A method for preparing high-purity large-particle fluorosilicate salt using acid-washed fluorine-containing waste liquid

Abstract

The application discloses a method for preparing high-purity large-particle fluorosilicate by using pickling fluorine-containing waste liquid, and belongs to the technical field of industrial waste acid resource utilization. The method comprises the following steps: introducing the fluorine-containing waste liquid into a PE / PP material reaction container; adding silicon dioxide according to the mass ratio of silicon dioxide to hydrofluoric acid of 1.17-1.20:1 to generate fluorosilicic acid by reaction; reacting with a potassium chloride / sodium chloride solution at 40 DEG C, adding 1% fluorosilicate seed crystal to induce the generation of large-particle crystals; performing pressure reduction and suction filtration at 0.8-1.5 MPa to obtain fluorosilicate products with a purity of greater than or equal to 99.5%; and adding hydrochloric acid and aluminum hydroxide to the filtrate after calcium chloride precipitation and electrodialysis impurity removal to generate a polyaluminum chloride solution by reaction at 140 DEG C. The application realizes the full-component recovery of fluorine and hydrochloric acid resources, solves the problems of small crystal particle size, difficult filtration and production of hazardous waste in the traditional process, shortens the fluorosilicate suction filtration time to 4 minutes, and makes the polyaluminum chloride product meet the national standard, thereby achieving remarkable environmental protection and economic benefits, and being suitable for the resource utilization treatment of various pickling fluorine-containing waste liquids.

Description

Technical Field

[0001] This invention belongs to the field of industrial waste acid resource utilization technology, specifically involving a method for preparing high-purity large-particle fluorosilicates using fluorine-containing pickling waste liquid and realizing the full-component resource recovery of fluorine and hydrochloric acid in the waste liquid. It is applicable to the harmless and high-value treatment of fluorine-containing pickling waste liquid generated by industries such as metal surface treatment and semiconductor silicon wafer processing. Background Technology

[0002] Pickling is an indispensable key process in modern manufacturing, widely used in steel metallurgy, non-ferrous metal processing, and semiconductor silicon wafer fabrication. This process typically employs a multi-component mixed acid system composed of hydrofluoric acid, hydrochloric acid, sulfuric acid, and nitric acid. The acid reacts chemically with oxides and impurities on the workpiece surface to achieve surface cleaning and refinement.

[0003] During pickling, the effective components of the acid are gradually consumed, while a complex and highly corrosive fluoride-containing waste liquid is generated. Typical characteristics of this type of waste liquid include: high concentrations of fluoride ions (existing in the form of hydrofluoric acid), unreacted hydrochloric acid, sulfuric acid, nitric acid, and other acidic components, as well as small amounts of metal ion impurities. Due to the strong corrosiveness and biotoxicity of fluoride ions, fluoride-containing waste liquid is classified as hazardous waste, and its treatment and disposal have always been a major challenge for the industry.

[0004] Currently, the mainstream process for treating fluoride-containing wastewater in the industry is calcium salt precipitation + lime neutralization. The core process involves adding calcium salt agents such as calcium chloride and calcium hydroxide to the wastewater, causing fluoride ions to combine with calcium ions to form calcium fluoride precipitate; then adding lime slurry to neutralize the residual acid in the wastewater, adjusting the pH to neutral before discharge. However, this process has many insurmountable drawbacks: Serious waste of resources: The fluorine and hydrochloric acid resources with recycling value in the waste liquid are not utilized and are all lost with the waste residue and neutralization liquid, which is inconsistent with the development concept of circular economy.

[0005] Large amount of hazardous waste generated: For every ton of fluoride-containing waste liquid treated, 0.3 to 0.5 tons of calcium fluoride sludge and neutralization residue are generated. These hazardous wastes require professional disposal, which not only increases the environmental protection costs of enterprises, but also poses a risk of secondary pollution.

[0006] High operating costs: The dosage of calcium salts and lime slurry is huge, and the cost of treating 1 ton of waste liquid is as high as several hundred yuan, which is a heavy economic burden for enterprises that generate a large amount of waste liquid.

[0007] Limited treatment effect: The fine particle size of calcium fluoride precipitate makes it easy to form colloids, which are difficult to completely settle and filter, resulting in the effluent fluoride concentration being difficult to consistently meet the standards; the neutralized waste liquid still contains a large amount of salt, and direct discharge will cause salt pollution to the water body.

[0008] In recent years, with increasingly stringent environmental protection requirements and a growing emphasis on resource recycling, some companies have attempted to recover fluorine resources from fluorine-containing wastewater to produce fluorosilicate products. Fluorosilicates are important inorganic chemical raw materials, widely used in glass manufacturing, ceramic glazes, pesticide production, concrete admixtures, and other fields, and are in high demand in the market.

[0009] However, existing fluorosilicate preparation technologies suffer from a key bottleneck: the fluorosilicate crystals precipitated from the reaction have excessively small particle sizes (typically less than 10 μm). This problem triggers a series of chain reactions: Ultrafine crystals can easily clog pipes, valves, and filtration equipment, causing the production system to be unable to operate continuously and stably. During the filtration process, some ultrafine crystals penetrate the filter membrane and enter the filtrate, which not only reduces the yield of fluorosilicate products, but also introduces fluorosilicate impurities, seriously affecting the purity of subsequent by-products. Fine crystals have high surface energy and easily adsorb a large number of impurity ions, making it difficult to improve product purity and meet the requirements of high-end applications.

[0010] Meanwhile, existing technologies only focus on the recovery of fluorine resources, neglecting the reuse of hydrochloric acid resources in waste liquid. The filtrate is directly discharged after neutralization, which still results in resource waste and environmental pollution.

[0011] With the rapid development of strategic emerging industries such as semiconductors and new energy, the production capacity of semiconductor silicon wafers, photovoltaic cells, and other products has continued to expand, leading to a sharp increase in the generation of fluoride-containing pickling wastewater. Statistics show that my country generates over ten million tons of fluoride-containing pickling wastewater annually, and its treatment and resource utilization have become key factors restricting the green and sustainable development of related industries.

[0012] Against this backdrop, developing a technology for treating fluoride-containing wastewater that can simultaneously achieve high-value recovery of fluorine and hydrochloric acid resources, generate no hazardous waste, and produce high-purity products is of significant practical and strategic value. This invention addresses this industry pain point by proposing an innovative technical solution. Summary of the Invention

[0013] The purpose of this invention is to provide a method for preparing high-purity large-particle fluorosilicates using fluorine-containing pickling wastewater, thereby solving the problems of resource waste, hazardous waste generation, small product particle size, and filtration difficulties in existing fluorine-containing pickling wastewater treatment processes, achieving full-component recovery of fluorine and hydrochloric acid resources, reducing treatment costs, and eliminating secondary pollution.

[0014] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing high-purity, large-particle fluorosilicates using acid-washing fluorine-containing waste liquid includes the following steps: Step 1: The fluoride-containing wastewater generated from the pickling process is directly transported to the reaction vessel without wastewater treatment; the fluoride-containing wastewater contains hydrofluoric acid and hydrochloric acid; the fluoride ion concentration in the wastewater is measured. Step 2: Add silicon dioxide to the reaction vessel from Step 1, stir and react. The reaction equation is 6HF + SiO2 → H2SiF6 + 2H2O. After the reaction is complete, filter and keep the filtrate for later use. Step 3: Directly use chloride powder, or prepare a chloride powder solution with a mass concentration of 20%~32%, wherein the chloride is potassium chloride or sodium chloride; when potassium chloride is used, prepare a saturated potassium chloride solution with a mass concentration of not less than 20%; when sodium chloride is used, prepare an industrial-grade sodium chloride solution with a mass concentration of 25%~30%. Step 4: Add the chloride powder or chloride solution from Step 3 to the filtrate from Step 2, control the reaction temperature at 40℃, and keep the stirring speed below 100 rpm; if the room temperature is low, use heating to maintain a constant temperature. When the chloride salt is potassium chloride, potassium fluorosilicate is formed. The chemical reaction formula is: 2KCl + H2SiF6 → K2SiF6 + 2HCl. When the chloride salt is sodium chloride, sodium fluorosilicate is produced. The chemical reaction formula is: 2NaCl + H2SiF6 → Na2SiF6 + 2HCl. Step 5: Add the pre-prepared fluorosilicate seed crystals to the fluorosilicate precursor solution obtained in Step 4, and continue stirring until the reaction is complete to induce the formation of large-particle fluorosilicate crystals. Step 6: Inject the reaction solution from Step 5 into a vacuum filtration device for vacuum filtration under reduced pressure. Wash the filtered solid with a small amount of water several times to obtain a fluorosilicate solid with a purity greater than 99.5%. The filtrate is a hydrochloric acid solution containing trace amounts of inorganic salts. Step 7: Add calcium chloride solution to the hydrochloric acid filtrate from Step 6. After the reaction, filter to remove sulfate ions. The reaction equation is SO42-. 2- + CaCl2→CaSO4+2Cl - Then, electrodialysis is used to remove nitrate and residual fluoride ions from the filtrate. Step 8: Add hydrochloric acid and aluminum hydroxide to the solution after purification in Step 7. The amount of hydrochloric acid added should be 20%~30% by mass, and the amount of aluminum hydroxide added should be 0.5~0.7 equivalents of the hydrochloric acid in the solution. Place the mixture in a sealed container and heat it to 140℃. During the heating process, the water vapor evaporated should be condensed and refluxed into the reaction system. The reaction equation is (6-n)HCl + 2Al(OH)3 → (6-n)H2O + Al2(OH) n Cl (6-n) After the reaction is complete, cool to 90°C and filter under reduced pressure to obtain a polyaluminum chloride solution.

[0015] Preferably, the reaction vessel used in step one is made of polyethylene (PE) or polypropylene (PP).

[0016] Preferably, in step two, the mass ratio of silicon dioxide added to hydrofluoric acid in the waste liquid is 1.17~1.20:1, the stirring reaction time is 0.5~1 hour, and the stirring speed is 50~80 rpm.

[0017] Preferably, the mass of chloride added in step four is determined based on the fluoride ion content in the waste liquid.

[0018] Preferably, the amount of seed crystals added in step five is 1% of the theoretical yield of fluorosilicate.

[0019] Furthermore, the fluorosilicate seed crystals have a particle size of 50~100μm, and the seed crystals are prepared by dissolving industrial-grade fluorosilicate and recrystallizing it, and then sieving it to obtain crystals of the target particle size.

[0020] Preferably, the water used for washing in step six is ​​deionized water, and the amount of water used for each wash is 5% to 10% of the mass of the fluorosilicate solid. The washing method is rinsing to avoid the solid crystals being dispersed. The negative pressure of the vacuum filtration is 0.8 MPa to 1.5 MPa.

[0021] Preferably, the electrodialysis method described in step seven uses a Corning P1 electrodialysis device, with the current intensity controlled at 0.1A~0.15A, the electrodialysis time at 2~4 hours, and the fluoride ion concentration in the treated filtrate being less than 0.5mg / L and the nitrate ion concentration being less than 1mg / L.

[0022] Preferably, the aluminum hydroxide used in step eight is aluminum hydroxide powder of grade AH-2d conforming to the national standard GB / T4294-2010, with a particle size of 100~200 mesh, to ensure a complete reaction.

[0023] Preferably, the polyaluminum chloride solution described in step eight meets the requirements of the national standard GB / T22627-2022 "Water Treatment Agent Polyaluminum Chloride", with an alumina mass fraction of ≥8% and a basicity of 20%~50%.

[0024] Compared with the prior art, the beneficial effects of the present invention are: Achieving full-component resource recovery: This invention converts fluoride ions in fluorine-containing waste liquid into high-value fluorosilicate products with a purity of ≥99.5%, and hydrochloric acid into polyaluminum chloride products that meet national standards, thus realizing the high-value utilization of all valuable components in the waste liquid and completely solving the problem of resource waste in traditional processes.

[0025] Addressing the industry pain point of excessively small crystal particle size: By adding 1% fluorosilicate seed crystals, large-particle fluorosilicate crystals are induced, increasing the particle size from less than 10μm to 50~100μm, significantly improving filtration efficiency. Examples demonstrate that adding 1% seed crystals reduces filtration time from "unfilterable" to 4 minutes, while also preventing crystal penetration and contamination of the filtrate, ensuring the purity of subsequent polyaluminum chloride (PAC) products.

[0026] Eliminating Hazardous Waste Generation: This invention replaces the traditional "calcium salt precipitation + lime neutralization" process, generating no hazardous waste such as calcium fluoride sludge or neutralization residue throughout the entire process. Only a small amount of recyclable silica and calcium sulfate filter residue is produced, achieving harmless treatment of fluoride-containing waste liquid and reducing the hazardous waste disposal costs for enterprises.

[0027] Reduced equipment corrosion and wear: The reaction vessel made of PE / PP material specifically solves the problem of the strong corrosiveness of fluorine-containing waste liquid, extending the service life of the equipment to more than 5 years and significantly reducing the operating and maintenance costs of the equipment.

[0028] Significant economic benefits: The raw materials used in this invention are industrial-grade potassium chloride / sodium chloride and ordinary silicon dioxide, which are inexpensive; the products, fluorosilicate and polyaluminum chloride, are both chemical products in high market demand and can be sold directly to generate economic benefits. Calculations show that treating 1 ton of fluoride-containing waste liquid can generate a profit of 200-500 yuan, demonstrating strong market competitiveness.

[0029] Stable and reliable process: This invention clarifies the key process parameters for each step, forming a technical solution that can be replicated industrially, ensuring stable and reliable product quality. The purity fluctuation of fluorosilicate is ≤0.3%, and the polyaluminum chloride product meets national standards, making it suitable for large-scale industrial production. Detailed Implementation

[0030] This invention aims to address the problems of resource waste, large hazardous waste generation, small product particle size, difficult filtration, and low purity of by-products in existing fluorinated pickling wastewater treatment processes. It provides a method for preparing high-purity, large-particle fluorosilicates using fluorinated pickling wastewater, achieving the following objectives: Simultaneously recover fluorine and hydrochloric acid resources from waste liquid to prepare high-purity fluorosilicate and polyaluminum chloride products, achieving full-component resource utilization; To address the issue of excessively small fluorosilicate crystal size, large-particle crystals are prepared using seed-induction technology, thereby improving filtration efficiency and product purity. To avoid the generation of hazardous waste, achieve the harmless treatment of waste liquid, and reduce the environmental protection costs of enterprises; Optimize process parameters to form a technical solution that can be replicated industrially, ensuring stable and reliable product quality.

[0031] In the exemplary implementation scheme, the following technical solution is adopted.

[0032] This invention provides a method for preparing high-purity, large-particle fluorosilicates using acid-washing fluorine-containing wastewater. This method, through precise process design and parameter control, achieves the complete resource recovery of the fluorine-containing wastewater. The specific steps are as follows: Waste liquid pretreatment and fluoride ion concentration detection The fluorinated wastewater generated from the pickling process is directly transported to a reaction vessel made of polyethylene (PE) or polypropylene (PP), preventing the wastewater from entering the existing wastewater treatment system. PE / PP materials have excellent corrosion resistance, effectively resisting the erosion of strong acids such as hydrofluoric acid and hydrochloric acid, thus extending the service life of the equipment.

[0033] The electrode method was used to detect the fluoride ion concentration in the waste liquid. The specific operating steps were as follows: 1 mL of the original waste liquid was taken and diluted 100 times with deionized water to obtain the test solution; the fluoride ion selective electrode and the reference electrode were inserted into the test solution, and the ion meter was connected. After the reading stabilized, the fluoride ion concentration was recorded; the fluoride ion concentration in the original waste liquid was calculated based on the dilution factor, which provided a basis for determining the subsequent process parameters.

[0034] The fluorine-containing waste liquid contains hydrofluoric acid and hydrochloric acid, wherein the fluoride ion concentration is 5~20g / L and the hydrochloric acid concentration is 10~30g / L.

[0035] Fluorosilicic acid conversion reaction Based on the hydrofluoric acid content in the waste liquid, silica powder is added to the reaction vessel at a mass ratio of silica to hydrofluoric acid of 1.17~1.20:1. Industrial-grade quartz powder with a particle size of 100~200 mesh is used to ensure sufficient contact with the waste liquid.

[0036] Start the stirring device and control the stirring speed at 50-80 rpm. Stir the reaction for 0.5-1 hour to allow the hydrofluoric acid and silicon dioxide to react completely to form fluorosilicic acid. The reaction equation is: 6HF + SiO2 → H2SiF6 + 2H2O.

[0037] After the reaction is complete, a plate and frame filter press is used for filtration. The filter residue is unreacted silicon dioxide, which can be recycled and reused. The filtrate is a mixed solution of fluorosilicic acid and hydrochloric acid, which is reserved for later use.

[0038] In this step, the amount of silica added is slightly higher than the theoretical value, in order to ensure that the hydrofluoric acid reacts completely and to avoid residual fluoride ions.

[0039] Preparation of chloride salt solution Prepare a chloride solution with a mass concentration of 20%~32%, using potassium chloride or sodium chloride as the chloride. The specific preparation method is as follows: Preparation of potassium chloride solution: Take industrial-grade potassium chloride powder, add it to room temperature deionized water, and stir until saturated to obtain a saturated potassium chloride solution with a mass concentration of not less than 20%; the mass of potassium chloride added is determined according to the fluoride ion content in the waste liquid to ensure that the molar ratio of potassium chloride to fluorosilicic acid is 2.0~2.2:1.

[0040] Preparation of sodium chloride solution: Take industrial grade sodium chloride solid, add it to room temperature deionized water, stir to dissolve, and prepare a sodium chloride solution with a mass concentration of 25%~30%; the amount of sodium chloride added is also determined according to the fluoride ion content to ensure that the molar ratio of sodium chloride to fluorosilicic acid is 2.0~2.2:1.

[0041] Synthesis of fluorosilicate precursor solutions The prepared chloride salt solution is slowly added to the mixed solution of fluorosilicic acid and hydrochloric acid while stirring, and the stirring speed is controlled to be less than 100 rpm.

[0042] The reaction temperature is controlled at 40℃, which is the optimal temperature for fluorosilicate crystallization. This temperature ensures the reaction rate while preventing excessively small crystal size due to excessively high temperatures. In winter or other conditions with low room temperature, steam or electric heating is used to maintain a constant reaction temperature.

[0043] Chloride salts undergo a metathesis reaction with fluorosilicic acid to produce a fluorosilicate precursor solution. The reaction equation is as follows: When potassium chloride is selected as the chloride salt powder, potassium fluorosilicate is generated accordingly. The chemical reaction formula is: 2KCl + H2SiF6 → K2SiF6 + 2HCl. When sodium chloride is used as the chloride salt powder, sodium fluorosilicate is generated. The chemical reaction formula is: 2NaCl + H2SiF6 → Na2SiF6 + 2HCl. A small amount of heat is released during the reaction, and the temperature can be regulated by the jacket cooling system of the reaction vessel to ensure that the reaction temperature is stable at 40°C.

[0044] Seed-induced large-particle crystallization Pre-prepared fluorosilicate seed crystals were added to the fluorosilicate precursor solution, with the amount of seed crystals added being 1% of the theoretical yield of fluorosilicate.

[0045] The method for preparing the seed crystals is as follows: take industrial-grade fluorosilicate products, add them to deionized water, and heat to dissolve them until saturation; slowly cool the saturated solution to allow the fluorosilicate to crystallize and precipitate; use a standard sieve to screen and select crystals with a particle size of 50~100μm as seed crystals.

[0046] After adding the seed crystals, continue stirring for 3-7 minutes. The seed crystals act as the nucleus for crystal growth, inducing fluorosilicate ions in the solution to deposit and grow on their surface, forming large crystal particles. The stirring speed should be controlled at 50-80 rpm to avoid excessive stirring that could cause crystal breakage.

[0047] Solid-liquid separation and fluorosilicate purification The reaction solution is injected into a vacuum filtration device for vacuum filtration under reduced pressure. The negative pressure of the vacuum filtration is controlled at 0.8MPa~1.5MPa. This negative pressure range can ensure filtration efficiency while preventing the crystals from being broken.

[0048] After filtration, the solid is washed multiple times with a small amount of deionized water, 3 to 5 times, with each wash using 5% to 10% of the fluorosilicate solid mass. The washing method is rinsing, with a slow and uniform water flow to avoid breaking up the solid crystals.

[0049] After washing, white solid crystals are obtained, which are high-purity fluorosilicate products with a purity greater than 99.5% and a crystal particle size of 50~100μm, meeting the requirements of high-end application fields.

[0050] The filtrate is a hydrochloric acid solution with a concentration of 8% to 20% and a pH value of less than 1, containing a small amount of impurity ions such as sulfate and nitrate, and is prepared for use.

[0051] Hydrochloric acid filtrate for impurity removal Add calcium chloride solution to the hydrochloric acid filtrate. The mass concentration of the calcium chloride solution is 30%. The amount added is determined according to the sulfate content in the filtrate to ensure that the molar ratio of calcium ions to sulfate ions is 1.1~1.2:1.

[0052] After stirring for 5 minutes, calcium ions combine with sulfate ions to form calcium sulfate precipitate. The reaction equation is: SO42- 2- +CaCl2→CaSO4+2Cl - .

[0053] After the reaction is complete, the mixture is filtered using a plate and frame filter press. The filter residue is calcium sulfate, which can be recycled as a raw material for building materials. The filtrate is a hydrochloric acid solution to remove sulfate ions.

[0054] The filtrate is introduced into a Corning P1 electrodialysis unit for electrodialysis treatment. The current intensity of the electrodialysis unit is controlled at 0.1A~0.15A, and the electrodialysis time is 2~4 hours. Under the action of the electric field, nitrate ions and residual fluoride ions in the solution permeate through the ion exchange membrane and enter the concentrate chamber, thereby achieving separation from hydrochloric acid.

[0055] After electrodialysis treatment, the concentration of fluoride ions in the filtrate is less than 0.5 mg / L and the concentration of nitrate ions is less than 1 mg / L, resulting in a high-purity hydrochloric acid solution.

[0056] Preparation of polyaluminum chloride Add concentrated hydrochloric acid to the purified hydrochloric acid solution to adjust the mass fraction of hydrochloric acid in the solution to 20%~30%.

[0057] Add aluminum hydroxide powder. The aluminum hydroxide selected is the product of grade AH-2d that conforms to the national standard GB / T4294-2010, with a particle size of 100~200 mesh. The amount added is 0.5~0.7 equivalents of hydrochloric acid in the solution.

[0058] Place the mixture in a sealed container and heat to 140 °C. During heating, start the stirring device at a speed of 50-80 rpm to ensure complete reaction between aluminum hydroxide and hydrochloric acid. The reaction equation is: (6-n)HCl + 2Al(OH)3 → (6-n)H2O + Al2(OH) n Cl (6-n) .

[0059] During the heating process, the water in the solution evaporates to form water vapor. The water vapor is condensed by the condenser and then flows back into the reaction system to ensure a stable solution concentration.

[0060] The reaction time is 1.5 to 2 hours. After the reaction is complete, the solution is cooled to 90 °C and filtered using a vacuum filtration device. The filter residue is unreacted inorganic salts such as aluminum hydroxide, which can be recycled and reused. The filtrate is a light yellow polyaluminum chloride solution.

[0061] The polyaluminum chloride solution meets the requirements of the national standard GB / T22627-2022 "Water Treatment Agent Polyaluminum Chloride", with an alumina mass fraction of ≥8% and a basicity of 20%~50%, and can be sold directly as a water treatment agent.

[0062] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0063] Example 1: Seedless Comparison Experiment To achieve fluoride ion recovery and hydrochloric acid treatment of pickling waste liquid, follow these steps in sequence: 1. Take 50.97 g of pickling process waste liquid using hydrochloric acid and hydrofluoric acid as pickling solution, and determine the content of fluorosilicic acid to be 10.77 g, HF to be 5.12 g, and HCl to be 9.66 g; 2. Prepare 56.83 g of a 20% sodium chloride solution at room temperature (20 degrees Celsius); 3. Add 2.06g of SiO2 to the pickling waste liquid, stir continuously and maintain the reaction time for 1 hour; 4. Filter the above mixture using filter paper. Vacuum filtration requires a vacuum pump connected to the filtration bottle to create negative pressure, with a maximum negative pressure of one atmosphere. The filter residue is SiO2, and the main components of the filtrate are fluorosilicic acid and hydrochloric acid. 5. Pour the above filtrate and sodium chloride solution into the reaction vessel in equal proportions, stirring while pouring. Measure the temperature of the reaction mixture. The temperature of the reaction mixture is 40 degrees Celsius. Continue stirring and maintain the reaction time for 3 minutes. You will see turbid white crystals suspended in the lower part of the solution. 6. Filter the reaction solution using filter paper. Vacuum filtration requires a vacuum pump connected to the filtration flask to create negative pressure, with a maximum negative pressure of one atmosphere. During filtration, sodium fluorosilicate crystals clog the filter paper, causing filtration failure, making it impossible to obtain relatively pure sodium fluorosilicate crystals, and the experiment cannot proceed.

[0064] Conclusion: Without seed crystals, the fluorosilicate crystals are too small to achieve solid-liquid separation, making subsequent processes impossible.

[0065] Example 2: Preparation of sodium fluorosilicate by adding 0.5% seed crystals To achieve fluoride ion recovery and hydrochloric acid treatment of pickling waste liquid, follow these steps in sequence: 1. Take 51.24 g of pickling process waste liquid using hydrochloric acid and hydrofluoric acid as pickling solution, and determine the content of fluorosilicic acid to be 10.84 g, HF to be 5.17 g, and HCl to be 9.71 g; 2. Prepare 64.95 g of a 20% sodium chloride solution at room temperature (20 degrees Celsius); 3. Add 2.15g of SiO2 to the pickling waste liquid, stir continuously and maintain the reaction time for 1 hour; 4. Filter the above mixture using filter paper. Vacuum filtration requires a vacuum pump connected to the filtration bottle to create negative pressure, with a maximum negative pressure of one atmosphere. The filter residue is SiO2, and the main components of the filtrate are fluorosilicic acid and hydrochloric acid. 5. Pour the above filtrate and sodium chloride solution into a container containing 0.14 g (0.5%) of pre-prepared sodium fluorosilicate in equal proportions. Stir while pouring and measure the temperature of the reaction mixture. The temperature of the reaction mixture is 40 degrees Celsius. Continue stirring and maintain the reaction time for 3 minutes. Turbid white crystals can be seen suspended in the lower part of the solution. 6. Filter the reaction solution using filter paper. Vacuum filtration requires a vacuum pump connected to the filtration flask to create negative pressure, with a maximum negative pressure of one atmosphere. Wash with water three times. The filtration time is 29 minutes. The filter residue is a white crystalline solid, weighing 13.15 g, which is identified as 99.6% sodium fluorosilicate solid. The filtrate is a yellow liquid, weighing 152.77 g, with HCl as the main component. 7. Add 8.62 g of 30% calcium chloride solution to the above filtrate and react for 5 minutes. Then, use a vacuum pump connected to a vacuum filtration flask to create a negative pressure for filtration. The maximum negative pressure reaches one atmosphere. The filter residue is CaSO4. 8. Introduce the above filtrate into the Corning P1 electrodialysis pilot plant, maintaining a current intensity of 0.15 A; 9. A total of 149.26 g of the purified solution was collected, and the mass of hydrogen chloride was 25.43 g. The inorganic salt content did not exceed 0.85%. 10. Take 19.71 g of aluminum hydroxide powder of grade AH-2d that meets the national standard GB / T4294-2010 for later use.

[0066] 11. Add aluminum hydroxide powder to the purified solution and stir evenly. Place it in a sealed container under normal pressure and heat to 140 degrees Celsius. Stir during the heating process to ensure that the aluminum hydroxide reacts fully with the hydrochloric acid. 12. The reaction time is 1.5 hours. To maintain the temperature and solution quality, the water vapor evaporated during the heating process is condensed and then refluxed back into the reaction mixture.

[0067] 13. After the reaction is complete, allow the liquid to cool to 90 degrees Celsius, then filter it using filter paper. The filtration process requires negative pressure. Use a vacuum pump connected to the filtration flask to apply negative pressure, with a maximum negative pressure of one atmosphere.

[0068] 14. The filter residue mainly consisted of inorganic salts such as aluminum hydroxide. After drying, its weight was 3.33 g. Filtration yielded 136.47 mL of clear liquid, with a pH of 2.9 and a specific gravity of 1.14 g / cm³. 3 The main component of the concentrated solution is polyaluminum chloride solution (PAC). According to the national standard GB / T22627-2022 "Water Treatment Agents - Polyaluminum Chloride", the mass of alumina is 8.53 g, the mass solubility of "PAC" is 8.45%, and the basicity is 27.62%, which meets the national standard requirements.

[0069] Example 3: Preparation of sodium fluorosilicate by adding 0.6% seed crystals 1. Take 48.98 g of pickling process waste liquid using hydrochloric acid and hydrofluoric acid as pickling solution, and determine the content of fluorosilicic acid to be 10.36 g, HF to be 4.11 g, and HCl to be 7.93 g; 2. Prepare 49.66 g of a 25% sodium chloride solution at room temperature (20 degrees Celsius); 3. Add 2.06 g of SiO2 to the pickling waste liquid, stir continuously and maintain the reaction time for 1 hour; 4. Filter the above mixture using filter paper. Vacuum filtration requires a vacuum pump connected to the filtration bottle to create negative pressure, with a maximum negative pressure of one atmosphere. The filter residue is SiO2, and the main components of the filtrate are fluorosilicic acid and hydrochloric acid. 5. Pour the above filtrate and sodium chloride solution into a container containing 0.16 g (0.6%) of pre-prepared sodium fluorosilicate in equal proportions. Stir while pouring. The temperature of the reaction mixture is 40 degrees Celsius. Continue stirring and maintain the reaction time for 5 minutes. Turbid white crystals will be visible suspended at the bottom of the solution. 6. Filter the reaction solution by vacuum filtration. Vacuum filtration requires a vacuum pump connected to the filtration flask to create negative pressure, with a maximum negative pressure of one atmosphere. Wash with water 3 times. The filtration time is 17 minutes. The filter residue is a white crystalline solid, weighing 12.78 g, and is identified as 99.6% sodium fluorosilicate solid. The filtrate is a yellow liquid, weighing 127.01 g, and its main component is hydrochloric acid. 7. Add 8.3 g of 30% calcium chloride solution to the above filtrate, react for 5 minutes, and then use a vacuum pump connected to a vacuum filtration flask to create a negative pressure for filtration. The maximum negative pressure reaches one atmosphere. The filter residue is CaSO4. 8. Introduce the above filtrate into the Corning P1 electrodialysis pilot plant, maintaining a current intensity of 0.1 A; 9. A total of 123.50 g of the purified solution was collected. The mass of hydrogen chloride was 22.63 g, and the inorganic salt content did not exceed 0.8%. 10. Take 18.02 g of aluminum hydroxide powder of grade AH-2d that meets the national standard GB / T4294-2010 for later use.

[0070] 11. Add aluminum hydroxide powder to the purified solution and stir evenly. Place it in a sealed container under normal pressure and heat to 140 degrees Celsius. Stir during the heating process to ensure that the aluminum hydroxide reacts fully with the hydrochloric acid. 12. The reaction time is 2 hours. To maintain the temperature and solution quality, the water vapor evaporated during the heating process is condensed and then refluxed back into the reaction mixture.

[0071] 13. After the reaction is complete, allow the liquid to cool to 90 degrees Celsius, then filter it using filter paper. The filtration process requires negative pressure. Use a vacuum pump connected to the filtration flask to apply negative pressure, with a maximum negative pressure of one atmosphere.

[0072] 14. The filter residue mainly consisted of inorganic salts such as aluminum hydroxide. After drying, it weighed 2.24 g. 118 mL of clear liquid was obtained after filtration. The pH value was measured to be 2.8, and the specific gravity of the solution was 1.17 g / cm³. 3The main component of the concentrated solution is polyaluminum chloride solution (PAC). According to the national standard GB / T22627-2022 "Water Treatment Agents - Polyaluminum Chloride", the mass of alumina is 10.51 g, the mass solubility of "PAC" is 8.83%, and the basicity is 30.67%, which meets the national standard requirements.

[0073] Example 4: Preparation of potassium fluorosilicate by adding 0.8% seed crystals 1. Take 100.37 g of pickling process waste liquid using hydrochloric acid and hydrofluoric acid as pickling solution, and determine the content of fluorosilicic acid to be 21.44 g, HF to be 8.75 g, and HCl to be 17.43 g; 2. Prepare 103.42 g of a 25% potassium chloride solution at room temperature (25 degrees Celsius); 3. Add 5.44 g of SiO2 to the pickling waste liquid, stir continuously and maintain the reaction time for 1 hour; 4. Filter the above mixture using filter paper. Vacuum filtration requires a vacuum pump connected to the filtration bottle to create negative pressure, with a maximum negative pressure of one atmosphere. The filter residue is SiO2, and the main components of the filtrate are fluorosilicic acid and hydrochloric acid. 5. Pour the above filtrate and potassium chloride solution into a container containing 0.38 g (0.8%) of pre-prepared potassium fluorosilicate in equal proportions. Stir while pouring and measure the temperature of the reaction mixture. The temperature of the reaction mixture is 40 degrees Celsius. Continue stirring and maintain the reaction time for 7 minutes. Turbid white crystals can be seen suspended in the lower part of the solution. 6. Filter the reaction solution using filter paper. A vacuum pump connected to the filtration flask is required to create negative pressure, with a maximum negative pressure of one atmosphere. Wash the solution three times with water. The filtration time is 9 minutes. The filter residue is a white crystalline solid, weighing 26.22 g, and is identified as 99.6% potassium fluorosilicate solid. The filtrate is a yellow liquid, weighing 263.72 g, and its main component is HCl. 7. Add 15.99 g of 30% calcium chloride solution to the above filtrate and react for 5 minutes. Then, use a vacuum pump connected to a vacuum filtration flask to apply negative pressure for filtration. The maximum negative pressure reaches one atmosphere. The filter residue is CaSO4. 8. Introduce the above filtrate into the Corning P1 electrodialysis pilot plant, maintaining a current intensity of 0.1 A; 9. A total of 266.11 g of the purified solution was collected, and the mass of hydrogen chloride was 45.94 g. The inorganic salt content did not exceed 0.9%. 10. Take 40.17 g of aluminum hydroxide powder of grade AH-2d that meets the national standard GB / T4294-2010 for later use.

[0074] 11. Add aluminum hydroxide powder to the purified solution and stir evenly. Place it in a sealed container under normal pressure and heat to 140 degrees Celsius. Stir during the heating process to ensure that the aluminum hydroxide reacts fully with the hydrochloric acid. 12. The reaction time is 2 hours. To maintain the temperature and solution quality, the water vapor evaporated during the heating process is condensed and then refluxed back into the reaction mixture.

[0075] 13. After the reaction is complete, allow the liquid to cool to 90 degrees Celsius, then filter it using filter paper. The filtration process requires negative pressure. Use a vacuum pump connected to the filtration flask to apply negative pressure, with a maximum negative pressure of one atmosphere.

[0076] 14. The filter residue mainly consisted of inorganic salts such as aluminum hydroxide. After drying, it weighed 6.27 g. 220.97 mL of clear liquid was obtained after filtration. The pH value was measured to be 2.9, and the specific gravity of the solution was 1.16 g / cm³. 3 The main component of the concentrated solution is polyaluminum chloride solution (PAC). According to the national standard GB / T22627-2022 "Water Treatment Agent Polyaluminum Chloride", the mass of alumina is 16.74 g, the mass solubility of "PAC" is 10.41%, and the basicity is 39.02%, which meets the requirements of the national standard.

[0077] Example 5: Preparation of sodium fluorosilicate by adding 1% seed crystals To achieve fluoride ion recovery and hydrochloric acid treatment of pickling waste liquid, follow these steps in sequence: 1. Take 52.29 g of pickling process waste liquid using hydrochloric acid and hydrofluoric acid as pickling solution, and determine the content of fluorosilicic acid to be 16.33 g, HF to be 4.39 g, and HCl to be 0.85 g; 2. Prepare 53.63 g of a 25% sodium chloride solution at room temperature (20 degrees Celsius); 3. Add 2.2g of SiO2 to the pickling waste liquid, stir continuously and maintain the reaction time for 0.5 hours; 4. Filter the above mixture using filter paper. Vacuum filtration requires a vacuum pump connected to the filtration bottle to create negative pressure, with a maximum negative pressure of one atmosphere. The filter residue is SiO2, and the main components of the filtrate are fluorosilicic acid and hydrochloric acid. 5. Pour the above filtrate and sodium chloride solution into a container containing 0.22 g (1%) of pre-prepared sodium fluorosilicate in equal proportions. Stir while pouring and measure the temperature of the reaction mixture. The temperature of the reaction mixture is 40 degrees Celsius. Continue stirring and maintain the reaction time for 5 minutes. Turbid white crystals can be seen suspended in the lower part of the solution. 6. Filter the reaction solution using filter paper. Vacuum filtration requires a vacuum pump connected to the filtration flask to create negative pressure, with a maximum negative pressure of one atmosphere. Wash with water 3 times. The filtration time is 4 minutes. The filter residue is a white crystalline solid, weighing 13.76 g, which is identified as 99.6% sodium fluorosilicate solid. The filtrate is a yellow liquid, weighing 126.26 g, with HCl as the main component. 7. Add 7.49 g of 30% calcium chloride solution to the above filtrate and react for 5 minutes. Then, use a vacuum pump connected to a vacuum filtration flask to apply negative pressure for filtration. The maximum negative pressure reaches one atmosphere. The filter residue is CaSO4. 8. Introduce the above filtrate into the Corning P1 electrodialysis pilot plant, maintaining a current intensity of 0.1 A; 9. A total of 124.48 g of the purified solution was collected, and the mass of hydrogen chloride was 8.27 g. The inorganic salt content did not exceed 0.85%. 10. Take 42.42 g of 12 mol / L hydrochloric acid solution and 20.43 g of aluminum hydroxide powder of grade AH-2d that meets the national standard GB / T4294-2010 for later use.

[0078] 11. Add the above hydrochloric acid solution and aluminum hydroxide powder to the purified solution and stir evenly. Place it in a sealed container under normal pressure and heat to 140 degrees Celsius. Stir during the heating process to ensure that the aluminum hydroxide and hydrochloric acid react completely. 12. The reaction time is 2 hours. To maintain the temperature and solution quality, the water vapor evaporated during the heating process is condensed and then refluxed back into the reaction mixture.

[0079] 13. After the reaction is complete, allow the liquid to cool to 90 degrees Celsius, then filter it using filter paper. The filtration process requires negative pressure. Use a vacuum pump connected to the filtration flask to apply negative pressure, with a maximum negative pressure of one atmosphere.

[0080] 14. The filter residue mainly consisted of inorganic salts such as aluminum hydroxide. After drying, its weight was 1.92 g. Filtration yielded 146.26 mL of clear liquid, with a pH of 2.3 and a specific gravity of 1.26 g / cm³. 3 The main component of the concentrated solution is polyaluminum chloride solution (PAC). According to the national standard GB / T22627-2022 "Water Treatment Agent Polyaluminum Chloride", the mass of alumina is 12.43 g, the mass solubility of "PAC" is 11.62%, and the basicity is 47.38%, which meets the national standard requirements.

[0081] Table 1: Effect of Seed Addition Amount on Fluorosilicate Filtration Time and Performance Testing of PAC Products The technical solution of this invention has strong industrial applicability and can be directly applied to the treatment of fluoride-containing pickling wastewater in industries such as metal surface treatment and semiconductor silicon wafer processing. The method of this invention is as follows: 1. By preparing fluorosilicates, resources can be recycled and utilized, thus achieving the recovery and utilization of fluoride ion resources and avoiding the regeneration of hazardous waste in the wastewater treatment process, which would harm the environment. 2. By using a specific feeding method and 1% pre-prepared fluorosilicate particles, large-particle fluorosilicates can be prepared, avoiding the phenomenon that small-particle fluorosilicates easily clog the funnel and hinder filtration. More importantly, the induced large-particle fluorosilicate product has higher purity, which is beneficial for filtration and indirectly improves the purity of polyaluminum chloride (PAC).

[0082] 3. By removing sulfuric acid and nitric acid from the reaction system through specific methods, the prepared polyaluminum chloride (PAC) can be made with higher purity.

[0083] 4. By preparing polyaluminum chloride (PAC), hydrochloric acid can be recycled and reused, avoiding the generation of large amounts of solid waste and increased environmental burden caused by adding large amounts of "alkali" for neutralization in the sewage treatment process. 5. A method for recycling resources by preparing fluorosilicate and polyaluminum chloride solution (PAC) from pickling process waste liquid using hydrochloric acid and hydrofluoric acid reduces the production cost of the pickling process, solves the environmental protection problem of pickling waste liquid, reduces the treatment difficulty and cost of downstream municipal sewage treatment plants, and can also increase income while recycling resources.

[0084] The equipment used in this invention is all conventional chemical equipment, requiring no special customization; the raw materials are industrial-grade chemicals, which are inexpensive and readily available; the process parameters are clear, the operation is simple, and it is easy for workers to master; the product quality is stable and reliable, and market demand is strong. The implementation of this invention can not only solve the environmental problems of enterprises but also create significant economic benefits, promoting the industry's transformation from "end-of-pipe treatment" to "source resource utilization," and is of great significance to achieving "dual carbon" goals and green sustainable development.

[0085] Any aspects of this invention not described in detail are well-known to those skilled in the art.

[0086] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications and equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing high-purity, large-particle fluorosilicates using acid-washing fluorine-containing waste liquid, characterized in that, Includes the following steps: Step 1: The fluoride-containing wastewater generated from the pickling process is directly transported to the reaction vessel without wastewater treatment; the fluoride-containing wastewater contains hydrofluoric acid and hydrochloric acid; the fluoride ion concentration in the wastewater is measured. Step 2: Add silicon dioxide to the reaction vessel from Step 1, stir and react. The reaction equation is 6HF + SiO2 → H2SiF6 + 2H2O. After the reaction is complete, filter and keep the filtrate for later use. Step 3: Directly use chloride powder, or prepare a chloride powder solution with a mass concentration of 20%~32%, wherein the chloride is potassium chloride or sodium chloride; when potassium chloride is used, prepare a saturated potassium chloride solution with a mass concentration of not less than 20%; when sodium chloride is used, prepare an industrial-grade sodium chloride solution with a mass concentration of 25%~30%. Step 4: Add the chloride powder or chloride solution from Step 3 to the filtrate from Step 2, control the reaction temperature at 40℃, and keep the stirring speed below 100 rpm; if the room temperature is low, use heating to maintain a constant temperature. When the chloride salt is potassium chloride, potassium fluorosilicate is formed. The chemical reaction formula is: 2KCl + H2SiF6 → K2SiF6 + 2HCl. When the chloride salt is sodium chloride, sodium fluorosilicate is produced. The chemical reaction formula is: 2NaCl + H2SiF6 → Na2SiF6 + 2HCl. Step 5: Add the pre-prepared fluorosilicate seed crystals to the fluorosilicate precursor solution obtained in Step 4, and continue stirring until the reaction is complete to induce the formation of large-particle fluorosilicate crystals. Step 6: Inject the reaction solution from Step 5 into a vacuum filtration device for vacuum filtration under reduced pressure. Wash the filtered solid with a small amount of water several times to obtain a fluorosilicate solid with a purity greater than 99.5%. The filtrate is a hydrochloric acid solution containing trace amounts of inorganic salts. Step 7: Add calcium chloride solution to the hydrochloric acid filtrate from Step 6. After the reaction, filter to remove sulfate ions. The reaction equation is SO42-. 2- + CaCl2→CaSO4+2Cl - Then, electrodialysis is used to remove nitrate and residual fluoride ions from the filtrate. Step 8: Add hydrochloric acid and aluminum hydroxide to the solution after purification in Step 7. The amount of hydrochloric acid added should be 20%~30% by mass, and the amount of aluminum hydroxide added should be 0.5~0.7 equivalents of the hydrochloric acid in the solution. Place the mixture in a sealed container and heat it to 140℃. During the heating process, the water vapor evaporated should be condensed and refluxed into the reaction system. The reaction equation is (6-n)HCl + 2Al(OH)3 → (6-n)H2O + Al2(OH) n Cl (6-n) After the reaction is complete, cool to 90°C and filter under reduced pressure to obtain a polyaluminum chloride solution.

2. The method according to claim 1, characterized in that, The reaction vessel used in step one is made of polyethylene (PE) or polypropylene (PP).

3. The method according to claim 1, characterized in that, In step two, the mass ratio of silicon dioxide added to hydrofluoric acid in the waste liquid is 1.17~1.20:1, the stirring reaction time is 0.5~1 hour, and the stirring speed is 50~80 rpm.

4. The method according to claim 1, characterized in that, The amount of chloride salt added in step four is determined based on the fluoride ion content in the waste liquid.

5. The method according to claim 1, characterized in that, The amount of seed crystals added in step five is 1% of the theoretical yield of fluorosilicate.

6. The method according to claim 5, characterized in that, The fluorosilicate seed crystals have a particle size of 50~100μm. The seed crystals are prepared by dissolving industrial-grade fluorosilicate, recrystallizing it, and then sieving it to obtain crystals of the target particle size.

7. The method according to claim 1, characterized in that, The water washing in step six uses deionized water. The amount of water used for each wash is 5% to 10% of the mass of the fluorosilicate solid. The washing method is rinsing to avoid the solid crystals being dispersed. The negative pressure of the vacuum filtration is 0.8 MPa to 1.5 MPa.

8. The method according to claim 1, characterized in that, The electrodialysis method described in step seven uses a Corning P1 electrodialysis device, with the current intensity controlled at 0.1A~0.15A and the electrodialysis time at 2~4 hours. After treatment, the fluoride ion concentration in the filtrate is less than 0.5mg / L and the nitrate ion concentration is less than 1mg / L.

9. The method according to claim 1, characterized in that, The aluminum hydroxide used in step eight is aluminum hydroxide powder of grade AH-2d conforming to the national standard GB / T4294-2010, with a particle size of 100~200 mesh, to ensure a complete reaction.

10. The method according to claim 1, characterized in that, The polyaluminum chloride solution described in step eight meets the requirements of the national standard GB / T22627-2022 "Water Treatment Agent Polyaluminum Chloride", with an alumina mass fraction of ≥8% and a basicity of 20%~50%.

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