GB2076682A

GB2076682A - Centrifugal filters

Info

Publication number: GB2076682A
Application number: GB8106603A
Authority: GB
Original assignee: Uhde GmbH
Current assignee: ThyssenKrupp Industrial Solutions AG
Priority date: 1980-03-26
Filing date: 1981-03-03
Publication date: 1981-12-09
Also published as: GB2076682B; DE3011622A1; FR2479012B3; DE3011622C2; FR2479012A1; NL8100831A

Abstract

A filter comprising a stack of filter discs which are periodically rotated to remove cake centrifugally has a cake discharge device comprising two or more scrapers (6 and 7) which are so inclined relative to radial lines as to direct the filter cake in the filter casing (1) to an annular zone which includes the discharge opening (9) in the casing floor. The scrapers may be arranged on a wheel-like device (5) having a rim (10), spokes (12) and a hub (11) which is positioned on a central shaft (2) in the filter casing (1) beneath the filter discs and rotates with them. Intermediate radially oriented scrapers may be included. The position and inclination of scrapers may be adjustable. <IMAGE>

Description

SPECIFICATION Centrifugal filters The present invention relates to centrifugal filters.

Centrifugal filters, also known as disk-type pressure filters, having a plurality of filter disks arranged on a hollow shaft become coated during filtration with the material suspended in the liquid which coating is periodically removed by centrifugation, the filter residue falling to the bottom of the filter casing in which there is a discharge opening. The residue on the filter disks usually has a pasty or even a solid consistency.

With such filter devices, the residue collects at the base of the device inside the casing and it is important to remove that residue from the casing of the filter within a short time, to reduce the length of the shut-down periods. If prolonged cleaning periods to remove the filter residue are required, it would be necessary to have another, back-up filter in operation to provide continuous filtration.

It has previously been proposed, in German Gebrauchsmuster No. 6 809 712 to provide rotating discharge paddles at the bottom of such centrifugal filters, said paddles rotating much more slowly than the shaft of the filter. With the aid of an additional gear, which is usually of the planetary type, the speed of the filter shaft may be selected such that the centrifugal force transmitted to the residue is just sufficient to enable it to drop to the bottom of the filter casing easily and thus reach the discharge opening.

However, the presence in the filter casing of filter elements moving at different speeds results in relative motions, which means that additional seals are imperative in the filter casing.

German Gebrauchsmuster No. 6 925 523 describes a centrifugal filter which is equipped with discharge scrapers which are physically attached to the filter shaft. A filter of this type has at least one outer scraper and one inner scraper.

This discharge device is connected by simple means to the drive of the hollow shaft of the filter and rotates during the centrifugal phase at the same, high speed as the filter disks. The outer scraper, which passes over the discharge opening, is inclined forward in the direction of rotation to press the residue into the discharge opening as it passes over said opening. To prevent excessive forces acting on the discharge scrapers in the event of a high accumulation of residue within a relatively short time, circular openings are provided in the discharge scrapers, which means that part of the residue passes through these openings into the space behind the scrapers and is not pushed in the direction of the discharge opening.This arrangement of the discharge scrapers results in the residue being continually centrifuged in an inward and outward direction with only a proportion of the residue being discharged from the filter casing when the outer scraper passes over the discharge opening.

There is a need for a centrifugal filter which gives a continuous, uniform discharge of the centrifuged residue without part of the residue rotating several times on the bottom of the filter casing around the axis of rotation of the filter.

The present invention provides a centrifugal filter in which filter cake is removed from a filter surface by centrifugation and is discharged through a discharge opening wherein the centrifugal filter is provided with two or more scraping members capable of rotation about a central axis, there being at least one outer and one inner scraping member, the outer scraping member having the outer edge as the leading edge and the inner scraping member having the inner edge as the leading edge. Suitably there is at least one additional scraping member present which is so positioned as to pass over the discharge opening when moved. Holes are not necessary in the scraping members.

Suitably the cake discharge device consists of a star-shaped scraper equipped with a number of radial arms, at least one discharge scraper being attached to each arm, the outer scrapers having the outer edge as the leading edge and the inner scrapers having the inner edge as the leading edge.

Suitably the scraping members are arranged on one or more arms extending radially from the centre of the base, preferably such that there is at least one scraping member on each arm and especially such that there is at least one outer scraping member and at least one inner scraping member on the same arm. It is another embodiment of the invention that scraping members are so arranged on the arms that they are staggered from the outer edge of the base to the centre of the base in the shape of a spiral of at least one flight.

According to a further embodiment of the invention, the position of the scraping members is adjustable in order to obtain the optimum discharge period for the various residues, the degree of inclination of the edges of each of the scraping members being the same or different.

The advantages obtained with the invention lie particularly in the fact that a residue flow is achieved in the bottom part of the filter casing, the maximum depth of the material being on the radius of the discharge opening. Contrary to the hitherto known types, the residue is not centrifuged alternately from the outside to the inside and vice versa to fall or be pressed into the discharge opening, which is suitably eccentric, when it happens to pass it, but is continually directed into the annular area above the discharge opening. It then drops automatically into the discharge opening.

One form of centrifugal filter of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a sectional view of the centrifugal filter; Figure 2 shows a horizontal cross-sectional view of the centrifugal filter taken above a filter scraper unit; and Figure 3 shows a horizontal cross-sectional view of the centrifugal filter as in Figure 2 with a modified filter scraper unit.

Referring first to Figure 1, a cylindrical filter casing (1) is in the form of a vertical pressure vessel. A filter package contained therein comprises a central shaft (2) which is supported by bearings in the vessel lid and in the bottom (4) of the casing (1) and on which a plurality of filter disks (3) having intermediate seals are arranged.

A scraper unit (5), which is rigidly fixed to the central shaft (2), is located beneath the filter package. The lower edge of each of the discharge scrapers, outer discharge scrapers (6) and inner discharge scrapers (7), attached to the scraper base (5) is adapted to the shape of the bottom (4) of the casing (1 ) with a minimum tolerance.

Beneath the scraper base (5) and displaced to one side is a discharge opening (9).

Referring now to Figure 2, the scraper unit (5) comprises an annular rim (10) which is concentric with the casing (1) and which is supported from a hub (11) surrounding the shaft (2) by a plurality of spokes (12). The outer discharge scrapers (6) are positioned under the rim on alternate spokes and are angled in the direction of rotation of the shaft (2) (as shown by the arrow). The inner discharge scrapers (7) extend from the hub (11) at the base of the same spokes which carry the outer discharge scrapers (6) and are also angled in the direction of rotation though to a lesser degree than the outer discharge scrapers (6). The discharge opening (9) is displaced to one side of the casing (1).

Referring now to Figure 3, the scraper unit (5) made up of rim (10), hub (11) and spokes (12) has an additional set of intermediate scrapers (8) positioned parallel to and adjacent the spokes which do not carry the outer discharge scrapers (6). The casing (1), central shaft (2) and displaced discharge opening (9) are shown.

During filtration a suspension is introduced at the top of the centrifugal filter and runs down over the filter disks, the filter disks become coated with filter residue (also called the filter cake). The cake may be subjected to aftertreatment, for example washing, and then discharged. In order to remove the cake, the filter package is rotated by means of an external drive motor with the result that the cake on the filter disks (3) is centrifuged outwards against the wall of the casing (1). From there, the cake drops down and collects on the bottom (4) of the casing (1). The discharge scrapers (6 and 7) convey the cake which accumulates on the bottom (4) towards the cake discharge opening (9) and the cake than leaves the casing through this opening. In detail, the filter cake is conveyed by the outer discharge scrapers (6) in the direction of rotation to the annular discharge area incorporating the discharge opening (9) and is also conveyed by the inner discharge scrapers (7) likewise in the direction of rotation to the annular discharge area. The solids flow reaches its maximum height at the radius of the discharge opening (9). The provision of the intermediate discharge scrapers (8) promotes the transport of the cake within the annular discharge area to the discharge opening (9) as those scrapers are positioned to travel over that opening. As can be seen from Figures 2 and 3, the trailing edges of the outer and inner discharge scrapers (6 and 7) extend slightly over the perimeter of the discharge opening.

Claims

1. A centrifugal filter in which filter cake is removed from a filter surface by centrifugation and is discharged through a discharge opening wherein the centrifugal filter is provided with two or more scraping members capable of rotation about a central axis, there being at least one outer and one inner scraping member the outer scraping member having the outer edge as the leading edge and the inner scraping member having the inner edge as the leading edge.

2. A centrifugal filter as claimed in claim 1, wherein at least one additional scraping member is present which is so positioned as to pass over the discharge opening when moved.

3. A centrifugal filter as claimed in claim 1 or claim 2, wherein the scraping members are arranged on one or more arms extending radially from the centre of the base.

4. A centrifugal filter as claimed in claim 3, wherein there is at least one scraping member on each arm.

5. A centrifugal filter as claimed in claim 3 or claim 4, wherein there is at least one outer scraping member and at least one inner scraping member on the same arm.

6. A centrifugal filter as claimed in any one of claims 3 to 5, wherein the scraping members are so arranged on the arms that they are staggered from the outer edge of the base to the centre of the base in the shape of a spiral of at least one flight.

7. A centrifugal filter as claimed in any one of claims 1 to 6, wherein the alignment of the scraping members is adjustable.

8. A centrifugal filter as claimed in any one of claims 1 to 7, wherein the positioning of the scraping members is adjustable.

9. A centrifugal filter as claimed in claim 1, which is substantially as described herein with reference to and as illustrated by the accompanying drawings.

10. A disk-type pressure filter having a centrifugal cake discharge device and a discharge opening at the bottom of the casing, wherein the cake-discharging device consists of a star-shaped scraper having a plurality of radial arms, to which at least one discharge scraper is attached, the outer scraper having the outer edge as the leading edge and the inner scraper having the inner edge as the leading edge.