Method for selectively recovering volatile PT or PD and system thereof

Abstract

The present disclosure relates to a process for selectively recovering Pd, Pt, or a combination of Pd and Pt from a gas phase comprising volatile noble metals. The process comprises the step of contacting the gas phase comprising a volatile noble metal with a recovery system to recover Pd, Pt or a combination of Pd and Pt, wherein the recovery system is coated with metallic Ag. The present disclosure further relates to a system for catalytic conversion of ammonia to nitric acid. The present disclosure further relates to a method for modifying a system for the catalytic conversion of ammonia to nitric oxide, the system comprising a catalytic system comprising one or more noble metals selected from the group consisting of Pd, Pt and Rh.

Description

Technical Field

[0001] The present disclosure is in the field of methods and related systems for recovering precious metals evaporated in the gas phase.

[0002] introduce

[0003] Some reactions require catalysts because these reactions are thermodynamically unfavorable or not very favorable. Such catalysts are usually based on precious metals, such as the so-called platinum group metals. The six platinum group metals are ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt). Other metals that catalysts can be based on include copper (Cu), silver (Ag), gold (Au), mercury (Hg), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn) and rhenium (Rh).

[0004] An example of a reaction involving the use of a catalyst comprising a noble metal is the oxidation of ammonia to nitrogen monoxide in the first step of producing nitric acid according to the Ostwald process. Ammonia oxidation catalysts typically comprise platinum and rhodium in alloy form to assist in the oxidation of ammonia, i.e. the fixing of oxygen to the nitrogen atom of ammonia.

[0005] The catalyst and the corresponding support structure (usually referred to as a burner basket) in the ammonia burner are subjected to temperatures of up to 950°C. Therefore, it is a challenge for platinum to evaporate and be lost in the gas phase. Typically, since platinum is expensive, a recovery screen including palladium is placed in the ammonia oxidation burner to recover the platinum lost in the gas phase. These recovery screens are located near and below the catalyst screen, that is, downstream of the catalyst screen, and the gas flow flows from the top to the bottom of the ammonia oxidation burner. Therefore, the recovery screen is also exposed to temperatures of up to 950°C, and the expensive palladium in the palladium-based recovery screen is also typically lost in the gas phase in the ammonia oxidation burner. Background Art

[0006] GB1343637 relates to a method for recovering platinum group metals entrained in a hot gas stream (Engelhard Minerals & Chemicals Corp, 1971) and related apparatus, wherein the gas passes through a getter device, which is an inert ceramic honeycomb structure coated with a getter containing gold to absorb platinum. The document defines gettering as a method of removing platinum group metals from a gas stream by surface absorption, and states that gold, silver, palladium, rhodium and platinum and combinations thereof are effective getter materials for platinum group metals. However, in the method / apparatus contemplated in GB1343637, it is further disclosed that silver or silver-containing alloys cannot be used because the silver will migrate into the ceramic honeycomb structure and have a detrimental effect on it.

[0007] EP63450 (Johnson Matthey, 1982) generally discloses a getter device and related methods for recovering precious metals lost in precious metal-containing catalysts operating at high temperatures, wherein the getter comprises an aggregate or assembly of non-woven fibers made of a metal selected from the group consisting of ruthenium, palladium, iridium, platinum, gold, silver, rhodium and alloys containing one or more of the metals. The document focuses mainly on Pd / Au alloys and does not mention the use of silver to recover volatilized palladium.

[0008] GB668935 (Evonik Degussa GmbH, 1949) relates to a method and related apparatus for recovering platinum from volatilized platinum, which volatilized platinum originates from a catalyst used, for example, to oxidize ammonia to nitrogen oxides. Although the document focuses mainly on the use of gold as a platinum capture material, it mentions that silver can also be used as a platinum trap. In this context, GB668935 claims a method for recovering platinum, in which platinum is captured on the surface of a baffle, which is placed at a temperature of at least 700°C, and some of the baffles have a silver coating or an alloy coating of silver with gold, palladium or platinum. However, although it has been found that silver coatings on metal supports (such as Ni) are stable, the use of silver also has some disadvantages (such as stability).

[0009] US20130149207 (Johnson Matthey, 2012) relates to an exhaust system device, which includes a platinum and palladium catalyst, a downstream SCR catalyst and a component capable of capturing gas-phase platinum group metals and / or alloying with gas-phase platinum group metals, wherein the component is typically a metal selected from the group consisting of gold, palladium and silver, preferably a Pd / Au alloy.

[0010] Therefore, there remains a need for a solution to selectively recover the palladium and platinum lost in the catalytic gauze. Summary of the invention

[0011] The present application addresses one or more of the above needs. The present inventors have surprisingly discovered that metallic silver can selectively capture palladium and platinum, but not rhodium, from a gas phase containing these volatile precious metals.

[0012] In one aspect of the present disclosure, a method for recovering Pd, Pt, or a combination of Pd and Pt from a gas phase containing volatile precious metals is disclosed. The method comprises the following steps:

[0013] Providing a vapour phase comprising a volatile precious metal, the vapour phase comprising at least (i) Rh and (ii) Pt and / or Pd;

[0014] • Contacting the gas phase containing the volatile precious metal with a recovery system to recover Pd, Pt or a combination of Pd and Pt, wherein the recovery system is coated with metallic Ag to selectively remove Pt and / or Pd from the gas phase containing the volatile precious metal.

[0015] The inventors have found that using only Ag metal as the metal present in the recovery screen can selectively recover platinum and palladium, rather than the remaining platinum group metals. Since these metals are expensive, it is very beneficial to recover them. The inventors have found that although palladium and platinum oxides are formed, these oxides in the gas phase can be recovered by the recovery system: the oxides of palladium and platinum are unstable, and the corresponding metals can then diffuse and form alloys with the Ag in the recovery system, thereby recovering platinum and palladium. Compared with Pt and Pd, other platinum group metals (such as in the catalytic screen for converting ammonia into nitric acid) are basically unrecoverable: the oxides of platinum group metals other than Pt and Pd are stable, and these metals cannot diffuse in the Ag recovery system, so alloys will not be formed, and metal recovery cannot be performed.

[0016] In one embodiment of the method according to the present disclosure, the gas phase comprising volatile precious metals comprises at least Pt and Rh; or at least Pd, Pt, and Rh.

[0017] In certain embodiments of the methods of the present disclosure, the amount of Pt and / or Pd recovered by the recovery system coated with metallic Ag is at least 2 times, preferably at least 5 times, the amount of Rh recovered by the recovery system coated with metallic Ag.

[0018] In one embodiment of the method according to the present disclosure, the gas phase containing the volatile precious metal is contacted with the recovery system at a temperature in the range of 550° C. to 950° C. In one embodiment of the method according to the present disclosure, the gas phase containing the volatile precious metal is contacted with the recovery system at a temperature in the range of 550° C. to below 700° C., or 600° C. to below 700° C., or 650° C. to below 700° C.

[0019] In one embodiment of the method according to the present disclosure, the method further comprises the following steps:

[0020] - operating a catalytic system, in particular a catalytic system comprising a catalyst comprising at least Pt and Rh, in a system for catalytically converting ammonia into nitrogen monoxide, wherein a volatile noble metal evaporates from the catalytic system, thereby generating a gas phase comprising the volatile noble metal.

[0021] In one embodiment of the method according to the present disclosure, the volatile noble metals Pt and Rh or the volatile noble metals Pt, Pd and Rh are evaporated from the catalytic system.

[0022] In one embodiment of the method according to the present disclosure, the catalytic system is operated at a temperature in the range of 750 to 950°C, such as in the range of 750 to 850°C, or in the range of 800 to 850°C, or in the range of 750 to 800°C.

[0023] In one embodiment of the method according to the present disclosure, the catalytic system is operated at a temperature in the range of 800°C to 950°C, and the recovery system includes a plurality of woven or knitted metal Ag meshes stacked together, or a multi-layer ceramic or glass carrier coated with metal Ag, or a ceramic monolithic structure, or a combination thereof.

[0024] In one aspect of the present disclosure, a method for converting a system for catalytically converting ammonia into nitric oxide into the system of the present disclosure is disclosed, the system comprising:

[0025] A catalytic system comprising:

[0026] o a catalyst comprising one or more noble metals selected from the group consisting of Pd, Pt and Rh, in particular a catalyst comprising at least Pt and Rh, and optionally Pd; and

[0027] o a Pd-containing recovery system for recovering volatile precious metals that evaporate from the catalytic system during the conversion of ammonia to nitric oxide;

[0028] The method comprises the following steps:

[0029] • Introducing a recovery system downstream of the catalytic system, the recovery system comprising a woven or knitted metallic Ag mesh, a ceramic or glass support coated with metallic Ag, or a combination of both.

[0030] In one embodiment of the method for transformation according to the present disclosure, the method further comprises the following steps:

[0031] Remove the recovery system containing Pd.

[0032] Further related aspects of the present disclosure provide the use of metallic silver for selectively removing Pt and / or Pd from a gas phase containing volatile precious metals, wherein the gas phase contains at least volatile (i) Rh and (ii) Pt and / or Pd. In certain embodiments, the gas phase containing volatile precious metals is produced by the catalytic oxidation of ammonia to nitric oxide. More specifically, the metallic silver is contained in a recovery system, wherein the recovery system comprises a metallic Ag mesh or a ceramic or glass carrier coated with metallic Ag, or a combination of the two, in particular wherein the recovery system comprises a plurality of woven or knitted metallic Ag meshes stacked together, or multiple layers of ceramic or glass carriers coated with metallic Ag, or a combination of the two. DETAILED DESCRIPTION

[0033] Throughout the description and claims of this application document, the word "comprise" and its variations mean "including but not limited to", and it is not intended to (and does not) exclude other parts, additives, components, integral bodies or steps. Throughout the description and claims of this disclosure, unless the context requires otherwise, the singular reference includes the plural. In particular, where the indefinite article is used, the disclosure will be understood to contemplate the plural as well as the singular, unless the context requires otherwise.

[0034] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the present disclosure will be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All features disclosed in the features of the present disclosure (including the description, claims, abstract and drawings) and / or all steps of the steps of any method or process disclosed thereby may be combined in any combination, except for combinations in which at least some of the features and / or steps are mutually exclusive. The present disclosure is not limited to the details of any foregoing embodiments. The present disclosure extends to any novel feature or any combination of novel features in the features disclosed in the present disclosure (including the description, claims, abstract and drawings), or to any novel step or any combination of novel steps in the steps of any method or process disclosed thereby.

[0035] Numerical values ​​recited with reference to the accompanying drawings include all values ​​and fractions within these ranges, as well as the cited endpoints. The terms "range from ... to ..." or "in the range of ... to ..." or "up to" as used when referring to a range of measurable values ​​(such as a parameter, amount, time period, etc.) are intended to include the limits associated with the disclosed ranges.

[0036] When the term "about" is applied to a particular value or range, the value or range is to be interpreted as being as accurate as the method used to measure the value or range.

[0037] As defined herein, the term "noble metal" refers to the platinum group metals, namely, ruthenium, rhodium, palladium, osmium, iridium and platinum.

[0038] As defined herein, the catalytic system generally comprises a catalyst, in particular a catalyst comprising one or more platinum group metals, and optionally comprises a recovery system, in particular a Pd-based recovery system known in the art, i.e. a recovery system comprising at least 70 wt% or at least 80 wt% Pd based on the total weight of the metals constituting the recovery system. More specifically, the catalytic system defined herein comprises a catalytic gauze, in particular comprising one or more platinum group metals, even more specifically at least Pt and Rh, and a recovery gauze containing Pd.

[0039] As defined herein, the term "selectivity", particularly in the context of selective recovery of a first precious metal versus a second precious metal, refers to at least 2 to 2.5 times more of the first precious metal than the second precious metal, such as recovered by a recovery system according to the present disclosure. Preferably, the term "selectivity" or "selective recovery" refers to at least 5 times or at least 10 times more of the first precious metal recovered by a recovery system according to the present disclosure relative to the second precious metal. Selectivity or selective recovery can be expressed in terms of weight % or atomic % based on the total weight or total atoms of metallic silver and captured precious metals, respectively, in a metallic silver-based recovery system.

[0040] A "recovery system" as contemplated herein refers to a system or element, particularly a metal-based system or element, designed to capture and recover volatile precious metals from the gas phase, thereby removing volatile precious metals from the gas phase. As used herein, the term "recover" or synonyms such as capture or captured are used to indicate that the elements recovered or captured by the recovery system according to the present application are incorporated into metallic silver. The recovery or capture of a particular element by the recovery system according to the present application can be determined by scanning electron microscopy (SEM) with energy dispersive X-ray fluorescence analysis (EDS) and / or inductively coupled plasma (ICP) spectroscopy, particularly by comparing data obtained by these techniques before and after the recovery system according to the present application is contacted with a gas containing one or more volatile platinum group metals, particularly Rh, Pt and / or Pd.

[0041] The recovery system comprising metallic silver or coated with metallic silver as contemplated herein is used synonymously with the term "Ag-based recovery system", wherein the major part of the metal constituting the recovery system, i.e. at least 70 wt% or at least 80 wt% up to 100%, is silver. The recovery system comprising metallic silver or coated with metallic silver as contemplated herein typically contains at least 90 wt% metallic silver, in particular at least 95 wt% metallic silver, more in particular at least 97 wt%, at least 98 wt% or even at least 99 wt% metallic silver, such as at least 99.5 wt% metallic silver. Based on the total weight of the metal constituting the wire mesh or coating. It will be appreciated that the composition of the recovery system comprising metallic silver will change during operation, in particular due to the capture of volatile precious metals and evaporation losses of metallic silver.

[0042] In one aspect of the present disclosure, a method for selectively recovering Pd, Pt, or a combination of Pd and Pt from a gas phase containing volatile precious metals is disclosed. The method comprises the step of contacting the gas phase containing volatile precious metals with a recovery system to recover Pd, Pt, or a combination of Pd and Pt, wherein the recovery system is coated with metallic Ag, thereby selectively removing Pt and / or Pd from the gas phase containing volatile precious metals. Typically, prior to this step, the method comprises providing a gas phase containing volatile precious metals, in particular wherein the gas phase comprises at least (i) Rh and (ii) Pt and / or Pd.

[0043] The inventor finds that only using Ag metal as the metal present in the recovery system (such as recovery wire mesh), platinum and palladium can be selectively recovered, rather than the remaining platinum group metals. Because these metals are expensive, it is very beneficial to recycle them. The inventor finds that, although palladium and platinum oxides are formed, these oxides in the gas phase can be recovered by the recovery system: the oxides of palladium and platinum are unstable, and the corresponding metals can then diffuse and form alloys with the Ag in the recovery system, thereby selectively recovering platinum and palladium from the Ag recovery system. On the contrary, the oxides of other precious metals are stable, so they can not form alloys with the Ag in the recovery wire mesh, so they can not be recovered from the Ag recovery system. Specifically, the amount of Pt and / or Pd recovered by the recovery system coated with metallic Ag is at least 2 times, preferably at least 5 times or even at least 10 times, of the amount of Rh recovered by the recovery system coated with metallic Ag.

[0044] In one embodiment of the method according to the present disclosure, the gas phase comprises volatile precious metals, comprises Pt and Rh; or comprises Pd, Pt, and Rh.

[0045] The production of nitric acid by oxidation of ammonia according to the Ostwald process is a widely used industrial process. As mentioned above, the catalytic gauze as part of the ammonia oxidation burner includes platinum and rhodium for catalytic conversion of ammonia into nitrogen monoxide. Therefore, the system of the present disclosure is very suitable for implementation in an ammonia oxidation burner to selectively recover platinum metal lost after evaporation from the catalytic gauze, but not rhodium.

[0046] As mentioned above, the recovery wire mesh based on Pd is usually integrated in a conventional ammonia oxidation burner. That is, the recovery wire mesh based on Pd is combined with a conventional catalytic wire mesh containing Pt and Rh, and the recovery wire mesh and the catalytic wire mesh form a catalytic system together. In the presence of the recovery wire mesh based on Pd, when ammonia is catalytically converted into nitrogen monoxide at a temperature of 800°C or higher, such as up to 950°C, the Pt evaporated from the catalytic wire mesh is recovered by the Pd recovery wire mesh. The inventors have found that the recovery system coated with Ag can be combined with a catalytic system including a catalytic Pt and Rh wire mesh and a recovery system based on Pd (such as a recovery wire mesh based on Pd): not only the recovery rate of Pt is improved by the presence of the recovery system based on Pd and coated with Ag, but the presence of the recovery system coated with Ag can further recover the Pd evaporated from the recovery wire mesh based on Pd. In other words, Pt and Pt can be selectively recovered, but rhodium cannot be recovered, and the recovery of precious Pt metal from the recovery wire mesh based on Pd does not cause the loss of precious Pd metal.

[0047] In addition, the inventors have observed that Pt oxide is less stable than Pd oxide in the gas phase, which means that at temperatures below 800°C, Pd oxide will not diffuse in the silver recovery system, so Pt can be selectively recovered.

[0048] In one embodiment of the method according to the present disclosure, the gas phase containing the volatile precious metals is contacted with the recovery system at a temperature in the range of 550°C to 950°C.

[0049] The inventors have demonstrated that the method of the present disclosure allows the gas phase produced by the recovery system to be produced over a wide temperature range. Therefore, the method of the present disclosure can be applied to a wide range of catalytic processes. Specifically, the inventors have demonstrated that the catalytic conversion of ammonia to nitric acid can be carried out at a temperature as low as 750°C, while maintaining a conversion rate of ammonia to nitrogen monoxide above 90%, and subsequently achieving a gas phase temperature as low as 550°C. The gas phase directly flowing out of the catalyst system for the catalytic conversion can actually be further adjusted, for example, by using a heat exchange system, so that the temperature is as low as 550°C when contacting the recovery system.

[0050] When a gas phase with a reduced temperature is generated, the energy consumption of the production process can be reduced by generating the gas phase at a reduced temperature. In addition, the evaporation of all precious Pd, Pt and Ag metals is reduced, thereby reducing the loss of these metals in the production process. In addition, in the process of reducing the temperature of the gas phase, more heat can be recovered from the gas phase.

[0051] In one embodiment of the method according to the present disclosure, the gas phase containing volatile precious metals is contacted with the recovery system at a temperature ranging from 550°C to less than 700°C, or from 600°C to less than 700°C, or from 650°C to less than 700°C.

[0052] As described above, generating a gas phase with a lower temperature by producing the gas phase at a lower temperature can minimize the loss of precious metals and facilitate additional heat recovery. For Ag, it is particularly beneficial to generate a gas phase with a temperature range of 500 to less than 700°C, or 600 to less than 700°C, or 650 to less than 700°C, so as to reduce the evaporation of Ag and thus reduce the loss of Ag, so that the recovery of precious Pd and Pt metals from the recovery system is not accompanied by the loss of Ag metal.

[0053] In one embodiment of the method according to the present disclosure, the gas phase comprising the volatile precious metal is contacted with the recovery system at a temperature in the range of 750°C to 950°C or 750 to 930°C.

[0054] In one embodiment of the method according to the present disclosure, the method further comprises the step of operating the catalytic system in a system for catalytically converting ammonia to nitric oxide, wherein the volatile precious metal evaporates from the catalytic system, thereby producing a gas phase comprising the volatile precious metal.

[0055] The production of nitric acid by oxidation of ammonia according to the Ostwald process is a widely used industrial process. As mentioned above, the catalytic gauze as part of the ammonia oxidation burner includes platinum and rhodium for catalytic conversion of ammonia into nitric acid. Therefore, the system of the present disclosure is very suitable for implementation in an ammonia oxidation burner to selectively recover platinum metal from the catalytic gauze after evaporation.

[0056] In one embodiment of the method according to the present disclosure, the volatile noble metals Pt and Rh or the volatile noble metals Pt, Pd and Rh are evaporated from the catalytic system.

[0057] As described above, a Pd-based or Pd-containing recovery screen is typically integrated in an ammonia oxidation burner in a conventional manner. That is, the Pd-based or Pd-containing recovery screen is combined with a conventional Pt- and Rh-containing catalytic screen, and the recovery screen and the catalytic screen together form a catalytic system. In the presence of a Pd-based recovery screen, when ammonia is catalytically converted into nitric acid at a temperature of 800°C or higher, such as up to 950°C, the Pt evaporated from the catalytic screen is recovered by the Pd-based recovery screen. The inventors have found that the Ag-coated or Ag-based recovery system according to the present disclosure can be combined with a catalytic system of a recovery system including a catalytic Pt and Rh screen and a Pd-based recovery screen: not only is the recovery rate of Pt increased by the presence of a Pd-based and Ag-coated recovery system, but the presence of the Ag-coated recovery system can also recover the Pd evaporated from the Pd-based recovery screen. In other words, both Pt and Pt can be selectively recovered, and the recovery of precious Pt metal from the Pd recovery mesh does not result in the loss of precious Pd metal.

[0058] In addition, the inventors have observed that Pt oxide is less stable than Pd oxide in the gas phase, which means that at temperatures below 800°C, Pd oxide will not diffuse in the silver recovery system, so Pt can be selectively recovered.

[0059] In one embodiment of the method according to the present disclosure, the catalytic system is operated at a temperature in the range of 750 to 950°C, or in the range of 750 to 850°C, or in the range of 800 to 850°C, or in the range of 750 to 800°C.

[0060] The inventors have demonstrated that the method of the present disclosure can be achieved by operating the catalytic system over a wide temperature range. Therefore, the method of the present disclosure can be applied to a wide range of catalytic processes. Specifically, the inventors have demonstrated that the catalytic conversion of ammonia to nitric acid can be carried out at temperatures as low as 750° C. while maintaining a conversion rate of ammonia to nitric oxide above 90%.

[0061] When the catalytic conversion is carried out at temperatures as low as 750°C, the energy consumption of the production process is reduced. In addition, the evaporation of Pd, Rh, Pt and Ag metals is reduced, thereby reducing the losses of these metals in the production process. In addition, a lower temperature gas phase is produced, which minimizes the loss of silver in the recovery system.

[0062] In one embodiment of the method according to the present disclosure, the catalytic system is operated at a temperature in the range of 750 to 950°C, or at 750 to 850°C, or at 800 to 850°C, or at 750 to 800°C, or at 850 to 950°C, or at a temperature in the range of 850 to 930°C, and the recovery system comprises a plurality of woven or knitted metallic Ag wire meshes stacked together, or a multilayer ceramic or glass support coated with metallic Ag, or a ceramic monolithic structure, or a combination thereof. Advantageously, it is observed that the silver wire does not expand during operation, which limits the development of pressure drop over the silver-based water collection system.

[0063] The inventors have found that when the recovery system includes a plurality of woven or knitted metal Ag screens stacked together, or a ceramic or glass carrier coated with metal Ag in multiple layers, or a ceramic monolithic structure, or a combination thereof, the Ag loss in the evaporation stage is reduced. Without being bound by theory, it is believed that when several recovery elements coated with Ag are stacked together, some elements are upstream relative to other elements, that is, they contact the gas flow generated by the catalytic conversion earlier than other elements. Therefore, volatile Ag becomes abundant in the gas phase, so the evaporation loss of Ag in the downstream recovery element coated with Ag is reduced.

[0064] Therefore, for catalytic processes conducted at temperatures between 800 and 950°C, it would be beneficial to practice the method of the present disclosure using a recovery system comprising a plurality of woven or knitted metallic Ag meshes, or a multilayer (ceramic or glass support coated with metallic Ag, or a ceramic monolithic structure, or a combination thereof, stacked together.

[0065] Ceramic tiles are particularly suitable as layers, and Raschig rings and particles are particularly suitable as supports coated with silver.

[0066] One aspect of the present disclosure generally provides a system including a catalyst comprising at least Pt and Rh; and a recovery system downstream of the catalyst, wherein the recovery system contains metallic Ag, and in particular is coated with metallic Ag.

[0067] In a specific embodiment of the present disclosure, a system for catalytically converting ammonia into nitric acid is disclosed. The system comprises a catalytic system, the catalytic system comprising a catalyst, comprising one or more precious metals selected from the group consisting of Pd, Pt and Rh, in particular comprising at least Pt and Rh; and a recovery system located downstream of the catalytic system for recovering Pd, Pt or a combination of Pd and Pt, the recovery system being coated with metallic Ag.

[0068] The system is characterized by a catalytic system that can operate in the temperature range of 750 to 950°C.

[0069] Optionally, the catalytic system may further include a Pd-containing recovery system for recovering volatile precious metals evaporated from the catalyst. It is understood that the metallic Ag-based recovery system according to the present disclosure is located downstream of the catalyst and the Pd-containing recovery system.

[0070] In one embodiment of the system according to the present disclosure, the catalytic system is an ammonia oxidation burner, whereby nitric oxide is produced by oxidation of ammonia. In other words, in certain embodiments, the catalytic system is a system for catalytically converting ammonia into nitric oxide to produce nitric acid. Therefore, the present invention contemplates an ammonia oxidation burner including a catalytic system according to the present disclosure.

[0071] The inventors have found that by using only Ag metal as the metal in the recovery wire mesh, platinum and palladium can be selectively recovered without recovering the remaining platinum group metals, particularly rhodium. Since these metals are expensive, it is very beneficial to recover them. The inventors have found that although palladium and platinum oxides are formed, these oxides in the gas phase can be recovered by an Ag-based recovery system: the oxides of palladium and platinum are unstable, and the corresponding metals can then diffuse and form alloys with the Ag in the recovery system, thereby selectively recovering platinum and palladium from the Ag recovery system. In contrast, the oxides of other precious metals are stable, so they cannot form alloys with the Ag in the recovery wire mesh, so they cannot be recovered from the Ag recovery system.

[0072] In addition, the inventors have found that the system can be operated at a temperature as low as 750°C to achieve the desired catalytic conversion rate, such as a catalytic conversion rate of ammonia to nitrogen monoxide of more than 90%. The system can still operate at a temperature as high as 950°C, so a wide range of catalytic reactions can be performed using the system of the present disclosure.

[0073] Furthermore, when the catalytic conversion is carried out at temperatures as low as 750°C, the energy consumption of the system operation is reduced. In addition, the evaporation of Pd, Pt and Ag metals is reduced, thereby reducing the loss of these metals during the production process.

[0074] In one embodiment of a system according to the present disclosure, the catalytic system may be operated in a temperature range of 750 to 850°C, or 800 to 850°C, or 750 to 800°C.

[0075] As mentioned above, catalytic conversion at lower temperatures can minimize the loss of precious metals. For silver, operating at a temperature in the range of 750 to 850°C or 800 to 850°C or 750 to 800°C is particularly beneficial in order to reduce the evaporation of Ag and thereby reduce the loss of Ag so that the recovery of precious Pd and Pt metals from the recovery system is not accompanied by the loss of precious Ag metal.

[0076] In one embodiment of a system according to the present disclosure, the catalytic system may operate at a temperature of 850 to 950°C, and the recovery system includes multiple woven or knitted metal Ag meshes stacked together, or multiple layers of ceramic or glass carriers coated with metal Ag, or a combination of both.

[0077] Ceramic tiles are particularly suitable as layers, and Raschig rings and particles are particularly suitable as supports coated with silver.

[0078] The inventors have found that when the recovery system includes a plurality of woven or knitted metal Ag screens stacked together, or a plurality of ceramic or glass carriers coated with metal Ag, or a combination of the two, the Ag loss in the evaporation stage is reduced. Without being bound by theory, it is believed that when several Ag-coated recovery elements are stacked together, some elements are upstream relative to other elements, that is, they are in contact with the gas flow generated by the catalytic conversion earlier than other elements. Therefore, Ag becomes abundant in the gas phase, so the evaporation loss of Ag in the downstream Ag-coated recovery element is reduced.

[0079] Therefore, in catalytic processes at temperatures above 800° C., it is beneficial to use a recovery system comprising multiple woven or knitted metallic Ag meshes stacked together, or multiple layers of ceramic or glass supports coated with metallic Ag, or a combination of both. Furthermore, considering the importance of nitric acid production using Pt-Rh catalyst meshes and Pd recovery systems, it is beneficial to operate the catalytic system between 850 and 950° C. in order to oxidize ammonia to nitric oxide.

[0080] In one embodiment of the system according to the present disclosure, the recycling system includes 3 to 50 woven or knitted metal Ag screens, 10 to 30 layers of ceramic or glass carriers coated with metal Ag, or a combination of woven or knitted metal Ag screens and ceramic or glass carrier layers coated with metal Ag, wherein the number of combined screens and layers ranges from 3 to 50.

[0081] It has been found that when the number of Ag-coated recycling elements stacked together ranges from 3 to 50, Ag loss is reduced when the recycling system is exposed to the gas phase in the temperature range of 800 to 950°C.

[0082] In one embodiment of the system according to the present disclosure, the wire mesh has a wire diameter ranging from 150 to 400 microns.

[0083] The use of such a mesh can increase the surface area, thereby allowing optimal contact between the gas and the mesh, thereby optimally capturing the precious metals platinum, palladium, or a mixture of platinum and palladium in the gas phase and subsequently forming Ag-Pt and / or Ag-Pd alloys to recover Pt and / or Pd precious metals in the gas phase.

[0084] A person skilled in the art can optimize the diameter without difficulty so that the amount of silver present on the wire mesh is sufficient to achieve the target silver recovery rate. The system of the present disclosure is flexible because the diameters of the different wire meshes present in the system can have different diameters, and the diameter decreases in the downstream direction. This means that the most upstream wire mesh has a larger diameter to provide some sacrificial silver, that is, silver that is about to evaporate and contribute to the silver saturation of the vapor phase, thereby reducing the evaporation of silver in the downstream wire mesh.

[0085] In one aspect of the present disclosure, a method for converting a system for catalytically converting ammonia into nitric oxide into a system of the present disclosure is disclosed, the system comprising a catalytic system, the catalytic system comprising: (i) a catalyst comprising one or more precious metals selected from the group consisting of Pd, Pt and Rh, in particular a catalyst comprising at least Pt and Rh and optionally Pd, and (ii) a Pd-containing recovery system for recovering volatile precious metals evaporated from the catalytic system during the conversion of ammonia into nitric oxide. The method includes the step of introducing a recovery system containing metallic Ag, in particular coated with metallic Ag. In certain embodiments, the recovery system containing metallic Ag comprises a woven or knitted metallic Ag mesh, a ceramic or glass carrier coated with metallic Ag, or a combination of the two.

[0086] As described above, a Pd-containing recovery wire mesh is usually integrated in a conventional ammonia oxidation burner for catalytic conversion of ammonia into nitric acid. That is, the Pd-containing recovery wire mesh is combined with a conventional catalytic wire mesh containing Pt and Rh, and the recovery wire mesh and the catalytic wire mesh together form a catalytic system. In the presence of a Pd-containing recovery wire mesh, when ammonia is catalytically converted into nitric acid at a temperature of 800°C or higher, such as up to 950°C, the Pt evaporated from the catalytic wire mesh is recovered by the Pd recovery wire mesh. The inventors have found that a recovery system coated with Ag can be combined with a catalytic system including a catalytic Pt and Rh wire mesh and a recovery wire mesh containing Pd: not only the recovery rate of Pt is increased by the presence of Pd and the recovery system coated with Ag, but also the presence of the recovery system coated with Ag can further recover the Pd evaporated from the recovery wire mesh containing Pd. In other words, both Pt and Pt can be selectively recovered, and the recovery of precious Pt metal from the Pd recovery wire mesh does not result in the loss of precious Pd metal.

[0087] In one embodiment of the method for transformation according to the present disclosure, the method further comprises the step of removing the recovery system containing Pd.

[0088] Alternatively, since Ag is suitable for recovering the volatile precious metal Pt, only Pt and Rh based catalytic mesh can be used as part of the catalytic system to convert ammonia to nitric oxide and then nitric oxide to nitric acid without using Pd recovery mesh, thus simplifying the system.

[0089] Further related aspects of the present disclosure provide the use of metallic silver for selectively removing Pt and / or Pd from a gas phase containing volatile precious metals, wherein the gas phase contains at least volatile (i) Rh and (ii) Pt and / or Pd. In certain embodiments, the gas phase containing volatile precious metals is produced by the catalytic oxidation of ammonia to nitric oxide. More specifically, the metallic silver is contained in a recovery system, wherein the recovery system comprises a metallic Ag mesh or a ceramic or glass carrier coated with metallic Ag, or a combination of the two, in particular wherein the recovery system comprises a plurality of woven or knitted metallic Ag meshes stacked together, or multiple layers of ceramic or glass carriers coated with metallic Ag, or a combination of the two.

[0090] Advantageously, the use of metallic Ag as the only metal present in a recovery system (such as a recovery screen) with a catalyst comprising Rh and Pt and optionally Pd downstream enables selective recovery of platinum and palladium rather than the remaining platinum group metals, in particular Rh. Since these metals are expensive, it is very beneficial to recover them. Specifically, the amount of Pt and / or Pd recovered by a recovery system coated with metallic Ag is at least 2 times, preferably at least 5 times or even at least 10 times the amount of Rh recovered by a recovery system coated with metallic Ag.

[0091] Example

[0092] Three sets of Ag mesh (each set includes 3 Ag meshes in sequence) were installed in a 9 cm pipeline at three different positions downstream of the catalytic wire mesh for converting ammonia to nitric oxide and operating at about 900 ° C, and they were exposed to three temperatures: 668 ° C, 598 ° C, and 566 ° C. The catalytic wire mesh is composed of Pt and Rh. A Pd-based recovery wire mesh is also part of the catalytic system. The Ag mesh is from Goodfellows Metals and is also a 190 μm plain weave with a wire diameter of 400 μm and 62 cells per square centimeter.

[0093] Each silver mesh is sandwiched between a pair of Megapyr wire meshes. This is to ensure that there is no risk of fusion between the silver meshes, so that a good quality balance of each silver mesh can be obtained. The stack of three meshes is rotated 15 relative to each other. o .

[0094] The Ag mesh exposed to 668°C lasted 30.8 days. The Ag mesh exposed to 598°C lasted 35.3 days. The Ag mesh exposed to 566°C lasted 27.1 days.

[0095] The recovered webs were weighed and chemically analyzed by SEM / EDS analysis and ICP, which are known to those skilled in the art. Tables 1, 2 and 3 summarize the mass change and chemical analysis of two tests for webs subjected to temperatures of 668°C, 598°C and 566°C, respectively.

[0096] Table 1. Analysis of Ag mesh subjected to 668°C

[0097]

[0098] Table 2. Analysis of Ag mesh subjected to 598°C

[0099]

[0100] Table 3. Analysis of Ag mesh subjected to 566°C

[0101]

[0102] The higher the temperature, the greater the Ag loss. The Ag loss decreased with each subsequent Ag screen. This is probably because the upstream screen partially saturates the gas flow with Ag vapor, which reduces the Ag loss in the downstream screen. In general, the weight and atomic % of Pt recovered is greater than that of Pd. On an atomic % basis, about 25% more Pt was recovered on the Ag screen than Pd. Rh was not captured in either test.

[0103] In the sample exposed to 668°C, some clear gradients in the capture process are observed, with higher concentrations of Pt and Pd in ​​the upstream mesh. This can be explained by the higher loss of silver in the upper mesh. At 598°C, the composition gradient is essentially absent.

[0104] Visually, the upstream side of the mesh is darker, while the downstream side remains bright. SEM analysis shows that the alloy has been restructured, but there is no obvious linear expansion that is common in water collection mesh containing Pd.

[0105] The severity of the surface reconstruction is slightly less than what we observed in the Pd recovery wire mesh. EDS analysis shows Pt and Pd contents in the range of a few atomic % compared to the bulk composition determined by ICP, indicating surface enrichment of the captured PGMs. The lack of swelling of the silver wires may be beneficial as the development of a pressure drop would not be expected to be observed on silver based water harvesting systems.

Claims

1. A method for recovering Pd, Pt or a combination of Pd and Pt from a gas phase containing volatile precious metals, the method comprising the steps of: Providing a vapour phase comprising a volatile precious metal, the vapour phase comprising at least (i) Rh and (ii) Pt and / or Pd; • contacting the gas phase comprising volatile precious metals with a recovery system to recover Pd, Pt or a combination of Pd and Pt, wherein the recovery system is coated with metallic Ag to selectively remove Pt and / or Pd from the gas phase comprising volatile precious metals.

2. The method of claim 1, wherein the gas phase comprising volatile precious metals comprises at least Pt and Rh.

3. The method according to claim 1 or 2, wherein the gas phase comprising volatile precious metals comprises at least Pd, Pt and Rh.

4. The method according to any one of claims 1 to 3, wherein the amount of Pt and / or Pd recovered by the recovery system coated with metallic Ag is at least 2 times, preferably at least 5 times, the amount of Rh recovered by the recovery system coated with metallic Ag.

5. The process according to any one of claims 1 to 4, wherein the gas phase comprising volatile precious metals is contacted with the recovery system at a temperature in the range of 550°C to 950°C.

6. The method of claim 5, wherein the gas phase comprising volatile precious metals is contacted with the recovery system at a temperature in the range of 550°C to less than 700°C, or 600°C to less than 700°C, or 650°C to less than 700°C.

7. The method according to any one of claims 1 to 6, further comprising the following steps: - operating a catalytic system, in particular a catalytic system comprising a catalyst containing at least Pt and Rh, in a system for the catalytic conversion of ammonia into nitrogen monoxide, wherein a volatile noble metal is evaporated from said catalytic system, thereby producing said gas phase comprising the volatile noble metal.

8. The method according to claim 6, wherein the volatile noble metals Pt and Rh or the volatile noble metals Pt, Pd and Rh are evaporated from the catalytic system.

9. The process according to claim 7 or 8, wherein the catalytic system is operated at a temperature in the range of 750 to 950°C.

10. The method according to claim 9, wherein the catalytic system is operated at a temperature in the range of 750 to 850°C, in particular in the range of 750 to 800°C.

11. The method according to any one of claims 7 to 9, wherein the catalytic system is operated at a temperature in the range of 800°C to 950°C, and wherein the recovery system comprises a plurality of woven or knitted metal Ag meshes stacked together, or a plurality of ceramic or glass supports coated with metal Ag, or a combination of both.

12. A method for modifying a system for catalytically converting ammonia to nitric oxide, the system comprising: A catalytic system comprising: o a catalyst comprising at least Pt and Rh and optionally Pd; and o a Pd-containing recovery system for recovering volatile precious metals evaporated from the catalytic system during the conversion of ammonia to nitric oxide; The method comprises the following steps: • Introducing a recovery system downstream of the catalytic system, the recovery system comprising a woven or knitted metallic Ag mesh, a ceramic or glass support coated with metallic Ag, or a combination of both.

13. The method according to claim 12, further comprising the steps of: • Removal of the Pd-containing recovery system.

14. Use of metallic silver for the selective removal of Pt and / or Pd from a gas phase comprising volatile noble metals, wherein the gas phase comprises at least volatile (i) Rh and (ii) Pt and / or Pd.

15. Use according to claim 14, wherein the gas phase comprising volatile noble metals is produced by the catalytic oxidation of ammonia to nitrogen monoxide.

16. The use according to claim 14 or 15, wherein the metallic silver is included in a recycling system, wherein the recycling system comprises a metallic Ag mesh or a ceramic or glass carrier coated with metallic Ag, or a combination of the two, in particular wherein the recycling system comprises a plurality of woven or knitted metallic Ag meshes stacked together, or a plurality of layers of ceramic or glass carriers coated with metallic Ag, or a combination of the two.

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