Sieving device

Abstract

Title: Screening Device A screening device (3) comprising a frame (7), two screening screens (11, 12), and an electroacoustic converter (8) fixed to the frame so as to transmit ultrasound to the two screening screens, the second screening screen having a mesh size strictly smaller than a mesh size of the first screening screen, the first screening screen extending in a first plane (P1), the second screening screen extending in a second plane (P2), the two screening screens and the frame defining an internal chamber (14) for receiving byproducts from the first screening screen, the screening device comprising an opening (15) configured to discharge byproducts from the second screening screen contained in the internal chamber, said opening comprising at least one edge extending in the second plane. Figure to be published with the abbreviation: Figure 1

Description

Title of the invention: Sieving device Technical field of the invention

[0001] The invention relates to the field of screening devices. The invention also relates to a screening installation comprising such a screening device. The invention also relates to a method for screening a powder using such a screening device. State of the prior art

[0002] For the preparation of powders calibrated according to the size of the particles they contain, it is customary to use a sieving device comprising a sieving screen having a given mesh size. The powder treated with such a sieving device is separated into two fractions corresponding on the one hand to the rejects and on the other hand to the passers of the sieving screen. When a more precise calibration of the powder is required, one of the two fractions obtained can be sieved again by means of a second sieving device having a different mesh size.

[0003] Thus, the preparation of powders calibrated according to at least three different particle sizes requires a complex installation comprising different sieving devices and means for moving the intermediate products. Such installations are both expensive and bulky. Presentation of the invention

[0004] The aim of the invention is to provide a screening device which overcomes the above drawbacks and improves the screening devices and screening installations known from the prior art.

[0005] More specifically, a first objective of the invention is a compact sieving device, simple to manufacture and use, and making it possible to calibrate a powder according to at least three particle size categories. Summary of the invention

[0006] The invention relates to a screening device comprising a frame, a first screening screen stretched over the frame, a second screening screen stretched over the frame, and an electroacoustic converter fixed to the frame so as to transmit ultrasound to the first screening screen and to the second screening screen, the second screening screen comprising a mesh size strictly smaller than a mesh size of the first screening screen, the first screening screen extending in a first plane, the second screening screen extending in a second plane distinct from the first plane, the two screening screens and the frame defining an internal chamber of the screening device intended to receive passers-by of the first screening screen. screening, the screening device comprising an opening configured to discharge rejects from the second screening screen contained in the internal chamber, said opening comprising at least one edge extending in the second plane.

[0007] Said opening may be formed by a third screening screen fixed to the second screening screen and extending in the second plane, the third screening screen comprising a mesh size strictly greater than the mesh size of the first screening screen.

[0008] The second screening screen may be bonded to the third screening screen.

[0009] The mesh size of the first sieving screen may be between 30 pm and 1000 pm inclusive, in particular between 1000 pm and 250 pm inclusive. The mesh size of the second sieving screen may be at least 5 pm, in particular between 25 pm and 1000 pm inclusive. The mesh size of the third sieving screen may be strictly greater than 1000 pm, in particular strictly greater than 250 pm.

[0010] The second screening screen may partially cover the third screening screen.

[0011] The frame may comprise a set of segments forming a baffle, said opening being arranged in the frame, at one end of the baffle.

[0012] Said first plane may be parallel to said second plane.

[0013] The first plane and / or the second plane may form a non-zero angle with a horizontal plane.

[0014] The invention also relates to a screening installation comprising a screening device as defined above, a first recovery tank configured to receive the rejects from the first screening screen, a second recovery tank configured to receive the passers-by from the first screening screen rejected by the second screening screen, a third recovery tank configured to receive the passers-by from the second screening screen.

[0015] The invention also relates to a method of sieving a powder by means of a sieving device as defined above, the method comprising: - a step of bringing the powder to the surface of the first sieving screen, - a step of ultrasonic vibration of the first sieving screen and the second sieving screen by means of the electroacoustic converter, - a step of separating the powder into three fractions, a first fraction corresponding to the rejects from the first sieving screen, a second fraction corresponding to the passers-by from the first sieving screen rejected by the second sieving screen, and a third fraction corresponding to the passers-by from the second sieving screen. Presentation of figures

[0016] These objects, characteristics and advantages of the present invention will be explained in detail in the following description of a particular embodiment made without limitation in relation to the attached figures among which:

[0017] [Fig.l] is a schematic side view of a screening installation comprising a screening device according to one embodiment of the invention.

[0018] [Fig.2] is a schematic top view of a first screening screen of the screening device of [Fig.l].

[0019] [Fig. 3] is a schematic top view of a second screening screen and a third screening screen of the screening device of [Fig. 1].

[0020] [Fig.4] is a schematic top view of a second screening screen and a third screening screen according to a first variant embodiment of the invention.

[0021] [Fig. 5] is a schematic top view of a second screening screen and a third screening screen according to a second alternative embodiment of the invention.

[0022] [Fig.6] is a schematic top view of a second screening screen according to a third alternative embodiment of the invention. Detailed description

[0023] [Fig.l] schematically illustrates a screening installation 1 according to a first embodiment of the invention. The screening installation 1 is intended to separate a powder, that is to say a cluster of particles, into three distinct fractions each comprising particles of different sizes: a first fraction is intended to be formed with the largest particles, a second fraction is intended to be formed with particles of intermediate size and a third fraction is intended to be formed with the smallest particles. For this purpose, the screening installation 1 comprises an inlet 2 (for example in the form of a chute), a screening device 3 according to an embodiment of the invention, and three recovery tanks 4, 5, 6 each intended to receive a fraction of the powder introduced at the inlet of the screening installation.The inlet 2 is arranged in the upper part of the screening installation 1 while the collection tanks 4, 5, 6 are arranged in the lower part of the screening installation 1. The screening installation 1 rests on a horizontal ground. The terms "lower" and "upper" refer to an arrangement relative to a vertical axis. The screening device 3 is held above the collection tanks 4, 5, 6 by a structure.

[0024] The screening device 3 comprises a frame 7, an electroacoustic converter 8, a first screening screen 11, and a second screening screen 12. The two screening screens 11, 12 are stretched over the frame 7. The frame 7 may be, for example: a metal structure, in particular an aluminum structure, it comprises sufficient rigidity to withstand the tension exerted by the screening screens 11, 12. The frame 7 may for example have a rectangular shape and be made up of profiles rigidly fixed to each other. Alternatively, the frame 7 could have any other shape, in particular a polygonal shape or a circular shape.

[0025] The electroacoustic converter 8 is fixed to the frame 7 so as to transmit ultrasound to the first sieving screen 11 and to the second sieving screen 12. In other words, the sieving screens 11, 12 are intended to be vibrated by means of the frame 7 which supports them. Advantageously, a single electroacoustic converter 8 is used to vibrate the two sieving screens 11, 12. Preferably, the electroacoustic converter 8 is configured to vibrate the sieving screens 11, 12 at a frequency between 25 kHz and 45 kHz inclusive, in particular between 30 kHz and 40 kHz inclusive, or even a frequency of the order of 35 kHz. The electroacoustic converter can also be configured to emit ultrasound periodically, by alternating phases of vibration of the sieving screens and rest phases where the sieving screens do not vibrate.The electroacoustic converter can also be configured to emit ultrasounds whose frequency varies in the range from 25kHz to 45kHz, or even in the range from 30kHz to 40kHz. The electroacoustic converter 8 is electrically connected to a generator 9 integrated or not in the screening installation.

[0026] The screening device 3 may further comprise damping elements 10, such as for example foam blocks, arranged at the interface with the structure which supports it. Thus the vibration waves generated by the electroacoustic converter 8 are not transmitted to the entire screening installation 1 but are confined to the screening device 3.

[0027] The sieve screens 11, 12 are shown schematically and respectively in Figures 2 and 3. They comprise a network of meshes calibrated to separate particles according to their size. The sieve screens 11, 12 may be woven wire cloths, in particular cloths provided with square meshes. The sieve screens may therefore be sieve cloths. Other types of mesh than square meshes are conceivable. The wires constituting each sieve cloth may be metal wires, in particular steel or stainless steel, or nylon wires. The mesh size of the first sieve screen 11 may for example be between 30 μm and 1000 μm inclusive, in particular between 100 μm and 250 μm inclusive. The second sieve screen 12 comprises a mesh size strictly smaller than a mesh size of the first sieve screen 11.The mesh size of the second sieve screen may be at least 5 pm, for example between 5 pm and 100 pm inclusive, in particular between 25 pm and 100 pm inclusive, it being understood that the size of . mesh of the second sieving screen is strictly smaller than the mesh size of the first sieving screen.

[0028] The screening screens 11, 12 are stretched on the frame 7 so that the ultrasound generated by the electroacoustic converter 8 propagate correctly. The tension of each screening screen may be, for example, between 40% and 80% of the elastic limit of the screening screen, preferably between 50% and 60% of the elastic limit of the screen. The screening screens 11, 12 may in particular be fixed by gluing to said frame 7, in particular by means of a shear-resistant adhesive. Alternatively, other fixing means such as stapling or welding could be envisaged. All edges of the first screening screen 11 are attached directly to the frame 7. In particular, the first screening screen 11 may comprise a rectangular shape and have its four sides attached to the frame 7. Conversely, and as we will see in more detail later, only some of the edges of the second screening screen are attached directly to the frame 7.In particular, the second screening screen 12 may comprise a rectangular shape and have only three of its four sides attached directly to the frame 7.

[0029] The first screening screen 11 extends in a first plane PI and the second screening screen 12 extends in a second plane P2 distinct from the first plane PL. More particularly, the two screening screens can be fixed on two opposite sides of the frame 7. The two screening screens 11, 12 and the frame 7 define an internal chamber 14 of the screening device 3. In other words, the internal chamber 14 is delimited by the two screening screens 11, 12 and the frame 7. The second screening screen 12 is arranged so as to receive the passers-by of the first screening screen 11. In particular, the second screening screen 12 extends below the first screening screen 11. The distance separating the first plane PI from the second plane P2 can be, for example, of the order of a few centimeters or a few tens of centimeters.

[0030] Advantageously, the first plane P1 and the second plane P2 form a non-zero angle A1 with the horizontal plane. The angle A1 is preferably less than or equal to 20°, in particular between 5° and 10°. As we will also see later, the sloping arrangement of the sieving screens 11, 12 allows the particles present on their surface to progress under the combined effect of ultrasound and gravity. This makes it possible to exploit the entire surface of the sieving screens and to avoid local accumulations leading to clogging of the sieving screens. Advantageously, the electroacoustic converter 8 is then fixed on a side of an upper segment of the frame 7. The planes P1 and P2 can be parallel to each other, which facilitates the design of the frame 7 or, alternatively, they can each form a different angle with the horizontal plane, thus conferring specific kinematics to the particles. present on their surface. According to another variant, the planes PI and P2 could also extend horizontally. In this case, the movement of particles on the surface of the sieve screens could, for example, be induced by an asymmetric mechanical vibration, according to the principle of the vibrating corridor.

[0031] The screening device 3 further comprises an opening 15 configured to evacuate the rejects from the second screening screen 12, which are contained in the internal chamber 14. This opening 15 comprises at least one edge extending in the second plane P2, that is to say in the plane in which the second screening screen 12 extends. Thus, the rejects from the second screening screen 12 can be evacuated from the internal chamber 14 by passing through the opening 15. No retention volume for the rejects is thus formed in the internal chamber 14.

[0032] According to a preferred embodiment of the invention, the opening 15 is formed by a third screening screen 13 fixed to the second screening screen 12, and extending in the second plane P2. Consequently, all the edges of the opening 15 extend in the second plane P2. The third screening screen 13 comprises a mesh size strictly greater than the mesh size of the first screening screen. For example, if the mesh size of the first screening screen is between 100 pm and 250 pm inclusive, the mesh size of the third screening screen may be strictly greater than 250 pm. Preferably, the mesh size of the third screening screen is significantly greater than the mesh size of the first screening screen, for example between two times and five times the mesh size of the first screening screen.

[0033] Advantageously, the third screening screen 13 is glued to the second screening screen 12, in particular by means of a shear-resistant adhesive, for example the same adhesive as that used to fix the first screening screen 11 and the second screening screen 12 to the frame 7. Thus the second screening screen remains perfectly taut: it comprises three sides fixed directly to the frame 7 and a fourth side fixed to the frame 7 via the third screening screen 13. The third screening screen is also taut on the frame 7 and its tension can be substantially equal to the tension of the second screening screen. Alternatively, other fixing means could be envisaged to fix the second screening screen to the third screening screen such as for example stapling or welding.

[0034] There is therefore an overlap zone 16 in which the second sieving screen 12 and the third sieving screen 13 extend. At this overlap zone 16 the second sieving screen 12 covers the third sieving screen 13. In other words, the second sieving screen 12 extends above the third sieving screen. Particles progressing from the second sieving screen to the third sieving screen 13 thus cross a descending step of which the height corresponds to the thickness of the second sieve screen 12. Outside the overlap zone 16, the second sieve screen does not extend above the third sieve screen 13. The third sieve screen 13 may possibly extend under the entire surface of the second sieve screen 12. A damping element 10 or even any other masking element may possibly extend at the level of the overlap zone 16, thus preventing any powder particle from passing through this zone. In the event that the plane P2 forms a non-zero angle Al with the horizontal plane, the third sieve screen 13 is advantageously positioned below the second sieve screen 12. Thus, the rejects present on the surface of the second sieve screen 12 can progress towards the third sieve screen 13 under the combined effect of gravity and ultrasound.

[0035] A first recovery tank 4 is configured to receive the rejects from the first screening screen 11. This first recovery tank can therefore be arranged below a lower edge of the first screening screen 11. A second recovery tank 5 is configured to receive the passers-by from the first screening screen 11 rejected by the second screening screen 12. The second recovery tank 5 is arranged below the outlet 15, that is to say below the third screening screen 13. Finally, a third recovery tank 6 is configured to receive the passers-by from the second screening screen 12, and is therefore arranged below the second screening screen 12. These recovery tanks 4, 5, 6 can comprise vertical walls which extend to the damping elements 10. The recovery tanks are thus isolated from each other.The second recovery tank 5 is positioned between the first recovery tank 4 and the third recovery tank 6. According to an alternative embodiment, the first recovery tank and the second recovery tank could form a single recovery tank. In other words, the outlet of the first screening screen could be connected to the outlet formed by the third screening screen. The screening installation thus obtained would thus simply make it possible to discharge the second screening screen with too high a flow rate, thus limiting the risk of clogging of the second screening screen.

[0036] According to another alternative embodiment of the invention, the screening device 3 could be integrated into a screening installation comprising other screening equipment, in particular for screening even more finely the passers-by from the second screening screen 12. In particular, the arrangement of the screening screens 11, 12 and 13 can be reproduced below the second screening screen 12. A fourth screening screen and a fifth screening screen are then provided which delimit, with the second screening screen, a second internal chamber. The mesh size of the fourth screening screen would be strictly smaller than the mesh size of the second screening screen. The mesh size of the fifth screening screen screening would be strictly greater than the mesh size of the second screening screen. The operation of the second, fourth and fifth screening screens would be similar to the operation of the first, second and third screening screens. Such an installation could include four or even more collection tanks.

[0037] To sift powder using the sieving installation 1, the procedure can be as follows: in a first step, powder is brought onto the surface of the first sieving screen 11. In particular, the powder can be poured into the sieving installation 11 via the inlet 2. The powder is thus poured into the upper part of the first sieving screen 11. The powder can be formed, for example, by a set of metallic, and / or ceramic, and / or organic particles.

[0038] Then, in a second step, the electroacoustic converter 8 is activated, which transmits ultrasound to the first sieving screen 11. This causes the powder to be gradually distributed over the surface of the first sieving screen and the finest particles to pass through the mesh of the first sieving screen. At each vibration, the particles in contact with the first sieving screen are projected above the first sieving screen in a direction generally perpendicular to the first sieving screen. Thus, the projected particles gradually advance towards the lower edge of the first sieving screen. When the particles fall back onto the first sieving screen, they may possibly pass through its mesh.The particles that do not pass through the mesh are thrown into the air again during a subsequent vibration of the first sieve screen and thus have a new chance to pass through its mesh. Advantageously, the slope and length of the first sieve screen, as well as the frequency of the ultrasound are different operating parameters that can be adapted in order to obtain complete sieving of the powder present on the surface, before it reaches the lower edge of the first sieve screen. The ultrasonic waves, given their frequency and amplitude, make it possible to promote the passage of passers-by through the first sieve screen. An alternation of vibration phases of the first sieve screen and rest phases where the first sieve screen does not vibrate allows the particles to pass even more easily through the mesh of the first sieve screen.Indeed, if several particles form a pile above a mesh of the first sieving screen during a rest phase, this pile collapses during the following vibration phase, which causes some of the particles in this pile to pass through the mesh. The same operation applies to the second sieving screen 12 which receives the passers-by from the first sieving screen.

[0039] Finally, a first fraction of the powder, corresponding to the rejects from the first sieving screen 11, advances onto the first sieving screen under the combined action ultrasound and gravity. This first fraction thus moves towards the lower edge of the first sieving screen from where it falls into the first recovery tank 4. The remaining part of the powder, corresponding to the passers-by of the first sieving screen, passes through the meshes of the first sieving screen into the internal chamber 14, onto the surface of the second sieving screen 12. The vibration of the second sieving screen also makes it possible to separate the powder present in the internal chamber into two fractions: a second fraction, corresponding to the rejects of the second sieving screen, remains on the surface of the second sieving screen and moves towards the opening 15 under the combined action of ultrasound and gravity. Since the opening 15 has a common edge with the second plane P2, no powder retention is formed in the internal chamber and in particular on the surface of the second sieve screen. This eliminates the risk of clogging the second sieve screen. The second fraction passes without difficulty through the meshes of the third sieve screen since their size is greater than the mesh size of the first sieve screen. The third sieve screen 13 does not have any rejection due to the large size of its meshes. Thus, the third sieve screen does not act as a sieve but as a discharge opening and a function of transmitting the tension between the second sieve screen 12 and the frame 7. The second fraction is thus recovered in the second recovery tank 5. Finally, the third fraction, corresponding to the passers-by of the second sieve screen, is recovered in the third recovery tank 6.

[0040] As a remark, it is possible that a part of the powder present on the surface of the first sieving screen 11 falls directly onto the third sieving screen 13 without passing through the second sieving screen. The quantity of powder following this path can be minimized, or even canceled, by adapting in particular the length of the first sieving screen, and / or the frequency and / or the amplitude of the ultrasonic waves, and / or the slope along which the first sieving screen extends, and / or the quantity of powder introduced at the inlet of the sieving installation.

[0041] Thanks to the invention, there is therefore a simple means to manufacture and use for calibrating a powder according to at least three particle size categories. The rejects from the first sieving screen and the second sieving screen are automatically evacuated, which makes it possible to maintain functional sieving surfaces. The efficiency of the sieving device is thus maintained as the sieving process is carried out.

[0042] In connection with Figures 4 to 6, different embodiment variants of the invention are now described. The references used for the description of the main embodiment are used to designate the common elements of the embodiment variants of the screening device. First of all, as can be seen in [Fig.4] , the overlapping zone 16 is not necessarily rectilinear. On the contrary, it can adopt any geometric shape such as for example the shape of a V ([Fig.4]) or even a circular shape ([Fig.5]). When the overlapping zone 16 adopts a circular shape, the third screening screen can be arranged around the second screening screen as shown in [Fig.6], or conversely, the second screening screen can be arranged around the third screening screen.

[0043] [Fig. 6] illustrates yet another alternative embodiment of the screening device 3. According to this alternative embodiment, the opening 15 is not made with a screening screen provided with a larger mesh size but is formed in the frame 7. The frame 7 thus comprises a discontinuity at the level of the opening 15 so that the second screening screen 12 is not fixed over its entire periphery. On the contrary, the second screening screen comprises an edge 17 not fixed to the frame 7, at the level of the opening 15. This edge 17 represents a relatively small proportion of the perimeter of the second screening screen so that the latter still remains sufficiently taut and correctly transmits the ultrasounds from the electroacoustic converter 8. Advantageously, the frame comprises a set of walls forming a baffle 18 and the opening 15 is arranged at one end of the baffle 18.In particular, the frame 7 can be formed by five segments 19, 20, 21, 22, 23. A first lower segment 19 is connected to a left lateral segment 20 and a second lower segment 21 is connected to a right lateral segment 22. The left lateral segment 20 and the right lateral segment 22 are further connected via an upper segment 23. The opening 15 is formed between the two lower segments 19 and 21. Thus, the rejects from the second screening screen can be evacuated outside the internal chamber 14 by passing through the opening 15. This opening 15 includes a common edge with the second screening screen 12, which makes it possible to evacuate the rejects from the second screening screen 12 without a retention zone.The lower segments 19 and 21 may optionally be inclined relative to the horizontal so as to more easily guide the rejects from the second screening screen 12 towards the outlet 15 under the effect of the combined action of gravity and ultrasound. The second screening screen 12 is fixed, in particular glued to the five segments 19, 20, 21, 22, 23. The baffle 18 provides an extended fixing support for the second screening screen 12 which compensates for the absence of fixing of the screening screen at the opening 15. The baffle 18 thus makes it possible to guarantee sufficient tension of the second screening screen 12 and proper operation thereof.

Claims

Claims

1. A screening device (3) comprising a frame (7), a first screening screen (11) stretched over the frame, a second screening screen (12) stretched over the frame, and an electroacoustic converter (8) fixed to the frame so as to transmit ultrasound to the first screening screen and to the second screening screen, the second screening screen comprising a mesh size strictly smaller than a mesh size of the first screening screen, the first screening screen extending in a first plane (PI), the second screening screen extending in a second plane (P2) distinct from the first plane, the two screening screens and the frame defining an internal chamber (14) of the screening device intended to receive passers-by of the first screening screen, the screening device comprising an opening (15) configured to evacuate rejects of the second screening screen contained in the internal chamber,said opening comprising at least one edge extending in the second plane.,

2. Screening device (3) according to the preceding claim, characterized in that said opening (15) is formed by a third screening screen (13) fixed to the second screening screen (12) and extending in the second plane (P2), the third screening screen comprising a mesh size strictly greater than the mesh size of the first screening screen.

3. Screening device (3) according to the preceding claim, characterized in that the second screening screen (12) is glued to the third screening screen (13).

4. Sieving device (3) according to one of claims 2 or 3, characterized in that: - the mesh size of the first sieving screen (11) is between 30 pm and 1000 pm inclusive, in particular between 1000 pm and 250 pm inclusive, and / or in that - the mesh size of the second sieving screen (12) is at least equal to 5 pm, in particular between 25 pm and 1000 pm inclusive, and / or in that - the mesh size of the third sieving screen (13) is strictly greater than 1000 pm, in particular strictly greater than 250 pm.

5. Screening device (3) according to one of claims 2 to 4, characterized in that the second screening screen (12) covers par- initially the third screening screen (13).

6. Screening device (3) according to claim 1, characterized in that the frame (7) comprises a set of segments (19, 21) forming a baffle (18), said opening (15) being arranged in the frame, at one end of the baffle.

7. Sieving device (3) according to one of the preceding claims, characterized in that said first plane (PI) is parallel to said second plane (P2).

8. Sieving device according to one of the preceding claims, characterized in that the first plane (PI) and / or the second plane (P2) form a non-zero angle (Al) with a horizontal plane.

9. Screening installation (1) comprising a screening device (3) according to one of the preceding claims, a first recovery tank (4) configured to receive the rejects from the first screening screen (11), a second recovery tank (5) configured to receive the passers-by from the first screening screen rejected by the second screening screen (12), a third recovery tank (6) configured to receive the passers-by from the second screening screen.

10. A method of sieving a powder by means of a sieving device (3) according to one of claims 1 to 8 comprising: - a step of supplying the powder to the surface of the first sieving screen (11), - a step of ultrasonically vibrating the first sieving screen (11) and the second sieving screen (12) by means of the electroacoustic converter (8), - a step of separating the powder into three fractions, a first fraction corresponding to the rejects from the first sieving screen, a second fraction corresponding to the passers-by from the first sieving screen rejected by the second sieving screen, and a third fraction corresponding to the passers-by from the second sieving screen.

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