CA2063041A1 - Process for the production of gas-permeable nets of noble metals for catalytic processes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Wires of platinum containing 4 to 12% rhodium or 4 to 12% palladium and rhodium and wires of palladium containing 2 to 15% nickel or 2 to 15% copper or 2 to 15% nickel and copper which must have a diameter of 50 to 120 µm, a tensile strength of 900 to 1050 N/mm2 and an elastic limit of 0.5 to 3% are used for the knitting of round nets for use as catalysts in the oxidation of ammonia and as recovery nets for platinum metals. A flat knitting machine with 7 to 14 needles per inch and a sinking depth (loop length) of 2 to 6 mm is also used.

Description

2 ~? $ ,~ ~? ~ 1 This invention relates to a process for the production of gas-permeable nets of noble metals for catalytic processes, more particularly for the catalytic oxidation of ammonia with air or for recovery of the platinum metals which volatilize in the catalytic oxidation of ammonia by knitting of wires of platinum or palladium alloys on knitting machines.

The use of noble metal catalysts in the form of gas-permeable nets is an established method of operating certaincatalytic processes with optimal yields. A well-known example is the use of platinum/rhodium catalyst nets in the oxidation of ammonia for the production of nitric acid.

The net form of the catalysts affords several advantages over other catalyst configurations. For example, the net form provides for large surface development coupled with high mechanical strength. In wire drawing and wire weaving, manufacturers have established production methods for processing the catalytically active metals and metal alloys.
These catalyst nets are made in web form on looms and are cut to size in circular form in accordance with the dimensions of the oxidation plants. This results in the accumulation of considerable noble metal waste which has to be reprocessed.
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However, nets have the advantage that, with their loops, they form a homogeneous structure of metal wires and free ',- throughflow openings for the reaction gases.
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There are no inhomogeneities, for example in the form of seams or welds which impede uniform gas flow.

Nets made in the same way are also used in the recovery of no~le metals, for example in the oxidation of ammonia. A

2~3~ i1 large part of the noble metals which volatilize from the platinum/rhodium catalyst nets during the process is - collected in nets of palladium alloys.

Woven nets have a number of disadvantages attributable to their production process, including the time required to set up the loom, weaving in considerable lengths and cutting out of the circular nets from rectangular webs with approx.
35% waste.

High production costs are incurred by long set-up times and the high percentage of waste and by the large amounts of expensive noble metals tied up on the loom which necessitates considerable capital outlay.

~he woven nets can also be replaced by other gas-permeable configurations. For example, perforated metal foils and metal fibre nonwovens (DE-PS 15 94 716), metal felts (DE-PS 28 29 035) or folded tubular wire nets (DE-OS 22 48 811) are known.
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However, these configurations have never been successfully used in practice, above all because they adversely affect the flow conditions prevailing in the reaction zone.
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Knitted nets have recently been mentioned in connection with the recovery of noble metals in the oxidation of ammonia (EP-PS 0 077 121). However, no particulars of their form or production are provided.

`~ EP-OS 0 364 153 describes a process for the production ; of nets of noble metal alloys, more particularly for the catalytic oxidation of ammonia and the recovery of noble 2~3~ ~ ~

metals, in which wires of noble metal alloys are knitted together with an auxiliary thread of natural fibres or a synthetic material on a knitting machine. For weights per unit area of more than 200 g/m2, pure noble metal alloy wires cannot be knitted without auxiliary threads because the wires break and tear during the knitting process, the tensile strength, wire diameter, ductility and surface friction factor of the material ~eing said to play a part. The disadvantage of including an auxiliary thread in the knitting process is that it has to be removed before the nets are used, which can be done by burning or dissolving. Both methods are complicated and can lead to contamination of the metal alloys which can adversely affect their catalytic activity or recovery level.

Accordingly, the present invention provides a process for the production of gas-permeable nets of noble metals for catalytic processes, more particularly for the catalytic oxidation of ammonia or for recovery of the platinum metals which volatilize in the catalytic oxidation of ammonia by knitting of wires of platinum or palladium alloys on knitting ; machines, in which no auxiliary thread would have to be knitted and the weights per unit area of more than 300 g/m2 required for the particular process could still be achieved.

More particularly, the invention provides a process for the production of gas-permeable noble metal for a catalytic process, comprising: knitting a wire on a knitting machine, wherein: (a) said wire is a platinum-rhodium alloy containing 4 to 12 weight percent rhodium, a platinum-palladium-rhodium alloy containing 4 to 12 weight percent palladium and rhodium, a palladium-nickel alloy containing 2 to 15 weight percent nickel, a palladium-copper alloy containing 2 to 15 weight percent copper, or a palladium-2 ~

nickel-copper alloy containing 2 to 15 weight percent nickel and coppper; (b) said wire has a diameter of from 50 to 120 ~m, a tensile strength, Rm, of from 900 to 1,050 N/mm2, and an elastic limit, A, of from 0.5 to 3.0 percent; and (c) said knitting machine is a flat knitting machine with a gauge, number of needles per inch, of from 7 to 14, and a sinking depth, loop length, of from 2 to 6 mm.

In many cases, it is of advantage to knit two and more wires toqether.

Providing these wire and knitting machine parameters are observed, nets having satisfactorily formed loops can be knitted - surprisingly without auxiliary threads - from platinum/rhodium wires, for example containing 10~ by weight rhodium, or from palladium/nickel wires, for example containing S% by weight nickel, and do not produce any inhomogeneities when used in gas streams.
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Wire breaks are not discernible or do not occur during the knitting process. Weights per unit area of more than 300 - 20 g/m2 can readily be achieved.

The use of flat knitting machines affords the further advantage that the knitted nets can be directly made in their final dimensions, i.e. in the exact diameter required for the plant, so that there is no accumulation of waste for subsequent reprocessing as in the usual circular cutting of woven nets.
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The following ~xamples are intended to illustrate the invention:

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Example 1 A flat knittinq machine with 12 needles per inch was used for the production of a 2300 mm diameter round catalyst net for the oxidation of ammonia. The net was knitted from a 76 ~m diameter wire of 90 platinum/10 rhodium rhodium, the wire being used in doubled form. It had a tensile strength of 930 N/mm2 and an elastic limit of 1%. The sinking depth durinq knitting was 2.8 mm. A net having a weight per unit area of approximately 600 g/m2 was obtained. The net-produced excellent results when used in ammonia oxidation - plants.

- Example 2 A two-bed flat knitting machine with 10 needles per inch lS was used for the production of circular recovery nets for the oxidation of ammonia. The wire (composition: 95 palladium/5 nickel) had a diameter of 90 ~m, a tensile strength of 920 N/mm and an elastic limit of 1%. The sinking depth during knitting was 3.9 mm. A 3700 mm diameter net with a weight per unit area of 500 g/m2 was obtained when the wire was used in doubled form. The recovery level of platinum metals in ammonia oxidation plants was at least as high as that achieved with corresponding woven nets.

Claims (6)

1. A process for the production of a gas-permeable noble metal net for a catalytic process, comprising:
knitting a wire on a knitting machine, wherein:

(a) said wire is a platinum-rhodium alloy containing 4 to 12 weight percent rhodium, a platinum-palladium-rhodium alloy containing 4 to 12 weight percent palladium and rhodium, a palladium-nickel alloy containing 2 to 15 weight percent nickel, a palladium-copper alloy containing 2 to 15 weight percent copper, or a palladium-nickel-copper alloy containing 2 to 15 weight percent nickel and copper;

(b) said wire has a diameter of from 50 to 120 µm, a tensile strength, Rm, of from 900 to 1,050 N/mm2, and an elastic limit, A, of from 0.5 to 3.0 percent; and (c) said knitting machine is a flat knitting machine with a gauge, number of needles per inch, of from 7 to 14, and a sinking depth, loop length, of from 2 to 6 mm.

2. The process of claim 1, wherein said net is adapted for the catalytic oxidation of ammonia, or the recovery of platinum metals which volatilize during the catalytic oxidation of ammonia.

3. The process of claim 2, wherein said net has a weight of at least 300 g/m2.

4. The process of claim 3, wherein at least two wires are knitted together.

5. The process of claim 4, wherein the net has a circular shape.

6. A gas-permeable noble metal net produced by the process of any one of claims 1 to 5.