A system and method for removing fluoride and recovering calcium fluoride using a tubular membrane contactor.

Abstract

This invention relates to the field of fluoride wastewater treatment technology, and more particularly to a system for defluorinating and recovering calcium fluoride using a tubular membrane contactor. The system comprises a first security filter, a tubular membrane contactor, a second security filter, a material separation membrane, a centrifuge, and a dryer. These components are connected sequentially from front to back. The permeate from the material separation membrane and the supernatant from the centrifuge are returned to the tubular membrane contactor via its calcium-containing solution inlet. Using a tubular membrane contactor for defluorination avoids direct contact between the calcium-containing solution and the fluoride-containing wastewater, thus preventing an increase in the hardness of the wastewater and the formation of complex fluoride-containing sludge. This results in higher purity calcium fluoride, a wider range of applications, and higher economic benefits. It also eliminates the need for large amounts of lime, coagulants, and flocculants, reducing costs and the risk of environmental pollution.

Description

Technical Field

[0001] This invention relates to the field of fluoride-containing wastewater treatment technology, and in particular to a system and method for removing fluoride and recovering calcium fluoride using a tubular membrane contactor. Background Technology

[0002] Fluoride-containing wastewater has a wide range of sources. It is generated in various production processes, including the smelting of non-ferrous and rare earth metals, aluminum electrolytic refining, glass and ceramic manufacturing, fluorosilicate production, pesticide production, stainless steel pickling, and the manufacturing of electronic products such as semiconductors and liquid crystal displays. If this fluoride-containing wastewater is discharged without treatment, it is likely to adhere to soil, water, and organic matter, increasing exposure risks and posing a threat to human health and safety.

[0003] Recovering calcium fluoride (CaF2) from high-concentration fluoride-containing wastewater not only cuts off a significant pathway for fluoride pollution into the natural environment, reducing the risk of fluoride pollution and benefiting environmental protection and ecological security, but also provides the recovered calcium fluoride with extremely high resource utilization value. After a series of purification and processing steps, calcium fluoride can be reused in various stages of industrial production. This not only significantly reduces dependence on natural fluorite resources and alleviates resource shortages, but also lowers raw material procurement costs for enterprises, improves economic efficiency, and forms a virtuous cycle model that benefits both the environment and the economy. This contributes to promoting sustainable industrial development and implementing the modern concept of resource recycling.

[0004] The traditional method for treating high-concentration fluoride-containing industrial wastewater is lime flocculation sedimentation, with the basic process as follows: fluoride-containing wastewater → reaction tank (adding lime, coagulant, or flocculant and stirring) → thickening tank (sludge is treated periodically) → supernatant is defluorinated wastewater. However, the traditional lime flocculation sedimentation method for treating high-concentration fluoride-containing industrial wastewater has drawbacks, including large amounts of calcium fluoride sludge production, high water content, limited comprehensive utilization, and difficulties in treatment and disposal. These drawbacks are detailed below: Fluorine-containing sludge is difficult to treat and dispose of: Fluorine-containing sludge has certain corrosive, toxic, and other hazardous characteristics. The traditional treatment method is landfill, which may cause secondary pollution. Although according to the 2016 edition of the "National Hazardous Waste List" (hereinafter referred to as the List), calcium fluoride sludge generated from wastewater treatment is no longer classified as hazardous waste, its low calorific value and high leaching risk mean that it is generally not accepted by disposal units, resulting in a continuous increase in the accumulation of calcium fluoride sludge.

[0005] Adding excessive lime increases wastewater hardness, increasing the burden on subsequent hardness removal processes: In the traditional lime flocculation and sedimentation method for treating fluoride-containing wastewater, operators often add excessive lime to ensure ideal fluoride ion removal and rapid sludge settling and separation. From a chemical reaction perspective, lime (mainly composed of calcium oxide or calcium hydroxide) reacts with fluoride ions in the wastewater to form calcium fluoride precipitate, a process that follows stoichiometry. However, in practice, due to the complexity of wastewater composition, fluctuations in reaction conditions, and considerations for ensuring treatment effectiveness, the amount of lime added often exceeds the theoretically required amount. When excessive lime enters the wastewater system, in addition to reacting with fluoride ions, a large amount of calcium ions dissolve in the water, significantly increasing the hardness of the wastewater and hindering subsequent treatment and purification.

[0006] Calcium fluoride has low purity, limiting its comprehensive utilization: In terms of resource utilization, calcium fluoride sludge faces numerous limitations. Although calcium fluoride theoretically has some application value, such as serving as a raw material or additive in some industrial fields, the low purity of calcium fluoride in sludge, often containing large amounts of unreacted lime, other metal ions, and organic matter, makes it difficult to directly apply to industrial production processes with high purity requirements. For example, the fluorochemical industry typically requires a CaF2 grade greater than 97%, while the content of harmful impurities such as CaCO3 and SiO2 must be controlled below 1%.

[0007] Hollow fiber membrane contactors have a high packing density, resulting in relatively narrow flow channels for fluid between the fiber bundles. Calcium fluoride is poorly soluble in water; as the reaction proceeds and the number of calcium fluoride particles gradually increases, it easily accumulates in these narrow channels, causing blockage and ultimately affecting the performance of the entire membrane contactor. Summary of the Invention

[0008] The purpose of this invention is to address the shortcomings of existing technologies by proposing a system and method for removing fluoride and recovering calcium fluoride using a tubular membrane contactor.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A system for removing fluoride and recovering calcium fluoride using a tubular membrane contactor includes a first security filter, a tubular membrane contactor, a second security filter, a material separation membrane, a centrifuge, and a dryer. The first security filter, the tubular membrane contactor, the second security filter, the material separation membrane, the centrifuge, and the dryer are connected in sequence from front to back. The permeate from the material separation membrane and the supernatant from the centrifuge are returned to the tubular membrane contactor through the calcium-containing solution inlet of the tubular membrane contactor.

[0010] Furthermore, it also includes wastewater lift pumps, calcium-containing solution tanks, calcium-containing solution lift pumps, calcium fluoride recovery tanks, calcium fluoride solution lift pumps, calcium fluoride concentrate tanks, centrifuge inlet pumps, and dryer sludge inlet pumps; The output end of the wastewater booster pump is connected to the inlet of the first security filter, and the outlet of the first security filter is connected to the fluoride-containing wastewater inlet of the tubular membrane contactor. The tubular membrane contactor is also provided with a calcium fluoride solution outlet, a calcium-containing solution inlet, and a defluorination wastewater outlet. The outlet of the calcium-containing solution tank is connected to the suction end of the calcium-containing solution booster pump, and the output end of the calcium-containing solution booster pump is connected to the calcium-containing solution inlet of the tubular membrane contactor. The outlet of the calcium fluoride recovery tank is connected to the suction end of the calcium fluoride solution booster pump, and the output end of the calcium fluoride solution booster pump is connected to the second security filter. The inlet of the first security filter is connected to the second security filter, and the outlet of the second security filter is connected to the inlet of the material separation membrane. The material separation membrane has a concentrate outlet and a product water outlet. Its product water outlet is connected to the inlet of the calcium-containing solution tank. The concentrate outlet of the material separation membrane is connected to the inlet of the calcium fluoride concentrate tank. The outlet of the calcium fluoride concentrate tank is connected to the suction end of the centrifuge inlet pump. The output end of the centrifuge inlet pump is connected to the inlet of the centrifuge. The supernatant outlet of the centrifuge is connected to the inlet of the calcium-containing solution tank. The slurry outlet of the centrifuge is connected to the inlet of the dryer.

[0011] Preferably, the filtration accuracy of the first security filter is 5 to 10 micrometers.

[0012] Preferably, the tubular membrane reactor has a membrane tube diameter of 3–8 mm and a pore size of 1–30 nm.

[0013] Preferably, the filtration accuracy of the second security filter is 5 to 10 micrometers.

[0014] A method for removing fluoride and recovering calcium fluoride using a tubular membrane contactor includes the following steps: S1. High-concentration fluoride-containing wastewater passes through a security filter and enters a tubular membrane contactor; S2. Inside the tubular membrane contactor, fluoride ions permeate through the membrane and react with calcium ions on the other side of the membrane to form calcium fluoride particles, which drive the defluorination of wastewater. S3, calcium fluoride solution is concentrated by material separation membrane filtration, and the product water is reused to continue reacting with fluoride ions; S4. The concentrated water from the material separation membrane is centrifuged and dried to obtain calcium fluoride powder.

[0015] Preferably, in step S2, the calcium chloride solution provides calcium ions.

[0016] Preferably, in step S2, the calcium hydroxide solution provides calcium ions.

[0017] The beneficial effects of this invention are: 1. High purity of calcium fluoride: The calcium-containing solution reacts with the fluoride-containing wastewater within a tubular membrane contactor, eliminating the need for direct contact between the two solutions. This design avoids the complex chemical reactions and cross-contamination issues that can result from direct mixing. The membrane acts as a selective barrier, allowing fluoride ions to migrate from the fluoride-containing wastewater to the calcium-containing solution side driven by the concentration gradient. Other metal ions and organic matter in the fluoride-containing wastewater are restricted by the membrane layer, making it difficult for them to enter the region where the calcium-containing solution reacts with fluoride ions. Therefore, the resulting calcium fluoride precipitate contains almost no other metal ions or organic matter, or their content can be controlled at very low levels, with a calcium fluoride concentration exceeding 98%.

[0018] 2. Does not increase wastewater hardness: Due to the barrier between the calcium-containing solution and the fluoride-containing wastewater, calcium ions do not pass through the membrane layer into the fluoride-containing wastewater, and will not cause a significant increase in wastewater hardness.

[0019] 3. No fluoride-containing sludge is generated: The calcium fluoride solution is concentrated, centrifuged and dried to become high-purity calcium fluoride powder, which can be widely used in various fields and helps to alleviate the pressure of fluorite resource shortage.

[0020] 4. Reduced chemical dosage and lower environmental pollution risk: The treatment significantly reduces the amount of lime, coagulants, and flocculants added, lowering the cost of chemicals used in fluoride-containing wastewater treatment. Furthermore, reduced dosage means less of these chemicals entering the wastewater and environment, lowering the risk of chemical pollution to aquatic bodies.

[0021] 5. More resistant to fouling than hollow fiber membranes: Compared to ordinary hollow fiber membranes, tubular membrane separation systems can withstand higher sludge concentrations and more extreme pH environments. During the fluoride ion recovery and calcium fluoride particle generation process, hollow fiber membranes, due to their narrow channels and large specific surface area, experience increased particle-to-particle interactions, resulting in weaker anti-fouling capabilities. In contrast, the calcium fluoride particles in tubular membranes have greater diffusion space and will not rapidly accumulate on the membrane, preventing clogging. Attached Figure Description

[0022] Figure 1 The flowchart below shows a system for removing fluoride and recovering calcium fluoride using a tubular membrane contactor, as proposed in this invention. Figure 2 This is a flowchart of a method for removing fluoride and recovering calcium fluoride using a tubular membrane contactor, as proposed in this invention.

[0023] In the diagram: 1-Wastewater lift pump; 2-First security filter; 3-Tube membrane contactor; 4-Calcium solution tank; 5-Calcium solution lift pump; 6-Calcium fluoride recovery tank; 7-Calcium fluoride solution lift pump; 8-Second security filter; 9-Material separation membrane; 10-Calcium fluoride concentrate tank; 11-Centrifuge inlet pump; 12-Centrifuge; 13-Dryer sludge inlet pump; 14-Dryer. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0025] Example 1: Reference Figure 1 A system for removing fluoride and recovering calcium fluoride using a tubular membrane contactor includes a wastewater lift pump 1, a first security filter 2, a tubular membrane contactor 3, a calcium-containing solution tank 4, a calcium-containing solution lift pump 5, a calcium fluoride recovery tank 6, a calcium fluoride solution lift pump 7, a second security filter 8, a material separation membrane 9, a calcium fluoride concentrate tank 10, a centrifuge inlet pump 11, a centrifuge 12, a dryer sludge inlet pump 13, and a dryer 14.

[0026] The output end of wastewater lift pump 1 is connected to the inlet of the first security filter 2. The outlet of the first security filter 2 is connected to the fluoride-containing wastewater inlet of the tubular membrane contactor 3. The tubular membrane contactor 3 is also equipped with a calcium fluoride solution outlet, a calcium-containing solution inlet, and a defluorination wastewater outlet. The outlet of the calcium-containing solution tank 4 is connected to the suction end of the calcium-containing solution lift pump 5. The output end of the calcium-containing solution lift pump 5 is connected to the calcium-containing solution inlet of the tubular membrane contactor 3. The outlet of the calcium fluoride recovery tank 6 is connected to the suction end of the calcium fluoride solution lift pump 7. The output end of the calcium fluoride solution lift pump 7 is connected to the second security filter. The inlet of the second security filter 8 is connected to the inlet of the material separation membrane 9. The material separation membrane 9 is provided with a concentrated water outlet and a product water outlet. Its product water outlet is connected to the inlet of the calcium-containing solution tank 4. The concentrated water outlet of the material separation membrane 9 is connected to the inlet of the calcium fluoride concentrated water tank 10. The outlet of the calcium fluoride concentrated water tank 10 is connected to the suction end of the centrifuge inlet pump 11. The output end of the centrifuge inlet pump 11 is connected to the inlet of the centrifuge 12. The supernatant outlet of the centrifuge 12 is connected to the inlet of the calcium-containing solution tank 4. The slurry outlet of the centrifuge 12 is connected to the inlet of the dryer 14.

[0027] In this embodiment, the calcium-containing solution is a calcium chloride solution.

[0028] High-concentration fluoride-containing wastewater enters the first security filter 2 (filtration accuracy 5-10 microns) via wastewater lift pump 1, and then enters the tubular membrane reactor 3. A high-concentration calcium chloride solution is prepared in the calcium solution tank 4 and injected into the tubular membrane reactor 3 via calcium solution lift pump 5.

[0029] In the tubular membrane reactor 3, fluoride-containing wastewater flows through the tube side, while calcium chloride solution flows through the shell side. The membrane tube diameter is 3-8 mm, and the pore size is 1-30 nm. Fluoride ions in the wastewater permeate through the membrane and react with calcium ions in the calcium chloride solution to generate CaF2, which is insoluble in water and is collected in the calcium fluoride recovery tank 6.

[0030] The calcium fluoride solution enters the second security filter 8 (filtration accuracy 5-10 microns) through the calcium fluoride solution booster pump 7, and then enters the material separation membrane 9 for separation and concentration. The clarified product water is transported to the calcium-containing solution tank 4 for reuse, and the concentrated calcium fluoride concentrate is collected in the calcium fluoride concentrate tank 10.

[0031] The calcium fluoride concentrate is pumped from centrifuge 11 to centrifuge 12 via centrifuge inlet pump. The supernatant is pumped to calcium solution tank 4 for reuse. The deposited calcium fluoride is pumped from dryer mud pump 13 to dryer 14. After dehydration and drying, calcium fluoride powder is finally obtained.

[0032] Reference Figure 2 A method for removing fluoride and recovering calcium fluoride using a tubular membrane contactor includes the following steps: S1. High-concentration fluoride-containing wastewater passes through a security filter and enters a tubular membrane contactor; S2. Inside the tubular membrane contactor, fluoride ions permeate through the membrane and react with calcium ions on the other side of the membrane to form calcium fluoride particles, which drive the defluorination of wastewater. S3, calcium fluoride solution is concentrated by material separation membrane filtration, and the product water is reused to continue reacting with fluoride ions; S4. The concentrated water from the material separation membrane is centrifuged and dried to obtain calcium fluoride powder.

[0033] Example 2:

[0034] Unlike Example 1, the calcium-containing solution is a calcium hydroxide solution. When treating acidic fluoride-containing wastewater, it recovers calcium fluoride while neutralizing H+ in the wastewater, thus adjusting the pH to a certain extent and reducing the amount of sodium hydroxide, a commonly used pH adjuster, required.

[0035] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A system for removing fluoride and recovering calcium fluoride using a tubular membrane contactor, characterized in that, It includes a first security filter (2), a tubular membrane contactor (3), a second security filter (8), a material separation membrane (9), a centrifuge (12), and a dryer (14), as well as a wastewater lift pump (1), a calcium-containing solution tank (4), a calcium-containing solution lift pump (5), a calcium fluoride recovery tank (6), a calcium fluoride solution lift pump (7), a calcium fluoride concentrate tank (10), a centrifuge inlet pump (11), and a dryer sludge inlet pump (13); The first security filter (2), tubular membrane contactor (3), second security filter (8), material separation membrane (9), centrifuge (12) and dryer (14) are connected in sequence from front to back. The product water of the material separation membrane (9) and the supernatant of the centrifuge (12) are returned to the tubular membrane contactor (3) through the calcium-containing solution inlet of the tubular membrane contactor (3). The diameter of the membrane tube of the tubular membrane contactor (3) is 3-8 mm, and the aperture is 1-30 nm. The output end of the wastewater booster pump (1) is connected to the inlet of the first security filter (2), and the outlet of the first security filter (2) is connected to the fluoride-containing wastewater inlet of the tubular membrane contactor (3). The tubular membrane contactor (3) is also provided with a calcium fluoride solution outlet, a calcium-containing solution inlet, and a defluorination wastewater outlet. The outlet of the calcium-containing solution tank (4) is connected to the suction end of the calcium-containing solution booster pump (5), and the output end of the calcium-containing solution booster pump (5) is connected to the calcium-containing solution inlet of the tubular membrane contactor (3). The outlet of the calcium fluoride recovery tank (6) is connected to the suction end of the calcium fluoride solution booster pump (7), and the output end of the calcium fluoride solution booster pump (7) is connected to the second security filter (8). The outlet of the second security filter (8) is connected to the inlet of the material separation membrane (9). The material separation membrane (9) is provided with a concentrated water outlet and a product water outlet. Its product water outlet is connected to the inlet of the calcium-containing solution tank (4). The concentrated water outlet of the material separation membrane (9) is connected to the inlet of the calcium fluoride concentrated water tank (10). The outlet of the calcium fluoride concentrated water tank (10) is connected to the suction end of the centrifuge inlet pump (11). The output end of the centrifuge inlet pump (11) is connected to the inlet of the centrifuge (12). The supernatant outlet of the centrifuge (12) is connected to the inlet of the calcium-containing solution tank (4). The slurry outlet of the centrifuge (12) is connected to the inlet of the dryer (14).

2. The system for removing fluoride and recovering calcium fluoride using a tubular membrane contactor according to claim 1, characterized in that, The first security filter (2) has a filtration accuracy of 5 to 10 micrometers.

3. The system for removing fluoride and recovering calcium fluoride using a tubular membrane contactor according to claim 1, characterized in that, The second security filter (8) has a filtration accuracy of 5 to 10 micrometers.

4. A method for removing fluoride and recovering calcium fluoride using the system described in claim 1, characterized in that, Includes the following steps: S1. High-concentration fluoride-containing wastewater passes through a security filter and enters a tubular membrane contactor; S2. Inside the tubular membrane contactor, fluoride ions permeate through the membrane and react with calcium ions on the other side of the membrane to form calcium fluoride particles, which drive the defluorination of wastewater. S3, calcium fluoride solution is concentrated by material separation membrane filtration, and the product water is reused to continue reacting with fluoride ions; S4. The concentrated water from the material separation membrane is centrifuged and dried to obtain calcium fluoride powder.

5. A method for removing fluoride and recovering calcium fluoride using a tubular membrane contactor according to claim 4, characterized in that, In step S2, the calcium chloride solution provides calcium ions.

6. The method for removing fluoride and recovering calcium fluoride using a tubular membrane contactor according to claim 4, characterized in that, In step S2, the calcium hydroxide solution provides calcium ions.

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