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WHAT WE CLAIM IS:-
1. Process for the production of nitrous oxide comprising inserting into an aqueous nitric acid solution two electrodes of which the one is of a metal of the kind specified and the other is of a conductive material that is inert to nitric acid under the operational conditions, connecting the electrode of the metal of the kind specified as cathode and the inert electrode as anode to a source of d.c. to produce an electrolytic circuit, selecting values for the dissolution parameters (as hereinbefore defined) which yields a combination that does not exceed a threshold combination (as hereinbefore defined), allowing an electric current to flow in said circuit and withdrawing a nitrous oxide containing gas mixture from near the cathode.
2. Process according to claim 1, wherein the current density on the cathode is within the range of 1 to 150 amp/dmi.
3. Process according to claim 1, wherein the nitric acid concentration is within the range of 1 to 8N.
4. Process according to claim 1, substantially as described.
5. A nitrous oxide-containing gas mixture, whenever obtained by a process according to any one of claims 1 to 4.
FORRESTER, KETLEY & CO., Chartered Patent Agents, Jessel Chambers, 88/90 Chancery Lane, London, W.C.2, and Rutland House, 148 Edmund Street, Birmingham, 3.
Agents for the Applicants.
Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1969.
Published by the Patent Office, 25 Southampton Buildings, London, W.C.2, from which copies may be obtained.
is 1,1SO,S10
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PATENT SPECIFICATION
DRAWINGS ATTACHED 1,150,510 Date of Application and filing Complete Specification: 21 Aug., 1967.
No. 38381/67.
Application-made in Israel (No. 26453) on 5 Sept., 1966.
Complete Specification Published: 30 April, 1969.
) Crown Copyright 1969.
Index at acceptance:-C7 B(2G, A2A4, A2B1, A9) Int. Ca.:-C 01 b 21/22 COMPLETE SPECIFICATION
Electrolytic Production of Nitrous Oxide We, SZMUEL RAvIv, an Israel citizen of 8 Deborah Street, Beersheba, Israel, and SHIMON MALKIELY, an Israel citizen of Shikun H Ledugma, 229/21, Beersheba, Israel, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-
The present invention concerns the manufacture of nitrous oxide (N20). Nitrous oxide serves, among others, as rocket fuel and is in high demand. An efficient and economic method for its production is therefore an important industrial requirement.
A variety of electro-negative metals and metal alloys are known that are-passivated by treatment with nitric acid and are normally either not dissolved at all or dissolved only very slowly by this acid. Examples of such metals and metal alloys are various kinds of stainless steel, Inconel (Registered Trade Mark for an alloy containing approximately 78% Ni, 15% Cr and 7% Fe), aluminium, chromium, nickel, uranium, titanium, zirconium, and their alloys. All electro-negative metals and metal alloys which are characterized by a passivation upon treatment with nitric acid will be referred to hereinafter for short as " metals of the kind specified ".
It is also known that the resistance of metals of the kind specified to corrosive acidic attack can be increased by so-called cathodic protection. Such a protection consists in connecting the metal as cathode into an electrolytic circuit and the rate of corrosion of metals of the kind specified so protected, that is the rate of their attack by acid, is reduced considerably.
It has surprisingly been discovered that if a metal of the kind specified is inserted into nitric acid and simultaneously therewith connected as cathode into an electrolytic circuit, and the current density on the cathode as well as the acid concentration and tempera[Price 4s. 6d.] ture are suitably selected, the metal is dissolved. This discovery forms the subject of co-pending Application No. 44104/67 (Serial No. 1,129,750).
Such a cathodic dissolution of metals of the kind specified is dependent on three operational parameters namely, the current density on the cathode, the nitric acid concentration and the temperature. These three parameters will be referred to hereinafter for short as "dissolution parameters ".
For each metal of the kind specified there exist various combinations of dissolution parameters each characterized by the fact that while it does not yield any appreciable dissolution of the metal, an increase of the value of any of the parameters will cause dissolution. Any such combination of values for the dissolution parameters which does not yet produce any appreciable dissolution of the metal while an increase of any of the values causes dissolution, which combination, in other words, constitutes the demarcation between non-dissolution and dissolution, will be referred to hereinafter for short as " threshold combination ".
Obviously for each metal there exists a plurality of threshold combinations. Thus for a given current density on the cathode and a given temperature a very definite acid concentration has to be selected in order to result in a combination of current density, temperature and acid concentration that will constitute a threshold combination. If while keeping, for example, the temperature constant the current density on the cathode is changed, the acid concentration must be changed as well in order to obtain in this way another threshold combination.
In accordance with the present invention it has surprisingly been found that if a metal of the kind specified is inserted into an aqueous nitric acid solution and connected as cathode to a source of d.c., and a combination of the dissolution parameters is selected which does c Vu:
Pi not exceed a threshold combination N20 is produced at the cathode.
Consequently the invention consists in a process for the production of nitrous oxide comprising inserting into an aqueous nitric acid solution two electrodes of which the one is of a metal of the kind specified and the other is of a conductive material that is inert to nitric acid under the operational conditions, connecting the electrode of the metal of the kind specified as cathode and the inert electrode as anode to a source of d.c. thereby to produce an electrolytic circuit, selecting values for the dissolution parameters (as hereinbefore defined) which yield a combination that does not exceed a threshold combination (as hereinbefore defined), allowing an electric current to flow in said circuit and withdrawing a nitrous oxide-containing gas mixture from near the cathode.
Examples of materials for the inert anode are the noble metals and their alloys, such as, for example, platinum.
It is believed that the production of nitrous oxide on the cathode is due to a cyclic passivation and depassivation of the cathode. It is further believed that during the passivation oxygen and nitrogen are adsorbed in gaseous form on the surface of the cathode while during the de-passivation they are desorbed with the simultaneous formation of nitrous oxide.
The method according to the invention is superior to all known methods for the production of nitrous oxide both as regards equipment and energetic requirements, and is therefore considerably cheaper. This method could not have been anticipated from anythink known from the literature.
As a rule there is obtained in accordance with the invention at the cathode a mixture containing in addition to nitrous oxide also elementary nitrogen, oxygen and hydrogen in varying proportions depending on the reaction conditions. For many purposes this mixture can be used as it is while for other purposes purified nitrous oxide may be isolated from that mixture by methods know per se.
In the accompanying drawing there is illustrated, by way of example only, a laboratory scale apparatus for carrying out the invention.
The apparatus here illustrated comprises two communicating vessels 1 and 2 serving, respectively, as anode and cathode compartments. Vessels 1 and 2 communicate through a duct 3 fitted with control valves 4 and 5 and linked to a funnel 6 serving for the supply of nitric acid. Compartment 1 is fitted with an inert electrode 7 serving as anode and compartment 2 is fitted with an electrode 14 of a metal of the kind specified serving as cathode.
Compartment 1 is further fitted with an elongated vessel 8 comprising an inlet valve 9 and an outlet valve 10. Vessel 8 serves for collecting the gaseous anode products.
Likewise, compartment 2 is fitted with two identical elongated vessels 1a and 11 b controlled, respectively by inlet valves 12a and 12b and outlet valves 13a and 13b.
Vessels 11a and 11b serve for collecting the gaseous cathode products and are designed for alternating use so that the apparatus may operate continuously.
The invention is illustrated by the following Examples without being limited thereto.
In all these Examples an apparatus of the form described above was used, having a platinum anode.
ExAMPLE 1
N20 was produced from 2.2N HN03 at 20 using as cathode stainless steel AISI-304. Current densities of 2, 10 and 15 amp/dm2 were used and the results are given in the Table below:
1,150,510 1,150,510 EXAMPLE 2
Concentration of HNO3 - 3.3N Temp. - 20 C.
Cathode - stainless steel AISI-304 Composition by volume of the gas in the cathode Current density compartment on the cathode in amp/dm2 N2O N2 02 H2 Total 61.5 20 0.2 14 96 69 26 2.8 0 98 88 9.2 1.8 1 100 82 11 4.2 1 99 EXAMPLE 3
Concentration of HNO, - 5N Temp. 20 C.
Cathode - stainless steel AISI-304 Composition by volume of the gas in the cathode Current density compartment on the cathode in amp/dm2 N.0 N2 02 H2 Total 72 28 0.1 0 100.1 75 24 0.8 0 99.8 27 55 4.2 0 86.2 44 45 5.2 2.5 96.7 62 30 10.3 0 102 1,150,510 EXAMPLE 4
Concentration of HNO3 - 2N Temp. - 30 C.
Cathode - nuclear uranium (common designation for purified uranium) Composition by volume of the gas in the cathode Current density compartment on the cathode in amp/dmi2 N20 N2 02 H2 Total 32 11 2 54 99 34 13 2 50 99 37 12 2 45 96 41 12 2 46 101 33 34 3 29 99 EXAMPLE 5
Concentration of HNO3 - 2N Temp. - 30 C.
Cathode - nuclear uranium a) Composition by volume of the gas in the cathode Current density compartment on the cathode in amp/dm2 N20 N2 02 H2 Total 26.4 10.5 0.25 48 85.15 47 11.2 0.1 11.5 69.8 68 11.9 1.8 10 91.7 b) Composition of the gas in the cathode compartment Current density expressed in ml/Faraday on the cathode in amp/dm2 N20 N2 02 H2 645 256 6 1173 1600 416 3.8 440 1507 264 40 222 I I ( within wide limits. Thus the current densities on the cathode may be varied within the range of 1-150 amp/dmi2. The operational temperature can be selected up to near the boiling point but as a rule lower temperatures will be preferred because of the corrosiveness of boiling nitric acid. The preferred range of the nitric acid concentration is from 1 to 8N.
EXAMPLE 6
Concentration of HNO2-2N Temp.-300C Cathode-titanium 99.5% Current density on the cathode 40 amp/dm2.
The composition of the gas obtained in the cathode compartment, expressed in parts by volume was as follows:
N,0 N2 02 H2 CO, - 41.2 - 17.5 - 0.2 - 0 - 42 EXAMPLE 7
Concentration of HNO,-2N Temp.-300C Cathode-Zircalloy 2 Current density on the cathode-40 amp/dm2 The composition of the gas obtained in the cathode compartment, expressed in parts by volume was as follows:
N,0 N2 02 H2 - 21 - 21.2 - 7.2 - 52 EXAMPLE 8
Concentration of HNO,-2N Temp.-300C Cathode-Zircalloy 2 Current density on the cathode amp/dm2 The composition of the gas obtained in the cathode compartment, expressed in parts by volume was as follows:
N,0 N, 02 H2 - 30 - 10 - 1 - 60 As can be seen from the foregoing Examples the various dissolution parameters-are variable
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