METHOD AND APPARATUS FOR GRINDING AND PULVERIZATION

Abstract: This invention relates to a method and apparatus for grinding and pulverization so as to produce fine powder such as silica powder as the raw material of a filler for sealing a semiconductor, for example. A disc is fixed coaxially to the lower end of a screw shaft put into a processing cylinder and the lower surface of the disc is an inverted conical surface. Therefore, wear of a screw is prevented by the disc and centripetal force resulting from the inverted conical surface acts to prevent sway of the screw shaft. If a plurality of screw shafts are disposed in the processing cylinder, the flow of fluid inside the processing cylinder does not become uniform and grinding efficiency can be improved. A partition is disposed between the screw shafts and communication holes are formed on the upper and lower surfaces of each partition so as to minimize short-circuiting between the outlet and inlet of the processing cylinder. Furthermore, if the rotating speed of each screw shaft, the height of each screw and the screw pitch are made different, the flow of fluid inside the processing cylinder becomes more complicated and grinding efficiency can be improved.

Description: The present invention relates to a continuously operable multi-chamber apparatus for pulverizing a material to be pulverized by use of a pulverizing medium and by rotating a vertically extending screw shaft mounted in a vertical shell. A prior art single chamber apparatus is shown in Fig. 1 of the accompanying drawings and comprises an upright shell 1 filled with a pulverizing medium b such as steel balls, and a vertically extending screw shaft 2 mounted in the shell 1. The material m to be pulverized is fed into the shell 1 with the screw shaft 2 in rotation to circulate it in the shell 1. When the material m is pulverized into a particulate product c having a desired particle size by the friction with the pulverizing medium b and between the particles of the material, it is entrained on the flow of air or water through the shell 1 to leave the shell. Other parts shown are a circulating fan 3, a product collector 4 such as a bag filter and a cyclone, and a rotary valve 5 for feeding the material m to be pulverized into the shell 1. With such prior art pulverizers, part of the material m fed into the shell through its inlet 9a tends to be discharged directly to its outlet 9b without being pulverized. Thus it is necessary to provide a collector 9 such as a cyclone between the outlet 9b and a product collector as shown in the drawing to collect the product c flowing out of the outlet 9b and feed it back into the shell 1. If the fluid in the shell 1 is liquid, a collector 9 such as a settling classifier has to be provided as shown in Fig. 2 to collect only the coarser product c and feed it back into the shell 1 by means of a pump. The provision of the collector 9 will not only make the machine bulky but also complicate the fluid control. Moreover, it is necessary to increase the driving force to drive the collector 9 which does not serve to pulverize the material. Thus the provision of the collector will increase the running cost and impair the pulverizing efficiency in comparison with the case in which no short circuiting of material takes place. Another problem with a prior art pulverizer is that the pulverizing medium b tends to flow in a rather simple, concerted manner because only one screw shaft 2 is provided in the shell 1. The material m is thus liable to be discharged unpulverized from the shell, resulting in an increase in the content of coarser particles in the product c . It is an object of the present invention to prevent or minimize the discharge of material without being pulverized, and to improve the pulverizing efficiency. DE-A-2124701 discloses a continuously operable multi-chamber apparatus for pulverizing a material into a particulate product, comprising a shell having an inlet and a discharge port and containing a pulverizing medium, and a plurality of vertical screw shafts rotatably mounted in said shell for agitating said pulverizing medium to pulverize the material fed into said shell by friction between the particles of the material and with said pulverizing medium into pulverized particles, said discharge port being positioned for removing said pulverized particles from said shell and a respective partitioning wall provided between adjacent screw shafts to form chambers within said shell. In DE-A-2124701 flow of the material being pulverized in each chamber is from a lower region thereof to an upper region thereof and flow of this material between successive chambers is via an opening in the shell at an upper portion of an upstream chamber, an opening in the shell at a lower portion of a downstream chamber and tubing external to said shell between said openings, said first mentioned opening having a grid there across for preventing passage therethrough of the pulverizing medium. JP-A-4210302 discloses a continuously operable multi-chamber apparatus for pulverizing a material into a particulate product, comprising a shell having an inlet and a discharge port and containing a pulverizing medium, and two horizontal screw shafts rotatably mounted in said shell for agitating said pulverizing medium to pulverize the material fed into said shell by friction between the particulars of the material and with said pulverizing medium into pulverised particles, said discharge port being positioned for removing said pulverized particles from said shell and a partitioning wall being provided between the screw shafts to define chambers within said shell, there being a hole provided at each end of the partitioning wall to allow flow communication between the chambers on each side of said wall, said holes being sized to allow passage therethrough of said pulverizing medium. The present invention provides a continuously operable multi-chamber apparatus as defined in the last but two preceding paragraph characterised in that a hole is provided in the or each partitioning wall to allow flow communication between the chambers on each side of said wall, said hole being sized to allow passage therethrough of said pulverizing medium and positioned such that all flow of said material (m) from the inlet to the discharge port occurs sequentially through said chambers and from an upper to a lower region of at least one of said chambers. The screw shafts may have a disk secured to the bottom thereof, the disk having a bottom surface shaped in the form of an inverted cone. The screw shafts may have different pitches, heights or revolving speeds from one another to circulate the material and the pulverizing medium smoothly so that no dead space will be left in the shell. In order that the invention may be well understood, some embodiments thereof which are given by way of example only, will now be described with reference to the accompanying drawings, in which: Figs. 1 and 2 are schematic views of prior art apparatus discussed above; Figs. 3 to 8 are respective schematic views of apparatus forming embodiments of the invention; Fig. 9 shows a modified screw shaft; Fig. 9a is a sectional view of the bottom part of the modified screw shaft; Fig. 9b is a plan view of the same; and Fig. 10 is a view similar to Fig. 9a showing an alternative modification. Like reference numerals have been used in Figs. 3 to 8 to designate like parts. Each of the continuously operable multi-chamber apparatus shown in Figs. 3 to 8 comprises a shell 11 having an inlet 18 and a discharge port 19 and containing a pulverizing medium b. A plurality of vertical screw shafts 12 are rotatably mounted in the shell for agitating the pulverizing medium to pulverize material fed into the shell through the inlet 18 by friction between the particles of the material and with the pulverizing medium into pulverized particles. The discharge port 19 is positioned for removing the pulverized particles from the shell 11. A respective partitioning wall 13 is provided between adjacent screw shafts 12 to form chambers within the shell and a hole 14 is provided in the or each partitioning wall 13 to allow flow communication between the chambers on each side of that wall 13. This hole 14 is sized and positioned to allow passage therethrough of the pulverizing medium b as well as the material being pulverized. The partitioning walls 13 in the embodiments not only prevent or minimize material being discharged unpulverized, but by virtue of the holes 14, the partitioning walls will also serve to circulate the pulverizing medium as well as the material to be pulverized more smoothly, leaving no dead space where the material and the medium are stuck. Referring in more detail to the apparatus shown in Fig. 3 a partitioning wall 13 is provided to separate two screw shafts 12 from each other and is formed with a hole 14 in a lower part thereof. The material m to be pulverized is fed into the top of a first or upstream chamber (left hand side of Fig. 3) in the shell 11 and pulverized by the first screw shaft 12 therein. The material in the first chamber will flow little by little into the second or downstream chamber through the hole 14 and be further pulverized by the second screw shaft 12. This will allow all the material to be pulverized uniformly without the fear of the coarse material being fed to the outlet port 19, which is in the upper part of the second chamber, without being fully pulverized. In the apparatus shown in Fig. 4 three or more screw shafts 12 arranged side by side may be provided in the shell with respective partitioning walls 13 arranged between adjacent screw shafts 12. The holes 14 are formed alternately under and over the respective walls 13 so that the slurry will flow across the shell 11 in a zigzag manner as shown by arrows between the inlet 18 and outlet port 19. In the apparatus shown in Fig. 5, there are two screw shafts 12 provided and in addition to the hole 14 in the partitioning wall 13 between these shafts which is provided in a lower part of the wall, the wall 13 is formed with a further hole 15 in its upper part above the pulverizing medium. The screw shaft 12 in the downstream chamber is adapted to be rotated faster than the screw shaft in the upstream (ie. left hand) chamber. Owing to the difference in the speed of revolution between the two screw shafts, the slurry level in the upstream chamber will become higher than that of the downstream chamber. This will cause the slurry to circulate between the chambers through the holes 14 and 15 in the partitioning wall 13. This arrangement will be especially efficient in pulverizing a material having a poor flowability. In this embodiment, the shell 11 is formed with an outlet port 19 disposed below the hole 15. The outlet 19 is covered with a screen 25 to prevent the leakage of the pulverizing medium b . The screen 25 may be a porous plate or a bar screen. A difference in the slurry level between the two chambers will be created by locating the rear screw shafts 12 nearer to the partitioning wall 13 than the front screw shaft 12. Means for moving the screw shafts 12 toward and away from the partitioning wall 13 may be provided to adjust the distance between the screw shaft 12 and the wall 13. It may be adjusted continuously during operation. Also, the screw shafts 12 may be provided with means for variably controlling their revolving speeds independently of each other. Also, the front screw shaft may have its blade pitch smaller than that of the rear screw shaft to create the aforementioned difference in the slurry level. Fig. 6 shows an apparatus in which the screw shafts 12 have the top end of their respective blades terminated at different levels. This arrangement will permit a smooth flow of the pulverizing medium b in the shell 11, thus improving the efficiency of pulverization. In this embodiment, either one of the upstream and downstream screw shafts may have its blade higher than the other. Fig. 7 shows an apparatus in which the inlet 18 is at the lower part of the upstream chamber, the outlet port 19 is at the lower part of the downstream chamber and the partitioning wall 13 between the chambers has the hole 14 in its upper part. The material m in the upstream chamber will flow into the chamber through the hole 14. In the apparatus shown in Fig. 8, the upstream screw shaft 12 has its bottom end terminated short of the bottom end of the downstream screw shaft. The upstream chamber of the shell is correspondingly shallower than the downstream chamber, the shell being formed with a stepped bottom. The partitioning wall 13 is formed at both its upper and lower parts with holes 15 and 14, respectively. Thus, the pulverizing medium b can be circulated smoothly between the front and rear half portions of the shell. The shell 11 may have a stepped bottom as in this embodiment even in the embodiments in which the slurry is not adapted to be circulated between the front and rear half portions e.g. as with the pulverizer shown in Fig. 3. All the above-mentioned embodiments are of a wet type using water as a fluid, but any of the embodiments may be used for a dry type of pulverizer using air as a fluid. The screw shafts may have different pitches from each other or even each screw shaft may have different pitches at different portions thereof. The screw shafts 12 of the embodiments may have a disk 20 secured to the bottoms thereof as shown in Fig. 9, 9A and 9B. As illustrated the disk 20 has a lower surface shaped in the form of an inverted cone and of a diameter corresponding to that of outside diameter of the blade 26 at the bottom of the shaft. The disk 20 has a liner 21 moulded of ceramics and bolted to its bottom. The liner 21 serves to protect the disk 20 from wear. According to the kind of material to be pulverized and the process, the liner 21 may be made of a wear-resistant rubber or metal instead of ceramics. The disk 20 and the liner 21 may be made of a wear-resistant material such as cast iron containing a large amount of chrome and may be integral with each other. Also, the disk 20 may be made of ordinary cast iron or steel so that its bottom face will have an inverted conical shape and have the bottom face covered with a wear-resistant layer formed by the heat spraying of ceramic, the coating of ceramic powder or the fusing of a highly wear-resistant alloy. The pulverizing apparatus illustrated can be used for the production of a fine or granular material such as silica powder and other materials e.g. materials for a filler for sealing semiconductors, high-quality glass, lens, synthetic resin additives, artificial crystal, fiber, ceramics, agricultural chemicals, dentures and abrasives. While the screw shafts 12 are rotating, a centripetal force will act on the disks 20 owing to the inverted conical shape of their bottom face, thus allowing the screw shafts to rotate without running out of true. The blade 26 of each screw shaft 12 has such a diameter at its lower portion as to decrease gradually toward its bottom end. This will allow the blade to wear uniformly over the entire length. If its diameter is uniform from end to end, the blade 26 will wear more rapidly at its lower portion than at the upper part. This is because the lower portion of the blade 26 mainly serves to form an initial movement of the medium. Fig. 10 shows a modification in which a screw shaft 12 is hollow and has its bottom end inserted into a hole 22 formed in a disk 20 so as to feed fluid into the shell through the hollow screw shaft 12, as in the device of Fig. 1 which is a conventional apparatus. The disk 20 in this modification having its bottom face shaped in an inverted truncated cone will provide the same effects as with the disk in the first embodiment. It will be appreciated that in each of the embodiments, the hole 14 provided in the or each partitioning wall 13 is positioned such that all flow of the material m from the inlet 18 to the discharge port 19 occurs sequentially through the chambers and from an upper to a lower region of at least one of those chambers.

Claims: A continuously operable multi-chamber apparatus for pulverizing a material (m) into a particulate product, comprising a shell (11) having an inlet (18) and a discharge port (19) and containing a pulverizing medium, and a plurality of vertical screw shafts (12) rotatably mounted in said shell for agitating said pulverizing medium to pulverize the material fed into said shell by friction between the particles of the material and with said pulverizing medium into pulverised particles, said discharge port being positioned for removing said pulverized particles from said shell and a respective partitioning wall (13) provided between adjacent screw shafts to form chambers within said shell, characterised in that a hole (14) is provided in the or each partitioning wall (13) to allow flow communication between the chambers on each side of said wall, said hole being sized to allow passage therethrough of said pulverizing medium and positioned such that all flow of said material (m) from the inlet to the discharge port occurs sequentially through said chambers and from an upper to a lower region of at least one of said chambers. An apparatus as claimed in claim 1 wherein there are two said screw shafts (12) and said hole (14) is provided in a lower part of said partitioning wall (13) between said shafts, and said partitioning wall has a further hole (15) in an upper part thereof above the pulverizing medium (b). An apparatus as claimed in claim 1 wherein there are two said screw shafts (12) and said partitioning wall (13) between said shafts has said hole (14) in a lower part thereof, said inlet (18) is the top of the chamber on one side of the partitioning wall and the outlet port (19) is in the upper part of the chamber on the other side of the partitioning wall. An apparatus as claimed in claim 1 wherein there are two said screw shafts (12) and said partitioning wall (13) between said shafts has said hole in an upper part thereof, said inlet (18) is at the lower part of the chamber on one side of the partitioning wall and the outlet port is at the lower part of the chamber on the other side of the partitioning wall. An apparatus as claimed in claim 1 wherein there are at least three of said screw shafts arranged side by side, the holes (14) in said partitioning walls (13) between said shafts being formed alternately under and over said walls providing a zig zag path between said inlet (18) and outlet port (19). An apparatus as claimed in claim 1, wherein said hole (14) is provided in an upper or lower part of the or each partitioning wall. An apparatus as claimed in any one of the preceding claims wherein the respective shafts have screw blades with different pitches. An apparatus as claimed in claim 1 comprising means for driving said screw shafts (12) at different speeds of rotation. An apparatus as claimed in claim 1 or 6, wherein each said screw shaft (12) has a disk (20) secured to the bottom of said shaft and has a bottom surface in the shape of an inverted cone.

Inventors: GANSE AKIRA, IHARA YOSHITAKA, IKEBUCHI IWAO, NAKANO MAMORU, TAKEI HISASHI, YAMADA SHOZO, GANSE, AKIRA, IHARA, YOSHITAKA, IKEBUCHI, IWAO, NAKANO, MAMORU, TAKEI, HISASHI, YAMADA, SHOZO
Applicants: KUBOTA LTD, KUBOTA, LTD
Application year: 1989
Main classification: B02C17
IPC codes: B02C17