Process for the purification of polymer-based materials

Abstract

The present invention relates to a process for reducing or completely eliminating foreign compounds and pollutants present in polymer pellets made from materials deriving from separate waste collection or industrial waste, so that the recovered polymeric material is ready for industrial reuse and does not generate odours in reuse processes and does not contain toxic residues.

Description

[0001] PROCESS FOR THE PURIFICATION OF POLYMER-BASED MATERIALS

[0002] ***** ***** *****

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a process for reducing or completely eliminating foreign compounds and pollutants present in polymer pellets made from materials deriving from separate waste collection, industrial waste, and in general from polymeric material subjected to a recovery process.

[0005] STATE OF THE ART

[0006] There are currently known technologies aimed at making the recycling of materials deriving from separate waste collection of both civil and industrial waste more economical and advantageous, including polymer-based materials.

[0007] In addition to energy recovery through direct incineration, a practice currently discouraged which will be increasingly discouraged in the future, other technologies for recycling polymer-based materials are known, which can be divided into three main categories:

[0008] 1 ) pyrolysis of the material and recovery of a gaseous fuel;

[0009] 2) depolymerization of the starting material and recovery of monomers to be used for the production process of new polymers; and

[0010] 3) washing and elimination of volatile compounds responsible for toxicity and bad odours of the material, which is then reused directly in the production cycle.

[0011] Applications of the first method are reported, for example, in US patent application 2012 / 0261247 A1 and in patent US 4759300; the first of these documents describes a process for treating plastic waste to provide at least one specific fuel product; the second document describes a process in which waste materials to be pyrolyzed are efficiently dehydrated via microwaves, and then irradiated with a very high intensity laser until methane and other combustible gases are obtained in amounts sufficient to support the pyrolysis reaction in the plant’s burner. Processes of depolymerization of the starting material and recovery of the monomers to be used for the production process of new polymers are described for example in patent US 10160741 B2, relating to a method for the recovery of monomers in particular from polyurethanes (including thermosetting polyurethanes); in patent application WO 1997 / 049652 A1 , relating to a process for the recovery of depolymerization products from polymers such as polyesters, polyamides and polyesteramides, especially when the starting polymer content is lower than about 98%; and in the patents US 6136869 and US 6191177 B1 , which relate to a process (and the related plant) for the recovery of polyester base material usable for new products starting from recycled polyester, which includes the phases of depolymerizing polyester into its monomers and half-esters, separating monomers and half-esters from other secondary materials, and adding monomers to the recovered compounds to produce a low molecular weight polyester.

[0012] In addition to pyrolysis and depolymerization technologies mentioned above, technologies for the recovery of polymer-based materials deriving from the plastic fraction of separate waste collection, which involve washing and removing the volatile compounds from the material and its direct reuse in the productive cycle, are also finding increasing space.

[0013] Processes for the purification of plastics are generally known.

[0014] For example, the patent EP 2780141 B1 relates to a process for the treatment of polyolefin granules in order to remove volatile components. The method comprises the phases of preparing a bed of granules in a liquid inside a treatment vessel; withdrawing a liquid flow, containing hydrocarbons, from the treatment vessel; introducing a first ascending steam flow into the treatment vessel, in which the steam has a temperature from Tb to Tb+10 °C, where Tb is the boiling point of the liquid at the applied pressure; withdrawing a second steam flow, containing volatile hydrocarbon compounds, from the treatment vessel; and recovering the granules from said treatment vessel. The method of this document, however, is only directed to the purification of virgin polymeric material in order to remove production reagent residues, and not to the purification of recycled polymeric materials. For this reason, the pollutants considered in this document are of a fixed chemical nature and limited to a few species, so that it is easy to design the purification process; vice versa, in the cleaning and purification processes of post-consumer plastic materials, the pollutants cover a very broad spectrum of substances and are often not even known, so it is much more complex to define an effective method for purifying recycled plastic materials.

[0015] In this field, US patent 5858105 describes a method for cleaning plastic beverage bottles, which comprises pre-treating the bottles with a concentrated formulation with more than 0.5% by weight of an alkaline agent, then followed by removal of the concentrated solution used in the cleaning in one or more subsequent phases.

[0016] Patent application JP-S-5731529 A describes a method in which the plastic product to be recycled is crushed into fine granules, foreign matter is removed from the fine granules, and then the fine granules are washed with alkaline washing.

[0017] Patent application WO 2020 / 245476 A1 describes a process which comprises a washing phase of the recycled starting material and a stripping phase of the material with a gaseous stream of hot air or steam to favour the draining of volatile compounds from the treated material before the regranulation or reuse phase.

[0018] Finally, patent application EP 3501785 A1 describes a process for recovering polymeric materials to be recycled which comprises a first washing phase of the starting material with a basic aqueous solution, drying the washed material with a hot air stream, and a final extrusion and extraction phase under vacuum, followed by filtration, of the material obtained in the previous step; these last two steps form the normal operating practice of extruders, which typically have vent points for the gases and steam generated during the melting of the material through suction ducts connected to the extruder screw jacket.

[0019] The Applicant observed that, despite intense research activity in the sector, the recycling technologies of polymer-based materials deriving from the plastic fraction of separate waste collection, which involve the washing and removal of volatile compounds from the material and their reuse directly in the production cycle, currently have a series of technical and functional limitations and give not entirely satisfactory results.

[0020] In particular, experimentation carried out by the Applicant has demonstrated that with known methods it is possible to quickly remove the volatile compounds present in the surface layers of the granule from a recovered polymer in granular form, while the time to eliminate also the portion trapped in the innermost material of the granule increase with a quadratic progression as the diameter of the granule itself increases, in accordance with the law of gas diffusion through a poorly permeable solid. The material obtained with known methods does not show any odour when it leaves the process, but when this is then reworked (melted and injected) the gases trapped in the internal portion are released creating a product with generally unpleasant odours which invalidate its use in many applications.

[0021] Furthermore, with the washing systems currently on the market, and in general with all the systems that do not involve depolymerization of the material and subsequent reconstruction of the polymer, the solid or liquid substances trapped in the polymer which, due to interactions with the environment, can undergo chemical alterations and subsequently release volatile compounds with a bad smell or even toxic, are not removed. A typical example is the recycling of the plastic portion of tetrapak-type containers used for the sale of long-life milk. The plastic material absorbs milk fats and proteins, which remain inside the material during the extrusion phase of the recycling process. The material obtained is then used to produce new objects, but over time, following exposure to the atmosphere, they interact with bacteria and yeasts which decompose casein, lactose and fats, conferring the classic smell of rancid milk to the product.

[0022] In particular, the Applicant has observed that one of the major problems encountered using these technologies relates to difficulties in the effective and rapid removal of residues absorbed in the recovered material during and after primary use, particularly in the innermost portions of the material subjected to the regeneration process. In fact, if on the one hand the presence of absorbed residues - even in amounts generally considered negligible - precludes their use, for example, for the creation of packaging with substances for human use, on the other hand it is not economically sustainable for recycling technologies to significantly extend processing time and costs in order to eliminate these residues.

[0023] The need is therefore still felt in the sector to have a technology available for the recycling of polymer-based materials deriving from the plastic fraction of separate waste collection which enables to overcome the issues of the prior art.

[0024] The object of the present invention is to provide a recycling technology for polymer-based materials deriving from the plastic fraction of separate waste collection capable of removing the residues absorbed by the material during and after primary use in the most effective and rapid manner possible, so as to guarantee economically sustainable processing time and costs.

[0025] SUMMARY OF THE INVENTION

[0026] This and other objects are achieved with the present invention, which in a first aspect thereof relates to a process for the recycling of a polymer-based material deriving from the plastic fraction of separate waste collection, comprising the steps of:

[0027] (a) preparing an amount of starting material comprising at least one polymeric material deriving from the plastic fraction of separate waste collection;

[0028] (b) adding to the polymeric material of the previous step a blowing agent selected among: b.1 ) a hydrogencarbonate of an alkali or alkaline earth metal in an amount between 0.1 and 0.5% by weight with respect to the weight of the polymeric material; b.2) a pair of materials consisting of a carbonate and an acidifying compound, wherein the carbonate is present in an amount between 0.2 and 1 % by weight with respect to the weight of the polymeric material; and b.3), a liquid compound with a boiling point lower than 100 °C, injected directly into the extruder head, wherein said liquid compound is added in an amount between 0.3 and 0.6% by weight with respect to the weight of the polymeric material, obtaining in all three cases a mixture;

[0029] (c) extruding the mixture of step (b) obtaining pellets of recycled material;

[0030] (d) carrying out a first extraction by boiling of the pellets obtained in step (c) with a basic aqueous solution, at a pH between 8 and 13, of an oxidizing compound selected among hydrogen peroxide, sodium perborate and sodium percarbonate, at a temperature between 50 °C and 20 °C lower than the softening temperature of the polymeric material, and with an amount of oxidizing compound of between 0.1 and 2% in the case of hydrogen peroxide, between 0.5 and 5% in the case of sodium perborate and between 0.4 and 4% in the case of sodium percarbonate, with respect to the weight of the polymeric material;

[0031] (e) carrying out a second liquid extraction by boiling of the polymeric material treated in step (d) under the same pressure and temperature conditions as in step (d) with a solution containing a hydrogencarbonate of an alkali or alkaline earth metal and an oxidizing compound selected among hydrogen peroxide, sodium perborate and sodium percarbonate, with an amount of oxidizing compound of between 0.1 and 0.8% in the case of hydrogen peroxide, between 0.4 and 4% in the case of sodium perborate, and between 0.3 and 3% in the case of sodium percarbonate, with respect to the weight of the polymeric material;

[0032] (f) separating the polymeric material obtained in step (e) from the present liquid phase;

[0033] (g) subjecting the dried material from step (f) to vacuum extraction at a residual pressure lower than 10 mbar to remove volatile substances from the recovered polymeric material.

[0034] In a second aspect thereof, the present invention relates to a plant for the recycling of a polymer-based material deriving from the plastic fraction of separate waste collection, comprising:

[0035] (A) a unit for extruding the polymer and the reactant according to the methods of step (b) of the process;

[0036] (B) a unit in which the first boiling extraction of step (d) is carried out with a basic aqueous solution of an oxidizing compound;

[0037] (C) a unit in which the second liquid extraction of step (e) is carried out with a solution of hydrogencarbonate of an alkali metal and hydrogen peroxide;

[0038] (D) a unit for separating the polymeric material from the liquid phase used in the previous step by filtration and / or centrifugation (e);

[0039] (E) a vacuum extraction unit for volatile substances suitable for receiving and processing the polymeric material separated in unit (D).

[0040] BRIEF DESCRIPTION OF THE FIGURES

[0041] Figure 1 schematically represents a plant for carrying out the process of the present invention in a first embodiment;

[0042] Figure 2 schematically represents a plant for carrying out the process of the present invention in a second embodiment.

[0043] DETAILED DESCRIPTION OF THE INVENTION

[0044] The Applicant has surprisingly found that, compared to existing technologies which provide for the washing of polymer-based materials and simple deodorization, for example by stripping the material with a gaseous stream of hot air or steam, the process of the present invention allows to obtain a particularly purified polymer material ready for industrial reuse, and such as not to generate odours or presence of toxic residues, such as benzene, in the finished product obtained from said reuse.

[0045] The process of the invention is able to significantly reduce even these types of unwanted substances, up to over 98%, obtaining a higher quality and therefore higher value recycled product; to the knowledge of the inventors, this result is not possible with any other known method of washing and eliminating volatile compounds from plastic materials.

[0046] The present invention may have in one or more of its aspects one or more of the preferred characteristics reported below, which can be combined with each other depending on the application needs.

[0047] The process according to the present invention involves step (a) of preparing an amount of a starting material comprising at least one polymer-based material deriving from the plastic fraction of separate waste collection.

[0048] Preferably said starting material is in the form of flakes obtained by grinding it or as an agglomerate of a plurality of flakes.

[0049] Said amount of starting material may be prepared according to any of the methods known for the purpose to the person skilled in the art.

[0050] The material of step (a) of the process of the invention comprises at least one polymer-based material deriving from the plastic fraction of separate waste collection; more preferably said starting material comprises at least 70% by weight, with respect to the total weight of the grinded material, of said at least one polymer-based material deriving from the plastic fraction of separate waste collection or from industrial waste; even more preferably, said starting material consists of said at least one polymer-based material deriving from the plastic fraction of the separate waste collection.

[0051] The polymer-based material may derive from the plastic fraction of separate waste collection of both civil and industrial waste and typically, but not necessarily or exclusively, contains packaging waste such as food trays and containers, vials and bottles.

[0052] Preferably, the polymer-based material deriving from the plastic fraction of separate waste collection is subjected to one or more preliminary pre-treatment and selection operations before step (a) of the process according to the present invention. Said one or more preliminary operations may, for example, comprise sieving to eliminate any foreign bodies (residues of glass, metal, ceramic, wood, etc.) and washings to remove organic residues present on the surface; the preliminary operations may also comprise the unpacking of previously prepared material and / or direct grinding of said material. These pre-treatments are normally carried out in all plastic material recovery processes and therefore are not part of the present invention.

[0053] In step (b) of the process of the invention, a blowing agent is added to the polymeric material of step (a), which can be one or more hydrogencarbonates of alkali or alkaline earth metals, a pair of materials consisting of one or more carbonates and one or more acidifying compounds, or one or more liquid compounds, according to the possibilities b.1 )-b.3) previously illustrated. In this description and in the claims, “blowing agent” means any compound or composition performing the function of generating gas in the extrusion step (c). Hydrogencarbonates are also known in chemistry by the common name of bicarbonates, which will be used in the rest of the description.

[0054] Blowing agents (or components thereof, when these are made up of mixtures of several compounds) can already be in the form of a solution before being added to the polymeric material but, in case they exist in solid form, they are preferably added in the form of powders, a condition that allows the size of the relevant tanks and therefore of the plant in which the process is carried out to be reduced.

[0055] In cases b.1 ) and b.2) the bicarbonate or carbonate are added in an amount between 0.1 and 0.5% by weight for the bicarbonates and between 0.2 and 1 % for the carbonates with respect to the weight of the polymeric material. Within these ranges, the higher the molecular weight of the salt the higher the percentage of bicarbonates or carbonates. In the case of using bicarbonates, sodium bicarbonate is preferred due to its wide commercial availability. In the case of using a carbonate / acidifying compound pair, the preferred carbonates are those of sodium, potassium, magnesium or calcium (or mixtures thereof), while among the acidifying compounds the preferred ones are solid organic acids, such as > C8 monocarboxylic acids, C2-C7 dicarboxylic acids ( / .e. oxalic to pimelic diacids), and polycarboxylic acids such as citric acid; the acidifying agent is used in a molar ratio of between 0.5 and 1.5 with respect to the carbonate used.

[0056] In case b.3) the blowing agent is selected among CO2, refrigerants from the hydrofluoro-olefin family (indicated in the field by the acronym HFO) such as trans-1 - chloro-3, 3, 3-trifluoro-1 -propene (known as R-1233zd) or trans-1 ,3,3,3-tetrafluoro-l - propene (known as R-1234ze), hydrocarbons with a boiling point below 100 °C or mixtures thereof. In the case of liquid blowing agents, these are injected directly onto the extruder head in amounts between 0.3 and 0.6% by weight with respect to the weight of the polymer to be treated.

[0057] In all three cases b.1 )-b.3) a mixture is obtained, in the first two cases at the extruder inlet, in the third case directly at the material outlet head.

[0058] An advantage in using bicarbonates compared to liquid blowing agents (such as HFO or CO2) occurs in the case of polyolefin recycling. If these materials are highly stressed during the extrusion process, in the presence of oxygen at temperatures above 200 °C they begin to degrade producing large amounts of acetic acid, which is trapped in the material and released over time giving a pungent and unpleasant odour to the material. The presence of bases deriving from bicarbonates decomposition process (for example, NaOH in the case of sodium bicarbonate) allows the stabilization of the acid in sodium acetate, blocking the release of odour over time.

[0059] Remaining within the expanding agent addition ranges indicated above, it is possible to verify with preliminary tests the optimal amount of said compound to be used with the particular fraction of recycled plastic to be treated; these tests consist of extruding a small plastic portion in a laboratory extruder and verifying the reduction of the volatile species contained in the product obtained after carrying out steps (d)-(g), in their turn carried out on a laboratory scale.

[0060] In step (c) the mixture obtained in step (b) is extruded. This step is carried out in a single or double screw extruder or with other mechanical mixing systems capable of bringing the material beyond the softening point and triggering the decomposition reactions of the reagents in cases b.1 ) and b.2), or to bring the compounds of case b.3) to the gaseous state, operating at a temperature higher than 110 °C and depending on the specific polymer or mixture of polymers treated.

[0061] When the material is known, the softening temperature of a polymeric material is known in the literature; alternatively, if the material is not known, this temperature can be estimated with an indicative test on a small sample of the same using the method defined in the UNI EN 1427 standard relating to polymeric materials.

[0062] Step (c) is indicated herein as a separate step for clarity of exposition, but the same could be carried out essentially contextual to step (b); in other words, it is possible to feed the component selected to carry out step (b) (bicarbonate, carbonate / acidifying agent, or blowing compound) into a loading hopper of the extruder into which the polymeric material to be recycled has already been introduced, and proceed with the two components continuously, immediately after adding the component from step (b).

[0063] The mixture is extruded in pellets with a diameter between 3 and 10 mm and a length between 4 and 10 mm at the exit from the extruder, according to methods well known in the field; the pellets are then introduced into a reactor where the next step is carried out.

[0064] Step (d) consists of a first boiling extraction of the mixture obtained in step (c) with a basic aqueous solution of an oxidizing compound as defined above. This step generally lasts between 30 and 120 minutes.

[0065] To carry out the step, the pellets are introduced into a reactor containing preheated water, for example at a temperature between 50 and 20 °C below the softening temperature of the material, in a volume approximately equivalent to that of the pellets themselves.

[0066] In the case of hydrogen peroxide, its amount is of between 0.1 and 2% by weight with respect to the weight of the polymeric material depending on the polymer type and the amount of pollutants. Hydrogen peroxide (H2O2) is not all added at the beginning of the reaction, but continuously under light flow conditions, with the main aim of avoiding an uncontrolled reaction by the developed oxygen; the inventors have also observed that the addition of H2O2 in constant flow mode over time actually halves the reagent consumption by reducing the portion of active oxygen dispersed.

[0067] Hydrogen peroxide is preferably used in the form of an aqueous solution at 35 - 40% by weight.

[0068] The base is added in an amount as such to have a pH of the solution between 8 and 13, and preferably between 8 and 11. The base is preferably sodium bicarbonate, NaHCOs, calcium hydroxide, Ca(OH)2, or even more preferably a mixture of the two compounds.

[0069] The pH, basic but not too high, makes it possible to obtain an optimal balance between the need not to disadvantage the oxidation reactions of the pollutants in the plastic material by H2O2, and to slow down its decomposition to give the oxidant time to reach the molecules to be decomposed.

[0070] A further effect that is obtained using Ca(OH)2 is to be able to sequester (in the form of the respective calcium salts) elements such as fluorine, phosphorus and the like, deriving from the decomposition of toxic compounds present in polymers which can be pesticides or insecticides often present in recovered polymers previously used in agriculture. This effect, when desired, can also be obtained by adding other compounds to the extraction solution, such as aluminium hydroxide, AI(OH)s, which does not contribute in obtaining the basic pH.

[0071] In the case of sodium perborate, one operates in conditions similar to those described above with reference to H2O2, with the amounts of component in relation to the polymeric material indicated above.

[0072] In the case of sodium percarbonate, on the other hand, a basic component (NaHCOs and / or Ca(OH)2) is not necessary, because this oxidizing compound is also basic and is sufficient on its own, in the amounts indicated above, to bring the pH of the solution in the range useful for carrying out step (d).

[0073] The temperature at which this step is carried out is between 50 °C and 20 °C lower than the softening temperature of the polymeric material. Depending on the type of polymer to be recycled, the softening temperature can vary from about 40 °C in the case of polycaprolactone (PCL) up to values greater than 200 °C in the case of some polyamides; consequently, the maximum temperature at which this step can be carried out can vary between about 20 and 200 °C, and preferably between 60 and 150 °C. Once the temperature is defined, the pressure for carrying out step (d) is selected so as to have an extraction process by the aqueous solution under boiling conditions; the pressure values at which it is possible to obtain boiling water at the selected operating temperature can easily be determined by a technician by referring to the phase diagram of water.

[0074] The solution in the reactor is kept under stirring, using one of the possible known techniques, such as mechanical stirring, extracting solution recirculation pump or by ultrasound; it is also possible to operate with combined techniques, in particular coupling the assistance of ultrasound with one of the other techniques to increase extraction efficiency.

[0075] In this step, as mentioned above, the decomposition through oxidation of the pollutants present in the recycled polymeric material takes place.

[0076] At the end of this first liquid extraction operation, the liquid phase is eliminated, the polymer is rinsed with water and sent to the next step.

[0077] In step (e), a second liquid extraction is carried out with a boiling solution under the same pressure and temperature conditions on the polymeric material treated in step (d).

[0078] In this case, a basic solution of an oxidizing compound is used, which may comprise hydrogen peroxide or sodium perborate together with an alkali metal bicarbonate (preferred sodium bicarbonate), or a sodium percarbonate solution. The amount of oxidizing compound used in step (e) is lower than that in step (d); in the case of using a hydrogen peroxide solution, its amount is between 0.1 and 0.8% by weight, preferably about 0.5% by weight, with respect to the weight of the polymeric material; in the case of use of sodium perborate or percarbonate their amount is, respectively, between 0.4 and 4% and between 0.3 and 3% by weight, with respect to the weight of the polymeric material.

[0079] This step completes the extraction of the polymer deriving from recycling and conversion of residual pollutants into compounds that are easier to eliminate in the downstream steps. In particular, the following are carried out:

[0080] - saponification of fats with subsequent solubilisation;

[0081] - breakdown of esters due to the alkaline environment;

[0082] - replacement of halide functional groups with OH groups;

[0083] - reduction in the size of complex polluting substances, which facilitates the subsequent stripping operation of the same from the liquid phase.

[0084] Step (e) lasts between 60 and 120 minutes.

[0085] After the second liquid extraction step (e), the liquid phase is eliminated, the polymer is rinsed with water and sent to the next step, (f), of eliminating rinse water. This step can be performed according to various methods well known in the field and which do not require a detailed description, for example filtration, press-filtration or, preferably, centrifugation. In the preferred case of centrifugation, this is typically carried out with rotation speeds between 100 and 900 rpm, for times between 1 and 10 minutes.

[0086] Finally, the polymeric material recovered at the end of step (f) is subjected, in step (g), to a vacuum extraction operation of the volatile substances indicated in the field as VOC (Volatile Organic Compounds), still present in the material itself.

[0087] The still wet material derived from the previous step is transferred to one or more stripping towers, heated to a temperature about 20 °C lower than the softening temperature of the plastic material, and subjected to vacuum extraction for a period varying between 30 and 120 minutes.

[0088] The final vacuum level reached in this operation is less than 10 mbar, and preferably less than 1 mbar. This vacuum level is reached gradually, in subsequent stages.

[0089] In a first phase, a low vacuum pump is used, generally a liquid ring pump with a water liquid ring, which brings the pressure to a value of about 950 mbar; this value strongly depends on the temperature and the flow of residual pollutants. Pumps of this type are able to better withstand the strong flow of pollutants that occurs in the first part of the step.

[0090] When the pressure value in the system settles on the first pump equilibrium pressure, the next pump is actuated, which is preferably also of the liquid ring type but with the oil ring for high vacuum.

[0091] This second pump can bring the pressure inside the extraction chamber containing the polymer down to a residual pressure of less than 1 mbar.

[0092] The extraction system can be equipped with a sampling of the extracted gases to verify the actual completion of the process based on the analysis of some target compounds with gas-chromatography technology or alternatively with infrared spectroscopy.

[0093] At the end of this step, the polymeric material is recovered and is ready to be sent to reuse processes for producing new products.

[0094] In a second aspect thereof, the invention relates to a plant for carrying out the process described above. The system comprises:

[0095] (A) a unit 3 for extruding the polymer and the reactant according to the methods of step (b) of the process;

[0096] (B) a unit in which the first boiling extraction of step (d) is carried out with a basic aqueous solution of oxygen peroxide;

[0097] (C) a unit in which the second liquid extraction of step (e) is carried out with a solution of hydrogencarbonate of an alkali metal and hydrogen peroxide;

[0098] (D) a unit for separating the polymeric material from the liquid phase used in the previous step by filtration and / or centrifugation (e);

[0099] (E) a vacuum extraction unit for volatile substances suitable for receiving and processing the polymeric material separated in unit (D).

[0100] More in detail, the plant comprises units in which operations corresponding to the operational and transformation steps of the invention are carried out, tanks for the reagents used in the process, valves for the correct movement of the reagents in the plant and dosing units for the same; in the following description, valves and dosing units, well known to those skilled in the art, are not mentioned.

[0101] Fig. 1 shows in schematic form a first possible embodiment of the plant of the invention, in which it is possible to carry out the process described above when the oxidizing compound is hydrogen peroxide or sodium perborate.

[0102] With reference to Fig. 1 , the first unit of the plant, (A), comprises a first tank, 1 , for the polymer to be treated, a second tank, 2, and an extruder 3. The second tank 2 contains the hydrogencarbonate of an alkaline or alkaline earth metal for carrying out step (b) according to mode b.1 , or the carbonate / acidifying agent pair for carrying out mode b.2, or finally a liquid compound for carrying out mode b.3; in the case b.3, the supply of the blowing agent to the extruder is obtained by regulating the flow with a variable valve or a flow meter, or via a pump.

[0103] By appropriately regulating a dispenser (not shown) downstream from tank 1 and a valve, flow meter or pump downstream of tank 2, the polymer and reagent required for the step necessary for step (b) according to one of the methods b.1 - b.3 are fed in the desired ratios to the extruder 3 through the lines L1 and L2, respectively, for carrying out step (c) of the process of the invention. At the outlet of extruder 3, the mixture is transformed into pellets which are then solidified by forced cooling.

[0104] Unit (B) comprises a reactor 4 and two tanks 5 and 6, respectively containing the aqueous solution of hydrogen peroxide or sodium perborate (in powder form or in turn in solution) and a basic compound, solid or in solution, to bring the pH of the reagent system to a value between 8 and 11 . The mixture pellets exiting the extruder 3 are fed, through line L3, to reactor 4 in which the first extraction by boiling is carried out (step (d) of the process). The hydrogen peroxide solution or the sodium perborate in tank 5 and the basic solution in tank 6 are fed to reactor 4 through lines L4 and L5, respectively.

[0105] The pellets that have undergone the first liquid extraction in unit (B) are sent to the second liquid extraction which takes place in unit (C) via line L6 exiting reactor 4; the washing liquid phase exiting reactor 4 is instead sent via line L7, which flows into a collector line L8 common to various units of the system, to the recovery treatments.

[0106] Unit (C) comprises a second reactor 7, tank 5 which is also part of unit (B) and a second tank 8; tanks 5 and 8 respectively contain the oxidant (hydrogen peroxide solution or sodium perborate, solid or in solution) and the basic compound, an alkaline or alkaline earth metal bicarbonate (also solid or in solution) and are connected to reactor 7 through lines L9 and L10, respectively.

[0107] The pellets that have undergone the second liquid extraction in unit (C) are sent, through line L11 , to the separation unit (D), while the washing waters are sent via line L12 to the collector line L8 for post-process treatment.

[0108] The separation unit (D) consists of an apparatus which is represented in the figure as a centrifuge (9), but as mentioned it could be a filter or a press-filter. The liquid separated in the unit (D) is sent through the collector line L8 to the post-process treatment, while the pellets are sent via line L13 to one or more stripping towers 10 (only one is shown in the figure), where the vacuum removal of the volatile components still present in the pellets of recycled polymeric material is carried out. The pellets thus purified from VOCs are recovered through line L14, while VOCs are extracted from tower 10 through lines L15 and L16 connected to intermediate vacuum and high vacuum pumps (not shown in the figure).

[0109] In an alternative embodiment, illustrated in Fig. 2, the plant is suitable for carrying out the process of the invention in the case in which the oxidizing compound is sodium percarbonate that, as previously mentioned, is also a basic and soluble compound; in Figures 1 and 2, elements having the same reference number have the same function. In the system of Fig. 2 it is not necessary to provide separate tanks for the addition of basic compounds, and the tank 5’, containing sodium percarbonate (also in this case solid or in solution form), performs the functions of the tanks 5, 6 and 8 of Fig. 1 .

[0110] The invention will be further illustrated by the following examples.

[0111] MATERIALS, EXPERIMENTAL CONDITIONS AND METHODS

[0112] EXAMPLE 1

[0113] 10 kg of flakes of PE, PP and mixed material of the two polymers deriving from the selection of the separate waste collection of plastic material, was previously sieved to eliminate any foreign bodies (residues of glass, metals, ceramics, wood, etc.) and subjected to preliminary washing to remove organic residues present on the surface.

[0114] The material thus treated was placed into a 20-liter dosing hopper to feed the extrusion apparatus.

[0115] A second 5-liter dosing hopper was filled with 4 kg of NaHCOs of 99.3% purity.

[0116] The material was extruded with a laboratory extruder (model MD30, Bausano & Figli S.p.A., Rivarolo Canavese (TO), Italy) under the following conditions:

[0117] - polymer feed: 2 kg / min;

[0118] - NaHCOs feed: 8 gr / min;

[0119] - material inlet temperature: room T;

[0120] - material outlet temperature before pellet head cutting: 240 °C;

[0121] - head cutting with air cooling.

[0122] The material obtained was placed into a 30-liter container together with 12 liters of water, 8 g of Ca(OH)2 and 28 g of a 35% aqueous H2O2 solution uniformly dosed in the first 30 minutes of temperature treatment.

[0123] The solution was brought to boiling by electrical heating and maintained at 98 - 100 °C e 990 - 1020 hPa for 60 minutes under constant mechanical stirring. The container was equipped with a lid, in this case not gastight, and the evaporated water was replenished during the process.

[0124] At the end of the previous phase, the perforated basket containing the pellets was removed, these were rinsed with water and the basket was placed in a second container similar to the previous one in which 12 liters of water, 6 g of NaHCOs and 20 g of 35% aqueous H2O2 solution, uniformly dosed in the first 30 minutes of treatment at temperature, were added.

[0125] The solution was brought to boiling by electrical heating and maintained at 98 - 100 °C and 990 - 1020 hPa for 120 minutes under constant mechanical stirring. The container was equipped with a lid, in this case not gastight, and the evaporated water was replenished during the process.

[0126] At the end of this phase, the perforated basket containing the pellets was extracted, these were rinsed with water and dried, draining the excess water.

[0127] The material was then transferred to a heated, gastight hopper where it was first dried for 120 min at 70 °C with an air flow of 10 liters / m inute to remove moisture, and subsequently subjected to vacuum stripping in a two-phase operation, carried out under the following conditions:

[0128] - phase 1 : 60 min at 80 °C with residual chamber pressure equal to 50-100 mbar;

[0129] - phase 2: 60 min at 80 °C with residual chamber pressure equal to 1 -4 mbar.

[0130] The indicated pressure levels were obtained in phase 1 with a liquid ring pump with water liquid ring (model TMR, Pompetravaini SpA, Castano Primo (Ml), Italy) and in phase 2 with a two-stage liquid ring pump for high vacuum (model RC4M, D.V.P. Vacuum Technology SpA, San Pietro in Casale (BO), Italy).

[0131] EXAMPLE 2

[0132] For comparison, a sample was prepared starting from the same initial material and following a procedure identical to that described in Example 1 but carrying out a simple extrusion of the recycled material without any addition of chemical compounds (oxidizing and basic compounds) which constitute the characteristic of the present invention, and without proceeding with the liquid phase extractions.

[0133] EXAMPLE 3

[0134] Gas release tests were carried out on the samples prepared in Examples 1 and 2. The tests were carried out using GC-MS measurements with solid phase microextraction (SPME) sampling carried out at a temperature of 100 °C.

[0135] The following equipment was used for the tests:

[0136] - analytical scale to the ten-thousandth of a gram;

[0137] - TD GC-MS / MS system Centers: Agilent Technologies - 7820A; Agilent Technologies-MDD 5977B;

[0138] - column SUPELCO SPB-624, I.D. 20 m x 0.18 mm, df 1.00 pm;

[0139] - Divinylbenzene / carboxen / polydimethylsiloxane (DVB / CAR / PDMS) SPME system for use with autosampler, needle size 23 ga, metal alloy fiber, length 1 cm.

[0140] The analysis procedure involved a preliminary sampling of the instrument headspace, under the following conditions:

[0141] - sample incubation: 5 min at 100 °C;

[0142] - headspace sampling volume: 5 mL;

[0143] - cryogenic trap: Material Emission, Markes International; loading temperature 10 °C; desorption temperature 250 °C; total split: 13.5:1.

[0144] The conditions under which the chromatographic tests were carried out are the following:

[0145] - initial column temperature: 40 °C for 5 minutes;

[0146] - temperature program: 10 °C / min up to 80 °C, 25 °C / min up to 220 °C, isotherm at 220 °C for 5 min;

[0147] - carrier gas flow rate (He): 1 .2 mL / min.

[0148] The mass spectrometer parameters were as follows:

[0149] - source temperature: 230 °C;

[0150] - interface temperature: 150 °C;

[0151] - solvent delay time: 3.0 min;

[0152] - acquisition mode: Full Scan (mass range 50-450 m / z).

[0153] The identification of the eight most significant compounds was carried out by evaluating the areas of the chromatographic peaks measurable in the GC-MS chromatogram obtained from the analysis of the treated sample (produced in Example 1 ) and the untreated sample (Example 2). An analysis of a control blank was also carried out to verify the presence of any spurious contributions to the recorded signals. The identification of the compounds was carried out by comparing the experimentally mass spectrum obtained with the mass spectrum available in the NIST database (version 2. Of, April 2009).

[0154] The results of the GC-MS analyses are shown in the following table: The data in the table demonstrate that following the treatment, practically all the compounds responsible for the odour released by the material are quantitatively reduced and are not released as volatile compounds during the reworking process.

Claims

CLAIMS1 . Process for the recycling of a polymer-based material deriving from the plastic fraction of separate waste collection, comprising the steps of:(a) preparing an amount of starting material comprising at least one polymeric material deriving from the plastic fraction of separate waste collection;(b) adding to the polymeric material of the previous step a blowing agent selected among: b.1 ) a hydrogencarbonate of an alkali or alkaline earth metal in an amount between 0.1 and 0.5% by weight with respect to the weight of the polymeric material; b.2) a pair of materials consisting of a carbonate and an acidifying compound, wherein the carbonate is present in an amount between 0.2 and 1 % by weight with respect to the weight of the polymeric material; and b.3), a liquid compound with a boiling point lower than 100 °C, injected directly into the extruder head, wherein said liquid compound is added in an amount between 0.3 and 0.6% by weight with respect to the weight of the polymeric material, obtaining in all three cases a mixture;(c) extruding the mixture of step (b) obtaining pellets of recycled material;(d) carrying out a first extraction by boiling of the pellets obtained in step (c) with a basic aqueous solution, at a pH between 8 and 13, of an oxidizing compound selected among hydrogen peroxide, sodium perborate and sodium percarbonate, at a temperature between 50 °C and 20 °C lower than the softening temperature of the polymeric material, and with an amount of oxidizing compound of between 0.1 and 2% in the case of hydrogen peroxide, between 0.5 and 5% in the case of sodium perborate, and between 0.4 and 4% in the case of sodium percarbonate, with respect to the weight of the polymeric material;(e) carrying out a second liquid extraction by boiling of the polymeric material treated in step (d) under the same pressure and temperature conditions as instep (d) with a solution containing a hydrogencarbonate of an alkali or alkaline earth metal and an oxidizing compound selected among hydrogen peroxide, sodium perborate and sodium percarbonate, with an amount of oxidizing compound of between 0.1 and 0.8% in the case of hydrogen peroxide, between 0.4 and 4% in the case of sodium perborate, and between 0.3 and 3% in the case of sodium percarbonate, with respect to the weight of the polymeric material;(f) separating the polymeric material obtained in step (e) from the present liquid phase;(g) subjecting the dried material from step (f) to vacuum extraction at a residual pressure lower than 10 mbar to remove volatile substances from the recovered polymeric material.

2. Process according to claim 1 , wherein in step (b):- in case b.1 ) said hydrogencarbonate is sodium hydrogencarbonate;- in case b.2) said carbonate is selected among sodium, potassium, magnesium and calcium carbonate or mixtures thereof, the acidifying compound is selected among > C8 monocarboxylic acids, C2-C7 dicarboxylic acids, citric acid and mixtures thereof, and is used in a molar ratio of between 0.5 and 1 .5 with respect to the carbonate; and- in case b.3) the blowing agent is selected among CO2, refrigerants from the hydrofluoro-olefin family, hydrocarbons with a boiling point below 100 °C and mixtures thereof.

3. Process according to any one of claims 1 or 2, wherein step (c) is performed subsequently to or simultaneously with step (b).

4. Process according to any one of the preceding claims, wherein step (d) is carriedout at a pH of between 8 and 11 under stirring by mechanical stirring, stirring with a recirculation pump, ultrasound or a combination of these techniques.

5. Process according to any one of the preceding claims, wherein step (d) is carried out with an aqueous solution of hydrogen peroxide at 35 - 40% by weight to which sodium hydrogencarbonate, calcium hydroxide or a mixture thereof is added, and the hydrogen peroxide aqueous solution is added under continuous flow conditions.

6. Process according to any one of claims 1 -4, wherein step (d) is carried out with an aqueous solution of sodium perborate to which sodium hydrogencarbonate, calcium hydroxide or a mixture thereof is added.

7. Process according to any one of the preceding claims, wherein step (f) is carried out by filtration, press-filtration or centrifugation.

8. Process according to any one of the preceding claims, wherein step (g) is carried out in one or more stripping towers, at a temperature about 20 °C lower than the softening temperature of the plastic material, and said pressure lower than 10 mbar is reached in at least two subsequent phases, a first phase carried out with a low vacuum pump which brings the pressure to a value of about 950 mbar, and a second phase carried out with a high vacuum pump, which is activated when the pressure obtained in the system with the low vacuum pump has reached an equilibrium value.

9. Process according to any one of the preceding claims, wherein step (a) is preceded by one or more preliminary operations for pre-treatment of the polymer- based material deriving from the plastic fraction of separate waste collection, saidone or more preliminary operations selected among sieving to eliminate foreign bodies and washings to remove organic residues present on the surface of the polymer-based material.

10. Plant for the recycling of a polymer-based material deriving from the plastic fraction of separate waste collection, comprising:(A) a unit for extruding the polymer and the reactant according to the methods of step (b) of the process;(B) a unit (4) in which the first boiling extraction of step (d) is carried out with a basic aqueous solution of an oxidizing compound;(C) a unit (7) in which the second liquid extraction of step (e) is carried out with a solution of hydrogencarbonate of an alkali metal and hydrogen peroxide;(D) a unit (9) for separating the polymeric material from the liquid phase used in the previous step by filtration and / or centrifugation (e); and(E) a vacuum extraction unit (10) for volatile substances suitable for receiving and processing the polymeric material separated in unit (D).

Images