WO1992021427A1

WO1992021427A1 - Apparatus for filtering liquids

Info

Publication number: WO1992021427A1
Application number: PCT/SE1991/000397
Authority: WO
Inventors: Rune Helmer Frykhult
Original assignee: Ingenjörsfirman R. Frykhult Ab
Priority date: 1991-06-05
Filing date: 1991-06-05
Publication date: 1992-12-10

Abstract

In a rotary disc filtering apparatus, the filter discs (3) are partially immersed in a body of a liquid to be filtered. Above the body of liquid, there is spray nozzle means (13) for spraying jets (17) of high pressure fluid for removing formed cakes (7) of course particles from the sides of the filter discs. Guide members (14) are attached to the filter discs and arranged, such that said jets of fluid are guided by the guide members to hit against joints where the radially outer peripheral portions of the cakes adheres to the filter discs, as the guide members passes the jets of fluid during rotation of the filter discs.

Description

APPARATUS FOR FILTERING LIQUIDS

The present invention relates to apparatuses for filtering liquids, such as fibre pulp suspensions. More particularly the invention relates to an improvement in the removal of fibre cakes in the type of filtering apparatuses having rotatable filter discs.

A conventional filtering apparatus of the above-mentioned type includes at least one hollow filter disc adapted to be partially immersed in a body of a liquid to be filtered and arranged substantially transverse to a horizontal axis extending centrally through the filter disc. The filter disc has two opposed walls of filter material. There is drive means for rotating the filter disc about said horizontal axis. Means is provided for creating a pressure difference between said body of liquid to be filtered and the interior of the filter disc. During rotation of the filter disc, a fine fraction of the liquid is forced through the filter material into the hollow filter disc, and cakes of course particles, such as fibres are formed on the outside of the filter material. Spray nozzle means is situated at the radially outer periphery of the filter disc above said liquid body for spraying jets of high pressure fluid onto the opposed walls of filter material, to loosen said formed cakes of course particles form the filter material during rotation of the filter disc.

In the operation of the conventional filtering apparatuses when dewatering fibre pulp suspensions, a high capacity is desired, which means a high rotational speed. However, it is also often desired to provide the dewatered fibre cakes with a high dry solids content, which is achieved if the fibre cakes are allowed to remain on the filter material as long as possible from the moment when the filter disc carries the fibre cakes from the body of suspension as the filter disc rotates. Accordingly, to achieve said high dry solids content the rotational speed should be low.

Because of the large diameter of the filter disc, usually about 4 to 5 metres, it is inevitable in practice to prevent the radially outer end of the filter disc from oscillating axially more or less during rotation. As a consequence the spray nozzle means is conventionally adapted to spray broad jets of water to flush off the fibre cakes, in order to ensure the removal of the fibre cakes from an oscillating filter disc. The use of jets of water for fibre cake removal is a drawback, because it decreases the dry solids content of the fibre cakes. This could of course be avoided if jets of air could be used. However, in practice such jets of air have to be very sharp to hit the backside of the formed fibre cakes, in order to perform a proper removal operation, which is impossible in connection with an axially oscillating filter disc.

The principal object of the present invention is to provide a filtering apparatus of the type described above, which can be operated to produce a higher dry solids content of the cakes of course particles than hitherto has been possible.

Another object of the present invention is to provide a filtering apparatus of this type, which can be operated a a higher rotational speed than that of a conventional apparatus.

These objects are fulfilled by a filtering apparatus described initially, which is characterized by guide mean attached to the filter disc for rotation therewith and arranged, such that at each side of the filter disc the jets of fluid from the spray nozzle means are guided by the guide means in direction against a joint where the radially outer peripheral portion of a cake of course particles adheres to the filter material, as the guide means passes said jets of fluid during rotation of the filter disc.

The advantage is achieved that the jets of fluid hit said joint between the filter material and the cake of course particles independently of the occurring oscillation of the filter disc. Therefore, the volume of the jets of fluid, such as water, can be reduced resulting in a higher dry solids content of the cake of course particles. Also, the use of jets of air is now made possible, which results in an even higher dry solids content of the cake of course particles.

As an alternative to obtaining a higher dry solids content of the course particles, the filtering apparatus of the invention may be operated at a higher rotational speed, which increases the capacity of the apparatus.

The invention is described in more detail in the following by way of example with reference to the accompanying drawings, in which Figure 1 is a cross-section through a filtering apparatus of the invention,

Figure 2 is a side view along line II-II of Figure 1,

Figure 3 is a detail of a filter disc in the apparatus shown in Figure 1,

Figure 4 is a detail of an alternative embodiment of a filter disc for the apparatus shown in Figure 1,

Figure 5 is an enlarged view of a detail of a filter disc in the apparatus shown in Figure 1,

Figure 6 is a section along line VI-VI of Figure 5,

Figure 7 is a section along line VII-VII of Figure 5,

Figure 8 is a view from above along line VIII-VIII of Figure 5,

Figure 9 is a front view along line IX-IX of Figure 5,

Figure 10 is a view similiar to Figure 8 but showing an alternative embodiment of the guide means, and

Figure 11 is a section similar to Figure 6 but showing another alternative embodiment of the guide means.

The filtering apparatus shown in Figs. 1 and 2 comprises a container 1 for a fibre suspension and a rotatable hollow rotor 2 journalled in the container 1 about a horizontal axis. The rotor 2 is composed of three annular vertical filter discs 3, which are arranged in parallel relation- ship to one another. Each filter disc 3 has an annular series of hollow filter bodies 4 with walls of filter cloth 5. The interior of the filter bodies 4 communicate with the interior of the hollow rotor 2. (As an alterna- tive, the interior of the filter bodies 4 may communicate with a source of vacuum via a valve device.)

The filter cloth 5 on each filter disc 3, is supported by perforated plates 6. During operation, fibre cakes 7 are formed on the outside of the filter cloths 5 (Figs. 5 and 6).

The fibre suspension to be filtered is supplied to the apparatus through an inlet 8, the created dewatered fraction of the suspension is discharged from the apparatus through an outlet 9, and the created filtrate is discharged from the apparatus through an outlet 10. A drive means 11 is arranged for rotating the rotor 2. A distribution pipe 12 for conducting a fluid, such as water or air, is arranged at top of the filter discs 3. The distribution pipe 12 is provided with six spray nozzles 13, one spray nozzle 13 at each side of each filter disc 3, for spraying jets of high pressure fluid against the filter cloth 5 on each filter disc 3.

A guide member 14 is attached to each filter body 4 and is formed with surfaces 15,16 at mutual sides of the filter body 4 for guiding the jets 17 of high pressure fluid against radially outer peripheral joints where the formed fibre cakes 7 adhere to the sides of the filter body 4. Each guide member 14 is made of plastic material and is formed in one single piece by for instance injection molding.

In Figs. 5 and 7 there are shown additional surfaces 18, which are adapted to guide -the jets of high pressure fluid in the circumferential direction against the leading tips 19 of the fibre cakes 7 in relation to the direction of movement of the filter discs.

As an alternative, the guide members may be composed by pieces of pressed steel plates, as shown in Fig. 10. In this case, each guide member is suitably composed of three plate pieces, namely two side guiding pieces 20 and 21, and one connection piece 22, which are assembled by welding. These guide members are suitably attached to the radially outer periphery of the filter discs 3 by means of bolts 23-25.

The guide members shown in Fig. 11 consists of two side guiding pieces 26 and 27, which are provided with radially inwards extending suppporting elements 28 and 29 for supporting the fibre cakes as they are partially loosened from the filter cloths 5. These suppporting elements 28,29 are particularly of advantage when using jets of high pressure air to avoid the loosened fibre cakes from blowing away in an uncontrolled manner.

In some cases it may be suitable to divide each outer side of each filter body 4 into a number of operable filtering sections 33 by means of strip elements 31, in order to facilitate cake removal (Fig.3 ). In such a case, a guide member 14 is arranged at each filtering section 33.

As an alternative, the guide members 30 may have the same circumferential extensions as the radially outer circumferential extensions of the filter bodies 4 (Fig. 4). The length of the circumferential extensions of the guide members is due to the type of fibre cake to be removed. In operation, the fibre suspension to be dewatered is supplied to the container 1 through the inlet 8. By the influence of hydrostatic pressure (or a subpressure in the filter bodies 4 created by a source of vacuum during a part of each revolution of the filter discs 3) fibres adhere to the filter cloths 5 forming cakes 7 of fibres on mutual sides of the filter bodies 4. These formed fibre cakes 7 are carried up above the fibre suspension in the container 1 and when they reach the top of the revolution of the filter discs they are removed by means of the jets 17 of high pressure fluid from the spray nozzles 13. Since the guide members 14 direct the jets of fluid at intended positions independently of any ocurring axial oscillation of the filter discs 3, the upper edges of fibre cakes 7 initially are folded outwards (see the dash-dotted position 32 in Figs. 6 and 11). After said initial folding of the fibre cakes 7 the jets 17 of fluid hit the back sides of the fibre cakes 7, whereby any particular axial precision of the direction of the jets 17 of fluid no longer is required. Therefore, in most cases it would be sufficient to provide guide members having short circum¬ ferential extensions, provided that they are placed at the leading tips of the fibre cakes in relation to the direction of movement.

Claims

1. An apparatus for filtering liquids, such as fibre pulp suspensions, including at least one hollow filter disc (3) adapted to be partially immersed in a body of a liquid to be filtered and arranged substantially transverse to a horizontal axis extending centrally through the filter disc, said filter disc having two opposed walls of filter material (5), drive means (11) for rotating the filter disc about said horizontal axis, means (1,8,9) for creating a pressure difference between said body of liquid to be filtered and the interior of the filter disc, whereby during rotation of the filter disc a fine fraction of the liquid is forced through the filter material into the hollow filter disc and cakes (7) of course particles are formed on the outside of the filter material, and spray nozzle means (13) situated at the radially outer periphery of the filter disc above said liquid body for spraying jets of high pressure fluid onto the opposed walls of filter material, to loosen said created formed cakes of course particles from the filter material during rotation of the filter disc, c h a r a c t e r i z e d by guide means (14;20,21;26,27;30) attached to the filter disc (3) for rotation therewith and arranged, such that at each side of the filter disc the jets (17) of fluid from the spray nozzle means (13) are guided by the guide means in direction against a joint where the radially outer peripheral portion of a cake (7) of course particles adheres to the filter material (5), as the guide means passes said jets of fluid during rotation of the filter disc.

2. An apparatus according to claim 1, c h a r a c t e¬ r i z e d in that the guide means (14;20,21;26,27;30) comprises a surface (15,16) at each side of the filter disc (3) directed obliquely towards the filter disc, for guiding said jets (17) of fluid.

3. An apparatus according to claim 2, c h a r a c t e¬ r i z e d in that the guide means comprises support portions (28,29) for supporting said radially outer portions (32) of the cakes (7) of course particles, as the cakes are loosened from the filter material (5) during operation.

4. An apparatus according to any one of claims 1-3, in which the filter disc (3) is divided into a plurality of separate hollow filter bodies (4) disposed in series in the circumferential direction of the filter disc, c h a r a c t e r i z e d in that the guide means comprises separate guide members (14), each filter body being provided with at least one of said guide members.

5. An apparatus according to claim 4, c h a r a c t e¬ r i z e d in that the guide members (14;20,21;26,27) are situated at the leading parts of the filter bodies (4) in relation to the direction of rotation of the filter disc (3).