US2690880A - Rectilinear pulverizer - Google Patents

Description

Oct. 5, 1954 J. B. CHATELAIN 2,690,880

' RECTILINEAR FULVERIZER Filed April 10. 1951 5 Sheets-Sheet 1 1s\ T fi T H INVENTOR.

JOHN B. CHATELAIN ATTOR NEY Get. 5, 1954 J. B. CHATELAIN RECTILINEAR PULVERIZER Fild April 10, 1951 5 Sheets-Sheet 2 S R E L F H W W Imwf mNm OZEwuUxm .rzmummn.

FEED RATE IN LBS./HR./C.F.M. FREE AIR INVENTOR.

JOHN B CHATELAI N AT ORNEY Oct 5, 1954 J. B. CHATELAIN 2,690,880

RECTILINEAR PULVERIZER Filed April 10, 1951 5 Sheets-Sheet 3 INVENTOR. JOHN B. CHATELAIN ATTORNEY Oct. 5, 1954 J. a. CHATELAIN 2,690,380

RECTILINEAR PULVERIZER Filed April 10, 1951 5 Sheets-Sheet 4 f/ g. 6. INVENTOR.

JOHN B. CHATELAIN ATTORNEY at. 5, H54 4. B. CHATELAIN 2,690,880

RECTILINEAR PULVERIZER Filed April 10, 1951 5 Sheets-Sheet 5 H4 IOl WITHOUT FILLERS WITH FIL RS AVERAGE PARTICLE SIZE- MICRONS FEED RATE IN LBSv/HR./C.F.M. FREE AIR INVENTOR.

JOHN B. CHATELAIN AT ToRNEY Patented Oct. 5, 1954 UNITED STATEh .EJATE @FFEQE RE CTILINEAR PULVEBIZER Ware Application April 10, 1951, Serial No. 220,193

(El. mil-39) 5 Claims.

This invention relates to pulverizers and more particularly to the design of the grinding chamber for fluid jet operated pulverizing mills.

Fluid jet operated pulverizing mills of various shapes and designs have been known and used for many years. The present invention is directed to the improvement of the efiiciency of this type of pulverizing mill. The basic criteria for determining the efficiency of these mills is the particle size produced, the uniformity of the particle size, the pressure and volume of air required for operation and the rate the material can be passed through the mill. It is not sufficient merely to improve the grinding character istics of the mill, in addition, the classification must be improved, if a suitable product is to be obtained.

In existing designs of pulverizers, difiiculty has been experienced due to the grinding chamber accumulating material between the grinding jets,

in its peripheral portions. This material accumulates until it extends into the main circulating stream of grinding fluid. When this occurs, the accumulated material is rapidly entrained into the circulating stream of grinding fluid, much of it entering the classifying vortex and passing to the collector without proper reduction in size. This results in an unsatisfactory product, due to the presence of a high percentage of oversize particles. This accumulation and periodic ejection, of the accumulated material, occurs in the standard, fluid jet, pulverizing mill as a rhythmic cycle.

It is frequently more desirable to produce a product of uniform size than to grind the material as fine as possible. Many materials processed in this type of mill should not be reduced to the smallest particle size possible, but rather they should be reduced to particles of intermediate size. In the case of many of these materials, however, it is critical that the particle size be uniform. An example of such a product is penicillin, wherein the maintaining of an effective level in the blood for therapeutic purposes is largely dependent upon obtaining a product having a uniform particle size falling within a relatively narrow range. My invention is directed to obtaining a product having a high percentage of particles of uniform size while increasing the capacity of the mill.

To obtain this objective, it has heretofore been conventional practice to increase the number of jets of grinding fiuid entering the grinding chamber. This, however, decreases the efficiency of the grinding action. Although the particle size Z of the final product is more uniform, the rate of feed of the material through the pulverizer must be reduced to prevent the final product from being too large.

My invention overcomes this difficulty by reshaping the grinding chamber so that little or no area remains for the accumulation of material. This may be done by placing segment or filler blocks between the jets in the conventional circular grinding chamber or by initially so designing the chamber that its walls are noncircular and eliminate the areas of accumulation.

It is, therefore, a primary object of my invention to redesign the internal shape of the grinding chamber of fluid jet, pulverizing mills whereby the material accumulating, peripheral pockets are eliminated.

It is a further object of my invention to reduce load fluctuations in the grinding chamber.

It is an additional object of my invention to increase the capacity of these mills and at the same time to increase their grinding efiiciency.

It is a further, additional object of my invention to provide a pulverizing mill capable of producing a product of more uniform size.

These and other objects and advantages of my invention will be immediately seen by those acquainted with the design and operation of fluid jet, pulverizing mills upon reading the following specification and the accompanying drawings.

In the drawings:

Figure 1 is a central, sectional, elevation View of a mill equipped with my segments.

Figure 2 is a sectional view taken along the plane 11-11 of Figure 1, not showing the means for anchoring the segments to the mill.

Figure 3 is a fragmentary, enlarged, sectional View of the nozzle installation in the mill shown in Figures 1 and 2.

Figure 4 is a central, sectional, elevation view of a mill equipped with filler segments to provide a substantially square grinding chamber, and taken along the plane IVIV of Figure 5.

Figure 5 is a sectional View taken along the plane V-V of Figure 4.

Figure 6 is a sectional, elevation view of a mill having a square grinding chamber and no fillers, taken along the plane VI--VI of Figure 7.

Figure 7 is a sectional view taken along the plane VII--VII of Figure 6.

Figure 8 is a central, sectional view of a mill having a triangular grinding chamber.

Figure 9 is an enlarged, sectional, elevation View taken along the plane IXIX of Figure 8.

Figure 10 is a central, sectional View of a modified mill parallel to one of the end plates of the mill.

Figure 11 is a performance graph.

Figure 12 is a performance graph.

In executing the objects and purposes of my invention, I have provided a fluid jet pulverizing mill having a noncircular grinding chamber. The flat segments of the grinding chamber walls are spaced between the jets of grinding fluid entering the grinding chamber. Depending upon the size of the grinding chamber and the number of jets used, the shape of the grinding chamber may be a polygon, square or triangle.

In the following description, the terms inwardly and outwardly are frequently used and shall be taken to mean inward1y as toward the geometric center of the mill and outwardly away therefrom.

Although the views of the mills are shown and described as though the general plane of the mill were horizontal, this is merely for simplicity and convenience and is not to be considered as a limitation. Generally, the mills may be operated in either a horizontal or vertical plane without modification of their structure.

Referring now specifically to the drawings, the numeral 1 indicates a fluid jet mill having a central grinding chamber 2 enclosed on each of its ends by plates 3 and 4 and about its circumferential periphery by the circular bafiie 5 (Figures 1 and 2). The end plates 3 and 3 each project a substantial distance beyond the circular baflle 5, and, about their periphery, are joined by the circular wall 6. An annular chamher I is defined between the end plates 3 and circular baiile 5 and circular wall 6. A grinding fluid inlet port 8 provides means for admitting the grinding fluid to the annular chamber 1 (Figure 2). The material to be pulverized is admitted through the conduit 9 which communicates with the grinding chamber 2 through the plate 3. This conduit is positioned to feed the material into the vortex of grinding fluid moving within the grinding chamber, which vortex of grinding fluid will be described more fully hereinafter. It will be understood that the conduit 9 is merely a schematic showing of the material feeding means. Several difierent types of material feeding means may be used according to the requirements of the pulverizer. Since the material feeding means forms no part of this invention a more detailed description is not believed necessary.

A plurality of replaceable nozzles [5 are mounted through openings [6, in the circular bafile 5, substantially midway between the plates 3 and 4 (Figures 1, 2 and 3). The openings !6 in the circular baiile 5 and the nozzles 15, are each tapered, with the end of smaller diameter positioned inwardly of the mill, whereby the nozzles l5 will be held in position by engagement of the nozzle surface with the walls of the opening it. Each of the nozzles i5 is provided with a central passageway I! for grinding fluid. The size of the passageway l1 will depend upon the size of the grinding chamber, the number of nozzles employed, the pressure of the grinding fluid and the velocity desired. The face of the inward end of each of the nozzles is perpendicular to the passageway l1.

Coaxial with each of the nozzles I5, an access opening I8 is provided through the circular wall 6. The access openings i8 are of larger diameter than the nozzles 15, whereby the nozzle may be passed through it during servicing or replacement. The internal walls of each of the access openings it are threaded for seating the plugs IS.

The number of nozzles provided depends in part upon the size of the mill and in part upon the type of grinding desired and the material to be treated. The shape of the grinding chamber, normally, is dependent upon the number of nozzles. In the large size mill, l, eight, equally spaced nozzles l5 are provided. The axis of each of the nozzles I5 is inclined to the circular baifie 5 and tangent to the circle 28 of circulating grinding fluid and entrained material within the grinding chamber 2. The diameter of the circle 20 is that which provides the most efiicient vortex for grinding and classifying. Within the circle 26 and concentric therewith, is the ofitake conduit 2| for leading the fines and the spent grinding fluid from the grinding chamber to a suitable collector of conventional design. At the collector, the fines and the grinding fluid are separated. The offtake conduit 2| is of substantially smaller diameter than the circle 20 and projects into the grinding chamber 2 a distance more than onehalf the spacing between the plates 3 and 4.

The circular grinding chamber is rendered 0ctagonal by the installation of the constricting segments or fillers 25. Each of the fillers 25 is arcuate along one side to snugly engage the circular baflle 5 and straight along the side facing into the grinding chamber 2. The fillers 25 are each provided with a hole to permit a nozzle Hi to project through it to the flllers inward surface. The flllers are held in place by means of the screws 28 (Figure 3). Each of the fillers 25 seats tightly against the end plates 3 and 4, whereby no material can collect between these parts or work behind the fillers.

The pulverizing mill 3%, shown in Figures 4 and 5, is quite similar to the pulverizing mill 1, except that it has a substantially square grinding chamber 3!. Like the pulverizing mill I, it has a grinding chamber 3| surrounded by an annular chamber 32. The annular chamber 32 and grinding chamber 31 are separated by a circular baiile 33. The annular chamber is enclosed by the outer wall 34 and the end plates 35 and 36. A suitable inlet port 38 is provided through the outer wall 34 for the grinding fluid.

Four, equally spaced, nozzles 31 connect the annular chamber 32 with the grinding chamber 3| tln'ough the holes 44. Each of the nozzles is tapered whereby they may be removed for cleaning or replacement. Access holes 39 are provided through the outer wall 3 to permit installation and removal of the nozzles. Each of the access holes is closed by a threaded plug 48. The axis of each of the nozzles 31 is at a right angle to the axis of each of its adjacent nozzles, and all are tangent to a circle 45 within the grinding chamber.

The nozzles 31 each have a concentric passageway 4] for the admission of grinding fluid to the grinding chamber. The discharge oriflce 42, of the nozzles 31, is substantially smaller than the passageway 4! to increase the velocity of the jet of grinding fluid entering the grinding chamber. One corner 43 of each of the nozzles 31 is chamfered so that no portion of the nozzle will project into the circulating stream within the grinding chamber. The major portion of the inward face of each of the nozzles 31, however, is perpendicular to the passageway M.

The grinding chamber 3| is equipped with four constricting segments or fillers 50, each fitting snugly against the circular baflie 33 and the end plates and 38. Each of the fillers 58 has one end at one of the openings 83 for a nozzle 31 and its flat side parallel to the axis of the passageway 48 of the same nozzle. The fillers 44 are secured to the circular baflle 33 by screws 41.

Material to be pulverized is fed into the grinding chamber 3i by means of the screw feed 48. The screw feed is located approximately on the circle representing the primary path of circulating grinding fluid and material. It will be recognized that any suitable feeding mechanism may be substituted for the screw type feed. The fine, pulverized material is removed from the grinding chamber 3I by the oiftake conduit 48, located at the center of the grinding chamber and extending through the end plate 36. The offtake conduit connects with any suitable, conventional separator. The diameter of the ohtake conduit 48 is substantially less than that of the circle 85. The oiftake conduit projects into the grinding chamber slightly more than one-half the distance between the end plates 35 and 38.

The pulverizing mill 88 (Figures 6 and 7) is quite similar to the mill 38 except that the fillers 58 have been eliminated and the grinding chamber 81 is a true square instead of being substantially square. The fillers are replaced by a jacket 88 having an internal square opening defining the grinding chamber 8I. end plates 82 and 83, joined about their periphery by an outer wall 88. A grinding fluid inlet means 85 is provided through the outer Wall 84. The jacket 88 has a circular outer surface. An annular grinding fluid chamber 88 is defined between the jacket 86 and the outer wall 84. A replaceable nozzle 89 is mounted at each corner of the grinding chamber BI. Each of the nozzles is tangent to the circle 98 representing the circular path traced by the grinding fluid in the grinding chamber. An outlet conduit 9| connected to a suitable, conventional collector provides means for withdrawing the fines from the grinding chamber.

Access openings 92 are provided through the outer wall 88 for each of the nozzles 89 to permit replacement. Each of the access openings 82 is closed by means of a plug 93. Any suitable means may be provided for feeding material into the grinding chamber 8| such as the screw feed 94.

The pulverizing mill I88 is designed to have a substantially triangular grinding chamber. The mill I88 includes a pair of slightly concave end plates IM and I82, separated by a pair of concentric rings, the circular bafiie I83 and the outer wall I84. A grinding chamber I85 is enclosed within the circular bafile I83 between the end plates MI and I82. An annular chamber I86 is defined between the end plates I8I and I82, the circular bafile I83 and the outer wall I84. A grinding fluid inlet port I I8 is provided through the outer wall I84.

Three small orifices I87 pass at an angle through the circular bafile I83 for admitting grinding fluid from the annular chamber I88 to the grinding chamber I05. The centerline of each of the orifices I81 is inclined at the same angle and in the same direction whereby they are each tangent to a circle I88 of substantially lesser diameter than that of the circular baflle I83. Within the circle I88, and of substantially lesser diameter, is the offtake conduit I89 which is connected to a suitable, conventional collector. It will be understood that the orifices I81 The mill 88 has could be enlarged and removable nozzles used to direct the grinding fluid as is shown in mills I and 38.

The grinding chamber is equipped with three constricting segments or fillers I I4, spaced about the periphery of the grinding chamber and attached to the circular baiile I83 by means of screws II5. Each of the fillers H4 is cross-sectionally, wedge shaped to seat snugly against the concave surfaces of the end plates I8I and I82. The fiat surface of each of the fillers I I4, is parallel to the axis of one of the orifices I81 but spaced a short distance radially outwardly from the circle I88. The end plates I8I and I82 may be made straight so that the inward faces of the plates are parallel. When this construction is employed, the fillers II4 are made with parallel end faces eliminating the wedge shaped construction.

In the construction of each of these mills, it will be understood that the nozzles, although preferably replaceable, are not necessarily so. They may be made permanent. When replaceable nozzles are used, means for anchoring the nozzles to the circular bafile other than a frictional fit may be used. One substitute method of attachment could be by means of threading.

In the construction of each of the above described mills the filler segments may be designed to have a slightly curved inward face. This design, however, is not considered preferable. Any such curvature should be slight, otherwise much of the benefit of utilizing the segments will be eliminated. Another feature of construction appearing throughout the above description is the use of a collector designed as a separate element distinct from the pulverizing mill. It is also possible to mount the collector directly on the pulverizing mill as an attachment to it. Such a construction is not illustrated because whether the collector is an independent element or an attachment to the mill is immaterial to the invention disclosed herein.

OPERATION The operation of each of the pulverizing mills I, 38, 88* and I88 is similar. Therefore, a single description will sufiice for all. In general, the mills each operate on the same principles and in the same general manner as conventional, fluid jet pulverizers. A pressurized grinding fluid, normally air or steam, is admitted to the annular chamber. Although air and steam are the most commonly used grinding fluids, it will be recognized that any of the grinding fluid may be employed if the operation requires it. The grinding fluid is then injected into the grinding chamber through the nozzles, in the case of mills i, 38 and 88 or through the orifices in the case of the mill I88. In each of the mills the grinding fluid enters the grinding chamber at a high velocity. Since each of the entering streams or jets of grinding fluid is inclined in the same direction, a high velocity, circular path is established in the grinding chamber. All of the jets may be tangent to a single circle or some of the jets may be tangent to one circle and the other jets tangent to another circle or circles. The circulating stream entrains the particles of material fed into the grinding chamber. The particles are reduced in size by the impact of the grinding fluid and by collision between particles. The smaller particles, commonly known as fines, enter into the vortex formed between the primary path of circulating grinding fluid and the oiftake conduit and are carried oif to a collector. This vortex serves as the classifier for the particles.

The larger particles, commonly known as heavies, are by centrifugal force, caused to migrate toward the periphery of the grinding chamber. In mills of conventional design, these heavies tend to accumulate in masses between the primary path or paths of circulating grinding fluid and the walls of the grinding chamber in the spaces between the jets. These masses of oversize particles grow in size until they extend into the path of circulating grinding fluid. These masses of heavy particles then break free and pass into the circulating stream. Since the eflicient operation of one of these mills requires a constant balance between the quantity and velocity of grinding fluid and the rate at which the material is processed through the mill, this overloading of the circulating stream seriously disrupts the balance between the quantity of entrained material and the volume of grinding fluid. This overloading temporarily destroys classifying vortex, permitting large quantities of the heavies to escape into the off-take conduit without proper reduction in size. The result is a product of widely varying particle size.

By placing the fillers in the grinding chamber, all or a substantial portion of the area within which this material is able to accumulate is eliminated. Thus, the loading of the grinding chamber will remain substantially constant, and a final product of more uniform size will be produced. All of the material passing through the mill is reduced to a substantially uniform size. The periodic flooding of heavy particles into the classifying vertex is eliminated. This uniformity of the grinding chamber load factor cannot be accomplished without the use of the fillers and, without this uniformity of load factor, the uniformity of product cannot be obtained. The fillers are situated radially outwardly of the primary path of the vortex of grinding fluid. When the stream of grinding fluid is caused to im-- pinge directly upon the fillers, excessive wear of the fillers and contamination of the material occurs.

The use of the fillers or their functional equivalent, the noncircular grinding chamber permits the number of jets to be reduced. The smaller the number of jets employed, the greater the grinding efliciency of the mill. However, heretofore the efficiency in grinding resulting from the elimination of some of the jets was more than offset by the reduction in classification efliciency. As the number of jets was reduced the total area for accumulation of material about the periphery of the grinding chamber increased and thus, the problem of periodic overloading of the grinding chamber became more acute. With the use of the fillers, however, the number of jets may be decreased without increasing the area of accumulation. Thus, while the grinding efiiciency of the mill is increased, the classification of the mill may also be increased. The fillers permit the load in the grinding chamber to be reduced to a minimum, thus, utilizing a maximum portion of the energy of the grinding fluid for accelerating the particles. This causes a substantial increase in the overall eficiency of the mill.

Excessive wear of the grinding chamber walls has always been a serious problem in fluid jet pulverizers, particularly when there is impingement of the fluid jet against the grinding chamber walls. Even the use of special, hard, wear resistant alloys does not eliminate this problem, merely alleviating it. Where the wearing surfaces of the grinding chamber walls are an integral part of the pulverizers construction, the replacement cost is high. With the use of the fillers, this difficulty is overcome because only the fillers need be made of the special alloy and these fillers can be easily replaced.

Example I Two separate test runs were made using cold air to grind calcite. Both runs were made with air at the same pressure and admitted to the grinding chamber at the same rate. The first test run was made in a conventional mill having an 8 inch diameter circular grinding chamber equipped with four No. 29 drilled nozzles in the peripheral wall tangent to a 5 /2 inch diameter circle. The nozzles were drilled openings, no replaceable nozzle structure being used.

The second test run was made with a mill having a 6 inch square grinding chamber equipped with four replaceable nozzles tangent to a 5 /2 inch diameter circle. Each of the replaceable nozzles had a No. 29 drilled hole. The inward face of each of the nozzles was perpendicular to the axis of the hole through the nozzle.

The results of these two test runs are tabulated below:

Percentage of Product in Exper minut First Test Run Second Test Run It is seen from this comparison that a very substantial improvement in the uniformity of the product was obtained. The comparative improvement in uniformity increased as the feed rate was increased. Some of the improvement may have been due to the use of the square end nozzles in the mill having the square grinding chamber. However, this factor will account only for a minor portion of the improved results.

Example II Two test runs were made grinding 99 sulphur and 1% triton X-l66. In both runs air was used as the grinding fluid at a flow rate of approximately 103 cubic feet per minute, free air. Both runs were made in a vertical mill having an 8 inch diameter grinding chamber. The first run was made without fillers. The second run was made with fillers as shown in Figures 4 and 5. Both mills were equipped with four replaceable nozzles tangent to a 6 inch diameter circle. The improvement in the product, from the standpoint of uniformity of particle size, is graphically presented in Figure 11. The improvement in the product from the standpoint of reduction of the average particle size is graphically presented in Figure 12.

Example III Two test runs were made grinding procaine penicillin, with the objective of producing a product having a particle size between 20 and 50 microns and a maximum particle size of 100 microns- The first run was made with a. two inch Product Size Distribution in Microns Maximum Lesszghan 20-50 50-120 Percent Percent Perccm First run 120 50 45 Second run 5O 30 70 0 From this test it is seen that not only may the grinding rate be accelerated but a product of more uniform particle size can be obtained, reducing the quantity of both undersize and oversize particles.

The improvement in the product due to the addition of the fillers is much more than an incidental improvement. The improvement not only produces a better product at the slower feed rates but the degree of improvement between the two products increases as the feed rate is increased.

MODIFICATIONS It will be recognized that the fillers described in the mills i, 30 and I00 may either be used as standard equipment in new mills of circular de sign or may be used for converting existing circular mills. It is also possible to design new mills equipped with noncircular grinding chambers without employing the fillers. Such a mill is shown in Figures 6 and '7. It is also possible to construct a mill having a noncircular grinding chamber using plate material for the jacket defining the grinding chamber. Such a mill I20 (Figure 10) will have the conventional end plates and a circular outer wall 123 provided with a grinding fluid inlet I24. The wall I25, however, separating the grinding chamber I26 from the grinding fluid chamber I2? is formed to provide the desired shape of the grinding chamber. This wall i25 can be shaped to provide any one of the desired, noncircular, geometric shapes. In this way, the fillers can be dispensed with and the same improvement in operation obtained.

These and other modifications of my invention may be made without departing from the principle of my invention. Each of the modifications is to be considered as included in the hereinafter appended claims unless these claims by their language expressly state otherwise.

I claim:

1. In a fluid jet pulverizer having a material inlet conduit and an offtake conduit, the combination including: a rectilinear grinding chamber; a plurality of jets for admitting gas under pressure to said grinding chamber, each of said jets being arranged tangential to a theoretical circle of greater diameter than said offtake conduit and concentric therewith, to form a classifying vortex within said grinding chamber.

In a fluid jet pulverizer having a material inlet conduit and an oiftake conduit the combination including: a wall defining a rectilinear grinding chamber and. a plurality of jets therethrough for admitting gas under pressure to said grinding 10 chamber, each of said jets being tangential to a theoretical circle of greater diameter than said offtake conduit and concentric therewith to form a classifying vortex within said grinding cham- ,ber; and each of the straight portions of said walls of said grinding chamber being substantially parallel to the axis of one of said jets.

3. In a fluid jet pulverizer having a material in- ;let conduit and an oiftake conduit the combination including: a wall defining a rectilinear rind- ,ing chamber; a member for substantially closing .each of the ends of said grinding chamber; a plurality of jets through said walls for admitting a gas under pressure to said grinding chamber, .forming a classifying vortex within said grinding chamber, the number of jets being equal to the number of straight portions of the said walls of said grinding chamber and arranged tangential to a theoretical circle of greater diameter than said ofitake conduit; each of the said straight portions of said grinding chamber being substantially parallel to the axis of one of the said jets.

4. In a fluid jet pulverizer having a circular Wall defining a grinding chamber therein and members substantially closing the ends of said grinding chamber except for inlet and offtake conduits, a plurality of jets through said circular wall for admitting grinding fluid to said grinding ,chamber, each of said jets tangent to a circle of greater diameter than said ofitake conduit within said grindin chamber, the improvement in said pulverizer including: a plurality of segmental fillers mounted about the periphery of ,said grinding chamber, the arcuate face of each of said fillers being mounted against said circular Wall, the chord face of each of said fillers being substantially parallel to one of said jets and radially outwardly of said circle; the number of .said fillers and of said jets being equal; and, said jets each extending through one of said filler plates and into said grinding chamber arranged to form a classifying vortex within said grinding chamber.

5. In a pneumatic pulverizer having a material inlet conduit and an oiftake conduit the combination including a wall defining a rectilinear grinding chamber and a plurality of openings through said wall for admitting gas under pressure to said grinding chamber; a removable nozzle having a central passageway therethrough mounted in 1 each of said openings; the face of said nozzle directed toward said grindin chamber being per- ,pendicular to said central passageway; the central passageway of each of said jet being tangential to a circle of greater diameter than said oiftake conduit and arranged to form a classifying vortex within said grinding chamber.

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