US3378140A - Method and apparatus for pneumatically separating solid particles - Google Patents

Description

April 1968 w. WOCHNOWSKI ETAL 3,

METHOD AND APPARATUS FOR PNEUMATICALLY SEPARATING SOLID PARTICLES Original Filed March 27, 1952 V 5 Sheets-Sheet l INVENTORS 4 [Jamar Mu we April 16, 1968 w WOCHNOWSKI ETAL 3,378,140

METHOD AND APPARATUS FOR PNEUMATICALLY SEPARATING SOLID PARTI CLES Original Filed March 27, 1952 5 Sheets-Sheet 2 INVENTOR5 A, 2: (Z4414 4w: (wwa p 1968 w. WOCHNOWSKI ETAL. 3,378,140

METHOD AND APPARATUS FOR PNEUMATICALLY SEPARATING sous PARTICLES Original Filed March 27, 1952 5 Sheets-Sheet 5 mom United States Patent 3,378,140 METHOD AND APPARATUS FOR PNEUMATI- CALLY SEPARATING SOLID PARTICLES Waldemar Wochnowski, Hamburg, Heinz Harte, Hamburg-Bergedorf, and Johannes Iwen, Reinbek, near Hamburg, Germany, assignors to Hanni Werke, Koerber & Co., KG, H'amburg-llergedorf, Germany Original application Mar. 27, 1962, Ser. No. 182,802, now Patent No. 3,265,209, dated Aug. 9, 1966. Divided and this application Aug. 9, 1966, Ser. No. 571,236 Claims priority, application Germany, Oct. 31, 1960, H 40,822; Mar. 27, 1961, H 42,131 9 Claims. (Cl. 209-137) ABSTRACT OF THE DISCLOSURE A separator for pneumatically separating solid particles in a housing having an inlet at its bottom through which a gaseous suspension of solids is projected along an incline-d path, a plurality of bafl les defining inclined passages through which portions containing light particles of the suspension are diverted and evacuated from the top of the housing. The heavy particles, which drop out of the suspension in an area of the housing below the ats sages, are evacuated at the bottom of the housing.

This application is a division of our application Ser. No. 182,802, filed Mar. 27, 1962, now US. Patent 3,265,209 issued Aug. 9, 1966.

The present invention relates to a method and ap paratus for separating lighter particles from heavier particles, particularly for separating lighter tobacco articles from a mixture of lighter and heavier tobacco particles.

An important object of our invention is to provide a method of continuously separating lighter particles from heavier particles which form a mixture with the lighter particles so that the heavier and/ or lighter particles may be continuously conveyed to a further processing station and that the separation of lighter particles from heavier particles is completed within short periods of time.

Another object of the invention is to provide a method of the just outlined characteristics which is especially suited for separating tobacco ribs from tobacco leaf laminae and which may be carried out at such a speed that the consistency of processed material remains unchanged or changes very little during the separating operation.

A ttuther object of the invention is to provide a method which may be carried out in a fully automatic way and which may be resorted to for separation of large quantities of tobacco leaf laminae from tobacco ribs so as to satisfy requirements for mass-manufacture of tobaccocontaining products.

An additional object of the invention is to provide an apparatus which may be utilized to carry out the above outlined method and which is constructed and assembled in such a way that it may be rapidly and conveniently converted for use with different types of mixtures including lighter and/or heavier particles of different specific weight, moisture content, configuration, thickness, flexibility or other characteristics, and which requires little attention once it is adjusted and started to carry out the method.

Still another object of the invention is to provide an apparatus of the just outlined characteristics for sep arating ribs from tobacco leaf laminae: with the help of one or more air currents which is constructed and assembled in such a way that the laminae are separated from the ribs within very short periods of time and that the consistency, particularly the moisture content, of the laminae is not appreciably affected during short-lasting contact with air.

A concomitant object of the invention is to provide an apparatus for separating tobacco ribs from tobacco leaf laminae by subjecting a mixture of ribs and laminae to the action of one or more air currents which is constructed in such a way that the mixture is subdivided into lighter and heavier fractions in one or more stages to make sure that any heavier particles which might have been entrained by the bulk of the lighter particles are also separated from the lighter particles, or vice versa.

An additional object of the invention is to provide an improved assembly for separating the mixture of tobacco leaf laminae and ribs immediately after and at the same rate at which the mixture is being formed in a stemming or destalking machine.

Briefly stated, one feature of the present invention resides in the provision of a method of separating lighter particles from a mixture of lighter and heavier particles. The method comprises the steps of introducing into the lower part of an inclined ascending path an air current containing a mixture of lighter and heavier particles and conveying the air current at such a speed that the current entrains the mixture into an intermediate part of the inclined path, subdividing the air current leaving the intermediate part of such path into a plurality of partial currents and conveying the partial currents through separate portions or passages of the upper part of the inclined path at such a speed that the partial currents entrain only lighter particles whereby the heavier particles descend by gravity and leave the inclined path, separating the lighter particles from the partial air currents, and collecting the heavier particles.

The passages in the upper part of the inclined path are preferably configurated in such a way that the partial currents are accelerated on their way to the separating zone.

In accordance with another feature of the present invention, the descending heavier particles are preferably conveyed through a second current of air which travels counter-current to heavier particles at such a speed that it entrains at least some lighter particles which might have been entrained with the heavier particles. This second current is preferably merged with at least one of the partial currents so that all of the lighter particles can be separated from air in a common separating zone.

The lower and intermediate parts of the inclined path preferably diverge upwardly so that the first air current expands and its speed decreases sufiicicntly to allow for gravitational descent of heavier particles. The second air current is preferably conveyed in a second upwardly diverging path so that its velocity also decreases to allow for substantially unimpeded descent of heavier particles.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its ntode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the fo'lowing detailed description of certain specific embodiments with reference to the accompanying drawings, in which:

FIG. 1 is a somewhat schematic perspective view of an apparatus which embodies one form of our invention;

FIG. 2 is a partly elevational and partly vertical sectional view of a separator assembly which forms part of the apparatus shown in FIG. 1;

FQG. 3 illustrates a vector diagram indicating the mannor in which gravity and o;her forces act upon lighter particles in the separator assembly of BIG. 2;

FIG. 3a illustrates a vector diagram indicating the action of various forces upon the heavier particles which enter the separator assembly of FIG. 2; and

FIG. 4 is a partly elevational and partly vertical sectional view of a modified apparatus.

Referring first to FIG. 1, there is shown a separating apparatus which comprises a housing 2 resembiing an up right tower and forming part of a separator assembly (to be described in greater detail in connection with FIG. 2), a stemming or destalking machine 202 whose hopper 203 is provided with an outlet 204 through which a mixture of tobacco leaf laminae (lighter particles) and ribs (heavier particles) produced by the machine 262 is adrnilted into a first current of air flowing through a supply duct 9. Unstemmed tobacco leaves are delivered by a conveyor 212 which discharges into a chute 201 provided on the machine 202. An evacuating duct 18 conveys air and lighter tobacco particles into an air extractor 267 provided wih an air lock 268 adapted to discharge a stream of lighter particles onto the upper stringer of a take-off conveyor 299.

The apparatus of FIG. 1 also comprises a source of compressed air, here shown as a blower 266, whose outlet is connected to a pressure conduit 205 connecting to the supply duct 9. The inlet of the blower 296 is connected with a suction conduit 210 which draws air from the evacuating duct 18. A branch conduit 211 connects the outlet of the blower 206 with a secondary separator chamher in the housing 2.

FIG. 2 illustrates all details of the separator assembly which is accommodated in the housing 2. The upper pOrtion of this housing accommodates a series of specially configurated baffles which define three passages 53, 54, 55 constituting the upper part of an ascending inclined path whose intermediate part is constituted by a primary separator chamber 1. The intake ends 50, 51. 52 of the passages 53, 54, 55 receive lighter tobacco particles which are entrained by an ascending air current admitted through the supply duct 9. The lower portions 56, 57, 58 of the walls 59, 60, 61 (respectively bounding the left-hand sides of the passages 53, 54, 55) are inclined with reference to a vertical plane. The lower portions 57, 58 of the walls 60, 6.1 are formed with wedge- shaped guide surfaces 64, 65 for lighter particles which ascend with three partial air currents toward a duct and/or into the evacuating duct 18. The right-hand sides of the passages 53, 54 are respectively bounded by walls 62, 63. The discharge ends 66, 67, 68 of the passages 53, 54, and the discharge end of the evacuating duct 15 communicate with the evacuating duct 13. The wall 59 makes an acute angle with an outer wall 29, and the wall 69 bounding the righthand side of the duct 53 makes an acute angle with the left-hand wall of the median passage 54. Furthermore, the right-hand wall 7t? of the median passage 54 makes an acute angle with the left-hand wall 61 of the third passage 55. The other wall 71 of the passage 55 makes an acute angle with the lower wall 72 of the duct 15.

A valve 73 is articulately connected to the upper end portion of the wall 71 by a hinge 74 so that it may regulate the cross-scctional area of the duct 15. A similar valve 75 is secured to the upper end portions of the walls 61, by a hinge 76, and a thi d hinge '78 connects a valve 77 to the upper end portions of the walls 60, 69. The valves 73, 75, 77 are adjustab e by means of suitable linkages 24% one of which is shown in FIG. 1. The uppermost valve 73 extends deeper into the evacuating duct 13 than the valves 75, 77 and is function is to change the amounts of air which are drawn through the passages 53-55. The distribution of the partial air currents flowing through the passages 53, 54, 55 may be regulated by appropriate adjustment of the valves and 77. The valves 73, 75 and 77 enable the operators to adjust the apparatus for separation of different types of mixtures.

The operation of the separator assembly shown in FIG. 2 is as follows:

The mixture of lighter and heavier tobacco particles is admitted into the ascending inclined air current which enters the lower part 1a of the aforementioned path through the supply duct 9. The heavier particles are permitted to descend by gravi y as indicated by the arrow 80 and enter the lower part of a secondary separator chamber 10. The velocity of partial air currents fiowihg through the passages 53, 54, 55 is selected in such a way that they entrain a substantial percentage of lighter tobacco particles and advance the lighter particles directly into the evacuating duct 18. The lighter particles in the remainder of the ascending air current pass along a wa'l 13 and are lifted into the intake end of the duct 15 to advance toward and into the evacuating duct 18. The second air current which is admitted into the secondary separator chamber 10 via branch conduit 211 flows countercurrent to heavier particles and merges with the first air current. This second current lifts such lighter particles which might have been entrained by the descending heavier particles. The heavier particles are evacuated through an air lock 5 driven by a motor 6 and the lighter particles are withdrawn from the evacuating duct 18 todescend onto the conveyor 209 shown in FIG. 1. Any heavier particles which might have been lifted to the level of the intake ends 50, 51, 52 of the respective passages 53, 54, 55 are slowed down near the walls which define these passages and descend into the secondary separator chamber 10.

The vector diagram of FIG. 3 illustrates the manner in which the air currents in the separator chambers 1 and 10 of the assembly shown in FIG. 2 act upon the lighter tobacco particles. The force represented by the vector b is the force with which the partial currents urge the lighter particles into the passages 53, 54, 55. The vector g represents the force of gravity, the vector a represents the force acting upon the mixture of lighter and heavier particles by the current of air which is admitted through the supply duct 9, the vector 0 indicates the resultant of forces represented by the vectors a and g, and the vector 2 represents the resultant of forces indicated by the vectors b and c. The resultant vector e makes a small acute angle with the vector a and points into the passages 5355. The vector c points into the upper part of the secondary separator chamber 10. The inclination of the vector a is the same as that of the wall portions 57, 58, 59.

The diagram of FIG. 3a shows the manner in which the air currents in the separator chambers 1 and 10 and other forces act upon the heavier tobacco particles. The force represented by the vector g is the force of gravity, and the vector a indicates the force produced by the air current admitted through the supply duct 9. The vector c' is the resultant of the forces represented by vectors g and a, and this vector points toward the lower part of the secondary separator chamber 10. The vector b indicates the force acting upon heavier particles in the chamber 1 in a direction of the passages 5355, and the vector f is the resultant of forces represented by the vectors b, c. This vector f also points toward the lower part of the secondary separator chamber 10.

FIG. 4 illustrates a second apparatus for separating lighter tobacco particles from heavier particles. This apparatus comprises a destalking assembly 301 including a destalking machine 301' mounted on a frame 301a and receiving unstemmed tobacco leaves from a non-illustrated conveyor which discharges into a chute 319. The mixture 347 of lighter and heavier tobacco particles produced by the machine 301' is discharged into a hopper 302 whose outlet 303 admits the mixture into a supply duct 304 connected to a separator assembly 305. The intake end of the supply duct 304 is connected to a pressure conduit 310 which is connected to the outlet of a source of compressed air here shown as a blower 309. The intake of the blower 309 in connected with a suction conduit 308 which is further connected to an air extractor 307. The latter communicates with the upper end of the separator assembly 305 through an evacuating duct 306. The duct 306 delivers lighter tobacco particles to the air extractor 307 which discharges lighter particles through an air lock 317 and onto the upper stringer of a take-off conveyor 318.

The blower 309 discharges surplus air through a nozzle 311 which is controlled by a valve 312, the latter having a manually actuated handgrip portion 312a. At the junction of the pressure conduit 310 and supply duct 304, there is provided a regulating device in the form of a rotary valve 313 which may be turned by means of a handle 313a to thereby adjust the strength of the current of compressed air which passes from the outlet of the blower 309 and through the supply duct 304 to entrain a stream of the mixture 347 into the separator assembly 305.

At the upstream side of the regulating valve 313, the pressure conduit 310 communicates with the intake end of a pressure pipe 314 which conveys a second current of compressed air to a pair of upwardly extending branch pipes 315, 316 connected to the separator assembly 305.

The separator assembly 305 comprises a housing 321 which resembles an upright tower. This housing is mounted on a frame 320 and includes an upwardly divering inclined funnel-shaped mixture-receiving channel or diffusor 322 bounded at its underside by a convex lower wall 325 and at its upper side by a convex upper wall 324. The channel 322 constitutes the lower part of an inclined ascending path whose intermediate part is constituted by a primary separator chamber 323. The upper zone of the space in the housing 321 accommodates three profiled baffles 329, 330, 331. These bafiles define between themselves a pair of arcuate passages 327, 328 communicating with the evacuating duct 306. The baflle 329 and the wall 324 define between themselves a third passage 326, and a fourth passage 346 is provided between the baflie 331 and the top Wall 3210 of the housing 321. The passages 326 and 346 communicate with the evacuating duct 306 and constitute the upper part of the aforementioned path. At the upper ends of the baffles 329-331, there are provided valves 332, 333, 334 fixed to horizontal shafts 332a, 333a, 334a which can be rotated by suitable handles (not shown) located externally of the housing 321.

The shafts 332a-334a enable the operator to adjust the respective valves 332-334 and to thereupon maintain the valves in selected positions in order to determine the rate of air flow through the passages 326320 and 346.

The blower 309, pressure conduit 310, supply duct 304 and pressure pipe 314 together constitute a pneumatic conveyor whose function is to advance the mixture 347 into the primary separator chamber 323 and to convey blasts of compressed air into a secondary separator chamher 335.

The upper region of the primary separator chamber 323 communicates with the upper region 336 of the secondary separator chamber 335, and this upper region 336 is in communication with the passage 346. The lower region of the secondary separator chamber 335 is bounded by a funnel-shaped upwardly diverging wall 337 which includes two mutually inclined perforated or apertured sections or screens 330, 339. These screens converge toward an air lock 340 which serves as a means for evacuating heavier particles from the secondary chamber 335. The screen 333 separates the chamber 335 from a plenum chamber 342 which communicates with the branch pipe 315 and is further bounded by the lower wall 325 of the channel 322, by the bottom wall 341 of the housing 321, and by the front and rear end walls 321a, 321b of the housing. The walls 321a, 321b also bound the separator chambers 323, 335 and the evacuating duct 306. The front Wall 321a is broken away in FIG. 4 to expose the interior of the separator assembly 305.

The right-hand end wall 343 of the housing 321, the screen 339, the walls 321a, 321b, and the bottom wall 341 define between themselves a second plenum. chamber 344 which communicates with the branch pipe 316. The flow of air through the branch pipes 316, 315 is respectively regulated by valves 348, 349 which may be adjusted by hand. The air lock 340 discharges heavier tobacco particles onto the upper stringer of a second takeoff conveyor 345- which is mounted below the bottom wall 341 and is provided with inclined side walls 345a, 34 5b forming a trough and serving as a means for guiding the heavier particles onto the upper stringer of this conveyor.

The apparatus of FIG. 4 operates as follows:

Unstemmed tobacco leaves are admitted through the cliute319 and enter the destalking machine 301 wherein the heavier particles (mainly ribs R) are separated from lighter tobacco particles (mainly tobacco leaf laminae L). The resulting mixture 347 is discharged into the hopper 302 to form a continuous stream which passes through the outlet 303 into the supply duct 304. The current of air passing through the supply duct 304 entrains the mixture into and through the channel 322. While advancing through the supply duct 304 and channel 322, certain heavier components of this mixture (almost exclusively ribs R) descend toward and advance along the lower wall 325 to be discharged into the secondary separator chamber 335. This is due to the fact that the ribs are heavier than the leaf laminae. Of course, some ribs might remain entangled between the lighter particles as the mixture passes through the supply duct 304 and through the channel 322; such ribs are then advanced into the primary separator chamber 323. The right-hand end portion of the lower wall 325 is nearly horizontal to otter little resistance to forward movement of heavier particles into the secondary separator chamber 335. Such heavier particles drop onto the screen 338 and advance toward the air lock 340 to be evacuated onto the take-off conveyor 345. Any lighter particles which might have been entrained by the heavier particles are separated therefrom by jets Y of air passing from the plenum chamber 342 through the apertures in the screen 338 so that such lighter particles rise in the secondary separator chamber 335 and enter the upper region 336 to be drawn into the passage 346 and thence into the evacuating duct 306. Thus, a preliminary separation of heavier tobacco particles from lighter particles takes place right in the lower part (channel 322.) of the aforementioned inclined path, and such separation is thereupon continued in the primary separator chamber 323. The inertia of heavier tobacco particles which have entered the chamber 323 (mainly ribs R which remain in the partially sorted mixture) causes such heavier particles to advance into the secondary separator chamber 335 Where they drop by gravity and slide along the screens 338, 339 toward and into the air lock 340 to be evacuated onto the conveyor 345. Any heavier particles which are entrained by the lighter particles toward the passages 326, 327, 328 and 346 are slowed down near the inclined surfaces of the baffles 329, 330, 331 and by the top wall 3210 of the housing 321 so that such heavier particles begin to move countercurreut to the flow of air and descend into the lower region of the secondary separator chamber 335. The lighter particles L advance with the partial air currents flowing through the passages 326-323 and 346 and accumulate in the air extractor 307. It was observed that the major percentage of heavier particles is separated from lighter particles while the mixture moves with the ascending inclined air current advancing toward and into the passages 326-328 and 346 so that the secondary separator chamber 335 is mainly a settling compartment for the heavier particles R. In FIG. 4, the current of 'air discharged by the pneumatic conveyor and the portions of this current travelling through the passages 3264628 and 346 are identified by reference characters X. The jets Y of air admitted from the plenum chambers 342, 344 through the screens 338, 339 form a second current YY which ascends through the secondary separator chamber 335 countercurrent to descending heavier particles and thereupon merges with the currents X to form therewith a single current Z advancing into the evacuation duct 396 and air extractor 397. Any lighter particles L which might have entered the secondary separator chamber 335 are lifted by the current X flowing into the passage 346 and by the second current YY so that the particles descending by gravity onto the wall 337 consist exclusively or nearly entirely of ribs R. The separating action in the secondary chamber 335 is surprisingly effective and this is attributed to the fact that the volume of this chamber is comparatively large and also because the chamber 335 receives a smaller percentage of the original mixture 347. Therefore, the material which enters the upwardly diverging chamber 335 is free to expand so that the lighter particles are fully exposed and may be subjected, individually, to the lifting action of the air current YY. Hardly any leaf laminae will descend into the air lock 340.

The lighter particles which accumulate in the air extractor 307 are separated from the air current which latter is evacuated through the suction conduit 308, and the condensed stream of lighter particles is then evacuated through the air lock 317 to descend onto the conveyor 318. This conveyor advances the lighter particles to a further processing station, not shown. The air which is withdrawn by the blower 309 through the suction conduit 308 is thereupon recirculated through the pressure conduit 310 and enters the supply duct 304 as well as the pipe 314 to bring about separation of a next batch of the mixture 347.

It is to be understood that, while we have described the operation of the separating apparatus in connection with segregation of tobacco leaf laminae from tobacco ribs, this apparatus is equally useful for treatment of other types of material wherein a heavier solid fraction is admixed to a lighter solid fraction.

The current X is strong enough to move the lighter particles L through the lower part 322 of the inclined path and into the upper regions of the chambers 323, 335 but is weak enough to permit gravitational descent of heavier particles R into the lower region of the secondary separator chamber 335. On the other hand, the current YY is just strong enough to counteract the gravity of lighter particles L which might have entered the lower region of the secondary separator chamber 335 and to entrain such lighter particles into the passage 346. The descent of heavier particles R is affected by the second current YY very little or not at all.

The passages 3263 28 and 346 converge toward the evacuating duct 306 to accelerate the lighter particles therein. The channel 322 and chamber 323 diverge upwardly so that the speed of the air current admitted by the supply conduit 304 decreases sufficiently to allow for gravitational descent of the heavier particles which leave the inclined path and enter the secondary separator chamber 335.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and spe- 8 cific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is:

l. Apparatus for separating lighter particles from a mixture of lighter and heavier particles, comprising a housing defining an ascending path having lower intermediate and upper parts; a plurality of bailles provided in said housing to divide said upper part into a plurality of separate passages which are inclined with respect to and communicate with said intermediate part to divert a portion of the lighter particles in said mixture, one of said baflies defining with a portion of said housing an additional passage for the remainder of the lighter particles in said mixture; means for admitting into the lower part of said path an air current containing said mixture of lighter and heavier particles at such a speed that the mixture ascends into said intermediate part but that the heavier particles descend in said intermediate part by gravity while the lighter particles continue to rise with partial air currents which result from subdivision of said air current on entry into said passages, said intermediate part including a separator chamber wherein the heavier particles are separated from the lighter particles whereby the lighter particles travel with said partial air currents in said passages; evacuating means for withdrawing said partial currents and the lighter particles from all of said passages; and means for evacuating heavier particles from said separator chamber.

2. Apparatus as set forth in claim 1, wherein said separator chamber diverges upwardly to effect gradual reduction in the speed of said air current.

3. Apparatus as set forth in claim 2, wherein said housing further includes a secondary separator chamber p0- sitioned to receive heavier particles which leave said first mentioned chamber and in communication with at least one of said separate passages, and further comprising means for admitting into said secondary chamber a second ascending current of air at such a speed that the second current entrains into the upper part of said path at least some lighter particles which might have been entrained by the heavier particles.

4. Apparatus as set forth in claim 3, wherein said housing comprises apertured wall means bounding the lower part of said secondary chamber and said second air current is admitted through such apertured wall means.

5. A method of separating lighter particles from a mixture of lighter and heavier particles, comprising the steps of introducing into the lower part of an inclined ascending path an air current containing a mixture of lighter and heavier particles and conveying the air current at such a speed that the current entrains the mixture into an intermediate part of said path; decelerating the air current in said intermediate part to effect gravitational descent of heavier particles so that the thus decelerated current is at least substantially free of heavier particles; subdividing the de'celerated air current and the lighter particles remaining therein into a plurality of partial currents by conveying the air of said decelerated current through separate portions of the upper part of said p'ath at a speed which suffices to entrain the lighter particles; separating the lighter particles from the partial air currents; and evacuating the heavier particles from the intermediate part of said path.

6. A method as set forth in claim 5, further comprising the step of accelerating the partial currents in the upper part of said p'ath.

7. A method as set forth in claim 6, further comprising the step of merging said second current with at least one of said partial currents.

8. A method as set forth in claim 7, wherein the lower and intermediate parts of said inclined path diverge upwardly so that the first mentioned cur-rent exp-ands and is decelerated sufiiciently to allow for gravitational descent of heavier particles, said second current being conveyed in a second upwardly diverging path so -that its velocity decreases sufficiently to allow for substantially unimpeded descent of heavier particles.

9. A method as set forth in claim 5, further comprising the step of conveying said descending heavier particles through a second current of air which travels counter-current to heavier particles at such a speed that it entrains at least some lighter particles which might have been entrained by the heavier particles.

References Cited UNITED STATES PATENTS 1,143,587 6/1915 Marsden 209-139 X 4,877,861 9/1932 Hatch 2()9137 X 2,728,632 12/1955 Ma'theson 209157 X 3,010,576 1 1/1961 Harte 209-133 FOREIGN PATENTS 40,716 5/ 1937 Netherlands.

FRANK W. LU'ITER, Primary Examiner.

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