GB2129327A - Centrifugal strainer cleaning - Google Patents

Abstract

The residual layer of cake (6), which remains on the drum of a centrifugal strainer after the usual cake-removal step, is periodically removed by directing a flat jet of liquid (8), which may be recycled filtrate, at an angle to the drum in a direction opposed to its rotation. This repeatedly breaks up the layer and washes out the finer particles which clog the filter cloth (5). The angle of the jet and the radial distance of jet pipe (10) from the drum surface are independently adjustable. <IMAGE>

Description

SPECIFICATION Centrifuges The invention relates to centrifuges.

Particularly, it relates to the regeneration of the residual layer formed on the filtration surface of the centrifuge drum during rotation of the drum as a result of centrifugal force. Such a layer is compact and normally largely insoluble.

In the filtering of a suspension by means of a rotary centrifuge drum a cake of solid matter forms against the filtration surfaces, which can be removed from the filtration surface as a product in various ways. A so-called residual layer is normally left however which, through embedding of fine grain constituents becomes progressively more dense and impenetrable, and in time obstructs the outlet of the filtrate through the filtration surface, finally preventing it. The filtration process must then be interrupted and the residual layer regenerated, in many cases removed or eliminated, in order to re-establish the penetrability of the filtration surface for the filtrate.

When the material of the residual layer is soluble, this task can be accomplished relatively simply. A washing device is commonly used, such devices usually being available in installations with centrifuge apparatus. Such a washing device is used to spray the residual layer with a suitable solvent until the residual layer has dissolved. This of course takes a certain amount of time, during which the centrifuge must be inoperative. When the material of the residual layer is insoluble, regeneration thereof is more difficult. In this case the centrifuge is stopped and the residual layer removed manually by cutting, scraping or the like.

It is also known, as is the case for example in the so-called syphon centrifuges, to regenerate the residual layer by a stream of fluid which flows against the direction of filtrate flow through the filter surface, removes the residual layer from the filter surface and rinses it away. This procedure obviously requires relatively costly additional equipment.

The present invention is directed at the recovery of a compact and largely insoluble residual layer from a centrifuge drum, and seeks to provide a technique which requires minimal, sometimes no interruption in the operation of the centrifuge, and avoids the use of manual labour.

To this end, the method according to the invention comprises directing a substantially flat jet of fluid at the internal surface of the centrifuge drum at an acute angle to the drum surface as it rotates, and against its direction of rotation to generate a force greater than the centrifugal urging the material of the residual layer against the surface and thereby lift the layer from the surface: the material of the layer being disrupted, and reforming behind the jet as an additional filtration layer on the surface.

A centrifuge adapted to accommodate the above technique can be formed with a rotatable drum and a conduit extending within the drum bearing at least one nozzle for directing a substantially flat jet of fluid at an acute angle to the drum surface, means being provided for selectively delivering a fluid under pressure to the conduit to feed said at least one nozzle. It will be appreciated that an existing centrifuge can be readily and inexpensively adapted to include equipment enabling it to operate in accordance with the above method.

The jet of fluid is normally delivered in the form of a fan from each of a plurality of nozzles, the fans merging at or proximate the drum surface to form a flat jet at the surface. The angle at which the jet impinges on the surface can be varied, as can the spacing of the source of the jet from the surface.

This may be accommodated by mounting nozzles in a conduit which extends parallel to the drum axis and is mounted with respect to the drum for rotation about an axis spaced from both the drum and conduit axes, such rotation simultaneously altering the spacing of the conduit from the drum surface and the angle at which said jet is directed.

If needed, provision may also be made for axial movement of the jet. For example, the axial dimension of the jet where it contacts the drum may be less than the axial length of the filtration surface of the drum, the method including the step of moving the pipe axially within the drum to direct said jet against the drum surface over its entire axial length.

As the residual layer is lifted from the drum surface in accordance with the invention, disrupted and reformed, the fine grain constituents in the layer are exposed and rinsed away with the fluid, which may be recycled filtrate, through the filtration surface. The newly formed residual layer thus has a lower specific through flow resistance, and is therefore favourably penetrable for the filtrate. In this manner, the residual layer is regenerated.

The invention will now be described by way of example and with reference to the accompanying drawing wherein: Figure 1 shows an axial iongitudinal section through a centrifuge according to the invention; Figure 2 shows a partial section through the centrifuge, taken on line I-I in Figure 1; and Figure 3 shows a partial view onto the lid of the centrifuge in the direction of the arrow 11 in Figure 1.

The centrifuge illustrated has a housing 1 in which a centrifuge drum 2 is rotatably mounted.

The radially outer wall 3 of the centrifuge drum is provided with openings 4 for the removal of the filtrate, which is collected in the housing and is carried away therefrom. The centrifuge drum is lined with a supporting fabric and a filter cloth 5, which forms the actual filtration surface. On the filtration surface the product grows into a solid material cake, which from time to time is peeled from the filtration surface by a peeling device (not shown) and is removed from the centrifuge. After the peeling of the product, however, a residual layer 6 remains on the filtration surface, which layer becomes increasingly dense and impenetrable in time through the embedding of fine grain constituents, so that it obstructs or prevents the through flow of the filtrate through the filtration surface.Depending on the material to be treated, this can be an insoluble residual layer, which is pressed against the filtration surface 5 on rotation of the surface under the effect of centrifugal force. In order to re-establish the penetrability of the filtration surface 5 for the filtrate, this residual layer 6 must be regenerated.

The destruction of the residual layer 6 is achieved in that a flat jet of fluid 8, in the manner of a fan, forming an intersecting plane, is directed at an acute angle of intersection 1 8 against the residual layer. Thereby the jet of fluid 8 is directed against the direction of rotation of the filtration surface 5 or of the centrifuge drum 2, which is represented by the arrow 9 in Figure 2. The pressure of the jet of fluid onto the residual layer is greater than the pressure brought about by the centrifugal force, by which the residual layer 6 is pressed against the rotating filtration surface.

The jet of fluid acts against the residual layer like a scraper surface and is able to lift the residual layer from the filtration surface without, however, damaging the filtration surface. The lifted residual layer is broken up and shifted, and this restructured material deposits itself again on the filtration surface under the effect of the centrifugal force behind the jet of fluid, where it builds up a regenerated penetrable base layer on the filtration surface. This restructuring of the material on the filtration surface takes place in the rotary motion of the filtration surface, as is represented by the arrow 9 in Figure 2, behind the jet of fluid 8. The process can be carried out at normal operational speeds of the centrifuge drum several times successively in a relatively short period of time.

For example, at a drum speed of approximately 900 r.p.m., the residual layer is shifted 15 times per second. Therefore the drum speed does not neecl to be altered during the process, and if desired, regeneration may be carried out continuously.

In the procedure described, no inhomogeneities result in the distribution of the material along the filter surface and consequently no imbalances are brought about, since the scraped up or raised residual layer material deposits itself again under centrifugal force. The multiple shifting under the effect of centrifugal force also distributes the material uniformly and symmetrically on the filtration surface.

In the illustrated embodiment a pipe 10 is provided, which is arranged parallel to the drum axis 11 and projects into the interior of the centrifuge drum. The pipe is equipped with appropriate nozzles 12; i.e., so-called flat jet nozzles, which form the jet of fluid in a fan-like flat shape. The distance of the pipe 20 in relation to the filtration surface 5 or to the residual layer 6, and also the direction of the jet of fluid 8 formed by the nozzles 12 against the residual layer, can be adjustable by rotation of the pipe 10.The pipe 10 extends into the centrifuge drum through a lid 1 3 of the centrifuge housing and is rotatably mounted and, if required eccentrically mounted in a disc 14, which in turn is rotatably mounted in the lid 1 3. By rotation of the disc 14 in both directions which are indicated by reference numeral 1 5 in Figure 3, the distance of the pipe 10 to the residual layer 6 can be adjusted. A lever 1 6 is fastened to the pipe 10, by means of which the pipe is able to be rotated in the disc 14, specifically in the direction of the arrows which are designated 1 7 in Figure 3. Through this rotation of the pipe 10, the direction of the jet of fluid formed by the nozzles 12 can be adjusted at an acute angle of intersection 1 8 to the residual layer 6.The pipe 10 is also provided with a valve 19, with which the current of a fluid into or in the pipe 20 can be regulated. The pressure fluid is pumped into the pipe 10 by a high pressure pump not shown in the drawing. This pump must be designed such that it is able to supply a sufficient quantity of fluid at a sufficient pressure; e.g., over 10 bar. The fluid may be liquid or gaseous.

In the embodiment described, the pipe 10 extends over the entire length of the centrifuge drum 2. The length of the section of the pipe 10 which is equipped with the nozzles 12 corresponds to the length of the filtration surface of the centrifuge drum. The nozzles 12 are arranged in a row along the pipe 10. In certain cases it is though, possible to equip the pipe 10 for example with nozzles arranged in two or more rows.

It is also possible that only a partial section of the pipe 10 would be equipped with the nozzles.

The pipe would then be displaceable parallel to the axis 11 of the centrifuge drum in axial direction so that the residual layer can be regenerated over the entire internal surface of the drum.

Claims (14)

1. A method of regenerating a residual layer of material deposited on the internal surface of a centrifuge drum while the drum is in rotation, which method comprises directing a substantially flat jet of fluid at the internal drum surface at an acute angle to the drum surface and against its direction of rotation to generate a force greater than the centrifugal force urging the material against the surface and thereby lift the layer from the surface; the material of the layer being disrupted and re-forming behind the jet as an additional filtration layer on the internal drum surface.

2. A method according to Claim 1 wherein the jet is delivered from a nozzle in the form of a fan.

3. A method according to Claim 1 or Claim 2 wherein the fluid comprises recycled filtrate from the centrifuge.

4. A method according to any preceding Claim wherein the jet of fluid is delivered from a pipe extending within the drum parallel to the rotational axis thereof.

5. A method according to Claim 4 wherein the axial dimension of the jet where it contacts the drum is substantially equal to the axial length of the filtration surface of the drum.

6. A method according to Claim 4 wherein the axial dimension of the jet where it contacts the drum is less than the axial length of the filtration surface of the drum, the method including the step of moving the pipe axially within the drum to direct said jet against the drum surface over its entire axial length.

7. A method according to any preceding Claim wherein the jet is directed at the drum surface intermittently to regenerate the residual layer.

8. A method according to any preceding Claim wherein the centrifuge drum is rotated at its operational speed while the jet is directed at its internal surface.

g A method of regenerating a residual layer of material deposited on the internal surface of a centrifuge drum substantially as described herein with reference to the accompanying drawing.

10. A centrifuge having a rotatable drum and a conduit extending within the drum bearing at least one nozzle for directing a substantially flat jet of fluid at an acute angle to the drum surface, means being provided for selectively delivering a fluid under pressure to the conduit to feed said at least one nozzle.

11. A centrifuge according to Claim 10 wherein said at least one nozzle is adapted to form a jet with an axial dimension less than that of the drum surface, and including means for moving said at least one nozzle axially within the drum to direct said jet at the surface of the drum over its entire axial length.

12. A centrifuge according to Claim 10 or Claim 11 including means for altering the angle to the drum surface at which said jet is directed.

1 3. A centrifuge according to any of Claims 10 to 1 2 including means for altering the spacing of the conduit from the drum surface.

14. A centrifuge according to Claim 12 and Claim 13 wherein the conduit extends parallel to the drum axis and is mounted with respect to the drum for rotation about an axis spaced from both the drum and conduit axes, such rotation simultaneously altering the spacing of the conduit from the drum surface and the angle at which said jet is directed.

1 5. A centrifuge substantially as described herein with reference to the accompanying drawing.