1. Field of the Invention
The present invention relates to a method for producing powder and a fluidized bed pulverizing apparatus.
2. Description of the Related Art
A toner used for an electrophotographic image forming apparatus is formed of fine particles having relatively uniform particle sizes of micron order. As an apparatus for producing such fine particles (powder) of micron order, a fluidized bed pulverizing apparatus (also called as an air flow pulverizing apparatus) is known. The fluidized bed pulverizing apparatus is constituted with a pulverization chamber (fluidized bed container), in which pulverization of a powder material is performed by allowing the powder material to collide against each other, a plurality of fluid jetting nozzles for jetting fluid in the pulverization chamber so as to entrain the powder material in the fluid, followed by colliding against each other so that the powder material entrained therein also collide each other, and then forming a fluidized bed in which the powder material further collide and are pulverized, and a centrifugal classification rotor which classifies the finely-pulverized powder, and is provided at the upper part of the pulverization chamber. In a typical fluidized bed pulverizing apparatus, the powder material supplied into the pulverization chamber are entrained in air flows which are jetted from a plurality of pulverizing nozzles, respectively, so as to collide against each other, and the powder material along with the air flow collide against each other, and then are pulverized. The air flow entirely fluidizes the powder material in the pulverization chamber, so as to accelerate pulverization caused by collision between the powder materials. Part of the powder material which has been pulverized and fluidized is guided to the area near a rotating rotor provided at the upper part of the pulverization chamber, and the particles of powder material each having a certain particle size or smaller are guided inside the rotor along with the fluid flow, and then powder as a final product (hereinafter referred to as product powder) is taken out from an outlet. The particles of the powder each having a certain particle size or larger are returned back to the outer periphery of the rotor by the centrifugal separation effect of the rotating roller, and are again returned back to the pulverization chamber, and then subjected to pulverization therein.
FIG. 1 shows a cross-sectional view of a conventional fluidized bed pulverizing apparatus. With reference to FIG. 1, a structure of the conventional fluidized bed pulverizing apparatus and a method for producing powder will be described below. In FIG. 1, 1 denotes a powder material supply inlet, from which a powder material is supplied, 2 denotes an outlet which discharges pulverized powder as a product along with exhaust air, 3 denotes a centrifugal classification rotor which classifies the pulverized powder, 4 denotes a pulverization chamber in a fluidized bed container, 5 denotes fluid jetting nozzles whose jetting openings are arranged inside the pulverization chamber 4, and which face each other and jet fluid, 6 denotes a motor driving the centrifugal classification rotor 3. The external appearance of the main body of the entire fluidized bed pulverizing apparatus is a substantially cylindrical housing.
The operation of the fluidized bed pulverizing apparatus shown in FIG. 1 is as follows. At first, before operation of the apparatus, inside the pulverization chamber 4 a certain amount of the powder material is charged. Next, compressed air is jetted from each of the two fluid jetting nozzles 5 facing each other, the air jetted from each of the two fluid jetting nozzles 5 forms jetted air flow. The jetted air flow entrains the powder material which is present in the pulverization chamber 4, so as to transport the powder material. The two jetted air flows entraining the powder material collide against each other near the center of the pulverization chamber 4, so as to form air flow upward, downward, leftward and rightward directions inside the pulverization chamber 4. These air flows further entrain the powder material in the pulverization chamber 4, so as to form a fluidized bed of the powder material in the pulverization chamber 4. On the other hand, the powder material entrained in the jetted air flows collide against each other along with the collision of a plurality of jetted air flows, and are pulverized. Further, in the fluidized bed, the collision and pulverization of the powder material are repeated.
Air in the pulverization chamber 4 passes from the outer periphery of the centrifugal classification rotor 3 located at the upper part of the pulverization chamber 4, through a gap between the rotors provided in the centrifugal classification rotor 3, and is guided to the outlet 2 connected to the centrifugal classification rotor 3, and then discharged from the outlet 2 to the outside. The powder material forming the fluidized bed is raised along with the exhaust air to the upper part inside the pulverization chamber 4, and enter the gap between the rotors from near the outer periphery of the centrifugal classification rotor 3. The centrifugal classification rotor 3 rotates at a certain rotating speed, among the powder material along with the air flow which reach the gap between the rotors, the powder material each having a certain particle size or larger is blown away to the outside of the centrifugal classification rotor 3 by centrifugal force. The particles of powder material each having a particle size smaller than a certain particle size along with the air flow are guided from the centrifugal classification rotor 3 to the outlet 2, and then discharged to the outside. The particles of powder material each having a certain particle size or larger are blown away to the outside of the centrifugal classification rotor 3, fall down in the pulverization chamber 4, and then are pulverized again in the fluidized bed.
From the powder material supply inlet 1, the powder material in an amount corresponding to the amount of powder discharged from the outlet 2 are supplied to the pulverization chamber 4, and the amount of the powder material in the pulverization chamber 4 is kept constant. Thus, in the fluidized bed pulverizing apparatus, the particles of powder material each having a desired particle size can be continuously produced. Meanwhile, the particle sizes of the particles of the powder material discharged from the outlet 2 can be controlled by adjusting the rotating speed of the centrifugal classification rotor 3. The pulverizing speed of the powder material, namely production speed of the pulverized powder material can be controlled by adjustment of the speed and flow rate of the air flow jetted from the fluid jetting nozzle 5.
In the fluidized bed pulverizing apparatus, the powder material is repeatedly pulverized in the pulverization chamber, in order to obtain the particles of product powder each having a desired particle size. In this case, when the production speed of the product powder is intended to increase, it is necessary to increase air flow rate jetted from the fluid jetting nozzle 5 so as to increase the pulverization efficiency of the powder material. However, in the case where the air flow rate jetted from the fluid jetting nozzle 5 is increased, the amount of exhaust air is increased, decreasing the classification efficiency of the centrifugal classification rotor 3. As a result, the average particle size of the product powder may become large, or the particles of powder material each having a large particle size may be easily mixed in the product powder. The average particle size of the particles of product powder can be controlled by adjustment of the rotating speed of the centrifugal classification rotor 3 to some degree. However, it is not easy to prevent the large size particles of the powder material from being mixed in the product powder. Therefore, as a countermeasure for the problem of the mixture of the large size particles of the powder material in the product powder, there has been known a method of providing a baffle plate at the upper part of the pulverization chamber 4, by which the course particles are prevented from mixing in the product powder. However, this method may decrease pulverization efficiency, probably causing decrease in production speed.
Moreover, there has been proposed a fluidized bed pulverizing apparatus (also referred to as “an air flow pulverizing apparatus”), for the purpose of improvement of pulverization efficiency of the fluidized bed pulverizing apparatus, adjustment of particle size of the product powder, and stabilization of product quality.
For example, Japanese Patent Application Publication (JP-B) No. 07-4557 discloses an air flow pulverization method, in which the pulverization efficiency of the powder material is improved by using a pulverization medium having a relatively large particle size.
Japanese Patent Application Laid-Open (JP-A) No. 2002-126560 discloses an air flow pulverizer, in which the pulverization efficiency is improved by adjusting the internal pressure of a pulverization chamber to negative pressure, or rising temperature in the pulverization chamber.
Japanese Patent (JP-B) No. 4025179 discloses an air flow pulverizer, in which a secondary collision unit for powder material which has collided by jetted air flow is provided so as to increase probability of collision between the powder materials, thereby increasing the pulverization efficiency.
JP-B No. 4291685 discloses an air flow pulverizer, in which compressed air jetted from a jetting nozzle is heated so as to improve the pulverization efficiency of the powder material, and the particle size of the product powder is optimized.
JP-A No. 2006-297305 discloses an air flow pulverizer, in which a space blocking member is provided in the inner wall of the pulverization chamber, particularly around a jetting nozzle, so as to decrease a dead space in the fluidized bed during the formation of the fluidized bed, thereby increasing the pulverization efficiency.
JP-B No. 2503826 discloses an air flow pulverizing method, in which a bypass directly leading from the pulverization chamber to the channel for discharging the final powder is provided so as to control the particle size distribution of the product powder.
JP-A No. 05-146704 discloses an air flow pulverizing method, in which a load current value of a motor for driving a classification rotor of a classifier is calculated as an integrated value of a predetermined time, and based on the value the supply amount of the powder material is adjusted so as to stabilize the particle size of the product powder.
JP-A No. 3995335 discloses an air flow pulverizer which controls the quality of the product powder in such a manner that the density of fluidized powder material in the pulverization chamber of the air flow pulverizer and the amount of powder material deposited in the lower part of the pulverization chamber are measured, and according to the density and the amount, the taking out of the deposited powder material and the supply of the raw material of the powder are controlled.
By using the above-described fluidized bed pulverizing apparatuses (air flow pulverizing apparatuses) or the fluidized bed pulverization methods, a certain effect is obtained for the purpose of improvement of pulverization efficiency, adjustment of product quality, product quality stabilization. However, any of the fluidized bed pulverizing apparatuses (air flow pulverizing apparatuses) and the fluidized bed pulverization methods aims to improve pulverization efficiency, and adjust and stabilize of product quality only during steady operation. Therefore, they still have problems in terms of the adjustment of product quality and the stabilization of quality during the initial operation of the fluidized bed pulverizing apparatus.
At the beginning of the operation of the fluidized bed pulverizing apparatus, a powder material is entirely in non-pulverized state. When air is jetted from a jetting nozzle, the powder material present in the pulverization chamber is whirled up in the air jetted from the jetting nozzle, and the powder material is started to collide and form a fluidized bed. In the unsteady state during the initial formation of the fluidized bed, not only the abundance ratio of the pulverized particles of powder material each having a certain particle size or smaller is low, but also the ratio of the non-pulverized particles of the powder material each having a large particle size introduced into a centrifugal classification rotor provided at the upper part of the pulverization chamber is high. In such unsteady operation state, the particle size of the product powder discharged from the outlet along with the exhaust air from the centrifugal classification rotor tends to be large. Thus, during the initial operation of the apparatus, the quality of product powder is not stable. In the case where the quality of the product powder is emphasized, the discharged product powder is discarded or recycled as an off-specification product for a certain time during the initial operation of the fluidized bed pulverizing apparatus, until the quality of the product powder finally becomes stable. In the fluidized bed pulverizing apparatus which is expected to produce a large number of product lots, every time when operation restarts for changing a product lot, off-specification products are formed, causing significant decrease in the production efficiency.