US2051107A - Process and apparatus for air classification - Google Patents

Description

Aug. 18, 1936. RKROURK; ,-051,1o7.

PROCESS AND APPARATUS FOR AIR CLASSIFICATION Filed Dec. 17, 1951' INVENTOR Rupert lfg fiourjae.

Patented Aug. 18, 1936 UNITED STATES PROCESS AND APPARATUS FOR AIR CLASSIFICATION Rupert K. Rourke, Lompoc, CaliL, asslgnor to Johns-Manville Corporation, New York, N. Y., a corporation of New York Application December 17, 1931, Serial No. 581,639

6 Claims. (Cl. 209-145) This invention relates to a process and apparatus for the separation, by air classification, o! a relatively small coarse fraction from a finely divided powdered material, such as comminuted diatomaceous earth or fullers earth. In this process the powdered material is thoroughly dispersed in a rapidly moving air stream which is caused to change direction suddenly. The larger particles, because their mass is relatively large in proportion to their cross-sectional area, oiler greater resistance to change in direction of motion than does the main body of fine particles and are thereby separated from the air stream containing the main body of fine particles. A feature of this invention in its preferred embodiment is a secondary air stream which is caused to strike the primary air stream containing the suspended particles more or less at a right angle, at a point near or preferably just beyond the point of deflection and in that direction toward which the primary stream is defiected. This secondary air stream serves to wash out from the coarse particles a certain amount of desirable fines which would otherwise be included in the coarse fraction and thereby produces a sharper separation than could be obtained otherwise.

This invention relates particularly. to the removal of a small proportion of grit and coarse agglomerates, such as would be retained on a 150 mesh screen, from a finely divided diatomaceous earth powder or the like. A finely divided material of this nature ofiers unusual difiiculties to classification by usual air separation methods, because of its extreme fineness, because all of the particles are of essentially the same density, and because of the fact that such particles show a pronounced tendency to agglomerate to form larger particles. In this invention, effective dispersion of all the particles is maintained because of the large volume of air and the high velocity at which it is moving. Effective separation is secured by reason of the large difierences in momentum of coarse and fine particles moving at this high velocity and also because of the secondary transverse air stream which washes out from the coarse particles a large proportion of such fine particles as have persisted undeflected in their direction of motion and which would otherwise be included in the coarse fraction. This separation, furthermore, is accomplished without substantial injury to the structure of the particles, which point is particularly important in the case of diatomaceous earth filter aids, fillers, etc. in which the utility of the product depends to a large degree on preservation of the characteristic delicate diatomaceous structure.

An apparatus by which I may produce such a separation of flne from coarse material in milled diatomaceous earth, for example, is shown in Figs. 1, 2, and 3.

Fig. 1 represents a side elevation of the apparatus, Fig. 2 a vertical section on line 22 of Fig. 3, and Fig. 3 a sectional plan view on line 3-3 of Fig. 2. In the figures, like reference numbers indicate like parts.

In Fig. 1, I indicates the intake, with adjustable damper la' through which milled or partially milled mineral, as, for example, diatomaceous earth is fed to a high speed blower 2 or other means for producing and delivering, at high speed, an aeriform suspension of a mineral powder, through the connecting pipe 3, within the narrow end of the truncated cone 4. 5 is a closed cone disposed centrally within said truncated cone and defining therewith an annular opening between the circumferences of their bases. The inner cone 5 is supported by the rod 6 which fits, at the lower end, adjustably into a seat or stufiing box I and, at the upper end, into the spider 8 provided with adjusting nut 8a. This inner cone has its base at approximately the same level as or slightly higher than that of the outer cone and has a more narrow base and flatter pitch than the outer cone. 9 is a cylindrical hopper communicating with the said annular opening between the two cones and having an opening ill at its bottom which is closed by the gate ll. I2 is a funnel-shaped member resting in a tee connection to the exhaust pipe l3 and held adjustably therein by the set screws I 2a and l2b, which may be reached through the handhole II with cover plate l8. The holes M are secondary or lateral air inlets or ports for admitting a lateral aeriform current adjacent to the annular opening between the two cones. The size of these ports may be adjusted by sliding the diaphragm I 4a upward or downward. I 5 is an inverted, relatively fiat cone closing the bottom of cone 5. I6 is an exhaust blower, which with its connections and adjustable damper lBa, provides exhausting means with entrance thereto disposed within the hopper 9 adjacent to the annular opening between the bases of the said cones and adapted to defiect inwardly the aeriform stream issuing therefrom, create a partial vacuum within the cyclone, and draw air in thru the lateral air ports It in the hopper 9. As stated, the funnel member I2 is adjustable, upward or downward; it may be made to open just below the annular space between the two cones 4 and 5. The entrance to the funnel or connection to the exhauster fan is suitably of diameter somewhat smaller than that of the said annular space between the two cones and is disposed below the said space. Also, the proportion of primary air coming in with the feed,

through pipe 3, and secondary air admitted laterally, through the openings l4, may be varied by varying the size or speed oi the fans 2 and IS, the number and size of the openings [4, the setting of one or both of the dampers, and also the position of the inner cone with respect to the outer cone, by moving the supporting rod 6 upward or downward.

In the operation of the device, primary air car-- rying the particles to be classified, passed through a blower 2 or other means to form an intimate suspension of the particles in the air. The resulting mixture enters the conical member 4 through pipe 3, passes down through the space between the outer cone 4 and the inner cone 5 and is there shaped into an annular form moving in substantially non-sinuous manner and is then drawn under the base of cone 5 into the funnelshaped member II, by inward suction produced by the exhauster fan l6 connected to the pipe l3. This causes sharp deflection of the stream of annular cross section towards the direction of radii of the said cross section, as, for example, towards the inside of the annular band, as illustrated. This deflection is flatwise of the band, that is, in the direction (towards the direction) of the shortest dimension of the stream in any localized area. Secondary air is drawn in through the lateral openings l4, suitably at almost a right angle to the original direction of the primary air stream, and passed also into the funnel l2. Coarse particles continue downward, outside the funnel member l2, and are caught in hopper 9, while the aeriform stream and flne particles are deflected, as for example, inwardly at an angle of less than 90 from the direction in which the stream issues from the annular opening, into the funnel member I! and thence through pipe l3 and the fan [6 to a suitable cyclonic dust collector (not shown).

The apparatus is suitably arranged so that the cones 4 and 5 (Fig. 1) are in an upright position. In such a case the narrow end or the truncated cone becomes the upper end.

For continuous operation gate l may be replaced by a rotary seal.

As stated, I have found this invention useful in classifying comminuted or milled mineral substances. Thus, I have classified satisfactorily a diatomaceous earth that was calcined subsequent to being milled to a degree of fineness approximating that of the filter-aid known as Filter- Cel. I have also classified a diatomaceous product consisting of calcined milled diatomaceous earth and corresponding closely to the commercial product known as Hyflo Super-Gel, particularly the Hyflo Super-Gel" as first made and containing a certain percentage of relatively coarse glassy aggregates or of soft lumps. The invention has been found especially useful in separating particles too large to pass through a 150- mesh screen from those that are finer.

The thoroughness of separation of fines from coarse may be controlled by changing (a) the proportion of secondary and primary air, as described above, (12) the width of the annular opening between cones 4 and 5, (c) the width of opening between the cone and the funnel member l2, (d) the position of the latter member with respect to that of the air ports l4, (e) the speed of movement of the aeriform suspension, and (f) the proportion of dust in-the said suspension. In general, thoroughness of separation of coarse particles from the fines in the primary aeriform stream is favored, for example, by a high pro-, portion of primary to secondary air, narrow openings only between the bases of cones 4 and 5 and between the base of the inner cone and the top of the funnel member, and a high speed oi movement of the aeriform suspension. The proportion of flnes going with coarse material, on the other hand, is decreased by thoroughly washing the separated stream of coarse particles with the lateral air stream.

The following details or construction and operation illustrate a'speciflc example of the practice of my invention. Into an outer truncated cone, of vertical height 90 inches and of diameter 18 inches at the top and 54 inches at the bottom, there is blown a suspension or 100 pounds calcined milled diatomaceous earth in 9,400 cu. ft. air per minute. Adjustments of the apparatus are so made that there is 1.52 sq. ft. of area of the annular opening between the inner and outer cones 4 and 5, at the closest point, and 1.76 sq. ft. of area of the annular opening between the tunnel member I! and the base of the inner cone, also at the closest point. Lateral or secondary air is admitted at the rate of 4,100 cu. ft. per minute.

From the preceding data, it may be calculated that the thickness of the band of primary aeriform suspension issuing from the annular opening between the outer and inner cones is less than 3 inches or about 1.4 inches and the linear speed approximately 103 ft. per second. The mixed primary and lateral air streams pass the most narrow part of the opening into the funnel member l2 at a linear speed somewhat higher than 103ft. per second.

The coarse fraction was collected in the hopper 9, the flnes or major fraction in a cyclonic dust separator. Analysis of the flnes showed a percentage of particles that failed to pass through a 150-mesh screen in a wet screening test that was approximately only half of the percentage of such coarse particles contained in the material before passing through the air classifier.

In other typical operations with the same 4112- paratus the amount of the primary aeriform suspension blown in per minute has varied from 6,200 to 8,500 cu. ft. per minute; the linear speed through the annular opening between cones 4 and 5, from 64 to 93 feet per second; the content of suspended diatomaceous earth from 85 to 110 pounds; the lateral or secondary air stream from 2,400 to 7,600 cu. ft., the area of annular opening between the inner and outer cones, at the closest point, from 0.83 to 1.68 sq. ft., and the area of opening between the base of the inner cone and the funnel member, also at the closest point, from 1.34 to 1.68 sq. ft. The percentage oi material flner than 150 mesh that is collected with the coarse, in hopper 9, varies from 13 to 95 per cent of the total weight of the coarse fraction. When the lateral or secondary air stream is used in amount in excess of 5,250 cu. it. a minute, the separated coarse fraction contains only 13 to 28 percent of lines, thus showing the effectiveness of treating the separated coarse fraction with an adequate lateral air stream or blast, to wash out the flnes.

The above description of apparatus and process are given by way of illustration, not limitation, of the invention. Many changes in the apparatus illustrated may be made without departing from the spirit of the invention. Thus the pitch of the conical members 4 and 5 may be varied or they may be substituted by other members of circular cross section, as, for example, by inner and outer concentric pipes, the inner pipe being closed at the upper end and suitably also at the lower. Or, a rectangular slot that is relatively long in pro portion to its width may be used and the thin stream or band of air-dust suspension passed through it in substantially unidirectional manner and then deflected fiatwise, suitably with a lateral blast of air impinging on it adjacent to the point of deflection and in the general direction towards which the band is deflected. However, I have found the use of cones very desirable in distributing the air-dust stream and in making it into an annular form or band, moving at a high speed in a course that is substantially unidirectional, with a minimum of eddies or cross currents. Difierent means of collecting the classified particles may be used.

In separating particles by this invention, the particles to be classified should enter the classifier proper at high velocity and with a very large weight of air in proportion to the suspended solid, to minimize aggregation of individual particles into clusters. Instead of an air suspension there may be used an aeriform suspension, as, for example, a suspension of particles of solid in an aeriform stream such as one of flue gas,

The terms substantially non-sinuous" and unidirectional, as applied to the manner of movement of the shaped stream of gas, are intended to indicate a degree of freedom from turbulence that is not realized when the stream of gas is being subjected continuously to change of direction, as in a spirally shaped pipe. An intimate suspension is one in which the particles to be classified are separately suspended, in nonaggregated form. In the preferred embodiment of the invention, the shape of the gas stream at the point of deflection is annular, that is, the shape of a ring of little thickness or shortest dimension, corresponding to the distance between the circumference of the bases of the cones 4 and 5. After the classification of the original suspension by sharp deflection is completed and the separated coarse particles are washed by the secondary air stream, the secondary air used in washing is blended, as in the member l2, with the portion of the original suspension remaining subsequent to sits sharp deflection; in this manner fines in the primary and secondary air streams are composited.

The term band, as used herein, means a shaped mass of material of relatively great length and little thickness at a given point.

The details that have been given are for the purpose of illustration, not restriction. It is intended that variations within the spirit of the invention should be included within the scope of the claims.

What I claim is:

1. A classifier of the character described comprising a hollow truncated cone, a closed cone disposed centrally within said truncated cone and defining therewith an annular opening between the circumferences of their bases, a hopper communicating with the annular opening, means of delivering, at high speed, an aeriform suspension of a mineral powder within the narrow end of the truncated cone, air exhausting means disposed adjacent said annular opening adapted to deflect the aeriform stream issuing therefrom, and means for admitting a lateral aeriform current into the first mentioned aeriform stream adjacent the zone of deflection.

2. A classifier of the character described comprising a hollow truncated cone, a closed cone disposed centrally within said truncated cone and defining therewith an annular opening between the circumferences of their bases, a hopper communicating with the annular opening, means of delivering, at high speed, an aeriform suspension of a mineral powder within the narrow end of the truncated cone, and air exhausting means disposed adjacent said annular opening adapted to deflect the aeriform stream issuing therefrom, whereby a separation of the coarse particles from the aeriform stream is effected.

3. In a classifier of the type described, means for forming an intimate suspension in a gas of finely divided solid material containing fine and relatively coarse particles, means for shaping the suspension into an annular form moving rapidly in substantially unidirectional manner, means for deflecting sharply the gas content of the suspension in the said annular form, whereby separation of the relatively coarse particles from the fine particles is effected, means for washing the separated coarse particles with a secondary gas stream, and means for blending the secondary gas used in the washing with the portion of the original suspension remaining subsequent to the sharp deflection of the said original suspension.

4. In classifying particles of various sizes of finely divided diatomaceous earth, the method which comprises forming the said particles into a dilute suspension in an aeriform stream, forming the suspension into a thin band moving at a linear speed in excess of sixty-four feet per second, and sharply deflecting the band towards the direction of the shortest dimension thereof at any given localized portion, whereby separation of the particles of various sizes is effected.

.5. In classifying particles of various sizes of finely divided diatomaceous earth, the method which comprises forming the said particles into a dilute suspension in an aeriform stream, including aeriform material in proportion of the order of at least 6,400 cubic feet for each 110 pounds of diatomaceous earth, forming the said stream into a thin band of high speed, and sharply deflecting the band fiatwise while maintaining high speed thereof.

6. A classifier of the character described comprising an upright hollow member of circular cross section, a smaller upright hollow element of circular cross section, disposed within the said member and terminating at its lower end approximately at the same level as the lower end of the member and defining an annular opening therewith, means for delivering, through the said opening, an aeriform suspension of the material to be classified, moving at high speed in substantially unidirectional manner, an exhauster fan and connections thereto including a circular entrance member disposed adjacent to and below the said annular opening and being of diameter smaller than that of the said opening, a hopper member disposed around and generally below the said opening and communicating therewith, and air ports in the hopper for admitting secondary air laterally to the said annular opening, the said exhauster fan being adapted to draw the said suspension in the form of a band through the said opening, to cause sharp deflection of the band into the said circular entrance member, and to draw auxiliary air through the said ports and through the separated coarse particles and then blend the auxiliary air with the deflected stream.

RUPERT K. BOURKE.

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