Method and system for step-by-step recovery of phosphorus and fluorine in phosphogypsum leachate

Abstract

The invention provides a method and system for step-by-step recovery of phosphorus and fluorine in ardealite leachate, and relates to the technical field of industrial wastewater treatment and resource.The method for step-by-step recovery of phosphorus and fluorine in ardealite leachate comprises the steps that (a) pretreated leachate is introduced into a first-stage reaction environment, and calcium phosphate crystals are separated out; (b) carrying out primary solid-liquid separation on the separated crystal slurry to obtain a solid product containing calcium phosphate and dephosphorized mother liquor; (c) introducing the dephosphorized mother liquor into a secondary reaction environment to separate out calcium fluoride crystals; and (d) carrying out secondary solid-liquid separation on the calcium fluoride crystal slurry to obtain a solid product containing calcium fluoride and treated effluent. According to the method, the pretreated phosphogypsum leachate is sequentially subjected to primary calcium phosphate selective crystallization, primary solid-liquid separation, secondary calcium fluoride directional crystallization and secondary solid-liquid separation, so that step-by-step efficient recovery of two valuable elements of phosphorus and fluorine is realized.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment and resource utilization technology, and in particular to a method and system for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate. Background Technology

[0002] Phosphogypsum is a major byproduct of wet-process phosphoric acid production, producing approximately 5 tons of phosphogypsum per ton of phosphoric acid, resulting in hundreds of millions of tons of emissions globally annually. Its leachate is highly hazardous acidic wastewater (pH 1.5-3.5), containing high concentrations of phosphates and fluorides. Direct discharge would severely pollute the environment: the strong acidity corrodes soil and water bodies, phosphates cause eutrophication, and fluorides accumulate through the food chain, harming human health and leading to diseases such as fluorosis. From a resource perspective, phosphorus and fluoride in the leachate are important strategic resources. Phosphorus is an essential element for agriculture, and fluoride is widely used in high-tech industries. my country faces a shortage of high-quality phosphate rock and relies on imports for fluoride resources. Recycling and utilizing leachate can alleviate resource pressure, promote the development of a circular economy, and achieve a green transformation of the phosphorus chemical industry. Effective treatment of leachate, possessing both environmental and resource value, is key to the sustainable development of the industry.

[0003] Currently, mainstream technologies for treating phosphogypsum leachate still face many bottlenecks, failing to achieve a balance between environmental and economic benefits.

[0004] (1) Traditional neutralization and precipitation method. This is the most widely used method, which involves directly adding neutralizing agents such as lime (Ca(OH)2) or sodium hydroxide to the leachate. The biggest drawback of this method is the co-precipitation of phosphorus and fluoride. During the neutralization process, phosphate (PO4) ions are released into the leachate. 3- ) and fluoride ions (F - Phosphorus and fluoride ions react simultaneously with calcium ions to form complex and unstable phosphorus- and fluoride-containing sludge (commonly known as "phosphorus- and fluoride sludge"). This type of sludge has low purity and high impurity content, and has no resource utilization value. According to the "National Hazardous Waste List," this type of waste is often classified as hazardous waste (HW49). Ultimately, the pollution control process generates new hazardous waste, requiring high-cost safe landfilling. This not only results in the permanent waste of phosphorus and fluoride resources but also poses a risk of secondary pollution, creating a dilemma of "using waste to treat waste, and the more it is treated, the more waste there is."

[0005] (2) Adsorption and flocculation methods. Adsorption methods use adsorbents such as activated alumina and modified zeolite to remove phosphorus / fluoride; flocculation methods use flocculants such as aluminum salts and iron salts to generate precipitates. These two methods are generally suitable for the advanced treatment of low-concentration wastewater. For high-concentration phosphogypsum leachate, there are problems such as high treatment costs, large consumption of adsorbents / flocculators, and difficulty in regeneration. More importantly, they essentially transfer pollutants from the liquid phase to the solid phase, and the resulting adsorbents or chemical sludge enriched with pollutants also face disposal problems, making resource recovery impossible and difficult to apply as the main process for in-situ resource recovery of leachate.

[0006] (3) Electrochemical precipitation and other methods. These include electrocoagulation, chemical crystallization, and other techniques. Although some methods have shown certain treatment effects under laboratory conditions, they generally suffer from high energy consumption, complex equipment maintenance, and poor process stability, making it difficult to achieve large-scale industrial application. Most importantly, these techniques rarely solve the problem of selective separation of phosphorus and fluoride ions at their root, and the purity of the products obtained cannot meet the standards for industrial raw materials.

[0007] In summary, the core shortcomings of existing treatment technologies lie in one point: "emphasizing removal while neglecting recovery," lacking effective means for the efficient separation and high-value recovery of phosphorus and fluorine, two valuable elements. These technologies either generate hazardous waste with no utilization value or have prohibitively high operating costs, failing to transform environmental governance from a "cost center" to a "value center."

[0008] In view of this, the present invention is proposed. Summary of the Invention

[0009] One of the objectives of this invention is to provide a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate, so as to at least solve one of the technical problems existing in the prior art.

[0010] The second objective of this invention is to provide a stepwise recovery system for phosphorus and fluorine in phosphogypsum leachate.

[0011] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate, comprising: (a) Introduce the pretreated leachate into the primary reaction environment to cause calcium phosphate to crystallize and precipitate; (b) The precipitated crystal slurry is subjected to primary solid-liquid separation to obtain a solid product containing calcium phosphate and a dephosphorization mother liquor; (c) Introduce the dephosphorization mother liquor into a secondary reaction environment to allow calcium fluoride to crystallize and precipitate; (d) The calcium fluoride crystallization slurry is subjected to two-stage solid-liquid separation to obtain a solid product containing calcium fluoride and treated effluent.

[0012] Furthermore, the pH value of the primary reaction environment is 4.5-5.5; Preferably, the pH value of the primary reaction environment is 5.0 ± 0.2; Preferably, in step (a), a calcium source is added to the reaction system to carry out selective crystallization of calcium phosphate; Preferably, the calcium source in step (a) includes at least one of calcium chloride and lime milk.

[0013] Furthermore, the pH value of the secondary reaction environment is 9.0-11.0; Preferably, the pH value of the secondary reaction environment is 10.0 ± 0.3; Preferably, in step (c), a calcium source is added to adjust the pH value and supplement the calcium ions required for calcium fluoride crystallization; Preferably, the calcium source in step (c) includes at least one of calcium chloride and lime milk; Preferably, a reaction promoter is also added in step (c); the reaction promoter includes one or more of calcium-magnesium complex salts, inorganic flocculants, and polymeric coagulants. Preferably, the calcium-magnesium complex salt includes at least one of magnesium chloride and magnesium sulfate, and calcium chloride; Preferably, the inorganic flocculant includes at least one of polyaluminum chloride and polyferric sulfate; Preferably, the polymeric coagulant includes anionic or nonionic polyacrylamide.

[0014] Furthermore, the pretreatment includes: homogenizing the phosphogypsum leachate, and then removing suspended solids and colloidal substances therein; Preferably, the pretreatment process further includes oxidizing the leachate before removing suspended solids and colloidal substances to oxidize ferrous ions to ferric ions and remove them.

[0015] Furthermore, the total phosphorus concentration in the treated effluent is less than 0.5 mg / L, and the total fluoride concentration is less than 1 mg / L.

[0016] In a second aspect, the present invention provides a method for the stepwise recovery of phosphorus and fluorine in phosphogypsum leachate, comprising a primary reaction unit, a primary solid-liquid separation unit, a secondary reaction unit, and a secondary solid-liquid separation unit connected in sequence. The primary reaction unit is used to promote the crystallization and precipitation of calcium phosphate from the pretreated leachate; The primary solid-liquid separation unit is used to separate the slurry output from the primary reaction unit into a solid product containing calcium phosphate and a dephosphorization mother liquor. The secondary reaction unit is used to receive the dephosphorization mother liquor and promote the crystallization of calcium fluoride. The secondary solid-liquid separation unit is used to separate the slurry output from the secondary reaction unit into a solid product containing calcium fluoride and treated effluent.

[0017] Furthermore, the phosphorus and fluorine stepwise recovery system in the phosphogypsum leachate also includes a pretreatment unit; The pretreatment unit includes a conditioning tank, a media filter, and a safety filter connected in sequence. The pretreatment unit also includes a pre-neutralization / oxidation reaction tank.

[0018] Furthermore, both the primary and secondary reaction units are fluidized bed reactors, equipped with a water distribution structure and a stirring device to maintain the fluidized state of the crystals and promote uniform growth. The primary solid-liquid separation unit is a fully automatic plate and frame filter press or a horizontal screw sedimentation centrifuge; the secondary solid-liquid separation unit is a combination system of a high-efficiency deep cone thickener and a fully automatic plate and frame filter press.

[0019] Furthermore, the phosphorus and fluorine stepwise recovery system in the phosphogypsum leachate also includes a reaction promoter addition unit, which is connected to the secondary reaction unit and is used to add an additive to the secondary reaction unit to promote the precipitation of calcium fluoride.

[0020] Furthermore, the phosphorus and fluorine stepwise recovery system in the phosphogypsum leachate also includes a central control unit, which is electrically connected to the primary reaction unit and the secondary reaction unit, and is used to monitor the reaction process and regulate the operating parameters.

[0021] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a method for the stepwise recovery of phosphorus and fluoride from phosphogypsum leachate. By sequentially subjecting the pretreated leachate to primary selective crystallization and solid-liquid separation of calcium phosphate, and secondary directional crystallization and solid-liquid separation of calcium fluoride, this method achieves the stepwise and efficient recovery of these two valuable elements. This method effectively solves the technical problem of complex product composition and difficulty in resource utilization caused by phosphorus and fluoride co-precipitation in traditional neutralization methods, ultimately leading to low-value hazardous waste, thus avoiding the dilemma of "treating waste with waste." Its core lies in precisely controlling the pH environment of the system at different reaction stages, utilizing the solubility difference between calcium phosphate and calcium fluoride under different acidic and alkaline conditions to achieve stepwise and selective crystallization and separation of the two. This yields high-purity calcium phosphate and calcium fluoride solid products, directly realizing resource utilization. The entire process is clear and stable, significantly reducing the load on subsequent treatments while deeply removing pollutants, and the final effluent quality consistently meets discharge standards. This invention offers significant environmental and economic benefits, providing a reliable technical approach for the synergistic realization of harmless treatment of phosphogypsum leachate and high-value recovery of its valuable elements. Attached Figure Description

[0022] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the phosphorus and fluoride stepwise recovery system in phosphogypsum leachate provided in an embodiment of the present invention; Figure 2 A flowchart of the stepwise recovery method of phosphorus and fluorine in phosphogypsum leachate provided by the present invention; Figure 3 The image shows the physical equipment of the stepwise recovery system for phosphorus and fluorine in phosphogypsum leachate provided in an embodiment of the present invention.

[0024] Icons: 100 - Primary reaction unit; 200 - Primary solid-liquid separation unit; 300 - Secondary reaction unit; 400 - Secondary solid-liquid separation unit; 410 - High-efficiency deep cone thickener; 420 - Fully automatic plate and frame filter press; 1 - CaCl2 solution storage tank; 2 - NaOH solution storage tank; 3 - Lime slurry storage tank; 4 - Reaction promoter storage tank. Detailed Implementation

[0025] Unless otherwise defined herein, the scientific and technical terms used in conjunction with this invention shall have the meanings commonly understood by one of ordinary skill in the art. The meaning and scope of terms shall be clear; however, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or foreign definitions. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "comprising" and other forms is non-limiting.

[0026] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. 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.

[0027] like Figure 2 As shown, the first aspect of the present invention provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate, comprising: (a) The pretreated leachate is introduced into the primary reaction environment, a calcium source is added, and calcium phosphate is selectively crystallized. (b) The precipitated crystal slurry is subjected to primary solid-liquid separation to obtain calcium phosphate product with a purity ≥98% and dephosphorization mother liquor; (c) Introduce the dephosphorization mother liquor into a secondary reaction environment, add calcium source to supplement calcium fluoride crystallization; (d) The calcium fluoride crystallization slurry is subjected to two-stage solid-liquid separation to obtain calcium fluoride product with a purity of ≥95% and effluent with a total fluoride concentration of less than 1 mg / L that meets the standards.

[0028] This invention provides a method for the stepwise recovery of phosphorus and fluoride from phosphogypsum leachate. Specifically, it is a crystallization method for the stepwise recovery of high-purity calcium phosphate and calcium fluoride from phosphogypsum stockpile leachate, achieving precise separation of phosphorus and fluoride and recovery of high-purity products. This invention employs a stepwise selective crystallization method under pH gradient control. By precisely controlling the pH value of the first-stage reaction to 4.5-5.5 (preferably pH≈5.0) and the pH value of the second-stage reaction to 9.0-11.0 (preferably pH≈10.0), calcium phosphate with a purity of ≥98% and calcium fluoride with a purity of ≥95% can be recovered respectively, turning waste into treasure and creating significant economic benefits. This stepwise selective crystallization under pH gradient control achieves stepwise and selective crystallization of calcium phosphate and calcium fluoride from the source, completely solving the problem of phosphorus and fluoride co-precipitation in traditional neutralization methods, laying the foundation for high-purity recovery.

[0029] The method for stepwise recovery of phosphorus and fluoride from phosphogypsum leachate provided by this invention achieves deep removal of pollutants, with the final effluent containing total phosphorus and total fluoride concentrations consistently below 1 mg / L, easily meeting the most stringent environmental emission standards. Relying on a central control system for precise and coordinated control of key parameters, the system operates stably and reliably, significantly reducing labor costs and operational complexity, and exhibiting a high degree of automation.

[0030] In some preferred embodiments, the pH value of the primary reaction environment is 4.5-5.5, for example, it can be 4.5, 5, 5.5, etc.; Preferably, the pH value of the primary reaction environment is 5.0 ± 0.2; Preferably, in step (a), a calcium source is added to the reaction system to carry out selective crystallization of calcium phosphate; Preferably, the calcium source in step (a) includes at least one of calcium chloride and lime milk.

[0031] In some preferred embodiments, the pH value of the secondary reaction environment is 9.0-11.0, for example, it can be 9.0, 10.0, 11.0, etc.; Preferably, the pH value of the secondary reaction environment is 10.0 ± 0.3; Preferably, in step (c), a calcium source is added to adjust the pH value and supplement the calcium ions required for calcium fluoride crystallization; Preferably, the calcium source in step (c) includes at least one of calcium chloride and lime milk; Preferably, a reaction promoter is also added in step (c); the reaction promoter includes one or more of calcium-magnesium complex salts, inorganic flocculants, and polymeric coagulants. Preferably, the calcium-magnesium complex salt includes at least one of magnesium chloride and magnesium sulfate, and calcium chloride; Preferably, the inorganic flocculant includes at least one of polyaluminum chloride and polyferric sulfate; Preferably, the polymeric coagulant includes anionic or nonionic polyacrylamide.

[0032] In some preferred embodiments, the pretreatment includes: homogenizing the phosphogypsum leachate, followed by removing suspended solids and colloidal substances therein; Preferably, the pretreatment process further includes oxidizing the leachate before removing suspended solids and colloidal substances to oxidize ferrous ions to ferric ions and remove them.

[0033] In some preferred embodiments, the total phosphorus concentration in the treated effluent is less than 0.5 mg / L and the total fluoride concentration is less than 1 mg / L.

[0034] like Figure 2As shown, a second aspect of the present invention provides a method for the stepwise recovery of phosphorus and fluoride from phosphogypsum leachate, comprising a primary reaction unit 100, a primary solid-liquid separation unit 200, a secondary reaction unit 300, and a secondary solid-liquid separation unit 400 connected in sequence; the primary reaction unit 100 is used to promote the crystallization and precipitation of calcium phosphate from the pretreated leachate; the primary solid-liquid separation unit 200 is used to separate the slurry output from the primary reaction unit 100 into a solid product containing calcium phosphate and a dephosphorization mother liquor; the secondary reaction unit 300 is used to receive the dephosphorization mother liquor and promote the crystallization and precipitation of calcium fluoride; the secondary solid-liquid separation unit 400 is used to separate the slurry output from the secondary reaction unit 300 into a solid product containing calcium fluoride and treated effluent.

[0035] In some preferred embodiments, the phosphorus and fluorine stepwise recovery system in the phosphogypsum leachate further includes a pretreatment unit; the pretreatment unit includes an equalization tank, a media filter and a safety filter connected in sequence; the pretreatment unit also includes a pre-neutralization / oxidation reaction tank.

[0036] In some preferred embodiments, the phosphorus and fluorine stepwise recovery system in phosphogypsum leachate further includes a central control unit, which is electrically connected to the primary reaction unit 100 and the secondary reaction unit 300, and is used to monitor the reaction process and regulate the operating parameters.

[0037] Optionally, in this embodiment, the primary reaction unit 100 is a primary crystallization reactor, the primary solid-liquid separation unit 200 is a primary solid-liquid separation device, the secondary reaction unit 300 is a secondary crystallization reactor, and the secondary solid-liquid separation unit 400 is a secondary solid-liquid separation device.

[0038] The phosphorus and fluoride stepwise recovery system for phosphogypsum leachate provided in this embodiment includes, in sequence, a pretreatment unit, a primary crystallization reactor, a primary solid-liquid separation device, a secondary crystallization reactor, a secondary solid-liquid separation device, and a central control system. Its core lies in the use of pH gradient controlled crystallization technology: firstly, in the primary crystallization reactor, the pH is controlled at 4.5-5.5 for selective crystallization and recovery of calcium phosphate with a purity ≥98%; subsequently, in the secondary crystallization reactor, the pH is controlled at 9.0-11.0 for crystallization and recovery of calcium fluoride with a purity ≥95%, and reaction promoters such as calcium-magnesium composite salts are added to improve efficiency. The final effluent has total phosphorus and total fluoride concentrations both below 1 mg / L, realizing the transformation of phosphogypsum leachate from "pollution control" to "resource creation," solving the industry problem of hazardous waste generation from phosphorus and fluoride co-precipitation in traditional methods, and demonstrating significant economic and environmental benefits.

[0039] In some preferred embodiments, the primary reaction unit 100 and the secondary reaction unit 300 are both fluidized bed reactors, equipped with a water distribution structure and a stirring device to maintain the fluidized state of the crystals and promote uniform growth. The primary solid-liquid separation unit 200 is a fully automatic plate and frame filter press 420 or a horizontal spiral sedimentation centrifuge; the secondary solid-liquid separation unit 400 is a combination system of a high-efficiency deep cone thickener 410 and a fully automatic plate and frame filter press 420.

[0040] In some preferred embodiments, the phosphorus and fluorine stepwise recovery system in phosphogypsum leachate further includes a reaction promoter addition unit, which is connected to the secondary reaction unit 300 and is used to add an additive to the secondary reaction unit 300 to promote the precipitation of calcium fluoride.

[0041] The following section provides a detailed description of the method for stepwise recovery of phosphorus and fluorine from phosphogypsum leachate provided in this embodiment, using the stepwise recovery system for phosphorus and fluorine in phosphogypsum leachate as an example.

[0042] 1. Pretreatment unit in the stepwise recovery system of phosphorus and fluorine in phosphogypsum leachate: The pretreatment unit is a prerequisite for the stable operation of the entire process and for ensuring the purity of the final product. It is the cornerstone for ensuring the stable operation of the system. Its core purpose is to provide stable and clean water conditions for the subsequent core crystallization stage, which is achieved through three major functions: homogenization, impurity removal, and conditioning.

[0043] First, the leachate enters the equalization tank, where its large volume (usually designed for 8-24 hour processing capacity) and mechanical stirring effectively homogenize the fluctuations in the quality and quantity of the raw water, ensuring a continuous and stable feed to the system. This is the foundation for achieving precise pH control in the future.

[0044] Subsequently, the homogenized wastewater enters a multi-stage filtration and solid-liquid separation process. After removing large floating objects through a screen, if the suspended solids content is high, preliminary mud-water separation can be performed in a sedimentation tank. Afterward, the wastewater enters the core media filter (such as multi-layer filter media made of quartz sand or anthracite) for deep removal of fine suspended particles and colloidal substances. Finally, before entering the precision crystallization reactor, a bag or cartridge safety filter is installed as a final safeguard, trapping particles >10-50μm to protect the subsequent expensive pH sensor and dosing system from clogging and contamination.

[0045] In addition, inexpensive alkali (such as lime slurry) can be added to initially adjust the highly acidic wastewater (pH < 1.5) to 2.5-4.0, thereby significantly reducing the alkali consumption cost of the core crystallization unit. Furthermore, by adding an oxidant (such as hydrogen peroxide), the ferrous ions (Fe2+) in the water can be removed. 2+ ) is converted into iron ions (Fe 3+This process allows iron ions to precipitate and be removed in advance, effectively preventing co-precipitation of iron ions with calcium phosphate in the crystallization unit. This is crucial for ultimately recovering calcium phosphate products with a purity of ≥98%.

[0046] In summary, after treatment by the pretreatment unit, the effluent ultimately achieves high standards with suspended solids (SS) below 20 mg / L, total iron content less than 5 mg / L, and stable pH. It is by no means an auxiliary facility, but rather the core support ensuring the efficient and economical high-purity recovery of phosphorus and fluoride through the "pH gradient controlled crystallization" process in this embodiment, directly determining the success or failure and efficiency of the entire resource recovery system.

[0047] 2. Primary reaction unit 100 (i.e., primary crystallization reactor) in the stepwise recovery system of phosphorus and fluorine in phosphogypsum leachate: The primary crystallization reactor is also a calcium phosphate crystallization unit, which integrates a sophisticated system of precision sensing, intelligent control and crystal engineering; its core mission is to achieve highly selective and high-purity crystallization and recovery of calcium phosphate from complex components.

[0048] To preferentially extract calcium phosphate from leachate containing both phosphorus and fluorine, the reactor employs a "precise pH window control" strategy. Its core structure includes a corrosion-resistant fluidized bed reactor body equipped with an internal stirring system to create a uniform reaction and crystal growth environment.

[0049] Optionally, the primary crystallization reactor integrates an intelligent control loop consisting of a high-precision pH sensor, a metering pump, and a central control system (PLC / DCS). The pumps supply calcium chloride and sodium hydroxide solutions separately, entering the distribution zone from the bottom of the fluidized bed via independent pipelines to ensure the purity of the calcium phosphate product and adjust the pH value. This system strictly stabilizes the pH of the reaction environment within a narrow range of 4.5 to 5.5, with an optimal setpoint of 5.0 ± 0.2. Within this specific pH window, the solubility of calcium phosphate is extremely low, while the solubility of calcium fluoride is relatively high. Therefore, both thermodynamically and kinetically, calcium phosphate crystallization is preferentially induced, while effectively inhibiting the precipitation of calcium fluoride and other impurities, thus solving the co-precipitation problem at its source.

[0050] Under optimized process conditions, the reactor introduces seed crystals and provides a hydraulic residence time of 30-90 minutes to ensure the orderly growth of calcium phosphate solute on the seed crystal surface, forming large, uniform, and easily separable crystals. Ultimately, the reactor produces a slurry of calcium phosphate crystals and mother liquor, with the calcium phosphate purity consistently exceeding 98%. The mother liquor, which has undergone deep dephosphorization but remains rich in fluoride ions, is then transported to the subsequent secondary crystallization unit, creating conditions for the recovery of calcium fluoride. The successful operation of this reactor represents a primary and crucial technological breakthrough for the entire system in achieving efficient separation and resource recovery of phosphorus and fluoride.

[0051] 3. Primary solid-liquid separation unit 200 (i.e., primary solid-liquid separation device) in the stepwise recovery system of phosphorus and fluorine in phosphogypsum leachate: The primary solid-liquid separation unit is a crucial bridge connecting the stepwise recovery processes of phosphorus and fluorine. Its core mission is to efficiently separate the calcium phosphate crystal slurry output from the primary crystallization reactor into solid and liquid components. This unit is not merely a simple dehydration device, but also a "gatekeeper" of the final product purity and a "stabilizer" for downstream processes. It aims to obtain high-purity, low-moisture calcium phosphate solid product while providing clear feed for the subsequent fluorine recovery section.

[0052] In terms of equipment selection, the fully automatic plate and frame filter press 420 is the first choice due to its superior overall performance. Its working principle involves pumping slurry into parallel filter chambers. Under pressure, the liquid penetrates the filter cloth to become a clarified "dephosphorization mother liquor," while the solids are retained to form a "filter cake." The outstanding advantage of this equipment lies in its powerful filter cake washing function. By injecting clean water or dilute alkaline solution for through-washing, it effectively replaces and removes soluble fluorides, chlorides, and other impurities remaining on the surface and in the gaps between calcium phosphate crystals. This is a crucial step in ensuring that the final calcium phosphate product consistently achieves a purity of ≥98%. Simultaneously, its high-pressure drying function can reduce the moisture content of the filter cake to below 25%, greatly improving product quality and storage and transportation performance.

[0053] After processing by this device, the system yields two products: first, a high-purity calcium phosphate filter cake, which can be sold as a valuable industrial raw material; and second, a clear dephosphorization mother liquor, with its total phosphorus concentration significantly reduced to below 50 mg / L, which is then transported to the secondary crystallization reactor to create pure and stable reaction conditions for the recovery of high-purity calcium fluoride. Therefore, the precise and efficient operation of this separation device is a crucial guarantee for achieving a win-win situation for both environmental and economic benefits in the entire resource recovery process.

[0054] 4. Secondary reaction unit 300 (i.e., secondary crystallization reactor) in the stepwise recovery system of phosphorus and fluorine in phosphogypsum leachate: The secondary crystallization reactor is a calcium fluoride crystallization unit and is the final link in this system to achieve closed-loop resource recovery and deep purification. Its core mission is to recover fluoride ions in the form of high-purity calcium fluoride from the mother liquor that has completed dephosphorization and ensure that the final effluent fully meets the standards.

[0055] This reactor structurally continues the precise control design of the primary crystallizer, but its core strategy shifts to operation under strongly alkaline conditions (optimal pH 10.0 ± 0.3). By adding lime slurry, the system simultaneously raises the pH and replenishes the calcium source, offering both economic and high-efficiency advantages. To ensure extremely low effluent fluoride concentration (<1 mg / L) and the formation of easily separable crystals, the reactor typically integrates key optimization measures: adding a dedicated "reaction promoter," which significantly improves sedimentation efficiency and speed by altering crystal nucleation and growth kinetics. The reaction promoter can be a calcium-magnesium complex salt (a mixture of calcium chloride (CaCl2) and magnesium chloride (MgCl2) or magnesium sulfate (MgSO4) solution), an inorganic flocculant (polyaluminum chloride (PAC), polyferric sulfate (PFS)), or a polymeric coagulant aid (anionic or nonionic polyacrylamide (PAM)), preferably a calcium-magnesium complex salt.

[0056] In this precisely controlled reaction environment, residual fluoride ions in the solution efficiently combine with calcium ions, transforming into high-purity calcium fluoride precipitate. Ultimately, this unit produces two products: first, a calcium fluoride slurry transported to subsequent separation processes, which, after dehydration and drying, yields calcium fluoride with a purity ≥95%, realizing the resource utilization of fluoride; second, clear final effluent, with total fluoride and total phosphorus concentrations consistently reduced to below 1 mg / L, not only meeting the most stringent environmental emission standards but also marking a successful transformation of the entire process from "pollution control" to "resource creation."

[0057] 5. Secondary solid-liquid separation unit 400 (i.e., secondary solid-liquid separation device) in the stepwise recovery system of phosphorus and fluorine in phosphogypsum leachate: The secondary solid-liquid separation unit is the final stage of the entire system's process flow and the ultimate guarantee for achieving emission standards. It bears the dual mission of recovering high-purity calcium fluoride and producing clean effluent. Given that calcium fluoride crystals are usually quite small, this unit often employs a combination process of "high-efficiency deep cone thickener 410 + fully automatic plate and frame filter press 420" to achieve the best separation effect.

[0058] Its workflow begins with the flocculation and concentration stage: the slurry from the secondary crystallization reactor first enters the high-efficiency deep cone thickener 410, where a trace amount of flocculant (such as anionic PAM) is added to cause the fine calcium fluoride particles to agglomerate into larger flocs, thereby accelerating gravity settling. This step significantly increases the slurry concentration, and its underflow solids content can be increased to 10-20%, greatly reducing the load on subsequent equipment; at the same time, the resulting supernatant is already very clear.

[0059] The final dehydration stage then begins: the concentrated underflow is pumped by a high-pressure pump into a fully automatic plate and frame filter press 420 for pressing and filtration. Here, through strong positive pressure, almost all suspended solids are completely retained, forming a pure calcium fluoride filter cake, while the liquid that penetrates the filter cloth becomes a clear liquid with excellent water quality. This process can be equipped with a washing system to further purify the product through water washing, ensuring that the purity of calcium fluoride consistently reaches over 95%.

[0060] After processing by this device, the system ultimately produces two products: one is a high-purity calcium fluoride product with controllable moisture content, realizing the value of fluoride resource recovery; the other is the final effluent that fully meets the standards, with the key indicator of total fluoride concentration consistently below 1 mg / L, indicating that the phosphogypsum leachate has been completely converted into a useful resource and thoroughly purified.

[0061] 6. Central control system in the stepwise recovery system of phosphorus and fluorine in phosphogypsum leachate: The central control system is the "intelligent brain" and command center of the entire phosphorus and fluorine stepwise recovery crystallization unit, and is the key guarantee for realizing fully automated operation and the core technology of "pH gradient control". The system is based on a programmable logic controller (PLC) and collects process data in real time through high-precision sensors integrated into each level of equipment (such as hydrofluoric acid resistant pH electrodes, flow meters, level gauges, etc.) and precisely drives the actuators (such as metering pumps, agitators, valves, etc.) to respond.

[0062] Its core function is to execute a precise pH gradient control strategy for the two-stage crystallization reactor. Through an advanced PID control algorithm, the system dynamically adjusts the pH value of the first-stage crystallization reactor at the millisecond level, strictly stabilizing it within the optimal range of 5.0±0.1, creating an ideal environment for the selective crystallization of calcium phosphate. Simultaneously, it independently and precisely controls the pH value of the second-stage crystallization reactor within the strongly alkaline window of 10.0±0.2, ensuring efficient precipitation and deep purification of calcium fluoride. This process is fully automated and requires no manual intervention.

[0063] Furthermore, the system is responsible for the sequential control, equipment interlocking, and safety protection of the entire process. It coordinates every step of the operation from water intake, chemical dosing, reaction, to solid-liquid separation, and provides immediate alarms and interlock shutdowns for abnormal operating conditions (such as pH out-of-control or equipment failure) to ensure system safety. All process parameters and operating data are automatically recorded and analyzed, providing solid data support for process optimization, fault diagnosis, and economic accounting, ultimately ensuring that the entire resource recovery system can continuously and stably produce high-purity products and achieve effluent standards.

[0064] The present invention will be further illustrated below by way of examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0065] like Figure 3 As shown, the stepwise recovery and crystallization apparatus and method for phosphorus and fluorine in phosphogypsum leachate provided by the present invention can be used in a unit with a daily processing capacity of 36 cubic meters (i.e., 1.5 m³). 3 The invention has been practically applied in a pilot-scale demonstration system ( / h). This embodiment is used to specifically illustrate the implementation and operational effects of the technical solution of the present invention under engineering conditions.

[0066] The water quality of the leachate used in the following examples and comparative examples is as follows: Before treatment, the water quality of the phosphogypsum leachate was sampled and monitored in multiple batches. The concentration range of its main components is shown in Table 1. This range represents the typical influent water quality conditions of each embodiment and comparative example of the present invention.

[0067] Table 1

[0068] Example 1 This embodiment provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate, comprising pretreatment of the phosphogypsum leachate, a primary reaction, a primary solid-liquid separation, a secondary reaction, and a secondary solid-liquid separation, the specific steps of which are as follows: Step 1: Pretreatment is carried out in the pretreatment unit, which includes an equalization tank, a pre-oxidation reaction tank, and a multi-stage fine filtration device connected in sequence. After this series of pretreatments, the effluent SS < 15 mg / L, total iron < 5 mg / L, and pH stabilized between 2.5 and 3.0; the equalization tank is designed with an effective volume of 30 m³. 3 The hydraulic retention time (HRT) is 20 hours. Two submersible mixers, each with a power of 1.5kW, are installed in the tank to ensure uniform water quality and quantity through continuous stirring, thus avoiding interference from shock loads on subsequent precise pH control. Regarding the pre-neutralization tank: To reduce the consumption cost of expensive alkaline agents (such as NaOH) in the subsequent core crystallization unit, a pre-neutralization tank is set up after the effluent from the equalization tank (the equalization tank, pre-neutralization tank, and pre-oxidation reaction tank are connected in sequence). Inexpensive lime slurry is added via metering pumps to initially neutralize and adjust the pH of the strongly acidic influent (pH < 1.5) to the range of 2.5-4.0. This step significantly reduces the neutralization load on the subsequent primary crystallization reactor and is an important measure for achieving economical and efficient operation. Regarding the pre-oxidation reaction tank: At the effluent end of the equalization tank, 30% concentration hydrogen peroxide is continuously added via a metering pump at a dosage of 120 mg / L. The hydrogen peroxide will neutralize dissolved ferrous ions (Fe...). 2+ It is rapidly oxidized to iron ions (Fe). 3+ The reaction time was controlled at 25 minutes. After oxidation, Fe... 3+Under subsequent weakly acidic to neutral conditions, it will hydrolyze to form ferric hydroxide flocs. Monitoring showed that the total iron concentration in the effluent after pre-oxidation steadily decreased to below 5 mg / L. Regarding the multi-stage fine filtration device: First stage: Multi-media filter. Filled with carefully selected quartz sand and anthracite, forming a gradient filter layer with coarser material at the top and finer material at the bottom. Designed filtration speed of 8 m / h, effectively removing most suspended solids and ferric hydroxide flocs. Second stage: Safety filter. Employing a 10μm precision bag filter to ensure that particles larger than 10μm are retained.

[0069] Step 2: The pretreated leachate is introduced into the primary reaction environment to induce calcium phosphate crystallization. The primary reaction environment is a primary crystallization reactor, specifically a fluidized bed crystallization reactor with an effective volume of 2.0 m³. 3 The phosphogypsum leachate was diluted at 1.5 m³. 3 A flow rate of [flow rate] / h enters from the bottom distributor, creating a uniform upward flow through the distribution holes. The system maintains the reaction temperature at 35±2℃ via jacket heat exchange, which is the optimal temperature range for calcium phosphate crystal growth. An internal slow-speed stirrer (75 rpm) further promotes fluidization stability and prevents crystal deposition. During the reaction, a high-precision, hydrofluoric acid-resistant pH electrode is used to monitor the solution pH value in real time. The central control system (PLC), based on the setpoint pH = 5.0 and considering the measurement deviation, dynamically adjusts the two metering pumps using a PID algorithm. Calcium source pump: Adds 10% CaCl2 solution (stored in CaCl2 solution storage tank 1) at a flow rate of 15 L / h. CaCl2 was chosen because of its good solubility, lack of introduction of additional pH interference, and ability to precisely control the Ca / P molar ratio; Alkali pump: Adds 20% NaOH solution (stored in NaOH solution storage tank 2) at a flow rate of 10 L / h. NaOH is used for precise neutralization of acidity, with a fast response and accurate control. In actual operation, the PLC successfully controlled the pH fluctuation of the reactor within an extremely narrow range of 5.0 ± 0.1. Crystallization process and products: Under the above conditions, phosphate ions preferentially combine with calcium ions, growing orderly on the surface of existing seed crystals in the fluidized bed to form large, uniform crystals with a particle size mainly distributed between 50-150 μm. The hydraulic residence time is 80 minutes. The resulting crystal slurry has a solids content of 15-20%.

[0070] Step 3: The precipitated crystal slurry undergoes primary solid-liquid separation to obtain a solid product containing calcium phosphate and a dephosphorization mother liquor. Specifically, the slurry is pumped from the bottom of the reactor through the crystal slurry inlet to a fully automatic plate and frame filter press 420. The pressure during the filtration stage is 0.6 MPa, which is then increased to 1.2 MPa for pressing. To further improve product purity, a through-wash is performed using 0.6 times the filter cake volume of clean water, effectively removing soluble fluorides and chlorides adhering to the crystal surface. Finally, a calcium phosphate filter cake with a moisture content of 23% is obtained. Third-party testing shows that its Ca3(PO4)2 purity is as high as 98.5%, with excellent whiteness, making it suitable as a high-quality industrial raw material. The total phosphorus concentration of the separated dephosphorization mother liquor is reduced to 28 mg / L, with a phosphorus recovery rate exceeding 98%, but the fluoride concentration remains at around 780 mg / L, verifying the success of selective crystallization.

[0071] Step 4: Introduce the dephosphorization mother liquor into the secondary reaction environment to allow calcium fluoride to crystallize out. Specifically: the dephosphorization mother liquor enters the secondary crystallization reactor (also 2.0 m). 3 (Effective volume). The process objective here shifts to the deep precipitation of fluorides under a strongly alkaline environment. During this reaction, 15% lime slurry (Ca(OH)2) is mainly added (stored in lime slurry storage tank 3) at a flow rate of 28 L / h. The lime slurry also serves to provide OH... - Raise pH and supplement calcium 2+ It has dual functions and its cost is far lower than that of NaOH; To achieve deep fluoride removal (target <1 mg / L), a dedicated reaction promoter (stored in reaction promoter storage tank 4)—a calcium-magnesium composite salt (with a Ca:Mg molar ratio of 1:0.4)—was added to the system at a dosage of 60 mg / L. The introduction of magnesium ions significantly enhances the removal effect of residual fluoride ions through co-precipitation and adsorption. The PLC system precisely controls the pH value of the secondary reactor at 10.0 ± 0.2.

[0072] Step 5: Perform a two-stage solid-liquid separation on the calcium fluoride crystal slurry obtained in Step 4 to obtain a solid product containing calcium fluoride and treated effluent. Specifically: Given the small size and slow settling characteristics of calcium fluoride crystals, the secondary solid-liquid separation adopts a combined process of "high-efficiency deep cone thickener 410 + fully automatic plate and frame filter press 420".

[0073] First, the crystallization slurry enters a thickening tank, where 1.2 mg / L of anionic PAM is added. The bridging effect of PAM causes the fine calcium fluoride particles to form large, dense flocs, significantly increasing the settling velocity and concentrating the underflow concentration to 18%. The concentrated underflow then enters a plate and frame filter press for dewatering at 0.7 MPa, yielding a calcium fluoride filter cake with a moisture content of 26%. Analysis shows that its CaF2 purity reaches 95.8%, achieving the quality of fluorite ore.

[0074] The supernatant from the secondary system and the clarified liquid from the filter press are combined to form the final effluent. Continuous monitoring data shows that the total phosphorus in the effluent is 0.35 mg / L and the total fluoride is 0.68 mg / L, which is better than the design target and the indirect emission limits of ≤20 mg / L for total phosphorus and ≤20 mg / L for total fluoride in the national emission standard ("Emission Standard of Water Pollutants for Phosphate Fertilizer Industry" (GB15580-2011) for both "existing enterprises" and "new enterprises".

[0075] Example 2 This embodiment provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate. The difference from Embodiment 1 is that the reaction pH in step 2 is controlled at 4.5-4.7, and the reaction pH in step 4 is controlled at 9.0-9.2.

[0076] Example 3 This embodiment provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate. The difference from Embodiment 1 is that the reaction pH in step 2 is controlled at 5.2-5.5, and the reaction pH in step 4 is controlled at 10.7-11.0.

[0077] Example 4 This embodiment provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate, which differs from Embodiment 1 in that: This embodiment provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate. The difference from Embodiment 1 is that the reaction pH in step 2 is controlled at 4.0-4.4, and the reaction pH in step 4 is controlled at 8.5-8.9.

[0078] Example 5 This embodiment provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate. The difference from Embodiment 1 is that the reaction pH in step 2 is controlled at 5.6-6.0, and the reaction pH in step 4 is controlled at 11.1-11.5.

[0079] Example 6 This embodiment provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate. The difference from Embodiment 1 is that a pre-oxidation reaction tank is not set up in step 1, that is, pre-oxidation is not performed.

[0080] Comparative Example 1 This comparative example provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate, which differs from Example 1 in that step 1 is not performed.

[0081] Comparative Example 2 This comparative example provides a method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate. The difference from Example 1 is that steps 2-4 are replaced with a one-time precipitation, that is, a large amount of lime is added at the same time to raise the pH to above 10, and phosphorus and fluorine are precipitated simultaneously.

[0082] Test methods: The water quality indicators and product purity analysis methods involved in the embodiments and comparative examples of this invention are as follows. All analyses comply with current national standards or industry general specifications: pH value: measured directly on-site using a calibrated portable pH meter (accuracy ±0.01).

[0083] Total phosphorus (TP): After sampling, the sample was digested with sulfuric acid-potassium persulfate and then measured at a wavelength of 700 nm using the ammonium molybdate spectrophotometric method (according to GB 11893-89 "Determination of Total Phosphorus in Water by Ammonium Molybdate Spectrophotometric Method").

[0084] Total fluoride (TF): After sampling, total ionic strength adjustment buffer (TISAB) was added, and the fluoride was determined by the fluoride ion-selective electrode method (according to GB 7484-87 "Determination of Fluoride in Water by Ion-Selective Electrode Method").

[0085] Suspended solids (SS): Take an appropriate amount of water sample and filter it through a 0.45 μm filter membrane. Dry the filter membrane in an oven at 105±5℃ until constant weight. Calculate the weight difference (according to GB 11901-89 "Determination of Suspended Solids in Water by Gravimetric Method").

[0086] Total iron: After nitric acid digestion, the water sample was determined by the o-phenanthroline spectrophotometric method (according to HJ / T 345-2007 "Determination of Iron in Water by o-phenanthroline Spectrophotometric Method (Trial)") or atomic absorption spectrometry.

[0087] Purity of calcium phosphate product: After drying and grinding the solid product, phase qualitative and semi-quantitative analysis was performed by X-ray diffraction (XRD), and the results were verified by chemical titration methods (such as EDTA titration of calcium and quinoline phosphomolybdate gravimetric method for phosphorus determination). The effective content of Ca3(PO4)2 was calculated.

[0088] Purity of calcium fluoride product: After drying and grinding the solid product, phase analysis was performed by X-ray diffraction (XRD), and verified by chemical analysis methods (such as EDTA titration of calcium, ion-selective electrode method or distillation-titration method for fluoride determination), and the effective content of CaF2 was calculated.

[0089] Phosphorus recovery rate calculation: It is obtained through material balance based on the total phosphorus mass of the influent, the phosphorus mass of the calcium phosphate product, and the total phosphorus mass of the effluent.

[0090] All water quality samples were analyzed in triplicate, and the arithmetic mean was taken; the samples for solid product analysis were representative.

[0091] The test results are shown in Table 2.

[0092] Table 2

[0093] (1) Validation of the effectiveness of the stepwise crystallization strategy Example 1 3. Within the recommended pH range (Level 1, 4.5) 5.5, Level 2 9.0 Both methods achieved excellent results: phosphorus recovery rate >95%, total phosphorus in effluent <0.5 mg / L, total fluoride <1 mg / L, and purity of both products >94%, proving the feasibility, efficiency, and stability of the "stepwise selective crystallization under pH gradient control" strategy.

[0094] Comparative Example 2 uses traditional one-time sedimentation, which can also reduce phosphorus and fluoride concentrations, but the product is mixed sludge with no resource value, and the effluent indicators are far worse than those of the Example, directly proving the dual advantages of stepwise crystallization in resource recovery and deep purification.

[0095] (2) The key role of pH control In Examples 4 and 5, after the pH deviated from the recommended range, all indicators decreased significantly. Note: Both excessively low (<4.5) and excessively high (>5.5) pH values ​​in the primary stage affect the selective precipitation of calcium phosphate, leading to a decrease in phosphorus recovery and product purity. When the secondary pH is insufficient (<9.0), calcium fluoride precipitation is incomplete, while excessively high pH (>11.0) may cause co-precipitation of other impurities, affecting the purity of CaF2.

[0096] pH≈5.0 (Level 1) and pH≈10.0 (Level 2) are the optimal control points for this process, which can ensure product purity and effluent quality while achieving efficient recovery.

[0097] (3) Importance of pretreatment Compared with Example 1, Example 6 (without pre-oxidation) showed a slight decrease in product purity and effluent indicators, indicating that pre-oxidation can effectively remove Fe. 2+ Interference, improving the purity of calcium phosphate crystals.

[0098] The indicators of Comparative Example 1 (without pretreatment) deteriorated significantly, especially the low product purity and high effluent concentration, highlighting the necessity of pretreatment (homogenization, oxidation, filtration) to ensure the stable operation of the crystallization process and the high purity of the product.

[0099] (4) Achieving both resource utilization and emission compliance All embodiments can stably obtain Ca3(PO4)2 and CaF2 with a purity >90% under optimized conditions, realizing the conversion of pollutants into products.

[0100] The final effluent contains less than 0.5 mg / L of total phosphorus and less than 1 mg / L of total fluoride, meeting the requirements of the "Emission Standard of Water Pollutants for Phosphate Fertilizer Industry" (GB15580). Indirect emission limits (2011).

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for the stepwise recovery of phosphorus and fluorine from phosphogypsum leachate, characterized in that, include: (a) Introduce the pretreated leachate into the primary reaction environment to cause calcium phosphate to crystallize and precipitate; (b) The precipitated crystal slurry is subjected to primary solid-liquid separation to obtain a solid product containing calcium phosphate and a dephosphorization mother liquor; (c) Introduce the dephosphorization mother liquor into a secondary reaction environment to allow calcium fluoride to crystallize and precipitate; (d) The calcium fluoride crystallization slurry is subjected to two-stage solid-liquid separation to obtain a solid product containing calcium fluoride and treated effluent.

2. The method for stepwise recovery of phosphorus and fluorine from phosphogypsum leachate according to claim 1, characterized in that, The pH value of the primary reaction environment is 4.5-5.5; The pH value of the primary reaction environment is 5.0 ± 0.2; In step (a), a calcium source is added to the reaction system to carry out selective crystallization of calcium phosphate; The calcium source in step (a) includes at least one of calcium chloride and lime milk.

3. The method for stepwise recovery of phosphorus and fluorine from phosphogypsum leachate according to claim 1, characterized in that, The pH value of the secondary reaction environment is 9.0-11.0; The pH value of the secondary reaction environment is 10.0 ± 0.3; In step (c), a calcium source is added to adjust the pH value and replenish the calcium ions required for calcium fluoride crystallization. The calcium source in step (c) includes at least one of calcium chloride and lime milk; In step (c), a reaction promoter is also added; the reaction promoter includes one or more of calcium-magnesium complex salts, inorganic flocculants, and polymeric coagulants. The calcium-magnesium complex salt includes at least one of magnesium chloride and magnesium sulfate, as well as calcium chloride; The inorganic flocculant includes at least one of polyaluminum chloride and polyferric sulfate; The polymeric coagulant includes anionic or nonionic polyacrylamide.

4. The method for stepwise recovery of phosphorus and fluorine from phosphogypsum leachate according to claim 1, characterized in that, The pretreatment includes: homogenizing the phosphogypsum leachate, and then removing suspended solids and colloidal substances therein; The pretreatment process includes oxidizing the leachate before removing suspended solids and colloidal substances to oxidize ferrous ions to ferric ions and remove them.

5. The method for stepwise recovery of phosphorus and fluorine from phosphogypsum leachate according to claim 1, characterized in that, The total phosphorus concentration in the treated effluent is less than 0.5 mg / L, and the total fluoride concentration is less than 1 mg / L.

6. A phosphorus and fluoride stepwise recovery system for phosphogypsum leachate based on the method for stepwise recovery of phosphorus and fluoride from phosphogypsum leachate according to any one of claims 1-5, characterized in that, It includes a primary reaction unit, a primary solid-liquid separation unit, a secondary reaction unit, and a secondary solid-liquid separation unit connected in sequence; The primary reaction unit is used to promote the crystallization and precipitation of calcium phosphate from the pretreated leachate; The primary solid-liquid separation unit is used to separate the slurry output from the primary reaction unit into a solid product containing calcium phosphate and a dephosphorization mother liquor. The secondary reaction unit is used to receive the dephosphorization mother liquor and promote the crystallization of calcium fluoride. The secondary solid-liquid separation unit is used to separate the slurry output from the secondary reaction unit into a solid product containing calcium fluoride and treated effluent.

7. The stepwise recovery system for phosphorus and fluoride in phosphogypsum leachate according to claim 6, characterized in that, It also includes a preprocessing unit; The pretreatment unit includes a conditioning tank, a media filter, and a safety filter connected in sequence. The pretreatment unit also includes a pre-neutralization / oxidation reaction tank.

8. The stepwise recovery system for phosphorus and fluoride in phosphogypsum leachate according to claim 6, characterized in that, Both the primary and secondary reaction units are fluidized bed reactors, equipped with water distribution structures and stirring devices to maintain the fluidized state of the crystals and promote uniform growth. The primary solid-liquid separation unit is a fully automatic plate and frame filter press or a horizontal screw sedimentation centrifuge; the secondary solid-liquid separation unit is a combination system of a high-efficiency deep cone thickener and a fully automatic plate and frame filter press.

9. The stepwise recovery system for phosphorus and fluoride in phosphogypsum leachate according to claim 6, characterized in that, It also includes a reaction promoter addition unit, which is connected to the secondary reaction unit and is used to add an additive that promotes the formation of calcium fluoride precipitation to the secondary reaction unit.

10. The stepwise recovery system for phosphorus and fluorine in phosphogypsum leachate according to claim 6, characterized in that, It also includes a central control unit, which is electrically connected to the primary reaction unit and the secondary reaction unit, and is used to monitor the reaction process and regulate the operating parameters.

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