Enhanced treatment method and device for recycling fluorine resources in semiconductor processing wastewater

Abstract

The invention belongs to the technical field of industrial wastewater treatment and resource recovery, and particularly relates to an enhanced treatment method for recovering fluorine resources in semiconductor processing fluorine-containing wastewater. And step-by-step efficient recovery of sodium fluosilicate and calcium fluoride is realized through cooperation of precipitant compounding and pH regulation. The method comprises the following steps: firstly, adding a NaCl-NaHCO3 composite precipitator and sodium fluosilicate seed crystals to induce crystallization and recover hexagonal prism-shaped sodium fluosilicate single crystals; and after filtering, adding a CaCl2-Ca (OH) 2 composite precipitator and calcium fluoride seed crystals into the filtrate to generate large-particle calcium fluoride. And the impurity content in the recovered product respectively reaches the industrial sodium fluosilicate superior product standard and the metallurgical grade calcium fluoride secondary product standard. And the treated wastewater is neutral or weakly alkaline, so that the subsequent mixed treatment with other wastewater is facilitated.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of industrial wastewater treatment and resource recovery, and particularly relates to a strengthened treatment method for fluorine resource recovery in semiconductor processing fluorine-containing wastewater. BACKGROUND

[0002] In recent years, China's photovoltaic industry has developed rapidly, and the industrial scale has continued to expand. With the increase in production capacity, fluorine-containing wastewater generated in the process of semiconductor (photovoltaic, chip, etc.) processing and manufacturing has become a pain point in the industry. If such wastewater is not effectively treated before being directly discharged, fluorides will accumulate in the soil for a long time and eventually seep into groundwater, causing serious environmental and health problems.

[0003] Fluoride, as an important strategic resource, has irreplaceable value in the fields of semiconductor manufacturing, optical devices, medicine, and pesticides. Among them, fluorite (CaF2) as the main fluorine source has faced global depletion, and high-quality fluorite mines in China have a low proportion, making it difficult to exploit and utilize. Sodium fluorosilicate (Na2SiF6) is a highly effective insecticide, preservative, and glass etching agent raw material.

[0004] Therefore, the recycling of fluorine resources is of great significance. Concentrated acid wastewater in photovoltaic wastewater usually has a large amount of fluorine, fewer impurity ions, and small water volume, so it is usually first subjected to fluorine resource recovery.

[0005] Chemical precipitation is considered the most widely used method for resource recovery of fluorine-containing wastewater. Induced crystallization process is an optimization of traditional precipitation reaction. It is suitable for resource recovery of fluorine in concentrated acid wastewater. For high-concentration fluorine-containing wastewater, fluorine removal and recovery are usually achieved by adding calcium salts (Ca(OH)2, CaCl2, etc.) or aluminum salts. Chinese patent CN112624388A uses sodium metaaluminate to remove fluorine ions and silicon dioxide and silicon-containing compounds in water, obtaining silicon-containing and fluorine-containing crystals. Although it can effectively remove fluorine ions and silicon-containing compounds, it cannot obtain silicon-containing and fluorine-containing crystals separately and cannot be resourcefully utilized. Chinese patent CN117700041A adjusts the pH of the influent by detecting the pH of the effluent to improve efficiency, but an additional influent pH adjustment module is required. Chinese patent CN106746113A recovers fluorosilicate and calcium fluoride from concentrated acid photovoltaic wastewater, but cannot control the pH of the wastewater. In addition, calcium chloride is a common precipitant for fluorine-containing photovoltaic wastewater to recover fluorine resources. However, due to the low pH of concentrated acid wastewater and the high concentration of fluorosilicate, existing technologies have problems such as poor recovery effect (low recovery rate), high impurity content of products, low product purity, and small particle size, which cannot meet the industrial grade standard.

[0006] Therefore, it is necessary to develop a green treatment method capable of simultaneously achieving high fluorine recovery rate (>95%), high product purity (>99%) and controllable crystal morphology in an acidic high-silicon environment. SUMMARY

[0007] In view of the above problems existing in the prior art, the present application provides a technology for adjusting pH and strengthening fluoride recovery by compounding precipitants. By changing the adding ratio of the alkaline precipitant and the conventional precipitant, the pH of the concentrated acid wastewater is adjusted to the optimal state, so that high-quality fluoride products are recovered from the concentrated acid wastewater containing fluorine in steps, and the resource recovery of fluorine is realized; at the same time, the pH of the effluent is adjusted to be close to neutral.

[0008] The first aspect of the present application provides a method for strengthening the treatment of fluorine resources in semiconductor processing wastewater, comprising the following steps: S1. Adding sodium fluosilicate seeds and NaCl-NaHCO3 composite precipitants to the fluorine-containing silicon acid semiconductor processing wastewater, and controlling the reaction end point pH value to be 3.4-4.2; S2. Separating the generated sodium fluosilicate crystals to obtain a first treatment liquid; S3. Adding calcium fluoride seeds and CaCl2-Ca(OH)2 composite precipitants to the first treatment liquid, and controlling the reaction end point pH value to be 5.5-9.6; S4. Separating the generated calcium fluoride crystals.

[0009] In some embodiments, the molar ratio of NaCl to NaHCO3 in the NaCl-NaHCO3 composite precipitants in step S1 is 0.8:1-1.2:1; In some embodiments, the molar ratio of CaCl2 to Ca(OH)2 in the CaCl2-Ca(OH)2 composite precipitants in step S3 is 1:1.5-1:3.

[0010] In some embodiments, the adding amount of sodium fluosilicate seeds in step S1 is 8-12 g / L, In some embodiments, the adding amount of calcium fluoride seeds in step S3 is 4-6 g / L.

[0011] In some embodiments, in step S1, the stirring rate is 200-380 r / min, and the reflux rate is 5-15 mL / min; In some embodiments, in step S3, the stirring rate is 380-420 r / min, and the reflux rate is 8-12 mL / min.

[0012] In some embodiments, the total sodium ion adding amount of the NaCl-NaHCO3 composite precipitants in step S1 is 100%-120% in excess of the molar concentration of silicon in the wastewater, preferably 100%.

[0013] In some embodiments, the excess ratio of the total calcium ion dosage of the CaCl2-Ca(OH)2 complex precipitant in step S3 to the molar concentration of fluoride ions in the wastewater is 60%-100%.

[0014] In some embodiments, the reaction time in step S1 is 50-70 minutes, and the reaction time in step S3 is 50-70 minutes.

[0015] In some embodiments, in step S1, the stirring rate is 200-400 r / min; preferably 300 r / min.

[0016] In some embodiments, in step S1, the reflux rate is 5-15 mL / min; preferably 5 mL / min.

[0017] In some embodiments, in step S3, the stirring rate is 200-400 r / min; preferably 300 r / min.

[0018] In some embodiments, in step S3, the reflux rate is 10-30 mL / min; preferably 10 mL / min.

[0019] In some preferred embodiments, in step S1, the control reaction end point pH value is 3.82±0.4; In some preferred embodiments, in step S3, the control reaction end point pH value is 8.54±0.5; In a preferred embodiment, in step S1, the control reaction end point pH value is 3.82±0.1, and the obtained sodium fluorosilicate crystal is a hexagonal prism single crystal structure, with a purity of ≥99.5% and a Cl⁻ content of ≤0.15%.

[0020] In a preferred embodiment, in step S3, the control reaction end point pH value is 8.54±0.1; In some embodiments, in step S2, the recovered sodium fluorosilicate crystal is a regular hexagonal prism single crystal, and the D50 is ≥43 μm; In some embodiments, in step S4, the recovered calcium fluoride crystal has a D50 particle size of ≥23 μm, and the proportion below 1 μm is ≤3%.

[0021] The fluorine-containing silicic acid-containing semiconductor processing wastewater has a fluoride ion concentration of ≥5000 mg / L, a silicon concentration of ≥1000 mg / L, and an initial pH of ≤3.0.

[0022] The second aspect of the present application provides a device for recovering fluoride resources from strong acid wastewater in semiconductor processing, which is used to implement the method described above, and the device comprises: Reaction vessel: with a cylindrical upper section and a conical lower section; Stirring unit: comprising a stirring shaft extending vertically along the axis of the reaction vessel, the stirring shaft penetrating the liquid phase of the vessel body and having a terminal end connected to a stirring paddle; Dosing system: connecting the precipitant storage tank to the top of the reaction vessel; Reflux system: connecting the liquid inlet to the upper part of the vessel and the liquid outlet to the bottom of the vessel, forming an anti-settling circulation loop; The cylindrical section of the reaction vessel is provided with a sampling port, and the conical section of the reaction vessel is provided with a discharge port.

[0023] In some embodiments, the recovery system is a sequencing batch type, after the reaction in step S1 is completed, the reaction liquid is discharged from the discharge port, and the sodium fluorosilicate crystals and the first treatment liquid are obtained by filtration, after the reactor is cleaned, the first treatment liquid is readded to the reaction vessel, and the reaction in step S3 is carried out.

[0024] Beneficial effects: The application provides a method for strengthening the treatment of fluorine resources in semiconductor processing wastewater and a device for implementing the method. For concentrated acid wastewater in the wastewater, the recovery of fluorine resources is strengthened by the complexing and synergistic pH control of precipitants, the efficient treatment of concentrated acid fluorine-containing wastewater in semiconductor processing and the recovery of fluorine resources are realized, and the recovery rate of fluorine in the wastewater is greatly improved.

[0025] (1) By adding an alkaline precipitant instead of part of the conventional precipitant, the pH of the concentrated acid wastewater is adjusted, and the pollutant removal effect is improved, and the best precipitant complex combination is selected: NaCl+NaHCO3 as a precipitant for removing fluorosilicate; CaCl2+Ca(OH)2 as a precipitant for removing fluorine ions.

[0026] (2) By using NaCl+NaHCO3 as a precipitant for removing fluorosilicate to adjust the pH, the removal rate of SiF6 2- can reach 90.03%, and the purity of the recovered sodium fluorosilicate product reaches 99.55%; by using CaCl2+Ca(OH)2 as a precipitant for removing fluorine ions to adjust the pH, the removal effect of F - can reach 97.96%, and the purity of the recovered calcium fluoride can reach 86.03%.

[0027] (3) The method is used for fluorine resource recovery of actual wastewater, and the impurity content in the recovered product reaches the industrial sodium fluorosilicate superior product standard and the metallurgical grade calcium fluoride secondary product standard, respectively.

[0028] (4) The wastewater obtained after the process treatment of the application is neutral or weakly alkaline, which is conducive to subsequent mixed treatment with other wastewater.

[0029] (5) Crystal size advantage: the application realizes the breakthrough optimization of crystal size and morphology through the compounding of precipitants and the precise control of pH. Sodium fluosilicate: form smooth and complete hexagonal prism single crystals, and the proportion of more than 45 mu m is 51.25%; the purity is greater than 99.5%, which meets the standard of optical grade raw materials; calcium fluoride: generate dense cubic crystals (D50 is greater than or equal to 23 mu m), which can be directly used as a metallurgical fluxing agent. BRIEF DESCRIPTION OF DRAWINGS

[0030] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of the specification, illustrate embodiments of the application and are used to explain the application, and do not constitute a limitation on the application. In the drawings: Figure 1 Flow chart of the fluorine resource recovery method of the application; Figure 2 Schematic diagram of the fluorine resource recovery device of the application; Figure 3 Schematic diagram of the fluorine resource recovery method of the application; Figure 4 a The influence of the precipitant dosage on the removal of fluoride ions by induced crystallization; b The influence of the precipitant dosage on the removal rate of fluoride ions and the concentration of sodium ions; Figure 5 a The influence of different precipitant compounding on the removal effect of fluoride ions; b The removal efficiency of fluoride ions and the pH after reaction under different precipitant compounding conditions; Figure 6 XRD and chlorine content analysis of the recovered products under different pH conditions; Figure 7 SEM images of the recovered products of Comparative Example 1 (a), Example 2 (b) and Example 6 (c) in step (1); Figure 8 a The influence of the precipitant dosage on the removal of fluoride ions by induced crystallization; b The influence of the precipitant dosage on the removal rate of fluoride ions and the concentration of sodium ions; Figure 9 a The removal efficiency of fluoride ions under different precipitant compounding conditions; b The particle size distribution of the recovered products under different precipitant compounding conditions; Figure 10 a The concentration changes of F- and Ca2+ under different pH conditions; b Crystallization kinetics analysis under different precipitant compounding conditions; c XRD analysis of the recovered products under different precipitant compounding conditions. Figure 11 a SEM images of the recovered products under the condition of pH=3.68 after reaction; b SEM images of the recovered products under the condition of pH=8.54 after reaction; c SEM images of the recovered products under the condition of pH=10.08 after reaction; Figure 12aSilicon content in solution after recovery of calcium fluoride by induced crystallization under different pH conditions; bSilicon content in recovery product under different pH conditions; cChlorine content in recovery product under different pH conditions; Figure 13 aRemoval of fluorosilicate by induced crystallization of actual wastewater; bRemoval of fluoride by induced crystallization of actual wastewater. DETAILED DESCRIPTION

[0031] In order to make the above objectives, features and advantages of the present application more apparent, specific embodiments of the present application are described in detail below with reference to the description and examples. In the following description, a large number of specific details are set forth in order to provide a sufficient understanding of the present application, but the present application can also be implemented in other manners different from those described herein, and those skilled in the art can make similar generalizations without departing from the concept of the present application, so the present application is not limited to the specific examples disclosed below. In the following examples, the experimental methods are described, and unless otherwise specified, all are conventional methods; and the reagents and instruments, unless otherwise specified, can be obtained commercially.

[0032] The experimental instruments used in the examples mainly include: analytical balance, pH meter, fluoride ion electrode, stirring paddle, ultrapure water machine, oven, spectrophotometer, vacuum filtration machine, ultrasonic cleaner.

[0033] The experimental reagents mainly include: hydrofluoric acid, fluorosilicic acid, sodium fluorosilicate, sodium fluoride, sodium chloride, sodium bicarbonate, sodium hydroxide, calcium chloride, calcium hydroxide, etc.

[0034] Simulated wastewater: fluorosilicic acid and hydrofluoric acid solution were added to water to obtain simulated wastewater with a fluorine ion concentration of 9000 mg / L and a silicon concentration of 2000 mg / L.

[0035] Actual wastewater: wastewater from a certain photovoltaic enterprise in Inner Mongolia: initial pH 2.30, initial Si content 1972.29 mg / L, initial F content 5676.25 mg / L.

[0036] The analysis methods used in the examples are as follows: 0.22 μm filter head was used for filtration, and the remaining silicon content was determined by silicon molybdenum blue spectrophotometry; 0.45 μm filter head was used for filtration, and the fluoride ion content was determined by fluoride ion selective electrode; The purity of recovered sodium fluorosilicate was determined by titration method; The purity of recovered calcium fluoride was determined by fluoride ion selective electrode; The chloride content in the recovery product was determined by silver nitrate titration method; The phosphate content in the recovery product was determined by phosphomolybdenum blue spectrophotometry; The particle size distribution of the recovered products was analyzed using a laser particle size analyzer; the recovered samples were analyzed by SEM and XRD.

[0037] In one embodiment, the present invention provides a sequencing batch reactor (SBR) for fluorine resource recovery, such as... Figure 2 As shown, its main body is a conical-bottomed cylindrical reaction vessel. An adjustable speed stirring paddle is axially connected to the top, and the side wall is equipped with a sampling port with an integrated pH / fluoride ion probe and a seed crystal addition port. A discharge valve is installed at the bottom. The reaction vessel is connected to an external reflux pump through a closed-loop pipeline to form an anti-settling circulation loop. At the same time, it is connected to the NaCl-NaHCO3 composite precipitant storage tank and the CaCl2-Ca(OH)2 composite precipitant storage tank through a metering pump system. The dosage, stirring rate, and reflux speed are linked and controlled by a programmable controller.

[0038] The operating procedure of the device is as follows: Figure 3 As shown: First, fluoride-containing wastewater is injected into the reactor, and sodium fluorosilicate seed crystals and primary composite precipitant are added. The reaction is carried out at pH 3.7-4.0 to generate hexagonal prism sodium fluorosilicate crystals. After the reaction is completed and the mixture is allowed to settle, the discharge valve is opened to discharge the crystals. After the liquid is drained, the reactor is cleaned. After completion, primary treated wastewater is injected again, and calcium fluoride seed crystals and secondary composite precipitant are added to form large-particle calcium fluoride precipitates. The high-purity product is directly discharged through the fully open discharge valve, without the need for centrifugation.

[0039] Example 1 (1) Removal of fluorosilicic acid from wastewater: 1 L of simulated wastewater (pH=2.5) was placed in the reactor, 10 g of sodium fluorosilicate seed crystals were added, and an excess of 100% sodium salt was added (the molar ratio of sodium bicarbonate to sodium chloride was 1:4.7; the sodium bicarbonate dosage was 4.2 g / L). The reactor was run for 1 h with a stirring rate of 300 r / min and a reflux rate of 5 mL / min. During the reaction, water samples were taken from the beginning of the reaction and filtered through a 0.22 μm pore size filter membrane. The SiF6 content in the water at different time points was determined by the silicomolybdenum blue spectrophotometric method. 2- Concentration. pH after reaction = 3.47.

[0040] After the reaction was completed, the wastewater was filtered, and the recovered particulate matter was dried in a 60°C oven for 24 hours to obtain the recovered sodium fluorosilicate product.

[0041] The performance parameters of the recovered sodium fluorosilicate are shown in the table below: Table 1-1 Table 1-2 Example 2 (1) Fluorosilicic acid removal in wastewater: Take 1 L of simulated wastewater (pH = 2.5) to the reactor, add 10 g of sodium fluorosilicate seed, add sodium salt to maintain an excess ratio of 100% (the molar ratio of sodium bicarbonate and sodium chloride is 1:1.87), operate the reactor at a stirring rate of 300 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, take water samples from the beginning of the reaction, filter them through a filter membrane with a pore size of 0.22 μm, and then determine the SiF6 2- concentration at different time points using the silicon molybdenum blue spectrophotometric method. After the reaction, the pH is 3.82.

[0042] After the reaction, filter the wastewater, dry the recovered particulate matter in a 60°C oven for 24 h, and obtain the recovered sodium fluorosilicate product; the SiF6 2- removal rate in the treated wastewater is 90.03%; and the purity of the recovered sodium fluorosilicate product reaches 99.55%.

[0043] Table 2-1 Table 2-2 (2) Filter the wastewater after removing the fluorosilicic acid in step (1), take 1 L of the filtered wastewater to the reactor, add 5 g of calcium fluoride seed, add an excess of calcium salt (the molar ratio of calcium chloride to calcium hydroxide is 1:1.5, and the calcium hydroxide is added at a dosage of 14.954 g / L), and operate the reactor at a stirring rate of 400 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, take water samples from the beginning of the reaction, filter them through a filter membrane with a pore size of 0.45 μm, and then determine the F- concentration in the water at different time points using a fluorine ion meter. After the reaction, the pH is 8.54.

[0044] After the reaction, filter the wastewater, dry the recovered particulate matter in a 60°C oven for 24 h, and obtain the recovered calcium fluoride product.

[0045] Table 2-3 Table 2-4 Example 3 (1) Fluorosilicic acid removal in wastewater: Take 1 L simulated wastewater (pH = 2.5) to the reactor, add 10 g of sodium fluorosilicate seed, add sodium salt (the molar ratio of sodium bicarbonate and sodium chloride is 1:0.9; the sodium bicarbonate dosage is 12.6 g / L), operate the reactor at a stirring speed of 300 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, take water samples from the beginning of the reaction, filter them through a filter membrane with a pore size of 0.22 μm, and then determine the SiF6 2- concentration at different time points using the silicon molybdenum blue spectrophotometric method. After the reaction, filter the wastewater, dry the recovered particulate matter in a 60°C oven for 24 h, and obtain the recovered sodium fluorosilicate product. The pH after the reaction is 4.17.

[0046] Table 3-1 Table 3-2 Example 4 Step (1) is the same as Example 2; (2) Filter the wastewater after removing the fluorosilicic acid in step (1), take 1 L of the filtered wastewater to the reactor, add 5 g of calcium fluoride seed, and add excess calcium salt with an excess ratio of 60% (the molar ratio of calcium chloride to calcium hydroxide is 1:2, and the calcium hydroxide dosage is 16.615 g / L), operate the reactor at a stirring speed of 400 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, take water samples from the beginning of the reaction, filter them through a filter membrane with a pore size of 0.45 μm, and then determine the F- concentration in water at different time points using a fluorine ion meter. The pH after the reaction is 9.56.

[0047] Filter the wastewater after the reaction, dry the recovered particulate matter in a 60°C oven for 24 h, and obtain the recovered calcium fluoride product.

[0048] Table 4-3 Table 4-4 Example 5 Step (1) is the same as Example 2; (2) The wastewater after removing fluosilicic acid in step (1) was filtered, 1 L of the filtered wastewater was taken into the reactor, 5 g of calcium fluoride seed was added, and an excess of calcium salt (the molar ratio of calcium chloride to calcium hydroxide was 1:3, and the amount of calcium hydroxide added was 18.692 g / L) was added. The reactor was operated at a stirring rate of 400 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, water samples were taken from the beginning of the reaction, filtered through a filter membrane with a pore size of 0.45 μm, and the F - concentration in the water at different time points was determined by fluorine ion meter. After the reaction, the pH was 10.8.

[0049] The wastewater after the reaction was filtered, and the recovered particulate matter was dried in a 60°C oven for 24 h to obtain the recovered calcium fluoride product.

[0050] Table 5-3 Table 5-4 Example 6 (1) Fluorosilicic acid removal in wastewater: 1 L of simulated wastewater (pH = 2.5) was taken into the reactor, 10 g of sodium fluosilicate seed was added, and sodium salt (excess ratio of 100%) was added, in which the amount of NaHCO3 added was 16.8 g / L. The reactor was operated at a stirring rate of 300 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, water samples were taken from the beginning of the reaction, filtered through a filter membrane with a pore size of 0.22 μm, and the SiF6 2- concentration in the water at different time points was determined by silicon-molybdenum blue spectrophotometry; after the reaction, the pH was 4.38.

[0051] The wastewater after the reaction was filtered, and the recovered particulate matter was dried in a 60°C oven for 24 h to obtain the recovered sodium fluosilicate product.

[0052] Table 6-1 Table 6-2 Example 7 (1) Fluorosilicic acid removal in wastewater: 1 L of simulated wastewater (pH = 2.5) was taken into the reactor, 10 g of sodium fluosilicate seed was added, and sodium chloride (the excess ratio was controlled at 40%, 50%, 80%, and 120%, respectively) was added. The reactor was operated at a stirring rate of 300 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, water samples were taken from the beginning of the reaction, filtered through a filter membrane with a pore size of 0.22 μm, and the SiF6 2-The wastewater after reaction was filtered, and the recovered particulate matter was placed in a 60°C oven to dry for 24 h to obtain the recovered sodium fluorosilicate product.

[0053] As shown in Figure 4 : increasing the dosage of the precipitant can improve the removal efficiency of fluorosilicate, but as the dosage increases, the removal efficiency decreases, which may be due to the excessive Na + concentration, the number of precipitation sites provided by the crystal surface is limited, and the solution is in a supersaturated state, which inhibits the growth rate of the crystal nucleus. When the excess ratio of the precipitant dosage is 100%, there is a higher removal efficiency of fluorosilicate, and the remaining Na + content is low.

[0054] Comparative Example 1 Single Precipitant NaCl (1) Removal of fluorosilicic acid in wastewater: 1 L of simulated wastewater (pH = 2.5) was placed in a reactor, 10 g of sodium fluorosilicate seed crystals were added, and NaCl was added to maintain an excess ratio of 100%. The reactor was operated at a stirring rate of 300 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, water samples were taken from the beginning of the reaction, filtered through a filter membrane with a pore size of 0.22 μm, and the SiF6 2- concentration in the water at different time points was determined by the silicomolybdenum blue spectrophotometric method. The pH after the reaction was 3.04.

[0055] The wastewater after reaction was filtered, and the recovered particulate matter was placed in a 60°C oven to dry for 24 h to obtain the recovered sodium fluorosilicate product.

[0056] Table 7-1 Table 7-2 Comparative Example 2 NaCl + NaOH (1) Removal of fluorosilicic acid in wastewater: 1 L of simulated wastewater (pH = 2.5) was placed in a reactor, 10 g of sodium fluorosilicate seed crystals were added, and sodium salts were added to maintain an excess ratio of 100% (the molar ratio of sodium hydroxide to sodium chloride was 1:1.87), and the reactor was operated at a stirring rate of 300 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, water samples were taken from the beginning of the reaction, filtered through a filter membrane with a pore size of 0.22 μm, and the SiF6 2- concentration in the water at different time points was determined by the silicomolybdenum blue spectrophotometric method. The pH after the reaction was 3.04.

[0057] As shown in Figure 5As shown, the efficiency of NaOH and NaCl as precipitants in removing fluorosilicate is significantly lower than that of the compound precipitant (NaHCO3 and NaCl) of this invention, and also significantly lower than that of NaCl as a precipitant. It was also observed that the sodium fluorosilicate product obtained using NaOH and NaCl as precipitants had fewer large-diameter particles than that obtained using other compound precipitants. The reason why NaHCO3 and NaCl are more efficient at removing fluorosilicate is that sodium bicarbonate acts as a pH buffer after its addition. Under acidic conditions, sodium bicarbonate not only provides Na... + It also consumed a certain amount of H + This promotes the dissociation of fluorosilicic acid to generate more fluorosilicate ions, thereby promoting the formation of sodium fluorosilicate precipitate.

[0058] like Figure 6 Data shows that using NaHCO3 to replace part of NaCl as a precipitant reduced the nucleation rate of sodium fluorosilicate and increased the crystal growth rate; the low nucleation rate and high growth rate promoted the formation of large crystal particles. XRD analysis revealed that Na2SiF6 crystal faces were observed under different pH conditions. The largest crystal size and better crystallinity were observed at pH 3.82. With increasing pH, the chloride content in the recovered product gradually decreased.

[0059] like Figure 7 In Comparative Example 1, amorphous crystals formed under low pH conditions; as the pH increased after the reaction, the morphology of the recovered products in Examples 2 and 6 changed from rough and porous to smooth and flat, exhibiting a hexagonal prism shape; this was due to the H+ under low pH conditions. + It readily adsorbs onto the crystal surface, interfering with the arrangement of ions on the crystal surface, leading to the formation of rough and porous crystals. Under low pH conditions, it exhibits a lower crystal growth rate and a higher nucleation rate, and the H2 adsorbed on the crystal surface... + This process protonates the crystals, weakens the electrostatic repulsion between particles, leading to explosive nucleation and the formation of amorphous particles. As pH increases, H... + With reduced interference, it is easier to form crystal particles with fewer surface defects and better dispersion.

[0060] Table 8 As shown in Table 8, the best fluorosilicate removal effect was achieved under the conditions of Example 2 (pH=3.82 after the reaction), because when pH>3.8, all H2SiF6 in the solution dissociates into SiF6. 2- This provided sufficient reactants for crystallization, promoting the crystallization of SiF6. 2- with Na + Precipitation; however, excessively high pH values ​​may also cause SiF6 to precipitate. 2- Hydrolysis of SiF6, thereby inhibiting its hydrolysis. 2- Remove.

[0061] Example 8 Step (1) same as Example 2; (2) The wastewater after removing fluorosilicic acid in step (1) was filtered, 1 L of the filtered wastewater was taken into the reactor, 5 g of calcium fluoride seed was added, and excess calcium chloride was added (the excess of calcium was controlled at 20%, 60%, 100%, 140%, and 180%, respectively), the reactor was operated at a stirring rate of 400 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, water samples were taken from the beginning of the reaction, filtered through a filter membrane with a pore size of 0.45 μm, and the F- concentration in the water at different time points was determined using a fluorine ion meter. After the reaction, the wastewater was filtered, the recovered particulate matter was dried in a 60°C oven for 24 h, and the recovered calcium fluoride product was obtained.

[0062] As shown in Figure 8 , increasing the dosage of the precipitant can improve the removal effect of fluorine ions, but as the dosage increases, the removal efficiency improves at a decreasing rate. Under the condition of excessive Ca + concentration, the number of precipitation sites provided by the surface of the seed is limited, and the solution is in a supersaturated state, which inhibits the growth rate of the crystal nucleus. And excessive dosage of precipitant leads to an increase in the residual concentration of Ca + in the solution.

[0063] Comparative Example 3 Single precipitant CaCl2 Step (1) same as Example 2; (2) The wastewater after removing fluorosilicic acid in step (1) was filtered, 1 L of the filtered wastewater was taken into the reactor, 5 g of calcium fluoride seed was added, and excess calcium chloride was added (the excess of calcium was controlled at 20%, 60%, 100%, 140%, and 180%, respectively), the reactor was operated at a stirring rate of 400 r / min and a reflux rate of 5 mL / min for 1 h. During the reaction, water samples were taken from the beginning of the reaction, filtered through a filter membrane with a pore size of 0.45 μm, and the F- concentration in the water at different time points was determined using a fluorine ion meter. After the reaction, the wastewater was filtered, the recovered particulate matter was dried in a 60°C oven for 24 h, and the recovered calcium fluoride product was obtained. - concentration. After the reaction, the pH was 3.52.

[0064] After the reaction, the wastewater was filtered, the recovered particulate matter was dried in a 60°C oven for 24 h, and the recovered calcium fluoride product was obtained. The particle size data is shown in the following table.

[0065] Table 9-3 Table 9-4 Example 9 Step (1) same as Example 2 The excess ratio of calcium in step (2) is 60%, and the ratio of calcium chloride to calcium hydroxide is 3:1, 2:1, and 1:1, respectively. The remaining reaction conditions are the same as in Example 2; the pH after the reaction is 3.68, 4.48, and 5.65, respectively.

[0066] As shown in Figure 9 : CaCl2: Ca(OH)2 is 1:1~1:3, which has a high fluorine ion removal efficiency, but the high proportion of Ca(OH)2 addition will lead to a decrease in the purity of the recovered product, which may be due to the low solubility of Ca(OH)2, resulting in impurities in the recovered product. When CaCl2: Ca(OH)2 is 1:2~1:3, the recovered product has a high proportion of large particles. The D50 particle size is 23.14 μm.

[0067] As shown in Figure 10 , the reaction after the pH is 5.65~10.80 has a high fluorine ion removal efficiency. With the increase of the proportion of Ca(OH)2 addition, the growth rate of calcium fluoride crystals increases, and the nucleation rate slows down, so that the product recovered under high pH conditions has a high proportion of large particles. From the XRD analysis, it can be concluded that under different pH conditions, the crystal surface of CaF2 is observed.

[0068] As shown in Figure 11 , under the condition of low pH=3.68, more small particle products are formed, under the condition of pH=8.54, larger particle distribution is observed, and under the condition of pH=10.08, more small particle products are observed. Under the condition of low pH, excessive H + may be preferentially adsorbed on the surface of the crystal, hindering the diffusion and arrangement of ions in that direction, resulting in hindered crystal growth. At the same time, it may cause the protonation of the crystal surface, weakening the electrostatic attraction between the crystal and Ca 2+ in the solution, inhibiting the ordered deposition of Ca 2 on the crystal surface, resulting in a decrease in the growth rate of the crystal and the formation of small particle products. Under the condition of high pH, the increase of F - concentration may trigger rapid nucleation and generate a large number of small crystal nuclei due to the high initial supersaturation. And the increase of OH - may cause Ca 2+ to generate Ca(OH)2 or CaOH + , which is consumed, and the concentration of free Ca 2 ⁺ decreases sharply, and the surface of the crystal nucleus does not have enough ion supply, and the growth rate decreases, ultimately leading to the formation of small particle products.

[0069] As shown in Figure 12 , the silicon content in the solution after the recovery of calcium fluoride by induced crystallization under different conditions; and the silicon content / chlorine content in the recovered product. The high pH of the solution increases the remaining silicon content, and the silicon content in the recovered product decreases, which is due to the fact that under alkaline conditions, more OH- The presence of the remaining SiF6 inhibited 2- With Ca 2+ The combination of SiF6 reduces 2- The removal of chlorides and the formation of CaSiF6. As pH increases, the chloride content in the recovered product gradually decreases.

[0070] Table 10 Example 10: Actual Wastewater Verification Wastewater: Wastewater from a photovoltaic factory in Inner Mongolia (pH=2.30, Si=1972.29 mg / L, F⁻=5676.25 mg / L) (1) Removal of fluorosilicic acid from wastewater: The parameters for this step are the same as in Example 2, maintaining an excess sodium salt ratio of 100% and controlling the pH after the reaction to 3.82.

[0071] (2) Fluoride ion removal: Specific parameters are the same as in Example 2, maintaining Ca 2+ The excess ratio was 60%, and the pH was controlled at 8.54 after the reaction.

[0072] Results: The removal rate of fluorosilicic acid in the actual wastewater after step (1) reached 80.28%; the removal rate of fluoride ions in step (2) reached 98.08%; the purity of the recovered sodium fluorosilicate reached 98.59%, and the purity of calcium fluoride reached 83.31%. The impurity content in the recovered products met the standards for superior grade industrial sodium fluorosilicate and secondary grade metallurgical calcium fluoride, respectively. The particle size data are shown in the table below.

[0073] Table 11-1 Impurity Content of Sodium Fluorosilicate Products Recovered from Actual Wastewater Table 11-2 Impurity Content of Calcium Fluoride Products Recovered from Actual Wastewater In summary, the data comparison demonstrates that the sodium fluorosilicate recovered by the method of this invention exhibits a breakthrough optimization in crystal size and morphology compared to conventional methods. It produces smooth, complete hexagonal prism single crystals while maintaining a high proportion of large particles, with a D50 particle size exceeding 43 μm. The recovered calcium fluoride shows a significant advantage in particle size compared to conventional methods, with a D50 particle size ≥23 μm (compared to only 12 μm in the comparative example). Simultaneously, it has a low proportion of small particles, with particles smaller than 1 μm accounting for less than 3% of the mass (compared to over 10% in the comparative example).

[0074] The application provides a resource recycling method of photovoltaic fluorine-containing wastewater, and realizes stepwise efficient recovery of sodium fluorosilicate and calcium fluoride through complexing of a precipitant and pH regulation.

[0075] The application replaces part of the conventional precipitants with alkaline precipitants (NaHCO3 / Ca(OH)2) to realize pH self-buffering regulation, determines the optimal pH window (3.82±0.1 for recovering sodium fluorosilicate and 8.54±0.1 for recovering calcium fluoride), and makes the impurity content of the product reach the industrial superior product standard, so that the treated wastewater is neutral / weakly alkaline and can be directly mixed for treatment.

[0076] The application obtains the best SiF6 2- removal effect (90.03%) and the purity of the recovered sodium fluorosilicate reaches 99.55%; and when the pH is adjusted to 8.54 through complexing of precipitants, the method has a higher F - removal effect (97.96%); and the purity of the recovered calcium fluoride reaches 86.03%.

[0077] It should be noted that the above examples are only used to illustrate the technical solutions of the application and not to limit the application, and although the application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the application can be modified or replaced equivalently without departing from the spirit and scope of the technical solutions of the application, and all should be covered in the scope of the application.

Claims

1. An enhanced treatment method for fluoride resource recovery from semiconductor processing wastewater, characterized in that: Includes the following steps: S1. Add sodium fluorosilicate seed crystals and NaCl-NaHCO3 composite precipitant to photovoltaic wastewater containing fluorosilicic acid, and control the pH value at the final reaction endpoint to be 3.4-4.2; S2. Separate the generated sodium fluorosilicate crystals to obtain the first treatment solution; S3. Add calcium fluoride seed crystals and CaCl2-Ca(OH)2 composite precipitant to the first treatment solution, and control the pH value at the reaction endpoint to be 5.5-9.6; S4. Separate the calcium fluoride crystals generated.

2. The method according to claim 1, characterized in that: In step S1, the molar ratio of NaCl to NaHCO3 in the NaCl-NaHCO3 composite precipitant is 0.8:1-1.2:

1. And / or, the molar ratio of the CaCl2-Ca(OH)2 composite precipitant in step S3 is 1:1.5-1:

3.

3. The method according to claim 1, characterized in that: In step S1, the amount of sodium fluorosilicate seed crystals added is 8-12 g / L. And / or, the amount of calcium fluoride seed crystals added in step S3 is 4-6 g / L.

4. The method according to claim 1, characterized in that: In step S1, the stirring rate is 200-380 r / min and the reflux rate is 5-15 mL / min; And / or, in step S3, the stirring rate is 380-420 r / min and the reflux rate is 8-12 mL / min.

5. The method according to claim 1, characterized in that: The total sodium ion dosage of the NaCl-NaHCO3 composite precipitant in step S1 is 100%-120% excess relative to the silicon molar concentration in the wastewater, preferably 100%. In step S3, the total calcium ion dosage of the CaCl2-Ca(OH)2 composite precipitant is 60%-100% excess relative to the molar concentration of fluoride ions in the wastewater.

6. The method according to claim 1, characterized in that: The reaction time for step S1 is 50-70 minutes. And / or, the reaction time for step S3 is 50-70 minutes.

7. The method according to claim 1, characterized in that: In step S1, the final pH value of the reaction is controlled to be 3.82 ± 0.4; And / or, in step S3, the final pH value of the reaction is controlled to be 8.54 ± 0.

5.

8. The method according to claim 1, characterized in that: In step S2, the recovered sodium fluorosilicate crystals are regular hexagonal prism single crystals with D50≥43μm; And / or, in step S4, the recovered calcium fluoride crystals have a D50 particle size ≥ 23 μm, and the proportion of particles smaller than 1 μm is ≤ 3%.

9. A device for recovering fluoride resources from strong acid wastewater in semiconductor processing, the recovery device being used to implement the method according to any one of claims 1-6, characterized in that, include Reaction vessel: has a cylindrical upper section and a conical lower section; Stirring unit: includes a stirring shaft extending vertically along the central axis of the reaction vessel, the stirring shaft passing through the liquid phase of the vessel body, and the end connected to a stirring paddle; Dosing system: connects the precipitant storage tank to the top of the reaction vessel; Reflux system: The inlet end is connected to the top of the container, and the outlet end is connected to the bottom of the container, forming an anti-settling circulation loop; The cylindrical section of the reaction vessel has a sampling port on its side wall, and the conical section of the reaction vessel has a discharge port.

10. The apparatus according to claim 9, characterized in that: The recovery system is a sequencing batch reactor. After the reaction in step S1 is completed, the reaction liquid is discharged from the outlet, filtered to obtain sodium fluorosilicate crystals and a first treatment liquid. After cleaning the reactor, the first treatment liquid is added back into the reaction vessel to carry out the reaction in step S3.

Images