Preparation method of nano silicon dioxide

Abstract

The invention relates to the technical field of resource utilization of solid waste fly ash, and discloses a preparation method of nano silicon dioxide. The method comprises the following steps: mixing pulverized coal ash subjected to grinding and activating treatment with a soluble alkali metal salt solution, reacting under a microwave condition, and separating to obtain a sodium silicate solution; and dropwise adding an acid solution in the microwave environment again until the solution is neutral to generate silica gel, centrifuging, filtering and drying to obtain a nano silicon dioxide product. According to the method, resource utilization of the fly ash is achieved, the high-value nano-silica is synthesized, the process is simple and rapid, less equipment is used, and the method is a fly ash resource utilization method with great prospects.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of resource utilization of solid waste fly ash, and particularly relates to a preparation method of nano silicon dioxide. BACKGROUND

[0002] Fly ash is a fine particle waste produced in the combustion process of coal-fired power plants, mainly formed by the non-combustible minerals in coal melting and cooling at high temperature. With the development of the thermal power industry, the annual output of fly ash is increasing year by year. If not properly handled, fly ash will occupy land, pollute air and water, and even harm human health.

[0003] Fly ash is low in price, and its main components are SiO2 and Al2O3, containing a small amount of Fe2O3, CaO, K2O and other inorganic oxides. Among them, SiO2 mainly exists in the form of amorphous silica, quartz and mullite, and the content is generally 40-60wt%, which can be used as a cheap silicon source. Therefore, extracting and preparing nano silicon dioxide from fly ash can effectively utilize solid waste fly ash, reduce the cost of raw materials, effectively activate desilicated fly ash, increase the proportion of aluminum elements and make it easier to be removed, which is helpful for further recovery of alumina.

[0004] At present, the traditional method for extracting silicon dioxide from fly ash mainly adopts the method of adding additives for roasting activation and adjusting pH with inorganic acid to prepare silicon dioxide, which has high energy consumption and strict requirements for equipment. Another common method is to first immerse the silicon dioxide in fly ash with a high-quality concentrated NaOH solution to generate sodium silicate solution, dilute the sodium silicate, and pass carbon dioxide to produce carbonic acid. The sodium silicate in the solution generates silicic acid and sodium carbonate solution under the action of carbonic acid. The sodium carbonate solution is added with quicklime to react to regenerate NaOH, which is then recycled after concentration. Although this technology is mature and complete, it has a long process, high energy and various additive consumption, and many equipment, resulting in high final cost.

[0005] Therefore, it is urgent to develop a preparation method of nano silicon dioxide with high extraction rate of silicon dioxide and uniform and controllable product particle size. SUMMARY

[0006] The purpose of the present application is to overcome the problems existing in the prior art, and to provide a method for extracting and synthesizing nano silicon dioxide by using microwave-assisted method, which aims to provide a fast and efficient method for preparing nano silicon dioxide for the resource utilization of fly ash, and to improve the production efficiency.

[0007] In order to achieve the above purpose, the present application provides a preparation method of nano silicon dioxide, wherein the method comprises the following steps:

[0008] (1) mixing fly ash, soluble alkali metal salt and water under a first microwave condition to generate a mixed product of desilicated fly ash and sodium silicate solution;

[0009] The first microwave condition comprises a power of 400-800w, a time of 0.25-2h and a temperature of 50-100℃;

[0010] (2) separating the mixed product obtained in step (1) to obtain desilicated fly ash as filter residue and sodium silicate extraction solution as filtrate;

[0011] (3) under a second microwave condition, adding an acid solution to the sodium silicate extraction solution obtained in step (2) to adjust the pH to 7-8;

[0012] The second microwave condition comprises a power of 300-600w, a time of 1-30min and a temperature of 30-80℃;

[0013] (4) centrifuging, washing and drying the obtained mixture to obtain nano-silicon dioxide.

[0014] Through the above technical solution, the beneficial technical effects obtained by the present application are as follows:

[0015] (1) The present application uses industrial solid waste fly ash as raw material and adopts microwave-assisted form to extract and prepare nano-silicon dioxide. In the extraction process, under the action of microwave, the charged particles (ions) dissolved in sodium hydroxide oscillate before and after under the influence of the microwave field, and the electric field of the microwave exerts torque on the electric dipole; molecules (such as water) with dipole moment try to align these dipoles with the electric component of the electromagnetic field, so they keep rotating, collide with the adjacent silicon dioxide, and can quickly dissolve most of the amorphous silicon dioxide. In the preparation process, the microwave also makes the silicon dioxide generate quickly and limit its growth through ion oscillation, and finally produces nano-silicon dioxide.

[0016] (2) The nano-silicon dioxide product prepared by the present application has uniform particle size and very fine particles. And the extraction of silicon dioxide from fly ash and the synthesis time of nano-silicon dioxide product are greatly shortened, and the production efficiency is improved.

[0017] (3) The whole process of the present application is simple, uses less equipment, has small investment, low cost, does not emit any toxic and harmful substances, and has small waste liquid discharge. The energy-saving effect is remarkable, the energy does not need to be transmitted in the microwave, the heat energy consumption is greatly reduced, the heat energy utilization rate is high, and the energy is greatly saved. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The process flowchart of the present application.

[0019] Figure 2 Scanning electron microscope image of the nanosilica prepared in Example 2 of the present application. DETAILED DESCRIPTION

[0020] The endpoints of the ranges and any values claimed herein are not to be understood as limited. They are understood to be approximate values. It is specifically intended that the endpoints of the ranges provided in the present application are inclusive of any and all

[0021] The present application provides a method for preparing nanosilica, wherein the method comprises the following steps:

[0022] (1) mixing fly ash, soluble alkali metal salt and water under a first microwave condition to generate a mixed product of desilicated fly ash and sodium silicate solution;

[0023] wherein the first microwave condition comprises: power of 400-800w, time of 0.25-2h, and temperature of 50-100℃;

[0024] (2) separating the mixed product obtained in step (1) to obtain desilicated fly ash as filter residue and sodium silicate extraction solution as filtrate;

[0025] (3) under a second microwave condition, adding an acid solution to the sodium silicate extraction solution obtained in step (2) to adjust the pH to 7-8;

[0026] wherein the second microwave condition comprises: power of 300-600w, time of 1-30min, and temperature of 30-80℃;

[0027] (4) centrifuging, washing and drying the obtained mixture to obtain nanosilica.

[0028] The principle of microwave-assisted synthesis is to use microwave radiation to directly act on the reaction system, accelerate chemical reactions through dielectric heating and dipole polarization mechanism: the microwave electric field makes the polar molecules oscillate at high frequency, generates heat through intermolecular friction, and realizes rapid and uniform heating; the microwave electric field promotes the rapid rearrangement of molecular dipole moment, reduces the reaction activation energy, and improves the reaction rate. And microwave heating is a bulk heating, which avoids thermal gradient effect and makes the reaction more efficient and controllable. The present application improves the extraction rate of silica by the method, and the prepared silica product has uniform particle size and controllable size.

[0029] In the present application, the power of the first microwave condition is, for example, 400 w, 500 w, 600 w, 700 w, 800 w, and any value within the range between any two of the above values; the time is, for example, 0.25 h, 0.5 h, 1 h, 2 h, and any value within the range between any two of the above values; and the temperature is, for example, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, and any value within the range between any two of the above values.

[0030] In the present application, the power of the second microwave condition is, for example, 300 w, 400 w, 500 w, 600 w, and any value within the range between any two of the above values; the time is, for example, 1 min, 3 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, and any value within the range between any two of the above values; and the temperature is, for example, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, and any value within the range between any two of the above values. In addition, the second microwave condition can include a suitable rotation speed to make the reaction solution uniform.

[0031] In the present application, the concentration of the acid solution can be 0.5-1 mol / L, which can adjust the pH to a suitable value.

[0032] In some embodiments of the present application, in step (1), the fly ash is fly ash after a grinding activation treatment.

[0033] In some embodiments of the present application, the grinding activation treatment is performed for 3 h.

[0034] In some embodiments of the present application, the volume average diameter of the fly ash is less than 10 μm.

[0035] In some embodiments of the present application, the soluble alkali metal salt is selected from at least one of sodium hydroxide and potassium hydroxide, and is preferably sodium hydroxide.

[0036] In some embodiments of the present application, in step (1), the mass ratio of the fly ash to the soluble alkali metal salt is 1:0.5-2, for example, 1:0.5, 1:1, 1:2, and any value within the range between any two of the above values, and is preferably 1:1.

[0037] In some embodiments of the present application, the mass ratio of the sum of the fly ash and the soluble alkali metal salt to water is 1:1-5, for example, 1:1, 1:2, 1:3, 1:4, 1:5, and any value within the range between any two of the above values, and is preferably 1:3.

[0038] In some embodiments of the present application, in step (1), a normal-pressure microwave synthesis reaction workstation is used to provide the first microwave condition.

[0039] In some embodiments of the present application, the first microwave condition comprises: power of 700w, time of 0.5h, and temperature of 90℃.

[0040] In some embodiments of the present application, the desilicated fly ash obtained in step (2) is used for extracting alumina.

[0041] In some embodiments of the present application, in step (3), the acid of the acid solution is selected from at least one of sulfuric acid, hydrochloric acid, citric acid and oxalic acid, and is preferably citric acid.

[0042] In some embodiments of the present application, in step (3), a normal pressure microwave synthesis reaction workstation is used to provide the second microwave condition.

[0043] In some embodiments of the present application, the second microwave condition comprises: power of 400w, temperature of 70℃, and time of 5min.

[0044] In some embodiments of the present application, the centrifugation in step (4) comprises: re-dispersing the solid phase of the previous centrifugation in water and centrifuging again, repeating the centrifugation operation for 6 times to obtain a final solid phase, and then washing and drying to obtain the nano-silicon dioxide.

[0045] According to a particularly preferred embodiment of the present application, a method for preparing nano-silicon dioxide, wherein the method comprises the following steps:

[0046] (1) mixing the fly ash subjected to the activation treatment by pulverization and sodium hydroxide, and the mass ratio of the fly ash to sodium hydroxide being 1:0.5-2.0;

[0047] (2) pouring the mixture of the fly ash and sodium hydroxide into water and stirring thoroughly, and the mass ratio of the mixture to water being 1:1-5;

[0048] (3) during the stirring, placing the material in a normal pressure microwave synthesis reaction workstation to react, the power being 400-700w, the reaction time being 0.25-2h, and the reaction being carried out at 50-100℃ to generate a mixed product of desilicated fly ash and sodium silicate solution;

[0049] (4) separating the mixed product in step (3), and the filter residue being the desilicated fly ash and the filtrate being the sodium silicate extraction solution;

[0050] (5) placing the sodium silicate extraction solution in the normal pressure microwave synthesis reaction workstation again, the power being 300-600w, the reaction time being 1-30min, the temperature being 30-80℃, and the pH being adjusted to neutral under stirring by using an acid solution;

[0051] (6) centrifuging, washing and drying the obtained mixture to obtain the nano-silicon dioxide.

[0052] The application will be described in detail below by way of examples.

[0053] The specific conditions not mentioned in the following examples and comparative examples are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments not mentioned by the manufacturer are the conventional products that can be obtained by the commercial route.

[0054] The fly ash used in each embodiment of the application has the following composition: the content of silicon dioxide is 46.213%, the content of aluminum oxide is 34.053%, the content of iron oxide is 4.634%, the content of calcium oxide is 3.041%, and the balance is other components.

[0055] The atmospheric microwave synthesis reaction workstation used in the application is of the MAS-II type and is manufactured by Shanghai Xinyi Microwave Chemical Technology Co., Ltd.

[0056] Example 1

[0057] This example is used to illustrate the preparation of nano-silicon dioxide. The process flow is shown in Figure 1 .

[0058] (1) The fly ash (volume average diameter less than 10 μm) activated by grinding for 3 h is mixed with sodium hydroxide, and the mass ratio of fly ash to sodium hydroxide is 1:0.5;

[0059] (2) The mixture of fly ash and sodium hydroxide is poured into water and stirred thoroughly, and the mass ratio of the mixture to water is 1:2;

[0060] (3) During the stirring process, the material is placed in the atmospheric microwave synthesis reaction workstation for reaction, the power is 500 w, the reaction time is 0.25 h, and the desilicated fly ash and sodium silicate solution are generated at 90°C;

[0061] (4) The mixture in step (3) is separated, the filter residue is desilicated fly ash, and the filtrate is sodium silicate extract;

[0062] (5) The sodium silicate extract is placed in the atmospheric microwave synthesis reaction workstation again, the power is 300 w, the rotation speed is 250 r / min, and after adjusting the pH to 7-8 with a citric acid solution at 70°C, it is continuously treated in the microwave environment for 3 min;

[0063] (6) The obtained mixture is centrifuged, washed, and dried to obtain nano-silicon dioxide.

[0064] Example 2

[0065] This example is used to illustrate the preparation of nano-silicon dioxide.

[0066] (1) Mix fly ash (volume average diameter less than 10 μm) and sodium hydroxide, the mass ratio of fly ash to sodium hydroxide being 1:1, after 3 hours of activation treatment by pulverization;

[0067] (2) Pour the mixture of fly ash and sodium hydroxide into water and stir thoroughly, the mass ratio of the mixture to water being 1:3;

[0068] (3) During stirring, place the material in a normal-pressure microwave synthesis reaction workstation, the power being 700 w, the reaction time being 0.5 hours, and the reaction temperature being 90°C, to generate a mixed product of desilicated fly ash and sodium silicate solution;

[0069] (4) Separate the mixed product in step (3), the filter residue being desilicated fly ash, and the filtrate being a sodium silicate extract;

[0070] (5) Place the sodium silicate extract again in the normal-pressure microwave synthesis reaction workstation, the power being 400 w, the temperature being 60°C, the rotation speed being 350 r / min, and the pH being adjusted to 7-8 by a citric acid solution, and then continue the reaction in a microwave environment for 5 minutes;

[0071] (6) Centrifuge, wash, and dry the obtained mixture to obtain nano-silicon dioxide. Figure 2 It can be seen that the generated nano-silicon dioxide particles are substantially spherical, uniform in size, and have a particle size of 40-60 nm.

[0072] Example 3

[0073] This example is used to illustrate the preparation of nano-silicon dioxide.

[0074] (1) Mix fly ash (volume average diameter less than 10 μm) and sodium hydroxide, the mass ratio of fly ash to sodium hydroxide being 1:2, after 3 hours of activation treatment by pulverization;

[0075] (2) Pour the mixture of fly ash and sodium hydroxide into water and stir thoroughly, the mass ratio of the mixture to water being 1:3;

[0076] (3) During stirring, place the material in a normal-pressure microwave synthesis reaction workstation, the power being 700 w, the reaction time being 0.5 hours, and the reaction temperature being 90°C, to generate a mixed product of desilicated fly ash and sodium silicate solution;

[0077] (4) Separate the mixed product in step (3), the filter residue being desilicated fly ash, and the filtrate being a sodium silicate extract;

[0078] (5) Place the sodium silicate extract again in the normal-pressure microwave synthesis reaction workstation, the power being 400 w, the temperature being 60°C, the rotation speed being 350 r / min, and the pH being adjusted to 7-8 by a citric acid solution, and then continue the reaction in a microwave environment for 10 minutes;

[0079] (6) The obtained mixture was centrifuged, washed, and dried to obtain nanosilica.

[0080] Example 4

[0081] Nanosilica was prepared according to the method of Example 2, except that the time of step (3) was 1 h.

[0082] Example 5

[0083] Nanosilica was prepared according to the method of Example 2, except that the power of step (5) was 500 w.

[0084] Comparative Example 1

[0085] Nanosilica was prepared according to the method of Example 2, except that steps (3) and (5) were not performed using the normal pressure microwave synthesis reaction workstation, and the reaction was heated using an oil bath in all reaction steps.

[0086] Comparative Example 2

[0087] Nanosilica was prepared according to the method of Example 2, except that the power of step (3) was 900 w.

[0088] Comparative Example 3

[0089] Nanosilica was prepared according to the method of Example 2, except that the power of step (5) was 700 w.

[0090] Comparative Example 4

[0091] Nanosilica was prepared according to the method of Example 2, except that the power of step (5) was 200 w.

[0092] Comparative Example 5

[0093] Nanosilica was prepared according to the method of Example 2, except that the power of step (3) was 300 w.

[0094] Test Example 1

[0095] The nanosilica prepared in Examples 1-5 and Comparative Examples 1-4 of the present application were respectively subjected to the following tests;

[0096] Silica extraction rate: the proportion of nanosilica product obtained finally to the silica in fly ash raw material.

[0097] Nanosilica particle size range: counted by scanning electron microscope images.

[0098] The results are shown in Tables 1 and 2.

[0099] Table 1

[0100]

[0101] As can be seen from Table 1, the silica extraction rate of Examples 1-4 of the present application is significantly improved compared with Comparative Example 1. This shows that the present application has a positive effect on the alkali solution leaching of silica in fly ash and the effect is obvious. In Comparative Example 2, the fly ash adheres to the inner wall of the container in a hard sheet shape and the solution is almost completely evaporated under the microwave environment, so the SiO2 extraction rate cannot be determined. Comparative Example 5 shows that too low microwave power will reduce the silica extraction rate.

[0102] Table 2

[0103]

[0104] As can be seen from Table 2, the particle size range of silica in Examples 1-3 and 5 is significantly reduced compared with Comparative Examples, and the particle size is also reduced to a certain extent. This shows that the present application has a certain control effect on the particle size and particle size range of nano-silica. The particle size range of Comparative Example 3 is relatively wide, which may be caused by the collision of high-speed particles in the microwave field to cause polymerization and agglomeration. The particle size range of Comparative Example 4 is relatively wide, which may be caused by incomplete reaction under the microwave environment and continued reaction after leaving the microwave environment.

[0105] The above describes the preferred embodiments of the present application, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be considered as disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A method for producing nanosilica, characterized by, The method comprises the following steps: (1) mixing fly ash, soluble alkali metal salt and water under first microwave conditions to generate a mixed product of desilicated fly ash and sodium silicate solution; wherein the first microwave conditions comprise: power of 400-800w, time of 0.25-2h, and temperature of 50-100℃; (2) separating the mixed product obtained in step (1) to obtain desilicated fly ash as filter residue and sodium silicate extraction solution as filtrate; (3) under second microwave conditions, adding an acid solution to the sodium silicate extraction solution obtained in step (2) to adjust the pH to 7-8; wherein the second microwave conditions comprise: power of 300-600w, time of 1-30min, and temperature of 30-80℃; (4) centrifuging, washing and drying the obtained mixture to obtain nano-silicon dioxide.

2. The production method according to claim 1, wherein, In step (1), the fly ash is fly ash after grinding activation treatment; Preferably, the grinding activation treatment time is 3h. Preferably, the volume average diameter of the fly ash is less than 10μm.

3. The production method according to claim 1 or 2, wherein The soluble alkali metal salt is selected from at least one of sodium hydroxide and potassium hydroxide, and is preferably sodium hydroxide.

4. The production process according to any one of claims 1 to 3, wherein, In step (1), the mass ratio of fly ash to soluble alkali metal salt is 1:0.5-2, and is preferably 1:

1.

5. The production process according to any one of claims 1 to 4, wherein, The mass ratio of the sum of fly ash and soluble alkali metal salt to water is 1:1-5, and is preferably 1:

3.

6. The production process according to any one of claims 1 to 5, wherein, In step (1), a normal pressure microwave synthesis reaction workstation is used to provide the first microwave conditions; Preferably, the first microwave conditions comprise: power of 700w, time of 0.5h, and temperature of 90℃.

7. The production process according to any one of claims 1 to 6, wherein The desilicated fly ash obtained in step (2) is used to extract alumina.

8. The production process according to any one of claims 1 to 7, wherein In step (3), the acid of the acid solution is selected from at least one of sulfuric acid, hydrochloric acid, citric acid and oxalic acid, and is preferably citric acid.

9. The production process according to any one of claims 1 to 8, wherein, In step (3), a normal pressure microwave synthesis reaction workstation is used to provide the second microwave conditions; Preferably, the second microwave conditions comprise: power of 400w, temperature of 70℃, and time of 5min.

10. The production process according to any one of claims 1 to 9, wherein, The centrifugation in step (4) comprises: re-dispersing the solid phase after previous centrifugation in water and centrifuging again, repeating the centrifugation operation 6 times to obtain a final solid phase, and then washing and drying to obtain nano-silicon dioxide.

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