DEWATERING DEVICE

Abstract: The present invention relates to a dewatering device for dewatering material suspensions. The device comprises two cylindric rotary rolls (1), which between themselves form a nip (2), through which the material is intended to pass downward from above and thereby be dewatered. According to known art a doctor abuts the liquid permeable roll (1) after the nip. The doctor and also the roll (1) get worn. Problems also often arise due to re-wetting of the dewatered material. According to the invention the doctor is replaced by a screen device (11) located below the nip and comprising at least one screen (12). The screen extends along the entire length of the roll, and its uppermost portion (13) is located closest to the roll (1). The distance between the uppermost portion (13) and the roll (1) is so great that a filtrate liquid film (10) formed after the nip can pass inbetween the roll (1) and uppermost portion (13) of the screen before the filtrate liquid film (10) leaves the roll (1). The screen (12) is located so that the filtrate after having left the roll (1) is prevented from re-wetting the dewatered material.

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 and be dewatered simultaneously. The material suspension can be, for example, a pulp suspension, and the dewatering device can be a washing press or a dewatering press. A dewatering device according to known art is described in the following, 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 achieve high capacity, the total open hole area shall be great, and at the same time the holes must be small, in order to prevent fine material, for example small fibres, from following along with the liquid through the holes. In order to solve these problems, the rolls can be provided with a wire cloth on the outside of the perforated shell metal sheet. The wire cloth allows liquid flow also along the shell surface and usually is formed of plastic wires. This implies that the perforation can be designed greater so as to yield higher capacity without the risk of increased passage of fine material. 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 transfer of the material from the rolls takes place by means of a doctor device and gravity. A doctor abuts the shell surface of each roll and extends along the entire length of the roll. The roll can have a length of up to 8 m. The doctor is intended to ensure the transfer of the material from the rolls and mechanically scrapes off the material. When the rolls are covered with wire, the doctors are arranged at a small distance, usually 0,1 - 0,5 mm, from the roll so that the doctors do not damage the wire cloth. A dewatering device of this kind is described, for example, in the Swedish patent specification SE-504 Oil. When only one of the rolls is liqud permeable, it is designed as described above and, consequently, has one doctor device for transferring the material. The roll, which is not liquid permeable, is in this case mainly provided for rendering the formation of a nip possible. At dewatering devices problems arise with wear and deflection of the doctors. The requirements on the doctors are greater, and the doctors shall abut the roll without the force between doctor and roll being so great that the doctor and roll are subjected to high mechanical stress. In spite of highly accurately designed and rigid doctor devices problems arise with wear of the doctors and rolls. Due to wear, the doctors must be exchanged often. They are, besides, expensive with regard to their construction and manufacture. Wear of the rolls results in deterioration of their function and shortening of their life. When the roll is covered with a wire, it is desired to maintain a space of tenths of a millimeter between the doctor and wire. This is not made easier by the fact that the doctors get worn, and in certain cases problems arise because the wire is damaged by the doctors. At a dewatering device of the kind described there arise also problems by 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 metal sheet and re- wet the material, whereby the washing result and dry matter content are deteriorated. The invention has the object to solve the aforesaid problems in that the doctor devices are replaced by screen devices. Experiments have shown that the filtrate running back after the nip preferably deposits as a liquid film on the roll. The liquid film then follows along with the roll in its rotation. The doctor, 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 invention implies, that a screen device is arranged so that the distance between the screen device and roll is of such magnitude that the liquid film on the roll passes between the screen device and roll, and thereafter, without coming into contact with the dewatered material, runs down into a filtrate collecting means. Re- wetting is thereby reduced considerably. Due to the fact that the doctor is removed, the mechanical effect between roll and doctor, and thereby also all costs in connection with the exchange of worn doctors are eliminated. The risk of damage of the wire cloth or roll is eliminated, too. The requirements on accuracy at the design of the screen device and its rigidity are much lower than on a doctor device, because the screen device is located at a relatively great distance from the roll. The invention, therefore, results in a reduction of the costs related to construction and manufacture compared to the use of the doctor device. By 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 30-35%. The characterizing features of the invention are apparent from the attached claims. The invention is described in greater detail in the following with reference to the accompanying drawings illustrating an embodiment of the invention. Fig. 1 is a cross-section of the dewatering device according to the invention, Fig. 2 shows the screen device according to Fig. 1 on an enlarged scale, Figs. 3 and 4 show other embodiments of the screen device. The 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, preferably a perforated metal sheet, with or without wire cloth, which is attached on a roll body. The upper part of the rolls 1 is surrounded by a casing 4, so that a space 5 is formed above the rolls 1. At least one inlet 6 is connected to the casing 4 for the supply of material suspension. In the space 5 a higher pressure is maintained than in and below the rolls 1. The pressure preferably is brought about by a pump, which feeds the material suspension to the space 5. Due to the overpressure in the space 5, liquid is pressed from the material suspension into the roll 1. The liquid is pressed through the liquid permeable shell surface 3, and at the same time the material deposits 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 on the pulp web. After the nip 2 the dewatered material is transferred from the rolls by gravity. The material should thereby have such a dry matter content that it leaves 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 assists in drawing with itself the subsequent material web 8. When the material suspension is a pulp suspension, the dry matter content after the nip should be above about 20% 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 tearing screw. The space 5, in order to be put under over-pressure, is sealed from the ambient by sealings 9 against the shell surfaces of the rolls 1 and sealings (not shown) against the end walls of the rolls. Between the rolls 1 the material web 8 in the nip 2 acts as sealing. Part of the liquid (filtrate), which has been 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 forms a liquid film 10 of filtrate on the roll 1. The filtrate liquid film 10 follows some distance along in the rotation of the roll 1 and thereafer leaves the roll 1 and drops down, as shown in Fig. 2. Below the nip 2, for each roll 1 at least one screen device 11 is located. Every screen device 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 device 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 device 11 and its screen is adjustable for adjusting its position in relation to the roll 1. The screen 12 is arranged so that the uppermost portion 13 of the screen is at such a distance from the roll 1, that the filtrate liquid film 10 passes between the roll 1 and screen substantially in such a way, that the uppermost portion 13 of the screen does not come into contact with the filtrate liquid film 10. The dewatered material/material web 8 passes on that side of the uppermost portion 13 of the screen which is the other side to the filtrate liquid film 10. This is shown in Fig. 2. If the uppermost portion 13 of the screen comes into contact with the filtrate liquid film 10 on the roll 1, the filtrate liquid film 10 can leave the roll 1 earlier than it would have left if it undisturbed would have followed along in the rotation of the roll 1. The filtrate liquid film 10 can in this case leave the roll already before it has passed in between the uppermost portion 13 of the screen and roll 1, and then pass on the same side of the screen 12 as the dewatered material and, thus, re-wet the same. The filtrate liquid film 10 is thinnest closest to the nip 2 and increases in thickness farther away from the nip. This implies that the farther upward to the nip the screen 12 is located, the closer to the roll the screen 12 can be located. The screen 12 shall be located so that the filtrate liquid film 10 passes in between the uppermost portion 13 of the screen and roll 1 before the filtrate liquid film 10 leaves the roll 1. The filtrate is thereafter led down into a filtrate collecting means 14 without coming into contact with the dewatered material. This is done preferably by means of the screen device 11, for example in that the screen 12 extends a distance down into the filtrate collecting means 14, as shown in Fig. 2. When the dewatered material/material web 8 after the nip 2 drops down to the transport means 7, it passes the screen 12, which shall be designed and located so, that the material/material web 8 passing therebetween does not stop in its falling movement in such a way, that a material plug is formed. The material shall substantially freely fall downward between the screens 12. The distance A between the screens 12 must, thus, be at least constant, but preferably increase with the distance from the nip 2. The surface of the screen 12 toward the material preferably is formed so as to offer as little friction resistance as possible. The screen 12 shall also be designed and located so, that the filtrate passing between the roll 1 and its screen 12 is not stopped. The distance B between the roll 1 and its screen 12, thus, preferably should increase with the distance from the nip 2. The material web 8 can deviate slightly to the screens 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 to the roll 1 as possible, paying regard to the thickness of the filtrate liquid film 10. Furthermore, at the start 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 does not leave the roll 1, and that a material web 8 is not formed instantly. The material then can be thrown into different directions due to the rotation of the rolls 1 and also follow along with the rolls in their rotation. If it is desired to minimize the amount of material, which at the start passes between the screen 12 and roll 1, the uppermost portion 13 of the screen should also from this aspect be located as close as possible to the roll 1, regard being paid to the thickness of the filtrate liquid film 10. The position of the screens in relation to the rolls 1 and nip 2 is an optimization of the distance between the screens 12, paying regard a.o. to where the filtrate liquid film 10 leaves the roll 1, the thickness of the filtrate liquid film 10 and the way in which the material behaves after the nip 2. The distance between the roll 1 and uppermost portion 13 of its screen can be 1-lOOmm, but preferably 5-30mm. The screen 12 in Fig. 2 has an angled profile, but the screen, of course, can be formed in a different way. Two other examples of the configuration of the screens 12 are shown in Figs. 3 and 4 where Fig. 3 shows a screen with curved profile, and Fig. 4 shows a screen with hollow wedge-shaped profile. The uppermost portion 13 of the screen preferably is designed so as to prevent possibly deposited material from accumulating and building-up on said portion. A build-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 of the dewatering device. A suitable configuration of the uppermost portion 13 of the screen is the one shown in Fig. 2. It is there bevelled, and possibly deposited material is thereby lead away with the filtrate. The uppermost portion 13 of the screen can also, for example, be rounded as in Fig. 3 or sharp as in Fig. 4. 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 screen device is in this case provided only for the liquid permeable roll. The invention can also be used in a device intended for washing a material suspension. The material suspension there is treated after the nip in the aforesaid manner, but before the nip, for example, forms a web on the liquid permeable roll or rolls, to which web washing liquid is supplied. The invention, of course, is not restricted to the embodiments set forth above, but can be varied within the scope of the claims with reference to description and Figures.

Claims: Claims 1. A device for dewatering material suspensions, which device comprises two cylindric rotary rolls (1), where at least one of the rolls (1) is liquid permeable, and where the rolls (1) between themselves form a nip (2), through which the material is intended to pass downward from above while simultaneously being dewatered, characterized in that for the liquid permeable roll (1) a screen device (11) with at least one screen (12) is provided below the nip (2), where the screen extends along the entire length of the roll, with its uppermost portion (13) located closest to the roll (1), that the distance between the uppermost portion (13) of the screen and roll (1) is so great that a liquid film (10) of filtrate formed on the roll after the nip (2) can pass between the roll and uppermost portion (13) of the screen before the filtrate liquid film (10) leaves the roll, while the dewatered material passes on the other side of the uppermost portion (13) of the screen, and that the screen device (11) is located so that the filtrate after having left the roll (1) is prevented from coming into contact with the dewatered material. 2. A device as defined in claim 1, characterized in that the distance between the roll (1) and uppermost portion (13) of the screen is 1-lOOmm. 3. A device as defined in any one of the preceding claims, characterized in that the distance B between the roll (1) and its screen (12) increases with the distance from the nip (2). 4. A device as defined in any one of the preceding claims, characterized in that both rolls (1) are liquid permeable. A device as defined in claim 4, characterized in that the distance (A) between the screens (12) of the screen device (11) increased with the distance from the nip (2).

Inventors: DANIELSSON MAGNUS, LUNDBERG JOERGEN, DANIELSSON, MAGNUS, LUNDBERG, JOERGEN
Applicants: DANIELSSON MAGNUS, LUNDBERG JOERGEN, VALMET FIBERTECH AB, DANIELSSON, MAGNUS, LUNDBERG, JOERGEN
Application year: 1999
Main classification: D21C9
IPC codes: B01D33, B30B9, D21C9