CaP crystal phosphorus recovery method and device based on aeration air floatation

Abstract

The invention discloses a CaP crystallized phosphorus recovery method based on aeration air floatation, which comprises the following steps: phosphorus-containing sewage subjected to conventional seed crystal induced crystallization treatment in a fluidization crystallization region enters an air floatation region in a horizontal flow state for aeration air floatation, in the air floatation region, CaP microcrystals and lost seed crystals in the sewage are quickly agglomerated with microbubbles to form flocculation clusters, and the flocculation clusters are separated from the sewage; under the continuous jacking action of the microbubbles, a stable scum area is formed at the top of the air floatation area, so that the separation of CaP microcrystals and seed crystals in the phosphorus-containing sewage is realized; wherein flocculation groups in the scum area complete the recombination of a floc structure under the stirring action of microbubbles and the shearing action caused by microbubble breakage to generate CaP crystal particles, a part of the generated CaP crystal particles are regularly pumped to the bottom of the fluidization and crystallization area to supplement lost crystal seeds, and the rest of the generated CaP crystal particles are regularly discharged as phosphorus recovery products. The method solves the problem of phosphorus recovery reduction caused by CaP microcrystal generation and seed crystal loss in the recovery process.

Description

Technical Field

[0001] This invention relates to the field of phosphorus recovery and utilization technology in wastewater, and particularly to a method and apparatus for recovering CaP crystallized phosphorus based on aerated flotation. Background Technology

[0002] Phosphorus is one of the major pollutants in the aquatic environment. If phosphorus in large quantities of wastewater is not effectively removed, it can enter the aquatic environment, leading to eutrophication, algal blooms, and severe water pollution. At the same time, phosphorus is an important natural resource, widely used in industry and agriculture. Due to the finite, non-renewable nature of phosphorus resources and their extremely uneven global distribution, a global phosphorus resource crisis is inevitable. It is predicted that by 2050, the world will face a severe phosphorus resource crisis, potentially reducing food production capacity by more than 25%. Therefore, recovering phosphorus resources from wastewater is of great significance.

[0003] Methods for recovering phosphorus from wastewater include chemical crystallization, electrochemical deposition, ion exchange, and membrane separation. Among these, chemical crystallization phosphorus recovery, represented by CaP, separates phosphorus from wastewater in solid form and is considered true wastewater phosphorus recovery. Wastewater CaP crystallization is currently widely used, primarily employing fluidized bed induced crystallization devices with added seed crystals. However, practice shows that although the crystallization rate of phosphorus in wastewater is very high, the phosphorus recovery effect is often limited. This is because, on the one hand, while fluidized bed induced crystallization can effectively suppress homogeneous crystallization, homogeneous crystallization inevitably occurs, producing a certain amount of CaP microcrystals, which weakens the phosphorus recovery effect; on the other hand, the fluidized bed adopts an upward flow pattern, making it easy for seed crystals to be lost with the water flow, leading to a continuous decrease in phosphorus recovery efficiency.

[0004] To overcome the impact of CaP microcrystals on phosphorus recovery, Chinese invention patent CN110395824B proposes to suppress CaP microcrystal formation by significantly increasing the seed crystal dosage and reducing the seed crystal particle size. The seed crystal dosage is as high as 50 g / L, with a particle size of only 75 μm. Although this patented technology can effectively suppress CaP microcrystal formation, the excessive seed crystal dosage results in high costs, and the small seed crystal size makes it prone to loss, requiring continuous replenishment.

[0005] To address the problem of seed crystal loss during the crystallization of sparingly soluble salts, Chinese invention patent CN120647094 A proposed a seed crystal recycling method. This method uses a seed crystal separation sieve to separate 2-3 mm of the product from the crystallization product as recycled seed crystals. Although this patented technology can ensure that the crystallization system has a sufficiently high seed crystal concentration, it requires the addition of an extra seed crystal separation device. Moreover, this technology cannot solve the problem of the formation of microcrystals with poor settling properties.

[0006] Therefore, developing a wastewater CaP crystallization phosphorus recovery method and device that can overcome the interference of CaP microcrystals on phosphorus recovery, solve the problem of seed crystal loss, and is simple to operate, low in cost, and stable in operation is of great practical significance. Summary of the Invention

[0007] To simultaneously address the interference of CaP microcrystals on phosphorus recovery and the loss of seed crystals during the CaP crystallization phosphorus recovery process in wastewater, this invention provides a CaP crystallization phosphorus recovery method and apparatus based on aerated flotation. The aerated flotation separates the CaP microcrystals generated in the CaP crystallization system from the lost seed crystals, and then returns the separated seed crystals to the CaP crystallization system, thereby achieving efficient recovery of phosphorus from wastewater by CaP crystallization.

[0008] To achieve the above objectives, this application adopts the following technical solution: A method for CaP crystallization phosphorus recovery based on aerated flotation involves introducing phosphorus-containing wastewater, after conventional seed-induced crystallization treatment in a fluidized crystallization zone, into an aerated flotation zone in a horizontal flow state. In the flotation zone, CaP microcrystals and lost seed crystals in the wastewater rapidly agglomerate with microbubbles to form flocs. Under the continuous support of the microbubbles, a stable scum zone is formed at the top of the flotation zone, thereby achieving the separation of CaP microcrystals and seed crystals in the phosphorus-containing wastewater. The flocs in the scum zone undergo floc restructuring under the stirring action of microbubbles and the shearing action caused by the rupture of microbubbles, generating CaP crystal particles. A portion of the generated CaP crystal particles is periodically pumped to the bottom of the fluidized crystallization zone to replenish the lost seed crystals, while the remainder is periodically discharged as phosphorus recovery products.

[0009] Specifically, the microbubble diameter is 30~50 μm. This is because controlling the microbubble within this range will benefit the CaP microcrystals and seed crystals and the microbubble flocculation. When the microbubble diameter is less than 30 μm, the number of CaP microcrystals and seed crystals aggregated by a single microbubble is limited. When it is greater than 50 μm, the microbubble is prone to rupture.

[0010] Specifically, when the air-to-water volume ratio in the flotation zone is greater than or equal to 10%, and less than 10%, the solid-liquid separation performance of the flotation is limited.

[0011] Specifically, in the fluidized bed crystallization zone, hydroxyapatite with a particle size of 100-150 μm is selected as the seed crystal, and a one-time addition method is adopted, with an addition amount of 10-20 g / L. When the seed crystal particle size is less than 100 μm, the seed crystals are prone to agglomeration, reducing the induction effect. When the particle size is greater than 150 μm, the induction ability is significantly reduced. The induction ability is limited when the seed crystal addition amount is less than 10 g / L, and the cost is too high when it is higher than 20 g / L.

[0012] Specifically, the hydraulic residence time in the fluidized crystallization zone is 10-30 minutes, and the height is 3-4 meters. Within this range, the seed-induced crystallization process is complete. Below this range, the induced crystallization process is insufficient; above this range, it increases equipment costs and floor space requirements.

[0013] Specifically, the hydraulic retention time in the air flotation zone is 1-2 hours. Within this range, the solid-liquid separation effect of air flotation is fully realized. Below this range, the solid-liquid separation effect is limited, and above this range, it increases equipment costs and floor space requirements. Specifically, the height ratio of the flotation zone to the scum zone is 3:1; the length-to-height ratio of the flotation zone is greater than or equal to 4. Within this range, the solid-liquid separation effect of flotation is optimal. Below this range, the solid-liquid separation effect of flotation is limited; above this range, it increases equipment costs and floor space requirements.

[0014] Through extensive experimentation, the inventors discovered that during conventional seed-induced crystallization treatment, the optimal Ca / P molar ratio for phosphorus-containing wastewater entering the fluidized crystallization zone is (1.8~2.5):1. Maintaining a pH of 8.0~9.0 within this range is ideal. Within this pH range, not only is the microcrystal formation rate faster, but the generated microcrystals are also suitable for air flotation separation. When the pH is below 8.0, the crystallization rate is relatively low, leading to a sharp decrease in phosphorus recovery. When the pH is above 9.0, the increased CaP microcrystal production significantly increases the required air-to-water ratio for subsequent air flotation, thus improving recovery.

[0015] Specifically, the wastewater in the fluidized crystallization zone first overflows into the diversion zone, and then enters the air flotation zone from the bottom side of the diversion zone.

[0016] Specifically, the hydraulic residence time in the diversion zone is 30-60 s.

[0017] This invention also provides a wastewater CaP crystallization phosphorus recovery device, including a processor. The processor is divided into a fluidized crystallization zone, a flow guiding zone, and an air flotation zone by a first baffle and a second baffle. The first baffle has an overflow port connecting the fluidized crystallization zone and the flow guiding zone. The lower side of the second baffle has an orifice connecting the flow guiding zone and the air flotation zone. An inlet pipe is located at the bottom of the fluidized crystallization zone, and an alkali addition pipe and a Ca addition pipe are provided on the inlet pipe. 2+ The air flotation zone has a microporous aeration disc at its bottom, a drain pipe on the side of the air flotation zone opposite to the second baffle, a surface scraper at the top of the air flotation zone to scrape scum into the scum trough, a first scum discharge pipe and a circulation pipe on the scum trough, and a circulation pump on the circulation pipe connected to the water inlet pipe.

[0018] The inventors discovered that when using CaP crystallization to recover phosphorus from wastewater, the formation of CaP microcrystals in the crystallization system is unavoidable. CaP microcrystals have poor solid-liquid separation performance and are easily carried out with the effluent. Therefore, although the phosphorus crystallization rate is very high, the recovery rate is very limited. Furthermore, due to collisions, friction, and compression, the seed crystals in the CaP crystallization system will break to some extent. These broken seed crystals easily flow out of the crystallization zone with the effluent, reducing the seed crystal concentration and thus weakening the phosphorus recovery effect.

[0019] The inventors' research also revealed that although CaP microcrystals have poor sedimentation and separation performance, if sufficient crystallization reaction time is ensured, CaP microcrystals can gradually grow into CaP crystalline particles through aggregation and maturation under hydraulic or mechanical stirring. Furthermore, the longer the time, the larger the particle size of the crystalline particles, the better the sedimentation performance, and the lower the corresponding water content. However, within the timescale of water treatment, the crystallization reaction time is limited, often not exceeding 1 hour.

[0020] The inventors' research also revealed that in actual production processes, the volume and phosphorus concentration of phosphorus-containing wastewater often fluctuate, impacting the stable operation of the CaP crystallization phosphorus recovery process. One effective measure to address these water quality and volume fluctuations is to adjust the seed concentration and particle size in the fluidized crystallization zone. Higher seed concentrations and smaller particle sizes result in a larger surface area for the seed crystals, providing more active crystallization sites.

[0021] This invention proposes a method and apparatus for recovering CaP crystallized phosphorus from wastewater based on aerated flotation. In the fluidized crystallization zone of the apparatus, phosphorus-containing wastewater undergoes conventional upward-flow fluidized bed seed-induced crystallization. Subsequently, the wastewater carrying CaP microcrystals and lost seed crystals is adjusted to a horizontal flow and sent to the flotation zone for aerated flotation. The CaP microcrystals and seed crystals rapidly agglomerate with microbubbles to form flocs and float to the scum zone at the top of the flotation zone. Due to the continuous supporting effect of the microbubbles, the CaP microcrystals and seed crystals separated by flotation form a stable scum layer in the scum zone. In the scum zone, CaP microcrystals and seed crystals have sufficient time to crystallize and reorganize, generating new CaP crystal products. Specifically: First, the flocs break down due to the shearing action caused by the rupture of microbubbles, releasing CaP microcrystals and seed crystals. Second, under the continuous stirring action of microbubbles, some of the released CaP microcrystals agglomerate into CaP crystal particles due to frequent collisions, i.e., the agglomeration process. Third, another part of the CaP microcrystals dissolves due to stirring, releasing crystal-forming ions. These ions crystallize on the surface of the aforementioned CaP crystal particles and seed crystals, generating CaP crystal particles with larger particle sizes, i.e., the ripening process. Finally, some of the ripened CaP crystal particles are pumped to the fluidized crystallization zone to replenish seed crystals, while the other part is discharged from the device as phosphorus recovery products.

[0022] Furthermore, in this invention, the particle size of the matured CaP crystals in the scum zone can be controlled by the scum discharge cycle in the scum zone and the air-to-water ratio in the flotation zone. Extending the scum discharge cycle and increasing the air-to-water ratio can increase the CaP crystal particle size, and vice versa. Similarly, the particle size of the circulating seed crystals can also be controlled by the circulation cycle and the air-to-water ratio in the flotation zone.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: Innovation Point 1: Utilizing aerated flotation to achieve the synergistic capture and separation of CaP microcrystals and lost seed crystals.

[0024] Traditional fluidized beds rely on gravity settling, which is ineffective for small-sized CaP microcrystals with poor settling performance, leading to their loss with the effluent and reducing recovery rates. Simultaneously, tiny seed crystals are also lost. This application creatively introduces a microbubble flotation system, utilizing the efficient separation characteristics of flotation technology for fine particles (especially those with densities similar to water) to actively "capture" CaP microcrystals and lost seed crystals from the water. This achieves a shift from "passive settling" to "active flotation," fundamentally solving the core problem of CaP microcrystal loss while simultaneously recovering valuable seed crystals.

[0025] Innovation Point Two: Achieving "in-situ ripening" and structural reorganization of CaP microcrystals in the air flotation zone.

[0026] This application's aerated flotation is not merely a simple solid-liquid separation, but rather transforms the flotation / scum zone into a "secondary reactor." The shear force generated by the rupture of microbubbles breaks down the flotation flocs, releasing CaP microcrystals and seed crystals. Under the continuous stirring of the microbubbles, some of the CaP microcrystals dissolve, releasing crystal-forming ions (Ca...). 2+ and HPO4 - PO4 3- The released crystal-forming ions recrystallize on the surface of another part of the incompletely dissolved CaP microcrystal agglomerates or the recovered seed crystals, achieving particle maturation and particle size growth. This transforms the originally "waste" and difficult-to-process CaP microcrystals into valuable CaP crystal products with larger and more stable particle sizes, realizing "turning waste into treasure" and improving the quality of the products.

[0027] Innovation Point 3: Achieving self-balancing and long-term stability of the system through product recycling

[0028] This application directly recirculates a portion of the CaP crystal particles matured in the scum zone back to the fluidized bed crystallization zone to replenish lost seed crystals, achieving self-replenishment of seed crystals. After system startup, little or no additional seed crystals are required, reducing operating costs and complexity. Furthermore, by controlling the recirculation flow rate and cycle, the seed crystal concentration and particle size distribution within the fluidized bed can be dynamically adjusted, making the system more adaptable to fluctuations in influent water quality and quantity. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the wastewater CaP crystallization phosphorus recovery device provided by the present invention. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] A wastewater CaP crystallization phosphorus recovery method based on aerated flotation involves introducing phosphorus-containing wastewater, after conventional seed-induced crystallization treatment in a fluidized crystallization zone, into an aerated flotation zone in a horizontal flow state. In the flotation zone, CaP microcrystals and lost seed crystals in the wastewater rapidly agglomerate with microbubbles to form flocs. Under the continuous support of the microbubbles, a stable scum zone is formed at the top of the flotation zone, thereby achieving the separation of CaP microcrystals and seed crystals in the phosphorus-containing wastewater. The flocs in the scum zone undergo floc restructuring under the stirring action of microbubbles and the shearing action caused by the rupture of microbubbles, generating CaP crystal particles. A portion of the generated CaP crystal particles is periodically pumped to the bottom of the fluidized crystallization zone to replenish the lost seed crystals, while the remainder is periodically discharged as phosphorus recovery products.

[0032] The process of floc structure reorganization is as follows: Step 1: Under the continuous support of microbubbles, the flocs form a stable scum zone at the top of the flotation zone.

[0033] Step 2: Under the shearing action caused by the rupture of microbubbles within the floc, the floc breaks down, and CaP microcrystals and seeds separate.

[0034] Step 3: CaP microcrystals from the broken flocs dissolve under the stirring action of microbubbles, releasing crystal-forming ions Ca. 2+ and HPO4 - PO4 3- wait.

[0035] Step 4: The crystal-forming ions from Step 3 undergo surface-induced crystallization on the seed crystal surface from Step 2 to generate CaP crystal particles.

[0036] Traditional fluidized beds rely on gravity settling, which is ineffective for small-sized CaP microcrystals with poor settling performance, leading to their loss with the effluent and reducing recovery rates. Simultaneously, tiny seed crystals are also lost. This application creatively introduces a microbubble flotation system, utilizing the efficient separation characteristics of flotation technology for fine particles (especially those with densities similar to water) to actively "capture" CaP microcrystals and lost seed crystals from the water. This represents a paradigm shift from "passive settling" to "active flotation," fundamentally solving the core problem of CaP microcrystal loss while simultaneously recovering valuable seed crystals.

[0037] This application's aerated flotation is not merely a simple solid-liquid separation, but rather transforms the flotation / scum zone into a "secondary reactor." In this "secondary reactor," the flocs undergo a "dissolution-recrystallization" maturation reaction driven by both "shearing action caused by microbubble rupture" and "microbubble agitation." Specifically, the shear force generated by microbubble rupture breaks down the flocs, releasing CaP microcrystals and seed crystals. Under continuous microbubble agitation, some of the CaP microcrystals dissolve, releasing crystal-forming ions (Ca...). 2+ and HPO4 - PO4 3- The released crystal-forming ions recrystallize on the surface of another part of the incompletely dissolved CaP microcrystal agglomerates or the recovered seed crystals, achieving particle maturation and particle size growth. This transforms the originally "waste" and difficult-to-process CaP microcrystals into valuable CaP crystal products with larger and more stable particle sizes, realizing "turning waste into treasure" and improving the quality of the products.

[0038] See Figure 1 The wastewater CaP crystallization phosphorus recovery device provided by this invention includes a processor. The processor is rectangular in shape and is divided into a fluidized crystallization zone A, a flow guiding zone B, and an air flotation zone C by a first baffle 4 and a second baffle 5. The first baffle 4 has an overflow port connecting the fluidized crystallization zone A and the flow guiding zone B. The lower side of the second baffle 5 has an orifice connecting the flow guiding zone B and the air flotation zone C. The bottom of the fluidized crystallization zone A is provided with an inlet pipe 1, and the inlet pipe 1 is provided with an alkali addition pipe 2 and a Ca addition pipe. 2+ Pipeline 3, the bottom of the flotation zone C is provided with a microporous aeration disc 7, the microporous aeration disc 7 is connected to the aeration system 6, the side of the flotation zone C opposite to the second baffle 5 is provided with a drain pipe 8, the top of the flotation zone C is provided with a surface scraper 9 to scrape the scum into the scum tank E, the scum tank E is provided with a first scum discharge pipe 10 and a circulation pipe 12, the circulation pipe 12 is provided with a circulation pump 11 and is connected to the water inlet pipe 1.

[0039] The wastewater treatment process of the aforementioned CaP crystallization phosphorus recovery device is as follows: Before entering the device through the inlet pipe, the phosphorus-containing wastewater is mixed with NaOH solution from alkali addition pipe 2 and Ca addition solution from the alkali addition pipe 2.2+ Ca in pipe 3 2+ After mixing, the mixture enters the fluidized crystallization zone A of the phosphorus recovery unit in an upward flow. Hydroxyphosphate particles are added as seed crystals to fluidized crystallization zone A in a single step. Under the support of the upward flow, the seed crystals are stably suspended in the fluidized crystallization zone. Induced by the seed crystals, the crystal-forming ions Ca... 2+ and HPO4 - PO4 3- This is equivalent to surface crystallization on the seed crystal surface. At the same time, some of the crystal-forming ions generate CaP microcrystals in the solution. The phosphorus-containing wastewater, carrying CaP microcrystals and lost seed crystals, passes through the first baffle 4 and enters the guiding zone B. Then, under the action of the holes in the second baffle 5, it enters the flotation zone C in the form of a horizontal flow.

[0040] High-pressure gas from aeration system 6 generates microbubbles under the action of microporous aeration discs 7. CaP microcrystals and seed crystals entering flotation zone C rapidly form aggregates with the microbubbles and are then flotated to scum zone D, achieving solid-liquid separation from the phosphorus-containing wastewater. The phosphorus-containing wastewater containing separated CaP microcrystals and seed crystals has a significantly reduced phosphorus concentration and is discharged from drain pipe 8. The flocs entering scum zone D undergo structural reorganization under the combined effects of the supporting, stirring, and shearing actions caused by the rupture of microbubbles, generating CaP crystalline particles. These CaP crystalline particles are then fed into scum tank E by a surface scraper 9.

[0041] The CaP crystal particles entering the scum tank E are partially discharged and recovered as crystal products through the scum discharge pipe 10, while the other part, driven by the circulation pump 11, passes through the circulation pipe 12 and mixes with the phosphorus-containing wastewater from the inlet pipe 1 before being sent to the fluidized crystallization zone A to replenish lost seed crystals. Furthermore, a second scum discharge pipe 13 is provided at the bottom of the fluidized crystallization zone A to discharge the crystal recovery products that settle in the fluidized crystallization zone A.

[0042] In this embodiment, a perforated baffle is used to adjust the flow pattern of phosphorus-containing wastewater, after conventional seed-induced crystallization treatment, from an upward flow to a horizontal flow. Microbubbles generated by a microbubble aeration system are then used to perform air flotation solid-liquid separation of CaP microcrystals and lost seed crystals carried in the phosphorus-containing wastewater. The CaP microcrystals and lost seed crystals that have achieved solid-liquid separation with the phosphorus-containing wastewater undergo structural reorganization under the shearing action caused by the support, stirring, and collapse of the microbubbles, generating CaP crystal products. A portion of these crystals is recovered as phosphorus recovery products, while the remainder is recycled back to replenish the lost seed crystals. Ultimately, the CaP microcrystals generated by the phosphorus-containing wastewater CaP crystallization system are also recovered, and the lost seed crystals are replenished, thus ensuring the efficient and stable performance of the phosphorus recovery device in recovering phosphorus from phosphorus-containing wastewater.

[0043] In this embodiment, the guide zone is not a simple water passage, but an indispensable and meticulously designed core component that enables the efficient coupling of the two functional modules of "crystallization" and "air flotation" in this invention. This is because the water flow in the fluidized crystallization zone A is upward, aiming to support the seed crystals, form a fluidized state, and provide a contact surface for crystal growth. The air flotation zone C, however, requires a stable, near-static hydraulic environment so that microbubbles can effectively collide with and adhere to CaP microcrystals and lost seed crystals, and float smoothly. If the water flow is upward, it will directly impact, interfere with, or even destroy the upward path of the bubble-particle flocs, significantly reducing air flotation efficiency. The guide zone B, through its physical structure (the openings between it and the air flotation zone), forces the water flow to change direction. After passing the first baffle at the top of zone A, the water flows downward and then enters the air flotation zone horizontally through the openings at the bottom of the second baffle. This design ensures that the water flows horizontally along the entire width of the air flotation zone at approximately the same flow rate and velocity, laying a solid foundation for subsequent efficient air flotation separation.

[0044] The specific working process of the above-mentioned wastewater CaP crystallization phosphorus recovery device is as follows: Step 1: Adjust the pH of the phosphorus-containing wastewater to alkaline and add crystallizing agent Ca. 2+ After mixing with the recycled seed crystals, the phosphorus-containing wastewater enters the fluidized crystallization zone from the bottom of the phosphorus recovery unit in an upward flow pattern. After the crystallization reaction, the phosphorus-containing wastewater, carrying CaP microcrystals and lost seed crystals, crosses the baffle and enters the guide zone in a downward flow pattern. As the crystallization process proceeds, the particle size of the crystallized products continuously increases, and they eventually settle by gravity to the bottom slag discharge pipe and are discharged, thus achieving phosphorus recovery.

[0045] Step 2: The phosphorus-containing wastewater treated in Step 1 flows horizontally through the holes at the lower end of the second baffle into the flotation zone equipped with a microbubble aeration system at the bottom. In the flotation zone, CaP microcrystals and lost seed crystals in the wastewater rapidly flocculate with the microbubbles generated by the microbubble aeration system, forming clumps that float to the scum zone above the flotation zone, thus achieving solid-liquid separation of CaP microcrystals and lost seed crystals.

[0046] Step 3: The flocs obtained after step 2 are stably suspended in the scum zone under the continuous air flotation support. Under the stirring action of microbubbles and the shearing action caused by the rupture of microbubbles, the floc structure is reorganized, and CaP crystal particles are generated.

[0047] Through extensive experimentation, the inventors discovered that adjusting the pH of phosphorus-containing wastewater to 8.0-9.0 is optimal. Within this range, not only is the microcrystal formation rate faster, but the generated microcrystals are also suitable for air flotation separation. When the pH is below 8.0, the crystallization rate is low, leading to a sharp decline in phosphorus recovery. When the pH is above 9.0, the increased CaP microcrystal production results in a significant increase in the required air-to-water ratio for subsequent air flotation, thus improving recovery.

[0048] Preferably, the Ca / P molar ratio of the phosphorus-containing wastewater entering the fluidized crystallization zone should be controlled at (1.8~2.5):1.

[0049] Further research by the inventors revealed that a microbubble diameter of 30–50 μm is beneficial for the aggregation of CaP microcrystals and seed crystals, as well as the formation of microbubble flocculations. When the microbubble diameter is less than 30 μm, the number of CaP microcrystals and seed crystals aggregated from a single microbubble is limited; when it is greater than 50 μm, the microbubbles are prone to rupture, thus affecting the treatment effect.

[0050] In practical applications, the inventors found that when the air-to-water ratio in the flotation zone is greater than 10% (by volume), the flotation separation efficiency of CaP microcrystals and seed crystals can reach over 90%. When the air-to-water volume ratio is less than 10%, the solid-liquid separation performance of the flotation decreases.

[0051] Preferably, the hydraulic residence time of the fluidized crystallization zone is 10-30 min, and the height is 3-4 m.

[0052] Preferably, the seed crystals in the fluidized crystallization zone are selected from calcium hydroxyphosphate with a particle size of 100-150 μm, and are added in a single application at a dosage of 10-20 g / L. When the seed crystal particle size is less than 100 μm, the seed crystals are prone to agglomeration, reducing the induction effect. When the particle size is greater than 150 μm, the induction ability is significantly reduced. The induction ability is limited when the seed crystal dosage is less than 10 g / L, while the cost is too high when the dosage is higher than 20 g / L.

[0053] Specifically, the hydraulic residence time in the fluidized crystallization zone is 10-30 min and the height is 3-4 m. The hydraulic residence time in the guide zone is 30-60 s. Within this range, the seed-induced crystallization process is complete. Below this range, the induced crystallization process is insufficient. Above this range, the equipment cost and floor space increase.

[0054] Specifically, the hydraulic retention time in the air flotation zone is 1-2 hours. Within this range, the solid-liquid separation effect of air flotation is fully realized. Below this range, the solid-liquid separation effect is limited, and above this range, it increases equipment costs and floor space requirements. Specifically, the height ratio of the flotation zone to the scum zone is 3:1; the length-to-height ratio of the flotation zone is greater than or equal to 4. Within this range, the solid-liquid separation effect of flotation is optimal. Below this range, the solid-liquid separation effect of flotation is limited; above this range, it increases equipment costs and floor space requirements.

[0055] This embodiment has the following advantages: By utilizing microbubbles generated by a microbubble aeration system to perform air flotation separation of CaP microcrystals, the high-efficiency phosphorus recovery rate of wastewater is ensured.

[0056] 2. By using microbubbles generated by the microbubble aeration system to separate the crystal seeds lost in the fluidized crystallization zone by air flotation and then circulating them back to the fluidized crystallization zone, the crystallization rate of phosphorus in wastewater is improved.

[0057] 3. The phosphorus-containing wastewater is controlled to undergo solid-liquid separation by air flotation in a horizontal flow manner. With the help of the supporting effect of microbubbles, the separated CaP microcrystals and seed crystals are stably suspended in the scum layer, thereby achieving the control of the crystallization reaction time without changing the hydraulic residence time.

[0058] 4. By jointly controlling the air-to-water ratio in the flotation zone and the slag discharge cycle in the slag zone, the particle size and moisture content of CaP crystallization products can be controlled, resulting in crystallization recovery products with controllable particle size and low moisture content.

[0059] 5. By controlling the seed crystal circulation cycle and circulation volume, the concentration and particle size of the seed crystals in the fluidized crystallization zone can be adjusted, thereby improving the adaptability to changes in influent water quality and fluctuations in water volume.

[0060] 6. Seed crystals in the fluidized crystallization zone are added in one go, without the need for additional replenishment.

[0061] 7. The phosphorus recovery process uses calcium hydroxyphosphate as a seed crystal and does not require the addition of chemical reagents other than crystal-forming ions, resulting in high purity of the phosphorus recovery product.

[0062] The following will provide a detailed explanation of this application with specific application examples.

[0063] Application Example 1 The influent is biogas slurry from a centralized rural biogas digester, with a phosphorus concentration of 12-27 mg / L, including PO4. 3- The phosphorus content is over 90%, and the pH value is 6.6~6.9. The average influent flow rate of the on-site phosphorus recovery unit is 0.8 m³. 3 / h, alkaline solution and Ca 2+All sources are Ca(OH)₂. Before entering the phosphorus recovery unit, the pH of the biogas slurry is adjusted to 8.0, and the Ca / P molar ratio is adjusted to 2.0~2.5. 100 μm hydroxyapatite is used as the seed crystal at a dosage of 20 g / L. The microbubbles generated by the microporous aeration disc have a diameter of approximately 50 μm, and the air-to-water ratio is controlled above 25%. The fluidized crystallization zone has a height of 3 m and a hydraulic retention time of 30 min; the guide zone has a hydraulic retention time of 30 s; and the flotation zone has a hydraulic retention time of 2 h. The length-to-height ratio of the flotation zone is not less than 4. The seed crystal circulation cycle is controlled at 2 days, with each circulation consisting of 3~12 kg of CaP crystal particles.

[0064] The results of the unit's operation show that the phosphorus concentration in the effluent is consistently below 1 mg / L, and the phosphorus recovery rate is consistently above 90%. The average particle size of the recovered CaP crystals is 500 μm, and the water content is less than 40%.

[0065] Application Example 2 The influent is the effluent from the secondary sedimentation tank of a municipal wastewater treatment plant, with a phosphorus concentration of 0.5~1.5 mg / L, including PO4. 3- The phosphorus content is over 80%, and the pH value is 6.9~7.2. The average influent flow rate of the on-site phosphorus recovery unit is 2 m³. 3 / h, the alkaline solution used is NaOH, Ca 2+ The source is Ca(OH)₂. Before entering the phosphorus recovery unit, the pH of the secondary sedimentation tank effluent is adjusted to 9.0, and the Ca / P molar ratio is adjusted to 2.0~2.5. 100 μm hydroxyapatite is used as the seed crystal at a dosage of 20 g / L. The microbubbles generated by the microporous aeration disc have a diameter of approximately 30 μm, and the air-to-water ratio is controlled above 15%. The fluidized crystallization zone has a height of 3 m and a hydraulic retention time of 30 min; the guide zone has a hydraulic retention time of 30 s; and the flotation zone has a hydraulic retention time of 2 h. The length-to-height ratio of the flotation zone is not less than 4. The seed crystal circulation cycle is controlled at 3 days, with each circulation consisting of 0.2~0.6 kg of CaP crystal particles.

[0066] The results of the unit's operation show that the phosphorus concentration in the effluent is consistently below 0.3 mg / L, and the phosphorus recovery rate is consistently above 75%. The average particle size of the recovered CaP crystals is 400 μm, and the water content is less than 40%.

[0067] Application Example 3 The influent is phosphorus-containing wastewater from a phosphate chemical plant, with a phosphorus concentration of 600-850 mg / L, including PO4. 3- The phosphorus content is over 95%, and the pH value is 5.5~6.2. The average influent flow rate of the on-site phosphorus recovery unit is 2 m³. 3 / h, the alkaline solution used is NaOH, Ca 2+The source is Ca(OH)₂. Before entering the phosphorus recovery unit, the pH of the phosphorus-containing wastewater is adjusted to 8.5, and the Ca / P molar ratio is adjusted to 1.8~2.0. 100 μm hydroxyapatite is used as the seed crystal at a dosage of 20 g / L. The microbubbles generated by the microporous aeration disc have a diameter of approximately 30 μm, and the air-to-water ratio is controlled above 15%. The fluidized crystallization zone has a height of 3 m and a hydraulic retention time of 30 min; the guide zone has a hydraulic retention time of 30 s; and the flotation zone has a hydraulic retention time of 2 h. The length-to-height ratio of the flotation zone is not less than 4. The seed crystal circulation cycle is controlled at 5 days, with each circulation consisting of 10~12 kg of CaP crystal particles.

[0068] The results of the unit's operation show that the phosphorus concentration in the effluent is consistently below 2 mg / L, and the phosphorus recovery rate is consistently above 99%. The average particle size of the recovered CaP crystals is 700 μm, and the water content is less than 40%.

[0069] The above embodiments are merely illustrative examples to clearly illustrate the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all embodiments here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for recovering CaP crystallized phosphorus based on aerated flotation, characterized in that: Phosphorus-containing wastewater, after conventional seed-induced crystallization treatment in the fluidized crystallization zone, enters the flotation zone in a horizontal flow state for aeration and flotation. CaP microcrystals and lost seed crystals in the wastewater rapidly agglomerate with microbubbles to form flocs. Under the continuous support of the microbubbles, a stable scum zone is formed at the top of the flotation zone, thereby achieving the separation of CaP microcrystals and seed crystals in the phosphorus-containing wastewater. The flocs in the scum zone undergo floc restructuring under the stirring action of microbubbles and the shearing action caused by the rupture of microbubbles, generating CaP crystal particles. A portion of the generated CaP crystal particles is periodically pumped to the bottom of the fluidized crystallization zone to replenish the lost seed crystals, while the remainder is periodically discharged as phosphorus recovery products.

2. The method for recovering CaP crystalline phosphorus according to claim 1, characterized in that: The microbubble diameter is 30~50 μm.

3. The method for recovering CaP crystalline phosphorus according to claim 1, characterized in that: The air-to-water volume ratio in the flotation zone is greater than or equal to 10%.

4. The method for recovering CaP crystalline phosphorus according to claim 1, characterized in that: Hydroxyphosphate with a particle size of 100-150 μm is selected as the seed crystal in the fluidized crystallization zone and is added in a single application mode at a dosage of 10-20 g / L.

5. The method for recovering CaP crystalline phosphorus according to claim 1, characterized in that: The hydraulic residence time in the fluidized crystallization zone is 10-30 minutes, and the height is 3-4 m.

6. The method for recovering CaP crystalline phosphorus according to claim 1, characterized in that: The wastewater in the fluidized crystallization zone first overflows into the diversion zone, and then enters the air flotation zone from the bottom side of the diversion zone.

7. The method for recovering CaP crystalline phosphorus according to claim 6, characterized in that: The hydraulic residence time in the diversion zone is 30-60 s, and the hydraulic residence time in the air flotation zone is 1-2 h.

8. The method for recovering CaP crystalline phosphorus according to claim 1, characterized in that: The height ratio of the air flotation zone to the scum zone is 3:1; the length-to-height ratio of the air flotation zone is greater than or equal to 4.

9. The method for recovering CaP crystalline phosphorus according to claim 1, characterized in that: During conventional seed-induced crystallization treatment, the phosphorus-containing wastewater entering the fluidized crystallization zone has a Ca / P molar ratio of (1.8~2.5):1, and the pH value in the fluidized crystallization zone is controlled at 8.0~9.

0.

10. A phosphorus recovery apparatus for implementing the CaP crystallization phosphorus recovery method according to any one of claims 1-9, characterized in that: The processor includes a fluidized bed crystallization zone, a flow guiding zone, and an air flotation zone, which are divided by a first baffle and a second baffle. The first baffle has an overflow port connecting the fluidized bed crystallization zone and the flow guiding zone. The lower side of the second baffle has an orifice connecting the flow guiding zone and the air flotation zone. The bottom of the fluidized bed crystallization zone has an inlet pipe, and the inlet pipe has an alkali addition pipe and a Ca addition pipe. 2+ The pipeline includes a microporous aeration disc at the bottom of the flotation zone, a drain pipe on the side of the flotation zone opposite to the second baffle, a surface scraper at the top of the flotation zone to scrape scum into the scum trough, a first scum discharge pipe and a circulation pipe on the scum trough, and a circulation pump on the circulation pipe connected to the water inlet pipe.

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