A method of recovering iridium in a platinum group metal refining process

Abstract

A method of recovering iridium in a platinum group metal refining process by dissolving iridium from a solid iridium containing material into an aqueous solvent to produce an iridium containing solution, wherein the solution is acidic and / or is acidified by addition of an acid (e.g. HCl) after dissolution of the iridium; adding an aliphatic polyamine (e.g. diethylenetriamine or ethylenediamine) to the solution to precipitate the iridium as an iridium polyamine salt; and separating the precipitated iridium polyamine salt from the solution. A majority of the iridium is dissolved in its +3 oxidation state and remains in this state throughout the precipitation and separation steps. The sold iridium (e.g. an iridate or an iridium oxide) can be reduced from its +4 oxidation state when it dissolved using an oxalate or oxalic acid reducing agent. The solution comprises substantially no rhodium or ruthenium.

Description

Field The present specification relates to a method of refining / recycling iridium. The method may be applied to a range of iridium containing refining / recycling feeds including those generated from waste catalyst coated membrane materials from fuel cell and hydrogen producing electrolyser applications. Background Platinum group metal refining processes can typically involve one or more pyrometallurgical processes, including smelting, to separate platinum group metals from other materials in the feed, followed by acid dissolution of the platinum group metals and a series of hydrometallurgical processes to separate and purify the individual platinum group metals (platinum, palladium, rhodium, iridium, and / or ruthenium). Such processes are known in the art. A known process for refining of an iridium refining stream uses ammonium chloride to produce ammonium hexachloroiridate. This requires the prior and simultaneous addition of hydrogen peroxide for many hours under reflux in order to force Ir (III) to convert to Ir (IV). This tends to frequently be ineffective at removing iridium from solution down to a sufficiently low concentration, requiring further additions of hydrogen peroxide and ammonium chloride. Even with additional doses, the final iridium concentration left in solution is usually quite high compared to many of the final precipitation steps for other platinum group metals. Ammonium hexachloroiridate is also partially water soluble, so washing to remove sodium can also result in significant iridium losses. WO2023099076 discloses another platinum group metal refining process which uses aliphatic polyamines to precipitate rhodium in oxidising conditions at a redox potential of >950 to 1050 mV. It is also disclosed that the redox potential can then be reduced by addition of a reducing agent, such as a stannous or ferrous salt, to allow excess polyamine to cause precipitation of Ir (III). However, such a method of adding aliphatic polyamines in oxidising conditions and then reducing the dissolved iridium (IV) to iridium (III) by addition of metal salt reducing agents can affect the final iridium sponge purity which can be contaminated by the use of metal salt reducing agents. Demand for iridium is projected to increase due, at least in part, to the increasing use of iridium in fuel cell and hydrogen producing electrolyser applications. As such, there is a need to provide improved methods for refining / recycling of iridium. The present specification aims to address this need. Summary According to the present specification there is provided a method of recovering iridium in a platinum group metal refining process, the method comprising: dissolving iridium from a solid iridium containing material into an aqueous solvent to produce an iridium containing solution, wherein the iridium containing solution is acidic and / or is acidified by addition of an acid after dissolution of the iridium; adding an aliphatic polyamine to the acidic iridium containing solution to precipitate the iridium as an iridium polyamine salt; and separating the precipitated iridium polyamine salt from the acidic solution, characterized in that a majority of the iridium is dissolved in its +3 oxidation state and remains in its +3 oxidation state throughout the precipitation and separation steps. In relation to the above, it has been found that iridium can be dissolved in its +3 oxidation state and subsequently precipitated in its +3 state using an aliphatic polyamine. This differs from the process describe in WO2023099076 which requires iridium to be in its +4 oxidation state first and then subsequently reduced to its +3 oxidation state using an iron or tin based reducing agent. In contrast, the present method does not require oxidising conditions in the first instance, nor does it require any additional transition or post-transition metal containing reducing agents to be introduced to reduce the iridium, thereby avoiding contamination in the final iridium sponge compromising purity. Furthermore, it has also been found that the method is more effective at precipitating iridium from solution when compared to a process which uses ammonium chloride to produce ammonium hexachloroiridate. Further still, the resultant iridium polyamine salt has been found to be less soluble than ammonium hexachloroiridate during subsequent washing steps resulting in lower losses of iridium. In summary, the present method provides a more efficient route to recovering higher yields of high purity iridium with reduced lead times, reduced losses, reduced reagent usage, and reduced waste effluent production. 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 according to the present specification showing the main steps in the process; and Figure 2 shows a flow diagram of a more detailed example. Detailed Description As described in the summary section and illustrated in Figure 1, the present specification provides a method of recovering iridium in a platinum group metal refining process, the method comprising: dissolving iridium from a solid iridium containing material into an aqueous solvent to produce an iridium containing solution, wherein the iridium containing solution is acidic and / or is acidified by addition of an acid after dissolution of the iridium; adding an aliphatic polyamine (e.g., diethylenetriamine or ethylenediamine) to the acidic iridium containing solution to precipitate the iridium as an iridium polyamine salt; and separating the precipitated iridium polyamine salt from the acidic solution (e.g., using a solidliquid separation technique such as filtration), characterized in that a majority of the iridium is dissolved in its +3 oxidation state and remains in its +3 oxidation state throughout the precipitation and separation steps (e.g., at least 70, 80, 90, 95, or 97 wt% of the iridium in the acidic iridium containing solution is dissolved and maintained in its +3 oxidation state). The iridium in the solid iridium containing material can be in its +4 oxidation state and the iridium is dissolved under reductive conditions to produce the iridium containing solution in which the iridium is in its +3 oxidation state, the aqueous solvent comprising a reducing agent. Advantageously, the reducing agent does not comprise a transition metal salt or a post-transition metal salt and most advantageously no metal or metal containing salt is used for the iridium reduction. This is because such metal based reducing agents can contaminate the final iridium product. Accordingly, it is preferred that the reducing agent is a non-metal-based reagent, or, if a metal is present, then that is an alkali or alkaline earth metal which can be readily washed from an iridium precipitate (e.g., a sodium salt). Preferred reducing agents are selected from an oxalate, ammonium oxalate, or oxalic acid. The solid iridium containing material may comprise an iridate or an iridium oxide. Advantageously, the solid iridium containing material and / or the iridium containing solution after dissolution comprises substantially no rhodium and / or ruthenium, optionally less than 100 ppm, 50 ppm, 20 ppm, 10 ppm, or 5 ppm of rhodium and / or ruthenium. This negates the requirement for an oxidative dissolve and precipitation prior to reduction and precipitation of iridium. After the step of dissolving the iridium to produce the iridium containing solution and prior to the step of adding an aliphatic polyamine to the iridium containing solution to precipitate the iridium as the iridium polyamine salt, the iridium containing solution can be acidified with HCI. This is advantageous to ensure that the iridium is in chloro-complex form prior to precipitation. The aliphatic polyamine can also be added to the iridium containing solution as an HCI solution of the aliphatic polyamine to ensure a high concentration of Cl'. In order to retain the iridium in its +3 oxidation state throughout the dissolving, precipitation and separation steps, a redox potential of the acidic solution can be maintained at a value of: no more than 850 mV, 750 mV, 650 mV, or 550 mV; no less than 100 mV, 200 mV, 300 mV or 400 mV; or within a range defined by any combination of the aforementioned upper and lower limits. It has also been found that this is possible without the requirement to add a transition metal salt or post-transition metal salt reducing agent (e.g., a stannous or ferrous salt) to the solution before, during, and / or after the dissolving step. For example, it has been found that iridium can be dissolved in its +3 oxidation state, without the addition of a metal reducing agent, and instead use a non-metal reducing agent such as one which is selected from an oxalate, ammonium oxalate, or oxalic acid. Such reducing agents can reduce Ir (IV) in an ammonium hexachloroiridate salt to Ir (III) which allows the iridium to dissolve very easily and not precipitate back out. As a reductant in a metal side stream, ammonium oxalate or oxalic acid is advantageous because it is simply removed as CO2 when consumed, and so does not introduce any impurities compared to metallic reducing agents such as stannous or ferrous salts. The present methodology is particularly useful in an iridium side stream where a crude iridate has already been separated from other platinum group metals such as rhodium and / or ruthenium. That is, the crude iridate and / or the acidic iridium containing solution comprise substantially no rhodium and / or ruthenium. In that case, it is not required, for example, to provide an oxidizing environment as described in WO2023099076 to precipitate rhodium and then introduce a stannous or ferrous reducing agent for an iridium precipitation. The present methodology is therefore advantageous at producing high yields of high purity iridium in a dedicated iridium side stream process as the addition of a transition / post-transition metal salt reducing agent, which can contaminate the final iridium product, is not required. In addition to producing high precipitation yields, the present methodology is also advantageous in producing an iridium polyamine salt which is insoluble in washing processes, particularly those using water. As such, after separating the precipitated iridium polyamine salt from the acidic solution, the precipitated iridium polyamine salt can be washed, optionally in water, and this washing step does not result in significant iridium losses via dissolution. As such, the present process is advantageous in ensuring that a high purity, washed, iridium product can be achieved without introducing losses. The iridium dissolution step can be performed with water and a non-metallic reducing agent such as ammonium oxalate or oxalic acid to achieve a solution of iridium in its +3 oxidation state. After the step of dissolving the iridium and prior to the step of adding an aliphatic polyamine to the acidic iridium containing solution to precipitate the iridium as the iridium polyamine salt, the iridium containing solution can be diluted with HCI (e.g., HCI having a molarity of at least 7, 8, 9, 10, 11, or 12). This approach can be used to produce chlorinated iridium species and precipitate an iridium polyamine chlorocomplex salt. Additionally, or alternatively, the aliphatic polyamine can be added to the acidic iridium containing solution as an HCI solution of the aliphatic polyamine. For example, an iridate can be initially dissolved in water with oxalate. In that case, for the subsequent aliphatic polyamine precipitation (where fully chloride-complexed Ir is advantageous) the Cl' concentration can be increased by addition of HCI. The aliphatic polyamine (e.g., DETA) can also be added as an HCI solution (e.g., DETA.3HCI). During the step of adding an aliphatic polyamine to the acidic iridium containing solution to precipitate the iridium as the iridium polyamine salt, the acidic iridium containing solution can be heated (with stirring) at a temperature of: at least 50°C, 60°C, 70°C, 80°C, or 90°C; no more than 100°C, 98°C, or 95°C; or within a range defined by any combination of the aforementioned lower and upper limits. It has been found that high yields of iridium can be precipitated efficiently under such conditions. Figure 2 shows a flow diagram of a more detailed example following the above-described methodology. The method comprises the following steps: • Dissolve iridium from an iridate into a solution comprising ammonium oxalate or oxalic acid in water to produce a solution of lr(III). • Add HCI to the solution to chloro-complex the Ir(lll). • Add a solution of an aliphatic polyamine (e.g., DETA or ED) in HCI to the solution while heating to precipitate the iridium as an iridium polyamine salt. • Separate the precipitated iridium polyamine salt from the solution (e.g., via filtration). • Wash the precipitate in water. According to certain processes, one or both of zinc and sodium sulphide are added to the iridium containing solution prior to adding the aliphatic polyamine. Such processes within the side stream are aimed at removing particular impurities. Zn metal is added to reduce any other PGMs to metal via a cementation / footing type method. These are removed by filtration. Sodium sulfide precipitates the dissolved zinc from the previous step along with lead, which is the only other significant impurity left after Zn. Filtration separates the clean Ir liquorfrom the residue. The addition of zinc and subsequently sodium sulphide does not cause the formation of any Ir(IV), therefore the liquor is more suited to this type of precipitant than one which requires Ir(IV). In relation to the above, it may be noted that an ammonium chloride precipitation is not selective for Ir, which is why these additional purification steps are required to achieve a high level of purity before the final iridium precipitation step. However, since an aliphatic polyamine precipitation can be more selective for Ir over, for example, Pt and Pd (in particular, but over most other metals besides Rh), at least some of the prior purification steps may not be necessary when using a more selective final precipitation. The benefit of this would be large as the zinc process can cause some iridium losses. As such, it is also envisaged that the aliphatic polyamine precipitation can be implemented earlier in the flowsheet to obviate the need for zinc and / or sulphide addition, saving further time and further reducing iridium losses. Examples A sample of iridium liquor (1 L) was obtained from an iridium side stream of a platinum group metal refinery. The sample was taken after crude iridate had been subjected to an oxalic dissolve, addition of zinc and sodium sulphide, and concentrated via a boil down. The sample was submitted to analytical labs for ICP-OES analysis to determine Ir concentration and titrated against 1 M sodium hydroxide to determine free acidity. The sample contained 40.7 g / L Ir with a free acidity of 2.1 M. A sample of DETA.3HCI solution (250 mL) was obtained and titrated against 1 M sodium hydroxide to determine DETA concentration. This was calculated as 282 g / L and used to calculate the required volume of DETA.3HCI solution to precipitate all the Ir. The calculation was made to 110% stoichiometry such that the DETA was provided in excess. The iridium liquor was added to a flat-bottom baffled flange vessel fitted with an overhead stirrer. This was diluted with 37% HCI (100 mL) and 9 M HCI (50 mL). The solution was stirred at 250 rpm and heated to 95°C, the temperature being controlled by an internal PTFE thermocouple. Once at temperature, addition of DETA.3HCI solution (18 mL) was started at a rate of 0.2 mL / min. Once the addition was complete a sample was taken, which was pale green / yellow. The heating was maintained for 30 minutes, and another sample was taken which had an appearance identical to the previous sample. The heating was stopped and, once cool, the slurry was filtered over a Whatman™ 541 filter paper and washed with demineralised water. The washings were combined with the mother liquor. The solid was dried over vacuum to afford a green-brown crystalline powder (10.65 g). The pale green solution of combined mother liquor and washings was collected (295 mL). A portion of the solid was submitted for impurity analysis (ICP-MS) and the combined mother liquor and washings was submitted for ICP-OES analysis. To determine the solubility of the Ir-DETAsalt in water, a small portion of the solid (1.81 g) was weighed out into a 100 mL beaker. Demineralised water (20 mL) was added to this beaker and the contents were stirred for 20 minutes. After this time, the solid was filtered over a Whatman™ 542 filter paper. The residual solid was rinsed in with further demineralized water and then allowed to dry in air. Appearance of solid and water after separation was unchanged. The total washings (27 mL) were submitted for a low-level effluent analysis (ICP-MS) and the washed solid (1.64 g recovered from paper) was submitted for impurity analysis (ICP-MS). Results The table below contains the results of ICP-OES analysis of the initial start liquor, the combined liquor and washings post Ir precipitation, and the solubility trial washings (mg / L values quoted, i.e, ppm). The residual Ir concentration in the combined mother liquor and washings (416 mg / L) corresponds to 3% of the total Ir input. That is, 97% of iridium was precipitated from the liquor. Due to the very pale colour of the mother liquor, it is believed the remaining iridium is Ir (III), and therefore would precipitate with relative ease if further additions of DETA.3HCI had been carried out. As such, it is considered that substantially all the iridium can be recovered using this method. Start Liquor Combined liquor &washings post precipitation Solubility trial j washings Volume (mL) 100 295 20 i Pt 4 3 0 J ! Pd 0 1 0 J Rh 0 0 0 J Ir 40771 416 6 j Ru 0 1 0 i Au 0 ! 0 0 1 Ag 0 0 0 1 Os 0 0 0 1 Cu 6348 o 0 i Ni 1 2 0 j Fe 7 i 9 0 J ! Pb 153 i o 0 i Co 0 i 0 0 J 11 ~1 o 0 i Zn 1 ] 2 0 1 Se 1 0 0 i Te 0 4 0 J Al 2 0 0 J As 32 3 0 J B 1 1 0 i Ba 0 o 0 i Bi 0 0 o ! In 0 0 0 1 Sb 14 0 0 J Si 14 11 1 j Sn 2 0 0 1 Ca 39 10 0 I Total Precious 40775 421 6 j Metals Total ICP 47401 464 7 Impurities For reference, the mean residual iridium concentration in the mother liquor of a process which uses ammonium chloride to produce ammonium hexachloroiridate is much higher and further additions of hydrogen peroxide and / or ammonium chloride are usually required on top of the 12-hour preparation with hydrogen peroxide which is carried out prior to every batch. As such, it is clear that the process of the present specification provides significant improvements in terms of efficiency to recover iridium at high yields with reduced lead times, reduced energy demand, reduced requirement for reagents, and reduced waste production. The washing trial demonstrated the very limited solubility of Ir-DETA in water (6 mg / L Ir). By comparison, washings of pure ammonium hexachloroiridate have been found to contain up to 5000 mg / L Ir. As such, it is clear that the process of the present specification provides significant improvements in terms of enabling washing of the recovered iridium salt to achieve a high purity product without significant losses of iridium in the washing process. 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 recovering iridium in a platinum group metal refining process, the method comprising:dissolving iridium from a solid iridium containing material into an aqueous solvent to produce an iridium containing solution, wherein the iridium containing solution is acidic and / or is acidified by addition of an acid after dissolution of the iridium;adding an aliphatic polyamine to the acidic iridium containing solution to precipitate the iridium as an iridium polyamine salt; andseparating the precipitated iridium polyamine salt from the acidic solution,characterized in that a majority of the iridium is dissolved in its +3 oxidation state and remains in its +3 oxidation state throughout the precipitation and separation steps.

2. A method according to claim 1,wherein the iridium in the solid iridium containing material is in its +4 oxidation state and the iridium is dissolved under reductive conditions to produce the iridium containing solution in which the iridium is in its +3 oxidation state, the aqueous solvent comprising a reducing agent.

3. A method according to claim 2,wherein the reducing agent does not comprise a transition metal salt or a post-transition metal salt.

4. A method according to claim 2 or 3,wherein the reducing agent does not comprise a metal salt.

5. A method according to any one of claims 2 to 4,wherein the reducing agent is an oxalate, ammonium oxalate, or oxalic acid.

6. A method according to any preceding claim,wherein the solid iridium containing material comprises an iridate or an iridium oxide.

7. A method according to any preceding claim,wherein the iridium containing solution comprises substantially no rhodium and / or ruthenium, optionally less than 100 ppm, 50 ppm, 20 ppm, 10 ppm, or 5 ppm.

8. A method according to any preceding claim,wherein, after the step of dissolving the iridium to produce the iridium containing solution and prior to the step of adding an aliphatic polyamine to the iridium containing solution to precipitate the iridium as the iridium polyamine salt, the iridium containing solution is acidified with HCI.

9. A method according to any preceding claim,wherein the aliphatic polyamine is selected from diethylenetriamine or ethylenediamine.

10. A method according to any preceding claim,wherein the aliphatic polyamine is added to the iridium containing solution as an HCI solution of the aliphatic polyamine.

11. A method according to any preceding claim,wherein, during the step of adding the aliphatic polyamine to the iridium containing solution to precipitate the iridium as the iridium polyamine salt, the iridium containing solution is heated at a temperature of: at least 50°C, 60°C, 70°C, 80°C, or 90°C; no more than 100°C, 98°C, or 95°C; or within a range defined by any combination of the aforementioned lower and upper limits.

12. A method according to any preceding claim,wherein, throughout the dissolving, precipitation and separation steps, a redox potential of the solution is maintained at a value of: no more than 850 mV, 750 mV, 650 mV, or 550 mV; no less than 100 mV, 200 mV, 300 mV or 400 mV; or within a range defined by any combination of the aforementioned upper and lower limits.

13. A method according to any preceding claim,wherein at least 70, 80, 90, 95, or 97 wt% of the iridium in the iridium containing solution is dissolved and maintained in its +3 oxidation state.

14. A method according to any preceding claim,wherein the precipitated iridium polyamine salt is separated from the acidic solution using a solid-liquid separation technique, optionally filtration.

15. A method according to any preceding claim,wherein after separating the precipitated iridium polyamine salt from the acidic solution, the precipitated iridium polyamine salt is washed, optionally in water.11

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