Patent Publication Number: US-11045816-B2

Title: Jet mill

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit, under 35 USC § 119(e), of U.S. Provisional Patent Application No. 62/829,352, filed Apr. 4, 2019. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to jet mills for reducing the size of solid particles and particularly to improvements in so-called “axial” or “spiral” jet mills, i.e., jet mills in which gas jets distributed about the perimeter of a housing cause the solid particles to circulate within the housing about a central axis, and in which particles exit from the housing at a central location along the direction of the central axis. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 2,032,827, granted on Mar. 3, 1936, describes a jet mill for pulverization and for providing material in finely divided form. The jet mill includes a housing having a circular, cylindrical, vertical wall with a multiplicity of angularly spaced nozzles arranged to release gaseous fluid at a high velocity in directions tangent to an imaginary cylinder located within the interior of the jet mill and coaxial with the cylindrical wall. The streams of gaseous fluid set up and maintain an inwardly spiralling vortex, and at the same time cut across and through the outer portion of the vortex, causing particles of material engaged by the streams of high velocity gaseous fluid to be hurled inward and into pulverizing impingement with other particles. Small particles are collected at a central location within the jet mill while larger particles continue to circulate. 
     Conventional axial jet mills, lack the ability to achieve efficient reduction of particles to the fine particle sizes required in modern pharmaceutical and biopharmaceutical products. In the case of larger diameter axial jet mills, the particles rapidly move inward to locations remote from the nozzles, where the kinetic energy of the gas streams exiting from the nozzles is insufficient to produce fine particles. Smaller diameter axial jet mills, on the other hand, can achieve better particle size reduction, but are less efficient because of their inability to process large volumes of material. 
     SUMMARY OF THE INVENTION 
     The jet mill in accordance with the invention comprises a housing, an internal structure within the housing for establishing an annular raceway for the flow of gas and particles of solid material, a plurality of nozzles for introducing gas into the raceway, an inlet passage for introduction of raw feed into the raceway, and an outlet passage for delivery of comminuted solid material out of the housing. 
     The housing has a top wall, a bottom wall, and an outer wall extending from the top wall to the bottom wall, thereby forming a closure. The outer wall has a substantially circular, cylindrical, inner surface uniformly spaced from a central axis. 
     The nozzles are disposed around the perimeter of the outer wall, and project jets of gas at high velocity inward through the outer wall. Each of the nozzles is arranged to project a jet of gas, e.g., air, in a direction on a side of the central axis such that the jets of gas projected by the nozzles can produce a unidirectional circulating flow of gas and particles in the raceway within the housing. 
     The internal structure has a substantially circular cylindrical outer surface coaxial with, spaced from, and opposed to, the inner surface of the outer wall of the housing. The diameter of the cylindrical outer surface of the internal structure is sufficiently large that the directions of the jets of fluid emitted by the nozzles intersect the cylindrical outer surface. The inner wall of the housing, and the outer surface of the internal structure, form boundaries of the annular raceway, which guides and confines the movement of circulating fluid and entrained particles of solid material. The internal structure limits inward movement of particles of solid material toward the central axis, and thereby increases the concentration of the particles of solid material in close proximity to the nozzles, where the kinetic energy of the gas streams is at its maximum. 
     The jets of gas hurl particles circulating in the raceway against one another, and also against the wall of the internal structure. Preferably, each of the nozzles is arranged to project a jet of fluid in a direction tangent to an imaginary cylinder, coaxial with, but having a diameter smaller than the diameter of, the cylindrical outer surface of the internal structure. 
     By carrying out all of the size reduction, i.e., grinding and micronization, in a circulating stream of gas and entrained particles within a raceway close to nozzles that continuously release jets of gas at high velocity, the particles are caused to collide with one another and with the wall of the internal structure with high kinetic energy, and with more effective and efficient size reduction than can be achieved with a conventional jet mill. The jet mill in accordance with the invention has the ability to achieve the fine particle distributions currently required for pharmaceutical and biopharmaceutical products that were heretofore possible only with low-efficiency, small diameter, axial jet mills. 
     In a preferred embodiment of the invention, the internal structure has a hollow interior space, and the inlet passage is arranged to deliver raw feed, consisting of solid material along with a material-carrying fluid, into the hollow interior space. From the hollow interior space, the raw feed passes through a gap between an edge of the internal structure and the bottom wall of the housing into the annular raceway. The gap can be configured so that the raw feed enters the raceway at substantially all locations along the length of inner boundary of the raceway, i.e., at all locations except at the locations of supporting structure connecting the internal structure to the bottom wall of the housing. Thus the raw feed can enter the raceway throughout substantially all of a 360° range, i.e., throughout substantially the entire length of the inner border of the raceway. 
     In the preferred embodiment, the internal structure also has a top surface below, and spaced from, the top wall of the housing. An outlet extends through the top wall of the housing, at a central location in the top wall. Particles ground in the raceway by the action of the jets of fluid move, through the space between top surface of the internal structure and the top wall of the housing and exit through the outlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical axial cross-sectional view of a jet mill in accordance with the invention; and 
         FIG. 2  is a horizontal section thereof, taken on section plane  2 - 2  in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the drawings, the jet mill in accordance with the invention includes a housing  10  having a top wall  12 , a bottom wall  14  and a vertical outer wall  16  having a substantially circular, cylindrical inner surface  18  having a central axis  20 , from which surface  18  is uniformly spaced. Typically the jet mill will be arranged so that the central axis is vertical. However the jet mill can be operated in other orientations. The wall  16  of the housing is provided with a set of openings distributed around its perimeter. These openings serve as nozzles for directing jets of gas into a raceway inside the housing. Four such openings,  22 ,  24 ,  26  and  28 , are shown in  FIG. 2 . Openings  22  and  26  are also visible in  FIG. 1 . Openings  22 ,  24 ,  26  and  28  are at the same level. These nozzles are spaced at 90° intervals on the perimeter of vertical wall  16 . Additional openings  30  and  32 , located at a level below the level of openings  22 - 28  are visible in  FIG. 1 . Openings  30  and  32  are spaced from each other by a 90° interval, and are positioned at an angular spacing of 45° from openings  22  and  26 . Two more such openings (not shown) at the same level as that of openings  30  and  32  are provided in wall  16 , and are spaced at 90° intervals from openings  30  and  32 . Thus, there is a total of eight openings, comprising four openings at each of two levels, in an arrangement such that the openings alternate from one level to the other and are spaced at angular intervals of 45°. Other arrangements of the openings can be utilized. For example jet mill can have eight openings all at the same level. 
     As seen in  FIG. 2 , openings  22 ,  24 ,  26  and  28  extend through wall  18  respectively on axes  23 ,  25 ,  27  and  29 , that are tangent to an imaginary circular cylinder  34  coaxial with vertical axis  20 . Openings  30  and  32 , and the other two openings (not shown) at the same level, similarly extend though wall  18  on axes that are tangent to imaginary cylinder  34 . With respect to a frame of reference for each opening, defined by a vertical plane in which the central axis  20  lies and which extends though the opening, the axis of each of the openings is directed toward the same side of the central axis  20 , i.e., toward the right of the axis, so that jets of gas exiting from the openings produce a circulating flow within the housing that is counterclockwise when viewed from above as in  FIG. 2 . In alternative embodiments, the openings can be arranged to produce a clockwise flow. 
     The axes of the openings can be directed obliquely upward as shown in  FIG. 1 . Alternatively, the axes of the openings can be horizontal or directed obliquely downward. 
     The part of wall  16  in which the openings are provided is surrounded by a circular manifold  36  into which a gas, e.g., air or superheated steam, is delivered under pressure though in inlet  38 . 
     An internal structure  40 , provided inside the housing  10 , has a substantially circular, cylindrical outer surface  42 , which is coaxial with the vertical central axis  20 , uniformly spaced from the inner surface  18  of the housing wall  16 , and opposed to the openings in wall  16  so that it is intersected by the axes of the openings. The diameter of the cylindrical outer surface  42  should be sufficiently large that the directions of the jets of fluid emitted by the nozzles intersect the cylindrical outer surface. 
     The internal structure is preferably composed of a ceramic material, e.g., alumina, which is substantially inert, less susceptible to deterioration than metal, and therefore more suitable for use in producing pharmaceutical products. 
     The internal structure  40  is preferably hollow, with a closed top wall  44  and a bottom opening  46 . The structure is supported from the bottom wall on brackets  48 , also preferably made of ceramic material, that maintain a space  50  between the lower edge of wall  42  and the bottom wall  14  of the housing. Solid material to be treated in the jet mill enters along with air or other carrying fluid through a channel  52 , which extends upward through bottom wall  14  and into the interior of the internal structure, and has an outlet opening  53  in direct communication with the interior of the internal structure. The material then flows through space  50  underneath the wall  42  into the space between walls  42  and  16 , which serves as the raceway for circulation of carrying gas and particles. 
     Particles are exposed to maximum kinetic energy because they are maintained in proximity to the gas jets, and they are reduced in size by collision with one another and by impact with the cylindrical wall  42  of the internal structure as they circulate at high velocity in the raceway. Smaller particles flow along with the carrying gas upward and over the top wall  44  of the internal structure, where they exit from the mill through a centrally located passage  54  extending vertically through top wall  12  of the housing. Passage  54  can be provided with a vertically adjustable tubular cylinder  56 , for controlling flow through the outlet. The tubular cylinder can be secured at any desired height within a limited range by means of one or more thumbscrews, one of which is thumbscrew  58  in  FIG. 1 . Markings can be provided on the tubular cylinder to facilitate adjustment of the clearance between the lower end of cylinder  56  and the top wall  44  of the internal structure. 
     The clearance between the lower end of cylinder and the top wall of the internal structure affects the rate of flow of gas and particles from the raceway. With a smaller clearance, the rate of flow toward the outlet is reduced and inertia will maintain larger particles in the raceway so that only particles in a limited range of sizes escape the influence of inertia and pass through the outlet. If the clearance is enlarged by raising the cylinder  56 , the rate of flow is increased, and the size range of the particles passing through the outlet increases. 
     The internal structure  40  maintains the circulating particles in close proximity to the jets of gas emitted through the jet nozzles in housing wall  16 , thereby utilizing the kinetic energy of the gas jets more efficiently than current axial jet mills in which the particles are free to move radially inward toward a central location remote from the nozzles. For most effective grinding, the diameter of the outer wall of the internal structure, and the diameter of the inner surface of the outer wall of the housing are preferably such that the width of the raceway is approximately 6 cm. The jet mill in accordance with the invention can be made in various diameters and heights for different feed rates. In those jet mills having greater heights, i.e., jet mills in which the raceway has a large dimension in the direction of the central axis of the mill, the jet nozzles are preferably arranged at two or more levels to ensure rapid circulation of gas and entrained particles in all parts of the raceway. 
     Current designs, in which larger diameter jet mills can produce higher throughput rates, cannot produce fine particle sizes and fine particle size distributions that are possible with smaller diameter jet mills. The jet mill in accordance with the invention, however, because it incorporates a raceway grinding zone makes it possible to achieve fine article sizes and fine particle size distributions at much higher throughput rates. 
     Various modifications can be made to the apparatus described above. For example, the height of the internal structure can be increased or decreased, and, instead of mounting the internal structure from the bottom wall  14  of the housing on brackets, the bottom of the internal structure can be in direct contact with the bottom wall of the housing, and passages can be provided in the cylindrical side wall of the internal structure for movement of gas and particles into the raceway. The number and arrangement of nozzles can also be modified. For example, although in the embodiment described above the nozzles are constituted by openings though the outer wall of the housing alternative forms of nozzles can be utilized, e.g., special inserts that are replaceably fitted into openings in the outer wall, or nozzles that extend through openings in the outer wall, each nozzle being connected to a source of compressed air through a separate tube. These and other modifications can be made to the apparatus described without departing from the scope of the invention, which is defined in the following claims.