METHOD AND APPARATUS FOR PULVERIZING PARTICULATE MATERIAL

Abstract: A method and apparatus for pulverizing a material such as sand to a desired particle size. The apparatus has a pulverizing shell in which the material to be pulverized is fed and pulverized. The pulverized fluid-carried material overflows the shell so as to be fed into a settling classifier, where the coarser material settles and is discharged through its bottom opening back into the shell for repulverization. The finer material overflows the classifier and is fed to an intermediate bin, which has its bottom connected to the shell through a valve. When the bin is filled up with the material, the valve is opened so that the material in the bin is returned to the shell. The material is thus circulated through the shell, classifier and intermediate bin. The shell is adapted to be alternately connected to the intermediate bin and a product collector bin by a changeover valve. When the material overflowing the shell is pulverized to a desired particle, size, the shell is now brought into communication with the collector bin.

Description: The present invention relates to a method and apparatus for pulverizing sand into a particulate product. There are two known methods for pulverizing such a material as sand. In one of the two methods the material is fed and pulverized in batches, and in the other method the material is pulverized continuously. In the former method, the material to be pulverized contained in a supply bin is fed into a pulverizer by a predetermined amount. The pulverizer is then actuated to grind the material to a desired particle size. It is then stopped and the pulverized product is discharged into an end product collector bin. In the latter or continuous method, the material to be pulverized contained in the supply bin is continuously fed into the pulverizer and pulverized into a product, which overflows the pulverizer and is fed into the collector bin. A classifier 4 may be provided between the pulverizer and the collector bin to collect the pulverized product of a larger particle size than the predermined value and return it to the pulverizer. In the batch method, the particle size of the pulverized product can be easily controlled by adjusting the operating time for the pulverizer, thus reducing the ratio of unacceptable coarser product to fully pulverized product. Also, if the material is pulverized in a liquid, the particle size of the pulverized product is not influenced by the concentration of the liquid in the pulverizer. But this method is time-consuming and thus inefficient, because the pulverizer has to be stopped each time when feeding the material and when discharging the product. Also, it will be troublesome to open and close the inlet and outlet ports of the pulverizer when feeding and discharging the material. Furthermore, in order to pulverize the material to such an extent that all the particles of the pulverized product will have smaller particle sizes than a predetermined value, not only the coarser product but also the one which has already been pulverized to the desired particle size will be pulverized. This will not only lead to the waste of time and energy but also cause part of the material to be over-pulverized too finely to be acceptable. Also, since the output of the pulverized product per hour including the downtime of the pulverizer is determined by the volume of the pulverizer, a pulverizer of a large volume is needed to increase the output of the product. This incurs economical losses. U.S. Patent specifications 4224354 and 3998938 proposed to circulate the material to be pulverized between the pulverizer and the collector bin. With this method, the pulverizing efficiency can be improved without increasing the volume of the pulverizer. But this method has one drawback that after each cycle of pulverization, the pulverizer has to be stopped until the pulverized product in the collector bin is emptied. In other words, this method is a batch method in a broader sense and thus inefficient. On the other hand, the continuous method is efficient, because the material to be pulverized as well as the pulverized product can be fed and discharged continuously. The pulverized product is less likely to be over-pulverized because it is discharged continuously. Especially with the apparatus in which fluid is used to classify the material by fluid flow, the product pulverized to a desired particle size is classified and carried by the fluid flow. Thus, the material is less liable to be over-pulverized and the pulverizing efficiency is high. But in the product fed into the collector bin, there is a tendency that a rather large amount of particles of larger sizes than required is mixed. This will lower the commercial value of the product. The provision of the classifier may be a partial solution to this problem. But if it is desired to obtain an ultra-fine powdery product of 10-0.1 µm (microns) in particle size, it is necessary to use an expensive high-grade classifier. Such a classifier is difficult to handle. Further, the pulverizing efficiency is related to the concentration of the fluid used for classification. The higher the concentration, the higher the thickness and the lower the precision of classification. The finer the particle size of the material to be classified, the more conspicuous this tendency will be. Namely, the classification point is limited by the concentration of the fluid. US-A-3 298 618 shows an apparatus for pulverizing a material and a method according to the preamble of claim 1. The apparatus includes a pulverizer having an input line for supplying the material to be pulverized and a discharge line for discharging the pulverized material to a bin. In the bin the pulverized material is either discharged or recirculated via the supply line to the pulverizer. Therefore, this apparatus is operable either in the one-pass mode or in a circulating batch mode as described above. It is the object of the present invention to provide a method and apparatus for producing a particulate material which has the advantages of both of the abovementioned two methods and in which the material can be entirely and uniformly be pulverized into a particulate. This object is achieved by a method as defined in claim 1 and by an apparatus as defined in claim 3; the dependent claims are related to further developments of the invention. According to the present invention, the material can be pulverized in a short period of time because the pulverizer is operated without intermission. Further, the pulverizing capacity, which is the sum of the volume of the pulverizer and that of the intermediate bin, can be increased inexpensively by increasing the volume of the less expensive intermediate bin. The provision of the classifier will serve to improve the pulverizing efficiency. The material left in the shell may be repulverized to prevent coarser particles from being mixed in the end product, thus improving its quality. 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. 1 is a schematic view of an entire system; according to the invention; Fig. 2 is a timing chart showing the operating procedure for the system of Fig. 1. As shown in Fig. 1, a pulverizer 10 has a cylindrical shell 11 filled with a pulverizing medium to a required height and having a screw shaft 12 rotatably mounted along its axis. The material a to be pulverized is introduced into the shell 11 with the screw shaft 12 rotating so as to be agitated in the shell in such a manner as shown by arrows. The material a is thus pulverized into a particulate product c by the grinding between its particles and between the material a and the pulverizing medium b . The shell 11 is formed in its lower portion with a water inlet 13. Water is introduced through the inlet 13 and flows upwardly in the shell. The particulate product c is classified by the upward current to be carried out of the shell 11 through an outlet 14 at its top into a settling classifier 51. The coarser particles of the material c , which have settled down in the classifier 51, are fed into the shell 11 by a pump p through the inlet 13 so as to be pulverized again. The shell 11 has its bottom connected to an auxiliary bin 80 through an on-off valve V1. Opening the valve V1 will cause the water and the material a to be pulverized (as well as the pulverized particulate product c ) in the shell 11 to fall by gravity into the auxiliary bin 80. The contents in the shell 11 may be forcibly drawn into the bin 80 by means of a pump. The settling classifier 51 includes an overflow pipe 17 connected to an intermediate bin 21 and an end product collector bin 31 through on-off valves V2 and V3, respectively. The slurry water overflowing the shell 11 (contains the pulverized particulate product c ) will be directed into the intermediate bin 21 if the valve V2 is open with the valve V3 closed or into the collector bin 31 if the valve V2 is closed with the valve V3 open. The intermediate bin 21 is connected to the inlet port 13 of the shell 11 through a return pipe 16 provided with a pump P1 and an on-off valve V4. By opening the valve V4 and actuating the pump P1, the slurry water containing the particulate product c (hereinafter merely referred to as water) will be returned into the shell 11. The intermediate bin 21 is also connected to the auxiliary bin 80 through a liquid supply pipe 81 provided with a pump P3 and an on-off valve V5. The water in the auxiliary bin 80 is fed into the intermediate bin 21 by means of the pump P3. Letters T indicate agitators. Now, the operation of the preferred embodiment will be described with reference to the timing chart of Fig. 2 in which solid lines mean that the respective parts are in operation or opened. In the first place, the valve V2 is opened, the pulverizer 10 is activated and the material a to be pulverized is fed into the shell 11 together with water. The material a is pulverized and classified while flowing in the up-and-down currents in the shell. The pulverized particulate product c overflows the shell to be fed into the classifier 51. The coarser particulate product c will settle in the classifier 51 and will be fed back into the shell 11, whereas the finer product c will overflow the classifier 51 so as to be fed into the intermediate bin 21. When the intermediate bin 21 becomes full, the supply of the material a to be pulverized is stopped and the on-off valve V4 is opened to feed the water in the intermediate bin 21 into the shell 11 by means of the pump P1. The material will be then circulated through the pulverizer 10, classifier 51, intermediate bin 21 to be further pulverized. When the particulate product c overflowing the shell 11 has been pulverized to a predetermined particle size, the on-off valve V2 is closed and the valve V3 is opened to direct the liquid containing the particulate product c into the end product collector bin 31. The pump P1 is deactivated and the valve V4 is closed when the intermediate bin 21 becomes empty. Then the valve V1 is opened to transfer the water in the shell 11 into the auxiliary bin 80. The water in the shell 11 may be drawn out by means of a pump so as to be fed into the collector bin 31. But the water in the shell 11 contains unpulverized coarse particles which are unacceptable as an end product, because part of the water is not circulated through the apparatus but remains settled in the shell and the piping. Thus it is preferable to transfer the water in the shell to the auxiliary bin 80 so as to feed it back to the shell during the next pulverizing cycle. The above-described operation can be repeated without stopping the pulverizer 10. The apparatus of the preferred embodiment was used to pulverize zircon sands in a liquid having a concentration of 60 per cent. It was confirmed that the sands were entirely and uniformly pulverized to a particulate product less than 3 microns in particle size.

Claims: A method for pulverizing a material into a particulate product by agitating the material to be pulverized in a fluid flow together with a pulverizing medium to pulverize the material by the friction between the particles of the material and between the material and the pulverizing medium, characterized in that a pulverizer (10) is adapted to be selectively connected with an intermediate bin (21) or a product collector bin (31), so that said pulverizer (10) is firstly connected with said intermediate bin (21) so as to circulate the fluid-carried material between said intermediate bin (21) and said pulverizer (10) until the material is pulverized to a desired particle size, and said pulverizer is then connected with said product collector bin (31) to direct the product pulverized to the desired particle size to said collector bin (31) and wherein the material and fluid left in the pulverizer at the end of every pulverizing cycle are directed into an auxiliary bin (80) and are fed back into said intermediate bin (21) at the beginning of the next pulverizing cycle. A method for pulverizing a material into a particulate product as claimed in claim 1, wherein the material is classified on its way from said pulverizer to said intermediate bin into finer and coarser materials according to particle size to return the coarser material into said pulverizer (10). An apparatus for pulverizing a material into a particulate product, comprising a pulverizer (10) having a pulverizing shell (11) in which the material to be pulverized is agitated in a fluid flow together with a pulverizing medium to pulverize the material by the friction between its particles and between the material and the pulverizing medium, an intermediate bin (21) and a product collector bin (31) adapted to be selectively connected with said pulverizer, a valve means (V₂, V₃) for selecting the connection so that the pulverized product in said shell (11) will be carried by the fluid into said intermediate bin (21) or said collector bin, and a return pipe (16) connecting said intermediate bin with said pulverizer to feed the material in said intermediate bin (21) back into said pulverizer (10), whereby the fluid-carried material can be circulated between said pulverizer (10) and said intermediate bin (21), wherein said pulverizing shell (11) is provided at its bottom with an on-off valve (V₁), and an auxiliary bin (80) is connected with said shell through said on-off valve (V₁) to allow the material in said shell (11) to be discharged into said auxiliary bin (80) and is connected with said intermediate bin (21) so that the material in said auxiliary bin (80) can be fed back into said intermediate bin. An apparatus as claimed in claim 4, further comprising a classifier (51) provided between said pulverizer and said valve means (V₂, V₃) and having its overflow port connected to said valve means (V₂, V₃) and its underflow port connected to said pulverizer (10).

Inventors: GANSE, AKIRA C/O KYUHOJI FACTORY OF KUBOTA, LTD.,, IHARA,YOSHITAKA,C/O KYUHOJI FACTORY OF KUBOTA LTD., IKEBUCHI, IWAO C/O KYUHOJI FACTORY OF KUBOTA LTD.,, KANEYASU,TOMIO C/O KYUHOJI FACTORY OF KUBOTA LTD.,
Applicants: KUBOTA LTD.
Application year: 1989
Main classification: B02C23
IPC codes: B02C25, B02C23, B02C21, B02C17