Method and apparatus for treating combustible and/or reactive particles, method of operating a system for producing a three-dimensional work piece and system for producing a three-dimensional work piece

Abstract

In a method for treating combustible and/or reactive particles (34) which have been separated from a gas stream (32) by means of a separation device (36) an oxidizing agent is supplied to an atmosphere surrounding the particles (34) so as to cause a passivating oxidation of at least a part of the particles (34). A content of the oxidizing agent in the atmosphere surrounding the particles (34) is detected and the supply of the oxidizing agent to the atmosphere surrounding the particles (34) is controlled in dependence on the detected content of the oxidizing agent in the atmosphere surrounding the particles (34).

Claims

1. A method for treating combustible and/or reactive particles which have been separated from a gas stream by means of a separation device, the method comprising: supplying an oxidizing agent to an atmosphere surrounding the particles separated from the gas stream so as to cause a passivating oxidation of at least a part of the particles; detecting a content of the oxidizing agent in the atmosphere surrounding the particles; and controlling the supply of the oxidizing agent to the atmosphere surrounding the particles in dependence on the detected content of the oxidizing agent in the atmosphere surrounding the particles.

2. The method according to claim 1, wherein the particles separated from the gas stream are supplied to a collecting vessel, wherein the oxidizing agent is supplied to the collecting vessel, wherein a content of the oxidizing agent in the collecting vessel is detected, and wherein a supply of the oxidizing agent to the collecting vessel is controlled in dependence on the detected content of the oxidizing agent in the collecting vessel.

3. The method according to claim 1, wherein the oxidizing agent and a diluting agent are separately supplied to the atmosphere surrounding the particles so as to form an oxidizing mixture in the atmosphere surrounding the particles and/or wherein the oxidizing agent and a diluting agent are premixed in a mixing chamber so as to form an oxidizing mixture in the mixing chamber before the oxidizing mixture is supplied to the atmosphere surrounding the particles, wherein the supply of oxidizing agent and the supply of diluting agent to the atmosphere surrounding the particles and/or to the mixing chamber in particular are controlled in such a manner that the content of the oxidizing agent in the oxidizing mixture increases over time.

4. The method according to claim 1, wherein the supply of the oxidizing agent to the atmosphere surrounding the particles is controlled in such a manner that the content of gaseous oxygen in the atmosphere surrounding the particles, at least during a first period of time following a start of the supply of the oxidizing agent to the atmosphere surrounding the particles is maintained at a level which allows a passivating oxidation of at least a part of the particles, but which is lower than an ambient oxygen content; and/or wherein the supply of the oxidizing agent to the atmosphere surrounding the particles is controlled in such a manner that the content of gaseous oxygen in the atmosphere surrounding the particles, at least during a first period of time following a start of the supply of the oxidizing agent to the atmosphere surrounding the particles is maintained at a level which allows a passivating oxidation of at least a part of the particles, but which is lower than a Limiting Oxygen Concentration for combustion of that particles from a particular material; and/or wherein the supply of the oxidizing agent to the atmosphere surrounding the particles is controlled in such a manner that a content of gaseous oxygen in the atmosphere surrounding the particles, at least during a second period of time preceding a completion of the supply of the oxidizing agent to the atmosphere surrounding the particles, approximates an ambient oxygen content, in particular approximately 21%.

5. The method according to claim 1, further comprising: controlling the supply of the oxidizing agent to the atmosphere surrounding the particles in dependence on a pressure in the atmosphere surrounding the particles, wherein the supply of the oxidizing agent to the atmosphere surrounding the particles in particular is controlled in such a manner that the pressure in the atmosphere surrounding the particles does not exceed a threshold value.

6. The method according to claim 5, wherein the oxidizing agent is supplied to the atmosphere surrounding the particles in cycles; following a supply cycle of the oxidizing agent, a variation of the pressure in the atmosphere surrounding the particles is determined; and a further supply cycle of the oxidizing agent to the atmosphere surrounding the particles is controlled in dependence on the determined pressure variation in the atmosphere surrounding the particles.

7. The method according to claim 1, further comprising: stirring and/or revolving the particles separated from the gas stream.

8. An apparatus for treating combustible and/or reactive particles which have been separated from a gas stream by means of a separation device, the apparatus comprising: an oxidizing agent supply system configured to supply an oxidizing agent to an atmosphere surrounding the particles separated from the gas stream so as to cause a passivating oxidation of at least a part of the particles; an oxidizing agent detector configured to detect a content of the oxidizing agent in the atmosphere surrounding the particles; and a control unit configured to control the supply of the oxidizing agent to the atmosphere surrounding the particles in dependence on the content of the oxidizing agent in the atmosphere surrounding the particles which is detected by means of the oxidizing agent detector.

9. The apparatus according to claim 8, further comprising: a collecting vessel being connectable to the separation device and being configured to accommodate the particles separated from the gas stream by means of the separation device, wherein the oxidizing agent supply system is configured to supply the oxidizing agent to the collecting vessel, the oxidizing agent detector is configured to detect a content of the oxidizing agent in the collecting vessel, and the control unit is configured to control the supply of the oxidizing agent to the collecting vessel in dependence on the content of the oxidizing agent in the collecting vessel which is detected by means of the oxidizing agent detector.

10. The apparatus according to claim 8, wherein the oxidizing agent supply system is configured to separately supply the oxidizing agent and a diluting agent to the atmosphere surrounding the particles so as to form an oxidizing mixture in the atmosphere surrounding the particles and/or wherein the oxidizing agent supply system comprises a mixing chamber comprising an oxidizing agent inlet connected to an oxidizing agent source, a diluting agent inlet connected to a diluting agent source and an oxidizing mixture outlet connected to the atmosphere surrounding the particles.

11. The apparatus according to claim 8, wherein the control unit is configured to control the supply of the oxidizing agent and the supply of diluting agent to the atmosphere surrounding the particles and/or to the mixing chamber in such a manner that the content of the oxidizing agent in the oxidizing mixture increases over time; and/or to control the supply of the oxidizing agent to the atmosphere surrounding the particles in such a manner that the content of gaseous oxygen in the atmosphere surrounding the particles, at least during a first period of time following a start of the supply of the oxidizing agent to the atmosphere surrounding the particles is maintained at a level which allows a passivating oxidation of at least a part of the particles, but which is lower than an ambient oxygen content; and/or to control the supply of the oxidizing agent to the atmosphere surrounding the particles in such a manner that the content of gaseous oxygen in the atmosphere surrounding the particles, at least during a first period of time following a start of the supply of the oxidizing agent to the atmosphere surrounding the particles is maintained at a level which allows a passivating oxidation of at least a part of the particles, but which is lower than a Limiting Oxygen Concentration for combustion of that particles from a particular material; and/or to control the supply of the oxidizing agent to the atmosphere surrounding the particles in such a manner that a content of gaseous oxygen in the atmosphere surrounding the particles, at least during a second period of time preceding a completion of the supply of the oxidizing agent to the atmosphere surrounding the particles, approximates an ambient oxygen content, in particular approximately 21%.

12. The apparatus according to claim 8, further comprising: at least one pressure sensor configured to detect the pressure in the atmosphere surrounding the particles, wherein the control unit is configured to control the supply of the oxidizing agent to the atmosphere surrounding the particles in dependence on the pressure in the atmosphere surrounding the particles which is detected by means of the at least one pressure sensor, wherein the control unit in particular is configured to control the supply of the oxidizing agent to the atmosphere surrounding the particles in such a manner that the pressure in the atmosphere surrounding the particles does not exceed a threshold value.

13. The apparatus according to claim 12, wherein the control unit is configured to control the supply of the oxidizing agent the atmosphere surrounding the particles in such a manner that the oxidizing agent is supplied to the atmosphere surrounding the particles in cycles; following a supply cycle of the oxidizing agent, to determine a variation of the pressure in the atmosphere surrounding the particles based on a signal received from the at least one pressure sensor; and to control a further supply cycle of the oxidizing agent to the atmosphere surrounding the particles in dependence on the determined pressure variation in the atmosphere surrounding the particles.

14. The apparatus according to claim 1, further comprising: a stirring and/or revolving device for stirring and/or revolving the particles separated from the gas stream.

15. A method of operating a system for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation, the method comprising: supplying a gas stream to a process chamber of the system; directing the gas stream through the process chamber, wherein the gas stream, while being directed through the process chamber takes up combustible and/or reactive particles; discharging the gas stream containing the combustible and/or reactive particles from the process chamber; separating the combustible and/or reactive particles from the gas stream by means of a separation device; and treating the combustible and/or reactive particles separated from the gas stream in accordance with the method as defined in claim 1.

16. A system for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation, the system comprising: a gas supply system configured to supply a gas stream to a process chamber of the system and to direct the gas stream through the process chamber, wherein the gas stream, while being directed through the process chamber takes up combustible and/or reactive particles; a gas discharge system configured to discharge the gas stream containing the combustible and/or reactive particles from the process chamber; a separation device configured to separate the combustible and/or reactive particles from the gas stream; and the apparatus for treating combustible and/or reactive particles as defined in claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Preferred embodiments of the invention will be described in greater detail with reference to the appended schematic drawings, wherein

[0047] FIG. 1 shows a system for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation which is equipped with gas circuit for directing the gas stream through a process chamber of the system,

[0048] FIG. 2 shows a detailed view of an apparatus for treating combustible and/or reactive particles contained in a gas stream which is discharged from the process chamber of the system according to FIG. 1,

[0049] FIG. 3 shows a diagram indicating the development of a content of gaseous oxygen in a collecting vessel of the apparatus according to FIG. 2 during a process of treating the combustible and/or reactive particles in the apparatus of FIG. 2,

[0050] FIG. 4 shows a diagram indicating the development of a pressure in the collecting vessel of the apparatus according to FIG. 2 during a process of treating the combustible and/or reactive particles in the apparatus of FIG. 2, and

[0051] FIG. 5 shows a diagram indicating the development of a temperature in the collecting vessel of the apparatus according to FIG. 2 during a process of treating the combustible and/or reactive particles in the apparatus of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] FIG. 1 shows a system 100 for producing a three-dimensional work piece by an additive layering process. The system 100 comprises a process chamber 12 accommodating carrier 14 and a powder application device 16 for applying a raw material powder onto the carrier 14. The process chamber 12 is sealable against the ambient atmosphere, i.e. the environment surrounding the process chamber 12. The system 100 further comprises an irradiation device 18 for selectively irradiating electromagnetic or particle radiation onto the raw material powder applied onto the carrier 14.

[0053] A gas circuit 20 is provided so as to establish a controlled gas atmosphere within the process chamber 12. The gas circuit 20 contains a gas source 22. The gas source 22 in particular is designed in the form of an inert gas source, for example in the form of an argon or nitrogen source. The process chamber 12 comprises a gas inlet 24 for supplying gas to the process chamber 12. A gas outlet 26 serves to discharge gas from the process chamber 12, A circulation line 28 connects the gas outlet 26 to the gas inlet 24. Further, the circulation line 28 is connected to the gas source 22. A conveying device 30 serves to convey the gas stream through the circulation line 28.

[0054] Upon being directed through the process chamber 12, a gas stream 32 supplied to the process chamber 12 via the gas inlet 24 takes up combustible and/or reactive particles 34 such as raw material powder particles, welding smoke and/or soot particles. Thus, the gas source 22, the gas inlet 24, the circulation line 28 and the conveying device 30 define a gas supply system which is configured to supply the gas stream 32 to the process chamber 12, whereas the gas outlets 26, the circulation line 28 and the conveying device 30 define a gas discharge system which is configured to discharge the gas stream 32 containing the combustible and/or reactive particles 34 from the process chamber 12, The gas/particle mixture exiting the process chamber 12 via the gas outlet 26 is treated in a separation device 36 which serves to separate the particles 34 contained in the gas stream 32 from the gas stream 32 before being recirculated to the process chamber 12 via the circulation line 28 and the gas inlet 24.

[0055] The separation device 36 is disposed in the circulation line 28 upstream of the conveying device 30. The purified gas stream 32 exiting the separation device 36 is recirculated to the process chamber 12 via the circulation line 28 and the gas inlet 24. In the embodiment shown in the drawings, the separation device 36 comprises a filter 38 for filtering the particles 34 from the gas stream 32. It is, however, also conceivable that the separation device 36 comprises a plurality of filters and/or one or more cyclone(s).

[0056] The separation device 36 is connected to a collecting vessel 40 of an apparatus 10 for treating the combustible and/or reactive particles 34 which have been separated from the gas stream 32 by means of the separation device 36 via a connecting line 42. The collecting vessel 40 serves to accommodate the particulate material 34 that is trapped in the filter 38 of the separation device 36 when the gas stream 32 is directed through the filter 38 and that is removed from the filter 38 upon continuously or periodically cleaning the filter 38, for example by a “back-flush” method. The supply of particles 34 from the filter 38 to the collecting vessel 40 is controlled by means of a valve 44 which is disposed in the connecting line 42, Operation of the valve 44 is controlled by means of an electronic control unit 46, It is, however, also conceivable that the valve 44 is designed in the form of a manually controllable valve. A filling level of the collecting vessel 40 is controlled by means of a filling level sensor 48 and a scale 50. The scale 50 is designed in the form of three-point scale. Signals that are output by the filling level sensor 48 and the scale 50 are transmitted to the electronic control unit 46. A mechanical stirring and/or revolving device 51 is detachably accommodated in the collecting vessel 40.

[0057] Further, the apparatus 10 comprises an oxidizing agent supply system 52 which is configured to supply an oxidizing agent to an atmosphere surrounding the particles 34, i.e. to an atmosphere within the collecting vessel 40. In the embodiment of the apparatus 10 shown in the drawings, the oxidizing agent which is supplied to the atmosphere surrounding the particles 34 by means of the oxidizing agent supply system 50 is gaseous oxygen. Specifically, gaseous oxygen which acts as the oxidizing agent is supplied to the atmosphere surrounding the particles 34 by means of the oxidizing agent supply system 50 in a diluted form as an ingredient of an oxidizing mixture which also includes a diluting agent.

[0058] The oxidizing agent supply system 52 comprises a mixing chamber 54 which serves to mix the oxidizing agent with the diluting agent in order to obtain the oxidizing mixture. The mixing chamber 54 comprises an oxidizing agent inlet 56 which is connected to an oxidizing agent source 58. In the embodiment of the apparatus 10 shown in the drawings, the oxidizing agent source 58 is designed in the form of an air source which provides air having an oxygen content of approximately 21%, An oxidizing agent supply valve 60 is provided for controlling the supply of oxidizing agent to the mixing chamber 54 via the oxidizing agent inlet 56.

[0059] Further, the mixing chamber 56 comprises a diluting agent inlet 62 which is connected to a diluting agent source 64. The diluting agent source 64 is designed in the form an inert gas source, in particular an Argon source containing 99% pure Argon, A diluting agent supply valve 66 is provided for controlling the supply of diluting agent to the mixing chamber 54 via the diluting agent inlet 62, During operation of the apparatus 10, the oxidizing agent and the diluting agent are supplied to the mixing chamber 54 of the oxygen agent supply system 52 with an increased pressure, i.e. with a pressure which exceeds the ambient pressure. As a result, the oxidizing agent, i.e. the air containing the oxidizing agent gaseous oxygen, and the diluting agent Argon are mixed in a turbulent flow which develops inside the mixing chamber 54.

[0060] In order to assist the mixing of the oxidizing agent with the diluting agent in the mixing chamber 54 and in order to prevent that the pressure within the mixing chamber 54 exceeds a desired maximum value, the mixing chamber 54 is provided with a pressure control valve 68 which allows the discharge of excess oxidizing mixture from the mixing chamber 54. The pressure control valve 68 may be designed in the form of an automatic pressure control valve which automatically opens in case the pressure in the mixing chamber 56 exceeds a threshold value. It is, however, also conceivable that the pressure control valve 68 is controlled by means of the control unit 46 based on a measured pressure value which detected by means of a pressure sensor (not shown) within the mixing chamber 54.

[0061] The oxidizing mixture, i.e. the mixture of air and Argon is discharged from the mixing chamber 54 via an oxidizing mixture outlet 70. The oxidizing mixture outlet 70 is connected to the atmosphere surrounding the particles 34, i.e. the collecting vessel 40 via an oxidizing mixture supply line 72. An oxidizing mixture discharge valve 74 is provided in the oxidizing mixture supply line 72 for controlling the discharge of the oxidizing mixture from the mixing chamber 54 to the atmosphere surrounding the particles 34. The supply of the oxidizing agent to the mixing chamber 54, the supply of the diluting agent to the mixing chamber 54 and the discharge of the oxidizing mixture from the mixing chamber 54 happen under the control of the control unit 46.

[0062] In particular, the control unit 46 controls the valves 60, 66, 74 in a suitable way in order to control the supply of the oxidizing agent to the mixing chamber 54, the supply of the diluting agent to the mixing chamber 54 and the discharge of the oxidizing mixture from the mixing chamber 54 as desired.

[0063] The apparatus 10 further comprises an oxidizing agent detector 76 which is arranged within the collecting vessel 40 and which is configured to detect a content of the oxidizing agent in the atmosphere surrounding the particles 34, i.e. in collecting vessel 40. In the embodiment of the apparatus 10 shown in the drawings, the oxidizing agent detector 76 is designed in the form of an oxygen detector which is configured to detect the amount of gaseous oxygen present in the collecting vessel 40.

[0064] Further, two pressure sensors 78, 80 are arranged within the collecting vessel 40 in order to detect the pressure prevailing in the atmosphere surrounding the particles 34, i.e. in the collecting vessel 40. A first pressure sensor 78 is arranged in the region of an oxidizing agent inlet 82 of the collecting vessel 40 via which the collecting vessel 40 is connected to the oxidizing agent supply system 52, i.e. the oxidizing mixture supply line 72. A second pressure sensor 80 is arranged in the region of a pressure control valve 84 via which excess gas can be discharged from the interior of the collecting vessel 40. The pressure control valve 84 may be designed in the form of an automatic valve which automatically opens in case the pressure in the collecting vessel 40 exceeds a threshold value. It is, however, also conceivable to operate the pressure control valve 84 under the control of the control unit 46 in dependence on the pressure values detected by at least one of the pressure sensors 78, 80.

[0065] Finally, three temperature sensors 86, 88, 90 are arranged within the collecting vessel 40 in order to detect the pressure prevailing in the atmosphere surrounding the particles 34, i.e. in the collecting vessel 40. A first temperature sensor 86 is arranged in a bottom region of the collecting vessel 40, a second temperature sensor 88 is arranged in a middle region of the collecting vessel 40 and a third temperature sensor 90 is arranged in a top region of the collecting vessel 40, Like the signals of the filling level sensor 48, the signals of the oxygen agent detector 76 and the signals of the pressure sensors 78, 80 also the signals of the temperature sensors 68, 88, 90 are transmitted to the control unit 46.

[0066] The particles 34 that are separated from the gas stream 32 by means of the separation device 36 are supplied to the collecting vessel 40. The supply of particles 34 to the collecting vessel 40 happens under the control of the control unit 46 and is started by opening the valve 44. The particles 34 may be supplied to the collecting vessel 40 either continuously or periodically, while the filling level of the collecting vessel 40 is continuously monitored by means of the filling level sensor 48 and the scale 50. When the particles 34 are accommodated in the collecting vessel 40, the particles 34 are stirred and/or revolved by means of the stirring and/or revolving device 51.

[0067] The supply of particles 34 to the collecting vessel 40 may be interrupted as required at any time, e.g., upon completion of the production of the three dimensional workpiece. However, at least when the filling level of the collecting vessel 40 with particles 34 has reached a first predetermined value that is determined by means of the filling level sensor 48 and the scale 50, the supply of particles 34 to the collecting vessel 40 is interrupted. In particular, the interruption of the supply of particles 34 to the collecting vessel 40 is achieved by closing the valve 44 under the control of the control unit 46 in response to the signals transmitted to the control unit 46 from the filling level sensor 48 and the scale 50.

[0068] Thereafter, oxidizing agent is supplied to the atmosphere surrounding the particles 34, i.e. to the collecting vessel 40 so as to cause a passivating oxidation of at least a part of the particles 34 contained in the collecting vessel 40. Specifically, the combustible and/or reactive particles 34 received within the collecting vessel 40, upon coming into contact with the oxidizing agent contained in the oxidizing mixture which is supplied to the atmosphere surrounding the particles 34 from the mixing chamber 54 of the oxidizing agent supply system 52, react with the oxidizing agent so as to form a passivating surface layer around the particles. The progress of the oxidation of the particles 34 is monitored by detecting the content of the oxidizing agent atmosphere surrounding the particles 34 by means of the oxidizing agent detector 76, Specifically, the oxidizing agent detector 76 is used to monitor the consumption of oxidizing agent in the oxidation reaction in order to form a passivating surface layer on the combustible and/or reactive particles 34.

[0069] The supply of the oxidation agent to the atmosphere surrounding the particles 34, by means of the control unit 46, is controlled in dependence on the detected content of the oxidizing agent in the atmosphere surrounding the particles 34 and hence in dependence on the status and the progress of the oxidation reaction occurring in the collecting vessel 40. As a result, a controlled oxidation and hence passivation of the combustible and/or reactive particles 34 is achieved. The supply of oxidizing agent to the atmosphere surrounding the particles 34 is controlled by suitably controlling the valve 74 which is arranged in the oxidizing mixture supply line 72 connecting the mixing chamber 54 of the oxidizing agent supply system 52 to the collecting vessel 40. In addition, the supply of oxidizing agent to the atmosphere surrounding the particles 34 is controlled by suitably controlling the content of the oxidizing agent in oxidizing mixture prepared in the mixing chamber 54 of the oxidizing agent supply system 52 as will be described in more detail below.

[0070] As becomes apparent from FIG. 3, the supply of the oxidizing agent to the atmosphere surrounding the particles 34 is controlled in such a manner that the content of gaseous oxygen in the atmosphere surrounding the particles 34, during a first period of time t1 following a start of the supply of the oxidizing agent to the atmosphere surrounding the particles 34, is maintained at a level which allows a passivating oxidation of at least a part of the particles, but which is lower than an ambient oxygen content or even lower than the Limiting Oxygen Concentration (LOC) for combustion of the particles of a particular material. This may be achieved by supplying a certain amount of oxidizing agent to the atmosphere surrounding the particles 34 and then interrupting the supply of the oxidizing agent to the atmosphere surrounding the particles 34 until the oxidizing agent in the atmosphere surrounding the particles 34 is consumed to a certain extent by the passivating oxidation of the combustible and/or reactive particles 34.

[0071] When the content of gaseous oxygen in the atmosphere surrounding the particles 34 has reached a desired level, the supply of the oxidizing agent to the atmosphere surrounding the particles 34 may be repeatedly started and interrupted as needed, Thus, during the first period of time t1 following the start of the supply of the oxidizing agent to the atmosphere surrounding the particles 34, several cycles of supplying oxygen agent to the atmosphere surrounding the particles 34 and interrupting the supply of oxygen agent of the atmosphere surrounding the particles 34 may be carried out.

[0072] To the contrary, during a second period of time t2 following the first period of time t1 and preceding a completion of the supply of the oxidizing agent to the atmosphere surrounding the particles 34, the supply of the oxidizing agent to the atmosphere surrounding the particles 34 is controlled in such a manner that the content of gaseous oxygen in the atmosphere surrounding the particles 34 increases and approximates the content of approximately 21%, i.e. the oxygen content of air. This reliably prevents an undesired reaction of the combustible and/or reactive particles 34 contained within the collecting vessel 40 when the particles 34, for example upon opening the collecting vessel 40, are exposed to air. During a later phase of the oxidation process, the reaction rate decreases due to the formation of passivating oxide layers on the surfaces of the combustible and/or reactive particles 34. Thus, an increase of the content of gaseous oxygen in the atmosphere surrounding the particles as shown in FIG. 3 ensures that there is always a sufficient amount of oxidizing agent present in the atmosphere surrounding the particles 34 in order to allow the formation of sufficiently thick passivating oxide layers on the surfaces of the particles 34 within a reasonable time.

[0073] Basically, the content of gaseous oxygen in the atmosphere surrounding the particles 34 may be increased by increasing the volume flow of oxidizing mixture to the atmosphere surrounding the particles 34, i.e. the collecting vessel 40. Preferably, however, the content of the oxidizing agent, i.e. gaseous oxygen in the oxidizing mixture is increased while the volume flow of oxidizing mixture to the atmosphere surrounding the particles 34 is maintained constant or only slightly increased. This is achieved by appropriately controlling the supply of oxidizing agent and the supply of diluting agent to the mixing chamber 54 of the oxidizing agent supply system 52.

[0074] It is, however, also conceivable that the supply of the oxidizing agent to the atmosphere surrounding the particles 34 is controlled in such a manner that a content of gaseous oxygen in the atmosphere surrounding the particles 34, during the second period of time t2 approximates an ambient oxygen content which differs from the oxygen content of air. This is advantageous in case the combustible and/or reactive particles 34 contained within the collecting vessel 40 should be exposed to an ambient atmosphere having an ambient oxygen content which differs from the oxygen content of air.

[0075] FIG. 4 shows the development of a pressure in the collecting vessel 40 in dependence on the time. The lower curve in the diagram of FIG. 4 shows the development of the pressure in a region of the oxidizing aging inlet 56 as measured by the first pressure sensor 78. The upper curve in the diagram of FIG. 4 shows the development of the pressure in a region of the pressure control valve 84 as measured by the second pressure sensor 80.

[0076] In general, the supply of the oxidizing agent to the atmosphere surrounding the particles is controlled in dependence on the pressure in the atmosphere surrounding the particles 34 in such a manner that pressure variations and pressure peaks are avoided as far as possible. Specifically, the supply of the oxidizing agent to the atmosphere surrounding the particles 34 is controlled in such a manner that the pressure in the atmosphere surrounding the particles 34 does not exceed a threshold value. The pressure in the collecting vessel 40 is controlled with the aid of the pressure control valve 84 and by suitably controlling the supply of oxidizing agent to the collecting vessel 40 in dependence on the pressure values detected by means of the pressure sensors 78, 80.

[0077] Basically, the oxidizing agent may be continuously supplied to the atmosphere surrounding the particles 34. Preferably, however, the oxidizing agent is supplied to the atmosphere surrounding the particles 34 in cycles. This is achieved by suitably controlling the valve 74. During a supply cycle of the oxidizing agent to the atmosphere surrounding the particles 34 a pressure equalisation in the collecting vessel 40 is achieved by suitably controlling the pressure control valve 84. Following the supply cycle of the oxidizing agent, i.e. after interrupting the supply of the oxidizing agent, a variation of the pressure hi the atmosphere surrounding the particles 34 is determined by the control unit 46 based on the signals provided by the pressure sensors 78, 80. A further supply cycle of the oxidizing agent to the atmosphere surrounding the particles 34 then is controlled by the control unit 46 in dependence on the determined pressure variation in the atmosphere surrounding the particles 34.

[0078] The development of a temperature in the collecting vessel 40 in dependence on the time is shown in FIG. 5. The lower curve in the diagram of FIG. 5 shows the development of the temperature in a bottom region of the collecting vessel 40 as measured by the first temperature sensor 86. The middle curve in the diagram of FIG. 5 shows the development of the temperature in a middle region of the collecting vessel 40 as measured by the second temperature sensor 88. The upper curve in the diagram of FIG. 5 shows the development of the temperature in a top region of the collecting vessel 40 as measured by the third temperature sensor 90.

[0079] The supply of the oxidizing agent to the atmosphere surrounding the particles 34, i.e. in the collecting vessel 40 is controlled in such a manner that temperature variations are avoided as far as possible. Specifically, the supply of the oxidizing agent to the atmosphere surrounding the particles 34 is controlled in such a manner that the temperature in the atmosphere surrounding the particles 34 does not exceed a threshold value. For example, the temperature in the atmosphere surrounding the particles 34 may be controlled in such a manner that it does not exceed 45° C. The temperature in the collecting vessel 40 is controlled by suitably controlling the supply of oxidizing agent to the collecting vessel 40 in dependence on the temperature values detected by means of the temperature sensors 86, 88, 90.

[0080] Upon completion of the passivating oxidation of the combustible and/or reactive particles 34 contained in the collecting vessel 40, i.e. for example when the content of gaseous oxygen in the atmosphere surrounding the particles 34, i.e. in the collecting vessel 40 is approximately 21%, the supply of the oxidizing agent to the atmosphere surrounding the particles 34 is finally interrupted. The full collecting vessel 40 then is replaced by a replacement collecting vessel 40a. Specifically, the replacement collecting vessel 40a is connected to the connecting line 42 in place of the collecting vessel 40. After being disconnected from the connecting line 42, the full collecting vessel 40 may be closed by means of a cover (not shown).

[0081] During the interruption of the supply of particles 34 to the collecting vessel 40, the flow of the particle loaded gas stream through the separation device 36 and the operation of the separation device 36 are continued, Thus, even during the Interruption of the supply of particles 34 to the collecting vessel 40, the separation device 36 continues separating particles 34 from the gas stream 32, These particles 34 are temporarily stored in an interim storage volume 92 which may be provided in the interior of the separation device 36 as shown in the drawing or which may be provided in a separate container. In particular, the particles 34 that are separated from the gas stream 32 during the interruption of the supply of particles 34 to the collecting vessel 40 maintain in the filter 38 of the separation device 36. Hence, continuous operation of the system 100 is made possible.