JP2001334157A - Pulverizing method for powder by low-temperature vibration mill - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pulverizing method for powder by a low-temperature vibration mill which is capable of pulverizing powder of hardly grindable material like crosslinked polytetrafluoroethyelene(PTFE) powder at a high yield. SOLUTION: The vibration mill generates vibration in a vessel 1 of a hollow cylindrical shape containing raw material powder 17 and many balls 22 by rotating a shaft 8 with unbalance weights mounted through bearings 7 in the central part of the vessel and pulverizes the raw material powder with impact and friction force by the vibration of the balls and agitating motion of this time. The entire part in the vessel is cooled down to <=-140 deg.C under the atmosphere gas therein and thereafter the raw material powder is extracted together with the atmosphere gas under vibration of the vessel, is recooled and is returned to the vessel. These operations are repeated until the raw material powder attains a prescribed grain size. Circulating system piping for the raw material powder and atmosphere gas connected between a lower port 13 and upper port 12 of the vessel is provided with branch piping across a valve 24 and the branch piping is provided with a bag filter 25, by which the powder pulverized to the prescribed grain size may be separated from the gas and may be taken out.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the field of the technology of finely pulverizing powders, and in particular, powders having rubber elasticity at room temperature and having high ductility and difficult to pulverize (for example, crosslinked PT).
FE) is finely pulverized to a particle size of about 5 μm or less.

[0002]

2. Description of the Related Art Conventionally, various techniques have been used for pulverizing powder, and the following are typical examples. (For example, published by Nikkan Kogyo Shimbun, Powder Engineering, P151, Shigeo Miwa.) 1) Jet mill (fluid energy mill); A method of accelerating and crushing particles by impact due to collision between opposing particles or collision between particles and a stationary collision plate.

2) Rotary mill; a raw material powder and a ball as a pulverizing medium are put into a cylindrical pulverizing chamber placed horizontally at normal temperature, and the ball is rotated around a central axis of a container to move the ball. Method of crushing raw material powder by impact and frictional force by

3) Vibration mill: A hollow cylindrical container placed horizontally at normal temperature is filled with raw material powder and balls serving as a pulverizing medium, and an annulus is connected to the container via a bearing at the center of the container. A method in which a shaft having a balance weight is rotated at a high speed by an external power, and the vibration of the shaft causes vibration and stirring movement of the ball, thereby imparting impact force and frictional force to the particles of the raw material powder to pulverize the raw material powder. .

[0005] 4) Low-temperature mill: A method of pulverizing raw material powder while cooling the whole inside of a mechanical pulverizing mill into which raw material powder is charged with liquid nitrogen or the like.

[0006]

SUMMARY OF THE INVENTION Powders having high ductility and rubber elasticity at a temperature higher than ordinary temperature, such as crosslinked PT
When the FE powder is pulverized by the above-described method, the following problem occurs.

[0007] 1) Jet mill; the impact of collision is reduced by the effect of rubber elasticity of the powder, and the particles are elongated by ductility. Therefore, it is difficult to pulverize the powder and the yield of the fine powder becomes extremely small. is there.

2) Rotary mill and 3) Vibration mill; flake or fibrous due to the ductility of the material, making it difficult to achieve the purpose of fine grinding.

4) Low-temperature mill: There has never been seen a practical example of a low-temperature mill capable of pulverizing the above-mentioned raw material powder from the structure of the apparatus.

Accordingly, an object of the present invention is to provide a method of finely pulverizing a powder using a low-temperature vibrating mill, which can produce a fine powder of a hard-to-pulverize material such as a crosslinked PTFE powder in a high yield.

[0011]

The method for finely pulverizing a powder by a low-temperature vibrating mill provided by the present invention is based on a hollow cylindrical container containing a raw material powder and a number of balls serving as a grinding medium for the raw material powder. By rotating a shaft with an unbalanced weight attached via a bearing at the center of this container, vibration is generated in the container, and the vibration and stirring motion of the ball at this time finely separate the raw material powder by impact and frictional force. After cooling to −140 ° C. or lower under the atmosphere gas of the entire interior of the container of the vibrating mill to be pulverized, the raw material powder is extracted together with the atmospheric gas while vibrating the container, recooled and returned to the container. This method consists of repeating this operation until the powder reaches a predetermined particle size.

As described above, when the raw material powder is cooled to −140 ° C. or less, embrittlement occurs, and pulverization becomes easy in this embrittlement state. Incidentally, PTFE exhibits a crystal structure change close to a glass transition at around -130 ° C. and is slightly embrittled, and crosslinked PTFE (not 100% crosslinked) loses the rubber elasticity it has at room temperature at that temperature. These synergistic effects facilitate the pulverization of the powder.

A vibrating mill having a high grinding ability is used as a grinding device. As the atmospheric gas, dry air or an inert gas is used, extracted from the container together with the raw material powder,
Using a low-temperature liquefied gas such as liquid nitrogen or liquefied natural gas or a refrigerator as a cold heat source, circulation is performed while cooling again through a heat exchanger, and grinding is repeated until a predetermined particle size is reached.

The amount of heat required to cool the atmospheric gas to be circulated is as follows, except for the amount required to initially cool the entire system.
The sum of the amount of heat required to cool the input raw material powder to the specified temperature, the amount of heat that enters the system from the outside, and the amount of internal heat generated by the system. Can be.

In the circulating system piping for the raw material powder and the atmosphere gas, which is connected between the lower port and the upper port of the container,
By providing a branch pipe via a valve and providing a bag filter in the branch pipe, powder finely pulverized to a predetermined particle size can be separated and taken out from gas.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 (a) systematically shows an embodiment of a method for finely pulverizing a powder by a low-temperature vibrating mill according to the present invention, and FIG. 2) shows a cross section along the line AA.

The vibrating mill of this embodiment has a crushing vessel 1 covered with a heat insulating material 3 around a hollow cylindrical vessel 2 having both ends closed.
Is supported by a pedestal 6 via a spring 5 at a stay 4.

A shaft 8 having an unbalanced weight 7 is inserted into a hollow portion at the center of the container 2,
The shaft 8 is attached to the container 2 by a bearing 9 so as to be relatively rotatable. The shaft 8 is linked at one end to a motor 11 via a flexible joint 10 so as to be rotated in the container 2.

The container 2 is provided with an upper port 12 and a lower port 13, which are connected by a pipe 15 via a flexible joint 14. A hopper 18 for supplying raw material powder is connected to the upper port 12 via a valve 16.

A blower 19, a valve 20, and a heat exchanger 21 connected to a cooling source 23 such as a liquefied gas or a refrigerator are sequentially provided from the lower port 13 side in the circulation system piping 15.
These piping systems should be well insulated.

In the above-mentioned piping system, at the outlet side of the blower 19, the valve 24,
A branch pipe is connected via 26, and a bag filter 25 for powder collection is connected to this pipe.

In the above system, the container 2 is filled with a small-diameter crushing ball (stainless steel ball) 22 of about 80% of the total internal volume of the container. When the shaft 8 with the unbalanced weight is driven to rotate to vibrate the container to the centrifugal effect of vibration 2 = about 6 to 10, the ball 23 vibrates in the container as shown in FIG. While rotating slowly in the direction of the arrow in the figure, the stirring motion of the entire ball is promoted.

In the above-described method of pulverizing powder in a facility having a vibration mill as a main body, the inside of the container 2 is subjected to -14 in the presence of an atmospheric gas such as dry air or an inert gas.
While cooling to 0 ° C. or lower, the valve 16 is opened and a predetermined amount of the raw material powder 17 is charged into the container 2 while the vibration of the container 2 is continued as described above, and the valve 16 is closed. The charged raw material powder is pulverized in a state where it is embrittled by cooling by an impact force and a frictional force due to the vibration and stirring of the ball 22. A part of the pulverized powder reaches the bottom of the container 2 and is sucked out from the lower port 13 by a blower 19 hermetically sealed with the atmospheric gas; after being extracted and recooled by the heat exchanger 21, the upper port It is returned into the container 2 via 12, and is again subjected to pulverization. By repeating this circulation, the powder is gradually pulverized to a predetermined size.

As described above, when fine pulverization is performed to a predetermined particle size, the valve 20 of the circulation system piping 15 is closed,
The valves 24 and 26 of the branch pipe are opened, and the finely pulverized raw material powder is mixed with the atmosphere gas in the bag filter 25.
Through. At this time, only finely pulverized powder is collected,
The atmosphere gas is returned to the circulation system piping 15 and is re-cooled by the heat exchanger 21 and returned. In this manner, when the collection of the finely pulverized powder has been performed by a predetermined amount, the valves 24 and 26 are closed and the bug is taken out, whereby the finely powdered powder can be taken out. At this time, valve 2
Opening 0 enables the operation of the apparatus even during the removal of the powder; the circulation of the powder is enabled.

The amount of cold heat to be supplied to the system by the heat exchanger 21 is as follows:
1) The amount of heat Q1 required to initially cool the entire system to a predetermined temperature, 2) The amount of heat q1 for cooling the raw material powder supplied to the system after the system has reached a steady state, and enters due to the temperature difference between the environment and the system. The quantity of heat q2 to cancel the heat q2 = q1 + q2 + q3, the sum of the quantity of heat q3 generated in the scenery, and the approximate size is as follows, assuming an apparatus that processes 2 kg of raw material powder at one time. .

Q1: Ball (SUS304, density 795)
0 kg / m ³ , specific heat 0.50 kJ / kgK at220
Assuming that the filling amount of K) is 50 kg and the weight of the container (SUS304) is 24 kg, the amount of heat required to cool the entire system from 20 ° C. to −150 ° C. is about 6300 kJ. When the amount of heat necessary for cooling the associated piping system is added, the following equation 1 is obtained. Even if cooling in one hour is assumed, a small refrigerator of about 2 kW can be used.

[0027]

(Equation 1)

Q2: 1) Assuming that a heat insulating material having a thermal conductivity of 0.04 W / mK is attached at a thickness of 5 cm, the amount of heat entering the interior through the heat insulating material is as shown in the following equation (2).

[0029]

(Equation 2)

2) Raw material powder (PTFE, density 2000 kg)
/ M ³ , specific heat 0.8 kJ / kgK) 2 kg / time,
If cooling is performed in 10 minutes, the required amount of heat will be as shown in the following Equation 3.

[0031]

(Equation 3)

3) The amount of internal heat generated is the frictional heat of the ball and the heat generated by the bearing.

[0033]

(Equation 4)

From the above, Q2 is given by the following equation (5). If a refrigerator of about 2 kW is prepared for initial cooling, the internal temperature of the system can be kept below the predetermined temperature (about -140 ° C.) even during operation. It is possible to keep.

[0035]

(Equation 5)

When liquefied natural gas is used as a cooling medium of the atmospheric gas to be circulated and cooled through a heat exchanger using liquefied natural gas through a heat exchanger, impurities are mixed into the liquefied natural gas. Instead, it can be used as it is as a fuel or raw material for the chemical industry in the next step, and by adding a pulverizing device to a liquefied natural gas plant, it is possible to greatly reduce the cooling unit consumption. When a refrigerator is used as a cooling medium, the apparatus can be greatly simplified.

As in this embodiment, since the crushing vessel 1 forming the main body of the vibrating mill is fixed, a flexible joint for absorbing vibration is used in the gas and raw material input and extraction ports to form a closed system. Can be easily realized.
Further, as described above, semi-continuous operation can be realized from the input of raw materials to extraction, and automatic continuous unmanned operation can be achieved.

[Other Embodiments and Modifications] The outer surface of a crushing vessel of a vibrating mill is sufficiently insulated, and liquid nitrogen is directly charged into the inside thereof together with balls and raw material powder, and is evaporated and cooled.
It is possible to use a method of constantly replenishing the shortage. In this case, the heat exchanger 21 and the cold heat source 23 are not required, but a cooling liquid nitrogen supply device is required. For example, in the case of the above-described condition device, 1.6 kW in a steady state in the case of the above-described condition device. It needs to be supplied as a consumable of about 29 kg / h.

In the case of using the raw material which is not liable to be contaminated by iron, the inner surface of the system including the pulverizing container may be coated with ceramic, and the ceramic ball may be used as the pulverizing ball.

[0040]

As described above, according to the present invention, there is provided a method of finely pulverizing a powder with a low-temperature vibrating mill, which can perform a fine powder of a hardly crushable material such as a crosslinked PTFE powder in a high yield. The objective of the period: the objective can be achieved, and the effect of implementing this can be considered to be enormous.

[Brief description of the drawings]

FIG. 1 shows an embodiment of a method for finely pulverizing a powder by a low-temperature vibration mill according to the present invention, wherein (a) is an explanatory view showing an apparatus system, and (b) is an AA sectional view of the same as (a).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Crushing container 2 Hollow cylindrical container 3 Heat insulation material 4 Stay 5 Spring 6 Pedestal 7 Unbalance weight 8 Shaft 9 Bearing 10 Flexible joint 11 Motor 12 Upper port 13 Lower port 14 Flexible joint 15 Piping 16 Valve 17 Raw material powder 18 Hopper 19 Blower Reference Signs List 20 Valve 21 Heat exchanger 22 Ball 23 Cold heat source 24 Valve 25 Bag filter 26 Valve

Claims (2)

[Claims] 1. A hollow cylindrical container for holding raw material powder and a large number of balls serving as a crushing medium for the base material, and a shaft with an unbalanced weight mounted via a bearing at the center of the container is rotated. By using a vibrating mill that causes the container to vibrate and finely pulverize the raw material powder by the impact and frictional force by the vibration and stirring motion of the ball at this time, -140 ° C. under the atmosphere gas of the entire interior of the container After cooling to below, the raw material powder is extracted together with the atmospheric gas while vibrating the container, recooled and returned to the container, and this operation is repeated until the raw material powder has a predetermined particle size. Fine grinding method of powder by vibration mill. 2. A piping system for circulating raw material powder and atmospheric gas, which is connected between a lower port and an upper port of the container,
A branch pipe is provided through a valve, a bag filter is provided in the branch pipe, and powder finely pulverized to a predetermined particle size is separated from a gas and taken out. Fine grinding method.