US2552603A

US2552603A - Apparatus and method to comminute solid particles in gas

Info

Publication number: US2552603A
Application number: US46507A
Authority: US
Inventors: Herbert G Tanner
Original assignee: Individual
Current assignee: Individual
Priority date: 1948-08-27
Filing date: 1948-08-27
Publication date: 1951-05-15
Anticipated expiration: 1968-05-15

Description

H. G. TANNER May 15, 1951 APPARATUS AND METHOD TO COMMINUTE SOLID PARTICLES IN GAS 2 Sheets-Sheet 1 Filed Aug. 27, 1948 Shiv Herbert G. Tunnr,

awk/ 1% ATTOR N EY H. e. TANNER 2,552,603

APPARATUS AND METHOD TO COMMINUTE SOLID PARTICLES IN GAS May.15, 1951 2 Sheets-Sheet 2 Filed Aug. 27, 1948 FIG. 2

FIG. 3

INVENTOR HERBER'IY G. TANNER,

ATTORNEY Patented May 15, 1951 UNITED STATES PATENT OFLFHCE.

APPARATUS AND METHOD TO COMMINU'IE SOLID PARTICLES IN GAS (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) 7 Claims.

The invention described herein may be manufactured and used by or for the Government, for governmental purposes, without the payment to me of any royalty thereon.

This invention deals with comminution of finely divided solid particles by means of highly turbulent gas. It relates to improved apparatus and methods for applying such particles to turbulent gas to break them into extremely fine dusts.

Heretofore currents of air have been used quite commonly to separate or classify particles of one size or weight from another and even to grind the particles to some extent. Jets of air in prior grinding mills or colloid mills rub solid particles against each other or else throw the particles against abutting elements of the grinder. Often the gas recircuclates the material to be ground as it is blown around in the apparatus. This recirculation indicates defects such as inefficient grinding and such as classification or great variation in particle size. Gas currents become less effective as particle size decreases and as lightness of the material increases. The use of gas in burdensome quantities has been heretofore necessary to obtain notable reduction in size.

Such dilution in turn introduces difficulties in separating fine particles from relatively large volumes of gas, requiring elaborate filtering systems. Consequently jet grinding or, in general, comminution depending on air currents, has been of limited use to reduce solid particles to extremely small size, as for example sizes less than about microns.

Another problem evident in comminuting solids, even with gas jets or gas streams, is the problem of heat. Heat develops when a particle is shattered by impact. The intensity of this heat may be considerable, at least locally. With some substances avoidance of local temperature increase becomes critical. Many substances deteriorate or are destroyed by the heat developed locally in pulverization. A suitable example is penicillin, for which some types of grinder are entirely unsuited and even jet-type machines are inefficient to conserve antibiotic activity. Many substances other than pharmaceuticals are even more sensitive or labile,- so that the heat problem is burdensome and in some cases is critical.

In pulverizing, deteriorative effects result also from oxidation in some instances, or local static electricity in others, or even from mechanical shock in others. Such efiects are difficul-t to avoid because they develop within the individual particles that are affected.

A broad object of this invention is to comminute small solid particles effectively and with minimum deleterious effect, even to extremely fine particles or the order of 30 microns or less. A subsidiary object is to accomplish such pulverization that a large portion of these fine particles are of any desired size and are of substantially uniform size. Another object is to produce these fine dusts, with relatively small volumes of gas, so that these dust particles can be readily recovered.

A further object is to obtain substantially homogeneous products in pulverizing solid particles of differing qualities. This is important with pharmaceuticals. For example, vitamin mixtures must be uniform so that uniform dosages may be capsulated. Not only must segregation of some solids from other be avoided, but deterioration of some constituents also must be prevented when rending to small sizes. Many colloid mills are deficient in these respects. To obtain uniform products of ultra-fine solids, has formerly been extremely diflicult, even practically impossible without altering the activity of delicate components.

Various other advantages of this invention will become apparent as exposition of this invention proceeds. It will be understood that suitable equivalents are contemplated in the illustrations and the definitions of the attached claims.

In the attached drawing illustrating a preferred embodiment of this invention, Figure 1 shows generalized apparatus diagrammatically, particularly including a smoothfaced, high-speed comminuting wheel 25. Figs. 2-6 inclusive show various illustrative forms of rotor peripheral face and annular operating zone. In these figures the operating zone is shown in fragmentary view somewhat enlarged relatively to the rest of the structure.

This invention applies extremely turbulent gas to solid particles to tear the particles into very fine dusts. The term gas as used herein is intended to include vapors. Avoided are mechanical impact elements; even impacts of particles against each other are minimized.

The apparatus shown in the drawing illustrates various principles of structure and of operation under this invention. The apparatus is designed to produce extreme turbulence in a shallow comminuting zone of gas 22. Means are provided to conduct small solid particles through this violently turbulent space for comminution, but with relatively little volume of gas. Many benefits of the invention are made possible by moving a surface bounding the comminution zone at speeds of about 108 to 700 feet per second to produce turbulence. This surface is preferably smooth, but the arrangement diagrammed is effective to pulverize solids to very fine dusts.

The preferred assembly of the apparatus illustrated in Figure 1, includes a tank 38 for supplying gas under pressure in communication with a feed valve l3, pressure control means 3|, and inlet conduits i2 and Ill. Connected to the inlet conduits i2 and iii by means of flexible hose in upper and lower portions 35, is the agitating chamber H provided with an upper inlet hopper 32 having removable and scalable closure means 29. Chamber l i is mounted for reciprocal movement as on the rollers St or other suitable support, and is agitated as by means of the eccentric mechanism 36 driven bysuitable motor not shown. The lower portion of the agitating chamber I! is provided with an outlet hopper 33 which is connected by the lower hose portion 35 to the inlet tube iii. Inlet tube if} communicates with the housing 23 of the grinder is at the inlet port 22 and is in communication through the zone 22 with the outlet tube 83 which is connected to the housing 23 at the outlet port 4 3. The outer end of the outlet tube is is in sealed communication with the trap chamber l4.

Although various dust recovery means may be used a very important advantage of this invention is that the dust produced, though ultrafine, may be recovered satisfactorily by simple trap 14 in cooperation with bag filter I when required. This advantage results from the exceedingly low volume of gas required by the present invention to convey the solid material through the mill and into the dust recovery apparatus.

Trap chamber H3 is joined by connection to the filter housing 38 and is in communication with the filter bag I5 by means of the conduit 31. The filter housing 38 is connected to the outlet tube 39 and the outlet control valve 5?.

The grinder comprises the housing 23 in which the wheel 24 is carried by its shaft 25 for high speed rotation in housing bearings 2E5 and 21. Shaft 25 is in operative connection with its pulley 28 which in turn is driven by a main drive motor not shown. The wheel housing 23, and main drive motor are designed to maintain wheel speeds at any desired peripheral speed up to 700 feet per second or more.

The peripheral surface 2! of the wheel 24 is positioned so that there is a zone 22 between the housing 23 and the surface 25, and so that there is a side clearance i6 between the sides of the wheel 24 and the sides of the housing 23. The sides of the housing 23' may be tapered toward the periphery of the wheel or they may be made substantially normal to shaft 25 as shown in Figure l. The whole unit ill may be submerged in a tank 553 containing a temperature controlled environment 21 as a preferred means for maintaining predetermined temperature control throughout the operation of the unit til. Bath 4! is diagrammed in Figure 1 as surrounding pulley 23. This can be used for a gas bath but when bath 4%! is liquid, pulley 28 would preferably be arranged to operate outside of tank 59.

Surface 2! is curved cylindrically, but may be curved otherwise. Shaft 25 is supported massively in housing 239 and in high speed bearings 26 and 27 arranged to be driven by direct con- 'nection from motor or turbine, not shown.

Speed of wheel 2% may also be stepped up, as by pulley 28 from a driving pulley. Instead of a pulley drive any suitable, smooth-running driving means adapted for high speed may be used.

Emphasis is placed on solid mounting and sturdy.

between 200 and 500 feet per second or about 0.03 inch for producing dust averaging below about 20 microns particle size when the peripheral speed of wheel as is of the order of to 300 feet per second. A portion of the wheel face 2|, housing 23 and gas turbulence zone 22 is shown broken away and enlarged for clarity. Spacing up to maximum of about 0.05 inch is suitable.

The optimum of clearance depth of zone 22 depends upon the character of the material to be ground,'the degree of subdivision desired and the rate of production desired. For any fixed clearance depth, and for a given material the peripheral speed of the wheel 2 and/or the rate of feed of solid will determine the degree of subdivision obtained. For example if a material having low internal cohesion, such as graphite, is to be ground to medium fineness in which the particles will have an average diameter of 30 microns, the clearance depth of zone 22 may be as large as 0.05 inch, with the peripheral wheel speed of wheel 2 as low as 100 feet per second,

and a feed rate of 50 lbs. of graphite per hour.

Finer subdivision will be obtained by increasing the wheel speed or reducing the feed rate, or both. Likewise, if it is desired to grind a harder material such as ypsum to a high degree of fineness such that the particles would have an average diameter of 2 microns, eiiicient conditions would be a clearance depth of zone 22 of 0.015 inch, a peripheral wheel speed of 400 feet per second and a feed rate of 15 lbs. per hour. These conditions are dictated by the circumstances that the intensity of the turbulence produced in zone 22 is increased either by increase in peripheral speed of wheel 24%, or by decrease in the clearance depth of zone 22, or both. Also, the length of time which a given particle remains in zone 22 and hence is subjected to the action of turbulence, depends upon the rate of feed. The longer this time, the smaller the size of the comminuted particle and the more nearly uniform will be the sizes of the particles. These intensit and time factors are independently controllable and, therefore, high eiliciency of comminution of a wide variety of materials may be attained by adjusting these controls in accordance withthe disclosures set forth.

Operation desired pressure and flow within the mill if], as

described herein. Main motor drive is adjusted to impart the desired R. P. M. Agitator means 36 and main motor drive to pulley 28 are energized.

' invention.

The particles 5| to be comminuted are introduced into chamber ll through hopper 32, and cover 20 is sealed back in place and valves l and H are next adjusted to deliver gas at the rate and at the pressure desired. Gas then passes through pipe 52, upper hose portion '35, and enters chamber El. Chamber ll is agitated sufficiently to cause particles to fall into hopper 33 at the desired rate. Particles 5i fiow with gas down through lower hopper at, lower hose portion 35, inlet conduit or duct i0, and inlet port 42 into the comminuting zone 22.

After comminution to the desired particle size, the gas stream carrying the newly comminuted particles passes through outlet port 43, outlet conduit or duct 13, into trap Hi which removes a major portion of the newly comminuted particles 55, through tube 31 and into filter bag which removes the remainder of the newly comminuted particles from the gas, which is finally discharged through outlet tube 39 and exhaust valve l1. Subsequently, the motors are stopped, valve !8 closed, valve ii opened, covers 53 and 54 removed from filter chamber 38 and trap M. respectively. The

comminuted particles

55 and 52 are removed from trap M and from filter bag l5 respectively.

Example I A specific example will emphasize features of this invention shown in Figure 1.

Cylindrical wheel 2 of high tensile strength, about 3.25 inches in diameter, was attached tightly to a steel shaft 25 of diameter. This wheel had a peripheral face 2i nearly one inch wide. The steel shaft was sturdily mounted in well-fitted and accurately aligned and welllubricated high-speed bearings 26 and 2'! and connected to an air-driven turbine motor which rotated the wheel 24 at any desired speed up to 40,000 or more R. P. M. This is a relatively new order of speed in rotating surfaces. Such speed requires a high degree of care in design and of workmanship to avoid rupturing the machine itself. These speeds provide many new and useful results.

The wheel 2:! and its shaft 25 and the housing 2'3 were made of steel in order to obtain the requisite strength, but other materials may be utilized. For one example, wheel 24 may be composed of fibre glass cloth, laminated and resin-bonded to obtain a wheel having a high ratio of tensile strength to density. The peripheral face 2| is relatively smooth. The width of this peripheral face 2! may bear a ratio to diameter of the wheel 213 from about 1 to 0.5 to about 1 to 6 for most practicable purposes.

The peripheral face 2i of the rotating member is technically smooth, from tool finished surface to lapped surface. Such surfaces accomplish unusual results at the high speeds utilized in this For example, face 2| of steel was brought first to a matte or satiny finish and comminuting effects were studied of the resulting turbulence of air in space 22 at the described ultra speeds of wheel face M. This was a satisfactory and useful surface, for when solid particles were fed through the turbulence chamber 22 the particles were comminuted to an extremely fine dust. Then face 2| was smoothed to an additional degree and coated with electro-deposited chromium to produce a hard, non-corrosive substantially highly polished finish. Solid particles fed to zone 22 when this surface was used were still comminuted to an equally fine dust. Moreover, when the solid was very labile, such as penicillin,

the antibiotic activity of the resulting dust was equal to that of the input material. Heretofore penicillin has lost in antibotic activity on being comminuted. Projections or roughness greater than about 0.01 inch deep on the peripheral face 2| of wheel 24 tend to deteriorate the quality of such solids as penicillin because of the local heat effects of impact at these tremendous speeds. Moreover, such roughnesses tend to clog and to unbalance the wheel. This is true with waxy substances such as DDT insecticide. Consequently a surface of a high degree of smoothness is preferred for the moving face 2! of this apparatus. At the ultra-speeds of this invention such a surface produces very high turbulence of gas in the peripheral clearance space 22.

The sides of wheel 24 and of housing 2'3 usually are straight to form straight clearance between them. Impact may be minimized by tapering this clearance slightly and by making side space I6 somewhat wider between walls than the peripheral clearance 22. The location and size of ports 42 and Z3 and conduits iii and it may vary. With the wheel discussed in the example, inlet port 42 and outlet port 43 were a half inch in diameter, with their centers iocated inch below the peripheral face 22. One inlet port and one outlet port suihce, though various numbers and arrangements are useful also. Ports 02 and 43 were connected respectively to conduits l0 and 13 each of internal diameter. Outlet duct is communicates with outlet valve i'i.

Example II As an example of processing very labile material under this invention, comminuticn of peni cillin is described. Feed chamber it was charged with dry, granular penicillin of size passing a 20 mesh sieve. A source of compressed, dry nitrogen was connected to gas inlet 52 through valve and pressure control indicator 33. Valve 58 was adjusted so that pressure in conduit 62 was 3 p. s. i. g., and the gas flow was throttled to 0.5 cubic feet a minute by valve ill in the discharge line 30. Discharge line it was connected to a conventional trap and bag filter. The wheel 24 which was 3.25 inches in diameter and 1 inch wide was brought to a peripheral speed of about 500 feet per second, corresponding to 35,000 R. P. M. The feed chamber H was vibrated to cause penicillin to flow into the grinder it at the rate of about 20 grams a minute. The comminuted product was collected readily in the trap l4 and bag filter E5. The output product was as active antibiotically as the input material. The mass median diameter of the particles produced was 6.3 microns in this example, while the frequency distribution of diameters was such that percent of the particles were of diameters less than 4 microns.

Certain observations may emphasize the important advance in the art resulting from this novel pulverization by turbulence of gas at a surface moving at ultra speeds in a comminution. chamber as illustrated at 22 and described herein.

First, only relatively little gas flows through the machine. This flow need by only sufficient to carry the particles through the machine and the assembly described. For example, penicillin comminution required only 0.5 cubic feet minute of gas through-put, instead of the order of cubic feet per minute of gas for comminution by gas jet. Since this volume of gas is small, difficulties of recovering fine dust are minimized. Those familiar with bag filters or with collec- 7 tion of dusts will appreciate the importance of being able to use a bag filter at all in this case, or of being able to substitute other collectin means if desired.

Another advantage of the small amount of gas deserves considerable emphasis. The flow of the small amount of gas can contribute only insignificantly to the turbulence of gas in the machine. The work done on the suspended material being comminuted is governed by the mechanical energy supplied to the rotating member which produces the required turbulence. This ener y is readily controlled. Therefore, a high degree of control of the processing is readily obtained. This avoids the lack of control in attempting to obtain gas turbulence by release of large quantities of costly compressed gas.

A further advantage of small through-put of gas is that it permits materials containing one or more volatile constituents to be processed without significant loss of volatile matter. Different batches of such materials can be blended or be comminuted together to obtain a product of uniform composition, while economic loss of Volatile constituents carried away by exhaust gas is minimized.

The small volume of gas through-put permits even costly gases to be used as the atmosphere in which the solid particles are carried through the machine. Thus materials heretofore prohibitive become economical. For example, use of nitrogen in small quantity makes feasible the comminution of protein material such as egg albumen that would deteriorate in air. The finer the particle is pulverized, the more surface is bared and the greater the oxidation that would occur in air. Furthermore, with materials that require dry gas, or any given humidity, or require pure oxygen, or can utilize any special gas, the novel low gas through-put makes feasible the use or" such gas in the present invention.

An additional further advantage is that the pressure at which comminution occurs can be varied readily or can be held at any desired level, simplj by adjusting valves is and 57. Or similarly, pressure less than atmospheric can be obtained by reducing or varying the flow f compressed gas from tank 353 by means of inlet gas valve 18 and exhausting gas through the outlet ll. Combinations or alterations or these adjustments of valves I! and i8 produce suitable difierential pressures within the pressure range of subatmospheric to supenatmospheric.

Another advantage of this invention lies in its ease of temperature control. When the appa ratus is placed in a thermally controlled environment, such as in a gas bath or a liquid bath H, conduction of heat occurs readily through the housing that confines the working area. Since the working zone 22 is of small cross-section such heat control Xtends readily to the particles being comminuted therein. Moreover, this control is both sensitive and substantially complete because of the extremely high turbulence in the working area. Predetermined temperatures of the particles being comminuted thus are readily maintained.

To protect individual solid particle against local heating from chance impact along the side of the wheel 2 1, side clearance it greater than that of the intercommunicating peripheral comminution zone 22 avoids drastic throw or sweep of particles outwardly. This also minimizes impacts against the housing 23 and augments the effect of locating a particle inlet port 42 near the periphery 2! to further reduce this throw.

The present invention also permits the compressibility of the fluid to impede the outward throw of the particles. This cushioning action also reduces mechanical shock and local heating of particles. Centrifugal densification or" gas outwardly tends to minimize movement of the solid material except in the narrow zone of turbulence at the super-speed surface 2 l. Consequently reduction to super-fine colloidal dust is substantially confined to the forces of the turbulent gas itself.

Comminution occurs in the space between the rotor periphery 2i and the housing 23 by action of the turbulent gas on the individual solid particles. To be distinguished from impact of particle against particle, is this rending apart by forces of gas acting either in shear or in tension. Such rending apart is evidenced by the fact that with this invention the lower the ratio of solids to gas the finer the comininution obtained. The entire arrangement described makes possible substantially isothermal conditions in reducing solid particles to dust.

It becomes apparent from the foregoing that the present invention provides independent regulation of turbulence, temperature and pressure within the turbulent comminution zone 22, as well as of particle feed.

The effects of static electricity are also avoided. The intimate contact of processed solids with electrically conducting parts of the machine provides adequate conduction to eliminate accumulation of static electricity. Not only does this reduce hazards of explosion, but facilitates separation of such fine particles from the eiiluent. Mutual repulsion, and consequent suspension, oi dust particles heretofore worked against collecting super-inc, colloidal dusts.

Size of particles fed to this apparatus varies with conditions and materials. In general, feed size of the order of 103 microns or less is desirable, but even as large as 20 mesh particle feed is suitable with some materials. Particles are readreduced to less than 2! microns diameter by this invention.

Though this invention is not limited by theory, it appears from research that the speed of the moving peripheral surface at and the narrow cross-section of the working space 22 produce exceedingly high turbulence of gas. It also appears th'at the many high velocity gradients, corresponding to high energy gradients, subject a particle to many intense and changing stresses in shear and intension. When the stresses exseed the cohesiveforce within the particle, the particle subdivides. Evidently at comminuting speeds of this apparatus, the gas in the peripheral working space is not simply carried along with the moving surface but breaks up into local and individual rotating masses or rollers. These spin on their individual axes of rotation at much higher rates than even the wheel surface, dependent on their own effective diameters relatively to the rotated wheel, as with gears. Solid particles caught in the spin of these whirling gases rotate at tremendous angular rates to develop disrupting centrifugal forces.

The rate of rotation of the wheel 25, determining the degree of turbulence of the gas and consequently the particle size obtained, varies with.

clliierent peripheral clearances. Small clearance andhigh speed subdivide solid particles most eirectively. Rate of flow and drift through the apparatus 50 also affect particle size and sizeuniformly of product since lowering the feed-rate increases duration of time to obtain small pai ticle production. In general, peripheral surface speeds of the wheel above one hundred feet per second are suitable for comminuting solids to very fine particles. Peripheral clearance may be from about 0.01 inch (0.25 millimeter or 250 microns) to about 0.05 inch. In Example I, where the wheel was 3.25 inches in diameter and about one inch wide at its peripheral face, a feed rate of about 20 grams a minute of solid penicillin suspended in. gas flowing about 0.5 to two cubic feet of gas a minute was about optimum. These conditions of course vary with different materials and sizes. Too fast a feed clogs the machine and this clogging point is readily observed. The apparatus of Example I showed a rather sharp beginning of cornminuting char 'acteristics at about 12,000 R. P. M. (or a peripheral speed of 1'70 feet per second) increasing rapidly to about 13,000 R. P. M., and increasing slowly thereafter to 42,000 R. P. M.

At the speeds used in this invention, it is essential that all details, workmanship and design be of highest quality so as to avoid rupture in the machine from the centrifugal forces involved. Stresses may develop that exceed strengths of known materials, particularly if unbalanced parts are rotated at these speeds. Projections of various sorts tend to serious unbalance, for the order of magnitude of these speeds and stresses far exceeds those of prior comminuting machines. Consequently, the smooth,

satiny-surfaced cylindrical wheel as described herein is required to obtain the necessary high speed surface.

The apparatus of the present invention has been used successfully for comminuting a variety of solids. From this use it has been found that the processing of these materials has fallen within the following preferred ranges: in the comminuting zone 22, a pressure range of from .5 to atmospheres absolute pressure was found desirable; the bath or environment zone at ii was maintained at temperature ranging from C. to +80 C.; the particle feed delivered by agitator 36 to hopper (-33 is preferred at rates ranging from one gram of solid particles per minute to 30 and more pounds per hour; the rate of feed for gas from source as to port i2 is preferred at a range of from .2 to cubic feet per minute;

the depth or thickness of zone 22 is preferred not to exceed of an inch, the optimum thickness being from .01 to .08 of an inch; the pcriferal face 2! is preferably provided with a width of from about to two times the diameter of wheel 2%; and the periferal speed of wheel 2 is preferred within the range of from 100 to 700 feet pcrsecond.

Comminution at temperatures well below freeaing increases the brittleness of certain solid and thereby increases the rate at which they may be subdivided. Other solids, however, are comminuted more emciently in environments maintained at the above-mentioned higher tempera ture ranges. Environments excessively dry or excessively humid are also sometimes desirable for the oomzninution of certain solids. Various ga. es or mixtures of gases have also been found desirable for other solids. All solids are easily and quickly recovered after comminution by the simple recovery means exemplified at M and The depth or thickness of zone 2?. may be 1naintained at predetermined distances by varying the diameter of wheel E l; or by inserting thin liners of sheet material in zone 22 on the underside of housing 23 adjacent to surface 2!; or by the use of a conical surfaced wheel in place of cylindrical surface 2|. This conical wheel is adjustable along its axis and operates in a conical zone rather than in zone 22, as shown. Combinations of these means, as well as other means of regulation, may also be used if desired.

This invention and the manner of making and using it is hereindescribed in full, clear, concise and exact terms so that those skilled in the art may make and use it. Its principle and preferred mode of application have been explained. However, it is intended that this invention includes all modifications and embodiments within the spirit and scope of the appended claims.

1. The method of comi nuting finely divided particles into substantia smaller particles, comprising agitating finely divided particles in a gas under compression, thereby suspending said particles insaid gas, passing said gas with said particles in suspension through a narrow annular turbulent having a width not in excess of 1 of an inch and being bounded on its outside by a rigic stationary surface and on its inside by a hard smooth surface moving at a speed of not less than feet per second.

2. A process for reducing solid particles to fine dust comprising advancing in a narrow zone gas containing solid particles, confining the zone between two wall not more than 0.12 inch apart, causing relative movement between the two walls one to the other of at least 100 feet per second, thereby causing high turbulence close to a wall surface and pulverizing the solid particles carried in the gas to fine dust of the order of less than ten microns.

3. A process for reducing solid particles to fine dust comprising advancing in a narrow zone gas containing solid particles, confining the zone between two Walls about 0.01 to 0.12 inch apart, causing relative movement between the two walls one to the other of about 300 feet per second, thereb causing high turbulence close to a wall surface and pulverizing the solid particles carried in the gas to fine dust of the order of less than ten microns.

4. A. process for reducing solid particles to fine dust comprising advancin in a narrow zone gas containing solid particles, confining the zone as an annulus between twoarcuate concentric walls about 0.01 to 0.12 inch apart, causing relative rotative movement between the two walls one to the other transversely of the general direction of the gas travel and narrow zone of at least 100 feet per second, thereby causing high turbulence close to a wall surface and pulverizing the solid particles carried in the gas to fine dust of the order of less than ten microns.

5. Apparatus for comniinuting solid particles comprising in combination a casing containing a substantially cylindrical recess, a substantially cylindrical rotor mounted in said casing and provided with a smooth circumferential surface arranged for rotary movement in said recess and disposed in" the recess to form with the casing an operating annular peripheral zone clear along the smooth rotor surface and having uniform radial depth to give clearance of the order of 0.01 to 0.12 inch, driving means arranged to provide said movement at a peripheral speed of at least 100 feet per second, there being a gaseous atmosphere in the peripheral operating zone, a compressed gas supply and conduit therefrom, and means for mixing solid particles in suspension in gas in the conduit, said conduit having an opening for feeding gas and. particles into the recess in close proximity to said peripheral operating zone, and outlet means at the opposite side of the rotor in close proximity to said peripheral zone.

6. Apparatus for comminuting solid particles comprising in combination a casing containing a recess, a substantially cylindrical rotor mounted in aid casing and provided with a highly polished circumferential surface arranged for rotary movement in said recess and disposed in the recess to form with the casing an operating annular peripheral zone clear along the polished rotor surface and having uniform radial depth to give clearance of the order of 0.01 to 0.12 inch, driving means arranged to provide said movement at a peripheral speed. of at least 100 feet per second,

there being a gaseous atmosphere in the peripheral operating zone, a compressed gas supply and conduit therefrom, and means for mixing solid particles in suspension in gas in the conduit, said conduit having an opening for feeding gas and particles into the recess in close proximity to said peripheral operating zone and outlet means at the opposite side of the rotor in close proximity to said peripheral zone.

7. Apparatus for reducing solid particles to fine dust comprising in combination a casing con taining a recess, a rotor mounted centrally therein, bearings at each end of the rotor and means to drive the rotor, all adapted and being of size and material and. being balanced dynamically for rotor peripheral speeds of at least 100 feet per second, the rotor having a circular and smooth peripheral face, the casing recess and the rotor peripheral face conforming to each other and defining a clear annular operating zone between rotor and easing having uniform radial depth to give about 0.01 to 0.12 inch clearance along the peripheral face, there being a gaseous atmosphere in the peripheral operating zone, a source 12 r of supply of gas, a conduit therefrom having an opening into the casing recess at one side of the rotor in close proximity to the annular operating zone, means to feed gas from the conduit into the annular operating zone, means to feed solid par- REFERENCES CITED The following references are of record in the file of this patent: V

UNITED STATES PATENTS Number Name Date Re. 1,381 Kingsland Jan. 6, 1863 213,471 Toufiiin Mar. 18, 1879 244,829 Smith July 26, 1831 405,281 Taggart June 18, 1889 1,054,881 Thomas Mar. 4, 1913 1,573,017 Podszus Feb. 16, 1926 1,575,717 Plauson Mar. 9, 1926 1,685,956 Podszus Oct. 2, 1928 1,718,184 Ostermann June 18, 1929 1,729,471 Bear Sept. 24, 1929 1,812,727 Symons June 30, 1931 1,931,555 Mosley Oct. 24, 1933 1,980,589 Acree Nov. 13, 1934 1,993,762 Tolman Mar. 12, 1935 2,316,124 Sheldon Apr. 6, 1943 2,319,192 Sheldon May 11, 1943 FOREIGN PATENTS Number Country Date 400,441 Great Britain Oct. 26, 1933 OTHER REFERENCES Chemical Engrs. Handbook by John H. Perry, pages 1493-1494, first edition, 1934. (Copy in Division 25.)