Method for cleaning tank and method for producing silica particles

Abstract

To provide a tank cleaning method, enabling reduction of load on the tank and the whole production apparatus as well as reduction of environmental burden, while enhancing efficiency in producing silica.SOLUTION: A method for cleaning a tank, comprising step (1) of separating a solvent from a dispersion of silica particles, and step (2) of dissolving coarse silica particles generated during the solvent separation in step (1) with an alkaline liquid.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for cleaning a tank and a method for producing silica particles. [Background technology]

[0002] Silica particles are used as desiccants, adsorbents, fillers in resins and rubber materials, catalysts, paints, adhesives, and anti-fouling agents. It is an industrial material used in a wide range of fields, including explosives and glass materials. Nano-sized silica is mainly used in various applications as colloidal silica dispersed in a dispersion medium. It is said.

[0003] Nano-sized silica particles used as colloidal silica are produced by Depending on the difference, it may be made by thermal decomposition of silicon tetrachloride (fumed silica, etc.), or silica such as water glass. By deionization of alkali, hydrolysis reaction and condensation reaction of alkoxysilane (generally Known methods include those based on the sol-gel method.

[0004] Many studies have been conducted on silica sols and methods for producing silica sols. For example, Patent Document 1 discloses a method for producing alkoxysilane or its condensation product in a specific reaction solution containing water. It is disclosed that silica sol can be produced by hydrolysis and polycondensation of the compound. In addition, in Patent Document 2, methyl orthosilicate is used among alkoxysilanes, and silanes are formed at relatively low temperatures. A method for producing colloidal silica by hydrolysis of hydroxypropyl methylcellulose to hydroxypropyl methylcellulose and then condensation of the hydroxypropyl methylcellulose is disclosed. are. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2020-189787 [Patent Document 2] Japanese Patent Application Publication No. 11-60232 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-071659 Summary of the Invention [Problem to be solved by the invention]

[0006] Generally, silica particles and silica sols obtained by the above methods contain the same These coarse particles are generated by the hydrolysis of alkoxysilanes and This phenomenon is particularly evident when silica particles are produced by a condensation reaction. Not only in the silica sol, but also in the reactor for producing silica particles and the solvent for adjusting the concentration of the silica sol. It may precipitate on the walls of the solvent exchange tank and the storage tank where the silica sol is stored.

[0007] Coarse particles deposited on the wall surface have a negative effect on the repeated use of each tank and must be disposed of appropriately. If this is not done, there is a possibility that the production efficiency and quality maintenance of silica particles and silica sol will be significantly hindered. do.

[0008] In contrast to this, the silica sol and colloidal silica preparations disclosed in the above Patent Documents 1 and 2 On the other hand, according to Patent Document 3, the general It is disclosed that coarse particles contained in silica sol can be removed by filtration. There is no disclosure about the coarse particles that precipitate on the wall of the tank. The method for handling the large particles has not yet been optimized.

[0009] Coarse particles deposited on the wall of the tank can be physically removed or dissolved and cleaned by chemical reaction. However, in either method, the inside of the tank The environmental impact of the waste and disposal process is significant, and depending on the time required for these processes, the silica particles This also affects the manufacturing efficiency of the product, so we must reduce the burden on the tank and the environment while also addressing the negative impact on manufacturing efficiency. There is a need for a non-invasive treatment method.

[0010] Therefore, an object of the present invention is to provide a method for cleaning a tank on which coarse particles have precipitated and adhered. Another object of the present invention is to provide a method for repeatedly producing silica particles in a tank in which coarse particles have been washed. do. [Means for solving the problem]

[0011] As a result of extensive research into the above-mentioned problems, the present inventors have found that an alkaline liquid can be used to dissolve silica particles. and by using it as a cleaning agent for tanks, the above problems can be solved, and the present invention has been completed. This led to the decision to do so.

[0012] The gist of the present invention is as follows. [1] A method for cleaning a tank, comprising the following steps (1) and (2): Step (1): A step of separating the solvent from the dispersion of silica particles. Step (2): The coarse silica particles generated during the separation of the solvent in the step (1) are treated with alkali. A process of dissolving the material in an aqueous solution. [2] The method according to [1], wherein the step (1) is a step of separating a solvent containing an alcohol. How to clean the tank. [3] The cleaning method of the tank according to [1] or [2], wherein the pH of the alkaline liquid is 10 or more. Cleaning method. [4] Any of [1] to [3], wherein the alkaline liquid is an aqueous solution of sodium hydroxide. A method for cleaning the tank described above. [5] In the step (2), the pH of the solution contained in the tank is adjusted from the beginning to the end of the step. The pH of the alkaline solution initially added is maintained within ±1 during this time.[1] A method for cleaning a tank according to any one of [4] to [4]. [6] In the step (2), the temperature of the solution contained in the tank is 0°C to 100°C. The method for cleaning a tank according to any one of [1] to [5]. [7] The time from the beginning to the end of the step (2) is within 10 hours. [1 [6]. The method for cleaning a tank according to any one of [1] to [6]. [8] The solution remaining in the tank after the step (2) is colorless and transparent [1] to [7]. A method for cleaning a tank according to any one of the above. [9] The average secondary particle diameter of the silica particles contained in the dispersion of the silica particles is 10 nm to The method for cleaning a tank according to any one of [1] to [8], wherein the particle size is 1000 nm.

[10] Any of [1] to [9], wherein the coarse silica particles contain particles of 0.01 g or more. A method for cleaning a tank described in any one of the preceding claims.

[11] The method for cleaning a tank according to any one of [1] to

[0010] , wherein the dispersion of silica particles contains an alcohol and an alkaline compound.

[12] The method for cleaning a tank according to

[11] , wherein the alkaline compound is ammonia.

[13] Any of [1] to

[12] , which includes the following step (0) before the step (1): A method for cleaning a tank according to claim 1. Step (0): A solution (A) containing water is mixed with a solution (B) containing tetraalkoxysilane and, if necessary, A step of adding solution (C) as needed.

[14] A tank cleaned by a method including the cleaning method according to any one of [1] to

[13] is used. The method for producing silica particles includes the steps of: [Effects of the Invention]

[0013] According to this embodiment, the load on the tank and the entire apparatus for producing silica particles as well as the environmental load are reduced. In addition to reducing the cost of cleaning, it also reduces the impact on products when used repeatedly. Furthermore, the cleaning efficiency is high, making it possible to clean the tank without any trouble. Efficiency can also be improved. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below, but the present invention is not limited to the following embodiments. It should be noted that the present invention can be practiced with various modifications within the scope of its gist. When using the expression "~", it is used as an expression that includes the numerical values ​​or physical properties before and after it.

[0015] [How to clean the tank] The method for cleaning a tank according to this embodiment includes the following steps (1) and (2). Step (1): A step of separating the solvent from the dispersion of silica particles. Step (2): The coarse silica particles generated during the separation of the solvent in the step (1) are treated with alkali. A process of dissolving in an aqueous liquid.

[0016] Each step in this embodiment will be described below.

[0017] <Process (1)> Step (1) in this embodiment is a step of separating the solvent from the dispersion of silica particles. At this time, the solvent may be separated in its entirety or in part. Any method is acceptable, but examples include distillation by heating, evaporation by long-term storage, and membrane concentration. In particular, coarse silica particles are precipitated remarkably during distillation by heating.

[0018] The step of separating the solvent from the dispersion of silica particles by distillation under heating is It is possible to adjust the components of the dispersion liquid and change the physical properties of the silica particles themselves. This is one of the methods frequently used in industry. In step (1), the particle production cycle can be accelerated and production efficiency can be improved. It is preferable to implement it.

[0019] When the dispersion of silica particles in the tank is heated, the method is not particularly limited. For example, It is possible to heat the device by attaching a jacket to the outside of the device. The distillation conditions of the dispersion medium can be controlled by the sheath temperature, and the higher the jacket height, the better The higher the temperature, the more the distillation rate can be improved, and the production efficiency of silica particles and silica sol can be improved. can be improved.

[0020] On the other hand, increasing the distillation rate and causing coarse silica particles to precipitate and adhere to the tank wall are There is a trade-off between these two factors, especially when raising the jacket too high or raising the temperature too high. When this happens, the temperature of the wall of the tank that is not filled with solution rises to 60°C or higher, and the The dispersion of silica particles dries easily, and coarse particles precipitate and adhere. However, by adopting the cleaning method of the tank according to the present embodiment described later, the coarse particles adhering to the tank can be easily removed. This method can efficiently remove large particles while minimizing the burden on the tank. As a result, the production efficiency of silica particles and silica sol can also be improved.

[0021] The dispersion medium for the dispersion of the silica particles is not particularly limited, but may be alcohol, water, alkali, or the like. It is preferable that the composition contains at least one of the following compounds: It is preferable that the solvent contains the above-mentioned components, and therefore the physical properties of the silica particles produced, such as the particle size, can be controlled. Control and reduction of coarse particle generation, excellent (or effective) in reducing the size of coarse particles (obtained).

[0022] The average primary particle diameter of the silica particles in the dispersion of silica particles is preferably 5 nm to 500 nm. The average primary particle diameter of the silica particles is preferably 5 nm or less, and more preferably 10 nm to 300 nm. If the silica sol is above 100%, the storage stability in the tank of the silica sol is excellent, and the cost of the cleaning operation described later can be reduced. In addition, when the average primary particle diameter of the silica particles is 500 nm or less, coarse particles are prevented from being formed. It is excellent at reducing the amount of particles generated and reducing the size of coarse particles, and the amount of alkali required to dissolve coarse particles This reduces the load on the tank and increases the production efficiency of silica particles.

[0023] The average primary particle size of silica particles is measured by the BET method. The specific surface area of ​​the silica particles was measured using a measuring device, and the average primary particle diameter was calculated using the following formula (1). Calculate. Average primary particle diameter (nm) = 6000 / (specific surface area (m 2 / g) x density (g / cm 3 ) ) (1)

[0024] The average secondary particle diameter of the silica particles in the dispersion of silica particles is 10 nm to 1000 nm. The average secondary particle diameter of the silica particles is preferably 10 nm to 600 nm, and more preferably 20 nm to 600 nm. When the silica sol has a viscosity of 1000 MPa or more, the storage stability of the silica sol in the tank is excellent, and the cost of the cleaning operation described below is reduced. If the average secondary particle diameter of silica particles is 1000 nm or less, coarse particles will be formed. It is excellent at reducing the amount of particles generated and reducing the size of coarse particles, and the amount of alkali required to dissolve coarse particles This reduces the load on the tank and increases the production efficiency of silica particles.

[0025] The average secondary particle diameter of silica particles is measured by the DLS method. Measure using a diameter measuring device.

[0026] The dispersion of silica particles is not particularly limited as long as it contains silica particles. Deionization of alkali silicate such as water glass by thermal decomposition of silicon chloride (fumed silica, etc.) by ion, hydrolysis and condensation reactions of alkoxysilanes (generally known as the "sol-gel method" These include deposits of saturated silica components in geothermal hot water, and However, those produced by hydrolysis and condensation of alkoxysilane are preferred, and tetraalkoxysilane is preferred. More preferred is one obtained by hydrolysis and condensation of a cooxysilane.

[0027] The hydrolysis and condensation reaction of the tetraalkoxysilane is carried out under known conditions and methods. For example, a method such as the following step (0) can be considered. Step (0): A solution (A) containing water is mixed with a solution (B) containing tetraalkoxysilane and, if necessary, A step of adding solution (C) as needed. In this embodiment, step (0) may be included before step (1). 0) will be explained.

[0028] <Process (0)> In step (0), tetraalkoxysilane is subjected to a hydrolysis reaction and a condensation reaction, and then a tetraalkoxysilane is subjected to a hydrolysis reaction and a condensation reaction. ) is a step of obtaining a dispersion of silica particles.

[0029] Solution (A) contains water. By including water, the amount of tetraalkoxysilane added later can be reduced. Hydrolysis and condensation reactions can occur.

[0030] Solution (A) can promote the hydrolysis and condensation reaction of tetraalkoxysilane. From this viewpoint, it is preferable that the catalyst contains an alkali catalyst. Examples of the alkali catalyst include ethylenediamine, diethylenetriamine, triethylenediamine, and the like. ethylenetetramine, ammonia, urea, ethanolamine, tetramethylammonium hydroxide These alkali catalysts may be used alone or in combination of two or more. Among these alkali catalysts, those with catalytic action, controllability of particle shape, and metal mixing ability are preferred. Ammonia is preferred from the viewpoints of suppressing intrusion and ease of removal after the reaction.

[0031] The solution (A) preferably contains an alcohol. Alcohols include, for example, methanol, ethanol, propanol, isopropanol, ethylene glycol, etc. These alcohols may be used alone. Among these alcohols, tetraalkoxysilane is particularly preferred. It is easily soluble, and the by-products used in the hydrolysis and condensation reactions are the same as those used in the hydrolysis and condensation reactions. Methanol and ethanol are more preferred because of their superior convenience. Preferred.

[0032] Solution (A) can promote the hydrolysis of alkoxysilane, so water is used. Furthermore, the solution (A) may contain a solvent other than alcohol and water.

[0033] The concentration of the alkali catalyst in the solution (A) is preferably 0 to 10.0 mass %. Particle aggregation is further suppressed, improving dispersion stability of silica particles in silica sol and storage stability in the tank. From the viewpoint of excellent performance and reduction in cost and time for the cleaning operation described below, The concentration is preferably 0% by mass or more, more preferably 0.1% by mass or more. From this viewpoint, the concentration of the alkali catalyst is preferably 10.0 mass % or less, more preferably 8.0 mass % or less. More preferable.

[0034] The concentration of the alcohol in the solution (A) is preferably 10% by mass to 96% by mass. From the viewpoint of dispersibility of tetraalkoxysilane in the reaction solution, the alcohol concentration is 10 mass%. The content of silicic acid produced by the hydrolysis reaction is preferably 69 mass % or more, and more preferably 69 mass % or more. From the viewpoint of dispersibility in the reaction solution, the alcohol concentration is preferably 96% by mass or less, and more preferably 94% by mass or less. It is more preferable that the content is % by mass or less.

[0035] The concentration of water in the solution (A) is preferably 3% by mass to 90% by mass. From the viewpoint of dispersibility of the silicic acid produced in the reaction liquid, the concentration of water is preferably 3% by mass or more. More preferably, the dispersibility of the tetraalkoxysilane in the reaction liquid is 5 mass % or more. From this viewpoint, the concentration of water is preferably 90% by mass or less, and more preferably 50% by mass or less.

[0036] The concentration of the solvent other than alcohol and water in solution (A) is the ratio of the alkali catalyst, alcohol, and water. It is preferable to use the concentration of the remainder.

[0037] The solution (B) contains a tetraalkoxysilane. Examples of tetraalkoxysilanes include tetramethoxysilane, tetraethoxysilane, Examples of the tetrasilane include tetrapropoxysilane and tetraisopropoxysilane. The alkoxysilane may be used alone or in combination of two or more kinds. Among tetraalkoxysilanes, it has a fast hydrolysis reaction, is less likely to leave unreacted residues, and is easy to produce. Since it has excellent properties and can easily produce stable silica sol, tetramethoxysilane is Of these, tetraethoxysilane is preferred, and tetramethoxysilane is more preferred.

[0038] Solution (B) is a solvent because it has excellent dispersibility of tetraalkoxysilane in the reaction solution. It is preferred that the compound contains:

[0039] The solvent in the solution (B) is, for example, methanol, ethanol, propanol, isopropanol, or the like. alcohols such as alcohol and ethylene glycol; ketones such as acetone and methyl ethyl ketone and esters such as ethyl acetate. These solvents may be used alone. Among these solvents, the solvents used in the hydrolysis reaction and the condensation reaction are preferably used in combination. Alcohol is preferred because the product produced and the by-product are the same and it is convenient for manufacturing. Of these, methanol and ethanol are more preferred, and methanol is even more preferred.

[0040] The concentration of the tetraalkoxysilane in the solution (B) is preferably 76 to 89 mass %. In order to reduce the amount of solvent used and increase the productivity of silica particles, The concentration of silane is preferably 76% by mass or more, more preferably 77% by mass or more. From the viewpoint of dispersibility of tetraalkoxysilane in the liquid, the concentration of tetraalkoxysilane is , preferably 89% by mass or less, and more preferably 88% by mass or less.

[0041] The concentration of the solvent in the solution (B) is preferably 11% by mass to 24% by mass. From the viewpoint of dispersibility of tetraalkoxysilane in the solvent, the concentration of the solvent is preferably 11% by mass or more. In addition, the amount of the solvent used can be reduced, and the production of silica particles can be improved. From the viewpoint of improving the stability, the concentration of the solvent is preferably 24% by mass or less, more preferably 23% by mass or less. In addition, the concentration of the solvent in the solution (B) is preferably It is preferred that the concentration is parts.

[0042] The solution (C) is a solution containing water, and preferably further contains an alkali catalyst. The alkali catalyst in the solution (C) is, for example, ethylenediamine, diethylenetriamine, Triethylenetetramine, ammonia, urea, ethanolamine, tetramethyl hydroxide These alkali catalysts may be used alone or in combination of two or more. Among these alkali catalysts, those with excellent catalytic activity and particle shape are It is easy to control, can suppress metal contamination, is highly volatile, and is suitable for hydrolysis and condensation reactions. Ammonia is preferred because it is easily removable after reaction.

[0043] The solution (C) may contain a solvent other than water. The solvent other than water in the solution (C) is, for example, methanol, ethanol, propanol, ethanol ... Examples of the solvent include alcohols such as isopropanol and ethylene glycol. One type may be used alone, or two or more types may be used in combination.

[0044] The concentration of the alkali catalyst in the solution (C) is preferably 0% by mass to 25% by mass. The solvent may be 0 mass %, i.e., not contained, but if an alkali catalyst is contained, its concentration is It is preferable that the content is 1 mass % or more. In addition, from the viewpoint of preventing the reaction from proceeding excessively quickly and providing excellent reaction controllability, Therefore, the concentration of the alkali catalyst is preferably 25% by mass or less, and more preferably 20% by mass or less.

[0045] The concentration of water in the solution (C) is preferably 75% by mass to 100% by mass. From the viewpoint of dispersibility of the silicic acid produced by the reaction in the reaction liquid, the concentration of water is preferably 85 mass % or more. The concentration of water is preferably 100% by mass, i.e., the concentration of the solution is 95% by mass or more. (C) may consist of only water, but if it contains other components, it should be 99% by mass or less. is preferred.

[0046] The concentration of the solvent other than water in the solution (C) is preferably the same as the concentration of the balance of water and the alkali catalyst. I wish.

[0047] The reaction temperature of the hydrolysis reaction and condensation reaction of tetraalkoxysilane, i.e., The temperature of the reaction solution is preferably 15 to 50°C. Here, the reaction does not proceed too slowly and is easy to control. From the viewpoint of excellent performance, the temperature of the reaction solution is preferably 15°C or higher, more preferably 20°C or higher. In addition, from the viewpoint of the balance between the hydrolysis reaction rate and the condensation reaction rate, the temperature of the reaction solution should be 50°C or higher. Preferably below 45°C, more preferably below 45°C.

[0048] The alcohol and alkaline catalyst contained in the dispersion of silica particles obtained in this step (0) The alkaline compound and water used as the solvent are separated in the above step (1). As mentioned above, it is the dispersion medium in the dispersion liquid. This improves the dispersion stability of the tetraalkoxysilane used as the material, suppressing the generation of coarse particles, and From the viewpoint of reducing the cost and time of the cleaning operation, the dispersion liquid of the silica particles is preferably an alcohol. It is preferable that the solution contains an alkaline compound.

[0049] <Process (2)> In the present embodiment, step (2) is carried out to remove the silica generated during the separation of the solvent in step (1). This is a process in which large particles of mosquitoes are dissolved using alkaline liquid. In this step (2), coarse particles of silica are dissolved to remove the coarse particles that have deposited and adhered to the walls of the tank. The tanks can be repeatedly used stably, and the production of silica particles and silica sol is When carrying out step (2), the production efficiency and quality can be maintained. The dispersion of separated silica particles may remain in the tank, but it may be left in another container or device. may be moved to

[0050] The alkaline liquid is not particularly limited as long as it can dissolve silica. For example, Examples include ammonia, ethylenediamine, and aqueous sodium hydroxide solution. From the viewpoint of cost and efficiency, an aqueous sodium hydroxide solution is particularly preferred.

[0051] The pH of the alkaline liquid is preferably in the range of 10 to 14, and more preferably in the range of 12 to 14. It is more preferable that the pH is within the range of 14. If the pH is above the lower limit, the solubility of silica will decrease. If the pH is below the upper limit, the load on the equipment increases. The low temperature can extend the life of the equipment used to clean the tank.

[0052] In step (2), the pH of the solution contained in the tank is controlled from the beginning to the end of step (2). It is preferable to maintain the pH within a range of ±1 of the initially added alkaline solution. It is more preferable to maintain it within a range of 0.5, and particularly preferable to maintain it within a range of ±0.2. By maintaining a specific pH range, alkaline solutions with a higher pH than the range being maintained can be prevented. It begins to dissolve in a soluble liquid and dissolves as quickly or faster than when the solution is left standing or stirred. This reduces the total amount of alkali required for complete dissolution. Although the mechanism is unclear, if silica is considered on a molecular level, it will not dissolve if the pH is not maintained. Immediately after starting the dissolution, the dissolution rate is fast, but as the pH gradually decreases, one silica molecule The alkaline component in contact with the surface decreased, the dissolution rate slowed down, and eventually the dissolution stopped. On the other hand, when maintaining the pH, the alkaline component that comes into contact with one silica molecule Since it can be maintained in a high state, it can be dissolved without slowing down the dissolution speed. It is thought that...

[0053] In step (2), the temperature of the solution contained in the tank is preferably 0°C to 100°C. The temperature is more preferably 30 to 90°C, and particularly preferably 60 to 80°C. When the temperature of the solution contained in the tank is equal to or higher than the lower limit, the silica dissolving ability by alkali is excellent. If the temperature of the solution contained in the tank is below the upper limit, the equipment may become contaminated due to evaporation or vaporization of the solution. Excellent for reducing dyes.

[0054] The time required from start to finish of the step (2) is 0.25 to 1 hour. It is preferably within 0 hours, more preferably 0.5 to 5 hours, and more preferably 1 hour. If the required time is equal to or longer than the lower limit, the silica may not be completely dissolved. If the required time is equal to or less than the upper limit, the operation cost is excellent.

[0055] The solution remaining after the completion of the step (2) is preferably colorless and transparent, white and translucent, or white. It is preferable that the solution is colorless and transparent. When the solution is colorless and transparent, the cost of treating the solution is excellent. In the present invention, colorless and transparent means that the color is colorless in visible light with a wavelength of 400 nm to 700 nm. The transmittance of the film is 85% or more. The transmittance can be calculated using the following formula. Transmittance (%) = (transmitted light / incident light) x 100

[0056] The coarse silica particles may contain particles of 0.01 g or more. There is a trade-off between increasing the production efficiency of silica sol and the precipitation and adhesion of coarse particles. However, even if particles of 0.01g or more are deposited or attached to the wall, this implementation By adopting this type of tank cleaning method, the production efficiency of silica particles and silica sol is improved. To rise.

[0057] The material, shape, size, etc. of the tank in this embodiment are not particularly limited. For example, SUS and glass, with a fluorine coating or glass lining on the inside. Among these, glass, fluorine coating and glass lining are preferred from the viewpoint of reducing the metal content. The silica-based coating of the present invention can also be used to prevent these coatings from dissolving and being consumed. It is suitable for adopting the solution method.

[0058] [Method of manufacturing silica particles] In the method for producing silica particles according to the present embodiment, the tank cleaned by the above <Method for cleaning tank> is The present invention relates to a method for producing silica particles, in which silica particles are produced using a silica particle producing method.

[0059] The method for producing silica particles is not particularly limited, but for example, the method described in the above step (0) can be used. The method for producing silica particles and the method for cleaning the tank in this embodiment may be interchangeable. Alternatively, the production of silica particles may be carried out several times and then the tank may be washed. The manufacturing cycle may be repeated. [Example]

[0060] The present invention will be described in more detail below using examples. Unless otherwise stated, the following examples are not intended to be limiting.

[0061] Evaluation Method <pH measurement> The coarse silica particles generated during the separation of the solvent from the dispersion liquid of silica particles in each example were removed by alkali. The pH of the solution was measured using a benchtop pH meter. Specifically, the sample solution is placed in an appropriate container and stirred while measuring the pH / ION METHOD. Measurement was carried out using a TER F-72 (model name, manufactured by HORIBA).

[0062] [Example 1] Solution (A) was prepared by mixing pure water at a concentration of 7.5 mass %, ammonia at a concentration of 1.2 mass %, and the remaining The solution was prepared so that the remaining part was methanol. Solution (B) was prepared by mixing tetramethoxysilane and methanol at a ratio of 85:15 (by mass). A solution was prepared. As solution (C), a 4 mass % aqueous ammonia solution was prepared. Then, the solution (A) was added to a reaction vessel equipped with a thermometer, a stirrer, a supply pipe, and a distillation line. The temperature of the reaction solution was adjusted to 23°C, and the above solution ( 58 parts by volume of B) and 16 parts by volume of the above solution (C) were added at equal speeds, and methanol was added. A dispersion of silica particles containing the above-mentioned cellulose acetate was obtained (step (0)).

[0063] The silica particle dispersion obtained above was diluted to a content of about 20% by mass. The temperature is increased to separate the solution containing ammonia and methanol in the dispersion of silica particles. The mixture was separated and collected (step (1)).

[0064] After the above step (1) is completed, a simple evaluation method is to measure the amount of silica deposited and attached to the wall of the tank. First, add 100g of water to a glass container and heat to 60℃. Then, one large silica particle was added and a sodium hydroxide solution with a pH of 13 was added. The solution was stirred while adding sodium hydroxide solution every 15 minutes until the target pH reached 13. The mixture was stirred until the internal solution became transparent and no coarse particles were visible (step (2)). The large particles were completely dissolved in 67 minutes.

[0065] [Reference example 1] 0.9 g of one particle of the coarse silica obtained in step (1) of Example 1 was used. ) First, 100 g of water was added to a glass container and heated to 60°C. Then, the pH 13 of the aqueous sodium hydroxide solution was added and stirring was continued. The coarse silica particles were completely dissolved in 390 minutes.

[0066] [Reference example 2] 0.9 g of one particle of the coarse silica obtained in step (1) of Example 1 was used. ) First, 100 g of water was added to a glass container and heated to 60°C. Then, the pH 14 of the aqueous sodium hydroxide solution was added and stirring was continued. The coarse silica particles were completely dissolved in 120 minutes.

[0067] [Reference example 3] 0.9 g of one particle of the coarse silica obtained in step (1) of Example 1 was used. ) First, 100 g of water was added to a glass container and heated to 80°C. Then, the pH 13 of the aqueous sodium hydroxide solution was added and stirring was continued. The coarse silica particles were completely dissolved in 200 minutes. The conditions for Example 1 and Reference Examples 1 to 3 are shown in Table 1.

[0068] [Table 1]

[0069] From the comparison between the working examples and the reference examples, it was found that rather than increasing the pH to dissolve silica, it was better to add alkali during the process. Adding it to maintain the pH and dissolving it will reduce the risk of contact with high pH liquid in a shorter time. The silica can be completely dissolved at a temperature that is relatively easy to handle. The amount of alkali used can also be reduced. [Industrial Applicability]

[0070] According to the method for cleaning the tank according to the present invention, silica particles adhered to or remaining in the tank during production can be removed. Furthermore, the tanks and equipment used for producing silica particles can be efficiently washed. In addition to reducing the burden on the body, it also reduces the environmental burden, reduces the cost of cleaning, and allows for repeated use. It has little effect on the product during use. As a result, it improves the production efficiency of silica particles for various applications. This can improve the quality and reduce the cost.

Claims

1. A method for cleaning a tank, comprising the following steps (1) and (2). Step (1): A step of separating the solvent from the dispersion of silica particles. Step (2): The coarse silica particles generated during the separation of the solvent in the step (1) are treated with an alkali. A process of dissolving the material in an aqueous solution.

2. The process of claim 1, wherein the step (1) is a step of separating a solvent containing alcohol. Cleaning method.

3. The method for cleaning a tank according to claim 1 , wherein the alkaline liquid has a pH of 10 or higher.

4. 2. The method of claim 1, wherein the alkaline liquid is an aqueous sodium hydroxide solution. 。

5. In the step (2), the pH of the solution contained in the tank is adjusted from the beginning to the end of the step. The pH of the alkaline solution initially charged is maintained within a range of ±1 during the charging period. How to clean the tank.

6. In the step (2), the temperature of the solution contained in the tank is 0°C to 100°C. Item 1. A method for cleaning a tank according to item 1.

7. 2. The method according to claim 1, wherein the time from start to finish of step (2) is 10 hours or less. A method for cleaning the tank described above.

8. 2. The method of claim 1, wherein the solution remaining in the tank after step (2) is colorless and transparent. Cleaning method.

9. The average secondary particle diameter of the silica particles contained in the dispersion of the silica particles is 10 nm to 100 10. The method of claim 1, wherein the surface roughness of the tank is 0 nm.

10. The method for cleaning a tank according to claim 1 , wherein the coarse silica particles comprise particles of 0.1 g or more.

11. 10. The tank of claim 1, wherein the dispersion of silica particles comprises an alcohol and an alkaline compound. How to clean.

12. 12. The method of claim 11, wherein the alkaline compound is ammonia.

13. The method for cleaning a tank according to claim 1, further comprising the following step (0) before step (1): Step (0): A solution (A) containing water is added with a solution (B) containing tetraalkoxysilane and, if necessary, A step of adding solution (C) as needed.

14. Silica particles are produced using a tank cleaned by a method including the cleaning method of any one of claims 1 to 13. Method for producing silica particles.

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