Combustion improver and method for its preparation

Abstract

The application provides a combustion improver and a preparation method thereof, the combustion improver comprises the following raw materials in the unit of mass fraction: 3-7 parts of nano cerium oxide, 10-20 parts of potassium nitrate, 8-15 parts of iron oxide, 40-60 parts of modified diatomite, 10-20 parts of sodium carboxymethyl cellulose, 1-5 parts of zinc borate and 1-3 parts of sodium dodecyl benzene sulfonate. The application uses nano cerium oxide as a catalyst to accelerate the combustion reaction; potassium nitrate and iron oxide provide necessary oxygen elements to promote the complete combustion of fuel; the modified diatomite has high adsorption and stability; the sodium carboxymethyl cellulose serves as a binder to enhance the stability and dispersibility of the combustion improver; the zinc borate and the sodium dodecyl benzene sulfonate respectively play the roles of flame retardant and surfactant. Through the combination of the above raw materials, the combustion efficiency is improved and the emission of harmful substances is reduced.

Description

Technical Field

[0001] This application relates to the field of fuel additive technology, and in particular to a combustion improver and its preparation method. Background Technology

[0002] The main pollutants produced during coal combustion include carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen monoxide, and particulate matter. These pollutants contribute to air pollution and are a major factor in smog formation. Therefore, improving coal combustion efficiency, reducing pollutant emissions, and achieving clean and efficient coal combustion not only has significant economic benefits but also major social implications. Using combustion accelerators is one of the more effective measures to achieve this goal. Adding combustion accelerators to coal can significantly improve its combustion characteristics, namely, increasing the release rate of volatile matter, lowering the ignition point, accelerating the combustion rate of coke, and shortening the combustion time, thereby improving coal combustion efficiency. By adding a small amount of accelerator to coal, complete combustion can be promoted while reducing dust and pollutant emissions.

[0003] Traditional combustion improvers such as potassium permanganate and sodium nitrate suffer from short reaction times (<48 hours) and easy decomposition and inactivation at high temperatures. Therefore, how to extend the release period of the active components of combustion improvers and improve catalytic efficiency under low-temperature (<500℃) conditions has become an urgent problem to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a combustion improver and its preparation method, which achieves efficient combustion of fuel through rare earth-transition metal synergistic catalysis and physical slow release technology.

[0005] Specifically, the first aspect of this application provides a combustion aid, comprising the following raw materials in parts by mass: 3-7 parts of nano-cerium oxide, 10-20 parts of potassium nitrate, 8-15 parts of iron oxide, 40-60 parts of modified diatomaceous earth, 10-20 parts of sodium carboxymethyl cellulose, 1-5 parts of zinc borate, and 1-3 parts of sodium dodecylbenzene sulfonate.

[0006] Furthermore, the particle size of the nano-cerium oxide is 20-50 nm.

[0007] Furthermore, the modified diatomaceous earth has a pore size of 10-50 nm.

[0008] A second aspect of this application provides a method for preparing the aforementioned combustion improver, comprising the following steps:

[0009] S1: Potassium nitrate and nano-cerium oxide are mixed and spray-dried to obtain a pre-composite active powder;

[0010] S2: Modified diatomaceous earth, sodium carboxymethyl cellulose, pre-composite active powder, iron oxide, zinc borate, and sodium dodecylbenzene sulfonate are mixed to obtain a mixture.

[0011] S3: Granulate and coat the mixture.

[0012] Further, in step S1, potassium nitrate is dissolved in water to obtain a potassium nitrate solution with a mass concentration of 28-30%, then nano-cerium oxide powder is added and ultrasonically dispersed for 30-40 minutes to obtain a mixed solution; and / or

[0013] The frequency of the ultrasonic dispersion is 40-50kHz.

[0014] Further, the inlet temperature of the spray dryer in step S1 is 160-190°C; and / or

[0015] The outlet temperature of the spray dryer is 70-90℃.

[0016] Further, the modification method of the modified diatomaceous earth in step S2 is as follows: calcining the diatomaceous earth at 600-650℃, followed by ball milling; and / or

[0017] The calcination time is 1-3 hours; and / or

[0018] Ball milling until the diatomaceous earth particle size is 200-250 mesh.

[0019] Further, the mixing speed in step S2 is 700-850 rpm; and / or

[0020] The mixing temperature is 50-60℃; and / or

[0021] Mixing time is 20-30 minutes.

[0022] Further, the granulation in step S3 involves spraying a binder into the mixture and simultaneously feeding it into a fluidized bed dryer to form granules; and / or

[0023] The adhesive is sprayed at a rate of 0.4-0.6 L / min; and / or

[0024] The inlet air temperature for the fluidized bed granulation is 75-85℃; and / or

[0025] The air velocity in the fluidized bed granulation is 1.1-1.5 m / s.

[0026] Furthermore, the coating in step S3 involves spraying a fumed silica suspension into the particles to form a silica coating layer with a thickness of ≤50μm on the particle surface, followed by drying.

[0027] The present invention has the following beneficial effects:

[0028] The synergistic effect of the components in the combustion aid formulation of this invention not only improves the combustion efficiency of coal but also significantly extends the release cycle of the active components of the combustion aid. Specifically, the composite of nano-cerium oxide and potassium nitrate, formed by spray drying into a pre-composite active powder, exhibits excellent catalytic performance, effectively lowering the ignition point of coal and accelerating the combustion rate. Modified diatomaceous earth, as a carrier, with its unique pore structure, facilitates the uniform dispersion and slow release of the active components, improving the stability of the combustion aid. Furthermore, nano-cerium oxide acts as an electron acceptor, and iron oxide as an electron donor; during combustion, the electron transfer between them promotes the combustion chain reaction, thereby improving combustion efficiency. Potassium nitrate is embedded in the pores of the modified diatomaceous earth to achieve gradient release. Simultaneously, the rare earth properties of nano-cerium oxide combined with the transition metal properties of iron oxide, through synergistic catalysis, effectively improves the combustion efficiency of coal and reduces the emission of harmful gases. The addition of sodium carboxymethyl cellulose enhances the adhesion and formability of the combustion aid, making the granulation process smoother. Zinc borate and sodium dodecylbenzene sulfonate respectively play a role in flame retardancy and dispersion, further improving the overall performance of the combustion aid.

[0029] Furthermore, the combustion aid preparation method of the present invention is simple in process, convenient in operation, and easy to realize industrial production, thus having high practical value. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0031] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0032] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0033] An embodiment of the first aspect of the present invention provides a combustion aid comprising, by weight parts, the following raw materials: 3-7 parts nano-cerium oxide, 10-20 parts potassium nitrate, 8-15 parts iron oxide, 40-60 parts modified diatomaceous earth, 10-20 parts sodium carboxymethyl cellulose, 1-5 parts zinc borate, and 1-3 parts sodium dodecylbenzene sulfonate.

[0034] The nano-cerium oxide has a particle size of 20-50 nm. The mass fraction of the nano-cerium oxide is any value or combination of 3, 4, 5, 6, and 7 parts. A mass fraction of nano-cerium oxide higher than 7 parts results in excessively high costs and no significant improvement in combustion efficiency; a mass fraction of nano-cerium oxide lower than 3 parts results in insufficient catalytic effect and cannot effectively promote coal combustion.

[0035] The modification method of the modified diatomaceous earth is as follows: diatomaceous earth is calcined at 600-650℃ for 1-3 hours to remove organic impurities and increase porosity, so that the pore size distribution is optimized to 10-50nm. After calcination, the diatomaceous earth is put into a ball mill and ball milled with zirconia grinding balls at a ball-to-material ratio of 5:1 until the particle size of the diatomaceous earth is 200-250 mesh, thus obtaining modified diatomaceous earth.

[0036] The modified diatomaceous earth has a pore size of 10-50 nm. Modified diatomaceous earth within this pore size range can effectively load and slowly release active components, improving the stability and durability of the combustion improver. The mass fraction of modified diatomaceous earth can be any value or combination of 40, 45, 50, 55, or 60 parts. If the mass fraction of modified diatomaceous earth is too high, the combustion improver may become too viscous, hindering its dispersion and application; if the mass fraction is too low, it may not be able to fully load the active components, affecting the combustion-enhancing effect.

[0037] The potassium nitrate is present in a mass fraction of 10-20 parts. Within this range, potassium nitrate provides good oxidizing properties, promoting coal combustion. The mass fraction of potassium nitrate can be any value or combination of 10, 12, 15, 18, or 20 parts. Too much potassium nitrate may lead to an overly reactive combustion promoter, potentially causing safety hazards; too little may fail to provide sufficient oxidizing properties, affecting the combustion-promoting effect.

[0038] The iron oxide comprises 8-15 parts by mass. As an electron donor, the iron oxide synergistically works with nano-cerium oxide during combustion to promote the combustion chain reaction. The mass fraction of the iron oxide can be any value or combination of 8, 10, 12, 14, or 15 parts. An appropriate amount of iron oxide can significantly improve combustion efficiency.

[0039] The sodium carboxymethyl cellulose is present in an amount of 10-20 parts by weight. The addition of sodium carboxymethyl cellulose enhances the binding and molding properties of the combustion improver, making the granulation process smoother. The mass fraction of sodium carboxymethyl cellulose can be any value or combination of 10, 12, 15, 18, or 20 parts. An appropriate amount of sodium carboxymethyl cellulose ensures the stability and processability of the combustion improver; too little may lead to difficulties in molding the combustion improver, affecting its performance.

[0040] The synergistic effect of the components in the combustion aid formulation of this invention not only improves the combustion efficiency of coal but also significantly extends the release cycle of the active components of the combustion aid. Specifically, the composite of nano-cerium oxide and potassium nitrate, formed by spray drying into a pre-composite active powder, exhibits excellent catalytic performance, effectively lowering the ignition point of coal and accelerating the combustion rate. Modified diatomaceous earth, as a carrier, with its unique pore structure, facilitates the uniform dispersion and slow release of the active components, improving the stability of the combustion aid. Furthermore, nano-cerium oxide acts as an electron acceptor, and iron oxide as an electron donor; during combustion, the electron transfer between them promotes the combustion chain reaction, thereby improving combustion efficiency. Potassium nitrate is embedded in the pores of the modified diatomaceous earth to achieve gradient release. Simultaneously, the rare earth properties of nano-cerium oxide combined with the transition metal properties of iron oxide, through synergistic catalysis, effectively improves the combustion efficiency of coal and reduces the emission of harmful gases. The addition of sodium carboxymethyl cellulose enhances the adhesion and formability of the combustion aid, making the granulation process smoother. Zinc borate and sodium dodecylbenzene sulfonate respectively play a role in flame retardancy and dispersion, further improving the overall performance of the combustion aid.

[0041] An embodiment of the second aspect of this application provides a method for preparing a combustion-supporting agent, comprising the following steps:

[0042] S1: Potassium nitrate and nano-cerium oxide are mixed and spray-dried to obtain a pre-composite active powder;

[0043] S2: Modified diatomaceous earth, sodium carboxymethyl cellulose, pre-composite active powder, iron oxide, zinc borate, and sodium dodecylbenzene sulfonate are mixed to obtain a mixture.

[0044] S3: Granulate and coat the mixture.

[0045] In step S1, potassium nitrate is dissolved in water to obtain a potassium nitrate solution with a mass concentration of 28-30%. Then, nano-cerium oxide powder is added and ultrasonically dispersed for 30-40 minutes to obtain a mixed solution. The ultrasonic dispersion frequency is 40-50 kHz.

[0046] The mixture is spray-dried at an inlet temperature of 160-190°C and an outlet temperature of 70-90°C to obtain a pre-composite active powder.

[0047] In step S2, the diatomaceous earth needs to be modified before mixing. The modification method is as follows: calcining the diatomaceous earth at 600-650℃ for 1-3 hours to remove organic impurities and increase porosity, so that the pore size distribution is optimized to 10-50nm. After calcination, the diatomaceous earth is put into a ball mill and ball milled with zirconia grinding balls at a ball-to-material ratio of 5:1 until the particle size of the diatomaceous earth is 200-250 mesh, thus obtaining modified diatomaceous earth.

[0048] In step S2, modified diatomaceous earth, sodium carboxymethyl cellulose, pre-composite active powder, iron oxide, zinc borate, and sodium dodecylbenzene sulfonate are sequentially added to a twin-shaft paddle mixer at a speed of 700-850 rpm. The mixing temperature is 50-60℃ to prevent premature decomposition of potassium nitrate. The mixing time is 20-30 minutes to ensure uniform dispersion of the components.

[0049] In step S3, the granulation involves spraying an adhesive into the mixture. The adhesive is 4-6 parts by weight of a polyvinyl alcohol (PVA) solution (10wt%), and the spraying rate of the adhesive is 0.4-0.6 L / min. Simultaneously, the mixture enters a fluidized bed dryer to form particles. The inlet air temperature of the fluidized bed granulation is 75-85℃, and the air velocity is 1.1-1.5 m / s. The particle size is controlled at 0.5-1.5 mm.

[0050] Furthermore, in step S3, before coating the particles, a 20-mesh vibrating screen is used to remove excessively large or fine particles, and then fumed silica is used for coating.

[0051] The sieved particles are placed in a rotary coating machine and a fumed silica suspension is sprayed in evenly. The silica suspension has a solid content of 5%, forming a silica coating layer with a thickness of ≤50μm on the particle surface. After coating, the particles are dried a second time by circulating hot air at 60℃ for 1 hour.

[0052] Example 1

[0053] A combustion aid, in parts by mass, comprises the following raw materials: 5 parts nano-cerium oxide, 15 parts potassium nitrate, 10 parts iron oxide, 50 parts modified diatomaceous earth, 15 parts sodium carboxymethyl cellulose, 3 parts zinc borate, and 2 parts sodium dodecylbenzene sulfonate.

[0054] Example 2

[0055] A combustion aid, in parts by mass, comprises the following raw materials: 3-7 parts nano-cerium oxide, 12 parts potassium nitrate, 8 parts iron oxide, 60 parts modified diatomaceous earth, 18 parts sodium carboxymethyl cellulose, 1 part zinc borate, and 3 parts sodium dodecylbenzene sulfonate.

[0056] Example 3

[0057] A combustion aid, in parts by mass, comprises the following raw materials: 3-7 parts nano-cerium oxide, 10 parts potassium nitrate, 15 parts iron oxide, 55 parts modified diatomaceous earth, 10 parts sodium carboxymethyl cellulose, 5 parts zinc borate, and 2.5 parts sodium dodecylbenzene sulfonate.

[0058] Example 4

[0059] A combustion aid, in parts by mass, comprises the following raw materials: 3-7 parts nano-cerium oxide, 20 parts potassium nitrate, 12 parts iron oxide, 40 parts modified diatomaceous earth, 20 parts sodium carboxymethyl cellulose, 4 parts zinc borate, and 1 part sodium dodecylbenzene sulfonate.

[0060] Example 5

[0061] A method for preparing a combustion accelerant includes the following steps:

[0062] S1: Dissolve potassium nitrate in water to obtain a potassium nitrate solution with a mass concentration of 28-30%, then add nano-cerium oxide powder, and ultrasonically disperse for 30 minutes to obtain a mixed solution. The ultrasonic dispersion frequency is 40kHz.

[0063] The mixture is spray-dried at an inlet temperature of 180°C and an outlet temperature of 85°C to obtain a pre-composite active powder.

[0064] S2: Diatomaceous earth is modified by calcining it at 600℃ for 2 hours to remove organic impurities and increase porosity, optimizing the pore size distribution to 10-50nm. After calcination, the diatomaceous earth is fed into a ball mill and ball milled with zirconia grinding balls at a ball-to-material ratio of 5:1 until the particle size of the diatomaceous earth is 200-250 mesh, thus obtaining modified diatomaceous earth.

[0065] The modified diatomaceous earth, sodium carboxymethyl cellulose, pre-composite active powder, iron oxide, zinc borate, and sodium dodecylbenzene sulfonate from Example 1 were sequentially added into a twin-shaft paddle mixer at a speed of 800 rpm. The mixing temperature was 60°C to prevent premature decomposition of potassium nitrate. The mixing time was 30 min to ensure uniform dispersion of the components, resulting in a mixture.

[0066] S3: Spray an adhesive into the mixture, the adhesive being 5 parts by mass of a polyvinyl alcohol (PVA) solution (concentration 10wt%), the spraying rate of the adhesive being 0.5L / min; simultaneously, the mixture enters a fluidized bed dryer to form granules, the inlet air temperature of the fluidized bed granulation being 80℃ and the air velocity being 1.5m / s; the particle size of the formed granules is controlled within 0.5-1.5mm;

[0067] Before coating the particles, a 20-mesh vibrating screen is used to remove oversized or overly fine particles. The screened particles are then placed in a rotary coating machine and a fumed silica suspension is sprayed in evenly. The silica suspension has a solid content of 5%, forming a silica coating layer with a thickness of ≤50μm on the particle surface. After coating, the particles are dried a second time by circulating hot air at 60℃ for 1 hour.

[0068] Example 6

[0069] This embodiment is basically the same as embodiment 5, except that the inlet temperature of the spray dryer in step S1 is 170°C and the outlet temperature of the spray dryer is 70°C.

[0070] Example 7

[0071] This embodiment is basically the same as embodiment 5, except that the mixing speed in step S2 is 750 rpm, the temperature is 55°C, and the time is 20 min.

[0072] Example 8

[0073] This embodiment is basically the same as embodiment 5, except that the inlet air temperature of the fluidized bed granulation in step S3 is 85°C and the air velocity is 1.3 m / s.

[0074] Example 9

[0075] This embodiment is basically the same as embodiment 5, except that the raw materials in embodiment 2 are used in step S2.

[0076] Comparative Example 1

[0077] This embodiment is basically the same as embodiment 5, except that the raw materials do not contain nano-cerium oxide.

[0078] Comparative Example 2

[0079] This embodiment is basically the same as embodiment 5, except that the mass fraction of potassium nitrate in the raw materials is 25 parts.

[0080] Comparative Example 3

[0081] This embodiment is basically the same as embodiment 5, except that sodium carboxymethyl cellulose is replaced with starch binder.

[0082] Comparative Example 4

[0083] This embodiment is basically the same as embodiment 5, except that the modified diatomaceous earth is replaced with ordinary kaolin.

[0084] Comparative Example 5

[0085] This embodiment is basically the same as embodiment 5, except that in step S2, modified diatomaceous earth, sodium carboxymethyl cellulose, pre-composite active powder, iron oxide, zinc borate, and sodium dodecylbenzene sulfonate are added to a twin-shaft paddle mixer and the mixing temperature is 80°C.

[0086] Comparative Example 6

[0087] This embodiment is basically the same as embodiment 5, except that the particles are not coated with silica vapor in step S3.

[0088] The performance of the combustion improvers prepared in Examples 5-9 and Comparative Examples 1-6 was tested. The coal saving rate was tested according to GB / T31097-2014 Evaluation Method for Combustion Improvement Effect of Coal Combustion Improvers. Under the same working conditions, the combustion improver was mixed with pulverized coal (1:10), and the flame duration from ignition to extinction was recorded. The results are shown in the table below:

[0089]

[0090]

[0091] As shown in the table above, the combustion improver obtained by this invention, through the use of specific components and proportions and a specific preparation process, significantly improves both the coal saving rate and release time compared to the comparative examples. In Example 5, the coal saving rate reached 18.7%, and the release time was 240 min. Compared to the coal saving rate of 12.3% and release time of 90 min in Comparative Example 1 (without nano-cerium oxide), and the coal saving rate of 9.7% and release time of 40 min in Comparative Example 4 (replacing modified diatomaceous earth with ordinary kaolin), it can be seen that the combustion improver of this invention has significant advantages in improving combustion efficiency and extending the release cycle. In addition, Examples 6-9 also showed good coal saving effects and long release times, further verifying the effectiveness and stability of the combustion improver of this invention.

[0092] In Comparative Example 1, the absence of nano-cerium oxide resulted in a significant decrease in both coal-saving rate and release time. This was because the lack of nano-cerium oxide catalyzed the combustion chain reaction, leading to insufficient combustion efficiency and a shortened release cycle of the active components. In Comparative Example 2, the excessively high potassium nitrate content, while providing sufficient oxidizing properties, could make the combustion aid overly reactive, making the combustion process difficult to control and resulting in an excessively long release time, which is detrimental to practical applications. In Comparative Example 3, replacing sodium carboxymethyl cellulose with starch binder weakened adhesion and formability, reduced combustion aid stability, and increased the likelihood of incomplete combustion, leading to a decrease in coal-saving rate. In Comparative Example 4, replacing modified diatomaceous earth with ordinary kaolin significantly reduced the combustion aid's effectiveness due to the unfavorable pore structure of ordinary kaolin, which hindered the uniform dispersion and slow release of the active components. In Comparative Example 5, the excessively high mixing temperature could cause premature decomposition of potassium nitrate, affecting the performance of the combustion aid. In Comparative Example 6, the lack of silica vapor-phase coating on the particles weakened the stability and durability of the combustion aid, shortening the release time.

[0093] In summary, the combustion improver of the present invention achieves the dual advantages of high-efficiency combustion and sustained release through a carefully designed component ratio and preparation process, and has significant application value.

[0094] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A combustion improver characterized by comprising, By mass fraction, the following raw materials are included: nano cerium oxide 3-7 parts, potassium nitrate 10-20 parts, iron oxide 8-15 parts, modified diatomite 40-60 parts, sodium carboxymethyl cellulose 10-20 parts, zinc borate 1-5 parts, and sodium dodecyl benzene sulfonate 1-3 parts; The preparation method of the combustion improver comprises the following steps: S1: dissolving potassium nitrate in water, mixing with nano cerium oxide, and performing spray drying to obtain a pre-composite active powder; S2: mixing modified diatomite, sodium carboxymethyl cellulose, the pre-composite active powder, iron oxide, zinc borate, and sodium dodecyl benzene sulfonate to obtain a mixture; The modification method of the modified diatomite is: calcining diatomite at 600-650°C, and then ball milling; S3: granulating and coating the mixture.

2. The combustion improver according to claim 1, characterized by The particle size of the nano cerium oxide is 20-50 nm.

3. The combustion improver according to claim 1, characterized by The pore size of the modified diatomite is 10-50 nm.

4. The combustion improver of claim 1, wherein In step S1, potassium nitrate is dissolved in water to obtain a potassium nitrate solution with a mass concentration of 28-30%, and then nano cerium oxide powder is added and ultrasonic dispersed for 30-40 min to obtain a mixture; The frequency of the ultrasonic dispersion is 40-50 kHz.

5. The combustion improver of claim 1, wherein In step S1, the inlet temperature of the spray drying is 160-190°C; The outlet temperature of the spray drying is 70-90°C.

6. The combustion improver of claim 1, wherein In the modification method of the modified diatomite in step S2, the calcination time of the diatomite is 1-3 h; and the ball milling is performed until the particle size is 200-250 mesh.

7. The combustion improver of claim 1 wherein, In step S2, the rotating speed of the mixing is 700-850 rpm; The temperature of the mixing is 50-60°C; The mixing time is 20-30 min.

8. The combustion improver of claim 1, wherein In step S3, the granulation is to spray a binder into the mixture while synchronously entering a fluidized bed dryer to form particles; the spraying rate of the binder is 0.4-0.6 L / min; The inlet air temperature of the fluidized bed dryer is 75-85°C; The air speed of the fluidized bed dryer is 1.1-1.5 m / s.

9. The combustion improver of claim 1, wherein In step S3, the coating is to spray a gas-phase silicon dioxide suspension into the particles to form a silicon dioxide coating layer with a thickness of ≤50 μm on the surface of the particles, and then perform drying treatment.

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