Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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-
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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.
