Method for producing silicon nitride powder

Abstract

To improve the efficiency of producing a silicon nitride powder by improving the cleaning step after acid treatment.SOLUTION: There is provided a method for producing a silicon nitride powder which includes a cleaning step of supplying a liquid to be treated comprising a silicon nitride powder, an acidic substance and impurities to a solid-liquid separator (excluding decantation), followed by obtaining a silicon nitride powder-containing composition with reduced acidic substance and impurities, wherein the solid-liquid separator is a continuous solid-liquid separator, which performs operations of concentrating the liquid to be treated, adding a cleaning liquid after concentration and concentrating again continuously.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing silicon nitride 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 step is required in which the silicon nitride fine powder is immersed in an acidic solution to dissolve the impurities, followed by the removal of 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 impurities. [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] The waste liquid generated by the decantation process contains dissolved impurities and the acidic substances used in the acid treatment, which requires work and costs for waste liquid treatment. Furthermore, if the waste liquid contains ultrafine powder that does not easily settle by gravity, production losses will occur. Therefore, reducing these factors will improve the efficiency of silicon nitride powder production. The present invention aims to improve the efficiency of silicon nitride powder production by improving the cleaning process after acid treatment. [Means for solving the problem]

[0007] The present invention has the following aspects. [1] A method for producing silicon nitride powder, comprising a washing step of supplying a liquid to be treated containing silicon nitride powder, acidic substances, and impurities to a solid-liquid separator (excluding decantation) to obtain a silicon nitride powder-containing composition in which the acidic substances and impurities have been reduced, wherein the solid-liquid separator is a continuous solid-liquid separator, and the liquid to be treated is concentrated, followed by adding a washing liquid after concentration, and then concentrating again, the steps being carried out continuously. [2] The method for producing silicon nitride powder according to [1], wherein the continuous solid-liquid separator is a cross-flow filtration device or a centrifuge. [3] After the washing step, the silicon nitride powder has an alpha conversion rate of 88% or more, an average particle size of 1 μm or less, and a specific surface area of ​​5 to 15 m 2 / g, oxygen content of 0.5 to 1.5 mass%, iron content of 300 mass ppm or less, chlorine content of 70 mass ppm or less, and fluorine content of 900 mass ppm or less. [Effects of the Invention]

[0008] According to the present invention, the efficiency of silicon nitride powder production can be improved by improving the cleaning process after acid treatment. 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." The "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 silicon nitride powder is the total amount of oxygen obtained by heating the silicon nitride powder to 2000°C in a helium atmosphere and detecting the amount of oxygen in the released gas.

[0012] The iron content of the silicon nitride powder is a value measured using an X-ray fluorescence analyzer (manufactured by Rigaku Corporation, device name: Primas II). The chlorine and fluorine contents of silicon nitride powder are determined by heating the silicon nitride powder, dissolving the generated gas in water, and measuring the amount of Cl ions and F ions in the water using an ion chromatograph. These values ​​are sometimes referred to as the "chlorine (Cl) content" and the "fluorine (F) content," respectively.

[0013] <Method for producing silicon nitride powder> The method for producing silicon nitride powder of 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 solid-liquid separator to obtain a silicon nitride powder-containing composition in which the acidic substances and impurities have been reduced. 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.

[0014] <Silicon nitride powder manufacturing process> In the present invention, the process for producing silicon nitride powder is not particularly limited. A preferred embodiment of the process for producing silicon nitride powder will be described. The process for producing silicon nitride powder of this embodiment includes a nitriding step of metal 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] (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.

[0021] 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.

[0022] (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.

[0023] The media in a media agitation mill may be either metallic or ceramic. From the viewpoint of sufficiently pulverizing 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.

[0024] 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.

[0025] 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.

[0026] (Silicon nitride powder before acid treatment) D of silicon nitride powder obtained by the fine grinding process 50is 1 μm or less, preferably 0.9 μm or less, and more preferably 0.8 μm or less. 50 The lower limit of the thickness 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. Silicon nitride powder D 50 When the value is equal to or less than the upper limit, it becomes easier to produce a silicon nitride sintered body.

[0027] 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.

[0028] <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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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 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

[0033] <Cleaning process> In the washing process, the liquid to be treated is fed to a solid-liquid separator, concentrated, and then the washing liquid is added after concentration, followed by another concentration. This washing process is repeated. This produces a silicon nitride powder-containing composition with reduced levels of acidic substances and impurities in the liquid to be treated. Because gravity settling requires a long treatment time, decantation is not used to improve efficiency. In this embodiment, a continuous solid-liquid separator is used. Examples of the continuous solid-liquid separator include a cross-flow filtration device and a continuous centrifugal separator.

[0034] 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 material of the filter material and other components that come into contact with the liquid to be treated may be any acid-resistant material, such as ceramics, or acid-resistant resins such as polyvinylidene fluoride (PVDF) and polypropylene (PP).

[0035] In the cross-flow method, the liquid to be treated flows parallel to the filtering surface of the filter media, and pressure is applied from the inside to the outside of the filtering surface. This prevents solids from accumulating on the inside of the filtering surface, allowing the liquid to pass through the filter media and concentrate the liquid. 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 filtration amount. When the membrane pore size is equal to or greater than the above lower limit, a sufficient filtration flux is easily obtained, whereas when the membrane pore size is equal to or less than the above upper limit, the ultrafine powder of silicon nitride 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 filter cloth's air permeability 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 ultrafine powder of silicon nitride is less likely to pass through the filtration membrane, making it easier to reduce the amount of solids in the waste liquid.

[0036] A continuous centrifuge is equipped with a mechanism for continuously supplying the liquid to be treated to a rotating separator, applying centrifugal force to separate the solid and liquid, and discharging the concentrated liquid and separated liquid from their respective outlets. Commercially available continuous centrifuges can be used. They may be either horizontal or vertical. The material of the part of the continuous centrifuge that comes into contact with the liquid to be treated may be any material that is acid-resistant. For example, an acid-resistant alloy may be used, and Hastelloy is preferred. The centrifugal force is preferably set so that the solids concentration of the precipitate formed by centrifugation is 65 to 80% by mass, preferably 68 to 75% by mass. Water is added to the outlet for discharging this precipitate, and the concentrate is discharged from the separator in the form of a concentrated liquid with a solids concentration of 20 to 45% by mass, preferably 30 to 40% by mass. If the solids concentration of the concentrated liquid is equal to or less than the upper limit mentioned above, the viscosity of the concentrated liquid is reduced, improving the liquid's transportability to the next process. The centrifugal force (centrifugal acceleration) of the separator is, for example, preferably 1000 to 3000 G, and more preferably 2000 to 2500 G.

[0037] Examples of the cleaning solution include water and alkaline solutions. 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.

[0038] In the washing step, it is preferable to carry out a first step using water as a washing liquid, followed by a second step using an alkaline solution as a washing liquid for washing and neutralization. The pH of the liquid to be treated at the start of the first 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 first step is the same as the pH of the liquid to be treated at the start of the second 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, metal impurities such as iron tend to remain in the silicon nitride powder, so the pH is preferably equal to or greater than the lower limit. The pH (end point) of the treatment liquid at the end of the second step is preferably 7 to 10, more preferably 8 to 9. If it exceeds the upper limit, alkaline components tend to remain in the silicon nitride powder after drying, so it is preferably equal to or less than the upper limit.

[0039] It is possible to wash by supplying only water until the pH (end point) is reached without providing the second step, but providing the second step is preferable compared to the first step alone in terms of shortening the processing time and reducing the amount of waste liquid. Furthermore, if washing is carried out only in the second step, metal impurities such as iron will be reprecipitated and will not be removed sufficiently, which is undesirable.

[0040] 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.

[0041] <Drying process> The silicon nitride powder-containing composition obtained in the washing step can be dried to obtain silicon nitride powder washed after acid treatment (hereinafter also referred to as "silicon nitride powder after washing step"). 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

[0042] 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.

[0043] 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.

[0044] 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 %.

[0045] 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.

[0046] <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.

[0047] 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 %.

[0048] 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.

[0049] 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. [Example]

[0050] The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

[0051] (D 50 ) D 50 was determined by measuring the particle size distribution by a 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 silicon nitride powder after cleaning 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 silicon nitride powder contained in the liquid to be treated 50 is the silicon nitride powder D after the above-mentioned washing process after drying and crushing the silicon nitride slurry (silicon nitride slurry after wet milling) before adding the acidic solution. 50 was measured by the same method.

[0052] (specific surface area) The BET specific surface area of ​​the silicon nitride powder was measured by the single-point BET method using nitrogen gas in accordance with JIS R 1626:1996, "Method for measuring specific surface area of ​​fine ceramic powders by gas adsorption BET method." The silicon nitride slurry after wet milling was thoroughly dried in a tray dryer, then crushed in a mortar and measured using the above method. The silicon nitride powder-containing composition after acid treatment and washing was dried in a spray dryer and then measured using the above method.

[0053] (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.

[0054] (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.

[0055] (solids) The solid content concentration (unit: mass %) of the liquid to be treated was calculated using the formula: (weight of silicon nitride powder used in wet pulverization) ÷ (weight of silicon nitride slurry obtained by wet pulverization + weight of the mixed solution of hydrofluoric acid and hydrochloric acid added to the silicon nitride slurry) × 100.

[0056] 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.

[0057] <Production Example 1: Preparation of treatment liquid> The liquid to be treated used in each example was prepared by producing silicon nitride by direct nitriding, wet-pulverizing it to obtain a slurry containing silicon nitride powder (silicon nitride slurry), and then treating it with acid. Specifically, a molded body (bulk density: 1.4 g / cm) was produced using metal silicon powder. 3The 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.

[0058] 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.

[0059] 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 3 (first step). The average filtration flux in the first step is shown in Table 1 (the same applies below). Next, instead of water, 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 (second step). The average filtration flux in the second step is shown in Table 1 (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. The obtained silicon nitride powder D 50 The specific surface area, iron content (Fe content), oxygen content, chlorine content (Cl content), fluorine content (F 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 the acidic substances and iron in the treated liquid were highly removed. The total amount of waste liquid, the amount of solids in the waste liquid, the overall average filtration flux, and the treatment capacity in this example are shown in Table 1 (the same applies hereinafter). 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 and the total amount of aqueous ammonia supplied in the second step. The treatment capacity was 1 m 3 The total treatment time is the sum of the treatment times for the first and second steps. The overall average filtration flux was calculated by dividing the total wastewater volume by the filter media area and the total treatment time.

[0060] <Example 2> 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. The first and second steps were carried out in the same manner as in Example 1 to obtain a silicon nitride powder-containing composition after washing. The obtained silicon nitride powder-containing composition was dried in the same manner as in Example 1 to obtain silicon nitride powder after the washing step.

[0061] Example 3 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 A polypropylene filter cloth (double weave) of s was used. The first and second steps were carried out in the same manner as in Example 1 to obtain a silicon nitride powder-containing composition after washing. The obtained silicon nitride powder-containing composition was dried in the same manner as in Example 1 to obtain silicon nitride powder after the washing step.

[0062] Example 4 In this example, a continuous centrifugal separator was used to separate the liquid to be treated into solid and liquid, and also to carry out the washing step. 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 a centrifuge, where solid-liquid separation was carried out under centrifugal force conditions such that the liquid to be treated, which had a solid content of 20% by mass, was concentrated to a solid content of 70% by mass.The separated liquid was removed as waste liquid, and the concentrated liquid 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 centrifuge, 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 liquid was concentrated again using the centrifuge. This operation was repeated continuously. While maintaining constant centrifugal force conditions, washing with water was continued until the pH of the liquid in the storage tank reached 3 (step 1). Next, instead of water, 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 in the storage tank reached 9 (second step). When the pH of the liquid in the storage tank 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 35 mass %. The resulting silicon nitride powder-containing composition was dried at 300°C to obtain silicon nitride powder after the washing step. The obtained silicon nitride powder D 50 The specific surface area, iron content (Fe content), oxygen content, chlorine content (Cl content), fluorine content (F content), and alpha conversion rate are shown in Table 1. 50 is the D of silicon nitride powder in the liquid to be treated 50 It was confirmed that the acidic substances and iron in the treated liquid were highly removed. The total amount of waste liquid, the amount of solids in the waste liquid, and the treatment capacity in this example are shown in Table 1. 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 and the total amount of aqueous ammonia supplied in the second step. The treatment capacity was 1 m 3 The treatment time refers to the treatment time required to treat the liquid to be treated, and is the sum of the treatment times for the first step and the second step (total treatment time).

[0063] <Comparative Example 1> In this example, decantation (gravitational settling method) was used for cleaning after the acid treatment. Specifically, a tank equipped with a stirring blade is placed in a 1m 3 After decantation (sedimentation and removal of supernatant) was performed once, the liquid volume in the tank reached 1 m 3 Thereafter, decantation was repeated until the pH of the liquid in the tank before removing the supernatant reached 3, and washing with water was carried out (first step). Next, after removing the supernatant, the liquid volume in the tank is 1 m 3An aqueous ammonia solution with a concentration of 0.08% by mass was supplied so that the pH of the liquid in the tank before removing the supernatant reached 9. Thereafter, decantation was repeated until the pH of the liquid in the tank before removing the supernatant reached 9 (second step). When the pH of the liquid in the tank before removing the supernatant reached 9, the liquid in the tank was used as a silicon nitride powder-containing composition. The solids concentration of the silicon nitride powder-containing composition was the same as that of the liquid to be treated. The resulting silicon nitride powder-containing composition was dried at 300°C to obtain silicon nitride powder after the washing step. The obtained silicon nitride powder D 50 Table 1 shows the specific surface area, iron content (Fe content), oxygen content, chlorine content (Cl content), fluorine content (F content), and gelatinization rate. The total amount of waste liquid, the amount of solids in the waste liquid, and the treatment capacity in this example are shown in Table 1. 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 and the total amount of aqueous ammonia supplied in the second step. The treatment capacity was 1 m 3 This means the treatment time required to treat the liquid to be treated, and is the average value of the operation records.

[0064] [Table 1]

[0065] In all of the solid-liquid separators of 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, compared to Comparative Example 1, Examples 1 and 2 had reduced solid content in the waste liquid, and Examples 3 and 4 had reduced total waste liquid volume. In particular, Examples 1 and 2 had zero solid content in the waste liquid. This reduces the work and costs involved in waste liquid treatment, improving the efficiency of silicon nitride powder production. Furthermore, Examples 1 to 4 were able to shorten the total treatment time compared to Comparative Example 1.

Claims

1. a washing step of supplying a liquid to be treated containing silicon nitride powder, an acidic substance, and impurities to a solid-liquid separator (excluding decantation) to obtain a silicon nitride powder-containing composition in which the acidic substance and the impurities have been reduced; The solid-liquid separator is a continuous solid-liquid separator, and the process of concentrating the liquid to be treated, adding a washing liquid after the concentration, and concentrating again is continuously carried out.

2. 2. The method for producing silicon nitride powder according to claim 1, wherein the continuous solid-liquid separator is a cross-flow filtration device or a centrifugal separator.

3. After the washing step, the silicon nitride powder has an alpha conversion rate of 88% or more, an average particle size of 1 μm or less, and a specific surface area of ​​5 to 15 m ２ / g, oxygen content of 0.5 to 1.5 mass%, iron content of 300 mass ppm or less, chlorine content of 70 mass ppm or less, and fluorine content of 900 mass ppm or less.

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