Process for producing optical-grade calcium fluoride from photovoltaic wastewater and calcium fluoride induced crystallization system

Abstract

The invention discloses a process for producing optical-grade calcium fluoride from photovoltaic wastewater and a calcium fluoride induced crystallization system, which comprises the following steps of: firstly, pumping concentrated acid fluorine-containing wastewater into a nanofiltration membrane system to intercept fluosilicate ions and polymers thereof, aluminum, iron, copper and nickel high-valence metal ions, hydrofluoric acid molecules and fluorine ions to form acidic fluoride produced water through a nanofiltration membrane; the method comprises the following steps: adding a high-purity optical-grade calcium fluoride seed crystal into acidic fluoride produced water, adding a high-purity calcium chloride solution, stirring and crystallizing, standing and aging after the reaction is completed to obtain calcium fluoride crystals, and washing the calcium fluoride crystals with high-purity water and absolute ethyl alcohol for multiple times to obtain the calcium fluoride crystals. According to the method, the quality of the product is accurately regulated and controlled by controlling the crystallization path, the calcium fluoride material which is high in purity and crystallinity and can be used in the optical field is directly and controllably prepared, the traditional mode that the calcium fluoride material is only used for hazardous waste treatment is overturned, and the method is suitable for industrial production. The high value of the waste is realized.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a process and system for producing optical-grade calcium fluoride from photovoltaic wastewater. Background Technology

[0002] The photovoltaic industry uses large amounts of hydrofluoric acid during production, especially in the etching and cleaning processes of silicon wafers, resulting in highly acidic and toxic acidic fluoride-containing wastewater. This wastewater mainly contains hydrofluoric acid (HF), fluorosilicic acid (H2SiF6), nitric acid, and trace amounts of metal ion impurities.

[0003] Currently, the conventional treatment method for this type of wastewater in the industry is mainly "chemical precipitation," which involves adding excess calcium salts (such as industrial lime or industrial calcium chloride) to generate calcium fluoride (CaF2) precipitate, thereby removing fluoride. Lime crystallization is also used for fluoride recovery, yielding crystalline calcium fluoride products. However, these methods suffer from relatively low purity of the recovered calcium fluoride product, failing to meet the standards for high-end applications. The analysis is as follows: Because fluoride-containing wastewater contains some fluorosilicic acid, during alkali neutralization or precipitation, it easily generates highly difficult-to-treat silica gel (SiO2·nH2O) or calcium fluorosilicate (CaSiF6), etc. These substances co-precipitate and encapsulate with CaF2, reducing product purity and becoming one of the core technical bottlenecks hindering the acquisition of high-purity CaF2. Meanwhile, acidic fluoride-containing wastewater contains small amounts of high-valence transition metal ions (aluminum, iron, copper, nickel, etc., with concentrations ranging from 0.1 mg / L to tens of mg / L). Conventional defluorination reactions are generally carried out under near-neutral conditions, which easily leads to the formation of metal hydroxide precipitates, further reducing the purity of CaF2. The purity of the calcium fluoride product generally obtained does not exceed 95%. In addition, existing technologies are difficult to meet the raw material grade and process control requirements of high-end applications. Summary of the Invention

[0004] To overcome the above-mentioned defects, the present invention provides a process and system for producing optical-grade calcium fluoride from photovoltaic wastewater. This process and system can produce calcium fluoride crystals with purity, crystal integrity, and ultraviolet transmittance that meet the standards for ordinary optical-grade applications using concentrated acidic fluoride wastewater generated during photovoltaic production.

[0005] The technical solution adopted by this invention to solve its technical problem is: a process for producing optical-grade calcium fluoride from photovoltaic wastewater, comprising the following steps:

[0006] Step 1: The concentrated acidic fluoride wastewater generated during the photovoltaic production process is fed into a nanofiltration membrane system. The nanofiltration membrane retains large molecular weight, negatively charged fluorosilicate ions and their polymers, as well as high-valence metal ions such as aluminum, iron, copper, and nickel. Hydrofluoric acid molecules and fluoride ions pass through the nanofiltration membrane to form purified acidic fluoride permeate water with HF / F- as the main fluoride source, thus achieving source separation of fluoride from silicon and high-valence metal ions.

[0007] Step 2: Add high-purity optical-grade calcium fluoride seed crystals to the acidic fluoride product water obtained in Step 1;

[0008] Step 3: Add high-purity calcium chloride solution to the system containing optical-grade calcium fluoride seed crystals, and carry out precipitation crystallization reaction under stirring conditions. During the calcium fluoride-induced crystallization process, continuous and slow stirring is required to ensure uniform dispersion of the seed crystals.

[0009] Step 4: After the precipitation and crystallization reaction is completed, stop stirring and allow it to stand for aging. The newly generated calcium fluoride will undergo heterogeneous nucleation and directional epitaxial growth on the surface of the pre-added optical-grade calcium fluoride seed crystals.

[0010] Step 5: Wash the calcium fluoride crystals obtained in Step 4 repeatedly with high-purity water with a resistivity ≥18MΩ·cm until the conductivity of the washing solution is stable. Then wash once with anhydrous ethanol. By thoroughly cleaning the calcium fluoride crystals, various impurities attached to the surface of the calcium fluoride crystals are removed.

[0011] Step Six: Dry the calcium fluoride crystals washed in Step Five at 100-105℃ for 2-4 hours;

[0012] Step 7: The dried calcium fluoride crystals are calcined in a tube furnace filled with high-purity nitrogen at a temperature of 400°C for 2-3 hours to remove organic adsorbates and moisture adhering to the surface of the calcium fluoride crystals, thereby obtaining a high-purity, optical-grade calcium fluoride product.

[0013] This invention utilizes acidic nanofiltration to efficiently separate silicon and metallic impurities from complex concentrated acidic fluoride wastewater. It then employs high-purity optical-grade calcium fluoride seed crystals to induce calcium fluoride crystallization, enabling the direct and controllable preparation of high-purity, highly crystalline calcium fluoride materials suitable for optical applications. The core objective of this invention is to transform the "wastewater treatment" process into a "high-end material synthesis" process.

[0014] As a further improvement of the present invention, the nanofiltration membrane used in step one is a strong acid resistant nanofiltration membrane with a molecular weight cutoff between 200 and 1000 Da and an operating pressure of 0.5 to 3.0 MPa.

[0015] As a further improvement of the present invention, in step two, the optical-grade calcium fluoride seed crystal has a particle size range of 30-80 micrometers and a purity of not less than 99.99% (4N). The newly generated calcium fluoride is guided to undergo directional crystallization growth by the optical-grade calcium fluoride seed crystal to form large-particle optical-grade calcium fluoride crystals.

[0016] As a further improvement of the present invention, the mass ratio of the amount of optical-grade calcium fluoride seed crystals added to the total amount of fluoride (calculated as fluoride ions) expected to be recoverable in the wastewater is in the range of 1:5 to 1:20. By using the amount of optical-grade calcium fluoride seed crystals added to match the total amount of fluoride in the wastewater, all calcium fluoride grows on the surface of the optical-grade calcium fluoride seed crystals during induced crystallization, forming calcium fluoride crystals with uniform particle size. Thus, the product quality can be precisely controlled by controlling the crystallization path.

[0017] As a further improvement of the present invention, the high-purity calcium chloride solution used in step three uses electronic grade or high-purity grade calcium chloride with a purity ≥99.99%, and the molar ratio of calcium ions added to the calcium chloride solution to fluoride ions in the acidic fluoride product water is controlled between 1:1.8 and 1:2.2 to ensure that the reaction is sufficient and not excessive.

[0018] As a further improvement of the present invention, in step three, the calcium fluoride crystallization reaction time is controlled at 1 to 3 hours, and in step four, the aging time is controlled at 1 to 3 hours.

[0019] As a further improvement of the present invention, the aging treatment in step four is carried out at a temperature of room temperature to 60°C to accelerate crystal optimization.

[0020] A calcium fluoride induced crystallization system includes a concentrated acid fluoride wastewater collection tank, a nanofiltration membrane system, a seed crystal addition device, a calcium fluoride crystallization reactor, a reagent mixing tank, a filter, a crystal transfer device, a first cleaning tank, a second cleaning tank, a dryer, a tubular furnace, several booster pumps, and a control system. The concentrated acid fluoride wastewater collection tank is capable of collecting and storing concentrated acid fluoride wastewater. The outlet of the collection tank is connected to the inlet of the nanofiltration membrane system via a pipeline. The concentrated acid fluoride wastewater in the collection tank can be pumped into the nanofiltration membrane system via booster pumps. The permeate from the nanofiltration membrane system reacts with the calcium fluoride crystallizer via a pipeline. The inlet of the reactor is connected to a booster pump, which pumps the acidic fluoride permeate from the nanofiltration membrane system into the calcium fluoride crystallization reactor. A seed crystal addition device quantitatively adds optical-grade calcium fluoride seeds to the reactor. A high-purity calcium chloride solution prepared in the reagent mixing tank is pumped into the reactor via the booster pump. The reactor is equipped with a stirrer that continuously stirs the mixed liquid. The discharge port of the reactor is connected to the filter inlet via a pipe, allowing the discharge port to periodically open and discharge calcium fluoride crystals. The filter removes water from the calcium fluoride crystals discharged from the calcium fluoride crystallization reactor. The crystal transfer device sequentially sends the filtered calcium fluoride crystals to a first washing tank, a second washing tank, a dryer, and a tubular furnace. The first washing tank is equipped with a high-purity water inlet and a drain outlet. The high-purity water inlet allows for the metered addition of high-purity water to the first washing tank, while the drain outlet discharges the washing liquid after washing. The first washing tank performs at least one water wash on the filtered calcium fluoride crystals. The second washing tank is equipped with an anhydrous ethanol inlet and an ethanol outlet. The anhydrous ethanol inlet allows for the addition of water to the second washing tank. Anhydrous ethanol is added internally in a measured amount, and the ethanol outlet is sufficient to discharge the ethanol waste liquid after cleaning. The second cleaning tank can perform at least one ethanol cleaning on the water-washed calcium fluoride crystals. The dryer can heat and dry the ethanol-cleaned calcium fluoride crystals. High-purity nitrogen gas is passed through the tubular furnace, which can calcine the dried calcium fluoride crystals to remove organic adsorbents and moisture from the surface of the calcium fluoride crystals. The control system controls the operation of the nanofiltration membrane system, seed crystal addition device, calcium fluoride crystallization reactor, filter, crystal transfer device, first cleaning tank, second cleaning tank, dryer, tubular furnace, and various booster pumps.

[0021] As a further improvement of the present invention, a water bath is also provided, on which a first heater and a first temperature sensor are provided. The first heater can heat the water in the water bath, and the first temperature sensor can detect the water temperature in the water bath in real time and feed it back to the control system. The control system controls the first heater to start and stop. The calcium fluoride crystallization reactor is located in the water bath.

[0022] As a further improvement of the present invention, the drain outlet of the first cleaning tank is also provided with a washing liquid conductivity detection device. The conductivity detection device can detect the conductivity of the washing liquid discharged from the first cleaning tank and feed the detection result back to the control system. The dryer and the tubular furnace are respectively provided with a third temperature sensor and a fourth temperature sensor. The third temperature sensor and the fourth temperature sensor can detect the temperature inside the dryer and the tubular furnace in real time and feed it back to the control system. The control system controls the start and stop of the heating device inside the dryer and the tubular furnace.

[0023] The beneficial effects of this invention are as follows: This invention efficiently separates silicon and metal elements from complex photovoltaic fluoride-containing wastewater using acidic nanofiltration technology, solving the fundamental problem of silica gel contamination products in traditional methods. This lays the foundation for obtaining high-purity calcium fluoride. Furthermore, by introducing high-purity calcium fluoride seed crystals into the wastewater treatment system and using induced crystallization to guide the directional crystallization growth of newly generated calcium fluoride, it effectively suppresses impurity encapsulation caused by homogeneous nucleation. This allows the product to inherit the high crystallinity and high purity of the seed crystals. By controlling the crystallization path, the product quality can be precisely regulated, directly and controllably preparing high-purity, high-crystallinity calcium fluoride materials suitable for optical applications. This directly transforms photovoltaic fluoride-containing wastewater into high-value-added optical-grade calcium fluoride raw materials, overturning the traditional model of treating it only as hazardous waste, creating economic benefits, and realizing the high-value utilization of waste. Attached Figure Description

[0024] Figure 1 This is a flowchart of the processing of the present invention;

[0025] Figure 2 This is a schematic diagram of the system structure of the present invention. Detailed Implementation

[0026] To make the advantages, technical solutions, and innovations of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the protection scope of this invention.

[0027] Example:

[0028] Take 1 liter of concentrated acidic fluoride wastewater from the photovoltaic silicon wafer cleaning process. The pH is approximately 1, the total fluoride concentration is about 2%, the silicon concentration is 52 mg / L, and the transition metal content is as follows: aluminum ions 3.35 mg / L, iron ions 1.2 mg / L, copper ions 0.65 mg / L, and nickel 0.3 mg / L.

[0029] The process for producing optical-grade calcium fluoride from photovoltaic wastewater according to the present invention is used to treat the above-mentioned concentrated acidic fluoride wastewater. The specific steps are as follows:

[0030] 1. Acidic nanofiltration: Concentrated acidic fluoride wastewater was filtered using an acid-resistant nanofiltration membrane (molecular weight cutoff 400 Da) at a pressure of 1.5 MPa. The nanofiltration permeate was collected, and sampling analysis showed that the removal rate of silicon was >99.5%. The acidic permeate obtained was mainly composed of HF and a small amount of F-. At the same time, the content of transition metals in the permeate was <50 ppb.

[0031] 2. Induced crystallization: Add 5 grams of pre-prepared optical-grade calcium fluoride microcrystals with a particle size of about 50 micrometers and a purity of 99.995% (4N5) to the acidic fluoride product water, and stir slowly to disperse the seed crystals evenly.

[0032] 3. Precipitation and aging: Prepare a 0.5 mol / L high-purity electronic-grade calcium chloride (purity 99.999%) solution. Based on the fluoride content of the filtrate (which was determined to be approximately 0.9 mol), calculate and slowly add 1.8 L of calcium chloride solution (to make the Ca:F molar ratio approximately 1:2). After the addition is complete, continue stirring for 1 hour, then stop stirring and let the reaction system stand in a 50℃ water bath for 1 hour to age.

[0033] 4. Post-processing: After aging, the solid product is filtered and repeatedly washed with high-purity water (resistivity ≥18MΩ·cm) until the conductivity of the washing liquid is stable. Then it is washed once with anhydrous ethanol. The solid is dried at 100℃ for 2 hours and then calcined at 400℃ for 2 hours in a tube furnace with high-purity nitrogen to remove possible organic adsorbates and moisture, thus obtaining the final calcium fluoride product.

[0034] Comparative example:

[0035] One liter of wastewater from the same source was taken and treated using the traditional lime precipitation method. Excess lime slurry was added directly to neutralize the pH to approximately 7, and the mixture was stirred for 1 hour before being allowed to settle. Filtration yielded a mud-like CaF2 precipitate, which was dried at 100°C for 2 hours to obtain calcium fluoride sludge product.

[0036] The comparative data of the embodiments and comparative examples are shown in Table 1:

[0037] Comparison Projects Example Comparative Example Calcium fluoride purity 99.95％ 88.9％ Silicon content 0.015％ 2.4％ Transition metal content <50ppb >100ppm UV transmittance >85％ <10％

[0038] Table 1

[0039] By comparing the calcium fluoride product prepared using the process of this invention with the calcium fluoride product prepared using the traditional lime precipitation method (comparison of examples and comparative examples), it can be seen that the purity of the calcium fluoride product prepared using the process of this invention is much higher than that of the calcium fluoride product prepared using the traditional lime precipitation method. Furthermore, the crystal form integrity and ultraviolet transmittance of the calcium fluoride crystal prepared using this invention are much higher than those of the calcium fluoride product prepared using the traditional lime precipitation method. The calcium fluoride product prepared using the process of this invention meets the standards for general optical applications.

Claims

1. A process for producing optical-grade calcium fluoride from photovoltaic wastewater, characterized in that: Includes the following steps: Step 1: The concentrated acidic fluoride wastewater generated during the photovoltaic production process is fed into a nanofiltration membrane system. The nanofiltration membrane retains fluorosilicate ions and their polymers, as well as high-valence metal ions such as aluminum, iron, copper, and nickel. Hydrofluoric acid molecules and fluoride ions pass through the nanofiltration membrane to form acidic fluoride-containing water. Step 2: Add high-purity optical-grade calcium fluoride seed crystals to the acidic fluoride product water obtained in Step 1; Step 3: Add a high-purity calcium chloride solution to the system containing optical-grade calcium fluoride seed crystals, and carry out a precipitation crystallization reaction under stirring conditions; Step 4: After the precipitation and crystallization reaction is completed, stop stirring and allow it to stand for aging. The newly generated calcium fluoride will undergo heterogeneous nucleation and directional epitaxial growth on the surface of the pre-added optical-grade calcium fluoride seed crystals. Step 5: Wash the calcium fluoride crystals obtained in Step 4 repeatedly with high-purity water with a resistivity ≥18MΩ·cm until the conductivity of the washing solution is stable, and then wash once with anhydrous ethanol. Step 6: Dry the calcium fluoride crystals washed in Step 5 at 100-105℃ for 2-4 hours; Step 7: The dried calcium fluoride crystals are calcined in a tube furnace filled with high-purity nitrogen at a temperature of 400°C for 2-3 hours to remove organic adsorbates and moisture adhering to the surface of the calcium fluoride crystals, thus obtaining a high-purity, optical-grade calcium fluoride product.

2. The process for producing optical-grade calcium fluoride from photovoltaic wastewater according to claim 1, characterized in that: The nanofiltration membrane used in step one is a strong acid-resistant nanofiltration membrane with a molecular weight cutoff between 200 and 1000 Da and an operating pressure of 0.5 to 3.0 MPa.

3. The process for producing optical-grade calcium fluoride from photovoltaic wastewater according to claim 1, characterized in that: In step two, the optical-grade calcium fluoride seed crystals used as seed crystals have a particle size range of 30-80 micrometers and a purity of not less than 99.99% (4N).

4. The process for producing optical-grade calcium fluoride from photovoltaic wastewater according to claim 1 or 3, characterized in that: The mass ratio of the amount of optical-grade calcium fluoride seed crystals added to the total amount of fluoride (calculated as fluoride ions) expected to be recovered from the wastewater is in the range of 1:5 to 1:

20.

5. The process for producing optical-grade calcium fluoride from photovoltaic wastewater according to claim 1, characterized in that: The high-purity calcium chloride solution used in step three uses electronic grade or high-purity calcium chloride with a purity ≥99.99%. The molar ratio of calcium ions added to the calcium chloride solution to fluoride ions in the acidic fluoride product water is controlled between 1:1.8 and 1:2.

2.

6. The process for producing optical-grade calcium fluoride from photovoltaic wastewater according to claim 1, characterized in that: In step three, the calcium fluoride crystallization reaction time is controlled at 1 to 3 hours, and in step four, the aging time is controlled at 1 to 3 hours.

7. The process for producing optical-grade calcium fluoride from photovoltaic wastewater according to claim 1, characterized in that: The aging process in step four is carried out at a temperature of room temperature to 60°C.

8. A calcium fluoride induced crystallization system used in the process of producing optical-grade calcium fluoride from photovoltaic wastewater according to any one of claims 1-7, characterized in that: The system includes a concentrated acid-containing fluoride wastewater collection tank, a nanofiltration membrane system, a seed crystal addition device, a calcium fluoride crystallization reactor, a reagent mixing tank, a filter, a crystal transfer device, a first cleaning tank, a second cleaning tank, a dryer, a tubular furnace, several booster pumps, and a control system. The concentrated acid-containing fluoride wastewater collection tank is capable of collecting and storing concentrated acid-containing fluoride wastewater. The outlet of the collection tank is connected to the inlet of the nanofiltration membrane system via a pipeline. The concentrated acid-containing fluoride wastewater in the collection tank can be pumped into the nanofiltration membrane system via booster pumps. The permeate side of the nanofiltration membrane system is connected to the inlet of the calcium fluoride crystallization reactor via a pipeline. A booster pump delivers acidic fluoride permeate from the nanofiltration membrane system into a calcium fluoride crystallization reactor. A seed crystal addition device quantitatively adds optical-grade calcium fluoride seeds to the reactor. A high-purity calcium chloride solution prepared in a reagent mixing tank is pumped into the reactor via the booster pump. The reactor is equipped with a stirrer for continuous mixing of the liquid mixture. The reactor's discharge port is connected to the filter inlet via a pipe, allowing for periodic opening to discharge calcium fluoride crystals. The filter removes fluoride crystals. The calcium fluoride crystals discharged from the calcium fluoride crystallization reactor are filtered to remove water. A crystal transfer device sequentially transports the filtered calcium fluoride crystals to a first washing tank, a second washing tank, a dryer, and a tubular furnace. The first washing tank is equipped with a high-purity water inlet and an outlet. The high-purity water inlet allows for the metered addition of high-purity water to the first washing tank, while the outlet discharges the washing liquid after washing. The first washing tank performs at least one water wash on the filtered calcium fluoride crystals. The second washing tank is equipped with an anhydrous ethanol inlet and an ethanol outlet. The anhydrous ethanol inlet allows for the metered addition of high-purity water to the second washing tank. Anhydrous ethanol is added, and the ethanol outlet is sufficient to discharge the ethanol waste liquid after cleaning. The second cleaning tank can perform at least one ethanol cleaning on the water-washed calcium fluoride crystals. The dryer can heat and dry the calcium fluoride crystals after ethanol cleaning. High-purity nitrogen gas is passed through the tubular furnace, which can calcine the dried calcium fluoride crystals to remove organic adsorbents and moisture from the surface of the calcium fluoride crystals. The control system controls the operation of the nanofiltration membrane system, seed crystal addition device, calcium fluoride crystallization reactor, filter, crystal transfer device, first cleaning tank, second cleaning tank, dryer, tubular furnace and each lift pump.

9. The calcium fluoride-induced crystallization system according to claim 8, characterized in that: It also includes a water bath, on which a first heater and a first temperature sensor are installed. The first heater can heat the water in the water bath, and the first temperature sensor can detect the water temperature in the water bath in real time and feed it back to the control system. The control system controls the first heater to start and stop. The calcium fluoride crystallization reactor is located in the water bath.

10. The calcium fluoride-induced crystallization system according to claim 8, characterized in that: The drain outlet of the first cleaning tank is also equipped with a washing liquid conductivity detection device. The conductivity detection device can detect the conductivity of the washing liquid discharged from the first cleaning tank and feed the detection result back to the control system. The dryer and the tubular furnace are respectively equipped with a third temperature sensor and a fourth temperature sensor. The third temperature sensor and the fourth temperature sensor can detect the temperature inside the dryer and the tubular furnace in real time and feed it back to the control system. The control system controls the start and stop of the heating device inside the dryer and the tubular furnace.

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