DEWATERING DEVICE

Abstract: The present invention relates to a device for dewatering material suspensions. It comprises two cylindric rotary rolls (1) where at least one of the rolls (1) is liquid permeable. The rolls (1) form between themselves a nip (2), through which the material is intended to pass downward from above while being dewatered at the same time. The invention has the object to solve the problems of pressed-out liquid (filtrate) re-wetting the material. For the liquid permeable roll (1) below the nip (2) at least one abduction surface (20) extending along the entire roll (1) is located along which surface at least parts of the filtrate are intended to run. The filtrate is led into a gill (17), which extends along the entire length of the roll (1) and comprises a liquid inlet (18) and a return flow protection (19).

Description: Dewatering device This invention relates to a device for dewatering material suspensions. The device comprises two co-operating cylindric rotary rolls, where at least one of the rolls is liquid permeable. The rolls form between themselves a nip, through which the material is intended to pass downward from above while being dewatered at the same time. The material suspension can be, for example, a pulp suspension, and the dewatering device can then be a washing press or a dewatering press. In the following a dewatering device according to known art will be described, where both rolls are liquid permeable. The rolls are formed with liquid permeable shell surfaces, which consist of a perforated metal sheet attached on a roll body. In order to gain high capacity, the total open hole area shall be great, and at the same time the holes must be small to prevent fine material, for example small fibers, from following along with the liquid through the holes. For solving these problems, the rolls can be provided with a wire cloth on the outside of the perforated shell sheet. The wire cloth allows liquid flow also along the shell surface and usually is formed of plastic wires. This implies that the perforations can be given a greater size in order to yield a higher capacity without the risk of an increase in the passage of fine material. The wire cloth is used above all when the pulp suspension to be treated contains much fine material, which is the case, for example, with mechanically produced pulps. The rolls are mounted rotary on two parallel shafts. The material suspension is supplied to the device, so that it is moved by the rotation of the rolls downward from above through the nip, whereby compression and dewatering of the material to desired dry matter content takes place. After the nip, the pick-up of the material from the rolls takes place with the help of a doctor means and gravity force. The material drops down into a transport means and is conveyed away from the dewatering device. Against the shell surface of each roll a doctor blade abuts, which extends along the entire length of the roll. The roll can have a length of up to about 8 m. The object of the doctor blade is to ensure the pick-up of the material from the rolls and to scrape it off mechanically. In the case of rolls with wire cloth, the doctor blades must be arranged at a small distance, usually 0,1 - 0,5 mm, from the roll in order to prevent the doctor blades from cutting the wire cloth to pieces. A dewatering device of this kind is described, for example, in the patent specification Sweden SE 504 Oi l. In the case when only one of the rolls is liquid permeable, it is formed as described above and consequently has a doctor means for picking up the material. The liquid impermeable roll is in this case provided substantially to make the formation of a nip possible. At a dewatering device of the described kind problems arise with the re- wetting of the dewatered material. When the dewatered material leaves the nip, the compressing force decreases. Pressed-off liquid (filtrate) can then run back through the perforated sheet and re- wet the material, which deteriorates the washing result and dry matter content. At a dewatering device of the described kind problems also arise due to the wear of the doctor blades and deflection of the same. The doctor means is subject to high requirements, the doctor blades shall abut the roll, but the force between blade and roll must not be so great that the blade and roll are not subjected to great mechanical stress. In spite of accurately designed and rigid doctor means, problems arise with wear of the doctor blades and rolls. The doctor blades must be exchanged often because of wear. They are expensive with regard both to their design and manufacture. Wear of the rolls, besides, causes deterioration of their operation and shortening of their life. When a roll is covered with a wire, an interspace of tenths of a millimeter must be maintained between the doctor blade and wire. One wants the doctor blade to be located as close to the wire as possible, but without cutting the wire to pieces. This is not made easier by the fact that the doctor blades get worn, and it happens that the wire is cut to pieces by the doctor blades. The present invention offers a solution of the aforesaid problems. The functioning of the invention presupposes that the material suspension to be dewatered after the nip has such a dry matter content, that it gets free from the roll, which is facilitated if the dewatered material forms a coherent material web after the nip. Experiments have shown that the filtrate, which runs back after the nip, preferably places itself like a film of liquid on the roll. This liquid film then follows along with the roll at its rotation. The doctor blade, which according to known art abuts the roll, scrapes off this film, so that it follows along with the dewatered material and re-wets the same. The liquid runs along the doctor blade and down into the transport means where it re- wets the material. In order to prevent the liquid running along the doctor blade from re- wetting the material, a liquid abduction means, a gill, is provided. The liquid running along the doctor blade is led into the gill instead of being allowed to run down into the transport means. The re-wetting is thereby reduced considerably. When great amounts of liquid are scraped off the rolls, it may happen that part of the liquid is released from the doctor blade before the liquid has been led into the gill. According to the invention, the doctor blade can then be replaced by a screen means. The screen means is arranged so that the distance between the screen means and roll is so great, that the filtrate liquid film on the roll passes between the screen means and roll and thereafter, without getting into contact with the dewatered material, runs down into a filtrate collection means. The function of the gill in this case is to catch liquid which, for example at an occasionally thicker filtrate liquid film, does not pass between the screen means and roll, but runs along the screen means on its side facing the material web. Due to the fact that the doctor blade in this case is removed, the mechanical effect between roll and doctor blade is eliminated, and thereby also all costs in connection with exchange of worn doctor blades are eliminated. There is further no risk that the . wire cloth or roll can be damaged. The requirement on accuracy at the forming of the screen means and on its rigidity are much lower than at a doctor means, because the screen means is located at a relatively great distance from the roll. The invention, therefore, results in a reduction of costs related to design and manufacture compared to the use of doctor means. With a device according to the invention the dry matter content of pulp can be increased from 2,5 - 8% to 20 - 40%, preferably from 3 - 8% to 30 - 40%. The characterizing features of the invention are defined in the attached claims. The invention is described in greater detail in the following, with reference to the accompanying drawings, in which Fig. 1 is a cross-section of a dewatering means according to the invention, Fig. 2 shows on an enlarged scale a portion of the dewatering device in Fig. 1, Fig. 3 shows a different embodiment of the gill, Fig. 4 shows a portion of a dewatering device according to the invention, with a screen means. The dewatering device shown in Fig. 1 comprises two co-operating cylindric rotary liquid permeable press rolls 1, which between themselves form a press nip 2. For controlling the size of the nip 2, at least one of the rolls 1 is adjustable. The shell surface of the rolls 1 consists of a liquid permeable shell 3, suitably a perforated metal sheet with or without wire cloth, which is attached on a roll body. The upper portion of the rolls 1 is enclosed in a casing 4 so that a space 5 is formed above the rolls 1. To the casing 4 at least one inlet 6 for the supply of material suspension is connected. In the space 5 a higher pressure is maintained than in and below the rolls 1. The pressure suitably is established by a pump which feeds the material suspension to the space 5. Due to the over pressure in the space 5, liquid from the material suspension is pressed into the roll 1. The liquid is pressed through the liquid permeable shell surface 3, and at the same time the material is deposited on the surface of the shell surface 3. The material is then moved by the rotation of the rolls 1 through the nip 2 where it is dewatered to a dry matter content, which depends a.o. on the properties of the material and on the force applied to the pulp web. After the nip 2 the pick-up of the dewatered material from rolls takes place with the help of the gravity force. The material should thereby have such a dry matter content, that it is released from the rolls 1. This is facilitated if the material after the nip 2 forms a coherent material web 8. The weight of the material web 8 contributes to draw with itself subsequent material web 8. In the case that the material suspension is a pulp suspension, the dry matter content after the nip should be above about 20%, but preferably above 30%, so that the material shall form a coherent material web 8. After the nip 2 the material drops down into a transport means 7, for example a tearscrew. In order to be subjected to overpressure, the space 5 is sealed from the ambient by seals 9 against the shell surfaces of the rolls 1 and seals (not shown), which are applied to the end walls of the rolls. Between the rolls 1 the material web 8 in the nip 2 acts as a seal. Part of the liquid (filtrate) pressed out of the material suspension and in through the liquid permeable shell surface 3 of the roll 1 runs back after the nip 2 and forais a liquid film 10 of filtrate on the roll 1. The liquid film participates in the rotation of the roll 1. A doctor blade 16, which extends along the entire length of the roll, scrapes the liquid film off the roll 1. The liquid runs along an abduction surface 20 into a gill 17. The abduction surface 20 and gill 17 extend along the entire length of the roll 1. At the embodiment shown , the upper portion of the abduction surface 20 consists of the doctor blade 16. The doctor blade 16 can, of course, be composed of several doctor blades, which together extend along the entire length of the roll. The gill 17 has a liquid inlet 18, in this case a gap, which extends along the entire length of the roll 1. The gill 17 is formed with a return flow protection 19, in this case a longitudinal wall, so that the liquid, which has run in via the liquid inlet 18, cannot run back and re- wet the material. The liquid is let from the gill 17 to a filtrate collection means 14. The abduction surface 20 should be formed so that the liquid does not get released and drops down into the transport means 7. The abduction surface 20, thus, should have a relatively plane surface and not incline too much. The gill 17 can also, for example as in Fig. 3, be formed with a rod screen plate as liquid inlet 18, i.e. with a plurality of longitudinal rods 21, which between themselves form narrow gaps. In Fig. 3 the rods are substantially horizontal, but can also be oblique. A plurality of gills, of course, can also be arranged for collecting the liquid. An abduction surface shall then be provided to lead the liquid into the respective gill. As stated before, the filtrate liquid film 10 participates in the rotation of the roll 1. The liquid film, however, finally is released from the roll 1 and drops down. The doctor blades 16, thus, shall be located so close to the nip 2 that the filtrate liquid film not by itself can be released from the roll 1 and drop down into the transport means 7 and thereby re-wet the material. The doctor blades 16 can be replaced by screen means 11, as shown in Fig. 4. For each roll 1 at least one screen means is provided below the nip 2. Every screen means comprises a screen 12, which extends along the entire length of the roll 1, its uppermost portion 13 being located closest to the roll 1. The screen means 11, of course, can comprise several screens 12, which are arranged so that they together extend along the entire length of the roll. Every screen means 11 and its screen 12 is adjustable to adjust , its portion in relation to the roll 1. The screen 12 is positioned so that its uppermost portion 13 is at such a distance from the roll 1, that the filtrate liquid 10 passes between the roll 1 and screen substantially without the uppermost portion 13 of the screen getting into contact with the filtrate liquid film 10. The dewatered material/material web 8 passes on the opposite side of the uppermost portion 13 of the screen as the filtrate liquid film 10, as shown in Fig. 4. The screen 1 shall be located in a place so that the filtrate liquid film passes in between the uppermost portion 13 of the screen and the roll 1, before the filtrate liquid film 10 is released from the roll 1. The filtrate is thereafter led down into a filtrate collection means 14 without getting into contact with the dewatered material. This is done suitably with the help of the screen means 11, for example, as shown in Fig. 4, in that the screen 12 extends a distance down into the filtrate collection means 14. The screens 12 shall also be formed and arranged so that the filtrate passing between the roll 1 and its screen 12 is not stopped up. The distance B between the roll 1 and its screen 12 should suitably increase with the distance from the nip 2. If the uppermost portion 13 of the screen gets into contact with the filtrate liquid film 10 on the roll 1, the filtrate liquid film 10 at least partially can be released from the roll 1 and then pass on the same side of the screen 12 as the dewatered material. This liquid runs in this case along the abduction surface 20 and into the gill 17, in the same way as described with reference to Figs. 1, 2 and 3. If it is desired that all the liquid or in any case the major part of the liquid shall pass between the roll 1 and screen means 11, it must be considered that the filtrate liquid film 10 is thinnest closest to the nip 2 and then increases in thickness farther away therefrom. This implies that the farther up to the nip the screen 12 is located, the closer to the roll the screen 12 can be placed. The material web 8 can also deviate somewhat to the screen 12. In order to prevent the material web from moving in between the roll 1 and its screen 12, in this case the uppermost portion 13 of the screen should be located as close as possible to the roll 1, with regard to the thickness of the filtrate liquid film 10. Furthermore, at the start-up of a dewatering device according to the invention the dry matter content of the material after the nip 2 can be so low that the material is not released from the roll 1 , and that a material web 8 is not formed immediately. The material can then due to the rotation of the rolls 1 be thrown in different directions and even follow along with the rolls in their rotation. If it is wanted to minimize the amount of material, which at the start-up passes between the screen 12 and roll 1, also from this point of view the uppermost portion 13 of the screen must be positioned as close as possible to the roll 1, with regard to the thickness of the filtrate liquid film 10. The position of the screens 12 in relation to the rolls 1 and nip 2 will be an optimizing of the distance between the screens 12, with regard a.o. to the place where the filtrate liquid film 10 is released from the roll 1, the thickness of the filtrate film 10 and the way in which the material behaves after the nip 2. The distance between the roll 1 and the uppermost portion 13 of its screen should be at least 2 mm, but suitably at least 3 mm and preferably at least 5 mm. If it is desired to avoid to the greatest possible extent that filtrate liquid runs on the same side of the screen 12 as the material passes, the distance between the roll 1 and the uppermost portion 13 of its screen should be more than 20 mm, but suitably more than 30 mm. The uppermost portion 13 of the screen suitably is formed so that it prevents possibly deposited material from collecting and building up thereon. A building up of material on the uppermost portion 13 of the screen can in the worst case result in the formation of a plug. The risk thereof is greatest in connection with the start up of the dewatering device. A suitable design of the uppermost portion 13 of the screen is the one shown in Fig. 4. It is chamfered, and possibly deposited material is thereby led away with the filtrate. The uppermost portion 13 of the screen can, for example, also be rounded or sharp. The decision whether doctor blades or screen means shall be used, depends a.o. on how much liquid is found on the roll after the nip. In the case of large amounts of liquid it is suitable to use a screen means. In the case of a roll with wire cloth a screen means is also to be preferred. The dewatering device can also, for example, comprise two rolls, of which only one is liquid permeable. The roll which is not liquid permeable can in this case have a considerably smaller diameter than the liquid permeable roll. A gill is in this case provided only for the liquid permeable roll. The same applies to the screen means. The invention can also be used at a device intended for washing a material suspension. The material suspension then is treated after the nip in the aforedescribed way, while before the nip it forms, for example, a web on the liquid permeable roll or rolls, to which web washing liquid is applied. The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the claims with reference to description and drawings.

Claims: Claim 1. A device for dewatering material suspensions, comprising two cylindric rotary rolls (1), where at least one of the rolls (1) is liquid permeable, and the rolls (1) between themselves form a nip (2), through which the material is intended to pass downward from above while at the same time being dewatered, characterized i n that for the liquid permeable roll (1) below the nip (2) at least one abduction surface (20) extending along the entire roll (1) is located, along which surface at least parts of pressed-out liquid (filtrate) are intended to run and be led into a gill (17), which extends along the entire length of the roll (1) and comprises a liquid inlet (18) and a return flow protection (19) to prevent the pressed-out liquid from re- wetting the dewatered material. 2. A device as defined in claim 1, characterized in that the upper portion of the abduction surface (20) consists of at least one doctor blade (16) for scraping off a liquid film (10) of filtrate formed on the roll (1) after the nip (2), before the filtrate liquid film (10) is released from the roll. 3. A device as defined in claim 1, characterized in that the upper portion of the abduction surface (20) consists of at least one screen means (11) with at least one screen (12), with its uppermost portion (13) being located closest to the roll (1), that the distance between the uppermost portion (13) of the screen and the roll (1) is so great, that a liquid film (10) of filtrate formed on the roll (1) after the nip (2) can pass in between the roll and the uppermost portion (13) of the screen, before the filtrate liquid film (10) is released from the roll (1), while the dewatered material passes on the other side of the uppermost portion (13) of the screen. 4. A device as defined in claim 3, characterized in that the distance between the roll (1) and uppermost portion (13) of the screen is at least 2 mm. 5. A device as defined in any one of the preceding claims characterized in that both rolls (1) are liquid permeable. 6. A device as defined in any one of the preceding claims, characterized in that the liquid inlet (18) is formed as a gap extending along the entire length of the roll (1). 7. A device as defined in one of the claims 1 to 5 inclusive, characterized in thattheliquidinlet (18) is formed as a plurality of substantially horizontal longitudinal rods (21), which between themselves form narrow gaps.

Inventors: DANIELSSON MAGNUS, LUNDBERG JOERGEN T, DANIELSSON, MAGNUS, LUNDBERG, JOERGEN, T.
Applicants: DANIELSSON MAGNUS, LUNDBERG JOERGEN T, METSO PAPER INC, DANIELSSON, MAGNUS, LUNDBERG, JOERGEN, T., METSO PAPER, INC
Application year: 2001
Main classification: D21C9
IPC codes: B30B9, D21C9