A method of treating a process waste stream contaminated with fluorinated hydrocarbons

Abstract

A method comprises adding a fluoride anion scavenger to the waste stream, adding a base and then heating to de-fluorinate fluorinated hydrocarbons and liberating the fluoride ions, which are scavenge

Description

Field This specification relates to methods of treating process waste streams contaminated with fluorinated hydrocarbons. The methodology as described herein is particularly applicable to waste streams generated during recycling of waste ionomer components including waste catalyst coated membranes such as those used in fuel cells and hydrogen producing water electrolysers. The method is also applicable to waste streams generated during manufacture of ionomer components including catalyst coated membranes such as those used in fuel cells and hydrogen producing water electrolysers. Background Fuel cell and hydrogen producing water electrolyser production is set for rapid growth as investment is placed into the global hydrogen economy. Catalyst coated membranes (CCMs) are a major functional component of both fuel cells and electrolysers. Such CCMs generally comprise a conductive polymer membrane coated on either side by a catalyst containing layer. The CCMs are configured to drive oxidation and reduction reactions and support proton and electron transport, these processes been required for the fuel cell and electrolyser technologies to function. While variations in CCM component materials and configurations exist according to functional performance requirements in end use applications, they generally contain several components of value including one or more platinum group metal (PGM) catalysts and one or more proton conducting polymers. Typically, the membrane is formed of one or more ionomers such as perfluorosulfonic-acid (PFSA) ionomers. Ionomer may also be provided in one or both of the catalyst layers. The ionomer in the catalyst layers may be the same or different to the ionomer in the main membrane component and / or in the other catalyst layer(s). A CCM may comprise two different catalysts, one for driving an oxidation reaction on one side of the CCM and one for driving a reduction reaction on the other side of the CCM. A CCM may also comprise a recombination catalyst which is provided to catalyse the recombination of hydrogen and oxygen to form water, reducing the quantity of hydrogen crossing the membrane and mixing with oxygen to form a potentially explosive mixture. A CCM may also include a metal oxide (e.g., CeOj) as a peroxide scavenger. CCM catalysts can be based on platinum group metals such as platinum, ruthenium, iridium, palladium, or mixtures thereof. The platinum group metals may be provided in elemental (metallic) form, in compound form (e.g., an oxide, such as an iridium oxide catalyst), or as a PGM-base metal alloy (e.g., PtCo). Furthermore, the PGM catalyst materials may be supported on a substrate material (e.g., carbon, such as a platinum-on-carbon catalyst comprising particles of carbon on which platinum is disposed or PtCo-on-carbon). Catalyst coated membranes (CCMs) can also be provided in combination with additional functional layers to form multi-layer membrane electrode assemblies (MEAs). Such MEAs may have 3, 5, or 7 layers for example. With the increase in CCM manufacture for fuel cells and electrolysers, there is an associated increase in CCM waste materials, including a significant volume of scrap material created during CCM manufacture (e.g., due to failure at quality control) and also an increase in end-of-life (EoL) CCMs. Since CCMs contain several components which are rare and / or valuable, including platinum group metals (notably Pt, Pd, Ir and Ru) and ionomer (both in the membrane and catalyst layers), there is a growing demand for methods of recycling such components from waste CCM materials. One current method to recover PGMs from production scrap and end-of-life CCM material involves incineration. The incineration process yields a PGM rich (typically Pt and Ir) ash which is processed via conventional PGM refining routes. However, the incineration process releases harmful and toxic gases such as CO2 and HF from the polymers that are part of the membrane. Both these gases have negative impacts as they pollute the atmosphere, increase the greenhouse effect, and / or have harmful effects in the human body. As such, there is a need for a cleaner process which reduces or eliminates the emission of these gases. In addition to the above, the incineration method destroys the ionomer component which also has significant value. As such, it would also be desirable to provide a process which is capable of recovering both PGM and ionomer components as well as providing a process which is cleaner, safer, and more environmentally friendly. Processes for recovering perfluorosulphonic acid ionomer are known. See, for example, WO2016 / 156815 and US7255798. Furthermore, processes for recovering individual PGM catalyst components are known. See, for example, US7709135. However, to enable fuel cells and electrolysers to become more sustainable technologies, there is a need for commercially viable and environmentally friendly routes to manufacture CCM materials and also to recover, separate, and recycle both the PGMs and the ionomer components from waste CCM materials including production scrap and end-of-life material. It is an aim of the present specification to address this problem. Summary of Invention One potential problem when manufacturing or recycling CCM materials is the generation of process waste streams which are contaminated with fluorinated hydrocarbons such as per- and / or polyfluoroalkyl substances (PFAS). For example, the recycling process of PFSA ionomer may leave ionomer remnants with the PTFE reinforcement degradation products and small molecule degradation product. As such, a method of treating such waste streams to remove such contaminants is required. It has been noted that prior art documents such as US2023002297 and WO2010039820 describe depolymerisation techniques for treating fluorinated polymer waste materials. However, it is desired to provide a more accessible route to mineralisation of PFSA and PFAS wastes from a PFSA ionomer recycling processes than these previously described depolymerisation techniques. It has also been noted that prior art documents describe the hydrothermal alkaline treatment of PFASs present in Aqueous Film-Forming Foam used in firefighting. See, for example, "Destruction of PFAS in AFFF-impacted fire training pit water, with a continuous hydrothermal alkaline treatment reactor, Chemosphere, Volume 314, February 2023, 137681" and "Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Aqueous Film-Forming Foam, Environ. Sci. Technol. 2021, 55, 5, 3283-3295, February 8, 2021)". This literature has shown that small PFAS molecules can be mineralised in alkaline solutions under hydrothermal conditions. In light of the above, the present specification provides a method of treating a process waste stream contaminated with fluorinated hydrocarbons, the method comprising: adding a fluoride anion scavenger to the process waste stream; adding a base to the process waste stream; and heating the process waste stream after addition of the fluoride anion scavenger and the base to defluorinated the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger. The base and fluoride anion scavenger are two different components which can be added in either order or together. The base can be selected to optimize liberation of fluoride anions from the fluorinated hydrocarbons while the fluoride anion scavenger can be selected to optimize the scavenging of liberated fluoride anions. An advantage of the present approach is that the fluoride anions liberated from the fluorinated hydrocarbons can be captured and mineralized to achieve a safe output waste stream. Brief Description of the Drawings For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a flow diagram of a method for treating a process waste stream contaminated with fluorinated hydrocarbons according to the present specification; and Figure 2 shows a flow diagram of a method for recycling a waste catalyst coated membrane component according to the present specification. Detailed Description As described in the summary section and shown in the flow diagram of Figure 1, the present specification provides a method of treating a process waste stream contaminated with fluorinated hydrocarbons, the method comprising: adding a fluoride anion scavenger to the process waste stream; adding a base to the process waste stream; and heating the process waste stream after addition of the fluoride anion scavenger and the base to defluorinate the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger. The fluorinated hydrocarbons contaminating such process waste stream may be per- and / or polyfluoroalkyl substances (PFAS). The base can be a hydroxide such as sodium hydroxide or lithium hydroxide. The pH of the process waste stream after addition of the base can be: at least 8, 9, or 10; no more than 14,13, or 12; or within a range defined by any combination of the aforementioned lower and upper limits. The process waste stream can be heated to a temperature of: at least 200°C, 250°C, or 300°C; no more than 500°C, 400°C, 350°C; or within a range defined by any combination of the aforementioned lower and upper limits. Furthermore, the process waste stream can be heated in a pressurized vessel at a pressure of: at least 1, 5, 10, or 15 MPa; no more than 30, 25, or 20 MPa; or within a range defined by any combination of the aforementioned lower and upper limits. Various methods for scavenging the fluoride anions are possible. For example, the fluoride scavenger can form a soluble fluoride species or be a precipitant or an adsorbent which forms a solid material comprising the fluoride anions. Suitable soluble fluoride species are soluble fluoride salts (e.g., NaF, KF) which can be subsequently washed from ionomer dispersion and / or solid residues such as carbon support or PTFE reinforcement. Suitable precipitants include metal compounds which react with the fluoride anions in solution to form a solid metal fluoride salt which is precipitated from solution. Such metal compounds may be selected from one or more of: an alkaline earth metal compound; a calcium compound; a magnesium compound; a strontium compound; a transition metal compound; a titanium compound; a post-transition metal compound; an aluminium compound; a hydroxide. For example, a calcium compound such as calcium hydroxide can be used to mineralise the liberated fluoride anions as CaFj. The precipitated metal fluoride can then be separated from the process waste stream via filtration or centrifugation. In the case of using an adsorbent as the fluoride scavenger, the adsorbent can be a solid, insoluble metal compound which adsorbs fluoride anions. Such an adsorbent metal compound can be selected from one or more of: a metal oxide; magnesium oxide; calcium oxide; aluminium oxide; titanium oxide; a mixed metal oxide. The adsorbent can be separated from the process waste stream via filtration or centrifugation after adsorbing the fluoride anions. The above-described methodology can be used to treat process waste streams from a waste ionomer membrane recycling process and / or from an ionomer membrane manufacturing process (e.g., a catalyst coated ionomer membrane recycling process and / or a catalyst coated ionomer membrane manufacturing process). The recycling process for PFSA ionomer may leave ionomer remnants with the PTFE reinforcement degradation products and small molecule degradation product. Using the present approach provides a safe and reusable output stream rather than PFAS solutions or PFAS immobilised on solid supports which would require incineration. Furthermore, this method provides a more accessible route to mineralisation of PFSA and PFAS wastes from the PFSA ionomer recycling processes than previously described depolymerisation techniques described in US2023002297 and WO2010039820. An advantageous option is to use a source of calcium which forms insoluble CaF species to ensure that the fluoride liberated from the PFAS substance is mineralised. Ca(OH) or CaO can be used as soluble and insoluble precursors. Figure 2 shows a flow diagram of a method for recycling a catalyst coated membrane according to the present specification. The catalyst coated membrane is subjected to acid leaching as described in WO2023 / 247913 to remove and recover platinum group metal components. The acid leaching may, for example, include an oxidative acid leach to recover platinum and a reductive acid leach to recover iridium. The solid ionomer membrane material is then treated with a base such as NaOH or LiOH to convert the ionomer to salt form without dispersing the solid ionomer membrane material. The ionomer salt is more readily dispersed when in such salt form. Furthermore, as the saltation is performed without undue heating to disperse the ionomer, excess base can readily be removed via solid-liquid separation to avoid contaminating further process steps. The solid ionomer membrane material is then dispersed in a solvent such as water, an alcohol, or a mixture of water and an alcohol. This is typically done in an autoclave at elevated pressure. The resultant ionomer dispersion will typically include membrane reinforcement materials such as ePTFE and catalyst support materials such as carbon. These can be removed from the ionomer dispersion by, for example, centrifugation or filtration. The ionomer can then be subjected to an ion-exchange step to re-protonate the acid groups of the ionomer and freeze dried to yield a solid ionomer product ready for re-use. One or more of the aforementioned process steps can generate a process waste stream contaminated with PFAS and the present methodology can be used to ensure that the PFAS is stripped of fluorine by a basic hydrothermal treatment and that liberated fluoride anions are mineralized, e.g., using a fluoride scavenger such as a calcium compound to mineralise the liberated fluoride as CaFj. According to certain embodiments, the step of heating the process waste stream may be done in the presence of a platinum group metal (e.g., metallic Pt) in addition to a base and a fluoride anion scavenger. It has been found that platinum group metals can promote defluorination of fluorinated hydrocarbons. According to certain embodiments, additional process steps can be applied to the process waste stream after the step of heating the process waste stream to defluorinated the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger. For example, once side chains have been cleaved from the ionomer under hydrothermal conditions in the presence of excess base and optionally platinum group metal (e.g., metallic Pt), the resulting PFAS fragments will been mineralised under basic conditions by the fluoride sink and / or washed from the largely PTFE material using membrane filtration. The remaining PTFE backbone of the ionomer (and PTFE reinforcement) will be free from sulfate, which means that it can be subject to a single-stage thermal depolymerisation process described in WO2010039820A2 to generate TFE building block for PTFE remanufacture. If not free from sulfate, a dual stage thermal decomposition would be necessary as described in WO2021130626A1, which is more energy intensive and yield is compromised. As such, combining the presently described methodology with a single-stage thermal depolymerisation process can result in a more efficient route to generating TFE building blocks from recycle streams for PTFE remanufacture. While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of treating a process waste stream contaminated with fluorinated hydrocarbons, the method comprising:adding a fluoride anion scavenger to the process waste stream;adding a base to the process waste stream; andheating the process waste stream after addition of the fluoride anion scavenger and the base to defluorinate the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger.

2. A method according to claim 1,wherein the fluorinated hydrocarbons are per- and / or poly-fluoroalkyl substances (PFAS).

3. A method according to claim 1 or 2,wherein the base is a hydroxide.

4. A method according to claim 3,wherein the base is sodium hydroxide or lithium hydroxide.

5. A method according to any preceding claim,wherein the pH of the process waste stream after addition of the base is: at least 8, 9, or 10; no more than 14,13, or 12; or within a range defined by any combination of the aforementioned lower and upper limits.

6. A method according to any preceding claim,wherein the process waste stream is heated to a temperature of: at least 200°C, 250°C, or 300°C; no more than 500°C, 400°C, 350°C; or within a range defined by any combination of the aforementioned lower and upper limits.

7. A method according to any preceding claim,wherein the process waste stream is heated in a pressurized vessel at a pressure of: at least 1, 5,10, or 15 MPa; no more than 30, 25, or 20 MPa; or within a range defined by any combination of the aforementioned lower and upper limits.

8. A method according to any preceding claim,wherein the fluoride scavenger is a precipitant or adsorbent which forms a solid material comprising the fluoride anions.

9. A method according to claim 8,wherein the precipitant is a metal compound which reacts with the fluoride anions in solution to form a solid metal fluoride salt which is precipitated from solution.10 A method according to claim 9,wherein the metal compound is selected from one or more of: an alkaline earth metal compound; a calcium compound; a magnesium compound; a strontium compound; a transition metal compound; a titanium compound; a post-transition metal compound; an aluminium compound; a hydroxide.

11. A method according to claim 9 or 10,wherein the precipitated metal fluoride is separated from the process waste stream via filtration or centrifugation.

12. A method according to claim 8,wherein the adsorbent is a solid, insoluble metal compound which adsorbs fluoride anions.

13. A method according to claim 12,wherein the solid metal compound is selected from one or more of: a metal oxide; magnesium oxide; calcium oxide; aluminium oxide; titanium oxide; a mixed metal oxide.

14. A method according to claim 12 or 13,wherein the adsorbent is separated from the process waste stream via filtration or centrifugation after adsorbing the fluoride anions.

15. A method according to any preceding claim,wherein the process waste stream is a waste stream from a waste ionomer membrane recycling process.

16. A method according to any one of claims 1 to 14, wherein the process waste stream is a waste stream from an ionomer membrane manufacturing process.

17. A method according to any one of claims 15 or 16, wherein the ionomer membrane is a catalyst coated ionomer membrane.

18. A method according to any preceding claim, wherein heating of the process waste stream is done in the presence of a platinum group metal in addition to the base and the fluoride anion scavenger.

19. A method according to any preceding claim,wherein after heating the process waste stream to defluorinate the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger, remaining defluorinated hydrocarbons are subjected to thermal depolymerization.T +44(0)30 0300 2000A

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