Pulse air flotation and continuous air flotation combined fluorine-containing wastewater fluorine recovery method and device

Abstract

The invention discloses a pulse air floatation and continuous air floatation combined fluorine-containing wastewater fluorine recovery method and device, and the recovery method comprises the following steps: adjusting the pH value of fluorine-containing wastewater to alkalescence, and then reacting with calcium salt to obtain wastewater containing calcium fluoride microcrystals; the calcium fluoride microcrystals are intercepted by using a stable scum layer formed under the combined action of pulse air floatation and gravity, and the intercepted calcium fluoride is cured into a large-particle-size calcium fluoride crystal product in the scum layer and then settled and recycled; according to the method, calcium fluoride microcrystals penetrating through a scum layer are captured through continuous air flotation, the captured calcium fluoride microcrystals are agglomerated into agglomerates, and the agglomerates are pumped to the scum layer through a sludge pump so as to supplement particle number loss caused by calcium fluoride crystallization product recovery. Aiming at the problems of low recovery rate and high energy consumption during fluorine recovery of fluorine-containing wastewater calcium fluoride crystals, the pulse air flotation and continuous air flotation combined fluorine recovery method is innovatively adopted, and the recovery method is high in fluorine recovery rate, high in product purity and low in energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of fluoride recovery and treatment technology in wastewater. Specifically, this invention relates to a method and apparatus for fluoride recovery from fluoride-containing wastewater using a combination of pulse flotation and continuous flotation. Background Technology

[0002] my country is rich in rare earth mineral resources. However, the rare earth smelting process generates large amounts of high-fluoride wastewater, which seriously endangers human health and the environment. Fluoride concentrations in drinking water exceeding 3 mg / L can lead to fluorosis (skeletal deformities and dental abnormalities), cognitive impairment, infertility, and damage to organs such as endocrine glands, thyroid glands, and kidneys.

[0003] Meanwhile, fluoride compounds are an important class of industrial raw materials with wide applications. Therefore, fluoride recovery from fluoride-containing wastewater has gradually gained attention. The traditional method for removing and recovering fluoride from industrial wastewater is chemical precipitation. However, the calcium fluoride precipitates formed by the traditional calcium salt precipitation method are fine particles (0.1~10 μm), difficult to settle, and easily penetrate the filter, leading to excessive fluoride concentration in the effluent.

[0004] To overcome this problem, invention patent CN108383206A proposed a "combined use of calcium salt and flocculant," which increased the settling performance of calcium fluoride. However, it did not solve the problem of sludge resource utilization, and the purity of the product could not be guaranteed due to the addition of coagulant. Invention patent CN117228871A uses crystallization-membrane separation technology, which can significantly reduce the concentration of fluoride ions and obtain relatively pure calcium fluoride crystals. However, its equipment is complex and has a high risk of membrane fouling, making it difficult to use in practical applications.

[0005] Adding seed crystals to fluoride-containing wastewater transforms the calcium fluoride crystallization mode from homogeneous to heterogeneous (induced) crystallization, resulting in crystalline products with controllable particle size and excellent settling properties. However, the heterogeneous crystallization mode requires an upflow fluidized bed reactor. When the upflow velocity is too high, the seed crystals will be washed out of the fluidized bed; when the upflow velocity is too low, the seed crystals tend to settle to the bottom. Moreover, even with a large amount of seed crystals added, homogeneous crystallization inevitably occurs in the fluidized bed, producing a certain amount of calcium fluoride microcrystals with poor settling properties.

[0006] It is evident that the key to achieving efficient recovery and utilization of fluoride from fluoride-containing wastewater through calcium fluoride crystallization lies in solving the problem of poor sedimentation performance of calcium fluoride microcrystals, which makes it difficult to improve the recovery rate. In particular, this problem needs to be solved without adding coagulants or other chemical agents, so as to improve the purity and quality of the recovered products.

[0007] In view of this, the present invention is hereby proposed. Summary of the Invention

[0008] The purpose of this invention is to propose a method and apparatus for fluoride recovery from fluoride-containing wastewater using a combination of pulse flotation and continuous flotation, in order to solve the problem of difficulty in improving the recovery rate caused by the poor settling performance of calcium fluoride microcrystals during the recovery of fluoride from calcium fluoride crystals in fluoride-containing wastewater.

[0009] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: In a first aspect, embodiments of the present invention provide a method for fluoride recovery from fluoride-containing wastewater using a combination of pulse flotation and continuous flotation, comprising the following steps: (1) After adjusting the pH of the fluoride-containing wastewater to weak alkalinity, it is mixed with calcium salt to carry out a homogeneous crystallization reaction of calcium fluoride, and wastewater containing calcium fluoride microcrystals is obtained. (2) The wastewater containing calcium fluoride microcrystals is pumped into the air flotation crystallizer. When the wastewater passes through the scum zone of the air flotation crystallizer from top to bottom, most of the calcium fluoride microcrystals in the wastewater are intercepted by the scum layer composed of high-concentration calcium fluoride crystal particles in the scum zone. The remaining small amount of calcium fluoride microcrystals escape into the pulse air flotation zone of the air flotation crystallizer and flow into the continuous air flotation zone of the air flotation crystallizer with the wastewater. (3) The calcium fluoride microcrystals intercepted in the scum layer dissolve to generate crystal-forming ions Ca. 2+ and F - Subsequently, the crystal-forming ions recrystallize on the surface of the calcium fluoride crystal particles in the slag layer, increasing the particle size of the calcium fluoride crystal particles. Then, under the action of gravity, they sink to the bottom slag discharge area of ​​the air flotation crystallizer and are discharged, thus realizing the recovery of fluorine. (4) The escaped calcium fluoride microcrystals obtained in step (2) are captured by microbubbles in the continuous air flotation zone and float to the agglomeration zone of the air flotation crystallizer. After agglomeration occurs in the agglomeration zone, they are pumped to the scum zone of the air flotation crystallizer to serve as new seed crystals to compensate for the loss of seed crystals caused by the recovery of calcium fluoride crystal particles. Finally, the treated residual water is discharged in compliance with standards.

[0010] In some embodiments, in step (1), the pH of the fluoride-containing wastewater is adjusted to 7.5~8.5; And / or, the F in the fluoride-containing wastewater - With the Ca in the calcium salt 2+ The molar ratio is 1:(1.1~1.3); And / or, the reaction time of the homogeneous crystallization reaction is 10~20 s.

[0011] In some embodiments, in step (2), the hydraulic residence time of the pulse air flotation zone is 0.5~1 h; And / or, the dissolved gas pressure in the pulsed air flotation zone is 0.5 MPa; And / or, the air-to-water volume ratio in the pulsed air flotation zone is 10% or more, the single air flotation time is 1 min, and the air flotation interval is 10 min.

[0012] In some embodiments, in step (4), the hydraulic residence time of the continuous air flotation zone is 5 to 10 minutes; And / or, the dissolved air pressure in the continuous flotation zone is 0.5 MPa and the air-to-water volume ratio is >2%.

[0013] Secondly, embodiments of the present invention also propose a fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation. The fluoride recovery device is used to implement the fluoride recovery method described in the first aspect and includes an alkaline storage tank, a calcium salt storage tank, a static mixer, a flotation crystallizer, and a pressure dissolved air tank. The flotation crystallizer includes a scum zone, a pulse flotation zone, a slag discharge zone, an agglomeration zone, and a continuous flotation zone. The scum zone, the pulse flotation zone, and the slag discharge zone are connected sequentially from top to bottom. The agglomeration zone is located at the top of the continuous flotation zone, and the continuous flotation zone is connected to the pulse flotation zone. The outlet of the alkaline storage tank is connected to the inlet of the static mixer, the outlet of the calcium salt storage tank is connected to the inlet of the static mixer, and the outlet of the static mixer is connected to the top inlet of the flotation crystallizer.

[0014] In some embodiments, both the pulse flotation zone and the continuous flotation zone are provided with a high-pressure dissolved air water release component. The high-pressure dissolved air water release component is composed of several perforated tubes arranged in parallel, and each perforated tube is provided with an aeration head at its top. The inlet of the high-pressure dissolved air water release component is connected to the outlet of the pressure vessel tank.

[0015] In some embodiments, the height ratio of the pulse flotation zone to the scum zone is 1:(3~5).

[0016] In some embodiments, the height ratio of the continuous air flotation zone to the agglomeration zone is (3~5):1.

[0017] In some embodiments, the pulsed air flotation zone and the continuous air flotation zone are separated by a perforated baffle.

[0018] In some embodiments, a slag discharge trough is provided at the bottom of the slag discharge zone, and the slag discharge trough is inclinedly connected to one side wall of the air flotation crystallizer.

[0019] The advantages and beneficial effects of the embodiments of the present invention are as follows: (1) High fluoride recovery efficiency: First, the high-concentration scum layer in the scum zone is used to intercept calcium fluoride microcrystals. Second, the calcium fluoride microcrystals that escape through the high-concentration scum layer are captured by continuous air flotation and solid-liquid separation is carried out, which realizes the efficient interception of calcium fluoride microcrystals and the high fluoride recovery rate.

[0020] (2) Excellent settling performance of fluorine recovery products: The combined action of pulsed air flotation and gravity stabilizes the scum layer in the scum zone, prolongs the crystallization reaction time, realizes the "dissolution-recrystallization" maturation process, and generates crystallized recovery products with large particle size and excellent settling performance, which facilitates solid-liquid separation.

[0021] (3) Low energy consumption for fluorine recovery: Pulse flotation and continuous flotation are used together. Pulse flotation only requires intermittent aeration, which greatly reduces the energy consumption required for aeration compared with conventional flotation. At the same time, after being intercepted by the scum layer, only a very small amount of calcium fluoride microcrystals escape. The air-water ratio required for continuous flotation to capture the escaped microcrystals is greatly reduced, further reducing the energy consumption required for aeration.

[0022] (4) High purity of recycled products: No seed crystals are required during the crystallization process, and no chemical reagents are added during the solid-liquid separation process of the crystallized products. The obtained calcium fluoride crystal products have high purity and great recycling value. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation according to an embodiment of the present invention.

[0024] Attached reference numerals: 1-Alkali storage tank, 2-Calcium salt storage tank, 3-Static mixer, 4-Air flotation crystallizer, 401-Scum zone, 4011-Scum layer, 402-Pulse air flotation zone, 403-Slag discharge zone, 4031-Slag discharge trough, 404 Agglomeration zone, 405-Continuous air flotation zone, 406-Perforated pipe, 407-Aeration head, 408-Perforated baffle, 5-Pressure dissolved air tank, 6-Pressure pump, 7-Water pump, 8-Sludge pump. Detailed Implementation

[0025] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0026] In this invention, when a value is described as a range, it should be understood that such disclosure includes disclosure of all possible subranges within that range, as well as specific numerical values ​​falling within that range, regardless of whether specific numerical values ​​or specific subranges are explicitly specified.

[0027] In this invention, the terms “comprising” and “including” and their various variations mean that other elements or wholes may be included but are not specifically described.

[0028] In this invention, the term "and / or" is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can represent three situations: A exists alone, A and B exist simultaneously, and B exists alone.

[0029] This invention is based on the inventor's discoveries and understanding of the following facts and problems: The inventors discovered that when using calcium fluoride crystallization to recover fluoride from fluoride-containing wastewater, although the crystallization rate of fluoride is very high, the recovery rate is often very limited. The main reason for this is that the crystallized products are often composed of fine-grained calcium fluoride microcrystals, resulting in poor solid-liquid separation. The fundamental reason for the small particle size of the crystallized products is insufficient crystallization reaction time, allowing the crystals insufficient time to fully develop. Crystallization reaction time refers to the solid-phase residence time in the crystallization reactor. In actual production processes, the solid-phase residence time (i.e., crystallization reaction time) in the crystallization reactor is determined by the hydraulic residence time. Extending the hydraulic residence time of the crystallization reactor can increase the solid-phase residence time (i.e., the crystallization reaction duration). However, in water treatment processes, extending the hydraulic residence time increases the reactor's footprint and cost. Therefore, considering overall cost, in actual calcium fluoride crystallization processes, the hydraulic residence time and solid-phase residence time generally do not exceed 2 hours.

[0030] Further research by the inventors revealed that extending the crystallization reaction time can effectively increase the particle size of the crystallized product and improve its solid-liquid separation performance. On one hand, under hydraulic or mechanical stirring, frequent collisions between calcium fluoride crystals cause flocculation and agglomeration. On the other hand, according to Ostwaldell's ripening theory, smaller calcium fluoride crystals, due to their higher surface free energy, will dissolve, yielding F... - and Ca 2+ Crystallization will occur on the surface of microcrystals with larger particle sizes, which will result in a gradual increase in the average particle size of calcium fluoride crystal products in the crystallization system, and the overall sedimentation performance will also be improved.

[0031] Based on this, in order to obtain a solid phase residence time (i.e., crystallization reaction time) that is longer than the hydraulic residence time of the crystallization reaction device, and at the same time solve the problem of high energy consumption caused by continuous aeration in conventional air flotation solid-liquid separation, the present invention innovatively adopts a technical solution that combines pulse air flotation and continuous air flotation. First, a static mixer completes the homogeneous nucleation process of calcium fluoride, obtaining calcium fluoride microcrystals, creating conditions for the efficient operation of subsequent pulse-type and continuous air flotation. Second, using pulse-type air flotation and the gravity of the scum layer itself, the high-concentration scum layer is stabilized within the scum zone, undergoing vertical reciprocating motion, thereby intercepting most of the calcium fluoride microcrystals. Third, the intercepted calcium fluoride microcrystals move with the scum layer within the scum zone, with a residence time far exceeding the hydraulic residence time of the device, allowing for a thorough "dissolution-recrystallization" maturation process, generating large-particle-size calcium fluoride crystals with excellent settling properties, which are then recovered by gravity settling away from the scum layer. Finally, continuous air flotation captures calcium fluoride microcrystals that penetrate the scum layer, achieving solid-liquid separation between wastewater and calcium fluoride microcrystals. The separated liquid meets discharge standards, while the separated calcium fluoride microcrystals agglomerate in the agglomeration layer and are then pumped back to the scum layer to compensate for the particle number loss caused by the recovery of calcium fluoride crystal products.

[0032] In a first aspect, embodiments of the present invention provide a method for fluoride recovery from fluoride-containing wastewater using a combination of pulse flotation and continuous flotation, comprising the following steps: (1) After adjusting the pH of the fluoride-containing wastewater to weak alkalinity, it is mixed with calcium salt to carry out a homogeneous crystallization reaction of calcium fluoride, and wastewater containing calcium fluoride microcrystals is obtained. (2) The wastewater containing calcium fluoride microcrystals is pumped into the air flotation crystallizer. When the wastewater passes through the scum zone of the air flotation crystallizer from top to bottom, most of the calcium fluoride microcrystals in the wastewater are intercepted by the scum layer composed of high-concentration calcium fluoride crystal particles in the scum zone. The remaining small amount of calcium fluoride microcrystals escape into the pulse air flotation zone of the air flotation crystallizer and flow into the continuous air flotation zone of the air flotation crystallizer with the wastewater. (3) The calcium fluoride microcrystals intercepted in the scum layer dissolve to generate crystal-forming ions Ca. 2+ and F - Subsequently, the crystal-forming ions recrystallize on the surface of the calcium fluoride crystal particles in the slag layer, increasing the particle size of the calcium fluoride crystal particles. Then, under the action of gravity, they sink to the bottom slag discharge area of ​​the air flotation crystallizer and are discharged, thus realizing the recovery of fluorine. (4) The escaped calcium fluoride microcrystals obtained in step (2) are captured by microbubbles in the continuous air flotation zone and float to the agglomeration zone of the air flotation crystallizer. After agglomeration occurs in the agglomeration zone, they are pumped to the scum zone of the air flotation crystallizer to serve as new seed crystals to compensate for the loss of seed crystals caused by the recovery of calcium fluoride crystal particles. Finally, the treated residual water is discharged in compliance with standards.

[0033] It is understood that in the fluorine recovery method of this invention, the calcium fluoride crystal particles (i.e., calcium fluoride crystals) in the scum layer only require additional addition of calcium fluoride crystals during the initial startup of the device. Thereafter, the device will continuously generate calcium fluoride crystals on its own and circulate them back to the scum layer, maintaining a high concentration of calcium fluoride crystal particles within the scum layer.

[0034] In some embodiments, in step (1), the pH of the fluoride-containing wastewater is adjusted to 7.5-8.5; the inventors have found through research that when the pH of the fluoride-containing wastewater is below 7.5, F - and Ca 2+ Before crystallization can occur, the fluoride penetrates the scum layer, causing a sharp drop in fluoride recovery rate. Furthermore, when the pH of the fluoride-containing wastewater is higher than 8.5, some calcium will be released. 2+ The formation of Ca(OH)2 crystals not only reduces the fluoride recovery rate, but also lowers the purity of the recovered product due to the contamination of Ca(OH)2. Therefore, it is more appropriate to control the pH value of the fluoride-containing wastewater within the range of 7.5 to 8.5 in this embodiment of the invention. This not only ensures a faster formation rate of microcrystals, but also makes the generated microcrystals more easily intercepted by the scum layer.

[0035] And / or, the F in the fluoride-containing wastewater - With the Ca in the calcium salt 2+ The molar ratio is 1:(1.1~1.3); And / or, the reaction time of the homogeneous crystallization reaction is 10~20 s.

[0036] In some embodiments, in step (2), the hydraulic residence time of the pulse air flotation zone is 0.5~1 h; And / or, the dissolved air pressure in the pulse flotation zone is 0.5 MPa, which ensures that the diameter of the generated microbubbles is about 30 μm, which is most conducive to the scum layer being stable in the scum zone and carrying out vertical reciprocating motion. And / or, the air-to-water volume ratio in the pulsed air flotation zone is 10% or more, the single air flotation time is 1 min, and the air flotation interval is 10 min.

[0037] In step (3), the calcium fluoride microcrystals undergo a "dissolution-recrystallization" maturation process within the scum layer, specifically the dissolution of the microcrystals to generate the crystal-forming ions Ca. 2+ and F - Subsequently, the crystal-forming ions recrystallize on the surface of the calcium fluoride crystal particles in the slag layer, causing the calcium fluoride particles to gradually grow.

[0038] In some embodiments, in step (4), the hydraulic residence time of the continuous air flotation zone is 5 to 10 minutes; And / or, the dissolved air pressure in the continuous flotation zone is 0.5 MPa and the air-to-water volume ratio is >2%, thus achieving the best capture effect on calcium fluoride microcrystals.

[0039] Secondly, such as Figure 1 As shown in the figure, this embodiment of the invention also proposes a fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation. The fluoride recovery device is used to implement the fluoride recovery method described in the first aspect, and includes an alkaline storage tank 1, a calcium salt storage tank 2, a static mixer 3, a flotation crystallizer 4, and a pressure dissolved air tank 5. The flotation crystallizer 4 includes a scum zone 401, a pulse flotation zone 402, a slag discharge zone 403, an agglomeration zone 404, and a continuous flotation zone 405. The scum zone 401, the pulse flotation zone 402, and the slag discharge zone 403 are connected sequentially from top to bottom. The agglomeration zone 404 is located at the top of the continuous flotation zone 405, and the continuous flotation zone 405 is connected to the pulse flotation zone 402. The outlet of the alkaline storage tank 1 is connected to the inlet of the static mixer 3, the outlet of the calcium salt storage tank 2 is connected to the inlet of the static mixer 3, and the outlet of the static mixer 3 is connected to the top inlet of the flotation crystallizer 4. A pressure pump 6 is provided on the connecting pipeline.

[0040] It should be noted that the aforementioned static mixer 3 is composed of one or more parallel static mixing components, and the number of parallel components is not particularly limited; those skilled in the art can determine this based on the volume of fluoride-containing wastewater. The mixing time of the static mixer 3 (i.e., the mixing time of F in the fluoride-containing wastewater) - With Ca in calcium salt storage tank 2+ The duration of the homogeneous crystallization reaction of calcium fluoride should be controlled to be 10-20 s.

[0041] It should also be noted that the alkaline solution in alkaline storage tank 1 is not particularly limited, and those skilled in the art can store alkaline solution according to actual needs. For example, the alkaline solution in alkaline storage tank 1 can be Ca(OH)2 solution, NaOH solution, etc. Similarly, the calcium salt in calcium salt storage tank 2 is not particularly limited, and those skilled in the art can select it according to actual needs. For example, the calcium salt can be CaCl2, etc.

[0042] In some embodiments, both the pulse flotation zone 402 and the continuous flotation zone 405 are provided with high-pressure dissolved air water release components. The high-pressure dissolved air water release components are composed of a plurality of perforated tubes 406 arranged in parallel. Each perforated tube 406 is provided with an aeration head 407 at its top to generate microbubbles. The inlet of the high-pressure dissolved air water release components is connected to the outlet of the pressure vessel tank 5, and a water pump 7 is provided on the connecting pipeline.

[0043] High-pressure dissolved air water in the pressure dissolved air tank 5 is pumped by the water pump 7 into the perforated pipe 406 of the high-pressure dissolved air water release component, and then released through the aeration head 407, generating a large number of microbubbles. When the lower boundary of the scum layer 4011 moves to the lower boundary of the scum zone 401 (i.e., the upper boundary of the pulse flotation zone 402), the aeration head in the pulse flotation zone 402 is activated, driving the entire scum layer 4011 upward until the upper boundary of the scum layer 4011 moves to the upper boundary of the scum zone 401, at which point the pulse flotation stops. Afterward, the scum layer 4011 begins to move downward under the influence of gravity until its lower boundary reaches the lower boundary of the scum zone 401, initiating a new round of pulse flotation. After the microbubbles generated by the aeration heads in the continuous flotation zone 405 capture the calcium fluoride microcrystals, they are flotated into the agglomeration zone 404. In the agglomeration zone 404, the microbubbles rupture, and the calcium fluoride microcrystals collide frequently with each other, causing them to agglomerate and grow. Then, the agglomerates in the agglomeration zone 404 are pumped into the scum zone 401 by the sludge pump 8 to replenish the reduced particle number concentration after the calcium fluoride crystal particles are recovered.

[0044] In some embodiments, the height ratio of the pulse flotation zone 402 to the scum zone 401 is 1:(3~5).

[0045] In some embodiments, the height ratio of the continuous air flotation zone 405 to the agglomeration zone 404 is (3~5):1.

[0046] In some embodiments, the pulse air flotation zone 402 and the continuous air flotation zone 405 are separated by a perforated baffle 408, which has a plurality of water holes.

[0047] It should be noted that the fluoride-containing wastewater flows downwards in the scum zone 401 and the pulse flotation zone 402, and horizontally in the agglomeration zone 404 and the continuous flotation zone 405. The fluoride-containing wastewater from the static mixer 3 enters the scum zone 401 through the inlet located at the top of the scum zone 401. The calcium fluoride microcrystals in the fluoride-containing wastewater are intercepted by the high-concentration scum layer 4011 in the scum zone 401, and undergo a "dissolution-recrystallization" maturation process, which promotes the increase in the particle size of the calcium fluoride crystals. The calcium fluoride crystals with increased particle size settle and are then discharged for recycling.

[0048] In some embodiments, a slag discharge trough 4031 is provided at the bottom of the slag discharge zone 403. The slag discharge trough 4031 is inclinedly connected to one side wall of the air flotation crystallizer 4 to facilitate the downward sliding of calcium fluoride crystal particles into the slag discharge trough 4031 for collection. It is understood that if the slag discharge trough 4031 is horizontally connected to one side wall of the air flotation crystallizer 4, an additional slag scraper is required to discharge and recycle the calcium fluoride crystal particles.

[0049] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. Experimental methods without specific conditions in the embodiments are conventional methods and conditions well known in the art.

[0050] Example 1 The fluoride-containing wastewater recovered and treated in this embodiment is high-fluoride mine water with a fluoride concentration of 50 mg / L, pH=6.2, and an average influent flow rate of 2 m³ / L. 3 / h. The method for recovering fluoride from fluoride-containing wastewater includes the following steps: (1) The pH of the high-fluoride mine water was adjusted to 8.0 using Ca(OH)2 solution in the alkaline storage tank and mixed with CaCl2 transported from the calcium salt storage tank (the Ca / F molar ratio was controlled to be 1.1:1) and transported to a static mixer for homogeneous crystallization reaction of calcium fluoride. After 10 s of reaction, wastewater containing calcium fluoride microcrystals was obtained. (2) The wastewater containing calcium fluoride microcrystals obtained in step (1) is pumped to the air flotation crystallizer by a pressurized pump. When the wastewater passes through the scum zone of the air flotation crystallizer from top to bottom, most of the calcium fluoride microcrystals in the wastewater are intercepted by the scum layer composed of high-concentration calcium fluoride crystal particles in the scum zone. The remaining very small amount of calcium fluoride microcrystals penetrate the scum layer and escape to the pulse air flotation zone of the air flotation crystallizer. Then, they flow into the continuous air flotation zone of the air flotation crystallizer along with the wastewater through the perforated baffle. (3) The calcium fluoride microcrystals intercepted in the scum layer dissolve to generate crystal-forming ions Ca. 2+ and F - Subsequently, the crystallizing ions recrystallize on the surface of the calcium fluoride crystal particles in the slag layer, causing the particle size of the calcium fluoride crystal particles to gradually increase. When the pulse air flotation is insufficient to keep the calcium fluoride crystal particles suspended in the slag layer, the calcium fluoride crystal particles sink to the bottom slag discharge area of ​​the air flotation crystallizer under the action of gravity and are discharged through the slag discharge tank, thus realizing the recovery of fluorine. (4) The escaped calcium fluoride microcrystals obtained in step (2) are captured by microbubbles in the continuous flotation zone and float to the agglomeration zone of the flotation crystallizer, realizing the solid-liquid separation of fluoride wastewater. The residual water after solid-liquid separation is discharged through the bottom drain of the flotation crystallizer. After the escaped calcium fluoride microcrystals in the agglomeration zone agglomerate, they are pumped by the sludge pump to the scum zone of the flotation crystallizer to serve as new seed crystals to compensate for the loss of seed crystals caused by the recovery of calcium fluoride crystal particles.

[0051] In this embodiment, the hydraulic residence time in the pulsed air flotation zone is controlled at 0.5 h, and the hydraulic residence time in the continuous air flotation zone is controlled at 5 min; the height ratio of the pulsed air flotation zone to the scum zone is 1:4; the air-to-water volume ratio in the pulsed air flotation zone is 12%, the single air flotation time is 1 min, and the air flotation interval is 10 min; the air-to-water volume ratio in the continuous air flotation zone is 3%; the dissolved air pressure in both the pulsed and continuous air flotation zones is 0.5 MPa, and the diameter of the generated microbubbles is about 30 μm.

[0052] Experimental results show that after treatment using the recovery method described in this embodiment, the fluoride concentration in the effluent is consistently below 1 mg / L, and the fluoride recovery rate is consistently above 95%. The average particle size of the recovered calcium fluoride crystals is 500 μm, the water content is less than 40%, and the purity is consistently above 98%.

[0053] Example 2 The fluoride-containing wastewater recovered and treated in this embodiment is industrial wastewater containing fluoride, with a fluoride concentration of 8-12 mg / L, pH=7.2, and an average influent flow rate of 2 m³ / L. 3 / h. The method for recovering fluoride-containing wastewater is basically the same as that in Example 1, except that: in step (1) of this example, the Ca / F molar ratio is 1.3:1; the height ratio of the pulse flotation zone to the scum zone is 1:5; the single flotation time of the pulse flotation zone is 1.2 min; and the air-water volume ratio of the continuous flotation zone is 4%.

[0054] Experimental results show that after treatment using the recovery method described in this embodiment, the fluoride concentration in the effluent remains stable below 0.6 mg / L, and the fluoride recovery rate remains stable above 90%. The average particle size of the recovered calcium fluoride crystals is 500 μm, the water content is less than 40%, and the purity remains stable above 98%.

[0055] Example 3 In this embodiment, the fluoride-containing wastewater being recycled and treated was groundwater containing fluoride, with a fluoride concentration of 2-3.2 mg / L, pH=6.9, and an average influent flow rate of 2 m³ / L. 3 / h. The method for recovering fluoride-containing wastewater is basically the same as that in Example 2, except that: in step (1) of this example, NaOH solution is used to adjust the pH of the fluoride-containing groundwater to 8.0; the hydraulic retention time of the pulse flotation zone is controlled to be 1 h; the single flotation time of the pulse flotation zone is 2 min; and the air-water volume ratio of the continuous flotation zone is 3%.

[0056] Experimental results show that after treatment using the recovery method described in this embodiment, the fluoride concentration in the effluent is consistently below 0.5 mg / L, and the fluoride recovery rate is consistently above 75%. The average particle size of the recovered calcium fluoride crystals is 400 μm, the water content is less than 40%, and the purity is consistently above 98%.

[0057] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0058] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" or "several" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0059] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0060] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0061] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0062] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for fluoride recovery from fluoride-containing wastewater using a combination of pulse flotation and continuous flotation, characterized in that, Includes the following steps: (1) After adjusting the pH of the fluoride-containing wastewater to weak alkalinity, it is mixed with calcium salt to carry out a homogeneous crystallization reaction of calcium fluoride, and wastewater containing calcium fluoride microcrystals is obtained. (2) The wastewater containing calcium fluoride microcrystals is pumped into the air flotation crystallizer. When the wastewater passes through the scum zone of the air flotation crystallizer from top to bottom, most of the calcium fluoride microcrystals in the wastewater are intercepted by the scum layer composed of high-concentration calcium fluoride crystal particles in the scum zone. The remaining small amount of calcium fluoride microcrystals escape into the pulse air flotation zone of the air flotation crystallizer and flow into the continuous air flotation zone of the air flotation crystallizer with the wastewater. (3) The calcium fluoride microcrystals intercepted in the scum layer dissolve to generate crystal-forming ions Ca. 2+ and F - Subsequently, the crystal-forming ions recrystallize on the surface of the calcium fluoride crystal particles in the slag layer, increasing the particle size of the calcium fluoride crystal particles. Then, under the action of gravity, they sink to the bottom slag discharge area of ​​the air flotation crystallizer and are discharged, thus realizing the recovery of fluorine. (4) The escaped calcium fluoride microcrystals obtained in step (2) are captured by microbubbles in the continuous air flotation zone and float to the agglomeration zone of the air flotation crystallizer. After agglomeration occurs in the agglomeration zone, they are pumped to the scum zone of the air flotation crystallizer to serve as new seed crystals to compensate for the loss of seed crystals caused by the recovery of calcium fluoride crystal particles. Finally, the treated residual water is discharged in compliance with standards.

2. The method for fluoride recovery from fluoride-containing wastewater using a combination of pulse flotation and continuous flotation according to claim 1, characterized in that, In step (1), the pH of the fluoride-containing wastewater is adjusted to 7.5~8.5; And / or, the F in the fluoride-containing wastewater - With the Ca in the calcium salt 2+ The molar ratio is 1:(1.1~1.3); And / or, the reaction time of the homogeneous crystallization reaction is 10~20 s.

3. The method for fluoride recovery from fluoride-containing wastewater using a combination of pulse flotation and continuous flotation as described in claim 1, characterized in that, In step (2), the hydraulic residence time of the pulse air flotation zone is 0.5~1 h; And / or, the dissolved gas pressure in the pulsed air flotation zone is 0.5 MPa; And / or, the air-to-water volume ratio in the pulsed air flotation zone is 10% or more, the single air flotation time is 1 min, and the air flotation interval is 10 min.

4. The method for fluoride recovery from fluoride-containing wastewater using a combination of pulse flotation and continuous flotation according to claim 1, characterized in that, In step (4), the hydraulic residence time of the continuous air flotation zone is 5 to 10 minutes. And / or, the dissolved air pressure in the continuous flotation zone is 0.5 MPa and the air-to-water volume ratio is >2%.

5. A fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation, characterized in that, The fluorine recovery device is used to implement the fluorine recovery method according to any one of claims 1-4, and includes an alkali storage tank, a calcium salt storage tank, a static mixer, an air flotation crystallizer, and a pressure dissolved air tank; the air flotation crystallizer includes a scum zone, a pulse air flotation zone, a slag discharge zone, an agglomeration zone, and a continuous air flotation zone, wherein the scum zone, the pulse air flotation zone, and the slag discharge zone are connected sequentially from top to bottom, the agglomeration zone is located at the top of the continuous air flotation zone, and the continuous air flotation zone is connected to the pulse air flotation zone; the outlet of the alkali storage tank is connected to the inlet of the static mixer, the outlet of the calcium salt storage tank is connected to the inlet of the static mixer, and the outlet of the static mixer is connected to the top inlet of the air flotation crystallizer.

6. The fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation as described in claim 5, characterized in that, Both the pulse flotation zone and the continuous flotation zone are equipped with high-pressure dissolved air water release components. Each high-pressure dissolved air water release component consists of several perforated tubes arranged in parallel, and each perforated tube has an aeration head at its top. The inlet of the high-pressure dissolved air water release component is connected to the outlet of the pressure vessel.

7. The fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation as described in claim 5, characterized in that, The height ratio of the pulse flotation zone to the scum zone is 1:(3~5).

8. The fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation as described in claim 5, characterized in that, The height ratio of the continuous air flotation zone to the agglomeration zone is (3~5):

1.

9. The fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation as described in claim 5, characterized in that, The pulsed air flotation zone and the continuous air flotation zone are separated by a perforated baffle.

10. The fluoride recovery device for fluoride-containing wastewater using a combination of pulse flotation and continuous flotation as described in claim 5, characterized in that, The bottom of the slag discharge area is provided with a slag discharge trough, which is inclinedly connected to one side wall of the air flotation crystallizer.

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