Volatile and noble metal recovery system made of stacked silver-containing elements

Abstract

The present disclosure relates to a recovery system for recovering volatile noble metals from a gas phase, in particular for selectively recovering Pt and / or Pd but not Rh from a gas phase. The recovery system is characterized in that it comprises a plurality of woven or knitted metallic silver wire webs, or a plurality of layers of ceramic or glass carriers coated with metallic silver, or a combination of both, stacked together. The present disclosure further relates to a method for recovering volatile noble metals from a gas phase. The present disclosure further relates to a system for the catalytic conversion of ammonia to nitric oxide, comprising a recovery system according to the recovery system of the present disclosure. The present disclosure further relates to the use of a system of the present disclosure for the catalytic conversion of ammonia to nitric oxide. The present disclosure further relates to a method for retrofitting 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 systems for recovering evaporated precious metals in the gas phase.

[0002] introduce

[0003] There are several reactions that require catalysts because they are thermodynamically unfavorable or less favorable. Such catalysts are usually based on precious heavy metals, such as the so-called platinum group metals. The six platinum group metals are ruthenium, rhodium, palladium, osmium, iridium and platinum. Other metals that can serve as the basis for catalysts include copper, silver, gold, mercury, iron, cobalt, nickel, zinc and rhenium.

[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 nitric acid production according to the Ostwald process. Ammonia oxidation catalysts generally comprise platinum and rhodium in alloy form in order to assist the oxidation of ammonia, i.e. the fixing of oxygen to the nitrogen atom of ammonia.

[0005] The catalyst and corresponding support structure in the ammonia oxidation burner (commonly referred to as the burner basket) can withstand temperatures as high as 950°C. Therefore, platinum evaporation and loss in the gas phase is a challenge. Typically, since platinum is expensive, there is a recovery wire mesh in the ammonia oxidation burner containing palladium for recovering the platinum lost in the gas phase. These recovery wire meshes are located near the catalyst wire mesh and below the catalyst wire mesh, i.e., downstream of the catalyst wire mesh, where the gas flow flows from the top to the bottom of the ammonia oxidation burner. Therefore, the recovery wire mesh is also exposed to temperatures as high as 950°C, and the expensive palladium from the palladium-based recovery wire mesh is typically also lost in the gas phase of the ammonia oxidation burner. Background Art

[0006] GB1343637 relates to a process 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 in the form of an inert ceramic honeycomb structure coated with a getter containing gold to absorb platinum. The document defines gettering as a process in which platinum group metals are removed from a gas stream by absorption on a surface, and therefore states that gold, silver, palladium, rhodium and platinum and combinations thereof are effective getter materials for platinum group metals. However, in the process / 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 deleterious effect on it.

[0007] EP63450 (Johnson Matthey, 1982) generally discloses a getter device and a related process for recovering precious metals lost in precious metal-containing catalysts operating at elevated temperatures, wherein the getter comprises agglomerates or aggregates 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. This document focuses primarily 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 process and related apparatus for the recovery of platinum from volatilized platinum, which originates from a catalyst used, for example, to oxidize ammonia to nitrogen oxides. Although it focuses primarily 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 process for the recovery of platinum, in which platinum is captured on the surface of baffles placed at a temperature of at least 700°C, and some of the baffles have a coating of silver or a coating of an alloy of silver with gold, palladium or platinum. However, although it has been found that silver coatings on metal supports (e.g., Ni) are stable, the use of silver also has some disadvantages (e.g., stability).

[0009] US20130149207 (Johnson Matthey, 2012) relates to an exhaust system device comprising a platinum and palladium catalyst, and 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 this 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 that allows for flexibility of the reclaimer over a wide temperature range without interfering with other components of the system including the catalytic gauze. Summary of the invention

[0011] The present application addresses one or more of the needs indicated above. The inventors unexpectedly discovered that metallic silver can selectively capture Pd and Pt, but not Rh, from a gas phase containing these volatile precious metals. Furthermore, by providing a recovery system containing metallic Ag, which includes a plurality of woven or knitted metallic silver screens or multiple layers of ceramic or glass carriers coated with metallic silver stacked together, evaporation losses of Ag from downstream screens or layers are minimized, presumably because the gas phase is partially saturated with Ag evaporated from upstream screens or layers.

[0012] In one aspect of the present disclosure, a system is disclosed, comprising: a catalyst comprising at least Pt and Rh and optionally Pd; and a recovery system comprising metallic Ag for recovering volatile precious metals from a gas phase, the system being particularly useful for selectively recovering volatile Pt and / or Pd from a gas phase comprising Pt, Rh and optionally Pd. The system is characterized in that the recovery system comprising metallic silver comprises a plurality of woven or knitted metallic silver wire meshes stacked together, or a plurality of ceramic or glass supports coated with metallic silver, or a ceramic monolithic structure, or a combination thereof.

[0013] The inventors have found that when the recovery system includes a plurality of woven or knitted metal Ag wire meshes, or a plurality of ceramic or glass carriers coated with metal Ag, or a combination of the two, stacked together, the evaporation loss of Ag in the recovery system containing metal Ag is minimized, presumably because when a plurality of Ag recovery elements are stacked together, some elements are upstream relative to other elements, i.e., they are in contact with the gas stream generated by the catalytic conversion earlier than some other elements. As a result, the gas phase is enriched with volatilized Ag, and thus the evaporation loss of Ag in the downstream Ag recovery element is reduced.

[0014] In certain embodiments, the system further comprises a recovery system comprising Pd, located downstream of the catalyst and upstream of the recovery system comprising metallic Ag.

[0015] In one embodiment of the system according to the present disclosure, the recovery system containing metallic Ag comprises at least 3 woven or knitted metallic silver meshes, or at least 3 ceramic or glass carriers coated with metallic silver, or a combination of layers of woven or knitted metallic silver meshes and ceramic or glass carriers coated with metallic silver, wherein the number of combined meshes and layers is at least 3.

[0016] In one embodiment of the system according to the present disclosure, the recovery system containing metallic Ag includes from 3 to 50 woven or knitted metallic silver wire meshes, or from 10 to 30 ceramic or glass carriers coated with metallic silver, or a combination of layers of woven or knitted metallic silver wire meshes and ceramic or glass carriers coated with metallic silver, and the number of combined wire meshes and layers is in the range of from 3 to 50.

[0017] In one embodiment of the system according to the present disclosure, the metallic silver wire mesh has a wire diameter ranging from 150 to 400 microns, or the thickness of the metallic silver coating on the ceramic or glass carrier ranges from 150 to 400 microns.

[0018] In a particular embodiment of the system of the present disclosure, the silver wire meshes of the plurality of woven or knitted metallic silver wire meshes have different wire diameters, and / or the layers of the ceramic or glass carrier in the multilayer ceramic or glass carrier coated with metallic silver have coatings of metallic silver with different coating thicknesses. More specifically, the wire diameter of the metallic silver wire mesh and / or the coating thickness of the ceramic or glass carrier coated with metallic silver decreases in the downstream direction. Advantageously, the wire mesh with a thicker wire diameter or the layer with a thicker metallic silver coating can provide sacrificial silver, thereby promoting saturation of the gas phase with volatile Ag, thereby minimizing evaporative Ag losses in the more downstream portion of the recovery system containing Ag.

[0019] In certain embodiments, the system of the present disclosure is an ammonia oxidation reactor (also referred to as an ammonia oxidation burner) or a portion thereof.

[0020] In one aspect of the present disclosure, a method for recovering volatile precious metals from a gas phase is disclosed. The method comprises the following steps:

[0021] operating the catalyst in one embodiment of the system of the present disclosure so as to produce a gas phase comprising volatile precious metals, in particular so as to produce a gas phase comprising Pt and Rh and optionally Pd; and

[0022] • Contacting the gas phase comprising volatile precious metals with the recovery system comprising metallic silver in the system of the present disclosure, more particularly, further comprising selectively removing Pt and / or Pd (but not Rh) from the gas phase comprising precious metals.

[0023] In one embodiment of the method according to the present disclosure, the gas phase containing the volatile precious metal has a temperature in the range from 750°C to about 950°C.

[0024] In one embodiment of the method according to the present disclosure, the gas phase containing the volatile precious metal has a temperature ranging from 850°C to 950°C.

[0025] In one embodiment of the process of the present disclosure, step (a) comprises operating a catalyst for the catalytic conversion of ammonia into nitric oxide, wherein volatile precious metals are evaporated from the catalyst and optionally from a recovery system comprising Pd, thereby producing a gas phase comprising volatile precious metals, in particular a gas phase comprising Rh and Pt and optionally Pd.

[0026] In one aspect of the present disclosure, a system for catalytically converting ammonia into nitric oxide is disclosed. The system comprises:

[0027] a catalytic system comprising a catalyst containing Pt and Rh and optionally Pd and operating at a temperature ranging from 750°C to 950°C; and a recovery system optionally comprising Pd; and downstream thereof;

[0028] • A recovery system comprising silver as specified in this disclosure.

[0029] In one embodiment, the system may operate at a temperature ranging from 850°C to 950°C or from 850°C to 930°C.

[0030] In one aspect of the present disclosure, use of the system of the present disclosure for catalytic conversion of ammonia to nitric oxide in the production of nitric acid is disclosed.

[0031] In one aspect of the present disclosure, a method for modifying a system for catalytically converting ammonia to nitric oxide is disclosed, the system comprising:

[0032] o a catalyst comprising one or more noble metals selected from the group consisting of Pd, Pt and Rh, in particular comprising Pt and Rh and optionally Pd; and

[0033] o a recovery system comprising Pd for recovering evaporated volatile precious metals, in particular volatile Pt, during the conversion of ammonia into nitric oxide in the catalytic system;

[0034] The system is transformed into the system disclosed in the present invention. The method comprises the following steps:

[0035] Introducing a recovery system containing metallic silver for recovering volatile precious metals from a gas phase, wherein the recovery system containing metallic silver comprises a plurality of woven or knitted metallic silver meshes stacked together, or a plurality of ceramic or glass supports coated with metallic silver, or a ceramic monolithic structure, or a combination thereof.

[0036] In one embodiment of the method for transformation according to the present disclosure, the method further comprises the following steps:

[0037] • Remove the recovery system containing Pd. DETAILED DESCRIPTION

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] As defined herein, the term "noble metal" is generally used interchangeably with the term "platinum group metals" (ie, ruthenium, rhodium, palladium, osmium, iridium, and platinum).

[0043] As defined herein, the catalytic system generally comprises a catalyst, in particular a catalyst comprising one or more platinum group metals, and optionally a recovery system, in particular a recovery system comprising Pd as known in the art. More specifically, the catalytic system as defined herein comprises a catalytic gauze, in particular comprising one or more platinum group metals, even more specifically comprising at least Pt and Rh, and a recovery gauze comprising Pd.

[0044] As defined herein, the term "selectivity", particularly in the context of selective recovery of a first precious metal over a second precious metal, means that the first precious metal (i.e., the first platinum group metal) recovered by a recovery system according to the present disclosure is at least 2 to 2.5 times the second precious metal (i.e., the second platinum group metal). Preferably, the term "selectivity" or "selectively recovering" means that at least 5 times or at least 10 times the first precious metal relative to the second precious metal is recovered by a recovery system according to the present disclosure. Selectivity or selective recovery can be expressed as wt% or atomic%, respectively, based on the total weight or total atoms of metallic silver constituting the recovery system.

[0045] A "recovery system" as contemplated herein refers to a system or assembly designed to capture and recover volatile precious metals from the gas phase, thereby removing volatile precious metals from the gas phase. In a "recovery system comprising or containing Pd" (also referred to herein as a "Pd-containing recovery system"), Pd is primarily responsible for (i.e., constituting at least 70 wt% or at least 80 wt% of the recovery system metals) capturing volatile precious metals, particularly Pt (particularly by forming an alloy containing Pd). The term "recovery system according to the present disclosure" is used interchangeably with a recovery system comprising metallic Ag or a recovery system coated with metallic Ag.

[0046] As used herein, the term "recover" / "recovery" or similar terms are used to mean that an element recovered or captured by a recovery system according to the present application is incorporated into metallic silver. The recovery or capture of a particular element by a 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 by inductively coupled plasma (ICP) spectroscopy, in particular by comparing data obtained by these techniques before and after contacting a recovery system according to the present application with a gas containing one or more volatile platinum group metals, in particular Rh and Pt and / or Pd.

[0047] In one aspect of the present disclosure, a system is disclosed, comprising: a catalyst comprising a precious metal, in particular comprising Pt, Rh and / or Pd, more in particular comprising at least Pt and Rh and optionally Pd; and a recovery system comprising metallic Ag for recovering volatile precious metals from the gas phase. The system is characterized in that the recovery system comprising metallic Ag comprises a plurality of woven or knitted metallic silver wire meshes stacked together, or a ceramic or glass carrier coated with metallic silver in multiple layers, or a ceramic monolithic structure, or a combination thereof.

[0048] Ceramic tiles are particularly suitable as layers, while Raschig rings and spheres are particularly suitable as supports coated with silver.

[0049] A 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 proportion (i.e. at least 70 wt% or at least 80 wt% up to 100%) of the metal is silver. A recovery system comprising metallic silver or coated with metallic silver as contemplated herein typically comprises at least 90 wt% of metallic silver, in particular at least 95 wt% of metallic silver, more in particular at least 97 wt%, at least 98 wt% or even at least 99 wt% of 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 due to evaporation losses of metallic silver.

[0050] The inventors have found that when a recovery system containing metallic Ag includes a plurality of woven or knitted metallic Ag screens stacked together, or a ceramic or glass carrier coated with metallic Ag in multiple layers, or a combination of the two, the evaporation loss of Ag during the operation of the recovery system containing metallic Ag is minimized. Generally, when there are several Ag-containing recovery elements stacked together, some elements are upstream relative to other elements, i.e., they are earlier in contact with the gas flow generated by the catalytic conversion mediated by the catalyst than some other Ag-containing recovery elements. As a result, the gas phase is rich in volatile Ag, and therefore the evaporation loss of Ag in the downstream Ag recovery element is reduced.

[0051] In one embodiment of the system according to the present disclosure, the recovery system containing metallic Ag comprises at least 3 woven or knitted metallic silver meshes, or at least 3 ceramic or glass carriers coated with metallic silver, or a combination of layers of woven or knitted metallic silver meshes and ceramic or glass carriers coated with metallic silver, wherein the number of combined meshes and layers is at least 3.

[0052] When at least 3 Ag-coated recycling elements constitute the Ag-coated recycling system, the loss of silver is reduced. Therefore, in order to reduce the loss of Ag, it is beneficial to use the following items: at least 3 woven or knitted metallic silver wire meshes, or at least 3 ceramic or glass carriers coated with metallic silver, or a combination of layers of woven or knitted metallic silver wire meshes and ceramic or glass carriers coated with metallic silver, wherein the number of combined wire meshes and layers is at least 3.

[0053] In one embodiment of the system according to the present disclosure, the recovery system containing metallic Ag includes from 3 to 50 woven or knitted metallic silver wire meshes, or from 10 to 30 ceramic or glass carriers coated with metallic silver, or a combination of layers of woven or knitted metallic silver wire meshes and ceramic or glass carriers coated with metallic silver, and the number of combined wire meshes and layers is in the range of from 3 to 50.

[0054] Optionally, the system may further include a Pd-containing recovery system downstream of the catalyst for recovering volatile precious metals, such as Pt, evaporated from the catalyst. It is understood that the metallic Ag-based recovery system according to the present disclosure is located downstream of both the catalyst and the Pd-containing recovery system.

[0055] In one embodiment of a system according to the present disclosure, the system is an ammonia oxidizing burner or a portion thereof, whereby nitric oxide is produced by oxidation of ammonia. In other words, in certain embodiments, the catalytic system is a system that catalytically converts ammonia into nitric oxide to produce nitric acid. Thus, the present invention contemplates an ammonia oxidizing burner including a system according to the present disclosure.

[0056] In one embodiment of the system according to the present disclosure, the silver wire mesh has a wire diameter ranging from 150 to 400 μm, or the thickness of the metallic silver coating on the ceramic or glass carrier ranges from 150 to 400 μm.

[0057] Use of such a mesh may provide increased surface area for optimal contact of the gas with the mesh, resulting in optimal capture of the precious metal in the gas phase and subsequent alloying of the metal with Ag for recovery of the precious metal.

[0058] Those skilled in the art will have no difficulty optimizing the diameter of the silver wire mesh or the thickness of the coating 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 different wire meshes present in a recovery system containing metallic Ag can have different diameters and / or the layer of the ceramic or glass support coated with metallic silver can have different Ag coating thicknesses.

[0059] In a particular embodiment, the wire diameter of the metallic silver mesh and / or the coating thickness of the ceramic or glass carrier coated with metallic silver decreases in the downstream direction. This means that the most upstream Ag mesh or Ag-coated carrier has a larger diameter or a thicker Ag coating, respectively, to provide some sacrificial silver (silver that will evaporate and contribute to the saturation of volatile silver in the gas phase), thereby reducing the evaporation loss of more downstream silver from the Ag mesh or Ag-coated carrier of the Ag-containing recovery system. In other words, the wire diameter of the metallic silver mesh and / or the coating thickness of the ceramic or glass carrier coated with metallic silver decreases with increasing distance from the catalyst.

[0060] In one aspect of the present disclosure, a method for recovering volatile precious metals from a gas phase is disclosed. The method comprises the following steps: (a) operating a catalyst in a system according to the present disclosure, thereby producing a gas phase comprising volatile precious metals, in particular a gas phase comprising Rh and Pt and optionally Pd; and

[0061] (b) contacting the gas phase comprising the volatile precious metal with a recovery system comprising metallic silver in a system according to the present disclosure.

[0062] In particular, the catalyst comprises one or more noble metals, in particular Pt and Rh and optionally Pd.

[0063] The recovery system containing metallic Ag disclosed herein is capable of selectively recovering precious metals, in particular Pt and Pd, but not Rh, from the gas phase. It is particularly beneficial to use the recovery system disclosed herein in the process of generating a phase containing precious metals, by contacting the generated gas phase with the recovery system disclosed herein, so as to recover the precious metals in the gas phase, in particular for selectively recovering Pt and / or Pd (compared to Rh).

[0064] In one embodiment of the method according to the present disclosure, the gas phase containing the volatile precious metal has a temperature in the range from 750°C to about 950°C.

[0065] The inventors have demonstrated that the disclosed method can be performed over a wide temperature range to produce a gas phase through a catalytic system and pass it through a recovery system. Thus, the disclosed method can be applied to a wide range of catalytic processes.

[0066] When catalysis is performed at temperatures as low as 750°C, the energy consumption of the production process is reduced. In addition, the evaporation of Pd, Pt and Ag metals is reduced, which in turn reduces the loss of these metals during the production process.

[0067] In one embodiment of the method according to the present disclosure, the gas phase comprising the volatile precious metal has a temperature in the range of from 850°C to 950°C or from 850°C to 930°C.

[0068] As explained above, in the presence of multiple woven or knitted metallic Ag screens stacked together, or multiple layers of ceramic or glass carriers coated with metallic Ag, monolithic structures, or combinations thereof, the loss of Ag in the evaporation phase is reduced. Presumably, when there are several Ag recovery elements stacked on top of each other, some elements are upstream relative to other elements, i.e., they come into contact with the gas stream generated by the catalytic conversion earlier than some other elements. As a result, the gas phase is enriched with Ag, and therefore the evaporation losses of Ag in the downstream Ag recovery elements are reduced.

[0069] Therefore, for catalytic processes carried out at temperatures up to 850° C. to 950° C. or from 850° C. to 930° C., it is beneficial to practice the method of the present disclosure using a recovery system comprising a plurality of woven or knitted metal Ag meshes stacked together, or a ceramic or glass carrier coated with metal Ag in multiple layers, or a monolithic structure, or a combination thereof. This means that it is particularly beneficial to use the system of the present disclosure in catalytic processes carried out at temperatures ranging from 850° C. to 950° C. or from 850° C. to 930° C.

[0070] In one embodiment of the method according to the present disclosure, the method, in particular step (a) of the method, further comprises the following step: operating a catalyst for the catalytic conversion of ammonia into nitrogen monoxide, wherein volatile precious metals are evaporated from the catalytic system, in particular from the catalyst and optionally from a recovery system comprising Pd, thereby producing a gas phase comprising volatile precious metals, in particular producing a gas phase comprising Pt and Rh and optionally Pd.

[0071] The production of nitric acid by oxidation of ammonia according to the Ostwald process is a widely used industrial process, the first step being oxidation of ammonia to nitrogen monoxide on a catalytic gauze containing precious metals at a temperature ranging from 800° C. to 950° C. Therefore, the process of the present disclosure is well designed for implementation in an ammonia oxidation burner in order to recover the precious metals lost from the catalytic gauze after evaporation.

[0072] In one embodiment of the method according to the present disclosure, the method is used to recover volatile precious metals evaporated during the catalytic conversion of ammonia into nitrogen monoxide from a catalytic system used therein, wherein the catalytic system comprises Pt and Rh.

[0073] The catalytic gauze as part of the ammonia oxidizing burner contains platinum and rhodium. Therefore, it is beneficial to practice the method of the present disclosure in an ammonia oxidizing burner using a catalytic gauze containing Pt and Rh.

[0074] In one embodiment of the method according to the present disclosure, the method is used to recover volatile precious metals evaporated during the catalytic conversion of ammonia into nitrogen monoxide from a catalytic system used therein, wherein the catalytic system comprises Pt, Rh and Pd.

[0075] In a conventional ammonia oxidation burner, a Pd-containing recovery screen is included together with a Pt and Rh catalytic screen. In this way, the Pd in ​​the recovery screen captures the volatile platinum evaporated during the catalytic conversion, which allows Pt recovery. However, the precious volatile Pd is lost in the gas phase. By further utilizing the recovery system coated with Ag according to the present disclosure during the conversion of ammonia into nitrogen monoxide in the production of nitric acid, the evaporated Pd and the Pt not captured by the Pd recovery screen can also be recovered by forming Pd-Ag, Pt-Ag or Pt-Pd-Ag alloys.

[0076] In one aspect of the present disclosure, a system for catalytically converting ammonia into nitric oxide is disclosed. The system comprises: a catalytic system comprising Pt and Rh and operating at a temperature ranging from 750°C to 950°C; and, downstream thereof; a recovery system according to the present disclosure comprising a recovery system of metallic Ag.

[0077] The system of the present disclosure allows the production of nitric acid from ammonia using conventional Pt and Rh based catalytic gauze, typically at temperatures in the range of 800°C to 950°C. The inventors have found that the process of the present disclosure can be performed over a wide temperature range for producing a gas phase from the catalytic system and passing it through a recovery system comprising metallic silver. Furthermore, the inventors have demonstrated that it is possible to catalytically convert ammonia to nitric oxide in the production of nitric acid at temperatures as low as 750°C while maintaining greater than 90% conversion of ammonia to nitric oxide.

[0078] 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 the precious metals is reduced, which in turn reduces the loss of these metals in the production process. By the presence of the recovery system containing metallic Ag of the present disclosure, the precious Pt metal can be recovered from the catalytic wire mesh in any case.

[0079] In one embodiment of the system according to the present disclosure, the catalytic system further comprises Pd, such as in the form of a recovery system comprising Pd, in particular a recovery mesh comprising Pd.

[0080] In a conventional ammonia oxidation burner, a Pd-containing recovery screen is included together with a Pt and Rh catalytic screen. In this way, the Pd in ​​the recovery screen captures the volatile platinum evaporated during the catalytic conversion, which allows Pt recovery. However, the precious volatile Pd is lost in the gas phase. By further utilizing an Ag-coated recovery system during the conversion of ammonia to nitric acid, the evaporated Pd and the Pt not captured by the Pd recovery screen can also be recovered by forming a Pd-Ag, Pt-Ag or Pt-Pd-Ag alloy.

[0081] In one embodiment of a system according to the present disclosure, the system may operate at a temperature ranging from 850°C to 950°C or from 850°C to 930°C.

[0082] This temperature range is particularly suitable for converting ammonia to nitric oxide and achieving conversion selectivities greater than 96% while minimizing the disadvantages associated with high temperature operation, such as energy consumption and equipment damage.

[0083] In one aspect of the present disclosure, use of the system of the present disclosure for catalytic conversion of ammonia to nitric oxide in the production of nitric acid is disclosed.

[0084] In one aspect of the present disclosure, a method for modifying a system for catalytically converting ammonia to nitric oxide is disclosed, the system comprising a catalytic system comprising a catalyst containing one or more precious metals selected from the group consisting of Pd, Pt and Rh, in particular comprising Pt and Rh and optionally Pd; and a Pd-containing recovery system for recovering volatile precious metals evaporated during the conversion of ammonia to nitric oxide in the catalytic system; the system is modified into the system of the present disclosure. The method comprises the step of introducing a recovery system containing metallic Ag according to the recovery system.

[0085] As described above, the Pd-containing recovery wire mesh is typically integrated in a conventional ammonia oxidation burner for catalytic conversion of ammonia into nitrogen monoxide in the production of nitric acid. That is, the Pd-containing recovery wire mesh is combined with a conventional catalytic wire mesh containing Pt and Rh, and the Pd-containing recovery wire mesh forms a catalytic system with the catalytic wire mesh. In the presence of the 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-containing recovery wire mesh. The inventors have found that the Ag-coated recovery system can be combined with a catalytic system comprising a catalytic Pt and Rh wire mesh and a Pd recovery wire mesh: not only can Pt recovery be increased by the presence of both Pd and Ag-containing recovery systems (such as Pd and Ag-coated recovery systems), but also the presence of the Ag-coated recovery system according to the present disclosure further leads to the recovery of Pd evaporated from the Pd-containing recovery wire mesh. In other words, both Pt and Pt are recovered, and the recovery of precious Pt metal from the Pd-containing recovery screen is not associated with the loss of precious Pd metal.

[0086] 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.

[0087] Alternatively, since Ag is suitable for recovering the volatile precious Pt metal, it is possible to use only Pt and Rh based catalytic screens as part of the catalytic system (i.e. in the absence of a Pd containing recovery screen) to convert ammonia into nitric oxide in the production of nitric acid, thereby simplifying the system.

[0088] Example

[0089] Three groups of Ag mesh (each group in turn containing 3 Ag meshes) were installed in a 22 cm tube at three different locations downstream of a catalytic mesh used to convert ammonia to nitric oxide and operated at about 840°C, exposing it to a temperature of about 100% of the catalytic mesh. The catalytic mesh contained Pt and Rh, which together accounted for 5% by weight of the mesh. The Ag mesh originated from KARasmussen and was an 85 μm warp knitted mesh. After 22 days of combustion activity, in which 6.83 t of ammonia was burned to produce about 25.3 t of nitric acid, the mesh was recovered for analysis. The mass changes and mesh composition are summarized in Table 1. The 5 wt% Pt / Rh combustion mesh lost 4.22 g. Pt and Pd were captured on the Ag mesh, but Rh was not recovered and the Rh loss accounted for only 5% of the weight of the Pt loss. The Ag loss of the second and third groups was reduced (see Figure 1 ). Since the gas phase is partially saturated with Ag from the upstream screen, the evaporation loss of Ag in the downstream screen is reduced. The concentration of platinum in the Ag screens decreases from the upper screen to the lower screen. However, the highest amount of platinum was found in the second set of screens (see Figure 2 This may be related to the large loss of Ag on the Ag wire. The efficiency of Pt recovery on the Ag wire naturally shows the same trend as the quality of Pt recovered on the wire (see Figure 3 ).

[0090] The recovery system used in this example recovered about 28% of the Pt lost from the catalytic screens, with the upper, middle and lower groups of Ag screens recovering 8.2%, 12.6% and 7.2%, respectively.

[0091] Table 1. Analysis of Ag screen

[0092]

Claims

1. A system comprising: - a catalyst comprising at least Pt and Rh and optionally Pd; as well as - A recovery system containing metallic silver for recovering volatile precious metals from the gas phase, characterized in that the recovery system containing metallic silver comprises a plurality of woven or knitted metallic silver meshes stacked together, or a plurality of ceramic or glass carriers coated with metallic silver, or a ceramic monolithic structure, or a combination thereof.

2. The system of claim 1, further comprising a recovery system comprising Pd, wherein the recovery system comprising Pd is located downstream of the catalyst and upstream of the recovery system comprising metallic silver.

3. A system according to claim 1 or 2, wherein the recovery system containing metallic silver comprises at least 3 woven or knitted metallic silver meshes, or at least 3 ceramic or glass carriers coated with metallic silver, or a combination of layers of woven or knitted metallic silver meshes and ceramic or glass carriers coated with metallic silver, wherein the number of combined meshes and layers is at least 3.

4. The system of claim 3, wherein the recovery system comprising metallic silver comprises 3 to 50 woven or knitted metallic silver meshes, or 10 to 30 ceramic or glass carriers coated with metallic silver, or a combination of layers of woven or knitted metallic silver meshes and ceramic or glass carriers coated with metallic silver, the number of combined meshes and layers being in the range of 3 to 50.

5. The system according to any one of claims 1 to 4, wherein the metallic silver wire mesh has a wire diameter in the range of 150 to 400 μm, or wherein the thickness of the metallic silver coating on the ceramic or glass carrier is in the range of 150 to 400 μm.

6. The system according to any one of claims 1 to 5, wherein the silver wire meshes in the plurality of woven or knitted metallic silver wire meshes have different wire diameters, and / or wherein the layers of the ceramic or glass carrier in the multilayer ceramic or glass carrier coated with metallic silver have coatings of metallic silver with different coating thicknesses. 7 . The system of claim 6 , wherein the wire diameter of the metallic silver wire mesh and / or the coating thickness of the ceramic or glass carrier coated with metallic silver decreases in a downstream direction.

8. The system according to any one of claims 1 to 7, wherein the system is an ammonia oxidation reactor or a portion thereof.

9. A method for recovering volatile precious metals from a gas phase, the method comprising the steps of: (a) operating a catalyst in a system according to any one of claims 1 to 8, thereby producing a gas phase comprising a volatile precious metal; as well as (b) contacting the gas phase comprising volatile precious metals with the recovery system comprising metallic silver in a catalytic system according to any one of claims 1 to 8.

10. A method according to claim 9, wherein in step (a) a gas phase comprising (i) Rh and (ii) Pt and / or Pd is produced, and wherein in step (b) Pt and / or Pd is selectively removed from the gas phase comprising volatile precious metals.

11. The method according to claim 9 or 10, wherein the gas phase comprising the volatile precious metal has a temperature in the range of 750°C to about 950°C, in particular in the range of 850°C to 950°C or 850°C to 930°C.

12. The process according to any one of claims 9 to 11, wherein the catalyst comprises Pt and Rh.

13. The process according to any one of claims 9 to 12, wherein step (a) comprises operating a catalyst for the catalytic conversion of ammonia into nitrogen monoxide, wherein volatile precious metals are evaporated from the catalyst and optionally from a recovery system comprising Pd, thereby producing a gas phase comprising volatile precious metals, in particular a gas phase comprising Rh and Pt and optionally Pd.

14. Use of the system according to any one of claims 1 to 8 for the catalytic conversion of ammonia into nitrogen monoxide in the production of nitric acid.

15. A method for modifying a system for catalytically converting ammonia to nitric oxide, the system comprising: A catalytic system comprising: o a catalyst comprising one or more noble metals selected from the group consisting of Pd, Pt and Rh; and o a Pd-containing recovery system for recovering evaporated volatile Pt during the conversion of ammonia into nitric oxide in the catalytic system; The system is transformed into a system according to any one of claims 1 to 8, wherein the method comprises the following steps: A recovery system containing metallic silver for recovering volatile precious metals from the gas phase is introduced downstream of the catalytic system, wherein the recovery system containing metallic silver comprises a plurality of woven or knitted metallic silver meshes stacked together, or a plurality of ceramic or glass supports coated with metallic silver, or a ceramic monolithic structure, or a combination thereof.

16. The method according to claim 15, further comprising the steps of: • Remove the Pd containing recovery system.

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