A targeted step-by-step defluorination and fluorine resource recycling closed-loop treatment method for phosphogypsum

Abstract

The application discloses a method for phosphogypsum stepwise deep defluorination and fluorine resource recovery, which comprises the following steps: firstly, washing the phosphogypsum to remove soluble fluorine; then, treating the washed phosphogypsum by using an ultrasonic reinforced ferric sulfate-citric acid composite complexing system; the citric acid promotes the dissolution of the insoluble CaF2 and the complexing migration of fluorine through the double complexation with Fe3+ and Ca2+, and the ultrasonic cavitation effect greatly strengthens the mass transfer process; then, acid washing and water washing are performed to remove residual fluorine and impurities, and finally, deep defluorination is realized. The method has the advantages of high defluorination efficiency, resource recovery and environmental friendliness, and provides a reliable technical path for high-value utilization of the phosphogypsum.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization and pollution control technology, specifically a targeted cascade defluorination and closed-loop treatment method for phosphogypsum and fluorine resource recovery. Background Technology

[0002] Phosphogypsum is a large-scale industrial solid waste generated during the wet-process phosphoric acid production process, producing approximately 4.5-5 tons of phosphogypsum for every ton of phosphoric acid produced. The main component of phosphogypsum is calcium sulfate dihydrate (CaSO4·2H2O), typically comprising 70%–90% of its composition. It also contains small amounts of calcium sulfate hemihydrate, anhydrous calcium sulfate, and various phosphorus and fluorine compounds, organic impurities, and other insoluble compounds. Global phosphogypsum stockpiles are enormous and continue to grow at a rate of tens of millions of tons annually. The long-term storage of such massive amounts of phosphogypsum not only occupies vast amounts of land resources but also poses a potential pollution risk to water bodies, soil, and the atmosphere. In particular, the slow leaching of fluorides poses a serious threat to the ecological environment and human health.

[0003] Achieving large-scale resource utilization of phosphogypsum is essential to mitigating its environmental risks and promoting the sustainable development of the phosphate chemical industry. However, the high content of soluble and insoluble fluorides in phosphogypsum is a key bottleneck restricting its safe utilization. These fluoride impurities not only severely degrade the performance of phosphogypsum-based building materials (such as cement retarders and gypsum boards), leading to reduced strength and abnormal setting, but also slowly release fluorides during product use, causing indoor air pollution or long-term environmental toxicity. Therefore, developing efficient and economical deep defluorination technology is a core prerequisite for transforming phosphogypsum from "waste" into "resource".

[0004] Currently, research on fluoride removal from phosphogypsum mainly focuses on single methods, such as water washing to remove soluble fluoride, or precipitation / complexation using aluminum or iron salts. However, these methods generally have limitations: water washing is ineffective at removing lattice fluoride in the form of sparingly soluble calcium fluoride (CaF2); and conventional iron salt complexation methods are prone to deactivation due to premature hydrolysis of Fe³⁺ under acidic conditions, forming hydroxide precipitates (i.e., "iron sludge"), resulting in unstable defluorination efficiency, high reagent consumption, and the potential introduction of new impurities. Furthermore, existing technologies are mostly "end-of-pipe" removal methods, focusing only on the purification of the phosphogypsum itself while neglecting the secondary pollution problem of fluoride-containing wastewater generated during the defluorination process, failing to form a complete green technology closed loop. Summary of the Invention

[0005] Based on the deficiencies in the existing technology, the purpose of this invention is to provide a method for tiered deep defluorination and fluorine resource recovery from phosphogypsum, thereby solving the problems of low removal efficiency of sparingly soluble fluoride, easy deactivation of iron salts, high reagent consumption, and serious secondary pollution in existing phosphogypsum defluorination technologies. This invention achieves its objective through the following technical solution: A targeted, cascaded defluorination and closed-loop fluorine resource recovery method for phosphogypsum includes the following steps: Step 1: Pre-washing to remove soluble fluorides and impurities: Add phosphogypsum to deionized water, mix, shake and wash, filter, collect the filtrate, and retain the filter residue as the pre-washed phosphogypsum. Step 2, Complexation of sparingly soluble fluoride: To the pre-washed phosphogypsum obtained in Step 1, a composite solution consisting of 0.125-0.375 M ferric sulfate solution and 0.04-0.06 M citric acid solution mixed at a volume ratio of 2:1 is added to obtain a mixture. The solid-liquid ratio of the pre-washed phosphogypsum to the composite solution is 1:3 g / mL. The mixture is ultrasonically reacted at 40-60℃ for 20-40 min at an ultrasonic frequency of 40-60 Hz. After the reaction, the mixture is filtered to separate the filter residue and the filtrate. The resulting filtrate is the complexation waste liquid. Step 3: Recover fluoride ions from the complexed waste liquid obtained in Step 2 to obtain calcium fluoride; Step 4: Acid washing to deeply remove insoluble fluorides: Add the filter residue obtained in step 2 to a 10%-30% sulfuric acid solution at a solid-liquid ratio of 1:3 g / mL, and stir magnetically for 1 hour at 55-75℃ and 300-400 r / min. After the reaction, filter the solution, collect the filtrate, and retain the filter residue. Step 5, Secondary washing: Add deionized water to the filter residue obtained in Step 4 at a solid-liquid ratio of 1:3 g / mL, stir and wash for 0.5~1.5h, filter, collect the filtrate, and obtain the treated phosphogypsum filter residue. Step 6: Air dry the phosphogypsum filter residue; Step 7: Reuse the filtrate from Step 5 for the shaking and washing in Step 1.

[0006] Further optimization, the specific operation of step one is as follows: take a certain mass of fresh phosphogypsum, add deionized water according to the solid-liquid ratio of 1:4-1:2 g / mL, and shake in a vortex shaker at a speed of 110-160 r / min for 1 hour; after the reaction, filter the mixture, collect the filtrate, and retain the filter residue.

[0007] Further optimization involves step two, in which the mixture is subjected to ultrasonic reaction at 50°C for 30 min at an ultrasonic frequency of 50 Hz.

[0008] Further optimization involves the following steps: slowly adding quicklime slurry to the complexed waste liquid obtained in step two, stirring the reaction, adjusting the pH to 10-11, so that fluoride ions precipitate in the form of calcium fluoride; after standing, filtering, washing, and drying the precipitate to obtain the calcium fluoride product.

[0009] Furthermore, in step four: add a 20% sulfuric acid solution and react with magnetic stirring at 65℃ and 350 r / min for 1 hour.

[0010] Furthermore, in step five, the specific operation of the second washing is as follows: add deionized water at a solid-liquid ratio of 1:3 and g / mL, and stir and wash for 1 hour.

[0011] The innovation of this invention lies in: 1. Innovative Mechanism: A synergistic and targeted fluoride removal mechanism of "citric acid and ferric sulfate" through "complexation-hydrolysis" is introduced. Citric acid forms a stable and soluble complex with Fe³⁺, effectively inhibiting its premature hydrolysis and ensuring that Fe³⁺ fully reacts with sparingly soluble CaF₂ to generate a soluble fluoride-iron complex. Simultaneously, the complexation effect of citric acid on Ca²⁺ further promotes the shift of the CaF₂ dissolution equilibrium, achieving highly efficient extraction of lattice fluoride.

[0012] 2. Process Intensification: Innovatively, ultrasonic field-thermal field coupling technology is used to replace traditional stirring. The ultrasonic cavitation effect can powerfully destroy the fluoride coating layer on the surface of phosphogypsum particles, greatly enhancing the solid-liquid mass transfer efficiency, thereby significantly improving the defluorination reaction rate and depth, and reducing the reaction temperature and time.

[0013] 3. Innovative Process Design: A "three-stage targeted, step-by-step deep defluorination" process based on fluorine form transformation was established. This process precisely targets fluorine in different states (soluble fluorine, adsorbed fluorine, sparingly soluble CaF2, etc.) in phosphogypsum, achieving precise defluorination from coarse to fine, step-by-step refinement through three sequential steps: "physical water washing," "chemical complexation," and "acid washing and refining," avoiding ineffective consumption of reagents. Experimental results show that this process can reduce water-soluble fluorine in phosphogypsum by up to 99.62% and total fluorine content by up to 96.42%.

[0014] 4. Green Circular Innovation: A green closed-loop technology for "waste liquid resource utilization and recycling" has been developed. On the one hand, lime is added to high-concentration fluorine-containing complexed waste liquid to recover high-purity CaF2 (fluorite), realizing the transformation of harmful component fluorine into valuable products; on the other hand, the end-of-pipe washing water is reused in the front-end process, constructing an internal cycle of water resources, reducing fresh water consumption and wastewater discharge from the source, and truly practicing the green concept of "zero waste" and resource recycling.

[0015] The beneficial effects of this invention are as follows: 1) Precise and efficient process: A three-stage targeted defluorination process of "water washing-complexing-acid washing" was constructed to remove different forms of fluoride step by step, with defluorination rates of water-soluble fluoride and total fluoride as high as 99.62% and 96.42%, respectively.

[0016] 2) Unique mechanism: Utilizing the "iron-citric acid" synergistic complexation mechanism, it effectively inhibits the formation of iron sludge and significantly improves the extraction efficiency of lattice fluorine.

[0017] 3) Process enhancement: Introduce ultrasonic-thermal field coupling technology to shorten reaction time and improve defluorination depth.

[0018] 4) Green Recycling: This invention enables the precipitation and recovery of high-purity CaF2 (fluorite) products from fluoride-containing wastewater, and the reuse of end-of-pipe washing water in upstream processes, forming a green closed-loop system of wastewater resource utilization and water resource recycling, fundamentally reducing the discharge of waste gas, wastewater, and solid waste. This invention combines the advantages of high defluorination efficiency, resource recovery, and environmental friendliness, providing a reliable technical path for the high-value utilization of phosphogypsum. Attached Figure Description

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0020] Figure 1 This is a process flow diagram of the present invention; Figure 2 SEM images of phosphogypsum raw material and defluorinated phosphogypsum from Example 1; Figure 3 SEM images of phosphogypsum raw material and defluorinated phosphogypsum from Example 2; Figure 4 The images show SEM images of the phosphogypsum raw material and defluorinated phosphogypsum from Example 2. Detailed Implementation

[0021] The method of this invention first washes phosphogypsum with water to remove soluble fluoride. Then, it innovatively employs an ultrasonically enhanced ferric sulfate-citric acid composite complexing system to treat the washed phosphogypsum. Citric acid, through its dual complexing effect with Fe³⁺ and Ca²⁺, significantly promotes the dissolution of insoluble CaF₂ and the complexation and migration of fluoride. Simultaneously, the ultrasonic cavitation effect greatly enhances the mass transfer process. Following this, acid washing and water washing are performed to remove residual fluoride and impurities, ultimately achieving deep defluorination. Specific examples are as follows.

[0022] Example 1: A targeted, cascaded defluorination and closed-loop fluorine resource recovery method for phosphogypsum includes the following steps: Step 1: Pre-washing to remove soluble fluorides and impurities: Take a certain mass of fresh phosphogypsum and add deionized water at a solid-liquid ratio of 1:4 (g / mL). Shake in a vortex shaker at 160 r / min for 1 hour to remove soluble fluorides and some impurities. After the reaction, filter the mixture and collect the filtrate (referred to as "first wash waste liquid"), retaining the filter residue (phosphogypsum after pre-washing).

[0023] Step 2: Complexation of sparingly soluble fluorides: A composite solution consisting of 0.125 M ferric sulfate solution and 0.04 M citric acid solution mixed in a volume ratio of 2:1 is added to the filter residue obtained in Step 1. The solid-liquid ratio of the pre-washed phosphogypsum to the composite solution is controlled at 1:3 (g / mL). The mixture is placed in an ultrasonic reactor and reacted at 60℃ for 40 min with an ultrasonic frequency of 60 Hz to complex and convert the sparingly soluble fluorides. After the reaction, the mixture is filtered to separate the filter residue (defluorinated phosphogypsum) and the filtrate (referred to as "complexation waste liquid").

[0024] Step 3: Recovering fluoride ions from the complexed waste liquid to obtain calcium fluoride: Slowly add quicklime (CaO) slurry to the complexed waste liquid obtained in Step 2, stir the reaction, and adjust the pH to alkaline (pH 10-11) to precipitate fluoride ions as calcium fluoride (CaF2). After standing, filter, wash, and dry the precipitate to obtain high-purity calcium fluoride (fluorite) product.

[0025] Step 4: Acid washing for deep removal of insoluble fluorides: The defluorinated phosphogypsum filter residue obtained in Step 2 is added to a 30% sulfuric acid solution at a solid-liquid ratio of 1:3 (g / mL). The mixture is stirred on a magnetic stirrer at 75℃ and 400 r / min for 1 hour to deeply remove insoluble fluorides. After the reaction, the mixture is filtered, and the filtrate is collected (referred to as "acid washing waste liquid"), while the filter residue (acid-washed phosphogypsum) is retained.

[0026] Step 5, Secondary Washing: Add deionized water to the acid-washed phosphogypsum filter residue obtained in Step 4 at a solid-liquid ratio of 1:3 (g / mL), and stir and wash for 1.5 h to remove residual acid and soluble salts. After filtration, collect the filtrate (referred to as "secondary washing waste liquid") to obtain the final treated phosphogypsum filter residue.

[0027] Step 6: Air drying of phosphogypsum filter residue: The final phosphogypsum filter residue obtained in step 5 is placed in an air drying chamber for natural air drying. Then, its water-soluble fluoride and total fluoride content are determined according to relevant national standards (such as HJ 873-2017) to verify the defluorination effect of this method.

[0028] Step 7: Reuse the washing water (filtrate) from Step 5 for the shaking water washing in Step 1 to achieve water resource recycling.

[0029] The results showed that, after measuring the water-soluble fluorine and total fluorine in the phosphogypsum in step six, it was finally concluded that, compared with the raw phosphogypsum, the water-soluble fluorine in the treated phosphogypsum was reduced by 95.73% and the total fluorine was reduced by 92.54%.

[0030] Example 2: A targeted, cascaded defluorination and closed-loop fluorine resource recovery method for phosphogypsum includes the following steps: Step 1: Pre-washing to remove soluble fluorides and impurities: Take a certain mass of phosphogypsum raw material, add deionized water at a solid-liquid ratio of 1:3 (g / mL), and shake in a vortex shaker at 140 r / min for 1 hour to remove soluble fluorides and some impurities. After the reaction, filter the mixture, collect the filtrate (referred to as "first wash waste liquid"), and retain the filter residue (phosphogypsum after pre-washing).

[0031] Step 2: Complexation of sparingly soluble fluorides: A composite solution consisting of 0.25M ferric sulfate solution and 0.05M citric acid solution mixed in a volume ratio of 2:1 is added to the filter residue obtained in Step 1. The solid-liquid ratio of the pre-washed phosphogypsum to the composite solution is controlled at 1:3 (g / mL). The mixture is placed in an ultrasonic reactor and reacted at 50℃ for 30 min at an ultrasonic frequency of 50 Hz to complex and convert the sparingly soluble fluorides. After the reaction, the mixture is filtered to separate the filter residue (defluorinated phosphogypsum) and the filtrate (referred to as "complexation waste liquid").

[0032] Step 3: Recovering fluoride ions from the complexed waste liquid to obtain calcium fluoride: Slowly add quicklime (CaO) slurry to the complexed waste liquid obtained in Step 2, stir the reaction, and adjust the pH to alkaline (pH 10-11) to precipitate fluoride ions as calcium fluoride (CaF2). After standing, filter, wash, and dry the precipitate to obtain high-purity calcium fluoride (fluorite) product.

[0033] Step 4: Acid washing for deep removal of insoluble fluorides: The defluorinated phosphogypsum filter residue obtained in Step 2 is added to a 20% sulfuric acid solution at a solid-liquid ratio of 1:3 (g / mL). The mixture is stirred on a magnetic stirrer at 65℃ and 350 r / min for 1 hour to deeply remove insoluble fluorides. After the reaction, the mixture is filtered, and the filtrate is collected (referred to as "acid washing waste liquid"), while the filter residue (acid-washed phosphogypsum) is retained.

[0034] Step 5, Secondary Washing: Add deionized water to the acid-washed phosphogypsum filter residue obtained in Step 4 at a solid-liquid ratio of 1:3 (g / mL), and stir and wash for 1 hour to remove residual acid and soluble salts. After filtration, collect the filtrate (referred to as "secondary washing waste liquid") to obtain the final treated phosphogypsum filter residue.

[0035] Step 6: Air drying of phosphogypsum filter residue: The final phosphogypsum filter residue obtained in step 5 is placed in an air drying chamber for natural air drying. Then, its water-soluble fluoride and total fluoride content are determined according to relevant national standards (such as HJ 873-2017) to verify the defluorination effect of this method.

[0036] Step 7: Reuse the washing water (filtrate) from Step 5 for the shaking water washing in Step 1 to achieve water resource recycling.

[0037] The results showed that, after measuring the water-soluble fluorine and total fluorine in the phosphogypsum in step six, it was finally concluded that, compared with the raw phosphogypsum, the water-soluble fluorine in the treated phosphogypsum was reduced by 99.62% and the total fluorine was reduced by 96.42%.

[0038] Example 3: A targeted, cascaded defluorination and closed-loop fluorine resource recovery method for phosphogypsum includes the following steps: Step 1: Pre-washing to remove soluble fluorides and impurities: Take a certain mass of fresh phosphogypsum and add deionized water at a solid-liquid ratio of 1:2 (g / mL). Shake in a vortex shaker at 110 r / min for 1 hour to remove soluble fluorides and some impurities. After the reaction, filter the mixture and collect the filtrate (referred to as "first wash waste liquid"), retaining the filter residue (phosphogypsum after pre-washing).

[0039] Step 2: Complexation of sparingly soluble fluorides: A composite solution consisting of 0.375M ferric sulfate solution and 0.06M citric acid solution mixed in a volume ratio of 2:1 is added to the filter residue obtained in Step 1. The solid-liquid ratio of the pre-washed phosphogypsum to the composite solution is controlled at 1:3 (g / mL). The mixture is placed in an ultrasonic reactor and reacted at 40℃ for 20 min at an ultrasonic frequency of 40 Hz to complex and convert the sparingly soluble fluorides. After the reaction, the mixture is filtered to separate the filter residue (defluorinated phosphogypsum) and the filtrate (referred to as "complexation waste liquid").

[0040] Step 3: Recovering fluoride ions from the complexed waste liquid to obtain calcium fluoride: Slowly add quicklime (CaO) slurry to the complexed waste liquid obtained in Step 2, stir the reaction, and adjust the pH to alkaline (pH 10-11) to precipitate fluoride ions as calcium fluoride (CaF2). After standing and aging, filter, wash, and dry the precipitate to obtain high-purity calcium fluoride (fluorite) product.

[0041] Step 4: Acid washing for deep removal of insoluble fluorides: The defluorinated phosphogypsum filter residue obtained in Step 2 is added to a 10% sulfuric acid solution at a solid-liquid ratio of 1:3. The mixture is stirred on a magnetic stirrer at 55°C and 300 r / min for 1 hour to deeply remove insoluble fluorides. After the reaction, the mixture is filtered, and the filtrate is collected (referred to as "acid washing waste liquid"), while the filter residue (acid-washed phosphogypsum) is retained.

[0042] Step 5, Secondary Washing: Add deionized water to the acid-washed phosphogypsum filter residue obtained in Step 4 at a solid-liquid ratio of 1:3 (g / mL), and stir and wash for 0.5 h to remove residual acid and soluble salts. After filtration, collect the filtrate (referred to as "secondary washing waste liquid") to obtain the final treated phosphogypsum filter residue.

[0043] Step 6: Air drying of phosphogypsum filter residue: The final phosphogypsum filter residue obtained in step 5 is placed in an air drying chamber for natural air drying. Then, its soluble fluoride and total fluoride content are determined according to relevant national standards (such as HJ 873-2017) to verify the defluorination effect of this method.

[0044] Step 7: Reuse the washing water (filtrate) from Step 5 for the shaking water washing in Step 1 to achieve water resource recycling.

[0045] The results showed that, after measuring the water-soluble fluorine and total fluorine in the phosphogypsum in step six, it was finally concluded that, compared with the raw phosphogypsum, the water-soluble fluorine in the treated phosphogypsum was reduced by 93.25% and the total fluorine was reduced by 90.07%.

[0046] Comparative Example 1: This comparative example differs from Example 2 only in that citric acid solution was not added in step two; all other steps are the same as in Example 2. Step two is performed as follows: Step 2: Add 0.25 M ferric sulfate solution to the filter residue obtained in Step 1, controlling the solid-liquid volume ratio of the pre-washed phosphogypsum to the ferric sulfate solution to be 1:3 (g / mL). Place the mixture in an ultrasonic reactor and react at 50℃ for 30 min with the ultrasonic frequency set to 50 Hz to precipitate and transform the sparingly soluble fluoride. After the reaction is complete, perform vacuum filtration to separate the filter residue (defluorinated phosphogypsum) and the filtrate.

[0047] The results showed that, after measuring the water-soluble fluorine and total fluorine in the phosphogypsum in step six, it was finally concluded that, compared with the raw phosphogypsum, the water-soluble fluorine in the treated phosphogypsum was reduced by 98.21% and the total fluorine was reduced by 86.53%.

[0048] Comparative Example 2: This comparative example differs from Example 2 only in step two, where ultrasound is not used; instead, oscillation is employed. The other steps are the same as in Example 2. Step two is performed as follows: Step 2: Add a composite solution, consisting of a 0.25M ferric sulfate solution and a 0.05M citric acid solution mixed in a volume ratio of 2:1, to the filter residue obtained in Step 1. Maintain the solid-liquid ratio of the pre-washed phosphogypsum to the composite solution at 1:3 (g / mL). Place the mixture in a cyclotron shaker and shake at 110 r / min for 30 min at room temperature to complex and convert the sparingly soluble fluorides. After the reaction is complete, perform vacuum filtration to separate the filter residue (defluorinated phosphogypsum) and the filtrate (referred to as "complexed waste liquid").

[0049] The results showed that, after measuring the water-soluble fluorine and total fluorine in the phosphogypsum in step six, it was finally concluded that, compared with the raw phosphogypsum, the water-soluble fluorine in the treated phosphogypsum was reduced by 98.88% and the total fluorine was reduced by 87.67%.

[0050] Comparative Example 3: Compared with Example 2, this comparative example lacks two steps: step one (pre-washing) and step seven. The specific operation of this comparative example is as follows: Step 1: Take a certain mass of phosphogypsum raw material and add it to a composite solution made of 0.25M ferric sulfate solution and 0.05M citric acid solution in a volume ratio of 2:1, controlling the solid-liquid ratio to be 1:3 (g / mL). Place the mixture in an ultrasonic reactor and react at 50℃ for 30 min with the ultrasonic frequency set to 50 Hz to complex and convert the sparingly soluble fluoride. After the reaction is complete, perform vacuum filtration to separate the filter residue (defluorinated phosphogypsum) and the filtrate (referred to as "complexation waste liquid").

[0051] Step 2: Slowly add quicklime (CaO) slurry to the complexed waste liquid obtained in Step 1, stir the reaction, and adjust the pH to alkaline (pH 10-11) to precipitate fluoride ions as calcium fluoride (CaF2). After standing and aging, filter, wash, and dry the precipitate to obtain high-purity calcium fluoride (fluorite) product.

[0052] Step 3: Add the defluorinated phosphogypsum filter residue obtained in Step 2 to a 20% sulfuric acid solution at a solid-liquid ratio of 1:3 (g / mL). Stir the mixture on a magnetic stirrer at 65℃ and 350 r / min for 1 hour to thoroughly remove the insoluble fluorides. After the reaction, filter the mixture and collect the filtrate (referred to as "acid washing waste liquid"), retaining the filter residue (acid-washed phosphogypsum).

[0053] Step 4: Add deionized water to the acid-washed phosphogypsum residue obtained in Step 3 at a solid-liquid ratio of 1:3 (g / mL), and stir and wash for 1 hour to remove residual acid and soluble salts. After filtration, collect the filtrate to obtain the final treated phosphogypsum residue.

[0054] Step 5: Place the final phosphogypsum filter residue obtained in Step 4 into an air-drying chamber for natural air drying, and then determine its water-soluble fluoride and total fluoride content according to relevant national standards (such as HJ 873-2017) to verify the defluorination effect of this method.

[0055] The results showed that, after measuring the water-soluble fluorine and total fluorine in the phosphogypsum in step five, it was finally concluded that, compared with the raw phosphogypsum, the water-soluble fluorine in the treated phosphogypsum was reduced by 86.43% and the total fluorine was reduced by 90.22%.

[0056] Comparative Example 4: Compared with Example 2, this comparative example lacks steps two and three. The specific operation of this comparative example is as follows: Step 1: Take a certain mass of fresh phosphogypsum and add deionized water at a solid-liquid ratio of 1:3 (g / mL). Shake the mixture in a vortex mixer at 140 r / min for 1 hour to remove soluble fluorides and some impurities. After the reaction, filter the mixture and collect the filtrate (referred to as "first wash waste liquid"), retaining the filter residue (pre-washed phosphogypsum).

[0057] Step 2: Add the phosphogypsum filter residue obtained in Step 1 to a 20% sulfuric acid solution at a solid-liquid ratio of 1:3 (g / mL). Stir the mixture on a magnetic stirrer at 65℃ and 350 r / min for 1 hour to thoroughly remove the insoluble fluoride. After the reaction, filter the mixture and collect the filtrate (referred to as "acid washing waste liquid"), retaining the filter residue (acid-washed phosphogypsum).

[0058] Step 3: Add deionized water to the acid-washed phosphogypsum filter residue obtained in Step 2 at a solid-liquid ratio of 1:3 (g / mL), and stir and wash for 1 hour to remove residual acid and soluble salts. After filtration, collect the filtrate (referred to as "secondary washing waste liquid") to obtain the final treated phosphogypsum filter residue.

[0059] Step 4: Place the final phosphogypsum filter residue obtained in Step 3 into an air-drying chamber for natural air drying, and then determine its water-soluble fluoride and total fluoride content according to relevant national standards (such as HJ 873-2017) to verify the defluorination effect of this method.

[0060] Step 5: Reuse the washing water from Step 3 for the oscillating water washing in Step 1 to achieve water resource recycling.

[0061] The results showed that, after measuring the water-soluble fluorine and total fluorine in the phosphogypsum in step four, it was finally concluded that, compared with the raw phosphogypsum, the water-soluble fluorine in the treated phosphogypsum was reduced by 98.79% and the total fluorine was reduced by 81.65%.

[0062] Comparative Example 5: Compared to Example 2, this comparative example lacks step four, the acid washing step. The specific operation of this comparative example is as follows: Step 1: Take a certain mass of phosphogypsum raw material and add deionized water at a solid-liquid ratio of 1:3 (g / mL). Shake the mixture in a vortex shaker at 140 r / min for 1 hour to remove soluble fluorides and some impurities. After the reaction, filter the mixture and collect the filtrate (referred to as "first washing waste liquid"), retaining the filter residue (pre-washed phosphogypsum).

[0063] Step 2: Add a composite solution, consisting of a 0.25 M ferric sulfate solution and a 0.05 M citric acid solution mixed in a volume ratio of 2:1, to the filter residue obtained in Step 1. Maintain the solid-liquid ratio of the pre-washed phosphogypsum to the composite solution at 1:3 (g / mL). Place the mixture in an ultrasonic reactor and react at 50°C for 30 minutes with an ultrasonic frequency of 50 Hz to complex and convert the sparingly soluble fluorides. After the reaction, perform vacuum filtration to separate the filter residue (defluorinated phosphogypsum) and the filtrate (referred to as "complexed waste liquid").

[0064] Step 3: Slowly add quicklime (CaO) slurry to the complexed waste liquid obtained in Step 2, stir the reaction, and adjust the pH to alkaline (pH 10-11) to precipitate fluoride ions as calcium fluoride (CaF2). After standing, filter, wash, and dry the precipitate to obtain high-purity calcium fluoride (fluorite) product.

[0065] Step 4: Add the complexed phosphogypsum filter residue obtained in Step 2 to deionized water at a solid-liquid ratio of 1:3 (g / mL), and stir and wash for 1 hour. After filtration, collect the filtrate (referred to as "secondary washing waste liquid") to obtain the final treated phosphogypsum filter residue.

[0066] Step 5: Place the final phosphogypsum filter residue obtained in Step 4 into an air-drying chamber for natural air drying, and then determine its water-soluble fluoride and total fluoride content according to relevant national standards (such as HJ 873-2017) to verify the defluorination effect of this method.

[0067] Step 6: Reuse the washing water from Step 4 for the oscillating water washing in Step 1 to achieve water resource recycling.

[0068] The results showed that, after measuring the water-soluble fluorine and total fluorine in the phosphogypsum in step five, it was finally concluded that, compared with the raw phosphogypsum, the water-soluble fluorine in the treated phosphogypsum was reduced by 97.89% and the total fluorine was reduced by 80.65%.

[0069] Finally, it should be noted that the above description is only used to illustrate the technical solutions of the present invention and is not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention.

Claims

1. A targeted, cascaded defluorination and closed-loop fluorine resource recovery method for phosphogypsum, characterized in that: Includes the following steps, Step 1: Pre-washing to remove soluble fluorides and impurities: Add phosphogypsum to deionized water, mix, shake and wash, filter, collect the filtrate, and retain the filter residue as the pre-washed phosphogypsum. Step 2, Complexation of sparingly soluble fluoride: To the pre-washed phosphogypsum obtained in Step 1, a composite solution consisting of 0.125-0.375 M ferric sulfate solution and 0.04-0.06 M citric acid solution mixed at a volume ratio of 2:1 is added to obtain a mixture. The solid-liquid ratio of the pre-washed phosphogypsum to the composite solution is 1:3 g / mL. The mixture is ultrasonically reacted at 40-60℃ for 20-40 min at an ultrasonic frequency of 40-60 Hz. After the reaction, the mixture is filtered to separate the filter residue and the filtrate. The resulting filtrate is the complexation waste liquid. Step 3: Recover fluoride ions from the complexed waste liquid obtained in Step 2 to obtain calcium fluoride; Step 4: Acid washing to deeply remove insoluble fluorides: Add the filter residue obtained in step 2 to a 10%-30% sulfuric acid solution at a solid-liquid ratio of 1:3 g / mL, and stir magnetically for 1 hour at 55-75℃ and 300-400 r / min. After the reaction, filter the solution, collect the filtrate, and retain the filter residue. Step 5, Secondary washing: Add deionized water to the filter residue obtained in Step 4 at a solid-liquid ratio of 1:3 g / mL, stir and wash for 0.5~1.5 h, filter, collect the filtrate, and obtain the treated phosphogypsum filter residue. Step 6: Air dry the phosphogypsum filter residue; Step 7: Reuse the filtrate from Step 5 for the shaking and washing in Step 1.

2. The targeted cascade defluorination and fluorine resource recovery closed-loop treatment method for phosphogypsum according to claim 1, characterized in that: The specific operation of step one is as follows: Take a certain mass of fresh phosphogypsum, add deionized water according to the solid-liquid ratio of 1:4-1:2 g / mL, and shake in a vortex shaker at a speed of 110-160 r / min for 1 hour; after the reaction, filter the mixture, collect the filtrate, and retain the filter residue.

3. The targeted cascade defluorination and fluorine resource recovery closed-loop treatment method for phosphogypsum according to claim 1, characterized in that: In step two, the mixture is subjected to ultrasonic reaction at 50°C for 30 min at an ultrasonic frequency of 50 Hz.

4. The targeted cascade defluorination and fluorine resource recovery closed-loop treatment method for phosphogypsum according to claim 1, characterized in that: The specific operation of step three is as follows: slowly add quicklime slurry to the complexed waste liquid obtained in step two, stir the reaction, adjust the pH to 10-11, so that fluoride ions precipitate in the form of calcium fluoride; after standing, filter, wash and dry the precipitate to obtain calcium fluoride product.

5. The targeted cascade defluorination and closed-loop fluorine resource recovery method for phosphogypsum according to claim 1, characterized in that: In step four: add a 20% sulfuric acid solution and stir magnetically for 1 hour at 65℃ and 350 r / min.

6. The targeted cascade defluorination and fluorine resource recovery closed-loop treatment method for phosphogypsum according to claim 1, characterized in that: In step five, deionized water was added at a solid-liquid ratio of 1:3 and g / mL, and the mixture was stirred and washed for 1 hour.

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