GB2082634A - Heat treatment method - Google Patents

Abstract

A gas mixture produced in an exothermic generator (20), suitable for use in forming a protective or reactive atmosphere in the heat treatment of metals, typically contains carbon dioxide and/or water vapour. In order to reduce the level of carbon dioxide and/or water vapour therein, the mixture is heat exchanged (14) with a cryogenic liquid (typically liquid nitrogen) so as to freeze out of the mixture at least some of the water vapour and/or carbon dioxide. The mixture is then passed to a treatment chamber of a furnace (2) in which metal is heat treated, mixed with at least some of the nitrogen vaporised by the heat exchange. <IMAGE>

Description

SPECIFICATION Heat treatment method This invention relates to a heat treatment method. It is particularly concerned with the removal of carbon dioxide and/orwatervapourfrom a gas mixture which is to be supplied to a furnace in which metal is heat treated. The term "heat treatment" as used herein is intended to embrace processes such as sintering or brazing as well as those processes such as carburising, carbonitriding, hardening, annealing, and normalising that are normally classed as being heat treatment processes.

In one example of the method according to the invention the levels of water vapour and carbon dioxide in a gas mixture produced by an exothermic generator are substantially reduced before such gas mixture is passed to the heat treatment furnace.

Exothermic generators are widely used in the production of protective furnace atmospheres for such processes an annealing, normalising, hardening, brazing, and sintering of metals. In an exothermic generator a fuel gas, normally propane or natural gas, is partially oxidised. The resultant gas mixture comprises nitrogen, carbon monoxide and impurities. The impurities include carbon dioxide, water vapour and oxygen. Typically, at least 5% by volume of carbon dioxide and water vapour is formed. For many potential and actual uses of the resulting gas mixture, such levels of carbon dioxide and water vapour are undesirably high, tending to make the gas mixture decarburising and/or oxidising to the metal being treated in the furnace. Moreover, the relatively high levels of carbon dioxide and water vapour tend to corrode pipes used to conduct the gas mixture to the furnace.

Conventional methods of reducing levels of carbon dioxide and water vapour involve stripping out these constituents from the gas mixture by means of adsorbent beds or liquid adsorbents. However, the size of installation required to effect removal of carbon dioxide and water vapour in such manner markedly increases the total capital and operating cost of the plant.

It is an aim of the present invention to provide a method of heat treating metal in a furnace, in which at least some water vapour and/or at least some carbon dioxide is removed from a gas mixture including water vapour and/or carbon dioxide, which method does not require relatively large and expensive-to-run equipment for removing the water vapour and/or carbon dioxide.

According to the present invention there is provided a method of heat treating metal in a furnace, including the steps of removing at least some water vapour and/or at least some carbon dioxide from a gas mixture including water vapour and/or carbon dioxide, and then admitting the gas mixture to the furnace so as to form an atmosphere therein of chosen composition, wherein the removal of the water vapour and/or carbon dioxide is effected by freezing water vapour and/or carbon dioxide by means of a cryogenic medium, at least some of the cryogenic medium being supplied to the furnace after the heat exchange. By the term cryogenic medium as used herein is meant liquid nitrogen or its cold vapour or a noble gas having a temperature of below -100 C.

The method according to the present invention is particularly suited to removing substantially all carbon dioxide and water vapour from a gas mixture produced by an exothermic generator.

Thus, although this is not preferred, it is possible to freeze carbon dioxide and/orwatervapour by direct contact with the cryogenic medium. Instead, it is preferred to effect such freeing by indirect heat exchange with the cryogenic medium and then mix the cryogenic medium with the purified gas mixture.

One advantage of mixing a gas mixture with the nitrogen, be it in the heat treatment furnace or upstream of the heat treatment furnace, is that it enables a non-oxidising or non-decarburising atmosphere of carrier gas for heat treatment to be produced economically. Another advantage is that it is possible to vary the flow rate of the gas produced by varying the rate at which nitrogen is combined with the gas mixture. This enables the method according to the present invention to be used in heat treatment where there is a varying demand for gas that forms the atmosphere in the heat treatment furnace. Moreover, the amount of water vapour and carbon dioxide that is removed may be varied. For example, in some treatments of metal it may be desirable to remove at least some of the water vapour but none of the carbon dioxide.This can be done by choosing an appropriate flow rate for the cryogenic medium.

Preferably, the freezing out of the carbon dioxide and/or water vapour is effected in a 'freeze-out' heat exchanger having a first passage defined by a pipe or pipes of heat conductive material extending therethrough. The liquid nitrogen or other cryogenic medium is admitted to the first passage. There is preferably a second passage in heat exchange relationship with the first passage. The second passage is preferably defined by a pipe or pipes through which the pipe or pipes of the first passage extend. The gas mixture is admitted to the second passage. As the liquid nitrogen passes through the first passage so it absorbs heat from the gas mixture (which is typically passed countercurrently to the cryogenic medium) in the second passage and effects freezing out of the carbon dioxide and/or water vapour.Preferably, the pipe or pipes defining the first passage have internal and external fins so as to facilitate transfer of heat from the gas mixture to the cryogenic medium. The passages preferably follow winding or tortuous paths for the fluid that passes therethrough. Instead of using pipes, it is possible to use an arrangement of plates to define passages for the cryogenic medium through the 'freeze-out' heat exchanger.

It is not generally possible to use the 'freeze out' heat exchanger indefinitely as, in time, the amount of solid carbon dioxide and ice deposited on heat exchange surfaces will be such that the gas mixture leaving such container will include an undesirably high proportion of carbon dioxide and/or water vapour. Accordingly, it is desirable to regenerate the heat exchanger from time to time. There are therefore preferably at least two such heat exchangers, one being used to separate carbon dioxide and/or water vapour from the gas mixture while the other or another is being regenerated.

Regeneration is preferably effected by passing compressed air or other relatively warm gas through the second passage so as to melt the ice and cause the solid carbon dioxide to sublime. The water will typically collect at the bottom of the drum or other container. Preferably the drum or other container has at its bottom taps which may be opened to drain such water from the container. The compressed air of the gas may be vented from the drum after it has passed therethrough.

It is preferred that the cryogenic medium (e.g.

leaving the freeze-out heat exchanger (or one of the freeze-out heat exchangers)) is used to precool the gas mixture before it enters a 'freeze-out' heat exchanger. This is preferably done by indirect heat exchange. For example, one part of the gas mixture may be precooled by passage through a preliminary or precooling heat exchanger upstream of the freeze-out heat exchanger forthe time being in an active mode (ie. being employed to freeze out the carbon dioxide and/or water vapour and not be regenerated). Cooling for the preliminary heat exchanger is provided by the cryogenic medium. The said one part of the gas mixture and the cryogenic medium are then passed to their respective inlets to the 'active' freeze-out heat exchanger.Another or the other part of the gas mixture is precooled by being passed through the passage for cryogenic medium in the freeze out heat exchanger that, for the time being, is being regenerated. Such part of the gas mixture is then united with the said one part thereof upstream of the active freeze-out heat exchanger. Such an arrangement helps to increase the value of the maximum flow rate of gas mixture with which the method according to the invention can cope.

Preferably, the freeze out heat exchanger or heat exchangers together with any pre-cooling heat exchanger, are contained in a suitable housing, which, if desired, can be adapted to be bolted on or otherwise mounted to a source of the gas mixture to be treated, for example, an exothermic generator.

An alternative method of freezing the carbon dioxide and/or water vapour includes the step of passing the gas mixture into a vertical column into which liquid nitrogen is sprayed or otherwise introduced. In such a method the purity of the gas leaving the column is controlled by monitoring its temperature. For example, a temperature sensor may be employed to measure the temperature of this gas and be linked to a controller which controls the flow of liquid nitrogen into the column such that the exhaust gas temperature is maintained at a constant level, for example - 1 000C. Particles of solid carbon dioxide and ice fall to the bottom of the column from which they may from time to time be passed into a hopper associated with the bottom of the column.If desired, the column may be provided with baffle plates or such means as glass beads in order to facilitate the precipitation of carbon dioxide and water vapour in the solid phase. Alternatively, or additionally, a cyclone may be associated with the column for this purpose.

Another alternative method of freezing the carbon dioxide and/or water vapour is to pass the gas mixture through a fluidised bed which is cooled by the cryogenic medium. Such cooling can be effected by passing the cryogenic medium through heat exchange pipes or the like in the bed or by directly introducing the cryogenic medium into the bed (or by a combination of both these means). If desired, the cryogenic medium may be used as a fluidising medium or as part of the fluidising medium. The introduction of the cryogenic medium may be controlled in accordance with the temperature of the gas leaving the fluidised beds.

Ayetfurther method of freezing the carbon dioxide and/or water vapour is to pass the gas mixture through a bath of liquid nitrogen or other liquid cryogenic medium.

The method according to the present invention is not limited to the treatment of gas produced by an exothermic generator before such gas is admitted to a heat treatment furnace. It may alternatively, for example, be used to remove any water vapour and/or carbon dioxide from the gas mixture formed by thermally cracking methanol or other alcohol or organic liquid comprising a compound of carbon, hydrogen and oxygen prior to the admission of such mixture to a heat treatment furnace. Another possible use of the method according to the present invention is to take a gas mixture leaving one or more radiant tube heaters employed to raise the temperature internally of a heat treatment furnace and to remove at least some of the carbon dioxide and/or water vapour from such gas. After this treatment, the gas mixture may be used so as to form a protective atmosphere in the heat treatment furnace.Yet another possible use of the method according to the present invention is in the removal of water vapour from hydrogen produced by the reduction of steam so as to produce a gas that may be used in the heat treatment of metals.

The method according to the present invention will now be described further by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing apparatus for performing the method according to the invention; Figure 2 is a circuit diagram illustrating apparatus for freezing out carbon dioxide and water vapour from a gas mixture produced by the exothermic generator shown in Figure 1; Figure 3 is a schematic side elevation of a freeze-out heat exchanger forming part of the apparatus shown in Figure 2; Figure 4 is a schematic section showing generally the arrangement of the passages within the freezeout exchanger shown in Figure 3; Figure 5 is a section through the nitrogen passage employed in the heat exchanger shown in Figure 4.

Referring to Figure 1 of the accompanying drawings, a heat treatment furnace 2 has inlets 4, 6 and 8 for the supply of gases thereto so as to form a protective atmosphere. Inlets 4 and 6 communicate with outlets 10 and 12 respectively of a gas purification unit 14. The inlet 8 communicates with a source of hydrocarbon such as natural gas or propane or with a source of hydrogen (or with some other gas which may be used in the formation of a protective atmosphere within the furnace 2). The purification unit 14 has a first inlet 16 for gas generated by an exothermic generator 20 and a second inlet 18 for liquid nitrogen.

The exothermic generator 20 is shown only schematically in Figure 1. It is a conventional piece of equipment which comprises a gas generating section 22 in which a mixture of fuel gas and air is burnt and a cooling section 24 in which gas mixture is cooled to a temperature near to ambient by, for example, means of water.

In operation, the composition of the gas mixture produced by the exothermic generator 20 depends on the relative proportions of fuel gas and air that are admitted to the burners (not shown) of the generator. However, the amount of air used is always less than the stoichiometric requirement for complete oxidation of the fuel gas, to carbon dioxide and water vapour. Thus, a gas mixture comprising nitrogen, carbon monoxide, hydrogen, water vapour, carbon dioxide and oxygen is produced. The relative proportions of these constituents depends on the precise ratio of fuel gas to air employed.

Typically, a "lean" gas mixture or a "rich" gas mixture may be produced, the lean gas mixture containing a relatively small proportion of constituents other than nitrogen in comparison with the rich mixture.

The gas mixture that is produced in the section 22 of the exothermic generator 20 is then cooled in section 24 to a temperature not greatly above ambient. The cooled gas mixture is then passed into the freezing unit in which at least some of the carbon dioxide and water vapour are frozen out of the gas mixture. Such freezing is effected by heat exchange with liquid nitrogen. Resulting nitrogen vapour is passed to the inlet 6 of the furnace 2 and the gas mixture from which the carbon dioxide and water vapour have at least partially been removed is passed to the furnace 2 through the inlet 4.

The composition chosen for the furnace atmosphere formed from the gases admitted through the inlets 4,6 and 8 will depend on the nature of the treatment being performed in the furnace 2. If a relatively neutral atmosphere, (ie, one that is neither carburising nor decarburising to the metal being heat treated) is required, it will typically not be necessary to add hydrocarbon in the form of natural gas or propane to the furnace atmosphere through the inlet 8. If, however, a carburising atmosphere is required then such an addition may be desirable or necessary.

The unit 14 in which the freezing out of the water vapour and carbon dioxide is carried out is further illustrated in Figures 2 to 5. As shown in Figure 2, the unit 14 contains two freeze-out heat exchangers 30 and 32 and a preliminary or precooling heat exchanger 34. The general arrangement of the circuit for effecting freeze-out of the carbon dioxide and water vapour is shown in Figure 2. The freeze-out heat exchanger 30 has fluid inlet conduits 36 and 38, and the freeze-out heat exchanger 32 has fluid inlet conduits 40 and 42. The freeze-out heat exchanger 30 has a first fluid outlet conduit 44 communicating with the inlet conduit 36 and a second fluid outlet conduit 46 communicating with the inlet conduit 38.

The freeze-out heat exchanger 32 has a first fluid outlet 48 communicating with the inlet conduit 40 and the second fluid outlet conduit 50 communicating with the inlet conduit 42. The inlet conduits 36 and 42 are connected to a liquid nitrogen inlet 56 by means of a conduit 54. In the conduit 36 is an on-off valve 58 and in the conduit 42 is an on-off valve 60.

An inlet 62 for exothermic gas has an on-off valve 64 in it and communicates with one set of heat exchange passages of the preliminary heat exchanger 34 at one end thereof, the heat exchange passages at the other end communicating with a connecting conduit 66 via a heat exchanger outlet 68. One end of the connecting conduit 66 forms a union with the inlet conduit 38 of the heat exchanger 30 and the other end of the connecting conduit 66 makes a union with the inlet conduit 40 of the heat exchange 32. An on-off valve 70 is located in the conduit 66 intermediate the union of that conduit with the conduit 38 and the union of that conduit with the conduit 68. A second on-off valve 72 is also located in the conduit 66. The valve 72 is positioned intermediate the union of the conduit 66 with the conduit 40 and the union of the conduit 66 with the heat exchanger outlet 68.

The freeze-out unit also has an inlet 74 for compressed air. Inlet 74 ends in a connecting conduit 76 which extends between the inlet conduit 38 of the heat exchanger 30 and the inlet conduit 40 of the heat exchanger 32. Conduits 38 and 40 join with the connecting conduit 76 at locations upstream of their union with the connecting conduit 66. In the conduit 38 intermediate its union with the connecting conduit 76 and its union with the connecting conduit 66 is an on-off valve 80. In the inlet conduit 40 of the heat exchanger 32 intermediate its union with the connecting conduit 76 and the connecting conduit 66 is a valve 82.

The outlet conduits 44 and 50 come to an end in the respective ends of a connecting conduit 84. In the conduit 44 is an on-off valve 86 and in the conduit 50 is an on-off valve 88. Intermediate its ends the conduit 84 communicates with a conduit 90 leading to the inlet to the other set of heat exchange passages in the heat exchanger 34. The outlets of these passages communicate with a conduit 92 that feeds an outlet 94 from which nitrogen may be withdrawn from the freeze-out unit.

A conduit 77 connects a region of the inlet conduit 36 downstream of the valve 68 to a region of the inlet conduit 42 downstream of the valve 60. The conduit 77 also communicates with the inlet 62 to the peliminary heat exchanger 34 at a region of the inlet downstream of the valve 64.

The outlets 46 and 48 of the heat exchangers 30 and 32 respectively communicate with either end respectively of a common conduit 96. Conduit 96 at an intermediate location thereof has a union with an outlet 98 for purified exothermic gas. In the conduit 46 is an on-off valve 100 and in the conduit 48 is an on-off valve 102. Associated with the inlet conduit 46 is a vent passage 104. In the vent passage 104 is an on-off valve 106. Associated with the outlet conduit 48 is a vent passage 108. In the vent passage 108 is an on-offvalve 110. The vent passages 104 and 108 are situated upstream of the valves 100 and 102 respectively. Communicating with the outlet conduit 44 of a region upstream of the valve 86 is a conduit 112. Conduit 112 terminates in the conduit 66 at a region thereof intermediate the valve 72 and the union of the heat exchanger outlet 68 with the conduit 66.In the conduit 112 is an on-off valve 114 and downstream of the valve 114a one-way or non-return valve 116 which prevents return of gas from the conduit 66 to the conduit 44. The conduit 118 communicates at one of its ends with a region of the conduit 50 upstream of the valve 88 and at its other end with a region of the conduit 66 intermediate the valve 70 and the union of the heat exchanger outlet 68 with the conduit 66.

In the conduit 118 is an on-off valve 120. Downstream of the valve 120 is non-return or one-way valve 122 which prevents return of gas from the conduit 66 to the conduit 50.

In operation, a stream of exothermic gas enters the unit through the inlet 62. One part of the stream is precooled in heat exchanger 34 by heat exchanger with vaporised nitrogen returning from the heat exchanger 30 or the heat exchanger 32. The other part of the stream is precooled in the freeze-out heat exchanger to which the liquid nitrogen is not supplied. The two parts of the stream are then recombined and the pre-cooled exothermic gas is then admitted to the appropriate one of the heat exchanger 30 and the heat exchanger 32 in which carbon dioxide and water vapour are frozen out of the exothermic gas by heat exchange with liquid nitrogen. The resulting vaporised nitrogen is returned to the preliminary heat exchanger 34 and subsequently to the heat treatment furnace shown in Figure 1.

At the start of a cycle of operations, only on-off valves 64,68,70,75,82,86, 100, 110 and 120 are open. The other on-off valves are all shut. Accordingly, one part of the stream of exothermic gas is admitted to the heat exchanger 34 for pre-cooling and the pre-cooled exothermic gas passes through valve 70 into the inlet conduit 38 and then passes through the heat exchanger 30. Upstream of the valve 70 a second part of the incoming exothermic gas stream is united with the first part, so that all the exothermic gas flows through the heat exchanger 30. Liquid nitrogen is admitted to the unit through the inlet 56 and passes through the valve 58 and enters the drum 30 through the inlet conduit 36. In the heat exchanger 30 the liquid nitrogen absorbs heat from the exothermic gas and consequently carbon dioxide and water vapour are frozen out.The resultant nitrogen vapour passes out of the drum 30 through the conduit 44, flows through the valve 86 and returns to the heat exchanger 34 and passes to the outlet 94 via the conduit 92. The exothermic gas leaves the heat exchanger 30 and enters the outlet conduit 46. It then passes through the valve 100 and into the outlet 98.

While the freeze-out heat exchanger 30 is being used to remove carbon dioxide and water vapour from the exothermic gas, the other freeze-out heat exchange 32 is being regenerated. Compressed air is passed into the inlet conduit 40 from the inlet 74. The compressed air sublimes carbon dioxide and melts water vapour in the heat exchanger 32 and passes out of the unit through the vent passage 108. The other part of the exothermic gas stream is passed through the other heat exchange passage in the heat exchanger 32 and is thus precooled. It then passes out of the heat exchanger 32 through the outlet conduit 50 and then to the conduit 118. Finally, it rejoins the first part of the exothermic gas in the conduit 66.

Carbon dioxide and water vapour frozen out of the exothermic gas tend to form ice around the heat exchange surfaces in the heat exchanger 30. Accordingly, the heat exchange gradually becomes less efficient and it eventually becomes necessary to regenerate the heat exchanger 30. The various on-off valves are then operated so as to switch the flow of nitrogen and exothermic gas to the drum 32 and to allow relatively warm compressed air to sublime the solid carbon dioxide and melt the ice. The resulting water can be drained off from the drum 30 as will be shown later.

In order to effect this change in the fluid flow paths through the apparatus shown in Figure 2, the valves, 68,70,75,82,86, 100, 110 and 120 are closed, and simultaneously the valves 60,72,73, 80,88, 102, 106 and 114 open. This marks the start of the second part of the operating cycle. The first part of the exothermic gas after leaving the heat exchanger 34 now flows through the valve 72 into the inlet conduit 40, passes through the heat exchanger 32 in which the carbon dioxide and water vapour are frozen out, leaves the heat exchanger 32 through the outlet conduit 48, passes through the valve 102 to the outlet 98.The liquid nitrogen passes from the inlet 56 through the valve 60 into the heat exchanger 32 via the inlet conduit 42, provides the necessary refrigeration for the freezing out of the water vapour and carbon dioxide in the drum 32, leaves the heat exchanger 32 as vapour and passes into the conduit 50, through the valve 88 and then into the heat exchanger 34. The nitrogen leaving the heat exchanger 34 then passes through the conduit 92 to the outlet 94. The compressed air enters the apparatus through the inlet 74, passes through the valve 80 in the inlet conduit 38 and then passes through the heat exchange passages for the exothermic gas in the heat exchanger 50, thereby melting ice and subliming solid carbon dioxide. The compressed air then passes out of the heat exchanger 30 and enters the outlet conduit 46 and is vented through valve 106 in the vent line 104. The other part of the exothermic gas passes through the valve 73 and is then precooled in the heat exchanger 30 and reunited with the first part in the passage 66 after passing through the valve 114.

In time, it becomes necessary to regenerate the heat exchanger 32 and the cycle is repeated.

When starting-up the apparatus shown in Figure 2, only valves 64, 68, 70,86 and 100 are opened as the heat exchangers 30 and 32 have not yet been cooled down. Once the gas mixture having the heat exchanger 30 with a concentration of impurity at or above a chosen value, the flows of gas mixture and liquid nitrogen are switched to the heat exchanger 32, while the heat exchanger 30 is regenerated as described above. Operation of the apparatus is thereafter as described above.

All the on-off valves may,if desired, be automatically operated. For example, they may be solenoid valves. The opening and closing of the solenoid valves may be controlled by means of suitably programmed timers. Alternatively, the control of the opening and closing of the valves may be governed by means of a sensor, for example, in the outlet 98 which senses the carbon dioxide and/or water vapour content of the exothermic gas leaving the apparatus shown in Figure 2. If such an arrangement is adopted, once the level of the carbon dioxide and/or water vapour reaches a chosen value, the heat exchanger being regenerated can be switched to its freezing-out mode, and vice versa.

The construction of the freeze-out heat exchanger is illustrated in Figures 3 to 5. The gas inlets and outlets are all situated at the top of the heat exchangers. In Figure 3, the heat exchanger 30 is illustrated. In communication with passages for the exothermic gas are drainpipes 130. Each drainpipe has tap 132 which may be opened manually.

The internal passages of the heat exchanger 30 are illustrated in Figure 4. There is a nitrogen pipe 140 (only a part of which if formed). As shown in Figure 5 the nitrogen pipe has internal and external fins 142 and 144 respectively. The pipe and the fins are made of heat conductive metal which is not embrittled by the liquid nitrogen. The passage 140 comprises a plurality of vertical legs 146 connected or integral with U-shaped portions 148. The arrangement is that the nitrogen flows up one leg and then down the next leg and so on. Typically, the total path length of the nitrogen passage is at least 6 metres and may be much longer depending on the gas flow rates with which the freeze-out unit has to cope. The legs 146 of the passages 100 are coaxial with wider-bore passages 150 for exothermic gas. The passages 150 are closed at both ends by means of sealing plates 152 and 154. Connections 160 enable gas to pass from one passage 150 to the next. The arrangement of the passages for nitrogen and exothermic gas is that the exothermic gas travels countercurrently to the nitrogen. Associated with each passage 158 is a drainpipe as previously described with reference to Figure 3.

The apparatus shown in Figures 2 to 5 is the sugbject of our patent application entitled "Method and Apparatus for Purifying a Gas Mixture" having the same date of filing as this application.

Claims (23)

1. A method of heat treating metal in a furnace, including the steps of removing at least water vapour and/or at least some carbon dioxide from a gas mixture including water vapour and/or carbon dioxide, and then admitting the gas mixture to the furnace so as to form an atmosphere therein of chosen composition, wherein the removal of the water vapour and/or carbon dioxide is effected by freezing water vapour and/or carbon dioxide by means of a cryogenic medium, at least some of the cryogenic medium being supplied to the furnace after the heat exchange.

2. A method as claimed in claim 1, in which the cryogenic medium is liquid nitrogen.

3. A method as claimed in claim 1 or claim 2, in which the gas mixture is produced by an exothermic generator.

4. A method as claimed in any one of the preceding claims, in which the freezing is effected by indirect heat exchange of the cryogenic medium with the gas mixture.

5. A method as claimed in any one of the preceding claims, in which the freezing out of the carbon dioxide and/or water vapour is effected in a "freeze-out" heat exchanger having a first passage, defined by a pipe or pipes of heat conductive material, to which the cryogenic medium is admitted and a second passage in heat exchange relationship with the first to which the gas mixture is admitted.

6. A method as claimed in claim 5, in which the second passage is defined by a pipe or pipes through which the pipe or pipes of the first passage extend.

7. A method as claimed in claim 5 or claim 6, in which the pipe or pipes defining the first passage have internal and external heat exchange fins.

8. A method as claimed in any one of claims 5 to 7, in which the passages follow winding or tortuous paths.

9. A method as claimed in any one of claims 5 to 8, in which there are two freeze-out heat exchangers, one being used to freeze-out the water vapour and/or carbon dioxide while the other is being regenerated.

10. A method as claimed in claim 9, in which regeneration is effected by passing relatively warm gas through the second passage.

11. A method as claimed in any one of claims 5 to 10, in which the cryogenic medium is used to pre-cool the gas mixture before it enters a "freezeout" heat exchanger.

12. A method as claimed in claim 9 or claim 10, in which one part of the gas mixture is precooled by passage through a preliminary heat exchanger upstream of the active freeze-out heat exchanger for the time being employed to freeze-out the carbon dioxide and/or water vapour, while the other part of the gas mixture is precooled by being passed through the passage for cryogenic medium in the freeze-out heat exchanger that, for the time being, is being regenerated and the two parts of the gas mixture are then united upstream of the active freeze-out heat exchanger.

13. A method as claimed in claim 12, in which the cryogenic medium is used to provide cooling for the preliminary heat exchanger leaving the active freezeout heat exchanger.

14. A method as claimed in any one of claims 1 to 3, in which the gas mixture is passed into a vertical column into which the liquid nitrogen is sprayed or otherwise introduced so as to freeze-out the carbon dioxide and/or water vapour.

15. A method as claimed in claim 14, in which the temperature of the gas mixture leaving the column is monitored by a temperature sensor linked to a controller which controls the flow of the liquid nitrogen into the column such that the exhaust gas temperature is maintained at a constant level.

16. A method as claimed in claim 14 or claim 15, in which particles of solid carbon dioxide and ice falling to the bottom of the column are from time to time passed into a hopper associated with the bottom of the column.

17. A method as claimed in any one of claims 14 to 16, in which the column is provided with baffle plates or means such as glass beads so as to facilitate precipitation of carbon dioxide and water vapour in the solid phase.

18. A method as claimed in any one of claims 14 to 17, in which a cylone is associated with the column so as to facilitate precipitation of carbon dioxide and water vapour in the solid phase.

19. A method as claimed in any one of claims 1 to 13, in which the gas mixture is passed through a fluidised bed which is cooled by the cryogenic medium.

20. A method as claimed in claim 19, in which cooling of the bed is effected by passing the cryogenic medium through heat exchange pipes or the like in the bed.

21. A method as claimed in claim 19 or 20, in which cooling of the bed is effected by introducing the cryogenic medium directly into the bed.

22. A method as claimed in any one of claims 1 to 3, in which the gas mixture is passed through a bath of liquid nitrogen or other cryogenic medium.

23. A method of heat treating metal in a furnace, substantially as herein described with reference to the accompanying drawings.