JPS6241179B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing chromium oxide having a large particle size, and its purpose is to provide chromium oxide suitable for thermal spraying materials, refractory materials, abrasive materials, and the like. Powdered chromium oxide, which has conventionally been produced industrially, is obtained by thermal decomposition of chromic anhydride or heating and calcination of chromium hydroxide, but in both cases, the primary particles are in the form of very small fine powder. It is. Recently, for surface modification of metals and ceramics,
Thermal spray coating is performed using a plasma jet, and there are various materials used as the spray agent, one of which is chromium oxide. It is said that it is important for thermal spray chromium oxide used for such purposes to satisfy the following requirements. (1) A chromium oxide coating with high chemical purity and desired performance can be obtained on the surface of the base material. (2) It has good fluidity as a powder and can be smoothly supplied at a constant rate without clogging the pores of the plasma gun during use. (3) During thermal spraying, the proportion of chromium oxide powder ejected with plasma gas that reaches the surface of the target substrate without scattering or loss and effectively forms a coating, that is, the yield of thermal spraying is high. thing. In general, in order to satisfy these requirements, especially (2) and (3) above, the particle shape of chromium oxide must be as close to spherical as possible, the particle surface must be smooth, the particle size distribution must be uniform, and each individual particle must be as close to spherical as possible. It is thought that it is necessary to have a certain mass (size) or more. This is because fine particles with a particle size of about 5 μm or less have small inertia and are easily scattered and lost during injection. However, conventional chromium oxide for thermal spraying is made by heating ordinary chromium oxide powder to a high temperature above the melting point of approximately 2,400°C, melting it, cooling it to solidify it, pulverizing it, and sifting it to obtain the desired product. Rotate the particle size part of 〓〓〓〓
Because it is made by a method of combusting, it is easy for contaminants to be mixed in and a decrease in chemical purity due to erosion of the furnace materials and electrodes during manufacturing, and there is also an increase in contamination and a decrease in yield due to crushing and classification. Not only does it have the disadvantage of easily causing deterioration, but microscopic observation shows that the particles are in the form of angular, irregularly shaped fragments, and the surface is irregular and has rough fractured surfaces, making it difficult to assemble as a powder. Fluidity is not necessarily sufficient. On the other hand, methods for producing large single crystals of chromium oxide have been known for some time, but for example, when fine chromium oxide powder is heated at 1200 to 1500°C in an air atmosphere, it grows into a rather coarse chromium oxide. There are reports that (Ceramic Industry Association Magazine 87 [5] 1979, 253-258
Page) Another method is to heat and sinter a mixture of chromium oxide and alkali chromate. (Japanese Unexamined Patent Publication No. 53-26296) However, the former method requires an extremely long time to grow to coarse particles, and there is actually an upper limit to the size of the particles, and the latter method Since the purpose is to recover flaky chromium oxide that has been evaporated and condensed by using an extremely large amount of alkali chromate, twice the amount of raw material chromium oxide, the yield as chromium oxide is low. However, there is a problem in that the container is extremely small and corrosion of the container is difficult to avoid. In view of the above-mentioned problems, the present inventors investigated the production of chromium oxide single crystals with large grain size, and found that it is possible to easily produce chromium oxide single crystals at relatively low temperatures and in a short time by using a limited amount of flux. The present invention was completed based on the discovery that coarse chromium oxide particles can be obtained. That is, the gist of the present invention is that chromium oxide powder contains at least 20% by weight of at least one or more metals selected from alkali metal salts or alkaline earth metal salts based on Cr ₂ O ₃ . This method for producing chromium oxide with large particle size is characterized by heating and baking a salt-added compound at a temperature of 1200 to 1600°C, and then washing the obtained baked product with water to remove soluble salts. The raw material chromium oxide is usually industrially available powdered chromium oxide, but there are also
Its precursors, chromic anhydride, chromium hydroxide,
It is also possible to reuse ammonium chromate, ammonium dichromate, or a portion of chromium oxide recovered by the production method of the present invention. On the other hand, as metal salts added to coarsen the particle size of chromium oxide, alkali metal salts and alkaline earth metal salts are used, and their carbonates, hydroxides, chromates, and dichromate Salt, chloride or sulfate. Particularly preferred are sodium,
One or more salts selected from the above salts of potassium, lithium or barium are preferable. The amount of such a metal salt to be used based on the raw material chromium oxide may be at most 20% by weight based on Cr ₂ O ₃ . The reason for this is that crystal growth of chromium oxide is expected as the amount of addition increases, but at around 20% by weight, it reaches saturation and crystal growth cannot be expected for that amount of addition, and the yield of chromium oxide decreases significantly. This is because the corrosion of the firing container and furnace materials will also increase. Therefore, manufacturing conditions vary depending on the purpose of use of chromium oxide and the desired particle size, but in most cases, the amount added is 1.
A range of 15% by weight is preferred. In order to heat and bake the above-mentioned mixture of the two types of raw materials, it is sufficient to heat and bake the mixture at a temperature of 1200 to 1600°C for a desired period of time. The firing time does not need to be particularly limited, but the longer it is, the greater the crystal growth, so it may be set in consideration of energy efficiency. Next, after heating and firing, the fired product is washed with water or warm water to remove soluble salts. The purpose of this water washing is to remove chromium oxide partially oxidized and converted into chromate or dichromate and metal salts added as crystal growth agents. After washing with water, the product is dried and classified as desired. Incidentally, classification can be carried out by elutriation as well as washing with water. Thus, according to the present invention, a coarse chromium oxide single crystal can be produced industrially and advantageously by relatively simple operations, and its purity is sufficiently suitable for various uses. That is, according to the method of the present invention, unlike the melting method, the product does not need to be pulverized and falls into the particle size range of commercially available chromium oxide for thermal spraying, and also has good fluidity because the shape is close to spherical. It is. Moreover, according to this method, rough particle size adjustment can be performed only by adjusting the firing temperature or firing time, which is a further advantage. 〓〓〓〓
Even if the product requires a more stringent particle size range in practical terms, if an elutriation step is provided after the leaching step of the fired product, the product will have a uniform spherical shape, making it easy to classify by elutriation. And, moreover,
Until now, the coarse and fine parts after collecting materials with the required particle size range as thermal spray materials have generally been hard,
Even though it has excellent heat resistance and corrosion resistance, it has been hesitant to use expensive chromium oxide, which is obtained by melting and crushing, to obtain materials with the required particle size. It can be supplied and utilized, and its contribution to these industries is extremely large. Examples will be described below. Examples 1 to 6 Put 30 g of a raw material prepared by blending a predetermined amount of commercially available chromium oxide powder (average particle size 0.6 μ) and soda ash into an aluminium crucible with an internal volume of 30 ml, cover it, put it in a siliconite electric furnace, and heat it in an air atmosphere at 1500°C. After firing for 5 hours, the product was left to cool until the next day to obtain a fired product.
Water-soluble components of the calcined product were leached out with hot water, removed by solid/liquid separation, and dried to obtain coarse chromium oxide. The yield, particle size, and fluidity of each of the obtained chromium oxides were measured.
The results shown in Table 1 were obtained. The particle size was measured using the Coulter counter method, and the fluidity was measured using the angle of repose determined by the injection method. (30mm ring is used)

[Table] Furthermore, an electron micrograph of the chromium oxide obtained in Example 6 is shown in FIG. Examples 7 to 12 Regarding the same raw material chromium oxide used in Example 1, predetermined amounts of various metal salts were added in place of the added metal salt, soda ash, and the mixture was calcined at 1500°C for 5 hours.
Coarse chromium oxide was obtained in the same manner. When the particle size and angle of repose of the chromium oxide were determined, they were as shown in Table 2.

[Table] An electron micrograph of the chromium oxide obtained in Example 11 is shown in FIG. Comparative Examples 1 to 3 The same chromium oxide as in Example 1 was heated and fired for 5 hours without adding any metal salt, and then the properties of the chromium oxide were measured, and the results shown in Table 3 were obtained. 〓〓〓〓

[Table] As described above, the chromium oxide according to the present invention is a coarse single crystal and has fluidity, so it has excellent properties as a thermal spray agent. Figure 3 shows an electron micrograph of chromium oxide.

[Brief explanation of the drawing]

Figures 1 and 2 are 400x scanning electron micrographs showing the grain structure of chromium oxide crystals in an embodiment of the present invention, respectively, and Figure 3 is a 400x scanning electron micrograph showing the grain structure of commercially available chromium oxide. This is a scanning electron micrograph. 〓〓〓〓

Claims (1)

[Claims] 1. Chromium oxide powder contains at most 20% of Cr ₂ O ₃
After heating and firing a compound containing at least one or two or more metal salts selected from alkali metal salts or alkaline earth metal salts in an amount of % by weight at a temperature of 1200 to 1600°C, the resulting fired product is washed with water. A method for producing chromium oxide with a large particle size, characterized by removing soluble salts. 2. The alkali metal salt is one or more salts selected from carbonates, hydroxides, chromates, dichromates, chlorides, and sulfates of sodium, potassium, or lithium. A method for producing chromium oxide having a large particle size according to claim 1.