Method for producing ceramic powder

Abstract

To provide a method for producing ceramic powder that improves the solid-liquid separation process and increases production efficiency.SOLUTION: A method for producing ceramic powder including a step of adding a flocculant to a to-be-treated liquid containing ceramic powder, followed by filtration.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing ceramic powder. [Background technology]

[0002] Silicon nitride sintered bodies are materials with excellent strength, hardness, toughness, heat resistance, corrosion resistance, thermal shock resistance, etc., and are therefore used in various industrial parts such as die-casting machines and melting furnaces, as well as insulating substrates for automotive parts, etc. As the silicon nitride powder that is the raw material for silicon nitride sintered bodies, silicon nitride powder with a high degree of alpha conversion is used in order to obtain high-quality sintered bodies.

[0003] Known methods for producing silicon nitride powder include the "direct nitridation method," in which metallic silicon is reacted at high temperature in a nitrogen or ammonia stream, the "silica reduction method," in which a mixed powder of silicon dioxide and carbon is reacted at high temperature in a nitrogen or ammonia stream, and the "vapor phase synthesis method," in which a silicon halide or monosilane is reacted with ammonia at high temperature. The direct nitridation method in particular is a production method that is widely used industrially.

[0004] To produce sintered silicon nitride, silicon nitride powder with a small particle size is required. Therefore, silicon nitride obtained by the direct nitriding method is then pulverized in a pulverizer. Pulverization is broadly divided into dry pulverization and wet pulverization, but it is generally known that wet pulverization is easier to produce fine particles. Although wet milling produces silicon nitride fine powder with a small particle size, the components of the wet mill may be worn during the process. Therefore, impurities (e.g., iron) derived from the components of the wet mill may be mixed into the silicon nitride fine powder. Furthermore, wet milling is performed in water, and a portion of the silicon nitride reacts with water to produce silica. These impurities degrade the quality of the silicon nitride sintered body produced from the silicon nitride fine powder. Therefore, a washing process is required in which the silicon nitride fine powder is immersed in an acidic solution to dissolve the impurities, followed by a solid-liquid separation process to remove the dissolved impurities. Patent Document 1 describes an example in which silicon nitride powder after acid treatment is washed with water and decantation by gravity settling is repeated to remove dissolved impurities from the powder. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 7-25603 Summary of the Invention [Problem to be solved by the invention]

[0006] However, decantation by gravity settling has the disadvantage of requiring a long processing time, and it is therefore necessary to shorten the time required for solid-liquid separation to improve production efficiency. Furthermore, similar problems exist not only in the above-mentioned method for producing silicon nitride powder, but also in any method for producing ceramic powders that involves solid-liquid separation. The present invention aims to improve the efficiency of ceramic powder production by improving the solid-liquid separation process. [Means for solving the problem]

[0007] The present invention has the following aspects. [1] A method for producing ceramic powder, comprising the steps of adding a flocculant to a liquid to be treated containing ceramic powder and filtering the liquid. [2] The method for producing a ceramic powder according to [1], wherein the flocculant is an inorganic flocculant. [3] The method for producing a ceramic powder according to [1] or [2], wherein the method for adding the flocculant is a method for adding an aqueous flocculant solution containing the flocculant, and the concentration of the flocculant in the aqueous flocculant solution is 0.005 to 0.15 mass%. [4] The method for producing a ceramic powder according to any one of [1] to [3], wherein the ceramic powder is silicon nitride powder, and the average particle size of the silicon nitride powder in the liquid to be treated before the addition of the flocculant is 0.6 to 0.8 μm. [Effects of the Invention]

[0008] According to the present invention, the solid-liquid separation process can be improved, thereby increasing the efficiency of ceramic powder production. DETAILED DESCRIPTION OF THE INVENTION

[0009] The meanings and definitions of terms used in this specification are as follows: A numerical range expressed by "to" means a numerical range in which the numbers before and after "to" are the lower and upper limits. In this specification, the upper and lower limits can be combined in any way. "Average particle size" refers to the 50% cumulative volume particle size (hereinafter referred to as "D 50 The particle size distribution can be obtained in accordance with JIS R 1629:1997 "Method for measuring particle size distribution of fine ceramic raw materials by laser diffraction and scattering method." "Specific surface area" is a value measured by the BET single-point method using nitrogen gas in accordance with JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method."

[0010] The "alpha phase ratio" refers to the ratio of alpha silicon nitride to the total silicon nitride, and is a value calculated by the following formula 1. αization rate (%)=[I α(102) +I α(210) ] / [I α(102) +I α(210) +I β(101) +I β(210) ]×100 formula 1 In the formula 1, I α(102) is the intensity of the diffraction peak of the (102) plane of the α-phase of silicon nitride, and I α(210) is the intensity of the diffraction peak of the (210) plane of the α-phase of silicon nitride, and I β(101) is the intensity of the diffraction peak of the (102) plane of the β phase of silicon nitride, and I β(210) is the intensity of the diffraction peak of the (210) plane of the β phase of silicon nitride. Each diffraction peak can be obtained from the XRD pattern obtained by powder X-ray diffraction measurement.

[0011] The oxygen content of the ceramic powder is the total amount of oxygen obtained by heating the ceramic powder to 2000°C in a helium atmosphere and detecting the amount of oxygen in the released gas.

[0012] <Method for producing ceramic powder> The method for producing a ceramic powder according to this embodiment includes a step of adding a flocculant to a liquid to be treated that contains ceramic powder and filtering the liquid (hereinafter also referred to as the "main filtration step"). The method for producing the ceramic powder is not particularly limited, and any method can be used as long as it produces a ceramic powder through a solid-liquid separation process. By using this filtration process in the solid-liquid separation process, production efficiency can be improved.

[0013] The type of ceramic is not particularly limited, and examples thereof include silicon nitride, boron nitride, aluminum oxide, and aluminum nitride. The particle size of the ceramic powder contained in the liquid to be treated is not particularly limited, and the average particle size of the ceramic powder in the liquid to be treated before the addition of the coagulant may be, for example, 0.1 to 10 μm.

[0014] The content of ceramic powder in the liquid to be treated may be within a range that allows the filtration step to be carried out, and may be, for example, 5 to 50 mass %. The liquid to be treated may contain components other than the ceramic powder. For example, it may contain impurities generated in the process of preparing the liquid to be treated, and may further contain components added in the process of preparing the liquid to be treated.

[0015] The filtration method is not particularly limited. When the material to be treated contains an acidic substance, the material of the member that comes into contact with the liquid to be treated, such as the filter medium, may be acid-resistant. Examples include ceramics, and acid-resistant resins such as polyvinylidene fluoride (PVDF) and polypropylene (PP).

[0016] It is preferable to use a continuous filtration device as the filtration device. The continuous filtration device continuously performs the operations of filtering and concentrating the liquid to be treated, adding a washing liquid after concentration, and concentrating again. An example of a continuous filtration device is a cross-flow filtration device. In the cross-flow method, the liquid to be treated flows parallel to the filtration surface of the filter medium while applying pressure from the inside to the outside of the filtration surface. This prevents solids from accumulating inside the filtration surface, allowing the liquid to pass through the filter medium and concentrate the liquid to be treated.

[0017] Examples of cross-flow type filtration devices include a filter type cross-flow filtration device equipped with a filtration membrane as a filter medium, and a rotary type cross-flow filtration device equipped with a filter cloth as a filter medium. The pore size of the filtration membrane is determined by the D 50 The pore size is preferably 0.01 to 1.00 times, more preferably 0.02 to 0.50 times, and even more preferably 0.03 to 0.30 times, relative to the total pore size. When the membrane pore size is equal to or greater than the lower limit, a sufficient filtration flux is easily obtained, whereas when the membrane pore size is equal to or less than the upper limit, the ceramic ultrafine powder is less likely to pass through the filtration membrane, making it easy to reduce the amount of solids in the waste liquid. The transmembrane pressure difference of the filtration membrane is, for example, preferably 0.10 to 0.50 MPa, more preferably 0.15 to 0.40 MPa, and even more preferably 0.20 to 0.30 MPa. The optimum range for the permeability of the filter cloth varies depending on the particle size of the ceramic powder. For example, if the particle size of the powder is 0.4 μm to 1.0 μm, the permeability of the filter cloth is 0.10 to 0.50 cc / cm. 2 ·s is preferred, 0.15 to 0.45cc / cm 2 ·s is more preferable, 0.20 to 0.40cc / cm 2 When the membrane pore size is equal to or larger than the above lower limit, a sufficient filtration flux is easily obtained, whereas when the membrane pore size is equal to or smaller than the above upper limit, the ceramic ultrafine powder is less likely to pass through the filtration membrane, making it easier to reduce the amount of solids in the waste liquid.

[0018] The flocculant is preferably an inorganic flocculant, and the inorganic flocculant is preferably at least one selected from the group consisting of ammonium ammonium sulfate, ammonium acetate, aluminum sulfate, iron sulfate, and the like.

[0019] The preferred method for adding the flocculant is to add an aqueous solution of the flocculant to the liquid to be treated. If the concentration of the flocculant relative to the mass of the aqueous flocculant solution (unit: mass %, hereinafter also referred to as "addition concentration") is too high, the flocculant will remain in the ceramic powder after drying, and if it is too low, sufficient flocculation effect will not be obtained. Therefore, it is preferable to keep the concentration within a range that does not cause these problems. For example, 0.005 to 0.15 mass % is preferred, 0.01 to 0.14 mass % is more preferred, and 0.05 to 0.13 mass % is even more preferred.

[0020] The pH of the liquid to be treated in the step of adding the flocculant preferably includes at least a part of the pH range in which the dispersibility of the ceramic powder is enhanced. When the dispersibility of the ceramic powder in the liquid to be treated increases, clogging occurs and the filtration flux decreases if the pore size of the filter medium is small, whereas if the pore size of the filter medium is large, the ceramic powder passes through the filter medium, increasing the amount of solids in the waste liquid. By adding a flocculant to the liquid to be treated in a pH range where dispersibility increases, and promoting flocculation, it is possible to obtain an increase in filtration flux, a decrease in the amount of solids in the waste liquid, or both of these effects.

[0021] <Method for producing silicon nitride powder> As a preferred embodiment of the method for producing ceramic powder of the present invention, an example of a method for producing silicon nitride powder will be described. The method for producing silicon nitride powder in this embodiment includes a washing step in which a liquid to be treated containing silicon nitride powder, acidic substances, and impurities is supplied to a continuous filtration device, and a silicon nitride powder-containing composition in which the acidic substances and impurities have been reduced is obtained, and the washing step includes a step of adding a coagulant to the liquid to be treated and filtering it (main filtration step). The silicon nitride powder in the liquid to be treated is preferably silicon nitride powder produced by wet-pulverizing silicon nitride obtained by direct nitriding and then treating it with acid.

[0022] <Silicon nitride powder manufacturing process> In this embodiment, the manufacturing process of the silicon nitride powder is not particularly limited. A preferred embodiment of the manufacturing process of the silicon nitride powder will be described. The process for producing silicon nitride powder of this embodiment includes a nitriding step of metallic silicon and a pulverization step. The pulverization step preferably includes a coarse pulverization step for obtaining a coarse powder and a fine pulverization step for producing a fine powder by wet pulverization. (Nitriding process of metal silicon) In the nitriding step of metal silicon, raw material powder containing metal silicon powder is fired in an atmosphere containing nitrogen to nitride it. The metal silicon powder may be prepared by pulverizing metal silicon lumps or particles. Examples of pulverizing equipment include a hammer mill, a pin mill, a ball mill, a vibration mill, and a jet mill.

[0023] The purity of the metal silicon powder is preferably 98% by mass or more, more preferably 99% by mass or more. The metal silicon powder may contain impurities derived from metal silicon lumps or particles and impurities derived from the grinding device.

[0024] The metallic silicon powder may be used as the raw material powder as it is, or the raw material powder may be prepared by blending metallic silicon powder, fluorite, and other metal powders or metal compound powders (e.g., chromium compounds, nickel compounds). The content of metallic silicon powder per 100 parts by mass of the raw material powder is preferably 92 parts by mass or more, more preferably 95 parts by mass or more, and even more preferably 97 parts by mass or more.

[0025] When the raw material powder contains fluorite, the content of fluorite per 100 parts by mass of metal silicon powder is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.8 parts by mass or more. If the content of fluorite is above the lower limit, nitridation of the metal silicon is likely to proceed. If the content of fluorite per 100 parts by mass of metal silicon powder is above the upper limit, the content of calcium and fluorine elements in the resulting silicon nitride powder can be reduced.

[0026] In the nitriding process for metal silicon, the raw material powder may be fired as is, or the raw material powder may be formed into a compact and then fired. Nitriding is performed by firing the raw material powder or a compact of the raw material powder (hereinafter also referred to as "raw material powder, etc.") in a firing furnace under a nitrogen-containing atmosphere. The firing furnace may be a continuous furnace or a batch furnace. Examples of batch furnaces include electric furnaces and rotary kilns. Examples of continuous furnaces include container-transporting tunnel-type pusher furnaces and roller hearth kilns. When firing the raw material powder as is, a continuous furnace is preferred. A continuous furnace allows the container containing the raw material powder to be continuously heated within the furnace. Since the raw material powder can be fired without being formed, the nitriding reaction proceeds efficiently. This allows silicon nitride to be obtained in a short period of time.

[0027] The resulting silicon nitride has a gelatinization rate of preferably 88% or more, more preferably 89% or more, and even more preferably 90% or more. When the gelatinization rate is equal to or greater than the lower limit, grain growth during sintering can be promoted when a sintered body is produced using the silicon nitride powder. As a result, a sufficiently densified silicon nitride sintered body can be produced. Therefore, a silicon nitride sintered body with even better high-temperature strength and high thermal conductivity can be obtained. The alpha-conversion rate of silicon nitride is substantially determined by the nitriding step, and therefore the preferred ranges of the alpha-conversion rate of silicon nitride in the coarse powder, the silicon nitride powder after pulverization, the silicon nitride powder after acid treatment, and the silicon nitride powder after the washing step, which will be described later, are also the same as those described above.

[0028] (coarse grinding process) In the coarse pulverization step, the molded or powdered silicon nitride obtained in the metal silicon nitriding step is pulverized. The pulverization may be carried out using, for example, a coarse pulverizer, a ball mill, a vibration mill, etc. The coarse pulverization is preferably carried out in a dry manner.

[0029] The coarse powder may contain elements other than silicon, nitrogen, iron, and oxygen. Examples of the other elements include calcium derived from fluorite, halogens, the metal powders, the metal compound powders, and metal elements other than silicon. The total content of the other elements relative to the total mass of the coarse powder is preferably 2.0 mass% or less, more preferably 1.5 mass% or less, and even more preferably 1.0 mass% or less.

[0030] (Fine grinding process) In the fine pulverization step, the coarse powder is fed to a media agitation mill and wet-pulverized. Wet milling refers to milling a mixture of coarse powder and a solvent, i.e., a slurry, in which water can be used as the solvent.

[0031] The media in a media agitation mill may be either metallic or ceramic. From the viewpoint of sufficiently pulverizing the coarse powder, metallic media are preferred. As the metal, iron and steel containing iron as the main component are preferred. Only one type of media may be used, or two or more types may be used in combination.

[0032] The coarse powder and the solvent may be directly fed to the media agitation mill, or a slurry may be prepared in advance and then fed to the media agitation mill.

[0033] The silicon nitride powder after pulverization may contain elements other than silicon, nitrogen, iron, and oxygen. Examples of other elements include calcium derived from fluorite, halogens, the metal powders, the metal compound powders, and metal elements other than silicon. The total content of other elements relative to the total mass of the silicon nitride powder is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and even more preferably 0.5 mass% or less.

[0034] (Silicon nitride powder before acid treatment) D of silicon nitride powder obtained by the fine grinding process 50 is 1 μm or less, preferably 0.9 μm or less, and more preferably 0.8 μm or less. 50 The lower limit of the D of the silicon nitride powder is not particularly limited, but may be, for example, 0.4 μm or more, 0.5 μm or more, or 0.6 μm or more. 50 When the value is equal to or less than the upper limit, it becomes easier to produce a silicon nitride sintered body. In particular, in the cleaning process described below, the smaller the particle size, the more likely it is that clogging of the filtration membrane will occur, and the application of the present invention is effective. 50 It is more preferable that the thickness is 0.6 to 0.8 μm.

[0035] The specific surface area of ​​the silicon nitride powder obtained in the fine grinding process is 30 to 70 m 2 / g is preferred, and 30 to 55m 2 / g is more preferable, and 30 to 45m 2 / g is more preferred.

[0036] <Acid treatment process> The silicon nitride powder obtained through wet milling is subjected to an acid treatment by adding an acid solution to the silicon nitride powder, which dissolves and removes impurities such as metals and silica contained in the silicon nitride powder.

[0037] Examples of acidic substances contained in the acidic solution include inorganic acids such as hydrogen fluoride, hydrogen chloride, nitric acid, and sulfuric acid. Hydrochloric acid (aqueous hydrogen chloride solution) is preferably used to dissolve metals contained in the silicon nitride powder after wet pulverization. Hydrofluoric acid (aqueous hydrogen fluoride solution) is preferably used to dissolve silica contained in the silicon nitride powder after wet pulverization. That is, the acid treatment step is preferably carried out using a mixture of hydrofluoric acid and hydrochloric acid. The acid treatment step may be carried out by stirring an acid treatment slurry containing silicon nitride powder and an acidic solution, or by allowing the mixture to stand still without stirring.

[0038] The acid-treated slurry contains the silicon nitride powder after the acid treatment, acidic substances, and impurities. After the acid treatment, the acid-treated slurry is used as the liquid to be treated, and the acidic substances and impurities are removed in the subsequent washing step.

[0039] The total concentration of acidic substances such as hydrofluoric acid and hydrogen chloride relative to the total mass of the acid-treated slurry is preferably 7.0 to 10.0 mass %, more preferably 7.5 to 9.5 mass %, and even more preferably 8.0 to 9.0 mass %. When the concentration of the acidic substance is equal to or higher than the lower limit, dissolution of impurities such as iron and silica contained in the silicon nitride powder is promoted.When the concentration of the acidic substance is equal to or lower than the upper limit, the acidic substance is easily removed by washing.

[0040] The solid content of the acid-treated slurry used as the liquid to be treated is preferably 10 to 45 mass %, more preferably 15 to 40 mass %, and even more preferably 20 to 35 mass %. The pH of the acid-treated slurry (ie, the liquid to be treated) after completion of the acid treatment is preferably 0 to 2.0, more preferably 0 to 1.5, and even more preferably 0 to 1.0. The D of silicon nitride powder before and after the acid treatment process 50 The D of silicon nitride powder in the acid-treated slurry hardly changes. 50 The preferred size of D of the silicon nitride powder obtained in the fine grinding process is 50 is the same as

[0041] <Cleaning process> In the washing step, the liquid to be treated is fed to a continuous filtration device, concentrated, and then washing liquid is added after concentration, followed by another concentration step, to obtain a silicon nitride powder-containing composition in which the acidic substances and impurities in the liquid to be treated have been reduced. As a continuous filtration device, a cross-flow type filtration device is preferred because it is easy to suppress a decrease in filtration flux. For example, when the average value (overall average) of the filtration flux in the washing step is 50 to 800 L / m 2 hr, preferably 100 to 500 L / m 2 ·hr can be used.

[0042] In the cleaning process of this embodiment, it is preferable to carry out a first process using water as the cleaning liquid, followed by a second process (main filtration process) using an aqueous coagulant solution as the cleaning liquid, and then a third process using an alkaline solution as the cleaning liquid to perform cleaning and neutralization. Examples of alkaline solutions include aqueous ammonia, aqueous sodium hydroxide, and aqueous calcium hydroxide solutions. Among these, aqueous ammonia is preferred because it leaves little residue on the silicon nitride powder after drying. The pH of the alkaline solution is preferably 9 to 12, and more preferably 10 to 11.

[0043] The pH of the liquid to be treated at the start of the first step may be the same as the pH of the slurry at the end of the acid treatment step. The pH of the liquid to be treated at the end of the first step is the same as the pH of the liquid to be treated at the start of the second step, and is preferably in the same range as the pH of the liquid to be treated at the start of the above-mentioned main filtration step. The pH of the liquid to be treated at the end of the second step is the same as the pH of the liquid to be treated at the start of the third step, and is preferably in the same range as the pH of the liquid to be treated at the end of the main filtration step described above. The pH (end point) of the treatment liquid at the end of the third step is preferably 7 to 10, more preferably 8 to 9. If the pH exceeds the upper limit, alkaline components tend to remain in the silicon nitride powder after drying.

[0044] After the second step, instead of the third step using an alkaline solution, it is possible to supply only water and wash until the pH (end point) is reached, but providing the third step is preferable in terms of shortening the treatment time and reducing the amount of waste liquid.

[0045] The solids concentration of the silicon nitride powder-containing composition after the washing step is preferably 15 to 40 mass %, more preferably 20 to 35 mass %. A concentration equal to or higher than the lower limit is preferred in terms of reducing the thermal energy required for drying, while a concentration equal to or lower than the upper limit is preferred in terms of ease of transport to the next step.

[0046] <Drying process> The silicon nitride powder-containing composition obtained in the washing step can be dried to obtain silicon nitride powder. The drying temperature is preferably 100 to 140°C, more preferably 110 to 130°C. The D of silicon nitride powder before and after the cleaning process 50 The D of silicon nitride powder after the washing process remains almost unchanged. 50 The preferred range of is the D of the silicon nitride powder before the acid treatment. 50 and the D of the silicon nitride powder obtained in the fine grinding step is the same as the preferred range of 50 The preferred range is the same as that of

[0047] The specific surface area of ​​the silicon nitride powder after the washing process is 5 to 15 m 2 / g is preferred, and 6 to 14m 2 / g is more preferable, and 7 to 13m 2 / g is more preferred.

[0048] The iron content relative to the total mass of the silicon nitride powder after the washing step is preferably 300 mass ppm or less, more preferably 200 mass ppm or less, and even more preferably 150 mass ppm or less. The lower limit of the iron content relative to the total mass of the silicon nitride powder after the washing step is not particularly limited, but may be, for example, 10 mass ppm or more, 30 mass ppm or more, or 50 mass ppm or more. When the iron content relative to the total mass of the silicon nitride powder after the washing step is equal to or less than the upper limit, the thermal conductivity and mechanical properties of the silicon nitride sintered body produced using the silicon nitride powder tend to be improved.

[0049] The oxygen content relative to the total mass of the silicon nitride powder after the washing step is preferably 0.5 to 1.5 mass %, more preferably 0.6 to 1.4 mass %, and even more preferably 0.7 to 1.3 mass %. The oxygen content of silicon nitride powder originates from oxides present in the primary particles of silicon nitride. The oxygen content of silicon nitride powder can be controlled by adjusting the conditions of the steps in the silicon nitride powder production process where oxidation occurs. For example, it can be controlled by adjusting the component composition of the firing atmosphere in the firing step and the concentration of the acidic substance used in the acid treatment step.

[0050] The silicon nitride powder after the washing step may contain elements other than silicon, nitrogen, and oxygen. Examples of other elements include calcium and halogen elements derived from the fluorite, and iron and metal elements other than silicon derived from the metal powder, the metal compound powder, and the milling apparatus. The total content of other elements relative to the total mass of the silicon nitride powder after the washing step is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and even more preferably 0.5 mass% or less.

[0051] <Application> The silicon nitride fine powder of this embodiment has a small average particle size and therefore has excellent sinterability, and can therefore be used as a sintering material for silicon nitride sintered bodies.

[0052] The sintering raw material may contain an oxide-based sintering aid in addition to the silicon nitride fine powder. Examples of the oxide-based sintering aid include yttrium oxide, magnesia, and alumina. The content of the oxide-based sintering aid in the sintering raw material is, for example, 3 to 10 mass %.

[0053] The silicon nitride sintered body is produced by a process of producing a molded body and a process of firing. In the process of producing a molded body, for example, a molding pressure of 3.0 to 30 MPa is applied to obtain a molded body. The molded body may be produced by uniaxial pressing or by CIP molding. Furthermore, the molded body may be fired while being molded by hot pressing. The molded body may be fired in an inert gas atmosphere such as nitrogen or argon. The pressure during firing may be 0.7 to 1 MPa. The firing temperature is preferably 1860 to 2100°C, more preferably 1880 to 2000°C. The firing time at the firing temperature is preferably 6 to 20 hours, more preferably 8 to 16 hours. The rate of temperature rise to the firing temperature is preferably, for example, 1.0 to 10.0°C / hour.

[0054] The silicon nitride fine powder of this embodiment has a small average particle size and a low content of impurities such as iron, and therefore the resulting silicon nitride sintered body has excellent thermal conductivity and mechanical properties.

[0055] <Boron nitride powder manufacturing method> As a preferred embodiment of the method for producing ceramic powder of the present invention, an example of a method for producing boron nitride powder will be described. The method for producing boron nitride powder in this embodiment includes a washing step in which a liquid to be treated containing boron nitride powder, acidic substances, and impurities is supplied to a continuous filtration device, and a boron nitride powder-containing composition in which the acidic substances and impurities have been reduced is obtained, and the washing step includes a step of adding a coagulant to the liquid to be treated and filtering the liquid (main filtration step).

[0056] The boron nitride powder in the solution to be treated is preferably boron nitride powder produced by crushing boron nitride obtained by a synthesis method involving the reaction of boron oxide with an organic nitrogen compound, followed by acid treatment. Known methods can be used to synthesize boron nitride. For example, the method for producing hexagonal boron nitride described in Japanese Patent No. 7002196 can be used.

[0057] (Boron nitride powder before acid treatment) D of boron nitride powder obtained in the crushing process 50The diameter of the boron nitride powder is preferably 3 to 20 μm, and more preferably 5 to 15 μm. 50 If the content is less than the lower limit, the spreadability and hiding power of the cosmetic raw material will be insufficient when used as a cosmetic raw material, whereas if the content exceeds the upper limit, the coating spreadability and hiding power will not be a problem, but the glitter in appearance will be too strong, making the cosmetic raw material undesirable. The boron nitride powder obtained in the crushing step may contain elements other than boron and nitrogen. Examples of such elements include oxygen derived from the water-soluble boron compound and alkali metal elements derived from the sintering aid. The oxygen content derived from such elements relative to the total mass of the boron nitride powder is preferably 8% by mass or less, more preferably 6% by mass or less, and even more preferably 4% by mass or less.

[0058] <Acid treatment process> The boron nitride powder obtained in the crushing step is subjected to an acid treatment by immersing it in an acidic solution. By immersing the boron nitride powder in an acidic solution, impurities contained in the boron nitride powder, such as water-soluble boron compounds and alkali metal compounds, can be dissolved and removed.

[0059] Examples of the acidic substance contained in the acidic solution include inorganic acids such as nitric acid, hydrochloric acid, etc. To dissolve an alkali metal compound, it is preferable to use nitric acid.

[0060] The acid-treated slurry, which is obtained by immersing boron nitride powder in an acidic solution, contains the boron nitride powder after the acid treatment, acidic substances, and impurities. After the acid treatment, the acid-treated slurry is used as the treated liquid, and the acidic substances and impurities are removed in the subsequent washing step.

[0061] The total concentration of acidic substances such as nitric acid relative to the total mass of the acid-treated slurry is preferably 2 to 9 mass %, more preferably 3 to 8 mass %, and even more preferably 4 to 7 mass %. When the concentration of the acidic substance is equal to or higher than the lower limit, dissolution of impurities contained in the boron nitride powder is promoted, whereas when the concentration of the acidic substance is equal to or lower than the upper limit, removal of the acidic substance by washing is facilitated.

[0062] The solid content concentration of the acid-treated slurry is preferably from 5 to 40% by mass, more preferably from 7 to 30% by mass, and even more preferably from 9 to 20% by mass. The pH of the acid-treated slurry (ie, the liquid to be treated) after completion of the acid treatment is preferably 1.0 to 3.0, more preferably 1.3 to 2.5, and even more preferably 1.5 to 2.0. The D of boron nitride powder before and after the acid treatment process 50 The D of boron nitride powder in the acid-treated slurry hardly changes. 50 The preferred size of D of the boron nitride powder obtained in the crushing step is 50 is the same as

[0063] <Cleaning process> In the washing step, the liquid to be treated is fed to a continuous filtration device, concentrated, and then washing liquid is added to the concentrated liquid, followed by another concentration. This washing operation is carried out continuously to obtain a boron nitride powder-containing composition in which the acidic substances and impurities in the liquid to be treated have been reduced. As a continuous filtration device, a cross-flow type filtration device is preferred because it is easy to suppress a decrease in filtration flux. For example, when the average value (overall average) of the filtration flux in the washing step is 250 to 100 L / m 2 hr, preferably 230 to 180 L / m 2 ·hr can be used.

[0064] When using filter cloth as the filter material, the air permeability of the filter cloth should be 45 to 75 cm 3 / cm 2 ·s is preferred, 50-70cm 3 / cm 2 · s is more preferable, 55-65cm 3 / cm 2 When the air permeability is equal to or greater than the above lower limit, a sufficient filtration flux is easily obtained, and when the air permeability is equal to or less than the above upper limit, the amount of solids in the waste liquid is easily reduced.

[0065] The washing step includes a main filtration step using an aqueous coagulant solution as a washing solution. Furthermore, a step using water as a washing solution (hereinafter also referred to as a "pre-filtration step") may be included before the main filtration step. After the main filtration step, a neutralization step (hereinafter also referred to as a "post-filtration step") may be included using an alkaline solution as a washing solution. The pH of the liquid to be treated at the start of the washing step may be the same as the pH of the acid-treated slurry at the end of the acid treatment step. The pH of the liquid to be treated at the end of the washing step (end point) is preferably 6 to 8, more preferably 6.5 to 7.5. If the pH exceeds the upper limit, alkaline components are likely to remain in the boron nitride powder after drying, so it is preferable that the pH be equal to or less than the upper limit.

[0066] When a pre-filtration step is performed, the pH of the liquid to be treated at the end of the pre-filtration step is the same as the pH of the liquid to be treated at the start of the main filtration step, and is preferably 2.0 to 4.0, more preferably 2.5 to 3.5. If the pH is less than the lower limit, oxygen elements derived from the water-soluble boron compound and alkali metal elements derived from the sintering aid tend to remain in the boron nitride powder, so the pH is preferably equal to or greater than the lower limit.

[0067] If the pH at the end of this filtration step is lower (smaller) than the pH (end point), it is preferable to carry out a post-filtration step to adjust the pH after thoroughly removing impurities from the treated liquid in this filtration step.

[0068] The solids concentration of the boron nitride powder-containing composition after the washing step is preferably 6 to 40 mass %, more preferably 7 to 20 mass %, and even more preferably 8 to 10 mass %. A concentration equal to or greater than the lower limit is preferred in terms of reducing the thermal energy required for drying, while a concentration equal to or less than the upper limit is preferred in terms of ease of transport to the next step.

[0069] <Drying process> The boron nitride powder-containing composition obtained in the washing step can be dried to obtain the boron nitride powder after acid treatment. The drying temperature is preferably 100 to 140°C, more preferably 110 to 130°C. The washing process reduces the D of the boron nitride powder. 50The D of the boron nitride powder after acid treatment is 50 The thickness is preferably 8.0 to 9.0 μm, more preferably 7.5 to 8.5 μm.

[0070] The specific surface area of ​​the boron nitride powder after acid treatment is 1.5 to 3.0 m 2 / g is preferred, and 1.8 to 2.7m 2 / g is more preferable, and 2.0 to 2.5m 2 / g is more preferred.

[0071] The oxygen content relative to the total mass of the boron nitride powder after the acid treatment is preferably 0.8 mass % or less, more preferably 0.7 mass % or less, and even more preferably 0.6 mass % or less.

[0072] The boron nitride powder after the acid treatment may contain elements other than boron, nitrogen, and oxygen. The content of the other elements relative to the total mass of the boron nitride powder after the acid treatment is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and even more preferably 0.5 mass% or less.

[0073] When using boron nitride powder as a cosmetic ingredient, it is preferable that the amount of eluted boron (the amount of boron dissolved from boron nitride powder in water at 50°C in 1 hour) measured in accordance with the Standards for Quasi-drug Ingredients 2006 is low in order to prevent skin irritation. The amount of eluted boron from the boron nitride powder after acid treatment is preferably 20 mg / L or less, more preferably 15 mg / L or less, and even more preferably 10 mg / L or less, relative to the mass of the boron nitride powder. When the amount of eluted boron is below the upper limit, skin irritation can be sufficiently prevented.

[0074] <Application> The boron nitride powder of this embodiment can be suitably used as a raw material for cosmetics, a solid lubricant, a mold release agent, a filler for resins and rubbers, a raw material for heat-resistant insulating sintered bodies, and the like. Examples of cosmetics include foundations (powder foundations, liquid foundations, cream foundations), face powders, point makeup, eye shadows, eyeliners, nail polishes, lipsticks, blushers, mascaras, and the like. [Example]

[0075] The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

[0076] (D 50 ) D 50 was determined by measuring the particle size distribution by laser diffraction / scattering method. The measurement was carried out in accordance with the method described in JIS R 1629:1997 "Method for measuring particle size distribution of fine ceramic raw materials by laser diffraction / scattering method." D of ceramic powder after washing process 50 was measured by the method described above. In the measurement, 60 mg of the target powder was weighed into a 500 mL container. To this, 2 mL of a 20 mass % aqueous solution of sodium hexametaphosphate as a dispersant and 200 g of water were added to prepare a dispersion. This container was set in an ultrasonic disperser (manufactured by Sharp Corporation) so that the entire portion containing the dispersion was immersed, and ultrasonic dispersion was performed for 1 minute. The particle size distribution measurement described above was performed using the sample after ultrasonic dispersion. D of ceramic powder contained in the liquid to be treated 50 is the D of the ceramic powder after the above-mentioned washing process after drying and crushing the slurry (slurry after wet milling) before adding the acid solution. 50 was measured by the same method.

[0077] (specific surface area) The BET specific surface area of ​​the ceramic powder was measured by the single-point BET method using nitrogen gas in accordance with JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method." The wet-milled slurry was thoroughly dried in a tray dryer, then crushed in a mortar and measured using the above method. The powder-containing composition after acid treatment and washing was dried in a spray dryer and then measured using the above method.

[0078] (Chlorine, fluorine, oxygen and iron content) The chlorine content and fluorine content of the silicon nitride powder were measured by the method described above. Specifically, the silicon nitride powder was heated using an automatic sample combustion apparatus (manufactured by Mitsubishi Chemical Corporation, apparatus name: AQF-2100H type), and the generated gas was dissolved in water. Cl ions and F ions in the water were measured using an ion chromatograph (manufactured by Thermo Fisher Scientific, apparatus name: ICS-2100). Based on the obtained measured values, the chlorine (Cl) content and fluorine (F) content of the silicon nitride powder were calculated. The oxygen content of the silicon nitride powder was determined as the total oxygen content using the method described above. Specifically, using an oxygen / nitrogen analyzer (Horiba, Ltd., model EMGA-920), the silicon nitride powder was heated in a helium atmosphere from 20°C to 2000°C at a heating rate of 8°C / s, and the amount of oxygen released was quantified to determine the oxygen content (mass%) of the entire silicon nitride powder. The iron content of the silicon nitride powder was measured using an X-ray fluorescence analyzer (manufactured by Rigaku Corporation, device name: Primas II). The iron content in the liquid to be treated was measured by drying and crushing the silicon nitride slurry (silicon nitride slurry after wet pulverization) before the addition of the acidic solution, using the same method as for the iron content in the silicon nitride powder described above.

[0079] (amount of dissolved boron) For the boron powder to be measured, the amount of boron eluted into water immediately after production was extracted using a method basically based on the "External Hara Standards 2006" and measured using an ICP atomic emission spectrometer. Specifically, 2.5 g of boron powder immediately after production was placed in a fluororesin beaker, 10 mL of ethanol was added, and the mixture was thoroughly stirred. Then, 40 mL of water was added and the mixture was thoroughly stirred. A fluororesin watch glass was placed on top, and the mixture was heated at 50°C for 1 hour. After cooling, the contents of the beaker were filtered, and the filtrate was combined with a small amount of water to wash off the residue. The recovered liquid was then filtered through a 0.22 μm membrane filter and transferred to a quartz beaker. 2 mL of sulfuric acid (47.5% by mass) was added to the quartz beaker. The mixture was then boiled on a hot plate for 10 minutes. After cooling, the resulting solution was poured into a polyethylene measuring flask, the beaker was rinsed with a small amount of water, and the rinse water was transferred to the polyethylene measuring flask. Water was added to the polyethylene measuring flask to make a final volume of exactly 100 mL. This was used as the sample solution. The boron content of the sample solution was measured using an ICP atomic emission spectrometer (Shimadzu Corporation, ICPE-9000).

[0080] (gelatinization rate) The alpha phase ratio was measured by the method described above. XRD patterns were obtained by powder X-ray diffraction measurement. Specifically, an XRD pattern was obtained using a powder X-ray diffractometer (Rigaku Corporation, device name: Ultima IV) by filling a dedicated substrate with the target powder and measuring it using a Cu-Kα source under the conditions of a diffraction angle 2θ = 10 to 80°, a sampling width of 0.02°, and a scan speed of 10° / min. Next, using integrated powder X-ray analysis software "Rietveld method software" (MDI, integrated powder X-ray software Jade+9), the alpha phase ratio was calculated from the diffraction peak intensity of the (102) plane of the alpha phase of silicon nitride, the diffraction peak intensity of the (210) plane of the alpha phase of silicon nitride, the diffraction peak intensity of the (102) plane of the β phase of silicon nitride, and the diffraction peak intensity of the (210) plane of the β phase of silicon nitride in the XRD pattern using the above-mentioned formula 1.

[0081] (solids) The solids concentration (unit: mass %) of the liquid to be treated containing silicon nitride powder was calculated using the formula: (input weight of silicon nitride powder subjected to wet pulverization) ÷ (weight of silicon nitride slurry obtained by wet pulverization + weight of the mixed liquid of hydrofluoric acid and hydrochloric acid added to the silicon nitride slurry) × 100. The solids concentration (unit: mass%) of the liquid to be treated containing boron nitride powder was calculated using the formula (weight of boron nitride powder after crushing) ÷ (total weight of boron nitride powder after crushing and dilute nitric acid) × 100.

[0082] The amount of solids in the wastewater (unit: mg / L) was measured using a method that conforms to the method for measuring the concentration of suspended solids specified in JIS K 0102:2019 "Testing Methods for Industrial Wastewater." Specifically, the wastewater to be measured was filtered through filter paper with a pore size of 1 μm and a diameter of 25 to 50 mm, and the solids amount (unit: mg / L) was calculated by dividing the dry weight of the material remaining on the filter paper by the volume of the filtered liquid.

[0083] <Production Example 1: Preparation of treatment liquid containing silicon nitride powder> The liquid to be treated used in the examples in which the ceramic powder was silicon nitride powder was prepared by producing silicon nitride by a direct nitriding method, wet-pulverizing the silicon nitride powder to obtain a slurry containing the silicon nitride powder, and then treating the slurry with an acid. Specifically, a molded body (bulk density: 1.4 g / cm) was produced using metal silicon powder. 3 The resulting compact was placed in an electric furnace and fired at 1400°C for 60 hours to obtain a fired body containing silicon nitride. A mixed gas of nitrogen and hydrogen (a mixed gas of N2 and H2 in a volume ratio of 80:20 under standard conditions) was supplied as the firing atmosphere. The resulting fired body was crushed using a dry jaw crusher and then crushed using a dry ball mill to obtain a coarse powder containing silicon nitride. The resulting coarse powder was fed to a media agitation mill and wet-pulverized using water as a solvent to obtain a silicon nitride slurry containing silicon nitride powder. The media of the media agitation mill were made of an iron-containing alloy. The resulting silicon nitride slurry contains at least iron as an impurity in addition to the silicon nitride powder.

[0084] The silicon nitride slurry obtained by wet pulverization was placed in an acid treatment tank, and a mixed solution of hydrofluoric acid and hydrochloric acid was added. The mixture was stirred at room temperature for 6 hours to carry out acid treatment, thereby obtaining a solution to be treated. The solution to be treated contains the silicon nitride powder after the acid treatment, as well as the acidic substance used in the acid treatment and at least iron as an impurity. The solid concentration of the liquid to be treated used in each example, the content of hydrochloric acid, the content of hydrofluoric acid, the iron content (Fe content), and the D of silicon nitride powder in the liquid to be treated 50 The results are shown in Table 1. The hydrochloric acid content and hydrofluoric acid content of the liquid to be treated were calculated from the content of acidic substances in the acid solution used in the acid treatment step.

[0085] Example 1 In this example, a cross-flow filtration device with a ceramic membrane filter medium was used to separate the liquid to be treated into solid and liquid, and also to carry out a washing step. The ceramic membrane used was a tubular membrane made of alumina with a pore size of 0.1 μm. Specifically, 1 m 3 The liquid to be treated was stored in the storage tank, and the liquid in the storage tank was continuously supplied to the filter medium. The permeated liquid that had passed through the filter medium was removed as waste liquid, and the concentrated liquid that had not passed through the filter medium was returned to the storage tank, thereby circulating the liquid. At the same time as the start of supplying the liquid to be treated to the filter media, water was supplied so that the liquid volume in the storage tank remained constant. That is, water was added to the concentrated liquid returned to the storage tank, and the operation of concentrating the liquid again using the filter media was continuously carried out. In this way, washing with water was carried out until the pH of the liquid supplied to the filter media reached 0 to 1 (first step). The treatment time, total amount of water supplied, and average filtration flux required for the first step are shown in Table 2 (the same applies below). Next, instead of water, a 0.05% by mass aqueous solution of ammonium sulfate was supplied to the storage tank as a flocculant solution so that the amount of liquid in the storage tank remained constant. In this way, washing was carried out while adding the flocculant until the pH of the liquid supplied to the filter medium reached 3 (second step). The treatment time required for the second step, the total amount of the flocculant solution supplied, and the average filtration flux are shown in Table 2 (the same applies below). Next, instead of the coagulant solution, an ammonia solution with a concentration of 0.08% by mass was added to the storage tank until the liquid volume (1 m 3) was supplied to the storage tank so that the pH was constant. In this way, neutralization and washing using aqueous ammonia solution were carried out until the pH of the liquid supplied to the filter medium reached 9 (third step). The treatment time required for the third step, the total amount of aqueous ammonia solution supplied, and the average filtration flux are shown in Table 2 (the same applies below). When the pH of the liquid supplied to the filter medium reached 9, the supply of liquid to the storage tank was stopped, and a washed silicon nitride powder-containing composition was obtained. The solids concentration of the silicon nitride powder-containing composition was the same as that of the liquid to be treated. The silicon nitride powder-containing composition obtained in the washing step was dried at 300° C. to obtain silicon nitride powder after the washing step.

[0086] The obtained silicon nitride powder D 50 The specific surface area, iron content (Fe content), oxygen content, and alpha conversion rate are shown in Table 1 (the same applies below). 50 is the D of silicon nitride powder in the liquid to be treated 50 It was confirmed that iron, an impurity in the treated liquid, was highly removed. The total amount of waste liquid, the amount of solids in the waste liquid, the overall average filtration flux, and the processing capacity in all processes are shown in Table 1 (the same applies below). The amount of solids in the wastewater was measured by the method described above. The total amount of wastewater was the sum of the total amount of water supplied in the first step, the total amount of aqueous coagulant solution supplied in the second step, and the total amount of aqueous ammonia solution supplied in the third step. The treatment capacity was 1 m 3 The total treatment time is the sum of the treatment times for the first, second, and third steps. The overall average filtration flux was calculated by dividing the total wastewater volume by the filter media area and the total treatment time.

[0087] <Example 2> In the second step of Example 1, the aqueous flocculant solution was changed to an aqueous ammonium sulfate solution with a concentration of 0.005% by mass. Other than that, the procedure was the same as in Example 1. In this example, the overall average filtration flux is 324 L / m 2 hr, and 429 L / m in Example 1 2The reason for this is thought to be that the amount of flocculant added in Example 1 was greater than that in Example 2, resulting in a greater flocculation effect.

[0088] Example 3 In Example 1, the pore size of the ceramic membrane was changed to 50 nm.Other than that, the procedure was the same as in Example 1. In this example, the overall average filtration flux is 353 L / m 2 hr, and 429 L / m in Example 1 2 The reason for this is thought to be that the membrane pore size in Example 1 was larger than that in Example 3, so the filtrate was more easily discharged.

[0089] Example 4 In Example 1, the flocculant aqueous solution was changed to an ammonium acetate aqueous solution with a concentration of 0.08% by mass, but the rest was the same as in Example 1.

[0090] <Example 5> In Example 1, the aqueous coagulant solution was changed to an aqueous aluminum sulfate solution with a concentration of 0.13 mass %, but the rest was the same as in Example 1.

[0091] <Reference Example 1: No flocculant> This example is an example in which the second step in Example 1 was not carried out. In the first step of this example, washing with water was carried out until the pH of the liquid supplied to the filter medium reached 3. The rest of the procedure was the same as in the first step of Example 1. After the first step, the second step was not performed, and an aqueous ammonia solution with a concentration of 0.08% by mass was supplied to the storage tank so that the amount of liquid in the storage tank remained constant. In this way, neutralization and washing using the aqueous ammonia solution were carried out until the pH of the liquid supplied to the filter medium reached 9 (third step). When the pH of the liquid supplied to the filter medium reached 9, a washed silicon nitride powder-containing composition was obtained in the same manner as in Example 1, and further dried to obtain washed silicon nitride powder.

[0092] Example 6 In this example, a cross-flow filtration device in which the filter material is a resin membrane was used to separate the liquid to be treated into solid and liquid, and also to carry out a washing step. The resin membrane used was a hollow fiber membrane made of polyvinylidene fluoride (PVDF) with a pore size of 0.1 μm. A washed silicon nitride powder-containing composition was obtained by carrying out the first, second, and third steps in the same manner as in Example 1. The obtained silicon nitride powder-containing composition was dried in the same manner as in Example 1 to obtain a washed silicon nitride powder.

[0093] <Reference Example 2: No flocculant> This example is an example in which the second step in Example 6 was not carried out. In the first step of this example, washing with water was carried out until the pH of the liquid supplied to the filter medium reached 3. The rest of the procedure was the same as in the first step of Example 6. After the first step, the second step was not performed, and an aqueous ammonia solution with a concentration of 0.08% by mass was supplied to the storage tank so that the amount of liquid in the storage tank remained constant. In this way, neutralization and washing using the aqueous ammonia solution were carried out until the pH of the liquid supplied to the filter medium reached 9 (third step). When the pH of the liquid supplied to the filter medium reached 9, a washed silicon nitride powder-containing composition was obtained in the same manner as in Example 1, and further dried to obtain washed silicon nitride powder.

[0094] Example 7 In this example, a rotary cross-flow filtration device was used to separate the liquid to be treated into solid and liquid, and also to carry out a washing step. The filter media has a permeability of 0.3 cm 3 / cm 2 ·s, filter area (effective filtration area) 1.0m 2 A polypropylene filter cloth (double woven) was used. A washed silicon nitride powder-containing composition was obtained by carrying out the first, second, and third steps in the same manner as in Example 1. The obtained silicon nitride powder-containing composition was dried in the same manner as in Example 1 to obtain a washed silicon nitride powder.

[0095] <Reference Example 3: No flocculant> This example is an example in which the second step in Example 7 was not carried out. In the first step of this example, washing with water was carried out until the pH of the liquid supplied to the filter medium reached 3. The rest of the procedure was the same as in the first step of Example 7. After the first step, the second step was not performed, and an aqueous ammonia solution with a concentration of 0.08% by mass was supplied to the storage tank so that the amount of liquid in the storage tank remained constant. In this way, neutralization and washing using the aqueous ammonia solution were carried out until the pH of the liquid supplied to the filter medium reached 9 (third step). When the pH of the liquid supplied to the filter medium reached 9, a washed silicon nitride powder-containing composition was obtained in the same manner as in Example 1, and further dried to obtain washed silicon nitride powder.

[0096] <Production Example 2: Preparation of treatment liquid containing boron nitride powder> The liquid to be treated used in the example where the ceramic powder was boron nitride powder was prepared by reacting boron oxide with an organic nitrogen compound to produce boron nitride, which was then crushed and treated with acid. Specifically, 100 g of boric acid powder (purity of 99.8% by mass or more) and 90 g of melamine powder (purity of 99.0% by mass or more) were mixed for 10 minutes in an alumina mortar. The resulting mixture was humidified at 80°C and a relative humidity of 95% for 1 hour, and then dried at 120°C for 1 hour. This was placed in a hexagonal boron nitride container (volume approximately 500 cm). 3 Nitrogen gas was supplied into the furnace, and the temperature was raised from room temperature to 1000°C. After holding at 1000°C for 2 hours, heating was stopped and the material was naturally cooled to 100°C. The electric furnace was then opened, and a fired product was obtained. To 100 g of the obtained fired product, 3 g of sodium carbonate (purity 99.5 mass% or more) was added and mixed for 10 minutes in an alumina mortar. The product was then placed in the electric furnace, nitrogen gas was supplied into the furnace, the temperature was raised to 1700°C, and the product was held at 1700°C for 4 hours. The heating was then stopped, the product was naturally cooled to 100°C, and the electric furnace was opened to obtain crude hexagonal boron nitride (before crushing). This was crushed in an alumina mortar for 3 minutes to produce crude hexagonal boron nitride powder. In order to remove impurities from the obtained powder, the powder was added to dilute nitric acid with a concentration of 5% by mass to form an acid treatment slurry, which was stirred at room temperature for 60 minutes to perform an acid treatment, thereby obtaining a liquid to be treated. The solid concentration of the liquid to be treated, the nitric acid content, the oxygen content, and the D of the boron nitride powder in the liquid to be treated 50 The results are shown in Table 1. The nitric acid content of the liquid to be treated was calculated from the nitric acid content in the acidic solution used in the acid treatment step.

[0097] Example 8 In this example, a rotary cross-flow filtration device was used to separate the liquid to be treated into solid and liquid, and a washing step was also carried out. Only an aqueous solution of a flocculant was used as the washing liquid. The filter material has a permeability of 60 cm 3 / cm 2 A polypropylene filter cloth (double weave) of s was used. Specifically, 0.5 m of water is added to the storage tank of the filtration equipment. 3 The liquid to be treated was stored in the storage tank, and the liquid in the storage tank was continuously supplied to the filter medium. The permeated liquid that had passed through the filter medium was removed as waste liquid, and the concentrated liquid that had not passed through the filter medium was returned to the storage tank, thereby circulating the liquid. At the same time as the start of supplying the liquid to be treated to the filter medium, an aqueous solution of flocculant was supplied so that the liquid volume in the storage tank remained constant. The aqueous solution of flocculant used was an aqueous solution of ammonium sulfate with a concentration of 0.05% by mass. That is, the concentrated liquid returned to the storage tank was added with an aqueous coagulant solution, and the process of concentrating the liquid again using the filter media was repeated. In this way, washing using the aqueous coagulant solution was carried out until the pH of the liquid supplied to the filter media reached 1.5 to 7 (main filtration process). The processing time required for the main filtration process (all processes), the total amount of aqueous coagulant solution supplied, and the average filtration flux are shown in Table 2. When the pH of the liquid supplied to the filter medium reached 7, the supply of liquid to the storage tank was stopped, and a washed boron nitride powder-containing composition was obtained. The solids concentration of the boron nitride powder-containing composition was the same as that of the liquid to be treated. The obtained boron nitride powder-containing composition was dried at 300°C to obtain washed boron nitride powder. The obtained boron nitride powder D 50The specific surface area, oxygen content, and amount of dissolved boron are shown in Table 1. 50 is the D of boron nitride powder in the treated liquid 50 It was smaller. It was confirmed that oxygen elements derived from water-soluble boron compounds, which are impurities in the treated liquid, were removed to a high degree. The total amount of waste liquid, the amount of solids in the waste liquid, the overall average filtration flux, and the treatment capacity in all processes are shown in Table 1. The amount of solids in the waste liquid was measured using the method described above. The total amount of waste liquid was the total amount of coagulant aqueous solution supplied in this filtration process. The treatment capacity was 0.5 m 3 The overall average filtration flux was calculated by dividing the total amount of waste liquid by the filter media area and the total treatment time.

[0098] <Reference Example 4: No flocculant> This example is an example in which water was used instead of the aqueous coagulant solution in Example 8. That is, at the same time as the start of supplying the liquid to be treated to the filter medium, water was supplied so that the amount of liquid in the storage tank remained constant, and washing was carried out using only water until the pH of the liquid supplied to the filter medium reached 7 (first step). The treatment time required for the first step (all steps), the total amount of coagulant aqueous solution supplied, and the average filtration flux are shown in Table 2. When the pH of the liquid supplied to the filter medium reached 7, the supply of liquid to the storage tank was stopped, and a washed boron nitride powder-containing composition was obtained. The solids concentration of the boron nitride powder-containing composition was the same as that of the liquid to be treated. The obtained boron nitride powder-containing composition was dried at 300°C to obtain washed boron nitride powder. The obtained boron nitride powder D 50 The specific surface area, oxygen content, and amount of eluted boron are shown in Table 1. The total amount of waste liquid, the amount of solids in the waste liquid, the overall average of the filtration flux, and the treatment capacity in all steps are shown in Table 1. The amount of solids in the waste liquid was measured by the method described above. The total amount of waste liquid was the total amount of water supplied in the first step. The treatment capacity was 0.5 m 3 The overall average filtration flux was calculated by dividing the total amount of waste liquid by the filter media area and the total treatment time.

[0099] [Table 1]

[0100] [Table 2]

[0101] In all of the filtration devices of Examples 1 to 8 and Reference Examples 1 to 4, no corrosion was observed in the members that came into contact with the liquid to be treated, and the acid resistance was good. As shown in the table, Examples 1 to 5 had increased filtration flux and reduced total processing time compared to Reference Example 1. This improves the efficiency of silicon nitride powder production. Furthermore, compared with Reference Example 2, Example 6 had an increased filtration flux and a reduced total processing time, which improves the efficiency of silicon nitride powder production. Furthermore, the amount of solids in the waste liquid was reduced in Example 7 compared to Reference Example 3. This reduces the work and costs involved in treating the waste liquid, improving the efficiency of silicon nitride powder production. Furthermore, compared with Reference Example 4, Example 8 had an increased filtration flux and a reduced total processing time, which improves the efficiency of producing boron nitride powder.

Claims

1. A method for producing ceramic powder, comprising the steps of adding a flocculant to a liquid to be treated that contains ceramic powder and filtering the liquid.

2. 2. The method for producing a ceramic powder according to claim 1, wherein the flocculant is an inorganic flocculant.

3. 2. The method for adding the flocculant is a method for adding an aqueous flocculant solution containing the flocculant, and the concentration of the flocculant in the aqueous flocculant solution is 0.005 to 0.15 mass%.

3. The method for producing a ceramic powder according to claim 1.

4. 2. The method for producing a ceramic powder according to claim 1, wherein the ceramic powder is silicon nitride powder, and the average particle size of the silicon nitride powder in the liquid to be treated before adding the flocculant is 0.6 to 0.8 μm.

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