How to operate a screw press and a screw press

Abstract

To provide a screw press operation method where steady operation can be quickly attained by avoiding corotation between a plug cake and a second screw.SOLUTION: In a screw press operation method, a low differential velocity operation and a differential velocity adjustment operation are implemented before starting steady operation that a first screw 3 and a second screw 4 are rotated at a predetermined rotation speed. The low differential velocity operation is an operation to lower the hardness of a plug cake in a plug cake formation region 1B in which the second screw 4 is disposed to hardness where the plug cake does not corotate with the second screw 4. The differential velocity adjustment operation is an operation to gradually lower the rotation speed of the second screw 4 at the low differential velocity operation to the rotation speed of the second screw 4 at the steady operation.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for operating a screw press that compresses a liquid-containing material such as sludge to separate liquid from the liquid-containing material, and to the screw press. [Background technology]

[0002] Screw presses have been known as devices for compressing sludge (liquid-containing material) discharged from liquid treatment facilities such as water supply and sewage treatment plants and sewage treatment plants to separate water from the sludge (i.e., dewater it). This screw press includes a filter cylinder formed from a screen (perforated plate) and a screw disposed inside the filter cylinder. The screw has a screw shaft concentrically disposed with the filter cylinder and screw blades fixed to the outer surface of the screw shaft. A rotation mechanism connected to the screw shaft rotates the screw blades to compress and dewater the sludge introduced into the filter cylinder. A backpressure plate is disposed at the downstream open end of the filter cylinder to block the sludge. This backpressure plate retains the cake (dewatered sludge) conveyed by the rotating screw blades, forming a plug of cake. This plug applies backpressure to the cake conveyed later, further compressing the cake and reducing the moisture content of the sludge in the filter cylinder.

[0003] Patent Document 1 describes a screw press having a first screw and a second screw arranged in series inside a filter cylinder. The second screw of this screw press is configured to be rotated by a drive device different from the drive device for the first screw, allowing the first screw and the second screw to rotate at different speeds and in any direction. Therefore, by individually controlling the rotation speeds and directions of the first screw and the second screw, a plug cake can be formed inside the filter cylinder without providing a back pressure plate, and further, the sludge inside the filter cylinder can be efficiently squeezed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-51582 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as a result of extensive research by the inventors, it was found that even with the screw press described in Patent Document 1, depending on the properties of the sludge fed into the screw press, co-rotation may occur between the second screw and the cake (plug cake) in the plug cake formation region where the second screw is located during startup of the device. It was found that the risk of co-rotation between the plug cake and the second screw increases particularly when sludge with a low fibrous content is fed into the screw press and / or when a strong back pressure is applied all at once to the sludge fed into the screw press. Note that, in this specification, the "start-up operation of the device" refers to the operation of the screw press to form a plug cake with a desired moisture content in the plug formation region. If co-rotation occurs between the plug cake and the second screw during startup of the device, it takes a long time to transition to steady-state operation in which the screw press discharges a plug cake with the desired moisture content.

[0006] On the other hand, if a weak back pressure is applied to the sludge fed into the screw press during startup, the moisture content of the sludge can be reduced relatively slowly, preventing co-rotation between the plug cake and the second screw. However, even in this case, it takes a long time from startup to steady-state operation.

[0007] Therefore, an object of the present invention is to provide a method for operating a screw press that can quickly achieve steady-state operation by avoiding co-rotation of the plug cake and the second screw, and to provide a screw press that can implement such an operating method. [Means for solving the problem]

[0008] In one aspect, there is provided a method for operating a screw press including a filter cylinder into which a liquid-containing material is introduced, a first screw and a second screw that are arranged concentrically with the filter cylinder within the filter cylinder and transport the liquid-containing material in a predetermined transport direction, a first rotation mechanism that rotates the first screw, and a second rotation mechanism that rotates the second screw independently of the first screw, wherein before starting steady-state operation in which the first screw and the second screw rotate at predetermined rotational speeds, a low differential speed operation and a differential speed adjustment operation are performed, the low differential speed operation is an operation that reduces the hardness of the plug cake in a plug cake formation region where the second screw is located to a hardness at which the plug cake does not rotate together with the second screw, and the differential speed adjustment operation is an operation that gradually reduces the rotational speed of the second screw during the low differential speed operation to the rotational speed of the second screw during the steady-state operation.

[0009] In one embodiment, before the low differential speed operation, the pressure of the plug cake in the plug formation region is confirmed, and if the pressure of the plug cake is lower than a predetermined threshold, a sludge filling operation is carried out. The sludge filling operation is an operation in which only the first screw is rotated at a predetermined rotational speed without rotating the second screw, or the first screw is rotated at the predetermined rotational speed while rotating the second screw at a rotational speed lower than the rotational speed during the low differential speed operation, until the pressure of the plug cake exceeds the threshold. In one embodiment, the rotation speed of the second screw during the low differential speed operation is in the range of 0.6 to 0.9 times the rotation speed of the first screw during the steady operation. In one embodiment, the operation time of the low differential speed operation is in the range of 1 to 20 minutes.

[0010] In one embodiment, the number of stages by which the rotation speed of the second screw is reduced from the rotation speed during the low differential speed operation to the rotation speed of the second screw during the steady operation during the differential speed adjustment operation is in the range of 3 to 10. In one embodiment, the operation time for each stage of the differential speed adjustment operation is in the range of 1 to 20 minutes.

[0011] In one aspect, there is provided a screw press comprising: a filter cylinder into which a liquid-containing material is introduced; a first screw and a second screw that are arranged concentrically with the filter cylinder within the filter cylinder and transport the liquid-containing material in a predetermined transport direction; a first rotation mechanism that rotates the first screw; a second rotation mechanism that rotates the second screw independently of the first screw; and a control unit that controls the operation of the first rotation mechanism and the second rotation mechanism, wherein the control unit performs a low differential speed operation and a differential speed adjustment operation before starting a steady operation in which the first screw and the second screw rotate at a predetermined rotation speed, wherein the low differential speed operation is an operation that reduces the hardness of a plug cake in a plug cake formation region where the second screw is located to a hardness at which the plug cake does not rotate together with the second screw, and the differential speed adjustment operation is an operation that gradually reduces the rotational speed of the second screw during the low differential speed operation to the rotational speed of the second screw during the steady operation. [Effects of the Invention]

[0012] According to the present invention, in the differential speed adjustment process, the rotational speed of the second screw is gradually reduced to the target rotational speed, which is the rotational speed of the second screw during steady operation, thereby gradually reducing the hardness of the entire plug cake present between the spirally extending second screw flights. As a result, co-rotation of the plug cake and the second screw 4 can be effectively prevented. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing a screw press according to one embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view of the second screw shown in FIG. [Figure 3] FIG. 3 is a schematic diagram for explaining the number of turns of the second screw flight. [Figure 4] FIG. 4 is a flowchart showing a method of operating a screw press according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing a screw press according to one embodiment. The screw press shown in FIG. 1 includes a cylindrical screen casing (filter tube) 1, a first screw 3 and a second screw 4 arranged concentrically within the screen casing 1 and transporting sludge (liquid-containing material) in a predetermined transport direction D, a first rotation mechanism 7 for rotating the first screw 3, and a second rotation mechanism 20 for rotating the second screw 4 independently of the first screw 3. The screen casing 1 is formed of a screen (perforated plate) such as a punched metal. A sludge inlet 2 is formed at the upstream end of the screen casing 1. The sludge introduced into the screen casing 1 through the inlet 2 is transported in the predetermined transport direction D within the screen casing 1 by the rotating first screw 3 and second screw 4. The screw press further includes a control unit 6 for controlling the operation of the first rotation mechanism 7 and the second rotation mechanism 20.

[0015] The second screw 4 is connected to the first screw 3 so as to be rotatable independently of the first screw 3. The first screw 3 and the second screw 4 extend through the screen casing 1 and the discharge chamber 33, respectively. The discharge chamber 33 is connected to the screen casing 1. A plug cake, described later, is discharged from the screen casing 1 into this discharge chamber 33. The axial length of the second screw 4 is shorter than the axial length of the first screw 3. The first screw 3 has a first screw shaft 3A with a truncated cone shape (tapered shape) whose diameter gradually increases downstream in the sludge transfer direction D, and a first screw blade 3B fixed to the outer surface of the first screw shaft 3A. The second screw 4 has a cylindrical second screw shaft 4A and a second screw blade 4B fixed to the outer surface of the second screw shaft 4A.

[0016] The upstream end of the screen casing 1 is sealed by a blocking wall 8. One end of the first screw shaft 3A (the upstream end in the transfer direction D) extends through this blocking wall 8. A water sealing device 10 is installed on this blocking wall 8 to seal the gap between the blocking wall 8 and the first screw shaft 3A. The upstream end of the first screw shaft 3A extending through the blocking wall 8 is rotatably supported by bearings 11 and 12 installed on a base (not shown) while being restricted from moving in the axial direction. Note that one of the bearings 11 and 12 may be omitted.

[0017] The upstream end of the first screw shaft 3A is connected to a first rotation mechanism 7 for rotating the first screw 3. In this embodiment, the first rotation mechanism 7 includes a first driver (e.g., an electric motor) 14, a sprocket 15 fixed to the rotation shaft of the first driver 14, a sprocket 16 fixed to the first screw shaft 3A, and a chain 17 wound around these sprockets 15 and 16. The sprocket 16 is located between the bearings 11 and 12. When the first driver 14 of the first rotation mechanism 7 is driven, the sprocket 15 fixed to the rotation shaft of the first driver 14 rotates, which in turn rotates the sprocket 16 fixed to the first screw shaft 3A via the chain 17. As a result, the first screw 3 is rotated by the first rotation mechanism 7. The first driver 14 is connected to a control unit 6, which is configured to control the operation of the first driver 14.

[0018] Although not shown, the rotation shaft of the first driving device 14 may be connected to the first screw shaft 3A via a reducer, or may be directly connected to the first screw shaft 3A.

[0019] FIG. 2 is a schematic cross-sectional view of the second screw 4 shown in FIG. 1. As shown in FIG. 2, the second screw shaft 4A has a hollow structure. A reduced-diameter portion 3C is formed at the other end (the downstream end in the transfer direction D) of the first screw shaft 3A, and is inserted into the cylindrical second screw shaft 4A. By forming the reduced-diameter portion 3C on the first screw shaft 3A, a wall surface 3D perpendicular to the axial direction of the first screw shaft 3A is formed on the first screw shaft 3A. The reduced-diameter portion 3C is inserted into the cylindrical second screw shaft 4A and is rotatably supported by slide bearings 30 and 31 fixed to the inner wall 4C of the second screw shaft 4A. With this configuration, the first screw 3 is rotatably connected to the second screw 4.

[0020] 2, with the reduced diameter portion 3C of the first screw shaft 3A inserted inside the second screw shaft 4A, the upstream end of the second screw shaft 4A is in contact with the wall surface 3D of the first screw shaft 3A. In one embodiment, a small gap may be formed between the upstream end of the second screw shaft 4A and the wall surface 3D of the first screw shaft 3A. In this case, the plain bearing 30 and / or the reduced diameter portion 3C may be provided with a seal structure (e.g., a labyrinth structure) that prevents sludge in the screen casing 1 from passing through the gap between the plain bearing 30 and the reduced diameter portion 3C.

[0021] As shown in Figures 1 and 2, the second screw shaft 4A of the second screw 4 is arranged concentrically with the first screw shaft 3A. The outer diameter of the second screw shaft 4A is the same as the maximum diameter of the first screw shaft 3A. The second screw shaft 4A extends through the wall 33A of the discharge chamber 33. The upstream end of the second screw shaft 4A is rotatably supported by the first screw shaft 3A via the sliding bearings 30 and 31, while the downstream end of the second screw shaft 4A is rotatably supported by bearings 22 and 23 installed on a base (not shown) while being restricted from moving axially. The bearing 23 may be omitted.

[0022] The downstream end of the second screw shaft 4A is connected to a second rotation mechanism 20 for rotating the second screw 4. In this embodiment, the second rotation mechanism 20 includes a second driver (e.g., an electric motor) 24, a sprocket 25 fixed to the rotation shaft of the second driver 24, a sprocket 26 fixed to the second screw shaft 4A, and a chain 27 wound around these sprockets 25, 26. The sprocket 26 is located between bearings 22, 23. When the second driver 24 of the second rotation mechanism 20 is driven, the sprocket 25 fixed to the rotation shaft of the second driver 24 rotates, which in turn rotates the sprocket 26 fixed to the second screw shaft 4A via the chain 27. As a result, the second screw 4 is rotated by the second rotation mechanism 20.

[0023] The second driver 24 is connected to the control unit 6. The second driver 24 has a built-in inverter (not shown), and the control unit 6 is configured to be able to control the operation of the second driver 24 via the inverter. That is, the control unit 6 can control the rotation speed and rotation direction of the second driver 24 via the inverter. The second driver 24 can rotate the second screw 4 independently of the first screw 3. It is preferable that the first driver 14 also has a built-in inverter that can change the rotation speed and rotation direction of the first driver 14.

[0024] Although not shown, the rotation shaft of the second driving device 24 may be connected to the second screw shaft 4A via a reducer, or may be directly connected to the second screw shaft 4A.

[0025] The first screw blade 3B extends spirally along the axial direction of the first screw shaft 3A, and the second screw blade 4B extends spirally along the axial direction of the second screw shaft 4A. The combined length of the portion of the first screw 3 to which the first screw blade 3B is fixed and the portion of the second screw 4 to which the second screw blade 4B is fixed is equal to or longer than the axial length of the screen casing 1.

[0026] A small gap is formed between the inner surface of the screen casing 1 and the first screw blade 3B, allowing the first screw blade 3B to rotate without coming into contact with the screen casing 1. Similarly, a small gap is formed between the inner surface of the screen casing 1 and the second screw blade 4B, allowing the second screw blade 4B to rotate without coming into contact with the screen casing 1. Sludge introduced into the screen casing 1 from an inlet 2 formed at the upstream end of the screen casing 1 can be transported by the rotating first screw blade 3B and second screw blade 4B toward the discharge chamber 33 (i.e., in the transport direction D).

[0027] In this embodiment, the winding direction (i.e., the spiral direction) of the second screw blade 4B is opposite to that of the first screw blade 3B. Therefore, when sending the sludge introduced from the inlet 2 to the discharge chamber 33, as shown in FIG. 1, the second screw 4 is rotated in the opposite direction to the first screw 3.

[0028] The winding direction of the second screw blade 4B may be the same as that of the first screw blade 3B. In this case, when sending the sludge introduced from the inlet 2 to the discharge chamber 33, the second screw 4 is rotated in the same direction as the first screw 3.

[0029] As shown in FIG. 2, the pitch P1 of the second screw blade 4B is smaller than the pitch P2 of the first screw blade 3B (i.e., P1 < P2). Further, the number of turns of the second screw blade 4B is less than 3 turns. FIG. 3 is a schematic diagram for explaining the number of turns of the second screw blade 4B. As shown in FIG. 3, the number of turns of the second screw blade 4B extending spirally is counted as 1 turn when the second screw blade 4B advances 360° around the first screw shaft 3A from the starting point S1 to the point S2. In the second screw 4 shown in FIG. 3, the number of turns of the second screw blade 4B is 2 turns.

[0030] As shown in FIG. 1, the screen casing 1 is divided into a dewatering region 1A in which the first screw 3 is disposed and a plug-forming region 1B in which the second screw 4 is disposed. The space through which the sludge is transported in the dewatering region 1A is formed by the inner surface of the screen casing 1, the first screw blade 3B, and the first screw shaft 3A. The cross-sectional area of ​​this transport space gradually decreases along the sludge transport direction D, as shown in FIG. 1. Therefore, as the sludge introduced through the inlet 2 is transported through this transport space by the first screw blade 3B, the sludge is squeezed and dewatered. The filtrate that passes through the screen (perforated plate) of the screen casing 1 is collected in a filtrate receiver 38 disposed below the screen casing 1. A drain 39 is connected to the filtrate receiver 38, and the filtrate collected by the filtrate receiver 38 is discharged from the screw press via the drain 39.

[0031] The space through which the sludge is transferred in the plug-forming region 1B is formed by the inner surface of the screen casing 1, the second screw blade 4B, and the second screw shaft 4A. As shown in FIG. 1, the cross-sectional area of ​​this transfer space is constant. In the plug-forming region 1B, a plug cake is formed from the sludge (i.e., cake) dewatered in the dewatering region 1A. The method for forming the plug cake will be described later.

[0032] As shown in FIG. 1, the screw press has a pressure sensor 29 disposed in the plug-forming region 1B. The pressure sensor 29 is a sensor for measuring the pressure of the sludge in the plug-forming region 1B, i.e., the pressure of the (plug) cake. The pressure sensor 29 is connected to the control unit 6 via a signal line (not shown) and is configured to be able to transmit its measured value to the control unit 6. The control unit 6 can monitor the pressure of the (plug) cake in the plug-forming region 1B based on the measured value of the pressure sensor 29.

[0033] Next, an example of a method for operating the screw press shown in Fig. 1 will be described. Fig. 4 is a flowchart showing a method for operating the screw press according to one embodiment. More specifically, Fig. 4 is a flowchart mainly showing the start-up operation of the screw press shown in Fig. 1.

[0034] 4, the control unit 6 first obtains the pressure of the cake (or sludge) in the plug formation region 1B using the pressure sensor 29 and determines whether the cake pressure is equal to or greater than a predetermined threshold value (S101). The predetermined threshold value is a value that is set in advance depending on the properties of the sludge fed into the screw press, and is set to, for example, 5 kPa, in order to determine whether a sufficient amount of sludge is loaded into the entire plug formation region 1B to produce a plug cake. This predetermined threshold value is stored in advance in the control unit 6.

[0035] If the measurement value of pressure sensor 29 is smaller than the predetermined threshold value ("No" in S101), control unit 6 determines that sludge has not been sufficiently filled in plug formation region 1B, and executes a sludge filling operation process (S102). The sludge filling operation process is a process of filling sludge into plug formation region 1B. In the sludge filling operation process, control unit 6 rotates only first screw 3 at a predetermined rotational speed without rotating second screw 4 until the measurement value of pressure sensor 29 exceeds the predetermined threshold value. This operation fills sludge into plug formation region 1B.

[0036] In one embodiment, the control unit 6 may rotate the second screw 4 at a relatively low speed during the sludge filling operation step. For example, the control unit 6 may rotate the second screw 4 at a rotational speed lower than the rotational speed during low differential speed operation, which will be described later, during the sludge filling step. In this case, the control unit 6 also rotates the first screw 3 at a predetermined rotational speed set for the sludge filling step.

[0037] If the measurement value of the pressure sensor 29 is equal to or greater than a predetermined threshold value ("Yes" in S101), or if the sludge filling operation step is completed, the control unit 6 carries out a low differential speed operation step (S102). The low differential speed operation step is a step of reducing the hardness (or moisture content) of the plug cake formed in the plug cake formation region 1B to a level at which the plug cake does not rotate together with the second screw 4, and is a step of operating the screw press while applying a weak back pressure to the plug cake. Therefore, the low differential speed operation step may also be referred to as a weak back pressure operation step.

[0038] During low differential speed operation, the rotational speed of the second screw 4 is adjusted so that the plug cake in the plug cake formation region 1B is not over-dewatered. Specifically, the control unit 6 adjusts the rotational speed of the second screw 4 to a range of 0.6 to 0.9 times the rotational speed of the first screw 3 at the start of steady-state operation, which will be described later. This operation allows the plug cake in the plug cake formation region 1B to maintain a moisture content that is lower than that during the sludge loading operation step, but that is still somewhat high.

[0039] The rotational speed of the second screw 4 during low differential speed operation is predetermined according to the properties of the sludge supplied to the screw press and is pre-stored in the control unit 6. For example, the rotational speed of the second screw 4 during low differential speed operation is adjusted within a range of 0.6 to 0.9 times, preferably 0.65 to 0.8 times, the rotational speed of the first screw 3 at the start of steady operation so that the moisture content of the plug cake in the plug cake formation region 1B is maintained at 80% or higher, preferably in the range of 81 to 84%. If the rotational speed of the second screw 4 is too fast, it will take a long time for the plug cake to reach the desired hardness, and if the rotational speed of the second screw 4 is too slow, the plug cake may become too hard.

[0040] The low differential speed operation is performed for a predetermined time. The operation time (duration) of this low differential speed operation is stored in advance in the control unit 6. If the operation time of the low differential speed operation is extended, the plug cake can be made to have a desired hardness, but the time required for the screw press to start steady-state operation will be extended. On the other hand, if the operation time of the low differential speed operation is shortened, there is a risk that the plug cake will not reach a hardness suitable for starting the differential speed adjustment process described below.

[0041] Therefore, the operation time of the low differential speed operation is set in the range of 1 to 20 minutes, preferably in the range of 5 to 10 minutes, based on experiments and / or simulations conducted depending on the properties of the sludge supplied to the screw press. Alternatively, the operation time of the low differential speed operation may be set by the rotation speed of the second screw 4 based on the rotation speed of the second screw 4, the length of the plug cake forming region 1B, the number of turns of the second screw flight 4B, and the pitch of the second screw flight 4B. For example, the low differential speed operation may be performed for 1 to 5 rotations, preferably 1.5 to 3 rotations, of the second screw 4.

[0042] When the low differential speed operation is completed, the control unit 6 carries out a differential speed adjustment operation process (S104). The differential speed adjustment operation process is a process in which the screw press is operated while gradually reducing the rotational speed of the second screw 4 during low differential speed operation to a target rotational speed, which is the rotational speed of the second screw 4 during steady operation. In this differential speed adjustment process, the back pressure applied to the plug cake is gradually increased as the rotational speed of the second screw 4 is gradually reduced in stages. Therefore, the low differential speed operation process may also be referred to as a back pressure adjustment process.

[0043] If the rotational speed of the second screw 4 during low differential speed operation were to be reduced to the target rotational speed all at once, the back pressure on the plug cake in contact with the second screw flight 4B would increase suddenly, resulting in only the plug cake in contact with the second screw flight 4B becoming hard. This would increase the risk of the second screw flight 4B and the harder plug cake co-rotating with the softer plug cake present between the spirally extending second screw flight 4B. Therefore, in the differential speed adjustment process, the rotational speed of the second screw 4 is gradually reduced to the target rotational speed, gradually reducing the hardness of the entire plug cake present between the spirally extending second screw flight 4B. This operation effectively prevents the plug cake from co-rotating with the second screw 4.

[0044] If co-rotation occurs between the plug cake and the second screw 4, it takes time to reduce the moisture content of the plug cake discharged from the plug-forming region 1B to the desired value. According to the method for operating a screw press according to this embodiment, co-rotation between the plug cake and the second screw 4 can be effectively prevented, and the operating state of the screw press can be quickly brought to a steady state.

[0045] In the differential speed adjustment operation step, the number of stages by which the rotation speed of the second screw 4 is reduced from the rotation speed during low differential speed operation to the target rotation speed (hereinafter simply referred to as the "number of stages"), the operation time of each stage, and the reduction amount of the rotation speed of the second screw 4 when moving to the next stage (hereinafter simply referred to as the "reduction amount") are set in advance based on the properties of the sludge supplied to the screw press, the difference between the rotation speed during low differential speed operation and the target rotation speed, the operation time at each stage, etc. The number of stages, the operation time at each stage, and the reduction amount are stored in advance in the control unit 6.

[0046] As a result of extensive research by the inventors into the differential speed adjustment operation process, they found that setting the number of stages to three or more effectively prevents the occurrence of co-rotation between the plug cake and the second screw 4. Furthermore, they found that setting the operation time for each stage to one minute or more effectively prevents the occurrence of co-rotation between the plug cake and the second screw 4. On the other hand, the more the number of stages and the longer the operation time for each stage are increased, the more effectively the occurrence of co-rotation between the plug cake and the second screw 4 can be prevented, but it takes longer to bring the screw press into a steady state. Therefore, it is preferable to set the number of stages in the range of 3 to 10, preferably in the range of 5 to 8, and to set the operation time for each stage in the range of 1 to 20 minutes, preferably in the range of 3 to 10 minutes.

[0047] The amount of reduction in the rotational speed of the second screw 4 when moving to the next stage may be the same or different. Similarly, the operation time of each stage may be the same or different.

[0048] When the differential speed adjustment operation step is completed, the control unit 6 starts steady operation (S105). In steady operation, the rotational speeds of the first screw 3 and the second screw 4 are set so that a plug cake whose moisture content has been reduced to a desired value or less is discharged from the plug formation region 1B.

[0049] In one embodiment, after the operating state of the screw press transitions to steady operation, the control unit 6 may adjust the amount of plug cake discharged into the discharge chamber 33 by changing the rotational speed of the second screw 4. More specifically, the control unit 6 may decrease the rotational speed of the second screw 4 to decrease the amount of plug cake discharged, or the control unit 6 may increase the rotational speed of the second screw 4 to increase the amount of plug cake discharged. When the amount of plug cake discharged decreases, subsequent cakes remain in the dewatering region 1A, and the back pressure applied to the subsequent cakes increases. Therefore, by decreasing the rotational speed of the second screw 4, the control unit 6 can decrease the moisture content of the subsequent cakes. In one embodiment, the back pressure applied to the subsequent cakes may be adjusted by performing intermittent operation in which the second screw 4 is alternately rotated and stopped.

[0050] The above-described embodiments have been described for the purpose of enabling a person of ordinary skill in the art to practice the present invention. Various modifications of the above-described embodiments would naturally be possible for a person skilled in the art, and the technical concept of the present invention may also be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is to be interpreted in the broadest scope in accordance with the technical concept defined by the claims. [Explanation of symbols]

[0051] 1 Screen casing (filter tube) 1A Dehydration area 1B Plug formation region 2 Inlet 3 First screw 3A First screw shaft 3B First screw blade 3C Reduced diameter part 3D wall 4 Second screw 4A Second screw shaft 4B No. 2 screw blade 4C Inner wall 6 Control Unit 7 First rotation mechanism 8 Blocking wall 10 Water sealing device 11,12 Bearings 14 First Drive Unit 15,16 sprocket 17 Chain 20 Second rotation mechanism 22,23 Bearings 24 Second Drive 25,26 sprocket 27 Chain 29 Pressure Sensor 30,31 Plain bearings 33 Exhaust chamber 36 Bearings 38 Filtrate receiver 39 Drain

Claims

1. A method for operating a screw press comprising: a filter cylinder into which a liquid-containing material is introduced; a first screw and a second screw that are arranged concentrically with the filter cylinder within the filter cylinder and transport the liquid-containing material in a predetermined transport direction; a first rotation mechanism that rotates the first screw; and a second rotation mechanism that rotates the second screw independently of the first screw, Before starting a steady operation in which the first screw and the second screw are rotated at a predetermined rotation speed, a low differential speed operation and a differential speed adjustment operation are performed; the low differential speed operation is an operation for reducing the hardness of the plug cake in a plug cake formation region where the second screw is disposed to a hardness at which the plug cake does not rotate together with the second screw, and is an operation in which the rotational speed of the second screw is adjusted within a range of 0.6 to 0.9 times the rotational speed of the first screw at the start of the steady operation so that the moisture content of the plug cake in the plug cake formation region is maintained at 80% or more; The differential speed adjustment operation is an operation in which the rotation speed of the second screw during the low differential speed operation is gradually reduced to the rotation speed of the second screw during the steady operation.

2. Before the low differential speed operation, the pressure of the plug cake in the plug formation region is confirmed; When the pressure of the plug cake is lower than a predetermined threshold, a sludge filling operation is performed; 2. The method for operating a screw press according to claim 1, wherein the sludge filling operation is an operation in which only the first screw is rotated at a predetermined rotational speed without rotating the second screw until the pressure of the plug cake exceeds the threshold value, or an operation in which the first screw is rotated at the predetermined rotational speed while rotating the second screw at a rotational speed lower than the rotational speed during the low differential speed operation.

3. 2. The method for operating a screw press according to claim 1, wherein the operation time of the low differential speed operation is in the range of 1 to 20 minutes.

4. 2. The method for operating a screw press according to claim 1, wherein the number of stages by which the rotational speed of the second screw is reduced from the rotational speed during the low differential speed operation to the rotational speed of the second screw during the steady operation in the differential speed adjustment operation is in the range of 3 to 10.

5. 5. The method for operating a screw press according to claim 4, wherein the operation time for each stage of the differential speed adjustment operation is in the range of 1 to 20 minutes.

6. a filter cylinder into which the liquid content is introduced; a first screw and a second screw arranged concentrically with the filter cylinder within the filter cylinder, the first screw and the second screw transporting the liquid-containing material in a predetermined transport direction; a first rotation mechanism that rotates the first screw; a second rotation mechanism that rotates the second screw independently of the first screw; a control unit that controls operations of the first rotation mechanism and the second rotation mechanism, the control unit performs a low differential speed operation and a differential speed adjustment operation before starting a steady operation in which the first screw and the second screw are rotated at a predetermined rotation speed, the low differential speed operation is an operation for reducing the hardness of the plug cake in a plug cake formation region where the second screw is disposed to a hardness at which the plug cake does not rotate together with the second screw, and is an operation in which the rotational speed of the second screw is adjusted within a range of 0.6 to 0.9 times the rotational speed of the first screw at the start of the steady operation so that the moisture content of the plug cake in the plug cake formation region is maintained at 80% or more; The differential speed adjustment operation is an operation in which the rotation speed of the second screw during the low differential speed operation is gradually reduced to the rotation speed of the second screw during the steady operation.

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