US2392866A - Method and apparatus for comminuting or drying materials - Google Patents

Description

Jan. 15; 1946.

N. N. STEPHANOFF METHOD AND APPARATUS FOR COMMINUTINC OR DRYING MATERIALS Filed Feb. 28, 1940 3 Sheets-Sheet l piazza f =EVJ.

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N. N." STEPHANOFF I METHOD AND APPARATUS FOR COMMINUTING OR DRYING MATERIALS Filed Feb. 28, 1940 '3 Sheets-Sheet 2 Cr v Arm/f era.

3 Sheets-Sheet 3 N. N. STEPHANOFF Filed Feb. 28, 1940 METHOD AND APPARATUS FOR COMMINUTING OR DRYING MATERIALS Jan. 15, 1 946.

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METHOD AND APPARATUS FOR COMMINUT- IN G R DRYING MATERIALS Nicholas N. Stephanofl', Bryn Mawr. Pa., assignor to Thermo-Plastics Corporation, Camden, N. J

a corporation or New Jersey 7 Application February 28, 1940, Serial No. 321,328

'(Cl. 24l5)- 10 Claims.

This invention relates to a'method and p ratus for commuting and/or drying materials, these terms being used in a broad sense to include the breaking up of large particles, the grinding of particles, and the drying of material in the form of droplets or particles and the production of solid particles by cooling a molten liquid or by reaction with, or of, a liquid, e. g., polymerization, and more particularly relates to a method and apparatus for effecting such results by the action of high velocity gas or vapor jets. This application is in part a continuation. of my application Serial No. 145,421, filed May 29, 1937.

While reference is made herein particularly to grinding or comminution of solid and dry materials, it will be understood that the invention is equally applicable .to drying in a broad sense, as will be evident from consideration of the disclosure of my application Serial No. 199,687, filed April 2, 1938. It will also be evident that there may be adapted to the present invention the principles set forth in my application Serial No. 235,139, filed October 15, 1938.

The present apparatus is concerned primarily with the utilization of staging for securing the most emcient use of the heat and pressure energy of the elastic fluid used for the production of high velocity jets. In the production of grinding or the breaking up of droplets to effect the most efficient drying, it i essential ,for best results,

including most eifective entrainment of the material in jets, to produce as much turbulence as possible in the jets into which the material to be comminuted or dried is introduced. While from the standpoint of securing maximum kinetic energy of gases from a nozzle the best thermal efllciency is obtained when the fluid is fully expanded in a converging-diverging nozzle, the fiow from such nozzles is relatively non-turbulent as compared with the flow from nozzles of an abrupt type. These latter nozzles produce sufiicient eddies and whirls for entrapping the'material and increasing the frequency of impacts of particles on each other.

If at least a critical pressure drop is provided through a nozzle, a. high velocity which will be the acoustic velocity for the elastic fluid at the temperature and pressure conditions xisting in the jet will be secured. If 'a greater drop is provided and the nozzle suitably formed to take advantage of complete expansion with such drop, higher velocities are secured, but the increase in velocity is not efiective to produce any corresponding increase in the extent of grinding.-

In accordance with the present invention, an

arrangement is provided for expanding the fluid efliciently in stages to the lowest practical final pressure, producing at the same time a pressure drop in each stage to a final pressure only slightly below the critical pressure corresponding to that at the entrance of the nozzle, thereby obtaining very high jet velocities in each stage and producing, since abrupt or over-expanded nozzles can be used in at least some or the stages, the most eflective turbulence for impact grinding of particles. For this purpose there are used several chambers, the material passing from one chamber into the other with successive expansion of the fluid in the stages to the final exhaust pressure. Successive stages of the type just indicated require, of course, the use of chambers which are sealed. to maintain the fluid under the required pressures for delivery to successive stages.

In accordance with the present invention, to

secure the maximum eificiency, there may be provided jacketing for the chambers in which the grinding takes place and also for reheating of the elastic fluid between the stages, preferably by the introduction of small amounts of high temperature and high pressure fluid which additionally serves to increase the velocity of flow.

It is a further object of the invention to pro vide a selective action whereby large particles are retained in a stage inwhich they occur and only finer particles pass to the next succeeding stage for further comminution. In this way, extremely fine products are secured.

These and other objects of the invention. will become more apparent from the following description, read in conjunction with the accompanying drawings, in which:

Figure 1 is a vertical sectional view through a preferred form of multiple stage apparatus designed particularly for efiecting fine grinding;

Figure -2 is a vertical section taken on the plane indicated at 22 in Figure 1;

Figure 3 is a sectional plan view taken on the broken surface the trace of which is indicated at 33 in Figure 1; and

Figure 4 is a section taken through the central region of an interstage nozzle, as indicated by the trace 1i4 in Figure 3.

The apparatus which is disclosed comprises a first stage chamber 2, preferably in the form of an oval enclosure having substantially plane vertical faces, as indicated in the figures. While this enclosure is not shown as tubular in form, a tubular type of chamber may be provided and, more particularly, one of the type specifically illustrated in my application Serial No. 235,139,

.pipes i 6.

referred to above. Whether the central portion of the chamber is or is not available for flow, nexertheless a centrifugal separation will occur as the fluid carrying material circulates along the oval bounding walls, thereby efiecting return 01' particles to the region of high velocity jets.

The material to be comminuted or dried is inber 2 to the extent or the velocity head resulting from the recirculation.

The chamber 26, like the chamber 2, is desirably oval in form, and may be tubular. The jets troduced through the passage 4, being dispersed into the chamber by means of the jet from a nozzle i, which aspirates it from a connection 8 communicating with a supply tank i0, which may be provided with a suitable valve as indicated at i4 to provide for the assage of material into chamber Ill from a hopper l2 without interfering with the presure conditions existing in the first stage which may be under rather high pressure. The pressure in the chamber ID will, of course, be substantially less than that in chamber 2 by reason of the aspirator action in nozzle 6.

Directing elastic fluid into the chamber 2 are a series of nozzles l8 receiving elastic fluid from These nozzles are preferably so arranged that their jets impinge on each other and are desirably of abrupt type to secure a maximum of turbulence in the jets to effect the greatest number of impacts of the particles on each other. The nozzles are so directed, as indicated in the figures, that the jets have a component in an approximately tangential direction with respect to the lower curved wall ofthe chamber 2, thus fluid within the chamber. This recirculation takes place at high velocity with resultant tendency to throw larger particles selectively outwardiy, whereby, upon recirculation they pass into the high velocity jets for'regrinding.

Extending from one side wall of the chamber 2 at a level above that of the nozzles i8, are nozzles 20, which, it will be noted from Figure 3, which illustrates one of these, bear some resemblance to turbine buckets. From the arrangement with respect to the direction of recirculation in chamber 2, it will be evident that to enter these nozzles, the elastic fluid and particles carried by it must reverse their direction of flow. Preferably the nozzles provided at 20 are so shaped interiorly as to provide a-breaking of the fluid away from the wall on the concave sides of these nozzles, whereby there are set up eddies aiding in the production of turbulence and producing still more effective grinding by reason of their presence in a regionof intensely high velocity. Into this concave side of the nozzle passage there may be introduced additional high pressure elastic fluid in relatively small quantity from a chamber 22 through openings 24, illustrated in Figures 3 and 4. It will be noted that the vertical crosssection of these nozzles at any point is essentially rectangular, resembling thereby turbine buckets of conventional type or theinterstage nozzles used in turbine practice. By the introduction of the high pressure and high temperature elastic fluid in the small auxiliary nozzle openings, the fluid is reheated to some extent between the stages and the velocity of flow is increased. The design of the nozzles 20 is such that, taking into account the introduction of additional fluid and reheating, the action is esentially that of abrupt nozzles, thereby projecting into the second stage chamber 26 jets having intense turbulent flow. It will be noted that, due to the recirculation in chamber 2, the initial pressure at each of the nozzles 20 will be less than the static pressure of the champroviding for a recirculatory movement of the discharged from the nozzles 20 are directed in a general tangential direction into the chamber 26 with the resultant production of high velocity recirculation in this chamber with centrifugal separation of heavier particles of material.

Assuming thatthe complete expansion may be effected in three stages,a third chamber 30 may constitute the last of the chambers. Into this there are projected high velocity jets from nozzles 28, which are directed from the chamber 28 in a direction opposite the flow therein, so that here again a centrifugal separation takes place at the entrance to the nozzle. The nozzles 28 desirably expand the elastic fluid to the final pressure, and are accordingly of a converging-diverging type. In order to secure a highly turbulent flow, it is desirable that these nozzles be of an over-expanded type, rather than of a De Laval type, which would produce a jet relatively ineilective for grinding purposes. In this nozzle, as in the case of nozzle 20, auxiliary high pressure elastic fluid may be introduced through openings 32 to increase the temperature and,to some extent, the presure to secure very high velocities. The gas so introduced also substantially increases the velocity of flow. It will be evident that the nozzles described are of an illustrative nature and that it is quite possible to utilize other forms of nozzles as described in my application Serial No. 235.139.

The third stage chamber 30 is also preferably oval in form and, as will be evident from Figure 2, nozzles 28 are directed from a location above the bottom of the chamber 28 tangentially into the lowermost portion of the chamber 30. Thus again centrifugal action tends to prevent the passage of large particles from the chamber 26 into the chamber 30.

From a central portion of the chamber 30 there extends an outlet 34, communicating with the upper portion 36 of a conventional separator 38. From the separator the exhaust gases pass into a pipe 40 and may be partially released to the atmosphere through a valve controlled passage 42, or may be passed through a valve controlled passage 44 to a jacket 46 surrounding the three stages of the apparatus and provided with an exhaust outlet 48 and a drain 5|) for condensate in the event that a condensible vapor is being used as the elastic fluid or drying is being effected and condensation of the liquid being evaporated may occur.

In the apparatus specifically described, it will be notedthatthroughout the grinding stages there are a large number of tendencies for centrifugal separation to occur. First the location of nozzles 20 with respect to the bottom of chamber 2 tends to prevent entrance into them of larger particles which recirculate along the walls of chamber 2. Secondly, particles entering the nozzle 20. must undergo an abrupt change of direction by reason of the fashion in which the nozzles 20 extend from the chamber wall. Here again occurs a tendency towardthe centrifugal rejection of large particles. Two similar actions again occur to prevent the entrance of larger particles into the nozzles 28 and finally the central position of the tube 34 in which chamber 30 tends to prevent passage of large particles from the third stage. Thus only the finest of particles can reach theseparator and larger particles are subject to repeated recirculations in the various stages.

As was noted above, the pressure at the entrances to the interstage nozzles will be somewhat less than the static pressures in the chambers due to a Pitot tube effect at their entrances. They are designed to take this into account and give at least critical pressure drops to secure acoustic velocities where they discharge into the next chambers. If separation is not so essential, a reversal of flow to enter the nozzles need not be provided, but the nozzles may extend from the chambers in the direction of circulatory flow therethrough. In such case, the fluid will have an initial velocity of approach tending toward an increase in the entrance pressure at the nozzle. Of course, if the nozzles have suiiicient capacity, there is no conversion of the kinetic approach energy into pressure, but the velocity through the nozzle is increased to a substantial degree by this velocity of approach. For a given overall pressure drop, therefore, more stages may possibly be provided in case such discharge'in the direction of flow is effected. v

The desirability of stages will be evident from consideration of some typical figures. Assume, for instance, that steam is being used as the elastic fluid at a pressure of 105 pounds per square inch and at a temperature of 420 F. or 88 F. superheat. If an acoustic velocity is acquired in the nozzle, the pressure at the most contracted part of the jet will be 61 pounds per square inch. The resulting superheat will be 22 F., and the temperature 316 F. There will be 7.25 cubic feet of steam per pound. The original heat content of the steam will be 1235 B. t. u.s per pound, while the heat content at the pressure of 61 pounds will be 1188 B. t. u.s. Thus a difference of 47 B. t. u.s will produce a velocity of 1530 feet per second. The steam will still further expand in the chamber into which it discharges and at the exhaust in a typical single stage grinder will be approximately at atmospheric pressure, still having, however, a temperature of 300 F. This latter expansion in the chamber and through the exhaust will not perform any useful work. If the pressure is raised to 175 pounds per square inch, the velocity at the tip of the nozzle is raised only to 1545 feet per second, so that there is very little increase in the kinetic energy of the jet.

By properly selecting the rate of expansion in the nozzles of successive stages of a multiple stage apparatus so as to distribute the total pressure drop between the stages and to obtain the highest velocities in each jet thus produced, it is possible to secure the most efiicient grinding action. For instance, assuming steam at 300 pounds pressure used in the first stage superheated to 800 F., this will give 17 4 pounds pressure at the tip of the first nozzles if they are of abrupt type. At the entrance to the second nozzles, the pressure may have dropped to 150 pounds in the mill itself. The velocity attained in abrupt nozzles in the first stage will be about 1850 feet per second. In the second stage the same type of nozzles will give 1730 feet per second if a critical pressure drop occurs. Thus approximately the same grinding effect can be expected if the second stage nozzles are arranged similar to those of the first stage. At the tips of the second nozzles the pressure will still be 82 pounds per square inch sufficient to provide nozzles of a third stage with more than a critical pressure drop.

It may be pointed out that while abrupt nozthe turbulence necessary to insure proper entrainment of the material, the material leaving this stage will be already finely ground and entrained in the fiuid, so that between the first and second stages there may be used a type of nozzle adapted to produce quite high velocities but with less turbulence, such as, for example, an overexpanded nozzle. It may be noted that turbulence is desirable even if very high velocities are secured to insure a maximum number of impacts of the particles. However, turbulence here is not absolutely necessary since the particles in the jets will impinge upon other particles recirculating in the chamber into which the jet discharges. High velocity may therefore be the primary end to be considered, with turbulence a secondary matter. There is, therefore, considerable freedom of choice in the design of the interstage nozzles.

As illustrated in the figures, the volumes of the successive chambers may be increased to take into account the very substantial increase in specific volume of the elastic fluid, if it is desired that the circulating velocities should not increase proportionately. Likewise, either larger numbers of nozzles of similar size or similar numbers of nozzles of largersize are used in the successive stages to takecare of the increasing volume of gas to be handled. It will be evident that the size of the apparatus and pressures and temperatures used, as well as the relative arrangements of the nozzles, will depend very largely upon the desired final product. At times, very .fine products are not desirable, in which case high circulatory ve- I locities and reverse flow of entrance to nozzles should be avoided, so that moderately large particles may pass in succession through the stages.

It will be evident from the above that staging effects improvements in both the thermodynamic efficiency, i. e'., utilization of a much greater proportion of the original energy to produce high velocities, and in the mechanical features of grinding, i. e., the handling of the material, over the operation of single stage apparatus, and the two advantages may be used independently to some extent. For example, if very fine grinding is not necessary, material may be fed into each of successive stages and separation of moderately ground material effected in each, in which case the carrying of some material from one stage to the next may be merely incidental. In such apparatus, for example, a separator may be provided between the stages with suitable escape valve arrangement for removal of material without loss of pressure. Thus full use is made of the heat and pressure energy with very full conversion of it to kinetic energy.

The mechanical advantage of staging lies principally in the entrainment of the material to effect proper grinding. Entrainment is best effected in a turbulent jet from an abrupt nozzle when the material is thrown thereinto by centrifugal action. Less turbulent jets will entrain material thrown into them centrifugally when the particles are large, but will do so relatively less effectively when the particles are small. In the stages subsequent to the first, however, the entrainment is complete and hence very high jet velocities may be attained with little regard for turbulence to impart to the particles high kinetic energies to secure effective impact with particles of larger size undergoing recirculation. These very high velocities are particularly secured when the nozzles extend in the direction of recirculation from zles ar quite important in the first stage to secure a preceding stage so that a high approach velocity is provided.

It may be noted that multiple staging also results in possible utilization of heat resulting from impact in the breaking up of particles. The kinetic energy which the particles have prior to impact is in part transformed into heat which raises the temperature of the gas. The resulting additional heat energy is then transformable into kinetic energy in the subsequent nozzles by expansion.

Shock waves may be produced by overexpansion into a region of pressure lower than that normal for the nozzle discharge. .The sudden variations of pressure on particles in such waves may be utilized to break up by explosive action porous materials containing evaporable liquid, for example, wet asbestos, or the like. In such case, entrainment and heating of the material to a high temperature may occur in the first stage in which the pressure remains above that permitting boiling. In passing into the second stage, there will occur a pressure drop such that boiling may occur at the temperature acquired in the first stage. Desirably, for most violent explosive action, the temperature in the first stage should be raised to 'such extent that there is present the heat necessary to provide fully the latent heat of vaporization required in the evaporation.

In this same general fashion, a substance may be maintained in liquid phase in one stage and passed into vapor phase in a subsequent stage at lower pressure.

What I claim and desire to protect by Letters Patent is:

1. The method of pulverising material by the use of an elastic fluid comprising entrainlng the material in a jet of the fluid from a nozzle constructed and arranged to expand the elastic fluid at least to the critical pressure with respect to the initial pressure so that the jet acquires at least the acoustic velocity for the pressure and temperature conditions of the jet to efiect comminution of the material, applying heat to the fluid from said jet, and causing it to form a jet flowing from a second nozzle with a velocity at least the acoustic velocity for the pressure and temperature conditions of the second jet.

2. An apparatus comprising a pair of upwardly elongated chambers each of which has outer boundary walls having curved intercepts with planes extending in the direction of said elongation, means for introducing into the first of said chambers material to be pulverized, at least one nozzle arranged to discharge an elastic fluid into the first chamber with a substantial tangential component of velocity with respect to the curved outer boundary walls of said first chamber to entrain particles of the material and effect recirculation thereof along the chamber boundary walls, the curved flow of fluid along said walls serving to eflect centrifugal separation of suspended particles so that lighter particles predominate over heavier ones in regions inward of said outer boundary walls, means for leading fluid containing suspended material to the second chamber from a region of the first chamber in which centriiugally separated light particles predominate and in a direction at an angle of at least substantially 90 with the direction of recirculation in the first chamber, the last named means comprising at least one nozzle arranged to discharge elastic fluid into the second chamber with a substantial tangential component of velocity with respect to the outer boundary walls of the second chamber, means for supplying to the first mentioned nozzle elastic fluid at a pressure exceeding the final pressure in the second chamber by substantially more than the critical pressure drop from the supply pressure, and means for removing the material from the fluid from the second chamber.

3. An apparatus comprising a pair of upwardly elongated chambers each of which has outer boundary walls having curved intercepts with planes extending in the direction of said elon gation, means for introducing into the first of said chambers material to be pulverized, at least one nozzle arranged to discharge an elastic fluid into the first chamber with a substantial tangential component of velocity with respect to the curved outer boundary walls of said first chamher to entrain particles of the material and efl'ect recirculation thereof along the chamber boundary walls, the curved flow of fluid along said walls serving to effect centrifugal separation of suspended particles so that lighter particles predominate over heavier ones in regions inward of said outer boundary walls, means for leading fluid containing suspended material to th second chamber from a region of the first chamber in which centrifugally separated light particles predominate and in a direction at an angle of at least substantially 90 with the direction of recirculation in the first chamber, the last named means comprising at least one nozzle arranged to discharge elastic fluid into the second chamber with a substantial tangential component of velocity with respect to the outer boundary walls of the second chamber, means for supplying to the first mentioned nozzle elastic fluid at a Pressure exceeding the final pressure in the second chamber by substantially more than the critical pressure drop from the supply pressure, and

40 means for removing the material from the fluid from the second chamber, the first mentioned nozzle being of abrupt type and arranged to impart to the elastic fluid substantial turbulence and a velocity approximately the acoustic velocity for the pressure and temperature conditions in the nozzle jet to effect pulverizing of the material by reason of the turbulent conditions within the jet.

4. An apparatus comprising a pair of upwardly elongated chambers each of which has outer boundary walls having curved intercepts with planes extending in the direction of said elongation, means for introducing into the first of said chambers material to be pulverized, at least one nozzle arranged to discharge an elastic fluid intothe first chamber with a substantial tangential component of velocity with respect to the curved outer boundary walls of said first chamber to entrain particles of the material and effect recirculation thereof along the chamber boundary walls, the curved flow of fluid along said walls serving to effect centrifugal separation of suspended particles so that lighter particles predominate over heavier ones in regions inward of said outer boundary walls, means for leading fluid containing suspended material to the second chamber from a region of the first chamber in which centrifugally separated light particles predominate and in a direction at an angle of at least substantially With the direction of recirculation in the first chamber, the last named means comprising at least one nozzle having a curved axis in the direction of flow therethrough and arranged to discharge elastic fluid into the second chamber with a substantial tangential component of velocity with respect to the outer boundary walls of the second chamber, means for supplying to the first mentioned nozzle elastic fluid at a pressure exceeding the final pressure in the second chamber by substantially more than the critical pressure drop fromthe supply pressure, and means for removing the material from the fluid from the second chamber. I

5. An apparatus comprising a pair of upwardly elongated chambers each of which has outer boundary walls having curved intercepts with planes extending in the direction of said elongation, means for introducing into the first of said chambers material to be pulverized, at least one nozzle arranged to discharge an elastic fluid into the first chamber with a substantial tangential component of velocity with respect to the curved outer boundary walls of said first chamber to entrain particles of the material and eifect recirculation thereof along the chamber boundary walls, the curved flow of fluid along said walls serving to efi'ect centrifugal separation of suspended particles so that lighter particles predominate over heavier ones in regions inward of said outer boundar walls, means for leading fluid containing suspended material to the second chamber from a region of the first chamber in which centrifugally separated light particles predominate and in a direction at an angle of at least substantially 90 with the direction of recirculation in the first chamber, the last named means comprising at least one nozzle arranged to discharge elastic fluid into the second chamber with a substantial tangential component of velocity with respect to the outer boundary walls of the second chamber, means for supplying to the first mentioned nozzle elastic fluid at a pressure exceeding the final pressure in the second chamher by substantially more than the critical pressure drop from the supply pressure, and means for removing the material from. the fluid from the second chamber, the first mentioned nozzle being of abrupt type and arranged to impart to bers each of which has curved outer boundary walls, means for introducing into the first of said chambers material to be pulverized, at least one nozzle arranged to discharge an elastic. fluid into the first chamber with a substantial tangential component of velocit with respect to the curved outer boundary walls of said first chamber to entrain particles of the material and efiect recirculation thereof along the chamber boundary walls, the curved fiow of fluid along said walls serving to effect centrifugal separation of suspended particles so that lighter particles predominate over heavier ones in regions inward of said outer boundary walls, means for leading fluid containing suspended material to the second chamber from a region of the first chamber in which centrifugally separated light particles predominate and in a direction at an angle of at least substantially 90 with the direction of recirculation in the first chamber, the last named means comprising at least one nozzle arranged to discharge elastic fluid into the second chamber with a substantial tangential component of velocity with respect to the outer boundary walls of the second chamber, means for supplying to the first mentioned nozzle elastic fluid at a pressure exceeding the final pressure in the second chamber by substantially more than the critical pressure drop from the supply pressure, and means for removing the material from the fluid from the second chamber.

7. The method ofpulverizing material by the use of an elastic fluid working between predetermined initial and flnal pressures, the difierence between the initial and final pressures being substantially greater than the critical pressure drop from the initial pressure, comprising entraining particles of the material in a turbulent jet produced by expansion of tm elastic fluid substantially to the critical pressure and having a velocity approximately the acoustic velocity for the temperature and pressure conditions of the jet so that pulverizing of the material occurs due to the turbulence of the jet, causing the elastic fluid from the jet carrying entrained particles to follow a curved path to effect centrifugal classification of particles, and then further expanding the elastic fluid carrying centrifugally separated lighter particles and substantially free of heavier particles to form a second jet having a velocity at least the acoustic velocity for the temperature and pressure conditions of the second jet.

8. An apparatus comprising a pair of chambers each of which has curved outer boundary walls, means for introducing into the first of said chambers material to be pulverized, at least one nozzle arranged to discharge an elastic fluid into the first chamber with a substantial tangential component of velocity with respect to the curved outer boundary walls of said first chamber to entrain particles of the material and effect recirculation thereof along the chamber boundary walls, the curved flow of fluid along said walls serving to effect centrifugal separation of suspended particles so that lighter particles predominate over heavier ones in regions inward of said outer :boundary walls, means for leading fluid containing suspended material to the second chamber from a region of the first chamber in which centrifugally separated light particles predominate and in a direction at an angle of at least substantially 90 with the direction of recirculation in the first chamber, the last named means comprising at least one nozzle arranged to discharge elastic fluid into the second chamber with a substantial tangential component of velocity with respect to the outer boundary walls of the second chamber, means for supplying to the first mentioned nozzle elastic fluid at a pressure exceeding the final pressure in the second chamber by substantially more than the critical pressure drop from the supply pressure, means for feeding auxiliary elastic fluid to the second nozzle, and

means for removing the material from the fluid from the second chamber.

9. The method of pulverizing material by the use of an elastic fluid working between predetermined initial and final pressures, the difference between the initial and final pressures being substantially greater than the critical pressure drop from the initial pressure, comprising entraining particles of the material in a turbulent jet produced by expansion of the elastic fluid substantially t0 the critical pressure and having a velocity approximately the acoustic velocity for the temperature and pressure conditions of the jet so that pulverizing of the material occurs'due to the turbulence of the jet, causing the elastic fluid from the jet carrying entrained particles to follow a curved path to effect centrifugal classification of particles, and then further expanding the elastic fluid substantially free of particles of large size to form a second jet having a velocity at least the acoustic velocity for the temperature and pressure conditions of the second jet to efi'ect further pulverizing of materials.

10. An apparatus for pulverising material by the use of an elastic fluid working between predetermined initial and final pressures, the difference between the initial and final pressures being substantially greater than the critical pressure drop from the initial pressure, comprising a primary pulverizing chamber, having curved flow-directing walls, means for introducing material to be pulverized into the chamber, a nozzle arranged to discharge the elastic fluid into the chamber to entrain particles of the material and carry said particles through a substantial curved path of flow within said chamber to effect centrifugal separation of particles of different sizes, means for supplying the elastic fluid to the nozzle at said initial pressure, the nozzle being conaseasce structed and arranged to expand the elastic fluid substantially to the critical pressure with respect to the initial pressure to impart to the elastic fluid substantial turbulence and a velocity approximately the acoustic velocity fcr the temperature and pressure conditions of the nozzle jet to eflect pulverizing ot the material by reason of the turbulent conditions within the jet, the jet being directed in the direction of the walls of the apparatus to avoid substantial impact of the entrained particles with parts of the apparatus, a subsequent pulverizing chamber, a second nozzle arranged to discharge elastic fluid into the subsequent chamber, the first chamber having a region of relatively smooth flow substantially spaced from any elastic fluid jet and containing predominantly centrifugally separated light particles, means for supplying fluid containing suspended centrifugally separated, light material solely from said smooth flow region of the first chamber to said second nozzle, the second nozzle being constructed and arranged to expand the elastic fluid further and impart to it a velocity at least the acoustic velocity for the temperature and pressure conditions of the second nozzle jet, and means for removing the material from the fluid from the subsequent chamber.

NICHOLAS N. STEPHANOFF.

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