A method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride

Abstract

This invention relates to the field of fluorochemicals, specifically to a method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride. The method includes: gas molecular sieve adsorption purification, SiF4 water absorption to produce primary fluorosilicic acid, and crystallization purification to obtain high-purity fluorosilicic acid. Fresh water is recycled to the absorption system, and the concentrated liquid is recycled back to the absorption system, achieving closed-loop recycling of waste liquid and significantly improving resource utilization and product purity.

Description

Technical Field

[0001] This invention relates to the field of fluorochemicals, specifically to a method for preparing high-purity fluorosilicic acid from silicon tetrafluoride, an industrial byproduct. Background Technology

[0002] With the rapid development of the semiconductor, photovoltaic, and fluorochemical industries, the efficient resource utilization of silicon tetrafluoride, an industrial byproduct, has become a core challenge in the global environmental protection and circular economy fields. Meanwhile, high-purity fluorosilicic acid, as a core raw material for electronic-grade silicon, high-end fluorides, and catalysts, also requires green preparation technologies that are crucial for the upgrading of the chemical industry. Existing processes have several shortcomings: incomplete purification of silicon tetrafluoride, insufficient purity of high-purity fluorosilicic acid, and high wastewater treatment costs. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride. This method utilizes electrodialysis, anion exchange membranes, low-temperature membrane concentration, and dynamic cooling crystallization to achieve efficient conversion of silicon tetrafluoride into high-purity fluorosilicic acid.

[0004] The specific technical solution of the present invention is as follows: A method for preparing high-purity fluorosilicic acid from silicon tetrafluoride, an industrial byproduct, includes the following steps: Step S1: Gas molecular sieve adsorption purification: The SiF4 gas overflowing from industrial hydrogen fluoride production is filtered and dried with concentrated acid before being passed into a molecular sieve adsorption tower. Hydrogen fluoride is adsorbed at low temperature to obtain high-purity SiF4 gas. The packing material of the molecular sieve adsorption tower is activated carbon and cerium-based MOF composite agent. Step S2: SiF4 water absorption to produce fluorosilicic acid primary product: The SiF4 gas purified in step S1 is passed into a single-stage packed absorption tower for countercurrent absorption. After in-situ acidic complexation conversion, a fluorosilicic acid primary product solution is generated. Step S3: Crystallization and purification of high-purity fluorosilicic acid: The solution obtained in step S2 is filtered, and the filtrate is concentrated under normal pressure and low temperature membrane to obtain a concentrated solution; then cooled and crystallized, and filtered to obtain high-purity fluorosilicic acid crystals.

[0005] As a preferred embodiment, step S4 is also included: Fluorine-silicon recovery: The filter residue obtained from step S3 is reacted with the hydrogen fluoride desorbed from the molecular sieve adsorption tower after being heated to ≥65℃ in a reaction vessel, and then pumped into the electrodialysis tank.

[0006] Preferably, in step S1, the HF content in the SiF4 gas is 0.25-3%, and the packing density in the molecular sieve adsorption tower is 10-15 kg / m³. 3 The temperature inside the molecular sieve adsorption tower is -15 to 5°C.

[0007] Preferably, in step S1, the packing material of the molecular sieve adsorption tower is activated carbon and cerium-based MOF composite agent, with a mass percentage of 80%~100%: 20%~0%.

[0008] Preferably, in step S2, the absorbent for countercurrent absorption is deionized water at a temperature of 20-30°C, and the concentration of the generated fluorosilicic acid is 5-10%.

[0009] Preferably, in step S2, the in-situ acidic complexation conversion is carried out in an acidic environment by adding a complexing agent to carry out the reaction. The complexing agent is sodium citrate, sodium gluconate, or sodium perfluorobutyl sulfonate with a total acid content of 0.15~0.5%.

[0010] Preferably, in step S3, the crystallization is gradient crystallization, the gradient crystallization temperature is -5~0℃, the graded gradient cooling rate is 0.1~0.3℃ / min, and the concentration of the obtained fluorosilicic acid crystals is ≥79%; in the atmospheric pressure low temperature membrane concentration, the distillation concentration temperature is ≤60℃, the micro negative pressure is -1000Pa ~ -300Pa, and the pore size of the PVDF ultrafiltration membrane is 1~1.5nm.

[0011] Preferably, in step S3, a ceramic membrane cross-flow filter and a nanofiltration membrane are used in series for filtration, with the cross-flow velocity controlled at 3~5 m / s and the membrane pore size ≤0.05 μm; the filtration pressure is 0.15~0.3 MPa, and the total membrane area is 1.9~2.1 m². 2 .

[0012] Preferably, in step S4, both the mother liquor from the crystal filtration in step S3 and the solution after the reaction in the reactor in step S4 are pumped into the electrodialysis tank, and the F of the solution... - The concentration is increased from 300~500mg / L to 900~1500mg / L through an electrodialysis device for in-situ acidification and complexation conversion; fresh water with conductivity <20 μS / cm is separated by electrodialysis and then reacted with silicon tetrafluoride gas from a single-stage packed absorption tower. Attached Figure Description

[0013] Figure 1 This is a process flow diagram of one specific embodiment of the present invention. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased.

[0015] Unless otherwise stated, all percentages in this invention represent mass fractions. Ratios are mass percentages, and concentrations are mass concentrations.

[0016] Unless otherwise specified, all materials, instruments, and equipment used below are conventional materials, instruments, and equipment or obtained through commercial channels; all testing methods used are existing methods unless otherwise specified.

[0017] In existing technologies, the purification effect of HF impurities in silicon tetrafluoride gas is not good. Traditional molecular sieves have poor selectivity and low efficiency in adsorbing HF. The novel nanopore modified molecular sieve technology has not formed a closed-loop system and still poses a risk of waste liquid discharge.

[0018] In traditional fluorosilicic acid preparation processes, packed tower methods result in products with high impurity content and fluorine residue in the mother liquor, posing compliance risks. Gradient cooling crystallization is energy-intensive and has a heavy burden of waste liquid treatment. Although recent research has achieved breakthroughs in purity through multi-stage concentration and dynamic cooling technologies, the problem of waste liquid recycling has not been completely solved.

[0019] To address the aforementioned pain points of incomplete purification of silicon tetrafluoride, insufficient purity of high-purity fluorosilicic acid, and high wastewater treatment costs, this invention provides a method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride, comprising the following steps: Step S1: Gas molecular sieve adsorption purification: The SiF4 gas overflowing from industrial hydrogen fluoride production is filtered and dried with concentrated acid before being passed into a molecular sieve adsorption tower. Hydrogen fluoride is adsorbed at low temperature to obtain high-purity SiF4 gas. The packing material of the molecular sieve adsorption tower is activated carbon and cerium-based MOF composite agent. Step S2: SiF4 water absorption to produce fluorosilicic acid primary product: The SiF4 gas purified in step S1 is passed into a single-stage packed absorption tower for countercurrent absorption. After in-situ acidic complexation conversion, a fluorosilicic acid primary product solution is generated. Step S3: Crystallization and purification of high-purity fluorosilicic acid: The solution obtained in step S2 is filtered, and the filtrate is concentrated under normal pressure and low temperature membrane to obtain a concentrated solution; then cooled and crystallized, and filtered to obtain high-purity fluorosilicic acid crystals.

[0020] This invention employs a composite of activated carbon and cerium-based MOF as a molecular sieve, integrating low-temperature adsorption and purification of SiF4, single-stage water absorption, gradient crystallization, and electrodialysis mother liquor reuse. This process yields high-purity fluorosilicic acid crystals with a purity ≥79%, achieving a zero-waste system with a fluorine recovery rate >99.0%. The high-purity fluorosilicic acid crystals contain approximately 18% water of crystallization, meeting the requirements for electronic-grade hydrofluoric acid precursors. This method eliminates the risk of membrane fouling, features a high degree of equipment automation, adapts to gas concentration fluctuations, significantly improves resource efficiency and process stability, and optimizes and improves the high-value utilization process of industrial hydrogen fluoride gas.

[0021] Industrial hydrogen fluoride production mainly involves the pyrolysis of fluorosilicic acid and concentrated sulfuric acid. Atmospheric pressure, low-temperature membrane concentration can be carried out using existing methods, such as the method described in CN121342036A.

[0022] One implementation method also includes step S4: Fluorosilicone recovery: The filter residue obtained from step S3 is reacted with the hydrogen fluoride desorbed from the molecular sieve adsorption tower by heating to ≥65℃ in a reactor, and then pumped into an electrodialysis tank. The low temperature in step S3 is conducive to the condensation and adsorption of hydrogen fluoride gas, and the heating in step S4 releases it from the packing material and returns it to the reactor to participate in the reaction.

[0023] In one embodiment, in step S1, the HF content in the SiF4 gas is 0.25-3%, and the packing density in the molecular sieve adsorption tower is 10-15 kg / m³. 3 The temperature inside the molecular sieve adsorption tower is maintained between -15°C and 5°C. This packing density achieves a balance between adsorption capacity and gas pressure drop, ensuring a high removal rate while reducing system energy consumption by approximately 25%. Within this temperature range, the physical adsorption competition of SiF4 on the molecular sieve surface can be effectively suppressed, reducing the SiF4 loss rate to below 0.5%.

[0024] In one embodiment, in step S1, the packing material of the molecular sieve adsorption tower is a composite of activated carbon and cerium-based MOF, with a mass ratio of 80%~100%:20%~0%. This ratio of composite packing material significantly improves the adsorption effect.

[0025] In one implementation method, in step S2, the absorbent for countercurrent absorption is deionized water at a temperature of 20~30°C, and the concentration of fluorosilicic acid generated is 5~10%, which fully absorbs silicon tetrafluoride gas while reducing the thermal decomposition of fluorosilicic acid.

[0026] In one implementation method, in step S2, the in-situ acidic complexation conversion involves adding a complexing agent, such as sodium citrate, sodium gluconate, or sodium perfluorobutyl sulfonate, at a ratio of 0.15-0.5% under an acidic environment. The addition of the complexing agent effectively converts the silica gel produced in the single-stage absorption tower into a soluble silicate ion solution, extending the clogging cycle of the next process membrane.

[0027] In one implementation method, in step S3, the crystallization is gradient crystallization, with a gradient crystallization temperature of -5 to 0°C and a graded gradient cooling rate of 0.1 to 0.3°C / min. The resulting fluorosilicic acid crystals (pure H2SiF6·2H2O) have a concentration ≥79%. In the atmospheric pressure low-temperature membrane concentration, the distillation concentration temperature is ≤60°C, the slight negative pressure is -1000Pa to -300Pa, and the PVDF ultrafiltration membrane has a pore size of 1 to 1.5 nm.

[0028] By employing atmospheric pressure and low temperature concentration and gradient cooling crystallization, the problem of high impurity content in products from traditional packed tower processes is solved, effectively reducing fluoride ions and impurities in the mother liquor.

[0029] In one implementation method, in step S3, a ceramic membrane cross-flow filter and a nanofiltration membrane are connected in series for filtration. The cross-flow velocity is controlled at 3~5 m / s, and the membrane pore size is ≤0.05 μm. The filtration pressure used in the filtration equipment is 0.15~0.3 MPa, and the total area of ​​the filter membrane is 1.9~2.1 m². 2 The above filtration equipment, when used in combination, significantly extends the clogging cycle compared to traditional single-unit filtration equipment. It employs a two-stage series connection of cross-flow ceramic membrane and nanofiltration membrane to effectively trap large molecular impurities in the fluorosilicic acid solution.

[0030] In one embodiment, in step S4, both the mother liquor from the crystal filtration in step S3 and the solution after the reaction in the reactor in step S4 are pumped into the electrodialysis tank, and the F of the solution... - The concentration, initially 300-500 mg / L, is increased to 900-1500 mg / L via electrodialysis and used in the in-situ acidification and complexation conversion stage. Fresh water with a conductivity <20 μS / cm is separated by electrodialysis and reacted with silicon tetrafluoride gas from a single-stage packed absorption tower. Electrodialysis further increases the purity of fluorosilicic acid crystals to over 79%, while simultaneously establishing a closed-loop wastewater resource recovery system, achieving a freshwater reuse rate of over 90% and a concentrate recovery rate of over 95%, thus eliminating the risk of wastewater discharge.

[0031] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a method for preparing high-purity fluorosilicic acid from industrial by-product silicon tetrafluoride.

[0032] The devices used in these examples are all commercially available products: Ceramic membrane cross-flow filter (HK-CLFL) Nanofiltration membrane (HJC ARNF) Electrodialysis unit (LANRAN® series, non-standard equipment from Hangzhou Lanran) Reactor (conventional PTFE reactor) Example 1

[0033] like Figure 1 As shown, the method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride in this embodiment includes the following steps: Step S1: SiF4 gas containing 1.5% HF, after being dried and dust-removed by 98% sulfuric acid, is introduced into the atmosphere at a filling rate of 12.5 kg / m³. 3The molecular sieve adsorption tower uses activated carbon and cerium-based MOF (JK-15-001) composite agent as packing material in a ratio of 80%:20% to adsorb HF at a low temperature of -5℃, and obtains high-purity SiF4 gas with HF residue of 0.03%.

[0034] Step S2: The high-purity SiF4 gas obtained in step S1 is passed into a single-stage packed absorption tower with PP material and Heil rings. It is absorbed countercurrently with deionized water at 30°C. Sodium perfluorobutyl sulfonate with a total acid content of 0.5% (total acid content converted to pure fluorosilicic acid mass) is added to the buffer tank at the bottom of the tower. After warm acidification and complexation conversion, a fluorosilicic acid solution with a concentration of 8.3% is generated.

[0035] Step S3: The solution obtained in step S2 is flowed through a ceramic membrane cross-flow filter at a pipeline speed of 3 m / s to further convert the SiO2 colloid into soluble SiF6. 2- The complex ions are then filtered through a nanofiltration membrane with a pore size of 0.05 μm, at a filtration pressure of 0.12 MPa, and with a total membrane area of ​​2 m². 2 .

[0036] The filtrate obtained after filtration was concentrated using a low-temperature membrane at atmospheric pressure to obtain a primary product of concentrated fluorosilicic acid with a concentration of 45%. The low-temperature membrane concentration at atmospheric pressure was carried out according to CN121342036A, with a distillation concentration temperature of 58℃, a slight negative pressure of -500Pa, and a PVDF ultrafiltration membrane with a pore size of 1nm.

[0037] The concentrated solution was then subjected to gradient cooling and crystallization in a crystallizer at a cooling rate of 0.20℃ / min at -2.5℃. After filtration, high-purity fluorosilicic acid crystals with a purity of 79.1% were obtained.

[0038] Step S4: The silica gel filter residue obtained from step S3 is reacted with the hydrogen fluoride desorbed from the molecular sieve adsorption tower after being heated to 65°C in a reaction vessel.

[0039] Step S5: The mother liquor from the crystallization filtration in Step S3 and the solution from the reaction in the reactor in Step S4 are both pumped into an electrodialysis tank. After electrodialysis treatment, fresh water with a conductivity of 15 μS / cm is obtained and reused in the countercurrent absorption in Step S2; simultaneously, F... - The concentrate, whose concentration was increased from 350 mg / L to 1200 mg / L, was refluxed to the in-situ acidic complexation conversion section in step S2. The concentrate was hydrofluoric acid.

[0040] Example 2

[0041] like Figure 1 As shown, the method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride in this embodiment includes the following steps: Step S1: SiF4 gas containing 0.35% HF, after being dried and dust-removed by 98% sulfuric acid, is introduced into the gas at a packing density of 10.2 kg / m³.3 The molecular sieve adsorption tower, with activated carbon and cerium-based MOF composite filler in a ratio of 85%:15%, adsorbs HF at a low temperature of -12℃ to obtain high-purity SiF4 gas with HF residue of 0.02%.

[0042] Step S2: The high-purity SiF4 gas obtained in step S1 is passed into a single-stage packed absorption tower with PP material and Haier rings. It is absorbed countercurrently with deionized water at 22°C. Sodium perfluorobutyl sulfonate with a total acid content of 0.2% (total acid content converted to pure fluorosilicic acid mass) is added to the buffer tank at the bottom of the tower. After a warm acidification-complexation conversion process, a fluorosilicic acid solution with a concentration of 5.8% is generated.

[0043] Step S3: The solution obtained in step S2 is passed through a ceramic membrane cross-flow filter at a pipeline speed of 3.2 m / s to further convert the SiO2 colloid into soluble SiF6. 2- The complex ions are then filtered through a nanofiltration membrane with a pore size of 0.03 μm, at a filtration pressure of 0.28 MPa, and with a total membrane area of ​​2 m². 2 .

[0044] The filtrate obtained by filtration was concentrated at low temperature to obtain a primary product of concentrated fluorosilicic acid with a concentration of 45%. The low-temperature membrane concentration under normal pressure was carried out according to CN121342036A, with a distillation concentration temperature of 59℃, a slight negative pressure of -480Pa, and a PVDF ultrafiltration membrane with a pore size of 1nm.

[0045] The concentrated solution was then subjected to gradient cooling and crystallization in a crystallizer at a cooling rate of 0.12℃ / min at -4.5℃. After filtration, high-purity fluorosilicic acid crystals with a purity of 79.05% were obtained.

[0046] Step S4: The silica gel filter residue obtained from step S3 reacts with the hydrogen fluoride released from the molecular sieve adsorption tower after being heated to 65°C in the reaction vessel.

[0047] Step S5: The mother liquor from the crystallization filtration in Step S3 and the solution from the reaction in the reactor in Step S4 are both pumped into an electrodialysis tank. After electrodialysis treatment, fresh water with a conductivity of 18 μS / cm is obtained and reused in the countercurrent absorption in Step S2; simultaneously, F... - The concentrate, whose concentration was increased from 320 mg / L to 1050 mg / L, was refluxed to the in-situ acidic complexation conversion section of step S2. The concentrate was hydrofluoric acid.

[0048] Example 3

[0049] like Figure 1 As shown, the method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride in this embodiment includes the following steps: Step S1: SiF4 gas containing 2.8% HF, after being dried and dust-removed by 98% sulfuric acid, is introduced into the system at a filling rate of 14.8 kg / m³.3 A molecular sieve adsorption tower, with activated carbon and cerium-based MOF composite filler in a ratio of 92%:8%, adsorbs HF at a low temperature of 3℃ to obtain high-purity SiF4 gas with 0.05% HF residue.

[0050] Step S2: The high-purity SiF4 gas obtained in step S1 is passed into a single-stage packed absorption tower with PP material and Heil rings. It is absorbed countercurrently with deionized water at 29°C. Sodium perfluorobutyl sulfonate with a total acid content of 0.48% (total acid content converted to pure fluorosilicic acid mass) is added to the buffer tank at the bottom of the tower. After a warm acidification-complexation conversion process, a fluorosilicic acid solution with a concentration of 9.7% is generated.

[0051] Step S3: The solution obtained in step S2 is passed through a ceramic membrane cross-flow filter at a pipeline speed of 4.8 m / s to further convert the SiO2 colloid into soluble SiF6. 2- The complex ions are then filtered through a nanofiltration membrane with a pore size of 0.05 μm. The filtration pressure is 0.15 MPa, and the total area of ​​the filtration membrane is 2 m². 2 .

[0052] The filtrate obtained by filtration was concentrated at room temperature and low temperature to obtain a primary product of concentrated fluorosilicic acid with a concentration of 45%. The low-temperature membrane concentration under normal pressure was carried out according to CN121342036A, with a distillation concentration temperature of 58℃, a slight negative pressure of -600Pa, and a PVDF ultrafiltration membrane with a pore size of 1nm.

[0053] The concentrated solution was then subjected to gradient cooling and crystallization in a crystallizer at a cooling rate of 0.28℃ / min at -0.8℃. After filtration, high-purity fluorosilicic acid crystals with a purity of 79% were obtained.

[0054] Step S4: The silica gel filter residue obtained from step S3 reacts with the hydrogen fluoride released from the molecular sieve adsorption tower after being heated to 65°C in the reaction vessel.

[0055] Step S5: The mother liquor from the crystallization filtration in Step S3 and the solution from the reaction in the reactor in Step S4 are both pumped into an electrodialysis tank. After electrodialysis treatment, fresh water with a conductivity of 12 μS / cm is obtained and reused in the countercurrent absorption in Step S2; simultaneously, F... - The concentrate, whose concentration was increased from 480 mg / L to 1450 mg / L, was refluxed to the in-situ acidic complexation conversion section of step S2. The concentrate was hydrofluoric acid.

[0056] Example 4

[0057] like Figure 1 As shown, the method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride in this embodiment includes the following steps: Step S1: SiF4 gas containing 1.2% HF, after being dried and dust-removed by 98% sulfuric acid, is introduced into the atmosphere at a filling rate of 12.0 kg / m³.3 The molecular sieve adsorption tower, with activated carbon and cerium-based MOF composite filler in a ratio of 88%:12%, adsorbs HF at a low temperature of -8℃ to obtain high-purity SiF4 gas with HF residue of 0.03%.

[0058] Step S2: The high-purity SiF4 gas obtained in step S1 is passed into a single-stage packed absorption tower with PP material and Haier rings. It is absorbed countercurrently with deionized water at 26°C. Sodium perfluorobutyl sulfonate with a total acid content of 0.35% (total acid content converted to pure fluorosilicic acid mass) is added to the buffer tank at the bottom of the tower. After a warm acidification-complexation conversion process, a fluorosilicic acid solution with a concentration of 7.5% is generated.

[0059] Step S3: The solution obtained in step S2 is passed through a ceramic membrane cross-flow filter at a pipeline speed of 4.0 m / s to further convert the SiO2 colloid into soluble SiF6. 2- The complex ions are then filtered through a nanofiltration membrane with a pore size of 0.04 μm, at a filtration pressure of 0.19 MPa, and with a total membrane area of ​​2 m². 2 .

[0060] The filtrate obtained by filtration was concentrated at low temperature to obtain a primary product of concentrated fluorosilicic acid with a concentration of 45%. The low-temperature membrane concentration under normal pressure was carried out according to CN121342036A, with a distillation concentration temperature of 60℃, a slight negative pressure of -450Pa, and a PVDF ultrafiltration membrane with a pore size of 1nm.

[0061] The concentrated solution was then subjected to gradient cooling and crystallization in a crystallizer at a cooling rate of 0.11℃ / min at -4.8℃. After filtration, high-purity fluorosilicic acid crystals with a purity of 79.12% were obtained.

[0062] Step S4: The silica gel filter residue obtained from step S3 reacts with the hydrogen fluoride released from the molecular sieve adsorption tower after being heated to 65°C in the reaction vessel.

[0063] Step S5: The mother liquor from the crystallization filtration in Step S3 and the solution from the reaction in the reactor in Step S4 are both pumped into an electrodialysis tank. After electrodialysis treatment, fresh water with a conductivity of 14 μS / cm is obtained and reused in the countercurrent absorption in Step S2; simultaneously, F... - The concentrate, whose concentration was increased from 380 mg / L to 1180 mg / L, was refluxed to the in-situ acidic complexation conversion section of step S2. The concentrate was hydrofluoric acid.

[0064] Example 5

[0065] like Figure 1 As shown, the method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride in this embodiment includes the following steps: Step S1: SiF4 gas containing 1.8% HF is dried and dust-removed with 98% sulfuric acid, and then introduced into the atmosphere at a filling rate of 13.2 kg / m³.3 The molecular sieve adsorption tower, with activated carbon and cerium-based MOF composite filler in a ratio of 90%:10%, adsorbs HF at a low temperature of -2℃ to obtain high-purity SiF4 gas with HF residue of 0.04%.

[0066] Step S2: The high-purity SiF4 gas obtained in step S1 is passed into a single-stage packed absorption tower with PP material and Heil rings. It is absorbed countercurrently with deionized water at 28°C. Sodium citrate of 0.42% of the total acid (total acid based on the mass of pure fluorosilicic acid) is added to the buffer tank at the bottom of the tower. After a warm acidification-complexation conversion process, a fluorosilicic acid solution with a concentration of 8.9% is generated.

[0067] Step S3: The solution obtained in step S2 is passed through a ceramic membrane cross-flow filter at a pipeline speed of 4.5 m / s to further convert the SiO2 colloid into soluble SiF6. 2- The complex ions are then filtered through a nanofiltration membrane with a pore size of 0.05 μm, at a filtration pressure of 0.12 MPa, and with a total membrane area of ​​2 m². 2 .

[0068] The filtrate obtained by filtration was concentrated at low temperature to obtain a primary product of concentrated fluorosilicic acid with a concentration of 45%. The low-temperature membrane concentration under normal pressure was carried out according to CN121342036A, with a distillation concentration temperature of 60℃, a slight negative pressure of -700Pa, and a PVDF ultrafiltration membrane with a pore size of 1nm.

[0069] The concentrated solution was then subjected to gradient cooling and crystallization in a crystallizer at a cooling rate of 0.29℃ / min at -0.3℃. After filtration, high-purity fluorosilicic acid crystals with a purity of 79.08% were obtained.

[0070] Step S4: The silica gel filter residue obtained from step S3 reacts with the hydrogen fluoride released from the molecular sieve adsorption tower after being heated to 65°C in the reaction vessel.

[0071] Step S5: The mother liquor from the crystallization filtration in Step S3 and the solution from the reaction in the reactor in Step S4 are both pumped into an electrodialysis tank. After electrodialysis treatment, fresh water with a conductivity of 13 μS / cm is obtained and reused in the countercurrent absorption in Step S2; simultaneously, F... - The concentrate, whose concentration was increased from 420 mg / L to 1320 mg / L, was refluxed to the in-situ acidic complexation conversion section of step S2. The concentrate was hydrofluoric acid.

[0072] Comparative Example 1 The difference from Example 3 is that no complexing agent is added in step S2.

[0073] Comparative Example 2 The difference from Example 3 is that in step S1, the adsorption tower packing material is replaced with a single activated carbon packing material.

[0074] Comparative Example 3 The difference from Example 3 is that the nanofiltration membrane pore size in step S3 is replaced with a nanofiltration membrane with a pore size of 0.06 μm.

[0075] Comparative Example 4 The difference from Example 3 is that the cooling rate is increased in step S3, and gradient cooling crystallization is carried out in the crystallizer at a cooling rate of 0.7℃ / min.

[0076] The comparative products of the example were analyzed and tested according to the standard "HGT2832-2020 Industrial Fluorosilicic Acid" after low-temperature dilution. The results are shown in Table 1 below. Table 1. Experimental results of fluorosilicic acid crystal purity and nanofiltration membrane equipment pressure under different operating conditions. As shown in the table, in Examples 1-5, the product indicators under different working conditions all met the qualified standards (HF residue <0.05%, crystal purity ≥79%). Comparative Examples 1-3 show that the purity of fluorosilicic acid crystals decreased after the absence of complexing agents, the use of a single filler, and the replacement of the large-pore nanofiltration membrane. Abnormal working conditions affected normal continuous operation, resulting in large fluctuations in the purity of fluorosilicic acid crystals. Therefore, the beneficial effects of the present invention are significantly improved.

[0077] Comparative Example 4 shows that the purity of the obtained fluorosilicic acid crystals decreased significantly due to the rapid local cooling, which led to an increase in the formation of metastable polymers.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing high-purity fluorosilicic acid from industrial byproduct silicon tetrafluoride, characterized in that: Includes the following steps: Step S1: Gas molecular sieve adsorption purification: The SiF4 gas overflowing from industrial hydrogen fluoride production is filtered and dried with concentrated acid before being passed into a molecular sieve adsorption tower. Hydrogen fluoride is adsorbed at low temperature to obtain high-purity SiF4 gas. The packing material of the molecular sieve adsorption tower is activated carbon and cerium-based MOF composite agent. Step S2: SiF4 water absorption to produce fluorosilicic acid primary product: The SiF4 gas purified in step S1 is passed into a single-stage packed absorption tower for countercurrent absorption. After in-situ acidic complexation conversion, a fluorosilicic acid primary product solution is generated. Step S3: Crystallization and purification of high-purity fluorosilicic acid: The solution obtained in step S2 is filtered, and the filtrate is concentrated under normal pressure and low temperature membrane to obtain a concentrated solution; then cooled and crystallized, and filtered to obtain high-purity fluorosilicic acid crystals.

2. The method according to claim 1, characterized in that: It also includes step S4: Fluorosilicone recovery: The filter residue obtained from step S3 is reacted with the hydrogen fluoride desorbed from the molecular sieve adsorption tower after being heated to ≥65℃ in the reaction vessel and then pumped into the electrodialysis tank.

3. The method according to claim 1, characterized in that: In step S1, the HF content in the SiF4 gas is 0.25-3%, and the packing density in the molecular sieve adsorption tower is 10-15 kg / m³. 3 The temperature inside the molecular sieve adsorption tower is -15 to 5°C.

4. The method according to claim 1, characterized in that: In step S1, the packing material of the molecular sieve adsorption tower is activated carbon and cerium-based MOF composite agent, with a mass percentage of 80%~100%: 20%~0%.

5. The method according to claim 1, characterized in that: In step S2, the absorbent for countercurrent absorption is deionized water at a temperature of 20-30°C, and the concentration of the generated fluorosilicic acid is 5-10%.

6. The method according to claim 1, characterized in that: In step S2, the in-situ acidic complexation conversion is carried out in an acidic environment by adding a complexing agent to carry out the reaction. The complexing agent is sodium citrate, sodium gluconate or sodium perfluorobutyl sulfonate with a total acid content of 0.15~0.5%.

7. The method according to claim 1, characterized in that: In step S3, the crystallization is gradient crystallization, with a gradient crystallization temperature of -5~0℃ and a graded gradient cooling rate of 0.1~0.3℃ / min, resulting in a fluorosilicic acid crystal concentration ≥79%; in the atmospheric pressure low temperature membrane concentration, the distillation concentration temperature is ≤60℃, the micro negative pressure is -1000Pa ~ -300Pa, and the PVDF ultrafiltration membrane pore size is 1~1.5nm.

8. The method according to claim 1, characterized in that: In step S3, a ceramic membrane cross-flow filter and a nanofiltration membrane are used in series for filtration, with the cross-flow velocity controlled at 3~5m / s and the membrane pore size ≤0.05μm; The filtration pressure is 0.15~0.3 MPa, and the total area of ​​the filter membrane is 1.9~2.1 m². 2 .

9. The method according to claim 2, characterized in that: In step S4, both the mother liquor from the crystal filtration in step S3 and the solution from the reaction in the reactor in step S4 are pumped into the electrodialysis tank. The F of the solution... - The concentration is increased from 300~500mg / L to 900~1500mg / L through an electrodialysis device for in-situ acidification and complexation conversion; fresh water with conductivity <20 μS / cm is separated by electrodialysis and then reacted with silicon tetrafluoride gas from a single-stage packed absorption tower.

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