The invention is in the field of recycling and processing of plastic material. In particular, the invention is directed to a method for dechlorination of a chlorine-containing polymer comprised in a plastic material such as plastic waste, followed by an optional chloride recovery process. The invention is further directed to a dechlorinated plastic material obtainable by the method.

It is widely acknowledged that plastic waste has a negative impact on the environment. Methods to recycle plastics are therefore continuously sought after in attempt to achieve a circular economy (i.e. a system of closed loops in which renewable sources are used and used materials lose their value as little as possible).

Typically, a recycling process starts after a plastic product has reached its end-of-life and is considered waste. The waste is collected and sorted. Sorting typically comprises mechanical sorting first by type of material (e.g. plastics, paper), after which the separate plastics stream is sorted by type of polymer, for instance using infrared spectroscopy. This results in several mono streams and a waste stream. A mono stream ideally comprises substantially one type of polymer or a plurality of similar types of polymers. However mono streams typically do not reach a high enough purity for the materials to be used or to be interchangeable with virgin materials.

One example of plastics that are challenging to reuse and recycle are plastics comprising chlorinated polymers, such as polyvinyl chloride (PVC). PVC is a very versatile material that has numerous applications, in i.a. pipes, flooring and window frames. While there are several methods proposed to increase the recyclability of chlorinated polymers, still a large part of the used plastics is incinerated. This however, gives a problem with chlorine in the flue gas, such as enhanced corrosion and emission of dioxides.

Several of the methods that have been proposed to increase the recyclability include mechanical recycling wherein the polymer chains are not broken down. However, for mechanical recycling, the provided waste stream needs to be clean (i.e. pure). Moreover, an additional problem is that even when PVC material is ‘clean’, it contains various co-polymers, plasticizers, fire retardants and lubricants, in various ratios and composition depending on the intended application. With mechanical recycling these materials are well blended, thereby becoming a homogeneous mixture, but not conforming to any standard; or they are not well blended and not homogeneous, which gives rise to inconsistent product specs.

Other methods include feedstock recycling, wherein the polymer chains are broken. This includes gasification, pyrolysis and dehydrochlorination. Gasification and pyrolysis typically release hydrogen chloride in the hot gas phase, resulting in highly corrosive materials and risk of forming chlorinated polycyclic aromatic hydrocarbons.

One gasification method dedicated for high-chlorine-containing feedstock is applied commercially as the Ebara Ube Industries Processes (EUP) in Japan. This includes a relatively low-temperature gasification (600-800 °C) and a second high-temperature gasification (around 1350 °C) at a pressure of roughly 10 bar to allow PVC to convert into carbon dioxide and syngas. However, this requires high temperatures and a significant investment, and a large-scale operation is needed to be profitable. And the method still suffers from corrosive HC1 flue gases.

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

Another type of method includes dehydrochlorination. This process can be applied in water or in an ionic liquid (see i.a. Zhao et al. The 5 ^th ISFR, October 11-14, 2009, Chengdu, China). Ionic liquids are however expensive.

One example of dechlorination in water is described in US3826789. Herein, PVC is added to an aqueous solution of a basic inorganic material to react and release chlorine from the PVC.

Another particular example of dechlorination of PVC in mixed plastic-containing waste in water is described in WO02/074845. The method uses mixed plastics that are heated batch-wised in an aqueous environment. The chlorine is transferred from the plastic to an alkaline aqueous phase wherein the base is used to minimize corrosion and to accelerate the dechlorination reaction. Drawbacks thereof include the formation of a chloride salt as a side product, which has minimal commercial value. Moreover, use of alkaline base is found to disadvantageously increase the oxygen-to-carbon ratio in the dechlorinated product.

Li et al., Applied Science 7 (2017) 256 describe hydrothermal treatment of PVC with a lower alkaline dosage. Compared to neutral conditions, the usage of NaOH was found helpful for chlorine removal from PVC. A drawback of using NaOH however, is that it is undesired in chloride recovery processes that can be applied on the water born chloride stream obtained from the dechlorination process.

It is an object of the present invention to provide an improved method for dechlorinating of plastic material containing a chlorine- containing polymer that overcomes at least part of the above-mentioned drawbacks. In particular, the method according to the present invention may be used to provide a dechlorinated plastic material with a low oxygencontent. An additional advantage of the present invention is that a hydrogen chloride solution is formed, which has a high commercial value.

Thus, in a first aspect the invention is directed to a method for dechlorination of a plastic material containing a chlorine-containing polymer, wherein the method comprises providing a plastic material, preferably a plastic waste, and water in a vessel to obtain an aqueous plastic mixture. The plastic material comprises the 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 (herein also referred to as dechlorinated plastic material). Notably, during the superheating, the pH of the water in the aqueous plastic mixture is maintained at at most 4.

The invention can be used to dechlorinate any type of plastic material comprising chlorine-containing polymer. The plastic material typically comprises plastic waste, which may instance include unpolluted or polluted PVC waste and/or electricity cables with PCT insulation. The plastic waste that can suitably be used for the present invention can be provided after it has been mechanically separated and provided as a mono stream. The plastic material may have been subjected to a pre-treatment step, such as shredding. It was found that the size of the plastic material does not significantly influence the dechlorination process. However, the size is typically such that sufficient material is contacted with the water to allow for the dechlorination. Accordingly, preferably, the plastic material comprises, more preferably consists essentially of, pieces having a smallest dimension of at most 10 mm, preferably at most 3 mm, more preferably at most 2 mm, most preferably at most 1 mm. At the other hand, the plastic material preferably has a largest dimension of more than 0.5 mm, more preferably more than 1 mm. The terms smallest dimension and largest dimension refer to the different dimensions of the pieces, i.e. to the width, length and thickness of the pieces. Hence, the smallest of these dimensions (typically the thickness) is at most 10 mm, etc., while the largest of these dimension (typically the length) is more than 0.5 mm, etc. It was found for example that the present invention is particularly suitable for the treatment of non-powdery plastic materials, e.g. of materials comprising particles having particles sizes of more than 0.5 mm, preferably more than 1 mm. Such materials are accordingly also preferred.

The plastic material can comprise other polymers or additives besides the chlorine-containing polymer. These additives are e.g. fireretardants, plasticizers, fillers and the like. These additives may make up to about 30 wt% of the plastic material. As such, in a typical embodiment 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. There is however, no technically determined minimum for the amount of chlorine-containing polymer that is contained in the plastic material. The minimum in practice will be driven by economics and commercial reasons. Accordingly, preferably, the plastic material has a chlorine content of at least 10 wt%, based on the total weight of the plastic material.

The aforementioned additives in the plastic material may dissolve in the aqueous environment during the method. Other polymers that can be present in the plastic material may for instance be polyolefins. Advantageously, polyolefins typically have a lower density (below 1 g/cm ³ at 20 °C) than water. These polymers therefore tend to float, while the chlorine-containing polymers typically have a higher density (above 1 g/cm ³ at 20 °C) than water, thus typically sinking. The layers may accordingly be easily retrieved independently.

The chlorine-containing polymer preferably comprises polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) and/or chlorinated polyethene (CPE), preferably PVC. Typically, the chlorine-containing polymer has a chlorine content between 20 wt% and 90 wt%, preferably between 30 wt% and 80 wt%, based on the total weight of the chlorine- containing polymer. For examples, 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.

In a particular embodiment, the plastic material can be fed to a vessel already comprising water.

The vessel for use in the invention is not particularly limiting but should be able to withstand the conditions used in the method of the present invention. For instance, an acid-resistant coating such as a perfluorinated coating (e.g. a Teflon™ coating) may be provided to ensure that corrosion of the vessel is minimized.

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, here water, is heated to a temperature higher than its boiling point under atmospheric pressure. Boiling of the liquid at the superheated pressure is prevented with an increased pressure. The pressure may in the method according to the present invention be autogenous. Superheating the water can be done with the plastic material already in the water or prior to feeding the plastic material to the water. The water may also be heated or superheated before it is fed into the vessel. It is typically preferred to have the water superheated before mixing it with the plastic material as this may require less energy.

During the superheating of the aqueous plastic mixture, the water is maintained at the superheated temperature for a particular residence time. The residence time allows for the chlorine to be removed from the plastic material. The optimal residence time may depend on a variety of aspects such as temperature as well as the size and composition of the plastic material. Typically, the residence time is at least 5 minutes, preferably at least 15 minutes. Superheating the aqueous plastic mixture to a temperature of at least 220 °C, particularly at least 230 °C, more particularly between 230 °C and 250 °C allows for the highest and fastest chlorine removal from the chlorine-containing polymers. Preferably, the upper limit is around 260 °C, as a higher temperature requires more energy, while a further increase in temperature is typically not associated with a substantially higher degree of chlorine removal.

The present inventors surprisingly found that adding a base before or during the superheating is not required and may even be disadvantageous. As mentioned above, the pH of the water during the superheating is at most 7. An acidic environment is thus used in the method according to the present invention. It may be appreciated that the pH of the water may start at pH 4 but may drop during superheating due to the formation of hydrogen chloride (HC1). The HC1 typically dissolves in the aqueous environment, acidifying the water and lowering the pH.

Further, prior or at the start of superheating, the aqueous plastic mixture has a pH of at most 4 by the addition of an acid such as sulfuric acid or hydrochloric acid, to the vessel. This pH may also be lower, (e.g. less than 3, or even less than 0) by the addition of an acid,. It was surprisingly found that a lower pH does not adversely affect dechlorination. Accordingly, the water of the aqueous plastic mixture has a pH of at most 4 during the superheating. There is in principle no limit to which pH value the acidity of the water may be allowed to drop as a consequence of the formation of the HC1 and/or the addition of the acid. However, it may be appreciated that for chemical engineering reasons, the lowest pH during the process may be limited to for instance 0 or 1, in order to avoid corrosion of equipment, for example. It may however also be appreciated that corrosion can be avoided by using appropriated equipment (for instance, coated with acid-resistant coating, vide supra) and that the water may be allowed to become saturated with HC1 and that any further HC1 may be allowed to evaporate out of the solution.

Thus, favorably, using the present invention, no chloride salt is formed. Instead, by maintaining a pH of less than 4, HC1 is formed. And favorably, HC1 has a positive commercial value and can be recovered and purified by any conventional means in the art.

Another surprising advantage of the present invention is that by maintaining the pH below 7, oxygen is not or only minimally incorporated in the plastic material during the method. Without wishing to be bound by theory, the present inventors believe that dechlorination is associated with formation of carbon-carbon double bonds and crosslinks between carbon atoms. This in contrast to dechlorination in water at higher pH values Accordingly, the dechlorinated plastic material as obtained by the present method may 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 polymers in i.a. pyrolysis, gasification, naphtha cracking wherein oxygen is highly undesired. Typically, any oxygen present originates from additives such as plasticizers in the plastic material.

The at least partially dechlorinated plastic material may be retrieved from the acidic aqueous solution by any suitable means. Exemplary means are sieving, filtration and the like. In certain embodiment, when a high degree of dechlorination is reached, the dechlorinated plastic material may have a density of less than 1 g/cm ³ and will float, thus separating itself. However, this is generally limited to plastic materials containing a relatively high amounts of chlorine-containing polymers and relatively low amounts of fillers and other additives that have a higher density than 1 g/cm ³ .

In a preferred embodiment according to the present invention, at least 50% of the chlorine present in the plastic material is removed. With the method of the present invention, a removal of at least 80%, typically even at least 90% can be achieved, based on the amount of chlorine originally present in the plastic material. The removal may be increased by an increased residence time and/or an increased temperature. Typically, the at least partially dechlorinated plastic material that is obtainable by this 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. While this chlorine content may still be too high for direct further processing in i.a. pyrolysis, gasification, naphtha cracking, it does reduce the need for posttreatment and dilution before further processing.

The carbon-to-oxygen ratio in the dechlorinated plastic material is typically less than 0.2, preferably less than 0.15, more preferably 0.1 or less.

It can be appreciated that the present dechlorination method may be performed at a suitable liquid to solid (L/S) ratio in the reactor vessel. The liquid to solid ratio herein refers to the weight ratio of the water to the plastic material in the vessel. Depending on the plastic material composition and the chlorine content of the chlorine-containing polymer and/or of the plastic material, more or less water is preferred. The L/S ratio in the vessel is therefore preferably at least 10:1, preferably at least 15:1, more preferably at least 20:1, most preferably the ratio is between 20:1 and 40:1.

The method may be performed batch wise, but a continuous method is preferred. As such, the method is preferably carried out in a continuous-flow reactor. It may also be preferred to perform the method more than once, i.e. retrieve the at least partially dechlorinated plastic material, wash and subsequently mix the at least partially dechlorinated plastic material as plastic material with a fresh aqueous environment and superheat the resulting mixture. This may lead to more chlorine to be recovered from the chlorine-containing polymers. The acidic environment in which the present dechlorination takes place is particularly advantageous when considering the downstream chloride recovery process that can take place. Chloride recovery can take place by electrolysis or ion exchange. Electrolysis is however very energy consuming, for which reason ion exchange processes are preferred. However, chloride ion exchange processes cannot take place under caustic conditions. Moreover, the chloride lean stream that is obtained after chloride recovery is preferably recycled into the dechlorination process. Advantageously, the present method for dechlorination can thus quite readily be incorporated with chloride extraction in a chloride recovery process.

Accordingly, a further aspect of the precent invention is directed to a chloride recovery process, which process comprises the method for dechlorination as described herein, which is followed by chloride extraction. More specifically, the step of superheating the aqueous plastic mixture in the method for dechlorination results in the dechlorination plastic material and a chloride rich liquid. The dechlorination plastic material can be separated from the chloride-rich liquid using conventional solid-liquid separation. This separation leads to a chloride-rich liquid stream, which is subjected to the chloride extraction. Chloride extraction preferably comprises ion exchange, more preferably ion exchange comprising contacting the chloride-rich liquid stream and 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.

By subjecting the chloride-rich liquid stream to the chloride extraction, a chloride-lean liquid stream is obtained. This chloride-lean liquid stream is typically acidic. In typical embodiments, this chloride-lean liquid stream has a pH of at most 4, which makes it suitable for directly recycling it back into the method for dechlorination according to the present invention. Accordingly, a preferred embodiment of the method for chloride recovery from plastic material comprises extracting chloride from said chloride-rich liquid stream which extraction leads to the chloride-lean liquid stream, and which chloride-lean liquid stream is recycled back into the vessel used in the method for dechlorination.

Figure 1 shows a particular embodiment of the present invention. Figure 1 shows that the plastic material (1) comprising the chlorine- containing polymer and water (2) are led into the vessel (10). After superheating the aqueous plastic mixture, a stream (3) comprising the dechlorinated plastic and the chloride-rich liquid is obtained. This stream (3) is led to the solid-liquid separator (20), in which the dechlorinated plastic (4) is separated from the chloride-rich liquid stream (5). This chloride-rich liquid stream (5) is led into the chloride extractor (30), wherein chloride is extracted from stream (5). This extraction leads to an extracted chloride stream (6) and the chloride-lean liquid stream (7). This chloride- lean liquid stream (7) is recycled back to the vessel (10).

For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.

The present invention can be illustrated by the following nonlimiting examples.

Examples

A commercially available grey PVC pipe was sawn in pieces (rings, R). The rings were added to water in an autoclave and the water was superheated to a desired temperature for 30 min. The reached temperature is indicated in Table 1.

In listed examples 2 to 7, about 2 gram of PVC and 75 ml of demineralized water was used. In certain examples, only demineralized (demi) water was used without the addition of acid and/or base. In certain other examples, sulfuric acid (A) in an amount of 1.2 gram. In yet other (comparative) examples, sodium hydroxide (B) in an amount of 2.1 gram was added. The examples with only demineralized water, acidify strongly during the test, due to the release of HC1 from the PVC and subsequent dissolution in the water. The end pH is very close to the pH of the examples that start with H2SO4.

For all examples 2-7, the composition in terms of atomic content was determined using Ion Chromatography for Cl, and an elemental analyzer for C, H, N and O (FlashSmart™ of Intersciences, based on the Dumas method). In addition, the pH-value of the water was measured after superheating. The results are provided in Table 1.

The results show that in all examples, typically 85-90% of Cl is removed with 250 °C being the most effective temperature. The treatment in water, alkaline and acid give comparable results in Cl-removal rate, with treatment in NaOH is slightly less effective.

The O/C ratios show a difference between the treatment in alkaline and the treatment in acid. When starting in demineralized water, the liquid acidifies quickly due to the release and dissolution of HC1. In the feedstock, the O/C molar ratio is 0.10-0.11. When treatment takes place in in alkaline, the O/C ratio doubles to 0.19-0.22. When the treatment is started in demineralized water, the O/C ratio remains at 0.10 in the case of rings, but drop to 0.04 in the case of powder. In H2SO4 solution, the O/C ratio increases slightly to 0.12.

Table 1 gram of Cl recovered per kilogram of PVC treated PVC window frame waste was milled below 4 mm. About 3.75 gram of the milled material was added to 75 ml of demi- water in an autoclave and the water was superheated to 250 °C for 30 min (example 9). The results are presented in Table 2.

In listed examples 10 to 12, an aqueous solution of HC1 in demineralized water was used with decreased initial pH (pH = 4.0, 3.0 and 2.0, respectively). The end pH was between 0.7 and 0.8.

The results show that, typically 85-91% of Cl is removed from PVC under acidic conditions, with the O incorporation in the solid structure being very limited provided that the process starts at significant acidic conditions, while the dechlorination efficiency being only slightly affected at extreme low initial pH value (pH < 2.0).

The results show that is preferable that the dechlorination of PVC is done with starting low pH values (pH < 4.0) so no oxygen is incorporated in the polymer structure and the chlorine can be further recovered as HC1 from the liquid limiting the pH working interval between 0.7 and 4.0.

Table 2 gram of Cl recovered per kilogram of PVC treated