Solid material comminution or disintegration – Processes – Miscellaneous
Reexamination Certificate
2002-07-09
2004-05-18
Rosenbaum, Mark (Department: 3725)
Solid material comminution or disintegration
Processes
Miscellaneous
C241S036000
Reexamination Certificate
active
06736343
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a milling apparatus and a milling method wherein an object material is milled by colliding the object material against each other within a mill. The present invention also relates to a sintered body manufactured from a powder obtained by the above milling method.
2. Description of the Related Art
A conventional milling apparatus of the above kind comprises a mill for milling of the object material, a raw material feeder for supply of the object material to the mill, and a collecting tank for collection of a powder obtained by the mill. The mill includes a nozzle for injection of a high-velocity blast of an inert gas, and a classifying rotor for classified collection of the milled powder.
In the mill, the object material fed by the raw material feeder are collided against each other into small grains by the high-velocity blast of the inert gas injected from the nozzle. The milled powder is taken out by the classifying rotor and then collected in the collecting tank.
During the above-described operation, in order to obtain a uniform grain size distribution in the powder collected in the collecting tank, a constant milling condition, i.e. a constant condition for the collision in the mill, must be maintained. Then, in order to maintain the constant milling condition, the amount of the object material present in the mill must be maintained at a constant level.
Conventionally, in order to maintain the amount of the object material present in the mill at a constant level, the amount of the object material fed by the raw material feeder is varied by detecting weight change of the collecting tank.
Another method is to detect a load current to an electric motor which drives the classifying rotor. This detection method uses a principle that the load to the electric motor is increased by an increased amount of the object material colliding against the classifying rotor when there is an increase in the amount of the object material in circulation within the mill. Thus, the amount of the object material fed by the raw material feeder is reduced when the load to the electric motor becomes not smaller than a predetermined value.
Still another method of control is to provide a sensor for detecting a level of the raw material in the mill in order to determine if the amount of the object material in the mill has reached a predetermined level. In this method the feed of the object material from the raw material feeder must be continued unless the predetermined level is detected.
However, according to these conventional methods, it is difficult to maintain the amount of the object material remaining in the mill at a constant level.
Specifically, according to the method of detecting the weight of the collecting tank, the amount of the object material actually remaining in the mill can only be detected with a certain time lag. Therefore, since a decrease in the amount of the object material from a predetermined level cannot be detected soon, there is always a delay in supply timing of the object material. Furthermore, the object material is always overfed even if there is already enough amount of the object material supplied.
On the other hand, according to the method of detecting the load current to the electric motor which drives the classifying rotor, although there is less problem of the time lag than in the method of detecting the weight of the collecting tank, it is impossible to make detection that allows precise measurement of a change in the amount of the object material.
According to the method of providing the raw-material level sensor in the mill, it is possible to determine if the amount of the object material has reached the predetermined level, without the time lag. However, it is impossible to determine how much of the object material should be added when the level has not yet reached to the predetermined level. Therefore, there is a possibility that the amount of feed of the object material is being increased gradually in an attempt to achieve the predetermined level, yet actually the mill is discharging a greater amount, further reducing the amount of the object material present in the mill from the predetermined amount. If a large amount of the object material is fed at one time in an attempt to eliminate such a possibility as above, the amount in the mill can easily go far beyond the predetermined level. Further, the level sensor can not detect the level of the object material accurately, since the object material is blown up in the mill.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a milling apparatus and a milling method capable of obtaining a powder having a desired grain size distribution, by maintaining the amount of the object material in the mill at a constant level, and to provide a compact and a sintered body manufactured from the powder obtained by this milling method.
According to an aspect of the present invention, there is provided a milling apparatus comprising; a mill for milling of an object material, a raw material feeder for supply of the object material to the mill, a collecting tank for collection of a powder obtained by the mill, a weight detector for detection of a weight of the mill holding the object material remaining therein, and a controlling portion for control of an amount of supply of the object material to the mill based on an output from the weight detector, for maintenance of a constant amount of the object material present in the mill.
According to another aspect of the present invention, there is provided a milling method using a mill milling a supplied object material and discharging an obtained powder out of the mill, comprising; a first step of detecting a weight of the mill holding the object material remaining therein, and a second step of controlling an amount of supply of the object material to the mill based on a result of the detection in the first step, for maintenance of a constant amount of the object material present in the mill.
According to the present invention, the weight of the object material remaining in the mill is detected by detecting the weight of the mill holding the remaining object material. Thus, change in the weight of the object material remaining in the mill can be detected without time delay. Therefore, according to the present invention, as compared to a method, for example, of measuring the weight of the collecting tank which collects the powder discharged from the mill, the amount of feed of the object material into the mill can be varied more appropriately without time delay. As a result, it becomes possible to eliminate hunting caused by over-feeding, and maintain the amount of the object material in the mill at a constant level. Further, by detecting the weight, the weight deviation from a target value can be detected. Therefore, the amount of supply can be varied in accordance with the deviation, making possible to bring the amount of the object material in the mill close to the desired amount more quickly. As a result, the powder having a uniform grain size distribution can be obtained.
It should be noted here that in the present specification the term the “object material remaining in the mill” means the object material sedimented in the mill.
According to the present invention, preferably, the controlling portion controls the amount of supply of the object material by a proportional-plus-integral-plus-derivative control. By controlling the amount of supply of the object material by the proportional-plus-integral-plus-derivative control as above, the amount of the object material in the mill can be brought closer to the desired value, making possible to reduce fluctuation. Therefore, the grain diameter of the powder can be maintained within a constant range.
Further, preferably, the weight detector includes a plurality of detecting devices. By detecting the weight of the mill using the plurality of the detecting devices, it becomes possible to accurately detect the weig
Okayama Katsumi
Oota Akiyasu
Tsujimoto Akihito
Armstrong, Kratz, Quintos Hanson & Brooks, LLP.
Rosenbaum Mark
Sumitomo Special Metals Co. Ltd.
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