Method and Apparatus for Classifying Particles

This invention relates to method and apparatus for classifying particles.

Various types of methods and apparatus for classifying particles have been proposed which classify fine particles according to their grain size. Among these prior art methods and apparatus, the one having the following construction can classify at a high efficiency. This type of apparatus comprises a cylindrical housing, a rotary disc including rotating classifying members mounted on the top of the cylindrical housing for classifying fine particles, means for creating a whirling upward flow of air in the cylindrical housing, means for dispersing fine particles to be classified in the upward whirling flow, and means located at the bottom of the cylindrical housing for taking out classified fine particles. With this type of apparatus, since the whole interior of the cylindrical housing is used to classify the particles according to the whirling upward flow and the gravity, fine particles can be efficiently classified according to their grain size. Such a prior art method and apparatus, however, requires independent driving mechanisms for dispersing the fine particles in the whirling upward flow and for classifying the particles, thus complicating the driving mechanism. Furthermore, disposition of various members in the cylindrical housing and movements of such members create turbulence in the classifying air which not only impairs the classifying effect but also increases the running cost.

Accordingly, it is an object of this invention to provide an improved method and apparatus capable of efficiently classifying particles according to their particle size having a simple construction.

Another object of this invention is to provide an improved method and apparatus capable of efficiently dispersing particles to be classified in the air in a classifying apparatus withut using an independent dispersing device.

According to one aspect of this invention there is provided a method of classifying particles according to their particle size of the type wherein fine particles are separated by fine particle separating means disposed in an upper portion of a vertical cylindrical housing and air containing the remaining fine particles is discharged out of the housing, the particles to be classified being dispersed in a whirling upward flow of air created at a lower portion of the housing for separating coarse particles, and the separated coarse particles are discharged out of the housing from the bottom of the housing, characterized by the steps of supplying upwardly the air together with the particles to be classified, converting the air containing the particles into the whirling upward flow at the lower portion of the housing, radially ejecting air at a portion beneath the whirling upward flow to separate and deposit separated particles on an inner surface of the housing, separating fine particles from the layer of deposited particles with the radially ejected air for conveying upwardly separated fine particles by the whirling upward flow of the air, causing the coarse particles remaining on the inner surface to fall down under gravity, and discharging the fallen coarse particles out of the housing through the bottom thereof.

According to another aspect of this invention, there is provided a particle classifying apparatus of the type wherein a fine particle separating means is provided in an upper portion of a vertical cylindrical housing to discharge air containing the remaining fine particles out of the housing, with means provided at a lower portion of the housing for forming a whirling upward flow of the air in the housing and for separating coarse particles, and means provided at the bottom of the housing for discharging separated coarse particles characterized in that there are provided vertical pipe means which convey upwardly air together with the particles to be classified, whirling upward flow-forming nozzle means concentric with the housing and connected to an upper end of the vertical pipe means, coarse particle classifying means located beneath the whirling flow forming nozzle means and having a larger diameter than the nozzle means, and means for supplying air to the coarse particle classifying means, the air ejected by the coarse particle classifying means being directed to an inner surface of the housing for separating and blowing upwardly fine particles from a layer of particles deposited on the inner surface of the housing.

Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which

FIG. 1 is a perspective view, partly broken away, showing one embodiment of the classifying apparatus according to this invention;

FIG. 2 is a cross-sectional view showing a nozzle for forming a whirling upward flow;

FIG. 3 is a cross-sectional view showing a coarse particle classifying mechanism;

FIG. 4 is a partial longitudinal sectional view showing the nozzle and the coarse particle classifying mechanism shown in FIGS. 2 and 3;

FIG. 5 is a perspective view, partly broken away, showing a modified embodiment of this invention;

FIG. 6 is a partial vertical view similar to FIG. 4;

FIG. 7 is a cross-sectional view showing another embodiment of this invention in which both halves are cut at different levels;

FIG. 8 is a partial longitudinal sectional view showing the modified embodiment shown in FIG. 7;

FIG. 9 is a cross-sectional view similar to FIG. 7 and showing still another embodiment of this invention;

FIG. 10 is a partial vertical sectional view similar to FIG. 8;

FIG. 11 is a plan view showing yet another embodiment of this invention comprising two vertical cylinders;

FIG. 12 is an enlarged side view of the embodiment shown in FIG. 11; and

FIG. 13 is an enlarged sectional view useful to explain the interface layer separating effect of the coarse particle classifying mechanism on the wall surface of the cylindrical housing.

A preferred embodiment of this invention shown in FIGS. 1 through 4 comprises a cylindrical housing 10, transverse supporting beams 11 at the bottom of the housing 10, and an ejection mechanism 1 including a nozzle 12 supported by the supporting beams 11 and adapted to form a whirling air flow as shown in FIG. 2, and a coarse particle classifying mechanism shown in FIG. 3. The upper portion of the cylindrical housing is enlarged as at 10a and annular rings 7 are provided for the inner surface of the enlarged portion. Rotary radial classifying vanes 9 are located beneath the respective annular rings 7. The classifying vanes 9 are secured to the peripheries of the discs 8 mounted on a rotary shaft 20 to classify fine particles. The radial classifying vanes 9 may be secured to the peripheries of the discs 8 or pivotally secured thereto such that when the discs 8 rotate the classifying vanes will automatically assume radially extending positions.

Thus, the fine particle classifying mechanism is contained in the top portion of the cylindrical housing and the whirling flow created by the whirling flow forming nozzle 12 rises upwardly in the cylindrical housing. The whirling upward flow disperses and classifies the particles, and fine particles are separated by the rotating classifying vanes 9. Coarse particles are taken out through a discharge pipe 24 attached to one side of the inclined bottom plate 10b of the cylindrical housing 10. According to this invention there is provided an upward flow supply pipe 6 which vertically extends through the inclined bottom plate 10b, and through the coarse particle classifying mechanism 13, and the upper end of the pipe 6 opens at the central portion of the whirling flow forming nozzle 12. The particles to be classified are supplied into the cylindrical housing 10 through the pipe 6 together with fluid, usually air. The particles are dispersed by the nozzle 12. Coarse particles are classified by the coarse particle classifying mechanism 13 and fall down along the inner surface of the housing 10.

More particularly, an intermediate plate 14 is interposed between the whirling flow forming nozzle 12 and the coarse particle classifying mechanism 13. The whirling flow forming nozzle 12 is provided with a plurality of guide vanes 16 between its top plate 15 and an intermediate plate 14, the guide vanes 16 being equally spaced in the circumferential direction and considerably inclined with respect to the radial direction. Accordingly, the air and the particles contained therein supplied through the pipe 6 pass through the gaps between the guide vanes 16 in directions shown by arrows in FIG. 2. The air thus ejected forms a whirling upward flow along the inner surface of the cylindrical housing 10. As shown in FIG. 1, the diameter of the top plate 15 is smaller than that of the intermediate plate 14. Moreover, as shown in FIGS. 1 and 4, the guide vanes 16 are inclined with respect to the vertical so as to readily form the whirling upward flow.

The coarse particle classifying mechanism 13 disposed beneath the intermediate plate 14 is formed with a distributing chamber 17 at its bottom portion, and air is tangentially blown into the distributing chamber 17 as shown in FIG. 3. through a pipe 4. As shown in FIG. 3, circumferentially spaced guide vanes 19 which are parallel with the guide vanes 16 are disposed between the intermediate plate 14 and a bottom plate 18 which is parallel therewith. As shown, the guide vanes 16 and 19 are suitably curved for controlling the direction of ejection of a mixture of air and particles or air,, respectively.

The classifying apparatus shown in FIGS. 1 to 4 may be modified as shown in FIGS. 5 and 6. In this modification, the height of the whirling flow forming nozzle 12 is made to be smaller than that shown in FIGS. 1 through 4. Thus, the guide vanes 16 are inclined only with respect to the radial direction and are not inclined with respect to the vertical. A fine particle discharge pipe 21 is bent at right angles and driving means 22, for example an electric motor, is mounted on the discharge pipe 21 for rotating the discs 8. The other mechanisms are identical to those shown in FIGS. 1 through 4.

In the embodiment shown in FIGS. 1 through 6, the guide vanes of the coarse particle classifying mechanism may be omitted. Such simplified construction is shown in FIGS. 7 and 8, in which the pipe 4 admits air tangentially into the distribution chamber 17 which is disposed eccentrically with respect to the pipe 6 so that the radial sectional area of the distribution chamber decreases gradually from the inlet opening of the pipe 4. With this construction, the admitted air whirls in the distribution chamber and then radially blown outwardly through an annular opening 13 connected to the upper periphery of the distributing chamber 17. The eccentric arrangement of the distributing chamber 17 about the central pipe 6 ensures uniform discharge of the air throughout the entire periphery of the distributing chamber 17.

Also the guide vanes 16 may be omitted from the whirling flow forming nozzle 12. In still another modification of this invention shown in FIGS. 9 and 10, both guide vanes 16 and 19 of the whirling flow forming nozzle 12 and of the coarse particle classifying mechanism 13 are omitted. In this modification another distributing chamber 27 is added beneath the distributing chamber 17 and a pipe 6 for admitting a mixture of air and particles to be classified is tangentially connected to the additional distributing chamber 27. The radial sectional area of the distributing chamber 27 is also gradually decreased, in other words, the additional distributing chamber 27 is eccentrically disposed with respect to a central post 15a that supports the top plate so as to uniformly discharge the whirling flow of the mixture through an annular opening beneath the top plate 15, as shown by arrows.

Elimination of the guide vanes 16 and 19 not only simplifies the construction but also decreases pressure loss of the air and makes the flow thereof smoother.

When a plurality of classifying apparatus shown in FIGS. 1 through 10 are combined, their utility can be increased greatly. Such embodiment is shown in FIGS. 11 and 12 in which two classifying apparatus A and B are used to operate in cascade. More particularly, the apparatus A separates coarse particles, and a mixture of air and fine particles is introduced into the bottom of the other classifying apparatus B. Where two classifying apparatus A and B are connected in cascade, at least the apparatus B should have a construction according to this invention, whereas the other apparatus A may have a construction of this invention or any other construction. With this modification, the particles are subjected to multi-stage classifying treatment without requiring any energy for supplying the mixture from one classifying apparatus to the other.

Whether a single classifying apparatus is used as shown in FIGS. 1.about.10 or two cascade connected classifying apparatus are used as shown in FIGS. 11 and 12, a mixture of air and fine particles discharged through a discharge pipe 21 is conveyed to such well known fine particle removing apparatus as a cyclone separator C. In this case the air discharged from the top opening 31 of the cyclone 31 is conveyed, through a pipe 32, to the inlet port of a blower 33 which supplies compressed air to the inlet pipe 6 of the other classifying apparatus A for creating the whirling upward flow. With this construction, the air is circulated through both classifying apparatus A and B. Where only one classification apparatus is used the air discharged from the cyclone is recirculated through the classifying apparatus. Although the fine particles discharged through the discharge pipe 21 can be separated by means other than a cyclone such as by a back filter or an electric precipitator, it was found that use of a cyclone in combination with a blower is most effective for stably establishing the required pressure condition (usually a negative pressure) in the classifying apparatus A and/or B. It was also found that, without the cyclone, the pressure in the classifying apparatus varys substantially depending upon the condition of supplying the particles to be classified i.e., their quality, quantity, etc. and upon the temperature condition, and that such variation in the internal pressure affects the classifying efficiency, whereas when the cyclone is incorporated it was confirmed that the pressure variation was decreased to about 1/10 or less, meaning stable and efficient classification can be made according to this embodiment.

In the embodiment shown in FIGS. 11 and 12 a back filter 36 is connected to a discharge pipe 34 of the blower 33 via a conduit 35 including a valve V.sub.4 for removing fine particles floating in the circulating air, thus avoiding degradation of the classifying efficiency due to increase in the concentration of the fine particles in the circulating air.

The operation of the classifying apparatus shown in FIGS. 1 through 10 will be described hereunder by using concrete data. The air supplied to the pipe under a pressure 10.about.20 mmHg is ejected by the nozzle for form a whirling upward flow in the housing 10. The particles introduced into the housing 10 together with the air generally form layers along the inner surface of the housing 10 as shown in FIG. 13 due to the whirling upward flow. Thus, the grain size gradually decreases from the inner surface of the housing 10 toward its central portion, and the deposited layers of the coarse particles fall down under the gravity. The air is ejected against the lower portions of the particle layers by the coarse particle classifying mechanism 13 as shown in FIG. 13. The quantity of the air ejected by the coarse particle classifying mechanism is smaller by 1/2 (preferably 10 to 30%) than that ejected from the nozzle 12, but the speed of the air ejected by the mechanism 13 is higher by 4.about.38% (preferably 6 to 32%) than that of the air ejected by the nozzle 12. Layers of the fine particles are separated and blown upwardly by the air ejected by the coarse particle classifying mechanism 13 and by the whirling upward flow created by the nozzle 12.

The mechanism of classifying fine particles has been well known in the art. More particularly, as the diameter of the upper portion of the housing 10 is increased as shown in FIG. 1, the speed of air is decreased so that coarser particles among blown up particles are projected against the inner wall of the enlarged diameter portion to deposit thereon. As the air flow projected by the coarse particle classifying mechanism is reflected inwardly and upwardly as shown by an arrow shown in FIG. 4 the speed of air flow near the inner wall of the housing 10 is lower than that in the central portion. Consequently, the coarser particles deposited on the inner wall of the enlarged diameter portion fall down and are discharged through the pipe 24.

In the embodiment shown in FIGS. 11 and 12, the quantities and speeds of the air ejected from the coarse particle classifying mechanism 13 and the whirling upward flow forming nozzle 12 can be adjusted to any desired values by adjusting valves V.sub.1, V.sub.2, V.sub.3 and V.sub.4. Preferred pressure conditions for a cascade connection of two classifying apparatus A and B are as follows.

in apparatus A: -5 mmHg

in apparatus B: <200 mmHg

in cyclon separator C: -550 mmHg

in conduit 32 leading to blower 33: -600 mmHg

in discharge conduit 34 of blower: +20 mmHg

Of course, the pressure of the air circulating through the classifying apparatus A and B and the cyclone increases when the degree of opening of the valve V.sub.4 included in the pipe 35 leading to the back filter 36 is decreased and vice versa.

As above described, since ejected air is utilized for classifying coarse particles, the construction of the apparatus can be simplified. Since the course particle classifying mechanism 13 of the present invention is designed as being stationery, the quantity and pressure of the ejected air do not change so that the layers of the deposited particles are separated and fall down. Moreover, as the particles to be classified are supplied to a whirling upward flow forming device together with air supplied thereto no independent device is required for dispersing the particles to be classified in the apparatus which also simplifies the construction. Moreover, as there is no member that interferes or prevents the whirling upward flow, the efficiency of separation of the coarser particles in the region of the whirling upward flow can be improved. Where two or more classifying apparatus are cascade connected, not only the apparatus for supplying particles to be classified can be simplified, but also it is possible to classify the particles into 3 or more classes according to their particle size.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.