MIXING SILO FOR BULK MATERIAL, PRODUCTION PLANT WITH A MIXING SILO OF THIS TYPE AND METHOD FOR OPERATING A MIXING SILO OF THIS TYPE

Abstract

A mixing silo for bulk material comprises a silo container, a mixing installation mounted in the silo container for mixing the bulk material, at least one shut-off element for shutting off the mixing installation, wherein the mixing silo has a minimum extraction rate and the silo container, with the mixing installation shut off, has a residual cross-sectional area which ensures a mass flow of the bulk material that is greater than or equal to the minimum extraction rate of the mixing silo.

Claims

1. A mixing silo for bulk material comprising a silo container, a mixing installation mounted in the silo container for mixing the bulk material, at least one shut-off element for shutting off the mixing installation, wherein the at least one shut-off element is movable between a closed position, in which a bulk material flow through the mixing installation is prevented and the mixing silo has a flow-through function, and an open position, in which a bulk material flow through the mixing installation is possible and the mixing silo has a mixing function, wherein the at least one shut-off element is arranged at least one of at and in the mixing installation, wherein the at least one shut-off element is arranged at the outlet of the mixing installation, wherein the mixing silo has a minimum extraction rate of at least 20 t/h for plastic bulk material comprising at least one of powder with an average particle size of between 50 μm and 2000 μm and granulate with an average particle size of 1500 μm to 6000 82 m, wherein the silo container, with the mixing installation shut off, has a residual cross-sectional area that is limiting to the silo container in such a manner that a mass flow of the bulk material ensures the flow-through function and is greater than or equal to the minimum extraction rate of the mixing silo, wherein the limiting residual cross-sectional area represents a minimum cross-sectional area of the silo container along the flow direction of the bulk material.

2. The mixing silo according to claim 1, wherein the silo container has a base container and a bottom section.

3. (canceled)

4. The mixing silo according to claim 1, wherein the mixing installation comprises at least one of at least one mixing tube and at least one mixing cone.

5. The mixing silo according to claim 4, wherein at least one of the at least one mixing tube and the at least one mixing cone open into a collecting pot.

6. The mixing silo according to claim 1, further comprising a shut-off drive connected to the at least one shut-off element for driven actuation of the at least one shut-off element.

7. The mixing silo according to claim 6, further comprising a control unit in signal connection with the shut-off drive for automated actuation of the at least one shut-off element.

8. The mixing silo according to claim 1, wherein the at least one shut-off element is designed as a flap disc.

9. A production plant having a production reactor for producing bulk material, a mixing silo according to claim 1, a feed unit for feeding bulk material.

10. The production plant according to claim 9, further comprising a recirculation unit connecting the discharge unit to the feed unit for recirculation of the bulk material.

11. A method for operating a mixing silo according to claim 1, comprising the steps of feeding bulk material into the mixing silo, mixing of the bulk material in the mixing silo by means of the mixing installation, shutting off the mixing installation by means of the at least one shut-off element, wherein the at least one shut-off element is movable between a closed position, in which a bulk material flow through the mixing installation is prevented and the mixing silo has a flow-through function, and an open position, in which a bulk material flow through the mixing installation is possible and the mixing silo has a mixing function, wherein the at least one shut-off element is arranged at least one of at and in the mixing installation, wherein the at least one shut-off element is arranged at the outlet of the mixing installation, discharging the bulk material from the mixing silo with the mixing installation shut off, over the limiting residual cross-sectional area and the outlet cross-sectional area at the outlet of the silo container in such a manner that a mass flow of the bulk material ensures the flow-through function and one of is greater than and equal to the minimum extraction rate of at least 20 t/h for plastic bulk material comprising at least one of powder having an average particle size between 50 μm and 2000 μm and/or granulate having an average particle size of 1500 μm to 6000 μm of the mixing silo.

12. The method according to claim 11, wherein the shut-off takes place when a change of at least one of a bulk material type and a bulk material quality class is pending.

13. The method according to claim 11, wherein a plurality of shut-off elements are used for shutting off the mixing installation.

14. The method according to claim 11, wherein the at least one shut-off element is opened again after a variably adjustable changeover time (t) has elapsed.

15. The method according to claim 11, wherein the maximum dwell time of the bulk material in the mixing silo with the mixing installation shut off is 1.0 times to 1.4 times a maximum dwell time of an otherwise identical silo container without mixing installation.

16. The mixing silo according to claim 2, wherein the base container is designed so as to be cylindrical.

17. The mixing silo according to claim 2, wherein the bottom section is conical.

18. The mixing silo according to claim 4, wherein the at least one shut-off element is arranged at least one of at and in a collecting pot.

19. The production plant according to claim 9, wherein said production plant has the feed unit for feeding bulk material from the production reactor into the mixing silo.

20. The method according to claim 11, wherein the shut-off takes place when a change of at least one of a bulk material type and a bulk material quality class is pending at the beginning of the change.

21. The method according to claim 11, wherein all shut-off elements are used for shutting off the mixing installation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0091] FIG. 1 shows a schematic illustration of a production plant for bulk material with a production reactor and a mixing silo according to the invention,

[0092] FIG. 2 shows a schematic longitudinal sectional illustration through the mixing silo according to FIG. 1, which is designed as a cone mixer,

[0093] FIG. 3 shows an illustration corresponding to FIG. 2 of a mixing silo according to a further embodiment with a flap disc as a shut-off element, which is arranged in the outlet region of the mixing installation,

[0094] FIG. 4 shows an enlarged cross-sectional illustration of the mixing silo according to section line IV-IV in FIG. 3,

[0095] FIGS. 5 to 7 show different configurations of a side edge of the flap disc in FIG. 4,

[0096] FIG. 8 shows an illustration corresponding to FIG. 3 of a mixing silo according to a further embodiment in which the shut-off element is arranged at the lower end of the outlet pot,

[0097] FIG. 9 shows an illustration corresponding to FIG. 3 of a mixing silo in the form of a tube mixer with a shut-off element in the outlet region of a collecting pot,

[0098] FIG. 10 shows an illustration corresponding to FIG. 9 of a tube mixer according to a further embodiment, in which shut-off elements are arranged on the mixing tubes upstream of the collecting pot,

[0099] FIG. 11 shows an illustration corresponding to FIG. 9 of a tube mixer having a central mixing tube and a plurality of shut-off elements,

[0100] FIG. 12 shows an illustration corresponding to FIG. 11 of a tube mixer having a central mixing tube, wherein the shut-off elements are arranged at the inlet of the mixing tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0101] A production plant shown in FIG. 1, designated as a whole as 1, is used for the production of bulk material, in particular plastic granulate, in particular polyolefin granulate. The production plant 1 comprises a production reactor 2 in which bulk material is produced. The production reactor 2 is in particular a polymerization reactor and/or an extruder. The production reactor 2 is connected to a mixing silo 4 by means of a feed unit 3. The feed unit 3 serves to feed the bulk material into the mixing silo 4. The feeding can in particular be carried out purely gravimetrically. Pneumatic conveying can be used additionally or alternatively. In this case, the feed unit 3 is designed as part of a pneumatic conveying system.

[0102] In the mixing silo 4, the bulk material is mixed in a mixing operation and discharged for further use. The bulk material is discharged from the mixing silo 4 by means of a discharge unit. The discharge can be carried out in particular purely gravimetrically, for example by discharging the bulk material into a transport container 6. In this case, the discharge unit 5 is formed as an outlet opening of the mixing silo 4. In addition or alternatively, discharge can take place by means of pneumatic conveyance into a storage container 7, in particular a silo. In this case, the discharge unit 5 is formed as part of a pneumatic conveying system 8 from the mixing silo 4 into the storage container 7. A recirculation unit 9 in the form of a recirculation line is arranged in the region of the discharge unit 5. The recirculation unit 9 makes it possible to recirculate bulk material that has been discharged from the mixing silo 4 back into the mixing silo 4 in the region of the feed unit 3. For this purpose, the recirculation line, as shown in FIG. 1, can open separately from the feed unit 3 into an upper opening of the mixing silo 4. It is also possible that the recirculation line is connected to a feed line of the feed unit 3.

[0103] In the following, the mixing silo 4 in FIG. 1 is explained in more detail with reference to FIG. 2. The mixing silo 4 has a silo container 11 with a longitudinal axis 10. The silo container 11 comprises a cylindrical base container 12 and a conical bottom section 13 which is connected to the base container 12 at the lower end thereof. The silo container 11 has an upper inlet opening 14, in particular arranged centrically with respect to the longitudinal axis 10, and a lower outlet opening 15. The cross-sectional area of the outlet opening 15 corresponds to the cross-sectional area of the lower end of the conical bottom section 13. The outlet opening 15 is arranged at the lower end of the bottom section 13. The outlet opening 15 is arranged in particular concentrically to the longitudinal axis 10.

[0104] A plurality of mixing installations are arranged in the silo container 11, in particular permanently installed in the silo container 11. A first mixing installation is a central mixing tube 16 arranged concentrically to the longitudinal axis 10. The lower end of the mixing tube 16 forms the mixing tube outlet region 17, at which a mixing tube shut-off element 18 is arranged.

[0105] A further mixing installation is formed by a mixing cone 19, which in particular has a plurality of flow zones with different flow speeds. The mixing cone 19 tapers along the longitudinal axis 10 towards the discharge opening 15. The mixing cone 19 can have a plurality of sectors in the circumferential direction with respect to the longitudinal axis 10, which sectors are separated from each other by separating plates. The separating plates are oriented in particular vertically and radially with respect to the longitudinal axis 10. The mixing silo 4 according to FIG. 2 is also called a cone mixer.

[0106] The mixing cone 19 has a cone outlet region 20 at its lower end, on which a mixing cone shut-off element 21 is arranged.

[0107] Along the longitudinal axis 10, the mixing installations 16, 19, i.e. the mixing tube 16 and the mixing cone 19, are arranged overlapping at least in some regions. This means that the mixing cone 19 is arranged around the centrally arranged mixing tube 16.

[0108] The shut-off elements 18, 21 can be moved between a closed position shown in FIG. 2 and an open position. In the closed position shown, the mixing installations 16, 19 are closed. A bulk material flow through the mixing installations 16, 19 is prevented.

[0109] In the open position, a bulk material flow through the mixing internals 16, 19 is possible.

[0110] In particular, the shut-off elements 18, 21 can be actuated independently of each other.

[0111] In axial direction with respect to the longitudinal axis 10, the mixing tube 16 protrudes with the outlet region 17 downwards at the mixing cone 19. The outlet region 17 of the mixing tube 16 is arranged closer to the outlet opening 15 than the outlet region 20 of the mixing cone 19.

[0112] In the following, a method for operating the mixing silo 4 during a product change in the production plant 1 is explained in more detail with reference to FIGS. 1 and 2.

[0113] In the event of a product change, in particular a change of the bulk material type and/or the bulk material quality class, the production reactor 2 is converted to the new bulk material type and/or the new bulk material quality class. This changeover typically takes at least one hour, in particular several hours. In the production of plastic granulate, in particular polyolefin granulate, the mixing silo is operated continuously. In standard operation, the mixing silo 4 is in a mixing operation in which bulk material in the mixing silo 4 can enter the mixing installations 16, 19 and a wide dwell time distribution is achieved due to the different flow speeds. The shut-off elements 18, 21 are moved into the closed position and thus the mixing installations 16, 19 are closed. In the region of the closed mixing installations 16, 19, an accumulation region 22 is formed in which the bulk material stands, i.e. does not flow. Outside the accumulation region 22, a flow region 23 is formed in which the bulk material flows gravimetrically through the mixing silo 4 in mass flow, i.e. according to the “first-in-first-out” principle. The flow direction 24 of the flowing bulk material is symbolically marked in FIG. 2. The bulk material flows downwards in an outer edge region 25 around the centrally arranged mixing installations 16, 19. In the radial direction with respect to the longitudinal axis 10, the edge region 25 is bounded on its outside by the bottom section 13 and on its inside by the mixing cone 19.

[0114] The mixing silo 4 has a minimum residual cross-sectional area 26 which, according to the embodiment example shown, is designed to be annular. The residual cross-sectional area 26 is oriented in a plane perpendicular to the longitudinal axis 10. The residual cross-sectional area 26 represents the edge region 25 at an axial position of the shut-off element 18, which is arranged closest to the discharge opening 15.

[0115] The residual cross-sectional area 26 is large enough to ensure a mass flow of the bulk material that is greater than or equal to the minimum extraction rate of the mixing silo 4. This ensures that the mass flow in flow-through operation through the mixing silo 4 does not cause any limitation of the process performance of the production plant 1.

[0116] A subsequent opening of the shut-off elements 18, 21 takes place after a calculated transition period of the mixing silo 4 has elapsed.

[0117] The transition period in the mixing silo 4 is also referred to as the dwell time. The dwell time is the time required until the product change is completed in the mixing silo 4 itself, i.e. there is no longer any product in the mixing silo 4 that was in the mixing silo 4 before the change, but only product that is to be available after the change.

[0118] In particular, the shut-off elements 18, 21 are closed before the product change begins. The bulk material flows exclusively along the flow region 23, i.e. where there are no mixing installations 16, 19. The bulk material flows uniformly in a mass flow in the sense of a plug flow. The product to be added, which enters the mixing silo 4 via the feed opening 14, sinks downwards at a uniform speed in the mixing silo 4 over the cross-sectional area, i.e. without creating a dwell time distribution. Mixing of new product with old product is prevented. The opening of the shut-off elements takes place after the dwell time of the bulk product in the mixing silo 4 has elapsed. After the dwell time has elapsed, it can be assumed that no more product of the product previously in the mixing silo 4 is present. In particular, the time required for a product change can be made very short and, in particular, almost without transition.

[0119] Product that is in the mixing installations 16, 19 when the mixing installations 16, 19 are shut off can be emptied from the mixing silo 4 by opening the shut-off elements 18, 21 with the last transition product.

[0120] In the following, a further embodiment of the invention is described with reference to FIGS. 3 to 7. Constructively identical parts are given the same reference numerals as in the previous embodiment, the description of which is hereby referred to. Constructively different but functionally similar parts are given the same reference numerals with a trailing letter a.

[0121] In the mixing silo 4a, which is also designed as a cone mixer, a cylindrical extension section 27 is formed on the bottom section 13a at its lower end. The extension section 27 forms a mixing silo outlet region. An end cone 34 is flanged to the lower end of the mixing silo outlet region 27.

[0122] In the mixing silo outlet region 27, a cylindrical extension 28 is arranged below and connected to the mixing installations 16, 19. The cylindrical extension 28 is designed to be tubular. The extension 28 is also referred to as a discharge pot or a collecting pot. A cone end piece 35 is attached to the collecting pot 28. The outlet regions 17 and 20 of the mixing installations 16 and 19 open into the cylindrical extension 28, which has an extension outlet 29 at its lower end opposite the mixing installations 16, 19. The extension outlet 29 forms a common outlet region for the mixing installations 16, 19 according to the embodiment example shown.

[0123] A shut-off element 30, in particular one single shut-off element, is arranged in the extension 28. The shut-off element 30 is designed as a flap disc, which is shown in the open position in FIG. 3. The shut-off element 30 is arranged axially with respect to the longitudinal axis 10 at a distance from the outlet region 17 of the mixing tube 16, so that a collision-free rotation of the flap disc is possible. The flap disc 30 is attached to a flap disc shaft 31. The flap disc shaft 31 runs perpendicular to the longitudinal axis 10 and is guided laterally out of the mixing silo 4a. Corresponding bearings 32 are provided for this purpose on the extension 28 and on the mixing silo outlet region 27.

[0124] The end of the flap disc shaft 31 facing away from the flap disc 30 is connected to a shut-off drive 33. The shut-off drive 33 is in particular an electric motor. By means of the shut-off drive 33, the flap disc shaft 31 and thus the flap disc 30 can be rotated. A shift from the open position shown in FIG. 3 to the closed position is carried out by a 90° rotation around the flap disc shaft 31.

[0125] The shut-off drive is in signal connection with a control unit 36. The signal connection can be wired, as indicated in FIG. 3. The signal connection between the shut-off drive 33 and the control unit 36 can also be wireless.

[0126] The design of the flap disc 30 is explained in more detail below with reference to FIG. 4. FIG. 4 shows the flap disc 30 in the closed position.

[0127] The flap disc 30 is adapted to the extension 28. In particular, the outer diameter D.sub.a of the flap disc 30 is adapted to the extension 28. In particular, the flap disc 30 is adapted to the extension 28 in such a manner that an annular gap 37 with a gap width S results between the outer diameter D.sub.a of the flap disc 30 and the inner diameter D.sub.i of the extension 28. It is advantageous if the annular gap 37 has a gap width S which is 0.3 to 20 times, in particular 0.4 to 10 times and in particular 0.5 to 5 times the average grain size of the bulk material.

[0128] The flap disc 30 is essentially designed as a cylindrical disc with an upper side surface 38 which, in the closed position according to FIG. 4, faces the outlet regions 17, 20 of the mixing installations 16, 19. It is advantageous if the upper side surface 38 has a flattening with an angle α in an outer edge region. The flattening may extend along the entire circumference of the flap disc 30 or at least in regions along the circumference of the flap disc 30. A plurality of regions of a flattening separated from each other may be provided along the circumference. The angle α is in particular between 10° and 70°, in particular between 15° and 45° and in particular between 20° and 30°. A corresponding design of the flap disc is shown in FIG. 5.

[0129] Alternatively, it is conceivable that a lower side surface 39 opposite the upper side surface 38 also has a corresponding flattening. The flattenings on the upper side surface 38 and the lower side surface 39 can also be designed with different angles. A flap disc 30 flattened on both sides is shown in FIG. 6.

[0130] FIG. 7 shows a flap disc 30 in which the side surfaces 38, 39 are rounded in the outer edge region. The rounding can be elliptical, as shown in FIG. 7. Alternatively, a circular or differently shaped rounding is also possible.

[0131] The annular residual cross-sectional area 26 is dimensioned in such a manner that the bulk material can flow in mass flow through the mixing silo 4a of the shut-off mixing installations 16, 19. In particular, the residual cross-sectional area 26 is so large that a mass flow of the bulk material is ensured which is greater than or equal to the minimum extraction rate of the mixing silo 4a, in particular at least double, in particular at least 3 times, in particular at least 5 times, in particular at least 10 times and in particular at most 20 times the minimum extraction rate.

[0132] The operation of the mixing silo 4a is explained in more detail below. Initially, the mixing silo 4a operates in a standard mode, i.e. in a mixing mode. When a product change begins, product leaves the extruder that does not (yet) have the product characteristic that is to be set. In the mixing silo 4a, the mixing installations 16, 19 are shut off by means of the flap disc 30 by shifting the flap disc 30 from the open position shown in FIG. 3 to the closed position shown in FIG. 4. The mixing silo 4aoperates in mass flow according to the “first in-first out” principle. The product still being discharged from the mixing silo 4a is so-called “old” product and can be fed to a corresponding storage container. The transition product produced as a result of the product change can be discharged from the mixing silo 4a into a separate storage container. Once the product change has been completed and all transition product has been discharged from the mixing silo 4a, the mixing silo 4a is returned from mass flow mode to mixing mode by shifting the flap disc 30 to the open position. First, a mixed product is discharged from the mixing silo 4a, which is a mixture of “new” product and the “old” product stored in the mixing installations 16, 19. This mixed product can also be discharged into the separate container for the transition product. It is also conceivable to provide an additional storage container for this mixed product.

[0133] When the mixed product has been completely discharged from the mixing silo 4a, there is only “new” product in the mixing silo 4a.

[0134] The “new” product is mixed in the mixing silo 4a and can be discharged into a storage container provided for this purpose.

[0135] Due to the fact that the shut-off drive 33 is connected to the control unit 36, the sequence, i.e. the change between the mixing operation and the flow-through operation, of the mixing silo 4a can be controlled and in particular regulated. In particular, the control unit 36 is in signal connection with the production reactor 2, in particular with an extruder, wherein a control signal is transmitted from the extruder to the control unit whenever the production of the “old” product and/or the transition product is completed.

[0136] A further embodiment of the invention is described below with reference to FIG. 8. Constructively identical parts are given the same reference numerals as in the previous embodiments, the description of which is hereby referred to. Constructively different but functionally similar parts are given the same reference numerals with a trailing letter b.

[0137] The mixing silo 4b corresponds essentially to the previous embodiment in FIG. 3. One difference is that the shut-off element 30b is arranged at the lower end of the extension 28 with the cone end piece 35. The shut-off element 30b is shown purely schematically. The shut-off element 30b can be designed as a flap disc.

[0138] The mixing silo 4b has an outlet diameter D.sub.0 at the outlet opening 15. The annular residual cross-sectional area 26 has an annular gap width B which corresponds to the difference between the inner diameter D.sub.r of the mixing silo outlet region 27 in the plane of the residual cross-sectional area 26 and the outer diameter of the extension 28 with cone end piece 35 in this region. The average annular gap length L is understood to be the average circumference of the annular residual cross-sectional area 26.

[0139] The base container 12 has an internal diameter D.sub.S of 4.2 m. The mixing silo 4b has a net volume of 130 m.sup.3. The minimum extraction rate for the mixing silo 4b is set at 80 t/h of polyolefin pellets. The polyolefin pellets have a bulk material density of 550 kg/m.sup.3 and a particle diameter of 3.5 mm. Accordingly, an empirical discharge coefficient C=0.58 and the empirical particle coefficient k=1.6 result.

[0140] The other geometric data of the mixing silo 4b are:

r=0.545 m, D.sub.0=0.31 m, W=0.0454 m and L=1.566 m.

[0141] According to the Beverloo equation (1), the maximum mass flow through the outlet diameter D.sub.0 of the mixing silo 4b is 184 t/h, which mass flow is greater than the minimum extraction rate, so that there is no limitation for the mixing silo 4b when the mixing installations 16, 19 are open.

[0142] If the mixing installations 16, 19 are shut off by the shut-off element 30b and the bulk material flows exclusively over the residual cross-sectional area, this results in a mass flow over the residual cross-sectional area 26 according to Nedderman's equation of 80.3 t/h.

[0143] The mixing silo 4b with the geometric data mentioned allows a mass flow over the residual cross-sectional area 26 that is greater than the minimum extraction rate.

[0144] In the following, a further embodiment of the invention is described with reference to FIG. 9. Constructively identical parts are given the same reference numerals as in the previous embodiments, the description of which is hereby referred to. Constructively different but functionally similar parts are given the same reference numerals with a trailing letter c.

[0145] The mixing silo 4c is designed as a so-called tube mixer. According to the embodiment example shown, the tube mixer has two mixing tubes 40, each of which represents a mixing installation. The mixing tubes 40 are arranged in particular on the inner wall of the silo container 11 and are in particular fastened thereto. The mixing tubes 40 are arranged diametrically opposite with respect to the longitudinal axis 10. Fewer or more than two mixing tubes 40 may also be provided. The arrangement of the mixing tubes 40 relative to one another, in particular a spacing of the mixing tubes 40 in the circumferential direction about the longitudinal axis 10, can be selected differently.

[0146] The mixing tubes 40 open into the collecting pot 28. The shut-off element 30c is arranged at the lower end of the collecting pot, which can be designed in particular as an adapted flap disc. According to the embodiment example shown, the collecting pot 28 is configured to be cylindrical. It is conceivable to taper the outlet of the collecting pot 28 conically, in particular in order to be able to design the shut-off element 30c with a small construction.

[0147] The mixing tubes 40 each have at least one lateral opening 41 facing the interior space of the silo container 11. Bulk material can pass through the openings 41 from the silo container 11, in particular the base container 12, into a mixing tube 40. According to the embodiment example shown, the openings 41 in the mixing tubes 40 are each arranged at the same height, i.e. at the same axial position with respect to the longitudinal axis 10. It is conceivable that the openings 41 are arranged at different axial positions with respect to the longitudinal axis 10. In particular, it is conceivable that a plurality of openings 41 are provided on a mixing tube 40. A plurality of openings 41 on a mixing tube 40 can be arranged differently at the mixing tube 40 with respect to the axial position of the longitudinal axis 10. It is also conceivable that a plurality of openings 41 are arranged at the mixing tube 40 at the same height with respect to the longitudinal axis 10, but at different circumferential positions of the mixing tube 40.

[0148] The mixing tubes 40 each have a circular cross-section. Other cross-sectional shapes are possible.

[0149] In the following, a further embodiment of the invention is described with reference to FIG. 10. Constructively identical parts are given the same reference numerals as in the previous embodiments, the description of which is hereby referred to. Constructively different but functionally similar parts are given the same reference numerals with a trailing letter d.

[0150] The mixing silo 4d is a tube mixer. The mixing tubes 40 run partly inside and partly outside the silo container 11.

[0151] One difference compared to the previous embodiment is that shut-off elements 42 are each arranged inside the mixing tube 40. The shut-off elements 42 are each arranged upstream of the collecting pot 28. Such an arrangement of the shut-off elements 42 is advantageous in the embodiment shown, since the cone outlet 43 of the bottom section 13 also opens into the collecting pot 28. This ensures that when the mixing tubes 40 are shut off, the mixing operation is switched off and a uniform discharge in the mass flow from the mixing silo 4d is maintained, since the cone outlet 43 is free, i.e. not shut off.

[0152] For the design of the mixing silo 4d, in particular for the size of the outlet diameter D.sub.r, the rearranged Beverloo equation (1) can be used. The data for the mixing silo 4d are according to the example shown:

{dot over (M)}=80 t/h, C=0.58, η=550 kg/m.sup.3, k=1.6, d=3.5 mm.

[0153] Accordingly, there is a minimum size for the outlet diameter of 0.224 m, so that the mass flow in flow-through operation is greater than or equal to the minimum extraction rate.

[0154] In the following, a further embodiment of the invention is described with reference to FIG. 11. Constructively identical parts are given the same reference numerals as in the previous embodiments, the description of which is hereby referred to. Constructively different but functionally similar parts are given the same reference numerals with a trailing letter e.

[0155] The mixing silo 4e is designed as a tube mixer having a central mixing tube 16e.

[0156] The shut-off element 30e is arranged at the lower end of the mixing tube 16e. At the lower end, the mixing tube 16e has a cone-shaped end piece 44. In particular, the shut-off element 30e is arranged at the end of the conical end piece 44. The central mixing tube 16e protrudes into the conically tapered outlet region 34 of the mixing silo 4e. In particular, the shut-off element 30e is arranged at the lower outlet opening 15 of the mixing silo 4e.

[0157] A plurality of openings 41 are arranged at the mixing tube 16e, in particular at different positions in the axial direction and in the circumferential direction with respect to the longitudinal axis 10. It is optionally possible to close at least one of the openings 41 with an additional shut-off element 45 in order to prevent bulk material from entering the mixing tube 16 from the silo container 11. It is also conceivable to provide all openings 41 with shut-off elements 45. In this case, it is conceivable to dispense with the lower shut-off element 30e.

[0158] According to the embodiment example shown, a further shut-off element 46 is provided in the mixing tube 16e, which shut-off element 46 is arranged upstream with respect to the shut-off element 30e. The shut-off element 46 serves in particular to prevent a bulk material flow in the mixing tube 16e through the openings 41 arranged above the shut-off element 46. In particular, the shut-off elements 45, 46 enable the mixing behaviour of the mixing silo 4e to be influenced during the mixing operation.

[0159] In the following, a further embodiment of the invention is described with reference to FIG. 12. Constructively identical parts are given the same reference numerals as in the previous embodiments, the description of which is hereby referred to. Parts that are structurally different but functionally the same are given the same reference numerals with a trailing letter f.

[0160] The mixing silo 4f is designed as a tube mixer with a central mixing tube 16f. The mixing tube 16f has an overall height, i.e. a longitudinal extension along the longitudinal axis 10, which essentially corresponds to the overall height of the mixing cone of the cone mixer according to FIG. 2. In particular, the overall height of the mixing tube 16f is between 80% and 120% of the overall height of the mixing cone, in particular between 90% and 110%.

[0161] At least one opening 41, in particular a plurality of openings 41, is provided on the mixing tube 16f, which form the inlet of the mixing tube 16f. The openings 41 are arranged at an end of the mixing tube 16f opposite the lower outlet opening 15. The openings 41 are arranged in the jacket wall of the mixing tube 16f. Additionally or alternatively, at least one opening can be provided at the face side of the upper end of the mixing tube 16f.

[0162] In particular, the mixing tube 16f is closed at its upper end 47 opposite the lower discharge opening 15. A bonnet 48, which is displaceable relative to the mixing tube 16f, serves as a shut-off element. The bonnet 48 has a cylindrical ring section 49, the inner diameter of which is at least as large as the outer diameter of the mixing tube 16f. In the arrangement shown in FIG. 12, the ring section 49 is in overlap with the openings 41, which means that bulk material from the silo container 11 is prevented from flowing into the mixing tube 16f. In the arrangement shown, the mixing tube 16f is in the shut-off state due to the shut-off element 48.

[0163] The bonnet 48 can be displaced along the longitudinal axis 10 by means of a lifting drive 50. The lifting drive 50 is in particular a linear lifting drive, in particular a pneumatic drive. By actuating the lifting drive, the bonnet 48 is displaced in a direction 52 away from the mixing tube 16f, i.e. in a direction away from the lower discharge opening 15. This releases the openings 41 from the ring section 49 so that a bulk material flow via the openings 41 into the mixing tube 16f is possible.

[0164] The bonnet 48 has an upper conical section 51. This ensures that the bulk material in the silo container 11 can flow along the bonnet 48 without jamming. In particular, the bonnet 48 is made in one piece. The linear actuating element 50 engages in particular with the conical section 51 of the bonnet 48.

[0165] Alternatively, it is also possible to provide openings in the cylinder section 49 that substantially correspond to the openings 41 in the mixing tube 16f. A displacement of the bonnet 48 between the open and the shut-off arrangement is then possible by rotating the bonnet 48 about the longitudinal axis 10. When the openings of the bonnet 48 and the openings 41 of the mixing tube 16f are at least partially aligned, a bulk material flow into the mixing tube 16f is possible. In this case, the use of the lifting drive 50 is not necessary. The lifting drive can be replaced accordingly by a rotary drive, which enables rotation of the bonnet 48 relative to the mixing tube 16f.

[0166] The lifting movement and/or possible rotary movements of the bonnet 48 are shown schematically by movement arrows 52 in FIG. 12.

[0167] Alternatively, it is also possible to close the upper end 47 of the mixing tube 16f by means of a static installation, for example a conical bonnet. Shut-off elements 45 can then be arranged at the openings 41, as explained with reference to the previous embodiment.

[0168] According to the embodiment shown, in addition to the shut-off element 48 at the inlet of the mixing tube 16f, the shut-off element 30f is arranged at the lower end of the mixing tube 16f, i.e. at the outlet. This shut-off element 30f can also be omitted, in particular if the inlet can be shut off by means of at least one shut-off element 45, 48.

[0169] The main advantage of the arrangement of the shut-off elements 45, 48 at the inlet of the mixing tube 16f is that stagnating product in the mixing tube 16f can be avoided. This minimizes the risk and in particular prevents stagnating bulk material from getting stuck in the mixing tube 16f and not being able to be released again, or only incurring great effort.