Method for the pretreatment of phosphogypsum for backfilling and land use and preparation system

Abstract

The application provides a phosphogypsum pretreatment method and preparation system for filling backfill and land use, and relates to the technical field of solid waste harmless treatment and resource utilization. Firstly, a turbulent flow-laminar flow partition is constructed in a thickener feedwell, high shear membrane breaking and low shear growth are decoupled, high-concentration underflow and low-turbidity overflow are efficiently produced; then, pH gradient induced crystallization is carried out on the overflow water, phosphorus and fluorine are accurately recovered in steps as high-purity iron phosphate and calcium fluoride; then, the high-concentration underflow is subjected to two-stage activation and rheological reconstruction, a pseudoplastic paste is constructed at a medium or low concentration, and low-cost high-performance filling is realized; finally, through deep coupling and inventory blending interfaces of the water-solid system, the treatment of newly produced phosphogypsum and the historical storage of aged phosphogypsum are unified in the same system, and the pain points of traditional processes, such as concentration difficulty, resource waste, high cost and secondary pollution, are solved from the root.

Description

Technical Field

[0001] This invention relates to the field of solid waste harmless treatment and resource utilization technology, specifically to a method and preparation system for the pretreatment of phosphogypsum for backfilling and land use. Background Technology

[0002] Phosphogypsum, a core solid waste residue in wet-process phosphoric acid production, generates approximately 5 tons of phosphogypsum for every ton of phosphoric acid produced. my country, a major global producer of phosphoric acid chemicals, produces 80-86 million tons annually, with accumulated stockpiles exceeding 1 billion tons. However, the comprehensive utilization rate is less than 45%. The long-term stockpiling of this large amount of waste residue not only occupies vast amounts of land, but its highly acidic nature (pH 2-4) and the presence of soluble phosphorus, fluorine, and heavy metals also easily lead to groundwater acidification and excessive levels of total phosphorus and fluoride through acidic leachate, posing significant environmental risks. With the implementation of policies such as the "Action Plan for Comprehensive Utilization of Phosphogypsum" and the "Technical Specification for Pollution Control of Utilization and Harmless Storage of Phosphogypsum" (HJ1315-2025), which clearly stipulate the mandatory requirements of 100% harmless treatment rate of newly added phosphogypsum and dynamic balance between comprehensive consumption and generation, the traditional wet storage and dam-type storage models no longer meet the compliance requirements. The combination of harmless disposal and resource conversion of phosphogypsum with dry discharge and paste backfilling technology has become the only path for the industry's transformation and upgrading.

[0003] Current mainstream phosphogypsum treatment technologies have significant shortcomings. Firstly, the challenges of fine particle settling and concentration are prominent. Phosphogypsum particles are small, with a high proportion of particles as small as -20μm. Due to their small mass, large specific surface area, and high surface energy, they struggle to overcome fluid resistance in ordinary thickeners, resulting in poor flocculation and slow settling. This leads to overflow becoming turbid and underflow concentrations failing to consistently exceed 40%, necessitating the subsequent installation of large-scale filter press equipment, significantly increasing investment and energy consumption, and making it unsuitable for large-scale continuous production. Secondly, the overflow / filtrate treatment process is rudimentary. The acidic overflow water generated during the concentration stage contains high concentrations of phosphate and fluoride ions. Existing one-step lime neutralization only adjusts the pH, but causes phosphorus, fluoride, and calcium to co-precipitate, forming complex mixed sludge. This wastes valuable resources, increases secondary solid waste disposal costs, and hinders high-value recovery. Third, the modification and curing are incomplete and pose a risk of acid reversion. Existing processes mostly use single-stage low-speed mechanical stirring, relying on the surface coating of cement and lime, which cannot destroy the acidic hydration film on the surface of phosphogypsum crystals and organic impurities within the crystal lattice. The modifier does not react sufficiently with the internal acidic substances, and the cured body is prone to acid reversion after long-term storage. The phosphorus and fluorine curing rates are low, and the leachate indicators are difficult to meet the HJ1315-2025 standard. Fourth, the filling paste is costly and has insufficient mechanical properties. The low concentration in the first stage requires the addition of 20% to 30% cement and other cementitious materials, resulting in high costs. Furthermore, the lack of gradation and rheological control leads to high bleeding rate and easy segregation of the paste. The 28-day strength is often below 1 MPa, which cannot meet the mechanical requirements for large-scale goaf filling and ecological restoration.

[0004] In summary, existing technologies for the harmless treatment and resource utilization of phosphogypsum generally suffer from multiple technical problems, including low concentration efficiency, inefficient resource recovery, incomplete solidification, high overall costs, and substandard mechanical properties. These technologies fail to meet the latest environmental policies and technical standards, and cannot support the industry's demand for low-cost, large-scale, and green disposal. Therefore, developing a new, end-to-end harmless treatment process that integrates efficient gradient concentration, stepwise phosphorus and fluorine recovery, and deep modification and solidification has become an urgent need to overcome existing technological bottlenecks and promote the high-quality development of the phosphate chemical and related mining industries. Summary of the Invention

[0005] In view of the technical problems existing in the background art, the present invention provides a method and preparation system for pretreatment of phosphogypsum for backfilling and land use.

[0006] In a first aspect, embodiments of the present invention provide a phosphogypsum preparation system for backfilling and land use, comprising: The gradient fluid dynamics thickening unit includes a thickener with a feed well featuring a turbulent-laminar coupling structure, used to thicken phosphogypsum slurry into underflow thick slurry and generate overflow water; The filtrate resource recovery gradient separation unit is connected to the overflow port of the thickener and is used to perform stepwise recovery treatment of the overflow water, sequentially recovering phosphorus products and fluorine products. A two-stage activated paste preparation unit is connected to the underflow outlet of the thickener and is used to mix and stir the underflow slurry with a modifier to prepare a paste backfill material. The turbulent-laminar coupled structure feed well is located above the feed inlet of the thickener and includes: The high-shear turbulent mixing zone located at the top of the feed well body has a turbulent structure inside, which is used to make the phosphogypsum slurry turbulent to destroy the hydration film on the particle surface. The low-shear laminar flow growth zone is located at the bottom of the feed well body and has a flow guiding structure inside to reduce the slurry flow velocity and change the flow state to laminar flow in order to promote the growth of micro flocs.

[0007] As a further improvement of the present invention, the turbulence structure is a spiral turbulence plate with an inclination angle of 35°-45°; the flow guiding structure is a reverse flow guiding expansion structure.

[0008] As a further improvement of the present invention, the spiral baffle extends spirally downward along the inner wall of the feed well body to form a continuous spiral fluid channel, and the ratio of the pitch of the spiral fluid channel to the diameter of the feed well body is 1:2 to 1:3.

[0009] As a further improvement of the present invention, the radial width of the spiral baffle is 0.10 to 0.20 times the diameter of the feed well body.

[0010] As a further improvement of the present invention, the spiral baffle has a thickness of 5~10mm and is made of acid-resistant stainless steel (316L) or acid-resistant polymer composite plate.

[0011] As a further improvement of the present invention, the total number of spiral baffles in the high-shear turbulent mixing zone is 3 to 5.

[0012] As a further improvement of the present invention, the spiral direction of the spiral baffle is consistent with the spiral direction of the feed slurry flow to enhance the circumferential shear force; The inlet of the spiral fluid channel is smoothly connected to the feed pipe, and an arc transition section is provided at the outlet to avoid dead zones and achieve a smooth flow transition to the low-shear laminar growth region.

[0013] As a further improvement of the present invention, the reverse flow expansion structure includes a plurality of guide plates disposed on the inner wall of the feed well body; the extension axis of all the guide plates is coaxially disposed with the central axis of the feed well body to form an annular flow guiding surface; the guide plates are inclined relative to the inner wall of the feed well body and the inclination direction is such that the slurry forms a centripetal flow that converges toward the central axis in the feed well body, or forms a swirling flow that is opposite to the direction of the slurry flow, so as to achieve a smooth transition of the flow state from turbulent flow to laminar flow.

[0014] As a further improvement of the present invention, the inclination angle of the guide plate is 15°~30°, wherein the inclination angle is the angle between the surface of the guide plate and the vertical central axis (or vertical wall surface) of the feed well body.

[0015] As a further improvement of the present invention, the guide plates are evenly distributed circumferentially along the inner wall of the feed well body, with a quantity of 6 to 12 pieces, a radial width of 0.08 to 0.15 times the diameter of the feed well body, and a thickness of 5 to 12 mm.

[0016] As a further improvement of the present invention, a smooth arc transition section is provided between the lower edge of the guide plate and the outlet of the low-shear laminar growth zone to avoid fluid dead zones and secondary turbulence.

[0017] As a further improvement of the present invention, the feed well includes a plurality of symmetrically arranged feed well bodies; the upper part of the feed well body is a cylindrical section and the lower part is a conical contraction section, the high-shear turbulent mixing zone is located in the cylindrical section, and the low-shear laminar growth zone spans the connection between the cylindrical section and the conical contraction section.

[0018] As a further improvement of the present invention, the ratio of the height to the diameter of the cylindrical section is 1.0 to 1.8:1, and the cone angle of the tapered constriction section is 10° to 25° (preferably 15° to 20°).

[0019] As a further improvement of the present invention, the high-shear turbulent mixing zone is located within the cylindrical section, and its height accounts for 60% to 80% of the total height of the cylindrical section; The low-shear laminar flow growth zone spans the connection between the lower part of the cylindrical section and the upper part of the conical contraction section, and its height accounts for 20% to 40% of the total height of the feed well body.

[0020] As a further improvement of the present invention, a smooth arc transition section (with a transition radius of 0.05 to 0.10 times the diameter of the feed well body) is provided at the connection between the cylindrical section and the conical contraction section to achieve a smooth transition of the flow from turbulent to laminar flow and reduce dead zones.

[0021] As a further improvement of the present invention, the filtrate resource utilization gradient separation unit includes a primary phosphorus removal reactor, a primary solid-liquid separator, a secondary fluoride removal reactor, and a secondary solid-liquid separator connected in series. The primary phosphorus removal reactor is equipped with a first pH control device and an iron source addition device; the secondary fluoride removal reactor is equipped with a second pH control device and a calcium source addition device. Both the primary and secondary solid-liquid separators are filter presses or micro-flocculation separators.

[0022] As a further improvement of the present invention, the dual-stage activated paste preparation unit includes: A high-shear forced activation mixer with a feed inlet connected to the underflow outlet of the thickener and equipped with a modifier dosing port; The inlet of the secondary high-speed flexible rheological reconfiguration machine is connected to the outlet of the primary high-shear forced activation mixer, and the outlet is used to connect to the filling pipeline.

[0023] As a further improvement of the present invention, it also includes an aged phosphogypsum feeding device, the discharge port of which is connected to the two-stage activated paste preparation unit for mixing the aged phosphogypsum with the underflow slurry.

[0024] As a further improvement of the present invention, the two-stage activated paste preparation unit also includes a composite modifier metering and addition system, including a cementitious aggregate bin, an alkaline activator bin, and a rheology modifier bin.

[0025] Secondly, embodiments of the present invention provide a method for pretreatment of phosphogypsum for backfilling and land use, comprising the following steps: S1, Fluid Dynamics Gradient Concentration: Low-concentration phosphogypsum slurry is fed into a thickener with a turbulent-laminar coupling structure in the gradient fluid dynamics concentration unit for concentration. The feed velocity is controlled at 2.0~3.0 m / s, and the flocculant dosage is 30~50 g / t. The flocculant is efficiently dispersed and the hydration film on the particle surface is broken through the upper high-shear turbulent mixing zone. The flow velocity is then controlled to be reduced to <0.5 m / s, and the micro-flocs are orderly grown and compacted through the lower low-shear laminar growth zone, finally obtaining a high-concentration underflow slurry and low-turbidity overflow water. S2, Overflow liquid gradient separation and resource utilization: The overflow water is transported to the filtrate resource utilization gradient separation unit for stepwise precipitation treatment with pH gradient control: First, the pH is adjusted to 3.0~4.0 and an iron source is added to carry out selective phosphorus precipitation reaction and recover phosphate products in the primary phosphorus removal reactor. Then, the pH is adjusted to 5.5~6.0 and a calcium source is added to carry out induced fluoride precipitation reaction and recover fluoride products in the secondary fluoride removal reactor. The terminal liquid phase is reused. S3, Underflow dual-stage activated paste preparation: The underflow slurry is transported to the dual-stage activated paste preparation unit, where it undergoes a first-stage high-shear forced activation treatment to remove the impurity layer on the crystal surface and chemically modify it. Then, a second-stage flexible rheological reconstruction treatment is performed to construct a pseudo-plastic paste structure, thereby obtaining a paste backfill material that meets the filling performance requirements.

[0026] As a further improvement of the present invention, the first-stage high-shear forced activation step adopts horizontal forced stirring at 60~100 rpm, with a residence time of 30~60s, and the composite modifier is added; the second-stage flexible rheological reconstruction step adopts high-speed flexible stirring at 250~450 rpm, so that the slurry changes from a liquid state to a pseudoplastic paste state with a yield stress >200Pa.

[0027] As a further improvement of the present invention, the specific process of fluid dynamic gradient concentration in step S1 is as follows: The high-shear turbulent mixing step involves controlling the slurry inlet velocity at 2.0–3.0 m / s. Under the action of a helical baffle (inclination angle 35°–45°, pitch / diameter ratio 1:2–1:3), high-frequency turbulence is generated, with an average shear rate of 800–1500 s⁻¹ (peak value reaching 2000 s⁻¹) and a residence time of approximately 10–25 s. This high-speed shearing enables efficient dispersion of the flocculant (dosage 30–50 g / t) and complete removal of the hydration film on the surface of phosphogypsum particles (film breakage rate >90%), providing a good microscopic basis for subsequent floc formation. The low-shear laminar growth step utilizes a reverse guide plate (inclination angle 15°~30°, preferably 20°~25°) to rapidly reduce the slurry velocity to 0.3~0.5 m / s (average <0.45 m / s), gradually transitioning the flow pattern from turbulent to laminar flow. The average shear rate is 50~200 s⁻¹ (preferably 80~150 s⁻¹), with a residence time of approximately 15~30 s. This low-shear environment protects the already formed micro-flocs from breakage while simultaneously promoting orderly collision, growth, and compaction of the micro-flocs (average floc size increase of 25%~40%).

[0028] As a further improvement of the present invention, the ointment, with a solid content of 55% to 65%, has a 28-day compressive strength ≥3.0 MPa and a water bleeding rate <5%.

[0029] As a further improvement of the present invention, it also includes the step of transporting aged phosphogypsum and the underflow concentrate together to the two-stage activated paste preparation unit for mixing treatment; according to the dry basis mass ratio, aged phosphogypsum: underflow concentrate = (2~4):1, aged phosphogypsum is added to the underflow concentrate to adjust the system water balance and synergistically absorb the historically accumulated aged phosphogypsum.

[0030] As a further improvement of the present invention, the iron source includes ferric sulfate or polyferric sulfate, and the calcium source includes lime milk.

[0031] As a further improvement of the present invention, the cementitious aggregate is slag powder; the alkaline activator is carbide slag or quicklime; and the rheology modifier is cellulose ether or modified polyacrylamide.

[0032] As a further improvement of the present invention, the paste backfill material has the following properties: solid content: 55%~65%; yield stress >200Pa; water bleeding rate: <5%; 28-day compressive strength: ≥3.0MPa; total phosphorus concentration in leachate <0.2mg / L, fluoride concentration <2.0mg / L.

[0033] Beneficial effects: 1. This invention breaks through the traditional wastewater treatment model, transforming "harmless disposal of mixed sludge" into "stepwise separation of phosphorus and fluorine, and high-value resource utilization," significantly improving the overall economic benefits of the project and solving the industry's dual problems of resource waste and high pollution control costs. Addressing the core pain point of the existing one-step lime neutralization process, which is prone to causing the simultaneous co-precipitation of phosphorus, fluorine, and calcium, generating complex mixed solid waste / hazardous sludge, this not only results in a large loss of high-quality phosphorus and fluorine resources but also incurs high secondary treatment costs, increasing the production burden on enterprises. This invention proposes a selective enrichment mechanism for phosphates and a controlled crystallization gradient separation mechanism for fluorides. By precisely controlling the pH value in stages, phosphorus is first enriched and recovered at pH 3-4, and then fluoride is removed by directional crystallization at pH 5.5-6. Combined with a dedicated reagent system, high-purity iron phosphate (a precursor material for lithium-ion battery cathodes) and industrial-grade calcium fluoride are successfully recovered from acidic wastewater, realizing the transformation of wastewater treatment from a "cost-input type" to a "resource-output type". According to actual measurements, the recovery rates of phosphorus and fluoride both reach over 85%, resulting in a dual improvement in resource utilization and economic benefits.

[0034] 2. This invention overcomes the hydrodynamic bottleneck in the concentration treatment of fine-particle phosphogypsum, solving the common industry problems of "slow settling, easy turbidity, and low underflow concentration" in traditional processes, and optimizing the efficiency of the entire solid-liquid separation process. Phosphogypsum particles are fine (high proportion of -20μm) and have a large specific surface area. When processed using traditional thickeners, problems such as slow settling rate, excessive overflow turbidity, and difficulty in consistently achieving an underflow concentration of 40%~50% are common, directly increasing the energy consumption and cost of subsequent dewatering operations. To address this problem, this invention develops a turbulent-laminar coupled gradient flocculation feed well. The upper high-shear turbulence enables rapid diffusion of the reagent and breakup of the particle hydration film, while the lower stable laminar flow ensures the orderly growth of micro-flocs. Without increasing the floor space, it significantly improves the capture efficiency of fine particles and the material compression performance. Actual operation data shows that the underflow concentration can be stably maintained at 55%~60%, and the overflow turbidity is reduced to below 100 NTU, providing a stable and qualified material guarantee for low-cost paste filling.

[0035] 3. This invention solves the technical problems of easy segregation and insufficient strength in low-concentration slurry filling, optimizes the paste filling process, and significantly reduces the cost of cementitious materials. Traditional filling processes require adding more than 15% cement or forcibly increasing the slurry concentration to over 65% to avoid segregation and bleeding in low-concentration (<60%) slurries. The former results in excessively high cementing costs, while the latter leads to high pumping resistance and difficult transportation, hindering large-scale application. This invention employs chemical rheological reconstruction technology to construct a pseudoplastic microstructure in the slurry within a medium concentration range of 55%~60% using a composite modifier, fundamentally eliminating bleeding and segregation problems. A two-stage activation process, involving both powerful membrane breaking and structural reconstruction, fully stimulates the cementitious activity of phosphogypsum. With a 30%~50% reduction in cementitious material usage, the prepared filling paste achieves a stable compressive strength of over 3.0 MPa after 28 days, balancing filling quality and cost control, and meeting the requirements of large-scale filling operations in mines.

[0036] 4. This invention achieves deep curing of modified phosphogypsum, completely eliminating the environmental pollution risks associated with long-term storage and resource utilization, and meeting stringent environmental standards. Existing conventional modification processes involve only simple physical mixing, failing to break the acidic hydration film on the surface of phosphogypsum crystals. The curing reaction remains only on the surface, easily leading to acid reversion during storage, resulting in excessive phosphorus and fluoride leaching concentrations and causing secondary pollution. This invention employs a forced activation film-breaking process, using high-speed mechanical shearing force to completely peel off the impurity layer and acidic hydration film on the crystal surface, allowing the curing agent to directly act on the lattice defect sites, achieving comprehensive deep curing. Testing shows that the total phosphorus in the leachate of the treated cured body is <0.2 mg / L and fluoride is <2.0 mg / L, indicators far exceeding the "Integrated Wastewater Discharge Standard" and current industry standards, truly achieving harmless disposal of phosphogypsum and completely eliminating environmental hazards.

[0037] 5. This invention constructs a water-solid integrated processing system to achieve the coordinated utilization of newly added phosphogypsum and existing aged phosphogypsum, aligning with the national policy of "production based on waste." Traditional processes involve independent operation of water treatment and backfilling systems, leading to problems such as redundant equipment investment, complex operation and maintenance, and ineffective utilization of existing solid waste, resulting in long-term stockpiling of phosphogypsum in storage areas. This system deeply couples overflow water treatment with phosphogypsum preparation, sharing a reaction control and monitoring system. It also reserves an interface for the coordinated utilization of aged phosphogypsum, allowing for flexible adjustment of existing phosphogypsum based on slurry concentration fluctuations. This not only stabilizes the system's water balance but also gradually utilizes historical solid waste, achieving a virtuous cycle of immediate disposal of newly added solid waste and gradual digestion of existing solid waste. This promotes efficient and coordinated utilization of phosphogypsum resources throughout the entire process, meeting the industry's requirements for green and low-carbon development.

[0038] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and in order to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0039] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the present invention will be briefly described below. Obviously, the drawings described below are merely some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0040] Figure 1 This is a schematic diagram of the structure of the feed well body of the phosphogypsum harmless paste backfill material preparation system with turbulent-laminar flow coupling structure provided in the embodiment of the present invention; Figure 2 This is a side view of the feed well body of the phosphogypsum harmless paste backfill material preparation system with turbulent-laminar flow coupling structure provided in this embodiment of the invention; Figure 3 This is a schematic diagram of the structure of the phosphogypsum harmless paste backfill material preparation system with turbulent-laminar flow coupling structure feeding well body and feed pipe provided in the embodiment of the present invention; Figure 4 This is a side view of the feed well body and distributor of the phosphogypsum harmless paste backfill material preparation system provided in this embodiment of the invention, which has a turbulent-laminar flow coupled structure. Figure 5 This is a side view of the thickener of the feed well with a turbulent-laminar flow coupled structure provided in the embodiment of the present invention for preparing harmless phosphogypsum paste backfill material; Figure 6 This is a schematic flowchart of the method for preparing harmless phosphogypsum paste backfill material provided in this embodiment of the invention.

[0041] Explanation of reference numerals in the attached figures: 10. Feed well; 11. Spiral baffle; 12. Reverse flow expansion structure; 13. Distributor; 14. Feed pipe; 15. Cylindrical section; 16. Conical contraction section; 17. Guide plate; 18. High shear turbulent mixing zone; 19. Low shear laminar growth zone; 20. Thickener. Detailed Implementation

[0042] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. These embodiments are merely illustrative of the technical solution of the present invention and are therefore intended to limit the scope of protection of the present invention.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the invention, are intended to cover non-exclusive inclusion.

[0044] In the description of the embodiments of this invention, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this invention, "multiple" means two or more, unless otherwise explicitly defined.

[0045] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0046] In the description of the embodiments of this invention, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0047] In the description of the embodiments of the present invention, the term "multiple" refers to two or more (including two), similarly, "multiple groups" refers to two or more (including two groups), and "multiple pieces" refers to two or more (including two pieces).

[0048] In the description of the embodiments of the present invention, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of the present invention.

[0049] In the description of the embodiments of the present invention, unless otherwise explicitly specified and limited, the technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of the present invention according to the specific circumstances.

[0050] To address the numerous technical problems inherent in existing technologies for the harmless treatment and resource utilization of phosphogypsum, such as low concentration efficiency, inefficient resource recovery, incomplete solidification, high overall cost, and substandard mechanical properties, this invention provides a phosphogypsum pretreatment method and preparation system for backfilling and land use. By coupling physical flow patterns and chemical gradients in the space-time, the harmless treatment and resource utilization of phosphogypsum are integrated into a progressive and synergistic system. First, a turbulent-laminar flow partition is constructed in the thickener feed well, utilizing the decoupling of high-shear membrane breaking and low-shear growth to efficiently produce high-concentration underflow and low-turbidity overflow. Then, pH gradient-induced crystallization is applied to the overflow water to precisely recover phosphorus and fluorine in steps into high-purity iron phosphate and calcium fluoride. Next, the high-concentration underflow is activated and rheologically reconstructed through two stages to construct pseudoplastic paste at medium to low concentrations (55%~60%), achieving low-cost and high-performance filling. Finally, through deep coupling of the water-solid system and the interface for blending existing stock, the treatment of newly generated phosphogypsum and the disposal of historical stockpiles are unified in the same system, fundamentally solving the pain points of traditional processes such as difficult concentration, resource waste, high cost, and secondary pollution.

[0051] Example 1 Please see Figures 1 to 5 As shown, Embodiment 1 of the present invention provides a phosphogypsum preparation system for backfilling and land use, which mainly includes: a gradient fluid dynamics concentration unit, a filtrate resource utilization gradient separation unit, and a two-stage activated paste preparation unit.

[0052] The gradient fluid dynamics thickening unit includes a thickener 20 with a feed well 10 having a turbulent-laminar coupling structure, used to thicken phosphogypsum slurry into underflow thick slurry and generate overflow water.

[0053] The filtrate resource recovery gradient separation unit is connected to the overflow port of the thickener 20 and is used to perform stepwise recovery treatment of the overflow water, sequentially recovering phosphorus products and fluorine products.

[0054] The dual-stage activated paste preparation unit is connected to the underflow outlet of the thickener 20 and is used to mix and stir the underflow slurry with the modifier to prepare a paste backfill material.

[0055] Please see Figures 3 to 5 As shown, the feed well 10 with a turbulent-laminar coupling structure is located above the feed inlet of the thickener 20, and includes: The high-shear turbulent mixing zone 18 is located on the upper part of the feed well body and has a turbulence structure inside, which is used to make the phosphogypsum slurry form turbulence to destroy the hydration film on the particle surface. The low-shear laminar flow growth zone 19 is located at the bottom of the feed well body and has a flow guiding structure inside to reduce the slurry flow velocity and change the flow state to laminar flow to promote the growth of micro flocs.

[0056] Specifically, the turbulence structure is a spiral turbulence plate 11 with an inclination angle of 35°~45°; the flow guiding structure is a reverse flow guiding expansion structure 12.

[0057] The spiral baffle 11 extends spirally downward along the inner wall of the feed well body, forming a continuous spiral fluid channel; the ratio of the spiral fluid channel pitch to the diameter of the feed well body is 1:2 to 1:3. The spiral direction of the spiral baffle 11 is consistent with the spiral direction of the feed slurry to enhance the circumferential shear force; the inlet of the spiral fluid channel is smoothly connected to the feed pipe 14, and the outlet is provided with an arc transition section to avoid dead zones and achieve a smooth flow transition to the low-shear laminar growth zone 19.

[0058] The reverse flow expansion structure 12 includes several guide plates 17 disposed on the inner wall of the feed well body; the extension axis of all guide plates 17 is coaxial with the central axis of the feed well body, forming an annular guide surface; the guide plates 17 are inclined relative to the inner wall of the feed well body, the inclination direction being such that the slurry forms a centripetal flow converging towards the central axis within the feed well body, or forms a reverse vortex flow opposite to the slurry flow from the high-shear turbulent mixing zone 18; the inclination angle of the guide plates 17 is 15°~30° (preferably 20°~25°), wherein the inclination angle is the angle between the surface of the guide plate 17 and the vertical central axis of the feed well body. A smooth arc transition section is provided between the lower edge of the guide plate 17 and the outlet of the low-shear laminar growth zone 19 to avoid fluid dead zones and secondary turbulence.

[0059] In this embodiment, the feed well body has a diameter D=2.0m, the high shear turbulent mixing zone 18 has a height of 1.5D, the spiral baffle 11 is made of 316L stainless steel plate with a thickness of 8mm, a radial width of 0.15D, a pitch of 0.8m (pitch to diameter ratio 1:2.5), and a total of 4 spiral turns.

[0060] After the slurry enters the spiral channel through the feed pipe 14, it forms high-frequency turbulence under the action of the spiral baffle 11 with an inclination angle of 40°, and the measured shear rate reaches 800~1200 s. -1 The hydration film on the particle surface was broken at a rate of >95%, laying the foundation for the subsequent compaction of flocs in the laminar flow growth zone.

[0061] In some embodiments, the feed well 10 includes a plurality of symmetrically arranged feed well bodies; the upper part of the feed well body is a cylindrical section 15, and the lower part is a conical contraction section 16. The high-shear turbulent mixing zone 18 is located inside the cylindrical section 15, and the low-shear laminar growth zone 19 spans the connection between the cylindrical section 15 and the conical contraction section 16.

[0062] The ratio of the height to the diameter of the cylindrical section 15 is 1.0 to 1.8:1, and the cone angle of the tapered constriction section 16 is 10° to 25° (preferably 15° to 20°).

[0063] The high-shear turbulent mixing zone 18 is located within the cylindrical section 15, and its height accounts for 60% to 80% of the total height of the cylindrical section 15; the low-shear laminar growth zone 19 spans the connection between the lower part of the cylindrical section 15 and the upper part of the conical contraction section 16, and its height accounts for 20% to 40% of the total height of the feed well body; A smooth arc transition section (with a transition radius of 0.05 to 0.10 times the diameter of the feed well body) is provided at the connection between the cylindrical section 15 and the conical contraction section 16 to achieve a smooth transition of the flow from turbulent to laminar flow and reduce dead zones.

[0064] The feeding well 10 also includes a distributor 13 disposed above several feeding well bodies; the feeding well bodies and the distributor 13 are connected by a feed pipe 14 to achieve flow connection of phosphogypsum slurry. Specifically, the top end of the feed pipe 14 is disposed at the bottom of the distributor 13, and the tail end is disposed at the inlet of the spiral fluid channel. The phosphogypsum slurry is fed from the distributor 13 and then flows through the feed pipe 14 to the spiral fluid channel of the high shear turbulent mixing zone 18 of each feeding well body.

[0065] The filtrate resource utilization gradient separation unit includes a primary phosphorus removal reactor, a primary solid-liquid separator, a secondary fluoride removal reactor, a secondary solid-liquid separator, and a terminal water reuse tank connected in series.

[0066] The primary phosphorus removal reactor (high-efficiency reactor A) is equipped with a first pH control device and an iron source addition device, which are used to adjust the pH in the reactor to 3.0~4.0 so that phosphorus is precipitated and recovered in the form of iron phosphate; The secondary defluorination reactor (high-efficiency reactor B) is equipped with a second pH control device and a calcium source dosing device, which are used to adjust the pH in the reactor to 5.5~6.0 so that fluorine is precipitated and recovered in the form of calcium fluoride; Both the primary and secondary solid-liquid separators are filter presses or micro-flocculation separators.

[0067] The dual-stage activated paste preparation unit mainly includes: The first-stage high-shear forced activation mixer has its feed inlet connected to the underflow outlet of the thickener 20 and is equipped with a modifier dosing port. It is used to perform high-shear mixing on the underflow thick slurry to break down the residual particle surface coating and initiate a preliminary hydration reaction. The secondary high-speed flexible rheological reconstruction machine (secondary high-speed flexible mixer) has its inlet connected to the outlet of the primary high-shear forced activation mixer. The outlet is used to connect to the filling pipe and is used to perform rheological reconstruction on the material after primary mixing, so that it forms a paste with pseudoplastic fluid properties.

[0068] The shear rate of the first-stage high-shear forced activation mixer is higher than that of the second-stage high-speed flexible rheological reconfiguration machine.

[0069] In some other embodiments, the two-stage activated paste preparation unit further includes a composite modifier metering and adding system connected to the modifier dosing port, mainly comprising a cementitious aggregate bin, an alkaline activator bin, and a rheology modifier bin.

[0070] In some other embodiments, the phosphogypsum harmless paste backfill material preparation system further includes an aged phosphogypsum feeding device, the discharge port of which is connected to the two-stage activated paste preparation unit, for mixing the aged phosphogypsum with the underflow slurry.

[0071] Example 2 Based on the aforementioned phosphogypsum preparation system for backfilling and land application, Example 2 provides a phosphogypsum pretreatment method for backfilling and land application, specifically applied to resource-based backfilling under pure bottom flow conditions. The raw material is slurry discharged from a phosphate chemical plant, with a solid content of 12%, pH 2.2, soluble phosphorus of 0.7%, and soluble fluorine of 0.35%. Please refer to... Figure 6 As shown, the specific steps are as follows: S1, Phospholipid Gradient Concentration of Phosphogypsum Slurry: Low-concentration phospholipid slurry (10%~15%) produced by the concentrator is pumped into the thickener 20 of the fluid dynamic concentration unit, which has a turbulent-laminar coupling structure in the feed well 10, for concentration. The flocculant is efficiently dispersed and the hydration film on the particle surface is broken through the upper high-shear turbulent mixing zone 18. Flocculation and compaction are achieved through the lower low-shear laminar growth zone 19, ultimately yielding a high-concentration underflow slurry and overflow water; specifically: S11, Turbulent Film Breaking and Capture (Upper Region): The feed velocity is controlled at 2.5 m / s. High-frequency turbulence is formed under the action of the spiral baffle 11 (40° inclination angle, pitch / diameter ratio 1:2.5, radial width 0.15D, 4 turns), with an average shear rate of 800~1500 s⁻¹ (peak value up to 2000 s⁻¹) and a residence time of approximately 20 s. 35 g / t of a high-molecular-weight flocculant (anionic polyacrylamide) is added, utilizing high shear force to break the hydration film on the surface of the phosphogypsum crystals, allowing the agent to rapidly disperse and capture fine particles. In this embodiment, the underflow concentration is stabilized at 58%, and the overflow turbidity is 100 NTU, providing a good microscopic basis for the orderly growth and densification of micro-flocs in the subsequent laminar flow growth zone.

[0072] S12, Laminar Crystal Agglomeration (Lower Region): The slurry enters the expansion zone and, under the action of the reverse guide plate 17 (20° inclination), the slurry velocity rapidly decreases to 0.4 m / s. The flow pattern transitions from turbulent to laminar flow, with an average shear rate of 80–150 s⁻¹ and a residence time of approximately 20 s. This low-shear environment protects the already formed micro-flocs from breakage while simultaneously promoting orderly collision, growth, and densification of the micro-flocs (average floc size increase of 25%–40%). The transition to laminar flow protects the already formed micro-flocs from being broken down, and through positive dynamic flocculation, the crystals overlap to form dense, large flocs.

[0073] S13, paste thickening and compaction: Under the action of the mud layer pressure at the bottom of the thickener 20 (mud layer height > 15m), the underflow concentration is stabilized at 55%~60%, and the underflow is directly pumped to the pulping unit.

[0074] S2, Overflow Gradient Separation and Resource Utilization: The overflow water is transported to the filtrate resource utilization gradient separation unit for stepwise precipitation treatment with pH gradient control: first, a selective phosphorus precipitation reaction is performed to recover phosphate products, then an induced fluoride precipitation reaction is performed to recover fluoride products, and the final liquid phase is reused; the P and F gradient separation and resource utilization of the overflow water involves separating the acidic overflow water (pH 2~3, containing PO4) generated by the thickener 20. 3- F - Introduce a gradient recycling system. Specifically: S21, Priority Phosphorus Removal (FePO4 Recovery): In the primary phosphorus removal reactor, ferric sulfate or polyferric sulfate is added as an iron source. Sodium hydroxide solution is added simultaneously, and the pH value is precisely controlled at 3.0. After reacting for 15 minutes, high-purity ferric phosphate filter cake (lithium battery precursor raw material) is recovered by pressure filtration, and the filtrate enters the next stage. In this embodiment, primary phosphorus removal involves adding liquid ferric sulfate + 30% NaOH solution, controlling the pH to 3.5. The recovered product is ferric phosphate (purity >92%).

[0075] S22, Induced Defluorination (CaF2 Recovery): In the secondary defluorination reactor, lime slurry or calcium chloride is added to adjust the pH to 5.5. Calcium fluoride precipitate is generated using the fluoride ion-induced crystallization mechanism. The calcium fluoride filter cake (a fluorochemical raw material) is recovered by pressure filtration. In this embodiment, secondary defluorination involves adding 10% lime slurry to control the pH at 5.8. The recovered product is calcium fluoride (purity >85%).

[0076] S23, Terminal Reuse: The pH of the supernatant is adjusted back to neutral and returned to the beneficiation plant for recycling, achieving zero emissions.

[0077] S3, Underflow Two-Stage Activated Paste Preparation (Pure Underflow Chemical Rheological Control and Paste Filling): The underflow slurry is transported to the paste preparation unit, where a first-stage high-shear forced activation treatment is performed to remove the impurity layer on the crystal surface and chemically modify it. A second-stage flexible rheological reconstruction treatment is then performed to construct a pseudoplastic paste structure, obtaining a paste backfill material that meets the filling performance requirements. Specifically: S31, Primary Forced Activation: A composite modifier is added to a forced mixer. The modifier formulation includes: cementitious aggregate (slag powder), alkaline activator (carbide slag / quicklime), and rheology modifier (cellulose ether / modified PAM, 0.05%~0.1%). High-speed shearing (residence time 45s) disrupts the lattice impurity layer, initiating a hydration reaction. In this embodiment, the raw material ratio is: 58% concentrated underflow (100 parts) + slag powder (8 parts) + carbide slag (2 parts) + rheology modifier (0.05 parts).

[0078] S32, Secondary Rheological Reconstruction: The slurry enters a secondary high-speed flexible mixer. Under the action of the rheology modifier, the slurry transforms from a "liquid" state to a "pseudoplastic paste" state. Even at a relatively low concentration of 58%, the slurry possesses a yield stress of >200 Pa, achieving stable transport without bleeding or segregation.

[0079] In this embodiment, the two-stage stirring process is specifically set as follows: first-stage forced stirring (45s, stirring rate controlled at 80rpm) + second-stage flexible stirring (30s, stirring rate controlled at 350rpm).

[0080] S33, Pumping Filling: The prepared paste is pumped to the goaf area underground, and the strength meets the design requirements after 28 days.

[0081] In this embodiment 2, the product properties of the filling paste in step S3 are: slump of 24cm, water bleeding rate after standing 3%, and compressive strength of 3.2MPa after 28 days.

[0082] Example 3 This embodiment 3 provides a method for the pretreatment of phosphogypsum for backfilling and land use, synergistically addressing the issue of aged phosphogypsum. To regulate the system's water balance and utilize existing stock in the storage area, aged material is added during the slurry preparation stage.

[0083] The difference from Example 2 is that aged phosphogypsum with a moisture content of 25% is added before the secondary mixer at a dry mass ratio of 3:1 (underflow: aged material).

[0084] The concentration of the filling mixture prepared in Example 3 was increased to 64%, and the compressive strength after 28 days was increased to 4.5 MPa, indicating that the system and method have excellent raw material adaptability.

[0085] Comparative Example 1 Comparative Example 1 provides the existing conventional process, namely, conventional preparation using a common thickener (without optimized feed well), one-step lime neutralization, and cement solidification process.

[0086] The concentrated underflow concentration prepared in Comparative Example 1 was only 42%, and the overflow was turbid. Water treatment generated a large amount of mixed sludge (hazardous waste) containing phosphogypsum, with no resource recovery value. Furthermore, the filling paste required the addition of 20% cement to barely achieve a strength of 2 MPa, and the bleeding rate was as high as 12%.

[0087] Table 1. Performance comparison test data of the present invention process and existing processes. In summary, this invention provides a method and preparation system for the pretreatment of phosphogypsum for backfilling and land use. First, a turbulent-laminar flow partition is constructed within the thickener feed well, utilizing the decoupling of high-shear membrane rupture and low-shear growth to efficiently produce high-concentration underflow and low-turbidity overflow. Then, pH gradient-induced crystallization is applied to the overflow water to precisely recover phosphorus and fluorine in steps into high-purity iron phosphate and calcium fluoride. Next, the high-concentration underflow is subjected to two-stage activation and rheological reconstruction to construct a pseudoplastic paste at medium-low concentrations (55%~60%), achieving low-cost, high-performance backfilling. Finally, through deep coupling of the water-solid system and the interface for existing stockpiling, the treatment of newly generated phosphogypsum and the disposal of historical stockpiles are unified within the same system, fundamentally solving the pain points of traditional processes such as difficult concentration, resource waste, high cost, and secondary pollution.

[0088] It should be noted that the present invention is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments that have the same structure and perform the same effects as the technical concept within the scope of the present invention are included within the scope of the present invention. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of the present invention, are also included within the scope of the present invention.

Claims

1. A phosphogypsum preparation system for backfilling and land application, characterized in that, include: The gradient fluid dynamics thickening unit includes a thickener with a feed well featuring a turbulent-laminar coupling structure, used to thicken phosphogypsum slurry into underflow thick slurry and generate overflow water; The filtrate resource recovery gradient separation unit is connected to the overflow port of the thickener and is used to perform stepwise recovery treatment of the overflow water, sequentially recovering phosphorus products and fluorine products. A two-stage activated paste preparation unit is connected to the underflow outlet of the thickener and is used to mix and stir the underflow slurry with a modifier to prepare a paste backfill material. The turbulent-laminar coupled structure feed well is located above the feed inlet of the thickener and includes: The high-shear turbulent mixing zone located at the top of the feed well body has a turbulent structure inside, which is used to make the phosphogypsum slurry turbulent to destroy the hydration film on the particle surface. The low-shear laminar flow growth zone is located at the bottom of the feed well body and has a flow guiding structure inside to reduce the slurry flow velocity and change the flow state to laminar flow in order to promote the growth of micro flocs.

2. The phosphogypsum preparation system for backfilling and land use according to claim 1, characterized in that, The turbulence structure is a spiral turbulence plate with an inclination angle of 35°~45°; the flow guiding structure is a reverse flow guiding expansion structure.

3. The phosphogypsum preparation system for backfilling and land use according to claim 2, characterized in that, The spiral baffle extends spirally downward along the inner wall of the feed well body to form a continuous spiral fluid channel; the ratio of the spiral fluid channel pitch to the diameter of the feed well body is 1:2 to 1:

3. The ratio of the radial width of the spiral baffle to the diameter of the feed well body is 0.10~0.20; The total number of spiral baffles in the high-shear turbulent mixing zone is 3 to 5.

4. The phosphogypsum preparation system for backfilling and land use according to claim 2, characterized in that, The reverse flow expansion structure includes several guide plates disposed on the inner wall of the feed well body; the extension axis of all the guide plates is coaxial with the central axis of the feed well body, forming an annular flow guiding surface; The guide plate is inclined relative to the inner wall of the feed well body. The inclination direction is such that the slurry in the feed well body forms a centripetal flow that converges towards the central axis, or forms a reverse swirling flow that is opposite to the slurry flow from the high shear turbulent mixing zone, so as to achieve a smooth transition of the flow state from turbulent to laminar flow. The angle between the surface of the guide plate and the vertical wall of the feed well body is 15°~30°; The ratio of the radial width of the guide plate to the diameter of the feed well body is 0.08~0.

15.

5. The phosphogypsum preparation system for backfilling and land use according to any one of claims 2 to 4, characterized in that, The feeding well includes several symmetrically arranged feeding well bodies; The upper part of the feeding well body is a cylindrical section, and the lower part is a conical converging section; wherein, the ratio of the height to the diameter of the cylindrical section is 1.0~1.8:1, and the cone angle of the conical converging section is 10°~25°; The high-shear turbulent mixing zone is located within the cylindrical section, accounting for 60% to 80% of the total height of the cylindrical section; The low-shear laminar flow growth zone spans the connection between the lower part of the cylindrical section and the upper part of the conical contraction section, and its height accounts for 20% to 40% of the total height of the feed well body.

6. The phosphogypsum preparation system for backfilling and land use according to claim 1, characterized in that, The filtrate resource utilization gradient separation unit includes a primary phosphorus removal reactor, a primary solid-liquid separator, a secondary fluoride removal reactor, and a secondary solid-liquid separator connected in series. The primary phosphorus removal reactor is equipped with a first pH control device and an iron source addition device; the secondary fluoride removal reactor is equipped with a second pH control device and a calcium source addition device. Both the primary solid-liquid separator and the secondary solid-liquid separator are filter presses or micro-flocculation separators; Alternatively, the two-stage activated paste preparation unit includes: A high-shear forced activation mixer with a feed inlet connected to the underflow outlet of the thickener and equipped with a modifier dosing port; The inlet of the secondary high-speed flexible rheological reconfiguration machine is connected to the outlet of the primary high-shear forced activation mixer, and the outlet is used to connect to the filling pipeline.

7. The phosphogypsum preparation system for backfilling and land use according to claim 1, characterized in that, It also includes an aged phosphogypsum feeding device, the discharge port of which is connected to the two-stage activated paste preparation unit, for mixing the aged phosphogypsum with the underflow slurry.

8. A method for pretreating phosphogypsum for backfilling and land application, comprising preparation using the system described in any one of claims 1-8, characterized in that, Includes the following steps: S1, Gradient Fluid Dynamics Concentration: Low-concentration phosphogypsum slurry is fed into a thickener with a turbulent-laminar coupling structure in the gradient fluid dynamics concentration unit for concentration. The feed velocity is controlled at 2.0~3.0 m / s, and the flocculant dosage is 30~50 g / t. The flocculant is efficiently dispersed and the hydration film on the particle surface is broken through the upper high-shear turbulent mixing zone. The flow velocity is then controlled to be reduced to <0.5 m / s, and the micro-flocs are orderly grown and compacted through the lower low-shear laminar growth zone, finally obtaining a high-concentration underflow slurry and low-turbidity overflow water. S2, Overflow liquid gradient separation and resource utilization: The overflow water is transported to the filtrate resource utilization gradient separation unit for stepwise precipitation treatment with pH gradient control: First, the pH is adjusted to 3.0~4.0 and an iron source is added to carry out selective phosphorus precipitation reaction and recover phosphate products in the primary phosphorus removal reactor. Then, the pH is adjusted to 5.5~6.0 and a calcium source is added to carry out induced fluoride precipitation reaction and recover fluoride products in the secondary fluoride removal reactor. The terminal liquid phase is reused. S3, Underflow dual-stage activated paste preparation: The underflow slurry is transported to the dual-stage activated paste preparation unit, where it undergoes a first-stage high-shear forced activation treatment to remove the impurity layer on the crystal surface and chemically modify it. Then, a second-stage flexible rheological reconstruction treatment is performed to construct a pseudo-plastic paste structure, thereby obtaining a paste backfill material that meets the filling performance requirements.

9. The method for pretreatment of phosphogypsum for backfilling and land use according to claim 8, characterized in that, In step S3, the specific process of the two-stage activation process is as follows: The first-stage high-shear forced activation step employs horizontal forced stirring at a stirring rate of 60-100 rpm and a residence time of 30-60 s, during which a composite modifier is added; the composite modifier includes cementitious aggregate, alkaline activator, and rheology modifier; The secondary flexible rheological reconstruction step employs high-speed flexible stirring at a stirring rate of 250~450 rpm to transform the slurry from a liquid state to a pseudoplastic paste state with a yield stress >200Pa.

10. The method for pretreatment of phosphogypsum for backfilling and land use according to claim 8, characterized in that, It also includes the step of transporting aged phosphogypsum and the underflow concentrate together to the two-stage activated paste preparation unit for mixing treatment; and adding aged phosphogypsum to the underflow concentrate at a dry basis mass ratio of (2~4):1 to adjust the system water balance and synergistically absorb the historically accumulated aged phosphogypsum.

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