Decanter centrifuge

Abstract: A decanter centrifuge used for liquid-solids separation of a feed solution containing suspended solids by means of a centrifugal force. The decanter centrifuge is characterized by at least one dip weir fixed to the external periphery of the wall of a straight shell of a screw conveyor on the solid discharge port-side away from a slurry feeding port, while there is a distance between the external periphery of the dip weir and the internal periphery of a rotating bowl, and an overflow hole formed at the internal periphery-side of the dip weir so that liquid goes through the overflow hole, while the external peripheral edge of the overflow hole locates closer to the rotating axis of the screw conveyor compared with the external edge of a liquid discharge port or the internal edge of a weir board provided on the liquid discharge port. The water content in the feed solution is decreased efficiently in the straight shell provided with the dip weir having the overflow hole.

Description: Background of the Invention 1. Field of the Invention This invention relates to a decanter centrifuge having an excellent ability of liquid-solids separation. 2. Prior Art Decanter centrifuges are sedimentation centrifuges used in clarification, dewatering and classification for the mixture of solids and liquid (slurry). Generally, the decanter centrifuge has a screw conveyor in a rotating bowl. The decanter centrifuge has a structure allowing that the solids in the slurry introduced from a slurry feeding pipe inserted into the screw conveyor are sedimented on the inner surface of the rotating bowl due to centrifugal force. Then, the solids are scraped together toward one end of the rotating bowl to be discharged by the screw conveyor rotating with a predetermined rotative speed being different from that of the rotating bowl. Simultaneously, the separated liquid is discharged automatically from the other end of the rotating bowl by liquid pressure due to the centrifugal force. In conventional apparatuses which have heretofore been employed in the decanter centrifuges of the kind, dip weirs (baffles) are generally not provided. Because, if the dip weirs are provided, efficient dewatering can not be attained. However, some decanter centrifuges provided with dip weirs are proposed in order to obtain the specific reasons respectively. (a) Japanese Patent Application Laid-Open No. JP-A-59-169550 discloses a decanter centrifuge where a dip weir for its cake-lay is provided and many blades are formed between the dip weir and a cake discharge port in order to obtain orifice effect. (b) Japanese Patent Application No. JP-B-62-43745 discloses a decanter centrifuge provided with a dip weir near a clarified liquid discharge port. Therefore, even if solid particles are attached on bubbles of separated water so as to float, the solid particles are prevented from being discharged due to the dip weir. (c) Japanese Patent Application No. JP-B-1-19941 discloses a decanter centrifuge having a baffle disposed at the border between a conical portion and a straight shell. Then, the baffle can be rotatively adjusted to change a clearance between the baffle and a bowl so that efficient concentration can be performed. (d) Japanese Utility Model Application Laid-Open No. JP-U-57-35849 discloses a decanter centrifuge having a baffle disposed at the border between a conical portion and a straight shell. Then, the conical portion and the straight shell can be separated clearly with the baffle so that efficient dewatering can be performed in the conical portion. In each conventional centrifuge, the baffle or the dip weir is provided in order to improve liquid-solids separating efficiency in the conical portion, while the straight shell has a mechanism for transporting the solids using conveyor means. That is to say, in the conventional centrifuge, the straight shell does not have mechanism of improving the concentrating efficiency for a feed solution and the dewatering efficiency for a cake. Therefore, in the prior art, the water content in the cake depends on the degree of a centrifugal force produced in the straight shell, the length of the residents time of the feed solution in the straight shell and dewatering efficiency for the feed solution in the conical portion. Summary of the Invention It is therefore the main object of the present invention to provide a centrifuge provided with a straight shell which has a mechanism for improving dewatering efficiency for a cake. According to the present invention, a decanter centrifuge comprises: a rotating bowl, which is provided with a solid discharge port and a clarified liquid discharge port; a screw conveyor, which comprises a straight shell and a conical portion and which is formed coaxially with the rotating bowl so as to be included in the rotating bowl, while the rotating bowl and the screw conveyor are rotated in the same direction with a differential rotative speed and while a feed solution to be separated is introduced into a ring-shaped space formed between the rotating bowl and the screw conveyor and is continuously separated to be solids and liquid by means of a centrifugal force so that the solids are discharged from the solid discharge port and the liquid is discharged from the clarified liquid discharge port; a slurry feeding port, which is formed on the wall of the straight shell of the screw conveyor and from which the feed solution to be separated is fed to the ring-shaped space; at least one dip weir, which is fixed to the external periphery of the wall of the straight shell of the screw conveyor on the solid discharge port-side away from the slurry feeding port, while there is a distance between the external periphery of the dip weir and the internal periphery of the rotating bowl; and an overflow hole, which is formed at the internal periphery-side of the dip weir so that the liquid goes through the overflow hole, while the external peripheral edge of the overflow hole locates closer to the rotating axis of the screw conveyor compared with the external edge of the clarified liquid discharge port or the internal edge of a weir board provided on the clarified liquid discharge port. Preferably, plural number of dip weirs are fixed to the external periphery of the wall of the straight shell of the screw conveyor and the dip weirs have the overflow holes respectively so that the overflow hole decreases in the length on the radius direction as the dip weir locates closer to the solid discharge port. When plural number of dip weirs are fixed, it is also preferable that the distance between the external periphery of the dip weir and the internal periphery of the rotating bowl becomes shorter as the dip weir locates closer to the solid discharge port. Further, the conical portion of the screw conveyor is provided so as to connect to the solid discharge port-side end of the straight shell of the screw conveyor and if the external edge of the dip weir locates closer to the rotating axis of the screw conveyor compared with the periphery of the straight shell-side end of the conical protion, this end substantially acts as the dip weir. Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein preferred embodiments of the present invention are clearly shown. Brief Description of the Drawings Fig. 1 is a longitudinal cross section of the decanter centrifuge related to the present invention; Fig. 2 is a cross sectional view taken on line II-II of Fig. 1; Fig. 3 is a schematic illustration of the essential part of a decanter centrifuge related to the present invention; Fig. 4 is a cross section showing the structure of the essential part of a decanter centrifuge related to the present invention; Fig. 5 and 6 illustrate the structures of the essential parts of the conventional centrifuges respectively. Description of the Preferred Embodiment Now, the present invention is described more particulary. As shown in Fig. 1, the decanter centrifuge related to the present invention has the structure stated below. A rotating bowl 2 and a screw conveyor 3 are included in a casing 1. The rotating bowl 2 is rotated with a predetermined rotative speed by a rotational force, which is obtained from a driving motor (not shown) through a bearing 22 by means of a pulley and a pulley belt drum 21. On the other hand, another rotational force is transformed through a gear unit 4 to a shaft end portion 6 supported by a bearing 5. Thus, the screw conveyor 3 and the rotating bowl 2 can be rotated in the same direction with a predetermined differential rotative speed. The above rotating bowl 2 is provided with a side wall 20 at the one end of its longitudinal direction while its another end is totally open. The mixture of liquid and solids such as sludge can be fed through a feed pipe 7, which is inserted through the center portion of the side wall 20 with allowance. A ring-shaped clarified liquid discharge port 8 is formed at the internal periphery of the side wall 20. A ring-shaped weir board 10 is formed so as to cover the clarified liquid discharge port 8 partly in order to control the level of the clarified liquid. Thus, only the clarified liquid can be discharged from the discharge port 8. On the other hand, at the opposite side of the longitudinal direction of the centrifuge to the clarified liquid discharge port 8, a space formed between the rotating bowl 2 and the screw conveyor 3 is used as a solid discharge port 9. The screw conveyor 3 comprises a straight shell 3A and a conical portion 3B. A partition wall 34 is provided in the straight shell 3A so as to cross it at its substantial center. Due to this partition wall 34, the flow of the introduced slurry can be changed. A slurry feeding port 35 is formed on the wall of the straight shell 3A so that the slurry can be fed through a ring-shaped space formed between the rotating bowl 2 and the screw conveyor 3. Screw blades 30 are fixed to the external periphery of the wall of the straight shell 3A so as to encircle the straight shell 3A. Due to these screw blades 30, the solids are transported toward the solid discharge port 9. The first dip weir 32 and the second dip weir 33 are fixed to the external periphery of the wall of the straight shell 3A so as to encircle the straight shell 3A on the solid discharge port-side away from the slurry feeding port 35. There is a distance between the external periphery of the dip weirs 32, 33 and the internal periphery of the rotating bowl 2. Overflow holes 32a, 33a are formed at the internal periphery-sides of the dip weirs 32, 33 respectively so that the liquid goes through the overflow holes 32a, 33a. The external peripheral edge of the overflow hole 32a, 33a locates closer to the axis of the screw conveyor 3 compared with the internal edge of the weir board 10 provided on the clarified liquid discharge port 8. If the weir board 10 is not provided, the external peripheral edge of the overflow hole 32a, 33a locates closer to the axis of the screw conveyor 3 compared with the external edge of the clarified liquid discharge port 8. Further, as shown in Fig. 4, the distance l₂ between the external edge of the second dip weir 33 and the rotating bowl 2 is shorter than the distance l₁ between the external edge of the first dip weir 32 and the rotating bowl 2. Then, as shown in Fig. 3, the overflow hole 33a of the second dip weir 33 locates internal side as compared with the overflow hole 32a of the first dip weir 32. As shown in Fig. 1, the conical portion 3B has the shape of truncated cone tapered toward the solid discharge port 9. The screw blades 31 are fixed to the external periphery of the wall of the conical portion 3B so that the above mentioned sedimented solids are transformed in the form of cake. As shown in Fig. 4, the distance l₃ between the straight shell-side peripheral end of the conical portion 3B and the rotating bowl 2 is smaller than the distance l₂ Next, the operation of the liquid-solids separation with the centrifuge having the structure stated above will be explained referring to Fig. 3 where its structure is shown schematically. When the slurry such as sludge is introduced from the slurry feeding port 35 into the space formed between the rotating bowl 2 and the screw conveyor 3 and is sedimented toward the internal surface of the rotating bowl 2 by means of the centifugal force, the concentration of the solid layer is higher on the internal surface -side of the rotating bowl 2 than on the rotating axis-side. Then, a force is applied to the solid layer so that the solid layer can be transported toward the solid discharge port 9 with the screw conveyor 3. In this situation, the solid layer goes through the space formed between the first dip weir 32 and the rotating bowl 2 against the resistance, thereby a consolidation force is applied to the solid layer. Therefore, only a heavy layer, which has large solid content and small water content, can go through the first dip weir 32. After going through the first dip weir 32, this heavy layer residents between the first dip weir 32 and the screw blade 30 disposed at the solid discharge port-side of the first dip weir 32. In this resident time, a centrifugal force is applied to the heavy layer so that the separation of the light layer from the heavy layer is further advanced. Thus, the separated light layer is overflown the overflow hole 32a so as to be returned toward the slurry feeding port 35. Precisely, due to the provision of the first dip weir 32 having the overflow hole 32a, the solid layer can be consolidated, while the slurry having large water content can be returned so that only the heavy layer can be transported toward the solid discharge port 9. Next, the same operation is carried out for the heavy layer reaching at the second dip weir 33 so that the heavy layer is further dewatered. On the other hand, the final liquid-solids separation is attained at the space formed between the straight shell-side peripheral end of the conical portion 3B and the rotating bowl 2. Continuously, the concentrated or dewatered cake is discharged from the solid discharge port 9. The water content in the solid layer decreases in the stages of the dip weirs 32, 33, as the solid layer is transported closer to the solid discharge port 9. In this case, the external peripheral edge of the overflow hole 32a, 33a locates closer to the rotating axis of the screw conveyor 3 compared with the internal edge of the weir board 10 provided on the clarified liquid discharge port 8. Accordingly, the slurry having large water content is overflown so as to be returned toward the clarified liquid discharge port 8. These phenomena are carried out in the field of the centrifugal force before and after the dip weirs 32, 33. Therefore, the liquid layer having large water content is transported back toward the liquid discharge port 8 with tile screw conveyor 3 as if a force is applied to the liquid layer by the dewatered solid layer. The separated liquid is partly overflown the overflow holes 32a, 33a of the dip weirs 32, 33 and the weir board 10 provided on the clarified liquid discharge port 8. Then, the liquid is discharged from this port 8. On the other hand, in the conventional centrifuges, as shown in Figs. 5 and 6, the above mentioned consolidation effect can not be performed, because, the dip weir is not provided on the straight shell 3A. In Fig. 1, when the level of the overflow hole 33a of the second dip weir 33 is the same or lower compared with the level of the solid discharge port 9, the partition wall 37 can be used as the third dip weir. In the present invention, one dip weir is enough. However, three or more than three dip weirs can be applied. The shape of the overflow hole can be selected optionally. Each dip weir is fixed to the straight shell 3A by welding so as not to be removed. Alternatively, when the dip weir is ring-shaped and its internal periphery has larger diameter than that of the straight shell 3A, it can be fixed to the straight shell 3A with a stay member. In this case, the overflow hole is also ring-shaped. The effect of the present invention is shown more clearly by examples. In the following examples, mixed raw sewage sludge is used. Each applied decanter centrifuge has a bowl diameter of 460 mm⊘ and the bowl length of 1200 mmL. The examples are performed with three kinds of decanter centrifuges; two kinds of conventional apparatuses and one kind of apparatus of the present invention. First, one apparatus of the present invention is compared with the conventional apparatuses. As shown in Fig. 5, the first conventional apparatus has a dry zone in the conical portion and a dip weir or another equipment like this is not provided in the screw. Then, as shown in Fig. 6, the second conventional apparatus has a partition wall 37 at the inlet of the conical portion 3B and a dry zone is not provided. On the other hand, the apparatus of the present invention, as shown in Fig. 4, two dip weirs are provided in the straight shell 3A. The partition mechanism is provided at the inlet of the conical portion 3B, like the second conventional apparatus. The distance l₁ between the first dip weir 32 and the rotating bowl 2 is 50 mm. The distance l₂ formed between the second dip weir 33 and the rotating bowl 2 is 35 mm. The distance l₃ between the partition wall 37 and the rotating bowl 2 is 30 mm. On the condition that the level of the external edge of the solid discharge port is standardized, the weir levels are determined as follows. The overflow level of the external peripheral edge of the overflow hole 33a of the second dip weir 33 is the same as the standard, the overflow level of the external peripheral edge of the overflow hole 32a of the first dip weir 32 is 1.5 mm below the standard (-1.5mm) and the level of the external edge of the clarified liquid discharge port is 3 mm below the standard (-3mm). With the above three kinds of the decanter centrifuges, the mixed raw sewage sludge is dewatered respectively. The results obtained are shown in Table 1 below. It is clear from Table 1 that the centrifuge of the present invention shows following effects. Comparing with the first conventional apparatus, on the condition that the throughput is the same, the water content in the cake can be decreased by ca. 4 % with the apparatus of the present invention. On the condition of the same water content, much sludge cart be processed by more than ca. 30%. Then, comparing with the second conventional apparatus, on the condition that the throughput is the same, the water content in the cake can be decreased by ca. 2 % with the apparatus of the present invention. On the condition of the same water content, much sludge can be processed by more than ca. 30%. Next, as for the apparatus of the present invention, if the conditions such as the number of dip weir, provision of the overflow hole, the overflow level of the weir are changed, dewatering efficiency is changed as shown in Table 2 below. The operation conditions are stated below; Sludge The mixed raw sewage sludge Sludge concentration 2.5 to 2.6 % Throughput 6 m²/H. Apparatus Dip Weir Level of Weir (mm) Distance (mm) Water Content in Cake (%) Recovery Rate (%) Numbers Location Overflow Hole h₁ h₂ l₁ l₂ No. 1 1 B X - 1.5 - 35 77.9 98.1 No. 2 1 B O - 1.5 - 35 76.2 98.4 No. 3 2 A, B X 1.5 1.5 50 35 76.9 98.2 No. 4 2 A, B O 1.5 1.5 50 35 74.5 98.7 No. 5 2 A, B O 0 0 50 35 77.0 99.0 Note; A indicates the position A shown in Fig. 4. B indicates the position B shown in Fig. 4. X indicates that the dip weir does not have the overflow hole. O indicates that the dip weir has at least one overflow cavity. h₁ indicates the level of the external edge of the clarified liquid discharge port. h₂ indicates the level of the external edge of the overflow hole of the first dip weir. Comparing with the centrifuge which does not have the overflow hole, the centrifuge which has the overflow hole can be used with more efficient dewatering. Precisely, the water content in the cake can be decreased by ca. 2.0 %. This effect can be attained regardless to the number of dip weir. Although the overflow hole is provided, when there is no difference between the levels of the weirs (as apparatus No.5 in Table 2), the water content in the cake is increased as compared with the case where there is level difference. Therefore, it is understood that the liquid layer having large water content is overflown so as to be returned toward the clarified liquid discharge port due to the level difference. Comparing with the centrifuge provided with single dip weir, the centrifuge provided with two dip weirs can be used with more efficient dewatering.

Claims: A decanter centrifuge comprising: a rotating bowl (2), which is provided with a solid discharge port (9) and a clarified liquid discharge port (8); a screw conveyor (3), which comprises a straight shell (3A) and a conical portion (3B) and which is formed coaxially with said rotating bowl (2) so as to be included in said rotating bowl (2), while said rotating bowl (2) and said screw conveyor (3) are rotated in the same direction with a differential rotative speed and while a feed solution to be separated is introduced into a ring-shaped space formed between said rotating bowl (2) and said screw conveyor (3) and is continuously separated to be solids and liquid by means of a centrifugal force so that said solids are discharged from said solid discharge port (9) and said liquid is discharged from said clarified liquid discharge port (8); a slurry feeding port (35), which is formed on the wall of said straight shell (3A) and from which said feed solution to be separated is fed to said ring-shaped space; at least one dip weir (32, 33), which is fixed to the external periphery of said wall of said straight shell (3A) of said screw conveyor (3) on the solid discharge port-side away from said slurry feeding port (35), while there is a distance (l₁, l₂) between the external periphery of said dip weir (32, 33) and the internal periphery of said rotating bowl (2) wherein the radial distance between the external peripherie of said dip weir (32, 33) and the internal side of said rotating bowl (2) is such, that the resulting ring-shaped space constricts the movement of a solid layer being forced by said screw conveyor (3) through it, causing a consolidation of said solid layer; characterized by: at least one overflow hole (32a, 33a), which is formed at the internal periphery-side of said dip weir (32, 33) so that said liquid goes through said overflow hole (32a, 33a), while the external peripheral edge of said overflow hole (32a, 33a) locates closer to the rotating axis of said screw conveyor (3) compared with the external edge of said clarified liquid discharge port (8) or the internal edge of a weir board (10) provided on said clarified liquid discharge port (8). A decanter centrifuge according to claim 1, wherein plural number of dip weirs (32, 33) are fixed to the external periphery of said wall of said straight shall (3A) of said screw conveyor (3) and said dip weirs (32, 33) have the overflow holes (32a, 33a) respectively so that said overflow hole (32a, 33a) decreases in the length on the radius direction as said dip weir (32, 33) locates closer to said solid discharge port (9). A decanter centrifuge according to claim 1, wherein plural number of dip weirs (32, 33) are fixed to the external peripherie of said wall of said straight shell (3A) of said screw conveyor (3) and said distance (l₁, l₂) between the external peripherie of said dip weir (32, 33) and the internal peripherie of said rotating bowl (2) becomes shorter as said dip weir (32, 33) locates closer to said solid discharge port (9). A decanter centrifuge according to claim 1, wherein said conical portion (3B) is formed at the solid discharge port-side end of said straight shell (3A) of said screw conveyor (3) and the external edge of said dip weir (32, 33) locates closer to said rotating axis of said screw conveyor (3) compared with the periphery of the straight shell-side end of said conical portion (3B).

Inventors: MIYANO KEIICHIRO C O TSUKISHIM, NISHIDA KATSUNORI C O TSUKISHI, SANO HIROSHI, MIYANO, KEIICHIRO C/O TSUKISHIMA KIKAI CO.,LTD, NISHIDA, KATSUNORI C/O TSUKISHIMA KIKAI CO.,LTD., SANO, HIROSHI
Applicants: TSUKISHIMA KIKAI CO, TSUKISHIMA KIKAI CO. LTD.
Application year: 1991
Main classification: B04B1
IPC codes: B04B1