Platinum group metal recovery from catalyst coated membranes

Abstract

A method of recovering a platinum group metal from a catalyst coated membrane, the method comprising: incinerating a catalyst coated membrane to produce a platinum group metal containing ash; treating the platinum group metal containing ash with hydrochloric acid and an oxidant to leach the platinum group metal from the ash and produce a platinum group metal solution without smelting of the ash; and separating the platinum group metal solution from the ash.

Description

[0001] PLATINUM GROUP METAL RECOVERY FROM CATALYST COATED MEMBRANES

[0002] Field

[0003] This specification relates to recycling methods for recovering platinum group metal from catalyst coated membranes such as those used in fuel cells and hydrogen producing water electrolysers.

[0004] Background

[0005] 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 (ionomer) membrane coated on either side by a platinum group metal (PGM) catalyst containing layer. The CCMs are configured to drive oxidation and reduction reactions and support proton and electron transport, these processes being required for the fuel cell and electrolyser technologies to function. A CCM may also comprise a platinum group metal recombination catalyst disposed within the membrane 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.

[0006] 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 / or 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.

[0007] One current method to recover PGMs from production scrap and end-of-life CCM material involves incineration. The incineration process yields a PGM rich ash which is processed via conventional PGM refining routes which involve pyrometallurgical processes such as smelting followed by hydrometallurgical processes to separate and purify the PGMs.

[0008] One disadvantage of the aforementioned approach is that the incineration method destroys the ionomer component which has significant value. Whilst processes for recycling both PGM and ionomer components of CCM materials are being developed they are not yet widely available for scaled industrial usage to process the rapidly increasing amount of CCM waste materials at time of writing.

[0009] Another disadvantage of the aforementioned approach is that processing of the PGM material through a conventional refining route (incineration followed by pyrometallurgical and hydrometallurgical processes to separate and purify the PGMs) involves significant time, energy, and cost. Conventional PGM refining in this manner is an energy intensive process and can result in slow recovery of PGMs from CCM waste materials.

[0010] It is an aim of the present specification to address the aforementioned issues.

[0011] Summary The present specification provides a method of recovering a platinum group metal from a catalyst coated membrane, the method comprising: incinerating a catalyst coated membrane to produce a platinum group metal containing ash; treating the platinum group metal containing ash with hydrochloric acid and an oxidant to leach the platinum group metal from the ash and produce a platinum group metal solution without smelting of the platinum group metal containing ash; and separating the platinum group metal solution from the ash.

[0012] This approach provides a method of recovering platinum group metal from waste catalyst coated membrane materials while avoiding conventional pyrometallurgical PGM refining processes involving smelting. By directly using an oxidative acid leach on the platinum group metal containing ash produced by incinerating CCM material, the method reduces energy costs and the time required to recover the PGM material for re-use when compared with convention PGM refining via smelting. Furthermore, such a process can be readily scaled while more complex processes for recovering both ionomer and PGM components are made more available. As such, the present processes can address the immediate problem of increasing CCM waste materials with an approach which is more efficient, quick, and lower cost than current scaled industrial processes.

[0013] Brief Description of the Drawings

[0014] Figure 1 shows a flow sheet for recycling of PGM from CCM materials using conventional PGM refining processes.

[0015] Figure 2 shows a flow sheet for recycling of PGM from CCM materials according to the present specification.

[0016] Figure 3 shows another flow sheet for recycling of PGM from CCM materials according to the present specification with several additional steps.

[0017] Detailed Description

[0018] As illustrated in Figure 1, conventional platinum group metal refining for recycling of platinum group metals from catalyst coated membrane waste materials involves both pyrometallurgical / smelting processes and hydrometallurgical processes, which is energy intensive and can result in long lead times for recovery of the platinum group metal.

[0019] The CCM waste material is first incinerated to produce a platinum group metal containing ash. If the ash is routed through a conventional PGM refinery with other types of PGM waste materials, the ash is then evaluated for its PGM content. Evaluation is a critical stage in the PGM refining process. During this step, PGM content of the customer's material is determined to enable agreement as to what quantity or value of PGM will by returned and the lead time for refining. In an alternative, the CCM waste material can be evaluated for PGM content prior to incinerating to produce the ash.

[0020] The PGM rich ash recovered after incineration of the CCM waste material is then mixed with other PGM waste materials and smelted to produce one or more mixed PGM bullions. It is to be noted that once terms are agreed, feeds are no longer handled on a customer-by-customer basis, but rather are combined together in much larger batches for smelting. In this pyrometallurical process material is melted at temperatures over 1200°C for around 12 hours in large furnaces to separate the non-metallic components and produce one or more bullion feeds which can contain all five PGM metals which are typically recovered (Pt, Pd, Ru, Rh, Ir).

[0021] The mixed PGM bullion is then subjected to chemical leaching to yield a mixed PGM solution. Typically, the bullion feeds will undergo multiple leaches to concentrate the PGMs. Gold and / or silver can also be separated from the PGMs at this stage.

[0022] The mixed PGM solution is then subjected to a series of chemical separation and purification processes (hydrometallurgical processing). Chemical separation is a highly complex multistage process in which the five PGMs are separated into their final product forms, commonly a PGM sponge, although other forms are produced depending on customer requirements. The chemical separation process involves dissolution and a series of solvent extractions, evaporation, precipitation and filtration steps to produce highly pure salts for each individual PGM. These salts then undergo heat treatment to convert them to the final pure platinum group metal.

[0023] It will be appreciated that the aforementioned process is complex, energy intensive, and time consuming. Given the rapidly increasing quantity of PGM containing catalyst coated membrane waste materials, the present specification provides a dedicated CCM recycling process which is less energy intensive and more rapidly recovers the PGM material from CCM waste materials. Figure 2 shows a flow sheet illustrating the main steps in the process. The CCM waste materials is incinerated to produce a platinum group metal containing ash. The platinum group metal containing ash is then directly treated with hydrochloric acid and an oxidant to leach the platinum group metal from the ash and produce a platinum group metal solution without smelting of the ash. The platinum group metal solution is then separated from the ash, e.g., via filtration. This approach thus avoids energy intensive pyrometallurgical / smelting processes and simplifies any hydrometallurgical separation / purification steps as CCM waste materials will not typically include all five PGM metals.

[0024] The step of treating the platinum group metal containing ash typically comprises heating the hydrochloric acid. For example, the hydrochloric acid can be heated to a temperature of: at least 40°C, 50°C, or 60°C; no more than 95°C, 85°C, or 75°C; or within a range defined by any combination of the aforementioned lower and upper limits. The hydrochloric acid may have a concentration of: at least 6M, 8M, or 10M; no more than 12M, 10M, or 8M; or within a range defined by any combination of the aforementioned lower and upper limits. Furthermore, the oxidant can be selected from one or more of chlorine, a chlorate (e.g., sodium chlorate), hydrogen peroxide, or nitric acid. An oxidant such as chlorine can be generated in situ rather than being added as a reagent. Depending on the composition of the ash, the oxidant may need to be added to the HCI before addition of the ash due to the presence of base metals, preventing the risk of hydrogen formation.

[0025] Figure 3 shows another flow sheet for recycling of PGM from CCM materials according to the present specification with several additional steps. The PGM content of CCM waste material can be evaluated. This is particularly required if the CCM waste material is to be subsequently mixed with other CCM waste materials (e.g., from different customer sources). The CCM waste materials can then be incinerated to produce a platinum group metal containing ash. As previously mentioned, in an alternative the CCM waste material can be incinerated first and then the ash can be evaluated for PGM content. However, this order of steps will not account for any loss of PGM material in the incineration process or allow mixing of materials from different sources prior to incineration. In yet another alternative, the PGM content can be evaluated before and after incineration. This may be appropriate, for example, if the incineration process is outsourced in order to assess PGM losses in the outsourced incineration process. The platinum group metal containing ash is then directly treated with hydrochloric acid and an oxidant to leach the platinum group metal from the ash and produce a platinum group metal solution without smelting of the ash, and the platinum group metal solution is then separated from the ash. Details of the leaching solution and conditions are as described previously in relation to Figure 2, and examples are given later in this description.

[0026] The platinum group metal solution can then be treated to remove any base metal impurities and / or any non-platinum PGM. The platinum group metal solution can then be concentrated and converted to chloroplatinic acid.

[0027] Base metal impurities, as well as any other PGMs, can be separated using solid phase extraction techniques or liquid-liquid phase extraction techniques. Alternatively, or additionally, the treatment to remove base metal impurities may comprise addition of a base (e.g., sodium bicarbonate or sodium hydroxide) to increase the pH of the solution and precipitate the base metal impurities which are separated via a solid-liquid separation process. For example, the pH of the solution prior to addition of the base may be less than 5 and the pH of the solution after addition of the base is in a range 5 to 10, 8 to 10, 9 to 10, or around 10.

[0028] Typically, for CCM waste materials the primary PGM component is platinum. However, CCM waste materials may include other PGMs. For example, CCM materials for electrolysers typically comprise both platinum and iridium. When the platinum group metal solution comprises platinum and at least one other platinum group metal, the method may further comprise separating the platinum from the at least one other platinum group metal. For example, the platinum can be separated from other platinum group metals using a solid-phase extraction technique.

[0029] After separating the platinum group metal solution from the ash, and any treatments to remove base metal impurities and / or other platinum group metals, the platinum group metal solution can be concentrated and converted to chloroplatinic acid. The chloroplatinic acid can then be used to manufacture new platinum products.

[0030] Examples

[0031] Leaching in peroxide / HCI

[0032] Fuel cell catalyst coated membranes were incinerated to produce a PGM containing ash. The PGM containing ash was analysed (ICP / XRF) to determine elemental content (including platinum content). The PGM containing ash was then subjected to a direct leaching process using peroxide and HCI and the resultant solution was analysed to determine elemental content (including platinum content).

[0033] Leaching Method

[0034] A round bottomed flask was set up on a hot plate equipped with a temperature probe, stirrer bar, condenser, and stopper. 5.12 g of PGM containing ash was added to the flask, washing out the remainder of the ash from the weighing vessel with 12M HCI. 295 mL 12M HCI was added to this vessel whilst stirring at 600 rpm and heated to 70°C. On reaching temperature, 1.4 mL of 30 wt% peroxide was added at 1 rpm (0.15 mL / min) via a Watson Marlow pump. After the first minute, the rate of addition was increased to 7 rpm for the remainder of the addition. At this point the timer was started for a 50-minute period. The mixture was then left to cool and was filtered via a Buchner funnel under vacuum yielding a bright yellow solution (278 mL; pH 1.5; acid molarity 9.6257 M; ORP 394.3 mV (Ag / AgCI)). The solution was analysed for elemental content (including platinum).

[0035] Results The platinum recovery yield in the acidic solution after leaching with peroxide and HCI was found to be 65 wt%. The solution also contained base metals which were also leached into solution.

[0036] Leaching in sodium chlorate / HCI

[0037] Fuel cell catalyst coated membranes were incinerated to produce a PGM containing ash. The PGM containing ash was then subjected to a direct leaching process using sodium chlorate and HCI. Leaching was carried out in both glass and PTFE reaction vessels to determine if etching of the glass vessel causes any contamination as compared to a PTFE vessel. Prior to leaching, the PGM containing ash was analysed (ICP / XRF) to determine elemental content (including platinum content).

[0038] Leaching Method (Glass Vessel)

[0039] A round bottomed flask was set up on a hot plate equipped with a temperature probe, stirrer bar, condenser, and stopper. 5.0183 g of PGM containing ash was added to the flask. 300 mL of 6M HCI was added to this vessel whilst stirring at 600 rpm and heated to 70°C. On reaching temperature, 2.3 mL (2eq) NaCIOs solution (450 g / L) was added at 1 rpm (0.15 mL / min) via a Watson Marlow pump. Bubbling was observed on each drop of addition, chlorine gas evolved. On completion of the addition, the timer was started for a 50-minute period. The mixture was left to cool and was filtered via a Buchner funnel under vacuum yielding a bright yellow solution (295 mL; ORP 553.8 mV (Ag / AgCI)). The solution was analysed for elemental content (including platinum).

[0040] Leaching Method (PTFE Vessel)

[0041] A PTFE beaker was clamped onto a hotplate with a PTFE temperature probe and stirrer bead, and PTFE watch glass placed on top. 5.56 g ash was added to the beaker, with 297 mL 12M HCI. This was heated to 70°C. 2.6 mL (2 eq.) of NaCIOs (450 g / L) solution was added at 1 rpm (0.15 mL / min) via a Watson Marlow pump. This was allowed to heat for 50 minutes and then cooled to around 40°C before filtering as before. The resultant solution was analysed for elemental content (including platinum).

[0042] Results

[0043] The platinum recovery yield in the acidic solution after leaching with sodium chlorate and HCI in the glass vessel was found to be 89 wt%. The platinum recovery yield in the acidic solution after leaching with sodium chlorate and HCI in the PTFE vessel was found to be 78 wt%. The lower yield for the leaching experiment in the PTFE vessel was attributed to the lack of condenser on the PTFE beaker and the thickness of the base meant that stirring was less efficient. However, both of these yields were higher than the yield when reacting the ash with the peroxide / HCI (65 wt%).

[0044] Similar amounts of Si were found in the solutions for both glass and PTFE vessels, indicating that the glass vessel was not substantially etched. This is further supported by the lack of B in the solution.

[0045] Based on these results, the present process will enable direct leaching of CCM ash to be carried out with approximately 90% of the Pt being recovered directly upfront from the CCM ash. The remaining residue can enter a standard smelting and PGM refining process. Any base metal impurities and other PGMs can be separated using known processes as described previously. Further optimisation of this process should result in greater Pt recovery.

[0046] 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

Claims1. A method of recovering a platinum group metal from a catalyst coated membrane, the method comprising: incinerating a catalyst coated membrane to produce a platinum group metal containing ash; treating the platinum group metal containing ash with hydrochloric acid and an oxidant to leach the platinum group metal from the ash and produce a platinum group metal solution without smelting of the ash; and separating the platinum group metal solution from the ash.

2. A method according to claim 1, wherein the step of treating the platinum group metal containing ash comprises heating the hydrochloric acid.

3. A method according to claim 2, wherein the hydrochloric acid is heated to a temperature of: at least 40°C, 50°C, or 60°C; no more than 95°C, 85°C, or 75°C; or within a range defined by any combination of the aforementioned lower and upper limits.

4. A method according to any preceding claim, wherein the hydrochloric acid has a concentration of: at least 6M, 8M, or 10M; no more than 12M, 10M, or 8M; or within a range defined by any combination of the aforementioned lower and upper limits.

5. A method according to any preceding claim, wherein oxidant is selected from one or more of chlorine, a chlorate, hydrogen peroxide, or nitric acid.

6. A method according to claim 5, wherein oxidant is sodium chlorate.

7. A method according to any preceding claim, wherein the oxidant is added to the HCI before addition of the platinum group metal containing ash.

8. A method according to any preceding claim, wherein after separating the platinum group metal solution from the ash, the platinum group metal solution is treated to remove base metal impurities.

9. A method according to claim 8, wherein the treatment to remove base metal impurities is a solid phase extraction or a liquid phase extraction.

10. A method according to claim 9, wherein the treatment to remove base metal impurities comprises addition of a base to increase the pH of the solution and precipitate the base metal impurities which are separated via a solid-liquid separation process.

11. A method according to claim 10, wherein the pH of the solution prior to addition of the base is less than 5 and the pH of the solution after addition of the base is in a range 5 to 10.

12. A method according to claim 10 or 11, wherein the base is selected from sodium bicarbonate or sodium hydroxide.

13. A method according to any preceding claim, wherein the platinum group metal solution comprises platinum.

14. A method according to claim 13, wherein the platinum group metal solution comprises platinum and at least one other platinum group metal, and the method further comprises separating the platinum from the at least one other platinum group metal.

15. A method according to claim 14, wherein the platinum is separating from the at least one other platinum group metal using a solid-phase extraction technique.

16. A method according to any preceding claim,wherein after separating the platinum group metal solution from the ash, and any treatments to remove base metal impurities and / or other platinum group metals, the platinum group metal solution is concentrated to yield a platinum containing solution.

17. A method according to claim 16, wherein the platinum is converted to chloroplatinic acid.

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