THICKENER

Abstract: A thickener for fiber suspensions and the like includes a plurality of rotatable filter disks (3) mutually spaced in axial direction and being adapted to be partially submerged in a liquid or suspension (5) to be filtered. Each filter disk comprises a plurality of filter sections (4) communicating with axial ducts (2) at the outer peripheries of the filter disks (3) for removal of filtrate. The axial filtrate ducts (2) are open to the atmosphere at at least one end (2') thereof for the purpose of conducting air through these ducts into the interior of the filter sections to replace the filtrate flowing out. Advantageously, there is communication between all filtrate ducts through the annular filter disk holder (1) which carries each filter section (4).

Description: THICKENER The present invention relates to a thickener, i.e. an apparatus for raising the dry solids content in suspensions, such as fibe suspensions, by means of filtering. In particular there is intended such an apparatus including a plurality of rotatable filter disks mutually spaced in axial direction and being adapted to be partially submerged in a liquid or a suspension t be filtered, each disk comprising a plurality of filter section communicating with axial ducts arranged at the outer peripherie of the filter disks for removal of filtrate. Apparatuses of this kind are known inter alia rom SE-C-8305817-2 (corresponding to EP-B1-0160069 and US-A-4 634 529) and have the good property of affording a very large filte surface with a relatively small volume. However, when it comes to achieving thickening of a fiber suspension, i.e. raising the dry solids content from typically 0,5% to typically 5%, there i the demand of very high capacity, i.e. a large filtrate flow through the apparatus. One way of increasing the flow capacity is to increase the revolutionary rate. However, a high revolutionary rate results in that the filter sections do not have time to be emptied of filtrate before the sections reach the position where the filter cake is to be removed. The reason for this is that the filter cake that is built up has time to b too dense to pass through sufficient air to replace the filtrat flowing out. It can thus occur that the filtrate wets the filte cake remaining on the filter section, or that the filtrate together with the filter cake runs down into the collection vessel intended for the filter cake. This drawback is usually designated re-wetting or "backwash". The known apparatuses are moreover expensive to manufacture, inter alia because they require a vacuum valve and a vacuum source for establishing a sufficiently large pressure difference between the suspension and the filtrate discharge duct. The object of the present invention is to provide an apparatus of the kind initially stated, which allows a high revolutionary rate and thereby high capacity, and which does not require an outside vacuum source, but is simple and cheap in manufacture. This has been achieved by the invention having been given the distinguishing features disclosed in the following claims. The invention will now be described in the following with reference to the accompanying drawings, wherein Fig. 1 is a cross section through a thickener with a plurality of annularly arranged filter sectors, Fig. 2 is an axial section through the thickener according to Fig. 1 , Fig. 3 is a detail to a somewhat enlarged scale of a filter rotor of a thickener according to Fig. 1, and Fig. 4 is an axial section through a developed embodiment of the thickener according to the invention. A thickener in accordance with the invention includes, like filters of the kind initially stated, a filter rotor having a plurality of annular filter disk holders 1 arranged axially after each other, and filtrate ducts 2 axially connecting the holders 1. Fig. 1 is a cross section through such a filter rotor, which is rotatable counter-clockwise in the figure, and which carries an annular filter disk 3 comprising a plurality of filter βections 4. The interior of each filter section 4 is in communication with an associated axial filtrate duct 2, namely the closest trailing filtrate duct 2 in the direction of rotation. All the filter disks of the filter are partially submerged in the liquid or suspension 5 to be thickened (filtered) and which is received in a container 6. The container 6 rests on a foundation 7,8. Through the central opening of the filter disks extends a collecting trough 9 for solid material, such as cellulose fibres, which are deposited on the filter sections during thickening and are caused to fall, optionally with the aid of a jet 10, from the filter sections when they ar above the trough 9. Numeral 11 denotes an oscillating spray means for washing off particles from the filter sections. 12 denotes an inlet for suspension and 13 a casing over the rotor, which has two inspection hatches 14 and 15, of which the hatch 15 is shown in an open position. In Fig. 2 there is illustrated how the rotor at one end thereof by means of a drive shaft 16 is journalled in a bearing 18 carried by the rotor housing 17. A drive motor, gearing etc. ar indicated at 19. At the other, open end of the rotor, through which the collection trough 9 is inserted, the rotor is terminated by a carrier ring 20 which is carried by and slides on sliding pads 21 mounted in the rotor housing 17. As it has now been described, the thickener in accordance with the invention corresponds in all essential to a known filter according to the above. However, in such a filter the axial filtrate ducts 2 in the closed end of the rotor (to the right in Fig. 2), are directed substantially radially inwards, and close to the center directed axially outwards through the container wall to a suction box, which is in communication with a so-called barometric leg or other vacuum source. Upon rotation of the rotor, the axial filtrate ducts, and thus the filter sections which are lowered below the suspension surface, are put one at a time in communication with the suction box for sucking the filtrate through the filter sections to the axial filtrate ducts and further to the suction box. Connection to the suction box ceases when the filter sections associated with an axial filtrate duct are at a certain point above the suspension surface . When it is a question of filtering with relatively low flows (u to 15000 1/min) and low revolutionary rates (0,2 - 0,6 rp ) the known filter is excellent. At increasing flows and revolutionar rates there occurs, inter alia, what is usually designated "bottle effect", signifying that a filter section rising above the suspension surface does not have time to empty its filtrate contents into the associated axial filtrate duct during the tim the filtrate duct is at a level under some part of the associated filter section. The reason for this is that air does not have time to flow to a sufficient degree into the filter section, through its filter means, such as filter cloth with a fiber layer deposited thereon, to replace the filtrate flowing out, the filtrate then running back through the filter means an re-wetting the filter cake. In accordance with the invention, for remedying this problem, the axial filtrate ducts have direct communication with the atmosphere. More specifically, the ducts open out into the atmosphere outside the suspension container. In this way, air can flow into the filtrate ducts as soon as their entire cross section is not occupied by filtrate flowing out, or, is situate beneath a possible filtrate surface at the outlet. It will be seen from Fig. 2 how this can be arranged in practice. A circular, liquid tight disk 22 is unrotatably attached to the shaft 16. The axial filtrate ducts 2 are mounted in the disk 22 close to its outer periphery and open out with open ends 2' on the drive side of the disk. A sealing strip 23 sealing against the disk 22 extends along the inside of the container 6 at leas up to the maximum suspension level 5'. In the illustrated example, the sealing strip engages with a sliding seal against an annular, axially directed flange 24 at the outer periphery o the disk 22. The disk 22 and the sealing strip 23 thus define the suspension-containing volume of the container, and in practice constitute a rotating container wall. Outside this rotating container wall 22 an outlet 25 for the filtrate iε arranged in the bottom of the container 6 or outside the container itself. Atmosphere pressure prevails in the entire thickener, i.e. at the suspension surface inside the rotor as well as outside the disk 22 and at the outlet 25. As will be understood from the foregoing, upon rotation of the rotor there is created a static pressure difference between the suspension in the container 6 and the interior of the filter sections 4, since the latter are in communication with the atmosphere via the open ends of the axial filtrate ducts 2. This pressure difference goes from zero, when a filter section is lowered into the suspension, via a maximum at the bottom of the container, and back to zero when a filter section is rised above the suspension surface. The pressure difference forces the filtrate through the filter means of the filter sections, solid particles such as cellulose fibres then being deposited on the filter means. The varying subpressure, independent of an exterior constant vacuum source, brings about the good effect that the pressure difference first increases in time with the increasing layer of solid particles on the filter means. Hereby the flow is kept at a substantially uniform level during the firεt half of the time during which a filter section is located under the suspension surface. During the remainder of this time, the pressure difference decreases, and thereby the flow through the filter means, which gives the filtrate a possibility of running off via the axial filtrate ducts, while the filter section rises above the suspension surface. In a thickener in accordance with the invention, this running-off can take place entirely without replacement air through the filter means, since replacement air is supplied through the axial filtrate ducts 2 opening out in free air. For regulating the static pressure difference, means can be arranged for varying the filtrate level at the outlet 25, e.g. a regulatable valve, which can be controlled by a level-sensing transducer . A filter section 4 normally has an outlet to the filter disk holder 1 only in the part of the section closest to the associated axial filtrate duct 2, and the filter disk holder has communication only with this filtrate duct. To ensure that the maximum flow capacity of the axial filtrate ducts is utilized, it is now proposed that there shall be mutual communication between the axial filtrate ducts 2. In practice this is achieved by the filter disk holder 1 functioning as a filtrate conductor along its entire circumference and having communication with all axial filtrate ducts 2. This iε schematically illustrated in Fig. 3. Accordingly, the filtrate ducts situated lowest at each time will run filled with filtrate that iε flowing off, while the filter sections 4 and associated filter ducts 2 approaching or departing from the suspension surface 5', will be able to directly pass on their filtrate contents to a duct situated at a lower level. By this accelerated emptying of the rising axial filtrate ducts, an early supply of replacement air through the axial filtrate ducts can be ensured. Instead of being open at one end, as illustrated in Fig. 2, the axial filtrate ducts 2 can be open at both ends. Such an embodiment is illustrated in Fig. 4. The principal difference between this embodiment and the firεt one is that both end walls 22 and 22' of the filter rotor must constitute rotating container walls. Accordingly, there is also a sealing strip 23' and a flange 24' for sealing the other container end, as well as an outlet 25'. If the suspension level in the container is to be the normal one, i.e. close to the upper edge of the collection trough 9, a seal must also be arranged around the trough. In this example, an at least partially (i.e. at least under the suspension level) circular plate 26 is sealingly and rigidly mounted on the trough, said plate carrying at at its outer periphery a sealing strip 27, similar to the sealing strips 23 and 23'. This strip provides a sliding seal against the edge of the circular opening for the collection trough 9 made in the rotating container wall 22'.

Claims: C L A I M S 1. Thickener for fiber suspensions and the like, including a plurality of rotatable filter disks (3) mutually spaced in axial direction and being adapted to be partially submerged in a liquid or suspension (5) to be filtered, each disk comprising a plurality of filter sections (4) communicating with axial ducts (2) at the outer peripheries of the filter disks (3) for removal of filtrate, c h a r a c t e r i z e d i n that the axial filtrate ducts (2) are open to the atmosphere at at least one end (2') thereof. 2. Thickener as claimed in claim 1, c h a r a c t e r i z e d i n that the axial filtrate ducts (2) in said end (2') open out towards an outlet (25), which likewise is in communication with the atmosphere. 3. Thickener as claimed in claim 2, c h a r a c t e r i z e d i n that the filtrate level in the outlet (25) is regulatable. 4. Thickener as claimed in anyone of claims 1-3, c h a r a c t e r i z e d i n that mutual communication exists between the axial filtrate ducts (2). 5. Thickener as claimed in claim 4, c h a r a c t e r i z e d i n that the mutual communication is established through the annular filter disk holder (1) which carries each filter disk (4) .

Inventors: NILSSON BJARNE, NILSSON HARRY, NILSSON, BJARNE, NILSSON, HARRY
Applicants: NILSSON BJARNE, NILSSON HARRY, NILSSON, BJARNE, NILSSON, HARRY
Application year: 1989
Main classification: B01D33
IPC codes: B01D24, B01D33