EP0404496A1

EP0404496A1 - Heat treatment of metals

Info

Publication number: EP0404496A1
Application number: EP90306645A
Authority: EP
Inventors: Robert Franks; Paul Francis Stratton; Colin John Precious
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1989-06-22
Filing date: 1990-06-19
Publication date: 1990-12-27
Also published as: JPH03166343A; ZA904677B; AU621230B2; AU5759290A; CA2019565A1; GB8914366D0; KR910001073A

Abstract

A metal less readily oxidisable than iron is annealed in an atmosphere that is formed by separating (in for example a pressure swing adsorption apparatus 2) nitrogen from air to form a gas mixture comprising at least 95% by volume of nitrogen and a minor proportion of oxygen impurity, and then reacting the oxygen impurity with hydrogen in a catalytic reactor 4. The resulting atmosphere, which is admitted to an annealing furnace 6, comprises nitrogen, hydrogen and water vapour.

Description

This invention relates to the heat treatment of metals. In particular, it relates to the heat treatment of metals which are less readily oxidisable than iron. Such metals include cobalt, nickel, lead, copper, palladium, silver and gold, alloys of such metals, and alloys of mercury.
In manufacturing articles made of metals less oxidisable than iron, it is typically desirable to subject such articles to the step of annealing. Although the articles are relatively difficult to oxidise, it is still nonetheless necessary to maintain a reducing or non-oxidising atmosphere in the furnace used to perform the annealing operation. It is known that in theory nitrogen may be used to form an atmosphere that is inert for the purposes of annealing. Typically, the nitrogen may be supplied from a source of nitrogen which has been separated from air by distillation at cryogenic temperatures and need only contain parts per million of reactive impurities such as oxygen. Such nitrogen can be used in a heat treatment shop as the atmosphere for a range of different heat treatments. In recent years, it has been found that there are certain economic advantages in producing the nitrogen on the site of its use by non-cryogenic means rather than off-site by a cryogenic distillation process and then transporting the nitrogen product to its site of use. There are two main ambient temperature methods which may be used to separate nitrogen from air. The first is by pressure swing adsorption which entails adsorbing oxygen from the air on an adsorbent to produce a nitrogen product and then periodically regenerating the adsorbent by subjecting it to a pressure lower than that at which adsorption takes place. The alternative method is to separate air by means of semi-permeable membranes. Known semi-permeable membranes suitable for the separation of air permit oxygen to diffuse through them at a much more rapid rate than nitrogen with the result that the non-permeate gas becomes enriched in nitrogen.
Such non-cryogenic methods are able to be used to produce a nitrogen product containing in the order of 1% by volume of oxygen more cheaply than cryogenic methods may be used to separate nitrogen from air, provided the cost of transporting the cryogenically produced nitrogen to its site of use is taken into consideration. For many industrial processes the fact that the non-cryogenically produced nitrogen contains in the order of 1% of oxygen as an impurity is not a drawback. However, we have found that in the annealing of metals less readily oxidisable than iron, such an oxygen concentration is indeed a drawback. Although at first sight it may be thought that a suitable annealing atmosphere may be produced by mixing hydrogen with the non-cryogenically produced nitrogen, in for example the annealing of copper, which takes place at temperatures of 400 to 800 C, the reaction between hydrogen and oxygen proceeds sufficiently slowly at temperatures in at least the lower part of this temperature range for there to be a substantial risk that oxidation of the copper will still take place notwithstanding the addition of a stoichiometric excess of hydrogen to the atmosphere.
It is known to purify nitrogen containing about 1 % by volume of oxygen by subjecting it to a process in which the oxygen is first catalytically reacted with hydrogen and then the resulting water vapour is adsorbed by means of an adsorbent or getter. The reliance on an adsorption step to purify the nitrogen requires the use of a kind of apparatus in which there are two parallel adsorption stages, one of which is used while the other is regenerated, so as to make possible continuous production of the purified gas. The need for the adsorption stage adds considerably to the capital and running cost of the apparatus and tends to eliminate the economic advantage that would otherwise result from the production of nitrogen on-site by non-cryogenic means.
There is thus a need for a method and apparatus of annealing articles of metal less oxidisable than iron which renders the use of non-cryogenically produced nitrogen containing an appreciable quantity of oxygen impurity attractive from the economic point of view.
According to the present invention there is provided a method of annealing an article of a metal less oxidisable than iron, comprising the steps of subjecting the article to an annealing temperature in a heat treatment furnace, separating nitrogen from air at the site of the annealing furnace to produce a gas mixture containing at least 95% by volume of nitrogen and a minor proportion of oxygen impurity, catalytically reacting the oxygen impurity with a stoichiometric excess of hydrogen to form water vapour, and passing the resulting gas mixture comprising nitrogen, water vapour and unreacted hydrogen into the furnace to create an annealing atmosphere.
Instead of using a stoichiometric excess of hydrogen the precise stoichiometric amount of hydrogen as required by the reactions:

2H₂ + 0₂ --> 2H₂0
may be employed. Accordingly the resulting gas mixture then contains no oxygen and no hydrogen.

The invention also provides apparatus for annealing an article of a metal less oxidisable than iron comprising an annealing furnace, means for separating a gas mixture comprising at least 95% by volume of nitrogen and oxygen impurity from air, and a catalytic reactor for catalytically reacting the oxygen impurity with a stoichiometric excess of hydrogen to form a gas mixture comprising nitrogen, water vapour and unreacted hydrogen, wherein said reactor has an outlet in communication with the annealing furnace so as to enable a suitable annealing atmosphere to be created in the furnace.
The method and apparatus according to the invention are particularly suited for the bright annealing of the metals less oxidisable than iron.
Preferably, the nitrogen product contains 0.5-3% by volume of oxygen upstream of its catalytic reaction with hydrogen. In general only a small if any stoichiometric excess of hydrogen is required. For example, in the bright annealing of copper at a temperature in the order of 600 C, suitable annealing conditions can be maintained provided that the ratio of the partial pressures of hydrogen to water vapour in the annealing atmosphere does not fall below 1 x 10-⁶.
The catalytic reaction preferably takes place over a platinum or palladium catalyst. Alternatively, copper or nickel catalysts may be used. The catalyst is typically heated by the reaction between hydrogen and oxygen to a temperature of up to 200 C.
The method and apparatus according to invention will now be described by way of example with reference to the accompanying drawings, in which

Figure 1 is a schematic diagram of apparatus for the bright annealing of copper, and
Figure 2 is a schematic drawing illustrating apparatus according to the invention including a continuous mesh belt furnace.

In Figure 1 of the drawings, there is shown a plant 2 for separating nitrogen from air by pressure-swing adsorption. Suitable plants and apparatus for this purpose are for example disclosed in Patent Specifications 2073043A and UK 2195097A. The resulting nitrogen typically contains from 0.5 to 3% by volume of oxygen impurity. The nitrogen stream is passed through a catalytic reactor 4 in which its oxygen impurity is reacted with a small stoichiometric excess of hydrogen over a palladium or platinum catalyst at a reaction temperature. The resulting gas mixture comprises nitrogen, hydrogen and water vapour. It is admitted to the heat treatment furnace 6 which may be of a batch or continuous kind in which an article made of copper or an alloy of copper such as bronze, copper-nickel, or brass containing up to 15% by weight of zinc, is annealed by immersion in an annealing atmosphere at a temperature of 400 ` -800` C for a period of time typically in the order of 10 minutes to 2 hours. By maintaining the ratio of the partial pressure of hydrogen to the partial pressure of water vapour in the atmosphere and hence in the nitrogen supplied from the catalytic reactor 4 at a value not less than 1 x 10-⁶ it is possible to maintain conditions in the atmosphere in which the copper is not deoxidised. Therefore its bright surface is maintained during the annealing.
Referring to Figure 2 of the drawings, there is shown a conventional continuous mesh belt furnace 10 having an inlet zone 12, 1 metre in length, a hot or heated zone 14, 5.67 metres in length, and a cooling zone 16, 6.86 metres in length. The furnace 10 is provided at its respective ends with baffles 18 or the like to impede the ingress of air from outside the furnace into its interior. The hot zone 14 and the cooling zone 16 have respectively inlets 20 and 22 for gas.
The inlets 20 and 22 are each alternatively able to be placed in communication for source 24 of gas mixture comprising hydrogen, water vapour and nitrogen. The source 24 comprises a plant 26 for separating nitrogen from air by pressure swing adsorption, and a catalytic reactor 28 for reacting oxygen impurity in the nitrogen with hydrogen. A commercially available catalyst comprising palladium supported on alumina is used. Typically, the catalyst is heated by the reaction between hydrogen and oxygen and the gas mixture leaves the reactor 28 at a temperature in the range of 50 to 150 C.
In operation, a gas mixture comprising nitrogen, water vapour and hydrogen from the catalytic reactor 28 is passed into the furnace 10 through either or both the inlets 20 and 22. The hot zone 14 is heated to a chosen temperature typically in the range 500 to 800° C. Copper work to be bright annealed is loaded onto the belt (not shown) of the furnace and the belt is then advanced slowly through the furnace at a speed such that the work typically has a residence time in the furnace of from 30 minutes to 1 hour. As the work passes through the hot zone 14 so it is raised approximately to the temperature of the hot zone 14. On leaving the hot zone 14 the work passes into the cooling zone 16 in which it is cooled by contact with the relatively cold atmosphere therein. Typically, the work is at a temperature between ambient and 50° C when it leaves the furnace 10. By employing an atmosphere in accordance with the invention, it can be ensured that bright annealed work leaves the furnace 10.
A number of experimental tests of the apparatus shown in Figure 2 were performed. The results of the tests are set out in Tables 1 to 4 below.
Table 1 shows the results of comparative experiments in which various different proportions of hydrogen were mixed with nitrogen from a PSA plant (0.5% oxygen impurity) and the resulting mixture passed into the furnace without a catalytic reaction between hydrogen and oxygen being performed. It was a feature of the furnace used that entry to the hot zone was by way of a muffle made of an alloy with high nickel and chrome content which had the property of acting as a getter for free oxygen. Accordingly, it was found that substantially all the oxygen entering the furnace with the gas mixture was removed therefrom. It was therefore possible to obtain bright work (at 750. C) even when the oxygen level was nominally in excess of the stoichiometric requirement for reaction with hydrogen. Results are given in Table 1 for the oxygen concentration, hydrogen concentration and dew point in both the hot zone 14 and the cooling zone 16 of the furnace 10 at the different hydrogen levels.
The results set out in Table 2 relate to a set of experiments similar to those of Table 1 save that the mixture of nitrogen, oxygen and hydrogen was reacted in the catalytic reactor 28 upstream of being admitted to the hot zone of the furnace through the inlet 20.
The Experiments illustrated in Table 3 and 4 are respectively comparable to those set out in Tables 1 and 2 with the exception that the gas mixture is not introduced directly into the hot zone 14 through the inlet 20. Rather, it is introduced directly into the cooling zone 16 through the inlet 22. Table 3 shows experiments in which the gas mixture by-passed the catalytic reactor 28, whereas Table 4 relates to experiments in which the catalytic reactor was used. The low dew points obtained in the experiments illustrated by Table 3 indicate that when the gas mixture is supplied directly to the cooling zone, a failure to react the oxygen with at least a stoichiometric quantity of hydrogen in the catalytic reactor 28 will tend to result in work that is not bright even if the hot zone is ooerated at a temperature as high as 750 C.
It must be further borne in mind that if there is used a furnace 10 which does not have a muffle that acts as a getter for oxygen and if the hot zone 14 is operated at a temperature substantially lower than 750 C, there is a substantial risk that copper work will not be given a bright finish.

NOTE

Oxygen concentrations were measured using an oxygen probe. The temperature of the gas from the hot zone at the sampling point was 750° C: the temperature of the gas from the cooling zone at the sampling point was 200°C. Most results are shown in Millivolts (mV). The actual oxygen concentration can be calculated using the formula

E ₌ 0.0215.T. loge (20.95/% 0₂)
where
E is the oxygen probe reading in mV
T is the temperature in Kelvin at the oxygen probe location.
%O₂ is the concentration of oxygen expressed as a percentage by volume.

Claims

1. A method of annealing an article of a metal less oxidisable than iron, comprising the steps of subjecting the article to an annealing temperature in a heat treatment furnace, separating nitrogen from air at the site of the annealing furnace to produce a gas mixture containing at least 95% by volume of nitrogen and a minor proportion of oxygen impurity, catalytically reacting the oxygen impurity with a stoichiometric excess of hydrogen to form water vapour, and passing the resulting gas mixture comprising nitrogen, water vapour and unreacted hydrogen into the furnace to create an annealing atmosphere.

2. A method of annealing an article of a metal less oxidisable than iron, comprising the steps of subjecting the article to an annealing temperature in a heat treatment furnace, separating nitrogen from air at the site of the annealing furnace to produce a gas mixture containing at least 95% by volume of nitrogen and a minor proportion of oxygen impurity, catalytically reacting the oxygen impurity with a stoichiometric quantity of hydrogen to form water vapour, and passing the resulting gas mixture comprising nitrogen and water vapour into the furnace to create an annealing atmosphere.

3. A method according to Claim 1 or Claim 2, in which the nitrogen product contains 0.5-3% by volume of oxygen upstream of its catalytic reaction with hydrogen.

4. A method according to any one of the preceding claims, in which the catalytic reaction takes place over a platinum or palladium catalyst.

5. A method according to any one of the preceding claims, in which the metal is cobalt, nickel, lead, copper, palladium, silver or gold, or an alloy of at least one of such metals, or as alloy of mercury.

6. A method according to any one of the preceding claims, in which the metal is bright annealed.

7. A method according to any one of the preceding claims, in which the resulting gas mixture is introduced directly into the cooling zone of a continuous furnace.

8. Apparatus for annealing an article of a metal less oxidisable than iron comprising an annealing furnace, means for separating a gas mixture comprising at least 95% by volume of nitrogen and a minor proportion of oxygen impurity from air, and a catalytic reactor for catalytically reacting the oxygen impurity with a stoichiometric excess of hydrogen to form a gas mixture comprising nitrogen, water vapour and unreacted hydrogen, wherein said reactor has an outlet in communication with the annealing furnace so as to enable a suitable annealing atmosphere to be created in the furnace.

9. Apparatus according to claim 8, in which the catalytic reactor includes a platinum or palladium catalyst.