Filtration method for solution containing particles

Abstract

To provide a method for filtering a particle-containing solution which enables a filtrate to be obtained with high yield, while maintaining high filtration rate.SOLUTION: A filtration method for a solution containing particles comprises the steps for: (a) performing wet classification to the solution containing particles to divide into a light liquid including fine particles and a heavy liquid including coarse particles; (b) separating the coarse particles in the heavy liquid with a filter medium; (c) causing the fine particles in the light liquid to grow; and (d) returning the solution containing the particles grown in step (c) to step (a).SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for filtering a solution containing particles. [Background technology]

[0002] It is widely known that methylaluminoxane, a condensation product of water and trimethylaluminum, exhibits high activity in ethylene polymerization as a cocatalyst for metallocene catalysts (Non-Patent Document 1). In the reaction of water and trimethylaluminum, the formation of solids or gels in the form of trimethylaluminum that has been subjected to excessive hydrolysis is unavoidable, resulting in a decrease in the reaction yield based on aluminum. In addition, the gels themselves accumulate in the production line or tank, hindering the smooth operation of the production plant and causing a decrease in the yield and purity of the final product. Furthermore, the presence of impurities in polyolefins has adverse effects such as a decrease in the functionality of the polyolefins themselves, coloration, and physiological activity on the human body, so it is desirable that the cocatalyst methylaluminoxane itself does not contain impurities.

[0003] As a method for removing gel-like matter, that is, aluminum-containing gel, for example, sedimentation separation is known (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 9-507515 [Non-patent literature]

[0005] [Non-Patent Document 1] H. Sinn, W. Kaminsky. Adv. Organomet. Chem. 18, 99 (1980). Summary of the Invention [Problem to be solved by the invention]

[0006] However, the method described in Patent Document 1 is undesirable from the viewpoint of industrial production due to poor operability.

[0007] Cross-flow filtration is known as a method for removing solids and gels. Cross-flow filtration is a method of filtering while suppressing cake deposition on the surface of the filter media by creating a flow parallel to the filter media. Although cross-flow filtration suppresses cake deposition on the surface of the filter media, the filtration efficiency decreases when fine particles accumulate in the pores of the filter media, and there is a risk that the pores of the filter media may eventually become clogged.

[0008] An object of the present invention is to provide a method for filtering a solution containing particles, which can obtain a filtrate in high yield while maintaining a high filtration rate. [Means for solving the problem]

[0009] The present inventors discovered that by wet-classifying a solution containing particles to separate it into a light liquid containing fine particles and a heavy liquid containing coarse particles, then separating the coarse particles in the heavy liquid using a filter medium, while allowing the fine particles in the light liquid to grow and then returning them to the wet classification process again, it is possible to prevent the pores in the filter medium from becoming clogged and also to increase the yield of the filtrate, which led to the completion of the present invention.

[0010] This application includes the following inventions. [Aspect 1] 1. A method for filtering a solution containing particles, comprising the steps of: (a) wet classifying a solution containing particles to obtain a light liquid containing fine particles and a heavy liquid containing coarse particles; (b) separating the coarse particles from the heavy liquid using a filter; (c) growing the fine particles in the light liquid; (d) returning the solution containing the particles grown in step (c) to step (a); A method comprising: [Aspect 2] 2. The method of claim 1, wherein the particle growth in step (c) is achieved by at least one method selected from the group consisting of aggregation and crystallization. [Aspect 3] 3. The method of claim 1 or 2, wherein the particles in the solution in step (a) comprise a metal oxide. [Aspect 4] The method according to embodiment 3, wherein the metal oxide is a metal oxide partially substituted with an alkyl group. [Aspect 5] The method of claim 4, wherein the metal oxide is an alkylaluminoxane. [Aspect 6] The method according to any one of Aspects 1 to 5, wherein the minimum particle size of the coarse particles in the heavy liquid in the step (a) is 1.2 times or more the pore size of the filter medium. [Aspect 7] The method according to any one of aspects 1 to 6, wherein the coarse particles in the heavy liquid in the step (a) have a minimum particle size of 0.6 μm or more. [Aspect 8] The method according to any one of Aspects 1 to 7, wherein the wet classification in the step (a) is carried out by at least one method selected from the group consisting of centrifugal classification and sedimentation classification. [Aspect 9] The method according to any one of aspects 1 to 8, wherein the separation using the filter medium in the step (b) is carried out by cross-flow filtration. Effect of the Invention

[0011] According to the present invention, in filtering a solution containing particles, a filtrate can be obtained in high yield while maintaining a high filtration rate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] In the present disclosure, the minimum particle size and average particle size of particles refer to the minimum particle size and average particle size determined by a laser diffraction particle size measurement device (temperature: 25° C., particle refractive index: 1.52, measurement solvent: toluene).

[0013] The present invention provides a method for filtering a solution containing particles. In one embodiment, the filtering method comprises the steps of: (a) wet classifying a solution containing particles to obtain a light liquid containing fine particles and a heavy liquid containing coarse particles; (b) separating coarse particles from the heavy liquid using a filter; (c) growing the fine particles in the light liquid; (d) returning the solution containing the particles grown in step (c) to step (a); Includes.

[0014] In this embodiment, in step (a), the solution containing particles is wet classified into a light liquid containing fine particles and a heavy liquid containing coarse particles, and then in step (b), the coarse particles in the heavy liquid are separated using a filter medium. On the other hand, in step (c), the fine particles in the light liquid are grown, and then in step (d), the solution containing the particles grown in step (c) is returned to step (a). In step (b), a heavy liquid with a reduced content of fine particles that may clog the pores of the filter medium is provided for separation using the filter medium, so that the clogging of the pores of the filter medium can be suppressed. As a result, the decrease in the filtration rate can be suppressed, and the throughput of the filtration process can be increased or the filtration device can be made smaller. In addition, by returning the solution containing the particles grown in step (c) to step (a), the yield of the filtrate can be increased while suppressing the clogging of the pores of the filter medium.

[0015] <Process (a)> In step (a), a solution containing particles is wet classified to obtain a light liquid containing fine particles and a heavy liquid containing coarse particles. The solution containing particles is not particularly limited as long as the particles have the property of growing under a specific environment.

[0016] The wet classification is not particularly limited, but is preferably performed by at least one selected from the group consisting of centrifugal classification and sedimentation classification. Centrifugal classification and sedimentation classification are industrially advantageous in terms of high processing speed. Centrifugal classification and sedimentation classification can be performed using a commonly known device. Examples of centrifugal classification devices include a liquid cyclone and a cylindrical centrifuge.

[0017] In one embodiment, the particles in the solution include a metal oxide, such as an oxide including at least one metal selected from the group consisting of aluminum, zinc, magnesium, titanium, and lithium.

[0018] In one embodiment, the metal oxide is partially substituted with alkyl groups, including, for example, alkylaluminoxanes, alkylzinc oxides, alkylmagnesium oxides, and alkyltitanium oxides.

[0019] The solute of the solution is not particularly limited, but may be, for example, an alkylaluminoxane, a trialkylaluminum, an alkylzinc, an alkylmagnesium, an alkyltitanium, and an alkyllithium.

[0020] The solvent of the solution is not particularly limited as long as it dissolves the solute, and examples thereof include aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, ketones, esters, sulfoxides, amides, and water. The solvent may be used alone or in combination of two or more.

[0021] The minimum particle size of the particles in the solution before wet classification is not particularly limited. The minimum particle size of the particles in the solution may be, for example, less than the pore size of the filter medium described later in step (b). When filtering a solution containing particles with a minimum particle size less than the pore size of the filter medium, the pores of the filter medium are easily clogged, and the filtration rate decreases over time. According to the method of the present disclosure, even such a solution can be filtered efficiently with high throughput.

[0022] The concentration of the particles in the solution is not particularly limited, but can be, for example, 0.0001% to 30% by mass, preferably 0.0001% to 10% by mass, and more preferably 0.0001% to 2% by mass.

[0023] By the wet classification, a light liquid containing fine particles and a heavy liquid containing coarse particles are obtained. The average particle size of the fine particles in the light liquid is smaller than the average particle size of the coarse particles in the heavy liquid. The difference between the average particle size of the fine particles in the light liquid and the average particle size of the coarse particles in the heavy liquid is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more.

[0024] The minimum particle size of the coarse particles in the heavy liquid is preferably 1.2 times or more, more preferably 1.5 times or more, and even more preferably 2 times or more, the pore size of the filter medium. By making the minimum particle size of the coarse particles in the heavy liquid 1.2 times or more the pore size of the filter medium, clogging of the pores of the filter medium can be effectively suppressed in step (b).

[0025] The minimum particle size of the coarse particles in the heavy liquid is preferably 0.6 μm or more, more preferably 0.8 μm or more, and even more preferably 1 μm or more. By making the minimum particle size of the coarse particles in the heavy liquid 0.6 μm or more, clogging of the pores of the filter medium can be effectively suppressed in step (b).

[0026] In one embodiment, the solutes of the solution are methylaluminoxane and trimethylaluminum, the solvent of the solution is the solvent used in the production of methylaluminoxane, and the particles contained in the solution are particles of a gel-like substance produced during the production of methylaluminoxane. The gel-like substance includes methylaluminoxane having a high molecular weight or a crosslinked structure that precipitates from the solvent.

[0027] Methylaluminoxane is a condensation product obtained by partial hydrolysis of highly reactive trimethylaluminum, and has a structural unit represented by the formula: [(CH3)AlO]. Methylaluminoxane is generally a mixture of two or more compounds with different molecular weights, and the compound has a linear structure, a cyclic structure, or a combination thereof. The average degree of oligomerization (average degree of polymerization) of methylaluminoxane is generally 60 or less. Methylaluminoxane may also contain a branched structure or a crosslinked structure. It is believed that there is a close relationship between the composition and structure of methylaluminoxane and its stability, but as described above, methylaluminoxane has a very complicated structure, so it is difficult to identify the difference in stability of methylaluminoxane compositions from the viewpoint of the composition and structure of methylaluminoxane.

[0028] Methylaluminoxane can be obtained by reacting trimethylaluminum with water. The reaction is preferably carried out in a solvent. Examples of the solvent include aliphatic and aromatic hydrocarbons that are inactive against trimethylaluminum and methylaluminoxane. Examples of the aliphatic hydrocarbon include saturated hydrocarbon compounds such as n-pentane, n-hexane, n-heptane, n-octane, isooctane, and refined kerosene; and cyclic hydrocarbon compounds such as cyclopentane, cyclohexane, methylcyclohexane, ethylcyclohexane, and cycloheptane. Examples of the aromatic hydrocarbon include benzene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, propylbenzene, and cumene. The solvent is preferably toluene.

[0029] The reaction temperature is generally 0°C to 100°C, preferably 0°C to 40°C.

[0030] The reaction time is generally 0.5 to 100 hours, and preferably 1 to 10 hours.

[0031] The molar ratio of trimethylaluminum to water (moles of trimethylaluminum / moles of water) is preferably 1.0 to 5.0, more preferably 1.2 to 4.0, and further preferably 1.5 to 3.0.

[0032] It is preferable that trimethylaluminum is mixed with the solvent to form a trimethylaluminum solution, and then reacted with water. The trimethylaluminum concentration in the trimethylaluminum solution is preferably 1% by mass to 20% by mass, and more preferably 5% by mass to 15% by mass.

[0033] Water is preferably added in the form of an aqueous dispersion. By using an aqueous dispersion, the addition rate can be easily adjusted. The aqueous dispersion can be prepared by mixing the above-mentioned solvent with water. The medium of the aqueous dispersion is preferably the same as the solvent of the trimethylaluminum solution. The water concentration in the aqueous dispersion is preferably 1% by mass to 20% by mass, more preferably 5% by mass to 15% by mass.

[0034] The reaction between trimethylaluminum and water to produce methylaluminoxane is preferably carried out using a stirring device. The stirring device is not particularly limited, but it is preferable to use a high-speed homogenizer. By using the high-speed homogenizer, trimethylaluminum and water can be reacted quickly. The rotation speed of the high-speed homogenizer can be, for example, 1000 rpm to 20000 rpm.

[0035] The low molecular weight methylaluminoxane produced in the initial stage of the reaction of trimethylaluminum with water is still highly reactive with water, just like trimethylaluminum, so if the reaction is carried out in a state where the entire amount of water is already present in the reaction field, the reaction will proceed irregularly and a gel-like substance will easily be produced. Therefore, it is preferable to gradually add water or an aqueous dispersion to the trimethylaluminum solution. This allows the reaction to be carried out while controlling the amount of water coexisting with trimethylaluminum or low molecular weight methylaluminoxane, and as a result, the production of a gel-like substance can be somewhat suppressed.

[0036] <Step (b)> In step (b), the coarse particles in the heavy liquid are separated using a filter medium. The separation method using a filter medium is not particularly limited, but examples thereof include cross-flow filtration, ultrafiltration, pressure filtration, vacuum filtration, and centrifugal filtration.

[0037] The pore size of the filter medium is not particularly limited, but can be, for example, 0.01 μm to 100 μm, preferably 0.03 μm to 10 μm, and more preferably 0.05 μm to 1 μm.

[0038] The material of the filter medium may include an inorganic material or an organic material. The inorganic material preferably includes a ceramic material such as aluminum oxide, zirconium oxide, or titanium oxide, stainless steel, or glass. The organic material preferably includes polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, cellulose acetate, polyacrylonitrile, polyimide, polysulfone, polyethersulfone, aromatic polyamide, or polyvinyl alcohol.

[0039] When the solvent of the solution containing the particles is an organic solvent such as toluene, it is preferable to use a ceramic filter that is durable against the solvent as the filter material.

[0040] Separation using a filter medium is preferably performed by cross-flow filtration because it is less likely to clog. The filter medium used in cross-flow filtration may be an ultrafiltration membrane or a reverse osmosis membrane. The filter medium used in cross-flow filtration is preferably a ceramic filter.

[0041] The filter medium may be washed using a washing liquid as necessary. By washing, particles clogged in the pores of the filter medium can be removed. Preferably, the filter medium is washed by backwashing in which the washing liquid flows in the opposite direction to the filtration direction of the filter medium. The washing liquid is preferably one that dissolves particles. Examples of the washing liquid include acidic solutions such as water, hydrochloric acid aqueous solution, sulfuric acid aqueous solution, and nitric acid aqueous solution, aromatic hydrocarbons such as benzene, toluene, and xylene, amides such as dimethylformamide and formamide, amines such as N-methylpyrrolidone and 2-pyrrolidone, ethers such as tetrahydrofuran, tetraglyme, and 1,4-dioxane, carbonates such as dimethyl carbonate and diethyl carbonate, and carbamates such as methyl carbamate and ethyl carbamate, and combinations of two or more of these.

[0042] When the particles are particles of a gel-like substance produced during the production of methylaluminoxane, the cleaning liquid is preferably at least one selected from the group consisting of toluene, dimethylformamide, N-methylpyrrolidone, tetrahydrofuran, tetraglyme, and 1,4-dioxane, which has high solubility for the gel-like substance, and more preferably toluene.

[0043] The temperature of the cleaning solution is not particularly limited, but is preferably a temperature at which dissolution of the particles is promoted. The temperature of the cleaning solution can be, for example, from 25°C to the boiling point of the cleaning solution, preferably from 40°C to the boiling point, and more preferably from 40°C to 60°C.

[0044] The filtrate obtained in step (b) may be concentrated as necessary. In an embodiment in which the filtrate contains methylaluminoxane, it is preferable to concentrate the filtrate so that the product concentration can be adjusted in a subsequent step. The concentration equipment is not particularly limited, but is preferably a natural circulation type, calandria type, forced circulation type, or jacket coil type concentration equipment.

[0045] The concentration is preferably carried out under reduced pressure. The concentration pressure is preferably 1 kPaA (absolute pressure) to 50 kPaA, more preferably 2.6 kPaA to 7.5 kPaA.

[0046] The concentration is generally carried out at a temperature range of 0°C (273K) to 110°C (383K). The concentration temperature is preferably 60°C (333K) or lower, more preferably 40°C (313K) or lower. In one embodiment, the concentration temperature is 0°C (273K) to 60°C (333K), preferably 20°C (293K) to 40°C (313K).

[0047] The concentration time is not particularly limited, but is preferably 1 hour to 100 hours, and more preferably 8 hours to 20 hours.

[0048] The concentration may be continuous concentration or batch concentration. In an embodiment in which the filtrate contains methylaluminoxane, it is preferable to carry out batch concentration after continuous concentration. Continuous concentration is concentration performed while maintaining the volume of the solution constant, and concentration is performed while continuously supplying the solution to the concentration tank. Batch concentration can be performed, for example, for the purpose of precisely adjusting the concentration of the methylaluminoxane solution. In an embodiment in which continuous concentration and batch concentration are performed, the concentration time means the total time of continuous concentration and batch concentration.

[0049] <Process (c)> In step (c), the fine particles in the light liquid are grown. The growth of the fine particles may be carried out in a reactor in which the solution in step (a) is produced, or in a curing tank separate from the reactor. The particle growth may be carried out by reacting the fine particles with components in the reactor or components in the light liquid, by agitating or leaving the light liquid to agglomerate the fine particles, or by crystallization due to a change in temperature or concentration of the light liquid.

[0050] For example, in an embodiment in which the light liquid contains methylaluminoxane, the fine particles grow by reacting with methylaluminoxane, water, or residual trimethylaluminum, or two or more of these, in the light liquid, or the fine particles aggregate with each other by stirring to grow into particles.

[0051] The particle growth is preferably performed by at least one selected from the group consisting of aggregation and crystallization. Aggregation and crystallization are advantageous in that they do not involve consumption of the solute in the solution, for example, the target product, methylaluminoxane, and therefore can increase the yield of the target product.

[0052] <Step (d)> In step (d), the solution containing the particles grown in step (c) is returned to step (a). The properties of the solution containing the particles grown in step (c), such as the composition, concentration and average particle size of the particles, the type of solute and solvent, etc., are as described for the solution initially subjected to wet classification in step (a). The properties of the solution containing the particles grown in step (c) may be the same as or different from the properties of the solution initially subjected to wet classification in step (a).

[0053] Steps (a) to (d) can be repeated two or more times. In one embodiment of the filtration method, when the concentration of fine particles in the light liquid obtained after the wet classification in step (a) is equal to or less than a predetermined value, the heavy liquid is separated in step (b) and the light liquid is subjected to a separation treatment using a filter material to obtain a filtrate, thereby completing the filtration method. The light liquid separation treatment may be performed in a separate step from the heavy liquid separation treatment in step (b), or the light liquid may be combined with the heavy liquid again in the same step. That is, the filtration method of this embodiment is completed in the shortest six steps of steps (a) to (d) of the first time, step (a) of the second time, and a heavy liquid and light liquid separation step, or seven steps of steps (a) to (d) of the first time, steps (a) and (b) of the second time, and a light liquid separation step.

[0054] All documents mentioned herein are incorporated by reference in their entirety.

[0055] The following examples of the present invention are for illustrative purposes only and do not limit the technical scope of the present invention. The technical scope of the present invention is limited only by the claims. The present invention can be modified, for example, by adding, deleting, and replacing the constituent elements of the present invention, without departing from the spirit of the present invention. EXAMPLES

[0056] The present invention will be described in more detail below with reference to examples, but these are not intended to limit the present invention in any way.

[0057] [Comparative Example 1] Using the method described in U.S. Patent No. 4,730,072, a toluene solution of methylaluminoxane (25.3 g, trimethylaluminum 6.6 mass%, methylaluminoxane 5.4 mass%, gel concentration 0.26 mass%, gel particle ratio of 1 μm or less 0.62 volume%, minimum particle size 0.45 μm) was produced under a nitrogen atmosphere. The resulting solution was filtered under reduced pressure at 30 kPaA using a membrane filter (PTFE, pore size 0.5 μm, Φ47 mm), and the reduced pressure filtration was completed in 66 seconds, yielding 24.0 g of filtrate (trimethylaluminum 6.6 mass%, methylaluminoxane 5.4 mass%). The processing speed per unit area was 0.937 m 3 / (m 2 ·hr).

[0058] A pretreatment was carried out to evaluate the effect of filtration time during the vacuum filtration operation on the gelation of methylaluminoxane. 2 , processing volume 45.0m 3 Based on this calculation, the filtrate obtained by performing vacuum filtration multiple times as pretreatment (total of 140 g) was sealed in a glass bottle under a nitrogen atmosphere and left to stand at 25°C for 48 hours to give it a thermal history.

[0059] The filtrate that had been subjected to the above-mentioned thermal history was charged in a 100 mL separable flask at 23.0 g (about 26 mL) and in a dropping funnel at 91.0 g (about 101 mL). Next, the filtrate was dropped while successively adjusting the amount of feed from the dropping funnel so that the solution volume in the separable flask was maintained at about 23 mL, and continuous concentration was carried out for 9 hours at a vacuum degree of 7.5 kPaA and an internal temperature of 40°C (313K). Thereafter, batch concentration was carried out for 0.6 hours at the same vacuum degree and internal temperature as in the continuous concentration while adding toluene from time to time so that the methylaluminoxane concentration did not exceed 35 mass%, to obtain 17.5 g of a methylaluminoxane concentrated solution (methylaluminoxane composition). The composition of the methylaluminoxane concentrated solution was as follows: 1 The H-NMR analysis revealed that the components were trimethylaluminum (1.37 g, 0.0190 mol), methylaluminoxane (6.06 g), and toluene (10.1 g, 0.109 mol). The resulting methylaluminoxane composition was sealed in a glass container under a nitrogen atmosphere and stored at -20°C, and precipitation of a gel was confirmed 370 days after storage.

[0060] [Example 1] The toluene solution of methylaluminoxane of Comparative Example 1 was wet classified at a feed pressure of 0.4 MPaG using a liquid cyclone (manufactured by Nippon Chemical Machinery Co., Ltd., feed section inner diameter: 2.3 mm, downstream section inner diameter: 1.5 mm, upstream section inner diameter: 1.75 mm, body diameter: 9.5 mm). The heavy liquid obtained from the downstream section (25.3 g, gel concentration 0.42 mass%, gel particle ratio of 1 μm or less particle size 0.29 vol%, minimum particle size 0.63 μm) was filtered under reduced pressure in the same manner as in Comparative Example 1. The vacuum filtration was completed in 43 seconds, and 24.3 g of filtrate (6.6 mass% trimethylaluminum, 5.4 mass% methylaluminoxane) was obtained. The processing speed per unit area was 1.44 m 3 / (m 2 ·hr). Example 1 corresponds to step (a) and step (b). The gel concentration of the heavy liquid (0.42 mass%) is higher than that of the toluene solution of methylaluminoxane in Comparative Example 1 (0.26 mass%), but the reduced pressure filtration was completed in a shorter time.

[0061] Next, the above-mentioned filtrate (total of 140 g) obtained by carrying out the reduced pressure filtration several times was subjected to a heat history in the same manner as in Comparative Example 1, except that the pretreatment standing time was set to 31 hours.

[0062] The filtrate that had been subjected to the above-mentioned thermal history was charged in a 100 mL separable flask at 23.0 g (about 26 mL) and in a dropping funnel at 91.0 g (about 101 mL). Next, the filtrate was dropped while successively adjusting the amount of feed from the dropping funnel so that the solution volume in the separable flask was maintained at about 23 mL, and continuous concentration was carried out for 9 hours at a vacuum degree of 7.5 kPaA and an internal temperature of 40°C (313K). Thereafter, batch concentration was carried out for 0.6 hours at the same vacuum degree and internal temperature as in the continuous concentration while adding toluene from time to time so that the methylaluminoxane concentration did not exceed 35 mass%, to obtain 17.0 g of a methylaluminoxane concentrated solution (methylaluminoxane composition). The composition of the methylaluminoxane concentrated solution was as follows: 1 The H-NMR analysis revealed that the components were trimethylaluminum (1.30 g, 0.0180 mol), methylaluminoxane (5.75 g), and toluene (9.95 g, 0.108 mol). The resulting methylaluminoxane composition was sealed in a glass container under a nitrogen atmosphere and stored at -20°C, and precipitation of a gel was confirmed 460 days after storage.

[0063] [Example 2] The heavy liquid of Example 1 was diluted with the toluene solution of methylaluminoxane so as to have a gel concentration equivalent to that of the toluene solution of methylaluminoxane of Comparative Example 1. The resulting diluted liquid (25.2 g, gel concentration 0.27 mass %, gel particle ratio of 1 μm or less particle size 0.29 vol %, minimum particle size 0.63 μm) was filtered under reduced pressure in the same manner as in Comparative Example 1, and the vacuum filtration was completed in 20 seconds, yielding 24.5 g of filtrate (6.6 mass % trimethylaluminum, 5.4 mass % methylaluminoxane). The processing speed per unit area was 3.09 m 3 / (m 2 ·hr). Example 2 corresponds to steps (a) and (b). The gel concentration of the diluted solution (0.27% by mass) was equivalent to that of the toluene solution of methylaluminoxane in Comparative Example 1 (0.26% by mass), and the reduced pressure filtration was completed in ⅓ or less of the time.

[0064] Next, the above-mentioned filtrate (total of 140 g) obtained by carrying out the reduced pressure filtration several times was subjected to a heat history in the same manner as in Comparative Example 1, except that the pretreatment standing time was changed to 15 hours.

[0065] The filtrate that had been subjected to the above-mentioned thermal history was charged in a 100 mL separable flask at 23.0 g (about 26 mL) and in a dropping funnel at 91.0 g (about 101 mL). Next, the filtrate was dropped while successively adjusting the amount of feed from the dropping funnel so that the solution volume in the separable flask was maintained at about 23 mL, and continuous concentration was carried out for 9 hours at a vacuum degree of 7.5 kPaA and an internal temperature of 40°C (313K). Thereafter, batch concentration was carried out for 0.6 hours at the same vacuum degree and internal temperature as in the continuous concentration while adding toluene from time to time so that the methylaluminoxane concentration did not exceed 35 mass%, thereby obtaining 18.3 g of a methylaluminoxane concentrated solution (methylaluminoxane composition). The composition of the methylaluminoxane concentrated solution was as follows: 1 The H-NMR analysis revealed that the components were trimethylaluminum (1.42 g, 0.0197 mol), methylaluminoxane (6.30 g), and toluene (10.6 g, 0.115 mol). The resulting methylaluminoxane composition was sealed in a glass container under a nitrogen atmosphere and stored at -20°C, and precipitation of a gel was confirmed 500 days after storage.

[0066] [Example 3] When the toluene solution of methylaluminoxane of Comparative Example 1 was stirred at 25°C for 6 hours, the proportion of gel particles with a particle size of 1 μm or less decreased from 0.62 vol% to 0.30 vol% due to particle growth. The resulting solution (25.2 g, gel concentration 0.26 mass%, minimum particle size 0.63 μm) was filtered under reduced pressure in the same manner as in Comparative Example 1, and the vacuum filtration was completed in 22 seconds, yielding 24.3 g of filtrate (6.6 mass% trimethylaluminum, 5.4 mass% methylaluminoxane). The processing speed per unit area was 2.81 m 3 / (m 2·hr). Example 3 is to confirm the effect of particle growth in step (c) on the filtration efficiency in step (b). In Example 3, the solution containing the grown particles was returned to step (a) in step (d) and vacuum filtration was directly performed in step (b) without wet classification. However, if the solution containing the grown particles in Example 3 was subjected to wet classification in step (a) and then vacuum filtration in step (b), it is clear from a comparison between Examples 1 and 2 and Comparative Example 1 that the time to complete vacuum filtration would be 22 seconds or less.

[0067] Next, the above-mentioned filtrate (total of 140 g) obtained by carrying out the reduced pressure filtration several times was subjected to a heat history in the same manner as in Comparative Example 1, except that the pretreatment standing time was changed to 16 hours.

[0068] The filtrate that had been subjected to the above-mentioned thermal history was charged in a 100 mL separable flask at 23.0 g (about 26 mL) and in a dropping funnel at 91.0 g (about 101 mL). Next, the filtrate was dropped while successively adjusting the amount of feed from the dropping funnel so that the solution volume in the separable flask was maintained at about 23 mL, and continuous concentration was carried out for 9 hours at a vacuum degree of 7.5 kPaA and an internal temperature of 40°C (313K). Thereafter, batch concentration was carried out for 0.6 hours at the same vacuum degree and internal temperature as in the continuous concentration while adding toluene from time to time so that the methylaluminoxane concentration did not exceed 35 mass%, to obtain 17.8 g of a methylaluminoxane concentrated solution (methylaluminoxane composition). The composition of the methylaluminoxane concentrated solution was as follows: 1 The H-NMR analysis revealed that the components were trimethylaluminum (1.40 g, 0.0194 mol), methylaluminoxane (6.18 g), and toluene (10.2 g, 0.111 mol). The resulting methylaluminoxane composition was sealed in a glass container under a nitrogen atmosphere and stored at -20°C, and precipitation of gel was confirmed 500 days after storage.

[0069] Although the embodiments of the present invention have been described above, the present invention is not limited to these, and the devices, materials, various conditions, etc. used can be appropriately changed without departing from the spirit of the invention.

Claims

1. 1. A method for filtering a solution containing particles, comprising the steps of: (a) wet classifying a solution containing particles to obtain a light liquid containing fine particles and a heavy liquid containing coarse particles; (b) separating the coarse particles from the heavy liquid using a filter; (c) growing the fine particles in the light liquid; (d) returning the solution containing the particles grown in step (c) to step (a); A method comprising:

2. The method according to claim 1 , wherein the particle growth in step (c) is achieved by at least one selected from the group consisting of aggregation and crystallization.

3. 3. The method of claim 1 or 2, wherein the particles in the solution of step (a) comprise a metal oxide.

4. The method according to claim 3, wherein the metal oxide is a metal oxide partially substituted with an alkyl group.

5. The method of claim 4, wherein the metal oxide is an alkylaluminoxane.

6. 3. The method according to claim 1, wherein the minimum particle size of the coarse particles in the heavy liquid in step (a) is 1.2 times or more the pore size of the filter medium.

7. 3. The method according to claim 1, wherein the minimum particle size of the coarse particles in the heavy liquid in step (a) is 0.6 μm or more.

8. The method according to claim 1 or 2, wherein the wet classification in the step (a) is carried out by at least one method selected from the group consisting of centrifugal classification and sedimentation classification.

9. 3. The method according to claim 1, wherein the separation using the filter medium in step (b) is carried out by cross-flow filtration.