WO1991012067A1

WO1991012067A1 - Rotating disc filter

Info

Publication number: WO1991012067A1
Application number: PCT/SE1991/000117
Authority: WO
Inventors: Folke Jakobson; Harry Nilsson
Original assignee: Celleco-Hedemora Ab
Priority date: 1990-02-16
Filing date: 1991-02-18
Publication date: 1991-08-22
Also published as: SE9000557L; SE466189B; SE465857B; SE9000557D0; SE9101845L; SE9101845D0

Abstract

A rotating filter, including at least one filter disc (19) arranged at the outer periphery of a fluid tight drum (7) adapted to internally receive liquid (39) to be filtered, to lead the liquid to the interior of the filter disc (19), to rotate about a collecting trough (21) for solid material deposited in the filter disc and falling therefrom, and to be partially submerged in a container (1) for filtrate (40), said filter disc (19, 20) comprising a plurality of filter sectors (16). The liquid tight drum (7) has a regularly polygonal section having flat outer limiting surfaces (7'), and each filter sector (16) has a flat connecting surface (15) for sealed mounting against one of the flat limiting surfaces (7') of the drum.

Description

ROTATING DISC FILTER

The present invention concerns a rotating filter, including at least one filter disc arranged at the outer periphery of a fluid tight drum adapted to internally receive suspension to be filtered, to lead the suspension to the interior of the filter disc, to rotate about a collecting trough for solid material deposited on the filter disc and falling therefrom and to be partially submerged in a container for filtrate.

A filter of similar kind is known from Swedish Patent

Specification 224,131. In such a filter filtration takes place from the interior of the filter disc outwardly to the

container, solid material thus depositing inside the filter disc. This filtering direction is primarily used when

filtering very small particles, so called fines, requiring the very finely meshed filter cloth to be sprayed from the back side, i.e., in the opposite direction of the filtering

direction, in order to be released from particles clogging the filter cloth. The filter known from the patent specification mentioned has a cylindrical drum and several parallel filter discs having annular filter cloth portions as side surfaces. Each such filter disc, thus, comprises two relatively large annular filter cloth pieces.

Apart from the difficulty to keep such large filter cloth pieces stretched, as is necessary in order to resist the filtering pressure as well as the oppositely directed spray pressure, a possible damage of the cloth involves exchange of the entire cloth. It would be desirable, thus, to be able to divide the cloth into smaller parts, such as sectors, as is common in filters where the filtration takes place from the outside towards the inside. In the kind of filtration now concerned, where the suspension or other liquid to be filtered is contained within a drum, naturally, due to the internal pressure, problems arise in the sealing between the drum and a filter sector, which problems do not exist in the other kind of filter. If a cylindrical drum known from the patent specification mentioned should be provided with filter discs, each comprising a plurality of filter sectors, the labour and cost

contribution to achieve the absolute tightness between the drum and all the filter sectors necessary when filtering so called fines would be quite too high to make such a structure profitable, since the drum as well as each filter sector must be extremely accurately worked in order to achieve corresponding arcuate shapes of the connecting areas and thereby the conditions for a satisfactory sealing.

The object of the present invention, therefore, is to provide a filter of the kind initially stated, in which a filter disc comprising a plurality of filter sectors can be utilized and in which the sealing problem is substantially set aside. In accordance with the present invention this has been achieved in that the drum has been given a regularly polygonal section having flat outer limiting surfaces and in that each filter sector is provided with a flat connecting surface to be sealingly mounted against one of the flat limiting surfaces of the drum. It is evident that the sealing between the flat drum surfaces and the flat filter sector surfaces involves a considerably less problem than if these surfaces were curved.

The invention will be described in the following, reference being made to the accompanying schematic drawings, wherein:

- Fig. 1 is an axial section through a disc filter according to the invention,

- Fig. 2 is an end view of the disc filter according to Fig. 1 seen from the left in said Figure, - Fig. 3 is a portion of a cross section at an enlarged scale through the disc filter according to Fig. 1, the section being taken outside the left filter disc of Fig. 1,

- Fig. 4 shows a filter sector seen in the axial direction of the filter, and

- Fig. 5 is a view of the filter sector according to Fig. 4 seen from its inlet end.

The disc filter according to the present invention is, as far as its basic structure is concerned, constructed substantially in correspondence with a conventional drum filter having filter discs. Thus, in a machine stand 1 having a forward end- wall 2 and a backward end wall 3 is arranged a filtrate vessel 4. Portions of the end walls 2 and 3 also constitute the for- ward and backward walls of the vessel. By means of bearings 5 a shaft 6 rotatably driveable in a non-shown manner is

journaled on a bracket attached to the backward end wall 3. On the inner end of this shaft extending through the backward end wall 3 a hollow shaft or drum 7 is non-rotatably mounted, viz., by means of a wall 8 tightly closing the end of the drum facing the backward end wall 3. According to the present invention, the drum has a regularly polygonal shape, in the example shown being hexagonal (Figs. 2 and 3). Of course, other cross sectional shapes can exist, such as octagonal. In any case, according to the present invention, the periphery of the drum 7 comprises flat limiting surfaces 7'. The end of the drum 7 facing the end wall 2 is open and is terminated by a circular ring 9 slide-sealingly contacted by a likewise annular lip seal 10 carried by an annular cylinder 11 arranged at the end wall 2. More precisely, the cylinder 11 is mounted at the inside of a door 12 tightly closing the forward end wall 2.

In the embodiment shown in Fig.3 the drum 7 is built up of three plates 13 each circumferentially extending over two sides of the drum, i.e., having two flat surfaces 7', while, in the embodiment shown in Fig. 4, it is composed of six flat plates 13', each covering one side of the drum 7. In an embodiment not shown, the drum is composed of two plates, each, thus, comprising one half of the drum, i.e., three flat surfaces 7'. In all cases the plates are tightly interconnected along the edges of the drum, e.g., by means of flanges 14 extending in the longitudinal direction of the plates. Thus, the drum is tight per se, apart form its open end which, however, is sealed by means of the seaing

arrangement described above.

In the flat surfaces 7' of the plates 13 and 13' are openings 7'', and, on the outside of the plates, over these openings filter sectors 16 are mounted by means of flat fastening flanges 15. One such fastening flange is shown in Fig. 4 together with a corresponding plate 13' and separately in Fig. 5. As appears, the filter sector 16 in a conventional manner substantially has the shape of a circular sector having a truncated tip and comprises a non-shown framework on which a filter cloth 17 is arranged. Radially outwardly the filter sector is closed, while it radially inwardly has an opening 18 the shape of which corresponds to the shape of the openings 7'' in the plates 13 and 13' (the opening of plate 13' not being shown in Fig. 4). In a non-shown manner the flat

fastening flanges 15 of the filter sectors 16 are sealed against the flat surfaces 7' of the plates 13 and 13'. As also appears from Figs. 4 and 5, the opening of the filter sector - and, consequently, the corresponding opening 7'' of the plate 13' - extend perpendicularly to the direction of the axis of the drum over a substantial portion of the flat surface 7'. Hereby is achieved a very stable mounting of the filter sector to the drum, and the openings 7'' and 18 can be given great through flow areas. In the example shown, the six-sided drum 7 carries totally tv/elve filter sectors arranged as two filter discs 19 and 20, each comprising six filter sectors 16, and each filter sector extending over a substantial portion of the width of one of the flat surfaces 7' of the drum.

A collecting trough 21 extending into the drum 7 somewhat beyond the inner filter disc 20 is rigidly and liquid tightly mounted onto the inside of the door 12. The bottom 22 of the collecting trough is somewhat inclining from the inner end towards the door 12 where it merges with an outlet 23 for solid particles. Also rigidly and liquid tightly mounted at the door 12 is an inlet tube 24 for the. liquid to be filtered. The inlet tube 24 extends into the drum at least to the inner filter disc 20. Preferably, the inlet tube 24 has openings or tube pieces 25 at its periphery located opposite to the filter discs 19 and 20, respectively, to direct the incoming fluid towards the respective filter disc. Preferably, these openings are directed obliquely upwards and outwards. In Fig. 3 are indicated bars 26 and 27 by means of which the inner end of the inlet tube 24 is supported by the collecting trough 21.

A filtrate outlet 28 extends from the vessel 4 through the end wall 2.

On the machine stand is mounted a spray device 35 serving to direct jets of water via an inlet 36, an axially directed conduit 37 and conduits 38 connected thereto and extending along the entire radial extension of the filter discs and having nozzles directed towards the filter sectors, towards the filter cloth 17 of the filter sectors for the purpose of cleaning it from particles clogging the openings of the filter cloth, but also for knocking off larger particles deposited on the inside of the filter cloth. The function of the disc filter according to the present invention is the following. The liquid or suspension 39 to be filtered is introduced through inlet tube 24 and its openings 25 into the interior of the liquid tight drum 7. Due to the location of the inlet tube 24 next to the collection trough and the outlet 23 (on the left hand side of the latter

according to Figs. 2 and 3) and relatively close to the periphery of the drum 7, the liquid 39 is directed by the openings 25 towards the opening 18 of a filter sector 16 which, during its rotation, lowers itself below the liquid level. The normal operational level of the liquid 39 is indicated at L₁ (Fig. 3) and is always below the upper edge of the collecting trough 21. The liquid will now flow into the filter sectors located below the liquid level L₁ through the openings 7'' in the drum 7 and the openings 18 in the filter sectors, and, under filtration through the filter cloth thereof, out to the vessel 4 and therefrom to the outlet 28. By controlling the normal operational level L₂ of the filtrate 40 in the vessel 4 such that it is always below the level L₁, the filtering process takes place solely depending on the static pressure difference between the levels L₁ and L₂.

Naturally, some filtration of liquid remaining in a filter sector takes place during the time it raises above the liquid level L₂. When the edge of a filter sector leading in the direction of rotation is in a position approximately two o'clock, i.e., where the spray conduit 38 is located in Fig. 2, the spray device 29 sprays the outside of the filter cloth 17. Particles hereby released from the filter cloth fall, or, follow the water down through the interior of the filter sector and is collected in the collecting trough 21, from where they are discharged through the outlet 23.

Claims

1. A rotating filter, including at least one filter disc (19, 20) arranged at the outer periphery of a fluid tight drum (7) adapted to internally receive liquid (39) to be filtered, to lead the liquid to the interior of the filter disc (19, 20), to rotate about a collecting trough (21) for solid material deposited in the filter disc and falling therefrom and to be partially submerged in a container (1) for filtrate (40), said filter disc (19, 20) comprising a plurality of filter sectors (16), c h a r a c t e r i z e d i n that the liquid tight drum (7) has a regularly polygonal section having flat outer limiting surfaces (7'), and in that each filter sector (16) has a flat connecting surface (15) for sealed mounting against one of the flat limiting surfaces (7') of the drum.

2. A rotating filter according to claim 1, wherein each of the flat limiting surfaces (7') of the drum is provided with at least one opening (7'') the shape of which corresponds to an opening (18) in each filter sector (16) for leading liquid to be filtered from the drum towards the interior of the filter sector, c h a r a c t e r i z e d i n that the openings (7'') in the flat limiting surfaces (7') of the drum extend perpendicularly to the direction of the drum axis over a substantial portion of the width of the flat limiting surfaces (7').

3. A rotating filter according to claim 1 or 2,

c h a r a c t e r i z e d i n that an inlet conduit (24) for suspension emerging into the drum (7) extends into the drum and is provided with an outlet opening (25) opposite to each filter disc (19, 20).

4. A rotating filter according to claim 3,

c h a r a c t e r i z e d i n that each outlet opening (25) is directed obliquely uppwards and radially outwards towards a corresponding filter disc (19, 20).