Abstract:
A precoat renewal system for a filter, such as a disc filter is provided. The renewal system includes a plurality of nozzles coupled to spray piping and valving with each nozzle being positioned for spraying an area of one side of a disc. The nozzles and at least a portion of the spray piping are movable such that the nozzles may be displaced relative to the discs. The nozzles and piping are arranged in groups or zones with each zone covering a specified percentage of the discs in the filter and each zone representing about 40% or less of the surface area of the filter media of the discs in the filter. Each zone is configured such that high pressure fluid may be supplied to that particular zone for removal of a precoat layer on the disc. The system may be configured so that other zones may be concurrently washed with a lower pressure fluid. The zones may be sequentially or otherwise segmentally provided with high pressure fluid until the precoat layers of each respective zone have been removed and renewed. A filter system and a method of treating a precoat layer on a disc-type filter including a plurality of filter discs are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 10/126,864, filed Apr. 19, 2002, now U.S. Pat. No. 6,833,077, issued Dec. 21, 2004, which claims the benefit of U.S. provisional patent application, Ser. No. 60/285,858 filed Apr. 23, 2001, and U.S. provisional patent application, Ser. No. 60/302,724 filed Jul. 3, 2001. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to precoat filters such as disc filters having a layer of precoat thereon and, more particularly, to the washing and renewal of a precoat layer on such filters.  
         [0004]     State of the Art: Filtration equipment such as disc or drum filters are known in the art for their use in separating solid particles from a liquid in which they are suspended. Such filters typically pass the particle-containing liquid through a selected filter media which allows the passage of liquid therethrough while retaining the particles on a surface of the filter media for subsequent collection. The liquid passing through the filter media is typically referred to as filtrate while the particles on the surface of the filter media are typically referred to as cake.  
         [0005]     The filter media used in such a process may be formed from a variety of different materials but is often of a cloth- or fabric-type material, including, for example, wire mesh, formed of natural or synthetic materials. Additionally, filter media may include a built-up layer of cake on the filtration surface of a drum, disc, or other filter element. This layer of cake is referred to as a precoat. It is noted that, depending on the type of material being processed, the precoat layer may be formed from a material that is the same as or similar to the material being filtered. For example, if lime mud is being filtered and removed from a liquid, lime mud may also be used as the precoat. However, the precoat does not have to be formed of the same material as that which is being filtered.  
         [0006]     The precoat layer is typically formed on top of a filter cloth, with the two elements working in conjunction to separate filtrate from the process cake. The precoat layer is a permeable layer which allows filtrate to pass through while capturing additional particles on a surface of the precoat layer. The filtered particles are typically removed, often by a scraping device, while the precoat layer remains on the filter media for continued filtering.  
         [0007]     After a precoat layer has been in use for an extended amount of time, the precoat layer becomes less permeable and thus reduces the efficiency of the filtering operation by allowing less filtrate to pass therethrough. When the precoat layer is in such a condition, it becomes desirable to remove and regenerate the precoat layer by removing the existing precoat layer of material in its entirety and forming a new precoat layer in its place.  
         [0008]     One type of apparatus used in removing and regenerating precoat layers on a filter is disclosed in U.S. Pat. No. 5,759,397 to Larsson et al. (issued Jun. 2, 1998), the disclosure of which is hereby incorporated herein by reference.  FIGS. 1 and 2  hereof are representative of the apparatus disclosed in the Larsson patent.  FIGS. 1 and 2  show a disc filter  10  including a plurality of hollow discs  12  having sidewalls  14  of a filter material  16 . The hollow discs  12  are coupled to a hollow axle  18  and are in fluid communication therewith via holes  20  formed along the hollow axle  18 .  
         [0009]     The hollow discs  12  are positioned within a container  22 , such as a tank, which includes an inlet  24  for introducing a slurry  26  (i.e., a particle-containing liquid) into the container  22 . A vacuum  28  draws the filtrate through filter material  16  with particles from the drawn liquid accumulating on the surface of the filter material  16  to form a cake.  
         [0010]     As shown in  FIG. 2 , the container  22  is filled slightly less than halfway with the slurry. The discs  12  are configured to rotate counterclockwise through the slurry  26 . In operation, a portion of the discs  12  enters into the slurry  26  to draw filtrate from the slurry  26  and then emerge with a cake formed of accumulated particles. A scraper  30  is used to remove the accumulated cake from the discs  12  while leaving a specified thickness of precoat material. The cake removed from the discs  12  is collected in a chute or bin  32  and transferred from the filter  10  for further processing or disposal, as the case may be.  
         [0011]     When it is desired to remove and regenerate the precoat, an oscillating spray pipe  34  having a spray nozzle  36  coupled thereto moves between the periphery and center of the disc  12  removing the precoat layer by spraying the precoat layer with a pressurized fluid. The Larsson patent describes such removal and regeneration as taking place in two steps with half of the discs being stripped of the their precoat layer followed by replacement of such layer, while the precoat layers of the other half of the discs are not sprayed.  
         [0012]     It is believed that such an arrangement may expose too much surface area of the fabric filter media by removal of the precoat layer therefrom, thus resulting in the loss of vacuum crucial to operation of the filter. If the vacuum is broken, it is likely that the precoat layers remaining on the additional discs (i.e., those not having the precoat layers renewed) will drop their precoat layers, causing a serious and expensive interruption in the operation of the filter.  
         [0013]     In addition to potential loss of vacuum, removal of the precoat layer using high pressure fluid may cause damage to the underlying filter media, particularly over time. Thus, while it may be desirable to remove and regenerate precoat at fairly frequent intervals, such frequency may necessitate a consequent undesirable frequency of downtime of the filter system for replacement of the filter media.  
         [0014]     U.S. Pat. No. 5,897,788 to Ketolainen et al. (issued Apr. 27, 1999) discloses the use of a spray nozzle oriented at an acute angle to the surface of a filter drum of a drum-type filter to remove an outer, more porous and softer portion of the precoat layer on a substantially continuous basis between removals of the precoat layer in its entirety down to the filter wire.  
         [0015]     Other filters have likewise used swinging spray showers to assist in washing of process material on precoat layers such as that disclosed in U.S. Pat. No. 6,063,294 to Martensson et al. (issued May 16, 2000), the disclosure of which is hereby incorporated herein by reference. The Martensson patent discloses a spray assembly having multiple nozzles in each spray pipe and wherein the speed at which the spray pipe traverses the face of a disc is varied according to its radial position relative to the disc. An additional feature disclosed in the Martensson patent is an independent washing shower which is optionally used to soak process material on the exterior of the precoat and thus increase the efficiency of cake removal using the scraper. In essence, the washing shower includes a spray pipe and one or more nozzles directed at the filtering surface of a given filter (i.e., the filtering surface of a drum or a disc). Using a disc as an example, and considering the disc to be divided into a plurality of segments, the wash assembly sprays water on the precoat surface of a given disc segment subsequent to the disc segment being rotated through the pool of slurry. The wash water flows through the filter media, including the precoat, forcing any filtrate potentially remaining in the cake and precoat layers to be drawn through the filter by the vacuum. Without the washing apparatus, some filtrate liquid might potentially be discharged with the cake layer, thus causing some inefficiency with the operation.  
         [0016]     While the washing shower provides some increased efficiency in production of filtrate, such shower assemblies require piping and valving in addition to that required for the precoat removal system. Additionally, the inclusion of a washing shower may add constraints to the overall design of the filter as the shower heads take up additional space and must be designed so as to not interfere with any moving components of the filter.  
         [0017]     Accordingly, it would be advantageous to provide a precoat removal apparatus and system which allows for maximum efficiency in replacing the precoat layer on filter media of filter discs of a disc-type filter without losing the vacuum (or pressure differential) formed across the filter media. Additionally, it would be advantageous to provide a precoat removal apparatus and system which also incorporates a precoat renewal feature in a disc-type filter to minimize the necessity to remove and regenerate the precoat layer and to potentially reduce the frequency of repair of the filter media.  
       BRIEF SUMMARY OF THE INVENTION  
       [0018]     The present invention comprises a precoat renewal system for a filter such as a disc filter. The precoat renewal system includes a plurality of nozzles coupled to spray piping and valving with each nozzle being positioned for spraying an area of one side of a disc. The nozzles and at least a portion of the spray piping may be movable such that the nozzles may be displaced relative to the discs. The nozzles and piping may be arranged in groups or zones with each zone covering a specified percentage of the discs in the filter. Each zone may represent about 40% or less of the total surface area of the filter media of the plurality of filter discs in the filter. It may be preferred that each zone be configured to effect fluid impingement on between about 15 and 25% of the total surface area of the filter media of the plurality of filter discs.  
         [0019]     Each zone may be configured such that high pressure fluid may be supplied to that particular zone for removal of a precoat layer on the disc. The precoat renewal system may be configured so that other zones may be concurrently washed with a lower pressure fluid. The zones may be sequentially or otherwise segmentally provided with high pressure fluid until the precoat layers of each respective zone have been removed and renewed.  
         [0020]     A disc-type filter system incorporating the precoat renewal system as well as a method of treating a precoat layer on a disc-type filter including a plurality of filter discs are also encompassed by the present invention. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0021]     The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0022]      FIG. 1  is a cross-sectional side view of a prior art disc filter;  
         [0023]      FIG. 2  is a cross-sectional end view of the prior art disc filter shown in  FIG. 1 ;  
         [0024]      FIG. 3  is a cross-sectional end view of a filter and precoat renewal apparatus according to one embodiment of the invention;  
         [0025]      FIG. 4  is a cross-sectional end view of the filter and precoat renewal apparatus shown in  FIG. 1 ;  
         [0026]      FIG. 5  is an enlarged detail of a portion of the precoat renewal apparatus as indicated in  FIG. 4 ;  
         [0027]      FIG. 6  is an enlarged detail of a portion of the precoat renewal apparatus as indicated in  FIG. 4 ;  
         [0028]      FIG. 7  is an enlarged cross-sectional detail of the precoat renewal apparatus at section line  7 - 7  as indicated in  FIG. 4 ;  
         [0029]      FIG. 8  is a schematic showing valving and piping of the precoat renewal apparatus according to one embodiment of the invention; and  
         [0030]      FIG. 9  is a cross-sectional end view of a filter according to a further embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     Referring to  FIGS. 3 and 4 , a disc filter  100  is shown which is generally constructed in a manner similar to the filter shown in  FIGS. 1 and 2 . The disc filter  100  includes a plurality of discs  102  coupled to a hollow shaft  104  allowing for fluid communication therebetween. The discs  102  are formed of sidewalls  106  having a filter media  108  thereon which may include a filter cloth made, for example, of a wire mesh as well as a layer of precoat material thereover. The discs  102  are housed in a vessel or container  110 , which may include a tank portion  112  and a hood or lid portion  114 . The vessel  110  may or may not be pressurized depending on specific applications and processing requirements. A drive system  116  is coupled to one end of the hollow shaft  104  to rotate the discs  102  and a vacuum source  118  is coupled to the other end of the hollow shaft  104 . The vacuum source  118  may not be required if the vessel  110  enclosing the discs  102  is pressurized at a sufficient level.  
         [0032]     As seen in  FIG. 3 , with the discs  102  rotating counterclockwise, a plurality of sections  120  formed on the disc  102  each individually rotate downward into a slurry  122  and upward out of the slurry  122 . As the sections  120  rotate into the slurry  122 , a pressure differential between the interior of the vessel  110  and the interior of the hollow shaft  104  (such as may be produced by the vacuum source  118 ) causes the slurry  122  to flow through the sidewalls  106  of the disc  102  with filtrate passing through the filter media  108  and particulates being formed as a cake on the surface of the filter media  108  (i.e., on the surface of the precoat layer).  
         [0033]     A scraper assembly  124  is positioned adjacent the falling side of each disc  102  (i.e., adjacent the portion of the disc  102  rotating down toward the slurry  122 ) along with a chute or bin  126  for collection of the cake scraped from the filter media  108 . The collection bins  126  may be in communication with a conveyor system, such as a screw conveyor or the like (not shown), for transportation of the discharged cake as is known in the art.  
         [0034]     The disc filter  100  further includes a regenerative shower system  128  which is used to remove and regenerate or, alternatively, renew the precoat layers of the discs  102  in a sequential or other segmented fashion. The regenerative shower system  128  includes a plurality of spray pipes  130 , each extending along at least one side of a disc  102  and having a nozzle  132  at the distal end thereof. Each nozzle  132  is positioned and configured to spray a portion of an adjacent sidewall  106  of a disc  102  and the filter media  108  associated therewith and either remove a portion of the thickness of the layer of precoat material from filter media  108  or, in the alternative, strip the precoat material in its entirety from its associated filter media  108  on sidewall  106  and clean the filter media  108 . It is noted that most of the spray pipes  130  are configured to spray the sidewalls  106  of two independent but adjacent discs  102  while the spray pipes  130  on the ends of the disc filter  100  are configured to only spray one sidewall  106  of one disc  102  each. This arrangement may be seen in  FIG. 5 , where an end spray pipe  130 ′ includes a single nozzle  132  while an interior spray pipe  130 ″ includes two nozzles  132 , each oriented toward the sidewall  106  of a separate disc  102 .  
         [0035]     Referring again to  FIGS. 3 and 4 , the spray pipes  130  are coupled to a support  134  which may be formed from a structural member such as an I-beam and which is, in turn, coupled to bearings  136  at either end of the vessel  110 . The bearings  136  allow the support  134  and associated spray pipes  130  to oscillate through a predetermined arc which extends from the outer edge of the discs  102  to the inner edge of the discs  102 , as is best seen in  FIG. 3 . The oscillating pattern of the spray pipes  130  allows renewal or regeneration of the precoat layers to take place at one small section of filter media at a time. The support  134  and spray pipes  130  are motivated through the oscillating pattern by a drive  135  such as a servo motor coupled to the support  134 . It is noted that a displacement mechanism of another configuration may also be used to move the nozzles  132  relative to the discs  102 . For example, the nozzles  132  and spray pipes  130  may be coupled to a linear or curvilinear track positioned between the discs  102 . Such a mechanism would allow displacement of the nozzles  132  and might be desirable if overhead space was limited within the vessel  110 . In such a case, it might be desirable to form at least a portion of the spray pipe  130  from flexible material. In addition, it is contemplated that fixed spray pipes, each with a plurality of spray nozzles disposed along its length and appropriately sized to distribute liquid flow, may be placed between each of the discs  102  and longitudinally outboard of the end discs  102  to renew and regenerate the precoat layers thereon, as further described below with respect to  FIG. 9 .  
         [0036]     While not required, an auxiliary wash shower assembly  138  for adding liquid to process material on the precoat layer may be installed on the disc filter  100  and may include one or more spray headers  140  having a plurality of spray nozzles  142  installed therein. Such a wash shower, if desirable for given process parameters and if design constraints allow, may enhance the recovery of filtrate as discussed above.  
         [0037]     The regenerative shower assembly  128  is arranged in sections or zones  144 A- 144 E with each zone representing a percentage of total disc filter media surface area within a predetermined range. For example, the disc filter  100  shown includes a total of ten (10) discs  102 , or twenty (20) sidewalls  106 . Each sidewall  106  has an associated spray pipe  130  and nozzle  132 . However, if each of the spray pipes  130  were operated such that the precoat layers were being simultaneously stripped off of each sidewall  106 , it is likely that the vacuum formed within hollow shaft  104  (or the pressure differential formed between the vessel  110  and hollow shaft  104 ) might be lost, resulting in the loss of all precoat layers and cake layers formed on the sidewalls  106  of the discs  102 . Thus, only a portion of the discs  102  may have their associated precoat layers removed and regenerated on filter media  108  at a given time.  
         [0038]     In the disc filter  100  shown in  FIG. 4 , each zone  144 A- 144 E represents a zone in which removal and regeneration of the precoat layer may be performed at a given time. For example, the precoat layers in zone  144 A, which includes three (3) different spray pipes  130  and five (5) different disc sidewalls  106 , might be removed and generated at a given time while the precoat layers in each of the other zones  144 B- 144 E were not being renewed. Thus, in operation, zone  144 A might first be subjected to precoat removal and regeneration, followed sequentially by zones  144 B,  144 C,  144 D and finally  144 E. It is noted, however, that by monitoring the production of the disc filter  100 , it may be possible to select a particular zone for precoat removal and regeneration without the need to sequentially remove and regenerate the precoat layer in the remaining zones.  
         [0039]     When the regenerative shower system  128  is not being used to remove and regenerate the precoat layers in a given zone, it may be used to renew the precoat layers in every zone  144 A- 144 E. Alternatively, when a given zone, for example  144 A, has the precoat layers being removed and regenerated, the remaining zones  144 B- 144 E may be renewed using the regenerative shower system  128 . This is accomplished by selectively communicating the spray pipes  130  of each zone  144 A- 144 E to separate supply sources for removal and regeneration of precoat layers (with associated cleaning of the underlying filter media  108 ) and for renewal of the precoat layers. For example, referring to  FIGS. 4 and 6 - 8 , separate feed piping  146 A- 146 E extends to the spray pipes  130  of each zone  144 A- 144 E. Each zone is thus connected to a high pressure source  148  with a valve  150  being coupled therebetween. The high pressure source may include, for example, water at a pressure of approximately 300 to 1000 psi for removal of precoat layers of a given zone  144 A- 144 E and cleaning of the filter media  108  therein. A second source  152  is also coupled to the feed piping  146  which may be, for example, plant water at a pressure of approximately 60 psi. A second valve  154  may be coupled between the second source  152  and the feed piping  146  if so desired. The piping extending downstream from valves  150  and  154  meets at a common connection, for example, a “T” connection  156  as shown, communicating with feed piping  146 . The valves  150  and  154  may comprise, for example, solenoid-actuated full port ball valves.  
         [0040]     Thus, in operation, the second valve  154  may open, allowing the second source  152  to supply a particular zone with relatively low pressure water for the renewal of precoat layers thereof by removing only a partial thickness of the outer portion or layer of the precoat, such outer layer being the first to clog with particulates and compromise filtration efficiency. By removing only an outer portion of the precoat, the precoat is renewed and efficiency restored without stripping the precoat layer completely. However, when it is desired to remove and regenerate the precoat layers in a particular zone, either by operator decision or by virtue of control logic through sensing of various filtering parameters, the second valve  154  (if so equipped) will shut off the second supply  152  from the piping and the first valve  150  will open, allowing the high pressure source  148  to supply the feed piping  146  with relatively higher pressure water to remove the precoat layers of the associated zone. As can be seen in  FIG. 7 , different sections of feed piping  146 A- 146 E are separately valved and may thus be utilized to selectively direct fluid to each zone  144 A- 144 E respectively. It is, of course, also contemplated that disc filter I  00  may operate in a mode wherein neither renewal nor removal and regeneration of a precoat layer is being effected on any of the discs  102 , the frequency of need for renewal of the precoat layer as well as for the removal and regeneration thereof being dependent on a number of operational parameters of disc filter  100  which may vary according to the filtration operation for which disc filter  100  is employed.  
         [0041]     Referring back to  FIGS. 3 and 4 , and particularly  FIG. 4 , the spray zones  144 A- 144 E and their arrangement are designed to allow for efficient removal and regeneration of precoat layers without losing the vacuum (or other pressure differential) required for filter operation. For example, as noted above, the first zone  144 A represents five (5) sidewalls  106  of the filtering surface area. This is equivalent to about 25% of the total filter media surface area for the depicted disc filter  100 . For the configuration shown, no zone represents more than about 25% of the total filter media surface area or less than about 15% (zone  144 E) of the filter media surface area. Most of the zones shown ( 144 B- 144 D) represent about 20% of the total filter media surface area. While the zones of a given filter may be designed to work with greater than about 25% of the total filter media surface area, it is believed that a zone representing about 50% of the total filter media surface area would allow the vacuum (or pressure differential) to be lost. Thus, a zone representing about 35 to 40% of total filter media surface area may represent the upper limit of efficiency for complete removal of precoat down to and including washing of the filter media  108  without an endangering loss of vacuum or pressure differential.  
         [0042]      FIG. 9  of the drawings illustrates a further exemplary embodiment of the disc filter  200  of the present invention wherein previously described components and features now depicted in  FIG. 9  are identified by reference numerals having the same numbers as with respect to disc filter  100 . The disc filter  200  includes a plurality of discs  102  coupled to a hollow shaft  104  allowing for fluid communication therebetween. The discs  102  are formed of sidewalls  106  having a filter media  108  thereon which may include a filter cloth made, for example, of a wire mesh as well as a layer of precoat material thereover. The discs  102  are housed in a vessel or container, which may include a tank portion and a hood or lid portion. The vessel may or may not be pressurized depending on specific applications and processing requirements. A drive system is coupled to one end of the hollow shaft  104  to rotate the discs  102  and a vacuum source is coupled to the other end of the hollow shaft  104 . The vacuum source may not be required if the vessel enclosing the discs  102  is pressurized at a sufficient level.  
         [0043]     With the discs  102  rotating clockwise, a plurality of sections  120  formed on the disc  102  each individually rotate downward into a slurry  122  and upward out of the slurry  122 . As the sections  120  rotate into the slurry  122 , a pressure differential between the interior of the vessel and the interior of the hollow shaft  104  (such as may be produced by the vacuum source) causes the slurry  122  to flow through the sidewalls  106  of the disc  102  with filtrate passing through the filter media  108  and particulates being formed as a cake on the surface of the filter media  108  (i.e., on the surface of the precoat layer).  
         [0044]     A scraper assembly  124  is positioned adjacent the falling side of each disc  102  (i.e., adjacent the portion of the disc  102  rotating down toward the slurry  122 ) along with a chute or bin (not shown in  FIG. 9 ) for collection of the cake scraped from the filter media  108 . The collection bins may be in communication with a conveyor system, such as a screw conveyor or the like (not shown) for transportation of the discharged cake as is known in the art. A regenerative shower assembly  128 , which may comprise fixed or movable spray pipes  130  bearing spray nozzles  132 , is placed below scraper assembly  124  and above the surface of slurry  122 . As previously described with respect to disc filter  100 , the regenerative shower system  128  may be used to periodically renew the precoat layers on the filter media  108  of filter discs  102  by removing outer portions of the precoat layers using a relatively lower pressure fluid, or remove the precoat layers in their entirety and clean the underlying filter media  106  using a relatively higher pressure fluid.  
         [0045]     It will be appreciated that such an arrangement, with regenerative shower assembly  128  on the falling side of each disc  102 , will expose a relatively small portion of the surface area of filter media  108  of each disc  102  during removal of the precoat layer for regeneration. Thus, it may not be necessary to divide the discs  102  into a plurality of zones for removal of the precoat layer, as the total filter media surface area exposed at any one time is insufficient to cause a detrimental loss of pressure differential between the interiors and exteriors of filter discs  102 , since a maximum of only about 30° of arc out of 360° on each filter disc  102  may be exposed before being resubmerged in slurry  122 . In contrast, if regenerative shower system  128  is placed on a rising side of a filter disc  102  (as filter media  108  moves upwardly out of slurry  122 ), as much as about 40% of a given filter disc&#39;s total filter media surface area may be exposed during precoat layer removal, causing an unacceptable loss of pressure differential.  
         [0046]     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.