is limited because mechanical sorting allows the purity of the stream of a given type of polymer to be improved, but thereby generally does not allow the complete removal of impurities, such as additives, such as fillers, colorants, pigments and metals, which are at least partially trapped in the polymer matrix.

[0004] "Chemical" recycling is generally directed towards at least partially reforming the monomers via an array of complex processes. For example, plastic waste may undergo a pyrolysis process, and the recovered pyrolysis oil may generally, after purification, be at least partially converted, for example, to olefins by steam cracking. These olefins may then be polymerized. This type of array may be suitable for feedstocks that have undergone little or no sorting or for waste from sorting centers, but it generally requires a large consumption of energy, particularly due to the high-temperature treatment.

[0005] Another route for recycling plastic waste consists of at least partially dissolving the plastic, particularly thermoplastics. This is for the purpose of purifying them by removing polymers and / or impurities other than the targeted one or more types of polymers in the feedstock, for example, additives such as fillers, colorants, pigments, and metals.

[0006] Some studies have thus presented various methods for treating plastic waste by dissolution and purification. Patent Document 1 describes a specific method for purifying a polymer feedstock, particularly one obtained from plastic waste, which is carried out by dissolving the polymer in a solvent under specific temperature and pressure conditions and then placing the resulting polymer solution in contact with a solid.

[0007] Patent Document 2 proposes a method for dissolving plastic in a solvent at a dissolution temperature close to the boiling point of the solvent for a part of it. However, the method of Patent Document 2 does not enable efficient treatment of impurities other than polymers.

[0008] Patent Document 3 proposes a treatment method by liquefying thermoplastics in a solvent, followed by separating out insoluble substances and / or gases. The method of Patent Document 3 does not enable efficient treatment of impurities soluble in the solvent.

[0009] The present invention aims to overcome these drawbacks and contribute to the recycling of plastics. More specifically, the present invention processes a plastic feedstock based on polyethylene, particularly one obtained from plastic waste, to at least partially eliminate the impurities it contains, and in particular, to at least partially eliminate the additives it contains and that are conventionally added to plastics, and proposes an effective, simple, and economically feasible method for this purpose, and the plastic feedstock, more specifically plastic waste, can be economically upgraded. The present invention actually attempts to efficiently separate impurities from the polymers contained in used plastics, particularly from polyethylene, and recover purified polyethylene, and for example, it can be used as a polymer base in the manufacture of new plastic products, particularly in place of virgin resin.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Means for Solving the Problems

[0011] (Summary of the Invention) The present invention relates to a method for purifying a plastic feedstock containing polyethylene, the method including the following steps: a) Dissolution step; including placing the plastic feedstock in contact with a dissolution solvent; the dissolution solvent contains at least one hydrocarbon-based compound and has a boiling point of -15 to 100 °C; the dissolution temperature at that time is 120 °C to 220 °C, and the dissolution pressure is 1.0 to 25.0 MPa (absolute); obtaining at least one crude polymer solution; b) Step of purifying the crude polymer solution to obtain a purified polymer solution; including the following sub-steps: b1) Sub-step of separating out insoluble matter; and / or b2) Sub-step of washing by contact with a concentrated solution; and / or b3) Sub-step of extraction by contact with an extraction solvent; and / or b4) Sub-step of adsorbing impurities by contact with an adsorbent; and then c) Solvent-polymer separation step; using at least one supercritical separation section; the temperature when the section is operated is 160 to 300 °C, and the pressure (P supercritical) at that time is 2.7 to 10.0 MPa (absolute); at least one solvent recovery section follows the section; obtaining at least one purified polyethylene fraction.

[0012] The advantage of the method of the present invention lies in proposing a method for efficiently processing a polyethylene-based plastic feedstock, in particular, polyethylene-based plastic waste, especially that obtained from collection and sorting channels, recovering the polyethylene it contains, and enabling it to be recycled for any type of use. The method according to the present invention makes it possible to obtain a stream of purified polyethylene, which advantageously contains impurities, in particular additives and solvents, especially dissolution solvents, and the contents of these are negligible or at least sufficiently low relative to the stream of purified polyethylene and can be introduced into any type of plastic formulation in place of virgin polyethylene resin. For example, the stream of purified polyethylene obtained at the end of the method according to the present invention advantageously contains less than 5% by weight of impurities, very preferably less than 1% by weight of impurities, and very preferably less than 5% by weight of solvents (especially dissolution solvents), preferably less than 1% by weight of solvents, preferably less than 0.1% by weight of solvents.

[0013] The method according to the present invention thus proposes a simple scheme corresponding to a series of operations, by which at least a part of the impurities, in particular at least a part of the additives, can be removed from the polyethylene-based plastic waste, and it is possible to recover purified polyethylene, advantageously containing little or even almost no solvent, and economically upgrade the plastic waste by recycling the purified polyethylene. Advantageously, depending on the conditions used in the steps of the method, the additives present in the plastic feedstock may be soluble or insoluble in the solvent used throughout the method according to the present invention, enabling efficient purification and separation of the polymer.

[0014] Furthermore, a series of operations are proposed which are carried out under the operating conditions that are optimal for efficiently separating impurities and solvents from polyethylene by the method according to the invention, in particular under the operating conditions that are optimal with respect to temperature and pressure, but under reasonable operating conditions, and thus limit the energy consumption of the method, and as a result, make the method economically advantageous.

[0015] The present invention also has the advantage of participating in the recycling of plastics and protecting fossil resources by enabling the economic upgrading of plastic waste. Specifically, it enables the purification of plastic waste for the purpose of obtaining a purified polyethylene fraction with a reduced impurity content, which are, in particular, decolorized and deodorized polyethylene fractions, which may be reused to form new plastic objects. The resulting purified polyethylene fraction may therefore be used directly in formulations, as a mixture with additives, such as colorants, pigments or other polymers, instead of or as a mixture with virgin resin, for the purpose of obtaining plastic products with aesthetic, mechanical or rheological processing properties that promote their reuse and their economic upgrading.

[0016] The present invention also enables the efficient separation of polyethylene, advantageously at a lower cost, from the solvents used (in particular, the dissolving solvents), while at the same time limiting the thermal degradation of polyethylene. Therefore, the solvents used, in particular the dissolving solvents, for treating the plastic feedstock can be at least partially recovered and recycled into one of the steps of the method, thus avoiding excessive solvent consumption, which is an environmental and economic advantage of the method.

[0017] Therefore, the present invention aims to purify a plastic feedstock, in particular plastic waste, to obtain purified polyethylene, such that it can be used in any application, in particular instead of virgin resin.

[0018] More specifically, the present invention proposes a method that includes a dissolution step, then at least one purification step, and then an optimized solvent-polymer separation, with the aim of obtaining a stream of purified polyethylene.

Embodiments for Carrying Out the Invention

[0019] (Description of Embodiments) According to the present invention, the expressions "comprised between... and..." and "between... and...", when used in relation to A and B, are synonymous and mean that the two limit values (A, B) of the interval are included in the described range of values. If this is not the case and the two limit values are not included in the described range, such clarification will be provided by the present invention.

[0020] For the purposes of the present invention, various ranges of parameters for a given process, for example, pressure ranges and temperature ranges, may be used alone or in combination. For example, within the scope of the present invention, a range of suitable pressure values can be combined with a range of more suitable temperature values.

[0021] In the following text, specific embodiments of the present invention may be described. They may be implemented together in combination without limitation if they are separate or technically feasible.

[0022] According to the present invention, pressure is absolute pressure and is given in MPa (absolute) (MPa absolute) (or absolute MPa (MPa abs)).

[0023] The terms "upstream" and "downstream" should be understood in accordance with the general flow of one or more fluids or one or more streams under consideration in the method.

[0024] The term "additive" is a term conventionally used in the field of polymers, particularly in the field of polymer formulations. Additives introduced into a polymer formulation can be, for example, plasticizers, fillers (which are organic or inorganic solid compounds used to modify the physical, thermal, mechanical and / or electrical properties of the polymer material or to reduce its cost), reinforcing agents, colorants, pigments, curing agents, flame retardants, combustion retardants, stabilizers, antioxidants, UV absorbers, antistatic agents, etc.

[0025] The additives correspond to at least a part of the impurities in the plastic feedstock to be treated and can be at least partially removed by the method according to the invention. Other types of impurities can be use-related impurities, such as, for example, metal impurities, paper / cardboard, biomass, polymers other than the targeted polyethylene (e.g., polypropylene), etc.

[0026] Therefore, according to the invention, additives containing impurities that can be at least partially removed by the method according to the invention are conventionally used in polymer formulations, generally originating from the life cycle of plastic objects and materials and / or from the circuits of waste collection and sorting. The said impurities can be of a metallic, organic or mineral type: they can be packaging residues, food residues or compostable residues (biomass). These use-related impurities may include glass, wood, cardboard, paper, aluminum, iron, metal, tires, rubber, silicone, hard polymers, thermoplastic polymers other than polyethylene, thermosetting polymers, household, chemical or cosmetic products, used oil and water.

[0027] According to the present invention, the polymer solution is a solution containing a dissolving solvent and at least a targeted polyethylene, the targeted polyethylene is dissolved in the dissolving solvent (i.e., in particular, solvated and dispersed), and the dissolved polyethylene is present from the beginning in the feedstock. According to the present invention, the polymer solution is a solution containing a dissolving solvent and at least a targeted polyethylene, the polyethylene is dissolved in the dissolving solvent (i.e., in particular, solvated and dispersed), and the dissolved polyethylene is present from the beginning in the feedstock. The polymer solution may 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 present invention received, the polymer solution may therefore contain impurities in the form of insoluble particles, the insoluble particles are preferably suspended in the polymer solution, and may also contain soluble impurities, the soluble impurities are dissolved in the dissolving solvent, and / or optionally, may contain another liquid phase that does not mix with the polymer solution.

[0028] The critical temperature and critical pressure of the solvent, particularly the 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 the pure substance are, respectively, the temperature and pressure of the critical point of the pure substance. As is well known to those skilled in the art, above the critical point, the pure substance under consideration is in a supercritical form or in a supercritical state: it may be called a supercritical fluid.

[0029] The present invention relates to a method for purifying a plastic feedstock, preferably composed of plastic waste and containing polyethylene, the method comprising the following steps, preferably consisting of: a) Dissolution step; including bringing the plastic feedstock into contact with a dissolution solvent; the dissolution solvent contains at least one hydrocarbon-based compound, which is preferably aliphatic, preferably paraffinic, and has a boiling point of -15 to 100 °C, preferably 8 to 100 °C, preferentially 25 to 69 °C, preferably 25 to 61 °C, most preferably 25 to 40 °C; the dissolution temperature is 120 °C to 220 °C, preferably 130 to 200 °C, preferentially 150 °C to 200 °C, and the dissolution pressure is 1.0 to 25.0 absolute MPa, preferably 1.0 to 20.0 absolute MPa, preferentially 5.0 to 18.0 absolute MPa, preferably 10.0 to 17.0 absolute MPa; obtaining at least one crude polymer solution; and then b) Step of purifying the crude polymer solution; including at least one of the following sub-steps: b1) Sub-step of separating out insolubles; obtaining at least one clarified polymer solution and preferably an insoluble fraction; and / or b2) Sub-step of washing by contact with a concentrated solution; obtaining at least one washed polymer solution and preferably a wash effluent; and / or b3) Sub-step of extraction by contact with an extraction solvent; obtaining at least one extracted polymer solution and preferably a spent solvent; and / or b4) Sub-step of adsorbing impurities by contact with an adsorbent; obtaining at least one purified polymer solution; This purification step enables the obtaining of a purified polymer solution, which preferably corresponds to a clarified and / or washed and / or extracted and / or refined polymer solution; and then c) Solvent-polymer separation step; using at least one supercritical separation section; the temperature during operation is 160 - 300 °C, preferably 190 - 250 °C, preferentially 200 - 230 °C, and the pressure (Psupercritical) at that time is 2.7 - 10.0 absolute MPa, preferably 3.0 - 6.0 absolute MPa, preferentially 3.0 - 5.0 absolute MPa, preferably 3.0 - 4.0 absolute MPa, followed by at least one solvent recovery section, and the temperature at which the section is specifically operated is 160 - 300 °C, and the pressure is between Psupercritical and 0.000005 MPa (i.e., 5 Pa), preferentially 2.7 MPa - 0.000005 MPa, especially 1.0 MPa - 0.000005 MPa; obtaining at least one fraction of purified polyethylene and preferably a solvent fraction.

[0030] (Feedstock) The feedstock of the method according to the present invention is known as a plastic feedstock, and the feedstock contains plastic, and the plastic itself more specifically contains polyethylene. Preferably, the plastic feedstock contains 50 wt% - 100 wt%, preferably 70 wt% - 100 wt% of plastic.

[0031] The plastic contained in the feedstock of the method according to the present invention is based on polyethylene and generally includes production waste and / or end-of-life plastic objects, especially household plastic waste, plastic waste from the construction industry, plastic waste from vehicles or any type of transportation or electrical and electronic equipment waste. Preferably, the plastic waste is from collection and sorting channels. Generally, the plastic or plastic material contains a polymer, which after shaping is mixed with additives for the purpose of forming various materials and objects (injection molded parts, tubes, films, fibers, fabrics, mastics, coatings, etc.). The additives used in plastics can be organic or inorganic compounds. They are, for example, fillers, colorants, pigments, plasticizers, property modifiers, flame retardants, etc.

[0032] Preferably, the feedstock for the method according to the invention contains 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 feedstock. The method according to the invention is thus most particularly directed towards recovering and purifying the polyethylene contained in the feedstock so that it can be reused in various applications.

[0033] The plastic feedstock may contain impurities, such as polymers other than polyethylene, in particular thermoplastic polymers, additives, preferably those used to compound plastic materials, and may also contain use-related impurities, which generally originate from the life cycle of the materials and plastic objects and / or from the waste collection and sorting circuits. The plastic feedstock for the method according to 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 feedstock may contain, for example, at least 5% by weight of impurities.

[0034] The plastic feedstock may advantageously be pretreated before the present method, removing at least some and generally all of the "coarse" impurities, i.e., impurities in the form of particles with a size of 10 mm or more, preferably 5 mm or more, more preferably 1 mm or more, such as wood, paper, biomass, iron, aluminum, glass, etc., generally shaping it into the form of divided solids (or particles) to facilitate the treatment in the present method. This pretreatment may include a grinding step, a washing step at atmospheric pressure, and / or a drying step. This pretreatment may be carried out at different sites, for example, at a waste collection and sorting center, or at the same site where the treatment method according to the present invention is carried out. Preferably, by this pretreatment, the impurity content can be reduced to less than 20% by weight, preferably less than 15% by weight, preferably less than 10% by weight, and the percentage is given relative to the weight of the plastic feedstock to be treated by the method according to the present invention. At the end of the pretreatment, the feedstock is generally stored in the form of divided solids, for example, in the form of a ground material or powder, to facilitate handling and transportation to the method.

[0035] (Dissolution step a)) According to the present invention, the method includes a dissolution step a), in which the plastic feedstock is placed in contact with a dissolution solvent to obtain at least one, preferably one crude polymer solution. Specifically, by this step, it is advantageously possible to dissolve at least a part, preferably all, of the polyethylene in the plastic feedstock.

[0036] The term "dissolution" should be understood to mean any phenomenon leading to the formation of at least one polymer solution, in particular a polyethylene solution, i.e., a liquid containing a polymer, in particular polyethylene, dissolved in a solvent, more specifically in a dissolution solvent. A person skilled in the art is well aware of the phenomena involved in polymer dissolution, including at least mixing, dispersion, homogenization, solvation, and the disentangling of polymer chains, more specifically polyethylene chains.

[0037] During and at the end of dissolution step a), depending on the pressure and temperature conditions, it is possible to maintain the dissolution solvent at least partially, preferably entirely, in liquid form, optimizing the dissolution of the targeted polyethylene.

[0038] Due to the nature of the dissolution solvent, advantageously, it is possible to use operating conditions, in particular temperature and pressure conditions, especially pressure conditions, which are, firstly, reasonable for ensuring the maintenance of at least a partial, preferably total, liquid phase of the dissolution solvent not only in dissolution step a), but also, advantageously, in purification step b), and thus enable optimal dissolution of the targeted polyethylene and advantageously efficient purification of the polymer solution, and secondly, are reasonable for ensuring the transition of at least a portion of the said dissolution solvent to a supercritical state in solvent-polymer separation step c), enabling demixing and thus separation of at least a portion of the dissolution solvent and, optionally, evaporation of at least a portion of the residual dissolution solvent, thereby making it possible to achieve a very low solvent content in the purified polyethylene recovered at the end of the process (advantageously, the content is less than 5% by weight of the solvent relative to the total weight of the purified polyethylene fraction, preferably 1% by weight of the solvent, preferably less than 0.1% by weight of the solvent). In fact, solvents that are composed of very light alkanes with a boiling point below -15°C, such as propane, and which can be particularly advantageous for their relatively mild critical conditions (temperature and pressure), would require the use of high pressures to maintain the dissolution solvent at least partially, preferably entirely, in liquid form throughout dissolution step a) and purification step b), which would involve significant costs, especially investment costs. Conversely, the use of heavy solvents, such as alkanes with a boiling point above 100°C, would require very stringent operating conditions in step c) to achieve the critical conditions of the said heavy solvent and to obtain the solvent in at least a partially supercritical state.

[0039] Advantageously, the dissolving solvent comprises, preferably consists of, at least one hydrocarbon-based compound, which is advantageously aliphatic, preferably paraffinic (i.e., saturated), preferably at least one alkane, with a boiling temperature of -15 to 100 °C, preferably 8 to 100 °C, preferably 25 to 69 °C, preferably 25 to 61 °C, most preferably 25 to 40 °C. Preferably, the dissolving solvent mainly, preferably at least 80% by weight, preferentially at least 95% by weight, preferably up to 98% by weight of hydrocarbon-based compounds, which are advantageously aliphatic, preferably paraffinic (or alkane) (maximum 100%, percentages being expressed relative to the total weight of the dissolving solvent), with a boiling point of -15 to 100 °C, preferably 8 to 100 °C, preferentially 25 to 69 °C, preferably 25 to 61 °C, most preferably 25 to 40 °C. Most advantageously, the critical temperature of the hydrocarbon-based compound (advantageously aliphatic, preferably paraffinic, forming the main amount of the dissolving solvent) (the temperature at the critical point of the high-purity hydrocarbon-based compound) is 130 to 285 °C, preferably 158 to 285 °C, preferentially 185 to 245 °C, preferably 185 to 230 °C, most preferably 185 to 200 °C. In great detail, the critical pressure of the main paraffinic hydrocarbon-based compound of the dissolving solvent is 2.5 to 5.0 MPa, preferably 2.7 to 4.6 MPa, preferentially 3.0 to 3.8 MPa, most preferably 3.0 to 3.5 MPa. According to a preferred embodiment, the dissolving solvent mainly, preferably at least 80% by weight, preferentially at least 95% by weight, preferably at least 98% by weight of aliphatic paraffinic hydrocarbon-based compounds, which are preferably linear or branched, with a boiling point of -15 to 100 °C, preferably 8 to 100 °C, preferentially 25 to 69 °C, preferably 25 to 61 °C, most preferably 25 to 40 °C, containing 4 to 7 carbon atoms (i.e., C4-C7), preferably 5, 6 or 7 carbon atoms (C5, C6 or C7 respectively), preferably containing 5 or 6 carbon atoms (C5 or C6), most preferentially containing 5 carbon atoms (C5).

[0040] Advantageously, the dissolution temperature when performing the dissolution step a) is 120°C to 220°C, preferably 130°C to 200°C, most preferably 150°C to 200°C, and the dissolution pressure is 1.0 to 25.0 MPa (absolute), preferably 1.0 to 20.0 MPa (absolute), more preferably 5.0 to 18.0 MPa (absolute), preferably 10.0 to 17.0 MPa (absolute). More specifically, the temperature and pressure may vary over the entire process a) from the conditions of introduction of the plastic feedstock and / or the dissolution solvent, e.g., ambient conditions, i.e., a temperature of 10 to 30°C and atmospheric pressure (0.1 MPa), to the dissolution conditions, i.e., the dissolution temperature, in particular 120°C to 220°C, preferably 130 to 200°C, most preferably 150°C to 200°C and the dissolution pressure, in particular 1.0 to 25.0 MPa (absolute), preferably 1.0 to 20.0 MPa (absolute), more preferably 5.0 to 18.0 MPa (absolute), preferably 10.0 to 17.0 MPa (absolute) until they are achieved. Most advantageously, at the end of the dissolution step a), the flow of the dissolved polymer, in particular the polymer solution, is at the dissolution temperature and the dissolution pressure.

[0041] By restricting the temperature in step a) to 220°C or lower, preferably 200°C or lower, it is possible not only to avoid or limit the thermal degradation of the polymer, in particular polyethylene, but also to limit the energy requirements of the process, and thus contribute to limiting the operating costs and carbon dioxide emissions of the process. Preferably, the dissolution temperature is above the melting point of polyethylene to facilitate its dissolution.

[0042] In parallel, the dissolution pressure is advantageously higher than the saturated vapor pressure of the dissolution solvent at the dissolution temperature, so that the dissolution solvent is at least partially, preferably entirely, in liquid form at the dissolution temperature, optimizing the dissolution of the targeted polyethylene.

[0043] Advantageously, the temperature and pressure conditions of the dissolution achieved in step a) are adjusted such that the mixture (dissolution solvent + polyethylene) is homogeneous and, most preferably, single-phase, and the said mixture may contain insoluble impurities suspended therein.

[0044] Preferably, the weight ratio between the plastic feedstock and the dissolution solvent ((feedstock / solvent)), or the ratio between the mass flow rate of the plastic feedstock and the mass flow rate of the dissolution solvent at the inlet of the dissolution step a), is from 0.01 to 2.0, preferably from 0.05 to 1.0, preferably from 0.10 to 0.8.

[0045] Advantageously, the dissolution step a) is carried out over a residence time of from 1 to 600 minutes, preferably from 2 to 300 minutes, preferably from 5 to 180 minutes. The residence time is understood to be the residence time at the dissolution temperature and pressure in step a), i.e., the time of implementation of the plastic feedstock with the dissolution solvent at the dissolution temperature and pressure.

[0046] Advantageously, the dissolution solvent comprises and preferably consists of a stream of recycled solvent obtained from the supply of fresh solvent and / or from subsequent steps of the process, preferably from the solvent-polymer separation step c).

[0047] Contacting the dissolving solvent with the plastic feedstock to dissolve at least partially, preferably entirely, the polyethylene of the plastic feedstock in the dissolving solvent may be carried out in a line and / or in a device and / or between two devices. Therefore, step a) advantageously includes at least one dissolving device and optionally includes at least one feedstock preparation device, mixing device and / or transport device. These devices and / or apparatuses may be, for example, static mixers, extruders, pumps, reactors, co-current or counter-current columns, or combinations of lines and installations. Devices for transport, especially for the transport of fluids, such as liquids or solids, are well known to those skilled in the art. By way of non-limiting example, the transport device may include pumps, extruders, vibrating tubes, endless screws or valves. The devices and / or apparatuses may include or be combined with a heating system (such as an oven, exchanger, trace, etc.) to achieve the conditions required for dissolution. The dissolution step a) may be carried out continuously, in batch mode or in fed-batch mode.

[0048] The dissolution step a) feeds the plastic feedstock, in particular in the form of one or more streams of the plastic feedstock, at least, and the dissolving solvent, in particular in the form of one or more streams of the dissolving solvent, is preferably fed by one or more transport devices. One or more streams of the plastic feedstock may be different from one or more streams of the dissolving solvent. Part or all of the plastic feedstock may be fed to step a) as a mixture with part or all of the dissolving solvent, and the remainder of the solvent and / or feedstock may optionally be fed separately to step a).

[0049] While the plastic feedstock is in contact with the dissolution solvent, the dissolution solvent is preferably at least partially, preferably entirely, in liquid form, whereas the plastic feedstock containing polyethylene may be in solid or liquid form and optionally contains solid particles in suspension. The plastic feedstock may in some cases be injected into the dissolution equipment in the form of a suspension in the dissolution solvent as a mixture with the dissolution solvent, and the preparation and injection of the suspension may be continuous or batchwise.

[0050] According to a particular embodiment of the invention, step a) may be carried out using an extruder and optionally at least one other dissolution device. In this case, the plastic feedstock may in some cases be fed to the extruder together with at least part of the dissolution solvent, and at the outlet of the extruder, at least part, preferably all, of the targeted polyethylene contained in the feedstock is in molten form (or at least partially dissolved form). The plastic feedstock may in some cases be mixed with at least part of the dissolution solvent and then injected into a dissolution device, such as a reactor, in at least partially molten form (or partially dissolved form). The plastic feedstock leaves the extruder in at least partially molten form (and / or partially dissolved form), and this may be pumped by a pump dedicated to viscous fluids, often known as a melt pump or a gear pump. The plastic feedstock may in at least partially molten form (or partially dissolved form) be filtered using a filtration device at the outlet of the extruder, optionally in addition to a melt pump, for the purpose of removing the coarsest particles; generally, the mesh size of this filter is from 10 microns to 1 mm, preferably from 20 to 200 microns.

[0051] Preferably, step a) includes an extruder into which the dissolution solvent is preferably injected at several points, promoting shear and thus promoting intimate mixing between the dissolution solvent and the plastic feedstock, which contributes to dissolving the polyethylene.

[0052] In some cases, the present treatment method may include an intermediate adsorption step a’), which is located during the dissolution step a) or directly downstream of the dissolution step a). This includes the introduction of an adsorbent in the form of divided particles, preferably, for example, alumina, silica, silica-alumina, activated carbon or bleaching earth, into the crude polymer solution obtained at the end of step a) or, in some cases, during the dissolution step a). The adsorbent may be removed during the purification step b), for example, during a sub-step b1) of separating out insoluble substances and / or a washing sub-step b2). This optional adsorption step a’) in the presence of the adsorbent in divided form makes it possible to optimize the purification of the polymer solution.

[0053] The crude polymer solution obtained at the end of the dissolution step a) contains at least a dissolution solvent, and the polyethylene that the present invention intends to recover and purify is dissolved in the dissolution solvent. Generally, the crude polymer solution also contains soluble impurities that are soluble in the dissolution solvent. The crude polymer solution may optionally contain insoluble impurities or compounds in suspension. The crude polymer solution obtained at the end of step a) may optionally contain polymers other than the target polyethylene, for example, in a molten form.

[0054] Under such operating conditions, especially at the end of the dissolution step carried out under operating conditions regarding temperature and pressure, the polyethylene of the plastic feedstock is preferably wholly or partly dissolved in the dissolution solvent; the obtained polyethylene solution (i.e., the crude polymer solution) can undergo a purification step b) and then a solvent-polymer separation step c), recovering the polyethylene from the plastic feedstock in a purified form, with the impurity and residual solvent contents being very low and suitable for any type of subsequent application. Therefore, the method according to the present invention enables the recovery of polyethylene from plastic waste in an optimal manner and overall under reasonable operating conditions (especially, a sufficiently restricted, i.e., restricted dissolution pressure), and thus, the energy consumption is controlled and, as a result, the cost is restricted.

[0055] (Step b) of purifying the polymer solution) The purification method according to the present invention includes a step of purifying the crude polymer solution obtained from step a). This purification step b) includes at least one of the following sub-steps b1), b2), b3) and b4): b1) Sub-step of separating out insoluble matter, b2) Sub-step of washing by contact with a concentrated solution, b3) Sub-step of extraction by contact with an extraction solvent, b4) Sub-step of adsorbing impurities by contact with an adsorbent.

[0056] The various sub-steps b1), b2), b3) and b4) that may be carried out in purification step b) may be operated continuously, in batch mode or in fed-batch mode.

[0057] Preferably, purification step b) includes at least one sub-step b1) of separating out insoluble matter. Purification step b) preferably includes several (i.e., at least two) sub-steps selected from sub-steps b1), b2), b3) and b4) in parallel, preferably including at least one sub-step b1) of separating out insoluble matter and, for example, at least one adsorption sub-step b4) in that order very advantageously. By combining at least two sub-steps selected from b1), b2), b3) and b4), it is advantageously possible to optimally purify the polymer solution.

[0058] The polymer solution obtained at the end of step b) is a purified polymer solution and contains polyethylene dissolved at least in a dissolution solvent. This purified polymer solution may correspond to a clarified polymer solution obtained from the sub-step b1) of separating out insoluble matter, a washed polymer solution obtained from the washing sub-step b2), an extracted polymer solution obtained from the extraction sub-step b3) or a refined purified polymer solution obtained from the sub-step b4) of adsorbing impurities.

[0059] (Sub-step b1) of separating out insoluble matter) This purification method may include a sub-step b1) of separating out insoluble matter by solid-liquid separation, and advantageously, at least one polymer solution and preferably an insoluble fraction are obtained, and the polymer solution is clarified (i.e., free from at least a part, preferably all, of the insoluble matter contained in the crude polymer solution). The insoluble fraction advantageously contains at least a part, preferably all, of the insoluble impurities, especially in suspension in the crude polymer solution obtained from step a).

[0060] Therefore, by the sub-step b1) of separating out insoluble matter, it is possible to remove at least a part, preferably all, of the particles of the insoluble compound in the dissolving solvent that are present in suspension in the crude polymer solution obtained from step a) or from an optional step a’). The insoluble compounds (or impurities) removed during the sub-step b1) of separating out insoluble matter are, for example, pigments, mineral compounds, packaging residues (glass, wood, cardboard, paper, aluminum) and insoluble polymers.

[0061] When it is carried out, this separation sub-step b1) advantageously, in addition to removing at least a part of the insoluble impurities, makes it possible to limit operational problems in the processing steps downstream of such a sub-step b1), especially clogging and / or erosion, etc., while at the same time contributing to the purification of the plastic feedstock.

[0062] The temperature at which the sub-step b1) of separating out insoluble matter is advantageously carried out is preferably 120 °C to 220 °C, preferentially 130 °C to 200 °C, very preferentially 150 °C to 200 °C, and the pressure is 1.0 to 25.0 MPa (absolute), preferably 1.0 to 20.0 MPa (absolute), preferentially 5.0 to 18.0 MPa (absolute), preferably 10.0 to 17.0 MPa (absolute). Very advantageously, the sub-step b1) of separating out insoluble matter is carried out under the temperature and pressure conditions at the outlet of the dissolving step a), i.e., at the above-mentioned dissolving temperature and dissolving pressure.

[0063] When it is incorporated into the present method, the sub-step b1) of separating out the insoluble matter is preferably fed with the crude polymer solution obtained from step a) or from an optional intermediate adsorption step a’). According to another embodiment, the sub-step b1) may be fed with the washed polymer solution obtained from the washing sub-step b2).

[0064] Advantageously, the sub-step b1) includes at least one solid-liquid separation section (or a solid-liquid-liquid separation section; 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 and / or polymers having properties different from the targeted polyethylene in a liquid form and are hardly or not soluble at all). The solid-liquid separation section includes at least one piece of solid-liquid separation equipment, for example, a separation vessel, a decanter, a centrifugal decanter, a centrifuge, a filter, a sand filter, a tangential filter, in particular, those using a membrane and / or a depth filter, a vortex current separator, an electrostatic separator, a triboelectric separator, preferably including a decanter, a filter, a sand filter and / or an electrostatic separator. Advantageously, a self-cleaning filter may be used, and the cleaning or clogging elimination enabling the removal of insoluble matter is carried out using a solvent stream. Preferably, the sub-step b1) includes at least one precipitation section preferably containing at least one decanter and / or at least one filtration section. In the sub-step b1), a filter aid (for example, diatomaceous earth or sand) may be added, optionally, before decanting and / or filtration.

[0065] The removal of the insoluble fraction may be facilitated by equipment for transporting and / or removing traces of the solvent that may be present in the insoluble fraction, such as a conveyor, a vibrating tube, an endless screw, an extruder or a stripper. The sub-step b1) may, therefore, include equipment for transporting and / or removing traces of the solvent to remove the insoluble fraction. Advantageously, at least a part of the solvent recovered in the sub-step b1) is recycled to the present method.

[0066] According to certain embodiments, the sub-step b1) of separating out insolubles includes at least 2 and generally less than 5 solid-liquid separation devices in series and / or in parallel. The presence of at least 2 solid-liquid separation devices in series enables the improvement of the removal of insolubles, while the presence of parallel equipment enables the management of the maintenance and / or clogging elimination operations of said equipment.

[0067] Certain insoluble compounds that are conventionally added during the blending of polymers, particularly certain pigments and mineral fillers, may be in the form of particles with a size of less than 1 μm. This applies, for example, to titanium dioxide, calcium carbonate, and carbon black. According to certain embodiments of sub-step b1), the sub-step b1) of separating out insolubles preferably includes an electrostatic separator, thereby enabling the efficient removal of at least a part of the insoluble particles with a size of less than 1 μm. According to another certain embodiment of sub-step b1), the sub-step b1) of separating out insolubles includes a sand filter to remove particles of different sizes, particularly particles with a size of less than 1 μm. According to yet another certain embodiment, the sub-step b1) of separating out insolubles includes a tangential filter, particularly one using a membrane and / or a depth filter, optionally containing a filter aid such as diatomaceous earth, etc.

[0068] Depending on the nature of the feedstock, the polymer solution, preferably the crude polymer solution, fed to sub-step b1) may optionally contain a second liquid phase, for example, a liquid phase consisting of a molten polymer, and these polymers are different in nature from those of polyethylene. According to another certain embodiment, sub-step b1) preferably includes a solid-liquid-liquid separation section, using equipment for separating out two liquid phases and one solid phase, preferably by at least 1 two-phase or three-phase separator.

[0069] (Washing sub-step b2)) This purification method may optionally include a sub-step b2) of washing with a concentrated solution, and advantageously, at least one washed polymer solution and preferably a wash effluent are obtained. The washed polymer solution obtained at the end of sub-step b2) advantageously contains polyethylene to be recovered and purified dissolved in a dissolution solvent. Optionally, the washed polymer solution may further contain residual impurities, which are in particular soluble in the dissolution solvent and / or, optionally, may further contain traces of the wash solvent (i.e., traces of the concentrated solution) if sub-step b2) is carried out.

[0070] When these two sub-steps are integrated into the purification step b), the washing sub-step b2) may be integrated upstream or downstream, preferably downstream, of the sub-step b1) that separates out the insoluble matter.

[0071] When it is integrated into the method, the washing sub-step b2) is fed with a concentrated solution and a crude polymer solution obtained from step a) or an optional intermediate adsorption step a’), or alternatively a clarified polymer solution obtained from b1). The polymer solution fed to the washing sub-step b2), in particular the crude or clarified polymer solution, may contain impurities in the form of suspended insoluble compounds and / or dissolved compounds. These suspended or dissolved compounds may be partially or entirely removed during the washing sub-step b2) by dissolution or precipitation and / or entrainment in the concentrated solution. Therefore, if it is carried out, this sub-step b2) contributes to the treatment of the plastic feedstock and, more particularly, to the purification of the polymer solution.

[0072] The cleaning sub-step b2) preferably includes placing the polymer solution fed to the sub-step b2), i.e., the crude or clarified polymer solution, in contact with the dense solution. Preferably, the dense solution has a higher density than the polymer solution (i.e., the mixture contains at least the target polyethylene and the dissolution solvent, and the target polyethylene is dissolved in the solvent). In particular, the density of the dense solution is preferably 0.85 or more, preferably 0.9 or more, preferentially 1.0 or more, and preferably 1.5 or less. The dense solution may be an aqueous solution, which preferably contains at least 50 wt% water, preferably at least 75 wt% water, most preferably at least 90 wt% water. The pH of the aqueous solution may be adjusted using an acid or a base to promote the dissolution of certain compounds. The dense solution may optionally contain an organic solvent and is preferably a solution consisting thereof. The density is preferably 0.85 or more, preferably 0.9 or more, preferentially 1.0 or more. The polyethylene of the plastic feedstock remains insoluble under the temperature and pressure conditions of the sub-step b2). For example, the organic solvent is selected from sulfolane or N-methylpyrrolidone (NMP), optionally as a mixture with water. Most preferably, the dense solution is an aqueous solution, preferably containing at least 50 wt% water, preferably at least 75 wt% water, most preferably at least 90 wt% water.

[0073] When the cleaning sub-step b2) is preferably carried out, the temperature is preferably 120°C to 220°C, preferentially 130°C to 200°C, most preferentially 150°C to 200°C, and the pressure at that time is 1.0 to 25.0 MPa (absolute), preferably 1.0 to 20.0 MPa (absolute), preferentially 5.0 to 18.0 MPa (absolute), preferably 10.0 to 17.0 MPa (absolute). Most preferably, the cleaning sub-step b2) is carried out at the dissolution temperature and dissolution pressure.

[0074] In the case where it is incorporated into this method in the washing sub-step b2), the mass ratio (concentrated solution / polymer solution) between the mass flow rate of the concentrated solution and the mass flow rate of the polymer solution fed to the sub-step b2) is preferably from 0.05 to 20.0, preferably from 0.1 to 10.0, preferably from 0.5 to 3.0. Placing it so as to be in contact between the polymer solution and the concentrated solution may be carried out at several points in the equipment used, i.e., through several injections of the polymer solution and / or the concentrated solution at different points along the equipment; what is taken into account in the calculation of the mass ratio (concentrated solution / polymer solution) is the total of the injected flows.

[0075] The sub-step b2) may be carried out in one or more washing apparatuses enabling it to be placed in contact with the concentrated solution and / or by separation equipment enabling at least one washing effluent and at least one washed polymer solution to be recovered. This equipment is well-known and includes, 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, pulsed columns, etc., and each type of equipment may include one or more apparatuses and is used alone or in combination with another type of equipment.

[0076] According to a preferred embodiment, the washing sub-step b2) is carried out in a counter-current washing column in which the concentrated solution is preferably injected, on the one hand, preferably into the upper half, preferably the upper third of the column closest to the top of the column, and on the other hand, the crude or clarified polymer solution is preferably injected, preferably into the lower half, preferably the lower third of the column closest to the bottom of the column. According to this embodiment, it is possible to recover at least one washed polymer solution and advantageously the washing effluent.

[0077] According to a very particular embodiment, the flow at the inlet and / or outlet of the washing column may be split and injected at several injection points along the column and / or withdrawn at several withdrawal points along the column.

[0078] According to another embodiment, the washing sub-step b2) is carried out in a mixer-decanter comprising a stirred mixing zone arranged so as to bring the concentrated solution into contact with the crude or clarified polymer solution, and a decantation zone enabling the recovered washed polymer solution and advantageously the washing effluent to be recovered.

[0079] At the end of the washing sub-step b2), a washing effluent is advantageously obtained, which contains, in particular, compounds dissolved in the concentrated solution and / or insoluble compounds entrained in the washing effluent. The washing effluent may be reprocessed in a washing effluent treatment section, on the one hand, at least partially separating out the dissolved and / or entrained compounds, and optionally purifying the washing effluent to obtain a purified concentrated solution, and on the other hand, at least partially recycling a portion of the purified concentrated solution. This washing effluent treatment section may include one or more pieces of equipment known for solid-liquid separation, such as separation vessels, decanters, centrifugal decanters, centrifuges or filters. The washing effluent may be sent outside the process, for example, to a used water treatment station when the concentrated solution is an aqueous solution.

[0080] (Extraction sub-step b3)) Step b) of the method according to the invention may include a sub-step b3) of extraction by placing it in contact with an extraction solvent, to obtain at least one extracted polymer solution and, preferably, a used solvent, in particular one filled with impurities. The extracted polymer solution obtained at the end of sub-step b3) advantageously contains polyethylene, which the invention seeks to recover and purify, dissolved in a dissolution solvent. Optionally, the extracted polymer solution may further contain residual impurities that are particularly soluble in the dissolution solvent and / or traces of the concentrated solution and / or the extraction solvent if one or more sub-steps b2) and / or b3) are carried out.

[0081] When it is integrated into the method according to the invention, extraction sub-step b3) is advantageously located between the dissolution step a) and the solvent-polymer separation step b), preferably downstream of the sub-step b1) that separates out the insoluble matter. Optionally, if the adsorption sub-step b4) is also integrated into step b), or if the adsorption sub-step b4) is located upstream or downstream of it.

[0082] The extraction sub-step b3) is preferably fed with an extraction solvent and a polymer solution, in particular the crude polymer solution obtained from step a), the clarified polymer solution obtained from sub-step b1), the washed polymer solution obtained from sub-step b2) or the refined polymer solution obtained from the adsorption sub-step b4). Preferably, the extraction sub-step b3) is fed with an extraction solvent and the clarified polymer solution obtained from sub-step b1), the washed polymer solution obtained from sub-step b2), or optionally the refined polymer solution obtained from the adsorption sub-step b4). The polymer solution fed to sub-step b3), preferably the clarified polymer solution, the washed polymer solution or the refined polymer solution, may therefore, in some cases, contain dissolved compounds or dissolved impurities in addition to polyethylene. These dissolved compounds may be partially or completely removed by placing them in contact with the extraction solvent during the extraction sub-step b3). Very advantageously, the combination of sub-step b1) for separating out the insolubles in the extraction sub-step b3) and optionally the washing sub-step b2) and / or the adsorption sub-step b4) enables improved purification of the polymer solution and exploits the affinity of the impurities for both the extraction solvent and, optionally, the concentrated solution and / or the adsorbent.

[0083] When it is incorporated into the process according to the invention, the extraction sub-step b3) preferably includes at least one extraction section, preferably from 1 to 5 extraction sections, very preferably 1 extraction section.

[0084] The mass ratio (extraction solvent / polymer solution) between the mass flow rate of the extraction solvent and the mass flow rate of the polymer solution fed to sub-step b3), preferably a clarified polymer solution, a washed polymer solution or a refined polymer solution, is advantageously from 0.05 to 20.0, preferably from 0.1 to 10.0, preferably from 0.2 to 5.0. Placing the polymer solution fed to sub-step b3), preferably a clarified polymer solution, a washed polymer solution or a refined polymer solution, in contact with the extraction solvent may be carried out at several points in the extraction section, i.e. via several injections of the polymer solution and / or the extraction solvent at different points along the extraction section; what is taken into account in the calculation of the mass ratio (extraction solvent / polymer solution) is the sum of the flows being injected.

[0085] In the extraction sub-process b3), the extraction solvent used preferably contains an organic solvent or a mixture of organic solvents. Preferably, the extraction solvent contains at least one hydrocarbon-based compound, preferably consists of it, which is preferably an aliphatic, preferably paraffinic, preferably at least one alkane, and the boiling point is -15 to 100 ° C, preferably 8 to 100 ° C, preferentially 25 to 69 ° C, preferably 25 to 61 ° C, most preferably 25 to 40 ° C. Preferably, the extraction solvent mainly contains, preferably at least 80% by weight, preferentially at least 95% by weight, preferably up to at least 98% by weight of preferably paraffinic aliphatic hydrocarbon-based compounds (or alkanes) (100% being the maximum, and the percentage is expressed relative to the total weight of the dissolving solvent), and the boiling point is -15 to 100 ° C, preferably 8 to 100 ° C, preferentially 25 to 69 ° C, preferably 25 to 61 ° C, most preferably 25 to 40 ° C. Most preferably, the hydrocarbon-based compound is preferably aliphatic, preferably paraffinic, forms the majority of the extraction solvent, and the critical temperature of this (the temperature at the critical point of the high-purity hydrocarbon-based compound) is 130 to 285 ° C, preferably 158 to 285 ° C, preferentially 185 to 245 ° C, preferably 185 to 230 ° C, most preferably 185 to 200 ° C. According to a preferred embodiment, the extraction solvent mainly contains, preferably at least 80% by weight, preferentially at least 95% by weight, preferably up to at least 98% by weight of aliphatic paraffinic hydrocarbon-based compounds, which are preferably linear or branched, and the boiling point is -15 to 100 ° C, preferably 8 to 100 ° C, preferentially 25 to 69 ° C, preferably 25 to 61 ° C, most preferably 25 to 40 ° C, contains 4 to 7 carbon atoms (C4-C7), preferably 5, 6 or 7 carbon atoms (each C5, C6 or C7), preferentially contains 5 or 6 carbon atoms (C5 or C6), and most preferentially contains 5 carbon atoms (C5).

[0086] Most preferably, the extraction solvent used in b3) is the same solvent as the dissolution solvent used in step a), and in some cases, is in a different physical state (for example, the extraction solvent is in a supercritical form, while the dissolution solvent is in a liquid form), facilitating the management of the solvents, particularly their purification and their possible recycling to the dissolution step a) and in particular to the extraction sub-step b3). Another advantage of using the same dissolution and extraction solvents in the same or different physical states is that it facilitates the management of the solvents included in the method according to the invention, particularly the recovery of the solvents, their treatment and their recycling to at least one of the steps of the method, in addition to particularly limiting the energy consumption and cost resulting from the treatment and purification of the solvents.

[0087] One or more extraction sections of b3) may include one or more pieces of extraction equipment and are arranged to be in contact with a separation facility for recovering the extraction solvent and / or at least one used solvent, particularly one filled with impurities, and the extracted polymer solution. This equipment is well-known and includes, 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, pulsed columns, etc., and each type of equipment may include one or more pieces of equipment used alone or in combination with another type of equipment.

[0088] According to a preferred embodiment of b3), the extraction is carried out in a countercurrent extraction column where, on the one hand, the extraction solvent is injected and, on the other hand, the polymer solution fed to sub-step b3) is injected. According to this embodiment, it is possible to recover, on the one hand, at least one extracted polymer solution and, on the other hand, the used solvent, in particular the one filled with impurities. Preferably, the polymer solution fed to b3), preferably a clarified, washed or refined polymer solution, is injected into the upper half of the column closest to the top of the countercurrent extraction column, preferably the upper third, while the extraction solvent is preferably injected into the lower half of the column closest to the bottom of the countercurrent extraction column, preferably the lower third.

[0089] The flow at the inlet and / or outlet of the countercurrent extraction column may be split at several injection points and / or extraction points along the column.

[0090] According to another embodiment of b3), the extraction is carried out in a mixer - settler which advantageously comprises, on the one hand, a stirred mixing zone for placing the extraction solvent in 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 which makes it possible to recover, on the one hand, the extracted polymer solution and, on the other hand, the used solvent.

[0091] Advantageously, the extraction sub-step b3) is carried out under temperature and pressure conditions different from those of the dissolution step a).

[0092] According to a preferred embodiment of b3), the extraction sub-step b3) includes a liquid / liquid extraction section. Preferably, the liquid / liquid extraction section is operated at 120°C to 220°C, preferentially 130°C to 200°C, and most preferentially 150°C to 200°C, and the pressure at that time is 1.0 to 25.0 MPa (absolute), preferably 1.0 to 20.0 MPa (absolute), preferentially 5.0 to 18.0 MPa (absolute), preferably 10.0 to 17.0 MPa (absolute). In any case, in this embodiment, the temperature and pressure conditions are adjusted so that the extraction solvent is in a liquid form, and the dissolution solvent is also preferably in a liquid form. Most advantageously, the liquid / liquid extraction is carried out under temperature and pressure conditions different from those of the dissolution in step a), especially when the extraction solvent is the same as the dissolution solvent, specifically at a temperature above the dissolution temperature and / or a pressure below the dissolution pressure, resulting in a two-phase zone in the diagram of the corresponding polymer-solvent mixture.

[0093] According to another preferred embodiment of b3), the extraction sub-step b3) includes a section for extraction under specific temperature and pressure conditions where the extraction solvent is preferably at least partially in a supercritical form. Such extraction may be referred to as supercritical extraction. In this embodiment, the extraction is advantageously carried out mainly (i.e., preferably at least 50% by weight, preferentially at least 70% by weight, preferably at least 90% by weight) by placing the polymer solution fed to b3), preferably a clarified, washed or refined polymer solution, in contact with the extraction solvent under temperature and pressure conditions that enable the formation of a supercritical phase composed mainly of the extraction solvent. In other words, in this embodiment, the extraction is carried out by placing the polymer solution fed to b3), preferably a clarified, washed or refined polymer solution, in contact with the extraction solvent that is at least partially, preferably entirely, in a supercritical form. Such a supercritical extraction sub-step b3) advantageously enables efficient purification of the polymer solution, particularly due to the very high affinity of organic impurities, such as some additives, especially certain colorants or plasticizers, for the supercritical phase. The use of the extraction solvent in a supercritical form also makes it possible to create a substantial density difference between the supercritical phase and the polymer solution in liquid form, which facilitates demixing and separation by decantation between the two phases, i.e., the supercritical phase and the liquid phase, and thus contributes to the efficiency of the purification of the polymer solution.

[0094] In this other preferred embodiment, the extraction solvent used in the sub-step b3) mainly, preferably at least 80% by weight, preferentially at least 95% by weight, preferably up to at least 98% by weight, preferably paraffinic aliphatic hydrocarbon-based compounds (or alkanes) (where 100% is the maximum and the percentage is expressed relative to the total weight of the dissolution solvent), and the critical temperature is preferably 130 to 285 °C, preferably 158 to 285 °C, preferentially 185 to 245 °C, very preferentially 185 to 230 °C, preferably 185 to 200 °C. Very preferably, in such a supercritical extraction sub-step b3), the extraction solvent mainly, preferably at least 80% by weight, preferentially at least 95% by weight, preferably up to at least 98% by weight of paraffinic aliphatic hydrocarbon-based compounds, the boiling point is -15 to 100 °C, preferably 8 to 100 °C, preferentially 25 to 69 °C, very preferentially 25 to 61 °C, preferably 25 to 40 °C, contains 4 to 7 carbon atoms (i.e., C4-C7), preferably 5, 6 or 7 carbon atoms (C5, C6 or C7 respectively), preferentially contains 5 or 6 carbon atoms (C5 or C6), and very preferentially contains 5 carbon atoms (C5). Very particularly, the critical pressure of the extraction solvent mainly composed of paraffinic aliphatic hydrocarbon-based compounds is 2.5 to 5.0 MPa, preferably 2.7 to 4.6 MPa, preferentially 3.0 to 3.8 MPa, and most preferably 3.0 to 3.5 MPa.

[0095] Advantageously, when the supercritical extraction sub-step b3) of this specific embodiment is carried out, the temperature is preferably 160°C to 300°C, preferentially 190 to 250°C, preferably 200°C to 230°C, and the pressure at that time is preferably 2.7 to 10.0 MPa (absolute), preferentially 3.0 to 6.0 MPa (absolute), preferably 3.0 to 5.0 MPa (absolute), most preferably 3.0 to 4.0 MPa (absolute). According to a very special mode of this embodiment of sub-step b3), the pressure when supercritical extraction is carried out is very advantageously between the critical pressure (CP(extraction solvent)) of a mainly paraffinic aliphatic hydrocarbon-based compound of the extraction solvent (i.e., preferably having a boiling point of -15 to 100°C, preferably 8 to 100°C, preferentially 25 to 69°C, most preferentially 25 to 61°C, preferably 25 to 40°C, containing 4 to 7 carbon atoms, preferably 5, 6 or 7 carbon atoms, preferentially containing 5 or 6 carbon atoms, most preferentially containing 5 carbon atoms, a mainly paraffinic aliphatic hydrocarbon-based compound) and a pressure 3.0 MPa higher than the critical pressure of the mainly paraffinic aliphatic hydrocarbon-based compound of the extraction solvent (i.e., CP(extraction solvent) + 3.0 MPa), preferably between the critical pressure (CP(extraction solvent)) of the mainly paraffinic aliphatic hydrocarbon-based compound of the extraction solvent and a pressure 1.5 MPa higher than the critical pressure of the mainly paraffinic aliphatic hydrocarbon-based compound extraction solvent of the extraction solvent (i.e., CP(extraction solvent) + 1.5 MPa), preferably between the critical pressure (CP(extraction solvent)) of the mainly paraffinic aliphatic hydrocarbon-based compound of the extraction solvent and a pressure 0.5 MPa higher than the critical pressure of the mainly paraffinic aliphatic hydrocarbon-based compound of the extraction solvent (i.e., equal to CP(extraction solvent) + 0.5 MPa), and the pressure is absolute pressure.In any case, in this embodiment, the adjustment of the temperature and pressure conditions is carried out, in particular, in an adjustment section that is carried out upstream of the extraction section in the extraction sub-step b3). As a result, the extraction solvent is at least partially in a supercritical state in the extraction section, and the adjustment of the temperature and pressure of the extraction solvent in the adjustment section is advantageously carried out by means known to those skilled in the art (for example, using pumps and / or valves and / or turbines and / or heat exchangers and / or ovens).

[0096] In a highly preferred embodiment of b3), the extraction sub-step b3) includes supercritical extraction. Apart from the fact that the extraction solvent is at least partially in a supercritical phase, the extraction solvent is the same as the dissolution solvent (or contains the same main compound as the dissolution solvent and, in some cases, impurities).

[0097] Advantageously, at the end of the extraction sub-step b3), the resulting used solvent is particularly filled with impurities. It may be reprocessed in an organic treatment section, which, on the one hand, makes it possible to at least partially separate out the impurities and purify the solvent to obtain a purified extraction solvent, and on the other hand, makes it possible to recycle at least a part of the purified extraction solvent to the inlet of the extraction sub-step b3) and / or, in the case where the dissolution solvent and the extraction solvent are the same, to the inlet of the dissolution step a). The used solvent may be treated according to any method known to those skilled in the art, for example, one or more methods from the following: distillation, evaporation, extraction, adsorption, crystallization and precipitation of insoluble substances, or purge.

[0098] (Adsorption sub-step b4)) The process step b) of the treatment method according to the present invention may include an adsorption sub-step b4) to obtain at least one refined and purified polymer solution. The refined and purified polymer solution obtained at the end of the sub-step b4) advantageously contains the polyethylene that the present invention intends to recover and purify, dissolved in the dissolution solvent.

[0099] When 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). It may be carried out upstream of the sub-step b1) for separating out insolubles and / or the washing sub-step b2), and may in particular correspond to an optional intermediate adsorption step a’). Preferably, it is carried out downstream of the sub-step b1) for separating out insolubles and, optionally, downstream of the washing sub-step b2), which sub-step b2) itself is preferably downstream of the sub-step b1). It may 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 thereto, in particular the crude polymer solution obtained from step a), the clarified polymer solution obtained from b1), the washed polymer solution obtained from b2) or alternatively the extracted polymer solution obtained from b3), in contact with one or more adsorbents.

[0100] The adsorption sub-step b4) advantageously includes an adsorption section which is operated in the presence of at least one adsorbent which is preferably solid and is in particular in the form of a fixed bed, a fluidized bed (or slurry, i.e. in the form of particles introduced into and entrained with the stream to be purified) or a boiling bed, preferably in the form of a fixed bed or a fluidized bed. Each adsorbent used in the sub-step b4) is 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 a fluidized bed, and the flow-through of the stream can be upward or downward.

[0101] Advantageously, when it is incorporated into the present method, the temperature at which the adsorption sub-step b4) is carried out is preferably 120 °C to 220 °C, preferentially 130 °C to 200 °C, and most preferentially 150 °C to 200 °C, and the pressure at that time is 1.0 to 25.0 MPa (absolute), preferably 1.0 to 20.0 MPa (absolute), preferentially 5.0 to 18.0 MPa (absolute), preferably 10.0 to 17.0 MPa (absolute). Most advantageously, the adsorption sub-step b4) is carried out under the temperature and pressure conditions of the dissolution in step a), that is, at the dissolution temperature and dissolution pressure. Preferably, in the optional sub-step b4), the hourly space velocity (HSV) corresponds to the ratio between the volume flow rate of the polymer solution fed to b4) and the volume of the adsorbent, and advantageously, it is operated in b4) at 0.05 to 10 h -1 , preferentially 0.1 to 5.0 h -1 .

[0102] According to a specific embodiment of the sub-step b4), the adsorption section may include one or more fixed beds of one or more adsorbents, for example, in the form of an adsorption column, preferably at least two adsorption columns, preferentially 2 to 4 adsorption columns, and contains one or more of the above-mentioned adsorbents. When the adsorption section includes two adsorption columns, one operating mode may be what is referred to as a "swing" operation in technical terms, where one of the columns is online, that is, in service, while the other column is in reserve. When the adsorbent in the online column is exhausted, this column is isolated, while the reserve column is put online, that is, into service. The used adsorbent can then be regenerated in situ and / or replaced with fresh adsorbent, and the column containing it can be put online again when the other column is isolated.

[0103] Another mode of functionalization of this specific embodiment of b4) involves one or more fixed beds of one or more adsorbents and functionalizing at least two columns in series. When the adsorbent in the column placed at the head is exhausted, this first column is isolated and the used adsorbent is either regenerated on-site or replaced with fresh adsorbent. The column is then returned online in the last position and this is repeated. This operation is known as the variable array mode or by the term PRS (Permutable Reactor System) or also as "lead and lag". The combination of at least two adsorption columns makes it possible to overcome the potentially rapid poisoning and / or clogging of the adsorbent due to the combined action of impurities, contaminants and insolubles that may be present in the stream to be treated. The reason for this is that the presence of at least two adsorption columns facilitates the replacement and / or regeneration of the adsorbent, preferably without stopping the treatment, and also makes it possible to control costs and limit the consumption of the adsorbent.

[0104] According to this specific embodiment of the adsorption sub-step b4) in the fixed bed of one or more adsorbents, the sub-step b4) is preferably carried out downstream of the sub-step b1) for separating out insolubles 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) for separating out insolubles and / or the washing sub-step b2) and, optionally, the extraction sub-step b3) with the adsorption sub-step b4) enables improved purification of the polymer solution by taking advantage of the affinity of the residual impurities for both the adsorbent and the extraction solvent and, optionally, the concentrated solution.

[0105] According to another embodiment, the adsorption section of b4) may consist of adding adsorbent particles to a polymer solution, in particular a crude polymer solution, and the particles may be separated from the polymer solution via a step of removing the adsorbent particles located downstream of the adsorption section. The removal of the adsorbent particles may advantageously correspond to the sub-step b1) of separating out the insolubles or the washing sub-step b2). Such an implementation of the adsorption sub-step b4) advantageously corresponds to the optional intermediate adsorption step a’) described earlier herein by introducing the adsorbent particles and subsequently performing a solid-liquid separation.

[0106] (Step c) of solvent-polymer separation) According to the present invention, the method includes a solvent-polymer separation step c) to obtain at least one purified polyethylene fraction and preferably a solvent fraction.

[0107] The solvent-polymer separation step c) advantageously includes at least one supercritical separation section, followed in series by at least one solvent recovery section, preferably 1 to 5 solvent recovery sections. The solvent-polymer separation step c), more particularly the supercritical separation section, especially the first supercritical separation section, is fed with the purified polymer solution obtained from the purification step b).

[0108] The solvent-polymer separation step c) is thus initially directed towards, and preferably mainly, and even more preferably entirely, recovering polyethylene by separating out one or more solvents, in particular the dissolution solvent, contained in the purified polymer solution fed to step c). The polyethylene is at least partially, preferably mainly, preferentially and even more preferably entirely freed from impurities and the dissolution solvent and, optionally, one or more other solvents used in the process (i.e. the extraction solvent and / or the concentrated solution). The term "predominantly" is understood to mean at least 50% by weight, preferentially 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 one or more solvents, in particular the dissolution solvent, contained in the purified polymer solution fed to step c), and optionally the extraction solvent and / or the concentrated solution. Any method known to the person skilled in the art for separating a solvent from a polymer may be carried out, in particular any method enabling a phase change of the polymer or of one or more solvents. The one or more solvents may be separated out, for example, by evaporation, stripping, demixing, density difference, in particular by decantation or centrifugation, etc.

[0109] The fraction of purified polyethylene obtained at the end of step c) may correspond to a concentrated polyethylene solution or to liquid (i.e. molten) or solid purified polyethylene. The solvent-polymer separation step c) may optionally include a conditioning section for conditioning the recovered purified polyethylene in solid form, and more particularly in the form of solid granules. In this possible conditioning section, the recovered purified polyethylene is advantageously cooled to a temperature below the melting point of polyethylene to obtain a fraction containing polyethylene in solid form.

[0110] The solvent-polymer separation step c) is also directed towards at least partially, preferably mainly, preferentially entirely recovering one or more solvents contained in the purified polymer solution fed to step c), in particular the dissolution solvent, optionally the extraction solvent and / or the concentrated solution. The term "mainly" should be understood to mean at least 50% by weight, preferentially preferably at least 70% by weight, preferably at least 90% by weight, very preferably at least 95% by weight relative to the weight of one or more solvents contained in the purified polymer solution fed to step c). Therefore, step c) also advantageously makes it possible to obtain at least one solvent fraction. The solvent-polymer separation step c) is also directed, in some cases, towards purifying the recovered solvent fraction and recycling it, in particular upstream of the dissolution step a), optionally upstream of the sub-step b2) and / or the sub-step b3).

[0111] The solvent-polymer separation step c) thus includes a supercritical separation section, whereby at least a part of the dissolved solvent, and optionally the extraction solvent and the concentrated solution, and in some cases also a part of the residual impurities not removed during step b), are brought to supercritical conditions, i.e., above the critical point of one or more solvents to be separated out, in particular above the critical point of the dissolved solvent, more particularly above the critical point of the mainly hydrocarbon-based compounds of the dissolved solvent, under temperature and pressure conditions adjusted accordingly, so that advantageously at least a part of the solvent, in particular the dissolved solvent, can be easily separated out and recovered. This supercritical separation section particularly includes a fluid system composed of a supercritical phase mainly containing the solvent, in particular the dissolved solvent, and a liquid phase containing polyethylene. The term "mainly" here means 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 flow under consideration, i.e., the supercritical phase. The separation may be referred to as the supercritical separation of one or more solvents. By the supercritical separation of one or more solvents, on the one hand, at least a part of one or more solvents, in particular the dissolved solvent, and on the other hand, polyethylene or a concentrated polyethylene solution can be efficiently separated. The supercritical separation is advantageously made possible by a significant density difference between the two phases, namely the supercritical phase mainly containing the solvent, in particular the dissolved solvent, and the liquid phase containing polyethylene. Furthermore, by the supercritical separation of one or more solvents, advantageously, the energy and environmental costs are significantly reduced compared to the simple evaporation of the solvent. This is because there is no latent heat of evaporation during the transition to the supercritical state.

[0112] When the supercritical separation section is operated advantageously, the temperature is 160°C to 300°C, preferably 190 to 250°C, preferably 200°C to 230°C, and the pressure (Psupercritical) at that time is 2.7 to 10.0 MPa (absolute), preferably 3.0 to 6.0 MPa (absolute), preferably 3.0 to 5.0 MPa (absolute), preferably 3.0 to 4.0 MPa (absolute).

[0113] According to certain embodiments, the pressure (Psupercritical) at which the supercritical separation section of step c) is carried out is between the critical pressure (CP(solvent)) of the main hydrocarbon-based compound of the solvent in solution and a pressure 3.0 MPa higher than the critical pressure of the main hydrocarbon-based compound of the solvent in solution (i.e., CP(solvent) + 3.0 MPa), preferably between the critical pressure (CP(solvent)) of the main hydrocarbon-based compound of the solvent in solution and a pressure 1.5 MPa higher than the critical pressure of the main hydrocarbon-based compound of the solvent in solution (i.e., CP(solvent) + 1.5 MPa), more preferably between the critical pressure (CP(solvent)) of the main hydrocarbon-based compound of the solvent in solution and a pressure 0.5 MPa higher than the critical pressure of the main hydrocarbon-based compound of the solvent in solution (i.e., CP(solvent) + 0.5 MPa), where the pressure is absolute pressure, and the main hydrocarbon-based compound of the solvent in solution is advantageously an aliphatic, preferably a paraffinic hydrocarbon-based compound, having a boiling point of -15 to 100 °C, preferably 8 to 100 °C, more preferably 25 to 69 °C, most preferably 25 to 61 °C, preferably 25 to 40 °C, preferably containing 4 to 7 carbon atoms, preferably 5, 6 or 7 carbon atoms, more preferably 5 or 6 carbon atoms, most preferably 5 carbon atoms, as detailed in the description of step a) above.

[0114] The supercritical separation section of step c) is preferably carried out by demixing the liquid phase (containing polyethylene) and the supercritical phase (composed of the solvent) and then decanting. Advantageously, the supercritical phase from the supercritical separation section at least partially constitutes the solvent fraction obtained at the end of step c). The liquid phase containing polyethylene is preferably sent to a solvent recovery section or a series of solvent recovery sections.

[0115] Step c) may optionally include one or a plurality of consecutive supercritical separation sections, in particular 1 to 5, more particularly 1, 2 or 3. The liquid phase containing polyethylene and obtained from the supercritical separation section may thus be fed to another subsequent supercritical separation section, and the liquid phase from the last supercritical separation section is preferably sent to one or a series of solvent recovery sections. Most preferably, step c) includes one supercritical separation section.

[0116] Most advantageously, the supercritical separation of the solvent further reduces the residual impurity content of the purified polyethylene fraction.

[0117] Preferably, the supercritical separation section, optionally a series of supercritical separation sections, is followed by at least one, preferably 1 to 5, preferably consecutive solvent recovery sections. The first solvent recovery section is fed with the liquid phase containing polyethylene obtained from the supercritical separation section, optionally a series of supercritical separation sections, in particular from the last supercritical separation section. In cases where the separation process includes at least two solvent recovery sections, each of the subsequent solvent recovery sections, i.e., from the second solvent recovery section onwards, is fed with the liquid phase containing polyethylene obtained from the preceding solvent recovery section. For example, the second solvent recovery section is fed with the liquid phase containing the target thermoplastic polymer obtained from the first solvent recovery section. The liquid phase containing polyethylene obtained from the last solvent recovery section constitutes the purified polyethylene fraction obtained at the end of step c).

[0118] The phase or combination of phases containing only the solvent obtained from the solvent recovery section, together with the supercritical phase obtained from the supercritical separation section, or in some cases a series of supercritical separation sections, constitutes one or more solvent fractions advantageously recovered at the end of step c). The phase or combination of phases containing only the solvent obtained from the solvent recovery section is preferably in gaseous form, can be condensed, and in some cases is mixed with the supercritical phase obtained from the supercritical separation section, and the temperature and pressure conditions are preferably pre-adjusted to be in liquid form.

[0119] When each solvent recovery section is carried out, the temperature is preferably 160 - 300 °C (preferably, the temperature is higher than the melting point of polyethylene), and the pressure is between the pressure (Psupercritical) used in one or more supercritical separation sections and 0.000005 MPa (i.e., 5 Pa). Preferably, when each solvent recovery section is carried out, the temperature is 160 - 300 °C, and the pressure at that time is between the pressure of the preceding section of step c) and 0.000005 MPa. Therefore, when step c) includes a supercritical separation section and several (at least two) solvent recovery sections, the pressure P(S1) when the first solvent recovery section S1 directly following the supercritical separation section is carried out is between the pressure (Psupercritical) used in the supercritical separation section (preferably the immediately preceding section) and 0.000005 MPa; the pressure P(S2) when the second solvent recovery section S2 directly following the solvent recovery section S1 is carried out is between the pressure P(S1) used in the first solvent recovery section S1 and 0.000005 MPa, and the same applies to subsequent sections. According to a preferred embodiment, when each solvent recovery section is carried out, the temperature is preferably 160 - 300 °C, the pressure is between the pressure of the preceding section of step c) and 0.000005 MPa, preferably, the pressure at that time is 10.0 MPa - 0.000005 MPa, preferentially 5.0 MPa - 0.000005 MPa, preferably 2.7 MPa - 0.000005 MPa. Preferably, the temperature and pressure conditions are adjusted in each solvent recovery section, and preferably modify the volatility of one or more solvents still present in the polymer phase in the form of a concentrated polyethylene solution or in the form of molten or solid polyethylene.

[0120] In cases where several different solvents are used in the purification method according to the invention, in particular in the dissolution step a), and possibly in the extraction sub-step b3), step c) may include several solvent recovery sections, for example 2, 3 or 4 solvent recovery sections, and separately, successively and / or continuously recover various solvents, in particular the dissolution solvent, and possibly the extraction solvent.

[0121] Advantageously, the section for supercritical separation and solvent recovery in step c) may be operated continuously, in batch or fed-batch mode.

[0122] Most advantageously, the solvent fraction recovered at the end of step c) may be treated in an organic treatment section arranged at the end of step c) to purify it and obtain at least one purified solvent, in particular at least one purified dissolution solvent, and advantageously recycle it to the dissolution step a), possibly to the washing sub-step b2) or the extraction sub-step b3). The optional organic treatment section at the end of step c) may use any method known to those skilled in the art, for example, distillation, evaporation, liquid-liquid extraction, adsorption, crystallization and precipitation of insoluble substances, or one or more methods from among purge.

[0123] The method according to the invention thus makes it possible to obtain a purified stream of polyethylene from plastic waste, which may be used in any application, for example, instead of virgin resin. The content of impurities and residual solvents in the purified stream of polyethylene obtained via the method according to the invention, i.e., the purified polyethylene fraction, is thus sufficiently low to be used in any application. Preferably, the purified polyethylene stream obtained at the end of the method according to the invention advantageously contains less than 5% by weight of impurities, most advantageously less than 1% by weight of impurities, and most advantageously less than 5% by weight of residual solvents (in particular, the dissolution solvent), preferably less than 1% by weight of residual solvents, preferably less than 0.1% by weight of residual solvents.

[0124] According to a preferred embodiment of the present invention, a method for purifying a plastic feedstock containing polyethylene includes the following steps, preferably consisting of: a) A step of dissolving in a dissolution solvent; this dissolution solvent contains at least one aliphatic, paraffinic hydrocarbon-based compound, and the boiling point is -15 to 100 °C, preferably 8 to 100 °C, preferentially 25 to 69 °C, highly preferentially 25 to 61 °C, preferably 25 to 40 °C; the dissolution temperature when carried out is 150 °C to 250 °C, preferentially 160 to 225 °C, highly preferentially 165 °C to 210 °C, preferably 170 °C to 195 °C, and the dissolution pressure is 1.0 to 18.0 MPa (absolute), preferably 1.0 to 12.0 MPa (absolute), preferentially 3.0 to 11.0 MPa (absolute), preferably 5.0 to 11.0 MPa (absolute), highly preferably 6.0 to 10.0 MPa (absolute); obtaining at least one crude polymer solution; and then, b) A step of purifying the polymer solution; including the following sub-steps; b1) A sub-step of separating out insolubles; obtaining a clarified polymer solution and an insoluble fraction; and then b4) A sub-step of adsorbing impurities by contacting the clarified polymer solution with an adsorbent; obtaining at least one refined and purified polymer solution; and then c) A solvent-polymer separation step; using at least one supercritical separation section; the temperature when operated is 160 to 300 °C, preferably 190 to 250 °C, preferentially 200 to 230 °C, and the pressure (P supercritical) at that time is 2.7 to 10.0 absolute MPa, preferably 3.0 to 6.0 absolute MPa, preferentially 3.0 to 5.0 absolute MPa, preferably 3.0 to 4.0 absolute MPa; followed by at least one solvent recovery section, and the temperature when operated is 160 to 300 °C, and the pressure is between the pressure of the supercritical separation section (P supercritical) and 0.000005 MPa (i.e., 5 Pa); obtaining at least one fraction of purified polyethylene.

[0125] According to another aspect, the present invention relates to a device for purifying a plastic feedstock containing polyethylene, said device comprising, preferably consisting of: a) a section for dissolving the plastic feedstock in a dissolution solvent; the dissolution solvent advantageously contains at least one paraffinic aliphatic hydrocarbon-based compound; the dissolution temperature during operation is 150 - 250 °C, preferably 160 - 225 °C, more preferably 165 - 210 °C, preferably 170 - 195 °C, and the dissolution pressure at that time is 1.0 - 18.0 MPa (absolute), preferably 1.0 - 12.0 MPa (absolute), preferably 3.0 - 11.0 MPa (absolute), preferably 5.0 - 11.0 MPa (absolute), more preferably 6.0 - 10.0 MPa (absolute); at least one crude polymer solution is obtained; and then b) a section for purifying the polymer solution; including: b1) a sub-section for separating insolubles; and / or b2) a washing sub-section by contact with a concentrated solution; and / or b3) an extraction sub-section by contact with an extraction solvent; and / or b4) an impurity adsorption sub-section by contact with an adsorbent; and then c) a solvent-polymer separation section; including at least one supercritical separation section; the temperature during operation is 160 - 300 °C, preferably 190 - 250 °C, more preferably 200 - 230 °C, and the pressure (Psupercritical) at that time is 2.7 - 10.0 absolute MPa, preferably 3.0 - 6.0 absolute MPa, preferably 3.0 - 5.0 absolute MPa, preferably 3.0 - 4.0 absolute MPa; followed by at least one solvent recovery section; the temperature during operation is 160 - 300 °C, and the pressure is between the pressure of the supercritical separation section (Psupercritical) and 0.000005 MPa (i.e., 5 Pa); at least one purified polyethylene fraction is obtained.

[0126] Preferably, the purification section b) includes: b1) Insoluble matter separation sub-step; obtaining a clarified polymer solution and an insoluble fraction; and then b4) Sub-step of adsorbing impurities by contacting the clarified polymer solution with an adsorbent; obtaining at least one refined and purified polymer solution.

[0127] The following examples illustrate the present invention, particularly specific embodiments thereof, but do not limit the scope of the present invention.

[0128] (Example) (Example 1 (in accordance with the present invention)) (Dissolution step a)) A feedstock containing 95% by weight of polyethylene (PE) from plastic waste is fed in flake form to an extruder heated to 180°C. As it leaves the extruder, the feedstock is in at least a partially molten form, which is mixed with a solvent containing 99% n-pentane preheated to 180°C at a solvent / feedstock mass ratio of 9:1. The mixture of solvent and feedstock is introduced into a stirred reactor, heated to 180°C, and maintained at 16 MPa (absolute) for a residence time of 1 hour. A polymer solution is then obtained.

[0129] The polymer solution from the dissolution step a) is then subjected to a purification step b): The polymer solution is continuously withdrawn from the stirred reactor and passed through three filters arranged in series, maintained at 180°C, with cut-off diameters of 500 μm, 100 μm, and 10 μm respectively (in this order). The pressure drop across the filters is 0.05 MPa.

[0130] At the outlet of the series of filters, the clarified polymer solution is contacted for 2 hours in an adsorption section containing a bed of activated carbon particles and then passed through a filter retaining the activated carbon particles. This adsorption section is operated at 180°C. As a result, a pressure drop of 0.2 MPa occurs.

[0131] The purified polymer solution from purification step b) is then subjected to a solvent-polymer separation step c) which includes a supercritical section: The purified polymer solution from the adsorption section is then heated to 210 °C and the pressure is slightly lower than 16 MPa (the value obtained by subtracting the pressure drop induced in the section of purification step b) from the dissolution pressure). The polymer solution is then expanded to 4 MPa (absolute) and injected into a decanter. This decanter is maintained at 4 absolute MPa and 210 °C for a residence time of 5 minutes. Two phases are formed: an upper supercritical phase mainly containing the n-pentane solvent and a lower liquid phase containing polyethylene dissolved in the n-pentane solvent. The upper phase is withdrawn from the upper part of the decanter.

[0132] The lower liquid phase is then subjected to evaporation of the residual solvent in two consecutive evaporation sections: first at a temperature of 210 °C and a pressure of 0.5 MPa for 5 minutes and second at a temperature of 210 °C and a pressure of 0.01 MPa for 2 minutes.

[0133] At the end of the method, a solid A composed of purified polyethylene (PE) is obtained at atmospheric standard conditions. Solid A is analyzed.

[0134] The obtained solid A is almost colorless and almost translucent and contains less than 5% by weight of impurities and less than 1% by weight of n-pentane.

[0135] (Example 2 (non-compliant)) In this Example 2, the same feedstock is processed and the dissolution step a) and the purification step b) are carried out in the same manner as described in Example 1.

[0136] The purified polymer solution from purification step b) is subjected to a solvent-polymer separation step that does not include a supercritical section: The purified polymer solution from the suction section is maintained at 180 °C, expanded to 2 MPa (absolute), and then injected into a decanter. This decanter is maintained at 2 absolute MPa and 180 °C for a residence time of 5 minutes. Two phases are formed: the gaseous upper phase consists of the n-pentane solvent, and the liquid lower phase contains polyethylene dissolved in the n-pentane solvent. The gas phase is withdrawn from the upper part of the decanter.

[0137] The lower liquid phase is then subjected to evaporation of the residual solvent, 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.

[0138] At the end of the process, a solid B composed of purified polyethylene (PE) is obtained at atmospheric standard conditions. Solid B is analyzed.

[0139] The obtained solid B is almost colorless and almost translucent, containing less than 5 wt% impurities and less than 1 wt% n-pentane.

[0140] However, the content of impurities (organic compounds excluding the dissolved solvent) in solid B is higher than the content measured in solid A obtained in Example 1 that conforms to the present invention.

[0141] Furthermore, according to Example 2, the energy consumption required for polymer-solvent separation is greater than the energy consumption required for the polymer-solvent separation in the method described in Example 1, i.e., when the polymer-solvent separation includes a supercritical phase section.

Claims

1. A method for purifying plastic supply raw materials containing polyethylene, comprising the following steps: a) Dissolution step; which includes placing the plastic supply material in contact with a dissolution solvent; the dissolution solvent comprises at least one hydrocarbon-based compound with a boiling point of -15 to 100°C; the dissolution temperature is 120°C to 220°C and the dissolution pressure is 1.0 to 25.0 MPa absolute pressure; at least one crude polymer solution is obtained; b) A step of purifying the crude polymer solution; obtaining the purified polymer solution; comprising the following sub-steps: b1) A sub-step for separating insoluble material; and / or b2) A sub-step of cleaning by contact with a high-density solution; and / or b3) Sub-step of extraction by contact with extraction solvent; and / or b4) A sub-step of adsorption of impurities by contact with an adsorbent; and then c) Solvent-polymer separation step; using at least one supercritical separation section; operating at a temperature of 160–300°C and a pressure (P supercritical) of 2.7–10.0 MPa absolute pressure; followed by at least one solvent recovery section; obtaining at least one purified polyethylene fraction.

2. The method according to claim 1, wherein the plastic supply material contains polyethylene in an amount of at least 80% by weight, preferably at least 85% by weight, and preferably at least 90% by weight, relative to the total weight of the plastic supply material.

3. The method according to claim 1 or 2, wherein the dissolving solvent comprises an aliphatic hydrocarbon-based compound, the boiling point of which is 8°C to 100°C, preferably 25°C to 69°C, more preferably 25°C to 61°C, and more preferably 25°C to 40°C.

4. The method according to claim 1 or 2, wherein the dissolution temperature when performing dissolution step a) is 130°C to 200°C, preferably 150°C to 200°C.

5. The method according to claim 1 or 2, wherein the dissolution pressure when performing dissolution step a) is 1.0 to 20.0 MPa absolute pressure, preferably 5.0 to 18.0 MPa absolute pressure, and preferably 10.0 to 17.0 MPa absolute pressure.

6. The method according to claim 1 or 2, wherein the purification step b) comprises at least a sub-step b1) for separating insoluble matter, and optionally a sub-step b2) for washing by contact with a high-density solution, and / or a sub-step b3) for extraction by contact with an extraction solvent, and / or a sub-step b4) for adsorption of impurities by contact with an adsorbent.

7. The method according to claim 1 or 2, wherein the temperature when operating the supercritical separation section in step c) is 190 to 250°C, preferably 200 to 230°C.

8. The method according to claim 1 or 2, wherein the pressure (P supercritical) when operating the supercritical separation section in step c) is 3.0 to 6.0 MPa absolute pressure, preferably 3.0 to 5.0 MPa absolute pressure, and more preferably 3.0 to 4.0 MPa absolute pressure.

9. The method according to claim 1 or 2, wherein step c) comprises 1 to 5 solvent recovery sections, the temperature when operating each solvent recovery section is 160 to 300°C, and the pressure is between the pressure of the supercritical separation section (P supercritical) and 0.000005 MPa, preferably 2.7 MPa to 0.000005 MPa.

10. A device for purifying plastic supply raw materials, including polyethylene, comprising: a) A section for dissolving the plastic supply material in a dissolving solvent; the dissolution temperature during the operation is 150 to 250°C, and the dissolution pressure is 1.0 to 18.0 MPa absolute pressure; at least one crude polymer solution is obtained; and then b) Section for purifying polymer solutions; including: b1) Subsection for separating insoluble matter; and / or b2) Washing of subsections by contact with high-density solution; and / or b3) Extraction subsection by contact with extraction solvent; and / or b4) Subsection for impurity adsorption by contact with adsorbent; and then c) Solvent-polymer separation section; comprising at least one supercritical separation section; operating temperature is 160–300°C and pressure (P supercritical) is 2.7–10.0 MPa absolute pressure; followed by at least one solvent recovery section, operating temperature is 160–300°C and pressure is between the pressure (P supercritical) of the supercritical separation section and 0.000005 MPa (i.e., 5 Pa); at least one purified polyethylene fraction is obtained.