Method of treating an iridium containing effluent stream

Abstract

A method of treating an iridium containing effluent stream to remove and recover iridium, the method comprising: adding a base to the iridium containing effluent stream to increase pH of the effluent

Description

Field This specification relates to a method of treating an iridium containing effluent stream to remove and recover iridium. The method is particularly useful in treating an effluent stream from an iridium oxide manufacturing process. Background Demand for iridium oxide (IrOx) is projected to increase rapidly. Iridium oxide catalyst material is a key material used in hydrogen producing water electrolysers. However, since iridium is rare and expensive, it is important that iridium is used efficiently, recycled, and losses of iridium during manufacturing processes and use are minimized. Iridium oxide manufacturing processes, such as those used for the manufacture of electrochemical grade iridium oxide for electrolyser catalyst production, generate effluent streams containing soluble iridium species. It is desired to recover and reuse iridium from such effluent streams to minimize iridium losses during iridium oxide manufacture. One method to recover iridium from an effluent stream is to treat the effluent stream with a reducing agent, such as hydrazine or NaBH4, to reduce lrn+ species in solution to lr° which can then be separated and recovered from the liquid effluent stream. However, such methods of treating effluent from iridium oxide manufacturing processes have been found to be not sufficiently effective at recovering iridium and / or such treatments release hydrogen as a side product requiring complex and expensive equipment to deal with hydrogen evolution, and / or such treatments use toxic reagents which present a health and safety risk and / or an environmental hazard. It is an aim of the present specification to address these problems. In particular, it is an aim to provide a method suitable for recovering iridium from effluent streams, such as those generated during iridium oxide manufacture, which is safe, efficient, has high recovery yields, provides fast iridium recovery rates, is low cost, environmentally friendly, and is reliable and easy to implement without requiring expensive and complex equipment. Summary The present specification provides a method of treating an iridium containing effluent stream to remove and recover iridium, the method comprising: adding a base to the iridium containing effluent stream to increase pH of the effluent stream and precipitate the iridium as a solid iridium salt; and separating the solid iridium salt from the effluent stream. It has been found that such a method can be used to recover iridium from effluent streams, such as those from iridium oxide manufacturing effluent streams, at high yields and rates. Furthermore, the method can be implemented safely without the use of highly toxic reagents or complex equipment and is reliable, environmentally friendly, and cost effective. The method is particularly useful where the pH of the effluent stream prior to addition of the base is acidic (e.g., less than 6, 5, 4, or 3.5). Such acidic effluent streams are generated from certain types of iridium oxide manufacturing processes which are advantageous for manufacturing catalyst materials used in hydrogen producing water electrolysers. Advantageously, the base is added to the effluent stream to raise the pH of the effluent stream to: no less than 5, 6, 7, or 8; no more than 13,12,11,10,9 or 8; or within a range defined by any combination of the aforementioned lower and upper limits. Results have indicated that while iridium recovery yields increase when increasing the pH of the effluent stream, if the pH of the effluent stream is increased beyond a certain amount, recovery yields may begin to fall again. That is, there may be an optimum range of pH for optimal iridium recovery yields. Avoiding very high pH also has the additional advantage of generating a less corrosive effluent stream. The base can be selected from one or more of: a hydroxide; an organic hydroxide; ammonium hydroxide; a metal hydroxide; an alkali metal hydroxide; an alkali earth metal hydroxide; sodium hydroxide; and lithium hydroxide. For example, sodium hydroxide can be used as the reagent which is cheap and readily available. It has further been found that iridium can be recovered at high yields and rates using the present method without the necessity for heating, therefore reducing energy requirements. As such, the base can be added to the effluent stream at below room temperature, at room temperature, or otherwise without excessive heating. For example, the base can be added to the effluent stream at a temperature of: at least 0°C, 10°C, 20°C, 30°C, 40°C or 50°C; no more than 90°C, 80°C, 70°C, or 60°C; or within a range defined by any combination of the aforementioned lower and upper limits. While not being bound by theory, prior to addition of the base, at least a portion of the iridium may be present in the effluent stream as dissolved H2lr(OH)g. Such an iridium species may be present in effluent streams such as those from certain types of iridium oxide manufacturing processes which are advantageous for manufacturing catalyst materials used in hydrogen producing water electrolysers. In such effluent streams, after addition of the base, at least a portion of the iridium can be converted to solid lr(OH)4. Such a reaction scheme also provides an explanation as to why increasing the pH of the effluent beyond a certain limit may lead to a drop in the recovery yield of iridium due to the formation of soluble iridium species such as Mx[lr(OH)6] at high pH (where M is a metal and x is 1 or 2 or a value between 1 and 2). As such, the pH is advantageously controlled within a window to form high yields of solid iridium species (e.g., lr(OH)4) in preference to soluble species of iridium at lower and higher pH. Optionally, after addition of the base to the effluent stream to raise the pH of the effluent, a reducing agent is added to the effluent stream to further increase precipitation of iridium from the effluent stream. The reducing agent may be selected from one or more of: a hydride; a borohydride; NaBH4; NH2NH2; a peroxide; and hydrogen peroxide. While the present methodology can provide an alternative to using such reducing agents, the present methodology can be combined with such an approach while still enabling a reduction in the amount of such reducing reagents required to achieve high yields of iridium recovery. Once precipitated in the effluent stream, the solid iridium salt can be separated from the effluent stream using a solid-liquid separation technique such as by one or more of filtration, centrifugation, and / or another solid-liquid separation technique to recover the solid iridium salt for further processing. As previously indicated, the iridium containing effluent stream can be from an iridium oxide manufacturing process. In which case, the recovered iridium salt from the effluent can be reused in the iridium oxide manufacturing process or used for another application. The present specification also provides a system for performing the method as described herein, the system comprising: an iridium oxide synthesis section for manufacturing iridium oxide, said iridium oxide synthesis section generating an iridium oxide product and an iridium containing effluent stream; and an effluent processing section for recovering iridium from the effluent stream, the effluent processing section being configured to add a base to the effluent stream to increase pH of the effluent stream and precipitate the iridium as a solid iridium salt, the effluent processing section being further configured to separate the solid iridium salt from the effluent stream. The present methodology thus provides a means to recover iridium from effluent streams, such as those generated during iridium oxide manufacture, which is safe, efficient, has high recovery yields, provides fast iridium recovery rates, is low cost, environmentally friendly, and is reliable and easy to implement without requiring expensive and complex equipment. Detailed Description As described in the summary section, the present specification provides a method (and associated system) for treating an iridium containing effluent stream to remove and recover iridium, the method comprising: adding a base to the iridium containing effluent stream to increase pH of the effluent stream and precipitate the iridium as a solid iridium salt; and separating the solid iridium salt from the effluent stream. Preferred features of embodiments of this method are also set out in the preceding summary section and will not be repeated here for conciseness. 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 in more detail by way of example only. Experimental General procedure Effluent mixtures (0.5 -1.0 I) were set to stir with optional heating (e.g., up to 90°C). A pH probe was submerged in the solution and the base was added dropwise to adjust the pH. Typically, the solution becomes cloudy with the formation of a white / pale yellow precipitate. Once the required pH was reached, for some experiments either NaBH4or NH2NH2 were added. The precipitate was then filtered off and the filtrate was collected for analysis. Results pH treatment tests at 90 °C with NaBH4, NaOH and NH2NH2 Tests were carried out on some effluent mixtures using different reducing agents. In each case, the effluent mixture was heated to 90°C with NaOH added dropwise until it reached a pH of 9. In one experiment, 9.8 g of 12% NaBH4 solution was subsequently added. In a second experiment, 13.9 g of 7.5% NH2NH2 solution was subsequently added. In a third experiment, nothing else was added after the NaOH to increase the pH to 9. The mixture was filtered to remove the precipitate, with a sample of the filtrate taken for analysis of remaining impurities. Iridium content data for the untreated and treated effluent mixtures is shown below. Ir content (mg I1) Effluent (untreated) 67.90 NaOH / NaBH4 treated effluent 1.07 NaOH / NH2NH2 NaOH treated treated effluent | effluent 37.83 4.91 The most effective method, resulting in just 1.07 mg I1 of Ir remaining in the effluent, appears to be with the use of NaOH followed by NaBH4, whereas the addition of NaOH followed by NH2NH2 seems to be the least effective. It is worth noting that in the trial in which only NaOH was added to increase the pH, a significant amount of Ir was removed, with only 4.91 mg I1 remaining in the effluent. As such, it is concluded that effluent can be effectively treated with only addition of a base such as NaOH or by treatment with a base followed by treatment with a reducing agent such as NaBH4. pH treatment at room temperature using 1.0 M NaOH solution To probe the effect of pH on the extent of Ir removal, an effluent (containing 67.19 mg I1 Ir) was treated at room temperature with 1.0 M NaOH solution added dropwise to increase pH. In one experiment, the pH was raised to 10.35 and analysed. In a further set of experiments samples were taken in increments as the pH was increased, which were separately filtered and analysed. From the results of the experiment in which the pH was raised to 10.35, a significant amount of Ir was removed from the solution with only 3.87 mg I1 remaining from the 67.19 mg I1 present in the untreated effluent. It is also worth noting that this is similar to the amount of Ir recovered from the earlier experiment carried out at 90°C, suggesting that heating does not necessarily facilitate additional Ir recovery. For the further experiment in which samples were taken and filtered as the pH was raised in increments, Ir content in the effluent samples after filtration decreased as pH was increased up to a point (around a pH of 8 to 9), and then a further increase in pH resulted in an increase in Ir content remaining in the effluent after filtration. This would suggest that soluble iridium species at low pH are converted to insoluble iridium species as pH is raised up to a point, butthereafter on further increasing the pH insoluble iridium species may be converted back to soluble iridium species. That is, increasing the pH too much may inhibit the removal of Ir from solution. Further lrOx effluent treatment trials Further experiments have been performed on effluent with 60 ppm of Ir present. The effluent was treated with 1.0 M NaOH solution until it reached a pH of 9. The results correspond closely to the previous trials, with 4.4 ppm of Ir detected in the filtrate after removing the precipitate that formed. When the effluent had its pH increased to 12.34, there was 26.3 ppm of Ir present in the filtrate, again suggesting that increasing the pH too much can inhibit the removal of Ir from solution. Conclusions Trials have revealed that raising the pH of effluent from an iridium oxide manufacturing process with the addition of a base such as NaOH provides a method to recover a significant amount of Ir from the effluent (e.g., greater than 90%). This is achievable at room temperature, with similar results obtained as when carried out at 90°C. The addition of a reducing agent such as NaBH4to the effluent after raising the pH can further increase the amount of Ir that can be removed. Furthermore, results indicate a trend of increasing iridium recovery with increasing pH up to a certain point after which further increasing pH leads to a reduction in iridium recovery. The treatment method has been shown to be particularly useful when applied to acidic effluent streams. Such acidic effluent streams from an iridium oxide manufacturing method may comprise dissolved H2lr(OH)g which is converted to solid lr(OH)4 on addition of base and then which is converted to soluble iridium species such as Mx[lr(OH)6] at high pH (where M is a metal and x is 1 or 2 or a value between 1 and 2). As such, the pH is advantageously controlled within a window to form high yields of solid lr(OH)4 in preference to soluble species of iridium. 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 an iridium containing effluent stream to remove and recover iridium, the method comprising:adding a base to the iridium containing effluent stream to increase pH of the effluent stream and precipitate the iridium as a solid iridium salt; andseparating the solid iridium salt from the effluent stream.

2. A method according to claim 1,wherein the pH of the effluent stream prior to addition of the base is less than 6, 5, 4, or 3.5.

3. A method according to claim 1 or 2,wherein the base is added to the effluent stream to raise the pH of the effluent stream to: no less than 5, 6, 7, or 8; no more than 13,12,11,10,9 or 8; 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 base is selected from one or more of: a hydroxide; an organic hydroxide; ammonium hydroxide; a metal hydroxide; an alkali metal hydroxide; an alkali earth metal hydroxide; sodium hydroxide; and lithium hydroxide.

5. A method according to any preceding claim,wherein the base is added to the effluent stream at a temperature of: at least 0°C, 10°C, 20°C, 30°C, 40°C or 50°C; no more than 90°C, 80°C, 70°C, or 60°C; 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 base is added to the effluent stream at room temperature.

7. A method according to any preceding claim,wherein, prior to addition of the base, at least a portion of the iridium is present in the effluent stream as dissolved H2lr(OH)6.

8. A method according to any preceding claim,wherein, after addition of the base, at least a portion of the iridium is solid lr(OH)4.

9. A method according to any preceding claim,wherein, after addition of the base to the effluent stream to raise the pH of the effluent, a reducing agent is added to the effluent stream to increase precipitation of iridium from the effluent stream.

10. A method according to claim 9,wherein the reducing agent is selected from one or more of: a hydride; a borohydride; NaBH4; NH2NH2; a peroxide; and hydrogen peroxide.

11. A method according to any preceding claim,wherein the solid iridium salt is separated from the effluent stream by one or more of filtration, centrifugation, and / or another solid-liquid separation technique to recover the solid iridium salt for further processing.

12. A method according to any preceding claim,wherein the iridium containing effluent stream is from an iridium oxide manufacturing process.

13. A method according to claim 12,wherein the recovered iridium salt from the effluent is re-used in the iridium oxide manufacturing process.

14. A system for performing the method according to any preceding claim, the system comprising:an iridium oxide synthesis section for manufacturing iridium oxide, said iridium oxide synthesis section generating an iridium oxide product and an iridium containing effluent stream; andan effluent processing section for recovering iridium from the effluent stream, the effluent processing section being configured to add a base to the effluent stream to increase pH of the effluent stream and precipitate the iridium as a solid iridium salt, the effluent processing section being further configured to separate the solid iridium salt from the effluent stream.