Phosphogypsum purification method based on pelletizing and circulating wetting-drying deep desalination

Abstract

The invention discloses an ardealite purification method based on balling and circulating wetting-drying deep desalination, which comprises the following steps: firstly, making ardealite and a binder into regular spheres with through pores, then carrying out multiple times of'wetting-drying-stripping 'circulation on the regular spheres in a circulating desalination device, wetting and dissolving salt in the spheres through a low liquid-solid ratio, and carrying out deep desalination through the circulating wetting-drying. Controllable drying is utilized to drive the salt solution to migrate to the surface of the sphere through the capillary action for crystallization, and finally surface salt is stripped in a spraying or mechanical mode. Through multiple times of the gradient strengthening process of dissolution-migration-precipitation-stripping, soluble impurities in the ardealite spheres can be deeply removed, and the phenomenon of salt flooding is fundamentally eradicated. According to the method, the problems of high water consumption, difficulty in solid-liquid separation, incomplete desalination and the like of a traditional washing process are solved, the purified spheres can be directly used as a high-added-value product, and efficient and low-cost ardealite resource purification is realized.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization and environmental protection technology, and specifically relates to a phosphogypsum purification method based on pelleting and circulating wetting-drying deep desalination. Background Technology

[0002] Phosphogypsum is a large-scale industrial solid waste generated during the wet-process phosphoric acid production process. Its large stockpile and low comprehensive utilization rate have become a prominent environmental problem restricting the green development of the phosphoric acid chemical industry. To achieve the resource utilization of phosphogypsum, it is essential to effectively remove soluble impurities (such as fluorides, phosphates, and various metal ions) to avoid product performance degradation and environmental risks in subsequent applications such as building materials and soil conditioners. Currently, the industrial practice commonly uses water washing to desalinate and purify phosphogypsum. This involves bringing a large amount of washing water into full contact with the phosphogypsum slurry to dissolve and remove soluble salts, followed by solid-liquid separation processes such as sedimentation and filtration to obtain purified gypsum.

[0003] However, traditional water washing processes typically consume large amounts of water, and the resulting high-salinity wastewater is costly to treat. Furthermore, due to the small particle size and complex internal pore structure of phosphogypsum, conventional water washing is insufficient to completely replace residual salts deep within the pores. The resulting slurry system also exhibits high viscosity, poor settling performance, low solid-liquid separation efficiency, and high energy consumption, leading to low economic benefits. In practical engineering, when phosphogypsum treated with water is molded into spherical vegetation substrates, landscaping materials, and other products, residual salts migrate to the surface and crystallize under natural environmental conditions such as diurnal temperature variations and alternating wet and dry periods, resulting in a phenomenon known as "salt blooming." This severely affects the product's appearance stability, mechanical properties, and long-term durability, limiting its application in high-end building materials and ecological restoration.

[0004] Therefore, how to provide a phosphogypsum purification method based on pelleting and cyclic wetting-drying deep desalination to achieve integrated preparation of deep phosphogypsum purification and product, effectively suppress salt blooming, and reduce overall costs is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a method for purifying phosphogypsum based on pelleting and cyclic wetting-drying deep desalination, so as to solve at least one of the above-mentioned technical problems.

[0006] To achieve the above objectives, the first aspect of the present invention provides a method for purifying phosphogypsum based on pelletizing and cyclic wetting-drying deep desalination. The method includes: mixing phosphogypsum raw materials with a binder and preparing porous green pellets using a pelletizing device; feeding the green pellets into a circulating desalination device for several cycles; each cycle includes: penetrating and wetting the green pellets with a washing liquid to fill the internal capillary pores of the green pellets; drying the wetted green pellets under controlled temperature and humidity to allow the internal salt solution of the green pellets to migrate and evaporate to the surface of the green pellets through capillary action, and crystallize and precipitate on the surface of the green pellets; performing surface treatment on the dried green pellets to remove the precipitated surface crystallized salt; ending the cycle when the salt concentration inside and on the surface of the green pellets tends to be consistent, obtaining deeply desalinated pellets; and rinsing and stabilizing the deeply desalinated pellets, followed by drying to obtain purified phosphogypsum pellets.

[0007] In the first aspect, the porosity of the green pellets is 25%-45%, and the average pore size is 5-50 μm.

[0008] In the first aspect, the binder includes polyacrylamide, hydroxypropyl methylcellulose, or phosphogypsum-based solid waste cementitious materials.

[0009] In the first aspect, the circulating desalination device is a tower structure containing several layers of support units, and each layer of support unit performs one cycle treatment; each layer of support unit is independently provided with a penetration wetting zone, a temperature and humidity controlled drying zone and a surface salt removal zone, so as to sequentially penetrate and wet the green pellets, control the temperature and humidity for drying, and remove surface crystallized salt.

[0010] In the first aspect, during the penetration wetting process, the liquid-to-solid ratio of the washing liquid to the green pellets is 0.1:1 to 0.5:1.

[0011] In the first aspect, the temperature and humidity controlled drying process conditions include: a temperature of 30-80℃, and the drying time ending when there is no obvious liquid water on the surface of the green pellets and the humidity at the center of the green pellets is 5%-15% higher than that on the surface.

[0012] In the first aspect, the surface treatment includes high-pressure water mist spraying, airflow carrying water mist spraying, mechanical brushing combined with low-pressure water rinsing, or lifting-lowering immersion.

[0013] The second aspect of this invention provides a phosphogypsum purification system based on pelletizing and circulating wetting-drying deep desalination. The system is used to implement the phosphogypsum purification method based on pelletizing and circulating wetting-drying deep desalination described in the first aspect. The system includes: a mixing pelletizing unit for mixing phosphogypsum raw materials with a binder to prepare green pellets; and a circulating wetting-drying deep desalination unit, the internal structure of which is a multi-layer tower structure, each layer having a permeation wetting zone, a temperature and humidity controlled drying zone, and a surface salt removal zone arranged sequentially. The internal structure includes a sphere-carrying and conveying mechanism to allow the green spheres to move sequentially through each layer and functional zone and complete multiple cycles. Seals are installed between each functional zone to prevent airflow cross-contamination. A washing liquid circulation and treatment unit provides washing liquid to the penetration and wetting zone and the surface salt removal zone, and treats the collected high-salt wastewater. A final purification and product processing unit performs final rinsing and drying on the deeply desalinated spheres that have completed the desalination cycle, resulting in purified phosphogypsum spheres. A control unit monitors and regulates the temperature and humidity of each zone.

[0014] In the second aspect, in the circulating humidification-drying depth desalination unit, the flow direction of the green pellets between adjacent layers is opposite to or cross-shaped with the flow direction of the hot air.

[0015] The third aspect of the present invention provides an application of phosphogypsum spheres in phosphogypsum vegetation substrates, landscape materials or high-end building materials, wherein the phosphogypsum spheres are prepared by any one of the phosphogypsum purification methods based on sphere formation and cyclic wetting-drying deep desalination as described in the first aspect, and the total removal rate of soluble impurities of the phosphogypsum spheres is ≥95%.

[0016] Beneficial effects: This invention provides a method for purifying phosphogypsum based on pelletizing and cyclic wetting-drying deep desalination. First, phosphogypsum raw materials are mixed with a binder and formed into green pellets with interconnected pores using a pelletizing device, providing the foundation for subsequent capillary migration. Second, the green pellets are fed into a circulating desalination device for several cycles until the salt concentration inside and on the surface of the green pellets tends to be uniform, at which point the cycle is terminated, resulting in deeply desalinated pellets. Each cycle includes: first, using a washing solution to penetrate and wet the green pellets to fill the interior of the pellets. The capillary action allows soluble salts within the internal capillaries to dissolve and migrate to the surface. Subsequently, temperature and humidity controlled drying causes water evaporation, and salt crystallizes and precipitates on the surface of the green pellets. The surface is then sprayed to remove the precipitated surface crystals. Through this repeated cycle of "dissolution-migration-precipitation-stripping," the salt concentration inside and on the surface of the green pellets becomes more uniform, achieving deep removal of soluble impurities from the interior of the green pellets, resulting in deeply desalinated pellets. Finally, the deeply desalinated pellets are rinsed and stabilized, and dried to obtain purified phosphogypsum pellets. This invention creatively proposes a "first forming, then deep gradient desalination" process based on capillary migration. Through multiple cycles of "dissolution-migration-precipitation-stripping," the salts deeply embedded within the green pellets are systematically "extracted" to the surface and removed, achieving a deep desalination effect that inhibits "salt blooming" at its source. The method provided by this invention not only integrates purification and product molding, fundamentally avoiding the high-cost and high-energy-consumption solid-liquid separation problem in traditional water washing processes, but also allows the prepared purified phosphogypsum spheres to be directly used as highly stable phosphogypsum planting substrates, landscape materials, and high-end building materials, reducing overall costs and promoting the high-value utilization of phosphogypsum resources. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this specification or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart of a phosphogypsum purification method based on pelletizing and circulating wetting-drying deep desalination in this invention; Figure 2 This is a schematic diagram of the unit function of a phosphogypsum purification system based on pelletizing and circulating wetting-drying for deep desalination in this invention; Figure 3 This is a partial structural diagram of a phosphogypsum purification system based on pelletizing and circulating wetting-drying for deep desalination, as described in this invention. Figure 1 ; Figure 4 This is a partial structural diagram of a phosphogypsum purification system based on pelletizing and circulating wetting-drying for deep desalination, as described in this invention. Figure 2 ; Figure label: 1. Mixing and pelletizing unit; 2. Circulating wetting-drying deep desalination unit; 21. Penetration wetting zone; 22. Temperature and humidity controlled drying zone; 23. Surface salt removal zone; 24. Support plate; 3. Washing liquid circulation and treatment unit; 4. Final purification and product treatment unit; 5. Control unit. Detailed Implementation

[0019] The present invention will be described in detail below with reference to specific embodiments and examples, thereby making the advantages and various effects of the present invention more clearly apparent. Those skilled in the art should understand that these specific embodiments and examples are for illustrative purposes only and are not intended to limit the present invention.

[0020] Throughout this specification, unless otherwise specified, the terminology used herein should be understood as having the meaning commonly used in the art. Therefore, 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. In the event of any conflict, this specification shall prevail.

[0021] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be obtained by purchasing them from the market or by existing methods.

[0022] Example 1 Please see Figure 1 This invention provides a method for purifying phosphogypsum based on pelletizing and cyclic wetting-drying deep desalination. The method includes: mixing phosphogypsum raw materials with a binder and preparing porous green pellets using a pelletizing device; feeding the green pellets into a circulating desalination device for several cycles; each cycle includes: penetrating and wetting the green pellets with a washing liquid to fill the internal capillary pores of the green pellets; drying the wetted green pellets under controlled temperature and humidity to allow the internal salt solution of the green pellets to migrate and evaporate to the surface of the green pellets through capillary action, and crystallize and precipitate on the surface of the green pellets; performing surface treatment on the dried green pellets to remove the precipitated surface crystallized salt; ending the cycle when the salt concentration inside and on the surface of the green pellets tends to be consistent, obtaining deeply desalinated pellets; rinsing and stabilizing the deeply desalinated pellets, and drying them to obtain purified phosphogypsum pellets.

[0023] Specifically, this invention provides a method for purifying phosphogypsum based on pelletizing and cyclic wetting-drying deep desalination. First, phosphogypsum raw materials are mixed with a binder and formed into green pellets with interconnected pores using a pelletizing device, providing the foundation for subsequent capillary migration. Second, the green pellets are fed into a circulating desalination device for several cycles until the salt concentration inside and on the surface of the green pellets tends to be uniform, at which point the cycle is terminated, resulting in deeply desalinated pellets. Each cycle includes: first, using a washing solution to penetrate and wet the green pellets to fill them. The internal capillaries allow soluble salts within the capillaries to dissolve and migrate to the surface. Subsequently, temperature and humidity controlled drying causes water evaporation, and salt crystallizes and precipitates on the surface of the green pellets. The surface is then sprayed to remove the precipitated surface crystals. Through this repeated cycle of "dissolution-migration-precipitation-stripping," the salt concentration inside and on the surface of the green pellets becomes more uniform, achieving deep removal of soluble impurities from the interior of the green pellets, resulting in deeply desalinated pellets. Finally, the deeply desalinated pellets are rinsed and stabilized, and dried to obtain purified phosphogypsum pellets. This invention creatively proposes a "first forming, then deep gradient desalination" process based on capillary migration. Through multiple cycles of "dissolution-migration-precipitation-stripping," the salts deeply embedded within the green pellets are systematically "extracted" to the surface and removed, achieving a deep desalination effect that inhibits "salt blooming" at its source. The method provided by this invention not only integrates purification and product molding, fundamentally avoiding the high-cost and high-energy-consumption solid-liquid separation problem in traditional water washing processes, but also allows the prepared purified phosphogypsum spheres to be directly used as highly stable phosphogypsum planting substrates, landscape materials, and high-end building materials, reducing overall costs and promoting the high-value utilization of phosphogypsum resources.

[0024] In some possible implementations, the porosity of the green pellets is 25%-45%, and the average pore size is 5-50 μm.

[0025] In this application, by adjusting the type and amount of binder or the pelletizing pressure, the porosity of the green pellets is controlled between 25% and 45%, and the average pore size is controlled between 5 and 50 μm, so as to ensure that the green pellets have sufficient through channels and specific surface area, thereby ensuring that the subsequent capillary migration effect can be carried out efficiently, and realizing the effective migration and control of moisture and dissolved salts in the green pellets.

[0026] In some possible implementations, the binder includes polyacrylamide, hydroxypropyl methylcellulose, or phosphogypsum-based solid waste cementitious materials.

[0027] In this application, when the final purified phosphogypsum spheres are used as a vegetation substrate, polyacrylamide or hydroxypropyl methylcellulose is selected as a binder and mixed with phosphogypsum raw materials to prepare raw spheres; when the final purified phosphogypsum spheres are used as landscape materials or building materials, phosphogypsum-based solid waste cementitious materials are selected as a binder and mixed with phosphogypsum raw materials to prepare raw spheres.

[0028] In some possible implementations, the circulating desalination device is a tower structure containing several layers of support units, with each layer of support unit performing one cycle of processing; each layer of support unit is independently provided with a penetration wetting zone, a temperature and humidity controlled drying zone, and a surface salt removal zone, so as to sequentially penetrate and wet the green pellets, control the temperature and humidity for drying, and remove surface crystallized salt.

[0029] Specifically, the circulating desalination device provided in this application is a tower structure, which has several layers of support units inside. Each support unit can perform a complete circulating desalination process on its own. The spherical material between adjacent layers can be transferred by gravity or a lifting system. In addition, each support unit is independently equipped with a penetration wetting zone, a temperature and humidity controlled drying zone, and a surface salt stripping zone to sequentially perform penetration wetting, temperature and humidity controlled drying, and surface crystallization salt stripping on the green pellets, thereby completing the deep removal of soluble impurities inside the green pellets.

[0030] In some possible implementations, during the penetration wetting process, the liquid-to-solid ratio of the washing liquid to the green pellets is 0.1:1 to 0.5:1.

[0031] In this application, the washing solution permeates and wets the prepared green pellets through atomized spraying, low-pressure spraying, or steam permeation, and controls the liquid-to-solid ratio between 0.1:1 and 0.5:1 to ensure that the pellet material is wetted but does not generate free-flowing liquid. A lower liquid-to-solid ratio significantly reduces water consumption and facilitates the treatment of high-concentration wastewater; while atomized spraying or steam permeation allows the washing solution to fill the capillaries inside the green pellets to dissolve or migrate salts.

[0032] In some possible implementations, the temperature and humidity controlled drying process conditions include: a temperature of 30-80°C, and the drying time ending when there is no obvious liquid water on the surface of the green pellets and the humidity at the center of the green pellets is 5%-15% higher than that on the surface.

[0033] In this application, the temperature and humidity controlled drying zone occupies almost 70-80% of the area in each layer of the support unit to accommodate the installation of a circulating hot air system, temperature sensors, and humidity sensors. The temperature and humidity of the temperature and humidity controlled drying zone are monitored using these sensors. By controlling the humidity at the center of the green pellets to be higher than the surface humidity, a continuous driving force is provided for capillary migration, allowing the salt solution inside the green pellets to continuously migrate to the surface, thereby achieving deep and thorough desalination of the green pellets.

[0034] In some possible implementations, the surface treatment includes high-pressure water mist spraying, airflow-carried water mist spraying, mechanical brushing combined with low-pressure water rinsing, or lifting-lowering immersion.

[0035] In this application, when phosphogypsum-based solid waste cementitious material is used as a binder, surface treatment can be carried out by high-pressure water mist spraying to remove the crystalline salt precipitated on the surface of the spheres; while when polyacrylamide or hydroxypropyl methylcellulose is used as a binder, the crystalline salt precipitated on the surface of the spheres can be removed by airflow water spraying, mechanical brushing combined with low-pressure water rinsing, or multiple immersion methods of lifting and lowering; the waste liquid after surface treatment is collected by a special water collection tank set at the bottom and combined with the high-concentration wastewater generated in the penetration and wetting zone for treatment.

[0036] It should be further explained that each layer of the carrying unit is equipped with a carrying plate at the bottom to receive the green pellets and move them sequentially within each functional area. A rotary drive mechanism is installed at the center to drive the carrying plate carrying the green pellets to rotate sequentially in the penetration wetting zone, the temperature and humidity controlled drying zone, and the surface salt removal zone, thereby effectively removing soluble impurities from the inside of the green pellets. In addition, after each layer of the carrying unit completes the desalination process, the pellets are transferred to the next layer via a lift or gravity unloading device, achieving at least 3-5 deep desalination cycles. At the end of the surface salt removal zone of each layer, a lift interface or gravity unloading hole is installed to transfer the pellets that have completed the desalination cycle of this layer to the next layer for a new cycle of desalination. That is, at the end of the surface salt removal zone of each layer, a movable fan-shaped tilting plate or spiral chute is installed. After the pellets fall from the upper surface salt removal zone, they enter the lower penetration wetting zone through a buffer chute to begin the next deep desalination process. Furthermore, soft seals or air curtains are installed in the permeation and humidification zones and temperature and humidity controlled drying zones of each load-bearing unit to prevent heat loss or cross-contamination of moisture.

[0037] Example 2 Please see Figure 2-4This invention provides a phosphogypsum purification system based on pelletizing and circulating wetting-drying deep desalination. The system is used to implement the phosphogypsum purification method based on pelletizing and circulating wetting-drying deep desalination described in the first aspect. The system includes: a mixing pelletizing unit 1 for mixing phosphogypsum raw materials with a binder to prepare green pellets; and a circulating wetting-drying deep desalination unit 2, the internal structure of which is a multi-layered tower structure. Each layer sequentially includes a penetration wetting zone 21, a temperature and humidity controlled drying zone 22, and a surface salt removal zone 23. The structure is internally equipped with a sphere carrying and conveying mechanism to allow the green spheres to move sequentially through each layer and functional zone and complete multiple cycles. Seals are provided between each functional zone to prevent airflow cross-contamination. The washing liquid circulation and treatment unit 3 provides washing liquid to the penetration wetting zone 21 and the surface salt stripping zone 23, and treats the collected high-salt wastewater. The final purification and product treatment unit 4 is used to perform final rinsing and drying on the deep desalinated spheres that have completed the desalination cycle to obtain purified phosphogypsum spheres. The control unit 5 is used to monitor and adjust the temperature and humidity of each zone.

[0038] Specifically, this invention provides a phosphogypsum purification system based on pelletizing and circulating wetting-drying deep desalination, used to implement the aforementioned phosphogypsum deep desalination purification method. The system includes a mixing and pelletizing unit 1, a circulating wetting-drying deep desalination unit 2, a washing liquid circulation and treatment unit 3, and a final purification and product treatment unit 4. The mixing and pelletizing unit 1 is used to uniformly mix phosphogypsum raw materials and binders to form spherical green pellets with a through-pore structure. The circulating wetting-drying deep desalination unit 2 is a multi-layer tower structure, with multiple independent support layers arranged from top to bottom. Each support layer sequentially arranges a permeation wetting zone 21, a temperature and humidity controlled drying zone 22, and a surface salt removal zone 23. A support plate 24 driven by a rotary drive mechanism is installed on each support layer, allowing the green pellets to complete one desalination cycle within a single layer by sequentially passing through the permeation wetting zone 21, the temperature and humidity controlled drying zone 22, and the surface salt removal zone 23. Adjacent layers are separated by gravity discharge holes. The spheres are transferred by a conveying mechanism such as a lifting mechanism for multiple cycles. Seals are installed in adjacent functional areas to prevent airflow cross-flow and ensure effective desalination. The washing liquid circulation and treatment unit 3 provides a low liquid-to-solid ratio washing liquid to the penetration wetting zone 21 and a rinseable washing liquid to the surface salt stripping zone 23, which are then collected for centralized treatment. The final purification and product processing unit 4 is located at the bottom of the tower structure and is used to perform surface purification and final drying on the deeply desalinated spheres that have completed the desalination cycle, resulting in purified phosphogypsum spheres that can be directly utilized. The control unit 5 monitors and regulates the temperature and humidity of each area to ensure stable operation of the circulating wetting-drying deep desalination process under a preset temperature and humidity gradient, achieving integrated continuous operation of phosphogypsum desalination purification and product molding.

[0039] In some possible embodiments, the hot air system is typically preferentially connected to the temperature and humidity controlled drying zone 22 at the bottom of the tower structure, where the air temperature is highest and the drying capacity is strongest. Each layer of the temperature and humidity controlled drying zone 22 has a return air vent at its top to guide the waste heat air released from the lower layer into the bottom air inlet of the upper layer's temperature and humidity controlled drying zone 22 via pipes. A humidity sensor is installed at the connection between the pipes and the temperature and humidity controlled drying zone 22. If the humidity of the air drawn from the lower layer is too high, it is condensed and dehumidified through a heat exchanger before entering the upper layer, or supplemented with drying hot air. The spherical material descends layer by layer from the top, coming into contact with the gradually heating and drying hot air, forming a complete temperature / humidity gradient, maximizing the utilization of capillary migration dynamics.

[0040] In addition, each bearing plate 24 on each bearing layer is equipped with a sealing ring at its edge to prevent hot air from leaking directly from the interlayer gaps without passing through the temperature and humidity controlled drying zone 22. Double-layer airlock valves or flexible baffles are installed in the transfer channel from the upper layer to the lower layer to ensure that the spherical material falls while preventing hot air from escaping along the solid channel. Furthermore, a heat exchanger is installed at the top return air inlet of the tower top structure to preheat the washing liquid using the discharged hot and humid air.

[0041] The control unit 5 may include an electrical control cabinet and a control panel, which can centrally monitor and automatically adjust the temperature, humidity, material flow and equipment operating status of the entire process to ensure stable operation of the device.

[0042] In some possible embodiments, the washing liquid can also be circulated in reverse. The waste liquid with fewer soluble impurities after rinsing the surface salt stripping zone 23 at the bottom of the tower structure can be raised to the upper layer as the washing liquid in the penetration wetting zone 21 to penetrate and wet the spherical material.

[0043] It should be noted that since this second embodiment and the first embodiment are embodiments under the same inventive concept and their structures are completely identical, the structures in the second embodiment that are substantially the same as those in the first embodiment will not be described in detail. For the parts not described in detail, please refer to the first embodiment.

[0044] Example 3 This example provides a method for preparing highly stable plant substrate spheres using the above-described purification method, including the following steps: (1) Pre-treatment and molding Take phosphogypsum with a moisture content of about 20% (soluble P2O5 content 1.0%, soluble F content 0.4%) and mix it with 0.5% hydroxypropyl methylcellulose (HPMC, as a binder and water-retaining agent by dry basis weight); use a disc pelletizer to control the spray volume and tilt angle to make green pellets with a diameter of 12 mm; and test by mercury intrusion porosimetry, the porosity of the green pellets is 35%, and the pore size is distributed in 10-30 μm to improve capillary action.

[0045] (2) Circulating wetting-drying deep desalination treatment The prepared green pellets are transferred to the top first layer of a five-layer circulating desalination tower structure via a feeding machine. The process inside the tower includes: each layer is divided into three functional zones: a penetration and wetting zone 21 (equipped with ultrasonic atomizing nozzles), a temperature and humidity controlled drying zone 22 (equipped with a circulating hot air system with temperature and humidity sensors), and a surface salt removal zone 23 (equipped with rotating brushes and water mist nozzles). These three functional zones are installed on the support plate 24 of each layer. A rotating drive mechanism is installed at the center of the support plate 24 to drive the green pellets through the penetration and wetting zone 21, the temperature and humidity controlled drying zone 22, and the surface salt removal zone 23 in sequence to complete a single desalination process. Then, the pellets are transferred to the next layer via an elevator to continue the wet-drying deep desalination process. The wet-dry deep desalination process for each layer includes: In the penetration and wetting zone 21, the green pellets are sprayed with water containing 0.1% dispersant for 30 seconds, with a liquid-to-solid ratio of 0.15:1, so that the surface of the green pellets is uniformly wetted and the water begins to penetrate into the interior. In the temperature and humidity controlled drying zone 22, the temperature is set at 60℃ and the relative humidity is 50%. The wetted green pellets are transferred to the temperature and humidity controlled drying zone by rotating the support plate. They are dried under hot air for 25 minutes. The pore water solution inside the pellets continues to migrate to the surface of the dried pellets and evaporate under the drive of capillary force and humidity gradient, and crystallizes on the surface of the green pellets. In the surface salt stripping zone 23, the bearing plate 24 continues to rotate, transferring the dried green pellets to the surface salt stripping zone. First, the surface of the pellets is gently brushed with a rotating nylon brush to remove some of the crystalline salt. Then, the pellets are sprayed with water mist for 5 seconds to thoroughly wash away and remove the white salt frost that has precipitated on the surface. The wastewater from the washing process is collected and discharged into the high-salt wastewater pool. After each "wetting-drying-stripping" cycle is completed, the water-soluble conductivity of the sphere surface and interior is tested by sampling. The difference between the two values ​​is used to assess the desalination progress. The cycle ends when the salt concentration on the sphere surface and interior tends to be consistent. In this embodiment, the total removal rate of soluble impurities after 5 complete "wetting-drying-stripping" cycles is higher than 95%.

[0046] (3) Final purification and stabilization treatment The deep desalination spheres discharged from the bottom of the circulating desalination tower structure device are subjected to rinsing and stabilization treatment. First, they are rinsed with clean water to remove any trace residual salts that may be attached. Then, they are dried at 60°C until the moisture content is <3% to obtain purified phosphogypsum spheres, which can be used as highly stable plant substrate spheres. Accelerated salt bloom test (30 dry and wet cycles) shows no obvious salt precipitation on the surface, and its performance is far superior to the product obtained by traditional water washing.

[0047] Example 4 This embodiment provides a method for preparing lightweight aggregate for construction using the above-described purification method, including the following steps: (1) Pre-treatment and molding Take phosphogypsum with a moisture content of about 20% (soluble P2O5 content 1.0%, soluble F content 0.4%) and mix it with 5% by dry weight of phosphogypsum-based cementitious material (as a binder); use a disc pelletizer to control the spray volume and tilt angle to make green pellets with a diameter of 25 mm; and test by mercury intrusion porosimetry, the porosity of the green pellets is 35%, and the pore size distribution is 10-30 μm.

[0048] (2) Circulating wetting-drying deep desalination treatment The prepared green pellets are transferred to the top first layer of a three-layer circulating desalination tower structure via a feeder. Compared with Example 3, the difference is that the cycle time is extended; low-pressure steam permeation is used to wet the green pellets in the permeation and wetting zone 21; hot air at 80°C is used to dry the permeated and wetted green pellets in the temperature and humidity controlled drying zone 22; and high-pressure water mist (pressure 3 bar) is used to spray the dried green pellets for 3 seconds in the surface salt removal zone 23 to remove surface crystalline salt. After three cycles, the deep desalination treatment of the green pellets is completed, and deep desalinated pellets are obtained.

[0049] (3) Final purification and stabilization treatment After rinsing and drying the deeply desalinated spheres, they are cured for 7 days to allow them to fully hydrate and harden, resulting in high-strength, low-alkalinity, and salt-free lightweight aggregates.

[0050] In summary, compared with the prior art, the present invention has the following advantages: (1) Deep desalination to eliminate the risk of salt bloom: This invention directly targets the fundamental mechanism of salt bloom in phosphogypsum products, namely "capillary migration". Through multiple cycles of "wetting-drying-peeling", the capillary force is artificially strengthened and utilized to systematically and gradiently "extract" the soluble salts hidden in the micron-level interconnected pores inside the sphere to the surface and peel them off. After 3-5 cycles, the salt concentration inside the sphere and the surface layer tend to be uniform, and the removal rate of soluble impurities can reach more than 95%. This fundamentally eliminates the driving force of salt bloom in the product under temperature and humidity change environment and significantly improves the long-term stability of the product.

[0051] (2) High efficiency in water saving and wastewater reduction: The present invention adopts a low liquid-to-solid ratio atomized spraying or steam permeation in the permeation and wetting zone, which only provides the water required for pore filling and salt dissolution, avoiding the large amount of diluted wastewater generated by traditional full immersion washing; the wastewater generated is a high concentration of salt solution, with a smaller volume than traditional washing process, and a high salt enrichment, which is convenient for subsequent evaporation and crystallization or resource recovery, greatly reducing wastewater treatment costs and environmental burden.

[0052] (3) Purification and molding are integrated to achieve direct resource utilization: This invention integrates desalination and purification with product molding process, and simultaneously completes the two goals of "deep desalination" and "regular sphere molding" in a single continuous process; the resulting purified phosphogypsum spheres have stable structure and controllable strength, and can be directly used as high-stability plant substrate, garden landscape material or lightweight building aggregate, eliminating the intermediate links such as pressure filtration, drying and remolding required after traditional water washing process, significantly reducing the overall treatment cost and improving the high-value utilization efficiency of phosphogypsum.

[0053] (4) High process integration and easy to scale up: Based on a multi-layer tower structure, the circulating desalination unit achieves tiered utilization of thermal energy and water resources through the counter-flow of hot air and spheres and the staged counter-circulation of washing liquid; each functional area adopts soft-sealed curtains, airlock valves and other structures to prevent airflow short-circuiting and ensure process stability. The system provided by this invention can realize continuous or semi-continuous automated operation of "mixing and pelletizing - circulating desalination - final treatment", and is suitable for industrial-scale phosphogypsum deep purification and resource utilization projects.

[0054] Finally, it should be noted that the above embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the scope of the technology disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, 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. All should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

[0055] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for purifying phosphogypsum based on pelleting and cyclic wetting-drying deep desalination, characterized in that, The method includes: The phosphogypsum raw material is mixed with a binder and then processed into porous green pellets using a pelletizing device. The green pellets are fed into a circulating desalination device for several cycles of processing. Each cycle includes: permeating and wetting the green pellets with a washing solution to fill the internal capillary pores of the green pellets; drying the wetted green pellets under controlled temperature and humidity to allow the internal salt solution of the green pellets to migrate and evaporate to the surface of the green pellets through capillary action, and crystallize and precipitate on the surface of the green pellets; and surface treating the dried green pellets to remove the precipitated surface crystallized salt. When the salt concentration inside and on the surface of the green pellets tends to be consistent, the cycle process ends, and deeply desalinated pellets are obtained. The deep desalination spheres are subjected to rinsing and stabilization treatment, and then dried to obtain purified phosphogypsum spheres.

2. The phosphogypsum purification method based on pelletizing and circulating wetting-drying for deep desalination according to claim 1, characterized in that, The porosity of the green pellets is 25%-45%, and the average pore size is 5-50 μm.

3. The phosphogypsum purification method based on pelletizing and circulating wetting-drying for deep desalination according to claim 1, characterized in that, The binder includes polyacrylamide, hydroxypropyl methylcellulose, or phosphogypsum-based solid waste cementitious materials.

4. The phosphogypsum purification method based on pelletizing and circulating wetting-drying for deep desalination according to claim 1, characterized in that, The circulating desalination device is a tower structure containing several layers of support units, and each layer of support unit performs one cycle treatment. Each layer of support unit is independently equipped with a penetration wetting zone, a temperature and humidity controlled drying zone, and a surface salt removal zone, so as to sequentially penetrate and wet the green pellets, control the temperature and humidity for drying, and remove surface crystallized salt.

5. The phosphogypsum purification method based on pelletizing and circulating wetting-drying for deep desalination according to claim 1, characterized in that, During the penetration wetting process, the liquid-to-solid ratio of the washing liquid to the green pellets is 0.1:1 to 0.5:

1.

6. The phosphogypsum purification method based on pelletizing and circulating wetting-drying for deep desalination according to claim 1, characterized in that, The temperature and humidity controlled drying process conditions include: a temperature of 30-80℃, and the drying time ending when there is no obvious liquid water on the surface of the green pellets and the humidity at the center of the green pellets is 5%-15% higher than that on the surface.

7. The phosphogypsum purification method based on pelletizing and circulating wetting-drying for deep desalination according to claim 1, characterized in that, The surface treatment includes high-pressure water mist spraying, airflow carrying water mist spraying, mechanical brushing combined with low-pressure water rinsing, or lifting-lowering immersion.

8. A phosphogypsum purification system based on pelletizing and circulating wetting-drying deep desalination, characterized in that, The system is used to implement the phosphogypsum purification method based on pelletizing and circulating wetting-drying deep desalination as described in any one of claims 1-7, the system comprising: A mixed pelletizing unit is used to mix phosphogypsum raw materials with binders to prepare green pellets; The circulating humidification-drying deep desalination unit has a multi-layer tower structure inside. Each layer is arranged with a permeation humidification zone, a temperature and humidity controlled drying zone, and a surface salt removal zone. The multi-layer tower structure is equipped with a ball bearing and conveying mechanism to make the green balls move sequentially in each layer and each functional zone and complete multiple cycles of processing. Seals are provided between each functional zone to prevent airflow cross-flow. The washing liquid circulation and treatment unit provides washing liquid to the penetration wetting zone and the surface salt stripping zone respectively, and treats the collected high-salt wastewater; The final purification and product processing unit is used to perform final rinsing and drying on the deep desalination spheres that have completed the desalination cycle, to obtain purified phosphogypsum spheres. The control unit is used to monitor and adjust the temperature and humidity of each area.

9. The phosphogypsum purification system based on pelletizing and circulating wetting-drying for deep desalination according to claim 8, characterized in that, In the circulating humidification-drying deep desalination unit, the flow direction of the green pellets between adjacent layers is opposite to or crosses the flow direction of the hot air.

10. The application of phosphogypsum spheres in phosphogypsum planting substrates, landscaping materials, or high-end building materials, characterized in that, The phosphogypsum spheres are prepared by the phosphogypsum purification method based on sphere formation and cyclic wetting-drying deep desalination as described in any one of claims 1-7, and the total removal rate of soluble impurities in the phosphogypsum spheres is ≥95%.

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