Dechlorination of plastic material in water using overheating

Abstract

The present invention is in the field of recycled plastic materials. In particular, the present invention relates to a method for dechlorinating a chlorine-containing polymer contained in a plastic material using overheating. The invention also relates to a dechlorinated polymer obtainable by this process. The process is carried out in water, during which the pH is at most 4.

Description

Technical Field

[0001] The present invention belongs to the field of recycling and processing of plastic materials. In particular, the present invention relates to a method for dechlorinating chlorine-containing polymers contained in plastic materials such as plastic waste, followed by an optional chloride recovery process. The present invention also relates to a dechlorinated plastic material obtainable by this method. Background Art

[0002] It is widely acknowledged that plastic waste has a negative impact on the environment. Therefore, people are constantly looking for ways to recycle plastics in an attempt to achieve a circular economy (i.e., a closed-loop system in which renewable resources are used and used materials lose as little of their value as possible).

[0003] Typically, the recycling process begins after a plastic product has reached the end of its useful life and is considered waste. The waste is collected and sorted. Sorting typically involves first mechanical sorting by material type (e.g., plastic, paper) and then sorting the separated plastic streams by polymer type, for example using infrared spectroscopy. This results in the creation of several single streams and waste streams. Ideally, a single stream consists essentially of one type of polymer or a plurality of polymers of similar types. However, single streams are often not of sufficiently high purity for materials to be used or interchangeable with pure materials.

[0004] An example of a plastic that is difficult to reuse and recycle is a plastic containing chlorinated polymers such as polyvinyl chloride (PVC). PVC is a very versatile material with a variety of applications, especially in pipes, flooring and window frames. Although several methods have been proposed to improve the recyclability of chlorinated polymers, a large part of used plastics is still incinerated. However, this creates problems caused by chlorine in the flue gas, such as enhanced corrosion and dioxide emissions.

[0005] Several methods that have been proposed to improve recyclability include mechanical recycling, in which the polymer chains are not broken. However, for mechanical recycling, the waste stream provided needs to be clean (i.e., pure). In addition, other problems are that even if the PVC material is "clean", it contains a variety of copolymers, plasticizers, flame retardants, and lubricants in various proportions and compositions based on the intended application. With mechanical recycling, these materials are well blended to become a homogeneous mixture, but do not meet any standards; or they are not well blended and are not homogeneous, which leads to inconsistent product specifications.

[0006] Other methods include feedstock recycling where polymer chains are broken. These include gasification, pyrolysis and dehydrochlorination. Gasification and pyrolysis usually release hydrogen chloride in the hot gas phase, resulting in the production of highly corrosive materials and the risk of forming chlorinated PAHs.

[0007] A gasification process specifically for high chlorine content feedstocks is commercially available in Japan as Ebara Ube Industries Processes (EUP). This involves a relatively low temperature gasification (600-800°C) and a second high temperature gasification (about 1350°C) at a pressure of approximately 10 bar to allow the PVC to be converted into carbon dioxide and synthesis gas. However, this requires high temperatures and significant investment, and needs to be operated on a large scale to be profitable. Also, the process still suffers from corrosive HCl flue gases.

[0008] Another gasification method suitable for PVC is the Sumitomo process, which uses a high temperature packed bed of about 2000°C and a low temperature fluidized bed of about 800-1100°C to produce a synthesis gas stream (see Yamamoto et al. Sustainable Development of Energy, Water and Environmental Systems, 2007, pp. 536-547). However, when PVC is used as a feed, it requires more additional coke or wood as a carbon source due to its relatively low calorific value. In addition, it requires very high temperatures.

[0009] Another class of methods involves dehydrochlorination. This method can be applied in water or in ionic liquids (see in particular Zhao et al. The 5th ISFR, October 11-14, 2009, Chengdu, China). However, ionic liquids are expensive.

[0010] An example of dechlorination in water is described in US 3826789. Here, PVC is added to an aqueous solution of an alkaline inorganic material to react and release chlorine from the PVC.

[0011] Another specific example of dechlorinating PVC in plastic waste mixed in water is described in WO02 / 074845. The method uses mixed plastics heated in batches in an aqueous environment. Chlorine is transferred from the plastics to an alkaline aqueous phase, where the alkali is used to minimize corrosion and accelerate the dechlorination reaction. The method disadvantages include the formation of chloride salts with minimal commercial value as by-products. In addition, it was found that the use of an alkaline substrate disadvantageously increases the oxygen-to-carbon ratio in the dechlorinated product.

[0012] Li et al., Applied Science 7 (2017) 256 describes hydrothermal treatment of PVC at lower alkali dosages. The use of NaOH was found to be helpful for removing chlorine from PVC compared to neutral conditions. However, the use of NaOH has the disadvantage that it is undesirable in chloride recovery processes, which can be applied to chloride streams that are water born from dechlorination processes. Summary of the invention

[0013] The object of the present invention is to provide an improved process for dechlorinating plastic materials containing chlorine-containing polymers, which overcomes at least some of the above-mentioned disadvantages. In particular, the process according to the present invention can be used to provide dechlorinated plastic materials with a low oxygen content. A further advantage of the present invention is the formation of a hydrogen chloride solution with high commercial value.

[0014] Thus, in a first aspect, the present invention relates to a method for dechlorinating a plastic material containing a chlorine-containing polymer, wherein the method comprises providing a plastic material, preferably plastic waste, and water in a container to obtain an aqueous plastic mixture. The plastic material comprises a chlorine-containing polymer. The method further comprises superheating the aqueous plastic mixture to a temperature of at least 210° C. to obtain an at least partially dechlorinated plastic material (also referred to herein as dechlorinated plastic material). Notably, during the superheating, the pH of the water in the aqueous plastic mixture is maintained at at most 4.

[0015] The present invention can be used to dechlorinate any type of plastic material containing chlorine-containing polymers. Plastic materials generally include plastic waste, which can include, for example, uncontaminated or contaminated PVC waste and / or cables with PCT insulation. Plastic waste that can be appropriately used in the present invention can be provided after it is mechanically separated and provided as a single stream. Plastic materials can be subjected to a pretreatment step, such as shredding. It has been found that the size of the plastic material does not significantly affect the dechlorination process. However, the size generally allows enough material to contact with water to allow dechlorination. Therefore, preferably, the plastic material includes the following, more preferably substantially consisting of: fragments with a minimum size of at most 10mm, preferably at most 3mm, more preferably at most 2mm, and most preferably at most 1mm. On the other hand, the plastic material preferably has a maximum size greater than 0.5mm, more preferably greater than 1mm. The terms minimum size and maximum size refer to the different sizes of the fragments, i.e. the width, length and thickness of the fragments. Therefore, the minimum size (usually thickness) of these sizes is at most 10mm, etc., and the maximum size (usually length) of these sizes is greater than 0.5mm, etc. For example, the present invention has been found to be particularly suitable for the treatment of non-powdered plastic materials, for example materials comprising particles having a particle size greater than 0.5 mm, preferably greater than 1 mm. Therefore, these materials are also preferred.

[0016] The plastic material may contain other polymers or additives in addition to the chlorine-containing polymer. These additives are, for example, flame retardants, plasticizers, fillers, etc. These additives may account for up to about 30wt% of the plastic material. Therefore, in a typical embodiment, based on the gross weight of the plastic material, the plastic material comprises at least 50wt%, preferably at least 70wt% of the chlorine-containing polymer. However, there is no technically determined minimum value for the amount of the chlorine-containing polymer contained in the plastic material. In practice, the minimum value will be driven by economic and commercial reasons. Therefore, preferably, based on the gross weight of the plastic material, the plastic material has a chlorine content of at least 10wt%.

[0017] During the process, the above-mentioned additives in the plastic material can be dissolved in the aqueous environment. Other polymers that may be present in the plastic material may be, for example, polyolefins. Advantageously, polyolefins generally have a lower density than water (less than 1 g / cm at 20°C). 3 ). Therefore, these polymers tend to float, and chlorine-containing polymers generally have a higher density than water (greater than 1 g / cm at 20 °C). 3 ), and therefore usually sink. Therefore, these layers can be easily recovered separately.

[0018] The chlorine-containing polymer preferably comprises polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) and / or chlorinated polyethylene (CPE), preferably PVC. Typically, the chlorine-containing polymer has a chlorine content of 20 wt% to 90 wt%, preferably 30 wt% to 80 wt%, based on the total weight of the chlorine-containing polymer. For example, pure PVC has a Cl content of 57 wt%, PVDC of 73 wt%, and CPE in the range of 34-44 wt%, based on the total weight of the polymer.

[0019] In a particular embodiment, the plastic material may be fed into a container already containing water.

[0020] The container used in the present invention is not particularly limited, but should be able to withstand the conditions used in the method of the present invention. For example, an acid-resistant coating such as a perfluorinated coating (e.g., Teflon TM coating) to ensure that container corrosion is minimized.

[0021] The water in the aqueous plastic mixture is superheated to a temperature of at least 210°C. Superheating is a term used to indicate that a liquid (water in this article) is heated to a temperature above its boiling point at atmospheric pressure. The liquid is prevented from boiling under superheat pressure by increasing the pressure. In the method according to the present invention, the pressure can be autogenous. The superheating of the water can be carried out when the plastic material is already in the water or before the plastic material is fed into the water. The water can also be heated or superheated before it is fed into the container. It is usually preferred to superheat the water before mixing it with the plastic material because this may require less energy.

[0022] During the superheating of the aqueous plastic mixture, the water is kept at the superheat temperature for a specific residence time. The residence time allows the removal of chlorine from the plastic material. The optimal residence time may depend on a number of aspects, such as the temperature and the size and composition of the plastic material. Typically, the residence time is at least 5 minutes, preferably at least 15 minutes.

[0023] Superheating the aqueous plastic mixture to a temperature of at least 220° C., specifically at least 230° C., more specifically 230° C. to 250° C. allows for the highest and fastest removal of chlorine from the chlorine-containing polymer. Preferably, the upper limit is about 260° C., since higher temperatures require more energy, and further increases in temperature are generally not associated with significantly higher degrees of chlorine removal.

[0024] The inventors have surprisingly found that adding alkali before or during superheating is not necessary and may even be disadvantageous. As mentioned above, the pH value of the water during superheating is at most 7. Therefore, an acidic environment is used in the method according to the present invention. It is understood that the pH of the water can start from pH 4, but decreases during superheating due to the formation of hydrogen chloride (HCl). HCl is usually dissolved in an aqueous environment, thereby acidifying the water and reducing the pH.

[0025] In addition, before or at the beginning of the overheating, the aqueous plastic mixture is made to have a pH of at most 4 by adding an acid, such as sulfuric acid or hydrochloric acid, to the container. By adding an acid, the pH can also be lower (e.g., less than 3 or even less than 0). It was unexpectedly found that a lower pH does not adversely affect dechlorination. Therefore, the water of the aqueous plastic mixture has a pH of at most 4 during the overheating period. In principle, there is no limit to the pH value at which the acidity of the water can be allowed to decrease due to the formation of HCl and / or the addition of acid. However, it is understood that for chemical engineering reasons, the minimum pH during the method can be limited to, for example, 0 or 1, to avoid, for example, corrosion of equipment. However, it is also understood that corrosion can be avoided by using appropriate equipment (e.g., coated with an acid-resistant coating, see above), and water can be allowed to be saturated with HCl, and any other HCl can be allowed to evaporate from the solution.

[0026] Thus, advantageously, using the present invention, chloride salts are not formed. Instead, HCl is formed by maintaining a pH of less than 4. And, advantageously, HCl has a positive commercial value and can be recovered and purified by any conventional method in the art.

[0027] Another unexpected advantage of the present invention is that by maintaining a pH less than 7, oxygen is not incorporated or only minimally incorporated into the plastic material during the process. Without wishing to be bound by theory, the inventors believe that dechlorination is associated with the formation of carbon-carbon double bonds and crosslinking between carbon atoms. This is in contrast to dechlorination in water at a higher pH value. Therefore, the dechlorinated plastic material obtained by the process of the present invention can have an oxygen-to-carbon (O / C) ratio of less than 0.2, preferably less than 0.15, more preferably 0.1 or less. This is particularly advantageous for further processing of the dechlorinated polymer, especially in pyrolysis, gasification, naphtha cracking, where oxygen is highly undesirable. Typically, any oxygen present originates from additives in the plastic material, such as plasticizers.

[0028] The at least partially dechlorinated plastic material may be recovered from the acidic aqueous solution by any suitable means. Exemplary means are screening, filtering, etc. In certain embodiments, when a high degree of dechlorination is achieved, the dechlorinated plastic material may have a carbon dioxide content of less than 1 g / cm 3 density and will float, thereby separating itself. However, this is usually limited to materials containing relatively large amounts of chlorine-containing polymers and relatively small amounts of materials with a density greater than 1 g / cm 3 Fillers and other additives in plastic materials.

[0029] In a preferred embodiment according to the invention, at least 50% of the chlorine present in the plastic material is removed. By the method of the invention, at least 80%, usually even at least 90% removal can be achieved based on the amount of chlorine initially present in the plastic material. Removal can be increased by increasing the residence time and / or increasing the temperature. Typically, the at least partially dechlorinated plastic material obtainable by the method has a chlorine content of less than 20 wt%, preferably less than 10 wt%, more preferably less than 5 wt%, most preferably less than 1 wt%, based on the total weight of the dechlorinated plastic material. Although this chlorine content may still be too high for direct further processing, especially in pyrolysis, gasification, naphtha cracking, it does reduce the need for post-treatment and dilution before further processing.

[0030] The carbon to oxygen ratio in the dechlorinated plastic material is generally less than 0.2, preferably less than 0.15, more preferably 0.1 or less.

[0031] It will be appreciated that the dechlorination process of the present invention can be carried out in a reactor vessel at a suitable liquid-to-solid ratio (L / S). In this context, the liquid-to-solid ratio refers to the weight ratio of water to plastic material in the vessel. Depending on the composition of the plastic material and the chlorine content of the chlorine-containing polymer and / or plastic material, more or less water is preferred. Thus, the L / S ratio in the vessel is preferably at least 10:1, preferably at least 15:1, more preferably at least 20:1, and most preferably 20:1 to 40:1.

[0032] The process can be carried out batchwise, but a continuous process is preferred. Therefore, the process is preferably carried out in a continuous flow reactor. It is also preferred to carry out the process more than once, i.e. recovering the at least partially dechlorinated plastic material, washing and subsequently mixing the at least partially dechlorinated plastic material as plastic material with a fresh aqueous environment, and superheating the resulting mixture. This may result in more chlorine being recovered from the chlorine-containing polymer.

[0033] When considering the downstream chloride recovery process that may occur, the acidic environment in which the dechlorination of the present invention occurs is particularly advantageous. Chloride recovery can be carried out by electrolysis or ion exchange. However, electrolysis is very energy-consuming, so the ion exchange method is preferred. However, the chloride ion exchange process cannot be carried out under alkaline conditions. In addition, the lean chloride stream obtained after chloride recovery is preferably recycled into the dechlorination process. Advantageously, the dechlorination method of the present invention can therefore be combined with the chloride extraction in the chloride recovery process very easily.

[0034] Therefore, other aspects of the present invention relate to chloride recovery methods, which methods include dechlorination as described herein, followed by a method for chloride extraction. More specifically, the step of overheating the aqueous plastic mixture in the dechlorination method results in the production of a dechlorinated plastic material and a chloride-rich liquid. Conventional solid-liquid separation can be used to separate the dechlorinated plastic material from the chloride-rich liquid. This separation results in the production of a chloride-rich liquid stream, which is subjected to chloride extraction. The chloride extraction preferably includes ion exchange, more preferably includes an ion exchange in which the chloride-rich liquid stream is contacted with an anion exchange resin. This chloride extraction is preferably carried out under acidic conditions, preferably at a pH of at most 4, preferably at most 2.

[0035] By subjecting the chloride-rich liquid stream to chloride extraction, a chloride-depleted liquid stream is obtained. This chloride-depleted liquid stream is typically acidic. In a typical embodiment, this chloride-depleted liquid stream has a pH of at most 4, which makes it suitable for being directly recycled back to the dechlorination process according to the present invention. Therefore, a preferred embodiment of the method for recovering chloride from plastic material comprises extracting chloride from the chloride-rich liquid stream, which extraction results in the production of a chloride-depleted liquid stream, and recycling the chloride-depleted liquid stream back to the container used in the dechlorination process. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 A specific embodiment according to the present invention is shown. Figure 1 A plastic material (1) comprising a chlorine-containing polymer and water (2) are shown introduced into a vessel (10). After superheating the aqueous plastic mixture, a stream (3) comprising dechlorinated plastic and chloride-rich liquid is obtained. The stream (3) is introduced into a solid-liquid separator (20) wherein the dechlorinated plastic (4) is separated from a chloride-rich liquid stream (5). The chloride-rich liquid stream (5) is introduced into a chloride extractor (30) wherein chloride is extracted from the stream (5). This extraction results in the production of an extracted chloride stream (6) and a chloride-depleted liquid stream (7). The chloride-depleted liquid stream (7) is recycled back into the vessel (10).

[0037] For purposes of clarity and concise description, features are described herein as part of the same or separate embodiments, however, it will be understood that the scope of the present invention may include embodiments having all or some combination of the described features. DETAILED DESCRIPTION

[0038] The present invention is illustrated by the following non-limiting examples.

[0039] Example

[0040] Commercially available grey PVC pipes were sawn into pieces (rings, R). The rings were added to water in an autoclave and the water was superheated to the desired temperature for 30 min. The temperatures reached are shown in Table 1.

[0041] In the listed Examples 2 to 7, about 2 grams of PVC and 75 ml of demineralized water were used. In certain examples, only demineralized (desalted) water was used without the addition of acid and / or base. In certain other examples, the amount of sulfuric acid (A) was 1.2 grams. In still other (comparative) examples, an amount of 2.1 grams of sodium hydroxide (B) was added. Due to the release of HCl from the PVC and the subsequent dissolution in water, the examples using only demineralized water were strongly acidified during the test. The final pH was very close to that of the PVC solution with H 2 O. 2 SO 4The pH of the examples above was used as described above.

[0042] For all Examples 2-7, ion chromatography (for Cl) and elemental analyzer (FlashSmart TM The composition in terms of atomic content was determined based on the Dumas method) (for C, H, N and O). In addition, the pH value of the water was measured after superheating. The results are provided in Table 1.

[0043] The results showed that in all examples typically 85-90% of the Cl was removed, with 250° C. being the most effective temperature. Treatment in water, base and acid provided comparable results in Cl removal, with treatment in NaOH being slightly less effective.

[0044] The O / C ratio shows a difference between the treatment in alkali and in acid. When starting in demineralized water, the liquid acidifies rapidly due to the release and dissolution of HCl. In the raw material, the molar ratio of O / C is 0.10-0.11. When the treatment is carried out in alkali, the O / C ratio doubles to 0.19-0.22. When starting the treatment in demineralized water, the O / C ratio remains at 0.10 for the rings, while it drops to 0.04 for the powder. In H 2 SO 4 In solution, the O / C ratio increased slightly to 0.12.

[0045] Table 1

[0046]

[0047] * Grams of Cl recovered per kg of PVC processed

[0048] PVC window frame waste was ground to below 4 mm. About 3.75 grams of ground material was added to 75 ml of desalted water in an autoclave and the water was superheated to 250°C for 30 min (Example 9). The results are provided in Table 2.

[0049] In the listed examples 10 to 12, aqueous solutions of HCl in demineralized water were used at reduced initial pH (pH = 4.0, 3.0 and 2.0 respectively). The final pH was between 0.7 and 0.8.

[0050] The results show that under acidic conditions, typically 85–91% of Cl is removed from PVC, that if the process is initiated under significantly acidic conditions, the incorporation of O into the solid structure is very limited, and that at very low initial pH values ​​(pH < 2.0), the dechlorination efficiency is only slightly affected.

[0051] The results show that preferably, the dechlorination of PVC is done with an initial low pH value (pH < 4.0), so that no oxygen is incorporated into the polymer structure and the chlorine can be further recovered from the liquid as HCl, limiting the working interval of pH to 0.7 to 4.0.

[0052] Table 2

[0053]

[0054] * Grams of Cl recovered per kg of PVC processed.

Claims

1. A method for dechlorinating a plastic material comprising a chlorine-containing polymer, wherein the method comprises: - providing the plastic material comprising the chlorine-containing polymer and water in a container to provide an aqueous plastic mixture having a pH of less than 4; as well as - superheating the aqueous plastic mixture to a temperature of at least 210° C., during which the pH of the water in the aqueous plastic mixture is maintained at most 4, to obtain a dechlorinated plastic material.

2. The method according to the preceding claim, wherein the plastic material comprises at least 50 wt%, preferably at least 70 wt%, of the chlorine-containing polymer, based on the total weight of the plastic material.

3. A method according to any one of the preceding claims, wherein the plastic material is a plastic waste stream.

4. The method according to any one of the preceding claims, wherein the chlorine-containing polymer comprises a chlorinated polyolefin, preferably polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), chlorinated polyethylene (CPE) and / or copolymers thereof, most preferably PVC.

5. The method according to any one of the preceding claims, wherein the chlorine-containing polymer has a chlorine content of at least 20 wt%, preferably 20 wt% to 90 wt%, more preferably 30 wt% to 80 wt%, based on the total weight of the chlorine-containing polymer.

6. A method according to any one of the preceding claims, wherein the plastic material has a chlorine content of at least 10 wt%, preferably at least 20 wt%, more preferably at least 40 wt%, based on the total weight of the plastic material.

7. The method according to any one of the preceding claims, wherein the aqueous plastic mixture is superheated to a temperature of at least 220°C, preferably at least 230°C, more preferably from 230°C to 260°C.

8. The method of any one of the preceding claims, wherein providing the plastic material and water to the container comprises feeding plastic water into the container containing preheated water.

9. The method according to any one of the preceding claims, wherein the aqueous plastic mixture is superheated for at least 5 minutes, preferably at least 15 minutes.

10. The method according to any one of the preceding claims, wherein the plastic material comprises fragments having a smallest dimension of at most 10 mm, preferably at most 3 mm, more preferably at most 2 mm, even more preferably at most 1 mm.

11. A method according to any one of the preceding claims, wherein the weight ratio of water to the plastic material in the container (L / S ratio) is at least 10:1, preferably at least 15:1, more preferably at least 20:1, most preferably the ratio is from 20:1 to 40:

1.

12. A method according to any one of the preceding claims, wherein the dechlorinated plastic material has a chlorine content of less than 20 wt%, preferably less than 10 wt%, more preferably less than 5 wt%, most preferably less than 1 wt%, based on the total weight of the dechlorinated plastic material.

13. A method according to any one of the preceding claims, wherein the dechlorinated plastic material has an oxygen to carbon (O / C) ratio of less than 0.2, preferably less than 0.15, more preferably 0.1 or less.

14. A method for recovering chloride from a plastic material comprising a chlorine-containing polymer, said method comprising the method for dechlorination of a plastic material according to any one of the preceding claims, wherein superheating the aqueous plastic mixture results in the dechlorinated plastic material and a chloride-rich liquid stream, wherein the method for recovering chloride further comprises extracting chloride from the chloride-rich liquid stream.

15. A method for recovering chlorides from plastic materials according to claim 14, wherein extracting chlorides from the chloride-rich liquid stream comprises ion exchange, preferably using an anion exchange resin.

16. Process for recovering chloride from plastic material according to any one of claims 14-15, wherein the extraction of chloride from the chloride-rich liquid stream is carried out at a pH of at most 4, preferably at most 2.

17. A process for recovering chloride from plastic material according to any one of claims 14 to 16, wherein the extraction of chloride from the chloride-rich liquid stream results in a chloride-depleted liquid stream which is recycled back to the vessel used in the process for dechlorination.

18. A dechlorinated plastic material obtainable by a process according to any one of the preceding claims, wherein the dechlorinated plastic material has a chlorine content of less than 5 wt% and / or an oxygen-to-carbon (O / C) ratio of less than 0.2, preferably less than 0.15, more preferably 0.1 or less, based on the total weight of the dechlorinated plastic material.

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