JPH0526725B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing fine particulate bismuth oxide from an aqueous bismuth nitrate solution. [Prior Art] Conventionally, methods for producing this kind of bismuth oxide include: (1) a wet method in which an alkali is added to an aqueous solution of bismuth nitrate or chloride to separate it from the mother liquor as bismuth subnitrate or bismuth hydroxide, and then calcining it; . (2) There is a dry method in which bismuth is forcibly oxidized in an electric furnace and then volatilized and collected as bismuth oxide. However, in the case of method (1) above, the product contains alkali metals and nitric acid groups, and the diameter of the particles is as large as 2 to 10 μm. In addition, although the method (2) does not introduce impurities, it also has the drawback that the diameter of the particles is large, 5 to 10 μm. [Problems to be Solved by the Invention] The purpose of the present invention is to provide particles suitable for additives for capacitor parts, etc., with a diameter of 0.3 to 1 μm and an average particle diameter of 0.6 μm.
An object of the present invention is to provide a method for producing bismuth oxide that is approximately spherical in shape and has good purity. [Means for solving the problem] In order to achieve this objective, the inventor of the present application, as a result of intensive research, added a sodium bicarbonate aqueous solution to a bismuth nitrate aqueous solution under appropriate conditions, and thoroughly washed the generated precipitate. The present invention was achieved by experimentally finding that the desired bismuth oxide can be obtained by performing subsequent suction filtration to remove as much of the aqueous solution as possible and then calcining it at a predetermined temperature. That is, in the method of the present invention, an aqueous bismuth nitrate solution containing a small excess of nitric acid is maintained at a temperature of 30°C or lower, preferably 10 to 30°C, and an aqueous alkali bicarbonate solution is added to the solution while stirring to give a final pH of 7 to 8. It is preferably added by the dropwise method at a relatively fast rate (equivalent of the above alkali per 1 g of bismuth is added over about 1 minute). The concentration of bismuth nitrate and the aqueous alkali bicarbonate solution are not specified, but are 10 to 90 g/b as bismuth and 10 to 90 g/b as sodium bicarbonate, respectively.
g/ is preferred. The precipitate generated by the addition of the above alkali is
First, it is separated from the aqueous solution by a suction filtration method, and then repulped with a large amount of hot water, preferably at about 50°C, two to three times or more, and the final suction filtration is carried out so that the moisture content of the precipitate is 60% by weight or less. ,
Next, it is baked at 350-400℃. [Function] In the method of the present invention, an alkali bicarbonate is particularly used as the alkali because fine crystals of bismuth subcarbonate cannot be obtained with other alkalis. The reason why the bismuth precipitation temperature is set to 30° C. or lower is that if the treatment temperature is high, the reaction rate will improve, but the primary particles will stick together and become coarse particles. It is for the same reason that it is preferable to make the dropping rate relatively fast. The purpose of containing excess nitric acid in the bismuth nitrate aqueous solution used as a raw material is to suppress the hydrolysis of bismuth, but if there are too many free nitrate radicals, not only will alkali be wasted, but a large Care must be taken because alkali nitrate salts are generated and as a result, bismuth oxide with good purity cannot be obtained. The amount of sodium bicarbonate or potassium salt added is
It is preferable that the amount is slightly more than the usual 1.0 equivalent, but
It is important that the pH of the slurry of bismuth subcarbonate after the addition of the alkali is between 7 and 8, and the final pH is
If it is lower than 7.0, secondary agglomeration of precipitates is thought to occur, but the particle size increases, although not linearly with the pH value, and the object of the present invention cannot be achieved. Even if too much alkaline bicarbonate solution is added, the PH
Although it does not exceed 8.0, excessive amounts of alkali can be harmful. The precipitate obtained in this step is washed preferably using a large amount of hot water at 50°C or higher, and at least 2
The repulping process is repeated, and the subsequent solid-liquid separation is carried out by suction filtration, at least the final suction filtration is carried out until the moisture content of the precipitate is 60% by weight or less, and impurities are almost completely removed. The precipitate thus obtained is dried at a temperature of 100°C or higher and then calcined at 350-400°C for 1 hour or more. If the firing temperature is low, the bismuth will not be oxidized sufficiently, and if the temperature is too high, the fine particles of bismuth oxide that are formed will fuse with each other, and this will have a strong correlation with the rise in temperature, resulting in coarse grain size of the product. Even if the sintering temperature slightly exceeds 400°C, when observed with a scanning electron microscope, it can be seen that fine particles begin to fuse, so extreme care must be taken in controlling the sintering temperature. According to the method of the present invention, the purity is improved as shown in the examples.
99.98% by weight or more, thickness approximately 0.1μm, average particle size 0.5
-0.7 μm, and the particle size range is 0.4 to 0.9 μm, resulting in fine-grained bismuth oxide. [Example] Examples will be described below. Example 1 After adding 50 ml of ion water to 500 g of 99.99% by weight bismuth, concentrated nitric acid (840 g/) in an amount of 1.3 equivalents to bismuth was added and dissolved by heating to prepare a stock solution. A fraction of the above stock solution was diluted with water, and 70g of bismuth/500g of bismuth nitrate aqueous solution was prepared.
ml was kept at 24℃ while stirring with a propeller type stirrer, and 90 g/aqueous sodium bicarbonate solution 2 was added dropwise using a roller pump at a uniform rate for 35 minutes.
7.2, a slurry containing fine precipitates of bismuth subcarbonate was obtained. The above slurry is immediately subjected to solid-liquid separation using a suction filtration method, and the resulting precipitate is mixed with warm water 5 at 50°C, stirred, and suction-filtered, so-called repulp washing, which is repeated three times. Suction was carried out until the concentration was approximately 50% by weight. The obtained precipitate was dried at 105°C and then calcined for 1 hour in an Elema furnace maintained at 400°C. The average particle size was 0.6 μm, the purity was 99.98% by weight, and impurities were 0.01% by weight of Na and 0.008% by weight of NO. 35.9 g of bismuth oxide containing ₃ was obtained. When observed with a scanning electron microscope, the shape of bismuth oxide was 0.1 μm thick, and the particle size range was 0.4 to 0.8 μm.
The particles were spherical with an average particle size of 0.6 μm, and had excellent dispersibility. The yield was 92%. Example 2 Bismuth oxide was produced in the same manner as in Example 1 except that the final pH was set to a predetermined value. The results are shown in Table 1.

[Table] As is clear from Table 1, the final pH was lowered.
Nos. 2 and 3 had coarse grains with an average grain size of 1.0 μm or more, but otherwise showed satisfactory grain sizes. Example 3 Bismuth oxide was produced in the same yield as in Example 1 in the same manner as in Example 1, except that the temperature at which the precipitate was generated or the temperature at which the precipitate was calcined was set to a predetermined value. The results are shown in Table 2.

〔Effect of the invention〕

According to the method of the present invention, fine particulate bismuth oxide with a narrow particle size range can be produced stably and efficiently by almost preventing the contamination of alkali metals and nitric acid groups, which were the drawbacks of the conventional wet method. .

Claims (1)

[Claims] 1. A bismuth nitrate aqueous solution containing excess nitric acid is heated at 30°C.
While keeping the temperature below and stirring, add the final pH to 7 to 8.
Drop an aqueous alkali bicarbonate solution dropwise, wash the formed precipitate using the repulp method, and then perform a final suction filtration so that the moisture content of the precipitate is 60% by weight or less. A method for producing fine-grained bismuth oxide, which comprises firing the bismuth oxide. 2. The method for producing fine particulate bismuth oxide according to claim 1, which is carried out by maintaining an aqueous bismuth nitrate solution at 10 to 30°C.