WO1993000979A1

WO1993000979A1 - Method of drying a filter cake using a surface layer

Info

Publication number: WO1993000979A1
Application number: PCT/EP1992/001584
Authority: WO
Inventors: Werner Stahl
Original assignee: Maschinenfabrik Andritz Actiengesellschaft
Priority date: 1991-07-12
Filing date: 1992-07-13
Publication date: 1993-01-21
Also published as: DE4123144C1; AU2320292A

Abstract

The invention proposes a method of producing substantially dry filter cakes. The invention calls for the filter cake to be covered with a surface layer after it has been formed, the surface layer enabling the filter cake to be pre-consolidated by compression at pressures which are higher than the capillary admission pressure of the material in the filter cake.

Description

Process for dehumidifying a filter with a top layer

The invention relates to a method for dehumidifying a filter cake on a filter medium, in particular by means of press filtration, and a device which is preferably suitable for carrying out this method.

The problem with such methods is that the compressibility of the filter cake increases continuously as the particle distribution becomes finer. Likewise, the susceptibility of the filter cake to cracking increases during the subsequent drying process by means of vacuum or pressure filtration. This behavior occurs to a particularly high degree when the sludge or suspensions to be dehumidified are flocculated and then a filter cake is produced which is then dehumidified by pressure filtration, that is to say blown through.

These cracks in the filter cake lead to phenomena such as incorrect air suction and washing water losses, which are associated with a higher expenditure of energy. In order to avoid these phenomena, two ways are followed in a known manner:

(a) The cake-forming filtration is no longer carried out openly. This means that a gas for dehumidification is not directly applied to the filter cake, but that the filter cake is dehumidified in a closed room. These processes are used in filter presses and filter presses with membranes. With high pump pressure the Sludge or the suspension is already compressed during cake formation so that the solid particles are pressed together so tightly that they touch. A later shrinking of the filter cake therefore does not occur when dehumidifying.

(b) The use of a press belt in the drum filter compresses the thickness of the filter cake and increases the capillary inlet pressure due to the reduced porosity. These methods are used in vacuum filtration and are e.g. described in DE 38 29 936 AI. There a rubber band is pressed onto the filter cake, air being able to be sucked through the compressed filter cake after the rubber band has run out, without cracking occurring, since the particles of the filter cake have already been pressed very strongly together by the pre-compression.

If one wants to transfer this process of pre-compressing the filter cake with a press belt from vacuum to pressure filtration, it must be taken into account that the effect of the gas differential pressure on the rubber band increases significantly. The pressing of the filter cake by the guide rollers of the rubber band becomes secondary here, because the surface pressure of the rubber band on the filter cake due to the differential pressure during pressure pressing acts in a relatively long time in contrast to the pressing of the rollers. If a press belt were used in pressure filtration, the expenditure on equipment would therefore be reduced since the guide rollers for the press belt would no longer have to be designed to apply pressure to the filter cake. Nevertheless, the use of the press belt, in particular rubber band, is associated with disadvantages. A press belt, even when using guide rollers that do not exert any contact pressure on the filter cake, is associated with a higher outlay in terms of apparatus, which relates in particular to the lateral guidance of the belt.

In an economically viable way, press belts can only be used for drum or belt filters, since the belt has to roll in one direction on the filter cloth. The use e.g. on disc filters is therefore not possible.

The use of press belts becomes problematic if the filter cake thickness is not uniform, or if a matrix filter cloth is used to obtain a filter cake divided into several "cookies". Especially with this highly compressible filter cake, which therefore tends to crack, pre-compression is necessary.

However, for the reasons mentioned above, this is not possible either with the press belt or in the open process, since the capillary inlet pressure is greatly reduced due to the flocked material.

It is therefore an object of the invention to provide a method of the type mentioned above and an apparatus for carrying out the method, which enable efficient dehumidification of filter cakes with a low capillary inlet pressure and with little outlay on equipment.

This object is achieved according to the invention by the method according to claim 1 and the device according to claim 12. Advantageous further developments are the subject of the corresponding subclaims. The idea of the invention is to apply a layer on the filter cake, the capillary inlet pressure of which is higher than that of the filter cake. This allows the filter cake to be pressurized with a dehumidification pressure which is far above the capillary inlet pressure of the filter cake. Due to the higher dehumidification pressure, the filter cake is better compacted and cracking during the subsequent blowing through of the filter cake is avoided.

The invention can be implemented in various ways. For example, a foil, for example a plastic or aluminum foil, is placed on the filter cake. The filter cake can then be removed from the filter medium with the film.

The film can be used for both continuous and discontinuous filter processes. The use of a film in a continuous process on a drum or belt filter appears to be particularly advantageous. The use of foils does not only make sense for vacuum filters, but also for pressure filters.

Another possibility of applying a layer with a smaller pore size, which is associated with a correspondingly higher capillary inlet pressure, is to apply a further suspension layer as a top layer to the filter cake.

Here, the application of a suspension of a fine clay mineral, such as kaolin or bentonite, is advisable. However, the cover layer does not necessarily have to consist of a further suspension. Applying a more viscous liquid to the filter cake would also lead to an increase in the pressing or dehumidifying pressure. The additional Layer can, for example, be sprayed on in a spray mist or layered in another immersion bath. The cover layer with the fine porosity could also be attached to the filter cake by foaming the suspension. The disadvantage of these suspension layers is that a foreign substance is applied to the filter cake, which either has to be left in the filter cow or has to be removed again by further treatment, for example a thermal treatment.

In an advantageous development of the invention, the cover layer with a low capillary inlet pressure consists of the same material as the filter cake. This is done in the following way:

The suspension stream is e.g. divided in a separating or classifying device. This can be done, for example, by feeding the suspension stream to a separating or classifying device, the overflow of which is used for the top layer and the underflow of which is used for the filter cake. To increase the cake formation rate, e.g. a flocculant is added to the partial flow in the underflow, while a thickening agent is added to the partial flow in the overflow, which increases the viscosity and prevents the applied suspension from running off the filter cake before a layer is formed.

The addition of a flocculant and / or the classification of the suspension have the effect that the partial flow in the underflow after the cake formation is relatively large-pored and thus has a low capillary inlet pressure but a high cake formation rate. In contrast, the partial flow in the overflow is very fine-grained and forms a layer on the filter cake, the pore size distribution of which is shifted towards lower pore radii. This layer therefore has a much larger capillary inlet pressure than the filter cake itself.

Therefore, a much higher pressing or dehumidifying pressure can be applied to the filter cake than the capillary inlet pressure of the filter cake would allow. Due to the higher pressure, the filter cake is pre-compressed better and can therefore be dehumidified to a lower res moisture without cracking.

This method can also be used in the production of divided filter cakes using matrix filter cloths. In these matrix filter cloths, the filter medium is divided by impermeable webs. The thickness of the filter cake is greatly reduced, if not interrupted, in the area of these webs. While dehumidification of these filter cakes with a press belt was almost impossible, such variable-thickness filter cakes can now also be dehumidified with higher pressure.

A small amount of less than about 5% of the suspension is required for the top layer. This part is branched off from the feed line before flocculation and is evenly applied to the surface of the filter cake emerging from the suspension on its surface. The suspension for the top layer can be sprayed, atomized, dripped or agitated. The suspension is sucked in on the surface of the filter cake and forms a thin solid layer on the actual filter cake. Since this material is very fine and not flocked, it is much denser than the filter cake itself and thus has a much higher capillary inlet pressure. Because of its constant moisture content, this layer remains plastic and, because of the constant saturation of the deformation of the cake underneath, it can consequences.

There are a number of possible variants of how the suspension to be sprayed could be better processed in order to achieve a largely air-impermeable coating or "varnishing" of the filter cake. Too thin a suspension would have to be thickened in this sense. This could e.g. can be realized by concentrating the corresponding suspension.

Many filter cakes are still thixotropic even after they have been compressed or desaturated by air blowing. Such filter cakes can be liquefied again into a paste by vigorous shearing. This could be sprayed with or without thinning with the suspension for the top layer. When using a separating or classifying device, the fine fraction of the solid is used in accordance with the particle size distribution in order to produce the top layer. If you want to increase the pressing or dehumidifying pressure even further to improve the squeezing effect, it may be that the finer grain distribution of the cover layer is not sufficient to increase the capillary inlet pressure. In this case, the pre-classified partial flow would have to be further reduced in a fine mill, which would result in a corresponding reduction in the pore size and consequently an increase in the capillary inlet pressure. Additives that counteract Newtonian flow behavior (such as bentonite or fiber materials) can also be added to the suspension.

To get a good pre-compression of the filter cake, de;,. Compression pressure for the pre-compression of the cake higher than _. the capillary inlet pressure of the filter cake, but set lower than the capillary inlet pressure of the cover layer. After pre-compression the filter cake is dehumidified by applying a gas which is above the capillary inlet pressure of the cover layer. The precompressed filter kitchen is then blown through by the gas and thus blown dry. Technically, this is realized with a drum pressure filter in such a way that different pressure differences are applied to the filter cake in different angular ranges of the filter.

Since the gas pressure in the boiler for all areas of Kuchenbil- ung, the compression phase and dehumidification ie der¬ same, the gradation can be done by a back pressure in the various individual phases zu¬ parent ^"separators is constructed separately. Pressure ¬ Regulation can take place via control valves.

In this way it is also possible to set different pressure differences between the suspension space and the filtrate space during the cake formation phase. A lower pressure difference is preferably applied at the beginning of the cake formation in order to counteract blocking of the filter cloth or collapse of the first cake layer. Later the layers filtered on it can be formed with higher pressure differences.

In this sense, it is therefore advantageous to provide each zone for the cake formation, the top layer application, the compression and the dehumidification with at least one, preferably several separate separators.

The invention is described below, for example, in the schematic drawing. In this show: 1 shows a side view of a drum filter for producing a filter cake with a cover layer;

FIG. 2 shows a further embodiment of the drum filter from FIG. 1;

Fig. 3 shows an arrangement for the suspension supply to the drum filter according to FIGS. 1 and 2 and

4 shows further embodiments of suspension feeds to 7 gene according to FIG. 3rd

1 shows a drum filter 10 with a cylindrical filter drum 12 which is partially immersed in a suspension bath 14. The suspension bath 14 is connected to a suspension feed 16. In the direction of rotation A of the filter drum 12, the suspension bath 14 is followed by a sprinkler system 18 which is connected to a second suspension feed 20. The largely dehumidified filter cake is removed from the filter drum 12 by means of a scraper or deflector 22.

The drum filter 10 operates continuously, certain process steps being carried out in certain sectors 24 to 38 of the filter drum 12.

In sectors 24 to 28, a filter cake is formed on the filter medium of the filter drum 12 spanning the lateral surface. This cake formation phase is divided into three sections 24 to 28, in which cake formation takes place with different differential pressures. The outer surface of the filter drum 12 is formed by a filter medium. On the outer surface of this filter medium is a constant pressure of 4 bar. On the inside of the filter medium there is a pressure of 3.5 bar in sector 24, a pressure of 3.0 bar in sector 26 and a pressure of 2.0 bar in sector 28. This results in cake formation on the filter medium with an increasing differential pressure of 0.5 to 2 bar. In this way, a filter cake has formed in the direction of rotation A of the filter drum 12 at the end of the sector 28, the capillary inlet pressure of which is relatively low due to the particle size distribution of the suspension in the suspension bath 14 and due to the addition of a flocculant.

In order to be able to work with higher pressures in the subsequent compression and dehumidification cycles, a second suspension is applied uniformly to the surface of the filter cake in the sector 30 by means of the sprinkler system 18. This second suspension forms a second layer on the filter cake with a higher capillary inlet pressure, so that the application of higher pressures of 2 to 3 bar in the subsequent compression zone 32 is possible. At the end of the compression zone 32, the filter cake is pre-compressed so well that when the cake is blown through in the subsequent dehumidification or desaturation zone 34, no drying cracks occur in the filter cake. In the dehumidification or desaturation zone 34, the full differential pressure of 4 bar is applied to the filter cake. If a vacuum is applied to the inside of the filter medium, the differential pressure can even be increased to 4.8 bar.

The suspension for the top layer can preferably be adjusted in its grain or pore size distribution so that it forms a top layer whose capillary inlet pressure lies between the differential pressure for the pre-compression and the differential pressure in the dehumidification zone. In the last sector 36, the filter cake is aerated in order to reduce mechanical tensions. At the end of the sector 36, the filter cake is detached from the outer surface of the filter drum 12 by a scraper or deflector 22.

Sector 38 represents a neutral zone with regard to filter cake production.

FIG. 2 shows a drum filter 40, in which, in contrast to the drum filter 10 from FIG. 1, two suspension baths 42, 44 connected one behind the other in the direction of rotation A are provided.

The application of the cover layer with a finer porosity is accordingly not achieved by spraying or sprinkling as in FIG. 1, but by layering in the suspension bath 44. The suspension baths 42, 44 are not necessarily separated from one another, but preferably by a sealing strip 46, so that the suspensions do not mix, which would greatly reduce the function of the cover layer.

3 to 7 show how the suspension feed for the filter cake 16 and the cover layer 20 can be formed.

3, a suspension stream 50 is divided into two sub-streams 52, 54. The first partial flow 52 is set via a control valve 56 and serves as a suspension feed 20 for the cover layer to be applied to the filter cake. The second partial stream 54 is fed to a stirred tank 60 and mixed with a flocculant 62. By adding the flocculant, a particularly porous filter cake with a greatly increased cake formation rate built up. Such a filter cake has a very low capillary inlet pressure, which is why the filter cake is then covered with a high capillary inlet pressure by the top layer. The mixture of the second partial stream 54 with the flocculant 62 is passed through a control valve 64 and serves as a feed 16 for the suspension bath 14 or 42.

In this suspension feed shown in FIG. 3 and also in the suspension feeds shown below, there is a great advantage in that the cover layer consists of the same material as the filter cake itself, ie no foreign substances are introduced into the filter cake. In all cases, a suspension flow is divided, the two partial flows being changed by a different treatment in such a way that they form layers with different porosity.

Parts identical to FIG. 3 are provided with identical reference symbols.

In FIG. 4, a partial flow 68 is branched off from a suspension flow 50 via a pump 66 and is fed to a hydrocyclone 70. In the hydrocyclone 70, the suspension is separated according to grain size, the coarse fraction being fed to the suspension stream 50 through the underflow 72. The overflow 74 with the fine fraction is fed via a pump 56 to the sprinkler system 18 or the suspension bath 44 to the drum filter 10 or 40. A flocculant 62 is added to the suspension stream 50 in the mixer 60. The mixture of suspension stream 50 and flocculant 62 is in turn fed to the suspension bath 14 or 42. By adding the flocculant and by adding Mixing the underflow 72 of the hydrocyclone produces a coarse-pored filter cake which counteracts blockage of the filter medium and has a high cake formation rate.

5, the entire suspension stream 50 is fed to the hydrocyclone 70. The overflow is in turn set by a pump 56 and serves as a feed 20 for the suspension bath 44 or the sprinkler system 18.

The underflow 72 is provided with a flocculant 62 and serves as a suspension feed 16 for the filter cake formation.

FIG. 6 differs from FIG. 5 in that no flocculant is added to the underflow 72 of the hydrocyclone. The fine fraction of the suspension stream 50 present in the overflow 74 is finely ground in a mill 76 and fed to the sprinkler system 18 or the suspension bath 44 via a control valve 76.

7 shows a technically very simple way of obtaining suspensions which lead to differently porous layers. The suspension stream 50 is here divided into a first and a second sub-stream 52, 54, the first sub-stream being fed to a mill 76 via a pump 56. There the pore size of the solids in the suspension is reduced, which ultimately leads to a layer with a finer porosity, that is to say with a higher capillary inlet pressure, during filter cake production. The second partial flow 54 serves as a suspension feed 16 for the suspension bath 14 or 42 without further treatment. The sprinkler device 18 from FIG. 1 is preferably covered with a hood so that the suspension for the cover layer does not contaminate the suspension for the filter cake and other devices of the drum filter 10.

The method is suitable for both continuous and discontinuous filters, e.g. Pressure filters and candle filters. The top layer process can also be used in centrifuges in which the centrifugal acceleration is superimposed on a gas differential pressure.

Claims

Expectations

1. Process for dehumidifying a filter cake on a filter medium, in particular by means of continuous press filtration, in which a cover layer is applied to the filter cake, the capillary inlet pressure of which is higher than that of the filter cake, and in which the A pressure is applied to the filter cake which is higher than the capillary inlet pressure of the filter cake and lower than the capillary inlet pressure of the cover layer.

2. The method according to claim 1, characterized in that a thin film, in particular, is used as the cover layer

Plastic film on which filter cake is applied.

3. The method according to claim 1, characterized in that a suspension with a fine sludge or clay mineral is used as the top layer.

4. The method according to claim 1, characterized in that the cover layer and the filter cake consist of the same material.

5. The method according to claim 4, characterized in that a flocculant is added to the material for the filter cake.

6. The method according to any one of claims 3 to 5, characterized in that the cover layer is sprayed onto the filter cake or is layered in a further suspension bath.

7. The method according to any one of the preceding claims, characterized in that the filter cake is formed with different differential pressures.

8. The method according to any one of claims 4 to 7, characterized in that a suspension is fed to a separating or classifying device and that the overflow (74) is used for the top layer and the underflow (72) for the filter cake.

9. The method according to any one of claims 2 to 8, characterized in that the partial stream located in the overflow is thickened.

10. The method according to any one of claims 2 to 9, characterized in that an additive for influencing the rheological behavior is added to the partial flow located in the overflow.

11. The method according to any one of the preceding claims, characterized in that the filter cake is dehumidified after the pre-compression with the pressure by applying a dehumidification pressure, the pre-compression pressure being lower and the dehumidification pressure higher than the capillary inlet pressure of the top layer.

12. Device for producing a filter cake with low residual moisture, particularly suitable for carrying out the method according to one of the preceding claims, with a filter (10) having a filter medium for forming a filter cake, and with two suspension application devices (14, 18) on the filter medium of the filter (10) for the supply of different fluids and / or suspensions.

13. The apparatus according to claim 12, characterized in that the suspension application devices (14, 18; 42, 44) are connected to the overflow and underflow (70, 72) of a separating or classifying device (68).

14. The apparatus according to claim 12 or 13, characterized in that the suspension feed means (14,18; 42,44) is a device for dividing the Suspen¬ sionsstrom (50) in two partial streams (52,54) angeord¬ net that in a thickener and / or a fine mill (76) is / are arranged in the first partial flow and that a mixing device (60) for mixing the second partial flow (54) with a flocculant (62) is provided in the second partial flow.

15. Device according to one of claims 12 to 14, characterized in that the filter is designed as a drum filter (12), the outer circumferential surface of which passes through a suspension bath (14), that on the outside of the peripheral surface of the filter drum (12) a constant Pressure is present and that sector-shaped filtrate chambers (24, 26, 28, 32, 34) are formed in the interior of the filter drum (12), in which a predeterminable pressure can be set independently of one another.