Fine screening machine and method for operating a fine screening machine

Abstract

Method for operating a fine sieve machine (1) with at least one sieve deck (10) for receiving at least one sieve lining (21) and at least one ultrasonic vibration exciter (23) for generating ultrasonic vibrations which are transmitted to the sieve lining (21), characterized in that the at least one ultrasonic vibration exciter (23) is operated in pulsed mode, wherein the ultrasonic vibration exciter (23) repeatedly switches between a first operating mode with a first vibration amplitude and a second operating mode with a second vibration amplitude that is smaller than the first.

Description

[0001] The invention relates to a fine screening machine with at least one screen deck for receiving at least one screen lining and at least one ultrasonic vibration generator for generating vibrations that are transmitted to the screen lining. The invention further relates to a method for operating such a fine screening machine.

[0002] Fine screening machines are used for separating or classifying material with particle sizes ranging from a few tens of micrometers to a few millimeters. Unlike screening machines with a freely oscillating screen box, the screen decks in these fine screening machines are usually statically fixed. The screen panels are fixed within the screen deck at least at two of their edges, for example, by being clamped in at least one direction within the screen deck using a clamping device. Two or more vibration exciters, also called vibration generators, are typically arranged below the screen panels. These exciter bars, for example, have transverse excitation strips mounted on a vibrating drive beam. The excitation strips can be excited to vibrate in various ways.

[0003] For example, a screening machine with an unbalanced motor for generating vibrations is known from German patent application DE 200 08 762 U1. A special feature of this machine is that the unbalanced mass of the motor can be varied so that the amplitude of the vibration excitation can be set to at least two different values. This makes it possible, for example, to regularly set cleaning intervals in which the vibration excitation occurs with an amplitude that differs from normal screening operation. During these intervals, stuck grains ("stuck grains") are removed due to the altered vibration conditions of the screen surface.

[0004] Document WO 2013 / 152765 A1 describes a screening machine of the type mentioned above, in which an ultrasonic vibration generator is coupled to the screen material to excite it to vibrations at a high frequency between 30 and 38 kHz (kilohertz). In exemplary embodiments, excitation with ultrasound is advantageously described in addition to excitation with lower frequencies in the range of 100 Hz (hertz). The ultrasonic excitation enhances the self-cleaning effect of the screen material and reduces adhesion of the screened material to the screen material.

[0005] In practice, ultrasonic excitation of a sieve is currently only used for smaller fine sieving machines, for example, in the field of analytical chemistry. For fine sieving machines with higher throughput, ultrasound is generally only used to supplement sieve cleaning. However, the use of different vibration excitation systems increases the design complexity and cost of a fine sieving machine.

[0006] It is therefore an object of the present invention to provide a fine screening machine of the type mentioned above, which is also suitable for screening larger material throughputs, in which screening is effectively carried out by ultrasonic excitation of the screen surface. It is a further object to provide an operating method for such a fine screening machine.

[0007] This problem is solved by a fine screening machine or an operating method for such a fine screening machine with the features of the respective independent claim. Advantageous embodiments and further developments are the subject of the dependent claims.

[0008] In an inventive method for operating a fine sieve machine of the type mentioned at the outset, the at least one ultrasonic vibration generator is operated in pulsed mode by repeatedly switching between a first operating mode with a first vibration amplitude and a second operating mode with a second, smaller vibration amplitude of the at least one ultrasonic vibration generator compared to the first.

[0009] In the first operating mode, the larger initial amplitude of the ultrasonic vibration generator causes the individual particles of the material being screened to bounce. This movement allows the material to be moved laterally (e.g., due to an inclination of the screen surface) and also causes it to impact the screen surface at a high frequency. Consequently, the particles of the material have a greater number of opportunities to pass through the mesh of the screen surface. The high frequency of the ultrasonic vibration generation, in the range of a few to several tens of kilohertz, results in an effective screening process.

[0010] During operation of ultrasonic vibration generators, a layer or cloud of a fine fraction of the material being screened can form. Due to internal vibrations during operation, this layer, characterized by a large initial amplitude, remains permanently suspended above the screen surface. Consequently, some of the material particles remain suspended, either continuously or for extended periods compared to the average residence time of particles on the screen surface. As a result, these particles rarely come into contact with the screen surface and cannot pass through it, even if their particle size would otherwise allow them to do so.

[0011] The inventive, regular operation of the at least one ultrasonic vibration generator with the smaller second amplitude causes the cloud of these grains to settle, thus enabling the sieving of these grains as well. The regular reduction of the ultrasonic excitation amplitude therefore leads to an even more effective sieving process and a higher material throughput through the sieve surface.

[0012] The second, smaller vibration amplitude can also be zero. The at least one ultrasonic vibration generator is thus switched back and forth between an active operating phase (first operating mode) and a standby phase (second operating mode).

[0013] Preferably, the first and second operating modes alternate regularly in this process. Furthermore, the at least one ultrasonic vibration exciter is preferably operated in the first operating mode for a first period of time that is shorter than the average residence time of unscreened material particles on the screen surface. This ensures that the material particles experience the second operating mode, in which settling on the screen surface is supported, during their average residence time on the screen surface. A suitable value for the first period of time depends on various parameters of the screening process, including the properties of the material being screened (density, size distribution, etc.). However, it has been shown that in typical fine screening processes, a first period of time between 0.1 s (seconds) and 10 s, and particularly between 0.5 s and 3 s, is advantageous.

[0014] A suitable value for the second time period is advantageously chosen to be at least long enough to allow a large proportion of the suspended particles of the material to settle. Conversely, a second time period that is too long reduces the material throughput. The optimal value for the second time period also depends on the parameters of the sieving process, with advantageous times between 0.01 s and 5 s, and particularly between 0.05 s and 1 s.

[0015] By increasing the efficiency and material throughput of the process according to the invention, it can advantageously be carried out exclusively with ultrasonic vibration generators. In this way, manufacturing, operating, and maintenance costs are reduced compared to a fine screening machine in which ultrasonic vibration generators are used in addition to, for example, unbalanced motors.

[0016] Another advantage is that, due to the increased efficiency of the screening process, screen panels with an essentially square mesh geometry can be used. This applies even when the screen panels are installed at a steep incline relative to a horizontal orientation. In such cases, according to the prior art, screen panels with longitudinal meshes are often used, which are longer in the direction of the incline than in the transverse direction. However, these are significantly more expensive than screen panels with a square mesh geometry and regularly result in a lower screening quality, as elongated particles (so-called "fish") can also pass through the screen panel.

[0017] A fine screening machine according to the invention, comprising at least one screen deck for receiving at least one screen coating and at least one ultrasonic vibration exciter, is characterized in that it is configured to carry out such a process. The advantages mentioned in connection with the process result from this.

[0018] In an advantageous embodiment of the fine sieving machine, two ultrasonic vibration exciters are provided for each sieve layer to excite vibration; these are spaced apart from each other in the longitudinal direction and act on the respective sieve layer.

[0019] Preferably, the vibration of the ultrasonic vibration exciter is transmitted indirectly, for example via a vibration transducer in the form of a frame with a rectangular base. The screen material is then tensioned by an externally arranged clamping device and thereby pressed onto the vibration transducer. This allows transmission to a screen material that is not rigidly connected to a frame, simplifying logistics and storage of the screen materials. Furthermore, in the event of a screen material change, the changeover time is significantly reduced, as the vibration transducer does not need to be bonded to the screen material.

[0020] In a further advantageous embodiment of the fine screening machine, the at least one screen surface has an inclination of at least 20° and preferably at least 30° to a horizontal. Preferably, a screen surface with a substantially square mesh geometry is used.

[0021] A fine screening machine according to the invention can be designed as a single-deck or multi-deck screening machine. Within a screening deck, screening units can be arranged one behind the other, each comprising a screen lining with at least one ultrasonic vibration exciter. At least two of the screen linings of a screening deck can be inclined at different angles in order to achieve, on the one hand, the best possible material transport and, on the other hand, the best possible separation of the different fractions.

[0022] The invention is explained in more detail below with reference to exemplary embodiments and the accompanying figures. The figures show: Fig. 1 a schematic isometric view of a multi-deck fine screening machine; Fig. 2 a schematic isometric view of a single-deck fine screen machine; and Fig. 3 a schematic longitudinal section through a sieve coating with ultrasonic vibration generators.

[0023] Fig. Figure 1 shows a fine screening machine 1 in an isometric schematic representation in a first embodiment. The same reference numerals denote identical or equivalent elements in this and all subsequent figures.

[0024] In the illustrated embodiment, the fine screening machine 1 comprises three screen decks 10 arranged one above the other. The number of three screen decks 10 is exemplary. In alternative embodiments of the fine screening machine 1, the number of screen decks 10 can also be less than or greater than three. A patented operating method can also be used in a single-deck fine screening machine (see [reference]). Fig. 2).

[0025] The screen decks 10 are arranged at an angle to a horizontal orientation, shown here at an angle of approximately 45 degrees. An inlet chute 2 is provided for feeding the material to be screened onto the uppermost screen deck 10. The screened or classified material is discharged to the side or downwards via discharge chutes 3. The upper screen deck 10 is covered by a cover 4 to minimize the ingress of contaminants into the material being screened. The fine screening machine 1 is mounted on a frame (not shown) by means of mounting feet 5 located on the lower screen deck.

[0026] In the Fig. Figure 2 is another example of a fine screening machine 1, a single-deck fine screening machine. Its basic structure corresponds to that shown in Fig. 2 Fine sieve machine 1 shown of the first embodiment.

[0027] At the in Fig. The fine screening machine 1 shown in Figure 2 has a screen deck 10 with side panels 11, between which three screening units 20 are inserted as an example. The screening units 20 are arranged one behind the other in the longitudinal direction of the screen deck 10, so that a flow of material to be screened from the inlet chute 2 passes the screening units 20 one after the other in the Fig. The material passes through the screen deck 10 from the upper right to the lower left. The inclination of the screen units 20 varies, with the first screen unit 20 in the direction of travel of the material being screened having a steeper inclination than the second, and the second in turn having a steeper inclination than the third screen unit 20. Apart from the inclination, the screen units 20 each have the same design. It should be noted that the number of three screen units 20 in the screen deck 10 shown here is purely exemplary. A patented operating procedure can also be implemented with a single screen unit and a screen coating.

[0028] The side panels 11 of the screen deck 10 each have access openings on each side corresponding to the respective screen unit 20. The edges of the access openings are fitted with a circumferential profile seal. Each access opening can be closed with a flap 12.

[0029] In the exemplary embodiment of the Fig. The cover 4, which covers the sieve deck 10 from above, is multi-part, so that each sieve unit 20 can be covered with its own cover 4. In the example shown, only the cover 4 belonging to the first sieve unit 20 is in place. The covers 4 for the other two sieve units 20 are removed to show the structure of the sieve units 20.

[0030] Each of the sieve units 20 has a sieve lining 21 extending over the entire surface of the sieve unit 20. This is in the Fig. Figure 2 shows only the lower of the sieve units 20. For the middle of the sieve units 20, the sieve lining 21 is omitted in order to show the components arranged below the sieve lining 21.

[0031] Excitation transmitters 22 are provided below the screen layer 21; in this embodiment, two excitation transmitters 22 are provided, one for each screen layer 21. In this embodiment, these transmitters are designed as frames with a rectangular base. The corners of the frame are preferably rounded.

[0032] Each of the excitation transmitters 22 is coupled to an ultrasonic vibration generator 23. Vibrations generated by the ultrasonic generator in the ultrasonic range between a few and several tens of kilohertz are transmitted to the excitation transmitters 22 and from there to the screen material 21. The number of excitation transmitters 22 and, correspondingly, two ultrasonic vibration generators 23 per screen material 21 can be varied in alternative configurations of the fine screening machine. Any number of ultrasonic vibration generators 23 per excitation transmitter 22 is possible. One or more excitation transmitters 22 can be used per screen material 21. The number of excitation transmitters 22 and ultrasonic vibration generators 23 depends on the total sound power required to be transmitted to the screen material and the material being screened, as well as on the size of the screen material 21.Furthermore, the sound energy should be distributed as homogeneously as possible across the surface of the screen covering 21. Areas with insufficient sound excitation can lead to undesirable material accumulation.

[0033] Each sieve unit 20 also has a receiving and clamping device with which the sieve lining 21 is attached and clamped within the sieve unit 20. This receiving and clamping device is located in the Fig. 2 not marked separately.

[0034] In the Fig. Figure 3 shows a longitudinal section through the screen material 21 of a screening unit 20. This figure illustrates that the screen material 21 is tensioned longitudinally between a receiving strip 24, to which it is connected via the material flow, and a tensioning strip 25, into which it is hooked. The receiving strip 24 and the tensioning strip 25 are part of the aforementioned receiving and tensioning device. In the illustrated example, a screen material 21 with the attached receiving strip 24 is inserted into the fine screening machine 1. In this example, however, the screen material 21 is not mounted on a separate frame, but rather obtains its shape and stability through the tensioning process. Preferably, sealing strips are arranged on the screen deck 10, for example, on the side panels 11, which slightly overlap the screen material 21 at the edges to prevent material from passing laterally past the screen material 21 and downwards through the screen plane.The sealing strip mentioned above can be in two parts and can clamp the screen covering in sections or along its entire length.

[0035] Below the sieve layer 21, the two are already from the area associated with Fig. The excitation transmitters 22 mentioned above are arranged. These are positioned so that the sieve lining 21 presses on the excitation transmitters 22 after tensioning, thus establishing good mechanical contact between the excitation transmitter 22 and the sieve lining 21, which leads to effective ultrasound transmission from the excitation transmitters 22 to the sieve lining 21.

[0036] Each excitation transmitter 22 is coupled to an ultrasonic vibration generator 23. The ultrasonic vibration generator 23 can be a piezoelectric transducer or an electromagnetically operating transducer.

[0037] Each of the ultrasonic vibration generators 23 is connected to an ultrasonic generator 31, which supplies the electrical signal necessary to operate the ultrasonic vibration generator 23. In an alternative embodiment, it is possible to operate both ultrasonic vibration generators 23 using only a single, common generator 31. The generators 31 are shown below the ultrasonic exciters 23 for illustrative and schematic purposes only. In practice, they are located outside the fine screening machine 1 in a corresponding control arrangement for the fine screening machine 1.

[0038] In the case of a registered operating procedure for a fine screening machine, for example for those associated with the Fig. In the fine sieve machine 1 described in 1-3, the at least one ultrasonic vibration generator 23 is not operated continuously, but pulsed.

[0039] At the in Fig.In the arrangement shown in Figure 3, this is achieved by a control unit 30, which is superior to the generators 31 and outputs a corresponding pulse signal for pulsed operation of the generators 31. The control unit 30 can be configured externally or internally within the ultrasonic generators 31, as shown.

[0040] The pulsed operation of the ultrasonic generators 31 causes the ultrasonic vibration generators 23 to operate for a specific initial duration, which can range from a few tenths of a second to several seconds. This is followed by a pause of a second duration, which can range from a few hundredths of a second to several seconds. This second duration is subsequently referred to as the pause time. The operating modes – active operation and pause – alternate regularly and repeatedly.

[0041] The active operation of the ultrasonic vibration generators 23 causes the individual grains of the material being screened to vibrate and / or bounce. This movement causes the material to move laterally and to impact the screen lining 21 at a high frequency, thus increasing the likelihood of passing through a mesh of the screen lining 21. The high frequency of the vibration excitation results in an effective screening process.

[0042] During active operation of the ultrasonic vibration generators 23, a layer or cloud of a very fine fraction of the material being screened can form, which, due to internal vibrations during the activation time of the ultrasonic generator 23, is almost permanently located above the screen lining 21. Because the particles of the material being screened are suspended, they do not come into contact with the screen lining, or only rarely, and therefore cannot pass through it, even if their particle size would require them to do so.

[0043] The regular interruption of the active operation of the ultrasonic vibration generators 23 according to the invention causes the cloud of these grains to settle, thus enabling the sieving of these particles as well. The regular switching off (or operation at a lower amplitude) of the ultrasonic vibration generators 23 therefore leads to an even more effective sieving process and a higher material throughput through the sieve lining 21.

[0044] The on and off times of the ultrasonic vibration generators 23 should be selected based on the material being screened and its particle size distribution, such that the material experiences at least one, but preferably several, pauses within the average time it spends within the area of ​​a screen coating 21. The pause time itself should be long enough to allow the material to settle due to gravity. In practical operation, both on and off times in the range of a few hundred milliseconds to a few seconds have proven particularly advantageous.

[0045] The increased screening throughput achieved by the patented method advantageously allows the use of screen linings 21 at a relatively steep inclination of 10°, 20°, or more, even in fine screening machines, and enables the use of screen linings 21 with a square mesh geometry. To achieve sufficiently high screening throughputs without the patented pulse method, linings with elongated meshes have been used, particularly in fine screening machines with steeply inclined screen linings. In these linings, the mesh dimensions are larger in the longitudinal direction than in the transverse direction. However, these screen linings are comparatively expensive and have a shorter service life compared to those with square mesh dimensions. Reference symbol list 1 fine sieve machine 2 inlet chutes 3 discharge shafts 4 Cover 5 Mounting foot 10 screen deck 11 Side panel 12 Access hatch 20 sieve units 21 sieve coating 22 Excitation transmitters 23 Ultrasonic vibration generators 24 recording strip 25 tension strip 30 Control unit 31 Ultrasonic generator

Claims

[1] Method for operating a fine screening machine (1) with at least one screen deck (10) for receiving at least one screen lining (21) and at least one ultrasonic vibration exciter (23) for generating ultrasonic vibrations which are transmitted to the screen lining (21), characterized by , that the at least one ultrasonic vibration exciter (23) is operated in pulsed mode, wherein the ultrasonic vibration exciter (23) repeatedly switches between a first operating mode with a first vibration amplitude and a second operating mode with a second vibration amplitude that is smaller than the first. [2] Method according to claim 1, wherein the second smaller vibration amplitude is zero. [3] Method according to claim 1 or 2, wherein the first and second operating modes alternate regularly. [4] Method according to one of claims 1 to 3, wherein the at least one ultrasonic vibration exciter (23) is operated in the first operating mode for a first period of time which is shorter than the mean residence time of unscreened particles on the screen surface. [5] Method according to claim 4, wherein the first time duration is between 0.1 s and 10 s and preferably between 0.5 s and 3 s. [6] Method according to any one of claims 1 to 5, wherein the at least one ultrasonic vibration exciter (23) is operated in the second operating mode for a second duration which is between 0.01 s and 5 s and preferably between 0.05 s and 1 s. [7] Method according to any one of claims 1 to 6, wherein the sieve coating (21) is excited exclusively by ultrasound. [8] Method according to any one of claims 1 to 6, wherein the sieve lining (21) has a substantially square mesh geometry. [9] Fine screening machine (1) with at least one screen deck (10) for receiving at least one screen lining (21) and at least one ultrasonic vibration exciter (23), characterized by , that the fine sieving machine (1) is set up to carry out a method according to one of the preceding claims. [10] Fine sieve machine (1) according to claim 9, in which two ultrasonic vibration exciters (23) are provided for each sieve layer (21) for vibration excitation, the which act on the respective sieve layer (21) at longitudinal intervals from each other. [11] Fine sieving machine (1) according to claim 9 or 10, wherein the at least one sieve layer (21) has an inclination of at least 10° and preferably at least 30° relative to a horizontal. [12] Fine sieving machine (1) according to one of claims 9 to 11, wherein the at least one sieve lining (21) has a substantially square mesh geometry. [13] Fine sieving machine (1) according to one of claims 9 to 12, comprising at least two sieve units (20) arranged one behind the other in the longitudinal direction within a sieve deck (10), each comprising a sieve lining (21) with at least one ultrasonic vibration exciter (23). [14] Fine sieving machine (1) according to claim 13, in which at least two of the sieve linings (21) of a sieve deck (10) are inclined to different degrees.

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