Abstract:
A furnace filtration system for improving the speed and efficiency of a molten metal centrifugal impeller pump contained within a non-ferrous molten metal furnace. The system includes a filtration well located upstream of the pump well. The filtration well includes a filter having a plurality of through filter passages that are sized to prevent any solid contaminants that are larger than the pump&#39;s impeller openings from passing therethrough. By preventing any contaminants that cannot pass through the pump, the pump can be run at a higher speed. A furnaces efficiency increases with the centrifugal pump&#39;s efficiency increases with its centrifugal pump&#39;s speed along with the flow and pressure. Increased flow and pressure increases a furnace&#39;s efficiency by a) increasing the melting rate; b) decreasing the energy lost by reducing the metal temperature stratification; and c) reducing the contaminants increases the quality of the metal produced while reducing the amount of nitrogen and chlorine needed to clean the metal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/968,825, filed Aug. 29, 2007, titled Furnace Filtration System For Molten Metal, the disclosure of which is expressly incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates to the separation of solids from liquids and, more particularly, to filtering dross and other solid contaminants from molten metal. 
   BACKGROUND OF THE INVENTION 
   A typical molten metal facility includes a furnace with a pump for moving molten metal. During the processing of molten metals, such as aluminum, the molten metal is normally continuously circulated through the furnace by a centrifugal circulation pump to equalize the temperature of the molten bath. These pumps contain a rotating impeller that draws in and accelerates the molten metal creating a laminar-type flow within the furnace. 
   A well-known problem with such processes, however, is the accumulation of dross within the metal bath. Dross is a mass of solid impurities floating on and within a molten metal bath. It usually appears within molten metals or alloys having a relatively low melting point, such as tin, lead, zinc or aluminum, or by oxidation of these metals. Other impurities, such as pieces of the furnace&#39;s refractory material may also be found within such a molten metal bath. 
   The dross can range in size from small particles to relatively large pieces or chunks. The smaller dross material, while undesirable, does not normally pose a threat to the operation of the furnace and its circulation pump(s). However, the larger pieces of dross can, and often do, get pulled into the pump and cause the pump to become jammed, causing catastrophic failure to occur. 
   As a result of this problem, furnace operators frequently must run their circulation pumps at a relatively low speed, such as approximately 250-300 rpm. This slower speed, while reducing the damage to the pump components if a larger piece of dross becomes lodged therein, results in the undesirable condition of the pump motor being operated at much less than peak efficiency. That is, through the use of a frequency converter, a motor can produce the necessary torque at these lower speeds, but resulting in only using 10-15% of the available motor horsepower. 
   Another common solution to the pump-damaging large dross pieces within a furnace is to install larger and larger pumps having impellers that will receive and transfer all but the largest contaminants. These pumps, however, are expensive and they waste energy by continually pushing the dross through the system. Furthermore, as the dross is circulated through the furnace along with the molten metal, the dross pieces tend to accumulate together or conglomerate to create larger and larger pieces. Eventually, these growing pieces of dross may reach a size that will jam within even a very large impeller. 
   Currently, there are filters that are placed at the furnace discharge. These filters prevent dross from exiting the furnace—not from entering the pumps. Additionally, they do not provide for a user to collect the filtered contaminants from the system. Instead, the contaminants are left free to settle throughout the furnace. 
   In view of the current inefficient use of molten metal pumps, there is a need for a system for filtering the larger dross pieces in the furnace from entering a molten metal pump, thereby allowing the pump to operate at higher speeds and increasing efficiency within the system. Additionally, there is a need for such a filtering system that enables a user to quickly clean out the accumulated filtered contaminants from the furnace. 
   SUMMARY OF THE INVENTION 
   The present invention provides a filtration well adapted to filter solid contaminants from a bath of molten metal that are too large to pass through the impeller openings of a centrifugal impeller pump located within a pump well. The filtration well includes at least two walls that cooperate to define an enclosure having an upstream section and a downstream section, wherein the upstream section includes an inlet arch and the downstream section includes an outlet arch. The outlet arch is in direct fluid communication with the pump well. A filter covers the outlet arch and includes bore means which prevent solid contaminants that are sized larger than the impeller openings from passing therethrough, but allow solid contaminants that are smaller than the impeller openings to flow through. 
   The present invention also provides a filtration system for a molten metal bath having a pump well containing a centrifugal impeller pump. The impeller pump having impeller openings which receive and pump molten metal and any solid contaminants within the molten metal that are smaller than the impeller openings. The filtration system includes an upstream filtration well that is fluidly coupled upstream to the pump well. The filtration well including an inlet wall having an enlarged inlet opening and a spaced outlet wall having an enlarged outlet opening, wherein the filtration well is in fluid communication with the pump well through the outlet opening. The filtration system also includes a furnace filter having a body that is sized to cover the outlet opening. The body includes a plurality of filter bores or openings, each of the filter bores or openings being smaller than the impeller openings, whereby contaminants larger than the filter bores are retained within the upstream filtration well and molten metal and contaminants smaller than the filter bores pass through the furnace filter and into the pump well. The filtration system also includes means for removably retaining the furnace filter over the outlet opening. 
   The present invention further provides a method of increasing the operating speed of a centrifugal pump within a bath of molten metal. This method includes the steps of: forming a filtration well directly upstream of the pump well, wherein the filtration well includes a pair of spaced walls, wherein one of the walls includes an inlet arch and the other wall includes an outlet arch, the outlet arch being in fluid communication with said pump well; providing a furnace filter; and preventing all solid contaminants that are larger than the pump&#39;s impeller openings from entering the pump well by covering the outlet arch with the furnace filter. 
   The present invention is a molten metal filtering system that prevents over-sized contaminants from being pulled into a molten metal recirculation pump. This system includes a filter well that is disposed upstream of the furnace&#39;s pump well. The filter well is generally defined by two pairs of spaced opposed walls. These walls act as an inlet and outlet to the filter well as each has a through opening or arch. The inlet wall is in fluid communication with the furnace, while the outlet wall is in fluid communication with the pump well. 
   In the preferred embodiment, the outlet wall opening is covered by a filter plate. The outlet wall includes a generally flat filter plate having a plurality of through filter bores that allow molten metal to pass through the filter and into the pump well. Each filter bore is sized to prevent any solid contaminants that are larger than the pump impeller&#39;s openings to pass through the filter and into the pump well. 
   By filtering all of the potentially pump-damaging pieces of dross from entering the pump well, the pumps located within the pump well may be freely operated at a much higher speed, which dramatically increases the flow rate of the molten metal bath within the furnace. Running the pump at this more efficient speed will typically require the output from the pump to be diffused to optimize the penetration into the charge well, additionally any pump-safe sized solid contaminants are also preferably removed from the furnace. 
   A centrifugal pump&#39;s efficiency increases with rotational speed along with the flow and pressure. Increased flow and pressure increases a furnace&#39;s efficiency by increasing the melting rate; decreasing the energy lost by reducing the metal temperature stratification; and reducing the contaminants, which increases the quality of the metal produced thereby reducing the amount of nitrogen and chlorine needed to clean the metal. 
   It is an advantage of the present invention to provide a filtration system that will prevent large pieces of dross from damaging a furnace&#39;s molten metal pump, thereby allowing the pump to run at a higher, more efficient speed. 
   It is another advantage of the present invention that the filters can be readily removed and replaced from the filtration system to reduce furnace downtime. 
   It is still another advantage of the present invention that the filtration system provides a dross filtration well that collects and retains the filtered dross in an easily accessible location. 
   These and other objects, features and advantages of the present invention will become apparent from the following description when viewed in accordance with the accompanying drawings and appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The description refers to the accompanying drawings in which like reference characters refer to like parts throughout the several views, and in which: 
       FIG. 1  is a cut-away perspective view of the preferred molten metal filtration well; 
       FIG. 2  is a top view of the upstream filtration well; 
       FIG. 3  is a front view of the pump well inlet filter plate; 
       FIG. 4  is a partial side view of the pump well inlet filter covering the pump well inlet arch; 
       FIG. 5  is a top sectional view of a high flow centrifugal impeller pump; 
       FIG. 6  is a partial top sectional view of the through bore shape for the filtration system&#39;s filter; 
       FIG. 7  is a cut-away perspective view of the molten metal filtration well and an alternate configuration of the filter; 
       FIG. 8  is a perspective view of the alternate configuration of the filter shown in  FIG. 7 ; 
       FIG. 9  is a top view of the upstream filtration well shown in  FIG. 7  with an alternate configuration filter installed; 
       FIG. 10  is a partial top view of an alternate filter retention bracket; 
       FIG. 11  is a sectional view through line A-A in  FIG. 10 ; 
       FIG. 12  is a partial side sectional view of an alternate filter having a angled lower filter bores; 
       FIG. 13  is a partially exploded perspective view of an alternate filter having inert graphite filter tubes; 
       FIG. 14  is an assembled perspective view of the alternate filter shown in  FIG. 13 ; 
       FIG. 15  is a perspective view of an another alternate configuration of the filter plate; 
       FIG. 16  is a front view of still another alternate configuration of the filter plate, including inert graphite filter grates; 
       FIG. 17  is sectional view of an inert gas saturated, insulated, graphite filter tube; 
       FIG. 18  is a perspective view of yet another alternate configuration of the filter; 
       FIG. 19  is a cut-away perspective view of a molten metal recirculation and transfer system, including an upstream filtration well and a downstream filter/transfer well; 
       FIG. 20  is a rear perspective view of the filter/gate valve used in the system illustrated in  FIG. 19 ; and 
       FIG. 21  is another alternate embodiment illustrating a furnace filtration system formed by adapting a pre-existing furnace pump well. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIGS. 1-4 , there is shown a preferred embodiment of the present invention. The present invention is molten metal filtration system  10  for a central melting or holding furnace, such as furnace  12 . 
   A conventional furnace  12  is generally shaped as a fluid retaining enclosure. This enclosure includes a heating area or hearth  13 , a pump well  14  that contains a molten metal circulation and/or a transfer pump  16  and a charge well  18 . A bath  20  of molten metal is contained within furnace  12 . A series of arches or gates fluidly connect the hearth, pump well and charge well allowing the molten metal to flow through the furnace. The bath  20  is heated in the hearth  13 , pulled into the pump well  14  by pump  16  and accelerated out from the pump and into the charge well  18 . Additional raw material sought to be melted is inserted into the furnace at charge well  18 . 
   Pump  16  is typically a high flow centrifugal impeller pump adapted to be immersed in molten metal. Pump  16  rotates an impeller  22  to draw in and expels the molten metal forming bath  20 . One example of such a pump is the type disclosed in my pending U.S. patent application Ser. No. 11/337,266 entitled HIGH FLOW/DUAL INDUCER/HIGH EFFICIENCY IMPELER FOR LIQUID APPLICATIONS INCLUDING MOLTEN METAL which is incorporated herein by reference, word for word and paragraph for paragraph. It should be appreciated that while pump  16  is being described as a centrifugal impeller-type of pump, it can be substantially any style pump suitable for use in a molten metal environment. 
   Bath  20 , while being primarily composed of the intended molten metal material, also contains solid contaminants or dross that flows throughout furnace  12 . Dross varies in size from particulate-type matter to large pieces  24  that can be at least partially pulled into pump  16 . These larger pieces of dross  24  oftentimes jam the rotating impeller  22 , resulting in damage to or clogging of the pump. 
   Filtration system  10  includes a filtration well  28  that is located within furnace  12  between hearth  13  and pump well  14 . Filtration well  28  is preferably a rectilinear enclosure defined by two pairs of opposed vertical walls  29 - 30  and  31 - 32 . Walls  29 - 32  are formed from the same material as the other furnace enclosure walls, typically a castable refractory cement. Two of the opposed walls  29  and  30  include passages or arches  33 ,  34 . 
   Filtration well  28  is disposed relative to the rest of furnace  12  such that wall  29  separates well  28  from hearth  13 , while its interior region  35  is in fluid communication with hearth  13  via arch  33 . Wall  30  separates well  28  from pump well  14 , while interior region  35  is in fluid communication with pump well  14  via arch  34 . Walls  31 ,  32  complete well  28  and separate the two arched walls  29 ,  30 . As will be described in greater detail below, the distance walls  29 ,  30  are spaced is sufficient to allow filtration well  28  to be readily cleaned out and to retain a significant amount of dross  24 . 
   Filtration well  28  includes at least one filter  36 . Filter  36  is preferably a rectangular flat sheet or plate body  37  formed from a ceramic material, such as silicon carbide or silicon nitride reaction-bonded silicon carbide. Filter  36  includes a plurality of substantially identical through bores  38 , which are arranged in an array and cooperate to form a rough screen or filter in body  37 . Importantly, each through bore  38  is sized slightly smaller than the impeller inlet opening  40  of the pump  16 . In this manner, the diameter of through bore  38  is wholly dependent upon the inlet opening  40  of the pump  16  located within the furnace  12  being filtered. In a typical furnace  12 , inlet opening  40  will be approximately 1½ inches in diameter, and the through bores  38  for such an inlet size would be approximately 1 to 1¼ inches in diameter. In one non-limiting embodiment, the area of the upstream openings  38   a  is approximately 90% of the size of the impeller inlet opening  40  for the pump  16  located in pump well  14 . 
   It should be appreciated that by substantially covering the filter body  37  with through bores  38 , the flow of the molten metal  20  is not impeded, with negligible pressure drop across the arch-covering filter  36 . 
   In one embodiment, as shown in  FIG. 6 , each through bore, denoted  38 ′, has a frusto-conical shape where the opening  38   a ′ at the upstream face  41  (i.e., the side that faces the bath  20  within region  35 ) of plate  36  tapers outwardly to the downstream opening  38   b ′. In this embodiment, opening  38   a ′ is substantially the same size as opening  38   a  and will block any solid contaminants larger than impeller inlet opening  40 . This tapered shape of the through bores  38 ′ reduces the possibility of the filter clogging and further reduces the difference in fluid pressure across filter  36 . 
   Filter plate  36  is disposed within filtration well  28  and is sized to cover the filtration well&#39;s outlet arch  34  (i.e., the pump well&#39;s inlet arch). It should be understood that the size and shape of filter plate  36  may vary as long as it substantially covers arch  34 . In an embodiment illustrated in  FIG. 2 , wall  30  includes a filter slot  42  that is formed down through the top surface  30   a  of wall  30 . Slot  42  is shaped complementary to and is sized to receive filter plate  36 . Slot  42  intersects and is substantially co-planar with arch  34 . Slot  42  extends longitudinally beyond arch  34  along wall  30  forming a pair of vertical channels  43  that abuttingly receive the opposite edges of filter plate  36  and cooperate to retain the filter plate within filtration well  28 . In one embodiment, the upstream face of wall  30  includes an opening  44  approximately the same width as arch  34  and extends up through the top surface  30   a  of wall  30 . Opening  44  is aligned with and intersects slot  42  and presents the substantially the entire upstream face  41  of filter  36  to the metal bath in filtration well  28 . 
   As best shown in  FIG. 4 , filter  36  preferably projects from its retention means above the upper surface  30   a  of wall  30 , such that the top edge  36   a  may be grasped to remove filter  36  from furnace filtration system  10 . Means for withdrawing the filter, such as apertures  39  formed through the filter  36  proximate to its top edge  36   a , are preferably provided to facilitate removal of filter  36  from the furnace  12 . 
   In the embodiment illustrated in  FIG. 1 , arches  33 ,  34  are formed in the walls  29 ,  30 . Each arch  33 ,  34  preferably is an opening extending up the wall from the floor  45  of the filtration well  28 . It should be understood that the location, shape and/or size of these arches may vary and that the filters and filter retaining means, such as slot  42 , may vary accordingly. 
   Referring now to  FIGS. 7-9 , another embodiment of filtration well  28  replaces filter  36  with a box filter  46 . Filter  46  includes a rectangular box-like body that is formed from three upstream plate-like filter walls  47 , which are each similar in construction to filter  36 . Each filter wall  47  includes a plurality of identical through bores  38 , which are identical to the through bores formed in filter  36 . A fourth downstream filter wall  48  completes the box-like configuration to define an internal chamber  49 . It should be appreciated that the four-walled box configuration is exemplary in nature and that substantially any number of walls could be used to form box filter  46 . 
   In the preferred embodiment, downstream filter wall  48  is sized to be received in slot  42 , while upstream walls  47  project into filtration well  28 . That is, the outermost edges of downstream wall  48  extend beyond the upstream walls  47  forming a pair of outer mounting flanges  50  which are received within channels  43 . 
   Downstream filter wall  48  includes an array of identical through bores  51 . These bores  51  are smaller than bores  38 . The reduced size of bores  51  enables filter wall  48  to screen or filter even smaller particles of dross from the bath of molten metal, which may dynamically impact and damage the rotating impeller at higher rotating velocities. 
   Each through bore  38  in filter  46  is approximately 10% to 20% smaller than impeller inlet opening  40  to cause dross  24  that would otherwise damage the downstream pump  16  to be retained in filtration well  28 . In one embodiment, box filter  46  includes a closed bottom  52  that interconnects each of the walls  47 ,  48  and allows the filter  46  to be periodically removed from well  28  to be emptied/cleaned and subsequently re-installed. In one embodiment, box filter  46  is approximately one-half to two-thirds the size of the filtration well  28 . 
   Box filter  46  thereby provides dual filtration by first screening out any pieces of dross that are larger than the impeller inlet opening  40  via upstream through bores  38  in walls  47 . These larger pieces of dross are retained within filtration well  28 . The finer filter bores  51  formed in downstream wall  48  further screens out even smaller pieces of dross to further protect the pump. That is, bores  51  block pieces of dross that will pass through the pump  16 , but may cause damage to the impeller  22  when the pump is run at higher velocities. These smaller pieces of dross are retained with retention chamber  49 . The pump  16  can then be accelerated to higher RPMs. 
   The above description illustrates how furnace filtration system  10 , through filters  36 ,  46  and filtration well  28  prevents the larger non-pumpable pieces of dross  24  from entering pump well  14 , thereby preventing dross  24  from damaging or clogging the pump. In turn, system  10  enables a user to run pump  16  at a higher speed. By eliminating the chance of a dross  24  jamming pump  16 , the pump can be run at a speed that is at or near its peak efficiency, instead of a typical slower speed that may save the pump if such a contaminant was pulled into the pump (i.e., by running at a slower speed, the pump may only stop turning instead of resulting in a catastrophic/breakage event). 
   Further, by retaining all of the potentially pump-damaging dross pieces  24  within filtration well  28 , filtration system  10  can be readily cleaned by shoveling or scraping the floor  45  between the spaced upstream and downstream walls  29 ,  30 . 
   In other embodiments, shown in  FIGS. 10 and 11 , the filter slot  42  of filtration well  28  are replaced or supplemented by guide brackets  54  that are anchored to the top  30   a  of wall  30  proximate to the arch  34 . These brackets  54  receive a filter and hold the filter plates  36  or filter box  46  over the arch opening. 
   In one embodiment, filters  36 ,  46  and their corresponding retention means  42 ,  54  are complementarily shaped so that the filters cannot be placed backwards. 
   In still another embodiment, illustrated in  FIG. 12 , the bottom-most row or rows or through bores  38 ″ are angled upward from upstream face  41  to the downstream face of the filter  36 ,  46 . This upward angle directs the flow of molten metal bath  20  slightly upward and causes the flowing bath  20  to exert a downward force on the filter  36 ,  46 . This downward force resists any upheaval of filter  36 ,  46  as bath  20  flows through the through bores, while ensuring that the upwardly directed flow through bores  38 ″ is dispersed by the remaining through bores  38  to prevent any ripples on the surface of bath  20 , which could result in increased oxidation of the molten metal. 
   Referring now to  FIGS. 13-18  and as disclosed in my previous U.S. Pat. No. 6,168,753 entitled INERT PUMP LEG ADAPTED FOR IMMERSION IN MOLTEN METAL which is incorporated herein by reference, word for word and paragraph for paragraph, injecting an inert gas into a graphite body having sufficient porosity to house the inert gas prevents the entry of either air or molten metal inside the flow tube—filtration plate. As best shown in  FIG. 17 , a graphite tube  60  having an axial internal passage  61  is coated or covered with a layer  62  of suitable refractory cement material mixed with boron nitride paint. 
   A layer of nylon or fiberglass tape  63  covers the cement coated tube and is preferably wrapped around the tube in a helical pattern to ensure coverage of the tube. An outer layer  64 , also a mix of cement and boron nitride paint coats the tape layer  63 . The tape layer  63  is cemented by a combination of the refractory cement and boron nitride paint which constitutes inner and outer layers  62  and  64 . 
   As disclosed in my previous patent, an inert gas (e.g., nitrogen or argon) is pumped into passage  61  and the graphite is sufficiently porous to house the inert gas and prevent the entry of either air or molten metal to prevent the graphite from burning in the molten bath  20 . 
   It should be appreciated that multiple graphite tube  60  may be interconnected such that each internal conduit  61  is in fluid communication and where each insulating layer (e.g., layers  62 ,  63 , and  64 ) substantially cover the molten metal-contacting surfaces, typically the outermost surfaces. 
     FIGS. 13 and 14  illustrate an alternate filtration plate, denoted  136 , having a generally rectangular frame  137  defining an interior cavity  138 . Frame  137  is sandwiched between two plates  140 ,  142 , which are similar in construction to filter plate  36 , having a plurality of bores  143  formed therethrough. Like plate  36 , plates  140 ,  142  are preferably formed from a molten aluminum heat resistant ceramic material, such as silicon carbide, silicon nitride reaction-bonded silicon carbide, or insulated-nitrogen saturated graphite to avoid burning at the metal line  20 A as discussed above and as disclosed in my previous U.S. Pat. No. 6,168,753. It should be appreciated that each bore  143  in the upstream plate  140  is axially aligned with a bore  143  in the downstream plate  142 . 
   A graphite flow tube  148  is mounted within cavity  138  between the plates  140 ,  142 . Flow tube  148  is of a generally tubular construction and spans the gap between each aligned bore  143  of the opposing plates  140 ,  142 , such that through bore  146  formed in each flow tube traverse the thickness of filtration plate  136 . In the preferred embodiment each flow tube fits within bores  143  and is cemented to plates  140 ,  142  using refractory cement. A complementarily shaped ceramic or graphite frame  137  is disposed within and cemented to each plate  140 ,  142  to prevent the molten aluminum from coming inside the filter body pressurized with nitrogen. Filtration plate  136  includes a fitting  148  having a gas-receiving passage  149  which is in fluid communication with cavity  138  for receiving an inert gas, such as nitrogen or argon, from a source  150  through conduit means  152 . Nitrogen is preferably injected into cavity  138  and is received by and saturates the graphite flow tubes  145  to prevent the tubes exposed above the molten metal line  20 A of furnace  12  from burning. In this embodiment, flow tubes  145  are shaped substantially the same as filter bores  38 . 
   Referring now to  FIG. 15 , an alternate embodiment of a filtration plate, denoted  236 , is shown having a plurality of spaced parallel bars or tubes  237  that are coupled together at their opposite ends by a pair of support members  238 ,  239 . Each bar or tube  237  is a generally cylindrical bar or tube formed from a heat resistant silicon carbide ceramic material or graphite which is properly insulated. With respect to the preferred graphite tube embodiment, the insulated graphite tubes are nitrogen saturated. The tubes  237  are spaced apart such that the gap  240  between adjacent tubes  237  is slightly smaller than the impeller inlet opening  22  of pump  16 . In this manner, plate  236  operates in substantially the same manner as filter plate  36 . The upper and lower support members  238 ,  239  include a plurality of spaced apertures  241  which are shaped complementary to the tubes  237 . Tubes  237  are preferably fixed to support members  238 ,  239  with refractory cement. 
   In this embodiment, both the upper and lower support bars  238 ,  239  include internal passages  241  which are in fluid communication with the passages  61  of tubes  237 . The upper support bar  238  includes a fitting  148  which is in fluid communication with passages  61  and  241  for receiving an inert gas, such as nitrogen. The upper support bar  238  further includes retrieval apertures  39 , which enable the plate  236  to be readily retrieved. It should be appreciated that these apertures  39  are not connected with the nitrogen passage  241 . It should further be appreciated that plate  236  is sized to be readily received within retention means, such as slot  42  or guide brackets  54  located proximate to furnace arch  33 . While the embodiment illustrated in  FIG. 15  depicts the filter bars  237  running vertically, it should be appreciated that bars  237  can be oriented at substantially any angle. 
   Referring now to  FIG. 16  an alternate embodiment of the filtration plate  236 , denoted  336  is illustrated incorporating the inert graphite tubing described above. Filtration plate  336  includes a plurality of spaced graphite tubes  337  that span and interconnect a pair of graphite support members  338 ,  339 . Tubes  337  and members  338 ,  339  are insulated by applying a layer  62  of refractory cement mixed with boron nitride paint. Layer  62  bonds the layer  63  of fiberglass or nylon tape to the graphite bodies  337 - 339 . The tape layer  63  is coated with another layer  64  of suitable refractory cement and boron nitride paint mixture. Each graphite body  337 - 339  includes an internal conduit  342  (i.e., passages  61 ) which are fluidly interconnected and which receives an inert gas, such as nitrogen, through a fitting  148  mounted to one of the support members. Each tube  337  is spaced apart from the adjacent tube  337  a distance forming a gap  343  that is smaller than the impeller inlet opening  22 . Like filter plate  136 , filter plate  336  is sized to substantially cover inlet arch  33 , effective to prevent over-sized pieces of dross  24  within the furnace from entering the pump well  14 . 
   Referring now to  FIG. 18  yet another embodiment of a molten metal furnace filter, denoted  436  is shown. Filter  436 , like filter  46  is a box or basket filter having three upstream walls  447  and one downstream wall  448  which cooperate to define an inner cavity or space  449 . Each wall in filter  436 , unlike filter box  46 , is formed from an inert gas graphite filter body, similar to filter plate  336 . 
   That is, each of the three upstream walls  447  and the downstream wall  448  is formed from insulated graphite tubes, which are substantially the same as tubes  60 . The walls of filter  436  have an internal passage  450  (i.e., interconnected passages  61 ) that is injected with an inert gas, such as nitrogen. In the preferred embodiment, the internal passages within each wall are fluidly connected and receive nitrogen gas through a common fitting, such as fitting  148 . Like filter box  46 , the upstream walls are configured to prevent over-sized pieces of dross from entering space  449 , such that the plurality of parallel filter tubes  451  are spaced apart a distance that is slightly smaller than the impeller inlet opening  22 . The downstream wall  448  includes parallel filter tubes  452  which are spaced even closer together than tubes  450  to further filter even smaller pieces of dross from entering the pump well  14 . 
   In one embodiment, filter  436  includes a bottom plate  453  which includes a plurality of drain holes  454 , which aid in draining the molten metal from filter  436  when filter  436  is removed from furnace  12 . 
   Referring now to  FIGS. 19 and 20  an alternate embodiment of filtration system  10  is shown where filtration well  28  is supplemented with a secondary transfer filtration unit  500  that is proximate to the pump well outlet arch  502  and charge well  18 . Filtration unit  500  includes a box filter/gate valve  504  that is similar in construction to filters  36  and  46 . This filter  504 , however, is configured to screen out or filter much smaller solid contaminants, which readily pass through pump  16 . In one embodiment, filter  504  includes a plurality of through bores  506  that are approximately 1/16th to ⅛th inches in size. 
   Filtration unit  500  is disposed downstream of the outlet  70  of pump  16 . As shown in  FIG. 19 , unit  500  preferably includes a transfer well  507  that is defined by spaced dual walls  508 ,  509 . Well  507  is located between pump well  14  and charge well  18 . 
   The upstream wall  508  contains pump well outlet arch  502  and includes filter retention means, such as a filter guide slot  510 , which is similar in construction to slots  42  described above, such that filter  504  covers arch  502 . Similarly, downstream wall  509  contains charge well inlet arch  512  and includes filter retention means, such as filter guide slot  514 , which is substantially the same as and faces slot  510 . A transfer spout or outlet  516  is formed in the upper portion of the outer wall of well  507 . 
   As shown in  FIG. 20 , filter  504  has a box-like configuration having three upstream filter walls  517  and a fourth downstream solid wall  518 . Filter walls  517  are similar to walls  47  of filter box  46 , with each walls  517  configured as a fine particle filter using a plurality of through bores  506 . 
   As the filter box  504  is lowered along slots  510 ,  514 , solid wall  518  will close off the recirculation arch  512  (i.e., the arch passing into the charge well  18 ). Molten metal will begin filling transfer well  506  being filtered by a fine particle filter bores  506  in walls  517 . When the metal reaches the level of spout  516 , it will begin to transfer the finely filtered molten metal out of the furnace  12 . In this embodiment a bottom wall  520  interconnects the four walls  517 ,  518   
   As the box filter/gate valve  504  is lifted, the pump  16  will resume recirculating the molten metal through the arches  502  and  512  in a normal manner with the bottom  520  of the filter box  504  as a top guide for the flow. 
   Referring now to  FIG. 21  another embodiment of furnace filtration system  10  is illustrated, having a more compact filter well  28  which may be formed by adapting a pre-existing furnace  12  by reducing the size of pump well  14  by adding an additional wall  600  having another inlet arch  34  and filter retention means. In this embodiment, wall  600  is located between the original pump well return arch and the pump  16 . The original pump well return arch operates as the hearth return arch  33  discussed above. 
   From the foregoing description, one skilled in the art will readily recognize that the present invention is directed to a furnace molten metal filtration well, a system utilizing such a filtration well, and methods for using the same to improve pump speed and efficiency. While the present invention has been described with particular reference to various preferred embodiments, one skilled in the art will recognize from the foregoing discussion and accompanying drawing and claims that changes, modifications and variations can be made in the present invention without departing from the spirit and scope thereof.