Abstract:
A method and device for dewatering most liquid/solid solutions and slurries is described. A dewatering device having a dewatering pump in fluid communication with a filter rack with vertically arrayed filter elements that is carried by a container. To begin the dewatering process, the filter rack is lowered to the bottom of the container and the liquid/solid slurry is introduced. Next, the dewatering pump draws the liquid from the slurry into the filter rack and then to the pump. When dewatering efficiency decreases, the dewatering operation is momentarily stopped and the filter elements of the filter rack are backpulsed with air and raised to dislodge the filter cakes that have formed. The filter rack is then relowered and the dewatering operation continues. These steps of dewatering, backpulsing, raising the filter rack, and lowering filter rack are continued until the container is full of dewatered solids. Alternatively, the filter rack is affixed to the top of the container and the liquid/solid slurries are introduced until the container is mostly full. Next dewatering operations commence and continue until the container is full of dewatered solids.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to dewatering methods and devices, and, in particular, to a method and device having removable filter elements for dewatering most liquid/solid solutions and slurries.  
         BACKGROUND OF THE INVENTION  
         [0002]    The handling of hazardous wastes, such as radioactive wastes, generated by various industrial, medical, and electrical power generating activities is both a complex and troublesome undertaking considering the potentially devastating effects of exposure to such wastes. To limit the exposure of employees who process and handle this waste is of particular importance. It is also important to process these wastes so that they remain stable during their transport and disposal. For example, radioactive wastes need processing until the radionuclides present decay to nominal levels. Accordingly, various techniques and devices have been developed to effectively process wastes.  
           [0003]    In the case of certain radioactive wastes, such as ion exchange media, slurry wastes and sludges, one processing technique is to dewater the slurries and sludges. In order to be acceptable for disposal, radioactive wastes must be dried to less than one-half percent water by volume of waste. It is known to dewater most liquid/solid solutions and slurries by introducing these slurries to containers having a variety of fixed filter elements such as filter sheets attached to the container floor and walls, and multiple filter elements secured into fixed piping headers or racks. These dewatering devices can typically be cleaned of filter cakes that form on the filter elements during the dewatering process by briefly reversing the direction of flow, or “back blowing,” so that a partially cleaned or regenerated filter surface can restore the dewatering rates.  
           [0004]    Previous dewatering devices took days or weeks to transfer and fully dewater waste slurries. Very fine or colloidal solids removed by powdered ion exchange resins may take 24 to 144 hours to transfer and dewater. Furthermore, typical filter backwash or backflush operations utilize such large quantities of air or liquids that the dewatered solids are agitated, partially refluidized, liquefied and remixed with the balance of the container contents. Even when the dewatering operations continue, there is still a substantial filter cake on the surface of the filter and the fines have been remixed with the rest of the container contents.  
           [0005]    Therefore, there exists a need for a dewatering method and device that will more effectively and quickly transfer and dewater slurries, such as ion exchange resin slurries.  
         SUMMARY OF THE INVENTION  
         [0006]    According to its major aspects and briefly stated, the present invention is generally a device and method for dewatering liquid/solid solutions and slurries. The dewatering device includes a removable filter rack and a lifting means. In particular, the device includes a container, a filter rack assembly, a filter rack lifting means, a dewatering or vacuum pump, and an air or water source for backpulsing or backflushing. These components cooperate to transfer and dewater waste slurries quickly and effectively.  
           [0007]    The dewatering method includes the following steps: 1) lowering a filter rack to the bottom of a container; 2) slurrying wastes into the container; 3) vacuum pumping water from the slurried wastes; 4) stopping the dewatering operations when dewatering efficiency decreases from the initial rate and backpulsing the filter elements of the filter rack; 5) raising the filter rack off the bottom of the container and then optionally backpulsing the filter elements again. This process cycle of lowering filter rack, slurrying wastes, vacuum pumping, backpulsing/backflushing, raising the filter rack, and backpulsing again, is continued until the container is full of dewatered solids. Once the container is full of solids, the filter rack is removed for use in another container. Alternatively, the filter rack is left in the container and rests on top of the dewatered solids.  
           [0008]    An alternative method of the present invention includes the following steps: 1) affixing the filter rack near the top of the container; 2) slurrying wastes into the container; 3) vacuum pumping water from the slurried wastes; 4) backpulsing the filter elements of the filter rack; 5) allowing the dewatered solids to slide down off the filter elements and settle by gravity to the bottom of the container; and 6) optionally backpulsing the filter elements. This process of slurrying wastes, vacuum pumping, backpulsing, allowing solids to settle, and backpulsing is continued until the container is full of dewatered solids. Once the container is full of solids, the filter rack is removed. Alternatively, the filter rack is left in the containers and rests on top of the dewatered solids.  
           [0009]    A feature of the present invention is the use of backpulsing during the dewatering cycles. The backpulsing of a small volume and low pressure of air or water serves to break the filter free of the dewatered filter cake on the surface of the filter. Backpulsing at this small volume and low pressure does not mix or stir solids. Backpulsing also serves to optimize the dewatering efficiency. When the dewatering rate slows, dewatering is momentarily stopped and the filter rack is backpulsed. Thereafter, dewatering proceeds at a more efficient rate.  
           [0010]    Another feature of the present invention is the use of the combined backpulsing and raising of the filter rack. The feature serves to effectively clean the filter elements while leaving all previously dewatered materials in a solids cake. As stated above, the backpulsing has such a small volume and pressure that no solids are mixed or stirred. Consequently, the dewatered solids remain in tact and also serve to scrape the surface of the filter elements as the latter are being lifted.  
           [0011]    Yet another feature of the present invention is the use of the removable filter rack. Typical dewatering filters are fixed and may only be appropriate for one-time use. The filter rack of the present invention has the ability of being fully cleaned and reused. By moving the filter rack up during the dewatering cycles, the solids cakes are left at the bottom of the container and actually help to scrape the finer particles from the filter elements.  
           [0012]    Still another important feature of the present invention is the use of the lifting mechanism. The lifting mechanism not only helps to fully clean the filter rack by lifting it, but also enables the filter rack to produce highly dewatered solids that completely fill the container with only one set of filter elements. By lifting and then relowering the rack during the dewatering cycles, the container eventually becomes full of the solids cakes that are dislodged from the filter elements.  
           [0013]    Yet another important feature of the present invention is the arrangement of the filter elements on the filter rack. Because of particular symmetry and spacing, the filter element arrangement is designed so that the slurries are dewatered more effectively and evenly. Further, the orientation of the filter elements helps to more effectively clean the filter elements. As the rack is lifted, the vertically oriented filter elements are scraped by the solids cakes that are left behind.  
           [0014]    Other features and their advantages will be apparent to those skilled in the art of dewatering devices and methods from a careful reading of the Detailed Description of the Preferred Embodiments accompanied by the following drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    In the drawings,  
         [0016]    [0016]FIG. 1A is a cross-sectional perspective view of a dewatering device having a lowered filter rack, according to a preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 1B is a cross-sectional perspective view of a dewatering device having a raised filter rack, according to a preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 2 is a top view of a filter rack of a dewatering device, according to a preferred embodiment of the present invention;  
         [0019]    [0019]FIG. 3 is a side detailed view of a small pipe branch of a filter rack of a dewatering device, according to a preferred embodiment of the present invention;  
         [0020]    [0020]FIG. 4 is a side detailed view of a large pipe branch of a filter rack of a dewatering device, according to a preferred embodiment of the present invention;  
         [0021]    [0021]FIG. 5 is a side view of a lifting mechanism of a dewatering device, according to a preferred embodiment of the present invention;  
         [0022]    [0022]FIG. 6 is a cross-sectional perspective view of a dewatering device having a fixed filter rack, according to a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]    [0023]FIGS. 1A and 1B illustrate a preferred embodiment of the invention. FIG. 1A shows a perspective view of a movable dewatering device  10 . Specifically, FIG. 1A shows the dewatering device in a lowered position and FIG. 1B shows the dewatering device in a raised position.  
         [0024]    As shown, dewatering device  10  includes a container  12 , a filter rack  14 , a lifting mechanism  16 , and a dewatering pump  55  external to container  12  that is connected to filter rack  14  by a pipe  62 . These components cooperate to transfer and dewater waste slurries quickly and effectively. Filter rack  14  is connected to lifting mechanism  16  by cables  28  and is dimensioned to fit within container  12 . Preferably, filter rack  14  extends along only a portion of container  12 . Depending on the particular stage of the dewatering process, filter rack  14  is moved axially in container  12  according to the desired direction. To begin the dewatering process, filter rack  14  is lowered by reversing lifting mechanism  16  to the bottom of container  12  whereupon wastes are slurried into container  12 . Wastes continue being slurried into container  12  until the height of filter rack  14  is covered. At this time the dewatering of the slurries commences by starting dewatering pump  55 . Once the slurries have been dewatered, filter rack  14  is then raised by lifting mechanism  16  for cleaning and the process continues in layers from the bottom of container  12  to the top.  
         [0025]    [0025]FIGS. 1A and 1B show container  12  as cylinder shaped; however, container  12  can be shaped into any shape as long as it has a uniform horizontal cross section. Preferably, container  12  is made of plastic or metal. Container  12  is provided with an opening  18  for receiving the top of lifting mechanism and for facilitating the filling and dewatering of the contents of container  12 . Although FIGS. 1A and 1B show that opening  18  of container  12  receives pipe  62  that is connected to dewatering and backpulsing connection  30 , pipe  62  could alternatively pass through the side wall of container  12 . Container  12  is preferably a container particularly suited for storage of radioactive materials.  
         [0026]    Filter rack  14  is shown in further detail in FIGS.  2 - 4 . As illustrated, the main structural component of filter rack  14  is a pipe header  20  that defines a plane. Along pipe header  20  are alternating small and large pipe branches  22 ,  23  (also shown in FIGS. 3 and 4, respectively) that extend from pipe header  20  and lie in the plane defined by pipe header  20 . Preferably, small and large pipe branches  22 ,  23 , are evenly spaced along pipe header  20 . Pipe header  20  further includes filter elements  24 , lifting lugs  26  with cables  28  attached, and a dewatering and backpulsing connection  30  that is in fluid communication through pipe  62  with dewatering pump  55  and an air and liquid source  60 . To help guide filter rack  14  along the interior of container  12 , filter rack  14  is further provided with wheels  50  that are affixed along the sides of pipe header  20 . As discussed above, the arrangement of filter rack  14  is a particular feature of the present invention. The symmetry and spacing of pipe branches  22 ,  23 , and filter elements  24  of filter rack  14  contribute to a more effective and evenly distributed dewatering process.  
         [0027]    [0027]FIGS. 3 and 4 show side cross-sectional views of portions of filter rack  14 . These figures illustrate the interconnection of pipe header  20  to smaller pipes  32  with multiple fittings  34 . FIGS. 3 and 4 further illustrate side views of filter elements  24 . Preferably, filter elements  24  are cylinder shaped and are connected to fittings  34  of filter rack  14 . As the piping of filter rack  14  is all interconnected, and therefore in fluid communication, the dewatering operations affect each filter element  24  simultaneously. Accordingly, when dewatering pump  55  or vacuum source external to container  12  and attached to dewatering and backpulsing connection  30  by pipe  62  pulls a vacuum on filter rack  14 , solids are collected on the exterior of each filter element  24 . Similarly, when filter elements  24  need to be cleaned, a backflow or backpulse of air through dewatering and backpulsing connection  30  dislodges the formed solids cakes from each filter element  24  simultaneously. Solids “cakes” or filter “cakes” refers to a dense accumulation of dewatered solids around filter elements  24 .  
         [0028]    As further discussed above, the orientation of filter elements  24  is an important feature of the present invention. The figures illustrate (in particular FIGS. 3 and 4) that filter elements  24  are vertically oriented so that the major axes of filter elements  24  are perpendicular to the plane defined by pipe header  20  of filter rack  14 . The particular orientation of filter elements  24  minimizes the disturbance of the solids cakes when filter rack  14  is raised. Additionally, the solids cakes help to scrape the remaining fine and colloidal material from filter elements  24  during the raising of filter rack  14 .  
         [0029]    Lifting mechanism  16  is shown in detail in FIG. 5. Preferably, lifting mechanism  16  includes a winch  36  that is affixed to container  12  and powered by a drive shaft  38 . Winch  36  has a drum  40  around which are wound cables  28  attached to lifting lugs  26  of filter rack  14 . Depending on the stage of the dewatering process filter rack  14  is moved axially in container  12  by turning winch  36  around a major axis  42  (shown in FIGS. 1A and 1B). If drum  40  of winch  36  is rotated in one direction around major axis  42 , cables  28  are wound around drum  40  and filter rack  14  is raised. Similarly, if drum  40  is rotated in the opposite direction around main axis  42 , cables become unwound and filter rack  14  is lowered. Although numerous cables may be used in connection with winch  36 , the preferred embodiment includes three cables  28 . As shown in FIG. 5, each of multiple cables  28  extends out from winch  36  and are perpendicular, or approximately perpendicular, to major axis  42  of winch  36  and are passed through a pulley  44 .  
         [0030]    Although FIGS. 1A, 1B, and  5  depict extension arms  46  to which pulley  44  is attached, extension arms  46  are optional. Alternatively, pulley  44  could be directly attached to the top of container  12 . In this alternative embodiment, lifting mechanism  16  would be raised accordingly so that cables  28  extend out from winch  36  and are perpendicular, or approximately perpendicular, to major axis  42  of winch  36 .  
         [0031]    As discussed above, the dewatering process begins when filter rack  14  is lowered into container and wastes are slurried into container  12  until filter elements  24  of filter rack  14  are covered. Next, dewatering of the slurried wastes takes place by starting dewatering pump  55  or vacuum source external to container  12 . Dewatering pump  55  pulls a vacuum on filter rack  14  and filter elements  24  drawing water from the slurry into the filter rack piping and then to dewatering pump  55  through pipe  62 . This dewatering action removes liquids, concentrating the wastes into a dense, thickened sludge or solids cake. Alternatively, container  12  could be pressurized to push the water through filter elements  24 .  
         [0032]    As discussed above, the use of backpulsing is a particular feature of the present invention. Throughout the dewatering process, filter elements  24  of filter rack  14  are cleaned by means of reverse flow or backpulses of air or clean liquid through dewatering and backpulsing connection  30 . The backpulsing helps to clean filter rack  14  and filter elements  24  so as to optimize the dewatering efficiency. Once the rate of liquid removal substantially decreases from the initial rate, the dewatering operation is momentarily stopped and filter rack  14  is backpulsed with a short burst of air or liquid to release the pressure differential across filter elements  24 . This step results in essentially zero pressure differential across the filter and merely serves to break the vacuum and dislodge the solids cakes that have formed around filter elements  24  during the dewatering. The solids cakes retain their solid form and are not mixed or stirred. These cakes typically include fine particles, gels, colloidal materials, as well as other materials.  
         [0033]    As further discussed above, the use of backpulsing in combination with lifting is also a particular feature of the present invention. Lifting mechanism  16  raises filter rack  14  off the bottom of container  12  up the full length of filter elements  24 . Once filter rack  14  is raised, the settled and dewatered solids are left undisturbed and in place. Because the backpulse preceding the lifting has such a small volume and low pressure, the solids cakes are left intact. Depending on the type of waste that makes up the formed solids cakes, the exact volume and pressure required to dislodge the solids cakes may require experimentation by those skilled in the art. However, in the present invention preferred ranges for volume and pressure required to merely dislodge the solids cakes formed of such materials as fine particles, gels, colloidal materials are 1-2 cubic feet and 30-50 psig (pounds force of pressure per square inch gauge), respectively. The solids cakes thereafter scrape the sides of filter elements  24  as they are being raised. Consequently, the lifting of filter rack  14  actually helps clean filter elements  24 . Optionally, one or more backpulses by air and liquid source  60  can be done once filter rack  14  is raised to fully clean it and to start a new dewatering cycle. The dewatering, backpulsing, raising, and further backpulsing steps are continuously repeated until container  12  is full of dewatered solids. At this point, dewatering device  10  and dewatering pump  55  and air and liquid source  60  are disconnected. Filter rack  14  is thereafter removed and introduced into an empty container. Alternatively, filter rack  14  is just left in container  12  to rest on top of the dewatered solids.  
         [0034]    [0034]FIG. 6 illustrates an alternative preferred embodiment of the present invention. As shown, filter rack  14  is affixed near the top of container  12 . To initiate the dewatering process, wastes are slurried into container  12  until container  12  is approximately full. Next, dewatering of the slurried wastes takes place by starting dewatering pump  55  or vacuum source external to container  12 . Dewatering pump  55  pulls a vacuum on filter rack  14  and filter elements  24  drawing water from the slurry into the filter rack  14  piping and then to dewatering pump  55 . During dewatering, the solids accumulate and form a dense, dewatered cake on filter elements  24  that are initially backpulsed by air and liquid source  60 . In this event, the solid filter cake is released as a large dewatered mass that is heavier than the surrounding solution or slurry on the bottom of the container. The dewatered solids then slide down off filter elements  24  and settle by gravity to the bottom of container  12  leaving clean filter elements  24 . As discussed above, filter elements  24  are vertically oriented so that the major axes of filter elements  24  are perpendicular to the plane defined by pipe header  20  of filter rack  14 . The particular orientation of filter elements  24  minimizes the disturbance of the solids cakes when they slide down filter elements  24 . Additionally, the solids cakes may help to scrape the remaining fine and colloidal material from filter elements  24  as the cakes are sliding off filter elements  24 . Filter elements  24  are then backflushed a final time and the dewatering operation is continued until container  12  is full of dewatered solids. Once container  12  is filled, filter rack  14  is optionally removed and introduced into another container to begin a new dewatering process. Alternatively, filter rack  14  is just left in container  12  to rest on top of the dewatered solids.  
         [0035]    It will be apparent to those skilled in the art that many changes and substitutions can be made to the preferred embodiment herein described without departing from the spirit and scope of the present invention as defied by the appended claims.