Method for producing silica-containing composition

Abstract

To provide a manufacturing method of a silica-containing composition having high purity and large silanol group density at a low cost with high safety.SOLUTION: A manufacturing method of a silica-containing composition includes: a slurry preparation step of blending a silicon-containing waste with an aqueous solution having a sodium hydroxide concentration of 1-24 wt.% for preparation of a silicate-containing slurry; a fractionation step of fractionating a silicate-containing extract and a residue from the slurry; a crystallization step of bringing a silicate-containing liquid involving the extract into contact with a CO2-containing gas at a temperature of 30-90°C for crystallization of a silica particle; and a cleaning step of cleaning a solid phase involving the silica particle. In the crystallization step, the silicate-containing liquid is brought into contact with the CO2-containing gas in such a way that a CO2 contact quantity per 1 L of the silicate-containing liquid becomes 120-400 L.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a silica-containing composition. [Background technology]

[0002] Tens of millions of tons of silicon-containing waste, such as coal ash, incineration ash, slag, and waste glass, are generated annually, and any that cannot be recycled are disposed of by landfill. Since silicon, which is contained in large amounts in waste, is an industrially useful component, if silicon could be recovered from these wastes in a reusable form, it is expected to reduce the amount of silicon disposed of and contribute to the creation of a recycling-oriented society.

[0003] As a technique for recovering silica from silicon-containing waste, for example, Patent Documents 1 and 2 disclose a technique in which coal ash is used as a raw material, silica components are extracted using a high-concentration aqueous sodium hydroxide solution, such as 40% by mass or more or 25% by mass or more, under heating conditions of 70 to 150°C, and then carbon dioxide gas is passed through the extract to obtain a silica crystallization liquid, which is then subjected to solid-liquid separation to recover silica.

[0004] Patent Document 1 aims to produce artificial aggregate from alumina-rich residue, and discloses a technique for treating the residue after extracting silica from coal ash by heating and solidifying it and molding it, or by adding cement and water and granulating it. Patent Document 2 aims to recover alumina components along with silica components, and discloses a technique for producing Al2O3 from the residue after extracting the silica component and using the residue as a filler material or cement raw material.

[0005] Applications of silica powder include fillers for polymeric materials such as synthetic rubber, catalysts, carriers, adsorbents, moisture absorbents, concrete admixtures, antiblocking materials, etc. When using recovered silica powder as fillers for polymeric materials such as synthetic rubber, the purity of the silica powder, its BET specific surface area, and the silanol group density, which greatly affect the bonding strength between the silica powder and polymers, are important. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-67526 [Patent Document 2] Special Publication No. 2009-519829 Summary of the Invention [Problem to be solved by the invention]

[0007] As described in Patent Documents 1 and 2, conventional silica extraction processes involve treatment with a high-temperature, high-concentration aqueous sodium hydroxide solution to achieve a high silica yield, but safety remains an issue. Furthermore, increasing the sodium hydroxide concentration poses the problem of high Na content in the extract, which can be an interfering factor when crystallizing silica particles, and the problem of extracting large amounts of silicon-containing waste-derived components other than silica. Thus, when waste is used as a raw material, components other than silica are contained in the extract. Therefore, it is expected that the conditions for crystallizing silica particles with the desired purity, BET specific surface area, and silanol group density will differ from those for crystallizing silica particles from industrial raw materials, etc. Patent Documents 1 and 2 make no mention of this point. Furthermore, when a high-concentration aqueous sodium hydroxide solution is used, a large amount of NaO, a cement-repellent component, remains in the extraction residue, making it difficult to use the resulting product as a raw material for cement.

[0008] The present invention provides a method for producing a silica-containing composition having high purity and a high silanol group density at low cost and with high safety. [Means for solving the problem]

[0009] In one aspect, the present invention provides a method for producing a silica-containing composition, comprising: a slurry preparation step of mixing silicon-containing waste with an aqueous solution having a sodium hydroxide concentration of 1 to 24 mass% to prepare a slurry containing silicate; a separation step of separating an extract containing silicate and a residue from the slurry; a crystallization step of contacting a silicate-containing liquid containing the extract with a CO2-containing gas at a temperature of 30 to 90°C to crystallize silica particles; and a washing step of washing the solid phase containing silica particles, wherein in the crystallization step, the silicate-containing liquid is contacted with a CO2-containing gas so that the amount of CO2 contacted per 1 L of the silicate-containing liquid is 120 to 400 L.

[0010] In the above-described manufacturing method, a silicate-containing slurry is prepared using silicon-containing waste and an aqueous solution containing sodium hydroxide at a lower concentration than conventional methods. A silicate-containing liquid, including an extract containing silicate obtained from the slurry, is then contacted with CO2-containing gas under specific conditions to crystallize silica particles. In this way, using silicon-containing waste and a low-concentration aqueous sodium hydroxide solution as raw materials, a silica-containing composition containing silica particles can be produced at low cost and with high safety. Furthermore, the silica-containing composition obtained by this manufacturing method has high purity due to the reduced amount of impurities other than silicate contained in the extract and the washing process, and has a high silanol group density. Such silica-containing compositions can be particularly useful as fillers for polymeric materials such as synthetic rubber. Meanwhile, the residue can be effectively utilized, for example, as a cement raw material. However, the uses of the silica-containing composition and residue are not limited to these.

[0011] The above production method preferably includes an aging step in which the suspension containing silica particles obtained in the crystallization step is maintained at 50 to 90° C. This makes it possible to produce a silica-containing composition having a higher silanol group density.

[0012] In the above-mentioned separation step, it is preferable to obtain a residue having a Na2O content of 0.01 to 5 mass % from the slurry. Since the Na2O content of such a residue is sufficiently reduced, it can be suitably used, for example, as a cement raw material.

[0013] In the slurry preparation step, it is preferable to react the silicon-containing waste with sodium hydroxide by heating the slurry to 50 to 200° C. This improves the extraction rate of silicate from the silicon-containing waste, and increases the yield of the final silica-containing composition.

[0014] The washing step preferably includes using an acid and then water in this order to wash the solid phase containing silica particles. This can further increase the purity and BET specific surface area of ​​the silica-containing composition. The reasons for this effect are presumably that sodium carbonate is dissolved and removed, and unreacted components are reduced.

[0015] The silicon-containing waste preferably contains coal ash, which allows for the production of a silica-containing composition with higher purity. Furthermore, the amount of alkali in the residue can be reduced, making the residue more suitable for use as a cement raw material.

[0016] The CO2-containing gas preferably includes waste gas generated in the factory, which allows the silica to be produced at a lower production cost.

[0017] In the above-mentioned production method, it is preferable to use the residue as a cement raw material. Since an aqueous solution with a low concentration of sodium hydroxide is used when preparing the slurry, the amount of Na2O remaining in the residue can be reduced. This allows the residue to be suitably used as a cement raw material. By effectively utilizing the residue in this way, the production cost of the silica-containing composition can be further reduced. [Effects of the Invention]

[0018] According to the present invention, a method for producing a silica-containing composition having high purity and a high silanol group density can be provided at low cost and with high safety. Such a silica-containing composition is useful as a filler material for polymer materials such as synthetic rubber. Furthermore, the effective utilization of silicon-containing waste can be further promoted. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a diagram illustrating an example of a method for producing silica. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the drawings as needed. However, the following embodiment is an example for explaining the present invention, and is not intended to limit the present invention to the following content.

[0021] A method for producing a silica-containing composition according to one embodiment includes a slurry preparation step of mixing a silicon-containing waste material with an aqueous solution containing sodium hydroxide at a concentration of 1 to 24% by mass to prepare a slurry containing silicate, a separation step of separating an extract containing silicate and a residue from the slurry, a crystallization step of contacting the extract with a CO2-containing gas to crystallize silica particles, and a washing step of washing the solid phase containing the silica particles. Figure 1 shows an example of the production method according to this embodiment.

[0022] In the slurry preparation step, silicon-containing waste and an aqueous solution containing sodium hydroxide at a concentration of 1 to 24% by mass are used as raw materials. The silicon-containing waste may contain at least one selected from the group consisting of coal ash, incineration ash, slag, and waste glass. Of these, from the viewpoint of improving the purity of silica and reducing the amount of alkali in the residue, it is preferable that the silicon-containing waste contains coal ash. There are no particular limitations on the coal ash, as long as it is produced by burning coal.

[0023] Coal ash may be, for example, ash generated when pulverized coal is burned in a coal-fired power plant. More specifically, examples include fly ash collected by an electrostatic precipitator and clinker ash collected by dropping from a combustion boiler. Fly ash, in particular, is a fine particle and highly reactive with aqueous sodium hydroxide solution. Therefore, it is preferable that the silicon-containing waste contains fly ash.

[0024] The silicon content of the silicon-containing waste, calculated as SiO2, is preferably 30 to 80 mass%, more preferably 40 to 80 mass%, and even more preferably 60 to 80 mass%. If the silicon content is within the above range, the silicate component required for silica production can be sufficiently secured. Furthermore, since the silicon content in the residue can be maintained at a certain level, it can be suitably used as a cement raw material.

[0025] The chemical components of the silicon-containing waste are preferably, based on the dry mass, 1 to 40 mass% Al2O3, 0 to 5 mass% Fe2O3, 0 to 5 mass% CaO, 0 to 5 mass% MgO, 0 to 5 mass% SO3, 0 to 5 mass% Na2O, and 0 to 5 mass% KO. With silicon-containing waste of such properties, fewer impurities precipitate during silica crystallization, making it easier to obtain a high-purity silica-containing composition.

[0026] The average particle size of the silicon-containing waste is preferably 0.1 to 100 μm, more preferably 0.5 to 50 μm, and even more preferably 1 to 10 μm. If the particle size of the silicon-containing waste is within the above range, silicate components can be efficiently extracted. The above average particle size is the median diameter (D50) determined by a laser diffraction / scattering method.

[0027] The aqueous solution having a sodium hydroxide concentration of 1 to 24% by mass (aqueous sodium hydroxide solution) may be an industrially produced aqueous sodium hydroxide solution as is, or may be prepared by mixing an aqueous sodium hydroxide solution with water to a predetermined concentration. Alternatively, solid sodium hydroxide in granular or powder form may be mixed with water to prepare an aqueous solution of a predetermined concentration.

[0028] The sodium hydroxide concentration in the aqueous sodium hydroxide solution is 1 to 24% by mass. From the viewpoint of sufficiently extracting silicate components, this sodium hydroxide concentration is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more. On the other hand, from the viewpoint of suppressing the extraction of impurities such as Ca and increasing the purity of silica, and from the viewpoint of suppressing the remaining Na derived from the aqueous sodium hydroxide solution in the residue and making it suitable for use as a cement raw material, the sodium hydroxide concentration is preferably 20% by mass or less, and more preferably 18% by mass or less.

[0029] In the slurry preparation process, the raw materials described above are mixed to prepare a slurry containing silicate. In this slurry preparation process, by adjusting the sodium hydroxide concentration of the sodium hydroxide aqueous solution mixed with the silicon-containing waste to fall within the above-mentioned range, silicate can be extracted with high purity while suppressing the extraction of impurities such as Ca. The reason for this is unclear; for example, when the silicon-containing waste is heated in an aqueous solution containing sodium hydroxide at a concentration exceeding 24% by mass, the amount of Na in the aqueous solution becomes excessive, which makes it easier for Ca in the Ca-containing minerals contained in the silicon-containing waste to be replaced by Na in the aqueous solution. This results in a high Ca extraction rate. On the other hand, when the concentration of the sodium hydroxide aqueous solution is 24% by mass or less, the amount of Na in the aqueous solution becomes appropriate, suppressing the replacement of Ca-containing minerals and the like with Na in the aqueous solution, which is presumably responsible for a low Ca extraction rate.

[0030] The blending ratio of the sodium hydroxide aqueous solution to the silicon-containing waste when preparing the slurry, i.e., the liquid / solid ratio, is preferably 1 to 20 by mass, more preferably 2 to 15, and even more preferably 2.5 to 13. If the liquid / solid ratio is within the above range, it becomes easy to adjust the molar ratio of Na contained in the sodium hydroxide to Si contained in the silicon-containing waste to a range of 1.0 to 2.0 while ensuring the fluidity of the slurry. This prevents the amount of Na in the extract from becoming excessive, making it easier to obtain high-purity silica.

[0031] The slurry is preferably heated from the viewpoint of promoting the reaction between the silicon-containing waste and sodium hydroxide. The slurry may be heated while mixing and stirring the slurry, and, if necessary, while applying pressure. The heating temperature of the slurry is preferably 50 to 200°C. This allows silicates to be sufficiently extracted from the silicon-containing waste. From the viewpoint of further promoting the extraction of silicates, the heating temperature of the slurry is preferably 65°C or higher, more preferably 80°C or higher. On the other hand, from the viewpoint of simplifying the equipment, the heating temperature of the slurry is preferably 150°C or lower, more preferably 100°C or lower.

[0032] The heating time of the slurry in the above temperature range is preferably 0.5 to 4 hours, more preferably 1 to 3.5 hours, and even more preferably 2 to 3.5 hours, which allows silicates to be extracted efficiently and sufficiently from the silicon-containing waste.

[0033] In the separation step, an extract containing silicate and a residue are separated from the slurry. The slurry may be separated into an extract containing silicate and a residue that is a solid content. For example, the slurry may be separated into the extract and the residue using a known dehydrator. Examples of dehydrators include a filter press, a belt press, a roll press, a centrifugal dehydrator, a rotary filter, and a ceramic filter. However, the separation means is not limited to these.

[0034] The extraction rate of silicate extracted from silicon-containing waste into the extract is preferably 5 to 60%, more preferably 10 to 50%, and even more preferably 15 to 40%. If the silicate extraction rate is within the above range, the silicate necessary for silica production can be sufficiently secured, thereby increasing the yield of high-purity silica. Furthermore, since the residue also contains a certain amount of silicate, the residue can be suitably used as a cement raw material. In this specification, the silicate extraction rate is a value obtained by dividing the silicon content in the extract from which silicate has been extracted by the silicon content in the coal ash. Details of the derivation method will be described in the Examples below.

[0035] The percentage (residual rate) of silicate contained in the silicon-containing waste remaining in the residue is preferably 40 to 95%, more preferably 50 to 90%, and even more preferably 60 to 85%. If the residual rate of silicate is within the above range, the silicate necessary for producing the silica-containing composition can be sufficiently secured, thereby increasing the yield of the silica-containing composition. Furthermore, since the residue also contains a certain amount of silicate, the residue can be suitably used as a cement raw material. Note that when the slurry is separated into an extract and a residue, the residual rate of silicate in the residue can be calculated by subtracting the above-mentioned silicate extraction rate from 100(%).

[0036] The residue separated in the separation step may be used as a cement raw material as is, or may be used as a cement raw material after pretreatment such as washing. Examples of the washing method include through-flow water washing. Examples of the washing water include tap water, industrial purified water, and industrial wastewater. However, the washing method and washing water are not limited to these. The NaO content of the cement can be adjusted by washing the residue and appropriately changing the NaO content contained in the residue.

[0037] The extraction rate of Ca extracted from silicon-containing waste into the slurry extract is preferably 0.1 to 5%, more preferably 0.1 to 2%, and even more preferably 0.1 to 1%. If the extraction rate of Ca is within the above range, the amount of calcium carbonate crystallizing during silica crystallization can be reduced, and the purity of silica can be increased. In this specification, the extraction rate of Ca is a value obtained by dividing the calcium content in the extract from which silicate has been extracted by the calcium content in the coal ash. Details of the derivation method will be described in the Examples below.

[0038] From the viewpoint of obtaining silica with a sufficiently high purity, the Na concentration of the extract is preferably 300 g / L or less, more preferably 200 g / L or less, and even more preferably 100 g / L or less. The lower limit of the Na concentration may be, for example, 10 g / L.

[0039] This extract may be used as a silicate-containing liquid in the crystallization step as is, or the extract may be diluted with water to prepare a silicate-containing liquid for use in the crystallization step. Examples of dilution water include tap water, industrial purified water, industrial wastewater, and water obtained by rinsing and filtering residues. However, the dilution water is not limited to these. By diluting the extract and appropriately adjusting the silicate concentration, the primary particle size of the crystallized silica particles can be adjusted.

[0040] The Si concentration of the silicate-containing liquid used in the crystallization step is adjusted to a value that satisfies the BET specific surface area (e.g., 100 to 450 m) suitable for use as a filler for polymer materials such as synthetic rubber. 2 From the viewpoint of obtaining a powdered silica-containing composition (silica powder) having a BET specific surface area (BET specific surface area), the silicate concentration is preferably 3 to 30 g / L, more preferably 10 to 25 g / L, and even more preferably 15 to 20 g / L. When the silicate concentration of the silicate-containing liquid is within the above range, handling becomes easy when contacting with a CO2-containing gas, and a powdered silica-containing composition having a desired BET specific surface area can be smoothly produced.

[0041] The content of Na in the residue in terms of Na2O (Na2O content) is preferably 5% by mass or less, more preferably 4% by mass or less, and even more preferably 0.01 to 4% by mass. If the Na2O content in the residue is within the above range, the residue can be suitably used as a cement raw material. In this embodiment, since a low-concentration aqueous sodium hydroxide solution is used in the slurry preparation step, the Na2O content of the residue can be smoothly adjusted to be within the above range.

[0042] The content of Si in the residue in terms of SiO2 (SiO2 content) is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 35% by mass or more. If the SiO2 content in the residue is within the above range, the residue can be suitably used as a cement raw material. The SiO2 content in the residue is preferably 70% by mass or less, more preferably 60% by mass or less. If the SiO2 content in the residue is within the above range, the SiO2 yield can be sufficiently increased. The SiO2 content of the residue can be determined by converting the Si content into SiO2.

[0043] In the crystallization step, silica particles are crystallized by contacting the silicate-containing liquid with a CO2-containing gas. The method for contacting the silicate-containing liquid with the CO2-containing gas is not particularly limited, and for example, the CO2-containing gas may be bubbled through the silicate-containing liquid, or a contact tower or the like may be used to bring the descending silicate-containing liquid into countercurrent contact with the ascending CO2-containing gas.

[0044] In the crystallization step, the silicate-containing liquid is contacted with a CO2-containing gas so that the amount of CO2 contacted per 1 L of the silicate-containing liquid is 120 to 400 L. The above-mentioned amount of CO2 contacted is preferably 150 to 350 L, more preferably 200 to 300 L. By setting the amount of CO2 contacted per 1 L of the silicate-containing liquid within the above range, it is possible to easily crystallize silica particles having a BET specific surface area and silanol group density suitable for use as a filler for polymer materials such as synthetic rubber. In this specification, the volume of the gas is the volume at standard conditions (pressure: 101.325 kPa, temperature: 0°C (273.15 K)), and the volume of the liquid is the volume at a pressure of 101.325 kPa and a temperature of 20°C.

[0045] The inventors speculate that the reason why silica with a high silanol group density can be obtained by setting the contact amount of CO2 per 1 L of silicate-containing liquid within the above range is as follows. The silanol group density is calculated using the formula: silanol group amount ÷ BET specific surface area. Therefore, to increase the silanol group density, it is necessary to increase the silanol group amount or reduce the BET specific surface area. If the contact amount of CO2 is too small, the amount of CO2 contacted is too small relative to the amount of silicate in the silicate-containing liquid, preventing the crystal growth reaction of silica particles from proceeding, resulting in fine silica particles and a low silanol group density. On the other hand, if the contact amount of CO2 is set within the above range, the crystal growth reaction of silica particles proceeds, resulting in particle coarsening, thereby reducing the BET specific surface area and increasing the silanol group density. It is speculated that if the contact amount of CO2 is too large, the amount of CO2 will be in excess of the amount of silicate in the silicate-containing liquid, making it difficult to achieve an improvement in silanol group density. In this way, by controlling the amount of CO2 contacted per liter of silicate-containing liquid to a specific amount, it is possible to produce a silica-containing composition having a BET specific surface area and silanol group density suitable as a filler for polymer materials such as synthetic rubber.

[0046] The temperature of the silicate-containing liquid in the crystallization step is 30 to 90°C, preferably 40 to 85°C, and more preferably 50 to 80°C. If the temperature is too low, the rate of the nucleation reaction of silica particles slows down, causing the crystal growth reaction to become active, which tends to result in an excessively small BET specific surface area. On the other hand, if the temperature is too high, the rate of the nucleation reaction of silica particles increases, resulting in the production of a large amount of fine silica particles, which tends to result in an excessively large BET specific surface area.

[0047] The flow rate of the CO2-containing gas is preferably 1 to 30 L / min, more preferably 3 to 20 L / min, and even more preferably 5 to 15 L / min per liter of silicate-containing liquid. When the flow rate is within the above range, silica particles having an appropriate BET specific surface area can be crystallized. Furthermore, by appropriately changing the flow rate within the above range, the primary particle size of the silica particles can be adjusted to a desired value.

[0048] The time for passing the CO2-containing gas may be adjusted appropriately so that the amount of CO2 contacted per 1 L of silicate-containing liquid falls within the above-mentioned range at the above-mentioned aeration flow rate.

[0049] From the viewpoint of cost reduction, the CO2-containing gas used in the crystallization step preferably contains exhaust gas discharged from a factory. The exhaust gas discharged from a factory preferably contains at least one selected from the group consisting of boiler exhaust gas, cement kiln exhaust gas, chlorine bypass exhaust gas, and synthetic gas exhaust gas from a chemical plant. From the viewpoint of improving CO2 purity, the CO2-containing gas may contain industrial gas.

[0050] The CO2 concentration of the CO2-containing gas is preferably 10 to 100% by volume, more preferably 10 to 98% by volume, and even more preferably 30 to 90% by volume. Using a CO2-containing gas having a CO2 concentration within the above range allows for smoother production of high-purity silica particles. In the crystallization step, silica particles are generated, and the silicate-containing liquid becomes a suspension (slurry). The silicate-containing liquid is contacted with the CO2-containing gas until the pH of the suspension reaches preferably 7 to 12, more preferably 8 to 10. This allows for sufficient crystallization of the silica particles, increasing the yield of the silica particles. The silica particles may be recovered by solid-liquid separation of the resulting silica-containing slurry. This results in a powdered silica-containing composition. The sodium carbonate component may be recovered from the alkaline solution obtained by recovering the silica particles through the solid-liquid separation and reused as part of the raw material in the extraction step.

[0051] In the washing step, the solid phase containing silica particles contained in the suspension obtained in the crystallization step is washed. Washing may be performed by solvent substitution of the suspension. Specifically, solid-liquid separation and addition of a washing liquid may be repeated. Solid-liquid separation may be performed by centrifugation or filtration.

[0052] For washing, water is preferably used, and acid and water are preferably used. When washing with acid and water, it is particularly preferable to wash the solid phase containing silica particles with acid and then with water. By having a procedure of washing with acid and then with water, impurities contained in the solid phase can be reduced with a smaller amount of washing. In addition, a silica-containing composition with higher purity and a larger specific surface area can be produced. The reasons for this include the fact that salts such as carbonates that crystallize together with the silica particles are easily dissolved by acid and are easily removed by subsequent washing with water, the amount of crystallized silica particles increases due to the reaction of unreacted silicate with acid, and the fact that blocked pores are exposed due to the removal of impurities.

[0053] The acid used for washing may include at least one selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid. These may be used alone or in combination of two or more. Among these, it is preferable to use hydrochloric acid for washing from the viewpoint of further increasing the purity of the silica-containing composition. In this specification, dilute hydrochloric acid is also included in hydrochloric acid. The purity of the silica-containing composition refers to the content of silica in the solid content of the silica-containing composition.

[0054] In the washing step, the purity of the silica-containing composition can be improved by repeatedly performing solvent substitution, which involves adding a solvent and performing solid-liquid separation. The number of times that the solvent substitution is repeated is preferably 3 times or more, more preferably 4 times or more, and even more preferably 5 times or more. By increasing the number of times of repetition, the main impurities in the silica-containing composition can be removed, and a high-purity silica-containing composition can be recovered.

[0055] In one example, as shown in FIG. 1, an aging step may be carried out before the washing step. In this case, the washing step involves washing the solid phase (solid content) containing aged silica particles contained in the suspension obtained in the aging step. The washing step is as described above. In the aging step, the suspension containing silica particles obtained in the crystallization step is maintained at 50 to 90°C for aging. By performing the aging step, dissolution and precipitation of the fine silica particles obtained in the crystallization step occurs, and the silica particles undergo crystal growth and become coarse. This makes it possible to produce silica particles with a higher silanol group density. The holding time in the above temperature range in the aging step may be, for example, 30 to 150 minutes.

[0056] The silica-containing composition produced by the method for producing a silica-containing composition of this embodiment may be in the form of a powder, a slurry, or a cake. The SiO2 purity of the silica-containing composition is preferably 85% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 97% by mass or more, based on the dry mass. Such a high-purity silica-containing composition can be suitably used as a raw material for fillers, paints, adhesives, abrasives, fine ceramics, etc. However, its applications are not limited to the above examples.

[0057] The BET specific surface area of ​​the powdered silica-containing composition produced by the method for producing a silica-containing composition of the present embodiment is preferably 50 to 450 m 2 / g, more preferably 70 to 250m 2 / g. Silica having such a BET specific surface area can be suitably used as a filler material, an adsorbent, a moisture absorbent, an admixture for concrete, an anti-blocking material, etc.

[0058] The silanol group density of the silica-containing composition produced by the method for producing a silica-containing composition of the present embodiment is preferably 1.0 / nm 2 More preferably, it is 2.0 particles / nm 2 More preferably, it is 3.0 particles / nm 2 The silica-containing composition can be suitably used as a filler for polymeric materials such as synthetic rubber. The silanol group density in this specification can be measured by the method described in the Examples.

[0059] As shown in FIG. 1, the method for producing a silica-containing composition according to this embodiment may include a calcination step in which the residue separated from the slurry is introduced into a cement kiln, such as a rotary kiln, as one of the cement raw materials. The residue separated from the slurry in the calcination step contains Si and Al and has a low NaO content. Because the NaO content is low, the amount of volatile matter generated during calcination in the cement kiln can be reduced. This reduces the amount of dust generated during calcination, thereby reducing the load on the equipment. Therefore, cement clinker can be produced stably.

[0060] Other cement raw materials (limestone, silica stone, clay, construction soil, blast furnace slag, steelmaking slag, etc.) may be introduced into the cement kiln together with the residue. In the cement kiln, the cement raw materials are burned to obtain cement clinker. The cement clinker may be pulverized while being mixed with gypsum in, for example, a pulverizer (finishing mill), thereby obtaining cement (cement composition). Fly ash, slag powder, etc. may also be blended as necessary. The resulting cement composition may be Portland cement or a blended cement.

[0061] According to the method for producing a silica-containing composition of the present embodiment, the silica-containing composition can be produced with high safety. In addition, the blending conditions can be easily set, and cement clinker and cement compositions with stable quality can be produced.

[0062] 1 shows an example of producing a cement clinker and a cement composition together with silica powder, which is a type of silica-containing composition, but it is not essential to produce a cement clinker and a cement composition. For example, only silica powder may be produced, and the residue may be used for other purposes.

[0063] One example of a method for producing cement clinker includes a calcination step after the separation step, in which the residue is used as a raw material for a cement kiln. The slurry preparation step, separation step, and calcination step in this production method can be performed in the same manner as the slurry preparation step and separation step in the above-mentioned silica production method. Therefore, the above-mentioned description of the method for producing a silica-containing composition can also be applied to the production method of cement clinker in this example.

[0064] In the method for producing cement clinker of this example, a residue having a Na2O content of 0.01 to 5 mass% may be used as a cement raw material. This reduces the amount of volatile matter generated when the cement raw material is fired in a cement kiln in the firing process. This reduces the amount of dust generated during firing and the load on the equipment. This allows for stable production of cement clinker.

[0065] One example of a method for producing a cement composition includes a blending step of blending cement clinker and gypsum after the calcination step in the above-mentioned method for producing cement clinker. The blending may be performed while grinding the cement clinker using a conventional grinding machine (finishing mill). The resulting cement composition may be Portland cement or a blended cement. According to this method for producing a cement composition, it is easy to set the blending conditions, and a cement composition with stable quality can be produced.

[0066] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. In one aspect, the present disclosure can provide a slurry preparation step of mixing silicon-containing waste with an aqueous solution containing sodium hydroxide at a concentration of 1 to 24% by mass to prepare a slurry containing silicate; a separation step of separating an extract containing silicate and a residue from the slurry to obtain a residue having an Na2O content of 0.01 to 5% by mass; and a method for producing cement clinker using the residue as a cement raw material. Since the Na2O content of the residue is sufficiently reduced, the generation of volatile alkali components is suppressed when the residue is introduced into a cement kiln. This reduces the generation and precipitation of volatile matter, thereby reducing the load on the cement kiln. Therefore, cement clinker can be produced stably. [Example]

[0067] The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0068] [1: Examination of silicate extraction conditions] Several experiments were conducted in which at least one of the conditions, namely the concentration of the sodium hydroxide aqueous solution mixed with the silicon-containing waste, the heating temperature when preparing the slurry, and the heating time, was varied. In this way, the effects of each condition on the extraction rate of silicate and Ca from the silicon-containing waste, the Na concentration in the extract, and the NaO content in the residue were examined. The specific procedures and results are described below.

[0069] The silicon-containing waste used was coal ash (fly ash, manufactured by Ube Industries, Ltd.) generated from a coal-fired power plant that burns pulverized coal. The ignition loss and chemical composition of the coal ash used are shown in Table 1. The values ​​shown in Table 1 were measured using the following method.

[0070] Ignition loss of coal ash (Ig.loss): Measured in accordance with the ignition loss measurement method specified in JIS R 5202 "Methods for chemical analysis of cement." - SiO2, Al2O3, Fe2O3, CaO, MgO, SO3, Na2O, K2O contents of coal ash: Measured in accordance with JIS M 8853 "Method for chemical analysis of aluminosilicate raw materials for ceramics." · Average particle size of coal ash: The median diameter (D50) determined by laser diffraction / scattering method was 27.9 μm.

[0071] [Table 1]

[0072] Example 1 Sodium hydroxide (Fujifilm Wako Pure Chemical Industries, Ltd., first-grade reagent, granular form) was mixed with distilled water to prepare a 3.7% by mass aqueous solution of sodium hydroxide. 50 g of coal ash and 550 g of a 3.7% by mass aqueous solution of sodium hydroxide were weighed. These were mixed at 200 rpm using a stirrer (Shinto Scientific Co., Ltd., Three-One Motor Type 600G) in a container adjusted to 80°C, and the mixture was allowed to react for 1 hour. The mass ratio of the aqueous solution of sodium hydroxide to the coal ash was as shown in the "liquid / solid ratio" in Table 2. The resulting slurry was filtered using commercially available filter paper (circular quantitative filter paper No. 5C) and a suction filtration device (Advantec Toyo Co., Ltd.) to separate the extract and the residue (solids).

[0073] The concentrations of Si and Ca in the extract were quantified using an ICP optical emission spectrometer (Hitachi High-Tech Science Corporation, model PS3520UVDDII). The concentration of Na in the extract was quantified using an atomic absorption spectrophotometer (Shimadzu Corporation, model AA-7000). The following formulas were used to calculate (1) the silicate extraction rate, (2) the Ca extraction rate, and (3) the NaO content in the residue. The mass of NaO in (3) was calculated by converting the quantitative analysis result of Na. The results are shown in Table 2.

[0074] (1) Silicate extraction rate (%) = Si in extract (g) / {mass of coal ash (g) × Si content of coal ash (mass%)} × 100 (2) Ca extraction rate (%) = Ca in extract (g) / {mass of coal ash (g) × Ca content of coal ash (mass%)} × 100 (3) NaO content in residue (mass%) = {NaO (g) when blending raw materials - NaO (g) contained in extract} / mass of residue (g) × 100

[0075] (Examples 2 to 4 and Comparative Example 1) An experiment was carried out in the same manner as in Example 1, except that the concentration of the aqueous sodium hydroxide solution was changed as shown in Table 2, to obtain an extract and a residue. Measurements and calculations were carried out in the same manner as in Example 1 to determine the extraction rates of silicate and Ca, and the NaO content in the residue. The results are shown in Table 2.

[0076] From the results of Examples 1 to 4 and Comparative Example 1, it was confirmed that the higher the sodium hydroxide concentration in the sodium hydroxide aqueous solution, the higher the silicate extraction rate. However, it was also confirmed that the higher the sodium hydroxide concentration, the higher the Na concentration in the extract. Furthermore, in Comparative Example 1, where the sodium hydroxide concentration was 40 mass%, the Ca extraction rate was also extremely high. The Ca component precipitates as CaCO3 during crystallization, which causes a decrease in the purity of the obtained silica. For this reason, a low CaO extraction rate is preferable.

[0077] In Examples 1 to 3, in which an aqueous sodium hydroxide solution with a low sodium hydroxide concentration (24 mass % or less) was used, the Na2O content in the residue was sufficiently low.

[0078] (Examples 5 to 8) In Examples 5 and 6, the heating temperature during slurry preparation was changed from that of Example 4, and in Examples 7 and 8, the heating time during slurry preparation was changed from that of Example 4. Other conditions were the same as in Example 4, and slurry preparation and separation operations were carried out to obtain the extracts and residues of Examples 5 to 8. In Examples 5 to 8, measurements and calculations were carried out in the same manner as in Example 4 to determine the extraction rates of silicate and Ca, the Na concentration in the extracts, and the NaO content in the residue. The results are shown in Table 2.

[0079] [Table 2]

[0080] From Examples 4 to 8, it was confirmed that, under conditions where the sodium hydroxide concentration is constant, by changing the heating temperature and / or heating time of the slurry, it is possible to adjust the extraction rate of silicate while suppressing the Na concentration in the extract and the extraction rate of Ca. In Example 6, where the heating temperature was set to 95°C, the highest extraction rate of silicate was achieved.

[0081] [2: Examination of crystallization conditions] Example 9 <Slurry preparation process and fractionation process> 200 g of the coal ash shown in Table 1 was mixed with 500 g of a sodium hydroxide solution with a sodium hydroxide concentration of 16% by mass, and the mixture was allowed to react while stirring at 200 rpm and 95°C for 3.5 hours using a stirrer (manufactured by Shinto Scientific Co., Ltd., Three-One Motor Type 600G). The resulting slurry was filtered using commercially available filter paper (circular quantitative filter paper No. 5C) and a suction filtration device (manufactured by Advantec Toyo Co., Ltd.), and the extract and residue (solids) were separated. After separation, 0.3 L of the extract containing silicate components was diluted with distilled water to a liquid volume of 1 L to obtain a silicate-containing liquid. The Si concentration in this silicate-containing liquid was 18 g / L.

[0082] <Crystallization process> The resulting silicate-containing liquid (diluted extract) was transferred to a crystallization reactor. While mixing at 450 rpm using a stirrer, carbon dioxide gas (manufactured by Air Liquide Japan, CO2 concentration: 99.5% by volume or higher) was bubbled through at 8.4 L / min for 30 minutes, so that the contact volume of CO2 per 1 L of silicate-containing liquid was 252 L. In this way, the silicate-containing liquid and carbon dioxide gas were in contact for 30 minutes. The temperature of the silicate-containing liquid during the carbon dioxide gas bubbling was 50°C. This resulted in the crystallization of silica particles, yielding a suspension containing silica particles.

[0083] <Cleaning process> After the reaction was completed, the suspension was centrifuged for solid-liquid separation. To wash the solid phase (solid content) containing silica particles, part of the liquid phase of the silica-containing slurry was replaced with dilute hydrochloric acid, and the HCl concentration of the entire liquid phase was adjusted to 9.5 to 10.5 w / v% and stirred. Subsequently, the replacement with distilled water by centrifugation was repeated until the electrical conductivity of the liquid phase reached 0.1 mS / cm or less, and then the solid content was obtained by drying.

[0084] The solid matter obtained after drying was crushed using a planetary mill (Ito Seisakusho, Model LA-PO.1) at 340 rpm for 8 minutes and recovered as a powdered silica-containing composition (silica powder). The physical properties of the silica powder thus produced (purity, BET specific surface area, and silanol group density) were measured using the following methods. The results are shown in Table 3.

[0085] The SiO2 purity of the silica powder was determined by quantifying the insoluble Si in the silica powder using the perchloric acid dehydration gravimetric method. The acid-dissolved Si was also quantified using an ICP optical emission spectrometer (Hitachi High-Tech Science Corporation, Model: PS3520UVDDII). The SiO2 purity of the silica powder was calculated by adding up the respective quantitative values.

[0086] The BET specific surface area of ​​the silica powder was determined using the following procedure. The silica powder was heated at 110°C for 30 minutes in a nitrogen atmosphere to remove moisture. After removing moisture in this manner, the BET specific surface area of ​​the silica powder was measured using a specific surface area and pore distribution analyzer (Microtrack BEL Corporation, device name: BEL-SORP-mini).

[0087] The silanol group density of silica powder was measured and calculated using the Sears method as follows: 1.5 g of silica powder and 30 g of sodium chloride were added to 100 mL of distilled water, and the resulting silica slurry was stirred using a magnetic stirrer to completely dissolve the sodium chloride. Distilled water and 0.1 mol / L dilute hydrochloric acid or 0.1 mol / L aqueous sodium hydroxide solution were added at 25°C, and the silica slurry was adjusted to a total volume of 150 mL and a pH of 4. This was used as the test solution. While stirring the test solution with a magnetic stirrer, 0.1 mol / L aqueous sodium hydroxide solution was added dropwise using a burette at a rate of 2 mL / min, and the titration volume V (L) of the 0.1 mol / L aqueous sodium hydroxide solution required to change the pH of the test solution from 4.0 to 9.0 was measured. The silanol group density ρ (number / nm) of the silica powder was calculated. 2 ) was calculated using the following formula:

[0088] ρ=(0.1V×NA) / (W×SBET) V: Titration volume (L) of 0.1 mol / L sodium hydroxide solution added NA: Avogadro's number (units / mol) W: Amount of silica powder collected (1.5g) SBET: BET specific surface area (nm) of silica powder 2 / g)

[0089] The method for measuring and calculating the silanol group density of the above-mentioned silica powder was based on "GW Sears, Jr., Analytical Chemistry, Vol. 28, No. 12, pp. 1981-1983 (1956)" and "Haba Shinichi, Development of Abrasives for Semiconductor Integrated Circuit Processing, Doctoral Dissertation, Kochi University of Technology, pp. 42-45, March 2004."

[0090] Example 10 Silica powder was produced using the same procedure as in Example 9, except that in the crystallization step, carbon dioxide gas was bubbled at 4.2 L / min for 40 minutes so that the contact amount of CO2 per 1 L of silicate-containing liquid was 168 L. The physical properties of the silica powder were evaluated using the same procedure as in Example 9. The results are shown in Table 3.

[0091] Example 11 The slurry preparation step and separation step described in Example 9 were repeated three times to obtain 3 L of silicate-containing liquid. Silica powder was produced using the same procedure as in Example 9, except that carbon dioxide gas was bubbled at 8.4 L / min for 60 minutes so that the contact amount of CO2 per 1 L of silicate-containing liquid was 168 L. The physical properties of the silica powder were evaluated using the same procedure as in Example 9. The results are shown in Table 3.

[0092] Example 12 A silica powder was produced in the same manner as in Example 9, except that in the crystallization step, the rotation speed of the stirrer was set to 200 rpm and the temperature of the silicate-containing liquid during carbon dioxide gas bubbling was set to 80°C. The physical properties of the silica powder were evaluated in the same manner as in Example 9. The results are shown in Table 3.

[0093] (Comparative Example 2) Silica powder was produced using the same procedure as in Example 9, except that in the crystallization step, carbon dioxide gas was bubbled at 8.4 L / min for 10 minutes so that the contact amount of CO2 per 1 L of silicate-containing liquid was 84 L. The physical properties of the silica powder were evaluated using the same procedure as in Example 9. The results are shown in Table 3.

[0094] (Reference example 1) The physical properties of wet silica (Ultrasil 7000GR, manufactured by Evonic), which is commercially available as a silica filler for synthetic rubber, were evaluated in the same manner as in Example 9. The results are shown in Table 3.

[0095] [Table 3]

[0096] As shown in Table 3, the silanol group density of the resulting silica powder tended to increase with increasing CO2 contact volume per liter of silicate-containing liquid. When the CO2 contact volume per liter of silicate-containing liquid was 168 L, silica powder with a silanol group density approximately 1.5 times that of the commercially available product (Reference Example 1) was obtained (Examples 10 and 11). Furthermore, by increasing the temperature of the silicate-containing liquid during carbon dioxide gas aeration to 80°C, silica powder with a higher silanol group density was obtained (Example 12).

[0097] [3. Consideration of aging conditions] Example 13 A suspension containing silica particles was obtained using the same procedure as in Example 9. After the bubbling of carbon dioxide gas was completed, the suspension containing the crystallized silica particles was heated to 80°C in a container and held for 60 minutes while stirring at 200 rpm (aging step). The suspension after holding was subjected to the same washing step as in Example 9. The purity, BET specific surface area, and silanol group density of the silica powder were then measured using the same procedure as in Example 9. The results are shown in Table 4.

[0098] Example 14 A suspension containing silica particles was obtained using the same procedure as in Example 12. After the bubbling of carbon dioxide gas was completed, the suspension containing the crystallized silica particles was heated to 80°C in a container and held for 120 minutes while stirring at 200 rpm (aging step). The aged suspension was subjected to the same washing step as in Example 9. The purity, BET specific surface area, and silanol group density of the silica powder were then measured using the same procedure as in Example 9. The results are shown in Table 4.

[0099] [Table 4]

[0100] As shown in Table 4, by carrying out the aging step after the crystallization step, a silica powder having a higher silanol group density could be obtained.

Claims

1. a slurry preparation step of mixing the silicon-containing waste with an aqueous solution having a sodium hydroxide concentration of 1 to 24 mass % to prepare a slurry containing silicate; a separation step of separating an extract containing the silicate and a residue having a Na 2 O content of 0.01 to 5% by mass from the slurry; The silicate-containing liquid containing the extract and CO ２ a crystallization step of contacting the silica particles with a containing gas to crystallize the silica particles; a washing step of washing the solid phase containing the silica particles, In the crystallization step, CO per 1 L of the silicate-containing liquid ２ The silicate-containing liquid and CO are mixed together so that the contact volume is 120 to 400 L. ２ contacting the mixture with a gas containing the mixture; The method for producing a silica-containing composition, wherein the washing step produces a silica-containing composition having a silanol group density of 1.0 groups / nm 2 or more.

2. A slurry preparation step of mixing silicon-containing waste with an aqueous solution of sodium hydroxide having a concentration of 1 to 24 mass % to prepare a slurry containing silicate; a separation step of separating an extract containing the silicate and a residue from the slurry; a crystallization step of contacting a silicate-containing liquid containing the extract with a CO 2 -containing gas at a temperature of 30 to 90°C to crystallize silica particles; a washing step of washing the solid phase containing the silica particles, In the crystallization step, the silicate-containing liquid is contacted with a CO 2 -containing gas so that the amount of CO 2 contacted per 1 L of the silicate-containing liquid is 120 to 400 L; The method for producing a silica-containing composition, wherein the washing step includes using an acid and water in this order to wash the solid phase, and a silica-containing composition having a silanol group density of 1.0 groups / nm 2 or more is obtained.

3. A slurry preparation step of mixing silicon-containing waste including coal ash with an aqueous solution of sodium hydroxide having a concentration of 1 to 24 mass % to prepare a slurry containing silicate; a separation step of separating an extract containing the silicate and a residue from the slurry; a crystallization step of contacting a silicate-containing liquid containing the extract with a CO 2 -containing gas at a temperature of 30 to 90°C to crystallize silica particles; a washing step of washing the solid phase containing the silica particles, In the crystallization step, the silicate-containing liquid is contacted with a CO 2 -containing gas so that the amount of CO 2 contacted per 1 L of the silicate-containing liquid is 120 to 400 L; The method for producing a silica-containing composition, wherein the washing step produces a silica-containing composition having a silanol group density of 1.0 groups / nm 2 or more.

4. A slurry preparation step of mixing silicon-containing waste with an aqueous solution of sodium hydroxide having a concentration of 1 to 24 mass % to prepare a slurry containing silicate; a separation step of separating an extract containing the silicate and a residue to be used as a cement raw material from the slurry; a crystallization step of contacting a silicate-containing liquid containing the extract with a CO 2 -containing gas at a temperature of 30 to 90°C to crystallize silica particles; a washing step of washing the solid phase containing the silica particles, In the crystallization step, the silicate-containing liquid is contacted with a CO 2 -containing gas so that the amount of CO 2 contacted per 1 L of the silicate-containing liquid is 120 to 400 L; The method for producing a silica-containing composition, wherein the washing step produces a silica-containing composition having a silanol group density of 1.0 groups / nm 2 or more.

5. The method for producing a silica-containing composition according to claim 2, wherein in the separation step, a residue having an Na 2 O content of 0.01 to 5 mass % is obtained from the slurry.

6. A method for producing a silica-containing composition described in claim 3 or 4, wherein the washing step includes using an acid and water in this order to wash the solid phase.

7. A method for producing a silica-containing composition as described in claim 4, wherein the silicon-containing waste includes coal ash.

8. The method for producing a silica-containing composition according to any one of claims 1 to 7, further comprising an aging step of maintaining the suspension containing the silica particles obtained in the crystallization step at 50 to 90°C.

9. The method for producing a silica-containing composition according to any one of claims 1 to 8, wherein in the slurry preparation step, the slurry is heated to 50 to 200°C to react the silicon-containing waste with the sodium hydroxide.

10. The CO ２ The method for producing a silica-containing composition according to any one of claims 1 to 9, wherein the contained gas includes an exhaust gas generated in a factory.

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