WO2001011094A1

WO2001011094A1 - Recovery of precious metal

Info

Publication number: WO2001011094A1
Application number: PCT/US2000/021605
Authority: WO
Inventors: Joe Calamusa, Iii; Craig B. Ostroff; Gary B. A. Schuster
Original assignee: Engelhard Corporation
Priority date: 1999-08-10
Filing date: 2000-08-08
Publication date: 2001-02-15
Also published as: AU6761100A

Abstract

A foraminate element for recovery of platinum and/or palladium and/or rhodium lost from a platinum and/or palladium and/or rhodium-containing catalyst comprising a substrate containing palladium or a palladium alloy thereon in a thickness of about 0.005 to about 0.03 cm. The palladium or palladium alloy may be coated, plated or clad onto the substrate or is preferably deposited on a surface of the substrate by a thermal spray process, thick film application or a sputter coating process. The substrate may be a solid structure possibly made from graphite, glass, silicon, a ceramic, a metal or a metal alloy and a composite of a metal and non-metal having a plurality of openings therein or a fiber or wire screen.If the substrate is a screen, it preferably has a mesh size of less than 1 to about 320 fibers or wires/cm and an area of at least about 900 cm2. Preferably, the substrate comprises a base metal which is stable at temperatures as high as 1200 °C and may be a metal or alloy such as an aluminum-chromium-iron alloy, a stainless steel alloy, a nickel-chromium alloy, a nickel-chromium-iron alloy, a nickel-chromium-iron-aluminum alloy, a nickel-base superalloy, an iron-base superalloy or a cobalt-base high temperature alloy. The foraminate element is especially useful for recovering platinum and/or palladium and/or rhodium lost during the course of an ammonia oxidation process.

Description

RECOVERY OF PRECIOUS METAL

Field of the Invention

The invention relates to a foraminate element for recovery of precious metals such as platinum and/or palladium and/or rhodium lost from catalysts containing such precious metals comprising a substrate containing palladium or a palladium alloy thereon. The foraminate element is especially useful for recovering platinum and/or palladium and/or rhodium lost during the course of an ammonia oxidation process.

Background of the Invention Nitric acid is produced commercially by passing ammonia and air across a gauze woven or knitted from platinum or a platinum alloy such as platinum-rhodium or platinum-palladium-rhodium. The ammonia is initially converted to nitric oxide upon contact with the precious metal gauze and the nitric oxide is subsequently oxidized and absorbed to form nitric acid. The oxidation of ammonia to nitric oxide is exothermic and causes the temperature of the precious metal catalyst to rise to 800 to 1,000°C. During the oxidation step, some of the precious metal is volatilized from the surface of the wire gauze. The rate of loss is dependent upon the temperature, pressure and flow rate of gases across the catalyst surface. The cost of the precious metal lost from the catalyst during the ammonia oxidation is a significant part of the cost of operating the nitric acid plant. Foraminate elements for recovery of precious metals lost from catalysts containing such metals which are employed in the production of nitric acid by ammonia oxidation are known in the prior art, see, e.g., U.S. Patents 4,239,833 and 4,511,539. These prior art foraminate elements, commonly referred to as "getters," all consist of palladium or a palladium alloy such as palladium-gold fabricated into a wire gauze with the wires disposed in various manners, e.g., evenly woven or knitted in random fashion.

The prior art foraminate elements work well in capturing the lost precious metals, but they are nevertheless disadvantageous since they are all fabricated entirely from expensive palladium or palladium alloys. On the other hand, the foraminate elements of the present invention are fabricated from an inexpensive substrate containing the palladium or a palladium alloy thereon. The foraminate elements of the present invention thus provide several advantages over the prior art foraminate elements: (1) the foraminate elements of the present invention offer increased palladium or palladium-alloy utilization efficiency since the amount of palladium or palladium-alloy present on the substrate is a fraction of that employed in the prior art foraminate elements; (2) the foraminate elements of the present invention offer greater design flexibility, i.e., the prior art foraminate elements have design limitations due to the need to fabricate the elements entirely out of palladium or a palladium alloy as distinguished from those of the present invention in which a variety of substrates capable of being fabricated in a variety of design configurations. Some of these design configurations may be practical only with substrate materials that offer superior mechanical performance as compared to palladium alloys; and (3) the foraminate elements of the present invention are more quickly activated and therefore more quickly recover the lost precious metals due to the fact that they have a relatively rough surface to which the lost precious metals quickly adhere.

The following illustration is given as to the greater utilization efficiency of the palladium in accordance with the present invention. A screen containing 10 wires/cm prepared from palladium wire of a diameter of 0.5mm would weigh approximately 0.47g/cm². An approximately equivalent-sized screen can be prepared from a base metal having a wire diameter of about 0.33mm and applying a coating of about 0.011cm of palladium on one face of the screen. The effective diameter is again approximately 0.5mm, but the amount of palladium used on the base metal screen is only 0.13g/cm². This represents a 72% reduction in the amount of palladium employed in the preparation of the foraminate element of the present invention.

Summary of the Invention The invention pertains to a foraminate element, for recovery of precious metals such as platinum and/or palladium and/or rhodium lost from a catalyst containing such metals comprising substrate containing palladium or a palladium alloy thereon as well as to processes for preparing the foraminate element and for using the foraminate element to capture the lost precious metals. Details of the Invention For the purposes of the present invention, it is to be understood that the term "foraminate" element means a structure having a plurality of openings therein. Thus the foraminate element may be, e.g., a structure having a plurality of holes of various shapes (e.g., circular, oval, triangular, rectangular, etc.) in a regular or random pattern. Alternatively, the foraminate element may be a screen with fibers or wires therein of the same or differing thicknesses, arranged in a regular or random pattern.

The foraminate element of the present invention employed for the recovery of precious metals (e.g., platinum and/or palladium and/or rhodium) lost from a catalyst containing such metals comprises a substrate containing palladium or a palladium alloy thereon. The nature of the substrate is not critical; its only limitation is that it must be capable of withstanding the operating temperature and environment in the process to which the foraminate element will be exposed in the course of recovering the lost precious metals. Thus, the substrate may comprise a solid structure having a plurality of openings therein or a fiber or wire screen and may be non-metallic in nature, e.g., graphite, glass, silicon, ceramics, etc.; alternatively, the substrate may comprise a metal or metal alloy; yet another type of suitable substrate is the composite of a metal and non- metal. The substrate will contain palladium or a palladium alloy thereon such as palladium-nickel, palladium-cobalt, palladium-copper, palladium-platinum, palladium- gold, palladium-rhodium, palladium-rhenium and palladium-iridium. As indicated above, a wide variety of materials may serve as the substrate for the foraminate element of the present invention with the only criterion being that the substrate comprises a material which will be stable in the environment in which the foraminate element is intended to operate in order to recover lost platinum and/or palladium and/or rhodium. As such, it is desirable that the substrate comprises a non- metal, metal or composite which is stable at temperatures as high as 1,200°C.

Preferably, the substrate comprises a metal alloy such as an aluminum- chromium-iron alloy, a stainless steel alloy, a nickel-chromium alloy, a nickel-chromium- iron alloy, a mckel-chromium-iron-aluminum alloy, a nickel-base superalloy, an iron-base superalloy and a cobalt-base high temperature alloy. The foregoing alloys are readily commercially available and are typically sold under the following brand names: Kanthal^®, Megapyr^®, FecralloyV^ayπes , Haynes-25θ , Hastello^® , Hastelloy , Nichrome and Inconel^®.

In the case of a forarninate elements in the form of a screen(s), the foraminate element will have a mesh size of less than 1 to about 320 fibers or wires/cm and an area of at least about 900 cm². The screen(s) may be metallic or non-metallic in nature and may be used in any shape desired, e.g., circular, oblong, rectangular, square, etc., depending on factors such as the configuration of the tube though which the stream of lost precious metal flows, the flow speed of such stream, the concentration of the lost precious metal in such stream, etc. The screen(s) may consist of fibers or wires of the same or differing thicknesses and may be woven or knitted in a regular or random pattern. In general, the palladium or palladium alloy is present on a surface of the substrate in the form of a plating, a cladding or a coating in a thickness of about 0.005 to about 0.03 cm, preferably in a thickness of 0.007 to 0.015 cm.

The palladium or palladium alloy may be plated onto the substrate using either a well-known electroplating or electroless plating processes. Typically, the palladium would be plated onto the surface of the substrate by electroplating from an aqueous or fused salt bath.

Cladding is a physical union of two or more materials, i.e., the palladium or a palladium alloy and the substrate, which are mechanically cold or hot-worked to cause intimate contact between the two materials. Depending on the particular choice of the substrate material and the palladium or the particular palladium alloy chosen for fabrication of the foraminate element of the present invention, the resulting composite may or may not be metallurgically bonded.

Preferably, the palladium or palladium alloy is deposited as a coating on at least one face of the substrate by well-known processes such as thermal spray processes, thick film applications or sputter coating processes. Thermal spray processes, e.g., plasma-arc spraying, combustion flame spraying, detonation-gun spraying and high velocity oxyfuel spraying, are particularly suitable for the preparation of the foraminate elements of this invention. A particularly preferred process for depositing the palladium or palladium alloy as a coating on a surface of the substrate is the plasma-arc spraying process which is described below. The application of palladium or palladium alloy to the substrate by a thick film process involves blending of the palladium or palladium in powder form with a suitable binder and one or more solvents to make a paste, and applying the paste, in a quantity which will result in a thickness of the palladium or palladium alloy within the range indicated above, to the substrate by brushing, rolling, dipping or screen printing. After drying at temperatures ranging from ambient to about 50 °C, the coated substrate is fired at a higher temperature to effect a bond between the substrate and the coating.

Sputter coating is a process which deposits material, i.e., the palladium or palladium alloy atom by atom from a target to the substrate to be coated. By fabricating a segmented target, comprising two or more elements, a deposition can be made that is a uniformly dispersed alloy of the constituents. The sputtering process is typically performed in the presence of reduced pressures of inert gases. The sputtering process makes it possible to attain very thin as well as thicker coatings and may be used for deposition of a coating of palladium or a palladium alloy on the surface of the substrate with a thickness in the range indicated above. It is preferred that the deposition of the palladium or palladium alloy takes place on that surface of the substrate which will be oriented in a direction facing a stream from which the platinum and/or palladium and/or rhodium are to be recovered. In the case where the substrate is to be coated by one of the thermal spray processes described below, the surface of the substrate which is to receive the coating may be oriented at an angle 0 to 90 °relative to the device which generates the spray. However, by spraying at more than one orientation at an angle of about 45° relative to the device which generates the spray it will cause the coating to wrap around the wire. This is advantageous in that it increases the coated surface area, and will also mechanically lock the coating onto the surface.

The surface of the substrate which is to contain the palladium or palladium alloy desirably is planar in nature and clean, i.e., the surface should be free of dirt, grease, oxides and other contaminants. Preferably, the surface of the substrate which is to contain the palladium or palladium alloy is roughened by grit blasting to improve adherence of the palladium or palladium alloy on such surface. Typical grit blasting parameters are as follows: Air Pressure: about 5.6 kg/cm²

Nozzle Exit Diameter: about 0.8 cm Grit Size: about 46-70 mesh Preferred Grit Material: alumina

Blasting Time: surface subjected to particle flow for about 1 second Thermal spray process are of four general types: combustion flame spraying (using palladium or a palladium alloy in the form of a powder or a wire as a coating source), plasma-arc spraying, detonation-gun spraying and high velocity oxyfuel spraying. The preferred processes are the combustion flame spraying process and the plasma-arc spraying process. The particularly preferred process for preparation of the foraminate element of the present invention is the plasma-arc process.

In a typical combustion flame spraying process, the coating material, i.e., the palladium or a palladium alloy is fed, in the form of a wire, rod, or powder into an oxyacetylene flame at about 1,500°C. The coating material is melted and then atomized by a compressed-air blast which accelerates the particles to a velocity of approximately 165 m/sec. The compressed air also serves to cool the substrate during the coating process, thereby maintaining a part temperature below about 200 °C. Typically, when used as a powder, the palladium or palladium alloy uses an inert carrier gas to feed the powder having a particle size below about 200 mesh into the oxyacetylene mixture. Although this process is the least expensive of the thermal spray processes, the bond strength obtained is lower and the porosity is higher than that produced by the plasma-arc or the detonation- gun methods. The high velocity oxyfuel spraying process employs an internal combustion

(rocket) jet to generate hypersonic gas velocities of about 1,830 m/sec, more than five times the speed of sound. Typical combustion fuels include propylene, acetylene, propane and hydrogen gases. When burned in an atmosphere or in conjunction with pure oxygen, these fuels produce gas temperatures in excess of 2,760°C. The combustion ignition, gas control and power feed are fundamentally simple in the high velocity oxyfuel spraying system. A pilot flame, typically operating on hydrogen and oxygen is ignited manually, and the flow rates of two main jet gases are controlled by a flow meter. Electrically- operated solenoids activate the main combustion jet. The flow of powder, i.e., the palladium or a palladium alloy in powdered form, is electrically controlled and feed rates are monitored automatically. Typically, the microstructures resulting from high velocity oxyfuel spraying are equal to, or better than, those of the highest quality plasma- arc sprayed coatings. The high velocity oxyfuel-sprayed coatings exhibit no cracking, spalling or delamination after heating to temperatures as high as 1,095 °C and possess higher coating bond strength, lower oxide content and improved wear resistance. The high velocity oxyfuel spraying process has a deposit efficiency of 75% compared with 45% for plasma-arc spraying and has only one-half as many spraying parameters to control compared with plasma-arc spraying.

The detonation-gun process employs the controlled detonation of any oxyacetylene-gas mixture to produce high temperatures and extremely high particle velocities which coat with exceptionally high bond strength and hardness and with low porosity. In this process, a mixture of oxygen and acetylene is fed into a combustion chamber in the rear of a long-barreled gun. The coating material, i.e., the palladium or a palladium alloy, in the form of a powder of about 325 mesh, is added to the gases, and the gaseous-powder mixture is then ignited by a spark plug. The resulting detonation produces a high velocity shock front which travels down the barrel at ten times the speed of sound, accelerating the particles, which have been heated to a plastic state by the detonation, to a muzzle velocity of about 760 m/sec. This velocity is the equivalent of twenty-five times the kinetic energy of powder particles in a combustion flame-spraying device. The high kinetic energy of each powder particle is converted to additional heat upon impact with the substrate, thereby producing a metallurgical/mechanical bond, e.g., a tensile bond strength in excess of about 1700 kg/cm² is typically obtained. Unlike plasma-arc devices which operate continuously, the detonation-gun fires four to eight times per second, thereby forming a laminar coating of the palladium or a palladium alloy coating on the substrate. Typically, the gun-to-workpiece distance is 5 to 10 cm and the detonation deposits a pattern on the substrate approximately 2.5 cm in diameter and a thickness of about 0.0006 cm. Successive detonations can build up coatings of the palladium or a palladium alloy on the substrate to a thickness in the desired range of about 0.005 to about 0.03 cm.

A typical process for depositing a coating of palladium or palladium alloy on the surface of the substrate by plasma-arc spraying will now be described. The substrate, e.g., a base metal screen, is supported in a frame such that a palladium or palladium alloy in powdered form can be directed at the substrate. Multiple substrates, e.g., multiple base metal screens, can be simultaneously coated by placing one behind the other, thereby increasing the efficiency of the spray operations. Spraying over large areas is performed by rotating the substrate or by moving the plasma gun manually or by robotic means. Very large substrates can be cut apart into sections, coated with the palladium or palladium alloy and subsequently re-assembled by welding.

Typical plasma-arc spray gun parameters are as follows: Pd or Pd Alloy Powder Mesh Size: about -120 to about +325, preferably -120 to +200 Preferred Type of Plasma- Arc Spray Gun: "Miller Thermal SGI 00" or equivalent Gas Flow Rates: Primary (Ar) - about 4.2 m³/hr Secondary (He) - about 0.8 m³/hr Carrier (He) - about 2 m³/hr Accelerating Voltage: about 41 volts Current: about 550 amps Cooling Jets: about 2.8 kg/cm² Gun Distance: about 9 to 10 cm

Typical Spray Motion Parameters are as follows: The preferred gun traverse speed is in the range of about 3 to about 20 cm/sec; the particular speed to be employed will depend on the area of the substrate to be coated, e.g., in the case of a base metal screen, the principal parameters would be the mesh size of the screen and the diameter of the base metal wire. Deposition efficiency may be increased by heating the substrate.

The speed at which the gun in a robotic mode traverses the substrate should be slow enough such that the substrate gets sufficiently hot to provide good deposition efficiency.

Of course, thicker substrates, e.g., thicker wire screens, require slower traverse speeds.

The gun is preferably set at an angle of about 45 "relative to the plane of the substrate. At such angle, the substrate is desirably sprayed four times from four directions. All four spray directions are preferably at 45° relative to the plane of the substrate and parallel to the four symmetry directions of the substrate. The use of this preferably spray procedure will result in a coating of palladium or palladium alloy that wraps around much of the substrate, thereby providing a mechanical bonding to the substrate. The thickness of the Pd or Pd alloy coating on the substrate will vary along the circumference of the wires. Preferably, the maximum coating thickness is about 0.03 cm. Accordingly, to the extent feasible, the Pd or Pd alloy powder feed rate, gun traversal speed and the number of coating passes desirably are adjusted such that the maximum coating thickness of the Pd or Pd alloy on the substrate will be in the range of about 0.005 to 0.03 cm.

The following non-limiting example serves to illustrate the foraminate element of the present invention.

A Kanthal^® screen was employed as the base metal substrate in this example. The screen was circular in nature, with a diameter of about 104 cm, and contained approximately 7 wires/cm, with the Kanthal^® wire having a diameter of about 0.38 mm. "Kanthal" is the registered trademark for an aluminum-chromium-iron alloy which contains approximately 70 wt.% iron, 4.5 wt.% aluminum, 22 wt.% chromium, 2 wt.% cobalt and minor amounts of sulfur, phosphorus, manganese and magnesium.

The screen was coated over a circular area having a diameter of approximately 99 cm with palladium using the plasma-arc spray process described above such that a coating thickness of 0.01cm of palladium resulted; the palladium coating added 488 g of weight to the screen. The screen was placed in operation as a "getter" in a nitric acid manufacturing plant which utilized a catalytic gauze prepared from a platinum-rhodium- palladium alloy. The palladium-coated screen remained in use in the plant as a "getter" for a period of 90 days of operation and was then removed and evaluated.

After removal of the screen, a total of 728 g of precious metal was recovered by mechanically peeling off the coating. It was estimated that 15-30 g of precious metal remained on the screen. The assay of the recovered precious metal indicated a composition of 55 wt.% platinum (400 g), 44 wt.% palladium (320 g) and 1 wt.% rhodium (8 g). It is significant that the palladium coating remained adhered to the base metal substrate throughout the 90-day period of exposure to the nitric acid manufacturing operations, but could be readily removed for re-use in the fabrication of a new platinum- palladium-rhodium catalytic screen for further nitric acid manufacturing operations.

Claims

WHAT IS CLAIMED IS:

1. A foraminate element for recovery of platinum and/or palladium and/or rhodium lost from a platinum and/or palladium and/or rhodium-containing catalyst comprising a substrate containing palladium or a palladium alloy thereon.

2. The element of claim 1 wherein the substrate comprises a solid structure having a plurality of openings therein or a fiber or wire screen

3. The element of claim 1 wherein the substrate comprises a material selected from the group consisting of graphite, glass, silicon, a ceramic, a metal or metal alloy and a composite of a metal and a non-metal.

4. The element of claim 3 wherein the substrate comprises a metal or metal alloy which is stable at temperatures as high as 1,200°C.

5. The element of claim 4 wherein the metal alloy is selected from the group consisting of an aluminum-chromium-iron alloy, a stainless steel alloy, a nickel-chromium alloy, a nickel-chromium-iron alloy, a nickel-chromium-iron-aluminum alloy, a nickel- base superalloy, an iron-base superalloy and a cobalt-base high temperature alloy.

6. The element of claim 1 wherein the palladium alloy is selected from the group consisting of palladium-nickel, palladium-cobalt, palladium-copper, palladium-platinum, palladium-gold, palladium-rhodium, palladium-rhenium and palladium-iridium.

7. The element of claim 1 wherein the palladium or palladium alloy is present on the substrate in a thickness in the range of about 0.005 to about 0.03 cm.

8. The element of claim 7 wherein the palladium or palladium alloy is present on the substrate in a thickness of 0.007 to 0.015 cm.

9. The element of claim 1 wherein the palladium or palladium alloy is present as a coating, a plating or a cladding on a surface of the substrate.

10. The element of claim 7 wherein the coating, plating or cladding is present on that surface of the substrate which will be oriented in a direction facing a stream from which the platinum and/or palladium and/or rhodium is to be recovered. 4530

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11. The element of claim 9 wherein the palladium or palladium alloy is present as a coating which is deposited on a surface of the substrate by a thermal spray process, thick film application or a sputter coating process.

12. The element of claim 11 wherein the thermal spray process is selected from the group of processes consisting of plasma-arc spraying, combustion flame spraying, detonation-gun spraying and high velocity oxyfuel spraying.

13. The element of claim 12 wherein the thermal spray process comprises plasma- arc spraying.

14. The element of claim 11 wherein the surface of the substrate which is to receive the coating by plasma-arc spraying is oriented at an angle of 0 to about 90° relative to a device which generates the plasma spray.

15. The element of claim 14 wherein the angle is about 45° relative to a device which generates the plasma spray.

16. The element of claim 9 wherein the surface of the substrate which is to receive the palladium or palladium alloy is grit-blasted prior to coating, plating or cladding of the surface.

17. The element of claim 1 wherein the substrate comprises a screen having a mesh size in the range of less than 1 to about 320 fibers or wires/cm and an area of at least about 900 cm².

18. A method for recovery of platinum, palladium and/or rhodium lost from a catalyst containing platinum and/or palladium and/or rhodium which comprises contacting a stream containing the lost platinum and/or palladium and/or rhodium with a foraminate element comprising a substrate containing palladium or a palladium alloy.

19. The method of claim 18 wherein the platinum and/or palladium and/or rhodium are lost in the course of an ammonia oxidation process.

20. The method of claim 18 wherein the substrate comprises a solid structure having a plurality of openings therein or a fiber or wire screen 4530

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21. The method of claim 18 wherein the substrate comprises a material selected from the group consisting of graphite, glass, silicon, a ceramic, a metal or metal alloy and a composite of a metal and a non-metal.

22. The method of claim 21 wherein the substrate comprises a metal or metal alloy which is stable at temperatures as high as 1,200°C.

23. The method of claim 22 wherein the metal alloy is selected from the group consisting of an alurmnum-chromium-iron alloy, a stainless steel alloy, a nickel-chromium alloy, a nickel-chromium-iron alloy, a nickel-chromium-iron-aluminum alloy, a nickel- base superalloy, an iron-base superalloy and a cobalt-base high temperature alloy.

24. The method of claim 18 wherein the palladium alloy is selected from the group consisting of palladium-nickel, palladium-cobalt, palladium-copper, palladium- platinum, palladium-gold, palladium-rhodium, palladium-rhenium and palladium-iridium.

25. The method of claim 18 wherein the palladium or palladium alloy is present on the substrate in a thickness in the range of about 0.005 to about 0.03 cm.

26. The method of claim 25 wherein the palladium or palladium alloy is present on the substrate in a thickness of 0.007 to 0.015 cm.

27. The method of claim 18 wherein the palladium or palladium alloy is present as a coating, a plating or a cladding on a surface of the substrate.

28. The method of claim 27 wherein the coating, plating or cladding is present on that surface of the substrate which will be oriented in a direction facing a stream from which the platinum, palladium and/or rhodium is to be recovered.

29. The method of claim 27 wherein the palladium or palladium alloy is present as a coating which is deposited on a surface of the substrate by a thermal spray process, thick film application or a sputter coating process.

30. The method of claim 29 wherein the thermal spray process is selected from the group of processes consisting of plasma-arc spraying, combustion flame spraying, detonation-gun spraying and high velocity oxyfuel spraying.

31. The method of claim 30 wherein the thermal spray process comprises plasma- arc spraying. 4530

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32. The method of claim 29 wherein the surface of the substrate which is to receive the coating by plasma-arc spraying is oriented at an angle of 0 to about 90° relative to a device which generates the plasma spray.

33. The method of claim 32 wherein the angle is about 45° relative to a device which generates the plasma spray.

34. The method of claim 27 wherein the surface of the substrate which is to receive the palladium or palladium alloy is grit-blasted prior to coating, plating or cladding of the surface.

35. The method of claim 18 wherein the substrate comprises a screen having a mesh size in the range of less than 1 to about 320 fibers or wires/cm and an area of at least about 900 cm².