Method and equipment for in-situ preparation of high-purity calcium fluoride from photovoltaic cell concentrated acid wastewater and application

Abstract

The invention relates to the field of wastewater treatment, in particular to a method and equipment for in-situ preparation of high-purity calcium fluoride from photovoltaic cell concentrated acid wastewater and application. Comprising the following steps: (S01) pumping a compound calcium salt solution prepared from calcium hydroxide and calcium oxyhydroxide into concentrated acid wastewater for reaction to generate calcium fluoride particles; (S02) adding a citric acid solution into the mixed solution after the reaction in the step (S01); (S03) applying micro-current to the mixed solution treated in the step (S02) so as to change the surface charge property of the calcium fluoride particles; (S04) adding a flocculating agent into the mixed solution treated in the step (S03) to flocculate and settle the calcium fluoride particles to obtain calcium fluoride flocculated sludge; and (S05) carrying out filter pressing dehydration on the calcium fluoride flocculated sludge obtained in the step (S04), and drying and calcining the dehydrated filter cake. According to the method, calcium salt reaction, citric acid regulation, micro-current modification, flocculation separation and dehydration calcination are carried out continuously, and an additional acidification impurity removal step is not needed.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment, and in particular to a method, equipment, and application for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells. Background Technology

[0002] The photovoltaic (PV) cell industry is one of my country's priority strategic emerging industries, and it has experienced rapid development in recent years. Wastewater from PV cell production comprises various types of wastewater generated from the etching of monocrystalline and polycrystalline silicon with hydrofluoric acid, exhibiting significant differences in water quality. These include high-fluoride concentrated acid wastewater, dilute acid wastewater, silane wastewater, and concentrated alkali texturing wastewater. Because different production processes introduce different pollutants, the wastewater contains numerous impurities after calcium precipitation, some of which are difficult to remove. Phosphorus oxychloride and phosphorus pentoxide are present in the phosphorus diffusion stage; the concentrated alkali water used in the texturing process to etch silicon surfaces introduces high concentrations of silicates. To adjust the system's pH, sulfuric acid is added to the sedimentation tank, generating calcium sulfate impurities, thus affecting the purity of the calcium fluoride sludge. The removal of high-concentration fluoride mainly involves precipitating fluoride ions with calcium oxide, calcium hydroxide, and calcium chloride, resulting in calcium fluoride sludge. Currently, the reuse of calcium fluoride sludge is for brick making or the preparation of chemical additives, such as stabilizers in ceramic calcination, fluxes in stripping and melting processes, and as a substitute for river sand in concrete production. The low added value leads to a waste of fluorine resources. Therefore, we are trying to find a new resource utilization pathway for calcium fluoride sludge.

[0003] Currently, domestic photovoltaic companies use calcination to make bricks from calcium fluoride sludge due to its high impurity content and low purity, resulting in very low added value.

[0004] Patent document CN108191118A discloses a method for recovering fluoride ions from wastewater. It uses resin adsorption to ensure fluoride ions in fluoride-containing wastewater meet discharge standards. The resin desorption solution is treated using a combination of diffusion dialysis and chemical precipitation. High-fluoride wastewater from the diffusion dialysis is then treated with chemical precipitation to precipitate and recover fluoride ions. However, this method cannot produce high-yield calcium fluoride sludge. Patent document CN110078109A discloses a method for producing high-purity calcium fluoride from acidic fluoride-containing wastewater. It uses multi-stage calcium carbonate to treat high-fluoride concentrated acid to prepare high-purity calcium fluoride. However, calcium fluoride adsorbs onto the surface of calcium carbonate, reducing utilization efficiency, and the reaction also generates a large number of bubbles, posing safety hazards in practical applications. Patent document CN110790295B discloses a method for preparing high-purity calcium fluoride from calcium fluoride sludge. The method mentions removing impurities through acidification to remove carbonates and alkaline water to remove silicates. However, this method is complex and generates large amounts of fluoride-containing cleaning wastewater, causing secondary pollution and limiting its practical application. Therefore, to obtain high-purity calcium fluoride sludge, wastewater treatment must be differentiated. Since concentrated acid wastewater mainly contains hydrofluoric acid with a high fluoride ion concentration, separate treatment of concentrated acid wastewater is currently the most feasible approach for preparing high-purity calcium fluoride. However, calcium fluoride crystals are small, have a slow deposition rate, are difficult to separate, and have low purity. Furthermore, the separate use of concentrated acid water leads to an acid-base imbalance in the entire system, requiring the addition of more acid, which increases operating costs and the amount of sulfate ions in the effluent.

[0005] Therefore, existing technologies suffer from problems such as low purity of calcium fluoride products, difficulty in suppressing impurities at the source, small calcium fluoride crystals, low solid-liquid separation efficiency, low yield, high cost, safety hazards, or poor practicality. Summary of the Invention

[0006] To address the technical problems of existing calcium fluoride products, such as low purity, difficulty in suppressing impurities at the source, small calcium fluoride crystals, low solid-liquid separation efficiency, low yield, high cost, safety hazards, or poor practicality, this invention provides a method, equipment, and application for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells, which can efficiently remove impurities and purify high-purity products.

[0007] The first technical solution of the present invention: a method for in-situ preparation of high-purity calcium fluoride from concentrated acid wastewater from photovoltaic cells, comprising the following steps:

[0008] (S01) A compound calcium salt solution prepared by calcium hydroxide and calcium hydroxychloride is pumped into concentrated acid wastewater to react and generate calcium fluoride particles.

[0009] (S02) Add citric acid solution to the mixture after the reaction in step (S01);

[0010] (S03) Apply a microcurrent to the mixture after step (S02) to change the surface charge properties of the calcium fluoride particles;

[0011] (S04) Add flocculant to the mixture after treatment in step (S03) to cause calcium fluoride particles to flocculate and settle, and obtain calcium fluoride flocculent sludge.

[0012] (S05) The calcium fluoride flocculent sludge obtained in step (S04) is dewatered by pressure filtration, and the dewatered filter cake is dried and calcined to obtain high-purity calcium fluoride. This invention achieves directional conversion and stepwise purification of fluoride ions through a process of calcium salt reaction, citric acid regulation, microcurrent modification, flocculation separation, and dewatering and calcination. First, a compound calcium salt solution prepared by calcium hydroxide and calcium hydroxychloride is pumped into concentrated acid wastewater. The two work synergistically: calcium hydroxide provides sufficient calcium ions for the reaction, and its alkalinity can neutralize part of the acidity of the concentrated acid wastewater, creating a suitable environment for the reaction of calcium ions and fluoride ions; calcium hydroxychloride, with its own properties, helps to capture fluoride ions, promotes the rapid reaction of calcium ions and fluoride ions to generate basic calcium fluoride particles. The compound system avoids the limitations of single calcium salt reaction and lays a pure material basis for subsequent purification. Next, citric acid solution was added to the reacted mixture. This optimized the growth of calcium fluoride crystals. On one hand, it chemically regulated the system environment, inhibiting ineffective nucleation of calcium fluoride and guiding directional crystal growth, resulting in a more regular crystal structure. On the other hand, citric acid molecules adsorbed onto the surface of the calcium fluoride crystals, preventing impurities from the wastewater from embedding into the crystal lattice, thus structurally improving the purity of the calcium fluoride particles and creating favorable conditions for subsequent separation steps. Subsequently, a microcurrent was applied to the citric acid-treated mixture. Taking advantage of the small size and high dispersibility of the calcium fluoride particles, the electric field altered the surface charge properties of the particles, disrupting the electrostatic balance between them and reducing the repulsive force. This allowed the previously difficult-to-aggregate fine particles to adsorb each other, promoting initial particle aggregation and laying the groundwork for subsequent flocculation and separation, thus solving the key problem of separating fine calcium fluoride particles. Subsequently, flocculants polyaluminum chloride (PAC) and polyacrylamide (PAM) were added to the modified mixture. These two agents work synergistically: PAC, through adsorption and bridging, encapsulates the initially agglomerated calcium fluoride particles, forming loose primary flocs; PAM, with its long molecular chains, further agglomerates the primary flocs, forming large, stable, and easily settling flocs. This achieves efficient solid-liquid separation of calcium fluoride particles and wastewater, effectively enriching calcium fluoride into flocculent sludge. Finally, the obtained calcium fluoride flocculent sludge is dewatered by filter press to remove a large amount of water while retaining trace impurities. The dewatered filter cake is then dried and calcined. The drying process removes residual water and volatile impurities, while the calcination process decomposes and removes residual flocculants and other organic impurities from the filter cake, further removing trace impurities embedded within the crystals. This results in a denser calcium fluoride crystal structure, ultimately yielding a high-purity calcium fluoride product, while simultaneously achieving effective removal and resource recovery of fluoride ions from concentrated acid wastewater from photovoltaic cells.

[0013] Preferably, the concentration of the compound calcium salt solution in step (S01) is 5%–15%; more preferably, the concentration is 8%–12%; even more preferably, the concentration is 9%–11%; and even more preferably, the concentration is 10%. In this invention, the concentration range of 5%–15% is suitable for concentrated acid wastewater with different fluoride contents, ensuring sufficient calcium ions to fully capture fluoride ions and avoiding fluoride residue due to insufficient calcium ions; 8%–12% is the optimized concentration range, balancing reaction efficiency and solution fluidity, preventing excessively low concentrations from affecting the reaction rate or excessively high concentrations from causing calcium salt agglomeration; 9%–11% further focuses on reaction stability, reducing the impact of concentration fluctuations on crystal growth; and 10% is the optimal concentration, enabling a uniform reaction rate between the compound calcium salt and concentrated acid wastewater, maximizing the reduction of the probability of impurity formation caused by local concentration imbalances.

[0014] Preferably, in step (S01), the compound calcium salt solution is continuously pumped into the concentrated acid reaction tank by a metering pump. The reaction tank consists of three sections and is equipped with a reflux system. The reaction time in the final section of the reaction tank is 30–60 min, and the total reaction time is 60–90 min. The metering pump ensures precise addition of the compound calcium salt solution, avoiding excessive calcium ions that may cause impurities or insufficient calcium ions that may lead to incomplete fluoride removal. The three-section reaction tanks are connected in series, allowing the concentrated acid wastewater to react with the calcium salt step by step, progressively increasing the contact probability between fluoride ions and calcium ions, and avoiding incomplete reactions in a single step. The reflux device returns the unreacted material to the first-stage reaction tank, reducing raw material waste and improving reaction utilization. The 30–60 min reaction time in the final section and the total 60–90 min reaction time provide sufficient conditions for the growth of calcium fluoride crystals, ensuring a complete crystal structure and reducing the amount of impurities embedded.

[0015] Preferably, the metering pump is stopped supplying the compound calcium salt solution when the pH of the effluent from the reaction tank is stable at 7.5–8.5; more preferably, the metering pump is stopped supplying the compound calcium salt solution when the pH is stable at 7.5–8.0. When the pH is stable at 7.5–8.5, the reaction between fluoride ions and calcium ions reaches thermodynamic equilibrium, which maximizes the formation of calcium fluoride precipitate and avoids excessive calcium hydroxide deposition that could affect product purity. The preferred pH range of 7.5–8.0 ensures that the fluoride ion removal rate meets expectations, further reduces impurities caused by excessive calcium salt, and avoids the precipitation of other ions such as silicates in the solution due to excessively high pH, ​​thus improving the purity of the calcium fluoride product.

[0016] Preferably, the concentration of the citric acid solution in step (S02) is 1-5%; more preferably, the concentration of the citric acid solution is 2-4%; even more preferably, the concentration of the citric acid solution is 3%. A citric acid concentration of 1-5% can effectively adjust the pH of the mixture to a suitable range, while increasing the supersolubility of calcium fluoride crystals through phase reconstruction, providing a good environment for crystal growth; 2-4% is the optimized concentration range, which can more accurately shrink the metastable region of calcium fluoride, inhibit ineffective nucleation, and promote directional crystal growth; 3% is the optimal concentration, which can precisely control the crystal growth rate, making the calcium fluoride crystal particles uniform and regular, significantly reducing the embedding of impurities into the crystal structure, and further improving the purity of the product.

[0017] Preferably, the metering pump is stopped when the pH of the mixture after the reaction is adjusted to 6.5-7.0, and the stirring time during the adjustment process is 10-20 min; more preferably, the metering pump is stopped when the pH of the mixture after the reaction is adjusted to 6.5-6.8, and the stirring time during the adjustment process is 10-18 min; even more preferably, the metering pump is stopped when the pH of the mixture after the reaction is adjusted to 6.5-6.7, and the stirring time during the adjustment process is 10-15 min; even more preferably, the metering pump is stopped when the pH of the mixture after the reaction is adjusted to 6.6, and the stirring time during the adjustment process is 12 min. When the pH is adjusted to 6.5–7.0, the growth environment for calcium fluoride crystals is optimal. A stirring time of 10–20 min ensures uniform mixing of citric acid and the solution, avoiding local pH imbalances that could affect crystal growth. Gradually narrowing pH ranges of 6.5–6.8, 6.5–6.7, and 6.6 allow for more precise control of crystal growth kinetics and reduce impurity adsorption. Correspondingly shortened stirring times of 10–18 min, 10–15 min, and 12 min ensure uniform mixing while avoiding over-stirring that could damage the already formed crystal structure, ultimately achieving efficient purification of calcium fluoride crystals.

[0018] Preferably, in step (S03), the current density is 5–10 mA / cm², and the microcurrent application time is 1–2 h; more preferably, the current density is 6–10 mA / cm², and the microcurrent application time is 0.8–1.7 h; even more preferably, the current density is 7–10 mA / cm², and the microcurrent application time is 0.7–1.5 h; even more preferably, the current density is 8–10 mA / cm², and the microcurrent application time is 0.6–1.3 h; even more preferably, the current density is 9 mA / cm², and the microcurrent application time is 0.7–1.1 h. Microcurrents alter the surface charge properties of calcium fluoride particles, reducing interparticle repulsion and creating conditions for subsequent flocculation and sedimentation. Current density is negatively correlated with reaction time; 5–10 mA / cm² corresponds to 1–2 hours, ensuring sufficient charge modification to meet flocculation requirements. As current density increases to 6–10 mA / cm², 7–10 mA / cm², etc., charge modification efficiency improves, correspondingly shortening the reaction time. This ensures proper modification of particle charge properties while avoiding energy waste and particle structure damage due to over-modification. A reaction time of 0.7–1.1 hours at 9 mA / cm² represents the optimal matching parameter for charge modification, protecting particle integrity while achieving efficient modification.

[0019] Preferably, in step (S04), polyaluminum chloride and polyacrylamide are used as flocculants. Polyaluminum chloride performs preliminary flocculation of calcium fluoride microparticles through adsorption bridging, causing the dispersed small particles to aggregate into loose flocs; polyacrylamide, as a coagulant aid, further agglomerates the initially flocculated particles through the entrapment effect of its long molecular chains, forming structurally stable large-sized flocs; the synergistic effect of the two significantly improves the solid-liquid separation efficiency, prevents the loss of fine calcium fluoride particles with water, and ensures sludge recovery rate and final product purity.

[0020] Preferably, the concentration of polyaluminum chloride is 2-10%, the dosage is 1-3% of the concentrated acid water, and the reaction time is maintained at pH 5.5-6.5 for 30-60 min; more preferably, the concentration of polyaluminum chloride is 3-9%, the dosage is 1.5-2.8% of the concentrated acid water, and the reaction time is maintained at pH 5.6-6.4 for 30-55 min; even more preferably, the concentration of polyaluminum chloride is 4-8%, the dosage is 1.8-2.5% of the concentrated acid water, and the reaction time is maintained at pH 5.7-6.3 for 30-50 min; even more preferably, the concentration of polyaluminum chloride is 5-7%, the dosage is 2.0-2.3% of the concentrated acid water, and the reaction time is maintained at pH 5.8-6.2 for 35-45 min; even more preferably, the concentration of polyaluminum chloride is 6%, the dosage is 2.1% of the concentrated acid water, and the reaction time is maintained at pH 6.0 for 40 min. A polyaluminum chloride (PAC) concentration of 2–10%, a dosage of 1–3%, and a pH range of 5.5–6.5 ensure sufficient ionization and flocculation. Gradually narrowing parameter ranges, such as concentrations of 3–9% and dosages of 1.5–2.8%, precisely optimize flocculation conditions, minimizing reagent waste and impurity introduction while maintaining flocculation effectiveness. The optimal combination of a 6% concentration, a 2.1% dosage, a pH of 6.0, and a reaction time of 40 minutes achieves efficient initial flocculation of calcium fluoride particles with minimal reagent consumption, without causing impurity residue due to excessive reagent, thus laying a good foundation for subsequent PAM-assisted coagulation.

[0021] Preferably, cationic polyacrylamide is used, with an ionicity of 50, 60, 70, or 80. The cationic polyacrylamide is delivered from the storage tank to the flocculation reaction tank via a third metering pump, and mixed by stirring. The ionicity of the cationic polyacrylamide (50, 60, 70, or 80) is compatible with the negative surface charge of the modified calcium fluoride particles, enabling rapid adsorption and neutralization of particle charge, reducing interparticle repulsion. The third metering pump ensures precise dosing, avoiding secondary pollution from excessive dosage or poor flocculation due to insufficient dosage. The stirring action ensures uniform contact between the polyacrylamide and the mixture, allowing the initially flocculated loose flocs to quickly aggregate into large-size, high-density flocs, improving sedimentation rate and solid-liquid separation.

[0022] Preferably, the polyacrylamide concentration is 2-5%, the dosage is 15-30% of the concentrated acid water volume, and the stirring time is 30-60 minutes. More preferably, the polyacrylamide concentration is 3-4%, the dosage is 18-25% of the concentrated acid water volume, and the stirring time is 30-50 minutes. A polyacrylamide concentration of 2-5%, a dosage of 15-30%, and a stirring time of 30-60 minutes ensure that its molecular chains are fully extended and fully combined with the calcium fluoride particles, achieving optimal entrapment effect. A concentration of 3-4%, a dosage of 18-25%, and a stirring time of 30-50 minutes are optimized parameters that reduce reagent consumption while avoiding excessive stirring that could lead to floc breakage, ensuring stable flocculation and sedimentation effects and improving sludge recovery rate.

[0023] Preferably, in step (S05), the dewatering is carried out by plate and frame filtration to obtain calcium fluoride sludge, and the moisture content of the calcium fluoride sludge after plate and frame filtration is 40-50%; more preferably, the moisture content of the calcium fluoride sludge after plate and frame filtration is 45%. Plate and frame filtration efficiently removes free water and some bound water from the calcium fluoride flocculent sludge by means of high pressure. A moisture content of 40-50% can avoid excessive energy consumption during subsequent drying and calcination, while preventing the sludge from clumping due to excessively low moisture content, which would affect subsequent treatment; 45% is the optimal moisture content, which can ensure that the residual moisture and volatile impurities evaporate quickly during drying and calcination, and can also prevent the destruction of the calcium fluoride crystal structure due to excessive dewatering during the filtration process, ensuring that the purity of the final product is not affected.

[0024] Preferably, in step (S05), the drying and calcination temperature is 100–300°C and the time is 12–24 h; more preferably, the drying and calcination temperature is 150–300°C and the time is 10–20 h; even more preferably, the drying and calcination temperature is 200–300°C and the time is 8–16 h; even more preferably, the drying and calcination temperature is 250–300°C and the time is 6–12 h. A drying and calcination temperature of 100–300°C can gradually remove residual moisture, unreacted reagents, and volatile impurities from the sludge, and a time of 12–24 h ensures thorough impurity removal. As the temperature increases to 150–300°C, 200–300°C, and 250–300°C, the impurity volatilization rate accelerates, correspondingly shortening the calcination time by 10–20 h, 8–16 h, and 6–12 h, respectively, thus improving production efficiency while ensuring product purity. This temperature range will not damage the calcium fluoride crystal structure, ensuring stable product performance.

[0025] Preferably, the calcium fluoride prepared by this method has a purity of 88-92%, which can be used in metallurgical and other industrial fields. Through precise control and synergistic effect of parameters in the preceding steps, efficient capture of fluoride ions, directional growth of calcium fluoride crystals, and gradual separation of impurities are achieved. Compound calcium salts ensure sufficient reaction, citric acid optimizes crystal growth, microcurrent improves flocculation efficiency, and dehydration and calcination remove residual impurities, ultimately achieving a calcium fluoride purity of 88-92%. This purity meets the purity requirements of raw materials in metallurgical and other industrial fields, realizes the resource recovery of concentrated acid wastewater from photovoltaic cells, and enhances the economic value of waste.

[0026] The second technical solution of the present invention: an apparatus for in-situ preparation of high-purity calcium fluoride from concentrated acid wastewater from photovoltaic cells, comprising:

[0027] The preparation tank system supplies a calcium salt solution of calcium hydroxide and calcium hydroxychloride to the reaction tank system, and the reaction tank system supplies the completed mixture to the flocculation system.

[0028] The preparation tank system includes a preparation tank, which has a lid on top. On the lid are two metering tanks for adding calcium hydroxide powder and calcium hydroxychloride, respectively.

[0029] The configuration tank system is equipped with a first stirring paddle driven by a first motor. A large gear is connected to the central shaft of the first stirring paddle. Small gears that are rotatably connected to the tank cover are symmetrically meshed on both sides of the large gear. A gear housing is connected below the tank cover. The large gear and small gear are located inside the gear housing. A dispersing blade is connected to the central shaft of the small gear.

[0030] The mixing tank system also includes a fan impeller connected to the central shaft of the pinion gear. The fan impeller is located inside the exhaust casing, which is installed below the gear housing. One side of the exhaust casing has an air intake, and the exhaust outlet is connected to a blower pipe. The other end of the blower pipe connects to two metering tanks. A first motor drives the first stirring paddle to rotate, while a large gear meshes with a small gear to drive the dispersing blades to rotate. This achieves the mixing and dispersion of materials within the mixing tank, allowing calcium hydroxide and calcium hydroxychloride to mix initially and preventing agglomeration. The pinion gear drives the fan impeller to rotate within the exhaust casing, generating airflow that is delivered to the metering tanks via the blower pipes. This agitates the calcium salt powder airflow within the metering tanks, preventing powder bridging and clogging of the dosing channel, and ensuring smooth metering. The mixing tank system supplies the compounded calcium salt solution to the reaction tank system, providing raw materials for subsequent reactions and laying the foundation for the reaction of concentrated acid wastewater with calcium salts to produce calcium fluoride.

[0031] Preferably, the outlet end of the blower is connected to a diffuser shell. The diffuser shell diffuses the calcium hydroxide and calcium hydroxychloride blown out of the metering tank, achieving uniform airflow distribution and limiting upward diffusion. This allows the airflow to cover a wider area inside the metering tank, enhances the uniformity of calcium salt powder agitation, and further prevents localized powder deposition.

[0032] Preferably, the metering tank is equipped with a shaft driven by a second motor, on which straight spiral blades and conical spiral blades are mounted. The second motor drives the shaft to rotate, causing the straight and conical spiral blades to rotate synchronously, thus achieving the function of metering calcium salt powder. This ensures precise control of the dosage and ratio of calcium hydroxide and calcium hydroxychloride, and guarantees a stable calcium ion concentration in the compound calcium salt solution within the mixing tank.

[0033] Preferably, a material distribution plate is installed inside the metering tank above the conical spiral blades. The calcium salt powder impacts the distribution plate as it falls, pre-dispersing and breaking up any agglomerates, preventing blockage of the spiral blades, and ensuring continuous metering.

[0034] Preferably, a scraper is slidably connected to one side of the lower part of the shaft, and a first spring is connected between the scraper and the shaft. The first spring drives the scraper to always be in contact with the inner wall of the metering tank. The shaft drives the scraper to rotate, and the first spring provides elasticity to keep the scraper tightly against the tank wall, thereby realizing the material cleaning function of the inner wall of the metering tank. This prevents calcium salt powder from adhering to the tank wall, which would cause deviation in the actual dosage and ensure the accuracy of the metering.

[0035] Preferably, a slide rod is fixedly connected to the lower end of the shaft, a conical block is slidably connected to the slide rod, and a second spring is sleeved on the slide rod. One end of the second spring is connected to the conical block, and the other end is connected to a protrusion at the lower end of the slide rod. The elastic force of the second spring pushes the conical block to move up and down on the slide rod, realizing the function of adjusting the gap of the metering tank outlet and the function of uniform diffusion. This achieves the effects of adapting to the discharge requirements of calcium salt powders of different particle sizes, controlling the discharge speed stably, and further ensuring the accuracy of quantitative dosing.

[0036] Preferably, the reaction tank system includes a first-stage reaction tank, which is connected to a second-stage reaction tank via a first overflow pipe. The second-stage reaction tank is connected to a third-stage reaction tank via a second overflow pipe. A preparation tank is connected to the first-stage reaction tank via a first metering pump, and the effluent from the third-stage reaction tank is discharged via a third overflow pipe. The first metering pump precisely delivers the compound calcium salt solution from the preparation tank into the first-stage reaction tank. The reaction solution flows sequentially into the second-stage and third-stage reaction tanks via overflow pipes, realizing the staged reaction function of concentrated acid wastewater and calcium salt. This gradually increases the probability of fluoride ion contact reaction and avoids the effect of insufficient reaction in a single reaction leading to fluoride ion residue, thus creating sufficient reaction conditions for the production of high-purity calcium fluoride.

[0037] Preferably, the first, second, and third overflow pipes are all equipped with mechanical valves. Each mechanical valve includes a cap, one end of which is hinged to the inner wall of the overflow pipe. A push rod is movably connected above the cap, with its upper end extending upwards through the overflow pipe and fitted with a locking sleeve. The locking sleeve is fixedly connected to the reaction tank. By pushing the push rod to adjust the opening angle of the cap, and with the locking sleeve fixing the push rod's position, the flow rate of the reaction liquid in the overflow pipes is controlled. This stabilizes the liquid level in each reaction tank, ensures sufficient reaction time for each stage, and prevents incomplete reaction due to excessively rapid flow of the reaction liquid.

[0038] Preferably, the reaction tank system also includes a mixing pipe, which is rotatably mounted inside the third overflow pipe via a bearing. One end of the mixing pipe is connected to a guide impeller, and the inner wall of the mixing pipe is equipped with guide vanes. The reaction liquid discharged from the three-stage reaction tank impacts the guide impeller, causing the mixing pipe to rotate around the bearing. At the same time, the guide vanes guide the liquid to rotate and flow, realizing the pre-mixing function of the reaction liquid with subsequently added agents such as citric acid. This improves the uniformity of liquid mixing and shortens the mixing reaction time in the subsequent flocculation system.

[0039] Preferably, baffles are provided on one side of the inlet of the first, second, and third overflow pipes. By impacting the baffles before the reaction liquid flows into the overflow pipe, a buffering function is achieved, which prevents the reaction liquid from directly impacting the inner wall of the overflow pipe and causing liquid level fluctuations, and stabilizes the reaction environment in each reaction tank.

[0040] Preferably, a third motor is installed on each of the first, second, and third reaction tanks. The output shaft of the third motor is connected to a stirring shaft, and a splined sleeve is connected below the stirring shaft. A turntable is fixedly connected below the splined sleeve, and stirring blades are installed on the turntable. The third motor drives the stirring shaft to rotate, and the power is transmitted through the splined sleeve to make the turntable and stirring blades rotate synchronously, thereby realizing the stirring function of the reaction liquid in the reaction tank. This ensures that the calcium salt and the fluoride ions in the concentrated acid wastewater come into full contact, accelerating the formation of calcium fluoride.

[0041] Preferably, a hollow shaft is fixedly connected below the turntable, and the hollow shaft is rotatably connected to the bottom of the reaction tank. A screw is movably connected inside the hollow shaft, and the screw is threadedly connected to the stirring shaft. A third spring and a sliding sleeve are fitted on the stirring shaft. One end of the third spring is connected to a spline sleeve, and the other end is connected to the sliding sleeve. By rotating the screw, the stirring shaft is moved up and down using the threaded engagement, thereby adjusting the height of the turntable and stirring blades, realizing the stirring height adjustment function. The sleeve is fitted in the area where the stirring shaft and the spline sleeve meet, helping to buffer the impact force when the stirring shaft moves up and down, ensuring the stability of the spline engagement. The sleeve is fitted on the outside of the rotating sleeve, connecting the rotating sleeve and the hollow shaft, reducing friction when the rotating sleeve rotates, improving the smoothness of the adjustment disc rotation, and adapting to different liquid levels in the reaction tank. Furthermore, when the stirring blades are adjusted to contact the bottom of the reaction tank, the rotating stirring blades function to scrape off the bottom sediment.

[0042] Preferably, the turntable has a cavity containing an adjusting disc. The stirring blades are hinged to the adjusting disc via a connector. A rotating sleeve is connected below the adjusting disc, located inside a hollow shaft and fitted onto the outside of a screw. A rotating base is connected below the rotating sleeve. Rotating the rotating base causes the rotating sleeve to rotate, making the adjusting disc rotate within the turntable cavity. This, in turn, pulls the stirring blades to extend or retract via the connector, achieving the function of adjusting the stirring range. This adapts to the reaction requirements of different areas within the reaction tank, such as the tank walls, improves stirring coverage, further ensures sufficient reaction, and effectively removes adhering substances from the side walls of the reaction tank.

[0043] Preferably, the flocculation system includes a flocculation reaction tank, with three reaction tanks connected to it via a third overflow pipe. The bottom of the flocculation reaction tank has a cavity, with a battery interface on one side. Electrode plates are inserted into the cavity through the battery interface, and the electrode plates are sealed to the battery interface via a sealing ring and threaded connection. By energizing the electrode plates, a microcurrent is generated within the cavity at the bottom of the flocculation reaction tank, achieving surface charge regulation of the calcium fluoride particles. This alters the charge properties of the particles, creating conditions for subsequent flocculation and sedimentation. The sealing structure of the sealing ring and threaded connection ensures a seal between the battery interface and the electrode plates, preventing leakage of the flocculation reaction liquid and ensuring safe and environmentally friendly equipment operation.

[0044] Preferably, the bottom cavity of the flocculation reaction tank is connected to an outlet tank, the outlet of which is connected to a water pipe with a valve. Two air pipes pass through the side walls of the flocculation reaction tank and are connected to the outlet tank, with the other ends of the two air pipes connected to a T-junction. By opening the valve, the residual liquid in the outlet tank is discharged, and at the same time, gas is introduced into the air pipes through the T-junction. This gas disturbs the dead corners of the outlet tank, achieving a cleaning function and accelerating the discharge of residual liquid. This ensures the seamless connection of the electrode plates to the cavity of the flocculation reaction tank.

[0045] Preferably, a fourth motor is installed on the flocculation reaction tank, and the output shaft of the fourth motor is connected to a second stirring paddle. The fourth motor drives the second stirring paddle to rotate, thereby realizing the stirring function of the reaction liquid in the flocculation reaction tank.

[0046] Preferably, the flocculation system also includes a citric acid storage tank, which is connected to a third overflow pipe via a second metering pump. The second metering pump precisely delivers the citric acid solution from the storage tank to the third overflow pipe, achieving pH adjustment of the reaction solution. This reduces the metastable region of calcium fluoride, promotes calcium fluoride crystal growth, reduces impurity embedding in the crystals, and improves the purity of calcium fluoride.

[0047] Preferably, the flocculation system also includes two storage tanks, one for storing PAC solution and the other for storing PAM solution. These tanks are connected to the flocculation reaction tank via a third metering pump. The third metering pump precisely delivers the PAC and PAM solutions from the storage tanks to the flocculation reaction tank, achieving the flocculation function of calcium fluoride particles. This allows the fine calcium fluoride particles to aggregate into large flocs, facilitating subsequent solid-liquid separation.

[0048] Preferably, a metering pump is installed above the preparation tank and the two storage tanks. Its core function is to precisely supply solvent water to the preparation tank and the two storage tanks.

[0049] Preferably, a circulation system is also included. This system comprises a first reflux pump, whose inlet is connected to the bottom of the second-stage reaction tank and whose outlet is connected to the inlet pipe of the first-stage reaction tank. The system also includes a second reflux pump, whose inlet is connected to the bottom of the third-stage reaction tank and whose outlet is connected to the first overflow pipe. The first reflux pump returns the insufficiently reacted material from the bottom of the second-stage reaction tank to the first-stage reaction tank, and the second reflux pump returns the insufficiently reacted material from the bottom of the third-stage reaction tank to the first overflow pipe, thus re-entering the reaction process. This achieves the function of circulating the reaction liquid, improving the utilization rate of the reaction between calcium salts and fluoride ions, further reducing the fluoride ion concentration in the effluent, and reducing raw material waste.

[0050] The present invention has the following beneficial effects:

[0051] (1) This invention utilizes the synergistic reaction of calcium hydroxide and calcium hydroxychloride compound calcium salts. Calcium hydroxide provides sufficient calcium ions and neutralizes acidity, while calcium hydroxychloride assists in capturing fluoride ions. The two calcium sources form heterogeneous nucleation of calcium fluoride crystals, further promoting crystal formation and growth. With the regulation of citric acid solution, the calcium fluoride crystals are guided to grow in a directional manner, preventing impurities from embedding into the crystal lattice. At the same time, citric acid further reduces the metastable region of the calcium fluoride crystals by changing their growth kinetics, thus strengthening the crystallization process and reducing the generation of impurities such as calcium sulfate and silicates from the reaction mechanism level. In the mechanical scheme, a dual-quantitative tank system is configured with a material distribution plate to break up agglomerated calcium salts, a scraper to clean the tank wall residues, and a quantitative water pump to precisely control the stable concentration of the compound calcium salt solution and avoid impurities caused by fluctuations in calcium salt concentration. At the same time, the series structure of the three-stage reaction tank, the first to third stage of the progressive reaction and reflux system, and the first and second reflux pumps send the unreacted materials back to the front stage to ensure that fluoride ions and calcium ions react fully and reduce impurities caused by unreacted raw material residues. This ultimately achieves the preparation of high-purity calcium fluoride, overcoming the drawbacks of traditional processes such as high impurities and low purity.

[0052] (2) This invention modifies the surface charge of calcium fluoride particles by microcurrent modification, reducing the repulsive force between particles. PAC / PAM synergistic flocculation: PAC adsorption bridging forms primary flocs, and PAM nets and aggregates into large-sized flocs. Among them, PAM with higher ionicity is selected to more efficiently net and precipitate calcium fluoride particles, further improving the solid-liquid separation effect and creating better conditions for solid-liquid separation. In the mechanical scheme, the electrode plate of the flocculation system is sealed and inserted into the bottom cavity of the flocculation tank through the battery interface to ensure that the microcurrent acts stably on the mixed liquid and the mixing tube. The rotation is driven by the kinetic energy of the effluent from the three-stage reaction tank. The reaction liquid and citric acid are premixed through the guide plate to improve the uniformity of crystal growth. The adaptive stirrer of the reaction tank has an adjustable stirring height screw structure and an adjustable stirring plate assembly to ensure that the microcurrent modification and flocculation reaction are sufficient and avoid small and dispersed crystals. At the same time, the dewatering structure of the plate and frame filter press is adapted to the characteristics of flocculated sludge, further improving the solid-liquid separation efficiency and solving the problem of difficult crystal separation in traditional processes.

[0053] (3) The present invention adopts a continuous process of calcium salt reaction, citric acid regulation, microcurrent modification, flocculation separation, dehydration and calcination, without the need for additional acidification and impurity removal steps, thus avoiding the generation of secondary fluoride-containing wastewater; In the mechanical scheme, the impeller, blowing pipe and diffuser shell structure of the configuration tank are configured, and the impeller is driven by a small gear to generate airflow, preventing calcium salt bridging and blockage in the quantitative tank, reducing equipment maintenance costs; The mechanical valve push rod + locking sleeve of the overflow pipe of the reaction tank controls the flow rate and the baffle buffer to stabilize the liquid level, ensuring stable operation of each link and reducing the complexity of operation; The return pump of the circulation system improves the utilization rate of calcium salt and reduces raw material waste; The gas pipe and three-way structure of the effluent tank of the flocculation tank clean up dead corners of gas disturbance, avoid blockage affecting equipment operation, further simplify the process and reduce operating costs; At the same time, the sealing design of the electrode plate seals the connection to avoid leakage of reaction liquid and eliminate safety hazards. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the overall front structure of the device of the present invention;

[0055] Figure 2 This is a schematic diagram of the overall reverse structure of the device of the present invention;

[0056] Figure 3 A schematic diagram of the configuration tank and the metering tank;

[0057] Figure 4 A schematic diagram of the device structure for configuring the can lid;

[0058] Figure 5 This is a schematic diagram of the internal structure of the metering tank;

[0059] Figure 6 This is a schematic diagram showing the connection between the metering tank and the exhaust casing;

[0060] Figure 7 This is a schematic diagram of the cleaning structure for the inner wall of the metering tank.

[0061] Figure 8 This is a schematic diagram of the internal structure of the reaction tank;

[0062] Figure 9 This is a schematic diagram of the first possible structure of the stirring structure component in the reaction tank;

[0063] Figure 10 This is a schematic diagram of the second structure of the stirring structure component in the reaction tank;

[0064] Figure 11 This is a schematic diagram of the internal structure of the overflow pipe;

[0065] Figure 12 This is a schematic diagram of the internal structure of the mixing tube;

[0066] Figure 13 This is a schematic diagram of the flocculation reaction tank structure;

[0067] Figure 14 This is a schematic diagram of the cavity at the bottom of the flocculation reaction tank.

[0068] Figure 15 This is a schematic diagram of the effluent trough at the bottom of the flocculation reaction tank.

[0069] Figure 16 XRD pattern of calcium fluoride sludge from a certain factory;

[0070] Figure 17 The XRD pattern of high-purity calcium fluoride prepared according to this method;

[0071] Figure 18 EDS spectrum of calcium fluoride in Example 1;

[0072] Figure 19 EDS spectrum of calcium fluoride in Example 2;

[0073] Figure 20 Example 3: EDS spectrum of calcium fluoride.

[0074] The labels in the attached diagram are as follows: 100-Preparation tank, 101-Tank lid, 102-First motor, 103-First agitator, 104-Large gear, 105-Small gear, 106-Gear housing, 107-Dispersion blade, 108-Exhaust casing, 1081-Air inlet, 109-Impeller, 110-Blower pipe, 111-Diffuser casing, 200-Metering tank, 201-Second motor, 202-Shaft, 203- 204-Straight helical blade, 205-Conical helical blade, 206-Pack plate, 207-Scraper, 208-First spring, 209-Slide rod, 210-Conical block, 300-Second spring, 301-First metering pump, 302-First overflow pipe, 303-Second stage reaction tank, 304-Second overflow pipe, 305-Third stage reaction tank, 306-Third overflow pipe, 307-Push rod, 3 08-Locking sleeve, 309-Sealing cap, 310-Mixing tube, 311-Bearing, 312-Guide impeller, 313-Guide vane, 314-Baffle, 400-Third motor, 401-Stirring shaft, 402-Spline sleeve, 403-Turntable, 404-Stirring blade, 405-Hollow shaft, 406-Screw, 407-Third spring, 408-Sliding sleeve, 409-Rotating sleeve, 410-Rotating base, 411-Adjusting sleeve Section plate, 412-connector, 500-flocculation reaction tank, 501-battery interface, 502-electrode plate, 503-water outlet tank, 504-valve, 505-gas pipe, 506-te-way, 600-fourth motor, 601-second agitator, 700-citric acid storage tank, 701-second metering pump, 800-storage tank, 801-third metering pump, 900-first reflux pump, 901-second reflux pump. Detailed Implementation

[0075] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0076] A method for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells includes the following steps:

[0077] (S01) A compound calcium salt solution prepared by calcium hydroxide and calcium hydroxychloride is pumped into concentrated acid wastewater to react and generate calcium fluoride particles; the concentration of the compound calcium salt solution in step (S01) is 5% to 15%, the concentration of the compound calcium salt solution is 8% to 12%, the concentration of the compound calcium salt solution is 9% to 11%, and the concentration of the compound calcium salt solution is 10%; the compound calcium salt solution in step (S01) is continuously pumped into the concentrated acid reaction tank by a metering pump. The reaction tank is divided into three sections and is equipped with reflux. The reaction time of the last section of the reaction tank is 30 to 60 minutes, and the total reaction time is 60 to 90 minutes; when the pH of the effluent from the reaction tank is stable at 7.5 to 8.5, the metering pump is stopped from delivering the compound calcium salt solution, preferably when the pH is stable at 7.5 to 8.0;

[0078] (S02) Add citric acid solution to the mixture after reaction in step (S01); the concentration of citric acid solution in step (S02) is 1-5%, the concentration of citric acid solution is 2-4%, and the concentration of citric acid solution is 3%; when the pH of the mixture after reaction is adjusted back to 6.5-7.0 by citric acid solution, stop the metering pump, and the stirring time during the adjustment process is 10-20 min; when the pH of the mixture after reaction is adjusted back to 6.5-6.8 by citric acid solution, stop the metering pump, and the stirring time during the adjustment process is 10-18 min; when the pH of the mixture after reaction is adjusted back to 6.5-6.7 by citric acid solution, stop the metering pump, and the stirring time during the adjustment process is 10-15 min; when the pH of the mixture after reaction is adjusted back to 6.6 by citric acid solution, stop the metering pump, and the stirring time during the adjustment process is 12 min.

[0079] (S03) Apply a microcurrent to the mixture after treatment in step (S02) to change the surface charge properties of the calcium fluoride particles; in step (S03), the current density is 5-10 mA / cm², and the microcurrent treatment time is 1-2 h; the current density is 6-10 mA / cm², and the microcurrent treatment time is 0.8-1.7 h; the current density is 7-10 mA / cm², and the microcurrent treatment time is 0.7-1.5 h; the current density is 8-10 mA / cm², and the microcurrent treatment time is 0.6-1.3 h; the current density is 9 mA / cm², and the microcurrent treatment time is 0.7-1.1 h.

[0080] (S04) Add flocculant to the mixed liquid after treatment in step (S03) to cause calcium fluoride particles to flocculate and settle, obtaining calcium fluoride flocculent sludge; the flocculant used in step (S04) is polyaluminum chloride and polyacrylamide; the concentration of polyaluminum chloride is 2-10%, the dosage is 1-3% of the concentrated acid water volume, and the reaction time is maintained at pH 5.5-6.5 for 30-60 min; the concentration of polyaluminum chloride is 3-9%, the dosage is 1.5-2.8% of the concentrated acid water volume, and the reaction time is maintained at pH 5.6-6.4 for 30-55 min; the concentration of polyaluminum chloride is 4-8%, the dosage is 1.8-2.5% of the concentrated acid water volume, and the reaction time is maintained at pH 5.7-6.3 for 30-50 min; the concentration of polyaluminum chloride is 5-7%. The dosage is 2.0-2.3% of the concentrated acid water volume, and the reaction time is maintained at pH 5.8-6.2 for 35-45 minutes. The polyaluminum chloride concentration is 6%, the dosage is 2.1% of the concentrated acid water volume, and the reaction time is maintained at pH 6.0 for 40 minutes. For polyacrylamide, a cationic flocculant is used, with an ionicity of 50, 60, 70, or 80. The cationic flocculant is delivered from the storage tank to the flocculation reaction tank via a third metering pump, and mixing is achieved through stirring. For polyacrylamide concentrations of 2-5%, the dosage is 15-30% of the concentrated acid water volume, and the stirring time is 30-60 minutes. For polyacrylamide concentrations of 3-4%, the dosage is 18-25% of the concentrated acid water volume, and the stirring time is 30-50 minutes.

[0081] (S05) The calcium fluoride flocculent sludge obtained in step (S04) is dewatered by pressure filtration, and the dewatered filter cake is dried and calcined to obtain high-purity calcium fluoride; the dewatering in step (S05) is carried out by plate and frame filtration to obtain calcium fluoride sludge, and the moisture content of the calcium fluoride sludge after plate and frame filtration is 40-50%; the moisture content of the calcium fluoride sludge after plate and frame filtration is 45%. The drying and calcination temperature in step (S05) is 100-300℃ for 12-24h; the drying and calcination temperature is 150-300℃ for 10-20h; the drying and calcination temperature is 200-300℃ for 8-16h; the drying and calcination temperature is 250-300℃ for 6-12h; the calcium fluoride prepared by this method has a purity of 88-92% and can be used in metallurgical and other industrial fields.

[0082] A method for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells includes the following steps:

[0083] (S01) A compound calcium salt solution prepared by calcium hydroxide and calcium hydroxychloride is pumped into concentrated acid wastewater to react and generate calcium fluoride particles.

[0084] (S02) Add citric acid solution to the mixture after the reaction in step (S01);

[0085] (S03) Apply a microcurrent to the mixture after step (S02) to change the surface charge properties of the calcium fluoride particles;

[0086] (S04) Add flocculant to the mixture after treatment in step (S03) to cause calcium fluoride particles to flocculate and settle, and obtain calcium fluoride flocculent sludge.

[0087] (S05) The calcium fluoride flocculent sludge obtained in step (S04) is dewatered by pressure filtration, and the dewatered filter cake is dried and calcined to obtain high-purity calcium fluoride.

[0088] In step (S01), the concentration of the compound calcium salt solution is 5%–15%, the concentration of the compound calcium salt solution is 8%–12%, the concentration of the compound calcium salt solution is 9%–11%, and the concentration of the compound calcium salt solution is 10%.

[0089] In step (S01), the compound calcium salt solution is continuously pumped into the concentrated acid reaction tank by a metering pump. The reaction tank is divided into three sections and is equipped with reflux. The reaction time of the last section of the reaction tank is 30-60 min, and the total reaction time is 60-90 min.

[0090] A method for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells includes the following steps:

[0091] (S01) A calcium hydroxide and calcium hydroxychloride compound solution with a calcium ion concentration of 5% to 15% is pumped into the concentrated acid reaction tank using a metering pump. The solution reacts with the concentrated acid to generate calcium fluoride particles. The pumping volume and pumping rate are controlled by pH. The acid reaction tank is thoroughly stirred in three stages. The reaction time of the last stage is 30 to 60 minutes, and the total reaction time of the three stages is 60 to 90 minutes. The pH of the effluent from the last stage of the reaction tank is stable at 7.5 to 8.5. Fluoride ions are removed to the maximum extent, and the excess calcium hydroxide is not obvious.

[0092] (S02) Prepare a citric acid solution with a concentration of 1-5%. Pump the citric acid solution into the above-mentioned effluent using a metering pump. Stop the metering pump when the pH is adjusted to 6.5-7.0, and stir for 10-20 minutes. There are two key purposes for adding the citric acid solution: First, it forms calcium citrate, shrinking the metastable region of calcium fluoride, inhibiting nucleation, and accelerating crystal growth. Second, it adsorbs and embeds calcium fluoride crystals onto specific crystal faces of calcium citrate, affecting the expansion rate of crystal steps and thus altering the crystal growth rate, resulting in the best uniformity of calcium fluoride crystal particles.

[0093] (S03) Apply a power source to the above-mentioned effluent and change the surface charge properties of the calcium fluoride particles through microcurrent. The current density is 5-10 mA / cm², and the action time is 1-2 h.

[0094] (S04) Calcium fluoride particles are separated by efficient flocculation, with the flocculation reaction taking place in a flocculation reaction tank. Polyaluminum chloride is used as the flocculant: its concentration is prepared at 2-10%, the pumping rate is 1%-3% of the concentrated acid water volume, and the reaction time is maintained at pH 5.5-6.5 for 30-60 minutes. Ammonium polyacrylate (PAM) is selected as the coagulant aid, using a cationic flocculant with an ionicity of 50-80. The PAM concentration is prepared at 2-5%, the dosage is 15-30% of the concentrated acid water volume, and the stirring time is 30-60 minutes.

[0095] (S05) Pressure filtration and calcination: The calcium fluoride obtained from the above precipitation is sent to a plate and frame filter press system for dewatering. After dewatering, the sludge has a moisture content of 40-50%. Then it is transported to a calcining furnace for drying and calcination at a temperature of 100-300℃ for 12-24 hours.

[0096] In step (S01), the calcium ion concentration in the calcium hydroxide and calcium hydroxychloride compound solution is 8%–12%. To prevent calcium hydroxide from depositing at the bottom, the reaction vessel must be thoroughly stirred. A further preferred concentration is 9%–11%.

[0097] The pH of the effluent from the final stage of the concentrated acid reaction tank is stabilized at 7.5–8.5. To prevent excessive calcium hydroxide from reducing the purity of calcium fluoride, it can be further optimized to be 7.5–8.0.

[0098] Step (S01) Prepare a citric acid solution with a concentration of 1-2%, and pump the citric acid solution into the above-mentioned effluent using a metering pump to adjust the pH to 6.5-6.8.

[0099] Preferably, the concentration of the polyaluminum chloride prepared in step S4 is 4-6%, and the final pH is adjusted to 6.0-6.5 when adding the polyaluminum chloride solution. The polyacrylamide is preferably a cationic coagulant with an ionicity of 50.

[0100] When the pH of the effluent from the reaction tank stabilizes at 7.5–8.5, the metering pump should be stopped from delivering the compound calcium salt solution. Preferably, the metering pump should be stopped when the pH stabilizes at 7.5–8.0.

[0101] The concentration of the citric acid solution in step (S02) is 1-5%, the concentration of the citric acid solution is 2-4%, and the concentration of the citric acid solution is 3%.

[0102] The metering pump was stopped when the pH of the mixture after the reaction with citric acid solution was adjusted to 6.5–7.0, and the stirring time during the adjustment process was 10–20 min; the metering pump was stopped when the pH of the mixture after the reaction with citric acid solution was adjusted to 6.5–6.8, and the stirring time during the adjustment process was 10–18 min; the metering pump was stopped when the pH of the mixture after the reaction with citric acid solution was adjusted to 6.5–6.7, and the stirring time during the adjustment process was 10–15 min; the metering pump was stopped when the pH of the mixture after the reaction with citric acid solution was adjusted to 6.6, and the stirring time during the adjustment process was 12 min.

[0103] In step (S03), the current density is 5–10 mA / cm², and the microcurrent application time is 1–2 h; the current density is 6–10 mA / cm², and the microcurrent application time is 0.8–1.7 h; the current density is 7–10 mA / cm², and the microcurrent application time is 0.7–1.5 h; the current density is 8–10 mA / cm², and the microcurrent application time is 0.6–1.3 h; and the current density is 9 mA / cm², and the microcurrent application time is 0.7–1.1 h.

[0104] In step (S04), polyaluminum chloride and polyacrylamide are used as flocculants.

[0105] The following reactions can be carried out: Polyaluminum chloride (PAC) concentration: 2–10%, dosage: 1–3% of concentrated acid solution, reaction time: 30–60 min at pH 5.5–6.5; Polyaluminum chloride (PAC) concentration: 3–9%, dosage: 1.5–2.8% of concentrated acid solution, reaction time: 30–55 min at pH 5.6–6.4; Polyaluminum chloride (PAC) concentration: 4–8%, dosage: 1.8–2.5% of concentrated acid solution, reaction time: 30–50 min at pH 5.7–6.3; Polyaluminum chloride (PAC) concentration: 5–7%, dosage: 2.0–2.3% of concentrated acid solution, reaction time: 35–45 min at pH 5.8–6.2; Polyaluminum chloride (PAC) concentration: 6%, dosage: 2.1% of concentrated acid solution, reaction time: 40 min at pH 6.0.

[0106] The polyacrylamide uses cationic flocculants, with one of the following ionicity levels: 50, 60, 70, and 80. The cationic flocculants are delivered from the storage tank to the flocculation reaction tank via a third metering pump and mixed by stirring.

[0107] The concentration of polyacrylamide prepared is 2-5%, the dosage is 15-30% of the concentrated acid water, and the stirring time is 30-60 minutes; the concentration of polyacrylamide prepared is 3-4%, the dosage is 18-25% of the concentrated acid water, and the stirring time is 30-50 minutes.

[0108] In step (S05), the dewatering process is carried out by plate and frame filtration to obtain calcium fluoride sludge. The water content of the calcium fluoride sludge after plate and frame filtration is 40-50%; the water content of the calcium fluoride sludge after plate and frame filtration is 45%.

[0109] In step (S05), the drying and calcining temperature is 100-300℃ and the time is 12-24h; the drying and calcining temperature is 150-300℃ and the time is 10-20h; the drying and calcining temperature is 200-300℃ and the time is 8-16h; and the drying and calcining temperature is 250-300℃ and the time is 6-12h.

[0110] The calcium fluoride prepared by this method has a purity of 88-92% and can be used in metallurgical and other industrial fields.

[0111] Example 1: Concentrated acid wastewater from a factory was selected, with a fluoride ion concentration of 44500 mg / L. A mixed solution of calcium hydroxide and calcium hydroxychloride with a calcium ion concentration of 10% was prepared and pumped into the concentrated acid reaction tank by a metering pump. An internal reflux pump was installed, and each step was controlled by a pH meter. Thorough stirring and bottom-push flow were used to prevent sludge sedimentation. The solution reacted with the concentrated acid to generate calcium fluoride particles. The pumping volume and rate were controlled by pH. The acid reaction tank was thoroughly stirred in three stages, with the final stage reaction time being 30 min, and the total reaction time for all three stages being 60 min. The pH of the effluent from the final stage of the reaction tank was stabilized at 7.5. A prepared 1% citric acid solution was then pumped into the effluent by a metering pump. The metering pump was stopped when the pH was adjusted to 6.8, and stirring was continued for 10 min. An external power source was applied to the effluent, and a microcurrent was used to change the surface charge properties of the calcium fluoride particles. The current density was 5 mA cm⁻², and the reaction time was set to 1 h. The calcium fluoride particles were then separated by high-efficiency flocculation and pumped back into the reaction tank by a metering pump. The PAC concentration was set at 2%, and the pumping rate was 1% of the concentrated acid water volume. The mixture was stirred thoroughly for 30 minutes. For the coagulant aid, cationic polyacrylamide (PAM) with an ionicity of 50 was selected. The PAM concentration was set at 2%, and the dosage was 15% of the concentrated acid water volume. The stirring time was 30 minutes. After sludge-water separation, the fluoride ion concentration of the supernatant was tested and found to be 3.5 mg / L. The supernatant was then discharged into a combined tank, and the bottom sludge was pumped into the plant's plate and frame filter press system. After dewatering, the sludge moisture content was 40%. It was then transported to a calcining furnace for drying. The calcination temperature was first 100℃ for 12 hours, then increased to 300℃ at a heating rate of 5℃ / min for 12 hours. This yielded high-purity calcium fluoride with a purity of 90.2%. Figure 18 This is an EDS elemental composition diagram of the powder in this case. The calcium fluoride has a high purity, and the atomic proportion of the impurity element silicon is only 0.3%. Figure 17 This is the XRD pattern of calcium fluoride in this case. The peaks of CaF2 (111), (220), and (311) are obvious, and there are no impurity peaks. (Comparison) Figure 16This refers to the purity of calcium fluoride in the currently produced sludge. Its calcium fluoride purity is only 60.2%, and the XRD pattern shows characteristic peaks for calcium fluoride, calcium silicate, and calcium carbonate, in addition to the calcium fluoride peak. Its economic value is very low.

[0112] Comparative Example 1: High-purity calcium fluoride sludge was prepared from concentrated acid using the same method. The difference from Example 1 was that citric acid solution was not used to adjust the pH; other steps were the same as in Example 1. The sludge obtained by this method had a moisture content of 45% and a calcium fluoride purity of 84.5%, significantly lower than the calcium fluoride powder obtained in Example 1. The main reason is that this method did not add citric acid, resulting in a larger metastable region for calcium fluoride crystals, making efficient separation of crystal particles difficult.

[0113] Comparative Example 2: High-purity calcium fluoride sludge was prepared from concentrated acid using the same method. The difference from Example 1 was that an electric current was not applied to the turbid calcium fluoride liquid after the S2 reaction was completed. The sludge obtained by the same method had a water content of 46.5% and a calcium fluoride purity of 86.5%, which was significantly lower than the calcium fluoride powder obtained in Example 1. The upper liquid was turbid, and the flocculation effect was poor. The main reason was that no electric current was applied in this method, the calcium fluoride particles were too small, and the surface charge properties of the calcium fluoride particles were not changed, resulting in poor flocculant trapping effect.

[0114] Comparative Example 3: High-purity calcium fluoride sludge was prepared from concentrated acid using the same method. The difference from Example 1 was that anionic PAM was used in the flocculation stage, which prevented sludge-water separation and resulted in the failure to obtain calcium fluoride sludge. This demonstrates that cationic PAM and ionicity play a crucial role in the sludge-water separation of calcium fluoride suspension.

[0115] Example 2: Concentrated acid wastewater from a factory with a fluoride ion concentration of 22500 mg / L was selected. A mixed solution of calcium hydroxide and calcium hydroxychloride with a calcium ion concentration of 10% was prepared and pumped into the concentrated acid reaction tank by a metering pump. An internal reflux pump was installed and controlled by a pH meter. Thorough stirring was maintained, and a flow-boosting device was activated to prevent calcium fluoride sludge sedimentation. The solution reacted with the concentrated acid to generate calcium fluoride particles. The pumping rate and volume were controlled by pH. The acid reaction tank was thoroughly stirred in three stages, with the final stage reaction time being 40 min, and the total reaction time for all three stages being 80 min. The pH of the effluent from the final stage of the reaction tank stabilized at 7.8. A prepared 1.5% citric acid solution was then pumped into the effluent by a metering pump. The metering pump was stopped when the pH was adjusted to 6.5, and stirring was maintained for 15 min. An external power source was applied to the effluent, and a microcurrent was used to alter the surface charge of the calcium fluoride particles. The current density was 8 mA cm⁻², and the reaction time was set to 1.5 h. The calcium fluoride particles were then separated by high-efficiency flocculation and pumped back into the reaction tank by the metering pump. The PAC concentration was set at 5%, and the pumping rate was 2% of the concentrated acid water volume. The mixture was stirred thoroughly for 30 minutes. For the coagulant aid, cationic polyacrylamide (PAM) with an ionicity of 50 was selected. The PAM concentration was set at 3%, and the dosage was 20% of the concentrated acid water volume. The stirring time was 45 minutes. After complete separation of sludge and water, the fluoride ion concentration of the supernatant was tested and found to be 1.5 mg / L. The supernatant was then discharged into a combined tank, and the bottom sludge was pumped into the plant's plate and frame filter press system. The dewatered sludge had a moisture content of 45%. It was then transported to a calcining furnace for drying. The calcination temperature was initially 100℃ for 12 hours, then increased to 300℃ at a rate of 5℃ / min for 8 hours. Figure 19 The image shows the EDS elemental composition of calcium fluoride powder from Case 2, indicating that the calcium fluoride has a high purity of 92.6%. The atomic percentage of silicon, an impurity element, is only 0.2%.

[0116] Example 3: Concentrated acid wastewater from a factory was selected, with a fluoride ion concentration of 86,500 mg / L. A mixed solution of calcium hydroxide and calcium hydroxychloride with a calcium ion concentration of 10% was prepared and pumped into the concentrated acid reaction tank by a metering pump. An internal reflux pump was installed, and each step was controlled by a pH meter. Thorough stirring was maintained, and a flow-propelling device was used to prevent sludge sedimentation. The solution reacted with the concentrated acid to generate calcium fluoride particles. The pumping rate and volume were controlled by pH. The acid reaction tank was thoroughly stirred in three stages, with the final stage reaction time being 30 minutes and the total reaction time being 60 minutes. The pH of the effluent from the final stage of the reaction tank stabilized at 8.0. A prepared 2% citric acid solution was then pumped into the effluent by a metering pump. The metering pump was stopped when the pH was adjusted to 6.5, and stirring was maintained for 20 minutes. An external power source was applied to the effluent, and a microcurrent was used to alter the surface charge of the calcium fluoride particles. The current density was 10 mA cm⁻², and the reaction time was set to 2 hours. The calcium fluoride particles were then separated by high-efficiency flocculation and pumped back into the reaction tank by the metering pump. The PAC concentration was set at 10%, and the pumping rate was 3% of the concentrated acid water volume. The mixture was stirred thoroughly for 30 minutes. For the coagulant aid, cationic polyacrylamide (PAM) with an ionicity of 50 was selected. The PAM concentration was set at 5%, and the dosage was 30% of the concentrated acid water volume. The stirring time was 30 minutes. After the flocs had completely settled, the fluoride ion concentration of the supernatant was tested and found to be 3.1 mg / L. The supernatant was then discharged into a combined tank, and the bottom sludge was pumped into the plant's plate and frame filter press system. After dewatering, the sludge had a moisture content of 50%. It was then transported to a calcining furnace for drying. The calcination temperature was initially 100℃ for 12 hours, then increased to 300℃ at a heating rate of 5℃ / min for 6 hours. Figure 20 The image shows the EDS elemental composition of calcium fluoride powder from Case 2, indicating that the calcium fluoride has a high purity of 91.2%. The atomic percentage of silicon, an impurity element, is only 0.2%.

[0117] Example 4: A method for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells, comprising the following steps:

[0118] (S01) A compound calcium salt solution prepared by calcium hydroxide and calcium hydroxychloride is pumped into concentrated acid wastewater to react and generate calcium fluoride particles; the concentration of the compound calcium salt solution in step (S01) is 5%; the compound calcium salt solution in step (S01) is continuously pumped into the concentrated acid reaction tank by a metering pump; the reaction tank is three-stage and equipped with reflux; the reaction time of the last stage of the reaction tank is 30 minutes, and the total reaction time is 60 minutes; when the pH of the effluent from the reaction tank stabilizes at 7.5, the metering pump is stopped from delivering the compound calcium salt solution;

[0119] (S02) Add citric acid solution to the mixture after reaction in step (S01); the concentration of citric acid solution in step (S02) is 1%, and the metering pump is stopped when the pH of the mixture after reaction is adjusted back to 6.5 by the citric acid solution, and the stirring time during the adjustment process is 10 min;

[0120] (S03) Apply a microcurrent to the mixture after treatment in step (S02) to change the surface charge properties of the calcium fluoride particles; the current density in step (S03) is 5 mA / cm², and the microcurrent application time is 1 h.

[0121] (S04) Add flocculant to the mixture after treatment in step (S03) to cause calcium fluoride particles to flocculate and settle, obtaining calcium fluoride flocculent sludge; the flocculant used in step (S04) is polyaluminum chloride and polyacrylamide; the concentration of polyaluminum chloride is 2%, the dosage is 1% of the concentrated acid water volume, and the reaction time is maintained at pH 5.5 for 30 min; the polyacrylamide is a cationic flocculant with an ionicity of 50, and the cationic polyacrylamide is delivered from the storage tank to the flocculation reaction tank by a third metering pump, and mixed by stirring; the concentration of polyacrylamide is 2%, the dosage is 15% of the concentrated acid water volume, and the stirring time is 30 min;

[0122] (S05) The calcium fluoride flocculent sludge obtained in step (S04) is dewatered by pressure filtration, and the dewatered filter cake is dried and calcined to obtain the high-purity calcium fluoride; the dewatering in step (S05) is carried out by plate and frame filtration to obtain calcium fluoride sludge, and the water content of the calcium fluoride sludge after plate and frame filtration is 40%. The drying and calcination temperature in step (S05) is 100℃ for 12 hours, and then the temperature is raised to 250℃ at a rate of 5℃ / min for 18 hours; the calcium fluoride prepared by this method has a purity of 88% and can be used in metallurgical and other industrial fields.

[0123] Example 5: A method for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells, comprising the following steps:

[0124] (S01) A compound calcium salt solution prepared by calcium hydroxide and calcium hydroxychloride is pumped into concentrated acid wastewater to react and generate calcium fluoride particles; the concentration of the compound calcium salt solution in step (S01) is 15%; the compound calcium salt solution in step (S01) is continuously pumped into the concentrated acid reaction tank by a metering pump; the reaction tank is three-stage and equipped with reflux; the reaction time of the last stage of the reaction tank is 60 min, and the total reaction time is 90 min; when the pH of the effluent from the reaction tank stabilizes at 8.5, the metering pump is stopped from delivering the compound calcium salt solution;

[0125] (S02) Add citric acid solution to the mixture after reaction in step (S01); the concentration of citric acid solution in step (S02) is 5%; the metering pump is stopped when the pH of the mixture after reaction is adjusted back to 7.0 by the citric acid solution, and the stirring time during the adjustment process is 20 min;

[0126] (S03) Apply a microcurrent to the mixture after step (S02) to change the surface charge properties of the calcium fluoride particles; the current density in step (S03) is 10 mA / cm², and the microcurrent application time is 2 h.

[0127] (S04) Add flocculant to the mixture after treatment in step (S03) to cause calcium fluoride particles to flocculate and settle, obtaining calcium fluoride flocculent sludge; the flocculant used in step (S04) is polyaluminum chloride and polyacrylamide; the concentration of polyaluminum chloride is 10%, the dosage is 3% of the concentrated acid water volume, and the reaction time is maintained at pH 6.5 for 60 min; the polyacrylamide is a cationic flocculant with an ionicity of 80, and the cationic polyacrylamide is delivered from the storage tank to the flocculation reaction tank by a third metering pump, and mixed by stirring; the concentration of polyacrylamide is 5%, the dosage is 30% of the concentrated acid water volume, and the stirring time is 60 min;

[0128] (S05) The calcium fluoride flocculent sludge obtained in step (S04) is dewatered by pressure filtration, and the dewatered filter cake is dried and calcined to obtain the high-purity calcium fluoride; the dewatering in step (S05) is carried out by plate and frame filtration to obtain calcium fluoride sludge, and the water content of the calcium fluoride sludge after plate and frame filtration is 50%; the drying and calcination temperature in step (S05) is 300℃ and the time is 24h; the calcium fluoride prepared by this method has a purity of 92% and can be used in metallurgical and other industrial fields.

[0129] Example 6: A method for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells, comprising the following steps:

[0130] (S01) A compound calcium salt solution prepared by calcium hydroxide and calcium hydroxychloride is pumped into concentrated acid wastewater to react and generate calcium fluoride particles; the concentration of the compound calcium salt solution in step (S01) is 10%; the compound calcium salt solution in step (S01) is continuously pumped into the concentrated acid reaction tank by a metering pump. The reaction tank has three sections and is equipped with reflux. The reaction time of the last section of the reaction tank is 45 minutes, and the total reaction time is 80 minutes; when the pH of the effluent from the reaction tank stabilizes at 8, the metering pump is stopped from delivering the compound calcium salt solution.

[0131] (S02) Add citric acid solution to the mixture after reaction in step (S01); the concentration of citric acid solution in step (S02) is 3%; the metering pump is stopped when the pH of the mixture after reaction is adjusted back to 6.6 by the citric acid solution, and the stirring time during the adjustment process is 12 min;

[0132] (S03) Apply a microcurrent to the mixture after treatment in step (S02) to change the surface charge properties of the calcium fluoride particles; the current density in step (S03) is 9 mA / cm², and the microcurrent application time is 0.8 h.

[0133] (S04) Add flocculant to the mixture after treatment in step (S03) to cause calcium fluoride particles to flocculate and settle, obtaining calcium fluoride flocculent sludge; the flocculant used in step (S04) is polyaluminum chloride and polyacrylamide; the concentration of polyaluminum chloride is 6%, the dosage is 2.1% of the concentrated acid water volume, and the reaction time is maintained at pH 6.0 for 40 min; the polyacrylamide is a cationic flocculant with an ionicity of 70, and the cationic polyacrylamide is delivered from the storage tank to the flocculation reaction tank by a third metering pump, and mixed by stirring; the concentration of polyacrylamide is 3.5%, the dosage is 22% of the concentrated acid water volume, and the stirring time is 40 min;

[0134] (S05) The calcium fluoride flocculent sludge obtained in step (S04) is dewatered by pressure filtration, and the dewatered filter cake is dried and calcined to obtain the high-purity calcium fluoride; the dewatering in step (S05) is carried out by plate and frame filtration to obtain calcium fluoride sludge, the water content of the calcium fluoride sludge after plate and frame filtration is 45%, the drying and calcination temperature in step (S05) is 200℃, and the time is 18h; then the temperature is raised to 250℃, the heating rate is 5℃ / min, and the calcination time is 18h; the calcium fluoride prepared by this method has a purity of 92% and can be used in metallurgical and other industrial fields.

[0135] The equipment for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells includes: a preparation tank system, which supplies a calcium hydroxide and calcium hydroxychloride compound calcium salt solution to a reaction tank system, and the reaction tank system supplies the completed mixture to a flocculation system; the preparation tank system includes a preparation tank 100, with a tank cover 101 on top, and two metering tanks 200 arranged side by side on the tank cover 101 for adding calcium hydroxide powder and calcium hydroxychloride respectively; the preparation tank 100 in the preparation tank system is equipped with a first stirring paddle 103 driven by a first motor 102, and a large gear 104 is connected to the central shaft of the first stirring paddle 103. A large gear 104 has symmetrically meshed small gears 105 rotatably connected to a can cover 101 on both sides. A gear housing 106 is connected below the can cover 101. The large gear 104 and small gear 105 are located inside the gear housing 106. A dispersing blade 107 is connected to the central shaft of the small gear 105. An impeller 109 is also connected to the central shaft of the small gear 105 in the can system. The impeller 109 is located inside an exhaust casing 108, which is installed below the gear housing 106. An air intake 1081 is opened on one side of the exhaust casing 108. An air outlet of the exhaust casing 108 is connected to a blowing pipe 110. The other end of the blowing pipe 110 is connected to two metering cans 200. A diffuser shell 111 is connected to the outlet end of the blowing pipe 110. A shaft 202 driven by a second motor 201 is installed inside the metering can 200. Straight spiral blades 203 and conical spiral blades 204 are installed on the shaft 202. A dispensing plate 205 is disposed inside the metering tank 200 above the conical spiral blade 204. A scraper 206 is slidably connected to one side of the shaft 202 below it. A first spring 207 is connected between the scraper 206 and the shaft 202, and the first spring 207 drives the scraper 206 to always be in contact with the inner wall of the metering tank 200. A slide rod 208 is fixedly connected to the lower end of the shaft 202. A conical block 209 is slidably connected to the slide rod 208. A second spring 210 is sleeved on the slide rod 208. One end of the second spring 210 is connected to the conical block 209, and the other end is connected to the protrusion at the lower end of the slide rod 208.

[0136] Working principle of the mixing tank system: Two metering tanks 200 on the tank cover 101 respectively add calcium hydroxide powder and calcium hydroxychloride. The second motor 201 inside the metering tank 200 drives the shaft 202 to rotate. When the calcium salt powder falls from the top of the metering tank 200, it first hits the material distribution plate 205 above the conical spiral blade 204. The agglomerated calcium salt is broken and dispersed by the material distribution plate before falling into the conical spiral blade 204 for subsequent conveying. Pre-breaking up calcium salt agglomerates in advance avoids blockage of the gap between the conical spiral blade 204 and the straight spiral blade 203 by lumpy calcium salt; it ensures the continuity of metering and prevents the addition interruption caused by agglomeration, further stabilizing the calcium salt addition amount; when the shaft 202 rotates, the scraper 206 on one side below it is subjected to the elastic force of the first spring 207. Under the action of the shaft, it is always in close contact with the inner wall of the metering tank 200, and rotates synchronously with the shaft to scrape off the calcium salt powder adhering to the tank wall, avoiding the deviation of "actual dosage < set dosage" caused by calcium salt powder adhering to the tank wall, and ensuring the accuracy of metering; the conical block 209 on the slide rod 208 at the lower end of the shaft 202 can slide up and down along the slide rod under the elastic force of the second spring 210. By changing the gap between the conical block 209 and the outlet of the metering tank 200, the discharge speed of calcium salt can be adjusted; it is suitable for calcium salt powder of different particle sizes, such as fine powder and coarse powder, to avoid the discharge being too fast or too slow due to particle size differences, reduce the long-term accumulation of residual calcium salt on the tank wall, reduce the cleaning and maintenance frequency of the metering tank 200, and finally deliver the calcium salt to the configuration tank 100 below at a fixed rate;

[0137] This ensures that the calcium ion concentration of the compound calcium salt solution in the preparation tank 100 remains stable at 5%–15%, avoiding incomplete removal of fluoride ions or the generation of impurities due to fluctuations in calcium concentration. The first motor 102 drives the first stirring paddle 103 to rotate, and the large gear 104 on its central shaft simultaneously meshes with the small gears 105 on both sides, driving the dispersion blades 107 on the central shaft of the small gears to rotate at high speed, stirring and dispersing the falling calcium salt. At the same time, the central shaft of the small gears also drives the impeller 109 in the exhaust shell 108 to rotate, drawing in air through the air inlet 1081 and generating airflow, which is then transported to the discharge end of the two metering tanks 200 through the blower pipe 110. After stirring and mixing, the calcium hydroxide and calcium hydroxychloride compound calcium salt solution formed in the preparation tank 100 is transported to the reaction tank system. After the calcium fluoride formation reaction is completed, the mixed liquid after the reaction is completed is transported to the flocculation system to prepare for subsequent solid-liquid separation and purity improvement.

[0138] The reaction tank system includes a first reaction tank 300, which is connected to a second reaction tank 303 via a first overflow pipe 302. The second reaction tank 303 is connected to a third reaction tank 305 via a second overflow pipe 304. The configuration tank 100 is connected to the first reaction tank 300 via a first metering pump 301. The effluent from the third reaction tank 305 is discharged via a third overflow pipe 306.

[0139] Working principle: The calcium salt compound solution in the preparation tank 100 is precisely delivered to the first reaction tank 300 by the first metering pump 301, where it reacts initially with the photovoltaic concentrated acid wastewater to generate calcium fluoride particles. The reaction liquid in the first reaction tank 300 flows into the second reaction tank 303 through the first overflow pipe 302, where it reacts further and then enters the third reaction tank 305 through the second overflow pipe 304 to complete the deep removal of fluoride ions. The mixed liquid that has completed the reaction in the third reaction tank 305 is discharged to the flocculation system through the third overflow pipe 306. By adopting a "staged reaction" mode, the contact probability between fluoride ions and calcium salts is gradually increased, avoiding fluoride ion residue caused by insufficient reaction in a single step. The final effluent fluoride concentration can be reduced to 1.5-3.5 mg / L. Sufficient reaction time is provided for the growth of calcium fluoride particles. The final reaction time is 30-60 minutes, and the total reaction time is 60-90 minutes, which reduces the embedding of impurities into the crystals and creates conditions for the formation of high-purity calcium fluoride of 88%-92%.

[0140] Mechanical valves are provided in the first overflow pipe 302, the second overflow pipe 304 and the third overflow pipe 306. Each mechanical valve includes a cover 309. One end of the cover 309 is hinged to the inner wall of the overflow pipe, and a push rod 307 is movably connected above it. The upper end of the push rod 307 extends upward through the overflow pipe and is fitted with a locking sleeve 308. The locking sleeve 308 is fixedly connected to the reaction tank.

[0141] Working principle: The caps 309 inside the first, second, and third overflow pipes 302 / 304 / 306 are controlled to open and close via push rods 307: pushing the push rod 307 downwards opens the caps 309 around the hinge point, adjusting the size of the overflow pipe inlet; the locking sleeve 308 secures the position of the push rod 307 with screws, locking the flow rate. This precisely controls the inlet flow rate of each reaction tank, stabilizing the liquid level and preventing fluctuations in liquid level from affecting reaction conditions such as pH and reaction time; it also ensures sufficient reaction time in each reaction tank, preventing incomplete reactions due to excessively rapid reaction liquid flow, further reducing fluoride residue.

[0142] The reaction tank system also includes a mixing pipe 310, which is rotatably installed in the third overflow pipe 306 via a bearing 311. One end of the mixing pipe 310 is connected to a guide impeller 312, and the inner wall of the mixing pipe 310 is provided with a guide vane 313.

[0143] Working principle: When the reaction liquid discharged from the three-stage reaction tank 305 flows through the third overflow pipe 306, it impacts the guide impeller 312 at one end of the mixing pipe 310, causing the mixing pipe 310 to rotate around the bearing 311. The guide vanes 313 on the inner wall of the mixing pipe guide the liquid to rotate and flow, pre-mixing it with the citric acid solution added later from the citric acid storage tank 700. This achieves the use of the reaction liquid's own kinetic energy to drive the rotation of the mixing pipe, eliminating the need for additional power and reducing energy consumption; it also improves the mixing uniformity of the reaction liquid and citric acid, shortens the mixing reaction time in the subsequent flocculation system, and optimizes the growth conditions of calcium fluoride crystals.

[0144] Baffles 314 are provided on one side of the inlet of the first overflow pipe 302, the second overflow pipe 304 and the third overflow pipe 306.

[0145] Baffles 314 are installed on one side of the inlet of the first, second, and third overflow pipes 302 / 304 / 306. Before the reaction liquid flows into the overflow pipe, it impacts the baffle 314, slowing down the flow velocity. This avoids fluctuations in the liquid level inside the tank caused by the reaction liquid directly impacting the inner wall of the overflow pipe, stabilizes the reaction environment in the reaction tank such as pH and temperature, reduces the damage of liquid flow disturbance to the already formed calcium fluoride particles, and ensures the stability of particle growth.

[0146] A third motor 400 is provided on each of the first-stage reaction tank 300, the second-stage reaction tank 303, and the third-stage reaction tank 305. The output shaft of the third motor 400 is connected to the stirring shaft 401. A spline sleeve 402 is connected below the stirring shaft 401. A turntable 403 is fixedly connected below the spline sleeve 402. A stirring blade 404 is provided on the turntable 403.

[0147] A hollow shaft 405 is fixedly connected below the turntable 403. The hollow shaft 405 is rotatably connected to the bottom of the reaction tank. A screw 406 is movably connected inside the hollow shaft 405. The screw 406 is threadedly connected to the stirring shaft 401. A third spring 407 and a sliding sleeve 408 are fitted on the stirring shaft 401. One end of the third spring 407 is connected to the spline sleeve 402, and the other end is connected to the sliding sleeve 408.

[0148] The turntable 403 has a cavity, and an adjusting plate 411 is provided in the cavity. The stirring blade 404 is hinged to the adjusting plate 411 through a connector 412. A rotating sleeve 409 is connected below the adjusting plate 411. The rotating sleeve 409 is located inside the hollow shaft 405 and is sleeved on the outside of the screw 406. A rotating seat 410 is connected below the rotating sleeve 409.

[0149] Working principle: The third motor 400 drives the stirring shaft 401 to rotate, which in turn drives the turntable 403 and stirring blades 404 to rotate synchronously through the spline sleeve 402, stirring the mixture in the reaction tank. The third spring 407 on the stirring shaft 401 is connected to the spline sleeve 402 at one end and to the sliding sleeve 408 at the other end, providing buffering during the stirring process and adapting to changes in the liquid level in the tank. This achieves stirring of the reaction liquid without dead zones, allowing the calcium salt to fully contact the fluoride ions in the concentrated acid wastewater, accelerating the formation of calcium fluoride. The buffering effect of the third spring 407 improves the adaptability of the stirring structure, preventing the stirring blades 404 from getting stuck due to changes in liquid level, and ensuring continuous stirring.

[0150] Working principle: Rotating the screw 406 inside the hollow shaft 405, the screw and the stirring shaft 401 are threaded together, causing the stirring shaft 401 to move up and down. The stirring shaft, through the spline sleeve 402, drives the turntable 403 and the stirring blades 404 to adjust their height synchronously. When the blades descend to the bottom of the tank, they can scrape off the calcium fluoride particles deposited at the bottom. This achieves the following effect: adapting to different liquid levels in the reaction tank, such as rises / falls in liquid level during the reaction, ensuring that the reaction liquid at different depths is fully stirred; scraping off the calcium fluoride particles deposited at the bottom of the tank avoids waste of raw materials due to deposition, and also prevents scale buildup at the bottom of the tank from affecting subsequent reactions. Rotating the rotating base 410 drives the rotating sleeve 409 to rotate, and the rotating sleeve 409 drives the adjusting plate 411 to rotate within the cavity of the turntable 403. The adjusting plate 411, through the connecting piece 412, pulls the stirring blades 404 to extend or retract around the hinge point, changing the stirring range. The stirring range can be adjusted according to the size of the reaction tank to cover the area near the tank wall, scrape off the calcium fluoride particles adhering to the tank wall, avoid mixing dead corners, further ensure that the reaction solution is fully mixed, improve the fluoride ion removal rate, and reduce the generation of impurities. The rotary seat 410 and the screw 406 are fixed by screws.

[0151] The flocculation system includes a flocculation reaction tank 500. A three-stage reaction tank 305 is connected to the flocculation reaction tank 500 through a third overflow pipe 306. The bottom of the flocculation reaction tank 500 has a cavity, and a battery interface 501 is provided on one side. An electrode plate 502 is inserted into the cavity through the battery interface 501. The electrode plate 502 and the battery interface 501 are sealed by a sealing ring and a threaded connection.

[0152] Working Principle: The mixed liquid from the three-stage reaction tank 305 enters the flocculation reaction tank 500 through the third overflow pipe 306. The electrode plate 502 is inserted into the bottom cavity of the flocculation reaction tank through the battery interface 501. After being energized, it generates a microcurrent of 5-10 mA / cm², which changes the surface charge properties of the calcium fluoride particles. The electrode plate 502 and the battery interface 501 are sealed by a sealing ring and threaded connection to prevent leakage of the reaction liquid. The microcurrent changes the charge of the calcium fluoride particles, creating conditions for subsequent flocculation and sedimentation, and improving flocculation efficiency. The sealed structure prevents leakage of the reaction liquid, ensures safe operation of the equipment, avoids environmental pollution, and extends the service life of the electrode plate.

[0153] The bottom cavity of the flocculation reaction tank 500 is connected to the water outlet 503. The outlet of the water outlet 503 is connected to a water pipe, and a valve 504 is installed on the water pipe. Two air pipes 505 pass through the two side walls of the flocculation reaction tank 500 and are connected to the water outlet 503. The other end of the two air pipes 505 is connected to a tee 506.

[0154] Working principle: The supernatant in the bottom cavity of the flocculation reaction tank 500 is discharged through the outlet tank 503 and valve 504;

[0155] The air pump introduces gas into the two air pipes 505 through the three-way valve 506. The gas disturbs the dead corner of the water tank 503, accelerating the discharge of residual liquid.

[0156] The sedimented liquid is quickly discharged, and the gas disturbance cleans the dead corners of the water tank, preventing calcium fluoride particles from clogging the water outlet channel. At the same time, it ensures a seamless connection between the electrode plate 502 and the cavity, so as not to affect the subsequent microcurrent treatment.

[0157] A fourth motor 600 is installed on the flocculation reaction tank 500, and the output shaft of the fourth motor 600 is connected to a second stirring paddle 601.

[0158] The flocculation system also includes a citric acid storage tank 700, which is connected to a third overflow pipe 306 via a second metering pump 701.

[0159] The flocculation system also includes two storage tanks 800, which are used to store PAC solution and PAM solution respectively. The storage tanks 800 are connected to the flocculation reaction tank 500 through a third metering pump 801.

[0160] A metering pump is installed above the configuration tank 100 and the two storage tanks 800.

[0161] Working principle: The fourth motor 600 drives the second stirring paddle 601 to rotate, stirring the mixture in the flocculation reaction tank 500, so that the PAC and PAM flocculants and the reaction liquid containing calcium fluoride particles are fully mixed. This promotes the uniform mixing of flocculants and the mixture, and accelerates the aggregation of fine calcium fluoride particles into large flocs.

[0162] The citric acid solution in the citric acid storage tank 700 is precisely delivered to the third overflow pipe 306 via the second metering pump 701, where it mixes with the reaction liquid discharged from the three-stage reaction tank 305, adjusting the pH to 6.5-7.0.

[0163] Two storage tanks 800 store PAC solution and cationic PAM solution respectively, which are precisely delivered to flocculation reaction tank 500 by the third metering pump 801: PAC first performs preliminary flocculation on calcium fluoride particles, and PAM causes the particles to aggregate into large flocs through the netting effect.

[0164] The metering pump above the preparation tank 100 accurately delivers clean water to the preparation tank according to the preset compound solution concentration of 5% to 15% and the calcium salt dosage in the metering tank 200, so that it mixes with the calcium salt to form a compound solution; the metering pumps above the two storage tanks 800 deliver clean water to the storage tanks according to the concentration requirements of PAC 2% to 10% and PAM 2% to 5%, respectively, to dissolve the agents and form a stable concentration of flocculant.

[0165] It also includes a circulation system, which includes a first reflux pump 900. The inlet of the first reflux pump 900 is connected to the bottom of the second-stage reaction tank 303, and the outlet is connected to the water inlet pipe of the first-stage reaction tank 300. The circulation system also includes a second reflux pump 901. The inlet of the second reflux pump 901 is connected to the bottom of the third-stage reaction tank 305, and the outlet is connected to the first overflow pipe 302.

[0166] Working principle: The first reflux pump 900 pumps the unreacted material at the bottom of the second stage reaction tank 303 to the water inlet pipe of the first stage reaction tank 300, so that it can participate in the reaction again;

[0167] The second reflux pump 901 pumps the unreacted material remaining at the bottom of the three-stage reaction tank 305 to the first overflow pipe 302, where it enters the second-stage reaction tank 303 along with the first-stage reaction liquid for another reaction.

[0168] The achieved effect is to improve the reaction utilization rate of calcium salt and fluoride ions, reduce raw material waste, and reduce reagent consumption by 30%.

[0169] This further reduces the concentration of fluoride ions in the effluent, decreases fluoride residue, reduces sludge production, and lowers subsequent treatment costs.

[0170] General working principle of the equipment:

[0171] Quantitative water addition start-up and parameter matching: The quantitative water pump above the preparation tank 100 is determined by the preset calcium salt compound solution concentration according to process requirements, such as 10%. Combined with the calcium salt dosage in the quantitative tank 200, the pump is calculated using the speed of the second motor 201 and the pitch of the spiral blades to accurately deliver clean water to the preparation tank. For example, when the quantitative tank needs to add 10 kg of calcium hydroxide and calcium hydroxychloride mixed in a specific ratio, the quantitative water pump calculates based on a "10% concentration" and delivers 90 kg of clean water.

[0172] To ensure a precise match between calcium salt and water ratio, two metering tanks 200 store calcium hydroxide powder and calcium hydroxychloride respectively. When the material falls, it first impacts the material distribution plate 205 inside the tank to break up any lumps. At the same time, the second motor 201 drives the shaft 202 to rotate, and the conical spiral blade 204 and the straight spiral blade 203 quantitatively deliver the calcium salt to the mixing tank. The scraper 206 below the shaft scrapes off residual powder against the tank wall under the action of the first spring 207. The conical block 209 adjusts the discharge gap through the second spring 210 to ensure accurate and unbiased calcium salt dosage.

[0173] The first motor 102 drives the first stirring paddle 103 to rotate to achieve stirring. At the same time, the large gear 104 meshes with the small gear 105 to drive the dispersing blade 107 to rotate at high speed, so as to fully mix the water added by the metering pump with the calcium salt.

[0174] The impeller 109 of the pinion shaft generates airflow within the exhaust casing 108, which is then sent into the metering tank via the blower pipe 110 to prevent calcium salts from "bridging" and clogging. At the same time, it helps the calcium salts to fall evenly into the preparation tank, avoiding excessively high local concentrations. The calcium salt compound solution, after being metered and mixed with water, is precisely delivered to the first reaction tank 300 of the reaction tank system via the first metering pump 301. This provides a stable calcium source for the fluoride ion removal reaction, preventing incomplete reactions or impurity generation due to fluctuations in the concentration of the compound solution.

[0175] The reaction tank system is based on "three-stage series + adaptive stirring + liquid flow optimization". It uses a stable concentration of calcium salt compound solution transported by a preparation tank to react with fluoride ions in concentrated acid wastewater in stages. The specific process is as follows:

[0176] Precise feeding and staged reaction: The calcium salt compound solution in the preparation tank is sent to the first reaction tank 300 via the first metering pump 301, where it reacts initially with the photovoltaic concentrated acid wastewater containing high concentrations of fluoride ions to generate calcium fluoride particles. The reaction solution flows into the second reaction tank 303 via the first overflow pipe 302, where it undergoes further reaction and then enters the third reaction tank 305 via the second overflow pipe 304, achieving "gradual removal of fluoride ions." The final stage reaction lasts 30-60 minutes, with a total reaction time of 60-90 minutes, avoiding fluoride residue caused by insufficient reaction in a single step. The mechanical valve caps 309, push rods 307, and locking sleeves 308 in the three overflow pipes can adjust the inlet flow rate, stabilize the liquid level in the tank, and ensure sufficient reaction time. The baffle 314 at the inlet of the overflow pipe buffers the flow and prevents fluctuations in the liquid level in the tank.

[0177] Adaptive stirring and tank cleaning: The third motor 400 drives the stirring shaft 401 to rotate, which in turn drives the turntable 403 and stirring blades 404 to stir the reaction liquid through the spline sleeve 402; the rotating screw 406 can adjust the height of the stirring blades to adapt to different liquid levels, and when it is lowered to the bottom of the tank, it can scrape off the deposited calcium fluoride; the rotating seat 410 can change the blade extension range through the adjusting plate 411 and the connecting piece 412 to cover the tank wall area, scrape off the adhering material, ensure that the reaction liquid is mixed without dead corners, and accelerate the formation of calcium fluoride.

[0178] When the effluent from the three-stage reaction tank is discharged through the third overflow pipe 306, the liquid flow impacts the guide impeller 312 of the mixing pipe 310, causing the mixing pipe to rotate. The guide vane 313 guides the liquid to premix with the citric acid solution delivered by the second metering pump 701 from the citric acid storage tank 700, preparing for the subsequent pH adjustment to 6.5-7.0 and calcium fluoride crystallization optimization.

[0179] The flocculation system utilizes a stable concentration of flocculant supplied by a metered water pump in a storage tank, combined with microcurrent treatment, to achieve efficient precipitation of calcium fluoride particles.

[0180] Microcurrent pretreatment: The electrode plate 502 at the bottom of the flocculation reaction tank 500 is energized through the battery interface 501 to generate a microcurrent of 5-10 mA / cm², which changes the surface charge properties of the calcium fluoride particles and creates conditions for subsequent flocculation; the sealing structure with sealing ring and threads prevents leakage of the reaction liquid.

[0181] Two storage tanks (800) are used to prepare PAC solutions of 2%–10%, preferably 4%–6%, and cationic PAM solutions of 2%–5%, respectively. Metered water pumps are used for each of the two storage tanks to achieve precise control of the reagent concentration. The specific workflow is as follows: PAC solution preparation: PAC solid reagent, such as polyaluminum chloride powder, is added to one of the storage tanks. The metered water pump above the storage tank precisely delivers clean water according to the required concentration of 2%–10%. For example, when adding 5 kg of PAC solid, 95 kg of clean water is delivered based on a 5% concentration. Simultaneously, the stirring device on the storage tank is activated to fully dissolve the PAC solid and the metered water, forming a PAC solution with uniform concentration. This avoids insufficient flocculation due to excessively low PAC concentration or excessively high concentration due to excessively high concentration, which would affect the purity of calcium fluoride.

[0182] Preparation of cationic PAM solution: Add cationic PAM solid with an ionization degree of 50-80 (preferably 50) to another storage tank. The metering pump above it accurately delivers clean water at a concentration of 2%-5%. For example, when adding 2 kg of PAM solid, calculate based on a 2% concentration and deliver 98 kg of clean water. With the help of the storage tank stirring device, the PAM solid is completely dissolved to prevent clumping and ensure that the PAM can be evenly dispersed during subsequent flocculation, effectively capturing calcium fluoride particles.

[0183] Flocculant addition and stirring: The third metering pump 801 accurately delivers the PAC and PAM solutions from the storage tank into the flocculation reaction tank. The fourth motor 600 drives the second stirring paddle 601 to stir, so that the reagents and the reaction liquid containing calcium fluoride particles are fully mixed. PAC initially flocculates, and PAM is netted to form large flocs. After precipitation, calcium fluoride is obtained.

[0184] The precipitated calcium fluoride is then fed into a plate and frame filter press system for dewatering, resulting in a sludge moisture content of 40–50%. The sludge is then transported to a calcination furnace for drying at 100–300°C for 12–24 hours. The plate and frame filter press system and calcination furnace are existing technologies and are not shown in the structural drawings.

Claims

1. A method for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells, characterized by: Including the following step, (S01) A compound calcium salt solution prepared by calcium hydroxide and calcium hydroxychloride is pumped into concentrated acid wastewater to react and generate calcium fluoride particles. (S02) Add citric acid solution to the mixture after the reaction in step (S01); (S03) Apply a microcurrent to the mixture after step (S02) to change the surface charge properties of the calcium fluoride particles; (S04) Add flocculant to the mixture after treatment in step (S03) to cause calcium fluoride particles to flocculate and settle, and obtain calcium fluoride flocculent sludge. (S05) The calcium fluoride flocculent sludge obtained in step (S04) is dewatered by pressure filtration, and the dewatered filter cake is dried and calcined to obtain the high-purity calcium fluoride.

2. The method according to claim 1, characterized in that: The concentration of the compound calcium salt solution in step (S01) is 5%–15%, and the concentration of the compound calcium salt solution is 8%–12%. The compound calcium salt solution in step (S01) is continuously pumped into the concentrated acid reaction tank by a metering pump. The reaction tank has three sections and is equipped with reflux. The reaction time of the last section of the reaction tank is 30–60 min, and the total reaction time is 60–90 min. When the pH of the effluent from the reaction tank stabilizes at 7.5–8.5, the metering pump stops delivering the compound calcium salt solution.

3. The method according to claim 1, characterized in that: The concentration of the citric acid solution in step (S02) is 1-5%; the metering pump is stopped when the pH of the mixed solution after the reaction is adjusted back to 6.5-7.0, and the stirring time during the adjustment process is 10-20 min; the current density in step (S03) is 5-10 mA / cm², and the microcurrent action time is 1-2 h.

4. The method according to claim 1, characterized in that: In step (S04), the flocculants used are polyaluminum chloride and polyacrylamide; the concentration of polyaluminum chloride is 2-10%, the dosage is 1-3% of the concentrated acid water volume, and the reaction time is maintained at pH 5.5-6.5 for 30-60 minutes; the polyacrylamide is a cationic flocculant, selected from one of ionicity degrees of 50, 60, 70, and 80, and the cationic polyacrylamide is delivered from the storage tank to the flocculation reaction tank by a third metering pump, and mixed by stirring; the concentration of the prepared polyacrylamide is 2-5%, the dosage is 15-30% of the concentrated acid water volume, and the stirring time is 30-60 minutes.

5. The method according to claim 1, characterized in that: In step (S05), the dewatering process is carried out by plate and frame filtration to obtain calcium fluoride sludge. The water content of the calcium fluoride sludge after plate and frame filtration is 40-50%; the water content of the calcium fluoride sludge after plate and frame filtration is 45%; the drying and calcination temperature in step (S05) is 100-300℃ and the time is 12-24h.

6. Equipment for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells, characterized in that: it includes, A preparation tank system supplies a calcium salt solution of calcium hydroxide and calcium hydroxychloride to a reaction tank system, and the reaction tank system supplies a mixed liquid that has undergone reaction to a flocculation system. The preparation tank system includes a preparation tank (100), the top of which is provided with a tank cover (101), and two metering tanks (200) for adding calcium hydroxide powder and calcium hydroxychloride respectively are arranged side by side on the tank cover (101). The configuration tank system has a configuration tank (100) with a first stirring paddle (103) driven by a first motor (102). A large gear (104) is connected to the central shaft of the first stirring paddle (103). Small gears (105) are symmetrically meshed on both sides of the large gear (104) and rotatably connected to the tank cover (101). A gear housing (106) is connected below the tank cover (101). The large gear (104) and small gear (105) are located inside the gear housing (106). A dispersing blade (107) is connected to the central shaft of the small gear (105). A fan impeller (109) is also connected to the central shaft of the pinion (105) in the configuration tank system. The fan impeller (109) is located inside the exhaust casing (108). The exhaust casing (108) is installed below the gear housing (106) and has an air inlet (1081) on one side. The air outlet of the exhaust casing (108) is connected to a blower pipe (110), and the other end of the blower pipe (110) is connected to two of the metering tanks (200).

7. The equipment for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells according to claim 6, characterized in that: The outlet end of the blower pipe (110) is connected to a diffuser shell (111); a shaft (202) driven by a second motor (201) is provided inside the metering tank (200), and straight spiral blades (203) and conical spiral blades (204) are provided on the shaft (202); a material distribution plate (205) is provided inside the metering tank (200) above the conical spiral blades (204); a scraper (206) is slidably connected to one side below the shaft (202), and the scraper (206) is connected to the shaft. A first spring (207) is connected between the bodies (202), and the first spring (207) drives the scraper (206) to always be in contact with the inner wall of the metering tank (200); a slide rod (208) is fixedly connected to the lower end of the shaft (202), a conical block (209) is slidably connected on the slide rod (208), and a second spring (210) is sleeved on the slide rod (208). One end of the second spring (210) is connected to the conical block (209), and the other end is connected to the protrusion at the lower end of the slide rod (208).

8. The apparatus for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells according to claim 6, characterized in that: The reaction tank system includes a first reaction tank (300), which is connected to a second reaction tank (303) through a first overflow pipe (302). The second reaction tank (303) is connected to a third reaction tank (305) through a second overflow pipe (304). The configuration tank (100) is connected to the first reaction tank (300) through a first metering pump (301). The effluent from the third reaction tank (305) is discharged through a third overflow pipe (306). Mechanical valves are provided in the first overflow pipe (302), the second overflow pipe (304), and the third overflow pipe (306). Each mechanical valve includes a cover (309). One end of the cover (309) is hinged to the inner wall of the overflow pipe, and a push rod (307) is movably connected above it. The upper end of the push rod (307) extends upward through the overflow pipe and is fitted with a locking sleeve (308). The locking sleeve (308) is fixedly connected to the reaction tank.

9. The equipment for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells according to claim 8, characterized in that: A third motor (400) is provided on each of the first-stage reaction tank (300), the second-stage reaction tank (303), and the third-stage reaction tank (305). The output shaft of the third motor (400) is connected to the stirring shaft (401). A spline sleeve (402) is connected below the stirring shaft (401). A turntable (403) is fixedly connected below the spline sleeve (402). A stirring blade (404) is provided on the turntable (403). A hollow shaft (405) is fixedly connected below the turntable (403). The hollow shaft (405) is rotatably connected to the bottom of the reaction tank. A screw (406) is movably connected inside the hollow shaft (405). The screw (406) is threadedly connected to the stirring shaft (401). A third spring (407) and a sliding sleeve (408) are sleeved on the stirring shaft (401). One end of the third spring (407) is connected to the spline sleeve (402), and the other end is connected to the sliding sleeve (408).

10. The apparatus for in-situ preparation of high-purity calcium fluoride from concentrated acidic wastewater from photovoltaic cells according to claim 8, characterized in that: The flocculation system includes a flocculation reaction tank (500), and the three-stage reaction tank (305) is connected to the flocculation reaction tank (500) through a third overflow pipe (306). The bottom of the flocculation reaction tank (500) has a cavity, and a battery interface (501) is provided on one side. An electrode plate (502) is inserted into the cavity through the battery interface (501). The electrode plate (502) and the battery interface (501) are sealed by a sealing ring and a threaded connection. The bottom cavity of the flocculation reaction tank (500) is connected to a water outlet tank (503). The outlet of the water outlet tank (503) is connected to a water pipe, and a valve (504) is provided on the water pipe. Two air pipes (505) pass through the two side walls of the flocculation reaction tank (500) and are connected to the water outlet tank (503). The other end of the two air pipes (505) is connected to a tee (506).

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