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An air sieving apparatus capable of sieving particles in a particle size range below 50 μm includes a cylindrical screening case defining an internal space and provided with a screen dividing the internal space into an upper space and a lower space. Powdered material is fed through a powdered materi

An air sieving apparatus capable of sieving particles in a particle size range below 50 μm includes a cylindrical screening case defining an internal space and provided with a screen dividing the internal space into an upper space and a lower space. Powdered material is fed through a powdered material feed opening onto a bottom plate disposed to cover the lower open end of the screening case with gaps formed therebetween. Air in the screening case is sucked upward by a suction blower. The fed powdered material is dispersed by currents of air sucked through the gaps and the dispersed powdered material is sucked upward. Fine particles that have passed through the screen 3 flow through a fine particle recovery duct into a cyclone and a fine particle container. Coarse particles remaining in the lower space drop through the coarse particle recovery opening into a coarse particle container.

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1. An air sieving method comprising the steps of: feeding a powdered material onto a single bottom plate disposed to cover a lower open end of a screening case, defining therein an internal space divided into an upper space and a lower space by a screen, with at least one gap formed in side parts of

1. An air sieving method comprising the steps of: feeding a powdered material onto a single bottom plate disposed to cover a lower open end of a screening case, defining therein an internal space divided into an upper space and a lower space by a screen, with at least one gap formed in side parts of the screening case between a lower end surface of the screening case and the bottom plate;generating ascending air currents in the upper space of the screening case by suction produced by a suction device;generating currents of outside air entering the screening case through the at least one gap in the side parts of the screening case into the lower space by the ascending air currents in the upper space, wherein the screening case is configured so that the currents of outside air enter the screening case in a direction substantially parallel to the bottom plate;causing the currents of outside air sucked into the lower space to flow between the bottom plate and the screen to produce swirling air currents in the lower space;causing the powdered material fed onto the bottom plate to be carried and dispersed by the swirling air currents in the lower space; andcausing the dispersed powered material to flow through the screen into the upper space to sieve the dispersed powdered material. 2. The air sieving method according to claim 1, wherein the swirling air currents include upward air currents extending to a lower surface of the sieving screen, air currents extending along the lower surface of the sieving screen, and downward air currents extending away from the sieving screen along an inner surface of the screening case. 3. An air sieving apparatus comprising: a cylindrical screening case defining an internal space therein and provided with a screen dividing the internal space into an upper space and a lower space;a single bottom plate disposed to cover a lower open end of the lower space of the cylindrical screening case, with at least one gap formed in a side part of the screening case between a lower end surface of the screening case and the bottom plate, wherein the screening case is configured so that air enter the screening case through said at least one gap travels in a direction substantially parallel to the bottom plate;a material feed device having a material feed opening which opens into the lower space of the screening case, to feed a powdered material onto the bottom plate; anda suction device for generating ascending air currents in the upper space of the screening case in such a manner as to cause outside air to be sucked into the lower space through the gap, to flow over the bottom plate to induce currents and dispersion of the powdered material fed onto the bottom plate, and to cause air in which the powdered material is dispersed to be sucked upward through said screen. 4. The air sieving apparatus according to claim 3, further comprising: a fine particle recovery device disposed between the screen and the suction device to recover fine particles being sucked by the suction device; anda coarse particle container protruding downward from a coarse particle recovery opening formed in the bottom plate. 5. The air sieving apparatus according to claim 4, wherein the lower space in the screening case is defined by a lower case having the shape of a circular cylinder, the upper space in the screening case is defined by an upper case having the shape of an upwardly tapered cone, the material feed device is formed in one of the lower case and the bottom plate, and the coarse particle recovery opening formed in the bottom plate is on a center axis of the cylindrical lower case. 6. The air sieving apparatus according to claim 4, wherein: the screening case has the shape of a longitudinally elongate rectangular cylinder,the bottom plate has the shape of a longitudinally elongate rectangle, the bottom plate is disposed to cover the lower open end of the screening case with at said least one gap formed between the bottom plate and the lower open end of the screening case,an upper open end of the screening case is covered with a top plate having the shape of a longitudinally elongate rectangle, the material feed device is formed in one of a front wall of the longitudinally elongate lower case and a front end part of the bottom plate,the suction device sucks air in the screening case upward from a rear end part of the longitudinally elongate top plate,and the coarse particle recovery opening is formed in a rear end part of the longitudinally elongate bottom plate. 7. The air sieving apparatus according to claim 6, wherein the screening case and the bottom plate are declined rearward. 8. An air sieving method comprising the steps of: feeding a powdered material onto a single bottom plate disposed to cover a lower open end of a screening case defining therein an internal space divided into an upper space and a lower space by a screen with at least one gap formed in side parts of the screening case between a lower end surface of the screening case and the bottom plate;generating ascending air currents in the upper space of the screening case by suction produced by a suction device; generating currents of outside air through the at least one gap in the side parts of the screening case into the lower space by the ascending air currents in the upper space; causing the currents of outside air sucked into the lower space to flow between the bottom plate and the screen to produce swirling air currents in the lower space; causing the powdered material fed onto the bottom plate to be carried and dispersed by the swirling air currents in the lower space; and causing the dispersed powdered material to flow through the screen into the upper space to sieve the dispersed powdered material;wherein oppositely located gaps are provided in opposite side parts of the screening case and the currents of outside air sucked through the oppositely located gaps flow to merge with each other over the bottom plate. 9. An air sieving apparatus comprising: a cylindrical screening case defining an internal space therein and provided with a screen dividing the internal space into an upper space and a lower space;a single bottom plate disposed to cover a lower open end of the lower space of the cylindrical screening case with at least one gap formed in a side part of the screening case between a lower end surface of the screening case and the bottom plate;a material feed device having a material feed opening which opens into the lower space of the screening case, to feed a powdered material onto the bottom plate; anda suction device for generating ascending air currents in the upper space of the screening case in such a manner as to cause outside air to be sucked into the lower space through the gap, to flow over the bottom plate to induce currents and dispersion of the powdered material fed onto the bottom plate, and to cause air in which the powdered material is dispersed to be sucked upward through said screen;wherein oppositely located gaps are provided in opposite side parts of the screening case and said suction device is configured to cause the outside air sucked through the oppositely located gaps to merge with each other over the bottom plate.