AU630639B2

AU630639B2 - Process for producing a heat treatment atmosphere by separation of air by adsorption

Info

Publication number: AU630639B2
Application number: AU45561/89A
Authority: AU
Inventors: Eric Duchateau; Philippe Queille
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 1988-11-24
Filing date: 1989-11-24
Publication date: 1992-11-05
Anticipated expiration: 2009-11-24
Also published as: AU4556189A; PT92407B; CA2003472A1; DE68917093D1; DE68917093T2; FR2639252B1; JP2698674B2; EP0370872A1; ATE109115T1; EP0370872B1; PT92407A; FR2639252A1; ZA898875B; ES2058581T3; JPH02227133A

Abstract

The invention relates to a process for producing an atmosphere for thermal treatment of metals, formed by providing a gaseous nitrogen stream, according to which a crude gaseous nitrogen stream having a residual oxygen content is first formed by air separation by selective adsorption, to which stream is added a stream of hydrogen at least sufficient to remove, by water vapour formation by catalytic reaction, the essential part of the residual oxygen, characterised in that the following measures are employed in combination: a) the air separation by selective adsorption is carried out so that the gaseous nitrogen stream has a residual oxygen content of between 0.1% and 3%; b) the catalytic reaction to form water vapour is brought about exclusively with catalysts whose use does not involve external application of heat and c) the catalytic reaction to form water vapour is carried out in such a way that the residual oxygen content is less than 30 vpm, with a water vapour content of between 0.2% and 6%, relative to the nitrogen stream, being set up. Exclusive application to the annealing or brazing of copper, decarburising annealing of steels, nitriding of steels, and sintering of copper, nickel, copper-nickel alloys, where appropriate with lead, and copper-zinc alloy.

Description

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AUSTRALIA

Patents Act Fo6 0 3 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority fl 9 Related Art: 4 Applicant(s): L'Air Liquide, Societe Anonyme pour l'Etude et 1'Exploitation des Procedes Georges Claude Quai d'Orsay, 75007 Paris, FRANCE Address for Servie is: Address for Service is: a S S PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Complete Specification for the invention entitled: PROCESS FOR PRODUCING A HEAT TREATMENT ATNOSPHERE PB SEPARATION OF AIR BY

ADSORPTION

Our Ref 154955 POF Code: 1290/43509 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): 1 6006 P-1 T1 PROCESS FOR PRODUCING A HEAT TREATMENT ATMOSPHERE BY SEPARATION OF AIR BY ADSORPTION The present invention relates to a process for producing an atmosphere for heat treating metals formed by the supply of a nitrogen gas current with if required one or more of the following constituents: hydrogen, methanol, hydrocarbon.

The composition of such heat treatment atmospheres must -be substantially devoid of oxygen and generally only allows 10 the presence of relatively low contents of water vapour, ~moreover defined from one application to the other. This is the rcason why, in a very large majority of appliation this type, one starts with very pure nitrogen produced by a cryogenic distillation of air, the residual oxygen content 615 of which does not exceed 10 v.p.m. (10 volumes per million).

As this nitrogen, termed cryogenic nitrogen, often has a high production cost, other industrial sources, in particular the separation of air by selective adsorption or permeation, have been investigated. But, in these cases, in order to obtain nitrogen production costs which are advantageous relative to those of cryogenic nitrogen, it has been necessary to privilege the yield to the detriment of the purity, so that the nitrogen which is competitive as concerns adsorption or permeation usually contains a residual oxygen content of 0.5% to 3% by volume.

This relative impurity however renders very difficult the use of this type of nitrogen for producing a heat I I rrrr 2 treatment atmosphere compatible with good results. In practice, there has been proposed nitrogen according to the selective adsorption process solely for the production of atmosphere from a mixture of nitrogen and metha.iol, as S described in the article "Heat Treating Processes with Nitrogen and Methanol based atmospheres" M. KOSTELITZ and al Heat Treating". Volume 2 Number 1-35, and in the 0 j French patents Nos. 79.05599, 82.09328, 85.12380 and ooe* 85.12379 in the name of the Applicant.

S.'0 Although this type of atmosphere obtained from relatively impure nitrogen and methanol is usually intended for heating before hardening, carbonitriding and carburiziny stL 0 l, it Sis above all in carburizing that the use of nitrogen of the ii adsorption or permeation technique has been employed I 15 industrially, owing to the high temperature it requires, on the order of 9000 C, which temperature promotes the reaction I of the residual oxygen carried along by the nitrogen with 0 the chemical species of the hydrocarbon type added in the basic atmosphere.

i 20 Furthermore, systems exist for producing nitrogen of I high purity other than those employing the cryogenic method.

These relatively complex systems have as a starting point an impure nitrogen generator of the type mentioned hereinbefore, with which there is associated a unit known under the name DEOXO which permits attaining a purity similar to that of cryogenic nitrogen, i.e. a residual oxygen content lower than 10 v.p.m. Such a system is for

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-3example described in the European patent application EP-A- 0.075,663.

These systems are not widely used, since this highly pure nitrogen tien results in a production cost close to that of cryogenic nitrogen, while the production plants employing adsorption or permeation do not have the advantages of flexibility and simplicity of the cryogenic .nitrogen production plants.

eeoc There has been proposed in the document DE-A-2,844,167 a heat treatment of metals in a protective atmosphere in a 0000 furnace capable of being operated under a vacuum, comprising starting with a gas mixture containing the nitrogen which is passed over an adsorber which traps the constituents other o than the nitrogen, supplying this nitrogen to the furnace previously put under a vacuum. Experience has however shown that if one starts with a N /0 mixture, the residual oxygen o2o 2 content subsequent to the adsorption is quite prohibitive for a large number of applications in which the anti- 00o0 oxidation protection effect is determinant.

In view of this situation, the Applicant has however come to the conclusion that it was possible, by a series of production optimizations, to adapt the industrial adsorption nitrogen to a number of heat treatment applications selected in a strict and limitative manner, by starting with the production process according to which there is first of all formed in the known manner by separation of air by selective adsorption, a raw nitrogen gas current having a residual

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4 oxygen content, to which there is added a flow of hydrogen which is at least sufficient to eliminate by water vapour forming catalytic reaction, the essential part of the residual oxygen, and the-invention is characterized by the following operational conditions: a) conducting the separation of the air by selective adsorption in such manner that the nitrogen gas current has a residual oxygen content of between 0.1% an 3%; i *i b) effecting the water vapour forming catalytic 0 reaction exclusively with catalysts whose operation is o. exempt of an exterior heat supply; c) conducting the water vapour forming catalyLic reaction in such manner that the residual oxygen content is S lower than 30 with establishment of a water vapour content of between 0.2% and 6% relative to the nitrogen flow; The whole being adapted to any one of the following applications: dl) the addition of the hydrogen is effected with an V 20 excess flow so that the composite current after catalytic Sreaction has a hydrogen content of 2% to 5% relative to the total flow and said gas current is employed exclusively for forming an atmosphere for annealing copper or brazing copper; d2) the addition of the hydrogen is effected with an excess flow so that the composite current after catalytic reaction has a hydrogen content of 2% to 75% relative to the 16 L IB~R-9~ ill 1IP I IP I I I L i 1 J- 5 total flow and said gas current is employed exclusively for forming an atmosphere for decarburizing annealing steel; d3) the addition of the hydrogen is effected at a minimum value j-ust sufficient to ensure the conversion into water vapour and said atmosphere is produced by introduction j of said gas current and a current of 15% to 50% ammonia relative to the total flow and of 2% to 5% nitrogen i protoxide relative to the total flow, for nitriding steel; d4) the addition of hydrogen is effected with an excess 10 flow so that the composite current formed has a hydrogen content of 2% to 15% and said gas current is employed exclusively for forming an atmosphere for sintering copper, nickel, alloys thereof, if required with lead; d5) the addition of the hydrogen is effected with an excess flow so that the current formed has a hydrogen content of 2% to 15%, while the hydrogen/water vapour ratio remains higher than four and said gas current is employed for forming an atmosphere for sintering a copper-tin alloy.

i The complex compromise mentioned hereinbefore results from taking into account several factors. Thus, if the oxygen content of the adsorber producing raw nitrogen is limited to this permits covering a certain number of heat treatment applications which allow water vapour contents which may attain as is the case in the annealing or brazing of copper, 'he decarburizing annealing, nitriding of steel, or the sintering of certain non-ferrous metals. 'If it is proposed to operate the adsorber with a I -e 6 residual oxygen content at least equal to it is not only because the desire to improve the separation quality of the adsorber would result in a cost of the nitrogen which is incompatible with the envisiaged applications, but also because these applications require, or at least support without risk, water vapour contents equal to at least 0.2%.

It must also be noted that, in applications proposed by the invention, it is not possible to benefit from the presence of hydrogen in the treatment atmosphere in the hope 10 that the oxygen content is reduced in a certain manner in this region to a very low value compatible with the application, since when, in the course of treatment, the temperature drops below 400-500° C, the kinetics of the formation of water vapour from hydrogen and oxygen is 15 insufficiently rapid and the residual oxygen content is then j|i sufficient to oxidize the metal, in particular in the cooling zones of continuous furnaces. It is therefore essential to eliminate the oxygen before the treatment gas i is introduced into the furnace.

n 20 But not only the oxygen content of the raw nitrogen must i not exceed 3 if the obligation of adding the hydrogen in economically inadmissible quantities is to be avoided, but also the water vapour forming catalytic reaction must be carried out with a relatively good efficiency so that the residual oxygen content does not exceed 30 v.p.m. It is however unnecessary and very costly to employ reactors of the DEOXO type with very high performances which eliminate I ii ii i .0 0 0 *00* 0 *00*0*~ 0*0 0 000* fi4 7 almost all of the oxygen and the water and in any case deliver a very pure treated gas. Such reactors of the DEOXO type are expensive to employ, since they require an appreciable exterior supply of heat. This is why the present invention proposes using catalytic reactors having an energetic auto-sufficiency, as some already known and employed in other applications, such as those employing palladium on a alumina support as a catalyst. Such catalysts operate without an exterior supply of heat and permit correct performances, whereby it is possible to avoid with certainty residual oxygen contents higher than v. p. in.

The invention will now be described in more detail by first of all examining the nitrogen generator.

15 This nitrogen generator is of the conventional type having two adsorbers incorporating adsorbents of the type having generally carbonaceous molecular sieves and employing cycles having an adsorption kinetic, one of the adsorbers being in a pressurizing stage and the other in a decompression stage, the initial part of which corresponds to the production of nitrogen. The air is compressed and introduced in an adsorber at a pressure on the order of 79 bars and the nitrogen withdrawn by rapid decompression is therefore available under pressure. Depending on the degree of withdrawal, the oxygen content is more or less high.

With a production withdrawal of 30%, while 70% is released to the air, raw nitrogen is obtained which has an oxygen c *00 0 *0*0 0**0 8 content of With a withdrawal limited to 15% of the entering air, while 85% is rejected to the air, raw nitrogen is obtained which has an oxygen content of The residual oxygen content is therefore determined for a group of adsorbers essentially by the degree of production withdrawal which characterizes the efficiency or yield of the system.

Advantageously, there will be disposed at the outlet of the generator having adsorbers a buffer reservoir both for 0 taking into account non-productive stages and equalize the slight fluctuations in the residual oxygen content.

Preferably there is also employed a continuallyoperating analyzer of the oxygen content of the raw nitrogen and a tank of pure liquid nitrogen for guaranteein the 5 security of supply and for, if necessary, modifying the quality of the adsorption nitrogen.

*000 *0*900 0 00 0 0 0 00 1 The catalytic reaction requires the prior intervention of a mixer of raw nitrogen and hydrogen, advantageously associated with a buffer chamber. The catalyst is so chosen as to permit an immediate and complete reaction of the oxygen at ambient temperature, with a residual oxygen content lower than 30 v.p.m. The catalyst which may be employed is of the alumina type with 0.5% of palladium which is capable of treating an hourly flow of about 5,000 to 10,000 times the volume of the reactor. This type of catalyst requires no prior h.ating of the gas and moreover does not involve a reactor starting up sequence with an

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9 initial rejection of gas to the open air.

This catalytic reactor may be inserted in a plant in different ways, for example: each heat treatment furnace includes a catalytic pot whose volume is adapted _to the flow of the furnace. This arrangement permits delivering to the furnace hydrogenated nitrogen with a water vapour content which is a function of the initial oxygen content of the nitrogen generator. The 0 reactors may be supplied at low pressure (lower than 1 .I0 relative bar), which results in simplifications in the realization.

or one and the same catalytic reactor supplies a plurality of furnaces and this reactor is then supplied with pressurized gas, generally of 5 to 7 bars, and the de- '0.*15 oxygenated gas issuing from the reactor has a water vapour quantity which is a function of the regulation of the 5 generator.

The addition of hydrogen in excess relative to the stoichiometric quantity is then a function of the treatments 00 to be carried out.

The applications concerned by the invention will now be examined.

Annealing and brazing of copper The treatment is usually carried out with cryogenic nitrogen and hydrogen (2 to With copper, it is essential to have a very low residual oxygen content in the treatment gas in order to avoid oxidation problems. On the i I r *o .i.

0**0c 0 *00* B 2 10 other hand, the control of the water vapour is unimportant.

The cryoget.ic nitrogen is therefore advantageously replaced by a tre ent gas from an adsorption generator which delivers raw nitrogen to-which hydrogen is added so that there is 2 to 5% of hydrogen after the catalytic reaction, this gas mixture containing water vapour whose quantity is a function of the initial oxygen content in the nitrogen produced by the generator.

Decarburizing annealings of steels 0 These annealings are usually achieved with cryogenic nitrogen and hydrogen. An injection of water vapour at high temperature permits increasing the dew point and ploiuLeb the decarburizing. The hydrogen and water vapour contents are so adjusted as to decarburize the parts without oxidizing them. The cryogenic nitrogen is advantageously replaced by a treatment gas generated by a nitrogen generator employing adsorption to which hydrogen is added so that there is obtained between 2 and 75% of hydrogen after the catalytic reaction, the gas mixture containing water 0 vapour whose content is a function of the initial oxygen content in the nitrogen.

Nitriding of steel The treatments for nitriding steels may be carried out with nitrogen, ammonia (15 to 50%) and 2 to 5% of nitrogen protoxide. These treatments are principally carried out in discontinuous furnaces. The injection of a nitrogen produced by an adsorption generator would be liable to (1 00 00 P~pi~ 0 11 result in an oxidizing of the parts upon cooling, and this may be avoided by adding hydrogen in a quantity which is just sufficient to ensure the catalytic conversion of the oxyyen into water vapour.

Sintering of copper, nickel and alloys thereof with if required lead Cryogenic nitrogen with hydrogen are usually employed for sintering these metals, there may be advantageously substituted therefor a mixture of permeation nitrogen and hydrogen so that 2 to 15% of hydrogen is obtained after the catalytic reaction.

Sintering of copper-tin alloys In the case of Cu-Sn alloys, the tin being a rather oxidizable element, it is in addition necessary to limit the water vapour content so that the ratio 0(v.p.m) is higher than four.

S2 0**S 0 0 0 0. 1 0*0*0* *09 0000 00 r~r r

Claims

1. Process for producing an atmosphere for heat treating metals formed by supplying a nitrogen gas current, if required with one or more of the following constituents: hydrogen, methane, hydrocarbon, comprising first ef£d forming, i= t iwt p by separation of air by selective S adsorption, a raw nitrogen gas current having a residual Soxygen content, to which there is added a hydrogen flow which is at least sufficient to)eliminate by water vapour 10 forming catalytic reaction, the aS-&1i L I- ta h residual oxygen, characterized by the combination of the following steps: a) conducting the separation of the air by selective S adsorption in such manner that the nitrogen gas current has a residual oxygen content of between 0.1% and 3% vo\v~-A b) effecting the water vapour forming catalytic reaction exclusively with catalysts whose use is exempt from S an exterior supply of heat; c) conducting the water vapour forming catalytic reaction in such manner that the residual oxygen content is lower than 30 with establishment of a water vapour content of between 0.2% and 6% relative to the nitrogen flow; d) said nitrogen gas current purified in this way serves as a basis for the production of an atmosphere for annealing copper, brazing copper, decarburizing annealing i~ steel, sintering copper, nickel or alloys thereof; if NT i 13 required with lead, sintering copper-tin alloy.

2. Process for producing an atmosphere for heat treating according to claim 1, comprising adding the hydrogen with an excess flow so that the composite current after catalytic reaction has a hydrogen content of 2% to relative to the total flow and employing said gas current exclusively for forming an atmosphere for annealing copper **Wa or brazing copper.

3. Process for producing an atmosphere for heat ,0 treating according to claim 1, comprising adding the j hydrogen with an excess flow so that the composite current after catalytic reaction has a hydrogen content of 9a t-n 7r% relative to the total flow, and employing said gas current I exclusively for forming an atmosphere for decarburizing annealing steel. 15 i

4. Process for producing an atmosphere for heat 0C4**0 treating according to claim 1, comprising adding the hydrogen at a minimal value which is just sufficient to i ensure the conversion into water vapour, and producing said 20 atmosphere by introducing said gas current and a current of to 50% of ammonia relative to the total flow and 2% to of nitrogen protoxide relative to the total flow, for nitriding steel.

Process for producing an atmosphere for heat treating according t, claim 1, comprising adding the hydrogen with an excess flow so that the current formed has a hydrogen content of 2% to 15%, and employing said gas 1 0 S S S. r.o S rC 14 current exclusively for forming an atmosphere for sintering copper, nickel, alloys thereof, if required with lead.

6. Process for producing an atmosphere for heat treating according to claim 1, comprising adding the hydrogen with an excess flow so that the current formed has a hydrogen content of 2% to 15% while the hydrogen/water vapour ratio remains higher than four, and employing the gas current exclusively for forming an atmosphere for sinteriny a copper-tin alloy.

7. A process for producing an atmosphere for heat treating substantially as hereinbefore described with reference to any one of the examples. DATED: 24th November, 1989 PHILLIPS, ORMONDE FITZPATRICK Attorneys for: L'AIR LIQUIDE, SOCIETE ANONYME POUR L'ETUDE ET L'EXPLOITATION DES PROCEDES GEORGES CLAUDE IA 0G 0S c 0 0 S o 0 @OCC 0 SO 11~