JPH0131428B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention relates to a method and apparatus for electrostatic classification of powder particles, and more specifically to silicide powder used in fine ceramics and magnetic storage media. Ultra-fine particle materials, such as magnetic powder used in so-called cutting-edge technology fields, which require extremely high degree of homogeneity in terms of particle size, can be classified into single-phase or multi-phase particles with high precision. This is related to the development of new technology.

(Prior art) In the following description, "ultrafine particles" refer to
1μm. Point to the following particles. In addition, "powder particle group" is 1 μm. It refers to a group consisting of particles including ultrafine particles distributed over a certain particle size range, including those with a unit particle size. Furthermore, the term "powder particle small group" refers to a small group comprised of particles including ultrafine particles distributed over a narrower particle size range within such a powder particle group. For example, a certain powder particle group has a diameter of 0.9 to 0.1 μm. If it is composed of ultrafine particles distributed over a particle size range of 0.7 to 0.5 μm. A small group composed of ultrafine particles distributed over a particle size range of is one of the powder particle groups.

Also, “high” or “low” regarding particle size.
is a comparative expression; for example, in the case of the above powder particle group, it is 0.9 to 0.7 μm. The grain size of is "high"
and 0.5 to 0.3 μm. The grain size of is said to be "low".

Ultrafine particle materials used in the above-mentioned cutting-edge technology fields are strongly required to be extremely homogeneous, that is, to have uniform particle sizes. For example, in the case of ultra-thin transparent plastics used for lenses, the electrical insulation properties and resolving power of the resulting lenses are greatly influenced by the homogeneity of the silicide powder used as the material. Furthermore, in the case of storage media used in computers, such as magnetic tapes and magnetic disks, the homogeneity of the magnetic powder used as the material greatly affects the storage density and noise generation of the resulting media. Ru.

In order to advance the field of application of ultrafine particles as materials for advanced technology as described above, it is essential to develop handling technologies such as crushing, classification, and transportation. development is important.

Known techniques for classifying powder particles include those that utilize centrifugal force, jet jets, and fluid elements. However, the limits of classification in these known techniques are limited to micron units at most, and do not extend to ultrafine particles in submicron units. Although some proposals have been made on a theoretical level, they all have many problems in terms of practical processing capacity and energy consumption, and have not been adopted on an industrial scale. It's not working.

As will be detailed later, this invention is based on the phenomenon that when ultrafine particles are charged in a corona discharge field, the ultrafine particles are deflected and moved in the direction of electrostatic discharge due to the action of electrostatic attraction and air-entrained ion flow. However, electrostatic precipitators are known as a technology that utilizes the electrostatic charge of particles. Conventional electrostatic precipitators generally have a vertical structure, in which a discharge electrode and a dust-collecting plate are placed opposite each other with a gap between them, and a dust-containing airflow is generated during discharge to fill the gap. The charged dust particles are deposited on the dust collection electrode plate, and the accumulated dust particles are removed by being brushed off at regular intervals, and the dust-removed airflow is sent out of the system. Indeed, according to this method, even when submicron-sized ultrafine particles are included in the dust-containing airflow, they can be charged and captured together with micron-sized dust. Of course, this is sufficient for the purpose of collecting dust, that is, removing dust from a dust-containing airflow. However, the function of conventional electrostatic precipitators is limited to this, and it cannot be expected to go beyond that and classify captured ultrafine particles or dust on the micron scale according to their particle size.

Furthermore, when classifying ultrafine particles, consideration must be given to the fact that ultrafine particles are very likely to fly up into the air. In other words, even if a group of powder particles is classified into several small groups of powder particles, if the subsequent processing is incorrect, the ultrafine particles will fly up into the air, and even if the small groups of powder particles once classified are However, they are mixed together again. In particular, as with the electrostatic precipitator mentioned above, handling that involves brushing off the accumulated material on the dust collecting electrode plate will cause a violent upheaval of ultrafine particles, so this must be avoided at all costs when the purpose is classification. . In addition, the rise of ultrafine particles into the air can lead to environmental pollution problems.

(Objective of the Invention) The object of the present invention is to provide ultrafine particles on an industrial scale that have a very high degree of homogeneity and are suitable as materials for advanced technology.

Another object of the present invention is to continuously classify powder particles consisting of ultrafine particles in a highly accurate, highly orderly manner while consuming very little power.

Still another object of the present invention is to carry out the classification of granular particles consisting of ultrafine particles without contaminating the working environment and without causing loss due to scattering of materials.

(Basic Structure of the Invention) According to the present invention, a group of powder particles including ultrafine particles over a wide particle size range is mixed into a conveying air flow, and a push-pull type rectification mechanism is used to disperse powder particles in a plane direction perpendicular to the traveling direction. A solid-gas multiphase flow with a uniform velocity distribution is formed over the air, and two or more electrostatic high voltage discharge electrode plates are installed vertically in parallel with one or more electrostatic high voltage discharge electrodes that are vertically placed facing each other in a closed classification chamber. This solid-gas multiphase flow is guided into the gap between the collection elements (classification zone) and allowed to proceed, and under electrical discharge, small groups of powder particles having a high particle size range are sequentially separated from the ongoing solid-gas multiphase flow, and each The collection elements are used to individually suction, capture and collect.

(Embodiment of the invention) Fig. 1 shows an example of an electrostatic classifier of the present invention, which has a vertically elongated structure, the main parts of which are a frame and a wall (these are omitted in the figure). ) is housed in a substantially closed classification chamber defined by and.

This classification device includes a supply section 1 that actively sends out a conveying airflow mixed with powder particles to be classified as a solid-gas mixed phase flow having a uniform velocity distribution in a plane direction perpendicular to the direction of travel. , a classification section 2 is provided above the solid-gas multiphase flow to separate small groups of powder particles according to a predetermined particle size range, and a classification section 2 is provided above the classification section 2 that separates small groups of powder particles according to a predetermined particle size range from the solid-gas mixed phase flow. It consists of a storage section 3 to be taken over.

Of course, this is an arrangement for performing classification while raising the solid-gas mixed phase flow; conversely, classification may be performed while lowering the solid-gas mixed phase flow. In this case, the classification in the supply section 1 The storage section 3 is located above the classification section 2 and the storage section 3 is located below the classification section 2. However, in the following description, an example will be explained in which classification is performed while raising the solid-gas multiphase flow.

At least the classification section 2 of the three parts described above is arranged in a closed classification chamber. Furthermore, the supply section 1 and the storage section 3 form a so-called push-pull type airflow by actively sending out and actively taking in the airflow.

The supply unit 1 has a rectifying discharge mechanism 11, a conveying air flow supply duct 13, and a mixing duct 12 connected to both. On the ceiling of the mixing duct 12, there is a hopper 14 that accommodates a group of powder particles covering a wide particle size range. The bottom is open. The powder particles in the hopper 14 are transferred to the mixing duct 12 in a predetermined amount per unit time by the action of, for example, a known rotary feeder.
The solid-gas mixed phase flow is mixed with the conveying air flow from the supply duct 13 to form a solid-gas multiphase flow.

As the rectifying discharge mechanism 11, for example, Patent No.
A flow velocity equalizing device at a duct discharge port, etc. disclosed in Japanese Patent No. 1027787 or Patent No. 1175637 is used. The solid-gas multiphase flow formed in the mixing duct 12 as described above has a uniform velocity distribution in a plane direction perpendicular to the direction of movement, and then is discharged upward.

The classification section 2 includes a discharge electrode plate 21 extending vertically,
Collection plates 22a to 22d that constitute the main body of a plurality of collection elements (four in the illustrated example) are fixedly arranged on the frame vertically adjacent to each other with a gap (classification area) S in between. have. Here, the classification zone S becomes a flow path for a solid-gas multiphase flow. Discharge electrode plate 21
is fixed to the frame by an arm 23 made of an insulator (or covered with an insulator), and electrically connected to a DC high voltage power source (not shown) by an appropriate known method. Further, as shown in FIG. 2, a large number of needle-shaped discharge electrode plates 211 are densely arranged to protrude from the surface facing the classification area S.

A large number of recovery holes are transparently formed in each of the recovery plates 22a to 22d, and the hood end 24a
~24d are connected. Each hood end 24a~
24d is connected by a duct (not shown) to a known separation and recovery section for small powder particles, which is equipped with an absorption fan and a high-performance filter.

The opening area of the recovery hole may be the same for all recovery plates, or may be the same within the same recovery plate but may be different between recovery plates. For example, the lower the recovery plate (upstream side in the direction of solid-gas mixed phase flow), the smaller the opening area of the recovery hole, and the upper (also downstream) the recovery plate, the smaller the opening area of the recovery hole. It may be designed to be large. The size of the opening area of the collection holes in each collection plate and the degree of change between the collection plates will depend on process conditions such as the form of the particle size distribution of the powder particles to be classified and the degree of classification required. This will be determined as appropriate. Further, the vertical dimensions of each collection plate do not necessarily have to be substantially the same as shown in the drawings, but may be determined as appropriate depending on the process conditions as described above.

The housing section 3 has a rectifying suction mechanism 31 that opens facing the classification area S, and a duct 32 that is connected to the rectifying suction mechanism 31. As the rectification suction mechanism 31, it is sufficient to use one having the same rectification mechanism as the rectification discharge mechanism 11 used in the supply section 1, and the classification can be performed with a uniform velocity distribution in the plane direction perpendicular to the direction of movement. It functions to suck the conveying airflow from the area into the duct 32.

Next, the operation of the electrostatic classifier having such a configuration will be explained.

As described above, the solid-gas multiphase flow containing the powder particles supplied from the hopper 14 flows into the classification zone S under electric discharge with a uniform velocity distribution in the plane direction perpendicular to the direction of movement. It is guided in an orderly manner by the push-pull method.

By the way, when we look at the behavior of ultrafine particles of various particle sizes that make up the powder particle group in a solid-gas mixed phase flow under an electric discharge, we find that they are not floating individually in the air flow, but are caused by the electric discharge. When they are subjected to positive loading, they collide under the influence of electrostatic forces such as Crohn's action force and dipole moment action force due to electrostatic polarization, causing so-called electrostatic aggregation. This electrostatic aggregation occurs between ultrafine particles with relatively similar particle sizes, and such ultrafine particles aggregate to form a type of agglomerate over a certain particle size range.

At this time, the smaller the diameter of the ultrafine particles (that is, the lower the particle size range), the smaller the tendency to be charged, so the amount of charge is smaller, and the electrostatic attraction acting on the particles is smaller. Therefore, the degree to which the ions are deflected from the direction of movement of the solid-gas multiphase flow by riding on the air-entrained ion flow progressing within the classification zone S is small. In other words, there is a strong tendency to go straight. Therefore, the smaller the diameter of the ultrafine particles, the longer the residence time in the classification zone S, and the more likely they are to form large agglomerates.

On the other hand, particles with a large diameter (that is, in a high particle size range) have a larger tendency to charge, so the amount of charge is larger, and the electrostatic attraction that acts on them is larger. Riding on the entrained ion flow, the degree of deflection from the advancing direction of the solid-gas multiphase flow is large.In other words, the tendency to move straight is small.Therefore, since the residence time in the classification zone S is short, the ions are deflected and moved as small lumps. .

This invention utilizes the difference in the straight-line propensity of ultrafine particles depending on their particle size within the classification zone S under discharge as described above.

For convenience of explanation, it is assumed here that the powder particle group in the hopper 14 is divided into groups .about. in descending order of particle size range. Then, since the ultrafine particles belonging to the group belong to the highest particle size range in the powder particle group, they tend to form the smallest nodule (referred to as "nodule", hereinafter the same),
Furthermore, the tendency of the vehicle to move straight in the classification zone S is the smallest. Since the ultrafine particles belonging to the group belong to the lowest particle size range among the powder particles, they are most likely to form the largest agglomerates, and have the greatest tendency to move straight through the classification zone S. The solid-gas mixed phase flow guided into the classification zone S between the discharge electrode plate 21 and the collection plates 22a to 22d contains such miscellaneous ultrafine particles with different behaviors.

Now, in FIG. 2, the lowest collection plate 22a
In the classification area Sa covered by , agglomerates consisting of ultrafine particles with the smallest tendency to move in a straight line (in the highest particle size range) ride the air-entrained ion flow, and as shown by the arrow in the figure, the agglomerates consisting of ultrafine particles with the smallest straight tendency to move in a straight line ride on the air-entrained ion flow, and as shown by the arrow in the figure, the solid-gas multiphase flow It is deflected from the traveling direction and moves toward the recovery plate 22a, reaches the recovery plate 22a, is sucked and captured, and is separated from the solid-gas multiphase flow. The baby booms are also deflected to some extent, but because of their tendency to move straight, at this stage they have not yet reached the collection plate side and are captured by suction.

Next, in the classification zone Sb covered by the second collection plate 22b from the bottom, agglomerates consisting of ultrafine particles of a group with a slightly larger tendency to move straight (in the second highest particle size range) ride the air-entrained ion flow,
The recovery plate 22b is deflected from the advancing direction of the solid-gas multiphase flow.
It moves toward the recovery plate 22b, where it is sucked and captured and separated from the solid-gas mixed phase flow. The nodules are also deflected to some extent, but because of their large tendency to move straight, they have not yet reached the collection plate side and are captured by suction at this stage.

Next, in the classification area C covered by the third collection plate 22c from the bottom, agglomerates made of ultrafine particles of a group with a greater tendency to move straight (in the third highest particle size range) ride the air-entrained ion flow,
The recovery plate 22c is deflected from the advancing direction of the solid-gas multiphase flow.
It moves towards the recovery plate 22c, where it is sucked and captured and separated from the solid-gas mixed phase flow. The nodules are also deflected, but because of their large tendency to move straight, at this stage they have not yet reached the collection plate side and are captured by suction.

Furthermore, in the classification zone Sd covered by the uppermost collection plate 22d, agglomerates made of ultrafine particles of the group with the greatest tendency to move straight (in the lowest particle size range) also ride the air-entrained ion flow and are deflected from the solid-gas multiphase flow. and moves toward the collection plate 22d, finally reaching the collection plate 22d and being suctioned and captured.
Separated.

The small groups of powder particles separated from the solid-gas mixed phase flow and individually sucked and captured by the collection plates 22a to 22d as described above are sent to the collection section from the respective hood ends 24a to 24d and collected as described above. be done. The collected ultrafine particles may be used as is, or may be subjected to even finer electrostatic classification. In this way, at least theoretically, all the ultrafine particles have been classified and separated from the solid-gas mixed phase flow, so that only the conveying air flow remains to be accommodated in the storage section 3. However, in reality, it may not be possible to say that there are no ultrafine particles in this carrier airflow, so if necessary, the particles may be passed through an appropriate filter in the storage section, or even finer electrostatic particles may be removed once they are contained. It may also be used for classification.

In the example described above, a perforated recovery plate was used as the recovery element, but the mode of the recovery element is not necessarily limited to this. For example, as shown in Figure 3A, a shelf opening 4 facing the classification area
Alternatively, a collection box 4 having a mesh opening 5 facing the classification area may be used as shown in FIG.
1 may be used. In addition, various collection elements having an air-permeable opening structure facing the classification area can be used, such as one having a nozzle-shaped or ring-shaped opening, or one having a grid-shaped opening. Furthermore, in the case of the recovery hole as described above, if a ring-shaped electrode or the like is attached to the periphery of the recovery hole, the effects of electrostatic attraction and air-entrained ion flow can be further enhanced.

By the way, in the above electrostatic classification, the discharge electrode plate 2
1 on each discharge electrode plate 211 and each opening (e.g. collection hole, grid opening or net mesh) on the collection element.
Therefore, it is desirable that the pitch of the openings on the recovery element correspond to the pitch of the discharge electrode needles on the discharge electrode plate 21.

Now, in the embodiment shown in FIGS. 1 and 2, 1
discharge electrode plates 21 and a plurality of recovery plates 22a to 22
Since classification is performed by electrostatic discharge in combination with d, the electrostatic applied voltage in each classification area is the same. Therefore, although it is possible to perform far more accurate and advanced classification than conventional technology, it requires even higher accuracy and is more flexible in responding to subtle changes in process conditions. Sometimes things are required.

Figure 4 shows an example of an embodiment that meets such requirements, in which the classification is further subdivided by taking advantage of the fact that electrostatic aggregation becomes more pronounced when the applied voltage is increased. This is what I am trying to do.
For simplicity, the same four-stage classification as in Figure 1 is shown in the figure, but in general, when the structure shown in Figure 1 is changed to the one shown in Figure 4, more stages of classification are performed. Is possible.

That is, in this embodiment, there are four collection plates 22.
4 discharge electrode plates 21a-2 for a-22d
1d are arranged vertically in parallel, each electrically connected to a different DC voltage power source. More specifically, when the solid-gas mixed phase flow is raised into the classification zone S, the upper discharge electrode plate is connected to a DC high voltage power source with a higher voltage. In this case, therefore, the applied voltage is higher in the upper classification zone. Furthermore, when the solid-gas multiphase flow is lowered within the classification zone S, the lower discharge electrode plates are connected to a DC high-voltage power source with a higher voltage. The applied voltage is therefore higher in the lower classification zone.

In this embodiment, the electrostatic aggregation is controlled by adjusting the electrostatic applied voltage, and furthermore, the tendency of the ultrafine particles to move straight in the solid-gas mixed phase airflow is controlled. Therefore, for example, in the embodiment of FIG. 1, one classification area where one stage of classification is performed is divided into two,
If two discharge electrode plates are arranged vertically in parallel corresponding to the two collecting plates arranged vertically in parallel and connected to different DC high-voltage power supplies, the classification can be subdivided into two stages and the degree of sophistication can be achieved.

Figure 5 shows another example of an embodiment that satisfies the above-mentioned requirements. This is an attempt to further refine the classification by taking advantage of the fact that electrocoagulation is promoted. For simplicity, the same 4 as in Figure 1 is used in the figure.
Although the classification is shown in stages, in general, if the structure shown in FIG. 1 is changed to the one shown in FIG. 5, it is possible to further perform classification in other stages.

That is, in the case of this embodiment, the cross-sectional area of the classification zone is gradually increased along the traveling direction of the solid-gas multiphase flow, but it may be increased in steps. More specifically, when the solid-gas mixed phase flow is raised within the classification zone S, the cross-sectional area of the classification zone S is made larger as it moves upwards as shown in the figure. Therefore, as long as the flow rate is constant, the lower the classification zone, the lower the velocity of the solid-gas mixed phase flow, the longer the residence time, and the easier it is to obtain the desired deflection movement effect. In addition, when the solid-gas multiphase flow is lowered within the classification area S, the lower the position, the more the classification area S
Increase the cross-sectional area of Therefore, if the flow rate is constant, the lower the classification zone, the lower the velocity of the solid-gas multiphase flow, the longer the residence time, and the more electrostatic aggregation will be promoted.

(Example) The voltage applied to the discharge is preferably in the range of 25 to 100 KV, and the width of the classification area is determined by the relationship with the applied voltage, but it is 40 to 400 mm. range is common. For example, the discharge electrode has a diameter of 1.6 mm. ×10mm. needle-like electrodes are used. The flow rate of the solid-gas mixed phase flow is preferably in the range of about 0.5 to 3.0 m/sec. The size of the coverage area of the entire recovery element is determined by the flow rate and flow rate of the solid-gas multiphase flow.
For example, the width is 375mm. ×Length 1340mm. It is the rank.

(Effects of the Invention) As is clear from the above, according to the present invention, a powder particle group consisting of particles containing ultrafine particles over a very wide range of particle sizes can be divided into ultrafine particles over different particle size ranges selected in advance. Since it is possible to orderly classify powder particles into a plurality of small groups containing powder particles, it is possible to supply ultrafine particle materials with high uniform particle size accuracy depending on the purpose of use.

In addition, since the classification operation is carried out in a substantially sealed classification chamber, there is no possibility of contaminating the working environment, and no excessive scattering of generally valuable and expensive ultrafine particle materials.

Furthermore, by appropriately adjusting and combining various process factors such as the characteristics of the conveying air flow, the intensity of the discharge, the number of stages of the discharge, the number of stages of the collection element, the cross-sectional area of the classification zone and its changes, we can achieve very high performance according to the requirements. It is possible to carry out a wide variety of classifications. Especially classification area S
To explain in detail the effect of adopting the push-pull rectification method in the S flow path, the flow in the S flow path is
When drifting or turbulent flow occurs, the conventional classification system based on successive deflection and movement according to the particle size is significantly impaired. That is, the most important point in order to obtain charge and electrostatic aggregation according to the particle size in an orderly manner is how the ultrafine particles are evenly dispersed and raised in the flow path of the classification zone S. In addition, as other benefits,
In the single push or pull dust-containing air flow, which was considered the most difficult in conventional electrostatic precipitators, the flow path between the discharge electrode plate and the dust collection electrode plate must be a conduit, and the flat electrode plate When sealing both sides of
There is a problem of perfecting the insulation between mutual electrode plates. In contrast, according to the present invention, the discharge electrode plate 21 and the recovery element are installed facing each other in the free space of the classification chamber, and both sides of the flow path of the classification area S are left open and air-insulated. is being carried out. In other words, push
Corresponding strength effect of pull airflow (pushing without side wind)
There is no concern that the classification chamber will be overfilled or overflowed due to a pull air curtain (as seen in pull air curtains), and only the necessary flow paths can be formed in a self-restrictive manner.

Furthermore, although a relatively high voltage is used for discharging, the current itself may be very small, so the power consumption is low, and therefore the running cost is low, making it economical.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an example of the essential parts of the electrostatic classifier of the present invention, FIG. 2 is a side view illustrating its classification action, and FIGS. 3A and B are perspective views showing an example of the collection element. 4 and 5 are side views showing other examples of essential parts of the electrostatic classifier of the present invention. 1... Supply section, 31... Rectification discharge mechanism, 14...
...Hopper, 2...Classification department, 21, 21a-21
d...discharge plate, 22a-22d...recovery plate, 3...
...accommodating section, 31... rectification suction mechanism.

Claims (1)

[Claims] 1. Powder particles including ultrafine particles over a wide particle size range are mixed into a conveying air flow to form a solid-gas multiphase flow, and the velocity distribution of this solid-gas multiphase flow is controlled by a push-pull rectification method. The above-mentioned solid-gas multiphase flow is made uniform over a plane direction perpendicular to the direction of movement, and is caused to proceed within a classification zone extending in the vertical direction that is defined and formed in a substantially sealed environment. , A discharge using electrostatic high voltage is applied to the above-mentioned solid-gas multiphase flow from one side of this classification area, and under this discharge, on the other side of the above-mentioned classification area,
A method for electrostatic classification of powder particles, characterized in that preselected small groups of powder particles having a high particle size range are sequentially and individually separated from an ongoing solid-gas mixed phase flow and captured and recovered. 2. The method according to claim 1, wherein in the classification zone, discharge is performed using an electrostatic high voltage that is higher toward the downstream side in the advancing direction of the solid-gas multiphase flow. 3 In the substantially sealed classification chamber, a discharge electrode plate 21 connected to an electrostatic high-voltage power source is provided extending vertically, and the discharge electrode plate 21 is connected to a vertically long classification area S in between. At least two collection elements each having a permeable opening structure facing the electrode plate and facing the classification area are arranged one above the other and each connected to an individual powder particle small group collection section, Facing one end in the longitudinal direction of S, a solid-gas mixed phase flow of powder particles containing ultrafine particles over a wide particle size range and a conveying air stream is discharged with a uniform velocity distribution over a plane direction perpendicular to the direction of movement of the powder particles. Rectifying mechanism 11
A supply section 1 is provided, and facing the other end in the longitudinal direction of the classification zone S, suctions the classified carrier air with a uniform velocity distribution over a plane direction perpendicular to its traveling direction. An electrostatic classification device for powder particles, characterized in that a storage section 3 is provided with a rectification mechanism 31 for straightening the particles. 4 The discharge electrode plate is divided into at least two or more discharge electrode plates 21a to 21d arranged in parallel in the vertical direction, and the discharge electrode on the downstream side with respect to the advancing direction of the solid-gas multiphase flow within the classification area. 4. The device according to claim 3, wherein the device is connected to an electrostatic high-voltage power supply whose voltage is higher than the plate. 5. Claim 3 or 4, characterized in that the cross-sectional area of the classification zone is configured such that the cross-sectional area of the classification zone increases toward the downstream side with respect to the advancing direction of the solid-gas mixed phase flow within the classification zone.
Apparatus described in section. 6. The device according to any one of claims 3 to 5, wherein the collection element is a collection plate 22a to 22d having a collection hole as an air-permeable opening structure. 7. The device according to any one of claims 3 to 5, wherein the collection element is a collection box 4 having a shelf-like opening as a ventilation opening structure. 8. The device according to any one of claims 3 to 5, wherein the collection element is a collection box 5 having a mesh opening as a ventilation opening structure. 9. Device according to any one of claims 3 to 5, characterized in that the collection element has a grid as the ventilation opening structure.