US3734413A

US3734413A - Fluidized bed jet mill

Info

Publication number: US3734413A
Application number: US00135919A
Authority: US
Inventors: F Kaiser
Original assignee: Alpine AG
Current assignee: Alpine AG
Priority date: 1970-08-14
Filing date: 1971-04-21
Publication date: 1973-05-22
Anticipated expiration: 1990-05-22
Also published as: DK147654B; DE2040519C2; DE2040519A1; DK147654C; AT322952B; GB1368284A; FR2104091A5

Abstract

The method and apparatus disclosed relates to an improved jet mill in which the material to be ground is fed into a chamber or zone containing a jet that emanates from a nozzle with high velocity, the material being in the state of a compact bed and the outlet of the nozzle being located beneath the surface of the bed. The bed is preferably a ''''moved compact bed'''' arranged so that the material to be ground is fed sideways into the jet. A sifter or classifier for the coarse and fine material produced in the grinding operation is arranged above the bed of material in the grinding chamber or zone. The compact bed of material may be moved in a variety of ways, and the volume occupied by it is filled up to the greater part with compact granular material. This could be defined as the ratio of wholly occupied volume to the volume of the compact matter, or by the term ''''compact current'''' as commonly employed in the pneumatic transportation art.

Description

Write States Patent 1 Kaiser [54] FLUIDIZED BED JET MXLL [21] Appl. No.: 135,919

[30] Foreign Application Priority Data Aug. 14, 1970 Germany ..P 20 40 519.0

[52] US. Cl ..24l/39, 241/5 [51] Int. Cl ..B02c 19/06 [58] Field of Search ..24l/l, 5, 18, 39, 241/40 [56] References Cited UNITED STATES PATENTS 3,311,307 3/1967 Lopker ..24l/39 X 1,948,609 2/1934 Andrews et a1 1 ..24l/5 1,791,100 2/1931 Lykken ..24l/5 2,072,492 3/1937 Anger .....24l/40 2,103,454 12/1937 Graemiger et a1 .241/40 45] May 22, 1973 Primary ExaminerGranville Y. Custer, Jr. AttorneyDean S. Edmonds et al.

57 I ABSTRACT bed. The bed is preferably a moved compact bed arranged so that the material to be ground is fed sideways into the jet. Asifter or classifier for the coarse and fine material produced in the grinding operation is arranged above the bed of material in the grinding chamber or zone The compact bed of material may be moved in a variety of ways, and the volume occupied by it is filled up to the greater part with compact granular material. This could be defined as the ratio of wholly occupied volume to the volume of the compact matter, or by the term compact current" as commonly employed in the pneumatic trans portation art.

10 Claims, 20 Drawing Figures Patented May 22, 1973 I 3,734,413

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IN VEN TOR FLUIDIZED BED JET MILL BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to jet mills and, in particular, to the mill-type in which the material to be comminuted is fed into a gas jet of high velocity wherein such material is disintegrated by interparticle impact.

2. Description of the Prior Art A jet mill and its operation according to the invention is distinct from known types of mills in which the material to be ground is speeded up in an injector or by means of a gas jet, and subsequently disintegrated by impact upon a baffle plate or by collision with material from a jet of the same kind moving in counter direction. Known types of jet mills have in their interior, in addition to the gas jet for obtaining size reduction, a circulating current of the disintegrating gas by which the material to be comminuted is constantly refed to the gas jets, and the material which has been sufficiently reduced in size is separated and carried out of the mill.

The methods employed in these prior known jet mills, however, have the following disadvantages:

They have an inherent, very high power consump-.

tion;

A very small quantity of material is brought into the jet, especially in fine grinding processes that require high velocities and classification of the ground material;

The jet energy is exploited very incompletely in actual grinding;

The efflux velocity of the jets at the nozzles is reduced by the counterpressure of the classifier, and because of high turbulence in the mill, the classifier or sifter operates inefficiently so that it is necessary to run it an extremely high velocity requiring considerable consumption of pressure to obtain sifting at a high fineness of grain.

Some of the drawbacks referred to are eliminated according to German Pat. Specification No. 1,164,806, of which the present inventor is a joint inventor. According to the disclosure of this German patent, the gas jets are directed from the top down onto the surface of a bed of material to be ground which is constantly renewed with the aid of a mechanical transport means. The design provided in this patent, however, has presented some mechanical difficulties which up to the present have not been overcome.

SUMMARY OF THE INVENTION In accordance with the present invention, the jet mill comprises a grinding chamber with a jet or jets mounted in its lower portion, the chamber providing a grinding zone in which the granular material to be ground is comprised of a compact bed in which the gas jet nozzle is located and from which a gas jet emanates with high velocity. In this arrangement granular material is in the state of a moved compact bed which is preferably moved sideways into the gas jet emanating from the nozzle which is located beneath the surface of the bed. The energy of the jet is used up to disintegrate the granular material in the bed, and while a sitter or classifier may be arranged above the bed, it is not operated by the impetus or velocity of the gas jet.

The term moved compact bed of material as used herein includes all varieties of motion of a granular material, the occupied volume of which is filled up to the greater part with compact matter. It could be defined as the ratio of the wholly occupied volume to the volume of the compact matter or by the term compact current" as usually used in pneumatic transportation. Examples for the motion in the compact bed are: the displacing of a solid bed as a whole with the aid of a moved support such as, e.g., a conveying belt;

the displacing of the material within the solid bed by means of a continuous conveyor, e.g., screw or chain conveyor, insofar as the formation of gaps in the material is prevented by a suitable design;

the slipping down of the contents of a bin by discharging;

the sliding down (in case of banking-up from below) or the shooting down (in case of unhindered movement) on an inclined chute or in an oblique conveyor pipe;

the fluidized bed;

the pneumatic pipe transport in the compact current (with free path of the particles considerably shorter than the pipe diameter).

Tests carried out in connection with the invention have shown that the realization of a high-loading of the jet with material and the missing or lack of power consumption for producing the circulating current increase the grinding efficiency. On the other hand, the higher consumption of mechanical or pneumatic energy by the separate sifter is insignificant.

The scope of the basic invention comprehends a plurality of configurations and arrangements such as, e.g., for wear protection, arrangement of nozzles, means for material circulation, sitters and propellants which can be interconnected in a unit-composition manner, the most advantageous of which are described in connection with the drawings.

According to an extended arrangement of the inventive jet mill, a wear protection can be obtained by en-- larging the air volume above the surface of the material bed so that air resistance and gravity are sufficient to retard the flying grains of material. The wear by impact upon the mill wall then will be tolerable. This arrangement is especially suitable for very fine feed material because of the short stopping distance of the grains in the air.

As a wear protection, an armour-plate may be provided, such as, e. g., an easily exchangeable baffle plate arranged vertically to the direction of the jet and above the surface of the material bed. This is particularly suitable as an additional provision in case of a precious product that admits of only a small charge of material, or if the charge occasionallyis not present, e. g., if the mill is emptied.

BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of the invention are illustrated schematically by way of example in the accompanying drawings in which the figures are vertical, sectional views respectively and further defined as follows:

FIG. 1 illustrates an embodiment of a jet mill with vibrator and rotary lock with chambers, in which the mill nozzle is directed perpendicularly upward;

FIG. 2 illustrates a .jet mill including a mill chamber having a perforated bottom for use in fluidizing the material being ground, the mill including nozzles providing an intersecting jet system and a standpipe lock;

FIG. 3 illustrates a mill chamber provided with a horizontal jet nozzle and with a screw conveyor for effecting inner circulation of granular material;

FIG. 4 illustrates ajet mill provided with a screw conveyor for feeding and inner circulation of material;

FIG. 5 illustrates a multi-jet mill provided with a material stirrer;

FIG. 6 illustrates ajet mill provided with a bucket elevator for circulation of granular material and with a baffle plate located above the material bed in the grinding chamber;

FIG. 7 illustrates a jet mill provided with pneumatic means for feeding material into the jet;

FIG. 8 illustrates a jet mill assembly including an air stream sifter (a spiral air classifier) in one unit;

FIG. 9 illustrates a stirrer-type sifter (with counteracting blades);

FIG. 10 illustrates a rising-tube sifter with several zigzag-shaped sifting channels arranged in parallel;

FIG. 11 illustrates a spiral air sifter with one rotating front wall and blades generating the rotary current;

FIG. 12 illustrates a sifter provided with airpropelling jet nozzles;

FIG. 13 illustrates a simple rising-tube sifter or classifier;

FIGS. 14 and 14a illustrate a combined jet mill rotortype classifier with zig-zag-shaped sifting channels, material admission along part of the rotor circumference and with a screw conveyor for returning the coarse material to the jet mill;

FIG. 15 illustrates a rotor with zig-zag-shaped separating channels and arranged for material admission along the entire circumference of the rotor;

FIG. 16 illustrates a nozzle provided with a jet needle controlled pneumatically;

FIG. 17 illustrates a nozzle structure having; an adjustable slot;

FIG. 18 illustrates a nozzle structure having an annular slot, and

FIG. 19 illustrates a jet mill provided with a baffle plate adapted to act as a level tracer.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the jet mill, represented in FIG. I, a single nozzle 3 having a perpendicular axis is located at the foot of the funnel-shaped lower part 1 of the mill bin or grinding chamber 2, which is supplied with compressed air from an air chamber 4. The feed material to be ground is brought through a charging hopper 6, a rotary lock with chambers 7 and a flexible delivery tube 8 into the spring-mounted bin 2 which is oscillated by a vibrator 9, thus causing the material to slide continuously downward into the cavity swept free in the bed 5 by the strong jet. This embodiment, in which the jet emanating from the nozzle is vertical and retarded by the material bed, is particularly advantageous since it allows rather small bin dimensions and mill contents without the risk of locating the inner wall of the bin, the feeding-t In the embodiment shown in FIG. 2, a nozzle plate 10 of the jet mill carries nozzles 3 placed in such a way that their jets'collide with each other at an acute angle. The nozzles are supplied with compressed air from the air chamber 4. The material to be ground, suitable to be fluidized, is brought into the mill chamber 2 through charging hopper 6 and a standpipe 11, and is fluidized byair which flows out of the annular air chamber 12 through the porous bottom 13 extending around the nozzle plate 10 into the material bed 5, thus causing the material of the bed to continuously flow into the jets. Two or more nozzles may be mounted in the plate 10, but at an acute angle to a control axis so that one nozzle is not damaged by grains from thejet of another nozzle.

In FIG. 3 a jet mill with a horizontal nozzle is shown in which the material bed 5 is dimensioned so high and wide that no particle of material can be forced through the material bed by the kinetic energy of the jet. In this case when a sheet-metal plate was fixed above the material bed, it did not show any wear phenomena. The mill contents is circulated by a screw conveyor 14 which presses the material sideways into the gas jet coming horizontally out of the nozzle 3, that is supplied with compressed air from air chamber 4.

In FIG. 4 the material is passed into the mill bin by a screw conveyor 14 which subsequently forces it sideways into the jet from nozzle 3, blowing perpendicularly upward, and being supplied with compressed air from air chamber 4. Material continuously slides from the bed towards and into the screw conveyor 14 which continuously forces material inwards into the jet as indicated by the sloping surface 16 of the material bed. A bin wall 15 is made conical in its lower part, as shown diagrammatically, in order to reduce mill contents. It is, of course, possible to furnish this jet mill and as well those which are shown in FIGS. 1 and 3 with several nozzles and, respectively, with pairs of nozzles, and eventually with several screw conveyors provided that the lower part of the mill chamber is of adequate size.

FIG. 5 illustrates an embodiment suitable as a largescale jet mill. It is provided with a stirrer l7 having a vertical axis 18 or drive shaft. The nozzles 3 of this mill which are supplied with compressed air from an air chamber 19, are placed as a ring around the stirrer 17. The material for the bed 5 is fed to the mill by the controlled double swing flap 20, of any known kind, through the duct 21.

The material 5 to be ground by the jet mill of FIG. 6 is carried up again and again by the bucket elevator 22 to the chute 23 on which it slides down hitting sideways the upward blowing jet of the nozzle 3 which is supplied with compressed air from the air chamber 4. This mill is furnished with an easily exchangeable wearprotecting or absorbing baffle plate 24 mounted vertically above the direction of the jet and above the bed of material in the grinding chamber. Raw material may be supplied to the bucket elevator.

In FIG. 7 ajet mill is shown in which the feed material is conveyed by a pneumatic transport means as a compact current (piston conveyance) and is injected sideways into the upwardly blowing jet issuing out of the nozzle 3 that is supplied with compressed air from the air chamber 4. The material to be ground is fed through a charging hopper 6 and the rotary lock with chambers 7 into a storage bin 25 from where it is brought out by a blow-through rotary cell lock 26 into a duct 27 which is supplied with compressed air through air inlet mouth 28. From the duct 27 the material is injected sideways into the jet from nozzle 3. The transport means effects both feeding and inner circulation. The height of the material surface 16 inside the mill bin 2 is kept at a constant value by an overflow duct 29 which connects mill bin 2 with storage bin 25.

A separator, sifter or classifier, such as illustrated in FIGS. 9 to 15, may be mounted on the mills of FIGS. 1 to 7. Such a combination unit is illustrated in FIG. 8 in which a stationary air separator is mounted at the top of the jet mill. As it is known, such an air separator or sifter consists, in the main, of a flat cylindrical sifting chamber 70 and the adjustable guide vanes 30. In the same way as shown in FIG. 4, the material to be processed is passed into the mill by a screw conveyor 14 and subsequently forced sideways into the jet from nozzle 3, blowing perpendicularly upward, and being supplied with compressed air from air chamber 4. The coarse material being separated within the spiral air sifter falls back through duct 31 to the screw conveyor which forces it into the jet again, whereas the fine grained material is carried away at the top of the sifter in the air stream through duct 32 for final separation. For the same reasons as given for FIG. 4, the bin wall 15 opposite the screw conveyor is made conical.

FIG. 9 illustrates a stirrer type sifter, of particular advantage for ground material of medium fineness, which alternatively can be combined with any of the jet mills according FIGS. 1 to 7. The rotatable stirrer 33 is, in this case, fitted with an exchangeable baffle plate 24 for wear protection. The coarse material thrown out from the stirrer 33 moves along the inside of wall 34 of the sifting chamber back into the bin of the jet mill beneath the sifter, whereas the fine material separated by the sifter is carried away through a duct 35 for later recovery from the air stream. The stirrer is known from scatter type wind sifters as having counter-acting blades. Its task is to put the air ascending in a cylindrical separating chamber into rotation.

FIG. illustrates a rising tube sifter or classifier, with several zig-zag-shaped sifting channels 36 arranged in parallel, which is mainly useful in cooperation with a mill for coarse grinding. The coarse material in the sifter tubes simply falls back into the mill bin below. The fine material discharging at the top of the sifting channels is carried away for recovery through duct 37. A sifter of this type is, e.g., known from German Pat. No. 1,482,427, or British Pat. No. 1,014,723.

The sifter of FIG. 11 is a spiral air sifter with one rotating front wall 38 and blades 39 for initiating the air rotation as in German Pat. No. 1,507,466. The coarse material in this sifter is simply thrown back into the bin of the jet mill beneath the sifter. The fine material discharged centrally from the top of the sifter is carried away through a duct 40.

FIG. 12 illustrates a stationary spiral air sifter in which rotation is effected by propelling jets 41. This sifter which makes possible classification to very high fineness, flings the separated coarse material back to the bin of the jet mill without the aid of movable structural parts. The separated fine material is centrally discharged at the top of the sifter and carried away through duct 42.

FIG. 13 shows a simple rising tube sifter 43 with a single sifting channel or chamber designed for mounting on ajet mill. The coarse material separated in this sifter simply falls back into the bin ofjet mill. The fine material is discharged through the conically contracted upper part 44 of the sifter and the duct 45.

FIGS. 14 and 14a show in two views a sifting rotor 47, like that of the applicants U. S. Pat. No. 3,089,595, with radial zigzag-shaped sifting channels 48, rotating round a horizontal axis, combined with a jet mill according to the present invention in which the material to be ground is passed into the mill by the screw conveyor 14, as shown for example in FIG. 8, and subsequently is forced sideways into the jet of nozzle 3 which has an upward direction and which is supplied with compressed air from air chamber 4. The classifier rotor 47, by which particle separation of the highest fineness and selectivity can be achieved, is fed from the mill along section 49 to the circumference of the rotor. The coarse material thrown out along section 50 of the rotor circumference falls back through the duct 51 to the screw conveyor 14 which forces it again into the jet 3, whereas the time material separated by the rotor is carried away centrally through duct 52.

The feed of ground material to the sifting rotor, rotating on the axis 53, with radial zig-zag-shaped sifting channels 48, shown in FIG. 15, is maintained along the whole circumference 54 of the rotor. The separated coarse material is thrown back into the chamber 55 from which it is returned to the jet mill. The fine material is carried away centrally of the rotor and discharged through a duct 56. An additional narrow blade wheel 57 is placed between the upper face of the rotor and the casing, the blades of which, having recesses 58, are closely matched with the collars 59 on casing wall 60. This blade wheel 57 is provided to prevent material from passing through the gap between the upper face of the sifting rotor and the casing wall 60.

FIG. 16 illustrates a closing means for the nozzles in the form of a jet needle 62 preferably controlled by a pneumatic switch or actuating means 61. This closing means prevents material from falling into the nozzles when the mill is stopped or when the supply of grinding gas fails.

The slot nozzle 63, shown in FIG. 17, which can be manufactured more cheaply and exactly than many small nozzles, allows the width of the slot 64 to be adjusted by shifting a plate 65 with the aid of a screw 66. The compressed air is supplied to and distributed by air chamber 4. The length of the slot is selected according to the jet area of the mill.

FIG. 18 illustrates a form of annular slot nozzle which can be used in mills where the material to be ground is fed to the nozzle symmetrically from outside, e.g., by fluidization, or from inside, e.g., by a stirring apparatus.

The nozzle structure is readily understood from the showing and may be used in place of a number of small nozzles.

FIG. 19 shows a mill provided with a baffle plate 67 acting as level tracer which is mounted above the material surface 68. A signal or control switch 71 is oper ated by the tracer arm if the impellent power of the ascending jet of air and material acting on the baffle plate exceeds a definite value, that can be fixed by adjusting the position of a sliding weight 69. The level tracer shown in FIG. 19, or other types of level tracers, such as radioactive emitters or weighing types, may be used so that it is possible to control the level of the body of material in the mill chamber and correlate it with the said return and grinding output. The control 71 may be used to actuate the feeder which may also be responsive to the return grinding output. In a non-fluidized bed of material in the grinding chamber, the jet discharging from the nozzle is retarded in the bed of material. If the baffle plate 67 is affected by a force, it is an indication that the jet may be breaking through the material bed because the level is too low. This force will actuate the switch 71 and cause an increase in the feed return.

In the preferred type of mill according to the present invention, a fluidized bed is employed and the material ascends therefrom upwardly in a strong jet. Tests have shown that a baffle plate such as that shown in FIG. 19, located at a predetermined height, receives an impellent power from the jet which is the greater, the higher the level of material in the chamber. If there is a decrease of power applied to the baffle plate below a certain point, this point may serve as a signal point for operating the control 71.

The material in the mill must generally be fed to the jet several times to complete reduction. The mill, therefore, runs with a high inner circulation rate by which the unground material thrown out by the jet is constantly re-fed to the jet. Also the coarse material that had been rejected by the sifter is brought back to the jet. Consequently, the following currents of material are circulating in a mill unit according to the present invention:

1. The material fed to the mill.

2. The transport of material of the bed into the jet, the conveyance inside the jet and the discharge back to the material bed.

3. The discharge out from the material bed to the classifier (caused by the above-described fountains of material without any additional provision).

I 4. The reflux of the coarse material back to the material bed or into the jet.

5. The flow of the fine material from the classifier to a separator.

The currents numbered 2, 3 and 4 represent the inner circulation.

To obtain this circulation, any suitable conveying devices can be used. For example, the material may be fed into the jet by a stirrer that necessarily must be designed in such a manner that it never can be hit by the jet. Expecially in case of large-scale mills a stirrer with a perpendicular axis and nozzles spaced around said stirrer may be used as illustrated in FIG. 5. Such a ring of nozzles that has a diameter of l m, in which the nozzle diameter is 5 mm and the circular pitch is mm, yields a clear opening of the nozzles of 63 cm by which, at a throughput of compressed air of 38 tons per hour and a jet power of about 1,300 kW can be obtained.

The advance in the art provided by the jet mill of the present invention can be recognized from typical test results presented as follows:

In this test a jet mill according to FIG. 4 with a bin diameter of 630 mm was operated in combination with a sifter according to FIG. 15, that has an outside diameter of 400 mm, a sifting rotor with zigzag-shaped radial sifting channels, revolving at a speed of 2,000 rpm. Limestone power was used as test material, having been obtained as the fine material from a usual wind sifter, with 50 per cent minus 12 microns and a specific surface of 4,000 Blaine. For comparison, part of the same material was ground in a conventional spiral jet mill of the type known first under the trademark MI- CRONIZER," having the characteristic features: Outside diameter of the grinding chamber 200 mm, six nozzles each with an opening of 2.5 mm, one injector nozzle with an inner width of 3.8 mm.

To determine the expenditure for the grinding process, the jet output that could be obtained by adiabatic expansion, was calculated out from the measured pressure of the compressed air and the cross section area of the nozzles. It is represented by the intelligible unit jet-kW."

To determine the grinding result the specific surface of the material before and after milling was evaluated by Blaine Test and therefrom the new surface was computed that was produced during 1 hour. intelligible unit is km lh.

The grinding efficiency is defined by the specific power consumption in jet-kwhlkm which is the ratio that the expenditure for the grinding process bears to the grinding result.

MILL TYPE Conventional According to the Invention Day/No. of test 3 l .7701] 4.8.7011 Pressure of compressed air 6 5.5 (kg/cm over air pressure) Throughout of air g/h) 24 730 Jet output (jet-kW) 8.3 28.8 Yield of material (kg/h) 15 63 Specific surface of 8000 9700 material (cm/g) Increase of surface (cm lgl 4000 5700 Specific power consumption I370 690 (jet-kWh/km) The height of material bed above the nozzle outlet in the new jet mill during the test was about 500 mm, the bed being fully fluidized with the material moving by itself to the nozzle. The mixture of air and material ascended as a strong jet with a diameter of about 5 cm vertically upward out from the bed, but at such a low velocity that no wear of the paint on the sifting rotor was visible at the area of impact. Wear was perceptible only on the periphery of the sifting rotor because of its circumferential speed. The values of mass transportation and velocity of the ascending jet, however, could be estimated inexactly at 35 t/h and 2.5 5 m/sec.

The gaseous grinding or fluid agent for disintegrating granular material by inter'particle impact according to the present invention may be compressed where as described either at ambient temperature or in a heated or cold condition. Water vapor at any suitable temperature or inert gas may be used. Since the jet mill according to this invention may be built up in a size that allows working with several thousand kilowats, the outlet of a gas turbine may be employed as the gaseous medium in a compressed and heated condition. A gas turbine would have many advantages over the use of steam boilers or internal combustion engines while at the same time providing an effective grinding aid.

The jet mill according to the present invention allows the material bed to be rated in height and width so large that the kinetic energy of the jet is unable to force any grain through the whole bed. For this purpose the mill may be filled up with so much material that wear, even on a plate above the bed, is not noticeable. A surplus material in the mill, that has no detrimental effect at all, may represent a valuable reserve in case of varying conditions of work. With a high bed as described the disintegrating gas emerges bubble-shaped throwing up fountains of material as in a fluidized bed. As these fountains are not produced by the kinetic energy of the jet, but by the pressure drop within the material bed, they move so slowly that they virtually cause no wear in the mill. This method of working presents the advantage of eliminating any wear phenomena in the jet mill and of fully utilizing the kinetic energy of the jet and of the accelerated particles for interparticle impact and for size reduction. In addition, there is a minimum of noise effects. In most cases it is not necessary to pile up excess material in the mill in order to use up the whole kinetic energy of the jet and of the particles of the material acted on by the jet.

In a non-fluidized bed the jet creates a cavity that is entered from the side by the grains of the material being conveyed by the transporting systems that are provided for the process of circulation in the mill chamber. These grains are accelerated by the jet and, at a sufficient height of the bed, they are thrust entirely into the unmoved part of the bed, thereby effecting the process of size reduction by consumption of the kinetic energy. The impellent power of the jet presses against the lower part of the bed, thereby diminishing the pressure of it upon the base of the mill. The weight of the bed within the range of action above the jet (e.g., a cone with an aperture with twice the size of the angle of friction) should be at least equal to the impetus of the jet, thus determining the minimum height of the bed. In practice it must necessarily be higher according to a certain safety factor to prevent shooting through gaps and chimneys.

Depending upon the kind of material being processed, the gas jet either ascends after being retarded, in diffuse state between the grains or forms clefts and chimneys from which grains may secondarily fly upwards. Their speed is, however, so low that they do not cause any wear since the only accelerating power acting upon them is the fall of pressure inside the bed.

The phenomena in a fluidized bed of material are quite different. Immediately after discharging from the nozzle, the jet sucks up the surrounding, fluidized material and immediately after passing a short distance the jet is saturated with the material; and, under these conditions, the main part of size reduction by inter-particle impact is likely to have taken place. Compared to the small cross-sectional area of the jet, the jet velocity is yet rather high. As more material is sucked in, the speed of the jet decreases further on and the crosssectional area increases. Contrary to the efflux of an air jet under water, where the airjet is broken up into bubbles under the influence of turbulence and surface tension, the jet under consideration retains its compactness along a considerable distance and it comes out from the fluidized material bed in a compact state. Though this emerging jet may still carry a large amount of the impellent power of the original gas jet, its velocity is so low and its mass so voluminous that it is no longer capable of causing any wear. Therefore, in a fluidized bed it is enough to pile up the material so high that the jet of material still ascending with part of the impetus of the original gas jet does not cause any wear at any area of contact with the mill surface. This relation to the spot of striking is important for the reason that the jet emerging from the bed already with a low speed is further retarded by gravity.

I claim:

1. A jet mill for comminuting or grinding granular materials by inter-particle impact, comprising a grinding chamber containing a bed of granular material to be disintegrated, means for supplying granular material to the grinding chamber to maintain the bed, an upwardly directed nozzle means located in the lower portion of the chamber for delivering a high velocity jet of gaseous material into the bed of granular material in the chamber, means for delivering gaseous material to the nozzle means to provide the jet, and means for moving granular material to be acted on by the jet into a side of the jet of gaseous material issuing from the nozzle means.

2. A jet mill as claimed in claim 1, wherein the nozzle means is set to direct a jet of gaseous material vertically into and through the bed of material in the chamber.

3. A jet mill as claimed in claim 1, wherein the nozzle means is provided with means for automatically closing its outlet in response to the stopping of the delivery of gaseous material thereto.

4. A jet mill as claimed in claim 1, wherein the nozzle means includes means for delivering a plurality of converging jet streams which collide at an acute angle in the bed of material.

5. A jet mill as claimed in claim 1, wherein the nozzle means includes a plurality of upwardly directed nozzles arranged in a ring, and a stirrer located inside the ring of nozzles for moving granular material into the jets issuing from the ring of nozzles.

6. A jet mill as claimed in claim 1, including means for introducing and distributing a gaseous means into the lower portion of the bed of material in the grinding chamber to fluidize the bed.

7. A jet mill for comminuting or grinding granular materials by inter-particle impact, comprising a grinding chamber containing a bed of granular material to be disintegrated, means for supplying granular material to the grinding chamber to maintain the bed, an upwardly directed nozzle means located in the lower portion of the chamber for delivering a high velocity jet of gaseous material into the bed of granular material in the chamber, means for delivering gaseous material to the nozzle means to provide the jet, a feed bin, an overflow from the mill chamber into the feed bin, and means for supplying granular material from the bin into the side of the jet of gaseous material issuing from the nozzle means.

8. A jet mill for comminuting or grinding granular materials by inter-particle impact, comprising a grinding chamber containing a bed of granular material to be disintegrated, means for supplying granular material to the grinding chamber to maintain the bed, an upwardly directed nozzle means located in the lower portion of the chamber for delivering a high velocity jet of gaseous material into the bed of granular material in the chamber, means for delivering gaseous material to the nozzle means to provide the jet, means for moving granular material to be acted on by the jet into a side of the jet of gaseous material issuing from the nozzle means, and means for vibrating the bed of granular material in the grinding chamber.

9. A jet mill for comminuting or grinding granular materials by inter-particle impact, comprising a grinding chamber containing a bed of granular material to be disintegrated, means for supplying granular material to the grinding chamber to maintain the bed, an upwardly directed nozzle means located in the lower portion of the chamber for delivering a high velocity jet of gaseous material into the bed of granular material in the chamber, means for delivering gaseous material to the 10. A jet mill as claimed in claim 9, wherein the level tracer includes a baffle plate located in the upper part of the grinding chamber and acted on by the jet issuing upwardly from the body of granular material in the granular material bed in the mill chamber, and a conchamber.

trol actuated by the movement of the level tracer.

Claims

9. A jet mill for comminuting or grinding granular materials by inter-particle impact, comprising a grinding chamber containing a bed of granular material to be disintegrated, means for supplying granular material to the grinding chamber to maintain the bed, an upwardly directed nozzle means located in the lower portion of the chamber for delivering a high velocity jet of gaseous material into the bed of granular material in the chamber, means for delivering gaseous material to the nozzle means to provide the jet, means for moving granulaR material to be acted on by the jet into a side of the jet of gaseous material issuing from the nozzle means, a level tracer for controlling the level of the granular material bed in the mill chamber, and a control actuated by the movement of the level tracer.

10. A jet mill as claimed in claim 9, wherein the level tracer includes a baffle plate located in the upper part of the grinding chamber and acted on by the jet issuing upwardly from the body of granular material in the chamber.