Process for decolorizing aqueous silicate solutions and silicate products obtained by the process

Abstract

The application provides a method for decolorizing a silicate aqueous solution, which comprises: providing a silicate aqueous solution; adding a persulfate to the silicate aqueous solution and treating under a first treatment condition for a first time; and then adding a sulfite to the silicate aqueous solution and treating under a second treatment condition for a second time. The method achieves an excellent decolorizing effect of the silicate aqueous solution in a simple and cost-saving manner. The application also provides a low-cost and high-quality silicate product and a silicon dioxide product synthesized by the method.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of chemical synthesis, more particularly to a method for decolorizing an aqueous silicate solution and a silicate product and a silicon dioxide product obtained by the method. BACKGROUND

[0002] White carbon black is a white powder-like amorphous silicon dioxide / silica / silicate product, an environmentally friendly industrial additive with excellent performance, widely used in various industrial, commercial and consumer applications, such as in the fields of rubber, plastics, textiles, electronic device packaging, optical devices, ceramics, papermaking, pesticides, fire protection, pigments, coatings, cosmetics, food, etc. as reinforcing agents, friction agents, thickening agents, matting agents, cementing agents, anti-caking agents, fillers, film opening agents, etc.

[0003] The traditional process for preparing white carbon black is to use sodium silicate, silicon tetrachloride, tetraethyl orthosilicate, silicon-containing ores (such as wollastonite, opal, halloysite, olivine, serpentine, kaolin, hard kaolin, coal gangue, fly ash, etc.) as silicon-containing raw materials, but these traditional processes have various defects in raw material cost, process complexity, product quality, environmental pollution, waste treatment and reuse, etc. which have been long-standing and need to be solved urgently.

[0004] In order to solve the above problems, using grain bran as raw material to produce white carbon black has become a very promising alternative technology. Grain bran (such as rice husk) is not only the main by-product of grain but also the main by-product of grain processing plants. A large amount of silicon contained in grain bran ash produced by burning grain bran exists in the form of amorphous silicon dioxide, which can be obtained by traditional precipitation method to obtain white carbon black. The main process is: making rice husk ash react with NaOH to leach the silicon dioxide in the rice husk ash to form a sodium silicate solution, the remaining components are carbon black, then making sodium silicate react with sulfuric acid to generate sodium sulfate and silicic acid, and finally drying and dehydrating the silicic acid to obtain white carbon black. Compared with the traditional white carbon black process using mineral silicon source, the process using grain bran as raw material has low cost, simple process and low pollution, and can effectively utilize agricultural waste.

[0005] However, compared with the white carbon black process using mineral sand as raw material, the raw material of sodium silicate from biomass, such as rice husk ash after burning, has a rather complex composition, resulting in a sodium silicate solution with a relatively deep color, which can affect the quality of the downstream synthesized white carbon black.

[0006] Since sodium silicate aqueous solution itself is an adsorbent, the prior art such as reference 1 (Lu Fangyi, Ren Yuhong. Research on rice hull ash for white carbon black and activated carbon [J]. Chemical Engineer, 1997 (04): 4-6.) is to improve the color of water glass by improving the process of the early carbonization stage. However, due to the fluctuation of raw material quality and the difficulty in stable and effective control of carbonization process, it is impossible to keep all water glasses stable in color.

[0007] Therefore, how to improve the color performance of silicate aqueous solution from biomass source by using simple, efficient and low-cost technology, thereby obtaining high-quality silicate products (such as sodium silicate or water glass) and silicon dioxide products (such as white carbon black), has been a long-standing problem for those skilled in the art. SUMMARY

[0008] In view of the above problems, the inventors of the present application have conducted in-depth research and successfully developed a simple, low-cost and effective process method. This method achieves excellent decolorization effect of silicate solution by a very simple and cost-saving method, thereby enabling the use of biomass-derived raw materials, more preferably grain bran, to synthesize high-quality silicate products (such as sodium silicate or water glass) by low-cost and simple process steps, and further to synthesize high-quality silicon dioxide products (such as white carbon black).

[0009] The first aspect of the present application provides a method for decolorizing silicate aqueous solution, which comprises the following steps:

[0010] Step 1: providing a silicate aqueous solution;

[0011] Step 2: adding a persulfate salt to the silicate aqueous solution and treating under a first treatment condition for a first time; and

[0012] Step 3: after step 2, adding a sulfite salt to the silicate aqueous solution and treating under a second treatment condition for a second time.

[0013] The second aspect of the present application provides a silicate product obtained by the method of the present application. Preferably, the silicate product is water glass or sodium silicate.

[0014] The third aspect of the present application provides a silicon dioxide product obtained by using the silicate product obtained by the method of the present application as raw material. Preferably, the silicon dioxide product is white carbon black. BRIEF DESCRIPTION OF DRAWINGS

[0015] The following paragraphs discuss various embodiments of this application in conjunction with the accompanying drawings. However, it should be noted that the embodiments shown in the drawings and described in detail below are merely some preferred embodiments of this application, and the scope of protection of this application is defined by the claims, and not limited to these preferred embodiments. Furthermore, for clarity, the reactors and various components shown in the accompanying drawings are not drawn to scale.

[0016] Figure 1 A process flow diagram of a method for decolorizing an aqueous silicate solution according to one embodiment of this application is shown;

[0017] Figure 2 A process flow diagram is shown for a method of synthesizing an aqueous silicate solution (water glass or sodium silicate) from grain bran according to one embodiment of this application;

[0018] Figure 3 The absorbance of the sodium silicate solutions after treatment in Examples 1-3 and Comparative Examples 1-5 is shown in summary. Detailed Implementation

[0019] The “range” disclosed in this document takes the form of a lower limit and an upper limit. It can be one or more lower limits and one or more upper limits, respectively. A given range is defined by selecting a lower limit and an upper limit. The selected lower and upper limits define the boundaries of the particular range. All ranges that can be defined in this way are inclusive and composable; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is also expected that ranges of 60-110 and 80-120 are also included. Furthermore, if the minimum range values ​​are listed as 1 and 2, and if the maximum range values ​​are listed as 3, 4, and 5, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5.

[0020] In this application, unless otherwise stated, the numerical range "ab" is a shortened representation of any combination of real numbers from a to b, where a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed in this document, and "0-5" is simply a shortened representation of these numerical combinations.

[0021] Unless otherwise specified in this application, all embodiments and preferred embodiments mentioned herein can be combined to form new technical solutions.

[0022] Unless otherwise specified, all technical features and preferred features mentioned herein can be combined to form new technical solutions.

[0023] In this application, unless otherwise specified, the term "comprising" as used herein can be either open-ended or closed-ended. For example, "comprising" may mean that it may also include other components not listed, or it may only include the listed components.

[0024] One embodiment of this application provides a method for decolorizing an aqueous silicate solution. For example, Figure 1 A process flow diagram illustrating a method for decolorizing a silicate solution (specifically, an aqueous silicate solution) according to one embodiment of this application is shown. Figure 1 As can be seen, the decolorization method includes step one: providing a silicate aqueous solution; step two: adding persulfate to the silicate aqueous solution and treating it under first treatment conditions for a first time; and step three: after step two, adding sulfite to the silicate aqueous solution and treating it under second treatment conditions for a second time.

[0025] Another embodiment of this application provides a silicate product prepared by the method of this application. This silicate product can be solid sodium silicate, hydrated sodium silicate, or an aqueous solution of sodium silicate, etc., which exhibits excellent color performance after being decolorized by the method of this invention. For example, Figure 2 A process flow diagram illustrating the method for synthesizing silicate products using biomass raw materials according to one embodiment of this application is shown. Figure 2 As can be seen, the method includes step I: forming rice husk ash by burning rice husk raw material, and treating the rice husk ash with an alkaline solution to provide a silicate aqueous solution; step II: adding persulfate to the silicate aqueous solution and treating it under first treatment conditions for a first time; step III: after step II, adding sulfite to the silicate aqueous solution and treating it under second treatment conditions for a second time. Through the above steps, silicate aqueous solutions derived from biomass raw materials (such as rice husks) with significant discoloration can be effectively decolorized to obtain high-quality silicate products with excellent color.

[0026] Another embodiment of this application provides a method for further synthesizing silica products using silicate products obtained by the method of this invention as raw materials. According to a specific embodiment of this application, the silica product is fumed silica. By using the silicate products of this application as raw materials, the fumed silica and other silica products synthesized by this invention also possess excellent color. The silica products of this application can be synthesized using conventional processes known in the art; an exemplary process is precipitation. The precipitation method may include precipitating and drying an aqueous silicate solution, that is, it can be... Figure 2Based on the steps shown, the following steps are further performed: Step IV: After step III, hydrated silica is precipitated from the silicate aqueous solution; and Step V: The hydrated silica precipitated in step IV is dried to form a silica product (e.g., precipitated silica). According to another embodiment of this application, one or more of the following steps may be performed before and / or after steps IV and V as needed: calcination, cooling, concentration, dilution, filtration, washing, pulping, etc.

[0027] According to an exemplary embodiment of this application, in the above Figure 1 Step 1 and above of the illustrated implementation method Figure 2 In step I of the embodiment shown, the step of providing the silicate aqueous solution may optionally include pretreatment of the silicon-containing raw material, such as washing, crushing, sorting, sieving, drying, calcining, etc., so that the morphology and particle size of the silicon-containing raw material are as favorable as possible for the subsequent acid dissolution or alkali dissolution operation, and the impurity components therein are minimized. However, depending on the specific circumstances, if the state of the silicon-containing raw material itself is very suitable for the subsequent acid dissolution or alkali dissolution operation, or simply for the purpose of saving costs and / or simplifying the process, the above pretreatment may not be performed.

[0028] The following description primarily focuses on silicate solutions prepared using grain bran ash as a silicon-containing raw material. However, this is merely illustrative, and the scope of protection of this application is not limited thereto. It should be specifically noted that the decolorization method of this application can actually be used to treat silicate solutions prepared using any silicon-containing raw material (such as grain bran or the silicon-containing mineral raw materials mentioned above, such as sodium silicate, silicon tetrachloride, tetraethyl orthosilicate, wollastonite, opal, halloysite, olivine, serpentine, kaolin, hard kaolin, coal gangue, fly ash, etc.), and similarly achieves the advantages of low cost, simple process, and good decolorization effect of this invention. Furthermore, the method of this application can also be used to decolorize commercially available silicate solutions or silicate solutions provided by other manufacturers or research institutions. According to another embodiment of this application, in addition to being used for synthesizing silica products (e.g., white carbon black), the decolorized silicate solution can also be used for other purposes, such as as a raw material for synthesizing zeolite molecular sieves, weathering / fireproof coatings, foundation grouting, quick-setting waterproofing agents, acid-resistant / heat-resistant mortars, acid-resistant / heat-resistant concrete, etc.

[0029] In exemplary embodiments of this application, the silicate aqueous solution is a biomass source, such as grain bran. The bran can be separated from the grains during operations such as threshing, milling, pressing, dehulling, and crushing. Depending on the specific processing technology, this grain bran can be relatively intact or already crushed or pulverized. The grain bran can include bran selected from the following grains: rice, upland rice, wheat, barley, oats, rye, buckwheat, corn, sorghum, millet, foxtail millet, yellow millet, soybean, broad bean, pea, mung bean, red bean, kidney bean, and combinations thereof. In the exemplary embodiments below, rice bran (rice husk) is used as an example to describe the method of this application; however, it should be understood that the grain bran described in this application is not limited to rice husk raw materials, but includes all the grain bran mentioned above.

[0030] For example, according to a specific exemplary embodiment, rice husks are used as a silicon-containing raw material to synthesize a silicate solution. First, the rice husks are calcined, which can be carried out in a desired atmosphere [e.g., an oxygen atmosphere, an air atmosphere, a conditioned atmosphere (the oxygen content can be adjusted to a low or high level) or an inert atmosphere], for example, in a processing boiler (staged combustion) to fully utilize the energy generated by combustion while obtaining rice husk ash; calcination (combustion) can be carried out in a muffle furnace; or high-temperature carbonization can be performed in a cyclone carbonization furnace to obtain rice husk ash; or any high-temperature calcination equipment or process known in the art or developed hereafter can be used to calcine the rice husks to obtain rice husk ash.

[0031] During its growth, rice absorbs silica from the soil as part of its bran. Generally, the silica content in rice husks before firing is about 16-20% by weight, while the silica content in the rice husk ash obtained after firing increases significantly, for example, to about 40-70% by weight, or 45-60% by weight, or about 50% by weight. The remaining components in rice husk ash are mainly carbon.

[0032] In the above Figure 2In step I of the illustrated embodiment, the rice husk ash is subsequently dissolved using an alkaline solution to obtain an aqueous silicate solution. According to one embodiment of this application, the alkaline solution is a solution of an alkaline reagent, such as an aqueous solution of an alkaline solvent. The alkaline reagent may be selected from: sodium hydroxide, potassium hydroxide, sodium bicarbonate, potassium bicarbonate, sodium carbonate, potassium carbonate, or a mixture of two or more of the above alkaline reagents. According to one embodiment of this application, based on the total weight of the alkaline solution, the content of the alkaline reagent in the alkaline solution may be 2-40% by weight, for example, 5-30% by weight, or 8-20% by weight, or 10-15% by weight, or 12-14% by weight. According to one embodiment of this application, the leaching operation can be carried out at room temperature or under heating conditions, for example, at a temperature of 15-150°C. Heating temperatures exceeding those of the alkaline solution can be achieved by heating under pressure, or the heating temperature can be 25-120°C, 30-100°C, 50-100°C, 60-100°C, 70-100°C, 80-100°C, or 90-100°C. According to a preferred embodiment of this application, the leaching treatment can be carried out under normal pressure by treating rice husk ash with a boiled alkaline solution. According to another embodiment of this application, the duration of the alkaline leaching treatment can be 10 minutes to 3 days, or 15 minutes to 48 hours, or 20 minutes to 24 hours, or 30 minutes to 16 hours, or 40 minutes to 12 hours, or 1-10 hours, or 2-8 hours, or 3-6 hours, or 4-5 hours. The aqueous silicate solution may optionally be concentrated or diluted, or optionally filtered, decanted, or vacuum-filtered to remove solid impurities (e.g., residual insoluble ash, carbon residue, etc.). Based on the total weight of the silicate solution obtained from the alkali dissolution operation, the silicate concentration may be from 1 wt% to 55 wt%, for example, 5 wt% to 50 wt%, or 8 wt% to 45 wt%, or 10 wt% to 40 wt%, or 12 wt% to 35 wt%, or 15 wt% to 30 wt%, or 17 wt% to 25 wt%, or 18-22 wt%, or within a range derived from any combination of the above two extreme values.

[0033] According to another embodiment of this application, the silicate in the silicate aqueous solution can be an alkali metal silicate, such as sodium silicate, potassium silicate, lithium silicate, rubidium silicate, cesium silicate, or any combination thereof. Most preferably, the silicate is sodium silicate, and more preferably, it does not contain any silicates other than sodium silicate. According to another embodiment of this application, the modulus (i.e., the SiO2:M2O molar ratio, where M is lithium, sodium, potassium, rubidium, cesium, or a combination thereof) of the silicate aqueous solution can be 2.20-3.60, for example 2.20-2.50, or 2.60-2.90, or 3.10-3.40, or 3.41-3.60, or can be within the range of values ​​obtained by combining any two of the above endpoints.

[0034] According to a non-limiting embodiment of this application, the silicate solution prepared from rice husks may contain a certain amount of impurities, thereby exhibiting a certain degree of color or turbidity. Therefore, absorbance can be used to effectively characterize the purity of the silicate solution. The absorbance of the silicate solution can be detected using a spectrophotometer at a specific wavelength or wavelength range. For example, in an exemplary embodiment of this application, an ultraviolet spectrophotometer or an ultraviolet-visible spectrophotometer can be used to detect the absorbance of the silicate solution at a suitable detection wavelength. For example, by... Figure 1 Step one or above of the embodiments shown Figure 2 The absorbance of the silicate solution provided in step I of the illustrated embodiment may be ≥0.8, for example ≥0.9, or ≥1.0, or ≥1.1, or ≥1.2, or ≥1.3, or ≥1.4, or ≥1.5, or ≥1.6, or ≥1.7, or ≥1.8, or the absorbance may be within the range obtained by combining any two of the above endpoints.

[0035] Next, in Figure 1 Step two or above of the illustrated implementation method Figure 2In step II of the illustrated embodiment, persulfate is subsequently added to the silicate solution, and the solution is treated for a first time under first treatment conditions. According to one embodiment of this application, the persulfate may be selected from sodium persulfate, potassium persulfate, ammonium persulfate, or a combination of two or three of the above, preferably sodium persulfate. According to one embodiment of this application, the persulfate may be added directly to the silicate aqueous solution in solid form. According to another embodiment of this application, the persulfate may be added to the silicate aqueous solution in aqueous form, and the concentration of the persulfate in the aqueous solution, based on the total weight of the persulfate aqueous solution, may be 0.05-20% by weight, for example, 0.1-18% by weight, or 0.2-17% by weight, or 0.5-15% by weight, or 0.8-14% by weight, or 1-12% by weight, or 2-10% by weight, or 3-8% by weight, or 4-7% by weight, or 5-6% by weight, or within a numerical range obtained by combining any two of the above endpoints. According to one embodiment of this application, based on 100% by mass of the total mass of the silicate aqueous solution, the amount of persulfate added thereto (based on the dry mass of the persulfate) can be 0.001-5% by mass, for example, 0.01-4.5% by mass, or 0.05-4.0% by mass, or 0.1-3.5% by mass, or 0.15-3.0% by mass, or 0.20-2.5% by mass, or 0.25-2.0% by mass, or 0.30-1.5% by mass, or 0.35-1.0% by mass, or 0.40-0.80% by mass, or 0.42-0.70% by mass, or 0.45-0.60% by mass, or 0.50-0.55% by mass, or within the range of values ​​obtained by combining any two of the above endpoints. According to another embodiment of this application, the first processing condition includes heating the silicate aqueous solution. For example, the heating temperature can be 20-200°C (wherein a heating temperature higher than the room temperature boiling point of the aqueous solution can be achieved by heating under pressure), for example, 30-180°C, or 40-170°C, or 50-165°C, or 55-160°C, or 60-150°C, or 65-120°C, or 70-105°C, or 75-100°C, or 80-100°C, or 85-100°C, or 90-100°C, or 95-100°C, or a numerical range obtained by combining any two of the above extreme values. According to a preferred embodiment of this application, the first processing includes heating the silicate aqueous solution (in which the persulfate has been added) to boiling under normal pressure.According to another embodiment of this application, the first processing lasts for a first time, which can be from 10 minutes to 24 hours, for example, 20 minutes to 20 hours, or 30 minutes to 18 hours, or 40 minutes to 16 hours, or 50 minutes to 12 hours, or 1-10 hours, or 1.2-8 hours, or 1.5-6 hours, or 1.8-5 hours, or 2-4 hours, or 2-3 hours, or within the range of values ​​obtained by combining any two of the above endpoints.

[0036] Next, in Figure 1 Steps three and above in the illustrated implementation method Figure 2In step III of the illustrated embodiment, sulfite is subsequently added to the silicate solution, and the solution is treated for a second time under second treatment conditions. According to one embodiment of this application, the sulfite may be selected from sodium persulfite, potassium sulfite, ammonium sulfite, or a combination of two or three of the above, preferably sodium sulfite. According to one embodiment of this application, the sulfite may be added directly to the silicate aqueous solution in solid form. According to another embodiment of this application, the sulfite may be added to the silicate aqueous solution in aqueous form, and the concentration of the sulfite in the aqueous solution, based on the total weight of the sulfite aqueous solution, may be 0.01-25% by weight, for example, 0.05-20% by weight, or 0.1-18% by weight, or 0.5-15% by weight, or 0.8-14% by weight, or 1-12% by weight, or 2-10% by weight, or 3-8% by weight, or 4-7% by weight, or 5-6% by weight, or may fall within the range of values ​​obtained by combining any two of the above endpoints. According to one embodiment of this application, based on the total mass of the silicate aqueous solution as 100% by mass, the amount of sulfite added thereto (based on the dry mass of sulfite) can be 0.001-5% by mass, for example, 0.01-4.5% by mass, or 0.02-4.0% by mass, or 0.03-3.5% by mass, or 0.04-3.0% by mass, or 0.05-2.5% by mass, or 0.05-2.0% by mass, or 0.05-1.5% by mass, or 0.05-1.0% by mass, or 0.05-0.80% by mass, or 0.05-0.50% by mass, or 0.05-0.20% by mass, or 0.05-0.10% by mass, or within the range of values ​​obtained by combining any two of the above endpoints. According to another embodiment of this application, the second processing condition includes heating the silicate aqueous solution. For example, the heating temperature can be 20-200℃ (where a heating temperature higher than the room temperature boiling point of the aqueous solution can be achieved by heating under pressure), for example, 30-180℃, or 40-170℃, or 50-165℃, or 55-160℃, or 60-150℃, or 65-120℃, or 70-105℃, or 75-100℃, or 80-100℃, or 85-100℃, or 90-100℃, or 95-100℃, or a numerical range obtained by combining any two of the above extreme values. According to a preferred embodiment of this application, the second processing includes heating the silicate aqueous solution (in which the persulfate and the sulfite have been added sequentially) to boiling under normal pressure.According to another embodiment of this application, the second process lasts for a second time, which can be from 10 minutes to 24 hours, for example, 20 minutes to 20 hours, or 30 minutes to 18 hours, or 40 minutes to 16 hours, or 50 minutes to 12 hours, or 1-10 hours, or 1.2-8 hours, or 1.5-6 hours, or 1.8-5 hours, or 2-4 hours, or 2-3 hours, or within a numerical range obtained by combining any two of the above endpoints.

[0037] According to this application Figure 1 Steps one through three shown or Figure 2 After steps I to III, an effectively decolorized silicate solution can be obtained. According to one embodiment of this application, after step III, the absorbance of the silicate solution can decrease significantly, for example, to ≤0.59, ≤0.55, ≤0.52, ≤0.50, ≤0.45, ≤0.40, ≤0.35, ≤0.30, ≤0.25, ≤0.20, ≤0.15, or ≤0.12. The lower limit of absorbance can be ≥0.01, ≥0.02, ≥0.03, ≥0.05, ≥0.07, ≥0.09, or ≥0.10, or the absorbance after treatment can be within the range of values ​​obtained by combining any two of the above endpoints.

[0038] A key technical feature of this application is that persulfate is first added to the silicate solution to be decolorized, followed by a first treatment period (first time period), and then sulfite is added to the silicate solution. That is, in this application, the components are added in a manner that prioritizes persulfate followed by sulfite, with a time interval (first time period) between the addition of persulfate and sulfite. According to one embodiment of this application, there is no point in time where persulfate and sulfite are added simultaneously, nor is sulfite added before persulfate.

[0039] As described above, after decolorizing the silicate solution according to the steps of this application, silica products, such as precipitated silica, can be synthesized from the decolorized silicate solution using conventional processing steps. In one embodiment of this application for synthesizing precipitated silica, after obtaining the decolorized silicate aqueous solution through steps I to III, steps IV and V are continued on the silicate aqueous solution to prepare precipitated silica. According to an exemplary embodiment, in step IV, by changing the conditions of the silicate aqueous solution, such as lowering the temperature of the silicate aqueous solution (and optionally adjusting its pH value), hydrated silica (SiO2·nH2O, where n can be a value of 0.5-15, can be a fraction or an integer, for example, 0.8-12, or 1.0-10, or 2-8, or 3-6, or 4-5, or a range of values ​​obtained by combining any two of the above endpoints) precipitates out. According to a preferred embodiment, hydrated silica is precipitated by lowering the temperature of the silicate aqueous solution to room temperature or lower, for example, by lowering the temperature to ≤30°C, or ≤25°C, or ≤20°C, or ≤15°C, or ≤12°C, or ≤10°C, or ≤8°C, or ≤5°C. According to another preferred embodiment of this application, the pH of the silicate aqueous solution is further adjusted in step IV to promote the precipitation of hydrated silica, for example, by adding an acid (e.g., dilute hydrochloric acid or dilute sulfuric acid) to adjust the pH of the silicate aqueous solution to ≤13, for example, ≤12, or ≤11, or ≤10. After the precipitation, the hydrated silica precipitate can be separated from the filtrate by filtration (e.g., vacuum filtration). According to a limiting embodiment, the filtrate can be directly recycled for alkaline leaching of rice husk ash (in the case where pH adjustment is not performed).

[0040] Next, in step V, the hydrated silica precipitate obtained in step IV is optionally washed and then dried to obtain the target silica product. According to one embodiment of this application, step V can be carried out in an air atmosphere, an oxidizing atmosphere, a reducing atmosphere, a vacuum environment, or an inert atmosphere. The drying step can be carried out at the desired temperature, such as 50-200°C, 60-180°C, 70-170°C, 80-150°C, or 100-120°C, using a heating device such as a vacuum oven or a convection oven. The silica obtained through step V has better color and more stable product quality due to the decolorization process specifically designed in this invention.

[0041] The present application is described below by way of specific embodiments, the purpose of which is to provide a better understanding of the content of the present application. It should be understood that these embodiments are merely illustrative and not restrictive. The reagents used in the embodiments, unless otherwise stated, are commercially available. The methods and conditions used in the embodiments, unless otherwise specified, are conventional methods and conditions.

[0042] Example

[0043] The following examples illustrate the specific implementation of the method of this application using the preparation of silicate aqueous solution from rice husk raw material, and comparative examples demonstrate the key role of the design of the process steps in this application. Unless otherwise stated, all chemical reagents used in the following examples are chemically pure, purchased from suppliers and used directly without further processing, and all water used is distilled water.

[0044] In the following examples, the color of the solution is characterized by absorbance, and the test steps are as follows: using a Shimadzu UV-2700 ultraviolet spectrophotometer, an appropriate amount of the liquid to be tested is taken and placed in a cuvette, and its absorbance is measured using a detection wavelength of 300 nm, with the same amount of redistilled water as the baseline.

[0045] Example 1:

[0046] In this embodiment, a transparent sodium silicate aqueous solution obtained by dissolving rice husk ash provided by Fenghai (Panjin) Rice Biotechnology Co., Ltd. was used as a raw material. The concentration of the sodium silicate aqueous solution was 20.6% by weight, the modulus was 2.9, and its absorbance was 1.4.

[0047] 100 g of the sodium silicate solution described above was added to a 250 mL three-necked flask equipped with a paddle stirrer and a reflux condenser. The sodium silicate solution was heated to a gentle boil with continuous stirring, and this gentle boil was maintained throughout the subsequent process. 0.5 g of sodium persulfate was added to the flask. After the sodium persulfate was added, the mixture in the flask was stirred continuously at a gentle boil for 2 hours. 0.05 g of sodium sulfite was added to the flask. After the sodium sulfite was added, the mixture in the flask was stirred continuously at a gentle boil for 2 hours. Then stirring was stopped, and the absorbance of the sodium silicate aqueous solution was measured to be 0.12.

[0048] Example 2:

[0049] 100 g of the sodium silicate solution with an absorbance of 1.4 as described in Example 1 was added to a 250 mL three-necked flask equipped with a paddle stirrer and a reflux condenser. The sodium silicate solution was heated to a gentle boil under continuous stirring, and the solution in the flask was kept at a gentle boil throughout the subsequent process. 0.8 g of sodium persulfate was added, and the material in the flask was stirred continuously under a gentle boil for 2 hours. Then, 0.1 g of sodium sulfite was added, and the material in the flask was stirred continuously under a gentle boil for 2 hours. The absorbance of the resulting sodium silicate solution was measured to be 0.11.

[0050] Example 3:

[0051] 100 g of sodium silicate solution prepared from rice husk ash (absorbance 1.4 as described in Example 1) was added to a 250 mL three-necked flask equipped with a paddle stirrer and a reflux condenser. The sodium silicate solution was heated to a gentle boil under continuous stirring, and this gentle boil was maintained throughout the subsequent process. 0.3 g of sodium persulfate was added, and the mixture was stirred under gentle boiling conditions for 2 hours. Then, 0.02 g of sodium sulfite was added, and the mixture was stirred under gentle boiling conditions for another 2 hours. The resulting sodium silicate solution had an absorbance of 0.13.

[0052] Comparative Example 1

[0053] In Comparative Example 1, the same steps and process conditions as in Example 1 were used, except that after adding 0.5 g of sodium persulfate aqueous solution, the material in the flask was continuously stirred under gentle boiling conditions for 2 hours. Then, instead of adding sodium sulfite aqueous solution, the material in the flask was stirred under gentle boiling conditions for another 2 hours. Stirring was then stopped, and the absorbance of the resulting sodium silicate solution was measured to be 0.6.

[0054] Comparative Example 2

[0055] 100 g of sodium silicate solution prepared from rice husk ash (absorbance 1.4 as described in Example 1) was added to a 250 mL three-necked flask equipped with a paddle stirrer and a reflux condenser. The sodium silicate solution was heated to a gentle boil under continuous stirring. 0.02 g of sodium sulfite was added. The material in the flask was stirred continuously under a gentle boil for 4 hours. The absorbance of the resulting sodium silicate solution was measured to be 1.4.

[0056] Comparative Example 3

[0057] In Comparative Example 3, 100 g of the sodium silicate solution prepared as described in Example 1 was added to a three-necked flask equipped with a paddle stirrer and a reflux condenser. The sodium silicate solution was heated to a gentle boil under continuous stirring, and the solution in the flask was kept at a gentle boil throughout the subsequent process. 0.02 g of sodium sulfite was added, and the mixture in the flask was stirred continuously under a gentle boil for 2 hours. Then, 0.5 g of sodium persulfate was added, and the mixture in the flask was stirred continuously under a gentle boil for 2 hours. Stirring was then stopped, and the absorbance of the resulting sodium silicate solution was measured to be 0.6.

[0058] Comparative Example 4

[0059] In Comparative Example 4, 100 g of the sodium silicate solution prepared as described in Example 1 was added to a 250 mL three-necked flask equipped with a paddle stirrer and a reflux condenser. The sodium silicate solution was heated to a gentle boil under continuous stirring, and this gentle boil was maintained throughout the subsequent process. 0.02 g of sodium sulfite and 0.5 g of sodium persulfate were then rapidly added to the reaction flask. After the addition was complete, the material in the flask was continuously stirred under a gentle boil for 4 hours. Stirring was then stopped, and the absorbance of the resulting sodium silicate solution was measured to be 0.8.

[0060] Comparative Example 5

[0061] 100 g of the sodium silicate solution prepared as described in Example 1 was added to a 250 mL three-necked flask equipped with a paddle stirrer, a reflux condenser, and a adding funnel. The sodium silicate solution was heated to a gentle boil under continuous stirring, and this gentle boil was maintained throughout the subsequent process. 2 g of a 30% sodium hypochlorite aqueous solution was added to the flask. After the addition was complete, the mixture in the flask was stirred continuously under a gentle boil for 2 hours. 0.05 g of the solution was added to the flask, and the mixture was stirred continuously under a gentle boil for 2 hours. The absorbance of the resulting sodium silicate solution was measured to be 0.9.

[0062] The absorbance of the sodium silicate solutions after treatment in Examples 1-3 and Comparative Examples 1-5 is summarized and plotted graphically. Figure 3 As can be seen, Comparative Examples 1-5 failed to achieve the desired sodium silicate solution purification and decolorization effect because either persulfate or sulfite was omitted (Comparative Examples 1-2), the order of their addition was changed (Comparative Example 3), they were not added sequentially (Comparative Example 4), or an oxidant different from that of the present invention was used (Comparative Example 5). In contrast, Examples 1-3 of the present invention achieved excellent solution purification and decolorization effects.

[0063] In addition, it is conceivable that using the sodium silicate solution of Examples 1-3 of the present invention, through conventional precipitation and drying operations, will also be able to synthesize high-quality silica products such as fumed silica.

[0064] The examples above are merely specific examples listed in this application, but the technical features of this application are not limited thereto. Any simple changes, equivalent substitutions, or other modifications made based on this application to solve essentially the same technical problem and achieve essentially the same technical effect are all covered within the protection scope of this application.

Claims

1. A method for decolorizing an aqueous silicate solution, the method comprising the following steps: Step 1: Provide an aqueous silicate solution; the aqueous silicate solution is derived from biomass; Step 2: Add persulfate to the silicate aqueous solution and treat it under the first treatment conditions for the first time; based on the total mass of the silicate aqueous solution, the amount of persulfate is 0.001-5% by mass, the first treatment conditions include a treatment temperature of 20-200°C, and the first time is equal to or greater than 10 minutes; as well as Step 3: After step 2, sulfite is added to the silicate aqueous solution and treated under the second treatment conditions for the second time. The amount of sulfite is 0.001-5% by mass based on the total mass of the silicate aqueous solution. The second treatment conditions include a treatment temperature of 20-120°C and the second time is equal to or greater than 10 minutes.

2. The method as described in claim 1, characterized in that, The persulfate is selected from sodium persulfate, potassium persulfate, ammonium persulfate, and combinations thereof.

3. The method as described in claim 1, characterized in that, The sulfite is selected from sodium sulfite, potassium sulfite, ammonium sulfite, and combinations thereof.

4. The method as described in claim 1, characterized in that, The persulfate is sodium persulfate, and the sulfite is sodium sulfite.

5. The method as described in claim 1, characterized in that, In step one, based on the total weight of the silicate aqueous solution, the silicate content in the silicate aqueous solution is 1-55% by weight. The silicate aqueous solution has a modulus of 2.20-3.60, where the modulus represents the SiO2:M2O molar ratio, and M is lithium, sodium, potassium, rubidium, cesium, or a combination thereof.

6. The method as described in claim 1, characterized in that, In step one, based on the total weight of the silicate aqueous solution, the silicate content in the silicate aqueous solution is 15-25% by weight.

7. The method as described in claim 1, characterized in that, The silicate aqueous solution provided in step one is derived from grain bran; the grain bran is selected from the bran of the following grains: rice, upland rice, wheat, barley, oats, rye, buckwheat, corn, sorghum, millet, foxtail millet, yellow millet, soybean, broad bean, pea, mung bean, red bean, kidney bean, and combinations thereof.

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