Ardealite innocent treatment method and system

Abstract

The invention relates to the technical field of ardealite treatment technology, and discloses a method and a system for harmless treatment of ardealite, which are characterized in that fresh ardealite discharged by wet-process phosphoric acid filtration is re-pulped and then sequentially passes through a first-section reaction tank (added with fly ash and cement), a tank A of a second-section reaction tank, a tank B of the second-section reaction tank and a tank C of the second-section reaction tank for continuous reaction; performing solid-liquid separation to obtain harmless phosphogypsum; after the filtrate is settled, supernate is reused for repulping and lime milk preparation, and thick slurry is returned to the reaction tank; the reaction waste gas is washed with phosphoric acid acidic water, and washing liquid is reused for ore grinding; through multi-stage reaction and accurate pH control, the phosphogypsum stably reaches the class I solid waste standard, and the curing time is shortened from traditional several days to several hours; the system is deeply coupled with a front-end phosphoric acid system, and water balance and resource circulation of the system are realized by optimizing and filtering a washing water source and recycling steam condensate water; the process has the advantages of low cost, stable quality and flexible operation.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of phosphorus chemical waste resource utilization, and particularly relates to a harmless treatment method and system for phosphogypsum. BACKGROUND

[0002] Phosphogypsum is a large amount of industrial solid waste generated in the production process of wet-process phosphoric acid. About 4.5 to 5 tons of phosphogypsum are generated per ton of phosphoric acid (P2O5) produced. Because it contains impurities such as residual phosphoric acid, fluorides, soluble heavy metals and organic matter, it has traditionally been classified as a general industrial solid waste of Class II, which needs to be managed according to the "Pollution Control Standard for General Industrial Solid Waste Storage and Disposal Sites" (GB 18599-2001).

[0003] The huge amount of phosphogypsum storage not only occupies a large amount of land, but also poses a significant environmental pollution risk. With the increasingly strict national ecological environment supervision and the implementation of the "production based on use" policy, local governments have set mandatory targets for the comprehensive utilization and harmless treatment of phosphogypsum. For example, the "Yunnan Province Work Plan for Comprehensive Utilization of Phosphogypsum" (issued on December 11, 2023) clearly requires that the comprehensive utilization rate of phosphogypsum be no less than 65% by 2024, and reach more than 75% by 2025, and all newly added phosphogypsum that cannot be comprehensively utilized must be completely treated harmlessly.

[0004] Currently, the common phosphogypsum harmless treatment process in industry uses the "water washing + lime neutralization" method. However, this traditional process has a series of technical bottlenecks that need to be solved urgently: Low reaction efficiency and long curing period: The lime neutralization reaction time is short, the reaction is not complete, and it needs to rely on natural storage curing for 7 to 15 days, which not only occupies a large amount of land, but also seriously restricts the treatment efficiency and continuous production.

[0005] Unstable treatment quality: Due to incomplete reaction and extensive control means, the quality of the treated phosphogypsum fluctuates greatly, the pH value of the leaching liquid is unstable, and it is difficult to stably meet the Class I solid waste standard or the building material application requirements, which restricts its resource utilization.

[0006] Large water consumption and system imbalance: The water washing process consumes a large amount of water, and the existing process is often disconnected from the front-end phosphoric acid production system, which fails to achieve system circulation of water resources, resulting in high fresh water consumption in the entire phosphoric acid-phosphogypsum system, continuous increase in the amount of backwater in the residue field, and new environmental risks and water balance problems.

[0007] Outstanding secondary pollution problem: The acid wastewater generated during the treatment process has low reuse rate, and part of it may cause secondary pollution; at the same time, the ammonia nitrogen gas released during the neutralization reaction process has the problem of unorganized emission, affecting the atmospheric environment of the plant and the surrounding area.

[0008] Therefore, in the face of the increasingly urgent environmental pressure and resource demand, the industry urgently needs to develop a new process which can realize efficient, stable and low-cost harmless treatment of phosphogypsum, and can be deeply coupled with the front-end phosphoric acid production system, so as to fundamentally solve the problems of water balance, material circulation and secondary pollution. SUMMARY

[0009] In view of the deficiencies of the prior art, the purpose of the present application is to provide a method and system for harmless treatment of phosphogypsum.

[0010] The scheme of the present application is as follows: A method and system for harmless treatment of phosphogypsum, comprising the following steps: 1) Re-slurry the phosphogypsum discharged from the filtration system of the wet-process phosphoric acid production to obtain a slurry with a solid content of 30-35%; 2) Deliver the slurry to a first-stage reaction tank, add fly ash and cement to the first-stage reaction tank, and stir to react; 3) The reacted slurry is overflowed through a chute to an A tank of a second-stage reaction tank, lime emulsion is added to the A tank to adjust the pH value to 10.5-11, the A tank is in turn communicated with a B tank and a C tank of the second-stage reaction tank through a chute overflow, and the slurry is reacted in the C tank to obtain a mixed material; 4) Pump the mixed material into a belt filter for solid-liquid separation to obtain harmless phosphogypsum and filtrate; 5) Deliver the filtrate to a sedimentation tank, add a flocculating agent for sedimentation, the supernatant produced by the sedimentation is used for re-slurry of phosphogypsum and preparation of lime emulsion, and the thick slurry after sedimentation is returned to the A tank of the second-stage reaction tank for reaction; 6) The waste gas produced during the reaction of the first-stage reaction tank and the second-stage reaction tank is introduced into a washing tower for washing, the washing tower uses acidic water produced by the wet-process phosphoric acid production as washing liquid, and when the pH value of the washing liquid is greater than 5.5, the washing liquid is replaced into a grinding device of the wet-process phosphoric acid production.

[0011] As a preferred technical scheme, the fly ash is added in an amount of 8-12 kg / t of wet basis phosphogypsum, and the cement is added in an amount of 1-2 kg / t of wet basis phosphogypsum in the step 2).

[0012] As a preferred technical scheme, the concentration of the lime emulsion is 4-6% in the step 3), and the pH value of the outlet of the C tank is controlled at 10-10.5.

[0013] As a preferred technical scheme, the tail gas washing replacement water is used for the secondary washing of the filtration system of the wet-process phosphoric acid production, and the process water is used for the tertiary washing; the filter cloth regeneration water of the filtration system is used for re-slurry of the phosphogypsum in the re-slurry tank; and the heat exchange mode of the filter cloth washing water is indirect heat exchange.

[0014] As a preferred technical solution, the harmless phosphogypsum is used as a filling material for ecological restoration and road base material.

[0015] This invention also discloses a system for the harmless treatment of phosphogypsum, including a re-slurry tank. The re-slurry tank is used for re-slurrying phosphogypsum produced by a wet-process phosphoric acid production filtration system. The re-slurry tank is connected to the inlet of a first-stage reaction tank via a phosphogypsum slurry pump. The top chute of the first-stage reaction tank is connected to the overflow of tank A in a second-stage reaction tank. Tanks A, B, and C of the second-stage reaction tank are sequentially connected via overflow chute. The outlet of tank C is connected to the inlet of a belt filter via a slurry pump. The filtrate outlet of the belt filter is connected to the inlet of a filtrate tank. The outlet of the filtrate tank is connected to a settling tank via a filtrate pump. The supernatant outlet of the settling tank is connected to a supernatant pump. The settling tank is connected to the inlet of the re-slurry tank and the lime slurry storage tank respectively. The discharge port of the settling tank is connected to the inlet of the A tank through a thick material pump. The top of the first-stage reaction tank and the second-stage reaction tank are equipped with air ducts. The air ducts are connected to the air inlet of the washing tower through a blower. The liquid inlet of the washing spray pipe of the washing tower is connected to the acidic slag water storage tank of the wet phosphoric acid production filtration system. The liquid outlet at the bottom of the washing tower is connected to the liquid inlet of the washing spray pipe through a washing pump. The displacement liquid outlet of the washing tower is connected to the wash water filtrate tank through a pipe. The wash water filtrate tank is connected to the water outlet of the grinding unit of the wet phosphoric acid production through a wash water filtrate pump. The lime slurry storage tank is connected to the lime slurry inlet of the A tank via a lime slurry pump.

[0016] As a preferred technical solution, the chute, the pipeline between the slurry pump and the belt filter, and the lime slurry storage tank are each equipped with a pH meter.

[0017] As a preferred technical solution, the system also includes a flocculant tank, which is connected to the feed inlet of the belt filter via a flocculant pump.

[0018] As a preferred technical solution, the secondary washing water inlet of the wet-process phosphoric acid production filtration system is connected to the extraction tail gas washing and replacement water pipeline; the filter cloth regeneration water outlet of the wet-process phosphoric acid production filtration system is connected to the reslurry tank.

[0019] As a preferred technical solution, the heat exchange method for filter cloth washing water is indirect heat exchange, which recovers steam condensate, reduces the amount of condensate carried into the phosphoric acid production filtration system, increases the water consumption in the slag yard, and achieves a steam consumption of 14-15 m³ / hour. 3 It can increase by 120,000 m² per year. 3 Water usage in the slag heap. Condensate is recycled back to sulfuric acid for reuse. Simultaneously, the amount of clean water entering the phosphogypsum treatment system is reduced.

[0020] Compared with the prior art, the advantages of the present invention are: Stable quality: By adopting a composite formula of "fly ash + cement + lime" and a multi-stage series zoned reaction process, the reaction is ensured to be full and the maturation is complete, so that the quality of the treated phosphogypsum can stably meet the Class I solid waste standard, laying a solid foundation for resource utilization.

[0021] Low cost and resource recycling: Utilizing inexpensive industrial solid waste such as fly ash and cement as part of the treatment agent reduces reagent costs. The filtrate and tail gas scrubbing liquid are all internally recycled, achieving "zero discharge" of wastewater and tiered utilization of resources.

[0022] System synergy yields significant comprehensive benefits: Through deep coupling and optimization with the upstream phosphoric acid production process, not only is the water consumption of the phosphogypsum harmless treatment system reduced, but the phosphorus yield of the entire phosphoric acid production system is also increased (by 0.51%), and high-quality steam condensate is recovered, resulting in significant economic and environmental benefits. Calculations show that the harmless treatment cost per ton of phosphogypsum can be reduced to 13.5 yuan, and electricity consumption is reduced by 4.91 kWh, resulting in comprehensive annual cost savings exceeding 2.4 million yuan.

[0023] Through process optimization, internal water circulation was achieved, effectively controlling the total water volume in the slag yard. Simultaneously, the provided "fly ash-cement-lime" formula and multi-stage reaction process ensured a stable and economical harmless treatment process. Implementation calculations showed that this process reduced the harmless treatment cost per ton of phosphogypsum to approximately RMB 13.5 (of which reagent costs were approximately RMB 5.8), reduced electricity consumption by approximately 4.91 kWh / t, and allowed for flexible adjustment of output according to demand. Furthermore, the associated optimization of the phosphoric acid filtration system (such as replacing the third-stage wash water with process water) brought an additional benefit of approximately 0.51% increase in phosphorus yield. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the system structure of the present invention.

[0026] In the diagram: 1-Re-slurry tank; 2-Phosphogypsum slurry pump; 3-First stage reaction tank; 4-Second stage reaction tank; 5-Slurry pump; 6-Belt filter; 7-Filtrate tank; 8-Filtrate pump; 9-Gypsum conveyor belt; 10-Lime slurry storage tank; 11-Fan; 12-Washing tower; 13-Washing pump; 14-Wash water filtrate tank; 15-Wash water filtrate pump; 16-Lime slurry pump; 17-Flocculant tank; 18-Flocculant pump; 19-Vacuum pump; 20-Circulating water pump; 21-Cooling tower; 22-Lime spiral conveyor belt; 23-Lime silo. Detailed Implementation

[0027] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0028] Example 1: 1) The phosphogypsum discharged from the wet-process phosphoric acid production filtration system is re-slurried to obtain a slurry with a solid content of 30-35%; 2) The slurry is transported to a first-stage reaction tank, and fly ash and cement are added to the first-stage reaction tank and stirred to react; 3) The slurry after reaction overflows through a chute into tank A of the second-stage reaction tank. Lime slurry is added to tank A to adjust the pH value to 10.5-11. Tank A is connected to tanks B and C of the second-stage reaction tank in sequence through a chute. The slurry completes the reaction in tank C to obtain a mixture. 4) The mixture is pumped into a belt filter for solid-liquid separation to obtain harmless phosphogypsum and filtrate; 5) The filtrate is sent to the settling tank, flocculant is added for settling, and the supernatant produced by settling is used for the re-slurrying of phosphogypsum and the preparation of lime slurry. The thick slurry after settling is returned to tank A of the two-stage reaction tank for reaction. 6) The waste gas generated during the reaction process in the first and second reaction tanks is introduced into the washing tower for washing. The washing tower uses acidic water produced by wet phosphoric acid production as the washing liquid. When the washing liquid is neutral, it is used to replace the grinding equipment of wet phosphoric acid production.

[0029] In step 2), the amount of fly ash added is 8 kg / t wet phosphogypsum, and the amount of cement added is 2 kg / t wet phosphogypsum.

[0030] In step 3), the concentration of lime slurry is 6%, and the pH of the outlet of tank C is controlled at 10 to 10.5.

[0031] Example 2: A system for harmlessly treating wet-process phosphogypsum includes a re-slurry tank 1 for re-slurrying phosphogypsum produced by a wet-process phosphoric acid production filtration system. The re-slurry tank 1 is connected to the inlet of a primary reaction tank 3 via a phosphogypsum slurry pump 2. The top chute of the primary reaction tank 3 is overflowed into tank A of a secondary reaction tank 4. Tanks A, B, and C of the secondary reaction tank 4 are sequentially connected via high-level chute overflow. The outlet of tank C is connected to the inlet of a belt filter 6 via a slurry pump 5. The filtrate outlet of the belt filter 6 is connected to the inlet of a filtrate tank 7. The outlet of the filtrate tank 7 is connected to a settling tank via a filtrate pump 8. The supernatant outlet of the settling tank is connected to... The inlets of the re-slurry tank 1 and the lime slurry storage tank 10 are connected. The discharge port of the settling tank is connected to the inlet of the A tank through a thick material pump. The tops of the first-stage reaction tank 3 and the second-stage reaction tank 4 are equipped with air ducts. The air ducts are connected to the air inlet of the washing tower 12 through a blower 11. The liquid inlet of the washing spray pipe of the washing tower 12 is connected to the acidic slag water storage tank of the wet phosphoric acid production filtration system. The liquid outlet at the bottom of the washing tower 12 is connected to the liquid inlet of the washing spray pipe through a washing pump 13. The displacement liquid outlet of the washing tower 12 is connected to the wash water filtrate tank 14 through a pipe. The wash water filtrate tank 14 is connected to the water inlet of the grinding device for wet phosphoric acid production through a wash water filtrate pump 15. The lime slurry storage tank 10 is connected to the lime slurry inlet of the tank A via a lime slurry pump 16.

[0032] The chute, the pipeline between the slurry pump 5 and the belt filter 6, and the lime slurry storage tank 10 are each equipped with a pH meter.

[0033] It also includes a flocculant tank 17, which is connected to the feed inlet of the belt filter 6 via a flocculant pump 18.

[0034] The secondary washing water inlet of the wet-process phosphoric acid production filtration system is connected to the tail gas washing and replacement water pipeline; the filter cloth regeneration water outlet of the wet-process phosphoric acid production filtration system is connected to the reslurry tank.

[0035] The heat exchange method for filter cloth washing water is indirect heat exchange, recovering steam condensate, reducing the amount of condensate carried into the phosphoric acid production filtration system, increasing the water consumption in the slag yard, and achieving a steam consumption of 14-15 m³ / hour. 3 It can increase by 120,000 m² per year. 3 Water usage in the slag heap. Condensate is recycled back to sulfuric acid for reuse. Simultaneously, the amount of clean water entering the phosphogypsum treatment system is reduced.

[0036] Example 3: This embodiment is based on the actual production conditions of a chemical company in Yunnan Province, and specifically implements and verifies the method and system for harmless treatment of phosphogypsum described in this invention.

[0037] 1. Implementation Method (1) Determination and implementation of harmless chemical agent formulation To address the issue of inconsistent quality, a composite formula of "fly ash + cement + lime" is adopted, with specific process control indicators as follows: Solid content of the re-slurry: controlled at 32% (range 30-35%).

[0038] Cement addition amount: Add at a fixed ratio of 2 kg / t wet phosphogypsum.

[0039] Fly ash addition amount: Add at a ratio of 10 kg / t wet phosphogypsum, using solid dry powder addition method.

[0040] Lime slurry: Prepare a lime slurry with a mass concentration of 5% and add it through a metering pump.

[0041] pH control point: The outlet of the second-stage reaction tank A: By adjusting the flow rate of lime slurry, the pH is precisely controlled at 10.8 (range 10.5-11).

[0042] The outlet of the C tank in the second stage reaction: as a monitoring indicator, it is stable at 10.3 (range 10-10.5).

[0043] Offline gypsum extract (tested by HJ 557 method): stable at 9.2-9.4 (range 9-9.5), meeting the Class I solid waste standard.

[0044] (2) Process optimization and implementation (solving problems such as long curing time and unstable quality) A new continuous harmless treatment device will be built, with the core reaction section consisting of four reaction tanks connected in series.

[0045] Process: Fresh phosphogypsum discharged from wet-process phosphoric acid filter → slurry tank (prepared into 32% slurry) → first-stage reaction tank (fly ash and cement added, stirred and reacted) → second-stage reaction tank A (5% lime slurry added, pH controlled at 10.8, reaction time 1h) → second-stage reaction tank B (no material added, reaction continues, reaction time 0.5h) → second-stage reaction tank C (no material added, final maturation and monitoring, pH=10.3, reaction time 0.5h).

[0046] Key optimizations: The setup of tanks B and C provides ample maturation time for the reaction, significantly shortening the total effective reaction time (total reaction time 2 hours), replacing the 7-15 days of natural storage maturation required by traditional processes.

[0047] An online pH meter was installed in the chute between the tanks to monitor and control the amount of lime slurry added in real time, which enabled precise and stable control of process parameters and fundamentally solved the problem of quality fluctuation.

[0048] After the reaction is complete, the slurry undergoes solid-liquid separation using a belt filter. The solid is harmless phosphogypsum, which is directly loaded onto trucks and transported to the ecological restoration site for use, while the filtrate proceeds to the next stage.

[0049] (3) Optimization and implementation of water balance system (solving high water consumption and system imbalance) To achieve water conservation and internal water resource recycling in the system, the following key modifications were made to the upstream phosphoric acid production filtration system and its supporting facilities: Upgrade of filtration and washing system: Secondary wash water: The original method of using some process water was changed to using "extraction tail gas washing and replacement water" entirely. This eliminated a portion of acidic wastewater in the system and reduced the consumption of fresh water.

[0050] The third wash water was changed from fresh water to "process water" circulated within the system. Although this change slightly increased the water-soluble phosphorus content in phosphogypsum, it significantly reduced phosphorus dilution and loss during the washing process, resulting in a 0.51% increase in the total phosphorus yield of the wet-process phosphoric acid system.

[0051] Filter cloth regeneration water: All of it is collected and introduced into the phosphogypsum resizing tank, and is no longer used as supplement for secondary washing water, which not only recovers phosphorus resources but also optimizes washing efficiency.

[0052] Filter cloth washing water system renovation: Change the heating method from direct steam heating to indirect heating using a plate heat exchanger.

[0053] Effect: Approximately 14.5m³ can be recycled per hour. 3 High-quality steam condensate. All of this condensate is returned to the sulfuric acid production unit for reuse.

[0054] Global impact: This renovation can reduce the annual area by approximately 120,000 m². 3 The external water (condensate) enters the phosphate-phosphogypsum system, significantly "opening up new sources" and freeing up capacity for more wastewater to be used in the slag yard, effectively solving the problems of system water imbalance and rising water level in the slag yard.

[0055] (4) Implementation of environmental protection collaborative treatment Each reaction tank is equipped with a gas collection duct at the top, and the ammonia nitrogen waste gas released from the reaction is uniformly drawn to the scrubbing tower by a fan.

[0056] Washing medium: Acidic water (pH<2) generated during the phosphoric acid production process is used for spray washing.

[0057] Resource recovery: When the washing liquid circulates to a pH > 5.5 (approximately 5.8 in actual measurements), it is displaced and transported to the grinding unit for phosphoric acid production as process water. This achieves compliant emissions of waste gas and additionally increases the amount of acidic water recovered, thus achieving the dual goals of "treating waste with waste" and resource recovery.

[0058] 2. Implementation Results and Data Validation After the implementation of this process, and following continuous and stable operation testing, the results are shown in Table 1 below: Table 1 3. Conclusion This embodiment fully demonstrates that the method and system for the harmless treatment of phosphogypsum provided by this invention solves the quality and curing problems through a composite formula of "fly ash-cement-lime" and a multi-stage reaction process, reconstructs the system's water balance through "front-end filtration and washing optimization and condensate recovery," and achieves environmental synergy through "tail gas acidic water washing and reuse." The entire process forms a complete solution that is efficient, stable, low-cost, and resource-recycling, successfully transforming phosphogypsum from Class II solid waste into Class I solid waste that can be directly utilized as a resource. This not only meets the stringent environmental policy requirements but also creates significant economic benefits of over 2.4 million yuan annually for enterprises, possessing extremely high promotional value.

[0059] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for the harmless treatment of phosphogypsum, characterized in that, Includes the following steps: 1) The phosphogypsum discharged from the wet-process phosphoric acid production filtration system is re-slurried to obtain a slurry with a solid content of 30-35%; 2) The slurry is transported to a first-stage reaction tank, and fly ash and cement are added to the first-stage reaction tank and stirred to react; 3) The slurry after reaction overflows through a chute into tank A of the second-stage reaction tank. Lime slurry is added to tank A to adjust the pH value to 10.5-11. Tank A is connected to tanks B and C of the second-stage reaction tank in sequence through overflow chute. The slurry completes the reaction in tank C to obtain a mixture. 4) The mixture is pumped into a belt filter for solid-liquid separation to obtain harmless phosphogypsum and filtrate; 5) The filtrate is sent to the settling tank, flocculant is added for settling, and the supernatant produced by settling is used for the re-slurrying of phosphogypsum and the preparation of lime slurry. The thick slurry after settling is returned to tank A of the two-stage reaction tank for reaction. 6) The waste gas generated during the reaction process in the first and second reaction tanks is introduced into a washing tower for washing. The washing tower uses acidic water from the wet phosphoric acid production process as the washing liquid. When the pH of the washing liquid is neutral, it is used to replace the grinding equipment of the wet phosphoric acid production process.

2. The method for harmless treatment of phosphogypsum as described in claim 1, characterized in that: In step 2), the amount of fly ash added is 8-12 kg / t wet phosphogypsum, and the amount of cement added is 1-2 kg / t wet phosphogypsum.

3. The method for harmless treatment of phosphogypsum as described in claim 1, characterized in that: In step 3), the mass concentration of lime slurry is 4-6%, and the pH of the outlet of tank C is controlled at 10-10.

5.

4. The method and system for harmless treatment of phosphogypsum as described in claim 1, characterized in that: The wet-process phosphoric acid production filtration system uses extraction tail gas washing replacement water for secondary washing and process water for tertiary washing; the filter cloth regeneration water is used for phosphogypsum resizing in the resizing tank; and the heat exchange method for the filter cloth washing water is indirect heat exchange.

5. The method for harmless treatment of phosphogypsum as described in claim 1, characterized in that: The harmless phosphogypsum is used as a filling material for ecological restoration and road base materials.

6. A system for the harmless treatment of phosphogypsum, characterized in that: The system includes a re-slurry tank for re-slurrying phosphogypsum produced by the wet-process phosphoric acid production filtration system. The re-slurry tank is connected to the inlet of a first-stage reaction tank via a phosphogypsum slurry pump. The top chute of the first-stage reaction tank is connected to the overflow of tank A of the second-stage reaction tank. Tanks A, B, and C of the second-stage reaction tank are connected sequentially via overflow chute. The outlet of tank C is connected to the feed inlet of a belt filter via a slurry pump. The filtrate outlet of the belt filter is connected to the filtrate tank inlet, the filtrate tank outlet is connected to the sedimentation tank via a filtrate pump, the supernatant outlet of the sedimentation tank is connected to the inlet of the re-slurry tank and the lime slurry storage tank via a supernatant pump, and the discharge outlet of the sedimentation tank is connected to the inlet of the A tank via a thick material pump. Both the first-stage and second-stage reaction tanks are equipped with air ducts at their tops. The air ducts are connected to the air inlet of the washing tower via a fan. The liquid inlet of the washing spray pipe of the washing tower is connected to the acidic slag water storage tank of the wet-process phosphoric acid production filtration system. The liquid outlet at the bottom of the washing tower is connected to the liquid inlet of the washing spray pipe via a washing pump. The displacement liquid outlet of the washing tower is connected to the wash water filtrate tank via a pipe. The wash water filtrate tank is connected to the water outlet of the grinding unit in the wet-process phosphoric acid production via a wash water filtrate pump. The lime slurry storage tank is connected to the lime slurry inlet of the A tank via a lime slurry pump.

7. The system for harmless treatment of phosphogypsum as described in claim 6, characterized in that: The chute, the pipeline between the slurry pump and the belt filter, and the lime slurry storage tank are each equipped with a pH meter.

8. The system for harmless treatment of phosphogypsum as described in claim 6, characterized in that: It also includes a flocculant tank, which is connected to the feed inlet of the belt filter via a flocculant pump.

9. The system for harmless treatment of phosphogypsum as described in claim 6, characterized in that: The secondary washing water inlet of the wet phosphoric acid production filtration system is connected to the extraction tail gas washing and replacement water pipeline; the filter cloth regeneration water outlet of the wet phosphoric acid production filtration system is connected to the reslurry tank.

10. The system for harmless treatment of phosphogypsum as described in claim 6, characterized in that: The heat exchange method for the filter cloth washing water is indirect heat exchange.

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