Abstract:
A powder classifying apparatus for classifying powder that has a granularity distribution and recovering fine powder has: a casing provided with two disc-shaped members and a surrounding wall member, a disc-shaped hollow section for classifying powder using a spinning airflow in the interior thereof being formed on the inner side of the casing; at least one powder supply opening for supplying powder into the disc-shaped hollow section; a discharge section for discharging air including fine powder discharged from the disc-shaped hollow section; a recovery unit formed in the thickness-wise center of the surrounding wall member of the casing and provided with a slit-shaped opening for recovering coarse powder discharged from the disc-shaped hollow section; and two air introduction units provided with a plurality of air introduction devices for introducing air into the disc-shaped hollow section in order to form the rotational airflow inside the disc-shaped hollow section.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a powder classifying apparatus that classifies powder having a particle size distribution according to a desired diameter (classification point) and, in particular, to a powder classifying apparatus capable of accurately classifying powder ranging preferably from about several micrometer size to about submicron size using a balance between a centrifugal force imparted to the powder by a whirling gas stream and a drag over long periods of time. 
       BACKGROUND ART 
       [0002]    There has been conventionally known a classifying apparatus that uses guide vanes or air nozzles to generate a whirling gas stream in a classifying chamber to impart a whirling motion to powder supplied into the classifying chamber for separating the powder into fine powder and coarse powder by a centrifugal force, the fine powder being collected from the central part of the whirling gas stream and the coarse powder being collected from a lower peripheral portion of the whirling gas stream. 
         [0003]    Aside from that, in recent years, with the advancement in technology of electronic components and the like such as a capacitor, there is demand for fine particles having a narrow particle size distribution. 
         [0004]    To cope with it, for instance, in Patent Literature 1, the applicant proposes a powder classifying apparatus in which a disc-like cavity serving as a classifying site for centrifuging powder having a particle size distribution is formed between two disc-like members; a plurality of guide vanes are arranged at an outer periphery of the disc-like cavity so as to extend from the outer periphery of the disc-like cavity toward an inner direction at a predetermined angle; a powder supply port for supplying the powder to the disc-like cavity is disposed at an upper disc-like member; a discharge unit for air streams including fine powder discharged from the central part of the disc-like cavity is disposed in the central part of the upper disc-like member; a collection unit for coarse powder discharged from the disc-like cavity is disposed between a lower portion of an outer edge portion of a lower disc-like member and an outer peripheral wall of the disc-like cavity; a plurality of first air nozzles for blowing compressed air into an inside of the disc-like cavity are arranged near the powder supply port and above the plurality of guide vanes on the outer peripheral wall of the disc-like cavity so as to extend along tangential directions of the wall; and a plurality of second air nozzles for blowing compressed air into an inside of the disc-like cavity are arranged at the collection unit of coarse powder and below the plurality of guide vanes on the outer peripheral wall of the disc-like cavity so as to extend along tangential directions of the wall. 
         [0005]    With this configuration, the powder classifying apparatus disclosed in Patent Literature 1 sucks and discharges air through the discharge unit by use of a blower to cause air sucked from the outside of the apparatus to pass through the guide vanes to thereby form a whirling gas stream in the disc-like cavity which serves as a centrifuge chamber (classifying site), thus imparting a whirling motion to the powder and separating the powder into coarse powder and fine powder by a centrifugal force. At this time, the apparatus blows compressed air into the inside of the disc-like cavity through the first air nozzles to cause the powder supplied from the powder supply port to join the whirling gas stream and blows compressed air to the lower portion of the outer edge portion of the disc-like cavity through the second air nozzles to return, to the disc-like cavity, fine powder mixed in coarse powder to be collected through a coarse powder collection port, thereby accurately sorting out fine powder of up to about several micrometers or submicron size. 
         [0006]    As a result, in Patent Literature 1, the powder classifying apparatus is attained which can accurately sort out (classify) fine powder of up to about several micrometers or submicron size, enables easy control of the particle size, and allows easy maintenance. 
         [0007]    Besides, for instance, in Patent Literature 2, the applicant proposes a powder classifying apparatus in which formed in a casing are a disc-like centrifuge chamber for centrifuging powder having a particle size distribution, and a ring-shaped powder dispersion chamber and a ring-shaped powder reclassifying chamber which are located separately at the opposite sides of the centrifuge chamber to be coaxial therewith and to communicate with the centrifuge chamber; the outer peripheral portion of the centrifuge chamber is closed by a peripheral wall; formed at the casing are a powder supply port for supplying powder into the powder dispersion chamber, a fine powder discharge port for discharging air stream including fine powder from the centrifuge chamber, and a coarse powder discharge port for discharging coarse powder from the powder reclassifying chamber; a plurality of first air nozzles for ejecting compressed air into the inside of the powder dispersion chamber and a plurality of second air nozzles for ejecting compressed air into the inside of the powder reclassifying chamber are arranged on the peripheral wall of the casing along a circumferential direction thereof to generate a first whirling gas stream for dispersing the powder in the powder dispersion chamber and a second whirling gas stream for allowing fine powder mixed in coarse powder in the powder reclassifying chamber to float and return to the centrifuge chamber; and a third whirling gas stream is generated using the above two whirling gas streams for classifying (centrifuging) the powder having a particle size distribution in the centrifuge chamber. 
         [0008]    With this configuration, the powder classifying apparatus disclosed in Patent Literature 2 generates the first whirling gas stream in the ring-shaped powder dispersion chamber by use of compressed air ejected from the first air nozzles to the powder dispersion chamber to allow the powder supplied through the powder supply port to be carried and dispersed by the first whirling gas stream while causing the powder to enter a disc-like cavity which serves as the centrifuge chamber and communicates with the powder dispersion chamber, and generates the second whirling gas stream in the ring-shaped powder reclassifying chamber by use of compressed air ejected from the second air nozzles to the powder reclassifying chamber to allow fine powder mixed in coarse powder to float and return to the centrifuge chamber while causing the floating fine powder to enter the disc-like cavity which serves as the centrifuge chamber and communicates with the powder reclassifying chamber, whereby the third whirling gas stream for classifying the powder is generated in the disc-like cavity to impart a whirling motion to the powder and separate the powder into coarse powder and fine powder by a centrifugal force, thereby accurately sorting out fine powder of up to about several micrometers or submicron size. 
         [0009]    As a result, Patent Literature 2 enables fine particles to be accurately sorted out.
   Patent Literature 1: JP 2009-34560 A   Patent Literature 2: JP 2011-45819 A   
 
       SUMMARY OF INVENTION 
     Technical Problems 
       [0012]    Meanwhile, in order to deal with much finer powder in recent years, it is required to form much stronger whirl (whirling gas stream) in a disc-like cavity serving as a classifying site. 
         [0013]    In the powder classifying apparatus disclosed in Patent Literature 1, however, since the whirling gas stream used for centrifugation is generated in the disc-like cavity serving as the classifying site by use of air which has passed through the guide vanes due to suction by the blower, and the inflow velocity of air entering through the guide vanes is far lower than that of air being ejected and entering from the air nozzles, even if the inflow velocity of air entering through the guide vanes is increased, the resulting increase in flow rate of the whirling gas stream for use in classification (centrifugation) is not enough, so that finer particles requiring a high flow rate of whirling gas stream cannot be sorted out, disadvantageously. 
         [0014]    Therefore, in the powder classifying apparatus disclosed in Patent Literature 1, it is necessary to considerably increase the amount of compressed air to be ejected from the first air nozzles. When a great amount of air is sucked through the conventional guide vanes, whirl (whirling gas stream) to be generated in the disc-like cavity serving as the classifying site is uniform. However, as the amount of air ejected from the first air nozzles is increased, the whirl (whirling gas stream) gets nonuniform accordingly, and as shown in  FIGS. 8(C) and 8(D) , powder is adhered to an upper wall surface of the disc-like cavity (a lower wall surface of the upper disc-like member) or a lower wall surface of the same (an upper wall surface of the lower disc-like member), resulting in great deterioration of classification accuracy. Such adhered powder increases with increasing periods of time of the classification operation, which may cause detachment of the adhered powder or the like, leading to such problems as deteriorated classification accuracy and mixing of coarse particles. 
         [0015]    In the powder classifying apparatus disclosed in Patent Literature 2, since the third whirl (whirling gas stream) is generated by the first and second whirls (whirling gas streams) generated by compressed air ejected from the first and second air nozzles, the amount of air in the third whirl (whirling gas stream) can be further increased compared to the powder classifying apparatus disclosed in Patent Literature 1. When, however, the amount of compressed air ejected from the first or second air nozzles is increased to sort out much finer particles, as with the case of the powder classifying apparatus disclosed in Patent Literature 1, the whirl (whirling gas stream) gets nonuniform, leading to deterioration of dispersion accuracy, and powder may be adhered to an upper wall surface of the disc-like cavity (a lower wall surface of an upper disc-like member), resulting in great deterioration of classification accuracy. 
         [0016]    The object of the present invention is to solve the above problems of the conventional art and provide a powder classifying apparatus that can generate an uniform whirling gas stream in a disc-like cavity serving as a classifying site and maintain the uniform whirling gas stream over long periods of time to thereby accurately sort out fine powder of up to about several micrometers or submicron size over long periods of time without allowing powder to adhere to a wall surface of the disc-like cavity, particularly to an upper or lower wall surface thereof. 
       Solution to Problems 
       [0017]    In order to attain the above objects, the present invention provides a powder classifying apparatus for classifying powder having a particle size distribution to collect fine powder having a size not larger than a predetermined particle size, comprising: a casing including two disc-like members arranged to be spaced apart at a predetermined interval and a peripheral wall member attached on outer peripheral sides of the two disc-like members, the casing having therein a disc-like cavity formed between the two disc-like members and at an inner side of the peripheral wall member, the disc-like cavity being a place where the powder is classified by a whirling gas stream generated therein; one or more powder supply ports disposed at an outside of at least one of the two disc-like members of the casing so as to communicate with an inner side of an outer edge portion of the disc-like cavity, and configured to supply the powder conveyed by air flow into the disc-like cavity; a discharge section formed at at least one of the two disc-like members of the casing so as to communicate with a central portion, in a radial direction, of the disc-like cavity, and configured to discharge air containing the fine powder to be discharged from the disc-like cavity; a collecting section formed at a middle portion, in a thickness direction, of the peripheral wall member of the casing so as to communicate with the outer edge portion of the disc-like cavity, and including a slit-type opening provided to collect coarse powder having a size larger than the predetermined particle size to be discharged from the disc-like cavity; and two air introducing sections separately provided at opposite sides, in the thickness direction, of the slit-type opening of the peripheral wall member of the casing, each including a plurality of air introducing devices arranged at the peripheral wall member of the casing at the outer edge portion of the disc-like cavity to extend along tangential directions of the outer edge portion, the plurality of air introducing devices being configured to introduce air into the disc-like cavity to generate the whirling gas stream in the disc-like cavity. 
         [0018]    Preferably, the two disc-like members comprise an upper disc-like member and a lower disc-like member, and the powder classifying apparatus further includes a second collecting section formed at at least one of the two disc-like members of the casing so as to communicate with the disc-like cavity, the second collecting section being configured to collect a part of the coarse powder discharged from the disc-like cavity. 
         [0019]    Preferably, the discharge section is composed of an inner cylindrical tube standing upright through the upper disc-like member of the casing and having an end projecting toward an inside of the disc-like cavity, the second collecting section is composed of an outer cylindrical tube with a diameter larger than that of the inner cylindrical tube, the outer cylindrical tube standing upright through the upper disc-like member of the casing and being disposed coaxially with the inner cylindrical tube, and an end of the outer cylindrical tube is positioned above the end of the inner cylindrical tube and communicates with the disc-like cavity. 
         [0020]    Alternatively, the second collecting section preferably includes a groove-shaped discharge path formed at a lower side of the lower disc-like member of the casing so as to communicate with the inner side of the outer edge portion of the disc-like cavity. 
         [0021]    Preferably, the slit-type opening of the collecting section has a taper shape that widens toward the disc-like cavity. 
         [0022]    Preferably, the discharge section is provided at each of the two disc-like members of the casing. 
         [0023]    It is preferable to further include a ring-shaped edge disposed at a center of at least one of opposing inner surfaces of the two disc-like members of the casing, the opposing inner surfaces constituting an upper surface and a lower surface of the disc-like cavity. 
         [0024]    Preferably, the powder supply ports are evenly arranged at the upper disc-like member of the two disc-like members of the casing and are inclined toward the inner side of the outer edge portion of the disc-like cavity along a whirling direction of the whirling gas stream, and the powder is conveyed by air flow generated by an ejector, and ejected and supplied with the air flow from the powder supply ports into the disc-like cavity in the whirling direction of the whirling gas stream. 
         [0025]    Preferably, the powder supply port opens to an inside of one of the air introducing devices of one of the two air introducing sections, and the powder is conveyed by air flow by an ejector effect that occurs owing to air introduced by the one of the air introducing devices and is supplied to the disc-like cavity. 
         [0026]    Preferably, the powder to be conveyed by air flow is distributed in advance by a distributor to a plurality of pipelines each connected to each of the powder supply ports by aid of compressed air. 
         [0027]    Preferably, each of the air introducing devices is an air nozzle for use in injecting compressed air into the disc-like cavity. 
       Advantageous Effects of Invention 
       [0028]    According to the present invention, there can be achieved the powder classifying apparatus that can generate an uniform whirling gas stream in a disc-like cavity serving as a classifying site and maintain the uniform whirling gas stream over long periods of time to thereby accurately sort out fine powder of up to about several micrometers or submicron size over long periods of time without allowing powder to adhere to a wall surface of the disc-like cavity, particularly to an upper or lower wall surface thereof, and that also enables easy control of the particle size and allows easy maintenance. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0029]      FIG. 1  is a cross-sectional view schematically showing a configuration of a powder classifying apparatus according to an embodiment of the invention. 
           [0030]      FIGS. 2(A) and 2(B)  are cross-sectional views taken along lines IIA-IIA and IIB-IIB of the powder classifying apparatus shown in  FIG. 1 , respectively. 
           [0031]      FIG. 3  is a cross-sectional view schematically showing another exemplary configuration of the powder classifying apparatus according to the embodiment of the invention. 
           [0032]      FIG. 4  is a cross-sectional view schematically showing a configuration of a powder classifying apparatus according to another embodiment of the invention. 
           [0033]      FIG. 5  is a cross-sectional view schematically showing a configuration of a powder classifying apparatus according to still another embodiment of the invention. 
           [0034]      FIG. 6  is a cross-sectional view schematically showing a configuration of a powder classifying apparatus according to still another embodiment of the invention. 
           [0035]      FIG. 7  is a schematic view showing a configuration of a whole classifying system using any of the powder classifying apparatuses of the invention. 
           [0036]      FIGS. 8(A) and 8(B)  are photographs substituted for drawings showing the states of upper and lower disc-like members of the powder classifying apparatus of the invention and  FIGS. 8(C) and 8(D)  are those of the conventional powder classifying apparatus. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0037]    The powder classifying apparatus of the invention will be now described in detail based on preferred embodiments illustrated in the attached drawings. 
       First Embodiment 
       [0038]      FIG. 1  is a cross-sectional view schematically showing a configuration of the powder classifying apparatus according to a first embodiment of the invention along a plane extending through the central axis of the powder classifying apparatus. 
         [0039]      FIGS. 2(A) and 2(B)  are cross-sectional views taken along lines IIA-IIA and IIB-IIB of the powder classifying apparatus shown in  FIG. 1 , respectively. 
         [0040]    A powder classifying apparatus  10  in the first embodiment of the invention shown in  FIG. 1  includes a casing  20  of substantially truncated cone shape with the top being oriented vertically downward. The casing  20  includes an upper disc-like member  12  and a lower disc-like member  14  that are arranged to face each other at a predetermined interval, an annular peripheral wall member  16  attached to the outer peripheries of the two disc-like members  12  and  14 , and a cone member  18  attached to a lower portion of the peripheral wall member  16 . A centrifuge chamber  22  constituted of a disc-like cavity which is substantially vertically symmetric is formed between the two disc-like members  12  and  14  and in the inside of the peripheral wall member  16 . 
         [0041]    As shown in  FIGS. 1 and 2 (A), the powder classifying apparatus  10  includes: a powder supply section  24  made up of a plurality of, e.g., six powder supply ports  24   a  evenly arranged on a circle having a predetermined radius from the center of the upper disc-like member  12  so as to communicate with the inner side of an outer edge portion of an upper portion of the centrifuge chamber  22 ; a fine powder collecting section  26  having a fine powder collecting port  26   a  located at the center of the upper disc-like member  12  so as to communicate with the center portion of the upper portion of the centrifuge chamber  22  for collecting, together with air, fine powder having a size not larger than a predetermined particle size (classification point); a coarse powder collecting section  30  having a slit-type annular opening  28  located at the middle, in the vertical direction, of the peripheral wall member  16  so as to communicate with a middle portion, in the vertical direction, of the outer edge portion of the centrifuge chamber  22  for collecting coarse powder having a size larger than the predetermined particle size (classification point); and first and second air introducing sections  32  and  34  that are arranged at the opposite sides, in the vertical direction, of the annular opening  28  of the peripheral wall member  16 , each of the air introducing sections  32  and  34  being composed of a plurality of, e.g., six air nozzles  32   a  or  34   a . The fine powder collecting section  26  and the coarse powder collecting section  30  constitute a discharge section and a collecting section of the invention, respectively, and the first air introducing section  32  and the second air introducing section  34  constitute air introducing devices of the invention. 
         [0042]    The upper disc-like member  12  is composed of an inner member  12   a  and an outer member  12   b  but may be formed as a single member integrally including these members. 
         [0043]    The inner member  12   a  is fixed to the bottom surface of the outer member  12   b  by fixing members such as bolts or screws and supported thereby. The bottom surface of the inner member  12   a  constitutes the upper surface of the centrifuge chamber  22 . The bottom surface of the inner member  12   a  is inclined upward in the vicinity of the outer peripheral portion and accordingly, the upper surface of the centrifuge chamber  22  is inclined upward in the vicinity of the outer edge portion. 
         [0044]    A ring-shaped edge portion  12   c  is formed at the center of the inner member  12   a  and at an opening end of the fine powder collecting port  26   a  facing the centrifuge chamber  22  to project toward the centrifuge chamber  22 . The fine powder collecting port  26   a  is constituted of a circular hole at the center of the inner member  12   a  and a cylindrical tube  26   b  disposed at the center of the outer member  12   b , and is connected to a suction blower  92  (see  FIG. 7 ) via an appropriate fine powder collecting filter  90  (see  FIG. 7 ) such as a bag filter. Consequently, air containing fine powder having been sorted out in the centrifuge chamber  22  is sucked by the suction blower  92  and discharged through the fine powder collecting port  26   a.    
         [0045]    The plurality of, e.g., six powder supply ports  24   a  are evenly arranged at an annular region of the outer member  12   b  between an outer peripheral edge portion of the inner member  12   a  and an inner peripheral edge portion of the peripheral wall member  16 , i.e., on a circle having a predetermined radius from the center of the upper member  12   b  as described above. The powder supply ports  24   a  are disposed along a whirling direction of a whirling gas stream in the centrifuge chamber  22  to be oriented from the outside of the upper disc-like member  12  (outer member  12   b ) to the inside of the centrifuge chamber  22  and to be inclined with respect to the top surface of the upper disc-like member  12  (outer member  12   b ). 
         [0046]    The powder supply ports  24   a  are arranged to be located at respective plural positions at regular intervals inside the outer edge portion of the centrifuge chamber  22  so that powder conveyed from a distributor  84  (see  FIG. 7 ) by means of a flow of compressed air is caused to join a whirling gas stream in the centrifuge chamber  22  in the whirling direction of the whirling gas stream. In other words, the powder conveyed by air flow is supplied or preferably ejected evenly from the plural positions in the same direction as the whirling direction of the whirling gas stream, i.e., in tangential directions of the whirling gas stream in the centrifuge chamber  22 . Consequently, compared with a conventional apparatus in which powder is supplied from a powder supply port in a direction perpendicular to the whirling direction of a whirling gas stream (supplied vertically downward) and the whirling gas stream is greatly disturbed in a centrifuge chamber accordingly, the disturbance of the whirling gas stream in the centrifuge chamber  22  caused by supply of powder from the powder supply ports  24   a  can be minimized. 
         [0047]    The lower disc-like member  14  is composed of an inner edge portion  14   a  having an inner surface (top surface) which is substantially symmetrical to the inner member  12   a  of the upper disc-like member  12 , and an outer edge portion  14   b  fixed at and supported by a lower peripheral wall member  16   b  of the peripheral wall member  16  to be described later. The top surface of the inner edge portion  14   a  is inclined downward in the vicinity of the outer peripheral portion in a symmetrical manner with respect to the bottom surface of the inner member  12   a , and accordingly, the bottom surface of the centrifuge chamber  22  is inclined downward in the vicinity of the outer edge portion. 
         [0048]    Thus, the centrifuge chamber  22  is a disc-like cavity that is substantially symmetrical in the vertical direction. 
         [0049]    A ring-shaped edge portion  14   c  is formed at the center of the inner edge portion  14   a  of the lower disc-like member  14  to project toward the centrifuge chamber  22  and face the ring-shaped edge portion  12   c  formed at the center of the inner member  12   a  of the upper disc-like member  12 . In other words, the edge portions  12   c  and  14   c  are arranged to face each other across the centrifuge chamber  22 . 
         [0050]    The ring-shaped edges portions  12   c  and  14   c  are factors determining the classification performance of the powder classifying apparatus  10 , and the attachment positions, the ring sizes and the edge heights thereof need to be set according to the type of powder to be classified, the type of fine powder to be collected, or the like. However, the present invention is not limited to the illustrated example. 
         [0051]    While the ring-shaped edge portions  12   c  and  14   c  are arranged to face each other across the centrifuge chamber  22  in the illustrated example, only one of the edge portions  12   c  and  14   c  may be provided. 
         [0052]    The peripheral wall member  16  is composed of an upper peripheral wall member  16   a  and the lower peripheral wall member  16   b  which are fixed by fixing members such as bolts at a predetermined interval. The upper peripheral wall member  16   a  has a top surface fixed to the bottom surface of the outer member  12   b  of the upper disc-like member  12  by fixing members such as bolts and supported thereby, and a bottom surface to and by which the top surface of the cone member  18  is fixed by fixing members such as bolts and supported. The lower peripheral wall member  16   b  has a bottom surface to and by which the outer edge portion  14   b  of the lower disc-like member  14  is fixed by fixing members such as bolts and supported. The configurations of the upper disc-like member  12 , the lower disc-like member  14 , the peripheral wall member  16  and the cone member  18  and the fixing and supporting states are not limited to the illustrated example. 
         [0053]    The slit-type annular opening  28  connected to a coarse powder collecting port  30   a  of the coarse powder collecting section  30  is formed between the upper and lower peripheral wall members  16   a  and  16   b  fixed to be spaced apart at a predetermined interval. 
         [0054]    The slit-type annular opening  28  is located at the middle, in the vertical direction, of the outer edge portion of the centrifuge chamber  22  and therefore, coarse powder affected by a large centrifugal force in a whirling gas stream in the centrifuge chamber  22  smoothly moves to the slit-type annular opening  28  and is drawn out of the centrifuge chamber  22 . Thus, coarse powder can be smoothly taken out from the centrifuge chamber  22  serving as the classifying site. 
         [0055]    The first and second air introducing sections  32  and  34  are respectively provided at the upper and lower peripheral wall members  16   a  and  16   b  of the peripheral wall member  16  at vertically symmetrical positions with respect to the slit-type annular opening  28 . 
         [0056]    The first air introducing section  32  is composed of the plurality of, e.g., six first air nozzles  32   a  that are arranged at the inner peripheral portion of the upper peripheral wall member  16   a  adjoining the centrifuge chamber  22 , so as to face the inside of the centrifuge chamber  22 , while the second air introducing section  34  is composed of the plurality of, e.g., six second air nozzles  34   a  that are arranged at the inner peripheral portion of the lower peripheral wall member  16   b  adjoining the centrifuge chamber  22 , so as to face the inside of the centrifuge chamber  22 . 
         [0057]    The first air nozzles  32   a  eject compressed air toward the bottom surface of the inner member  12   a  of the upper disc-like member  12  (a part where the slope of the outer periphery starts), and the second air nozzles  34   a  eject compressed air toward the top surface of the inner edge portion  14   a  of the lower disc-like member  14  (a part where the slope of the outer periphery starts). 
         [0058]    In the first air introducing section  32 , the first air nozzles  32   a  are formed at a nozzle member  32   b  and communicates with a space  32   c  that is formed by the outer member  12   b  of the upper disc-like member  12 , the upper peripheral wall member  16   a  of the peripheral wall member  16 , and the nozzle member  32   b  interposed between the outer member  12   b  and the upper peripheral wall member  16   a  and that serves as a compressed air gathering space. The space  32   c  communicates with a pipe  32   d  connected to the outer member  12   b . The pipe  32   d  is further connected to a compressed air supply source  82  (see  FIG. 7 ). The first air nozzles  32   a  are thus connected to the compressed air supply source  82 . 
         [0059]    On the other hand, in the second air introducing section  34 , the second air nozzles  34   a  are formed at a nozzle member  34   b  and communicates with a space  34   c  that is formed by the outer edge portion  14   b  of the lower disc-like member  14 , the lower peripheral wall member  16   b  of the peripheral wall member  16 , and the nozzle member  34   b  interposed between the outer edge portion  14   b  and the lower peripheral wall member  16   b  and that serves as a compressed air gathering space. The space  34   c  communicates with the space  32   c  of the first air introducing section  32  via a through-hole  34   e  in a communication member  34   d  interposed between the upper and lower peripheral wall members  16   a  and  16   b  of the peripheral wall member  16 . The through-hole  34   e  in the communication member  34   d  is of course configured so as not to communicate with the slit-type annular opening  28  between the upper and lower peripheral wall members  16   a  and  16   b . The second air nozzles  34   a  are thus connected to the compressed air supply source  82  (see  FIG. 7 ). 
         [0060]    In the first air introducing section  32 , as shown in  FIG. 2(A) , the six first air nozzles  32   a  are arranged on the outer periphery of the centrifuge chamber  22 , i.e., on a predetermined circle, to be spaced apart from each other at regular intervals in a circumferential direction and to extend along tangential directions of the circle, for instance, at a predetermined angle with respect to the tangential directions. 
         [0061]    In the same manner, in the second air introducing section  34 , as shown in  FIG. 2(B) , the six second air nozzles  34   a  are arranged on the outer periphery of the centrifuge chamber  22 , i.e., on a predetermined circle, to be spaced apart from each other at regular intervals in a circumferential direction and to extend along tangential directions of the circle, for instance, at a predetermined angle with respect to the tangential directions. 
         [0062]    The first and second air nozzles  32   a  and  34   a  are connected to the compressed air supply source  82  (see  FIG. 7 ) as described above, and ejection of compressed air from the first and second air nozzles  32   a  and  34   a  respectively generate symmetrical whirling gas streams whirling in the same direction at the upper and lower portions in the centrifuge chamber  22 . The thus generated symmetrical whirling gas streams at the upper and lower portions in the centrifuge chamber  22  generate a whirling gas stream at the middle, in the vertical direction, of the centrifuge chamber  22 , too, and consequently, a uniform whirling gas stream is generated in the whole centrifuge chamber  22 . 
         [0063]    Since the uniform whirling gas is thus generated in the whole centrifuge chamber  22 , coarse powder affected by a large centrifugal force can be smoothly discharged through the slit-type annular opening  28  positioned at the middle, in the vertical direction, of the outer peripheral portion of the centrifuge chamber  22 . Since coarse powder can be smoothly taken out from the centrifuge chamber  22  through the slit-type annular opening  28  as described above, a whirling gas stream generated in the centrifuge chamber  22  is not disturbed. 
         [0064]    As shown in  FIG. 2(A) , the six powder supply ports  24   a  are each disposed between two adjacent air nozzles  32   a  of the six first air nozzles  32   a  to be positioned along a whirling direction of a whirling gas stream generated at the upper portion in the centrifuge chamber  22  by the six air nozzles  32   a , i.e., to extend along tangential directions of the whirling gas stream, and are inclined downward. Due to this configuration, powder conveyed by air flow is supplied, together with air, through the six powder supply ports  24   a  from an obliquely upward position toward the same direction as the whirling direction of the whirling gas stream at the upper portion in the centrifuge chamber  22 . As a result, the powder is urged to be dispersed in the whirling gas stream at the upper portion in the centrifuge chamber  22 , and the disturbance of the whirling gas stream at the upper portion can be suppressed and minimized compared to the case of supplying powder by dropping the powder naturally from above in the vertical direction. 
         [0065]    While an interior region of the centrifuge chamber  22 , i.e., the disc-like cavity forms the classifying site (zone) for classifying supplied powder, it can be said that an area which receives therein compressed air ejected from the first air nozzles  32   a  at the upper portion in the centrifuge chamber  22  and which is supplied with powder doubles as a powder dispersion zone because powder supplied into the centrifuge chamber  22  is dispersed in this region. An area which receives therein compressed air ejected from the second air nozzles  34   a  at the lower portion in the centrifuge chamber  22  has a function to return powder that has not completely been classified and therefore includes both coarse and fine powders having not collected from the centrifuge chamber  22 , to the upper portion in the centrifuge chamber  22 . 
         [0066]    While in the illustrated example, the six first air nozzles  32   a  and the six second air nozzles  34   a  are evenly arranged on respective circles, and the six powder supply ports  24   a  are evenly arranged so that each is disposed between two adjacent air nozzles  32   a  of the six first air nozzles  32   a , the present invention is not limited thereto and the numbers, the arrangements and the like of the first and second air nozzles  32   a  and  34   a  and the powder supply ports  24   a  may be appropriately changed depending on the type of powder to be classified or other factors. 
         [0067]    As described above, the fine powder collecting section  26  includes the fine powder collecting port  26   a  constituted of the opening of the upper disc-like member  12  and the cylindrical tube  26   b , and the cylindrical tube  26   b  is connected to the suction blower  92  via the appropriate filter  90  such as a bag filter (see  FIG. 7 ). 
         [0068]    The coarse powder collecting section  30  includes: the slit-type annular opening  28  between the upper and lower peripheral wall members  16   a  and  16   b  of the peripheral wall member  16 ; a space  30   b  that is formed between the outer peripheral wall of the lower peripheral wall member  16   b  and the inner peripheral walls of the upper peripheral wall member  16   a  and the cone member  18  and communicates with the slit-type annular opening  28 ; an internal space  18   a  of the cone member  18  that communicates with the space  30   b ; and the coarse powder collecting port  30   a  at an end of the cone member  18 . 
         [0069]    The present invention may be configured so that a slit-type annular opening  28   a  formed between the upper and lower peripheral wall members  16   a  and  16   b  of the peripheral wall member  16  has a taper shape that widens toward the disc-like cavity serving as the centrifuge chamber  22  as is seen in a powder classifying apparatus  10 A in  FIG. 3 . In other words, the degree of opening of an entrance  28   b  of the slit-type annular opening  28   a  or the degree of opening of the annular opening  28   a  at its one end on the centrifuge chamber  22  side may be increased. 
         [0070]    In the powder classifying apparatus  10 A, coarse powder of large size can be moved to the slit-type annular opening  28   a  and drawn out of the centrifuge chamber  22  further smoothly, and thus the coarse powder can be smoothly taken out from the centrifuge chamber  22  serving as the classifying site. 
         [0071]    In the present invention, while powder is evenly supplied from the six powder supply ports  24   a  into the centrifuge chamber  22  as described with regard to the powder supply section  24  of the powder classifying apparatus  10  shown in  FIGS. 1 and 2 (A), the present invention is not limited thereto and powder may be conveyed by air flow through a powder supply section  25  having a powder supply port  25   a  opening toward one of the first air nozzles  32   a  of the nozzle member  32   b  and supplied into the centrifuge chamber  22  in a manner of using an ejector, as in the powder classifying apparatus  10 A shown in  FIG. 3 . 
         [0072]    The powder supply section  25  is composed of a hopper  25   b  having the powder supply port  25   a  at the lower end and storing powder. Powder in the hopper  25   b  is supplied, together with compressed air, from the powder supply port  25   a  at the lower end into the centrifuge chamber  22  by an ejector effect that occurs owing to compressed air in the first air nozzle  32   a . In the illustrated example, the powder supply section  25  is composed of the single hopper  25   b  having the single power supply port  25   a  but may be composed of a plurality of, e.g., six hoppers. 
         [0073]    While the fine powder collecting section  26  of the powder classifying apparatus  10  in  FIG. 1  is composed of a straight pipe having the same diameter as that of the fine powder collecting port  26   a , the present invention is not limited thereto and the fine powder collecting section  26  may be constituted of a portion having an inner diameter increasing to a diameter larger than the inner diameter of the fine powder collecting port  26   a  and a straight pipe portion having a large inner diameter as in the powder classifying apparatus  10 A shown in  FIG. 3 . 
         [0074]    The powder classifying apparatus according to the first embodiment of the invention is basically configured as above. 
         [0075]    Next, the operation of the powder classifying apparatus according to the first embodiment of the invention is described below. 
         [0076]    First, the suction blower  92  (see  FIG. 7 ) sucks air with a predetermined air volume from the inside of the centrifuge chamber  22  through the fine powder collecting port  26   a  of the fine powder collecting section  26 , and compressed air is supplied from the compressed air supply source  82  (see  FIG. 7 ) to each of the six first air nozzles  32   a  and each of the six second air nozzles  34   a  of the first and second air introducing sections  32  and  34 , whereby symmetrical whirling gas streams are generated at the upper and lower portions in the centrifuge chamber  22  and consequently, a whirling gas stream is generated in the whole centrifuge chamber  22 . 
         [0077]    Under this condition, powder having a particle size distribution as conveyed by air flow from the distributor (see  FIG. 7 ) is supplied through the six powder supply ports  24   a  of the powder supply section  24  at a predetermined flow rate, and subsequently, the powder is supplied, together with air, from an obliquely upward position to the upper portion in the centrifuge chamber  22  in the same direction as the whirling direction of the whirling gas stream, is exposed to the whirling gas stream to exhibit whirling motion, and whirls with the whirling gas stream in the centrifuge chamber  22 . 
         [0078]    Since the whirling gas streams which are vertically symmetrical to each other are generated in the centrifuge chamber  22  by ejection of compressed air from the first and second air nozzles  32   a  and  34   a , the powder is, while whirling, affected by centrifugal action in the centrifuge chamber  22 . 
         [0079]    As a result, fine powder having a size not larger than a classification point is sucked and discharged together with an air stream through the fine powder collecting port  26   a  by the aid of the ring-shaped edge portions  12   c  and  14   c  formed at the central part of the whirl in the centrifuge chamber  22 , and collected by the appropriate fine powder collecting filter  90  (see  FIG. 7 ) such as a bag filter. Thus, the fine powder can be sorted from the powder having a particle size distribution and collected. The thus collected fine powder scarcely contains coarse powder having a particle size larger than the classification point. 
         [0080]    On the other hand, coarse powder having a diameter larger than the classification point is affected by a large centrifugal force, is therefore smoothly moved radially outward of the whirling gas stream, smoothly enters the slit-type annular opening  28  of the coarse powder collecting section  30  formed at the middle, in the vertical direction, of the centrifuge chamber  22 , passes the space  30   b  and the internal space  18   a  of the cone member  18 , and is discharged through the coarse powder collecting port  30   a  to be collected. 
         [0081]    The remaining powder having not been discharged through the fine powder collecting port  26   a  or the slit-type annular opening  28  moves down to the lower portion in the centrifuge chamber  22 . Since the remaining powder often contains not only coarse powder having a diameter larger than the classification point but also fine powder having a diameter not larger than the classification point, the powder joins the whirling gas stream generated by ejection of compressed air from the second air nozzles  34   a  to be moved up to the upper portion in the centrifuge chamber  22  and is affected by centrifugal action, whereby coarse powder and fine powder are efficiently separated by a centrifugal force, and the fine powder is discharged through the fine powder collecting port  26   a  to be collected while the coarse powder enters the slit-type annular opening  28  and is discharged through the coarse powder collecting port  30   a  to be collected, as described above. 
         [0082]    For classification with a lower classification point, that is, for sorting out finer particles, it is necessary to increase the speed of a whirling gas stream (whirl) generated in the centrifuge chamber. In the conventional powder classifying apparatus using the guide vanes as described in Patent Literature 1, it is possible to sort out finer particles by forcibly causing a high flow rate of air to enter the centrifuge chamber through air nozzles for use in powder dispersion which are disposed at an upper portion of the centrifuge chamber. In this case, however, an upper whirling gas stream (whirl) generated through the air nozzles and a whirling gas stream (whirl) generated through the guide vanes in the centrifuge chamber greatly differ from each other in speed, in other words, a nonuniform whirling gas stream having different speeds is generated in the centrifuge chamber. Consequently, as shown in  FIGS. 8(C) and 8(D) , powder is adhered to the bottom surface of the upper disc-like member and the top surface of the lower disc-like member, and the amount of adhered powder increases with increasing speed difference. In addition, since such a nonuniform whirling gas stream is generated in the centrifuge chamber, the classification accuracy is deteriorated and therefore, it is difficult to accurately sort out submicron particles having a size smaller than, for example, 1 μm. 
         [0083]    Furthermore, in the conventional powder classifying apparatuses described in Patent Literatures 1 and 2, powder is supplied through a single powder supply port from above to fall in the vertical direction with respect to a whirling gas stream in the centrifuge chamber. Therefore, even though the powder is dispersed by an upper whirling gas stream generated through the air nozzles, a whirling gas stream for classification in the centrifuge chamber is disturbed and as a result, a nonuniform whirling gas stream having different speeds is generated in the centrifuge chamber, leading to the adhesion of powder to the bottom surface of the upper disc-like member and deterioration of classification accuracy. 
         [0084]    Furthermore, in the conventional powder classifying apparatuses described in Patent Literatures 1 and 2, applied is a method of causing coarse powder to fall to a lower peripheral portion in the centrifuge chamber to collect the coarse powder, and in order to enhance the classification efficiency, powder containing fine powder as fallen to a powder reclassifying zone located at the lower peripheral portion in the centrifuge chamber is returned to the centrifuge chamber by air nozzles. Accordingly, coarse powder (coarse particles) stays near the top surface of the lower disc-like member due to air blown through these air nozzles, leading to the adhesion of powder as well as uneven adhesion. 
         [0085]    In contrast, in the powder classifying apparatus  10  in the first embodiment, no guide vanes are used, the first and second air introducing sections  32  and  34  respectively including the first and second air nozzles  32   a  and  34   a  are provided at the upper and lower portions of the annular peripheral wall member  16  in the circumferential direction at the outer peripheral portion of the centrifuge chamber  22  of substantially disc-like shape, and a high flow rate of compressed air is forcibly caused to enter through the first and second air nozzles  32   a  and  34   a  to generate high flow rates of symmetrical whirling gas streams at the upper and lower portions in the centrifuge chamber  22  to thereby generate a uniform whirling gas stream in the centrifuge chamber  22 ; a slit-type annular opening  28  is provided at the middle, in the vertical direction, of the annular peripheral wall member  16  to take out coarse powder from the side of the centrifuge chamber  22  to thereby smoothly discharge the coarse powder from the centrifuge chamber  22  serving as the classifying site; and powder conveyed by air flow through the evenly-arranged powder supply ports is supplied from an obliquely upward position with respect to the whirling gas stream in the centrifuge chamber  22  so as to move along the whirling direction of the whirling gas stream, thereby suppressing and minimizing the disturbance of the whirling gas stream in the centrifuge chamber  22 . 
         [0086]    In particular, since the whirling gas stream in the centrifuge chamber  22  is uniform and flows at a high flow rate, as shown in  FIGS. 8  (A) and (B), this configuration prevents the adhesion of powder to the bottom surface of the upper disc-like member or the top surface of the lower disc-like member and the deterioration of classification accuracy, whereby submicron particles can be accurately and stably sorted out. 
         [0087]    With the powder classifying apparatus in this embodiment, fine particles such as submicron particles can be efficiently sorted out despite its tendency to cohere. Examples of classifiable powders include various types of powders ranging from low specific gravity powders such as silica powder and toners to high specific gravity powders such as powders of metals and alumina. 
         [0088]    In addition, since a movable member such as guide vanes is not used, a compact powder classifying apparatus can be attained. 
         [0089]    Next, an exemplary configuration of a powder classifying apparatus according to another embodiment of the invention is described. 
         [0090]      FIG. 4  is a cross-sectional view schematically showing a powder classifying apparatus according to a second embodiment of the invention. 
         [0091]    A powder classifying apparatus  50  in the embodiment as shown in  FIG. 4  has the same configuration as that of the powder classifying apparatus  10  shown in  FIG. 1  except having an annular collecting container  52  at the bottom surface of the lower disc-like member  14 , and the same constituent elements are assigned by the same reference signs and will not be explained. Different points are mainly described. 
         [0092]    As compared to the powder classifying apparatus  10  shown in  FIG. 1 , the powder classifying apparatus  50  shown in  FIG. 4  further includes the annular collecting container  52  which serves as an intermediate powder collecting section for collecting, from coarse powder having a size larger than a classification point (particle size) for fine powder, intermediate powder having a size not larger than a second classification point (particle size) which is larger than the classification point for fine powder, i.e., a first classification point (particle size). 
         [0093]    The annular collecting container  52  is provided on the bottom surface (underside) of an annular region bridging the inner and outer edge portions  14   a  and  14   b  of the lower disc-like member  14 . An annular inclined opening  54  is provided at the inner edge portion  14   a  to allow the inside of the centrifuge chamber  22  to communicate with the inside of the collecting container  52 . The inclined opening  54  is a groove-shaped discharge path that is inclined toward the outer periphery (in the radial direction) from the position which is located on the lower disc-like member  14  on the side facing the centrifuge chamber  22  and which corresponds to a point where the slope of the inner portion  12   a  of the upper disc-like member  12  starts toward the outer periphery, and that then reaches the inside of the collecting container  52  to thereby allow the collecting container  52  to communicate with an inner side of the outer edge portion  14   b.    
         [0094]    Powder supplied into the centrifuge chamber  22  is centrifuged from the central part of a whirling gas stream toward the outer peripheral portion thereof and separated according to the particle size by the whirling gas stream in the centrifuge chamber  22 . Consequently, the powder is separated into fine powder with a small particle size which goes to the central part of the whirling gas stream, coarse powder with a large particle size, such as coarse particles, which goes to the outer peripheral portion of the whirling gas stream, and coarse powder with an intermediate particle size which goes to a region between the central part and the outer peripheral portion of the whirling gas stream. 
         [0095]    As a result, the fine powder with a size not larger than the predetermined particle size (first particle size) is discharged, together with sucked air, from the central part of the whirling gas stream through the fine powder collecting port  26   a , and the coarse powder with a size larger than the second particle size, such as coarse particles, is easily discharged from the outer peripheral portion of the whirling gas stream through the slit-type annular opening  28  by a centrifugal force. The coarse powder with an intermediate particle size which is larger than the first particle size but smaller than the particle size of the coarse powder having a size larger than the second particle size, i.e., intermediate powder is to be discharged from the outer peripheral portion of the whirling gas stream through the slit-type annular opening  28  by a centrifugal force at the end. However, the intermediate powder may be caused to repeatedly fall to the top surface of the lower disc-like member  14  and again float by air ejected by the second air nozzles  34   a , and accordingly stay in the centrifuge chamber  22  for longer periods of time compared to the fine powder and the coarse powder having a size larger than the second particle size, hampering the improvement of powder classification efficiency. 
         [0096]    To cope with it, there is provided the collecting container  52  having the inclined opening  54  located at the annular region where the intermediate powder is prone to gather, whereby the intermediate powder, which is prone to gather in the centrifuge chamber  22 , is aggressively collected to the collecting container  52  through the inclined opening  54  when falling to the top surface of the lower disc-like member  14 . This configuration enables the classification of powder newly supplied, further improving powder classification efficiency. 
         [0097]    The collecting container  52  having the inclined opening  54  constitutes a second collecting section of the invention. 
         [0098]    With this configuration, the powder classifying apparatus  50  in the second embodiment of the invention can accurately sort out fine powder over long periods of time without allowing powder to adhere to a wall surface of the centrifuge chamber  22 , particularly to an upper or lower wall surface thereof. 
         [0099]    The powder classifying apparatus  50  in this embodiment may also be provided with, instead of the slit-type annular opening  28 , the slit-type annular opening  28   a  having a taper shape that widens toward the centrifuge chamber  22  as in the powder classifying apparatus  10 A shown in  FIG. 3 . 
         [0100]    Next, an exemplary configuration of a powder classifying apparatus according to still another embodiment of the invention is described. 
         [0101]      FIG. 5  is a cross-sectional view schematically showing a powder classifying apparatus according to a third embodiment of the invention. 
         [0102]    A powder classifying apparatus  60  in the embodiment shown in  FIG. 5  has the same configuration as that of the powder classifying apparatus  10  shown in  FIG. 1  except having an intermediate powder collecting section  62  at the outside of the fine powder collecting section  26  at the center of the upper disc-like member  12 , and the same constituent elements are assigned by the same reference signs and will not be explained. Different points are mainly described. 
         [0103]    As compared to powder classifying apparatus  10  shown in  FIG. 1 , the powder classifying apparatus  60  shown in  FIG. 5  includes the upper disc-like member  12  of integral type and the fine powder collecting section  26  which has an end portion being a ring-shaped edge  12   c  projecting toward the centrifuge chamber  22  at the center of the upper disc-like member  12  and which is composed of an inner tube (inner cylindrical tube)  26   d  constituting the fine particle collecting port  26   a , and further includes, at the outside of the fine particle collecting port  26   a  of the fine particle collecting section  26 , the intermediate powder collecting section  62  composed of an opening  64  of the upper disc-like member  12  and an outer tube (outer cylindrical tube)  62   b , the opening  64  constituting an intermediate powder collecting port  62   a  for collecting intermediate powder having a size not larger than the above-described second classification point (particle size). 
         [0104]    The outer tube  62   b  of the intermediate powder collecting section  62  is connected to extend from the opening  64  of the upper disc-like member  12  so as to have the same diameter as that of the opening  64 , and constitutes a double tube together with the inner tube  26   d  of the fine powder collecting section  26 . The intermediate powder collecting port  62   a  of the intermediate powder collecting section  62  is formed between the inner surface of the central opening  64  of the upper disc-like member  12  and the outer tube  62   b  and the outer surface of the inner tube  26   d  being the fine powder collecting port  26   a  of the fine powder collecting section  26 . The end of the intermediate powder collecting port  62   a  is constituted by the central opening  64  of the upper disc-like member  12  and positioned above an opening at the end of the inner tube  26   d  being the fine powder collecting port  26   a  of the fine powder collecting section  26 . In other words, the end of the inner tube  26   d  being the fine powder collecting port  26   a  projects beyond the end of the intermediate powder collecting port  62   a  toward the centrifuge chamber  22  and forms the ring-shaped edge  12   c.    
         [0105]    The intermediate powder collecting port  62   a  is connected to a suction blower (not shown) via an appropriate intermediate powder collecting filter (not shown) such as a bag filter, as with the fine powder collecting port  26   a.    
         [0106]    As described above, intermediate powder which is prone to gather in a whirling gas stream in the region between the central part and the outer peripheral portion of a whirling gas stream is collected together with sucked air through the intermediate powder collecting port  62   a . This configuration enables the classification of powder newly supplied, further improving powder classification efficiency. 
         [0107]    Thus, the powder classifying apparatus  60  in the third embodiment of the invention can accurately sort out fine powder over long periods of time without allowing powder to adhere to a wall surface of the centrifuge chamber  22 , particularly to an upper or lower wall surface thereof, as with the powder classifying apparatus  50  in the second embodiment described above. 
         [0108]    The powder classifying apparatus  60  in this embodiment may also be provided with, instead of the slit-type annular opening  28 , the slit-type annular opening  28   a  having a taper shape that widens toward the centrifuge chamber  22  as in the powder classifying apparatus  10 A shown in  FIG. 3 , the annular collecting container  52  for collecting intermediate powder as in the power classifying apparatus  50  shown in  FIG. 4 , or both thereof. 
         [0109]    Next, an exemplary configuration of a powder classifying apparatus according to still another embodiment of the invention is described. 
         [0110]      FIG. 6  is a cross-sectional view schematically showing a powder classifying apparatus according to a fourth embodiment of the invention. 
         [0111]    A powder classifying apparatus  70  in the embodiment shown in  FIG. 6  has the same configuration as that of the powder classifying apparatus  10  shown in  FIG. 1  except having a second fine powder collecting section  72  located at the center of the lower disc-like member  14  to correspond to the fine powder collecting section  26  at the center of the upper disc-like member  12 , and the same constituent elements are assigned by the same reference signs and will not be explained. Different points are mainly described. 
         [0112]    As compared to the powder classifying apparatus  10  shown in  FIG. 1 , the powder classifying apparatus  70  shown in  FIG. 6  includes the second fine powder collecting section  72  having a second fine powder collecting port  72   a  at the center of the inner edge portion  14   a  of the lower disc-like member  14  to be symmetrical to the fine powder collecting port  26   a  of the fine powder collecting section  26  at the center of the upper disc-like member  12 . Needless to say, the end of an opening portion of the fine powder collecting port  72   a  projects toward the centrifuge chamber  22  and forms the ring-shaped edge  14   c.    
         [0113]    While the fine powder collecting port  72   a  of the second fine powder collecting section  72  on the lower side of the centrifuge chamber  22  is symmetrical to the fine powder collecting port  26   a  of the fine powder collecting section  26  on the upper side of the centrifuge chamber  22 , an opening constituting the second fine powder collecting port  72   a  at the center of the inner edge portion  14   a  of the lower disc-like member  14  is connected to an extension cylindrical tube  72   b . The extension cylindrical tube  72   b  extends vertically downward, then is bent and horizontally extends outward of the cone member  18 , and is finally connected to, for instance, the suction blower  92  through the filter  90 . 
         [0114]    Thus, the fine powder collecting port  26   a  and the second fine powder collecting port  72   a  are arranged to be symmetrical with respect to the centrifuge chamber  22 . This configuration enhances symmetry properties of upper and lower whirling gas streams in the centrifuge chamber  22  and accordingly, a more uniform whirling gas stream can be achieved in the whole centrifuge chamber  22 . 
         [0115]    As a result, the powder classifying apparatus  70  in the fourth embodiment of the invention can accurately sort out fine powder over long periods of time without allowing powder to adhere to a wall surface of the centrifuge chamber  22 , particularly to an upper or lower wall surface thereof. 
         [0116]    The powder classifying apparatus  70  in this embodiment may also be provided with, instead of the slit-type annular opening  28 , the slit-type annular opening  28   a  having a taper shape that widens toward the centrifuge chamber  22  as in the powder classifying apparatus  10 A shown in  FIG. 3 , at least one of the annular collecting container  52  and the intermediate powder collecting section  62  for collecting intermediate powder as in the power classifying apparatuses  50  and  60  shown in  FIGS. 4 and 5 , or all thereof. 
         [0117]    The various powder classifying apparatuses in the embodiments of the invention described above can each constitute a classifying system shown in  FIG. 7 . 
         [0118]      FIG. 7  is a schematic view showing a configuration of a whole classifying system using any of the powder classifying apparatuses of the invention. 
         [0119]    The classifying system  80  shown in the drawing includes the powder classifying apparatus  10  in the first embodiment shown in  FIG. 1 , the compressed air supply source  82  for supplying compressed air to the pluralities of air nozzles  32   a  and  34   a  of the air introducing sections  32  and  34  of the powder classifying apparatus  10 , the distributor  84  for conveying, by air flow, powder to be classified to the powder supply ports  24   a  of the powder supply section  24  of the powder classifying apparatus  10 , a screw feeder  86  for supplying the powder to the distributor  84 , a compressed air supply source  88  for supplying compressed air used by the distributor  84  to convey by air flow the powder supplied from the screw feeder  86 , the fine powder collecting filter  90  such as a bag filter for collecting fine powder discharged through the fine powder collecting port  26   a  of the fine powder collecting section  26  of the powder classifying apparatus  10 , the suction blower  92  for sucking air containing fine powder through the fine powder collecting port  26   a , an orifice  94  disposed between the filter  90  and the suction blower  92  to measure the flow rate of air sucked by the suction blower  92 , a display  96  for displaying the air flow rate measured at the orifice  94 , and piping that constitutes pipelines interconnecting these components. 
         [0120]    In the classifying system  80 , firstly, compressed air is supplied from the compressed air supply source  82  to the air nozzles  32   a  and  34   a  of the air introducing sections  32  and  34  of the powder classifying apparatus  10  through the piping so that the compressed air is ejected into the centrifuge chamber  22  of the powder classifying apparatus  10  whereby symmetrical whirling gas streams are separately generated in the upper and lower portions to generate a uniform whirling gas stream in the whole centrifuge chamber  22 . 
         [0121]    Secondly, compressed air is supplied from the compressed air supply source  88  to an ejector  84   a  of the distributor  84  through the piping while powder is supplied from the screw feeder  86  to the distributor  84  so that the powder joins the compressed air ejected from the ejector  84   a  to be conveyed by air flow through the piping, and the powder conveyed by air flow is supplied to the powder supply ports  24   a  of the powder supply section  24  of the powder classifying apparatus  10  and ejected from an obliquely upward position toward the whirling gas stream in the centrifuge chamber  22  so as to move along the whirling direction of the whirling gas stream. 
         [0122]    The powder ejected into the centrifuge chamber  22  together with air is centrifuged by the whirling gas stream in the centrifuge chamber  22 . Fine powder is sucked together with air by the suction blower  92  through the fine powder collecting port  26   a  of the fine powder collecting section  26  of the powder classifying apparatus  10  and the piping, and then collected by the filter  90 . 
         [0123]    On the other hand, coarse powder is discharged through the slit-type annular opening  28  of the coarse powder collecting section  30  of the powder classifying apparatus  10 , passes the space  30   b  and the internal space  18   a  of the cone member  18 , and is collected through the coarse powder collecting port  30   a.    
       Example 
       [0124]    The powder classifying apparatus of the invention is specifically described below based on an example. 
         [0125]    For the example, a classification test was carried out by supplying metal powder having a median diameter of up to 1 μm in an amount of 1 kg/h in the classifying system  80  shown in  FIG. 7  employing the powder classifying apparatus  10  shown in  FIG. 1 . 
         [0126]    The disc-like cavity of the centrifuge chamber  22  of the powder classifying apparatus  10  had a diameter of 174 mm. 
         [0127]    The powder was supplied evenly through the six powder supply ports  24   a  into the centrifuge chamber  22  from an obliquely upward position with respect to a whirling gas stream. The powder supply was 1 kg/h as a whole. 
         [0128]    The amount of air sucked by the suction blower  92  was 2.5 m 3 /min, the ejection pressure of each of the upper and lower air nozzles  32   a  and  34   a  was 0.58 MPa, and the ejection rate thereof was 430 L/min. 
         [0129]    After the metal powder classification test carried out for 1 hour, the bottom surface of the upper disc-like member  12  (inner member) and the top surface of the lower disc-like member  14  were inspected. 
         [0130]    As a result, no adhesion of powder on the bottom surface of the upper disc-like member  12  and the top surface of the lower disc-like member  14  was found at all as shown in  FIGS. 8(A) and 8(B) . 
         [0131]    For a comparative example, a classification test was carried out by supplying metal powder having a median diameter of up to 1 μm in an amount of 1 kg/h in the classifying system  80  shown in  FIG. 7  employing a powder classifying apparatus shown in FIG. 1 of Patent Literature 1 in place of the powder classifying apparatus  10  shown in  FIG. 1 . 
         [0132]    A disc-like cavity of a centrifuge chamber of the powder classifying apparatus had a diameter of 174 mm. 
         [0133]    The powder was supplied into the centrifuge chamber through a single powder supply port from above to naturally fall in the vertical direction with respect to a whirling gas stream in the centrifuge chamber. The powder supply was 1 kg/h. 
         [0134]    The amount of air sucked by a suction blower was 2.0 m 3 /L/min; the ejection pressure and the ejection rate of each upper air nozzle for dispersing powder were respectively 0.65 MPa and 510 L/min, the ejection pressure and the ejection rate of each lower air nozzle for reclassification were respectively 0.5 MPa and 180 L/min, and the amount of air entering through guide vanes was 100 L/min. 
         [0135]    After the metal powder classification test carried out for 1 hour, the bottom surface of an upper disc-like member and the top surface of a lower disc-like member were inspected. 
         [0136]    As a result, it was seen that a considerable amount of powder was adhered to the upper disc-like member as shown in  FIG. 8(C) . It was also seen that powder was slightly adhered to the top surface of the lower disc-like member as shown in  FIG. 8(D)   
         [0137]    From the foregoing results, the effects of the invention are apparent. 
         [0138]    The embodiments and examples described above each illustrate one example of the invention and the invention is not limited thereto. It should be understood that various improvements and modifications are possible without departing from the scope and spirit of the invention. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               10 ,  10 A,  50 ,  60 ,  70  powder classifying apparatus 
               12  upper disc-like member 
               12   c ,  14   c  ring-shaped edge 
               14  lower disc-like member 
               16  peripheral wall member 
               18  cone member 
               20  casing 
               22  centrifuge chamber 
               24 ,  25  powder supply section 
               24   a ,  25   a  powder supply port 
               26 ,  72  fine powder collecting section 
               26   a ,  72   a  fine powder collecting port 
               28 ,  28   a  slit-type annular opening 
               30  coarse powder collecting section 
               30   a  coarse powder collecting port 
               32 ,  34  air introducing section 
               32   a ,  34   a  air nozzle 
               52  annular collecting container 
               62  intermediate powder collecting section 
               62   a  intermediate powder collecting port