FR3132654A1 - Screening device - Google Patents

Abstract

Title: Sieving device Sieving device (3) comprising a frame (7), two sieving screens (11, 12), and an electroacoustic converter (8) fixed to the frame so as to transmit ultrasound to the two sieving screens, the second sieving screen comprising a mesh size strictly smaller than a mesh size of the first sieving screen, the first sieving screen extending in a first plane (P1), the second sieving screen extending in a second plane (P2), the two sieving screens and the frame defining an internal chamber (14) intended to receive passers-by from the first sieving screen, the sieving device comprising an opening (15) configured to evacuate rejects from the second sieving screen screening contained in the internal chamber, said opening comprising at least one edge extending in the second plane. Figure to be published with abstract: Figure 1

Description

Sieving device Technical field of the invention

The invention relates to the field of sieving devices. The invention also relates to a screening installation comprising such a screening device. The invention also relates to a method of sieving a powder using such a sieving device.

State of the prior art

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 passes of the sieving screen. When a more precise calibration of the powder is required, one of the two fractions obtained can be sieved again using a second sieving device having a different mesh size.

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

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.

More precisely, 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.

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 attached to the frame so as to transmit ultrasound to the first screen sieving screen and to the second sieving screen, the second sieving screen comprising a mesh size strictly less than a mesh size of the first sieving screen, the first sieving screen extending in a first plane, the second sieving 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 from the first screening screen, the screening device comprising an opening configured to evacuate refusal of the second screening screen contained in the internal chamber, said opening comprising at least one edge extending in the second plane.

Said opening can 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.

The second sieving screen can be glued to the third sieving screen.

The mesh size of the first sieving screen can be between 30µm and 1000µm inclusive, in particular between 100µm and 250µm inclusive. The mesh size of the second sieving screen can be at least equal to 5µm, in particular between 25µm and 100µm inclusive. The mesh size of the third sieving screen can be strictly greater than 1000µm, in particular strictly greater than 250µm.

The second sieving screen can partially cover the third sieving screen.

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

Said first plane can be parallel to said second plane.

The first plane and/or the second plane can form a non-zero angle with a horizontal plane.

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 screen of sieving refused by the second sieving screen, a third recovery tank configured to receive the passers-by of the second sieving screen.

The invention also relates to a process for sieving a powder using a sieving device as defined above, the process comprising:
- a step of supplying 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 refusals of the first sieving screen, a second fraction corresponding to the passers-by of the first sieving screen refused by the second sieving screen, and a third fraction corresponding to the passers-by of the second sieving screen.

Presentation of figures

These objects, characteristics and advantages of the present invention will be explained in detail in the following description of a particular embodiment made on a non-limiting basis in relation to the attached figures among which:

There is a schematic side view of a screening installation comprising a screening device according to one embodiment of the invention.

There is a schematic top view of a first sieving screen of the sieving device of the .

There is a schematic top view of a second screening screen and a third screening screen of the screening device of the .

There is a schematic top view of a second screening screen and a third screening screen according to a first alternative embodiment of the invention.

There is a schematic top view of a second screening screen and a third screening screen according to a second alternative embodiment of the invention.

There is a schematic top view of a second screening screen according to a third alternative embodiment of the invention.

detailed description

There schematically illustrates a screening installation 1 according to a first embodiment of the invention. The sieving 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 particles of more large size, a second fraction is intended to be formed with particles of intermediate size and a third fraction is intended to be formed with particles of smaller size. 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 one embodiment of the invention, and three recovery tanks 4, 5, 6 each intended to receive a fraction of the powder introduced into the sieving installation. The inlet 2 is arranged in the upper part of the sieving installation 1 while the recovery tanks 4, 5, 6 are arranged in the lower part of the sieving installation 1. The sieving installation 1 rests on a floor horizontal. The terms "lower" and "upper" refer to an arrangement relative to a vertical axis. The screening device 3 is held above the recovery tanks 4, 5, 6 by a structure.

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 can be for example a metal structure, in particular an aluminum structure, it includes sufficient rigidity to withstand the tension exerted by the screening screens 11, 12. The frame 7 can for example have a rectangular shape and be made up of profiles rigidly fixed to each other. to others. Alternatively, the frame 7 could have any other shape, in particular a polygonal shape or a circular shape.

The electroacoustic converter 8 is fixed to the frame 7 so as to transmit ultrasound to the first screening screen 11 and to the second screening screen 12. In other words, the screening screens 11, 12 are intended to be vibrated via 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 included, in particular between 30kHz and 40kHz inclusive, or even a frequency of around 35kHz. The electroacoustic converter can also be configured to emit ultrasound periodically, alternating phases of vibration of the screening screens and rest phases where the screening screens do not vibrate. The electroacoustic converter can also be configured to emit ultrasound 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 into the screening installation.

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 limited to the screening device 3.

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

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

The first sieving screen 11 extends in a first plane P1 and the second sieving screen 12 extends in a second plane P2 distinct from the first plane P1. 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 sieving screens 11, 12 and the frame 7. The second sieving screen 12 is arranged so as to receive the loops of the first sieving screen 11. In particular, the second sieving screen 12 extends below the first sieving screen 11. The distance separating the first plane P1 from the second plane P2 can for example be of the order of a few centimeters or a few tens of centimeters.

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 one 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 P1 and P2 could also extend horizontally. In this case, the movement of particles on the surface of the sieving screens could for example be induced by an asymmetric mechanical vibration, according to the vibrating corridor principle.

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, c that is to say in the plane in which the second screening screen 12 extends. Thus, the rejects from the second sieving screen 12 can be evacuated from the internal chamber 14 via the opening 15. No retention volume for the rejects is thus formed in the internal chamber 14.

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 sieving screen 13 includes a mesh size strictly greater than the mesh size of the first sieving screen. For example, if the mesh size of the first sieving screen is between 100µm and 250µm inclusive, the mesh size of the third sieving screen can be strictly greater than 250µm. Preferably, the mesh size of the third sieving screen is significantly greater than the mesh size of the first sieving screen, for example between two times and five times the mesh size of the first sieving screen.

Advantageously, the third sieving screen 13 is glued to the second sieving screen 12, in particular by means of a shear-resistant glue, for example the same glue as that which is used to fix the first sieving screen 11 and the second screen sieving screen 12 to frame 7. Thus the second sieving 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 sieving screen 13. The third sieving screen is also stretched on the frame 7 and its tension can be substantially equal to the tension of the second sieving screen. Alternatively, other fixing means could be considered for fixing the second screening screen to the third screening screen such as for example stapling or welding.

There is therefore an overlapping zone 16 in which the second screening screen 12 and the third screening screen 13 extend. At the level of this overlapping zone 16 the second screening screen 12 covers the third screening screen 13. Otherwise said, the second sieving screen 12 extends above the third sieving screen. Particles progressing from the second sieving screen towards the third sieving screen 13 thus cross a downward step whose height corresponds to the thickness of the second sieving screen 12. Outside the overlapping zone 16, the second sieving screen does not does not extend above the third sieving screen 13. The third sieving screen 13 may possibly extend under the entire surface of the second sieving screen 12. A damping element 10 or even any other masking element may possibly extend extend at the level of the covering zone 16, thus preventing any powder particle from passing through this zone. In the hypothesis where the plane P2 forms a non-zero angle A1 with the horizontal plane, the third screening screen 13 is advantageously positioned below the second screening screen 12. Thus, the rejects present on the surface of the second screening screen 12 can progress towards the third sieving screen 13 under the combined effect of gravity and ultrasound.

A first recovery tank 4 is configured to receive the refusals from the first sieving screen 11. This first recovery tank can therefore be arranged below a lower edge of the first sieving screen 11. A second recovery tank 5 is configured to receive the passers-by from the first sieving screen 11 refused by the second sieving screen 12. The second recovery tank 5 is arranged below the outlet 15, that is to say below the third sieving screen 13. Finally, a third recovery tank 6 is configured to receive the passers-by of the second sieving screen 12, and is therefore arranged below the second sieving screen 12. These recovery tanks 4, 5, 6 can include vertical walls which are 'extend up 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 a variant embodiment, the first recovery tank and the second recovery tank could form one and the same recovery tank. In other words, the output of the first sieving screen could be connected to the output formed by the third sieving screen. The sieving installation thus obtained would thus simply allow the second sieving screen to be discharged from too high a flow rate, thus limiting the risk of clogging of the second sieving screen.

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 to screen even more finely the passers-by from the second screening screen 12. In particular we can reproduce the arrangement of the sieving screens 11, 12 and 13 below the second sieving screen 12. We then provide a fourth sieving screen and a fifth sieving screen which delimit, with the second sieving screen, a second internal chamber. The mesh size of the fourth sieving screen would be strictly less than the mesh size of the second sieving screen. The mesh size of the fifth sieving screen would be strictly greater than the mesh size of the second sieving screen. The operation of the second, fourth and fifth sieving screens would be similar to the operation of the first, second and third sieving screens. Such an installation could include four collection bins or even more.

To sift powder using the sieving installation 1, one can proceed 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 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 particles, and /or organic.

Then, in a second step, the electroacoustic converter 8 is activated which transmits ultrasound to the first sieving screen 11. This causes the progressive distribution of the powder on the surface of the first sieving screen and the passage of the finest particles through it. meshes 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 onto the first sieving screen, they may possibly pass through its mesh. The particles which do not pass through the meshes are projected into the air again during a subsequent vibration of the first sieving screen and thus benefit from a new chance to pass through its meshes. Advantageously, the slope and the length of the first sieving screen, as well as the frequency of the ultrasound are different operating parameters which can be adapted in order to obtain complete sieving of the powder present on the surface, before it reaches the edge bottom of the first sieving screen. The ultrasonic waves make it possible, given their frequency and amplitude, to encourage the passage of passers-by through the first screening screen. An alternation of vibration phases of the first sieving screen and rest phases where the first sieving screen does not vibrate allows the particles to pass even more easily through the meshes of the first sieving 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 part of the particles to pass through. of this pile through the mesh. The same operation applies to the second sieving screen 12 which receives the loops from the first sieving screen.

Finally, a first fraction of the powder, corresponding to the rejections of the first sieving screen 11, advances on the first sieving screen under the combined action of ultrasound and gravity. This first fraction thus moves towards the lower edge of the first sieving screen from which it falls into the first recovery tank 4. The remaining part of the powder, corresponding to the loops of the first sieving screen, passes through the meshes of the first sieving screen in the internal chamber 14, on 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 refusal 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. The opening 15 having 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 sieving screen. This eliminates the risk of clogging the second sieving screen. The second fraction passes without difficulty through the meshes of the third sieving screen since their size is greater than the size of the meshes of the first sieving screen. The third sieving screen 13 does not present any refusal due to the large size of its meshes. Thus, the third sieving screen does not act as a sieve but as an evacuation opening function and as a function of transmitting the tension between the second sieving screen 12 and the frame 7. The second fraction is thus recovered in the second tray recovery 5. Finally, the third fraction, corresponding to the loops of the second sieving screen, is recovered in the third recovery tank 6.

As a note, it is possible that 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, in particular by adapting 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 it is extends the first sieving screen, and/or the quantity of powder introduced into the sieving installation.

Thanks to the invention, we therefore have a means that is simple 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.

In relation to Figures 4 to 6, different embodiments 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 alternative embodiments of the sieving device. First of all, as can be seen in the , the covering zone 16 is not necessarily rectilinear. On the contrary, it can adopt any geometric shape such as for example the shape of a V ( ) or even a circular shape ( ). When the covering zone 16 adopts a circular shape, the third sieving screen can be arranged around the second sieving screen as shown in , or vice versa, the second sieving screen can be arranged around the third sieving screen.

There 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 includes a discontinuity at the opening 15 so that the second screening screen 12 is not fixed over its entire perimeter. On the contrary, the second sieving 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 sieving screen so that it remains all the same. even sufficiently tense and correctly transmits the ultrasound coming 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 lateral segment straight 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 passing through the opening 15. This opening 15 clearly includes a common edge with the second sieving screen 12, which makes it possible to evacuate the rejects from the second sieving screen 12 without a retention zone. The lower segments 19 and 21 can optionally be inclined relative to the horizontal so as to more easily guide the rejects from the second sieving screen 12 towards the outlet 15 under the effect of the combined action of gravity and ultrasound. The second sieving screen 12 is fixed, in particular glued to the five segments 19, 20, 21, 22, 23. The chicane 18 offers an extended fixing support for the second sieving screen 12 which compensates for the absence of fixing of the screen sieving at the level of the opening 15. The chicane 18 thus makes it possible to guarantee sufficient tension of the second sieving screen 12 and proper operation thereof.

Claims (10)

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 screening frame so as to transmit ultrasound to the first sieving screen and to the second sieving screen, the second sieving screen comprising a mesh size strictly less than a mesh size of the first sieving screen, the first sieving screen extending in a first plane (P1), 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 from the second screening screen contained in the internal chamber, said opening comprising at least one edge extending in the second plane. Sieving device (3) according to the preceding claim, characterized in that said opening (15) is formed by a third sieving screen (13) fixed to the second sieving screen (12) and extending in the second plane (P2 ), the third sieving screen comprising a mesh size strictly greater than the mesh size of the first sieving screen. Sieving device (3) according to the preceding claim, characterized in that the second sieving screen (12) is glued to the third sieving screen (13). 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 30µm and 1000µm inclusive, in particular between 100µm and 250µm inclusive, and/or in that
- the mesh size of the second sieving screen (12) is at least equal to 5µm, in particular between 25µm and 100µm inclusive, and/or in that
- the mesh size of the third sieving screen (13) is strictly greater than 1000µm, in particular strictly greater than 250µm. Sieving device (3) according to one of claims 2 to 4, characterized in that the second sieving screen (12) partially covers the third sieving screen (13). 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 a end of the chicane. Sieving device (3) according to one of the preceding claims, characterized in that said first plane (P1) is parallel to said second plane (P2). Sieving device according to one of the preceding claims, characterized in that the first plane (P1) and/or the second plane (P2) form a non-zero angle (A1) with a horizontal plane. Sieving installation (1) comprising a sieving device (3) according to one of the preceding claims, a first recovery tank (4) configured to receive the rejects from the first sieving screen (11), a second recovery tank ( 5) configured to receive the passers-by from the first sieving screen refused by the second sieving screen (12), a third recovery tray (6) configured to receive the passers-by from the second sieving screen. Method for sieving a powder using 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 vibrating by ultrasound 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 refusals of the first sieving screen, a second fraction corresponding to the passers-by of the first sieving screen refused by the second sieving screen, and a third fraction corresponding to the passers-by of the second sieving screen.