Abstract:
This filtration device  10  has a can body  20  having a reservoir  3  that connects to an opening  9  at the top, and a filtration unit  40  which is detachably installed in the reservoir. The filtration unit has a support plate  42 , and filtration tubes  41  of bottomed cylindrical shape fastened to the support plate. The side wall  22  of the can body  20  has a protruding locking part  30  for installation of the filtration unit so that the filtration tubes are suspended from the support plate  42 , with the openings  43  thereof facing upward. With the support plate  42  locked into the locking part  30 , pressing members  70  for pressing the support plate  42  downward from the upper surface side in sections thereof lying towards the peripheral edge S in relation to the fastening locations of the filtration tubes  41  are deployed, making it possible for the support plate  42  to be fastened in clamped fashion by the locking part  30  and the pressing members  70.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a molten metal filtration apparatus for removing inclusions and the like from various molten metals, such as aluminum and aluminum alloys. 
       BACKGROUND ART 
       [0002]    Inclusions and the like in molten metal containing aluminum, an aluminum alloy, and so on cause problems in casting, such as casting defects, and cracking of the resulting casting in rolling. Therefore, filtration of molten metal has been practiced to remove inclusions and the like. A filtration apparatus having one or more porous ceramic filtration tubes placed horizontally in a molten metal chamber is widely known. In a filtration apparatus of this type molten metal is made to flow from the outside to the inside of the tube to be deprived of inclusions, etc. and gain improved quality. Castings obtained from, for example, molten aluminum thus filtered have reduced cracking or scratches on rolling. Examples of such a filtration apparatus are described in Patent Literatures 1 and 2 below. 
         [0003]      FIG. 24  schematically illustrates the structure of such a conventional filtration apparatus  210 . The filtration apparatus  210  includes a housing  220  and a filtration unit  240 . The filtration unit  240  includes a filtration tube  241  having its both ends supported by a pair of support plates  242  (called closure heads). The filtration unit  240  is pressed to the inner wall of the housing  220  by a wedge  290  thereby to secure a seal for molten metal. The pair of support plates  242  are supported by respective bases  230 . 
         [0004]    The assignee common to this patent application previously proposed a filtration apparatus having filtration tubes disposed in a molten metal chamber not horizontally but vertically (see Patent Literature 3 below). The embodiment shown in  FIGS. 1 to 3  in Patent Literature 3 is configured in such a manner that, when the filtration unit is fitted into the housing, the longitudinal direction of the filtration tubes intersects the horizontal direction of the housing and their open end face upward. 
       CITATION LIST 
     Patent Literature 
       [0005]    Patent Literature 1: JP 5-195101A 
         [0006]    Patent Literature 2: JP 9-137235A 
         [0007]    Patent Literature 3: JP 2013-136812A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    In the case of a filtration system having filtration tubes  241  disposed horizontally as with the case of the filtration apparatus  210  shown in  FIG. 24 , molten metal tends to deposit and solidify on the base  230  with long-term use of the system, resulting in making a slight difference in height of the base  230  at the ends of the filtration tubes. That being the case, when the filtration unit  240  is pressed by the wedge  290  onto the inside wall of the housing  220 , the contact pressure of the support plate  242  with the base  230  varies between the facing pair of the support plates  242 . Consequently, the sealability of the filtration unit  240  reduces, which can lead to molten metal leakage. Once molten metal leakage occurs, non-filtered molten metal will be incorporated into filtered molten metal, and the whole molten metal flowing in the filtration system must be re-treated. Such re-treatment demands time and effort, causing a heavy loss. 
         [0009]    In the case of the filtration system of Patent Literature 3 having filtration tubes disposed vertically, molten metal enters the filtration tubes from the outside toward the inside of the filtration tube, flows upward in the tubes, overflows from the open end of each tube, and flows outside the system. If the molten metal upward flow lifts the support plate to which the filtration tubes are fixed, the molten metal can leak. To avoid this, Patent Literature 3 proposes providing the support plate with a weight and the like. 
         [0010]    However, the filtration apparatus of Patent Literature 3 still has room for improvement on the prevention of the support plate from being lifted more effectively. 
         [0011]    An object of the invention is to provide a molten metal filtration apparatus that eliminates the above discussed various disadvantages associated with conventional techniques. 
       Solution to Problem 
       [0012]    The present invention provides a molten metal filtration apparatus including:
       a housing having an opening in its upper portion and a molten metal chamber and   a filtration unit detachably fitted in the molten metal chamber of the housing,       
 
         [0015]    the filtration unit including a support plate and at least one bottomed cylindrical filtration tube which is made of a porous ceramic material and is fixed to the support plate along the thickness direction of the support plate, 
         [0016]    with the filtration unit being fitted in the housing, the filtration tube being configured in such a manner that the longitudinal direction of the filtration tube intersects the horizontal direction of the housing and the open end of the filtration tube faces upward, 
         [0017]    the housing having, on its inner wall, a catching portion for fitting the support plate therein, the catching portion projecting horizontally inward from the inner wall, 
         [0018]    the apparatus having a hold-down member holding down the support plate from the upper surface toward the lower side of the support plate at a location closer to the periphery of the support plate than the location at which the filtration tube is fixed, and 
         [0019]    the catching portion and the hold-down member being configured to hold the support plate therebetween under pressure. 
       Advantageous Effects of Invention 
       [0020]    The molten metal filtration system according to the invention easily allows for sealing the filtration unit and effectively prevents lift of the support plate and resultant leakage of molten metal. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  is an exploded perspective view of a molten metal filtration apparatus  10  according to an embodiment of the invention, with part of the housing  20  broken away 
           [0022]      FIG. 2  is a cut-away end view of the apparatus  10  after fitting a filtration unit  40  and before fitting a closure  52  (a heating unit  50 ) or a closure  92 , taken along line I-I of  FIG. 1  viewed in the direction of the arrows. 
           [0023]      FIG. 3  is a cut-away end view of the apparatus  10  in the same state as in  FIG. 2 , taken along line II-II of  FIG. 1  viewed in the direction of the arrows. 
           [0024]      FIG. 4  is a cross-sectional view of the apparatus  10  in the same state as in  FIGS. 2 and 3 , taken along line of  FIGS. 2 and 3  viewed in the direction of the arrows. 
           [0025]      FIG. 5( a )  and  FIG. 5( b )  are each a perspective view of the filtration unit  40  shown in  FIG. 1  viewed from below a support plate  42 , of which  FIG. 5( a )  represents the state non-engaged by a fitting tool, and  FIG. 5( b )  represents the state engaged by a fitting tool. 
           [0026]      FIG. 6  is a perspective view of the apparatus  10  of which the filtration unit  40  is being fitted in the housing  20 , with part of the housing cut away. 
           [0027]      FIG. 7( a ) ,  FIG. 7( b ) , and  FIG. 7( c )  are partially enlarged end views corresponding to  FIG. 3 , illustrating the assembly sequence from fitting the filtration unit  40  into the housing  20  to insertion of a wedge  70  into a recess  39  in the order of (a), (b), and (c). 
           [0028]      FIG. 8  is an exploded perspective view of a heating unit  50  shown in  FIG. 1 . 
           [0029]      FIG. 9  is equivalent to  FIG. 2 , illustrating the apparatus  10  fitted with the closure  52 . [ FIG. 10 ]  FIG. 10  is equivalent to  FIG. 1 , illustrating the apparatus  10  of which a first closure  52  is replaced with a second closure  92 . 
           [0030]      FIG. 11  is equivalent to  FIG. 9 , illustrating the apparatus  10  of which a first closure  52  is replaced with a second closure  92 . 
           [0031]      FIG. 12  is an exploded perspective view of a molten metal filtration apparatus  110  according to another embodiment of the invention. 
           [0032]      FIG. 13  is a cross-sectional view of the apparatus  110  before fitting a closure (heating unit), taken along line I′-I′ of  FIG. 12  viewed in the direction of the arrows. 
           [0033]      FIG. 14  is a cut-away end view of the apparatus  110  of the same state as in  FIG. 13 , taken along line IF-IF of  FIG. 12  viewed in the direction of the arrows. 
           [0034]      FIG. 15  is a partially enlarged cross-sectional view of  FIG. 14 , illustrating a housing opening  9  and its vicinity. 
           [0035]      FIG. 16  is a cross-sectional view of the apparatus in the same state as in  FIG. 13 , taken along line of  FIG. 13  viewed in the direction of the arrows. 
           [0036]      FIG. 17  is equivalent to  FIG. 13 , illustrating the apparatus fitted with a second closure. 
           [0037]      FIG. 18  is an exploded perspective view of a filtration apparatus  110 A according to still another embodiment of the invention. 
           [0038]      FIG. 19  is a cross-sectional view of the apparatus  110 A fitted with a heating unit, taken along line A-A of  FIG. 18 . 
           [0039]      FIG. 20  is a cross-sectional view of the apparatus  110 A before being fitted with the heating unit, taken along line B-B of  FIG. 19 . 
           [0040]      FIG. 21  is a cut-away end view of the apparatus  110 A fitted with a second closure in place of the heating unit, taken along line C-C of  FIG. 20 . 
           [0041]      FIG. 22  is equivalent to  FIG. 20 , illustrating a filtration apparatus  110 A′ having an engageable member for fixing a closure  90  to the housing  20  in right position. 
           [0042]      FIG. 23( a )  and  FIG. 23( b )  are each a cut-away end view of the apparatus  110 A′ fitted with a second closure in place of the heating unit, taken along line D-D of  FIG. 22  viewed in the direction of the arrows, of which  FIG. 23( a )  depicts the upper portion of the filtration apparatus  110 A′, and  FIG. 23( b )  is an enlarged fragmentary view of  FIG. 23( a ) . 
           [0043]      FIG. 24  is a cross-sectional view illustrating the structure of a conventional molten metal filtration apparatus  210 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0044]    The invention will be described on the basis of preferred embodiments thereof with reference to the accompanying drawings.  FIG. 1  is an exploded perspective view illustrating an embodiment of the molten metal filtration apparatus (hereinafter simply referred to as “filtration apparatus” or more simply “apparatus”). As illustrated in  FIG. 1 , the filtration apparatus  10  includes a housing  20  in which molten metal is retained and a filtration unit  40 . The housing  20  has an opening (a space)  9  (also called a housing opening  9 ) in the upper portion thereof. The filtration unit  40  includes a support plate  42  and at least one bottomed cylindrical filtration tube  41  which is made of a porous ceramic material and is fixed to the support plate  42  along the thickness direction of the support plate  42 . As illustrated in  FIGS. 2 and 3 , the filtration unit  40  is detachably fitted into the housing  20  through the housing opening  9 . The housing opening  9  is blockable by a closure  52  or  92  described below. The housing  20  includes an outer casing made of metal (e.g., iron) and a refractory liner. 
         [0045]    The filtration apparatus  10  in the state before being fitted with the closure  52  or  92  (i.e., before being fitted with a heating unit  50 ) will be described by way of  FIGS. 1 through 7 .  FIGS. 2 through 7  each illustrate the apparatus  10  in that state. In  FIGS. 1 and 2 , the depth direction of the housing  20  is indicated by arrow X. As used hereunder, the terms “horizontal plane” and “horizontal direction” mean a plane perpendicular to the depth direction X and a direction perpendicular to the depth direction X, respectively. 
         [0046]    The housing  20  illustrated in  FIG. 1  has a box shape with its top open and is formed of a bottom  21  of a rectangular plan view shape and a wall  22  upright from the four sides of the bottom. As illustrated in  FIGS. 1, 4 , etc., the bottom  21  is substantially oblong rectangular in a plan view. The upper end of the wall  22  is a flat surface parallel to the bottom  21 , forming the top  23  of the housing  20 . The housing opening  9  is made through the top  23 . 
         [0047]    As illustrated in  FIGS. 1 to 3 , the housing  20  has an internal space  4  defined by the bottom  21  and the wall  22 . The internal space  4  has, in the bottom ( 21 ) side, a chamber  3  for retaining molten metal and is open upward to provide the housing opening  9 . The chamber  3  is substantially rectangular in plan view, specifically substantially rectangular oblong in the same direction as the bottom  21 . The chamber  3  is configured to house the filtration unit  40  therein. In carrying out molten metal filtration, the chamber  3  is filled with molten metal. 
         [0048]    As illustrated in  FIGS. 1, 2, and 4 , the rectangular wall  22  surrounding the chamber  3  has a molten metal inlet  5  (hereinafter simply “inlet  5 ”) through which molten metal to be filtered is poured into the chamber  3 . The inlet  5  is provided with a chute  24 . The molten metal to be filtered is guided by the chute  24  and poured through the inlet  5  into the chamber  3 . The wall  22  also has a molten metal outlet  6  (hereinafter simply “outlet  6 ”), and the inlet  5  and the outlet  6  are opposite to each other with respect to the opening  9  and the chamber  3 . In a plan view, the mutually facing direction of the inlet  5  and the outlet  6  corresponds to the direction in which the short side of the bottom  21  extends, and the direction perpendicular to the mutually facing direction corresponds to the long side. 
         [0049]    The housing  20  has a hole  8  going through the wall  22  near the bottom  21  (see  FIG. 3 ). The hole  8  is for the withdrawal of the molten metal remaining in the chamber  3  after completion of the molten metal filtration operation. 
         [0050]    As illustrated in  FIGS. 1, 2, 4 , etc., the wall  22  of the bottom  20  has the outlet  6  through which filtered molten metal is discharged. As illustrated in  FIG. 2 , the outlet  6  is located so as to lead to the opening  43  of the filtration tube  41  when the filtration unit  40  is housed in the housing  20 . The outlet  6  leads to the housing opening  9  with its lower end  6   b  substantially even with the upper surface  42   b  of the support plate  42  of the filtration unit  40  fitted in the housing  20 . The outlet  6  is connected to a discharge gutter  26 . As illustrated in  FIG. 4 , the outlet  6  is located to face the inlet  6  in plan view with the chamber  3  and the housing opening  9  therebetween. Note that the positional relationship of the inlet  5  and the outlet  6  is not limited to the one illustrated. Molten metal filtered by the filtration unit  40  described below is discharged out of the filtration apparatus  10  from the outlet  6  through the gutter  26  and transferred to the next step. 
         [0051]    As shown in  FIGS. 1 to 3 , etc., the housing  20  has a catching portion  30  which projects horizontally inward from the wall  22  and which is for catching the support plate  42  of the filtration unit  40  when the unit  40  is lowered to be fitted in the housing  20 . The catching portion  30  is located above the chamber  3  and at the lower end of the housing opening  9 . It is preferred for the housing  20  and the catching portion  30  be an integrally formed ceramic body. 
         [0052]    In the example illustrated in  FIG. 1 , etc., the housing  20  has a projection  32  projecting horizontally and internally from the inner surface of the wall  22  defining the housing opening  9 . The projection  32  constructs the catching portion  30 . The projection  32  is continuous in the inner circumferential direction of the housing opening  9  and has a virtually flat upper surface  32   a . In the example of  FIG. 1 , etc., the projection  32  has an annular form along the whole inner circumference of the housing opening  9 . Because of the provision of the catching portion  30 , the internal space  4  has a decreasing width in a part thereof between the bottom  21  and the housing opening  9  upwardly when viewed in a direction perpendicular to the depth direction X. 
         [0053]    As illustrated in  FIGS. 2 and 3 , the upper surface  32   a  of the projection  32  is a flat surface substantially perpendicular to the depth direction X. On the upper surface  32   a  is placed packing  31 . The projection  32  catches, on its upper surface  32   a  via the packing  31 , the lower surface  42   a  of the support plate  42  of the filtration unit  40  described below. The packing  31  is made of a material having heat resistance and sealing capabilities, for example, heat-insulating fibers such as fibrous alumina or fibrous alumina silica. 
         [0054]    Even if metal deposits on the upper surface  32   a  of the projection  32  (catching portion  30 ), the position of the upper surface  32   a  in the depth direction X can be prevented from changing by timely removing the deposited metal. In this regard, the upper surface  32   a  preferably has a coated surface so that deposited and solidified molten metal may easily be removed therefrom. For example, in the case when the filtration apparatus  10  is used to filter aluminum or an aluminum alloy, the upper surface  32   a  may be coated with a ceramic material having poor wettability with molten aluminum, such as alumina, boron nitride, silicon nitride, or zirconia. With the coated surface, the projection  32  easily maintains a good seal against the filtration unit  40  for a long time of use of the apparatus  10 . When the coated upper surface  32   a  catches the support plate  42  of the filtration unit  40  via the packing  31 , it is possible to hold the support plate  42  under pressure between the upper surface  32   a  (via the packing  31 ) and a hereinafter described wedge  70  inserted into a hereinafter described recess  39  thereby to keep the support plate  42  at a fixed position in the depth direction X. It is preferred that such a coating be also applied to a hereinafter described edge-mating portion  33  and the recess  39 . 
         [0055]    As shown in  FIGS. 2 and 3 , the projection  32  has, at the end thereof, an end surface  32   b , which is substantially vertical to the horizontal plane. With the filtration unit  40  fitted in the housing  20 , the end surface  32   b  surrounds the lateral side of the filtration tubes  41 . The end surface  32   b  of the projection  32  substantially makes a circle in plan view. The shape defined by the end surface  32   b  is not limited to a circle and may be a generally polygonal shape, for example, a generally octagonal shape similar to the shape of the support plate  42  of the filtration unit  40 . 
         [0056]    As shown in  FIGS. 1 to 3 , the wall  22  has, on its inner surface, on and above the projection  32 , an edge-mating portion  33  shaped in a complementary fashion to the edge surface  42   c  of the support plate  42 . 
         [0057]    Here, the shape of the support plate  42  of the filtration unit  40  will be described. As illustrated in  FIGS. 1 to 3 , etc., the support plate  42  of the filtration unit  40  has a first main surface  42   a  facing downward when the filtration unit  40  is being fitted in the housing  20  (hereinafter also called a lower surface  42   a ) and a second main surface  42   b  located opposite to the lower surface  42   a  (hereinafter also called an upper surface  42   b ). The lower surface  42   a  and the upper surface  42   b  are substantially parallel to each other. When viewed from the side, the support plate  42  of the filtration unit  40  has a fittable configuration to be fitted in along the direction from the opening  43  side toward the lower end  41   b  side of the filtration tube  41 , i.e., in the direction from the upper surface  42   b  toward the lower surface  42   a  of the support plate  42 . When the support plate  42  is described as having a fittable configuration along the direction from the opening  43  side toward the lower end  41   b  side of the filtration tube  41  in a side view, it is meant that, when the support plate is viewed in a direction parallel to its main surface, it has a smaller horizontal dimension (a dimension in a direction parallel to the main surface of the support plate  42 ) on its lower surface  42   a  than on its upper surface  42   b . Preferably, the horizontal dimension of the support plate  42  decreases from the upper surface  42   b  side toward the lower surface  42   a  side (in the direction from the opening  43  toward the lower end  41   b ). Such a configuration may be an either continuously or discontinuously tapered configuration or a shouldered configuration. 
         [0058]    When the support plate  42  is described as having a fittable configuration as viewed in a direction, the shape of the support plate  42  may be such that the horizontal dimension does not decrease from the opening  43  side toward the lower end  41   b  side when viewed in another direction. Such a configuration may be a shape a projected planform of which is not larger than the shape of the upper surface  42   b , such as a shape the opposite ends of which seen in a side view are vertical to the main surfaces of the support plate  42  (the opposite ends both extend vertically from the opening  43  side toward the lower end  41   a  side. 
         [0059]    The support plate  42  may have only a part thereof configured to be fitted in, such as tapered or shouldered, in a side view. For example, in the case where the support plate  42  has a generally polygonal plan view shape, not all the edge surfaces should be tapered or shouldered. That is, some of the edge surfaces may not be tapered nor shouldered. A tapered or shouldered edge surface does not always have to be tapered or shouldered over its entire area and may be tapered or shouldered in only a part thereof, e.g., a lower or an upper portion thereof. 
         [0060]    The support plate  42  used in the example illustrated in  FIGS. 1 to 7 , etc. is configured to be fitted in from the upper surface  42   b  side toward the lower surface  42   a  side when viewed in every direction parallel to the horizontal plane. In  FIGS. 2 and 3 , all the edge surfaces  42   c  of the support plate  42  of the filtration unit  40  have an inwardly tapered surface inclined from the upper surface  42   b  side to the lower surface  42   a  side, so that the edge surfaces  42   c  of the support plate  42  have a tapered surface over substantially the entire area thereof. While the support plate  42  of the embodiment has a generally polygonal plan-view shape, specifically a generally octagonal plan-view shape (see  FIG. 4 ), the shape of the support plate  42  is not limited thereto. 
         [0061]    As viewed from the side, the edge-mating portion  33  of the wall  22  has an increasing inner distance in a direction from the bottom  21  to the top  23  of the wall  22  in conformity to the above described edge profile of the edge surface of the support plate  42 . That is, in the example illustrated in  FIG. 2 , the edge-mating portion  33  of the wall  22  has a surface reverse-tapered in a direction from the bottom  21  to the top  23  (inclined outwardly of the housing  20 ) complementarily to the tapered surface of the edge surface  42   c  of the support plate  42 . Since the support plate  42  of the embodiment has a tapered surface on substantially the entire area of its edge surface as stated above, the edge-mating portion  33  of the wall  22  has a reverse-tapered surface inclined outwardly of the housing  20  in the direction from the bottom  21  to the top  23  on substantially the entire area of its surface mating the edge surface of the support plate  42  so that the support plate  42  and the edge-mating portion  33  may snugly fit to each other. 
         [0062]    As illustrated in  FIGS. 1, 3, 7 , etc., the wall  22  has recesses  39  bored on substantially the same horizontal plane, the recesses  39  each extending horizontally outward from the inner surface of the wall  22 . The plurality of recesses  39  are formed along the inner circumference of the housing opening  9 . As illustrated in  FIGS. 3 and 7 , the position of the recesses  39  in the depth direction of the housing  20  is almost the same as that of the upper surface  42   b  of the support plate  42  of the filtration unit  40  as fitted on the catching portion  30 . 
         [0063]    As illustrated in  FIGS. 3 and 7 , the recess  39  has a bottom inner surface  39   a  along the horizontal plane of the housing  20 . It is preferred that the bottom inner  39   a  be positioned lower than the upper surface  42   b  of the support plate  42  fitted on the catching portion  30  for the following reason. Molten metal can deposit on the bottom inner surface  39   a  of the recess  39 . In such a case, if the bottom inner surface  39   a  is positioned on the same level as the upper surface  42   b  of the support plate  42 , there can remain solidified aluminum at a position higher than the upper surface  42   b , and the above discussed catching mechanism may fail to function. 
         [0064]    In the example illustrated in  FIGS. 3, 7 ( a ),  7 ( b ), etc., the recess  39  has the bottom inner surface  39   a  that is substantially parallel with the horizontal plane and a top inner surface  39   b  having a tapered surface that slopes outwardly downward and, as a whole, is configured to be fitted with a hereinafter described wedge  70 . The number of the recesses  39  formed in the wall  22  is preferably at least two, more preferably three or more, in order to prevent the support plate  42  from being lifted. There are three recesses  39  in the embodiment illustrated in  FIG. 4 . For the same purpose, it is preferred that at least one recess  39  be formed to extend in a direction perpendicular to the mutually facing direction of the inlet  5  and the outlet  6  in a plan view of the housing  20 . In the case where the support plate  42  has a generally polygonal plan-view shape, the recess  39  may be formed at a position facing a vertex of the polygon but is preferably provided at a position facing a side of the polygon, more preferably at substantially the midpoint between adjacent vertices. 
         [0065]    As illustrated in  FIG. 1 , etc., the filtration apparatus  10  has at least one wedge  70  inserted and fitted into the recess  39 . The wedge  70  has a rod shape longer in the direction of insertion into the recess  39  and has its width (a dimension in the depth direction X in the state fitted into the recess  39 ) decreasing toward its leading end to be inserted into the recess  39 . In the example shown in  FIG. 1 , etc., the wedge  70  as inserted into the recess  39  has a downwardly facing lower surface  70   a  that is substantially horizontally flat and a tapered upper surface  70   b  inclined downward (toward the side of the lower surface  70   a ) in the direction of insertion into the recess  39 . The wedge  70  has an upward projection  70   c  in its tailing end opposite to the leading end, the upward projection  70   c  projecting opposite to the lower surface  70   a . As illustrated in  FIG. 1 , an edge surface of the projection  70   c  and the upper surface  70   b  meet to make a concave corner  70   d . As illustrated in  FIG. 3 , etc., the part of the wall  22  that defines the housing opening  9  and the top inner surface  39   b  of the recess  39  meet to make a convex corner  39   c . As illustrated in  FIG. 3 , etc., the concave corner  70   d  and the convex corner  39   c  are complementary to each other when viewed from the side in a direction parallel to the horizontal plane and perpendicular to the direction of inserting the wedge  70  into the recess  39 . The wedge  70  is configured in such a manner that, with the convex corner  70   d  of the wedge  70  in contact with the concave corner  39   c  of the recess  39 , the lower surface  70   a  is substantially even with the upper surface  42   b  of the support plate  42 . As illustrated in  FIG. 4 , both the recess  39  and the wedge  70  inserted into the recess  39  have substantially a rectangular plan view shape, but the shape of them is not limited the one illustrated. The wedge  70  may be made of, for example, a ceramic material containing at least one of silicon carbide, silicon nitride, and zirconia. A silicon nitride-bonded silicon carbide refractory material is particularly suitable. The preferred number of the wedges  70  is the same as for the recesses  39 . 
         [0066]    As illustrated in  FIG. 1 , the housing  20  has a closure-receiving portion  29  above the projection  32  and the edge-mating portion  33 . The closure-receiving portion  29  forms an opening going through the depth direction with a generally circular plan-view shape. The closure-receiving portion  29  is open in substantially the center of the top  23  and defines the housing opening  9  together with the projection  32  and the edge-mating portion  33 . The housing opening  9  (specifically the closure-receiving portion  29 ) is complementary to a hereinafter described closure  52  or  92 . 
         [0067]    As illustrated in  FIGS. 1, 3, 6, and 7 , the edge-mating portion  33  of the housing  20  has a groove  37  for easily guiding a fitting tool to the catching portion  30  for housing the filtration unit  40  into the housing  20 . The groove  37  is a recess open to the internal space of the housing  20 , which is formed by recessing a circumferential part of the edge-mating portion  33  of the housing opening  9  from the edge-mating side toward the external of the housing  20 . As illustrated in  FIG. 6 , the groove  37  is formed by boring the edge-mating portion  33  (specifically a circumferential part of the edge-mating portion  33 ) from its upper surface  33   b  down to substantially the lower end of the edge-mating portion  33  in the depth direction X. As illustrated in  FIGS. 3, 7 ( c ), etc., the level of the bottom (lower surface)  37   a  of the groove  37  is preferably even with or above the lower end  33   a  of the edge-mating portion  33  to ensure prevention of reduction in sealing performance between the support plate  42  and the housing  20 . As shown in  FIGS. 1, 3, 6 , and  7 , it is preferred that the groove  37  not be formed on the closure-receiving portion  29  that is located above the edge-mating portion  33  in order to block the housing  20  by the closure  52  or  92  with a tight seal. The groove  37  has an end  37   b  (bottom portion of groove) facing inward of the housing  20 . While in  FIGS. 3, 6, and 7  the end  37   b  of the groove  37  is substantially even with the inner periphery of the closure-receiving portion  29 , the position of the end  37   b  is not limited thereto. The groove  37  may be formed so that its end  37   b  is external to the inner periphery of the closure-receiving portion  29  with respect to the radial direction of the housing  20  (the radial direction of the housing opening  9  in plan view). The end  37   b  of the groove  37  may be inclined similarly to the inner periphery of the closure-receiving portion  29  which is tapered downward, or may be vertical to the horizontal plane in side view. The width, length (distance from the end  37   b  to the opposite end), and the like of the groove  37  are subject to variation according the configuration of the fitting tool. 
         [0068]    As illustrated in  FIG. 4 , the groove  37  is one of grooves  37  provided at an interval in the circumferential direction of the housing opening  9 . In order to facilitate stably holding and transferring the filtration unit  40  by the fitting tool, it is preferred to make at least two, more preferably three or more, grooves  37  on the wall  22 . There are three grooves  37  in the example shown in  FIG. 4 . The groove  37  serves to guide a hooking member of the fitting tool engaging the support plate  42  to the catching portion  30  where the support plate  42  is to be located. Therefore, the wall  22  usually has as many grooves  37  as hereinafter described hook-engageable cuts  46  of the support plate  42 , in which the hooking member of the fitting tool is configured to engage, at positions mating the hook-engageable cuts  46 . As illustrated in  FIG. 4 , an adequate number of the grooves  37  and an adequate number of the recesses  39  can be arranged with ease by providing every groove  37  between adjacent recesses  39  in the circumferential direction of the housing opening  9 . 
         [0069]    The filtration unit  40  to be fitted in the housing  20  will then be described in detail. As illustrated in  FIGS. 1 and 2 , the support plate  42  of the filtration unit  40  is made of a ceramic material hardly reactive with molten metal. The filtration tube  41  is made of a porous ceramic material having pores allowing for passage and filtration of molten metal. As illustrated in  FIGS. 2 and 3 , the filtration tube  41  is a bottomed cylinder with one end  41   a  open (having an opening  43 ) and the other end  41   b  closed, which is configured to allow molten metal to penetrate from the outside into the inside of the tube  41  and to make the molten metal having penetrated into the inside of the tube  41  flow out from the opening  43 . While the filtration tube  41  generally has an annular shape in a transverse cross-section as in the example shown in  FIG. 1 , etc., the filtration tube  41  of the present embodiment is not limited to such a configuration. 
         [0070]    The filtration tube  41  is made of a refractory material having low reactivity with molten metal to be treated. In treating molten metal containing aluminum or an aluminum alloy, the filtration tube  41  may be made of silicon carbide ceramics, silicon nitride ceramics, alumina ceramics, zirconia ceramics, and so forth. Filtration tubes made of alumina ceramics are preferred taking into consideration elution of a ceramic component into molten aluminum and in terms of filtration tube production ease and cost reduction. 
         [0071]    As illustrated in  FIGS. 1 to 4 , etc., the support plate  42  has a plurality of openings  44  going through its thickness. In the filtration unit  40 , each filtration tube  41  is fixed to the support plate  42  along the thickness direction of the plate  42 . More specifically, each filtration tube  41  is affixed to the support plate  42  on its open end  41   a  (i.e., the end  41   a  having the opening  43 ) with its longitudinal direction substantially perpendicular to the main surface of the support plate  42 . Accordingly; the filtration tube  41  sticks out from only the lower surface  42   a  of the support plate  42 . The location at which each filtration tube  41  is affixed to the support plate  42  corresponds to the position of the opening  44  of the support plate  42  in a plan view of the filtration unit  40 . Therefore, when the filtration unit  40  is viewed from the upper surface  42   b  side of the support plate  42   b , the opening  44  of the support plate  42  and the hollow of the filtration tube  41  interconnect to each other. The filtration tube  41  may be fixed to the support plate  42  using, for example, ceramic cement, such as alumina cement. 
         [0072]    The filtration unit  40  can be fitted into the housing  20  usually by bringing down the filtration unit  40  through the housing opening  9  using a fitting tool external to the filtration apparatus  10 . Various types of transfer equipment may be used as a fitting tool. For example, a lifting and lowering device, such as a hoist, is conveniently used. The support plate  42  has a hook-engageable cut  46  in which the hooking members  80  of the fitting tool engage. 
         [0073]    As illustrated in  FIGS. 3 and 5 , the hook-engageable cut  46  of the support plate  42  of the embodiment is formed by boring a part of the peripheral edge portion S of the support plate  42  except a part containing the upper surface  42   b  of the support plate  42 . As used herein, the term “boring a part of the peripheral edge portion except a portion containing the upper surface  42   b ” is intended to mean that the hook-engageable cut  46  may be formed either one of or both of a part of the edge surface  42   c  and a part of the lower surface  42   a  of the support plate  42 . To improve a seal between the support plate  42  and the housing  20 , the cut is preferably made on only the edge surface  42   c  but not on the lower surface  42   a  of the support plate  42 , or the cut on the lower surface  42   a  is preferably as small as possible. Accordingly, the hook-engageable cut  46  is preferably a recess bored radially inwardly on the edge surface  42   c  at a location other than the upper surface  42   b  of the support plate  42 . The hook-engageable cut  46  may be formed by, for example, counterboring. The hook-engageable cut  46  is located radially distal to the openings  44  in plan view. 
         [0074]    In the example illustrated in  FIG. 5( a ) , each hook-engageable cut  46  has a top inner surface  46   a  facing down (toward the lower end  41   b  of the filtration tube  41 ) and an end surface  46   b  facing laterally outward of the support plate  42  and is configured to allow the hooking member  80  to engage on the top inner surface  46   a . However, the shape of the hook-engageable cut  46  is not limited to the example shown. For example, the hook-engageable cut  46  may be a longer-than-wide recess extending inward from the edge surface  42   c  of the support plate  42  as long as it does not interconnect to any opening  44 . The top inner surface  46   a  of the hook-engageable cut  46  may or may not be substantially parallel to the support plate  42 . For example, the top inner surface  46   a  may be inclined upwardly (toward the upper surface  42   b  of the support plate) from the edge surface  42   c  to the end surface  461 ), or the top inner surface  46   a  may have a part thereof recessed upwardly (toward the upper surface  42   b  of the support plate). The end surface  46   b  may or may not be a vertical surface which is vertical to the radial direction of the support plate  42 . For example, the end surface  46   b  may be a tapered surface which is inclined laterally outwardly from the upper surface ( 42   b ) side toward the lower surface ( 42   a ) side, or the end surface  46   b  may be shouldered or curved. 
         [0075]    The hook-engageable cut  46  is provided along the peripheral edge portion S of the support plate  42 . It is usually preferred that at least two, more preferably three or more, hook-engageable cuts be provided similarly to the grooves  37  of the housing  20 . There are three hook-engageable cuts  46  in the example shown in  FIGS. 3 to 5 , etc. In the case where the support plate  42  has a generally polygonal plan-view shape, each hook-engageable cut  46  may be formed at the vertex of the polygon but is preferably between adjacent vertices, more preferably at substantially the midpoint of a side between adjacent vertices. While the hook-engageable cut  46  is provided on the tapered edge surface  42   c  of the support plate  42  in the example shown in  FIGS. 3 to 5 , etc., when only part of the edge surface  42  is tapered or shouldered, the part of the edge surface  42  on which the hook-engageable cut  46  is formed may or may not be tapered or shouldered. 
         [0076]    In what follows, the assembling sequence of fitting the filtration unit  40  into the housing  20  by means of a fitting tool will be described with reference to  FIGS. 5 to 7 . 
         [0077]    As illustrated in  FIG. 5( b ) , each hooking member  80  of a fitting tool, such as a hoist crane, engages in each hook-engageable cut  46  of the support plate  42  with the filtration tubes  41  hanging downward in the depth direction X. By the engagement between the hooking members  80  and the hook-engageable cuts  46 , the filtration unit  40  is transported with its support plate  42  held stably. 
         [0078]    As illustrated in  FIG. 6 , the hooking members  80  of the fitting tool holding the filtration unit  40  are located above the grooves  37 . In this state, the filtration unit  40  is moved down in the housing opening  9 . As previously stated, since the grooves  37  are provided at positions corresponding to the hook-engageable cuts  46 , the hooking members  80  of the fitting tool engaging in the hook-engageable cuts  46  are allowed to move down in the grooves  37  without being obstructed by the edge-mating portion  33  of the housing  20  (e.g., without contacting the edge-mating portion  33 ) thereby to lower the support plate  42  stably and smoothly. 
         [0079]    Since the edge surface  42  of the support plate  42  has a liftable configuration, and the wall  22  of the housing  20  has, above the catching portion  30 , the edge-mating portion  33  complementary to the edge surface  42  as described supra, the lowering support plate  42  is easily brought into register with the edge-mating portion  33 . Thus, as illustrated in  FIG. 7( a ) , at the time when the support plate  42  reaches the catching portion  30 , the support plate  42  is easily and stably mounted on the catching portion  30  with the support plate  42  snugly fitted to the complementary edge-mating portion  33  of the wall  22 . The filtration apparatus  10  according to the present embodiment can thus be assembled easily by fitting the filtration unit  40  at a right position of the housing  20  without requiring computer-aided precise alignment. 
         [0080]    As illustrated in  FIG. 7( a ) , the support plate  42  contacts with and is mounted on the catching portion  30  (specifically, the upper surface  32   a  of the projection  32 ) via the packing  31  on its lower surface  42   a  of the peripheral edge portion S that is radially outward of the positions where the filtration tubes  41  are fixed, whereupon the support plate  42  is caught by the catching portion  30  and is prevented from further lowering. The filtration unit  40  is thus housed in the housing  20 , is suspended inside the housing  20 . To secure a seal between the edge surface  42   c  of the support plate  42  and the edge surface of the edge-mating portion  33 , it is preferred that the edge surface  42   c  of the support plate  42  and the edge surface of the edge-mating portion  33  be in contact with each other over their entire surface except the portions having the grooves  37 . 
         [0081]    All the filtration tubes  41  of the filtration unit  40  as housed in the housing  20  have their longitudinal direction coinciding with the same direction intersecting the horizontal plane of the housing  20  and their openings  43  facing upward. In the embodiment, each filtration tube  41  hangs down from the support plate  42  in the depth direction in the molten metal chamber  3  (see  FIGS. 2 and 3 ). In such a suspended fashion, the filtration unit  40  is disposed in the housing  40  with its two main surfaces  42   a  and  42   b  substantially coincide with the horizontal plane. 
         [0082]    A liquid-tight seal is achieved between the filtration unit  40  and the housing  20  by the packing  31  disposed on the upper surface of the projection  32 . The seal is achieved by the self-weight of the suspended filtration unit  40 . 
         [0083]    In the conventional filtration apparatus  210 , the seal of the filtration unit  240  is achieved by using a pair of support plates. In contrast, the filtration apparatus  10  achieves a seal using only one support plate  42 . Therefore, even in long-term use of the apparatus  10 , the contact condition between the support plate  42  and the catching portion  30  is easily kept uniform. Consequently, the sealing properties between the filtration unit  40  and the housing  20  hardly reduce even in long-term use of the apparatus  10 . In particular, since the catching portion  30  extends the whole inner circumference of the housing opening  9 , the support plate  42  is caught by a single member (catching portion  30 ), so that occurrence of slip between members will be avoided effectively. Since the housing  20  and the catching portion  30  are integrally formed of a ceramic material, a slip or displacement of the catching portion  30  in the housing  20  never occurs even in long-term use of the apparatus  10 . Therefore, the engagement of the support plate  42  and the catching portion  30  is maintained properly and easily for a long period of use. 
         [0084]    As illustrated in  FIG. 7( b ) , the distance L in the horizontal direction between the end  37   b  of the groove  37  and the support plate  42  as fitted in the housing  20  is longer than the width W of the leading end of the hooking member  80  in the same direction when the apparatus  10  is viewed from the side. Therefore, the hooking member  80  can easily be moved upward along the end  37   b  of the groove  37  to be taken out of the housing opening  9  as illustrated in  FIGS. 7( b ) and 7( c ) . 
         [0085]    After the support plate  42  is fitted on the catching portion  30 , the wedge  70  is inserted in the recess  39  formed on the inner side of the wall  22  as illustrated in  FIG. 7( c ) . As described earlier, the recess  39  is formed on a level substantially even with the upper surface  42   b  of the support plate  42  as fitted on the catching portion  30  in the depth direction. The support plate  42  of the filtration unit  40  is thus held under pressure between the wedge  70  inserted in the recess  39  and the catching portion  30  (specifically, the projection  32  via the packing  31 ). Specifically, a part of the support plate  42  that is in contact with the catching portion  30  and located in the peripheral edge portion S radially outward of the positions where the filtration tubes  41  are fixed is held under pressure between the catching portion  30  and a hold-down member, i.e., the wedge  70  inserted in the recess  39 . Because the wedge  70  thus holds down the upper surface  42   b  of the support plate  42 , the filtration unit  40  is effectively prevented from being lifted by the ascending force of molten metal flow at the commencement of filtration. Because the support plate  42  is held down under pressure not on its central portion but on a part of its peripheral edge portion S, it is prevented from breaking. As previously described, since the wedge  70  decrease its width, the position of the support plate  42  in the depth direction X can finely be adjusted with ease by adjusting the extent of insertion of the wedge  70  into the recess  31 . By the use of a plurality of wedges  70 , the peripheral edge portion S of the support plate  42  is pressed uniformly thereby to provide an increased seal of the support plate  42 . 
         [0086]    As described, since the bottom inner surface  39   a  of the recess  39  is located below the support plate  42  caught by the catching portion  30 , the lower surface  70   a  of the wedge  70  inserted into the recess  39  is out of contact with the bottom inner surface  39   a  of the recess  39  as illustrated in  FIGS. 3 and 7 ( c ). Thus, even if molten metal deposits on the bottom inner surface  39   a , the support plate hold-down mechanism by the wedge  70  continues functioning effectively. 
         [0087]    The mechanism for preventing lift of the filtration unit  40  including the recesses  39  and the wedges  70  according to the embodiment easily works continuously even in an environment liable to involve deposition of molten metal and therefore provides the advantage of easy maintenance of the apparatus. For example, even when molten metal enters the recess  39  having the wedge  70  fitted in and solidifies there, the recess  39  may easily be cleared off by applying to the recess  39  the same coating as applied to the upper surface  32   a  of the projection  32 . Even when molten metal deposits on the wedge  70 , it is easy to replace the wedge  70 . According to the embodiment, there is no need to provide a hold-down mechanism that pierces the closure  52  or  92  to prevent lift of the support plate  42 . This allows for relatively free structural design of the closure. 
         [0088]    In this regard, the filtration apparatus described in Patent Literature 3 adopts a weight  45  disposed on the support plate  42  as a lift preventive mechanism in the embodiment in which the opening of filtration tubes faces upward. However, the lift preventive action by the weight  45  is insufficient in some cases. 
         [0089]    As described above, the filtration apparatus  10  of the embodiment includes the housing  20  having the opening  9  in its upper portion and the filtration unit  40  fitted in the housing  20 . The filtration unit  40  includes at least one bottomed cylindrical filtration tube  41  made of a porous ceramic material, the filtration tube  41  being fixed with its longitudinal direction intersecting the horizontal direction of the housing  20  and with its open end upward. The apparatus  10  further includes an additional structure described below. 
         [0090]    The filtration apparatus  10  includes a heating unit  50  detachably fitted to the filtration unit  40 . The heating unit  50  includes a support  52  and at least one heater  51  fixed to the support  52 . The heater  51  is fixed to the support  52  at such a position as to be inserted in the filtration tube  41 . Because the filtration tube is heated from its inside by the heater  51  inserted therein, efficient heating is achieved, and the time for preheating the filtration tube is shortened. 
         [0091]    The heating unit  50  is fittable to the filtration tube  41  of the filtration unit  40  housed in the housing  20 . As illustrated in  FIGS. 1, 8, and 9 , the heating unit  50  has a plurality of heaters  51  fixedly supported by the support  52 . The number of the heaters  51  is preferably the same as the number of the filtration tubes  41 . Provided that the number of the heaters  51  is at least one, it may be smaller than the number of the filtration tubes  41  when, for example, the requisite heating temperature is not so high. 
         [0092]    The support  52  has a first closure  52  configured to fit in with the housing opening  9  to block the opening  9 . The first closure  52  has at least one heater  51  fixed therethrough. In the embodiment, the first closure  52  and the heater  51  constitute the heating unit  50  that is detachably fitted to the filtration unit  40 . As the support  52  fits in the housing opening  9 , the heating unit  50  is fixed to the filtration unit  40 . In the embodiment, upon closing the housing opening  9  with the first closure  52 , which is the support  52 , the heater  51  is inserted in the filtration tube  41 , whereby the heating unit  50  is fitted to the filtration unit  40 . 
         [0093]    As illustrated in  FIG. 8 , the heater  51  has a length and a cross-sectional shape capable of heating the filtration tube  41  along the longitudinal direction of the tube  41 . The heater  51  used in the embodiment is rod-shape, specifically a generally columnar shape. Materials that can make the heater  51  include, but are not limited to, silicon carbide heating elements and metallic heating elements. The cross-sectional shape of the heater  51  is such that allows the heater  51  to be inserted through the opening  44  of the support plate  42  and the opening  43  of the filtration tube  41 . For example, comparing the cross-sections of the heater  51  and the filtration tube  41  taken along a plane perpendicular to their longitudinal direction, it is preferred for at least the part of the heater  51  that is inserted in the filtration tube  41  to have a cross-section the maximum length of which is smaller than the diameter of the inscribed circle of each of the two openings  43  and  44 . As used herein, the term “maximum length” of a cross-section denotes the maximum distance between two points on the perimeter of the cross-section. 
         [0094]    As for a heater  51  having a generally columnar shape, the maximum length is the diameter of its cross-section. When the openings  43  and  44  are circular, the diameter of their inscribed circle is the inner diameter of these circular openings. The heater  51  may have a constant or varied cross-sectional shape, taken along a plane perpendicular to its longitudinal direction, over the whole length thereof. For example, the heater  51  may have a decreasing cross-sectional area toward its leading tip to be inserted in the filtration tube  41 . 
         [0095]    As illustrated in  FIGS. 1, 8, and 9 , the support  52  of the heating unit  50  as fitted to the filtration tube  41  has a lower surface  52   a  facing down in the depth direction X, an upper surface  52   b  facing up in the depth direction X, and a lateral surface  52   c  located between the upper surface  52   a  and the lower surface  52   b . The upper surface  52   a  and the lower surface  52   b  are substantially parallel with each other. The lateral surface  52   c  of the support  52  preferably has a tapered surface which is inclined inwardly from the upper surface  52   c  to the lower surface  52   b . That is, as illustrated in  FIG. 9 , when viewed from at least one direction parallel to the horizontal direction, it is preferred for the support  52  of the heating unit  50  fitted to the filtration tube  41  to have its width gradually decreasing downward in the depth direction X. The support  52  used in the embodiment decrease its width downward on its entire lateral side as viewed from every direction parallel to the horizontal direction. The angle between the tapered lateral surface  52   c  and the upper surface  52   b  of the support  52  may be either constant or varied when viewed from a plurality of directions parallel to the horizontal direction as long as the tapered lateral surface  52   c  is fittable to the inner surface of the housing opening  9  (closure-receiving portion  29 ). 
         [0096]    The heater  51  is fixed to the support  52  to stick out downward from the lower surface  52   a  of the support  52 . As illustrated in  FIG. 8 , the support  52  has as many through-holes  52   d  as the heaters  51 . Each through-hole  52   d  has a cross-sectional shape allowing for insertion and fixation of the heater  51 . The through-hole  52   d  goes through the support  52  in the direction substantially perpendicular to the upper surface  52   b  and lower surface  52   a  of the support  52 . The number of the through-holes  52   d  is the same as that of the heaters  51 . In plan view, positions of each through-hole  52  of the support  52  are corresponding to positions of each filtration tube  41  of the support plate  42 . The heater  51  is fixed to the support  52  as passed through the through-hole  52   d . The heater  51  may be fixed to the support  52  using, for example, ceramic cement, such as alumina cement. The heater  51  as fixed to the support  52  has its longitudinal direction substantially perpendicular to the upper surface  52   b  and the lower surface  52   a  of the support  52 . The closure  52  preferably includes a member made of an alumina castable from the standpoint of heat resistance and cost. For example, the heater  51  may be fixed in place to the support  52  using an unshown jig in order to prevent the heater  51  from dropping more securely. 
         [0097]    The heating unit  50  is fixed to the filtration tube  41  of the filtration unit  40  housed in the housing  20  as follows. The heating unit  50  is placed above the housing opening  9  in such a manner that the upper and the lower surface  52   b  and  52   a  of the support  52  are substantially perpendicular to the depth direction and the position of the heater  51  substantially corresponds to the position of the opening  44  of the support plate  42  and the opening  43  of the filtration tube  41  in plan view. In this state, the heating unit  50  is lowered in the depth direction X, whereby the heater  51  is inserted into the filtration tube  41  through the opening  44  of the support plate  42  and the opening  43  of the filtration tube  41 . In the embodiment, it is possible to insert the heater  51  into the filtration tube  41  with the direction of insertion, the longitudinal direction of the heater  51 , and the longitudinal direction of the filtration tube  41  substantially coincident with each other. When the heating unit  50  is described as being “fitted to” the filtration unit  40  or the filtration tube  41  in the embodiment, it is meant that the heater  51  is inserted in the filtration tube  41 . As illustrated in  FIG. 9 , the number of the heaters  51  of the heating unit  40  is preferably equal to or less than the number of the filtration tubes  41 , and every filtration tube  41  has the heater  41  inserted therein. 
         [0098]    As illustrated in  FIG. 9 , the support (closure)  52  being lowered in the depth direction X is fitted to the closure-receiving portion  29  of the housing opening  9  of the housing  20 , whereby the heating unit  50  is fixed to be kept at a given position with respect to the filtration unit  40 . As previously described, the lateral surface  52   c  of the support  52  has a tapered surface, and the inner surface  29   a  of the part of the wall  22  that forms the closure-receiving portion  29  is shaped complementary to the lateral surface  52   c . Therefore, as the heating unit  50  moves downward in the depth direction X, the lateral surface  52   c  of the support  52  comes into contact with the inner surface  29   a  of the closure-receiving portion  29  in the housing opening  9  as shown in  FIG. 9 . Further lowering of the support  52  in the direction X is thereby stopped and, at the same time, the support  52  blocks up the housing opening  9  to complete the function as a first closure. By fixing the heating unit  50  with respect to the filtration unit  40  in the manner described, it is possible to insert the heater  51  into the filtration tube  41  to a certain degree so that the leading tip (the lower end) of the heater  51  may be maintained at a given distance from the lower end  41   b  of the filtration tube  41 . 
         [0099]    In the state that the heating unit  50  is fixed to the filtration unit  40  and the filtration tube  41 , the inserted heater  51  heats the filtration tube  41 . In this state, the ratio of the inserted length a of the heater  51  in the filtration tube  41  to the longitudinal length A of the filtration tube  41  (a/A, see  FIG. 9 ) may be the same or different among the plurality of filtration tubes  41 . The a/A is preferably 0.2 or greater to sufficiently heat the filtration tube  41  and 0.99 or smaller to allow for smooth insertion into the filtration tube  41 . The a/A is more preferably 0.7 to 0.99. 
         [0100]    An embodiment in which the filtration apparatus  10  does not have the heating unit  50  fitted therein will next be illustrated. In  FIGS. 10 and 11 , the housing opening  9  is closed by a second closure  92  in place of the support  52  having the heaters  51  fixed therethrough. 
         [0101]    As illustrated in  FIGS. 10 and 11 , the second closure  92  has almost the same contour as the first closure  52  but has no through-hole  52   d  for fixing the heater  51 . That is, the second closure  92  is not configures to support the heater  51 . Accordingly, the filtration apparatus  10  is configured to have its housing opening  9  closed by the second closure  92  with no heater  51  inserted into the filtration tube  41 . 
         [0102]    The second closure  92  is preferably made of a shaped or unshaped refractory material having low reactivity with molten metal. In treating molten metal containing aluminum or an aluminum alloy, the second closure  92  is preferably made of a material containing a silicon nitride-bonded silicon carbide refractory material, a silicon nitride refractory material, an alumina refractory material, a zirconia refractory material, and so forth. In that case, the outer surface of the second closure  92  may be covered with a metallic casing. As illustrated in  FIG. 11 , the second closure  92  has packing  92   d  covering the lateral and lower surface thereof. The packing  92   d  can contain alumina fiber and provides a liquid-tight seal between the housing  20  and the second closure  92 . The seal is achieved by the self-weight of the second closure  92 . Packing containing alumina fiber similar to the packing  92   d  may be applied to the first closure  52 . 
         [0103]    As illustrated in  FIGS. 9 and 11 , both the first closure  52  and the second closure  92  fitted in to block the housing opening  9  have their lower surface located above the first main surface (upper surface)  42   b  of the support plate  42  of the filtration unit  40  at a certain distance. The lower surface of the closures  52  and  92  is almost even with the upper surface of the molten metal outlet  6 . During molten metal filtration operation, the level of the molten metal in the housing  20  is substantially even with the lower surface of the closures  52  and  92 . Therefore, the surface of the molten metal has a small contact area with oxygen in air and thereby protected from oxidation. In addition, such a configuration of the closures  52  and  92  is effective in preventing air convection above the molten metal surface that would draw heat from the molten metal. 
         [0104]    Having the above described structure, the filtration apparatus  10  of the embodiment provides the following advantages. With the heating unit  50  fitted to the filtration tube  41  in the housing  20  to insert the heater  51  into the filtration tube  41 , the filtration tube  41  can be heated from the inside. Therefore, the filtration tube  41  and its vicinity can be heated with good efficiency. For example, the filtration tube may be preheated efficiently by fitting the heating unit  50  to the filtration tube  41  before the commencement of molten metal filtration. Consequently, not only is reduced the time required for preheating but, because tube clogging is reduced, the molten metal casting time will be reduced. Since the support  52  of the heating unit  50  also functions as a first closure, it blocks the housing opening  9  to prevent air from drawing heat from the molten metal in the filtration tube  41 , the housing  20 , and the chamber  3  and to further improve heating efficiency. Moreover, needing fewer parts, the apparatus  10  is produced more easily. Since the heating unit  50  is fixed with respect to the filtration unit  40 , the efficiency of the operation of installing a heating mechanism is improved in heating the filtration tube  41 . 
         [0105]    With a conventional filtration system having the filtration tube  41  fitted horizontally, it is difficult to set the heater insertably into the filtration the  41  as in the filtration apparatus of the invention. If the structure for detachably inserting a heater into the horizontal filtration tube  41  is adopted in a conventional filtration apparatus, the housing opening  9  must be provided on the lateral side of the filtration apparatus, so that, unless the engagement between the housing opening  9  and the closure  52  is securely sealed, leakage of the molten metal would occur. Furthermore, if the filtration tube is set horizontally, is it impossible to detach the heating unit, which also functions as a closure of the opening, from the housing as containing molten metal without leakage of molten metal. Such usage is allowed with the apparatus of the invention having the filtration tube set vertically. Hence, the invention is a technique that is made feasible only by adopting the structure in which the filtration tube is disposed with its longitudinal direction intersecting the horizontal plane. 
         [0106]    The filtration apparatus  10  of the embodiment may use either one or both of the first closure  52  and the second closure  92 . 
         [0107]    In the case when both the first closure  52  and the second closure  92  are used, the apparatus  10  may be operated, for example, as follows. The housing opening  9  is first closed by the first closure  52 , and the housing  20  is heated by the heating unit  50  before the commencement of molten metal filtration. Thereafter, the first closure  52  is removed from the housing  20 , and the housing opening  9  is then blocked by the second closure  92 . According to this usage, molten metal filtration can be accomplished with no heater  51  inserted into the filtration tube  41  while preventing the filtration tube  41  from cooling by the atmosphere. To conduct molten metal filtration with the heating unit  50  removed from the housing  20 , i.e., with no heater  51  inserted in the filtration tube  41 , is preferred in order to avoid deposition of molten metal to the heater  51  or the support  52 , which will facilitate maintenance of the heating unit  50 . 
         [0108]    A preferred method of molten metal filtration by the use of the apparatus  10  according to the embodiment will be described. The filtration unit  40  is housed in the housing  20 . The housing opening  9  is closed and blocked by the first closure  52  thereby to fix the heating unit  50  relative to the filtration tube  41  to provide the state shown in  FIG. 9 . In this state, the heater  51  inserted in each filtration tube  41  is caused to generate heat thereby to preheat the housing  20  and the filtration tubes  41  to a predetermined temperature. The first closure  52  is then removed from the housing opening  9  to withdraw the heaters  51  from the filtration tubes  41  to provide the state shown in  FIGS. 2 and 3 . The housing opening  9  is closed by the second closure  92  to provide the state shown in  FIG. 11 . 
         [0109]    In that state, molten metal to be filtered is introduced to the chute  24  and made to flow in the chamber  3  through the inlet  5  as illustrated in  FIG. 11 . The molten metal in the chamber  3  penetrates from the outside into the inside of the filtration tubes  41 . The molten metal having entered the filtration tubes  41  rises in the filtration tubes and flows outside the filtration unit  40  through the opening  43  of the filtration tubes  41  and the openings  44  of the support plate  42  to complete filtration. The filtered molten metal flowing out of the filtration unit  40  flows in the gap between the upper surface  42   b  of the support plate  42  and the lower surface  92   a  of the second closure  92 , and is discharged from the outlet  6  and led to a next stage as guided by the gutter  26 . The molten state of the metal is maintained during the filtration by the heat generated by an unshown housing heater. 
         [0110]    In the case when the filtration apparatus  10  of the embodiment uses only the first closure  52 , molten metal can be filtered in the same manner as described above, except that the operation is carried out with the housing opening  9  blocked by the first closure  52  and with the heaters  51  inserted in the filtration tubes  41 . 
         [0111]    In the case when the filtration apparatus  1  of the embodiment uses only the second closure  92 , molten metal can be filtered in the same manner as described above, except that the preheating of the filtration tubes  41  and the housing  20  and the filtration of molten metal are performed with the housing opening  9  closed by the second closure  92 . In this case, the preheating and the heating of the apparatus during filtration are achieved by a heater (unshown) disposed in the housing  20 . 
         [0112]    Examples of metal that may be filtered by the use of the filtration apparatus  10  typically include aluminum and aluminum alloys. The filtration apparatus of the invention is also useful to filter other molten metals. 
         [0113]    A second and third embodiments of the invention will then be described. In what follows, the description of the second and third embodiments will be confined to the differences from the first embodiment. The structures or members corresponding or similar to those of the first embodiment are identified with the same reference numerals as in the first embodiment and will not be described redundantly except for differences. 
         [0114]    A filtration apparatus  110  according to the second embodiment of the invention will be described with reference to  FIGS. 12 through 17 . 
         [0115]    All the edge surfaces  42   c  of a support plate  142  of this embodiment are substantially flat and vertical to the upper surface  42   b  unlike the support plate of the first embodiment. As illustrated in  FIG. 13 , the housing  20  does not have the edge-mating portion  33 , which is provided over the whole inner circumference of the housing opening  9  in a complementary fashion to the edge surface of the support plate  42  in the first embodiment. In the present embodiment, the upper surface  32   a  of the catching portion  30  is substantially even with the lower surface of the outlet  6 . 
         [0116]    As illustrated in  FIGS. 12 and 14 to 16 , the housing  20  has a pair of projections  132  which projects horizontally inward above the catching portion  30  from the wall  22  and which faces each other via the housing opening  9 . As depicted in  FIG. 16 , the direction in which the two projections  132  face each other is substantially perpendicular to the direction in which the inlet  5  and the outlet  6  face each other. The projections  132  each have a flat front surface  132   a  parallel to the depth direction X. The facing front surfaces  132   a  of the pair of projections  132  are parallel to each other. The projections  132  are provided to inhibit the rotation of the filtration unit  40  fitted in the housing  20 . 
         [0117]    As illustrated in  FIGS. 12 and 14 , the pair of projections  132  is provided at the lower end of the housing opening  9 . Each projection  132  has a flat upper surface  132   b  parallel to the horizontal plane. When the housing opening  9  is blocked by the first closure  52  of the heating unit  50 , the lower surface  52   a  of the closure  52  abuts against the upper surface  132   b  of the projections  132 . 
         [0118]    As illustrated in  FIGS. 12 and 16 , at least one recess  39 , into which the wedge  70  is inserted, is bored on the front surface  132   a  of each projection  132  in the direction substantially perpendicular to the direction in which the inlet  5  and the outlet  6  face each other in a plan view of the housing  20 . The number of the recesses  39  formed on one of the projections  132  is preferably the same as that formed on another projection  132 . As illustrated in  FIG. 15 , in the state that the support plate  142  of the filtration unit  40  is caught by the catching portion  30 , the position of the recesses  39  in the depth direction is even with the upper surface (second main surface)  42   b  of the support plate  142 . In the second embodiment the bottom inner surface  39   a  of the recess  39  is substantially even with the upper surface  42   b  of the support plate. In the first embodiment the wedge  70  as inserted into the recess  39  contacts the top inner surface of the recess  39  on its upper surface but does not contact the bottom inner surface of the recess  39  on its lower surface. In the second embodiment, in contrast, the wedge  70  as inserted into the recess  39  contact both the top and bottom inner surfaces of each recess  39  on its upper and lower surfaces. In assembling the filtration apparatus of this embodiment, when the filtration unit  40  is moved down in the housing  20  until the first main surface  42   a  of the support plate  142  comes into contact with the upper surface  30   a  of the catching portion  30 , the support plate  142  is fitted into the upper surface  30   a  of the catching portion  30  in such a manner that opposite edge surfaces  42   c  thereof, which are parallel to each other, may face the front surface  132   a  of the two opposite projections  132  of the housing  20  as shown in  FIGS. 14 and 15 . The wedge  70  is then inserted into each recess  39  formed in the projections  132 , whereby the support plate  142  is fixedly held under pressure between the wedges  70  inserted into the recesses  39  and the catching portion  30 . 
         [0119]    The filtration apparatus  110  of the second embodiment produces the same effects as by the apparatus of the first embodiment, i.e., the filtration unit is easy to seal, lifting of the support plate and a resulting leakage of molten metal are prevented effectively, and yet the apparatus is simple and easy to maintain. 
         [0120]    In the case when the housing  20  is provided with a heater separate from the heating unit  50 , the heater may have the structure of housing heaters  80  illustrated in  FIGS. 12 and 14 to 16 . The housing heaters  80  each have a rod shape and are arranged with their longitudinal direction coincident with the depth direction X of the housing  20 . As illustrated in  FIG. 16 , a plurality of housing heaters  80  are disposed on opposite sides of the housing opening  9  in a plan view of the filtration apparatus  110 . The mutually facing direction of a pair of heaters  80  on opposite sides of the opening  9  is substantially perpendicular to the mutually facing direction of the inlet  5  and the outlet  6 . As illustrated in  FIG. 14 , each heater  80  has its upper portion going through the wall  22  and fixed to the wall  22  while having its lower portion exposed in the chamber  3 . The so arranged heaters  80  make it possible to maintain the filtration tubes  41  and the housing  20  during a filtration operation at a predetermined temperature even when the filtration is carried out with the heating unit  50  removed from the filtration tubes  41  as shown in  FIG. 17 . The housing heaters  80  may be used to preheat the apparatus before introducing molten metal similarly to the heating unit  50 . The heaters  80  are omitted in  FIGS. 13 and 17 . 
         [0121]    As illustrated in  FIGS. 12 through 17 , the second embodiment is further different from the first one in that packing is not provided between the catching portion  30  and the support plate  142 , that the housing  20  has no grooves, and that the support plate  142  does not have a hook-engageable cut. 
         [0122]    A filtration apparatus  110 A according to the third embodiment of the invention will be described with reference to  FIGS. 18 to 21 . 
         [0123]    In the third embodiment, the combination of the wedge  70  and the recess  39  as a hold-down member is replaced with a combination of a rod member  77  and an urging member  75 . 
         [0124]    As illustrated in  FIGS. 18 and 19 , the opening  9  of the housing  20  is detachably closed by a plate-shaped closure  90  that covers the top  23  of the housing  20 . The closure  90  is separably composed of a central portion  52  and a peripheral portion  84 . The central portion  52  is located to cover the filtration tubes  41  in a plan view of the apparatus  110 A, and the peripheral portion  94  surrounds the central portion  52  in a plan view. In the particular example shown in  FIGS. 18 and 19 , the central portion  52  is the same as the support  52  of the heating unit  50 . In the example shown in  FIG. 19 , the central portion  52  has a substantially uniform thickness, and the peripheral portion  94  also has a substantially uniform thickness. The thickness of the central portion  52  and that of the peripheral portion  94  are substantially equal but may be different. The inner periphery  94   a  of the peripheral portion  94  and the outer periphery  52   c  of the central portion  52  are complementary to each other. In order to prevent the central portion  52  from falling off the peripheral portion  94 , the outer periphery  52   c  of the central portion  52  and the inner periphery  94   a  of the peripheral portion  94  may be tapered and reverse-tapered, respectively, downward in the depth direction X. 
         [0125]    The closure  90  has a larger plan view area than the housing opening  9  and covers the entirety of the housing opening  9 . When the closure  90  is placed on the top  23  of the housing  20 , the closure  90  closes the housing opening  9 . The central portion  52  has a plan view area that is smaller than that of the central portion  52  and is located entirely inside the housing opening  9  in a plan view. 
         [0126]    As illustrated in  FIGS. 18 and 19 , the closure  90  has a through-hole  95  in the peripheral portion  94 . At least two, preferably 3 or more, through-holes  95  are provided along the inner circumference of the peripheral portion  94 . The direction that each through-hole  95  penetrates the closure  90  is the thickness direction of the closure  90 . The thickness direction of the closure  90  as closing the housing  20  is substantially coincident with the depth direction X of the housing  20 . The apparatus  110 A has rod members  77  which are inserted in the through-holes  95 , and the rod members  77  are inserted in the through-holes  95  on a one-to-one basis. The plurality of rod members  77  have substantially the same length. Each rod member  77  as inserted in the through-hole  95  of the closure  90  is located in the peripheral portion  94  with its longitudinal direction coincident with the depth direction X and is slidable in the through-hole  95  in the depth direction X. The urging member  75  is connected to each rod  77  and disposed on the upper surface of the peripheral portion  94 . The urging member  75  has a cylinder  78  fixed to the upper surface of the peripheral portion  94  by means of, for example, an unshown jig and an unshown screw. 
         [0127]    The rod member  77  may be either solid or hollow. In using a hollow rod member  77 , the lower end  77   a  of the rod member  77 , which contacts the support plate  42 , is preferably closed as illustrated in  FIG. 19  so as to prevent molten metal from entering the hollow inside of the rod  77 . In the example shown in  FIG. 19 , while the rod member  77  has an unchanged cross-sectional shape taken in the direction perpendicular to its longitudinal direction over the whole length thereof, the shape of the rod member  77  is not limited thereto. The rod member  77  may be provided, at its lower end  77   a , with a hold-down member whose cross-sectional shape differs from that of the rod member  77 . The material making up the rod member  77  may be, for example, silicon nitride when the metal to be filtered is aluminum. 
         [0128]    The urging member  75  is preferably configured to apply an adjusted and fixed urging force against the rod member  77 . Specific examples of such an urging member include a hydraulic cylinder, an air cylinder, a position-controllable motor, e.g., a stepping motor, and a manually operable screw. When a motor is used as the urging member  75 , the rod member  77  may be a ball screw. With these urging members, the hold-down force against the support plate  42  is easily adjustable, and by using a plurality of urging members, it is easy to uniformly hold down the peripheral edge portion S of the support plate  42 . 
         [0129]    The urging member  75  used in the example illustrated in  FIG. 19  is a hydraulic cylinder. The hydraulic cylinder  75  includes a handle  76 , a cylinder  78 , and a piston  79  in the cylinder  78 . The rod member  77  is connected to the piston  79 . The handle  76  is operated manually or automatically to feed oil into the cylinder  78  through an unshown pipe, whereby the piston  79  moves downward in the depth direction X to urge the rod member  77  in that direction. 
         [0130]    As illustrated in  FIGS. 18 and 19 , the rod members  77  hold down part of the peripheral edge portion S of the support plate  42  that is mounted on the projection  32  (the catching portion  30 ), whereby the support plate  42  is fixedly held under pressure between the rod members  77  urged by the urging members  75  and the catching portion  30 . The support plate  42  is thus effectively prevented from being lifted at the start of a filtration operation similarly to the embodiment in which a wedge  70  is used as a hold-down member. 
         [0131]    As illustrated in  FIG. 19 , the projection  32  is projecting inwardly of the housing  20  over the whole depth and inner periphery of the chamber  3 . Therefore, the projection  32  has high strength. 
         [0132]      FIG. 20  is a plan view of the housing  20  from which the central portion is removed. As illustrated, in the case when the support plate  42  has a polygonal plan view shape, the sites of the support plate  42  to be held down by the rod members  77  are located on a diagonal of the polygon provided that the sites are closer to the periphery of the polygon than the positions of fixing the filtration tubes  41 . This arrangement is favorable in terms of preventing imbalanced location of the hold-down sites and fixing the support plate  42  under uniformly applied force thereby to provide a more perfect seal. It is particularly preferred for the sites to be held down are located on the diagonals dividing the area of the support plate  42  into equal halves. As illustrated in  FIG. 20 , it is preferred for preventing the lift of the support plate  42  that the site to be held down by the rod member  77  not be located on a straight line connecting the inlet  5  and the outlet  6 . 
         [0133]    The structural material of the closure  90  is the same as described with respect to the first embodiment. Although the lift of the closure  90  while being urged by the hold-down member is usually prevented by its own weight, a member for fixing the closure  90  to the housing  20  may be provided as will be described later. 
         [0134]      FIG. 21  presents a cross-section of the apparatus  110 A taken along a different direction from  FIG. 19 . As illustrated in  FIG. 21 , there is a space between the upper surface  42   b  of the support plate  42  and the lower surface  90   a  of the closure  90 . The position of the outlet  6  in the depth direction X is such that the upper surface  42   b  of the support plate  42  is located between the upper end  6   a  and the lower end  6   b  of the outlet  6 . As illustrated in  FIG. 21 , the upper end  6   a  of the outlet  6  is located below the lower surface  90   a  of the closure  90 . Therefore, molten metal having been filtered is unlikely to adhere to the lower surface  90   a  of the closure  90  and is therefore prevented from entering the through-hole  95  of the closure  90  and the hereinafter described through-hole  58 . 
         [0135]    In the third embodiment, since the closure  90  is separably composed of the central portion  52  and the peripheral portion  94 , the first closure  52  having the heaters  51  is easily replaced with the second closure  92  with no need to remove the urging members  75  from the closure  90 . Further, the inside of the housing  20  may be cleaned without replacement of the filtration unit  40  by removing only the central portion  52  with no need to remove the urging members  75  from the closure. If needed, the central portion  52  may be replaced with a central portion  92  having no heaters. The central portion  92  has a through-hole  58  and a hot air generator  99  that is located on the upper surface side of the central portion  92  in an interconnected relation to the through-hole  58  as illustrated in  FIG. 21 . According to this configuration, hot air generated by the hot air generator  99  may be supplied to the inside of the housing  20  through the through-hole  58  during filtration operation. 
         [0136]    As described above, similarly to the apparatuses of the foregoing embodiments, the filtration apparatus  110 A of the third embodiment effectively prevents the lift of the support plate  42  that would occur when molten metal is filtered through a filtration tube  41  the open end of which faces upward. The third embodiment offers an additional advantage in that efforts such as masonry work for forming the recess  39  are not required, which also saves the physical effort of removing the metal deposited on the recess  39 . Removal of the metal deposited on the rod member  77  is easily achieved by previously coating the rod member  77  with the same coating as described with respect to the first embodiment. 
         [0137]    While the above described apparatus  110 A has no member for securing the closure  90  to the housing  20 , a fixing member  61  may be used to secure the closure  90  to the housing  20  as illustrated in  FIGS. 22 and 23 . The apparatus  110 A′ shown in  FIGS. 22 and 23  is the same as the apparatus  110 A except that the apparatus  110 A′ has the fixing member  61 . To provide the fixing member  61  further ensures prevention of the closure  90  from being lifted when the closure  90  is urged by the urging members  75 . The fixing member  61  is exemplified by a fastener  61  illustrated in  FIG. 23 . The fastener  61  includes a hook  62  having a concavity  68  and an engageable member  63  engageable with the hook  62 . The hook  62  is attached to the lower end of the exterior surface of the closure  90  by means of, e.g., an unshown screw and nut system, while the engageable member  63  is attached to the upper end of the exterior surface of the housing  20  at a position opposite to the hook  62  by an unshown screw and nut system. The engageable member  63  has a rod  66  having a projection  65  on either side thereof and a lever  67  connected to the rod  66 . Upon pivoting the lever  67  with the projections  65  fitted into the concavity  68  of the hook  62 , the closure  90  is fastened to the housing  20 . Preferred examples of such a fastener include Adjust Fastener® C-236 available from Takigen Manufacturing Co., Ltd. 
         [0138]    In the example shown in  FIG. 22 , a plurality of the fasteners  61  (engageable members  63 ) are provided along the outer circumferential direction of the wall  22 . The plurality of, preferably three or more, fasteners are preferably provided in the circumferential direction at a substantially regular interval. 
         [0139]    While the invention has been described on the basis of its preferred embodiments, the invention is not limited to these embodiments. For instance, while the recess  39  adopted in the first and the second embodiment has a tapered shape similarly to the wedge  70 , the shape of the recess  39  is not limited thereto. For example, the dimension of the recess  39  in the depth direction X may be substantially constant in the horizontal direction. The filtration apparatus does not always need to have as many wedges  70  as the recesses  39 . 
         [0140]    While in the first and the second embodiment the plurality of recesses  39  have the same shape, and the plurality of wedges  70  have the same shape, they may have different shapes. The plurality of wedges  70  may be made of different materials. 
         [0141]    While in the foregoing embodiments the catching portion  30  (the projection  32 ) is continuous in the inner circumferential direction of the housing opening  29 , it may be discontinuous in that direction. The catching portion  30  (the projection  32 ) does not need to be integral with the housing  20 . 
         [0142]    The support plate  42  does not need to have a generally polygonal plan view shape and may have a generally circular shape. 
         [0143]    The support plate  42 , when viewed from its side, does not need to have a fittable configuration to be fitted in along the direction from the side of its upper surface  42   b  toward the side of its lower surface  2   a . In that case, it is preferred that the support plate  42  not be tapered from the side of the lower surface  42   a  toward the side of the upper surface  42   b  when viewed in every direction perpendicular to the depth direction X. 
         [0144]    The filtration apparatus does not need to have a heating unit. For example, the apparatus may have the second closure  92  instead of the first closure  52 . 
         [0145]    The shapes of the closure and the housing opening (closure-receiving portion) of the apparatus  10  are not limited to those described above and may be any possible shape. For example, the closure and the housing opening may have a polygonal plan view shape, and their mating surfaces do not need to be tapered or may be shouldered instead of being tapered. 
         [0146]    While in the foregoing embodiments the filtration tube  41  is fixed with its longitudinal direction coincident with the depth direction X of the housing, the longitudinal direction of the tube  41  does not always need to be coincident with the depth direction X provided that it intersects the horizontal plane. Specifically, the angle formed between the longitudinal direction of the filtration tube  41  and the horizontal plane is preferably at least 45°, more preferably 55° or !Heater, even more preferably 80° or greater. 
         [0147]    In the cases where the filtration apparatus has a heating unit, for example, in the foregoing embodiments, in which the filtration unit  40  has at least one filtration tube  41 , the number of the heaters  51  of the heating unit  50  is enough to heat a part or all of the filtration tubes  41  so that the heater  51  is inserted into a part or all of the filtration tubes  41 . The number of the filtration tubes  41  may be one or more, and the number of the heaters  51  may be one or more. 
         [0148]    The number of the heaters  51  does not need to be equal to that of the filtration tubes  41  and may be smaller than that of the filtration tubes  41 . When there are two or more filtration tubes  41 , it is only necessary that at least one of them have the heater inserted therein. Not all the filtration tubes  41  need to have the heater inserted therein. 
         [0149]    There needs not be a one-for-one relationship between the filtration tube  41  and the heater  51 . For example, a plurality of heaters  51  may be inserted into one filtration tube  41 . The through-hole  52   d  and the heater  51  do not need to be in a one-for-one relationship, either. That is, a plurality of heaters  51  may be inserted through one through-hole  52   d . The number of the through-holes  52   d  does not need to be equal to that of the filtration tubes  41 . For example, the number of the through-holes  52   d  may be smaller than that of the filtration tubes  41 . Note that the number of the filtration tubes  41  should be equal to that of the openings  43 . 
         [0150]    While in the foregoing embodiments the plurality of heaters  51  are of the same shape and material, they may be of different shapes and materials. 
         [0151]    While the shape of the heater  51  is a rod in the foregoing embodiments, the shape of the heater is not limited thereto. For example, the rod-shaped heater may be replaced with a spiral heater. 
         [0152]    The manner of fixing the heater to the support is not limited to passing the heater through the through-hole as in the foregoing embodiments. For example, the heater may be fixed to the lower surface of the support by disposing a hold-down member on the lower surface of the support. 
         [0153]    The support  52  of the heating unit does not need to perform the function as a closure for blocking the housing opening  9 . For example, the housing  20  may have a catching portion for catching the support  52  and may be closed by a separate closure from the support. 
         [0154]    While in the foregoing embodiments, the support  52  and the inner surface of the wall  22  are tapered in a complementary fashion, so that the support  52  is configured to be fittable in the housing  20 , the fittable configuration of the support  52  in the housing  120  is not limited thereto. For example, the support  52  may have a shoulder on its edge surface, and the inside of the wall  22  may have a complementary shoulder so that they are fitted to each other, whereby the support  52  may be fitted in the housing  20 . 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  Molten metal filtration apparatus 
           20  Housing 
           21  Bottom 
           22  Wall 
           9  Housing opening 
           30  Catching portion 
           39  Recess 
           40  Filtration unit 
           41  Filtration tube 
           43  Opening of filtration tube 
           42  Support plate 
           42   a  Lower surface 
           42   b  Upper surface 
           42   c  Edge surface 
           50  Heating unit 
           51  Heater 
           52  Support 
           52   a  Lower surface 
           52   b  Upper surface 
           52   c  Lateral surface 
           70  Wedge 
           75  Urging member 
           77  Rod member 
           90  Closure 
           92  Second closure