JPH0565502A - Method for granulating tantalum powder - Google Patents

Abstract

PURPOSE:To obtain the tantalum powder having a low carbon content and a good flow property at a good yield. CONSTITUTION:A fluidized bed type granulating machine is used and the powder is first granulated by using an inorg. binder as a binder. The powder is then granulated by using an org. binder. The Ta powder having an adequate grain size distribution and the low carbon content is obtd. in this way and, therefore, this powder is optimum as the powder for sintered tantalum capacitors.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to tantalum powder, and more particularly to a method for producing tantalum powder useful as a material for electrolytic capacitors.

[0002]

2. Description of the Related Art An electrolytic oxide film of metal tantalum has a high dielectric constant and is widely used as a capacitor electrode. Metallic tantalum for capacitors is usually manufactured by a method of reducing potassium fluorotantalate (K ₂ TaF ₇ ) with sodium (Na), and a powder purified through several chemical treatments and heat treatments is obtained. The tantalum powder for capacitors has a complex outer shape in which the individual particles are rich in irregularities and, at the same time, has an extremely porous sponge-like shape, and the effective surface area is much larger than it looks. Since a larger surface area is required, it is said that the powder should have a fine particle size. The obtained tantalum powder is compression molded into pellets, heat-treated in a high vacuum at a high temperature of 1400 ° C. or higher to form a porous sintered body, which is subjected to chemical conversion treatment to form an oxide film. It is used as a capacitor anode.

Since the flowability of the powder becomes a problem when it is filled in a die for molding the pellets, a powder having good flowability is required. To solve this problem, the following means have been conventionally used. After heat treatment, sieve with a 250 mesh sieve
A method of re-heat-treating powder of mesh or less and then blending powder of 250 mesh or more (USP401730)
2). After heat treatment, sieving is performed with a sieve, and only the powder on the coarse grain side is used (USP4968481). A method of granulating fine powder into coarse particles using a granulator such as spray drying. As the binder, PVA, PVB
And other organic binders are used (JP-A-2-3470).
1).

[0004]

The prior art has the following problems. After heat treatment, sieve with a 250 mesh sieve
In the method of blending the powder of 250 mesh or more after re-heat-treating the powder of mesh or less, the ratio of the powder of 325 mesh or less decreases only about 5-10%,
The effect is insufficient. After heat treatment, sieving is performed with a 325-mesh sieve to obtain 325
In the method using only powder of mesh or more, the yield is deteriorated, resulting in cost increase. In the method of granulating with an organic binder using a granulator, the carbon concentration in the powder becomes 4 to 6 times the normal level even if a debinding step is performed after granulation, and electrical characteristics, especially leakage The current gets worse. It is known that if the carbon concentration in the powder exceeds 40 ppm, the electrical characteristics of the capacitor, especially the leakage current, are adversely affected. Therefore, it is necessary to keep the carbon concentration below 40 ppm. The use of organic binders is not preferred to keep the carbon concentration low. However, when water or the like is used as a binder, there is a drawback that the granulation collapses when dried.

Further, the spray dryer cannot granulate if the powder to be granulated contains a powder having a particle size of 50 μm or more, and the powder can be granulated by the tumbling granulator (tantalum powder forms porous agglomerated particles). Since it has been pulverized and becomes finer, a dense granulated powder is formed, and the strength of the molded body (hereinafter abbreviated as GS) when the powder after granulation is made into pellets also has a drawback. Further, it is desired that the coarse particles have good fluidity while maintaining an appropriate particle diameter in the process of moving to the compression molding step. The conventional granulation method has not obtained tantalum powder having sufficient characteristics in these respects.

[0006] As an index showing the degree of granulation, the ratio of powder of 325 mesh or less (hereinafter abbreviated as -325 #) and the value of FR (the number of grams of powder flowing down in one second in the measuring method according to JIS2504) are used. Often used. The proportion of the -325 # powder is important because if the proportion of the -325 # powder is unsuitable, the fluidity becomes poor (FR becomes large).

At present, it is required that the fluidity be such that it flows down from the hopper without applying vibration. For that purpose, FR must be 0.8 or more,
For that purpose, the ratio of −325 # needs to be 25% or less. Also, the molded body strength GS is 6 mm × 7.9 mm created using 1.0 g of powder, and the pressed body density is 4.5 g.
It is expressed by the crushing strength of pellets of / cc, but if GS is 4 kg or less, the pellets may be broken. Therefore, GS is required to be 4 kg or more. An object of the present invention is to obtain a tantalum powder having the above characteristics in good yield.

[0008]

In order to solve the above problems, the present invention employs the following means. In order not to reduce the strength of the granulated powder compact,
Use a fluid bed granulator. In order not to increase the carbon concentration of the powder, first, granulation is performed using only water as a binder. However, if the granulation is carried out only with water, the granules will be broken by the mechanical external force until the granules are taken out from the granulator and put into a dryer or a heat treatment furnace. Therefore, after water granulation, an organic binder such as PVA is sprayed, and the surface of the powder is thinly covered with the organic binder to complete the granulation. It has been found that the granulation satisfying the above two points can solve the problems of the prior art. The present invention will be described in detail below.

In the present invention, a fluidized bed granulator is used as the granulation method. The fluidized bed granulator is composed of a combination of a solid gas fluidized bed and a spray nozzle, and granulates by a coating action and an aggregating action when a binder is sprayed on a powder that flows in the layer. If a fluidized bed granulator is used, the powder will not be crushed into fine particles, so that a porous granulated powder can be obtained. Therefore, granulated powder with GS of 4 kg or more can be manufactured. The binder is used in two stages.
First, at the initial stage of granulation, an inorganic binder having a low adhesive strength is used. Water or phosphoric acid can be used as the inorganic binder. At the initial stage of granulation, it is sufficient that the adhesive force is weak but the cohesive force is exhibited, and it is necessary to use a carbon-free one. 50-80% of the granulation is done using an inorganic binder. Then, in the latter stage of granulation, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), methyl cellulose, carboxymethyl cellulose and the like can be used as the inorganic binder in the latter stage. These organic binders contain carbon and have strong adhesion. If the organic binder is used at the final stage of the granulation, the strength of the granulated product can be increased, so that it does not break even after drying. The organic binder need only be used on the surface of the granule. Since the organic binder is used as a supplement, the amount used is small, and the carbon concentration after heat treatment is much lower than in the past.

[0010]

The present invention utilizes the cohesiveness of the inorganic binder and the adhesive strength of the organic binder. The main granulation was performed with an inorganic binder containing no carbon, and the organic binder was used only in the surface layer portion to maintain the strength of the granulated body.

[0011]

EXAMPLES Next, the present invention will be described with reference to examples.

Example 1 1 kg of tantalum powder having a 325 # ratio of 50% was put into a fluidized bed granulator (STREA-1 manufactured by Aeromatic Co.). The hot air flow rate was set to 50 m ³ / min and the hot air temperature was set to 50 ° C. to flow the powder. First, water was sprayed for 10 minutes at a flow rate of 20 cc / min to granulate the powder. Then, PVA was sprayed at a flow rate of 20 cc / min for 5 minutes for granulation. The granulated powder is dried in a dryer and then heat treated in vacuum (145
0 ° C.). Crushing was performed after the heat treatment.

Example 2 1 kg of tantalum powder with a 325 # ratio of 50% was placed in a fluid bed granulator. The hot air flow rate was set to 50 m ³ / min and the hot air temperature was set to 50 ° C. to flow the powder. First, add phosphoric acid 20
The powder was granulated by spraying for 10 minutes at a flow rate of cc / min. Then, PVB was sprayed at a flow rate of 20 cc / min for 5 minutes for granulation. After drying the granulated powder with a dryer, heat treatment (1450 ° C.) was performed in vacuum. After the heat treatment, crushing was performed.

For comparison, the following three examples of granulation were carried out. Comparative Example 1 As a binder, water was sprayed at a flow rate of 20 cc / min for 10 minutes, and the same procedure as in Example 1 was carried out except that the powder was granulated. Comparative Example 2 PVA as a binder was sprayed at a flow rate of 20 cc / min for 10 minutes, and the same procedure as in Example 1 was carried out except that the powder was granulated. Comparative Example 3 1 kg of tantalum powder similar to that in Example 1 was granulated using a rolling granulator. 200 PVA as binder
I used cc. The granulated powder was treated as in Example 1.

For each sample of the example and the comparative example,
The ratio of 325 # powder, FR, GS, and carbon concentration after heat treatment were measured, and the results are shown in Table 1.

[0016]

[Table 1]

As shown in Table 1, the granulated tantalum powder obtained according to the present invention has a good fluidity which conventional powders do not have, so that it is easy to fill the pellet die with the powder. Become. Further, it can be seen that the pellet strength is high even though the carbon concentration is extremely low, which is suitable for a sintered capacitor.

[0018]

EFFECTS OF THE INVENTION According to the present invention, it is possible to obtain a tantalum powder having an appropriate particle size distribution with good pellet packing property and a low carbon content from an alkali-reduced powder in good yield. The economic effect it gives is extremely large.

Claims (3)

[Claims] 1. A method for granulating tantalum powder using a fluidized bed granulator, wherein an inorganic binder is first sprayed for granulation, and then an organic binder is sprayed for granulation. Granulation method of tantalum powder. 2. The method for granulating tantalum powder according to claim 1, wherein the inorganic binder is water. 3. The inorganic binder is phosphoric acid.
A method for granulating tantalum powder according to the item.