Wires incorporating a helical component, assemblies thereof, and

Details Wires incorporating a helical component, assemblies thereof, and Wires incorporating a helical component, assemblies thereof, and

1995-01-31

1997-12-23

Crowder, C. D.

Textiles: knitting

Fabrics or articles

Materials

66170, 139420R, 502439, 502527, 57216, 57212, 442 6, 442308, 442229, D04B 2112, D03D 1502, B01J 2340

Patent

active

056996802

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to novel threads containing at least one element in the form of a helical winding, to their assemblings, particularly in the form of woven fabrics, knits or felts, and to the use of these assemblies as catalyst and/or for recovering the precious metals.


BACKGROUND OF THE INVENTION

The industrial process for preparing nitric acid includes as essential step the oxidation of ammmonia into nitric oxide. This reaction is conducted industrially by passing a mixture of air and ammonia over a metal catalyst generally constituted by platinum or a platinum alloy. The precise conditions of reaction vary little from one installation to the other; the mixture of 10% ammonia and 90% air by volume is preheated to a temperature of 180.degree.-250.degree. C. before passage through the catalyst. The principal reaction: which may attain 96%. This exothermic reaction raises the temperature of the gases and maintains the catalyst at 850.degree.-900.degree. C. The composition of the gases is such that there remains an excess of oxygen after reaction; in the range of temperatures attained, this oxygen forms with the platinum a volatile oxide, which produces a loss of matter from the catalytic cloths. These platinum losses vary, depending on the operating conditions of the installations and are of the order of 50 to 400 mg of platinum per ton of nitric acid produced.
The metallic catalyst is generally in the form of cloths obtained by weaving linear threads. Numerous industrial installations, or burners, thus use platinum and rhodium alloys drawn into threads of 60 or 76 .mu.m diameter, then woven at a rate of 32 threads/cm warpwise and weftwise, to obtain a fabric comprising 1024 stitches/cm.sup.2. A catalytic bed is constituted by 3 to 40 superposed layers or cloths, this number essentially depending on the operational pressure and the mass flowrate of the gases reduced to the surface unit of the catalytic bed. The diameter of the catalytic cloths attains 5 m in certain burners.
The volatilized platinum may be partially picked up by means of palladium alloy cloths placed immediately beneath the layers of platinum cloths. These palladium alloy cloths are woven products, produced in the same way as the catalytic cloths.
The same catalytic cloths are also employed by the synthesis of hydrocyanic acid by the Andrussow process. The overall reaction is the following:
This exothermic reaction raises the temperature of the gases to 1100.degree. C. The operational conditions are such that there is no excess oxygen and that platinum oxide cannot be formed; the level of the platinum losses is, in this case, very low.
The form of embodiment of the platinum alloy catalysts has remained to this day virtually identical to that developed at the beginning of the 20th century. Certain improvements in the structure of the catalysts have been proposed, but they have not led to a long-lasting industrial exploitation.
French Patent 2 074 921 describes the replacement of about 1/3 to 2/3 of the precious metal cloths by a foraminous structure of non-precious metal, corrosion-resistant and such that the pressure drop is unchanged. This foraminous structure may be made in the form of a metal pad constituted by wires oriented at random.
European Patent 0 275 681 describes a catalyst pack comprising a foraminous layer of fibers of a metal of the platinum group or an alloy containing same and at least one layer of foraminous ceramic material having a coating of at least one of the metals of the platinum group.
French Patent 2 467 629 describes a catalytic bed which comprises an assembly or an agglomeration of fibers of a metal or an alloy of the platinum group.
It should be noted that the examples described in these Patents present the common point of requiring the permanent use of one or more platinum cloths of the prior art to which novel structures are added; the novel structures which are described in these Patents do not present a cohesion and sufficient mechanical properties to be self-supporti

REFERENCES:
patent: 3660024 (1972-05-01), Gillespie
patent: 4313998 (1982-02-01), Pivot et al.
patent: 4375426 (1983-03-01), Knapton et al.
patent: 4435373 (1984-03-01), Knapton et al.
patent: 4926910 (1990-05-01), Wade
patent: 5188813 (1993-02-01), Fairey et al.

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