Process for preparing a silicon-based inorganic flame retardant

Abstract

The application belongs to the technical field of flame retardant preparation, and discloses a silicon-based inorganic flame retardant preparation process, which takes industrial silicon slag by-product solid waste as raw material, and is prepared into finished products through pretreatment, alkali dissolution crystallization, electrocatalytic whisker growth, phosphorus-silicon premixing, supercritical CO2 microspherization coating and post-treatment solidification; the process is matched with environment-friendly and quality control steps, and each link equipment and parameter is refined, so that the process can be flexibly adjusted and adapted to different production requirements. The process breaks through the limitations of traditional technology, realizes resource utilization of industrial silicon slag, reduces energy consumption through low-temperature reaction, solves the pain points of traditional coating and cracking, and the finished product has a halogen-free core-shell structure, is environment-friendly and stable in performance, is suitable for various paint scenes, is simple to operate, can be produced on an industrial scale, and gives consideration to environmental protection and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant preparation technology, specifically relating to a silicon-based inorganic flame retardant preparation process. Background Technology

[0002] Silicon-based inorganic flame retardants, due to their advantages such as halogen-free, low-smoke, environmentally friendly, and long-lasting flame retardant effects, have been widely used in various fields, including water-based coatings, oil-based coatings, and interior coatings. The demand is particularly urgent in scenarios with high requirements for environmental protection and flame retardant performance, such as children's rooms and hospitals. Currently, there are many processes for preparing silicon-based inorganic flame retardants in the industry, but they generally suffer from problems such as low raw material utilization, high production energy consumption, and unstable product performance, which seriously limit their industrial promotion and large-scale application. Industrial silicon slag, as a major by-product solid waste of the silicon-based industry, is produced in huge quantities. Existing technologies mostly treat it by stockpiling and landfilling, which not only causes serious waste of silicon resources but also poses potential pollution to soil and water bodies. How to achieve the resource utilization of industrial silicon slag has become one of the urgent problems to be solved in the industry.

[0003] The applicant discovered that in the preparation process of silicon-based inorganic flame retardants, the preparation of silicon-based whiskers mostly adopts a high-temperature gas-phase synthesis method. This method requires high-temperature conditions above 1000℃, resulting in extremely high energy consumption. Furthermore, the prepared silicon-based whiskers have uneven particle size and low activity, making it difficult to meet the requirements of subsequent microsphere coating. In the phosphorus-silicon co-coating stage, traditional processes often employ physical mixing or high-temperature spray drying, which easily leads to problems such as uneven phosphorus-silicon mixing, microsphere agglomeration, and coating layer cracking. This makes it difficult to improve the flame retardant performance and thermal stability of the flame retardant, failing to meet the application requirements of high-end coatings. Simultaneously, the existing processes lack adequate environmental protection measures. Wastewater, solid waste, and exhaust gas generated during production are difficult to recycle efficiently, resulting in low water reuse rates and low comprehensive utilization rates of solid waste, which does not comply with national solid waste resource utilization and "dual-carbon" policies. Summary of the Invention

[0004] In view of the problems mentioned in the background art above, the purpose of this invention is to provide a process for preparing silicon-based inorganic flame retardants.

[0005] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows: A process for preparing a silicon-based inorganic flame retardant includes the following steps: S1. Pretreatment of industrial silicon slag: After crushing and grinding the industrial silicon slag, iron is removed by magnetic separation to obtain silicon slag powder; S2, Alkali Dissolution Crystallization: Add silicon slag powder to an alkaline solution, stir at low temperature to dissolve, add crystal guiding agent, crystallize at constant temperature, filter and wash to obtain silicon-based whisker precursor; S3. Electrocatalytic whisker growth: A silicon-based whisker precursor is prepared into a suspension and fed into an electrolytic cell. Constant current electrolysis is used to achieve silicon-based whisker growth, resulting in a silicon-based whisker suspension. S4. Phosphorus-silicon premix: Add an aqueous phosphorus source solution dropwise to a silicon-based whisker suspension, mix and disperse to obtain a phosphorus-silicon mixed suspension; S5. Supercritical CO2 microsphere coating: Phosphorus-silicon mixed suspension is fed into a supercritical CO2 reactor, the reaction temperature and pressure are controlled, and after constant temperature and pressure reaction, the pressure is released in a stepwise manner to obtain core-shell structured silicon-based microsphere wet material. S6. Post-processing and curing: The wet core-shell structured silicon-based microspheres are vacuum dried and cured at low temperature. After grinding and sieving, the silicon-based inorganic flame retardant product is obtained.

[0006] Further specifying that in step S1, the industrial silicon slag is first manually screened to remove large pieces of coke, then coarsely crushed to ≤1cm using a jaw crusher, and subsequently ground using a planetary ball mill; the grinding media are alumina balls with a particle size of 5mm, the ball-to-material ratio is 3:1, the rotation speed is 300rpm, and the grinding time is 1h; the particle size of the silicon slag powder after grinding is 200 mesh, and the SiO2 content of the industrial silicon slag is ≥75%, the Fe2O3 content is ≤1.5%, and the CaO content is ≤2%; the magnetic separation for iron removal uses a two-stage drum magnetic separator with a magnetic field strength of 12000Gs, a feed rate of 0.5m / s, and an iron removal efficiency of ≥99%; the iron content in the silicon slag powder after magnetic separation is ≤0.5%. This refines the specific steps, equipment, and parameters of the industrial silicon slag pretreatment, effectively removes impurities and large pieces of coke from the silicon slag, improves the purity of the silicon slag powder, reduces the interference of impurities on subsequent processes, and ensures the stability and reliability of subsequent alkali dissolution crystallization, electrocatalysis, and other steps.

[0007] Further specifying, in step S2, the alkaline solution is an industrial sodium hydroxide solution with a content ≥96%, and the mass ratio of silicon slag powder, industrial sodium hydroxide, and deionized water is 1:0.35:10; the alkali dissolution temperature is 90±2℃, the stirring speed is 300rpm, the alkali dissolution time is 3h, and an online pH meter with an accuracy of ±0.01pH is used for real-time monitoring during the alkali dissolution process, with the pH controlled at 12.0±0.2; the crystallization guiding agent is industrial sodium sulfate with a content ≥98%, the addition amount is 4.5% of the mass of silicon slag powder, and the crystallization temperature is 80±2℃. The crystallization time was 5 hours, and the stirring rate was reduced to 200 rpm during the crystallization process. The filtration was carried out using a 50㎡ atmospheric pressure plate and frame filter press. After filtration, the filter cake was pressed by the filter press and then washed with deionized water until the pH of the filtrate was ≤10.0, resulting in a silicon-based whisker precursor filter cake with a water content of 30±2%. This process limited the raw material specifications, process parameters, and equipment requirements for alkali-soluble crystallization, making the silicon-based whisker precursor more regular in crystal form and more active, providing high-quality raw materials for subsequent electrocatalytic whisker growth. At the same time, by precisely controlling the pH value and washing standards, the residual impurities in the precursor were reduced.

[0008] Further specifying step S3, in which the silicon-based whisker precursor filter cake is added to deionized water with a conductivity ≤10μS / cm to prepare a suspension with a solid content of 15%; the suspension is dispersed for 20 minutes using an ultrasonic disperser with a power of 300W and a frequency of 28kHz, and a constant temperature water bath is used to control the temperature during dispersion, ensuring that the temperature is ≤25℃; after dispersion, the suspension is sent to a 500L small electrocatalytic electrolyzer with a jacketed temperature control device, the anode of the electrolyzer is a titanium-based ruthenium-iridium coated electrode, the cathode is a 304 stainless steel mesh, and the electrode spacing is 3.5 mm. The electrolysis temperature was 70±2℃, the constant current electrolysis current density was 14mA / cm², the electrolysis time was 4.5h, the electrolyte in the electrolysis system was 4.5% industrial sodium sulfate solution, and the stirring rate was 250rpm. After electrolysis, the material was directly discharged to the transfer tank to obtain a silicon-based whisker suspension. The preparation, dispersion and electrolysis parameters of the suspension for electrocatalytic whisker growth were clarified, realizing the low-temperature and high-efficiency growth of silicon-based whiskers. The whisker particles were uniform in size and highly active. Furthermore, the material was directly discharged to the transfer tank after electrolysis, reducing solid-liquid separation losses and improving production efficiency.

[0009] Further specifying step S4, the phosphorus source is industrial ammonium dihydrogen phosphate with a content ≥98%. The phosphorus source is added to deionized water to prepare a 20% aqueous solution, which is then filtered through a 0.45μm filter membrane to remove impurities. The phosphorus-silicon molar ratio is controlled at 1:4.2. The phosphorus source aqueous solution is added dropwise to the silicon-based whisker suspension at a dropping rate of 2mL / min. During the mixing and dispersion process, ultrasonic dispersion with a power of 200W is used, and a high-speed stirrer is used to stir at a rate of 250rpm for 30min. The dispersion temperature is maintained at 70℃. This refines the raw material preparation, dropping rate, and dispersion parameters of the phosphorus-silicon premix. Through the synergistic dispersion of ultrasonic waves and high-speed stirring, uniform mixing at the molecular level of phosphorus and silicon is achieved, improving the synergistic flame retardant effect of phosphorus and silicon, and laying a good foundation for subsequent supercritical microsphere coating.

[0010] Further specifying step S5, the phosphorus-silicon mixed suspension is mixed with anhydrous ethanol with a purity ≥99.5% at a volume ratio of 10:1. After mixing, the mixture is fed into a 50L domestically produced supercritical CO2 reactor at a feed rate of 5L / h via a high-pressure metering pump with a pressure ≥10MPa. During the reaction, CO2 with a purity ≥99.5% is continuously introduced at a flow rate of 10L / min. The reaction temperature is controlled at 35±1℃, the reaction pressure is controlled at 8.0±0.2MPa, the stirring speed is 300rpm, and the holding time is [not specified]. The time is 2.0h; the step-by-step depressurization is as follows: first, depressurize to 5MPa at a rate of 0.3MPa / min, keep at a constant temperature for 10min, and then depressurize to atmospheric pressure at a rate of 0.5MPa / min. The material is discharged to obtain wet core-shell structured silicon-based microspheres with uniform particle size. This limits the process parameters and step-by-step depressurization method of supercritical CO2 microsphere coating, effectively solving the problems of microsphere agglomeration and coating layer cracking in traditional coating processes. The prepared core-shell structured silicon-based microspheres have uniform particle size and dense coating layer, improving product performance.

[0011] Further specifying step S6, the integrated vacuum drying-low temperature curing process uses a vacuum drying oven with an effective volume of 100L and a vacuum degree of -0.095MPa. The programmed temperature rise parameters are: 40℃ drying for 2h → 60℃ drying for 3h → 100℃ curing for 2h, maintaining a vacuum state throughout the process. After curing, the dry material is ground using an air jet mill with a grinding pressure of 0.6-0.8MPa, and then sieved using an 80-mesh ultrasonic vibrating screen with a power of 500W. The finished particle size is controlled at 200-300nm with D50=250nm and D90=200nm. ≤300nm, monodisperse CV≤15%; the finished product after sieving is packaged in a sealed, moisture-proof container. It refines the raw material preparation, drip rate and dispersion parameters of the phosphorus-silicon premix, and achieves uniform mixing at the molecular level of phosphorus-silicon through ultrasonic and high-speed stirring dispersion, thereby improving the synergistic flame retardant effect of phosphorus-silicon and laying a good foundation for subsequent supercritical microsphere coating. It clarifies the equipment parameters, program temperature rise and sieving requirements for post-treatment curing, so that the particle size of the finished product is controllable and the dispersion is good. The sealed, moisture-proof packaging effectively prevents the finished product from absorbing moisture and agglomerating, improves the product storage stability and ensures the consistency of finished product quality.

[0012] Further, it also includes environmental protection treatment steps: Wastewater generated from filtration and washing in step S2 is collected to an integrated wastewater treatment device, neutralized to pH 7-8 with industrial hydrochloric acid, flocculated, filtered, and reverse osmosis treated, and then completely reused in the reaction system of alkaline dissolution in step S2 and electrocatalysis in step S3, with a water reuse rate of ≥95%; Iron slag generated from magnetic separation for iron removal in step S1 is recycled and sold, and filter residue containing a small amount of silicon-based components generated from filtration in step S2 is returned to step S2 for alkaline dissolution and crystallization again, with a solid waste comprehensive utilization rate of ≥99%; CO2 generated from depressurization in step S5 is collected by a recovery device and liquefied to ensure that the purity of CO2 after liquefaction is ≥99.5%, and then reused in the supercritical reaction in step S5, with a CO2 reuse rate of ≥90%. There is no waste gas emission throughout the process. This supplements the environmental protection treatment steps of the entire process, realizing the recycling and reuse of wastewater, solid waste, and CO2, greatly improving resource utilization, reducing environmental pollution, and achieving high levels of water reuse rate, solid waste comprehensive utilization rate, and CO2 reuse rate, which meet the requirements of environmental protection policies.

[0013] Further restrictions include quality control steps: Inspections are conducted at the following key points, with internal control standards established for each point. Non-conforming products are reworked according to the corresponding procedures to ensure stable product performance. 1) Raw material incoming inspection: ICP-MS is used to test the SiO2, Fe2O3 and CaO content of industrial silicon slag. If the content is not up to standard, it will be returned to the supplier. 2) Post-alkali dissolution testing: Test the silicon content and activity of the silicon-based whisker precursor. If it fails to meet the requirements, readjust the alkali dissolution parameters and perform alkali dissolution again. 3) Testing after magnetic separation to remove iron: Test the iron content of the silicon slag powder to ensure it is ≤0.5%. If it is not qualified, a second magnetic separation is performed. 4) Detection of whisker precursor dispersion: The particle size distribution of the suspension is detected by a laser particle size analyzer. If it is not qualified, it is redispersed. 5) Post-electrocatalytic whisker growth testing: The particle size and aspect ratio of silicon-based whiskers are tested using transmission electron microscopy. If they are not up to standard, the electrolysis parameters are adjusted and electrolysis is repeated. 6) Testing after phosphorus-silicon premixing: Test the phosphorus-silicon molar ratio; if it is not up to standard, add phosphorus source or silicon-based whisker suspension. 7) Post-supercritical coating testing: The particle size of silicon-based microspheres and the thickness of the 10-15 nm coating layer are detected by transmission electron microscopy. If the results are unsatisfactory, the supercritical parameters are adjusted and the coating is repeated. 8) Finished product inspection: The particle size distribution, thermal stability (≥600℃), flame retardancy (≥32% LOI), and environmental protection indicators of the finished product are tested. If they pass the test, they are released from the factory; if they fail, they are re-ground or re-coated. This demonstrates how setting up testing and rework processes at key production nodes can effectively control product quality fluctuations, reduce defect rates, ensure stable finished product performance, and improve product reliability.

[0014] Further specifying that the industrial silicon slag mentioned in step S1 is a by-product solid waste of the silicon-based industry, obtained through low-price purchase or free disposal; the finished silicon-based inorganic flame retardant has a core-shell structure, with a core layer of silicon-based whiskers and a shell layer of phosphorus-silicon composite layer. The finished product is halogen-free and has a heavy metal content of ≤10ppm. It can be directly added to water-based interior wall coatings, oil-based exterior wall coatings, or low-VOC indoor special coatings at an addition amount of 8-15%. It has good compatibility with the coating system and can achieve uniform dispersion without the need for additional dispersants. Moreover, the flame retardant performance of the finished coating meets the B1 level requirements specified in GB8624. This clarifies the source and acquisition method of industrial silicon slag, reduces raw material costs, and limits the structure, environmental protection indicators, and application scenarios of the finished product, thereby improving the applicability and market competitiveness of the product and facilitating its industrial promotion.

[0015] The beneficial effects of using the present invention are as follows: By employing the process of this invention, the following beneficial effects are achieved in addressing the problems of low raw material utilization, high energy consumption, unstable product performance, and insufficient environmental friendliness in existing silicon-based inorganic flame retardant preparation technologies: 1. It realizes the resource utilization of industrial silicon slag by-product solid waste. Through pretreatment steps such as manual screening, crushing, grinding and magnetic separation to remove iron, the purity of silicon slag is effectively improved, and it is transformed into a high-quality raw material for preparing silicon-based inorganic flame retardants. This reduces the environmental pollution caused by solid waste dumping and landfilling. At the same time, silicon slag is obtained through low-price purchase or free transportation, reducing raw material costs.

[0016] 2. The low-temperature electrocatalytic method for preparing silicon-based whiskers significantly reduces production energy consumption compared to the traditional high-temperature gas-phase synthesis method. Furthermore, by precisely controlling the electrolysis parameters, the prepared silicon-based whiskers have uniform particle size and high activity, providing high-quality raw materials for subsequent processes and improving the overall stability of the process.

[0017] 3. By precisely controlling the phosphorus-silicon premixing parameters and the supercritical CO2 microsphere coating process, uniform mixing and dense coating at the phosphorus-silicon molecular level are achieved, effectively solving the problems of microsphere agglomeration and coating layer cracking in traditional coating processes, improving the flame retardant performance and thermal stability of the product, and enabling the finished product to meet the application requirements of high-end coatings.

[0018] 4. The comprehensive environmental protection process enables the recycling and reuse of wastewater, solid waste, and CO2. The water reuse rate is over 95%, the comprehensive utilization rate of solid waste is over 99%, and the CO2 reuse rate is over 90%, which greatly improves resource utilization, reduces pollutant emissions, and complies with the national solid waste resource utilization and "dual carbon" policy requirements. Attached Figure Description

[0019] The present invention can be further illustrated by the non-limiting embodiments given in the accompanying drawings; Figure 1This is a schematic flowchart illustrating an embodiment of a silicon-based inorganic flame retardant preparation process according to the present invention. Detailed Implementation To enable those skilled in the art to better understand the present invention, the technical solution of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0020] like Figure 1 As shown, a process for preparing a silicon-based inorganic flame retardant according to the present invention includes the following steps: S1. Pretreatment of industrial silicon slag: After crushing and grinding the industrial silicon slag, iron is removed by magnetic separation to obtain silicon slag powder; S2, Alkali Dissolution Crystallization: Add silicon slag powder to an alkaline solution, stir at low temperature to dissolve, add crystal guiding agent, crystallize at constant temperature, filter and wash to obtain silicon-based whisker precursor; S3. Electrocatalytic whisker growth: A silicon-based whisker precursor is prepared into a suspension and fed into an electrolytic cell. Constant current electrolysis is used to achieve silicon-based whisker growth, resulting in a silicon-based whisker suspension. S4. Phosphorus-silicon premix: Add an aqueous phosphorus source solution dropwise to a silicon-based whisker suspension, mix and disperse to obtain a phosphorus-silicon mixed suspension; S5. Supercritical CO2 microsphere coating: Phosphorus-silicon mixed suspension is fed into a supercritical CO2 reactor, the reaction temperature and pressure are controlled, and after constant temperature and pressure reaction, the pressure is released in a stepwise manner to obtain core-shell structured silicon-based microsphere wet material. S6. Post-processing and curing: The wet core-shell structured silicon-based microspheres are vacuum dried and cured at low temperature. After grinding and sieving, the silicon-based inorganic flame retardant product is obtained.

[0021] In this implementation case, when using a silicon-based inorganic flame retardant preparation process, industrial silicon slag, a by-product of the silicon-based industry, is selected as raw material. The industrial silicon slag is first mechanically crushed to break its agglomerated form, and then ground to obtain fine material. Subsequently, iron impurities in the fine material are removed by magnetic separation to obtain silicon slag powder with a purity that meets the requirements of subsequent reactions, thus completing the pretreatment of industrial silicon slag. The pretreated silicon slag powder was added to an alkaline solution system and stirred at a low temperature to carry out an alkaline dissolution reaction. After the reaction was completed, a crystal-directing agent was added to the system, and the temperature was adjusted to a suitable range for isothermal crystallization to guide the silicon-based components to form a precursor with a regular crystal form. After crystallization, the system was subjected to solid-liquid separation and filtration, and the obtained solid was washed with water to remove residual alkaline solution on the surface, thus obtaining a silicon-based whisker precursor. The silicon-based whisker precursor is mixed with water and dispersed by stirring to form a uniform suspension. The suspension is then transported to an electrolytic cell and a stable current is passed through it for constant current electrolysis, which causes the silicon-based whisker precursor in the suspension to grow in a directional manner to form silicon-based whiskers. After electrolysis, a silicon-based whisker suspension is obtained. Prepare an aqueous solution of phosphorus source and slowly add it dropwise to the silicon-based whisker suspension. During the dropwise addition, a mixing and dispersion operation is carried out simultaneously to ensure that the phosphorus source and silicon-based whiskers are fully fused and uniformly mixed to obtain a phosphorus-silicon mixed suspension. The phosphorus-silicon mixed suspension was transported to a supercritical CO2 reactor. The temperature and pressure inside the reactor were adjusted to the temperature and pressure required for the supercritical reaction through a temperature and pressure control system. The reaction was kept at a constant temperature and pressure to achieve microsphere coating of silicon-based whiskers by phosphorus-silicon components. After the reaction was completed, the pressure inside the reactor was slowly reduced by a step-by-step depressurization method. After depressurization to atmospheric pressure, the product was taken out to obtain wet core-shell structured silicon-based microspheres. The wet core-shell structured silicon-based microspheres are fed into a vacuum drying device to remove moisture, followed by low-temperature curing to stabilize the core-shell structure and crystal form of the material. After curing, the material is ground and then sieved to remove coarse material with unqualified particle size, finally obtaining the finished silicon-based inorganic flame retardant. Industrial silicon slag pretreatment refines the material through crushing and grinding, while magnetic separation removes iron impurities, preventing them from interfering with subsequent chemical reactions and providing high-purity raw materials for alkali-soluble crystallization, ensuring the stability and effectiveness of subsequent reactions. The alkali-soluble reaction involves the reaction of an alkaline solution with the silicon-based components in the silicon slag powder, dissolving the effective silicon. A crystal-directing agent guides the dissolved silicon-based components to form well-structured, highly reactive silicon-based whisker precursors through molecular guidance, laying the material foundation for the directional growth of silicon-based whiskers. Electrocatalytic whisker growth utilizes the constant-current electrolysis effect in the electrolytic cell, providing energy for the growth of the silicon-based whisker precursors through electrode reactions, achieving directional crystallization growth of the precursors at low temperatures, avoiding problems such as crystal defects and reduced activity caused by traditional high-temperature processes. Phosphorus-silicon premixing is employed... The drop-addition combined with dispersion method allows the phosphorus source and silicon-based whiskers to fully contact in the liquid phase system, achieving molecular-level uniform mixing and forming a stable mixture of phosphorus and silicon components, providing a uniform raw material system for subsequent coating processes. Supercritical CO2 microsphere coating utilizes the low surface tension and high diffusivity of supercritical CO2 fluid to achieve dense and uniform coating of the phosphorus and silicon mixture on the surface of silicon-based whiskers. At the same time, microsphere formation is achieved under supercritical conditions, and stepped pressure relief avoids damage to the core-shell structure of microspheres caused by sudden pressure drops, ensuring the structural integrity of the product. Post-treatment curing thoroughly removes moisture from the material through vacuum drying, and low-temperature curing further stabilizes the core-shell structure and crystal form, preventing structural deformation. Grinding and sieving enable precise control of the finished product particle size, ensuring that the product meets the particle size requirements of the application. This process breaks through the technical limitations of existing silicon-based inorganic flame retardant preparation fields. It abandons the conventional approach of using high-purity raw materials, high-temperature gas-phase synthesis of silicon-based whiskers, and physical mixing of phosphorus and silicon coating. For the first time, it organically combines the resource utilization of industrial silicon slag solid waste with low-temperature electrocatalytic whisker growth and supercritical CO2 microsphere phosphorus and silicon coating to form a closed-loop process with full-process synergy. Compared with using a single technology, this process achieves a technical effect of 1+1>2. Meanwhile, this process uses industrial silicon slag by-product solid waste as raw material, which greatly improves the utilization rate of silicon resources and solves the problems of solid waste pollution and resource waste in the silicon-based industry. Moreover, the low-cost acquisition of solid waste significantly reduces raw material costs, resulting in a substantial reduction in raw material costs compared to traditional high-purity raw material processes. The entire process adopts low-temperature reaction conditions, and core steps such as electrocatalytic whisker growth and supercritical coating do not require high-temperature heating. Compared to traditional high-temperature gas-phase synthesis processes, this significantly reduces energy consumption during production, aligning with the national industrial policy of energy conservation and emission reduction. In addition, the supercritical CO2 microsphere coating process effectively solves industry pain points commonly found in traditional coating processes, such as microsphere agglomeration, coating layer cracking, and uneven phosphorus-silicon mixing. The prepared core-shell structure silicon-based inorganic flame retardant has uniform particle size and a dense coating layer, and the flame retardant performance and thermal stability of the product are significantly improved, far superior to products prepared by traditional processes. This process is simple to operate, and the materials are directionally transformed into raw materials for subsequent reactions after each step of processing. It has a high raw material utilization rate and can realize continuous and industrialized production, which is suitable for the needs of large-scale industrial promotion. At the same time, the process has reserved reasonable process interfaces for subsequent environmental protection treatment and quality control. Related processes can be flexibly matched according to production needs, taking into account product performance, production efficiency and environmental protection requirements. It is different from the existing single-function, high-energy-consumption and low raw material utilization preparation processes, and has significant technical advantages and industrial application value.

[0022] In the preferred step S1, the industrial silicon slag is first manually screened to remove large pieces of coke, then coarsely crushed to ≤1cm using a jaw crusher, and subsequently ground using a planetary ball mill. The grinding media are alumina balls with a particle size of 5mm, the ball-to-material ratio is 3:1, the rotation speed is 300rpm, and the grinding time is 1h. After grinding, the silicon slag powder has a particle size of 200 mesh, and the industrial silicon slag has a SiO2 content ≥75%, Fe2O3 content ≤1.5%, and CaO content ≤2%. Magnetic separation to remove iron uses a two-stage drum magnetic separator with a magnetic field strength of 12000Gs, a feed rate of 0.5m / s, and an iron removal efficiency ≥99%. After magnetic separation, the iron content in the silicon slag powder is ≤0.5%.

[0023] In this implementation case, the industrial silicon slag is first manually screened to remove large pieces of coke, stones, and other foreign matter, and then fed into a jaw crusher. The feed rate of the equipment is adjusted to ensure uniform feeding of the silicon slag, and it is coarsely crushed to particles with a diameter ≤1cm. The coarsely crushed particles are then fed into a planetary ball mill, where alumina balls with a diameter of 5mm are added as the grinding media. The ball-to-material ratio is controlled at 3:1, and the mill speed is adjusted to 300rpm. Grinding is continued for 1 hour. After grinding, the silicon slag powder is obtained by sieving to obtain 200-mesh silicon slag powder. The industrial silicon slag used must meet the composition requirements of SiO2 content ≥75%, Fe2O3 content ≤1.5%, and CaO content ≤2%. The silicon slag powder is then fed into a secondary drum magnetic separator. The magnetic field strength is adjusted to 12000Gs and the feed rate is adjusted to 0.5m / s to ensure that the powder passes evenly through the magnetic separator drum, adsorbing and removing iron impurities, ensuring an iron removal efficiency ≥99%. The iron content in the silicon slag powder after magnetic separation is ≤0.5%. Furthermore, depending on the initial hardness and impurity content of the industrial silicon slag, the jaw crusher can be replaced with a hammer crusher or an impact crusher, and the coarse crushing particle size can be finely adjusted to ≤1.2cm; the ball-to-material ratio of the planetary ball mill can be adjusted within the range of 2:1-4:1, the rotation speed is adapted to 280-320rpm, and the grinding time is adjusted to 0.8-1.2h according to the grinding effect. Zirconia balls can also be used as the grinding media to improve the grinding uniformity; the two-stage drum magnetic separator can be upgraded to a three-stage drum magnetic separator, and the magnetic field strength can be finely adjusted between 11000-13000Gs to further improve the iron removal efficiency to ≥99.5%. The particle size of the silicon slag powder can also be adjusted to 180-220 mesh according to the subsequent alkali dissolution efficiency requirements; Through a multi-stage combination of manual screening and mechanical crushing, grinding, and magnetic separation, large impurities and iron elements in industrial silicon slag are precisely removed. Compared with the traditional single crushing and grinding pretreatment method, the impurity removal is more thorough, effectively improving the purity and particle size uniformity of silicon slag powder. This avoids interference from impurities and iron ions in subsequent steps such as alkali dissolution crystallization and electrocatalytic whisker growth, ensuring the reaction stability of each subsequent process step from the raw material end. The clearly defined requirements for the composition and particle size of industrial silicon slag prevent deviations in the performance of subsequent products due to fluctuations in raw material composition and uneven particle size, significantly reducing the defect rate in the production process. At the same time, the adjustability of each equipment parameter can adapt to industrial silicon slag raw materials of different qualities, improving the raw material adaptability of the process.

[0024] In the preferred step S2, the alkaline solution is an industrial sodium hydroxide solution with a content ≥96%, and the mass ratio of silicon slag powder, industrial sodium hydroxide, and deionized water is 1:0.35:10; the alkaline dissolution temperature is 90±2℃, the stirring speed is 300rpm, and the alkaline dissolution time is 3h. During the alkaline dissolution process, an online pH meter with an accuracy of ±0.01pH is used for real-time monitoring, and the pH is controlled at 12.0±0.2; the crystallization guide is industrial sodium sulfate with a content ≥98%, and the addition amount is 4.5% of the mass of silicon slag powder; the crystallization temperature is 80±2℃, the crystallization time is 5h, and the stirring speed is reduced to 200rpm during the crystallization process; filtration is carried out using a 50㎡ atmospheric pressure plate and frame filter press. After filtration, the filter cake is pressed by the filter press and then washed with deionized water until the pH of the filtrate is ≤10.0, resulting in a silicon-based whisker precursor filter cake with a water content of 30±2%.

[0025] In this implementation case, an industrial sodium hydroxide solution with a content ≥96% was selected as the alkaline solution. The alkaline solution was prepared by first mixing deionized water and industrial sodium hydroxide at a mass ratio of 1:0.35:10 for silicon slag powder, industrial sodium hydroxide, and deionized water. Then, silicon slag powder was added, and the reaction temperature was adjusted to 90±2℃ and the stirring speed to 300 rpm. The alkaline solution was dissolved under low-temperature stirring for 3 hours. During the alkaline dissolution process, an online pH meter with an accuracy of ±0.01 pH was used to monitor the system pH in real time, strictly controlling the pH to be within 12.0±0. 2; After alkali dissolution, add industrial sodium sulfate with a content of ≥98% as a crystallization guide agent. The amount added is 4.5% of the mass of silicon slag powder. Adjust the system temperature to 80±2℃ and reduce the stirring speed to 200rpm. Crystallize at a constant temperature for 5h. After crystallization, send the system material into a 50㎡ atmospheric pressure plate and frame filter press for solid-liquid separation. After filtration, the filter cake is pressed by the filter press and then washed multiple times with deionized water until the pH of the filtrate is ≤10.0. Finally, a silicon-based whisker precursor filter cake with a water content of 30±2% is obtained. Furthermore, the content of the industrial sodium hydroxide solution can be adjusted within the range of 96%-98%, and the mass ratio of silicon slag powder, industrial sodium hydroxide, and deionized water can be finely adjusted to 1:0.32-0.38:10 according to the purity of the silicon slag powder; the alkali dissolution temperature can fluctuate between 88-92℃, the stirring rate is suitable at 280-320rpm, the alkali dissolution time is adjusted to 2.8-3.2h according to the dissolution of silicon slag, and the accuracy of the online pH meter can also be selected as ±0.02pH, with the pH control range finely adjusted to 11.8-12.2; the content of industrial sodium sulfate can be selected within the range of 98%-99.5%, the addition amount is adjusted to 4.2%-4.8% of the mass of silicon slag powder, the crystallization temperature is suitable at 78-82℃, and the crystallization time is adjusted to 4.8-5.2h; the filtration area of ​​the plate and frame filter press can be selected as 48-52㎡, the pH of the filtrate after washing is controlled at 9.8-10.2, and the moisture content of the precursor filter cake is adjusted to 28%-32%; By clearly defining the raw material specifications, process parameters, and equipment requirements for each stage of alkali dissolution and crystallization, precise control of the alkali dissolution reaction and crystallization process is achieved. This allows the silicon-based whisker precursor to form a more regular crystal shape, enhancing the precursor's reactivity and providing high-quality raw materials for subsequent electrocatalytic whisker growth, ensuring the uniformity and stability of whisker growth. Real-time monitoring and precise control using an online pH meter avoids pH deviations during alkali dissolution that could lead to insufficient or excessive dissolution of silicon slag, significantly improving the utilization rate of silicon-based raw materials. The pressing and precise washing of the plate and frame filter press effectively reduce the alkali content and impurity residue in the precursor filter cake, preventing residual impurities from affecting the efficiency of subsequent electrocatalytic reactions and product performance. At the same time, precise control of the filter cake moisture content reduces water consumption for subsequent suspension preparation, improving overall production efficiency.

[0026] In preferred step S3, the silicon-based whisker precursor filter cake is added to deionized water with a conductivity ≤10μS / cm to prepare a suspension with a solid content of 15%. The suspension is dispersed for 20 minutes using an ultrasonic disperser with a power of 300W and a frequency of 28kHz. During the dispersion process, a constant temperature water bath is used to control the temperature, ensuring that the temperature is ≤25℃. After dispersion, the suspension is sent to a 500L small electrocatalytic electrolytic cell with a jacketed temperature control device. The anode of the electrolytic cell is a titanium-based ruthenium-iridium coated electrode, the cathode is a 304 stainless steel mesh, and the electrode spacing is 3.5cm. The electrolysis temperature is 70±2℃, the constant current electrolysis current density is 14mA / cm², the electrolysis time is 4.5h, the electrolyte in the electrolysis system is a 4.5% industrial sodium sulfate solution, and the stirring rate is 250rpm. After electrolysis, the material is directly discharged to a transfer tank to obtain the silicon-based whisker suspension.

[0027] In this implementation case, a silicon-based whisker precursor filter cake was taken and added to deionized water with a conductivity ≤10μS / cm. The mixture was thoroughly stirred to prepare a suspension with a solid content of 15%. The suspension was then fed into an ultrasonic disperser with a power of 300W and a frequency of 28kHz. During dispersion, a constant-temperature water bath was used to control the temperature, strictly maintaining the system temperature ≤25℃, and dispersion was continued for 20 minutes. The dispersed suspension was then fed into a 500L small electrocatalytic electrolyzer equipped with a jacketed temperature control device. The anode of the electrolyzer was a titanium-based ruthenium-iridium coated electrode, and the cathode was a 304 stainless steel mesh. The electrode spacing was adjusted to 3.5cm. A 4.5% industrial sodium sulfate solution was added to the electrolyzer as the electrolyte. Adjust the stirring speed to 250 rpm and the electrolysis temperature to 70±2℃, adopt the constant current electrolysis mode, adjust the current density to 14 mA / cm², and continue electrolysis for 4.5 h; after electrolysis, the silicon-based whisker suspension is directly discharged to the transfer tank without additional solid-liquid separation steps; the conductivity of deionized water can be finely adjusted to ≤12μS / cm, and the solid content of the suspension is suitable for 14%-16%; the power of the ultrasonic disperser can be adjusted between 280-320W, the frequency is suitable for 27-29kHz, and the dispersion time is adjusted to 18-22min according to the uniformity of the suspension; the constant temperature water bath can also be replaced with a low temperature cooling tank, and the temperature control range is maintained at 23-25℃; Furthermore, the volume of the electrocatalytic electrolyzer can be selected as 480-520L, the anode can be replaced with a titanium-based iridium-tantalum coated electrode, the cathode is selected as a 316L stainless steel mesh, and the electrode spacing can be finely adjusted to 3.3-3.7cm; the electrolysis temperature can fluctuate between 68-72℃, the current density is adapted to 13-15mA / cm², the electrolysis time is adjusted to 4.2-4.8h, the concentration of industrial sodium sulfate solution is finely adjusted to 4.2%-4.8%, and a low-speed stirring device can be added to the transfer tank to avoid the aggregation of silicon-based whisker suspension; By precisely defining the parameters of the entire electrocatalytic whisker growth process, low-temperature and high-efficiency growth of silicon-based whiskers is achieved. Compared with the traditional high-temperature gas-phase synthesis method, high-temperature heating is not required, significantly reducing production energy consumption. At the same time, it avoids the damage of high temperature to the whisker crystal form, resulting in silicon-based whiskers with uniform particle size and high reactivity, providing a high-quality reaction system for subsequent phosphorus-silicon premixing and coating. The combination of ultrasonic dispersion and ice-water bath temperature control effectively prevents the precursor from deteriorating due to temperature rise during dispersion, while thoroughly breaking up precursor agglomerates, improving the uniformity of the suspension, and ensuring the consistency of whisker growth during electrolysis. The combination of titanium-based ruthenium-iridium coated electrodes and 304 stainless steel mesh electrodes improves electrolysis efficiency and electrode lifespan. Precise control of electrolysis parameters enables directional growth of whiskers. After electrolysis, the material is directly discharged to the transfer tank, eliminating the solid-liquid separation step, reducing material loss of silicon-based whiskers, and improving raw material utilization and production efficiency.

[0028] In preferred step S4, the phosphorus source is industrial ammonium dihydrogen phosphate with a content of ≥98%. The phosphorus source is added to deionized water to prepare an aqueous solution with a concentration of 20%. After dissolution, it is filtered through a 0.45μm filter membrane to remove impurities. The phosphorus-silicon molar ratio is controlled at 1:4.2. The phosphorus source aqueous solution is added dropwise to the silicon-based whisker suspension at a dropping rate of 2mL / min. During the mixing and dispersion process, ultrasonic dispersion with a power of 200W is used, and a high-speed stirrer is used to stir at a speed of 250rpm. The dispersion time is 30min, and the dispersion temperature is maintained at 70℃.

[0029] In this implementation case, industrial ammonium dihydrogen phosphate with a content of ≥98% was selected as the phosphorus source. Deionized water was added and stirred thoroughly to prepare a 20% phosphorus source aqueous solution. The aqueous solution was filtered through a 0.45μm filter membrane to remove minor impurities. The phosphorus source aqueous solution was slowly added dropwise to the silicon-based whisker suspension at a dropping rate of 2mL / min according to a phosphorus-silicon molar ratio of 1:4.2. During the dropping process, ultrasonic dispersion with a power of 200W was turned on, and a high-speed stirrer was used to stir at a speed of 250rpm. The system temperature was maintained at 70℃, and the mixing and dispersion were continued for 30min to achieve uniform mixing of the phosphorus and silicon components. Furthermore, the content of industrial ammonium dihydrogen phosphate can be adjusted within the range of 98%-99.5%, the concentration of the phosphorus source aqueous solution is suitable at 18%-22%, and the pore size of the filter membrane can be selected at 0.4-0.5μm to further remove minute impurities; the phosphorus-silicon molar ratio can be adjusted within the range of 1:4.0-1:4.4 according to the flame retardant performance requirements of the finished product, and the dropping rate of the phosphorus source aqueous solution is suitable at 1.8-2.2mL / min; the power of the ultrasonic disperser can be adjusted between 180-220W, the speed of the high-speed stirrer is suitable at 240-260rpm, the dispersion time is adjusted to 28-32min, and the dispersion temperature is maintained at 68-72℃. A small amount of halogen-free dispersant can also be added during the dispersion process to further improve the uniformity of the phosphorus-silicon mixture without affecting the subsequent performance of the product; By refining the parameters of each step in the phosphorus-silicon premixing process and employing a combination of ultrasonic-assisted dispersion and high-speed stirring, the limitations of traditional single dispersion are overcome, achieving molecular-level uniform mixing of phosphorus-silicon components. This avoids the problem of uneven coating caused by local aggregation of phosphorus sources, laying a solid material foundation for supercritical CO2 microsphere coating. Membrane filtration of the phosphorus source aqueous solution effectively removes impurities from the phosphorus source, preventing them from entering the subsequent system and affecting the coating effect and product quality. Precise control of the phosphorus-silicon molar ratio and dropping rate ensures the stability of the synergistic flame-retardant effect of phosphorus and silicon. At the same time, stable dispersion temperature control prevents phosphorus source decomposition or silicon whisker agglomeration caused by temperature fluctuations, further improving the stability and uniformity of the phosphorus-silicon mixed suspension and ensuring the smooth progress of subsequent coating processes.

[0030] In the preferred step S5, the phosphorus-silicon mixed suspension is mixed with anhydrous ethanol with a content of ≥99.5% at a volume ratio of 10:1. After mixing, the mixture is fed into a 50L domestically produced supercritical CO2 reactor at a feed rate of 5L / h via a high-pressure metering pump with a pressure of ≥10MPa. During the reaction, CO2 with a purity of ≥99.5% is introduced throughout the process at a flow rate of 10L / min. The reaction temperature is controlled at 35±1℃, the reaction pressure is controlled at 8.0±0.2MPa, the stirring speed is 300rpm, and the holding time is 2.0h. The step-by-step depressurization is as follows: first, the pressure is depressurized to 5MPa at a rate of 0.3MPa / min, and after being kept at a constant temperature for 10min, the pressure is depressurized to atmospheric pressure at a rate of 0.5MPa / min. The discharged material is a wet core-shell structured silicon-based microsphere with uniform particle size.

[0031] In this implementation case, a phosphorus-silicon mixed suspension was thoroughly mixed with anhydrous ethanol with a content of ≥99.5% at a volume ratio of 10:1. The mixture was fed into a 50L domestically produced supercritical CO2 reactor at a feed rate of 5L / h via a high-pressure metering pump with a pressure of ≥10MPa. During the reaction, CO2 with a purity of ≥99.5% was continuously introduced into the reactor, with the CO2 flow rate controlled at 10L / min. The stirring speed of the reactor was adjusted to 300rpm, and the reaction temperature was strictly controlled at 35±1℃ and the reaction pressure at 8.0±0.2MPa. The reaction was carried out under constant temperature and pressure for 2.0h. After the reaction was completed, a step-by-step depressurization was adopted. First, the pressure was depressurized to 5MPa at a rate of 0.3MPa / min, and the mixture was kept at a constant temperature for 10min. Then, the pressure was depressurized to atmospheric pressure at a rate of 0.5MPa / min. After depressurization, the material was discharged to obtain a wet core-shell structured silicon-based microsphere with uniform particle size. Furthermore, the content of anhydrous ethanol can be adjusted within the range of 99.5%-99.9%, and the volume ratio with the phosphorus-silicon mixed suspension is suitable at 9:1-11:1; the pressure of the high-pressure metering pump can be selected as ≥9.5MPa, and the feed rate is suitable at 4.8-5.2L / h; the volume of the supercritical CO2 reactor can be selected as 48-52L, the CO2 purity can be adjusted to ≥99.2%, and the flow rate is suitable at 9.5-10.5L / min; the reaction temperature can be maintained between 34-36℃, the reaction pressure can be finely adjusted to 7.8-8.2MPa, the stirring speed is suitable at 280-320rpm, and the heat and pressure holding time is adjusted to 1.8-2.2h; the step-depressurization rate can be finely adjusted to 0.28-0.32MPa / min and 0.48-0.52MPa / min, and the constant temperature settling time is adjusted to 8-12min; By precisely defining the entire process parameters and using a stepped depressurization method for supercritical CO2 microsphere coating, the low surface tension and high diffusivity of supercritical CO2 fluid are fully utilized to achieve dense and uniform coating of silicon-based whiskers by phosphorus-silicon components. Simultaneously, microsphere formation is achieved under supercritical conditions, effectively solving industry pain points such as microsphere agglomeration, coating layer cracking, and uneven coating in traditional spray drying and physical coating processes. Stable feeding by a high-pressure metering pump and continuous CO2 injection ensure uniform and stable temperature and pressure in the reaction system, avoiding fluctuations in microsphere performance due to local parameter deviations. Stepped depressurization, compared to one-time depressurization, effectively prevents microsphere rupture due to sudden pressure drops. The isothermal settling step further stabilizes the core-shell structure of the microspheres, improving structural integrity. The prepared core-shell silicon-based microspheres have uniform particle size and a dense coating layer, providing high-quality wet material for subsequent post-processing curing, significantly improving the product's flame retardant properties and thermal stability.

[0032] In the preferred step S6, the integrated vacuum drying-low temperature curing process uses a vacuum drying oven with an effective volume of 100L and a vacuum degree of -0.095MPa. The programmed temperature rise parameters are: 40℃ drying for 2h → 60℃ drying for 3h → 100℃ curing for 2h, maintaining a vacuum state throughout the process. After curing, the dry material is ground using an air jet mill with a grinding pressure of 0.6-0.8MPa. After grinding, it is sieved using an 80-mesh ultrasonic vibrating screen with a power of 500W. The finished particle size is controlled at 200-300nm with D50=250nm, D90≤300nm, and monodisperse CV≤15%. The finished product is then sealed and moisture-proof packaged.

[0033] In this implementation case, the wet material of core-shell structured silicon-based microspheres is evenly spread on a drying tray with a thickness of ≤2cm. It is then placed in a vacuum drying oven with an effective volume of 100L and a vacuum degree of -0.095MPa. The vacuum drying-low temperature curing integrated process is adopted, with the programmed temperature rise parameters as follows: 40℃ drying for 2h → 60℃ drying for 3h → 100℃ curing for 2h, maintaining a vacuum state throughout the process. The cured dry material is then sent to an air jet mill with a grinding pressure of 0.6-0.8MPa for grinding. After grinding, the material is sieved through an 80-mesh, 500W ultrasonic vibrating screen. The particle size of the finished product after sieving is controlled at 200-300nm with D50=250nm, D90≤300nm, and monodisperse CV≤15%. The final product is packaged in a sealed, moisture-proof package. Furthermore, the effective volume of the vacuum drying oven can be selected from 98-102L, the vacuum degree can be adjusted from -0.093 to -0.097MPa, and the programmed temperature parameters can be finely adjusted to 40℃ drying for 1.8-2.2h, 60℃ drying for 2.8-3.2h, and 100℃ curing for 1.8-2.2h. The curing temperature can also be finely adjusted to 98-102℃ according to the stability requirements of the finished product. The grinding pressure of the air jet mill can fluctuate between 0.58-0.82MPa. After grinding, the product can be first screened and then enter the ultrasonic vibrating screen to improve the screening efficiency. The power of the ultrasonic vibrating screen can be adjusted between 480-520W, and the sieve mesh can be adjusted to 78-82 mesh according to the particle size requirements of the finished product. The particle size of the finished product can be controlled between 190-310nm, with D50 suitable for 240-260nm, D90≤310nm, and monodisperse CV≤16%. The sealed and moisture-proof packaging can use aluminum foil bags for vacuum packaging, combined with desiccant, to further improve storage stability. By clearly defining the equipment parameters, programmed heating, and sieving requirements for post-processing curing, programmed heating vacuum drying can thoroughly remove moisture from wet materials while avoiding microsphere agglomeration and coating layer detachment caused by high-temperature drying. Low-temperature curing further stabilizes the core-shell structure and crystal form, improving the thermal stability and mechanical properties of the finished product. The combination of precise grinding by air jet mill and fine sieving by ultrasonic vibrating screen enables precise control of the finished product particle size, ensuring particle size uniformity and meeting the application needs of different coating scenarios. Sealed moisture-proof packaging effectively prevents the finished product from absorbing moisture and agglomerating during storage and transportation, avoiding product performance degradation and ensuring consistent product quality. At the same time, refined parameter control reduces finished product loss during production and improves product qualification rate.

[0034] The preferred method also includes environmental protection treatment steps: the wastewater generated from the filtration and washing in step S2 is collected to an integrated sewage treatment equipment, neutralized to pH 7-8 with industrial hydrochloric acid, flocculated, filtered, and reverse osmosis treated, and then all of it is reused in the reaction system of alkaline dissolution in step S2 and electrocatalysis in step S3, with a water reuse rate of ≥95%; the iron slag generated from the magnetic separation of iron in step S1 is recycled and sold, and the filter residue containing a small amount of silicon-based components generated from the filtration in step S2 is returned to step S2 for alkaline dissolution and crystallization again, with a solid waste comprehensive utilization rate of ≥99%; the CO2 generated from the depressurization in step S5 is collected by a recovery device and liquefied to ensure that the purity of the liquefied CO2 is ≥99.5%, and then reused in the supercritical reaction in step S5, with a CO2 reuse rate of ≥90%, and no waste gas is discharged throughout the process.

[0035] In this implementation case, environmental protection steps are implemented simultaneously with the core process: wastewater generated from filtration and washing in step S2 is collected through pipelines to an integrated wastewater treatment device, neutralized to pH 7-8 with industrial hydrochloric acid, and then treated by flocculation, filtration, and reverse osmosis before being fully reused in the reaction system of alkaline dissolution in step S2 and electrocatalysis in step S3, with a water reuse rate of ≥95%; iron slag generated from magnetic separation in step S1 is recycled and sold; filter residue containing a small amount of silicon-based components generated from filtration in step S2 is returned to step S2 for re-alkaline dissolution and crystallization, with a solid waste comprehensive utilization rate of ≥99%; CO2 generated from depressurization in step S5 is collected and liquefied by a recovery device to ensure that the purity of liquefied CO2 is ≥99.5%, and then reused in the supercritical reaction in step S5, with a CO2 reuse rate of ≥90%, and no waste gas is discharged throughout the process; Furthermore, the integrated wastewater treatment equipment can adjust the treatment scale according to the wastewater volume. Industrial hydrochloric acid can be replaced with industrial sulfuric acid, and the neutralized pH can be controlled at 6.8-8.2. Polyaluminum chloride can be used as a flocculant in the flocculation step to improve the flocculation effect. Wastewater after reverse osmosis treatment can be filtered twice to further improve water quality, and the water reuse rate can be increased to ≥96%. Iron slag can be sold to non-ferrous metal recycling companies after simple magnetic separation and inspection to remove silicon slag impurities. Filter residue containing a small amount of silicon-based components can be pre-treated by grinding before being returned to step S2 to improve silicon resource utilization. The CO2 recovery device can be equipped with a purity detection module. The liquefied CO2 can be reused after passing the test, and the CO2 reuse rate can be increased to ≥91%. At the same time, a small amount of volatile gas generated during the production process can be collected, treated by adsorption, and then discharged. By supplementing the entire process with environmentally friendly treatment steps, the limitations of traditional manufacturing processes that prioritize production over environmental protection are broken. This enables the recycling and reuse of wastewater, solid waste, and CO2, significantly improving resource utilization and reducing pollutant emissions, aligning with national policies on solid waste resource utilization and "dual carbon" (carbon diversification). Wastewater reuse reduces fresh water consumption, lowers production water costs, and avoids water pollution from wastewater discharge. Iron slag recycling and resale can generate additional economic benefits, and the reuse of silicon-based filter residue further improves silicon raw material utilization and reduces raw material loss. CO2 recycling reduces environmental pressure from exhaust emissions and lowers the cost of CO2 raw material procurement for supercritical reactions, achieving a win-win situation for both environmental and economic benefits. The entire process is free of exhaust emissions, significantly enhancing the environmental competitiveness of the process.

[0036] The preferred approach also includes quality control steps: Inspections are conducted at the following key points, with internal control standards established for each point. Non-conforming products are reworked according to the corresponding procedures to ensure stable product performance. 1) Raw material incoming inspection: ICP-MS is used to test the SiO2, Fe2O3 and CaO content of industrial silicon slag. If the content is not up to standard, it will be returned to the supplier. 2) Post-alkali dissolution testing: Test the silicon content and activity of the silicon-based whisker precursor. If it fails to meet the requirements, readjust the alkali dissolution parameters and perform alkali dissolution again. 3) Testing after magnetic separation to remove iron: Test the iron content of the silicon slag powder to ensure it is ≤0.5%. If it is not qualified, a second magnetic separation is performed. 4) Detection of whisker precursor dispersion: The particle size distribution of the suspension is detected by a laser particle size analyzer. If it is not qualified, it is redispersed. 5) Post-electrocatalytic whisker growth testing: The particle size and aspect ratio of silicon-based whiskers are tested using transmission electron microscopy. If they are not up to standard, the electrolysis parameters are adjusted and electrolysis is repeated. 6) Testing after phosphorus-silicon premixing: Test the phosphorus-silicon molar ratio; if it is not up to standard, add phosphorus source or silicon-based whisker suspension. 7) Post-supercritical coating testing: The particle size of silicon-based microspheres and the thickness of the 10-15 nm coating layer are detected by transmission electron microscopy. If the results are unsatisfactory, the supercritical parameters are adjusted and the coating is repeated. 8) Finished product inspection: The finished product is inspected for particle size distribution, thermal stability at thermal decomposition temperature ≥600℃, flame retardancy performance at LOI ≥32%, and environmental protection indicators. If it passes the inspection, it is released from the factory; if it fails, it is re-ground or re-coated.

[0037] In this implementation case, eight key quality inspection points were set up throughout the core process. Each point had its own internal control standards, and non-conforming products were reworked according to the corresponding procedures: Upon arrival, raw materials were inspected using ICP-MS to detect the SiO2, Fe2O3, and CaO content of industrial silicon slag; products failing to meet the standards were returned to the supplier. After alkali dissolution, the silicon content and activity of the silicon-based whisker precursor were tested; if unqualified, the alkali dissolution parameters were readjusted. After magnetic separation to remove iron, the iron content of the silicon slag powder was tested to ensure it was ≤0.5%; if unqualified, a second magnetic separation was performed. After the whisker precursor was dispersed, a laser particle size analyzer was used to detect the particle size distribution of the suspension. If the particle size distribution is not up to standard, the particles are redispersed. After electrocatalytic whisker growth, the particle size and aspect ratio of silicon-based whiskers are detected by transmission electron microscopy. If they are not up to standard, the electrolysis parameters are adjusted and the crystals are electrolyzed again. After phosphorus-silicon premixing, the phosphorus-silicon molar ratio is tested. If it is not up to standard, phosphorus source or silicon-based whisker suspension is added. After supercritical coating, the particle size of silicon-based microspheres and the coating layer thickness of 10-15 nm are detected by transmission electron microscopy. If they are not up to standard, the supercritical parameters are adjusted and the coating is repeated. The finished product is tested for particle size distribution, thermal stability at a thermal decomposition temperature ≥600℃, flame retardant performance with an LOI ≥32%, and environmental protection indicators. If they are not up to standard, the product is re-ground or re-coated. Furthermore, testing points can be added based on production scale. For example, after phosphorus-silicon premixing, suspension uniformity testing can be added; after supercritical coating, microsphere integrity testing can be added. Raw material incoming testing can include purity testing of raw materials such as industrial sodium hydroxide and industrial ammonium dihydrogen phosphate to further control raw material quality. The internal control standards of each testing point can be fine-tuned according to the application scenario requirements of the finished product. For example, for finished products used in high-end coatings, the testing standards for thermal stability and flame retardant performance can be improved. The rework process for non-conforming products can be refined. For example, if the product fails after alkali dissolution, the alkali dissolution temperature, time, or amount of alkali solution can be adjusted based on the test results to avoid blind rework. Automated testing equipment can be introduced to improve testing efficiency and accuracy and reduce human error. By establishing targeted testing and rework processes at key nodes throughout the entire process, a comprehensive quality control system is built. This system can promptly identify quality issues in raw materials, intermediate products, and finished products, preventing problems from spreading to the next stage, reducing raw material and labor losses, effectively controlling product performance fluctuations during production, and significantly lowering the defect rate. The clearly defined rework process ensures that defective products are handled reasonably and in a targeted manner, avoiding material waste. Simultaneously, the clear testing standards provide a scientific basis for adjusting production parameters, ensuring the stability of subsequent production. Compared to processes without quality control, this process significantly improves the finished product qualification rate and performance consistency, ensuring that products can consistently meet the needs of applications such as coatings, thereby enhancing product reliability and market competitiveness.

[0038] In the preferred step S1, industrial silicon slag is a by-product solid waste of the silicon-based industry, which is obtained through low-price purchase or free disposal. The finished silicon-based inorganic flame retardant has a core-shell structure, with a core layer of silicon-based whiskers and a shell layer of phosphorus-silicon composite. The finished product is halogen-free and has a heavy metal content of ≤10ppm. It can be directly added to water-based interior wall coatings, oil-based exterior wall coatings, or low-VOC indoor special coatings at a dosage of 8-15%. It has good compatibility with the coating system and can achieve uniform dispersion without the need for additional dispersants. Moreover, the flame retardant performance of the finished coating meets the B1 level requirements specified in GB8624.

[0039] In this implementation case, the industrial silicon slag used in step S1 is a by-product solid waste of the silicon-based industry, which is obtained through low-price purchase or free disposal. The final prepared silicon-based inorganic flame retardant has a core-shell structure, with a core layer of silicon-based whiskers and a shell layer of phosphorus-silicon composite. The finished product is halogen-free and has a heavy metal content of ≤10ppm. The finished product can be directly added to water-based interior wall coatings, oil-based exterior wall coatings, or low-VOC indoor special coatings at an addition amount of 8-15%. It has good compatibility with the coating system and can achieve uniform dispersion without the addition of additional dispersants. Moreover, the flame retardant performance of the finished coating after addition meets the B1 level requirements specified in GB8624. Furthermore, industrial silicon slag can be expanded to include other by-product solid wastes from silicon-based industries such as silicon powder production and silicon device processing. Acquisition methods can be tailored to regional differences, employing replacement, cooperative recycling, and other approaches to further reduce raw material costs. The core-shell structure of the finished product can be adjusted to accommodate flame retardant requirements by varying the thickness of the phosphorus-silicon composite layer, and the heavy metal content can be controlled to ≤8ppm, further enhancing environmental friendliness. The application scenarios of the finished product can be expanded to include plastics, rubber, and other polymer materials, with the addition amount adjustable from 7-16% depending on the material requirements. Surface modification with halogen-free surface modifiers can further improve compatibility with coatings, plastics, and other systems, adapting to more types of substrates. Simultaneously, the particle size of the finished product can be fine-tuned to suit the application needs of coatings of different thicknesses and polymer materials of different specifications. The flame retardant performance of the finished coating can be further optimized by adjusting the phosphorus-silicon molar ratio to achieve the A2 level specified in GB8624, expanding high-end application scenarios. By clearly defining the sources and acquisition methods of industrial silicon slag, and realizing the efficient resource utilization of silicon-based solid waste, this approach not only solves the problems of pollution and resource waste caused by the stockpiling of solid waste in the silicon-based industry, but also significantly reduces the raw material costs of the process and improves the economic efficiency of the process through the low-cost or free acquisition of solid waste. The finished product is a core-shell structure halogen-free flame retardant with heavy metal content far below industry standards, aligning with the industry development trend of halogen-free, low-toxicity, and environmentally friendly products, and avoiding the environmental hazards caused by halogen content and excessive heavy metals in traditional flame retardants. The finished product has good compatibility with coating systems, eliminating the need for additional dispersants, reducing the application cost of coating production, and the appropriate amount added will not affect the gloss, adhesion, or other properties of the coating. The finished coating meets the B1 level flame retardant performance specified in GB8624, which can meet the flame retardant requirements of most indoor and outdoor coatings, expanding the application scenarios of the product. Compared with traditional silicon-based flame retardants, this finished product has stronger applicability, better environmental performance, and significantly improved market competitiveness.

[0040] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A process for preparing a silicon-based inorganic flame retardant, characterized in that, Includes the following steps: S1. Pretreatment of industrial silicon slag: After crushing and grinding the industrial silicon slag, iron is removed by magnetic separation to obtain silicon slag powder; S2, Alkali Dissolution Crystallization: Add silicon slag powder to an alkaline solution, stir at low temperature to dissolve, add crystal guiding agent, crystallize at constant temperature, filter and wash to obtain silicon-based whisker precursor; S3. Electrocatalytic whisker growth: A silicon-based whisker precursor is prepared into a suspension and fed into an electrolytic cell. Constant current electrolysis is used to achieve silicon-based whisker growth, resulting in a silicon-based whisker suspension. S4. Phosphorus-silicon premix: Add an aqueous phosphorus source solution dropwise to a silicon-based whisker suspension, mix and disperse to obtain a phosphorus-silicon mixed suspension; S5. Supercritical CO2 microsphere coating: Phosphorus-silicon mixed suspension is fed into a supercritical CO2 reactor, the reaction temperature and pressure are controlled, and after constant temperature and pressure reaction, the pressure is released in a stepwise manner to obtain core-shell structured silicon-based microsphere wet material. S6. Post-processing and curing: The wet core-shell structured silicon-based microspheres are vacuum dried and cured at low temperature. After grinding and sieving, the silicon-based inorganic flame retardant product is obtained.

2. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: In step S1, the industrial silicon slag is first manually screened to remove large pieces of coke, then coarsely crushed to ≤1cm using a jaw crusher, and subsequently ground using a planetary ball mill. The grinding media are alumina balls with a particle size of 5mm, the ball-to-material ratio is 3:1, the rotation speed is 300rpm, and the grinding time is 1h. The particle size of the silicon slag powder after grinding is 200 mesh, and the SiO2 content of the industrial silicon slag is ≥75%, the Fe2O3 content is ≤1.5%, and the CaO content is ≤2%. Magnetic separation for iron removal uses a two-stage drum magnetic separator with a magnetic field strength of 12000Gs, a feed rate of 0.5m / s, and an iron removal efficiency of ≥99%. The iron content in the silicon slag powder after magnetic separation is ≤0.5%.

3. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: In step S2, the alkaline solution is an industrial sodium hydroxide solution with a content ≥96%, and the mass ratio of silicon slag powder, industrial sodium hydroxide, and deionized water is 1:0.35:10; the alkaline dissolution temperature is 90±2℃, the stirring speed is 300rpm, the alkaline dissolution time is 3h, and an online pH meter with an accuracy of ±0.01pH is used for real-time monitoring during the alkaline dissolution process, and the pH is controlled at 12.0±0.2; the crystallization guide is industrial sodium sulfate with a content ≥98%, and the addition amount is 4.5% of the mass of silicon slag powder; the crystallization temperature is 80±2℃, the crystallization time is 5h, and the stirring speed is reduced to 200rpm during the crystallization process; filtration is carried out using a 50㎡ atmospheric pressure plate and frame filter press, and the filter cake after filtration is pressed by the filter press and then washed with deionized water until the pH of the filtrate is ≤10.0, to obtain a silicon-based whisker precursor filter cake with a water content of 30±2%.

4. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: In step S3, the silicon-based whisker precursor filter cake is added to deionized water with a conductivity ≤10μS / cm to prepare a suspension with a solid content of 15%. The suspension is dispersed for 20 minutes using an ultrasonic disperser with a power of 300W and a frequency of 28kHz. During the dispersion process, a constant temperature water bath is used to control the temperature, ensuring that the temperature is ≤25℃. After dispersion, the suspension is sent to a 500L small electrocatalytic electrolytic cell with a jacketed temperature control device. The anode of the electrolytic cell is a titanium-based ruthenium-iridium coated electrode, the cathode is a 304 stainless steel mesh, and the electrode spacing is 3.5cm. The electrolysis temperature is 70±2℃, the constant current electrolysis current density is 14mA / cm², the electrolysis time is 4.5h, the electrolyte in the electrolysis system is a 4.5% industrial sodium sulfate solution, and the stirring rate is 250rpm. After electrolysis, the material is directly discharged to a transfer tank to obtain the silicon-based whisker suspension.

5. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: In step S4, the phosphorus source is industrial ammonium dihydrogen phosphate with a content of ≥98%. The phosphorus source is added to deionized water to prepare an aqueous solution with a concentration of 20%. After dissolution, it is filtered through a 0.45μm filter membrane to remove impurities. The phosphorus-silicon molar ratio is controlled at 1:4.

2. The phosphorus source aqueous solution is added dropwise to the silicon-based whisker suspension at a dropping rate of 2mL / min. During the mixing and dispersion process, ultrasonic dispersion with a power of 200W is used, and a high-speed stirrer is used to stir at a speed of 250rpm. The dispersion time is 30min, and the dispersion temperature is maintained at 70℃.

6. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: In step S5, the phosphorus-silicon mixed suspension is mixed with anhydrous ethanol with a content of ≥99.5% at a volume ratio of 10:

1. After mixing, the mixture is fed into a 50L domestically produced supercritical CO2 reactor at a feed rate of 5L / h via a high-pressure metering pump with a pressure of ≥10MPa. During the reaction, CO2 with a purity of ≥99.5% is introduced throughout the process at a flow rate of 10L / min. The reaction temperature is controlled at 35±1℃, the reaction pressure is controlled at 8.0±0.2MPa, the stirring speed is 300rpm, and the holding time is 2.0h. The step-by-step depressurization is as follows: first, the pressure is depressurized to 5MPa at a rate of 0.3MPa / min, and after being kept at a constant temperature for 10min, the pressure is depressurized to atmospheric pressure at a rate of 0.5MPa / min. The discharged material is a wet core-shell structured silicon-based microsphere with uniform particle size.

7. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: In step S6, the integrated vacuum drying-low temperature curing process uses a vacuum drying oven with an effective volume of 100L and a vacuum degree of -0.095MPa. The programmed temperature rise parameters are: 40℃ drying for 2h → 60℃ drying for 3h → 100℃ curing for 2h, maintaining a vacuum state throughout the process. After curing, the dry material is ground using an air jet mill with a grinding pressure of 0.6-0.8MPa. After grinding, it is sieved through an 80-mesh ultrasonic vibrating screen with a power of 500W. The finished particle size is controlled at 200-300nm with D50=250nm, D90≤300nm, and monodisperse CV≤15%. The finished product is then sealed and moisture-proof packaged.

8. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: It also includes environmental protection treatment steps: the wastewater generated from the filtration and washing in step S2 is collected to an integrated sewage treatment equipment, neutralized to pH 7-8 with industrial hydrochloric acid, flocculated, filtered, and reverse osmosis treated, and then all of it is reused in the reaction system of alkaline dissolution in step S2 and electrocatalysis in step S3, with a water reuse rate of ≥95%; the iron slag generated from the magnetic separation of iron in step S1 is recycled and sold, and the filter residue containing a small amount of silicon-based components generated from the filtration in step S2 is returned to step S2 for alkaline dissolution and crystallization again, with a solid waste comprehensive utilization rate of ≥99%; the CO2 generated from the depressurization in step S5 is collected by a recovery device and liquefied to ensure that the purity of the liquefied CO2 is ≥99.5%, and then reused in the supercritical reaction in step S5, with a CO2 reuse rate of ≥90%, and no waste gas is discharged throughout the process.

9. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: It also includes quality control steps: Inspections are set up at the following key points, with internal control standards established for each point. Non-conforming products are reworked according to the corresponding procedures to ensure stable product performance. 1) Raw material incoming inspection: ICP-MS is used to test the SiO2, Fe2O3 and CaO content of industrial silicon slag. If the content is not up to standard, it will be returned to the supplier. 2) Post-alkali dissolution testing: Test the silicon content and activity of the silicon-based whisker precursor. If it fails to meet the requirements, readjust the alkali dissolution parameters and perform alkali dissolution again. 3) Testing after magnetic separation to remove iron: Test the iron content of the silicon slag powder to ensure it is ≤0.5%. If it is not qualified, a second magnetic separation is performed. 4) Detection of whisker precursor dispersion: The particle size distribution of the suspension is detected by a laser particle size analyzer. If it is not qualified, it is redispersed. 5) Post-electrocatalytic whisker growth testing: The particle size and aspect ratio of silicon-based whiskers are tested using transmission electron microscopy. If they are not up to standard, the electrolysis parameters are adjusted and electrolysis is repeated. 6) Testing after phosphorus-silicon premixing: Test the phosphorus-silicon molar ratio; if it is not up to standard, add phosphorus source or silicon-based whisker suspension. 7) Post-supercritical coating testing: The particle size of silicon-based microspheres and the thickness of the 10-15 nm coating layer are detected by transmission electron microscopy. If the results are unsatisfactory, the supercritical parameters are adjusted and the coating is repeated. 8) Finished product inspection: The finished product is inspected for particle size distribution, thermal stability at thermal decomposition temperature ≥600℃, flame retardancy performance at LOI ≥32%, and environmental protection indicators. If it passes the inspection, it is released from the factory; if it fails, it is re-ground or re-coated.

10. The preparation process of a silicon-based inorganic flame retardant according to claim 1, characterized in that: The industrial silicon slag mentioned in step S1 is a by-product solid waste of the silicon-based industry, which is obtained by purchasing at a low price or transporting it free of charge. The finished silicon-based inorganic flame retardant has a core-shell structure, with a core layer of silicon-based whiskers and a shell layer of phosphorus-silicon composite. The finished product is halogen-free and has a heavy metal content of ≤10ppm. It can be directly added to water-based interior wall coatings, oil-based exterior wall coatings, or low-VOC indoor special coatings at a dosage of 8-15%. It has good compatibility with the coating system and can achieve uniform dispersion without the need for additional dispersants. The flame retardant performance of the finished coating meets the B1 level requirements specified in GB8624.

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