Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D37/00—Processes of filtration
  • B01D37/03—Processes of filtration using flocculating agents
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D33/00—Filters with filtering elements which move during the filtering operation
  • B01D33/06—Filters with filtering elements which move during the filtering operation with rotary cylindrical filtering surfaces, e.g. hollow drums
  • B01D33/073—Filters with filtering elements which move during the filtering operation with rotary cylindrical filtering surfaces, e.g. hollow drums arranged for inward flow filtration
  • B01D33/09—Filters with filtering elements which move during the filtering operation with rotary cylindrical filtering surfaces, e.g. hollow drums arranged for inward flow filtration with surface cells independently connected to pressure distributors
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D33/00—Filters with filtering elements which move during the filtering operation
  • B01D33/58—Handling the filter cake in the filter for purposes other than for regenerating the filter cake remaining on the filtering element
  • B01D33/62—Handling the filter cake in the filter for purposes other than for regenerating the filter cake remaining on the filtering element for drying

Definitions

• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• a flocculant is added to the partial flow in the underflow
• 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.
• 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.
• This method can also be used in the production of divided filter cakes using 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.
• 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.
• 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.
• 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 can also be added to the suspension.
• 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.
• each zone for the cake formation, the top layer application, the compression and the dehumidification with at least one, preferably several separate separators.
• 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
• FIG. 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the filter cake is aerated in order to reduce mechanical tensions.
• 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.
• 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.
• 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.
• a partial flow 68 is branched off from a suspension flow 50 via a pump 66 and is fed to a hydrocyclone 70.
• the suspension 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.
• 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.
• 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.
• 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.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Treatment Of Sludge (AREA)

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Citations (4)

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• 1991
  • 1991-07-12 DE DE4123144A patent/DE4123144C1/de not_active Expired - Lifetime
• 1992
  • 1992-07-13 WO PCT/EP1992/001584 patent/WO1993000979A1/de active Application Filing
  • 1992-07-13 AU AU23202/92A patent/AU2320292A/en not_active Abandoned

Patent Citations (4)

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