Heterogeneous crystallization and anti-competitive adsorption coupled deep defluorination process

Abstract

The invention relates to the technical field of water treatment, in particular to a deep defluorination process with heterogeneous crystallization and anti-competitive adsorption coupled. The process comprises the following steps: sequentially adding an initial calcium feeding solution and a citrate modified solution into the fluorine-containing wastewater, regulating the pH value of a system in stages, then sequentially adding a secondary calcium feeding solution and a third calcium supplementing solution, mixing, standing and separating to obtain HC unit effluent and bottom sediment; carrying out resuspension and standing separation on the bottom sediment, and then discarding supernate to obtain seed crystal wet slurry; and deep fluorine removal is carried out on effluent of the HC unit. According to the method, the out-phase nucleation orderliness of the surface of the calcium fluoride seed crystal can be improved, the seed crystal with concentrated particle size and excellent settling performance is obtained, favorable water inlet conditions with low suspended solids and low residual calcium are provided for a subsequent adsorption unit, and stable and deep fluorine removal of fluorine-containing wastewater is realized.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a deep defluorination process coupled with heterogeneous crystallization and anti-competitive adsorption. Background Technology

[0002] Fluoride-containing wastewater is widely generated in semiconductor manufacturing, photovoltaic industry, metal smelting, electroplating, and chemical production. If fluoride ions are discharged into natural water bodies without effective treatment, they will not only disrupt the ecological balance and cause soil and groundwater pollution, but may also accumulate through the food chain, causing serious damage to human bones and teeth, and even leading to diseases such as fluorosis. With increasingly stringent national environmental protection requirements, the limits for fluoride ion concentration in industrial wastewater discharge standards have generally been raised to below 1.0 mg / L. Therefore, developing efficient, stable, and economical deep defluorination technologies has become a critical issue that relevant industries urgently need to address.

[0003] Currently, chemical precipitation is a common method for treating high-concentration fluoride wastewater. This involves adding precipitants such as calcium salts to the wastewater, causing fluoride ions to separate from the liquid phase as calcium fluoride. However, traditional precipitation methods have significant limitations: the resulting calcium fluoride precipitates are mostly amorphous or fine crystals with poor settling properties, leading to difficulties in solid-liquid separation. The effluent often contains a large amount of suspended solids, causing secondary exceedances of fluoride ion standards. To improve treatment efficiency, excessive amounts of calcium salts are often added in engineering projects, but this introduces new calcium ion pollution and increases subsequent treatment costs and sludge production.

[0004] To overcome the shortcomings of homogeneous precipitation, induced crystallization (or heterogeneous crystallization) technology has emerged. This technology involves adding seed crystals to the reaction system to provide a nucleation and growth carrier for calcium fluoride, allowing the precipitate to grow epitaxially around the seed crystals, thereby forming larger, denser, and easier-to-settle crystals.

[0005] However, existing induced crystallization processes still face a series of challenges in practice. On the one hand, the active sites on the surface of the crystal seed are limited, and as the number of cycles increases, the surface will be covered by heterogeneous deposits, and the activity will gradually decrease, resulting in a decline in defluorination efficiency. The crystal seed needs to be frequently replaced, which increases operating costs. On the other hand, the existing process controls the crystallization process in a relatively crude manner, making it difficult to form a stable, uniform and highly active epitaxial layer on the surface of the crystal seed. This results in a wide particle size distribution and a high content of fine powder in the newly generated crystal seed particles. These fine powders are easily lost during solid-liquid separation, which reduces the crystal seed utilization rate and deteriorates the effluent water quality.

[0006] Furthermore, a single crystallization process often struggles to stably reduce the fluoride ion concentration in the effluent to below 1.0 mg / L, typically requiring combination with advanced treatment units such as adsorption. However, residual calcium ions, suspended particles, and incompletely precipitated fluoride colloids in conventional crystallization effluent can easily clog the pores of the adsorbent or occupy its active sites, leading to a rapid decline in adsorption capacity, frequent regeneration, and high operating costs.

[0007] Therefore, how to prepare calcium fluoride seed crystals with excellent sedimentation performance, high surface activity and strong cycle stability through precise process control, and make them work efficiently with subsequent deep processing units, is a long-standing but unsolved technical problem in this field. Summary of the Invention

[0008] In view of this, the purpose of this invention is to propose a deep defluorination process that couples heterogeneous crystallization with anti-competitive adsorption, so as to overcome the problems of insufficient control of seed surface activity and poor particle stability in existing induced crystallization processes, which leads to low defluorination efficiency, difficulty in solid-liquid separation, and difficulty in efficient synergy with deep treatment.

[0009] To achieve the above objectives, this invention provides a deep defluorination process coupled with heterogeneous crystallization and anti-competitive adsorption, comprising the following steps:

[0010] An initial calcium dosing solution and a citrate-modified solution were sequentially added to the fluoride-containing wastewater. Sodium hydroxide solution and glacial acetic acid solution were then added sequentially to adjust the pH of the system. Subsequently, a second calcium dosing solution and a third calcium supplement solution were added. After mixing and settling, the effluent from the HC unit and bottom sediment were obtained. The bottom sediment was resuspended, settling, and the supernatant was discarded to obtain a seed slurry. The effluent from the HC unit was then introduced into an adsorption tank filled with anti-competitive deep defluorination packing for deep defluorination treatment.

[0011] Preferably, the fluoride-containing wastewater originates from the wafer etching and cleaning processes in the semiconductor and photovoltaic industries or the hydrofluoric acid etching and sanding processes in the glass manufacturing industry.

[0012] Preferably, the fluoride ion concentration in the fluoride-containing wastewater from the glass manufacturing industry is 500-3000 mg / L.

[0013] More preferably, the fluoride ion concentration in the fluoride-containing wastewater is 790-810 mg / L.

[0014] Preferably, the initial calcium dosing solution is an aqueous solution of calcium chloride, and the amount of calcium chloride added in the initial calcium dosing solution is 0.72-0.76g, based on a total weight of 1000g of fluoride-containing wastewater.

[0015] Preferably, the secondary calcium dosing solution is an aqueous solution of calcium chloride, and the amount of calcium chloride added in the secondary calcium dosing solution is 0.20-0.24g, based on a total weight of 1000g of fluoride-containing wastewater.

[0016] Preferably, the calcium supplementation solution is an aqueous solution of calcium chloride, and the amount of calcium chloride added in the calcium supplementation solution is 2.25-2.31g based on a total weight of 1000g of fluoride-containing wastewater.

[0017] Preferably, the citrate modification solution is an aqueous solution of sodium citrate, and the amount of sodium citrate added in the citrate modification solution is 0.16-0.20g based on a total weight of 1000g of fluoride-containing wastewater.

[0018] Preferably, the initial calcium solution is added within 18-22 seconds, and stirring is maintained for 1.5-2.5 minutes after addition.

[0019] Preferably, the citrate-modified solution is added and stirred for 0.8-1.2 minutes.

[0020] Preferably, the sodium hydroxide solution is added within 2.5-3.5 minutes, and stirring is maintained for 4-6 minutes after addition.

[0021] Preferably, the sodium hydroxide solution is added to adjust the pH of the system to 4.7-4.9.

[0022] Preferably, the concentration of the sodium hydroxide solution is 10 wt%.

[0023] Preferably, the glacial acetic acid solution is added within 40-50 seconds, and stirring is maintained for 80-100 seconds after addition.

[0024] Preferably, the purpose of adding the glacial acetic acid solution is to adjust the pH of the system to 4.4-4.6.

[0025] Preferably, the concentration of the glacial acetic acid solution is 10 wt%.

[0026] Preferably, the secondary calcium solution is added within 12-18 seconds, and stirring is maintained for 2.5-3.5 minutes after addition.

[0027] Preferably, sodium hydroxide solution is added simultaneously during the three calcium supplementation processes and within 2.5-3.5 minutes after the addition is completed, in order to adjust the pH of the system to be maintained at 5.8-6.1.

[0028] Preferably, the defluorination filler is obtained by preloading Lewatit MonoPlus TP 260 as the parent resin with zirconium salt solution.

[0029] Preferably, the concentration of the zirconium salt solution is 0.05-0.20 mol / L.

[0030] The beneficial effects of this invention are:

[0031] This invention achieves stable deep defluorination in fluoride-containing wastewater. By introducing citrate ions after the formation of primary calcium fluoride crystal nuclei, a semi-covering directional coating layer is formed. Combined with a short-term weak acid backflow to promote surface rearrangement, the calcium ions added in the second step are precisely locked onto the calcium-rich area of ​​the compressed surface, laying the structural foundation for the epitaxial growth of the main body. The multi-step synergy significantly improves the heterogeneous nucleation order on the crystal seed surface, and the fluoride ion concentration in the effluent can be stably controlled at a low level for a long time. The performance does not show significant degradation after continuous operation for multiple batches.

[0032] The modified calcium fluoride seed crystals prepared by this invention have a more concentrated particle size distribution and a larger average particle size, and the proportion of fine particles is significantly reduced. As the growth of calcium fluoride is guided to the surface of the seed crystals, the suspended fine crystals in the supernatant are greatly reduced, the settling velocity is significantly improved, and the turbidity is extremely low. The recovered seed crystal wet slurry can be directly used for the next batch without the need for complex regeneration or grading operations, which is conducive to continuous operation of the project.

[0033] This invention avoids explosive nucleation in the liquid phase by synergistically arranging stepwise calcium addition and postponing the main calcium supplementation, which promotes the deposition of calcium fluoride in a dense form on the seed crystal surface. The sludge particles are compact and have high mechanical strength. At the same time, the calcium ion utilization efficiency is significantly improved, the residual calcium ion concentration in the effluent is effectively reduced, and the waste of reagents and the risk of pipe scaling are reduced.

[0034] The crystallized water produced by this invention has the characteristics of low suspended solids and low residual calcium, which creates favorable influent conditions for the subsequent anti-competitive deep defluorination unit, effectively protects the active sites of the defluorination packing, prevents non-specific clogging and competitive adsorption, gives the adsorbent material a longer working cycle and a higher treatment bed volume, and significantly reduces the regeneration frequency and operating cost.

[0035] This invention constructs an integrated defluorination process that combines efficient precipitation and stable polishing through deep synergy between crystallization and adsorption units at the material and operational levels, providing a high-performance and stable solution for treating fluoride-containing wastewater. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0037] In this invention, the fluoride-containing wastewater used is preferably wastewater generated from hydrofluoric acid etching and frosting processes in the glass manufacturing industry, with a fluoride ion concentration of 790-810 mg / L.

[0038] Example 1: A deep defluorination process coupled with heterogeneous crystallization and anti-competitive adsorption, the specific process flow is as follows:

[0039] S1: First, slowly add 10.00g of sodium hydroxide to 90.00g of water, and cool to 25℃ to obtain a 10wt% sodium hydroxide solution; then add 10.00g of glacial acetic acid to 90.00g of water, mix well to obtain a 10wt% glacial acetic acid solution; then add 1000.00g of fluoride-containing wastewater (fluoride ion concentration of 800mg / L) to the reaction vessel.

[0040] S2: Dissolve 0.74g of calcium chloride in 20.00g of water to obtain the initial calcium addition solution; dissolve 0.22g of calcium chloride in 10.00g of water to obtain the second calcium addition solution; dissolve 2.28g of calcium chloride in 30.00g of water to obtain the third calcium addition solution; dissolve 0.18g of sodium citrate in 10.00g of water to obtain the citrate-modified solution;

[0041] S3: Under stirring at 300 rpm, the initial calcium dosing solution was added to the fluoride-containing wastewater within 20 seconds. After the addition was completed, stirring was continued for 2 minutes, and then citrate-modified solution was immediately added. Stirring was maintained for 1 minute. Then, sodium hydroxide solution (10 wt%) was added dropwise to the system within 3 minutes to adjust the pH of the system to 4.7-4.9. Stirring was continued for another 5 minutes, and then glacial acetic acid solution (10 wt%) was added dropwise to the system within 45 seconds to adjust the pH of the system to 4.4-4.6. This pH condition was maintained for 90 seconds. Then, the second calcium dosing solution was added to the system within 15 seconds and stirring was maintained for 3 minutes. Finally, the third calcium supplementation solution was added to the system. During the addition of the three calcium supplementation solutions and within 3 minutes after the addition was completed, sodium hydroxide solution (10 wt%) was added dropwise to adjust the pH of the system to maintain it at 5.8-6.1. The stirring speed was reduced to 250 rpm, and mixing was continued for 20 minutes. Then, the mixture was allowed to stand for 10 minutes. Solid-liquid separation was performed to obtain the effluent from the HC unit and the bottom sediment.

[0042] S4: Collect the bottom sediment obtained in S3, add water and gently resuspend once, repeat the standing for 10 minutes and discard the supernatant to obtain seed wet slurry;

[0043] S5: The effluent from the HC unit obtained in S3 is introduced into an adsorption tank filled with anti-competitive deep defluorination packing material. The packing material is Lewatit MonoPlus TP 260 as the parent resin, which is preloaded with 0.05 mol / L zirconium salt solution before being loaded into the column. The empty bed contact time is controlled to be 20 min to perform deep defluorination treatment on the effluent from the HC unit.

[0044] Example 2: A deep defluorination process coupled with heterogeneous crystallization and anti-competitive adsorption, the specific process flow is as follows:

[0045] S1: First, slowly add 10.00g of sodium hydroxide to 90.00g of water, and cool to 25℃ to obtain a 10wt% sodium hydroxide solution; then add 10.00g of glacial acetic acid to 90.00g of water, mix well to obtain a 10wt% glacial acetic acid solution; then add 1000.00g of fluoride-containing wastewater (fluoride ion concentration of 790mg / L) to the reaction vessel.

[0046] S2: Dissolve 0.72g of calcium chloride in 19.00mL of water to obtain the initial calcium dosing solution; dissolve another 0.20g of calcium chloride in 9.00mL of water to obtain the second calcium dosing solution; then dissolve 2.25g of calcium chloride in 29.00mL of water to obtain the third calcium dosing solution; dissolve 0.16g of sodium citrate in 9.00mL of water to obtain the citrate-modified solution;

[0047] S3: Under stirring at 290 rpm, the initial calcium dosing solution was added to the fluoride-containing wastewater within 18 s. After the addition was completed, stirring was continued for 1.5 min, and then citrate-modified solution was immediately added. Stirring was maintained for 0.8 min. Subsequently, sodium hydroxide solution was added dropwise to the system within 2.5 min to adjust the pH of the system to 4.7-4.9. Stirring was continued for another 4 min, and glacial acetic acid solution was added dropwise to the system within 40 s to adjust the pH of the system to 4.4-4.6. This pH condition was maintained for 80 s. Then, the second calcium dosing solution was added to the system within 12 s and stirring was maintained for 2.5 min. Finally, the third calcium supplementation solution was added to the system. During the addition of the three calcium supplementation solutions and within 2.5 min after the addition was completed, sodium hydroxide solution was added dropwise to adjust the pH of the system to maintain it at 5.8-6.1. The stirring speed was reduced to 240 rpm, and mixing was continued for 18 min. The mixture was then allowed to stand for 8 min for solid-liquid separation to obtain the effluent from the HC unit and the bottom sediment.

[0048] S4: Collect the bottom sediment obtained in S3, add water and gently resuspend once, repeat the standing for 8 minutes and discard the supernatant to obtain seed seed wet slurry;

[0049] S5: The effluent from the HC unit obtained in S3 is introduced into an adsorption tank filled with anti-competitive deep defluorination packing material. The packing material is Lewatit MonoPlus TP 260 as the parent resin, which is preloaded with 0.10 mol / L zirconium salt solution before being loaded into the column. The empty bed contact time is controlled to be 15 min to perform deep defluorination treatment on the effluent from the HC unit.

[0050] Example 3: A deep defluorination process coupled with heterogeneous crystallization and anti-competitive adsorption, the specific process flow is as follows:

[0051] S1: First, slowly add 10.00g of sodium hydroxide to 90.00g of water, and cool to 25℃ to obtain a 10wt% sodium hydroxide solution; then add 10.00g of glacial acetic acid to 90.00g of water, mix well to obtain a 10wt% glacial acetic acid solution; then add 1000.00g of fluoride-containing wastewater (fluoride ion concentration of 810mg / L) to the reaction vessel.

[0052] S2: Dissolve 0.76g of calcium chloride in 21.00g of water to obtain the initial calcium addition solution; dissolve 0.24g of calcium chloride in 11.00g of water to obtain the second calcium addition solution; dissolve 2.31g of calcium chloride in 31.00g of water to obtain the third calcium addition solution; dissolve 0.20g of sodium citrate in 11.00g of water to obtain the citrate-modified solution;

[0053] S3: Under stirring at 310 rpm, the initial calcium dosing solution was added to the fluoride-containing wastewater within 22 s. After the addition was completed, stirring was continued for 2.5 min, and then citrate-modified solution was immediately added. Stirring was maintained for 1.2 min. Subsequently, sodium hydroxide solution was added dropwise to the system within 3.5 min to adjust the pH of the system to 4.7-4.9. Stirring was continued for another 6 min, and then glacial acetic acid solution was added dropwise to the system within 50 s to adjust the pH of the system to 4.4-4.6. This pH condition was maintained for 100 s. Then, the second calcium dosing solution was added to the system within 18 s and stirring was maintained for 3.5 min. Finally, the third calcium supplementation solution was added to the system. During the addition of the three calcium supplementation solutions and within 3.5 min after the addition was completed, sodium hydroxide solution was added dropwise to adjust the pH of the system to maintain it at 5.8-6.1. The stirring speed was reduced to 260 rpm, and mixing was continued for 22 min. Then, the mixture was allowed to stand for 12 min. Solid-liquid separation was performed to obtain the effluent from the HC unit and the bottom sediment.

[0054] S4: Collect the bottom sediment obtained in S3, add water and gently resuspend once, repeat the standing for 12 minutes and discard the supernatant to obtain seed wet slurry;

[0055] S5: The effluent from the HC unit obtained in S3 is introduced into an adsorption tank filled with anti-competitive deep defluorination packing material. The packing material uses Lewatit MonoPlus TP 260 as the parent resin, which is preloaded with 0.20 mol / L zirconium salt solution before being loaded into the column. The empty bed contact time is controlled at 30 min to perform deep defluorination treatment on the effluent from the HC unit.

[0056] Comparative Example 1: The difference from Example 1 is that 0.18g of sodium citrate was directly added to the initial calcium dosing solution and dissolved together with 0.74g of calcium chloride in 20.00g of water to form a mixed solution. In subsequent operations, citrate modification solution was not added separately; the other conditions were the same as in Example 1.

[0057] Comparative Example 2: The difference from Example 1 is that 0.18g of sodium citrate was not added, and the other conditions were the same as in Example 1.

[0058] Comparative Example 3: The difference from Example 1 is that after adding sodium hydroxide solution dropwise to the system to adjust the pH to 4.7-4.9, stirring was continued for 5 minutes, and glacial acetic acid solution was not added to adjust the pH to 4.4-4.6 and held for 90 seconds; the other conditions were the same as in Example 1.

[0059] Comparative Example 4: The difference from Example 1 is that the initial calcium solution and the second calcium solution were omitted and replaced with equal amounts of the third calcium solution; the other conditions are the same as in Example 1.

[0060] Comparative Example 5: The difference from Example 1 is that after adjusting the pH of the system to 4.4-4.6 with glacial acetic acid solution and maintaining it for 90 seconds, the secondary calcium addition solution was added to the system within 15 seconds and stirred for 3 minutes, and then the citrate-modified solution was added to the system; the other conditions were the same as in Example 1.

[0061] Comparative Example 6: The difference from Example 1 is that after adjusting the pH of the system from 4.8 to 4.4-4.6 with glacial acetic acid solution and maintaining it for 90 seconds, calcium supplementation solution was added to the system three times. During the addition of the three calcium supplementation solutions and within 3 minutes after the addition was completed, sodium hydroxide solution was added dropwise to adjust the pH of the system to maintain it at 5.8-6.1; the other conditions were the same as in Example 1.

[0062] Comparative Example 7: The difference from Example 1 is that sodium hydroxide solution was added dropwise during the three additions of calcium solution and within 3 minutes after the addition was completed, in order to adjust the pH of the system to be maintained at 6.4-6.6; the other conditions were the same as in Example 1.

[0063] Performance testing

[0064] Take 2.0000g of wet slurry obtained from each of the examples and comparative examples, dry it to constant weight in a 40℃ forced-air drying oven, calculate the solids content of the wet slurry, dry the remaining wet slurry to constant weight at 40℃, lightly grind it, pass it through a 100-mesh sieve, and place it in a desiccator for later use. When used for application performance testing, take 1000.00g of freshly prepared fluoride-containing wastewater, add 1.00g of the corresponding seed crystals on a dry basis, and uniformly use the feeding sequence, stirring speed, pH adjustment program, mixing time, and settling conditions of Example 1 to complete the heterogeneous crystallization treatment; after settling, take the supernatant at 20mm below the liquid surface for testing, and use the bottom sediment for supplementary characterization of particles and crystal phases. Three sets of samples were prepared in parallel for each sample, and the test results were taken as the arithmetic mean of the three sets.

[0065] Particle size distribution determination: The dried samples obtained in the examples and comparative examples were tested according to GB / T 19077-2024. 0.2000g of sample was weighed and added to 100mL of water. After ultrasonic dispersion for 60s, the sample was transferred to the wet circulation tank of a laser particle size analyzer. The data was processed using the Mie model. The particle refractive index was set to 1.43, the absorptivity was set to 0.01, and the shading was controlled at 10%. The test was repeated 3 times and D10, D50 and D90 were recorded.

[0066] Determination of fluoride ions in the first batch of heterogeneous crystallization effluent: The seed crystals obtained in the examples and comparative examples were added to 1.00g of fluoride-containing wastewater per 1000.00g of dry basis, and the heterogeneous crystallization treatment conditions of Example 1 were uniformly used for treatment. After standing for 10 minutes, the supernatant was taken at 20mm below the liquid surface, filtered through a 0.45μm aqueous filter membrane, and the mass concentration of fluoride ions in the effluent was determined according to HJ 488-2009. When the sample concentration exceeded the range of the working curve, it was diluted with deionized water and re-measured, and the concentration was adjusted according to the dilution factor. Each sample was measured in parallel 3 times.

[0067] Stability evaluation of five consecutive batches of heterogeneous crystallization: Using the seed crystals obtained in the examples and comparative examples as starting seed crystals, five batches of fluoride-containing wastewater were treated consecutively. The mass of each batch of wastewater was 1000.00g. After each batch was treated, the bottom sediment was recovered by standing, discarding the supernatant, gently resuspending it once in deionized water and standing again as in Example 1. A small amount of wet slurry was taken to determine the solids content and the amount was adjusted to make up to 1.00g of the dosage on a dry basis for the next batch. The fluoride ion mass concentration of the supernatant after the treatment of the first to the fifth batch was determined according to HJ 488-2009.

[0068] Determination of suspended solids and turbidity in supernatant: The suspended solids and turbidity of each supernatant obtained from the above tests were determined. The suspended solids were determined according to GB / T 11901-1989. Specifically, 100.0 mL of supernatant was taken, filtered through a pre-weighed filter membrane according to the standard, and dried at 105℃ to constant weight. The mass concentration of suspended solids was calculated. The supernatant of the same batch was directly measured at 25℃ using a turbidimeter. Each sample was measured three times consecutively and the average value was taken.

[0069] Determination of pH and residual calcium ions in effluent: For each supernatant obtained from the above tests, the pH should be determined according to HJ 1147-2020. The determination should be completed within 5 minutes after sampling, and the test temperature should be controlled at 25℃. Then, another 50.0 mL of filtered supernatant should be taken and the calcium ion mass concentration should be determined according to GB / T 7476-1987. Each sample should be measured twice in parallel.

[0070] Determination of the final effluent and effective working capacity of deep defluorination: The supernatant of heterogeneous crystallization corresponding to each sample was directly used as the feed water for deep defluorination. Fixed-bed adsorption experiments were conducted under the same adsorption column conditions. The inner diameter of the adsorption column was 16 mm, and 100.0 mL of Lewatit MonoPlus TP 260 parent resin pre-loaded with zirconium ions was packed. The bed height was 49.0 cm. Before the bed was started, it was backwashed with deionized water until no obvious fine powder was found. Then, it was equilibrated with deionized water at pH 6.0 in the co-current flow for 2 BV. Subsequently, water was continuously fed from top to bottom at 25℃ for 20 min with an empty bed contact time. After 20 BV of stable operation, the effluent was collected and filtered through a 0.45 μm aqueous filter membrane. The effluent was then treated according to HJ 84-2016 "Water Quality - Inorganic Anions (F... - Cl - NO 2- ,Br - NO3 -1 PO4 -3 SO3 2- SO4 2- The concentration of fluoride ions in the effluent at the endpoint of deep defluorination was determined by ion chromatography, and the process continued until the concentration of fluoride ions in the effluent reached 1.0 mg / L. The cumulative treatment bed volume was then recorded as the effective working capacity. The test results are shown in Table 1.

[0071] Table 1 Performance Test Results

[0072]

[0073] Data Analysis: As can be seen from the data in Table 1, the calcium fluoride seeds prepared by this invention exhibit a consistent optimization trend in terms of particle size distribution, reusability stability, supernatant clarification, and final deep defluorination. With the continuous action of the initial calcium solution, citrate-modified solution, sodium hydroxide solution, glacial acetic acid solution, secondary calcium solution, and tertiary calcium supplementation solution in a predetermined sequence, the seed surface state is gradually refined and stabilized. Consequently, the formed calcium fluoride seeds have a more concentrated particle size and fewer fine powder particles, which is beneficial for the migration and continuous deposition of fluoride ions to the seed surface and for maintaining low suspended solids and turbidity in the supernatant after sedimentation. Simultaneously, the low residual calcium ion level indicates that the calcium source is not largely retained in the liquid phase but rather participates more fully in the orderly deposition on the seed surface. Furthermore, after further treatment with defluorination packing, the effluent remains at a low level, indicating that the initial heterogeneous crystallization does not increase the burden on subsequent treatments but instead creates more stable influent conditions for subsequent deep defluorination. Therefore, the entire process combines the synergistic advantages of rapid defluorination in the initial stage and stable polishing in the subsequent stage.

[0074] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 1 and 2, mixing sodium citrate with calcium chloride in advance, or omitting sodium citrate altogether, both result in a smaller particle size distribution and decreased continuous effluent stability. The main reason is that the true effectiveness of this invention lies not simply in the presence or absence of sodium citrate, but in the semi-coverage surface modification achieved through low-dose, delayed addition and subsequent pH enhancement, resulting in a synergistic effect.

[0075] As can be seen from the data in Table 1 for Example 1 and Comparative Example 3, simply canceling the short-term weak acid reversion of the glacial acetic acid solution causes a simultaneous deterioration in the effluent fluoride ions, supernatant suspended solids, and turbidity, and this difference is further amplified after multiple batches of cycles. The main reason is that although the short-term weak acid reversion is a short-term step, it plays a crucial role in the establishment of subsequent hierarchical interfaces.

[0076] As can be seen from the data in Example 1 and Comparative Example 4 in Table 1, even after the initial and secondary calcium addition solutions were removed and replaced with an equal amount of tertiary calcium addition solution, the total calcium dosage did not decrease. However, the particle size distribution deteriorated significantly, and the levels of fluoride ions in the effluent, suspended solids in the supernatant, turbidity, and residual calcium ions all worsened simultaneously. The main reason is that the main calcium addition can only form more fine particles in the liquid phase, which is difficult to transform into stable epitaxial growth. This indicates that the effect of the present invention does not come from the amount of calcium itself, but from the synergistic arrangement of stepwise calcium addition and the subsequent shift of the main calcium addition.

[0077] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 5 and 6, simply changing the order of sodium citrate and secondary calcium addition resulted in a significant decrease in performance; further, moving the overall calcium supplementation process forward further increased the degradation. The main reason is that the compression, locking, and then main body growth have a continuous progressive relationship, and arbitrarily changing the order cannot achieve the synergistic effect of this invention.

[0078] As can be seen from the data of Example 1 and Comparative Example 7 in Table 1, after raising the pH during the main body calcium supplementation stage, even if the subsequent application performance test still uses the heterogeneous crystallization conditions of Example 1, the continuous water effluent stability and solid-liquid separation performance of the obtained seed crystals still decreased.

[0079] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A deep defluorination process coupled with heterogeneous crystallization and anti-competitive adsorption, characterized in that, Includes the following steps: An initial calcium dosing solution and a citrate-modified solution were sequentially added to the fluoride-containing wastewater. Sodium hydroxide solution and glacial acetic acid solution were then added sequentially to adjust the pH of the system. Subsequently, a second calcium dosing solution and a third calcium supplement solution were added. After mixing and settling, the effluent from the HC unit and bottom sediment were obtained. The bottom sediment was resuspended, settling, and the supernatant was discarded to obtain a seed slurry. The effluent from the HC unit was then introduced into an adsorption tank filled with anti-competitive deep defluorination packing for deep defluorination treatment. The initial calcium dosing solution is an aqueous solution of calcium chloride. Based on a total weight of 1000g of fluoride-containing wastewater, the amount of calcium chloride added in the initial calcium dosing solution is 0.72-0.76g. The secondary calcium dosing solution is an aqueous solution of calcium chloride. Based on a total weight of 1000g of fluoride-containing wastewater, the amount of calcium chloride added in the secondary calcium dosing solution is 0.20-0.24g. The three-stage calcium supplementation solution is an aqueous solution of calcium chloride. Based on a total weight of 1000g of fluoride-containing wastewater, the dosage of calcium chloride in the three-stage calcium supplementation solution is 2.25-2.31g. The citrate-modified solution is an aqueous solution of sodium citrate. Based on a total weight of 1000g of fluoride-containing wastewater, the amount of sodium citrate added to the citrate-modified solution is 0.16-0.20g.

2. The deep defluorination process according to claim 1, characterized in that, The fluoride-containing wastewater originates from wafer etching and cleaning processes in the semiconductor and photovoltaic industries, or hydrofluoric acid etching and sanding processes in the glass manufacturing industry.

3. The deep defluorination process according to claim 1, characterized in that, The initial calcium solution is added within 18-22 seconds, and stirring is continued for 1.5-2.5 minutes after addition.

4. The deep defluorination process according to claim 1, characterized in that, After adding the citrate-modified solution, keep stirring for 0.8-1.2 minutes.

5. The deep defluorination process according to claim 1, characterized in that, The purpose of adding the sodium hydroxide solution is to adjust the pH of the system to 4.7-4.

9.

6. The deep defluorination process according to claim 1, characterized in that, The purpose of adding the glacial acetic acid solution is to adjust the pH of the system to 4.4-4.

6.

7. The deep defluorination process according to claim 1, characterized in that, The secondary calcium solution is added within 12-18 seconds, and stirring is maintained for 2.5-3.5 minutes after addition.

8. The deep defluorination process according to claim 1, characterized in that, Sodium hydroxide solution is added simultaneously during the three calcium supplementation processes and within 2.5-3.5 minutes after each addition to adjust the pH of the system to maintain it at 5.8-6.

1.

9. The deep defluorination process according to claim 1, characterized in that, The defluorination filler is obtained by preloading LewatitMonoPlus TP 260 as the base resin with zirconium salt solution.

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