Apparatus For Comminuting And Dispersing Solid Particles

Abstract

Apparatus for comminuting dry solid particles to a size of one micron or less comprises a vessel which contains a supply of balls agitated by one or more disks mounted on a shaft which is coaxial with and extends into the vessel. The comminuted material is evacuated by gravity or by conveying a gas through the interior of the vessel. The diameters of balls do not exceed 5 millimeters and the diameters of agitating disks are between 70 and 90 percent of internal diameter of the vessel.

Description

BACKGROUND OF THE INVENTION

The present invention relates to improvements in apparatus for comminuting and dispersing solid particles. More particularly, the invention relates to improvements in apparatus of the class including so-called ball mills, stream mills and like machines wherein dry solid particles are comminuted by balls or analogous dispersing elements which are agitated in the interior of a vessel. Still more particularly, the invention relates to improvements in apparatus which are especially suited for breaking up solid granular or like material into minute particles with a size in the range of one or more microns or a fraction of a micron. The definition "breaking up" is intended to embrace reducing the size of larger solid particles so that they yield smaller particles as well as separating agglomerations of smaller particles into a mass of discrete particles.

It was found that, once the material which is treated in conventional ball mills or analogous apparatus reaches a certain degree of fineness, no further refinement takes place even if the material remains in the apparatus for extended periods of time. Reference may be had, for example, to German Pat. Nos. 589,796, 592,105 and 642,127 which disclose such types of apparatus. It was further found that the time required to reduce the size of solid particles in conventional apparatus, especially if the particles are to be converted into a mass of finely comminuted material, is very long so that the operation of such apparatus is uneconomical. This is attributed to the fact that conventional apparatus utilize relatively large balls or analogous dispersing elements, normally with a diameter of about 20 millimeters. If such balls are used to break up particles to a size of about 0.001 millimeter, the ratio of the size of particles of the ultimate product to the size of the balls is about 1:20,000; therefore, the probability of satisfactory impact of comminuted material against the balls depends too much on the roughness of external surfaces of balls and on the speed at which the balls are agitated in the interior of the vessel, i.e., on frequency of impact of balls against one another. As a rule, the device which agitates the balls cannot be rotated at a speed which exceeds a predetermined value. It was further found that, in conventional apparatus, the efficiency does not depend on the energy of collisions between particles of comminuted material and the balls. The comminuting action is terminated when the comminuted particles cease to move with reference to each other and with reference to the balls. Some but not all of the above enumerated problems are eliminated when the dispersing elements fill only a certain portion of the interior of the vessel.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a novel and improved apparatus which is capable of comminuting solid particles to a size of one micron or less in a time-saving operation.

Another object of the invention is to provide an apparatus which can break up agglomerations of coherent particles or which can be used to break up discrete solid particles in a small area, faster than in conventional apparatus, and with a higher degree of efficiency.

A further object of the invention is to provide the apparatus with novel agitating means for balls or analogous dispersing elements.

Our invention is based on the recognition that the dispersing elements can comminute solid particles with greater efficiency and more economically if the mixture of solid particles and dispersing elements is agitated at a high speed, if the ratio of maximum dimensions of dispersing elements to maximum dimensions of admitted solid particles is less than in conventional apparatus, and if the dispersing elements and solid particles are maintained in motion by novel agitating means. The agitating means is designed in such a way that the entire contents of the vessel are kept in constant motion and that the contents of the vessel fill the latter when the apparatus is in use even though the combined volume of solids is substantially less than the capacity of the vessel when the apparatus is idle.

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

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partly schematic and partly vertical sectional view of a dispersing and comminuting apparatus which embodies one form of our invention;

FIG. 2 is a fragmentary schematic sectional view of a second apparatus;

FIG. 3 is a fragmentary schematic sectional view of a third apparatus which constitutes a modification of the apparatus shown in FIG. 2;

FIG. 4 is a fragmentary schematic sectional view of a fourth apparatus which is similar to the apparatus of FIG. 3;

FIG. 5 is a fragmentary schematic sectional view of a fifth apparatus comprising a rotary separator which is built into its vessel;

FIG. 6 is a fragmentary schematic sectional view of a sixth apparatus which comprises an externally mounted separator;

FIG. 7 is a fragmentary schematic sectional view of a seventh apparatus which constitutes a modification of the apparatus shown in FIG. 6;

FIG. 8 is a schematic partly sectional and partly elevational view of a further apparatus with two externally mounted separators;

FIG. 9 is a schematic sectional view of a multi-stage separator which can be utilized in the apparatus of FIGS. 1 to 8; and

FIG. 10 is a similar schematic sectional view of a modified multi-stage separator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, there is shown an apparatus which comprises a horizontal cylindrical vessel or container 1 which contains a supply of dispersing elements 30. Such dispersing elements (hereinafter called balls for short) may be of spherical, ovoid, polygonal or other regular or irregular shape and are kept in motion by agitating members 5 mounted on a horizontal drive shaft 4 which is coaxial with the vessel 1. Particles of sand can be used as dispersing elements. The shaft 4 extends through a stuffing box 31 in the left-hand end wall 2 of the vessel 1 and is journalled in bearings 32, 33 provided in brackets 34, 35 carried by a base member 36. The left-hand end of the shaft 4 is fixed to a pulley 37 which is driven by an electric motor or another suitable prime mover through the intermediary of V-belts, not shown. The vessel 1 is provided with an inlet 6 which is adjacent to the end wall 2 and serves to admit dry solid particles which are comminuted in the chamber 1a of the vessel and are thereupon evacuated by way of an outlet 8 adjacent to the right-hand end wall 3 of the vessel. A screen or filter 9 of fine mesh is installed in the outlet 8 to confine the balls 30 in the chamber 1a. Since the dispersed material is evacuated upwardly (by way of the outlet 8), the vessel 1 is preferably further provided with a second inlet 7 which admits a gas serving as a carrier to entrain comminuted particles through the interstices of the filter 9 and upwardly through the outlet 8. The inlet 7 is connected to a source 38 of compressed fluid, e.g., air or an inert gas (such as nitrogen).

The vessel 1 includes a jacket 39 provided with an inlet 40 and an outlet 41. The compartment defined by the jacket 39 serves to accommodate circulating heating or cooling fluid (gas or liquid) if and when a heating or cooling of solid particles is necessary. If the fluid entering at 40 is a heating medium, the apparatus further comprises a heat exchanger 42 which maintains the circulating medium at a desired temperature. The vessel 1 (or the gas which is admitted by way of inlet 7) is cooled if the particles admitted through the inlet 6 are soft at room temperature or when the treatment at room temperature or above room temperature would adversely affect the color, quality or other characteristics of treated material.

The mixture of balls 30 and solid particles preferably fills between 40 and 85 percent of the chamber 1a. The volumetric ratio of balls 30 to solid particles in the chamber 1a is between 0.3 and 10 to one, preferably between two and 10 to one, i.e., the volume occupied by balls is preferably a multiple of the volume occupied by solid particles. The remaining part of the chamber 1a (namely, between 15 and 60 percent) is filled with gas. When the shaft 4 is idle, the chamber 1a accommodates three layers including a lower layer of balls 30, a median layer of solid particles, and an upper layer of gas. Of course, finer solid particles penetrate into the gaps between the balls 30 in the lower layer. When the shaft 4 is rotated at a high speed, the gas, solid particles and balls 30 form a homogeneous mixture which fills the entire chamber 1a.

The means for feeding solid particles from a hopper 43 or another suitable source to the inlet 6 may comprise a screw conveyor 44 or the like. The conduit connecting the source 38 with the inlet 7 preferably accommodates an adjustable pressure regulating valve 45 so that the operator can select the rate of admission and pressure of the gaseous carrier as a function of the speed of comminuted particles which are to be evacuated through the interstices of the filter 9.

For example, the apparatus of FIG. 1 may employ a vessel 1 having an internal diameter of 300 millimeters and an axial length of 420 millimeters, a total of seven agitating members 5 each having a diameter of 260 millimeters, and the distance between adjoining agitating members may be 60 millimeters. The valve 45 may be adjusted to admit gas at the rate of 10 cubic meters per hour. The pressure of gas which is admitted via inlet 7 need not be high. Such gas must overcome the resistance of the filter 9 and the resistance of filter or filters (to be described in connection with FIGS. 8 to 10) which are used to separate the gaseous carrier from finely comminuted material. As a rule, and particularly when the outlet discharges comminuted material downwardly, the comminuting and dispersing operation is carried out at normal pressure. Nitrogen or another inert gas is employed when the apparatus is used in an area where an explosion is likely to occur or when the material to be comminuted should not be mixed with oxygen.

The interstices of the filter 9 should be small enough to prevent escape of freshly introduced solid particles and/or the escape of balls 30. It is normally sufficient if the size of interstices in the filter 90 is about 50 percent of the size of particles which are admitted by way of the inlet 6. The filter 9 may be a sheet of textile material or a sheet which is formed with slits or other types of openings. The size of particles which are admitted from the source 43 is preferably less than 500 microns, most preferably between 100 and 300 microns or even less than 100 microns. The fineness of comminuted material depends on the grinding time; the size of particles which are evacuated by way of the outlet 8 may be in the range of one micron or even less than one micron. The grinding time is variable and depends on the desired size of finely comminuted material. Such grinding time may vary between one or more minutes and one or more hours. In normal operation, the contents of the chamber 1a are maintained at or slightly above room temperature, for example, at 30.degree. C, provided that the material admitted via inlet 6 is solid at such temperature.

If desired or necessary, the apparatus may be mounted in such a way that the common axis of the shaft 4 and vessel 1 is inclined with reference to a horizontal plane. The angle of inclination with reference to such horizontal plane is preferably less than 60.degree., most preferably less than 45.degree. .

The ability of our apparatus to produce a stream of finely comminuted particles with a size in the range of one micron or less is attributed to the fact that we prefer to utilize relatively small balls or analogous dispersing elements 30. At the present time, we prefer to utilize dispersing elements having a maximum transverse dimension in the range of 0.1 to 5 millimeters, most preferably between 1-5 millimeters. As a rule, the ratio between the maximum dimensions of dispersing elements and the maximum dimensions of freshly admitted solid particles is between 10:1 and 10,000:1. A very satisfactory ratio is between 100:1 and 1,000:1. The motor or other prime mover which rotates the shaft 4 is preferably of the variable-speed type so that the speed of the agitating members 5 can be varied within a wide range. The rotational speed of these agitating members is preferably high to insure an intensive dispersing and comminuting action which results from repeated collision between the balls 30 and the solid particles. The fact that the mass of dispersing elements is relatively small does not affect the quality of dispersing and comminuting action. The dispersing speed is proportional to the frequency and intensity of collisions between the balls 30 and solid particles; the factors which affect such dispersing speed include the dimensions of balls 30, the ratio of the combined volume of balls 30 to the combined volume of solid particles in the chamber 1a, the ratio of the combined volume of balls 30 and solid particles to the volume of the chamber 1a, the specific weight of balls 30, the specific weight of solid particles, the size of solid particles which are admitted via inlet 6, and the size of comminuted material which is evacuated by way of the outlet 8. The ratio of specific weight of the balls 30 to specific weight of solid particles is normally between 0.1 and 8 to one. The nature of the material of the balls 30 depends on the nature of solid particles. Such material may be glass, ceramic, metal or plastic, e.g., polyvinyl chloride, polystyrene, Teflon (trademark) and/or others.

Another feature of our apparatus which contributes significantly to intensive dispersing and comminuting action resides in that the dimensions of agitating members are in a certain relationship to dimensions of the vessel 1. It was found that the diameters of agitating members 5 should be less than the diameter of the cylindrical internal surface of the vessel 1, preferably between 70 and 90 percent of such internal diameter. It was further found that, if the apparatus employs two or more agitating members, the axial distance between such agitating members should preferably correspond to between 20 and 50 percent of their diameter. The agitating members 5 preferably resemble or constitute disks which are provided with slots, holes or analogous recesses or openings and/or with ribs or other protuberances to insure satisfactory entrainment of balls 30 and solid particles when the drive shaft 4 rotates. It was found that disk-shaped agitating members undergo little wear, even at very high speeds in the range of 5-30 meters per second which are desirable to bring about satisfactory agitation of the contents of the chamber 11.

The apparatus can be operated continuously or intermittently and may be employed for comminuting and/or dispersing of different sizes and/or types of solid particles. The valve 45 may be closed to seal the inlet 7 from the source 38 if the nature of solid material is such that it can be evacuated without resorting to a gaseous carrier. As will be explained hereinafter, the gaseous carrier which is discharged by way of the outlet can be recirculated through the chamber 1a. Furthermore, at least those parts of the apparatus which are subjected to extensive wear can be provided with one or more coats or layers of highly wear-resistant material.

FIG. 2 illustrates schematically certain details of a modified apparatus. The main difference between the apparatus of FIGS. 1 and 2 is that the latter does not employ a gaseous carrier, chiefly because the outlet 108 is arranged to discharge comminuted material downwardly so that such material can be evacuated by gravity. All such parts of the modified apparatus which are clearly identical with or analogous to corresponding parts of the apparatus shown in FIG. 1 are denoted by similar reference numerals plus 100. The chamber of the vessel 101 accommodates a transverse partition 10 at least a portion of which constitutes a filter or sieve to provide interstices for passage of finely comminuted material which is thereupon caused to leave the vessel by way of the outlet 108. The partition 10 may form an integral part of the end wall 103 which is preferably detachably secured to the cylindrical shell of the vessel 101. In the embodiment of FIG. 2, the filter or sieve constitutes the upper part of the partition 10. This partition may consist of sheet metal which is formed with slits, round holes or other types of interstices for passage of finely comminuted material. When the shaft 104 rotates, the balls (not shown) cooperate with the agitating members 105 to automatically transport finely comminuted material from the inlet 106 toward the partition 10.

FIG. 3 shows a portion of a third apparatus wherein all such parts which are clearly identical with or analogous to the parts of the apparatus shown in FIG. 1 are denoted by similar numerals plus 200. The inlet 206 for solid particles is provided in the upper portion of the end wall 203 and the outlet 208 for finely comminuted material is located in the lowermost part of shell of the vessel 201 adjacent to the end wall 202. In this apparatus, the filter 9 or 10 is replaced by another confining device which prevents escape of dispersing elements and freshly admitted (non-communited) solid particles from the chamber of the vessel 201. This confining device comprises a ring 12 which is installed in the chamber of the vessel 201 and extends a relatively short distance toward the shaft 204, and a disk 11 which preferably constitutes the leftmost agitating member and defines with the adjoining end face of the ring 12 a narrow annular gap or an annulus of discrete gaps through which finely comminuted material is free to pass on its way toward the outlet 208. It will be seen that the gap or gaps for evacuation of communited material are defined by stationary and moving parts 12, 11.

The only important difference between the apparatus of FIGS. 3 and 4 is that the latter comprises a stationary ring 312 which is adjacent to the inner side of the disk 311, i.e., the disk 311 is located between the ring 312 and the end wall 302. All other parts of the apparatus shown in FIG. 4 are denoted by numerals similar to those employed in FIG. 3 plus 100. An advantage of the apparatus shown in FIG. 4 is that finely comminuted material passes through the gap or gaps between the parts 311, 312 under the action of centrifugal force.

In the apparatus of FIG. 5, the means for confining dispersing elements and freshly admitted solid particles in the chamber of the vessel 401 comprises a rotary separator 13 which is preferably (but not necessarily) installed in the end wall 403 and is driven by the shaft 404 or by a separate drive 13a (indicated by phantom lines). The separator 13 comprises a preferably conical rotor 13b which defines a narrow annular gap with a stationary tube 13c for passage of finely comminuted material into the outlet 408. The latter can form an integral part of the tube 13c. The separator 13 can perform the additional function of separating coarser and finer particles of the material which enters the tube 13c. All other parts of the apparatus of FIG. 5 are denoted by numerals similar to those employed in FIG. 1 plus 400.

The apparatus of FIG. 6 constitutes a further modification of the apparatus shown in FIG. 1. This apparatus comprises a separator 14 which is mounted externally of the vessel 501. The outlet 508 for finely comminuted material is formed with a perforated wall and extends into the separator 14 to receive the gaseous carrier and the finely comminuted material. All coarser particles are returned into the chamber of the vessel 501. The parts 502-507 correspond to parts 2-7 of the apparatus shown in FIG. 1.

FIG. 7 shows an apparatus wherein the separator 15 for gaseous carrier and comminuted material is mounted on the cylindrical shell of the vessel 601. The outlet 608 evacuates comminuted material. Coarser particles are returned into the vessel 601. The parts 602-607 correspond to parts 2-7 of the apparatus shown in FIG. 1.

FIG. 8 illustrates schematically an apparatus wherein the gaseous carrier is recirculated through the chamber of the vessel 701. The apparatus comprises two separators 16, 17 the first of which separates comminuted material and gas from dispersing elements and coarser solid particles and the second of which separates comminuted materials from the gaseous carrier. The separator 16 comprises a rotor 16a which is provided with a filter or sieve (e.g., a textile filter) having interstices just large enough to permit entry of gas and comminuted material into a conduit 16b which admits the mixture of gas and comminuted material into the housing 17c of the second separator 17. This housing accommodates a rotor 17a which is designed to retain comminuted solid material but to permit entry of gaseous carrier into a conduit 17b connected to the intake of a blower 18. The pressure side of this blower is connected with the inlet 707 of the vessel 701. Solid particles which cannot penetrate through the rotor 16a of the first separator 16 leave the latter's housing 16c by way of a return conduit 16d which discharges its contents through the end wall 703 and back into the chamber of the vessel 701. The outlet 708 for comminuted material is provided in the bottom portion of the housing 17c. The efficiency (separating action) of separators 16, 17 depends on the size of interstices in their rotors 16a, 17a and on rotational speed of such rotors. Each of these rotors can be driven by a pulley or the like, not shown, preferably from a variable-speed motor or transmission. The same motor or transmission can also serve to drive the rotor of the blower 18 and to rotate the drive shaft 704 for the agitating members 705. The numerals 702,706 respectively denote the left-hand end wall of the vessel 701 and the inlet for solid particles. As described in connection with and as shown in FIG. 1, the vessel of the apparatus can be heated or cooled, depending on the nature of solid particles and/or on certain other factors. It is also possible to employ in the apparatus of FIG. 8 (or in the apparatus of FIG. 1 or 6 or 7) a vessel which does not have a cooling or heating jacket and to provide instead a heat exchanger (shown at 17e in FIG. 8) which heats or cools the gaseous carrier upstream or downstream of the blower.

It is further clear that the separator 16 and/or 17 of FIG. 8 can be replaced by other types of separators without departing from the spirit of our invention. For example, the apparatus can employ a bag filter, a chamber filter or the like. Furthermore, the separators 16, 17 of FIG. 8 can be replaced by a multi-stage separator of the type shown in FIG. 9. This separator is designed to segregate coarse solid particles and dispersing elements from gases and comminuted material and to thereupon separate comminuted material from gases. The mixture of solid particles, comminuted material and gaseous carrier is admitted by suction into an outer housing 19a by way of an inlet 19. The housing 19a accommodates an upright hollow rotary outer filter 24 having interstices large enough to permit passage of gases and comminuted material into the interior of this filter. The solid particles which are too large are intercepted by the filter 24 and descend by gravity into an outlet 20 which returns them into the vessel, not shown. The mixture of gases and solid comminuted material is separated in the interior of the outer filter 24; thus, the gases are drawn into a pipe 22 through the interstices of a hollow rotary inner filter 23 which is coaxially mounted in the outer filter 24. The filter 23 intercepts the comminuted material which descends by gravity into an outlet 21 corresponding to the outlet 8 in the apparatus of FIG. 1. The numberals 25, 25a respectively denote gears which form part of two independent drives for the filters 23, 24, i.e., each of these filters can be rotated at a different speed. The upper portions of filters 23, 24 rotate in antifriction gearings 26, 26a. It is also possible to rotate the filters 23, 24 in different directions.

FIG. 10 illustrates a modified multi-stage separator. All parts of this separator which are clearly identical with or analogous to the parts of the separator shown in FIG. 9 are denoted by similar reference numerals plus 800. The main difference is that the filters 823, 824 are rigidly connected to each other so that they rotate at the same speed and in the same direction. The pipe 822 for the gaseous carrier can form part of the drive which rotates the filters 823, 824.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art.

Claims

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. Apparatus for comminuting and dispersing solid particles to form a mass of finely comminuted solid material, said apparatus comprising a vessel having an internal chamber provided with inlet means for admission of solid particles and outlet means for evacuation of comminuted material; agitating means comprising a drive shaft extending into said chamber and at least one substantially disk-shaped agitating member provided on said drive shaft in the interior of said vessel; a supply of discrete dispersing elements in said chamber, said elements being arranged to move in response to rotation of said agitating means to thereby convert solid particles admitted through said inlet means into finely comminuted material; a source of compressed gas; and second inlet means for admitting gas from said source into said chamber so that the thus admitted gas carries comminuted material toward and into said outlet means.

2. Apparatus as defined in claim 1, wherein said vessel comprises a cylindrical internal surface and wherein said shaft is coaxial with said vessel.

3. Apparatus as defined in claim 2, wherein the diameter of said disk-shaped agitating member is between 70 and 90 percent of the diameter of said chamber.

4. Apparatus as defined in claim 2, wherein the angle of inclination of the common axis of said shaft and said vessel with reference to a horizontal plane does not exceed 45 degrees.

5. Apparatus as defined in claim 1, wherein said agitating means comprises a plurality of agitating members spaced from each other in the axial direction of said shaft by distances corresponding to between 20 and 50 percent of the diameter of an agitating member.

6. Apparatus as defined in claim 1, wherein said outlet means is arranged to discharge comminuted material and gases upwardly from said chamber.

7. Apparatus as defined in claim 1, wherein the maximum dimensions of said dispersing elements are in the range of between 0.1 and 5 millimeters.

8. Apparatus as defined in claim 1, further comprising confining means for retaining the dispersing elements and solid particles whose size exceeds a predetermined value in said chamber.

9. Apparatus as defined in claim 8, wherein said confining means comprises a filter in the region of said outlet means.