Method for producing titanium dioxide sol using solvent recycling method and titanium dioxide sol

Abstract

To provide a method for producing titanium oxide sol with low environmental load and titanium oxide sol with high storage stability.SOLUTION: A method for producing a titanium oxide sol comprises a water sol preparation step of preparing a water sol containing water and titanium oxide particles, a surface treatment step of surface-treating the titanium oxide particles by mixing a surface treatment solution obtained by mixing a polar solvent and an organosilicon compound with the water sol to obtain a surface-treated sol, a solvent replacement step of replacing the water and the polar solvent contained in the surface-treated sol with another solvent to obtain a titanium oxide sol and a recovery solvent, and a purification step of purifying the recovery solvent to prepare a purified solvent. The purified solvent is reused for the surface treatment step or the solvent replacement step.SELECTED DRAWING: Figure 1

Description

Technical Field

[0005]

[0001] The present invention relates to a method for producing a titanium oxide sol using a solvent recycling method and the titanium oxide sol.

Background Art

[0002] Titanium oxide is used in various applications, such as pigments, ultraviolet ray blocking agents, catalysts, photocatalysts, catalyst carriers, adsorbents, ion exchangers, fillers, reinforcing agents, raw materials for ceramics, precursors of composite oxides such as perovskite-type composite oxides, and undercoat agents for magnetic tapes. It is used in various forms such as powders, pastes, sols, etc. according to these applications.

[0003] Patent Documents 1 to 3 disclose a titanium oxide sol and a method for producing the same. It is also disclosed that these production methods may include a surface treatment step of surface-treating titanium oxide particles with an organosilicon compound (also referred to as surface modification) to enhance the dispersibility in a solvent, and a solvent substitution step of substituting the solvent contained in the titanium oxide sol with another solvent. However, in the method for producing a titanium oxide sol including these steps, a large amount of waste liquid is generated, so the environmental load is high, and an increase in cost associated with waste disposal has been regarded as a problem.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to obtain a titanium oxide sol having high storage stability using a production method with a low environmental load. [Means for solving the problem]

[0006] The inventors diligently studied and conducted research to solve the above problems, and have completed the present invention. The present invention is as follows (1) to (13). (1) A water sol preparation step of preparing an aqueous sol containing water and titanium dioxide particles, A surface treatment step to obtain a surface treatment sol by mixing a surface treatment solution obtained by mixing a polar solvent and an organosilicon compound with the aqueous sol, and surface treating the titanium oxide particles with the aqueous sol. A solvent replacement step is performed by replacing the water and polar solvent contained in the surface treatment sol with another solvent to obtain a titanium oxide sol and a recovered solvent. The process includes a purification step of purifying the recovered solvent to prepare a purified solvent, The purified solvent is reused in the surface treatment step or the solvent replacement step. A method for producing titanium dioxide sol. (2) The method for producing titanium oxide sol according to (1) above, wherein the polar solvent is a polar organic solvent. (3) A method for producing titanium dioxide sol according to (1) or (2) above, wherein the recovered solvent contains water, Si, Ti, and a polar organic solvent, the water content is 10% by mass or more, the Si content is 1000 ppm or more, and the Ti content is 20 ppm or more. (4) The manufacturing method according to any one of (1) to (3) above, wherein the recovered solvent is purified to recover a polar organic solvent having a water content in the range of 1 to 5% by mass as the purified solvent. (5) The manufacturing method according to any one of (1) to (3) above, wherein the recovered solvent is purified to recover a polar organic solvent having a Si content of 50 ppm or less as the purified solvent. (6) The manufacturing method according to any one of (1) to (3) above, wherein the recovered solvent is purified to recover a polar organic solvent having a Ti content of 5 ppm or less as the purified solvent. (7) The manufacturing method according to any one of (1) to (6) above, wherein the polar organic solvent contained in the recovered solvent is evaporated under reduced pressure in the system during the purification step. (8) The manufacturing method according to any one of (1) to (7) above, wherein the recovered solvent is distilled at a temperature below the boiling point of the polar organic solvent and within -20°C. (9) A sol in which titanium dioxide particles are dispersed in a solvent, The total amount of impurity elements dissolved in the aforementioned solvent is 100 ppm or less in terms of atomic weight. The aforementioned impurity element is at least one element selected from the group consisting of Ti, Si, Na, K, Ca, Co, Al, Cr, Cu, Fe, Ni, Pb, Zr, and Zn. Titanium dioxide sol. (10) The titanium dioxide sol according to (9) above, wherein the solvent is a polar organic solvent. (11) The titanium oxide sol according to (9) or (10) above, wherein the polar solvent is an alcohol. (12) The titanium dioxide sol according to any one of (9) to (11) above, wherein the content of organosilicon compounds in the solvent is 20 ppm or less. (13) The titanium dioxide sol according to any of (9) to (12) above, wherein the water content in the solvent is in the range of 0.2% by mass to 8% by mass. [Effects of the Invention]

[0007] According to the present invention, a method for producing titanium dioxide sol with low environmental impact and a titanium dioxide sol with high storage stability are provided. [Brief explanation of the drawing]

[0008] [Figure 1] An example of a production flow for titanium oxide sol using the manufacturing method of the present invention. [Figure 2] An example of a process flow for purifying the recovered solvent. [Modes for carrying out the invention]

[0009] The present invention relates to a method for producing titanium dioxide sol, which includes a method for recovering waste liquid generated in a solvent replacement step and reusing it in the titanium dioxide sol production step. Specifically, it includes a method for producing titanium dioxide sol (hereinafter also referred to as "the method of production of the present invention") which includes: a water sol preparation step for preparing a water sol containing water and titanium dioxide particles; a surface treatment step for surface-treating the titanium dioxide particles by mixing a surface treatment solution obtained by mixing a polar solvent and an organosilicon compound with the water sol to obtain a surface treatment sol; a solvent replacement step for replacing the water and the polar solvent contained in the surface treatment sol with another solvent to obtain titanium dioxide sol and a recovered solvent; and a purification step for purifying the recovered solvent to prepare a purified solvent, wherein the purified solvent is reused in the surface treatment step or the solvent replacement step. An example of the production flow of titanium dioxide sol using the method of production of the present invention is shown in Figure 1. The method of production of the present invention will be described in detail below.

[0010] [Manufacturing method of the present invention] [Aqueous sol preparation process] The manufacturing method of the present invention includes a step of preparing an aqueous sol containing water and titanium dioxide particles. In the present invention, "aqueous sol" refers to a sol in which particles are dispersed in water. In this step, the aqueous sol can be prepared using conventionally known methods. For example, the aqueous sol can be prepared by hydrothermating a mixture containing titanium peroxide obtained by hydrolyzing titanium chloride in an aqueous solution at a temperature of 80 to 250°C in an autoclave, by hydrolyzing titanium alkoxide, or by dispersing titanium dioxide powder in water. Alternatively, if a commercially available titanium dioxide sol is purchased and the solvent contained in the sol is water, it can be used as is, or if the solvent is a solvent other than water, it can be replaced with water before use. Furthermore, the aqueous sol can also be prepared by referring to the manufacturing methods described in Patent Documents 1 to 3.

[0011] [Surface treatment process] The manufacturing method of the present invention includes a surface treatment step of mixing a surface treatment solution obtained by mixing a polar solvent and an organosilicon compound with the aqueous sol to surface-treat the titanium oxide particles and obtaining a surface-treated sol. In this step, the titanium oxide particles can be surface-treated with an organosilicon compound using a conventionally known method. For example, after mixing a surface treatment solution in which the organosilicon compound is dissolved in a polar solvent such as water or alcohol with the aqueous sol, it can be surface-treated by heating to a temperature of 40 to 60 °C and stirring for about 1 to 20 hours. In this method, the organosilicon compound is hydrolyzed by water, and the product generated by hydrolysis binds to the surface of the titanium oxide particles to cause surface treatment. At this time, by-products generated by the hydrolysis of the organosilicon compound and unreacted organosilicon compounds remain in the solvent of the surface-treated sol.

[0012] In this step, as the polar solvent, alcohols such as methanol, ethanol, butanol, propanol, isopropyl alcohol, and general polar solvents such as water can be used, and it is preferable to use alcohols. When a polar organic solvent such as alcohol is used, the titanium oxide particles after surface treatment are likely to maintain a dispersed state in the surface-treated sol. Since such an organic solvent has a high environmental load when discarded as it is, it is necessary to perform some treatment and then discard it, which is one of the factors contributing to cost increase. However, in the manufacturing method of the present invention, even if a polar organic solvent is used in this step, it is purified and reused in the purification step described later, so the effects of reducing the environmental load and cost are more显著地表现出来。

[0013] It should be noted that there is an incorrect expression in the original text "显著地表现出来", which should be "more significantly manifested". The corrected translation is as follows: The manufacturing method of the present invention includes a surface treatment step of mixing a surface treatment solution obtained by mixing a polar solvent and an organosilicon compound with the aqueous sol to surface-treat the titanium oxide particles and obtaining a surface-treated sol. In this step, the titanium oxide particles can be surface-treated with an organosilicon compound using a conventionally known method. For example, after mixing a surface treatment solution in which the organosilicon compound is dissolved in a polar solvent such as water or alcohol with the aqueous sol, it can be surface-treated by heating to a temperature of 40 to 60 °C and stirring for about 1 to 20 hours. In this method, the organosilicon compound is hydrolyzed by water, and the product generated by hydrolysis binds to the surface of the titanium oxide particles to cause surface treatment. At this time, by-products generated by the hydrolysis of the organosilicon compound and unreacted organosilicon compounds remain in the solvent of the surface-treated sol.

[0012] In this step, as the polar solvent, alcohols such as methanol, ethanol, butanol, propanol, isopropyl alcohol, and general polar solvents such as water can be used, and it is preferable to use alcohols. When a polar organic solvent such as alcohol is used, the titanium oxide particles after surface treatment are likely to maintain a dispersed state in the surface-treated sol. Since such an organic solvent has a high environmental load when discarded as it is, it is necessary to perform some treatment and then discard it, which is one of the factors contributing to cost increase. However, in the manufacturing method of the present invention, even if a polar organic solvent is used in this step, it is purified and reused in the purification step described later, so the effects of reducing the environmental load and cost are more significantly manifested.

[0013] In this step, as the organosilicon compound, monofunctional silanes such as trimethylethoxysilane, dimethylphenylethoxysilane, dimethylvinylethoxysilane, difunctional silanes such as dimethyldiethoxysilane, diphenyldiethoxysilane, trifunctional silanes such as methyltriethoxysilane, phenyltriethoxysilane, and tetrafunctional silanes such as tetraethoxysilane can be used, and it is preferable to use trifunctional silane or tetrafunctional silane. By surface-treating titanium oxide particles with these organosilicon compounds, the titanium oxide particles are more likely to disperse in an organic solvent.

[0014] 〔Solvent replacement step〕 The production method of the present invention includes a solvent replacement step of replacing the water and the polar solvent contained in the surface-treated sol with another solvent to obtain a titanium oxide sol and a recovered solvent. In this step, one of the purposes is to replace the solvent of the surface-treated sol with another solvent to remove by-products generated by hydrolysis of the organosilicon compound contained in the solvent of the surface-treated sol and unreacted organosilicon compounds. Therefore, the "another solvent" in this step may be of the same type as the solvent contained in the surface-treated sol. For example, titanium oxide sol may be obtained using water as another solvent, or titanium oxide sol may be obtained using a polar organic solvent. In this step, the water and the polar solvent contained in the surface-treated sol can be replaced with another solvent using a conventionally known method. For example, a method of removing a certain amount of the solvent contained in the surface-treated sol using a known method such as an ultrafiltration method or an evaporator and then adding another solvent can be used. The solvent removed at this time is recovered as a recovered solvent and purified in the purification step described later.

[0015] 〔Purification step〕 The manufacturing method of the present invention includes a purification step of purifying the recovered solvent to prepare a purified solvent. In this step, the solvent and impurities contained in the recovered solvent are removed. The solvent contained in the recovered solvent may include water, polar solvents, polar organic solvents, etc. The impurities contained in the recovered solvent refer to components other than the solvent, such as unreacted organosilicon compounds, by-products produced by the hydrolysis of organosilicon compounds, and titanium oxide particles that have not been filtered out. These impurities can be removed by conventionally known methods. For example, a purified solvent can be obtained by evaporating the recovered solvent under certain conditions and recovering the evaporated solvent. Figure 2 shows an example of a process flow for purifying the recovered solvent using an evaporator. The recovered solvent can be purified by methods other than evaporators, such as distillation or evaporators. The content of solvent and impurities contained in the recovered solvent is not particularly limited.

[0016] When purifying a recovered solvent that contains water, Si, Ti, and a polar organic solvent, and in which the water content is 10% by mass or more, it is preferable to recover a polar organic solvent with a water content in the range of 1 to 5% by mass as the purification solvent. Recovering and reusing a polar organic solvent with a water content in the aforementioned range as the purification solvent improves the stability of the titanium dioxide sol.

[0017] When purifying a recovered solvent containing water, Si, Ti, and a polar organic solvent, and having a Si content of 1000 ppm or more, it is preferable to recover a polar organic solvent with a Si content of 50 ppm or less as the purification solvent. Recovering and reusing a polar organic solvent with a Si content within the aforementioned range as the purification solvent improves the stability of the titanium dioxide sol.

[0018] When purifying a recovered solvent containing water, Si, Ti, and a polar organic solvent, and having a Ti content of 50 ppm or more, it is preferable to recover a polar organic solvent with a Ti content of 5 ppm or less as the purification solvent. Recovering and reusing a polar organic solvent with a Ti content within the aforementioned range as the purification solvent improves the stability of the titanium dioxide sol.

[0019] In this process, it is preferable to evaporate the polar organic solvent contained in the recovered solvent under reduced pressure within the system. By reducing the pressure within the system, the thermal energy required for evaporation is saved, allowing for purification. Furthermore, by adjusting the temperature of the recovered solvent within the system to be below the boiling point of the polar organic solvent and within -20°C from the boiling point, the content of the aforementioned impurities in the purified polar organic solvent can be reduced. In addition, by adjusting the vapor temperature to 15°C or lower when recovering the evaporated polar organic solvent, the water content in the recovered polar organic solvent can be reduced. Moreover, by lowering the vapor temperature, the polar organic solvent can be recovered efficiently without being discharged outside the system.

[0020] The manufacturing method of the present invention reuses the purified solvent in the surface treatment step or the solvent replacement step. In this case, the step may be part of a step in another flow-type (continuous) manufacturing method, or part of a step in a separate batch manufacturing method as shown in Figure 1. Furthermore, the entire amount of the purified solvent may be reused in the step, or a portion of it may be mixed with a commercially available solvent and reused. In the manufacturing method of the present invention, it is preferable that the amount of purified solvent used is at least 50% by mass or more of the total amount of solvent used as the polar solvent in the surface treatment step or as the other solvent in the solvent replacement step.

[0021] The present invention relates to a titanium dioxide sol in which titanium dioxide particles are dispersed in a solvent, and includes a high-purity titanium dioxide sol with few impurities. Specifically, it includes a titanium dioxide sol (hereinafter also referred to as "the titanium dioxide sol of the present invention") in which the total amount of impurity elements dissolved in the solvent is 100 ppm or less in terms of atomic weight, and the impurity elements are at least one element selected from the group consisting of Ti, Si, Na, K, Ca, Co, Al, Cr, Cu, Fe, Ni, Pb, Zr, and Zn. The titanium dioxide sol of the present invention has high storage stability by keeping the content of the impurity elements specified by the above elements below a certain amount. Such a titanium dioxide sol can be prepared, for example, using the manufacturing method of the present invention. Such a titanium dioxide sol can be used in a wide range of applications such as optical applications, polishing applications, and catalytic applications. The titanium dioxide sol of the present invention will be described in detail below.

[0022] The total amount of impurities is preferably 50 ppm or less, more preferably 30 ppm or less, even more preferably 20 ppm or less, and particularly preferably 10 ppm or less. The titanium dioxide sol of the present invention, in which the total amount of impurities is within the above range, has higher storage stability.

[0023] The solvent is preferably a polar organic solvent, more preferably a polar organic solvent with a solubility parameter in the range of 10 to 22, and particularly preferably an alcohol. When the solvent is a polar organic solvent, the water content is preferably in the range of 0.2% to 8% by mass, more preferably in the range of 0.2% to 6% by mass, and particularly preferably in the range of 0.2% to 1% by mass. The titanium dioxide sol of the present invention with a water content in the above range has higher storage stability.

[0024] The titanium dioxide particles have at least one crystal structure selected from anatase, rutile, and brookite types. The titanium dioxide sol of the present invention preferably has either anatase or rutile crystal structures.

[0025] The average particle size of the titanium oxide particles is preferably in the range of 5 nm to 100 nm, more preferably in the range of 5 nm to 50 nm, and particularly preferably in the range of 5 nm to 30 nm. Titanium oxide particles with an average particle size within the above range can be suitably used in the above-mentioned applications.

[0026] The titanium oxide particles may contain elements other than Ti and O. For example, they may contain elements such as Si, Al, Fe, and Sn in a total amount ranging from 1% to 20% by mass. Titanium oxide particles containing these elements tend to have lower photocatalytic activity and are suitable for use in mixtures with resins and the like.

[0027] The titanium dioxide particles contained in the titanium dioxide sol of the present invention may have a coating layer on their surface. For example, they may be coated with silica, zirconia, or alumina. Titanium dioxide particles having a coating layer disperse well in the solvent. Furthermore, since titanium dioxide particles having a coating layer are less susceptible to photocatalytic reactions, they are suitable for use in combination with materials that degrade in photocatalytic reactions, such as resins.

[0028] The titanium dioxide particles contained in the titanium dioxide sol of the present invention are preferably surface-treated with an organosilicon compound. [Examples]

[0029] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples. [Measurement method or evaluation method] Various measurements and evaluations were performed as follows:

[0030] [1] Compositional analysis (Ti, Si, Na, K, Ca, Co, Al, Cr, Cu, Fe, Ni, Pb, Zr, and Zn) The concentrations of Ti, Si, Na, K, Ca, Co, Al, Cr, Cu, Fe, Ni, Pb, Zr, and Zn in the solvent, recovered solvent, or purified solvent contained in the titanium dioxide sol were measured in atomic weight units using inductively coupled plasma (ICP) emission spectroscopy. The solvent in the titanium dioxide sol was filtered using ultrafilm filtration before being used as the measurement sample. For the particle composition analysis of the titanium dioxide sol, the remaining solids after evaporation were dissolved using an acid or similar substance before being used as the measurement sample. Values ​​below the detection limit (5 ppm or less in this invention) were considered as 0 ppm.

[0031] [2] Measurement of H2O contained in the solvent The concentration of H2O in the solvent, recovered solvent, or purified solvent contained in the titanium dioxide sol was measured using a Karl Fischer method moisture analyzer (Mitsubishi Chemical Analytec Co., Ltd., CA-200).

[0032] [3] Crystal structure analysis Titanium oxide sol was used as the sample for measurement. 2 g of solid content was collected in a porcelain crucible (Type B-2), dried at 110°C for 12 hours, and the residue was cooled to room temperature in a desiccator. Next, the residue was ground in a mortar, and powder X-ray diffraction was measured using a SmartLab X-ray diffractometer (manufactured by Rigaku Corporation). The measurement conditions and details of the data analysis are as follows. • Measurement conditions Measurement device: Powder X-ray diffraction analyzer SmartLab (manufactured by Rigaku Corporation) X-ray generator: 9kW open tube (CuKα source, voltage 45kV, current 200mA) Soller / PSC:5.0deg IS length: 10.0mm PSA: None Soller: 5.0deg IS:1 / 2 RS1: 13mm RS2: 20mm Scan step: 0.02deg Scan range: 5-70deg Scan speed: 5deg / min X-ray detector: High-speed one-dimensional X-ray detector (D / TeX Ultra 250) Measurement atmosphere: Under atmospheric pressure Sample stage: Al2O3 sample holder (bottomless) • Data analysis Analysis software: Integrated powder X-ray diffraction analysis software PDXL2 Version 2.7.2.0 (manufactured by Rigaku Corporation) Smoothing: Smoothing using B-Splne (X threshold 1.5) Background removal: Fitting method Kα2 removal: intensity ratio 0.497 Peak search: Second derivative method, σ-cut value = 3, σ-cut range 0.5~20.0 Profile fitting method: Fitting to measurement data Profile fitting peak shape: Split pseudo-Voigt function

[0033] [4] Average particle size measurement Titanium dioxide sol was used as the measurement sample, and the shape of the particles contained in the measurement sample was observed using a scanning electron microscope (SEM) (Hitachi High-Technologies Corporation, S-5500) at an accelerating voltage of 30 kV. The observation sample was prepared as follows: The measurement sample was diluted with water to a solid content concentration of 0.05 mass%, then coated onto a collodion film-coated metal grid (Oken Shoji Co., Ltd.), and the solvent was evaporated by irradiating it with a 250 W infrared lamp for 30 minutes to prepare the observation sample. The obtained SEM image was printed, and the particle diameter of 100 primary particles was measured with calipers, and the average value was taken as the average particle diameter. In the case of anisotropic particle shape, the longest axis was taken as the particle diameter.

[0034] [5]pH measurement Titanium dioxide sol was used as the measurement sample. 50 ml of this sol was placed in a cell, and the glass electrode of a pH meter (Horiba, F22), which had been calibrated with pH 4, 7, and 9 standard solutions, was inserted into the cell in a constant temperature bath maintained at 25°C, and the pH value was measured. In cases where the solvent of the titanium dioxide sol was an organic solvent, the titanium dioxide sol was diluted 10-fold with distilled water and used as the measurement sample.

[0035] [6] Viscosity measurement 20 ml of titanium dioxide sol was used as the sample, and viscosity was measured at room temperature using a viscometer (TV-10M, manufactured by Toki Sangyo Co., Ltd.). The viscometer rotor was set to measure viscosity in the following ranges: 1.0 to 10.0 mPa·s at 60 rpm, 10.0 to 20.0 mPa·s at 30 rpm, 20.0 to 50.0 mPa·s at 12 rpm, and 50.0 to 100.0 mPa·s at 6 rpm.

[0036] [7] Stability evaluation 50 ml of titanium dioxide sol was used as the measurement sample, and the sample was stored in a sealed container in a cool, dark place with an ambient temperature of 10°C. The stability of the titanium dioxide sol was evaluated by the number of days required for the viscosity to increase to 1.5 times or more compared to the viscosity on the first day.

[0037] [Preparation Example 1] [Water sol preparation process] 93.665 kg of an aqueous titanium tetrachloride solution containing 2% by mass of titanium tetrachloride (manufactured by Osaka Titanium Technologies Co., Ltd.) on a TiO2 basis, and 0.218 kg of an aqueous ferric chloride solution containing 4% by mass of ferric chloride (manufactured by Hayashi Pure Chemical Industries, Ltd.) on a Fe2O3 basis, were mixed. This mixture was then mixed with 36.295 kg of aqueous ammonia (manufactured by Ube Industries, Ltd.) containing 15% by mass of ammonia to prepare a slightly yellowish-brown slurry with a pH of 8.5. Next, this slurry was filtered, and the filtrate was washed with pure water to obtain 72.7 kg of iron-containing hydrated titanate cake containing iron with a solid content of 10% by mass.

[0038] Next, 1.51 kg of hydrogen peroxide solution containing 35% by mass of hydrogen peroxide (manufactured by Mitsubishi Gas Chemical Co., Ltd.) and 7.632 kg of pure water were added to this cake. The mixture was then stirred at 80°C for 1 hour, and then 159 kg of pure water was added to obtain 11.05 kg of an aqueous solution of iron-containing titanic acid peroxide, containing 2% by mass of titanium and iron, respectively, based on the amounts of titanium and iron contained therein converted to TiO2 and Fe2O3. This aqueous solution of iron-containing titanic acid peroxide was transparent yellowish-brown and had a pH of 8.5.

[0039] Next, 22.5 kg of the iron-containing titanium peroxide aqueous solution was mixed with 0.19 kg of silica sol (manufactured by JGC Catalysts & Chemicals Co., Ltd., SN-350) and 2.747 kg of pure water, and heated in an autoclave at 150°C for 6 hours. After the autoclave was cooled to room temperature, the mixture was concentrated using an ultrafiltration membrane apparatus to obtain 2.153 kg of aqueous sol with a solid content of 10% by mass.

[0040] [Surface treatment process] A surface treatment solution was prepared by dissolving 3.00 kg of commercially available methanol as a polar solvent and 0.13 kg of tetraethoxysilane (manufactured by Tama Chemical Industry Co., Ltd.) as an organosilicon compound. To this solution, 1.50 kg of the aqueous sol obtained in the previous step was added under stirring, and the mixture was heated at 50°C for 6 hours to prepare 4.50 kg of surface treatment sol.

[0041] [Solvent replacement process] Using an ultrafiltration membrane apparatus, the solvent in 4.50 kg of the surface treatment sol obtained in the previous step was replaced with another solvent (the methanol mentioned above). The resulting filtrate was recovered as the recovered solvent. Furthermore, the surface treatment sol was concentrated to obtain 0.50 kg of titanium dioxide sol with a solid content of 30% by mass and the recovered solvent. The total amount of methanol used in this step was 18.00 kg. The properties of the recovered solvent are shown in Table 1.

[0042] [Refining process] The recovered solvent obtained in the aforementioned solvent replacement process was subjected to simple distillation in the purification apparatus shown in Figure 2 to obtain the purified solvent. Specifically, after filling the evaporator with the recovered solvent, a valve was closed to form a circulation line. Then, the circulation pump was started, and the recovered solvent circulated in the order of circulation pump → heat exchanger → evaporator → circulation pump. Next, the vacuum pump was started, and the vacuum in the evaporator was reduced to -0.065 MPa. Then, the heat exchanger was heated to 70°C and distillation was started. Subsequently, the temperature of the recovered solvent in the evaporator rose to 57°C, and evaporation of methanol contained in the recovered solvent began. The methanol-containing vapor generated at this time was cooled in a condenser to obtain the purified solvent. The properties of the purified solvent are shown in Table 1.

[0043] [Example 1] Titanium dioxide sol was obtained in the same manner as in Preparation Example 1, except that the purified solvent obtained in the purification step was used instead of commercially available methanol in the solvent substitution step. The properties of this titanium dioxide sol are shown in Table 2.

[0044] [Example 2] Titanium dioxide sol was obtained in the same manner as in Preparation Example 1, except that in the solvent substitution step, purified solvent was used for the first half of the commercially available methanol. The properties of this titanium dioxide sol are shown in Table 2.

[0045] [Comparative Example 1] Titanium dioxide sol was obtained in the same manner as in Preparation Example 1, except that a recovered solvent was used instead of commercially available methanol in the solvent substitution step. The properties of the titanium dioxide sol are shown in Table 2.

[0046] [Table 1]

[0047] [Table 2]

Claims

1. A water sol preparation step for preparing an aqueous sol containing water and titanium dioxide particles, A surface treatment step to obtain a surface treatment sol by mixing a surface treatment solution obtained by mixing a polar organic solvent and an organosilicon compound with the aqueous sol, and surface treating the titanium oxide particles with the aqueous sol. A solvent replacement step is performed to obtain a titanium oxide sol and a recovered solvent by replacing the water and polar organic solvent contained in the surface treatment sol with another solvent. The process includes a purification step of purifying the recovered solvent to prepare a purified solvent, In the aforementioned purification process, The recovered solvent comprises water, Si, Ti, and the polar organic solvent, with a water content of 10% by mass or more. The purification solvent is a polar organic solvent having a water content in the range of 1 to 5% by mass. The purified solvent is reused in the surface treatment step or the solvent replacement step. A method for producing titanium dioxide sol.

2. The method for producing titanium oxide sol according to claim 1, wherein the recovered solvent has a Si content of 1000 ppm or more and a Ti content of 20 ppm or more.

3. The manufacturing method according to claim 1 or 2, wherein the recovered solvent is purified to recover a polar organic solvent having a Si content of 50 ppm or less as the purified solvent.

4. The manufacturing method according to claim 1 or 2, wherein the recovered solvent is purified to recover a polar organic solvent having a Ti content of 5 ppm or less as the purified solvent.

5. The manufacturing method according to claim 1 or 2, wherein in the purification step, the polar organic solvent contained in the recovered solvent is evaporated under reduced pressure within the system.

6. The manufacturing method according to claim 5, wherein the recovered solvent is distilled at a temperature below the boiling point of the polar organic solvent and within -20°C.

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