JPH0424285B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing silicon nitride powder with few impurities by acid treatment of silicon nitride powder. Conventionally, when wet refining silicon nitride powder that has been finely ground using a grinder, hydrochloric acid, hydrofluoric acid, sulfuric acid,
A slurry is made using an inorganic acid such as nitric acid or a mixed acid thereof, and impurities such as metals and silica contained in the silicon nitride powder are dissolved in the acid. After stirring and cleaning the slurry in an acidic region, only the silicon nitride powder is prepared. A method used industrially is to repeat sedimentation and separation (hereinafter referred to as decantation) several times, followed by filtration and washing using a filter press. However, although it is possible to remove metals as large as iron and impurities such as silica, anions such as halogens (hereinafter referred to as impurity anions) constituting the inorganic acid mixed in during acid treatment may be nitrided. The drawback was that a large amount remained in silicon. Also, with filtration means such as filter presses,
The handling of the cake is difficult, the planar area of the filter cloth is limited even when carried out on an industrial scale, and the yield of powder is poor. The present inventors have conducted various studies on methods for producing high-purity silicon nitride powder with a low content of impurities, and have made the PH of the slurry 3 or less using an inorganic acid, and reacted the impurities in the silicon nitride powder with the inorganic acid. Next, the silicon nitride powder is subjected to a so-called decantation process in which it is sedimented and separated, and ammonia is added to the slurry containing the silicon nitride powder from which impurities have been removed.
8.5 or more, sufficiently stirred, and then sedimented and separated and dried to provide a method for producing silicon nitride powder that can obtain silicon nitride powder with a high yield with less impurities and impurity anions in the silicon nitride powder. It is something. That is, the present invention is a method for producing silicon nitride powder with few impurities by acid-treating silicon nitride powder, by mixing silicon nitride powder containing fine powder of at least 0.7 μm or less with an inorganic acid aqueous solution to obtain a silicon nitride powder with a pH of 3 or less. After removing the impurities by reacting the inorganic acid aqueous solution of the slurry with the impurities contained in the silicon nitride powder, the silicon nitride powder is separated by sedimentation, and then the silicon nitride from which the impurities have been removed is Ammonia is added to the slurry containing the powder.
This is a method for producing silicon nitride powder with few impurities, which is characterized by forming a slurry with a pH of 8.5 or higher, and precipitating the silicon nitride powder in the slurry, separating it, and drying it. The present invention will be further explained in detail below. The present invention is a method for producing silicon nitride powder with few impurities by forming a slurry of silicon nitride powder and an acid aqueous solution and repeatedly decanting the slurry in an acidic region and an alkaline region. In particular, the present invention is characterized in that the raw material silicon nitride powder contains a specific fine powder. In the present invention, the silicon nitride powder used as a raw material contains particles of at least 0.7 μm or less. Those that do not contain particles of 0.7 μm or less are not suitable because although they will settle in the inorganic acid aqueous solution, the amount of impurities removed will be small. It is sufficient if the content of particles of 0.7 μm or less is as much as 1% in the raw material silicon nitride powder. Next, this raw material silicon nitride powder and an inorganic acid aqueous solution are made into a slurry having a pH of 3 or less. If the pH exceeds 3, it is not preferable because it takes time for the silicon nitride powder to settle and separation becomes difficult. In the present invention, one or more types of inorganic acids such as hydrochloric acid, nitric acid, sulfuric acid, and hydrofluoric acid are used. These acids vary depending on the type of impurities contained in the silicon nitride powder, but when iron, calcium, or silica is contained, hydrochloric acid, hydrofluoric acid, and a mixed acid are preferred. The reason for this is that iron, calcium, etc. become chlorides, and silica becomes hydrogen silicofluoride. When these are dissolved in an inorganic acid aqueous solution and then decanted, impurities are removed and silicon nitride powder is separated. be. By repeating such steps, impurities in the silicon nitride powder are removed. An alkali is added to the slurry from which impurities have been removed to adjust its pH to 8.5 or higher, and it is preferable to use ammonia as the alkali to be added. Other than ammonia, such as caustic soda and caustic potash, are not preferred because they contain alkali metal ions. The ammonia concentration of the slurry is
The amount is preferably 0.015 mol/or more, and there is no problem with the treatment temperature being around room temperature. Decantation in an alkaline region is carried out at a pH of 8.5 or above, but a pH below 8.5 is not preferred because it takes time for the silicon nitride powder to settle and separation becomes difficult. In the above decantation,
In addition to removing remaining impurity anions by acidic region decantation, impurities such as iron, calcium, and silica remaining in the raw material silicon nitride powder are also removed. Although known means can be used for this purpose, spray drying is preferred. According to the method of the present invention, impurities such as metals and silica contained in silicon nitride powder and impurity anions mixed in during acid treatment are largely removed, resulting in 98% high purity silicon nitride powder. It can be obtained in high yield. The product of the present invention is expected to be used in high-temperature structural components such as high-performance gas turbines. As production equipment for carrying out the present invention, it is sufficient to use an acid-resistant container made of polyethylene, polypropylene, etc. equipped with an agitator, and to serve as both an acid treatment tank and a cleaning tank, and there is no particular need to provide a separate cleaning device. isn't it. The effects of the present invention are as follows. (1) It becomes possible to remove impure anions, which could not be achieved with conventional methods. (2) Gas turbine components with further improved performance;
It is expected to be used in high-temperature structural parts and mechanical parts such as high-temperature bearings. (3) There is no need for complicated cleaning processes or cleaning equipment as in the past. (4) High purity silicon nitride powder can be obtained with a high yield of over 98%. (5) When a purified slurry is dried with a spray dryer, it is advantageous because the slurry concentration can be fed without adjustment, and the step of diluting the cake when filtered and washed using a filter press or the like can be omitted. (6) Equipment costs are low. The present invention will be explained below with reference to Examples. Example 1 98% of particles are 10 μm or less, and 0.7 μm
100 g of silicon nitride powder (A) shown in Table 1 containing the following particles was put into a polypropylene container containing a mixed acid aqueous solution 2 consisting of 1 5% hydrochloric acid aqueous solution and 1 5% hydrofluoric acid aqueous solution, and heated to 60°C. After stirring and washing for 1 hour, decantation in an acidic region was repeated 5 times until the pH reached 2.5. Then add 28% to that slurry
2 ml of ammonia water was added to adjust the pH to 10.4, and the alkaline region decantation was repeated three times, resulting in a pH of 9.2. When the purified slurry thus treated was spray-dried at 110°C using a spray fryer (manufactured by Yamato Scientific Co., Ltd.),
99.2g of silicon nitride powder was obtained. The measurement results of the chemical components, particle size distribution, and yield of this silicon nitride powder were
Shown in the table. Comparative Example 1 The same procedure as in Example 1 was carried out except that the slurry pH in the acidic region decantation was set to 4, but sedimentation separation was not possible. When aqueous ammonia was further added to bring the pH to 7.5, sedimentation separation was similarly impossible. Comparative Example 2 More than 98% of particles are 10 μm or less, and 0.7 μm
Example 1 was carried out in the same manner as in Example 1, except that silicon nitride powder (B) shown in Table 1 containing no particles smaller than m was used, and 97.7 g of silicon nitride powder was obtained. Table 2 shows the chemical component particle size distribution and yield measurement results of this silicon nitride powder. Comparative Example 3 The same procedure as in Example 1 was carried out except that filtration was performed using a filter press (manufactured by Toyo Kagaku Co., Ltd.) instead of alkaline region decantation, and the result was 89.6
g of silicon nitride powder was obtained. Table 2 shows the chemical composition, particle size distribution, and measurement results of the silicon nitride powder.

【table】

[Table] Example 2 The same procedure was carried out except that silicon nitride powder (B) in Table 1 was obtained by adjusting the grinding conditions of a jet mill and had a different content of particles of 0.7 μm or less. The same treatment as in Example 1 was performed. Table 3 shows the measurement results of the amounts of Fe and F impurities in the obtained silicon nitride powder.

[Table] The physical properties listed in Tables 1, 2, and 3 were measured by the following methods. (1) Fe, Ca...Compliant with JIS-G-1322. (2) O…O/N simultaneous analyzer (LECO (USA)
(manufactured by). (3) F...Pyrohydrolysis distillation, by Alftsson spectrophotometry. (4) Cl...By pyrohydrolysis distillation, mercuric thiocyanate neutralization titration method. (5) Particle size distribution…Particle size distribution (laser diffraction method, N&
Manufactured by Company L (UK), product name: Microtrack
SPA”).

Claims (1)

[Claims] 1 In the method of manufacturing silicon nitride powder with few impurities by acid treatment of silicon nitride powder, at least 0.7 μm
Silicon nitride powder containing the following fine powder and an inorganic acid aqueous solution were mixed to form a slurry with a pH of 3 or less, and the inorganic acid aqueous solution of the slurry was allowed to react with impurities contained in the silicon nitride powder to remove impurities. After that, the silicon nitride powder is separated by sedimentation, and then ammonia is added to the slurry containing the silicon nitride powder from which impurities have been removed to make a slurry with a pH of 8.5 or higher, and the silicon nitride powder in the slurry is sedimented. A method for producing silicon nitride powder with few impurities, which is characterized by separation and drying.