Abstract:
An apparatus and method of diluting a solid particle-containing slurry in conjunction with a sedimentation process. A slurry is introduced into the launder portion of a feedwell and flowed vertically downward through an eductor nozzle having its exit location placed below the liquid level of a volume of clarified liquor. The eductor nozzle is positioned adjacent a diluent inlet such that discharge of the slurry through the eductor nozzle creates a low pressure zone, drawing clarified liquor through the diluent inlet for mixing and dilution of the slurry. A flocculating reagent may additionally be introduced into the diluted slurry for producing a floc and expediting settling of the solid particles.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/361,985, filed Mar. 5, 2002, for SELF DILUTING FEEDWELL INCLUDING A VERTICAL EDUCTION MECHANISM AND METHOD OF DILUTION EMPLOYING SAME. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Field of the Invention: The present invention relates generally to dilution of an influent slurry stream entering into a settling tank or basin and, more particularly, to a method and apparatus of effecting self dilution of such slurry with improved control regarding the flow and flocculation of the slurry stream.  
           [0003]    State of the Art: Various techniques may be used in the separation of suspended solid particles from the liquid in which they are suspended. These techniques may include, for example, flotation, filtration, centrifugation, expression and sedimentation.  
           [0004]    Conventionally, the technique of sedimentation includes introducing a slurry (i.e., a liquid containing suspended solid particles) into a settling tank or basin of, for example, a clarifier or thickener, and allowing the suspended solid particles to settle by gravity to form a sludge or thickened mud on the bottom of the tank and a clarified liquor at the top of the tank. The sludge may then be collected and further processed or otherwise disposed of. Likewise, the supernatant liquid, also referred to as the clarified liquor, may be collected for further processing, disposal, or possibly reused to assist in a similar separation process.  
           [0005]    In introducing the influent stream of slurry into the settling tank it is desirable to control the flow of such a stream so as to avoid, or at least minimize, the disruption of the sludge formed along the bottom of the tank. Undue disruption of the sludge causes particles to become suspended within the supernatant liquid, once again resulting in an inefficient sedimentation process. In an effort to control the flow and distribution of the influent slurry, a feedwell may be positioned to receive the slurry as it is introduced into the settling tank. Conventionally, a feedwell includes an area or compartment within the settling tank but which is separated from the contents of the settling tank. The influent stream of slurry is then directed along a predetermined flow path to dissipate the kinetic energy associated with the flow of the influent stream of slurry. This is done so that the influent stream has reduced flow characteristics as it leaves the separated compartment and is intermixed with the contents of the settling tank. Furthermore, the feedwell may be configured to control the distribution of the influent stream into the settling tank such that the influent is not introduced at a single location within the settling tank. In essence, the feedwell reduces the velocity of the influent stream and provides increased distribution of the influent stream such that any associated turbulence, which would likely disrupt the sludge formed along the bottom of the settling tank, is reduced or eliminated.  
           [0006]    In addition to controlling the flow and distribution of the influent stream of slurry, the feedwell may also be utilized for introducing a flocculating reagent into the influent stream. The addition of a flocculating reagent is sometimes used to expedite the sedimentation process. The flocculating reagent conventionally has a polymeric structure and acts to form a coagulated mass of the suspended particles, the mass sometimes being referred to as a floc. The floc exhibits an increased density over the suspended solid particles and thus provides an improved rate of settling.  
           [0007]    While the use of a flocculating reagent may be beneficial in the sedimentation process, various factors may affect its efficiency. For example, it is important to thoroughly mix the flocculating reagent with the influent stream of slurry. Additionally, the concentration of suspended solid particles within the slurry must be taken into account. For example, if the concentration of suspended particles in the slurry is low, additional mixing may be required to ensure adequate interaction between the suspended particles and the flocculating reagent. On the other hand, if the concentration of suspended solid particles is high, the influent stream of slurry may need to be diluted in order to obtain a lower concentration level for optimum flocculation of the slurry to occur.  
           [0008]    Various methods have been used in the past to dilute the influent stream of slurry. For example, diluent, which may include clarified liquor obtained in the same or a previous sedimentation process, may be pumped to the feedwell and mixed with the influent stream of slurry to obtain to desired level of concentration. However, use of a pump requires additional piping, valving and monitoring equipment as well as additional energy. Such equipment, with the attendant operation and maintenance thereof, adds to the expense of the sedimentation process.  
           [0009]    An alternative method of diluting the influent stream of slurry includes constructing the feedwell to include a dilution channel positioned below the liquid level of the supernatant liquid in the settling tank such that an amount of clarified liquor spills into the dilution channel and mixes with the influent stream of slurry. However, with conventional spill over methods it is often difficult to accurately control the dilution ratio. This becomes particularly important as the concentration of solids in the influent stream changes during the process and adjustments to the dilution ratio must be made.  
           [0010]    Another more recent method of diluting the influent stream of slurry includes the use of an eductor to effect mixing of the diluent with the influent stream of slurry as well as to control the dilution ratio. Such a method is set forth in U.S. Pat. No. 5,643,463 issued Jul. 1, 1997, (to Wood et al.), the disclosure of which is incorporated by reference herein in its entirety. The Wood et al. patent discloses a self diluting feedwell which includes an eductor structure. The eductor structure, described therein, generally includes a walled channel including a launder portion for receiving the influent stream of slurry, at least one port for introduction of a diluent therethrough, a narrow through portion to bring about eduction of the diluent into the feed stream, and a discharge portion from which the diluted influent slurry is introduced into the basin.  
           [0011]    One problem associated with the dilution method and apparatus described in the Wood et al. patent is that classification of the solid particles or sanding may occur when the influent stream of slurry is introduced at low flow rates. When sanding occurs, particles begin settling prior to the slurry being introduced into the settling tank, causing build up on the floor of the walled channel within the feedwell. This in turn affects the flow characteristics within the feedwell, making it difficult to obtain the desired mixing, flow and distribution of the slurry into the tank. Additionally, such sanding may affect the solids concentration of the slurry entering the tank since at high flow rates the built up sludge at the bottom of the walled channel may become disturbed, with the attendant result of solids being reintroduced into the stream of slurry. In effect, the method and apparatus of the Wood et al. patent are limited in their ability to effectively provide dilution in low flow situations.  
           [0012]    Thus, it would be advantageous to provide a method and apparatus for diluting an influent stream of slurry which provides effective control of the diluent ratio without the need for extraneous and expensive mechanical equipment and which are not limited by the flow rate of the influent stream of slurry. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0013]    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:  
         [0014]    [0014]FIG. 1 is a sectional view of sedimentation apparatus incorporating diluting structure according to one embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a sectional view of a dilution structure according to one embodiment of the present invention;  
         [0016]    [0016]FIG. 3 is a sectional view of a dilution structure according to another embodiment of the present invention;  
         [0017]    [0017]FIG. 4 is a sectional view of a dilution structure according to another embodiment of the present invention;  
         [0018]    [0018]FIG. 5A is a plan view showing multiple dilution structures installed in a sedimentation apparatus according to yet another embodiment of the present invention;  
         [0019]    [0019]FIG. 5B is a sectional view of one of the dilution structures shown in FIG. 5A; and  
         [0020]    [0020]FIG. 5C is a perspective view of one of the dilution structures shown in the embodiment of FIG. 5A. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring to FIG. 1, an exemplary sedimentation apparatus  100  is shown which may be used, for example, as a clarifier or thickener. The sedimentation apparatus  100  includes a substantially circular tank  102  formed of one or more side walls  104  joined to a floor  106 . The floor  106  generally slopes toward a discharge location  108  which is shown to be at the center of the tank  102 , although other configurations may be possible.  
         [0022]    Positioned above the tank  102  is a bridge  110  or other structural apparatus to which a drive  112  is mounted. The drive  112  is operably coupled to a rotatable column  114  which, in turn, is coupled to a set of rake arms  116  positioned adjacent the tank floor  106 . The drive  112  may include a motor and a gear reducer appropriately sized and configured to provide the required torque for turning the rotatable column  114  and rake arms  116 . The rake arms  116  rotate with and about the column  114  within the tank  102  causing sludge, or thickened mud, formed along the floor  106  of the tank  102  to move toward the discharge location  108  for collection and/or disposal thereof. The tank  102  further holds clarified liquor which, after reaching a specified liquid level  118  within the tank  102 , spills over a weir or set of weirs  120  and into an adjacent collection trough  122 . Slurry is provided to the tank  102  through an influent feed pipe  124  which discharges into a diluting structure  126 .  
         [0023]    It is noted that while the exemplary sedimentation apparatus  100  is depicted as a circular tank with a bridge mounted column drive, various other tank and drive configurations may be utilized as will be apparent to and appreciated by one of ordinary skill in the art.  
         [0024]    Referring now to FIG. 2, one embodiment of a diluting structure  126 , also referred to herein as a feedwell, is shown in accordance with the present invention for use with the exemplary sedimentation apparatus  100 . The feedwell  126  includes a launder  128  into which the influent feed pipe  124  discharges. The launder  128  includes an upper portion thereof positioned above the liquid level  118  of the tank  102 . The liquid level  130  of the slurry in the launder  128  is also shown to be higher than the liquid level  118  of the clarified liquor in the tank  102 . By maintaining the liquid level  130  at a relatively higher elevation within the launder  128 , the slurry contained therein may be gravity fed through the feedwell  126  and into the tank  102  via the slurry&#39;s tendency to obtain an equilibrium with the liquid level  118  of the clarified liquor in the tank  102 .  
         [0025]    The launder  128  further includes a constricted discharge portion at its lower end which serves to create an eduction zone and which may be referred to as an eductor nozzle  132 . The constricted flow of the slurry at the eductor nozzle  132  helps to maintain the slurry&#39;s liquid level  130  at a position higher than that of the clarified liquor&#39;s liquid level  118 . The amount of liquid contained in the launder  128  provides a certain amount of pressure, or head, at the eductor nozzle  132  serving to increase the velocity of the slurry as it exits through the eductor nozzle  132 , thus increasing its kinetic energy. As the influent slurry passes through the eductor nozzle  132 , it creates a reduced pressure adjacent the discharge side of the eductor nozzle  132  via the acceleration of the influent slurry therethrough. The reduced pressure causes clarified liquor from tank  102  to be drawn into a mixing zone, such as within a mixing tube  134 , through a diluent inlet  136  positioned adjacent the eductor nozzle  132  and effects a momentum transfer between the influent slurry and clarified liquor. The influent slurry is thus mixed with the clarified liquor to create a diluted slurry.  
         [0026]    At this point, if so desired, a flocculating reagent may be added to the influent slurry via a flocculating header  138  having a plurality of nozzles  140  therein. Flocculating reagent may be dispensed via the nozzles  140  at a predetermined rate which may correlated with, for example, the rate of flow of the influent slurry, the concentration of the slurry, and the type of flocculating reagent being utilized. Additionally, if so desired, the flocculating header  138  may be constructed such that it is adjustable with respect to its position within the flow of slurry leaving the eductor nozzle  132 . Such adjustability may be effected both in terms of variation of distance from the eductor nozzle  132  and in terms of angle relative to the flow of the slurry exiting the eductor nozzle  132 . The area in which the flocculating reagent is added to the influent slurry and/or the diluted slurry may also be referred to as a flocculating zone.  
         [0027]    After the slurry has been diluted and flocculated, the diluted slurry passes through a discharge zone  141  of the feedwell  126  into the tank  102  for settling of the floc and clarifying of the liquor. The discharge zone  141  may be configured to divert the discharge of the diluted slurry such that it does not flow directly toward the bottom of the tank or sedimentation apparatus  100  in which it is disposed. Such diversion of the discharged slurry helps to keep any sludge formed at the bottom of the sedimentation apparatus from being resuspended within the clarified liquor. Thus, it may be desirable to configure the discharge zone  141  such that the discharged slurry exhibits a radially outward component as well as well as a downward component.  
         [0028]    It is noted that the use of an eductor nozzle  132  with a diluent inlet  136  positioned adjacent thereto as described herein provides a low-shear environment in which the flocculant may be thoroughly mixed with the slurry with minimal floc breakup. Reducing floc breakup greatly enhances the sedimentation process as the solid particles contained in the slurry settle more expeditiously when formed as a floc.  
         [0029]    The vertical nature of the eductor nozzle  132  positioned in the feedwell  126  provides various advantages over the use of more conventional dilution techniques such as, for example, the use of a horizontally disposed eductor nozzle. One advantage includes the marked reduction in sanding at low flow rates. By controlling to the flow such that it has a continual vertical component associated with it, the solid particles and/or floc have substantially no opportunity to settle within the feedwell  126  but instead settle along the floor  106  of the tank  102  as is intended. By substantially eliminating the possibility of sanding within the feedwell  126 , the influent slurry may be fed into the tank  102  via the feedwell  126  at very low flow rates and the diluent ratio may be infinitely adjusted for different types and concentrations of influent slurries.  
         [0030]    Additionally, the arrangement described with respect to FIG. 2 provides the advantages of requiring a smaller overall volume feedwell  126  while also providing increased mix tube  134  area over conventional feedwells using eductor-type configurations. The reduced size of the feedwell  126  allows for the use of fewer materials in its construction and additionally provides for greater flexibility in the customization and design of the overall sedimentation apparatus  100 . The increased mixing tube  134  area, relative to conventional eductor-type configurations, provides a decrease in shear and velocity of the slurry as it passes therethrough which, as mentioned above, allows for flocculating and mixing to occur without substantial breakup of the floc.  
         [0031]    Control of the dilution ratio may be accomplished in any of a number of ways with the present invention. One manner of controlling the dilution ratio is to control the flow rate of the influent slurry as it exits the influent feed pipe  124 . Controlling the flow rate of the influent slurry will help to determine the liquid level  130  of the influent slurry. A greater difference between the liquid level  130  of the slurry and the liquid level  118  of the clarified liquor provides greater pressure or head at the eductor nozzle  132 , thus increasing the velocity of influent slurry as it passes through the eductor nozzle  132 . Similarly, a decreased differential between the two liquid levels  118  and  130  results in a decreased influent slurry velocity at eductor nozzle  132 . An increase or decrease in the head, with a resulting increase or decrease of slurry velocity through the eductor nozzle  132  results in a greater or lesser amount of diluent being drawn through the diluent inlet  136  respectively.  
         [0032]    Therefore, controlling the influent flow rate controls the differential between the liquid levels  118  and  130 ; controlling the differential of the liquid levels  118  and  130  controls the head and velocity of the slurry at the eductor nozzle  132 ; and controlling the head and velocity of the slurry at the eductor nozzle  132  controls the ratio of diluent being mixed therewith.  
         [0033]    Alternatively, mechanical devices may be utilized to help control the diluent ratio. For example, a vertically adjustable baffle  142  may be installed at the diluent inlet  136  to either enlarge or reduce the size of the opening at the diluent inlet  136  such that a greater or lesser amount of clarified liquor from tank  102  may be allowed to pass therethrough. Alternatively, while perhaps more mechanically complex, the entire lower portion of the feedwell  126 , including the mixing tube  134  and the discharge zone  138 , may be made to be adjustable relative to the upper portion of the feedwell  126  including the launder  128  and the eductor nozzle  132 . For example, the lower portion of the feedwell  126  may be attached to the column  114 , which, as will be appreciated by those of ordinary skill in the art, may be adjusted vertically for positioning the rake arms  116  relative to the floor  106  of the tank  102 . By raising the lower portion of the feedwell  126 , less clarified liquor would be able to pass through the diluent inlet  136  and vice versa.  
         [0034]    It is noted that the launder  128 , and more generally the feedwell  126 , may be formed as a substantially annular-type member circumscribing the column  114  such that the column  114  passes through the slurry contained by the launder  128 . For example, the launder  128  may be formed as a cylinder with the eductor nozzle  132  being shaped as a shell of an inverted-truncated-cone coupled to the bottom of the launder  128  with both the launder  128  and eductor nozzle  132  circumscribing the column. Alternatively, the launder  128  may be constructed to feed a plurality of individual eductor nozzles  132  spaced and arranged in a defined pattern to distribute the slurry therefrom.  
         [0035]    Similarly, depending on the construction of the eductor nozzles  132 , the mixing tube  134  may be formed as a substantially annular member, or as a plurality of individual mixing tubes configured to receive slurry from the individual eductor nozzles  132 , as the case may be.  
         [0036]    Referring to FIG. 3, another embodiment of a feedwell  126  according to the present invention is shown. The feedwell  126 ′ includes similar components as the feedwell  126  described with respect to FIG. 2, with modifications in the launder  128 ′, eductor nozzle  132 ′ and mixing tube  134 ′. Particularly, the launder  128 ′ includes an interior wall  144  such that both the launder  128 ′ and the eductor nozzle  132 ′ are each substantially configured as an annulus surrounding the column  114  such that the column  114  does not penetrate through the slurry. Additionally, the mixing tube  134 ′ includes an interior wall  146  which is laterally spaced from the column  114  and such that the mixing tube  134 ′ is likewise substantially configured as an annulus surrounding the column  114 . The interior wall  146  terminates at an upper end slightly above the outlet of the eductor nozzle  132 ′ but is laterally spaced therefrom. Such a design allows the clarified liquor to be drawn upwards between the column  114  and the interior wall  146  of the mixing tube  134 ′ and into the mixing tube  134 ′ by virtue of the reduced pressure area created by the eductor nozzle  132 ′ resulting in increased mixing of the clarified liquor with the slurry as the slurry exits the eductor nozzle  132 ′.  
         [0037]    Referring to FIG. 4, another embodiment of a feedwell  126 ″ according to the present invention is shown. The feedwell  126 ″ includes a launder  128 ″ similar to that which is shown and described with respect to FIG. 3. Particularly, the launder  128 ″ includes an interior wall  144 ″ separating the slurry from the column  114 . However, the interior wall  144 ″ of the launder  128 ″ extends further downward and serves as the interior wall  146 ″ of the mixing tube  134 ″ as well. Thus, the launder  128 ″, the eductor nozzle  132 ″ and the mixing tube  134 ″ are each substantially configured as an annulus with a common interior wall  144 ″,  146 ″. It is additionally noted that the end of the mixing tube  134 ″ adjacent the discharge zone  138 ″ includes an expanded cross section. Such a design allows for additional reduction in velocity and pressure of the diluted slurry/floc as it exits from mixing tube  134 ″ into the tank  102 .  
         [0038]    It is noted that while certain embodiments have been described as being substantially configured as an annulus, such a description should not be considered as being limited to a circular structure. Rather, the geometric configuration may include other shapes such as, for example, ovals, elliptical configurations, square, or rectangular configurations which exhibit mutually independent internal and external peripheries.  
         [0039]    Referring to FIG. 5A, multiple diluting structures  226  are shown to be installed in a sedimentation apparatus  200  which includes a tank  202 , a bridge  210  or other structural support, and associated components such as described above with regard to other embodiments of the invention. The diluting structures  226  are shown to be positioned within the tank  202  at a location which corresponds generally to the diameter of a feedwell (shown in dashed lines  228 ). It is noted that the diluting structures  226  may be positioned at other locations within the tank  202  but that the configuration shown in FIG. 5A allows for replacement of a more conventional influent feed apparatus with the diluting structures of  226  of the present invention. Such a configuration allows for an existing sedimentation apparatus to be more easily retrofitted or converted with the installation of the diluting structures  226  of the present invention.  
         [0040]    Referring now to FIGS. 5B and 5C, sectional and perspective views, respectively, of the diluting structure  226  is shown. The diluting structure  226  includes a feed pipe  224  which may include a constricted outlet  225  or nozzle to introduce slurry into the diluting structure  226 . The feed pipe outlet  225  is positioned to discharge the slurry at a location proximate to a diluent inlet  236 . Thus, for example, as shown in FIG. 5B the feed pipe outlet  225  may be positioned such that it lies substantially along the same plane as the diluent inlet  236 . The diluent inlet  236  is generally configured to exhibit a greater cross-sectional area than the feed pipe outlet  225  and may exhibit a frustoconical geometry such as shown, although other geometries may be utilized.  
         [0041]    The influent slurry exiting the feed pipe  225  serves to draw clarified liquor to enter into the diluting structure  226  via the diluent inlet  236  in a manner similar to that described above with respect to other embodiments of the invention. Both the influent slurry and the clarified liquor then enter a mixing zone, which may comprise a mixing tube  234  wherein the diluent and influent slurry mix to form a diluted slurry. The mixed, diluted slurry then exits the diluting structure  226  via a discharge zone  241  and into a sedimentation apparatus. A diverter  250  may be positioned in the discharge zone so as to impart a radial component to the exiting diluted slurry thereby keeping the diluted slurry from jetting into the floor or bottom of an associated sedimentation apparatus positioned beneath the diluting structure  226 . Flocculating headers  238  may be positioned adjacent the diluent inlet  236  and/or the discharge zone  241  for introduction of a flocculating reagent.  
         [0042]    It is noted that the diluting structure  226  of the embodiment depicted in FIGS. 5A through 5C is positioned entirely beneath the liquid level  252 , or in other words is submerged in the liquor. Such a configuration allows the present invention to be installed in a sedimentation apparatus without a feedwell associated therewith as previously discussed with respect to FIG. 5A. Also, as discussed with respect to FIG. 5A, such a configuration allows for easier installation of a diluting structure according to the present invention in an existing sedimentation apparatus.  
         [0043]    Additionally, the diluting structure  226  shown in FIGS. 5A through 5C is more flexible in its installation, particularly with regard to locating the diluting structure within a sedimentation apparatus. Also, the configuration of the diluting structure  226  is more conducive to installing multiple diluting structures  226  in a single sedimentation apparatus such as is shown in FIG. 5A. The ability to provide multiple diluting structures  226  and the flexibility in the placement of the diluting structures  226  within a sedimentation apparatus allows for much greater control of the mixing of the slurry and ultimately the sedimentation of the sludge.  
         [0044]    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, this invention includes modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.