VERTICAL GRINDING MILL

Abstract: An improved vertical grinding mill is proposed which comprises a shell (10), a vertical screw shaft (11) rotatably mounted in the shell, a classifying means, a drive for the screw shaft, and means for forming a current of fluid for taking fine particles out of the shell. The inlet (21) for the fluid is provided in the center of the bottom of the shell (10) directly under the screw shaft (11). The fluid is uniformly distributed in all directions, carrying up the pulverized fine particles.

Description: This invention relates to a vertical grinding mill according the the preamble part of claim 1. A conventional grinding mill of this type disclosed in JP-A- 39-5584 is shown in Fig. 2. A vertical screw shaft 2 is rotatably mounted in a vertical shell l. Grinding medium b such as steel balls is filled in the shell l. While rotating the screw shaft 2, the material a to be pulverized is fed into the shell. The material is pulverized to fine particles by friction between the material and the grinding medium. The fine product particles c are carried away by air or water current out of the shell l. On the conventional grinding mill, the air or water current is introduced through an opening in the side wall of the shell into the bottom of the shell l and leaves the shell at its top together with the fine articles. Since the current is blown into the shell from one side of its bottom, the product particles will be unevenly distributed as shown in Fig. 2 with a dotted line and thus will not be smoothly discharged out of the shell. Therefore, the production efficiency was relatively low and the capacity was small for a large apparatus size. Another grinding mill shown in Fig. 3 is disclosed in EP-A- 85 11 2783. It comprises a hollow screw shaft 2 rotatably mounted and is adapted to cause a fluid to flow down through the screw shaft and spread uniformly in all directions from the open bottom end of the screw shaft into the shell 10. This arrangement assures a uniform distribution of the material to be treated, thus solving the abovementioned problem. However, this grinding mill has a shortcoming that various members for driving the screw shaft and for feeding a fluid thereinto have to be mounted on the top end of the screw shaft which projects beyond the top end of the shell. This complicates the structure of the grinding mill. A further vertical grinding mill is disclosed in DE-B1 216 078. In this grinding mill an inlet for the material to be pulverized is provided in the center of the bottom of the shell. The inlet comprises a non-return valve which is surrounded by a cylindrical covering means. A stirring member is mounted at the lower end of the screw shaft above the open covering means. By this arrangement it is possible to prevent grinding medium from enterring the non-return valve. However, this device is not related to an improvement in the distribution of the material to be pulverized in the vessel. It is therefore the object of the invention to provide a vertical grinding mill which assure a uniform distribution of the particles in the shell without the necessity of mounting various members on the screw shaft. This object is achieved by a vertical grinding mill comprising a shell for containing material to be pulverized and grinding medium, a vertical screw shaft rotatably mounted in said shell so as to extend through the top wall of said shell, means connected to said screw shaft for driving said screw shaft to agitate the material and the grinding medium and pulverize the material to fine particles, classifying means at the top of said shell for collecting the fine particles, means for forming a fluid current for taking the fine particles out of the shell, wherein an inlet for the fluid current is provided in the center of the bottom of said shell and a top-open covering means is provided at the bottom of said shell under said screw shaft around the opening of said inlet to prevent the material to be pulverized and the grinding medium from flowing into said inlet, and wherein said covering means is in the shape of a truncated cone and is formed with openings each covered by a penthouse. While rotating the screw shaft, the material to be pulverized is fed into the shell of the grinding mill as described above. The material is pulverized to fine particles by friction between the material and the grinding medium. When air or water current is blown into the shell through the inlet in the bottom of the shell, it flows up in the shell and leaves the shell at its top carrying the product particles. Other features and objects of the present invention will become apparent from the following description taken with reference to the accompanying drawings, in which: Fig. l is a vertical sectional view of a grinding mill; and Figs. 2 and 3 are vertical sectional views of prior art grinding mills. Referring first to Fig. l, a vertical grinding mill comprises a vertical cylindrical shell l0 having its top and bottom walls closed, and a screw shaft ll supported in the cylindrical shell l0 by means of a thrust bearing or the like. The screw shaft ll extends through the top wall of the shell l0 and is rotated by a motor (not shown). The screw shaft ll is hollow at part of its lower portion and is formed with a plurality of small openings 25 at the lower portion to adjust or retard the speed at which the fluid flows out of the bottom of the screw shaft ll. The number and position of the small holes may be decided as necessary. The small holes may be omitted. The shell l0 is provided at its upper portion with an inlet l2 for the grinding medium b and an inlet l3 for the material a to be pulverized. Through the inlet l2, the grinding medium such as ceramic, gravels or steel balls is supplied into the shell l0 up to the level L in Fig. l. Through the inlet l3, the material to be pulverized is fed into the shell l0 by a screw conveyor or the like, keeping airtightness. As the screw shaft ll rotates, the grinding medium b and the material a to be pulverized are agitated in the direction of arrow. As a result, the material is pulverized to fine particles c by friction between the material and the grinding medium. At the upper portion of the shell l0, a vane wheel l4 having blades l4a arranged at circumferentially equal intervals is rotatably mounted on the screw shaft ll. An annular member l5, triangular in section, is provided over the entire circumference of the inner wall of the shell l0 so as to be opposed to the vane wheel l4. When a motor 20 rotates the vane wheel l4, a swirling force is given to the air current passing between the vane wheel l4 and the annular member l5, so that the product particles are classified. Above the vane wheel l4 is formed a suction port l6 to which a suction fan l8 is connected through a product collector l7 such as a back filter or a cyclone. In the center of the bottom wall of the shell l0 is formed an air inlet 2l to which an air supply pipe 22 is connected. Over the air inlet 2l, a truncated conical member 23 is provided. When the suction fan l8 is started, the inside of the shell l0 will be put under negative pressure, so that the air current is admitted into the shell l0 through openings 24 in the member 23. The air current flows into the screw shaft ll, too, and through small holes 25 into the shell l0 and spread uniformly all around the shell. An outlet l9 is provided to take the grinding medium out of the shell l0. In operation, when the screw shaft ll starts to rotate and the material to the pulverized is fed into the shell l0, the shaft ll will agitate the material and the grinding medium, so that the material a will be pulverized to fine particles c by friction between them. On the other hand, when the suction fan l8 is started, air will flow through the air inlet 2l into the bottom of the shell l0 and spread uniformly in all directions. This air current will flow up in the shell l0, swirling up between the vane wheel l4 and the member l5. The air current passing therebetween carries up the fine particles, which pass through the suction port l6 out of the shell l0 and are collected in the collector l7. Coarse particles are separated by the swirling force produced by the vane wheel l4. Since the air admitted through the member 23 spreads uniformly in all directions and agitates the grinding medium and the material to be pulverized, they will not stay at the same position. In the grinding mill, pulverization under a dry condition is possible if hot air is introduced into the shell. The fluid for carrying the product particles c may be any other gas than air or a liquid such as water. The air outlet of a suction fan may be connected to the inlet port of the air supply pipe 22 to form a closed circuit. In this case, it is preferable to supply about two-thirds of the entire volume from the suction fan to the air supply pipe 22 so that even if the inside of the air supply pipe 22 is under positive pressure, the inside of the shell l0 will be kept under negative pressure. In this closed-circuit operation using air as the fluid, if the air becomes saturated with water, part of the exhaust of the suction fan l8 may be discharged and fresh air be supplied from the outlet from the air supply pipe 22 under the screw shaft ll to make up for the discharged volume. Although with the above embodiment, the inside of the shell l0 was out under negative pressure, it is naturally possible to gain a beneficial effect of the present invention with the inside of the shell l0 put under positive pressure by feeding pressurized fluid through the inlet 2l into the shell.

Claims: A vertical grinding mill comprising: A shell (10) for containing material (a) to be pulverized and grinding medium; a vertical screw shaft (11) rotatably mounted in said shell (10) so as to extend through the top wall of said shell (10); means connected to said screw shaft for driving said screw shaft to agitate the material and the grinding medium and pulverize the material to fine particles; classifying means (14) at the top of said shell for collecting the fine particles; means for forming a fluid current for taking the fine particles out of the shell; characterized in that an inlet (21) for the fluid current is provided in the center of the bottom of said shell (10); and that a top-open covering means (24) is provided at the bottom of said shell (10) under said screw shaft (11) around the opening of said inlet to prevent the material to be pulverized and the grinding medium from flowing into said inlet, wherein said covering means (24) is in the shape of a truncated cone and is formed with openings each covered by a penthouse.

Inventors: IKEBUCHI, IWAO, NAKANO, MAMORU
Applicants: KUBOTA LTD.
Application year: 1987
Main classification: B02C17
IPC codes: B02C17