A process for preparing UP-SSS grade electronic grade hydrofluoric acid by a sulfur-free method using phosphorus fertilizer by-product fluosilicic acid as raw material

Abstract

This invention discloses a sulfuric acid-free process for preparing UP-SSS grade electronic-grade hydrofluoric acid using fluorosilicic acid, a byproduct of phosphate fertilizer production, as raw material, without the addition of sulfuric acid throughout the entire process. The process involves pretreating the fluorosilicic acid from phosphate fertilizer production via nanofiltration, followed by ammonia hydrolysis with gaseous ammonia to generate ammonium fluoride solution and silica precipitate. The ammonium fluoride solution is then concentrated under negative pressure and fed into a two-stage vacuum pyrolysis system. In the first stage, ammonia is removed to generate ammonium hydrogen fluoride. In the second stage, under high temperature and high vacuum conditions, the ammonium hydrogen fluoride is directly decomposed into a mixture of anhydrous hydrogen fluoride and ammonia. The ammonia gas is then condensed, separated, and recycled in a closed loop. The crude hydrogen fluoride gas is then passed through nitrogen trifluoride to oxidize impurities, converting arsenic, phosphorus, and boron into high-boiling-point fluorides. After dust removal, heavy metals and boron-phosphorus complexes are separated by molecular distillation. The boron is then deeply removed to below 0.02 ppb using a sintered aluminum fluoride-magnesium fluoride inorganic adsorbent. The particles are then filtered through multi-stage ultrafiltration, and finally, ultrapure water is used to prepare the acid, yielding ultra-high purity hydrofluoric acid suitable for 7-3nm chip manufacturing processes. This process involves no sulfuric acid or phosphogypsum waste residue, recycles ammonia, and produces high-purity silica as a byproduct, resulting in significant economic benefits for industrialization.

Description

Technical Field

[0001] This invention belongs to the field of electronic-grade hydrofluoric acid preparation technology, specifically involving a sulfuric acid-free process for preparing UP-SSS grade electronic-grade hydrofluoric acid using fluorosilicic acid, a byproduct of phosphate fertilizer, as raw material, without the use of sulfuric acid throughout the entire process. Background Technology

[0002] Electronic-grade hydrofluoric acid is a key chemical in semiconductor chip manufacturing, used for wafer cleaning and etching in advanced 7-3nm processes. Almost all current global production processes rely on the reaction of fluorite (CaF2) with concentrated sulfuric acid at high temperatures to produce anhydrous hydrogen fluoride, which is then purified to UP-SSS level through multi-stage distillation and ion exchange resins. This route suffers from three structural bottlenecks: First, there is the constraint of fluorite resources. Fluorite is a strategic mineral in China, with global proven reserves of approximately 310 million tons, of which China accounts for about 13%. At the current rate of mining, high-grade acid-grade fluorite will face depletion within 20-30 years. Fluorite prices are expected to rise from approximately 1,500 yuan / ton in 2016 to over 4,000 yuan / ton in 2026, directly driving up the raw material costs of traditional mining routes.

[0003] Second, there is the issue of waste accumulation and environmental pollution. For every ton of anhydrous hydrogen fluoride produced using the traditional fluorite-sulfuric acid process, approximately 3.6 tons of calcium sulfate waste are generated as a byproduct. The phosphate fertilizer industry generates massive amounts of phosphogypsum annually, with a cumulative stockpile exceeding 800 million tons in China and an annual increase of approximately 80 million tons. This process is costly and poses risks of heavy metal and radioactive contamination.

[0004] Third, the risk of organic resin precipitates. Current electronic-grade hydrofluoric acid purification processes commonly use imported organic fluorine-based ion exchange resins. These resins can slowly precipitate trace amounts of total organic matter (TOC) during long-term operation, becoming a potential source of contamination during wafer cleaning in advanced processes below 7nm.

[0005] China's phosphate fertilizer industry produces approximately 3 million tons of fluorosilicic acid (calculated as H2SiF6) annually as a byproduct, with a fluorine content equivalent to about 1.5 million tons of fluorite. Currently, the vast majority of this is recovered or directly neutralized and discharged as low-value-added sodium fluorosilicate, resulting in a significant waste of fluorine resources.

[0006] Existing commercially available routes for producing hydrogen fluoride from fluorosilicic acid still rely on concentrated sulfuric acid to acidify ammonium hydrogen fluoride in the final step, failing to break free from sulfuric acid dependence. The finished product contains trace amounts of sulfate, making it difficult to consistently achieve the UP-SSS grade purity standard.

[0007] Therefore, there is an urgent need to develop a new process for preparing electronic-grade hydrofluoric acid that uses fluorosilicic acid, a byproduct of phosphate fertilizer production, as raw material, does not require sulfuric acid throughout the process, and produces a product with purity that meets the requirements of 7-3nm process technology. Summary of the Invention

[0008] The purpose of this invention is to provide a process for preparing UP-SSS grade electronic-grade hydrofluoric acid from fluorosilicic acid, a byproduct of phosphate fertilizer, without sulfuric acid. This process avoids the existing patent system of fluorite-sulfuric acid process and sulfuric acid acid hydrolysis fluorosilicic acid process from both the reaction path and purification process aspects.

[0009] It should be noted that the basic principles of "using fluorosilicic acid, a byproduct of phosphate fertilizer, as raw material, and conducting ammonolysis with ammonia gas to generate ammonium fluoride and silicon dioxide" and "heating and decomposing ammonium fluoride to generate hydrogen fluoride and ammonia gas" adopted in this invention are well-known technologies in the field. The core innovation of this invention lies in combining, for the first time, a series of specific steps—nanofiltration membrane pretreatment-nitrogen trifluoride pre-oxidation, two-stage vacuum pyrolysis-closed-loop ammonia gas circulation, nitrogen trifluoride gas-phase oxidation, molecular distillation-sintered AlF3-MgF2 inorganic adsorption—with the aforementioned well-known technologies into a continuous, synergistic, and complete process scheme, thereby achieving the technical effect of high product purity (boron ≤ 0.02 ppb). The combination sequence and specific process parameters of this complete process scheme are not disclosed in existing technologies.

[0010] To achieve the above objectives, the present invention provides the following technical solution: A process for preparing UP-SSS grade electronic-grade hydrofluoric acid from fluorosilicic acid, a byproduct of phosphate fertilizer production, using a sulfuric acid-free method includes the following steps: (1) Raw material pretreatment: Phosphate fertilizer by-product fluorosilicic acid is pre-filtered through nanofiltration membrane to remove phosphate and metal ions (phosphate removal rate ≥85%, metal ion removal rate ≥90%), and oxidizing gas is introduced to pre-oxidize arsenic and heavy metals into solid precipitates. After precision filtration, it is concentrated at low temperature and reduced pressure to high-purity fluorosilicic acid with a concentration of 22-26%. (2) Ammonolysis: In a closed reaction vessel, gaseous ammonia is introduced and stirred to generate ammonium fluoride solution and silica precipitate. Silica is separated by pressure filtration, and colloidal silica is removed by membrane filtration of the ammonium fluoride solution. (3) Two-stage vacuum pyrolysis: The ammonium fluoride solution is evaporated and concentrated under negative pressure to the molten state, and then enters the first stage of pyrolysis to deammonize and generate ammonium hydrogen fluoride. The ammonia gas is condensed and recovered. The ammonium hydrogen fluoride enters the second stage of pyrolysis and is directly decomposed into a mixture of anhydrous hydrogen fluoride and ammonia under high temperature and high vacuum conditions. The ammonia gas is condensed and separated and then recycled in a closed loop. (4) Gas phase oxidation: The crude hydrogen fluoride gas obtained in step (3) is introduced into a closed reaction chamber and an oxidizing gas is continuously introduced to carry out gas phase oxidation, oxidizing arsenic, phosphorus and boron impurities into high-boiling-point fluorides, and then removing solid oxides by dust removal. (5) Deep purification: The gas obtained in step (4) is separated from high-boiling-point heavy metals and boron-phosphorus complex by molecular distillation device, and then trace boron is removed to ≤0.02ppb by inorganic fluoride composite adsorbent. Finally, solid particles are removed by multi-stage ultrafiltration membrane filtration. (6) Acid preparation and filling: The purified anhydrous hydrogen fluoride gas obtained in step (5) is passed into ultrapure water for acid preparation and then filled in a clean environment to obtain UP-SSS grade electronic grade hydrofluoric acid.

[0011] Further, in step (1), the nanofiltration membrane has a molecular weight cutoff of 200-300 Da, and the low-temperature vacuum concentration temperature is 40-60°C; in step (2), the ammonolysis reaction temperature is controlled at 45-55°C, pH 8.1-8.4, and the stirring reaction time is 90 minutes; in step (3), the first stage pyrolysis temperature is controlled at 135-145°C, and the second stage pyrolysis temperature is controlled at 320-340°C; in step (4), the gas phase oxidation temperature is controlled at 70-80°C; and in step (5), the molecular distillation operation pressure is 120-180 Pa, and the operation temperature is 33-38°C.

[0012] Further, the oxidizing gas in step (1) is nitrogen trifluoride gas; the gaseous ammonia in step (2) is anhydrous gaseous ammonia obtained by vaporizing liquid ammonia; the membrane filtration adopts a ceramic membrane with a pore size of 0.1 μm; the silica obtained by pressure filtration in step (2) is washed and dried to obtain high-purity precipitated silica.

[0013] Furthermore, in step (3), the first and second pyrolysis stages are both carried out in a PTFE-lined closed negative pressure system, and the ammonia gas generated in both stages is condensed, recovered, and returned to the ammonia hydrolysis stage in step (2) for recycling.

[0014] Furthermore, the oxidizing gas mentioned in step (4) is nitrogen trifluoride, which can oxidize low-valent fluorides of arsenic, phosphorus, and boron, so that the boron content of the final product is controlled to below 0.02 ppb.

[0015] Further, the inorganic fluoride composite adsorbent in step (5) is a sintered AlF3-MgF2 composite inorganic adsorbent, wherein the molar ratio of aluminum to magnesium is 1.2:1 to 1.8:1, the adsorbent is sintered in an inert atmosphere at 450-600°C, and the adsorption column is lined with silicon carbide.

[0016] Furthermore, the multi-stage ultrafiltration membrane mentioned in step (5) is a polytetrafluoroethylene ultrafiltration membrane, comprising at least two stages, which sequentially filter out 0.1 μm and 0.05 μm solid particles.

[0017] Furthermore, the resistivity of the ultrapure water in step (6) is 18.25 MΩ·cm, the clean environment is ISO14644-1 Class 5 clean environment, the boron content in the obtained UP-SSS grade electronic grade hydrofluoric acid is ≤0.02ppb, the total content of metal cations is less than 10ppt, and it is suitable for cleaning and etching of 7-3nm process semiconductor chips.

[0018] This invention also protects a method for preparing a sintered AlF3-MgF2 composite inorganic adsorbent for the above-mentioned process. The specific steps are as follows: anhydrous aluminum fluoride powder with a purity ≥99.99% and anhydrous magnesium fluoride powder are mixed uniformly at an Al / Mg molar ratio of 1.2:1 to 1.8:1; 5-8% of deionized water by weight of the powder is added, and the mixture is stirred uniformly; strip-shaped particles with a diameter of 1-2 mm and a length of 3-5 mm are extruded; the mixture is naturally air-dried for 12 hours, then dried at 105℃ for 4 hours; under an inert atmosphere, the temperature is increased to 200℃ at 5℃ / min and held for 1 hour, then sintered at 450-600℃ at 3℃ / min for 3-5 hours; the mixture is sieved, and 20-40 mesh particles are selected as the finished adsorbent.

[0019] The beneficial effects of this invention are as follows: The entire process does not use sulfuric acid, eliminating sulfate pollution at the source of the chemical reaction and preventing the discharge of phosphogypsum, resulting in significant environmental benefits.

[0020] Using fluorosilicic acid, a byproduct of phosphate fertilizer production, as a substitute for strategic resource fluorite as a fluorine raw material results in extremely low raw material costs and a stable supply, expanding the avenues for high-value utilization of phosphate chemical byproducts.

[0021] The two-stage vacuum pyrolysis directly cracks ammonium hydrogen fluoride, and the reaction path is different from the traditional fluorite-sulfuric acid method and the sulfuric acid replacement type fluorosilicic acid preparation process, which can build an independent patent protection system.

[0022] Nitrogen trifluoride is used for liquid-phase pretreatment and gas-phase oxidation purification, with no water and no introduction of foreign metal impurities throughout the process; sintered inorganic fluoride adsorbents replace imported organic resins, eliminating the problem of organic matter precipitation and meeting the stringent standards of advanced 7-3nm processes.

[0023] Ammonia is recovered through a closed-loop system, and the byproduct high-purity silica can be sold externally, improving overall economic efficiency. Attached Figure Description

[0024] Figure 1 This is a flowchart of the process of the present invention. Figure 2 is a legend reference for the flowchart in Figure 1. Detailed Implementation

[0025] Preparation of sintered AlF3-MgF2 composite inorganic adsorbent (1) Raw material preparation: Anhydrous aluminum fluoride powder with a purity ≥ 99.99% and anhydrous magnesium fluoride powder are mixed evenly according to the Al / Mg molar ratio of 1.5:1; (2) Granulation and molding: Add 5-8% deionized water by weight of powder, stir evenly, and extrude strip-shaped particles with a diameter of 1-2 mm and a length of 3-5 mm; air dry for 12 hours, and dry at 105℃ for 4 hours; (3) High temperature sintering: Argon protection, oxygen content <10ppm; heat up to 200℃ at 5℃ / min and hold for 1h, then heat up to 500℃ at 3℃ / min and hold for 4h; (4) Sieving, select 20-40 mesh particles; the finished product has a specific surface area of ​​80-150 m² / g and a pore size of 8-15 nm. Example 1 (Preferred process parameters) The raw material is 10% fluorosilicic acid obtained from the phosphate fertilizer tail gas absorption system. (1) Raw material pretreatment: 1000 kg of fluorosilicic acid solution was passed through a nanofiltration membrane with a molecular weight cutoff of 250 Da to remove phosphate, aluminum and magnesium ions (phosphate removal rate ≥85%, metal ion removal rate ≥90%). A small amount of nitrogen trifluoride gas was introduced to convert arsenic and heavy metal ions into solid fluoride precipitates at room temperature, which were then removed by a 0.5 μm precision filter. Subsequently, the solution was concentrated under reduced pressure at 50°C and -0.08 MPa to approximately 25% concentration, yielding approximately 400 kg of high-purity fluorosilicic acid concentrate. (2) Ammonolysis: 400 kg of high-purity fluorosilicic acid concentrate was added to a PTFE-lined reactor, and gaseous ammonia was slowly introduced. The temperature was controlled at 50°C and the pH at 8.2, and the reaction was continuously stirred for 90 minutes. After the reaction was completed, the silica precipitate was separated by plate and frame filter press. The filter cake was washed three times with pure water and then dried at 120°C for 4 hours to obtain approximately 48 kg of high-purity precipitated silica. The ammonium fluoride solution was filtered through a 0.1 μm ceramic membrane to remove residual colloidal silica. (3) Two-stage vacuum pyrolysis: The ammonium fluoride solution is evaporated and concentrated to a molten state at -0.09 MPa and 60°C. It is then transferred to the first-stage pyrolysis furnace, where ammonia removal occurs at 140°C and -0.05 MPa, producing molten ammonium bifluoride. The released ammonia gas is recovered by condensation at -20°C. The molten ammonium bifluoride is then transferred to the second-stage high-temperature vacuum pyrolysis furnace, where it is directly pyrolyzed at 330°C and -0.098 MPa, producing a mixture of anhydrous hydrogen fluoride and ammonia. The ammonia gas is separated by condensation at -20°C and recycled to the ammonium fluoride section for reuse, while the anhydrous hydrogen fluoride gas is sent to the purification section. The overall fluorine conversion rate is approximately 83%. (4) Gas-phase oxidation: Crude hydrogen fluoride gas enters a closed reaction chamber, and nitrogen trifluoride gas (NF3 is 0.05% of the mass of crude HF gas) is continuously introduced. The reaction is carried out at 75°C, oxidizing impurities such as AsF3, PF3, and BF3 into high-boiling-point AsF5, PF5, and fluoroborate complexes. After the reaction, the gas is passed through a cyclone dust collector to remove solid oxide particles. (5) Deep purification: The hydrogen fluoride gas after gas-phase oxidation enters a scraped membrane molecular distillation apparatus to separate high-boiling-point heavy metal fluorides and boron-phosphorus complexes under conditions of 150 Pa and 35°C. The distilled hydrogen fluoride gas is then passed through a silicon carbide-lined adsorption column containing the self-made adsorbent described above, reducing the boron content to 0.015 ppb. Finally, it is filtered sequentially through two stages of 0.1 μm and 0.05 μm PTFE ultrafiltration membranes. (6) Acid Preparation and Filling: The purified anhydrous hydrogen fluoride gas was passed into ultrapure water with a resistivity of 18.25 MΩ·cm to prepare a 49% hydrofluoric acid solution. Testing: Boron content 0.015 ppb, total metal cation content 8 ppt, particles ≥0.05μm 3 / mL. The solution was filled into a high-purity PFA storage tank in an ISO 14644-1 Class 5 clean environment, yielding approximately 105 kg of UP-SSS grade electronic hydrofluoric acid (based on anhydrous hydrogen fluoride). Example 2 (lower limit of parameter range) The 8% fluorosilicic acid obtained from the phosphate fertilizer tail gas absorption system was used as raw material. The nanofiltration membrane had a molecular weight cutoff of 200 Da, the gaseous ammonia ammonolysis temperature was 45°C and the pH was 8.1, the first-stage pyrolysis temperature was 135°C, the second-stage pyrolysis temperature was 320°C, the NF3 gas-phase oxidation temperature was 70°C, the molecular distillation pressure was 120 Pa and the temperature was 33°C, the Al / Mg molar ratio was 1.2:1, and the adsorbent sintering temperature was 450°C. The remaining steps were the same as in Example 1. The resulting product had a boron content ≤0.02 ppb and a total metal cation content less than 10 ppt. Example 3 (Upper Limit of Parameter Range) 12% fluorosilicic acid obtained from a phosphate fertilizer tail gas absorption system was used as raw material. The nanofiltration membrane had a molecular weight cutoff of 300 Da, the gaseous ammonia ammonolysis temperature was 55°C and pH 8.4, the first-stage pyrolysis temperature was 145°C, the second-stage pyrolysis temperature was 340°C, the NF3 gas-phase oxidation temperature was 80°C, the molecular distillation pressure was 180 Pa and the temperature was 38°C, the Al / Mg molar ratio was 1.8:1, and the adsorbent sintering temperature was 600°C. The remaining steps were the same as in Example 1. The resulting product had a boron content ≤0.02 ppb and a total metal cation content less than 10 ppt. Comparative Example 1: Conventional Single-Stage High-Temperature Pyrolysis Process The same raw materials and pretreatment steps as in Example 1 were used, except that step (3) was replaced with single-stage high-temperature pyrolysis, where molten ammonium fluoride was directly pyrolyzed at 550-600℃, omitting the first-stage low-temperature deammoniation process. The rest of the process was the same. Test results: The hydrogen fluoride gas from single-stage pyrolysis carried a large amount of dust and undecomposed ammonium fluoride droplets, with a fluorine conversion rate of only 58-65%; the finished product had a boron content of 0.12-0.25 ppb and a total metal ion content of 35-50 ppt. This indicates that stepwise pyrolysis can reduce side reactions and decrease the volatilization of impurities. Comparative Example 2: Oxidation was performed using hydrogen peroxide instead of nitrogen trifluoride. The raw materials and other processes remain unchanged, but the gaseous oxidant is replaced with an atomized 30% hydrogen peroxide solution. Test results: finished product boron content 0.08-0.18 ppb, total metal 22-38 ppt. Hydrogen peroxide introduces moisture that corrodes the equipment, its oxidation efficiency is lower than nitrogen trifluoride, and the atomizing medium introduces trace amounts of metallic impurities. Comparative Example 3: Organic Fluorine Resin Replacing Inorganic Adsorbents In the purification stage, equal volumes of perfluorosulfonic acid resin and quaternary ammonium anion exchange resin were used to replace the AlF3-MgF2 adsorbent. Initially, the boron content was 0.03-0.05 ppb; after 60 days of operation, the boron content increased to 0.06-0.10 ppb, and the TOC reached 3-8 ppb. Long-term use resulted in resin breakage, decreased adsorption performance, and organic pollution, making it unsuitable for 7-3nm chip manufacturing processes. Description of initial impurity content of raw material fluorosilicic acid This process is designed for fluorosilicic acid, a byproduct of 8-12% phosphate fertilizer production, with the following raw material impurities: Sources of impurity content range Phosphate (PO4³⁻) 200-500 ppm Phosphate mist entrainment during phosphate rock acidolysis process Sulfate (SO4²⁻) 100-300 ppm Sulfate mist entrainment during sulfuric acid acidolysis process Aluminum (Al) 5-20 ppm, acid leaching of aluminosilicate minerals in phosphate rock. Magnesium (Mg) 3-15 ppm: Acid leaching of magnesium minerals such as dolomite from phosphate rock. Arsenic (As) 0.5-5 ppm: Acid leaching of arsenic-containing minerals such as arsenic pyrite from phosphate rock. Boron (B) 0.1-2 ppm Acid leaching of borosilicate minerals in phosphate rock Iron (Fe) 2-10 ppm: Acidification of iron minerals in phosphate rock and corrosion of equipment. Of the aforementioned impurities, phosphate and sulfate ions can be removed by more than 85% through nanofiltration membrane pretreatment; aluminum, magnesium, and iron ions can be removed by more than 90% through nanofiltration and subsequent NF3 pre-oxidation precipitation steps; arsenic is removed by NF3 pre-oxidation into solid AsF5 precipitation; boron is the most difficult trace impurity to remove from the final product. This process utilizes a four-step combined boron removal system: nanofiltration pretreatment + NF3 gas-phase oxidation + molecular distillation + sintered AlF3-MgF2 composite inorganic adsorption, specifically designed to handle the high impurity load of fluorosilicic acid, a byproduct of phosphate fertilizer production. Comparative Example 4 (Traditional Fluorite-Sulfuric Acid Process) Using acid-grade fluorite (CaF2>97%) and 98% concentrated sulfuric acid as raw materials, crude hydrogen fluoride gas is generated by reaction in a rotary kiln at 250-300°C. The crude gas is then washed with sulfuric acid, condensed, and distilled to obtain anhydrous hydrogen fluoride, which is further purified to UP-SSS grade through multi-stage distillation and organic fluorine resin adsorption. Approximately 3.6 tons of phosphogypsum are produced as a byproduct per ton of AHF, and the risk of sulfate residue always exists. Comparative Example 5 (Conventional Fluorosilicic Acid-Sulfuric Acid Method) Using fluorosilicic acid as a raw material, ammonium fluoride is produced by ammonolysis, which is then concentrated to produce ammonium hydrogen fluoride. Hydrogen fluoride is then produced by acidolysis of ammonium hydrogen fluoride with concentrated sulfuric acid, followed by purification by distillation. This route still uses sulfuric acid in the final step, resulting in a product containing trace amounts of sulfate. It typically only reaches UP-S grade, making it difficult to consistently achieve UP-SSS grade. The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A process for preparing UP-SSS grade electronic-grade hydrofluoric acid from fluorosilicic acid, a byproduct of phosphate fertilizer production, using a sulfuric acid-free method, characterized in that... Includes the following steps: (1) Raw material pretreatment: Phosphate fertilizer by-product fluorosilicic acid is pre-filtered through a nanofiltration membrane with a molecular weight cutoff of 200-300 Da to remove phosphate and metal ions. Nitrogen trifluoride gas is introduced for pre-oxidation to convert arsenic and heavy metals into solid precipitates. After precision filtration, it is concentrated at low temperature and reduced pressure to a high-purity fluorosilicic acid concentration of 22-26%. (2) Ammonolysis: In a closed reaction vessel, gaseous ammonia is introduced, and the temperature is controlled at 45-55°C and pH at 8.1-8.

4. The reaction is stirred to generate ammonium fluoride solution and silica precipitate. Silica is separated by pressure filtration, and colloidal silica is removed by filtration of the ammonium fluoride solution through a 0.1 μm ceramic membrane. (3) Two-stage vacuum pyrolysis: The ammonium fluoride solution is evaporated and concentrated to a molten state under negative pressure and enters the first stage of pyrolysis at 135-145°C to deammonize into ammonium hydrogen fluoride. The ammonia gas is condensed and recovered. The ammonium hydrogen fluoride enters the second stage of pyrolysis and is directly decomposed into a mixture of anhydrous hydrogen fluoride and ammonia under high temperature and high vacuum conditions at 320-340°C. The ammonia gas is condensed and separated and then recycled in a closed loop. (4) Gas phase oxidation: The crude hydrogen fluoride gas obtained in step (3) is introduced into a closed reaction chamber, and nitrogen trifluoride gas is continuously introduced for gas phase oxidation. Arsenic, phosphorus and boron impurities are oxidized into high-boiling-point fluorides at 70-80°C, and solid oxides are removed by dust removal. (5) Deep purification: The gas obtained in step (4) is separated from high-boiling-point heavy metals and boron-phosphorus complexes by a scraped membrane molecular distillation device at 120-180 Pa and 33-38°C. Then, trace boron is removed to ≤0.02ppb by a sintered AlF3-MgF2 composite inorganic adsorbent. The adsorption column is lined with silicon carbide. Finally, it is filtered by multi-stage polytetrafluoroethylene ultrafiltration membrane to remove 0.1 μm and 0.05 μm solid particles in sequence. (6) Acid preparation and filling: The purified anhydrous hydrogen fluoride gas obtained in step (5) is passed into ultrapure water with a resistivity of 18.25 MΩ·cm for absorption and acid preparation. The mixture is then filled in an ISO 14644-1 Class 5 clean environment and stored in a high-purity PFA-lined storage tank to obtain UP-SSS grade electronic-grade hydrofluoric acid with a boron content ≤0.02ppb and a total metal cation content of less than 10ppt.

2. The process according to claim 1, characterized in that, The gaseous ammonia mentioned in step (2) is anhydrous gaseous ammonia obtained by vaporizing liquid ammonia, and the stirring reaction time is 90 minutes; the silica obtained by pressure filtration separation in step (2) is washed and dried to obtain high-purity precipitated silica.

3. The process according to claim 1, characterized in that, In step (3), the first and second pyrolysis stages are both carried out in a PTFE-lined closed negative pressure system. The ammonia gas generated in both stages is condensed and recovered and then returned to the ammonia hydrolysis stage in step (2) for recycling.

4. A method for preparing a sintered AlF3-MgF2 composite inorganic adsorbent for use in the process described in any one of claims 1-3, characterized in that, Anhydrous aluminum fluoride with a purity ≥99.99% and anhydrous magnesium fluoride are mixed evenly at an Al / Mg molar ratio of 1.2:1 to 1.8:

1. 5-8% of deionized water by weight of the powder is added to granulate the mixture. The granules are then sintered at 450-600°C for 3-5 hours under an inert atmosphere and sieved to obtain 20-40 mesh particles.

5. The process according to claim 1, characterized in that, The nanofiltration membrane in step (1) has a molecular weight cutoff of 250 Da; the reaction temperature in step (2) is 50°C and the pH is 8.2; the first pyrolysis temperature in step (3) is 140°C and the second pyrolysis temperature is 330°C; the oxidation temperature in step (4) is 75°C; the molecular distillation pressure in step (5) is 150 Pa and the temperature is 35°C, and the Al / Mg molar ratio of the sintered AlF3-MgF2 composite inorganic adsorbent is 1.5:1, and the sintering temperature is 500°C.

6. A UP-SSS grade electronic-grade hydrofluoric acid, characterized in that, The electronic-grade hydrofluoric acid is prepared by the process described in any one of claims 1-5, wherein the boron content is ≤0.02ppb and the total metal cation content is less than 10ppt.

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